Unravelling the Mechanism of Electrochemically Induced Sol-Gel Depositions: pH Profiles Near Electrode and Influence on Film Growth

Abstract

Electrochemically induced sol-gel depositions have become a widespread, versatile method for fabricating hybrid and nanostructured oxides on conductive substrates. The process is based on the buildup of electrochemically generated OH- in the diffusion layer near the electrode surface. For the electrodeposition of silica thin films, these OH- ions catalyze the gelation of a kinetically stable precursor solution, thereby resulting in an electrochemically controlled process. The control of the diffusion layer has proven pivotal to depositing thin films while preventing the formation of aggregated by-products deeper in the solution. In this work, the silica sol-gel reactions and electrochemical OH- generation were critically analyzed and described to gain insight into the deposition mechanism. A general model is proposed that predicts the pH profile during both stationary and rotating disk electrode depositions under different conditions (i.e., current densities, times, and rotation rates). This model provides insights into the reactive zones where gelation occurs, and explains typical phenomena observed during deposition such as the dependence of film growth rates and aggregate formation on the deposition conditions. The insights and expressions obtained in this work are invaluable when designing future experiments using novel chemistries or setups.