Room temperature synthesis and characterization of CdO nanowires by chemical bath deposition (CBD) method
Introduction
One-dimensional nanoscale semiconducting metal oxide materials have attracted much attention due to their importance in understanding of the fundamental roles of low-dimensional effects, as well as their applications in nanodevices. Production of metal chalcogenide and/or oxide nanowires of uniform diameter has of special interest because it not increases only surface area, but also provides superior electrical conductivity when compared to other morphologies [1], [2]. The brown CdO is generally formed by burning of Cd in air. The CdO is insoluble in water, absorbs CO2 from air and can be reduced to the conducting oxides it has received very little attention; though it is one of the promising candidates for optoelectronic field [3]. The special features such as high conductivity, high transmission, and low band gap have made it applicable in photodiodes, phototransistors, photovoltaics, transparent electrodes, liquid crystal displays, IR detectors and anti-reflecting coatings [4]. Most of the reported methods for CdO formation includes sophisticated instrumentations including dc magnetron sputtering [5], [6], metal organic chemical vapor deposition [7], spray pyrolysis [8], [9], [10], [11], etc.
Literature survey confirmed that there are only few reports on the chemical synthesis (also known as chemical bath deposition, CBD) of CdO [12]. From the structural point of view, growing nanowires of high aspect ratio by low temperature chemical methods such as CBD can find extensive scope. Chemical bath deposition, which has been well known as prevalent low temperature aqueous method for directly depositing large-area thin films of semiconductors has advantages over above-mentioned techniques because it allows films to be deposited on nonplanar substrates that might not be chemically or mechanically stable at high temperatures [13]. Moreover, it requires no sophisticated instruments such as vacuum systems, etc., and the starting chemicals are commonly available and cheap. Also, the preparative parameters are easily controlled. The principles of direct deposition of film via CBD method is based on a gradual release of metal ions from supersaturation solution. A chelating agent is usually used to limit the hydrolysis of the metal ion and impart some stability to the bath, which would otherwise undergo rapid hydrolysis and precipitation. It is well known that there are distinct mechanisms or models leading to the formation of CBD films, notably: (a) adsorption and coagulation of colloids performed solution by homogeneous reaction (as usually called cluster-by- cluster process), and (b) ion-by-ion condensation at the surface of the substrate by heterogeneous reaction. In practice, both processes may occur and/or interact in the growing films. The predominance of one mechanism over another is controlled by the extent of heterogeneous and homogeneous nucleations. Key parameters include the degree of supersaturation of the solution and the catalytic activity of the surface of the substrate. Many metal chalcogenide [14] thin films have been deposited by CBD, however, only a few oxide including NiO, TiO2, ZnO and MnO thin films have been deposited via this technique [13], [15], [16], [17], [18].
Our main interest is to prepare interconnected CdO nanowire by using chemical bath deposition method which is applicable for gas sensing as well as dye sensitized solar cell applications. In this work, we report on structural, morphology, optical, and electrical properties of CdO thin films in nanowire form prepared by CBD method. The films were deposited onto glass substrate. Different deposition preparative parameters such as concentration, pH and deposition time are optimized to get CdO nanowires at room temperature.
Section snippets
Experimental details
Microslides glass of the dimensions 2.5 cm × 7.5 cm are used as substrate. Initially, the substrate were washed with double distilled water (DDW), boiled in chromic acid for 2 h and kept in it for 12 h. Again, the substrates were washed with detergent, rinsed in acetone and finally ultrasonically cleaned with DDW before deposition of thin film.
Unless otherwise noted, AR chemicals were obtained from commercial suppliers and used without further purification. The CdO nanowires network was grown on
Reaction mechanism
The CBD is based on the formation of solid phase from a solution, which involves two steps as nucleation and particle growth. In the nucleation, the clusters of molecules formed undergo rapid decomposition and particle combine to grow up to a certain thickness of the film by heterogeneous reactions at the substrate surface. For the deposition of CdO film, CdCl2 is used as source of cadmium. When the ammonia is added to it, white precipitate occurred, which got dissolved by additional ammonia.
Conclusions
In conclusion, an aqueous solution system for growing CdO nanowires network was investigated. The present method is simple, economic and easily reproducible for cadmium oxide nanowires. Through XRD examination, it has been concluded that amorphous CdO thin films could be converted into polycrystalline after annealed at 623 K. The optical transmittance measurement showed that the CdO films has flat surface, a high average transmittance over 80% in the visible region with presence of direct and
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