Rapid Screening by Scanning Electrochemical Microscopy (SECM) of Dopants for Bi₂WO₆ Improved Photocatalytic Water Oxidation with Zn Doping

Bi2WO6 microelectrode arrays on FTO glass substrates were fabricated by a picoliter solution dispensing technique using Bi(NO3)3 as the Bi source and (NH4)6H2W12O40 as the W source in ethylene glycol. The scanning electrochemical microscope modified by using an optical fiber in place of an ultramicroelectrode was employed for rapid screening of the Bi2WO6 arrays and to investigate the effect of 12 different dopants on the photocatalytic oxidation of SO3 2−. Among the different dopant compositions, addition of 12% Zn showed a photocurrent enhancement of up to 80% compared with that of the pure Bi2WO6. This result was further confirmed with bulk electrode studies for SO3 2− and water oxidation. UV− vis absorption, electrochemical impedance spectroscopy, scanning electron microscopy, and X-ray diffraction studies were carried out with the photocatalysts to elucidate the role of Zn in the bulk semiconductors. Absorbed photon-to-current efficiency and incident photon-to-current efficiency determinations further confirm the enhancement of photoelectrochemical behavior upon addition of Zn to Bi2WO6 photocatalysts. ■ INTRODUCTION The present study describes the preparation and rapid screening of microelectrode arrays of Bi2WO6 using scanning electrochemical microscopy (SECM) to find suitable dopants for better photoelectrochemical (PEC) performance of the semiconductor. When 12% Zn was introduced as a dopant to the Bi2WO6, the photocatalytic current of the semiconductor (SC) increased significantly. Photocatalyst-based semiconductor−liquid junction solar cells are now considered a promising route for harvesting chemical energy directly from sunlight, and a large number of candidate materials have been investigated. In PEC cells, a semiconductor is excited by light of energy higher than the band gap, which results in formation of electron−hole pairs. These are separated in the electric field at the interface with the liquid and drive a heterogeneous electron-transfer reaction. Photocatalysis has attracted much attention during recent years due to its potential to split water into H2 and O2 as well as for environmental purification through photodecomposition of different waste materials, dyes, polymers, and sterilization of bacteria. TiO2 is one of the most widely explored photocatalyst materials due to its high stability, nonpolluting nature, and low cost. However, the large band gap, Eg, of TiO2 [Eg = 3.2 eV (anatase) and 3.0 eV (rutile)] limits its efficiency with solar wavelengths, and extensive investigations continue to uncover suitable semiconductor materials with more appropriate band gaps to harvest more of the visible as well as in the near UV region of the solar spectrum. For example, binary and ternary semiconductor oxides, for example, BiVO4, 7,8 Ag3PO4, 9 Ag3VO4, 10 and many others, have been suggested for PEC applications. Bi2WO6, a member of the “Aurivillius” family of bismuthbased mixed oxides, represented by the general formula of (Bi2O2) [Am−1BmO3m+1] 2− (for Bi2WO6, A = Bi, B = W, and m = 1), are known to form layered structures consisting of the regular intergrowth of [Am−1BmO3m+1] 2−, that is, (WO4) 2− perovskite-like slabs and (Bi2O2) 2+ sheets. This group of compounds comprises some important ferroelectric materials and oxide anion conductors. In particular, Bi2WO6 oxide has attracted a great deal of attention due to its significant piezoelectric behavior, catalytic activity, and nonlinear dielectric susceptibility. The layered structure of this compound has been reported to make it suitable for charge-transfer processes because of restricted recombination of the photogenerated holes and electrons. However, most of the research has concentrated on the photocatalytic activity of Bi2WO6 “particles” toward the photodecomposition of dyes or other organic molecules and with a quite low PEC performance even in the presence of sacrificial reagents. This semiconductor photocatalyst has been primarily synthesized through solid-state reactions, the hydrothermal route, or by coprecipitation Received: August 30, 2012 Revised: March 26, 2013 Published: April 10, 2013 Article