10 Chemical Modification of TiO Surfaces with Methylsilanes and Characterization by IR Absorption Spectroscopy 2
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H A R R Y O. FINKLEA and R. VITHANAGE Virginia Polytechnic Institute and State University, Chemistry Department, Blacksburg, V A 24061
Infrared absorption spectra have been obtained of methylsilanes bonded to a TiO2 powder. The reacting silanes include Me4- SiX (n=1-4; X=Cl, OMe) and hexamethyldisilazane (HMDS). Reactions were performed on hydroxylated-but-anhydrous TiO2 surfaces i n the gas phase. IR spectra confirm the presence of a bonded silane layer. Terminal surface OH groups are found to react more readily than bridging OH groups. By-products of the modificat i o n adsorb tenaciously to the surface. The various silanes show only small differences in their ability to sequester surface OH groups. Following hydrolysis i n moist air, Si-OH groups are only observed for the tetrafunctional silanes. n
n
We are investigating the effects of binding non-electroactive molecules to electrode surfaces. The attached layer w i l l be suff i c i e n t l y thin (ca. 1 monolayer) that electron transfer across the electrode/electrolyte interface w i l l not be inhibited. However, other surface properties may be advantageously modified. For semiconductor electrodes, desirable changes include suppression of the photo-activated surface corrosion and s h i f t s i n the flatband potential. We are seeking to improve the performance of semiconductor liquid-junction solar c e l l s by these means. One highly successful form of surface modification i s s i l a n i z a t i o n (1) (equation 1). A silane containing a h y d r o l y t i c a l l y
0097-6156/82/0192-0185 $6.00/0 © 1982 American Chemical Society In Chemically Modified Surfaces in Catalysis and Electrocatalysis; Miller, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
CHEMICALLY MODIFIED SURFACES
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unstable bond w i l l react with a surface 0-H group to form a s i l y l ether bond to the substrate* Commercially available silanes offer a selection of leaving groups such as chlorides (X«C1), alcohols (X=OR), and amines (X=NHR). They also provide the p o s s i b i l i t y of forming one-to-three bonds between the s i l i c o n atom and the surface. We have systematically investigated a series of methylsilanes which span the three leaving groups mentioned and one-tofour hydrolytically unstable bonds (Me^nSiX^ n=l-4; X=C1, OMe; and hexamethyldisilazane - HMDS). These silanes were used to modi f y a Ti02 substrate. Knowledge of the composition and coverage of the attached layer i s v i t a l l y important to our investigations. Previous work on monolayers of silanes have employed x-ray photoelectron spectroscopy (XPS) as a probe (2-7). XPS confirms the presence of s i l i c o n after the reaction and also allows an estimation of the layer thickness. We have used an infrared spectroscopic technique 2.» 10) which provides complementary information on the nature of the surface layer. Experimental Ti02 powder (Degussa P-25) was used i n a l l experiments. I t consists of anastase particles with a mean diameter of 0.03 microns and a surface area of approximately 50 m^/g. The powder was heated i n a i r to 500°C for several hours and then stored i n moist a i r . Approximately 50 mg of powder was pressed into a translucent p e l l e t using a 13 mm diameter die and a hydraulic laboratory press. The pellet was mounted i n a vacuum IR c e l l (CaF2 windows) which could be sealed, detached from the vacuum l i n e , and inserted i n the spectrometer. A l l spectra were recorded on a Perkin-Elmer 283B Infrared Spectrophotometer with a P-E computer data station. P e l l e t s were predried by heating to 150°C for 2 hours under vacuum (
