Impact of Thiourea Addition on Morphological and Structural Characteristics of Electrodeposited ZnO Thin Films Using Nitrate Aqueous Solutions
The deposition of zinc oxide is investigated using zinc nitrate aqueous solutions. The effect of thiourea concentration has been first evaluated from a theoretical solution chemistry analysis by mean of solubility diagrams and repartition of the species showing that even if zinc-thiourea complexation constants are quite low, zinc ions are consistently complexed by the thiourea for pH<6. Moreover, standard equilibrium potentials have been estimated from the Gibbs free energy values at 80 °C in order to predict the reactions which are most likely to occur. Cyclic voltammetry studies are then carried out using SnO2:F (FTO) and ZnO:Al-coated glass as a function of the thiourea concentration. The main reduction wave corresponds to simultaneous nitrate reduction and ZnO electrodeposition (Reaction 1):
Zn2+ + NO3- + 2 e- → ZnO + NO2- (1)
A shift of the reduction wave is observed as soon as thiourea is added into the electrolyte. In order to visualize this trend, Fig. 1 shows the potential as a function of the thiourea concentration recorded for a fixed current density value in the voltamperometric curves.
The films have been grown at a potential of -1.5 V/MSE for 5 minutes for increasing thiourea concentrations between 0 M and 1 M on both the substrates. The films are characterized through XRD, SEM and EDX analysis. Results show that the concentration of thiourea has a significant influence on the morphology and growth rate of films. XRD analysis shows that the film is composed of well-crystallized ZnO. EDX analysis highlights that Zn and O are the main components of the films with a very low incorporation of sulfur. SEM analysis indicates that the surface morphology is clearly evolving from a classical platelet ZnO structure, to nodular morphology for increasing thiourea concentrations. Fig. 2 shows the cross section of a deposited ZnO film in presence of thiourea. The film is about 4 μm thick and presents a nanoporous structure. In fact, as soon as 0.05 M of thiourea is added in the electrolyte, the thickness of the films seems to be much larger than expected from the apparent Faradaic efficiency (Fig. 3). Based on these results it seems that thiourea could be a suitable additive for the formation of nanoporous films. The mechanism of the formation of the films will be discussed in detail.
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