Effect of oxygen nonstoichiometry on the photoelectrochemical performance of oxide-nanorod based TiO2/Sb2S3 and ZnO/Sb2S3 heterostructured photoanodes

Abstract

Single crystalline ZnO and TiO2 nanorods are grown on fluorine-doped tin oxide (FTO) substrates by hydrothermal method. The nanorods are annealed under air and reducing conditions to alter the oxygen nonstoichiometry and hence the mid-bandgap defect states. Such an annealing process is shown to impart significant change on the photoelectrochemical (PEC) performance of the photoelectrodes. Large photocurrent densities (J) of 0.78 mA cm−2 are obtained for air annealed (AA) TiO2 nanorods (TNR) compared to hydrogen annealed (HA) TNR (J = 0.36 mA cm−2). ZnO nanorods (ZNR), on the contrary, shows photocurrent density of 0.76 mA cm−2 and 0.36 mA cm−2 for ZNR-HA and ZNR-AA photoanodes, respectively. The contrasting difference in the PEC performance is attributed to the synergetic effect of interfacial impedance with electrolytes and the oxygen nonstoichiometry. Further, to overcome the limitation of light absorption by these materials owing to their wide bandgap, TiO2 and ZnO nanorods are coated with Sb2S3 by chemical bath deposition to form heterostructured TNR-AA/Sb2S3(CBD) and ZNR-HA/Sb2S3(CBD) thin films. Such heterostructures exhibit enhanced photocurrent values of ∼1.39 mA cm−2 and 3.36 mA cm−2 (at 1.6 V versus Ag/AgCl), respectively. The PEC performances of the nanorods are analyzed in terms of the annealing conditions and subsequent introduction of defect states in the bandgap. The present study shows the importance of oxygen defect control at the interface between the oxide and chalcogenide, and its role in the betterment of PEC performance in TiO2/Sb2S3 and ZnO/Sb2S3 heterostructure photoanodes.