Over All Water Splitting By Anti-Photocorrosive Core-Shell Composite Sulfide Photocatalyst Synthesized Via Acid Dissolution Process
Authors: Nagakawa Haruki , Nagata Morio 
Published: December 2020

Some of the most significant challenges of the 21st century are related to energy exhaustion and global warming. In order to overcome these issues, it is necessary to produce sustainable energy without carbon dioxide emissions. Since the discovery of the Honda-Fujishima effect (H2O + 2h
ν → 1/2 O2 + H2) in 1972, photocatalytic water splitting has attracted considerable attention as a solution to energy exhaustion. Since hydrogen is a clean source of energy that does not generate carbon dioxide when employed, water splitting has been considered an effective measure against global warming. The most commonly studied photocatalyst is TiO2. However, hydrogen production from water using TiO2 requires ultraviolet light, which deems it inefficient. In order to achieve a high conversion efficiency, which is considered suitable for practical applications, it is necessary to achieve a minimal absorption of visible light. To satisfy this requirement, a visible-light-responsive photocatalyst should be employed in the water splitting process. However, many of the photocatalysts with high visible light responsiveness, such as sulfide semiconductors and nitride semiconductors, have photocorrosive issues. This is an issue where hole carriers generated by photo-excitation cause the oxidative decomposition of the photocatalyst itself.
To date, two approaches have been reported to suppress photocorrosion. The first is to transfer photocorrosive holes to different photocatalysts. The transfer of holes to the valence band of another photocatalyst has been reported. (H. Nagakawa et al., ACS Omega, 2018, 3, 10, 12770-12777.) In the previous study, the hydrogen generation efficiency was increased by transporting the excited electrons and holes from CdS to SiC. This approach successfully suppressed the photocorrosion of CdS. The second photocorrosion suppression method is to protect the photocorrosion-susceptible photocatalyst with different stable photocatalysts, thereby reducing exposure to the solution and preventing elution. The current authors reported that this method promoted the water splitting reaction in a CdS composite photocatalyst. (H. Nagakawa et al., ACS Applied Energy Materials, 2018, 1, 12, 6730-6735.) In the study, we achieved an overall water splitting under visible light using a home-synthesized tri-composite, in which CdS was covered with a photocatalyst on both the oxidation and reduction sides. Previously, the use of CdS/SiC/TiO2 tri-composite photocatalysts was found to promote the consumption of holes on CdS and reduce the exposure of CdS to the solution, thereby suppressing photocorrosion. However, since oxidation of water cannot be achieved with the oxidizing power of SiC, in this study, we employed WO3 instead of SiC and realized stronger oxidizing power by constructing a Z-scheme. In addition, we employed CdWO4 as a photocatalyst on the reduction side in order to enable the transport of electrons excited on CdS to the conduction band of CdWO4. A highly controlled CdS/CdWO4 core–shell composite was synthesized by a simple method via the dissolution of CdS under acidic conditions. The eluted Cd2+ reacted with the dissolved WO4
2- in solution in situ, allowing selective deposition on the CdS surface for uniform shell synthesis. (H. Nagakawa et al., RSC Advances, 2020, 10, 1, 105-111.) As a result, recombination was suppressed and the hydrogen production ability was improved compared to that of the dispersion system of CdS and WO3. Furthermore, water splitting was achieved by reducing the CdS exposure and suppressing photocorrosion.
Hydrogen evolution experiments were conducted in vial bottles under Ar flow. First, 1 wt % Pt was deposited on the prepared photocatalyst using H2PtCl6·6H2O through photo-deposition under visible light (λ > 420 nm) in 30 vol% methanol aqueous solution. The synthesized CdS/WO3/CdWO4-Pt composite photocatalyst (0.03 g) and pure water (15 mL) were prepared in the reaction cell. A Xe lamp with a UV cutoff filter (λ > 420 nm) was used as a visible-light source. A 0.1 mL aliquot of the evolved gas was collected using a gastight syringe, and the sampled gas was injected into the gas chromatograph to determine the amounts of evolved hydrogen and oxygen. As a result, hydrogen and oxygen were constantly produced at a ratio of 2 : 1 while suppressing photocorrosion. This is the first report of water splitting by a powder-based photocatalyst using CdS under visible light.

Figure 1