Advances on Photocatalytic CO2 Reduction Based on CdS and CdSe Nano-Semiconductors

Abstract

Carbon dioxide (CO2) is one of the main greenhouse gases in the atmosphere. The conversion of CO2 into solar fuels (CO, HCOOH, CH4, CH3OH, etc.) using artificial photosynthetic systems is an ideal way to utilize CO2 as a resource and reduce CO2 emissions. A typical artificial photosynthetic system is composed of three key components: a photosensitizer (PS) to harvest visible light, a catalyst (C) to catalyze CO2 or protons into carbon-based fuels or H2, respectively, and a sacrificial electron donor (SED) to consume the holes generated in the PS. In most cases, the PS and catalyst are two different components of a system. However, some components that possess both light harvesting and redox catalysis functionalities, e.g., nano-semiconductors, are referred to as photocatalysts. During photocatalysis, the PS is typically excited by photons to generate excited electrons. The excited electrons in the PS are transferred to the catalyst to generate a reduced catalyst. The reduced catalyst is used as an active intermediate to perform CO2 binding and transformation. The PS can be recovered through a reaction with the SED. Nano-semiconductors have been used as photosensitizers and/or photocatalysts in photocatalytic CO2 reduction systems owing to their excellent photophysical and photochemical properties and photostability. CdS and CdSe nano-semiconductors, such as quantum dots, nanorods, and nanosheets, have been widely used in the construction of photocatalytic CO2 reduction systems. Systems based on CdS or CdSe nano-semiconductors can be classified into three categories. The first category is systems based on CdS or CdSe photocatalysts. In these systems, CdS or CdSe nano-semiconductors function as photocatalysts to catalyze CO2 reduction without a co-catalyst under visible-light irradiation. The CO2 reduction reaction occurs at the surface of the CdS or CdSe nano-semiconductors. The second category is systems based on CdS or CdSe composite photocatalysts. CdS or CdSe nano-semiconductors are combined with functional materials, such as reduced graphene oxide or TiO2, to prepare composite photocatalysts. These composite photocatalysts are expected to improve the lifetime of the charge separation state and inhibit the photocorrosion of the nano-semiconductors during photocatalysis. The third category is hybrid systems containing a CdS nano-semiconductor and molecular catalysts, such as nickel and cobalt complexes and iron porphyrin. In these hybrid systems, CdS functions as a photosensitizer and the CO2 reduction reaction occurs at the molecular catalyst. This review article introduces the construction of artificial photosynthetic systems and the photocatalytic mechanism of nano-semiconductors, and summarizes the representative works in the three aforementioned categories of systems. Finally, the challenges of nano-semiconductors for photocatalytic CO2 reduction are discussed.