Significance ZnO, as a representative wide-bandgap semiconductor, possesses high exciton binding energy, excellent optoelectronic properties, diverse controllable morphologies, and outstanding material compatibility. These characteristics make it highly attractive for micro-nano photonic applications such as ultraviolet photodetectors, light emitting diodes, photonic crystals, and microlasers. Compared with conventional high temperature routes such as chemical vapor deposition and physical vapor deposition, hydrothermal synthesis offers distinct advantages including low reaction temperature, process simplicity, low cost, environmental friendliness, and compatibility with flexible substrates and complex surfaces. These merits make it one of the most representative methods for fabricating ZnO micro-nanostructures. As photonic devices continue to evolve toward flexibility, miniaturization, array integration, and on-chip implementation, the development of low temperature, controllable, and scalable strategies for the high-quality growth and patterned construction of ZnO nanostructures has become an important research topic. Therefore, a systematic review of the growth behavior, morphology regulation mechanisms, and application progress of hydrothermally synthesized ZnO nanostructures is of great significance for promoting the transition of ZnO materials from fundamental research to engineering applications in high performance micro-nano photonic devices. Precursor solution chemistry, reaction temperature, reaction time, and seed layer characteristics strongly affect nucleation behavior, crystallographic orientation, and morphology evolution, which in turn determine the optoelectronic properties and application performance of ZnO-based systems. In this context, establishing clear relationships among hydrothermal conditions, structural features, and device performance is essential for the rational design of next generation ZnO-based photonic materials and devices.

Progress In recent years, research on hydrothermal ZnO micro-nanostructures has expanded from the growth of single morphologies to multiparameter synergistic regulation, region selective growth, and the construction of composite and heterostructured systems. First, in terms of basic growth control, existing studies demonstrate that precursor species and concentration jointly determine the morphology and crystallinity of ZnO by regulating the Zn²⁺/OH⁻ complexation equilibrium, nucleation kinetics, and preferential crystal growth. By tuning the hydrothermal system, a variety of ZnO architectures, including nanorods, nanowires, nanosheets, nanoflowers, self-assembled spheres, and sea-urchin-like structures, can be obtained in a controllable manner. Second, reaction time and temperature have been identified as key parameters governing c axis growth, array ordering, aspect ratio, and crystal defect evolution. Appropriate extension of reaction time generally favors the increase in nanostructure length and improvement in orientation, whereas temperature variation further influences ion diffusion, interfacial reaction rates, and morphology transformation. Third, the presence and quality of seed layers play a decisive role in crystallographic orientation, nucleation density, and array uniformity, thereby enabling the fabrication of highly aligned ZnO arrays under low temperature conditions. Meanwhile, remarkable advances have been achieved in region selective growth technologies. Through strategies such as microcontact printing, inkjet printing, and laser induced hydrothermal growth, researchers have realized patterned ZnO growth in designated regions on both rigid and flexible substrates. Among these methods, laser induced hydrothermal growth is especially promising because it enables rapid, digital, and oriented epitaxial growth in localized regions without requiring complex masks or high vacuum processing, and is therefore regarded as an important emerging strategy for on-chip integration and flexible device fabrication. Furthermore, by combining ZnO with other functional materials to construct composites or heterojunctions, continuous improvements have been achieved through heterojunction engineering, doping control, and composite incorporation, which effectively regulate interfacial charge transfer, defect states, carrier transport, and photoresponse behavior. As a result, hydrothermally synthesized ZnO nanostructures have been widely applied in ultraviolet photodetectors, light emitting diodes, photonic crystals, and microlasers, exhibiting strong application response characteristics, integration capability, and considerable application potential. These studies collectively indicate that hydrothermal synthesis is no longer limited to morphology control alone, but is increasingly serving as a practical platform for coupling material growth with device oriented functional optimization.

Conclusions and Prospects Overall, hydrothermal synthesis provides a low temperature, simple, economical, and scalable route for the controllable fabrication of ZnO micro-nanostructures and their application in micro-nano photonic devices. Existing studies have demonstrated that by coordinating precursor solution chemistry, kinetic processes, seed layer design, and region selective growth strategies, it is possible to achieve morphology tailoring, oriented growth, and functional integration of ZnO nanostructures to a considerable extent, thereby meeting the structural and performance requirements of diverse optoelectronic devices. Nevertheless, several critical challenges still need to be addressed. First, the coupled mechanisms of nucleation, growth, defect formation, and interface evolution in complex hydrothermal systems remain insufficiently understood. Second, region selective growth still requires further improvement in terms of spatial resolution, uniformity, reproducibility, and large area manufacturing stability. Third, in heterostructured systems, the effects of interface states, surface hydroxyl groups, and intrinsic defects on carrier separation, device stability, and long-term service performance still require more systematic experimental and theoretical investigation. Future studies should therefore focus on the development of high precision region selective growth techniques, the strengthening of in situ characterization and multiscale theoretical analysis, the deeper integration of hydrothermal synthesis with micro-nanofabrication technologies, and the acceleration of practical applications of ZnO and related composite nanostructures in high performance, low power, flexible, and wearable micro-nano photonic devices. By synthesizing the reported correlations between application response characteristics and the corresponding material configurations and growth conditions, this study is expected to provide stronger practical guidance for the scalable integration and performance optimization of ZnO nanomaterials in micro-nano photonics.