Surface Of Nanocrystallites Research Articles

Preparation and DSSC Performance of Mesoporous Film Photoanodes Based on Aqueous-Synthesized Anatase Nanocrystallites

In this work, anatase nanocrystallites are synthesized from an aqueous titanium(IV) chloride solution by hydrolysis and are used to fabricate mesoporous TiO 2 film photoanodes. The aqueous-synthesized titania nanocrystallites possess high specific surface area (250-350 m 2 /g), 4-5 nm crystallite size, enlarged bandgap, and enhanced surface hydroxylation. Current-voltage measurements of dye-sensitized solar cells (DSSCs) based on the aqueous-synthesized titania showed higher photocurrent (13.0 mA/cm 2 ) and conversion efficiency (5.0%) when compared to cells built from commercial Dyesol (10.7 mA/cm 2 , 4.8%) or P25 (8.5 mA/cm 2 , 4.1%) benchmark nanotitania materials. The enhanced performance is discussed in terms of improved dye loading and associated electron injection and transport facilitated by the nanocrystallite surface characteristics.

Magnetic Properties of the La<sub>0.50</sub>Ba<sub>0.50</sub>MnO<sub>3</sub> Nanomanganites

Nanocrystalline La0.50Ba0.50MnO3 manganite was synthesized by an optimized sol-gel method. The initial sample was subjected to step-by-step heat treatment under air atmosphere. The ion stoichiometry, the morphology of crystallites of ceramics, and the magnetic properties were studied. It is established that the average crystallite size increases with increasing annealing temperature. All of the samples studied are characterized by a perovskite-like cubic structure, with the unit cell parameter a increasing continuously with the average crystallite size. The most significant lattice compression occurs in the sample with an average crystallite size of ~ 30 nm. The increase in the average crystallite size causes a nonmonotonic increase in the Curie temperature and in the spontaneous magnetic moment. The anomalous behavior of the magnetic properties of the La0.50Ba0.50MnO3 manganites obtained is explained by the competition between two size effects, namely, the frustration of the indirect exchange interactions Mn3+ – O – Mn4+ on the nanocrystallite surface and the crystal lattice compression due to the crystallite surface tension.

Thermally Induced Nano-Structural and Optical Changes of nc-Si:H Deposited by Hot-Wire CVD.

We report on the thermally induced changes of the nano-structural and optical properties of hydrogenated nanocrystalline silicon in the temperature range 200–700 °C. The as-deposited sample has a high crystalline volume fraction of 53% with an average crystallite size of ~3.9 nm, where 66% of the total hydrogen is bonded as ≡Si–H monohydrides on the nano-crystallite surface. A growth in the native crystallite size and crystalline volume fraction occurs at annealing temperatures ≥400 °C, where hydrogen is initially removed from the crystallite grain boundaries followed by its removal from the amorphous network. The nucleation of smaller nano-crystallites at higher temperatures accounts for the enhanced porous structure and the increase in the optical band gap and average gap.

Blue/White Emission from Hydrogenated Amorphous Silicon Carbide Films Prepared by PECVD

AbstractPhotoluminescence (PL) from hydrogenated silicon carbide (SiC:H) films is studied at room temperature. The films were deposited by plasma-enhanced chemical vapor (PECVD) technique with and without substrate bias using methyltrichlorosilane as a main precursor. After the deposition the samples were annealed at various temperatures in vacuum. The films were characterized by atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The samples deposited without substrate bias (series A) were amorphous, whereas the samples deposited with negative substrate bias -100V (series B) were nanocrystalline. The one-peak (470 nm) and double-peak (415 and 437 nm) PL structures of the as-deposited samples A and B were observed, respectively. Annealing strongly enhanced intensity of PL of the samples B and trandformed PL spectrum from double-peak into broad featureless band with intensity at about 470 nm. The blue PL in as-deposited films B is supposed to be assigned to the radiative recombination in the sites located at the nanocrystallite surface, whereas the photo excitation of carries mostly occurs in nanocrystallite cores. A further increase in annealing temperature causes hydrogen effusion, which leads to an increase of the concentration of non-raidative recombination centers associated with dangling-bonds and as a result, to the quenching of PL.

Low Temperature Synthesis of Nanocrystallized Titanium Oxides with Layered or Tridimensional Frameworks, from [Ti8O12(H2O)24]Cl8·HCl·7H2O Hydrolysis

A low-temperature aqueous chemical growth process was developed to produce nanometric titanium oxide with controlled size and structural variety. Gentle hydrolysis of a commercial TiOCl2 solution, in a controlled relative humidity, leads to the formation of single crystals of [Ti8O12(H2O)24]Cl8·HCl·7H2O. Under autogenous pressure at 120 °C, the hydrolysis of the latter by tetramethylammonium hydroxide (noted TMAOH) is mainly governed by the R = Ti/TMAOH molar ratio and thus by the pH value. Two values are particularly important: R = 8/9 and 8/17. The former corresponds to the balance of the Cl− ions of the titanium oxychloride hydrate by TMA+ cations and the latter to the stoichiometric ratio for the formation of a layered oxo-hydroxide (TMA)2Ti2O4−x(CO3)x(OH)2·nH2O, x = 0.7, which is obtained under basic conditions. Above R = 8/9, that is, in an acidic medium, the solvothermal treatment at 120 °C directly leads to the crystallization of anatase and then rutile when the pH decreases. For intermediate R values, the amorphous dried solid is required to be rinsed and dried at 110 °C to lead to the crystallization of nanometric titania, either anatase or brookite. TEM studies show that titania nanoparticles exhibit superstructures involving a doubling of the cell parameters that could originate from the adsorption of carbonate, hydrogen carbonate, or hydroxyl groups on the nanocrystallite surface. During evaporation of the solvent, these crystallites can self-assemble to form micrometric platelets.

Effect of the size factor on the magnetic properties of manganite La0.50Ba0.50MnO3

Nanocrystalline manganite La0.50Ba0.50MnO3 was synthesized by an optimized sol-gel method. The initial sample was subjected to step-by-step heat treatment under air atmosphere. The ion stoichiometry, the morphology of crystallites of ceramics, and the magnetic properties were studied. It is established that the average crystallite size D increases from ∼30 nm to ∼7 μm with increasing annealing temperature. All of the samples studied are characterized by a perovskite-like cubic structure, with the unit cell parameter a increasing continuously from ∼3.787 to ∼3.904 A with the average crystallite size. The most significant lattice compression (≈3%) occurs in the sample with an average crystallite size of ∼30 nm. The increase in the average crystallite size causes a nonmonotonic increase in the Curic temperature T C from ∼264 to ∼331 K and in the spontaneous magnetic moment σ S from ∼1.52 to ∼3.31 μB/f.u. The anomalous behavior of the magnetic properties of the manganite La0.50Ba0.50MnO3 obtained is explained by the competition between two size effects, namely, the frustration of the indirect exchange interactions Mn3+-O-Mn4+ on the nanocrystallite surface and the crystal lattice compression due to the crystallite surface tension.

Adsorption of a Statherin Peptide Fragment on the Surface of Nanocrystallites of Hydroxyapatite

Statherin is an active inhibitor of calcium phosphate precipitation in the oral cavity. For many studies of the interaction between statherin and hydroxyapatite (HAp), the samples are prepared by a direct mixing of statherin or its fragment with well-crystalline HAp crystals. In this work, the HAp sample is precipitated in the presence of peptide fragment derived from the N-terminal 15 amino acids of statherin (SN-15). The in situ prepared HAp crystallites are nanosized, leading to a significant increase of the peptide amount adsorbed on the HAp surface. The enhancement in NMR sensitivity allows, for the first time, the measurement of a two-dimensional 13C-13C correlation spectrum for a 13C uniformly labeled peptide sample adsorbed on mineral surface. The measurement time is about 18.5 h at a field strength of 7.05 T. Preliminary results suggest that there may exist two different mechanisms for the interaction between SN-15 and HAp. In addition to the one which will cause a conformational change near the N-terminal, SN-15 may also be absorbed on the HAp surface by simple electrostatic interaction, without any significant conformational changes of the peptides.

Oxidation processes of surface hydrogenated silicon nanocrystallites prepared by pulsed laser ablation and their effects on the photoluminescence wavelength

Natural oxidation processes of surface hydrogenated silicon nanocrystallites prepared by pulsed laser ablation under various hydrogen gas pressures are discussed by measuring the vibrational frequency of Si–Hn units on the surface and intensity of Si–O–Si stretching vibration. The surfaces of nanocrystallites are predominantly composed of Si–H bonds and oxidation starts from backbonds of these bonds. The deposited nanocrystal films have a porous secondary structure which depends on the background gas pressure. The oxidation rate observed by infrared absorption measurements depended on this porous secondary structure. The oxidation process is discussed by the correlation between oxidation rate and porous structure of nanocrystal film. We found that Si–O bond density increases with covering the surface of the nanocrystallites during the diffusion of oxygen-related molecules through the void spaces in the porous structure. The surface oxidation of each nanocrystallite is not homogeneous; after the coverage of easy-to-oxidize sites, oxidation continues to gradually progress at the post-coverage stage. We point out that the oxidation process at coverage and post-coverage stages result in different photoluminescence (PL) wavelengths. Adsorption of the water molecule before oxidation also affects the PL wavelength. Defect PL centers which have light emission around 600 and 400 nm are generated during the coverage and post-coverage oxidation processes, respectively.

Role of singlet oxygen in formation of nanoporous silicon

The effect of singlet oxygen on the formation of nanoporous silicon in the case of photoelectrochemical etching of p-Si is studied. Pulsed electrolysis was used to discriminate between chemical and electrochemical processes. The pulsed etching of silicon was accompanied by pulsed illumination coinciding either with current pulses or with zero-current periods. An analysis of the results obtained demonstrated that, as the number of silicon nanocrystals increases under illumination, singlet oxygen starts to be generated from molecular oxygen dissolved in the electrolyte. This process leads to oxidation of the surface of silicon nanocrystallites and to a change in its passivation.

Development kinetics of silver clusters on silver halides

Silver nuclei are produced by pulse radiolysis at the surface of AgCl nanocrystallites in the presence of an electron donor, the methyl viologen, which induces the growth of silver nuclei. The experimental results observed on the increase of the silver atom concentration and on the decay of the donor concentration during this process, which is similar to the photographic development by an electron donor, are compared with the kinetics obtained from numerical simulation. The model assumes that the formation of silver clusters with a supercritical nuclearity is required before the start of an electron transfer reaction from the two reduced forms of the donor methyl viologen to the silver clusters. The reaction is controlled by the access of the donor to the surface sites of the AgCl crystallite. The rate constant values of the successive steps of the mechanism are derived from the adjustment of calculated kinetics to experimental signals under various conditions, using a single set of parameters which are fairly suitable under all conditions studied.

Inside Powders: A Theoretical Model of Interfaces between MgO Nanocrystallites

The electron- and hole-trapping and optical properties of a wide variety of interfaces between MgO nanocrystallites are investigated for the first time using a quantum-mechanical embedded-cluster method and time-dependent density functional theory. We conclude that delocalized holes can be transiently trapped at a large number of places within a powder. However, it is more energetically favorable for holes to trap on low-coordinated anions on the nanocrystallite surface, forming O- species. Electrons are trapped at few interfaces but are readily trapped by surface kink and corner sites. Contrary to common perception, our calculations of optical absorption spectra indicate that a variety of features buried within a powder can be exited with photon energies less than 5 eV, usually used to selectively excite low-coordinated surface sites.

Studies of optical and structural properties of CdSe/polymer nanocomposites: evidence of charge transfer and photostability

In this work, the role of conducting [poly (p-phenylinevinylene) (PPV)] and nonconducting (polystyrene) polymers on the properties of their respective composites with CdSe quantum dots of varied sizes has been investigated. The emission and structural properties of polymer–CdSe composites are found to be dependent on the crystallite size and morphology of CdSe nanocrystallites. Smaller CdSe quantum dots (size, ∼5 nm) ensures efficient charge transfer process across polymer–CdSe interface as evident by almost complete quenching of photoluminescence (PL) emission as compared to larger CdSe quantum dots (size, ∼7 nm). Presence of residual trioctylphosphine (TOP)/ tri-n-octylphosphine-oxide (TOPO) species and agglomeration of particles act as a hindrance for quenching of emission and hence charge transfer for larger CdSe nanocrystallites. Emission studies indicated an increased conjugation length for PPV polymers in different solvents (toluene, pyridine) and in solid state. Nonconducting polymer polystyrene shows charge transfer across polymer–CdSe interface as well. However, polystyrene polymer has a shorter chain length, which ensures maximum coverage on the surface of CdSe nanocrystallites and provides better photostability to CdSe QDs within the polymer matrix as compared to that for PPV–CdSe nanocomposites.

Photocatalytic Activity of Sol−Gel-Derived Nanocrystalline Titania

Nanocrystalline titania (TiO2) powders have been synthesized via sol−gel, using an alkoxide precursor, under different processing conditions, and their photocatalytic activity has been investigated as a function of processing and material parameters through the decomposition of the methylene blue (MB) dye under exposure to the ultraviolet (UV) radiation (λ = 200−400 nm) in an aqueous solution. The nanocrystalline TiO2 powders with different morphology, crystallinity, average nanocrystallite size, surface area, and phase structure are obtained by controlling the ratio of molar concentrations of water and alkoxide (R) within the range of 5−60 and calcining the as-synthesized amorphous powders at higher temperatures (400−800 °C). The nanocrystalline TiO2 powders have been characterized using the scanning electron microscope (SEM), X-ray diffraction (XRD), and the Brunauer, Emmett, and Teller (BET) surface area measurement techniques while their photocatalytic activity was monitored using a UV−visible spectro...

Dynamics of low-coordinated surface atoms on gold nanocrystallites

The authors highlight the importance of transient configurations of atoms on the surface of nanocrystallites, and present methodologies for their investigation. A Monte Carlo method has been developed and is used to simulate the thermodynamic equilibrium of nanometer sized Au nanocrystallites, both free and supported on a MgO(100) surface. The authors find that appreciable numbers of atoms transiently occupy adatom positions on Au(111) facets, even at room temperature. This type of dynamically appearing site is usually neglected in relation to catalysis but may have a significant activity (for CO oxidation, for example). They also observe a complex solid-solid roughening transition which involves a variety of transient local atom configurations on the surface of nanocrystallites.

Plume analysis during pulsed laser ablation of silicon in hydrogen gas

Recently, surface passivated Si nanocrystallites are prepared by pulsed laser ablation (PLA) of Si target in hydrogen gas. Plume analysis during PLA is one of the available methods to clarify a mechanism of surface hydrogenation. We observed plume emission by time- and space-resolved spectroscopy. The dominant species were exited neutral Si atoms and SiH species when the delay time is longer than 100 ns. The Si atoms and SiH species existed throughout the plume. This indicates that nanocrystallites grow in the mixture of Si and SiH vapor. The surfaces of Si nanocrystallites are considered to be hydrogenated by SiH species during the growth. It seems that surface hydrogenation is not due to the spatial separation of Si and H species but due to surface reaction with SiH and/or Si species.

A Facile Route to ZnS–Polymer Nanocomposite Optical Materials with High Nanophase Content via γ‐Ray Irradiation Initiated Bulk Polymerization

Studying the fabrication and composition of semiconductor nanoparticles in polymer matrices has attracted the interest of many scientists because nanoparticle–polymer composites may find applications in high-refractive-index materials, light-emitting diodes, photocatalysts, photovoltaic solar cells, and nonlinear optical devices. Control of the nanoparticle size and size distribution as well as the dispersion homogeneity in the polymer matrix are critical prerequisites for controlling the properties of the composites. However, nanoparticles are prone to aggregation in the polymer because of their high specific surface energies and inherently hydrophilic character. Therefore, it is still a technological challenge to incorporate inorganic nanoparticles into polymer matrices and thus to prepare transparent bulk nanocomposites with high nanoparticle content. Far fewer studies of nanoparticle–polymer bulk materials have been reported than of nanocomposite films. Semiconductor nanoparticles (PbS, CdSe, CdTe) have been introduced into bulk polymer matrices to prepare bulk nanocomposites, although the content of inorganic particles was low (< 5 wt %). In these studies, two main approaches have been developed: in situ formation of nanoparticles in a presynthesized polymer and bulk polymerization of an organic monomer in the presence of premade nanoparticles. The latter provides full synthetic control over both the nanoparticles and the matrices, and is a more effective and practical route for fabricating bulk polymer nanocomposites on a large scale. We have utilized UV radiation curing to prepare transparent ZnS–polymer nanocomposite films with high ZnS contents from a solution mixture of premade ZnS particles and acrylate macromers. This is an effective method for preparing nanocomposite films with nanoparticles dispersed homogeneously horizontally and vertically throughout the entire polymer matrix, but it is unsuitable for preparing thick bulk nanocomposites. In addition, the ZnS nanoparticles obtained by the usual method cannot be redispersed in monomer or solvent because of their incomplete surface modification with organic molecules. The application of c-ray irradiation in the preparation of bulk polymer nanocomposites has remarkable advantages, such as processing under ambient pressure at room temperature and quick polymerization of monomers. These valuable properties can be put to use in the fabrication of nanocomposites with homogeneously dispersed inorganic nanoparticles. In this communication, we report a novel, facile route for the preparation of transparent bulk nanocomposites with high ZnS content via c-ray irradiation initiated polymerization (Fig. 1). Our strategy involves the design and optimization of the nanoparticle surface and polymeric monomer as well as the selection of a suitable polymerization technology. First, we reasoned that the compatibility between nanoparticles and the polymer matrix is a prerequisite for synthesizing a bulk nanocomposite with high nanophase content. So, it is necessary to tailor the corresponding polymer matrix by the decoration of the nanoparticles with organic molecules. Furthermore, it is highly important that the monomer should act as both the ligand and solvent for the inorganic nanoparticles. It has been reported that polar organic solvent molecules, such as N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), co-ordinate to the surface of ZnS or CdS nanocrystallites and can effectively stabilize them. Thus, it is expected that a monomer with a similar structure to DMF can be selected as both the solvent and ligand for the nanoparticles. Here, we have selected N,N-dimethylacrylamide (DMAA), which can effectively disperse and stabilize ZnS particles, as the monomer. Finally, the c-ray irradiation technique is another crucial factor in the fabrication of a bulk polymer nanocomposite with high particle content. This polymerization method can induce a mild and rapid gelation process for the bulk polymeric system containing a high content of nanoparticles uniformly dispersed inside. We present a simple approach for the large-scale production of mercaptoethanol (ME)-capped ZnS nanoparticles in DMF from Zn(OAc)2 (Ac: acetate) and thiourea. The MEC O M M U N IC A TI O N S

Simulation and Bonding of Dopants in Nanocrystalline Diamond

The doping of the wide-band gap semiconductor diamond has led to the invention of many electronic and optoelectronic devices. Impurities can be introduced into diamond during chemical vapor deposition or high pressure-high temperature growth, resulting in materials with unusual physical and chemical properties. For electronic applications one of the main objectives in the doping of diamond is the production of p-type and n-type semiconductors materials; however, the study of dopants in diamond nanoparticles is considered important for use in nanodevices, or as qubits for quantum computing. Such devices require that bonding of dopants in nanodiamond must be positioned substitutionally at a lattice site, and must exhibit minimal or no possibility of diffusion to the nanocrystallite surface. In light of these requirements, a number of computational studies have been undertaken to examine the stability of various dopants in various forms of nanocrystalline diamond. Presented here is a review of some such studies, undertaken using quantum mechanical based simulation methods, to provide an overview of the crystal stability of doped nanodiamond for use in diamondoid nanodevices.

Fractal nature of porous silicon nanocrystallites

Experimental study of the hydrogen coverage of a nanoporous silicon-specific surface reveals a fractal nature of the surface of silicon nanocrystallites constituting the porous layer. A fractal model describing the nanocrystallite morphology is elaborated. The evolution of the nanocrystallites' fractal dimension $(2.1--2.4)$ along with porosity is deduced from a correlation of the model to the experimental measurements of hydrogen concentration.

Densification of Mn-doped tin oxide films by conventional heating and microwave heating treatment

Mn(II) doped SnO2 thin films used for shielding fluoride glasses against corrosion were investigated by x-ray absorption spectroscopy (EXAFS and XANE)S at the Sn and Mn K-edges. The effect of firing treatment on the densification of the films was studied. It has been evidenced a partial change of Mn valence from 2.3 to 2.6 upon heating which is attributed to a change of ratios of two Mn sites: grafted divalent Mn ions at the surface of SnO2 nanocrystallites and trivalent Mn ions embedded into a substitutional solid solution with Sn.

Effect of Cu2O on the fabrication of SiCp/Cu nanocomposites using coated particles and conventional sintering

Abstract SiC particulate-reinforced copper composites were fabricated using coated particles and conventional atmospheric sintering. Characterization of the copper coated SiC submicron particles and the composites sintered in nitrogen or ammonia atmosphere were conducted by XRD, TEM, SEM, and AES analysis. It was found that Cu2O islands formed on the surface of copper nano crystallites in the coated particles. Cu2O was reduced to copper during sintering in an ammonia atmosphere and this led to porous composites with flocculation of SiC grains and segregation from coagulated copper blocks. By contrast, Cu2O remained in the composites that were sintered in nitrogen atmosphere. This procedure was essential for the wetting of SiC by copper and for the densification process, and also facilitated the formation of nanostructured SiCp/copper composites.