Influence of Sacrificial Reagents on the Photodeposition Reaction of Cocatalysts

Cocatalyst depositing on photocatalysts with the oxidation of sacrificial reagents (photodeposition) has been widely used because of its simplicity. In most cases, alcohols are used as sacrificial reagents during photodeposition; however, the optimal types have yet to be fully investigated. In this study, methanol (MeOH) and triethanolamine (TEOA) are used as sacrificial reagents to prepare Pt‐ or Au‐deposited titanium dioxide. The use of different sacrificial reagents changes the deposition process. Pt deposited in the presence of MeOH and Au deposited in the presence of TEOA show higher cocatalytic activity. In this study, it is suggested that the consideration and selection of sacrificial reagents in terms of their reactivity with the cocatalyst are important for the development of optimal cocatalyst‐supported photocatalysts.


Introduction
Owing to the rapid increase in fossil fuel consumption, environmental pollution and resource depletion have increasingly become a serious problem worldwide.Therefore, the development of new energy sources that are environmentally friendly and sustainable is crucial.The direct conversion of abundant solar energy into hydrogen is one of the most promising methods to solve these problems, [1] and photocatalytic hydrogen production has attracted considerable attention since it was proposed in 1972. [2,3]Titanium dioxide (TiO 2 ), in particular, is widely used as a chemically stable, nontoxic, and highly reactive material. [4]Although pure TiO 2 does not produce hydrogen, metal supports such as Pt [5,6] and Au [7,8] are effective in improving photocatalytic activity by inducing high activity through the development of cocatalytic activity.Photodeposition with the oxidation of sacrificial reagents was used to support these cocatalysts.This technique has been widely used because it is simple and efficient, and provides a good metal-semiconductor contact through reduction by photoexcited electrons. [9,10]Furthermore, sacrificial reagents consume holes instead of the slow four-electron reaction of oxygen, allowing for efficient loading while suppressing electron-hole recombination.[13] Reportedly, even if the theoretical loading is the same, the type of sacrificial reagent changes the amount of deposition and particle size as a cocatalyst, affecting the hydrogen production activity. [14]In this study, methanol (MeOH), which has been commonly used in the past, and triethanolamine (TEOA), which has been extensively studied as a sacrificial reagent for hydrogen production [15][16][17] and CO 2 reduction reactions, [18,19] were selected for comparison.Pt and Au were selected as representative noble metals for the cocatalysts.

Hydrogen Production Activity
Figure 1 shows the hydrogen production activity of the cocatalyst-loaded TiO 2 .The hydrogen production rate was calculated from the time course of evolved hydrogen accumulation (Figure S1, Supporting Information).The order of activity did not change with the addition of different amounts of metal ions (Figure S2, Supporting Information).Hydrogen production was observed under all conditions, whereas slight hydrogen production was observed with bare TiO 2 , indicating that Pt or Au acted as a cocatalyst under all conditions.Notable, Pt/TiO 2 showed higher activity when prepared in the presence of MeOH, and Au/TiO 2 showed higher activity in the presence of TEOA.

DOI: 10.1002/aesr.202300295
Cocatalyst depositing on photocatalysts with the oxidation of sacrificial reagents (photodeposition) has been widely used because of its simplicity.In most cases, alcohols are used as sacrificial reagents during photodeposition; however, the optimal types have yet to be fully investigated.In this study, methanol (MeOH) and triethanolamine (TEOA) are used as sacrificial reagents to prepare Pt-or Au-deposited titanium dioxide.The use of different sacrificial reagents changes the deposition process.Pt deposited in the presence of MeOH and Au deposited in the presence of TEOA show higher cocatalytic activity.In this study, it is suggested that the consideration and selection of sacrificial reagents in terms of their reactivity with the cocatalyst are important for the development of optimal cocatalyst-supported photocatalysts.

Characteristics of Pt/TiO 2
Figure 2 shows the absorption properties of the prepared Pt/TiO 2 .The diffuse reflection spectra (DRSs) of different Pt ions preparation concentrations are shown in Figure S3a,b, Supporting Information.The increase in absorption intensity in the visible-light range for Pt/TiO 2 samples indicated Pt nanoparticles loading.For Pt-MeOH/TiO 2 , the absorption intensity increased compared to Pt-TEOA/TiO 2 , suggesting a higher loading.This is consistent with the ICP-OES results shown in Table 1; when 0.5 wt% (the same amount) of Pt precursor was prepared, the actual loading was 0.27 wt% for Pt-MeOH/TiO 2 and 0.05 wt% for Pt-TEOA/TiO 2 .
The observation of Pt nanoparticles via SEM was conducted using 3.0 wt% Pt-MeOH/TiO 2 and Pt-TEOA/TiO 2 to enhance the frequency of Pt detection (Figure 3).The results confirmed the dispersion and deposition of Pt fine particles on the surface of the TiO 2 photocatalyst.In particular, the detected intensity of Pt-MeOH in energy dispersive spectroscopy (EDS) was higher than that of Pt-TEOA, suggesting a higher precipitation yield.
To further clarify the detailed Pt loading state, the surface of Pt/TiO 2 was analyzed by XPS measurements (Figure 4).In the results, higher intensity was obtained for Pt-MeOH/TiO 2 with higher Pt loading.The peak positions correspond to Pt metal (Figure 4, red line, Pt 4f 7/2 71.2 eV, 4f 5/2 74.8 eV). [9]In contrast, for Pt-TEOA/TiO 2 with low Pt loading, a low intensity peak was detected on the high energy side.This peak shift is attributed to Pt 2þ formed by partial oxidation of Pt by light irradiation in the TEOA solution. [6]The Pt cocatalyst is preferably in the metallic state when acting as an electron acceptor.Therefore, the significantly lower hydrogen production activity of Pt-TEOA/TiO 2 is considered to be due to the small amount of Pt cocatalyst loaded on TiO 2 and its partially oxidized state.
To further investigate the deposition process of the Pt cocatalyst, the spectra of the reaction solutions were measured (Figure 5a).In the supernatant of H 2 [PtCl 6 ] in the MeOH solution before light irradiation, absorption derived from [PtCl 6 ] 2À was observed at 275 nm, [20] regardless of the presence or absence of      solution. [6]Due to the stabilization of Pt ions by complex formation, photoreduction was less likely to proceed, and absorption derived from Pt ions could be observed even after light irradiation.The adsorption of TEOA on the photocatalyst inhibits precipitation of Pt metal. [6]These effects lead to a significantly lower Pt loading in Pt-TEOA/TiO 2 than in the MeOH system (Table 1).

Characteristics of Au/TiO 2
Au-MeOH/TiO 2 and Au-TEOA/TiO 2 exhibited no significant difference in Au loading (Table 1).However, a comparison of their DRS shows a broader and stronger peak in absorption attributed to plasmon resonance derived from Au nanoparticles in Au-TEOA/TiO 2 (Figure 6).A similar tendency was observed for different stocking amounts of HAuCl 4 (Figure S3c,d, Supporting Information).Observation of Au nanoparticles by SEM was performed using 3.0 wt% Au-MeOH/TiO 2 and Au-TEOA/TiO 2 to increase the frequency of Au observation (Figure 7).The results showed that the average diameters of Au deposited in the presence of MeOH and TEOA were 73and 56 nm, respectively (Figure S4a, Supporting Information).When deposited in the presence of MeOH, spherical Au particles were deposited in contact with TiO 2 aggregates (Figure 7a and S4b, Supporting Information).In contrast, the Au nanoparticles deposited in the presence of TEOA showed a distorted shape and were highly dispersed and precipitated along the TiO 2 surfaces (Figure 7b and S4c, Supporting Information).Even with the same amount of Au, if smaller particles are precipitated with high dispersion, the effective absorption cross section is assumed to be larger.In addition, the distorted shape and contact with TiO 2 with a high refractive index shifts the plasmon resonance wavelength to the long wavelength side.Consequently, broad and strong Au-derived absorption was observed in Au-TEOA/TiO 2 .In Au-TEOA/TiO 2 , the large contact area between the Au cocatalyst and TiO 2 and the highly dispersed loading indicate that the electrons from TiO 2 were easily received.This may have increased the efficiency of hydrogen production activity.
To clarify the difference in the precipitation mechanism of Au nanoparticles, Au 4f XPS of Au/TiO 2 was performed before and after light irradiation (Figure 8).Before light irradiation, almost no Au-derived peaks were detected in the presence of MeOH, while strong peaks were detected in the presence of TEOA (peak energies, Au 4f 7/2 84.2 eV, 4f 5/2 87.7 eV).After light irradiation, in the presence of any of the sacrificial reagents, peaks attributed to Au metal (Au 4f 7/2 84.0 eV, 4f 5/2 87.6 eV) were found. [21]he pH of the TEOA reaction solution prepared in this study was 11.3, suggesting that gold hydroxide may have been precipitated before light irradiation. [22]This is suggested by the fact that the XPS spectrum is shifted to the higher energy side than that of Au metal.
In addition, to clarify the detailed Au precipitation process, the spectra of the solutions before and after light irradiation were measured (Figure 9).In the supernatant of a MeOH solution of H[AuCl 4 ] before light irradiation, [AuCl 4 ] À -derived absorption was observed at 310-320 nm (Figure 9a), [23] regardless of the presence of TiO 2 .After light irradiation, Au nanoparticles were precipitated in the solution by photoreduction using MeOH as the reducing agent, and plasmon resonance-derived extinction occurred at 560-570 nm while the [AuCl 4 ] À -derived absorption disappeared.After light irradiation of the TiO 2 -dispersed solution, the [AuCl 4 ] À derived absorption almost disappeared and no extinction by plasmon resonance of Au nanoparticles was observed, suggesting that the Au nanoparticles deposited on TiO 2 or in solution were separated with TiO 2 by centrifugation.
In the case of the TEOA sacrificial reagent, a broad extinction was observed around 580 nm before light irradiation (Figure 9b).This is considered to be absorption derived from the gold hydroxide precipitated in the TEOA solution.Upon photoirradiation, the photoreduction of gold hydroxide progressed, and a plasmon resonance peak originating from Au nanoparticles was observed at 530-540 nm.In the TiO 2 dispersion system, gold hydroxide was assumed to have deposited on TiO 2 , and no absorption derived from gold hydroxide or Au nanoparticles was observed in the supernatant with or without light irradiation.
From XPS (Figure 8) and the extinction spectra of the solutions (Figure 9b), the precipitation process of Au nanoparticles in the presence of TEOA occurs via gold hydroxide.Gold hydroxide precipitation occurs on the TiO 2 surface, which is then reduced by photoirradiation to complete the metal Au loading.Conversely, in the presence of MeOH, a general photodeposition reaction proceeds.In this case, the deposited Au acts as an electron acceptor and thus becomes a reduction site.Therefore, reductive growth of Au tends to proceed on the deposited Au nanoparticles, which tend to increase the particle size.This difference in the precipitation process is considered to be responsible for the difference in the shape and dispersibility of Au cocatalyst on TiO 2 (Figure 7).
In conclusion, the type of sacrificial reagent has a strong influence on the precursor metal ions in the process of depositing the cocatalyst.This results in different deposition processes as well as size, shape, contact area, and dispersibility of the finally obtained cocatalyst.Therefore, for the synthesis of suitable photocatalytic materials, it is important to select sacrificial reagents according to the kind of cocatalyst.In particular, the present study reveals that the combination of Pt-MeOH, which is more   easily proceeded by photodeposition reaction, and Au-TEOA, which has high dispersibility and is deposited via gold hydroxide, shows high hydrogen production activity.

Conclusion
For fabricating a composite of Pt and Au cocatalysts on TiO 2 , two sacrificial reagents, MeOH and TEOA, were selected for the photodeposition.The results showed that the deposition process was different for each combination of metal and sacrificial reagent and that the amount and state of loading were different for each combination.These features were also found to affect hydrogen production activity.In the present system, the hydrogen production activity was enhanced in the case of MeOH for Pt and TEOA for Au.Traditionally, sacrificial reagents have been selected based on their redox potential and activity in photocatalytic reactions.However, the results of this study indicate that a strong consideration of the reactivity of cocatalysts in the cocatalyst support is also important for improving the activity of cocatalystsupported photocatalysts.

Experimental Section
Materials and Characterization: Preparation of Cocatalyst-Loaded TiO 2 : TiO 2 (Nippon Aerosil Co. Ltd., Tokyo, Japan) was P25 and was used as purchased.TiO 2 (1.0 g) was dispersed in 100 mL of a 50 vol% MeOH (FUJIFILM Wako Pure Chemical Corporation, Tokyo, Japan) aqueous solution and stirred for 30 min.Then, H 2 PtCl 6 •6H 2 O (Kanto Chemical Co., Inc., Tokyo, Japan) was added at a constant volume such that the theoretical loadings were 0.1, 0.5, and 1.0 wt%, and the mixture was stirred for 30 min.After removing air with Ar, the solution was irradiated with a 300 W Xe lamp (λ > 300 nm) for 150 min.The products were collected by suction filtration and dried at 393 K for 24 h.An aqueous solution of 50 vol% TEOA (Chemical Co., Inc., Tokyo, Japan) was prepared in the same manner for the different sacrificial reagents; for Au loading, HAuCl 4 (Kanto Chemical Co., Inc., Tokyo, Japan) was added in the same manner.The prepared photocatalysts were labeled as Pt-MeOH/TiO 2 , Pt-TEOA/TiO 2 , Au-MeOH/TiO 2 , and Au-TEOA/TiO 2 , respectively.
Materials and Characterization: Photocatalytic Reaction: The prepared photocatalysts (50 mg) were dispersed in 5 mL of the reaction solution (50 vol% MeOH, TEOA) and stirred for 30 min in a test tube.Air was removed with Ar, and the reaction cell was sealed with a rubber cap before light irradiation.To maintain the same temperature in the cell, the test was performed in a quartz water bath and irradiated from the outside.A 300-W Xe lamp was used as the light source (λ > 300 nm).The generated gas was collected using a gastight syringe, and the amount of evolved hydrogen was measured by gas chromatography (GC-8 A, Shimadzu, Japan).The hydrogen production rate was calculated from the amount of hydrogen produced over 2 h.

Figure 1 .
Figure 1.Comparison of the hydrogen production rate over bare TiO 2 and 0.5 wt% cocatalyst-loaded TiO 2 prepared in the reaction solution.Photocatalyst: 50 mg; light source: 300 W Xe lamp (λ > 300 nm); and reaction solution: 5 mL of 50 vol% in water i) MeOH or ii) TEOA at 25 °C.

Figure 5 .
Figure 5. Extinction spectra of the H 2 PtCl 6 solution before and after light irradiation in the presence and absence of TiO 2 in a) MeOH solution and b) TEOA solution.The supernatant of the Pt/TiO 2 -dispersed solution was obtained by centrifugation and filtration.

Figure 8 .
Figure 8. Au 4f XPS spectra of the prepared Au-MeOH/TiO 2 and Au-TEOA/TiO 2 before and after light irradiation.

Figure 9 .
Figure 9. Extinction spectra of the HAuCl 4 solution before and after light irradiation in the presence and absence of TiO 2 in a) MeOH solution and b) TEOA solution.The supernatant of the Au/TiO 2 -dispersed solution was obtained by centrifugation and filtration.

Table 1 .
Determination of the elemental composition of prepared TiO 2 supporting cocatalysts by inductively coupled plasma optical emission spectroscopy (ICP-OES).