Electrochemically synthesized nanocrystalline spinel thin film for high performance supercapacitor

Spinels are not known for their supercapacitive nature. Here, we have explored electrochemically synthesized nanostructured NiCo₂O₄ spinel thin-film electrode for electrochemical supercapacitors. The nanostructured NiCo₂O₄ spinel thin film exhibited a high specific capacitance value of 580 F g⁻¹ and an energy density of 32 Wh kg⁻¹ at the power density of 4 kW kg⁻¹, accompanying with good cyclic stability.     

Analysis of semiconductor thin films deposited using a hollow cathode plasma torch

The hollow cathode plasma torch has been used for several years. One of the major applications has been the deposition of dielectric thin films. However, this technique has also been used to deposit metals where high-speed deposition is needed. It has proven to be useful in deposition of coatings onto the inside of substrates of complex shape, high-speed etching, and deposition of thin films at atmospheric pressure. In recent years, we have adapted the technique to deposit high-quality amorphous and polycrystalline semiconducting films. A large variety of measurement techniques have been employed to determine the film properties and the results are reported here.     

Electrodeposition and characterization of ZnSe semiconductor thin films

In this work, results on the preparation and characterization of ZnSe thin films obtained by electrodeposition are presented. Voltammetric curves were recorded in order to characterize the electrochemical behavior of the Zn⁺²/SeO₂ system on different substrates. Thin films were deposited potentiostatically from an unstirred, deareated aqueous solution onto titanium, glass substrates coated with fluorine doped tin oxide and ITO glass substrates. The effect of main parameters such as the deposition potential, SeO₂ concentration and annealing on film composition and structure were analyzed. The as-grown and treated layers were characterized by X-ray energy dispersive analysis, X-ray diffraction, scanning electron microscopy and photoelectrochemical studies. Optical measurements were done on these samples which gave a clear band edge near 2.6 eV quite close to the accepted room temperature value of 2.7 eV for ZnSe.     

Photoelectrochemical studies of chemically deposited nanocrystalline p-type HgS thin films

Nanocrystalline mercury sulfide (HgS) thin films were deposited by chemical bath deposition (CBD) method onto the glass and fluorine doped tin oxide (FTO) coated glass substrate from an aqueous alkaline bath (pH  8) at room temperature (300 K). Mercuric acetate and thiourea were used as Hg²⁺ and S²⁻ ion sources, respectively. The photoelectrochemical (PEC) studies of HgS films were carried out, and the nanocrystalline films were found to be photoactive in polyiodide solution. The PEC cell configuration was p-HgS/0.1 M (KOH–KI–I₂)/C. From the current–voltage (I–V) characteristics, it is concluded that the HgS films are of p-type electrical conductivity. The photovoltaic output characteristics were used to calculate the fill factor (ff) and solar conversion efficiency (η). The low value of η may be due to the high value of series resistance (R_s) and interface states in the cell, which are responsible for the recombination mechanism.     

Electrochemically induced electrochromic properties in nickel thin films deposited by DC magnetron sputtering

Nickel oxide thin films and other electrochromic materials receive particular attention due to the great variety of practical applications in energy conservation and in semitransparent optical devices. In this work, nickel thin films were produced by DC magnetron sputtering on ITO substrates. The nickel–ITO thin films were studied by electrochemical techniques, and electrochromic properties were induced in the films after several different cyclic voltammetry runs. The cyclic potential range was set from −400 to 600 mV and the scan rates were varied from 6.6 to 10 mV/s. The electrochromic phenomena was observed just after 80 cycles as derived from voltammograms and color changes in the nickel oxide films were observed close to 100 cycles. The optical properties of as-deposited films and of the ones tested in the electrochemical cell were determined by optical spectrophotometry in the visible range. The structural properties of the films were studied by X-ray diffractometry, scanning and transmission electron microscopy in conventional and high-resolution modes. The electrochemical properties were studied principally by the cyclic voltammetry technique. Noticeable differences in induced electrochromic behavior were observed between the nickel films deposited on two sets of ITO substrates, prepared by DC magnetron sputtering and spray pyrolysis.     

Using spectroscopy to probe the band energetics of transparent nanocrystalline semiconductor films

Transparent polycrystalline semiconductor films are formed on a conducting glass substrate by sintering nanocrystallites in air at high temperatures. Incorporation in an electrochemical cell, as the working electrode, permits potentiostatic control of the Fermi energy within the semiconductor film. On applying a potential more negative than the flatband potential electrons accumulate in the conduction band. Spectral changes assigned to accumulation of free carriers and the associated Burstein shift have been characterized. Calculation of the flatband potential is possible from the measured relationship between either the free electron absorbance or the Burstein shift, and the applied potential at several different pHs. These methods are finding applications in optimization of regenerative photoelectrochemical cells based on polycrystalline semiconductor films.     

An effective use of nanocrystalline CdO thin films in dye-sensitized solar cells

Thin films of cadmium oxide (CdO) were synthesized by layer-by-layer deposition method on indium doped tin oxide (ITO) substrates. Post-deposition annealing at 250 °C for 24 h produced pure phase CdO films by removal of trace amount of cadmium hydroxide, as confirmed from X-ray diffractogram. First time employment of CdO in place of TiO₂ in dye-sensitized solar cells is reported to check feasibility and cell performance. A dye-sensitized nanocrystalline CdO photo-electrode was obtained by adsorbing cis-dithiocyanato (4,4′-dicarboxylic acid-2,2′-bipyridide) ruthenium (II) (N3) dye by keeping at 45 °C for 20 h. The efficiency of dye-sensitized nanocrystalline CdO thin film solar cell was increased from 0.24% to 2.95% due to dye adsorption. This must be highest reported conversion efficiency for other metal oxides than TiO₂based dye-sensitized solar cells.     

Electrochemically assisted methane production in a biofilm reactor

Microbial electrolysis is a new technology for the production of value-added products, such as gaseous biofuels, from waste organic substrates. This study describes the performance of a methane-producing microbial electrolysis cell (MEC) operated at ambient temperature with a Geobacter sulfurreducens microbial bioanode and a methanogenic microbial biocathode. The cell was initially operated at a controlled cathode potential of −850 mV (vs. standard hydrogen electrode, SHE) in order to develop a methanogenic biofilm capable of reducing carbon dioxide to methane gas using abiotically produced hydrogen gas or directly the polarized electrode as electron donors. Subsequently, G. sulfurreducens was inoculated at the anode and the MEC was operated at a controlled anode potential of +500 mV, with acetate serving as electron donor. The rate of methane production at the cathode was found to be primarily limited by the acetate oxidation kinetics and in turn by G. sulfurreducens concentration at the anode of the MEC. Temperature had also a main impact on acetate oxidation kinetics, with an apparent activation energy of 58.1 kJ mol⁻¹.     

Photoresponse and H2 production of topographically controlled PEG assisted Sol–gel WO3 nanocrystalline thin films

WO₃ thin films were fabricated by sol–gel method using polyethylene glycol (PEG) as dispersing agent. Physical and photoelectrochemical properties of the synthesized nanocrystalline films were studied by varying weight ratio of PEG to tungsten precursor (x). Based on AFM observations and statistical modeling of the WO₃ surface, the thickness of the films increased by increasing the amount of x with a nearly linear fashion while the surface roughness reached to a saturated value. However, the film synthesized with x = 4 showed a chaotic surface behavior. Optical analysis revealed that by increasing the x, transmittance of the films decreased while their band gap energies remained unchanged. According to XRD results, variation of x did not change structure of the nanocrystalline film while XPS analysis indicated a better stoichiometry for the films with higher x values. A less charge transport life time was obtained for films with higher x values, but an enhanced photoresponse of the films and also hydrogen production via water splitting reaction were observed by increasing the amount of x. On the other hand, the charge transfer resistance of the samples reduced from 6.5 kΩ to 1.2 kΩ by addition of PEG to the sol from x = 0 to x = 2.     

Impacts of pulsed-laser assisted deposition on CIGS thin films and solar cells

We have proposed a novel laser-assisted-deposition (LAD) process for improving the crystalline quality of CIGS thin films and cell performance. The influences of laser power, Ga content in CIGS, substrate temperature, and photon energy of laser on CIGS thin films and solar cells have been investigated. In the LAD process a pulsed excimer laser and a pulsed Nd:YAG laser were irradiated onto the substrate surface during CIGS deposition by the three-stage process. The crystalline quality of CIGS thin films and cell performance, particularly open-circuit-voltage, improved by LAD process for all ranges of Ga content and at substrate temperature ranges of 400-550 °C. It was also found that the laser irradiation enhanced the diffusion of Ga into CIGS even at low substrate temperatures, which strongly affects the formation of double-graded bandgap. The PL decay time of LAD-CIGS solar cells was much longer than that of the fabricated by the three-stage process, which implies the reduced defects in CIGS absorber layer. The improved thin-film quality and cell performance became noticeable only when the laser wavelength was shorter than 266 nm (4.66 eV of photon energy). This result strongly suggests that the impacts of pulsed-laser irradiation are dominated by photon-energy rather than thermal-energy.     

High near-infrared transparent molybdenum-doped indium oxide thin films for nanocrystalline silicon solar cell applications

Molybdenum-doped indium oxide (IMO) thin films were deposited at 450 °C for varying molybdenum concentrations in the range of 0.5-2 at% by the spray pyrolysis technique. These films confirmed the cubic bixbyite structure of polycrystalline In₂O_3. The preferred growth orientation along the (2 2 2) plane shifts to (4 0 0) on higher Mo doping levels. The films doped with 0.5 at% Mo showed high mobility of 76.9 cm²/(V s). The high visible transmittance extends well into the near-infrared region. A possibility of using the produced IMO films in nanocrystalline (nc) silicon solar cell applications is discussed in this article. The morphological studies showed a change in the microstructure, which is consistent with the change in crystallographic orientation.     

Characteristics of nanocrystalline CdSe films

CdSe films have been pulse electrodeposited on evaporated gold substrates with different duty cycles. The deposition of nanocrystalline films was carried out at 100°C for 10 min with precursors of CdSO₄, SeO₂ and H₂SO₄. TEM studies indicated a crystal size in the range 10–50 nm depending upon the duty cycle. The size of the nanoparticle could be controlled by the pulse parameters. Variation of grain size and band gap with duty cycle are in close agreement with the results reported earlier for the films prepared by electrodeposition and chemical bath deposition. Luminescence studies indicated room temperature emission in the wavelength range 520–410 nm depending on the duty cycle, the excitation wavelength was 570 nm.     

Recent advances in polydopamine and its derivatives assisted electrocatalysis and photocatalysis

In recent years, polydopamine (PDA) has attracted extensive attention of researchers due to its unique chemical structure and functional groups. However, a comprehensive review of PDA-based materials in electrocatalysis and photocatalysis is still lacking. The aim of this review is to summarize recent progress and development of PDA-assisted electrocatalytic and photocatalytic reactions. Owing to its unique advantages in surface modification, it has been a good platform for the construction of electrocatalytic and photocatalytic materials. With the development of biological sources, PDA has been increasingly applied in the field of materials science. However, it is still lacking of a comprehensive review which focuses on PDA surface modification strategies and its derivative function for electrocatalysis and photocatalysis applications. Here in this article the preparation and polymerization mechanism of PDA and its derivatives has been firstly reviewed. Then their physical and chemical properties have been introduced, including chemical reactivity, optical property, conductivity, and cohesiveness. Thirdly, the electrocatalytic and photocatalytic activities of PDA and its derivatives have been demonstrated. Finally, the advances of their electrocatalytic and photocatalytic applications have been summarized including hydrogen production, pollutant degradation, and CO₂ reduction. We believe this synthetic study can provide critical information for mussel-inspired chemistry, which will be of great interest for researchers from chemistry, materials, biology, medicine, and interdisciplinary. This paper can provide readers with a general framework of PDA and its derivatives in the field of photocatalysis and electrocatalysis including hydrogen production, pollutant degradation, and CO₂ reduction.     

Low-temperature preparation of boron-doped nanocrystalline SiC:H films using mercury-sensitized photo-CVD technique

We investigated the dependence of hydrogenated boron-doped nanocrystalline silicon-carbide (p-nc-SiC:H) film characteristics against the substrate temperature (T_sub) by the transmission electron micrograph, Raman spectrum, and dark conductivity measurements. High quality of nanocrystalline growth at the low temperature of 120°C shows that the mercury-sensitized photo-assisted chemical vapor deposition (photo-CVD) technique is promising for a low-temperature fabrication of thin film solar cells onto flexible plastic substrates.     

Efficient solar photo-electrochemical hydrogen generation using nanocrystalline CeFeO3 synthesized by a modified microwave assisted method

Pure phase of CeFeO₃ perovskite was synthesized by using a modified microwave-assisted method and was systematically studied by photo-electrochemical (PEC) investigations for water splitting reaction. Characterization studies confirm the formation of crystalline orthorhombic single phase perovskite structure with space group Pbnm and having agglomerated sponge-like morphology with nano size grains. DRS shows broad absorption in UV–Visible region, while tauc plot also inferred estimated band gap of 1.9 eV. The photo-activity of their screen printed thin film was analyzed by PEC studies, which includes photocurrent, EIS spectra, MS-plot, J-V plots. On illumination, EIS analysis of CeFeO₃ reveals improved charge transfer at interfaces of semiconductor/electrolyte. The photocurrent density difference of CeFeO₃ was increased to 6.9 mA cm^?2 at an applied bias of 1.5 V vs (Ag/AgCl). PEC H₂ evolution shows significant cumulative hydrogen rate of 12.3 μmol cm^?2 h^?1. All these results reveal that the microwave-synthesized CeFeO₃ is a potential candidate for PEC application under the visible light illumination.     

Hydrogen sensing using gold nanoclusters supported on tungsten trioxide thin films

Controlled amounts of chemically synthesised gold (Au) nanoclusters were deposited onto tungsten trioxide (WO₃) nanostructured thin films as sensors for hydrogen. The Au/WO₃ thin films were characterised by XPS, XRD, SEM and TEM. Performance of Au/WO₃ films was tested at operating temperatures varying from room temperature to 450 °C. It was demonstrated that Au metal loading plays an important role in defining enhancement of the sensor response towards hydrogen. “Less is more” principle applies to the reported here sensors as materials made using lower concentration of Au nanoclusters demonstrated significantly better response. HRTEM images of the Au/WO₃ thin films provide evidence that the more active sensors are enriched with smaller Au nanoparticles (≤5 nm). Fast response towards H₂ within a wide range of industrially relevant concentrations, excellent baseline stability and signal reproducibility at optimized operating temperature demonstrate feasibility of this novel approach toward fabrication of sensors.     

Role of hydrogen in electrical and structural characteristics of bilayer CdTe/Mn diluted magnetic semiconductor thin films

Bilayer thin films of diluted magnetic semiconductor CdTe/Mn have been prepared using vacuum thermal evaporation method at pressure of 10⁻⁵ torr. Annealing of bilayer thin films has been performed in atmospheric condition at constant temperature 400 °C for 1 hour. Hydrogenation of as-grown and annealed bilayer thin films has been performed by keeping these in hydrogenation cell. Structural characteristics of as-grown and heat treated thin films have been performed by X-ray diffractometer. Current–voltage characteristics of both as-grown hydrogenated and annealed hydrogenated bilayer thin films have been studied to find out the effect of hydrogenation. Surface topography of as-grown and annealed bilayer thin films has been confirmed by optical microscopy.     

Thin semiconductor films for radiative cooling applications

Passive cooling systems use simple low-cost techniques to provide summer comfort in warm climates and can also be used to keep food, liquids and other materials at temperatures below ambient. Radiative cooling devices require a convective shield that should reject solar radiation but be transparent to mid-IR thermal radiation. In this paper, chemical solution deposition of thin semiconductor films (PbS and PbSe) onto polyethylene foils for radiative cooling applications is described. Optical and structural characterizations of the films were performed using UV–Vis, FTIR spectroscopies, X-ray diffraction and electron microscopy. Additionally, pigmented shields, which exhibit good radiation scattering properties, were prepared by incorporation of ZnS or ZnO into polyethylene. We also studied a combination of pigmented polyethylene foils coated with thin films of PbS that show improved optical properties for cooling purpose.     

Preparation of CuInS2 thin films using CBD CuxS films

This paper reports the conversion of CBD Cu_xS films into CuInS₂ films. XRD spectra of the as-prepared Cu_xS films show lack of crystallinity. When pure indium is evaporated over these films and the samples are subsequently annealed, the film improves in crystallinity and shows major peaks of CuInS₂. Absorption spectra and XPS depth profile of the samples support this result.