Fabricating CdS/BiVO4 and BiVO4/CdS heterostructured film photoelectrodes for photoelectrochemical applications

The photoelectrochemical (PEC) properties of heterostructured CdS/BiVO₄ and BiVO₄/CdS film electrodes on conducting glass for hydrogen production under visible light were investigated. These two types heterostructured film electrodes were prepared using spin coating method and ultrasonic spray pyrolysis method. The structural analyses of the prepared films were determined by using XRD, SEM, EDX and UV–vis. Photoelectrochemical measurements were carried out in a convenient three electrodes cell with 0.5 M Na₂SO₃ aqueous solution. In order to investigate band gap influence of electrode PEC property, a series ITO/Cd_1−xZn_xS/BiVO₄ and ITO/BiVO₄/Cd_1−xZn_xS (x = 0 ∼ 1) film electrodes were also synthesized. After PEC test, a maximum photocurrent density from ITO/CdS/BiVO₄ film electrode was confirmed. The maximum photocurrent density, 3 times and 113 times as that of single CdS film electrode and single BiVO₄ film electrode, respectively. Incident photon to current conversion (IPCE) of as prepared film electrodes were measured and the value were 65% (ITO/CdS/BiVO₄), 22% (single CdS film) and 10% (ITO/BiVO₄/CdS) at 480 nm with 0.3 V external bias. Comparison with ITO/BiVO₄/CdS electrode and single Cd_1−xZn_xS electrodes, the heterostructured ITO/CdS/BiVO₄ electrode can effectively suppress photogenerated electron-hole recombination and enhance light harvesting. Therefore, the ITO/CdS/BiVO₄ electrode gave the maximum photocurrent density and IPCE value.     

Highly stable CdS-modified short TiO2 nanotube array electrode for efficient visible-light hydrogen generation

A highly stable photoelectrocatalytic electrode made of CdS-modified short, robust, and highly-ordered TiO₂ nanotube array for efficient visible-light hydrogen generation was prepared via sonoelectrochemical anodization and sonoelectrochemical deposition method. The short nanotube electrode possesses excellent charge separation and transfer properties, while the sonoelectrochemical deposition method improves the combination between CdS and TiO₂ nanotubes, as well as the dispersion of CdS nanoparticles. Different characterization techniques were used to study the nanocomposite electrode. UV-vis absorption and photoelectrochemical measurements proved that the CdS coating extends the visible spectrum absorption and the solar spectrum-induced photocurrent response. Comparing the photoactivity of the CdS/TiO₂ electrode obtained using sonoelectrochemical deposition method with others that synthesized using plain electrochemical deposition, the current density of the former electrode is ∼1.2 times higher that of the latter when biased at 0.5 V. A ∼7-fold enhancement in photocurrent response is obtained using the sonoelectrochemically fabricated CdS/TiO₂ electrode in comparison with the pure TiO₂ nanotube electrode. Under AM1.5 illumination the composite photoelectrode generate hydrogen at a rate of 30.3 μmol h⁻¹ cm⁻², nearly 13 times higher than that of pure titania nanotube electrode. Recycle experiments demonstrated the excellent stability and reliability of CdS/TiO₂ electrode prepared by sonoelectrochemical deposition. This composite electrode, with its strong mechanical stability and excellent combination of CdS and TiO₂ nanotubes, offers promising applications in visible-light-driven renewable energy generation.     

Effects of surface modification on the PEC performance of electrochemically deposited n-CdTe films

CdTe thin films were electrodeposited on Ni substrates from aqueous solutions containing CdSO₄, TeO₂ and H₂SO₄ with an interchangeable rotating disk electrode. The variations in the composition of the CdTe films with cathodic potentials and heat treatment temperatures were studied by the polarographic method. The deposition and annealing parameters had been optimized to yield a good photoelectrochemical performance. After surface modification, the conversion efficiencies were 0.61% and 5.3% for the cells p-CdTe/SnCl₂ (sat.), 0.2M HCl/C and n-CdTe/1 M Na₂S, 1 M S, 1 M NaOH/C, respectively.     

Noble metal-free cobalt oxide (CoOx) nanoparticles loaded on titanium dioxide/cadmium sulfide composite for enhanced photocatalytic hydrogen production from water

In this present paper, cobalt oxide (CoO_x) is studied as an effective cocatalyst in a photocatalytic hydrogen production system. CoO_x-loaded titanium dioxide/cadmium sulfide (TiO₂/CdS) semiconductor composites were prepared by a simple solvothermal method and characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS), and X-ray photoelectron spectroscopy (XPS). Photocatalytic hydrogen production was studied using the as-synthesized photocatalysts in aqueous solution containing sodium sulfide (Na₂S)/sodium sulfite (Na₂SO₃) as hole scavengers under visible light irradiation (λ > 400 nm). The optimal cobalt content in CoO_x-loaded TiO₂/CdS composite is determined to be 2.1 wt% and the corresponding rate of hydrogen evolution is 660 μmol g⁻¹ h⁻¹, which is about 7 times higher than TiO₂/CdS and CdS photocatalysts under the same condition. Visible light-driven photocurrents of the semiconductor composites were further measured on a photoelectrochemical electrode, revealing that the photocorrosion of CdS can be prevented due to the presence of TiO₂–CoO_x.     

Dye sensitization of antimony-doped CdS photoelectrochemical solar cell

Sb-doped CdS single crystal was used as a photoanode to fabricate a photoelectrochemical solar (PECS) cell. The three organic dyes; eosin, thymol blue and rhodamin 6G were used as sensitizers in (PECS) cell. In the absence of the dye, the results showed that with Sb-doped CdS single crystal electrode, a higher power conversion efficiency 9.27% has been achieved compared to 5.7–7.4% for pure crystal. Application of the dye in PECS cell increases the efficiency to about 13%. The efficiency reaches its maximum value when the dye concentration is (2.5×10⁻⁵)M, sufficient to cover the surface of the semiconductor electrode with a continuous monolayer of the dye. Exceeding this value resulted in a gradual decrease of the efficiency from its maximum value. Mott–Schottky plots gave a doping density of 3.14×10¹⁷ cm⁻³ and a space charge width of 4.95×10⁻⁶ cm for the sample used. A flat-band potential equal to −0.84 V, independent of both frequency and pH, was also predicted. Cyclic voltammetry (c.v.) measurements showed an anodic current peak at 0.4 V vs. SCE. The disappearance of this peak after excess addition of the reducing agent Na₂S, indicates that this peak is due to the PEC corrosion of the semiconductor electrode.     

Co-deposition of TiO2/CdS films electrode for photo-electrochemical cells

A densely packed TiO₂ thin film onto an indium doped–tin oxide (ITO) substrate was synthesized at room temperature by chemical deposition and a CdS thin film was deposited onto the pre-deposited TiO₂ film by a doctor blade route (powder of CdS was obtained from chemical deposition). TiO₂/CdS film was annealed at 300 °C for 1 h in air for crystallinity improvement. The first grown TiO₂ film was nanocrystalline, whereas the CdS film was polycrystalline as evidenced by X-ray diffraction (XRD) and selected area electron diffraction (SAED). Scanning electron microscopy (SEM) images show formation of mono-dispersed CdS spherical grains onto compact, densely packed spherical nanocrystalline grains of TiO₂. The TiO₂/CdS bilayer film was used in a photo-electrochemical cell as a working electrode, and a platinum electrode as a counter electrode (0.1 M lithium iodide electrolyte) under 80 mW/cm² light illumination intensity.     

A 2D/1D TiO2 nanosheet/CdS nanorods heterostructure with enhanced photocatalytic water splitting performance for H2 evolution

TiO₂ with exposed (001) facets were composited with CdS nanorods to construct 2D/1D heterojunction. As comparison, P25 with mainly exposed (101) facets were employed to combine with CdS nanorods. The 2D/1D heterojunction of TiO₂ nanosheets and CdS nanorod displayed 3.7 times higher hydrogen generation than that of P25/CdS composites. The results indicated that TiO₂ with exposed (001) facets were favorable for enhancing the photocatalytic activity of CdS via optimizing the heterojunction between TiO₂ and CdS. Photoluminescence and photoelectrochemical characteristics results demonstrated that the 2D-TiO₂/1D-CdS heterojunction exhibits higher separation efficiency of photoinduced carriers and superior electron transfer ability. This work exemplifies that heterojunction modification is an effective strategy to improve the efficiency of the photocatalyst composites.     

Photoelectrochemical behavior of chemically deposited CdSe and coupled CdS/CdSe semiconductor films

Photoelectrochemical effects at chemically deposited CdSe thin films (2000 Å) coupled with as-prepared and air annealed (250°C) CdS films have been investigated by monitoring open-circuit voltage (V_oc) and short-circuit current density (I_sc) at varying incident light intensities and for different heat-treatments temperatures. Two consecutive chemical baths were used in the coupled system. Each bath has been optimized in earlier studies for the deposition of highly photosensitive CdS and CdSe thin films. The photoelectrochemical behavior of single and coupled films was investigated in ferricyanide redox couples. The enhanced short-circuit photocurrent of the as-deposited CdS/CdSe system, despite their lower photosensitivity, indicated that charge separation improved in the coupled system. The role of post-deposition thermal treatments in improving the photoelectrochemical cell characteristics and stability of coupled semiconductors was investigated. Excellent I–V properties were obtained for CdSe and CdS₂₅₀/CdSe photoelectrodes annealed at 280°C. For the coupled system: V_oc=960 mV; I_sc=8.6 mA/cm²; fill factor (ff)=0.53 and cell efficiency (η)=4.2%. The linearity of V_oc/ln(I_L) and I_sc/I_L plots supports the Schottky–Mott model for these interfaces. The stability of the coupled photoanode is superior to that of the CdSe only-film for the initial 3 h.     

Facile fabrication and optimization of bowl-like ZnO/CdS nano-composite thin films with hierarchical nanopores and nano-cracks for high-performance photoelectrochemistry

A special nano-structured composite ZnO/CdS thin film with hierarchical nanopores and nano-cracks has been synthesized by a facile two-step method for the first time, in which both loadings of ZnO and CdS are optimized. We first fabricated the hierarchical nanoporous ZnO thin film through rapid gas/liquid interface assembly and layer-by-layer transfers of bowl-like ZnO nanoparticles for thirteen times. The ZnO nanobowls are prepared by a simple solution chemical reaction without using any templates. After annealing, the assembled ZnO film is sensitized with CdS nanoparticles by successive ionic layer adsorption and reactions for six cycles. Nano-cracks form for the ZnO/CdS nano-composite film by calcination, which is due to the different thermal expansion behavior between the ZnO film and the CdS layer. The facilely optimized ZnO/CdS films can serve as a promising photoanode in a photoelectrochemical cell, and it can generate a saturated photocurrent density as high as 7.8 mA cm^?2 at ?0.9 V (vs. Hg|Hg₂SO₄|saturated K₂SO₄) under visible light illumination of 100 mW cm^?2 in an aqueous solution of 0.5 M Na₂S, corresponding to a solar-to-electricity conversion efficiency of 6.6%.     

Enhancement of photoelectrochemical activity of nanocrystalline CdS photoanode by surface modification with TiO2 for hydrogen production and electricity generation

Nanocrystalline CdS films on the FTO glass substrates using doctor-blade method were used as photoanodes in two different photoelectrochemical (PEC) cells for hydrogen production and electricity generation. The influence of surface modification by overcoating with a thin amorphous TiO₂ on the PEC performance of CdS films has also been investigated. It was found that TiO₂ content have a dominant effect on the performance of PEC cells. The optimized PEC cells with CdS/TiO₂ (1.8 wt.% TiO₂ content) electrode showed a 4-fold increase in hydrogen production and a five times enhancement of the cell efficiency (a maximum power conversion efficiency of 2.7%) compared to that of the unmodified one. Furthermore, surface modification has similar effect on these two PEC cells. The electrochemical investigation suggests that the TiO₂ layer on CdS reduces the interfacial charge recombination and induces a downward shift of the flat band potential in both PEC cells. This work reveals that the interfacial charge recombination is essentially critical for both hydrogen production and electricity generation.     

ZnO/CdS nanorod arrays decorated by layered double hydroxides for efficient solar water oxidation

Developing efficient photoanode without either hole scavenger or noble‐metal co‐catalyst is extremely important for solar water oxidation. However, in most cases, hole scavengers such as Na₂S and/or Na₂SO₃ are usually used to inhibit carrier recombination while blocking the real water oxidation process. To avoid this, non‐noble metal co‐catalyst, that is, CoNi layered double hydroxides (LDHs), has been successfully deposited onto ZnO/CdS core‐shell nanorod arrays for efficient photoelectrochemical water oxidation in KOH aqueous solution. By changing the deposition cycle numbers from 10 to 25 for CdS and tuning the electrodeposition charge quantities from 0.1 to 0.4 C for CoNi LDHs layers, the effect of thickness or loading of CdS and CoNi LDHs on light absorption and photocurrent was systematically studied. A transformation from M(OH)₂ to MOOH (M═Co, Ni) is found when the photoexcited holes in CdS are transferred to CoNi LDHs, making the co‐catalyst more active than before. These high active M³⁺ sites, combined with the high absorption over visible light, fast‐charge transfer along the 1D ZnO tunnels, and low‐charge transfer resistance at photoanode/electrolyte interfaces, endow the ZnO/CdS/CoNi LDHs nanoarrays photoanode with a photocurrent of 2.87 mA/cm² at 1.0 V_RHE and incident photon‐to‐current efficiency of 36% at 450 nm. The work provides an effective strategy toward efficient solar water oxidation free of hole scavengers. Copyright © 2017 John Wiley & Sons, Ltd.     

Metal chalcogenide based photocatalysts decorated with heteroatom doped reduced graphene oxide for photocatalytic and photoelectrochemical hydrogen production

Heteroatom (N, B and P) doped reduced graphene oxide (RGO)-metal chalcogenide nanocomposites (RGO-Cd_0.60Zn_0.40S) were prepared by the solvothermal method, and then they were characterized with X-ray diffraction, Raman spectroscopy, transmission electron microscopy, high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy, UV–Vis diffuse reflectance spectroscopy and photoluminescence techniques. Doping of RGO with heteroatoms of N, B and P increased charge-transfer capability of nanocomposites and thus, improved both photocatalytic and photoelectrochemical hydrogen production activities of them. N-doped RGO-Cd_0.60Zn_0.40S photocatalyst exhibited the highest photocatalytic hydrogen production rate (1114 μmolh⁻¹ g⁻¹) in photocatalytic (PC) system amongst other and it was 1.5 times higher than that of RGO-Cd_0.60Zn_0.40S photocatalyst. Having a current density of 0.92 mAcm⁻², photoelectrochemical hydrogen production activity of N-RGO-Cd_0.60Zn_0.40S electrode was found to be 3 times higher than RGO-Cd_0.60Zn_0.40S photoelectrode without any applied bias potential under visible light irradiation in photoelectrochemical system. In general, these results clearly showed that heteroatom doping of RGO led to promising materials for renewable hydrogen production in the photocatalytic and photoelectrochemical systems.     

Electrodeposition using non-aqueous solutions at 170 °C and characterisation of CdS, CdSxSe(1−x) and CdSe compounds for use in graded band gap solar cells

CdS, CdS_xSe_(1−x) and CdSe compounds have been grown at 170 °C using electrodeposition from an electrolyte containing ethylene glycol as the solvent. The materials were grown for x=0, 0.22, 0.50, 0.76 and 1.00, and the x values quoted here are obtained from the XRF measurements. The resulting materials were characterized by optical absorption method for determination of band gap variation, and by XRD for bulk structure variation. It has been demonstrated that the band gap could be varied from 1.7 eV for x=0 (CdSe) to 2.4 eV for x=1 (CdS) by varying the parameter x. Bulk structure remains as hexagonal, but the corresponding lattice spacing gradually increases as the smaller S⁻² ions are replaced by larger Se⁻² ions. The photoresponse shown in photoelectrochemical cell demonstrates that all compounds grown are suitable for solar cell applications.     

Simultaneously efficient light absorption and charge transport of CdS/TiO2 nanotube array toward improved photoelectrochemical performance

CdS has been widely used to modify TiO₂-based photoanodes for photoelectrochemical (PEC) water splitting. Due to the poor interface contact between chalcogenides and oxides, however, such CdS modified TiO₂ materials usually exhibit inefficient separation and transport of charges, leading to an unsatisfactory efficiency during the PEC water splitting process. Addressing this issue, we herein report a CdS/TiO₂ nanotube array (CdS/TNA) photoanode that was fabricated through a successive ion layer absorption and reaction (SILAR) method with an additional subsequent annealing. This post-annealing process is essential to enhance the interface contact between the CdS and the TNAs, resulting in an accelerated transfer of photogenerated electrons from the CdS to the TNAs. In addition, the post-annealing also improves the light absorption capability of the CdS/TNA photoanode. The simultaneous enhancement of charge transport and light absorption provided by the post-annealing is essential for improving the PEC performance of the CdS/TNA photoanode. The CdS/TNA photoanode obtained by this strategy exhibits a much enhanced PEC performance in water splitting, and its photocurrent density and solar-to-hydrogen conversion efficiency could reach 4.56 mA cm⁻² at 1.23 V vs. reversible hydrogen electrode and 5.61%, respectively. This simple but effective route can provide a general strategy for obtaining high-performance oxide-based photoelectrodes.     

CIS/CdS/ZnO/ZnO:Al modified photocathode for enhanced photoelectrochemical behavior under visible irradiation: Effects of pH and concentration of electrolyte solution

In this paper, the CuInS₂ films were firstly modified with CdS and CdS/ZnO/ZnO:Al/Au layers in order to improve the photoelectrochemical (PEC) water splitting efficiency. The CuInS₂ photoelectrode was synthesized by electrodeposition method as a facial and green method, on the FTO substrate. The effects of pH and concentration of Na₂S electrolyte solution on the photocurrent density of photoelectrode samples were studied. As a p-n junction photocathode, the CIS/CdS/ZnO/ZnO:Al/Au photoelectrode indicates the enhanced PEC activity. The photocurrent density of CIS/CdS/ZnO/ZnO:Al/Au photoelectrode reaches to 1.91 mA/cm², while is about 2.5 times higher than that for CuInS₂ film at pH = 8 (−0.6 V vs Ag/AgCl). The formation of a p-n junction at the CuInS₂ photoelectrode surface not only reduces the recombination of electron-hole pairs but also increases the PEC response and water splitting performance of the as-prepared CIS/CdS/ZnO/ZnO:Al/Au photoelectrode.     

Photocorrosion-less stable heterojunction photoanode for efficient visible-light driven solar hydrogen generation

In this work, a heterostructure CdS/TiO₂ nanotubes (TNT) photoelectrode is decorated with Ni nanoparticles (NPs) to enhance hydrogen generation via the photoelectrochemical method. Herein, we report a systematic study of the effect of Ni NPs heterostructure photoelectrode to improve light absorption and photoelectrochemical (PEC) performance. The fabricated photoelectrodes were evaluated for photoelectrochemical hydrogen generation under simulated sunlight. The optimized Ni/CdS/TNT photoelectrode exhibited an improved photocurrent density of 6.5 mA cm^?2 in poly-sulfide aqueous media at a low potential of 0 V. Owing to the enhanced photocurrent density, Ni NPs also played a significant role in improving the stability of the photoelectrode. The synergistic effect with semiconductor ternary junction incites the surface plasmon resonance (SPR) for light-harvesting to enhance photoelectrochemical hydrogen generation.     

Stabilizing effect in nano-titania functionalized CdS photoanode for sustained hydrogen generation

Efficient and stable photoanode has been fabricated by the surface functionalization of the nanostructured film. For this, the surface of spray deposited CdS thin film was modified through bi-functional molecule mediated chemisorption of TiO₂ nanoparticles (NP). Consequently, a systematic control over efficiency and photoanode stability against corrosion has been investigated. An in-depth quantitative analysis of the photocorrosion of these photoanodes is further studied using chronoamperometry, X-ray photoelectron spectroscopy and induced coupled plasma spectroscopy. TiO₂ NP modified photoanodes show an enhanced efficiency and a stability. For photoelectrochemical (PEC) systems, the stability factor (∑) has been defined for the first time based on the time dependent chronoamperometry, which clearly demonstrates that ∑_modified >> ∑_bare. The modified photoanode shows an improved Incident Photon to Current Efficiency of 22% than the bare CdS (∼8%) electrode. It gives an enhanced solar-to-hydrogen conversion efficiency of STH ∼ 0.7% w.r.t bare CdS (0.2%) under AM 1.5G solar simulator, at 0.2 V/SCE. Improved stability of more than nine hours and enhanced efficiency is attributed to the controlled passivation of CdS surface through TiO₂ NP (5 nm), and inhibition of the charge recombination. Superior and stable performance of modified photoelectrode has been validated by higher and stable hydrogen evolution over modified electrode.     

Photovoltaic cells based on pulsed electrochemically deposited SnS and photochemically deposited CdS and Cd1−xZnxS

CdS/SnS and Cd_1−xZn_xS/SnS solar cells were fabricated. SnS films were deposited by the pulsed electrochemical deposition method using an aqueous solution containing SnSO₄ and Na₂S₂O₃. CdS and Cd_1−xZn_xS window layers were deposited by using the photochemical deposition method using an aqueous solution containing CdSO₄, ZnSO₄ and Na₂S₂O₃. Both the techniques were simple, economical and advantageous for fabricating cheap solar cells. The fabricated cells showed rectification characteristics. The photovoltaic properties were measured under AM 1.5 illumination. The cells with the Cd_1−xZn_xS window layer show larger photocurrent than those with the CdS window layer.     

(Photo) electrochemical investigations on spray deposited n-CdIn 2Se4 thin film/polysulphide/c photoelectrochemical solar cell1

Cadmium indium selenide (n-CdIn ₂Se₄) thin films have been synthesized by spraying the mixture of an equimolar solutions of cadmium chloride [CdCl₂], indium trichloride [InCl₃] and selenourea [(NH₂)₂CSe] in aqueous media onto preheated fluorine doped tin oxide (FTO) coated glass substrates at optimized parameters of substrate temperature and solution concentration. The photoelectrochemical (PEC) cell configuration of n-CdIn₂Se/(l MNaOH + 1 MNa₂S + 1 M S)/C has been used for investigating the current—voltage (I–V) characteristics under dark and white light illumination, photovoltaic output, spectral response, photovoltaic rise and decay characteristics. The PEC study reveals the thin film of CdIn₂Se₄ exhibits n-type conductivity. The junction quality factor in dark (n _d) and light (n _l), series and shunt resistance (R _s and R _sh), fill factor (FF) and efficiency (η) for the cell have been estimated. The observed efficiency and FF of PEC solar cell is found to be 1.95 and 0.37% respectively. Mott-Schottky plot shows the flat-band potential (V _fb) of n-CdIn₂Se₄/(l M NaOH + 1 M Na₂S + 1 M S)/C cell to be—0.655 V/SCE.     

Photoluminescence characteristics of CdS layers deposited in a chemical bath and their correlation to CdS/CdTe solar cell performance

In this work, we study CdS films processed by chemical bath deposition (CBD) using different thiourea concentrations in the bath solution with post-thermal treatments using CdCl₂. We study the effects of the thiourea concentration on the photovoltaic performance of the CdS/CdTe solar cells, by the analysis of the I–V curve, for S/Cd ratios in the CBD solution from 3 to 8. In this range of S/Cd ratios the CdS/CdTe solar cells show variations of the open circuit voltage (V_oc), the short circuit current (J_sc) and the fill factor (FF). Other experimental data such as the optical transmittance and photoluminescence were obtained in order to correlate to the I–V characteristics of the solar cells. The best performance of CdS–CdTe solar cells made with CdS films obtained with a S/Cd ratio of 6 is explained in terms of the sulfur vacancies to sulfur interstitials ratio in the CBD–CdS layers.