Rupesh S. Devan

Efficient electrochromic performance of nanoparticulate WO3 thin films

This report highlights the suitability of electrodeposited nanoparticulate-WO3 (NP-WO3) electrodes for transmissive electrochromic devices (ECDs). The WO3 electrodes in the form of thin films are composed of 10–20 nm nanoparticles. An electrochromic (EC) device of dimensions 5 × 4 cm2 fabricated using NP-WO3 showed an Li insertion coefficient (x) of 0.43, which resulted in highest photopic transmittance modulation (88.51%), better Li-ion diffusion coefficient (∼3.16 × 10−9 cm2 s−1), fast electrochromic response time (5.2 s for coloration and 3.7 for bleaching) and excellent coloration efficiency (∼137 cm2 C−1). On reduction of WO3, the CIELAB 1931 2° color space coordinates show the transition from colorless to the deep blue state (Y = 97, a* = −1.93, b* = 0.46 and Y = 10, a* = 1.57, b* = −41.01) with steady decrease in relative luminance.

High room-temperature photoluminescence of one-dimensional Ta2O5 nanorod arrays

In this study we analyzed the structural and electronic properties of a new morphological form, one-dimensional (1D) Ta2O5 nanorod arrays, which were synthesized by hot filament metal vapor deposition. Field-emission scanning electron microscopy (FESEM) showed the 1D Ta2O5 nanorods to be arranged in a large-area high-density array about 50 nm wide and approximately 550 nm long. X-ray photoemission spectroscopy (XPS) revealed not only the electronic structures and chemical properties of the 1D Ta2O5 nanorods but also their stoichiometric Ta and O compositions. Photoluminescence (PL) spectra showed intensive green-light, yellow-light and red-light emissions at room temperature. These emissions simultaneously emerged from the trap levels of oxygen vacancies within the Ta2O5 bandgap. The emission results strongly indicate that the 1D Ta2O5 nanorods are good room-temperature visible-light emitters.

An Mn Doped Polyaniline Electrode for Electrochemical Supercapacitor

Undoped and Mn doped Polyaniline (PANI) thin films were deposited on stainless steel substrates by sonochemical method. Films deposition was done using dip coating technique. To study the Mn doping effect on the specific capacitance of PANI, concentration of Mn was varied from 0.4 to 1.6 wt %. The Fourier Transform-IR (FT-IR) and Raman spectroscopy techniques have been used for the phase identification and determination of the Mn in the PANI films. Surface morphology was examined by using Field Emission Scanning Electron Microscopy (FESEM) which showed nanofiber aggregate structure of undoped PANI and porous and well distributed nanofibers for the doped PANI. The supercapacitive behavior of the electrodes was tested in three electrode system with 1.0 M H 2 SO 4 electrolyte by using cyclic voltammetry. The specific capacitance value increases from 285 to 474 F g ―1 as the Mn concentration was increased. This work demonstrates a simple strategy of improving specific capacitance of the polymer and hence may be adopted easily for other dopants also. Thus the work will open a new avenue for designing low cost high performance devices for better supercapcitors.

Photoluminescence mechanisms of metallic Zn nanospheres, semiconducting ZnO nanoballoons, and metal-semiconductor Zn/ZnO nanospheres

We utilized a thermal radiation method to synthesize semiconducting hollow ZnO nanoballoons and metal-semiconductor concentric solid Zn/ZnO nanospheres from metallic solid Zn nanospheres. The chemical properties, crystalline structures, and photoluminescence mechanisms for the metallic solid Zn nanospheres, semiconducting hollow ZnO nanoballoons, and metal-semiconductor concentric solid Zn/ZnO nanospheres are presented. The PL emissions of the metallic Zn solid nanospheres are mainly dependent on the electron transitions between the Fermi level (EF) and the 3d band, while those of the semiconducting hollow ZnO nanoballoons are ascribed to the near band edge (NBE) and deep level electron transitions. The PL emissions of the metal-semiconductor concentric solid Zn/ZnO nanospheres are attributed to the electron transitions across the metal-semiconductor junction, from the EF to the valence and 3d bands, and from the interface states to the valence band. All three nanostructures are excellent room-temperature light emitters.

Electrochromic properties of dandelion flower like nickel oxide thin films

This report highlights a one pot, surfactantless, template free approach to grow novel dandelion flower like nickel oxide (NiO) thin films composed of nano-flakes with a highly porous structure. Good transmittance modulation, fast response time and excellent coloration efficiency make them a potential electrode material for electrochromic devices.

Hydrothermal synthesis of rutile TiO2 nanoflowers using Brønsted Acidic Ionic Liquid [BAIL]: Synthesis, characterization and growth mechanism

Herein we report a facile method to synthesize rutile TiO2 nanoflowers (TNF) comprising a bunch of aligned nanorods with uniform size and shape via a hydrothermal method in Bronsted Acidic Ionic Liquid [BAIL] room temperature ionic liquid (RTIL). This method has some advantages: the process is simple and single step; the reaction can be performed under low temperature. The TNFs are highly crystalline and free of aggregation.

Electrochromic properties of large-area and high-density arrays of transparent one-dimensional β-Ta2O5 nanorods on indium-tin-oxide thin-films

We report on the synthesis, crystalline structure, and electrochromic properties of transparent one-dimensional (1D) orthorhombic (β) Ta2O5 nanorods grown in a large-area high-density array. The transparent 1D β-Ta2O5 nanorod array was synthesized on a conducting indium-tin-oxide thin-film via hot-filament metal-oxide vapor deposition. The array contained ∼1900 β-Ta2O5 nanorods per square micrometer, which were on average, ∼17 nm wide and ∼300 nm long. The good coloration/bleaching cycles, large ion-diffusion coefficient (∼2.35×10−8 cm2/s), and high reversibility (∼79.8%) demonstrate that the 1D β-Ta2O5 nanorods to be a potential electrochromic material for electrochromic devices or smart windows.

Low-temperature phase transformation and phonon confinement in one-dimensional Ta2O5 nanorods

The thermochromic phase transformations of one-dimensional Ta2O5 nanorods have been analyzed at elevated temperatures ranging from 80 to 300 K. The nanorods, grown in a large-area high-density array, are 14–22 nm wide and approximately 500 nm long. The array contained ∼93.5% of the orthorhombic (β) phase and ∼6.5% of the tetragonal (α) phase. Low-temperature X-ray diffraction results showed complex and polymorphic thermochromic phase transformations of the β(001), α(101) and α(103) lattice planes of the nanorods, which incorporate (i) α-to-α (α–α), (ii) α–α–β and (iii) α–β phase transitions. In comparison with the Raman scattering of three-dimensional bulk powder and two-dimensional thin films of Ta2O5, there were concurrent Raman blue- and redshifts in the one-dimensional Ta2O5 nanorods, indicating that the molecular vibrations of the nanorods were confined owing to the reduction of size and dimension.

Impact of Nanosize on Supercapacitance: Study of 1D Nanorods and 2D Thin-Films of Nickel Oxide

We synthesized unique one-dimensional (1D) nanorods and two-dimensional (2D) thin-films of NiO on indium-tin-oxide thin-films using a hot-filament metal-oxide vapor deposition technique. The 1D nanorods have an average width and length of ∼100 and ∼500 nm, respectively, and the densely packed 2D thin-films have an average thickness of ∼500 nm. The 1D nanorods perform as parallel units for charge storing. However, the 2D thin-films act as one single unit for charge storing. The 2D thin-films possess a high specific capacitance of ∼746 F/g compared to 1D nanorods (∼230 F/g) using galvanostatic charge-discharge measurements at a current density of 3 A/g. Because the 1D NiO nanorods provide more plentiful surface areas than those of the 2D thin-films, they are fully active at the first few cycles. However, the capacitance retention of the 1D nanorods decays faster than that of the 2D thin-films. Also, the 1D NiO nanorods suffer from instability due to the fast electrochemical dissolution and high nanocontact resistance. Electrochemical impedance spectroscopy verifies that the low dimensionality of the 1D NiO nanorods induces the unavoidable effects that lead them to have poor supercapacitive performances. On the other hand, the slow electrochemical dissolution and small contact resistance in the 2D NiO thin-films favor to achieve high specific capacitance and great stability.

An efficient methodology for measurement of the average electrical properties of single one-dimensional NiO nanorods

We utilized a metal tantalum (Ta) ball-probe to measure the electrical properties of vertical-aligned one-dimensional (1D) nickel-oxide (NiO) nanorods. The 1D NiO nanorods (on average, ~105 nm wide and ~700 nm long) are synthesized using the hot-filament metal-oxide vapor deposition (HFMOVD) technique, and they are cubic phased and have a wide bandgap of 3.68 eV. When the 1D NiO nanorods are arranged in a large-area array in ohmic-contact with the Ta ball-probe, they acted as many parallel resistors. By means of a rigorous calculation, we can easily acquire the average resistance RNR and resistivity ρNR of a single NiO nanorod, which were approximately 3.1 × 1013 Ω and 4.9 × 107 Ω.cm, respectively.