Shape-Controlled Synthesis of Platinum-Based Nanocrystals and Their Electrocatalytic Applications in Fuel Cells

To achieve environmentally benign energy conversion with the carbon neutrality target via electrochemical reactions, the innovation of electrocatalysts plays a vital role in the enablement of renewable resources. Nowadays, Pt-based nanocrystals(NCs) have been identified as one class of the most promising candidates to efficiently catalyze both the half-reactions in hydrogen-and hydrocarbonbased fuel cells. Here, we thoroughly discuss the key achievement in developing shape-controlled Pt and Pt-based NCs, and their electrochemical applications in fuel cells. We begin with a mechanistic discussion on how the morphology can be precisely controlled in a colloidal system, followed by highlighting the advanced development of shape-controlled Pt, Pt-alloy, Pt-based core@shell NCs, Pt-based nanocages, and Pt-based intermetallic compounds. We then select some case studies on models of typical reactions(oxygen reduction reaction at the cathode and small molecular oxidation reaction at the anode) that are enhanced by the shape-controlled Pt-based nanocatalysts. Finally, we provide an outlook on the potential challenges of shape-controlled nanocatalysts and envision their perspective with suggestions.

Shape-controlled synthesis of novel precursor for fibrous Ni-Co alloy powders

A novel precursor of nickel-cobalt alloy powders with an appropriate Ni to Co molar ratio was prepared under selectively synthetic conditions. The composition and morphology of the precursor were characterized by X-ray diffractometry （XRD）, scanning electron microscopy （SEM）, Fourier transform infrared spectrometry （FT-IR） and energy dispersive spectrometry （EDS）. The effects of pH value, reaction temperature, metal ion concentrations and surfactant on the morphology and the dispersion of precursor were investigated. The results show that the morphology of precursor depends on ammonia content in the precursor. A fibriform precursor is a complicated ammonia-containing nickel-cobalt oxalate. The uniform shape-controlled fibrous precursor is obtained under the following optimum conditions： ammonia as complex agent as well as pH adjustor, oxalate as coprecipitator, 50-65 °C of reaction temperature, 0.5-0.8 mol/L of total concentration of Ni2＋ and Co2＋, PVP as dispersant, and pH 8.0-8.4.

Shape-Controlled Synthesis of Palladium Nanoparticles and Their SPR/SERS Properties

Morphological evolution of Pd nanoparticles was studied in a solution-phase synthesis using cetyltrimethylammonium bromide （CTAB） and CTAB/sodium citrate mixture as capping agents, respectively. The morphological diversity of Pd nanoparticles is the combined effect of different Pd twinned seeds formed in the nucleation stage and selectively enlarging one set of crystallographic facets in the growth stage, both of which can be affected by the concentrations of CTAB. Through changing the concentrations of CTAB and sodium ascorbate, Pd nanoparticles with different shapes were obtained. When citrate ions were introduced to manipulate the nucleation and growth process, star-shaped icosahedra and nanorods with pentagram cross-sections were obtained. Pd nanoparticles with different shapes have quite different surface plasmon resonance and surface-enhanced Raman scattering properties.

A Convenient Method for Preparing Shape-controlled ZnO Nanocrystals in a Polyol/Water Mixture System without Surfactants

A facile solution-phase route for the synthesis of shape-controlled ZnO nanocrystals in a polyol/water mixture system was developed. The obtained nanocrystals were characterized by X-ray diffraction, transmission electron microscopy and UV-visible absorption spectroscopy. The results indicate that modulating the adding ways of water has a significant effect on the shape of the obtained nanocrystals. The addition of small quantity of water can increase the growth rate of crystals and leads to the formation of different shapes. The resulting shapes of the novel structures are diverse, including spheres, cones, and teardrops, all of which are obtained without any additional surfactants. These studies concerning the shape evolution of nanocrystals should be valuable for further design and for greater understanding of advanced nanoscale building-block architectures.

Shape-controlled Synthesis of Porous SnO_2 Nanostructures via Morphologically Conserved Transformation from SnC_2O_4 Precursor Approach

Porous SnO_2 nanostructures with controlled shapes were synthesized by a facile morphologically conserved transformation from Sn C_2O_4 precursor approach. Well-defined Sn C_2O_4 nanostructures can be obtained through a solution-based precipitation process at ambient conditions without any surfactant. The formation mechanism of such microstructures was tentatively proposed on the basis of intrinsic crystal structure and the reaction conditions. We found that the morphologies of precursor were well maintained while numerous pores were formed during the annealing process. The combined techniques of X-ray diffraction, nitrogen absorption–desorption, field emission scanning electron microscopy, and(high-resolution) transmission electron microscopy were used to characterize the as-prepared SnO_2 products. Moreover, cyclic voltammetry(CV) study shows that the shape of CV presents a current response like roughly rectangular mirror images with respect to the zero-current line without obvious redox peaks, which indicating an ideal capacitive behavior of the SnO_2 electrodes. The photoluminescence(PL) spectrum study suggests that the as-obtained porous SnO_2 nanostructures might have a large number of defects, vacancies of oxygen, and local lattice disorder at the interface, interior and exterior surfaces.

Preparation of ultra-fine fibrous Fe-Ni alloy powder by coordinated co-precipitation-direct reduction process

The precursor prepared by coordinated co-precipitation was direct reduced by hydrogen to ultra-fine fibrous Fe-Ni alloy powder. The effects of concentrations of reactants, pH value, reaction temperature and additive on the preparation of precursor were systematically investigated. The structures, thermal decomposition processes and morphologies of the precursors were characterized by X-ray diffraction (XRD), thermal gravity-differential thermal analysis (TG-DTA) and scanning electron microscoy (SEM). The results show that using 2% polyvinylpyrrolidone (PVP) (in mass fraction) as additive, a well-dispersed precursor with a uniform morphology can be obtained in a solution with Fe2+ and Ni2+ total concentration (1:1) of 0.8 mol/L, pH value of 6.2 at 60 °C, and a pure and well dispersed fibrous iron-nickel powder can be prepared by direct reduction of this precursor in a mixed atmosphere of nitrogen and hydrogen at the temperature of 420 °C.

Refit Silver Nanostructures Using a Convergent Electron Beam

Using a superionic conductor AgI thin film and a direct current electric field, we synthesize silver nanowires in diameter of about lOOnm. In order to refit the prepared nanowires, the samples are irradiated by a convergent electron beam （200 k V） inside a transmission electron microscope to prepare new small silver nanostructures. The new nanostructures are investigated in situ by high-resolution transmission electron microscope. This electron- induced crystal growth method is useful for technical applications in fabrication of nanodevices.

Shape-controlled synthesis of liquid metal nanodroplets for photothermal therapy

The capping agents for liquid metal (LM) nanodroplets in aqueous solutions are restricted to thiol-containing and positively-charged molecules or macromolecules.However,both thiolate-metal complex and electrostatic interaction are liable to detachment upon strong mechanical forces such as sonication,leading to limited stability and applications.To address this,we utilized ultrasmall water soluble melanin nanoparticles (MNPs) as the capping agent,which exhibited strong metal binding capability with the oxide layer of gallium based LMs and resulted in enhanced stability.Interestingly,shape-controlled synthesis of LM nanodroplets can be achieved by the incorporation of MNPs.Various EGaln nanostructures including nanorice,nanosphere and nanorod were obtained by simply tuning the feed ratio,sonication time,and suspension temperature.Among these shapes,EGaln nanorice has the best photothermal conversion efficiency,which could be leveraged for photothermal therapy.

Shape-controlled growth of SrTiOz polyhedral submicro/ nanocrystals

A series of SrTiO3 polyhedral submicro/nanocrystals with systematic morphology evolution from cubic to edge-truncated cubic and truncated rhombic dodecahedra have been synthesized by using a series of alcohol molecules with different acidities as surfactants. The concentration and pKa value of the alcohols both play important roles in determining the size and shape of the SrTiO3 polyhedral submicro]nanocrystals. The adsorption energy of alcohol molecules on SrTiO3 [110] facets depends on their pKa values, which are therefore critical for morphology control. Using the same strategy, a series of BaTiO3 polyhedral submicro/nanocrystals with systematic morphology evolution have also been successfully prepared.

Shape-controlled Pt nanocubes directly grown on carbon supports and their electrocatalytic activity toward methanol oxidation

Synthesis of shape-controlled Pt nanocrystals is substantial and important for enhancing chemical and electrochemical reactions.However,the removal of capping agents,shape-controlling chemicals,on Pt surfaces is essential prior to conducting the catalytic reactions.Here we report a facile one-pot synthesis of Pt nanocubes directly grown on carbon supports(Pt nanocubes/C) with modulating the kinetic reaction factors for shaping the nanocrystals,but without adding any capping agents for preserving the clean Pt surfaces.Well-dispersed Pt nanocubes/C shows enhanced activity and long-term stability toward methanol oxidation reaction compared to the commercial Pt/C catalyst.

NUMERICAL SOLUTION TO SHAPE-CONTROL PROBLEMS OF WATER FLOW USING BEM-OPTIMIZATION METHOD

In this paper,some engineering problems arising from hydraulics and the design of hy- draulic structures are proposed,in accordance with the viewpoint of cybernetics,as the shape-control problems for distributed parameter system(DPS),whose solution is obtained using BEM-Optimization technique.As a classical example,the problem of flow over a weir is converted first into a shape-con- trol problem,which is then converted into an optimization problem to be solved.Computational results show that the BEM-Optimization method is an efficient way to solve shape-control problems for DPS, which not only can avoid the tedious process of trial and error or iteration,but also has high accuracy and less CPU consumption.For the problem of flow over a weir,the stream function,the free surface profile and the unknown flow-rate can be found simultaneously.

Size and shape effects of MnFe_(2)O_(4)nanoparticles as catalysts for reductive degradation of dye pollutants

The magnetic nanoparticles that are easy to recycle have tremendous potential as a suitable catalyst for environmental toxic dye pollutant degradation.Rationally engineering shapes and tailoring the size of nanocatalysts are regarded as an effective manner for enhancing performances.Herein,we successfully synthesized three kinds of MnFe_(2)O_(4)NPs with distinctive sizes and shapes as catalysts for reductive degradation of methylene blue,rhodamine 6G,rhodamine B,and methylene orange.It was found that the catalytic activities were dependent on the size and shape of the MnFe_(2)O_(4)NPs and highly related to the surface-to-volume ratio and atom arrangements.Besides,all these nanocatalysts exhibit selectivity to different organic dyes,which is beneficial for their practical application in dye pollutant treatment.Furthermore,the MnFe_(2)O_(4)NPs could be readily recovered by a magnet and reused more than ten times without appreciable loss of activity.The size and shape effects of MnFe_(2)O_(4)nanoparticles demonstrated in this work not only accelerate further understanding the nature of nanocatalysts but also contribute to the precise design of nanoparticles catalyst for pollutant degradation.

DNA-encoded morphological evolution of bimetallic Pd@Au core-shell nanoparticles from a high-indexed core

DNA-mediated synthesis of nanopartides is a powerful method to access exclusive shapes and surface properties. Previous studies employed seeds containing low-energy facets, such as a simple cubic palladium seed, in the synthesis of Pd-Au bimetallic nanoparticles; however, few studies have investigated whether DNA molecules are influential when a seed containing high-energy facets is used. Seeds enclosed by high-energy facets act as facile nucleation sites in nanopartide growth and could suppress the effect of DNA. We report the DNA-encoded control of the morphological evolution of bimetallic Pd@Au core-shell nanoparticles from a concave palladium nanocube seed containing high-indexed facets. Based on detailed spectroscopic and microscopic studies of time-dependent growth of bimetallic nanoparticles, we found that the DNA molecules containing 10 repeating units of thymine, guanine, cytosine, or adenine （referred to as T10, G10, C10, and A10, respectively） show a unique interaction with the surface of the seed and the precursor. The most important factor is the binding affinity of the nucleobase to the Pd surface; A10 shows the highest binding affinity and can stabilize the high energy surfaces of the seed. Initially, the growth of bases with lower binding affiru＇ties （T10, G10, and C10） is completely dictated by the seed＇s surface energy, but later growth can be influenced by different DNA sequences, providing four Pd@Au bimetallic nanopartides with unique morphologies. The effect of these DNA molecules with medium or low binding affinities can only be observed when more Au is deposited. We propose a scheme for DNA-controlled growth. These results provide insights into the factors governing the DNA-mediated growth of core-shell structures using seeds with high-energy sites, and the insights can be readily applied to other bimetallic systems.