Solid-State NMR Spectroscopic Investigation of TiO₂ Grown on Silica Nanoparticles by Solution Atomic Layer Deposition

Atomic layer deposition in solution (sALD) is just emerging as a technology for the preparation of thin films. Unlike ALD from the gas phase, it allows for mild reaction conditions in a solvent phase and at room temperature, thus decreasing the energy requirements of the process and widening the range of accessible precursor molecules. In this work, the deposition of thin films of titania on silica is investigated using titanium(IV) isopropoxide (TTIP) and water as precursors, which are alternatingly brought into contact with the support in a home-built plug flow reactor. The mechanism of covalent grafting of the precursor to the surface, subsequent hydrolysis, and reaction to a layer of titania are investigated in detail using magic angle spinning (MAS) solid-state nuclear magnetic resonance (NMR) spectroscopy. TTIP preferentially reacts with Q(2) groups of condensed silica. 2D solid-state NMR spectra allow to clearly show the successful grafting of this compound to the support by the appearance of a characteristic signal at -107 ppm, which is tentatively attributed to silicon nuclei in a Si-O-Ti bond, and to reveal the presence of titanol groups on the emerging TiO2 film.