Electrodeposition of CuInSe2 layers using a two-electrode system for applications in multi-layer graded bandgap solar cells

In order to develop low-cost, large area multi-layer graded bandgap solar cell structures, CuInSe₂ layers were grown using a simplified two-electrode system. Atomic force microscopy (AFM) shows the layers grown consist of nano- and micro- size particles. Photoelectrochemical (PEC) cell, X-ray photoelectron spectroscopy (XPS) and X-ray fluorescence (XRF) measurements confirm that it is possible to grow CuInSe₂ layers with p-, i- and n-type electrical conduction, as pre-determined for applications in multi-layer device structures. XRF, XPS and PEC measurements show that Cu-richness provides p-type conduction and In-richness provides n-type conduction in electrodeposited CuInSe₂ layers. It is also possible to grow materials with different bandgaps in the range 1.00–1.90 eV. The combination of these two properties allows growth of multi-layer structures and preliminary work on these devices show good rectifying properties and exhibit photovoltaic activity. These new developments will be presented in this paper.     

Investigation of n-type Cu2O layers prepared by a low cost chemical method for use in photo-voltaic thin film solar cells

A low cost and simple chemical method of boiling copper plates in CuSO₄ solution is used to prepare Cu₂O layers. X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), glow discharge optical emission spectroscopy (GDOES) and optical absorption have been used to characterise these layers. It has been found that the layers consist of Cu₂O phase with a thickness of about 1.4 μm for 60 minutes boiling in CuSO₄ solution. The largest grain sizes are in the order of 1 μm and the layers contain cubic Cu₂O phases. The layers are n-type in electrical conduction and the optical band gap observed is 2.2 eV.     

Electrodeposition of p–i–n type CuInSe2 multilayers for photovoltaic applications

Copper indium diselenide polycrystalline thin films of p-, i- and n-type electrical conductivity were grown using a one-step electrodeposition process in a single bath. The bulk structure and the stoichiometry of the layers were determined using X-ray diffraction and X-ray fluorescence. The material composition was correlated with the electrical conductivity type variation, detected by the photoelectrochemical cell. Atomic force microscopy analysis showed copper-rich films deposited at low cathodic potentials (0.6 V vs Ag/AgCl) are of spherical and granular morphology and the grain sizes were 0.3–0.5 μm, while stoichiometric CIS films deposited at 1.0 V vs Ag/AgCl have grain sizes of 0.1–0.4 μm. The initial studies of optoelectronic properties (V_oc, J_sc and FF) of the four-layer solar cell devices (glass/FTO/n-CdS/n-CIS/i-CIS/p-CIS/Au) are presented.     

Growth and characterization of stepwise flash evaporated CuInTe2 thin films

CuInTe₂ films grown by stepwise flash evaporation onto glass and silicon substrates held at 573 K were studied using X-ray diffraction (XRD), transmission electron microscopy (TEM), Rutherford backscattering spectrometry (RBS) and Raman spectroscopy. XRD and TEM studies showed the formation of single-phase polycrystalline CuInTe₂. Results of the RBS measurements showed the films to be near-stoichiometric and negligible diffusion of elements across the CuInTe₂/Si interface. Various lattice vibrational modes identified by Raman measurements were found to match well with those reported for single-crystal CuInTe₂, confirming the crystalline quality of the CuInTe₂ thin films.     

Fabrication of Pt nanoparticles on ethylene diamine functionalized graphene for formic acid electrooxidation

A novel and simple synthesis method of preparing ethylene diamine (ED) functionalized graphene (ED-Gh) decorated with Pt nanoparticle has been reported. Morphology, microstructure of the resulted material was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy (Raman), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Electrochemical studies toward formic acid electrooxidation were investigated by cyclic voltammetry (CV) and chronoamperometry (CA). These results showed that Pt nanoparticle with the crystallite size of about 5 nm was highly dispersed on ED-Gh and the catalyst exhibited good electrocatalytic activity and long-term stability. Therefore, ED-Gh can be a potential support for Pt in direct formic acid fuel cells.     

Enhancement of photoelectrochemical response by Au modified in TiO2 nanorods

We report on the design and synthesis of a novel Au/TiO₂/Au heterostructure and its implementation as a photoanode for photoelectrochemical (PEC) application. The Au/TiO₂/Au heterostructure was produced by assembling Au nanoparticles and TiO₂ nanorods (NRs) onto FTO substrate, followed by electrodepositing Au nanoparticles on the TiO₂NRs. Field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and electrochemical methods were adopted to characterize the prepared photoanodes. Compared to the system involving Au nanoparticles directly linked to TiO₂, this Au/TiO₂/Au heterostructure exhibits significant improved photoresponse as a photoanode, as demonstrated good performance in PECs. This study illustrates the importance of pre-deposited Au underlayers in influencing PEC properties of hybrid assembled nanostructures. As the Au/TiO₂NRs/Au photoanodes are easily fabricated and highly stable, Au/TiO₂NRs/Au can serve as a good substitution for TiO₂ in a variety of solar energy driven applications including PEC water splitting, photocatalysis, and solar cells.     

Hydrogen production from oxidative steam-reforming of n-propanol over Ni/Y2O3–ZrO2 catalysts

Oxidative steam reforming (OSR) of n-propanol was studied over new Ni catalysts (ca. 7% Ni wt/wt) supported on Y₂O₃–ZrO₂ oxides with different yttrium content (2–41 % Y₂O₃ wt/wt). Materials were characterized by X-ray diffraction, temperature-programmed reduction, X-ray photoelectron and Raman spectroscopy, scanning electron microscopy with energy dispersive X-ray analysis and high resolution transmission electron microscopy. Samples were used in calcined form and tested in the temperature range 673–773 K using a reactant feed of n-propanol/water/O₂ at a molar ratio 1/9/0.5. Hydrogen production is related with the support composition and Ni dispersion.     

Synthesis of nano-crystalline Sr2MgMoO6−δ anode material by a sol–gel thermolysis method

Nano-crystalline Sr₂MgMoO_6−δ (SMMO) powders were synthesized successfully by a novel sol–gel thermolysis method using a unique combination of polyvinyl alcohol (PVA) and urea. The decomposition behavior of gel precursor was studied by thermogravimetric-differential thermal analysis (TG/DTA) and the results showed that the double-perovskite phase of SMMO began to form at 1000 °C. The microstructure of the samples had been investigated by X-ray diffraction (XRD), transmission electron microscope (TEM), selected area electron diffraction (SAED), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). XRD patterns confirmed that well-crystalline double-perovskite SMMO powders were obtained by calcining at 1450 °C for 12 h. TEM morphological analysis showed that SMMO powders had a mean particle size around 50–100 nm. The SAED pattern and Raman spectroscopy showed that the SMMO powders were nano-polycrystalline well-developed A(B′_0.5B″_0.5)O₃ type perovskite material. The XPS results demonstrated that the Mo ions in SMMO had been reduced after exposure to H₂. The electric property was studied by four-probe method. The results showed that conductivity was 8.64 S cm⁻¹ in 5.0% H₂/Ar at 800 °C and the activation energies at low temperatures (400–640 °C) and high temperatures (640–800 °C) are about 21.43 and 6.59 kJ mol⁻¹, respectively.     

Enhanced photocatalytic water splitting activity of carbon-modified TiO2 composite materials synthesized by a green synthetic approach

We report a green and facile approach for the preparation of carbon-modified (C-modified) TiO₂ composite materials by hydrothermal synthesis followed by pyrolytic treatment. The resultant materials were characterized by powder X-ray diffraction (XRD), nitrogen physisorption studies, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy, and transmission electron microscopy (TEM). The photocatalytic performances of these materials were evaluated by calculating the amount of hydrogen evolved from the decomposition of water under solar simulated irradiation conditions. An improvement was achieved from no H₂ evolution at all with the bare TiO₂, to an evolution of 0.21 mL g⁻¹ h⁻¹ from a composite material modified with an optimum carbon loading of 3.62%. These results suggested that the interaction of carbon with predominantly rutile form of TiO₂ can promote shallow trapping of photogenerated electrons in the oxygen vacancies. This phenomenon consequently enhances the photocatalytic activity by minimizing charge carrier recombination, a characteristic demonstrated by fluorescence quenching of the TiO₂ emission.     

A synthesis of CO-tolerant Nb2O5-promoted Pt/C catalyst for direct methanol fuel cell; its physical and electrochemical characterization

A Pt-Nb₂O₅/C electrocatalyst was synthesized by a two-step process as an anode material in direct methanol fuel cell (DMFC). The Pt-Nb₂O₅/C catalysts heat-treated at different temperatures (400 and 500 °C) in flowing N₂ were characterized by various methods such as inductively coupled plasma-atomic emission spectroscopy, X-ray diffraction, transmission electron microscopy, and X-ray photoemission spectroscopy (XPS). The heat-treated Pt-Nb₂O₅/C catalyst at 400 °C showed the best electrochemical activity for CO and methanol oxidations among the prepared catalysts. The XPS results showed the electronic structure change of Pt, indicating a formation of interaction between Pt and Nb₂O₅. It is suggested that a synergistic effect between Pt and Nb₂O₅ enhances the electrocatalytic activity for CO and methanol oxidations. We believe that Nb₂O₅-promoted Pt/C catalyst may be regarded as one of the attractive candidates as an anode material in DMFC.     

Remarkable enhancement of photocatalytic hydrogen evolution over Cd0.5Zn0.5S by bismuth-doping

Bi³⁺ doped Cd_0.5Zn_0.5S photocatalysts were prepared by a simple hydrothermal method, and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscope (XPS), energy dispersive X-ray spectroscopy (EDX), BET and UV-Vis absorption spectroscope techniques. When Bi³⁺ doping content is lower, the doping ions lie at the surface lattice sites, whereas when the doping content is higher, the ions also enter the bulk lattice sites. Their photoactivities were evaluated by hydrogen evolution from aqueous solution containing Na₂S and Na₂SO₃ as a hole scavenger under visible light (λ ≥ 420 nm) irradiation. Bi³⁺ doping enhances markedly photocatalytic activity. When Bi³⁺ doping content is 0.10 mole %, the photocatalyst exhibits the highest activity, and the average apparent quantum yield amounts to 9.71% during 30 h irradiation. The possible mechanism was discussed.     

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.     

Investigation of WO3/ZnO thin-film heterojunction-based Schottky diodes for H2 gas sensing

A comparative study of Schottky diode hydrogen gas sensors based on Pd/WO₃/Si and Pd/WO₃/ZnO/Si structure is presented in this work. Atomic force microscopy and X-ray photoelectron spectroscopy reveal that the WO₃ sensing layer grown on ZnO has a rougher surface and better stoichiometric composition than the one grown on the Si substrate. Analysis of the I–V characteristics and dynamic response of the two sensors when exposed to different hydrogen concentrations and various temperatures indicate that with the addition of the ZnO layer, the diode can exhibit a larger voltage shift of 4.0 V, 10 times higher sensitivity, and shorter response and recovery times (105 s and 25 s, respectively) towards 10,000-ppm H₂/air at 423 K. Study on the energy band diagram of the diode suggests that the barrier height is modulated by the WO₃/ZnO heterojunction, which could be verified by the symmetrical sensing properties of the Pd/WO₃/ZnO/Si gas sensor with respect to applied voltage.     

Solar photocatalytic degradation of methylene blue in carbon-doped TiO2 nanoparticles suspension

Carbon-doped TiO₂ nanoparticles were prepared by sol–gel auto-combustion method and characterized by X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), Brunauer–Emmett–Teller method (BET), UV–vis diffuses reflectance spectroscopy (DRS). UV–vis diffuse reflectance spectra showed that carbon-doped TiO₂ exhibited obvious absorption in the visible light range. The visible light photocatalytic activity of carbon-doped TiO₂ was ascribed to the presence of oxygen vacancy state between the valence and the conduction bands because of the formation of Ti³⁺ species in the as-synthesized carbon-doped TiO₂. The sample calcined at 873 K showed the highest photocatalytic activity under solar irradiation. The effects of photocatalyst concentration, initial concentration of methylene blue, and pH value in aqueous solution were also presented.     

Direct Z-scheme anatase/rutile bi-phase nanocomposite TiO2 nanofiber photocatalyst with enhanced photocatalytic H2-production activity

Design and preparation of direct Z-scheme anatase/rutile TiO₂ nanofiber photocatalyst to enhance photocatalytic H₂-production activity via water splitting is of great importance from both theoretical and practical viewpoints. Herein, we develop a facile method for preparing anatase and rutile bi-phase TiO₂ nanofibers with changing rutile content via a slow and rapid cooling of calcined electrospun TiO₂ nanofibers. The phase structure and composition, surface morphology, specific surface area, surface chemical composition and element chemical states of TiO₂ nanofibers were analyzed by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), nitrogen adsorption and X-ray photoelectron spectroscopy (XPS). By a rapid cooling of 500 °C-calcined electrospun TiO₂ precursor, anatase/rutile bi-phase TiO₂ nanofibers with a roughly equal weight ratio of 55 wt.% anatase and 45 wt.% rutile were prepared. The enhanced H₂ production performance was observed in the above obtained anatase/rutile composite TiO₂ nanofibers. A Z-scheme photocatalytic mechanism is first proposed to explain the enhanced photocatalytic H₂-production activity of anatase/rutile bi-phase TiO₂ nanofibers, which is different from the traditional heterojunction electron–hole separation mechanism. This report highlights the importance of phase structure and composition on optimizing photocatalytic activity of TiO₂-based material.     

WO3−x nanowires based electrochromic devices

A simple method was developed to fabricate tungsten oxide (WO_3−x) nanowires based electrochromic devices. The WO_3−x nanowires are grown directly from tungsten oxide powders in a tube furnace. The WO_3−x nanowires have diameters ranging from 30 to 70 nm and lengths up to several micrometers. The WO_3−x nanowires based device has short bleach-coloration transition time and can be grown on a large scale directly onto an ITO-coated glass that makes it potential in many electrochromic applications. The structure, morphology, and composition of the WO_3−x nanowires were characterized using the scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and energy-dispersive spectrometer. The optical and electrochromic performance of the nanowires layer under lithium intercalation was studied in detail by UV–VIS–NIR spectroscope and cyclic voltameter.     

N/S dual-doped graphene with high defect density for enhanced supercapacitor properties

N/S dual-doped graphene was prepared by one-pot process using graphene oxide as raw material and thiourea and urea as reduction-dopants. The field emission scanning electron microscopy (FESEM), X-ray powder diffraction (XRD), Raman spectroscopy (Raman), nuclear magnetic resonance (NMR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and other means were used to characterize the microstructure and morphology of the samples. The electrochemical properties of the samples were tested by cyclic voltammetry, electrochemical alternating impedance and constant current charge-discharge techniques, and compared with graphene and nitrogen-doped graphene. Results show that the defect density of graphene can be increased more effectively by N/S dual doping than by nitrogen doping, and the contents of doped nitrogen and sulfur have a significant effect on the morphology and performance of the samples. The specific surface area of the best sample reaches 275.8 m² g⁻¹, and its conductivity is 477.6 S m⁻¹. When the window voltage is −1.2-0 V, the best sample shows superior specific capacitance of 386.5 F g⁻¹ and a high energy density of 69.6 Wh kg⁻¹ at a scan rate of 10 mV s⁻¹. At the current density of 10 A g⁻¹, after 5000 constant current charge/discharge cycles, the specific capacitance retention rate is 94.5%, showing excellent cyclic stability.     

Electrochemical properties of negative SiMox electrodes deposited on a roughened substrate for rechargeable lithium batteries

To replace conventional carbon, silicon has been widely proposed as a next-generation negative electrode (anode) material for lithium-ion batteries. In this study, Si and SiMo_x-alloy deposited by an RF-magnetron sputtering system is investigated by means of X-ray diffraction, ex situ Raman spectroscopy and transmission electron microscopy. Electrochemical tests are conducted and four different Si and SiMo_x-alloy electrodes and their structures and textual properties are characterized with X-ray photoelectron spectroscopy. The surface morphologies of the electrodes are also observed using field-emission scanning electron microscopy. The electrochemical properties of the electrodes are examined through cycling tests and electrochemical impedance spectroscopy. The results show that rough Cu foil and Mo as alloy materials help Si to retain its discharge capacity and overcome volume expansion during charging and discharging. After a few cycles, the Si electrode severely loses capacity, whereas the SiMo_x-alloy electrodes display good cycle retention and high capacity. The SiMo_0.79 electrode gives an initial capacity of 1319 mAh g⁻¹ that decreases to 1180 mAh g⁻¹ after 100 cycles (89.4%).     

Facile and green synthesis of titanate nanotube/graphene nanocomposites for photocatalytic H2 generation from water

A facile and green one-step method was used to prepare titanate nanotube/graphene (TNT/GR) photocatalysts via an alkaline hydrothermal process. The as-prepared samples were characterized by X-ray diffraction, transmission electron microscopy, Raman spectroscopy, ultraviolet–visible diffuse reflectance spectroscopy and photoluminescence emission spectroscopy. The photocatalytic performance was evaluated by H₂ generation from water splitting under Xe-lamp illumination. A significantly enhanced photocatalytic activity for H₂ evolution (12.1 μmol/h) was obtained over the compostion-optimized TNT/GR composite (with 1.0 wt% GR), two times higher than that of pure TNT (4.0 μmol/h). During hydrothermal reaction, the reduction of graphene oxide (GO) into GR without using any reducing agents and the formation of 1-D TNT were achieved simultaneously, which resulted in the direct growth of well-defined TNTs uniformly distributed on GR substrates.     

Structural destabilisation of MgH2 obtained by heavy ion irradiation

MgH₂ powder samples have been irradiated with 120 keV Ar⁺⁸ ions with different ion fluencies ranging from 10¹² to 10¹⁶ ions/cm². Irradiation effects are estimated by SRIM calculations, and investigated experimentally using Raman spectroscopy and X-ray diffraction (XRD) analysis. The observed changes of structure and vibrational spectra are elaborated, their consequences on hydrogen bonding in MgH₂ discussed, and influence on H-desorption properties investigated by Temperature Programmed Desorption (TPD) and Differential Scanning Calorimetry (DSC) techniques. It has been established that near-surface defects have a predominant influence on decreasing the H-desorption temperature. Variations of Raman, TPD and DSC spectra with irradiation conditions suggest that there are several mechanisms of dehydriding, and that they depend on defect concentration, interaction and ordering.