Metformin‐Templated Nanoporous ZnO and Covalent Organic Framework Heterojunction Photoanode for Photoelectrochemical Water Oxidation

Abstract Photoelectrochemical water‐splitting offers unique opportunity in the utilization of abundant solar light energy and water resources to produce hydrogen (renewable energy) and oxygen (clean environment) in the presence of a semiconductor photoanode. Zinc oxide (ZnO), a wide bandgap semiconductor is found to crystallize predominantly in the hexagonal wurtzite phase. Herein, we first report a new crystalline triclinic phase of ZnO by using N‐rich antidiabetic drug metformin as a template via hydrothermal synthesis with self‐assembled nanorod‐like particle morphology. We have fabricated a heterojunction nanocomposite charge carrier photoanode by coupling this porous ZnO with a covalent organic framework, which displayed highly enhanced photocurrent density of 0.62 mA/cm 2 at 0.2 V vs. RHE in photoelectrochemical water oxidation and excellent photon‐to‐current conversion efficiency at near‐neutral pH vis‐à‐vis bulk ZnO. This enhancement of the photocurrent for the porous ZnO/COF nanocomposite material over the corresponding bulk ZnO could be attributed to the visible light energy absorption by COF and subsequent efficient charge‐carrier mobility via porous ZnO surface.