Stoichiometric and non-stoichiometric tungsten doping effect in bismuth vanadate based photoactive material for photoelectrochemical water splitting Umesh Prasad, Jyoti Prakash * , Bruno Azeredo, Arunachala Kannan * The Polytechnic School, Ira A. Fulton Schools of Engineering, Arizona State University, Mesa, AZ 85212, USA article info Article history: Received 20 September 2018 Received in revised form 27 November 2018 Accepted 2 January 2019 Available online 6 January 2019 Keywords: Bismuth vanadate Tungsten doping Photoelectrochemical cell Oxygen evolution Water splitting abstract In photoelectrochemical (PEC) water splitting, BiVO 4 has attracted attention due to its favorable band gap but it suffers low PEC performance due to poor conductivity. The vast majority of publications on this system has examined doping of stoichiometric composition of tungsten (W) on this system to increase bulk and interfacial conductivity while managing the contaminant generation of crystallographic defects and recombination sites. In this paper, a deep investigation was carried out to examine the effect of non- stoichiometric W doping in BiVO 4 system. Stoichiometric and non-stoichiometric W-doped monoclinic BiVO 4 (i.e. Bi 1-(xþd) V 1-x W xþd O 4 ; BiV 1-x W xþd O 4 and BiV 1-y W y O 4; x ¼ 0.008; y ¼ 0.03 and d ¼ 0.005) were prepared using a facile dip coating technique. The stoichiometric composition contains charge balanced Bi, V and W atoms whereas non-stoichiometric compositions contain excess Bi and excess Bi and W. The non-stoichiometric composition BiV 1-x W xþd O 4 has shown better photoelectrochemical water splitting performance with respect to other compositions at 1.23 V vs RHE, under one sun illumination of elec- trode. The XRD and XPS results shows that non-stoichiometric doping with excess Bi or with excess Bi and W can possibly create an environment where V 5þ ions are substitutional replaced by W 6þ ions without generating other defects. But there was no significant difference in band gap of different compositional samples observed. Further electrochemical impedance technique was used to analyze change in bulk and surface charge mobility with W-doping in BiVO 4 . The electrochemical impedance analysis showed the presence of low interfacial resistance, lower charge transfer resistance and high charge donor/surface state density for non-stoichiometric composition BiV 1-x W xþd O 4 electrode. It is evident from and cyclic voltammetry that the addition of excess Bi and W from its stoichiometric quantity efficiently suppressed the formation of hole-electron pair recombination sites. The electro- chemical analytical results lead us to believe that the particular non-stoichiometric composition of BiV 1- x W xþd O 4 can significantly lower trap sites and enhances kinetics of charge transfer, leading to the better photoelectrochemical water splitting performance. © 2019 Elsevier Ltd. All rights reserved. 1. Introduction The ever-increasing global energy requirement around the world can be fulfilled using renewable energy sources [1e3]. Among various technologies, hydrogen fuel cell systems are very attractive as the efficiencies can reach as high as 90% [4]. However, the hydrogen is not available as a primary energy source in nature. In this context, photoelectrochemical (PEC) water splitting is one of the potential approaches for the generation of hydrogen [5e7]. There are numerous research groups developing novel photo- catalyst systems with high efficiency photo-to-hydrogen conver- sion, wide absorption of the solar spectrum and photo-corrosion resistance behavior [8e10]. In developing economical and effi- cient photo-catalysts, n-type monoclinic BiVO 4 has been widely reported due to its favorable band gap [11e13]. But the photo conversion efficiency of BiVO 4 catalyst is relatively low due to poor charge transport, fast surface charge recombination and sluggish water oxidation kinetics [14e17]. Doping with metals is one of the promising approaches for improving intrinsic properties of BiVO 4 * Corresponding authors. E-mail addresses: jprakas2@asu.edu (J. Prakash), amk@asu.edu (A. Kannan). Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta https://doi.org/10.1016/j.electacta.2019.01.013 0013-4686/© 2019 Elsevier Ltd. All rights reserved. Electrochimica Acta 299 (2019) 262e272