Synthesis of layered birnessite-type manganese oxide thin films on plastic substrates by chemical bath deposition for flexible transparent supercapacitors

Layered birnessite-type manganese oxide thin films are successfully fabricated on indium tin oxide coated polyethylene terephthalate substrates for flexible transparent supercapacitors by a facile, effective and inexpensive chemical bath deposition technology from an alkaline KMnO₄ aqueous solution at room temperature. The effects of deposition conditions, including KMnO₄ concentration, initial molar ratio of NH₃·H₂O and KMnO₄, bath temperature, and reaction time, on the electrochemical properties of MnO₂ thin films are investigated. Layered birnessite-type MnO₂ thin films deposited under optimum conditions display three-dimensional porous morphology, high hydrophilicity, and a transmittance of 77.4% at 550 nm. A special capacitance of 229.2 F g⁻¹ and a capacitance retention ratio of 83% are obtained from the films after 1000 cycles at 10 mV s⁻¹ in 1 M Na₂SO₄. Compressive and tensile bending tests show that as-prepared MnO₂ thin film electrodes possess excellent mechanical flexibility and electrochemical stability.     

Room-temperature synthesis of crystallized LiCoO2 thin films by electrochemical technique

LiCoO₂ thin films have been directly synthesized on cobalt substrate in LiOH solution at room temperature by electrochemical method. The obtained LiCoO₂ thin films were characterized by X-ray diffraction (XRD), scanning electronic microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The influence of electrochemical reaction time, current density and concentration of LiOH solution on the crystal structure and morphology of the obtained LiCoO₂ thin films was discussed emphatically. Our results show that the as-synthesized LiCoO₂ films all are pure hexagonal structure. The crystallinity, densification and uniformity of the films increase with increasing electrochemical reaction time, current density as well as concentration of LiOH solution and then decrease. The preferable electrochemical reaction conditions were optimized as: electrochemical reaction time is 50 h, current density is 1 mA cm⁻² and concentration of LiOH solution is 3 mol dm⁻³.     

Structural-controlled synthesis of manganese oxide nanostructures and their electrochemical properties

A structural-controlled synthesis of manganese oxide nanostructures from single Mn₃O₄ precursor via hydrothermal method is presented. The obtained products include α- and β-MnO₂ nanorods, γ-MnO₂ urchin-like microspheres, birnessite nanowires and nanosheets. The morphology evolution process and the effect of varying reaction parameter to the phase and morphology are systematically investigated. The formation mechanisms have been rationalized. The obtained nanostructures are as the model system for studying the electrochemical capacitance performances, which have been investigated by cyclic voltammetry.     

ZnO and TiO2 1D nanostructures for photocatalytic applications

ZnO and TiO₂ 1D nanostructures (nanorods and nanotubes) were prepared by low-cost, low-temperature, solution-based methods and their properties and photocatalytic performance were studied. ZnO nanorod samples with titania and alumina shells were also prepared by solution-based methods, and their properties and photocatalytic performance were compared to that of bare ZnO nanorods. We found that ZnO and TiO₂ exhibited comparable photocatalytic performance. Faster dye degradation under simulated solar illumination was observed for ZnO, while under UV illumination faster degradation was observed for TiO₂. ZnO nanorods with titania shells exhibited inferior photocatalytic performance, while for alumina shells the performance was similar to bare ZnO. Reasons for observed differences are discussed, and the effect of the shell on photocatalytic activity is attributed to the changes in native defects at the ZnO surface/shell interface.     

Synthesis mechanisms of lithium cobalt oxide prepared by hydrothermal–electrochemical method

Thermodynamic calculation method was adopted to predict the reaction mechanism of LiCoO₂ prepared by hydrothermal–electrochemical process. It was found that in the Co–LiOH–H₂O system, Co was oxidized to HCoO₂⁻, Co(OH)₂ (100 °C) or CoO (150 °C), CoOOH in sequence with the increase of electrode potential, then the ion-exchange reaction of CoOOH and lithium ion occurred and LiCoO₂ came into being. The optimum synthesis parameter was obtained through thermodynamic calculation and it was validated experimentally by cyclic voltammogram method.     

Advanced synthesis of nanostructured materials for environmental applications

The present study deals with Aerosol Spray Pyrolysis (ASP) synthesis of two families of nanostructured redox materials targeted to two different environmental applications: transition-metal-doped ferrites and base metal-doped cerium oxide, used for hydrogen production through solar-assisted water splitting and for catalytic soot oxidation, respectively. The synthesized powders were characterized with respect to their phase composition, morphology and particle size distribution by XRD, SEM and TEM analysis, which have shown their nanostructured character. Doped ferrites were evaluated, with respect to their hydrogen production activity from water dissociation, in an in-house built water-splitting testing rig. ASP materials proved to be very active water splitters demonstrating higher water conversion and hydrogen yields than materials of the same composition synthesized through Solid State Synthesis (SSS), with material performance depending on the dopants’ kind and stoichiometry. Base metal-doped cerium oxides were evaluated with respect to their direct soot oxidation activity, via Thermogravimetric Analysis (TGA), as well as on a diesel engine bench under realistic conditions. It was found that doping improves their activity and that they enhance soot oxidation at lower temperatures compared to materials synthesized through Liquid Phase Self-propagating High temperature Synthesis (LPSHS).     

Diphenylthiocarbazone (dithizone)-assisted solvothermal synthesis and optical properties of one-dimensional CdS nanostructures

One-dimensional (1D) cadmium sulfide (CdS) nanostructures with various aspect ratios were successfully synthesized by a diphenylthiocarbazone (dithizone)-assisted solvothermal method. The results showed that the dithizone-assisted synthesized samples had larger aspect ratio than that prepared in the absence of dithizone, and CdS nanowires with the highest aspect ratio were obtained with an appropriate dithizone amount (0.03 g/50 ml ethylenediamine in the present system). All the 1D CdS nanostructures were in hexagonal wurtzite phase. The as-synthesized large-scale CdS nanowires were in diameters ranging from 70 to 80 nm, length up to 20 μm, and aspect ratios of 250-285. Further characterization indicated that the CdS nanowires were single crystalline with a preferential growth orientation of [0 0 2], c-axis. Two optical absorption peaks were observed at about 488 nm and 502 nm for the CdS nanowire sample with high aspect ratio in the optical absorption spectroscopy, which could be attributed to the nanometer effect of nanowires. It was found that the additive dithizone was a crucial factor in controlling the morphology and optical properties of the 1D CdS nanostructures. The growth mechanism of 1D CdS nanostructure and the effects of dithizone in the present system were discussed.     

Deposition and characterization of CuInSe2 films for solar cells using an optimized chemical route

CuInSe₂ (CISe) thin films have been deposited on glass using successive ionic layer adsorption and reaction (SILAR). The as-deposited films are treated at 400 °C in argon atmosphere and etched in KCN solution to remove detrimental secondary phases. The preparation and temperature of the precursor solutions, the duration of the reaction cycles and the duration of the annealing stage have been optimized. The films have been characterized employing grazing incident X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and energy dispersive scanning spectroscopy. Relevant semiconductor parameters have been calculated. Photoelectrochemical tests confirm p-type conduction. The films are crystalline and the stoichiometry can be improved by renewing the precursor solution after completing half of the cycles, annealing for 90 min and later etching in KCN. The quality of the material seems to be promising for application in solar cell devices.     

Surfactant assisted electrodeposition of MnO2 thin films: Improved supercapacitive properties

In order to obtain a high specific capacitance, MnO₂ thin films have been electrodeposited in the presence of a neutral surfactant (Triton X-100). These films were further characterized by means of X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, field emission scanning electron microscopy (FESEM) and contact angle measurement. The XRD studies revealed that the electrodeposited MnO₂ films are amorphous and addition of Triton X-100 does not change its amorphous nature. The electrodeposited films of MnO₂ in the presence of the Triton X-100 possess greater porosity and hence greater surface area in relation to the films prepared in the absence of the surfactant. Wettability test showed that the MnO₂ film becomes superhydrophilic from hydrophilic due to Triton X-100. Supercapacitance properties of MnO₂ thin films studied by cyclic voltammetry, galvanostatic charge-discharge cycling and impedance spectroscopy showed maximum supercapacitance for MnO₂ films deposited in presence of Triton X-100 is 345 F g⁻¹.     

Solvothermal synthesis and characterization of tungsten oxides with controllable morphology and crystal phase

Tungsten oxides with various morphologies and crystal phases were synthesized by solvothermal reactions at 200 °C for 7-12 h using different solvents. The morphology and crystal phase of tungsten oxides changed depending on the solvents, i.e., spherical particles of ca. 1 μm in diameter consisting of nanowires, spindle shaped bundles of ca. 1 μm in length consisting of nanowires and accumulations consisting of micrometer sized plates and/or rods of tungsten oxides were obtained using ethanol, 1-propanol and water-ethanol mixed solution, respectively. When water-ethanol mixed solution was used, the crystallinity of tungsten oxide increased but the specific surface area greatly decreased. Crystallinity of tungsten oxides had more important effects on the NO degradation under light irradiation. The product using 42.9 vol.% water-ethanol mixed solvent consisted of the mixture of anhydrous tungsten oxide and hydrous tungsten oxide with preferential orientation of (0 0 2) plane and small band gap energy (2.43 eV), and showed higher photocatalytic degradation of NO even though it had a much smaller specific surface area than those prepared using ethanol and 1-propanol.     

Microwave assisted chemical bath deposited polyaniline films for supercapacitor application

In the present investigation, we are reporting the first time synthesis of polyaniline (PANI) thin films by microwave assisted chemical bath deposition (MW-CBD) method on the stainless steel substrate. The PANI thin films are prepared by the oxidation of aniline in the domestic microwave oven working with frequency 2.45 GHz. The PANI thin films are characterized for their structural, morphological and optical studies by means of X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM) and UV-vis spectrophotometer. The wettability study is carried out by measuring the contact angle. The supercapacitive behavior of PANI electrode is studied in 0.5 M H₂SO₄ using cyclic voltammetric (CV) measurements. The X-ray diffraction pattern showed the films are amorphous. Morphological study revealed PANI thin film is well covered over the entire substrate surface with less overgrown fine spherical granules. The optical band gap of PANI thin film is found to be 2.5 eV. The hydrophilic nature of the PANI thin films is observed from water contact angle measurement. A maximum specific capacitance is found to be 753 F g⁻¹ at the scan rate of 5 mV s⁻¹.     

Preparation and electrochemical study of cerium–silica sol–gel thin films

Design and development of suitable multilayered systems for delaying corrosion advance in metals requires that both the alteration mechanisms of the metal and the behaviour and properties of the protective coatings be known. Coatings prepared by the sol–gel method provide a good approach as protective layers on metallic surfaces. This kind of coatings can be prepared from pure chemical reagents at room temperature and atmospheric pressure, with compositions in a very wide range of environmentally non-aggressive precursors. Sol–gel coatings based on siloxane bonded units were prepared starting from an organic–inorganic hybrid system. The precursors were γ-methacryloxypropyltrimethoxysilane (MAP) and tetramethoxysilane (TMOS). Cerium nitrate hexahydrate in three different concentrations was added. Cerium salts may perform a similar protective effect to that carried out by the well-known lead oxides and chromium salts, even though in this case a negative environmental impact is not expected. Application of coatings upon pure zinc substrates and common glass slides were performed by spinning. Coated samples were heat treated at 40 °C for 6 days. Optical measurements (UV-Vis absorption and diffuse reflectance spectroscopies) pointed out that the coatings were colourless and transparent, reducing the diffuse reflectance of the metallic surface up to 60%. Optical and scanning electron microscopies (SEM) allowed observation of the texture and microstructure of the coated samples, both before and after the corrosion tests were carried out. Likewise, the remaining sols were kept to gelify at 60 °C for 4 days and then powdered to obtain suitable samples for analysing them by other characterisation techniques (Fourier transformed infrared, FTIR and differential thermal analysis, DTA). Electrochemical measurements were performed by impedance spectroscopy. This technique was used to clarify the anticorrosive protection role of cerium ions incorporated into the hybrid sol–gel network. The effect of cerium concentration on the impedance spectra was analysed, as well as the system behaviour against the corrosive medium (0.6 M NaCl aqueous solutions), as a function of exposure time. From the electrochemical point of view, the sol–gel films behave as a conversion coating on the metallic surface.     

Structural characteristics and resistive switching properties of thermally prepared TiO2 thin films

Polycrystalline TiO₂ thin films were formed on Pt(1 1 1)/Ti/SiO₂/Si by thermal oxidation of Ti films with temperatures ranging from 600 °C to 800 °C. Results of Raman spectra testing indicate that the structure of the oxidized TiO₂ films is rutile phase. The resistance switching behaviors (RSB) have been confirmed in Pt/TiO₂/Pt structures. A stable RSB with a narrow dispersion of the resistance states and switching voltages was observed in the sample fabricated with the oxidation temperature of 600 °C. The resistance ratios of high resistance states to low resistance states are larger than 10³ with the set and reset voltage as low as 2.5 V and 0.6 V, respectively.     

Annealing driven performance and optical effects in nanocrystalline SnO2 thin films induced by pulsed delivery

Tin dioxide thin films were prepared successfully by pulsed laser deposition techniques on glass substrates. The thin films were then annealed for 30 min from 50 °C to 550 °C at 50 °C intervals. The influence of the annealing temperature on the microstructure and optical properties of SnO₂ thin films was investigated using X-ray diffraction, optical transmittance and reflectance measurements. Various optical parameters, such as optical band gas energy, refractive index and optical conductivity were calculated from the optical transmittance and reflectance data recorded in the wavelength range 300-2500 nm. We found that the SnO₂ thin film annealed at temperatures up to 400 °C is a good window material for solar cell application. Our experimental results indicated that SnO₂ thin films with the high optical quality could be synthesized by pulsed laser deposition techniques.     

Yttrium aluminum garnet powders by nitrate decomposition and nitrate–urea solution combustion reactions—a comparative study

Precursors for yttrium aluminum garnet (Y₃Al₅O₁₂—YAG) were synthesized by simple decomposition of concentrated aqueous solution of nitrates and combustion of concentrated aqueous solution of nitrates with urea on a heater. The precursor formed by the former reaction was granules of agglomerated powder while that from the latter reaction was a voluminous and porous sponge-like mass. Both precursors were ground to powders and subjected to detailed thermogravimetric–differential thermal analysis and X-ray diffraction studies. The precursor from the simple decomposition of nitrates exhibited a total loss in weight of about 18% in stages (25 to 300 °C and 300 to 600 °C) accompanied by endotherms—characterized as processes of dehydration of absorbed moisture and decomposition of residual nitrates, respectively. The as formed precursor and that heated to 820 °C were amorphous. Crystallization to YAG phase occurred from an amorphous oxide characterized by an exotherm above 820 °C with no loss of weight. The precursor from nitrate–urea combustion reaction was found to exhibit a weight loss of 2.5% accompanied by a shallow endotherm in the range of 25 to 300 °C—characterized as the process of dehydration of absorbed moisture. No further weight loss or heat effect was noticed, confirming it to be chemically pure YAG. This as formed precursor was found to be crystalline YAG. The difference in chemical composition of the precursors formed by these two reactions is attributed to the difference in the actual reaction temperatures during their formation—lower reaction temperature for the endothermic decomposition of nitrates and higher reaction temperature for the exothermic combustion associated with the formation of a bright flame. The morphology of the precursor powder formed by the former reaction exhibited only cracks while that of the precursor from the latter reaction exhibited pores and voids. The precursor from the former reaction was calcined at 1100 °C to form into chemically pure YAG. Zeta potential variation with pH for the aqueous suspensions of the crystalline YAG powders from both the reactions exhibited a maximum value in the range of 40 to 50 mV around a pH of 4, indicating stability of these dispersions towards coagulation at this pH. Particle size distribution of wet ground powders (slurries with 20%, v/v, solid at a pH of 4) showed that the powder from combustion reaction could be formed into a finer size than that from simple nitrate decomposition, indicating the agglomerates of combustion reaction were softer.     

Growth and characteristics of Zn-Se-S thin layers by dip method

Zn-Se-S composite thin films of mixed cubic and hexagonal phases of ZnSe and ZnS are synthesized by dip process at room temperature. Polycrystalline nature of the films was observed from XRD. Optical band gap of Zn-Se-S thin film was found to be in between individual band gaps of ZnSe and ZnS. The electrical conductivity was found to be in the range of 10⁻⁵-10⁻² (Ω cm)⁻¹. Due to above properties, these films find applications as a buffer layer in solar cells.     

Combustion synthesis and characterization of highly crystalline single phase nickel ferrite nanoparticles

A combustion route of synthesizing highly crystalline single phase nickel ferrite (NiFe₂O₄) spinel nanoparticles using various amounts of dl-alanine as fuel has been reported in this paper. The role of the amount of fuel is found to be significant in the size control and phase purity of nano crystalline samples. The structural, thermal, morphological and magnetic studies have been carried out. XRD patterns reveal the formation of highly crystalline nano NiFe₂O₄ with high degree of phase purity when fuel concentration is maintained at 1 M and 2 M. FTIR spectra also prove the formation of pure nano NiFe₂O₄. The temperature and fuel effects are found to have strong influence on size, structural, and magnetic properties of materials. The magnetic measurements show that the nano NiFe₂O₄ samples exhibit soft ferromagnetism with the highest saturation magnetization Ms at room temperature as 42.53 emu/g.     

Hierarchical CuO hollow microspheres: Controlled synthesis for enhanced lithium storage performance

In this work, hierarchical CuO hollow microspheres were hydrothermally prepared without use of any surfactants or templates. By controlling the formation reaction conditions and monitoring the relevant reaction processes using time-dependent experiments, it is demonstrated that hierarchical CuO microspheres with hollow interiors were formed through self-wrapping of a single layer of radically oriented CuO nanorods, and that hierarchical spheres could be tuned to show different morphologies and microstructures. As a consequence, the formation mechanism was proposed to proceed via a combined process of self-assembly and Ostwald's ripening. Further, these hollow microspheres were initiated as the anode material in lithium ion batteries, which showed excellent cycle performance and enhanced lithium storage capacity, most likely because of the synergetic effect of small diffusion lengths in building blocks of nanorods and proper void space that buffers the volume expansion. The strategy reported in this work is reproducible, which may help to significantly improve the electrochemical performance of transition metal oxide-based anode materials via designing the hollow structures necessary for developing lithium ion batteries and the relevant technologies.     

One-dimensional growth of zinc oxide nanostructures from large micro-particles in a highly rapid synthesis

Large and irregularly-shaped zinc oxide (ZnO) micro-particles commonly found in a high-temperature vapor-phase process known as the catalyst-free combust-oxidized mesh (CFCOM) process, play a crucial role as nucleation hosts for ZnO one-dimensional (1D) nanostructures, especially nanometric wires and rods. Nanowires and nanorods tend to grow from the hillocks of the large micro-particles whereby these hillocks serve as nucleation sites for the acicular structures. Nanowires with aspect ratios exceeding 5 are the most common 1D structures that grow from pillar-like hillocks, while triangular hillocks are probable nucleation hosts for nanorods. The ZnO nanostructures possess a polycrystalline nature with photoluminescent emission in the UV band-edge and visible regimes. A novel and non-destructive electrical resistance measurement method is introduced in that the 1D ZnO nanostructures exhibited very high GΩ resistance that is over five times higher than that of commercial ZnO. A growth model is proposed to offer a probable explanation for the fascinating rapid growth of 1D nanostructures originating from large ZnO micro-particles. The ZnO particles in this work were synthesized using 5-ton industrial furnaces via a university-industry joint effort.     

Structural and electrical properties of SrBi2(Ta0.5Nb0.5)2O9 thin films

SrBi₂(Ta_0.5Nb_0.5)₂O₉ (SBTN) thin films were obtained by polymeric precursor method on Pt/Ti/SiO₂/Si(1 0 0) substrates. The film is dense and crack-free after annealing at 700 °C for 2 h in static air. Crystallinity and morphological characteristic were examined by X-ray diffraction (XRD), field emission scanning electron microscopy (FEG-SEM) and atomic force microscopy (AFM). The films displayed rounded grains with a superficial roughness of 3.5 nm. The dielectric permittivity was 122 with loss tangent of 0.040. The remanent polarization (P_r) and coercive field (E_c) were 5.1 μC/cm² and 96 kV/cm, respectively.