One-dimensional single atom arrays on ferroelectric nanosheets for enhanced CO2 photoreduction

Electrocatalytic Innovations at Atomic Scale: From Single-Atom to Periodic Ensembles for Sustainable Energy Conversion

Atomically dispersed catalysts, including single-atom, dual-atom, and periodic single-metal site catalysts, have revolutionized electrocatalysis by merging atomic precision with heterogeneous stability. This review traces their evolution from pioneering stabilization strategies to advanced microenvironment engineering, enabling breakthroughs in oxygen reduction, hydrogen evolution, and CO2 reduction. SACs maximize atom utilization but face multi-step reaction limits, addressed by DACs through synergistic dual-site mechanisms. PSMSCs further enhance activity via ordered atomic arrangements, ensuring uniform active sites and mechanistic clarity. Key breakthroughs include microenvironment engineering to tailor active sites, as well as advanced characterization techniques revealing dynamic restructuring under operando conditions. The transition from isolated atoms to ordered ensembles highlights the importance of atomic-level control in unlocking new catalytic mechanisms. This work underscores the transformative potential of ADCs in sustainable energy technologies and provides a roadmap for future research in rational catalyst design, dynamic behavior analysis, and scalable synthesis.

Two-Dimensional Perovskites for Photocatalytic CO2 Reduction

The photocatalytic conversion of Carbon dioxide (CO2) into valuable chemicals is one of the most promising approaches to addressing the CO2 emission problem. However, several issues still need to be resolved to increase the efficiency of photocatalytic reactions. Perovskites possess superior light absorption capacity, tunable band gaps, high defect tolerance, and diverse dimensionality. Among them, two-dimensional (2D) perovskites are more stable under photocatalytic conditions and have exciting excitonic characteristics compared to three-dimensional (3D) perovskites. 2D perovskites have unique physical and chemical properties, such as high stability, polaron formation, quantum well structures, and high exciton binding energies, which remain underexplored for photocatalytic CO2 reduction (pCO2RR). Tuning these properties is easier in 2D perovskites than in 3D perovskites by varying the layer thickness and spacer cations. Therefore, 2D perovskite photocatalysts are emerging as promising materials for reducing CO2 into valuable products. This review discusses the classification and synthesis methods of 2D perovskites, the unique properties that make them favorable for photocatalysis, and recent advances in applying 2D perovskites for pCO2RR by monitoring the operational methodology. It also emphasizes the potential for future developments in photocatalysis using 2D perovskites.

CS bonds mediated rapid charge transfer in hm-C4N3/CdS heterostructure for efficient photocatalytic CO2 reduction

The quest for stable and high-performance photocatalysts is pivotal in advancing the field of photocatalytic CO2 reduction. Traditional single-component semiconductors are often impeded by their inability to concurrently achieve a broad light absorption spectrum, efficient separation of photogenerated charge carriers, and enduring stability, thereby constraining their photocatalytic efficacy. In this study, we introduce an innovative hm-C4N3/CdS heterojunction that broadens the light absorption spectrum and significantly enhances the separation efficiency of photogenerated charge carriers. Notably, this type-II heterojunction design effectively curbs the photocorrosion of CdS, and the CS bonds within the structure expedite electron transfer. Under irradiation, the photocatalyst exhibits a marked increase in activity and stability, yielding CO and CH4 at rates of 67.5 μmol g-1 h-1 and 7.4 μmol g-1 h-1, respectively. This work provides valuable insights for promoting efficient and stable photocatalyst for CO2 reduction.

Oxygen vacancy mediated Pd-SA/TiO2 single-atom catalyst created via ultra-fast one-step synthesis for enhanced CO2 photoreduction

Single-atom catalysts (SACs) have garnered considerable interest in the field of heterogeneous catalysis. This study detail the synthesis of single-atom Pd catalysts supported on metal oxides using the Flame Spray Pyrolysis (FSP) method. This technique allows for the preparation of TiO2 with abundant oxygen vacancies by incorporating a hydrogen-rich atmosphere at high temperature (>2000 K) utilizing the quenching ring, which enhances the catalyst synthesis process. Additionally, the distribution and electronic structure of Pd were tailored in a hydrogen-rich atmosphere, which promoted the entrapment of Pd atoms within oxygen vacancies, preventing their aggregation into Pd nanoparticles and leading to the formation of Pd-SA/TiO2. Notably, Pd-SA/TiO2 achieves 92.51 % CO2-to-CO selectivity in the photocatalytic CO2 reduction reaction and exhibits an impressive catalytic activity of 56.84μmol g-1h-1. This research introduces a novel approach to modulate the anchoring process and optimize the microenvironment for single-atom metal synthesis, advancing the development of the-state-of-the-art SACs.

Narrow Bandgap Perovskite Enabled by Heterovalent Co-Doping for Visible-NIR Light Photocatalytic CO2 Reduction

Metal halide perovskites have garnered significant attention due to their vast potential in various optoelectronic applications. While their tunable bandgap properties allow for light absorption across the ultraviolet and much of the visible spectrum, the coverage in the near-infrared (NIR) region remains limited. Here, we demonstrate a heterovalent co-doping method for synthesizing Ag and Bi doped CsSnBr3 crystals with absorption edge up to 1300 nm, making it one of the narrowest bandgap perovskite materials. The incorporation of trivalent Bi (p) orbitals is responsible for the band narrowing, while the monovalent Ag stabilizes the entire perovskite lattice. Taking advantage of the new energy states within the bandgap, the absorption edge of the co-dopants is extended to NIR region, so they can efficiently utilize sunlight. Moreover, the co-dopants exhibit significantly better antioxidation capability than the pristine CsSnBr3. When applied to CO2 photoreduction, the co-dopants achieved highly selective CO production performance, with an apparent quantum yield (AQY) of 7.56 % at 700 nm, representing a 94 % improvement over CsSnBr3. Overall, this study provides effective strategies for optimizing tin-based perovskites and holds significant implications for future research in enhancing stability, reducing toxicity, and optimizing optoelectronic performance.

Electrostatic Confinement-Induced Excited Charge Transfer in Ionic Covalent Organic Framework Promoting CO2 Reduction

We demonstrate an electrostatic confinement-induced charge transfer pathway in a supramolecular photocatalyst comprising of an ionic covalent organic framework (COF) and cationic metal complexes. The dynamic electrostatic interactions not only attract cations around the COF to accept photogenerated electrons, but also allow for a retention of homogeneous catalytic characters of complexes, making a subtle balance. Accordingly, the electrostatic confinement effect facilitates the forward electron transfer from a photoexcited COF to cationic Co complex, realizing a remarkable photocatalytic CO2 reduction performance. Its catalytic efficiency is far superior to the supramolecular counterparts with Van-der-Waals or hydrogen bonding interactions. This work presents an insight for enhancing charge transfer in supramolecular systems, and provides an effective approach for construction of highly efficient photocatalysts.

Enigma of Sustainable CO2 Conversion to Renewable Fuels and Chemicals Through Photocatalysis, Electrocatalysis, and Photoelectrocatalysis: Design Strategies and Atomic Level Insights

Growing global population, escalating energy consumption, and climate change threaten future energy security. Fossil fuel combustion, primarily coal, oil, and natural gas, exacerbates the greenhouse effect driving global warming through CO2 emissions. To address such issues, research is focused on converting CO2 into valuable fuels and chemicals, which aims to reduce noxious CO2 and simultaneously bridge the gap between energy demands and sustainable supply. CO2 reduction has primarily been accomplished through three methodologies: photocatalysis, electrocatalysis, and photo-electrocatalysis. Review initially elucidates fundamental principles and kinetics that govern CO2 reduction across all three approaches. Subsequently, we have discussed emerging concepts such as role of hot carriers and plasmon-mediated processes in photocatalysis. In electrocatalysis process, we thoroughly discuss advanced design strategies including alloying, ligand-modified surfaces, and molecular tuning to regulate the specific nanostructures of metal-based compounds. Furthermore, it investigates impacts of distinct nanostructures to identify structure property-performance correlations and their mechanisms. Similarly, enhancement of photo-electrocatalytic efficiency is investigated using defect-engineered nanostructures, heterojunctions, and plasmonic metals. Finally, the review outlines potential and intricacies associated with design strategies to drive industrial-scale CO2 reduction. In summary, this comprehensive review offers a thorough analysis of current advances, challenges, and future perspectives for CO2 reduction to valuable products.

Strong Interfacial Interaction in Polarized Ferroelectric Heterostructured Nanosheets for Highly Efficient and Selective Photocatalytic CO2 Reduction

Heterojunctions are sustainable solutions for the photocatalytic CO2 reduction reaction (CO2RR) by regulating charge separation behavior at the interface. However, their efficiency and product selectivity are severely hindered by the inflexible and weak built-in electric field and the electronic structure of the two phases. Herein, ferroelectric-based heterojunctions between polarized bismuth ferrite (BFO(P)) and CdS are constructed to enhance the interfacial interactions and catalytic activity. The intrinsic polarization field depending on the ferroelectric state causes significant electrostatic potential difference and energy-band bending. This helps overcome the unsatisfactory redox potential that differs from the classical catalytic mechanism, and synergy from the heterostructure facilitates effective separation and transfer of photogenerated charges with an extended lifetime (>20 ns) and significantly enhanced photovoltage (1002 times that of BFO). The optimized charge carrier dynamics allow the heterojunction to achieve a much higher CO yield compared to state-of-the-art ferroelectric-based photocatalysts, and 85.46 and 23.47 times higher than those of pristine CdS and BFO, respectively. Moreover, it maintains an impressive 100% product selectivity together with excellent repeatability and cycling. This work not only sheds light on how a strong inherent polarity promotes the performance of heterojunction photocatalysts but also provides new insights for designing efficient photocatalytic CO2RR.

Photocatalytic Oxidative Coupling of Methane to Ethane Using CO2 as a Soft Oxidant over the Au/TiO2-Vo Nanosheets

Photocatalytic oxidative coupling of methane (OCM) offers an appealing route for converting greenhouse gas into valuable C2 hydrocarbons. However, O2, as the most commonly used oxidant, tends to result in inevitable overoxidation and waste of methane feedstock. Herein, we first report a photocatalytic OCM using CO2 as a soft oxidant for C2H6 production under mild conditions, where an efficient photocatalyst with unique interface sites is designed and constructed to facilitate CO2 adsorption and activation, while concurrently boosting CH4 dissociation. As a prototype, the Au quantum dots anchored on oxygen-deficient TiO2 nanosheets are fabricated, where the Au-Vo-Ti interface sites for CO2 adsorption and activation are collectively disclosed by in situ Kelvin probe force microscopy, quasi in situ X-ray photoelectron spectroscopy and theoretical calculations. Compared with single metal site, the Au-Vo-Ti interface sites exhibit the lower CO2 adsorption energy and decrease the energy barrier of the *CO2 hydrogenation step from 1.05 to 0.77 eV via Au-C and Ti-O dual-site bonding. The adsorbed CO2 on the photocatalyst reduces the energy barrier of *CH4 dissociation to *CH3 from 2.13 to 1.59 eV, contributing to CH4 oxidation. Additionally, in situ Fourier-transform infrared spectroscopy unveils the Au site facilitates ethane production by engaging in *CH3-Au interaction and accelerating CH3-CH3 coupling. Thus, the photocatalyst demonstrates a high C2H6 evolution rate of 2.60 mmol g-1 h-1 for OCM using CO2 as the soft oxidant, surpassing most of previously reported photocatalysts regardless of OCM and nonoxidative coupling of methane. This work highlights the importance of soft oxidants for improving oxidation reaction efficiency and provides atomic scale insight into the design of photocatalysts for CH4 conversion.

Single-Atom Suture

In atomically thin two-dimensional (2D) materials, grain boundaries (GBs) are ubiquitous, displaying a profound effect on the electronic structure of the host lattice. The random configuration of atoms within GBs introduces an arbitrary and unpredictable local electronic environment, which may hazard electron transport. Herein, by utilizing the Pt single-atom chains with an ultimate one-dimensional (1D) feature (width of a single atom and length up to tens of nanometers), we realized the suture of the electron pathway at GBs of diversified transition metal dichalcogenides (TMDCs). Theoretical calculations reveal that the construction of Pt single-atom sutures (SAS) prompts the emergence of electronic states proximal to the Fermi level, effectively modulating the transformation of the electronic structure from semiconductivity to metallicity. This transformation underscores the pivotal role of Pt SAS in reconfiguring the electron pathway. Benefiting from this, the Pt SAS-MoS2 emerges as an excellent catalyst, exhibiting an overpotential of 41 mV at 10 mA cm-2 and a Tafel slope of 54 mV dec-1 in hydrogen evolution reaction. Our results offer an understanding of the electron conduction pathway contributed by ultraordered atomic arrangement and the innovative mechanisms for future potential catalysts with an optimized architecture.

Electrochemical Nitrate Reduction to Ammonia on AuCu Single-Atom Alloy Aerogels under Wide Potential Window

Electrocatalytic nitrate reduction to ammonia (NO3RR) is very attractive for nitrate removal and ammonia production in industrial processes. However, the nitrate reduction reaction is characterized by intense hydrogen competition at strong reduction potentials, which greatly limits the Faraday efficiency at strong reduction potentials. Herein, we reported an AuxCu single-atom alloy aerogels (AuxCu SAAs) with three-dimensional network structure with significant nitrate reduction performance of Faraday efficiency (FE) higher than 90 % over a wide potential range (0 ~ -1 V RHE). The FE of the catalyst was close to 100 % at a high reduction potential of -0.8 VRHE, accompanying with NH3 yield reaching 6.21 mmol h1 cm2. More importantly, the catalyst maintained a long-term operation over 400 h at 400 mA cm2 for the NO3RR using a continuous flow system in a H-cell. Experimental and theoretical analysis demonstrate that the catalyst can lower the energy barrier for the hydrogenation reaction of *NO2, leading to a rapid consumption of the generated *H, facilitate the hydrogenation process of NO3RR, and inhibit the competitive HER at high overpotentials, which efficiently promotes the nitrate reduction reaction, especially in industrial applications.

Catalysis under electric-/magnetic-/electromagnetic-field coupling

The ultimate goal of catalysis is to control the cleavage and formation of chemical bonds at the molecular or even atomic level, enabling the customization of catalytic products. The essence of chemical bonding is the electromagnetic interaction between atoms, which makes it possible to directly manipulate the dynamic behavior of molecules and electrons in catalytic processes using external electric, magnetic and electromagnetic fields. In this tutorial review, we first introduce the feasibility and importance of field effects in regulating catalytic reaction processes and then outline the basic principles of electric-/magnetic-/electromagnetic-field interaction with matter, respectively. In each section, we further summarize the relevant important advances from two complementary perspectives: the macroscopic molecular motion (including translation, vibration and rotation) and the microscopic intramolecular electron state alteration (including spin polarization, transfer or excitation, and density of states redistribution). Finally, we discuss the challenges and opportunities for further development of catalysis under electric-/magnetic-/electromagnetic-field coupling.

Site-Specific for CO2 Photoreduction with Single-Atom Ni on Strained TiO2-x Derived from Bimetallic Metal-Organic Frameworks

The photocatalytic reduction of CO2 in water to produce fuels and chemicals is promising while challenging. However, many photocatalysts for accomplishing such challenging task usually suffer from unspecific catalytic active sites and the inefficient charge carrier's separation. Here, a site-specific single-atom Ni/TiO2-x catalyst is reported by in situ topological transformation of Ni-Ti-EG bimetallic metal-organic frameworks. The loading of nickel nanoparticles or individual atoms, which act as specific active sites, can be precisely regulated by chelating agents through the partial removal of nickel and adjacent oxygen atoms. Furthermore, the degree of lattice strain in Ni/TiO2-x catalysts, which improves the separation efficiency of charge carriers, can be modulated by fine-tuning the transformation process. By leveraging the anchored nickel atoms and the strained TiO2, the optimized NiSA0.27/TiO2-x shows a CO generation rate of 86.3 µmol g-1 h-1 (288 times higher than that of NiNPs/TiO2-x) and CO selectivity of up to 92.5% for CO2 reduction in a pure-water system. This work underscores the importance of tailoring lattice strain and creating specific single-atom active sites to facilitate the efficient and selective reduction of CO2.

Modulating the Moderate d-Band Center of Indium in InVO4 Nanobelts by Synergizing MnOx and Oxygen Vacancies for High-Efficiency CO2 Photoreduction

Modulating the electronic properties of transition metal sites in photocatalysts at the atomic level is essential for achieving high-activity carbon dioxide photoreduction (CO2PR). An electronic strategy is herein proposed to engineer In-d-band center of InVO4 by incorporating MnOx nanoparticles and oxygen vacancies (VO) into holey InVO4 nanobelts (MnOx/VO-InVO4), which synergistically modulates the In-d-band center to a moderate level and consequently leads to high-efficiency CO2PR. The MnOx/VO-InVO4 catalyst with optimized electronic property exhibits a single carbon evolution rate of up to 145.3 µmol g-1 h-1 and a carbon monoxide (CO) product selectivity of 92.6%, coming out in front of reported InVO4-based materials. It is discovered that the modulated electronic property favors the interaction between the In sites and their intermediates, which thereby improves the thermodynamics and kinetics of the CO2PR-to-CO reaction. This work not only demonstrates the effective engineering of the d orbital of the low-coordination In atoms to promote CO2PR, but also paves the way for the application of tuning d-band center to develop high-efficiency catalysts.

Clustering-Resistant Cu Single Atoms on Porous Au Nanoparticles Supported by TiO2 for Sustainable Photoconversion of CO2 into CH4

Photocatalysts based on single atoms (SAs) modification can lead to unprecedented reactivity with recent advances. However, the deactivation of SAs-modified photocatalysts remains a critical challenge in the field of photocatalytic CO2 reduction. In this study, we unveil the detrimental effect of CO intermediates on Cu single atoms (Cu-SAs) during photocatalytic CO2 reduction, leading to clustering and deactivation on TiO2. To address this, we developed a novel Cu-SAs anchored on Au porous nanoparticles (CuAu-SAPNPs-TiO2) via a vectored etching approach. This system not only enhances CH4 production with a rate of 748.8 μmol ⋅ g-1 ⋅ h-1 and 93.1 % selectivity but also mitigates Cu-SAs clustering, maintaining stability over 7 days. This sustained high performance, despite the exceptionally high efficiency and selectivity in CH4 production, highlights the CuAu-SAPNPs-TiO2 overarching superior photocatalytic properties. Consequently, this work underscores the potential of tailored SAs-based systems for efficient and durable CO2 reduction by reshaping surface adsorption dynamics and optimizing the thermodynamic behavior of the SAs.

Periodic Single-Metal Site Catalysts: Creating Homogeneous and Ordered Atomic-Precision Structures

Heterogeneous single-metal-site catalysts (SMSCs), often referred to as single-atom catalysts (SACs), demonstrate promising catalytic activity, selectivity, and stability across a wide spectrum of reactions due to their rationally designed microenvironments encompassing coordination geometry, binding ligands, and electronic configurations. However, the inherent disorderliness of SMSCs at both atomic scale and nanoscale poses challenges in deciphering working principles and establishing the correlations between microenvironments and the catalytic performances of SMSCs. The rearrangement of randomly dispersed single metals into homogeneous and atomic-precisely structured periodic single-metal site catalysts (PSMSCs) not only simplifies the chaos in SMSCs systems but also unveils new opportunities for manipulating catalytic performance and gaining profound insights into reaction mechanisms. Moreover, the synergistic effects of adjacent single metals and the integration effects of periodic single-metal arrangement further broaden the industrial application scope of SMSCs. This perspective offers a comprehensive overview of recent advancements and outlines prospective avenues for research in the design and characterizations of PSMSCs, while also acknowledging the formidable challenges encountered and the promising prospects that lie ahead.

Heteroatom Immobilization Engineering toward High-Performance Metal Anodes

Heteroatom immobilization engineering (HAIE) is becoming a forefront approach in materials science and engineering, focusing on the precise control and manipulation of atomic-level interactions within heterogeneous systems. HAIE has emerged as an efficient strategy to fabricate single-atom sites for enhancing the performance of metal-based batteries. Despite the significant progress achieved through HAIE in metal anodes for metal-based batteries, several critical challenges such as metal dendrites, side reactions, and sluggish reaction kinetics are still present. In this review, we delve into the fundamental principles underlying heteroatom immobilization engineering in metal anodes, aiming to elucidate its role in enhancing the electrochemical performance in batteries. We systematically investigate how HAIE facilitates uniform nucleation of metal in anodes, how HAIE inhibits side reactions at the metal anode-electrolyte interface, and the role of HAIE in promoting the desolvation of metal ions and accelerating reaction kinetics within metal-based batteries. Finally, we discuss various strategies for implementing HAIE in electrode materials, such as high-temperature pyrolysis, vacancy reduction, and molten-salt etching and anchoring. These strategies include selecting appropriate heteroatoms, optimizing immobilization methods, and constructing material architectures. They can be utilized to further refine the performance to enhance the capabilities of HAIE and facilitate its widespread application in next-generation metal-based battery technologies.

Plasma Chemical Looping: Unlocking High-Efficiency CO2 Conversion to Clean CO at Mild Temperatures

We propose a plasma chemical looping CO2 splitting (PCLCS) approach that enables highly efficient CO2 conversion into O2-free CO at mild temperatures. PCLCS achieves an impressive 84% CO2 conversion and a 1.3 mmol g-1 CO yield, with no O2 detected. Crucially, this strategy significantly lowers the temperature required for conventional chemical looping processes from 650 to 1000 °C to only 320 °C, demonstrating a robust synergy between plasma and the Ce0.7Zr0.3O2 oxygen carrier (OC). Systematic experiments and density functional theory (DFT) calculations unveil the pivotal role of plasma in activating and partially decomposing CO2, yielding a mixture of CO, O2/O, and electronically/vibrationally excited CO2*. Notably, these excited CO2* species then efficiently decompose over the oxygen vacancies of the OCs, with a substantially reduced activation barrier (0.86 eV) compared to ground-state CO2 (1.63 eV), contributing to the synergy. This work offers a promising and energy-efficient pathway for producing O2-free CO from inert CO2 through the tailored interplay of plasma and OCs.

Enhanced Generation of Reactive Oxygen Species via Piezoelectrics based on p-n Heterojunctions with Built-In Electric Field

Tuning the charge transfer processes through a built-in electric field is an effective way to accelerate the dynamics of electro- and photocatalytic reactions. However, the coupling of the built-in electric field of p-n heterojunctions and the microstrain-induced polarization on the impact of piezocatalysis has not been fully explored. Herein, we demonstrate the role of the built-in electric field of p-type BiOI/n-type BiVO4 heterojunctions in enhancing their piezocatalytic behaviors. The highly crystalline p-n heterojunction is synthesized by using a coprecipitation method under ambient aqueous conditions. Under ultrasonic irradiation in water exposed to air, the p-n heterojunctions exhibit significantly higher production rates of reactive species (·OH, ·O2-, and 1O2) as compared to isolated BiVO4 and BiOI. Also, the piezocatalytic rate of H2O2 production with the BiOI/BiVO4 heterojunction reaches 480 μmol g-1 h-1, which is 1.6- and 12-fold higher than those of BiVO4 and BiOI, respectively. Furthermore, the p-n heterojunction maintains a highly stable H2O2 production rate under ultrasonic irradiation for up to 5 h. The results from the experiments and equation-driven simulations of the strain and piezoelectric potential distributions indicate that the piezocatalytic reactivity of the p-n heterojunction resulted from the polarization intensity induced by periodic ultrasound, which is enhanced by the built-in electric field of the p-n heterojunctions. This study provides new insights into the design of piezocatalysts and opens up new prospects for applications in medicine, environmental remediation, and sonochemical sensors.