Surface Structure Of Nanocrystals Research Articles

Surface engineering of metallic nanocrystals via atomic structure and composition control for boosting electrocatalysis

Since the clean energy industry emerged, developing efficient nanocrystal catalysts has attracted ever-increasing attention. Recently, the utilization of metal nanocrystals as catalysts for electrochemical reactions is entering a new era with the development of theories and techniques that help incorporate surface chemistry into nanoscale materials. Current approaches in the field of nanocrystal catalysts include detailed analyses and modifications of the surface atoms of nanocrystals, with which optimal structures and compositions for target electrochemical reactions could be realized. This review presents two major strategies to engineer the surface structure of nanocrystals: control over the atomic arrangement and composition of nanocrystal surfaces. The first section mainly covers the modification of surface atom arrangements with various methods, including the induction of various facets, strains, and defects. The generation of anomalous crystal structures of nanocrystals is also discussed. The second section encompasses recent advances in controlling the composition of nanocrystal surfaces by bringing high entropy or periodicity to the metal elements in nanocrystals to attain high electrocatalytic activity and stability.

Toward Rationally Designing Surface Structures of Micro- and Nanocrystallites: Role of Supersaturation

Tailoring the surface structures of nanocrystals is an exciting research area on account of appealing surface-dependent properties in various applications. Although significant progress has been made in recent years, current synthetic approaches are mainly dependent upon trial and error because of the ambiguous roles of various influencing factors in complicated environments. Therefore, a general theory for predicting and guiding the rationally controlled synthesis of micro- and nanocrystallites with specific surface structures is highly desired. Of note, previous research attention was mainly focused on the crystal growth in near equilibrium conditions. However, in supersaturated growth environments (nonequilibrium conditions), the corresponding crystal growth theories are still limited. Recently, the supersaturation-controlled surface structure strategy, which is derived from thermodynamics and the Thomson-Gibbs equation, has opened up a new avenue for the control the surface structures of crystals. This strategy involves manipulating the supersaturation of growth units to control the surface structure of micro- and nanocrystallites, as the surface energy of exposed facets is correlated to the supersaturation of growth blocks. Based on the proposed theory, micro- and nanocrystallites with various surface structures, especially high-energy facets, have been successfully synthesized by our group and other researchers in past years. In order to draw lessons from previous studies, it is imperative to give a timely research account related to the supersaturation strategy and corresponding applications in controlling surface structures of different crystallites. In this Account, we explore the supersaturation-controlled surface structure strategy to construct functional nanomaterials with desired architectures. First, we highlight the role of supersaturation of growth units from theoretical analysis after a short introduction of fundamental principles for crystal growth. Then, some detailed cases concerning evolution of surface structures are presented to highlight the key experimental factors involved in manipulating the supersaturation of growth units during synthetic processes. These factors include solvents, reaction rates, and additives in wet chemical routes as well as overpotential in electrochemical routes. In addition, we briefly discuss the role of supersaturation in growth kinetics with focus on explaining the formation of spherical micro- and nanocrystallites at extremely high supersaturation. Finally, a general summary of the supersaturation-dependent surface structure control and future prospects in this field are provided. It is expected that this Account will deepen the current understanding on fundamental principles behind the control of surface structures of micro- and nanocrystallites, which can help us to construct desirable nanomaterials and promote their practical applications.

In-Situ Studies of Thermal Stability of Core–Frame Cubic Pd–Rh Nanocrystals at Elevated Temperatures

Nanosize metallic particles have long been recognized as important heterogeneous catalytic materials. Recent studies show that the atomic characteristics of metallic nanoparticles, including particle size, shape and surface composition, are critical to catalytic activity and selectivity. It has been proven that the catalytic activities of noble metals are highly dependent on their surface structures. Given that the surface structures of nanocrystals have a strong correlation with their morphologies, the control of nanocrystal morphology has become a central theme of research with an ultimate goal to tune the nanocrystal catalytic performance. Bimetallic nanocubes synthesized by seed-mediated growth have the advantage of coupling the catalytic properties of one metal with those of another metal and form multifunctional nanocrystals. Herein, we demonstrate a potential method to enhance the thermal stability of Pd-Rh core-frame nanocubes investigated by in-situ heating transmission electron microscopy. Pd-Rh bimetallic nanocubes were synthesized by site-specific growth. [1] Transmission electron microscopy (TEM), high-angle annular dark-field scanning (HAADF)-scanning transmission electron microscopy (STEM) and energy dispersive X-ray (EDS) mapping were performed in a JEOL ARM200F with a STEM aberration (Cs) corrector operated at 200 kV. In-situ heating experiments were performed using a MEMS-based localized heating specimen holder (by Protochips). Pd-Rh cubes obtained at different stages of a synthesis were investigated at elevated temperatures. Since the Pd nanocubes start to dramatically degrade at 500 °C, changing into spherical-like shape, we choose 500 °C as the heating treatment temperature to investigate the thermal stability of Pd-Rh core-frame nanocubes. [2] Figure 1 presents the results of the Pd-Rh nanocubes obtained in the initial stage of synthesis. Figures 1a-1c show in-situ HAADF-STEM images of Pd-Rh core-frame nanocubes recorded from the same region at room temperature (RT) and 500°C. The Pd-Rh nanocubes maintained the cubic shape very well up to annealing at 500 °C for 1 hour. The thermal stability of Pd nanocubes is significantly enhanced by coating high melting point metal (Rh) on the corners and edges. Figure 1d and 1e show the atomic HAADF-STEM images of typical nanocubes at RT and 500°C annealing 1 hr, respectively. The atomic numbers of Pd and Rh are 46 and 45, respectively. They are very close. Thus, there is no distinct contrast difference between core Pd and frame Rh in the HAADF images, as shown in Figure 1d-1e. STEM/EDS mapping was used to determine the distributions Pd and Rh in the nanocubes at RT and 500 °C. In summary, using in-situ heating transmission electron microscopy, we have demonstrated one effective approach to enhance thermal stability of Pd nanocubes at elevated temperatures by coating high melting point metal Rh on corners and edges to form core-frame bimetallic nanocubes. The Pd-Rh core-frame cubes maintained cubic shape after annealing 1 hour at 500 °C. Surface pre-melting of Pd is

Controlled Growth and Applications of Complex Metal Oxide ZnSn(OH)6 Polyhedra

We successfully controlled the crystallographic surface of ZnSn(OH)(6) crystals and systematically obtained ZnSn(OH)(6) crystals in different shapes including cubes, truncated cubes, cuboctahedrons, truncated octahedrons, and octahedrons using a simple solvothermal method in a methylcellulose (MC) ethanol/water solution. By simply adjusting the amount of the NaOH solution added to the reaction system, we observed the shape evolution of ZnSn(OH)(6) particles from cube to octahedron, with the sizes gradually increasing from about 200 nm to 1-2 μm. These results not only provide ZnSn(OH)(6) polyhedra bound by different lattice planes, but also make it possible to investigate the morphology-property relationship of ZnSn(OH)(6) particles with different morphologies obtained under similar conditions. The antibacterial activities of the as-prepared ZnSn(OH)(6) polyhedral particles were studied. It was found that the antibacterial activities of ZnSn(OH)(6) particles against Escherichia coli depend on the shape of the ZnSn(OH)(6) particles, demonstrating that the surface structure of nanocrystals affects the antibacterial activity. Additionally, the obtained ZnSn(OH)(6) polyhedra can be applied as precursors for Zn(2)SnO(4)/SnO(2) composites with different morphologies by calcining at 600 °C.

Hydrothermal synthesis and volatile organic compounds sensing properties of La–TiO 2 nanobelts

We report the microstructure and gas-sensing properties of La–TiO 2 nanobelts prepared by the hydrothermal method. In particular, we make a comparison of the volatile organic compounds (VOCs) sensing properties between La–TiO 2 nanobelts and La–TiO 2 nanospheres. The results show that La–TiO 2 nanobelts exhibit higher gas response as well as lower working temperature as compared to that of La–TiO 2 nanospheres, suggesting that the gas sensing properties of nanocrystals can be significantly improved by tailoring the shape and the surface structure of nanocrystals.

Shape-Enhanced Photocatalytic Activity of Single-Crystalline Anatase TiO2 (101) Nanobelts

Particle size is generally considered to be the primary factor in the design of nanocrystal photocatalysts, because the reduction of particle size increases the number of active sites. However, the benefit from the size reduction can be canceled by a higher electron-hole recombination rate due to the confined space in sphere-shaped nanoparticles. Here we report a mechanistic study on a novel nanobelt structure that overcomes the drawback of sphere-shaped nanoparticles. Single-crystalline anatase TiO(2) nanobelts with two dominant surfaces of (101) facet exhibit enhanced photocatalytic activity over the nanosphere counterparts with an identical crystal phase and similar specific surface area. The ab initio density functional theory (DFT) calculations show that the exposed (101) facet of the nanobelts yields an enhanced reactivity with molecular O(2), facilitating the generation of superoxide radical. Moreover, the nanobelts exhibit a lower electron-hole recombination rate than the nanospheres due to the following three reasons: (i) greater charge mobility in the nanobelts, which is enabled along the longitudinal dimension of the crystals; (ii) fewer localized states near the band edges and in the bandgap due to fewer unpassivated surface states in the nanobelts; and (iii) enhanced charge separation due to trapping of photogenerated electrons by chemisorbed molecular O(2) on the (101) facet. Our results suggest that the photocatalysis efficiency of nanocrystals can be significantly improved by tailoring the shape and the surface structure of nanocrystals, which provides a new concept for rational design and development of high-performance photocatalysts.

Coordination-dependent surface atomic contraction in nanocrystals revealed by coherent diffraction

Surface atoms have fewer interatomic bonds than those in the bulk that they often relax and reconstruct on extended two-dimensional surfaces. Far less is known about the surface structures of nanocrystals. Here, we show that coherent diffraction patterns recorded from individual nanocrystals are very sensitive to the atomic structure of nanocrystal surfaces. Nanocrystals of Au of 3-5 nm in diameter were studied by examining diffraction intensity oscillations around the Bragg peaks. Both results obtained from modelling the experimental data and molecular dynamics simulations strongly suggest inhomogeneous relaxations, involving large out-of-plane bond length contractions for the edge atoms (approximately 0.2 A); a significant contraction (approximately 0.13 A) for {100} surface atoms; and a much smaller contraction (approximately 0.05 A) for atoms in the middle of the {111} facets. These results denote a coordination/facet dependence that markedly differentiates the structural dynamics of nanocrystals from bulk crystalline surfaces.

Two-photon-assisted excited state absorption in nanocomposite films of PbS stabilized in asynthetic glue matrix

Strong nonlinear absorption is observed in nanocomposite films containing PbSnanocrystals of mean size of 3.3 nm stabilized in a commercial poly(vinyl acetate) glue by anovel and simple chemical route of synthesis. A significant blueshift of the opticalabsorption edge indicates strong quantum confinement. The mean nanocrystal size wascharacterized by x-ray diffraction and transmission electron microscopy. The surfacestructure of nanocrystals is analysed using infrared spectroscopy. The excitonic transitionsare probed by photoacoustic spectroscopy and the results are analysed on the basis oftheoretical calculations using envelope function formalism. Results of open aperturez-scan experiments suggest a model involving saturable absorption followed by two-photonabsorption at a lower concentration while the data for a higher concentration fittedsaturable absorption followed by three-photon absorption. Free carrier absorption due totwo-photon-assisted excited state absorption appears to be the predominant mechanism ofoptical nonlinearity.

First-order hyperpolarizability of ZnS nanocrystal quantum dots studied by hyper-Rayleigh scattering

Hyper-Rayleigh scattering technique was used to measure for the first time the first-order hyperpolarizability (β) of ZnS nanocrystals with 2.5nm mean diameter. Results show that the ‘per ZnS particle’ β value is 2.34×10−27esu and the ‘per ZnS formula unit’ β value is 1.63×10−28esu. An increase by at least two orders of magnitude in the β value per ZnS formula unit is found when compared with bulk ZnS. Two possible contributions originating from nanocrystal surface electric field and solvent field were experimentally excluded. Other two contributions, bulk-like contribution and surface contribution, are considered. Especially, the latter is emphasized due to the special surface structure of nanocrystals.