Zn1-xCoxO vs Ag-ZnO photoanodes design via combustion: Characterization and application in photoelectrocatalysis

In this work, different synthetic conditions were analyzed to prepare modified ZnO photoelectrodes, in order to prevent the recombination of their photogenerated charge carriers and thus allow the continuous formation of electrons and holes on photoelectrocatalytic reactions. Thus, an investigation was carried out by the chemical modification using cobalt and surface modification using silver, as well as the final heat treatment temperature, to monitor its effects onto the properties of this semiconductor oxide. In this context, Zn1−xCoxO and Ag-ZnO photoelectrodes were produced from combustion synthesis and prepared simply and efficiently, directly on the indium tin oxide (ITO) surface. These material modifications were efficient to improve the properties of pure ZnO, and the presence of silver on the ZnO surface provided significantly superior photoelectrochemical and photoelectrocatalytic responses for methylene blue discoloration. Zn1−xCoxO presented a current density of 132 μA cm−2 and kinetic discoloration constant of 9.05 × 10−3 min−1, and Ag-ZnO displayed a current density of 162 μA cm−2 and kinetic discoloration constant of 15.0 × 10−3 min−1. Among the ZnO modification conditions, the addition of 2.0% (mol L−1) of a metal nitrate precursor (either Co or Ag) and thermal treatment at 600 °C provided the best responses to the photoelectrodes.