Size effects of WO 3 nanocrystals for photooxidation of water in particulate suspension and photoelectrochemical film systems Suk Joon Hong, Hwichan Jun, Pramod H. Borse, Jae Sung Lee* Eco-friendly Catalysis and Energy Laboratory (NRL), Department of Chemical Engineering, Pohang University of Science and Technology, San 31, Hyoja-dong, Pohang 790-784, South Korea article info Article history: Received 18 December 2008 Received in revised form 3 February 2009 Accepted 3 February 2009 Available online 12 March 2009 Keywords: Tungsten trioxide Crystal size effects Photoelectrochemical films Water oxidation abstract Monoclinic WO 3 nanocrystals were synthesized by a hydrothermal reaction and post calcination. Their particle sizes were varied from 30 nm to 500 nm by changing calcination temperature from 500  C to 800  C. Photooxidation of water was studied in particulate suspension (PS) system and photoelectrochemical (PEC) film system. For PS system, WO 3 nanocrystals were suspended in 50 mM AgNO 3 solution to measure O 2 evolution rate. For PEC system, WO 3 films were fabricated by doctor blade method using synthesized nano- crystals. Photocurrent density was measured at AM 1.5 G (1 sun) solar condition in 0.5 M H 2 SO 4 . In PS system, the sample calcined at the highest temperature generated the largest amount of oxygen, whereas in PEC system the sample calcined at 600  C showed the maximum photocurrent. The two systems also showed opposite response to deposition of the Pt co-catalyst. These different behaviors were attributed to different mechanisms of charge separation in the two systems. ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. 1. Introduction Photocatalytic water splitting (PWS) using semiconductor photocatalysts has received much attention as an ideal way to produce hydrogen. It produces hydrogen directly from water and solar light, which are the most abundant natural resources and energy sources, respectively, available on earth. The PWS has been studied mainly via two systems; particulate suspension (PS) system and photoelectrochemical (PEC) cell system. In PS system, the photocatalyst powders are dispersed into water and oxygen and hydrogen are evolved simultaneously from the same photocatalyst particles. In contrast, PEC cell consists of two separated electrodes, of which at least one electrode is a photoactive semiconductor. In case of n-type semiconductor, anodic and cathodic reac- tions take place at the semiconductor and the other electrode (usually a metal), respectively. Since Fujishima and Honda first demonstrated PWS with TiO 2 electrode [1], many other semiconductors have been investigated as photoactive electrodes including SrTiO 3 , WO 3 and Fe 2 O 3 [2–5]. In spite of the large band gap energy which allows absorption of only UV light, TiO 2 is the most frequently investigated semiconductor because of chemical stability in wide pH range, electrochemical stability, and facility of preparation. Many studies have been dedicated to the effect of physico-chemical properties on the activity of TiO 2 photo- catalyst in PS system. According to several reports, an optimum crystal size of TiO 2 exists in nanoscale [6–8]. Small * Corresponding author. Tel.: þ82 54 279 2266; fax: þ82 54 279 5528. E-mail address: jlee@postech.ac.kr (J.S. Lee). Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he 0360-3199/$ – see front matter ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.02.006 international journal of hydrogen energy 34 (2009) 3234–3242