Imidazolium-modification enhances photocatalytic CO2 reduction on ZnSe quantum dots

Colloidal photocatalysts can utilize solar light for the conversion of CO₂ to carbon-based fuels, but controlling the product selectivity for CO₂ reduction remains challenging, in particular in aqueous solution. Here, we present an organic surface modification strategy to tune the product selectivity of colloidal ZnSe quantum dots (QDs) towards photocatalytic CO₂ reduction even in the absence of transition metal co-catalysts. Besides H₂, imidazolium-modified ZnSe QDs evolve up to 2.4 mmol_CO g_ZnSe⁻¹ (TON_QD > 370) after 10 h of visible light irradiation (AM 1.5G, λ > 400 nm) in aqueous ascorbate solution with a CO-selectivity of up to 20%. This represents a four-fold increase in CO-formation yield and 13-fold increase in CO-selectivity compared to non-functionalized ZnSe QDs. The binding of the thiolated imidazolium ligand to the QD surface is characterized quantitatively using ¹H-NMR spectroscopy and isothermal titration calorimetry, revealing that a subset of 12 to 17 ligands interacts strongly with the QDs. Transient absorption spectroscopy reveals an influence of the ligand on the intrinsic charge carrier dynamics through passivating Zn surface sites. Density functional theory calculations indicate that the imidazolium capping ligand plays a key role in stabilizing the surface-bound *CO₂⁻ intermediate, increasing the yield and selectivity toward CO production. Overall, this work unveils a powerful tool of using organic capping ligands to modify the chemical environment on colloids, thus enabling control over the product selectivity within photocatalyzed CO₂ reduction.

A photocatalyst system consisting of ZnSe quantum dots modified with a thiolated imidazolium capping ligand for visible light-driven reduction of aqueous CO₂ to CO is reported without the need for a metal complex co-catalyst.     

Mechanistic study of photocatalytic CO2 reduction using a Ru(ii)–Re(i) supramolecular photocatalyst

Supramolecular photocatalysts comprising [Ru(diimine)₃]²⁺ photosensitiser and fac-[Re(diimine)(CO)₃{OC(O)OC₂H₄NR₂}] catalyst units can be used to reduce CO₂ to CO with high selectivity, durability and efficiency. In the presence of triethanolamine, the Re catalyst unit efficiently takes up CO₂ to form a carbonate ester complex, and then direct photocatalytic reduction of a low concentration of CO₂, e.g., 10% CO₂, can be achieved using this type of supramolecular photocatalyst. In this work, the mechanism of the photocatalytic reduction of CO₂ was investigated applying such a supramolecular photocatalyst, RuC2Re with a carbonate ester ligand, using time-resolved visible and infrared spectroscopies and electrochemical methods. Using time-resolved spectroscopic measurements, the kinetics of the photochemical formation processes of the one-electron-reduced species RuC2(Re)−, which is an essential intermediate in the photocatalytic reaction, were clarified in detail and its electronic structure was elucidated. These studies also showed that RuC2(Re)− is stable for 10 ms in the reaction solution. Cyclic voltammograms measured at various scan rates besides temperature and kinetic analyses of RuC2(Re)− produced by steady-state irradiation indicated that the subsequent reaction of RuC2(Re)− proceeds with an observed first-order rate constant of approximately 1.8 s⁻¹ at 298 K and is a unimolecular reaction, independent of the concentrations of both CO₂ and RuC2(Re)−.

Formation processes and reactivity of an important intermediate of photocatalytic CO₂ reduction, one-electron reduced species of a Ru(ii)–Re(i) supramolecular photocatalyst with a carbonate ester ligand, were investigated in detail.     

Dual Ag/Co cocatalyst synergism for the highly effective photocatalytic conversion of CO2 by H2O over Al-SrTiO3

Loading Ag and Co dual cocatalysts on Al-doped SrTiO₃ (AgCo/Al-SrTiO₃) led to a significantly improved CO-formation rate and extremely high selectivity toward CO evolution (99.8%) using H₂O as an electron donor when irradiated with light at wavelengths above 300 nm. Furthermore, the CO-formation rate over AgCo/Al-SrTiO₃ (52.7 μmol h⁻¹) was a dozen times higher than that over Ag/Al-SrTiO₃ (4.7 μmol h⁻¹). The apparent quantum efficiency for CO evolution over AgCo/Al-SrTiO₃ was about 0.03% when photoirradiated at a wavelength at 365 nm, with a CO-evolution selectivity of 98.6% (7.4 μmol h⁻¹). The Ag and Co cocatalysts were found to function as reduction and oxidation sites for promoting the generation of CO and O₂, respectively, on the Al-SrTiO₃ surface.

Deposition Ag and Co dual cocatalysts onto Al-SrTiO₃ significantly improves its activity for photoreduction of CO₂ by H₂O, with extremely high selectivity to CO evolution (99.8%), in which Ag and Co enable CO₂ reduction and H₂O oxidation, respectively.     

Quasi-square-shaped cadmium hydroxide nanocatalysts for electrochemical CO2 reduction with high efficiency

Powered by a renewable electricity source, electrochemical CO₂ reduction reaction is a promising solution to facilitate the carbon balance. However, it is still a challenge to achieve a desired product with commercial current density and high efficiency. Herein we designed quasi-square-shaped cadmium hydroxide nanocatalysts for CO₂ electroreduction to CO. It was discovered that the catalyst is very active and selective for the reaction. The current density could be as high as 200 mA cm⁻² with a nearly 100% selectivity in a commonly used H-type cell using the ionic liquid-based electrolyte. In addition, the faradaic efficiency of CO could reach 90% at a very low overpotential of 100 mV. Density functional theory studies and control experiments reveal that the outstanding performance of the catalyst was attributed to its unique structure. It not only provides low Cd–O coordination, but also exposes high activity (002) facet, which requires lower energy for the formation of CO. Besides, the high concentration of CO can be achieved from the low concentration CO₂via an adsorption-electrolysis device.

Quasi-square cadmium hydroxide nanocrystals (Cdhy-QS) showed outstanding performance for electroreduction CO₂ to CO.     

The in situ study of surface species and structures of oxide-derived copper catalysts for electrochemical CO2 reduction

Oxide-derived copper (OD-Cu) has been discovered to be an effective catalyst for the electroreduction of CO₂ to C2+ products. The structure of OD-Cu and its surface species during the reaction process are interesting topics, which have not yet been clearly discussed. Herein, in situ surface-enhanced Raman spectroscopy (SERS), operando X-ray absorption spectroscopy (XAS), and ¹⁸O isotope labeling experiments were employed to investigate the surface species and structures of OD-Cu catalysts during CO₂ electroreduction. It was found that the OD-Cu catalysts were reduced to metallic Cu(0) in the reaction. CuO_x species existed on the catalyst surfaces during the CO₂RR, which resulted from the adsorption of preliminary intermediates (such as *CO₂ and *OCO⁻) on Cu instead of on the active sites of the catalyst. It was also found that abundant interfaces can be produced on OD-Cu, which can provide heterogeneous CO adsorption sites (strong binding sites and weak binding sites), leading to outstanding performance for obtaining C2+ products. The Faradaic efficiency (FE) for C2+ products reached as high as 83.8% with a current density of 341.5 mA cm⁻² at −0.9 V vs. RHE.

CuO_x species were shown to exist on OD-Cu during the CO₂RR, which resulted from the adsorption of preliminary intermediates (such as *CO₂ and *OCO⁻) on Cu instead of on the active sites of the catalyst.     

Supramolecular photocatalysts fixed on the inside of the polypyrrole layer in dye sensitized molecular photocathodes: application to photocatalytic CO2 reduction coupled with water oxidation

The development of systems for photocatalytic CO₂ reduction with water as a reductant and solar light as an energy source is one of the most important milestones on the way to artificial photosynthesis. Although such reduction can be performed using dye-sensitized molecular photocathodes comprising metal complexes as redox photosensitizers and catalyst units fixed on a p-type semiconductor electrode, the performance of the corresponding photoelectrochemical cells remains low, e.g., their highest incident photon-to-current conversion efficiency (IPCE) equals 1.2%. Herein, we report a novel dye-sensitized molecular photocathode for photocatalytic CO₂ reduction in water featuring a polypyrrole layer, [Ru(diimine)₃]²⁺ as a redox photosensitizer unit, and Ru(diimine)(CO)₂Cl₂ as the catalyst unit and reveal that the incorporation of the polypyrrole network significantly improves reactivity and durability relative to those of previously reported dye-sensitized molecular photocathodes. The irradiation of the novel photocathode with visible light under low applied bias stably induces the photocatalytic reduction of CO₂ to CO and HCOOH with high faradaic efficiency and selectivity (even in aqueous solution), and the highest IPCE is determined as 4.7%. The novel photocathode is coupled with n-type semiconductor photoanodes (CoO_x/BiVO₄ and RhO_x/TaON) to construct full cells that photocatalytically reduce CO₂ using water as the reductant upon visible light irradiation as the only energy input at zero bias. The artificial Z-scheme photoelectrochemical cell with the dye-sensitized molecular photocathode achieves the highest energy conversion efficiency of 8.3 × 10⁻²% under the irradiation of both electrodes with visible light, while a solar to chemical conversion efficiency of 4.2 × 10⁻²% is achieved for a tandem-type cell using a solar light simulator (AM 1.5, 100 mW cm⁻²).

A novel dye-sensitized molecular photocathode with polypyrrole networks exhibits high efficiency and durability for photocatalytic CO₂ reduction by using water as reductant and visible light as energy.     

Selective Photocatalytic Reduction of CO2 to CO Mediated by Silver Single Atoms Anchored on Tubular Carbon Nitride

Artificial photosynthesis is a promising strategy for converting carbon dioxide (CO₂) and water (H₂O) into fuels and value-added chemical products. However, photocatalysts usually suffered from low activity and product selectivity due to the sluggish dynamic transfer of photoexcited charge carriers. Herein, we describe anchoring of Ag single atoms on hollow porous polygonal C₃N₄ nanotubes (PCN) to form the photocatalyst Ag₁@PCN with Ag−N₃ coordination for CO₂ photoreduction using H₂O as the reductant. The as-synthesized Ag₁@PCN exhibits a high CO production rate of 0.32 μmol h⁻¹ (mass of catalyst: 2 mg), a high selectivity (>94 %), and an excellent stability in the long term. Experiments and density functional theory (DFT) reveal that the strong metal–support interactions (Ag−N₃) favor *CO₂ adsorption, *COOH generation and desorption, and accelerate dynamic transfer of photoexcited charge carriers between C₃N₄ and Ag single atoms, thereby accounting for the enhanced CO₂ photoreduction activity with a high CO selectivity. This work provides a deep insight into the important role of strong metal–support interactions in enhancing the photoactivity and CO selectivity of CO₂ photoreduction.     

Solvent-mediated outer-sphere CO2 electro-reduction mechanism over the Ag111 surface

The electrocatalytic CO₂ reduction reaction (CO₂RR) is one of the key technologies of the clean energy economy. Molecular-level understanding of the CO₂RR process is instrumental for the better design of electrodes operable at low overpotentials with high current density. The catalytic mechanism underlying the turnover and selectivity of the CO₂RR is modulated by the nature of the electrocatalyst, as well as the electrolyte liquid, and its ionic components that form the electrical double layer (EDL). Herein we demonstrate the critical non-innocent role of the EDL for the activation and conversion of CO₂ at a high cathodic bias for electrocatalytic conversion over a silver surface as a representative low-cost model cathode. By using a multiscale modeling approach we demonstrate that under such conditions a dense EDL is formed, which hinders the diffusion of CO₂ towards the Ag111 electrocatalyst surface. By combining DFT calculations and ab initio molecular dynamics simulations we identify favorable pathways for CO₂ reduction directly over the EDL without the need for adsorption to the catalyst surface. The dense EDL promotes homogeneous phase reduction of CO₂via electron transfer from the surface to the electrolyte. Such an outer-sphere mechanism favors the formation of formate as the CO₂RR product. The formate can undergo dehydration to CO via a transition state stabilized by solvated alkali cations in the EDL.

In addition to the commonly accepted inner-sphere mechanism for e⁻ transfer, we show that an outer-sphere electron transfer from the cathode to CO₂ is operable at high overpotentials.     

2D-C3N4 encapsulated perovskite nanocrystals for efficient photo-assisted thermocatalytic CO2 reduction

Very recently, halide perovskites, especially all-inorganic CsPbBr₃, have received ever-increasing attention in photocatalysis owing to their superior optoelectronic properties and thermal stability. However, there is a lack of study on their application in thermocatalysis and photo-thermocatalysis. Herein, we rationally designed a core–shell heterojunction formed by encapsulating CsPbBr₃ nanoparticles with the 2D C₃N₄ (m-CN) layer via a solid-state reaction (denoted as m-CN@CsPbBr₃). A series of experiments suggest that abundant adsorption and active sites of CO₂ molecules as well as polar surfaces were obtained by utilizing m-CN-coated CsPbBr₃, resulting in significant improvement in CO₂ capture and charge separation. It is found that the m-CN@CsPbBr₃ effectively drives the thermocatalytic reduction of CO₂ in H₂O vapor. By coupling light into the system, the activity for CO₂-to-CO reduction is further improved with a yield up to 42.8 μmol g⁻¹ h⁻¹ at 150 °C, which is 8.4 and 2.3 times those of pure photocatalysis (5.1 μmol g⁻¹ h⁻¹) and thermocatalysis (18.7 μmol g⁻¹ h⁻¹), respectively. This work expands the application of general halide perovskites and provides guidance for using perovskite-based catalysts for photo-assisted thermocatalytic CO₂ reduction.

A water-stable CsPbBr₃ catalyst is designed using core–shell encapsulation of the perovskite nanoparticle by 2D-C₃N₄ for photo-assisted thermocatalytic CO₂ reduction by H₂O. The m-CN@CsPbBr₃ heterojunction shows surprisingly high CO₂-to-CO yield.

The reduction of CO₂ into valuable hydrocarbon fuels via chemical catalytic processes to mitigate the greenhouse-effect has received wide attention.^1,2 Basically, the CO₂ molecule has a straight double-bonded non-polar structure, resulting in a huge activation energy barrier for the CO₂ reduction reaction (CRR).^3,4 Using a catalyst to adsorb and then activate the CO₂ molecules with free electrons is thus imperative.^5,6 Currently, there are three main types of catalytic CO₂ reduction reactions, i.e. thermocatalysis,⁷ electrocatalysis^8a,b and photocatalysis,^4b,c,5c where the catalysts used are referred to as thermocatalysts, electrocatalysts and photocatalysts, respectively. Traditionally, thermocatalysis is regarded as the conventional CRR process where the CO₂ molecules are well-activated and reduced by thermocatalysts using heat and the reductant agent H₂. Hence, thermocatalysis usually shows a relatively higher CRR efficiency compared to the other two cases.^7a,b,9a However, as it is very complex to safely transport and use H₂ for this process, it is more advantageous to use safe and free H₂O as the reductant if high thermocatalytic CRR efficiency can be achieved. Photocatalysis is regarded as an effective approach for CO₂ reduction owing to the merits of utilizing solar energy directly and low energy consumption. However, the efficiency of the photocatalytic CRR is still limited by the sluggish kinetics of CO₂ activation and H₂O dissolving.^4c,9b Very recently, Xu and co-workers reported a novel lead-free perovskite, Cs₃Sb₂I₉, for CO₂ reduction to CO and CH₄via photothermal synergistic catalysis without using any sacrificial agents or cocatalysts.^9c Unfortunately, the perovskite is not stable enough in the working solution. In addition, its activity is still limited. There is still some room to develop a simple method for synthesizing effective and stable photothermal catalysts.All-inorganic perovskites, for example cesium lead tribromide (CsPbBr₃), have emerged as a promising type of photocatalyst for the CRR, owing to their long photogenerated carrier diffusion length, tunable size, wide light-absorption range, etc.^5c,10 CsPbBr₃ perovskites have been widely explored in composites with other materials, such as g-C₃N₄,¹¹ graphene oxide^5a,12 and TiO₂ (ref. 13) to construct heterojunctions for photocatalytic CO₂ reduction. For example, Xu and his co-workers mixed CsPbBr₃ with NH_x-rich g-C₃N₄ nanosheets to construct a heterojunction with a fast carrier-transfer bridge for improved charge separation and hence enhanced photocatalytic activity for CO₂ reduction.^14a Similarly, Lu''s group reported the coating of CsPbBr₃ with graphdiyne as a physical protection layer to tackle the stability issue in photocatalytic CO₂ reduction in H₂O vapor, given that the perovskite shows a low tolerance towards water.^14b It should be mentioned that most of the reported halide perovskite nanocrystals (NCs) were prepared by the solution method using organic solvents, which is troublesome for large-scale production.^15a,b Besides, considering that thermocatalysis usually outperforms photocatalysis in CO₂ reduction, it would be intriguing to explore the catalytic activity of CsPbBr₃ under coupled thermal and irradiation effects. To the best of our knowledge, there are few reports of water-stable CsPbBr₃-based thermal CRR with H₂O as a reducing agent.Herein, using a molten-salt method, we elaborately encapsulate CsPbBr₃ with a 2D m-CN layer to construct a water-stable heterojunction for photo-assisted thermal catalytic CO₂ reduction. We found that under pure thermocatalysis conditions, m-CN@CsPbBr₃ showed the ability to drive the CRR reduction to CO using the CO₂ and H₂O as reactants. Moreover, with the introduction of simulated solar illumination, the corresponding reduction yield rises to 42.8 μmol g⁻¹ h⁻¹, which is 2.3 and 8.4 times those of pure thermocatalysts and photocatalysts, respectively. This is the first report on the fabrication of CsPbBr₃ perovskite NCs by means of a solid-phase reaction. This work expands the application of CsPbBr₃ perovskite and can help us better understand CO₂ reduction by H₂O.A molten-salt method, where ion salts act as a high-temperature liquid solvent to accelerate the dissolution of raw materials, the transport of reactants, and the directional assembly of basic units, was used to prepare perovskite catalysts with a specific morphology.^15c It is worth noting that melem can be obtained by thermal condensation of urea below 450 °C, while g-C₃N₄ can be acquired in the temperature range between 480 °C and 550 °C (Fig. S1a^†).^16a Therefore, the composition of the coating layer may be assigned to an intermediate product (named m-CN) possessing many edge NH_x groups between melem and g-C₃N₄, which is attributed to the higher local temperature (>450 °C) caused by the molten salt. To confirm this, a comparison study is performed at 420 °C and 470 °C for the synthesis of m-CN@CsPbBr₃ catalysts. As revealed in Fig. S1b,^† the best preparation temperature is 450 °C for m-CN@CsPbBr₃ catalysts, while the phase transition of CsPbBr₃ occurs at 470 °C. During the heat treatment, CsPbBr₃ was changed into the solution state, i.e. molten salt (Fig. S2^†). Meanwhile the polymerized monomers of carbon nitride adsorbed onto the surface of CsPbBr₃, and then the micron-sized CsPbBr₃ was “cut” into nanoparticles by encapsulation with m-CN coatings, as shown in Fig. 1a. To elucidate the formation process of m-CN@CsPbBr₃, different coatings with CsPbBr₃-to-urea mass ratios of 1 : 3, 1 : 5 and 1 : 10 were synthesized and are denoted as m-CN@CsPbBr₃-3, m-CN@CsPbBr₃-5 and m-CN@CsPbBr₃-10, respectively. The corresponding XRD and FTIR results also suggest the encapsulation of CsPbBr₃ with m-CN in these samples, as shown in Fig. S3a and b.^† It should be noted that the m-CN@CsPbBr₃-3 sample gives the typical peaks of g-C₃N₄ at 2-theta of 13 and 27°, while the broad XRD peaks of m-CN@CsPbBr₃-10 could be ascribed to melem, suggesting that a certain content of molten salt is required to improve the temperature profile for the thermal polymerization of the CN coating layers.^16b Fourier transform infrared (FTIR) spectroscopy of m-CN@CsPbBr₃-3 and m-CN@CsPbBr₃-10 showed similar peaks, where the peaks in the wide range from 1200 to 1700 cm⁻¹ are attributed to the skeleton signal of g-C₃N₄.^16c The absorption bands from 3000 to 3500 cm⁻¹ come from NH_x and –OH groups.^14a,16b In comparison to m-CN@CsPbBr₃-10, the normalized intensity of the band from 3000 to 3500 cm⁻¹ for m-CN@CsPbBr₃-3 is evidently decreased, which can be ascribed to the stronger interactions of edge NH_x groups and bromide anions of CsPbBr₃.¹⁶ It is also observed that the m-CN@CsPbBr₃-3 peaks shifted to lower values compared with m-CN@CsPbBr₃-10, as shown in the magnified FTIR spectra of Fig. S3,^† suggesting stronger interactions occurred with the introduction of more bromide anions.^14a,16d When the relative mass ratio of urea to CsPbBr₃ is changed, either the amount of loaded CsPbBr₃ particles is lower and sporadic (Fig. S4a,^† m-CN@CsPbBr₃-10) or the CsPbBr₃ particles are larger and exposed (Fig. S4b,^† m-CN@CsPbBr₃-3), whereas the m-CN@CsPbBr₃-5 sample displays completely uniform and coated CsPbBr₃ particles, suggesting that the appropriate ratio of urea to CsPbBr₃ is critical for optimal m-CN@CsPbBr₃ formation. Therefore, the m-CN@CsPbBr₃-5 catalyst with an optimized coating ratio was chosen for further study because it may be endowed with a more heterogeneous interface and excellent stability. For the synthesis of pure CsPbBr₃ for further composition, vacuum was employed to obtain less-defective CsPbBr₃. We also attempted to treat the mixture of CsPbBr₃ and urea in air. However, very little polymerization of urea to carbon nitride occurred, and thus, the N₂ condition was chosen for m-CN@CsPbBr₃ preparation. Therefore, we successfully encapsulated the perovskite CsPbBr₃ with a 2D m-CN layer by utilizing CsPbBr₃ as a molten salt under N₂ conditions. It is greatly emphasized that this is the first report on the fabrication of CsPbBr₃ perovskite NCs by means of a solid-phase reaction. Differing from the traditional synthesis approaches such as ball milling and solution processing strategies, this unique method can avoid some difficulties, including a complicated procedure and the formation of organic branches and undesired phases, showing the attractive application of this method for fabricating catalysts beyond perovskites.Open in a separate windowFig. 1(a) Illustration of the synthesis of m-CN@CsPbBr₃; (b) XRD patterns of m-CN@CsPbBr₃, the reference m-CN and CsPbBr₃; (c and d) TEM image and the corresponding HRTEM result of m-CN@CsPbBr₃, where the yellow arrows mark the thin m-CN layers.As a representative material, m-CN@CsPbBr₃-5 is selected to investigate the influence of encapsulating perovskite nanocrystals with 2D m-CN for boosting photo-assisted thermocatalytic CO₂ reduction. The XRD patterns in Fig. 1b show that the intensity of the peaks ascribed to CsPbBr₃ was obviously decreased after the m-CN encapsulation, suggesting that the crystallinity of CsPbBr₃ is reduced. Magnification of the diffraction region from 21.2 to 22° suggests that the m-CN@CsPbBr₃ sample exhibits a larger full-width at half maximum (FWHM) than bare CsPbBr₃ (Fig. S5^†), further indicating that the particle size was decreased by the calcination treatment. It should also be noted that the as-prepared CsPbBr₃ formed in a vacuum also showed a crystal-lattice orientation, given that the texture coefficient of the [200] peak is much higher than the standard value of 1, while it is less than 1 for CsPbBr₃ prepared in N₂ and air (Fig. S6a and b^†). It is noted that the texture coefficient of the [200] peak is in agreement with CsPbBr₃ prepared in N₂ after m-CN coating, which further proved the melt-crystallization process of CsPbBr₃. The FTIR spectra of m-CN@CsPbBr₃ and melem showed similar peaks (Fig. S7^†), and the normalized intensity of the band from 3000 to 3500 cm⁻¹ of m-CN@CsPbBr₃ is obviously decreased compared to the reference m-CN. Furthermore, compared with the pure m-CN layer, the peaks for m-CN@CsPbBr₃ located at 1250 cm⁻¹, corresponding to the typical stretching mode of aromatic C–N and C N heterocycles in m-CN, are systematically shifted to lower values in the magnified FTIR spectra in Fig. S7,^† suggesting that the edge NH_x groups interact with the bromide anion of CsPbBr₃via ionic bonding.^16,17aFig. 1c displays the transmission electron microscopy (TEM) image of the m-CN@CsPbBr₃ sample. Clearly, it shows that the CsPbBr₃ nanoparticles were uniformly encapsulated by the m-CN layer. The corresponding high-resolution transmission electron microscopy (HRTEM) images further show clear lattice fringes with a measured interplanar spacing of 0.3 nm, as shown in Fig. 1d, which can be assigned to the (002) plane of cubic CsPbBr₃. Additionally, it shows that nano-sized (ca. 8 nm) CsPbBr₃ particles were encapsulated by the m-CN layer in the m-CN@CsPbBr₃ catalyst (Fig. S8^†). In contrast, the reference CsPbBr₃, which was prepared without urea, is micron-sized (Fig. S9^†), further confirming that the particle size of CsPbBr₃ was changed by the in situ encapsulation with the m-CN layer. Fig. S10^† displays the scanning transmission electron microscopy (STEM) image with the corresponding elemental mapping results, which further confirm that the CsPbBr₃ nanoparticles are wrapped by the m-CN layer. The TEM images and the corresponding elemental mapping of the reference synthesized melem are shown in Fig. S11 and S12,^† respectively, and they resemble those of m-CN@CsPbBr₃, further confirming that the coating layer can be attributed to m-CN.X-ray photoelectron spectroscopy (XPS) was carried out to study the chemical environment of the elements in the m-CN@CsPbBr₃ heterojunction. Fig. S13a^† shows the C 1s XPS results of m-CN@CsPbBr₃ and reference m-CN samples. Two main peaks located at ca. 284.8 and 288.2 eV can be detected, and the first is assigned to the C–C in graphitic carbon, while the second comes from the N–C N coordination of triazine rings.^16,17b It should be noted that there is a slight shift of the N–C N signal of m-CN@CsPbBr₃ to higher binding energy as compared with the bare m-CN, indicating there is an interaction between CsPbBr₃ and m-CN. For the corresponding N 1s XPS curves, three main peaks centered at 398.7, 399.7 and 400.9 eV can be detected (Fig. S13b^†), and they can be assigned to C–N C, N–(C)₃ and C–N–H of the carbon nitride skeleton, respectively.¹⁷ However, the weak peak at ca. 404.4 eV is usually regarded as coming from positive charge localization in heterocycles.^17c It almost disappears in m-CN@CsPbBr₃ relative to bare m-CN, further suggesting that there is some chemical interaction between m-CN and CsPbBr₃ for neutralizing the positive charge.^14a Fig. S14a^† gives the Cs 3d XPS results of m-CN@CsPbBr₃ and bare CsPbBr₃. Two main peaks at 724.5 and 738.4 eV can be detected and come from the Cs 3d_5/2 and 3d_3/2 signals.^14,15 However, a slight shift to higher binding energy can be detected for m-CN@CsPbBr₃, further confirming the above mutual interaction between m-CN and CsPbBr₃. In the Pb 4f XPS result in Fig. S14b,^† there are two peaks at ca. 138.4 and 143.3 eV. Similarly, there is a shift to higher binding energy for m-CN@CsPbBr₃ compared with CsPbBr₃. Fig. S14c^† shows the Br 3d XPS results. Clearly, there are two fitted peaks located at ca. 68.3 and 69.3 eV. The sample of m-CN@CsPbBr₃ exhibits a ca. 0.2 eV shift to higher binding energy compared with bare CsPbBr₃. Both the shifts of Pb 4f and Br 3d in the XPS results suggest that chemical bonds of N–Br have been formed during the preparation process.^11b,14aThe UV-vis absorption spectra of the pristine m-CN and micro-sized CsPbBr₃ in Fig. 2a exhibit the typical absorption edges of CsPbBr₃ and m-CN at ca. 553 and 430 nm, corresponding to bandgaps of 2.24 and 2.88 eV, respectively. For the sample of m-CN@CsPbBr₃, two obvious edges were observed, which correspond to the light absorption edges of CsPbBr₃ and m-CN, indicating the successful combination of the two materials. Fig. S15^† displays the photoluminescence (PL) curves from 375 nm excitation. Strong emission at ca. 450 nm was marked for m-CN, and the peak at 524 nm is attributed to CsPbBr₃. There are two peaks in the PL curve of m-CN@CsPbBr₃, confirming the composition of the catalyst. Moreover, compared with the bare m-CN and CsPbBr₃, the PL intensity of m-CN@CsPbBr₃ is lower, which indicates suppressed carrier recombination in m-CN@CsPbBr₃. The time-resolved PL spectrum was further obtained to explore the carrier dynamics of m-CN@CsPbBr₃. Fig. S16a and b^† exhibit the transient PL spectra at 440 and 523 nm, where the signals can be considered as coming from m-CN and CsPbBr₃ (Fig. S15^†), respectively, and the corresponding fitted results are shown in Tables S1 and S2.^† Obviously, compared with pure m-CN at 440 nm (Fig. S16a^†), m-CN@CsPbBr₃ shows much faster decay after the introduction of CsPbBr₃. Interestingly, with respect to pure CsPbBr₃ at 523 nm (Fig. S16b^†), the opposite trend could be observed. The results testify that the photogenerated electrons of m-CN and holes of CsPbBr₃ will migrate towards each other under the effect of a built-in electric field, and the photogenerated electrons may be transported from CsPbBr₃ to the m-CN layer. The steady-state photovoltages (SPV) of the different samples were further compared to evidence the promoted charge separation in the m-CN@CsPbBr₃ heterojunction, as shown in Fig. 2b. Clearly, the samples of m-CN and CsPbBr₃ exhibited negligible SPV signals. Meanwhile, the m-CN@CsPbBr₃-5 sample demonstrated the highest positive photovoltage response compared with the other catalysts with coating layers (m-CN@CsPbBr₃-10 and m-CN@CsPbBr₃-3), suggesting that its surface is populated by a concentration of holes due to the high charge-separation efficiency (Fig. S17a^†).^18a Transient photovoltage (TPV) analysis was further carried out to study the migration dynamics of the photogenerated carriers.¹⁸ Two obvious positive response peaks (peaks 1 and 2) can be detected from the sample of m-CN@CsPbBr₃ under 400 nm illumination (Fig. S17b^†). Peak 1 can be assigned to a fast process related to the migration of photogenerated carriers in the built-in electric fields inside particles,^18b and thus the stronger intensity of peak 1 of m-CN@CsPbBr₃ suggests that more photogenerated carriers are transported to the interface between m-CN and CsPbBr₃. Peak 2 is thought to arise from carrier transport between particles.¹⁸ A further step to study the carrier transfer dynamics was to perform picosecond transient absorption spectroscopy (TA). Both the samples of bare CsPbBr₃ (Fig. S18a^†) and m-CN@CsPbBr₃ (Fig. S18b^†) exhibit obvious negative ground-state bleaching. Fig. 2c shows the corresponding TA kinetic plots monitored at 520 nm with a distinct difference in the delays. As listed in Table S3,^† the fitted average lifetime of m-CN@CsPbBr₃ (197.7 ps) is much longer than that of bare CsPbBr₃ (77.2 ps), consistent with the above transient PL and TPV results. It is concluded that the internal electric field between m-CN and CsPbBr₃ would promote the carrier transfer and separation in the m-CN@CsPbBr₃ heterojunction.¹⁹ According to the direction of carrier transfer and the strong built-in electric field, it can be inferred that an S-scheme heterojunction is formed between m-CN and CsPbBr₃.Open in a separate windowFig. 2(a) UV-Vis absorption spectra of m-CN@CsPbBr₃, the reference m-CN and CsPbBr₃; (b) surface photovoltage plots of the m-CN, CsPbBr₃ and m-CN@CsPbBr₃ samples; (c) transient absorption spectroscopy curves and lifetime fittings at 520 nm; (d) band structure of the m-CN@CsPbBr₃ heterojunction and the corresponding paths of CO₂ photo-reduction.In order to acquire stronger confirmation of the direction of carrier transfer, ultraviolet photoelectron spectroscopy (UPS) was performed to check the valence band (VB) potential and conduction band (CB) potential of the m-CN and CsPbBr₃ samples. The results and corresponding valence/conduction band potentials (vs. vacuum level) are given in Fig. S19^† and Table S4.^† The relative valence/conduction band positions of m-CN and CsPbBr₃ are shown in Fig. 2d, and they can be attributed to an S-scheme heterojunction, agreeing well with the aforementioned analysis. There is an offset of the bands of the m-CN and CsPbBr₃, and thus close contact between these two materials would cause band bending at the interface. Then, a built-in electric field will be formed making the m-CN and CsPbBr₃ centers of positive and negative charges, respectively. Moreover, the unique conduction/valence band structure of this heterojunction results in polar surfaces of m-CN (holes) and CsPbBr₃ (electrons). This built-in electric field would also facilitate the separation of photogenerated charges and thus suppress charge recombination.Considering that the m-CN@CsPbBr₃ catalyst possesses excellent photogenerated carrier separation ability, its photo-assisted thermocatalytic CO₂ reduction activity was then investigated. We evaluated the catalytic CO₂ reduction of the samples in a flow reactor with CO₂ and H₂O vapor as the reactants, such that the H₂O would continuously pass through the catalyst bed. During the flow reaction process, light and heat were applied to the reactor. Thermogravimetric analysis (TGA) was performed to check the temperature tolerance of the m-CN@CsPbBr₃ catalyst in nitrogen, as shown in Fig. S20.^† It indicates that the thermocatalytic CRR can take place at less than 200 °C because of the absence of any decomposition below this temperature. The instability of CsPbBr₃ to humidity has always been a huge obstacle for applications. The m-CN@CsPbBr₃ catalyst with the protection of the m-CN layer was then treated in pure water to check its chemical stability. As shown in Fig. S21a and b,^† after soaking in water for ∼0.5 and 17 h, there is almost no change in color; however, the bare CsPbBr₃ turns white after being in water for less than 1 h. The corresponding XRD results show no change from before to after the water immersion for ∼0.5 and 17 h. Meanwhile, bare CsPbBr₃ exhibited an obvious phase transition. Additionally, the water-treated samples were checked by TEM. As shown in Fig. S22,^† there is almost no change in morphology after the treatment of ∼0.5 and 17 h, in accordance with the above XRD results. Fig. S23^† shows the XPS results of the water-treated samples, and there are almost no changes for all the studied elements. These results suggest that m-CN@CsPbBr₃ exhibited superior water stability due to the m-CN encapsulation. Fig. 3a compares the CO₂ reduction activities of pristine m-CN, micro-sized CsPbBr₃ and m-CN@CsPbBr₃ under three different conditions, with our m-CN@CsPbBr₃ catalyst benefiting from superior durability. Under the pure simulated sunlight condition (Fig. 3a), the sample of m-CN@CsPbBr₃ displays the highest CO₂-to-CO yield of 8.15 μmol g⁻¹ h⁻¹, followed by CsPbBr₃ (4.5 μmol g⁻¹ h⁻¹) and m-CN (2.2 μmol g⁻¹ h⁻¹). For pure thermocatalysis at the temperature of 150 °C, the CO generation of m-CN@CsPbBr₃ is 22.4 μmol g⁻¹ h⁻¹, and those of the bare CsPbBr₃ and m-CN are 8.8 and 0 μmol g⁻¹ h⁻¹, respectively. More impressively, when the light and heat were coupled to drive photo-assisted thermocatalysis, the CO₂-to-CO yield of m-CN@CsPbBr₃ was increased to 42.8 μmol g⁻¹ h⁻¹, which is much higher than those of bare m-CN (5.1 μmol g⁻¹ h⁻¹) and CsPbBr₃ (18.7 μmol g⁻¹ h⁻¹). It is suggested that the light excitation would generate electrons and holes and these electrons and holes would migrate to the surfaces of CsPbBr₃ and m-CN, facilitating the thermocatalytic CRR process. Under all three conditions, i.e., photocatalysis, thermocatalysis and photo-thermocatalysis, the sample of bare CsPbBr₃ gives the lowest activity, further suggesting the significance of the heterojunction for promoting the CRR performance. The photo-thermocatalytic performance of the sample of m-CN@CsPbBr₃ is also comparable to those of the other systems, but exceeds those of most reported CsPbBr₃-based photocatalysts (Table S5^†). Moreover, we checked the impact of coating content on the catalytic activity of m-CN@CsPbBr₃ at 150 °C and 3 suns. As depicted in Fig. S24,^† in comparison with m-CN@CsPbBr₃-3 and m-CN@CsPbBr₃-10, m-CN@CsPbBr₃-5 delivers the highest CO₂-to-CO yield, possibly due to the existence of the enriched heterogeneous interface and strong built-in electric field. Meanwhile, the catalytic stability of m-CN@CsPbBr₃-5 was also tested during the 6 h period of operation (Fig. S25^†), and the result reveals that its reduction ability at the end is comparable to its original performance. Further, the XPS spectra before and after testing m-CN@CsPbBr₃-5 manifest that all elements are almost unchanged (Fig. S26^†), once again verifying its excellent catalytic stability, which is promising for practical application.Open in a separate windowFig. 3(a) The CO₂ reduction to CO performance under the three conditions of photocatalysis, thermocatalysis and photo-thermocatalysis. For the latter two, the temperature is set to 150 °C; (b) mass spectrometry (MS) result of ¹³CO produced over m-CN@CsPbBr₃ from the ¹³CO₂ isotope experiment under thermocatalysis (m/z, mass/charge ratio); (c) CO₂-TPD results of the reference m-CN and the m-CN@CsPbBr₃ catalyst.In addition, a small amount of H₂ was also measured as the reductive by-product from m-CN@CsPbBr₃ samples at different temperatures (Fig. S27^†). These results indicate that the coupling of photo and thermal effects profoundly promotes CO₂ reduction. Fig. 3b shows the result of an isotopic ¹³CO₂ labeling experiment under the thermocatalytic conditions. A clear peak at m/z = 29 (¹³CO) is observed, confirming that the generated CO originates from the thermocatalytic reduction of ¹³CO₂ rather than from contaminants. The corresponding screen shots of the raw mass spectra data are displayed in Fig. S28.^†To understand the promoted performance of m-CN@CsPbBr₃, the adsorption abilities of CO₂ and CO on m-CN and m-CN@CsPbBr₃ were studied. Fig. 3c exhibits the CO₂ temperature-programmed desorption (TPD) curves of m-CN and m-CN@CsPbBr₃, which yield the adsorbed amounts of CO₂ according to the integrated areas of the curves. It is found that m-CN@CsPbBr₃ has a higher adsorbed amount and thus more sites for CO₂ adsorption and activation compared to m-CM, which could be attributed to the polar surfaces of m-CN and CsPbBr₃. Moreover, the desorption peak of the m-CN@CsPbBr₃ catalyst is situated at higher temperature, suggesting that the corresponding adsorption sites can facilitate the CO₂ reduction due to a stronger interaction with the CO₂ molecules. In the CO₂-TPD curve of CsPbBr₃ in Fig. S29,^† a higher desorption temperature of ∼320 °C is observed compared with that of m-CN, which can be responsible for the enhancement of desorption for the m-CN@CsPbBr₃ catalyst. CO-TPD was also performed, and the results are shown in Fig. S30.^† Clearly, both the m-CN and m-CN@CsPbBr₃ samples showed weak CO adsorption, which is thought to benefit CO evolution. Measurements were further performed at two other temperatures (100 and 200 °C) to determine the effect of heating on the CRR efficiency. As shown in Fig. S31^† (100 °C) and Fig. S32^† (200 °C), the yields of CO₂-to-CO were lower than at 150 °C, which may be ascribed to the difference of CO₂ and CO adsorption at different temperatures. The result suggests that there is an optimal reaction temperature for suitable balance of adsorption/desorption of CO₂ and CO to obtain the highest performance of CO₂ reduction. In situ FTIR spectroscopy was further carried out to investigate the possible reaction pathways of the CRR under photo-assisted thermocatalysis conditions. It is evident that increased IR peaks emerge with increasing irradiation time from 0 to 30 minutes (Fig. 4a and b), in which the peaks at 1457 cm⁻¹ and 1646 cm⁻¹ could be assigned to b-HCO₃⁻, while the peaks at 1248 and 1695 cm⁻¹ came from the vibration of the carboxylate (CO₂⁻), and the peak at 1337 cm⁻¹ can be attributed to the bidentate carbonate (b-CO₃²⁻). Also, the peaks located at 1379 and 1507 cm⁻¹ matched well with monodentate carbonate groups (m-CO₃²⁻).²⁰ Meanwhile, for the photo-assisted thermocatalysis in Fig. 4b, almost the same peaks were observed except with relatively stronger peak intensities, corresponding to the improved CRR efficiency under the coupled photo-thermal effect. Note that the peak at 1695 increased in intensity and the peaks at 1457 cm⁻¹ and 1646 cm⁻¹ disappeared compared with the spectrum under bare thermocatalytic conditions, indicating that b-HCO₃⁻ ions on the surface are transformed to surface CO₂⁻ species instead with the assistance of light.Open in a separate windowFig. 4 In situ FTIR spectra of m-CN@CsPbBr₃ under (a) bare thermocatalytic and (b) photo-thermal conditions at ten-minute increments from 0 to 30 min (from top to bottom). (c) Schematic diagram of the possible reaction pathways of the CRR for the sample of m-CN@CsPbBr₃ under photo-thermal conditions.In addition, the m-CO₃²⁻ signals at 1507 cm⁻¹ were more obvious compared to those under bare thermocatalytic conditions, suggesting that the adsorption of the reaction intermediates was adjusted by light irradiation. This can be attributed to the strong light absorption of CsPbBr₃ according to the FTIR curves of the bare CsPbBr₃ sample under the above two conditions (Fig. S33a and b^†). This finding indicates the importance of encapsulating CsPbBr₃ for altering the formation of intermediates and improving CRR activity under photo-assisted thermocatalysis conditions. Moreover, the FTIR curves of the reference m-CN sample are similar to those of m-CN@CsPbBr₃ under bare thermocatalytic conditions (Fig. S34^†). Thus, the possible CRR pathways can be summarized as follows (Fig. 4c): (a) the H₂O molecules are first dissociated into H_ads and OH_ads groups when meeting hot carriers, i.e., holes generated from thermal or photo excitation; (b) the adsorbed CO₂ encounters OH_ads to generate HCO₃⁻ and m-CO₃²⁻ (step i); (c) then the conversion from surface m-CO₃²⁻ (step i) to CO₂⁻ species in the presence of H₂O is proposed, and the surface HCO₃⁻ (step i) species can be converted conveniently into CO₂⁻ under photo-assisted thermocatalysis conditions (step ii); for (c), the CO₂⁻ finally releases CO gas when it encounters H_ads and free electrons (step iii).Herein, a water-stable m-CN@CsPbBr₃ heterojunction was synthesized via a solid-state reaction, where bulk CsPbBr₃ micro-sized particles were melted and converted to nanoparticles with encapsulation by an m-CN coating during the calcination. The intimate contact between m-CN and CsPbBr₃ would induce band bending at the interface and form a built-in electric field, which would separate holes and electrons to m-CN and CsPbBr₃, respectively. As such a heterojunction with two charge poles, m-CN@CsPbBr₃ exhibited an excellent thermocatalytic CO₂-to-CO yield of 42.8 μmol g⁻¹ h⁻¹ under the assistance of irradiation, higher than that of pure photocatalysis (5.1 μmol g⁻¹ h⁻¹) or thermocatalysis (18.7 μmol g⁻¹ h⁻¹). This is the first report of photo-assisted thermocatalysis using CsPbBr₃-based materials. Our work thus expands the application of halide perovskites in CO₂ reduction.     

A reconstructed porous copper surface promotes selectivity and efficiency toward C2 products by electrocatalytic CO2 reduction

Electrocatalytic synthesis of multicarbon (C₂₊) products from CO₂ reduction suffers from poor selectivity and low energy efficiency. Herein, a facile oxidation–reduction cycling method is adopted to reconstruct the Cu electrode surface with the help of halide anions. The surface composed of entangled Cu nanowires with hierarchical pores is synthesized in the presence of I⁻, exhibiting a C₂ faradaic efficiency (FE) of 80% at −1.09 V vs. RHE. A partial current density of 21 mA cm⁻² is achieved with a C₂ half-cell power conversion efficiency (PCE) of 39% on this electrode. Such high selective C₂ production is found to mainly originate from CO intermediate enrichment inside hierarchical pores rather than the surface lattice effect of the Cu electrode.

The Cu electrode surface is reconstructed by a halide anion assisted method for promoting CO₂ reduction.     

Origin of the N-coordinated single-atom Ni sites in heterogeneous electrocatalysts for CO2 reduction reaction

Heterogeneous Ni–N–C single-atom catalysts (SACs) have attracted great research interest regarding their capability in facilitating the CO₂ reduction reaction (CO₂RR), with CO accounting for the major product. However, the fundamental nature of their active Ni sites remains controversial, since the typically proposed pyridinic-type Ni configurations are inactive, display low selectivity, and/or possess an unfavorable formation energy. Herein, we present a constant-potential first-principles and microkinetic model to study the CO₂RR at a solid–water interface, which shows that the electrode potential is crucial for governing CO₂ activation. A formation energy analysis on several NiN_xC_4−x (x = 1–4) moieties indicates that the predominant Ni moieties of Ni–N–C SACs are expected to have a formula of NiN₄. After determining the potential-dependent thermodynamic and kinetic energy of these Ni moieties, we discover that the energetically favorable pyrrolic-type NiN₄ moiety displays high activity for facilitating the selective CO₂RR over the competing H₂ evolution. Moreover, model polarization curves and Tafel analysis results exhibit reasonable agreement with existing experimental data. This work highlights the intrinsic tetrapyrrolic coordination of Ni for facilitating the CO₂RR and offers practical guidance for the rational improvement of SACs, and this model can be expanded to explore mechanisms of other electrocatalysis in aqueous solutions.

A constant-potential first-principles and microkinetic model is developed to uncover the nature of heterogeneous Ni–N–C catalysts. It highlights the crucial role of a pyrrolic-type NiN₄ moiety in electrochemical CO₂ reduction.     

Suppressing carboxylate nucleophilicity with inorganic salts enables selective electrocarboxylation without sacrificial anodes

Although electrocarboxylation reactions use CO₂ as a renewable synthon and can incorporate renewable electricity as a driving force, the overall sustainability and practicality of this process is limited by the use of sacrificial anodes such as magnesium and aluminum. Replacing these anodes for the carboxylation of organic halides is not trivial because the cations produced from their oxidation inhibit a variety of undesired nucleophilic reactions that form esters, carbonates, and alcohols. Herein, a strategy to maintain selectivity without a sacrificial anode is developed by adding a salt with an inorganic cation that blocks nucleophilic reactions. Using anhydrous MgBr₂ as a low-cost, soluble source of Mg²⁺ cations, carboxylation of a variety of aliphatic, benzylic, and aromatic halides was achieved with moderate to good (34–78%) yields without a sacrificial anode. Moreover, the yields from the sacrificial-anode-free process were often comparable or better than those from a traditional sacrificial-anode process. Examining a wide variety of substrates shows a correlation between known nucleophilic susceptibilities of carbon–halide bonds and selectivity loss in the absence of a Mg²⁺ source. The carboxylate anion product was also discovered to mitigate cathodic passivation by insoluble carbonates produced as byproducts from concomitant CO₂ reduction to CO, although this protection can eventually become insufficient when sacrificial anodes are used. These results are a key step toward sustainable and practical carboxylation by providing an electrolyte design guideline to obviate the need for sacrificial anodes.

Selective electrocarboxylation of nucleophilically susceptible organic halides without sacrificial anodes is enabled by inorganic salt additives, which suppress the nucleophilicity of anions in the electrolyte.     

EDOT-based conjugated polymers accessed via C–H direct arylation for efficient photocatalytic hydrogen production

3,4-Ethylene dioxythiophene (EDOT), as a monomer of commercial conductive poly(3,4-ethylene dioxythiophene) (PEDOT), has been facilely incorporated into a series of new π-conjugated polymer-based photocatalysts, i.e., BSO₂–EDOT, DBT–EDOT, Py–EDOT and DFB–EDOT, through atom-economic C–H direct arylation polymerization (DArP). The photocatalytic hydrogen production (PHP) test shows that donor–acceptor (D–A)-type BSO₂–EDOT renders the highest hydrogen evolution rate (HER) among the linear conjugated polymers (CPs) ever reported. A HER up to 0.95 mmol h⁻¹/6 mg under visible light irradiation and an unprecedented apparent quantum yield of 13.6% at 550 nm are successfully achieved. Note that the photocatalytic activities of the C–H/C–Br coupling-derived EDOT-based CPs are superior to those of their counterparts derived from the classical C–Sn/C–Br Stille coupling, demonstrating that EDOT is a promising electron-rich building block which can be facilely integrated into CP-based photocatalysts. Systematic studies reveal that the enhanced water wettability by the integration of polar BSO₂ with hydrophilic EDOT, the increased electron-donating ability by O–C p–π conjugation, the improved electron transfer by D–A architecture, broad light harvesting, and the nano-sized colloidal character in a H₂O/NMP mixed solvent rendered BSO₂–EDOT as one of the best CP photocatalysts toward PHP.

The excellent reactivity toward C–H direct arylation, water wettability and O–C p–π conjugation endow EDOT to be an attractive electron donor unit for CP photocatalysts, yielding an unprecedented hydrogen evolution rate up to 0.95 mmol h⁻¹/6 mg catalyst.     

Unexpected high selectivity for acetate formation from CO2 reduction with copper based 2D hybrid catalysts at ultralow potentials

Copper-based catalysts are efficient for CO₂ reduction affording commodity chemicals. However, Cu(i) active species are easily reduced to Cu(0) during the CO₂RR, leading to a rapid decay of catalytic performance. Herein, we report a hybrid-catalyst that firmly anchors 2D-Cu metallic dots on F-doped Cu_xO nanoplates (Cu_xOF), synthesized by electrochemical-transformation under the same conditions as the targeted CO₂RR. The as-prepared Cu/Cu_xOF hybrid showed unusual catalytic activity towards the CO₂RR for CH₃COO⁻ generation, with a high FE of 27% at extremely low potentials. The combined experimental and theoretical results show that nanoscale hybridization engenders an effective s,p-d coupling in Cu/Cu_xOF, raising the d-band center of Cu and thus enhancing electroactivity and selectivity for the acetate formation. This work highlights the use of electronic interactions to bias a hybrid catalyst towards a particular pathway, which is critical for tuning the activity and selectivity of copper-based catalysts for the CO₂RR.

A two-dimensional (2D) copper hybrid catalyst (Cu/Cu_xOF) composed of metallic Cu well dispersed on 2D F-doped Cu_xO nanoplates (Cu_xOF) is reported, which shows high catalytic activity toward the CO₂RR for acetate generation.     

A Molecular Z-Scheme Artificial Photosynthetic System Under the Bias-Free Condition for CO2 Reduction Coupled with Two-electron Water Oxidation: Photocatalytic Production of CO/HCOOH and H2O2

Bio-inspired molecular-engineered systems have been extensively investigated for the half-reactions of H₂O oxidation or CO₂ reduction with sacrificial electron donors/acceptors. However, there has yet to be reported a device for dye-sensitized molecular photoanodes coupled with molecular photocathodes in an aqueous solution without the use of sacrificial reagents. Herein, we will report the integration of Sn^IV- or Al^III-tetrapyridylporphyrin (SnTPyP or AlTPyP) decorated tin oxide particles (SnTPyP/SnO₂ or AlTPyP/SnO₂) photoanode with the dye-sensitized molecular photocathode on nickel oxide particles containing [Ru(diimine)₃]²⁺ as the light-harvesting unit and [Ru(diimine)(CO)₂Cl₂] as the catalyst unit covalently connected and fixed within poly-pyrrole layer (RuCAT-RuC₂-PolyPyr-PRu/NiO). The simultaneous irradiation of the two photoelectrodes with visible light resulted in H₂O₂ on the anode and CO, HCOOH, and H₂ on the cathode with high Faradaic efficiencies in purely aqueous conditions without any applied bias is the first example of artificial photosynthesis with only two-electron redox reactions.     

Plasmon-enabled N2 photofixation on partially reduced Ti3C2 MXene

Benefiting from the superior conductivity, rich surface chemistry and tunable bandgap, Ti₃C₂ MXene has become a frontier cocatalyst material for boosting the efficiency of semiconductor photocatalysts. It has been theoretically predicted to be an ideal material for N₂ fixation. However, the realization of N₂ photofixation with Ti₃C₂ as a host photocatalyst has so far remained experimentally challenging. Herein, we report on a sandwich-like plasmon- and an MXene-based photocatalyst made of Au nanospheres and layered Ti₃C₂, and demonstrate its efficient N₂ photofixation in pure water under ambient conditions. The abundant low-valence Ti (Ti^(4−x)+) sites in partially reduced Ti₃C₂ (r-Ti₃C₂) produced by surface engineering through H₂ thermal reduction effectively capture and activate N₂, while Au nanospheres offer plasmonic hot electrons to reduce the activated N₂ into NH₃. The Ti^(4−x)+ active sites and plasmon-generated hot electrons work in tandem to endow r-Ti₃C₂/Au with remarkably enhanced N₂ photofixation activity. Importantly, r-Ti₃C₂/Au exhibits ultrahigh selectivity without the occurrence of competing H₂ evolution. This work opens up a promising route for the rational design of efficient MXene-based photocatalysts.

N₂ photofixation in water is realized under ambient conditions using partially reduced Ti₃C₂ MXene that is interlaminated with Au nanospheres.     

Stereodivergent synthesis of β-iodoenol carbamates with CO2via photocatalysis

Photocatalytic conversion of carbon dioxide (CO₂) into value-added chemicals is of great significance from the viewpoint of green chemistry and sustainable development. Here, we report a stereodivergent synthesis of β-iodoenol carbamates through a photocatalytic three-component coupling of ethynylbenziodoxolones, CO₂ and amines. By choosing appropriate photocatalysts, both Z- and E-isomers of β-iodoenol carbamates, which are difficult to prepare using existing methods, can be obtained stereoselectively. This transformation featured mild conditions, excellent functional group compatibility and broad substrate scope. The potential synthetic utility of this protocol was demonstrated by late-stage modification of bioactive molecules and pharmaceuticals as well as by elaborating the products to access a wide range of valuable compounds. More importantly, this strategy could provide a general and practical method for stereodivergent construction of trisubstituted alkenes such as triarylalkenes, which represents a fascinating challenge in the field of organic chemistry research. A series of mechanism investigations revealed that the transformation might proceed through a charge-transfer complex which might be formed through a halogen bond.

Stereodivergent synthesis of β-iodoenol carbamates was achieved via a photocatalytic three-component coupling reaction of ethynylbenziodoxolones, CO₂ and amines.     

Three‐in‐One Oxygen Vacancies: Whole Visible‐Spectrum Absorption,Efficient Charge Separation,and Surface Site Activation for Robust CO2 Photoreduction

A facile and controllable in situ reduction strategy is used to create surface oxygen vacancies (OVs) on Aurivillius‐phase Sr₂Bi₂Nb₂TiO₁₂ nanosheets, which were prepared by a mineralizer‐assisted soft‐chemical method. Introduction of OVs on the surface of Sr₂Bi₂Nb₂TiO₁₂ extends photoresponse to cover the whole visible region and also tremendously promotes separation of photoinduced charge carriers. Adsorption and activation of CO₂ molecules on the surface of the catalyst are greatly enhanced. In the gas‐solid reaction system without co‐catalysts or sacrificial agents, OVs‐abundant Sr₂Bi₂Nb₂TiO₁₂ nanosheets show outstanding CO₂ photoreduction activity, producing CO with a rate of 17.11 μmol g^?1 h^?1, about 58 times higher than that of the bulk counterpart, surpassing most previously reported state‐of‐the‐art photocatalysts. Our study provides a three‐in‐one integrated solution to advance the performance of photocatalysts for solar‐energy conversion and generation of renewable energy.     

Three‐in‐One Oxygen Vacancies: Whole Visible‐Spectrum Absorption,Efficient Charge Separation,and Surface Site Activation for Robust CO2 Photoreduction

A facile and controllable in situ reduction strategy is used to create surface oxygen vacancies (OVs) on Aurivillius‐phase Sr₂Bi₂Nb₂TiO₁₂ nanosheets, which were prepared by a mineralizer‐assisted soft‐chemical method. Introduction of OVs on the surface of Sr₂Bi₂Nb₂TiO₁₂ extends photoresponse to cover the whole visible region and also tremendously promotes separation of photoinduced charge carriers. Adsorption and activation of CO₂ molecules on the surface of the catalyst are greatly enhanced. In the gas‐solid reaction system without co‐catalysts or sacrificial agents, OVs‐abundant Sr₂Bi₂Nb₂TiO₁₂ nanosheets show outstanding CO₂ photoreduction activity, producing CO with a rate of 17.11 μmol g^?1 h^?1, about 58 times higher than that of the bulk counterpart, surpassing most previously reported state‐of‐the‐art photocatalysts. Our study provides a three‐in‐one integrated solution to advance the performance of photocatalysts for solar‐energy conversion and generation of renewable energy.