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

I E Paulauskas,G M Brown,G E Jellison,L A Boatner

doi:10.4061/2011/563427

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.

Highlights

The increasing demand for energy, coupled with the ongoing decreasing availability of fossil fuels and the undesirable consequences of their use, is driving the need to develop alternative fuels such as hydrogen—that is, fuels that can be both abundant and environmentally safe
The production of hydrogen is primarily achieved by either the steam reforming of methane, which results in CO2 emissions, or water electrolysis, which currently requires the combustion of fossil fuels for the generation of electricity [1]
These results clearly show the presence of nonstoichiometric oxygen in some regions of the primarily metallic Zn structures found on the altered zinc oxide (ZnO) electrode surface

Summary

Introduction

The increasing demand for energy, coupled with the ongoing decreasing availability of fossil fuels and the undesirable consequences of their use, is driving the need to develop alternative fuels such as hydrogen—that is, fuels that can be both abundant and environmentally safe. Numerous studies have been conducted using semiconducting n-type oxides including titanium dioxide (TiO2), strontium titanate (SrTiO3), potassium tantalate (KTaO3), and zinc oxide (ZnO), among others [1, 4,5,6,7,8] These materials absorb solar energy or other optical energy sources and can drive the water electrolysis reaction (2H2O −h→v 2H2 + O2). The present work significantly extends the results of the prior studies of ZnO stability by focusing on the details of the surface reaction and degradation characteristics associated with the photoanodic conditions Both doped and undoped ZnO single-crystal photoanodes are investigated here—where the electronic properties of the single-crystal ZnO photoanodes have been modified through the use of various dopant elements and heat treatments. The role of these compositional variations in potentially improving both the ZnO surface stability in an operating PEC cell environment and the solar energy absorption properties has been investigated

Experimental Procedure

Electrochemical Measurements

Conclusions