Layered Ag/Ag 2 O/BiPO 4 /Bi 2 WO 6 heterostructures by two-step method for enhanced photocatalysis Xiaohong Hu a,b,1 , Qiansu Ma a,1 , Xinlong Wang b,c , Yingjun Yang b,c , Na Liu a , Cheng Zhang a , Naoki Kawazoe b , Guoping Chen b,c , Yingnan Yang a,⇑ a Graduate School of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Japan b Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Japan c Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Japan article info Article history: Received 6 November 2019 Revised 30 March 2020 Accepted 2 April 2020 Available online 13 April 2020 Keywords: Layered heterostructures Charge separation and transfer Two-step hydrothermal method Water oxidation Photocatalysis abstract Two-dimensionally layered structures are intriguing for the fundamental demand of opto-electronic devices and have broad applications due to their superior properties. In particular, layered heterostruc- tures as promising materials in photocatalysis, have lots of active atoms exposed on the surface, which largely affect the photocatalytic performance. In this work, two-dimensional heterostructures were syn- thesized by a two-step hydrothermal method, in which monolayer BiPO 4 chemically bonded on Bi 2 WO 6 nano-sheets with Ag/Ag 2 O nanoparticles (BWO-AP). The two-step method significantly improved photo- catalytic efficiency, compared with traditional one-step process. Compared to other controls, BWO-AP showed the highest quantum efficiency and photocatalytic performances for O 2 revolution in distilled water and Rh B degradation. It was ascribed to its fast photoinduced charge separation and transfer. Interestingly, there also was some amount of H 2 generated in distilled water, and the increased Ag 0 : Ag + of BWO-AP after photocatalytic reaction could explain the consumption of photo-induced electrons from water splitting. Ó 2020 Elsevier Inc. All rights reserved. 1. Introduction Semiconductors have been intensively considered as photocat- alysts for water splitting and organic degradation to deal with glo- bal energy and environmental issues. However, many semiconductors (such as TiO 2 , ZnS and SrTiO 3 ) are barely to be effi- ciently activated with irradiation of visible light due to their wide band gaps [1–3]. Fabrication of heterostructures (such as AgBr/ TiO 2 , CdS/TiO 2 and SrTiO 3 /Ag 3 PO 4 ) by loading co-catalysts on semi- conductors is one of the strategies to decrease original band gap and enhance photo-induced charge separation and transfer [4–7]. Compared to conventional heterostructures, layered heterostruc- tures could greatly enhance charge separation [8]. Moreover, active sites exposed on layered structures could largely affect the perfor- mance in solar energy conversion [9–11]. Since the great successes on fabrication of monolayer materials, layered heterostructures that are composed of different monolayer materials stacked by strong chemical bonds are considerably appealing [12,13]. The strong chemical bonding between two monolayers in heterostruc- tures could even induce new energy band structures. In addition, noble metals (Au, Ag and Pt) combined with semiconductors could efficiently extend the optical absorption spectra and improve pho- tocatalytic activities [14–16]. Therefore, layered heterostructures loaded with noble metals could not only facilitate charge separa- tion and transfer, but also be efficiently activated by solar light in practical application. Photocatalytic water splitting includes two half reactions, namely, hydrogen production and oxygen production. However, O 2 generation of water oxidation is energetically and mechanisti- cally demanding, requiring loss of 4e  from H 2 O molecules. In this process, band structures of catalysts dominate the water oxidation ability [17–19]. Recently, sheet-shaped Bi 2 WO 6 , with band gap of 2.78 eV, attracted much attention in photocatalysis used for high efficient water oxidation and degradation of organic pollutants [20,21]. Although layered Bi 2 WO 6 heterostructure is anticipated as a promising material in photocatalysis, Bi 2 WO 6 synthesized in conventional one-step process shows high photo-induced electron-hole recombination, which limits its photocatalytic effi- ciency [20]. In addition, BiPO 4 nanoparticles and nanorods has been reported with high efficiency for water splitting and organic degra- dation [22], which possesses higher photocatalytic activity than https://doi.org/10.1016/j.jcat.2020.04.002 0021-9517/Ó 2020 Elsevier Inc. All rights reserved. ⇑ Corresponding author. E-mail address: yo.innan.fu@u.tsukuba.ac.jp (Y. Yang). 1 These authors contributed equally to this work. Journal of Catalysis 387 (2020) 28–38 Contents lists available at ScienceDirect Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat