Effects of morphology on photocatalytic performance of Zinc oxide nanostructures synthesized by rapid microwave irradiation methods

In this study, two different chemical solution methods were used to synthesize Zinc oxide nanostructures via a simple and fast microwave assisted method. Afterwards, the photocatalytic performances of the produced ZnO powders were investigated using methylene blue (MB) photodegradation with UV lamp irradiation. The obtained ZnO nanostructures showed spherical and flower-like morphologies. The average crystallite size of the flower-like and spherical nanostructures were determined to be about 55 nm and 28 nm, respectively. X-ray diffraction (XRD), scanning electronic microscopy (SEM), Brunauer–Emmett–Teller (BET), room temperature photoluminescence (RT-PL) and UV–vis analysis were used for characterization of the synthesized ZnO powders. Using BET N₂-adsorption technique, the specific surface area of the flower-like and spherical ZnO nanostructures were found to be 22.9 m²/gr and 98 m²/gr, respectively. Both morphologies show similar band gap values. Finally, our results depict that the efficiency of photocatalytic performance in the Zinc oxide nanostructures with spherical morphology is greater than that found in the flower-like Zinc oxide nanostructures as well as bulk ZnO.     

Synthesis and optical properties of flower-like ZnO nanorods by thermal evaporation method

Flower-like ZnO nanorods have been synthesized by heating a mixture of ZnO/graphite powders using the thermal evaporation and vapor transport on Si (1 0 0) substrates without any catalyst. The structures, morphologies and optical properties of the products were characterized in detail by using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and Raman spectroscopy. The synthesized products consisted of large quantities of flower-like ZnO nanostructures in the form of uniform nanorods. The flower-like ZnO nanorods had high purity and well crystallized wurtzite structure, whose high crystalline quality was proved by Raman spectroscopy. The as-synthesized flower-like ZnO nanorods showed a strong ultraviolet emission at 386 nm and a weak and broad yellow-green emission in visible spectrum in its room temperature photoluminescence (PL) spectrum. In addition, the growth mechanism of the flower-like ZnO nanorods was discussed based on the reaction conditions.     

Apple – biomorphic synthesis of porous ZnO nanostructures for glucose direct electrochemical biosensor

Biomorphic porous ZnO nanostructures were successfully synthesized via an aqueous sol–gel soaking process using pieces of apple flesh and skin as templates and employed for glucose direct electrochemical biosensor. The structure and morphology of ZnO nanostructures were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). By modifying glassy carbon electrode with the biomorphic ZnO nanostructures and Nafion, two glucose biosensors were constructed and their direct electrochemistry of glucose oxidase (GOD) was successfully investigated by cyclic voltammetry (CV). The biomorphic porous ZnO nanostructures using apple skin template (S-ZnO) were more effective in facilitating the electron transfer of immobilized GOD than that of using flesh apple template (F-ZnO). This may be a result of the unique morphology and smaller average crystallite size of the S-ZnO nanostructure. GOD immobilized on Nafion-porous S-ZnO nanostructure composite display direct, reversible, and surface-controlled redox reaction with a detection limit of 10 μM, a response time of 7 s, high sensitivity of 23.4 μA/mM cm² and a fast heterogeneous electron transfer rate with a rate constant (k_s) of 3.9 s^?1. It was found that S-ZnO significantly has improved the direct electron transfer between GOD and glassy carbon electrode with good stability and reproducibility.     

Synthesis,characterization and photoluminescence of ZnO:Cd rosette-like nanostructures using solution process

Well crystalline undoped and Cd-doped ZnO rosette-like structures were successfully synthesized at low temperature (80 °C) via solution process technique during 30 min. Zinc nitrate, cadmium nitrate, sodium hydroxide and hexamine were used as starting materials. The morphology and microstructure were determined by field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX) and photoluminescence (PL) spectroscopy. X-ray diffraction indicated that the structure has a single phase with wurtzite structure. FESEM indicated that rosette like structures have been formed. This rosette consists of nanorods with length 210 and 460 nm and diameter 50 and 74 nm for undoped and Cd doped ZnO, respectively. HRTEM showed a decrease in the lattice parameter after the Cd doping. EDX showed that the amount of Cd incorporated into ZnO is 6.4 wt.%. Photoluminescence measurements taken on both doped and undoped samples showed that, in the Cd-doped ZnO nanostructures, the band-edge UV emission is blue shifted and the broad green emission intensity decreased.     

Evolution of ZnO nanostructures in sol–gel synthesis

Evolution of the microstructure and optical properties of ZnO nanoparticles in a mild sol–gel synthesis process is studied. The ZnO nanostructures were prepared by reacting zinc acetate dihydrate with NaOH in water at 50−60 °C. Evolution of ZnO nanostructures with reaction time is studied using UV–Vis spectroscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier transform infrared (FT-IR) spectroscopy techniques. During the process of Zn²⁺ hydroxylation, well defined rod-like crystals were formed within 15 min. Further hydroxylation leads to the formation of a gel-like structure within about 45 min. However, XRD, FT-IR and energy dispersive spectroscopy (EDS) confirmed that these initial products were zinc hydroxyl double salts (Zn-HDS), not ZnO. On ageing the reaction mixture, ZnO nanoparticles with wurtzite structure evolved.     

Synthesis of ZnO nanoplates decorated rhombus-shaped ZnO nanorods and their application in solar cells

Novel nanostructures of ZnF(OH) nanoplates decorated rhombus-shaped ZnF(OH) nanorods were fabricated. The obtained precursors were transformed by calcination to porous hierarchical ZnO nanostructures with the original morphologies retained. Field emission scanning electron microscope images exhibit that the nanoplates are grown in the interstices between the nanorods and on the top of the nanorods. The structure and composition of the obtained products have been confirmed by transmission electron microscope and X-ray diffraction measurements. The obtained ZnO nanostructures have been successfully used in solar cells. The light-to-electricity conversion results show that the complex nanostructures exhibit a power conversion efficiency of 1.36% with a photoelectrode thickness of 4.2 µm, which is comparable to those based on 40 µm vertically aligned hexagonal-shaped ZnO nanowire array photoelectrodes. These results indicate that the synthesized ZnO nanoplate decorated rhombus-shaped ZnO nanorod nanostructures are more suitable for application as a photoelectrode in solar cells.     

Synthesis and surface modification of ZnO:Cu nanoparticles by silica and PMMA

ZnO nanoparticles doped with Cu were synthesized by solid state reaction using different precursor routes and varying growth environment. Average crystallite size varied from 40 to 100 nm depending upon synthesis temperature, lower temperature favouring smaller particle size. Scanning electron microscope (SEM) images showed that particles synthesized at 250 °C were in the shape of nanorods but those synthesized at 900 °C had spherical shape. Luminescence emission showed marked dependence on the growth conditions varying from ultraviolet (UV) emission to green emission. For making the luminescent nanoparticles bio-compatible, a bioinorganic interface on ZnO:Cu nanoparticles was created by coating them with inert silica. Surface modification of ZnO:Cu was also done with lipophilic polymethylmethacrylate (PMMA). ZnO:Cu nanoparticles showed hexagonal wurtzite structure and the coating of silica was confirmed with the presence of two extra peaks due to silica in the XRD spectra. Thermogravimetric analysis (TGA) and FTIR spectroscopy indicated that PMMA molecules were adsorbed on the surface of ZnO:Cu nanoparticles. SEM images revealed that PMMA adsorption improved the dispersibilty of ZnO:Cu nanoparticles.     

Ultraviolet-assisted synthesis of hourglass-like ZnO microstructure through an ultrasonic and microwave combined technology

Well-defined hexagonal hourglass-like ZnO microstructures have been synthesized by using simultaneous ultrasound and microwave irradiation methods with the assistance of ultraviolet as well as double sonication (at 25 and 40 kHz). The structure and morphology of as-prepared samples were characterized by X-ray diffraction, scanning electron microscopy and photoluminescence spectrum. The results show the synergistic effect of ultrasound and microwave on the formation of ZnO microstructures. Meanwhile, the possible growth mechanism was proposed to elucidate the formation of hexagonal hourglass-like ZnO microstructures.     

Template-free solvothermal synthesis of 3D hierarchical nanostructured boehmite assembled by nanosheets

In the absence of template and surfactant, hierarchical nanostructured boehmite was synthesized via a simple solvothermal route using aluminum nitrate as aluminum source and isopropanol–toluene mixture as solvent. The crystal structures, morphologies and textural properties of products were characterized by X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and N₂ adsorption–desorption technique. The as-obtained hierarchical nanostructures consist of nanosheets keeping Brunauer–Emmett–Teller (BET) specific surface area and pore volume of ca. 264.7 m²/g and 1.2 cm³/g, respectively. The experimental results show that the longer reaction time and the lower reaction temperature are unfavorable to the formation of hierarchical nanostructures. Moreover, the properties of solvent have important influence on the morphology of product. The possible formation mechanism of boehmite hierarchical nanostructures was proposed and discussed.     

Synthesis of nanocrystalline ZnO nanobelts via pyrolytic decomposition of zinc acetate nanobelts and their gas sensing behavior

The pyrolytic decomposition of layered basic zinc acetate (LBZA) nanobelts (NBs) into nanocrystalline ZnO NBs is investigated using scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL). We also report on the gas sensing response of the resulting ZnO nanomaterial to CO. The LBZA NBs are grown at 65 °C in an aqueous solution of zinc acetate dihydrate. AFM and SEM results show as-grown products possess the characteristic layered structure of the LBZA crystals. XRD and XPS results show that annealing as-grown products at 210 °C in air causes a transformation from zinc acetate to nanocrystalline ZnO NBs via thermal decomposition. The ZnO crystalline domain size increases with temperature from 9.2 nm at 200 °C to 94 nm at 1000 °C, as measured from XRD. SEM shows evidence of sintering at 600 °C. The thickness of the NBs, determined via AFM, ranges from 10 to 50 nm and remains approximately constant with annealing temperature. XPS confirmed the chemical transformation from zinc acetate to ZnO and showed a significant remaining zinc hydroxide component for the ZnO NBs consistent with published results. PL measurements at room temperature show a blue shift in peak emission as the nanobelts change from LBZA to ZnO at 200 °C. Above this transition temperature, the ZnO nanobelts possess strong band edge emission at 390 nm and little broad band emission in the visible region. The AFM and SEM images reveal that the crystallites within the nanobelts orientate in rows along the long axis during annealing. This structure provides a high surface area to volume ratio of aligned nanoparticles which is beneficial for gas sensing applications. Gas sensors fabricated from 400 °C annealed nanobelts showed a response of 1.62 when exposed to 200 ppm of CO in dry air at 400 °C, as defined by the ratio of resistance before and during exposure. This indicates that ZnO nanostructures obtained by thermal decomposition of LBZA NBs could provide a cost effective route to high sensitivity gas sensors.     

Using sonochemistry for the fabrication of hollow ZnO microspheres

Hollow ZnO microspheres assembled by nanoparticles have been prepared by a sonochemical synthesis at room temperature using carbon spheres as template. The growth process of the precursor was investigated. The prepared hollow spheres were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HRTEM). The diameter of the obtained hollow spheres is about 500 nm, and the walls are composed of numerous ZnO aggregate nanocrystallines with diameters of 90 nm. A possible growth mechanism for the formation of ZnO microspheres has been proposed, in which carbon spheres play a crucial role in the formation of the wurtzite hollow ZnO microspheres. The specific structure of the hollow spheres may find applications in nanoelectronics, nanophotonics and nanomedicine.     

Controlled repeated chemical growth of ZnO films for dye-sensitized solar cells

The controlled growth of ZnO nanorods perpendicular to substrate surface i.e. c-axis by a repeated chemical deposition method for efficient dye-sensitized solar cell application is described. X-ray diffraction study shows the wurtzite structure of ZnO with high crystallinity. Intensity and newly evolved peaks of the ZnO are found to be thickness dependent. Dye loving flower-like globular architecture of ZnO is observed after 8 μm thickness. Dye-sensitized solar cell studies show the solar-to-electrical conversion efficiency of 2.21% for 11 μm ZnO electrode when illuminated with 80 mW/cm².     

Photocatalytic degradation of ciprofloxacin drug in water using ZnO nanoparticles

We report the synthesis of nanostructure ZnO semiconductor with ～2.1 nm diameter using a chemical precipitation method. The resulting nanoparticles were characterized by X-ray diffraction analysis (XRD), Fourier-transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The optical properties were investigated by UV–vis and fluorescence techniques. The absorption spectra exhibit a sharp absorption edge at ～334 nm corresponding to band gap of ～3.7 eV. The fluorescence spectra displayed a near-band-edge ultraviolet excitonic emission at ～410 nm and a green emission peak at ～525 nm, due to a transition of a photo-generated electron from the conduction band to a deeply trapped hole. The photocatalytic activity of the prepared ZnO nanoparticles has been investigated for the degradation of ciprofloxacin drug under UV light irradiation in aqueous solutions of different pH values. The results showed that the photocatalytic degradation process is effective at pH 7 and 10, but it is rather slow at pH 4. Higher degradation efficiency (～50%) of the drug was observed at pH 10 after 60 min. Photodegradation of the drug follows a pseudo-first-order kinetics.     

Experimental investigation on the structural,dielectric, ferroelectric and piezoelectric properties of La doped ZnO nanoparticles and their application in dye-sensitized solar cells

Pure and lanthanum (La) doped ZnO nanorods were synthesized via co-precipitation method. The structure and morphology of as grown ZnO and La-ZnO nanoparticles were studied using transmission electron microscopy (TEM) and powder X-ray diffraction (XRD) methods. The values of remnant polarization and coercive field were found to be 0.027 μC/cm² and 1.33 kV/cm, respectively, for as grown La-ZnO nanostructures. High Curie temperature (276 °C) for doped ZnO was observed in dielectric study. Piezoelectric coefficient at room temperature was found to be 101.30 pm/V. I-V characteristics were studied for both pure and doped ZnO nanoparticles. Photo-anodes of dye-sensitized solar cells (DSSCs) were made using ZnO and La-ZnO nanorods. The conversion efficiency and short circuit current density of La-ZnO nanorods based DSSC were 0.36% and 1.31 mA/cm², respectively, which were found to be largely enhanced when compared with that of pure ZnO based DSSC (0.20% and 0.94 mA/cm²).     

Spectral characterization of novel LiZnVO4 phosphor

This paper reports on the spectral analysis of novel Lithium Zinc Vanadate (LiZnVO₄) phosphor prepared by a solid state reaction method. Emission spectrum of LiZnVO₄ has shown a bright green emission at 533 nm with λ_exci = 359 nm. A bright green color emission has also been noticed directly from this phosphor under an UV source. For this luminescent phosphor, X-ray diffraction (XRD), Scanning electron microscopy (SEM), Energy dispersive X-ray spectroscopy (EDS), Fourier-transform IR spectroscopy (FTIR), Raman Spectrum, Comprehensive Differential Thermo gravimetric Analysis (TG–DTA) have also been carried out.     

A study on morphology control and optical properties of ZnO nanorods synthesized by microwave heating

In this study, we present morphology control investigations on zinc oxide (ZnO) nanorods synthesized by microwave heating of a mixture of zinc nitrate hexahydrate and hexamethylenetetramine (HMTA) precursors in deionized water (DI water). To study the morphology and structural variations of the obtained ZnO nanorods in different molar ratio of zinc nitrate hexahydrate to HMTA, X-ray diffraction (XRD), scanning electron microscopy (SEM) images, Raman scattering, and photoluminescence (PL) spectroscopy were measured. XRD and SEM images are utilized to examine the crystalline quality as well as the morphological properties of the ZnO nanorods. It is found that morphology control can be achieved by simply adjusting the reactant concentrations and the molar ratio of zinc nitrate hexahydrate to HMTA. Raman scattering and PL spectroscopy measurements were demonstrated to study the size- and shape-dependent optical response of the ZnO nanorods. The Raman scattering result shows that the intensity of LO mode at around 576 cm^?1 decreases with the increase in the molar ratio of zinc nitrate hexahydrate to HMTA, indicating the reduction of defect concentrations in the synthesized ZnO nanorods. Room temperature PL spectrum of the synthesized ZnO nanorods reveals an ultraviolet (UV) emission peak and a broad visible emission. An enhancement of UV emission appears in the PL spectra as the molar ratio of zinc nitrate hexahydrate to HMTA increases, indicating that the defect concentration of the synthesized ZnO nanorods can be reduced by increasing the molar ratio.     

Large-scale synthesis and growth habit of 3-D flower-like crystal of PbTe

In this paper, 3-D flower-like crystal of PbTe was successfully synthesized using Pb(CH₃COO)₂·3H₂O and Na₂TeO₃ as precursors under hydrothermal conditions, and characterized by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction pattern (XRD). The reaction parameters that influenced the evolution of PbTe synthesis and morphology were investigated. It was shown that the flower-like crystal of PbTe was composed of a nucleus with eight pods. A possible growth mechanism was proposed based on the calculation of the surface energies of PbTe and the SEM observation. Furthermore, the temperature-dependent transport properties of 3-D flower-like crystal of PbTe specimen have been evaluated with an average thermoelectric power of 120 S cm^?1 and electrical conductivity of 220 μV K^?1 at 740 K.     

Photoluminescence of chemical solution-derived amorphous ZnO layers prepared by low-temperature process

Growth characteristic and optical properties of the amorphous ZnO thin films prepared on soda-lime–silica glass substrates by chemical solution process at 100 and 200 °C were investigated by using X-ray diffraction analysis, scanning probe microscope, ultraviolet spectrophotometer, and photoluminescence. The films exhibited an amorphous pattern even when finally heat treated at 100–200 °C for 60 min. The photoluminescence spectrum of amorphous ZnO films shows a strong near-band-edge emission, while the visible emission is nearly quenched.     

Emission of Cu-related complexes in ZnO:Cu nanocrystals

Photoluminescence (PL), its temperature dependence, scanning electronic microscopy (SEM) and X ray diffraction (XRD) have been applied for the comparative study of varying the emission, morphology and crystal structure of ZnO and ZnO:Cu nanocrystals (NCs) versus technological routines, as well as the dependence of ZnO:Cu NC parameters on the Cu concentration. A set of ZnO and ZnO Cu NCs was prepared by the electrochemical (anodization) method at a permanent voltage and different etching durations with follows thermal annealing at 400 °C for 2 h in ambient air. The size of ZnO NCs decreases from 300 nm×540 nm down to 200 nm×320 nm with etching duration increasing. XRD study has confirmed that thermal annealing stimulates the ZnO oxidation and crystallization with the formation of wurtzite ZnO crystal lattice. XRD method has been used for monitoring the lattice parameters and for confirming the Cu doping of ZnO Cu NCs. In ZnO Cu NCs four defect related PL bands are detected with the PL peaks at 1.95–2.00 eV (A), 2.15-2.23  eV (B), 2.43–2.50 eV (C) and 2.61–2.69 eV (D). Highest PL intensities of orange, yellow and green emissions have been obtained in ZnO Cu NCs with the Cu concentration of 2.28 at%. At Cu concentration increasing (≥2.28 at%) the PL intensities of the bands A, B, C decrease and the new PL band peaked at 2.61–2.69 eV at 10 K appears in the PL spectrum. The variation of PL intensities for all PL bands versus temperature has been studied and the corresponding activation energies of PL thermal decay have been estimated. The type of Cu-related complexes is discussed using the correlation between the PL spectrum transformation and the variation of XRD parameters in ZnO Cu NCs.     

Syntheses and characterization of Mg(OH)2 and MgO nanostructures by ultrasonic method

Magnesium hydroxide nanostructures have been synthesized by the reaction of magnesium acetate with sodium hydroxide via sonochemical method. Reaction conditions such as the Mg²⁺ concentration, aging time and the ultrasonic device power show important roles in the size, morphology and growth process of the final products. The magnesium oxide nanoparticles have been prepared by calcination of magnesium hydroxide nanostructures at 400 °C. The magnesium hydroxide and magnesium oxide nanostructures were characterized by scanning electron microscopy (SEM), X-ray powder diffraction (XRD), thermal gravimetric (TG) and differential thermal analyses (DTA).