Band Gap CdS Research Articles

Fabrication of a composite electrode: CdS decorated Sb–SnO 2/TiO 2-NTs for efficient photoelectrochemical reactivity

A CdS modified Sb–SnO 2/TiO 2-NTs hybrid electrode (CdS/Sb–SnO 2/TiO 2-NTs) with tailored architecture is studied for its photoelectrochemical applications. The visible light driven heterojunction photoelectrode was fabricated by anodic oxidation of titanium foil to obtain TiO 2 nanotube arrays firstly, and then solvothermally deposited with antimony doped tin oxide (Sb–SnO 2) thin coating onto the TiO 2 nanotube walls, followed by the successive ionic layer adsorption and reaction (SILAR) process to deposit certain amount of CdS nanoparticles. The obtained heterostructure was characterized by FE-SEM, EDS, XRD, XPS and UV–Vis techniques. The photoelectrochemical performance of this hybrid electrode demonstrated that the photocurrent under UV–Vis light illumination was remarkedly enhanced up to 17.48 mA/cm 2 under 1.0 V vs. Ag/AgCl. The enhanced photocurrent can be partially attributed to the narrow bandgap CdS nanoparticles, which allows for more visible light harvesting. On the other hand, the effect of Sb doping content on the photo-electrochemical activities of the composites was crucial. Thus a detailed investigation was carried out and the results showed that the nominal value of 2%(molar ratio) antimony loading exhibited the best photo-electrochemical performance. It is expected that the novel CdS/Sb–SnO 2/TiO 2 nanotube array hybrid system could serve as promising photoanode for effective hydrogen evolution from water splitting to meet the urgent problems of energy demand.

A Molecule to Detect and Perturb the Confinement of Charge Carriers in Quantum Dots

This paper describes unprecedented bathochromic shifts (up to 970 meV) of the optical band gaps of CdS, CdSe, and PbS quantum dots (QDs) upon adsorption of an organic ligand, phenyldithiocarbamate (PTC), and the use of PTC to map the quantum confinement of specific charge carriers within the QDs as a function of their radius. For a given QD material and physical radius, R, the magnitude of the increase in apparent excitonic radius (ΔR) upon delocalization by PTC directly reflects the degree of quantum confinement of one or both charge carriers. The plots of ΔR vs R for CdSe and CdS show that exciton delocalization by PTC occurs specifically through the excitonic hole. Furthermore, the plot for CdSe, which spans a range of R over multiple confinement regimes for the hole, identifies the radius (R∼1.9 nm) at which the hole transitions between regimes of strong and intermediate confinement. This demonstration of ligand-induced delocalization of a specific charge carrier is a first step toward eliminating current-limiting resistive interfaces at organic-inorganic junctions within solid-state hybrid devices. Facilitating carrier-specific electronic coupling across heterogeneous interfaces is especially important for nanostructured devices, which comprise a high density of such interfaces.

Nanowires improved charge separation and light utilization in metal-oxide solar cells

The power conversion efficiencies of electrodeposited Cu2O/ZnO p-n junction based solar cells are significantly improved by sandwiching a layer of spin-coated CdS nanowires (NWs) in between the electrochemically deposited Cu2O and ZnO layers. With the inclusion of the CdS NWs, there is observed a 5 fold improvement in the conversion efficiency, from 0.12% to 0.6%, as compared with that of the plain Cu2O/ZnO cell. The improvement is attributed to the enlarged p-n interface area and enhanced light harvesting, charge separation, and electron transport made possible by incorporating the single crystalline, relatively low band gap CdS NWs.

Band alignment of Cd (1 − x) Zn xS produced by spray pyrolysis method

Cd (1 − x) Zn x S thin films have been grown on glass substrates by the spray pyrolysis method using CdCl 2 (0.05 M), ZnCl 2 (0.05 M) and H 2NCSNH 2 (0.05 M) solutions and a substrate temperature of 260 °C. The energy band gap, which depends on the mole fraction × in the spray solution used for preparing the Cd (1 − x) Zn x S thin films, was determined. The energy band gaps of CdS and ZnS were determined from absorbance measurements in the visible range as 2.445 eV and 3.75 eV, respectively, using Tauc theory. On the other hand, the values calculated using Elliott-Toyozawa theory were 2.486 eV and 3.87 eV, respectively. The exciton binding energies of Cd 0.8Zn 0.2S and ZnS determined using Elliott-Toyozawa theory were 38 meV and 40 meV, respectively. X-ray diffraction results showed that the Cd (1 − x) Zn x S thin films formed were polycrystalline with hexagonal grain structure. Atomic force microscopy studies showed that the surface roughness of the Cd (1 − x) Zn x S thin films was about 50 nm. Grain sizes of the Cd (1 − x) Zn x S thin films varied between 100 and 760 nm.

Spectroscopic Analysis and Synthesis of Wide Band Gap CdS Quantum Dots Using Colloidal Synthesis Technique at Low Temperature

A broad range of II-VI materials has been investigated in order to produce light in the full visible range for optoelectronic applications. The present investigation was carried out for the spectroscopic analysis and synthesis of wide band gap cadmium sulfide nanoparticles. Large-band gap semiconductors have the added advantage in that; they can support higher electric field before breaking down, which means that they can be used for high-power electronic devices.Synthesis has been carried out using colloidal synthesis technique at low temperature. The size, stabilization and optical properties were studied using UV-vis Spectrophotometer and Spectroflourometer. Further, the structural studies of synthesized powder were carried out using X-ray diffraction technique; which also confirms the formation of desired product. The capping ligand and the impurities present in the sample were characterized by Fourier transform infra red spectroscopy. Synthesized CdS powder dispersed in aqueous media gave the value of 193 nm for the onset wavelength using UV-vis spectrophotometer, which is significantly blue-shifted compared to bulk CdS and shows the quantum confinement effect. From the onset wavelength the radius of CdS quantum dot calculated using the Brus equation was found to be ca. 0.7 nm.

Surface modification of highly ordered TiO2 nanotube arrays for efficient photoelectrocatalytic water splitting

An efficient visible-light-sensitive heterostructure photoanode of CdS nanoparticles/ZnO shell/TiO2 nanotube (CdS/ZnO–TiNT) arrays were investigated for solar water splitting. Highly ordered arrays of TiNT were grown vertically on Ti foil by electrochemical anodization. Both aqueous solution routes were used in turn to coat a thin recombination barrier of ZnO shell and narrow band gap CdS nanoparticles to the surface of the TiNT arrays. As a result of strong absorption within solar spectrum and effective suppression of electron-hole pair recombination in CdS/ZnO–TiNT arrays, a significant improvement in solar-to-hydrogen conversion efficiency from 0.39% to 1.30% was obtained.

Water-in-CO2 Microemulsions as Nanoreactors for Synthesizing CdS and ZnS Nanoparticles in Supercritical CO2

Semiconductor nanopaparticles of CdS and ZnS were synthesized by mixing two water-in-CO2 microemulsions with one containing S2- ions and the other containing Cd2+ or Zn2+ ions in the water core. Nanoparticle formation was monitored in situ by measuring their absorption spectra in the UV−vis range using a high-pressure fiber-optic system. The size of the nanoparticles formed in the microemulsion was found to depend on the water-to-surfactant ratio (W). At W = 6 and 12, the band gaps of CdS were calculated to be 3.50 and 3.15 eV, corresponding to particle radii of 1.4 and 1.7 nm, respectively. The ZnS particles synthesized at W = 12 showed a band gap of 4.26 eV, corresponding to a mean particle radius of 1.6 nm. This microemulsion-plus-microemulsion approach offers a simple method for synthesizing various nanoparticles in supercritical CO2 using water-soluble reagents as starting materials.

Effect of different capping environments on the optical properties of CdS nanoparticles in reverse micelles

CdS (E=2.5 eV) nanoparticles were synthesized in reversed micelles and capped with materials of different band gaps and charge compensation ability: Cd(OH)2 (4.0 eV), ZnS (3.2 eV), and CdO (2.3 eV). The preparations allowed the examination and comparison of the efficiencies of confinement of the electron and hole by the various capping materials. The absorption and emission spectra of the capped CdS nanoparticles suggest the following: (1) Efficient capping and strong quantum confinement are observed for CdS particles of small size (<3.8 nm), showing well-resolved photoexcitation spectra; (2) the confinement efficiency improves with capping thickness; (3) although ZnS has smaller band gap than Cd(OH)2, it was more efficient in surface-charge compensation and it is the best capping agent of the three materials; and (4) the similar band gaps of CdO and CdS resulted spectroscopically in strong mixing between the excited states of these materials. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 439–450, 1999

Optical and structural investigations on spray-deposited CdS films

CdS and indium doped CdS thin films have been prepared by the spray pyrolysis method. The optical band gaps of CdS and doped CdS were found to be 2.35 and 2.39 eV, respectively. The carrier concentration of doped CdS, calculated from an optical method, was found to be 7.5×1018 cm-3. The X-ray diffraction (XRD) analysis revealed that the films were polycrystalline and exhibited hexagonal structure. In order to calculate theoretical XRD intensity values for CdS, the structure factor ⌊F(hkl)⌋2 was derived. The temperature correction factor was employed for both Cd and S to correct intensity values. The theoretically calculated XRD intensity values of (hkl) coincided with those of experimental values. © 1998 Kluwer Academic Publishers

Electrodepositions of CdS, ZnS and Cd 1-xZn xS films

Thin films of CdS, ZnS and Cd 1-xZn xS, (0< x<1) have been electrodeposited from acidic bath using CdSO 4, ZnSO 4 and Na 2S 2O 3 at pH between 2 to 2.5 onto different substrates. The structural and optical properties of these films have been studied. It was found that CdS, ZnS and Cd 1-x Zn x S film could be electrodeposited from acidic bath. The XRD patterns showed that the films consist of mixed phases of CdS and ZnS with presence of free Cd, Zn and S. The optical properties showed that bandgaps of CdS and ZnS are 2.4 and 3.55 eV respectively. The bandgaps of Cd 1-x Zn x S films varied between 2.4 to 3.55 eV, depending upon Zn content in the film.

Preparation and properties of graded band gap CdS xTe 1−x thin film solar cells

A novel polycrystalline thin film solar cell based on alloys of CdS and CdTe is proposed and investigated. The structure consists of an n-type CdS xTer 1−x (0.5< x < 1) surface layer with a graded composition and hence a band gap which varies between 2.4 eV and 1.5 eV. The base layer consists of homogeneous p-type CdTe or p-type CdS 0.5Te 0.5 of uniform band gap. An apparatus suitable for the vacuum deposition of the proposed structure is described which incorporates a novel technique to control the composition of the graded and mixed CdS xTer 1−x layers. The conductivity of both layers is controlled by simultaneous co-evaporation of suitable dopant materials. Preliminary results are presented, showing that in agreement with theoretical expectations the device exhibits a very wide spectral response. Copper electrodes used for contracting the p-type base layers were found, however, to be unsuitable because of the migration of copper atoms along the grain boundaries. Because of a lack of suitable contacting materials a solar cell with a sufficiently low series resistance has not as yet been produced.