Surface-alkoxylated silica can be prepared by reacting precipitated silica with high-boiling and polymeric alcohols. The structure of the products was investigated through 13C and 29Si MAS NMR experiments. The influence of the alkoxy group structure and density on the hydrolytic stability of the alkoxylated silicas was determined through contact with boiling water. Use of polymeric or branched alcohols increased the hydrolytic stability of surface-alkoxylated silicas compared to straight-chain alcohols. The rate of surface substitution is increased by both acid and base catalysis.

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Studies on the properties of zinc oxide (ZnO) have been well-reviewed, 1À3 with many applications proposed such as thin-film solar cells 1 and optoelectronic materials. 3 The performance of doped ZnO as a transparent conducting oxide (TCO) prepared using techniques such as magnetron sputtering, chemical vapor deposition, and pulsed laser deposition has provided a solid body of knowledge. 1 Zn is cheap and abundant, making doped ZnO a favored candidate to replace TCOs based on indium tin oxide (ITO) for thin-film photovoltaic applications. Typical dopants used to prepare ZnO-based TCOs include F, B, Al, Ga, In, and Sn. The solÀgel route has been proposed as a cost-effective alternative to vacuumassisted deposition of films for large-area TCOs, and a number of studies have examined the influence of processing details (heating profile, temperature, solvents, etc.) on film properties and performance. 4À20 However, despite the identification of optimum processing conditions, there is limited understanding as to how and why the processing variables influence electrical properties such as resistivity. Previous studies have shown, for example, that the texturing and resistivity of solutionprocessed films are sensitive to both the concentration 5 and heating profiles used to calcine the films. 6,7 Rapid thermal annealing appears to result in highly (002)-oriented films with low resistivities, 7,8 agreeing with other observations that higher conductivities and transmittances occur in preferred (002)-oriented films. 10 However, other studies appear to suggest that more random orientations are preferred. 11 There is general agreement that ZnO:Al TCOs with low resistivities appear to require around 1 at. % added Al dopant and postsynthesis annealing at ca. 400À500°C. 8À14 Estimates of lattice deformation made from X-ray diffraction (XRD) data using Bragg's equation to attempt to quantify the effective dopant concentration suggest that the effective Al concentration is much lower than the added Al concentration. 12 A dopant ion introduced to modify the electronic properties of a material needs to be incorporated into the crystal structure of the host material (either in lattice sites or interstitially). In the case of a ZnO:Al TCO, the Al 3+ ion is required to occupy a Zn 2+ lattice site in order to provide a free electron (charge carrier) and enhance the conductivity of the ZnO. A simplistic representation is shown, 1, in which an Al 3+ ion occupies the site of a Zn 2+ ion, producing a charged defect. 14 A quantum chemical approach has been applied to calculate the structural, electrical, and electronic properties of ZnO due to the Al doping and explains the increase in the n-type electrical conductivity. 15 To ABSTRACT: We report the discrimination of Al doping sites in solÀgel-formed ZnO powders by solid-state 27 Al nuclear magnetic resonance (NMR) spectrometry. A degree of control of dopant placement is demonstrated by modifying sol precursors and processing parameters. ZnO powders containing 1À8 at. % aluminum ions were ...