Growth of three-dimensional ZnO nanorods by electrochemical method for quantum dots-sensitized solar cells

There-dimensional (3D) superstructure was expected to fabricate high performance photoelectrodes of quantum dot-sensitized solar cells (QDSCs). In this paper, the ZnO 3D superstructure with multi-layer structure (3D ZnO nanorods) was grown on ITO glass by a novel electrochemical method at low temperature (60–90 °C). The 3D ZnO nanorods were composed of close-packed ZnO nanorod bundles with wide dimension distribution ranging from hundreds of nanometers to several micrometers. The effects of some important parameters, such as concentration of Zn(NO₃)₂, deposition temperature and concentration of ZnO nanoparticles (served as growth seeds for ZnO nanorods), on the morphology of 3D ZnO nanorods were also studied by scanning electron microscopy. Once being applied in QDSCs, the 3D ZnO nanorods showed more superior photoelectrochemical performance to ZnO nanorod array. The conversion efficiency of 1.42% achieved by the QDSC based on 3D ZnO nanorods was a very promising value for the QDSCs based on ZnO electrodes.     

A method for electrochemical growth of homogeneous nanocrystalline ZnO thin films at room temperature

We report the electrodeposition at room temperature (25 °C), in a potentiostatic mode, of cohesive nanocrystalline ZnO thin films from an oxygenated zinc chloride bath. It is shown that the bath saturation by molecular oxygen precursor is a key parameter to grow the oxide at low temperature. After low O₂ bubbling the solution is not saturated and the surface is more or less passivated by an amorphous Zn(OH)₂ veil-like thin layer. After intense and long molecular oxygen bubbling, the current density rapidly increases after an induction period of about 800 s. At the foot of the current onset, crystallized ZnO seeds appear entrapped in the initial amorphous layer. The film nucleation is a delayed process. The electrode is subsequently covered by a homogeneous ZnO film with structures of several hundreds of nanometers in length composed of nanocrystals with size of about 17 nm. The room-temperature photoluminescence spectrum of the film is dominated by a strong UV emission at 3.25 eV due to the recombination of excitons. The visible emission centered at 2.36 eV, due to deep defects, is less intense than the UV one showing the good structural quality of the ZnO nanocrystallized film. The films have interesting properties to be used as a seed layer for instance.     

Polystyrene sphere monolayer assisted electrochemical deposition of ZnO nanorods with controlable surface density

In this paper we report the zinc oxide nanorods (ZnO NRs) growth by electrochemical deposition onto polycrystalline gold electrodes modified with assemblies of polystyrene sphere monolayers (PSSMs). Growth occurs through the interstitial spaces between the hexagonally close packed spheres. ZnO NRs nucleate in the region where three adjacent spheres leave a space, being able to grow and projected over the PSSMs. The nanorod surface density (N_NR) shows a linear dependence with respect to a PS sphere diameter selected. XRD analysis shows these ZnO NRs are highly oriented along the (0 0 2) plane (c-axis). This open the possibility to have electronic devices with mechanically supported nanometric materials.     

Preparation of PbS thin films: A new electrochemical route for underpotential deposition

Nanostructured thin films of lead sulfide have been synthesized by a new electrochemical approach based on the underpotential deposition (UPD) of Pb and S from the saturated solution of PbS containing excess of PbS particles as a source of Pb²⁺ and S²⁻ at various temperatures.We have demonstrated that this new electrochemical route is a simple method with several advantages, including better control of the growth conditions and a one-step process to obtain the nanostructures of PbS. Scanning probe microscopy studies indicate that the growth of PbS nanofilms follows a two-dimensional layer-by-layer growth kinetics at the beginning of electrodeposition but a three-dimensional growth dominates after the formation of the first few layers. The results of morphological and structural investigations reveal that PbS nanostructures grown by this method are single-crystalline in cubic structure and have a preferential orientation along the [2 0 0] direction. The optical absorption spectra of PbS nanostructures show the blue-shift with respect to those of the bulk counterpart, which are attributed as quantum-size effect.     

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.     

Structural and optical properties of ZnO nanorods by electrochemical growth using multi-walled carbon nanotube-composed seed layers

We reported the enhancement of the structural and optical properties of electrochemically synthesized zinc oxide [ZnO] nanorod arrays [NRAs] using the multi-walled carbon nanotube [MWCNT]-composed seed layers, which were formed by spin-coating the aqueous seed solution containing MWCNTs on the indium tin oxide-coated glass substrate. The MWCNT-composed seed layer served as the efficient nucleation surface as well as the film with better electrical conductivity, thus leading to a more uniform high-density ZnO NRAs with an improved crystal quality during the electrochemical deposition process. For ZnO NRAs grown on the seed layer containing MWCNTs (2 wt.%), the photoluminescence peak intensity of the near-band-edge emission at a wavelength of approximately 375 nm was enhanced by 2.8 times compared with that of the ZnO nanorods grown without the seed layer due to the high crystallinity of ZnO NRAs and the surface plasmon-meditated emission enhancement by MWCNTs. The effect of the MWCNT-composed seed layer on the surface wettability was also investigated.     

Synthesis of dumbbell-like ZnO microcrystals via a simple solution route

ABSTRACT: Uniform dumbbell-like ZnO microcrystals had been successfully fabricated on a large scale via a facile solution technique under mild conditions. Obtained ZnO, with length of 1.2 to 1.6 mum and diameters of 350 to 600 nm, exhibited well-defined dumbbell-like morphology and hexagonal wurtzite structure and grew along the [001] direction. Effects of the reactant concentration on the sizes and morphologies of the ZnO products had been investigated, indicating that the reactant concentration played a crucial role in determining final sizes and shapes of the samples. In addition, the growth process of the dumbbell-like ZnO microcrystals was studied, and a possible formation mechanism was proposed. Furthermore, the optical properties of ZnO samples obtained at various reaction times were also investigated by photoluminescence (PL) spectroscopy. The PL spectra of the as-prepared dumbbell-like ZnO microcrystals showed a strong UV emission peak.     

Electrochemical route to the synthesis of ZnO microstructures: its nestlike structure and holding of Ag particles

Abstract

A simple and facile electrochemical route was developed for the shape-selective synthesis of large-scaled series of ZnO microstructures, including petal, flower, sphere, nest and clew aggregates of ZnO laminas at room temperature. This route is based on sodium citrate-directed crystallization. In the system, sodium citrate can greatly promote ZnO to nucleate and directly grow by selectively capping the specific ZnO facets because of its excellent adsorption ability. The morphology of ZnO is tuned by readily adjusting the concentration of sodium citrate and the electrodeposition time. Among the series structures, the remarkable ZnO nestlike structure can be used as a container to hold not only the interlaced ZnO laminas but also Ag nanoparticles in the center. The special heterostructures of nestlike ZnO holding Ag nanoparticles were found to display the superior properties on the surface-enhanced Raman scattering. This work has signified an important methodology to produce a wide assortment of desired microstructures of ZnO.

PACS

81 Materials science 81.07.-b nanoscale materials and structures Fabrication Characterization 81.15.-z Methods of deposition of films Coatings Film growth and epitaxy.     

Electrochemical deposition of branched hierarchical ZnO nanowire arrays and its photoelectrochemical properties

Branched hierarchical ZnO nanowire arrays are synthesized on fluorine-doped tin oxide (FTO) substrate via a two-step electrochemical deposition process, which involves the electrodeposition of ZnO nanowire arrays on conductive glass substrate, followed by the electrochemical growth of ZnO nanorod branches on the backbones of the primary ZnO nanowires. The formation mechanism of the branched hierarchical nanostructure is discussed. It is demonstrated that coating the primary nanowire arrays with ZnO nanoparticles seed layer plays a key role in synthesising the branched hierarchical ZnO nanostructure. By adjusting the concentration of Zn(CH₃COO)₂ colloid in coating process and the reaction time of the second-step deposition, the density and the length of the secondary nanorod branches in the hierarchical nanostructures can be both varied. Moreover, the photoelectrochemical properties of the dye-sensitized solar cell (DSSC) based on branched hierarchical ZnO nanowire arrays are investigated. Due to the enlargement of the internal surface area within the branched nanostructure photoelectrode, the DSSC consisting of branched hierarchical ZnO nanowire arrays yields a power conversion efficiency of 0.88%, which is almost twice higher than that of the DSSC fabricated using bare ZnO nanowire arrays.     

Influence of nanocrystalline diamond powder on the growth and photoluminescence of open-ended ZnO nanorods

This paper reports the influence of introducing nanocrystalline diamond powder on the growth and photoluminescence of ZnO nanorods fabricated by hydrothermal technique. The majority of ZnO nanorods show an open-ended feature due to the addition of nano-diamond powder in the reaction solution. It is speculated that the diamond nanocrystallines dropped on the top of the growing ZnO nanorods would suppress the further growth at the localized positions to form the open-ended nanorods. The photoluminescence spectra of the ZnO products show the band-edge-related UV emission of ZnO nanorods at ~ 380 nm, in addition to a broad visible band centered at about 650 nm, which may originate from the emissions related to defects in the open-ended ZnO nanorods. The open-ended ZnO nanorods and/or combined with diamond nanocrystallines would be favorable for potential applications including UV sensors, electron field emitter, and various photoelectric nanodevices.     

Effects of preparing conditions on the electrodeposition of well-aligned ZnO nanorod arrays

By using a potentiostatic electrodeposition method, well-aligned ZnO nanorod arrays (ZNAs) were synthesized under different conditions. The effects of preparing conditions on the electrodeposition of ZNAs were systematically studied by scanning electron microscopy (SEM), X-ray diffraction (XRD) and absorbance spectroscopy. It is indicated that the electrodeposition parameters, such as electrodeposition potential, electrolyte pH, concentration of precursors, temperature of solution and electrodeposition time, have significant influence on the morphology, diameter, density and growth rate of ZNAs. The ZNAs, with well-defined crystallization, can be only obtained when the applied potential is controlled from −0.4 to −1.0 V. The growth temperature has great impact on the morphology of ZnO nanostructure but it is weakly related to the band gap (E_g) of ZNAs. The rod diameters can be monitored to some extent only by changing the concentration of the precursors. The electrolyte pH value has relative influence on the diameter of ZNAs. With the growth time increasing, ZNAs with high aspect ratio can be gained.     

Ce-doped ZnO nanoparticles for efficient photocatalytic degradation of direct red-23 dye

This paper reports the simple, rapid and high-yield synthesis of Ce-doped ZnO nanoparticles using solution combustion method and its application for photocatalytic degradation. Several concentrations of Ce (0.5%, 1.39%, 2.55%, 3.28%, 3.71% and 4.14%) were doped into ZnO and the prepared nanoparticles were characterized by several techniques in terms of their morphological, structural, compositional, optical and photocatalytic properties. The detailed characterization studies revealed that the prepared Ce-doped ZnO nanoparticles are well-crystalline and possessing good optical properties. Further, the prepared Ce-doped ZnO nanoparticles were used as efficient photocatalyst for the photocatalytic degradation of harmful organic dye, i.e. direct red-23 (DR-23). From the detailed photocatalytic experiments, it was observed that Ce-doped ZnO nanoparticles are exhibiting appreciable photocatalytic activity and the degradation percentage was increased with increasing the concentration of Ce up to 3.28%; however, when the Ce concentration was further increased, the photocatalytic degradation was decreased. Thus, at optimum Ce concentration (3.28%), the prepared Ce-doped ZnO nanoparticles are exhibiting appreciable photocatalytic degradation (~99.5%) only in 70 min.     

离子束增强沉积制备p-ZnO薄膜

采用离子束增强沉积技术制备了ZnO薄膜,分析了退火温度、退火气氛对所制备ZnO薄膜的结构、电学特性和发光特性的影响。利用IBED法获得的薄膜p型N-In共掺ZnO薄膜在氮气下退火,随着退火温度的升高,薄膜电阻值先降低后升高,然后再降低。而在氧气下退火,即使退火温度只有400℃,薄膜的电阻很快变大。     

A simple route to vertical array of quasi-1D ZnO nanofilms on FTO surfaces: 1D-crystal growth of nanoseeds under ammonia-assisted hydrolysis process

A simple method for the synthesis of ZnO nanofilms composed of vertical array of quasi-1D ZnO nanostructures (quasi-NRs) on the surface was demonstrated via a 1D crystal growth of the attached nanoseeds under a rapid hydrolysis process of zinc salts in the presence of ammonia at room temperature. In a typical procedure, by simply controlling the concentration of zinc acetate and ammonia in the reaction, a high density of vertically oriented nanorod-like morphology could be successfully obtained in a relatively short growth period (approximately 4 to 5 min) and at a room-temperature process. The average diameter and the length of the nanostructures are approximately 30 and 110 nm, respectively. The as-prepared quasi-NRs products were pure ZnO phase in nature without the presence of any zinc complexes as confirmed by the XRD characterisation. Room-temperature optical absorption spectroscopy exhibits the presence of two separate excitonic characters inferring that the as-prepared ZnO quasi-NRs are high-crystallinity properties in nature. The mechanism of growth for the ZnO quasi-NRs will be proposed. Due to their simplicity, the method should become a potential alternative for a rapid and cost-effective preparation of high-quality ZnO quasi-NRs nanofilms for use in photovoltaic or photocatalytics applications.PACS: 81.07.Bc; 81.16.-c; 81.07.Gf.     

Enhanced optoelectronic properties of Ti-doped ZnO nanorods for photodetector applications

We report the effect of Ti-doping on structural, morphological, photoluminescence, optical and photoconductive properties of ZnO thin films. Pure and Ti(1, 3 and 5%)-doped ZnO thin films are deposited by the successive ionic layer adsorption and reaction (SILAR) method. The X-ray diffraction analysis revealed the single-phase hexagonal wurtzite ZnO structure of all the films. Scanning electron microscope images suggest the formation of rod shaped particles in Ti-doped ZnO thin films. Photoluminescence spectra of all the films show emission peaks centered at 398 nm, 413 nm, 438 nm, 477 nm and 522 nm wavelengths. Optical properties support the semiconducting nature of all the films. The optical bandgap values are estimated to be 3.29 eV, 3.26 eV, 3.19 eV and 3.23 eV for ZnO, ZnO:Ti(1%), ZnO:Ti(3%) and ZnO:Ti(5%) thin films, respectively. Photoconductivity study indicates that ZnO:Ti(3%) thin film exhibits high responsivity, external quantum efficiency and detectivity of 0.30 AW^-1, 97% and 5.49 × 10¹⁰ Jones, respectively, among all the films. The enhanced photoconductivity of Ti-doped ZnO thin films make them useful for optoelectronic applications.     

Nanoseed-assisted rapid formation of ultrathin ZnO nanorods for efficient room temperature NO2 detection

The influence of ZnO nanoseeds on the formation of ZnO nanorods from ε-Zn(OH)₂ in NaOH solution at 80 °C was investigated, using ZnO nanoparticles with a diameter of 4–10 nm as the seeds. The experimental results indicated that the presence of ZnO nanoseeds promoted the rapid heterogeneous formation of ultrathin ZnO nanorods. Compared with the ZnO submicron rods with a diameter of 0.5–1.0 µm, the ultrathin ZnO nanorods with a diameter of 10–15 nm were found to be more sensitive for detecting NO₂ at room temperature owing to their higher variation of channel conduction to the diameter.     

Selective growth of ZnO nanorods on microgap electrodes and their applications in UV sensors

Selective area growth of ZnO nanorods is accomplished on microgap electrodes (spacing of 6 μm) by using a facile wet chemical etching process. The growth of ZnO nanorods on a selected area of microgap electrode is carried out by hydrothermal synthesis forming nanorod bridge between two electrodes. This is an attractive, genuine, direct, and highly reproducible technique to grow nanowire/nanorod onto the electrodes on selected area. The ZnO nanorods were grown at 90°C on the pre-patterned electrode system without destroying the electrode surface structure interface and geometry. The ZnO nanorods were tested for their application in ultraviolet (UV) sensors. The photocurrent-to-dark (Iph/Id) ratio was 3.11. At an applied voltage of 5 V, the response and recovery time was 72 and 110 s, respectively, and the response reached 2 A/W. The deposited ZnO nanorods exhibited a UV photoresponse that is promising for future cost-effective and low-power electronic UV-sensing applications.     

Two-step growth of ZnO films on silicon by atomic layer deposition

Two-step growth of ZnO by atomic layer deposition at low temperatures was performed to grow quality ZnO films on silicon substrates: first, the growth of a buffer layer at 130 ?C and second, the growth of the main layer at 210 ?C. Structural and optical properties of the ZnO films deposited on ZnO-buffer/Si(111) were investigated as a function of buffer layer thickness. The films showed a strong UV emission at 380 nm and a weak green emission at 520–570 nm. The ZnO films deposited on a 327 å buffer layer showed overall the best surface morphology and structural and optical properties.     

Broadband anti-reflective properties of grown ZnO nanopyramidal structure on Si substrate via low-temperature electrochemical deposition

Anti-reflection coatings (ARCs) are widely used in various optical and optoelectronic devices to minimize the reflection of light. In this study, we demonstrated the fabrication of ZnO nanopyramidal structures on Si substrate via low-temperature electrochemical deposition. We also investigated the anti-reflection (AR) properties of these nanostructures compared with nanorods and planar ZnO texture on Si substrates. We changed the growth conditions, namely, growth temperature and applied current density, to modify the shape of the ZnO nanorod tips. Nanopyramidal structures with continuously varying refractive index profiles in a single layer were obtained. Reflectance spectra show that the nanopyramid-based texture reduced the reflection of light in a broad spectral range from 380 nm to 1000 nm and is much more effective than nanorod and planar textures. For nanopyramid arrays (NPAs) with average tip diameter of 20 nm, we achieved a 6.5% reflectance over a wide range of wavelengths, which is superior to an optimized single-layer ARC such as SiO₂ or TiO₂. These textured ZnO ARCs may be applied to a wide variety of photovoltaic devices and other anti-reflection applications with large areas because of their low temperature, fast growth, and simple fabrication.