Characterization and application of electrospun alumina nanofibers

Alumina nanofibers were prepared by a technique that combined the sol–gel and electrospinning methods. The solution to be electrospun was prepared by mixing aluminum isopropoxide (AIP) in ethanol, which was then refluxed in the presence of an acid catalyst and polyvinylpyrolidone (PVP) in ethanol. The characterization results showed that alumina nanofibers with diameters in the range of 102 to 378 nm were successfully prepared. On the basis of the results of the XRD and FT-IR, the alumina nanofibers calcined at 1,100°C were identified as comprising the α-alumina phase, and a series of phase transitions such as boehmite → γ-alumina → α-alumina were observed from 500°C to 1,200°C. The pore size of the obtained γ-alumina nanofibers is approximately 8 nm, and it means that they are mesoporous materials. The kinetic study demonstrated that MO adsorption on alumina nanofibers can be seen that the pseudo-second-order kinetic model fits better than the pseudo-first-order kinetic model.     

Microwave-assisted synthesis of layered basic zinc acetate nanosheets and their thermal decomposition into nanocrystalline ZnO

We have developed a low-cost technique using a conventional microwave oven to grow layered basic zinc acetate (LBZA) nanosheets (NSs) from a zinc acetate, zinc nitrate and HMTA solution in only 2 min. The as-grown crystals and their pyrolytic decomposition into ZnO nanocrystalline NSs are characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), atomic force microscopy (AFM), X-ray diffraction (XRD) and photoluminescence (PL). SEM and AFM measurements show that the LBZA NSs have typical lateral dimensions of 1 to 5 μm and thickness of 20 to 100 nm. Annealing in air from 200°C to 1,000°C results in the formation of ZnO nanocrystalline NSs, with a nanocrystallite size ranging from 16 nm at 200°C to 104 nm at 1,000°C, as determined by SEM. SEM shows evidence of sintering at 600°C. PL shows that the shape of the visible band is greatly affected by the annealing temperature and that the exciton band to defect band intensity ratio is maximum at 400°C and decreases by a factor of 15 after annealing at 600°C. The shape and thickness of the ZnO nanocrystalline NSs are the same as LBZA NSs. This structure provides a high surface-to-volume ratio of interconnected nanoparticles that is favorable for applications requiring high specific area and low resistivity such as gas sensing and dye-sensitized solar cells (DSCs). We show that resistive gas sensors fabricated with the ZnO NSs showed a response of 1.12 and 1.65 to 12.5 ppm and 200 ppm of CO at 350°C in dry air, respectively, and that DSCs also fabricated from the material had an overall efficiency of 1.3%.

PACS

81.07.-b; 62.23.Kn; 61.82.Fk     

Synthesis of zinc oxide nanostructures on graphene/glass substrate by electrochemical deposition: effects of current density and temperature

The electrochemical growth of zinc oxide (ZnO) nanostructures on graphene on glass using zinc nitrate hexahydrate was studied. The effects of current densities and temperatures on the morphological, structural, and optical properties of the ZnO structures were studied. Vertically aligned nanorods were obtained at a low temperature of 75°C, and the diameters increased with current density. Growth temperature seems to have a strong effect in generating well-defined hexagonal-shape nanorods with a smooth top edge surface. A film-like structure was observed for high current densities above -1.0 mA/cm² and temperatures above 80°C due to the coalescence between the neighboring nanorods with large diameter. The nanorods grown at a temperature of 75°C with a low current density of -0.1 mA/cm² exhibited the highest density of 1.45 × 10⁹ cm^-2. X-ray diffraction measurements revealed that the grown ZnO crystallites were highly oriented along the c-axis. The intensity ratio of the ultraviolet (UV) region emission to the visible region emission, I_UV/I_VIS, showed a decrement with the current densities for all grown samples. The samples grown at the current density below -0.5 mA/cm² showed high I_UV/I_VIS values closer to or higher than 1.0, suggesting their fewer structural defects. For all the ZnO/graphene structures, the high transmittance up to 65% was obtained at the light wavelength of 550 nm. Structural and optical properties of the grown ZnO structures seem to be effectively controlled by the current density rather than the growth temperature. ZnO nanorod/graphene hybrid structure on glass is expected to be a promising structure for solar cell which is a conceivable candidate to address the global need for an inexpensive alternative energy source.     

Tuning of structural,optical, and magnetic properties of ultrathin and thin ZnO nanowire arrays for nano device applications

One-dimensional (1-D) ultrathin (15 nm) and thin (100 nm) aligned 1-D (0001) and (0001¯) oriented zinc oxide (ZnO) nanowire (NW) arrays were fabricated on copper substrates by one-step electrochemical deposition inside the pores of polycarbonate membranes. The aspect ratio dependence of the compressive stress because of the lattice mismatch between NW array/substrate interface and crystallite size variations is investigated. X-ray diffraction results show that the polycrystalline ZnO NWs have a wurtzite structure with a = 3.24 Å, c = 5.20 Å, and [002] elongation. HRTEM and SAED pattern confirmed the polycrystalline nature of ultrathin ZnO NWs and lattice spacing of 0.58 nm. The crystallite size and compressive stress in as-grown 15- and 100-nm wires are 12.8 nm and 0.2248 GPa and 22.8 nm and 0.1359 GPa, which changed to 16.1 nm and 1.0307 GPa and 47.5 nm and 1.1677 GPa after annealing at 873 K in ultrahigh vacuum (UHV), respectively. Micro-Raman spectroscopy showed that the increase in E₂ (high) phonon frequency corresponds to much higher compressive stresses in ultrathin NW arrays. The minimum-maximum magnetization magnitude for the as-grown ultrathin and thin NW arrays are approximately 8.45 × 10⁻³ to 8.10 × 10⁻³ emu/g and approximately 2.22 × 10⁻⁷ to 2.190 × 10⁻⁷ emu/g, respectively. The magnetization in 15-nm NW arrays is about 4 orders of magnitude higher than that in the 100 nm arrays but can be reduced greatly by the UHV annealing. The origin of ultrathin and thin NW array ferromagnetism may be the exchange interactions between localized electron spin moments resulting from oxygen vacancies at the surfaces of ZnO NWs. The n-type conductivity of 15-nm NW array is higher by about a factor of 2 compared to that of the 100-nm ZnO NWs, and both can be greatly enhanced by UHV annealing. The ability to tune the stresses and the structural and relative occupancies of ZnO NWs in a wide range by annealing has important implications for the design of advanced photonic, electronic, and magneto-optic nano devices.     

Structural,electrical, and optical properties of Ti-doped ZnO films fabricated by atomic layer deposition

High-quality Ti-doped ZnO films were grown on Si, thermally grown SiO₂, and quartz substrates by atomic layer deposition (ALD) at 200°C with various Ti doping concentrations. Titanium isopropoxide, diethyl zinc, and deionized water were sources for Ti, Zn, and O, respectively. The Ti doping was then achieved by growing ZnO and TiO₂ alternately. A hampered growth mode of ZnO on TiO₂ layer was confirmed by comparing the thicknesses measured by spectroscopic ellipsometry with the expected. It was also found that the locations of the (100) diffraction peaks shift towards lower diffraction angles as Ti concentration increased. For all samples, optical transmittance over 80% was obtained in the visible region. The sample with ALD cycle ratio of ZnO/TiO₂ being 20 had the lowest resistivity of 8.874 × 10⁻⁴ Ω cm. In addition, carrier concentration of the prepared films underwent an evident increase and then decreased with the increase of Ti doping concentration.     

ZnO微／纳米纤维的静电纺丝及其表征

以聚乙烯吡咯烷酮(PVP)为络合剂与醋酸锌[Zn(CH3COO)2]反应制得前驱体溶液,用静电纺丝法制备了PVP／Zn(CH3COO)2纤维,经煅烧得到具有微孔结构的氧化锌(ZnO)微／纳米纤维。对所制备纤维分别采用差热-热重分析、红外光谱分析、X射线衍射分析、扫描电镜等手段进行了表征。结果表明: PVP/Zn(CH3COO)纤维表面光滑,直径约300-700 nm,经700℃煅烧后,可得到ZnO微／纳米纤维。     

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.     

Thin and long silver nanowires self-assembled in ionic liquids as a soft template: electrical and optical properties

Thin and long silver nanowires were successfully synthesized using the polyvinylpyrrolidone (PVP)-assisted polyol method in the presence of ionic liquids, tetrapropylammonium chloride and tetrapropylammonium bromide, which served as soft template salts. The first step involved the formation of Ag nanoparticles with a diameter of 40 to 50 nm through the reduction of silver nitrate. At the growing stage, the Ag nanoparticles were converted into thin and long one-dimensional wires, with uniform diameters of 30 ± 3 nm and lengths of up to 50 μm. These Ag nanowires showed an electrical conductivity of 0.3 × 10⁵ S/cm, while the sheet resistance of a two-dimensional percolating Ag nanowire network exhibited a value of 20 Ω/sq with an optical transmittance of 93% and a low haze value.     

Electrospun dendritic ZnO nanofibers and its photocatalysis application

A simple and efficient procedure has been developed to fabricate ZnO nanofibers with dendritic structure via electrospinning and subsequent calcination. The spinning solution is prepared by mixing polyvinylpyrrolidone (PVP) and zinc acetate into methanol. From SEM images, it can be observed that the ZnO product has a tendency of dendritic structure. The diameter of the dendritic branch is ～137 nm. The structure and physicochemical property of the prepared nanofibers are elucidated by TGA, SEM, XRD, FTIR, and PL, respectively. The photoluminescene measurement of the ZnO samples exhibit a broad visible emission band concentrate on around 430–580 nm. Meanwhile, the intensity is related to the content of surface oxygen vacancies, which probably influence photocatalytic activity of ZnO samples. Whereafter, the photocatalytical activity of the ZnO nanofibers is evaluated by quantifying the degradation of methyl blue. The result indicates that ZnO nanofibers annealed at 650°C show an excellent photocatalytic activity. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41627.     

ZnO纳米纤维制备及其性能表征

以PVP为前驱体溶液,采用静电纺丝法制备了PVP/Zn(CH3COO)2复合纤维,通过煅烧进一步制得了ZnO纳米纤维。并对所得样品进行了扫描电子显微镜(SEM)分析,TG-DTG分析,并对纳米ZnO纤维和纳米ZnO颗粒的抗菌性能进行了测试。     

Toxicity evaluation of zinc aluminium levodopa nanocomposite via oral route in repeated dose study

Nanotechnology, through nanomedicine, allowed drugs to be manipulated into nanoscale sizes for delivery to the different parts of the body, at the same time, retaining the valuable pharmacological properties of the drugs. However, efficient drug delivery and excellent release potential of these delivery systems may be hindered by possible untoward side effects. In this study, the sub-acute toxicity of oral zinc aluminium nanocomposite with and without levodopa was assessed using the Organization for Economic Co-operation and Development guidelines. No sign or symptom of toxicity was observed in orally treated rats with the nanocomposite at 5 and 500 mg/kg concentrations. Body weight gain, feeding, water intake, general survival and organosomatic index were not significantly different between control and treatment groups. Aspartate aminotransferase (AST) in 500 mg/kg levodopa nanocomposite (169 ± 30 U/L), 5 mg/kg levodopa nanocomposite (172 ± 49 U/L), and 500 mg/kg layered double hydroxides (LDH) nanocomposite (175 ± 25 U/L) were notably elevated compared to controls (143 ± 05 U/L); but the difference were not significant (p > 0.05). However, the differences in aspartate aminotransferase/alanine aminotransferase (AST/ALT) ratio of 500 mg/kg levodopa nanocomposite (0.32 ± 0.12) and 500 mg/kg LDH nanocomposite (0.34 ± 0.12) were statistically significant (p < 0.05) compared to the control (0.51 ± 0.07). Histology of the liver, spleen and brain was found to be of similar morphology in both control and experimental groups. The kidneys of 500-mg/kg-treated rats with levodopa nanocomposite and LDH nanocomposite were found to have slight inflammatory changes, notably leukocyte infiltration around the glomeruli. The ultra-structure of the neurons from the substantia nigra of nanocomposite-exposed group was similar to those receiving only normal saline. The observed result has suggested possible liver and renal toxicity in orally administered levodopa intercalated nanocomposite; it is also dose-dependent that needs further assessment.     

ZnO纳米纤维制备及其性能表征

以PVP为前驱体溶液,采用静电纺丝法制备了PVP/Zn(CH3COO)2复合纤维,通过煅烧进一步制得了ZnO纳米纤维。并对所得样品进行了扫描电子显微镜(SEM)分析,TG-DTG分析,并对纳米ZnO纤维和纳米ZnO颗粒的抗菌性能进行了测试。     

Seedless growth of zinc oxide flower-shaped structures on multilayer graphene by electrochemical deposition

A seedless growth of zinc oxide (ZnO) structures on multilayer (ML) graphene by electrochemical deposition without any pre-deposited ZnO seed layer or metal catalyst was studied. A high density of a mixture of vertically aligned/non-aligned ZnO rods and flower-shaped structures was obtained. ML graphene seems to generate the formation of flower-shaped structures due to the stacking boundaries. The nucleation of ZnO seems to be promoted at the stacking edges of ML graphene with the increase of applied current density, resulting in the formation of flower-shaped structures. The diameters of the rods/flower-shaped structures also increase with the applied current density. ZnO rods/flower-shaped structures with high aspect ratio over 5.0 and good crystallinity were obtained at the applied current densities of −0.5 and −1.0 mA/cm². The growth mechanism was proposed. The growth involves the formation of ZnO nucleation below 80°C and the enhancement of the growth of vertically non-aligned rods and flower-shaped structures at 80°C. Such ZnO/graphene hybrid structure provides several potential applications in sensing devices.     

Angle-dependent photodegradation over ZnO nanowire arrays on flexible paper substrates

In this study, we grew zinc oxide (ZnO) nanowire arrays on paper substrates using a two-step growth strategy. In the first step, we formed single-crystalline ZnO nanoparticles of uniform size distribution (ca. 4 nm) as seeds for the hydrothermal growth of the ZnO nanowire arrays. After spin-coating of these seeds onto paper, we grew ZnO nanowire arrays conformally on these substrates. The crystal structure of a ZnO nanowire revealed that the nanowires were single-crystalline and had grown along the c axis. Further visualization through annular bright field scanning transmission electron microscopy revealed that the hydrothermally grown ZnO nanowires possessed Zn polarity. From photocatalytic activity measurements of the ZnO nanowire (NW) arrays on paper substrate, we extracted rate constants of 0.415, 0.244, 0.195, and 0.08 s^-1 for the degradation of methylene blue at incident angles of 0°, 30°, 60°, and 75°, respectively; that is, the photocatalytic activity of these ZnO nanowire arrays was related to the cosine of the incident angle of the UV light. Accordingly, these materials have promising applications in the design of sterilization systems and light-harvesting devices.     

A catalyst-free growth of aluminum-doped ZnO nanorods by thermal evaporation

The growth of Al:ZnO nanorods on a silicon substrate using a low-temperature thermal evaporation method is reported. The samples were fabricated within a horizontal quartz tube under controlled supply of O₂ gas where Zn and Al powders were previously mixed and heated at 700°C. This allows the reactant vapors to deposit onto the substrate placed vertically above the source materials. Both the undoped and doped samples were characterized using scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), high-resolution transmission electron microscopy (HRTEM) and photoluminescence (PL) measurements. It was observed that randomly oriented nanowires were formed with varying nanostructures as the dopant concentrations were increased from 0.6 at.% to 11.3 at.% with the appearance of ‘pencil-like’ shape at 2.4 at.%, measuring between 260 to 350 nm and 720 nm in diameter and length, respectively. The HRTEM images revealed nanorods fringes of 0.46 nm wide, an equivalent to the lattice constant of ZnO and correspond to the (0001) fringes with regard to the growth direction. The as-prepared Al:ZnO samples exhibited a strong UV emission band located at approximately 389 nm (E _g  = 3.19 eV) with multiple other low intensity peaks appeared at wavelengths greater than 400 nm contributed by oxygen vacancies. The results showed the importance of Al doping that played an important role on the morphology and optical properties of ZnO nanostructures. This may led to potential nanodevices in sensor and biological applications.     

ZnO Nanofiber and Nanoparticle Synthesized Through Electrospinning and Their Photocatalytic Activity Under Visible Light

In this paper, we fabricate ZnO nanofibers and nanoparticles through electrospinning precursor solution zinc acetate(ZnAc)/cellulose acetate(CA) in mixed-solvent N , N -dimethylformamide/acetone. Depending on the posttreatment of precursor ZnAc/CA composite nanofibers, both ZnO nanofibers and nanoparticles were synthesized after calcination of precursor nanofibers. The morphology and crystal structure of the ZnO nanofiber and nanoparticle were characterized by scanning electron microscopy, transmission electron microscopy, atomic force microscopy, and X-ray diffraction. It was found that the mean diameter of the ZnO nanofiber and nanoparticle was ca. 78 and 30 nm, respectively. The photo-degradation of dye molecules such as Rhodamine B and acid fuchsin catalyzed by the ZnO nanofiber and nanoparticle was evaluated under the irradiation of visible light. Both morphological ZnO species showed strong photocatalytic activity. However, the ZnO nanofiber in the form of nanofibrous mats showed much higher efficiency than the nanoparticle although the latter has a smaller size than the former. The porous structure of ZnO nanofibrous mats is believed to improve the contacting surface areas between the catalyst and the dye molecules, while the aggregation of ZnO nanoparticle in the solution lowers the photocatalytic efficiency.     

Tailoring the grooved texture of electrospun polystyrene nanofibers by controlling the solvent system and relative humidity

In this study, we have successfully fabricated electrospun polystyrene (PS) nanofibers having a diameter of 326 ± 50 nm with a parallel grooved texture using a mixed solvent of tetrahydrofuran (THF) and N,N-dimethylformamide (DMF). We discovered that solvent system, solution concentration, and relative humidity were the three key factors to the formation of grooved texture and the diameter of nanofibers. We demonstrated that grooved nanofibers with desired properties (e.g., different numbers of grooves, widths between two adjacent grooves, and depths of grooves) could be electrospun under certain conditions. When THF/DMF ratio was higher than 2:1, the formation mechanism of single grooved texture should be attributed to the formation of voids on the jet surface at the early stage of electrospinning and subsequent elongation and solidification of the voids into a line surface structure. When THF/DMF ratio was 1:1, the formation mechanism of grooved texture should be ascribed to the formation of wrinkled surface on the jet surface at the early stage of electrospinning and subsequent elongation into a grooved texture. Such findings can serve as guidelines for the preparation of grooved nanofibers with desired secondary morphology.     

Facile synthesis and superior ethyl acetate sensing performance of Au decorated ZnO flower-like architectures

Au decorated ZnO flower-like architectures assembled from single crystal nanowires have been successfully synthesized. A facile thermal treatment route was employed, utilizing the composite gel of zinc acetate and polyvinyl pyrrolidone (PVP) as raw materials and was followed by a subsequent Au reduction process. PVP served as a surfactant and played a critical role in the generation of crystalline nanowires as well as for the formation of ZnO flower-like structure. Based on control experiments, the growth mechanism of ZnO flower-like structures was proposed. The diameter of ZnO crystalline nanowires was approximately 50–80 nm and the size of Au particles deposited on the surface of ZnO nanowires was approximately 5 nm. When tested as gas sensing material, the as-prepared Au decorated ZnO flower-like architecture exhibited superior gas sensing performance compared to ethyl acetate in terms of high response (approximately 102 at 100 ppm), short response and recovery times (10 s and 13 s, respectively), and low operating temperature (240 °C). The superior gas sensing performances are mainly attributed to the synergistic effects of ZnO crystalline nanowires and Au nanoparticles, as well as to the flower-like structure.     

Anomalous luminescence phenomena of indium-doped ZnO nanostructures grown on Si substrates by the hydrothermal method

In recent years, zinc oxide (ZnO) has become one of the most popular research materials due to its unique properties and various applications. ZnO is an intrinsic semiconductor, with a wide bandgap (3.37 eV) and large exciton binding energy (60 meV) making it suitable for many optical applications. In this experiment, the simple hydrothermal method is used to grow indium-doped ZnO nanostructures on a silicon wafer, which are then annealed at different temperatures (400°C to 1,000°C) in an abundant oxygen atmosphere. This study discusses the surface structure and optical characteristic of ZnO nanomaterials. The structure of the ZnO nanostructures is analyzed by X-ray diffraction, the superficial state by scanning electron microscopy, and the optical measurements which are carried out using the temperature-dependent photoluminescence (PL) spectra. In this study, we discuss the broad peak energy of the yellow-orange emission which shows tendency towards a blueshift with the temperature increase in the PL spectra. This differs from other common semiconductors which have an increase in their peak energy of deep-level emission along with measurement temperature.     

Open structure ZnO/CdSe core/shell nanoneedle arrays for solar cells

Open structure ZnO/CdSe core/shell nanoneedle arrays were prepared on a conducting glass (SnO₂:F) substrate by solution deposition and electrochemical techniques. A uniform CdSe shell layer with a grain size of approximately several tens of nanometers was formed on the surface of ZnO nanoneedle cores after annealing at 400°C for 1.5 h. Fabricated solar cells based on these nanostructures exhibited a high short-circuit current density of about 10.5 mA/cm² and an overall power conversion efficiency of 1.07% with solar illumination of 100 mW/cm². Incident photo-to-current conversion efficiencies higher than 75% were also obtained.