Autocatalytic Surface Research Articles

Study of the Kinetics of Nucleation and Growth of Silver Nanoparticles: UV-Visible Spectroscopy vs. Atomic Force Microscopy and Transmission Electron Microscopy

Silver nanoparticles have been the subjects of research interest because of their unique properties and extensive use in modern-day applications. A variety of preparation techniques have been reported for their synthesis but among them mostly recognized is a modified version of the Turkevich citrate method. In this study, we also chose this method of silver nanoparticle synthesis. The synthesized nanoparticles were characterized first, traditionally with UV-Visible spectroscopy and then their sizes and shapes were identified employing Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM). From the UV-Vis data were extracted the rate constants for the nucleation ($$k_1$$) and growth ($$k_2$$) of the silver nanoparticles, and then these constants were further used to fit those obtained from AFM and TEM. Using the values of $$k_1$$ and $$k_2$$ independently in the two experimental methods validated and supported the well-acknowledged two-step mechanism of slow nucleation and fast autocatalytic surface growth.

Mechanisms of controlled stabilizer-free synthesis of gold nanoparticles in liquid aerosol containing plasma.

The interaction between low-temperature plasma and liquid enables highly reactive solution phase chemistry and fast reaction kinetics. In this work, we demonstrate the rapid synthesis of stabilizer-free, spherical and crystalline gold nanoparticles (AuNP). More than 70% of gold ion complex (AuCl- 4) conversion is achieved within a droplet residence time in the plasma of ∼10 ms. The average size of the AuNPs increases with an increase in the droplet residence time and the particle synthesis showed a power threshold effect suggesting the applicability of the classical nucleation theory. Leveraging UV-vis absorption and emission spectroscopy, and nanoparticle size distributions obtained from TEM measurements, we showed that the AuCl- 4 conversion exceeded by 250 times the maximum faradaic efficiency. We identified important roles of both short-lived reducing species including solvated electrons and possibly vacuum ultraviolet (VUV) photons, and long-lived species, H2O2, in the reduction of AuCl- 4. A quantitative investigation was performed by a 1-D reaction-diffusion model which includes transport, plasma-enabled interfacial reduction of AuCl- 4, classical nucleation, monomer absorption and autocatalytic surface growth enabled by H2O2. The model shows good agreement with the experimental results. The timescale analysis of the simulation revealed that nucleation is enabled by fast reduction of gold ions, and autocatalytic growth mainly determines the particle size and is responsible for the majority of the ion precursor conversion while also explaining the excessively large faradaic efficiency found experimentally.

Autocatalytic Surface Reduction‐Assisted Synthesis of PtW Ultrathin Alloy Nanowires for Highly Efficient Hydrogen Evolution Reaction

AbstractAlloying noble metal catalysts with early transition metals (ETMs) has shown great promise by simultaneously boosting catalytic activity and durability because of their strong electronic interactions. However, the very negative reduction potential of ETMs has posed great challenges for the synthesis of the desired alloy catalysts, not to mention the structure‐controlled synthesis. Here an autocatalytic surface reduction‐assisted strategy is reported to realize the controllable synthesis of ultrathin PtW alloy nanowires (NWs). The experimental evidence and density functional theory (DFT) calculations demonstrate that the preformed Pt NWs in the synthesis serve as the catalyst to facilitate the reduction of Wx+ species through the autocatalytic surface reduction mechanism. Using the alkaline hydrogen evolution reaction (HER) as a model reaction, the as‐synthesized PtW NWs/C catalyst shows an ultralow overpotential of 18 mV at 10 mA cm–2 and a high mass activity of 6.13 A mg–1Pt at an overpotential of 100 mV, ranking it among the most active catalysts. The dual roles of alloyed W atoms are further uncovered by theoretical simulations, involving the ensemble effect for accelerating H2O dissociation and a ligand effect for optimizing the hydrogen adsorption strength.

The in-situ XAS study on the formation of Pd nanoparticles via thermal decomposition of palladium (II) acetate in hydroxyl functionalized ionic liquids

The formation of palladium nanoparticles (NPs) via thermal decomposition of palladium (II) acetate in hydroxyl functionalized ionic liquid (IL) [C2OHmim]+[NTf2]− was studied using in-situ energy dispersive x-ray absorption fine structure spectroscopy (DXAFS) and complementary methods such as transmission electron microscopy (TEM) and inductively coupled plasma mass spectrometry. Detailed XAFS spectra analysis unraveled that the palladium acetate trimer precursor underwent dissociation after being dispersed in ILs, which accelerated the thermal decomposition compared with other organic solvents. Based on the reaction kinetics, extened x-ray absorption fine structure spectroscopy fitting and TEM characterization results, the thermal decomposition process can be divided three successive stages namely the explosive nucleation, autocatalytic surface growth and NP attachment growth.

Electrochemical Mechanism of Microporous Autocatalytic Surface Formation on a High-Chromium Alloy in an Alkaline Solution

Electrochemical Mechanism of Microporous Autocatalytic Surface Formation on a High-Chromium Alloy in an Alkaline Solution

Ligand-Mediated Nanocluster Formation with Classical and Autocatalytic Growth

We present a systematic study of ligand-mediated nanocluster (NC) formation using a kinetic model, which provides atomic insight into sub-nanometer cluster (S-NC) and NC formation. Our model describes the role of ligand-mediated nucleation and growth in obtaining monodisperse NCs. Nucleation includes metal ion reduction, reversible ligand association to the metal ion/atom, and formation of dimer nuclei. Growth can occur through autocatalytic surface growth and ligand-associated monomer addition to the cluster, depending on the rate of metal ion to neutral metal atom conversion. Furthermore, we studied the effect of the initial concentration of metal ions on NC formation using fast and slow reducing agents in the presence of slowly and rapidly binding ligands. The model shows that fast nucleation, slow growth, and a high molar ratio of rapidly binding ligand to metal ions promote the formation of S-NCs and NCs. Our results can guide experiments in the synthesis of ultrasmall clusters.

Nanoparticle Formation Kinetics and Mechanistic Studies Important to Mechanism-Based Particle-Size Control: Evidence for Ligand-Based Slowing of the Autocatalytic Surface Growth Step Plus Postulated Mechanisms

Ligands are known to affect the formation, stabilization, size, and size-dispersion control of transition-metal and other nanoparticles, yet the kinetic and mechanistic basis for such ligand effects remains to be elucidated and then coupled to predictions for improved particle size and narrower particle size distribution syntheses. Toward this broad goal, the effect of the added excess ligand (L) and the stabilizer, L = POM9– (= the polyoxometalate, P2W15Nb3O629–) is studied for the formation of POM9–-stabilized Ir(0)n nanoparticles, {Ir(0)n·(POM9–)m}9m−, synthesized from an atomically characterized precatalyst (COD)Ir·POM8– under H2. First, the balanced reaction stoichiometry and characterization of the nanoparticle products are established. Next, the kinetics of nanoparticle formation is analyzed initially by the FW 2-step minimum mechanism consisting of slow, continuous nucleation, A → B (rate constant k1obs), and autocatalytic surface growth, A + B → 2B (rate constant k2obs), where A is nominally (COD...

Continuous and Scalable Synthesis of Pt Multipods with Enhanced Electrocatalytic Activity toward the Oxygen Reduction Reaction

AbstractPlatinum multipods are attractive for catalytic and electrocatalytic applications owing to their highly open, branched structure and thus high specific surface area. A number of methods have been reported for the synthesis of Pt multipods, but they are all limited in terms of throughput due to the use of batch reactors. Here we report the use of a fluidic device for the continuous and scalable synthesis of Pt multipods with sizes controlled in the ranges of 3–5 nm and 2–3 nm for the length and width, respectively, of the branched arms. The facile protocol involves the use of as a precursor to Pt and oleylamine as a solvent, surfactant, and temperature‐dependent reductant. When a solution of these two components is pumped into the polytetrafluoroethylene tube immersed in an oil bath and held at 180 °C, Pt multipods are formed through fast autocatalytic surface growth and small particles attachment. Compared with the batch‐based synthesis, the throughput of the production in the flow system can readily be increased to 17 mg of Pt per hour while retaining a tight control over the quality of the products. When supported on carbon, the Pt multipods exhibit enhanced activity toward oxygen reduction relative to the commercial Pt/C catalyst.

Au@Cu Core–Shell Nanocubes with Controllable Sizes in the Range of 20–30 nm for Applications in Catalysis and Plasmonics

Predominantly covered by a single type of {100} facets, Cu nanocubes are attractive catalytic material toward reactions such as electrochemical reduction of CO2. Here we report a seed-mediated approach to the facile synthesis of Au@Cu core–shell nanocubes with hexadecylamine and Cl– serving as capping agents toward the {100} facets of Cu and glucose as a reducing agent. The large (12%) lattice mismatch between Cu and Au led to the localized epitaxial growth of Cu shells on the Au seeds and the formation of nanocubes with randomly distributed Au cores. Compared to the same synthesis in the absence of Au seeds, the reduction of Cu(II) ions was greatly accelerated in the presence of Au seeds because of the autocatalytic surface reduction. It was also found that the structure and morphology of the products were highly dependent on the concentration of Cu(II) precursor in the reaction solution. Nanoplates rather than nanocubes were obtained when the concentration of Cu(II) precursor was reduced down to a certain level. By variation of the reaction time and/or the amount of Au seeds, the size of the Au@Cu nanocubes could be tuned in a range of 20–30 nm. The as-synthesized core–shell nanocubes exhibited a strong localized surface plasmon resonance peak at 581 nm, and the resonance was dominated by absorption rather than scattering. It is expected that the Au@Cu nanocubes with uniform and controllable sizes will find use in a variety of applications such as plasmonics and catalysis.

Kinetics of Ag300 nanoclusters formation: The catalytically effective nucleus via a steady‐state approach

AbstractThe kinetics of formation of silver nanoparticles consisting of nearly 300 metal atoms is investigated, which were prepared by reduction of silver nitrate with hydrazine in ethylene glycol at 25°C without any stabilizer other than the glycol solvent. The resulting sigmoidal kinetic curves are analyzed by using the 1997 Finke–Watzky two‐step mechanism of slow continuous nucleation with subsequent fast autocatalytic surface growth. The kinetics of homogeneous nucleation of metal nanoparticles was analyzed using the assumption about the stepwise adjunction of precursor and the quasi steady‐state approximation. The equations were proposed to calculate the concentration of the formed metal nanoparticles and their mean size from the experimentally determined values of the Finke–Watzky rate constants. It is shown that a stepwise nucleation process can be described in the terms of the catalytically effective nucleus concept and that the number of atoms in the catalytically effective nucleus can be estimated.

Quantitative analysis of the reduction kinetics of a Pt(II) precursor in the context of Pt nanocrystal synthesis

In this letter, we report a quantitative analysis of how a Pt(II) precursor is reduced to atoms at different temperatures for the formation of Pt nanocrystals with different morphologies and sizes. Our results suggest that in the early stage of a synthesis, the Pt(II) precursor is reduced to atoms exclusively in the solution phase, followed by homogeneous nucleation to generate nuclei and then seeds. At a relatively low reaction temperature such as 22°C, the growth of the seeds is dominated by autocatalytic surface reduction that involves the adsorption and then reduction of the Pt(II) precursor on the surface of the just-formed seeds. This particular growth pathway results in relatively large assemblies of Pt nanocrystals. When the reaction temperature is increased to 100°C, the dominant reduction pathway will be switched from surface to solution phase, producing much smaller assemblies of Pt nanocrystals. Our results also demonstrate that a similar trend applies to the seed-mediated growth of Pt nanocrystals in the presence of Pd nanocubes.

Nanoceria supported rhodium(0) nanoparticles as catalyst for hydrogen generation from methanolysis of ammonia borane

This work reports the preparation and catalytic use of nanoceria supported rhodium(0) nanoparticles, Rh(0)/nanoCeO2, as catalyst for hydrogen generation from the methanolysis of ammonia borane. Rh(0)/nanoCeO2 was in situ formed from the reduction of rhodium(II) octanoate on the surface of nanoceria during the catalytic methanolysis of ammonia borane at room temperature. The results of analysis using PXRD, TEM, STEM-EDS, XPS, SEM, SEM-EDX, N2 adsorption-desorption and ICP-OES reveal that rhodium(0) nanoparticles were well dispersed on the surface of nanoceria with an average size of 3.9 ± 0.6 nm. Rh(0)/nanoCeO2 shows high catalytic activity in the methanolysis of ammonia borane with a turnover frequency of 144 min−1. Note that hydrogen generation from the methanolysis of ammonia borane catalyzed by Rh(0)/nanoCeO2 is slightly less than 3.0 equivalent, due to the reduction of Ce4+ ions to Ce3+ ions on the surface of nanoceria during the methanolysis of ammonia borane. The reduction of Ce4+ ions leading to the formation of Ce3+ defects on the surface of nanoceria under the catalytic reaction conditions could be investigated by high resolution Ce 3d XPS analysis. Additionally, the formation kinetic of rhodium(0) nanoparticles could be studied by using the hydrogen generation from the methanolysis of ammonia borane as reporter reaction; thus, the rate constants for the slow nucleation, k1 and autocatalytic surface growth of rhodium(0) nanoparticles, k2 were determined. Our report also includes the results of kinetic study of the catalytic methanolysis of ammonia borane depending on rhodium concentration and temperature.

Autocatalytic surface reduction and its role in controlling seed-mediated growth of colloidal metal nanocrystals

The growth of colloidal metal nanocrystals typically involves an autocatalytic process, in which the salt precursor adsorbs onto the surface of a growing nanocrystal, followed by chemical reduction to atoms for their incorporation into the nanocrystal. Despite its universal role in the synthesis of colloidal nanocrystals, it is still poorly understood and controlled in terms of kinetics. Through the use of well-defined nanocrystals as seeds, including those with different types of facets, sizes, and internal twin structure, here we quantitatively analyze the kinetics of autocatalytic surface reduction in an effort to control the evolution of nanocrystals into predictable shapes. Our kinetic measurements demonstrate that the activation energy barrier to autocatalytic surface reduction is highly dependent on both the type of facet and the presence of twin boundary, corresponding to distinctive growth patterns and products. Interestingly, the autocatalytic process is effective not only in eliminating homogeneous nucleation but also in activating and sustaining the growth of octahedral nanocrystals. This work represents a major step forward toward achieving a quantitative understanding and control of the autocatalytic process involved in the synthesis of colloidal metal nanocrystals.

Quantitative Analysis of Different Formation Modes of Platinum Nanocrystals Controlled by Ligand Chemistry.

Well-defined metal nanocrystals play important roles in various fields, such as catalysis, medicine, and nanotechnology. They are often synthesized through kinetically controlled process in colloidal systems that contain metal precursors and surfactant molecules. The chemical functionality of surfactants as coordinating ligands to metal ions however remains a largely unsolved problem in this process. Understanding the metal-ligand complexation and its effect on formation kinetics at the molecular level is challenging but essential to the synthesis design of colloidal nanocrystals. Herein we report that spontaneous ligand replacement and anion exchange control the form of coordinated Pt-ligand intermediates in the system of platinum acetylacetonate [Pt(acac)2], primary aliphatic amine, and carboxylic acid ligands. The formed intermediates govern the formation mode of Pt nanocrystals, leading to either a pseudo two-step or a one-step mechanism by switching on or off an autocatalytic surface growth. This finding shows the importance of metal-ligand complexation at the prenucleation stage and represents a critical step forward for the designed synthesis of nanocrystal-based materials.

Nanoalumina-supported rhodium(0) nanoparticles as catalyst in hydrogen generation from the methanolysis of ammonia borane

Rhodium(0) nanoparticles were in situ formed from the reduction of rhodium(II) octanoate and supported on the surface of nanoalumina yielding Rh(0)/nanoAl2O3 which is highly active catalyst in hydrogen generation from the methanolysis of ammonia borane at room temperature. The kinetics of nanoparticle formation can be followed just by monitoring the volume of hydrogen gas evolved from the methanolysis of ammonia borane. The evaluation of the kinetic data gives valuable insights to the slow, continuous nucleation and autocatalytic surface growth steps of the formation of rhodium(0) nanoparticles. Rh(0)/nanoAl2O3 could be isolated and characterized by a combination of advanced analytical techniques including ATR-IR, PXRD, TEM, XPS, SEM, SEM-EDX and ICP-OES. The results reveal that rhodium(0) nanoparticles are highly dispersed on the surface of nanoalumina. The particle size of Rh(0)/nanoAl2O3 increases with the initial rhodium loading of nanoalumina. Rh(0)/nanoAl2O3 is highly active catalyst in hydrogen generation from the methanolysis of AB providing an exceptional initial turnover frequency of TOF=218min−1 at 25.0±0.5°C, which is the highest value ever reported for rhodium catalysts in hydrogen generation from the methanolysis of ammonia borane.

Bimetallic Au/Ag Core-Shell Superstructures with Tunable Surface Plasmon Resonance in the Near-Infrared Region and High Performance Surface-Enhanced Raman Scattering.

Due to the larger surface area and the synergistic effects between two noble metals, the bimetallic superstructures exhibit enhanced distinctive optical, catalytic, and photothermal performances and surface-enhanced Raman scattering (SERS) "hot-spot" effect, and thus have attracted great interest in various applications. Compared with the common Pd, Pt hierarchical structures coated onto Au nanoparticles (NPs), easily synthesized via fast autocatalytic surface growth arising from intrinsic properties of Pd and Pt metals, precisely controlling the hierarchical Ag growth onto Au NPs is rarely reported. In our present study, the reducing agent dopamine dithiocarbamate (DDTC) was covalently capped onto the first metal core (Au) to delicately control the growth model of the second metal (Ag). This results in heterogeneous nucleation and growth of Ag precursor on the surface of Au nanorods (NRs), and further formation of cornlike bimetallic Au/Ag core-shell superstructures, which usually cannot be achieved from traditional epitaxial growth. The thickness of the hierarchical Ag shell was finely tuned in a size range from 8 to 22 nm by simply varying the amount of the ratio between Ag ions and DDTC capped on Au NR core. The tunable Ag shell leads to anisotropic bimetallic Au/Ag core-shell superstructures, displaying two distinctive plasmonic resonances in the near-infrared region (NIR). In particular, the longitudinal surface plasmon resonance exhibits a broadly tunable range from 840 to 1277 nm. Additionally, the rich hot spots from obtained Au/Ag superstructures significantly enhance the SERS performance.

Simple mechanisms of early life – simulation model on the origin of semi-cells

The development of first cellular structures played an important role in the early evolution of life. Early evolution of life probably took place on a molecular level in a reactive environment. The iron-sulfur theory postulates the formation of cell-like structures on catalytic surfaces. Experiments show that H2S together with FeS and other metallic centers drive auto-catalytic surface reactions, in which organic molecules such as pyruvic and amino acids occur. It is questionable which mechanisms are needed to form cell-like structures under these conditions. To address this question, we implemented a model system featuring the fundamentals of molecular dynamics: heat, attraction, repulsion and formation of covalent bonds. Our basic model exhibits a series of essential processes: self-organization of lipid micelles and bilayers, formation of fluid filled cavities, flux of molecules along membranes, transport of energized groups towards sinks and whole colonies of cell-like structures on a larger scale. The results demonstrate that only a few features are sufficient for discovering hitherto non described phenomena of self-assembly and dynamics of cell-like structures as candidates for early evolving proto-cells.Significance statementThe quest for a possible origin of life continues to be one of the most fascinating problems in biology. In one theoretical scenario, early life originated from a solution of reactive chemicals in the ancient deep sea, similar to conditions as to be found in thermal vents. Experiments have shown that a variety of organic molecules, the building blocks of life, form under these conditions. Based on such experiments, the iron-sulfur theory postulates the growth of cell-like structures at certain catalytic surfaces. For an explanation and proof of such a process we have developed a computer model simulating molecular assembly of lipid bilayers and formation of semi-cell cavities. The results demonstrate the possibility of cell-like self-organization under appropriate physico-chemical conditions.

Rates of Fe(II)-Oxidation and Solubility of Bacteriogenic Iron Oxides

ABSTRACTThe oxidation of Fe(II) at circumneutral pH by natural bacterial consortia capable of oxidizing Fe(II) in flocculent mats of bacteriogenic iron oxides (BIOS) was investigated in a series of microcosm experiments under constant pH and pO2 conditions. The oxidation of Fe(II) was faster in the presence BIOS compared to chemical oxidation in solution with pseudo–first-order rate constants ranging from 0.036 to 0.248 min−1 for BIOS compared to 0.004 min−1 for the chemical control. The Fe(II)-oxidation rates exhibited a kinetic dependency on BIOS concentrations, measured in terms of a pseudo–second-order rate constant at 0.066 Lg−1min−1. From the asymptotic decline in Fe(total) in the microcosms, apparent rate constants for HFO dissolution (zero-order values 0.001 to 0.046 mgL−1min−1) and precipitation (second-order values 0.007 to .297 min−1) were obtained from the BIOS and control microcosms, which yield an apparent equilibrium BIOS solubility of 0.157 mgL−1 (i.e., 2.8 μM). These results reflect the complex interplay of Fe(II) oxidation reactions associated with BIOS including homogenous chemical oxidation in solution, chemical oxidation on HFO surfaces, and bacterially mediated oxidation. Furthermore, BIOS precipitated in response to Fe(II) oxidation exhibit a greater apparent solubility than more stable varieties of HFO. The implication is that the competitive advantage of Fe(II)-oxidizing bacteria over autocatalytic surface chemical Fe(II) oxidation on HFO might be enhanced by a reduction in the total amount of BIOS that is precipitated.

Formation Mechanism of Gold Nanoparticles Synthesized by Photoreduction in Aqueous Ethanol Solutions of Polymers Using In Situ Quick Scanning X-ray Absorption Fine Structure and Small-Angle X-ray Scattering

The photoreduction process of [AuCl4]− precursors in poly(N-vinyl-2-pyrrolidone) solution was investigated by utilizing a combination of in situ quick scanning X-ray absorption fine structure (QXAFS) and small-angle X-ray scattering (SAXS) measurements to elucidate the nucleation and subsequent aggregative particle growth of colloidal gold nanoparticles (Au NPs). The sequence of stages for the reduction–nucleation and association process (autocatalytic surface growth and aggregative particle growth) of Au atoms to generate Au NPs was observed during the photoreduction. QXAFS analysis revealed that the reduction of [AuCl2]− species to Au0 atoms is a slower step than that of [AuCl4]− to [AuCl2]−, and the reduction of [AuCl2]− to Au0 atoms and the association of Au0 atoms to produce Au nucleates concurrently proceeds during short-duration photoirradiation. A sigmoidal curve expressed by the solid-state kinetic model, specifically the Avrami-Erofe’ev model, was applicable to demonstrate the aggregative partic...

Rhodium(0) nanoparticles supported on nanosilica: Highly active and long lived catalyst in hydrogen generation from the methanolysis of ammonia borane

Nanosilica stabilized rhodium(0) nanoparticles (Rh(0)/nanoSiO2), in situ formed from the reduction of rhodium(II) octanoate impregnated on the surface of nanosilica, are active catalyst in hydrogen generation from the methanolysis of ammonia borane at room temperature. Monitoring the hydrogen evolution enables us to follow the kinetics of nanoparticles formation. The resulting sigmoidal kinetic curves are analyzed by using the 2-step mechanism of the slow, continuous nucleation and autocatalytic surface growth. By using the temperature dependent kinetic data, we could calculate the activation energy for the nucleation and autocatalytic surface growth of rhodium(0) nanoparticles as well as for the catalytic methanolysis of ammonia borane. Rh(0)/nanoSiO2 could be isolated and characterized by a combination of advanced analytical techniques including XRD, TEM, EDX, XPS, and N2 adsorption–desorption. The results reveal that rhodium(0) nanoparticles are highly dispersed on nanosilica surface and have tunable particle size depending on the initial metal concentration. An increase in the mean particle size of rhodium(0) nanoparticles is observed when the initial metal concentration increases. Rh(0)/nanoSiO2 are highly active and long lived catalyst in hydrogen generation from the methanolysis providing an exceptional initial turnover frequency of TOF=168min−1 (504min−1 corrected for the surface atoms) at 25.0±0.5°C, which is the highest value ever reported for rhodium catalysts. An inverse dependence of TOF on the initial rhodium concentration is observed and ascribed to the increasing size of rhodium(0) nanoparticles. Carbon disulfide poisoning and filtration experiments unequivocally demonstrate that rhodium(0) nanoparticles are the true heterogeneous catalyst in hydrogen generation from the methanolysis of ammonia borane.