Enhancing Photocatalytic Hydrogen Evolution with Oxygen Vacancy-Modified P/Ag/Ag₂O/Ag₃PO₄/TiO₂ by Using Optimized NaBH₄ Reduction Strategy

Abstract

The introduction of oxygen vacancies (OVs) is a promising strategy to enhance the hydrogen (H₂) evolution efficiency of photocatalysts. Sodium borohydride (NaBH₄) is widely used as a reducing agent to introduce OVs, particularly in composite materials. However, its impact on H₂ evolution remains underexplored. In this study, by employing various mass ratios of NaBH₄ to P/Ag/Ag₂O/Ag₃PO₄/TiO₂ (PAgT), OVs modified PAgT (R-PAgT) composites, which were synthesized and systematically characterized by XRD, FT-IR, and XPS. R-PAgT-10 with an optimal mass ratio exhibited a superior H₂ evolution efficiency and stability, maintaining its performance over 20 cycles under visible light irradiation, while the higher mass ratio of NaBH₄/PAgT led to the disruption of the crystal structure with excessive OVs amounts, resulting in poor stability. This study highlighted the importance of utilizing the optimal mass ratio of NaBH₄ to prepare OVs-PAgT for successful and stable H₂ evolution under visible light irradiation, which holds promise for developing efficient and durable photocatalysts for renewable energy applications.

1. Introduction

Global reliance on fossil fuels has led to critical issues, including energy crises and environmental pollution [1]. As a response to these challenges, the utilization of eco-friendly hydrogen (H₂) as a clean energy source has emerged as a promising approach for sustainable development and environmental protection [2,3]. In 1972, Fujishima introduced the concept of H₂ production through a photocatalytic water splitting process using titanium dioxide (TiO₂) [4], sparking growing interest due to its eco-friendly potential [5]. However, pure TiO₂ has limitations, such as the limited utilization of visible light and low charge transfer efficiency, which impede its practical application [6,7,8]. To address these limitations, our laboratory previously developed a novel solar light-driven composite, denoted as P/Ag/Ag₂O/Ag₃PO₄/TiO₂ (PAgT) [9]. In contrast to pure TiO₂, the PAgT composite demonstrated the enhanced utilization of visible light and accelerated electron separation/transformation rates, resulting in the excellent H₂ evolution under simulated solar light irradiation (the schematic diagram of PAgT is shown in Figure S1).

Furthermore, oxygen vacancy (OV) modification has been recognized as a promising strategy to further enhance photocatalytic activity. Bad’ura and co-authors reported that spin defects modified TiO₂ nanosystems, which exhibited a 1.5-times higher H₂ evolution rate than the original [10]. Moreover, Xu et al. also confirmed that OVs could efficiently improve the H₂ generation ability of TiO₂ under visible light irradiation [11].

OVs have the potential to enhance solar light utilization efficiency and accelerate the separation and transfer of electron–hole pairs in the host photocatalyst [12]. However, it is crucial to note that the quantity of OVs plays a significant role in enhancing the photocatalytic activity of modified catalysts. An insufficient number of OVs may not improve performance efficiently, while an excess of OVs can become recombination centers for electron–hole pairs, ultimately reducing the activity [13]. Therefore, it is paramount to introduce an optimal amount of OVs in the PAgT composite using a suitable method.

Among various methods, the chemical reduction method employing NaBH₄ as the reducing agent has been recognized as an attractive approach to introducing OVs into the host photocatalyst [14]. In this method, the cost-effective and easy-to-handle NaBH₄ serves as the reducing agent, generating active hydrogen atoms. These active hydrogen atoms subsequently react with oxygen atoms to create water and OVs. The quantity of active hydrogen atoms is positively correlated with the mass ratio of photocatalyst to NaBH₄, with a larger ratio leading to the release of a greater number of active hydrogen atoms and a higher concentration of oxygen vacancies in the host photocatalyst. Hence, it is essential to explore the ideal mass ratio of PAgT to NaBH₄ for the successful synthesis of the optimal OV ratio in the PAgT composite.

In this study, OV-modified PAgT composites using the chemical reduction method with NaBH₄ as the reducing agent were prepared. To investigate the impact on H₂ evolution, the different mass ratios of PAgT to NaBH₄ for OVs modification were employed. The H₂ evolution efficiency of the obtained catalysts was assessed under solar light irradiation and their stability was examined through cyclic experiments. Furthermore, comprehensive characterization using XRD, FT-IR, and XPS were conducted to evaluate the crystal structure and elemental composition of the different R-PagT photocatalysts. Based on these results, the potential effect of varying mass ratios of PAgT to NaBH₄ on the photocatalytic properties of the composite was proposed.

2. Results and Discussion

2.1. Photocatalytic H₂ Evolution Rate and Stability of R-PAgT Photocatalysts

2.1.1. First Cycle H₂ Evolution Rate of R-PAgT Photocatalysts

The H₂ evolution efficiencies of the R-PAgT catalysts were evaluated under simulated solar light irradiation using methanol as a hole-scavenger. As shown in Figure 1, the mass ratio of PAgT to NaBH₄ significantly influenced the H₂ evolution efficiency. It was evident that the rate of H₂ generation experienced a notable increase when the mass ratios of PAgT to NaBH₄ were 3:1 and 5:1. Specifically, the H₂ evolution rates for R-PAgT-3:1 and R-PAgT-5:1 were 1607 and 3347 μmol·g⁻¹·h⁻¹, respectively. These rates were approximately 3.25 and 6.78 times higher than that of pure PAgT (493 μmol·g⁻¹·h⁻¹). When the mass ratio of PAgT to NaBH₄ further increased to 7:1 and 10:1, it had a relatively modest positive impact on the H₂ evolution rate of the prepared catalysts. The H₂ evolution rates of R-PAgT-7:1 and R-PAgT-10:1 were 1027 and 917 μmol·g⁻¹·h⁻¹, respectively, which were approximately 2.08 and 1.86 times to pure PAgT. However, regarding the mass ratio of 15:1 for PAgT to NaBH₄, there was a detrimental effect on the H₂ generation rate of R-PAgT-15:1, at only about 337 μmol·g⁻¹·h⁻¹, which was significantly lower than original PAgT composite.

These aforementioned results demonstrated that R-PAgT-3:1, R-PAgT-5:1, R-PAgT-7:1, and R-PAgT-10:1 possessed a better H₂ evolution rate than pure PAgT. However, the practical application of photocatalysts heavily relies on their stability and durability. A key factor in this regard is their ability to maintain consistent performance in the long term. Hence, cyclic experiments were carried out on these four photocatalysts to assess their durability and stability.

2.1.2. Cyclic Experiment of R-PAgT Photocatalysts

The cyclic experiment involving R-PAgT-5:1 was depicted in Figure 2a. In the initial cycle, R-PAgT-5:1 achieved an impressive H₂ evolution rate of 3347 μmol·g⁻¹·h⁻¹. However, this value experienced a decline in the second cycle, dropping to 431 μmol·g⁻¹·h⁻¹ and further decreasing to 392 and 368 μmol·g⁻¹·h⁻¹ in the third and fourth cycles, respectively. The decline rate of R-PAgT-5:1 was about 89.1%, suggesting its limited stability for practical applications.

In Figure 2b, the cyclic experiment for R-PAgT-3:1 revealed a noticeable trend of decreasing H₂ evolution rate, akin to what was observed for R-PAgT-5:1. During the initial cycle, R-PAgT-3:1 generated approximately 1607 μmol·g⁻¹·h⁻¹ of H₂. However, after a single cycle, the H₂ evolution rate of R-PAgT-3:1 showed a steep decline, plummeting to 233 μmol·g⁻¹·h⁻¹, representing a significant 86.2% decrease. This decline strongly suggested that R-PAgT-3:1 exhibited poor stability when considered for practical application. In summary, although R-PAgT-5:1 and R-PAgT-3:1 initially demonstrated excellent H₂ yields in the first cycle, their substantial decline in performance during the second cycle indicated their inadequate stability for practical applications.

The results of the cyclic experiment using R-PAgT-7:1 as a photocatalyst are displayed in Figure 3a. It can clearly be observed that the H₂ evolution rate of R-PAgT-7:1 gradually decreased over the extended number of cycles compared with R-PAgT-5:1 and R-PAgT-3:1. The H₂ evolution rate decreased from 1024 μmol·g⁻¹·h⁻¹ in the first cycle to 900 μmol·g⁻¹·h⁻¹ in the second cycle. With further extended cycles, i.e., the third to the seventh cycle, the H₂ evolution efficiency slowly decreased to 740, 654, 556, 489, and 450 μmol·g⁻¹·h⁻¹, respectively. The H₂ evolution rate of R-PAgT-7:1 after seven cycles was higher than that of pure PAgT. After that, however, it sharply reduced to 260, and finally reached 158 μmol·g⁻¹·h⁻¹ in eighth and ninth cycles, respectively, which was lower than that of pure PAgT (493 μmol·g⁻¹·h⁻¹).

The stability results of R-PAgT-10:1 are displayed in Figure 3b. From the initial cycle to the fifth cycle, the H₂ evolution rates of R-PAgT-10:1 were 903, 823, 712, 602, and 563 μmol·g⁻¹·h⁻¹, respectively, exhibiting a gradual downward trend, which was similar to R-PAgT-7:1. However, in comparison to R-PAgT-5:1, R-PAgT-3:1, and R-PAgT-7:1, R-PAgT-10:1 was able to maintain its H₂ evolution rate at about 632 μmol·g⁻¹·h⁻¹ after 20 cycles, which indicated it good stability for practical applications.

Based on the above results, it can be summarized that R-PAgT-3:1 and R-PAgT-5:1 exhibited excellent H₂ evolution rates in the initial cycle, but experienced a dramatic decline in the second cycle, which suggests poor stability. It is noteworthy that R-PAgT-10:1 showed a superior H₂ evolution rate compared with pure PAgT, which was a result of the introduction of optimized OVs. Moreover, R-PAgT-10:1 was able to maintain a stable H₂ yield at about 632 μmol·g⁻¹·h⁻¹ after 20 cycles, which was much higher the related OV-modified TiO₂ reported (Table S1). In summary, the excellent and stable H₂ evolution performance of R-PAgT-10:1 suggested its potential for practical applications.

2.2. Characterization of R-PAgT Photocatalysts

2.2.1. Crystal Structures of R-PAgT Photocatalysts

The crystal structures of R-PAgT photocatalysts were analyzed through XRD to explain the results of the cyclic H₂ evolution experiments conducted for each photocatalyst. In Figure 4, the pure PAgT composite displayed two distinct diffraction peaks at around 25.4, 37.8, 48.0, 55.1, 62.7, 68.8, and 75.0 degrees, corresponding to the (101), (004), (200), (211), (204), (116), and (215) planes of the anatase TiO₂ phase (JCPDS # 21-1272), respectively [15]. Similarly, both R-PAgT-10:1 and R-PAgT-7:1 exhibited identical diffraction peak positions, indicating that they retained the same anatase TiO₂ phase as the original PAgT composite. This observation suggests that the low mass ratio of PAgT to NaBH₄ had minimal impact on the crystal structure of the resulting catalysts. In contrast, the XRD patterns of R-PAgT-5:1 and R-PAgT-3:1 displayed irregular line shapes, with the diffraction peaks of the anatase TiO₂ phase barely detectable in these two catalysts. This anomaly pointed to a significant alteration in the crystal structure of R-PAgT-5:1 and R-PAgT-3:1, likely resulting from an excessive ratio of PAgT to NaBH₄. On the other hand, the characteristic peaks of Ag species were hardly detectable in all the synthesized catalysts, likely due to their low concentrations in the fabrication process.

2.2.2. Chemical Bonds of R-PAgT Photocatalysts

Moreover, the chemical bonds of R-PAgT were measured using the FT-IR spectrum. As shown in Figure 5, R-PAgT-7:1 and R-PAgT-10:1 processed similar peaks at 3000–3750, 1626, and 400–700 cm⁻¹. The peaks at 3000–3750 and 1626 cm⁻¹ were related to the hydroxyl group and surface absorbed water, respectively [16,17]. Notably, the absorption band within 400–700 cm⁻¹ is associated with Ti–O stretching vibrations in the anatase TiO₂ lattice [16], indicating the retention of the TiO₂ phase in these two samples. This observation aligns well with the XRD results, further confirming the structural integrity of the TiO₂ component.

In contrast, for R-PAgT-3:1 and R-PAgT-5:1, the characteristic peak in the 400–700 cm⁻¹ range was either significantly weakened or nearly absent. This suggests that the TiO₂ structure was disrupted due to the excessive addition of NaBH₄ during the synthesis process, leading to the destruction or significant alteration of the TiO₂ lattice. These findings highlight the crucial role of NaBH₄ concentration in preserving the TiO₂ framework and emphasize the necessity of optimizing reaction conditions to maintain the structural and functional integrity of the composite material.

2.2.3. Elemental Composition of R-PAgT Photocatalysts

Furthermore, a detailed XPS analysis was performed to determine the elemental composition of R-PAgT catalysts. As depicted in Figure 6, the peaks corresponding to O, Ti, Ag, and P are clearly evidenced in both the pure PAgT and R-PAgT-7:1 and R-PAgT-10:1 sample. This observation suggested that R-PAgT-7:1 and R-PAgT-10:1 maintained the same elemental composition as pure PAgT. In contrast, the peaks for the Ti, Ag, and P elements were conspicuously absent in the XPS spectra of R-PAgT-5:1 and R-PAgT-3:1. This disparity between the small mass ratio (10:1 and 7:1) and the large mass ratio (5:1 and 3:1) of PAgT to NaBH₄ underscores the fact that the larger mass ratio had a detrimental effect on the elemental composition of the synthesized catalysts.

The high-resolution Ti 2p XPS spectra of the R-PAgT catalysts are presented in Figure 7. Notably, in the pure PAgT sample, the Ti species displayed two distinct peaks at 458.7 and 464.4 eV. These peaks were attributed to the presence of Ti⁴⁺ within the TiO₂ component in PAgT composite. Following the chemical reduction process, the Ti 2p XPS spectra of the R-PAgT catalysts exhibited significant alterations. In the case of R-PAgT-7:1 and R-PAgT-10:1, the Ti species could be discerned as four distinct peaks. The peaks at 458.7 and 464.4 eV were associated with Ti⁴⁺, while the peaks at 463.2 and 457.8 eV indicated Ti³⁺ [18,19]. The presence of Ti³⁺ confirmed the existence of OVs in R-PAgT [20]. However, when compared to R-PAgT-7:1 and R-PAgT-10:1, the Ti species in R-PAgT-5:1 and R-PAgT-3:1 were primarily centered at 463.2 and 457.8 eV, indicating the presence of Ti³⁺ and OVs in these two catalysts. Notably, in R-PAgT-3:1 and R-PAgT-5:1, no peaks attributed to Ti⁴⁺ (458.7 and 464.4 eV) were observed. This observation suggests that the use of NaBH4 as a reducing agent during the chemical reduction process could introduce OVs into the PAgT composite. Nonetheless, a higher mass ratio of PAgT to NaBH₄ led to the conversion of all Ti⁴⁺ into Ti³⁺ and an excess of OVs in R-PAgT-3:1 and R-PAgT-5:1.

Furthermore, the high-resolution O 1s XPS spectra of the synthesized catalysts are depicted in Figure 8. Evidently, the O species in pure PAgT, R-PAgT-10:1, and R-PAgT-7:1 displayed four discernible peaks. The peak at 529 eV was attributed to Ag–O, indicating the presence of Ag2O within the resulting catalysts [21]. Another peak at 531.3 eV originated from Ti–O–P, signifying the incorporation of phosphorus into the lattice of TiO₂. The peaks situated at 529.7 and 530.9 eV were contributed to lattice oxygen (Ti-O) and non-lattice oxygen (Ti-OH), respectively [21,22]. However, the O species in R-PAgT-5:1 and R-PAgT-3:1 could only be fitted into two peaks, specifically at 530.9 eV (corresponding to the Ti–OH bond) and 531.3 eV (attributed to the Ti–O–P bond). The disappearance of Ag–O and Ti–O bonds in R-PAgT-5:1 and R-PAgT-3:1 strongly indicates that the TiO₂ and Ag₂O components were significantly disrupted due to their reactions with excessive NaBH₄. Specifically, NaBH₄ served as a strong reducing agent, leading to the reduction in monovalent silver (Ag⁺) in Ag₂O to metallic silver (Ag⁰), thereby breaking the Ag–O bonds (further corroborated in Figure 9). Simultaneously, the TiO₂ lattice was also affected, likely through partial reduction or structural degradation, resulting in the loss of the characteristic Ti–O vibrational bands. These structural modifications suggest that an excessive NaBH₄ ratio during synthesis induces substantial compositional changes.

Additionally, the high-resolution Ag 3d XPS spectra for all the catalysts are presented in Figure 10. In the case of pure PAgT, the peaks at 374.2 and 368.2 eV were associated with metallic Ag⁰ [23], while the peaks at 373.4 and 367.4 eV corresponded to Ag⁺ ions presented in Ag₂O and Ag₃PO₄, respectively [15]. Following the reduction process with NaBH₄, the changes in Ag species in R-PAgT catalysts exhibited a pattern similar to that of the P species. When a small mass ratio of PAgT to NaBH₄ was used, the Ag 3d XPS spectra of R-PAgT-10:1 and R-PAgT-7:1 displayed the same four peaks as those in pure PAgT, located at 374.2, 368.2, 373.4, and 367.4 eV. The first two peaks indicated the presence of metallic Ag⁰, while the latter two peaks suggested the presence of Ag+ ions in Ag₂O and Ag₃PO₄. However, when compared to R-PAgT-10:1 and R-PAgT-7:1, the Ag 3d XPS spectra of R-PAgT-5:1 and R-PAgT-3:1 could only be fitted to two peaks at 374.2 and 368.2 eV, indicating the presence of metallic Ag⁰ in R-PAgT-5:1 and R-PAgT-3:1. These findings suggested that Ag⁺ ions in Ag₂O and Ag₃PO₄ were reduced to metallic Ag⁰, further indicating the destruction of the Ag₂O and Ag₃PO₄ components when a larger mass ratio of PAgT to NaBH₄ was employed.

The high-resolution P 2p XPS spectra of the as-synthesized catalysts are illustrated in Figure 10. It is evident that the P species in these catalysts displayed distinct behaviors depending on the mass ratio of PAgT to NaBH₄ used during the synthesis process. When a small mass ratio of PAgT to NaBH4 was employed, the P 2p XPS spectra of R-PAgT-10:1 and R-PAgT-7:1 exhibited a single peak at 133.2 eV, which corresponded to the presence of P⁵⁺ in Ag₃PO₄ [24]. However, in the case of a larger mass ratio of PAgT to NaBH₄, it was difficult to fit the P 2p XPS spectra of R-PAgT-5:1 and R-PAgT-3:1 to any P species, indicating the disruption of the Ag₃PO₄ component when a greater ratio of PAgT to NaBH₄ was employed.

In summary, the XRD and XPS results presented above highlight the substantial impact of the mass ratio of PAgT to NaBH₄ on both the crystal structure and elemental composition of the resulting catalysts. Specifically, R-PAgT-5:1 and R-PAgT-3:1 exhibited compromised crystal structures and an excessive presence of OVs, leading to diminished stability during cyclic experiments. In contrast, R-PAgT-7:1 and R-PAgT-10:1 maintained the same crystal structure and elemental composition as pure PAgT. Furthermore, the presence of Ti³⁺ in R-PAgT-7:1 and R-PAgT-10:1 indicated the successful introduction of OVs into these two catalysts, contributing to the enhanced H₂ evolution rate observed for these samples. Moreover, the relatively robust stability of R-PAgT-10:1 during cyclic experiments can be attributed to the appropriate amount of OVs in this catalyst.

2.3. Different Performances of R-PAgT-7:1 and R-PAgT-10:1

The cyclic experiments conducted with R-PAgT-7:1 and R-PAgT-10:1 revealed that the H₂ evolution rate of R-PAgT-7:1 gradually decreased with extended cyclic time, ultimately falling below that of pure PAgT. In contrast, R-PAgT-10:1 exhibited a different behavior. Remarkably, even after 20 cycles, its performance still outperformed the original PAgT. To elucidate the reasons behind these observations, a comprehensive XRD and XPS analysis was conducted both before and after the use of R-PAgT-7:1 and R-PAgT-10:1, respectively.

As depicted in Figure 11a, after nine cycles, the sample exhibited a lack of discernible peaks around 25.4 and 37.8 degrees, which were attributed to the (101) and (004) planes of the anatase TiO₂ phase. This observation strongly suggested that the crystal structure was compromised, and this occurred after only nine cycles. Furthermore, the chemical state of elements Ti, Ag, O, and P in the post-use R-PAgT-7:1 sample was illustrated in Figure 11b–e. It was apparent that the Ti⁴⁺ and Ag⁺ species presented in pure R-PAgT-7:1 were reduced to Ti³⁺ and Ag⁰ in the sample after nine cycles. Additionally, the presence of Ti-O and Ag-O species, indicative of TiO₂ and Ag₂O, was hardly observable in the sample after nine cycles. These findings strongly suggested that the TiO₂ and Ag₂O components were compromised in the used R-PAgT-7:1. Moreover, the absence of detectable P⁵⁺ peaks related to Ag₃PO₄ in the used R-PAgT-7:1 further supported the conclusion that its structure was disrupted. In summary, the results from XRD and XPS analyses collectively demonstrated the poor stability of R-PAgT-7:1, rendering it unsuitable for sustained H₂ evolution over an extended period.

The XRD patterns for R-PAgT-10:1, both before and after 20 cycles, were presented in Figure 12a. In contrast to R-PAgT-7:1, even after 20 cycles, the peaks corresponding to the (101) and (004) planes of the anatase TiO₂ phase remain distinctly visible in the used R-PAgT-10:1 sample. This observation strongly suggested that the crystal structure of R-PAgT-10:1 remained intact even after 20 cycles. Furthermore, the high-resolution XPS spectra for Ti 2p (Figure 12b) and O 1s (Figure 12c) showed that both the before and after 20 cycles, samples exhibited the same species of Ti (Ti⁴⁺ and Ti³⁺) and O (Ti-OH, Ti-O, Ti-O-P, and Ag-O). Additionally, as seen in Figure 12d,e, the composition of Ag species (Ag⁰ and Ag⁺) and P (P⁵⁺ in Ag₃PO₄) remained consistent in samples both before and after the 20th cycle. These results collectively confirmed the robust stability of R-PAgT-10:1, allowing it to maintain a stable performance during long-term H₂ evolution.

3. Materials and Methods

3.1. Materials

Tetrabutyl titanate (Ti(OC₄H₉)₄, Wako 1st grade, ≥95.0% purity), silver nitrate (AgNO₃, guaranteed reagent, ≥99.5% purity), and silver phosphate (Ag₃PO₄, ≥99% purity) were sourced from FUJIFILM Wako Pure Chemical Industries, Ltd., Ibaraki, Japan. Additionally, ethanol (99.5% purity) from Kishida Chemical Co., Ltd. (Osaka, Japan) was employed as the solvent in the hydrothermal process. HNO₃ (1M) was used for dissolving the silver species. Sodium borohydride (NaBH₄, Wako 1st grade, ≥95.0% purity) served as the reducing agent. Methanol (≥99.7% purity), also obtained from FUJIFILM Wako Pure Chemical Industries, Ltd., Japan, was utilized as a sacrificial agent in the hydrogen evolution reaction. Deionized water was produced using the Milli-Q^® water purification system (Merck, Rahway, NJ, USA).

3.2. Synthesis of the PAgT Photocatalyst

By using a hydrothermal method, the PAgT photocatalyst was synthesized [9]. In a typical procedure, Solution A was obtained by dissolving Ti(OC₄H₉)₄ (6 mL) into ethanol (46 mL) after stirring magnetically for 20 min. Subsequently, solution B was prepared, which consisted of 11 mL of AgNO₃, Ag₃PO₄, and HNO₃ (1M). Solution B was then slowly added dropwise to solution A with stirring vigorously to prepared solution C. The molar ratio of Ti to Ag was maintained at 10:1. Following continuous stirring for an additional 16 h; the solution C was transferred to a Teflon-lined autoclave (100 mL). After heating at 120 °C for 3 h and cooling down to room temperature, the pure PAgT photocatalyst was obtained and stored in the absence of light for subsequent steps.

3.3. Preparation of R-PAgT Composite with Different Mass Ratios of PAgT to NaBH₄

The OV-modified PAgT composite photocatalyst was prepared through the chemical reduction method using NaBH4 as reducing regent. The as-prepared PAgT (0.15 g) was well mixed with a certain amount of NaBH4. To clearly identify the reduced PAgT composites, the resulting products were designated as R-PAgT-X, where X represents the mass ratio of PAgT to NaBH₄. Specifically, the actual mass of NaBH₄ added for each ratio was as follows: 0.05 g for 3:1, 0.03 g for 5:1, 0.021 g for 7:1, 0.015 g for 10:1, and 0.01 g for 15:1. This systematic variation in NaBH₄ content allowed us to investigate its influence on the structural and chemical properties of the R-PAgT composites. Thereafter, the mixture was subjected to calcination in a tube furnace under a nitrogen atmosphere at 300 °C for 2 h, with a heating rate of 10 °C per minute. After cooling to room temperature, the obtained photocatalyst was washed with deionized water three times. Finally, the reduced PAgT powder was collected via centrifugation and was dried.

3.4. Characterizations

The crystal structure of R-PAgT was detected by XRD patterns by Rigaku Altima III Rint-2000 X-ray diffractometer with Cu Kα radiation range of 20–80°. The chemical composition of R-PAgT was measured through FT-IR (FT-IR-6800 JASCO instrument, JASCO Corporation, Tokyo, Japan) and XPS (JPS-9010MC photoelectron spectrometer, JEOL, Tokyo, Japan).

3.5. H₂ Evolution Experiment

The photocatalytic efficiency of the prepared R-PAgT photocatalysts for hydrogen (H₂) evolution was assessed using a closed, custom-built gas system. To simulate solar light, we employed a light source in the wavelength range of 300–780 nm (XC-100; SERIC., Ltd., Koshigaya, Saitama, Japan) positioned at a distance of 5 cm from the experimental setup. In each experimental run, the mixed solution of 7.5 mL methanol and 142.5 mL deionized water was the reaction medium. The dosage of PAgT and R-PAgT catalysts was 1 g/L. Prior to initiating the reaction, nitrogen gas was used to thoroughly replace the air in the system. Then, the system was exposed to simulated solar light (300–780 nm XC-100; SERIC., Ltd.) with an irradiance of 500 W/m² for a duration of 3 h. Following the irradiation, the H₂ gas generated during the reaction was collected at 30 min intervals and subsequently analyzed using a gas chromatography system (GC-8A; SHIMAZU, Tokyo, Japan). The gas chromatography system was equipped with a thermal conductivity detector and a Porapak Q column for the accurate detection and quantification of the produced H₂.

To assess the practical stability of R-PAgT, a series of ten consecutive cycles of H₂ generation experiments were carried out. As for the initial cycle, the procedure mirrored that of the individual experiment previously described. However, starting from the second round, the gaseous products generated in the preceding cycle were purged from the system using nitrogen (N₂) gas without introducing any new catalysts or methanol. This sequence of steps was then repeated for a total of ten cycles to evaluate the sustained performance and stability of R-PAgT.

4. Conclusions

In this research, the OV-modified P/Ag/Ag₂O/Ag₃PO₄/TiO₂ composite photocatalyst (R-PAgT) was prepared through the chemical reduction method using NaBH₄ as the reducing regent. The findings provided critical insights into the impact of varying mass ratios of PAgT to NaBH₄ on the performance and stability of the photocatalyst. The optimized composite, R-PAgT-10:1, achieved a significantly enhanced H₂ evolution rate compared to the pure PAgT. This improvement was attributed to the moderate introduction of OVs, which effectively enhanced light absorption and charge separation. However, a higher mass ratio of PAgT to NaBH₄, specifically in the cases of R-PAgT-5:1 and R-PAgT-3:1, led to excessive OVs and the destruction of the crystal structure. These adverse effects resulted in diminished stability and reduced photocatalytic performance. Furthermore, R-PAgT-10:1 demonstrated remarkable stability, maintaining its enhanced H₂ evolution rate over 20 cycles under visible light irradiation. In conclusion, our study highlighted the significance of introducing OVs into the PAgT composite to enhance its H₂ evolution efficiency under intense solar light irradiation. Furthermore, the appropriate mass ratio of PAgT to NaBH₄ can induce a moderate amount of OVs, which is beneficial for achieving superior and stable H₂ evolution. These findings hold promise for the development of efficient and robust photocatalysts for renewable energy applications.