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Materials and Interfaces

Photocatalytic Silica–Resin Coating for Environmental Protection of Paper as a Plastic Substitute
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Industrial & Engineering Chemistry Research

Cite this: Ind. Eng. Chem. Res. 2022, 61, 20, 6967–6972
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https://doi.org/10.1021/acs.iecr.2c00784
Published May 13, 2022

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Abstract

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We developed an easy silica–resin coating technique to compensate for paper’s weaknesses, including its lack of water resistance and strength, and proposed its use as an environmentally friendly alternative to plastic. We demonstrate here that 2 nm anatase TiO2 nanoparticles were finely dispersed in a coating film with a thickness of several micrometers formed on the paper’s cellulose fibers and exhibited moderate photocatalytic effects such as methylene blue degradation and antibacterial activity. Additionally, the porous silica–resin film has a high adsorptive capacity, efficiently capturing organic pollutants until they decompose via photocatalytic reactions. As a result, the stable silica–resin–TiO2 composite coating protects paper from the environment for an extended period of time, transforming it into an excellent plastic substitute with enhanced functionality.

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1. Introduction

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Environmental protection and a sustainable society require the development of a de-plasticization technology. (1) However, finding a substitute for cheap, easy-to-process, and chemically stable plastic is not easy. We focused on paper, a naturally occurring and environmentally friendly material, and developed a coating technology that enables paper to be used in place of plastic. (2,3) By using a low-viscosity alkoxysilane-based liquid and drying it at ambient temperature, a silica–resin layer several micrometers thick was formed around the cellulose fibers of the paper (Figure 1a). The siloxane-bonded film reinforces the paper while retaining its flexibility and texture, and the alkyl groups such as methyl groups left in the voids of the film provide the paper with water resistance (Figure 1b). Thus, coated paper can be used in place of the majority of plastics and degrades in an environmentally benign manner after disposal.

Figure 1

Figure 1. Silica–resin coating on cellulose fibers. (a) SEM image (left top) illustrates the morphology of paper composed of intertwined cellulose fibers that are a few tens of micrometers in diameter. The fibers’ edges appear relatively bright because of the enhanced backscattering of secondary electrons from Si, which is heavier than carbon, cellulose’s primary component. The corresponding elemental maps for carbon, silicon, and titanium, which were obtained using the energy-dispersive X-ray analysis, demonstrate the formation of a silica layer on the cellulose fibers: the brighter the image, the higher the concentration. Note that a trace of titanium is concentrated around the surface layer. (b) Water resistance demonstration: after 2 weeks in water at room temperature, an origami crane made of silica–resin-coated Japanese paper retained its shape (main panel), but an origami crane made of uncoated Japanese paper lost its shape after 24 h (inset).

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The silica–resin film formation reaction is basically an improved sol–gel method that does not require additional water or an acid/base catalyst, which was required in the conventional sol–gel method; (4) naturally, a high water concentration accelerates the decomposition of paper. It is believed that a trace amount of tetraisopropyl titanate (TPT, [(CH3)2CHO]4Ti) is required for the reaction. (2) Because TPT is more easily hydrolyzed than alkoxysilane, (5) it can act as an accelerator for the hydrolysis and polymerization of alkoxysilane, resulting in the quick formation of a silica–resin film in the presence of very little water from moisture and paper.
Although no experimental evidence has been provided, it is believed that the thus-consumed Ti will be integrated into the silica network or will remain in the film as titanium dioxide TiO2. If the coating contains TiO2, a photocatalytic effect is expected, leading to the development of novel functions as TiO2 nanoparticles exhibit superior photocatalytic activity (Supporting Information 1). (6−10) Indeed, previous research demonstrated that TiO2-containing paper prepared by combining a softwood craft pulp and TiO2 aqueous sol with a binder had a high photocatalytic activity, indicating that paper pulp serves as an excellent matrix. (11) The goal of this study is to characterize the presence and morphology of TiO2 in the coating film and to investigate unique capabilities of coated paper that add values and broaden possibilities of the silica–resin coating.

2. Experimental Section

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2.1. Coating Agents and Sample Preparations

We prepared a standard coating liquid that contained silane compounds and TPT at a concentration of 6.8 mol % per Si (liquid C1) (Supporting Information 2). We examined both a piece of handmade Japanese paper or filter paper treated with liquid C1 (paper C1) and a solidified bulk sample (bulk C1). Additionally, we prepared coating agents with TPT concentrations that were 2, 5, and 10 times that of standard C1 (liquids C2, C5, and C10, respectively).

2.2. Characterization Methods

The structures were characterized using powder X-ray diffraction (XRD) experiments, electron diffraction (ED), and transmission electron microscopy (TEM) (Supporting Information 3.1). The porosity of coating layers was determined using nitrogen adsorption experiments (Supporting Information 3.2). The X-ray absorption near-edge structure (XANES) spectra of Ti K-edge were obtained using synchrotron radiation (Supporting Information 3.3). The photocatalytic activity was determined by measuring the rate of decomposition of methylene blue (MB) (Supporting Information 3.4). Antibacterial performance tests were conducted using Staphylococcus aureus (Supporting Information 3.5).

3. Results and Discussion

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3.1. Porous Film Formation

The powder XRD patterns for bulk C1, C2, C5, and C10 are shown in Figure 2. There are two broad peaks whose diffraction angles correspond to the characteristic distances between Si atoms in the silica–resin network: the high-angle peak indicates the presence of short Si–Si bonds characteristic of crystalline or vitreous SiO2, while the low-angle peak indicates the presence of large voids containing methyl groups (Supporting Information 4).

Figure 2

Figure 2. Powder XRD patterns of a C1 bulk sample and of C2, C5, and C10 bulk samples with increasing TPT concentrations in coating liquids. The pattern of sample E obtained via the hydrolysis of TPT is also shown. The patterns are vertically offset for clarity. The arrow and vertical broken line indicate the location of the anatase 1 0 1 diffraction peak at 25.4°.

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The low density of bulk C1 (0.925 g cm–3), which is significantly less than that of ordinary silica glass (∼2.5 g cm–3), revealed the coated film’s porous structure as well. The surface area of paper C1 was determined using nitrogen adsorption experiments, the results of which are shown in Figure 3. In comparison to pristine filter paper, paper C1 adsorbs a significant amount of nitrogen as the pressure increases, implying the formation of a porous film with a large surface area on the cellulose fibers. The desorption curve is similar to the adsorption curve for pristine paper, but they diverge as the pressure decreases for paper C1, indicating that nitrogen gas is still adsorbing. This hysteresis is caused by the slow kinetics of nitrogen physisorption: (12) due to the small bottleneck in the micropores, nitrogen molecules cannot enter easily and are difficult to exit once inside; thus, an equilibrium adsorption state was not achieved for paper C1. Bulk C1, on the other hand, has a much lower adsorption capacity because the powdered sample was composed of large crystallites with a submicrometer size distribution.

Figure 3

Figure 3. Nitrogen physisorption. (a) Adsorption–desorption isotherms of coated paper (circles), uncoated paper (triangles), and bulk C1 (squares). The volume of the adsorbate (nitrogen) adsorbed at standard temperature and pressure is denoted by V, the equilibrium pressure of the adsorbate is denoted by P, and the saturated pressure is denoted by P0. The filled and open symbols represent the adsorption and desorption processes, respectively. (b) BET plots fitted with linear functions, the slopes of which yield the SSA.

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The specific surface area (SSA) and pore size were determined from the plot in Figure 3b using the Brunauer–Emmett–Teller (BET) theory. The SSA increased from 0.55 m2 for pristine paper to at least 4.3 m2 for paper C1 per unit volume of cubic centimeters; however, because an equilibrium state was not reached during the experiments, the actual SSA value of paper C1 must be significantly larger. The total volume and average diameter of pores in paper C1 (pristine paper) were determined to be 0.004 (0.007) cm3/g and 3.9 (23.1) nm, respectively; these values of pristine paper are in good agreement with those of dried cellulose. (13)

3.2. Anatase Nanoparticles

Figure 2 illustrates the powder XRD pattern of bulk C1 with a shoulder on the high-angle side of the high-angle peak. At similar diffraction angles, the shoulder grows with the increasing TPT content, suggesting that Ti compounds are responsible for its formation. The diffraction angle is identical to that of the main 1 0 1 peak for crystalline anatase TiO2 but not to those of rutile and brookite. The observed broadening indicates either a small particle size or a crystal structure that is highly disordered. Indeed, anatase nanoparticles prepared previously exhibited a single broad peak at ∼25°. (14,15) On the other hand, the XRD pattern of sample E obtained via hydrolysis of TPT under similar conditions in Figure 2 resembles that of anatase in the bulk, indicating the formation of larger crystallites with improved crystallinity, as previously observed. (14) Thus, under the mild reaction conditions used in the present study, TPT in the coating liquid may have been converted to anatase nanoparticles.
We used TEM to observe bulk C5 to confirm the presence of anatase nanoparticles and to investigate their size, dispersity, and distribution within the matrix. As shown in Figure 4, dark circular spots with a diameter of ∼2 nm are embedded in the granular contrast of the matrix. The dark contrast should be due to Ti atoms having a greater mass than that of Si atoms, while the granular contrast should be due to the amorphous structure of Si–O compounds. The enlarged photographs in Figure 4b,c show lattice images of anatase’s tetragonal structure. Therefore, anatase nanoparticles form and disperse uniformly throughout the silica–resin glass matrix. The diameter of the nanoparticles is always ∼2 nm, indicating a good dispersity. In comparison, as shown in Figure 4d, sample E from TPT hydrolysis contains well-crystallized anatase particles with extended (1 0 1) lattice fringes.

Figure 4

Figure 4. TEM images from bulk C5 and sample E. For bulk C5, a thin section of the fragment in (a) was observed at high magnification, as shown in (b,c). The image in sample E depicts densely packed and well-crystalized anatase nanoparticles (d). The lattice images in (c,d) with spacings of 0.35, 0.26, and 0.22 nm correspond to the anatase (1 0 1), (1 0 3), and (0 0 4)/(1 1 2) lattice planes, respectively. Insets show ED patterns of a holo pattern from an amorphous material in (b) and Debye rings from a crystalline material in (d).

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The XANES spectra of bulk C1 and paper C1 are nearly identical to each other and are closer to the standard anatase spectrum than the rutile and brookite spectra (Figure 5; Supporting Information 5). On the other hand, in the pre-edge spectra below ∼4.98 keV, the A2 peak is significantly enhanced in the C1 samples compared with the A1 and A3 peaks. (14−17) It is established that the A2 component increases sensitively with the increasing structural disorder, such as a lower Ti coordination number, as well as with the decreasing particle size. (14,16,17) The observed increase in A2 is even greater than that previously reported for anatase nanoparticles prepared using the standard sol–gel method, and their shapes resemble those previously reported for 2 nm size “amorphous” TiO2 particles prepared in a specific way at 0 °C by Zhang et al. (15)

Figure 5

Figure 5. XANES spectra of bulk C1, paper C1, and bulk E. For comparison, the spectra of the standard anatase, rutile, and brookite samples measured in the same experimental setup are also included. The inset enlarges the pre-edge region to demonstrate the presence of disordered anatase nanoparticles in both bulk C1 and paper C1.

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Therefore, we conclude that the TPT in the coating liquid resulted in highly disordered anatase nanoparticles. It is unknown why such amorphous-like anatase particles with a diameter of 2 nm form during our coating process. Perhaps the silane compounds play a role in the production. We hypothesize that hydrolyzed TPT is incorporated into the siloxane network during the initial reaction while it is still loose and that the Ti atom is discharged from the network as the reaction proceeds because Ti does not prefer the tetrahedral coordination of Si and instead prefers an octahedral coordination, as realized in anatase. As a result of the mild and slow reaction conditions, the Ti atoms aggregate to form anatase nanoparticles with a high degree of structural disorder that are embedded in the silica–resin network.

3.3. Photocatalytic Activity

MB degradation is frequently used to determine the photocatalytic activity of TiO2 and other semiconductors. (18−21) When a portion of the methyl groups are degraded by the catalytic reaction of TiO2, a hypochromic shift occurs, and the blue color of MB solutions becomes weaker. Finally, MB oxidizes to form water, carbon dioxide, and other inorganic molecules. (20)
We analyzed two samples. As shown in Figure 6, one is a film deposited on a glass substrate from coating liquid C1, and the other is paper C1. After 48 h, a significant difference is observed for C1 on glass: compared to a slight decrease for uncoated glass, the MB concentration becomes 32 and 1.6% of the starting composition of 10 μmol L–1 for coated glasses in the dark and in the light under UV illumination, respectively. The former reduction occurs as a result of adsorption onto a porous film, while the latter involves an additional photocatalytic reaction. Within a 3 h period, the variations are nearly linear in time, as illustrated in Figure 6b. The coefficients are obtained by fitting the data to the linear form: −3.74(3) and −9.35(2) nmol L–1 min–1, respectively, for dark and light conditions. The photodegradation index calculated as the difference between them is 5.6 nmol L–1 min–1, which is almost equal to the photocatalytic activity criteria of 5 nmol L–1 min–1 established by the Photocatalysis Industry Association of Japan (PIAJ). (22) Thus, our coating exhibits a moderate photocatalytic activity due to the presence of anatase nanoparticles; we did not observe a complete decomposition of MB.

Figure 6

Figure 6. Photocatalytic activity of the silica–resin coating. Time evolutions of the MB concentration in a C1 film on glass (a and b) and in paper C1 (b) are shown. The terms “dark” and “light” refer to measurements obtained in the dark and under 1 mW cm–2 UV light, respectively, while “blank” refers to a measurement obtained under UV light for uncoated glass. The lines serve as guides for the viewer’s eye. Another set of initial measurements for paper C1 are also plotted in (b). The green crosses represent data for uncoated blank paper exposed to UV light. The lines drawn on the “dark” and “light” data for C1 on glass are linear fits.

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Paper C1 exhibits larger initial reductions than the C1 film on glass, as plotted in Figure 6b. This is probably because of the enhanced adsorption to the porous film produced on the cellulose fibers of paper, as demonstrated by nitrogen adsorption experiments; the coated film on paper has a higher porosity than on glass. The difference between the dark and light curves is comparable to that of C1 on glass, indicating a similar photocatalytic activity, irrespective of the kind of the substrate. Note that the porous structure of the silica–resin matrix is advantageous for applications because it can capture pollutant molecules and keep them for a long time until they are degraded by the embedded TiO2 nanoparticles.

3.4. Antibiotic Susceptibility

The photochemical sterilization of microbial cells using TiO2 has been extensively studied. (9,10,23,24)Figure 7 illustrates the results of the antibacterial performance evaluation on our samples. After 8 h on uncoated paper (ref), the number of colonies of S. aureus increased from 9000 to 83,000 and 22,000 cfu in the dark and light, respectively; the mild UV light utilized (0.25 mW cm–2), which approximates the brightness near a window during the day, was insufficient to kill them. By comparison, significant reductions in colony numbers were observed for coated paper: the colony number decreased by 1 order of magnitude in the dark and by 2 orders of magnitude in the light; for paper C1, the number decreased from 9000 to 3000 and 80 cfu, respectively, in the dark and light. As a result, the coated paper is extremely antibiotic-resistant.

Figure 7

Figure 7. Antibiotic activity of the silica–resin coating. Evolution of the viable cell count (colony forming unit: cfu) for S. aureus in 50 × 50 mm2 papers; uncoated paper (ref) and coated papers D and C1 after 8 h in darkness and light. Paper D was prepared with liquid D composed mainly of tetraethyl orthosilicate (Supporting Information 2).

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The significant increase in activity when exposed to UV light indicates that the photocatalytic activity of the anatase nanoparticles in the covering film is critical for sterilization. The antibacterial activity value, SL, which is the standard index for determining antibiotic susceptibility (Supporting Information 6), is 2.4 (2.3) for paper C1 (D), which is greater than the PIAJ-defined threshold value of 2.0 for efficient photocatalytic activity but much less than the SL > 20 determined for efficient TiO2-based photocatalysts. (22) On the other hand, the light irradiation efficiency ΔS of paper C1 (D) is estimated to be 0.4 (0.9). Furthermore, these ΔS values exceed the PIAJ threshold value of 0.3. (22) Therefore, our silica–resin coating exhibits moderate photocatalytic activity, not a strong one as in conventional photocatalysts.

3.5. Performance of the Silica–Resin-Coated Paper

Our silica–resin coating film does not display a high photocatalytic effect, in part due to the low concentration of TiO2: the resulting TiO2 amounts only to 9.3 mg in the 50 × 50 mm2 paper used for the antibiotic test. The effect, however, is expected to last a long time due to the chemical and mechanical stability of the matrix silica coating, as well as its affinity for the substrate. In addition, the superior adsorptive property of the silica–resin film, owing to its porous structure, is advantageous for organic pollutant removal, and the persistence of the mild photocatalytic effect allows the decomposition reaction to proceed slowly. As a result, the substrate is protected for an extended period. Furthermore, the coated paper will be used as an organic pollutant remover that can be disposed harmlessly in nature after use.
The majority of commercially available photocatalytic materials are composed of anatase TiO2 nanoparticles. (25) For practical applications, a dispersant is required to keep them from aggregating. Additionally, a binder is required to adhere the TiO2 nanoparticles to the substrate surface, and an interfacial layer must be formed to prevent the substrate from decomposing due to the photocatalytic effect. However, because of TiO2’s overwhelming photocatalytic action, it is usually difficult to completely prevent the decomposition of these extra constituents, resulting in instability. Another issue is the high cost of manufacturing binders and interface layers. In stark contrast, the present coating enables the self-organization of a persistent protective film containing anatase nanoparticles with mild photocatalytic activity merely by applying a coating liquid. We believe that the genuine benefit of the photocatalytic effect stems from its protective function in a mild environment, not from its quick breakdown activity. Our silica–resin coating is well-suited for long-term use.
Our silica–resin coating technique is likely to find a wide range of applications. For instance, coated paper would provide waterproof and antimicrobial cutlery that is readily disposed of without causing environmental harm. A recent study demonstrated the formation of a protective silica–resin film containing an antimicrobial agent such as hinokitiol on the surface of nonwoven fabric used in medical masks. (26) This protective coating was found to have an excellent antimicrobial effect. We believe that our silica–resin coating technology can be applied to a wide variety of products, from everyday necessities to industrial products, to assist in the transition to a more sustainable and safe society through the use of paper or other materials rather than plastics.

4. Conclusions

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We report a novel silica–resin coating technique for paper that can change paper to plastic using a simple method at a low cost. The thus-obtained organic–inorganic hybrid materials can replace many plastic products and are easily decomposed in the natural environment without causing pollution. This technique will provide us with a breakthrough to solve the existing environmental problems.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.iecr.2c00784.

  • Titanium dioxides and its photocatalytic activity, coating agents and sample preparation, characterization techniques, XRD results, XANES spectra, antibiotic susceptibility, and references (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
    • Yoko Iwamiya - Choetsu Kaken Co., Ltd., Suehiro, Tsurumi, Yokohama, Kanagawa 230-0045, Japan
    • Daisuke Nishio-Hamane - Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
    • Kazuhiro Akutsu-Suyama - Neutron Science and Technology Center, Comprehensive Research Organization for Science and Society, Tokai, Ibaraki 319-1106, Japan;  Orcidhttps://orcid.org/0000-0002-4797-6604
    • Hiroshi Arima-Osonoi - Neutron Science and Technology Center, Comprehensive Research Organization for Science and Society, Tokai, Ibaraki 319-1106, Japan
    • Mitsuhiro Shibayama - Neutron Science and Technology Center, Comprehensive Research Organization for Science and Society, Tokai, Ibaraki 319-1106, Japan;  Orcidhttps://orcid.org/0000-0002-8683-5070
  • Author Contributions

    Y.I. conceived the project and conducted the design. D.N.-H. performed the XRD and SEM measurements. K.A.-S. and H.A.-O. performed nitrogen absorption and XANES experiments. Y.I., M.S., and Z.H. carried out the data analysis. All authors contributed to the manuscript preparation and discussion.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors are also grateful to K. Satomura, R. Hoshino, and R. Iwamiya for their helpful comments; T. Muroi, A. Fukui, and D. Hirai for their assistance in the XRD measurements; D. Aoki and Y. Akutsu for their assistance in the MB and antibiotic experiments; and K. Itoh and S. Takada for their assistance in the nitrogen adsorption measurements. They also thank M. Mochida and K. Taba for their help in improving the visual picture. The nitrogen adsorption measurements were conducted at the J-PARC center. The XANES experiments were conducted at the BL5S1 beamline of the Aichi Synchrotron Radiation Center, Aichi Science & Technology Foundation, Aichi, Japan (proposal no. 202104025). The sample preparation for the XANES experiments was conducted at the User Experiment Preparation Lab III operated by the Comprehensive Research Organization for Science and Society (CROSS).

References

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    We report on the synthesis of phase-pure TiO2 nanoparticles in anatase, rutile and brookite structures, using amorphous titania as a common starting material. Phase formation was achieved by hydrothermal treatment at elevated temps. with the appropriate reactants. Anatase nanoparticles were obtained using acetic acid, while phase-pure rutile and brookite nanoparticles were obtained with hydrochloric acid at a different concn. The nanomaterials were characterized using x-ray diffraction, UV-visible reflectance spectroscopy, dynamic light scattering, and transmission electron microscopy. We propose that anatase formation is dominated by surface energy effects, and that rutile and brookite formation follows a dissoln.-pptn. mechanism, where chains of sixfold-coordinated titanium complexes arrange into different crystal structures depending on the reactant chem. The particle growth kinetics under hydrothermal conditions are detd. by coarsening and aggregation-recrystn. processes, allowing control over the av. nanoparticle size.
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  8. 8
    Rahimi, N.; Pax, R. A.; Gray, E. M. Review of functional titanium oxides. I: TiO2 and its modifications. Prog. Solid State Chem. 2016, 44, 86– 105,  DOI: 10.1016/j.progsolidstchem.2016.07.002
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    Review of functional titanium oxides. I: TiO2 and its modifications
    Rahimi, Nazanin; Pax, Randolph A.; Gray, Evan MacA.
    Progress in Solid State Chemistry (2016), 44 (3), 86-105CODEN: PSSTAW; ISSN:0079-6786. (Elsevier Ltd.)
    A review. An extensive and wide-ranging literature about the polymorphs of titanium dioxide (TiO2) has accumulated during the last few decades, providing a very large resource of data on its properties, functionality and many present and potential industrial uses. This review focuses on the structural, kinetic, thermodn. and elec. properties of TiO2 from the viewpoint of the relationship between the crystal structure and its present or potential useful functionality, via the electronic structure. The reason for this focus is the fundamental relationship between the electronic band structure of this wide band-gap semiconductor and its interaction with light and chem. species. Intense interest in the photoactivity of TiO2 followed the demonstration by Fujishima and Honda in 1972 of its ability to dissocd. water using sunlight. Approaches to band gap engineering via chem. modifications are surveyed and correlated with band-structure calcns. using D. Functional Theory and Hartree Fock methods. In the last section, progress in TiO2 applications and prospects for new applications of this material are summarised.
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  9. 9
    Matsunaga, T.; Tomoda, R.; Nakajima, T.; Wake, H. Photoelectrochemical sterilization of microbial cells by semiconductor powders. FEMS Microbiol. Lett. 1985, 29, 211– 214,  DOI: 10.1111/j.1574-6968.1985.tb00864.x
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    Photoelectrochemical sterilization of microbial cells by semiconductor powders
    Matsunaga, Tadashi; Tomoda, Ryozo; Nakajima, Toshiaki; Wake, Hitoshi
    FEMS Microbiology Letters (1985), 29 (1-2), 211-14CODEN: FMLED7; ISSN:0378-1097.
    The novel concept of photochem. sterilization is described. Microbial cells were killed photoelectrochem. with semiconductor powder (Pt-loaded TiO2 [TiO2/Pt]). CoA in the whole cells was photoelectrochem. oxidized and, as a result, the respiration of cells was inhibited. Inhibition of respiration caused death of the cells. Lactobacillus acidophilus, Saccharomyces cerevisiae, And Escherichia coli (103 cells/mL, resp.) were completely sterilized when they were incubated with TiO2/Pt particles under metal halide lamp irradn. for 60-120 min.
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  10. 10
    Kikuchi, Y.; Sunada, K.; Iyoda, T.; Hashimoto, K.; Fujishima, A. Photocatalytic bactericidal effect of TiO2 thin films: dynamic view of the active oxygen species responsible for the effect. J. Photochem. Photobiol., A 1997, 106, 51– 56,  DOI: 10.1016/s1010-6030(97)00038-5
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    Photocatalytic bactericidal effect of TiO2 thin films: dynamic view of the active oxygen species responsible for the effect
    Kikuchi, Yoshihiko; Sunada, Kayano; Iyoda, Tomokazu; Hashimoto, Kazuhito; Fujishima, Akira
    Journal of Photochemistry and Photobiology, A: Chemistry (1997), 106 (1-3), 51-56CODEN: JPPCEJ; ISSN:1010-6030. (Elsevier)
    The role of active O spp. in the photocatalytic bactericidal effect was investigated using a thin transparent titanium dioxide (TiO2) film. The viable no. of Escherichia coli significantly decreased on the illuminated TiO2 film, and the bactericidal effect was obsd. even when E. coli was sepd. from the TiO2 surface with a 50-μm porous membrane. Mannitol, a hydroxyl radical scavenger, inhibited the effect only in the absence of the membrane. In contrast, catalase inhibited the effect in all cases. On the basis of these results, the long-range bactericidal effect of H2O2 was proposed, together with a cooperative effect due to other O spp.
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    Matsubara, H.; Takada, M.; Koyama, S.; Hashimoto, K.; Fujishima, A. Photoactive TiO2 Containing Paper: Preparation and Its Photocatalytic Activity under Weak UV Light Illumination. Chem. Lett. 1995, 24, 767– 768,  DOI: 10.1246/cl.1995.767
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    Bardestani, R.; Patience, G. S.; Kaliaguine, S. Experimental methods in chemical engineering: specific surface area and pore size distribution measurements─BET, BJH, and DFT. Can. J. Chem. Eng. 2019, 97, 2781– 2791,  DOI: 10.1002/cjce.23632
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    Experimental methods in chemical engineering: specific surface area and pore size distribution measurements-BET, BJH, and DFT
    Bardestani, Raoof; Patience, Gregory S.; Kaliaguine, Serge
    Canadian Journal of Chemical Engineering (2019), 97 (11), 2781-2791CODEN: CJCEA7; ISSN:0008-4034. (John Wiley & Sons, Inc.)
    A review. Gas physisorption is an exptl. technique based on equil. Van der Waals interactions between gas mols. and solid particles, that quantifies the sp. surface area (SSA), pore size distribution (PSD), and pore vol. of solids and powders. The performance of catalysts, absorbents, chromatog. column materials, and polymer resins depends on these morphol. properties. Here we introduce the basic principles and procedures of phys. adsorption, esp. nitrogen physisorption, as a guide to students and researchers unfamiliar with the field. The Brunauer-Emmett-Teller theory (BET) is a common approach to est. SSA that extends the Langmuir monolayer mol. adsorption model to multilayer layers. It relies on an equil. adsorption isotherm, measured at the normal b.p. of the adsorbate, eg, 77 K or 87 K for N2 and Ar, resp. Web of Science indexed 45 400 articles in 2016 and 2017 that mentioned N2 adsorption porosimetry-BET and BJH (Barrett-Joyner-Halenda) keywords. The VOSViewer bibliometric tool grouped these articles into four research clusters: adsorption, activated carbon in aq. solns. for removal of heavy metal ions; synthesis of nanoparticles and composites; catalysts performance in oxidn. and redn. processes; and photocatalytic degrdn. with TiO2. According to the literature, the accuracy of the d. function theory (DFT) method is higher than with the BJH theory and it is more reliable.
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    Stone, J. E.; Scallan, A. M. A Study of Cell Wall Structure by Nitrogen Adsorption. Pulp Pap. Mag. Can. 1965, 66, T-407
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    Cell wall structure studied by nitrogen adsorption
    Stone, J. E.; Scallan, A. M.
    Pulp & Paper Magazine of Canada (1965), 66 (8), T407-T414CODEN: PPMCAW; ISSN:0033-4103.
    Pierce-method analyses of the N adsorption-desorption isotherms of sprucewood and pulp fibers gave a median pore width of 32-38 A. and a most-common pore width of 16-20 A. Electron micrographs led to the adoption of slit-like pores rather than cylindrical pores as the basic structural unit. A model is postulated that consists of laterally assocd. microfibrils forming lamellae sepd. by the slitlike pores. The relation between N adsorption data and the postulated structure is examd. from 2 viewpoints: (1) starting with the small surface area of a water-dried fiber and basing calcns. on the surface increase occurring during swelling, and (2) starting with a small structural unit having a large sp. surface and basing the calcns. on the aggregation of these into larger groups with smaller sp. surfaces, while progressing through the wall-construction stage to the final dry fiber.
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    Luca, V.; Djajanti, S.; Howe, R. F. Structural and Electronic Properties of Sol–Gel Titanium Oxides Studied by X-ray Absorption Spectroscopy. J. Phys. Chem. B 1998, 102, 10650– 10657,  DOI: 10.1021/jp981644k
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    Structural and Electronic Properties of Sol-Gel Titanium Oxides Studied by X-ray Absorption Spectroscopy
    Luca, Vittorio; Djajanti, Samitha; Howe, Russell F.
    Journal of Physical Chemistry B (1998), 102 (52), 10650-10657CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
    X-ray absorption spectroscopy (XAS) was used to study the local Ti environment in TiO2 xerogel samples contg. nanoparticles of different sizes. The xerogels were prepd. by hydrolysis of Ti isopropoxide followed by peptization with HNO3 and size control was achieved through calcination in air at different temps. An amorphous ppt. obtained by hydrolysis of Ti isopropoxide prior to peptization with HNO3 also was studied. The x-ray absorption near edge structure (XANES) of the ppt. possesses a characteristic pre edge that is dominated by a transition designated as A2 at 4970.7 eV, and assigned to five coordinate Ti. This A2 component is detected in all of the studied xerogels. Its intensity decreases as the surface-to-vol. ratio of the TiO2 particles decreases, suggesting that it is assocd. with surface layers of the anatase particles. Other changes obsd. in the XANES of the xerogels as a function of particle size include broadening of 1s → np transitions in the postedge region. EXAFS shows that the smallest particles contain Ti in distorted coordination with contracted Ti-O bonds and probably reduced coordination no.
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  15. 15
    Zhang, H.; Chen, B.; Banfield, J. F.; Waychunas, G. A. Atomic structure of nanometer-sized amorphous TiO2. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 78, 214106,  DOI: 10.1103/physrevb.78.214106
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    Atomic structure of nanometer-sized amorphous TiO2
    Zhang, Hengzhong; Chen, Bin; Banfield, Jillian F.; Waychunas, Glenn A.
    Physical Review B: Condensed Matter and Materials Physics (2008), 78 (21), 214106/1-214106/12CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    Amorphous TiO2 is an important precursor for synthesis of single-phase nanocryst. anatase. We synthesized amorphous TiO2 by hydrolysis of Ti ethoxide at the ice point. Transmission electron microscopy examn. and nitrogen gas adsorption indicated that the particle size of the synthesized titania is ∼2 nm. Synchrotron wide-angle x-ray scattering (WAXS) was used to probe the at. correlations in this amorphous sample. At. pair-distribution function (PDF) derived from Fourier transform of the WAXS data was used for reverse Monte Carlo (RMC) simulations of the at. structure of the amorphous TiO2 nanoparticles. Mol.-dynamics simulations were used to generate input structures for the RMC. X-ray-absorption spectroscopy (XAS) simulations were used to screen candidate structures obtained from the RMC by comparing with exptl. XAS data. The structure model that best describes both the WAXS and XAS data shows that amorphous TiO2 particles consist of a highly distorted shell and a small strained anatase-like cryst. core. The av. coordination no. of Ti is 5.3 and most Ti-O bonds are populated around 1.940 Å. Relative to bulk TiO2, the redn. in the coordination no. is primarily due to the truncation of the Ti-O octahedra at the amorphous nanoparticle surface and the shortening of the Ti-O bond length to the bond contraction in the distorted shell. The pre-existence of the anatase-like core may be crit. to the formation of single-phase nanocryst. anatase in crystn. of amorphous TiO2 upon heating.
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    Chen, L. X.; Rajh, T.; Wang, Z.; Thurnauer, M. C. XAFS Studies of Surface Structures of TiO2 Nanoparticles and Photocatalytic Reduction of Metal Ions. J. Phys. Chem. B 1997, 101, 10688– 10697,  DOI: 10.1021/jp971930g
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    XAFS Studies of Surface Structures of TiO2 Nanoparticles and Photocatalytic Reduction of Metal Ions
    Chen, Lin X.; Rajh, Tijana; Wang, Zhiyu; Thurnauer, Marion C.
    Journal of Physical Chemistry B (1997), 101 (50), 10688-10697CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
    To probe the origin of the unique functions of titanium dioxide (TiO2) nanoparticles obsd. in photocatalytic reactions, structures of Ti atom sites in titanium dioxide (TiO2) nanoparticles with different sizes were studied by Ti K-edge XAFS (x-ray absorption fine structure). Compared to the bulk TiO2 structure, a shorter Ti-O distance from surface TiO2 resulting from Ti-OH bonding was obsd. The XAFS spectra also revealed an increasing disorder of the lattice with decreasing sizes of the nanoparticles based on a coordination no. decrease for the third-shell O atoms as well as changes in relative intensities of preedge peaks A1, A2, and A3. However, the Ti sites largely remain octahedral even in the 30 Å diam. particles. These results imply that the increasing no. of surface Ti sites as well as possible corner defects in small nanoparticles may be the main cause of the unique surface chem. exhibited by nanoparticles of TiO2. XAFS was also used in monitoring the photoredn. reaction products of Cu2+ and Hg2+ on TiO2 nanoparticle surfaces, with or without surface adsorbers, alanine (Ala) and thiolactic acid (TLA). Ala dramatically enhanced photoredn. of Cu2+ on TiO2 nanoparticle surfaces, whereas thiolactic acid did not affect or even hindered Hg2+ photoredn. Although both surface adsorbers chelated with the metal ions in the absence of TiO2 nanoparticles, this chelation was drastically changed in the Cu-Ala complex but was largely retained in the Hg-TLA complex when TiO2 was present. This may correlate with the different effects of the adsorbers on the photoredn. of the metal. Our exptl. results suggest that a proper balance between the affinities of the adsorber to the metal ions and to the surface Ti atoms of TiO2 may be one of the keys in selecting a surface adsorber for enhanced photoredn. efficiency.
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  17. 17
    Wu, Z. Y.; Zhang, J.; Ibrahim, K.; Xian, D. C.; Li, G.; Tao, Y.; Hu, T. D.; Bellucci, S.; Marcelli, A.; Zhang, Q. H.; Gao, L.; Chen, Z. Z. Structural determination of titanium-oxide nanoparticles by x-ray absorption spectroscopy. Appl. Phys. Lett. 2002, 80, 2973– 2975,  DOI: 10.1063/1.1470699
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    Structural determination of titanium-oxide nanoparticles by x-ray absorption spectroscopy
    Wu, Z. Y.; Zhang, J.; Ibrahim, K.; Xian, D. C.; Li, G.; Tao, Y.; Hu, T. D.; Bellucci, S.; Marcelli, A.; Zhang, Q. H.; Gao, L.; Chen, Z. Z.
    Applied Physics Letters (2002), 80 (16), 2973-2975CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    To understand and improve the applications of Ti oxide nanoparticles, it is extremely important to perform a detailed study of the surface and the interior structural properties of nanocryst. materials, such as rutile and anatase with diam. of few nm. X-ray absorption spectroscopy was used to identify the local Ti environment and characterize the related electronic structure. Exptl. results at the Ti K edge in both bulk and nanocrystal samples were combined to det. the lattice distortion via the characteristic pre-edge features and the variation in the multiple-scattering region of the XANES spectra. The correlation between peak intensities and surface-to-vol. ratio of nanoparticles is discussed.
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    Xu, C.; Rangaiah, G. P.; Zhao, X. S. Photocatalytic Degradation of Methylene Blue by Titanium Dioxide: Experimental and Modeling Study. Ind. Eng. Chem. Res. 2014, 53, 14641– 14649,  DOI: 10.1021/ie502367x
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    Photocatalytic Degradation of Methylene Blue by Titanium Dioxide: Experimental and Modeling Study
    Xu, Chen; Rangaiah, G. P.; Zhao, X. S.
    Industrial & Engineering Chemistry Research (2014), 53 (38), 14641-14649CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)
    The application of semiconductors in water treatment via photocatalysis of various pollutants has attracted much attention from researchers. In this work, photocatalytic degrdn. of methylene blue by P25 titanium dioxide was studied exptl. and then via simulation, physicochem., physicochem. The effects of lamp choice, concn. of catalyst, and methylene blue were analyzed. Desorption of methylene blue at the start of light radiation was obsd., and analyzed in detail for the first time. Both desorption and degrdn. processes were modeled, and exptl. data was fitted to a pseudo-first-order simulation, physicochem. with sufficient accuracy. The effects of catalyst and initial dye concn. on reaction rate consts. were discussed.
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  19. 19
    Yang, C.; Dong, W.; Cui, G.; Zhao, Y.; Shi, X.; Xia, X.; Tang, B.; Wang, W. Highly-efficient photocatalytic degradation of methylene blue by PoPD-modified TiO2 nanocomposites due to photosensitization-synergetic effect of TiO2 with PoPD. Sci. Rep. 2017, 7, 3973,  DOI: 10.1038/s41598-017-04398-x
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    19
    Highly-efficient photocatalytic degradation of methylene blue by PoPD-modified TiO 2 nanocomposites due to photosensitization-synergetic effect of TiO2 with PoPD
    Yang Chuanxi; Wang Weiliang; Yang Chuanxi; Dong Wenping; Cui Guanwei; Zhao Yingqiang; Shi Xifeng; Xia Xinyuan; Tang Bo
    Scientific reports (2017), 7 (1), 3973 ISSN:.
    Poly-o-phenylenediamine modified TiO2 nanocomposites were successfully synthesized via an 'in situ' oxidative polymerization method. The modified nanocomposites were characterized by BET, XRD, TEM, FT-IR, TGA, XPS, EA and UV-Vis DRS. The photocatalytic degradation of methylene blue was chosen as a model reaction to evaluate the photocatalytic activities of TiO2 and PoPD/TiO2. The results indicated that PoPD/TiO2 nanocomposites exhibited good photocatalytic activity and stability. The photocatalytic activity of PoPD/TiO2 increased as the initial pH increased because of electrostatic adsorption between the photocatalyst and MB as well as the generation of ·OH, whereas it exhibited an earlier increasing and later decreasing trend as the concentration of the photocatalyst increased owing to the absorption of visible light. The photocatalytic stability of the PoPD/TiO2 nanocomposite was dependent on the stability of its structure. Based on radical trapping experiments and ESR measurements, the origin of oxidizing ability of PoPD/TiO2 nanocomposites on photocatalytic degradation of MB was proposed, which taking into account of ·OH and ·O2(-) were the first and second important ROS, respectively. The possible photocatalytic mechanism and photocatalytic activity enhanced mechanism has been proposed, taking into account the photosensitization effect and synergetic effect of TiO2 with PoPD.
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    Dariani, R. S.; Esmaeili, A.; Mortezaali, A.; Dehghanpour, S. Photocatalytic reaction and degradation of methylene blue on TiO2 nano-sized particles. Optik 2016, 127, 7143– 7154,  DOI: 10.1016/j.ijleo.2016.04.026
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    Photocatalytic reaction and degradation of methylene blue on TiO2 nano-sized particles
    Dariani, R. S.; Esmaeili, A.; Mortezaali, A.; Dehghanpour, S.
    Optik (Munich, Germany) (2016), 127 (18), 7143-7154CODEN: OTIKAJ; ISSN:0030-4026. (Elsevier GmbH)
    Photocatalyst has been of considerable interest due to its new technol. for environmental pollution. Among the photocatalyst semiconductors, titanium dioxide is known as a photocatalyst for the removal of environmental contaminants. In this research, we first designed a new self-made lab. photocatalytic reactor and then photocatalytic properties of TiO2 nanoparticles are studied by using UV-A light and photodegrdn. of methylene blue as a water pollutant evaluated until the photocatalytic process optimizes with the process parameters. The parameters that were examd. include: initial dye concn., the mass of TiO2 nanoparticle, pH value, and TiO2 nano-sized particles. Absorbance spectra were measured using a spectrophotometer and the concn. of the test soln. was calcd. by using the Beer-Lambert Law. Our test results showed that the photocatalytic UV-A/TiO2 nanoparticle is a promising method for treating wastewater. The most important part is that with redn. of TiO2 particles from nano to micro scale, photodegrdn. time becomes very slowly and even did not reach to zero after 5 h. Also, photocatayst reaction efficency for 10 nm TiO2 nanoparticles improves to 90% within one hour and photodegrdn. completed after two hour, it means that the photocatalyst activity increases with increasing surface area.
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    Skoog, D. A.; Holler, F. J.; Crouch, S. R. Principles of Instrumental Analysis, 6th ed.; Thomson Brooks/Cole, 2007.
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    Photocatalysis Industry Association of Japan. https://www.piaj.gr.jp/en/.
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    Wei, C.; Lin, W. Y.; Zainal, Z.; Williams, N. E.; Zhu, K.; Kruzic, A. P.; Smith, R. L.; Rajeshwar, K. Bactericidal Activity of TiO2 Photocatalyst in Aqueous Media: Toward a Solar-Assisted Water Disinfection System. Environ. Sci. Technol. 1994, 28, 934– 938,  DOI: 10.1021/es00054a027
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    Bactericidal Activity of TiO2 Photocatalyst in Aqueous Media: Toward a Solar-Assisted Water Disinfection System
    Wei, Chang; Lin, Wen Yuan; Zainal, Zulkarnain; Williams, Nathan E.; Zhu, Kai; Kruzic, Andrew P.; Smith, Russell L.; Rajeshwar, Krishnan
    Environmental Science and Technology (1994), 28 (5), 934-8CODEN: ESTHAG; ISSN:0013-936X.
    Irradn. of suspensions of Escherichia coli (∼106 cells/mL) and TiO2 (anatase) with UV-visible light of wavelengths longer than 380 nm resulted in the killing of the bacteria within minutes. Oxygen was a prerequisite for the bactericidal properties of the photocatalyst. Bacterial killing was found to adhere to first-order kinetics. The rate const. was proportional to the square root of the concn. of TiO2 and proportional to the incident light intensity in the range ∼180-∼1660 μE s-1 m-2. The trends in these simulated lab. expts. were mimicked by outdoor tests conducted under the summer noonday sun in Texas. The implications of these results as well as those of previous investigations in terms of practical applicability to solar-assisted water treatment and disinfection at remote sites are discussed relative to water technologies currently considered as viable alternatives to chlorination.
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    Watts, R. J.; Kong, S.; Orr, M. P.; Miller, G. C.; Henry, B. E. Photocatalytic inactivation of coliform bacteria and viruses in secondary wastewater effluent. Water Res. 1995, 29, 95– 100,  DOI: 10.1016/0043-1354(94)e0122-m
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    Photocatalytic inactivation of coliform bacteria and viruses in secondary wastewater effluent
    Watts, Richard J.; Kong, Sungho; Orr, Margaret P.; Miller, Glenn C.; Henry, Berch E.
    Water Research (1995), 29 (1), 95-100CODEN: WATRAG; ISSN:0043-1354. (Elsevier)
    Coliform bacteria and poliovirus 1 inactivation was studied in secondary wastewater effluent contg. suspensions of TiO2 (250 mg/L) irradiated with either F40BL fluorescent lights or sunlight. Approx. 150 min were required to achieve 2-log inactivation of coliform bacteria under lab. lights, whereas the 2-log inactivation of poliovirus 1 occurred in ∼30 min. No differences in photocatalytic disinfection rates were found when assays were conducted at pH 5-8. Results showed that poliovirus 1 was effectively inactivated by TiO2 photocatalysis; rates were more rapid than for coliform bacteria inactivation. However, photocatalytic effluent disinfection using TiO2 under sunlight may be limited due to the relatively low inactivation rates and resulting long contact times compared to conventional disinfection methods.
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    Dong, H.; Zeng, G.; Tang, L.; Fan, C.; Zhang, C.; He, X.; He, Y. An overview on limitations of TiO2-based particles for photocatalytic degradation of organic pollutants and the corresponding countermeasures. Water Res. 2015, 79, 128– 146,  DOI: 10.1016/j.watres.2015.04.038
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    An overview on limitations of TiO2-based particles for photocatalytic degradation of organic pollutants and the corresponding countermeasures
    Dong, Haoran; Zeng, Guangming; Tang, Lin; Fan, Changzheng; Zhang, Chang; He, Xiaoxiao; He, Yan
    Water Research (2015), 79 (), 128-146CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
    The pollutants classified as "persistent org. pollutants (POPs)", are being subject to high concern among the scientific community due to their persistence in the environment. TiO2-based photocatalytic process has shown a great potential as a low-cost, environmentally friendly and sustainable treatment technol. to remove POPs in sewage to overcome the shortcomings of the conventional technologies. However, this technol. suffers from some main tech. barriers that impede its commercialization, i.e., the inefficient exploitation of visible light, low adsorption capacity for hydrophobic contaminants, uniform distribution in aq. suspension and post-recovery of the TiO2 particles after water treatment. To improve the photocatalytic efficiency of TiO2, many studies have been carried out with the aim of eliminating the limitations mentioned above. This review summarizes the recently developed countermeasures for improving the performance of TiO2-based photocatalytic degrdn. of org. pollutants with respect to the visible-light photocatalytic activity, adsorption capacity, stability and separability. The performance of various TiO2-based photocatalytic processes for POPs degrdn. and the underlying mechanisms were summarized and discussed. The future research needs for TiO2-based technol. are suggested accordingly. This review will significantly improve our understanding of the process of photocatalytic degrdn. of POPs by TiO2-based particles and provide useful information to scientists and engineers who work in this field.
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  26. 26
    Tsutsumi-Arai, C.; Iwamiya, Y.; Hoshino, R.; Terada-Ito, C.; Sejima, S.; Akutsu-Suyama, K.; Shibayama, M.; Hiroi, Z.; Tokuyama-Toda, R.; Iwamiya, R.; Ijichi, K.; Chiba, T.; Satomura, K. Surface Functionalization of Non-Woven Fabrics Using a Novel Silica–Resin-Coating Technology: Antiviral Treatment of Non-Woven Fabric Filters in Surgical Masks. Int. J. Environ. Res. 2022, 19, 3639,  DOI: 10.3390/ijerph19063639
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    26
    Surface Functionalization of Non-Woven Fabrics Using a Novel Silica-Resin Coating Technology: Antiviral Treatment of Non-Woven Fabric Filters in Surgical Masks
    Tsutsumi-Arai, Chiaki; Iwamiya, Yoko; Hoshino, Reiko; Terada-Ito, Chika; Sejima, Shunsuke; Akutsu-Suyama, Kazuhiro; Shibayama, Mitsuhiro; Hiroi, Zenji; Tokuyama-Toda, Reiko; Iwamiya, Ryugo; Ijichi, Kouhei; Chiba, Toshie; Satomura, Kazuhito
    International Journal of Environmental Research and Public Health (2022), 19 (6), 3639CODEN: IJERGQ; ISSN:1660-4601. (MDPI AG)
    Masks are effective for preventing the spread of COVID-19 and other respiratory infections. If antimicrobial properties can be applied to the non-woven fabric filters in masks, they can become a more effective countermeasure against human-to-human and environmental infections. We investigated the possibilities of carrying antimicrobial agents on the fiber surfaces of non-woven fabric filters by applying silica-resin coating technol., which can form silica-resin layers on such fabrics at normal temp. and pressure. SEM and electron probe microanal. showed that a silica-resin layer was formed on the fiber surface of non-woven fabric filters. Bioassays for coronavirus and quant. reverse transcription-polymerase chain reactions (RT-PCR) revealed that all antimicrobial agents tested loaded successfully onto non-woven fabric filters without losing their inactivation effects against the human coronavirus (inhibition efficacy: >99.999%). These results indicate that this technol. could be used to load a functional substance onto a non-woven fabric filter by vitrifying its surface. Silica-resin coating technol. also has the potential of becoming an important breakthrough not only in the prevention of infection but also in various fields, such as prevention of building aging, protection of various cultural properties, the realization of a plastic-free society, and prevention of environmental pollution.
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  • Abstract

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    Figure 1

    Figure 1. Silica–resin coating on cellulose fibers. (a) SEM image (left top) illustrates the morphology of paper composed of intertwined cellulose fibers that are a few tens of micrometers in diameter. The fibers’ edges appear relatively bright because of the enhanced backscattering of secondary electrons from Si, which is heavier than carbon, cellulose’s primary component. The corresponding elemental maps for carbon, silicon, and titanium, which were obtained using the energy-dispersive X-ray analysis, demonstrate the formation of a silica layer on the cellulose fibers: the brighter the image, the higher the concentration. Note that a trace of titanium is concentrated around the surface layer. (b) Water resistance demonstration: after 2 weeks in water at room temperature, an origami crane made of silica–resin-coated Japanese paper retained its shape (main panel), but an origami crane made of uncoated Japanese paper lost its shape after 24 h (inset).

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    Figure 2

    Figure 2. Powder XRD patterns of a C1 bulk sample and of C2, C5, and C10 bulk samples with increasing TPT concentrations in coating liquids. The pattern of sample E obtained via the hydrolysis of TPT is also shown. The patterns are vertically offset for clarity. The arrow and vertical broken line indicate the location of the anatase 1 0 1 diffraction peak at 25.4°.

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    Figure 3

    Figure 3. Nitrogen physisorption. (a) Adsorption–desorption isotherms of coated paper (circles), uncoated paper (triangles), and bulk C1 (squares). The volume of the adsorbate (nitrogen) adsorbed at standard temperature and pressure is denoted by V, the equilibrium pressure of the adsorbate is denoted by P, and the saturated pressure is denoted by P0. The filled and open symbols represent the adsorption and desorption processes, respectively. (b) BET plots fitted with linear functions, the slopes of which yield the SSA.

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    Figure 4

    Figure 4. TEM images from bulk C5 and sample E. For bulk C5, a thin section of the fragment in (a) was observed at high magnification, as shown in (b,c). The image in sample E depicts densely packed and well-crystalized anatase nanoparticles (d). The lattice images in (c,d) with spacings of 0.35, 0.26, and 0.22 nm correspond to the anatase (1 0 1), (1 0 3), and (0 0 4)/(1 1 2) lattice planes, respectively. Insets show ED patterns of a holo pattern from an amorphous material in (b) and Debye rings from a crystalline material in (d).

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    Figure 5

    Figure 5. XANES spectra of bulk C1, paper C1, and bulk E. For comparison, the spectra of the standard anatase, rutile, and brookite samples measured in the same experimental setup are also included. The inset enlarges the pre-edge region to demonstrate the presence of disordered anatase nanoparticles in both bulk C1 and paper C1.

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    Figure 6

    Figure 6. Photocatalytic activity of the silica–resin coating. Time evolutions of the MB concentration in a C1 film on glass (a and b) and in paper C1 (b) are shown. The terms “dark” and “light” refer to measurements obtained in the dark and under 1 mW cm–2 UV light, respectively, while “blank” refers to a measurement obtained under UV light for uncoated glass. The lines serve as guides for the viewer’s eye. Another set of initial measurements for paper C1 are also plotted in (b). The green crosses represent data for uncoated blank paper exposed to UV light. The lines drawn on the “dark” and “light” data for C1 on glass are linear fits.

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    Figure 7

    Figure 7. Antibiotic activity of the silica–resin coating. Evolution of the viable cell count (colony forming unit: cfu) for S. aureus in 50 × 50 mm2 papers; uncoated paper (ref) and coated papers D and C1 after 8 h in darkness and light. Paper D was prepared with liquid D composed mainly of tetraethyl orthosilicate (Supporting Information 2).

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      Luca, Vittorio; Djajanti, Samitha; Howe, Russell F.
      Journal of Physical Chemistry B (1998), 102 (52), 10650-10657CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
      X-ray absorption spectroscopy (XAS) was used to study the local Ti environment in TiO2 xerogel samples contg. nanoparticles of different sizes. The xerogels were prepd. by hydrolysis of Ti isopropoxide followed by peptization with HNO3 and size control was achieved through calcination in air at different temps. An amorphous ppt. obtained by hydrolysis of Ti isopropoxide prior to peptization with HNO3 also was studied. The x-ray absorption near edge structure (XANES) of the ppt. possesses a characteristic pre edge that is dominated by a transition designated as A2 at 4970.7 eV, and assigned to five coordinate Ti. This A2 component is detected in all of the studied xerogels. Its intensity decreases as the surface-to-vol. ratio of the TiO2 particles decreases, suggesting that it is assocd. with surface layers of the anatase particles. Other changes obsd. in the XANES of the xerogels as a function of particle size include broadening of 1s → np transitions in the postedge region. EXAFS shows that the smallest particles contain Ti in distorted coordination with contracted Ti-O bonds and probably reduced coordination no.
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    15. 15
      Zhang, H.; Chen, B.; Banfield, J. F.; Waychunas, G. A. Atomic structure of nanometer-sized amorphous TiO2. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 78, 214106,  DOI: 10.1103/physrevb.78.214106
      15
      Atomic structure of nanometer-sized amorphous TiO2
      Zhang, Hengzhong; Chen, Bin; Banfield, Jillian F.; Waychunas, Glenn A.
      Physical Review B: Condensed Matter and Materials Physics (2008), 78 (21), 214106/1-214106/12CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      Amorphous TiO2 is an important precursor for synthesis of single-phase nanocryst. anatase. We synthesized amorphous TiO2 by hydrolysis of Ti ethoxide at the ice point. Transmission electron microscopy examn. and nitrogen gas adsorption indicated that the particle size of the synthesized titania is ∼2 nm. Synchrotron wide-angle x-ray scattering (WAXS) was used to probe the at. correlations in this amorphous sample. At. pair-distribution function (PDF) derived from Fourier transform of the WAXS data was used for reverse Monte Carlo (RMC) simulations of the at. structure of the amorphous TiO2 nanoparticles. Mol.-dynamics simulations were used to generate input structures for the RMC. X-ray-absorption spectroscopy (XAS) simulations were used to screen candidate structures obtained from the RMC by comparing with exptl. XAS data. The structure model that best describes both the WAXS and XAS data shows that amorphous TiO2 particles consist of a highly distorted shell and a small strained anatase-like cryst. core. The av. coordination no. of Ti is 5.3 and most Ti-O bonds are populated around 1.940 Å. Relative to bulk TiO2, the redn. in the coordination no. is primarily due to the truncation of the Ti-O octahedra at the amorphous nanoparticle surface and the shortening of the Ti-O bond length to the bond contraction in the distorted shell. The pre-existence of the anatase-like core may be crit. to the formation of single-phase nanocryst. anatase in crystn. of amorphous TiO2 upon heating.
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    16. 16
      Chen, L. X.; Rajh, T.; Wang, Z.; Thurnauer, M. C. XAFS Studies of Surface Structures of TiO2 Nanoparticles and Photocatalytic Reduction of Metal Ions. J. Phys. Chem. B 1997, 101, 10688– 10697,  DOI: 10.1021/jp971930g
      16
      XAFS Studies of Surface Structures of TiO2 Nanoparticles and Photocatalytic Reduction of Metal Ions
      Chen, Lin X.; Rajh, Tijana; Wang, Zhiyu; Thurnauer, Marion C.
      Journal of Physical Chemistry B (1997), 101 (50), 10688-10697CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
      To probe the origin of the unique functions of titanium dioxide (TiO2) nanoparticles obsd. in photocatalytic reactions, structures of Ti atom sites in titanium dioxide (TiO2) nanoparticles with different sizes were studied by Ti K-edge XAFS (x-ray absorption fine structure). Compared to the bulk TiO2 structure, a shorter Ti-O distance from surface TiO2 resulting from Ti-OH bonding was obsd. The XAFS spectra also revealed an increasing disorder of the lattice with decreasing sizes of the nanoparticles based on a coordination no. decrease for the third-shell O atoms as well as changes in relative intensities of preedge peaks A1, A2, and A3. However, the Ti sites largely remain octahedral even in the 30 Å diam. particles. These results imply that the increasing no. of surface Ti sites as well as possible corner defects in small nanoparticles may be the main cause of the unique surface chem. exhibited by nanoparticles of TiO2. XAFS was also used in monitoring the photoredn. reaction products of Cu2+ and Hg2+ on TiO2 nanoparticle surfaces, with or without surface adsorbers, alanine (Ala) and thiolactic acid (TLA). Ala dramatically enhanced photoredn. of Cu2+ on TiO2 nanoparticle surfaces, whereas thiolactic acid did not affect or even hindered Hg2+ photoredn. Although both surface adsorbers chelated with the metal ions in the absence of TiO2 nanoparticles, this chelation was drastically changed in the Cu-Ala complex but was largely retained in the Hg-TLA complex when TiO2 was present. This may correlate with the different effects of the adsorbers on the photoredn. of the metal. Our exptl. results suggest that a proper balance between the affinities of the adsorber to the metal ions and to the surface Ti atoms of TiO2 may be one of the keys in selecting a surface adsorber for enhanced photoredn. efficiency.
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    17. 17
      Wu, Z. Y.; Zhang, J.; Ibrahim, K.; Xian, D. C.; Li, G.; Tao, Y.; Hu, T. D.; Bellucci, S.; Marcelli, A.; Zhang, Q. H.; Gao, L.; Chen, Z. Z. Structural determination of titanium-oxide nanoparticles by x-ray absorption spectroscopy. Appl. Phys. Lett. 2002, 80, 2973– 2975,  DOI: 10.1063/1.1470699
      17
      Structural determination of titanium-oxide nanoparticles by x-ray absorption spectroscopy
      Wu, Z. Y.; Zhang, J.; Ibrahim, K.; Xian, D. C.; Li, G.; Tao, Y.; Hu, T. D.; Bellucci, S.; Marcelli, A.; Zhang, Q. H.; Gao, L.; Chen, Z. Z.
      Applied Physics Letters (2002), 80 (16), 2973-2975CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      To understand and improve the applications of Ti oxide nanoparticles, it is extremely important to perform a detailed study of the surface and the interior structural properties of nanocryst. materials, such as rutile and anatase with diam. of few nm. X-ray absorption spectroscopy was used to identify the local Ti environment and characterize the related electronic structure. Exptl. results at the Ti K edge in both bulk and nanocrystal samples were combined to det. the lattice distortion via the characteristic pre-edge features and the variation in the multiple-scattering region of the XANES spectra. The correlation between peak intensities and surface-to-vol. ratio of nanoparticles is discussed.
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    18. 18
      Xu, C.; Rangaiah, G. P.; Zhao, X. S. Photocatalytic Degradation of Methylene Blue by Titanium Dioxide: Experimental and Modeling Study. Ind. Eng. Chem. Res. 2014, 53, 14641– 14649,  DOI: 10.1021/ie502367x
      18
      Photocatalytic Degradation of Methylene Blue by Titanium Dioxide: Experimental and Modeling Study
      Xu, Chen; Rangaiah, G. P.; Zhao, X. S.
      Industrial & Engineering Chemistry Research (2014), 53 (38), 14641-14649CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)
      The application of semiconductors in water treatment via photocatalysis of various pollutants has attracted much attention from researchers. In this work, photocatalytic degrdn. of methylene blue by P25 titanium dioxide was studied exptl. and then via simulation, physicochem., physicochem. The effects of lamp choice, concn. of catalyst, and methylene blue were analyzed. Desorption of methylene blue at the start of light radiation was obsd., and analyzed in detail for the first time. Both desorption and degrdn. processes were modeled, and exptl. data was fitted to a pseudo-first-order simulation, physicochem. with sufficient accuracy. The effects of catalyst and initial dye concn. on reaction rate consts. were discussed.
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    19. 19
      Yang, C.; Dong, W.; Cui, G.; Zhao, Y.; Shi, X.; Xia, X.; Tang, B.; Wang, W. Highly-efficient photocatalytic degradation of methylene blue by PoPD-modified TiO2 nanocomposites due to photosensitization-synergetic effect of TiO2 with PoPD. Sci. Rep. 2017, 7, 3973,  DOI: 10.1038/s41598-017-04398-x
      19
      Highly-efficient photocatalytic degradation of methylene blue by PoPD-modified TiO 2 nanocomposites due to photosensitization-synergetic effect of TiO2 with PoPD
      Yang Chuanxi; Wang Weiliang; Yang Chuanxi; Dong Wenping; Cui Guanwei; Zhao Yingqiang; Shi Xifeng; Xia Xinyuan; Tang Bo
      Scientific reports (2017), 7 (1), 3973 ISSN:.
      Poly-o-phenylenediamine modified TiO2 nanocomposites were successfully synthesized via an 'in situ' oxidative polymerization method. The modified nanocomposites were characterized by BET, XRD, TEM, FT-IR, TGA, XPS, EA and UV-Vis DRS. The photocatalytic degradation of methylene blue was chosen as a model reaction to evaluate the photocatalytic activities of TiO2 and PoPD/TiO2. The results indicated that PoPD/TiO2 nanocomposites exhibited good photocatalytic activity and stability. The photocatalytic activity of PoPD/TiO2 increased as the initial pH increased because of electrostatic adsorption between the photocatalyst and MB as well as the generation of ·OH, whereas it exhibited an earlier increasing and later decreasing trend as the concentration of the photocatalyst increased owing to the absorption of visible light. The photocatalytic stability of the PoPD/TiO2 nanocomposite was dependent on the stability of its structure. Based on radical trapping experiments and ESR measurements, the origin of oxidizing ability of PoPD/TiO2 nanocomposites on photocatalytic degradation of MB was proposed, which taking into account of ·OH and ·O2(-) were the first and second important ROS, respectively. The possible photocatalytic mechanism and photocatalytic activity enhanced mechanism has been proposed, taking into account the photosensitization effect and synergetic effect of TiO2 with PoPD.
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    20. 20
      Dariani, R. S.; Esmaeili, A.; Mortezaali, A.; Dehghanpour, S. Photocatalytic reaction and degradation of methylene blue on TiO2 nano-sized particles. Optik 2016, 127, 7143– 7154,  DOI: 10.1016/j.ijleo.2016.04.026
      20
      Photocatalytic reaction and degradation of methylene blue on TiO2 nano-sized particles
      Dariani, R. S.; Esmaeili, A.; Mortezaali, A.; Dehghanpour, S.
      Optik (Munich, Germany) (2016), 127 (18), 7143-7154CODEN: OTIKAJ; ISSN:0030-4026. (Elsevier GmbH)
      Photocatalyst has been of considerable interest due to its new technol. for environmental pollution. Among the photocatalyst semiconductors, titanium dioxide is known as a photocatalyst for the removal of environmental contaminants. In this research, we first designed a new self-made lab. photocatalytic reactor and then photocatalytic properties of TiO2 nanoparticles are studied by using UV-A light and photodegrdn. of methylene blue as a water pollutant evaluated until the photocatalytic process optimizes with the process parameters. The parameters that were examd. include: initial dye concn., the mass of TiO2 nanoparticle, pH value, and TiO2 nano-sized particles. Absorbance spectra were measured using a spectrophotometer and the concn. of the test soln. was calcd. by using the Beer-Lambert Law. Our test results showed that the photocatalytic UV-A/TiO2 nanoparticle is a promising method for treating wastewater. The most important part is that with redn. of TiO2 particles from nano to micro scale, photodegrdn. time becomes very slowly and even did not reach to zero after 5 h. Also, photocatayst reaction efficency for 10 nm TiO2 nanoparticles improves to 90% within one hour and photodegrdn. completed after two hour, it means that the photocatalyst activity increases with increasing surface area.
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    21. 21
      Skoog, D. A.; Holler, F. J.; Crouch, S. R. Principles of Instrumental Analysis, 6th ed.; Thomson Brooks/Cole, 2007.
      There is no corresponding record for this reference.
    22. 22
      Photocatalysis Industry Association of Japan. https://www.piaj.gr.jp/en/.
      There is no corresponding record for this reference.
    23. 23
      Wei, C.; Lin, W. Y.; Zainal, Z.; Williams, N. E.; Zhu, K.; Kruzic, A. P.; Smith, R. L.; Rajeshwar, K. Bactericidal Activity of TiO2 Photocatalyst in Aqueous Media: Toward a Solar-Assisted Water Disinfection System. Environ. Sci. Technol. 1994, 28, 934– 938,  DOI: 10.1021/es00054a027
      23
      Bactericidal Activity of TiO2 Photocatalyst in Aqueous Media: Toward a Solar-Assisted Water Disinfection System
      Wei, Chang; Lin, Wen Yuan; Zainal, Zulkarnain; Williams, Nathan E.; Zhu, Kai; Kruzic, Andrew P.; Smith, Russell L.; Rajeshwar, Krishnan
      Environmental Science and Technology (1994), 28 (5), 934-8CODEN: ESTHAG; ISSN:0013-936X.
      Irradn. of suspensions of Escherichia coli (∼106 cells/mL) and TiO2 (anatase) with UV-visible light of wavelengths longer than 380 nm resulted in the killing of the bacteria within minutes. Oxygen was a prerequisite for the bactericidal properties of the photocatalyst. Bacterial killing was found to adhere to first-order kinetics. The rate const. was proportional to the square root of the concn. of TiO2 and proportional to the incident light intensity in the range ∼180-∼1660 μE s-1 m-2. The trends in these simulated lab. expts. were mimicked by outdoor tests conducted under the summer noonday sun in Texas. The implications of these results as well as those of previous investigations in terms of practical applicability to solar-assisted water treatment and disinfection at remote sites are discussed relative to water technologies currently considered as viable alternatives to chlorination.
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    24. 24
      Watts, R. J.; Kong, S.; Orr, M. P.; Miller, G. C.; Henry, B. E. Photocatalytic inactivation of coliform bacteria and viruses in secondary wastewater effluent. Water Res. 1995, 29, 95– 100,  DOI: 10.1016/0043-1354(94)e0122-m
      24
      Photocatalytic inactivation of coliform bacteria and viruses in secondary wastewater effluent
      Watts, Richard J.; Kong, Sungho; Orr, Margaret P.; Miller, Glenn C.; Henry, Berch E.
      Water Research (1995), 29 (1), 95-100CODEN: WATRAG; ISSN:0043-1354. (Elsevier)
      Coliform bacteria and poliovirus 1 inactivation was studied in secondary wastewater effluent contg. suspensions of TiO2 (250 mg/L) irradiated with either F40BL fluorescent lights or sunlight. Approx. 150 min were required to achieve 2-log inactivation of coliform bacteria under lab. lights, whereas the 2-log inactivation of poliovirus 1 occurred in ∼30 min. No differences in photocatalytic disinfection rates were found when assays were conducted at pH 5-8. Results showed that poliovirus 1 was effectively inactivated by TiO2 photocatalysis; rates were more rapid than for coliform bacteria inactivation. However, photocatalytic effluent disinfection using TiO2 under sunlight may be limited due to the relatively low inactivation rates and resulting long contact times compared to conventional disinfection methods.
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    25. 25
      Dong, H.; Zeng, G.; Tang, L.; Fan, C.; Zhang, C.; He, X.; He, Y. An overview on limitations of TiO2-based particles for photocatalytic degradation of organic pollutants and the corresponding countermeasures. Water Res. 2015, 79, 128– 146,  DOI: 10.1016/j.watres.2015.04.038
      25
      An overview on limitations of TiO2-based particles for photocatalytic degradation of organic pollutants and the corresponding countermeasures
      Dong, Haoran; Zeng, Guangming; Tang, Lin; Fan, Changzheng; Zhang, Chang; He, Xiaoxiao; He, Yan
      Water Research (2015), 79 (), 128-146CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
      The pollutants classified as "persistent org. pollutants (POPs)", are being subject to high concern among the scientific community due to their persistence in the environment. TiO2-based photocatalytic process has shown a great potential as a low-cost, environmentally friendly and sustainable treatment technol. to remove POPs in sewage to overcome the shortcomings of the conventional technologies. However, this technol. suffers from some main tech. barriers that impede its commercialization, i.e., the inefficient exploitation of visible light, low adsorption capacity for hydrophobic contaminants, uniform distribution in aq. suspension and post-recovery of the TiO2 particles after water treatment. To improve the photocatalytic efficiency of TiO2, many studies have been carried out with the aim of eliminating the limitations mentioned above. This review summarizes the recently developed countermeasures for improving the performance of TiO2-based photocatalytic degrdn. of org. pollutants with respect to the visible-light photocatalytic activity, adsorption capacity, stability and separability. The performance of various TiO2-based photocatalytic processes for POPs degrdn. and the underlying mechanisms were summarized and discussed. The future research needs for TiO2-based technol. are suggested accordingly. This review will significantly improve our understanding of the process of photocatalytic degrdn. of POPs by TiO2-based particles and provide useful information to scientists and engineers who work in this field.
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    26. 26
      Tsutsumi-Arai, C.; Iwamiya, Y.; Hoshino, R.; Terada-Ito, C.; Sejima, S.; Akutsu-Suyama, K.; Shibayama, M.; Hiroi, Z.; Tokuyama-Toda, R.; Iwamiya, R.; Ijichi, K.; Chiba, T.; Satomura, K. Surface Functionalization of Non-Woven Fabrics Using a Novel Silica–Resin-Coating Technology: Antiviral Treatment of Non-Woven Fabric Filters in Surgical Masks. Int. J. Environ. Res. 2022, 19, 3639,  DOI: 10.3390/ijerph19063639
      26
      Surface Functionalization of Non-Woven Fabrics Using a Novel Silica-Resin Coating Technology: Antiviral Treatment of Non-Woven Fabric Filters in Surgical Masks
      Tsutsumi-Arai, Chiaki; Iwamiya, Yoko; Hoshino, Reiko; Terada-Ito, Chika; Sejima, Shunsuke; Akutsu-Suyama, Kazuhiro; Shibayama, Mitsuhiro; Hiroi, Zenji; Tokuyama-Toda, Reiko; Iwamiya, Ryugo; Ijichi, Kouhei; Chiba, Toshie; Satomura, Kazuhito
      International Journal of Environmental Research and Public Health (2022), 19 (6), 3639CODEN: IJERGQ; ISSN:1660-4601. (MDPI AG)
      Masks are effective for preventing the spread of COVID-19 and other respiratory infections. If antimicrobial properties can be applied to the non-woven fabric filters in masks, they can become a more effective countermeasure against human-to-human and environmental infections. We investigated the possibilities of carrying antimicrobial agents on the fiber surfaces of non-woven fabric filters by applying silica-resin coating technol., which can form silica-resin layers on such fabrics at normal temp. and pressure. SEM and electron probe microanal. showed that a silica-resin layer was formed on the fiber surface of non-woven fabric filters. Bioassays for coronavirus and quant. reverse transcription-polymerase chain reactions (RT-PCR) revealed that all antimicrobial agents tested loaded successfully onto non-woven fabric filters without losing their inactivation effects against the human coronavirus (inhibition efficacy: >99.999%). These results indicate that this technol. could be used to load a functional substance onto a non-woven fabric filter by vitrifying its surface. Silica-resin coating technol. also has the potential of becoming an important breakthrough not only in the prevention of infection but also in various fields, such as prevention of building aging, protection of various cultural properties, the realization of a plastic-free society, and prevention of environmental pollution.
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  • Supporting Information

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    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.iecr.2c00784.

    • Titanium dioxides and its photocatalytic activity, coating agents and sample preparation, characterization techniques, XRD results, XANES spectra, antibiotic susceptibility, and references (PDF)


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