No abstract

The surface functionalization of TiO2-based materials with alkylsilanes is attractive in several cutting-edge applications, such as photovoltaics, sensors, and nanocarriers for the controlled release of bioactive molecules. (3-Aminopropyl)triethoxysilane (APTES) is able to self-assemble to form monolayers on TiO2 surfaces, but its adsorption geometry and solar-induced photodegradation pathways are not well understood. We here employ advanced experimental (XPS, NEXAFS, AFM, HR-TEM, and FT-IR) and theoretical (plane-wave DFT) tools to investigate the preferential interaction mode of APTES on anatase TiO2. We demonstrate that monomeric APTES chemisorption should proceed through covalent Si–O–Ti bonds. Although dimerization of the silane through Si–O–Si bonds is possible, further polymerization on the surface is scarcely probable. Terminal amino groups are expected to be partially involved in strong charge-assisted hydrogen bonds with surface hydroxyl groups of TiO2, resulting in a reduced propensity to react with other species. Solar-induced mineralization proceeds through preferential cleavage of the alkyl groups, leading to the rapid loss of the terminal NH2 moieties, whereas the Si-bearing head of APTES undergoes slower oxidation and remains bound to the surface. The suitability of employing the silane as a linker with other chemical species is discussed in the context of controlled degradation of APTES monolayers for drug release and surface patterning.

nonstoichiometry will improve the electrocatalytic behavior of RuO2 films. Unlike normal semiconductor films, those of RuO2 are generally highly conducting; the main resistance component is probably due to intergranular contact resistance in these microcrystalline layers. S. Ardizzone, 15 A. Carugati, ~5 G. Lodi, ~5 and S. Trasatti: ~5The main point of Dr. Burke's remarks, as we understand it, is that there are no reasons to expect that the electrocatalytic activity of RuO2 electrodes should be influenced by nonstoichiometry since the active surface sites at high anodic potentials are invariably Ru(VI) species. While we do not feel like agreeing on this concept in principle, we contend that Dr. Burke's criticism is not properly addressed. Figure 1 on p. 1690 and the related comment in our paper clearly point out that "cracked" and "compact" electrodes differ in the surface morphology rather than in the nonstoichiometry. Figure 3 shows that no appreciable difference is observed within each group of electrodes although the nonstoichiometry varies largely, yet a difference possibly exists between the two groups, which may, thus, be related to the surface morphology. Therefore, the message from this paper is that morphology rather than nonstoichiometry is the crucial factor in electrocataIysis at RuO2 anodes.That the surface morphology can affect the electrocatalytic properties of RuO2 has been shown in our previous work ~6 on O3 evolution on some sets of electrodes. The effect in that case is admittedly more striking, and our conclusions have been confirmed in different laboratories. ~7' ,8 It has been neatly found TM that the degree of crystallinity has a definite effect on the O3 evolution mechanism. The point of zero charge of RuO2 samples has been found ,8, 38 to depend on the temperature of preparation and to be related, as expected from theories, with the crystal parameters of the oxide. 2' All of these observations emphasize the extreme sensitivity of the nature of the active sites to the morphology of the surface.Dr. Burke contends that the degree of hydration is not expected to be important in imparting the electrocatalytic properties. It is well established, however, that a number of properties of RuO2 are closely interrelated as a function of the temperature of preparation. ~6~ 22 Thus, the residual hydration decreases as T increases and at the same time the crystallinity increases. Hydration is presumably located in grain boundaries or at "inner" surfaces (pores, etc.). As the crystallites of RuO2 grow, defect-rich regions will shrink. That is what Fig. 9 in our paper is devised to point out.In Dr. Burke's opinion, we have not paid much attention to the above aspect apparently because we have not appreciated some points he touches upon in his comments. (i) Reversibility of the C12 reaction: This is easily proved by the fact that we were able to measure the exchange current from equilibrium i-E curves [cf. footnote 23]. The effect of mass transfer on the Tafel slope has been discussed by one of us in a pr...

No abstract