Visible Light-Sensitive Yttrium-Doped Indium Vanadate Photocatalysts for Hydrogen Evolution

Solar hydrogen (H 2 ) production using a photocatalyst through water splitting is one of the key technologies for utilization of renewable energy and environmental protection [1] . So, there have been extensive researches over decades for powdered photocatalysts due to the simplicity of the process and the possibility for large-scale H 2 production. However, most photocatalysts reported so far respond only to ultraviolet (UV) light, and thus highly-active and visible-light sensitive photocatalysts have been sought for an effective use of solar energy. The approaches used in these studies can be classified into two categories. The first is to control the band structure of wide-gap photocatalysts by doping foreign elements or making solid solutions to be sensitive to visible light [2] , and the second is to combine the two-types of visible-light sensitive photocatalysts in the presence or absence of a sacrificial agent, so called Z-scheme system [3-5] . Regarding the first approach, the solid solution of bismuth vanadate (BiVO 4 ) and yttrium vanadate (YVO 4 , Bi x Y 1-x VO 4 (BYV)) was reported to be the overall water-splitting photocatalyst with the bandgap (Eg) of ca. 3.0 eV [6] . Although the Eg of BiVO 4 is small (2.8 eV) and can be sensitive to visible light, YVO 4 has a wide bandgap energy (Eg=3.4 eV). Thus the solid solution of the two photocatalysts had the Eg of ca. 3.0 eV, which could not utilize visible light sufficiently. So, in the present study, we tried to utilize indium vanadate (InVO 4 ) in place of BiVO 4 and dope yttrium (Y) at an indium (In) site to form In 1-x Y x VO 4 . Then we expected that In 1-x Y x VO 4 would have a smaller Eg and be sensitive to visible light because the Eg of InVO 4 (1.9 eV) is smaller than that of BiVO 4 [7] . InVO 4 (denoted as IV) and In 1-x Y x VO 4 (x = 0.1, IYV) were prepared by a hydrothermal method. Commercially available indium oxide (In 2 O 3 ), yttrium oxide (Y 2 O 3 ), and vanadyl sulfate n-hydrate (VOSO 4 (nH 2 O)) were dissolved in 0.4 M of nitric acid aqueous solution for 30 m. The solution was treated hydrothermally at 200 °C for 72 h in teflon-lined stainless steel autoclave, and then cooled to room temperature. After the treatment, the obtained precipitates of IV, and IYV photocatalysts were washed with a sufficient amount of distilled water several times and dried at 100 °C for 2 h. After drying, they were heated at 300 °C in air for 2 h. Pt was deposited onto IV (Pt/IV) by photodeposition method. Crystal structures of the samples were determined by powder X-ray diffraction (XRD) and absorption spectra of the samples were recorded on a UV-visible spectrometer (UV-vis). IV and IYV were able to be assigned to a homogeneous orthorhombic phase of IV according to the JCPDS card #71-1689. Peak shifts to the lower angle were observed in IYV, compared to IV. This is reasonable because the ionic radius of Y (1.032 nm) is larger than that of In (0.930 nm). So, we considered that Y was successfully introduced at the In site. The onset of the UV-vis spectrum of IYV shifted to the lower wavelength than that of IV (Fig. 1) and the Eg of IYV and IV were estimated to be 2.0 eV and 1.9 eV, respectively. We evaluated the H 2 evolution in the presence of methanol as a sacrificial agent, caused by the half reaction of water in the presence of Pt/IV and IYV photocatalysts under visible light irradiation (> 420 nm). Pt/IV could not evolve H 2 at all. However, even though Pt was not loaded on IYV, IYV could evolve H 2 under visible irradiation. The potential of the conduction band minimum (CBM) of IV with Eg of 1.9 eV was located more positively than that of H 2 evolution (0 V vs. SHE) [7]. In contrast, the CBM potential of IYV shifted negatively beyond that of H 2 evolution. Thus IYV was capable of producing H 2 thermodynamically. We expect the further enhancement of the activity through the optimization of doping concentration and the aid of cocatalyst. [1] A. Fujishima et al., Nature 238, 37 (1972) [2] K. Maeda et al., Nature, 440, 16 (2006) [3] K. Sayama et al., J. Photo. Photo. A: Chem., 148, 71 (2002) [4] Y. Sasaki et al., J. Phys. Chem. C, 113, 17536 (2009) [5] H. Irie et al., J. Mater. Chem. A, 4, 3061 (2016). [6] H. Liu et al., J. Mater. Chem., 21, 16535 (2011) [7] X. Lin et al., J. Alloys Comp. 635, 256 (2015) Figure 1