Electrochimica Acta 104 (2013) 282–288 Contents lists available at SciVerse ScienceDirect Electrochimica Acta jou rn al hom ep age: www.elsevier.com/locate/elec tacta Structural and photoelectrochemical investigation of boron-modified nanostructured tungsten trioxide films Piotr J. Barczuk a , Agata Krolikowska b , Adam Lewera b , Krzysztof Miecznikowski b , Renata Solarska b,c , Jan Augustynski a,c,∗ a Centre for New Technologies, University of Warsaw, Zwirki i Wigury 93, 02-089 Warsaw, Poland b Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland c Institute of Electronic Materials Technology, Wolczynska 133, 01-919 Warsaw, Poland a r t i c l e i n f o Article history: Received 21 January 2013 Received in revised form 28 March 2013 Accepted 17 April 2013 Available online 29 April 2013 Keywords: Tungsten trioxide Photoanode Water splitting Boron doping Photoelectrochemistry a b s t r a c t We report a modification of nanostructured WO 3 films by doping with boron. The films were obtained by a direct one-step sol–gel route involving tungstic acid/polyethelene glycol precursor. Raman spec- troscopy and X-ray photoelectron spectroscopy (XPS) showed that the incorporation of boron results in the retention of a substantial amount of water and/or hydroxyl groups in the WO 3 lattice and at the surface of nanoparticles occurring despite high temperature (550 ◦ C) annealing of the films. Another con- sequence of boron doping is the largely increased roughness factor revealed by atomic force microscopy (AFM) imaging. Both kinds of films are highly porous and consist of partly sintered particles with sizes in the range of tens of nanometers. The photoelectrochemical (PEC) studies performed under simulated solar AM 1.5 illumination showed significantly enhanced water oxidation photocurrents for B-WO 3 pho- toanodes, by about 25% higher than those for the undoped WO 3 films of similar thickness. The low extent of recombination of photogenerated charges was confirmed by incident photon-to-current conversion efficiencies (IPCEs) reaching 70% in the region of visible wavelengths at 420 nm. The improved PEC prop- erties were attributed to the increased surface hydroxylation of B-WO 3 nanoparticles favoring water photo-oxidation reaction and to the larger surface area of the film exposed to the electrolyte. © 2013 Elsevier Ltd. All rights reserved. 1. Introduction Photoelectrochemistry provides a means to store solar energy through converting water to oxygen and hydrogen [1]. Most of the recent research on H 2 production via photoelectrochemical (PEC) water splitting has focused on the use of n-type semiconductor metal oxide materials employed as photoanodes [1,2]. Following the first report by Fujishima and Honda of photo-oxidation of water on a rutile titanium dioxide (TiO 2 ) photoanode illuminated with ultraviolet (UV) light [3], initiated an extensive search of photoelectrode materials that are able to capture a substantial part of the visible spectrum. Early investigations involving a variety of inorganic semiconductors showed that the only materials able actually to split water whilst avoiding photocorrosion are metal oxides [4]. Besides investigations of TiO 2 doped with a variety of elements [2], the recent work centered on hematite (-Fe 2 O 3 ) [5] and tungsten trioxide (WO 3 ) [6] thin-film photoanodes with band gaps of 2.1 eV, respectively, 2.5 eV. Since any among these ∗ Corresponding author at: Centre for New Technologies, University of Warsaw, Zwirki i Wigury 93, 02-089 Warsaw, Poland. E-mail address: Jan.Augustynski@unige.ch (J. Augustynski). materials having photoaction spectra covering visible wavelengths can perform unassisted water splitting, due to the position of the conduction band edges more positive than the H 2 evolution potential, continuing efforts are devoted to minimize the bias voltage required to perform visible light-driven photo-oxidation of water. Recently, unassisted solar production of H 2 has been demonstrated in a dual-absorber tandem device combining a semitransparent WO 3 photoanode and the latest version of the dye-sensitized solar cell (DSSC) providing a ca 1 V bias voltage to the photoelectrolyzer [7]. In such a tandem device, the WO 3 photoanode, deposited on the conductive glass substrate, absorbs the near-UV and blue-green portions of the solar spectrum and the DSS photovoltaic cell, placed behind the photoelectrolyzer, captures longer wavelengths transmitted by the WO 3 film. Efforts toward implementation of the solar water splitting include also the search of the low-cost, effective in terms of the energy payback, preparation methods of photoelectrode materials. This is illustrated by a recent use of solution-based colloidal syn- thesis approach, instead of chemical vapour deposition method, which offers better PEC performance, to form thin layer -Fe 2 O 3 photoanodes [8]. In this paper, we report a modification with boron of nano- structured WO 3 films formed using simple, one-step, sol–gel 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.04.107