Influence of Synthesis pH and Post-Synthetic Annealing on the Hole Density of CuGaO2 Mesoporous Films

Delafossite CuGaO2 has been investigated as a hole transport material for several solar conversion architectures, but a fundamental understanding of how the preparation of CuGaO2 electrodes impacts performance is lacking. A comparison of CuGaO2 nanocrystals was thus performed by synthesizing the material at different pH conditions (5 and 9) and annealing the resultant mesoporous films under O2, Ar, or H2/N2 conditions. The pH conditions of the synthesis had a profound impact on the morphology and the optical properties of the resultant nanocrystals, where pH 5 synthesis produced lightly colored large (500 nm to 1 μm) hexagonal plates with a significant number of smaller nanocrystals (<100 nm), and pH 9 synthesis produced a more homogeneous distribution of smaller (∼200 nm) hexagonal plates that were dark gray in color. We found that the electrochemical behavior of pH 9 films, which behaved as a degenerately doped metal oxide, could be modified by annealing under H2/N2 conditions, consistent with the reduction of defects induced by copper vacancies. Particles synthesized at pH 5, however, gave electrochemical behavior more consistent with a semiconductor material, which could also be impacted by annealing conditions. Elemental analysis via ICP-MS and EDS supported a larger concentration of copper vacancies for pH 5 nanocrystals (Cu:Ga = 0.88) than for pH 9 nanocrystals (Cu:Ga = 0.96), in contrast to what we expected, given the observed electrochemical behavior. Our results indicate a multifaceted impact on the charge transport properties of the films that stemmed from the differences in particle morphology, preferred anisotropic charge transport, and concentration of surface vs bulk CuII species within the nanocrystals. The implications of our findings for the performance of CuGaO2 photoelectrode architectures are also discussed.