Growth Of Metal Particles Research Articles

Hydrogen evolution during the electrodeposition of gold nanoparticles at Si(100) photoelectrodes impairs the analysis of current-time transients

The electrodeposition of noble metals, particularly gold, on semiconducting surfaces is a common step in the development of optoelectronic and catalytic devices. Theoretical models of electrodeposition transients can provide mechanistic insights on nucleation and growth of metal particles. Here we study to what extent these models can be applied to the growth of gold nanoparticles on semiconductor photoelectrodes, with the ultimate purpose of being able to control in two dimensions the electrodepostion process. We have examined current transients for the reduction of aurate salts at Si(100) photocathodes and have made adjustments both to the experimental parameters as well as to the available models so as to account for parallel adsorption steps. We have observed to what extent these models hold predictive power for a nucleation process on semiconducting photoelectrodes. We found that the hydrogen evolution reaction is significant even at very basic pH values, leading to a poor match between the modelled and actual outcomes in electrodeposition experiments. We have concluded that the catalytic activity of gold particles and semiconductor photoeffects make it difficult to rely on current transients alone to refine the experimental conditions for the growth of gold particles on Si(100) photocathodes. Specifically, it is proposed that hydrogen evolution causes turbulence leading to the displacement of particles and significant aggregation on the surface. A solution to this problem is to electrodeposit metals with high overpotentials for hydrogen evolution, such as copper, which allows us to control nucleation and ultimately to use illumination patterns to spatially address nanoparticle deposition on semiconducting surfaces.

Novel synthesis of hierarchical flower-like silver assemblies with assistance of natural organic acids for surface-enhanced Raman spectroscopy

Using ascorbic acid as reducing agent and different organic acids (e.g., oxalic acid, DL-malic acid and citric acid) as structure-directing agents, a novel class of hierarchical flower-like silver mesoparticles with tunable size and shape were synthesized in high yield. The chemical structures of organic acids with different carbon chain length and functional groups have been found to play critical roles in the process of assembling metal nanoparticles into hierarchical complex structures. The anisotropic morphologies of as-synthesized silver structures could readily be tuned by simply varying the concentrations of reagents and the growth temperature. This synthesis method provides a new synthetic strategy for the anisotropic growth and morphology control of metallic particles. The textured flower-like Ag mesoparticles with hierarchical three-dimensional structures could generate abundant surface plasmon resonance hot spots and exhibit an excellent surface-enhanced Raman scattering (SERS) activity, revealing the promising potential for practical applications in single-particle and particle-array SERS sensing.

Recovery of boron from high-boron iron concentrate using reduction roasting and magnetic separation

The comprehensive utilization of abundant high-boron iron concentrate is of particular significance to China, and the high-boron iron concentrate has not yet been utilized as a source for boron at an industrial scale due to its complex mineralogy and fine mineral dissemination. An innovative method was proposed for recovery of boron and iron from high-boron iron concentrate by reduction roasting and magnetic separation. The effects of reduction temperature and roasting time were investigated and their optimum conditions were determined. The mineralogical changes during roasting were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that the pyrrhotite (FeS) contained in the high-boron iron concentrate and the new-formed FeS-Fe solid solution softened or melted at high temperatures owing to their low melting points, and then decreased the metallic iron ratio and accelerated the growth of metallic iron particles. Meanwhile, the magnetite and szaibelyite were converted into metallic iron and suanite, respectively. Consequently, boron was readily enriched into the non-magnetic product and the metallic iron was aggregated to the magnetic concentrate by magnetic separation. Boron recovery of 88. 6% with corresponding B2 O3 content of 14. 5% and iron recovery of 95. 1% with an iron grade of 92. 7% were achieved when high-boron iron concentrate was reduced at 1125 °C for 150 min. Besides, the boron reactivity of the boron-rich non-magnetic product was up to 80. 8%.

Self-organisation processes and phase-structural states in hybrid metal-mesogenic nanosystems

ABSTRACTHybrid nanosystems including silver and copper and long-chain mesogenic derivatives of alkylcyanobiphenyls have been obtained by low temperature vacuum co-condensation of reagent vapors on cooled surfaces of quartz, KBr, CaF2 or polished copper under molecular beam conditions. Controlled thermal treatment of the samples allowed the directed formation of metal nanoparticles of definite size from 2 up to 100 nm. It was shown that manner of temperature treatment and molecular self-organisation of different liquid crystalline phases controlling of the size and morphology of nanoparticles and their aggregates formed in the system. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) data of the samples show the formation of orientationally ordered structures in nematic mesophases. Formation of flat 2D-aggregates has been found in layered smectic mesophases Optical absorbance spectra of silver/4-pentyl-4-cyanobiphenyl (Ag/5СВ) and copper/4-pentyl-4-cyanobiphenyl (Cu/5CB) samples encapsulated in polymer poly-para-xylylene at 300 K contain characteristic bands of plasmonic absorbance of metal nanoparticles at 420–440 nm and 560–600 nm. Rising of metal concentration in the sample led to the performable growth of rod-like metal particles with anisometric ratio l/d>20 and intensive absorbance at higher wavelengths (λ ≥ 650 nm)

Hydrogenolysis of Glycerol to 1,2-Propanediol and Ethylene Glycol over Ru-Co/ZrO2 Catalysts

A series of ZrO2 supported Ru-Co bimetallic catalysts were prepared and evaluated for the hydrogenolysis of glycerol. The Ru-Co/ZrO2 bimetallic catalyst combines the advantages of both Ru and Co, exhibiting high activity and good selectivity to 1,2-propanediol. The X-ray diffraction (XRD) and TEM results show that higher calcination temperature leads to lower reducibility of cobalt oxides and larger metal particle size, which is responsible for the decrease of glycerol conversion. Increasing the reduction temperature causes an inhibition effect on the catalytic activity, but it is beneficial to promote the 1,2-propanediol selectivity. The low temperature (<300 °C) reduction can prevent the growth of metal particles, resulting in higher activity. Co oxide is an important component for the good performance of Ru-Co/ZrO2. The reaction temperature, hydrogen pressure, and glycerol concentration have significant effects on the catalytic performance of the Ru-Co/ZrO2 catalyst.

Microstructure evolution during internal reduction of polycrystalline nickel-doped yttria-stabilized zirconia

The nucleation and growth of metallic particles within metal-doped oxides in reducing conditions is relevant to the processing of materials for catalysis, fuel cells, and structural applications. Here, the precipitation of metallic nickel during the internal reduction of nickel-doped yttria stabilized zirconia is studied with electron microscopy and SQUID magnetometry. It is shown that the microstructure evolution proceeds in three distinct stages, each with its own kinetics description, dependent on the porosity and grain size. 0.5 M percent NiO doped YSZ was synthesized, sintered, pressed into pellets, and then exposed to 1000 °C in 2% H2 for various times. Metallic Ni0 particles (>100 nm) are first formed in pores connected to grain boundaries; subsequently, metallic Ni0 particles (20–50 nm) precipitate at grain boundaries; and finally, superparamagnetic Ni0 particles (<10 nm) form within the bulk YSZ grains. The transitions between stages depend upon concentration gradients and electrostatic potentials that act upon the relevant transporting species, namely oxygen vacancies, electrons, nickel ions and zirconium vacancies.

Orthogonal Experiments on Direct Reduction of Carbon-bearing Pellets of Bayer Red Mud

To recycle residual iron efficiently in Bayer red mud, three-factor three-level orthogonal experiments on carbon-bearing pellets of Bayer red mud were conducted on the basis of their characteristics. The influences of CaO dosage, temperature and roasting time on total iron content and iron recovery of reduced iron powder were studied. Results showed that these factors slightly influenced iron recovery, but significantly influenced total iron content. The principal factor influencing total iron content was CaO dosage, followed by temperature and roasting time. An increase in CaO dosage could decrease total iron content, whereas an increase in temperature and an extension of roasting time could improve total iron content. The reduced iron powder with total iron content of 88.41% and iron recovery rate of 97.97% can be obtained under the optimal conditions of temperature of 1 275 °C, roasting time of 60 min and CaO dosage of 7.5%. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses of the reduced pellets showed that iron minerals in red mud were almost completely reduced to metallic iron. The principal factor influencing the total iron content of reduced iron powder was the grain size of metallic iron particles. An increase in CaO dosage hindered the growth of metallic iron particles, whereas an increase in temperature and an extension of roasting time could neutralise the effect of CaO dosage. Therefore, CaO dosage should be decreased when iron minerals in red mud can be adequately reduced into metallic iron.

Coprecipitation versus chemical vapour deposition to prepare Rh/Ni bimetallic catalysts

The Rh/Ni interaction in bimetallic Rh/Ni/Mg/Al hydrotalcite-type (HT) derived catalysts for the catalytic partial oxidation (CPO) of CH4 was studied modifying the preparation method. The conventional incorporation of both Rh and Ni by coprecipitation (CP) of HT precursors was compared with the addition of Rh by chemical vapour deposition (CVD) to reduced Ni/Mg/Al HT-derived catalysts. Two Ni loadings were selected (about 8 and 2wt.%), whereas the Rh content was kept constant (0.18wt.%). Ni- and Rh-monometallic catalysts were prepared as reference materials.The addition of Rh resulted in an increase in Ni reducibility and dispersion for both CP and CVD catalysts. The Rh/Ni interaction was stronger in CP catalysts, while in CVD ones Rh overspread either on the support or Ni particles, with a preferential deposition on the latter. The Ni loading, regardless of the Rh incorporation method, was the main factor to determine the formation and growth of metallic particles. CO-DRIFTS spectra revealed that the dispersing and reducibility effect of Rh was more evident in the samples with a lower Ni-loading.The small amount of Rh in the catalysts yielded to higher CH4 conversions, especially in low Ni-containing catalysts wherein the Ni0 oxidation was avoided. All the catalysts were activated during catalytic tests at high temperature by reduction of hardly reducible species. The strongest Rh/Ni interaction in CP catalysts yielded to better performances with a lower carbon formation and high resistance towards metallic particle sintering.

Antibacteria and anti-wear TaN–(Ag,Cu) nanocomposite thin films deposited on polyether ether ketone

Hard TaN–(Ag,Cu) nanocomposite films were deposited on PEEK (polyether ether ketone) substrates using reactive co-sputtering. The films were then annealed using RTA (Rapid Thermal Annealing) at 200°C to induce the nucleation and growth of soft metal particles in the TaN matrix and on the surface of the films. After examining the surface morphologies, structures, and mechanical properties of the samples, they were tested for their tribological properties under a normal load of 1N or 5N. It was found that the samples' tribological properties were much improved compared to uncoated PEEK, even though the substrate is relatively soft. This was especially apparent for heavier loads. Apparently, the solid lubricants (i.e., Ag and Cu particles) that emerged on coating's surface during the annealing process reduced the frictional force and wear rate of PEEK. These results were similar to those that were obtained using tool steel substrates. The coated samples were also tested for their anti-bacterial properties using Gram-negative Escherichia coli (E. coli) and Gram-positive Staphylococcus aureus (S. aureus) bacteria. It was found that the anti-bacterial efficiency of these samples was significant against both E. coli and S. aureus, even though the deposited samples were annealed through RTA at the relatively low temperature of 200°C. There was no peeling found between coatings and PEEK substrates after tribological and scratch testings.

Au photosensitized TiO2 ultrathin nanosheets with {001} exposed facets.

We report a general route for the direct growth of metal particles on TiO2 nanosheets with (001) exposed facets by an oxygen-vacancy-driven self-redox reaction. As there is no need for thermal treatment to remove stabilizing agents, the structure of the nanoparticles can be retained, preserving the active sites associated with high activity.

Selective growth of metal particles on ZnO nanopyramids via a one-pot synthesis

Hybrid nanostructures of metal (Cu, Au, Ag)-ZnO nanopyramids were synthesized. These hybrid nanostructures possess two distinct morphologies where the metal can be selectively attached to either the base or the tip of the ZnO pyramids. This is the first time that such morphologies are reported for Cu-ZnO and Ag-ZnO hybrid nanostructures.

A copper-phyllosilicate core-sheath nanoreactor for carbon–oxygen hydrogenolysis reactions

Hydrogenolysis of carbon-oxygen bonds is a versatile synthetic tool in organic synthesis. Copper-based catalysts have been intensively explored as the copper sites account for the highly selective hydrogenation of carbon-oxygen bonds. However, the inherent drawback of conventional copper-based catalysts is the deactivation by metal-particle growth and unstable surface Cu(0) and Cu(+) active species in the strongly reducing hydrogen and oxidizing carbon-oxygen atmosphere. Here we report the superior reactivity of a core (copper)-sheath (copper phyllosilicate) nanoreactor for carbon-oxygen hydrogenolysis of dimethyl oxalate with high efficiency (an ethanol yield of 91%) and steady performance (>300 h at 553 K). This nanoreactor, which possesses balanced and stable Cu(0) and Cu(+) active species, confinement effects, an intrinsically high surface area of Cu(0) and Cu(+) and a unique tunable tubular morphology, has potential applications in high-temperature hydrogenation reactions.

Composition dependence of the photochemical reduction of Ag+ by as-grown Pb(ZrxTi1−x)O3 films on indium tin oxide electrode

Photochemical growth of metal particles on ferroelectric films has usually been found to depend on polarization effect solely. This research exploits the interplay of the film/electrode interface barrier and depolarization field on the photoreduction of Ag+ to Ag onto Pb(Zr,Ti)O3 (PZT) films deposited on indium tin oxide (ITO) electrodes. Ag nanoparticles are observed on the as-grown polycrystalline PZT films without poling, while the particle size and density are closely related to the concentration of Zr in PZT and the poling direction. The enhancement on the photoelectrochemical properties of the ITO/PZT photocathode by the decoration of Ag nanoparticles is finally demonstrated.

Stability and activity of carbon nanofiber-supported catalysts in the aqueous phase reforming of ethylene glycol

Nickel, cobalt, copper and platinum nanoparticles supported on carbon nano-fibers were evaluated with respect to their stability, catalytic activity and selectivity in the aqueous phase reforming of ethylene glycol (230 °C, autogenous pressure, batch reactor). The initial surface-specific activities for ethylene glycol reforming were in a similar range but decreased in the order of Pt (15.5 h−1) >Co(13.0 h−1) >Ni(5.2 h−1) while the Cu catalyst only showed low dehydrogenation activity. The hydrogen molar selectivity decreased in the order of Pt (53%)>Co(21%)>Ni (15%) as a result of the production of methane over the latter two catalysts. Over the Co catalyst acids were formed in the liquid phase while alcohols were formed over Ni and Pt. Due to the low pH of the reaction mixture, especially in the case of Co (as a result of the formed acids), significant cobalt leaching occurs which resulted in a rapid deactivation of this catalyst. Investigations of the spent catalysts with various techniques showed that metal particle growth is responsible for the deactivation of the Pt and Ni catalysts. In addition, coking might also contribute to the deactivation of the Ni catalyst.

Reaction Behavior of Iron Minerals and Metallic Iron Particles Growth in Coal-Based Reduction of an Oolitic Iron Ore

The reaction behavior of iron minerals and growth of metallic iron particles in coal-based reduction of an oolitic iron ore were investigated by X-ray diffraction (XRD) and scanning electron microscopy/enegry dispersive spectroscoy (SEM/EDS), respectively. The results show that, the reduction of iron minerals occurred in the sequence of Fe2O3 → Fe3O4 → FeO → Fe. During the reduction, the main reducing reactions were indirect and the direct reactions that would also be crucial only occurred at the early stage. Because of high contents of SiO2 and Al2O3 in the ore, the solid phase reactions of FeO with SiO2 and Al2O3 took place. Small amounts of metallic iron (or FeO) were reduced from the ore in irregular shape and formed spherical particles when reduction time increased to 10 min. With reduction time increasing, the Fe diffused from small metallic particles toward the larger ones and coalesced with them. The whole reduction process could be analytically derived for the stages of chemical reaction, nucleat...

Growth of metallic iron particles during coal-based reduction of a rare earths-bearing iron ore

Coal-based reduction followed by magnetic separation is an innovative technology used in processing refractory iron ores. Given that size is essential to the separation of the metallic iron from the gangue, the morphology of metallic iron particles and their evolution during coal-based reduction are investigated in the present work. The size of the metallic iron particle increased from 128.76 μm to 565.70 μm when the reduction time increased from 20 min to 60 min at 1,225° C and C/O (i.e., the amount of coal addition in terms of the gram-atomic ratio of the fixed carbon in the coal added to the combined oxygen in iron oxides) of 2.5. Based on our microstructural and metallographic analysis, the iron ore reduction process can be divided into three stages: (1) metallic iron particle formation, (2) iron oxide reduction and metallic iron assembly and (3) metallic iron particle growth. Almost all iron oxides are reduced to metallic iron in the second stage. These newborn metallic iron particles are energetically active and able to assemble and grow, and are eventually separated from the gangue. The large metallic iron particles benefit the subsequent magnetic separation process.

Kinetics of the formation of solid phase in electrolyte for electroless nickel deposition

The effect of 1-(3-methyl-4-phenylpyrazolyl)-3-phenylthiourea (PPTU) on the kinetics of formation of particles of the solid phase in the bulk of electrolyte for nickel plating is investigated by photocolorimetry and photon correlation spectroscopy. It is established that PPTU favors the formation of larger particles as compared to the solution containing no additive and, depending on the concentration, can accelerate or slow down the formation and growth of metal particles and affect the location of the process.

XANES and EXAFS studies on metal nanoparticle growth and bimetallic interaction of Ni-based catalysts for CO2 reforming of CH4

Two groups of catalysts containing Ni and/or Co metals and MgAlOx as support material were made using coprecipitation and incipient wetness impregnation methods, respectively. The mechanism of metal particle growth during reduction of the monometallic Ni or Co and bimetallic Ni–Co catalysts was studied using X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure spectroscopy (EXAFS). The results show that the bimetallic Ni–Co catalysts made by coprecipitation method resulted in smaller metal nanoparticles upon reduction. TEM confirmed this observation and revealed that this catalyst has well-dispersed, more uniform metal particles. The metal reduction extent of Ni monometallic catalysts and the bimetallic catalysts made by impregnation was similar with respect to each metal. For the bimetallic Ni–Co catalysts made by coprecipitation, the interactions between two metals in reduction were observed such that Ni reduction was mitigated by Co, and Co reduction was promoted by Ni. And the first derivative of XANES of such made catalysts indicated stronger interaction between Ni and Co atoms and, perhaps, alloy formation. Time resolved analysis found that Ni reduction followed the second order kinetics at early stage and Co proceeded through the zero order all the way in catalyst reduction. The catalyst stability enhancement by Co atoms and the mechanism of Ni and Co interaction during reduction are explained.

Synthesis gas production in the combined CO2 reforming with partial oxidation of methane over Ce-promoted Ni/SiO2 catalysts

A series of nickel-based catalyst supported on silica (Ni/SiO2) with different loading of Ce/Ni (molar ratio ranging from 0.17 to 0.84) were prepared using conventional co-impregnation method and were applied to synthesis gas production in the combination of CO2 reforming with partial oxidation of methane. Among the cerium-containing catalysts, the cerium-rich ones exhibited the higher activity and stability than the cerium-low ones. The temperature-programmed reduction (TPR) and UV–vis diffuse reflectance spectroscopy (UV–vis DRS) analysis revealed that the addition of CeO2 reduced the chemical interaction between Ni and support, resulting in an increase in reducibility and dispersion of Ni. Over NiCe-x/SiO2 (x = 0.17, 0.50, 0.67, 0.84) catalysts, the reduction peak in TPR profiles shifted to the higher temperature with increasing Ce/Ni molar ratio, which was attributed to the smaller metallic nickel size of the reduced catalysts. The transmission electron microscopy (TEM) and X-ray diffraction (XRD) for the post-reaction catalysts confirmed that the promoter retained the metallic nickel species and prevented the metal particle growth at high reaction temperature. The NiCe-0.84/SiO2 catalyst with small Ni particle size exhibited the stable activity with the constant H2/CO molar ratio of 1.2 during 6-h reaction in the combination of CO2 reforming with partial oxidation of methane at 850 °C and atmospheric pressure.

Towards stable catalysts by controlling collective properties of supported metal nanoparticles

Supported metal nanoparticles play a pivotal role in areas such as nanoelectronics, energy storage/conversion and as catalysts for the sustainable production of fuels and chemicals. However, the tendency of nanoparticles to grow into larger crystallites is an impediment for stable performance. Exemplarily, loss of active surface area by metal particle growth is a major cause of deactivation for supported catalysts. In specific cases particle growth might be mitigated by tuning the properties of individual nanoparticles, such as size, composition and interaction with the support. Here we present an alternative strategy based on control over collective properties, revealing the pronounced impact of the three-dimensional nanospatial distribution of metal particles on catalyst stability. We employ silica-supported copper nanoparticles as catalysts for methanol synthesis as a showcase. Achieving near-maximum interparticle spacings, as accessed quantitatively by electron tomography, slows down deactivation up to an order of magnitude compared with a catalyst with a non-uniform nanoparticle distribution, or a reference Cu/ZnO/Al(2)O(3) catalyst. Our approach paves the way towards the rational design of practically relevant catalysts and other nanomaterials with enhanced stability and functionality, for applications such as sensors, gas storage, batteries and solar fuel production.