A method for electrochemical growth of homogeneous nanocrystalline ZnO thin films at room temperature
Introduction
In recent years, ZnO thin films have attracted tremendous attention as a promising material for many applications such as solar cells [1], [2], [3], [4], UV-light emitting diodes [5], UV-laser diodes [6], [7], gas sensing [8], field emission [9], [10], piezoelectrics [11], self-cleaning surfaces [12], [13], catalysis and photocatalysis [14]. Beside the gas-phase growth methods which require rather expensive equipment to achieve high temperature and/or low pressure, various solution-based grow techniques have been developed [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. They have emerged as promising from the viewpoints of low-cost, simplicity and scalability. Water is the most-commonly used solvent in the solution-based techniques and the syntheses are classically performed at a temperature higher than room temperature that is at 50 °C or higher. However, thermochemical calculations have demonstrated that in the zinc–water system ZnO is the most stable solid product at room temperature and that Zn(OH)2 is spontaneously dehydrated to form the oxide [16], [17].
Among the solution-based methods, electrodeposition is one of the most studied and most interesting due to the possibility of playing on a large variety of deposition parameters such as reactant concentration, applied current or potential, temperature, bath additives and so on [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. By adjusting these parameters the morphology and the crystallographic quality of the deposited material can be finely tailored [27]. The deposition mechanism consists in electrochemical generation of hydroxide ions by the reduction of a dissolved precursor. The precursor can be nitrate ions [19], [20], hydrogen peroxide [21], [22] or molecular oxygen [17], [18], [23], [24], [25], [26]. The latter has been chosen in the present work. It has in particular the advantage of not generating by-products by reduction. The reduction mechanism of O2 on ZnO electrode surface has been studied in detail elsewhere [26]. It occurs by a mixed two and four-electron pathway. A high cathodic overvoltage favors the latter pathway. The electrodeposition of ZnO is classically performed in the 70–80 °C temperature range. The method produces polycrystalline films made of large and well-crystallized ZnO grains [17] or ZnO rod or wire arrays [7], [12], [27], [28], [29], [30], in which each rod or wire is a single crystal.
Electrodeposition of ZnO at room temperature (RT) has been found difficult due to kinetic constants favorable to zinc hydroxide formation [17]. Studies on the low temperature deposition of ZnO are very scarce and the electrochemical production of ZnO films of good quality at room temperature remains challenging. A paper has reported that, after a long O2 bubbling of a zinc acetate solution containing a complexing agent, the instantaneous nucleation of ZnO is observed when a low cathodic overvoltage is applied. However, the deposited product was not clearly identified and the formation of well-crystallized ZnO under those conditions is doubtful [31]. Mei et al. [32] have described the electrochemical growth of ZnO at 20 °C using a zinc nitrate solution at high concentration. A two-electrode configuration was used and an applied potential of −1 V was pulsed. The as-deposited film presented a very weak near-band-edge UV photoluminescence (PL) and a strong visible PL due to deep defects (identified as singly ionized oxygen vacancies). The PL emission spectrum was typical of ZnO of poor structural quality. The film was textured with the c-axis oriented perpendicular to the surface. Lévy-Clément et al. produced ZnO films at RT made of small crystals by using molecular oxygen precursor and by applying a constant current density of −0.15 mA cm−2. The films were successfully used as a seed layer for ZnO nanowire array synthesis [29], [30].
The present work describes the growth conditions to synthesize ZnO films at room temperature with a high homogeneity and good morphological and crystallographic quality. The layers are made of well-crystallized grains with size of about 17 nm. The good quality of the material is certified by PL emission measurements at room temperature. They are dominated by the excitonic UV emission of ZnO. It is demonstrated that the film is formed by a two-step process. In the first step, a Zn(OH)2 amorphous thin layer of poor conductivity is produced on the conducting substrate. The second step is the formation of ZnO nuclei within the layer, probably by dehydration of hydroxide species, and growth of the nanocrystalline ZnO layer. We believe that the simple preparation method of such nanocrystalline ZnO thin films is of importance for seed layer preparation [33] as well as for superhydrophobic self-cleaning surface [12], [13] and eta [2] or dye sensitized [4] solar cell applications.
Section snippets
Experimental
Electrodeposition was carried out in a three-electrode cell. The counter-electrode was a platinum wire and the reference electrode was a saturated calomel electrode (SCE) (with a potential at +0.25 V vs NHE) placed in a separate compartment maintained at room temperature. The deposits were prepared on fluorine doped SnO2 coated glass substrates (FTO). The substrates were cleaned in an ultrasonic bath, 5 min in acetone, 5 min in ethanol and 2 min in 45% nitric acid. To ensure a deposition as
Results and discussion
Film formation on the electrode surface has been investigated for various intensities and durations of molecular oxygen bubbling before starting the experiment. Molecular oxygen has a dramatic effect on the shape of the chronoamperograms recorded during the film growth at constant applied potential. Fig. 1a shows that the deposition current rapidly decreases when the bubbling flow is low and short. The curve is then flat and the current exchanged is low. A similar behavior was reported
Conclusions
We have described a method for the facile electrodeposition of homogeneous nanocrystalline ZnO thin films. The obtained layers are of very good quality for ZnO films grown from solution at room temperature. The saturation of the solution by molecular oxygen is a key parameter to grow the oxide at RT. The nucleation-growth mechanism of the films is established. It is shown that the nucleation step is delayed with first the formation of a Zn(OH)2 amorphous layer and then of ZnO seeds in that
Acknowledgements
Th.P. is grateful to Dr. T. Dedova (University of Tallin, Estonia) for the preparation of several samples, to T. Le Bahers (LECIME, ENSCP, Paris) for XRD measurements and to P. Aschehoug (LCMCP, ENSCP, Paris) for PL measurements.
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