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Study and Modeling of the Kinetics of the Photocatalytic Destruction of Stearic Acid Islands on TiO2 Films
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C: Physical Properties of Materials and Interfaces

Study and Modeling of the Kinetics of the Photocatalytic Destruction of Stearic Acid Islands on TiO2 Films
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The Journal of Physical Chemistry C

Cite this: J. Phys. Chem. C 2023, 127, 25, 12194–12205
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https://doi.org/10.1021/acs.jpcc.3c00952
Published June 20, 2023

Copyright © 2023 The Authors. Published by American Chemical Society. This publication is licensed under

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Abstract

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The kinetics of the removal of stearic acid (SA) islands by photocatalytic coatings is controversial, with some reporting that the islands fade as their thickness, h, decreases with the irradiation time, t, but maintain a constant area, a, −da/dt = 0, and others reporting that −dh/dt = 0 and −da/dt = −constant, i.e., the islands shrink, rather than fade. This study attempts to understand the possible cause for these two very different observations through a study of the destruction of a cylindrical SA island and an array of such islands, on two different photocatalytic films, namely, Activ self-cleaning glass, and a P25 TiO2 coating on glass, which have established uniform and heterogeneous surface activities, respectively. In both cases, using optical microscopy and profilometry, it is shown that, irrespective of whether there is as a single cylindrical island or an array of islands, h decreases uniformly with t, −dh/dt = constant, and −da/dt = 0, so that the SA islands just fade. However, in a study of the photocatalyzed removal of SA islands with a volcano-shaped profile, rather than that of a cylinder, it is found that the islands shrink and fade. A simple 2D kinetic model is used to rationalize the results reported in this work. Possible reasons for the two very different kinetic behaviors are discussed. The relevance of this work to self-cleaning photocatalytic films is discussed briefly.

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Published as part of The Journal of Physical Chemistry virtual special issue “Michael R. Hoffmann Festschrift”.

1. Introduction

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At present, the biggest commercial market for photocatalytic products is self-cleaning glass, (1−3) the active component of which is a thin, ca. 15 nm thick, compact, i.e., non-porous, film of anatase TiO2. (4) Not surprisingly, therefore, an internationally recognized standard for classifying glass as photocatalytic, and self-cleaning, has been developed recently. This test is based on the measurement of the haze of the piece of glass under test that has been initially soiled, before and after irradiation. (5) A key component of the coating solution used to produce the initial haze on the glass sample is stearic acid (SA). SA is one of the most common saturated fatty acids found in nature and, in research, the photocatalyzed mineralization of SA is one of the most employed reactions used to demonstrate the self-cleaning action of new photocatalytic films. (6−20) The overall reaction process can be summarized as follows
CH3(CH2)16CO2H+26O2→photocatalysthν≥Ebg18CO2+18H2O
(1)
where Ebg is the band gap energy of the photocatalyst, which is ca. 3.2 eV for anatase TiO2, for example.
In most kinetic studies of the photocatalytic destruction of SA, the whole surface of the photocatalyst is covered with a layer of SA, i.e., a conformal coating of SA. It follows that during its photocatalyzed destruction, the total mass of SA, m, in the SA film at irradiation time, t, is given by the expression
mt=at·ht·ρ
(2)
where mt, at, and ht are the values of the mass, area, and thickness, h, of the SA film, respectively, at irradiation time t, and ρ is the density of SA. In such studies, it is usually found that the area of the SA film does not change during the photocatalytic process, i.e., at = a0, and that the kinetics approximate to zero order with respect to the concentration of SA, [SA]t (units: g cm–2), at time t, where [SA]t = m/a0, (4,20−26) so that the rate of removal of SA is described by the expression
−d[SA]t/dt=−d(mt/ao)=ko
(3)
the integrated form of which is
[SA]t(units:gcm−2)=[SA]o−k0t,⁣for0≤t≤[SA]0/k0
(4)
Where, [SA]0 is the initial concentration of SA (=m0/a0) and k0 is the zero-order rate constant which, in turn, is dependent upon the irradiance, φ, of the radiation absorbed by the photocatalyst (units: mW cm–2).
The observation of zero-order kinetics for reaction 1 is usually rationalized in terms of the following features: (i) all the photocatalytic sites are occupied by a stack of SA molecules that form an overall film of uniform thickness, h, (ii) the rate-determining step is the initial oxidation of SA (i.e., there are no long-lived intermediates), and (iii) all the photocatalytic sites are equally reactive. Note that in the few kinetic studies of this system in which φ has been varied, (26,27) k0 has been found to depend directly upon φ, which suggests that hole-trapping by the SA is very efficient, which is very likely given (i) and (ii). In some studies of this system, the reaction kinetics are not of zero order and are described by the following expression
−d[SA]t/dt=k[SA]tn
(5)
where n, the order of the reaction, is nearer unity. (10,28) The latter kinetic feature for reaction 1 has been attributed to a distribution of reactivities in the surface sites. (10,28) Previous work has established that, when the commercial self-cleaning glass product Activ is used as a photocatalytic film for mediating reaction 1, the kinetics are of zero order, i.e., n = 0. (10,28) The latter finding suggests that Activ comprises a surface with a very uniform reaction site photocatalytic activity, which is perhaps not too surprising given its large-scale method of production. (4,28)
In such studies of reaction 1, for a uniform coating of SA on a TiO2 film comprising sites of equal activity, in which n = 0, it follows that at all t, at is fixed at its initial value a0, so that the rate of SA removal, d[SA]/dt, can be expressed as follows
−d[SA]/dt=k0=−(dh/dt)ρ/a0
(6)
where −dh/dt is the rate of loss in the thickness of the SA film. Thus, a physical manifestation of the commonly observed zero-order photocatalyzed decay of [SA], on photocatalytic coatings, is a zero-order decay in SA film thickness, h, with a zero-order rate constant of k0/ρ. However, in practice, in kinetic studies of reaction 1, the variation of the thickness of the SA film as a function of t is rarely measured directly. Instead, most studies of reaction 1 are based on the measurement of the integrated absorbance, Absint(t), of the FT-IR spectrum of the SA film over the range 2700–300 cm–1, which is proportional to [SA]t, where an integrated absorbance value of 1 cm–1 is found to be equivalent to ca. 4.6 μg cm–2 of SA. (20−23)
Out in the field, a uniform film of SA on the surface of a photocatalytic self-cleaning product appears to be an unlikely scenario, and that a more likely one is that the surface will, initially at least, become contaminated by islands of the pollutant. However, even in this situation, it would seem reasonable to assume that the destruction of an array of the pollutant islands would proceed along the same lines as those found when the whole of the photocatalytic coating is covered with a film of a pollutant. For example, it would seem reasonable to expect that the overall decay of an array of identical, cylindrical SA islands of the same initial thickness, h0, dispersed over a photocatalytic coating of uniform activity would exhibit zero-order kinetics with each island showing no change in area a, but rather thickness, ht, that decreases linearly with t, i.e., −da/dt = 0 and −dh/dt = constant, ch. Evidence that this is indeed the case is provided by the work of Sawunyama et al. (29) in their study of the photocatalyst destruction of partial monolayers of SA, which formed islands with diameters spanning 1–20 μm. The results of this study revealed that, while the islands showed some evidence of random pitting, due to hot spots of reactivity on the surface of the photocatalyst, there was “no evidence of preferential reaction at island edges compared to the interior regions,” (29) so that the area of each island remained unchanged during the photocatalytic process, i.e., −da/dt = 0, or at = a0 at all t. Thus, in the latter work, the SA islands were found to fade with increasing irradiation time, t, and not shrink.
In striking contrast to the above findings, Ghazzal et al., (30) in a study of reaction 1 involving a wide distribution of SA islands on a sol–gel TiO2 film, found that at decreased at a constant rate with t, i.e. −da/dt = constant, ca, and −dh/dt = 0. (30) Thus, in the latter work, the SA islands were found to shrink with increasing irradiation time, t, and not fade. In this work, (30) evidence that −da/dt = constant, ca, is provided by, among other things, a plot of the size (in pixels, as assessed by optical microscopy) of three different typical SA islands, all with similar thicknesses, as a function of t, as illustrated in Figure 1. (30) Ghazzal et al. suggest that the reported novel kinetics arise because radical species formed throughout the uncovered TiO2 surface diffuse to the edges of the SA islands, (30) and so, for brevity, we shall refer to this mechanism and its associated kinetics as the edge activity, area dependent (EAAD) model.

Figure 1

Figure 1. Plot of area (in pixels from optical micrograph) vs irradiation time for three SA islands of similar initial thickness but different initial areas, on a sol–gel TiO2 film upon irradiation with 2.5 mW cm–2 UVA radiation. Insert diagram illustrates the reported outlines of the three different-sized islands at (from left to right) t = 0, 5, and 10 min, respectively. (30)

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The striking discrepancy between the findings of the above two studies (29,30) and the significance of the subject area when it comes to understanding the self-cleaning action of many photocatalytic products, not least self-cleaning glass, has prompted this study of the kinetics of reaction 1, using primarily a single cylindrical island of SA, and an array of such islands, deposited on a photocatalytic film. In this work, two different types of TiO2-based, photocatalytic films were tested, namely one with an established, uniform reactivity, Activ glass, and one with a more heterogenous surface reactivity.

2. Materials and Methods

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2.1. Materials Declaration

Unless stated otherwise, all chemicals were purchased from Merck and used as received. The SA used (Merck, 175366-1KG) was reagent grade with a purity of 95%. Activ self-cleaning glass was supplied by Pilkington Glass-NSG. The Aeroxide P25 TiO2 was purchased from Evonik, further details of which are given elsewhere. (31)

2.2. Preparation of P25 TiO2 Films on Glass

P25 TiO2 films on microscope slides were prepared using a drawdown method in which two strips of 3M Scotch Magic tape were placed either side of the microscope slide producing a ca. 2 cm wide, 60 μm deep trough. A P25 TiO2 paste was made, comprising 76 mg of P25 TiO2 dispersed in a mixed solvent solution made up of 20 mL acetic acid, 20 mL water, 3.98 mL ethanol, 0.43 g terpineol, and 0.74 g of 10% w/w ethyl cellulose in methanol. Further details regarding the preparation of this paste are given elsewhere. (32) A few drops of the paste were deposited at the top of the 3M Scotch tape trough and drawn down using a glass rod. The resulting 60 μm thick wet film of the paste was then annealed at 450 °C for 1 h (ramp rate = 10 °C min–1), to produce the 1.70 μm thick P25 TiO2 film used in this study.

2.3. Preparation of Sol–Gel TiO2 Films on Glass

The preparation of the sol–gel paste is detailed elsewhere. (7,33) Briefly, 4.65 g of glacial acetic acid were added to 20 mL of the precursor solution, titanium(IV) isopropoxide. 120 mL of deionized water, containing 1.08 g of nitric acid, were then added to the Ti(IV)/acetic acid solution, to produce a white dispersion of the hydrous oxide. This dispersion was used to grow colloidal TiO2 particles hydrothermally using an autoclave, 220 °C for 12 h. The resulting white precipitate was then redispersed using an ultrasonic probe (Lucas Dawe Ultrasonics, Soniprobe, London, England), and rotary evaporated until a weight percent of TiO2 of 10–12% was achieved, to which 50 wt % of polyethylene glycol was then added. The final white paste was mayonnaise-like in appearance and texture. The paste was then applied to microscope slides using the same drawdown method detailed above. The resulting 60 μm thick wet film of the sol–gel paste was then annealed at 450 °C for 1 h (ramp rate = 10 °C min–1), producing a ca. 2.8 μm thick sol–gel TiO2 film.

2.4. Deposition of Cylindrical SA Islands on Activ

2 mm diameter cylindrical dots of SA were deposited on Activ glass by first dip-coating a 2.5 cm × 7.5 cm slide of Activ in 0.1 M stearic acid in chloroform to produce a uniform coating of SA across the whole of the photocatalytic surface. In the work, the sample of Activ glass was immersed for 20 s into the SA/chloroform solution and then withdrawn at 1000 mm min–1 using a KSV NIMA Layer Builder (Biolin Scientific, Gothenburg, Sweden) dip-coater, thereby producing a ca. 150 nm thick film. The back of the resulting SA-coated Activ glass was wiped clean of SA using a cloth soaked in chloroform. An automated cutting machine (Cricut Explore Air 2, South Jordan, USA) was then used to cut 2 mm holes out of 100 μm thick PTFE adhesive tape (3M PTFE Film adhesive Tape 5490), which was then applied to the surface of the SA film covering the Activ glass. The holed PTFE adhesive tape film was pressed flat using a microscope slide to ensure good adhesion to the SA film. The tape was then quickly peeled off, leaving behind an array of 2 mm SA dots, ca. 160 nm thick, on the Activ glass.

2.5. Deposition of Cylindrical SA Islands on P25 TiO2 Films

2 mm diameter cylindrical dots of SA were deposited on a P25 TiO2 film by spray coating, using a Talon TS siphon-feed airbrush (Paasche, TS-3L, Kenosha, USA), 0.1 M stearic acid in chloroform, through a holed acetate template. The template was created by cutting 2 mm circles in the sheet of acetate using an automated cutting machine (Cricut Explore Air 2, South Jordan, USA). The airbrush was held 15 cm from the template, and 5 passes were used to deposit the SA, creating ca. 450 nm thick cylindrical dots.

2.6. Deposition of Volcano SA Islands on Sol–Gel TiO2 Films

Volcano-shaped islands were deposited on a sol–gel TiO2 film using a Talon TS siphon-feed airbrush (Paasche, TS-3L, Kenosha, USA). The airbrush was held parallel to the surface of the TiO2 film at a height of 5 cm, and 0.1 M SA in chloroform was sprayed for 5 s, which produced a number of volcano-shaped islands, ca. 50 μm wide and ca. 1 μm tall, on the sol–gel TiO2 film.

2.7. Irradiations, FT-IR Spectroscopy, Optical Microscopy, and Profilometry

Unless otherwise stated, all irradiations were performed using a 15 W, Analytik Jena UV bench lamp fitted with 254 nm (UVC) bulbs which provided an incident irradiance of 2 mW cm–2. The irradiance was measured using a UVX meter (Analytik Jena, Jena, Germany) fitted with a 254 nm sensor. All FTIR spectra were recorded using a Spectrum One FTIR (PerkinElmer, Massachusetts, USA). Digital photographs were taken using a Canon 77D fitted with a Canon EF-S 60 mm f2.8 USM Macro Lens. Optical microscopy was carried out using an Olympus Trinocular Microscope, SZ6045TR (Tokyo, Japan), fitted with a Kiralux 8.9 MP color CMOS camera, CS895CU (ThorLabs, New Jersey, USA). Profilometry measurements were carried out using a Dektak3ST surface profile measuring system (Veeco, California, USA). The SA dots were scanned over a distance of 5 mm at a rate of 100 μm s–1, with a stylus force of 3 mg.
Irradiations of the SA volcano islands were performed inside the profilometer. In this work, UVA light (365 nm, ca. 80 mW cm–2) was supplied by a 10 W 365 nm LED, ILH-XT01-S365-SC211-WIR200 (Intelligent LED Solutions, Berkshire, UK), through an optical fiber, BFY1000HS02 (ThorLabs, New Jersey, USA), held ca. 2–3 mm above the surface of the SA volcano island. The irradiance was measured using a C10427 UV power meter (Hamamatsu, Shizuoka, Japan).

2.8. Photocatalytic Film Characterization

Scanning electron microscopy was carried out using an FEI Quanta FEG - Environmental SEM Oxford Ex-ACT (FEI, Oregon, USA). Figure 2a shows the SEM image of the surface of Activ comprising TiO2 particles ca. 32 nm in diameter. As the thickness of the APCVD applied TiO2 coating on Activ glass is only ca. 15 nm, the surface consists of tightly packed broad domes of TiO2. (4) Figure 2b shows the network of loosely packed interconnected TiO2 particles, ca. 40 nm in diameter, of a mesoporous P25 TiO2 paste coating, which had a thickness of ca. 1.7 μm. (34) The SEM of the surface of a sol–gel TiO2 coating is shown in Figure 2c, from which it appears that it has a slightly tighter packed and more uniform mesoporous network compared to the P25 TiO2 coating, comprising particles ca. 44 nm in diameter. Other work showed that the sol–gel film was ca. 2.8 μm thick. (35)

Figure 2

Figure 2. SEM images of the surfaces of (a) Activ glass, (b) P25 TiO2 coating, and (c) sol–gel TiO2 coating, which reveal TiO2 particle sizes of ca. 32, 40, and 44 nm, respectively.

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3. 2D Kinetic Model

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In this work, all the SA islands prepared were symmetrical, usually cylindrical in shape, and so it follows that, instead of modeling the kinetics of the decay of such structures in three dimensions, it is possible to do it equally effectively, and much more simply and quickly, in two dimensions. This section describes a 2D kinetic model that was used to describe the results reported here for a conformal coating of SA (S1), a cylindrical island of SA (S2), and an array of such islands (S3). The basic assumptions of the 2D kinetic model are those usually proposed (4,20−26) for system S1, namely, that all photocatalytic sites are active, only occupied sites are effective, and the kinetics at each site is zero-order with respect to [SA], i.e., saturation kinetics.

3.1. General 2D Kinetic Model

The general 2D kinetic model is based on a line of 100 photocatalytic sites, i, each with a zero-order rate constant, ki, with an initial uniform covering of SA of thickness, hi,0. Note that the choice of 100 sites is arbitrary, with other work showing that identical model predictions are generated if the number of sites is increased to 1000 or more. In the 2D general kinetic model, it follows from eq 2 that at each reaction site, i, the initial concentration of SA will be
[SA]i,0=ρ·hi,0
(7)
where hi,0 is the initial thickness of the SA film at zero irradiation time, t, so that the initial, total concentration of SA, [SA]T,0, will be
[SA]T,0=∑i=0100[SA]i,0
(8)
Assuming each site, i, exhibits zero-order kinetics for reaction 1, it follows from eq 4 that at irradiation time, t, the total SA concentration will be
[SA]T,t=∑i=0100([SA]i,0−kit)
(9)
At this point, it is helpful to define a unitless time parameter, τ
τ=kavt/[SA]max
(10)
where kav is the average zero-order rate constant, and [SA]max is the maximum initial concentration of SA among all the 100 SA-occupied sites, which is associated with a maximum SA thickness, hmax. Combining eqs 8–10, it is possible to derive the following expression for fSA,τ, the fractional (or relative) total concentration of SA as a function of τ, i.e.
fSA,τ=[SA]T,τ/[SA]T,0=∑i=0100[SA]max(αi−βiτ)/∑i=0100[SA]i,0
(11)
where αi = [SA]i,0/[SA]max and βi = ki/kav. Note that due to the nature of zero-order kinetics, in carrying out the summations eqs 9 and 11, when the calculated term in parenthesis is negative, then, in the summation, its value is returned as zero.
Equation 11 of the above, 2D general kinetic model of reaction 1 is able to describe the kinetics of reaction 1, when the system is complicated by: (i) having variable amounts SA, i.e., where αi varies with i, and/or (ii) sites of variable activity, i.e., where βi varies with i. Note that the use of a dimensionless fractional parameter such as fSA,τ could hide light-related effects associated with the rate of absorption of the incident UV irradiance, φ. For example, effects such as reflection, scattering, and absorption could change during a reaction and so affect the rate. Fortunately, in this work, both the TiO2 films and SA coatings were largely non-scattering, and so, such light-related effects would most likely be minimal.

3.2. Simplified 2D Kinetic Model

As noted earlier, most of the work described here was focused on three simple systems, namely, SA deposited as either: (S1) a conformal film, (S2) a cylindrical island, or (S3) an array of cylindrical islands. In addition, since most work was carried out on Activ glass, it was assumed that all sites are equally active. As a result, the kinetics of SA removal for systems S1–S3, can be described by the following simplified version of eq 11
fSA,τ=[SA]T,τ[SA]T,0=∑i=0100[SA]i,0(1−τ)/∑i=0100[SA]i,0
(12)
since, at all sites i, kav = ki = k0 and [SA]max = [SA]i,0, i.e., αi = βi = 1. The above equation is for the simplified 2D kinetic model, whereas eq 11 is for the general 2D kinetic model.
Thus, if we assume that each of the 100 photocatalytic sites (of equal activity) are coated with a SA film of initial thickness, hi,0, the value of which is 100 (arbitrary units), then, from eq 9, given [SA]i,t is proportional to hi,t, the variation in the thickness of SA covering each site i, as a function of irradiation time τ, will be given by the following expression
hi,τ=hi,0(1−τ)
(13)
Using eq 11, the simplified 2D kinetic model was used to calculate the hi,τ vs site number, i, profiles at different irradiation times, τ, (insert diagram in Figure 3a), as well as the hi,τ vs τ profile (main diagram), illustrated in the main diagram in Figure 3a. These results show that the simplified 2D kinetic model predicts that the thickness of the SA film, hi,τ, will decrease uniformly with irradiation time, τ, whereas the width of the film, W, will remain unchanged. The simplified 2D model can be used to predict the variation in fSA,τ as a function of irradiation time, τ, using eq 12, and the results of this work are illustrated in Figure 3b. The latter show that the kinetics of destruction of the 2D SA film is zero order, as expected, given all sites are covered initially with the same amount of SA, and so the same h0, and have the same zero-order rate constant, k0.

Figure 3

Figure 3. 2D kinetic model-predicted plots, for a 2D sheet of SA covering 100 identical (ki = k0) photocatalytic sites, with each site, i, initially loaded with SA with hi,0 = 100 (a.u.), of: (a) (insert) SA film thickness, hi,τ, vs site number, i, and (main diagram) hi,τ vs τ, calculated using eq 13 of the 2D model; in the insert plot, the different calculated profiles correspond to τ values of (from top to bottom) 0, 0.2, 0.4, 0.6, and 0.8, respectively, and (b) variation in the fraction of SA, fSA,τ calculated using eq 12 of the 2D model, vs τ.

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The results illustrated in Figure 3 were generated using the simplified 2D kinetic model for a line of 100 columns/stacks of SA, of initial thickness 100 (a.u.), with each column/stack covering a different photocatalytic site, i, all with the same activity, i.e., ki = k0. However, as noted earlier, for reasons of symmetry, the kinetics features predicted by this simplified 2D kinetic model, as illustrated in Figure 3, will also be the same for a 3D system comprising a photocatalytic coating of uniform activity, with a SA deposit that is in the form of a conformal film (S1), a cylindrical island (S2), or an array of cylindrical islands (S3). Illustrations of the model-related SA cylindrical island and array of such islands are given in Figure S1a,b, respectively, in the electronic Supporting Information file that accompanies this paper.
Thus, given τ is proportional to irradiation time t, the results in Figure 3 show that the simplified 2D kinetic model predicts, for all examples of S1–S3, on Activ, the following common kinetic features: (a) no change in the SA film area with t, i.e., −da/dt = 0, (b) a linear decrease in SA film thickness with t, with a gradient -k0/(ρ·h0), i.e., −dh/dt = constant (ch), and (c) zero-order decay kinetics, i.e., n = 0, so that a plot fSA,t vs t will be a straight line with a negative gradient, −k0/[SA]0. Table 1 provides a summary of these predictions, which highlights that with increasing irradiation time, in all cases, the SA film/island/islands will fade (−dh/dt = ch, and NOT shrink, da/dt = 0).
Table 1. SA Island Photocatalytic Decay Models and Predictionsa
a

Where the island is cylinder shaped, of height h and area a, and ρ is the density of SA; at t = 0, a = a0 and h = h0 and n = order of reaction.

b

k = rate constant, which depends upon the irradiance, φ.

c

Using the general 2D kinetic model, with αi = 1 and βi varied so as to create volcano-shaped initial profile.

In this work, the 2D kinetic model is used exclusively to explain the fading Langmuir–Blodgett SA islands reported by Sawunyama et al. (29) and the results of numerous unique experiments, involving cylindrical islands and array of such islands and volcano-shaped islands─all of which are consistent with current theory and the model assumptions that all sites are equally active, only occupied sites are effective, and each site exhibits zero-order kinetics with respect to [SA]. Because of its very different underlying assumptions, the 2D kinetic model is not, and cannot be, used to predict the kinetics of the EAAD kinetic model proposed by Ghazzal et al. (30) The EAAD model is discussed in more detail in a later section, although Table 1 provides a summary of its underlying assumptions and predicted (30) kinetic features.

4. Results and Discussion

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4.1. Photocatalyzed Destruction of a Conformal Film of SA Deposited on a Photocatalytic Film: System S1

As noted earlier, system S1 represents the bulk of the studies of the kinetic studies of reaction 1 carried out to date, in which it is usually found that the kinetics of the removal of a uniform film of SA via reaction 1 is zero order, i.e., they fit eq 12 of the simplified 2D kinetic model. (4,20−26) Not surprisingly, given its uniform photocatalytic activity, Activ glass is often cited as an example of a photocatalytic film that yields zero-order kinetics for reaction 1. (36) Since the excellent fit of the simplified 2D kinetic model to the zero-order decay profiles for reaction 1 for real examples of S1 is well-established, (4,20−26) it did not merit further repeating in this study.

4.2. Photocatalyzed Destruction of a Single Cylindrical SA Island Deposited on Activ: System S2

A single cylindrical SA island, 2 mm in diameter, ca. 160 nm thick, was deposited on a piece of Activ self-cleaning glass and irradiated from above with UVC radiation (254 nm) emitted from two 15 W germicidal lamps (2 mW cm–2), during which the sample was removed at regular intervals and photographed under a microscope, and its height, ht, and width, Wt, were determined by profilometry.
The optical microscope images recorded of the SA cylindrical SA island as a function t are illustrated in Figure 4a and show that with increasing irradiation time, the image of the SA island becomes fainter, presumably due to a decrease in SA film thickness, and that this change was not accompanied by an apparent change in area of the SA island. These features were confirmed by the results of a parallel profilometry study of the same, single SA cylindrical island, illustrated in Figure 4b. The latter results show that −dh/dt = ch and −dW/dt (and so −da/dt) ≈ 0, both of which are features predicted by the simple 2D kinetic model, as illustrated by model-predicted hi,τ vs site, i, profiles, for different values of τ, in the insert diagram in Figure 3a and summarized for system S2 in Table 1.

Figure 4

Figure 4. (a) Photographic images recorded using a microscope and (b) thickness, ht, and width, Wt, values determined using a profilometer of a single cylindrical SA island on Activ as a function of irradiation time, t. The UVC light source used for this work comprised two 15 W germicidal lamps (2 mW cm–2).

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4.3. Photocatalyzed Destruction of an Array of Cylindrical SA Islands Deposited on Activ: System S3

In the previous section, optical microscopy and profilometry were used to validate the simplified 2D kinetic model-predicted features of the kinetics of reaction 1, −dh/dt = ch and −da/dt = 0, exhibited by a single cylindrical deposit of SA on a uniform photocatalytic coating. Unfortunately, given the very low FT-IR absorbance of a single cylindrical SA deposit, it was not possible to confirm, for this system, the 2D model-predicted linear decrease in the fractional amount of SA, i.e. fSA,t, as a function of irradiation time, τ (or t). However, it was possible to measure both the optical microscope images and the FT-IR absorbance of an array of cylindrical SA islands deposited on Activ as a function of t.
Figure 5a illustrates the recorded optical microscope images for an array of cylindrical SA islands on Activ irradiated with UVC as a function of t. These images show that, as with a single cylindrical SA island, the appearance of each island gets fainter with increasing t, but its area does not change. The insert diagram in Figure 5b illustrates the FT-IR absorbance spectra of the SA cylindrical island array on Activ, recorded at regular intervals during irradiation. At each time, t, a value for the integrated absorbance, Absint(t), was then calculated from the appropriate absorbance FT-IR spectrum and then used to calculate a value of fSA,t, where fSA,t = Absint(t)/Absint (t = 0), which in turn is equal to the model predicted parameter, ([SA]T,t/[SA]T,0). A plot of fSA,t vs t for this system is illustrated in the main diagram of Figure 5b and reveals an excellent fit to zero-order kinetics, in agreement with that predicted by the simplified 2D kinetic model (see Table 1.) The simplified 2D model predicts that the kinetics for the array should be identical to those exhibited by a single cylindrical SA island, and this is confirmed by noting that the fSA,t, vs t decay trace for an array illustrated in Figure 5b is nearly identical to that of the ht vs t trace, recorded using a profilometer, for a single cylindrical SA island illustrated in Figure 4b, with half-life, t1/2, values for these two systems of 7.5 and 8.7 min, respectively.

Figure 5

Figure 5. (a) Photographic images recorded as a function of t for an array of cylindrical SA islands on Activ and (b) insert plot, FT-IR spectra of the array recorded every 60 min during irradiation, and the main diagram of the fractional integrated absorbance, fSA,t, (=Absint(t)/Absint (t = 0) = [SA]T,t/[SA]T,0), determined using the spectra in the insert plot, vs t. The UVC light source used for this work comprised two 15 W germicidal lamps (2 mW cm–2).

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4.4. Photocatalyzed Destruction of a Single Cylindrical SA Island and Array Deposited on a Coating of P25 TiO2: Systems S2 and S3

One possible reason for the discrepancy between the kinetics of reaction 1 for a cylindrical SA island on Activ glass, for which, −dh/dt = ch and −da/dt = 0 (see Figure 4) and the findings of Ghazzal et al., (30) for SA islands on a sol–gel TiO2 film, where −dh/dt = 0 and −da/dt = ca (see Figure 1), is that in the latter work, the surface activity of the photocatalytic sol–gel film is likely to be much less uniform than that of Activ.
Previous work by this group (28) has established that while for a uniform film of SA deposited on Activ glass, n = 0, when coated on a film of P25 TiO2, n ≈ 1. The latter effect was due to a significant distribution in surface activity, i.e., a distribution in ki. (10,28)
To see how reaction site activity distribution affects the kinetics of reaction 1 for an array of cylindrical islands of SA, in this work, a P25 TiO2 film was tested under the same conditions as used above for Activ. In the study of system S2, the photocatalytic destruction of a single cylindrical SA island, using a P25 film, it was not possible to record the variation in the profile of the SA cylinder because the profiler needle tended to scratch the P25 TiO2 film. However, it was possible to monitor the photocatalyzed decay of a SA island on P25 TiO2 using optical microscopy, and the results of this work are illustrated in Figure 6. A brief inspection of the images in Figure 6 shows that with increasing t, while the SA island largely preserves its circular footprint, −da/dt = 0, there is some evidence of photocatalytic activity hot spots on the P25 TiO2 film leading to regions of SA within the island perimeter to disappear before the surrounding SA, so that the images of the SA island take on an increasingly pitted, as well as the usual fainter, appearance with increasing t.

Figure 6

Figure 6. Photographic images recorded using an optical microscope, as a function of irradiation time t, of a single cylindrical SA island on a P25 TiO2 film. The UVC light source used for this work comprised two 15 W germicidal lamps (2 mW cm–2).

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In a subsequent study, the FT-IR spectrum of an array of cylindrical SA islands, i.e., system S3, on a P25 TiO2 film was recorded as a function of t, and the results of this work are illustrated in the insert plot in Figure 7. The data in this plot were used to calculate the variation in fSA,t, (= Absint(t)/Absint (t = 0) = [SA]T,t/[SA]T,0), as a function of t, the results from which are illustrated in the main diagram in Figure 7. The latter plot shows that the kinetics of reaction 1 for an array of cylindrical SA islands on a film of P25 TiO2 are of a much higher order, i.e., n ≈ 1, than those observed when using Activ glass (see Figure 5), where n ≈ 0.

Figure 7

Figure 7. FT-IR spectra of the array recorded every 30 min during irradiation and main diagram of the fractional integrated absorbance, fSA,t, (= Absint(t)/Absint (t = 0) = [SA]T,t/[SA]T,0), determined using the spectra in the insert plot, vs t. The UVC light source used for this work comprised two 15 W germicidal lamps (2 mW cm–2).

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As noted earlier, in a previous paper, (28) such a deviation from zero-order kinetics for a film of SA on a P25 TiO2 photocatalytic coating was ascribed to a significant dispersion in surface activity, the presence of which is supported by the reaction hot spots in the optical micrographs illustrated in Figure 6. However, the optical micrograph images illustrated in Figure 6 also show that despite clear evidence of a dispersion of photocatalytic activity across the surface of the P25, the area of the cylindrical islands did not change with increasing t, i.e., −da/dt = 0. These observations are consistent with the 2D general kinetic model and not the EAAD model and the observation of Ghazzal et al. (30) that −da/dt = ca.

4.5. Photocatalyzed Destruction of a Single SA Volcano: System S4

In the above work, strenuous efforts were made in the production of the SA islands to ensure that they were of uniform thickness, so that not only the average initial thickness of each SA island was the same, h0, but also, within each island, there was no variation in SA film thickness, hi = h0 at all i. Thus, in this work, each SA island produced has a cylindrical shape, as illustrated in Figure S1 in the Supporting Information. In contrast, Ghazzal et al. (30) produced their SA islands by dip-coating the sol–gel TiO2 film in a methanolic solution of SA, thereby creating an array of different sized (i.e., different a0 and h0) and shaped islands, as illustrated by the optical micrograph outlines in Figure 1. In the latter work, the height of each island was assessed by the gray scale of a photograph of the micrograph, which, for reasons to be discussed later, appears at best a very crude scale. (30) If the method of assessing height was suspect, what if the SA islands produced by Ghazzal et al. were in fact volcano like in height profile and not table-topped? (30) Could such a situation, which is by no means unlikely, explain the observation that −da/dt = ca, and the possibly doubtful claim that −dh/dt = 0, the combination of which is the basis of the EAAD kinetic model (see Table 1)?
The kinetics of the photocatalytic destruction of a “volcano” shaped island of SA, on a TiO2 coating of uniform activity, are readily predicted using the general 2D kinetic model, eq 11, with the usual assumption that all the photocatalytic sites are equally active, βi = 1, but with a varying value of αi, so as to create a volcano-shaped 2D profile. The results of this work are illustrated in Figure 8, with the top right-hand side insert plot showing the predicted variation in the hi,τ vs site number, i, profile for an initially volcano-shaped profile, with increasing irradiation times, τ, from which it is clear that both the maximum thickness of the SA island, hmax,τ and the width/diameter, Wτ, decrease linearly with increasing τ, i.e., −dh/dt = ch and −dW/dt = constant. However, as the SA “volcano” will be circular in its 3D form, its area, a, is related to W via the expression, a = π(W/2)2, so that that −da/dt will be >0 but not constant, as it will decrease with increasing t. The general 2D kinetic model-predicted non-linear decay in the normalized island area (aτ/a0) as a function of irradiation time τ is illustrated in the bottom left-hand side insert plot in Figure 8.

Figure 8

Figure 8. 2D kinetic model-predicted decay curves for the photocatalyzed destruction of a “volcano” shaped island of SA on a photocatalytic film of uniform activity. The insert plot on the top right-hand side illustrates the predicted variation in hi,τ vs i profiles at irradiation times, τ, of (from top to bottom) 0, 0.2, 0.4, 0.6, 0.8, and 0.9, respectively. The main diagram and bottom left-hand side insert plots are the predicted variations in hmax,τ and normalized area, aτ/a0, vs τ, respectively. The data points were derived for the large (●), medium (■), and small (▲) SA islands, using the three data sets in Figure 1.

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Thus, according to the general 2D kinetic model, although a volcano-shaped island of SA would appear to shrink with increasing t, as observed by Ghazzal et al., (30) it would not shrink at a constant rate, and would simultaneously appear to fade, as −dh/dt = ch. These predictions do not appear to help explain the kinetic features reported by Ghazzal et al. (30) for their SA islands, namely −da/dt = constant, ca, and −dh/dt = 0, which form the basis of the EAAD model.
In order to test the 2D kinetic model predictions for a volcano-like SA island, such an island was prepared on a sol–gel TiO2 coating. The latter photocatalytic coating was used because it was sufficiently active that it could remove the SA islands within a 30 h irradiation period using 365 nm radiation, which Activ was unable to do because of its very thin TiO2 coating, ca. 15 nm. In addition, unlike the P25 TiO2 coating, the sol–gel TiO2 coating was sufficiently robust that it was not damaged by the needle of the profilometer. The experiment was also restricted to using 365 nm radiation, and not 254 nm, by the transmission characteristics of the fiber optic cable that needed to be used, in order to allow in situ irradiation of the SA volcano on the sol–gel TiO2 coating in the profilometer.
The volcano-shaped island of SA on a sol–gel TiO2 coating was then monitored by profilometry as the TiO2 coating was irradiated with 365 nm radiation delivered via a fiber optic cable. The variation in the profile of the SA island as a function of irradiation time is illustrated in Figure 9a and appears to confirm the 2D general model predictions that both the area and the maximum thickness will decrease with increasing t. The profiles illustrated in Figure 9a were used to construct plots of maximum SA film thickness, hmax, and SA island width, W, as a function of t, the results of which are illustrated in Figure 9b. These results show that, as predicted by the general 2D kinetic model, both the maximum thickness of the SA island, hmax,t and the width/diameter, Wt, decrease linearly with increasing t, i.e., −dh/dt = ch and −dW/dt = constant.

Figure 9

Figure 9. (a) Plot of the profile of a SA volcano-shaped island as a function of irradiation time, t, where t was (from top to bottom) 0, 10, 20, and 30 h, respectively; (b) based on the data in (a), subsequent plot hmax and W as a function of t. The irradiation was performed in situ in the profilometer using a fibre optic deliverying 365 nm radiation, with an irradiance of 80 mW cm–2.

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5. The EAAD Model

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As noted earlier, in their study of reaction 1, using islands of SA, Ghazzal et al. (30) reported −da/dt = constant, ca, and −dh/dt = 0 and rationalized these findings using an EAAD kinetic model, of which the underlying assumptions and predicted kinetic features are given in Table 1. It should be noted however, that in this model it is not at all obvious why the rate of reaction depends upon the area of the island, rather than its circumference, given the proposed mechanism relies on edge attack from “radical species formed throughout the uncovered surface and diffusing toward the islands edges”. (30)
Since the model assumes that the rate depends on the presence of an uncovered part of the photocatalytic surface, it follows that it will be zero when the photocatalytic film has a conformal coating of SA, i.e., system S1. This prediction of the EAAD model of no reaction, when the photocatalytic film is completely covered with SA, is clearly at odds with the fact that it has never been observed, and instead that for all examples of S1, the rate is >0. (4,20−26) Support for the EAAD model is further weakened by the fact that none of the results reported here support the predictions of the EAAD model; rather, all the data fit the predictions made by the 2D kinetic model in which only occupied sites are active and no preferential edge attack occurs.
If the EAAD kinetic model is not valid and the kinetics of SA islands are described only by the 2D kinetic model, what then could explain the observations that Ghazzal et al. (30) reported, −da/dt = constant, ca, and −dh/dt = 0? One possibility is that in the latter work, the islands of SA were volcano-shaped and not table-topped. We have seen that the 2D kinetic model predicts that such islands will shrink (and fade) with increasing t, and these predictions were confirmed in our profilometer study of such an island (see Figure 9). Support for the proposal that the SA islands studied by Ghazzal et al. (30) are volcano-shaped comes from the observation that the 2D general kinetic model-predicted decay in aτ/a0, illustrated in Figure 8 for a volcano-shaped island, provides a good fit to most of the at vs t data reported by Ghazzal et al. (30) for the three SA islands illustrated in Figure 1, as shown by the data points in the plot in Figure 8. (30) This fit is impressive, given that some deviation from the model would be expected since the model-predicted aτ/a0 vs τ plot is for a symmetrical volcano-shaped SA island on a photocatalytic film of uniform activity, whereas the data points derived from Figure 1, are for three, decidedly asymmetrically shaped, SA islands.
It would appear, therefore, that the major sticking point in reconciling the work of Ghazzal et al. (30) and the 2D kinetic model and all the results reported here is their observation that −dh/dt = 0, i.e., their claim that the islands shrink but apparently do not fade with increasing t. The latter observation lies counter to what we observed for volcano-shaped SA islands, as illustrated by the results in Figure 9, where −dh/dt = ch, as predicted by the general 2D kinetic model. Could it be that the gray-scale optical microscopy method that Ghazzal et al. (30) used to measure SA island height was not up to the task and that in fact −dh/dt was ≥0? Certainly, Ghazzal et al. (30) note that the dark and light gray levels of their optical micrographs lie in the very broad-thickness regions of 120–80 nm and <80, respectively. It follows that the error associated with the gray scale measurement of SA island height will be large. More striking evidence that optical microscopy is poorly suited for measuring h is obtained by comparing real height, as measured by AFM, and inverted gray scale “height” using data reported by Ghazzal et al. (30) for a line of SA, as illustrated by the results of such a comparison described in S2 in the Supporting Information. Taken together, the results cast doubt on the veracity of the original claim (30) that −dh/dt = 0. Thus, in the work of Ghazzal et al., (30) if −dh/dt was in fact >0, then this, coupled to the remaining observation that −da/dt = ca would be entirely consistent with the 2D kinetic model for volcano-shaped SA islands, which will shrink and fade with irradiation.

6. Conclusions

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The removal of a cylindrical SA island, or an array of islands, via reaction 1, mediated by a photocatalytic film, such as Activ self-cleaning glass, or a P25 TiO2 coating on glass, results in a decrease in their thickness, h, with t, but with da/dt = 0, i.e., the islands fade but do not shrink. This is opposite to what Ghazzal et al. (30) reported for SA islands on a sol–gel TiO2 film, namely, they shrink, rather than fade, i.e., dh/dt = 0 and −da/dt = ca. However, the latter observation of shrinking SA islands is observed when the SA islands are more volcano-like in profile, rather than cylindrically-shaped; however, these volcano-like SA islands also fade, i.e., −dh/dt = ch. A simple 2D kinetic model can be used to rationalize the results of this work, and to fit those from a previous study on the removal of SA islands using a sol–gel TiO2 film, (30) in which −da/dt = ca, but not to the additional claim that −dh/dt = 0, although the validity of the latter is questionable, given the demonstrably poor ability of the optical gray scale used to assess h.
The above findings suggest that it is likely that all the active photocatalytic sites on a self-cleaning film, covered with a pollutant such as SA, will be effective in destroying it, so that with islands of pollutants, its destruction at the edges will be no faster than in the bulk. It is suggested here that any apparent shrinking of a pollutant island with t is due to its thickness profile being more like a volcano rather than a cylinder.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.3c00952.

  • Schematic images of a cylindrical island and array of such islands based on the 2D kinetic model, and comparison of AFM and optical microscope height measurements (PDF)

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Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Author
  • Authors
    • Saleh Alofi - School of Chemistry and Chemical Engineering, Queens University Belfast, Stranmillis Road, Belfast BT9 5AG, U.K.
    • Christopher O’Rourke - School of Chemistry and Chemical Engineering, Queens University Belfast, Stranmillis Road, Belfast BT9 5AG, U.K.;  Orcidhttps://orcid.org/0000-0002-9767-8517
  • Notes
    The authors declare no competing financial interest.

    All data are provided in full in the Results section of this paper and the Supporting Information accompanying this paper.

Acknowledgments

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We thank the Catalysis Hub funded by EPSRC grant reference EP/R026645/1 for supporting this work and the Government of Saudi Arabia for funding the Ph.D. of S.A.

References

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This article references 36 other publications.

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    Alotaibi, Abdullah M.; Sathasivam, Sanjayan; Williamson, Benjamin A. D.; Kafizas, Andreas; Sotelo-Vazquez, Carlos; Taylor, Alaric; Scanlon, David O.; Parkin, Ivan P.
    Chemistry of Materials (2018), 30 (4), 1353-1361CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    Brookite is the least investigated phase of TiO2 due to the synthetic difficulty of obtaining the pure phase. Here, we present the first ever chem. vapor deposition synthesis of pure brookite TiO2 thin films. The films were highly cryst. and phase pure as detd. by X-ray diffraction and Raman spectroscopy studies. SEM studies showed the films to have a structured morphol. consisting of pyramidal features. The photocatalytic properties of the brookite film, tested using stearic acid under UVA (365 nm) irradn., were superior to both an anatase film grown under similar conditions and NSG Activ glass. Transient absorption spectroscopy showed that the innate electron-hole recombination dynamics are similar in brookite and anatase, akin to previous reports. The superior activity of the brookite film is hence attributed to the higher surface area compared to anatase.
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  10. 10
    Sawunyama, P.; Jiang, L.; Fujishima, A.; Hashimoto, K. Photodecomposition of a Langmuir-Blodgett Film of Stearic Acid on TiO2 Film Observed by In-situ Atomic Force Microscopy and FT-IR. J. Phys. Chem. B 1997, 101, 11000– 11003,  DOI: 10.1021/jp9730095
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    10
    Photodecomposition of a Langmuir-Blodgett Film of Stearic Acid on TiO2 Film Observed by in Situ Atomic Force Microscopy and FT-IR
    Sawunyama, Phillip; Jiang, Lei; Fujishima, Akira; Hashimoto, Kazuhito
    Journal of Physical Chemistry B (1997), 101 (51), 11000-11003CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
    We have probed the TiO2-mediated photomineralization of Langmuir-Blodgett (LB) films of stearic acid via at. force microscopy (AFM) and FT-IR. In situ AFM images revealed that at the submicrometer level the photodecompn. process of stearic acid mols. on a polycryst. anatase TiO2 film is inhomogeneous, with the various reaction initiation centers or nucleation regions being randomly distributed throughout the photocatalyst surface. Furthermore, parallel FT-IR results showed that the reaction follows pseudo-first-order kinetics. To rationalize the obsd. random LB film photoetching and buckling behavior, we invoked a simple reaction model that incorporates the reactivity of the TiO2 film and LB film disorganization phenomenon during the photodegrdn. process.
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  11. 11
    Minabe, T.; Tryk, D. A.; Sawunyama, P.; Kikuchi, Y.; Hashimoto, K.; Fujishima, A. TiO2-mediated Photodegradation of Liquid and Solid Organic Compounds. J. Photochem. Photobiol., A 2000, 137, 53– 62,  DOI: 10.1016/s1010-6030(00)00350-6
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    11
    TiO2-mediated photodegradation of liquid and solid organic compounds
    Minabe, T.; Tryk, D. A.; Sawunyama, P.; Kikuchi, Y.; Hashimoto, K.; Fujishima, A.
    Journal of Photochemistry and Photobiology, A: Chemistry (2000), 137 (1), 53-62CODEN: JPPCEJ; ISSN:1010-6030. (Elsevier Science S.A.)
    The photocatalytic degrdn. of several liq. and solid org. compds., including polymers, with mol. wts. covering a wide range from 600 to 500,000 was studied on TiO2 thin films on glass under UV illumination. Nearly exact agreement was found between the wt. losses of the solid compds. octadecane and stearic acid and the wts. of CO2 produced during photocatalytic degrdn. No other gas-phase degrdn. product was detected for these two compds. other than CO2, which means that potentially harmful products are not expected to pose a problem. For convenient comparison of degrdn. rates for various compds. and measurement methods, the values were converted to nos. of moles of carbon reacted per square centimeter per h. Under appropriate conditions (50° C, relative humidity 10% in air), octadecane was completely decompd. (<400 ng-cm-2). In contrast, stearic acid did not decomp. completely, even after more than 80 h of UV illumination. This may be due to the formation of a photocatalytically inert reaction product that blocks the TiO2 surface. The decompn. rates for all of the compds. examd. spanned less than two orders of magnitude, suggesting that the photocatalytic reactions involved are rather versatile.
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  12. 12
    Remillard, J. T.; McBride, J. R.; Nietering, K. E.; Drews, A. R.; Zhang, X. Real Time in Situ Spectroscopic Ellipsometry Studies of the Photocatalytic Oxidation of Stearic Acid on Titania Films. J. Phys. Chem. B 2000, 104, 4440– 4447,  DOI: 10.1021/jp9936300
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    12
    Real Time in Situ Spectroscopic Ellipsometry Studies of the Photocatalytic Oxidation of Stearic Acid on Titania Films
    Remillard, J. T.; McBride, J. R.; Nietering, K. E.; Drews, A. R.; Zhang, X.
    Journal of Physical Chemistry B (2000), 104 (18), 4440-4447CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
    The authors used in situ spectroscopic ellipsometry to measure the photocatalytic activity of TiO2 films on fused SiO2 and glass substrates. Amorphous and anatase TiO2 films with a variety of microstructures were prepd. by reactive sputtering and pyrolytic deposition. The TiO2 films were coated with thin, spin-cast films of stearic acid [Me(CH2)16COOH] to represent an org. contaminant. Photooxidn. rates were detd. from ellipsometric measurements of the redn. in stearic acid film thickness during exposure to UV irradn. at 313 or 365 nm. The photooxidn. rate is proportional to Iα, where I is the irradiance. The exponent α correlated with the TiO2 crystallinity, having values of ∼0.7 and 0.8 for amorphous and anatase films, resp. The largest photooxidn. rate was obsd. for the pyrolytically deposited anatase sample on which x-ray reflectometry and spectroscopic ellipsometry measurements detected the presence of a low-d. TiO2 surface layer. To assess the performance of these films in practical applications, the specimens were exposed to wavelength and irradiance conditions that simulated a solar UV spectrum. The most photocatalytically active sample had a stearic acid film removal rate of 22 nm/h, which would be suitable for self-cleaning window applications.
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  13. 13
    Smirnova, N.; Fesenko, T.; Zhukovsky, M.; Goworek, J.; Eremenko, A. Photodegradation of Stearic Acid Adsorbed on Superhydrophilic TiO2 Surface: In Situ FT-IR and LDI Study. Nanoscale Res. Lett. 2015, 10, 500,  DOI: 10.1186/s11671-015-1210-y
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    13
    Photodegradation of Stearic Acid Adsorbed on Superhydrophilic TiO2 Surface: In Situ FT-IR and LDI Study
    Smirnova Natalia; Fesenko Tatiana; Zhukovsky Maxim; Eremenko Anna; Goworek Jacek
    Nanoscale research letters (2015), 10 (1), 500 ISSN:1931-7573.
    TiO2 films prepared by template-assisted sol-gel method were characterized by X-ray diffraction spectroscopy, scanning and atomic force electron microscopy, and Fourier transform infrared (FT-IR) spectroscopy. Based on the hexane adsorption-desorption analysis, the films have a surface area of 390-540 m(2)/g with pore size distribution narrowly centered around 10 nm. Optimal component ratio and condition of heat treatment of mesoporous titania films have been found. Photocatalytic activity of the coatings was determined by the destruction of stearic acid layers, monitored using FT-IR spectroscopy and laser desorption-ionization (LDI) mass spectrometry. Under UV illumination, all the used films reach hydrophilicity with water contact angle of 0°. As the result, hydrophobic fat acid molecules undergo self-association and active desorption from the hydrophilic surface during mass-spectrometric experiment.
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  14. 14
    Alotaibi, A. M.; Promdet, P.; Hwang, G. B.; Li, J.; Nair, S. P.; Sathasivam, S.; Kafizas, A.; Carmalt, C. J.; Parkin, I. P. Zn and N Codoped TiO2 Thin Films: Photocatalytic and Bactericidal Activity. ACS Appl. Mater. Interfaces 2021, 13, 10480– 10489,  DOI: 10.1021/acsami.1c00304
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    14
    Zn and N Codoped TiO2 Thin Films: Photocatalytic and Bactericidal Activity
    Alotaibi, Abdullah M.; Promdet, Premrudee; Hwang, Gi Byoung; Li, Jianwei; Nair, Sean P.; Sathasivam, Sanjayan; Kafizas, Andreas; Carmalt, Claire J.; Parkin, Ivan P.
    ACS Applied Materials & Interfaces (2021), 13 (8), 10480-10489CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    We explore a series of Zn and N codoped TiO2 thin films grown using chem. vapor deposition. Films were prepd. with various concns. of Zn (0.4-2.9 at. % Zn vs Ti), and their impact on superoxide formation, photocatalytic activity, and bactericidal properties were detd. Superoxide (·O2-) formation was assessed using a 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-5-[(phenylamino)carbonyl]-2H-tetrazolium sodium salt (XTT) as an indicator, photocatalytic activity was detd. from the degrdn. of stearic acid under UVA light, and bactericidal activity was assessed using a Gram-neg. bacterium E. coli under both UVA and fluorescent light (similar to what is found in a clin. environment). The 0.4% Zn,N:TiO2 thin film demonstrated the highest formal quantum efficiency in degrading stearic acid (3.3 x 10-5 mols.·photon-1), while the 1.0% Zn,N:TiO2 film showed the highest bactericidal activity under both UVA and fluorescent light conditions (>3 log kill). The enhanced efficiency of the films was correlated with increased charge carrier lifetime, supported by transient absorption spectroscopy (TAS) measurements.
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  15. 15
    Alotaibi, A. M.; Williamson, B. A. D.; Sathasivam, S.; Kafizas, A.; Alqahtani, M.; Sotelo-Vazquez, C.; Buckeridge, J.; Wu, J.; Nair, S. P.; Scanlon, D. O.; Parkin, I. P. Enhanced Photocatalytic and Antibacterial Ability of Cu doped Anatase TiO2 Thin Films: Theory and Experiment. ACS Appl. Mater. Interfaces 2020, 12, 15348– 15361,  DOI: 10.1021/acsami.9b22056
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    15
    Enhanced photocatalytic and antibacterial ability of Cu-doped anatase TiO2 thin films: theory and experiment
    Alotaibi, Abdullah M.; Williamson, Benjamin A. D.; Sathasivam, Sanjayan; Kafizas, Andreas; Alqahtani, Mahdi; Sotelo-Vazquez, Carlos; Buckeridge, John; Wu, Jiang; Nair, Sean P.; Scanlon, David O.; Parkin, Ivan P.
    ACS Applied Materials & Interfaces (2020), 12 (13), 15348-15361CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Multifunctional thin films which can display both photocatalytic and antibacterial activity are of great interest industrially. Here, for the first time, we have used aerosol-assisted chem. vapor deposition to deposit highly photoactive thin films of Cu-doped anatase TiO2 on glass substrates. The films displayed much enhanced photocatalytic activity relative to pure anatase and showed excellent antibacterial (vs Staphylococcus aureus and Escherichia coli) ability. Using a combination of transient absorption spectroscopy, photoluminescence measurements, and hybrid d. functional theory calcns., we have gained nanoscopic insights into the improved properties of the Cu-doped TiO2 films. Our anal. has highlighted that the interactions between substitutional and interstitial Cu in the anatase lattice can explain the extended exciton lifetimes obsd. in the doped samples and the enhanced UV photoactivities obsd.
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  16. 16
    Peeters, H.; Keulemans, M.; Nuyts, G.; Vanmeert, F.; Li, C.; Minjauw, M.; Detavernier, C.; Bals, S.; Lenaerts, S.; Verbruggen, S. W. Plasmonic Gold-embedded TiO2 Thin Films as Photocatalytic Self-cleaning Coatings. Appl. Catal., B 2020, 267, 118654,  DOI: 10.1016/j.apcatb.2020.118654
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    16
    Plasmonic gold-embedded TiO2 thin films as photocatalytic self-cleaning coatings
    Peeters, Hannelore; Keulemans, Maarten; Nuyts, Gert; Vanmeert, Frederik; Li, Chen; Minjauw, Matthias; Detavernier, Christophe; Bals, Sara; Lenaerts, Silvia; Verbruggen, Sammy W.
    Applied Catalysis, B: Environmental (2020), 267 (), 118654CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
    Transparent photocatalytic TiO2 thin films hold great potential in the development of self-cleaning glass surfaces, but suffer from a poor visible light response that hinders the application under actual sunlight. To alleviate this problem, the photocatalytic film can be modified with plasmonic nanoparticles that interact very effectively with visible light. Since the plasmonic effect is strongly concd. in the near surroundings of the nanoparticle surface, an approach is presented to embed the plasmonic nanostructures in the TiO2 matrix itself, rather than deposit them loosely on the surface. This way the interaction interface is maximised and the plasmonic effect can be fully exploited. In this study, pre-fabricated gold nanoparticles are made compatible with the org. medium of a TiO2 sol-gel coating suspension, resulting in a one-pot coating suspension. After spin coating, homogeneous, smooth, highly transparent and photoactive gold-embedded anatase thin films are obtained.
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  17. 17
    Cosham, S.; Celorrio, V.; Kulak, A. N.; Hyett, G. Observation of Visible Light Activated Photocatalytic Degradation of Stearic Acid on Thin Films of Tantalum Oxynitride Synthesized by Aerosol Assisted Chemical Vapour Deposition. Dalton Trans. 2019, 48, 10619– 10627,  DOI: 10.1039/c8dt04638g
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    17
    Observation of visible light activated photocatalytic degradation of stearic acid on thin films of tantalum oxynitride synthesized by aerosol assisted chemical vapor deposition
    Cosham, Samuel D.; Celorrio, Veronica; Kulak, Alexander N.; Hyett, Geoffrey
    Dalton Transactions (2019), 48 (28), 10619-10627CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
    UV activated photocatalysts deposited using chem. vapor deposition have found com. success as self-cleaning coatings. However, only limited work has been conducted on the use of the more recently discovered visible light activated photocatalysis for this application. Tantalum oxynitride is an established visible light photocatalyst, and in this paper we have investigated the ability of thin films of tantalum oxynitride to photocatalytically degrade a model org. pollutant, stearic acid, and therefore assess the coatings potential for self-cleaning applications. Thin films of tantalum oxide were formed using aerosol assisted chem. vapor deposition (AACVD) of tantalum ethoxide, and then converted into tantalum oxynitride through ammonolysis at temps. between 550°C and 750°C. Investigation of the films using XRD, UV-vis spectroscopy and XAFS identify that amorphous tantalum oxynitride is formed during the ammonolysis, with complete conversion to TaON under conditions of 700°C for 24 h. The self-cleaning ability of this film was assessed using stearic acid as the model pollutant, with a degrdn. rate of 2.5(2) × 1013 mols. per min per cm2 when exposed to a 5-sun solar simulator, equipped with a UV cut-off filter. We therefore conclude that tantalum oxynitride thin films are able to act as self-cleaning coatings through visible light photocatalysis and that films of tantalum oxynitride can be synthesized using a scalable chem. vapor deposition route.
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  18. 18
    Jiamprasertboon, A.; Kafizas, A.; Sachs, M.; Ling, M.; Alotaibi, A. M.; Lu, Y.; Siritanon, T.; Parkin, I. P.; Carmalt, C. J. Heterojunction α-Fe2O3/ZnO Films with Enhanced Photocatalytic Properties Grown by Aerosol-assisted Chemical Vapour Deposition. Chem.─Eur. J. 2019, 25, 11337– 11345,  DOI: 10.1002/chem.201902175
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    18
    Heterojunction α-Fe2O3/ZnO films with enhanced photocatalytic properties grown by aerosol-assisted chemical vapor deposition
    Jiamprasertboon, Arreerat; Kafizas, Andreas; Sachs, Michael; Ling, Min; Alotaibi, Abdullah M.; Lu, Yao; Siritanon, Theeranun; Parkin, Ivan P.; Carmalt, Claire J.
    Chemistry - A European Journal (2019), 25 (48), 11337-11345CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Type I heterojunction films of α-Fe2O3/ZnO are reported here as a non-titania based photocatalyst, which shows remarkable enhancement in the photocatalytic properties towards stearic acid degrdn. under UVA-light exposure (λ=365 nm), with a quantum efficiency of ξ=4.42±1.54×10-4 mols. degraded/photon, which was about 16 times greater than that of α-Fe2O3, and 2.5 times greater than that of ZnO. Considering that the degrdn. of stearic acid requires 104 electron transfers for each mol., this represents an overall quantum efficiency of 4.60 % for the α-Fe2O3/ZnO heterojunction. Time-resolved transient absorption spectroscopy (TAS) revealed the charge-carrier behavior responsible for this increase in activity. Photogenerated electrons, formed in the ZnO layer, were transferred into the α-Fe2O3 layer on the pre-μs timescale, which reduced electron-hole recombination. This increased the lifetime of photogenerated holes formed in ZnO, which oxidise stearic acid. The heterojunction α-Fe2O3/ZnO films grown herein show potential environmental applications as coatings for self-cleaning windows and surfaces.
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  19. 19
    Li, D.; Bulou, S.; Gautier, N.; Elisabeth, S.; Goullet, A.; Richard-Plouet, M.; Choquet, P.; Granier, A. Nanostructure and Photocatalytic Properties of TiO2 Films Deposited at Low Temperature by Pulsed PECVD. Appl. Surf. Sci. 2019, 466, 63– 69,  DOI: 10.1016/j.apsusc.2018.09.230
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    19
    Nanostructure and photocatalytic properties of TiO2 films deposited at low temperature by pulsed PECVD
    Li, D.; Bulou, S.; Gautier, N.; Elisabeth, S.; Goullet, A.; Richard-Plouet, M.; Choquet, P.; Granier, A.
    Applied Surface Science (2019), 466 (), 63-69CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    The nanostructure and photocatalytic properties of TiO2 thin films deposited by PECVD on silicon substrates were investigated. The films were grown at low temp. (<120 °C) in an rf inductively coupled oxygen/titanium tetraisopropoxide plasma, in continuous and pulsed modes with different plasma-on time (via variation of the duty cycle, DC). All the films exhibit nano-columnar structures, but the redn. of plasma-on time by decreasing the duty cycle for pulsed mode leads to a more homogenous morphol. with a diminished column size, and a decrease in the surface roughness. TiO2 layers contg. a high amt. of anatase were grown at substrate temps. less than 100 °C corresponding to DC ≥ 40%, then the crystn. was hindered with the decrease of DC, even inducing amorphous films for DC ≤ 10%. Moreover, the films deposited below 100 °C with deposition conditions where 50% ≤ DC ≤ 75% were shown to present a high photocatalytic activity, likely due to the presence of anatase cryst. nanocolumns at the surface.
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  20. 20
    Paz, Y.; Luo, Z.; Rabenberg, L.; Heller, A. Photooxidative Self-cleaning Transparent Titanium Dioxide Films on Glass. J. Mater. Res. 1995, 10, 2842– 2848,  DOI: 10.1557/jmr.1995.2842
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    Photooxidative self-cleaning transparent titanium dioxide films on glass
    Paz, Y.; Luo, Z.; Rabenberg, L.; Heller, A.
    Journal of Materials Research (1995), 10 (11), 2842-8CODEN: JMREEE; ISSN:0884-2914. (Materials Research Society)
    In the context of studying the feasibility of photocatalytically self-cleaning windows and windshields, clear, abrasion resistant, photocatalytic films of TiO2 were formed on soda-lime glass and on vitreous silica by a sol-gel process. The rate of photooxidn. of contaminant deposits was estd. by measuring the rate of decrease in the integrated IR absorbance assocd. with the C-H stretching vibrations of a thin soln.-cast film of stearic acid under 365 nm (2.4 mW/cm2) or 254 nm (0.8 mW/cm2) irradn. Approx. 3 × 10-4 steric acid mols. were stripped per 365 nm photon in either front- or back-illuminated soda-lime glass, and 6 × 10-4 mols. when the films were coated on vitreous silica. For thin TiO2 films on vitreous silica, the rate of photooxidn., normalized by the no. of photons absorbed per unit area, was independent of the wavelength. In contrast, for films on soda lime glass, the rate of photooxidn., when similarly normalized, was higher for the less penetrating wavelength. The reduced photoactivity on glass at the deeply penetrating wavelength (365 nm), as was the greater photoefficiency on vitreous silica than on glass, are attributed to diffusion of sodium oxide from the glass into the inner glass-contacting zone of the TiO2 layer.
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  21. 21
    Fretwell, R.; Douglas, P. An Active, Robust and Transparent Nanocrystalline Anatase TiO2 Thin Film ─ Preparation, Characterisation and the Kinetics of Photodegradation of Model Pollutants. J. Photochem. Photobiol., A 2001, 143, 229– 240,  DOI: 10.1016/s1010-6030(01)00526-3
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    21
    An active, robust and transparent nanocrystalline anatase TiO2 thin film - preparation, characterization and the kinetics of photodegradation of model pollutants
    Fretwell, R.; Douglas, P.
    Journal of Photochemistry and Photobiology, A: Chemistry (2001), 143 (2-3), 229-240CODEN: JPPCEJ; ISSN:1010-6030. (Elsevier Science S.A.)
    The prepn., characterization, and photocatalytic activity of robust and transparent nanocryst. thin films of anatase TiO2 are described. The films, which are prepd. from a sol-gel can be either dip-coated or spin-coated, however, the latter have better optical and mech. properties, since dip coating results in the retention of a significant amt. of the org. stabilizers used in the sol-gel prepn. Quantum yields for the photooxidn. of 4-chlorophenol (Φ4CP) on these spin-coated films are similar to those reported previously for TiO2 films and dispersions, with Φ4CP = 0.5-1%. For films of different thicknesses, and therefore different absorption efficiencies, Φ4CP depends only upon the total no. of photons absorbed, i.e. Φ Is independent of both the absorbed photon flux and the distribution of charge carriers in the film. By way of contrast, the rate of photoredn. of Methylene blue is independent of film thickness and appears to be limited by the surface area of the film. Hence, quantum yields for Methylene blue redn. (ΦMBR) fall significantly from 1.7 to 0.5% as film thickness, and hence photoabsorption efficiency, is increased. These pure anatase films do not show any deleterious sodium ion effect for either 4-chlorophenol oxidn. or Methylene blue redn. Quantum yields for photooxidn. of stearic acid (ΦSA) deposited directly onto the films are relatively high with ΦSA = 1-4%. This, combined with the excellent optical and mech. properties of these films, suggests that they may be of particular interest for the development of 'self-cleaning windows'.
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  22. 22
    Manning, T. D.; Parkin, I. P.; Clark, R. J. H.; Sheel, D.; Pemble, M. E.; Vernadou, D. Intelligent Window Coatings: Atmospheric Pressure Chemical Vapour Deposition of Vanadium Oxides. J. Mater. Chem. 2002, 12, 2936– 2939,  DOI: 10.1039/b205427m
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    Intelligent window coatings: atmospheric pressure chemical vapour deposition of vanadium oxides
    Manning, Troy D.; Parkin, Ivan P.; Clark, Robin J. H.; Sheel, David; Pemble, Martyn E.; Vernadou, Dimitra
    Journal of Materials Chemistry (2002), 12 (10), 2936-2939CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
    Thin films of the vanadium oxides, V2O5, VO2, VOx (x = 2.00-2.50) and V6O13 were prepd. on glass substrates by atm. pressure chem. vapor deposition (APCVD) of vanadium tetrachloride and water at 400-550°C. The specific phase deposited was dependent on the substrate temp. and the reagent concns. The films were characterized by Raman microscopy, X-ray diffraction (XRD), RBS, SEM, energy dispersive anal. by X-rays (EDX), reflectance/transmittance and UV absorption spectroscopy. The VO2 films show reversible switching behavior at 68°C, assocd. with a phase change from monoclinic (MoO2 structure) to tetragonal (TiO2, rutile structure).
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    Mills, A.; Wang, J. Simultaneous Monitoring of the Destruction of Stearic Acid and Generation of Carbon Dioxide by Self-cleaning Semiconductor Photocatalytic Films. J. Photochem. Photobiol., A 2006, 182, 181– 186,  DOI: 10.1016/j.jphotochem.2006.02.010
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    23
    Simultaneous monitoring of the destruction of stearic acid and generation of carbon dioxide by self-cleaning semiconductor photocatalytic films
    Mills, Andrew; Wang, Jishun
    Journal of Photochemistry and Photobiology, A: Chemistry (2006), 182 (2), 181-186CODEN: JPPCEJ; ISSN:1010-6030. (Elsevier B.V.)
    The destruction of stearic acid (SA), the SA test, is a popular approach used to evaluate the activities of photocatalytic films. The destruction of SA via semiconductor photocatalysis is monitored simultaneously, using FT-IR spectroscopy, via the disappearance of SA and the appearance of CO2. Sol-gel and P25 films of titania are used as the semiconductor photocatalytic self-cleaning films. A conversion factor is used of 9.7 × 1015 mols. of SA cm-2 ≡ 1 cm-1 integrated areas of the peaks in the FT-IR of SA over the range 2700-3000 cm-1, which is three times that reported previously by others. As the SA disappeared the concomitant amt. of CO2 generated was >90% that expected throughout the photomineralization process for the sol-gel titania film. In contrast, the slightly more active, and scattering, P25 titania films generated CO2 levels that dipped as low as 69% during the course of the photoreaction, but eventually recovered to ≃100% that expected based on the amt. of SA present. The importance of these results with respect to SA test and the evaluation of new and existing self-cleaning films are discussed briefly.
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    Allain, E.; Besson, S.; Durand, C.; Moreau, M.; Gacoin, T.; Boilot, J. P. Transparent Mesoporous Nanocomposite Films for Self-Cleaning Applications. Adv. Funct. Mater. 2007, 17, 549– 554,  DOI: 10.1002/adfm.200600197
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    Transparent mesoporous nanocomposite films for self-cleaning applications
    Allain, Emmanuelle; Besson, Sophie; Durand, Clarisse; Moreau, Melanie; Gacoin, Thierry; Boilot, Jean-Pierre
    Advanced Functional Materials (2007), 17 (4), 549-554CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A versatile approach is studied for the elaboration of TiO2 based photocatalytic coatings for self-cleaning applications on transparent substrates. The basic principle of the synthesis relies on the use of preformed TiO2 colloidal particles that are further dispersed within a transparent silica binder with a mesoporous structure. Film porosity in the nanometer range is controlled by achieving the sol-gel silica condensation around self-organized micellar assemblies of a templating copolymer surfactant. The latter also acts as a stabilizer for the TiO2 particles, thus preserving their high dispersion within the film so that excellent optical properties are maintained even for high TiO2 loading (up to 50 %). Studies of photodegrdn. kinetics show that such mesoporous films are at least 15 times more active than films synthesized with a usual microporous silica binder. Moreover, the measured quantum-yield efficiency (1.1 %) is found to be among the highest reported up to now. Improved photoactivity of the films is discussed as resulting from the closer proximity between the org. mols. and the surface of the TiO2 crystallites as well as the improved diffusion rate of water and oxygen through the interconnected pore network.
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    Peruchon, L.; Puzenat, E.; Girard-Egrot, A.; Blum, L.; Herrmann, J. M.; Guillard, C. Characterization of self-cleaning glasses using Langmuir–Blodgett technique to control thickness of stearic acid multilayers: Importance of spectral emission to define standard test. J. Photochem. Photobiol., A 2008, 197, 170– 176,  DOI: 10.1016/j.jphotochem.2007.12.033
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    25
    Characterization of self-cleaning glasses using Langmuir-Blodgett technique to control thickness of stearic acid multilayers. Importance of spectral emission to define standard test.
    Peruchon, L.; Puzenat, E.; Girard-Egrot, A.; Blum, L.; Herrmann, J. M.; Guillard, C.
    Journal of Photochemistry and Photobiology, A: Chemistry (2008), 197 (2-3), 170-176CODEN: JPPCEJ; ISSN:1010-6030. (Elsevier B.V.)
    The photocatalytic properties of commercialized self-cleaning glasses have been evaluated by the degrdn. of stearic acid C17H35COOH (SA), selected as a model mol. representative of fatty dirt. The relative amt. of SA was detd. by measuring the FTIR peak area in the range 2752 and 2992 cm-1 corresponding to the C-H stretching bands. To est. the thickness of one layer, the deposit of SA was performed using the Langmuir-Blodgett (LB) method, which allows obtaining organized monomol. layers of amphiphilic orgs. on a substrate. The thickness of one SA layer, obtained by Langmuir-Blodgett deposition, was correlated to the FTIR signal. The photocatalytic degrdn. of a SA layer of a controlled thickness was carried out in a double jacket Pyrex reactor equipped with a UV Philips HPK 125 lamp. The photocatalytic degrdn. rate of the SA layer with different thicknesses was detd. The SA degrdn. rate was studied under UV-A and UV-B irradiations to det. the influence of the photon wavelength range on efficiency.
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    Peruchon, L.; Puzenat, E.; Herrmann, J. M.; Guillard, C. Photocatalytic efficiencies of self-cleaning glasses. Influence of physical factors. Photochem. Photobiol. Sci. 2009, 8, 1040– 1046,  DOI: 10.1039/b902139f
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    Photocatalytic efficiencies of self-cleaning glasses. Influence of physical factors
    Peruchon, L.; Puzenat, E.; Herrmann, J. M.; Guillard, C.
    Photochemical & Photobiological Sciences (2009), 8 (7), 1040-1046CODEN: PPSHCB; ISSN:1474-905X. (Royal Society of Chemistry)
    Two com. types of self-cleaning glass (SCG) have been tested to confirm the real photocatalytic nature of their properties. This was done by using four photocatalytic tests: (i) in the gas phase with the total oxidn. of acetylene; (ii) in water with the total degrdn. of malic acid, (iii) in water with the total degrdn. of Methylene blue, and (iv) in the solid phase with the total oxidative degrdn. of a layer of stearic acid deposited on the self-cleaning glass surface, in contact with the superficial titania coating. The influence of various factors (temp., humidity, wavelength, radiant flux, presence of inorg. particles stuck at the glass surface) was explained in line with the fundamentals of photocatalysis. The results helped to understand the behavior of self-cleaning glass.
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    Mills, A.; Lee, S.; Lepre, A.; Parkin, I. P.; O’Neill, S. A. Spectral and photocatalytic characteristics of TiO2 CVD films on quartz. Photochem. Photobiol. Sci. 2002, 1, 865– 868,  DOI: 10.1039/b205715h
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    27
    Spectral and photocatalytic characteristics of TiO2 CVD films on quartz
    Mills, Andrew; Lee, Soo-Keun; Lepre, Anne; Parkin, Ivan P.; O'Neill, Shane A.
    Photochemical & Photobiological Sciences (2002), 1 (11), 865-868CODEN: PPSHCB; ISSN:1474-905X. (Royal Society of Chemistry)
    A series of novel chem. vapor deposited (CVD) films of titanium(IV) oxide of different thicknesses, spanning the range 10-91 nm, are prepd. on quartz, via the reaction of titanium(IV) chloride and Et acetate, using a CVD technique. The films are clear, mech. robust and comprise thin layer of nanocryst. anatase titania of different thicknesses that absorb UV light. The UV-Visible spectral profiles of all the CVD TiO2 films of different thickness are the same and obey Lambert's law. A plot of the reciprocal length for the TiO2 coating vs. wavelength is reported. The photocatalytic activity of each film to mediate the destruction of a thin layer of stearic acid is investigated. The rate depends directly upon the fraction of light absorbed and the apparent quantum yield for the overall process is 0.00035, which appears low compared with that for sol-gel TiO2 films.
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    Alofi, S.; O’Rourke, C.; Mills, A. Modelling the Kinetics of Stearic Acid Destruction on TiO2 ‘Self-cleaning’ Photocatalytic Films. Appl. Catal., A 2022, 647, 118899,  DOI: 10.1016/j.apcata.2022.118899
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    28
    Modelling the kinetics of stearic acid destruction on TiO2 'self-cleaning' photocatalytic films
    Alofi, Saleh; O'Rourke, Christopher; Mills, Andrew
    Applied Catalysis, A: General (2022), 647 (), 118899CODEN: ACAGE4; ISSN:0926-860X. (Elsevier B.V.)
    The kinetics of stearic acid, SA, removal by photocatalytic films are modelled using a log-normal distribution in surface reactivity, which shows that as the distribution in reactivity, ρ, increases, so too does the apparent order of reaction, m, of the obsd. variation in SA concn., [SA], vs. irradn. time, t. The model is used to fit the obsd. [SA] vs. t profiles exhibited by three very different TiO2 films, namely Activ self-cleaning glass, and prepd. sol-gel TiO2 and P25 TiO2 films. These films show decay kinetics, with m = 0.3, 0.44 and 0.6, resp., which fit the model predicted decay profiles with distribution width, ρ, values, 0.4, 0.54 and 0.7, resp. A Ag photocatalyst indicator ink is used on these films to highlight the non-uniform distribution in surface reactivity on the micron scale. The use of the log-normal model to better understand the kinetics exhibited by photocatalytic films is discussed briefly.
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    Sawunyama, P.; Fujishima, A.; Hashimoto, K. Photocatalysis on TiO2 Surfaces Investigated by Atomic Force Microscopy: Photodegradation of Partial and Full Monolayers of Stearic Acid on TiO2(110). Langmuir 1999, 15, 3551– 3556,  DOI: 10.1021/la9814440
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    Photocatalysis on TiO2 Surfaces Investigated by Atomic Force Microscopy: Photodegradation of Partial and Full Monolayers of Stearic Acid on TiO2(110)
    Sawunyama, Phillip; Fujishima, Akira; Hashimoto, Kazuhito
    Langmuir (1999), 15 (10), 3551-3556CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    The authors studied the nature and surface morphol. changes assocd. with the photodegrdn. of stearic acid LB films on TiO2(110). Interestingly, submonolayers of stearic acid consisted of circular domains of various sizes - a feature very attractive for monitoring TiO2 photocatalysis by AFM. The authors noted that there was no bulk differential reactivity at island edges compared to the interior. This suggests that the rate of photodegrdn. of the stearic acid mols. is independent of their location in the island. Accordingly, the overall surface reactivity trends were similar for both partial films and complete films. Likewise, the obsd. inhomogeneous reactivity patterns appear to be a reflection of the transient distribution of the reaction centers.
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    Ghazzal, M. N.; Barthen, N.; Chaoui, N. Photodegradation Kinetics of Stearic Acid on UV-irradiated Titania Thin Film Separately Followed by Optical Microscopy and Fourier Transform Infrared Spectroscopy. Appl. Catal., B 2011, 103, 85– 90,  DOI: 10.1016/j.apcatb.2011.01.013
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    Photodegradation kinetics of stearic acid on UV-irradiated titania thin film separately followed by optical microscopy and Fourier transform infrared spectroscopy
    Ghazzal, M. N.; Barthen, N.; Chaoui, N.
    Applied Catalysis, B: Environmental (2011), 103 (1-2), 85-90CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
    We use optical microscopy for observing the photocatalytic decompn. of SA on TiO2 film during exposure to a simulated solar illumination. The TiO2 films were prepd. via sol-gel and deposited on a borosilicate glass substrate by dip-coating. A low amt. of SA was deposited by dipping the TiO2 film in a methanolic SA soln. Optical micrographs showed that the deposited SA forms islands with variable heights and sizes and that upon UV-irradn., these latter decompd. gradually from the edges towards the center. The SA photodecompn. as evaluated by measuring the integrated IR absorbance of SA in the 2700-3000 cm-1 region, proceeded according to a pseudo 1st-order kinetics with respect to SA. By contrast, it was of the zero-order type when monitoring the decompn. of an individual SA island by microscopy. We showed that the pseudo 1st-order kinetics is the result of a collective effect of many independent and parallel zero-order reactions with variable durations. This scenario describes well both the exptl. data and those reported in literature.
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    Jiang, X.; Manawan, M.; Feng, T.; Qian, R.; Zhao, T.; Zhou, G.; Kong, F.; Wang, Q.; Dai, S.; Pan, J. H. Anatase and rutile in evonik aeroxide P25: Heterojunctioned or individual nanoparticles?. Catal. Today 2018, 300, 12– 17,  DOI: 10.1016/j.cattod.2017.06.010
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    Anatase and rutile in evonik aeroxide P25: Heterojunctioned or individual nanoparticles?
    Jiang, Xiongzhen; Manawan, Maykel; Feng, Ting; Qian, Ruifeng; Zhao, Ting; Zhou, Guanda; Kong, Fantai; Wang, Qing; Dai, Songyuan; Pan, Jia Hong
    Catalysis Today (2018), 300 (), 12-17CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
    Evonik Aeroxide P25 (formerly Degussa P25) is a flame-made multiphasic TiO2 having nanoparticles contg. anatase and rutile, as well as a small amt. of amorphous TiO2. The past decades have witnessed the wide applications of P25 as a benchmark material for studying photocatalytic mechanism, materials and process. However, controversy remains regarding the microstructure of anatase and rutile: do they interwoven forming heterojunction structure or exist individually. To clarify it, we selected a medium alk., LiOH to erode bare P25 under a mild hydrothermal condition. Since rutile presents much higher resistances towards dissoln. by LiOH than anatase, it is reasonable to find during hydrothermal reaction that the ratio of anatase to rutile (A/R) gradually decrease if they exist individually. Reversely, the A/R value gradually increases at the beginning of the hydrothermal reaction, implying that rutile shows high activity towards dissoln. and phase transformation to lithium titanate. As calcd., around 15% rutile nanoparticles more likely exist on the surface of anatase with the formation of a heterojunction structure, although isolated nanoparticles with sole rutile phase coexist. In addn. to XRD anal., TEM measurement shows that the Moire fringes are frequently present, which further manifests that some anatase particles are covered with rutile clusters or thin overlayers.
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    Ito, S.; Chen, P.; Comte, P.; Nazeeruddin, M. K.; Liska, P.; Pechy, P.; Gratzel, M. Fabrication of Screen-printing Pastes from TiO2 Powders for Dye-sensitised Solar Cells. Prog. Photovoltaics 2007, 15, 603– 612,  DOI: 10.1002/pip.768
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    Fabrication of screen-printing pastes from TiO2 powders for dye-sensitized solar cells
    Ito, Seigo; Chen, Peter; Comte, Pascal; Nazeeruddin, Mohammad Khaja; Liska, Paul; Pechy, Peter; Graetzel, Michael
    Progress in Photovoltaics (2007), 15 (7), 603-612CODEN: PPHOED; ISSN:1062-7995. (John Wiley & Sons Ltd.)
    A prepn. technique of TiO2 screen-printing pastes from com.-available powders was described to fabricate the nanocryst. layers without cracking and peeling-off over 17 μm thickness for the photoactive electrodes of the dye-sensitized solar cells. A conversion efficiency of 8.7% was obtained by using a single-layer of a semi-transparent-TiO2 film. A conversion efficiency of 9.2% was obtained by using double-layers composed of transparent and light-scattering TiO2 films for a photon-trapping system.
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    Mills, A.; Hill, G.; Crow, M.; Hodgen, S. Thick Titania Films for Semiconductor Photocatalysis. J. Appl. Electrochem. 2005, 35, 641– 653,  DOI: 10.1007/s10800-005-1628-5
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    Thick titania films for semiconductor photocatalysis
    Mills, Andrew; Hill, George; Crow, Matthew; Hodgen, Stephanie
    Journal of Applied Electrochemistry (2005), 35 (7-8), 641-653CODEN: JAELBJ; ISSN:0021-891X. (Springer)
    A review. A brief overview of work carried out by this group on thick (>1 μm), optically clear, robust titania films prepd. by a sol-gel method, as well as new results regarding these films, are described. Such films are very active as photocatalysts and able to destroy stearic acid with a quantum yield of 0.32%. The activity of such films is largely unaffected by annealing temps. <760°, but is drastically reduced above this temp. The drop in photocatalyst activity of such films as a function of annealing temp. appears to correlate well with the change in porosity of the films and suggests that the latter parameter is very important in deciding the overall activity of such films. The importance of porosity in semiconductor photocatalyzed cold combustion may be due to the effect it has on access of oxygen to the active sites, rather like the effect the position of a fire grate (open or closed) has on the rate of burning, i.e., hot combustion, that takes place in a fireplace.
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    O’Rourke, C.; Mills, A. Probing P25 TiO2 Photocatalysis Using Photoinduced Absorption Spectroscopy (PIAS). Chem. Commun. 2021, 57, 1242– 1245,  DOI: 10.1039/d0cc07794a
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    Probing P25 TiO2 photocatalysis using photoinduced absorption spectroscopy (PIAS)
    O'Rourke, Christopher; Mills, Andrew
    Chemical Communications (Cambridge, United Kingdom) (2021), 57 (10), 1242-1245CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    Photoinduced absorption spectroscopy, PIAS, is used for the first time to probe the kinetics exhibited by the most commonly employed powder photocatalyst, P25 TiO2, in mesoporous film form, in the photocatalyzed oxidn. of a commonly used test org. pollutant, 4-chlorophenol, 4CP. The results show that PIAS can provide previously unobtainable, invaluable, direct kinetic, and mechanistic, information concerning photogenerated holes in powd. photocatalysts.
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    Mills, A.; Bingham, M.; O’Rourke, C. Ring Photocatalysis: Bands of Activation Surrounding Macro-Sized (≥1 mm Radius) Pt Islands Deposited on TiO2 Films. J. Phys. Chem. C 2020, 124, 13550– 13559,  DOI: 10.1021/acs.jpcc.0c02636
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    35
    Ring Photocatalysis: Bands of Activation Surrounding Macro-Sized (≥1 mm Radius) Pt Islands Deposited on TiO2 Films
    Mills, Andrew; Bingham, Michael; O'Rourke, Christopher
    Journal of Physical Chemistry C (2020), 124 (25), 13550-13559CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Clearly discernible rings of high photocatalytic activity, i.e., rings of activation, RoA, are formed around Pt macro-sized (radius: ro ≥ 1 mm radius) Pt islands, i.e., Pt "dots", deposited on TiO2 films, which have been covered with a visible pollutant, such as soot, or a dye, such as acid orange 7. The features of the RoA region appear very similar for soot- or dye-covered films. Thus, the radius of the RoA region, ra, appears to depend directly on ro, plus a const., typically 0.22-0.68 mm. The rate of photocatalytic removal of the surface pollutant inside the RoA is uniform and significantly greater, i.e., ca. 3.2-5x, than that outside the RoA region. RoA region formation is obsd. for (i) Pt dots which have been photodeposited or created through thermal redn., (ii) Pt dots on sol-gel and com. (paints and tiles) TiO2 films, and (iii) Pd dots, but less clearly for Au dots. A simple mechanistic rationale for RoA region formation is presented, and the potential importance of this feature, with regard to improving the performance of com. photocatalytic films, is discussed briefly.
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    Ollis, D. Removal kinetics of stearic acid discrete deposits on photocatalytic self-cleaning surfaces: Effect of deposit initial size distribution. Appl. Catal., B 2017, 209, 174– 182,  DOI: 10.1016/j.apcatb.2017.02.029
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    Removal kinetics of stearic acid discrete deposits on photocatalytic self-cleaning surfaces: Effect of deposit initial size distribution
    Ollis, David
    Applied Catalysis, B: Environmental (2017), 209 (), 174-182CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
    Earlier reports by Sawunyama et al. (1997, 1999) and Ghazzal et al. (2011) used at. force microscopy (AFM) and optical microscopy to characterize the structure of stearic acid (SA) deposits on polycryst. TiO2 films and Ti [110] crystal surfaces. Their prepn. methods and catalysts yielded island-like SA deposits, rather than continuous films, for stearic acid submonolayers and multi-layers. Ghazzal obsd. that individual islands showed disappearance kinetics to be zero order with respect to island area, but apparently first order with respect to total SA remaining. We develop here kinetic models which are consistent with these "island" findings and establish the guidelines for apparent kinetic order of reaction, n:1. Island deposits of a single size will exhibit apparent zero order kinetics., n = 0.2. Island deposits with a narrow distribution of island sizes will exhibit an apparent half order behavior, n = 0.5.3. Island deposits with a broad size distribution will exhibit an apparent reaction order of unity, n = 1. Our conclusion is that meaningful kinetic anal. of SA island photocatalyzed disappearance, and thus that of any oxidizable, carbonaceous pollutant which deposits in island form, rather than as a continuous film, requires knowledge of the initial island size distribution. The SA deposits may also undergo significant rearrangement as the titania surface is illuminated, quickly transforming the latter from hydrophobic to hydrophilic. Thus it is the reactant film and/or island distribution after initiation of illumination which is most relevant to kinetic modeling attempts.
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  • Abstract

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    Figure 1

    Figure 1. Plot of area (in pixels from optical micrograph) vs irradiation time for three SA islands of similar initial thickness but different initial areas, on a sol–gel TiO2 film upon irradiation with 2.5 mW cm–2 UVA radiation. Insert diagram illustrates the reported outlines of the three different-sized islands at (from left to right) t = 0, 5, and 10 min, respectively. (30)

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    Figure 2

    Figure 2. SEM images of the surfaces of (a) Activ glass, (b) P25 TiO2 coating, and (c) sol–gel TiO2 coating, which reveal TiO2 particle sizes of ca. 32, 40, and 44 nm, respectively.

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    Figure 3

    Figure 3. 2D kinetic model-predicted plots, for a 2D sheet of SA covering 100 identical (ki = k0) photocatalytic sites, with each site, i, initially loaded with SA with hi,0 = 100 (a.u.), of: (a) (insert) SA film thickness, hi,τ, vs site number, i, and (main diagram) hi,τ vs τ, calculated using eq 13 of the 2D model; in the insert plot, the different calculated profiles correspond to τ values of (from top to bottom) 0, 0.2, 0.4, 0.6, and 0.8, respectively, and (b) variation in the fraction of SA, fSA,τ calculated using eq 12 of the 2D model, vs τ.

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    Figure 4

    Figure 4. (a) Photographic images recorded using a microscope and (b) thickness, ht, and width, Wt, values determined using a profilometer of a single cylindrical SA island on Activ as a function of irradiation time, t. The UVC light source used for this work comprised two 15 W germicidal lamps (2 mW cm–2).

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    Figure 5

    Figure 5. (a) Photographic images recorded as a function of t for an array of cylindrical SA islands on Activ and (b) insert plot, FT-IR spectra of the array recorded every 60 min during irradiation, and the main diagram of the fractional integrated absorbance, fSA,t, (=Absint(t)/Absint (t = 0) = [SA]T,t/[SA]T,0), determined using the spectra in the insert plot, vs t. The UVC light source used for this work comprised two 15 W germicidal lamps (2 mW cm–2).

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    Figure 6

    Figure 6. Photographic images recorded using an optical microscope, as a function of irradiation time t, of a single cylindrical SA island on a P25 TiO2 film. The UVC light source used for this work comprised two 15 W germicidal lamps (2 mW cm–2).

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    Figure 7

    Figure 7. FT-IR spectra of the array recorded every 30 min during irradiation and main diagram of the fractional integrated absorbance, fSA,t, (= Absint(t)/Absint (t = 0) = [SA]T,t/[SA]T,0), determined using the spectra in the insert plot, vs t. The UVC light source used for this work comprised two 15 W germicidal lamps (2 mW cm–2).

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    Figure 8

    Figure 8. 2D kinetic model-predicted decay curves for the photocatalyzed destruction of a “volcano” shaped island of SA on a photocatalytic film of uniform activity. The insert plot on the top right-hand side illustrates the predicted variation in hi,τ vs i profiles at irradiation times, τ, of (from top to bottom) 0, 0.2, 0.4, 0.6, 0.8, and 0.9, respectively. The main diagram and bottom left-hand side insert plots are the predicted variations in hmax,τ and normalized area, aτ/a0, vs τ, respectively. The data points were derived for the large (●), medium (■), and small (▲) SA islands, using the three data sets in Figure 1.

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    Figure 9

    Figure 9. (a) Plot of the profile of a SA volcano-shaped island as a function of irradiation time, t, where t was (from top to bottom) 0, 10, 20, and 30 h, respectively; (b) based on the data in (a), subsequent plot hmax and W as a function of t. The irradiation was performed in situ in the profilometer using a fibre optic deliverying 365 nm radiation, with an irradiance of 80 mW cm–2.

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  • References

    This article references 36 other publications.

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      Pilkington, Self-cleaning Glass, https://www.pilkington.com/en-gb/uk/householders/types-of-glass/self-cleaning-glass (accessed: Feb, 2023)
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      Glass News, Guardian Launches Climaguard Blue Self-Cleaning Solar Control Glass for Conservatory Roof Applications, https://www.glassnews.co.uk/guardian-launches-climaguard-blue-self-cleaning-solar-control-glass-for-conservatory-roof-applications/(accessed Feb, 2023)
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      Mills, A.; Lepre, A.; Elliott, N.; Bhopal, S.; Parkin, I. P.; O’Neill, S. A. Characterisation of the Photocatalyst Pilkington Activ: A Reference Film Photocatalyst?. J. Photochem. Photobiol., A 2003, 160, 213– 224,  DOI: 10.1016/s1010-6030(03)00205-3
      4
      Characterization of the photocatalyst Pilkington Activ: a reference film photocatalyst?
      Mills, Andrew; Lepre, Anne; Elliott, Nicholas; Bhopal, Sharan; Parkin, Ivan P.; O'Neill, S. A.
      Journal of Photochemistry and Photobiology, A: Chemistry (2003), 160 (3), 213-224CODEN: JPPCEJ; ISSN:1010-6030. (Elsevier Science B.V.)
      Pilkington Glass Activ represents a possible suitable successor to P25 TiO2, esp. as a benchmark photocatalyst film for comparing other photocatalyst or PSH self-cleaning films. Activ is a glass product with a clear, colorless, effectively invisible, photocatalytic coating of titania that also exhibits PSH. Although not as active as a film of P25 TiO2, Activ vastly superior mech. stability, very reproducible activity and widespread com. availability makes it highly attractive as a ref. photocatalytic film. The photocatalytic and photo-induced superhydrophilitic (PSH) properties of Activ are studied in some detail and the results reported. Thus, the kinetics of stearic acid destruction (a 104 electron process) are zero order over the stearic acid range 4-129 monolayers and exhibit formal quantum efficiencies (FQE) of 0.7×10-5 and 10.2×10-5 mols. per photon when irradiated with light of 365±20 and 254 nm, resp.; the latter appears also to be the quantum yield for Activ at 254 nm. The kinetics of stearic acid destruction exhibit Langmuir-Hinshelwood-like satn. type kinetics as a function of oxygen partial pressure, with no destruction occurring in the absence of oxygen and the rate of destruction appearing the same in air and oxygen atmospheres. Further kinetic work revealed a Langmuir adsorption type const. for oxygen of 0.45±0.16 kPa-1 and an activation energy of 19±1 kJ mol-1. A study of the PSH properties of Activ reveals a high water contact angle (67°) before ultra-bandgap irradn. reduced to 0° after prolonged irradn. The kinetics of PSH are similar to those reported by others for sol-gel films using a low level of UV light. The kinetics of contact angle recovery in the dark appear monophasic and different to the biphasic kinetics reported recently by others for sol-gel films [J. Phys. Chem. B 107 (2003) 1028]. Overall, Activ appears a very suitable ref. material for semiconductor film photocatalysis.
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      British Standards Institution. BS EN 1096-5. Glass in Building. Coated Glass. Test Method and Classification for the Self-cleaning Performances of Coated Glass Surfaces . Retrieved from, 2016; https://shop.bsigroup.com/ProductDetail?pid=000000000030321556.
      There is no corresponding record for this reference.
    6. 6
      Mills, A.; Elliott, N.; Parkin, I. P.; O’Neill, S. A.; Clark, R. J. Novel TiO2 CVD Films for Semiconductor Photocatalysis. J. Photochem. Photobiol., A 2002, 151, 171– 179,  DOI: 10.1016/s1010-6030(02)00190-9
      6
      Novel TiO2 CVD films for semiconductor photocatalysis
      Mills, Andrew; Elliott, Nicholas; Parkin, Ivan P.; O'Neill, Shane A.; Clark, R. J.
      Journal of Photochemistry and Photobiology, A: Chemistry (2002), 151 (1-3), 171-179CODEN: JPPCEJ; ISSN:1010-6030. (Elsevier Science B.V.)
      A novel CVD film of titanium(IV) oxide has been prepd. on glass, via the reaction of titanium(IV) chloride and Et acetate, using a CVD technique. The film is clear, very robust mech. and comprised of a thin (24 nm) layer of nanocryst. anatase titania that absorbs light of λ<360 nm. The film is active in terms of photo-induced superhydrophilicity, and thus its water contact angle is reduced markedly (from 61 to 11°) upon irradn. with ultra-bandgap light. The film is also active photocatalytically and is able to destroy a deposited layer of stearic acid upon exposure to ultra-bandgap light. The photo-induced superhydrophilic and photocatalytic activities of this film are compared with those for one comprised of P25 particles.
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    7. 7
      Mills, A.; Elliott, N.; Hill, G.; Fallis, D.; Durrant, J. R.; Willis, R. L. Preparation and Characterisation of Novel Thick Sol-gel Titania Film Photocatalysts. Photochem. Photobiol. Sci. 2003, 2, 591– 596,  DOI: 10.1039/b212865a
      7
      Preparation and characterization of novel thick sol-gel titania film photocatalysts
      Mills, Andrew; Elliott, Nicholas; Hill, George; Fallis, David; Durrant, James R.; Willis, Richard L.
      Photochemical & Photobiological Sciences (2003), 2 (5), 591-596CODEN: PPSHCB; ISSN:1474-905X. (Royal Society of Chemistry)
      The prepn. and characterization of thick (9 μm), clear, mech. robust and photocatalytically active films of nanocryst. anatase titania are described. XRD and SEM anal. show the films comprise 13 nm particles of anatase TiO2. Thin (54 nm) films of the 'paste' TiO2, along with sol-gel titania films made by a more traditional route are also prepd. and characterized. All titania films mediate the photocatalytic destruction of stearic acid with a quantum yield of 0.0016 ± 0.0003, using either 365 nm (i.e. BLB) or 254 nm (germicidal) light. P25 TiO2 films also appear to mediate the same process with a similar formal quantum efficiency. Of all the films tested, the thick paste TiO2 films are the most ideally suited for use with near UV light, for reasons which are discussed. All the titania films tested exhibit photoinduced super-hydrophilicity.
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    8. 8
      Quesada-Cabrera, R.; Mills, A.; O’Rourke, C. Action Spectra of P25 TiO2 and a Visible Light Absorbing, Carbon-modified Titania in the Photocatalytic Degradation of Stearic Acid. Appl. Catal., B 2014, 150–151, 338– 344,  DOI: 10.1016/j.apcatb.2013.12.008
      8
      Action spectra of P25 TiO2 and a visible light absorbing, carbon-modified titania in the photocatalytic degradation of stearic acid
      Quesada-Cabrera, Raul; Mills, Andrew; O'Rourke, Christopher
      Applied Catalysis, B: Environmental (2014), 150-151 (), 338-344CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
      The photonic efficiencies of films of Evonik (formerly Degussa) P25 TiO2 and carbon-modified TiO2 Kronos VLP 7000 samples are reported as a function of excitation wavelength (300-430 nm; FWHM ∼ 7.5 nm), i.e. the action spectra, for the degrdn. of stearic acid, a model org. for the photocatalytic destruction of solid surface org. pollutants. For each of these semiconductor photocatalysts, at 365 nm (FWHM = 18 nm), the dependence of the rate of degrdn. of stearic acid, upon the irradiance, I, is detd. and the rate is found to be proportional to I0.65 and I0.82 for P25 and Kronos titania, resp. Assuming this relationship holds at all wavelengths, the action spectra for two different semiconductor photocatalysts is modified by plotting, (RSA (rate of stearic acid destruction, units: mols. cm-2 s-1)/Iθ) vs. wavelength of excitation (λexcit), and both differ noticeably from those of the original (unmodified) action spectra, which are plots of (RSA/I = photonic efficiency, ξ) vs. λexcit. The shape of the modified action spectrum for P25 TiO2 is consistent with that reported by others for other org. mineralization reactions and correlates well with diffuse reflectance data for P25 TiO2 (Kubelka-Munk plot), although there is some evidence that the active phase, in the photodegrdn. of stearic acid, is the anatase form present in P25. The unmodified and modified action spectra of the beige Kronos VLP 7000 TiO2 compd. exhibits little or no activity in the visible i.e. (λexcit > 400 nm) and a peak at 350 nm. The Kronos powder contains a yellow/brown conjugated, extractable, org. sensitizer which has been identified by others as the species responsible for its reported photocatalytic visible light activity. But, irradn. of the Kronos powder film, with and without a stearic acid coating, in air, using UVA or visible light, bleaches rapidly (<60 min) most, if not all, of the little color exhibited by the original Kronos powder. The photobleached form of the Kronos has a similar action spectrum to that of the unbleached form, which, in turn, appears very similar to that of P25 titania, at wavelengths >350 nm. It is proposed that the difference between the Kronos and P25 powder films at wavelengths <350 nm is due to a photodegrdn.-resistant, previously unidentified (but extractable using MeCN) UV-absorbing org. species in the former which screens the titania particles at these lower wavelengths. The implications of these observations are discussed briefly.
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    9. 9
      Alotaibi, A. M.; Sathasivam, S.; Williamson, B. A. D.; Kafizas, A.; Sotelo-Vazquez, C.; Taylor, A.; Scanlon, D. O.; Parkin, I. P. Chemical Vapor Deposition of Photocatalytically Active Pure Brookite TiO2 Thin Films. Chem. Mater. 2018, 30, 1353– 1361,  DOI: 10.1021/acs.chemmater.7b04944
      9
      Chemical Vapor Deposition of Photocatalytically Active Pure Brookite TiO2 Thin Films
      Alotaibi, Abdullah M.; Sathasivam, Sanjayan; Williamson, Benjamin A. D.; Kafizas, Andreas; Sotelo-Vazquez, Carlos; Taylor, Alaric; Scanlon, David O.; Parkin, Ivan P.
      Chemistry of Materials (2018), 30 (4), 1353-1361CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Brookite is the least investigated phase of TiO2 due to the synthetic difficulty of obtaining the pure phase. Here, we present the first ever chem. vapor deposition synthesis of pure brookite TiO2 thin films. The films were highly cryst. and phase pure as detd. by X-ray diffraction and Raman spectroscopy studies. SEM studies showed the films to have a structured morphol. consisting of pyramidal features. The photocatalytic properties of the brookite film, tested using stearic acid under UVA (365 nm) irradn., were superior to both an anatase film grown under similar conditions and NSG Activ glass. Transient absorption spectroscopy showed that the innate electron-hole recombination dynamics are similar in brookite and anatase, akin to previous reports. The superior activity of the brookite film is hence attributed to the higher surface area compared to anatase.
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    10. 10
      Sawunyama, P.; Jiang, L.; Fujishima, A.; Hashimoto, K. Photodecomposition of a Langmuir-Blodgett Film of Stearic Acid on TiO2 Film Observed by In-situ Atomic Force Microscopy and FT-IR. J. Phys. Chem. B 1997, 101, 11000– 11003,  DOI: 10.1021/jp9730095
      10
      Photodecomposition of a Langmuir-Blodgett Film of Stearic Acid on TiO2 Film Observed by in Situ Atomic Force Microscopy and FT-IR
      Sawunyama, Phillip; Jiang, Lei; Fujishima, Akira; Hashimoto, Kazuhito
      Journal of Physical Chemistry B (1997), 101 (51), 11000-11003CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
      We have probed the TiO2-mediated photomineralization of Langmuir-Blodgett (LB) films of stearic acid via at. force microscopy (AFM) and FT-IR. In situ AFM images revealed that at the submicrometer level the photodecompn. process of stearic acid mols. on a polycryst. anatase TiO2 film is inhomogeneous, with the various reaction initiation centers or nucleation regions being randomly distributed throughout the photocatalyst surface. Furthermore, parallel FT-IR results showed that the reaction follows pseudo-first-order kinetics. To rationalize the obsd. random LB film photoetching and buckling behavior, we invoked a simple reaction model that incorporates the reactivity of the TiO2 film and LB film disorganization phenomenon during the photodegrdn. process.
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    11. 11
      Minabe, T.; Tryk, D. A.; Sawunyama, P.; Kikuchi, Y.; Hashimoto, K.; Fujishima, A. TiO2-mediated Photodegradation of Liquid and Solid Organic Compounds. J. Photochem. Photobiol., A 2000, 137, 53– 62,  DOI: 10.1016/s1010-6030(00)00350-6
      11
      TiO2-mediated photodegradation of liquid and solid organic compounds
      Minabe, T.; Tryk, D. A.; Sawunyama, P.; Kikuchi, Y.; Hashimoto, K.; Fujishima, A.
      Journal of Photochemistry and Photobiology, A: Chemistry (2000), 137 (1), 53-62CODEN: JPPCEJ; ISSN:1010-6030. (Elsevier Science S.A.)
      The photocatalytic degrdn. of several liq. and solid org. compds., including polymers, with mol. wts. covering a wide range from 600 to 500,000 was studied on TiO2 thin films on glass under UV illumination. Nearly exact agreement was found between the wt. losses of the solid compds. octadecane and stearic acid and the wts. of CO2 produced during photocatalytic degrdn. No other gas-phase degrdn. product was detected for these two compds. other than CO2, which means that potentially harmful products are not expected to pose a problem. For convenient comparison of degrdn. rates for various compds. and measurement methods, the values were converted to nos. of moles of carbon reacted per square centimeter per h. Under appropriate conditions (50° C, relative humidity 10% in air), octadecane was completely decompd. (<400 ng-cm-2). In contrast, stearic acid did not decomp. completely, even after more than 80 h of UV illumination. This may be due to the formation of a photocatalytically inert reaction product that blocks the TiO2 surface. The decompn. rates for all of the compds. examd. spanned less than two orders of magnitude, suggesting that the photocatalytic reactions involved are rather versatile.
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    12. 12
      Remillard, J. T.; McBride, J. R.; Nietering, K. E.; Drews, A. R.; Zhang, X. Real Time in Situ Spectroscopic Ellipsometry Studies of the Photocatalytic Oxidation of Stearic Acid on Titania Films. J. Phys. Chem. B 2000, 104, 4440– 4447,  DOI: 10.1021/jp9936300
      12
      Real Time in Situ Spectroscopic Ellipsometry Studies of the Photocatalytic Oxidation of Stearic Acid on Titania Films
      Remillard, J. T.; McBride, J. R.; Nietering, K. E.; Drews, A. R.; Zhang, X.
      Journal of Physical Chemistry B (2000), 104 (18), 4440-4447CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
      The authors used in situ spectroscopic ellipsometry to measure the photocatalytic activity of TiO2 films on fused SiO2 and glass substrates. Amorphous and anatase TiO2 films with a variety of microstructures were prepd. by reactive sputtering and pyrolytic deposition. The TiO2 films were coated with thin, spin-cast films of stearic acid [Me(CH2)16COOH] to represent an org. contaminant. Photooxidn. rates were detd. from ellipsometric measurements of the redn. in stearic acid film thickness during exposure to UV irradn. at 313 or 365 nm. The photooxidn. rate is proportional to Iα, where I is the irradiance. The exponent α correlated with the TiO2 crystallinity, having values of ∼0.7 and 0.8 for amorphous and anatase films, resp. The largest photooxidn. rate was obsd. for the pyrolytically deposited anatase sample on which x-ray reflectometry and spectroscopic ellipsometry measurements detected the presence of a low-d. TiO2 surface layer. To assess the performance of these films in practical applications, the specimens were exposed to wavelength and irradiance conditions that simulated a solar UV spectrum. The most photocatalytically active sample had a stearic acid film removal rate of 22 nm/h, which would be suitable for self-cleaning window applications.
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    13. 13
      Smirnova, N.; Fesenko, T.; Zhukovsky, M.; Goworek, J.; Eremenko, A. Photodegradation of Stearic Acid Adsorbed on Superhydrophilic TiO2 Surface: In Situ FT-IR and LDI Study. Nanoscale Res. Lett. 2015, 10, 500,  DOI: 10.1186/s11671-015-1210-y
      13
      Photodegradation of Stearic Acid Adsorbed on Superhydrophilic TiO2 Surface: In Situ FT-IR and LDI Study
      Smirnova Natalia; Fesenko Tatiana; Zhukovsky Maxim; Eremenko Anna; Goworek Jacek
      Nanoscale research letters (2015), 10 (1), 500 ISSN:1931-7573.
      TiO2 films prepared by template-assisted sol-gel method were characterized by X-ray diffraction spectroscopy, scanning and atomic force electron microscopy, and Fourier transform infrared (FT-IR) spectroscopy. Based on the hexane adsorption-desorption analysis, the films have a surface area of 390-540 m(2)/g with pore size distribution narrowly centered around 10 nm. Optimal component ratio and condition of heat treatment of mesoporous titania films have been found. Photocatalytic activity of the coatings was determined by the destruction of stearic acid layers, monitored using FT-IR spectroscopy and laser desorption-ionization (LDI) mass spectrometry. Under UV illumination, all the used films reach hydrophilicity with water contact angle of 0°. As the result, hydrophobic fat acid molecules undergo self-association and active desorption from the hydrophilic surface during mass-spectrometric experiment.
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    14. 14
      Alotaibi, A. M.; Promdet, P.; Hwang, G. B.; Li, J.; Nair, S. P.; Sathasivam, S.; Kafizas, A.; Carmalt, C. J.; Parkin, I. P. Zn and N Codoped TiO2 Thin Films: Photocatalytic and Bactericidal Activity. ACS Appl. Mater. Interfaces 2021, 13, 10480– 10489,  DOI: 10.1021/acsami.1c00304
      14
      Zn and N Codoped TiO2 Thin Films: Photocatalytic and Bactericidal Activity
      Alotaibi, Abdullah M.; Promdet, Premrudee; Hwang, Gi Byoung; Li, Jianwei; Nair, Sean P.; Sathasivam, Sanjayan; Kafizas, Andreas; Carmalt, Claire J.; Parkin, Ivan P.
      ACS Applied Materials & Interfaces (2021), 13 (8), 10480-10489CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      We explore a series of Zn and N codoped TiO2 thin films grown using chem. vapor deposition. Films were prepd. with various concns. of Zn (0.4-2.9 at. % Zn vs Ti), and their impact on superoxide formation, photocatalytic activity, and bactericidal properties were detd. Superoxide (·O2-) formation was assessed using a 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-5-[(phenylamino)carbonyl]-2H-tetrazolium sodium salt (XTT) as an indicator, photocatalytic activity was detd. from the degrdn. of stearic acid under UVA light, and bactericidal activity was assessed using a Gram-neg. bacterium E. coli under both UVA and fluorescent light (similar to what is found in a clin. environment). The 0.4% Zn,N:TiO2 thin film demonstrated the highest formal quantum efficiency in degrading stearic acid (3.3 x 10-5 mols.·photon-1), while the 1.0% Zn,N:TiO2 film showed the highest bactericidal activity under both UVA and fluorescent light conditions (>3 log kill). The enhanced efficiency of the films was correlated with increased charge carrier lifetime, supported by transient absorption spectroscopy (TAS) measurements.
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    15. 15
      Alotaibi, A. M.; Williamson, B. A. D.; Sathasivam, S.; Kafizas, A.; Alqahtani, M.; Sotelo-Vazquez, C.; Buckeridge, J.; Wu, J.; Nair, S. P.; Scanlon, D. O.; Parkin, I. P. Enhanced Photocatalytic and Antibacterial Ability of Cu doped Anatase TiO2 Thin Films: Theory and Experiment. ACS Appl. Mater. Interfaces 2020, 12, 15348– 15361,  DOI: 10.1021/acsami.9b22056
      15
      Enhanced photocatalytic and antibacterial ability of Cu-doped anatase TiO2 thin films: theory and experiment
      Alotaibi, Abdullah M.; Williamson, Benjamin A. D.; Sathasivam, Sanjayan; Kafizas, Andreas; Alqahtani, Mahdi; Sotelo-Vazquez, Carlos; Buckeridge, John; Wu, Jiang; Nair, Sean P.; Scanlon, David O.; Parkin, Ivan P.
      ACS Applied Materials & Interfaces (2020), 12 (13), 15348-15361CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Multifunctional thin films which can display both photocatalytic and antibacterial activity are of great interest industrially. Here, for the first time, we have used aerosol-assisted chem. vapor deposition to deposit highly photoactive thin films of Cu-doped anatase TiO2 on glass substrates. The films displayed much enhanced photocatalytic activity relative to pure anatase and showed excellent antibacterial (vs Staphylococcus aureus and Escherichia coli) ability. Using a combination of transient absorption spectroscopy, photoluminescence measurements, and hybrid d. functional theory calcns., we have gained nanoscopic insights into the improved properties of the Cu-doped TiO2 films. Our anal. has highlighted that the interactions between substitutional and interstitial Cu in the anatase lattice can explain the extended exciton lifetimes obsd. in the doped samples and the enhanced UV photoactivities obsd.
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    16. 16
      Peeters, H.; Keulemans, M.; Nuyts, G.; Vanmeert, F.; Li, C.; Minjauw, M.; Detavernier, C.; Bals, S.; Lenaerts, S.; Verbruggen, S. W. Plasmonic Gold-embedded TiO2 Thin Films as Photocatalytic Self-cleaning Coatings. Appl. Catal., B 2020, 267, 118654,  DOI: 10.1016/j.apcatb.2020.118654
      16
      Plasmonic gold-embedded TiO2 thin films as photocatalytic self-cleaning coatings
      Peeters, Hannelore; Keulemans, Maarten; Nuyts, Gert; Vanmeert, Frederik; Li, Chen; Minjauw, Matthias; Detavernier, Christophe; Bals, Sara; Lenaerts, Silvia; Verbruggen, Sammy W.
      Applied Catalysis, B: Environmental (2020), 267 (), 118654CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
      Transparent photocatalytic TiO2 thin films hold great potential in the development of self-cleaning glass surfaces, but suffer from a poor visible light response that hinders the application under actual sunlight. To alleviate this problem, the photocatalytic film can be modified with plasmonic nanoparticles that interact very effectively with visible light. Since the plasmonic effect is strongly concd. in the near surroundings of the nanoparticle surface, an approach is presented to embed the plasmonic nanostructures in the TiO2 matrix itself, rather than deposit them loosely on the surface. This way the interaction interface is maximised and the plasmonic effect can be fully exploited. In this study, pre-fabricated gold nanoparticles are made compatible with the org. medium of a TiO2 sol-gel coating suspension, resulting in a one-pot coating suspension. After spin coating, homogeneous, smooth, highly transparent and photoactive gold-embedded anatase thin films are obtained.
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    17. 17
      Cosham, S.; Celorrio, V.; Kulak, A. N.; Hyett, G. Observation of Visible Light Activated Photocatalytic Degradation of Stearic Acid on Thin Films of Tantalum Oxynitride Synthesized by Aerosol Assisted Chemical Vapour Deposition. Dalton Trans. 2019, 48, 10619– 10627,  DOI: 10.1039/c8dt04638g
      17
      Observation of visible light activated photocatalytic degradation of stearic acid on thin films of tantalum oxynitride synthesized by aerosol assisted chemical vapor deposition
      Cosham, Samuel D.; Celorrio, Veronica; Kulak, Alexander N.; Hyett, Geoffrey
      Dalton Transactions (2019), 48 (28), 10619-10627CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
      UV activated photocatalysts deposited using chem. vapor deposition have found com. success as self-cleaning coatings. However, only limited work has been conducted on the use of the more recently discovered visible light activated photocatalysis for this application. Tantalum oxynitride is an established visible light photocatalyst, and in this paper we have investigated the ability of thin films of tantalum oxynitride to photocatalytically degrade a model org. pollutant, stearic acid, and therefore assess the coatings potential for self-cleaning applications. Thin films of tantalum oxide were formed using aerosol assisted chem. vapor deposition (AACVD) of tantalum ethoxide, and then converted into tantalum oxynitride through ammonolysis at temps. between 550°C and 750°C. Investigation of the films using XRD, UV-vis spectroscopy and XAFS identify that amorphous tantalum oxynitride is formed during the ammonolysis, with complete conversion to TaON under conditions of 700°C for 24 h. The self-cleaning ability of this film was assessed using stearic acid as the model pollutant, with a degrdn. rate of 2.5(2) × 1013 mols. per min per cm2 when exposed to a 5-sun solar simulator, equipped with a UV cut-off filter. We therefore conclude that tantalum oxynitride thin films are able to act as self-cleaning coatings through visible light photocatalysis and that films of tantalum oxynitride can be synthesized using a scalable chem. vapor deposition route.
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    18. 18
      Jiamprasertboon, A.; Kafizas, A.; Sachs, M.; Ling, M.; Alotaibi, A. M.; Lu, Y.; Siritanon, T.; Parkin, I. P.; Carmalt, C. J. Heterojunction α-Fe2O3/ZnO Films with Enhanced Photocatalytic Properties Grown by Aerosol-assisted Chemical Vapour Deposition. Chem.─Eur. J. 2019, 25, 11337– 11345,  DOI: 10.1002/chem.201902175
      18
      Heterojunction α-Fe2O3/ZnO films with enhanced photocatalytic properties grown by aerosol-assisted chemical vapor deposition
      Jiamprasertboon, Arreerat; Kafizas, Andreas; Sachs, Michael; Ling, Min; Alotaibi, Abdullah M.; Lu, Yao; Siritanon, Theeranun; Parkin, Ivan P.; Carmalt, Claire J.
      Chemistry - A European Journal (2019), 25 (48), 11337-11345CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Type I heterojunction films of α-Fe2O3/ZnO are reported here as a non-titania based photocatalyst, which shows remarkable enhancement in the photocatalytic properties towards stearic acid degrdn. under UVA-light exposure (λ=365 nm), with a quantum efficiency of ξ=4.42±1.54×10-4 mols. degraded/photon, which was about 16 times greater than that of α-Fe2O3, and 2.5 times greater than that of ZnO. Considering that the degrdn. of stearic acid requires 104 electron transfers for each mol., this represents an overall quantum efficiency of 4.60 % for the α-Fe2O3/ZnO heterojunction. Time-resolved transient absorption spectroscopy (TAS) revealed the charge-carrier behavior responsible for this increase in activity. Photogenerated electrons, formed in the ZnO layer, were transferred into the α-Fe2O3 layer on the pre-μs timescale, which reduced electron-hole recombination. This increased the lifetime of photogenerated holes formed in ZnO, which oxidise stearic acid. The heterojunction α-Fe2O3/ZnO films grown herein show potential environmental applications as coatings for self-cleaning windows and surfaces.
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    19. 19
      Li, D.; Bulou, S.; Gautier, N.; Elisabeth, S.; Goullet, A.; Richard-Plouet, M.; Choquet, P.; Granier, A. Nanostructure and Photocatalytic Properties of TiO2 Films Deposited at Low Temperature by Pulsed PECVD. Appl. Surf. Sci. 2019, 466, 63– 69,  DOI: 10.1016/j.apsusc.2018.09.230
      19
      Nanostructure and photocatalytic properties of TiO2 films deposited at low temperature by pulsed PECVD
      Li, D.; Bulou, S.; Gautier, N.; Elisabeth, S.; Goullet, A.; Richard-Plouet, M.; Choquet, P.; Granier, A.
      Applied Surface Science (2019), 466 (), 63-69CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      The nanostructure and photocatalytic properties of TiO2 thin films deposited by PECVD on silicon substrates were investigated. The films were grown at low temp. (<120 °C) in an rf inductively coupled oxygen/titanium tetraisopropoxide plasma, in continuous and pulsed modes with different plasma-on time (via variation of the duty cycle, DC). All the films exhibit nano-columnar structures, but the redn. of plasma-on time by decreasing the duty cycle for pulsed mode leads to a more homogenous morphol. with a diminished column size, and a decrease in the surface roughness. TiO2 layers contg. a high amt. of anatase were grown at substrate temps. less than 100 °C corresponding to DC ≥ 40%, then the crystn. was hindered with the decrease of DC, even inducing amorphous films for DC ≤ 10%. Moreover, the films deposited below 100 °C with deposition conditions where 50% ≤ DC ≤ 75% were shown to present a high photocatalytic activity, likely due to the presence of anatase cryst. nanocolumns at the surface.
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    20. 20
      Paz, Y.; Luo, Z.; Rabenberg, L.; Heller, A. Photooxidative Self-cleaning Transparent Titanium Dioxide Films on Glass. J. Mater. Res. 1995, 10, 2842– 2848,  DOI: 10.1557/jmr.1995.2842
      20
      Photooxidative self-cleaning transparent titanium dioxide films on glass
      Paz, Y.; Luo, Z.; Rabenberg, L.; Heller, A.
      Journal of Materials Research (1995), 10 (11), 2842-8CODEN: JMREEE; ISSN:0884-2914. (Materials Research Society)
      In the context of studying the feasibility of photocatalytically self-cleaning windows and windshields, clear, abrasion resistant, photocatalytic films of TiO2 were formed on soda-lime glass and on vitreous silica by a sol-gel process. The rate of photooxidn. of contaminant deposits was estd. by measuring the rate of decrease in the integrated IR absorbance assocd. with the C-H stretching vibrations of a thin soln.-cast film of stearic acid under 365 nm (2.4 mW/cm2) or 254 nm (0.8 mW/cm2) irradn. Approx. 3 × 10-4 steric acid mols. were stripped per 365 nm photon in either front- or back-illuminated soda-lime glass, and 6 × 10-4 mols. when the films were coated on vitreous silica. For thin TiO2 films on vitreous silica, the rate of photooxidn., normalized by the no. of photons absorbed per unit area, was independent of the wavelength. In contrast, for films on soda lime glass, the rate of photooxidn., when similarly normalized, was higher for the less penetrating wavelength. The reduced photoactivity on glass at the deeply penetrating wavelength (365 nm), as was the greater photoefficiency on vitreous silica than on glass, are attributed to diffusion of sodium oxide from the glass into the inner glass-contacting zone of the TiO2 layer.
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    21. 21
      Fretwell, R.; Douglas, P. An Active, Robust and Transparent Nanocrystalline Anatase TiO2 Thin Film ─ Preparation, Characterisation and the Kinetics of Photodegradation of Model Pollutants. J. Photochem. Photobiol., A 2001, 143, 229– 240,  DOI: 10.1016/s1010-6030(01)00526-3
      21
      An active, robust and transparent nanocrystalline anatase TiO2 thin film - preparation, characterization and the kinetics of photodegradation of model pollutants
      Fretwell, R.; Douglas, P.
      Journal of Photochemistry and Photobiology, A: Chemistry (2001), 143 (2-3), 229-240CODEN: JPPCEJ; ISSN:1010-6030. (Elsevier Science S.A.)
      The prepn., characterization, and photocatalytic activity of robust and transparent nanocryst. thin films of anatase TiO2 are described. The films, which are prepd. from a sol-gel can be either dip-coated or spin-coated, however, the latter have better optical and mech. properties, since dip coating results in the retention of a significant amt. of the org. stabilizers used in the sol-gel prepn. Quantum yields for the photooxidn. of 4-chlorophenol (Φ4CP) on these spin-coated films are similar to those reported previously for TiO2 films and dispersions, with Φ4CP = 0.5-1%. For films of different thicknesses, and therefore different absorption efficiencies, Φ4CP depends only upon the total no. of photons absorbed, i.e. Φ Is independent of both the absorbed photon flux and the distribution of charge carriers in the film. By way of contrast, the rate of photoredn. of Methylene blue is independent of film thickness and appears to be limited by the surface area of the film. Hence, quantum yields for Methylene blue redn. (ΦMBR) fall significantly from 1.7 to 0.5% as film thickness, and hence photoabsorption efficiency, is increased. These pure anatase films do not show any deleterious sodium ion effect for either 4-chlorophenol oxidn. or Methylene blue redn. Quantum yields for photooxidn. of stearic acid (ΦSA) deposited directly onto the films are relatively high with ΦSA = 1-4%. This, combined with the excellent optical and mech. properties of these films, suggests that they may be of particular interest for the development of 'self-cleaning windows'.
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    22. 22
      Manning, T. D.; Parkin, I. P.; Clark, R. J. H.; Sheel, D.; Pemble, M. E.; Vernadou, D. Intelligent Window Coatings: Atmospheric Pressure Chemical Vapour Deposition of Vanadium Oxides. J. Mater. Chem. 2002, 12, 2936– 2939,  DOI: 10.1039/b205427m
      22
      Intelligent window coatings: atmospheric pressure chemical vapour deposition of vanadium oxides
      Manning, Troy D.; Parkin, Ivan P.; Clark, Robin J. H.; Sheel, David; Pemble, Martyn E.; Vernadou, Dimitra
      Journal of Materials Chemistry (2002), 12 (10), 2936-2939CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
      Thin films of the vanadium oxides, V2O5, VO2, VOx (x = 2.00-2.50) and V6O13 were prepd. on glass substrates by atm. pressure chem. vapor deposition (APCVD) of vanadium tetrachloride and water at 400-550°C. The specific phase deposited was dependent on the substrate temp. and the reagent concns. The films were characterized by Raman microscopy, X-ray diffraction (XRD), RBS, SEM, energy dispersive anal. by X-rays (EDX), reflectance/transmittance and UV absorption spectroscopy. The VO2 films show reversible switching behavior at 68°C, assocd. with a phase change from monoclinic (MoO2 structure) to tetragonal (TiO2, rutile structure).
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    23. 23
      Mills, A.; Wang, J. Simultaneous Monitoring of the Destruction of Stearic Acid and Generation of Carbon Dioxide by Self-cleaning Semiconductor Photocatalytic Films. J. Photochem. Photobiol., A 2006, 182, 181– 186,  DOI: 10.1016/j.jphotochem.2006.02.010
      23
      Simultaneous monitoring of the destruction of stearic acid and generation of carbon dioxide by self-cleaning semiconductor photocatalytic films
      Mills, Andrew; Wang, Jishun
      Journal of Photochemistry and Photobiology, A: Chemistry (2006), 182 (2), 181-186CODEN: JPPCEJ; ISSN:1010-6030. (Elsevier B.V.)
      The destruction of stearic acid (SA), the SA test, is a popular approach used to evaluate the activities of photocatalytic films. The destruction of SA via semiconductor photocatalysis is monitored simultaneously, using FT-IR spectroscopy, via the disappearance of SA and the appearance of CO2. Sol-gel and P25 films of titania are used as the semiconductor photocatalytic self-cleaning films. A conversion factor is used of 9.7 × 1015 mols. of SA cm-2 ≡ 1 cm-1 integrated areas of the peaks in the FT-IR of SA over the range 2700-3000 cm-1, which is three times that reported previously by others. As the SA disappeared the concomitant amt. of CO2 generated was >90% that expected throughout the photomineralization process for the sol-gel titania film. In contrast, the slightly more active, and scattering, P25 titania films generated CO2 levels that dipped as low as 69% during the course of the photoreaction, but eventually recovered to ≃100% that expected based on the amt. of SA present. The importance of these results with respect to SA test and the evaluation of new and existing self-cleaning films are discussed briefly.
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    24. 24
      Allain, E.; Besson, S.; Durand, C.; Moreau, M.; Gacoin, T.; Boilot, J. P. Transparent Mesoporous Nanocomposite Films for Self-Cleaning Applications. Adv. Funct. Mater. 2007, 17, 549– 554,  DOI: 10.1002/adfm.200600197
      24
      Transparent mesoporous nanocomposite films for self-cleaning applications
      Allain, Emmanuelle; Besson, Sophie; Durand, Clarisse; Moreau, Melanie; Gacoin, Thierry; Boilot, Jean-Pierre
      Advanced Functional Materials (2007), 17 (4), 549-554CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A versatile approach is studied for the elaboration of TiO2 based photocatalytic coatings for self-cleaning applications on transparent substrates. The basic principle of the synthesis relies on the use of preformed TiO2 colloidal particles that are further dispersed within a transparent silica binder with a mesoporous structure. Film porosity in the nanometer range is controlled by achieving the sol-gel silica condensation around self-organized micellar assemblies of a templating copolymer surfactant. The latter also acts as a stabilizer for the TiO2 particles, thus preserving their high dispersion within the film so that excellent optical properties are maintained even for high TiO2 loading (up to 50 %). Studies of photodegrdn. kinetics show that such mesoporous films are at least 15 times more active than films synthesized with a usual microporous silica binder. Moreover, the measured quantum-yield efficiency (1.1 %) is found to be among the highest reported up to now. Improved photoactivity of the films is discussed as resulting from the closer proximity between the org. mols. and the surface of the TiO2 crystallites as well as the improved diffusion rate of water and oxygen through the interconnected pore network.
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    25. 25
      Peruchon, L.; Puzenat, E.; Girard-Egrot, A.; Blum, L.; Herrmann, J. M.; Guillard, C. Characterization of self-cleaning glasses using Langmuir–Blodgett technique to control thickness of stearic acid multilayers: Importance of spectral emission to define standard test. J. Photochem. Photobiol., A 2008, 197, 170– 176,  DOI: 10.1016/j.jphotochem.2007.12.033
      25
      Characterization of self-cleaning glasses using Langmuir-Blodgett technique to control thickness of stearic acid multilayers. Importance of spectral emission to define standard test.
      Peruchon, L.; Puzenat, E.; Girard-Egrot, A.; Blum, L.; Herrmann, J. M.; Guillard, C.
      Journal of Photochemistry and Photobiology, A: Chemistry (2008), 197 (2-3), 170-176CODEN: JPPCEJ; ISSN:1010-6030. (Elsevier B.V.)
      The photocatalytic properties of commercialized self-cleaning glasses have been evaluated by the degrdn. of stearic acid C17H35COOH (SA), selected as a model mol. representative of fatty dirt. The relative amt. of SA was detd. by measuring the FTIR peak area in the range 2752 and 2992 cm-1 corresponding to the C-H stretching bands. To est. the thickness of one layer, the deposit of SA was performed using the Langmuir-Blodgett (LB) method, which allows obtaining organized monomol. layers of amphiphilic orgs. on a substrate. The thickness of one SA layer, obtained by Langmuir-Blodgett deposition, was correlated to the FTIR signal. The photocatalytic degrdn. of a SA layer of a controlled thickness was carried out in a double jacket Pyrex reactor equipped with a UV Philips HPK 125 lamp. The photocatalytic degrdn. rate of the SA layer with different thicknesses was detd. The SA degrdn. rate was studied under UV-A and UV-B irradiations to det. the influence of the photon wavelength range on efficiency.
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    26. 26
      Peruchon, L.; Puzenat, E.; Herrmann, J. M.; Guillard, C. Photocatalytic efficiencies of self-cleaning glasses. Influence of physical factors. Photochem. Photobiol. Sci. 2009, 8, 1040– 1046,  DOI: 10.1039/b902139f
      26
      Photocatalytic efficiencies of self-cleaning glasses. Influence of physical factors
      Peruchon, L.; Puzenat, E.; Herrmann, J. M.; Guillard, C.
      Photochemical & Photobiological Sciences (2009), 8 (7), 1040-1046CODEN: PPSHCB; ISSN:1474-905X. (Royal Society of Chemistry)
      Two com. types of self-cleaning glass (SCG) have been tested to confirm the real photocatalytic nature of their properties. This was done by using four photocatalytic tests: (i) in the gas phase with the total oxidn. of acetylene; (ii) in water with the total degrdn. of malic acid, (iii) in water with the total degrdn. of Methylene blue, and (iv) in the solid phase with the total oxidative degrdn. of a layer of stearic acid deposited on the self-cleaning glass surface, in contact with the superficial titania coating. The influence of various factors (temp., humidity, wavelength, radiant flux, presence of inorg. particles stuck at the glass surface) was explained in line with the fundamentals of photocatalysis. The results helped to understand the behavior of self-cleaning glass.
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    27. 27
      Mills, A.; Lee, S.; Lepre, A.; Parkin, I. P.; O’Neill, S. A. Spectral and photocatalytic characteristics of TiO2 CVD films on quartz. Photochem. Photobiol. Sci. 2002, 1, 865– 868,  DOI: 10.1039/b205715h
      27
      Spectral and photocatalytic characteristics of TiO2 CVD films on quartz
      Mills, Andrew; Lee, Soo-Keun; Lepre, Anne; Parkin, Ivan P.; O'Neill, Shane A.
      Photochemical & Photobiological Sciences (2002), 1 (11), 865-868CODEN: PPSHCB; ISSN:1474-905X. (Royal Society of Chemistry)
      A series of novel chem. vapor deposited (CVD) films of titanium(IV) oxide of different thicknesses, spanning the range 10-91 nm, are prepd. on quartz, via the reaction of titanium(IV) chloride and Et acetate, using a CVD technique. The films are clear, mech. robust and comprise thin layer of nanocryst. anatase titania of different thicknesses that absorb UV light. The UV-Visible spectral profiles of all the CVD TiO2 films of different thickness are the same and obey Lambert's law. A plot of the reciprocal length for the TiO2 coating vs. wavelength is reported. The photocatalytic activity of each film to mediate the destruction of a thin layer of stearic acid is investigated. The rate depends directly upon the fraction of light absorbed and the apparent quantum yield for the overall process is 0.00035, which appears low compared with that for sol-gel TiO2 films.
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    28. 28
      Alofi, S.; O’Rourke, C.; Mills, A. Modelling the Kinetics of Stearic Acid Destruction on TiO2 ‘Self-cleaning’ Photocatalytic Films. Appl. Catal., A 2022, 647, 118899,  DOI: 10.1016/j.apcata.2022.118899
      28
      Modelling the kinetics of stearic acid destruction on TiO2 'self-cleaning' photocatalytic films
      Alofi, Saleh; O'Rourke, Christopher; Mills, Andrew
      Applied Catalysis, A: General (2022), 647 (), 118899CODEN: ACAGE4; ISSN:0926-860X. (Elsevier B.V.)
      The kinetics of stearic acid, SA, removal by photocatalytic films are modelled using a log-normal distribution in surface reactivity, which shows that as the distribution in reactivity, ρ, increases, so too does the apparent order of reaction, m, of the obsd. variation in SA concn., [SA], vs. irradn. time, t. The model is used to fit the obsd. [SA] vs. t profiles exhibited by three very different TiO2 films, namely Activ self-cleaning glass, and prepd. sol-gel TiO2 and P25 TiO2 films. These films show decay kinetics, with m = 0.3, 0.44 and 0.6, resp., which fit the model predicted decay profiles with distribution width, ρ, values, 0.4, 0.54 and 0.7, resp. A Ag photocatalyst indicator ink is used on these films to highlight the non-uniform distribution in surface reactivity on the micron scale. The use of the log-normal model to better understand the kinetics exhibited by photocatalytic films is discussed briefly.
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    29. 29
      Sawunyama, P.; Fujishima, A.; Hashimoto, K. Photocatalysis on TiO2 Surfaces Investigated by Atomic Force Microscopy: Photodegradation of Partial and Full Monolayers of Stearic Acid on TiO2(110). Langmuir 1999, 15, 3551– 3556,  DOI: 10.1021/la9814440
      29
      Photocatalysis on TiO2 Surfaces Investigated by Atomic Force Microscopy: Photodegradation of Partial and Full Monolayers of Stearic Acid on TiO2(110)
      Sawunyama, Phillip; Fujishima, Akira; Hashimoto, Kazuhito
      Langmuir (1999), 15 (10), 3551-3556CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      The authors studied the nature and surface morphol. changes assocd. with the photodegrdn. of stearic acid LB films on TiO2(110). Interestingly, submonolayers of stearic acid consisted of circular domains of various sizes - a feature very attractive for monitoring TiO2 photocatalysis by AFM. The authors noted that there was no bulk differential reactivity at island edges compared to the interior. This suggests that the rate of photodegrdn. of the stearic acid mols. is independent of their location in the island. Accordingly, the overall surface reactivity trends were similar for both partial films and complete films. Likewise, the obsd. inhomogeneous reactivity patterns appear to be a reflection of the transient distribution of the reaction centers.
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    30. 30
      Ghazzal, M. N.; Barthen, N.; Chaoui, N. Photodegradation Kinetics of Stearic Acid on UV-irradiated Titania Thin Film Separately Followed by Optical Microscopy and Fourier Transform Infrared Spectroscopy. Appl. Catal., B 2011, 103, 85– 90,  DOI: 10.1016/j.apcatb.2011.01.013
      30
      Photodegradation kinetics of stearic acid on UV-irradiated titania thin film separately followed by optical microscopy and Fourier transform infrared spectroscopy
      Ghazzal, M. N.; Barthen, N.; Chaoui, N.
      Applied Catalysis, B: Environmental (2011), 103 (1-2), 85-90CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
      We use optical microscopy for observing the photocatalytic decompn. of SA on TiO2 film during exposure to a simulated solar illumination. The TiO2 films were prepd. via sol-gel and deposited on a borosilicate glass substrate by dip-coating. A low amt. of SA was deposited by dipping the TiO2 film in a methanolic SA soln. Optical micrographs showed that the deposited SA forms islands with variable heights and sizes and that upon UV-irradn., these latter decompd. gradually from the edges towards the center. The SA photodecompn. as evaluated by measuring the integrated IR absorbance of SA in the 2700-3000 cm-1 region, proceeded according to a pseudo 1st-order kinetics with respect to SA. By contrast, it was of the zero-order type when monitoring the decompn. of an individual SA island by microscopy. We showed that the pseudo 1st-order kinetics is the result of a collective effect of many independent and parallel zero-order reactions with variable durations. This scenario describes well both the exptl. data and those reported in literature.
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    31. 31
      Jiang, X.; Manawan, M.; Feng, T.; Qian, R.; Zhao, T.; Zhou, G.; Kong, F.; Wang, Q.; Dai, S.; Pan, J. H. Anatase and rutile in evonik aeroxide P25: Heterojunctioned or individual nanoparticles?. Catal. Today 2018, 300, 12– 17,  DOI: 10.1016/j.cattod.2017.06.010
      31
      Anatase and rutile in evonik aeroxide P25: Heterojunctioned or individual nanoparticles?
      Jiang, Xiongzhen; Manawan, Maykel; Feng, Ting; Qian, Ruifeng; Zhao, Ting; Zhou, Guanda; Kong, Fantai; Wang, Qing; Dai, Songyuan; Pan, Jia Hong
      Catalysis Today (2018), 300 (), 12-17CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
      Evonik Aeroxide P25 (formerly Degussa P25) is a flame-made multiphasic TiO2 having nanoparticles contg. anatase and rutile, as well as a small amt. of amorphous TiO2. The past decades have witnessed the wide applications of P25 as a benchmark material for studying photocatalytic mechanism, materials and process. However, controversy remains regarding the microstructure of anatase and rutile: do they interwoven forming heterojunction structure or exist individually. To clarify it, we selected a medium alk., LiOH to erode bare P25 under a mild hydrothermal condition. Since rutile presents much higher resistances towards dissoln. by LiOH than anatase, it is reasonable to find during hydrothermal reaction that the ratio of anatase to rutile (A/R) gradually decrease if they exist individually. Reversely, the A/R value gradually increases at the beginning of the hydrothermal reaction, implying that rutile shows high activity towards dissoln. and phase transformation to lithium titanate. As calcd., around 15% rutile nanoparticles more likely exist on the surface of anatase with the formation of a heterojunction structure, although isolated nanoparticles with sole rutile phase coexist. In addn. to XRD anal., TEM measurement shows that the Moire fringes are frequently present, which further manifests that some anatase particles are covered with rutile clusters or thin overlayers.
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    32. 32
      Ito, S.; Chen, P.; Comte, P.; Nazeeruddin, M. K.; Liska, P.; Pechy, P.; Gratzel, M. Fabrication of Screen-printing Pastes from TiO2 Powders for Dye-sensitised Solar Cells. Prog. Photovoltaics 2007, 15, 603– 612,  DOI: 10.1002/pip.768
      32
      Fabrication of screen-printing pastes from TiO2 powders for dye-sensitized solar cells
      Ito, Seigo; Chen, Peter; Comte, Pascal; Nazeeruddin, Mohammad Khaja; Liska, Paul; Pechy, Peter; Graetzel, Michael
      Progress in Photovoltaics (2007), 15 (7), 603-612CODEN: PPHOED; ISSN:1062-7995. (John Wiley & Sons Ltd.)
      A prepn. technique of TiO2 screen-printing pastes from com.-available powders was described to fabricate the nanocryst. layers without cracking and peeling-off over 17 μm thickness for the photoactive electrodes of the dye-sensitized solar cells. A conversion efficiency of 8.7% was obtained by using a single-layer of a semi-transparent-TiO2 film. A conversion efficiency of 9.2% was obtained by using double-layers composed of transparent and light-scattering TiO2 films for a photon-trapping system.
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    33. 33
      Mills, A.; Hill, G.; Crow, M.; Hodgen, S. Thick Titania Films for Semiconductor Photocatalysis. J. Appl. Electrochem. 2005, 35, 641– 653,  DOI: 10.1007/s10800-005-1628-5
      33
      Thick titania films for semiconductor photocatalysis
      Mills, Andrew; Hill, George; Crow, Matthew; Hodgen, Stephanie
      Journal of Applied Electrochemistry (2005), 35 (7-8), 641-653CODEN: JAELBJ; ISSN:0021-891X. (Springer)
      A review. A brief overview of work carried out by this group on thick (>1 μm), optically clear, robust titania films prepd. by a sol-gel method, as well as new results regarding these films, are described. Such films are very active as photocatalysts and able to destroy stearic acid with a quantum yield of 0.32%. The activity of such films is largely unaffected by annealing temps. <760°, but is drastically reduced above this temp. The drop in photocatalyst activity of such films as a function of annealing temp. appears to correlate well with the change in porosity of the films and suggests that the latter parameter is very important in deciding the overall activity of such films. The importance of porosity in semiconductor photocatalyzed cold combustion may be due to the effect it has on access of oxygen to the active sites, rather like the effect the position of a fire grate (open or closed) has on the rate of burning, i.e., hot combustion, that takes place in a fireplace.
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    34. 34
      O’Rourke, C.; Mills, A. Probing P25 TiO2 Photocatalysis Using Photoinduced Absorption Spectroscopy (PIAS). Chem. Commun. 2021, 57, 1242– 1245,  DOI: 10.1039/d0cc07794a
      34
      Probing P25 TiO2 photocatalysis using photoinduced absorption spectroscopy (PIAS)
      O'Rourke, Christopher; Mills, Andrew
      Chemical Communications (Cambridge, United Kingdom) (2021), 57 (10), 1242-1245CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      Photoinduced absorption spectroscopy, PIAS, is used for the first time to probe the kinetics exhibited by the most commonly employed powder photocatalyst, P25 TiO2, in mesoporous film form, in the photocatalyzed oxidn. of a commonly used test org. pollutant, 4-chlorophenol, 4CP. The results show that PIAS can provide previously unobtainable, invaluable, direct kinetic, and mechanistic, information concerning photogenerated holes in powd. photocatalysts.
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    35. 35
      Mills, A.; Bingham, M.; O’Rourke, C. Ring Photocatalysis: Bands of Activation Surrounding Macro-Sized (≥1 mm Radius) Pt Islands Deposited on TiO2 Films. J. Phys. Chem. C 2020, 124, 13550– 13559,  DOI: 10.1021/acs.jpcc.0c02636
      35
      Ring Photocatalysis: Bands of Activation Surrounding Macro-Sized (≥1 mm Radius) Pt Islands Deposited on TiO2 Films
      Mills, Andrew; Bingham, Michael; O'Rourke, Christopher
      Journal of Physical Chemistry C (2020), 124 (25), 13550-13559CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Clearly discernible rings of high photocatalytic activity, i.e., rings of activation, RoA, are formed around Pt macro-sized (radius: ro ≥ 1 mm radius) Pt islands, i.e., Pt "dots", deposited on TiO2 films, which have been covered with a visible pollutant, such as soot, or a dye, such as acid orange 7. The features of the RoA region appear very similar for soot- or dye-covered films. Thus, the radius of the RoA region, ra, appears to depend directly on ro, plus a const., typically 0.22-0.68 mm. The rate of photocatalytic removal of the surface pollutant inside the RoA is uniform and significantly greater, i.e., ca. 3.2-5x, than that outside the RoA region. RoA region formation is obsd. for (i) Pt dots which have been photodeposited or created through thermal redn., (ii) Pt dots on sol-gel and com. (paints and tiles) TiO2 films, and (iii) Pd dots, but less clearly for Au dots. A simple mechanistic rationale for RoA region formation is presented, and the potential importance of this feature, with regard to improving the performance of com. photocatalytic films, is discussed briefly.
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    36. 36
      Ollis, D. Removal kinetics of stearic acid discrete deposits on photocatalytic self-cleaning surfaces: Effect of deposit initial size distribution. Appl. Catal., B 2017, 209, 174– 182,  DOI: 10.1016/j.apcatb.2017.02.029
      36
      Removal kinetics of stearic acid discrete deposits on photocatalytic self-cleaning surfaces: Effect of deposit initial size distribution
      Ollis, David
      Applied Catalysis, B: Environmental (2017), 209 (), 174-182CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
      Earlier reports by Sawunyama et al. (1997, 1999) and Ghazzal et al. (2011) used at. force microscopy (AFM) and optical microscopy to characterize the structure of stearic acid (SA) deposits on polycryst. TiO2 films and Ti [110] crystal surfaces. Their prepn. methods and catalysts yielded island-like SA deposits, rather than continuous films, for stearic acid submonolayers and multi-layers. Ghazzal obsd. that individual islands showed disappearance kinetics to be zero order with respect to island area, but apparently first order with respect to total SA remaining. We develop here kinetic models which are consistent with these "island" findings and establish the guidelines for apparent kinetic order of reaction, n:1. Island deposits of a single size will exhibit apparent zero order kinetics., n = 0.2. Island deposits with a narrow distribution of island sizes will exhibit an apparent half order behavior, n = 0.5.3. Island deposits with a broad size distribution will exhibit an apparent reaction order of unity, n = 1. Our conclusion is that meaningful kinetic anal. of SA island photocatalyzed disappearance, and thus that of any oxidizable, carbonaceous pollutant which deposits in island form, rather than as a continuous film, requires knowledge of the initial island size distribution. The SA deposits may also undergo significant rearrangement as the titania surface is illuminated, quickly transforming the latter from hydrophobic to hydrophilic. Thus it is the reactant film and/or island distribution after initiation of illumination which is most relevant to kinetic modeling attempts.
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  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.3c00952.

    • Schematic images of a cylindrical island and array of such islands based on the 2D kinetic model, and comparison of AFM and optical microscope height measurements (PDF)


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