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International Journal of Electrochemistry
Volume 2011, Article ID 563427, 10 pages
http://dx.doi.org/10.4061/2011/563427
Research Article

Photoelectrochemical Stability and Alteration Products of n-Type Single-Crystal ZnO Photoanodes

1Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996, USA
2Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6197, USA
3Center for Radiation Detection Materials and Systems, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6056, USA
4Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6119, USA

Received 24 November 2010; Accepted 22 January 2011

Academic Editor: Shen-Ming Chen

Copyright © 2011 I. E. Paulauskas et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The photoelectrochemical stability and surface-alteration characteristics of doped and undoped n-type ZnO single-crystal photoanode electrodes were investigated. The single-crystal ZnO photoanode properties were analyzed using current-voltage measurements plus spectral and time-dependent quantum-yield methods. These measurements revealed a distinct anodic peak and an accompanying cathodic surface degradation process at negative potentials. The features of this peak depended on time and the NaOH concentration in the electrolyte, but were independent of the presence of electrode illumination. Current measurements performed at the peak indicate that charging and discharging effects are apparently taking place at the semiconductor/electrolyte interface. This result is consistent with the significant reactive degradation that takes place on the ZnO single crystal photoanode surface and that ultimately leads to the reduction of the ZnO surface to Zn metal. The resulting Zn-metal reaction products create unusual, dendrite-like, surface alteration structural features that were analyzed using x-ray diffraction, energy-dispersive analysis, and scanning electron microscopy. ZnO doping methods were found to be effective in increasing the n-type character of the crystals. Higher doping levels result in smaller depletion widths and lower quantum yields, since the minority carrier diffusion lengths are very short in these materials.