Photocatalyst Design Principles for Photocatalytic Hydrogen Production and Benzyl Alcohol Oxidation with CdS Nanosheets

doi:10.21203/rs.3.rs-4876584/v1

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Photocatalyst Design Principles for Photocatalytic Hydrogen Production and Benzyl Alcohol Oxidation with CdS Nanosheets

https://doi.org/10.21203/rs.3.rs-4876584/v1

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To produce hydrogen using photocatalysts while maintaining environmentally friendly characteristics, research has focused on using benzyl alcohol (BzOH) as a hole scavenger to convert it into the valuable compound benzaldehyde. However, due to the relatively slow oxidation rate of BzOH compared to conventional hole scavengers, tailored photocatalyst designs are necessary. In this study, we prepared CdS nanosheets (NSs) and introduced Na₂SO₄-Na₂SO₃ and BzOH as hole scavengers to adjust the oxidation half-reaction rate (OHR) during the photocatalytic reaction. Various co-catalysts were introduced to examine how changes in the reduction half-reaction (RHR) and OHR rates influence hydrogen production. The results reveal that the selection of co-catalyst and hole scavenger significantly influences the rate-determining step (RDS) in the photocatalytic reaction. For bare CdS NSs, the slow RHR results in the RDS being the RHR, leading to similar hydrogen production rate regardless of the scavenger type. However, with Pt as a co-catalyst, the RDS shifts to the OHR due to the accelerated RHR, inducing hydrogen production rate highly sensitive to the type of scavenger. Consequently, hydrogen production is significantly reduced when using BzOH, which has a slower oxidation rate. These findings suggest that achieving high hydrogen yields is fundamentally challenging with BzOH due to its slow oxidation rate, even with the introduction of excellent co-catalysts. Therefore, it is imperative to develop photocatalyst materials with lower valence band level for high oxidation power.

Photocatalysis

CdS Nanosheets

Co-Catalysts

Hydrogen Production

Benzyl Alcohol Oxidation

Photocatalytic water splitting presents a promising route for sustainable hydrogen production, leveraging solar energy without the emission of carbon dioxide or the need for external energy inputs.[1, 2] Despite its potential, the commercial viability of this process is hindered by its inherently low efficiency. Various photocatalyst materials and structures have been developed, but the solar-to-hydrogen (STH) efficiency, which represents the conversion rate of solar energy to hydrogen energy, remains at only 1–2%.[3, 4] Recently, research results with STH efficiencies exceeding 9% have been reported, but these are limited to cases where concentrated solar light is used or the reactions are conducted at high temperatures.[5, 6] The fundamental limitation of low efficiency lies in the restricted range of the solar spectrum that can meet the chemical energy requirements for water splitting. Thermodynamically, the overall water splitting reaction requires a minimum energy of 1.23 eV, and photons with energy higher than this are abundant in the solar spectrum. However, from a kinetics perspective, the oxygen evolution reaction (OER), which requires multiple holes, demands photons with much higher energy.[7]

Photocatalysts with large band gaps that absorb UV light can drive the efficient OER but can only utilize the UV portion of the solar spectrum, which accounts for approximately 5% of the total solar energy, thus limiting their overall efficiency. Conversely, photocatalysts with smaller band gaps that can absorb visible light are unable to provide the necessary energy for complete overall water splitting. This trade-off between solar absorption and overall water splitting ability poses a fundamental challenge that prevents photocatalysts from achieving high efficiency.[8] As a result, most systems focus solely on the hydrogen evolution reaction (HER), using visible light-absorbing photocatalysts to maximize solar absorption while substituting the OER with the oxidation reaction of hole scavengers, known as sacrificial agents.[9, 10] These sacrificial agents are typically selected for their ability to rapidly oxidize by consuming the remaining holes in photocatalysts, thereby preventing photo-corrosion and sustaining the hydrogen production process. However, the use of hole scavengers undermines the environmental benefits of photocatalytic hydrogen production, as they are consumed during the reaction and need continuous replenishment.

To address this, recent research has concentrated on using reactants that act as hole scavenger and yield valuable products through the oxidation half-reaction (OHR), thus adding economic value while maintaining environmental benefits.[11] One notable example is the conversion of benzyl alcohol (BzOH) into benzaldehyde (BA) while simultaneously producing hydrogen from water.[12–14] BzOH can serve as a hole scavenger, producing BA—a compound with significant industrial applications in perfumes, dyes, and pharmaceuticals. Researchers have implemented multiple strategies, including the introduction of heterostructures and novel co-catalysts, to enhance both hydrogen production and value-added products.[11] However, these methods often adopt development strategies designed for traditional photocatalytic systems that use conventional hole scavengers, which facilitate rapid oxidation reactions, without accounting for the slower oxidation rate of BzOH. Consequently, this approach does not address the specific challenge posed by slower oxidation reaction of BzOH, requiring new design principles for photocatalysts that can effectively manage this rate.[15, 16] Understanding how changes in the oxidation rate affect the overall photocatalytic reaction is crucial for optimizing both hydrogen production and BzOH oxidation.

In this study, we investigated the effects of using different hole scavengers, specifically Na₂SO₄-Na₂SO₃ and benzyl alcohol (BzOH), to artificially adjust the OHR rate and its impact on photocatalytic hydrogen production. Additionally, we introduced various co-catalysts to observe how changes in the OHR and reduction half-reaction (RHR) rates affect hydrogen production. Ultimately, our goal is to provide insights into designing photocatalysts that can effectively utilize BzOH as a sacrificial agent, despite its relatively slower oxidation rate, to produce valuable by-products while optimizing hydrogen production.

2.1 Chemicals

The following chemicals were purchased from Sigma-Aldrich and used without further purification: cadmium acetate dihydrate (98%), oleic acid (OA, 90%), sulfur (S), sodium sulfide nonahydrate (Na₂S·9H₂O, ≥ 98%), N-methyl formamide (NMF, 99%), chloroplatinic acid hexahydrate (H₂PtCl₆, ≥ 37.5%), Ruthenium(III) chloride hydrate (RuCl₃·xH₂O), benzyl alcohol (BzOH, 99.8%), sodium sulfate (≥ 99%), sodium sulfide. Octadecene (ODE, 90%) was purchased from Thermo Fisher Scientific. Acetone, ethanol and hexane were purchased from Daejung. Chloroform (99.5%) was purchased from Samchun.

2.2 Synthesis of CdS nanosheets (NSs)

CdS nanosheets were synthesized following a method from the literature by Delikanli et al. with slight modifications.[17] Prior to synthesizing CdS NSs, 0.1 M ODE-S solution was prepared. Briefly, in a three-neck flask, 50 ml ODE solution containing 0.1679 g S was heated to 125 ℃ to form clear solution. After that, 1.0809 g of cadmium acetate dihydrate, 10 mL of 0.1M of ODE-S, 1.2 mL of OA, and 50 mL of ODE were loaded into a three-neck flask. The solution was stirred under an Ar atmosphere for 30 min at room temperature. Then, the mixture was degassed under vacuum at 85°C for 1 h. After degassing, the temperature was increased to 260°C at a rate of 12°C/min and maintained for 3 min before quenching the reaction. The resulting CdS NSs were precipitated using acetone as an anti-solvent via centrifugation at 5000 rpm for 5 min. The resulting precipitates were dispersed in chloroform, washed twice with ethanol, and stored in hexane.

2.3 Ligand exchange

Ligand exchange was performed by as described in previously reported method.[18] First, the 0.6127 g Na₂S·9H₂O dissolved in 50 ml NMF. Then, 10 ml of CdS NSs diluted in hexane was mixed with 10 ml of the prepared ligand solution in a 20 ml vial under ambient conditions. The mixture was then vigorously stirred for 3 h until the yellow-colored CdS NSs transferred from the hexane (organic phase) to the NMF (aqueous phase). The S²⁻ passivated CdS NSs were collected by centrifugation at 7500 rpm for 10 min after adding acetone. This process was repeated three times, and the final product was redispersed in water.

2.5 Photodeposition of co-catalyst on CdS NSs

The photodeposition method was used to deposit co-catalysts on CdS NSs. To deposit Pt and Ru as co-catalysts, H₂PtCl₆ and RuCl₃·xH₂O were used as precursors, respectively, and dissolved in water where CdS NSs were dispersed at a concentration of 10 mg/ml. The concentrations of H₂PtCl₆ and RuCl₃·xH₂O were adjusted to achieve a co-catalyst loading of 1 wt% relative to the CdS NSs. A mixture of 0.1 M sodium sulfate and 0.1 M sodium sulfide and a 0.1 M BzOH solution were used as hole scavengers. The mixture was irradiated for 1 hour using a solar simulator that provides 1 sun, 1.5 AM conditions to facilitate the growth of the co-catalysts.

2.6 Photocatalytic Hydrogen Evolution

First, S²⁻-capped CdS NSs were prepared in two different 12 ml aqueous solutions at a concentration of 10 mg/ml, each containing 0.1 M of either Na₂SO₄-Na₂SO₃ and BzOH as hole scavengers. The prepared photocatalyst solution contained in a gas-tight sealed vial was purged under Ar flow for 30 min in the dark and then irradiated under 1.5 AM light using a solar simulator for 2 hours for photocatalytic reaction. The generated hydrogen was detected using a microsensor (UniAmp x-5, UNISENSE). The microsensor was always calibrated before the experiment using a nitrogen/hydrogen mixed gas with 5% hydrogen.

2.7 Transient absorption spectroscopy

The pump and probe beams were derived from a pulsed regeneratively-amplified Ytterbium-doped potassium gadolinium tungstate (Yb) laser system (Pharos, Light Conversion), which emits photon pulses at a wavelength of 1028 nm, with a pulse duration of 196 fs, and operates at a repetition rate of 200 kHz. This laser served as the primary source for an Optical Parametric Amplifier (OPA) fitted with a second harmonic module (Orpheus, Light Conversion) to generate the pump beam. The pump beam was set to a wavelength of 320 nm and had a beam radius of roughly 130 µm. To prevent the generation of multi-excitons, the pump pulse intensity was controlled to be approximately 2 µJ/cm². A fraction of the primary beam, used as the probe beam, was directed through a delay line and focused onto a sapphire crystal to create a white-light continuum for transient absorption (TA) measurements (Harpia, Light Conversion). The probe beam, with a beam radius of 105 µm, was then focused onto the sample, overlapping with the pump beam area. After passing through the sample, the probe beam was collected by a detector (Kymera 193i, Andor).

2.8 Characterization

Absorption spectra of CdS NPLs were recorded using UV-vis spectrophotometer (Genesys 150, thermoscientific). Photoluminescence were obtained by spectrofluorometer (FP-8550ST, JASCO). The crystal structure of prepared CdS NPLs was analyzed by X-ray Diffractometer (XRD, LabX XRD-6100, Shimadzu).

3.1 Preparation and Characterization of CdS Nanosheets

To systematically investigate the impact of changes in the RHR and OHR on photocatalytic activity, it is essential to first prepare a reliable photocatalyst material with consistent energy levels and light absorption properties. We synthesized CdS NSs with uniform atomic-level thickness, ensuring a consistent bandgap and uniform light absorption. Figure 1a shows the absorption and photoluminescence (PL) spectra of the prepared CdS NSs. The absorption spectrum exhibits a distinct peak at 413 nm, indicating band-edge absorption and the successful synthesis of 6.5 ML CdS comprising seven cadmium layers and six sulfur layers.[18] The PL spectrum shows a slight band-edge emission at 430 nm due to Stokes shift, with most of the emission appearing as a broad trap-related emission from 450 nm to 800 nm, likely due to the presence of trap sites inevitably caused by the large lateral area.[19] Further analysis through XRD measurements confirmed the formation of a zincblende structure, essential for the growth of CdS in a two dimentional morphology.[20] Transmission electron microscopy (TEM) images revealed that the CdS had indeed grown into a two-dimensional nanosheet morphology. Due to high van der Waals forces, these nanosheets tend to curl, forming nanoscrolls.

For the prepared CdS NSs to be effective in hydrogen production with photocatalytic reaction, it was necessary to modify their surface from hydrophobic to hydrophilic to ensure colloidal stability in water. The original oleic acid capping ligands, which imparted hydrophobic characteristics, were replaced with S^2- atomic ligands through a ligand exchange process.[18] These shorter atomic ligands facilitate charge transfer to the reactants during the photocatalytic reaction, thereby enhancing photocatalytic activity, while minus charge of S^2- ensure colloidal stability. As shown in Figure S1, after the ligand exchange, the CdS NSs were successfully transferred from a non-polar solvent (hexane) to a polar solvent (NMF) and dispersed well, confirming the successful ligand exchange to S^2- ligands. Significant changes in optical properties were observed in the absorption spectrum after the ligand exchange, including a red shift and broadening of the 1s peak as shown in Figure S1. The red shift is attributed to the weakening of quantum confinement due to the addition of an extra sulfur layer to the originally cadmium-terminated 6.5 ML structure.[21] The broadening is caused by electron and hole wavefunction leakage and changes in surface strain, resulting from the replacement of bulky oleic acid capping ligands with atomic ligands.[22, 23] These changes in surface strain are evident in the transformation of the morphology from curled nanoscrolls to flat sheets, as shown in Figure 1d.

3.2 Photo-deposition of Co-catalysts

To investigate how changes in the RHR rate affect the influence of different hole scavengers, we introduced Pt and Ru, known as excellent co-catalysts for hydrogen production, into the CdS NSs.[4, 9] The CdS NSs were dispersed in a solution containing a hole scavenger and metal precursors, followed by light irradiation. This process, known as photodeposition, allows the reduction of metal cations by conduction band electrons, leading to the growth of metal nanoparticles on the surface of CdS NSs, while the remaining holes in the valence band are consumed by the hole scavenger.[24] As shown in FigureS2, after the photodeposition process, we observed an additional broad absorption feature extending from the NIR to the visible region, attributed to the d-sp interband transition of the metals.[25] XPS measurements provided further confirmation of the successful growth of metal nanoparticles on CdS NSs as shown in Figure 2a, b. For Pt/CdS NSs, the Pt 4f_7/2 and 4f_5/2 peaks, originating from spin-orbital splitting, were observed at 72.2 eV and 75.5 eV, respectively, indicating the formation of a metallic Pt-CdS heterostructure.[26, 27] In the XPS measurements of Ru/CdS NSs, the Ru 3d_5/2 peak was clearly observed at 279.6 eV.[28] Despite the overlap of the Ru 3d_3/2 signal at approximately 284.8 eV with the C 1s signal around 284 eV, deconvolution successfully distinguished between the two, confirming the successful growth of metallic Ru on the CdS NSs.

3.3 Transient Absorption Spectroscopy and Bleaching Recovery

Typically, when metal nanoparticles are introduced as co-catalysts in semiconductor nanoparticles, the electrons generated in the photocatalyst are rapidly transferred to the co-catalyst, facilitating charge separation and enhancing the photocatalytic reaction.[9] To examine this electron transfer, we conducted femtosecond laser-based transient absorption experiments. In these measurements, the bleaching recovery kinetics of the 1S transition reflect the carrier population at the band edge. Specifically, for cadmium chalcogenides, the conduction band is two-fold degenerate while the valence band is six-fold degenerate, making the bleaching recovery kinetics of 1S transition sensitive to electron population.[29] Thus, analyzing the bleaching recovery kinetics of the 1S peak allows us to exclusively monitor electron dynamics.

Figures 2c-e display the transient absorption spectra of bare CdS NSs, Pt/CdS NSs, and Ru/CdS NSs as a function of delay time. For all samples, a bleaching signal near 450 nm was observed, corresponding to the band-edge position (1S transition) after ligand exchange. Over time, this bleaching gradually recovers. Notably, the bleaching recovery occurs more rapidly after the introduction of the co-catalyst, which is due to the electrons in the conduction band being transferred to the co-catalyst. To quantitatively analyze bleaching recovery, we plotted the kinetics of the bleaching recovery at the band-edge transition region, as shown in Figure 2f. This data can be fitted to a tri-exponential equation (See Table S1). For bare CdS NSs, we observed rapid decay components with time constants of a few picoseconds (τ1) and several hundred picoseconds (τ2), followed by a slower decay on the order of nanoseconds (τ3). Given that radiative recombination in nanosheets or nanoplatelets occurs on a nanosecond timescale, the fast decays are attributed to electron trapping.[30] Upon introducing metal co-catalysts, the amplitude of the fast decay component increased significantly, irrespective of the metal type, indicating electron transfer from CdS NSs to the metal. Notably, the amplitude of this fast decay component increased by 24% for Pt/CdS NSs and by 34% for Ru/CdS NSs, suggesting that Ru acts as a more efficient electron acceptor. This is due to differences in the Schottky barrier height (SBH) arising from the work function differences between the metals.[31] When a metal forms a junction with an n-type semiconductor like CdS, band bending occurs at the interface due to Fermi level equilibrium, resulting in the formation of a Schottky barrier. In spherical nanoparticles, there is no physical space to create a depletion region to induce band bending, resulting in a flat band potential and merely a shift in band-edge energy levels. However, for nanosheets, their lateral 2D morphology allows for band bending and Schottky barrier formation. Ru, with a work function of -4.5 eV, has a lower SBH compared to Pt/ which has a work function of -5.1 eV, facilitating more efficient electron transfer from CdS to Ru than to Pt.

3.4 Photocatalytic Hydrogen Evolution

To investigate the effect of different co-catalysts and hole scavengers on the hydrogen production reaction, we conducted photocatalytic experiments using bare CdS NSs and CdS NSs with Pt and Ru cocatalysts. In this experiment, CdS NSs were dispersed in water at a concentration of 10 mg/mL and exposed to 1 sun illumination at 1.5 AM as shown in Figure S3. The amount of hydrogen produced through the photocatalytic reaction was monitored in real-time using a microelectrode. To facilitate the production of valuable products from the oxidation reaction, BzOH was introduced as hole scavenger. For comparision, we also introduced mixture of NaSO₄-NaSO₃, which are commonly used as hole scavengers.[9]

Figure 3 illustrates the hydrogen production rate over time for experiments using various types of co-catalysts and different scavengers. Interestingly, the hydrogen production rate varied significantly depending on the type of hole scavenger and co-catalyst. In the case of bare CdS NSs, the results demonstrate that the hydrogen production rate is 870 μmol/g/h when using hole scavengers and 991 μmol/g/h when using BzOH, indicating comparable hydrogen production in both cases. When Pt was used as the co-catalyst, hydrogen production increased by approximately 3 times with BzOH as the hole scavenger, while using the Na₂SO₄-Na₂SO₃ mixture resulted in a hydrogen production rate of 4,554 μmol/g/h, which is about 5 times higher than that of bare CdS NSs, demonstrating a dramatic increase in hydrogen production. When Ru was used as the cocatalyst, hydrogen production increased compared to bare CdS NSs, but it was generally lower than that with Pt, regardless of the type of hole scavenger used. Additionally, hydrogen production rate was similar when using Na₂SO₄-Na₂SO₃ and BzOH as hole scavengers.

Understanding why such significant differences in hydrogen production occur based on the type of cocatalyst and scavenger requires a deep understanding of the photocatalytic mechanism of hydrogen production on CdS NSs in the presence of hole scavengers. Initially, without a co-catalyst, when the photocatalyst absorbs light and generates excitons, electrons are generated in the conduction band and holes in the valence band. The electrons participate in the reduction reaction to produce hydrogen, while the holes react with the hole scavenger, oxidizing it and consuming the holes. This full reduction-oxidation reaction cycle ensures that no charge remains on the photocatalyst, maintaining its neutrality and enabling a sustainable photocatalytic reaction. If the reduction reaction is slower than the oxidation reaction, the holes on the CdS NSs are quickly consumed, and the hydrogen production reaction becomes the rate-determining step (RDS) as depicted in Scheme 1a. This is the case for bare CdS NSs, which have a relatively slow hydrogen production rate due to the absence of a co-catalyst.

The redox potential of the Na₂SO₄-Na₂SO₃ mixture is -0.104 V vs. NHE, and that of BzOH is 1.98 V vs. NHE, indicating that oxidation process of Na₂SO₄-Na₂SO₃ mixture is faster than that of BzOH.[15, 16] Generally, hole scavengers with a faster oxidation half-reaction are expected to enhance photocatalytic activity, but since the reduction half-reaction for hydrogen production is the RDS, the type of scavenger does not significantly impact the hydrogen production rate, resulting in comparable hydrogen production characteristics as shown in Figure 3c.

Introducing Pt significantly increases the hydrogen production rate due to excellent catalytic properties of Pt, resulted by the Gibbs free energy for proton absorption/desorption being close to zero.[32] In the case of Pt/CdS NSs, electrons generated in the conduction band are rapidly transferred to Pt, where they are quickly consumed in the hydrogen production reaction, leaving the CdS NSs rich in holes in constrast to case of bare CdS NSs. This change shifts the RDS from the RHR to the OHR, indicating that type of scavenger is sensitive to hydrogen generation rate as depicted in Scheme 1b. Indeed, using Na₂SO₄-Na₂SO₃ as the hole scavenger with Pt/CdS NSs results in the highest hydrogen production, whereas the slower oxidation rate of BzOH leads to significantly lower hydrogen production. This demonstrates that when a co-catalyst for efficienct hydrogen production is introduced, the oxidation rate of the hole scavenger becomes critically important.

Lastly, the higher hydrogen production observed for Ru/CdS NSs compared to bare CdS NCs indicates a clear catalytic effect. However, despite the transient absorption (TA) measurements showing more efficient electron transfer from CdS NSs to Ru than to Pt, the hydrogen generation rate is lower with Ru compared with that of Pt. This discrepancy is attributed to the relatively lower catalytic properties of Ru, due to the imbalance in Gibbs free energy for proton absorption/desorption.[32] This lower catalytic activity of Ru compared to Pt suggests that, similar to bare CdS NSs, the RDS shifts back from the OHR to the RHR as shown in Scheme 1c. In fact, the hydrogen production rate remains consistent regardless of the type of hole scavenger, supporting the shift of the RDS to the RHR.

Our findings underscore the complexity of optimizing photocatalytic systems for hydrogen production. Unlike systems using conventional hole scavengers, even the most effective co-catalysts cannot prevent the OHR from becoming the RDS when BzOH is introduced, thereby limiting hydrogen production. This suggests that merely developing co-catalysts with high hydrogen production capability or enhancing charge separation is insufficient when BzOH is used. Instead, it is crucial to design photocatalysts that can significantly increase the OHR rate. Implementing photocatalysts with higher oxidation power, even with the same bandgap, can effectively shift the RDS away from the OHR. This study provides key insights into designing advanced photocatalysts that can manage varied reduction and oxidation rates, thereby optimizing both hydrogen production and the generation of valuable by-products.

3.5 Stability of photocatalyst depending on scavanger type

We conducted stability tests on bare CdS NSs, Pt/CdS NSs, and Ru/CdS NSs by using Na₂SO₄-Na₂SO₃ and BzOH as scavengers during the hydrogen genration reaction. After the photocatalytic reactions, XPS analysis was performed to assess the stability of each sample as shown in Figure 4. The XPS spectra displayed characteristic peaks around 161 eV (S 2p_3/2) and 162 eV (S 2p_1/2), corresponding to binding energy of Cd and S, and a higher energy peak around 165 eV, which is indicative of CdSO₄ formation due to photo-corrosion of CdS NSs as a result of self-oxidation. As summarized in Figure 4d, we calculated the ratio of CdSO₄ to CdS peak areas to quantify the extent of oxidation, where a higher ratio signifies more extensive oxidation. The results revealed that Pt/CdS NSs, which had the highest hydrogen production rate, exhibited the least oxidation overall. In contrast, bare CdS NSs showed significant oxidation, corresponding to their lower hydrogen production rate, leading increased residual carrier in the CdS NSs.

A significant observation was the substantial increase in oxidation when BzOH was used as hole scavenger compared to Na₂SO₄-Na₂SO₃ across all samples. This increase is attributed to the slower oxidation rate of BzOH, which causes holes to remain on the CdS surface for a longer duration, leading to increased self-oxidation. This self-oxidation was evident from the color change of the photocatalyst material. After conducting photocatalytic experiments with BzOH, the originally yellow CdS NSs sample became a slightly darker yellow due to self-oxidation, whereas there was almost no change in the samples using Na₂SO₄-Na₂SO₃ as shown in Figure S4. Consequently, ensuring the stability of CdS NSs becomes more challenging with BzOH due to its slower oxidation rate compared to faster scavengers like Na2SO4-Na2SO3. Given the necessity of rapid hole removal for stability, designing photocatalysts with high oxidation power is crucial. Lowering the valence band level to enhance the oxidation power of photocatalysts can be a key strategy. This approach not only improves hydrogen production rates but also minimizes self-oxidation

In this study, we have demonstrated that the introduction of different co-catalysts and hole scavengers significantly impacts the photocatalytic hydrogen production efficiency of CdS nanosheets. Our results show that while Ru co-catalysts facilitate more efficient electron transfer, Pt co-catalysts achieve higher hydrogen yields due to their superior catalytic properties. Furthermore, using benzyl alcohol (BzOH) as a hole scavenger not only supports hydrogen production but also produces valuable benzaldehyde, adding economic value to the process.

A key insight from our study is the recognition that the oxidation half-reaction often becomes the rate-determining step (RDS) when using non-conventional hole scavengers, even with highly effective co-catalysts. This finding suggests that merely developing co-catalysts for enhanced hydrogen production or improving charge separation may not suffice. Instead, a strategic focus on designing photocatalysts with higher oxidation power, capable of significantly accelerating the oxidation half-reaction, is essential. This approach ensures that the RDS is not limited by the oxidation process, thereby optimizing the overall efficiency of the photocatalytic system.

Our research emphasizes the need for a balanced approach to reaction kinetics, highlighting that effective photocatalyst design must consider both the reduction and oxidation rates. By providing a comprehensive understanding of how co-catalysts and hole scavengers affect these processes, our study offers valuable guidelines for developing advanced photocatalysts. These insights are crucial for achieving more efficient and sustainable hydrogen production while generating valuable by-products, thereby paving the way for the next generation of photocatalytic technologies.

Supporting information

Additional experimental details and characterizations,

Author information

Corresponding authors

* Whi Dong Kim: [email protected]

Notes

The authors declare no competing financial interests.

ACKNOWLEDGMENTS

This study has been conducted with the support of the Korea Institute of Industrial Technology as "Development and commercialization for clean hydrogen production/storage and CO₂ monitoring system in the field of industrial complex (KITECH EH-24-0007)".

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CdSH2BzOHWDKSI.docx

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Photocatalyst Design Principles for Photocatalytic Hydrogen Production and Benzyl Alcohol Oxidation with CdS Nanosheets

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Abstract

Figures

1. Introduction

2. Experimental

2.1 Chemicals

2.2 Synthesis of CdS nanosheets (NSs)

2.3 Ligand exchange

2.5 Photodeposition of co-catalyst on CdS NSs

2.6 Photocatalytic Hydrogen Evolution

2.7 Transient absorption spectroscopy

2.8 Characterization

3. Results and discussion

4. Conclusion

Declarations

Supporting information

Notes

References

Supplementary Files

Status:

Version 1