Improvement of ZnO nanorod-based dye-sensitized solar cell efficiency by Al-doping

The current study investigates the performance of dye-sensitized solar cells (DSSCs) based on Al-doped and undoped ZnO nanorod arrays synthesized by a simple hydrothermal method. Current density-voltage (J-V) characterizations indicate that Al-doping in ZnO crystal structure can significantly improve current densities and the energy conversion efficiency (η) of ZnO nanorod-based DSSCs. The maximum η, 1.34%, was achieved in DSSC when Al-doped ZnO nanorod arrays were grown in 0.04 M zinc acetate dihydrate solution with 5 mM aluminum nitrate nonahydrate. This result represents a large increase of η in Al-doped ZnO nanorod-based DSSCs as compared to undoped (0.05%). The improved DSSC photovoltaic performance can be attributed to two main factors: (1) increased light harvesting efficiency due to a large amount of N719 adsorbed on the large surface area of Al-doped ZnO nanorod arrays, and (2) increased electrical conductivity due to A1³⁺ ion doped into the ZnO lattice at the divalent Zn²⁺ site, allowing electrons to move easily into the Al-doped ZnO conduction band.     

Alignment-controlled hydrothermal growth of well-aligned ZnO nanorod arrays

ZnO nanorod arrays (ZNAs) were prepared via a two-step seeding and solution hydrothermal growth process. Effects of preparing parameters such as seed layer, colloid concentration, substrate and precursor concentration, on the alignment control of ZNAs were systematically investigated. The deviation angle of ZnO nanorods was measured to evaluate the alignment of arrays. Results show that seed layer not only controls the vertical orientation of ZNAs, but also the compactness of ZNAs. Altering colloid concentration and substrate can influence the microstructure of ZnO seed layer and affect the ordered alignment of ZNAs. The precursor concentration has an insignificant effect on the alignment of ZNAs but has great impact on the morphology of ZNAs. Alignment-controlled and well-aligned ZnO nanorods with different diameter and aspect ratio can be obtained by properly controlling the preparing parameters. A growth mechanism was proposed for the growth of ZnO nanorods.     

A simple growth route towards ZnO thin films and nanorods

Highly orientated ZnO thin films and the self-organized ZnO nanorods can be easily prepared by a simple chemical vapor deposition method using zinc acetate as a source material at the growth temperature of 180 and 320 °C, respectively. The ZnO thin films deposited on Si (100) substrate have good crystallite quality with the thickness of 490 nm after annealing in oxygen at 800 °C. The ZnO nanorods grown along the [0001] direction have average diameter of 40 nm with length up to 700 nm. The growth mechanism for ZnO nanorods can be explained by a vapor-solid (VS) mechanism. Photoluminescence (PL) properties of ZnO thin films and self-organized nanorods were investigated. The luminescence mechanism for green band emission was attributed to oxygen vacancies and the surface states related to oxygen vacancy played a significant role in PL spectra of ZnO nanorods.     

Deposition and characterization of transparent and conductive sprayed ZnO:B thin films

Highly transparent and conductive Boron doped zinc oxide (ZnO:B) thin films were deposited using chemical spray pyrolysis (CSP) technique on glass substrate. The effect of variation of boron doping concentration in reducing solution on film properties was investigated. Low angle X-ray analysis showed that the films were polycrystalline fitting well with a hexagonal wurtzite structure and have preferred orientation in [002] direction. The films with resistivity 2.54×10⁻³ Ω-cm and optical transmittance >90% were obtained at optimized boron doping concentration. The optical band gap of ZnO:B films was found ∼3.27 eV from the optical transmittance spectra for the as-deposited films. Due to their excellent optical and electrical properties, ZnO:B films are promising contender for their potential use as transparent window layer and electrodes in solar cells.     

Surface modification with EDTA molecule: A feasible method to enhance the adsorption property of ZnO

Surface-functionalized zinc oxide (ZnO) nanoparticles were synthesized with ethylene diamine tetraacetic acid (EDTA) as a modification agent, which were used as adsorbents in the adsorption of Cu²⁺ at certain conditions. The transmission electron microscopy (TEM) results show that the average size of ZnO particles is about 45 nm, and it exhibits hexagonal wurtzite structure. Fourier transform infrared (FTIR) spectra reveal that the EDTA species are chemically bonded on the surface of ZnO. Compared with bare ZnO particles, the functionalized ZnO nanoparticles have a better activity in the Cu²⁺ adsorption. The maximum adsorption capacity of functionalized ZnO nanoparticles is 20.97 mg/g, while it is 17.93 mg/g for the bare ZnO. The adsorption isotherm of bare ZnO particles is in accordance with the Freundlich model, and the chemical adsorption is in a dominant position in the adsorption process of Cu²⁺ on functionalized ZnO particles.     

Characterization, sintering and dielectric properties of nanocrystalline zinc oxide prepared by a citric acid-based combustion route

Nanosized zinc oxide has been synthesized through a novel single step solution combustion route using citric acid as fuel. The X-ray diffraction (XRD) analysis revealed that the synthesized ZnO nanopowder has the pure wurtzite structure. The phase purity of the nanopowder has been confirmed using differential thermal analysis (DTA), thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FT-IR). The morphology and crystalline size of the as-prepared nanopowder characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that the powder consisted of a mixture of nanoparticles and nanorods. The nanocrystalline ZnO could be sintered to ∼97% of the theoretical density at 1200 °C in 4 h. The dielectric constant (ε_r) and dielectric loss (ε_i) of sintered ZnO pellets at 5 MHz were 1.38 and 9×10⁻², respectively, at room temperature.     

Growth of vertically aligned one-dimensional ZnO nanowire arrays on sol–gel derived ZnO thin films

Vertically aligned one-dimensional ZnO nanowire arrays have been synthesized by a hydrothermal method on sol–gel derived ZnO films. Sol–gel derived ZnO films and corresponding ZnO nanowire arrays have been characterized by X-ray diffraction and field-emission scanning electron microscopy. The effect of sol–gel derived ZnO film surface on the morphology of ZnO nanowire arrays has been investigated. The authors suggest from our investigation that sol–gel derived ZnO films affect the growth of one-dimensional ZnO nanostructures. Not only crystalline ZnO films but also amorphous ones can act as a scaffold for ZnO nucleus. Tilted ZnO micro-rods are grown on ZnO gel films, whereas vertically aligned ZnO nanowire arrays are grown on nanometer-sized ZnO grains. The average diameter of ZnO nanowire arrays are correlated strongly with the grain size of sol–gel derived ZnO films.     

Low-temperature synthesis of ZnO nanonails

Wurtzite ZnO nanonails on silicon substrate were successfully synthesized by thermal vapor transport and condensation method at a low temperature without a metal catalyst. Pure Zn powders were used as raw material and O₂/Ar powders as source gas. The products were characterized by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The results show that the deposited nanostructures include aligned ZnO nanonails. The ZnO nanonails, with crystalline cap and small-diameter shafts, grow along the c-axis. The optical properties have been revealed by photoluminescence spectra. We considered that the ZnO nanonails growth is a vapor-solid process.     

Property enhancement of transparent conducting zinc oxide thin films—Effect of simultaneous (Sn+F) doping

Undoped and simultaneously (Sn+F) doped ZnO thin films were fabricated using a simplified spray pyrolysis technique and the effects of Sn doping level on their electrical, structural, optical and surface morphological properties were studied. The XRD patterns confirmed the hexagonal wurtzite structure of ZnO. The minimum electrical resistivity of 0.45×10⁻² Ω cm was obtained for ZnO films having Sn+F doping levels of 8+20 at%. All the films exhibited average optical transmittance of 85% in the visible region, suitable for transparent electrode applications. The overall quality of the fabricated films was confirmed from photoluminescence (PL) studies. The PL and surface morphological studies along with the elemental analysis showed the increase of Sn diffusion into the ZnO lattice which was consistent with the concentration of Sn in the starting solution. The results of the analysis of physical properties of simultaneously doped ZnO films proved that these films might be considered as promising candidates for solar cells and other opto-electronic applications.     

Synthesis and Characterization of ZnO Nanoflowers Grown on AlN Films by Solution Deposition

ZnO nanoflowers are synthesized on AlN films by solution method. The synthesized nanoflowers are composed of nanorods, which are pyramidal and grow from a central point, thus forming structures that are flower-shaped as a whole. The nanoflowers have two typical morphologies： plate-like and bush-like. The XRD spectrum corresponds to the side planes of the ZnO nanorods made up of the nanoflowers. The micro-Raman spectrum of the ZnO nanoflowers exhibits the E2 （high） mode and the second order multiple-phonon mode. The photoluminescence spectrum of the ZnO nanoflowers exhibits ultraviolet emission centred at 375nm and a broad green emission centred at 526 nm.