Structural,optical and photocatalytic properties of hafnium doped zinc oxide nanophotocatalyst

A series of novel hafnium (Hf) doped ZnO nanophotocatalyst were synthesized using a simple sol–gel method with a doping content of up to 6 mol%. The structure, morphology and optical characteristics of the photocatalysts were characterized by XRD, SEM, TEM, FTIR, XPS, DRS and PL spectroscopy. The successful synthesis and chemical composition of pure and doped ZnO photocatalysts were confirmed by XRD and XPS. DRS confirmed that the spectral responses of the photocatalysts were shifted towards the visible light region and showed a reduction in band gap energy from 3.26 to 3.17 eV. Fluorescence emission spectra indicated that doped ZnO samples possess better charge separation capability than pure ZnO. The photocatalytic activity of Hf-doped ZnO was evaluated by the methylene blue (MB) degradation in aqueous solution under sunlight irradiation. Parameters such as irradiation time and doping content were found effective on the photoactivity of pure ZnO and Hf-doped ZnO. The photocatalysis experiments demonstrated that 2 mol% Hf-ZnO exhibited higher photocatalytic activity as compared to ZnO, ZnO commercial and other hafnium doped ZnO photocatalysts and also revealed that MB was effectively degraded by more than 85% within 120 min. The enhanced photoactivity might be attributed to effective charge separation and enhanced visible light absorption. It was concluded that the presence of hafnium within ZnO lattice could enhance the photocatalytic oxidation over pure ZnO.     

Sintering studies of nano-crystalline zinc oxide

Sintering and grain growth of nano-crystalline undoped ZnO has been studied in detail over a wide range of temperature and holding time. Below 800 °C, sintering of over 70% theoretical density is not observed, irrespective of particle size. At 900 °C for 6 h, the nano-crystalline sample sinters to 99% of theoretical density whereas the density for as received sample is 93% of theoretical density. However, at 1300 °C or higher, the densification is found to be much faster and after a few hours becomes independent of holding time. Grain growth studies reveal a similar feature of attaining saturation over holding time. The average saturated grain size is found to be ∼1.5 and ∼2.2 μm at 800 and 900 °C, respectively, while at 1300 °C or higher, it is in between 12 and 13 μm.     

In-situ study of electrophoretic deposition of zinc oxide nanosheets and nanorods

In the present work, the effects of two different morphologies of zinc oxide nanoparticles (nanosheets and nanorods) were investigated by in-situ measurement of deposition weight, and current density. ZnO nanosheets and nanorods were synthesized by microwave-assisted method using co-surfactant route. The average thickness of obtained nanosheets, and the average diameter of nanorods were measured to be about 26 nm and 139 nm, respectively. ZnO films were obtained by electrophoretic deposition from suspension of nanoparticles in ethanol under different voltages. Results indicated that ZnO nanosheets tend to have greater deposition rate than ZnO nanorods under similar conditions. The compactness of the film obtained from nanosheet suspension was higher than the one obtained from nanorod suspension. However, the film obtained from ZnO nanorods displayed more uniformity at different voltages in comparison to the film obtained from ZnO nanosheets, which can be due to different active surface area, and also different way of motion under hydrodynamic forces in the suspension.     

Synthesis and cathodoluminescence properties of porous wood (fir)-templated zinc oxide

Wood (fir)-templated ZnO with hierarchically porous structure has been successfully synthesized through a simple hydrothermal process. Morphology and porosity of the products were investigated by FESEM, TEM, and N₂ adsorption, respectively. The optical properties were measured by cathodoluminescence (CL) at room temperature. The morphologies of bulk and ground flake ZnO show an inheritance from the fir microstructure. Experimental results suggest that a higher calcination temperature will influence the grain size and porosity. The pore size decreases from 20 to 10 μm in the bulk ZnO, while increases from 50 nm to several micrometers in the flake ZnO when the calcination temperature changes from 600 to 1200 °C. CL spectra also show temperature-dependent properties at ultraviolet (UV) band and blue band. The intensity of visible emission originated from oxygen vacancies is proportional to the calcination temperature, while that of UV emission is inverse proportional due to quantum confinement effect.     

Hot pressing of nanocrystalline zinc oxide compacts: Densification and grain growth during sintering

Sintering behavior of nanocrystalline zinc oxide (ZnO) powder compacts using hot pressing method was investigated. The sintering conditions (temperature and total time) and results (density and grain size) of two-step sintering (TSS), conventional sintering (CS) and hot pressing (HP) methods were compared. The HP technique versus CS was shown to be a superior method to obtain higher final density (99%), lower sintering temperature, shorter total sintering time and rather fine grain size. The maximum density achieved via HP, TSS and CS methods were 99%, 98.3% and 97%, respectively. The final grain size of samples obtained by HP was greater than that of TSS method. However, the ultra-prolonged sintering total time and the lower final density (88 ks and 98.3%) are the drawbacks of TSS in comparison with the faster HP (17 ks and 99%) method.     

High-current measurement of the grain resistivity in zinc oxide varistor ceramics

A new pulse technique for grain resistivity measurement in varistor ceramics is suggested. Such technique allows obtaining more precise value of the grain resistivity due to the use of the concept of differential electrical resistance. This technique can be used in the current density range where the overheating of varistor sample is insignificant. The technique was verified using commercial ZnO varistors. Grain resistivities of 0.60±0.02 Ω cm at 293 K and of 3.40±0.13 Ω cm at 77 K were obtained. This result indicates the negative temperature coefficient of grain resistance in ZnO varistor in the range (77–293) K. The contribution of the grain boundaries to the current–voltage characteristic of ZnO varistor is estimated on the basis of the measured grain resistivity and the current–voltage data. It is shown that the electrical conduction in ZnO varistor is controlled by grains if the current density exceeds approximately 1000 А сm⁻².     

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.     

Morphology and optical analysis of defect levels in ultrasonically-sprayed zinc tin oxide thin films

We prepared zinc-doped tin oxide (ZTO) films of various concentrations of zinc using the ultrasonic spray pyrolysis technique. The surface morphology and roughness were studied by Field-Emission Scanning Electron Microscope and atomic force microscope. Systematic x-ray diffraction (XRD) measurements revealed that all films have tetragonal rutile structure and mixed preferred orientation along the (200) and (110) planes. The proper substitution of zinc ions into tin sites and the absence of any secondary phase in the films were proved by the analysis of the x-ray photoelectron spectroscopy (XPS) measurements. The detailed analysis of the optical absorption spectra revealed the presence of four direct optical transitions within the energy range 3.8–5.1 eV. The defect levels of the neutral and double-ionized oxygen vacancy states were determined within the energy bandgap by the comprehensive analysis of the photoluminescence spectra. The existence of high concentration of oxygen vacancies is supported by the XPS results. The observed transitions were used to sketch the band diagram of ZTO films.     

Growth and structural characteristics of perovskite-wurtzite oxide ceramics thin films

Wurtzite ZnO thin films were grown on single-crystal perovskite SrTiO₃(STO) (1 0 0) substrates at various temperatures. The ZnO/STO thin films thus formed exhibit a preferred (1 1 0)-orientation at a growth temperature of 600-700 °C. A high growth temperature enhances not only the (1 1 0)-texture of ZnO/STO thin films but also the crystalline quality of the film. (La_0.7Sr_0.3)MnO₃ (LSMO) thin films were subsequently grown on ZnO(1 1 0)/STO(1 0 0) substrates with various thicknesses, and were polycrystalline. A thicker LSMO film has a stronger (0 0 l)-preferred orientation than the thinner one. The lattice distortion of LSMO decreases as the LSMO thickness increases. Magnetization vs. temperature curves show that both crystalline quality and lattice distortion influence the magnetic properties of LSMO thin films. The physical properties of the manganite oxide can be modulated by forming a heterostructure with wurtzite ZnO.     

Recognition property and preparation of Staphylococcus aureus protein A-imprinted polyacrylamide polymers by inverse-phase suspension and bulk polymerization

Staphylococcus aureus protein A-imprinted polyacrylamide gel beads (SpA-IPGB) and imprinted polyacrylamide particles (SpA-IPGP) were synthesized by inverse-phase suspension polymerization and bulk polymerization, using SpA as template, acrylamide (AM) as functional monomers. Recognition capacity studies of the prepared SpA-IPGB and SpA-IPGP on SpA and S. aureus were conducted to determine recognition specificity. Computer imitation docking studies were conducted between template proteins and acrylamide monomer to reveal the recognition mechanism of SpA molecularly imprinted polymer. The results showed that the imprinted gel beads had high adsorption capacity and specificity to the SpA and S. aureus, and the adsorption quantity could reach 6.85 × 10⁻³ μmol/g and 10³-10⁴ CFU/g. The adsorption capacities of SpA-IPGB on SpA and S. aureus were higher than that of SpA-IPGP and Non-imprinted polyacrylamide gel beads (Non-IPGB). The recognition specificity of SpA-IPGB and SpA-IPGP on S. aureus was much higher than on Escherichia coli and Streptococcus thermophilus. The formation of complementary shape, and hydrogen bonding, electrostatic, and hydrophobic interactions between the imprinting cavities and the template proteins is the driving force for effective and specific recognition of protein imprinted polymer.     

Catalytic oxidation of benzene,toluene and p-xylene over colloidal gold supported on zinc oxide catalyst

Au was loaded (1.5 wt.%) on the supports (ZnO, Al₂O₃ and MgO) by a colloidal deposition method. For a range of low temperatures (50–300 °C), the catalytic activity of Au/ZnO was much greater than that of Au/Al₂O₃ and Au/MgO. In particular, for the Au/ZnO, the benzene conversion exceeded 80% at 150 °C. The results of catalyst characterization suggested that the high catalytic activity of the Au/ZnO might be attributed to the effects of strong metal–oxide interaction which is possibly originated from the small lattice parameter difference between Au {111} and ZnO {101} lattice planes.     

Effect of silica doping on the densification and grain growth in zinc oxide

The ability of silica (SiO₂) in controlling the densification and grain growth behavior of nano crystalline zinc oxide (ZnO) has been systematically studied. It has been observed that SiO₂ acts as a sintering inhibitor in the ZnO–SiO₂ system up to 4 wt.% limiting value beyond which densification behavior of the system remains almost unchanged, especially above 1100 °C. The addition of SiO₂ to ZnO retards grain growth which in turn results a finer ultimate grain size as compared to the undoped ZnO. However, stabilization in grain size occurs at ≥4 wt.% SiO₂ addition. It has been observed that SiO₂ incorporation changes the grain growth mechanism up to 4 wt.% addition, beyond which no remarkable changes was noticed. The grain growth (n) shows distinctly different slopes as a function of sintering time for the SiO₂ doped ZnO systems than undoped ZnO. The different slopes tend to indicate that different diffusion mechanisms and probably the formation of a secondary phase (Zn–Si–O) at the grain boundary control the densification and grain growth. The thermal expansion coefficient of the system has been found to decrease substantially beyond 4 wt.% SiO₂ addition to ZnO.     

Effect of oxygen flow rate on the ultraviolet sensing properties of zinc oxide nanocolumn arrays grown by radio frequency magnetron sputtering

Highly transparent metal–semiconductor–metal ultraviolet (UV) photoconductive sensors were fabricated using thin (less than 100 nm in thickness), dense, small-diameter ZnO nanocolumn arrays prepared via low-power, catalyst-free radio frequency (RF) magnetron sputtering at different oxygen flow rates ranging from 0 to 25 sccm. The FESEM images revealed the average nanocolumn diameter decreased with increasing oxygen flow rate. The transmittance spectra show that with the introduction of oxygen, the transmittance of the nanocolumn arrays in the visible region improves relative to that of a film prepared in the absence of oxygen with values greater than 95%. The UV responsivity and sensitivity were significantly improved for sputtered ZnO nanocolumn arrays prepared at oxygen flow rates up to 10 sccm, with the highest values of 9.70 mA/W and 2.20×10⁴. Furthermore, the responsivity and sensitivity decreased at oxygen flow rates greater than 10 sccm, which can be attributed to the increased electrical resistance of the nanocolumn arrays. Our findings indicate that a high-performance UV photoconductive sensor can be realised using very thin sputtered ZnO nanocolumn arrays and that such a sensor would exhibit high sensitivity.     

Synthesis, characterization and optical properties of Zn1−xTixO nanoparticles prepared via a high-energy ball milling technique

Zn_1−xTi_xO (x = 0, 0.01, 0.03 and 0.05) nanoparticles were prepared by high-energy ball milling at 400 rpm. The milled powders were characterized by X-ray diffractometer (XRD) and the results exhibited that Ti-doped ZnO nanoparticles consisted of single phase with hexagonal structure when the mixtures of ZnO and TiO₂ powders were milled for 20 h. The crystallite size reduced as a function of the doping content and milling time from 1 to 10 h then increased after milling for 20 h and when the annealing temperature increased. The strain changed inversely to the crystallite size. A wider band-gap was obtained by increasing the doping content and annealing temperature because of a reduction in defect concentration. Both ZnO- and Ti-doped ZnO nanoparticles caused damage to S. aureus, E. coli, P. mirabilis, S. typhi and P. aeruginosa.     

Impact of vanadium-, sulfur-, and dysprosium-doped zinc oxide nanoparticles on various properties of PVDF/functionalized-PMMA blend nanocomposites: Structural,optical, and morphological studies

The polymeric blend was fabricated with crystalline poly(vinylidene fluoride) (PVDF)/amorphous functionalized-poly(methyl methacrylate) (PMMA) in 70/30 w/w ratio by chemical mixing method. Functionalization of PMMA was achieved with 2-amino-5-nitrobenzoic acid. The prepared polymer blend was used as a matrix to synthesize nanocomposites with undoped/doped zinc oxide (ZnO) nanoparticles. Doping in ZnO was achieved with vanadium, sulfur, and dysprosium elements as a dopant. The structural, optical, electronic, and morphological properties of undoped/doped nanosized ZnO and blended nanocomposites were accessed through sophisticated analytical techniques, that is, Fourier transform infrared (FTIR), ultraviolet–visible (UV–vis), UV–vis–diffuse reflectance spectra, nuclear magnetic resonance, fluorescence spectroscopy, X-ray diffraction (XRD), transmission electron microscopy, and scanning electron microscopy. The FTIR band at 1165–1176 cm⁻¹ in functionalized-PMMA indicate the formation of aliphatic C-N bond along with aromatic ¹H chemical shift (δ) at 7.134, 7.829 and 8.210 ppm confirm the successfully functionalization of PMMA. The prominent XRD peak at 2θ = 20.8° in nanocomposites shown improvement in β-phase of PVDF. The results show that Dy doped ZnO nanoparticles create remarkable effect on various properties of nanocomposites. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47116.     

Structural,optical, and magnetic studies of manganese-doped zinc oxide hierarchical microspheres by self-assembly of nanoparticles

In this study, a series of manganese [Mn]-doped zinc oxide [ZnO] hierarchical microspheres [HMSs] are prepared by hydrothermal method only using zinc acetate and manganese acetate as precursors and ethylene glycol as solvent. X-ray diffraction indicates that all of the as-obtained samples including the highest Mn (7 mol%) in the crystal lattice of ZnO have a pure phase (hexagonal wurtzite structure). A broad Raman spectrum from as-synthesized doping samples ranges from 500 to 600 cm^-1, revealing the successful doping of paramagnetic Mn²⁺ ions in the host ZnO. Optical absorption analysis of the samples exhibits a blueshift in the absorption band edge with increasing dopant concentration, and corresponding photoluminescence spectra show that Mn doping suppresses both near-band edge UV emission and defect-related blue emission. In particular, magnetic measurements confirm robust room-temperature ferromagnetic behavior with a high Curie temperature exceeding 400 K, signifying that the as-formed Mn-doped ZnO HMSs will have immense potential in spintronic devices and spin-based electronic technologies.     

Analysis of structural,optical and magnetic properties of Fe/Co co-doped ZnO nanocrystals

In this paper, the structural, optical and magnetic properties of pure ZnO and Fe/Co co-doped ZnO nanoparticles are presented. Rietveld refinement of XRD pattern revealed the single phase wurtzite structure for prepared samples. FTIR study confirmed the formation of tetrahedral coordination between zinc and oxygen ions. SEM and TEM techniques were used to examine the morphology of samples. The absorption spectra showed the decrease in optical energy band gap with Fe/Co co-doping in ZnO. PL spectra demonstrated five peaks correspond to the ultraviolet region, violet, violet-blue, blue-green and green in the visible region. Emission peak in the UV region is attributed to near band-edge excitonic emission. Other four emission peaks in PL spectra are related to different defect states. M-H curve showed room temperature ferromagnetic (RTFM) behaviour of doped ZnO sample. This paper enhances the understanding of structural, optical and magnetic properties of Fe/Co co-doped ZnO nanocrystals for application in spintronics, solar cells, and ceramics.     

Layer-by-layer construction of multi-walled carbon nanotubes, zinc oxide, and gold nanoparticles integrated composite electrode for nitrite detection

A novel composite electrode of Au/ZnO/MWCNTs/GC has been constructed for the electrochemical detection of nitrite, where ZnO thin film and Au nanoparticles are electrodeposited through layer-by-layer onto MWCNTs/GC substrate. The resulting electrode is characterized by scanning electron microscopy and energy-dispersed X-ray spectroscopy. Its electrocatalytic activity toward the electro-oxidation of nitrite has been examined and compared to various modified electrodes, including MWCNTs/GC, Au/ZnO/GC, Au/MWCNTs/GC, and ZnO/MWCNTs/GC via cyclic voltammetry. The electrodeposition time for ZnO and the Au loading amount together with the solution pH are investigated to achieve optimal conditions for the electrode fabrication and nitrite detection. Linear relationship between current response and nitrite concentration is observed in the range from 7.8 × 10⁻⁷ to 4.0 × 10⁻⁴ M and the limit of the detection is 4.0 × 10⁻⁷ M (S/N = 3). The influence of various anions and cations on the nitrite detection has been studied. The proposed method is also employed for the determination of nitrite in real samples.     

Microstructure and electrical properties of Sc2O3-doped ZnO-Bi2O3-based varistor ceramics

The microstructure and electrical properties of ZnO-Bi₂O₃-based varistor ceramics doped with different Sc₂O₃ content sintered at 1100 °C were investigated. The results showed that the nonlinear coefficient of the varistor ceramics with Sc₂O₃ were in the range of 18-54, the threshold voltage in the range of 250-332 V/mm, the leakage current in the range of 0.1-23.0 μA, with addition of 0-1.00 mol% Sc₂O₃. The ZnO-Bi₂O₃-based varistor ceramics doped with Sc₂O₃ content of 0.12 mol% exhibited the highest nonlinearity, in which the nonlinear coefficient is 54, the threshold voltage and the leakage current is 278 V/mm and 2.9 μA, respectively. The results confirmed that doping with Sc₂O₃ was a very promising route for the production of the higher nonlinear coefficient of ZnO-Bi₂O₃-based varistor ceramics, and determining the proper amounts of addition of Sc₂O₃ was of great importance.     

Effect of substrate temperature on the structural,electrical, and optical properties of GZO/ZnO films deposited by radio frequency magnetron sputtering

Ga-doped ZnO (GZO)/ZnO bi-layered films were deposited on glass substrates by radio frequency magnetron sputtering at different substrate temperatures of 100, 200 and 300 °C to investigate the effects of substrate temperature on the structural, electrical, and optical properties of the films. Thicknesses of the GZO and ZnO buffer layer were kept constant at 85 and 15 nm by controlling the deposition times.