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due to the number of the so-called stepped atoms, which can be
found at the high-indexed crystal facets ((730), (411), etc.) and
are responsible for the enhanced electrocatalytic activity [49].
Thus, the motivation of the present work is to illuminate the same
aspects for the photocatalytic processes.
taking place within an electronic band system in which the band
gap has the same value, without being influenced by the presence
of platinum. Although their band gaps may not differ, the color
change of the material is obvious from the rest of the DRS spectra.
The situation slightly changed in the case of AR-based samples. By
depositing Pt onto the surface of AR, the band-gap energy values
are slightly changed from 2.96 to 2.91 and 2.82 eV (AR-Pt(s) and
AR-Pt(c) composites). The mentioned changes are also faintly visi-
ble in the first derivative spectra.
3.3. TiO2–Pt nanocomposites: characterization
The base photocatalysts’ crystal phase composition, crystal size,
and specific surface area values are summarized in Table 1. The
parameters obtained from the measurements coincide with
the ones given by the manufacturer or with those published in
the literature [50,51] (Fig. 2). By depositing platinum on the
surface of these materials, no structural changes were observed,
as expected. The next step in the characterization of these materi-
als was to literally study the morphology of these nanocomposites.
First the morphology of the polyhedral platinum nanoparticles
was examined by HRTEM while the lattice fringes were evaluated
based on Refs. [41,52]. The obtained micrographs are presented in
Fig. 3. As expected, the dominant shape of the nanocrystallites was
cuboctahedral/octahedral – 72% (a relatively small percentage of
tetrahedral – 5% – and some undefined polyhedral particles –
23% – were also noticed).2 The interplanar distances were evaluated
by FFT. The size distribution of these platinum nanocrystallites (both
spherical and polyhedral ones) was homogeneous, most of them
having a size of 4–6 nm (85%), as illustrated in Fig. 3.
The investigations in the cases of AA- and AR-based composites
already suggest that in the case of P25 (where both anatase and
rutile are present in a well-defined ratio) a mixture of the effects
should be observable. As expected, the presence of Pt modified
the optical properties of P25 significantly. The bare catalyst exhi-
bits two electron transition bands in the first derivative DRS spec-
tra, one assigned to anatase and the other to rutile (Fig. 5b). As
polyhedral nanoparticles are deposited (P25-Pt(c)) at the surface
of the material, the ratio of the two bands changes in favor of ana-
tase, while the peak positions do not vary. If the deposited Pt nano-
particles are spherical (P25-Pt(s)), then the ratio of the anatase/
rutile bands is even more balanced toward the anatase phase.
This means that in the case of P25-based composites the presence
of Pt denies/inhibits electron transitions within the rutile particles.
If this is true, then an activity decrease should be observable for
nonadsorbing pollutant degradation, such as phenol.
The deposition of the platinum nanoparticles at the surfaces of
the commercial titanias was also successful, as shown by Fig. 3.
While in the case of P25 it was quite easy to obtain high-quality
images of the deposition of platinum nanoparticles, the situation
was dire in the case of AA and AR due to their large crystal size
(200–300 nm). This is why only P25-related TEM micrographs
were presented.
3.5. The photocatalytic activity of the obtained nanocomposites
3.5.1. The photodegradation of phenol
Some hints regarding the possible importance of the Pt crystal
geometry are already given by the interesting changes observed
in the optical properties of the composite materials (see Fig. 6).3
In the first instance the P25-based composites’ activity was
evaluated. It is known that this commercial powder is a versatile
and quite efficient photocatalytic material, which can be seen also
in the present case by achieving 87% of phenol decomposition in
2 h. As platinum nanoparticles were deposited on P25, the activity
decreased significantly (achieving 72 and 52% of degraded phenol
for samples P25-Pt(s) and P25-Pt(c)). This activity drop in the case
of P25-based composites could have several causes. One could be
the efficiency of the electron transfer processes. One hint regarding
this was already given by the optical properties of the P25-based
composites. It was shown that when Pt nanoparticles were depos-
ited, the electron transition band (in the first derivative DRS spec-
tra) corresponding to the rutile phase diminishes significantly,
suggesting that a fraction of the electron transitions are ‘‘lost’’/
not happening at all. There is also a significant difference in phenol
degradation yield (72% for P25-Pt(s) vs. 52% for P25-Pt(c)) between
the two Pt-containing composites, and a further change can be
noticed in the ratio of the anatase and rutile electron transition
bands in favor of anatase in the case of composite P25-Pt(c). The
latter phenomenon raises the possibility of a special interaction
between rutile and Pt nanopolyhedra, which may be clarified in
the section regarding AR based composites.
3.4. The TiO2–Pt nanocomposites: optical properties
One of the first aspects that need investigation for materials
with photocatalytic potential is their optical properties. The first,
simplest approach was to examine the obtained nanocomposites’
color. One may expect that the color of the composite materials
should not change at all when the nanocrystals’ shape is varied,
because in each case we have the same material (the same optical
‘‘property set’’ should be observable) with the same composition.
However, as can be clearly seen in Fig. 4, just by changing the
shape of the platinum nanoparticles, while using the same base
catalyst (P25), an interesting change occurred in the investigated
nanocomposites’ color (intense creamy gray for sample P25-Pt(c),
conventional gray for P25-Pt(s)). These observations indicate that
a more detailed study of the optical properties of these materials
was inevitable.
To get quantified information about the optical peculiarities of
these materials, the DRS and the first-order derivative DRS spectra
were recorded (Fig. 5a) and the band-gap values calculated
(Table 1). The AA-based composites were examined in the first
step, to gain critical information when only a single crystalline
phase of titania was present in the composite. As Pt nanoparticles
are deposited onto the surface of AA, the band-gap value remains
constant. This can be even more precisely observed in the first
derivative spectra; the peak located at 375 nm (3.3 eV) in the case
of AA does not shift at all in the platinum-containing composites
(AA-Pt(s), AA-Pt(c)). This means that the possible electron transi-
tions between the valence band and the conduction band are
Pure AA itself proved to be quite active in the degradation of
phenol, although the manifested degradation yield is inferior to
that of P25 (63% vs. 87%). Based on the behavior of P25, it was
expected that after platinum deposition the activity would further
decrease, but surprisingly this was not the case. Both spherical and
polyhedral Pt nanoparticles enhanced with a factor of 1.5 the
3
Please note that the photocatalytic performance will be discussed based on the
photocatalytic efficiency given in the percentage of phenol removed. This was chosen
because in some of the cases the kinetics of the degradation changes abruptly; thus a
clear evaluation of the activity based on reaction rates would be uninformative (just
for comparison, the values are given in Table 1).
2
The shape distribution was estimated based on 10 TEM images – 150 particles
acquired from 10 randomly selected spots on the used copper grid.