Sulfur-Hz(CHx)y(z = 0,1) functionalized metal oxide nanostructure decorated interfaces: Evidence of Lewis base and Brönsted acid sites - Influence on chemical sensing

Nanostructure metal oxide decorated n-type extrinsic porous silicon (PS) semiconductor interfaces are modified through in-situ interaction with acidic ethane and butane thiols (EtSH, BuSH) and basic diethyl sulfide (Et2S). Highly sensitive conductometric sensor evaluations and X-ray Photoelectron Spectroscopy demonstrate the effect of sulfur group functionalization modifying the acidity of the metal oxides and their interaction with NH3. SEM micrographs demonstrate that the sulfur treated particles are less than 30 nm in size. EDAX studies confirm the chemical composition of the modified nanoparticles and suggest the surface interaction of the sulfides and thiols. The acidic thiols can form Brönsted acidic sites enhancing the acidity of the metal oxides, thus broadening the initial metal oxide acidity range. The sulfides interact to lower the Lewis acidity of nanostructured metal oxide sites. Conductometric response matrices with NH3 at room temperature, corresponding to the thiol and sulfide treated nanostructures of the metal oxides TiO2, SnOx, NixO, CuxO, and AuxO (x >> 1) are evaluated for a dominant electron transduction process forming the basis for reversible chemical sensing in the absence of chemical bond formation. Treatment with the acidic thiols enhances the metal center acidity. It is suggested that the thiols can interact to increase the Brönsted acidity of the doped metal oxide surface if they maintain SH bonds. This process may account for the shift in Lewis acidity as the Brönsted acid sites counter the decrease in Lewis acidity resulting from the interaction of S-(CHx)y groups. In contrast, treatment with basic Et2S decreases the Lewis acidity of the metal oxide sites, enhancing the basicity of the decorated interface. XPS measurements indicate a change in binding energy (BE) of the metal and oxygen centers. The observed changes in conductometric response do not represent a simple increase in surface acidity or basicity but involve a change in the mismatch of molecular electronic structure with the NH3 analyte. The nature of this interaction, coupled with the ability to transfer electrons to the extrinsic semiconductor, is explained using the developing IHSAB model.