Fine Tuning the Heterostructured Interfaces by Topological Transformation of Layered Double Hydroxide Nanosheets

Enhanced Hydroxyl Adsorption in Ultrathin NiO/Cr2 O3 In-Plane Heterostructures for Efficient Alkaline Methanol Oxidation Reaction

The exploration of advanced nickel-based electrocatalysts for alkaline methanol oxidation reaction (MOR) holds immense promise for value-added organic products coupled with hydrogen production, but still remain challenging. Herein, we construct ultrathin NiO/Cr2 O3 in-plane heterostructures to promote the alkaline MOR process. Experimental and theoretical studies reveal that NiO/Cr2 O3 in-plane heterostructures enable a favorable upshift of the d-band center and enhanced adsorption of hydroxyl species, leading to accelerated generation of active NiO(OH)ads species. Furthermore, ultrathin in-plane heterostructures endow the catalyst with good charge transfer ability and adsorption behavior of methanol molecules onto catalytic sites, contributing to the improvement of alkaline MOR kinetics. As a result, ultrathin NiO/Cr2 O3 in-plane heterostructures exhibit a remarkable MOR activity with a high current density of 221 mA cm-2 at 0.6 V vs Ag/AgCl, which is 7.1-fold larger than that of pure NiO nanosheets and comparable with other highly active catalysts reported so far. This work provides an effectual strategy to optimize the activity of nickel-based catalysts and highlights the dominate efficacy of ultrathin in-plane heterostructures in alkaline MOR.

Tuning the Interfaces of ZnO/ZnCr2 O4 Derived from Layered-Double-Hydroxide Precursors to Advance Nitrogen Photofixation

Drawing inspiration from the enzyme nitrogenase in nature, researchers are increasingly delving into semiconductor photocatalytic nitrogen fixation due to its similar surface catalytic processes. Herein, we reported a facile and efficient approach to achieving the regulation of ZnO/ZnCr2 O4 photocatalysts with ZnCr-layered double hydroxide (ZnCr-LDH) as precursors. By optimizing the composition ratio of Zn/Cr in ZnCr-LDH to tune interfaces, we can achieve an enhanced nitrogen photofixation performance (an ammonia evolution rate of 31.7 μmol g-1  h-1 using pure water as a proton source) under ambient conditions. Further, photo-electrochemical measurements and transient surface photovoltage spectroscopy revealed that the enhanced photocatalytic activity can be ascribed to the effective carrier separation efficiency, originating from the abundant composite interfaces. This work further demonstrated a promising and viable strategy for the synthesis of nanocomposite photocatalysts for nitrogen photofixation and other challenging photocatalytic reactions.

Recent advances of modification effect in Co3O4-based catalyst towards highly efficient photocatalysis

Tricobalt tetroxide (Co3O4) has been developed as a promising photocatalyst material for various applications. Several reports have been published on the self-modification of Co3O4 to achieve optimal photocatalytic performance. The pristine Co3O4 alone is inadequate for photocatalysis due to the rapid recombination process of photogenerated (PG) charge carriers. The modification of Co3O4 can be extended through the introduction of doping elements, incorporation of supporting materials, surface functionalization, metal loading, and combination with other photocatalysts. The addition of doping elements and support materials may enhance the photocatalysis process, although these modifications have a slight effect on decreasing the recombination process of PG charge carriers. On the other hand, combining Co3O4 with other semiconductors results in a different PG charge carrier mechanism, leading to a decrease in the recombination process and an increase in photocatalytic activity. Therefore, this work discusses recent modifications of Co3O4 and their effects on its photocatalytic performance. Additionally, the modification effects, such as enhanced surface area, generation of oxygen vacancies, tuning the band gap, and formation of heterojunctions, are reviewed to demonstrate the feasibility of separating PG charge carriers. Finally, the formation and mechanism of these modification effects are also reviewed based on theoretical and experimental approaches to validate their formation and the transfer process of charge carriers.

Volcano relationships and a new activity descriptor of 2D transition metal-Fe layered double hydroxides for efficient oxygen evolution reaction

Iron (Fe) sites play a critical role in boosting the catalytic activity of transition metal layered double hydroxide (LDH) electrocatalysts for the oxygen evolution reaction (OER), but the contribution of the Fe content to the catalysis of Fe-doped LDHs is still not well understood. Herein, a series of two-dimensional (2D) Fe-doped MFe-LDHs (M = Co, Ni, Cu, and Mn) was synthesized via a general molecular self-assembly method to track the role of Fe in their electrocatalytic OER activities. Besides the revelation of the intrinsic activity trend of NiFe > CoFe > MnFe > CuFe, volcano-shaped relationships among the catalytic activity descriptors, i.e., overpotential, Tafel slope, and turnover frequency (TOF), and the Fe-content in MFe-LDHs, were identified. Specifically, a ∼20% Fe content resulted in the highest OER performance for the LDH, while excess Fe compromised its activity. A similar volcano relationship was determined between the intermediate adsorption and Fe content via operando impedance spectroscopy (EIS) measurements, and it was shown that the intermediate adsorption capacitance (CPE_ad) can be a new activity descriptor for electrocatalysts. In this work, we not only performed a systematic study on the role of Fe in 2D Fe-doped LDHs but also offer some new insights into the activity descriptors for electrocatalysts.

Rational Regulation of Electronic Structure in Layered Double Hydroxide Via Vanadium Incorporation to Trigger Highly Selective CO2 Photoreduction to CH4

To realize excellent selectivity of CH₄ in CO₂ photoreduction (CO₂ PR) is highly desirable, yet which is challenging due to the limited active sites for CH₄ generation and severe electron-hole recombination on photocatalysts. Herein, based on the theoretically calculated effects of vanadium incorporation into the laminate of layered double hydroxides (LDHs), V into NiAl-LDH to synthesize a series of LDHs with various V contents is introduced. NiV-LDH is revealed to afford a high CH₄ selectivity (78.9%), and extremely low H₂ selectivity (only 0.4%) under λ > 400 nm irradiation. By further tuning the molar ratio of Ni to V, a CH₄ selectivity of as high as 90.1% is achieved on Ni₄ V-LDH, and H₂ is completely prohibited on Ni₂ V-LDH. Fine structural characterizations and comprehensive optical and electrochemical studies uncover V incorporation creates the lower-valence Ni species as active sites for generating CH₄ , and enhances the generation, separation, and transfer of photogenerated carriers.

Topological Transformation of Mg-Containing Layered Double Hydroxide Nanosheets for Efficient Photodriven CH4 Coupling

Direct conversion of methane (CH₄ ) to fuels and other high value-added chemicals is an attractive technology in the chemical industry; however, practical challenges to sustainable processes remain. Herein, we report the preparation of a heterostructured Co-doped MgO-based catalyst through topological transformation of a MgCo-layered double hydroxide (LDH) calcination from 200 to 1100 °C. Remarkably, the catalyst can activate CH₄ coupling to produce C₂ H₆ with a selectivity of 41.60 % within 3 h under full-spectrum irradiation through calcination of LDH at 800 °C. Characterization studies and catalytic results suggest that the highly dispersed active sites and large interfaces amongst the Co-doped MgO-based catalysts enable surface activation of CH₄ to methyl (CH₃ *), in turn promoting coupling of CH₃ * to C₂ H₆ . This study introduces a promising pathway for photodriven CH₄ coupling to give high value-added chemicals by using layered double hydroxides as a precursor.

Defect engineering of NiCo-layered double hydroxide hollow nanocages for highly selective photoreduction of CO2 to CH4 with suppressing H2 evolution

We found that abundant defects could be created in the HC-NiCo-LDH by constructing a hollow nanocage morphology. The HC-NiCo-LDH showed excellent CO₂ photoreduction performance that increasing the CH₄ selectivity while suppressing the H₂ evolution.

Topochemical synthesis of low-dimensional nanomaterials

Over the past several decades, nanomaterials have been extensively studied owing to having a series of unique physical and chemical properties that exceed those of conventional bulk materials. Researchers have developed a lot of strategies for the synthesis of low-dimensional nanomaterials. Among them, topochemical synthesis has attracted increasing attention because it can provide more new nanomaterials by improving and upgrading inexpensive and accessible nanomaterials. In this review, we summarize and analyze many existing topochemical synthesis methods, including selective etching, liquid phase reactions, high-temperature atmosphere reactions, electrochemically assisted methods, etc. The future direction of topochemical synthesis is also proposed.

Engineering polyoxometalate-intercalated layered double hydroxides for catalytic applications

Polyoxometalate-intercalated layered double hydroxide (POM-LDH) nanocomposites have received considerable attention in recent years because such nanocomposites not only inherit the intrinsic properties of POMs and LDHs but also exert significant synergistic effects during the catalytic process. In this frontier article, we present the latest advances on the POM-LDH nanocomposites ranging from new synthetic methods to catalytic applications. By making use of the host layer modification method and exfoliation assembly method, the as-prepared POM-LDH nanocomposites show a wide range of catalytic applications. The challenges and future opportunities are also discussed by highlighting some creative work on related POM- or LDH-based materials.

2-Methylimidazole directed ambient synthesis of zinc-cobalt LDH nanosheets for efficient oxygen evolution reaction

It is a highly desired yet challenging task to replace rare and expensive noble metal catalysts with inexpensive and earth-abundant metal ones in electrochemical sustainable chemistry field. Herein, the bimetallic zinc-cobalt layered double hydroxide nanosheets (ZnCo-LDH NS) have been facilely synthesized using 2-methylimidazole as a bifunctional alkali source (OH^-) and a morphological controlling reagent by the one-step room-temperature reaction. The mechanism study shows that the weak organic base of 2-methylimidazole-induced slow release of OH^- in water/ethanol is the controlling factor for the formation of the LDH nanosheets. The ZnCo-LDH NS modified on glassy carbon electrode exhibited an overpotential of 385 mV at 10 mA cm^-2 and a Tafel slope of 108 mV dec^-1 in 1.0 M KOH for oxygen evolution reaction, outperforming the corresponding the monometal oxides of Zn-OH, Co-OH, the NaOH produced ZnCo-LDH microparticles, and even the benchmark catalyst of RuO₂ at high current density. Characterization experiments indicated that the outstanding electrocatalytic activity can be ascribed to the nanosheet-induced surface area effect, excellent electronic conductivity, and the coupling effect between Zn²⁺ and Co²⁺ in the material. This work offers a facile and environmental-friendly method for synthesising LDH nanosheets bearing enhanced oxygen evolution reaction activity.

Devisable POM/Ni Foam Composite: Precisely Control Synthesis toward Enhanced Hydrogen Evolution Reaction at High pH

Polyoxometalates (POMs) are promising catalysts for the electrochemical hydrogen production from water owing to their high intrinsic catalytic activity and chemical tunability. However, poor electrical conductivity and easy detachment of the POMs from the electrode cause significant challenges under operating condition. Herein, a simple one-step hydrothermal method is reported to synthesize a series of Dexter-Silverton POM/Ni foam composites (denoted as NiM-POM/Ni; M=Co, Zn, Mn), in which the stable linkage between the POM catalysts and the Ni foam electrodes lead to high activity for the hydrogen evolution reaction (HER). Among them, the highest HER performance can be observed in the NiCo-POM/Ni, featuring an overpotential of 64 mV (at 10 mA cm-2 , vs. reversible hydrogen electrode), and a Tafel slope of 75 mV dec-1 in 1.0 m aqueous KOH. Moreover, the NiCo-POM/Ni catalyst showed a high faradaic efficiency ≈97 % for HER. Post-catalytic of NiCo-POM/Ni analyses showed virtually no mechanical or chemical degradation. The findings propose a facile and inexpensive method to design stable and effective POM-based catalysts for HER in alkaline water electrolysis.

Interfacial Coupling Effect on Electron Transport in Hierarchical TaON/Au/ZnCo-LDH Photoanode with Enhanced Photoelectrochemical Water Oxidation

Rational design and construction of interface heterostructures, which can simultaneously accelerate the photoinduced carrier separation and enhance the surface water oxidation kinetics, is of great necessity for photoelectrochemical (PEC) water oxidation. Herein, we report a new strategy for boosting the PEC water oxidation by introducing Schottky junction and semiconductor/water oxidation cocatalysts (SC/WOCs) junction into the TaON photocatalyst. Compared with pristine TaON photoanode, the hierarchical TaON/Au/ZnCo-LDH (LDH = layered double hydroxide) photoanode reveals a cathodic shift of 156 mV for the onset potential and 17.3-fold photocurrent density enhancement at 1.23 V vs RHE, as well as improved long-term stability. Diagnostic efficiencies of the TaON/Au/ZnCo-LDH photoanode demonstrate that the enhanced PEC performance is not dominated by surface electrochemical water oxidation kinetics but largely contributed by the improved charge separation and transfer, indicative of synergistic effects of Au and ZnCo-LDH. Theoretical calculations further reveal that the midgap states introduced by Au and ZnCo-LDH in TaON electronic structures bring about photoexcited electrons concentrated on TaON, while holes accumulated on ZnCo-LDH to achieve efficiently spatial charge separation, which is responsible for the boosted PEC water oxidation performance. The present work highlights the importance and elucidates the mechanism of interface heterojunction in PEC water oxidation, which can provide an efficient approach to design and fabricate a new structural photoanode.

Fabrication of Highly Dispersed Cu-Based Oxides as Desirable NH3-SCR Catalysts via Employing CNTs To Decorate the CuAl-Layered Double Hydroxides

In this study, three kinds of CuAl-LDO/CNT (LDO, layered double oxide) catalysts were prepared by the assembly of CNTs and CuAl-LDH (LDH, layered double hydroxides) as well as subsequently structural topological transformation. The effects of the assembly method on the surface structure property and the DeNO_x performance of the prepared samples were systematically investigated. It was found that three CuAl-LDO/CNT catalysts showed preferable NH₃-SCR catalytic performance compared with CuAl-LDO where the catalyst CuAl-LDO/CNTs(I) exhibited optimum NO_x conversion (>80%) and N₂ selectivity (>90%) within 180-300 °C. Such fine catalytic performance can be attributed to the proper surface acidity and redox ability of the catalyst, which might be correlated with the high dispersion of Cu-based active centers caused by the induced nucleation and effective separation action of LDH by carbon nanotubes. In addition, the outstanding H₂O and SO₂ resistance of the CuAl-LDO/CNTs(I) catalyst was also obtained because of the synergistic effect between CuAl-LDO and CNTs, which could greatly promote the activation and decomposition of ammonium sulfate at lower temperatures.

Two-dimensional-related catalytic materials for solar-driven conversion of COx into valuable chemical feedstocks

The discovery of improved chemical processes for CO and CO2 hydrogenation to valuable hydrocarbon fuels and alcohols is of paramount importance for the chemical industry. Such technologies have the potential to reduce anthropogenic CO2 emissions by adding value to a waste stream, whilst also reducing our consumption of fossil fuels. Current thermal catalytic technologies available for CO and CO2 hydrogenation are demanding in terms of energy input. Various alternative technologies are now being developed for COx hydrogenation, with solar-driven processes over two-dimensional (2D) and 2D-related composite materials being particularly attractive due to the abundance of solar energy on Earth and also the high selectivity of defect-engineered 2D materials towards specific valuable products under very mild reaction conditions. This review showcases recent advances in the solar-driven COx reduction to hydrocarbons over 2D-based materials. Optimization of 2D catalyst performance demands interdisciplinary research that embraces catalyst electronic structure manipulation and morphology control, surface/interface engineering, reactor engineering and density functional theory modelling studies. Through improved understanding of the structure-performance relationships in 2D-related catalysts which is achievable through the application of modern in situ characterization techniques, practical photo/photothermal/photoelectrochemical technologies for CO and CO2 reduction to high-valuable products such as olefins could be realized in the not-too-distant future.