This journal is © the Owner Societies 2016 Phys. Chem. Chem. Phys., 2016, 18, 23407--23411 | 23407 Cite this: Phys. Chem. Chem. Phys., 2016, 18, 23407 Electronic properties of highly-active Ag 3 AsO 4 photocatalyst and its band gap modulation: an insight from hybrid-density functional calculations † Pakpoom Reunchan, a Adisak Boonchun* a and Naoto Umezawa b The electronic structures of highly active Ag-based oxide photocatalysts Ag 3 AsO 4 and Ag 3 PO 4 are studied by hybrid-density functional calculations. It is revealed that Ag 3 AsO 4 and Ag 3 PO 4 are indirect band gap semiconductors. The Hartree–Fock mixing parameters are fitted for experimental band gaps of Ag 3 AsO 4 (1.88 eV) and Ag 3 PO 4 (2.43 eV). The smaller electron effective mass and the lower valence band edge of Ag 3 AsO 4 are likely to be responsible for the superior photocatalytic oxidation reaction to Ag 3 PO 4 . The comparable lattice constant and analogous crystal structure between the two materials allow the opportunities of fine-tuning the band gap of Ag 3 As x P 1x O 4 using a solid-solution approach. The development of Ag 3 As x P 1x O 4 should be promising for the discovery of novel visible-light sensitized photocatalysts. 1 Introduction Much attention has been paid to photocatalysts based on oxide semiconductors for degrading organic contaminants as well as splitting water into H 2 and O 2 . Since the discovery of the TiO 2 photocatalyst, many efforts have been made to reduce its band gap (3.2 eV) because it can only absorb ultraviolet light and utilizes about 5% of solar energy. A widely adopted approach is doping with foreign elements, either cations or anions, for sensitizing TiO 2 under visible light irradiation to increase the conversion efficiency of solar energy. 1 However, doping is often accompanied by the formation of trapping centers of photo- excited carriers and degrades the quantum efficiency; the ratio of the number of photoexcited carriers that actually participate in the photocatalytic reaction to the number of photoexcited carriers that are initially produced by photons. Development and synthesis of visible-light-responsive photocatalysts such as AgAl 1x Ga x O 2 , 2 CdS, 3 and BiVO 4 4 is another approach to tackle the aforementioned problem. Recently, silver orthophosphate (Ag 3 PO 4 ) was discovered to exhibit extremely high photocatalytic activity for O 2 evolution from water, and its quantum efficiency is nearly 90% under visible light irradiation. 5 This triggered several studies of the development of Ag 3 PO 4 in various aspects such as facet engineering, 6 external doping, 7 and combining with other materials such as graphene. 8 More recently, it has been reported that Ag 3 AsO 4 exhibits higher photocatalytic activity than Ag 3 PO 4 under visible-light irradiation. 9 The narrow band gap of Ag 3 AsO 4 (1.60–1.88 eV) 9,10 and the potential of the valence band edge that is sufficiently positive (2.2 eV referenced to normal hydrogen electrode potential, NHE) were attributed to the high oxidation ability of photogenerated holes that participate in decomposing the organic pollutants. A question arises as to why Ag 3 AsO 4 exhibits superior oxidation photocatalytic activity to Ag 3 PO 4 . Efforts have been made to understand the electronic and optical properties as well as the origin of high photocatalytic activity of Ag 3 PO 4 11–13 and Ag 3 AsO 4 under visible light. 14 Only a recent work reported a comparative theoretical and experimental study of silver oxosalts, including Ag 3 PO 4 and Ag 3 AsO 4 . 10 The origin of the band gap difference and qualitative trends in the photo- catalytic activity were discussed based on density functional theory calculations using a generalized gradient approximation (GGA). 10 However, the band offset and the band gap engineering that are the prime keys for enhancing photocatalytic activities have not been discussed elsewhere. Forming a solid solution of two semiconductors is a common approach toward band-gap engineering of photocatalysts. For example, an oxynitride (ZnO) x (GaN) 1x is known to split water into H 2 and O 2 under visible light irradiation. 15 A recent study showed that AgAlO 2 and AgGaO 2 can form solid solutions, and photoabsorption edges and photocatalytic performance of AgAl 1x Ga x O 2 are controlled by its composition ratio. 2 In this paper, we perform density-functional theory calculations using a screened hybrid-functional, which have been proven to a Department of Physics, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand. E-mail: fsciasb@ku.ac.th; Tel: +66856692765 b International Center for Materials Nanoarchitectonics, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan † Electronic supplementary information (ESI) available: Charge distribution of the valence band maximum and conduction band minimum of Ag 3 PO 4 and Ag 3 AsO 4 and formation enthalpy of Ag 3 As x P 1x O 4 solid solutions. See DOI: 10.1039/c6cp03633c Received 26th May 2016, Accepted 29th July 2016 DOI: 10.1039/c6cp03633c www.rsc.org/pccp PCCP PAPER Published on 29 July 2016. Downloaded by Durham University Library on 23/08/2016 16:22:27. View Article Online View Journal | View Issue