The modification of structural and optical properties of nano- and submicron ZnO powders by variation of solvothermal syntheses conditions

Nano- (30-60 nm) and submicron (100-350 nm) ZnO particles were synthesized using solvothermal method at 200 degrees C from an ethanolic solution of zinc acetate dihydrate, applying different reaction conditions, i.e., pH value of precursor and time of the reaction. The X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), UV-vis diffuse reflectance (DR), Raman spectroscopy, and photoluminescence (PL) spectroscopy have been employed for characterization of synthesized ZnO powders. It was shown that the structural, morphological, and optical properties are largely determined by reaction conditions during solvothermal synthesis. The particle crystallinity improves with the decrease of pH value and/or the increase of time of the reaction. The Raman and PL spectra analyses indicate that the oxygen interstitials are dominant intrinsic defects in solvothermally synthesized ZnO powders. It was observed that concentration of defects in wurtzite ZnO crystal lattices slightly changes with the variation of pH value of the precursor and time of the solvothermal reaction. The correlation between structural ordering and defect structure of particles and corresponding growth processes was discussed.

Morphological and electrical evolution of ZnO:Al thin films deposited by rf magnetron sputtering onto glass substrates

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Microstructural and Nonohmic Properties of ZnO center dot Pr6O11 CoO Polycrystalline System

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Surface modified Mg-doped ZnO QDs for biological imaging

Fundação Amparo à Pesquisa Estado de São Paulo (FAPESP)

Bio-hydrogen production based on catalytic reforming of volatiles generated by cellulose pyrolysis: an integrated process for ZnO reduction and zinc nanostructures fabrication

The paper presents a process of cellulose thermal degradation with bio-hydrogen generation and zinc nanostructures synthesis. Production of zinc nanowires and zinc nanoflowers was performed by a novel processes based on cellulose pyrolysis, volatiles reforming and direct reduction of ZnO. The bio-hydrogen generated in situ promoted the ZnO reduction with Zn nanostructures formation by vapor–solid (VS) route. The cellulose and cellulose/ZnO samples were characterized by thermal analyses (TG/DTG/DTA) and the gases evolved were analyzed by FTIR spectroscopy (TG/FTIR). The hydrogen was detected by TPR (Temperature Programmed Reaction) tests. The results showed that in the presence of ZnO the cellulose thermal degradation produced larger amounts of H2 when compared to pure cellulose. The process was also carried out in a tubular furnace with N2 atmosphere, at temperatures up to 900 °C, and different heating rates. The nanostructures growth was catalyst-free, without pressure reduction, at temperatures lower than those required in the carbothermal reduction of ZnO with fossil carbon. The nanostructures were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM). The optical properties were investigated by photoluminescence (PL). One mechanism was presented in an attempt to explain the synthesis of zinc nanostructures that are crystalline, were obtained without significant re-oxidation and whose morphologies are dependent on the heating rates of the process. This route presents a potential use as an industrial process taking into account the simple operational conditions, the low costs of cellulose and the importance of bio-hydrogen and nanostructured zinc.

Al-doping effect on the surface morphology of zno films grown by reactive rf magnetron sputtering

Zinc oxide (ZnO) and aluminum-doped zinc oxide (ZnO:Al) thin films were deposited onto glass and silicon substrates by RF magnetron sputtering using a zinc-aluminum target. Both films were deposited at a growth rate of 12.5 nm/min to a thickness of around 750 nm. In the visible region, the films exhibit optical transmittances which are greater than 80%. The optical energy gap of ZnO films increased from 3.28 eV to 3.36 eV upon doping with Al. This increase is related to the increase in carrier density from 5.9 × 1018 cm−3 to 2.6 × 1019 cm−3 . The RMS surface roughness of ZnO films grown on glass increased from 14 to 28 nm even with only 0.9% at Al content. XRD analysis revealed that the ZnO films are polycrystalline with preferential growth parallel to the (002) plane, which corresponds to the wurtzite structure of ZnO.

Effect of Zn Sputtering Rate on the Morphological and Optical Properties of ZnO Films

Zinc oxide (ZnO) and aluminum-doped zinc oxide (ZnO:Al) thin films were deposited onto glass and silicon substrates by RF magnetron sputtering using a zinc-aluminum target. Both films were deposited at a growth rate of 12.5 nm/min to a thickness of around 750 nm. In the visible region, the films exhibit optical transmittances which are greater than 80%. The optical energy gap of ZnO films increased from 3.28 eV to 3.36 eV upon doping with Al. This increase is related to the increase in carrier density from 5.9 × 1018 cm-3 to 2.6 × 1019 cm-3. The RMS surface roughness of ZnO films grown on glass increased from 14 to 28 nm even with only 0.9% at Al content. XRD analysis revealed that the ZnO films are polycrystalline with preferential growth parallel to the (002) plane, which corresponds to the wurtzite structure of ZnO.

Sonochemical effects on ZnO particles

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Investigação das propriedades ópticas de ZnO e ZnO:Al

Pós-graduação em Ciência e Tecnologia de Materiais - FC

Membranas de biocelulose como substrato para o crescimento de nanofios de ZnO: síntese e aplicação

Pós-graduação em Química - IQ

Membranas de biocelulose como substrato para o crescimento de nanofios de ZnO: síntese e aplicação

Pós-graduação em Química - IQ

Comparative electrical behavior at low and high current of SnO(2)- and ZnO-based varistors

The complete I-V characteristics of SnO(2)-based varistors, particularly of the Pianaro system SCNCr consisting in 98.9%SnO(2)+1%CoO+0.05%Nb(2)O(5)+0.05%Cr(2)O(3), all in mol%, have been seldom reported in the literature. A comparative study at low and high currents of the nonohmic behavior of SCNCr- and ZnO-based varistors (modified Matsuoka system) is proposed in this work. The SCNCr system showed higher nonlinearity coefficients in the whole range of measured current. The electrical breakdown field (E(b)) was twice as high for the SCNCr system (5400 V/cm) than for the ZnO varistor (2600 V/cm) due to a smaller average grain size of the former (4.5 mu m) with respect to the latter (8.5 mu m). Nevertheless, we consider that another important factor responsible for the high E(b) in the SCNCr system is the great number of electrically active interfaces (85%) as determined with electrostatic force microscopy (EFM). It was also established that the SCNCr system might be produced in disks of smaller dimensions than that of commercial ZnO-based product, with a 5.0 cm(-1) minimal area-volume (A/V) ratio. The SCNCr reached the saturation current in a short time because of the high resistivity of the grains, which is five times higher than that of the grains in ZnO-based varistors.

Comparative degradation of ZnO- and SnO(2)-based polycrystalline non-ohmic devices by current pulse stress

The degradation behaviour of SnO(2)-based varistors (SCNCr) due to current pulses (8/20 mu s) is reported here for the first time in comparison with the ZnO-based commercial varistors (ZnO). Puncturing and/or cracking failures were observed in ZnO-based varistors possessing inferior thermo-mechanical properties in comparison with that found in a SCNCr system free of failures. Both systems presented electric degradation related to the increase in the leakage current and decrease in the electric breakdown field, non-linear coefficient and average value of the potential barrier height. However, it was found that a more severe degradation occurred in the ZnO-based varistors concerning their non-ohmic behaviour, while in the SCNCr system, a strong non-ohmic behaviour remained after the degradation. These results indicate that the degradation in the metal oxide varistors is controlled by a defect diffusion process whose rate depends on the mobility, the concentration of meta-stable defects and the amount of electrically active interfaces. The improved behaviour of the SCNCr system is then inferred to be associated with the higher amount of electrically active interfaces (85%) and to a higher energy necessary to activate the diffusion of the specific defects.

Frequency upconversion properties of Tm3+ doped TeO2-ZnO glasses containing silver nanoparticles

Frequency upconversion (UC) properties of Tm3+ doped TeO2-ZnO glasses containing silver nanoparticles (NPs) were investigated. Infrared-to-visible and infrared-to-infrared UC processes associated to the Tm3+ ions were studied by exciting the samples with a cw 1050 nm ytterbium laser. The luminescence intensity as a function of laser intensity was also measured using a pulsed 1047 nm Nd3+:YVO laser in order to determine the number of photons participating in the UC processes. Enhancement of the UC signals for samples heat-treated during various time intervals is attributed to the growth of the local field in the vicinity of the NPs. PL enhancement by one-order of magnitude was observed in the whole spectrum of the samples heat-treated during 48 h. On the other hand PL quenching was observed for the samples heat-treated more than 48 h. (c) 2011 Elsevier B.V. All rights reserved.

Tuning Low-Spin to High-Spin Mn Pairs in 2-D ZnO by Injecting Holes

We have performed an ab initio theoretical investigation of substitutional Mn(Zn) atoms in planar structures of ZnO, viz., monolayer [(ZnO)(1)] and bilayer [(ZnO)(2)] systems. Due to the 2-D quantum confinement effects, in those Mn -doped (ZnO)(1) and (ZnO)(2) structures, the antiferromagnetic (AFM) coupling between (nearest neighbor) Mn(Zn) impurities have been strengthened when compared with the one in ZnO bulk systems. On the other hand, we find that the magnetic state of these systems can be tuned from AFM to FM by adding holes, which can be supplied by a p-type doping or even photoionization processes. Whereas, upon addition of electrons (n-type doping), the system keeps its AFM configuration.