Khalid Saeed

Although the origin and possible mechanisms for green and yellow emission from different zinc oxide (ZnO) forms have been extensively investigated, the same for red/orange PL emission from ZnO nanorods (nR) remains largely unaddressed. In this work, vertically aligned zinc oxide nanorods arrays (ZnO nR) were produced using hydrothermal process followed by plasma treatment in argon/sulfur hexafluoride (Ar/SF₆) gas mixture for different time. The annealed samples were highly crystalline with ~45 nm crystallite size, (002) preferred orientation, and a relatively low strain value of 1.45 × 10⁻³, as determined from X-ray diffraction pattern. As compared to as-deposited ZnO nR, the plasma treatment under certain conditions demonstrated enhancement in the room temperature photoluminescence (PL) emission intensity, in the visible orange/red spectral regime, by a factor of 2. The PL intensity enhancement induced by SF₆ plasma treatment may be attributed to surface chemistry modification as confirmed by X-ray photoelectron spectroscopy (XPS) studies. Several factors including presence of hydroxyl group on the ZnO surface, increased oxygen level in the ZnO lattice (O_L), generation of F–OH and F–Zn bonds and passivation of surface states and bulk defects are considered to be active towards red/orange emission in the PL spectrum. The PL spectra were deconvoluted into component Gaussian sub-peaks representing transitions from conduction-band minimum (CBM) to oxygen interstitials (O_i) and CBM to oxygen vacancies (V_O) with corresponding photon energies of 2.21 and 1.90 eV, respectively. The optimum plasma treatment route for ZnO nanostructures with resulting enhancement in the PL emission offers strong potential for photonic applications such as visible wavelength phosphors.

Silver (Ag) and/or aluminum (Al)-doped zinc oxide (ZnO:Ag, ZnO:Al) films with different concentrations were produced using sol-gel process and investigated for wettability and photocatalysis. Water contact angle (CA) measurements indicated the films to be hydrophilic with reduced solid/liquid interfacial surface energy upon metal doping. The films were highly transparent (>94%) with red or blue shift in the absorption edge depending on the dopant type (Ag or Al) owing to the Burstein–Moss effect. The ZnO:Ag and ZnO:Al films with 0.5 and 1.0 wt.% metal dopant showed high degradation efficiency in methylene blue (MB) solution under UV irradiation, mainly due to an increase in the photogenerated electron–hole pair recombination time and hydroxyl radicals (·OH) generation. The MB degradation followed pseudo-first-order reaction with maximum apparent reaction rate constant of 2.40 h⁻¹ for the 0.5 wt.% ZnO:Al film. ZnO films with 1.0 wt.% dopant demonstrated excellent photostability and recyclability even after several runs presumably due to reduced Zn²⁺ dissolution as well as blocking of the active surface area. ZnO:(Ag + Al) film containing 0.5 wt.% Al and Ag showed excellent UV photodegradation of MB and rhodamine blue (RhB) with high levels of photostability over five cycles.

Vanadium nitride (VN) thin films were produced via direct-current reactive magnetron sputtering technique followed by vacuum annealing. The treatment was carried out at different temperatures for any effect on their electrochemical (EC) stability, up to 10,000 charge–discharge cycles in 0.5 M K₂SO₄ solution. The film surface chemistry was investigated by using X-ray photoelectron spectroscope (XPS) and cyclic voltammetry (CV) techniques. For the as-deposited film, the oxide layer formed on the VN surface was unstable upon K₂SO₄ immersion treatment, along with ~23% reduction in the EC capacitance. Vacuum annealing under optimized conditions, however, made the oxide layer stable with almost no capacitance loss upon cycling for up to 10,000 cycles. Annealing treatment of the VN films makes them a potential candidate for long-term use in electrochemical capacitors.