Abstract
In this research, a model for electrodeposition of zinc oxide (ZnO) nanostructures over indium-doped tin-oxide (ITO) glass using pulsed current and zinc chloride as source of zinc was proposed. For the model, reactions kinetics rate constants were evaluated by obtaining the reaction product solid mass of the various species through time using an electrochemical quartz crystal microbalance (EQCM). To obtain a mathematical model of the electrodeposition using Ansys CFX 2D simulation software, the reaction kinetics rates were used to calculate mass transfer in the volume closest to the surface. The model was applied to the experimental electrodeposition conditions to validate its accuracy. Dense wurtzite nanostructures with controlled morphology were obtained on a indium-doped tin-oxide (ITO) glass. Sample characterization was performed using high-resolution field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) on focused ion beam milled (FIBed) sheets from wurtzite mono-crystals. Average crystallite size was evaluated by X-ray diffraction (XRD) using the Scherrer equation, and superficial areas were evaluated by Brunauer, Emmett, and Teller (BET) method. Through the experimental results, a chemical model was developed for the competing reactions based on the speciation of zinc considering pH evolution, and kinetic constants, on the oxygen rich aqueous environment. Owing to the model, an accurate prediction of thickness and type of electrodeposited layers, under given conditions, is achieved. This allows an excellent control of the optical properties of Wurtzite as a photon absorber, for an efficient separation of the electron-hole pair for conduction of the electric charges formed. The large surface area, and small wurtzite crystallites evenly distributed on the thin film electrodeposited over the ITO conductive layer are promising features for later dye-sensitized photovoltaic cell production.
Highlights
Zinc oxide, when in wurtzite crystalline form, presents very relevant optical properties in the visible spectrum
As analyzed in this paper, when electrodepositing zinc oxide (ZnO), from ZnCl2 and KCl, final surface morphology can be predicted through the proposed speciation computer model
This model is based on the kinetical analysis of all chemical speciation routes possible for ensuring a direct control on nucleation and growth of nanocrystals
Summary
Zinc oxide, when in wurtzite crystalline form, presents very relevant optical properties in the visible spectrum. Its semiconductor bandgap (3.33 eV), together with a high binding energy of (~60 meV) at 20 ◦ C allows very relevant photon absorption [1]. It exhibits outstanding chemical stability, favoring photo-corrosion resistance, at a very low cost. These properties make ZnO a perfect candidate for its industrial use in photocatalysts [2], ultraviolet (UV) detectors [3], lightemitting diodes [4], and dye-sensitized solar cells (DSSCs) [5]. ZnO samples were produced by electro-deposition using alternate current [6] for a better understanding and control of the speciation reactions taking place depending on pH and ion concentrations [7]
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