Untangling the Effect of Carbonaceous Materials on the Photoelectrochemical Performance of BaTaO2N

The water oxidation reaction is a rate-determining step in solar water splitting. The number of surviving photoexcited holes is one of the most influencing factors affecting the photoelectrochemical water oxidation efficiency of photocatalysts. The solar-to-hydrogen energy conversion efficiency of BaTaO2N is still far below the benchmark efficiency set for practical applications, notwithstanding its potential as a 600 nm-class photocatalyst in solar water splitting. To improve its efficiency in photoelectrochemical water splitting, this study offers a straightforward route to develop photocatalytic materials based on the combination of BaTaO2N and carbonaceous materials with different dimensions. The impact of diverse carbonaceous materials, such as fullerene, g-C3N4, graphene, carbon nanohorns, and carbon nanotubes, on the photoelectrochemical behavior of BaTaO2N has been examined. Notably, the use of graphene and g-C3N4 remarkably improves the photoelectrochemical performance of the composite photocatalysts through a higher photocurrent and acting as electron reservoirs. Consequently, a marked reduction in recombination rates, even at low overpotentials, leads to a higher accumulation of photoexcited holes, resulting in 2.6- and 1.7-fold increased BaTaO2N photocurrent densities using graphene and g-C3N4, respectively. The observed trends in the dark for the oxygen reduction reaction (ORR) potential align with the increase in the photocurrent density, revealing a good correlation between opposite phenomena. Importantly, the enhancement observed implies an underlying accumulation phenomenon. The verification of this concept lies in the evidence provided by oxygen reduction and is in line with photoredox flux matching during photocatalysis. This research underscores the intricate interplay between carbonaceous materials and oxynitride photocatalysts, offering a strategic approach to enhancing various photocatalytic capabilities.


INTRODUCTION
Photoelectrochemical (PEC) water splitting is one of the potential processes to generate green hydrogen by involving renewable energy. 1,2However, the water oxidation reaction on the photoanode requires the transfer of four electrons, in comparison to the two-electron-transfer water reduction reaction on the photocathode.Therefore, the sluggish water oxidation reaction is the rate-determining step that governs the rate of the water-splitting reaction. 3aTaO 2 N is a promising visible-light-active photocatalyst for water oxidation due to its capability to absorb visible light up to 660 nm, appropriate band-edge potential straddling the water oxidation reaction potential, good stability in concentrated alkaline solution, and nontoxicity. 4,5Under AM 1.5G simulated sunlight based on an incident photon-to-current conversion efficiency (IPCE) of 100% at <660 nm, the photocurrent density and solar-to-hydrogen (STH) conversion efficiency are assumed to reach approximately 18 mA cm −2 and 24%, respectively. 6,7To achieve higher efficiency in solar water splitting over BaTaO 2 N, various strategies, such as band-gap engineering via mono-and dual-substitution 8,9 and solid solutions, 10,11 controlling the defect density, 12−14 fabricating thin films, 15,16 tailoring the exposed surface, morphology, and size, 17−19 etc., were applied.Photocurrent densities of ∼0.03, 20 >1.2, 21 2.05, 22 4.2, 6 ∼4.5, 23 and 6.5 mA cm −224 at 1.2 V vs the reversible hydrogen electrode were progressively achieved for BaTaO 2 N, while incident photon-to-current efficiencies of 1% at 500 nm, 20 >4% at 400 nm, 21 13% at 420 nm, 22 ∼30% at 400 nm, 6 34−35% at 380−540 nm, 23 and ≈43% IPCE at 540 nm 24 at 1.2 V vs RHE steadily increased.Even though the half-cell solar-to-hydrogen energy conversion efficiency of BaTaO 2 N reached 1.4% at 0.88 V RHE , 24 it is still far from the benchmark efficiency set for practical application.Therefore, it is necessary to further explore new ways to improve its efficiency in photoelectrochemical water splitting.
As a straightforward approach, carbon-based nanomaterials have been involved in improving the water-splitting performance of various photocatalysts due to their excellent physicochemical, electrical, mechanical, and optical properties, structural diversity, low cost, and easy synthesis.Carbon-based nanomaterials have been applied as (i) supporting materials for increasing the adsorption sites of active centers and the homogeneous distribution of photocatalyst particles; (ii) cocatalysts for improving the charge separation, reducing the overpotential, providing the catalytic sites, and minimizing the activation energy of hydrogen; (iii) photosensitizers for enhancing the photoresponse of wide-band-gap photocatalysts to visible light with a longer wavelength; and (iv) photocatalysts.The photoanode based on hydrogenated TiO 2 nanorod arrays decorated with carbon quantum dots exhibited an IPCE value of ∼66.8% and a photocurrent density of ∼3.0 mA cm −2 at 1.23 V vs RHE under simulated sunlight, which are 6-fold higher than that of pristine TiO 2 , because the decorated carbon quantum dots acted as electron reservoirs to trap photoexcited electrons and enhanced solar light harvesting due to their upconversion effect. 25The ZnFe 2 O 4 photoanode showed an extremely weak transient photocurrent response, whereas the carbon quantum dot-modified ZnFe 2 O 4 photoanode exhibited an eight times higher transient photocurrent response because of the efficient separation of photoexcited charge carriers. 26The carbon quantum dot-modified Fe 2 O 3 photoanode demonstrated a 27-fold enhancement in photocurrent density at 0.23 V in comparison to the Fe 2 O 3 -based photoanode owing to the suppression of the recombination of photoexcited charge carriers and enhanced light absorption stemming from the upconversion of carbon quantum dots. 27ang et al. 28 enhanced the photoelectrochemical water oxidation reaction of the BiVO 4 photoanode by involving carbon spheres, and the photocurrent density and bulk carrier separation efficiency of carbon sphere-modified BiVO 4 were significantly higher than that of pristine BiVO 4 because carbon spheres acted as electron reservoirs, promoting efficient separation of photoexcited charge carriers.The photoelectrochemical performance of hexagonal and monoclinic WO 3 toward water oxidation under light irradiation was boosted by incorporating them with an amorphous nanoporous carbon additive that facilitated the majority carrier transport through the graphitic layers. 29arbonaceous materials with various dimensions have their own advantages over others and can influence photoelectrochemical performance differently. 30In this study, we aim to gain insights into the effect of carbonaceous materials, such as fullerene, g-C 3 N 4 , graphene, carbon nanohorns, and carbon nanotubes, on enhancing the photoelectrochemical performance of BaTaO 2 N for water oxidation.Particularly, the role of carbonaceous materials with various dimensions in the improvement of the light absorption, separation, and transfer of photoexcited charge carriers and photoelectrochemical water oxidation kinetics is discussed here.
2.2.Characterization.The X-ray diffraction (XRD) patterns were acquired with a PANalytical X'Pert PRO diffractometer with Cu Kα radiation.The microstructures of the samples were examined by scanning electron microscopy (SEM; JSM-7600F, JEOL).The bright-field and lattice images and selected-area electron diffraction (SAED) patterns were observed by transmission electron microscopy (TEM; EM-002B, TOPCON).The ultraviolet−visible (UV−Vis) diffuse reflectance spectra were recorded on an Evolution 220 UV−vis spectrometer (Thermo Fisher Scientific).

Photoelectrochemical Measurements.
To determine the photoelectrochemical behavior of the photoanodes, the working electrodes were prepared.First, the BaTaO 2 N/ carbonaceous material composites were prepared by mixing the as-synthesized BaTaO 2 N powders with 20 wt % fullerene (98%, chemPUR), 20 wt % g-C 3 N 4 , 32 20 wt % graphene (99.5%, chemPUR), 20 wt % carbon nanohorns (90%, Merck), or 20 wt % carbon nanotubes (90%, Strem Chemicals), and then, their corresponding suspensions (1.0 mg mL −1 ) were prepared using an ethanol/water mixture with a 1:1 ratio under ultrasonication for 30 min.The resulting suspensions were evenly deposited onto the Metrohm-DropSens (110) screenprinted electrodes by a dip-coating technique, 33,34 shielded by a glass beaker, and dried at 80 °C using a heat gun for 10 min.The main contact between BaTaO 2 N particles and carbonaceous materials is expected to be via electrostatics, and connectivity was verified through electrochemical tests.In fact, carbonaceous materials with <20 wt % did not give satisfactory results.Before the photoelectrochemical measurements, cyclic voltammetry (20 mV s −1 ) was conducted on the fabricated electrodes (starting at 0 V vs RHE, with an upper limit of 2.0 V vs RHE and a lower limit of −0.9 V vs RHE) in a deoxygenated supporting electrolyte (0.1 M NaOH), typically running 5 cycles or until a reproducible response was achieved.The photoelectrochemical tests were conducted in a 0.1 M NaOH aqueous solution, which was bubbled with N 2 for 10 min.The photoelectrochemical measurements were performed by using a light-emitting diode (LED) lamp with a light intensity of ∼100 mW cm −2 (Solar Light, G2 V).The counter electrode was a carbon ring concentric to the working electrode, and the reference electrode used was Ag/AgCl.The potential with respect to the Ag/AgCl electrode was converted to that relative to the reversible hydrogen electrode (RHE) using the Nernst relationship 35

E E (vs RHE)
(vs Ag/AgCl) 0.059 pH 0.197 The photoelectrochemical measurements were performed using a Metrohm-DropSens potentiostat (μSTAT200).The volume of the solution was 100 μL, and the geometric area exposed to light was 0.13 cm 2 .The photoelectrochemical measurements were performed in two modes: (i) potentiodynamic by linear scanning voltammetry (LSV) at 4 mV s −1 and (ii) potentiostatic by chronoamperometry (CA) at 1.2 and 1.5 V (vs RHE).The power density (P) was calculated according to eq 2 where J is the photocurrent density, E 0 = 1.23 V vs RHE, and E is the potential. 36In addition, for each fabricated electrode, cyclic voltammetry (CV) was performed under dark conditions at 20 mV s −1 in a 0.1 M NaOH aqueous solution with dissolved O 2 (∼1 mM).The O 2 concentration was fixed after air bubbling for 5 min at room temperature (25 °C) and measured with the HACH sensor.

RESULTS AND DISCUSSION
3.1.Material Characterization.The synthesized Ba-TaO 2 N powders were analyzed by X-ray diffraction.Figure 1 shows the XRD pattern of synthesized BaTaO 2 N powders along with an entry from the ICDD-PDF-2 powder pattern database.As shown, all of the intense reflections in the XRD pattern can be readily indexed to the cubic perovskite BaTaO 2 N phase with a space group of Pm3̅ m (No. 221) and unit cell parameters of a = b = c = 4.1128 Å and α = β = γ = 90°(ICDD PDF# 84−1748). 37No reflections assignable to the impurity crystalline phase are noted, indicating the high phase purity of the synthesized BaTaO 2 N powders.
Afterward, the synthesized BaTaO 2 N powders were mechanically mixed with carbonaceous materials having different dimensions and examined by scanning and transmission electron microscopies.The SEM images in Figure 2 show that the synthesized BaTaO 2 N powders have an idiomorphic crystal morphology and an average crystal size of 286 nm.Apparently, some BaTaO 2 N crystals are aggregated, creating a high number of grain boundaries that may hinder majority carrier charge transport. 38On the contrary, Yabuta et al. 39 pointed out that grain boundaries in aggregated particles do not act as recombination centers for photoexcited charge carriers but contribute to the prolongation of carrier lifetime.The BaTaO 2 N crystal is surrounded by facets.Particularly, the {110} planes exhibit edges or planes as facets as can be seen in Figure 3a.This morphological behavior suggests that BaTaO 2 N crystals have a high crystallinity.Despite their mechanical mixing, BaTaO 2 N crystals have close contact with the involved carbonaceous materials, such as fullerene, g-C 3 N 4 , graphene, carbon nanohorns, and carbon nanotubes (Figure 3).Such contacts are anticipated to facilitate the separation and transfer of photoexcited charge carriers in BaTaO 2 N crystals, promoting photoelectrochemical water oxidation performance.BaTaO 2 N powders (sample BT) indicate an onset of light absorption at a wavelength of approximately 665 nm, which corresponds to an optical band-gap energy of 1.87 eV.No background absorption beyond the absorption-edge wavelength is observed, suggesting the low defect density.Unlike the spectrum of the sample BT, the BT-FU and BT-CN samples exhibit two and three absorption edges in their corresponding UV−vis diffuse reflectance spectra, respectively.For BT-FU, three absorption edges located at the wavelengths of 700, 665, and 600 nm are associated with the light absorption edges of BaTaO 2 N powders and fullerene. 40For the BT-CN, two absorption edges positioned at the wavelengths of 665 and 460 nm are related to the light absorption edges of BaTaO 2 N powders and g-C 3 N 4 , respectively. 41The BT-GR, BT-NH, and BT-NT samples show light absorption with different intensities beyond the absorption-edge wavelength of BaTaO 2 N due to the response of graphene, 42 carbon nanohorns, 43 and carbon nanotubes 44 to visible light beyond 665 nm.This implies that the BT-GR, BT-NH, and BT-NT samples have the capability to absorb more visible light beyond 665 nm in comparison to other samples.

Photoelectrochemical Performance.
Combining the BaTaO 2 N powders with carbonaceous materials with different dimensions in the fabrication of photoanodes is a straightforward strategy to understand the photoelectrochemical behavior of photocatalysts that hinder the collection and transfer of photoexcited charge carriers. 28,29Figure 5a shows the LSV results of the BT, BT-FU, BT-CN, BT-GR, BT-NH, and BT-NT photoanodes.With the addition of each carbonaceous material, a relevant difference in the photocurrent densities was observed.Particularly, the difference in the photocurrent densities of BT-GR, BT-CN, and BT is obvious.Interestingly, no significant difference was observed for BT-FU, BT-NH, and BT-NT in the range of 0.6 and 1.2 V vs RHE.However, a trend in the photocurrent density in the range of 1.2 and 2.0 V vs RHE was elucidated.At high overpotentials, an increasing trend in the photocurrent densities was noted in the following order: 0.723 μA cm −2 (BT) < 0.802 μA cm −2 (BT-FU) < 0.816 μA cm −2 (BT-NT) < 0.874 μA cm −2 (BT-NH) < 1.279 μA cm −2 (BT-CN) < 1.879 μA cm −2 (BT-GR).Clearly, the BT-GR and BT-CN samples exhibited 2.6-fold and 1.7-fold higher photocurrent densities at 1.2 vs RHE compared to the BT sample.These results are consistent with previous reports on other types of photoanodes based on BiVO 4 /carbon spheres 28 and WO 3 /nanoporous carbon. 29Gomis-Berenguer et al. 29,45 observed a varying trend in the photoelectrochemical response of WO 3 /nanoporous carbon-based photoanodes for photocatalytic water oxidation with different weight ratios of nanoporous carbon,  and 20 wt % was found to be the most favorable content of nanoporous carbon.It is noteworthy to mention that the enhanced PEC performance of BT-CN, beyond the intrinsic properties of carbonaceous material, is anticipated not solely due to its typical characteristics but also because of the synergistic photoelectrochemical response arising from the inherent photocatalytic processes exhibited by g-C 3 N 4 . 33,46,47lso, the photoactive nature of g-C 3 N 4 contributes to the observed improvements, creating a distinctive charge accumulation region within the BT-CN composite. 46Consequently, the photocatalytic behavior of g-C 3 N 4 significantly influences and augments the overall photoelectrochemical response in the BT-CN composite, further enhancing its performance.Furthermore, it has been reported that in both darkness and light, 47,48 g-C 3 N 4 can promote the oxygen reduction reaction (ORR), and the arguments to be presented later on ORR remain consistent for the BT-CN.
It should be noted that at low overpotentials, the photocurrent density of BaTaO 2 N (sample BT) is limited by the recombination and dynamics of photoexcited charge carriers. 12With the incorporation of graphene and g-C 3 N 4 , the photocurrent density of BaTaO 2 N at lower overpotentials was improved, and the onset potentials for BT-GR and BT-CN were 0.8 and 0.9 V vs RHE, respectively.Therefore, the improved photocurrent density provides evidence that carbonaceous materials can lower the recombination rate of photoexcited charge carriers.Gomis-Berenguer et al. 29 enhanced the photoelectrochemical performance of hexagonal and monoclinic WO 3 by incorporating them with nanoporous carbon, resulting in an increased photocurrent even at low overpotentials due to the lowered recombination rate stemming from the delocalization of electrons in nanoporous carbon.Also, the IPCEs of hexagonal and monoclinic WO 3 were enhanced two and three times at different potentials and with different amounts of nanoporous carbon, respectively.In another study, Wang et al. 28 employed carbon spheres as an electron reservoir, which was fed by electrons photoexcited in the BiVO 4 photocatalyst, improving the photoelectrochemical performance substantially and leading to the photocurrent density of 2.20 mA cm −2 at 1.0 V vs RHE, which is about 6.5 times larger than the photocurrent density obtained for the BiVO 4 photoanode.Considering that the BT-GR and BT-CN samples exhibit a notably higher photocurrent even at low overpotentials, a comparative discussion with BT is made based on chronoamperometric results and power density curves versus photocurrent density.Other photocatalysts do not show a large difference in the resulting photocurrent at 1.2 V vs RHE.
Figure 5b presents the power density vs photocurrent density curves for BT, BT-GR, and BT-CN photoanodes.In all cases, a parabolic-shaped contour is resolved, indicating the existence of a photocurrent density value where power density is maximized.The obtained results lead to finding the best possible operating conditions in the photoelectrochemical water-splitting cells.Since the photocurrent density is greater at high overpotentials, the application of electrochemical assistance can be an effective strategy to improve the photoelectrochemical performance of BaTaO 2 N. The external assistance of a photoelectrochemical cell at the photocurrent density that maximizes the power density suggests operating conditions where the transformation of solar energy into hydrogen is greater. 36Another relevant aspect of Figure 5b is that the difference between the behavior of the fabricated photoanodes is increased, giving clear evidence that graphene and g-C 3 N 4 can outperform in enhancing the photoelectrochemical performance of BaTaO 2 N in comparison to other carbonaceous materials used in this study.In fact, the photocurrent density (power density) that maximized the solar-to-chemical energy conversion efficiency is 0.30 μA cm −2 (0.13 μW cm −2 ) for BT, which was increased to 0.91 μA cm −2 (0.22 μW cm −2 ) for BT-GR and 0.61 μA cm −2 (0.15 μW cm −2 ) for BT-CN.These conditions are the ones that must be imposed externally to implement these photocatalysts in reactors where the photocatalysis is electrochemically assisted so that the hydrogen evolution reaction (HER) can conveniently close the charge balance at an appropriate cathode.
Figure 5c,d shows the CA results of the BT, BT-GR, and BT-CN samples.In all cases, the photocurrent density as a function of time is higher under light irradiation than in the dark, defining a transient where the photocurrent spike is observed at the onset of light irradiation.It should be noted that when the light is turned off, the photocurrent density drops abruptly, registering a negative overshoot.The observed behavior in the CA results is characteristic of semiconductorelectrolyte interfaces, where the recombination of photoexcited charge carriers is important. 49A similar behavior was previously observed for BaTaO 2 N 6,12,24 and TiN-modified LaTiO 2 N 50 photoanodes.Therefore, the described qualitative evidence from the CA analysis confirms that the incorporation of graphene and g-C 3 N 4 into BaTaO 2 N changes the delicate balance between the electron transfer and recombination of photoexcited charge carriers.This favors an efficient electron transfer and a photocurrent collection.Particularly, the photocurrent density at low overpotentials increases because the photoexcited electrons pass into the carbonaceous materials, separating the charges and influencing the recombination rate.At high overpotentials, a greater tendency to transfer electrons to carbonaceous materials is evident.The obtained results are consistent with the behavior of graphene 51 and g-C 3 N 4 33 in photocatalysis.At this point, it is important to clarify that the recombination processes can manifest in various ways.One manifestation is evident in the overall changes in the net photocurrent: a higher photocurrent corresponds to a reduced recombination rate.Another aspect of recombination is reflected in the transient current profiles during the light− dark cycles.As described earlier, the appearance of spikes upon light illumination and a negative overshoot upon light extinction indicate recombination via surface and deep-level states.Notably, both BT-GR and BT-CN exhibit an increased photocurrent compared to that of BT, suggesting a reduction in recombination.However, the presence of peaks and negative overshoots in the transient response or CA results of BT-GR, BT-CN, and BT indicates that the recombination processes involving surface states persist.
To verify the effect of the potential on the complex dynamics of charge carriers and the stability of the electrodes giving a higher photocurrent (BT-GR and BT-CN), chronoamperometric tests were performed first at different potentials and then for a longer time.Figure 5c,5d shows the respective chronoamperometric responses obtained at 1.2 and 1.5 V (vs RHE) during a light−dark cycle.As mentioned, the shape of the current transient is associated with the balance between the electrons collected and those lost by recombination, affecting the kinetics of charge carriers.Higher bias increases the driving force to extract photoexcited electrons, thereby minimizing the current spikes and overshoots typically observed in the chronoamperometric response under light− dark cycles.In addition, Figure 5e shows the transient current at high polarization for over 60 min, enabling the observation of that current value.Although the photocurrent value is initially higher, it reaches a steady state, demonstrating the stability of the fabricated photoanodes over time scales longer than the conducted PEC measurements.
It is confirmed that the photoexcited electrons move from BaTaO 2 N to the carbonaceous material upon light irradiation, boosting the charge separation and favoring the accumulation of photoexcited holes in the valence band of BaTaO 2 N. Subsequently, the photoelectrochemical performance for water oxidation is enhanced.Apparently, the improvement in the photoelectrochemical performance of BaTaO 2 N depends on the contact between carbonaceous materials and BaTaO 2 N. Furthermore, the efficiency of carbonaceous materials may be related to their ability to accumulate, transfer, and donate photoexcited electrons due to their surface chemistry, dimensions, and electrocatalytic properties.
The discussion of the results turns to an interpretative analysis of the observed trends.In particular, the alignment between the oxygen reduction reaction (ORR) potential in the dark and the increased photocurrent density uncovers a rational connection between an apparently contrasting phenomenon.This observation becomes crucial when considering the observed improvement, which suggests an underlying accumulation phenomenon.If the performance of the composite photocatalyst is enhanced by charge accumulation in the carbonaceous phase after light excitation, then the same composite should be able to differentiate in the dark by performing reduction processes.To validate this concept, we turn to the compelling evidence provided by the ORR.Therefore, cyclic voltammetry (CV) was performed in the dark in the presence of the same electron acceptor (dissolved O 2 ) for carbonaceous material-modified BaTaO 2 N photoanodes.
The CV results are listed in Figure 5f.The electrochemical response arises from the juxtaposition of two signals: (i) the capacitive signal from the fabricated photoanodes 52 and (ii) the signal due to the oxygen reduction reaction (ORR). 53The ORR peak potential values were determined to be −0.46,−0.58, −0.60, −0.61, and −0.63 V vs RHE for BT-GR, BT-CN, BT-NH, BT-NT, and BT-FU, respectively.The obtained potential values are closer to the value of the reduction potential (one electron) of O 2 (−0.33 V vs RHE 53 ) and can be assigned to higher electrocatalytic activity for the abovementioned reaction. 54Then, the BaTaO 2 N/carbonaceous materials are expected to be a better electron donor compared to BaTaO 2 N. Thus, when the photoanodes fabricated based on the combination of BaTaO 2 N and carbonaceous material are illuminated and subjected to an electrochemical gradient, it is anticipated that the extraction of electrons can be favored.For all photoanodes, the intensifying tendency in the photocurrent density is consistent with the trend measured for the ORR potential in the dark.That is, the more positive the ORR potential, the more likely the BaTaO 2 N/carbonaceous materials are to be better electron donors.Therefore, the reduction processes during photocatalysis must be improved.The findings from this study can be used as a descriptor to optimize the design of novel photocatalytic materials based on photocatalysts and carbonaceous materials.
Finally, it is worth noting that the increase in the photocurrent and the decrease in the onset potential observed for BT-GR and BT-CN are related to the respective good and large contact between BaTaO 2 N and graphene or g-C 3 N 4 .In addition to the fact that both graphene and g-C 3 N 4 have high electron accumulation and charge retention properties, these characteristics are particularly associated with their twodimensional structures. 55,56Therefore, the depolarization of these carbonaceous materials, which are enriched with electrons during the photocatalytic process, depends on their electrocatalytic activity.It can be stated that the electrocatalytic performance of graphene and g-C 3 N 4 is relatively high in both ORR and water reduction reactions. 57,58The fact that BT-GR defines a higher photocurrent density than BT-CN, at both high and low overpotentials, may also be related to the electrochemical properties of graphene 59,60 and g-C 3 N 4 . 61,62It is inferred that the higher conductivity and electron transfer properties of graphene 59,60 in comparison to that of g-C 3 N 4 61 increase the efficiency of electron collection during the photoelectrochemical reaction in the presence of BaTaO 2 N (Figure 5a,5c,d).Thus, graphene assists in lowering the energy loss in BT-GR.In all cases, the observed transduction of the properties of graphene and g-C 3 N 4 to BT-GR and BT-CN photoanodes could enhance the photoelectrochemical performance of BaTaO 2 N.
The surface chemistry and interfacial effects between heterogeneous photocatalysts and surface water molecules are important for efficient solar water splitting.The electronic and structural properties of photocatalyst/water and photocatalyst/carbonaceous material/water interfaces depend on the electronic structure of a photocatalyst and band potentials matching with water redox potentials. 63The specific aqueous interface structure, chemical surface complexation, and the structure and morphology of nanocrystals have significant impacts on the band-edge positions of photoelectrode materials, Ohmic contacts, and Schottky barriers.In addition, the hydrophilic surfaces with surface oxygen vacancies at the terminated surfaces of metal oxides can form a hydrated layer on the surfaces and interfaces of photoelectrodes.However, it should be mentioned that it is still difficult to investigate the adsorption of water molecules using heterogeneous, multia-  tomic structure interface models by high-cost, first-principles molecular dynamics (FPMD) and density functional theory (DFT) simulations.Using implicit-solvent models with the Monte Carlo method provides a faster and inexpensive approach to exploring the interaction of the photocatalyst surface/interface with water molecules with regard to the surface photocatalytic reaction, which is strongly dependent on the stability of the photocatalyst, the adsorption of water molecules, and the formation of clusters on the photocatalyst surface. 64Therefore, the structure and adsorption energetics of the BaTaO 2 N/carbonaceous material interface and the interaction with water molecules were studied using the Monte Carlo method (BIOVIA, Adsorption Locator module) 65,66 and experimental structural data, 37,67 and the results are presented in Figures 6, 7, and 8.
The modeled composites consisted of BaTaO 2 N (BT) with a crystallographic plane of (110) and different carbonaceous materials: g-C 3 N 4 (CN) with a crystallographic plane of (002), multiwalled carbon nanotubes (NT), C60-fullerene (FU), carbon nanohorns (NHs), or graphene (GR) in the same periodic box.The predominant crystallographic planes of BaTaO 2 N and C 3 N 4 were confirmed by the XRD results.The layer of carbonaceous materials consisted of 250 carbon atoms, except for 240 carbon atoms for fullerene.The direct contact of carbonaceous materials and BaTaO 2 N is strongly related to the morphology of carbonaceous materials.The decrease in the values of the carbon surface affinity (E BT/C ) in the composite indicates the stability of the BT/carbon interface (Table 1).The BT-GR composite has the most stable interface interaction (−221.29 kcal mol −1 ) compared with BT-FU (−97.75 kcal mol −1 ), BT-CN (−156.71kcal mol −1 ), BT-NH (−58.75 kcal mol −1 ), and BT-NT (−40.25 kcal mol −1 ).The affinities of water molecules were evaluated by distribution field density (Figure 6), differential energy of adsorption of water molecules (Figure 7), and energy distribution of water molecules (Figure 8) adsorbed between the predominant surface of BaTaO 2 N (110) and carbonaceous materials with different morphologies: spherical (buckyballs) C 60 -fullerene, single-sheet graphene, two-dimensional (2D) layered graphitelike structure (g-C 3 N 4 (002)), tubular multiwalled carbon nanotube, and conical single-walled carbon nanohorn.
Figure 6 shows the interfaces between BaTaO 2 N and carbonaceous materials and the formation of a stable layer of adsorbed molecules on the composite surface.The distribution field density of adsorbed water molecules depends on the localization of the carbon nanoparticles.Carbon nanoparticles have a random spreading in the periodic box except for carbon nanotubes modeled in horizontal (Figure 6e) and vertical (Figure 6f) positions.Generally, the addition of carbonaceous materials increased the E ads value of BaTaO 2 N (−98.42kcal mol −1 ) (Table 1 and Figure 7b), and the highest E ads values were observed for the BT-NT composite with carbon nanotubes placed in horizontal and vertical positions to the surface of BaTaO 2 N. The calculated E ads values have the following order: −245.58 kcal mol −1 for BT-NT (vertical) < −202.52 kcal mol −1 for BT-NT (horizontal) < −179.57kcal mol −1 for BT-FU < −160.21kcal mol −1 for BT-GR < −151.76 kcal mol −1 for BT-CN < −127.56 kcal mol −1 for BT-NH < −98.43 kcal mol −1 for BT, while the differential adsorption has a different order: −1.61 kcal mol −1 for BT-GR < −0.60 kcal mol −1 for BT-CN < −0.18 kcal mol −1 for BT-FU < −0.06 kcal mol −1 for BT-NT (vertical) < −0.02 kcal mol −1 for BT-NT (horizontal) < −0.01 kcal mol −1 for BT = −0.01kcal mol −1 for BT-NH.The highest value of differential adsorption (dE ads / dN i ) is in good agreement with the experimental data (Figure 7a).Computational simulation and experimental data revealed that the water molecules can form stable hydrated multilayers  on the flat units of graphene. 68The highest peak observed at an adsorption energy of −2.15 kcal mol −1 (Figure 8) indicates a large number of water molecules adsorbed on the surface of the BT-GR composite.In the cases of BT-FU, BT-NH, and BT-NT composites, the presence of multiple peaks noted at different adsorption energies confirms a number of water molecules adsorbed on different adsorption sites in comparison to the BT-GR and BT-CN composites.However, the highest values of E ads for BT-NT, BT-NH, and BT-FU (Figure 7b) are the subject of controversy regarding the mode of adsorption of water molecules.−71 It is known that the carbon nanotube channel is strongly hydrophobic but can filled up with water molecules forming a hydrogen-bonded chain. 70The structure of carbon nanohorns has a closed horn tip at one side and an open end on the other side, allowing the water molecules to enter. 70Although fullerene is hydrophobic, the C60 nanoclusters have a hydrophilic nature that can increase the E ads value of BT-FU. 71The water molecules prefer to be adsorbed around the intrinsic vacancies of a single sheet of g-C 3 N 4 as clusters.The adsorption of water molecules on both sides of g-C 3 N 4 does not affect the flat structure.However, the adsorption of water molecules on one side of a single g-C 3 N 4 sheet leads to the transformation of the flat structure with indirect semiconductor properties to buckle the structure with direct semiconductor properties.Therefore, the band structure of g-C 3 N 4 is finally changed due to the adsorption of water molecules. 8A previous study 63 found that the exothermic adsorption of water molecules in the presence of chemisorbed oxygen causes the dissociation of water molecules on the hydroxylated surfaces of photocatalysts via a partial proton transfer mechanism at the interface.In addition, the surface anions can accept protons from water molecules, while equivalent OH ions form bonds with surface cations.The N-sites can also be protonated even more rapidly than the surface oxygen sites. 63Finally, the water molecules can be adsorbed dissociatively on the surface oxygen vacancies, leading to the formation of surface hydroxyls and oxidative intermediates (H* for H 2 evolution and HO*, O*, and HOO* for O 2 evolution). 8he dissociation of water molecules is dependent on the termination of the photocatalyst surface.BaTaO 2 N has negative charges on the oxygen-terminated surfaces and positive charges on the barium-terminated surfaces.The water molecules prefer to be adsorbed by interaction with surface oxygen and on the top side of the Ba atom on the (110) surface, forming the Ta−O•••H 2 O and H 2 O•••Ba bonds because the coordination of water molecules to the BaTaO 2 N surface is energetically more favorable than to carbonaceous materials due to electrostatic forces.The adsorption of water oxygen atoms located on the exposed cation and anion sites varies less proportionally to the number of oxygen atoms missing from the normal Ta(O,N 6 ) octahedral coordination and the Ba cation, which can be 12-fold surrounded by anions.The most stable surfaces are shown to be along the (110) crystallographic planes that have more metal cations exposed.The analysis of energy distribution of adsorbed water molecules (Figure 8) confirmed the shift to higher adsorption energy in the case of the BT-GR composite due to the interaction of the conjugated π-system of graphene and the BaTaO 2 N surface.Although the BT-NT (horizontal), BT-NH, and BT-FU composites also have the possibility to form similar interactions, the BT-GR composite has a more pronounced effect on the ability of water adsorption.
The adsorption of water molecules on the BaTaO 2 N/ carbonaceous material surfaces was found to be a favorable exothermic process.The surface reactivity of the BT-GR composite is significantly higher in comparison to that of BT-FU, BT-NT, and BT-NH, and the possible dissociation of water molecules is highly favorable on the BT-GR and BT-CN surfaces.The experimental results confirmed that the ORR potential is correlated with the photocurrent density and adsorption of water molecules at the interface between BaTaO 2 N and carbonaceous materials (Figure 7).Considering the localization of the distribution field density of adsorbed water molecules, it is clear that the adsorption of water molecules is favored onto surfaces of BT-GR and BT-CN composites.Thus, the incorporation of graphene can lead to a more positive ORR potential and better electron donors, which should be in line with the photoredox flux-matching conditions during the photocatalysis process, suggesting the improvement of the reduction processes during photocatalysis. 72Therefore, the BT-GR composite exhibited a higher water oxidation reaction (WOR) performance.

CONCLUSIONS
In this study, the photoelectrochemical performance of BaTaO 2 N was enhanced by involving carbonaceous materials, such as fullerene, g-C 3 N 4 , graphene, carbon nanohorns, and carbon nanotubes.The difference in the photoelectrochemical performance of BaTaO 2 N/carbonaceous materials and Ba-TaO 2 N could be attributed to the capability of each carbonaceous material for the collection and transfer of photoexcited electrons, thus decreasing the recombination of photoexcited charge carriers.Particularly, the photocurrent density of BaTaO 2 N was 2.6-and 1.7-fold increased using graphene and g-C 3 N 4 due to the efficient electron transfer, electron reservoir capacity, and accumulation of a greater number of photoexcited holes on the valence band.The comparison of trends between the ORR potential in the dark and the increase in the photocurrent density validates the charge accumulation process in the carbonaceous phase, highlighting that the subsequent photocurrent collection was not compromised.This implies that the more positive the ORR potential is, the better electron donors the BaTaO 2 N/ carbonaceous materials are expected to be, suggesting the improvement of the reduction processes during photocatalysis.Computational simulation of the adsorption of water molecules onto the surfaces of BaTaO 2 N/carbonaceous materials revealed that the incorporation of graphene can enhance the water oxidation reaction performance of BaTaO 2 N. The photoelectrochemical performance of other photocatalysts can also be improved using carbonaceous materials having different dimensions, morphologies, and surface and optoelectronic properties.

Figure 5 .
Figure 5. Photocurrent density vs potential (a), powder density vs photocurrent density (b), photocurrent density vs time under illumination−dark cycle at 1.2 V vs RHE (c) and 1.5 V vs RHE (d), photocurrent density vs time at 1.5 V vs RHE for 60 min (e), and normalized current vs potential (dark condition) (f).

Figure 7 .
Figure 7. (a) Relationship between differential energy of adsorption of water molecules, redox potential, and photocurrent density and (b) relationship between the energy of adsorption of water molecules, redox potential, and photocurrent density of BT-FU, BT-CN, BT-GR, BT-NH, and BT-NT composites.

Figure 8 .
Figure 8.(a) Energy distribution of adsorbed water molecules and (b) formation of water molecule layers on BT-FU, BT-CN, BT-GR, BT-NH, and BT-NT composites.The green, light blue, red, dark blue, and gray colors represent barium, tantalum, oxygen, nitrogen, and carbon, respectively.

Table 1 .
Energy Parameters of the Adsorption of Water Molecules