inorganic chemistry

water no desorption occurred within 24 h of irradiation time,
whereas in the presence of 0.1 M HCl desorption of [PtCl 6 ]^2 
was almost complete.
The diffuse reflectance spectra ofTH and 4.0%H 2 [PtCl 6 ]/TH
are compared in Fig. 2. The pronounced absorption of the
modified material in the visible region is tentatively assigned
to a metal-centered transition of the platinum(IV) chloride com-
ponent, by analogy with the solution spectrum of Na 2 [PtCl 6 ]
(18,28,29). Below 400 nm the steep absorption increase of the
modified sample originates from the band-gap transition of
TiO 2. For an indirect crystalline semiconductor, the band-gap
energy is obtained by extrapolation of the linear part of the plot
of (F(R 1 )hn)½versushn( 30 ). In the case of 4.0%H 2 [PtCl 6 ]/TH,
it amounts to 3.21 eV, what is slightly smaller than the value
of unmodifiedTH(3.27 eV). The absorption of these materials
extends down to 620–650 nm corresponding to about 2.0 eV.
To locate the approximate redox potentials of the reactive elec-
tron–hole pair, the quasi-Fermi levels of the powders were
measured as summarized above. The plots of photovoltage ver-
sus pH for TH and a series of H 2 [PtCl 6 ]/TH materials are
summarized in Fig. 3. From the inflection point (pHo), the
corresponding quasi-Fermi potentials at pH 7, as obtained via
Eq. (11) taking k¼0.059 V, are 0.58 V (P25) and 0.54 V
(TH). The flat-band potential of a single crystal of anatase was
reported to be0.59 V (pH 7) ( 31 ). Relative to the value ofTH
the quasi-Fermi level is shifted anodically by 0.05, 0.09, and

400 500 600 700 800

0.00

0.09

0.18

0.27

F(

R

)•

l(nm)

a

b

FIG. 2. Diffuse reflectance spectra ofTH(a) and 4.0%H 2 [PtCl 6 ]/TH
(b). The Kubelka–Munk function,F(R 1 ), is equivalent to absorbance.

380 HORST KISCH