Stacking Fault-Enriched MoNi4/MoO2 Enables High-Performance Hydrogen Evolution.

Producing green hydrogen in a cost-competitive manner via water electrolysis will make the long-held dream of hydrogen economy a reality. Although platinum (Pt)-based catalysts show good performance toward hydrogen evolution reaction (HER), the high cost and scarce abundance challenge their economic viability and sustainability. Here, a non-Pt, high-performance electrocatalyst for HER achieved by engineering high fractions of stacking fault (SF) defects for MoNi4/MoO2 nanosheets (d-MoNi) through a combined chemical and thermal reduction strategy is shown. The d-MoNi catalyst offers ultralow overpotentials of 78 and 121 mV for HER at current densities of 500 and 1000 mA cm-2 in 1 M KOH, respectively. The defect-rich d-MoNi exhibits four times higher turnover frequency than the benchmark 20% Pt/C, together with its excellent durability (> 100 h), making it one of the best-performing non-Pt catalysts for HER. The experimental and theoretical results reveal that the abundant SFs in d-MoNi induce a compressive strain, decreasing the proton adsorption energy and promoting the associated combination of *H into hydrogen and molecular hydrogen desorption, enhancing the HER performance. This work provides a new synthetic route to engineer defective metal and metal alloy electrocatalysts for emerging electrochemical energy conversion and storage applications.

Composition-driven morphological evolution of BaTiO3 nanowires for efficient piezocatalytic hydrogen production.

Hydrogen production from water by piezocatalysis is very attractive owing to its high energy efficiency and novelty. BaTiO3, a highly piezoelectric material, is particularly suitable for this application due to its high piezoelectric potential, non-toxic nature, and physicochemical stability. Owing to the critical role of morphology on properties, one-dimensional (1D) materials are expected to exhibit superior water-splitting performance and thus there is a need to optimise the processing conditions to develop outstanding piezocatalysts. In the present work, piezoelectric BaTiO3 nanowires (NWs) were hydrothermally synthesised with precursor Ba:Ti molar ratios of 1:1, 2:1, and 4:1. The morphology, defect chemistry, and hydrogen evolution reaction (HER) efficiency of the as-synthesised BaTiO3 NWs were systematically investigated. The results showed that the morphological features, aspect ratio, structural stability and defect contents of the 1D morphologies collectively have a significant impact on the HER efficiency. The morphological evolution mechanism of the 1D structures were described in terms of ion exchange and dissolution-growth processes of template-grown BaTiO3 NWs for different Ba:Ti molar ratios. Notably, the BaTiO3 NWs synthesised with Ba:Ti molar ratio of 2:1 displayed high crystallinity, good defect concentrations, and good structural integrity under ultrasonication, resulting in an outstanding HER efficiency of 149.24 µmol h-1g-1 which is the highest obtained for nanowire morphologies. These results highlight the importance of synthesis conditions for BaTiO3 NWs for generating excellent piezocatalytic water splitting performance. Additionally, post-ultrasonication tested BaTiO3 NWs demonstrated unexpected photocatalytic activity, with the BTO-1 sample (1:1 Ba:Ti) exhibiting 56% photodegradation of RhB in 2 h of UV irradiation.

Unveiling the mechanisms behind surface degradation of dental resin composites in simulated oral environments.

Dental resin composites are widely used as restorative materials due to their natural aesthetic and versatile properties. However, there has been limited research on the degradation mechanisms of these composites in gastric acid environments, which would be common in patients with gastroesophageal reflux. This study aims to investigate the degradation behavior of dental composites immersed in simulated oral environments, including acid, saliva, and water. Mechanical and morphological properties of the composites, upon immersion in the simulated environments, were thoroughly examined using hardness testing and SEM imaging. Qualitative analyses of the ions leached from the polymer matrix and fillers were conducted using XPS and ICP-MS. In addition, the thermodynamic stability of the inorganic fillers of the composites in aqueous solutions across a wide range of pH values was theoretically studied through construction of Pourbaix diagrams. This study proposed a mechanism for composite leaching involving interactions between the matrix's hydrophilic groups and the aqueous immersion media, leading to swelling and chemical degradation of the composites. Furthermore, it was demonstrated that filler leaching was followed by ion exchange with Ca and P, resulting in the formation of hard calcified layers on the composite surface. The current findings provide valuable insights into the development of new composite materials with improved durability and resistance to degradation, especially for patients suffering from gastroesophageal reflux.

Principles of Design and Synthesis of Metal Derivatives from MOFs.

Materials derived from metal-organic frameworks (MOFs) have demonstrated exceptional structural variety and complexity and can be synthesized using low-cost scalable methods. Although the inherent instability and low electrical conductivity of MOFs are largely responsible for their low uptake for catalysis and energy storage, a superior alternative is MOF-derived metal-based derivatives (MDs) as these can retain the complex nanostructures of MOFs while exhibiting stability and electrical conductivities of several orders of magnitude higher. The present work comprehensively reviews MDs in terms of synthesis and their nanostructural design, including oxides, sulfides, phosphides, nitrides, carbides, transition metals, and other minor species. The focal point of the approach is the identification and rationalization of the design parameters that lead to the generation of optimal compositions, structures, nanostructures, and resultant performance parameters. The aim of this approach is to provide an inclusive platform for the strategies to design and process these materials for specific applications. This work is complemented by detailed figures that both summarize the design and processing approaches that have been reported and indicate potential trajectories for development. The work is also supported by comprehensive and up-to-date tabular coverage of the reported studies.

Recent advances of metal telluride anodes for high-performance lithium/sodium-ion batteries.

Metal tellurides (MTs) have emerged as highly promising candidate anode materials for state-of-the-art lithium-ion batteries (LIBs) and sodium ion batteries (SIBs). This is owing to the unique crystal structure, high intrinsic conductivity, and high trap density of such materials. The present work delivers a detailed discussion on the latest research and progress associated with the use of MTs for LIBs/SIBs with a focus on reaction mechanisms, challenges, electrochemical performance, and synthesis strategies. Further, the prospects and future development of MT anode materials are discussed in terms of strategies to overcome the existing limitations. This review provides both an in-depth understanding of MTs and provides the driving force for expanding research on MTs for energy storage and conversion applications.

Design strategies for ceria nanomaterials: untangling key mechanistic concepts.

The morphologies of ceria nanocrystals play an essential role in determining their redox and catalytic performances in many applications, yet the effects of synthesis variables on the formation of ceria nanoparticles of different morphologies and their related growth mechanisms have not been systematised. The design of these morphologies is underpinned by a range of fundamental parameters, including crystallography, optical mineralogy, the stabilities of exposed crystallographic planes, CeO2-x stoichiometry, phase equilibria, thermodynamics, defect equilibria, and the crystal growth mechanisms. These features are formalised and the key analytical methods used for analysing defects, particularly the critical oxygen vacancies, are surveyed, with the aim of providing a source of design parameters for the synthesis of nanocrystals, specifically CeO2-x. However, the most important aspect in the design of CeO2-x nanocrystals is an understanding of the roles of the main variables used for synthesis. While there is a substantial body of data on CeO2-x morphologies fabricated using low cerium concentrations ([Ce]) under different experimental conditions, the present work fully maps the effects of the relevant variables on the resultant CeO2-x morphologies in terms of the commonly used raw materials [Ce] (and [NO3-] in Ce(NO3)3·6H2O) as feedstock, [NaOH] as precipitating agent, temperature, and time (as well as the complementary vapour pressure). Through the combination of consideration of the published literature and the generation of key experimental data to fill in the gaps, a complete mechanistic description of the development of the main CeO2-x morphologies is illustrated. Further, the mechanisms of the conversion of nanochains into the two variants of nanorods, square and hexagonal, have been elucidated through crystallographic reasoning. Other key conclusions for the crystal growth process are the critical roles of (1) the formation of Ce(OH)4 crystallite nanochains as the precursors of nanorods and (2) the disassembly of the nanorods into Ce(OH)4 crystallites and NO3--assisted reassembly into nanocubes (and nanospheres) as an unrecognised intermediate stage of crystal growth.

Impact of Surface Defects on LaNiO3 Perovskite Electrocatalysts for the Oxygen Evolution Reaction.

Perovskite oxides are regarded as promising electrocatalysts for water splitting due to their cost-effectiveness, high efficiency and durability in the oxygen evolution reaction (OER). Despite these advantages, a fundamental understanding of how critical structural parameters of perovskite electrocatalysts influence their activity and stability is lacking. Here, we investigate the impact of structural defects on OER performance for representative LaNiO3 perovskite electrocatalysts. Hydrogen reduction of 700 °C calcined LaNiO3 induces a high density of surface oxygen vacancies, and confers significantly enhanced OER activity and stability compared to unreduced LaNiO3 ; the former exhibit a low onset overpotential of 380 mV at 10 mA cm-2 and a small Tafel slope of 70.8 mV dec-1 . Oxygen vacancy formation is accompanied by mixed Ni2+ /Ni3+ valence states, which quantum-chemical DFT calculations reveal modify the perovskite electronic structure. Further, it reveals that the formation of oxygen vacancies is thermodynamically more favourable on the surface than in the bulk; it increases the electronic conductivity of reduced LaNiO3 in accordance with the enhanced OER activity that is observed.

Nanoscale design of 1D metal oxides derived from mixed Ni-MH battery/transition metal dust.

Controllable recycling of End-of-life rechargeable nickel-metal hydride (Ni-MH) batteries and by-products of steelmaking to added-value functional nanostructures is desired but challenging. The present work introduces an innovative and high-yield microrecycling strategy to simultaneous synthesis of TM alloy (i.e., Ni-based superalloy) and RE oxide (REO) nanostructures from obsolete Ni-MH batteries mixed with zinc-rich electric arc furnace dust (EAFD). This strategy involves integration of high-temperature thermal isolation followed by thermal nanowiring techniques. The impure thermally-isolated REOs were purified and transformed into one dimensional (1D) nanorods of hybrid REOs. Besides, during high-temperature thermal isolation, defect-rich ZnO with tailored structures of nanorods and nanoribbons were fabricated using controllable vapour deposition. The electrochemical performance of ZnO nanoribbons for oxygen evolution reaction (OER) revealed a considerable overpotential reduction of 131 mV (18%) compared to pure commercial nano-ZnO. This approach is transformational in providing a scalable and cost-effective pathway to facilitate recycling of the challenging, yet critical, waste materials into functional nanostructures for energy and environmental applications.

Multiwalled carbon nanotubes modified with MoO2 nanoparticles for voltammetric determination of the pesticide oxyfluorfen.

A glassy carbon electrode was functionalized by MoO2 nanoparticle-decorated multiwalled carbon nanotubes (MWCNTs) and examined as a working electrode in oxyfluorfen (OXY) detection by differential pulse stripping voltammetry (DPSV). Measurement parameters were as follows: initial potential - 0.1 V, end potential + 0.5 V, accumulation potential - 0.15 V, accumulation time 80 s, and scan rate 50 mV s-1. A stripping potential of + 0.315 V vs. Ag/AgCl was employed. The pPesticide oxyfluorfen was determined in model samples by DPSV with good reproducibility (RSD <2.4%) in the concentration range 2.5 to 34.5 ng mL-1, with r = 0.99 and a limit of detection of 1.5 ng mL-1. These results are in the same range as those of HPLC/DAD, which is used as the comparative method. Recovery for OXY determination in a real river water sample was 102%. Analyses in Briton-Robinson buffer has shown to be pH dependent with the best response at pH 6.0. Structural characterization of MoO2-MWCNT by Raman spectroscopy, field emission scanning electron microscopy, high-resolution transmission electron microscopy, and X-ray crystallography revealed a preserved MWCNT structure decorated with firmly attached clusters of MoO2 nanoparticles. Graphical abstract Glassy carbon electrode functionalized by MoO2 nanoparticle-decorated multiwalled carbon nanotubes is used as a working electrode in the voltammetric determination of pesticide oxyfluorfen in water.

Band gap engineering of Ce-doped anatase TiO2 through solid solubility mechanisms and new defect equilibria formalism.

The present work reports a detailed mechanistic interpretation of the role of the solubility of dopants and resultant midgap defect energies in band gap engineering. While there is a general perception that a single dopant is associated with single solubility and defect mechanisms, in reality, the potential for multiple solubility and defect mechanisms requires a more nuanced interpretation. Similarly, Kröger-Vink defect equilibria assume that stoichiometries during substitutional and interstitial solid solubility as well as Schottky and Frenkel pair formation are compensated by the diffusion of matrix ions to the grain boundaries or surface. However, this approach does not allow the possibility that stoichiometry is uncompensated, where diffusion of the matrix ion to lattice interstices occurs, followed by charge compensation by redox of this ion. Consequently, a modified defect equilibria formalism has been developed in order to allow description of this situation. Experimental data for the structural, chemical, semiconducting, and photocatalytic properties as a function of doping level are correlated with conceptual structural models, a comprehensive energy band diagram, and the corresponding defect equilibria. These correlations reveal the complex mechanisms of the interrelated solubility and defect formation mechanisms, which change significantly and irregularly as a function of small changes in doping level. The analyses confirm that the assumption of single mechanisms of solid solubility and defect formation may be simplifications of more complex processes. The generation of (1) a matrix of complementary characterisation and analytical data, (2) the calculation of a complete energy band diagram, (3) consideration of charge compensation mechanisms and redox beyond the limitations of Kröger-Vink approaches, and (4) the development of models of corresponding structural analogies combine to create a new approach to interpret and explain experimental data. These strategies allow deconstruction of these complex issues and thus targeting of optimal and possibly unique doping levels to achieve lattice configurations that may be energetically and structurally unfavorable. These approaches then can be applied to other doped semiconducting systems.

Coordination Polymer to Atomically Thin, Holey, Metal-Oxide Nanosheets for Tuning Band Alignment.

Holey 2D metal oxides have shown great promise as functional materials for energy storage and catalysts. Despite impressive performance, their processing is challenged by the requirement of templates plus capping agents or high temperatures; these materials also exhibit excessive thicknesses and low yields. The present work reports a metal-based coordination polymer (MCP) strategy to synthesize polycrystalline, holey, metal oxide (MO) nanosheets with thicknesses as low as two-unit cells. The process involves rapid exfoliation of bulk-layered, MCPs (Ce-, Ti-, Zr-based) into atomically thin MCPs at room temperature, followed by transformation into holey 2D MOs upon the removal of organic linkers in aqueous solution. Further, this work represents an extra step for decorating the holey nanosheets using precursors of transition metals to engineer their band alignments, establishing a route to optimize their photocatalysis. The work introduces a simple, high-yield, room-temperature, and template-free approach to synthesize ultrathin holey nanosheets with high-level functionalities.

Surface, Subsurface, and Bulk Oxygen Vacancies Quantified by Decoupling and Deconvolution of the Defect Structure of Redox-Active Nanoceria.

Oxygen vacancy concentrations are critical to the redox/photocatalytic performance of nanoceria, but their direct analysis is problematic under controlled atmospheres but essentially impossible under aqueous conditions. The present work provides three novel approaches to analyze these data from XPS data for the three main morphologies of nanoceria synthesized under aqueous conditions and tested using in vacuo analytical conditions. First, the total oxygen vacancy concentrations are decoupled quantitatively into surface-filled, subsurface-unfilled, and bulk values. Second, the relative surface areas are calculated for all exposed crystallographic planes. Third, XPS and redox performance data are deconvoluted according to the relative surface areas of these planes. Correlations based on two independent empirical results from volumetric surface XPS, combined with sequential deep XPS and independent EELS data, confirm that these approaches provide quantitative determinations of the different oxygen vacancy concentrations. Critically, the redox/photocatalytic performance depends not on the total oxygen vacancy concentration but on the concentration of the active sites on each plane in the form of subsurface-unfilled oxygen vacancies. This is verified by the pH-dependent performance, which can be increased significantly by exposing these vacancies to the surroundings. These approaches have significance to the design and engineering of semiconducting materials exposed to the environment.

Enhancement of Ce/Cr Codopant Solubility and Chemical Homogeneity in TiO2 Nanoparticles through Sol-Gel versus Pechini Syntheses.

Ce/Cr codoped TiO2 nanoparticles were synthesized using sol-gel and Pechini methods with heat treatment at 400 °C for 4 h. A conventional sol-gel process produced well-crystallized anatase, while Pechini synthesis yielded less-ordered mixed-phase anatase + rutile; this suggests that the latter method enhances Ce solubility and increases chemical homogeneity but destabilizes the TiO2 lattice. Greater structural disruption from the decomposition of the Pechini precursor formed more open agglomerated morphologies, while the lower levels of structural disruption from pyrolysis of the dried sol-gel precursor resulted in denser agglomerates of lower surface areas. Codoping and associated destabilization of the lattice reduced the binding energies in both powders. Cr4+ formation in sol-gel powders and Cr6+ formation in Pechini powders suggest that these valence changes derive from synergistic electron exchange from intervalence and/or multivalence charge transfer. Since Ce is too large to allow either substitutional or interstitial solid solubility, the concept of integrated solubility is introduced, in which the Ti site and an adjacent interstice are occupied by the large Ce ion. The photocatalytic performance data show that codoping was detrimental owing to the effects of reduced crystallinity from lattice destabilization and surface area. Two regimes of mechanistic behavior are seen, which are attributed to the unsaturated solid solutions at lower codopant levels and supersaturated solid solutions at higher levels. The present work demonstrates that the Pechini method offers a processing technique that is superior to sol-gel because the former facilitates solid solubility and consequent chemical homogeneity.