Mechanistic Study of the Electrodeposition of Nanoporous Self-Assembled ZnO/Eosin Y Hybrid Thin Films:  Effect of Eosin Concentration

Abstract

ZnO films prepared by one-step electrodeposition in the presence of dissolved eosin molecules present an internal nanoporous hybrid structure resulting from self-assembling processes occurring in solution between ZnO and eosin components. This study aims to better understand the underlying growth mechanism, which is still unexplained. The films were deposited by cathodic electrodeposition from an oxygen-saturated aqueous zinc chloride solution. The effects of the addition of 10 to 100 micromol.L(-1) eosin Y, as a sodium salt, on the growth rate and film properties, were systematically studied while all other parameters remained constant (concentrations of zinc salt and supporting electrolyte, applied potential of -1.4 V versus the mercurous sulfate electrode (MSE), temperature of 70 degrees C, rotating disk electrode at 300 rotations per min, and a glass-coated tin oxide electrode). It is shown that the addition of eosin provokes the formation of a nanoporous "cauliflower" structure whose nodule size and composition depend on the eosin concentration in the bath. The growth rate of the hybrid films increases markedly with the eosin concentration. The ZnO and eosin contents of the films are determined for each concentration by chemical analysis. Comparing with thickness determinations, it is shown that the total porosity increases up to 60-65% in volume fraction toward an eosin concentration of 100 micromol.L(-1). The empty pore volume fraction increases up to about 30% at an eosin concentration of about 20 micromol.L(-1) and then decreases. These correlations have been precisely established for the first time. It is shown that the global composition is fixed by the relative rate of deposition for zinc oxide, which is constant, and for the relative rate of eosin inclusion, which is proportional to the concentration in solution. This is explained on the basis of different steps in the growth mechanism, in particular, a diffusion effect limitation for both oxygen and eosin. This variation explains part of the increase in the growth rate. Another contribution is related by the structural effect on the nanoscale leading to the formation of the interpenetrated porous network. Competition between empty and eosin-filled parts of the pore network is evidenced. The formation of the porous network structure could be governed by a diffusion-limited aggregation mechanism. The system may represent a reference case of competing reactions in the electrochemical self-assembly of hybrid nanostructures.

Electrodeposition of Inorganic/Organic Hybrid Thin Films

“…Formation of a polymeric complex can be considered as one of the plausible reactions for such stabilization and could be the key for the formation of the nanowire structure shown above. [109] The significant thermodynamic stabilization against dye reoxidation also nicely explains the enhanced dye loading when dye reduction is involved. [107,113] As the complex does not absorb in the visible region, the hybrid thin film deposited with the reduced eosinY is colorless and can only be oxidized slowly in air to regenerate the color (Fig.…”

“…Highly transparent and very well adherent hybrid thin films with high dye loading could be obtained in systems that employed reduction of O 2 [107][108][109] and H 2 O 2 . [110] A perfectly crystalline nature is also preserved in these systems as evidenced by the fact that a single-crystalline ZnO/eosinY hybrid thin film could be obtained by epitaxial electrodeposition on GaN.…”

“…However, it slows down in the presence of Zn 2þ in solution, so that the electrodeposition of ZnO in the oxygen system receives mixed control, both by the charge transfer kinetics that is varied with the electrode potential and the mass transport that varies with the diffusion-layer thickness. [108,109] It is therefore very important to use a device to introduce homogeneous forced convection, such as a rotating electrode, so that homogeneous thickness of the diffusion layer is achieved to obtain uniform thin films at a high reproducibility. We have designed an electrode holder with a square recess into which a piece of TCO glass substrate can fit; it is then attached to a commercial rotatingelectrode system.…”

Electrodeposited Porous ZnO Sensitized by Organic Dyes – Promising Materials for Dye‐Sensitized Solar Cells with Potential Application in Large‐Scale Photovoltaics

“…The role of EY as a structure -directing agent ( SDA ) in the electrodeposition of ZnO could be developed to an even greater extent (Figure 4.2 b) when the deposition was performed from deposition solutions with O 2 as oxygen precursor for ZnO rather than NO 3 − [27, 261,262] . Also hybrid materials of TSPc with ZnO were deposited from O 2 -based solutions as in the example of TSPcNi leading to a range of materials stretching from monomers of TSPcNi embedded into ZnO crystals to amorphous networks of TSPcNi with corresponding contrasting optoelectronic properties [263] .…”

Nanostructured ZnO‐Based Surface with Reversible Electrochemically Adjustable Wettability

“…[35] The propensity of zinc oxide surfaces to react with organic or organometallic molecules containing carboxylic acid or silane functional groups has been already largely used to modify the surface free energy of this oxide [3,8,36] or to govern the formation of original nanotructures of solution-grown ZnO. [37][38][39] In the first step, an attempt was made to modify d-ZnO and r-ZnO film surfaces directly with ferrocene carboxylic acid molecules (1 in Fig. 2A).…”