Molecule/Semiconductor Hybrid Materials for Visible-Light CO2 Reduction: Design Principles and Interfacial Engineering

Abstract

ConspectusBecause of increasing concerns over the depletion of energy sources and the concomitant increase in CO2 emissions, much attention has been devoted to carbon capture and utilization technologies. Among the various methods and schemes proposed, visible-light-driven CO2 reduction in combination with water oxidation, one of the representative models of artificial photosynthesis, is an attractive solution because it enables abundant water and inexhaustible solar energy to be used to produce value-added chemicals.Molecular metal complexes and semiconductors are promising candidates for photocatalysts that can reduce CO2 to CO, formate, formaldehyde, or other hydrocarbons. Although both molecular metal complexes and semiconductors have strengths and weaknesses, their weaknesses (low oxidation ability and low selectivity for reduction reactions) can be overcome via the construction of a suitable molecule/semiconductor hybrid material. However, facilitating electron transfer at the molecule/semiconductor junction while suppressing unfavorable back electron transfer events is challenging. Consequently, the number of molecule/semiconductor hybrid systems that show a reasonable level of visible-light photocatalytic activity is limited, despite the development of a large number of visible-light-driven semiconductors and molecular photocatalysts (or catalysts).In this Account, we describe our approaches to developing hybrid photocatalysts and photoelectrodes for CO2 reduction. We have been developing both molecular (photo)catalysts and semiconductor photocatalysts individually, the latter of which are also designed for visible-light water splitting. For example, supramolecular photocatalysts that possess both photosensitizer and catalyst units in a single molecule can reduce CO2 to formate or CO in a homogeneous system, with high selectivity toward the desired product and high quantum yields of several tens of percent. However, nonoxide semiconductors such as C/N-based polymers and mixed-anion compounds exhibit a strong photooxidation ability under visible light. Carefully designed molecule/semiconductor hybrid materials achieve CO2 reduction under visible light with high product selectivity and stability even in an aqueous environment, where the concentration of CO2 is low but that of protons is high. Visible-light CO2 reduction combined with H2O oxidation is possible via the construction of a photoelectrochemical cell that comprises a molecular photocathode and an n-type semiconductor photoanode.Although our photosystems can be regarded as model systems for artificial photosynthesis, their light-energy conversion efficiencies are still unsatisfactory. To improve the efficiency, materials design, including interfacial engineering at the molecule/semiconductor junction, is important and is the general theme of the results highlighted in this Account.