Bandgap engineering of ZnO nanomaterials for enhanced electrochemical and photocatalytic efficiency

Zinc oxide (ZnO)-based hybrid nanocomposites are promising materials for electrochemical device development due to their tunable energy bandgap (E_g = 2.8–3.3 eV), high thermochemical stability, and enhanced electronic, mechanical, and piezoelectric properties. However, the wide E_g of ZnO restricts radiation absorption to the UV range, reducing efficiency in optoelectronic and photocatalytic applications. Bandgap tuning through metal and non-metal doping, structural defect incorporation, and heterojunction construction with lower bandgap materials improves visible light absorption and charge transfer efficiency. These modifications delay photoinduced electron-hole recombination, enhancing electrochemical properties and reducing charge transfer resistance. Bandgap tuning or bending mechanisms play a key role in ZnO heterojunction, while quantum confinement effects further influence bandgap shifts at the nanoscale. This review comprehensively covers various ZnO synthesis methods, including sol-gel, hydrothermal, vapor transport, and green synthesis techniques, and their impact on bandgap tuning and material properties. An overview is presented on the implications of ZnO bandgap engineering in electrochemical applications, including photocatalysis, gas sensing, dye-sensitized solar cells, and electrochemical energy-storing devices. The development of flexible, robust, and efficient ZnO-based electrochemical systems is important for summit the difficulties of next-generation smart and portable electronic devices.