6256 J. Phys. Chem. B, Vol. 106, No. 24, 2002
Sinlapadech et al.
•+
supports that some PC1 is ionized to PC1 thermally during
porated photoionizable molecules.32 The photoyield in Cr-
AlMCM-41/PC1 initially increases with the amount of Cr3+ ion-
exchanged into AlMCM-41. However, for Si/Cr ) 52 the
photoyield decreases. This may be explained by some structural
degradation at higher Cr content and/or by the formation of
secondary radicals.9,14 Therefore, the amount of Cr ions ion-
exchanged into extraframework sites of AlMCM-41 has an
optimal value to achieve maximum charge separation. One can
also conclude that Cr5+/Cr6+ in Cr-AlMCM-41 enhances
charge separation by accepting an electron from the electron
donor species, as illustrated in Figure 9. The optical spectra
sample preparation. From Figure 9, when Cr-AlMCM-41/PC1
is irradiated by 320 nm light for 60 min, the PC1 photoyield
significantly increases while the absorption of PC1 decreases
and also the two absorption peaks at 275 and 350 nm assigned
to Cr5+/Cr6+decrease. This shows that there is electron transfer
from PC1 to Cr5+/Cr6+ during photoirradiation.
Further study was focused on the effect of the chromium
amount in Cr-AlMCM-41 on the photoyield. The photoyield
of PC1 increases with increasing irradiation time. The
maximum net photoyield increase was obtained for Si/Cr ) 80.
At low Cr content (Si/Cr ) 121), the photoyield seems rather
constant after 10 min photoirradiation at room temperature. For
higher Cr content (Si/Cr ) 52), the photoyield increases after
10 min of photoirradiation and reaches a plateau after about 30
min. However, the net photoyield intensity increase after 10
min photoirradiation for Si/Cr ) 80 is larger than that for Si/
Cr ) 52. After 60 min irradiation, the photoyield intensities of
PC1•+ in Cr-AlMCM-41/PC1 increased 23% for Si/Cr ) 121,
36% for Si/Cr ) 80, and 20% for Si/Cr ) 52.
•+
•+
support that electron transfer occurs between PC1 and Cr5+
/
Cr6+. In Cr-AlMCM-41/PC1 calcined at 100 °C, the bands due
•+
to Cr3+ and PC1 overlap in the region 440-600 nm and do
not clearly indicate the nature of electron trapping sites in Cr-
AlMCM-41 calcined at 100 °C.
Incorporation of Al into the MCM-41 framework creates
Lewis and Bronsted acid sites.19 As plotted in Figure 5, the
more Al in AlMCM-41/PC1, the less the net photoyield. For
AlMCM-41, the hydroxyl groups of the framework may serve
as electron acceptors with production of hydrogen atoms.
However, no ESR signals due to trapped hydrogen atoms were
detected. This is consistent with an earlier report in which
trapped hydrogen atoms were not observed in material having
pore sizes similar to the one in the present study.9
Discussion
XRD and N2 adsorption experiments indicate that the cage
sizes of AlMCM-41 are large enough to incorporate 10-
methylphenothiazine (PC1), N,N,N′,N′-tetramethylbenzidine
(TMB), and pyrene (Py). ESR and diffuse reflectance UV-vis
spectra of some 10-methylphenothiazine cation radicals (g )
2.006 for ESR or 515 nm for DRS) were observed after sample
preparation and before photoirradiation. This indicates that the
impregnation procedure for PC1 into AlMCM-41 thermally
Conclusions
ESR and UV-vis spectroscopies were used to study photo-
ionization of 10-methylphenothiazine (PC1), N,N,N′,N′-tetram-
ethylbenzidine (TMB) and pyrene (Py) in Cr-AlMCM-41
mesoporous materials. It is found that PC1 incorporated into
Cr-AlMCM-41 is the best electron donor among these three
photoionizable molecules. Formation of cation radicals within
modified Cr-AlMCM-41 materials is due to electron transfer
between the electron donor molecules and chromium ions, with
Cr5+/Cr6+ being a better electron acceptor than Cr3+. It is
verified that the photoyield intensity depends on the amount of
chromium ions ion-exchanged into the mesoporous AlMCM-
41 materials, but an intermediate concentration is optimal. The
temperature before impregnation controls the chromium ion
valence state and the photoyield. Also the photoyield depends
of the type of photoionizable molecule.
oxidizes some of the PC1 into PC1 .
•+ 25,26,32 This is also true of
TMB and Py since ESR signals of TMB•+ and Py•+ at g )
2.006 are likewise observed after sample preparation.
The effect of heating Cr-AlMCM-41 before impregnation
with PC1, TMB, or Py on the photoionization efficiency is
considerable. Heating to 550 °C before impregnation causes the
oxidation of Cr3+ to Cr5+/Cr6+. Cr5+ is a better electron acceptor
according to its reduction potential. This fact is consistent with
•+
the ESR results where the net photoyield of PC1 in Cr-
AlMCM-41/PC1 heated at 550 °C is about 4 times greater than
when heated at 100 °C where the Cr ion still exists as Cr3+
.
The photoionization efficiency clearly depends on the type
of incorporated photoionizable molecule. Among the three
different organic molecules studied, PC1 is the best electron
donor with the highest net photoyield intensity and also the
longest radical lifetime. The high photoionization efficiency of
PC1 is not explained by the ionization potentials determined by
photoelectron spectroscopy, which are PC1 (7.15 eV46), TMB
(6.84 eV47), and Py (7.45 eV48). The expected order of
photoyields based on the ionization potentials is Cr-AlMCM-
41/TMB > Cr-AlMCM-41/PC1 > Cr-AlMCM-41/Py. How-
ever, the observed order is Cr-AlMCM-41/PC1 > Cr-
AlMCM-41/TMB > Cr-AlMCM-41/Py. The higher ionization
potential of pyrene may explain its low photoyield. The low
photoyield of TMB•+ may be due to the formation of some
Acknowledgment. This research was support by the Divi-
sion of Chemical Sciences, Office of Basic Energy Sciences,
Office of Energy Research, U.S. Department of Energy, the
Texas Advanced Research Program and the Environmental
Institute of Houston.
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nonparamagnetic (TMB)2 .
2+ 14,32 In addition, the photoionization
efficiency depends on the stability of the photoproduced radical
cations. It is found that TMB•+ and Py•+ decay 33% and 23%,
•+
respectively, compared to only 7% for PC1 in 24 h.
•+
The photoyield of PC1 increases in the order MCM-41 <
AlMCM-41 < Cr-AlMCM-41, indicating that the chromium
ion in Cr-AlMCM-41 enhances the photoionization reaction.
We previously found that the concentration of Ni2+ ion-
exchanged into AlMCM-41 enhanced the photoyield of incor-
(13) Ledney, M.; Dutta, P. K. J. Am. Chem. Soc. 1995, 117, 7687.
(14) Koodali, T. R.; Kevan, L. Phys. Chem. Chem. Phys. 2001, 3, 2921.