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increase the crystallinity and reduce the oxidation of Ag.[24]
However, the resultant Ag nanocrystallites are relatively large
(5–20 nm) and have no even distribution. Thus, an effective
method and a suitable reductant should be developed to
obtain a higher degree of crystallinity, even distribution, and
small-sized Ag nanocrystallites on the surface of TiO2. As for
the promotional mechanisms of a noble metal–semiconductor
heterojunction, no consensus is reached. Su et al.[30] attributed
the observed visible light-driven photocatalytic activity to the
hot electron transported from Ag to TiO2 owing to surface
plasma resonance (SPR); however, Liu et al.[23] found that their
TA product was inactive under visible light irradiation. Al-
though the evidence of the electron transport from the metal
to the semiconductor due to SPR has been presented,[31] the
increase is small compared with the bulk semiconductor exci-
tation effect.[8] Both the Schottky barrier, formed between the
metal and the illuminated semiconductor, and the upward
shift of the Fermi level due to the electron storage of noble
metal favour the charge separation and charge trans-
port.[21,32,33] Moreover, the smaller size noble metal particles
were found to induce a maximum shift of the Fermi level of
a noble metal–semiconductor heterojunction.[8] Thus, the small
noble metal particle would serve as an efficient electron trap-
per to increase the photocatalytic activity upon excitation of
the semiconductor in the noble metal–semiconductor hetero-
junction.
Figure 1. XRD patterns of TB, TS, THS, and TA.
depicted in the TS curve. The anatase phase was dominant,
and only a minor peak located at 2q=30.78 corresponding to
the brookite phase was found. For comparison purposes, the
amorphous TB was grown to THS with NH4F. No peak corre-
sponding to the brookite phase was observed on the THS
curve, which implied that a single anatase phase is present.
Thus, the presence of NH4F could inhibit the formation of the
brookite phase.[35] Three diffraction peaks located at 2q=38.1,
44.4, and 64.48 appeared on the TA curve, which should be as-
signed to Ag nanocrystallites (JCPDS 65-2871). The peak of Ag
centred at 38.18 overlapped with the peak of the anatase
phase centred at 38.58. No observable shift or widening of the
XRD peak was observed upon decoration of Ag, which sug-
gested that the decoration of Ag had not changed the THS
crystal structure. The above analysis demonstrated that TA het-
erostructures had been obtained by using the two-step hydro-
thermal method.
Herein, we synthesized TA heterostructures, in which the
ultra-small Ag nanocrystallties (2 nm) with high crystallinity
were distributed evenly on the surface of TiO2 hollow spheres
(THSs). In the synthesis of the hollow structure, the template-
free hydrothermal method was used to avoid the application
of the sacrificial template. For the deposition of Ag, DMF was
found to be an effective reductant to produce Ag having ex-
cellent dispersion with the help of polyvinylpyrrolidone (PVP)
by using a short-time (2 h) hydrothermal method. These TA
heterostructures demonstrated excellent photodegradation
under simulated sunlight irradiation. The degradation efficien-
cy of rhodamine B (RhB) could reach up to 100% within
20 min, which was much better than the results of some re-
ports on TA.[23,34] The high stability of TA heterostructures in
our investigated time range made it a promising material for
industrial applications.
To observe the morphology and configuration of TA hetero-
structures, FE-SEM, TEM, and STEM were used, and the results
are illustrated in Figure 2. After a slow hydrolysis of tita-
nium(IV) n-butoxide, the amorphous TB was formed (Fig-
ure 2a). These amorphous colloids were evenly distributed
with a diameter of approximately 500 nm. In the absence of
NH4F, the TB had been converted to TS by using the hydro-
thermal method (Figure 2b). The surface of TS was rougher
than that of the TB owing to the crystallization of TiO2 and the
growth of TiO2 crystallites. A THS with an open gap could be
obtained by using the hydrothermal method of TS in the pres-
ence of NH4F. The presence of Fꢀ induced a progressive redis-
tribution of mass from the interior to the exterior through
highly porous shells; hence, the hollow structure can be fabri-
cated. The panoramic image of THS is shown in Figure 2c,
which indicates that the well-formed hollow structures were
obtained on a large scale. A magnified image of THS is shown
in Figure 2d, in which the open gap structures can be clearly
seen. The decoration of Ag to THS did not change its hollow
structure. The typical SEM image of TA is shown in Figure 2e.
An open gap structure was retained upon the deposition of
Results and Discussion
Formation of TA heterostructures
Crystal structures, morphology, and configuration were investi-
gated by using XRD, field emission SEM (FE-SEM), TEM, and
scanning TEM (STEM) to monitor the evolution process of TA
heterostructures. As shown in Figure 1, the TiO2 bead (TB) pre-
pared through hydrolysis at room temperature demonstrated
no XRD peak (bottom curve), and the amorphous phase of the
TB was found, as expected. The amorphous TiO2 sphere trans-
formed to a TiO2 solid sphere (TS), which consisted of the ana-
tase (JCPDS 21-1272) and brookite (JCPDS 29-1360) phases
after the hydrothermal treatment in the absence of NH4F, as
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ChemCatChem 2014, 6, 1392 – 1400 1393