Small CdS Particles Research Articles

Constructing Defect-Mediated CdS/g-C3N4 by an In-situ interlocking strategy for Cocatalyst-free photocatalytic H2 production

The construction of heterojunction photocatalyst is an emerging strategy for realizing efficient photocatalytic hydrogen production, and the methodology of construction is critical for improving the interfacial charge carriers separation and transfer. A weakly coupling interface in a heterojunction leads to lower hydrogen production activity and stability. Herein, a facile strategy based on an in-situ interlocking reaction is developed to solve the interfacial contact issue. Litchi-like CdS is anchored at the defects of the g-C3N4 (CN) by an interlocking process, and the contact interface is further extended by some small necking CdS particles that assembled on the contact corner because of the surface tension. The as-formed CdS/g-C3N4 (CdS/CN) has a unique and strongly coupling interface, which can effectively improve interfacial separation and transfer kinetics of photogenerated charge carriers due to the synthetic advantages of interlocking binding, necking effect, and heterojunction. Impressively, the as-prepared CdS/CN photocatalyst yields over 36 folds of cocatalyst-free hydrogen generation rate than the pure CdS under the visible light irradiation. This work presents a case of practical study to realize the synthesis of defect-mediated heterojunction photocatalysts by a facile strategy.

Stability studies of CdS sensitized TiO2 nanotubes prepared using the SILAR method

Titania nanotubes sensitized with CdS nanoparticles of different sizes and amounts were prepared using the successive ionic layer (SILAR) deposition method. UV/visible spectra indicate a red shift in CdS absorption upon increasing the number of SILAR cycles this is due to an increase in amount and size of CdS particles on the surface of titania nanotube. The stability of CdS particle sensitized on the surface of titania nanotubes were monitored using X-ray photoelectron spectroscopy (XPS). The formation of CdSO4 species on the surface of CdS particles was evident from the XPS analysis. Smaller CdS particles, formed using a lower number of SILAR cycles, are more prone to oxidation than the larger particles formed using a greater number of SILAR cycles. The photocatalytic activity and solar cell efficiency is found to be dependent on the number of SILAR cycles employed.

Photoelectrochemical Properties of Vertically Aligned CuInS2 Nanorod Arrays Prepared via Template-Assisted Growth and Transfer.

Although copper-based chalcopyrite materials such as CuInS2 have been considered promising photocathodes for solar water splitting, the fabrication route for a nanostructure with vertical orientation has not yet been developed. Here, a fabrication route for vertically aligned CuInS2 nanorod arrays from an aqueous solution using anodic aluminum oxide template-assisted growth and transfer is presented. The nanorods exhibit a phase-pure CuInS2 chalcopyrite structure and cathodic photocurrent response without co-catalyst loading. Small particles of CdS and ZnS were conformally decorated onto CuInS2 nanorods using a successive ion layer adsorption and reaction method. With surface modification of CdS/ZnS, the photoelectrochemical properties of CuInS2 nanorod arrays are enhanced via flat-band potential shift, as determined by analyses of onset potential and Mott-Schottky plots.

An estimate of shape distribution of small CdS particles with luminescence spectra

The optical properties of CdS particles with different shapes are compared. The particles, characterized by spherical and rectangular shapes, were obtained with chemical and pounding methods, respectively. Then the particles were calibrated against their shapes. Obtained particles were investigated with the help of transmission electron microscopy, photoluminescence technique, and theoretical modeling. Absorption spectra were calculated via dissipative function in terms of local field approach. It was established that the comparison of experimental and theoretical results can give the additional information on particles, in particular, its shape and the shape distribution.

An estimate of shape-distribution of small CdS particles with luminescence spectra

The luminescence spectra of CdS particles obtained by sedimentation method are studied. The comparison of measured and calculated photoluminescence spectra was used as the base of the method defining the shape distribution of the particles. For this purpose one can use the symmetry rule for absorption and luminescence spectra. Absorption spectra were calculated via dissipative function in terms of local field theory. The experimental spectra of the photoluminescence were measured during the process of sedimentation in the CdS nano-composite powder. The modification of luminescence spectra detected during sedimentation time is associated with changing the dimension and shape distributions of CdS suspension. As a result, the particles shape distribution was estimated for sizes less then 1 μm.

Preparation of Aminodextran−CdS Nanoparticle Complexes and Biologically Active Antibody−Aminodextran−CdS Nanoparticle Conjugates

Stable aqueous dispersions consisting of CdS nanoparticles having modal diameters, ranging between 2 and 8 nm, were prepared with amino-derivatized polysaccharides (aminodextrans, hence abbreviated as Amdex) as the stabilizing agents. The size, stability, and luminescence intensity of such dispersions were shown to be dependent on the types of the cadmium salts and aminodextrans used, as well as on the reactant concentrations. Specifically, it was demonstrated that the degree of substitution of amino groups in the aminodextran molecules greatly affected the properties of the dispersions; i.e., with higher degree of substitution, smaller CdS particles and higher luminescence intensity were achieved. It was also shown that the Amdex−CdS nanoparticle complexes could be activated and conjugated with antibody by conventional means. Molecular weight ranges of the Amdex and their complexes with CdS nanoparticles and the purity of antibody−Amdex−CdS nanoparticle conjugates were determined by polyacrylamide gel el...

Nanosecond laser flash photolysis of thiophenolate-capped cadmium sulfide particles in acetonitrile

Abstract Thiophenolate-capped US particles (PhS-capped CdS) of two different particle sizes (∼18 A and ∼30 A) were subjected to nanosecond laser flash photolysis (299 or 355 nm) in acetonitrile in the presence and absence of a hole scavenger. The effects of particle size and of the identities of the capping agent and hole scavenger on the efficiency of the formation and decay of trapped holes at the surface of these particles were studied. The transient absorption of the thiophenyl radical (PhS), produced by hole trapping in the presence of diethyldithiocarbamate (Et 2 NCS 2 − ), differs from that reported for non-capped CdS colloids. The observed transi absorption spectrum shifted to shorter wavelengths with a decrease in particle size. The transient bleaching observed at short wavelengths in the large (∼30 A) PhS-capped CdS particles was not observed in the analogous small (∼18 A) particles. The decay of the transient absorption of the small (∼18 A) PhS-capped CdS particles is faster than that of large (∼30 A) ones. The dominant decay pathway for trapped holes is the reaction of surface-bound PhS- with adsorbed phenolate groups that exists in excess at the surface of the capped particle.

Structural phase behavior in II–VI semiconductor nanoparticles

Cubic structured CdS, CdSe, and CdTe, II–VI semiconductor nanoparticles have been synthesized using aqueous solution precipitation at room temperature. The ‘‘as-prepared’’ particles have a size of about 30 Å. Thermal annealing causes (a) an increase in particle size; (b) a structural transition from the cubic to the bulk, hexagonal structure for CdS and CdSe; and (c) no structural transition for CdTe. The unexpected cubic phase for small particles of CdS and CdSe may be due to either metastability or an equilibrium surface effect. The latter would imply a strong correlation between structure and the size and surface properties, and that there is a minimum size for hexagonal phase stability.

Small quantum-sized CdS particles assembled to form a regularly nanostructured porous film

Submicroporous CdS films were fabricated by a two-step replication process, in which porous anodic alumina was used as a template. The films were grown on a porous gold support of the same structure, resulting in a free-standing CdS/Au membrane. The diameter of the holes of the regular porous structure is in the order of 100 nm. Moreover, the replicated CdS film consists of small-sized quantized crystalline particles, the size of which are in the range of 5 nm. Thus, the film is ordered in the 5 and 100 nm domain forming a superstructure.

Growth Kinetics of CdS Nanocrystallites in Glass through Quantum‐Size Effects

AbstractGlasses containing very small particles of CdS are prepared and their properties studied. Changes in the optical absorption spectra of CdS glass result from quantum confinement effects. With very small particle size, large confinement energies of up to 4 eV are obtained, which have not been before. The blue‐shifts in the optical absorption spectra show that the growth rate of the particles at short times are diffusion controlled, and the activation energy for growth is calculated. The band gaps of the CdS nanocrystallites in glass are determined from photoluminescence measurements, which show a high‐energy peak, corresponding to the band gap, that had not been previously observed.

X-ray photoelectron/X-ray excited auger electron spectroscopic study of highly dispersed semiconductor CdS and CdO species in zeolites

The identification and study of the electronic properties and location of CdSx(x < 1) and CdO particles in faujasites have been performed using X-ray photoelectron spectroscopy (XPS), X-ray excited Auger electron spectroscopy (XAES) and fluorescence spectroscopy. These data were compared with results obtained by transmission electron microscopy (TEM), UV–VIS reflectance spectroscopy and adsorption measurements reported earlier for the same samples. Depending on the preparation procedure, CdS aggregates have different dispersions and can be located either inside the zeolite structure or on the external surface. Small CdS particles (1.5–2 nm) exhibit both XPS and fluorescence band shifts indicating a semiconductor size quantization (Q-size) effect. In addition, the non-stoichiometry of these species was also displayed in the fluorescence spectra. Larger CdS particles exhibit electronic and relaxation (polarization) properties rather close to those of bulk CdS. Strong surface enrichment with these particles was also observed. The CdO aggregates are formed during CdY treatment with NaOH and calcination. The identification of CdO is based on an abnormally low binding energy of Cd, which is characteristic for cadmium oxide only.

Spectroscopy of Semiconductor Nano-Crystals at High Pressure

ABSTRACTThe optical and vibrational properties of small CdSe and CdS particles embedded in a glass matrix and as a colloid have been studied as a function of pressure up to 90 kbar using Raman scattering and photoluminescence techniques. We will discuss the use of high pressure optical spectroscopy techniques, where the sample is contained in a diamond anvil.cell with optical access, to study the nature of the electronic states in semiconductor nanocrystals. Raman scattering is employed to establish the enhanced pressure stability of the wurtzite phase in the II-VI nanocrystalline composites. Photolurninescence is used to study the energies of electronic states. The wurtzite to rocksalt phase transition behavior in the nanocrystallite systems is quite different from that in the bulk material. This different behavior is attributed to a large number of defects (vacancies) in the nanocrystallite system. This work demonstrates that the main defects in the CdS glass composites are cadmium vacancies; while in CdSe Selenium vacancies exist in the conduction band. The pressure dependence of the Huang-Rhys parameter, characteristic for the strength of the electron-phonon coupling, will also be discussed for the nanocrystalline samples versus the bulk material.

Synthesis of cadmium sulfide in situ in reverse micelles. 2. Influence of the interface on the growth of the particles

AOT reverse micelles make possible the in situ synthesis of very small particles of CdS. The use of functional surfactants (such as cadmium lauryl sulfate or cadmium AOT) allows, by blocking one of the reactants at the interface of the micelles, semiconductor colloids to be obtained that are more stable and smaller than those obtained from ions solubilized in the water pool of the reverse micelle. A comparative study of the different conditions of synthesis is presented to optimize the production of very small aggregates without having to use a protecting agent. The behavior of these semiconductors with respect to photocorrosion is studied.

Optical and photoredox properties of small colloidal semiconductor particles in the presence of trivalent lanthanide ions

Size quantization effects upon the optical properties of small size semi-conductor particles ( d < 50 A ̊ ) have been documented in several systems. As the particle size decreases the band gap energy increases accompanied by a blue-shifted emission, and consequently the photoredox activity should increase. We are studying the correlation between the recombination luminescence of CdS particles and the redox potential of the conduction band probed in terms of oxidative luminescence quenching via electron transfer to Ln 3+ Ln 2+ and Ln 3+ Ln redox couples associated with different redox potentials. The electron transfer process from the conduction band of small CdS particles to electron acceptors ( ΔG ° el. tr < 0) is size dependent.

Photochemistry of colloidal semiconductors. 30. HPLC investigation of small CdS particles

HPLC is used to determine the size distribution of CdS colloids. These distributions agree with those obtained from electron microscopy. The method is applied to recognize “magic” agglomeration numbers and to follow thermal particle growth and photodissolution. In the thermal growth a gradual increase in size is ascribed to Ostwald ripening. However, in the beginning a rather abrupt increase in size takes place that is ascribed to particle combination. In the photodissolution of a sample containing two size distributions, the larger particles disappear more rapidly than the smaller ones, this effect being particularly pronounced when light of longer wavelengths is used where the small particles have little absorption.

Photochemistry of colloidal semiconductors 29. Fractionation of CdS sols of small particles by exclusion chromatography

AbstractCdS sols stabilized by sodium hexametaphosphate or poly‐phosphate‐15 were fractionated in a column of sephacryl gel and the fractions investigated by electron microscopy and spectrophotometrically. A linear relationship between the logarithm of the particle size and the elution time was observed. The method may be used to prepare sols of narrow size distribution and to analyse unknown samples with respect to the size of the particles. —The thermal growth of very small CdS particles was followed by chromatography. The particles grew rather abruptly from one size distribution to another, which may be explained by growth via particle combination.

ChemInform Abstract: Photophysical and Photochemical Properties of CdS with Limited Dimensions.

AbstractLaponite, porous Vycor glass, and zeolites (faujasite X, sodalite) are used to prepare very small particles of CdS and PbS in restricted and constrained environments.

Solubility and photocorrosion of small CdS particles

In this work small-particle CdS colloids of approximate diameter 250 A and below 25 A were prepared and an increase in solubility product from 7.2 × 10−27 for large particles to about 10−15 for small-particle colloids (d < 25 A) was found. Moreover, the observed change in the cathodic corrosion potential to a more positive value of −0.7 V (for particle sizes below 25 A) can be explained in terms of the increase in the solubility of the small particles. Cadmium metal formation in the cathodic corrosion process of small-particle colloids was found only in the presence of a sacrificial hole scavenger (triethanolamine), but hydrogen was produced even upon illumination of pure CdS colloid in water (Φ(H2) = 3.84%).

Photophysical and photochemical properties of CdS with limited dimensions

Laponite, porous Vycor glass and molecular sieves, faujasite X and sodalite, have been used to prepare small particles of CdS with limited dimensions. Laponite limits one dimension of the CdS particles by the interplaner spacing of 11.5 A. Porous Vycor glass limits two dimensions of the CdS particles to < 40 A. Molecular sieves were used to limit the particle size to the size of the cavity; however, the particles constructed were larger than the cavity dimensions. The absorption onset and main emission wavelength increase with increasing Cd2+ concentration. The spectral properties of PbS are also reported. Methyl viologen and Cu2+ quench two different emission bands arising from CdS on laponite in solution. Methyl viologen quenches the high-energy emission, thought to arise from exciton recombination, while Cu2+ efficiently quenches a low-energy emission thought to arise from S2– deficiencies. Methyl viologen shows dynamic-type kinetics, while the static-type kinetics observed from Cu2+ gives an estimate of the particle size. The spectroscopic properties of the particles are studied as a function of cadmium concentration and discussed in terms of particle size.

Size quantization in small colloidal CdS particles studied with stopped flow spectrometry

Size quantization effects have been observed in small particle size colloids (< 5 nm). Stopped-flow techniques have been used to observe the early stages of particle growth. Structure in the absorption spectra, which are believed to be due to quantization effects, have been detected in the first millisecond to 2.5 s of colloid formation. The structured spectra could be resolved into a series of Gaussian peaks of constant linewidth. The energy of the absorption maxima correspond surprisingly well to the predicted optical transitions for the theoretical model of carrier motion confined to a sphere of finite potential. The selection rules for optical transitions are found to be Δ n = 0, Δ l, ±1, ±2,… It was also found that the presence of different colloidal stabilizers leads to different modes of particle growth which can be explained with electrostatic and topological arguments.