A Gold-Inserted Zinc Rhodium Oxide and Bismuth Vanadium Oxide Heterojunction Photocatalyst for Overall Water Splitting Under Visible Light
Authors: Kamijo Kento , Kobayashi Ryoya , Irie Hiroshi 
Published: February 2020

Environmental and energy problems has been deteriorating due to mass consumption of the modern fossil fuel. Hydrogen (H2) has attracted growing attention as a next-generation clean and sustainable energy as it has potential to solve these problems[1]. One of the methods for clean H2 production includes a water-splitting reaction using the solar light and a photocatalyst originated from the first report of the Honda-Fujishima effect in 1972[1]. However, TiO2 and most of other photocatalysts respond to only UV light. The energy of the UV light occupies only 4 % of the total solar energy whereas the visible light (>400 nm) about half. So, in order to utilize the solar energy efficiently, numerous researches of the photocatalysts to utilize abundant visible light have been performed enthusiastically. Among such researches, a Z-scheme system is one of the methods to progress overall water-splitting under visible light irradiation[2]. Our group has recently reported the Z-scheme photocatalyst in which silver (Ag) was inserted between zinc rhodium oxide (ZnRh2O4, Eg=1.2 eV) and bismuth vanadate (Bi4V2O11, Eg=1.6~2.2 eV) as a H2 evolution photocatalyst and an O2 evolution photocatalyst, respectively (ZnRh2O4/Ag/Bi4V2O11)[3]. This photocatalyst was demonstrated to accomplish the overall-water splitting under visible light up to a wavelength of 700 nm. However, in this system, nitric acid treatment is required for removing the excess Ag to achieve overall pure-water splitting, because Ag acts as a sacrificial agent for O2 evolution. Such treatment damaged ZnRh2O4 and Bi4V2O11, leading to decreased photocatalytic activity. To avoid such damage and potentially further improve the activity, we replaced Ag with gold (Au), because Au is stable and does not act as a sacrificial agent. We have prepared the ZnRh2O4/Au/Bi4V2O11 photocatalyst as follows; To insert gold nanoparticles between the two photocatalysts efficiently, ZnRh2O4/Au/Bi4V2O11 was prepared by the two-step calcining method. The first step is that Bi4V2O11 and gold nanoparticle (AuNP; particle size:~100 nm) powders were mixed at a molar ratio of AuNP : Bi4V2O11= 1.5:2. The mixture was calcined at 850°C for 2 h under atmospheric conditions, and then thoroughly ground to obtain AuNP-loaded bismuth vanadate (Au/Bi4V2O11). Subsequently, this composite (Au/Bi4V2O11) and ZnRh2O4 powders were mixed at the ratio of ZnRh2O4 : AuNP : Bi4V2O11 = 1 : 0.15 : 0.2, and the resulting mixture was calcined at 850°C for 2 h under atmospheric conditions. For comparison, ZnRh2O4/Au/Bi4V2O11 was prepared by the one-step calcining method. ZnRh2O4, AuNP, and Bi4V2O11 powders were simultaneously mixed at a ratio of ZnRh2O4 : AuNP : Bi4V2O11 = 1 : 0.15 : 0.2. The mixture was heated at 850°C for 2 h. The sample was characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), and water splitting test. Z-scheme water-splitting reactions were carried out in a gas circulation system and amount of produced gases were measured by an online gaschromatography. In terms of water splitting test, 60mg of sample was suspended using a magnetic stirrer in 12 mL of pure water (distilled water without adding any chemicals). Argon gas (50 kPa) was induced into the system after deaeration. The reaction cell was illuminated by Xe lamp with a Y-44 cut-off filter (λ>420 nm) . ZnRh2O4/Au/Bi4V2O11 was confirmed to have three phases of Au, ZnRh2O4, and Bi4V2O11 using XRD. According to SEM observation, the Bi4V2O11 and ZnRh2O4 particles were distinguishable and were estimated to have sizes of ~3 μm and ~100 nm, respectively. Figure 1 shows the result of overall water splitting test over ZnRh2O4/Au/Bi4V2O11 obtained by the two-step calcining method under visible light ( >420 nm). H2 and O2 were evolved at a molar ratio of 2 : 1. So, this system was able to split pure water under visible light. By the analogy with ZnRh2O4/Au/Bi4V2O11, H2 and O2 were evolved by two-step photoexcitation over the ZnRh2O4 and Bi4V2O11 and charge transfer through the Au. In contrast, ZnRh2O4/Au/Bi4V2O11 obtained by the one-step calcining method could not evolve H2 and O2 at the molar ratio of 2 : 1 (not shown). Thus, the two-step method is more suitable to insert Au between ZnRh2O4 and Bi4V2O11. Ideally, the particle layers should be as small as possible to minimize the number of grain boundaries, which decrease the resistance to charge transfer. To further enhance the photocatalytic activity, we are now trying to use smaller Au.
[1] A. Fujishima et al., Nature, 238, 37 (1972) [2] K. Sayama et al., J. Photo. Photo. A: Chem., 148, 71 (2002) [3] H. Irie et al., J. Mater. Chem. A, 4, 3061 (2016)

Figure 1