Photochemical grafting of organic alkenes to single-crystal TiO2 surfaces: A mechanistic study
- Photochemical grafting of organic alkenes to single-crystal TiO2 surfaces: A mechanistic study
- Ryan Franking; 김희숙; Scott A. Chambers; Andrew N. Mangham; Robert J. Hamers
- Issue Date
- VOL 28, NO 33, 12085-12093
- The UV-induced photochemical grafting of terminal alkenes has
emerged as a versatile way to form molecular layers on semiconductor surfaces.
Recent studies have shown that grafting reactions can be initiated by
photoelectron emission into the reactant liquid as well as by excitation across
the semiconductor band gap, but the relative importance of these two processes is
expected to depend on the nature of the semiconductors, the reactant alkene and
the excitation wavelength. Here we report a study of the wavelength-dependent
photochemical grafting of alkenes onto single-crystal TiO2 samples. Trifluoroacetamide-
protected 10-aminododec-1-ene (TFAAD), 10-N-BOC-aminodec-1-
ene (t-BOC), and 1-dodecene were used as model alkenes. On rutile (110), photons with energy above the band gap but below
the expected work function are not effective at inducing grafting, while photons with energy sufficient to induce electronic
transitions from the TiO2 Fermi level to electronic acceptor states of the reactant molecules induce grafting. A comparison of
rutile (110), rutile (001), anatase (001), and anatase (101) samples shows slightly enhanced grafting for rutile but no difference
between crystal faces for a given crystal phase. Hydroxylation of the surface increases the reaction rate by lowering the work
function and thereby facilitating photoelectron ejection into the adjacent alkene. These results demonstrate that photoelectron
emission is the dominant mechanism responsible for grafting when using short-wavelength (~254 nm) light and suggest that
photoemission events beginning on mid-gap states may play a crucial role.
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