ZnO CdS Research Articles

Crystallite, optical, and photoelectrochemical properties of three-dimensional ZnO/CdS photoelectrodes synthesized by hydrothermal approach

A series of ZnO/CdS photoelectrodes with three-dimensional (3-D) structure were synthesized by two-step hydrothermal method. The photoelectrodes were characterized by x-ray diffraction, UV–vis absorption spectrum, and field emission scanning electron microscope. The results confirmed that all samples have similar hierarchical structure composed of CdS sheets and ZnO nanorod. In particular, the sample fabricated by the CdCl2 precursor as the Cd source gave the wider band gap and more porous structure. The photoelectrochemical performance of the samples was estimated by photocurrent density testing in the solution under irradiation. The sample prepared by the CdCl2 precursor exhibited higher photocurrent density than the other three samples due to its 3-D structure, wider band gap, and more porous morphology.

The origin of enhanced photoelectrochemical activity in metal-ion-doped ZnO/CdS quantum dots

To improve the performance of photoelectrochemical cells, various metal ions have been incorporated into different host semiconductor nanocrystals. However, such an approach raises two new questions that must be answered to design and optimize this new doping strategy: (i) How do we accurately evaluate and rank these dopants? and (ii) how do the dopant-related midgap states tailor the band structure of the host semiconductors and the dynamics of the photogenerated carriers? In the present work, we use nanosheet-nanorod ZnO as a free-standing substrate and CdS as a host for indium, copper, manganese, and cerium doping. Given the uniform physical properties and electronic structure of this environment, the photocurrent density and photoelectrochemical efficiency increase from ZnO–CdS (3.0 mA/cm2, 0.41%), ZnO–CdS–In (4.4 mA/cm2, 0.59%), ZnO–CdS–Cu (5.1 mA/cm2, 0.98%), ZnO–CdS–Mn (6.7 mA/cm2, 1.07%), to ZnO–CdS–Ce (8.9 mA/cm2, 1.9%). With the help of electrochemical cyclic voltammetry, open-circuit voltage-decay measurements, and fluorescence spectroscopy, the band edges tailored by the midgap states and the carrier dynamics are clearly determined. Furthermore, a complex relationship between these properties and the material performance is established for accurately deciphering the photoelectrochemical activity. By monitoring the ·OH concentration during irradiation, the dynamic photoelectrochemical catalytic activity is also clearly depicted by using ZnO–CdS–Mn and ZnO–CdS–Ce as model photocatalysts.

Cytotoxicity, Antioxidant, Antibacterial, and Photocatalytic Activities of ZnO-CdS Powders.

In this work, ZnO–CdS composite powders synthesized by a simple chemical precipitation method were thoroughly characterized. The morphological, structural, compositional, photocatalytical, and biological properties of the prepared composites were investigated in comparison with those of the pristine components and correlated with the CdS concentration. ZnO–CdS composites contain flower-like structures, their size being tuned by the CdS amount added during the chemical synthesis. The photocatalytic activity of the composites was analyzed under UV irradiation using powders impregnated with methylene blue; the tests confirming that the presence of CdS along the ZnO in composites can improve the dye discoloration. The biological properties such as antioxidant capacity, antibacterial activity, and cytotoxicity of the ZnO, CdS, and ZnO–CdS composites were evaluated. Thus, the obtained composites presented medium antioxidant effect, biocidal activity against Escherichia coli, and no toxicity (at concentrations less than 0.05 mg/mL for composites with a low CdS amount) for human fibroblast cells. Based on these results, such composites can be used as photocatalytic and/or biocidal additives for photoactive coatings, paints, or epoxy floors, which in their turn can provide a cleaner and healthier environment.

Theoretical investigation on the EPR parameters for VO2+ doped ZnO–CdS nanocomposites

The three optical absorption bands and electron paramagnetic resonance (EPR) parameters g factors (g||, g⊥) and the hyperfine structure constants (A||, A⊥) for VO2+ doped in the ZnO–CdS nanocomposites are theoretically investigated by using the perturbation formulas of these EPR parameters for a 3d1 ion under tetragonally compressed octahedral based on the cluster approach. In the calculation formulas, the relationships between local structure of the VO2+ dopants and the EPR parameters are established from the crystal-field parameters via the tetragonal distortion (characterized by relative tetragonal compression ratio τ), and the contributions to EPR parameters arising from the ligand orbital and spin-orbit coupling interactions via covalence effect are considered. Based on the studies, the V4+ dopants are found to show a relative compression ratio τ (≈ 2.1%) along the z-axis due to Jahn–Teller effect. Calculated results show reasonable agreement with the experimental data. Reasonableness of the proposed model is discussed.

A core@dual-shell nanorod array with a cascading band configuration for enhanced photocatalytic properties and anti-photocorrosion

ZnO–CdS–NiOx core@dual-shell nanorod arrays with cascade electron and hole transfer between them for high-performance photocatalytic H2 production.

Preparation and improved photoelectrochemical performance of ternary nano-heterostructure of Bi2S3/CdS/ZnO nanorods

Bi2S3/CdS co-sensitised ZnO nanorods (ZnO NRs) photoelectrode was successfully synthesised in four steps. ZnO nanoparticles were prepared using the sol-gel/spin-coating procedure; the hydrothermal method was used to prepare ZnO nanorods and CdS respectively; then, Bi2S3 was synthesised by SILAR method. The present study investigated the enhancement of photo response for CdS/ZnO NRs by applying a homogeneous covering of Bi2S3 with different cycles of dipping. Field emission-scanning of electron microscopic (FE-SEM), x-ray diffraction (XRD), Raman scattering, Ultraviolet-visible spectroscopy (UV–vis), and Photoelectrochemical performance were utilised to test the morphological, structural, optical, and photoactivity, respectively. The reason for the origin of improved photoelectrochemical performance of the Bi2S3/CdS/ZnO ternary films was basically attributed to the level of potential alignment between the CdS/ZnO and Bi2S3, which is beneficial for electron-hole pairs separation.

Visible light response and heterostructure of composite CdS@ZnS–ZnO to enhance its photocatalytic activity

In this work, CdS@ZnS–ZnO composites were prepared by the temperature-programmed hydrothermal method combined with the microwave-assisted hydrothermal method. The composite consisted mainly of cubic phase ZnS, hexagonal phase CdS and hexagonal phase ZnO, and presented a spherical structure with relatively uniform size and shape. There were heterostructures between CdS, ZnS and ZnO in the spherical structure. At the same time, the grain size, specific surface area, average pore diameter and pore volume of the composited material changed significantly, and its light absorption performance in the visible light region was obviously enhanced, and the photocatalytic activity was also greatly improved. Under ultraviolet light, CdS@ZnS–ZnO can degrade simulated pollutants by more than 90% within 20 min. In addition, the results of hydrogen evolution from photolysis of water showed that the ternary composite CdS@ZnS–ZnO had excellent hydrogen evolution capacity compared with pure CdS, ZnS and ZnO, whose hydrogen evolution amount can reach 3647 μmol g−1 in 8 h, was 298 times than that of P25. Moreover, it still maintained a high hydrogen evolution capacity after 4 cycles.

Hydrothermal synthesis of ZnO–CdS nanocomposites: Structural, optical and electrical behavior

ZnO–CdS nanocomposites with three different molar ratios of 25:75, 50:50 and 75:25 were synthesized by simple hydrothermal technique. Powder X-ray diffraction patterns confirmed the phase formation of ZnO and CdS in ZnO–CdS nanocomposites. The calculated crystallite size was found in the range of 33–38 nm and 15–21 nm for ZnO and CdS respectively. HRSEM images revealed flake-like morphology for all samples. Energy dispersive X-ray spectra confirmed the presence of all the elements. The estimated optical bandgap was found in the range of 3.71–3.35 eV. IR spectra confirmed the formation of stretching vibration in ZnO and CdS. Dielectric analysis was performed in order to study Ac conductivity, dielectric constant and dielectric loss. Photoconductivity studies revealed that ZnO–CdS nanocomposites material exhibited sound photo-response characteristics.

CdS nanocrystallites sensitized ZnO nanorods with plasmon enhanced photoelectrochemical performance

A novel architecture of CdS/ZnO nanorods with plasmonic silver (Ag) nanoparticles deposited at the interface of ZnO nanorods and CdS nanocrystallites, was designed as a photoanode for solar hydrogen generation, with photocurrent density achieving 4.7 mA/cm2 at 1.6 V (vs. RHE), which is 8 and 1.7 times as high as those of pure ZnO and CdS/ZnO nanorod films, respectively. Additionally, with optical absorption onset extended to ∼660 nm, CdS/Ag/ZnO nanorod film exhibits significantly increased incident photo-to-current efficiency (IPCE) in the whole optical absorption region, reaching 23.1% and 9.8% at 400 nm and 500 nm, respectively. The PEC enhancement can be attributed to the one-dimensional ZnO nanorod structure maintained for superior charge transfer, and the extended visible-light absorption of CdS nanocrystallites. Moreover, the incorporated plasmonic Ag nanoparticles could further promote the interfacial charge carrier transfer process and enhance the optical absorption ability, due to its excellent plasmon resonance effect.

Fabrication of CdS quantum dots sensitized ZnO nanorods/TiO2 nanosheets hierarchical heterostructure films for enhanced photoelectrochemical performance

A novel CdS/ZnO/TiO2 hierarchical heterostructure for photoelectrochemical (PEC) application was fabricated by growing ZnO nanorods (NRs) onto TiO2 nanosheets (NSs) with dominant high-energy {001} facets via hydrothermal method, subsequently coated CdS quantum-dots (QDs) sensitized layer via successive ionic layer adsorption and reaction (SILAR) technique. The as-prepared TiO2, ZnO/TiO2 and CdS/ZnO/TiO2 films were characterized by XRD, SEM, TEM, UV–vis absorption spectrum and EIS techniques to investigate the effects of ZnO NRs and CdS QDs on the structure, morphology, optical property and charge carrier transmission characteristics. The results demonstrated that the growth of ZnO NRs obviously enlarges the specific surface area, while the deposition of CdS QDs significantly enhances the light harvesting ability. And the heterojunctions formed among TiO2 NSs, ZnO NRs and CdS QDs effectively promote the photogenerated charge carrier transport and separation, simultaneously inhibit the photogenerated electron-hole recombination. Moreover, it is notable that ZnO NRs trend to grow on the low-energy {101} facets of TiO2 NSs due to the impact of crystal facet controlling agent, which can effectively shorten the electron conduction path-ways and facilitate the photogenerated electron-hole separation due to the lower band edge position of {101} facets than that of {001} facets. As a result, the photocurrent density of the optimized CdS/ZnO/TiO2 composite film enhances to an excellent value of 12.1 mA/cm2 at a potential of 0 V vs SCE, which is more than 27 times higher than that of bare TiO2 NSs films. In addition, the corresponding optimal condition and physical mechanism were also discussed in the paper.

Photodegradation enhancement of 2-chlorophenol using ZnO–CdS@CS nanocomposite under visible light

A facile micro-emulsion method used to prepare zinc oxide–cadmium sulfide (ZnO–CdS). Then, chitosan (CS) is utilized as a polymer matrix. Fourier-transform infrared spectroscopy, X-ray diffraction spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, dynamic light scattering, and photoluminescence is applied to evaluate the chemical structure, surface morphology, and the optical properties. Thermal gravimetric analysis is done to estimate the stability of the prepared nanomaterials. The photocatalytic activity of the synthesized nanoparticles was investigated via high-performance liquid chromatography analysis toward 2-chlorophenol degradation under visible light. Obviously, the spectrum of UV–visible for ZnO–CdS@CS nanostructure demonstrates a shift to longer wavelength (red shift) than the other prepared materials with the lowest band gap energy (2.10 eV). Also, the photocatalytic degradation of ZnO, ZnO@CS, ZnO–CdS, and ZnO–CdS@CS is 80%, 84%, 88%, and 98%, consecutively. The pseudo-first-order rate constant (K) for ZnO, ZnO@CS, ZnO–CdS, and ZnO–CdS@CS is 0.0039 min−1, 0.0048 min−1, 0.0058 min−1, and 0.0108 min−1, respectively. The CdS NPs and CS matrix (carbon and nitrogen “C–N”) introduce supplementary energy levels for excitation that causing a decrease in the band gap, increasing the extent of photodegradation in the visible region and assist for electron–hole separation. The reusability studied ZnO–CdS@CS manifested degradation under the same condition.

Enhanced photoelectrochemical performance with plasmon-induced hot electron injection of gold nanoparticle

Due to the capacity of efficiently harvesting and converting light energy, the property of localized surface plasmon resonance (LSPR) in the noble metal is employed as an efficient assistant in photoelectrochemical water splitting. In this study, the Au nanoparticles (NPs) were modified on the 1D ordered structure as the active sites of the SPR effect for the plasmon-enhanced photocatalytic performance. A maximum current density of 4.45 mA cm−2 at 1.2 V (versus RHE) was obtained in alkaline solution without a sacrificial agent for the ZnO–CdS–Au photoelectrode, which was 1.48 times higher than that of the ZnO–CdS nanorods without Au decoration, as well as the ~235% enhancement of the photoconversion efficiency. Compared to similar nanostructures previously reported, the present superior photoactivity response was ascribed to the injection process of the energetic hot electrons generated through the excitation and decay of surface plasmons in the Au NPs, resulting in the efficient separation and migration of photogenerated electron–hole pairs in ternary nanorod arrays. These excellent results have expanded the comprehensive understanding of hot electron migration in plasmonic NP-semiconductor nanocomposite, and provided a promising strategy for the rational design of the plasmonic-enhanced photocatalyst.

Time-resolution addressable photoelectrochemical strategy based on hollow-channel paper analytical devices

The construction of a photoelectrochemical (PEC) strategy for multi-component detection represents a great challenge in the field of sensing. To address these challenges, herein we presented a hollow-channel paper-based PEC analytical platform based on chemiluminescence (CL) addressable strategies excited PEC strategy for multiplexed sensing application. Sandwich-structured CdS quantum dots (QDs)/reduced graphene oxide (RGO)/ZnO nanorods arrays (NRAs) heterostructure where CdS serves as visible light sensitizers, RGO acts as an electron relay between ZnO NRAs and CdS QDs, were simply assembled on the gold nanoparticles modified paper working photoelectrode (Au-PWE). The PEC performance of the CdS/RGO/ZnO can be greatly improved benefiting from the formation of type II band alignment between CdS QDs and ZnO NR as well as the super charge collection and shuttling property of RGO. Multiplexed CL emission could be achieved through controlling the CL co-reagents transport. By the virtue of CL addressable technique and the excellent PEC activity of CdS/RGO/ZnO, a highly sensitive, and selective multiple microRNAs (miRNAs) quantification method was achieved. Such a tailored strategy would break the bottleneck of the current PEC detection techniques in multiplex tracing, as well as serve as a novel concept for designing multi-channel PEC sensing method.

UV–Visible light driven photocatalytic activities of CdS nanoparticles supported ZnO layers

CdS nanoparticles (NPs), ZnO layers, and CdS/ZnO composite were successfully prepared by a two-step chemical method. From the analysis of XRD, cubic CdS and the hexagonal ZnO phase were described for the synthesized samples. The CdS/ZnO composite exhibited larger surface area (99.9 m2 g−1) than pure CdS NPs (50.5 m2 g−1) and ZnO layers (67.3 m2 g−1), respectively. The band gap energies evaluated from the DRS results were 3.40, 2.13 and 1.78 eV for pure ZnO, CdS and CdS/ZnO composite respectively. The photocatalytic activity of the prepared materials was studied by the degradation of dye in aqueous solution under UV–Visible light exposure. CdS/ZnO composite exhibited the superior photocatalytic activity than ZnO and CdS. The photocatalytic performance was achieved 91.5% degradation in composite sample.

Electrophoretic deposition of single-source precursors as a general approach for the formation of hybrid nanorod array heterostructures

HypothesisSubjecting colloids to electric fields often results in (electrophoretic) deposition on conductive substrates. Dispersing a single-source precursor (SSP) of choice in an appropriate solvent, should allow its deposition on different substrates. The SSP-solvent interaction might play a role in the deposition (e.g., direction, rate, coverage). After thermal decomposition, the SSPs convert to the designed material, thus allowing formation of thin films or hybrid nanostructures. ExperimentsElectrophoretic deposition (EPD) was applied on two representative SSPs in different solvents. These SSPs were deposited onto substrates covered with vertically-aligned ZnO nanorod (NR) arrays. After thermal decomposition, hybrid nanostructures were obtained and their morphology and interfaces were characterized by electron microscopy, X-ray diffraction, UV–vis, and electrochemistry. FindingsTuning the organic dispersant-SSP interaction allows control over the final film morphology, which can result in coating and filling of NRs with metal-sulfides or metal-oxides after thermal decomposition of the SSP. These findings introduce a new facile method for a fast and large-scale uniform deposition of different (nanostructured) thin film semiconductors on a variety of substrates. We discuss the influence of the dispersant medium on the deposition of metallo-organic SSPs. As an example, the formed ZnO–CdS interface supports charge transfer upon illumination.

Sandwiched ZnO@Au@CdS nanorod arrays with enhanced visible-light-driven photocatalytical performance

The development of high-performance photocatalysts is central to efforts focused on taking advantage of solar energy to overcome environmental and energy crises. Integrating different functional materials artfully into nanostructures can deliver more efficient photocatalytic activity. Here, sandwiched ZnO@Au@CdS nanorod films were synthesized via successive ZnO nanorod electrodeposition, Au sputtering and CdS electrodeposition. The as-synthesized composites were characterized by UV–vis spectrophotometer, x-ray diffractometer, scanning and transmission electron microscopy. Their photocatalytic activity was assessed by degrading Rhodamine B solution under visible light irradiation. ZnO@Au@CdS exhibited better photocatalytic performance than ZnO@CdS throughout the visible light region, and the corresponding enhancement factor of Au nanoparticles was measured as a function of CdS loading amount, and it could reach 190% with CdS deposition for 1 min. The normalized rate constant could reach 0.387 h−1 for ZnO@Au@CdS-1min, which was equivalent to or better than results in reference photocatalysts. The enhancement mechanism of Au nanoparticles was estimated by comparing the monochromatic photocatalytic action spectra with the absorption spectrum of ZnO@Au@CdS, and it was mainly determined by incident photon energy. With selective excitation of Au nanoparticles by incident photons, the excited hot electrons in Au NPs are transferred to the conduction band of ZnO to boost photocatalytic reaction. With selective excitation of CdS, the enhanced interband absorption of CdS and relay station effect of Au nanoparticles should be responsible for the enhanced photocatalytic performance. Our work not only opens the door to the design of efficient supported photocatalysts, but also helps to understand the enhancement mechanism of LSPR effect on the photoelectric conversion of semiconductors.

Highly Efficient Photocatalyst Based on a CdS Quantum Dots/ZnO Nanosheets 0D/2D Heterojunction for Hydrogen Evolution from Water Splitting.

A novel CdS/ZnO heterojunction constructed of zero-dimensional (0D) CdS quantum dots (QDs) and two-dimensional (2D) ZnO nanosheets (NSs) was rationally designed for the first time. The 2D ZnO NSs were assembled into ZnO microflowers (MFs) via an ultrasonic-assisted hydrothermal procedure (100 °C, 12 h) in the presence of a NaOH solution (0.06 M), and CdS QDs were deposited on both sides of every ZnO NS in situ by using the successive ionic-layer absorption and reaction method. It was found that the ultrasonic treatment played an important role in the generation of ZnO NSs, while NaOH was responsible to the assembly of a flower-like structure. The obtained CdS/ZnO 0D/2D heterostructures exhibited remarkably enhanced photocatalytic activity for hydrogen evolution from water splitting in comparison with other CdS/ZnO heterostructures with different dimensional combinations such as 2D/2D, 0D/three-dimensional (3D), and 3D/0D. Among them, CdS/ZnO-12 (12 deposition cycles of CdS QDs) exhibited the highest hydrogen evolution rate of 22.12 mmol/g/h, which was 13 and 138 times higher than those of single CdS (1.68 mmol/g/h) and ZnO (0.16 mmol/g/h), respectively. The enhanced photocatalytic activity can be attributed to several positive factors, such as the formation of a Z-scheme photocatalytic system, the tiny size effect of 0D CdS QDs and 2D ZnO NSs, and the intimate contact between CdS QDs and ZnO NSs. The formation of a Z-scheme photocatalytic system remarkably promoted the separation and migration of photogenerated electron-hole pairs. The tiny size effect effectively decreased the recombination probability of electrons and holes. The intimate contact between the two semiconductors efficiently reduced the migration resistance of photogenerated carriers. Furthermore, CdS/ZnO-12 also presented excellent stability for photocatalytic hydrogen evolution without any decay within five cycles in 25 h.

Rational design of CdS@ZnO core-shell structure via atomic layer deposition for drastically enhanced photocatalytic H2 evolution with excellent photostability

Photocatalytic H2 evolution is highly attractive for converting abundant solar energy to valuable fuel. Herein, we report the use of an atomic layer deposition (ALD) technology to fabricate a new class of CdS@ZnO core-shell heterostructure. The rationally designed ultrathin ZnO shell not only allows the light to be absorbed by CdS core, but also provides an intimate heterojunction interface between ZnO shell and CdS core. The amount of ZnO shell coated on CdS core is finely tuned by the number of deposition cycles, and the obtained CdS@ZnO with 100 ALD deposition cycles displays the optimal photocatalytic H2 evolution rate of 11.36mmol/g/h. When Pt and PdS are used as the co-catalysts, the H2 evolution rates are further enhanced to 71.39 and 98.82mmol/g/h, respectively, which are 4.1 and 5.7 times higher than the highest reported value (17.40mmol/g/h) among CdS-ZnO catalyst systems. Detailed characterization reveals that the drastically enhanced photocatalytic activity can be attributed to not only efficient space separation of the photo-induced electrons and holes resulted from the formation of a direct Z-scheme photocatalytic system between crystalline ZnO and CdS, but also the intimate contact at molecular scale between the two semiconductors. Due to the coverage of ALD-prepared crystalline ZnO shell on CdS core, the CdS@ZnO core-shell structures exhibit excellent photostability.

Optimization of an electrode made from CdS–ZnO nanorods for hydrogen generation from photoelectrochemical splitting of water

Array structures of CdS-sensitized ZnO nanorods (NRs) were fabricated on conducting substrates of indium tin oxide (ITO) that serve as working electrodes in photoelectrochemical (PEC) cells for the generation of hydrogen by splitting of water. The ZnO NRs were synthesized by the hydrothermal method at low temperature with different growth times; structures of CdS-sensitized ZnO NRs were created by the dipping method with various dipping times to optimize the efficiency of splitting water. It was found that maximum photoconversion efficiency of an electrode made from CdS-sensitized ZnO NRs with a growth time of 3 h and a dipping time of 30 min under simulated solar irradiation of 100 mW cm−2 was about 2.7%. The rate of evolution of H2 gas generated from the water-splitting process was also measured. A maximum rate of 22 ml cm−2 was achieved after 1 h exposure, which is higher than with CdS–ZnO NR electrodes in previous studies.

Thin-film Cu(In,Ga)(Se,S)2 -based solar cell with (Cd,Zn)S buffer layer and Zn1−x Mg x O window layer

(Cd,Zn)S buffer layer and Zn1−xMgxO window layer were investigated to replace the traditional CdS buffer layer and ZnO window layer in Cu(In,Ga)(Se,S)2 (CIGSSe)-based solar cell. (Cd,Zn)S with band-gap energy (Eg) of approximately 2.6 eV was prepared by chemical bath deposition, and Zn1−xMgxO films with different [Mg]/([Mg] + [Zn]) ratios, x, were deposited by radio frequency magnetron co-sputtering of ZnO and MgO. The estimated optical Eg of Zn1−xMgxO films is linearly enhanced from 3.3 eV for pure ZnO (x = 0) to 4.1 eV for Zn0.6Mg0.4O (x = 0.4). The quality of the Zn1−xMgxO films, implied by Urbach energy, is severely deteriorated when x is above 0.211. Moreover, the temperature-dependent current density-voltage characteristics of the CIGSSe solar cells were conducted for the investigation of the heterointerface recombination mechanism. The external quantum efficiency of the CIGSSe solar cell with the (Cd,Zn)S buffer layer/Zn1−xMgxO window layer is improved in the wavelength range of 320–520 nm. Therefore, a gain in short-circuit current density up to about 5.7% was obtained, which is higher conversion efficiency of up to around 5.4% relative as compared with the solar cell with the traditional CdS buffer layer/ZnO window layer. The peak efficiency of 19.6% was demonstrated in CIGSSe solar cell with (Cd,Zn)S buffer layer and Zn1−xMgxO window layer, where x is optimized at 0.211. Copyright © 2017 John Wiley & Sons, Ltd.