Influence of the voltage waveform during nanocomposite layer deposition by aerosol-assisted atmospheric pressure Townsend discharge

This work examines the growth dynamics of TiO₂-SiO₂ nanocomposite coatings in plane-to-plane Dielectric Barrier Discharges (DBDs) at atmospheric pressure operated in a Townsend regime using nebulized TiO₂ colloidal suspension in hexamethyldisiloxane as the growth precursors. For low-frequency (LF) sinusoidal voltages applied to the DBD cell, with voltage amplitudes lower than the one required for discharge breakdown, Scanning Electron Microscopy of silicon substrates placed on the bottom DBD electrode reveals significant deposition of TiO₂ nanoparticles (NPs) close to the discharge entrance. On the other hand, at higher frequencies (HF), the number of TiO₂ NPs deposited strongly decreases due to their "trapping" in the oscillating voltage and their transport along the gas flow lines. Based on these findings, a combined LF-HF voltage waveform is proposed and used to achieve significant and spatially uniform deposition of TiO₂ NPs across the whole substrate surface. For higher voltage amplitudes, in the presence of hexamethyldisiloxane and nitrous oxide for plasma-enhanced chemical vapor deposition of inorganic layers, it is found that TiO₂ NPs become fully embedded into a silica-like matrix. Similar Raman spectra are obtained for as-prepared TiO₂ NPs and for nanocomposite TiO₂-SiO₂ coating, suggesting that plasma exposure does not significantly alter the crystalline structure of the TiO₂ NPs injected into the discharge.