High-index Facets Research Articles

High-index crystal plane of ZnO nanopyramidal structures: Stabilization, growth, and improved photocatalytic performance

While the low-index planes of Wurtzite ZnO, such as {0001} and {101¯0} are well-understood, the high-index crystal surfaces have not yet been thoroughly researched despite possessing structural characteristics that make them suitable for many important surface chemistry processes. The high surface energy of high-index ZnO crystal surfaces makes synthesis challenging to achieve due to their instability during crystal growth. In this work, we present a combined experimental and theoretical analysis of growth and photocatalytic activity of ZnO high-index crystal facets. Density functional theory calculations are performed to determine the thermodynamic conditions necessary to stabilize the high-energy semi-polar {112¯2} facets of pyramidal ZnO nanostructures grown via chemical vapor deposition (CVD). The photocatalytic properties of as-synthesized nanopyramidal structures for water splitting applications showed a 73% improved photocatalytic performance compared to CVD-grown hexagonal ZnO nanorods with dominating low-index {101¯0} facets.

Defect engineering for electrochemical nitrogen reduction reaction to ammonia

Abstract Electrochemical nitrogen reduction reaction (NRR) to ammonia (NH3) is considered as a promising alternative for the traditional Haber-Bosch process due to its lower energy consumption under ambient conditions. However, major obstacles still remain in improving the NRR activity and selectivity, mainly arising from the chemical inertness of N2 molecule, the sluggish reaction kinetics, and the competition between hydrogen evolution reaction (HER) and NRR. The defect engineering can regulate and modify the local coordination environment of electrocatalysts, which could be considered as effective strategies to promote the intrinsic activity. In this review, recent advances on defect engineering of nanostructured electrocatalysts for NRR, including vacancy, doping, single atom, amorphization and high-index facet, are summarized. Particularly, the strategies of defect engineering, the reaction mechanisms, and the reliable NH3 detection methods, are systematically discussed. Finally, the opportunities and challenges towards the rational design and synthesis of advanced electrocatalysts with controlled defects for NRR are proposed.

Single-Crystalline Mo-Nanowire-Mediated Directional Growth of High-Index-Faceted MoNi Electrocatalyst for Ultralong-Term Alkaline Hydrogen Evolution

As appealing alternatives to noble-metal-based electrocatalysts for catalyzing hydrogen evolution reaction (HER) in alkali electrolyzers, earth-abundant MoNi-based catalysts have attracted intensive attention. Unfortunately, the exploration of MoNi-based electrocatalysts with superior intrinsic activity and ultralong-term stability still remains a grand challenge. Here, ultralong high-index faceted Mo@MoNi core-shell nanowires were topochemically synthesized through the thermal reduction of Mo@NiMoO4 core-shell nanowires, where single-crystalline Mo support facilitates the topological transformation of NiMoO4 into high-index faceted MoNi. When the as-achieved Mo@MoNi core-shell nanowire film serve as a free-standing cathode in alkaline solutions, it exhibit a remarkably decreased HER overpotential of 18 mV at 10 mA cm-2 and a Tafel slope of ∼33 mV dec-1, which are much lower than those for the state-of-the-art earth-abundant electrocatalysts and even commercial Pt/C. Experimental and theoretical investigations reveal that the exposed high-index (331) facets of MoNi can considerably reduce the energy barriers of initial water dissociation and subsequent hydrogen combination steps, which synergistically accelerates the sluggish alkaline HER kinetics. Significantly, after a 70-day HER operation, the overpotential of Mo@MoNi electrocatalysts at 10 mA cm-2 decreases by only 4 mV. Therefore, this work sheds a bright light on the rational design of high-performance HER electrocatalysts and their practical utilization for alkaline electrolyzers.

Spiny Pd/PtFe core/shell nanotubes with rich high-index facets for efficient electrocatalysis

The performance of fuel-cell related electrocatalysis is highly dependent on the morphology, size and composition of a given catalyst. In terms of rational design of Pt-based catalyst, one-dimensional (1D) ultrafine Pt alloy nanowires (NWs) are considered as a commendable model for enhanced catalysis on account of their favorable mass/charge transfer and structural durability. However, in order to achieve the noble metal catalysts in higher efficiency and lower cost, building high-index facets and shaping hollow interiors should be integrated into 1D Pt alloy NWs, which has rarely been done so far. Here, we report the first synthesis of a class of spiny Pd/PtFe core/shell nanotubes (SPCNTs) constructed by cultivating PtFe alloy branches with rich high-index facets along the 1D removable Pd supports, which is driven by the galvanic dissolution of Pd substrates concomitant with Stranski-Krastanov (S-K) growth of Pt and Fe, for achieving highly efficient fuel-cells-related electrocatalysis. This new catalyst can even deliver electrochemical active surface area (ECSA) of 62.7 m2 gPt−1, comparable to that of commercial carbon-supported Pt nanoparticles. With respect to oxygen reduction catalysis, the SPCNTs showcase the remarkable mass and specific activity of 2.71 A mg−1 and 4.32 mA cm−2, 15.9 and 16.0 times higher than those of commercial Pt/C, respectively. Also, the catalysts exhibit extraordinary resistance to the activity decay and structural degradation during 50,000 potential cycles. Moreover, the SPCNTs serve as a category of efficient and stable catalysts towards anodic alcohol oxidation.

Highly active coral-like porous silver for electrochemical reduction of CO2 to CO

It is widely recognized that metal-based catalytic materials having a porous structure provides a great potential for electrochemical CO2 reduction. In this study, we prepared coral-like porous Ag (CP-Ag) using the electrodeposition approach at various deposition potentials at -0.9 (CP-Ag-0.9), -1.3 (CP-Ag-1.3), and -1.7 V (CP-Ag-1.7), respectively. Among them, CP-Ag-1.3 was revealed to possess the most enhanced CDRR activity and selectivity. The CO partial current density and the faradaic efficiency of CP-Ag-1.3 was 5.3 mA cm−2 and 96.38 %, respectively, which is a substantially high performance surpassingly compared to even the polycrystalline Au known as the most active materials for CO2 reduction. The distinctive CO2 reduction performance of CP-Ag-1.3 resulted from a large surface area, high index facets of the curved surfaces, and an extremely decreased charge transfer resistance. In addition, CP-Ag-1.3 showed an excellent durability without any morphology change in constant current application test for 24 h. The prepared catalysts are capable of enhancing the performance of Ag-based catalysts for the practical application by preparing with cost-effective material.

Catalyzed Kinetic Growth in Two-Dimensional MoS2.

It remains difficult to control the morphology of two-dimensional (2D) materials via direct chemical vapor deposition (CVD) growth. In particular, off-equilibrium (kinetic) growth may produce flakes with non-Wulff shapes (e.g., high-index edges, symmetrical shapes, etc.), which are potentially useful; however, a general controllable approach for the kinetic growth of 2D materials is currently lacking. In this work, we pushed the CVD growth of 2D MoS2 into deep kinetic regime, by using potassium chloride (KCl) as catalyst and plasma pretreatment on growth substrates. The unprecedented nonequilibrium high-index faceting and unusual high-symmetry shapes in 2D materials have been realized. The growth mechanism of high-index facets is rationalized based on the theory of kinetic instability on crystal surfaces. This new vapor-liquid-adatom-solid (VLAS) growth mechanism-synergistic capture of multiple vapor phase molecules by the catalyst particles on corners and the oversaturated adatom diffusion along adjacent edges can offer great opportunities for shape engineering on 2D materials. The high-quality, rapid, and controllable synthesis of high-index facets (edges) and other non-Wulff shapes of 2D transition metal dichalcogenides will benefit the developments in 2D materials.

Preparation of Unique Flower-like Pt-Ni Alloy Catalysts as the Cathode of a PEMFC by Electrodeposition Technique

The oxygen reduction reaction (ORR) at the cathode is the rate-determining step in a PEMFC. To enhance the ORR activity at the cathode, the unique flower-like platinum-nickel binary alloy catalysts were electrodeposited on the commercial carbon cloth by potentiostatic electrodeposition technique, the PtNi alloy effectively improved the oxygen absorption and desorption paths on the surface. The PtNi binary alloy catalysts with shorter lattice distance and effects of high-index facets would increase the electrochemical surface area (ECSA) of the catalysts and power density of the fuel cell. The samples of the pure platinum and commercial Pt/C catalyst were also prepared as comparison. Morphology, particle size and elemental composition are characterized by SEM, XRD and ICP-MS. In addition, the electrochemical characteristics of the PtNi catalysts were investigated via cyclic voltammetry (CV) and linear sweep voltammetry (LSV) analysis in 0.1 M perchloric acid solution with rotating disk electrode (RDE). In single cell test, the maximum peak power density of PEMFC with homemade PtNi alloy catalyst as the cathode reached 1031 mW/cm2, enhancing about 12% power density, compared to the commercial Pt/C.

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.

Unveiling One-Pot Template-Free Fabrication of Exquisite Multidimensional PtNi Multicube Nanoarchitectonics for the Efficient Electrochemical Oxidation of Ethanol and Methanol with a Great Tolerance for CO.

Multidimensional bimetallic Pt-based nanoarchitectonics are highly promising in electrochemical energy conversion technologies because of their fancy structural merits and accessible active sites; however, hitherto their precise template-free fabrication remains a great challenge. We report a template-free solvothermal one-pot approach for the rational design of cocentric PtNi multicube nanoarchitectonics via adjusting the oleylamine/oleic acid ratio with curcumin. The obtained multidimensional PtNi multicubes comprise multiple small interlace-stacked nanocube subunits assembled in spatially porous branched nanoarchitectonics and bound by high-index facets. The synthetic mechanism is driven by spontaneous isolation among prompt nucleation and oriented attachment epitaxial growth. These inimitable architectural and compositional merits of PtNi multicubes endowed the ethanol oxidation mass and specific activity by 5.6 and 9.03 times than the Pt/C catalyst, respectively, along with the enhancement of methanol oxidation mass activity by 2.3 times. Moreover, PtNi multicubes showed superior durability and a higher tolerance for CO poisoning than the Pt/C catalyst. This work may pave the way for tailored preparation of Pt-based nanoarchitectonics for myriad catalytic reactions.

Large Single‐Crystal Cu Foils with High‐Index Facets by Strain‐Engineered Anomalous Grain Growth

The rich and complex arrangements of metal atoms in high-index metal facets afford appealing physical and chemical properties, which attracts extensive research interest in material science for the applications in catalysis and surface chemistry. However, it is still a challenge to prepare large-area high-index single crystals in a controllable and cost-efficient manner. Herein, entire commercially available decimeter-sized polycrystalline Cu foils are successfully transformed into single crystals with a series of high-index facets, relying on a strain-engineered anomalous grain growth technique. The introduction of a moderate thermal-contact stress upon the Cu foil during the annealing leads to the formation of high-index grains dominated by the thermal strain of the Cu foils, rather than the (111) surface driven by the surface energy. Besides, the designed static gradient of the temperature enables the as-formed high-index grain seed to expand throughout the entire Cu foil. The as-received high-index Cu foils can serve as the templates for producing high-index single-crystal Cu-based alloys. This work provides an appealing material basis for the epitaxial growth of 2D materials, and the applications that require the unique surface structures of high-index metal foils and their alloys.

Facet-dependent antibacterial activity of Au nanocrystals

Engineered nanomaterials have attracted significantly attention as one of the most promising antimicrobial agents for against multidrug resistant infections. The toxicological responses of nanomaterials are closely related to their physicochemical properties, and establishment of a structure-activity relationship for nanomaterials at the nano-bio interface is of great significance for deep understanding antibacterial toxicity mechanisms of nanomaterials and designing safer antibacterial nanomaterials. In this study, the antibacterial behaviors of well-defined crystallographic facets of a series of Au nanocrystals, including {100}-facet cubes, {110}-facet rhombic dodecahedra, {111}-facet octahedra, {221}-facet trisoctahedra and {720}-facet concave cubes, was investigated, using the model bacteria Staphylococcus aureus. We find that Au nanocrystals display substantial facet-dependent antibacterial activities. The low-index facets of cubes, octahedra, and rhombic dodecahedra show considerable antibacterial activity, whereas the high-index facets of trisoctahedra and concave cubes remained inert under biological conditions. This result is in stark contrast to the previous paradigm that the high-index facets were considered to have higher bioactivity as compared with low-index facets. The antibacterial mechanism studies have shown that the facet-dependent antibacterial behaviors of Au nanocrystals are mainly caused by differential bacterial membrane damage as well as inhibition of cellular enzymatic activity and energy metabolism. The faceted Au nanocrystals are unique in that they do not induce generation of reactive oxygen species, as validated for most antibiotics and antimicrobial nanostructures. Our findings may provide a deeper understanding of facet-dependent toxicological responses and suggest the complexities of the nanomaterial-cell interactions, shedding some light on the development of high performance Au nanomaterials-based antibacterial therapeutics.

Seeded growth of large single-crystal copper foils with high-index facets.

The production of large single-crystal metal foils with various facet indices has long been a pursuit in materials science owing to their potential applications in crystal epitaxy, catalysis, electronics and thermal engineering1-5. For a given metal, there are only three sets of low-index facets ({100}, {110} and {111}). In comparison, high-index facets are in principle infinite and could afford richer surface structures and properties. However, the controlled preparation of single-crystal foils with high-index facets is challenging, because they are neither thermodynamically6,7 nor kinetically3 favourable compared to low-index facets6-18. Here we report a seeded growth technique for building a library of single-crystal copper foils with sizes of about 30×20 square centimetres and more than 30 kinds of facet. A mild pre-oxidation of polycrystalline copper foils, followed by annealing in a reducing atmosphere, leads to the growth of high-index copper facets that cover almost the entire foil and have the potential of growing to lengths of several metres. The creation of oxide surface layers on our foils means that surface energy minimization is not a key determinant of facet selection for growth, as is usually the case. Instead, facet selection is dictated randomly by the facet of the largest grain (irrespective of its surface energy), which consumes smaller grains and eliminates grain boundaries. Our high-index foils can be used as seeds for the growth of other Cu foils along either the in-plane or the out-of-plane direction. We show that this technique is also applicable to the growth of high-index single-crystal nickel foils, and we explore the possibility of using our high-index copper foils as substrates for the epitaxial growth of two-dimensional materials. Other applications are expected in selective catalysis, low-impedance electrical conduction and heat dissipation.

Synthesis of NiO Crystals Exposing Stable High-Index Facets.

Metal oxides exposing high-index facets are potentially impactful in catalysis and adsorption processes owing to under-coordinated ions and polarities that alter their interfacial properties compared to low-index facets. Here, we report molten-salt syntheses of NiO particles exposing a variety of crystal facets. We show that for a given anion (nitrate or chloride), the alkali cation has a notable impact on the formation of crystals exposing {311}, {611}, {100}, and {111} faces. Based on a parametric analysis of synthesis conditions, we postulate that the crystallization mechanism is governed by the formation of growth units consisting of NiII complexes whose coordination numbers are determined by temperature and the selection of anion (associated to the coordination sphere) and alkali cation (associated with the outer coordination sphere). Notably, our findings reveal that high-index facets are particularly favored in chloride media and are stable under prolonged periods of catalysis and steaming.

Concave PtCo nanocrosses for methanol oxidation reaction

Concave nanostructures with the high-index facets and highly exposed active sites are desirable in energy conversion technology, whereas the current synthetic strategies still remain challenging. Herein, we report a facile and effective strategy for the synthesis of concave PtCo nanocrosses (PtCo CNCs) with the assist of iminodiacetic acid (IDA). Owing to strong chelation of IDA with metal ions through carboxyl and imino groups, the IDA molecule plays a key role as the structure-directing agent in regulating the cross-like structure. The as-prepared PtCo CNCs are constructed by six arms in three-dimensional space and each protruding arm has concave surface bounded by high-index facets. As an electrocatalyst towards methanol oxidation reaction (MOR), the developed PtCo CNCs exhibit much enhanced specific and mass activity of 3.04 mA cmPt−2 and 692 mA mgPt−1 that are 3.1 and 2.6 times greater than those of commercial Pt black, respectively. The excellent electrocatalytic stability and improved tolerance toward COads are also demonstrated on PtCo CNCs catalyst. The remarkable electrocatalytic performance of PtCo CNCs towards MOR is highly related with their concave surface and high-index facets, as well as synergistic effect between Pt and Co atoms. The facile synthetic strategy and outstanding MOR performance endow PtCo CNCs with great application potential in fuel cells as an anode catalyst.

One-Step Synthesis of Supported High-Index Faceted Platinum–Cobalt Nanocatalysts for an Enhanced Oxygen Reduction Reaction

The slow rate of oxygen reduction reaction (ORR) is the main barrier limiting the commercialization of proton exchange membrane fuel cells (PEMFCS) on an industrial scale. Here we report a one-step...

Highly sensitive electrochemical detection of circulating tumor DNA in human blood based on urchin-like gold nanocrystal-multiple graphene aerogel and target DNA-induced recycling double amplification strategy

Detection of circulating tumor DNA (ctDNA) is important approach to risk stratification and treatment response monitoring of cancer patients, but current method lacks of enough sensitivity and repeatability. The paper repors shape-controlled synthesis of gold nanocrystals via reduction of HAuCl4 with ascorbic acid. The synergy of CTAC, KBr, KI and L-glutathione creates urchin-like gold nanocrystals (U–Au) with more exposed high-index facets. Preparation of electrochemical sensing platform for ctDNA involves modification of U–Au-multiple graphene aerogel for target DNA-induced recycle amplification. DNA probe 1 (P1) with methylene blue (MB) hybridizes with DNA probe 2 with ferrocene (Fc) to form duplex DNA, which was attached to U–Au through Au–S bond. The ctDNA hybridizes with hairpin DNA 1 to open hairpin structure, triggering target DNA-induced recycle. Utilization of target DNA-induced recycling allows one target DNA to approach many MB probes to electrode surface and to leave many Fc probes from electrode surface, promoting significant signal amplification. The detection signal is enhanced by catalyzed redox of Fc and MB. Electrochemical response increases with ctDNA concentration from 0.1 to 1 × 106 fM with detection limit of 0.033 fM. The biosensor provides ultrahigh sensitivity, specificity and stability and was successfully applied in detection of ctDNA in human blood.

Ultrathin PtCo nanorod assemblies with self-optimized surface for oxygen reduction reaction

Controllable synthesis of Pt-based materials at the nanoscale can efficiently tailor their electrocatalytic performance, enabling enhancement in both activity and durability. Herein, we report the synthesis of ultrathin PtCo nanorod assemblies (NRAs) terminated with high index facets through a grain-boundary corrosion approach. Atomic insight reveals that grain boundaries and other defects are formed in PtCo nanowire assemblies (NWAs) during the hydrothermal reaction. With a further increase in reaction time, selective dissolution at these sites generated PtCo NRAs composed of shorter nanorod assemblies with larger surface areas and high index facets termination. The carbon-supported catalyst of PtCo NRAs exhibits high mass activity of 0.914 A/mgPt at 0.9 V vs RHE for ORR, 2.4 times higher than those of the smooth PtCo NWAs (0.377 A/mgPt), 5.7 times higher than those of the commercial Pt/C (0.161 A/mgPt). Most significantly, the PtCo NRAs show high stability with about 25.6% loss of mass activity after 10,000 cycles, while only 22.3% loss of mass activity from 10,000 cycles to 50,000 cycles of accelerated durability test. The high durability is mainly attributed to the jagged surface resulted from the surface reconstruction of PtCo NRAs during the electrochemical process. This self-optimization phenomenon may lead to a new effective long-life electrocatalyst.

(Invited) Gold Nanoparticles for Key Reactions in Electrochemical Energy Conversion

Glucose oxidation (GOR) and oxygen reduction (ORR) are two intensively studied reactions for their concern in the development of sustainable energy sources. The oxidation of carbon monoxide (CO), considered as a poisoning reactant in organics conversion, is also a key reaction to be understood in electrocatalysis. Efficient catalysts should be designed for this purpose. Moreover, due to the surface reactions involved, the control of the catalysts' shape, size and structure is required to understand their activity and improve their stability. The outstanding properties exhibited by gold at nanoscale lead to various applications, including electrocatalysis which depends on the nanoparticles (NPs) surface structure and morphology [1]. Gold nanospheres (AuNSs) and pentatwinned gold nanorods (AuPTW NRs) were prepared by a revisited Turkevich synthesis and seed mediated methods respectively. Their surfaces were probed by the lead underpotential deposition (updPb) and the reduction of 4-mercaptobenzoic acid to reveal, respectively, the low and high index facets. The (100) facets of AuNSs are considered to be the active sites for glucose and CO oxidation in alkaline medium, while a AuPTW NRs@Pd core-shell material markedly promotes the ORR activity. Figure 1

Integrating Multiple Advantages into 1 Nm Pt3Ni Bimetallic Alloy Nanowires for Oxygen Reduction Reaction

Exploiting highly active and durable oxygen reduction reaction (ORR) electrocatalyst is still imperative for clean and efficient energy conversion device, such as fuel cells and metal-air batteries. For this purpose and maximize the utilization of noble Pt, we present here a facile, scalable strategy for the high-precise synthesis of 1 nm thick Pt3Ni bimetallic alloy nanowires (Pt3Ni BANWs). The seed-mediated growth mechanism of Pt3Ni BANWs is identified by examining the morphology and composition of the intermediate products collect at different reaction stages. As expected, the Pt3Ni BANWs delivered enhanced mass activity (0.546 A mg-1 Pt, exceeding the DOE 2020 target) in comparison to Pt nanowires assembly (Pt NWA, 0.098 A mg-1 Pt) and Pt/C (Pt, 0.135 A mg-1 Pt) due to the rational integration of multiple compositional and structural advantages, such as alloy feature, ultrathin 1D nanostructure and high-index facets. Moreover, the Pt3Ni BANWs displayed enhanced durability (37% MA retention) than Pt NWA and Pt after 50,000 potential cycles. All these results indicate that the ultrathin Pt3Ni BANWs are potential candidates for catalyzing ORR with acceptable activity and durability. The present work could not provide a facile strategy but also a general guidance for the design of superb performance Pt-based nanowire catalysts for ORR. Figure 1. Progressive formation mechanism of the ultrathin Pt3Ni BANWs. Keywords: Oxygen Reduction Reaction, Pt-Ni alloy, Nanowires, Seed-mediated growth

Preparation of Unique Flower-like Pt-Ni Alloy Catalysts As the Cathode of a PEMFC By Electrodeposition Technique

The hydrogen economy can play an important role in the low-carbon economy, and fuel cells technology is an essential part of the hydrogen economy. The proton exchange membrane fuel cell (PEMFC) is one kind of electrochemical reaction device with the advantages of low temperature operation, high conversion efficiency and noiseless, which is expected to widely utilizes in public transportations.The oxygen reduction reaction (ORR) at the cathode is the rate-determining step in a PEMFC. To enhance the ORR efficiency at the cathode, the unique flower-like platinum-nickel binary alloy catalysts were deposited on the carbon cloth by potentiostatic electrodeposition, effectively improve the oxygen absorption and desorption paths on the surface. The Pt-Ni binary alloy catalysts with the shorter lattice spacings and the effect of high-index facets would increase the electrochemical surface areas (ECSAs) of the catalyst and power density. The samples with the pure platinum and commercial Pt catalyst were also prepared as comparison.Morphology, particle size, chemical bonding, and elemental composition were characterized by SEM, TEM, XRD, XPS and ICP-MS. In addition, the electrochemical characteristics of the Pt-Ni catalysts were investigated via cyclic voltammetry (CV) and linear sweep voltammetry (LSV) analysis in 0.1 M perchloric acid solution. In single cell test, the peak power density of PEMFC with Pt-Ni cathode was 700 mW/cm2 .