Achieving metal-like catalysis from semiconductor for on-surface synthesis.

Free of posttransfer, on-surface synthesis (OSS) of single-atomic-layer nanostructures directly on semiconductors holds considerable potential for next-generation devices. However, due to the high diffusion barrier and abundant defects on semiconductor surfaces, extended and well-defined OSS on semiconductors has major difficulty. Furthermore, given semiconductors' limited thermal catalytic activity, initiating high-barrier reactions remains a significant challenge. Herein, using TiO2(011) as a prototype, we present an effective strategy for steering the molecule adsorption and reaction processes on semiconductors, delivering lengthy graphene nanoribbons with extendable widths. By introducing interstitial titanium (Tiint) and oxygen vacancies (Ov), we convert TiO2(011) from a passive supporting template into a metal-like catalytic platform. This regulation shifts electron density and surface dipoles, resulting in tunable catalytic activity together with varied molecule adsorption and diffusion. Cyclodehydrogenation, which is inefficient on pristine TiO2(011), is markedly improved on Tiint/Ov-doped TiO2. Even interribbon cyclodehydrogenation is achieved. The final product's dimensions, quality, and coverage are all controllable. Tiint doping outperforms Ov in producing regular and prolonged products, whereas excessive Tiint compromises molecule landing and coupling. This work demonstrates the crucial role of semiconductor substrates in OSS and advances OSS on semiconductors from an empirical trial-and-error methodology to a systematic and controllable paradigm.

Selective Photoreduction of CO2 to CH4 Triggered by Metal-Vacancy Pair Sites.

Selectively achieving the photoreduction of carbon dioxide (CO2) to methane (CH4) remains a significant challenge, which primarily arises from the complexity of the protonation process. In this work, we designed metal-vacancy pair sites in defective metal oxide semiconductors, which anchor the reactive intermediates with a bridged linkage for the selective protonation to produce CH4. As an example, oxygen-deficient Nb2O5 nanosheets are synthesized, in which the niobium-oxygen vacancy pair sites are demonstrated by X-ray photoelectron spectroscopy and electron paramagnetic resonance spectra. In situ Fourier transform infrared spectroscopy monitors the *CH3O intermediate, a key intermediate for CH4 production, during the CO2 photoreduction in oxygen-deficient Nb2O5 nanosheets. Importantly, the built metal-vacancy pair sites regulate the *CH3O formation step as a spontaneous process, making the reduction of CO2 to CH4 the preferred method. Therefore, the oxygen-deficient Nb2O5 nanosheets exhibit a CH4 formation rate of 19.14 µmol g-1 h-1, with an electron selectivity of ∼94.1%.

Nitrogen Doping-Roused Synergistic Active Sites in Perovskite Enabling Highly Selective CO2 Photoreduction into CH4.

The intricate protonation process in carbon dioxide reduction usually makes the product unpredictable. Thus, it is significant to control the reactive intermediates to manipulate the reaction steps. Here, we propose that the synergistic La-Ti active sites in the N-La2Ti2O7 nanosheets enable the highly selective carbon dioxide photoreduction into methane. In the photoreduction of CO2 over N-La2Ti2O7 nanosheets, in situ Fourier transform infrared spectra are utilized to monitor the *CH3O intermediate, pivotal for methane production, whereas such monitoring is not conducted for La2Ti2O7 nanosheets. Also, theoretical calculations testify to the increased charge densities on the Ti and La atoms and the regulated formation energy barrier of *CO and *CH3O intermediates by the constructed synergistic active sites. Accordingly, the methane formation rate of 7.97 µL h-1 exhibited by the N-La2Ti2O7 nanosheets, along with an electron selectivity of 96.6%, exceeds that of most previously reported catalysts under similar conditions.

Efficient Homojunction Tin Perovskite Solar Cells Enabled by Gradient Germanium Doping.

P-type self-doping is known to hamper tin-based perovskites for developing high-performance solar cells by increasing the background current density and carrier recombination processes. In this work, we propose a gradient homojunction structure with germanium doping that generates an internal electric field across the perovskite film to deplete the charge carriers. This structure reduces the dark current density of perovskite by over 2 orders of magnitude and trap density by an order of magnitude. The resultant tin-based perovskite solar cells exhibit a higher power conversion efficiency of 13.3% and excellent stability, maintaining 95% and 85% of their initial efficiencies after 250 min of continuous illumination and 3800 h of storage, respectively. We reveal the homojunction formation mechanism using density functional theory calculations and molecular level characterizations. Our work provides a reliable strategy for controlling the spatial energy levels in tin perovskite films and offers insights into designing intriguing lead-free perovskite optoelectronics.

Regulated Photocatalytic CO2-to-CH3OH Pathway by Synergetic Dual Active Sites of Interlayer.

Herein, composites of nanosheets with van der Waals contacts are employed to disclose how the interlayer-microenvironment affects the product selectivity of carbon dioxide (CO2) photoreduction. The concept of composites of nanosheets with dual active sites is introduced to manipulate the bonding configuration and promote the thermodynamic formation of methanol (CH3OH). As a prototype, the CoNi2S4-In2O3 composites of nanosheets are prepared, in which high-resolution transmission electron microscopy imaging, X-ray photoelectron spectroscopy spectra, and zeta potential tests confirm the presence of van der Waals contacts rather than chemical bonding between the In2O3 nanosheets and the CoNi2S4 nanosheets within the composite. The fabricated CoNi2S4-In2O3 composites of nanosheets exhibit the detection of the key intermediate *CH3O during CO2 photoreduction through in situ Fourier transform infrared spectra, while the In2O3 nanosheets and CoNi2S4 nanosheets alone do not show this capability, further verified by the density functional theory calculations. Accordingly, the CoNi2S4-In2O3 composites of nanosheets show the ability to produce CH3OH, whereas the CoNi2S4 and In2O3 nanosheets solely generate carbon monoxide products from CO2 photoreduction.

Breaking the Activity-Selectivity Trade-off for CH4-to-C2H6 Photoconversion.

Photocatalytic conversion of methane (CH4) to ethane (C2H6) has attracted extensive attention from academia and industry. Typically, the traditional oxidative coupling of CH4 (OCM) reaches a high C2H6 productivity, yet the inevitable overoxidation limits the target product selectivity. Although the traditional nonoxidative coupling of CH4 (NOCM) can improve the product selectivity, it still encounters unsatisfied activity, arising from being thermodynamically unfavorable. To break the activity-selectivity trade-off, we propose a conceptually new mechanism of H2O2-triggered CH4 coupling, where the H2O2-derived ·OH radicals are rapidly consumed for activating CH4 into ·CH3 radicals exothermically, which bypasses the endothermic steps of the direct CH4 activation by photoholes and the interaction between ·CH3 and ·OH radicals, affirmed by in situ characterization techniques, femtosecond transient absorption spectroscopy, and density-functional theory calculation. By this pathway, the designed Au-WO3 nanosheets achieve unprecedented C2H6 productivity of 76.3 mol molAu-1 h-1 with 95.2% selectivity, and TON of 1542.7 (TOF = 77.1 h-1) in a self-designed flow reactor, outperforming previously reported photocatalysts regardless of OCM and NOCM pathways. Also, under outdoor natural sunlight irradiation, the Au-WO3 nanosheets exhibit similar activity and selectivity toward C2H6 production, showing the possibility for practical applications. Interestingly, this strategy can be applied to other various photocatalysts (Au-WO3, Au-TiO2, Au-CeO2, Pd-WO3, and Ag-WO3), showing a certain universality. It is expected that the proposed mechanism adds another layer to our understanding of CH4-to-C2H6 conversion.

Selective Cleavage of α-Olefins to Produce Acetylene and Hydrogen.

Acetylene production from mixed α-olefins emerges as a potentially green and energy-efficient approach with significant scientific value in the selective cleavage of C-C bonds. On the Pd(100) surface, it is experimentally revealed that C2 to C4 α-olefins undergo selective thermal cleavage to form surface acetylene and hydrogen. The high selectivity toward acetylene is attributed to the 4-fold hollow sites which are adept at severing the terminal double bonds in α-olefins to produce acetylene. A challenge arises, however, because acetylene tends to stay at the Pd(100) surface. By using the surface alloying methodology with alien Au, the surface Pd d-band center has been successfully shifted away from the Fermi level to release surface-generated acetylene from α-olefins as a gaseous product. Our study actually provides a technological strategy to economically produce acetylene and hydrogen from α-olefins.

On-Surface Synthesis of Two-dimensional Carbon-based Networks via Hierarchical Ullmann Coupling Reactions.

The recent developed bottom-up on-surface synthesis offers unprecedent opportunities for the fabrication of two-dimensional (2D) carbon-based networks with atomic precision. Hierarchical coupling approach has been proposed as an efficient strategy for improving the corresponding reaction selectivity and quality of target structures. Herein, we report the synthesis of a nitrogen-doped carbon-based network on Ag(100) utilizing a hierarchical Ullmann coupling strategy. The accurate identification of reaction intermediates and products by scanning tunneling microscopy allows us to unravel the reaction mechanism. The synthetic process of 2D carbon-based networks is kinetics-driven, relying on the competition between dechlorination and C-C coupling. We expect that our discussion on the mechanism of hierarchical coupling may shed light on the rational design and precise synthesis of 2D carbon-based networks on surfaces.

Selective CO2 Photoreduction Enabled by Water-stable Cu-based Metal-organic Framework Nanoribbons.

The goal of photocatalytic CO2 reduction system is to achieve near 100 % selectivity for the desirable product with reasonably high yield and stability. Here, two-dimensional metal-organic frameworks are constructed with abundant and uniform monometallic active sites, aiming to be an emerged platform for efficient and selective CO2 reduction. As an example, water-stable Cu-based metal-organic framework nanoribbons with coordinatively unsaturated single CuII sites are first fabricated, evidenced by X-ray diffraction patterns and X-ray absorption spectroscopy. In situ Fourier-transform infrared spectra and Gibbs free energy calculations unravel the formation of the key intermediate COOH* and CO* is an exothermic and spontaneous process, whereas the competitive hydrogen evolution reaction is endothermic and non-spontaneous, which accounts for the selective CO2 reduction. As a result, in an aqueous solution containing 1 mol L-1 KHCO3 and without any sacrifice reagent, the water-stable Cu-based metal-organic framework nanoribbons exhibited an average CO yield of 82 µmol g-1 h-1 with the selectivity up to 97 % during 72 h cycling test, which is comparable to other reported photocatalysts under similar conditions.

Highly Active Photoreduction of Atmospheric-Concentration CO2 into CH3COOH over Palladium Particles on Nb2O5 Nanosheets.

The endeavor to drive CO2 photoreduction towards the synthesis of C2 products is largely thwarted by the colossal energy hurdle inherent in C-C coupling. Herein, we load active metal particles on metal oxide nanosheets to build the dual metal pair sites for steering C-C coupling to form C2 products. Taking Pd particles anchored on the Nb2O5 nanosheets as an example, the high-angle annular dark-field image and X-ray photoelectron spectroscopy demonstrate the presence of Pd-Nb metal pair sites on the Pd-Nb2O5 nanosheets. Density functional theory calculations reveal these sites exhibit a low reaction energy barrier of only 1.02 eV for C-C coupling, implying that the introduction of Pd particles effectively tailors the reaction step to form C2 products. Therefore, the Pd-Nb2O5 nanosheets achieve a CH3COOH evolution rate of 13.5 µmol g-1 h-1 in photoreduction of atmospheric-concentration CO2, outshining all other single photocatalysts reported to date under analogous conditions.

Electrosynthesis of an Improbable Directly Bonded Phosphorene-Fullerene Heterodimensional Hybrid toward Boosted Photocatalytic Hydrogen Evolution.

Phosphorene and fullerene are representative two-dimensional (2D) and zero-dimensional (0D) nanomaterials respectively, constructing their heterodimensional hybrid not only complements their physiochemical properties but also extends their applications via synergistic interactions. This is however challenging because of their diversities in dimension and chemical reactivity, and theoretical studies predicted that it is improbable to directly bond C60 onto the surface of phosphorene due to their strong repulsion. Here, we develop a facile electrosynthesis method to synthesize the first phosphorene-fullerene hybrid featuring fullerene surface bonding via P-C bonds. Few-layer black phosphorus nanosheets (BPNSs) obtained from electrochemical exfoliation react with C60 2- dianion prepared by electroreduction of C60, fulfilling formation of the "improbable" phosphorene-fullerene hybrid (BPNS-s-C60). Theoretical results reveal that the energy barrier for formation of [BPNS-s-C60]2- intermediate is significantly decreased by 1.88 eV, followed by an oxidization reaction to generate neutral BPNS-s-C60 hybrid. Surface bonding of C60 molecules not only improves significantly the ambient stability of BPNSs, but also boosts dramatically the visible light and near-infrared (NIR) photocatalytic hydrogen evolution rates, reaching 1466 and 1039 µmol h-1 g-1 respectively, which are both the highest values among all reported BP-based metal-free photocatalysts.

Light-Driven C-C Coupling for Targeted Synthesis of CH3 COOH with Nearly 100 % Selectivity from CO2.

Targeted synthesis of acetic acid (CH3 COOH) from CO2 photoreduction under mild conditions mainly limits by the kinetic challenge of the C-C coupling. Herein, we utilized doping engineering to build charge-asymmetrical metal pair sites for boosted C-C coupling, enhancing the activity and selectivity of CO2 photoreduction towards CH3 COOH. As a prototype, the Pd doped Co3 O4 atomic layers are synthesized, where the established charge-asymmetrical cobalt pair sites are verified by X-ray photoelectron spectroscopy and X-ray absorption near edge spectroscopy spectra. Theoretical calculations not only reveal the charge-asymmetrical cobalt pair sites caused by Pd atom doping, but also manifest the promoted C-C coupling of double *COOH intermediates through shortening of the coupled C-C bond distance from 1.54 to 1.52 Å and lowering their formation energy barrier from 0.77 to 0.33 eV. Importantly, the decreased reaction energy barrier from the protonation of two*COOH into *CO intermediates for the Pd-Co3 O4 atomic layer slab is 0.49 eV, higher than that of the Co3 O4 atomic layer slab (0.41 eV). Therefore, the Pd-Co3 O4 atomic layers exhibit the CH3 COOH evolution rate of ca. 13.8 µmol g-1 h-1 with near 100% selectivity, both of which outperform all previously reported single photocatalysts for CO2 photoreduction towards CH3 COOH under similar conditions.

Surface Activation by Single Ru Atoms for Enhanced High-Temperature CO2 Electrolysis.

Cathodic CO2 adsorption and activation is essential for high-temperature CO2 electrolysis in solid oxide electrolysis cells (SOECs). However, the component of oxygen ionic conductor in the cathode displays limited electrocatalytic activity. Herein, stable single Ruthenium (Ru) atoms are anchored on the surface of oxygen ionic conductor (Ce0.8 Sm0.2 O2-δ , SDC) via the strong covalent metal-support interaction, which evidently modifies the electronic structure of SDC surface for favorable oxygen vacancy formation and enhanced CO2 adsorption and activation, finally evoking the electrocatalytic activity of SDC for high-temperature CO2 electrolysis. Experimentally, SOEC with the Ru1 /SDC-La0.6 Sr0.4 Co0.2 Fe0.8 O3-δ cathode exhibits a current density as high as 2.39 A cm-2 at 1.6 V and 800 °C. This work expands the application of single atom catalyst to the high-temperature electrocatalytic reaction in SOEC and provides an efficient strategy to tailor the electronic structure and electrocatalytic activity of SOEC cathode at the atomic scale.

A Heterogeneous Single Atom Cobalt Catalyst for Highly Efficient Acceptorless Dehydrogenative Coupling Reactions.

A fundamental understanding of metal active sites in single-atom catalysts (SACs) is important and challenging in the development of high-performance catalyst systems. Here, a highly efficient and straightforward molten-salt-assisted approach is reported to create atomically dispersed cobalt atoms supported over vanadium pentoxide layered material, with each cobalt atom coordinated with four neighboring oxygen atoms. The liquid environment and the strong polarizing force of the molten salt at high temperatures potentially favor the weakening of VO bonding and the formation of CoO bonding on the vanadium oxide surface. This cobalt SAC achieves extraordinary catalytic efficiency in acceptorless dehydrogenative coupling of alcohols with amines to give imines, with more than 99% selectivity under almost 100% conversion within 3 h, along with a high turnover frequency (TOF) of 5882 h-1 , exceeding those of previously reported benchmarking catalysts. Moreover, it delivers excellent recyclability, reaction scalability, and substrate tolerance. Density functional theory (DFT) calculations further confirm that the optimized coordination environment and strong electronic metal-support interaction contribute significantly to the activation of reactants. The findings provide a feasible route to construct SACs at the atomic level for use in organic transformations.

Correlation between Interfacial Structures and Device Performance: The Double-Edged Sword Effect of Lead Iodide in Perovskite Solar Cells.

The interfacial electronic structure of perovskite layers and transport layers is critical for the performance and stability of perovskite solar cells (PSCs). The device performance of PSCs can generally be improved by adding a slight excess of lead iodide (PbI2 ) to the precursor solution. However, its underlying working mechanism is controversial. Here, we performed a comprehensive study of the electronic structures at the interface between CH3 NH3 PbI3 and C60 with and without the modification of PbI2 using in situ photoemission spectroscopy measurements. The correlation between the interfacial structures and the device performance was explored based on performance and stability tests. We found that there is an interfacial dipole reversal, and the downward band bending is larger at the CH3 NH3 PbI3 /C60 interface with the modification of PbI2 as compared to that without PbI2 . Therefore, PSCs with PbI2 modification exhibit faster charge carrier transport and slower carrier recombination. Nevertheless, the modification of PbI2 undermines the device stability due to aggravated iodide migration. Our findings provide a fundamental understanding of the CH3 NH3 PbI3 /C60 interfacial structure from the perspective of the atomic layer and insight into the double-edged sword effect of PbI2 as an additive.

Direct Polyethylene Photoreforming into Exclusive Liquid Fuel over Charge-Asymmetrical Dual Sites under Mild Conditions.

Direct polyethylene photoreforming to high-energy-density C2 fuels under mild conditions is of great significance and still faces a huge challenge, which is partly attributed to the extreme instability of *CH2CH2 adsorbed on the traditional catalysts with single catalytic sites. Herein, charge-asymmetrical dual sites are designed to boost the adsorption of *CH2CH2 for direct polyethylene photoreforming into C2 fuels under normal temperature and pressure. As a prototype, the synthetic Zr-doped CoFe2O4 quantum dots with charge-asymmetrical dual metal sites realize direct polyethylene photoreforming into acetic acid, with 100% selectivity of liquid fuel and the evolution rate of 1.10 mmol g-1 h-1, outperforming those of most previously reported photocatalysts under similar conditions. In situ X-ray photoelectron spectra, density-functional-theory calculations, and control experiments reveal the charge-asymmetrical Zr-Fe dual sites may act as the predominate catalytic sites, which can simultaneously bond with the *CH2CH2 intermediates for the following stepwise oxidation to form C2 products.

Surface Engineering on Commercial Cu Foil for Steering C2H4/CH4 Ratio in CO2 Electroreduction.

Designing catalysts with high selectivity toward C2 products in CO2 electroreduction is crucial to energy storage and sustainable development. Here, we propose a Cu foil kinetic model with abundant nanocavities possessing higher reaction rate constant k to steer the ratio of C2H4 to the competing CH4 during CO2 electroreduction. Chemical kinetic simulation demonstrates that the nanocavities could enrich the adsorbed CO surface concentration (θCOad), while the higher k helps to lower the C-C coupling barrier for CO intermediates, thus favoring the formation of C2H4. The commercial Cu foil treated with cyclic voltammetry is used to match this model, displaying a remarkable C2H4/CH4 ratio of 4.11, which is 18 times larger than that on the pristine Cu foil. This work offers a handy strategy for surface modification and provides new insights into the C-C coupling and the C2H4 selectivity in terms of mass transfer flux and energy barrier.

Pt Atomic Layers with Tensile Strain and Rich Defects Boost Ethanol Electrooxidation.

Surface and strain engineering are two effective strategies to improve performance; however, synergetic controls of surface and strain effects remains a grand challenge. Herein, we report a highly efficient and stable electrocatalyst with defect-rich Pt atomic layers coating an ordered Pt3Sn intermetallic core. Pt atomic layers enable the generation of 4.4% tensile strain along the [001] direction. Benefiting from synergetic controls of surface and strain engineering, Pt atomic-layer catalyst (Ptatomic-layer) achieves a remarkable enhancement on ethanol electrooxidation performance with excellent specific activity of 5.83 mA cm-2 and mass activity of 1166.6 mA mg Pt-1, which is 10.6 and 3.6 times higher than the commercial Pt/C, respectively. Moreover, the intermetallic core endows Ptatomic-layer with outstanding durability. In situ infrared reflection-absorption spectroscopy as well as density functional theory calculations reveal that tensile strain and rich defects of Ptatomci-layer facilitate to break C-C bond for complete ethanol oxidation for enhanced performance.

Monitoring Electron Flow in Nickel Single-Atom Catalysts during Nitrogen Photofixation.

An efficient catalytic system for nitrogen (N2) photofixation generally consists of light-harvesting units, active sites, and an electron-transfer bridge. In order to track photogenerated electron flow between different functional units, it is highly desired to develop in situ characterization techniques with element-specific capability, surface sensitivity, and detection of unoccupied states. In this work, we developed in situ synchrotron radiation soft X-ray absorption spectroscopy (in situ sXAS) to probe the variation of electronic structure for a reaction system during N2 photoreduction. Nickel single-atom and ceria nanoparticle comodified reduced graphene oxide (CeO2/Ni-G) was designed as a model catalyst. In situ sXAS directly reveals the dynamic interfacial charge transfer of photogenerated electrons under illumination and the consequent charge accumulation at the catalytic active sites for N2 activation. This work provides a powerful tool to monitor the electronic structure evolution of active sites under reaction conditions for photocatalysis and beyond.

Synthesis of a Porous [14]Annulene Graphene Nanoribbon and a Porous [30]Annulene Graphene Nanosheet on Metal Surfaces.

The electrical and mechanical properties of graphene-based materials can be tuned by the introduction of nanopores, which are sensitively related to the size, morphology, density, and location of nanopores. The synthesis of low-dimensional graphene nanostructures containing well-defined nonplanar nanopores has been challenging due to the intrinsic steric hindrance. Herein, we report the selective synthesis of one-dimensional (1D) graphene nanoribbons (GNRs) containing periodic nonplanar [14]annulene pores on Ag(111) and two-dimensional (2D) porous graphene nanosheet containing periodic nonplanar [30]annulene pores on Au(111), starting from a same precursor. The formation of distinct products on the two substrates originates from the different thermodynamics and kinetics of coupling reactions. The reaction mechanisms were confirmed by a series of control experiments, and the appropriate thermodynamic and kinetic parameters for optimizing the reaction pathways were proposed. In addition, the combined scanning tunneling spectroscopy (STS) and density functional theory (DFT) calculations revealed the electronic structures of porous graphene structures, demonstrating the impact of nonplanar pores on the π-conjugation of molecules.