A systematic review on fabrication techniques, optical and morphological properties of TiO2 and ZnO/rGO based heterostructures photodetectors

Titanium dioxide (TiO2) and Zinc Oxide (ZnO) are two wide-bandgap (Eg) materials (Eg ~3 to 3.3 eV) that have gained significant interest from researchers and are extensively studied in the field of optoelectronics due to their functionality in the UV region. In addition to having a bandgap corresponding to UV radiation, these metal oxides are also considered environmentally friendly, have a cost-effective synthesis and device fabrication mode, and are exhibiting strong physical and chemical stability against external environmental degradation. As observed, the bulk forms of TiO2 and ZnO operate in the UV region due to their favourable band gap. However, as they are downsized to the nanometre range, with varied sizes and morphologies, they can be tuned to work for a broader spectrum of wavelengths, i.e., in the UV-Vis region. Furthermore, in addition to nanostructured metal oxides, incorporating them as heterostructures can improve the overall working efficiency of fabricated devices. Reduced graphene oxide (rGO) is one such integrant that can be included in the heterostructure configuration, due to its stability and ability to boost efficiency owing to its tuneable low bandgap (Eg ~ 1 to 1.2 eV), dependent on its level of reduction, which aids in quicker charge transfer. The central research question that has been explored in this review is how the structural and morphological changes in TiO2, ZnO and rGO are influenced by the synthesis parameters, while also examining various fabrication techniques for constructing heterostructures and investigating how different heterostructure designs impact the UV-detection performance of TiO2, ZnO and ZnO or TiO2/rGO based photodetectors. In this review, we are presenting how the structural and morphological changes in TiO2, ZnO, and rGO affect the detection efficiency of photodetectors operating in the UV-Vis region, and how the heterostructures configured by these materials further influencing the performance.