Morphology and properties of ZnO nanostructures by electrochemical deposition: effect of the substrate treatment

Zinc Oxide (ZnO) nanostructures have been grown by electrochemical deposition on the indium-tin oxide (ITO) substrates. The influences and mechanism of pre-treatment of ITO on the morphology, density and size of ZnO nanostructures have been studied. Hexagonal dumbbell-like shape of ZnO bipods have been obtained because of the absence of nucleation sites on the unetched ITO substrate. An ITO etching process in dilute HCl results in the instantaneous and high density nucleation process, which was introduced to reduce the system energy and increase the growth density of ZnO nanostructures. Therefore, well-defined hexagonal and dense ZnO nanorods array are deposited on the etched ITO surface.     

Thickness of ITO thin film influences on fabricating ZnO nanorods applying for dye-sensitized solar cell

Background and purposeIn order to compare the substrates influence on the properties of ZnO films and nanostructures, in this paper, the ITO substrates with different thicknesses were investigated.MethodITO thin films of different thickness (200 nme500 nm) were deposited on glass substrates by DC sputtering, on which ZnO nanorods were fabricated from as-deposited ZnO films by reducing annealing method.ResultsIt was found that the structural and electrical properties of ITO films were significantly influenced by the ITO film thickness. The roughness of ITO films was increased with increase in thickness. The Hall mobility of ITO films was also increased with the increase of film thickness; in contrast, the resistivity was decreased. The highest Hall mobility of 29.2 cm²/V s and the lowest resistivity of 1.303 × 10⁻⁴ Ω cm were obtained from 500 nm-thick ITO film. The structural properties of ZnO nanorods were significantly influenced by the ITO film thickness. The density of ZnO nanorods gradually decreased with the increase in thickness of ITO film.ConclusionThe overall conversion efficiency of demonstrated dye-sensitized solar cell was 2.11% with a fill factor 0.526, indicating high potential to be used as photoanodes in dye-sensitized solar cell applications.     

Indium tin oxide nanorod electrodes for polymer photovoltaics

We have deposited indium tin oxide (ITO) nanorods on glass and glass/ITO substrates by DC sputtering and by e-beam deposition. The properties of the nanorods deposited by different methods and on different substrates have been investigated. The ITO nanorods were also used as an electrode in bulk heterojunction polymer solar cells. We found that the nanorod morphology and sheet resistance had a significant effect on the solar cell performance, with significant improvements in the efficiency compared to commercial ITO film substrates in all cases except for e-beam deposited nanorods on glass that had high sheet resistance. The best power conversion efficiency achieved was 3.2 % (for sputtered ITO nanorods on ITO), compared to 2.1 % for commercial ITO substrates.     

Preparation and optimization of epitaxial growth of transparent ZnO nanotip thin films by hydrothermal method

Zinc oxide (ZnO) nanotip thin films were prepared on ZnO coated nanocrystalline ITO/glass substrates by hydrothermal method. In order to obtain the ZnO nanotip arrays with high aspect ratio, the experimental conditions were optimized. The scanning electron microscope images showed that the surface morphology of ZnO thin films could be easily manipulated by changing the seed layer thickness and growth time. The ZnO nanotip thin films were grown epitaxially on ZnO seed layer coated ITO/glass substrates. The surface morphology of ZnO thin films on ITO/glass substrate changed from nanorods with a flat-top end to nanotips as the growth time was increased from 3 to 15 h. The ZnO thin films prepared under these deposition conditions were highly oriented along (002) direction. The as-prepared sample (15 h) was annealed at different temperatures (30, 100, 150, and 270 degrees C). The surface morphologies of annealed ZnO thin films did not show any remarkable change and the best crystallinity was observed at 100 degrees C. The photoluminescence spectra showed that the near band edge emission shifted to shorter wavelength as the annealing temperature was increased from 30 to 270 degrees C, it was due to the intrinsic stress in the films. This was confirmed by X-ray diffraction analyses. NPB thin films were prepared on ITO/clay and ITO/glass substrates by thermal evaporation method. The electrical properties of Ag/NPB/ITO/Clay showed the Ohmic characteristics (J proportional V(1.0)). The J-V characteristic of Ag/NPB/PMMA/ZnO/ITO/Glass showed good rectification behaviour with a diode-ideality factor of 1.36.     

Effect of buffer layer on solution deposited ZnO films

ZnO films were prepared by solution deposition method on various substrates: bare glass, TiO₂-buffer-coated glass, ITO glass and ZnO-buffer-coated glass. PEG addition was used to further investigate the effect of the patterned ZnO buffer on the morphology of the films. The structural morphology was investigated by using X-ray diffraction, scanning electron microscopy, field emission scanning electron microscopy analysis methods. The nature of the substrate was found to have effect on the morphology and crystal structure of the resultant films. All the films deposited on various substrates were c-oriented and the highest intensity of (002) diffraction peak appeared in the samples deposited on ZnO buffer. Addition of PEG to the buffer precursor affected the size distribution of ZnO grains on buffer layer, resulting in composite films with nano- and microcrystals which dispersed in each other. The homogeneous nucleation and heterogeneous nucleation on the two sides of the substrates were discussed based on the film morphology.     

Growth of highly oriented ZnO films by the two-step electrodeposition technique

Compact and transparent ZnO films were deposited on the ITO/glass substrates from zinc nitrate aqueous solution by the two-step electrodeposition technique. While the first potentiostatic step was used to produce ZnO seed layer, the ZnO film growth has been done galvanostatically. Effects of the potentiostatic parameters on the crystal structure, morphology and optical properties of ZnO films were investigated. Results show that ZnO films with highly c-axis preferred orientation can been obtained when the potentiostatic deposition at −1.2 V for 15 s has been applied. Such an observation might be attributed to the etching process of ITO substrate in the diluted HCl solution. The film exhibits smooth and compact morphology, high transmittance in the visible band (>80%) and sharp absorption edge (at ∼370 nm). The analysis on the growth mechanism indicates that the short potentiostatic process prior to the film growth can produce ZnO seed layer and substitute the initial nucleation process in the conventional one-step galvanostatic deposition, thus increasing the nucleation density and preventing the formation of loose structures.     

Dye-sensitized ZnO nanorod based photoelectrochemical solar cells with natural dyes extracted from Ixora coccinea,Mulberry and Beetroot

ZnO nanorods have been prepared on indium tin oxide (ITO) glass substrates coated with sol–gel based ZnO seed layer through chemical method. Natural dyes (Ixora coccinea, Mulberry and Beetroot) have been used for the fabrication of dye sensitized solar cells. The surface morphology studied using scanning electron microscope shows that the vertically aligned ZnO nanorods are of hexagonal shape. The solar cell efficiency has been calculated and is found to be 0.33, 0.41 and 0.28% for Ixora coccinea, Mulberry and Beetroot extract dye sensitized solar cells.     

Refractive indices of textured indium tin oxide and zinc oxide thin films

The refractive indices of textured indium tin oxide (ITO) and zinc oxide (ZnO) thin films were measured and compared. The ITO thin film grown on glass and ZnO buffered glass substrates by sputtering showed distinct differences; the refractive index of ITO on glass was about 0.05 higher than that of ITO on ZnO buffered glass in the whole visible spectrum. The ZnO thin film grown on glass and ITO buffered glass substrates by filtered vacuum arc also showed distinct differences; the refractive index of ZnO on glass was higher than that of ZnO on ITO buffered glass in the red and green region, but lower in the blue region. The largest refractive index difference of ZnO on glass and ITO buffered glass was about 0.1 in the visible spectrum. The refractive index variation was correlated with the crystal quality, surface morphology and conductivity of the thin films.     

Electric field influenced ZnO growth on nanoscale etched Si surface by continuous spray pyrolysis

This work reports ZnO growth by electric field influenced continuous spray pyrolysis reactor over seed layer assisted nanoscale etched Si surface. The comparative studies of ZnO growth in the presence and absence electric field on anisotropically etched Si surface are supported by XRD, SEM, EDX, FTIR and reflectance measurements. XRD measurements confirm preferable (002) oriented ZnO growth by certain amount of etching which is reduced with further anisotropic etching. The chlorine incorporation in ZnO thin film measured by EDX and FTIR may be attributed to the seed layer effect. This present investigation show that anisotropic etch pits surface energy profile has played an important role in deciding ZnO nanorods alignment and coalescence amongst ZnO species during self assembly at nucleation sites. It is observed that additional electric field force in spray droplets is enhancing population density, homogeneous growth and surface coverage of Si surface by ZnO nanorods for particular amount of etched Si surface and improved light coupling is confirmed by reflectance measurement. The relevant growth mechanisms for ZnO nanostructure layers comprising nanorods are also discussed.     

Study of nucleation and growth process of electrochemically synthesized ZnO nanostructures

The electrodeposition of ZnO nanostructures on ITO substrates was investigated by cyclic voltammetry, chronoamperometry and X-ray diffraction techniques. The potential deposition-dependent nucleation and growth mechanism of electrodeposited ZnO were studied by using the Scharifker-Hills nucleation model. From the analysis of the experimental current transients, the nucleation is in a good agreement with the instantaneous nucleation and three-dimensional (3D) diffusion-limited growth. X-ray diffraction measurements indicated that the as-grown films were of hexagonal wurtzite phase with a high crystalline quality.     

ZnO@CdS core–shell thin film: fabrication and enhancement of exciton life time by CdS nanoparticle

In the present study photoluminescence behavior of ZnO and ZnO@CdS core–shell nanorods film has been reported. ZnO nanorods were grown on the glass coated indium tin oxide (ITO) surface by seeding ZnO particle followed with nanorods growth. These nanorods were coated with CdS by chemical bath deposition techniques to have ZnO@CdS thin film and further annealed at 200 °C for their adherence to the ITO surface. The coating was characterized for surface morphology using SEM and optical behavior using UV–visible spectrophotometer. Energy dispersive X-ray (EDX) was used for compositional analysis and time resolve photoluminescence decay for excitons life time measurement. The absorption spectrum reveals that the absorption edge of ZnO@CdS core–shell heterostructure shifted to 480 nm in the visible region whereas ZnO nanorods have absorption maxima at 360 nm. The excitons lifetime of ZnO@CdS was found to be increased with the thickness of the CdS layer on ZnO nanorod. These ZnO@CdS core–shell nanostructures will be of great use in the field of photovoltaic cell and photocatalysis in a UV–visible region.     

ZnO单晶纳米棒阵列的水热法合成及光催化性能

采用水热法在各种衬底上低温、一步合成出一维有序ZnO单晶纳米棒阵列。以透明导电玻璃(ITO)、聚对苯二甲酸乙二醇酯(PET)以及铜作为衬底时,必须预先沉积一层ZnO薄膜作为形核层才能获得ZnO纳米棒阵列。然而,以黄铜作为衬底时,无需形核层也能获得纳米棒阵列。采用SEM、XRD、紫外吸收光谱等方法对纳米棒的结构以及形貌等进行了表征。此外还发现ZnO纳米棒阵列具有很好的紫外光催化活性,3h内对亚甲基蓝的降解率达到53%。尽管修饰CdSe层有助于提高ZnO纳米棒阵列在可见光波段的吸收,但其紫外光催化活性反而降低。并具体讨论了ZnO纳米棒阵列的合成与光催化性能。     

Large-scale electrodeposition of ZnO thin films with novel petal-like architectures

Large-scale and dense ZnO thin films with novel petal-like architectures were directly electrodeposited on the Au/ITO glass substrates from aqueous solution of Zn(NO₃)₂ and HMTA at a low temperature of 70 °C for the first time. Scanning electron microscopic (SEM) investigation revealed that the Au/ITO glass substrates are fully covered with densely distributed ZnO petal-like architectures. X-ray powder diffraction (XRD) pattern indicated that as-prepared unique films are highly crystalline and wurtzite hexagonal phase with extremely preferred orientation along [0001] direction. The optical band gap energy was found to be 3.85 eV for ZnO film with petal-like architectures on the Au/ITO substrate. Moreover, a strong and sharp ultraviolet (UV) emission at 386 nm but very weak defect-related deep level emission (DLE) in the room temperature photoluminescence (PL) spectrum also indicated that as-grown films are of good crystal quality. The possible growth mechanism for the novel petal-like architectures suggested that both the inherent highly anisotropic structure of ZnO and large lattice mismatch between ZnO and Au played crucial roles in determining final surface microstructures of the products.     

Catalytic effects of ZnO nanorods grown by sonochemical decomposition of zinc acetate dihydrate

In this study, we prepared ZnO nanorods by a sonochemical method using a zinc acetate dihydrate as a new precursor. Well-aligned high-quality ZnO nanorods were synthesized on FTO glass by the sonochemical decomposition of zinc acetate dihydrate using a ZnO thin-film as the catalytic layer. The ZnO thin-films were deposited on the FTO glass by a sputtering method. To investigate their catalytic effects on the ZnO nanorods, catalytic ZnO thin-films of 20 nm, 40 nm, and 60 nm thickness were prepared by adjusting the sputtering time. The ZnO nanorods grown on catalytic layers with different thicknesses were characterized by SEM, XRD, and PL. The ZnO nanorods grown on the catalytic layer of 40 nm thickness show the best crystal and spatial orientation and as a result display the best optical properties. It was found that a catalytic ZnO thin-film of 40 nm in thickness yields well-aligned high-quality ZnO nanorods, due to its small surface roughness and structural strain.     

Zinc oxide nanorods modified indium tin oxide surface for amperometric urea biosensor

ZnO nanorods (ZnONR) grown onto indium-tin-oxide (ITO) coated glass surface using zinc nitrate hexahydrate/hexamethylenetetramine (HMT) in aqueous phase has been utilized for urea biosensor. Urease (Urs) was immobilized onto ZnONR/ITO at physiological pH via electrostatic interactions between Urs and ZnO to fabricate Urs/ZnONR/ITO bioelectrode. ZnONR/ITO electrode has been characterized using XRD, FE-SEM techniques and Urs/ZnONR/ITO bioelectrode using electrochemistry. The XRD and FE-SEM measurements confirm the formation of ZnO nanorods in wurtzite structure. Cyclic voltammetric and amperometric measurements on the Urs/ZnONR/ITO biolectrode for urea concentrations in the range of 1-20 mM reveal 0.4 microA mM(-1) sensitivity, with a response time of 3 seconds, and a detection limit of 0.13 mM. The Michaelis-Menten constant (Km) was calculated to be 9.09 mM. Results indicate that ZnO nanorods provide suitable microenvironment for urease immobilization and can be utilized in biosensor design and other biological applications.     

Selective formation of GaN-based nanorod heterostructures on soda-lime glass substrates by a local heating method

We report on the fabrication of high-quality GaN on soda-lime glass substrates, heretofore precluded by both the intolerance of soda-lime glass to the high temperatures required for III-nitride growth and the lack of an epitaxial relationship with amorphous glass. The difficulties were circumvented by heteroepitaxial coating of GaN on ZnO nanorods via a local microheating method. Metal-organic chemical vapor deposition of ZnO nanorods and GaN layers using the microheater arrays produced high-quality GaN/ZnO coaxial nanorod heterostructures at only the desired regions on the soda-lime glass substrates. High-resolution transmission electron microscopy examination of the coaxial nanorod heterostructures indicated the formation of an abrupt, semicoherent interface. Photoluminescence and cathodoluminescence spectroscopy was also applied to confirm the high optical quality of the coaxial nanorod heterostructures. Mg-doped GaN/ZnO coaxial nanorod heterostructure arrays, whose GaN shell layers were grown with various different magnesocene flow rates, were further investigated by using photoluminescence spectroscopy for the p-type doping characteristics. The suggested method for fabrication of III-nitrides on glass substrates signifies potentials for low-cost and large-size optoelectronic device applications.     

Surface characterization of sol-gel derived indium tin oxide films on glass

Indium tin oxide (ITO) films containing different In : Sn atomic ratios, viz. 90 :10, 70 :30, 50 : 50, 30 :70, were deposited on two types of glass substrates by sol-gel spinning technique. XPS analysis of the films was done under as-received and after-sputtering conditions. The narrow spectra obtained for the Na1s, In3d, Sn3d and O1s have been discussed. Oxygen was found to exist in three chemical environments in as-received samples due to the existence of (i) environmental hydroxyl (-OH) group, (ii) crystalline ITO and (iii) amorphous ITO; but it was in two chemical environments, (ii) and (iii), after surface cleaning by sputtering. The presence of both tin metal and tin oxides was confirmed by the peak analysis of Sn3d. The In : Sn atomic ratio taken in the precursor sols did not change considerably in the case of developed films of low Sn content, but considerable change was observed in the films having high Sn content.     

Optical and field-emission properties of ZnO nanostructures deposited using high-pressure pulsed laser deposition

ZnO nanostructures were deposited on GaN (0001), Al2O3 (0001), and Si (100) substrates using a high-pressure pulsed laser deposition (PLD) method. Vertically aligned hexagonal-pyramidal ZnO nanorods were obtained on the Al2O3 and Si substrates whereas interlinked ZnO nanowalls were obtained on the GaN substrates. A growth mechanism has been proposed for the formation of ZnO nanowalls based on different growth rates of ZnO polar and nonpolar planes. Both ZnO nanorods and nanowalls exhibit a strong E2H vibration mode in the micro-Raman spectra. The corresponding fluorescence spectra of ZnO nanorods and nanowalls showed near band emission at 3.28 eV. The ZnO nanorods grown on the Si substrates exhibited better crystalline and optical properties compared with the ZnO structures grown on the GaN and Al2O3 substrates. The high aspect ratio, good vertical alignment, and better crystallinity of the ZnO nanorods with tapered tips exhibited promising field emission performance with a low turn-on field of 2 V/μm, a high current density of 7.7 mA/cm2, and a large field enhancement factor.     

Effects of seed layers on structural, morphological, and optical properties of ZnO nanorods

We studied the effects of seed layers on the structural and optical properties of ZnO nanorods. ZnO and Ag-doped ZnO (ZnO:Ag) seed layers were deposited on glass substrates by magnetron co-sputtering. ZnO nanorods were grown on these seed layers by the chemical bath deposition in an aqueous solution of Zn(NO3)2 and hexamethyltetramine. SEM micrographs clearly reveal that ZnO nanorods were successfully grown on both kinds of seed layers. The XRD patterns indicate that crystallization of ZnO nanorods is along the c-axis. Meanwhile, the packing density and the vertical alignment of the ZnO nanorods on the ZnO seed layer are better than those of the ZnO nanorods on ZnO:Ag. The enhanced growth of nanorods is thought to be due to the fact that the ZnO layer exhibits a higher crystalline quality than the ZnO:Ag layer. According to the low-temperature photoluminescence spectra, the ZnO nanorods on the ZnO seed layer show a narrow strong ultraviolet emission band centered at 369 nm, while those on ZnO:Ag exhibit multiple bands. These results are thought to be related with the crystallinity of ZnO nanorods, the morphologies of ZnO nanorods, and the reflectivities of seed layers. More detailed studies for clarification of the seed layer effect on the growth of ZnO nanorods are desirable.     

Effect of AZO seed layer on electrochemical growth and optical properties of ZnO nanorod arrays on ITO glass

We report the structural and optical properties of ZnO nanorod arrays (NRAs) grown by an electrochemical deposition process. The ZnO NRAs were grown on indium tin oxide (ITO) coated glass substrates with a thin sputtered Al-doped ZnO (AZO) seed layer and compared with ones directly grown without the seed layer. The growth condition dependence of ZnO NRAs was investigated for various synthetic parameters. The morphology and density of the ZnO NRAs were accordingly controlled by means of zinc nitrate concentration and growth time. From photoluminescence results, the ultraviolet emission was significantly enhanced after thermal treatment. For ZnO NRAs grown on ITO glass without the seed layer, the diffuse transmittance was enhanced despite the reduction in the total transmittance, indicating a high haze value. By using a thin AZO seed layer, the well-aligned ZnO NRAs on AZO/ITO glass are controllably and reproducibly synthesized by varying the growth parameters, exhibiting a total transmittance higher than 91% in the visible wavelength range as well as good optical and crystal quality.