Recently, the missing link between homogeneous and heterogeneous catalysis has been found and it was named single-atom catalysis (SAC). However, the SAC field still faces important challenges, one of which is controlling the bonding/coordination between the single atoms and the support in order to compensate for the increase in surface energy when the particle size is reduced due to atomic dispersion. Excellent candidates to meet this requirement are carbon nitride (CN)-based materials. Metal atoms can be firmly trapped in nitrogen-rich coordination sites in CN materials, which makes them a unique class of hosts for preparing single-atom catalysts (SACs). As one of the most promising two-dimensional supports to stabilize isolated metal atoms, CN materials have been increasingly employed for preparing SACs. Herein, we will cover the most recent advances in single-atoms supported by CN materials. In this review, the most important characterization techniques and the challenges faced in this topic will be discussed, and the commonly employed synthetic methods will be delineated for different CN materials. Finally, the catalytic performance of SACs based on carbon nitrides will be reviewed with a special focus on their photocatalytic applications. In particular, we will prove CN as a non-innocent support. The relationship between single-atoms and carbon nitride supports is two-way, where the single-atoms can change the electronic properties of the CN support, while the electronic features of the CN matrix can tune the catalytic activity of the single sites in photocatalytic reactions. Finally, we highlight the frontiers in the field, including analytical method development, truly controlled synthetic methods, allowing the fine control of loading and multi-element synthesis, and how understanding the two-way exchange behind single-atoms and CN supports can push this topic to the next level.

Carbon nitride based materials: more than just a support for single-atom catalysis.

Catalytic transformation of CO2 into valuable fuels/chemicals is a promising and economically profitable process because it offers an alternative toward fossil feedstocks and the benefit of transforming and cycling the...

Recent Advances of Single-Atom Catalysts in CO2 Conversion

We report an oxygen vacancy (Vo)-rich metallic MoO2-x nano-sea-urchin with partially occupied band, which exhibits super CO2 (even directly from the air) photoreduction performance under UV, visible and near-infrared (NIR) light illumination. The Vo-rich MoO2-x nano-sea-urchin displays a CH4 evolution rate of 12.2 and 5.8 μmol gcatalyst-1 h-1 under full spectrum and NIR light illumination in concentrated CO2, which is ca. 7- and 10-fold higher than the Vo-poor MoO2-x, respectively. More interestingly, the as-developed Vo-rich MoO2-x nano-sea-urchin can even reduce CO2 directly from the air with a CO evolution rate of 6.5 μmol gcatalyst-1 h-1 under NIR light illumination. Experiments together with theoretical calculations demonstrate that the oxygen vacancy in MoO2-x can facilitate CO2 adsorption/activation to generate *COOH as well as the subsequent protonation of *CO towards the formation of CH4 because of the formation of a highly stable Mo-C-O-Mo intermediate.

Identification of the Active Sites on Metallic MoO2-x Nano-Sea-Urchin for Atmospheric CO2 Photoreduction Under UV, Visible and Near-Infrared Light Illumination.

Catalysis is at the heart of modern-day chemical and pharmaceutical industries, and there is an urgent demand to develop metal-free, high surface area, and efficient catalysts in a scalable, reproducible and economic manner. Amongst the ever-expanding two-dimensional materials family, carbon nitride (CN) has emerged as the most researched material for catalytic applications due to its unique molecular structure with tunable visible range band gap, surface defects, basic sites, and nitrogen functionalities. These properties also endow it with anchoring capability with a large number of catalytically active sites and provide opportunities for doping, hybridization, sensitization, etc. To make considerable progress in the use of CN as a highly effective catalyst for various applications, it is critical to have an in-depth understanding of its synthesis, structure and surface sites. The present review provides an overview of the recent advances in synthetic approaches of CN, its physicochemical properties, and band gap engineering, with a focus on its exclusive usage in a variety of catalytic reactions, including hydrogen evolution reactions, overall water splitting, water oxidation, CO2 reduction, nitrogen reduction reactions, pollutant degradation, and organocatalysis. While the structural design and band gap engineering of catalysts are elaborated, the surface chemistry is dealt with in detail to demonstrate efficient catalytic performances. Burning challenges in catalytic design and future outlook are elucidated.

Multifunctional carbon nitride nanoarchitectures for catalysis.

Drawing inspiration from the natural process of photosynthesis found in plant leaves, scientists are exploring the use of photocatalysis to convert carbon dioxide (CO2) into valuable products using solar light and water. Photocatalytic CO2 conversion has emerged as one of the efficient green approaches to revitalize the environment from greenhouse gas pollution. Owing to its visible-range band gap, non-toxicity, ease of synthesis at economic costs and stability under light irradiation, g-C3N4 has emerged as the most explored photocatalyst. However, due to rampant exciton recombination owing to poor electrical conductivity, the efficiency of CO2 reduction falls short for g-C3N4 in its pure/pristine form. Therefore, the structural manoeuvre of g-C3N4 materials using N-rich configurations, heteroatom/single-atom doping and hybridization with various functional materials including metal oxides/sulfides, perovskite halides and metal complexes has been adopted, thereby overcoming their inherent drawbacks in photocatalytic CO2 reduction. In this timely review, we present an overview of the recent advances in surface/interface engineering of carbon nitrides for the conversion of CO2 to fuels and useful chemical by-products. More importance is given to the critical evaluation of surface manipulation in carbon nitrides and how it amplifies and affects their photocatalytic properties in CO2 reduction. Finally, we provide a comprehensive outlook into the future directions of these functionalised carbon nitrides for various applications. We strongly believe that this unique review will offer new knowledge on the surface property relationship of carbon nitride-based materials and their impact on enhancing their performance in various photocatalytic reactions and further create new opportunities for them in various areas. 

Carbon nitride based nanoarchitectonics for nature-inspired photocatalytic CO2 reduction

Artificial photosynthesis is a promising strategy for converting CO 2 and H 2 O into fuels and value-added products, while the low catalytic efficiency greatly restricts its practical applications. Herein, we demonstrated that graphitic carbon nitride with spatially confined Fe single-atom and potassium ion (FeN 4 /K-g-C 3 N 4 ) exhibited the high activity and selectivity for photocatalytic CO 2 reduction. Specifically, the conversion rates of CO 2 into CO could achieve up to 20.00 μmol g −1 h −1 with nearly 100% selectivity, more than 10 times higher performances than pristine g-C 3 N 4 . Comprehensive characterizations and theoretical calculations revealed that the single-atom Fe bonded with four N atoms in g-C 3 N 4 intralayer, which serve as the active center for absorption and activation of CO 2 molecules. The alkali K ions inserted the g-C 3 N 4 interlayers owing to their suitable diameters, which could effectively promote charge separation and transfer. Synergizing the spatial confinements of Fe single-atoms and K ions in g-C 3 N 4 remarkably promoted the photocatalytic activity and selectivity for CO 2 reduction into CO. Spatially confined iron single-atom (intralayer) and potassium (interlayer) ion could remarkably promote both activity and selectivity of g-C 3 N 4 for photocatalytic CO 2 reduction into CO. • One-step molten-salt-assisted route was developed for fabricating dual-spatially confined Fe single-atom and K + ion in C 3 N 4 . • The single-atom Fe acted as active center for significantly promoting CO 2 absorption and conversion. • The intercalated K ions remarkably promoted charge transfer between adjacent layers in C 3 N 4 . • The photocatalytic CO 2 reduction activity and selectivity have been significantly enhanced. 

Spatially confined iron single-atom and potassium ion in carbon nitride toward efficient CO2 reduction

Infrared lenses have a wide range of applications in thermal imaging and detection. Here, by designing a single layer metasurface consisting of anisotropic microstructures, we experimentally demonstrate a polarization-insensitive long-wavelength infrared achromatic metalens. The focal lengths of the fabricated metalens with a numerical aperture of 0.33 remain unchanged and maintain diffraction-limited performance for the incident wavelength varying from 8.5 to 11.5 μm, demonstrating the elimination of chromatic aberration with 30% operation bandwidth. We envision this metalens may pave the way to the development of ultracompact achromatic thermal imaging systems.

A polarization-insensitive infrared broadband achromatic metalens consisting of all-silicon anisotropic microstructures

The time evolution of the polarization of a rubidium atom spin ensemble driven by a resonant radio-frequency (RF) magnetic field is analyzed based on the rate equation. A simple optical pumping experimental system is constructed and the time response of the rubidium atomic ensemble is demonstrated by recording the transmitted intensity of pumping light. In the steady-state response, the polarization difference between the optical pumping steady state and the magnetic resonance steady state depends on the optical pumping power and RF magnetic intensity. We can obtain the optimal power value corresponding to the maximum polarization difference. In terms of transient response, where the intensity of RF magnetic field is too weak to observe Rabi oscillations, two decay processes between magnetic resonance and optical pumping steady states are monitored. The decay time from magnetic resonance steady state to optical pumping steady state depends on the optical pumping rate and the spin relaxation rate. The decay time from optical pumping steady state to magnetic resonance steady state depends on the optical pumping rate, the RF driving rate, and the spin relaxation rate. The scale factor of pumping rate to pumping power is obtained, in addition to that of RF driving rate to the RF magnetic field. It can provide an intuitive understanding of the spin dynamic evolution of the polarized atomic ensemble.

Time response of spin-polarized rubidium thermal gas with radio-frequency pulse driving

Polarimetry, refers to the measurement of polarization states, has triggered vital applications in polarization imaging, biological sensing, and quantum optics. Traditional polarimetry has been well established to characterize polarization states but with bulk optical elements. With the development of micro- and nano-optics, a polarimetry device with ultracompact architecture is highly on-demand. Herein, a silicon metasurface-based full-Stokes polarimetry device for long-wavelength infrared light is proposed and experimentally demonstrated. The metasurface is composed of silicon micropillar array, each of which can be regarded as a micro-scale birefringent wave-plate. Through a polarization-dependent phase profile design, the metasurface has the ability to split and focus different polarization states and, thus, is capable of achieving full-Stokes polarimetry. The demonstrated strategy holds the potential applications for infrared polarization detection and imaging in compact systems.

A silicon metasurface for full-Stokes polarimetry of infrared light

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Spatially confined iron single-atom and potassium ion in carbon nitride toward efficient CO2 reduction

A polarization-insensitive infrared broadband achromatic metalens consisting of all-silicon anisotropic microstructures

Time response of spin-polarized rubidium thermal gas with radio-frequency pulse driving

A silicon metasurface for full-Stokes polarimetry of infrared light