CdSe quantum dots sensitized nanoporous hematite for photoelectrochemical generation of hydrogen

A novel system of CdSe quantum dots (QDs) sensitized porous hematite (α-Fe₂O₃) films has been investigated as a potential photoelectrode for hydrogen generation via photoelectrochemical (PEC) splitting of water. Before sensitization, nanoporous hematite thin films were prepared by spray pyrolysis. Characterizations for crystalline phase formation, crystallite size, absorption spectra, and flatband potential were carried out to analyze PEC data. Loading time of sensitizer to hematite thin films was found to be crucial in affecting its PEC properties. Film having sensitizer loading time as 42 h exhibited best photocurrent density of 550 μA cm⁻² at 1.0 V versus SCE. Current study, for the first time, explores the possibility of using low band gap QDs sensitization on a low band gap film, hematite in PEC splitting of water.     

Role and prospects of green quantum dots in photoelectrochemical hydrogen generation: A review

Eco-friendly quantum dots (QDs) can be termed green QDs which stand as an attractive choice to modify the properties of known semiconductors in the direction of getting efficient photoelectrodes for solar-induced photoelectrochemical (PEC) splitting of water, due to their peculiar properties. Thus, it is of high significance to analyze their merit/demerit as an effective scaffold in PEC cell. QDs are known for their excellent optical properties however, the coupling of green QDs with semiconductor is not only useful in improving absorption characteristics but also promotes charge transfer. This review has undertaken the critical analysis on the worldwide research going on the green QDs modified photoelectrode with respect to their optical, electrical & photoelectrochemical properties, role, usefulness, efficiency, and finally the success in PEC system for hydrogen production. Various methods on the facile synthesis & sensitization techniques of green QDs available in the literature have also been discussed. Further, recent advances on the development of green QDs based photo-electrode, along with major challenges of using green QDs in this field have also been presented.     

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.     

Near-infrared CdSexTe1-x@CdS “giant” quantum dots for efficient photoelectrochemical hydrogen generation

Colloidal quantum dots (QDs) have attracted a lot of attention due to their unique optoelectronic properties. They have been widely used as building block materials for solar technologies such as solar cell, and photoelectrochemical (PEC) water splitting. Hydrogen generation by using QDs as photocatalysts has emerged a promising application in PEC devices. However, it is still very challenging to obtain high-efficiency PEC devices due to the limited absorption wavelength of QDs and the existence of surface traps which prohibit the efficient charge transfer. In this work, we synthesized ternary CdSe_xTe_1-x/CdS (CdSeTe/CdS) “giant” QDs to extend the light absorption to near infrared, matched well with Sun's spectrum. The as-synthesized CdSeTe/CdS “giant” QDs exhibit quasi-type II band alignment as confirmed by its long lifetime and red-shifted emission peak compared with bare CdSeTe QDs. The wide absorption range of “giant” core/shell QDs and their long lifetime can improve the efficient absorption of Sun's spectrum and charge transfer. As a proof-of-concept, a PEC device using QDs sensitized TiO₂ mesoporous thin film as a photoanode was used for hydrogen production. The corresponding photocurrent density was increased to 3.0 mA/cm² with the introduction of CdS shell, which is 1.5 times higher than the PEC device using CdSeTe QDs. This study indicates that ternary or polynary alloyed core/shell QDs can be used as promising optoelectronic materials for applications of PEC devices.     

Highly efficient photocatalytic conversion of gas phase CO2 by TiO2 nanotube array sensitized with CdS/ZnS quantum dots under visible light

With the massive consumption of fossil fuels, energy crisis and effectively reducing CO₂ to curb global warming have become urgent and severe problems in the world. Photocatalytic conversion of CO₂ technology which can convert CO₂ into combustible compounds by using solar energy can solve both of the problems mentioned above. However, the photocatalytic conversion of CO₂ exhibits too low efficiency, especially under visible light. So, in order to improve the photocatalytic efficiency, the composite photocatalysts of TiO₂ nanotube array (TNTA) sensitized by CdS/ZnS quantum dots (QDs) were successfully prepared by anodization method and successive ionic layer adsorption and reaction (SILAR) method in this work. And the composite photocatalysts exhibited a high performance for photocatalytic conversion of gas-phase CO₂ to methanol under visible light. X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscope (TEM), and X-ray photoelectric spectroscopy (XPS) were employed to characterize the ingredients and morphologies of the synthesized photocatalysts. And, UV–vis diffuse reflectance spectra (UV–Vis DRS) revealed that CdS/ZnS QDs enhanced the photo-absorption of composite photocatalyst in the visible light region. The main product methanol yield of CdS/ZnS-TNTA under visible light was 2.73 times that of bare TNTA when TNTA was treated by 10 SILAR cycles. Meanwhile, the product yield first increased before decreasing with the increase of the CO₂ flow rate. And the greatest product yield reached up to 255.49 nmol/(cm²-cat·h) with the increase of light intensity. The reaction mechanism was discussed in this paper. This high performance for photocatalytic reduction of CO₂ was primarily attributed to the CdS/ZnS QDs sensitization, which widens the response wavelength range of the catalyst to include visible light and partly inhibits the recombination of electron-hole pairs.     

Nanostructured Zn-Fe2O3 thin film modified by Fe-TiO2 for photoelectrochemical generation of hydrogen

Nanostructured semiconductor thin films of Zn-Fe₂O₃ modified with underlying layer of Fe-TiO₂ have been synthesized and studied as photoelectrode in photoelectrochemical (PEC) cell for generation of hydrogen through water splitting. The Zn-Fe₂O₃ thin film photoelectrodes were designed for best performance by tailoring thickness of the Fe-TiO₂ film. A maximum photocurrent density of 748 μA/cm² at 0.95 V/SCE and solar to hydrogen conversion efficiency of 0.47% was observed for 0.89 μm thick modified photoelectrode in 1 M NaOH as electrolyte and under 1.5 AM solar simulator. To analyse the PEC results the films were characterized for various physical and semiconducting properties using XRD, SEM, EDX and UV–Visible spectrophotometer. Zn-Fe₂O₃ thin films modified with Fe-TiO₂ exhibited improved visible light absorption. A noticeable change in surface morphology of the modified Zn-Fe₂O₃ film was observed as compared to the pristine Zn-Fe₂O₃ film. Flatband potential values calculated from Mott–Schottky curves also supported the PEC response.     

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.     

Surface modification of ZnO microrod arrays films by ion-exchange approach and their photoelectrochemical performances

ZnO microrod arrays films with the surface modification by two steps ion-exchange approach have been investigated as photoanodes in photoelectrochemical (PEC) cells. X-ray diffraction, Raman, scanning electron microscope, energy dispersive X-ray detector, UV–vis techniques and PEC measurement have been used in the pristine and surface modified ZnO microrod films. The results show that ZnS and CdS layer can be deposited on ZnO microrod surface through a two steps ion-exchange procedure. What's more, it is found that ion-exchange method is a simple approach to adjust CdS content on the samples surface via changing experimental temperature. Consequently, the PEC property of films can be improved through optimizing CdS content on the ZnO microrods surface. In this experiment, it is found that the optimized condition for preparing film is 70 °C (first step) and 100 °C (second step). These results suggest that surface tuning via ion-exchange method should represent a viable strategy to further improve the efficiency of ZnO microrods photoanodes.     

Improvement of photocatalytic hydrogen generation from CdSe/CdS/TiO2 nanotube-array coaxial heterogeneous structure

The fabrication and characterization of CdSe/CdS/TiO₂ nanotube-array coaxial heterogeneous structure that has potential applications in photocatalytic water splitting and toxic pollutants degradation are investigated. CdSe(top)/CdS(under) double-layer is conformally deposited onto TiO₂ nanotubes by successive ionic layer adsorption and reaction (SILAR) and electrochemical atomic layer deposition (ECALD), respectively, for the CdS under layer and the CdSe top layer. Such double sensitized TiO₂ nanotubular photoelectrode exhibits significant enhancements in photoconversion efficiency, visible light response, and efficient hydrogen generation. The detailed synthesis process and the surface morphology, phase structure, elemental analysis, and photoelectrochemical properties of the resulting films with the CdSe/CdS/TiO₂ nanotube-array coaxial heterogeneous structure are discussed. The photoconversion efficiency of 9.47% and hydrogen generation rate of 10.24 ml h⁻¹ cm⁻² were observed. Both values are a 7-fold enhancement compared with that of the pure TiO₂ nanotube. The as-prepared photoelectrode presents potential application for industrialized photocatalytic hydrogen generation in the future.     

On the synthesis of nanostructured TiO2 anatase phase and the development of the photoelectrochemical solar cell

Unlike the rutile; the anatase phase of TiO₂ has not been extensively employed for fabrication of PEC cells primarily due to the difficulty in the synthesis of a stable anatase structural variant. The present investigation is focused on the synthesis of the anatase phase and its use as a photoelectrode of high efficiency PEC Solar Cells. TiO₂, in the nanostructured form, has been prepared by the hydrolysis of Titanium (IV) isopropoxide solution. The nanostructured TiO₂ (anatase) stable phase has been synthesised by sintering the synthesised film at ∼500°C with a heating rate of 1°C/min for a duration of 3 h in argon. The films of nanostructured TiO₂ anatase phase have been used as photoelectrodes in PEC solar cells.An improvement in TiO₂(ns) anatase phase photoelectrode carried out in the present work corresponds to admixing In₂O₃ to improve the spectral response. It has been found that admixing In₂O₃ with TiO₂(ns) anatase phase improves the solar spectral response. The structural, microstructural, optical, and photoelectrochemical properties of the TiO₂(ns) anatase phase and TiO₂(ns) anatase-In₂O₃ photoelectrode have been studied. The response of TiO₂(ns) anatase phase bearing photoelectrode based PEC solar cell corresponds to V_OC ≈ 460 mV, I_SC ≈ 2.4 mA/cm² and for its In₂O₃ doped version, these are V_OC ≈ 640 mV and I_SC ≈ 10.4 mA/cm².     

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.     

PEC electrode of ZnO nanorods sensitized by CdS with different size and its photoelectric properties

The gradient PEC electrodes of ZnO nanorods sensitized by CdS with different size were fabricated via successive ionic layer adsorption and reaction (SILAR) method and applied in photochemical water splitting. The concentrations of reaction solution and SILAR cycles were investigated in the synthetic process and the working mechanism of the gradient PEC electrode was suggested. The results showed that the hydrogen generation efficiency of 4.88% was achieved for the ZnO/CdS gradient PEC electrode constructed by decreasing of the CdS quantum dots size on ZnO nanorods due to the improved absorption and appropriate energy gap structure, which was confirmed by enhanced absorption spectrum. The expected products have potential application in photoelectrochemical water splitting.     

Series resistance in n-MoSe2 (TMDC)-based PEC solar cells

Transition metal dichalcogenides (TMDCs) have been noticed as potentials for the PEC solar cells because they are inherently stable against the electrolytic environment. Since MoSe₂—a member of group VI TMDCs—possesses an optically matching band gap of around 1.4 eV, it holds relatively more promise as a better material for such devices. In this article, the authors report their investigations on PEC solar cells fabricated using n-MoSe₂ crystals grown by a direct vapour transport technique. The photoconversion characteristics of n-MoSe₂/I₂/I⁻/pt PEC solar cells were investigated under polychromatic illumination from an incandescent lamp at various intensities. Since the series resistance of TMDC-based PEC solar cells is expected to be high, it may be one of the major parameters blocking the available power on photoconversion from such devices. Efforts have been made here to estimate its value. In addition, the effect of thermal treatment of the photoelectrode on the series resistance was also investigated. It has been found that the series resistance decreases from 4.01 K ohms to 1.93 K ohmson controlled thermal treatment of the photoelectrode. This is accompanied by a marked increase in the photoconversion efficiency (from 3% to around 12%). Thus, it can be concluded that the contribution of the series resistance in TMDC-based PEC solar cells is quite significant and can be reduced by giving controlled thermal treatment to the photoelectrodes.     

ZnO/Cu2O-decorated rGO: Heterojunction photoelectrode with improved solar water splitting performance

In present work, we report a facile fabrication process to improve the photoelectrochemical (PEC) performance of ZnO-based photoelectrodes. In order to achieve that, the Cu₂O nanocubes are cathodic-deposited on the as-prepared ZnO nanorods. Then rGO nanosheets are electrodeposited on the ZnO/Cu₂O heterostructures. The fabricated photoelectrodes are systematically studied in detail by different characterization techniques such as powder X-ray diffraction, micro-Raman, X-ray photoelectron spectroscopy, ultraviolet diffused reflectance spectroscopy and photoluminescence spectroscopy analysis. Morphologies of the fabricated photoelectrodes are investigated through electron microscopy in scanning and transmission mode. To evaluate the PEC performance of the fabricated photoelectrodes, the line scan voltammetry (LSV) measurement is performed using a three-electrode system in 0.5-M Na₂SO₄ electrolyte solution under stimulated light illumination at 100 mW/cm² from a 300-W Xenon Arc lamp coupled with an AM 1.5G filter using a three-electrode system. The photocurrent measurement demonstrates that the photoelectrodes based on ZnO/Cu₂O/rGO possess enhanced PEC performance compared to the pristine ZnO and ZnO/Cu₂O photoelectrodes. The photocurrent density of ZnO/Cu₂O/rGO-15 photoelectrode (10.11 mA/cm²) is ∼9 and ∼3 times higher than the photoelectrodes based on pristine ZnO (1.06 mA/cm²) and ZnO/Cu₂O (3.22 mA/cm²). The enhanced PEC performance of ZnO/Cu₂O/rGO photoelectrode is attributed to the excellent light absorption properties of Cu₂O and excellent catalytic and charge transport properties of rGO. Experimental results reveal that the proposed functional nanomaterials have a great potential in water splitting applications.     

Bi shell-BiOI core microspheres modified TiO2 nanotube arrays photoanode: Improved effect of Bi shell on photoelectrochemical hydrogen evolution in seawater

Photoelectrochemical (PEC) seawater splitting can relive the shortage of purified water feedstocks. However, the corrosion from seawater on the photoelectrode becomes an uncertain influence factor for PEC performance. Developing an efficient and stable photoelectrode is a challenge. Herein, we present a Bi–BiOI shell-core microspheres modified TiO₂ nanotube arrays (TNA) photoanode prepared via solvothermal method, affording superior PEC hydrogen evolution activity in simulated seawater under AM1.5G light, which is 3.8 and 7.6 times than those of BiOI/TNA and TNA, respectively. Solar-to-hydrogen conversion efficiency of Bi–BiOI/TNA reaches to 2.21% with Faradaic efficiency up to 85.7%. Based on the optical and PEC measurements, it is verified that surface plasmon resonance effect of metallic Bi promotes transfer and separation of photogenerated charge and enhances visible-light absorption, thus benefiting higher PEC performance. Especially, Bi shell efficiently hinders the corrosion of BiOI by seawater. Our work provides a novel paradigm of photoanode for efficient and stable PEC seawater splitting.     

Photoelectrochemical hydrogen generation with nanostructured CdS/Ti–Ni–O composite photoanode

Composite photocatalysts have aroused great interest due to combination of favorable electronic and optical properties. Herein, novel CdS/Ti–Ni–O composite photoanodes were constructed through anodic fabrication of nanostructured Ni-doped TiO₂ (Ti–Ni–O) oxide films and CdS deposition by successive ionic layer adsorption and reaction (SILAR). The morphology and composition evolution, optical properties and photoelectrochemical (PEC) performance of the photoanodes were investigated. The composite nanofilms mainly consisted of micropores and nanotubes. The CdS/Ti–Ni–O composite photoanode demonstrated remarkable PEC hydrogen generation properties with a high photocurrent density (6.72 mA·cm^?2 at 0 V vs Ag/AgCl) which was 18.2 times to that of the bare Ti–Ni–O photoanode. The synergy of Ni-doping and CdS-coupling on the enhancement of PEC performance offers useful ideas to the exploitation of effective photocatalysts and contributes to the development of solar-driven PEC hydrogen generation.     

Efficient light harvesting by NiS/CdS/ZnS NPs incorporated in C,N-co-doped-TiO2 nanotube arrays as visible-light sensitive multilayer photoanode for solar applications

Here, we report a significant enhancement in photo-electrochemical activity of co-doped/modified TiO₂ nanotube arrays (TNAs). First, TiO₂ nanostructures were sensitized with nitrogen and carbon via a single step/low cost anodization process and then modified with Nis/CdS/ZnS nano particles (NPs) by the successive ionic layer adsorption and reaction (SILAR) method at room temperature. Photo-electrochemical properties and physical/chemical characteristics of the pure and sensitized/modified TNAs were investigated using field emission scanning electron microscopy (FESEM), XRD, XPS and EDX, comprehensively. Electrochemical measurements and UV–Vis DRS spectroscopy of the photo-electrodes showed that co-doping with anions and modification with different NPs result in the broadening of the absorption region of visible light and the reduction of band gap energy. The mechanism responsible for the enhanced photo-electrochemical activity of the C, N-co-doped/NiS, CdS, ZnS NPs modified TNAs for the water reduction reaction using aqueous solutions of Na₂S/Na₂SO₃ as sacrificial electrolyte under the whole spectrum of simulated solar light irradiation has been presented. The highest photocurrent in presence of sacrificial agent (Na₂S/Na₂SO₃) was obtained as 18.79 mA/cm², for the optimized SILAR loading cycles and dopants concentration. Furthermore, a high incident photon to current efficiency (IPCE) of about 82% for the optimum photo-anode had been achieved. These results confirm that the C, N-co-doped/NiS, CdS, ZnS NPs modified TNAs nanocomposite may offer a promising strategy to attain maximum efficiency in a variety of solar energy conversion systems, along with reduced photo-corrosion in the semiconductor-semiconductor heterojunction.     

Cu(In,Ga)Se2 thin film solar cells with buffer layer alternative to CdS

Progress in fabricating Cu(In,Ga)Se₂ (CIGS) solar cells with ZnS(O,OH) buffer layers prepared by chemical bath deposition (CBD) is discussed in this paper. Such buffer layers could potentially replace CdS in the CIGS solar cell. Total-area conversion efficiency of up to 18.6% has been reported previously using ZnS(O,OH) prepared by CBD. The reported 100 nm CBD ZnS(O,OH) layer was prepared by at least three consecutive depositions, which would make it a relatively expensive replacement for CdS. The recent development of a ZnS(O,OH) layer that enabled to obtain high-efficiency devices using a single-layer CBD is reported in this paper. A 14.4%-efficient device is obtained by using one-layer CBD ZnS(O,OH) on commercial-grade Shell Solar Cu(In,Ga)(S,Se)₂ (CIGSS) absorber and an up to 17.4% device is obtained by using two-layer CBD ZnS(O,OH) on an NREL CIGS absorber.     

Synthesis of uniform ZnO/ZnS/CdS nanorod films with ion-exchange approach and photoelectrochemical performances

The uniform ZnO/ZnS/CdS core–shell nanorod film was synthesized by a two-step ion-exchange method. The crystal structure, morphology, composition and optical property of as-prepared films were characterized by X-ray diffraction (XRD), Raman, Scanning electron microscope (SEM), Transmission electron microscope (TEM), Energy dispersive X-ray Detector (EDX) and UV–vis techniques. The results showed that the ZnO nanorod arrays can be used as sacrificial templates to synthesize uniform ZnS layer and further transform into CdS by simple ion-exchange approach. The CdS content in the films can be adjusted easily by changing the reaction temperature. The intimate contact between core and shell can be observed by high-resolution TEM image, which decrease the crystal boundary potential barrier and then facilitate the photogenerated charges transfer between the different phases. In this experiment, the ZnO/ZnS/CdS nanorod films were used in the photoelectrochemical (PEC) hydrogen production. And it is found that the film prepared at 70 °C (first step) and 100 °C (second step) gives a maximum photocurrent density and photo conversion efficiency.     

Preparation and characterization of (Zn,Cd)S photoelectrodes for hydrogen production

The authors report the preparation and characterization of (Zn,Cd)S (ZCS) photoelectrodes for hydrogen and oxygen production in a photoelectrolytic cell. ZCS photoelectrodes were prepared by mixing CdS and ZnS powders and sintering at high temperature in the presence of CdCl₂, as the flux. In some cases the ZCS was loaded with Mo during the preparation. These mixtures were characterized by structural analysis using X-ray diffraction (XRD), photosensitivity measurements in artificial light (halogen and xenon lamps)in solid state, and in a 0.1 M solution of Na₂SO₃. The results indicate that a screen printed CdS–ZnS mixture possesses high photosensitivity for its application as photocatalysts for water electrolysis.