Redox catalysis, including photocatalysis and (photo)electrocatalysis, may alleviate global warming and energy crises by removing excess CO2 from the atmosphere and converting it to value-added resources. Nano-to-atomic two-dimensional (2D) materials, clusters and single atoms are superior catalysts because of their engineerable ultrathin/small dimensions and large surface areas and have attracted worldwide research interest. Given the current gap between research and applications in CO2 reduction, our review systematically and constructively discusses nano-to-atomic surface strategies for catalysts reported to date. This work is expected to drive and benefit future research to rationally design surface strategies with multi-parameter synergistic impacts on the selectivity, activity and stability of next-generation CO2 reduction catalysts, thus opening new avenues for sustainable solutions to climate change, energy and environmental issues, and the potential industrial economy.

Surface strategies for catalytic CO2 reduction: from two-dimensional materials to nanoclusters to single atoms

Conjugated carbon nitride (CN) is an emerging and promising semiconductor photocatalyst for water photolysis owing to its unique properties. However, the traditional thermally induced polymerization of N-containing precursors typically produces melon-based CN solids with amorphous or semi-crystalline structures with only moderate photocatalytic performance. Many strategies have been developed to prepare crystalline CNs (CCNs), such as high-temperature and high-pressure routes, ionothermal synthesis, and microwave-assisted synthesis. In this Minireview, we summarize the progress that has been made in the synthesis of CCNs and their application in photocatalytic water splitting reactions. Three kinds of CCNs are mainly discussed according to their polymeric subunits. Challenges associated with CCNs and their future development are also included.

Crystalline Carbon Nitride Semiconductors for Photocatalytic Water Splitting.

This review focuses on the discussion of the latest progress and remaining challenges in selected metal-free photocatalysts for hydrogen production. The scope of this review is limited to the metal-free elemental photocatalysts (i.e. B, C, P, S, Si, Se etc.), binary photocatalysts (i.e. BC3, B4C, CxNy, h-BN etc.) and their heterojunction, ternary photocatalysts (i.e. BCN) and their heterojunction, and different types of organic photocatalysts (i.e. linear, covalent organic frameworks, microporous polymer, covalent triazine frameworks etc.) and their heterostructures. Following a succinct depiction of the latest progress in hydrogen evolution on these photocatalysts, discussion has been extended to the potential strategies that are deemed necessary to attain high quantum efficiency and high solar-to-hydrogen (STH) conversion efficiency. Issues with reproducibility and the disputes in reporting the hydrogen evolution rate have been also discussed with recommendations to overcome them. A few key factors are highlighted that may facilitate the scalability of the photocatalyst from microscale to macroscale production in meeting the targeted 10% STH. This review is concluded with additional perspectives regarding future research in fundamental materials aspects of high efficiency photocatalysts followed by six open questions that may need to be resolved by forming a global hydrogen taskforce in order to translate bench-top research into large-scale production of hydrogen.

Metal-free photocatalysts for hydrogen evolution.

Photocatalytic CO2 reduction over metal-organic framework-based materials

Defect engineering is a versatile approach to modulate band and electronic structures as well as materials performance. Herein, metal-organic frameworks (MOFs) featuring controlled structural defects, namely UiO-66-NH2 -X (X represents the molar equivalents of the modulator, acetic acid, with respect to the linker in synthesis), were synthesized to systematically investigate the effect of structural defects on photocatalytic properties. Remarkably, structural defects in MOFs are able to switch on the photocatalysis. The photocatalytic H2 production rate presents a volcano-type trend with increasing structural defects, where Pt@UiO-66-NH2 -100 exhibits the highest activity. Ultrafast transient absorption spectroscopy unveils that UiO-66-NH2 -100 with moderate structural defects possesses the fastest relaxation kinetics and the highest charge separation efficiency, while excessive defects retard the relaxation and reduce charge separation efficiency.

Switching on the Photocatalysis of Metal–Organic Frameworks by Engineering Structural Defects

Photosynthetic conversion of CO2 into fuel and chemicals is a promising but challenging technology. The bottleneck of this reaction lies in the activation of CO2 , owing to the chemical inertness of linear CO2 . Herein, we present a defect-engineering methodology to construct CO2 activation sites by implanting carbon vacancies (CVs) in the melon polymer (MP) matrix. Positron annihilation spectroscopy confirmed the location and density of the CVs in the MP skeleton. In situ diffuse reflectance infrared Fourier transform spectroscopy and a DFT study revealed that the CVs can function as active sites for CO2 activation while stabilizing COOH* intermediates, thereby boosting the reaction kinetics. As a result, the modified MP-TAP-CVs displayed a 45-fold improvement in CO2 -to-CO activity over the pristine MP. The apparent quantum efficiency of the MP-TAP-CVs was 4.8 % at 420 nm. This study sheds new light on the design of high-efficiency polymer semiconductors for CO2 conversion.

Carbon Vacancies in a Melon Polymeric Matrix Promote Photocatalytic Carbon Dioxide Conversion.

Metal-free carbonitride(CN) semiconductors are appealing light-transducers for photocatalytic redox reactions owing to the unique band gap and stability. To harness solar energy efficiently, CN catalysts that are active over a wider range of the visible spectrum are desired. Now a photochemical approach has been used to prepare a new-type triazine-based CN structure. The obtained CN shows extraordinary light-harvesting characteristics, with suitable semiconductor-redox potentials. The light absorption edge of the CN reaches up to 735 nm, which is significantly longer than that of the conventional CN semiconductor at about 460 nm. As expected, the CN can efficiently catalyze oxidation of alcohols and reduction of CO2 with visible light, even under red-light irradiation. The results represent an important step toward the development of red-light-responsive triazine-based structures for solar applications.

Photochemical Construction of Carbonitride Structures for Red-Light Redox Catalysis

Distorted carbon nitride nanosheets with activated n → π* transition and preferred textural properties for photocatalytic CO2 reduction

Photocatalytic water splitting necessitates robust cocatalysts to accelerate the oxygen evolution reaction (OER). However, most OER cocatalysts are based on noble metal oxides. Besides, the loose interface between semiconductor and cocatalyst results in inefficient charge transfer. The fabrication of photocatalysts with integrated light-harvesting and catalytic centers for OER is therefore desired. Herein, we provide a photocarving strategy to create nitrogen vacancies (NVs) on polymeric carbon nitride (PCN). It is confirmed that the embedded NVs can function as active sites to catalyze OER, while promoting the transfer of the photogenerated charge for OER. As a result, PCN-NVs without any extra noble-metal cocatalyst assistance exhibit an enhanced oxygen evolution rate compared with the pristine PCN. Additionally, the PCN obtained from other precursors can also be engineered by this photocarving method, while promoting oxygen photosynthesis. This work provides an avenue to design light-transducers with combined light-harvesting and catalytic configurations for oxygen synthesis chemistry.

Photocarving nitrogen vacancies in a polymeric carbon nitride for metal-free oxygen synthesis

Utilizing solar energy to convert CO2 into fuels and chemicals represents a promising solution to reduce reliance on fossil fuels, but it is hampered by the lack of highly-efficient catalysts. Herein, we report unique cobalt nitride/nitrogen-rich carbons (Co4N/NCs), which can work as noble-metal-free catalysts for CO2-to-CO conversion. The mass activity of Co4N/NCs is two orders of magnitude higher than those of previously reported CO2 photoreduction catalysts. The quantum yield for CO production at 450 nm reaches 7.2% with a turnover frequency per Co atom of 0.97 s-1. The electronic structure and coordinated environment of catalysts are analyzed by X-ray absorption fine structure spectroscopy and X-ray photoelectron spectroscopy. The reaction processes are investigated by in-situ diffuse reflectance infrared fourier transform spectroscopy and density functional theory calculations. Results suggest that the synergetic effects between the Co4N and the NCs can consolidate the adsorption and activation of CO2 and accelerate the interfacial electron-transfer kinetics between the Co4N/NCs catalysts and light-harvesting antenna, thereby resulting in the outstanding activity for synthesizing CO from CO2. This work offers the possibility to design Co-based host-guest topology for highly-efficient CO2 reduction.

Ruirui Wang

Papers

Carbon Vacancies in a Melon Polymeric Matrix Promote Photocatalytic Carbon Dioxide Conversion.

Photochemical Construction of Carbonitride Structures for Red-Light Redox Catalysis

Distorted carbon nitride nanosheets with activated n → π* transition and preferred textural properties for photocatalytic CO2 reduction

Photocarving nitrogen vacancies in a polymeric carbon nitride for metal-free oxygen synthesis

Cobalt Nitride Anchored on Nitrogen-Rich Carbons for Efficient Carbon Dioxide Reduction with Visible Light