Photocatalytic reduction of graphene oxides hybridized by ZnO nanoparticles in ethanol

Graphene oxide platelets synthesized by using a chemical exfoliation method were dispersed in a suspension of ZnO nanoparticles to fabricate ZnO/graphene oxide composite. Formation of graphene oxide platelets (with average thickness of ∼0.8 nm) hybridized by ZnO nanoparticles (with average diameter of ∼20 nm) was investigated. The 2D band in Raman spectrum confirmed formation of single-layer graphene oxides. The gradual photocatalytic reduction of the graphene oxide sheets in the ZnO/graphene oxide suspension of ethanol was studied by using X-ray photoelectron spectroscopy for different ultra violet (UV)–visible irradiation times. After 2 h irradiation, the relative concentration of the C–OH, CO and OC–OH bonds showed nearly 80% reduction relative to the corresponding concentrations before irradiation. The chemical reduction was accompanied by variations in the optical absorption of the ZnO/graphene (oxide) suspension, as its color changed from light brown to black. The current–voltage measurement showed that electrical sheets resistance of the ZnO/graphene oxide sheets decreased by increasing the irradiation time. Therefore, the ZnO nanoparticles in the ZnO/graphene oxide composite could be applied in gradual chemical reduction and consequently tuning the electrical conductivity of the graphene oxide platelets by variation of UV irradiation time in a photocatalytic process.     

Fabrication of spray derived nanostructured n-ZnO/p-Si heterojunction diode and investigation of its response to dark and light

Fluorine doped ZnO thin films were grown by chemical spray pyrolysis technique of zinc acetate and ammonium fluoride, and the effect of fluorine content on structural, optical and electrical properties were evaluated. The structural, morphological, optical properties of ZnO films were investigated by XRD (X-ray diffraction), AFM (Atomic force microscopy), SEM (Scanning electron microscop) and UV–Vis spectroscopy, respectively. According to results, it was observed that all films had polycrystalline texture with hexagonal wurtzite crystal structure and film surface were made up of nano-scale grains, varied by fluorine content. Optical properties showed that optical band gap energy of ZnO changed from 3.28 to 3.24 eV with F content. Shrinkage effect was assessed as the cause in the variation of optical band gap values. Finally, current-voltage (I-V) analysis was performed in Au/ZnO:F/p-Si device in dark and light conditions and certain diode parameters such as ideality factor, barrier height and series resistance were calculated and discussed in detail.     

Modification of optical and electrical properties of zinc oxide-coated porous silicon nanostructures induced by swift heavy ion

Morphological and optical characteristics of radio frequency-sputtered zinc aluminum oxide over porous silicon (PS) substrates were studied before and after irradiating composite films with 130 MeV of nickel ions at different fluences varying from 1 × 10¹² to 3 × 10¹³ ions/cm². The effect of irradiation on the composite structure was investigated by scanning electron microscopy, X-ray diffraction (XRD), photoluminescence (PL), and cathodoluminescence spectroscopy. Current–voltage characteristics of ZnO-PS heterojunctions were also measured. As compared to the granular crystallites of zinc oxide layer, Al-doped zinc oxide (ZnO) layer showed a flaky structure. The PL spectrum of the pristine composite structure consists of the emission from the ZnO layer as well as the near-infrared emission from the PS substrate. Due to an increase in the number of deep-level defects, possibly oxygen vacancies after swift ion irradiation, PS-Al-doped ZnO nanocomposites formed with high-porosity PS are shown to demonstrate a broadening in the PL emission band, leading to the white light emission. The broadening effect is found to increase with an increase in the ion fluence and porosity. XRD study revealed the relative resistance of the film against the irradiation, i.e., the irradiation of the structure failed to completely amorphize the structure, suggesting its possible application in optoelectronics and sensing applications under harsh radiation conditions.     

Electrochemical synthesis and in situ spectroelectrochemistry of conducting polymer nanocomposites. I. polyaniline/TiO2, polyaniline/ZnO,and polyaniline/TiO2+ZnO

The polyaniline (PAn), polyaniline/titanium dioxide (PAn/TiO₂), polyaniline/zinc oxide (PAn/ZnO), and a novel conducting polymer nanocomposites, polyaniline/titanium dioxide + zinc oxide (PAn/TiO₂+ZnO), were synthesized by in situ electropolymerization and potential cycling on gold electrode. The PAn and nanocomposite films were characterized by cyclic voltammetry, Fourier transform infra‐red (FTIR) spectroscopy, in situ resistivity measurements, in situ UV–Visible, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The differences between cathodic and anodic peaks of three redox couples were obtained for PAn and polymeric nanocomposite films. During cathodic and anodic scans, the shift of potential was observed for polymer nanocomposite films. The characteristic FTIR peaks of PAn were found to shift to lower wavelengthsin polymer nanocomposite films. These observed effects have been attributed to interaction of TiO₂, ZnO, and TiO₂+ZnO particles with PAn molecular chains. Significant differences from in situ resistivity of PAn and nanocomposite films were obtained. The resistance of PAn/TiO₂, PAn/ZnO, and PAn/TiO₂+ZnO films were found to be smaller than the PAn film. The in situ UV–Visible spectra for Pan and polymer nanocomposite films were studied. The results show the intermediate spectroscopic properties between PAn and polymer nanocomposite films. The morphological analyses of PAn and nanocomposite films have been investigated. The nanocomposites SEM and TEM micrographs suggest that the inorganic semiconductor particles were incorporated in organic conducting polymer, which consequently modifies the morphology of the films significantly. POLYM. COMPOS., 35:351–363, 2014. © 2013 Society of Plastics Engineers     

Electrodeposited nanoporous ZnO films exhibiting enhanced performance in dye-sensitized solar cells

Electrodeposition of nanoporous ZnO films and their applications to dye-sensitized solar cells (DSSCs) were investigated in the aim of developing cost-effective alternative synthetic methods and improving the ZnO-based DSSCs performance. ZnO films were grown by cathodic electrodeposition from an aqueous zinc nitrate solution containing polyvinylpyrrolidone (PVP) surfactant. PVP concentration had strong effects on the grain sizes and surface morphologies of ZnO films. Nanoporous ZnO film with grain size of 20-40 nm was obtained in the electrolyte containing 4 g/L PVP. The X-ray diffraction pattern showed that nanoporous ZnO films had a hexagonal wurtzite structure. Optical properties of such films were studied and the results indicated that the films had a band gap of 3.3 eV. DSSCs were fabricated from nanoporous ZnO films and the cell performance could be greatly improved with the increase of ZnO film thickness. The highest solar-to-electric energy conversion efficiency of 5.08% was obtained by using the electrodeposited double-layer ZnO films (8 μm thick nanoporous ZnO films on a 200 nm thick compact nanocrystalline ZnO film). The performance of such cell surpassed levels attained in previous studies on ZnO film-based DSSCs and was among the highest for DSSCs containing electrodeposited film components.     

Nucleation effects on structural and optical properties of electrodeposited zinc oxide on tin oxide

Zinc oxide was electrodeposited from oxygenated aqueous solutions of zinc chloride at 80 C on tin oxide covered glass substrates. A new activation treatment for the substrate is established. This consists in the initial formation, in the deposition solution, of a thin metallic zinc layer (5–50 nm) converted to ZnO by in situ reoxidation. Variable densities of nucleation centers (with values approaching 10¹⁰ cm^–2) are formed by this treatment. This allows control of the formation of a zinc oxide layer ranging from open deposits of isolated crystallites to compact and homogeneous layers. Compact layers have high specular transmission below the band gap value (3.5 eV), whereas open films exhibit extensive light scattering. The shapes of the current–time curves during deposition are discussed in terms of nucleation and structural effects. A possible influence of the semiconducting properties of the films is pointed out.     

Effect of doping concentration on the properties of aluminium doped zinc oxide thin films prepared by spray pyrolysis for transparent electrode applications

Zinc oxide possesses many interesting properties, such as modifiable conductivity, wide band gap, high excitonic binding energy, piezo-electric polarisation and cathodoluminiscence. In this study transparent conducting aluminium doped zinc oxide (ZnO:Al) thin films were deposited on float glass substrates by tailor made spray pyrolysis with adaptation for measuring the actual temperature of the substrate surface during deposition. The films were characterised and the effect of aluminium doping concentration [Al/Zn] on their optical, electrical and structural properties was investigated as a function of aluminium doping between 0 and 10 at.%. There was widening of optical band gap with increasing doping concentration. ZnO:Al films with low resistivity of 2.8 × 10⁻² Ω cm and high transmittance of over 85% at 550 nm which are crucial for opto-electrical applications were obtained at a doping ratio of 2 at.%.     

Morphologies of Sol–Gel Derived Thin Films of ZnO Using Different Precursor Materials and their Nanostructures

We have shown that the morphological features of the sol–gel derived thin films of ZnO depend strongly on the choice of the precursor materials. In particular, we have used zinc nitrate and zinc acetate as the precursor materials. While the films using zinc acetate showed a smoother topography, those prepared by using zinc nitrate exhibited dendritic character. Both types of films were found to be crystalline in nature. The crystallite dimensions were confined to the nanoscale. The crystallite size of the nanograins in the zinc nitrate derived films has been found to be smaller than the films grown by using zinc acetate as the precursor material. Selected area electron diffraction patterns in the case of both the precursor material has shown the presence of different rings corresponding to different planes of hexagonal ZnO crystal structure. The results have been discussed in terms of the fundamental considerations and basic chemistry governing the growth kinetics of these sol–gel derived ZnO films with both the precursor materials.

A green approach for the preparation of nanostructured zinc oxide: Characterization and promising antibacterial behaviour

In the present study, nanostructured zinc oxide (ZnO) films have been successfully synthesized using fruit extract of Viburnum opulus L. (VO) on glass slides by successive ionic layer adsorption and reaction (SILAR) procedure. The impact of VO concentrations on the structural, morphological, optical, electrical, and antibacterial attributes of ZnO films has been investigated in detail. The samples' XRD patterns present a hexagonal crystal structure with a preferential orientation along the (002) plane. The crystallite size values of ZnO samples were found to be in the ranges from 14.88 to 9.23 nm. The supplementation of VO to the synthesis solution remarkably affected the surface morphological features of the ZnO films. The optical results demonstrated that band gap energy values of the ZnO films at room temperature were decreased from 3.20 to 3.07 eV as a function of VO content in the bath solution. The films' electrical properties were determined by impedance analysis in the frequency range of 20 Hz ?1 MHz. Impedance-frequency measurements showed VO insertion to ZnO thin films cause an increase in impedance value at the low frequencies. Cole-Cole plots with a single semi-circle confirmed the contribution of grain and grain boundary for the electrical conduction process. The agar disk diffusion method was used to test the antibacterial properties of ZnO/VO inserted ZnO and inhibition zones were measured. VO inserted ZnO showed a stronger inhibitory effect on gram-positive bacteria Staphylococcus aureus (ATCC 25923) and gram-negative bacteria Escherichia coli (ATCC 35218) than ampicillin antibiotic used as a control group. In line with the promising bactericidal results of a new generation, VO inserted ZnO, the nanostructured product with this study, it can also be applied in multidrug-resistant clinical isolates obtained from patients.     

Effect of Au ion beam on structural,surface, optical and electrical properties of ZnO thin films prepared by RF sputtering

In the present work, ZnO thin films were irradiated with 700?keV Au⁺ ions at different fluence (1?× 10¹³, 1?× 10¹⁴, 2?× 10¹⁴ and 5?× 10¹⁴ ions/cm²). The structural, morphological, optical and electrical properties of pristine and irradiated ZnO thin films were characterized by X-ray diffraction (XRD), Fourier transforms infrared spectroscopy (FTIR), scanning electron microscope (SEM), spectroscopy ellipsometry (SE) and four point probe technique respectively. XRD results showed that the crystallite size decreased from pristine value at the fluence 1?×?10¹³ ions/cm², with further increase of ion fluence the crystallite size also increased due to which the crystallinity of thin films improved. SEM micrographs showed acicular structures appeared on the ZnO thin film surface at high fluence of 5?×?10¹⁴ ions/cm². FTIR showed absorption band splitting due to the growth of ZnO nanostructures. The optical study revealed that the optical band gap of ZnO thin films changed from 3.08?eV (pristine) to 2.94?eV at the high fluence (5?× 10¹⁴ ions/cm²). The electrical resistivity of ZnO thin film decreases with increasing ion fluence. All the results can be attributed to localized heating effect by ions irradiation of thin films and well correlated with each other.     

Boron-Doped Graphene/ZnO Nanoflower Heterojunction Composite with Superior Photocatalytic Activity

The boron-doped graphene (BG) is synthesized successfully by one pot reduce-doping of graphene oxide (GO) with borane tetrahydrofuran (BH₃·THF) and the novel BG/ZnO p–n heterojunction composite between p-type BG and n-type ZnO is obtained via a simple hydrothermal method. The samples are characterized by scanning electron microscopy, the Raman spectroscopy and the UV–Vis diffuse spectroscopy. The results confirm the formation of p–n junction between BG and ZnO. The photodegradation of MB demonstrate that the BG/ZnO composite has superior photocatalytic activity under UV–Vis or visible irradiation. The photocatalytic activity k value of BG/ZnO p–n heterojunction composite is 3.1 and 4.5 times as that of r-GO/ZnO and pure ZnO under white light and is 1.8 and 3.9 times under visible light, respectively. The superior photocatalytic activity of the BG/ZnO composite is ascribed to the formation of p–n heterojunction. The p–n heterojunction can promote the separation of electron/hole pairs and inhibit the recombination of electrons in conduction band and holes in valence band by transferring holes from the valence band of n-type ZnO to the valence band of p-type BG.     

Up-Conversion Fluorescence of Phosphorous and Nitrogen Co-Doped Carbon Quantum Dots (CDs) Coupled with Weak LED Light Source for Full-Spectrum Driven Photocatalytic Degradation via ZnO-CDs Nanocomposites

In order to achieve efficient, green, and low-cost photocatalysts, carbon materials are highly desirable alternatives to other sensitizers such as; dyes and chalcogenide quantum dots. In this paper, we describe green synthesis of phosphorous and nitrogen co doped carbon quantum dots (CDs) from a low-cost source. The CDs were used as photosensitizers to enhance the visible-NIR driven photocatalytic degradation of organic compounds under weak LED light illumination as an energy efficient light source. The CDs were highly fluorescent and exhibit excitation wavelength independent fluorescence. Electrochemical measurements of CDs were performed to assess the conduction and valences bands. Nanocomposites ZnO-CDs, interestingly, the CDs as a photosensitizer showed dramatic catalytic enhancement, as 80% of methylene blue was degraded under irradiation with a weak LED white light, visible to near IR light, for 10 h. While ZnO under same circumstances, could remove only 10%. The enhancement mechanism in addition to transfer of photoelectrons from the conduction band of CDs to the conduction band ZnO, could also be related to up-conversion properties of CDs, increase the absorptivity of the light. The presented finding showed that low cost source of carbon was used to make efficient nanocomposite photocatalyst for degradation of organic wastes.

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Nano zinc oxide as a UV-stabilizer for aromatic polyurethane coatings

Nano zinc oxide (ZnO) was co-deposited together with a cathodic electrodeposition paint onto phosphated normal steel panels. The films containing nano zinc oxide were compared with blank films regarding their stability against UV radiation. SEM micrographs show that nano-ZnO can stop the formation of cracks in the film. On the other hand, AFM, surface roughness and loss of gloss studies showed that the presence of nano zinc oxide particles reduces the photo-degradation of the aromatic polyurethane binder. It was also found that the presence of the nano-sized ZnO particles in the films reduces the tendency of the films to yellowing.     

Structure, optical and electrical properties of ZnO thin films on the flexible substrate by cathodic vacuum arc technology with different arc currents

ZnO thin ?lms were successfully deposited onto PET substrates prepared by using cathodic arc plasma deposition (CAPD) technique at a low temperature (<75 °C). Their structure, optical and electrical properties were investigated with various arc currents (40, 45, 50 and 55 A). ZnO (0 0 2) peak was clearly observed, and increased as the arc current increased from 40 A to 55 A. The calculated average crystallized sizes were around 15.9-17.7 nm. The films have an average transmittance over 85% in the visible region, and calculated values of the band gap around 3.33-3.31 eV with increase of the arc current. It was also found that a slight blue shift of optical transmission spectra was observable when decreasing the arc current. The deposited ZnO films had the lowest resistivity; about 3 × 10⁻³ Ω cm for the ZnO ?lm with the arc current of 40 A.     

Enhanced photoelectrochemical performance of Ag–ZnO thin films synthesized by spray pyrolysis technique

Silver doped zinc oxide (Ag–ZnO) thin films were deposited on glass and tin doped indium oxide (ITO) coated glass substrates by using pneumatic spray pyrolysis technique (SPT) at 450 °C from aqueous solutions of zinc acetate and silver nitrate precursors. The effect of silver doping on structural, morphological and optical properties of films was studied. The XRD spectra of the Ag–ZnO films indicate the polycrystalline nature having hexagonal crystal structure. SEM micrographs show the uniform distribution of spherical grains of about 80–90 nm grain size for the pure ZnO thin films. The Ag nanoparticles are clearly visualized in SEM images of Ag–ZnO samples. The optical band gap energy decreases as the percentage of silver doping increases. Surface Plasmon Resonance (SPR) related phenomena are observed and correlated to the optical properties of Ag–ZnO thin films. The overall photoelectrochemical (PEC) performance of the samples was investigated and discussed. Moreover, the samples are more photoactive as compare to the pure ZnO sample and the sample ZnOAg₁₅ shows the highest current. The photocurrent increases upto 249 μA cm⁻² and 303 μA cm⁻² in visible light and in UV illumination, respectively, and then decreases as the Ag doping increases into the film.     

Optimized weighted mobility induced high thermoelectric performance of ZnO-based multilayered thin films

As a transparent thermoelectric oxide, gallium-doped zinc oxide (GZO) has the potential to power wearable or portable electronics and may be used in the integrated circuits industry for chip cooling. Constructing ZnO–GZO interfaces has been proposed as an effective strategy for improving thermoelectric performance of GZO thin films. However, without the aid of band structure calculation for multilayered films, it is hard to directly elucidate the underlying mechanisms of carrier transport. Weighted mobility is an indicator that reveals the inherent electronic transport properties like carrier scattering, electronic band structure, and so on. Thus, to further investigate the effects of ZnO–GZO interfaces on electrical properties of GZO thin films, the structures containing different numbers of ZnO–GZO interfaces were designed and the correlations among numbers of ZnO–GZO interfaces, weighted mobility, and electrical properties were explored. It was found that with more ZnO–GZO interfaces, the weighted mobility increased, and the power factor values also improved as well. Consequently, an enhanced power factor value reached 439 μW m⁻¹ K⁻² at 623 K. This work demonstrated the beneficial effects of multiple interfaces on the improvements of electrical transport performance through analyzing weighted mobility, which laid a foundation for further optimization of thermoelectric performance.     

Epitaxial Aluminum-Doped Zinc Oxide Thin Films on Sapphire: II, Defect Equilibria and Electrical Properties

The electrical transport properties of epitaxial ZnO films grown on different orientations of sapphire substrates have been measured as a function of partial pressure of oxygen. After equilibration, the carrier concentration is found to change from a p ^-1/4_O2 to a p ^-3/8_O2 dependence with increasing oxygen partial pressure. The partial pressure dependence is shown to be consistent with zinc vacancies being the rate-controlling diffusive species. In addition, the carrier concentration in ZnO films grown on A-, C-, and M-plane sapphire are the same but that of R-plane sapphire is systematically lower. Electron Hall mobility measurements as a function of carrier concentration for all the substrate orientations exhibit a transition from "single-crystal" behavior at high carrier concentrations to "polycrystalline" behavior at low carrier concentrations. This behavior is attributed to the effective height of potential barriers formed at the low-angle grain boundaries in the epitaxial ZnO films. The trap density at the grain boundaries is deduced to be 7 × 10¹²/cm². The electron mobility, at constant carrier concentration, varies with the substrate orientation on which the ZnO films were grown. The difference is attributed to the difference in dislocation density in the films produced as a result of lattice mismatch with the different sapphire orientations.     

Effect of heterojunction on photocatalytic properties of multilayered ZnO-based thin films

In the present study, the effects of the heterojunctions on the optical and structural characteristics and the resulting photocatalytic properties of multilayered ZnO-based thin films were investigated. The junctions were composed of semiconducting ZnO nano-porous films coated on the In₂O₃ and SnO₂ counterpart layers. The multilayered ZnO films based on the triple-layered Ag-doped indium oxide (AIO)/tin oxide (TO)/zinc oxide (ZnO), indium oxide (IO)/Ag-doped tin oxide (ATO)/zinc oxide (ZnO), indium oxide (IO)/tin oxide (TO)/zinc oxide (ZnO) and tin oxide (TO)/indium oxide (IO)/zinc oxide (ZnO) have been fabricated by subsequent sol–gel dip coating. Their structural and optical properties combined with photocatalytic characteristics were examined toward degradation of Solantine Brown BRL (C.I. Direct Brown), an azo dye using in Iran textile industries as organic model under UV light irradiation. Effects of operational parameters such as initial concentration of azo dye, irradiation time, solution pH, absence and presence of Ag doping and consequent of sublayers on the photodegradation efficiencies of ZnO nultilayered thin films were also investigated and optimum conditions were established. It was found that the photocatalytic degradation of azo dye on the composite films followed pseudo-first order kinetics. Photocatalytic activity of AIO/TO/ZnO interface composite film was higher compared with other films and the following order was observed for films activities: AIO/TO/ZnO > IO/TO/ZnO > ATO/IO/ZnO > TO/IO/ZnO. Differences in the film efficiencies can be attributed to differences in crystallinity, interfacial lattice mismatch, and surface morphology. Besides, the presence of Ag doping between layers that may act as trap for electrons generated in the ZnO over layer thus preventing electron–hole recombination.