Gas Phase Reaction of Silane with Water at Different Temperatures and Supported by Plasma

The interaction of silane and water is discussed controversially in literature: some authors suggest monosilane and water react kinetically and sufficiently fast enough to remove water, while others state the reaction occurs only at elevated temperatures. This question is of technological interest for the removal of unavoidable water residues in Ar gases. Thermodynamic calculations show that virtually complete removal of water is expected with superstoichiometric silane addition. However, mass spectrometric and infrared spectroscopic experiments give evidence that the addition of monosilane to such an Ar gas at room temperature is unable to remove residual water, which disagrees with some current hypotheses in the literature. This holds even for very high SiH₄ concentrations up to 2 vol.-%. Silane reacts with water above temperatures of 555 °C, initiated by the thermal decomposition of silane. A cold dielectric barrier discharge-plasma used for silane and water dissociation enhances reactivity similar to elevated temperatures. Fourier-transformed infrared spectroscopy points toward silanol generation at temperatures between 400 and 550 °C, while quadrupole mass spectrometry indicates the creation of SiOH⁺, SiHOH⁺, SiH₂OH⁺, and SiH₃OH⁺. Cold plasmas generate smaller amounts of SiOH⁺, SiHOH⁺, and SiH₂OH⁺ compared to elevated temperatures. Reaction products are hydrogen and nanoscaled particles of non-stoichiometric silicon oxides. The silicon-oxide particles produced differ in elemental composition and shape depending on the prevailing water content during decomposition: SiO _x generated with residual water appears with relatively smooth surfaces, while the addition of water supports the formation of significantly rougher particle surfaces. Higher initial water contents correlate with higher oxygen contents of the particles.