Synthesis,characterization, and photoelectrochemical study of Cd1−xZnxS solid solution thin films deposited by spray pyrolysis for water splitting

A series of Cd_1−xZn_xS thin films were deposited onto indium-doped tin oxide (ITO) coated glass substrates by ultrasonic spray pyrolysis CdCl₂, ZnCl₂, and CS(NH₂)₂ aqueous solutions. The XRD patterns revealed that these films processed a wurtzite structure and a series of solid solutions of CdS and ZnS formed. The lattice constants decreased as the x value increased. From the transmittance and reflectance, the optical band gap was estimated to be between 2.45 eV and 3.72 eV, and the band gap increased as the x value increased according to a near linear relationship with the x value. The Mott-Schottky tests revealed that the flat potential shifted negatively as the x value increased. The photo responses agreed with the optical absorption of these films quite well. The current–potential measurements under chopped Xe lamp light irradiation show that the CdS deposited at 300 °C had best photoresponse. Its photoelectrochemical efficiency was estimated to be about 0.95% under 0.73 V bias from two electrodes current–potential tests.     

Pulse plated CdSxTe1−x films and their properties

CdS_xTe_1−x films were deposited on titanium and conducting glass substrates at room temperature using 0.25 M cadmium sulphate, the concentration of sodium thiosulphate and TeO₂ dissolved in sodium hydroxide was varied in the range of 0.01-0.05 M. The as deposited films exhibited hexagonal structure irrespective of the composition. The FWHM maximum of the x-ray diffraction peaks were found to decrease with increase of duty cycle. The optical energy gap values are in the range of 1.54-2.32 eV for films of different composition, it is observed that the band gap shifts towards CdS side as the concentration of CdS in the films increase. XPS studies indicated the formation of CdSTe solid solution. The grain size increases from 11.54 to 99.40 nm as the value of x increases from 0.2 to 0.8. The surface roughness is found to increase from 0.22 to 2.50 nm as the value of ‘x’ increases from 0.2 to 0.8. The resistivity is found to vary from 53 to 8 ohm cm as the ‘x’ value decreases from 1 to 0.     

Preparation and characterization of CdxZn1−xS thin films by spray pyrolysis technique for photovoltaic applications

Cd_xZn_(1−x)S (x = 0, 0.2, 0.4, 0.6, 0.8, and 1) thin films were deposited by the chemical spray pyrolysis technique using a less used combination of chemicals. Depositions were done at 573 K on cleaned glass substrates. The composition, surface morphology and structural properties of deposited films were studied using EDAX, SEM and X-ray diffraction technique. XRD studies reveal that all the films are crystalline with hexagonal (wurtzite) structure and inclusion of Cd into the structure of ZnS improved the crystallinity of the films. In the entire compositions, the (0 0 2) diffraction peak is prominent which gives lattice matching to the chalcogenide semiconductor such as CuIn_xGa_1−xSe₂ and CuIn (s_1−xSe_x)₂, which are used in photovoltaic devices. The value of lattice constant ‘a’ and ‘c’ have been observed to vary with composition from 0.382 to 0.415 nm and 0.625 to 0.675 nm, respectively. The band gap of the thin films varied from 3.32 to 2.41 eV as composition varied from x = 0.0 to 1.0. It was observed that presence of small amount of cadmium results in marked changes in the optical band gap of ZnS.     

Synthesis of (CuIn)xCd2(1−x)S2 photocatalysts for H2 evolution under visible light by using a low-temperature hydrothermal method

A new series visible-light driven photocatalysts (CuIn)_xCd_2(1−x)S₂ was successfully synthesized by a simple and facile, low-temperature hydrothermal method. The synthesized materials were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) surface area measurement, X-ray photoelectron spectroscopy (XPS) and ultraviolet-visible spectroscopy (UV–Vis DRS). The results show that the morphology of the photocatalysts changes with the increase of x from 0.01 to 0.3 and their band gap can be correspondingly tuned from 2.37 eV to 2.30 eV. The (CuIn)_xCd_2(1−x)S₂ nanocomposite show highly photocatalytic activities for H₂ evolution from aqueous solutions containing sacrificial reagents, SO₃²⁻ and S²⁻ under visible light. Substantially, (CuIn)_0.05Cd_1.9S₂ with the band gap of 2.36 eV exhibits the highest photocatalytic activity even without a Pt cocatalyst (649.9 μmol/(g h)). Theoretical calculations about electronic property of the (CuIn)_xCd_2(1−x)S₂ indicate that Cu 3d and In 5s5p states should be responsible for the photocatalytic activity. Moreover, the deposition of Pt on the doping sample results in a substantial improvement in H₂ evolution than the Pt-loaded pure CdS and the amount of H₂ produced (2456 μmol/(g h)) in the Pt-loaded doping system is much higher than that of the latter (40.2 μmol/(g h)). The (CuIn)_0.05Cd_1.9S₂ nanocomposite can keep the activity for a long time due to its stability in the photocatalytic process. Therefore, the doping of CuInS₂ not only facilitates the photocatalytic activity of CdS for H₂ evolution, but also improves its stability in photocatalytic process.     

Photoelectrochemical properties of CdSxSe1−x films

CdS_xSe_1−x films of different composition (0 < x < 1) were deposited by pulse plating technique at different duty cycles in the range of 10-50%. The films were polycrystalline and exhibited hexagonal structure. The band gap of the films varies from 1.68 to 2.39 eV as the concentration of CdS increases. Energy Dispersive analysis of X-rays (EDAX) measurements indicate that the composition of the films are nearly the same as that of the precursors considered for the deposition. Atomic force microscopy studies indicated that the grain size increased from 20 to 200 nm as the concentration of CdSe increased. Photoelectrochemical (PEC) cell studies indicated that the films of composition CdS_0.9Se_0.1 exhibited maximum photoactivity. Mott-Schottky studies indicated that the films exhibit n-type behaviour. Spectral response measurements indicated that the photocurrent maxima occurred at the wavelength value corresponding to the band gap of the films.     

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.     

Synthesis and characterization of nanocrystalline Zn1−xMxO (M = Ni, Cr) thin films for efficient photoelectrochemical splitting of water under UV irradiation

Nanocrystalline thin films of Zn_1−xM_xO (M = Ni, Cr) were deposited on glass substrate by sol-gel method. To a solution of zinc acetate 2-hydrate in dimethyl formamide, calculated quantities of nickel nitrate or chromium acetate were added. The clear solution, obtained after 2 h of continuous stirring, was coated on conducting glass (ITO plates). After preannealing at 250 °C to remove organic impurities, films were sintered at 400, 500 and 600 °C. XRD analysis reveals dominant evolution of hexagonal ZnO with a possible simultaneous growth of meta-stable cubic ZnO. AFM analysis indicated preferential growth of nanocrystallites along c-axis, while SEM analysis confirmed films having uniform morphology. Optical characterization led to two band gap values; one matching with the band gap of bulk ZnO and the second slightly higher, which suggest quantum confinement effect in nanocrystallites. Ni and Cr incorporation influenced the two band gap energies differently. Photoelectrochemical (PEC) splitting of water was attempted, using prepared thin films as working electrode, in conjunction with Pt counter electrode and saturated calomel reference electrode along with 150 W Xenon Arc light source and aqueous solution of NaOH (0.01 M). Results indicate Ni:ZnO films yielding improved photoresponse compared to Cr:ZnO films. Ni:ZnO (5 % at.) films sintered at 600 °C resulted in significantly enhanced photocurrent due to improved optical absorption and decrease in resistivity.     

Surface engineered active photocatalysts without noble metals: CuS–ZnxCd1−xS nanospheres by one-step synthesis

CuS(y)–Zn_xCd_1−xS (where 0.3 ≤ x ≤ 0.8, 0% ≤ y ≤ 15.8%) nanospheres with a Zn and Cu-rich surface were synthesized via a one-step method in ethylene glycol. Structural, morphological and optical properties of the samples have been investigated by XRD, TEM, XPS, ICP, N₂ physisorption and UV–vis DRS techniques. The cubic phased nanospheres of approximately 20–30 nm in diameter are comprised of nanocrystals of about 5 nm. Due to the reactivity difference among the metal ions in the organic solvent, the surface of the nanospheres was found enriched with Zn and Cu sulfide layers. In the absence of Cu, sample Zn_0.65Cd_0.35S gives a H₂ production rate of 29 μmol/h under the irradiation of a 300 W Xenon lamp with a cut-off filter (λ ≥ 420 nm) in aqueous solutions containing S²⁻ and SO₃²⁻. The activity, which can only be increased by four times by a traditional Pt cocatalyst for the as-prepared Zn_0.65Cd_0.35S sample, can be enhanced by around 20 times to 550 μmol/h and 624 μmol/h after adding 5.9 mol% and 11.1 mol% (metal basis) of Cu during the synthesis to form a Cu-rich surface. It is believed that the surface Cu²⁺ 3d impurity levels can function as the same role of the noble metal cocatalysts as charge accommodation sites for the charge separation, and hence increase the photocatalytic performance for water splitting.     

Structure, composition and optical properties of Cu2ZnSnS4 thin films deposited by Pulsed Laser Deposition method

Polycrystalline Cu₂ZnSnS₄ (CZTS) thin films have been directly deposited on heating Mo-coated glass substrates by Pulsed Laser Deposition (PLD) method. The results of energy dispersive X-ray spectroscopy (EDX) indicate that these CZTS thin films are Cu-rich and S-poor. The combination of X-ray diffraction (XRD) results and Raman spectroscopy reveals that these thin films exhibit strong preferential orientation of grains along [1 1 2] direction and small Cu_2−xS phase easily exists in CZTS thin films. The lattice parameters and grain sizes have been examined based on XRD patterns and Atom Force Microscopy (AFM). The band gap (E_g) of CZTS thin films, which are determined by reflection spectroscopy varies from 1.53 to 1.98 eV, depending on substrate temperature (T_sub). The optical absorption coefficient of CZTS thin film (T_sub=450 °C) measured by spectroscopic ellipsometry (SE) is above 10⁴ cm⁻¹.     

Cd1−xZnxS supported on SBA-16 as photocatalysts for water splitting under visible light: Influence of Zn concentration

Cd_1−xZn_xS solid solutions (x = 0.05–0.3) supported on mesoporous silica SBA-16 substrate with 3D cubic structure were investigated for hydrogen production from water splitting under visible light. The influence of Zn concentration (x) in the Cd_1−xZn_xS solid solution and support morphology were investigated. The bare SBA-16 substrate was synthetized by the hydrothermal method whereas the Cd_1−xZn_xS photocatalysts were prepared by coprecipitation of metal sulfides from aqueous solutions of Cd²⁺ and Zn²⁺ using Na₂S as precipitating agent. An attempt has been made to determine the photocatalyst structures using several techniques including elemental analysis, N₂ adsorption–desorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM), UV–Vis diffuse reflectance spectroscopy (UV–Vis DRS) and Raman spectroscopy. Surface characterization of the samples by XPS indicates that Cd_1−xZn_xS nanoparticles are unevenly distributed on both external surface and within the pore network. An increase of the band gap energy with increasing Zn loading up to x = 0.2 in the Cd_1−xZn_xS solid solution was observed. As a consequence, H₂ evolution increases gradually with an increase of the Zn loading in the photocatalysts from 0.05 to 0.2 wt% being the Cd_0.8Zn_0.2S/SBA-16 system the most active among the catalysts studied. The highest activity of this photocatalyst was explained in terms not only of its large band gap energy but also by the enhancement of the interaction between the particles of solid solution and the SBA-16 substrate.     

Photovoltaic solar cell properties of CdxZn1−xO films prepared by sol–gel method

Cd_xZn_1−xO films have been deposited by sol–gel spin-coating method onto glass substrates. The Cd/Zn ratio in solution was changed from 0 to 1. Zinc acetate dehydrates, cadmium acetate dehydrates, 2-methoxyethanol and monoethanolamine were used as a starting material (zinc and cadmium), solvent and stabilizer, respectively. The crystal structure and orientation of the films were investigated by X-ray diffraction (XRD) patterns. XRD patterns show that the films are polycrystalline nature. As x varies from 0 to 1, it was observed that the crystal structure changed from wurtzite (ZnO) to cubic (CdO) structure. The optical properties of these films have been investigated by means of the optical transmittance and reflectance spectra. A significant change in optical absorption edge, optical band gap and optical constant with variation in composition was observed.     

Sputtered Cd1−xZnxTe films for top junctions in tandem solar cells

Cd_1−xZn_xTe alloy films with 1.6 and 1.7 eV band gaps were deposited by RF magnetron sputtering from targets made either of mixed powders or alloys of CdTe and ZnTe (25% and 40%). High-quality polycrystalline films with the (1 1 1) preferred orientation were obtained. The films were characterized using X-ray diffraction (XRD), scanning electron microscopy, resistivity, optical absorption, Raman, and photoluminescence. The EDS, XRD, and optical absorption analysis indicated that the x-value of the as-grown films were typically 0.20 and 0.30 for films sputtered from 25% and 40% ZnTe containing targets, respectively. The as-deposited alloy films exhibit quite low photovoltaic performance when used to make cells with CdS as the hetero-junction partner. Therefore, we have studied various post-deposition treatments with vapors of chlorine-containing materials, CdCl₂ and ZnCl₂, in dry air or H₂/Ar ambient at 390 °C. The best performance of a Cd_1−xZn_xTe cell (, ) was found for treatment with vapors of the mixed CdCl₂+0.5%ZnCl₂ in an H₂/Ar ambient after pre-annealing at 520 °C in pure H₂/Ar.     

Nano-crystalline RuxSn1 − xO2 powder catalysts for oxygen evolution reaction in proton exchange membrane water electrolysers

Nano-crystalline powders of Ru_xSn_1 − xO₂ (1.0 ≥ x ≥ 0.2) were prepared as high performance electrocatalysts for oxygen evolution in polymer electrolyte membrane water electrolysers (PEMWE). A modified Adams fusion method was developed to produce these oxides. The Ru_xSn_1 − xO₂ powder catalysts were investigated with XRD, SEM, TEM, CV, and EIS. XRD showed a nano-crystalline rutile structure results over the whole composition range. The particle sizes determined by TEM were between 5 and 20 nm. With an increase in the Sn content in Ru_xSn_1 − xO₂ (x-value was decreased), the catalytic performance increased initially and then decreased dramatically. The catalyst Ru_0.6Sn_0.4O₂ demonstrated the best performance in general, which may be due to its smaller particle size and greater ratio of outer active surface area. Repetitive cyclic voltammograms demonstrated that the Ru_0.6Sn_0.4O₂ catalyst had better stability than pure RuO₂. Both the mass normalized current density and chronocoulometry at 1.4 V indicated that Ru_0.6Sn_0.4O₂ and RuO₂ had better performance at 70 °C than at 25 °C.     

Composition, surface topography, structure, Raman, and electrochemical/photoelectrochemical characterisation of CdxHg1−xTe films

Cd-rich Cd_xHg_{1 − x}Te films have been electrodeposited under potentiostatic conditions on conducting glass and Ti substrates from an acidic solution containing the respective ions as Cd²⁺:Hg²⁺:HTeO₂⁺ = 100:1:2. Six films one after another have been prepared from a single electrochemical cell. EDAX analysis of the air annealed films show decreasing Hg content in the deposit as the number of film preparation increases. SEM analysis indicate undulatory surface with Hg-rich clusters at the top surface. XRD analysis indicate the presence of Cd_xHg_{1 − x}Te along with . The Cd_xHg_{1 − x}Te alloy formation have been confirmed from Raman shift measurements which change with composition, x. The as-deposited films are n-type but converts to p-type after air annealing. Spectral response measurements gave band gap values that change with Hg content in the deposit. Band gap values ranging from 1.1 eV to 1.45 eV have been estimated. Photoelectrochemical solar cells using polysulphide electrolyte have been fabricated which gave an open-circuit photovoltage and short-circuit photocurrent, respectively, as 325 mV and 5.5 mA/cm² under 60 mW/cm² intensity of illumination.     

Highly conducting transparent nanocrystalline Cd1−xSnxS thin film synthesized by RF magnetron sputtering and studies on its optical, electrical and field emission properties

Transparent conducting Cd_1−xSn_xS thin films have been synthesized by radio frequency magnetron sputtering technique on glass and Si substrates for various tin concentrations in the films. X-ray diffraction studies showed broadening of peaks due to smaller crystal size of the Cd_1−xSn_xS films, and SEM micrographs showed fine particles with average size of 100 nm. Sn concentration in the films was varied from 0% to 12.6% as determined from energy-dispersive X-ray analysis. The room-temperature electrical conductivity was found to vary from 8.086 to 939.7 S cm⁻¹ and corresponding activation energy varied from 0.226 to 0.076 eV. The optimum Sn concentration for obtaining maximum conductivity was found to be 9.3%. The corresponding electrical conductivity was found to be 939.7 S cm⁻¹, and the mobility 49.7 cm² V⁻¹ s⁻¹. Hall measurement showed very high carrier concentrations in the films lying in the range of 8.0218×10¹⁸–1.7225×10²⁰ cm⁻³. The conducting Cd_1−xSn_xS thin films also showed good field emission properties with a turn on field 4.74–7.86 V μm⁻¹ with variation of electrode distance 60–100 μm. UV–Vis–NIR spectrophotometric studies of the films showed not needed the optical band gap energy increased from 2.62 to 2.80 eV with increase of Sn concentration in the range 0–12.6%. The optical band gap was Burstein–Moss shifted, and the corresponding carrier concentration obtained from the shift also well matched with that obtained from Hall measurement.     

Hybrid functionals studies of structural and electronic properties of ZnxCd(1−x)S and (ZnxCd1−x)(SexS1−x) solid solution photocatalysts

The structural and electronic properties of the wurtzite Zn_xCd_1−xS and (Zn_xCd_1−x)(Se_xS_1−x) alloys are calculated using density functional theory calculations with HSE06 hybrid exchange-correlation functional. Special quasirandom structures are used to describe the disordered alloys, for x = 0.125, 0.25, 0.375, 0.5, 0.625, 0.75 and 0.875, respectively. Our calculations reveal that Zn_xCd_1−xS alloy with the appropriate Zn doping concentration not only causes the elevation of the conduction band minimum energy, but also increase the mobility of photogenerated holes and electrons, which well explains the high photocatalytic activity and stability of Zn_0.2Cd_0.8S alloy under a long-term light irradiation. Compared with Zn_xCd_1−xS alloy, (Zn_xCd_1−x)(Se_xS_1−x) alloy holds greater potential to simultaneously meet band gap, band edge, and mobility criteria for water splitting. Theoretical results predict that (Zn_xCd_1−x)(Se_xS_1−x) alloy with the ZnSe concentration in the range from 0.38 to 0.75 could be a more promising candidate than Zn_0.2Cd_0.8S alloy for photoelectrochemical hydrogen production through water splitting.     

Optical, electrical and structural investigations on Cd1−xZnxSe sintered films for photovoltaic applications

II–VI polycrystalline semiconducting materials have come under increased scrutiny because of their wide use in the cost reduction of devices for photovoltaic applications. Cd_1−xZn_xSe is an important semiconducting alloy because of the tunability of its physical parameters such as band gap and lattice parameters by controlling its stoichiometry. Many more material characteristics of it would be altered and excellently controlled by controlling system composition x.Polycrystalline thin films of Cd_1−xZn_xSe with variable composition (0x1) have been deposited onto ultra-clean glass substrates by sintering process. The optical, structural and electrical transport properties of Cd_1−xZn_xSe thin films have been examined. The optical band gap and optical constants of these films were determined by using double beam spectrophotometer. The DC conductivity and activation energy of the films were measured in vacuum by two-probe technique. The Schottky junction of Cd_1−xZn_xSe with indium was made and the barrier height and ideality factor were determined using current–voltage characteristics. The nature of sample, crystal structure and lattice parameters were determined from X-ray diffraction patterns. The films were polycrystalline in nature having cubic zinc-blende structure over the whole range studied.Sintering is very simple and viable compared to other cost intensive methods. The results of the present investigation will be useful in characterizing the material, Cd_1−xZn_xSe, for its applications in photovoltaics.     

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.     

Cd1−xZnxS solid solutions supported on ordered mesoporous silica (SBA-15): Structural features and photocatalytic activity under visible light

A series of Cd_1−xZn_xS (x = 0.05–0.3) photocatalysts supported on ordered mesoporous silica (SBA-15) were prepared and investigated for hydrogen production from water splitting under visible light. Textural, structural and surface photocatalyst properties are determined by N₂ adsorption isotherms, UV–vis, Raman and XPS and related to the activity results in hydrogen production. Raman and XRD results indicated a mutual interaction between Cd and Zn, forming nanoparticles of Cd_1−xZn_xS solid solutions. All Cd_1−xZn_xS/SBA-15 samples showed relatively high activities for hydrogen evolution. The hydrogen production rate is found to increase gradually when the zinc concentration on photocatalysts increases from 0.05 to 0.2, achieving a maximum for the photocatalyst with zinc concentration equal to 0.2. Variation in photoactivity is discussed in terms of modification in the conduction band and light absorption ability of Cd_1−xZn_xS particles derived from the changes in the Zn concentration in the Cd_1−xZn_xS solid solution.     

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.