Enhanced Photoelectrochemical Water Oxidation Using TiO2-Co3O4 p–n Heterostructures Derived from in Situ-Loaded ZIF-67

Exposing catalytically active metal sites in metal–organic frameworks (MOFs) while maintaining porosity is beneficial for increasing electron transport to achieve better electrochemical energy conversion performance. Herein, we propose an in situ method for MOF formation and loading onto TiO2 nanorods (NR) using a simple solution-processable method followed by annealing to obtain TiO2-Co3O4. The as-prepared TiO2-ZIF-67 based photoanodes were annealed at 350, 450, and 550 °C to study the effect of carbonization on photo-electrochemical water oxidation. The successful loading of ZIF-67 on TiO2 and the formation of TiO2-Co3O4 heterojunction were confirmed by XRD, XPS, FE-SEM, and HRTEM analyses. TiO2-Co3O4-450 (the sample annealed at 450 °C) showed an enhanced photocurrent of 2.4 mA/cm2, which was 2.6 times larger than that of pristine TiO2. The improved photocurrent might be ascribed to the prepared p–n heterostructures (Co3O4 and TiO2), which promote electron–hole separation and charge transfer within the system and improve the photoelectrochemical performance. Moreover, the preparation of Co3O4 from the MOF carbonization process improved the electrical conductivity and significantly increased the number of exposed active sites and enhanced the photoresponse performance. The as-prepared ZIF-67 derived TiO2-Co3O4 based photoanodes demonstrate high PEC water oxidation, and the controlled carbonization method paves the way toward the synthesis of low-cost and efficient electrocatalysts.