Photoreduction of CO2 into sustainable and green solar fuels is generally believed to be an appealing solution to simultaneously overcome both environmental problems and energy crisis. The low selectivity of challenging multi-electron CO2 photoreduction reactions makes it one of the holy grails in heterogeneous photocatalysis. This Review highlights the important roles of cocatalysts in selective photocatalytic CO2 reduction into solar fuels using semiconductor catalysts. A special emphasis in this review is placed on the key role, design considerations and modification strategies of cocatalysts for CO2 photoreduction. Various cocatalysts, such as the biomimetic, metal-based, metal-free, and multifunctional ones, and their selectivity for CO2 photoreduction are summarized and discussed, along with the recent advances in this area. This Review provides useful information for the design of highly selective cocatalysts for photo(electro)reduction and electroreduction of CO2 and complements the existing reviews on various semiconductor photocatalysts.

Cocatalysts for Selective Photoreduction of CO2 into Solar Fuels.

Surface modification and enhanced photocatalytic CO2 reduction performance of TiO2: a review

As a promising approach to achieving two objectives with one strategy, photocatalytic CO2 conversion for C1/C2 “solar fuels” production can provide a package solution to the current global warming and growing energy demand by using inexhaustible solar energy and increasing atmospheric CO2. Although numerous efforts have been made to enhance the CO2 conversion efficiency through developing photocatalysts and CO2 reduction systems in recent years, some challenges still remain in improving the activity and selectivity of the CO2 photoreduction reactions. This review gives an overview of fundamental aspects and recent research advances of heterogeneous photocatalytic CO2 conversion systems in the last 3 years, and the catalysts are categorized as one-step excitation semiconductor systems, one-step excitation photosensitized semiconductor systems, and two-step excitation hybrid systems such as semiconductor heterojunction and Z-scheme systems. Also, some suggestions are given for further confirming that the ca...

Recent Advances in Heterogeneous Photocatalytic CO2 Conversion to Solar Fuels

Semiconductor nanowires (NWs) have been studied extensively for over two decades for their novel electronic, photonic, thermal, electrochemical and mechanical properties. This comprehensive review article summarizes major advances in the synthesis, characterization, and application of these materials in the past decade. Developments in the understanding of the fundamental principles of "bottom-up" growth mechanisms are presented, with an emphasis on rational control of the morphology, stoichiometry, and crystal structure of the materials. This is followed by a discussion of the application of nanowires in i) electronic, ii) sensor, iii) photonic, iv) thermoelectric, v) photovoltaic, vi) photoelectrochemical, vii) battery, viii) mechanical, and ix) biological applications. Throughout the discussion, a detailed explanation of the unique properties associated with the one-dimensional nanowire geometry will be presented, and the benefits of these properties for the various applications will be highlighted. The review concludes with a brief perspective on future research directions, and remaining barriers which must be overcome for the successful commercial application of these technologies.

/pdf/25th-anniversary-article-semiconductor-nanowires-synthesis-4ccslz11ke.pdf

25th Anniversary Article: Semiconductor Nanowires – Synthesis, Characterization, and Applications

Due to the large number of possible products and their similar reduction potentials, a significant challenge in CO2 photoreduction is achieving selectivity to a single product while maintaining high conversion efficiency. Controlling the reaction intermediates that form on the catalyst surface through careful catalyst design is therefore crucial. Here, we prepare atomically thin layers of sulfur-deficient CuIn5S8 that contain charge-enriched Cu–In dual sites, which are highly selective towards photocatalytic production of CH4 from CO2. We propose that the formation of a highly stable Cu–C–O–In intermediate at the Cu–In dual sites is the key feature determining selectivity. We suggest that this configuration not only lowers the overall activation energy barrier, but also converts the endoergic protonation step to an exoergic reaction process, thus changing the reaction pathway to form CH4 instead of CO. As a result, the CuIn5S8 single-unit-cell layers achieve near 100% selectivity for visible-light-driven CO2 reduction to CH4 over CO, with a rate of 8.7 μmol g−1 h−1. Many different molecules can form during photocatalytic reduction of CO2, so identifying catalyst structure–product selectivity relationships is vital. Here, the authors find that sulfur-deficient CuIn5S8 is highly selective to CH4 and suggest that the presence of Cu–In binding sites is key to this behaviour.

Selective visible-light-driven photocatalytic CO 2 reduction to CH 4 mediated by atomically thin CuIn 5 S 8 layers

We report on the first demonstration of stable photoelectrochemical water splitting and hydrogen generation on a double-band photoanode in acidic solution (hydrogen bromide), which is achieved by InGaN/GaN core/shell nanowire arrays grown on Si substrate using catalyst-free molecular beam epitaxy. The nanowires are doped n-type using Si to reduce the surface depletion region and increase current conduction. Relatively high incident-photon-to-current-conversion efficiency (up to ∼27%) is measured under ultraviolet and visible light irradiation. Under simulated sunlight illumination, steady evolution of molecular hydrogen is further demonstrated.

Highly stable photoelectrochemical water splitting and hydrogen generation using a double-band InGaN/GaN core/shell nanowire photoanode.

We report on the first demonstration of high-conversion-rate photochemical reduction of carbon dioxide (CO2) on gallium nitride (GaN) nanowire arrays into methane (CH4) and carbon monoxide (CO). It was observed that the reduction of CO2 to CO dominates on as-grown GaN nanowires under ultraviolet light irradiation. However, the production of CH4 is significantly increased by using the Rh/Cr2O3 core/shell cocatalyst, with an average rate of ∼3.5 μmol gcat–1 h–1 in 24 h. In this process, the rate of CO2 to CO conversion is suppressed by nearly an order of magnitude. The rate of photoreduction of CO2 to CH4 can be further enhanced and can reach ∼14.8 μmol gcat–1 h–1 by promoting Pt nanoparticles on the lateral m-plane surfaces of GaN nanowires, which is nearly an order of magnitude higher than that measured on as-grown GaN nanowire arrays. This work establishes the potential use of metal-nitride nanowire arrays as a highly efficient photocatalyst for the direct photoreduction of CO2 into chemical fuels. It al...

Wafer-Level Artificial Photosynthesis for CO2 Reduction into CH4 and CO Using GaN Nanowires

As a class of key building blocks in the chemical industry, aromatic compounds are mainly derived from the catalytic reforming of petroleum-based long chain hydrocarbons. The dehydroaromatization of methane can also be achieved by using zeolitic catalysts under relatively high temperature. Herein we demonstrate that Si-doped GaN nanowires (NWs) with a 97% rationally constructed m-plane can directly convert methane into benzene and molecular hydrogen under ultraviolet (UV) illumination at rt. Mechanistic studies suggest that the exposed m-plane of GaN exhibited particularly high activity toward methane C–H bond activation and the quantum efficiency increased linearly as a function of light intensity. The incorporation of a Si-donor or Mg-acceptor dopants into GaN also has a large influence on the photocatalytic performance.

Photoinduced Conversion of Methane into Benzene over GaN Nanowires

Syngas, the mixture of CO and H2, is a key feedstock to produce methanol and liquid fuels in industry, yet limited success has been made to develop clean syngas production using renewable solar energy. We demonstrated that syngas with a benchmark turnover number of 1330 and a desirable CO/H2 ratio of 1:2 could be attained from photoelectrochemical CO2 and H2O reduction in an aqueous medium by exploiting the synergistic co-catalytic effect between Cu and ZnO. The CO/H2 ratio in the syngas products was tuned in a large range between 2:1 and 1:4 with a total unity Faradaic efficiency. Moreover, a high Faradaic efficiency of 70 % for CO was acheived at underpotential of 180 mV, which is the lowest potential ever reported in an aqueous photoelectrochemical cell. It was found that the combination of Cu and ZnO offered complementary chemical properties that lead to special reaction channels not seen in Cu, or ZnO alone.

/pdf/tunable-syngas-production-from-co2-and-h2o-in-an-aqueous-macy3juppw.pdf

Tunable Syngas Production from CO2 and H2O in an Aqueous Photoelectrochemical Cell

H2 generation under sunlight offers great potential for a sustainable fuel production system. To achieve high efficiency solar-to-hydrogen conversion, multijunction photoelectrodes have been commonly employed to absorb a large portion of the solar spectrum and to provide energetic charge carriers for water splitting. However, the design and performance of such tandem devices has been fundamentally limited by the current matching between various absorbing layers. Here, by exploiting the lateral carrier extraction scheme of one-dimensional nanowire structures, we have demonstrated that a dual absorber photocathode, consisting of p-InGaN/tunnel junction/n-GaN nanowire arrays and a Si solar cell wafer, can operate efficiently without the strict current matching requirement. The monolithically integrated photocathode exhibits an applied bias photon-to-current efficiency of 8.7% at a potential of 0.33 V versus normal hydrogen electrode and nearly unity Faradaic efficiency for H2 generation. Such an adaptive mul...

Shizhao Fan

Papers

Highly stable photoelectrochemical water splitting and hydrogen generation using a double-band InGaN/GaN core/shell nanowire photoanode.

Wafer-Level Artificial Photosynthesis for CO2 Reduction into CH4 and CO Using GaN Nanowires

Photoinduced Conversion of Methane into Benzene over GaN Nanowires

Tunable Syngas Production from CO2 and H2O in an Aqueous Photoelectrochemical Cell

High Efficiency Solar-to-Hydrogen Conversion on a Monolithically Integrated InGaN/GaN/Si Adaptive Tunnel Junction Photocathode