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.

Construction of a ZnIn2S4/Au/CdS Tandem Heterojunction for Highly Efficient CO2 Photoreduction

“…In fact, heterojunctions can couple the advantages of two semiconductors to improve the light absorption efficiency and facilitate interfacial carrier transport. 6,7,16 Interestingly, the traditional type-II hetero- junction promotes the separation of photogenerated electrons (e − ) and holes (h + ) into different spaces. 18 Moreover, gold nanoparticles (Au NPs) have seen a surge of interest to be used in photocatalysts due to their excellent electrical conductivity, high selectivity, and optoelectronic properties.…”

“…Photocatalytic reduction of CO 2 is a sustainable technology that simulates the photosynthesis of green plants and converts CO 2 into valuable compounds. − The photocatalytic reduction technology has the advantage of being green and nonpolluting and has simple reaction conditions. In addition, the photoreduction of CO 2 is essentially an oxidation–reduction reaction process under the action of photogenerated carriers. − In the past few years, more and more semiconductors have been reported to be used for the photocatalytic reduction for CO 2 , such as TiO 2 , CuIn 5 S 8 , UiO-66, and g-C 3 N 4 . − Among them, ZnIn 2 S 4 as an eco-friendly photocatalyst with a typical layer structure has the advantages of an adjustable band gap, excellent photochemical stability, and low cost, so it is widely used in photocatalytic water hydrogen production, pollution degradation, CO 2 reduction reaction, etc. − Moreover, importantly, three-dimensional (3D) flower spherical ZnIn 2 S 4 has a suitable band structure that can not only make efficient use of visible light but also can convert CO 2 into valuable gaseous fuels; the excellent structure and specific surface area can provide more attachment sites for other semiconductors. , However, due to the rapid recombination of photogenerated carriers in the ZnIn 2 S 4 single-component semiconductor, a very small percentage of photogenerated electrons migrate to active sites to participate in the photocatalytic reduction reaction, resulting in a very low photocatalytic efficiency. , …”

“…To overcome the above difficulties, scientists have carried out a large number of theoretical studies and exploratory experiments, and many different strategies to increase the carrier migration rate have been proposed. − Combining other photocatalysts and building a heterojunction to prevent photogenerated carrier complexation in a single photocatalyst are a common and efficient approach. In fact, heterojunctions can couple the advantages of two semiconductors to improve the light absorption efficiency and facilitate interfacial carrier transport. ,, Interestingly, the traditional type-II heterojunction promotes the separation of photogenerated electrons (e – ) and holes (h + ) into different spaces . Moreover, gold nanoparticles (Au NPs) have seen a surge of interest to be used in photocatalysts due to their excellent electrical conductivity, high selectivity, and optoelectronic properties. − Especially, Au NPs’ fast electron transport ability greatly inhibit the recombination of carriers and reduce the disordered transport of photogenerated electrons .…”

Assembling CdSe Quantum Dots into Polymeric Micelles Formed by a Polyethylenimine-Based Amphiphilic Polymer to Enhance Efficiency and Selectivity of CO2-to-CO Photoreduction in Water

“…The candidate ligands are still limited, and sophisticated synthesis studies are needed. On another hand, the efficient and selective photocatalytic CO 2 reduction with QD photocatalysts in aqueous solution is still challenging; the reported activity and selectivity are far less than those of analogous systems in organic systems. , These motivate us to develop a new strategy to improve performance of photocatalytic CO 2 reduction in aqueous solution by using QD photocatalysts.…”

Reduced graphene oxide-modified Z-scheme g-C3N4/CdS photocatalyst with a staggered structure for the enhanced photoreduction of CO2

“…[18][19][20] The common 2D structure of g-C 3 N 4 has been widely used as a carrier material for building functional materials. [21][22][23] The construction of heterojunctions, element doping, introduction of defects, and other strategies have been used to improve the light absorption capacity, separation and transmission efficiency of photogenerated electrons in g-C 3 N 4 . [24][25][26][27] For example, in our previous work, the construction of a Z-scheme g-C 3 N 4 / Ag 3 VO 4 /rGO heterojunction effectively remedied the problem of the high recombination rate of g-C 3 N 4 electron-hole pairs and improved the utilization of photogenerated electrons in CO 2 reduction.…”