Shape-controlled Nanocrystals Research Articles

Facile and Green Synthesis of Well-Defined Nanocrystal Oxygen Evolution Catalysts by Rational Crystallization Regulation.

The development of catalysts for an economical and efficient oxygen evolution reaction (OER) is critical for clean and sustainable energy storage and conversion. Nickel-iron-based (NiFe) nanostructures are widely investigated as active OER catalysts and especially shape-controlled nanocrystals exhibit optimized surface structure and electronic properties. However, the structural control from amorphous to well-defined crystals is usually time-consuming and requires multiple stages. Here, a universal two-step precipitation-hydrothermal approach is reported to prepare a series of NiFe-based nanocrystals (e.g., hydroxides, sulfides, and molybdates) from amorphous precipitates. Their morphology and evolution of atomic and electronic structure during this process are studied using conclusive microscopy and spectroscopy techniques. The short-term, additive-free, and low-cost method allows for the control of the crystallinity of the materials and facilitates the generation of nanosheets, nanorods, or nano-octahedra with excellent water oxidation activity. The NiFe-based crystalline catalysts exhibit slightly compromised initial activity but more robust long-term stability than their amorphous counterparts during electrochemical operation. This facile, reliable, and universal synthesis method is promising in strategies for fabricating NiFe-based nanostructures as efficient and economically valuable OER electrocatalysts.

Tuning the Selective Ethanol Oxidation on Tensile-Trained Pt(110) Surface by Ir Single Atoms.

Development of efficient and robust electrocatalysts for complete oxidation of ethanol is critical for the commercialization of direct ethanol fuel cells. However, the complete oxidation of ethanol suffers from poor efficiency due to the low C1 pathway selectivity. Herein, single-atomic Ir (Ir1 ) on hcp-PtPb/fcc-Pt core-shell hexagonal nanoplates (PtPb@PtIr1 HNPs) enclosed by Pt(110) surface with a 7.2% tensile strain is constructed to drive complete electro-oxidation of ethanol. Benefiting from the construction of Ir1 sites, the PtPb@PtIr1 HNPs exhibit a Faraday efficiency of 57.93% for the C1 pathway, which is ≈8.3 times higher than that of the commercial Pt/C-JM. Furthermore, the PtPb@PtIr1 HNPs show a top-ranked electro-activity achieving 45.1-fold and 56.3-fold higher than the specific and mass activities of Pt/C-JM, respectively. Meanwhile, the durability can be significantly enhanced by the construction of Ir1 sites. Density functional theory calculations indicate that the strong synergy on the PtPb@PtIr1 HNPs surface significantly promotes the breaking of CC bond of CH2 CO* and facilitates CO oxidation and suppresses the deactivation of the catalyst. This work offers a unique single-atom approach using low-coordination active sites on shape-controlled nanocrystals to tune the selectivity and activity toward complicated catalytic reactions.

Preparing Alumina-Supported Gold Nanowires for Alcohol Oxidation.

The development of shape-controlled noble metal nanocrystals such as nanowires (NWs) is progressing steadily owing to their potentially novel catalytic properties and the ease with which they can be prepared by reducing the metal ions in a particular solution as capping agents. Recently, many reports have been presented on the preparation of shape-controlled Au nanocrystals, such as nanostars and nanoflowers, by a one-pot method using 2-[4-(2-hydroxyethyl)-1-piperazinyl] ethanesulfonic acid (HEPES) as capping and reducing agents. The catalytic activity is depressed due to the adsorption of the capping agent onto a Au surface. Since HEPES has low binding affinities on the Au surface, shape-controlled nanocrystals obtained using HEPES are effective for application as nanocatalysts because HEPES was easily removed from the Au surface. In this study, we report the preparation of AuNWs, with an average diameter of 7.7 nm and lengths of a few hundred nanometers, in an aqueous solution containing HEPES and sodium borohydride. A γ-Al2O3-supported AuNW (AuNW/γ-Al2O3) catalyst was obtained using catalytic supporters and a water extraction method that removed HEPES from the Au surface without morphological changes. AuNW/γ-Al2O3 was then utilized to catalyze the oxidation of 1-phenylethyl alcohol to acetophenone. The formation rate of acetophenone over AuNW/γ-Al2O3 was 3.2 times that over γ-Al2O3-supported spherical Au nanoparticles (AuNP/γ-Al2O3) with almost the same diameter.

Preparation and Catalytic Performance of Highly Stable Silica-Coated Gold Nanorods Supported on Alumina

Abstract Shape-controlled metal nanocrystals such as nanorods are attractive because of their potential novel catalytic properties. It is important to improve the stability of the shape-controlled nanocrystals to be applied as nanocatalysts. In this study, α-Al2O3-supported Au nanorods (AuNR/α-Al2O3) and silica-coated α-Al2O3-supported Au nanorods (SiO2/AuNR/α-Al2O3) were prepared as alcohol oxidation catalysts for the transformation of 1-phenylethyl alcohol to acetophenone. The formation rate of acetophenone over AuNR/α-Al2O3 is higher than that over α-Al2O3-supported spherical Au nanoparticles obtained by calcining AuNR/α-Al2O3. In addition, SiO2/AuNR/α-Al2O3 exhibits higher catalytic performance and thermal stability than those of AuNR/α-Al2O3 in alcohol oxidation.

Thermally stable surfactant-free ceria nanocubes in silica aerogel

Surfactant-mediated chemical routes allow one to synthesize highly engineered shape- and size-controlled nanocrystals. However, the occurrence of capping agents on the surface of the nanocrystals is undesirable for selected applications. Here, a novel approach to the production of shape-controlled nanocrystals which exhibit high thermal stability is demonstrated. Ceria nanocubes obtained by surfactant-mediated synthesis are embedded inside a highly porous silica aerogel and thermally treated to remove the capping agent. Powder X-ray Diffraction and Scanning Transmission Electron Microscopy show the homogeneous dispersion of the nanocubes within the aerogel matrix. Remarkably, both the size and the shape of the ceria nanocubes are retained not only throughout the aerogel syntheses but also upon thermal treatments up to 900 °C, while avoiding their agglomeration. The reactivity of ceria is measured by in situ High-Energy Resolution Fluorescence Detected - X-ray Absorption Near Edge Spectroscopy at the Ce L3 edge, and shows the reversibility of redox cycles of ceria nanocubes when they are embedded in the aerogel. This demonstrates that the enhanced reactivity due to their prominent {100} crystal facets is preserved. In contrast, unsupported ceria nanocubes begin to agglomerate as soon as the capping agent decomposes, leading to a degradation of their reactivity already at 275 °C.

The interplay between surface facet and reconstruction on isopropanol conversion over SrTiO3 nanocrystals

Strontium titanate (SrTiO3) is an extensively investigated perovskite for various applications due to its optical, electrical and chemical properties. To gain an in-depth understanding of the active sites involved in heterogeneous catalysis over the broadly used SrTiO3 (STO), we studied a model reaction, isopropanol conversion, on three differently shape-controlled nanocrystals: cube, truncated cube and dodecahedra. SEM, XRD and XPS confirmed the morphology, phase and composition of STO shapes. Low energy ion scattering (LEIS) revealed the occurrence of surface reconstruction over STO shapes during O2 pretreatment at different temperatures. Based on the catalytic activities, scanning transmission electron microscopy images and density functional theory calculations, the step sites on STO derived from surface reconstruction were proposed to be the active sites for isopropanol conversion. This was further confirmed by steady state isotopic kinetic analysis (SSITKA) which demonstrated similar intrinsic turnover frequencies (TOFs) for the differently reconstructed STO shapes. It is concluded that the crystal facets impose an indirect effect on the catalysis of STO via controlling the degrees of surface reconstruction: the less stable STO (1 1 0) facet (dodecahedra) leads to more step sites after reconstruction and hence higher overall reaction rate than the more stable (1 0 0) facet (cube). This work highlights the important interplay between the crystal facet and surface reconstruction in controlling the nature and density of active sties and thus catalysis over complex oxides.

Synthesis of water-dispersible, plate-like perovskites and their core-shell nanocrystals.

Shape-controlled halide perovskite nanocrystals are attractive as an emerging functional material; however, these nanocrystals are prepared using organic solvents containing alkylamines and there are few reports on the synthesis of water-dispersible halide perovskite nanocrystals. We report a simple method to prepare water-dispersible, plate-like perovskite nanocrystals by mixing a long-chain amidoamine derivative (C18AA) and potassium tetrachloropalladate (K2PdCl4) in water. The obtained nanocrystals have a 2D layered perovskite structure represented by the chemical formula (C18AAH2)PdCl4. Furthermore, because seed-mediated growth is useful for preparing shape-controlled nanocrystals, such as rods, plates, wires and cubes, we used the water-dispersible (C18AAH2)PdCl4 nanocrystals as seeds to grow (C18AAH2)PdCl4@Pt core–shell nanocrystals. The core–shell nanocrystals have rough surfaces due to the deposition of Pt on the (C18AAH2)PdCl4 seeds. In addition, plate-like (C18AAH2)PdCl4@Au core–shell nanocrystals were easily obtained using this seed-mediated growth method.

Shaping non-noble metal nanocrystals via colloidal chemistry.

Non-noble metal nanocrystals with well-defined shapes have been attracting increasingly more attention in the last decade as potential alternatives to noble metals, by virtue of their earth abundance combined with intriguing physical and chemical properties relevant for both fundamental studies and technological applications. Nevertheless, their synthesis is still primitive when compared to noble metals. In this contribution, we focus on third row transition metals Mn, Fe, Co, Ni and Cu that are recently gaining interest because of their catalytic properties. Along with providing an overview on the state-of-the-art, we discuss current synthetic strategies and challenges. Finally, we propose future directions to advance the synthetic development of shape-controlled non-noble metal nanocrystals in the upcoming years.

Insights into Reaction Intermediates to Predict Synthetic Pathways for Shape-Controlled Metal Nanocrystals.

Understanding nucleation phenomena is crucial across all branches of physical and natural sciences. Colloidal nanocrystals are among the most versatile and tunable synthetic nanomaterials. While huge steps have been made in their synthetic development, synthesis by design is still impeded by the lack of knowledge of reaction mechanisms. Here, we report on the investigation of the reaction intermediates in high temperature syntheses of copper nanocrystals by a variety of techniques, including X-ray absorption at a synchrotron source using a customized in situ cell. We reveal unique insights into the chemical nature of the reaction intermediates and into their role in determining the final shape of the metal nanocrystals. Overall, this study highlights the importance of understanding the chemistry behind nucleation as a key parameter to predict synthetic pathways for shape-controlled nanocrystals.

Spherical to truncated octahedral shape transformation of palladium nanocrystals driven by e-beam in aqueous solution

The crystallographic shapes of nanocrystals play critical roles in determining their physical and chemical properties. Liquid phase synthesis serves as one of the most important approaches for preparing shape-controlled nanocrystals, therefore, understanding the formation mechanisms of the thermodynamic equilibrium structures of nanocrystals in liquid solution is important. Using in situ liquid cell transmission electron microscopy (TEM), we observe for the first time the shape transformation of individual palladium nanocrystals from energy unfavored spherical shapes into equilibrium truncated octahedrons in aqueous solution. Via quantitative analysis of the shape evolution dynamics of an individual Pd nanocrystal, we find that about 10% of nanocrystal atoms were relocated during the shape transformation. The mass transport is attributed to the synergetic effect of electron beam irradiation and water environment.

Surface-Controlled Photocatalysis and Chemical Sensing of TiO2, α-Fe2O3, and Cu2O Nanocrystals

A relatively new approach to the design of photocatalytic and gas sensing materials is to use the shape-controlled nanocrystals with well-defined facets exposed to light or gas molecules. An abrupt increase in a number of papers on the synthesis and characterization of metal oxide semiconductors such as a TiO2, α-Fe2O3, Cu2O of low-dimensionality, applied to surface-controlled photocatalysis and gas sensing, has been recently observed. The aim of this paper is to review the work performed in this field of research. Here, the focus is on the mechanism and processes that affect the growth of nanocrystals, their morphological, electrical, and optical properties and finally their photocatalytic as well as gas sensing performance.

Different platinum crystal surfaces show very distinct protein denaturation capabilities.

Different platinum (Pt) surfaces of nanocrystals usually exhibit significant distinctions with regard to various biological, physical, and chemical characteristics, such as bio-recognition, surface wetting, and catalytic activities. In this study, we report for the first time that two shape-controlled Pt nanocrystals with the most common low-index surfaces, Pt(100) and Pt(111), show very dissimilar protein denaturation capabilities based on all-atom molecular dynamics simulations employing the widely used model protein, villin headpiece (HP35). We demonstrate that HP35 is well preserved on the Pt(100) crystal surface, whereas it is severely disrupted on the Pt(111) crystal surface. This surprising difference originates from the distinct water behavior in the first solvation shell (FSS) of the two Pt crystal surfaces. Within the FSS of the Pt(100) crystal surface, water molecules form a very compact and stable monolayer through a highly uniform rhombic hydrogen-bond network. This water monolayer prefers the adsorption of acidic residues (such as Glu and Asp) and acts as a shield to prevent other residues from directly coming into contact with the metal surface. On the other hand, the hydrogen bond network in the water monolayer in the FSS of the Pt(111) crystal surface is very sparse and quite defective, which makes it more vulnerable to the penetration of various residues, particularly those with planar side chains such as Phe, Trp and Arg due to strong dispersion interactions, leading to subsequent protein unfolding. The binding free energy calculations for some key amino acids on the two different crystal surfaces further uncover the molecular origin behind their distinct protein denaturation capability. Our study reveals the vital importance of interfacial water in determining the structure of proteins when binding to different metal crystal surfaces. The discovered molecular mechanisms may be helpful for the future development of a bio-assisted programmable synthetic strategy of sophisticated Pt nanostructures for biomedical applications.

Constructing Highly Porous Thermoelectric Monoliths with High-Performance and Improved Portability from Solution-Synthesized Shape-Controlled Nanocrystals

Thermoelectricity offers a viable and reliable solution to convert waste heat into electricity. To enhance the performance and portability of thermoelectric materials, the crystal grain and pore structure should be simultaneously manipulated to achieve high electrical conductivity (σ), low thermal conductivity (κ), high figure of merit (zT), and low relative density. However, they cannot be synchronously realized using nanocrystals with uncontrolled domain size and shape as building blocks. Here, we employ solution-synthesized PbS nanocrystals with large grain size, controllable shape and tunable spatial packing to realize the aforementioned structural tuning. The as-sintered highly porous and well crystalline monolith exhibits high σ, low κ, high zT (1.06 at 838 K) and low relative density (82%). The phonon transport is studied by density functional theory highlighting the crucial role of phonon-pore scattering in reducing κ to enhance zT. Our strategy may benefit thermoelectrics and shed light on other technical fields such as catalysis, gas sensing, photovoltaics, and so forth.

Theoretical Perspectives on the Influence of Solution-Phase Additives in Shape-Controlled Nanocrystal Synthesis

Shape-selective solution-phase nanocrystal growth is facilitated by capping agents, or structure-directing agents (SDAs), which guide shape evolution. It is often stated that these chemical additives impart shape selectivity by promoting nanocrystals with a majority of facets to which they bind most strongly. However, little is known on the mechanisms through which they impart shape selectivity. In this Feature Article, we highlight our recent studies aimed at understanding the thermodynamic and kinetic influences of SDAs using theory and computational tools. We review our studies of the poly(vinylpyrrolidone) (PVP)-mediated synthesis of {100}-faceted Ag nanocubes in ethylene glycol solution, which has been studied experimentally. Our studies of the interfacial free energy of Ag–PVP solution interfaces show that while solution-phase PVP does bind more strongly to Ag(100) than to Ag(111), this selectivity is not sufficient to thermodynamically change the Wulff shape of a PVP-covered Ag nanocrystal in solut...

Programmed Self-Assembly of Branched Nanocrystals with an Amphiphilic Surface Pattern.

Site-selective surface modification on the shape-controlled nanocrystals is a key approach in the programmed self-assembly of inorganic colloidal materials. This study demonstrates a simple methodology to gain self-assemblies of semiconductor nanocrystals with branched shapes through tip-to-tip attachment. Short-chained water-soluble cationic thiols are employed as a surface ligand for CdSe tetrapods and CdSe/CdS core/shell octapods. Because of the less affinity of arm-tip to the surface ligands compared to the arm-side wall, the tip-surface becomes uncapped to give a hydrophobic nature, affording an amphiphilic surface pattern. The amphiphilic tetrapods aggregated into porous agglomerates through tip-to-tip connection in water, while they afforded a hexagonally arranged Kagome-like two-dimensional (2D) assembly by the simple casting of aqueous dispersion with the aid of a convective self-assembly mechanism. A 2D net-like assembly was similarly obtained from amphiphilic octapods. A dissipative particle dynamics simulation using a planar tripod model with an amphiphilic surface pattern reproduced the formation of the Kagome-like assembly in a 2D confined space, demonstrating that the lateral diffusion of nanoparticles and the firm contacts between the hydrophobic tips play crucial roles in the self-assembly.

Designing Efficient Catalysts through Bimetallic Architecture: Rh@Pt Nanocubes as a Case Study

AbstractBimetallic nanocatalysts often have increased activities and stabilities over their monometallic counterparts due to surface strain effects and electron transfer between the two metals. Here, we demonstrate that the performance of a nanocatalyst can be precisely manipulated in shape‐controlled nanocrystals through a bimetallic core@shell architecture. This ability is achieved in a model core@shell Rh@Pt nanocube system through control of shell thickness. The enhanced performance with thin‐shelled nanocrystals is correlated with the weakening of surface–adsorbate interactions. In these thin‐shelled Rh@Pt nanocubes, the maximum current density achieved during formic acid oxidation was over 2 times greater than that achieved with similarly sized Pt nanocubes, with a decreased CO poisoning ratio as well. The strategy employed here should also enhance the performance of many other bimetallic nanomaterials composed of more cost‐effective metals too.

Highly Stable Silica-Coated Gold Nanoflowers Supported on Alumina.

Shape-controlled nanocrystals, such as nanowires and nanoflowers, are attractive because of their potential novel optical and catalytic properties. However, the dispersion and morphological stabilities of shape-controlled nanocrystals are easily destroyed by changing the dispersion solvent and temperature. Methods of support and the silica coating are known to improve the dispersion and morphological stabilities of metal nanocrystals. The silica-coating method often causes morphological changes in shape-controlled nanocrystals because the silica coating is formed in mixed solutions of water and organic solvents such as ethanol, and this results in aggregation due to changes in the dispersion solvent. Furthermore, ligand exchange, designed to improve the dispersion stability in the solvent, often causes morphological changes. This article introduces a method for the preparation of highly stable silica-coated Au nanoflowers (AuNFs) supported on Al2O3. The method of support prevents the aggregation and precipitation of AuNFs when the solvent is changed from water to water/ethanol. Through stability improvement, silica coating of AuNFs/Al2O3 was conducted in water/ethanol without ligand exchange that causes morphological changes. Furthermore, silica-coated AuNFs/Al2O3 exhibit high morphological stability under high-temperature conditions compared to uncoated AuNFs/Al2O3. These results are very useful when preparing highly morphologically stable, silica-coated, shape-controlled nanocrystals without ligand exchange.

Catalysts Facet Orientation and Surface Modifications for Improved Activity of Alcohol Oxidation

Many problems encountered in hydrogen-fuel-cell technology, particularly those of hydrogen storage and distribution, can be circumvented by replacing hydrogen with liquid fuels, such as methanol and ethanol. However, the electrocatalytic oxidation reactions of methanol and ethanol are slow and incomplete even on the best existing Pt-based catalysts, which greatly impeded the wide-spread application of direct alcohol fuel cell (DAFC). Our group demonstrated that one class of surfaces can be very active for the oxidation of methanol and ethanol [1]. These electrocatalysts consist of one Pt monolayer (PtML) under tensile strain when placed on extended or nanoparticle surfaces. Their activity and selectivity can be controlled with Pt-support interaction. Our experimental and DFT studies demonstrated a correlation between the strain induced by PtML-support interaction and the catalytic activity. Significant enhancement in the activity associated with the tensile strain and decreased activity associated with the compressive strain. Most recently, we deposited PtML on Au single crystals and shape-controlled nanocrystals enclosed with different crystallographic facets[1,2], and electrochemical behaviors (Figure 1a) suggested that the deposited PtML possessed facets matching to the underlying Au. Surface structure can determine catalytic properties and a large density of atomic steps on high-index facets is especially desirable for breaking chemical bonds such as C-C bond splitting in ethanol total oxidation. Such property of translating facet-specific features from substrates to the supported PtMLopens new opportunities to enhance catalytic performances. Another approach to improve activity involves modifying PtML surfaces with different co-catalysts. For example, Ru modified PtML/Au showed significantly enhanced activity for methanol oxidation and FTIR studies demonstrated that no adsorbed CO was formed during the reaction. The incorporation of co-catalysts such as Rh, Ir and Sn with PtML shifted onset potential of ethanol oxidation to more negative values (Figure 1b) and promoted a complete oxidation of ethanol to CO2. Details of electrochemical, structural and spectroscopic studies, discussion of reaction mechanisms and possibilities of these findings to resolve the major impediment hindering the development of practical DAFCs and to open new studies of C-C bond splitting in variety of important reactions will be presented in the meeting. Acknowledgements This work was financially supported by the US DOE, Office of Basic Energy Science, Material Science and Engineering Division, Division of Chemical Sciences, Geosciences and Biosciences Division (Contract No. DE-SC0012704).

Theory of the thermodynamic influence of solution-phase additives in shape-controlled nanocrystal synthesis.

Though many experimental studies have documented that certain solution-phase additives can play a key role in the shape-selective synthesis of metal nanocrystals, the origins and mechanisms of this shape selectivity are still unclear. One possible role of such molecules is to thermodynamically induce the equilibrium shape of a nanocrystal by altering the interfacial free energies of the facets. Using a multi-scheme thermodynamic integration method that we recently developed [J. Chem. Phys., 2016, 145, 194108], we calculate the solid-liquid interfacial free energies γsl and investigate the propensity to achieve equilibrium shapes in such syntheses. We first apply this method to Ag(100) and Ag(111) facets in ethylene glycol solution containing polyvinylpyrrolidone (PVP), to mimic the environment in polyol synthesis of Ag nanocrystals. We find that although PVP has a preferred binding to Ag(100), its selectivity is not sufficient to induce a thermodynamic preference for {100}-faceted nanocubes, as has been observed experimentally. This indicates that PVP promotes Ag nanocube formation kinetically rather than thermodynamically. We further quantify the thermodynamic influence of adsorbed solution-phase additives for generic molecules, by building a γsl ratio/nanocrystal shape map as a function of zero-temperature binding energies. This map can be used to gauge the efficacy of candidate additive molecules for producing targeted thermodynamic nanocrystal shapes. The results indicate that only additives with a strong facet selectivity can impart significant thermodynamic-shape change. Therefore, many of the nanocrystals observed in experiments are likely kinetic products.

The role of CO2 in dehydrogenation of ethylbenzene over pure α-Fe2O3 catalysts with different facets

The replacement of superheated steam with CO2 for ethylbenzene (EB) dehydrogenation to produce styrene (ST) is of great importance from both scientific and practical perspectives, while it is still a big challenge to design and synthesize catalysts for dehydrogenation in the presence of CO2. Research into shape-controlled nanocrystals has proved that well-defined facets are of great importance for understanding the catalytic phenomena due to their definite surface structure. In the present work, α-Fe2O3 nanocrystals enclosed by various crystal facets provide us the clue to understand the role of CO2 in dehydrogenation. O(III) (the triply coordinated surface oxygen atom) is found to play an important role in activating CO2 under reaction conditions, which is supported by both experimental and theoretical results. These results have deepened knowledge about the role of CO2 in dehydrogenation over pure α-Fe2O3 catalysts, which is significant in designing relative catalysts rationally.