Porous WO3 with Hierarchical Structure on FTO Substrates: Fabrication and Photoelectrochemical Water Oxidation Performance

Abstract

WO 3 is a type of oxide semiconductor material with bandgap of 2.5-2.8 eV and is considered a promising stuff that can be used in solar light-driven photocatalytic applications. In this work, WO 3 ·0.33H 2 O films on fluorine doped tin oxide substrates were firstly deposited hydrothermally and then were annealed at 500°C to form WO 3 . The as-prepared WO 3 films possess a hierarchical structure: short WO 3 nanorods are assembled into large clusters in a radial form and the large clusters are distributed uniformly on the substrate to form the films. By employing photoelectrochemical etching technique, the WO 3 nanorods were further treated to be nanopore-rich structure. When used as photoanodes, the porous WO 3 films showed much enhanced photoelectrochemical water splitting performance in comparison with the non-etched WO 3 films: under the illumination of simulated solar light and without using any oxygen evolution co-catalysts, the photocurrent increased from 0. I n recent years, WO 3 has attracted much attention as a photoanode material in photoelectrochemical (PEC) water splitting applications owing to its ~ 12% of solar spectrum absorption ability and the ideal band gap (E g = 2.5-2.8 eV) [1][2][3]. Compared to α-Fe 2 O 3 , another commonly used photoanode material, whose hole diffusion length is as short as ~ 2.5 nm, WO 3 possesses a moderate hole diffusion length of ~ 150 nm [4]. At the same time, the inherent mobility of WO 3 is ~ 12 cm 2 /(V·s) , which is higher than the value of 1 cm 2 / (V·s) for rutile TiO 2 [5]. These characteristics make WO 3 a very promising photoanode material for PEC water splitting applications. However, WO 3 also possesses drawbacks like sluggish kinetics of holes, slow charge transfer and rapid electron-hole recombination [6]. To overcome such drawbacks, many substantial efforts have been made, in which increasing the specific surface of the WO 3 nanostructures via morphology control is considered to be one direct and effective way [7]. For example, porous WO 3 flakes that were formed by treatment of the WO 3 flakes in ascorbic acid together with etching in poly(vinyl pyrrolidone) were reported to achieve much enhanced PEC performance in comparison with the non-etched WO 3 [8].In this work, we report a feasible photoelectrochemical etching method to achieve porous WO 3 film from nanorod-like WO 3 film on fluorine doped tin oxide (FTO) substrates. Photoelectrochemical water oxidation experiments showed that after photoelectrochemical etching the WO 3 films acquired much enhanced performance. This method could be used to prepare other porous semiconductor nanostructures and thus acquire novel properties. EXPERIMENTAL MaterialsNa 2 WO 4 (Damao Chemical Teagent Factory, Tianjin) and nitric acid (Xilong Scientific, Guandong), KH 2 PO 4 and Na 2 HPO 4 ·12H 2 O (Sinopharm Chemical Reagent Co.) were used without further purification. FTO substrates (Zhuhai Kaivo Optoelectronic Technology Co.) with dimensions of 25 mm×20 mm×1.6 mm were cleaned ultrasonically in a solution containing ac...