Synthesis of 4H/fcc Noble Multimetallic Nanoribbons for Electrocatalytic Hydrogen Evolution Reaction

Strain Engineering of Unconventional Crystal-Phase Noble Metal Nanocatalysts

The crystal phase, alongside the composition, morphology, architecture, facet, size, and dimensionality, has been recognized as a critical factor influencing the properties of noble metal nanomaterials in various applications. In particular, unconventional crystal phases can potentially enable fascinating properties in noble metal nanomaterials. Recent years have witnessed notable advances in the phase engineering of nanomaterials (PEN). Within the accessible strategies for phase engineering, the effect of strain cannot be ignored because strain can act not only as the driving force of phase transition but also as the origin of the diverse physicochemical properties of the unconventional crystal phase. In this review, we highlight the development of unconventional crystal-phase noble metal nanomaterials within strain engineering. We begin with a short introduction of the unconventional crystal phase and strain effect in noble metal nanomaterials. Next, the correlations of the structure and performance of strain-engineered unconventional crystal-phase noble metal nanomaterials in electrocatalysis are highlighted, as well as the phase transitions of noble metal nanomaterials induced by the strain effect. Lastly, the challenges and opportunities within this rapidly developing field (i.e., the strain engineering of unconventional crystal-phase noble metal nanocatalysts) are discussed.

Crystal Phase Engineering of Ultrathin Alloy Nanostructures for Highly Efficient Electroreduction of Nitrate to Ammonia

Electrocatalytic nitrate reduction reaction (NO3RR) toward ammonia synthesis is recognized as a sustainable strategy to balance the global nitrogen cycle. However, it still remains a great challenge to achieve highly efficient ammonia production due to the complex proton-coupled electron transfer process in NO3RR. Here, the controlled synthesis of RuMo alloy nanoflowers (NFs) with unconventional face-centered cubic (fcc) phase and hexagonal close-packed/fcc heterophase for highly efficient NO3RR is reported. Significantly, fcc RuMo NFs demonstrate high Faradaic efficiency of 95.2% and a large yield rate of 32.7 mg h-1 mgcat -1 toward ammonia production at 0 and -0.1 V (vs reversible hydrogen electrode), respectively. In situ characterizations and theoretical calculations have unraveled that fcc RuMo NFs possess the highest d-band center with superior electroactivity, which originates from the strong Ru─Mo interactions and the high intrinsic activity of the unconventional fcc phase. The optimal electronic structures of fcc RuMo NFs supply strong adsorption of key intermediates with suppression of the competitive hydrogen evolution, which further determines the remarkable NO3RR performance. The successful demonstration of high-performance zinc-nitrate batteries with fcc RuMo NFs suggests their substantial application potential in electrochemical energy supply systems.

Engineering improved strategies for spinel cathodes in high-performing zinc-ion batteries

The development of high-performing cathode materials for aqueous zinc-ion batteries (ZIBs) is highly important for the future large-scale energy storage. Owing to the distinctive framework structure, diversity of valences, and high electrochemical activity, spinel materials have been widely investigated and used for aqueous ZIBs. However, the stubborn issues of low electrical conductivity and sluggish kinetics plague their smooth applications in aqueous ZIBs, which stimulates the development of effective strategies to address these issues. This review highlights the recent advances of spinel-based cathode materials that include the configuration of aqueous ZIBs and corresponding reaction mechanisms. Subsequently, the classifications of spinel materials and their properties are also discussed. Then, the review mainly summarizes the effective strategies for elevating their electrochemical performance, including their morphology and structure design, defect engineering, heteroatom doping, and coupling with a conductive support. In the final section, several sound prospects in this fervent field are also proposed for future research and applications.

An In Situ Fabricated Hydrogel Polymer - Palladium Nanocomposite Electrocatalyst for the HER: Critical Role of the Polymer in Realizing High Efficiency and Stability

Development of general and simple designs of catalytic electrodes for the hydrogen evolution reaction (HER) is critical. The present work demonstrates the multiple roles played by a hydrogel polymer in the fabrication and activity enhancement of the nanoelectrocatalyst. A nanocomposite thin film of Pd with the insulating hydrogel, poly(2-hydroxyethyl methacrylate) (PHEMA), is fabricated through a facile in situ process, the polymer itself functioning as the reducing/stabilizing agent in the formation of Pd nanoparticles. Pd-PHEMA on Ni foam enables efficient HER in alkaline medium with a low overpotential; the polymer enables the electrocatalysis by its swelling and confinement of the electrolyte. Most significantly, when the electrode is subjected to an optimized cycling protocol, the overpotential decreases steadily, reaching an impressively low value of 36 mV (@10 mA cm-2 ). A low Tafel slope (68 mV dec-1 ), high exchange current density, Faradaic efficiency and TOF (3.27 mA cm-2 , 99 %, 122.7 h-1 ), and extended stability are achieved. Detailed investigations reveal the active role of the polymer in the evolution of the nanocatalyst, itself undergoing favorable morphological changes. The study illustrates the widened scope for developing efficient and stable catalytic electrodes with hydrogel polymers and unique features that promote the generation of green hydrogen.

Metal Alloys-Structured Electrocatalysts: Metal-Metal Interactions, Coordination Microenvironments, and Structural Property-Reactivity Relationships

Metal alloys-structured electrocatalysts (MAECs) have made essential contributions to accelerating the practical applications of electrocatalytic devices in renewable energy systems. However, due to the complex atomic structures, varied electronic states, and abundant supports, precisely decoding the metal-metal interactions and structure-activity relationships of MAECs still confronts great challenges, which is critical to direct the future engineering and optimization of MAECs. Here, this timely review comprehensively summarizes the latest advances in creating the MAECs, including the metal-metal interactions, coordination microenvironments, and structure-activity relationships. First, the fundamental classification, design, characterization, and structural reconstruction of MAECs are outlined. Then, the electrocatalytic merits and modulation strategies of recent breakthroughs for noble and non-noble metal-structured MAECs are thoroughly discussed, such as solid solution alloys, intermetallic alloys, and single-atom alloys. Particularly, unique insights into the bond interactions, theoretical understanding, and operando techniques for mechanism disclosure are given. Thereafter, the current states of diverse MAECs with a unique focus on structural property-reactivity relationships, reaction pathways, and performance comparisons are discussed. Finally, the future challenges and perspectives for MAECs are systematically discussed. It is believed that this comprehensive review can offer a substantial impact on stimulating the widespread utilization of metal alloys-structured materials in electrocatalysis.

Water Flow-Driven Coupling Process of Anodic Oxygen Evolution and Cathodic Oxygen Activation for Water Decontamination and Prevention of Chlorinated Byproducts

Electrochemical advanced oxidation process (EAOP) is a promising technology for decentralized water decontamination but is subject to parasitic anodic oxygen evolution and formation of toxic chlorinated byproducts in the presence of Cl-. To address this issue, we developed a novel electrolytic process by water flow-driven coupling of anodic oxygen evolution reaction (OER) and cathodic molecular oxygen activation (MOA). When water flows from anode to cathode, O2 produced from OER is carried by water through convection, followed by being activated by atomic hydrogen (H*) on Pd cathode to produce •OH. The water flow-driven OER/MOA process enables the anode to be polarized at low potential (1.7 V vs SHE) that is lower than that of conventional EAOP whose •OH is produced from direct water oxidation (>2.3 V vs SHE). At a flow rate of 30 mL min-1, the process could achieve 94.8% removal of 2,4-dichlorophenol (2,4-DCP) and 71.5% removal of chemical oxygen demand (COD) within 45 min at an anode potential of 1.7 V vs SHE and cathode potential of -0.5 V vs SHE. To achieve the comparable 2,4-DCP removal performance, 4.3-fold higher energy consumption was needed for the conventional EAOP with titanium suboxide anode (anode potential of 2.9 V vs SHE), but current efficiency declined by 3.5 folds. Unlike conventional EAOP, chlorate and perchlorate were not detected in the OER/MOA process, because low anode potential <2.0 V vs SHE was thermodynamically unfavorable for the formation of chlorinated byproducts by anodic oxidation, indicated by theoretical calculations and experimental data. This study provides a proof-in-concept demonstration of water flow-driven OER/MOA process, representing a paradigm shift of electrochemical technology for water decontamination and prevention of chlorinated byproducts, making electrochemical water decontamination more efficient, more economic, and more sustainable.

Recent Progress on Phase Engineering of Nanomaterials

As a key structural parameter, phase depicts the arrangement of atoms in materials. Normally, a nanomaterial exists in its thermodynamically stable crystal phase. With the development of nanotechnology, nanomaterials with unconventional crystal phases, which rarely exist in their bulk counterparts, or amorphous phase have been prepared using carefully controlled reaction conditions. Together these methods are beginning to enable phase engineering of nanomaterials (PEN), i.e., the synthesis of nanomaterials with unconventional phases and the transformation between different phases, to obtain desired properties and functions. This Review summarizes the research progress in the field of PEN. First, we present representative strategies for the direct synthesis of unconventional phases and modulation of phase transformation in diverse kinds of nanomaterials. We cover the synthesis of nanomaterials ranging from metal nanostructures such as Au, Ag, Cu, Pd, and Ru, and their alloys; metal oxides, borides, and carbides; to transition metal dichalcogenides (TMDs) and 2D layered materials. We review synthesis and growth methods ranging from wet-chemical reduction and seed-mediated epitaxial growth to chemical vapor deposition (CVD), high pressure phase transformation, and electron and ion-beam irradiation. After that, we summarize the significant influence of phase on the various properties of unconventional-phase nanomaterials. We also discuss the potential applications of the developed unconventional-phase nanomaterials in different areas including catalysis, electrochemical energy storage (batteries and supercapacitors), solar cells, optoelectronics, and sensing. Finally, we discuss existing challenges and future research directions in PEN.

Strain and Surface Engineering of Multicomponent Metallic Nanomaterials with Unconventional Phases

Multicomponent metallic nanomaterials with unconventional phases show great prospects in electrochemical energy storage and conversion, owing to unique crystal structures and abundant structural effects. In this review, we emphasize the progress in the strain and surface engineering of these novel nanomaterials. We start with a brief introduction of the structural configurations of these materials, based on the interaction types between the components. Next, the fundamentals of strain, strain effect in relevant metallic nanomaterials with unconventional phases, and their formation mechanisms are discussed. Then the progress in surface engineering of these multicomponent metallic nanomaterials is demonstrated from the aspects of morphology control, crystallinity control, surface modification, and surface reconstruction. Moreover, the applications of the strain- and surface-engineered unconventional nanomaterials mainly in electrocatalysis are also introduced, where in addition to the catalytic performance, the structure-performance correlations are highlighted. Finally, the challenges and opportunities in this promising field are prospected.

Intrinsic Optical Properties and Emerging Applications of Gold Nanostructures

The collective oscillation of free electrons at the nanoscale surface of gold nanostructures is closely modulated by tuning the size, shape/morphology, phase, composition, hybridization, assembly, and nanopatterning, along with the surroundings of the plasmonic surface located at a dielectric interface with air, liquid, and solid. This review first introduces the physical origin of the intrinsic optical properties of gold nanostructures and further summarizes stimuli-responsive changes in optical properties, metal-field-enhanced optical signals, luminescence spectral shaping, chiroptical response, and photogenerated hot carriers. The current success in the landscape of nanoscience and nanotechnology mainly originates from the abundant optical properties of gold nanostructures in the thermodynamically stable face-centered cubic (fcc) phase. It has been further extended by crystal phase engineering to prepare thermodynamically unfavorable phases (e.g., kinetically stable) and heterophases to modulate their intriguing phase-dependent optical properties. A broad range of promising applications, including but not limited to full-color displays, solar energy harvesting, photochemical reactions, optical sensing, and microscopic/biomedical imaging, have fostered parallel research on the multitude of physical effects occurring in gold nanostructures.

Recent Advances in Phase-Engineered Photocatalysts: Classification and Diversified Applications

Phase engineering is an emerging strategy for tuning the electronic states and catalytic functions of nanomaterials. Great interest has recently been captured by phase-engineered photocatalysts, including the unconventional phase, amorphous phase, and heterophase. Phase engineering of photocatalytic materials (including semiconductors and cocatalysts) can effectively affect the light absorption range, charge separation efficiency, or surface redox reactivity, resulting in different catalytic behavior. The applications for phase-engineered photocatalysts are widely reported, for example, hydrogen evolution, oxygen evolution, CO₂ reduction, and organic pollutant removal. This review will firstly provide a critical insight into the classification of phase engineering for photocatalysis. Then, the state-of-the-art development of phase engineering toward photocatalytic reactions will be presented, focusing on the synthesis and characterization methodologies for unique phase structure and the correlation between phase structure and photocatalytic performance. Finally, personal understanding of the current opportunities and challenges of phase engineering for photocatalysis will also be provided.

Interfacial built-in electric-field for boosting energy conversion electrocatalysis

The formation of a built-in electric field (BIEF) can induce electron-rich and electron-poor counterparts to synergistically modify electronic configurations and optimize the binding strengths with intermediates, thereby leading to outstanding electrocatalytic performance. Herein, a critical review regarding the concept, modulation strategies, and applications of BIEFs is comprehensively summarized, which begins with the fundamental concepts, together with the advantages of BIEF for boosting electrocatalytic reactions. Then, a systematic summary of the advanced strategies for the modulation of BIEF along with the in-detail mechanisms in its formation are also added. Finally, the applications of BIEF in driving electrocatalytic reactions and some cascade systems for illustrating the conclusive role from the induced BIEF are also systematically discussed, followed by perspectives on the future deployment and opportunity of the BIEF design.

Defect engineering of two-dimensional materials for advanced energy conversion and storage

In the global trend towards carbon neutrality, sustainable energy conversion and storage technologies are of vital significance to tackle the energy crisis and climate change. However, traditional electrode materials gradually reach their property limits. Two-dimensional (2D) materials featuring large aspect ratios and tunable surface properties exhibit tremendous potential for improving the performance of energy conversion and storage devices. To rationally control the physical and chemical properties for specific applications, defect engineering of 2D materials has been investigated extensively, and is becoming a versatile strategy to promote the electrode reaction kinetics. Simultaneously, exploring the in-depth mechanisms underlying defect action in electrode reactions is crucial to provide profound insight into structure tailoring and property optimization. In this review, we highlight the cutting-edge advances in defect engineering in 2D materials as well as their considerable effects in energy-related applications. Moreover, the confronting challenges and promising directions are discussed for the development of advanced energy conversion and storage systems.

Catalysis of Alloys: Classification, Principles, and Design for a Variety of Materials and Reactions

Alloying has long been used as a promising methodology to improve the catalytic performance of metallic materials. In recent years, the field of alloy catalysis has made remarkable progress with the emergence of a variety of novel alloy materials and their functions. Therefore, a comprehensive disciplinary framework for catalytic chemistry of alloys that provides a cross-sectional understanding of the broad research field is in high demand. In this review, we provide a comprehensive classification of various alloy materials based on metallurgy, thermodynamics, and inorganic chemistry and summarize the roles of alloying in catalysis and its principles with a brief introduction of the historical background of this research field. Furthermore, we explain how each type of alloy can be used as a catalyst material and how to design a functional catalyst for the target reaction by introducing representative case studies. This review includes two approaches, namely, from materials and reactions, to provide a better understanding of the catalytic chemistry of alloys. Our review offers a perspective on this research field and can be used encyclopedically according to the readers' individual interests.

Hierarchical Hollow CoWO4-Co(OH)2 Heterostructured Nanoboxes Enabling Efficient Water Oxidation Electrocatalysis

Rational design and construction of well-defined hollow heterostructured nanomaterials assembled by ultrathin nanosheets overtakes crucial role in developing high-efficiency oxygen evolution reaction (OER) electrocatalysts. Herein, a reliable metal-organic framework-mediated and cation-exchange strategy to tune the geometric structure and multicomponent heterostructures has been proposed for the fabrication of hollow CoWO₄-Co(OH)₂ hierarchical nanoboxes assembled by rich ultrathin nanosheets. Benefiting from the hierarchical hollow nanostructure, the CoWO₄-Co(OH)₂ nanoboxes offer plenty of metal active centers available for reaction intermediates. Moreover, the well-defined nanointerfaces between CoWO₄ and Co(OH)₂ can function as the bridge for boosting the efficient electron transfer from CoWO₄ to Co(OH)₂. As a consequence, the optimized CoWO₄-Co(OH)₂ nanoboxes can exhibit outstanding electrocatalytic performance toward OER by delivering 10 mA cm^-2 with a low overpotential of 280 mV and a small Tafel slope of 70.6 mV dec^-1 as well as outstanding electrochemical stability. More importantly, this CoWO₄-Co(OH)₂ heterostructured nanocatalyst can couple with Pt/C to drive overall water splitting to achieve 10 mA cm^-2 with a voltage of 1.57 V.

Insight into the effects of the crystal phase of Ru over ultrathin Ru@Pt core-shell nanosheets for methanol electrooxidation

Coating a second metal on the surface of ultrathin 2D nanosheets (NSs) could induce lattice strain and modify the electronic structure, thereby changing the surface reactivity. Herein, we report the effects of different crystal phases of Ru on the electrocatalytic performance of ultrathin Ru@Pt core-shell NSs for the methanol oxidation reaction (MOR). Importantly, Ru with a novel face-centered-cubic phase was found to have more effect on the electronic structure of Pt than Ru with a conventional hexagonal close-packed phase, thereby leading to improved electrocatalytic activity toward the MOR under acidic and basic conditions. It is believed that the strategy presented here would offer a new approach to the construction of bimetallic core-shell nanostructures with various promising applications.

Selective photocatalytic CO2 reduction in aerobic environment by microporous Pd-porphyrin-based polymers coated hollow TiO2

Direct photocatalytic CO₂ reduction from primary sources, such as flue gas and air, into fuels, is highly desired, but the thermodynamically favored O₂ reduction almost completely impedes this process. Herein, we report on the efficacy of a composite photocatalyst prepared by hyper-crosslinking porphyrin-based polymers on hollow TiO₂ surface and subsequent coordinating with Pd(II). Such composite exhibits high resistance against O₂ inhibition, leading to 12% conversion yield of CO₂ from air after 2-h UV-visible light irradiation. In contrast, the CO₂ reduction over Pd/TiO₂ without the polymer is severely inhibited by the presence of O₂ ( ≥ 0.2 %). This study presents a feasible strategy, building Pd(II) sites into CO₂-adsorptive polymers on hollow TiO₂ surface, for realizing CO₂ reduction with H₂O in an aerobic environment by the high CO₂/O₂ adsorption selectivity of polymers and efficient charge separation for CO₂ reduction and H₂O oxidation on Pd(II) sites and hollow TiO₂, respectively.

Emerging interstitial/substitutional modification of Pd-based nanomaterials with nonmetallic elements for electrocatalytic applications

Palladium (Pd)-based nanomaterials have been identified as potential candidates for various types of electrocatalytic reaction, but most of them typically exhibit unsatisfactory performances. Recently, extensive theoretical and experimental studies have demonstrated that the interstitial/substitutional modification of Pd-based nanomaterials with nonmetallic atoms (H, B, C, N, P, S) has a significant impact on their electronic structure and thus leads to the rapid development of one kind of promising catalyst for various electrochemical reactions. Considering the remarkable progress in this area, we highlight the most recent progress regarding the innovative synthesis and advanced characterization methods of nonmetallic atom-doped Pd-based nanomaterials and provide insights into their electrochemical applications. What's more, the unique structure- and component-dependent electrochemical performance and the underlying mechanisms are also discussed. Furthermore, a brief conclusion about the recent progress achieved in this field as well as future perspectives and challenges are provided.

Preparation of fcc-2H-fcc Heterophase Pd@Ir Nanostructures for High-Performance Electrochemical Hydrogen Evolution

With the development of phase engineering of nanomaterials (PEN), construction of noble-metal heterostructures with unconventional crystal phases, including heterophases, has been proposed as an attractive approach toward the rational design of highly efficient catalysts. However, it still remains challenging to realize the controlled preparation of such unconventional-phase noble-metal heterostructures and explore their crystal-phase-dependent applications. Here, various Pd@Ir core-shell nanostructures are synthesized with unconventional fcc-2H-fcc heterophase (2H: hexagonal close-packed; fcc: face-centered cubic) through a wet-chemical seeded method. As a result, heterophase Pd₆₆ @Ir₃₄ nanoparticles, Pd₄₅ @Ir₅₅ multibranched nanodendrites, and Pd₆₈ @Ir₂₂ Co₁₀ trimetallic nanoparticles are obtained via the phase-selective epitaxial growth of fcc-2H-fcc-heterophase Ir-based nanostructures on 2H-Pd seeds. Importantly, the heterophase Pd₄₅ @Ir₅₅ nanodendrites exhibit excellent catalytic performance toward electrochemical hydrogen evolution reaction (HER) under acidic conditions. An overpotential of only 11.0 mV is required to achieve a current density of 10 mA cm^-2 on Pd₄₅ @Ir₅₅ nanodendrites, which is lower than those of the conventional fcc-Pd₄₇ @Ir₅₃ counterparts, commercial Ir/C and Pt/C. This work not only demonstrates an appealing route to synthesize novel heterophase nanomaterials for promising applications in the emerging field of PEN, but also highlights the significant role of the crystal phase in determining their catalytic properties.

Band Structure Engineering of the W Replacement in ReSe2 nanosheets for Enhancing Hydrogen Evolution

Layered transition metal dichalcogenide Rhenium selenide (ReSe2) has attracted great attention as electrocatalyst for hydrogen evolution reaction (HER) due to its excellent stability and sufficient active sites. However, the intrinsic...

Synthesis of Pd3 Sn and PdCuSn Nanorods with L12 Phase for Highly Efficient Electrocatalytic Ethanol Oxidation

The crystal phase of nanomaterials is one of the key parameters determining their physicochemical properties and performance in various applications. However, it still remains a great challenge to synthesize nanomaterials with different crystal phases while maintaining the same composition, size, and morphology. Here, a facile, one-pot, wet-chemical method is reported to synthesize Pd₃ Sn nanorods with comparable size and morphology but different crystal phases, that is, an ordered intermetallic and a disordered alloy with L1₂ and face-centered cubic (fcc) phases, respectively. The crystal phase of the as-synthesized Pd₃ Sn nanorods is easily tuned by altering the types of tin precursors and solvents. Moreover, the approach can also be used to synthesize ternary PdCuSn nanorods with the L1₂ crystal phase. When used as electrocatalysts, the L1₂ Pd₃ Sn nanorods exhibit superior electrocatalytic performance toward the ethanol oxidation reaction (EOR) compared to their fcc counterpart. Impressively, compared to the L1₂ Pd₃ Sn nanorods, the ternary L1₂ PdCuSn nanorods exhibit more enhanced electrocatalytic performance toward the EOR, yielding a high mass current density up to 6.22 A mg_Pd ^-1 , which is superior to the commercial Pd/C catalyst and among the best reported Pd-based EOR electrocatalysts.

Fe2P encapsulated in carbon nanowalls decorated with well-dispersed Fe3C nanodots for efficient hydrogen evolution and oxygen reduction reactions

The development of cost-effective, high-efficiency bifunctional electrocatalysts as alternatives to the state-of-the-art Pt-based materials toward the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) is of great significance but still challenging. Herein, an advanced bifunctional electrocatalyst is presented, composed of Fe₂P encapsulated in carbon nanowalls decorated with well-dispersed Fe₃C nanodots (denoted as Fe₂P@Fe₃C/CNTs), which is achieved by a novel "inside-out" gas-solid reaction protocol. When functioning as a cathodic catalyst for water splitting, the Fe₂P@Fe₃C/CNT catalyst needs an ultralow overpotential of 83 mV to deliver a current density of 10 mA cm^-2, shows a small Tafel slope of 53 mV dec^-1 and ensures long-term stability for over 200 h in an alkaline electrolyte. Notably, the Fe₂P@Fe₃C/CNT catalyst exhibits an extremely impressive ORR performance with an onset potential (E_onset) of 1.060 V and a half-wave potential (E_1/2) of 0.930 V, excellent stability (≈94% activity retention after 36 000 s), and a strong methanol resistance ability, even far outperforming commercial Pt/C (E_onset = 0.955 V, E_1/2 = 0.825 V, ≈75% activity retention after less than 3500 s). Such outstanding HER and ORR performances are mainly ascribed to the improved corrosion resistance of the unique Fe₂P@C core-shell structures, the abundant catalytically active sites of ultrasmall Fe₃C nanodots incorporated in carbon nanowalls, and the good electrical conductivity of 2D graphitic carbon nanotubes used as a support.

Compressive Strain in N-Doped Palladium/Amorphous-Cobalt (II) Interface Facilitates Alkaline Hydrogen Evolution

The development of palladium-based catalysts for alkaline hydrogen evolution reaction (HER) is highly desired for renewable hydrogen energy systems, yet still challenging due to the strong palladium-hydrogen bond. Herein, the bottleneck is largely overcome by constructing a nitridation-induced compressively strained-interface N-doped palladium/amorphous cobalt (II) interface (N-Pd/A-Co(II)), which dramatically boosts HER performance in alkaline condition. The optimized catalyst with the compressive strain of 2.7% exhibits the higher activity with an overpotential of only 58 mV to achieve the current density of 10 mA cm^-2 , much better than those of pure Pd (327 mV), and the state-of-art Pt/C (78 mV). Notably, it also shows excellent stability with negligible decline during a 30 h stability test. Detailed analyses reveal that the strong absorption of H_ads on Pd can be efficiently reduced via the compressively strained N-doped Pd. And the amorphous Co(II) component accelerates the water dissociation. Consequently, the cooperative effect between the compressed N-doped Pd and amorphous Co(II) creates the impressive HER performance in alkaline condition, highlighting the importance of the functional interface to develop efficient electrocatalysts for HER and beyond.

Recent progress in water-splitting electrocatalysis mediated by 2D noble metal materials

Two-dimensional (2D) nanostructures have enabled noble-metal-based nanomaterials to be promising electrocatalysts toward overall water splitting due to their inherent structural advantages, including a high specific surface active area, numerous low-coordinated atoms, and a high density of defects and edges. Moreover, it is also disclosed that the electronic effect and strain effect within 2D nanostructures also benefit the further promotion of the electrocatalytic performance. In this review, we have focused on the recent progress in the fabrication of advanced electrocatalysts based on 2D noble-metal-based nanomaterials toward water splitting electrocatalysis. First, fundamental descriptions about water-splitting mechanisms, some promising engineering strategies, and major challenges in electrochemical water splitting are given. Then, the structural merits of 2D nanostructures for water splitting electrocatalysis are also highlighted, including abundant surface active sites, lattice distortion, abundant surface defects, electronic effects, and strain effects. Additionally, some representative water-splitting electrocatalysts have been discussed in detail to highlight the superiorities of 2D noble-metal-based nanomaterials for electrochemical water splitting. Finally, the underlying challenges and future opportunities for the fabrication of more advanced electrocatalysts for water splitting are also highlighted. We hope that this review article provides guidance for the fabrication of more efficient electrocatalysts for boosting industrial hydrogen production via water splitting.

Unconventional-Phase Crystalline Materials Constructed from Multiscale Building Blocks

Crystal phase, an intrinsic characteristic of crystalline materials, is one of the key parameters to determine their physicochemical properties. Recently, great progress has been made in the synthesis of nanomaterials with unconventional phases that are different from their thermodynamically stable bulk counterparts via various synthetic methods. A nanocrystalline material can also be viewed as an assembly of atoms with long-range order. When larger entities, such as nanoclusters, nanoparticles, and microparticles, are used as building blocks, supercrystalline materials with rich phases are obtained, some of which even have no analogues in the atomic and molecular crystals. The unconventional phases of nanocrystalline and supercrystalline materials endow them with distinctive properties as compared to their conventional counterparts. This Review highlights the state-of-the-art progress of nanocrystalline and supercrystalline materials with unconventional phases constructed from multiscale building blocks, including atoms, nanoclusters, spherical and anisotropic nanoparticles, and microparticles. Emerging strategies for engineering their crystal phases are introduced, with highlights on the governing parameters that are essential for the formation of unconventional phases. Phase-dependent properties and applications of nanocrystalline and supercrystalline materials are summarized. Finally, major challenges and opportunities in future research directions are proposed.

Advances in hydrogen production from electrocatalytic seawater splitting

As one of the most abundant resources on the Earth, seawater is not only a promising electrolyte for industrial hydrogen production through electrolysis, but also of great significance for the refining of edible salt. Despite the great potential for large-scale hydrogen production, the implementation of water electrolysis requires efficient and stable electrocatalysts that can maintain high activity for water splitting without chloride corrosion. Recent years have witnessed great achievements in the development of highly efficient electrocatalysts toward seawater splitting. Starting from the historical background to the most recent achievements, this review will provide insights into the current state, challenges, and future perspectives of hydrogen production through seawater electrolysis. In particular, the mechanisms of overall water splitting, key features of seawater electrolysis, noble-metal-free electrocatalysts for seawater electrolysis and the underlying mechanisms are also highlighted to provide guidance for fabricating more efficient electrocatalysts toward seawater splitting.

Phase Control of Noble Monometallic and Alloy Nanomaterials by Chemical Reduction Methods

In recent years, the phase control of monometallic and alloy nanomaterials has attracted great attention because of the potential to tune the physical and chemical properties of these species. In this Review, an overview of the latest research progress in phase-controlled monometallic and alloy nanomaterials is first given. Then, the phase-controlled synthesis using a chemical reduction method are discussed, and the formation mechanisms of these nanomaterials are specifically highlighted. Lastly, the challenges and future perspectives in this new research field are discussed.

Selective Epitaxial Growth of Rh Nanorods on 2H/fcc Heterophase Au Nanosheets to Form 1D/2D Rh-Au Heterostructures for Highly Efficient Hydrogen Evolution

Phase engineering of nanomaterials (PEN) enables the preparation of metal nanomaterials with unconventional phases that are different from their thermodynamically stable counterparts. These unconventional-phase nanomaterials can serve as templates to construct precisely controlled metallic heterostructures for wide applications. Nevertheless, how the unconventional phase of templates affects the nucleation and growth of secondary metals still requires systematic explorations. Here, two-dimensional (2D) square-like Au nanosheets with an unconventional 2H/face-centered cubic (fcc) heterophase, composing of two pairs of opposite edges with 2H/fcc heterophase and fcc phase, respectively, and two 2H/fcc heterophase basal planes, are prepared and then used as templates to grow one-dimensional (1D) Rh nanorods. The effect of different phases in different regions of the Au templates on the overgrowth of Rh nanorods has been systematically investigated. By tuning the reaction conditions, three types of 1D/2D Rh-Au heterostructures are prepared. In the type A heterostructure, Rh nanorods only grow on the fcc defects including stacking faults and/or twin boundaries (denoted as fcc-SF/T) and 2H phases in two 2H/fcc edges of the Au nanosheet. In the type B heterostructure, Rh nanorods grow on the fcc-SF/T and 2H phases in two 2H/fcc edges and two 2H/fcc basal planes of the Au nanosheet. In the type C heterostructure, Rh nanorods grow on four edges and two basal planes of the Au nanosheet. Furthermore, the type C heterostructure shows promising performance toward the electrochemical hydrogen evolution reaction (HER) in acidic media, which is among the best reported Rh-based and other noble-metal-based HER electrocatalysts.

Ultrathin Amorphous/Crystalline Heterophase Rh and Rh Alloy Nanosheets as Tandem Catalysts for Direct Indole Synthesis

Heterogeneous noble-metal-based catalysis plays an essential role in the production of fine chemicals. Rh-based catalysts are one of the most active candidates for indole synthesis. However, it is still highly desired to develop heterogeneous Rh-based catalysts with high activity and selectivity. In this work, a general, facile wet-chemical method is reported to synthesize ultrathin amorphous/crystalline heterophase Rh and Rh-based bimetallic alloy nanosheets (NSs), including RhCu, RhZn, and RhRu. Impressively, the amorphous/crystalline heterophase Rh NSs exhibit enhanced catalytic activity toward the direct synthesis of indole compared to the crystalline counterpart. Importantly, the obtained amorphous/crystalline heterophase RhCu alloy NSs can further enhance the selectivity to indole of >99.9% and the conversion is 100%. This work demonstrates the importance of phase engineering and metal alloying in the rational design and synthesis of tandem heterogeneous catalysts toward fine chemical synthesis.

Recent Progress of Ultrathin 2D Pd-Based Nanomaterials for Fuel Cell Electrocatalysis

Pd- and Pd-based catalysts have emerged as potential alternatives to Pt- and Pt-based catalysts for numerous electrocatalytic reactions, particularly fuel cell-related reactions, including the anodic fuel oxidation reaction (FOR) and cathodic oxygen reduction reaction (ORR). The creation of Pd- and Pd-based architectures with large surface areas, numerous low-coordinated atoms, and high density of defects and edges is the most promising strategy for improving the electrocatalytic performance of fuel cells. Recently, 2D Pd-based nanomaterials with single or few atom thickness have attracted increasing interest as potential candidates for both the ORR and FOR, owing to their remarkable advantages, including high intrinsic activity, high electron mobility, and straightforward surface functionalization. In this review, the recent advances in 2D Pd-based nanomaterials for the FOR and ORR are summarized. A fundamental understanding of the FOR and ORR is elaborated. Subsequently, the advantages and latest advances in 2D Pd-based nanomaterials for the FOR and ORR are scientifically and systematically summarized. A systematic discussion of the synthesis methods is also included which should guide researchers toward more efficient 2D Pd-based electrocatalysts. Lastly, the future outlook and trends in the development of 2D Pd-based nanomaterials toward fuel cell development are also presented.

Quasi-Epitaxial Growth of Magnetic Nanostructures on 4H-Au Nanoribbons

Phase engineering of nanomaterials is an effective strategy to tune the physicochemical properties of nanomaterials for various promising applications. Herein, by using the 4H-Au nanoribbons as templates, four novel magnetic nanostructures, namely 4H-Au @ 14H-Co nanobranches, 4H-Au @ 14H-Co nanoribbons, 4H-Au @ 2H-Co nanoribbons, and 4H-Au @ 2H-Ni nanoribbons, are synthesized based on the quasi-epitaxial growth. Different from the conventional epitaxial growth of metal nanomaterials, the obtained Co and Ni nanostructures possess different crystal phases from the Au template. Due to the large lattice mismatch between Au and the grown metals (i.e., Co and Ni), ordered misfit dislocations are generated at the Co/Au and Ni/Au interfaces. Notably, a new super-structure of Co is formed, denoted as 14H. Both 4H-Au @ 14H-Co nanobranches and nanoribbons are ferromagnetic at room temperature, showing similar Curie temperature. However, their magnetic behaviors exhibit distinct temperature dependence, resulting from the competition between spin and volume fluctuations as well as the unique geometry. This work paves the way to the templated synthesis of nanomaterials with unconventional crystal phases for the exploration of phase-dependent properties.

Can an InChI for Nano Address the Need for a Simplified Representation of Complex Nanomaterials across Experimental and Nanoinformatics Studies?

Chemoinformatics has developed efficient ways of representing chemical structures for small molecules as simple text strings, simplified molecular-input line-entry system (SMILES) and the IUPAC International Chemical Identifier (InChI), which are machine-readable. In particular, InChIs have been extended to encode formalized representations of mixtures and reactions, and work is ongoing to represent polymers and other macromolecules in this way. The next frontier is encoding the multi-component structures of nanomaterials (NMs) in a machine-readable format to enable linking of datasets for nanoinformatics and regulatory applications. A workshop organized by the H2020 research infrastructure NanoCommons and the nanoinformatics project NanoSolveIT analyzed issues involved in developing an InChI for NMs (NInChI). The layers needed to capture NM structures include but are not limited to: core composition (possibly multi-layered); surface topography; surface coatings or functionalization; doping with other chemicals; and representation of impurities. NM distributions (size, shape, composition, surface properties, etc.), types of chemical linkages connecting surface functionalization and coating molecules to the core, and various crystallographic forms exhibited by NMs also need to be considered. Six case studies were conducted to elucidate requirements for unambiguous description of NMs. The suggested NInChI layers are intended to stimulate further analysis that will lead to the first version of a "nano" extension to the InChI standard.

Crystal Phase Control of Gold Nanomaterials by Wet-Chemical Synthesis

Gold (Au), a transition metal with an atomic number of 79 in the periodic table of elements, was discovered in approximately 3000 B.C. Due to the ultrahigh chemical stability and brilliant golden color, Au had long been thought to be a most inert material and was widely utilized in art, jewelry, and finance. However, it has been found that Au becomes exceptionally active as a catalyst when its size shrinks to the nanometer scale. With continuous efforts toward the exploration of catalytic applications over the past decades, Au nanomaterials show critical importance in many catalytic processes. Besides catalysis, Au nanomaterials also possess other promising applications in plasmonics, sensing, biology and medicine, due to their unique localized surface plasmon resonance, intriguing biocompatibility, and superior stability. Unfortunately, the practical applications of Au nanomaterials could be limited because of the scarce reserves and high price of Au. Therefore, it is quite essential to further explore novel physicochemical properties and functions of Au nanomaterials so as to enhance their performance in different types of applications.Recently, phase engineering of nanomaterials (PEN), which involves the rearrangement of atoms in the unit cell, has emerged as a fantastic and effective strategy to adjust the intrinsic physicochemical properties of nanomaterials. In this Account, we give an overview of the recent progress on crystal phase control of Au nanomaterials using wet-chemical synthesis. Starting from a brief introduction of the research background, we first describe the development history of wet-chemical synthesis of Au nanomaterials and especially emphasize the key research findings. Subsequently, we introduce the typical Au nanomaterials with untraditional crystal phases and heterophases that have been observed, such as 2H, 4H, body-centered phases, and crystal-phase heterostructures. Importantly, crystal phase control of Au nanomaterials by wet-chemical synthesis is systematically described. After that, we highlight the importance of crystal phase control in Au nanomaterials by demonstrating the remarkable effect of crystal phases on their physicochemical properties (e.g., electronic and optical properties) and potential applications (e.g., catalysis). Finally, after a concise summary of recent advances in this emerging research field, some personal perspectives are provided on the challenges, opportunities, and research directions in the future.

CoMo-bimetallic N-doped porous carbon materials embedded with highly dispersed Pt nanoparticles as pH-universal hydrogen evolution reaction electrocatalysts

The hydrogen evolution reaction is a key half reaction for water electrolysis and is of great significance. Pt-based nanomaterials are promising candidates for HER electrocatalysts. However, the high price of platinum and poor durability impede their practical applications. Herein, a new CoMo-bimetallic hybrid zeolite imidazolate framework is employed to load Pt nanoparticles in a highly dispersed manner as a precursor to synthesize an efficient pH-universal HER electrocatalyst (PtCoMo@NC), which displays overpotentials of 26, 51, and 66 mV at a current density of 10 mA cm^-2 in acidic, basic, and neutral media, respectively. The strong synergistic effect of highly dispersed multi-type catalytic species, including cobalt, molybdenum carbide, and platinum (4.7%) promotes the catalytic activity in the HER process. Meanwhile, the aggregation of Pt nanoparticles is greatly restrained by the carbon matrix so that a brilliant long-time durability of 12 hours and a negligible current decrease in the LSV curve after 10 000 CV cycles are achieved.

Carbon Monoxide Gas Induced 4H-to-fcc Phase Transformation of Gold As Revealed by In-Situ Transmission Electron Microscopy

The hexagonal 4H phase gold nanostructures shows great potential for catalysis, optical, and biomedical fields. However, its phase stability remains largely unclear. Here, we report the 4H-to-face-centered cubic (fcc) phase transformation of gold induced by CO gas interactions and an electron beam observed through in-situ transmission electron microscopy (in-situ TEM). The atomic scale transformation mechanism is revealed experimentally and supported by first-principle calculations. Density functional theory calculations show that the 4H-to-fcc phase transformation processes via the transition of layer sliding with expanded layer spacing, which can be facilitated by both the adsorbed CO molecules and the extra electron provided by the electron beam. The transformation first takes place at the edges of the nanorods with the collective assistance of both CO and extra electrons, and then the inner portion of the bulk crystal follows with extra electrons as the lubricant. These results promote the understanding of the toxic effect of CO gas and shining light on the structural conversion and atomic migration of noble metal catalysts when they interact with CO molecules.

Catalytic Nanoframes and Beyond

The ever-increasing need for the production and expenditure of sustainable energy is a result of the astonishing rate of consumption of fossil fuels and the accompanying environmental problems. Emphasis is being directed to the generation of sustainable energy by the fuel cell and water splitting technologies. Accordingly, the development of highly efficient electrocatalysts has attracted significant interest, as the fuel cell and water splitting technologies are critically dependent on their performance. Among numerous catalyst designs under investigation, nanoframe catalysts have an intrinsically large surface area per volume and a tunable composition, which impacts the number of catalytically active sites and their intrinsic catalytic activity, respectively. Nevertheless, the structural integrity of the nanoframe during electrochemical operation is an ongoing concern. Some significant advances in the field of nanoframe catalysts have been recently accomplished, specifically geared to resolving the catalytic stability concerns and significantly boosting the intrinsic catalytic activity of the active sites. Herein, general synthetic concepts of nanoframe structures and their structure-dependent catalytic performance are summarized, along with recent notable advances in this field. A discussion on the remaining challenges and future directions, addressing the limitations of nanoframe catalysts, are also provided.

Key factors affecting Rayleigh instability of ultrathin 4H hexagonal gold nanoribbons

Rayleigh instability was originally used to describe the phenomenon of a cylindrical fluid jet that transforms into a chain of droplets. Very recently, it has been extended to metallic nanostructures like gold (Au) and silver (Ag) nanowires (NWs), as well as mixed alloy NWs by some thermodynamic processes. To date, the key factors affecting the Rayleigh instability have not been well studied. To clarify this, we systematically investigate the features of Rayleigh instability in ultrathin 4H hexagonal Au nanoribbons (NRBs) under electron beam (E-beam) irradiation. We prove that by decreasing the initial widths of 4H Au NRBs and the E-beam current density, as well as the irradiation time and intensity per unit area, the Rayleigh instability can be effectively restrained. Our work thus sheds light on how to effectively reduce or even eliminate the Rayleigh instability of one dimensional nanomaterials.

High-Index-Facet Metal-Alloy Nanoparticles as Fuel Cell Electrocatalysts

A method to introduce high-index facets into colloidally synthesized nanoparticles is used to produce compositionally uniform Pt-M (M = Ni, Co, and Cu) and Rh-M (M = Ni and Co) tetrahexahedral nanoparticles. The realization of this method allows for a systematic study of catalyst activity as a function of particle composition for various electrooxidation reactions of liquid fuels (formic acid, methanol, and ethanol). The individual contributions of their high-index facets, internal alloying of transition metals, and surface Bi modification to their electrocatalytic properties are experimentally explored, resulting in three key findings. First, the presence of high-index facets is favorable for improving the catalytic activity for all three classes of reactions studied. Second, the effect of transition metal alloying on catalytic activity differs from reaction to reaction. For methanol electrooxidation in an acid electrolyte, due to the contribution from surface Bi modification being negligible, transition metal alloying can significantly the improve overall catalytic efficiency. However, for the other studied reactions, where the surface Bi is highly favorable for improving catalytic activity, there is little influence from transition metal alloying. Finally, multimetallic tetrahexahedral particles have improved stabilities during prolonged operation compared to their monometallic counterparts due to the presence of the alloyed transition metal atoms.

Modern Chemical Routes for the Controlled Synthesis of Anisotropic Bimetallic Nanostructures and Their Application in Catalysis

Bimetallic nanoparticles (BNPs) have attracted greater attention compared to its monometallic counterpart because of their chemical/physical properties. The BNPs have a wide range of applications in the fields of health, energy, water, and environment. These properties could be tuned with a number of parameters such as compositions of the bimetallic systems, their preparation method, and morphology. Monodisperse and anisotropic BNPs have gained considerable interest and numerous efforts have been made for the controlled synthesis of bimetallic nanostructures (BNS) of different sizes and shapes. This review offers a brief summary of the various synthetic routes adopted for the synthesis of Palladium(Pd), Platinum(Pt), Nickel(Ni), Gold(Au), Silver(Ag), Iron(Fe), Cobalt(Co), Rhodium(Rh), and Copper(Cu) based transition metal bimetallic anisotropic nanostructures, growth mechanisms e.g., seed mediated co-reduction, hydrothermal, galvanic replacement reactions, and antigalvanic reaction, and their application in the field of catalysis. The effect of surfactant, reducing agent, metal precursors ratio, pH, and reaction temperature for the synthesis of anisotropic nanostructures has been explained with examples. This review further discusses how slight modifications in one of the parameters could alter the growth mechanism, resulting in different anisotropic nanostructures which highly influence the catalytic activity. The progress or modification implied in the synthesis techniques within recent years is focused on in this article. Furthermore, this article discussed the improved activity, stability, and catalytic performance of BNS compared to the monometallic performance. The synthetic strategies reported here established a deeper understanding of the mechanisms and development of sophisticated and controlled BNS for widespread application.

Phase engineering of nanomaterials

Phase has emerged as an important structural parameter - in addition to composition, morphology, architecture, facet, size and dimensionality - that determines the properties and functionalities of nanomaterials. In particular, unconventional phases in nanomaterials that are unattainable in the bulk state can potentially endow nanomaterials with intriguing properties and innovative applications. Great progress has been made in the phase engineering of nanomaterials (PEN), including synthesis of nanomaterials with unconventional phases and phase transformation of nanomaterials. This Review provides an overview on the recent progress in PEN. We discuss various strategies used to synthesize nanomaterials with unconventional phases and induce phase transformation of nanomaterials, by taking noble metals and layered transition metal dichalcogenides as typical examples. Moreover, we also highlight recent advances in the preparation of amorphous nanomaterials, amorphous-crystalline and crystal phase-based hetero-nanostructures. We also provide personal perspectives on challenges and opportunities in this emerging field, including exploration of phase-dependent properties and applications, rational design of phase-based heterostructures and extension of the concept of phase engineering to a wider range of materials.

In situ-generated NiCo@NiS nanoparticle anchored s-doped carbon nanotubes as dual electrocatalysts for oxygen reduction reaction and hydrogen evolution reaction

Developing low cost and highly robust electrocatalysts for oxygen evolution reaction, hydrogen evolution reaction (HER), and oxygen reduction reaction (ORR) is of great importance for the efficient conversion of sustainable energy sources. Herein, we report a facile pyrolysis strategy for the controllable synthesis of NiCo@NiS/S-CNTs with NiCo@NiS nanoparticles anchored on sulfur-doped carbon nanotubes (CNTs). The obtained NiCo@NiS/S-CNT electrocatalyst exhibits excellent dual-functional catalytic activities under an alkaline condition, an ORR performance with an onset potential of -30 mV, and a half-wave potential of -150 mV (versus Ag/AgCl) while the overpotential for the HER is -1.16 V (versus Ag/AgCl) at a current density of 10 mA cm^-2. It was found that the incorporation of sulfur can regulate the electronic structure of CNTs to accelerate the electron transfer performance and generate new catalytic sites, thus contributing to greatly enhancing both the activity and stability of the catalytic process. This work provides a promising way for the rational design of efficient and robust catalysts for sustainable energy conversion.