Epitaxial Growth of High-Energy Copper Facets for Promoting Hydrogen Evolution Reaction.

Copper is known as a conductive metal but an inert catalyst for the hydrogen evolution reaction due to its inappropriate electronic structure. In this work, an active copper catalyst is prepared with high-energy surfaces by adopting the friction stir welding (FSW) technique. FSW can mix the immiscible Fe and Cu materials homogenously and heat them to a high temperature. Resultantly, α-Fe transforms into γ-Fe, and low-energy γ-Fe (100) and (110) surfaces induce the epitaxial growth of high-energy Cu (110) and (100) planes, respectively. After the removal of γ-Fe by acid etching, the copper electrode exposes high-energy surface and exhibits excellent acidic HER activity, even being superior to Pt foil at high current densities (>66 mA cm-2 ). Density functional theory calculation reveals that the high-energy surface favors the adsorption of hydrogen intermediate, thus accelerating the hydrogen evolution reaction. The epitaxial growth induced by FSW opens a new avenue toward engineering high-performance catalysts. In addition, FSW makes it possible to massively fabricate low-cost catalyst, which is advantageous to industrial application.

Highly Conjugated Graphitic Carbon Nitride Nanofoam for Photocatalytic Hydrogen Evolution.

As a metal-free photocatalyst, graphitic carbon nitride (g-CN) shows great potential for photocatalytic water splitting, although its performance is significantly limited by structural defects due to incomplete polymerization. In the present work, we successfully synthesize highly conjugated g-CN nanofoam through an iodide substitution technique. The product possesses a high polymerization degree, low defect density, and large specific surface area; as a result, it achieves a hydrogen evolution rate of 9.06 mmol h-1 g-1 under visible light irradiation, with an apparent quantum efficiency (AQE) of 18.9% at 420 nm. Experimental analysis and theoretical calculations demonstrate that the recombination of photogenerated carriers at C-NHx defects was effectively depressed in the nanofoam, giving rise to the high photocatalytic activity.

Exposing Cu(100) Surface via Ion-Implantation-Induced Oxidization and Etching for Promoting Hydrogen Evolution Reaction.

Metallic materials with unique surface structure have attracted much attention due to their unique physical and chemical properties. However, it is hard to prepare bulk metallic materials with special crystal faces, especially at the nanoscale. Herein, we report an efficient method to adjust the surface structure of a Cu plate which combines ion implantation technology with the oxidation-etching process. The large number of vacancies generated by ion implantation induced the electrochemical oxidation of several atomic layers in depth; after chemical etching, the Cu(100) planes were exposed on the surface of the Cu plate. As a catalyst for acid hydrogen evolution reaction, the Cu plate with (100) planes merely needs 273 mV to deliver a current density of 10 mA/cm2 because the high-energy (100) surface has moderate hydrogen adsorption and desorption capability. This work provides an appealing strategy to engineer the surface structure of bulk metallic materials and improve their catalytic properties.

Regulating the work function of silver catalysts via surface engineering for enhanced CO2 electroreduction.

The work function can serve as a characteristic quantity to evaluate the catalytic activity due to its relationship with the surface structure of a material. However, what factors determine the influence of the work function on the electrochemical performance are still unclear. Herein, we elucidate the effect of the work function of Ag on the electrochemical reduction of CO2 to CO by controlling the ratio of exposed crystalline planes. To this end, the exposed surface of Ag powder was regulated by high-energy ball milling and its influence on CO2 reduction was investigated. The surface structure with more Ag(110) surface achieves higher activity and selectivity for CO production, resulting from the lower work function of Ag(110), which dramatically enhances the electron tunnelling probability during CO2 electroreduction. We found that a higher ratio of Ag(110) to Ag(100) leads to a lower work function and thus better electrochemical activity and selectivity. This study demonstrates a promising strategy to enhance the electrochemical performance of metal catalysts through tuning their work functions via regulating exposed crystalline planes.

Revealing the Dynamics and Roles of Iron Incorporation in Nickel Hydroxide Water Oxidation Catalysts.

The surface of an electrocatalyst undergoes dynamic chemical and structural transformations under electrochemical operating conditions. There is a dynamic exchange of metal cations between the electrocatalyst and electrolyte. Understanding how iron in the electrolyte gets incorporated in the nickel hydroxide electrocatalyst is critical for pinpointing the roles of Fe during water oxidation. Here, we report that iron incorporation and oxygen evolution reaction (OER) are highly coupled, especially at high working potentials. The iron incorporation rate is much higher at OER potentials than that at the OER dormant state (low potentials). At OER potentials, iron incorporation favors electrochemically more reactive edge sites, as visualized by synchrotron X-ray fluorescence microscopy. Using X-ray absorption spectroscopy and density functional theory calculations, we show that Fe incorporation can suppress the oxidation of Ni and enhance the Ni reducibility, leading to improved OER catalytic activity. Our findings provide a holistic approach to understanding and tailoring Fe incorporation dynamics across the electrocatalyst-electrolyte interface, thus controlling catalytic processes.

Valence-State Effect of Iridium Dopant in NiFe(OH)2 Catalyst for Hydrogen Evolution Reaction.

Engineering high-performance electrocatalysts is of great importance for energy conversion and storage. As an efficient strategy, element doping has long been adopted to improve catalytic activity, however, it has not been clarified how the valence state of dopant affects the catalytic mechanism and properties. Herein, it is reported that the valence state of a doping element plays a crucial role in improving catalytic performance. Specifically, in the case of iridium doped nickel-iron layer double hydroxide (NiFe-LDH), trivalent iridium ions (Ir3+ ) can boost hydrogen evolution reaction (HER) more efficiently than tetravalent iridium (Ir4+ ) ions. Ir3+ -doped NiFe-LDH delivers an ultralow overpotential (19 mV @ 10 mA cm-2 ) for HER, which is superior to Ir4+ doped NiFe-LDH (44 mV@10 mA cm-2 ) and even commercial Pt/C catalyst (40 mV@ 10 mA cm-2 ), and reaches the highest level ever reported for NiFe-LDH-based catalysts. Theoretical and experimental analyses reveal that Ir3+ ions donate more electrons to their neighboring O atoms than Ir4+ ions, which facilitates the water dissociation and hydrogen desorption, eventually boosting HER. The same valence-state effect is found for Ru and Pt dopants in NiFe-LDH, implying that chemical valence state should be considered as a common factor in modulating catalytic performance.

Electroreduction of Carbon Dioxide in Metallic Nanopores through a Pincer Mechanism.

Metallic catalysts with nanopores are advantageous on improving both activity and selectivity, while the reason behind that remains unclear all along. In this work, porous Zn nanoparticles (P-Zn) were adopted as a model catalyst to investigate the catalytic behavior of metallic nanopores. In situ X-ray absorption spectroscopy, in situ Fourier transform infrared spectroscopy, and density functional theory (DFT) analyses reveal that the concave surface of nanopores works like a pincer to capture and clamp CO2 and H2 O precursors simultaneously, thus lowering the energy barriers of CO2 electroreduction. Resultantly, the pincer mechanism endows P-Zn with a high Faradic efficiency (98.1 %) towards CO production at the potential of -0.95 V vs. RHE. Moreover, DFT calculation demonstrates that Co and Cu nanopores exhibit the pincer behavior as well, suggesting that this mechanism is universal for metallic nanopores.

A Hydrogen-Deficient Nickel-Cobalt Double Hydroxide for Photocatalytic Overall Water Splitting.

Developing highly efficient and low-cost photocatalysts for overall water splitting has long been a pursuit for converting solar power into clean hydrogen energy. Herein, we demonstrate that a nonstoichiometric nickel-cobalt double hydroxide can achieve overall water splitting by itself upon solar light irradiation, avoiding the consumption of noble-metal co-catalysts. We employed an intensive laser to ablate a NiCo alloy target immersed in alkaline solution, and produced so-called L-NiCo nanosheets with a nonstoichiometric composition and O2- /Co3+ ions exposed on the surface. The nonstoichiometric composition broadens the band gap, while O2- and Co3+ ions boost hydrogen and oxygen evolution, respectively. As such, the photocatalyst achieves a H2 evolution rate of 1.7 µmol h-1 under AM 1.5G sunlight irradiation and an apparent quantum yield (AQE) of 1.38 % at 380 nm.

Porous Copper Microspheres for Selective Production of Multicarbon Fuels via CO2 Electroreduction.

The electroreduction of carbon dioxide (CO2 ) toward high-value fuels can reduce the carbon footprint and store intermittent renewable energy. The iodide-ion-assisted synthesis of porous copper (P-Cu) microspheres with a moderate coordination number of 7.7, which is beneficial for the selective electroreduction of CO2 into multicarbon (C2+ ) chemicals is reported. P-Cu delivers a C2+ Faradaic efficiency of 78 ± 1% at a potential of -1.1 V versus a reversible hydrogen electrode, which is 32% higher than that of the compact Cu counterpart and approaches the record (79%) reported in the same cell configuration. In addition, P-Cu shows good stability without performance loss throughout a continuous operation of 10 h.

Porous Cobalt-Nickel Hydroxide Nanosheets with Active Cobalt Ions for Overall Water Splitting.

Low-cost and high-performance catalysts are of great significance for electrochemical water splitting. Here, it is reported that a laser-synthesized catalyst, porous Co0.75 Ni0.25 (OH)2 nanosheets, is highly active for catalyzing overall water splitting. The porous nanosheets exhibit low overpotentials for hydrogen evolution reaction (95 mV@10 mA cm-2 ) and oxygen evolution reaction (235 mV@10 mA cm-2 ). As both anode and cathode catalysts, the porous nanosheets achieve a current density of 10 mA cm-2 at an external voltage of 1.56 V, which is much lower than that of commercial Ir/C-Pt/C couple (1.62 V). Experimental and theoretical investigations reveal that numerous Co3+ ions are generated on the pore wall of nanosheets, and the unique atomic structure around Co3+ ions leads to appropriate electronic structure and adsorption energy of intermediates, thus accelerating hydrogen and oxygen evolution.

Strongly Coupled CoO Nanoclusters/CoFe LDHs Hybrid as a Synergistic Catalyst for Electrochemical Water Oxidation.

Exploiting high-performance, robust, and cost-effective electrocatalysts for the oxygen evolution reaction (OER) is crucial for electrochemical energy storage and conversion technologies. Engineering the interfacial structure of hybrid catalysts often induces synergistically enhanced electrocatalytic performance. Herein, a new strongly coupled heterogeneous catalyst with proper interfacial structures, i.e., CoO nanoclusters decorated on CoFe layered double hydroxides (LDHs) nanosheets, is prepared via a simple one-step pulsed laser ablation in liquid method. Thorough spectroscopic characterizations reveal that strong chemical couplings at the hybrid interface trigger charge transfer from CoII in the oxide to FeIII in the LDHs through the interfacial FeOCo bond, leading to considerable amounts of high oxidation state CoIII sites present in the hybrid. Interestingly, the CoO/CoFe LDHs exhibit pronounced synergistic effects in electrocatalytic water oxidation, with substantially enhanced intrinsic catalytic activity and stability relative to both components. The hybrid catalyst achieves remarkably low OER overpotential and Tafel slope in alkaline medium, outperforming that of Ru/C and manifesting itself among the most active Co-based OER catalysts.

Modest Oxygen-Defective Amorphous Manganese-Based Nanoparticle Mullite with Superior Overall Electrocatalytic Performance for Oxygen Reduction Reaction.

Manganese-based oxides have exhibited high promise as noncoinage alternatives to Pt/C for catalyzing oxygen reduction reaction (ORR) in basic solution and a mix of Mn3+/4+ valence is believed to be vital in achieving optimum ORR performance. Here, it is proposed that, distinct from the most studied perovskites and spinels, Mn-based mullites with equivalent molar ratio of Mn3+ and Mn4+ provide a unique platform to maximize the role of Mn valence in facile ORR kinetics by introducing modest content of oxygen deficiency, which is also beneficial to enhanced catalytic activity. Accordingly, amorphous mullite SmMn2 O5-δ nanoparticles with finely tuned concentration of oxygen vacancies are synthesized via a versatile top-down approach and the modest oxygen-defective sample with an Mn3+ /Mn4+ ratio of 1.78, i.e., Mn valence of 3.36 gives rise to a superior overall ORR activity among the highest reported for the family of Mn-based oxides, comparable to that of Pt/C. Altogether, this study opens up great opportunities for mullite-based catalysts to be a cost-effective alternative to Pt/C in diverse electrochemical energy storage and conversion systems.

Tuning Band Structure of Cadmium Chalcogenide Nanoflake Arrays via Alloying for Efficient Photoelectrochemical Hydrogen Evolution.

Owing to their high extinction coefficient and moderate band gap, cadmium chalcogenides are known as common semiconductors for photoelectric conversion. Nevertheless, no ideal cadmium chalcogenide with proper band structure is available yet for photoelectrochemical hydrogen evolution. In this work, we modified the band structure of CdTe via alloying with Se to achieve a ternary compound (CdSe0.8Te0.2) with n-type conduction, a narrower band gap, and a more negative band position compared to those of CdSe and CdTe. This novel material exhibits strong light absorption over a wider spectrum range and generates more vigorous electrons for hydrogen reduction. As a result, a photoelectrode based on nanoflake arrays of the new material could achieve a photocurrent density 2 times that of its CdSe counterpart, outperforming similar materials previously reported in the literature. Moreover, the quick transfer of holes achieved in the novel material was found to depress photocorrosion processes, which led to improved long-term working stability.

ZnFe2 O4 Leaves Grown on TiO2 Trees Enhance Photoelectrochemical Water Splitting.

TiO2 has excellent electrochemical properties but limited solar photocatalytic performance in light of its large bandgap. One important class of visible-wavelength sensitizers of TiO2 is based on ZnFe2 O4 , which has shown fully a doubling in performance relative to pure TiO2 . Prior efforts on this important front have relied on presynthesized nanoparticles of ZnFe2 O4 adsorbed on a TiO2 support; however, these have not yet achieved the full potential of this system since they do not provide a consistently maximized area of the charge-separating heterointerface per volume of sensitizing absorber. A novel atomic layer deposition (ALD)-enhanced synthesis of sensitizing ZnFe2 O4 leaves grown on the trunks of TiO2 trees is reported. These new materials exhibit fully a threefold enhancement in photoelectrochemical performance in water splitting compared to pristine TiO2 under visible illumination. The new materials synthesis strategy relies first on the selective growth of FeOOH nanosheets, 2D structures that shoot off from the sides of the TiO2 trees; these templates are then converted to ZnFe2 O4 with the aid of a novel ALD step, a strategy that preserves morphology while adding the Zn cation to achieve enhanced optical absorption and optimize the heterointerface band alignment.

ZnSe hollow nanospheres in mechanically stable near-IR antireflection coatings for ZnSe substrates.

Though possessing low absorption throughout a wide infrared (IR) spectral regime, owing to a high refractive index, zinc selenide substrates are generally covered by antireflection coatings (ARCs) for practical optical uses. However, achieving a high transmission of ZnSe substrates in the near-IR (NIR) region is still challenging. Herein, for the first time, colloidal ZnSe hollow nanospheres (HNSs) smaller than 100 nm were prepared and adopted to assemble ARCs for ZnSe substrates. The voiding kinetics of the HNSs was found to agree well with the nanoscale Kirkendall effect, and the self-diffusion of the Zn ion in the core was faster than its diffusion through the ZnSe shell. With single-index ARCs, the transmission of ZnSe substrates was remarkably enhanced in the NIR region, with up to an 18% increase at 840 nm. Besides, the ZnSe HNS-based ARCs showed superior mechanical stability even under violent ultrasonication in organic solutions. We expect that ZnSe HNSs will make it possible to construct graded-index ARCs to realize omnidirectional and broadband antireflection in IR, through further tuning of HNSs' void fraction.

Hierarchical, Ultrathin Single-Crystal Nanowires of CdS Conveniently Produced in Laser-Induced Thermal Field.

Hierarchical nanowires (HNWs) exhibit unique properties and have wide applications, while often suffering from imperfect structure. Herein, we report a facile strategy toward ultrathin CdS HNWs with monocrystal structure, where a continuous-wave (CW) Nd:YAG laser is employed to irradiate an oleic acid (OA) solution containing precursors and a light absorber. The high heating rate and large temperature gradient generated by the CW laser lead to the rapid formation of tiny zinc-blende CdS nanocrystals which then line up into nanowires with the help of OA molecules. Next, the nanowires experience a phase transformation from zinc-blende to wurtzite structure, and the transformation-induced stress creates terraces on their surface, which promotes the growth of side branches and eventually results in monocrystal HNWs with an ultrathin diameter of 24 nm. The one-step synthesis of HNWs is conducted in air and completes in just 40 s, thus being very simple and rapid. The prepared CdS HNWs display photocatalytic performance superior to their nanoparticle counterparts, thus showing promise for catalytic applications in the future.

Gain high-quality colloidal quantum dots directly from natural minerals.

Green and simple synthesis of high-quality colloidal quantum dots (CQDs) is of great importance and highly anticipated yet not fully implemented. Herein, we achieve the direct conversion of natural minerals to highly uniform, crystalline lead sulfide CQDs based on laser irradiation in liquid. The trivial fragmentation of mineral particles by an intense nanosecond laser was found to create a localized high degree of monomer supersaturation in oleic acid, initiating the LaMer growth of uniform CQDs. The photoconductive device made of these CQDs exhibits a competitive temporal response of photocurrent with those highly sensitive photodetectors based on PbS CQDs reported in the literature. Our synthesis strategy paves the way for the most environmentally friendly and convenient mass production of high-quality uniform CQDs.

Direct conversion of bulk metals to size-tailored, monodisperse spherical non-coinage-metal nanocrystals.

Monodisperse non-noble metal nanocrystals (NCs) that are highly uniform in shapes and particle size are much desired in various advanced applications, and are commonly prepared by either thermal decomposition or reduction, where reactive organometallic precursors or/and strong reducing agents are mandatory; however, these are usually toxic, costly, or suffer a lack of availability. Bulk Group 12 metals can now be converted into ligand-protected, highly crystalline, monodisperse spherical metal NCs with precisely controlled sizes without using any precursors and reducers. The method is based on low-power NIR-laser-induced size-selective layer-by-layer surface vaporization. The monodisperse Cd NCs show pronounced deep-UV (DUV) localized surface plasmon resonance making them highly competitive DUV-plasmonic materials. This approach will promote appreciably the emergence of a wide range of monodisperse technically important non-coinage metal NCs with compelling functionalities.

Photochemical Synthesis of Ultrafine Cubic Boron Nitride Nanoparticles under Ambient Conditions.

Cubic boron nitride (c-BN) is a super-hard material whose hardness increases dramatically with decreasing size. However, c-BN nanoparticles (NPs) with sizes less than 10 nm have never been obtained. Herein we report a simple strategy towards the synthesis of ultrafine c-BN NPs with an average size of 3.5 nm. The method, under ambient conditions, exploits a laser-induced photochemical effect and employs dioxane solution of ammonia borane (AB) as a liquid target. Meanwhile, total dehydrogenation of AB is realized by laser irradiation. Therefore, this approach shows great potential for the preparation of super-hard NPs as well as controllable dehydrogenation.