inorganic chemistry

same intensity as the 510 nm band. It rather may originate from
a rhodium-to-titanium charge-transfer transition. This is
corroborated by the fact that the silica analogue 2.0%RhCl 3 /
SiO 2 does not exhibit a strong absorption increase atl550 nm,
most likely because, different from titania, silica does not have a
low lying conduction band (Fig. 7, curve b). In the corresponding
difference spectrum an unsymmetrical absorption band is
observed at a maximum at about 380 nm. In the case of 2.0%
RhBr 3 /TH a similar comparison with 2.0%RhBr 3 /SiO 2 afforded
the MMCT maximum at about 390 nm ( 48 ).
Assuming that all samples are indirect crystalline
semiconductors, as is anatase, the band-gap energy can be
obtained by extrapolation of the linear part of a plot of [F(R 1 )
hn]1/2versus the energy of exciting light( 30 ). From this the band
gap of TH, 0.5, 1.0, 2.0, and 5.0%RhCl 3 /TH and of 2.0%RhBr 3 /
TH, can be calculated as 3.29, 3.26, 3.25, 3.22, 3.21, and
3.10 eV, respectively (Table I).

B. PHOTOCATALYTICACTIVITY

To investigate the photocatalytic activity, the disappearance
and mineralization of 4-CP was performed in the presence of
air. Surprisingly, the activity of 5.0%RhCl 3 /TH was very high

300 400 500 600

0.0

0.1

0.2

0.3

F(

R

)∞

l (nm)

c

a

d

b

380 nm

FIG. 7. Diffuse reflectance spectra of 2.0%RhCl 3 /TH (a), 2.0%RhCl 3 /
SiO 2 (b), TH (c). Spectrumd¼a(bþc).

VISIBLE LIGHT PHOTOCATALYSIS 387