ZnO Electrodeposition on Boron-Doped Diamond: Effects of Diamond Surface Terminations
Authors: Simon Nathalie , Vallée Anne , Gonçalves Anne-Marie , Gautier Pierrick , Etcheberry Arnaud 
Published: February 2020

Diamond and zinc oxide (ZnO) are two highly important, wide band gap, semiconductors with a great number of attractive properties. Recently the combination of n-type ZnO and p-type diamond has attracted much attention for different applications as the fabrication of heterojunctions [1], surface acoustic-wave (SAW) devices [2] or biosensors electrodes [3]. In these works, ZnO was mainly deposited on diamond surfaces by high temperature processes such as chemical vapor deposition (CVD) [4], magnetron sputtering [5] or atomic layer deposition (ALD) [6].
Electrochemical deposition (ECD) is emerging as an important method to prepare semiconductor thin films such as ZnO. This chemical route presents many advantages such as low cost, low temperature, soft processing of materials and possibility to obtain a large range of structures from dense layers up to well-defined nanostructures by means of appropriate growth parameters.
Since the last decades, the electrodeposition of ZnO has been successfully performed on many conductive substrates [7] leading to the growth of various structures but it was studied only one time on diamond from a Zn(NO3)2solution [8].
Whatever the process, the first step in the ECD of ZnO consists in the production of hydroxides near the electrode surface to raise the local pH. Three precursors can be used to produce the OH- ions: Hydrogen peroxide, nitrate salts or dissolved oxygen. Unlike zinc nitrate, using oxygen as a precursor allows the modulation of [OH-]/[Zn2+] ratio in the bath.
Recently our group has thus investigated the ECD of ZnO on Boron Doped Diamond (BDD) electrodes using the method based on the reduction of dissolved O2, developed by Peulon and Lincot [9]. The reduction of dissolved oxygen leads to the formation of hydroxide (step (1) or (1)’), by raising the local pH the precipitation of zinc hydroxide (step (2)) occurs in presence of Zn2+ions, which is finally transformed into zinc oxide by dehydration (step (3)).
O2 + 2 H2O + 4 e- → 4 OH- (1)
O2 + 2 H2O + 2 e- → H2O2 + 2 OH- (1)’
Zn2+ + 2 OH- → Zn(OH)2 (2)
Zn(OH)2 → ZnO + H2O (3)
We studied the influence of bath temperature and zinc chloride concentrations and evidenced that the ECD of zinc oxide was promising on as-grown H terminated BDD electrodes [10, 11].
In the present work, we focus on the influence of the BDD surface chemistry either H- or O-terminated obtained by different oxidation treatments (electroless [12] or anodic [13]) on the ZnO deposition process. Surface terminations are followed by contact angle measurments and XPS analyses. The morphologies and structures of ZnO deposits are characterized by XRD and SEM. Besides playing on deposit morphology, this work shows that the surface terminations of BDD (-CH, -CHx, C-OH, -COOH, -COC or -C=O) have also a strong effect on the ZnO deposit adhesion.
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