Electrochemical quartz crystal microbalance, X-ray photoelectron spectroscopy, and Raman spectroscopy analysis of SiCl4 reduction in ionic liquids

Abstract

The reaction mechanism of Si electrodeposition in the ionic liquid trimethyl-n-hexylammonium bis(trifluoromethylsulfonyl)imide (TMHATFSI) with SiCl 4 was investigated using an electrochemical quartz crystal microbalance (EQCM) and X-ray photoelectron spectroscopy (XPS). The deposited films were further characterized by Raman spectroscopy. The EQCM method measured the frequency change in situ during electrodeposition, whereas XPS provided the Si mass concentration in the electrodeposited films. By combining these results, the mass change due to SiCl 4 reduction was estimated. Then, the current efficiency for Si electrodeposition at various potentials was evaluated. The calculated current efficiency was almost 100% when it was assumed that SiCl 4 reduction progressed by four-electron electroreduction. The XPS and Raman results showed that the deposited films were composed primarily of amorphous Si with Si-Si bonds. Crystalline Si thin films were obtained after annealing in an Ar gas stream.

Analysis of Cathodic Reaction Process of SiCl4during Si Electrodeposition in Ionic Liquids

“…A Pt wire was used as the counter electrode. Based on our previous work, 8,9 we determined that a Ag/Ag + reference electrode would enable us to follow the detailed reduction processes in these reactions including the number of reduction processes undergone by the SiCl 4 as well as the possible formation of intermediate states. In this application of the Ag/Ag + reference electrode, an Ag wire is immersed into TMHATFSI dissolved 0.05 M AgTFSI (Aldorich) and the reference electrode is separated from the electrolyte of 0.5 M SiCl 4 /TMHATFSI by a vycor glass with a porous tip.…”

“…The detailed technique used in our EQCM analysis has been shown in our previous report. 8,9 XRR measurement.-The samples for XRR were fabricated by spinning the ionic liquid with 0.5 M SiCl 4 on a flat Au covered surface that served as the working electrode for the electrochemical deposition. The counter electrode was also a gold film with a 1 mm separation from the working electrode.…”

“…This behavior had been also corresponded to our previous study using Pt QRE. 9 These electrochemical measurements suggest that this Si electrodeposition involves several elementary steps with stable intermediates before the deposition; before reaching −2.2 V, electroactive species, such as SiCl 4 in our case, receive electrons to form some intermediate. Considering the fact that these steps were not observed in the blank ionic liquids as shown in Figure S2, they should derive from SiCl 4 , and they might be oxidized during the anodic CV scan.…”

“…[9][10][11] Previous studies have not been able to provide details on the reaction of SiCl 4 on the substrate surface at a molecular level. It is important to understand the mechanism for how these reactions proceed or what kind of intermediate are produced by focusing on the molecular level behavior at the solid-liquid interface.…”

“…The electrochemical quartz crystal microbalance (EQCM) measurement is one of techniques, which proves to be applicable for such interfacial analyses of Si deposition for detecting the formation of intermediates if the effects from solvent viscosity on measurement results were taken into account precisely. 8,9 X-ray reflectivity (XRR) has been used as a selective probe of the interface and has allowed us to suggest a detailed molecular level deposition mechanism including the presence of in-termediate states. [12][13][14] Density functional theory (DFT) calculation is also performed to investigate the theoretical aspects.…”