Fraction Of Surface Atoms Research Articles

A novel tetragonal pyramid-shaped porous ZnO nanostructure and its application in the biosensing of horseradish peroxidase

A novel nano-sized tetragonal pyramid-shaped porous ZnO (TPSP-ZnO) structure was, for the first time, prepared with a high morphological yield by a simple polyglycol-assisted wet chemical method. It was found that the polyglycol had a significant influence on the nucleation and pore formation in the ZnO nanostructure. The OH−concentration and the zinc counter-ion affected the morphology of the produced ZnO nanostructure. TPSP-ZnO can be used as an efficient matrix for immobilizing horseradish peroxidase (HRP) and applied to sense hydrogen peroxide (H2O2). Interestingly, it had better biosensing properties than solid spherical ZnO nanoparticles, which might result from the larger specific surface area of TPSP-ZnO, causing a higher HRP loading, and the tetragonal pyramid-shaped porous nanostructure having high fraction of surface atoms located on the corners and edges, resulting in an improved catalytic activity.

Magnetism in Polymers with Embedded Gold Nanoparticles

preserve the surprising magnetic properties induced at thenanoscale. NPs exhibit a large fraction of surface atoms, whichare different to those of the bulk volume mainly because ofthe broken symmetry at the surface, which alters their elec-tronic configuration and consequently their physical andchemical properties, depending on the electronic state. Al-though bulk materials also have surface atoms, they representa negligible fraction of the total, so their contribution to thephysical properties of the material is insignificant. Moreover,in the case of NPs, as the fraction of surface atoms stronglydepends on the particle size, so do their properties. Anotherinteresting feature of the surface atoms is that their propertiescan be modified by capping the NP with different chemicalspecies, which create bonds that induce a modification of thecharge distribution at the NP surface. A surprising example ofthese surface effects is the observation of magnetism in goldNPs capped with dodecanethiol, in spite of the diamagneticbehavior of the bulk gold, as noted above.

Catalysis with Transition Metal Nanoparticles in Colloidal Solution: Nanoparticle Shape Dependence and Stability

AbstractFor Abstract see ChemInform Abstract in Full Text.

Catalysis with Transition Metal Nanoparticles in Colloidal Solution: Nanoparticle Shape Dependence and Stability

While the nanocatalysis field has undergone an explosive growth during the past decade, there have been very few studies in the area of shape-dependent catalysis and the effect of the catalytic process on the shape and size of transition metal nanoparticles as well as their recycling potential. Metal nanoparticles of different shapes have different crystallographic facets and have different fraction of surface atoms on their corners and edges, which makes it interesting to study the effect of metal nanoparticle shape on the catalytic activity of various organic and inorganic reactions. Transition metal nanoparticles are attractive to use as catalysts due to their high surface-to-volume ratio compared to bulk catalytic materials, but their surface atoms could be so active that changes in the size and shape of the nanoparticles could occur during the course of their catalytic function, which could also affect their recycling potential. In this Feature Article, we review our work on the effect of the shape of the colloidal nanocatalyst on the catalytic activity as well as the effect of the catalytic process on the shape and size of the colloidal transition metal nanocatalysts and their recycling potential. These studies provide important clues on the mechanism of the reactions we studied and also can be very useful in the process of designing better catalysts in the future.

Using a Surface Complexation Model To Predict the Nature and Stability of Nanoparticles

Nanoparticles are discrete nanometer-scale assemblies of atoms and have dimensions between those characteristic of ions and those of macroscopic materials. These minerals commonly possess extremely large specific surface areas and surface adsorption capacities for foreign ions. Due to the large specific surface area and large fraction of surface atoms, the natures of nanoparticles are expected to be modified by the adsorption (surface complexation) process. In this paper,we discuss theoretically the stability of nanoparticles that make the surface complex with foreign ions. The principal theoretical assumption is that the surface complexation occurs at the bulk of the nanoparticles, as in a solid solution. The surface complexation affects two aspects of the intrinsic stability of the nanoparticles simultaneously: one is the composition of the nanoparticles; the other is the free energy of formation of nanoparticles. The solubility of hydrous ferric oxide (HFO) was estimated by using surface complexation modeling coupled with published data of the free energy of formation of the relevant components. The solubility modeling of surface-charged (H+ or OH- sorbed) HFO mechanistically and quantitatively explained the observed nonintegral behavior of the solubility of HFO. Moreover, solubility modeling of anion (SO4(2-), PO4(3-), and As(V)) sorption by HFO showed that the sorption process strongly influences the stability of the nanoparticles. This result implies that geochemical modeling leads to the erroneous prediction of a natural system if the effect of the sorption process is not taken into account.

Shape-Dependent Catalytic Activity of Platinum Nanoparticles in Colloidal Solution

The activation energies and the average rate constants are determined in the 298 K−318 K temperature range for the early stages of the nanocatalytic reaction between hexacyanoferrate (III) and thiosulfate ions using 4.8 ± 0.1 nm tetrahedral, 7.1 ± 0.2 nm cubic, and 4.9 ± 0.1 nm “near spherical” nanocrystals. These kinetic parameters are found to correlate with the calculated fraction of surface atoms located on the corners and edges in each size and shape.

Formation of stable Cu 2O nanocrystals in a new orthorhombic crystal structure

Substantially stable Cu 2O nanocrystals of 10–30 nm in size have been synthesised by an ion exchange Cu 2+→Cu→Cu + reaction in an aqueous solution (using a reducing agent NaBH 4) at 80–100°C. In so small particles, a large fraction of surface atoms, i.e. 20% or even more, which suffer with a reduced coordination number of the core atoms, support the formation of the low oxidation state Cu 2O oxide of copper in a stable structure. Otherwise, CuO is the most stable form of its oxides. The 10–30 nm Cu 2O nanocrystals have a modified X-ray diffractogram of FCC bulk Cu 2O structure. The diffractogram is very simple with a total of only seven peaks, over 0.25–0.12 nm d h k l interplanar spacing, which are successfully indexed assuming an orthorhombic crystal structure with lattice parameters a=0.421 nm, b=0.324 nm and c=0.361 nm (at 25 nm size of the sample).

Effects of oxygen dosing on Ca cluster yields and energy distributions

Sputtering from an oxygen-dosed calcium surface under 4 keV Ar + ion bombardment has been studied. Both the secondary neutral and positive secondary ion fluxes have been analyzed using time-of-flight mass spectrometry to characterize the abundance yields of atoms and molecules. The neutrals were probed using laser post-ionization by an ArF excimer laser. A large number of molecules of the form Ca m O n ( m=1–5 and n=0–5) were observed in both the neutral and positive ion channels. Measurements to characterize the sputter yield and abundance distributions of these species were performed at a series of oxygen doses ranging from 0.03 to 300 L. Energy distributions for the secondary neutral species have also been measured as a function of oxygen coverage. The results for neutral atomic calcium agree well with previous measurements. A simple model to describe mono-atomic oxide formation was used to relate oxygen dose to oxygen surface concentration. It was shown that sputter-cleaning the calcium surface could reduce the oxygen surface atom fraction concentration to about 0.01 and that at this level the sputter yield and abundance distribution for Ca clusters was essentially the same as for an oxygen-free calcium surface. The model was extended to larger molecules and found to describe their formation in the CaO system quite well. The model offers some insights into the unexpected result that substantial numbers of Ca 2 dimers form during the sputtering process.

X-ray absorption study on nanostructured zirconia and yttria

Nanostructured zirconia and yttria powders with grain sizes between 5 and 30 nm have been studied by extended X-ray absorption fine structure (Y and Zr K-edges). Due to the high specific surface area of the powders (75 to 120 m2/g) the fraction of surface atoms is drastically increased. The effective coordination number of the zirconium and yttrium atoms is reduced (up to 40%) in the powders. The decrease of the coordination number is most obvious in the cation-cation-distances. Fully dense ceramic samples with a grain size of 80 nm have nearly the same coordination number compared to samples with conventional grain sizes of about 1 μm.

Determination of metal particle sizes from EXAFS

Simulations of molecular dynamics carried out for Cu particles with sizes up to about 17 000 atoms, i.e. up to 70 Å in diameter, have shown that the surface atoms move in a much more anharmonic potential than the bulk atoms. Since the fraction of surface atoms increases with decreasing particle size, the pair distribution functions become more and more asymmetric when going to smaller particles. This anharmonicity may introduce significant errors in the apparent bond distance and in the coordination number when determined by use of the standard EXAFS analysis formalism which assumes that the thermal vibrations are harmonic. Such errors in the coordination numbers will result in large errors in the calculated particle sizes. Consequently, a method was developed to correct these errors. In the present work we have extended the simulation of molecular dynamics to other metals, such as Pt, in an attempt to make the above model generally applicable to all transition metals. Since the motion of the atoms in Pt particles is found to be more harmonic than the atomic motion in Cu particles, the corrections for Pt are smaller than for Cu.

Size-dependent magnetisation of Pd clusters and colloids

Magnetic susceptibility measurements of three different Pd clusters/colloids with diameters ranging from 25–150 Å are presented, and a comparison with bulk Pd is made. We observe a dramatic reduction of the susceptibility with particle size, and a corresponding decrease of the temperature dependence of χ. A simple model is presented explaining the reduction as a result of a drop in the local density of states from the center towards the surface of the cluster. The ensuing size dependence of the susceptibility results from the variation with cluster size of both the fraction of surface atoms and the Stoner enhancement factor.

Supported catalyst characterization

Small Ru- and Pt-containing mono- and bimetallic particles supported on silica were characterized using both theoretical thermodynamic simulations and experimental nuclear magnetic resonance (NMR) of chemisorbed hydrogen. The thermodynamic Monte Carlo simulations have provided detailed information on the morphologies, surface structures, and surface compositions of Ru–Cu and Pt–Cu bimetallic particles. For small Ru and Pt crystallites, a significant fraction of surface atoms populate defect-like edge and corner sites (e.g., 20% of total surface atoms for a particle with a dispersion of 0.31). These sites are preferentially occupied by copper. NMR of chemisorbed hydrogen is a surface-sensitive technique and has been used to measure the overall surface composition of the bimetallic particles and reveal subtle changes in the surface electronic environments due to addition of a second metal (copper) or a surface contaminant (chlorine or sulfur). The surface composition results determined from thermodynamic simulations are in agreement with those from proton NMR measurements for a series of 4% Ru–Cu/SiO2 and a series of 5% Pt–Cu/SiO2 bimetallic catalysts. Chlorine and sulfur are capable of inhibiting hydrogen chemisorption on Ru surfaces at a less-than-monolayer coverage. This observation and also the observed decrease in proton Knight shifts with increasing Cl coverage (less pronounced for S) on Ru surfaces were attributed to a short-ranged, ‘‘through-the-surface’’ electronic effect.

Studies of crystalline defects during the early stages of growth of Si on Si(100) at low temperatures by spot profile analysis of LEED (SPA-LEED)

The initial stages of epitaxy of Si on Si(100) have been studied in the low-temperature range from 290 up to 530 K. Using SPA-LEED (spot profile analysis of low-energy electron diffraction) growth imperfections could be detected. At substrate temperatures below 500 K we found that a significant fraction of surface atoms does not grow on lattice sites as given by substrate and surface reconstruction. The increasing number of atoms on non-lattice sites is connected with a drastic loss of superstructure. Only at T > 500 K all deposited atoms take part in the surface reconstruction and are found in perfect lattice sites.

The science of ultrafine aerosols

Abstract

Reply to “Comment on: ‘Surface miscibility gaps in CuAg alloys by Y. Liu and P. Wynblatt’ by J. du Plessis”

As pointed out by du Plessis in his comments [1] on our recent papers [2,3], the issues he raises do not pertain to the results reported in those papers, but rather to our "explanation". Our experimental paper [3] merely reported some interesting experimental results without providing any significant interpretation. Detailed interpretation is currently under preparation and will be developed in a future paper [4]. Our modeling paper [2] included some preliminary interpretation based on Helms' analysis [5] of this type of surface phase transition. In both of those papers, however, we clearly referred to the phenomena observed as phase transitions associated with a miscibility gap. This seems to be the principal bone of contention in du Plessis' comments, as he takes the view that no miscibility gap is involved in the transitions we have observed by both modeling and experimental approaches. At first sight it might seem that this disagreement is primarily a matter of semantics. Du Plessis insists that the terminology "miscibility gap" should be reserved for the case of a closed system when a single-phase solution becomes unstable, due to the appearance of a second minimum in the free energy versus composition curve below a certain "critical" temperature. The case we have studied in the papers in question [2,3] is also one where a single phase becomes unstable as a result of the development of a second minimum in the free energy versus composition curve, below some critical temperature; but, as correctly pointed out by du Plessis, the phenomenon we have observed occurs at a segregated surface, which must be considered to be an open system in contact with an infinite (bulk) reservoir of the components of the solution. Thus, it might appear as though the issue revolves around the definition of what might appropriately be referred to as a miscibility gap. However, we will demonstrate here that the locus of points at which the transition occurs in a system consisting of an open surface in contact with an infinite reservoir, coincides exactly with the miscibility gap which may be calculated for a closed two-dimensional system. In order to accomplish this, we must first clarify a misconception in du Plessis' discussion of the problem in his comments [1], and one which is discussed more fully in a previous publication [6]. The issue has to do with the so-called "double branch of solutions" (displayed in fig. 1 of ref. [1]) which du Plessis introduces in the description of the transition [6]. His argument given for the existence of the two branches can be explained by considering fig. 1, which represents a plot of the surface tension versus surface atom fraction (in du Plessis' formalism the extrema in surface tension are shown to correspond to extrema in the total free energy of the system). In the figure, curve T~ represents the system at temperature T z where free energies (surface tensions) at the two minima are equal, and curve T 2 represents the system at temperature T 2 where the region of negative curvature between the two minima has just disappeared. Du Plessis asserts that if the system exists in the state corresponding to the left

Reply to “comment on: ‘surface miscibility gaps in CuAg alloys by Y. Liu and P. Wynblatt’ by J. du Plessis”

As pointed out by du Plessis in his comments [1] on our recent papers [2,3], the issues he raises do not pertain to the results reported in those papers, but rather to our explanation. Our experimental paper [3] merely reported some interesting experimental results without providing any significant interpretation. Detailed interpretation is currently under preparation and will be developed in a future paper [4]. Our modeling paper [2] included some preliminary interpretation based on Helms' analysis [5] of this type of surface phase transition. In both of those papers, however, we clearly referred to the phenomena observed as phase transitions associated with a gap. This seems to be the principal bone of contention in du Plessis' comments, as he takes the view that no is involved in the transitions we have observed by both modeling and experimental approaches. At first sight it might seem that this disagreement is primarily a matter of semantics. Du Plessis insists that the terminology miscibility gap should be reserved for the case of a closed system when a single-phase solution becomes unstable, due to the appearance of a second minimum in the free energy versus composition curve below a certain temperature. The case we have studied in the papers in question [2,3] is also one where a single phase becomes unstable as a result of the development of a second minimum in the free energy versus composition curve, below some critical temperature; but, as correctly pointed out by du Plessis, the phenomenon we have observed occurs at a segregated surface, which must be considered to be an open system in contact with an infinite (bulk) reservoir of the components of the solution. Thus, it might appear as though the issue revolves around the definition of what might appropriately be referred to as a gap. However, we will demonstrate here that the locus of points at which the transition occurs in a system consisting of an open surface in contact with an infinite reservoir, coincides exactly with the which may be calculated for a closed two-dimensional system. In order to accomplish this, we must first clarify a misconception in du Plessis' discussion of the problem in his comments [1], and one which is discussed more fully in a previous publication [6]. The issue has to do with the so-called double branch of solutions (displayed in fig. 1 of ref. [1]) which du Plessis introduces in the description of the transition [6]. His argument given for the existence of the two branches can be explained by considering fig. 1, which represents a plot of the surface tension versus surface atom fraction (in du Plessis' formalism the extrema in surface tension are shown to correspond to extrema in the total free energy of the system). In the figure, curve T~ represents the system at temperature T z where free energies (surface tensions) at the two minima are equal, and curve T 2 represents the system at temperature T 2 where the region of negative curvature between the two minima has just disappeared. Du Plessis asserts that if the system exists in the state corresponding to the left

Particle Size Effects for Oxygen Reduction on Highly Dispersed Platinum in Acid Electrolytes

The particle size effect for oxygen reduction kinetics on highly dispersed Pt particles in acid electrolytes are discussed. It is suggested that the change in the fraction of surface atoms on the (100) and (111) crystal faces of Pt particles, which are assumed to be cubo‐octahedral structures, can be correlated to the mass activity (A/g Pt) and specific activity (μA/cm2 Pt) of highly dispersed Pt electrocatalysts. The maximum in mass activity that is observed at ∼3.5 nm in several studies is attributed to the maximum in the surface fraction of Pt atoms on the (100) and (111) crystal faces, which results from the change in surface coordination number with a change in the average particle size. The reduction of oxygen on supported Pt particles in acid electrolytes is classified as a demanding or structure‐sensitive reaction; the specific activity increases with an increase in particle size.

Binary alloy surface compositions from bulk alloy thermodynamic data

The surface composition of binary alloys is determined by minimizing alloy surface free energy with respect to atom exchange between the surface and the bulk. The theory is developed using a pairwise bound model of the solid with a broken bond surface. Parametric predictions for the atom fraction in the first four atomic layers for ideal and regular alloy solutions are presented. The predictions are based on pure element vaporization enthalpies and alloy solution activity coefficients. The surface atom fraction is different from the bulk atom fraction for all bulk compositions. The alloy element with the lower heat of vaporization is enriched with respect to its bulk atom fraction. The effects of chemisorption of a foreign species and alloy particle size on surface composition are also quantified. The theory is in good agreement with existing data on nickel-copper surface compositions.

Effect of pressure on the Lamb-Mössbauer fraction of surface atoms in F.C.C. lattices

Effect of pressure on the Lamb-Mössbauer fraction of surface atoms in F.C.C. lattices

Adsorption on evaporated tungsten films - II. The chemisorption of hydrogen and the catalytic parahydrogen conversion

The chemisorption of hydrogen in densely packed layers on tungsten is reversible even at liquid-air temperatures. Values for the fraction of surface atoms covered, θ, are obtained for various temperatures and pressures and the combination of isothermal heats for high θ values with previously published calorimetric heats for low θ values gives the heat curve for the whole chemisorption from θ = 0 to 1. The results are applied to the kinetics of the parahydrogen conversion, and it is claimed that reaction proceeds according to the equation 2W + H 2 ⇌ 2WH .