CdS QDs Research Articles

CdS quantum dot sensitized anatase TiO2 hierarchical nanostructures for photovoltaic application

We report on solar cells based on CdS QD sensitized anatase TiO2 hierarchical nanostructures prepared via a facile and controlled hydrothermal process. To adjust the size of CdS QDs, the TiO2 surfaces were first treated with thiolactic acid which provides a binding site for Cd2+ ions. CdS QDs with controllable size are grown onto the TiO2 hierarchical nanostructures. The CdS QDs/TiO2 hierarchical heterojunction nanocomposites display a red shift from the ultraviolet to the visible light region with a longer wavelength for the absorption edge. The matching of band edges between CdS and TiO2 to form a semiconductor heterojunction plays an important role in effective separation of light induced electrons and holes, providing a promising photoanode for its wide absorption spectrum, high electron injection efficiency, and fast electron transference. Under UV-vis irradiation, both CdS and TiO2 are excited. Under visible light irradiation, photons are captured by CdS QDs, with the photogenerated electron–hole pairs rapidly separated into electrons and holes at the interface between TiO2 and CdS QDs. In addition to this, more photoelectrons are generated from TiO2 by harvesting UV photons. TiO2 collects the photoelectrons from CdS and passes them through to the back contact. The holes (from both CdS and TiO2) generated in the process are transferred to the solid–liquid interface. Under one sun illumination, the solar cells based on CdS QDs/TiO2 hierarchical heterojunction nanocomposites with a smaller average size of 7.6 nm for CdS QDs show a maximum short circuit current density of 6.09 mA cm−2 and a conversion efficiency of 3.06%. The performance improvement of the solar cells based on CdS QDs/TiO2 hierarchical nanostructures can be attributed to the unique microstructure characteristics and the bandgap energy matching between the CdS QDs and TiO2.

Synergistic recombination suppression by an inorganic layer and organic dye molecules in highly photostable quantum dot sensitized solar cells

An inorganic layer and dye molecules have synergistically suppressed the recombination in a quantum dot sensitized solar cell (QDSSC), by the design of a structure featured TiO2-CdS-ZnS-N3 (N3: RuL2(NCS)2 (L = 2,2'-bipyridyl-4,4'-dicarboxylic acid)) hybrid photoanode. When fabricated into solar cells, a cobalt complex-based electrolyte rather than an iodine-based one was employed to obtain an impressive photostability for the devices. Raman and Photoluminescence (PL) measurements revealed that not only the CdS QDs were passivated by both the inorganic layer of ZnS and dye molecule of N3, but also N3 served as an efficient hole scavenger for the CdS QDs due to a type-II energetic alignment between the two sensitizers. This role of N3 as an intermediary in hole extraction from CdS QDs to the electrolyte was further proven by the significant photovoltaic performance improvement of the CdS sensitized solar cell after ZnS deposition and N3 co-sensitization. The overall efficiency of the solar cell incorporated with TiO2-CdS-ZnS-N3 film exceeded the sum of the single CdS QDs and N3 dye sensitized solar cells. This enhancement is ascribed mainly to the synergistic recombination suppression by the inorganic layer ZnS and N3 co-sensitization, leading to inhibited recombination and increased electron lifetime, as illustrated by the electrochemical impedance spectroscopy (EIS) analysis.

Au@poly(acrylic acid) plasmons and C60 improve the light harvesting capability of a TiO2/CdS/CdSeS photoanode

A quantum dot solar cell (QDSC) was developed using a photoanode with CdS and CdSeS QDs as co-sensitizers, fullerene (C60) clusters as electron conduits and Au nanoparticles capped by poly(acrylic acid) or Au@PAA as plasmons, which were all tethered to TiO2 as the semiconducting scaffold. The formation of CdSeS QDs and hexagon-shaped Au@PAA, each with a face-centered cubic lattice, was confirmed by high-resolution transmission electron microscopy. Evidence for rapid electron injection from CdS or CdSeS to C60 and Au@PAA, as well as for inter-dot electron transfer from CdS to CdSeS, were obtained from fluorescence quenching and emission decay lifetime analyses. The fastest electron deactivation pathway was achieved in the TiO2/CdS/CdSeS/C60/Au@PAA electrode, which demonstrated the ability of C60 and Au@PAA to serve as efficient electron sinks due to their high electronic conductivities and their favorably poised Fermi levels. Au@PAA particles induced absorbance increments of CdS and CdSeS QDs, thus indicating that plasmonic improvements could be exploited in the photoanode using the same. High electron transfer rates and plasmonic effects translated into a high overall power conversion efficiency of 3.61% for a QDSC constructed with this composite as the photoanode, aqueous S2− as the electrolyte and carbon-nanotube-based counter electrode. The efficiencies of cells without Au@PAA and C60 were lower by ∼19% and 23%, respectively, confirming the synergy between C60 and Au@PAA in producing larger photocurrents. Aligned conduction bands in the TiO2/CdS/CdSeS/C60/Au@PAA electrode that permitted electron transfer by cascade, broader spectral utilization by using CdS and CdSeS and near field plasmonic enhancement by Au@PAA due to its proximity with the QDs, cumulatively manifested in an incident photon to current conversion efficiency of 48%, which is 25% greater than that of the cell without Au@PAA. Our study shows that significant improvements in QDSC performances can be realized by simply developing more efficient designs for photoanode configurations.

Photocatalytic reduction of o-chloronitrobenzene under visible light irradiation over CdS quantum dot sensitized TiO2.

The photocatalytic activity of CdS/P25 hybrid catalysts was studied under visible-light irradiation. The CdS quantum dots sensitized P25 (CdS QDs-P25) showed extremely enhanced activity in the reduction of o-chloronitrobenzene (o-CNB) by comparing to CdS-P25 prepared by the direct deposition-precipitation method in the presence of HCOOH. The synergistic effects between CdS QDs and P25 were beneficial for the separation of photogenerated carriers in space and thus the combination of photoelectrons and holes was prevented, and the CdS QDs could provide more photocharges than CdS due to the particle size effect. Furthermore, the process of photocatalytic reduction in the present system was investigated, under visible-light irradiation, the photogenerated electrons transferred from the valence band (VB) to the conduction band (CB) of CdS QDs, and injected into the CB of inactivated P25. Meanwhile, the holes generated in the VB of CdS QDs could oxidize HCOO(-) to give ˙CO2(-) and H(+). Then, o-CNB was reduced to o-chloroaniline (o-CAN) by the couple of e(-) and ˙CO2(-) with H(+). It is a significant method and a green process for hydrogenation of nitro compounds, which may have great potential applications in the reduction of various organic chemicals.

CdSe–CdS quantum dots co-sensitized ZnO hierarchical hybrids for solar cells with enhanced photo-electrical conversion efficiency

We have developed a facile method to fabricate CdSe-CdS quantum dot sensitized hierarchical ZnO nanostructures for quantum dot sensitized solar cells (QDSCs) by combining a hydrothermal method, successive ionic layer adsorption and chemical reaction (SILAR) techniques. The method consists of the growth of the ZnO hierarchical structure on ITO substrates via a hydrothermal method and the layer deposition of double quantum dots CdSe and CdS by SILAR. The CdSe-CdS QDs co-sensitized ZnO hierarchical structures show enhanced light absorption in the entire visible light range. The photovoltaic performance of QDCSs based on CdSe-CdS QDs co-sensitized ZnO hierarchical structures was evaluated. As photoanodes for QDSCs, the CdSe-CdS QDs double-sensitized ZnO hierarchical structures demonstrate an increased Jsc and improved power conversion efficiency of up to 1.39%. Under light illumination, photons are captured by QDs, yielding electron-hole pairs that are rapidly separated into electrons and holes at the interface between the ZnO and the QDs. The electrons are transferred to the conduction band of ZnO and the holes are released by redox couples in the liquid polysulfide (S(2-)/Sx(2-)) electrolyte, resulting in greatly improved photo-electrical conversion efficiency of QDSCs. The results suggest that it is very promising and feasible to enhance light absorption, carrier generation, and effective carrier separation via band engineering by CdSe-CdS QDs co-sensitization, and the method reported here displays a great potential for applications to be scaled up.

PH-assisted shape-controlled synthesis of homogeneous hybrid CdS-cystine composites

Homogeneous hybrid CdS-cystine composites with different shapes such as side spindle-like, bundle-like, laminar and egg-like have been fabricated in mild conditions using CdS QDs as inorganic unit and l-cysteine as organic unit, in which l-cysteine was oxidated to generate cystine as an organic matrix. It is demonstrated that the morphologies of composites were controllable with pH of reactant solution and concentration of l-cysteine. The morphological evolution and photoluminescence properties of composites have also been explored.

Conjugation of chlorinated carbon nanotubes with quantum dots for electronic applications

Most of the reported methods for the development of carbon nanotube-quantum dot (CNT-QD) conjugates either use the application of cross-linkers or the treatment of CNT to form highly unstable acylated/thiolated precursor. The present work explores the synthesis of CNT-QD conjugates through simple electrostatic binding between the chlorinated CNTs and amine functionalized CdS QDs. This novel route helps in minimizing the chemical treatment to the CNTs surface. Absorption, infrared and Raman spectroscopic and microscopic studies have proved successful chlorination of the multiwalled CNT and their subsequent electrostatic attachment with QDs. The synthesized nanoconjugate has been electrodeposited as thin film that shows semiconducting behavior.

The influence of CdS quantum dots incorporation on the properties of CdO thin films

The aim of our work is to obtain nano-structured cadmium oxide (CdO) thin films by sol-gel spin coating method and to investigate the effects of cadmium sulfide quantum dots (CdS QDs) doping on the structural modification and surface morphology evolution. X-ray diffraction (XRD) results show that the intensities of the peaks of the crystalline phase increase with the increase in CdS QDs concentrations. From scanning electron microscopy (SEM) images, the distinct variations in the morphology of the thin films were also observed. In addition, the evolution of surface morphology, roughness and granularity has been characterized by atomic force microscopy (AFM). Moreover, we have performed the optical characteristics of the thin films such as transparency, energy band gap and Urbach tail. The optical band gap of the thin films increases from 2.23 to 2.51 eV with the increase in CdS QDs concentrations due to the Moss–Burstein effect. The enhanced values of the transparency, energy band gap and crystallity indicate that addition of CdS QDs can be used to modify the optical, structural and morphological properties of CdO thin films.

Post-annealing of CdS/ZnS-assembled TiO2 films for photoelectrochemical solar cells

CdS/ZnS-assembled TiO2 films are annealed at 350 °C in an inert Ar ambient before and after deposition of the ZnS overlayer and are denoted as the types I and II, respectively. As-grown and the annealed CdS-quantum-dot (QD)-assembled TiO2 samples without a ZnS overlayer were also prepared for comparison. Annealing of CdS-QD-assembled TiO2 without ZnS significantly reduced the electron lifetime due to the coalescence of CdS QDs on the TiO2 surface. The electron lifetime of the annealed CdS-QD-assembled TiO2 was recovered because of ZnS overlayer due to its being an intermediate layer and to the energy barrier effects at the TiO2/electrolyte and the CdS QD/electrolyte interfaces. The resultant photoelectrochemical solar cell (PEC) with the type I film exhibited better energy conversion efficiency than the PECs without the ZnS. The cell performance of the PEC with the type II film was further improved, as compared to that with the type I film. This can be attributed to the additional effect (improved interfacial contact at the CdS/ZnS interface) of the postannealing after the formation of the ZnS overlayer.

Conversion efficiency enhancement of CdS quantum dot-sensitized electrospun nanostructured TiO2 solar cells by organic dipole treatment

Here we fabricated quantum dot-sensitized solar cells (QDSCs) based on electrospinning of unique TiO2 nanostructures and subsequent CdS QDs deposition via successive ionic layer adsorption and reaction (SILAR) followed by dipole treatment (DT). It was found that by treating with 4-methoxy benzenethiol for 24h, an overall conversion efficiency of 1.17% was achieved under AM1.5 illumination which corresponds to a dramatic 100% enhancement compared with that of untreated cells. The significant photovoltaic improvement was attributed to the upward shifting of CdS QDs energy levels for efficient charge separation and effective electron transport in electrospun TiO2 nanostructures which is confirmed by electrochemical impedance spectroscopy.

Glutathione-assisted hydrothermal synthesis of CdS-decorated TiO2 nanorod arrays for quantum dot-sensitized solar cells

Abstract We report a facile one-step hydrothermal route for the synthesis of efficient CdS-decorated TiO 2 nanorod photoanodes with the aid of glutathione (GSH). The CdS QDs are covalently linked to the surface of TiO 2 nanorods through the GSH bifunctional molecule which also acts as stabilizer and sulfur source in this one-step fabrication. The results from FESEM and HRTEM images reveal that the CdS QDs are well dispersed on the surfaces of TiO 2 nanorods. Surface photovoltage measurements demonstrate that the CdS QDs sensitization enhances the visible spectral absorption of TiO 2 nanorod arrays, as well as their separation efficiency of photo-induced electron–hole pairs. As a result, the CdS-decorated TiO 2 nanorod heterostructure shows a maximum power conversion efficiency (PCE) of 0.278% under illumination at 100 mW cm −2 by employing polysulfide electrolyte.

3D-ZnO Nanorods Photoelectrodes for Quantum-Dot Sensitized Solar Cells

In this study, the three dimensional ZnO (3D-ZnO) nanorods were synthesized by a simple chemical solution method, which were used as photoelectrodes, and CdS QDs were deposited on the surface of the 3D-ZnO nanorods to act a light absorber by using the successive ionic layer adsorption and reaction (SILAR) method. The photovoltaic performances of the semiconductor quantum dots sensitized solar cells based on CdS QDs/3D-ZnO photoelectrodes were investigated. A maximum 5.06 mA/cm2short circuit current density and 1.03% conversion efficiency under one sun illumination has been achieved. These results demonstrate that the CdS QDs-sensitized 3D-ZnO nanorods photoelectrode has a potential application in solar cells.

Cysteamine CdS quantum dots decorated with Fe3+ as a fluorescence sensor for the detection of PPi

A new sensitive and selective fluorescence sensor for the detection of pyrophosphate (PPi) in aqueous media based on the Fe3+ decorated cysteamine CdS QDs ([Cys-CdS QDs]–Fe3+) was proposed. The presence of PPi can induce the fluorescence quenching of [Cys-CdS QDs]–Fe3+ due to the high formation constants between the phosphate group and Fe3+. Because the complex between Fe3+ and PPi acts as an efficient quencher, the concentration of PPi can be evaluated by tracking the degree of fluorescence quenching. The fabricated sensor was optimized to obtain the best sensor selectivity and sensitivity. Under optimal conditions, a linear relationship between the fluorescence response and the concentration of PPi was established in the range of 0.5–10μM. The limits of detection and quantitation for PPi were found to be 0.11 and 2.78μM, respectively. Furthermore, the proposed sensor exhibited high selectivity toward PPi relative to other common anions. The proposed sensor was successfully applied to the detection of PPi in urine samples with satisfactory results.

CdS/CdSe quantum dots co-sensitized solar cells with Cu2S counter electrode prepared by SILAR, spray pyrolysis and Zn–Cu alloy methods

Abstract Herein, CdS/CdSe co-sensitized TiO 2 photoanodes for QDSSCs were prepared by successive ionic layer adsorption and reaction (SILAR), spray pyrolysis and zinc–copper alloy processes. The HR-TEM, SEM, EDS, XRD, UV–vis and I – V curve analyses were performed to investigate the surface and structural properties of the prepared electrodes and the efficiencies of the fabricated QDSSs. Employing different methods for preparation of Cu 2 S counter electrode affected the performance of QDSSCs under one illumination of sun (100 mW/cm 2 ) so that various conversion efficiencies ( μ ) of 3.18, 0.341 and 0.266% were measured in alloy, SILAR and spray pyrolysis methods, respectively. Therefore, among these methods, the zinc–copper alloy process with higher efficiency is preferred that gives fill factor (ff) and short circuit density ( J SC ) values of 0.44 and 11.69 mA/cm 2 . The HR-TEM images showed that CdS and CdSe QDs are in close contact with TiO 2 nanoparticles and the sizes of CdS and CdSe QDs are about 5 and 6 nm, respectively. The energy-dispersive X-ray spectroscopy (EDS) measurement confirmed that CdS and CdSe QDs are successfully deposited on the surface of the TiO 2 film. The band gaps estimated from Tauc plots using UV–vis spectra vary from 3.1 eV (without CdS and CdSe, bare TiO 2 ) to 2.38 eV (TiO 2 /CdS (3)/CdSe). The SEM images of Cu 2 S counter electrodes prepared by zinc–copper alloy indicated nanosheets with high porosity that is much suitable for injection of electrolyte while in two other approaches (SILAR and spray pyrolysis), large (∼50–70 nm) and small (∼10–17 nm) nanoparticles were observed without high porosity.

Surface treatment properties of CdS quantum dot-sensitized solar cells

The dye-sensitized solar cells (DSSCs) are attractive due to their low cost and promising efficiency. One of the research perspectives in the respective field is to replace the expensive and photodegradable ruthenium metal-based dyes. Present work describes a simple, modified in situ route designed by mimicking the adsorption principle of dyes in DSSCs for surface modification and linking of CdS-Quantum Dots (QDs) to TiO2 electrode. An organic compound 2-mercaptoethanol (ME) was used as a surface modifying and linking agent. By following this route it was expected to get a well assembled layer of CdS QDs for better cell performance but performances were not as expected. The main reason for low photocurrent density is the partial coverage of QDs surface by ME and the spatial distance between QDs and TiO2 electrode. Additional surface treatment of the CdS QDs sensitized TiO2 electrode resulted in an increase in the photocurrent density and photovoltage. This indicates that ME is not an effective capping agent and thus partially covers the QDs surface. The remaining sites, not covered by ME were passivated by sulfur ions in the ionic solution.

Selective turn-on fluorescence sensor for Ag+ using cysteamine capped CdS quantum dots: Determination of free Ag+ in silver nanoparticles solution

Cadmium sulfide quantum dots capped with cysteamine (Cys–CdS QDs) were demonstrated as a selective fluorescence probe for sensing of free trace silver ions. The fluorescence intensity of the Cys–CdS QDs can be enhanced only in the presence of free Ag+ and the fluorescence spectrum was slightly red shift from the original spectra. In addition, the fluorescence intensities were linearly increased upon increasing Ag+ concentration. At the optimized condition for Ag+ detection, when adding other metal ions to the Cys–CdS QDs solution, fluorescence spectra of Cys–CdS QDs did not change significantly revealing good selectivity of the sensors towards Ag+. The working linear concentration range was found to be 0.1–1.5µM with LOD of 68nM. The proposed sensor was then applied to detect free Ag+ in the silver nanoparticles solution. The results showed that the proposed sensor can be efficiently used with good accuracy and precision providing the simple method for detection of free Ag+ in silver nanoparticles of quality control products.

TiO2-Au nanocomposite materials modified photoanode with dual sensitizer for solid-state dye-sensitized solar cell

The dual-photosensitizer consisting of cadmium sulphide quantum dots (CdS QDs) and basic blue-3 (BB-3) was employed in a solid-state dye-sensitized solar cell (DSSC) composed of aminosilicate sol-gel functionalized titanium dioxide-gold nanocomposite material (EDAS/(P25-Au)nps) photoanode and degussa-TiO2 (P25) nanoparticles incorporated poly(ethylene oxide) polymer electrolyte (PEO-P25-KI-I2). The UV-vis spectral analysis revealed that a large part of visible light is absorbed by the dual-photosensitizer (BB-3 + CdS QDs), particularly in the red region of the solar spectrum, and as a result the DSSC showed improved solar to electrical energy conversion efficiency of 0.37% under simulated AM 1.5G at 100 mW cm−2 solar irradiation. The photovoltaic performance of (BB-3 + CdS QDs) sensitized solid-state DSSC was compared to the BB-3 sensitized solar cell, in which the former exhibited around ∼3-fold increase in the overall solar to electrical energy conversion efficiency than that of the later. The solar to electrical energy conversion efficiency of the standard N719 dye sensitized DSSC was found to be higher than the dual sensitizer (BB-3 + CdS QDs) employed solid-state DSSC.

Detection of silver(I) ion based on mixed surfactant-adsorbed CdS quantum dots

Mixed cationic and anionic surfactants were adsorbed on cadmium sulfide quantum dots (CdS QDs) capped with mercaptoacetic acid. The CdS QDs can be extracted into acetonitrile with 98 % efficiency in a single step. Phase separation only occurs at a molar ratio of 1:1.5 between cationic and anionic surfactants. The surfactant-adsorbed QDs in acetonitrile solution display stronger and more stable photoluminescence than in water solution. The method was applied for determination of silver(I) ion based on its luminescence enhancement of the QDs. Under the optimum conditions, the relative fluorescence intensity is linearly proportional to the concentration of silver(I) ion in the range between 50 pmol L−1and 4 μmol L−1, with a 20 pmol L−1 detection limit. The relative standard deviation was 1.93 % for 9 replicate measurements of a 0.2 μmol L−1 solution of Ag(I).

Enhancement mechanism of photovoltaic performance for ZnO/CdS heterostructure photoelectrodes via post-annealing treatment

High-density and vertically-aligned cadmium sulfide quantum dots (CdS QDs) sensitized Zinc oxide nanorods (ZNRs) heterostructure as photoelectrode had been used to fabricate the quantum dots sensitized solar cells. The effect of post-annealing temperature was investigated on their photovoltaic performance. The power conversion efficiency was improved significantly after annealing treatment, which can be attributed to two factors: on the one hand, the increase in grain size of CdS quantum dots after annealing results in the enhancement of absorption intensity and expansion of absorption spectral range; on the other hand, the post annealing treatment can reduce the defect concentration and highly improve the crystallization quality of the ZNRs/CdS photoelectrodes. The highest power conversion efficiency of the solar cell can achieve 1.14% with a JSC of 5.27 mA/cm2 and a VOC of 0.51 V under the white light illumination intensity of 100 mW/cm2.

Preparation, optical properties and solar cell applications of CdS quantum dots synthesized by chemical bath deposition

We reported on temperature-dependent photoluminescence (PL) studies of CdS quantum dots (QDs, ~5 nm in diameter) grown onto TiO2 nanorod arrays by chemical bath deposition. By constructing a liquid-junction solar cell, a power conversion efficiency of 0.88 % was demonstrated. In addition, we observed anomalous emission behavior, specifically a ‘Λ’-shaped (blueshift–redshift) temperature dependence of the peak energy for CdS related PL with increasing temperature. From a study of the integrated PL intensity as a function of temperature, it was revealed that thermionic dissociation of excitons (carriers) out of local potential minima into higher energy states was the dominant mechanism leading to the quenching behavior of the QDs related PL. At 110 K, the localized excitons were totally dissociated and converted to the free excitons. Our results shed light on the exciton-dissociation process in CdS QDs and can be used for the proposed solar cell application.