Advancement in upconversion nanoparticles based NIR-driven photocatalysts

It is important to harness near-infrared (NIR) light in the plentiful and naturally sustainable sunlight to maximize the consumption rate of the uncontaminated resources on the planet. With an increase in unusual nonlinear optical properties of upconversion nanoparticles (UCNPs) to transform longer wavelength (lower-energy) photons into lower wavelength (high-energy) photons, they are the most promising candidates for integration with semiconductors to produce NIR driven photocatalytic systems. A comprehensive overview is presented in the current report on UCNPs based NIR-driven photocatalysts. This review discussed the various synthesis methods used for the preparation of the UCNPs, to control the photophysical properties. We emphasized various issues on how to improve the emission efficiency as well as the photocatalytic performance of the as-designed photocatalysts. The fundamental developments to overcome the recent challenges along with UCNPs and the requirements for developing the most efficient UCNPs-based NIR-light-responsive photocatalysts are discussed. Recent achievements on light absorption capability enhancement through the formation of hetero-nanostructure or coupling with plasmonic or semiconductor NPs improve the production of reactive oxygen species (ROS), because of their strong absorption in UV/visible range. The produced ROS are accessible to decompose the organic contaminants from the earth under NIR-induced irradiation of laser light. In heterogeneous photocatalytic reactions, the high efficiency of the UCNPs photocatalyst requires an adequate design that minimizes the loss of electrons during excitation and maximizes the absorption of photons. Important efforts are required to develop hetero-nanostructures between UCNPs and low-energy bandgap materials to fabricate heterogeneous photocatalysis. These heterogeneous junctions improve the absorption ability under UV/visible/NIR sunlight, which will further expand the migration of photo-induced charge transporters throughout the excitation state. The dualistic UCNPs-based nanocomposites with p-n & n-n hetero-nanostructure, and ternary UCNPs-nanocomposites fabricated with various heterostructures were extensively explored, as well as their mechanism for improved NIR light-harvesting and separation/relocation of charges are thoroughly discussed.