CdS Grains Research Articles

Hierarchically Periodic Macroporous CdS–ZnO Heterojunctions with Multiple Quantum Well-like Band Alignments for Efficient Photocatalytic Hydrogen Evolution without a Cocatalyst

Heterostructures constructed by conventional methods, particularly those randomly distributed manner on the catalyst surface as well as forming core–shell structures, lead to imbalance in carrier utilization, limitation of charge extraction/transfer, and hindrance of light harvesting. In this study, we constructed a new type of heterostructure consisting of alternate CdS and ZnO grains bridged with good interfaces in the hierarchically periodic macroporous (HPM) walls by the pyrolysis of CdZnS solid solution in a polymethyl methacrylate nanoreactor. Since the alternately arranged CdS–ZnO heterojunctions in the HPM form multiple quantum well-like (MQW-like) band alignments that favor the Z-scheme charge transfer, the photogenerated electrons and holes are spatially separate and accumulated on CdS and ZnO parts, respectively. Besides the desirable spatial separation of the photogenerated charges, the enabled excellent mass transfer in the macroporous structures effectively accelerates the utilization of the carriers. Owing to the synergistic effect of the MQW-like band alignments and the HPM structures, the photocatalytic H2 evolution rate of HPM CdS–ZnO is as high as 587.8 μmol h–1 without a cocatalyst. This work introduces a fascinating strategy for the creation of alternate heterojunctions to improve carrier utilization.

CdS-based ternary composite material for high-efficiency photocatalytic CO2 reduction via a cascade electron transfer

CdS conductor has a suitable band gap (~2.4 eV), and the energy band position is conducive to photocatalytic CO2 reduction. However, its photocatalytic efficiency is highly confined by its unfavored carrier transfer. In this study, a moderately in-situ growth method is harnessed to insert commercial TiO2 particles (P25) into the CdS grains in the first place, and the resulted CdS-P25 composite is further integrated with to a mixed-ligand-based metal organic framework (MOF), CuTCPP⊂UiO-66 (CTU), which is structure-analogous to regular UiO-66 MOF but with visible light response thanks to the involvement of copper(II) tetra(4-carboxyphenyl) porphyrin (CuTCPP) ligand. The eventually attained ternary composite (CTU/CdS-P25) by solvothermal approach presents a broadened light absorption and an impressive delivery for photo-induced carriers. The light-driven catalytic assessment of CTU/CdS-P25 for CO2 reduction furnishes the CO production rate as great as 2.38 μmol·h−1, as much as 3.9 and 8.8 times that of UiO/CdS-P25 and CdS-P25, respectively. According to the value determination of band edge for each component of the ternary composite, we proposed that the established heterojunction facilitates a cascade electron transfer mode within the structure, which highly suppresses the adverse recombination of carriers and thereafter significantly enhances the photocatalytic behavior. Our work provides an effective perspective for the melioration of CdS semiconductor.

Why Picasso’s yellow paint has faded to brown

In Pablo Picasso’s 1907 painting Femme (Époque des “Demoiselles d’Avignon”), a formerly yellow section in the upper left corner is now brownish while another yellow section in the painting has maintained its brilliant lemon hue. According to a new study, this difference is the result of Picasso using two different cadmium sulfide–based yellow paints: they contain different-sized CdS particles and have degraded at different rates (Anal. Chem. 2019, DOI: 10.1021/acs.analchem.8b04914). Daniela Comelli of the Politecnico di Milano and colleagues analyzed paint from the two different sections of the painting, using photoluminescence spectroscopy to look closely at the CdS particles. The emission spectra led the researchers to conclude that the degraded paint contains smaller CdS nanoparticles with more crystal defects, whereas the unfaded paint contains larger CdS grains, which may affect the rate of degradation. The study shows that analyzing the photoluminescence of pigment particles could help art conservators identify the

Inkjet printing and electrical characterisation of DNA-templated cadmium sulphide nanowires

Cadmium sulphide can be templated on λ-DNA molecules to form an aqueous dispersion of CdS/λ-DNA nanowires. Subsequent addition of ethylene glycol to 50% v/v is sufficient to formulate an ink suitable for printing using piezoelectric drop-on-demand technology. Printed droplet arrays show a coffee-ring morphology of individual deposits by fluorescence and Raman microscopy, but upon increasing the number of layers of printed material by repeated printing over each droplet, the dry deposit approaches closer to a disc shape. It is also possible to print parallel tracks by reducing the droplet separation in the array until neighbouring droplets overlap before they dry. The droplets coalesce to form a strip of width roughly equal to the diameter of the droplets. Evaporation-driven capillary flow sends the nanowires to the edges of the strip and when dry they form parallel tracks of CdS/λ-DNA nanowire bundles. Both droplets and tracks were printed onto Pt-on-glass interdigitated microelectrodes (10 μm width, 10 μm gap). The current–voltage characteristics of these two-terminal devices were approximately ohmic, but with some hysteresis. The conductance increased with temperature as a simple activated process with activation energies of 0.57 ± 0.02 eV (tracks) and 0.39 ± 0.02 eV (droplets). The impedance spectra of the printed films were consistent with hopping between CdS grains.

High Fidelity Polycrystalline CdTe/CdS Heterostructures via Molecular Dynamics

ABSTRACTMolecular dynamics simulations of polycrystalline growth of CdTe/CdS heterostructures have been performed. First, CdS was deposited on an amorphous CdS substrate, forming a polycrystalline film. Subsequently, CdTe was deposited on top of the polycrystalline CdS film. Cross-sectional images show grain formation at early stages of the CdS growth. During CdTe deposition, the CdS structure remains almost unchanged. Concurrently, CdTe grain boundary motion was detected after the first 24.4 nanoseconds of CdTe deposition. With the elapse of time, this grain boundary pins along the CdS/CdTe interface, leaving only a small region of epitaxial growth. CdTe grains are larger than CdS grains in agreement with experimental observations in the literature. Crystal phase analysis shows that zinc blende structure dominates over the wurtzite structure inside both CdS and CdTe grains. Composition analysis shows Te and S diffusion to the CdS and CdTe films, respectively. These simulated results may stimulate new ideas for studying and improving CdTe solar cell efficiency.

Nanostructured CdS thin films deposited by spray pyrolysis method

Influence of solution pH on the structural and optical properties of CdS films deposited by conventional spray pyrolysis technique was studied. X‐Ray Diffraction (XRD), Atomic Force Microscopy (AFM), Photoluminescence spectroscopy (PLS), and Spectroscopic Ellipsometry (SE) methods were used for the characterization of the deposited films. PL spectrum of the film deposited from the solution with pH = 10.2 shows broad‐band PL emission located at 460 nm (2.7 eV), which can be attributed to the quantum size effect at grain sizes of <10 nm. No shifts of ϵ1 and ϵ2 due to the quantum size effect are observed in dielectric function spectra, what can be caused by low concentration of nano‐sized (<10 nm) CdS grains. The change in the film properties with the pH of the solution was analyzed in terms of variation of grain sizes of the polycrystalline films.

Electrodeposition and characterisation of CdS thin films using thiourea precursor for application in solar cells

CdS thin films have been successfully electrodeposited on glass/FTO substrates using acidic and aqueous solution of CdCl2.xH2O and thiourea (SC(NH2)2). The electrodeposition of CdS thin films were carried out potentiostatically using a 2-electrode system. The prepared films were characterised using X-ray diffraction, Raman spectroscopy, Scanning electron microscopy, Atomic force microscopy, Photoelectrochemical cell measurements, Electrical resistivity measurements and UV–Vis spectrophotometry to study their structural, compositional, morphological, electrical and optical properties, respectively. The structural studies show that the as-deposited and annealed CdS layers are polycrystalline with hexagonal crystal structure and preferentially oriented along (200) planes. The optical studies indicate that the ED-CdS layers have direct bandgaps in the range (2.53–2.58) eV for the as-deposited and (2.42–2.48) eV after annealing at 400 °C for 20 min in air. The morphological studies show the good coverage of the FTO surface by the CdS grains. The average grain sizes for the as-deposited and annealed layers were in the range ∼(60–225) nm. These grains or clusters are made out of smaller nano crystallites with the sizes in the range ~(11–33) nm. The electrical resistivity shows reduction as thickness increases. The resistivity values for the as-deposited and annealed layers were in the range (0.82–4.92) × 105 Ωcm. The optimum growth voltage for the CdS thin films was found to be at the cathodic potential of 797 mV with respect to the graphite anode. No visible precipitations of elemental S or CdS particles were observed in the deposition electrolyte showing a stable bath using thiourea during the growth.

Electrical characterization of annealed chemical-bath-deposited CdS films and their application in superstrate configuration CdTe/CdS solar cells

Abstract Application of chemical-bath-deposited CdS in the superstrate configuration of CdTe/CdS solar cells involving CdCl 2 :O 2 heat treatment of CdTe/CdS structures at about 400 °C is problematic. Namely, the vertical capillary surfaces (grain boundaries) between the columnar CdS grains perform as fast diffusion channels leading to the emergence of short circuits between the absorber and front contact. It was assumed that the grain boundaries contain residual hydroxy-oxide type compounds and form electrical barriers between columnar grains in the lateral direction of the CdS layer and that the electrical methods should be indicative of the behavior of grain boundaries in the annealing process. All samples were characterized by temperature dependence of DC conductivity in a temperature range of 50–300 K, X-ray diffraction, and scanning electron microscope. It has been found that the deeper layers of H 2 and N 2 annealed CdS preserve residual hydroxide, which released the gas phase in the recrystallization process of the chloride processing and created porosity on the CdTe/CdS interface.

Preparation and Characterization of Porous and Composite Nanoparticulate Films of CdS at the Air/Water Interface

CdS nano-particulate films were prepared at the air/water interface under Langmuir monolayers of arachidic acid (AA) via interfacial reaction between <TEX>$Cd^{2+}$</TEX> ions in the subphase and <TEX>$H_2S$</TEX> molecules in the gaseous phase. The films were made up of fine CdS nanoparticles with hexagonal Wurtzite crystal structure after reaction. It was revealed that the formation of CdS nano-particulate films depends largely on the experimental conditions. When the films were ripened at room temperature or an increased temperature (<TEX>$60^{\circ}C$</TEX>) for one day, numerous holes were appeared due to the dissolution of smaller nanoparticles and the growth of bigger nanoparticles with an improved crystallinity. When the films were ripened further, CdS rodlike nanoparticles with cubic zinc blende crystal structure appeared due to the re-nucleation and growth of CdS nanoparticles at the stacking faults and defect structures of the hexagonal CdS grains. These structures were characterized by transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and X-ray diffraction (XRD). These results declare that CdS semiconductor nanoparticles formed at the air/water interface change their morphologies and crystal structures during the ripening process due to dissolution and recrystallization of the particles.

CdS Nanotubes and Y-Branched Nanochannels in AAM Fabricated by a Double Diffusion Route

Polycrystalline CdS nanotubes and a novel partition nanostructure were prepared in an anodic alumina membrane (AAM) template using a double diffusion method at room temperature. Transmission electron microscopy (TEM), selected area electron diffraction (SAED) and energy dispersive X-ray spectroscopy (EDX) investigations indicate that the nanotubes consisting of CdS grains have been synthesized in the AAM nanoholes. The influence of the reagent concentration on the morphology of the nanotubes has been systemically studied using a field emission scanning electron microscope (FE-SEM). It is confirmed that the Y-branched nanochannels can only formed under certain concentrations of the reactants ( and solutions). This novel Y-branched nanostructure may have potential applications for preparation of complicated nanostructure materials.

Effect of CdCl 2 annealing treatment on thin CdS films prepared by chemical bath deposition

In order to study the CdS recrystallization mechanism, a comparative study was carried out on thin films prepared by chemical bath deposition. The CdS films were annealed in air with or without a CdCl 2 coating layer. In-situ Raman spectra obtained during the annealing showed that both the air- and the CdCl 2-annealing did not cause rearrangement of the neighboring atoms in the CdS clusters below ~ 300 °C. CdS thin film was partially oxidated to CdO and CdSO 4 on the cluster surface when annealed in air. The oxides and the sulfur stoichiometric deficiency prevented the clusters to coalesce at higher temperatures. Coating thin CdS film with a thin CdCl 2 layer protected it from oxidation during annealing in air and promoted formation of Cl S and V Cd point defects in CdS. The anti-oxidation was attributed to the incorporation of a significant amount of Cl into CdS to form the Cl S, which prevented the oxygen in-diffusion and chemical bonding during the annealing. The anti-oxidation at the CdS nano-crystalline surface and the point defects formed in the CdS promoted coalescence of the neighboring clusters without the need of long-range redistribution of the atoms. Large CdS grains with good crystalline quality formed through recrystallization during the CdCl 2 heat treatment, which provided the solid basis for the subsequent CdTe growth and high efficient CdS/CdTe solar cell fabrication.

Transformation of the structural surface morphology and electronic structure of CdS nanostructured films prepared in glycerol-containing solution, under ultrasonic irradiation

Chemical bath deposition was improved by using glycerol, a very conventional surfactant, to control cadmium sulfide CdS in various sizes and structures, under ultrasonic irradiation conditions. Consequently, an effective route to prepare CdS thin film that consists of either CdS fibrils with an average diameter of 52 nm or monodispersed CdS grains 35 to 80 nm in size is proposed. X-ray diffraction (XRD) studies revealed that, in the crystallographic properties of the obtained CdS films, CdS may exist in two crystalline modifications—a hexagonal and a cubic phase—that are characterized by several diffraction peaks positioned at 2θ≈23.4°, 27.1° and 47.8°. Beside this, UV-Vis studies also revealed the positive effect of glycerol at grain boundaries, leading to detrimental reduction of the band bending towards the absorber surface and increasing the population of optical absorptions.

Implantation-assisted Co-doped CdS thin films: Structural, optical, and vibrational properties

This paper reports on structural, optical, vibrational, and morphological properties of cobalt-doped CdS thin films, prepared by 90 keV Co+ implantation at room temperature. In this work, we have used cobalt concentration in the range of 0.34–10.8 at. %. Cobalt doping does not lead to the formation of any secondary phase, either in the form of metallic clusters or impurity complexes. However, with increasing cobalt concentration a decrease in the optical band gap, from 2.39 to 2.26 eV, is observed. This reduction is addressed on the basis of band tailing due to the creation of localized energy states in association with Urbach energy calculations. In addition, implantation gives rise to grain growth and increase in the surface roughness. Size and shape fluctuations of individual CdS grains, at higher fluences, give rise to inhomogeneity in strain. The results are discussed in the light of ion-matter interaction in the keV regime.

Microscopic observation of the grain structure of photoetched CdSe xS 1 − x solid solution and CdSeCdS mixed crystal semiconductor electrodes

Grain structure of the polycrystalline CdSe x S 1− x solid solution and CdSeCdS mixed crystal semiconductor electrodes has been investigated by a photoelectrochemical etching (photoetching) technique. Photoetching produces a unique surface morphology on these electrode surfaces. After photoetching under ordinary anodic polarization, the grain boundary is selectively dissolve and characteristic etching patterns appear on each grain surface depending on the cyrstallographic orientations. Under weak anodic polarization the grain bulk is selectively dissolved and grain boundaries are left undissolved. This finding makes it possible to observe directly the grain boundaries as isolated specimens separated from the bulk. Chemical composition of the grain boundaries and the bulk of CdSe x S 1 − x solid solutions with various x values were examined by analyzing X-ray diffractograms and electron diffraction patterns. The results have shown that the grain boundary has almost the same composition of that of the grain bulk for each x of CdSe x S 1− x solid solutions, whereas grain boundaries between CdSe and CdS grains in the mixed crystal pellets consist mainly of CdSe and/or CdS and sometimes of CdSe x S 1− x solid solution.

Single-crystal transformation of sphalerite CdS to cubic phaseCu xS

Thin films, 800–2000Åthick, of single-crystal (100) sphalerite CdS, which were grown epitaxially on single-crystal (100) NaCl substrates and possessed a density of 10 12–13 defects/cm 2, were converted toCu xS by dipping them in a hot CuCl solution. Transmission electron microscopy of the converted samples revealed a newCu xS cubic phase of high quality (10 6–7 defects/cm 2), while the films remained single crystal despite the transformation process. In-situ TEM studies of the CdS toCu xS transformation were carried out by heating CdS films with CuCl layers on their surfaces. They revealed that the initiation of transformation centres is accompanied by a stress relaxation, in which defect-free CdS grains are created, while the final film quality is determined by the elimination of these grains in the transformation process. The newCu xS cubic phase was found to be stable up to 330°C, even in co-existence with CdS.

The photoconduction mechanisms of screen printed CdS films

The photoconduction mechanisms of screen printed CdS films were investigated by their structure and photoresponse characterizations. It was found that Cl ions were incorporated into the CdS grains and resulted in both the Vcd(Cls) 2 complex defect, corresponding to a 0.68 eV trap level, and the Vcd(Cl)s complex defect corresponding to a 0.4 eV level. These values are consistent with those calculated from a chemical quasi-equilibrium approach. In further studies of the CdS films on solar cell applications, these defects were suspected to be responsible for the reduction of V oc , thereby degrading the conversion efficiency of such devices.

Thermal and plasma-chemical conversion of cadmium into α-CdS

This report deals with investigations of the thermal and plasma-chemical conversion of cadmium into α-CdS in order to produce large CdS grains. Thermal conversion in an atmosphere containing H 2S led to CdS crystallites about 10 μm in size in cadmium layers of thickness 3 μm. Plasma-chemical conversion was only effective in the surface of the cadmium layer had already been transformed into CdS by thermal conversion. It was mainly locally effective, i.e. the stoichiometry in the CdS layers produced by thermal conversion could be altered, as demonstrated by the change in the sheet resistance. The combination of thermal and plasma-chemical conversion enables us to transform the upper part of a cadmium layer into α-CdS and to control the resistance in a well-defined manner. Since the lower part of the layer remains fundamentally metallic, the adhesion to the substrate is guaranteed, and at the same time the metallic part of the layer can be used as a rear electrode.