Electrodeposition of Cobalt Selenide Thin Films: An Electrochemical Quartz Crystal Microgravimetry Study

Abstract

The mechanism of electrodeposition of cobalt selenide (CoSe) thin films was investigated by the combined application of linear sweep voltammetry (LSV) and electrochemical quartz crystal microgravimetry (EQCM) on Pt-coated quartz electrodes. Cobalt selenide films were electrodeposited on the Pt surface from 0.1 M Na 2 SO 4 electrolyte solution containing 5 mM SeO 2 and 5 mM Co(CH 3 COO) 2 by linear sweep voltammetry. Four cathodic waves were observed during the linear scans and the reactions corresponding to these waves were investigated with LSV and EQCM. Combined stripping voltammetry and EQCM showed that CoSe was electrodeposited via two routes: (1) Underpotential deposition of Se followed by deposition of cobalt as CoSe; and (2) Reaction of Co(II) with electrogenerated Se(-II) to result in CoSe. Compositional analyses revealed that the electrodeposited films contained CoSe and free Se, depending on the deposition potential. However, no cobalt was found in these films because of chemical Transition metal chalcogenides, comprised of earth-abundant elements, have served as effective alternatives for expensive, noble metal catalysts for the hydrogen evolution reaction (HER), and the oxygen evolution reaction (OER).1 Especially, cobalt selenides have attracted much attention due to their favorable attributes such as good electrical conductivity, optimal bandgap (∼1.5 eV) in terms of match with the solar spectrum, and a high optical absorption coefficient. [2][3][4]6,7,10,16 Of these variant synthetic options, electrodeposition offers many advantages 17 such as cost-effectiveness, simplicity, easy scale-up, the use of relatively mild conditions, and the fact that (volatile) organometallic chemicals are not needed (as in techniques such as molecular beam epitaxy and atomic layer deposition). In addition, the composition and structure of electrodeposited films can be controlled by controlling pH, electrolyte composition, or deposition potential.In this work, we present a detailed study of the electrodeposition mechanism of cobalt selenide (CoSe) using combined linear sweep voltammetry and electrochemical quartz crystal microgravimetry (EQCM). We are only aware of one prior study on the electrodeposition mechanism for CoSe on fluorine-doped tin oxide (FTO) electrode using cyclic voltammetry. 16 On the other hand, the addition of a complementary mass-change probe such as EQCM to a voltammetrybased study, offers considerable advantage in a mechanistic sense as we demonstrate below. Compositional analysis of the electrodeposited films using combined stripping voltammetry and EQCM, is finally demonstrated. ExperimentalCobalt acetate tetrahydrate, selenium dioxide, and sodium sulfate were from Sigma-Aldrich, and used without further purification. For voltammetry and EQCM, an EG&G Princeton Applied Research * Electrochemical Society Fellow.z E-mail: myung@kku.ac.kr; rajeshwar@uta.edu 263A instrument equipped with Power Suite electrochemistry software, a Seiko EG&G model QCA 922 instrument and an oscillator module (QCA 922-...

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