Nanostructured ZnO Thin Films Research Articles

Manufacturing of Multifunctional Nanocrystalline ZnO Thin Films

Optical, surface and structural properties of ZnO thin films fabricated by reactive radio- frequency (rf) magnetron sputtering and sol-gel coating methods are comparatively investigated. The optical properties of films produced by both techniques have very similar characteristics, however; the surface morphology and degree of crystallinity have different behaviors. The nanostructure columnar zinc oxide thin films can be synthesized by sol-gel coating methods which can have numerous applications requiring larger surface area. Also, the process scalability and large-scale manufacturing of these materials are discussed. It indicated that the nanostructure ZnO thin films can be synthesized with sol-gel methods at wafer levels with nano-grains and improved surface properties compared with reactive rf magnetron sputtering deposition.

HYDROTHERMAL SYNTHESIS OF ROD AND CHRYSANTHEMUM LIKE NANOSTRUCTURED ZnO

Rod and chrysanthemum-like nanostructured ZnO thin film have been prepared hydrothermally in NaOH and NH3 solution utilizing zinc nitrate hexahydrate as the precursor. ZnO thin films were grown on ITO glass substrate with the seed layer of zinc oxide. Perpendicularly aligned arrays of corrugated ZnO nanorods were grown in NaOH solution, while the chrysanthemum like structure was obtained in ammonia solution. X-Ray diffraction patterns confirmed that both morphologies presenting wurtzite crystal of ZnO. The nanorod showed strong (002) orientation of ZnO.

Ultrathin TiO2 Films on ZnO Electron-Collecting Layers of Inverted Organic Solar Cell

Nanostructured ZnO thin films were fabricated on indium tin oxide (ITO) as hole-blocking and electron-collecting layers of inverted organic solar cells. In addition, TiO2 thin films were deposited on ZnO surfaces using atomic layer deposition. Ultrathin TiO2 layers with a mean thickness of <3 nm could enhance photovoltaic performance of the inverted organic solar cell; in particular, short-circuit current (Jsc) and power conversion efficiency (PCE) were increased by deposition of TiO2 layers. A higher thickness of TiO2 resulted in reduced photovoltaic performance. Studies on electronic structure using photoelectron spectroscopy showed that the TiO2 layers should act as barrier for the electron-collection on ZnO. However, recombination of electrons and holes on the surface of ZnO can be quenched by TiO2, resulting in an improved efficiency of the IOSCs. It is demonstrated that atomic layer deposition with a precise control of film thickness can be of potential application for fabrication of energy devices.

Fabrication of LP Gas Leakage Detector Systems Based on Modified Nanostructured ZnO Thin Film

The liquid petroleum (LP) gas leakage detector was successfully fabricated using modified nanostructured ZnO thin film as the sensitive layer. The detector consists of ZnO sensitive layer, a heater, a comparator, a transistor, an LED and a buzzer. The detector has the switch system where the comparator can change the current through the LED and buzzers when the LP gas is detected. The ZnO thin films were fabricated by sol gel method using chemical bath deposition technique at moderate temperature on an alumina substrate with electrode contacts. This sensors system allows us to produce LP gas sensor with a relatively low-cost procedure. The detector was tested with the LP gas leakages and had shown good response values as measured at the operational temperature of 200o C.

Conventional versus lattice photocatalysed reactions: Implications of the lattice oxygen participation in the liquid phase photocatalytic oxidation with nanostructured ZnO thin films on reaction products and mechanism at both 254 nm and 340 nm

This study aimed to investigate the relationship between surface morphologies of nanostructured ZnO thin films, the reaction intermediates and products produced, and the liquid phase and solid phase reaction mechanisms under both oxygen rich and oxygen limited conditions at two different wavelengths (254nm and 340nm). Four different ZnO morphologies were prepared by hydrothermal deposition on two different substrates – clean glass (CG) and direct current magnetron sputtered ZnO glass (MS). The two films grown on the MS templates were highly vertically aligned columnar structures and those grown without templates (CG) had little or no structural alignment. Methylene blue (MB) was used as the reactant, since its reaction intermediates and products are well defined and measurable, allowing predictable comparison between these catalysts.Results showed that there was a significant difference in MB degradation rates as well as reaction intermediate formation and destruction rates correlated to the morphologies and crystallinity at both UV wavelengths, with the highest reaction rates at 340nm. Reaction analysis indicates that there may be a competition between two different photocatalytic mechanisms: conventional photocatalysed radical oxidation and lattice oxygen driven oxidation. The dominant reaction mechanism present depended on the thin film morphology, crystallinity, availability of oxidant and the wavelength of the incident UV. The surface photocatalysed radical formation is thought to be predominant for more highly aligned and more crystalline morphologies, where there was plentiful oxygen and UV irradiation at 340nm. Lattice oxygen photodegradation is thought to be predominant for less aligned more amorphous morphologies and UV irradiation at 254nm. Based on these results a new MB ZnO photocatalysed oxidation mechanism is proposed.

ZnO Nanoestructured Layers Processing with Morphology Control by Pulsed Electrodeposition

The fabrication of nanostructured ZnO thin films is a critic process for a lot of applications of this semiconductor material. The final properties of this film depend fundamentally of the morphology of the sintered layer. In this paper a process is presented for the fabrication of ZnO nanostructured layers with morphology control by pulsed electrodeposition over ITO. Process optimization is achieved by pulsed electrodeposition and results are assessed after a careful characterization of both morphology and electrical properties. SEM is used for nucleation analysis on pulsed deposited samples. Optical properties like transmission spectra and Indirect Optical Band Gap are used to evaluate the quality of the obtained ZnO structures.

Influence of precursor concentration on structural, morphological and electrical properties of spray deposited ZnO thin films

AbstractNanostructured ZnO thin films were coated on glass substrate by spray pyrolysis using Zinc acetate dihydrate as precursor. Effect of precursor concentration on structural, morphological, optical and electrical properties of the films was investigated. The crystal structure and orientation of the ZnO thin films prepared with four different precursor solution concentrations were studied and it was observed that, the prepared films are polycrystalline in nature with hexagonal wurzite structure. The peaks are indexed to (100), (002), (101), (102) and (110) planes. Grain size and texture coefficient (TC) were calculated and the grain size found to increase with an increase in precursor concentration. Presence of Zn and O elements was confirmed with EDAX spectra. Optical absorption measurements were carried out in the wavelength region of 380 to 800 nm and the band gap decreases as precursor concentration increases. The current‐voltage characteristics were observed at room temperature and in dark. It was found that for the films deposited at four different precursor concentrations, the conductivity improves as precursor concentration increases. As trimethyl amine TMA is a good marker for food quality discrimination, sensing behavior of the films at an optimized operating temperature of 373 K, towards various concentrations of (TMA) was observed and reported. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Effect of nickel doping on the photocatalytic activity of ZnO thin films under UV and visible light

Nanostructured ZnO thin films with different concentrations of Ni2+ doping (0, 1, 5, 10 and 15wt.%) are prepared by the sol–gel method for the first time. The thin films are prepared from zinc acetate, 2-methoxyethanol and monoethanolamine on glass substrates by using dip coating method. The films comprise of ZnO nanocrystallites with hexagonal crystal structure, as revealed by X-ray diffraction. The film surface is with characteristic ganglia-like structure as observed by Scanning Electron Microscopy. Furthermore, the Ni-doped films are tested with respect to the photocatalysis in aqueous solutions of malachite green upon UV-light illumination, visible light and in darkness. The initial concentration of malachite green and the amount of catalyst are varied during the experiments. It is found that increasing of the amount of Ni2+ ions with respect to ZnO generally lowers the photocatalytic activity in comparison with the pure ZnO films. Nevertheless, all films exhibit a substantial activity under both, UV and visible light and in darkness as well, which is promising for the development of new ZnO photocatalysts by the sol–gel method.

Electrochemical Deposition Mechanism for ZnO Nanorods: Diffusion Coefficient and Growth Models

Fabrication of nanostructured ZnO thin films is a critical process for many applications based on semiconductor devices. So on understanding of the electrochemical deposition mechanism is also fundamental for knowing the optimal conditions on growth of ZnO nanorods by electrodeposition.In this paper the electrochemical mechanism for ZnO nanorods formation is studied. Results are based on the evolution of the diffusion coefficient using the Cotrell equation, and different growth models proposed by Scharifcker and Hills for nucleation and growth.

A highly sensitive humidity sensor based on DC reactive magnetron sputtered zinc oxide thin film

Nanostructured ZnO thin film was prepared by DC reactive magnetron sputtering and characterized by X-ray diffraction, SEM, AFM, FT-IR spectroscopy, UV–vis spectroscopy and photoluminescence measurements. Under the conditions employed, the sputtered ZnO film was found to be polycrystalline with grain size varying between 70 and 100 nm. The absorption and photoluminescence spectral data indicated the presence of defects in the form of zinc interstitials and oxygen vacancies which could promote adsorption of gaseous species. Impedance technique was used to confirm adsorption of water vapor onto the sensor film. The chemiresistor sensor showed nearly a four order variation in resistance in the relative humidity range between 6.3 and 84% with a response and recovery time of 3 and 12 s respectively. The device exhibited high stability offering scope for developing a practical humidity sensor.

Preparation of ZnO nanowires by electrochemical deposition

This work reports the results from the synthesis of nanostructured ZnO thin films via electrochemical deposition on glass substrates coated with F doped SnO2. The influence of the deposition parameters on the properties of the obtained ZnO films was studied. The Raman spectra of the ZnO films contain the typical for ZnO vibrational bands. The scanning electron microscope micrographs demonstrate that the films consist of ZnO nanowires. Growing of ZnO in the conditions with addition of H2O2 in lower concentration and without flowing air results in larger grain formation. The ZnO layers demonstrate high diffuse reflection.

CO Gas Sensing Characteristics of Nanostructured ZnO Thin Films

We investigated the carbon monoxide (CO) gas-sensing properties of nanostructured Al-doped zinc oxide thin films deposited on self-assembled Au nanodots (ZnO/Au thin films). The Al-doped ZnO thin film was deposited onto the structure by rf sputtering, resulting in a gas-sensing element comprising a ZnO-based active layer with an embedded Pt/Ti electrode covered by the self-assembled Au nanodots. Prior to the growth of the active ZnO layer, the Au nanodots were formed via annealing a thin Au layer with a thickness of 2 nm at a moderate temperature of <TEX>$500^{\circ}C$</TEX>. It was found that the ZnO/Au nanostructured thin film gas sensors showed a high maximum sensitivity to CO gas at <TEX>$250^{\circ}C$</TEX> and a low CO detection limit of 5 ppm in dry air. Furthermore, the ZnO/Au thin film CO gas sensors exhibited fast response and recovery behaviors. The observed excellent CO gas-sensing properties of the nanostructured ZnO/Au thin films can be ascribed to the Au nanodots, acting as both a nucleation layer for the formation of the ZnO nanostructure and a catalyst in the CO surface reaction. These results suggest that the ZnO thin films deposited on self-assembled Au nanodots are promising for practical high-performance CO gas sensors.

Reversible wettability of nanostructured ZnO thin films by sol–gel method

Nanostructured ZnO thin films were deposited on Si(1 1 1) and quartz substrate by sol–gel method. The thin films were annealed at 673 K, 873 K, and 1073 K for 60 min. Microstructure, surface topography, and water contact angle of the thin films have been measured by X-ray diffractometer, atomic force microscopy, and water contact angle apparatus. XRD results showed that the ZnO thin films are polycrystalline with hexagonal wurtzite structure. AFM studies revealed that rms roughness changes from 2.3 nm to 7.4 nm and the grain size grow up continuously with increasing annealing temperature. Wettability results indicated that hydrophobicity of the un-irradiated ZnO thin films enhances with annealing temperature increase. The hydrophobic ZnO surfaces could be reversibly switched to hydrophilic by alternation of UV illumination and dark storage (thermal treatment). By studying the magnitude and the contact angle reduction rate of the light-induced process, the contribution of surface roughness is discussed.

Photocatalytic action of ZnO thin films prepared by the sol–gel method

Nanostructured ZnO thin films were deposited on glass by the dip-coating sol–gel method. The films exhibited pronounced activity to destroy malachite green in water both under UV-light illumination and in the dark.

Preparation and characterization of nanostructured ZnO thin film by electrophoretic deposition from ZnO colloidal suspensions

Nanostructured ZnO thin films were electrophoretically deposited from ZnO colloidal suspensions. The microstructural properties of the nanoparticles were examined through XRD and TEM techniques. ZnO nanocrystallites in a hexagonal wurtzite phase with the size in the range of 4–8 nm were synthesized through colloid synthesis. The deposition parameters were studied with the aim of producing a crack-free, dense and smooth ZnO thin film. The deposit weight can be controlled by adjusting several deposition parameters. The average optical transmittance of the obtained ZnO film was higher than 90% with light wavelengths from 450 to 700 nm.

Characterization of Nanostructured ZnO Thin Films Deposited by Sol-Gel Spin Coating

ZnO thin films have been deposited onto the glass substrates by the sol-gel spin coating method at different chuck rotation rates. The effect of deposition parameters on the structural, optical and electrical properties of the ZnO thin films was investigated. The crystal structure and orientation of the ZnO thin films were investigated by X-ray diffraction (XRD) patterns. The optical absorbance and transmittance measurements were recorded by using a double beam spectrophotometer. The optical absorption studies reveal that the transition is direct band gap energy. The I-V plots of the ZnO thin films were carried out in dark and under UV-illumination. The result shows the obtained ZnO thin films can be used as a photovoltaic material.

Photoelectrochemical Study of Nanostructured ZnO Thin Films for Hydrogen Generation from Water Splitting

AbstractPhotoelectrochemical cells based on traditional and nanostructured ZnO thin films are investigated for hydrogen generation from water splitting. The ZnO thin films are fabricated using three different deposition geometries: normal pulsed laser deposition, pulsed laser oblique‐angle deposition, and electron‐beam glancing‐angle deposition. The nanostructured films are characterized by scanning electron microscopy, X‐ray diffraction, UV‐vis spectroscopy and photoelectrochemical techniques. Normal pulsed laser deposition produces dense thin films with ca. 200 nm grain sizes, while oblique‐angle deposition produces nanoplatelets with a fishscale morphology and individual features measuring ca. 900 by 450 nm on average. In contrast, glancing‐angle deposition generates a highly porous, interconnected network of spherical nanoparticles of 15–40 nm diameter. Mott‐Schottky plots show the flat band potential of pulsed laser deposition, oblique‐angle deposition, and glancing‐angle deposition samples to be −0.29, −0.28 and +0.20 V, respectively. Generation of photocurrent is observed at anodic potentials and no limiting photocurrents were observed with applied potentials up to 1.3 V for all photoelectrochemical cells. The effective photon‐to‐hydrogen efficiency is found to be 0.1%, 0.2% and 0.6% for pulsed laser deposition, oblique‐angle deposition and glancing‐angle deposition samples, respectively. The photoelectrochemical properties of the three types of films are understood to be a function of porosity, crystal defect concentration, charge transport properties and space charge layer characteristics.

Phonon assisted photoluminescence and surface optical mode of Zn embedded ZnO nanostructure

Zn embedded ZnO nanostructure thin films were prepared using a cost-effective novel vacuum-carbon-arc combined with the vacuum annealing technique. The photoluminescence spectra of the samples exhibit a strong near-band-edge emission around 377 nm and a weak defect band in the visible range accompanied by some multiphonon peaks at a higher energy. The shift in the optical absorption band arises due to potential fluctuation of the carrier density. Surface optical phonon mode predominant Raman spectra are observed, which is attributed to the surface potential (due to defects) present in the system. The predicted surface phonon frequency based on the dielectric continuum approach is in good agreement with that observed experimentally.

Structural evolution and visible photoluminescence of Zn–ZnO nanophosphor

AbstractThe Zn–ZnO nanostructure thin films are prepared by an inexpensive vacuum arc‐discharge method combined with vacuum annealing. The structural analysis of the grazing‐incidence X‐ray diffraction pattern using the Rietveld method showed the formation of Zn–ZnO in a single phase. The coexistence of Zn and ZnO in the thin‐film sample was also confirmed using transmission electron microscopy technique. Studies of optical properties by photoluminescence (PL) spectroscopy exhibit a strong violet emission accompanied by weak blue and green emission. The origin of defect emission of Zn–ZnO nanostructure was analyzed. Thus, the existence of metallic Zn nanocrystals embedded in ZnO islands provides a considerable change in the PL mechanism. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Preparation and characterization of nanostructured ZnO thin films for photoelectrochemical splitting of water

Nanostructured zinc oxide thin films (ZnO) were prepared on conducting glass support (SnO2: F overlayer) via sol-gel starting from colloidal solution of zinc acetate 2-hydrate in ethanol and 2-methoxy ethanol. Films were obtained by spin coating at 1500 rpm under room conditions (temperature, 28–35°C) and were subsequently sintered in air at three different temperatures (400, 500 and 600°C). The evolution of oxide coatings under thermal treatment was studied by glancing incidence X-ray diffraction and scanning electron microscopy. Average particle size, resistivity and bandgap energy were also determined. Photoelectrochemical properties of thin films and their suitability for splitting of water were investigated. Study suggests that thin films of ZnO, sintered at 600°C are better for photoconversion than the films sintered at 400 or 500°C. Plausible explanations have been provided.