a pore size from BET report; b cubic
unit cell parameter,
a0=\(\sqrt{2}\)d110; cmesopores void fraction; d diameter of the spherical
cavities; e the wall thickness
hw=\(\frac{\sqrt{3}a_{0}}{2}-d\).
Raman spectra of different supports are displayed in Fig. S3. For SBA-16
and AT-10 support (only modified by Al species), three peaks appearing
at 491, 604, and 977 cm-1 should be assigned to pure
silica.[24] An obvious peak at about 879
cm-1 appearing in the spectrum of AT-10 sample should
be assigned to the non-spinel γ-Al2O3phase.[25] As reported, two peaks at 491 and 604
cm-1 should be ascribed to the tri- and
tetracyclosiloxane rings generated by the condensation of surface (–OH)
groups. Meanwhile, the band at 977 cm-1 is resulted
from the surface Si–OH stretching mode.[26] As
for the supports modified by Al and Ti species, the peaks assigned to
silica and Al2O3 become too weak to be
observed due to strong intensities of peaks belonging to
TiO2 phase. Four characteristic peaks at 144
cm-1 399, 515 and 634 cm-1 observed
in the spectra of AT-7.5, AT-5, AT-2.5 and AT-0 samples are ascribed to
anatase phase.[27]
The UV–vis DRS spectra of various supports with different compositions
are shown in Fig. S4A. All supports exhibit different adsorption bands
with the change of addition amount of Ti sources. There are no
adsorption bands appearing in AT-10 samples due to Al and Si species are
transparent in the UV-vis DRS detected region. The absorption bands in
the samples containing Ti species are resulted from the ligand-to-metal
charge transfer from the O2- to Ti4+to form its charge-transfer excited state,
(Ti3+–O-).[28]For the samples incorporated into Ti atoms, the first peaks in the
adsorption range of 200-220 cm-1 is ascribed to the
charge-transfer transitions of oxygen to tetrahedrally coordinated
Ti4+ ions in the group of Ti(OSi)4,
which can demonstrate that Ti atoms are successfully incorporated into
the framework of SBA-16 materials.[29] The bands
ranged from 220 to 230 nm can be observed in AT-5, AT-2.5 and AT-0
samples, which are ascribed to the group of
Ti(OH)–(OSi)3 in framework of SBA-16
silica.[30] Meanwhile, obvious absorption bands in
the region of 300-330 nm can be observed in samples containing Ti atoms,
which can be ascribed to Ti-O-Ti bonds and the titanium sites with high
coordination numbers.[31] These results confirm
that the Ti species should be co-existed in the framework and external
framework as TiO2 crystal phases in the Al-Ti-SBA-16
composites, which is in accordance with Raman measurements. In addition,
band gap energy can be estimated from a plot of (α)1/2versus photon energy (hv). The band gap energy for TiO2phases can be obtained by extrapolating the linear part of the rising
curve to zero. Higher band gap energy will indicate smaller particle
size of TiO2 crystal inside the Al-Ti-SBA-16 composites.
The absorption coefficient α can be calculated from the following
equation of α=2303ρA/(lcm), in which ρ (TiO2) =3.9
g·cm-3, A is the absorption intensity, l is the
optical path length, c is the molar concentration for
TiO2 and m is molecular weight of
TiO2.[32] From Fig. S4B, the band
gap energies follows in the order of
AT-7.5
(3.00 eV) > AT-5 (2.94 eV) > AT-2.5 (2.80 eV)
> AT-0 (2.50 eV). Therefore, the particle size of
TiO2 crystal in the samples follows the reverse order
and increase with the additional amounts of Ti sources.
27Al NMR was also applied to obtain the existent form
for Al species in the Al-Ti-SBA-16 composite. It can be generally
recognized that the band appearing at about 6 ppm can be ascribed to the
octahedral structural unit AlO6, which can be treated as
extra-framework Al species. The band at about 33 ppm has been ascribed
to the extra-framework coordination of Al3+ as
pentahedral AlO5 unit. Meanwhile, the chemical shift at
about 53 ppm is assigned to tetrahedral structural unit
AlO4, in which aluminum is covalently connected with
four Si atoms through oxygen bridges.[33, 34] The27Al NMR spectrum of AT-5 support is shown in Fig. S5.
It presents three peaks belonging to AlO4,
AlO5 and AlO6 structural units,
indicating that the composites synthesized by this two-step method
possess different Al species in framework and extra-framework.
The XPS spectra for different supports are shown in Fig. S6. Ti 2p
envelop shows two characteristic peaks at about 485 and 464 eV, being
ascribed to Ti 2p3/2 and Ti 2p1/2 species, which is in accordance with
the reported literature.[35] It is obvious that
the intensity of the above two peaks increase with the Ti contents in
the supports. This result can confirm the appearance of
TiO2 phase on the surface of support, which can be seen
in Raman and XRD results. From Al 2p XPS envelop, it can also be seen
that the intensity of the characteristic peak increase with the Al
contents in the supports, which is similar with that of Ti 2p XPS
spectra. As shown in O1s XPS envelop, an obvious peak at 533.0 eV can be
assigned to the Si-O-Si bond. As the composition of Ti species reaches
10%, a weak peak at about 529 eV can be observed, which should be
assigned to the oxygen in the Ti-O-Si bond.[35,
36] This may demonstrate that Ti atoms have been successfully
incorporated into the framework of SBA-16 material. The binding energy
of AT-10 sample shifting to lower values compared with SBA-16 sample may
be caused by the Al incorporation of Al atoms into SBA-16 material,
which can be verified from 27Al NMR result. All Si 2p
XPS spectra show intensive peaks in the region ranged from 100 to 105
eV. For the spectra of supports containing Al atoms, the bands at about
101 eV, and 103 eV should be ascribed to the bonds of Si-O-Al and
Si-O-Si respectively.[37] The binding energy of
bands assigned to the Ti-O-Si bond is lower than that of
SiO2.[38] The Si 2p XPS spectra
exhibit that the binding energies of Al-Ti-SBA-16 composites shift to
lower values compared with SBA-16 silica, which should be due to the
incorporation of Al and Ti species into SBA-16 materials.
The SEM images of different supports are shown in Fig. S7. The SBA-16
pure silica presents regular particles with the morphology of
cross-linked sphere. The AT-0 sample modified by Al species through the
post-synthetic method exhibits a similar morphology with SBA-16 silica.
Compared with SBA-16 and AT-10 samples, the AT-7.5, AT-5 and AT-2.5
samples exhibit different morphologies with relatively lower regularity,
which should be caused by the addition of Ti sources. However, particles
with the shape of inerratic polyhedron can be observed in AT-0 sample.
In addition, the SEM mapping result of AT-5 sample is presented in the
Fig. S8. It can be seen that the Al and Ti atoms dispersed well in the
surface of AT-5 composite. The SEM EDS results of AT-0 and AT-5 samples
are displayed in Fig. S9. The Al composition in AT-0 sample is detected
as 11.4%, and Al and Ti compositions in the AT-5 sample are 6.4% and
5.1%, which are similar with theoretical value of Al and Ti
compositions. Hence, it can further confirm that the stepwise synthetic
method is effective to obtain serial Al-Ti-SBA-16 composites.
The TEM images of SBA-16 and Al-Ti-SBA-16 materials are displayed in
Fig. S10. The (111) or (100) crystal faces with highly ordered degree
can be clearly observed in all supports. Accompanied with small angel
XRD results, the TEM measurements can also demonstrate that serial
SBA-16 silica and Al-Ti-SBA-16 materials possesses highly ordered cubic
body-centred Im3m symmetry structure.[22]