Abstract
AbstractThe photocatalytic synthesis of solar fuels such as hydrogen and methane from water and carbon dioxide is a promising strategy to store abundant solar energy in order to overcome its intermittency. Although this approach has been studied for decades using inorganic semiconductor photocatalysts, organic semiconductors have only recently gained notable attention. The tunable energy levels of organic semiconductors can enable the design of photocatalysts with optimized solar light utilization. However, the solar conversion efficiency of organic semiconductor photocatalysts has so far been limited by their low quantum efficiencies. To address this issue, various photocatalyst design strategies including semiconductor energy level optimization, surface modification, and the fabrication of heterojunctions have been applied, resulting in substantial increases in photocatalytic efficiency. This progress report systematically describes the strategies employed to increase the efficiency of organic semiconductor photocatalysts for the generation of solar fuels from water and carbon dioxide. Particular attention is given to describing strategies to enhance quantum efficiency, and insights are provided on the mechanisms underlying their success to aid the rational design of future organic photocatalysts. Perspectives on the future challenges and promising research directions for the design of efficient organic photocatalysts for the generation of solar fuels are also provided.
Highlights
Perhaps for this reason, the organic semiconductors used to fabricate OER photocatalysts (OEPs) have so far been limited to carbon nitride (CN) and polymer networks containing heterocycles such as triazine and heptazine that are rich in pyridinic nitrogens.[132,142,143,144]
The ability to precisely tune the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of non-CN organic semiconductors is a major advantage that has only recently begun to be exploited for the photocatalytic generation of solar fuels
An ever-increasing number of photocatalysts based on a wide range of non-CN semiconductor classes including linear conjugated polymers and a variety of crosslinked polymer networks and frameworks are being tested for H2 evolution, O2 evolution, overall water splitting (OWS), and CO2 reduction
Summary
Photocatalysts are materials that absorb light and convert it to electrical charges that can drive reductions and oxidations of species at the photocatalyst surface. The two half-reactions can either be driven simultaneously by a single photocatalyst (Figure 1a) or occur individually on two separate photocatalysts that operate in series (Figure 1b,c).[22] The latter configuration is known as a Z-scheme and is analogous to natural photosynthesis.[22] Because both semiconductors in a Z-scheme must be photoexcited in order for both half-reactions to take place, Z-schemes must absorb twice as many photons to generate the same amount of product as a single semiconductor that can drive both half-reactions. The conversion of solar to chemical energy in all photocatalytic reactions in both single semiconductor and Z-scheme configurations consists of four major steps: 1) photoexcitation of semiconductor to generate an electron–hole pair, 2) charge separation, 3) charge transport to catalytic sites at the photocatalyst surface, and 4) electrocatalytic reduction and oxidation of species in the surrounding matrix (Figure 2).[18,25]. General strategies that are applicable to all photocatalysts will be discussed first, followed by a discussion of how specific strategies have been applied to increase the efficiency of photo catalysts for H2 evolution, overall water splitting (OWS), O2 evolution, and CO2 reduction
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