Defect engineering of metal-oxide interface for proximity of photooxidation and photoreduction

Solar light driven atomic and electronic transformations in a plasmonic Ni@NiO/NiCO3 photocatalyst revealed by ambient pressure X-ray photoelectron spectroscopy

This work employs ambient pressure X-ray photoelectron spectroscopy (APXPS) to delve into the atomic and electronic transformations of a core-shell Ni@NiO/NiCO3 photocatalyst - a model system for visible light active plasmonic photocatalysts used in water splitting for hydrogen production. This catalyst exhibits reversible structural and electronic changes in response to water vapor and solar simulator light. In this study, APXPS spectra were obtained under a 1 millibar water vapor pressure, employing a solar simulator with an AM 1.5 filter to measure spectral data under visible light illumination. The in situ APXPS spectra indicate that the metallic Ni core absorbs the light, exciting plasmons, and creates hot electrons that are subsequently utilized through hot electron injection in the hydrogen evolution reaction (HER) by NiCO3. Additionally, the data show that NiO undergoes reversible oxidation to NiOOH in the presence of water vapor and light. The present work also investigates the contribution of carbonate and its involvement in the photocatalytic reaction mechanism, shedding light on this seldom-explored aspect of photocatalysis. The APXPS results highlight the photochemical reduction of carbonates into -COOH, contributing to the deactivation of the photocatalyst. This work demonstrates the APXPS efficacy in examining photochemical reactions, charge transfer dynamics and intermediates in potential photocatalysts under near realistic conditions.

Air-Promoted Light-Driven Hydrogen Production from Bioethanol over Core/Shell Cr2O3@GaN Nanoarchitecture

Light-driven hydrogen production from biomass derivatives offers a path towards carbon neutrality. It is often however operated with the limitations of sluggish kinetics and severe coking. Herein, a disruptive air-promoted strategy is explored for efficient and durable light-driven hydrogen production from ethanol over a core/shell Cr2O3@GaN nanoarchitecture. The correlative computational and experimental investigations show ethanol is energetically favorable to be adsorbed on the Cr2O3@GaN interface, followed by dehydrogenation toward acetaldehyde and protons by photoexcited holes. The released protons are then consumed for H2 evolution by photogenerated electrons. Afterward, O2 can be evolved into active oxygen species and promote the deprotonation and C-C cleavage of the key C2 intermediate, thus significantly lowering the reaction energy barrier of hydrogen evolution and removing the carbon residual with inhibited overoxidation. Consequently, hydrogen is produced at a high rate of 76.9 mole H2 per gram Cr2O3@GaN per hour by only feeding ethanol, air, and light, leading to the achievement of a turnover number of 266,943,000 mole H2 per mole Cr2O3 over a long-term operation of 180 hours. Notably, an unprecedented light-to-hydrogen efficiency of 17.6 % is achieved under concentrated light illumination. The simultaneous generation of aldehyde from ethanol dehydrogenation enables the process more economically promising.

Twistedly hydrophobic basis with suitable aromatic metrics in covalent organic networks govern micropollutant decontamination

The pre-designable structure and unique architectures of covalent organic frameworks (COFs) render them attractive as active and porous medium for water crisis. However, the effect of functional basis with different metrics on the regulation of interfacial behavior in advanced oxidation decontamination remains a significant challenge. In this study, we pre-design and fabricate different molecular interfaces by creating ordered π skeletons, incorporating different pore sizes, and engineering hydrophilic or hydrophobic channels. These synergically break through the adsorption energy barrier and promote inner-surface renewal, achieving a high removal rate for typical antibiotic contaminants (like levofloxacin) by BTT-DATP-COF, compared with BTT-DADP-COF and BTT-DAB-COF. The experimental and theoretical calculations reveal that such functional basis engineering enable the hole-driven levofloxacin oxidation at the interface of BTT fragments to occur, accompanying with electron-mediated oxygen reduction on terphenyl motif to active radicals, endowing it facilitate the balanced extraction of holes and electrons.

Pine Dendritic Bi/BiOBr Photocatalyst for Efficient Degradation of Antibiotics

Constructing Bi/BiOX (X = Cl, Br) heterostructures with unique electron transfer channels enables charge carriers to transfer unidirectionally at the metal/semiconductor junction and inhibits the backflow of photogenerated carriers. Herein, novel pine dendritic Bi/BiOX (X = Cl, Br) nanoassemblies with multiple electron transfer channels have been successfully synthesized with the assistance of l-cysteine (l-Cys) through a one-step solvothermal method. Such a pine dendritic Bi/BiOBr photocatalyst shows excellent activity toward the degradation of many antibiotics such as tetracycline (TC), norfloxacin, and ciprofloxacin. In particular, its photocatalytic degradation activity of TC is higher than those of reference spherical Bi/BiOBr, lamellar BiOBr, and BiOBr/Bi/BiOBr double-sided nanosheet arrays. Comprehensive characterizations demonstrate that the pine dendritic structure can construct multiple electron transfer channels from BiOBr to metallic Bi, resulting in an obviously promoted separation efficiency of photogenerated carriers. The synthesis method that uses l-Cys to control the morphology provides a guidance to prepare special metal/semiconductor photocatalysts and would be helpful to design a highly efficient photocatalytic process.

Engineering interface structures for heterojunction photocatalysts

Solar photocatalysis is the most ideal solution to global energy concerns and environmental deterioration nowadays. The heterojunction combination has become one of the most successful and effective strategies to design and manufacture composite photocatalysts. Heterojunction structures are widely documented to markedly improve the photocatalytic behavior of materials by enhancing the separation and transfer of photogenerated charges, widening the light absorption range, and broadening redox potentials, which are attributed to the presence of both build-in electric fields at the interface of two different materials and the complementarity between different electron structures. So far, a large number of heterojunction photocatalytic materials have been reported and applied for water splitting, reduction of carbon dioxide and nitrogen, environmental cleaning, etc. This review outlines the recent accomplishments in the design and modification of interface structures in heterojunction photocatalysts, aiming to provide some useful perspectives for future research in this field.

Current Progress on Methods and Technologies for Catalytic Methane Activation at Low Temperatures

Methane (CH₄ ) is an attractive energy source and important greenhouse gas. Therefore, from the economic and environmental point of view, scientists are working hard to activate and convert CH₄ into various products or less harmful gas at low-temperature. Although the inert nature of CH bonds requires high dissociation energy at high temperatures, the efforts of researchers have demonstrated the feasibility of catalysts to activate CH₄ at low temperatures. In this review, the efficient catalysts designed to reduce the CH₄ oxidation temperature and improve conversion efficiencies are described. First, noble metals and transition metal-based catalysts are summarized for activating CH₄ in temperatures ranging from 50 to 500 °C. After that, the partial oxidation of CH₄ at relatively low temperatures, including thermocatalysis in the liquid phase, photocatalysis, electrocatalysis, and nonthermal plasma technologies, is briefly discussed. Finally, the challenges and perspectives are presented to provide a systematic guideline for designing and synthesizing the highly efficient catalysts in the complete/partial oxidation of CH₄ at low temperatures.

Sodium-Directed Photon-Induced Assembly Strategy for Preparing Multisite Catalysts with High Atomic Utilization Efficiency

Integrating different reaction sites offers new prospects to address the difficulties in single-atom catalysis, but the precise regulation of active sites at the atomic level remains challenging. Here, we demonstrate a sodium-directed photon-induced assembly (SPA) strategy for boosting the atomic utilization efficiency of single-atom catalysts (SACs) by constructing multifarious Au sites on TiO₂ substrate. Na⁺ was employed as the crucial cement to direct Au single atoms onto TiO₂, while the light-induced electron transfer from excited TiO₂ to Au(Na⁺) ensembles contributed to the self-assembly formation of Au nanoclusters. The synergism between plasmonic near-field and Schottky junction enabled the cascade electron transfer for charge separation, which was further enhanced by oxygen vacancies in TiO₂. Our dual-site photocatalysts exhibited a nearly 2 orders of magnitude improvement in the hydrogen evolution activity under simulated solar light, with a striking turnover frequency (TOF) value of 1533 h^-1 that exceeded other Au/TiO₂-based photocatalysts reported. Our SPA strategy can be easily extended to prepare a wide range of metal-coupled nanostructures with enhanced performance for diverse catalytic reactions. Thus, this study provides a well-defined platform to extend the boundaries of SACs for multisite catalysis through harnessing metal-support interactions.

Atomic and Electronic Structure of the Al2O3/Al Interface during Oxide Propagation Probed by Ab Initio Grand Canonical Monte Carlo

Identifying the structure of the Al₂O₃/Al interface is important for advancing its performance in a wide range of applications, including microelectronics, corrosion barriers, and superconducting qubits. However, beyond the study of a few select terminations of the interface using computational methods, and top-down, laterally averaged spectroscopic and microscopic analyses, the explicit structure of the interface and the initial stages of propagation of the interface into the metal are largely unresolved. In this study, we utilize ab initio grand canonical Monte Carlo to perform a physically motivated, unbiased exploration of the interfacial composition and configuration space. We find that at equilibrium, the interface is atomically sharp with aluminum vacancies and propagates in a layer-by-layer fashion, with aluminum excess in the oxide layer at the interfacial plane. Oxygen incorporation, aluminum vacancy formation, and aluminum vacancy annihilation are the building blocks of Al₂O₃ formation at the interface. The localized interfacial mid-gap states from under-coordinated aluminum atoms from the oxide and the immediate depletion of aluminum states near the Fermi level upon oxygen incorporation prevent oxygen dissolution ahead of the interface front and result in the layer-by-layer propagation of the interface. This is in sharp contrast to the ZrO₂/Zr system, which forms interfacial sub-oxides, and also explains the favorable self-healing nature of the Al₂O₃/Al system. The occupied interfacial mid-gap states also increase the calculated n-type Schottky barrier heights. Additionally, we identify that interfacial aluminum core-level shifts linearly depend on the aluminum coordination number, whereas interfacial oxygen core-level shifts depend on long-range ordering at the interface. The detailed geometric and electronic insights into the interface structure and evolution expand our understanding of this fundamental interface and have important implications for the engineering and design of Al₂O₃/Al-based corrosion coatings with enhanced barrier properties, controllable transistor technologies, and noise-free superconducting qubits.

Laser modification of Au-CuO-Au structures for improved electrical and electro-optical properties

CuO nanomaterials are one of the metal-oxides that received extensive investigations in recent years due to their versatility for applications in high-performance nano-devices. Tailoring the device performance through the engineering of properties in the CuO nanomaterials thus attracted lots of effort. In this paper, we show that nanosecond (ns) laser irradiation is effective in improving the electrical and optoelectrical properties in the copper oxide nanowires (CuO NWs). We find that ns laser irradiation can achieve joining between CuO NWs and interdigital gold electrodes. Meanwhile, the concentration and type of point defects in CuO can be controlled by ns laser irradiation as well. An increase in the concentration of defect centers, together with a reduction in the potential energy barrier at the Au/CuO interfaces due to laser irradiation increases electrical conductivity and enhances photo-conductivity. We demonstrate that the enhanced electrical and photo-conductivity achieved through ns laser irradiation can be beneficial for applications such as resistive switching and photo-detection.

Tailoring of electronic and surface structures boosts exciton-triggering photocatalysis for singlet oxygen generation

Arising from reduced dielectric screening, excitonic effects should be taken into account in ultrathin two-dimensional photocatalysts, and a significant challenge is achieving nontrivial excitonic regulation. However, the effect of structural modification on the regulation of the excitonic aspect is at a comparatively early stage. Herein, we report unusual effects of surface substitutional doping with Pt on electronic and surface characteristics of atomically thin layers of Bi₃O₄Br, thereby enhancing the propensity to generate ¹O₂ Electronically, the introduced Pt impurity states with a lower energy level can trap photoinduced singlet excitons, thus reducing the singlet-triplet energy gap by ∼48% and effectively facilitating the intersystem crossing process for efficient triplet excitons yield. Superficially, the chemisorption state of O₂ causes the changes in the magnetic moment (i.e., spin state) of O₂ through electron-mediated triplet energy transfer, resulting a spontaneous spin-flip process and highly specific ¹O₂ generation. These traits exemplify the opportunities that the surface engineering provides a unique strategy for excitonic regulation and will stimulate more research on exciton-triggering photocatalysis for solar energy conversion.

The Effect of Photoinduced Surface Oxygen Vacancies on the Charge Carrier Dynamics in TiO2 Films

Metal-oxide semiconductors (MOS) are widely utilized for catalytic and photocatalytic applications in which the dynamics of charged carriers (e.g., electrons, holes) play important roles. Under operation conditions, photoinduced surface oxygen vacancies (PI-SOV) can greatly impact the dynamics of charge carriers. However, current knowledge regarding the effect of PI-SOV on the dynamics of hole migration in MOS films, such as titanium dioxide, is solely based upon volume-averaged measurements and/or vacuum conditions. This limits the basic understanding of hole-vacancy interactions, as they are not capable of revealing time-resolved variations during operation. Here, we measured the effect of PI-SOV on the dynamics of hole migration using time-resolved atomic force microscopy. Our findings demonstrate that the time constant associated with hole migration is strongly affected by PI-SOV, in a reversible manner. These results will nucleate an insightful understanding of the physics of hole dynamics and thus enable emerging technologies, facilitated by engineering hole-vacancy interactions.

One-Pot Synthesis of Schiff Bases by Defect-Induced TiO2-x-Catalyzed Tandem Transformation from Alcohols and Nitro Compounds

Schiff bases that are generally formed from condensation reactions of aldehydes (or ketones) and amino groups could also be produced by a photodriven one-pot tandem reaction between alcohols and nitro compounds, in our case. Herein, TiO_2-x porous cages derived from NH₂-MIL-125 by a self-sacrificing template route are used to study the organic transformation and exhibit 100% conversion efficiency of nitrobenzene and 100% selectivity for Schiff bases in the system of benzyl alcohol (5 mL) and nitrobenzene (41 μL) upon light irradiation, but hydrogen by dehydrogenation of benzyl alcohol cannot be detected. Successful occurrence of the organic transformation is mainly attributed to Ti(III)-oxygen vacancy associates. Surface oxygen vacancy-related Ti(III) sites are responsible for binding with nitro groups, and low-coordinated Ti_5c sites selectively adsorb hydroxyl groups of benzyl alcohol. The Ti(III) and oxygen vacancy associates capture photogenerated electrons for achievement of multielectron reduction of nitrobenzene and the subsequent Schiff base condensation reaction with the as-formed benzaldehyde.

Controllable design of defect-rich hybrid iron oxide nanostructures on mesoporous carbon-based scaffold for pseudocapacitive applications

The controllable design of functional nanostructures for energy and environmental applications represents a critical yet challenging technology. The existing fabrication strategies focus mainly on increasing the number of accessible active sites. However, these techniques generally necessitate complex chemical agents and suffer from limited experimental conditions delivering high costs, low yields, and poor reproducibility. The present work reports a new strategy for controllable synthesis of a hybrid system including mixed iron oxide nanostructures enriched with non-stoichiometric Fe21.34O32 and Fe3+[Fe5/33+□1/32+]O4 phases, which possess a high concentration of oxygen and Fe2+ vacancies, and a mesoporous carbon-based scaffold (MCS), which was dervied from coffee residues, with graphitic surface and perforated architecture. The nanoperforates acted as trapping sites to localise the FexOy nanoparticles, thereby boosting the density of accessible active sites. Additionally, at the interfacial regions between the FexOy crystallites, a high density of oxygen vacancies with an oriented pattern was shown to create superlattice structures. The energy storage functionality of the defect-rich MCS/FexOy nanostructure with nanoperforated architecture was investigated, where the results exhibited a high gravimetric capacitance of 540 F g-1 at a current density of 1 A g-1 with outstanding capacitance retention of 73.6% after 14 000 cycles.

Pt–BiVO4/TiO2 composites as Z-scheme photocatalysts for hydrogen production from ethanol: the effect of BiVO4 and Pt on the photocatalytic efficiency

The photodeposition of platinum particles on the BiVO₄/TiO₂ composite surface promotes the H₂ production by reducing H⁺ species.

Gel Electrocatalysts: An Emerging Material Platform for Electrochemical Energy Conversion

Seeking sustainable and cost-effective energy sources is one of the significant challenges for the sustainable development of modern society. To date, considerable expectations have been held for technologies, such as fuel cells and electrolyzers, where the performance strongly depends on electrochemical conversion processes that can generate and store chemical energy through the breaking or formation of chemical bonds. However, those advanced technologies are severely limited by the efficiency, selectivity, and durability of electrocatalysis. Thanks to their hierarchically porous architecture, compositional and structural tunability, and ease of functionalization, the family of gel materials opens exciting opportunities for advanced energy technologies. Unique advances in gel materials based on controllable compositions and functions enable gel electrocatalysts to potentially break the limitations of current materials, enhancing the device performance of electrochemical energy. Here, recent developments and challenges for nanostructured gel-based materials for electrocatalysis applications are summarized. Future possibilities and challenges for gel electrocatalysts in terms of synthesis and applications are also discussed.

Enhanced Adhesion Energy at Oxide/Ag Interfaces for Low-Emissivity Glasses: Theoretical Insight into Doping and Vacancy Effects

Low-emissivity glasses rely on multistacked architectures with a thin silver layer sandwiched between oxide layers. The mechanical stability of the silver/oxide interfaces is a critical parameter that must be maximized. Here, we demonstrate by means of quantum-chemical calculations that a low work of adhesion at interfaces can be significantly increased via doping and by introducing vacancies in the oxide layer. For the sake of illustration, we focus on the ZrO₂(111)/Ag(111) interface exhibiting a poor adhesion in the pristine state and quantify the impact of introducing n-type dopants or p-type dopants in ZrO₂ and vacancies in oxygen atoms (nV_O; with n = 1, 2, 4, 8, 10, 16), zirconium atoms (mV_Zr; with m = 1, 2, 4, 8), or both (nV_O + mV_Zr; with m/n = 1:2, 1:4, 2:2, 2:4). In the case of doping, interfacial electron transfer promotes an increase in the work of adhesion, from initially 0.16 to ∼0.8 J m^-2 (n-type) and ∼2.0 J m^-2 (p-type) at 10% doping. A similar increase in the work of adhesion is obtained by introducing vacancies, e.g., V_O [V_Zr] in the oxide layer yields a work of adhesion of ∼1.5-2.0 J m^-2 at 10% vacancies. An increase is also observed when mixing V_O and V_Zr vacancies in a nonstoichiometric ratio (nV_O + mV_Zr; with 2n ≠ m), while a stoichiometric ratio of V_O and V_Zr has no impact on the interfacial properties.

Photogenerated Oxygen Vacancies in Hierarchical Ag/TiO2 Nanoflowers for Enhanced Photocatalytic Reactions

Oxygen vacancy (V_o) creation and morphology controlling make significant contributions to the electronic and structural regulation of metal oxide semiconductors, yet an investigation about convenient approaches for fabricating hierarchical catalyst with abundant oxygen vacancies still has significant challenges. Here, we report a unique method to create abundant oxygen vacancies in hierarchical Ag/TiO₂ nanoflowers during photocatalytic reaction, which is accompanied by light absorption variation and surface plasmon resonance (SPR) enhancement. Its high efficiency of photocatalytic H₂ evolution (the highest apparent quantum yield reaches 3.2% at 365 nm) and rhodamine B degradation can be considered as benefits from the synergistic effects of the well-arranged hierarchical structure, the photogenerated oxygen vacancies, and the SPR of cocatalyst Ag. This work proposes an effective strategy to optimize the synthesis of regular hierarchical structures and enriches the research on the vital function of oxygen vacancies in photocatalytic reactions.

Enabling Efficient Charge Separation for Optoelectronic Conversion via an Energy-Dependent Z-Scheme n-Semiconductor-Metal-p-Semiconductor Schottky Heterojunction

Achieving good charge separation while maintaining energetic electronic states in heterostructures is a challenge in designing efficient photocatalyst materials. Using first-principles calculations, we propose a Z-scheme S_n-m-S_p (n-semiconductor-metal-p-semiconductor) heterojunction as a viable avenue for achieving broad-spectrum sunlight absorption and, importantly, energy-dependent charge separation. As a proof-of-concept investigation, we investigated two ternary heterostructures, CdS-Au-PdO and SnO₂-W-Ag₂O, in which the electronic Fermi levels line up by virtue of the presence of an intermediate metal layer. A cascade of work functions in the relative order W_n < W_m < W_p drives electrons flowing from S_n to m and from m to S_p. The inner electric fields established at the S_n-m and m-S_p Schottky junctions selectively guide low-energy photoexcited electrons from S_n (CdS/SnO₂) and low-energy holes from S_p (PdO/Ag₂O) to the interposing Au or W metal, respectively. Importantly, relatively low Schottky barriers enforce charge separation by constraining high-energy photogenerated charges to the individual semiconductor layers. Operating together, these two mechanisms enable the achievement of highly efficient optoelectronic conversion.