Nickel oxide nanoparticle synthesis and photocatalytic applications: evolution from conventional methods to novel microfluidic approaches

Nickel oxide (NiO) nanoparticles are essential to developing a wide range of important industrial products, examples of which include electrodes, catalysts, and sensors, leading to diverse applications from electrochemical detection to energy storage and environmental remediation. NiO nanoparticles exhibit higher reaction selectivity under solar-driven conditions. Thus, they are good candidates for photocatalysts, which can generate strong oxidizing and reducing agents for photodegradation of organic pollutants and other target molecules under normal temperature and pressure conditions, giving rise to versatile applications for energy and environmental remediation. The conventional strategies of NiO nanoparticle synthesis can be broadly categorized into three themes: solid-phase method, liquid-phase method, and vapor-phase method. Recently, microfluidic reactors hold great promise for nanomaterial synthesis due to the thermal homogeneity across the reactor and rapid heat transfer ensured by the large ratio of surface area to volume. The exquisite control over the size, structure and composition of the droplet by microfluidic emulsification technology outperforms the traditional microemulsion method. Herein, we present an overview of the latest advances in fabrication of NiO nanoparticles using different approaches including both conventional methods and microfluidic methods, and focus on the fundamentals of each formation process with the main advantages and disadvantages discussed. This review also provides comparative overview of influence of synthesis conditions on size and morphologies of NiO nanoparticles. We also summarized the development of NiO-based photocatalysts in environmental applications. The perspectives for future research are also discussed. It can be envisioned that success in microfluidic method will continue to inspire novel approaches to drive the rapid evolution of the NiO synthesis technologies in future.