Application of Photoluminescence (PL) Spectroscopy to the Characterization of
Oxide Surfaces in Relation to their Functions under UV-VIS Light Irradiation
Masakazu ANPO
Vice President & Executive Director, Osaka Prefecture University, Osaka, JAPAN
anpo@chem.osakafu-u.ac.jp or anpo@osakafu-u.ac.jp
Photoluminescence (PL) spectroscopy, due to its high sensitivity and nondestructive nature,
has been applied to obtain significant information on the structure of active sites for oxides
involving surface ions in low coordination and oxide loadings of less than 0.1 wt% [1, 2].
MgO microcrystals degassed at high temperatures exhibit PL when excited by UV light
(3-4 eV) which is much lower than the band gap of MgO (8-9 eV) due to the charge-transfer
absorption involving surface ions in low coordination at the surface of well-degassed oxides.
We have found that such surface ions in low coordination play a significant role in
photocatalytic reactions such as the trans-cis isomerization of 2-butenes [3].
The PL behavior of TiO2 photocatalysts in the presence of various kinds of reactant
molecules was investigated [1]. The dependence of the PL intensity in atmosphere was
explained in terms of the surface band bending of TiO2 particles, its extent depending on the
electronegativity or electroaffinity of the molecules, and showing reversibility after
elimination of the reactant molecules under N2 flow [1]. The TiO2 photocatalyst surface
shows a high wettability under UV light irradiation, i.e., a "superhydrophilic” property. We
have investigated the nature of the wettability and analyzed the hydrogen bond networks in
H2O clusters adsorbed on the TiO2 surface by using near infrared (NIR) spectroscopy. The
distribution of the hydrogen bonds in the H2O clusters was found to decrease by UV light
irradiation, leading to a decrease in the surface tension of the H2O clusters and resulting in the
formation of an H2O thin layer on the TiO2 surface [4, 5].
Highly dispersed transition metal oxide species such as V, Mo, Ti, and Cr-oxides exhibit
PL attributed to their charge-transfer transitions involving a four-coordinated oxide species [1,
2, 6]. Various highly dispersed oxide species and their roles in the reaction on these oxides
under UV-Vis light irradiation have been investigated and our results will be presented.
The content of this presentation is well summarized in “Advances in Catalysis”, 44
(1999) and 52 (2009) (Academic Press), “Characterization of Solid Materials and
Heterogeneous Catalysts: From Structure to Surface Reactivity”, (2011) (Wiley-VCH Verlag
GmbH), “Nanoscale Materials in Chemistry” (2009) (John Wiley & Sons, Inc.) and
“Environmentally Benign Photocatalysts: Applications of Titanium Oxide-based Materials”
(2010) (Springer).