L. Si et al. / Journal of Alloys and Compounds 601 (2014) 88–93
89
reductant to prepare Ti3+-doped TiO2 nanoparticles. It was found
that Zn can affect the morphology and phase structure of TiO2,
and the surface adsorbed ZnO clusters were thought to be respon-
sible for the stability of the Ti3+-doped TiO2 [25]. However, the fab-
rication of Ti3+-doped TiO2 nanosheets with dominant (001) facets
in the presence of reductant through solution reaction has not been
reported.
[28,29]. The suspensions of TiO2 (1.0 g/L) containing coumarin (0.5 mmol/L) is
mixed under magnetic stirring, and then was shaken overnight. At given intervals
of visible-light irradiation under a 3 W LED lamp (wavelength of 420 10 nm),
small aliquots were withdrawn by a syringe, and filtered through a membrane (pore
size 0.45 lm). The filtrate was analyzed on a Hitachi F-7000 fluorescence spectro-
photometer by the excitation with the wavelength of 332 nm.
3. Results and discussion
Here in we report for the first time that a facile method
to synthesize Ti3+ self-doped TiO2 nanosheets with dominant
{001} facets with enhanced visible light photocatalytic activity
through one-pot hydrothermal reaction strategy using tetrabutyl
titanate (TBT) as titanium source, hydrofluoric acid (HF) as
structure-directing agent and zinc powder (Zn) as reductant. The
photocatalytic activity of the prepared Ti3+-doped TiO2 nanosheets
with dominant (001) facets was evaluated by a photolumines-
3.1. Morphology and phase structure
Fig. 1 shows TEM images of the photocatalysts. It can be seen
that the all of the samples were well defined sheet-shaped struc-
tures having a rectangular outline, which is consistent with the re-
ported TiO2 nanosheets with exposed (001) facets [2,28–30].
Fig. 1f shows the high resolution TEM image of a nanosheet for
R0.3 TiO2 sample, which was erected on copper grid. The lattice
spacing is of ca. 0.235 nm is corresponding to the {001} plane of
anatase TiO2, also indicating the top and bottom facets are the
{001} planes [30]. Although the presence of Zn powder shows neg-
ligible effect on the morphology of TiO2 nanosheets, the color of
the obtained powder turned from white to blue, and the blue col-
oration became darker as the amount of Zn powder added was in-
creased (inset of Fig. 1). Consistent with literature [25], the blue
coloration of the powder is due to Ti3+ ions coming from the reduc-
tion of Ti4+ by Zn.
The phase structure of TiO2 are of great importance for its pho-
tocatalytic activity. Fig. 2 shows the XRD patterns of the prepared
photocatalysts. A broad peak at 2h = 25.3°, corresponding to the
(101) plane diffraction of anatase TiO2 (JCPDS No. 21-1272), was
observed for all samples. Different from the reported by Huang
et al. [25], there was no rutile phase was formed in the obtained
TiO2 samples. This is due to the presence of HF, which can prevent
the formation of rutile and brookite phase TiO2 [31,32].
cence (PL) technique using coumarin as
a probe molecule
(Scheme 1) under visible light irradiation [26,27].
2. Experimental
2.1. Sample preparation
The samples were prepared using tetrabutyl titanate (TBT) as titanium source,
hydrofluoric acid (HF) as structure-directing agent, zinc powder (Zn) as reductant.
Specifically, 24 mmol of zinc powder was slowly added into 80 mmol of tetrabutyl
titanate (TBT) under magnetic stirring. After stirring for 30 min to make the zinc
powder uniformly dispersed, 180 mmol of HF solution (40 wt.%) was dropwise
added into the suspension and the resulted solution was continuously stirred for
another 1 h. The mixture was then transferred to a dried 100-mL Teflon-lined auto-
clave and kept at 200 °C for 24 h. After being cooled to room temperature, the pre-
cipitates were collected. To removal the excessive zinc powder, the collected
precipitates were washed with 10 ml of hydrochloric acid under magnetic stirring,
and followed by thoroughly washed with distilled water until the pH value of the
filtrate is about 7. Then obtained powder was denoted as R0.3, where 0.3 represents
the Zn/TBT molar ratio. For comparison, a serials of samples were also prepared un-
der other identical conditions except the amount of Zn powder (Table 1).
2.2. Characterization
3.2. Raman spectroscopy and XPS analysis
The morphology of the photocatalyst was observed on a transmission electron
microscopy (TEM) (Tecnai G20, USA) using an acceleration voltage of 200 kV. The X-
ray diffraction (XRD) patterns were obtained on a D8-advance X-ray diffractometer
(German Bruker) using Cu K
ated voltage and applied current were 15 kV and 20 mA, respectively. Raman spec-
To further verify the phase transition of the Ti3+ self-doped TiO2
nanosheets, we also conducted Raman spectroscopy and XPS. It is
well known that Raman spectroscopy, which is originated from the
vibration of molecular bonds with high measuring sensitivity, is a
simple, efficient, well-established technique and accurate alterna-
tive approach to study the significant structural changes in titania
[33]. As shown in Fig. 3, three Raman peaks of anatase TiO2 are ob-
served, that is, 391 cmÀ1 (B1g), 510 cmÀ1 (A1g and B1g), 633 cmÀ1
(Eg), respectively. There are no Raman peaks of rutile or brookite
phase TiO2 was found, also indicating the pure anatase phase
TiO2 nanosheets. It can also be observed that the intensities of
three Raman peaks decrease with increase in Zn/TBT molar ratio,
which is ascribed to increased number of oxygen vacancies in the
lattice structure of TiO2 nanosheets due to the formation of Ti3+
[20,21]. Oxygen deficiency can transfer its extra two electrons to
the adjacent two Ti4+ atoms to form Ti3+, and the presence of the
oxygen vacancies lead to the change of atomic coordination num-
bers and bonding length of the Ti–O–Ti network in Ti3+ self-doped
TiO2 nanosheets. Therefore, it is not strange to see that the inten-
sities of Raman peaks decrease with increase in the Zn/TBT molar
a
radiation at a scan rate of 0.02° 2h sÀ1. The acceler-
trum was recorded at room temperature using
a micro-Raman spectrometer
(Renishaw InVia) in the backscattering geometry with a 514.5 nm Ar+ laser as an
excitation source. X-ray photoelectron spectroscopy (XPS) measurements were
done with a Kratos XSAM800 XPS system with Mg Ka source and a charge neutral-
izer, all the binding energies were referenced to the C 1s peak at 284.8 eV of the sur-
face adventitious carbon. UV–vis diffuse reflectance spectroscopy (DRS) was carried
out on a Hitachi U-3010 UV–vis spectrophotometer, and BaSO4 was used as the ref-
erence sample. The BET surface area (SBET) of the powders was analyzed by using
nitrogen adsorption in
a nitrogen adsorption apparatus (Micromeritics ASAP
2020, USA). The BET surface area was determined by a multipoint BET method using
the adsorption data in the relative pressure (P/P0) range of 0.05–0.3. Pore volume
(PV) and average pore size (APS) were determined by nitrogen adsorption volume
at the relative pressure of 0.994. All the samples were degassed at 180 °C prior to
the nitrogen adsorption measurements.
2.3. Photocatalytic activity
The photocatalytic activity of the photocatalyst was evaluated by a photolumi-
nescence (PL) technique using coumarin as a probe molecule, which readily reacted
with ÅOH radicals to produce highly fluorescent product, 7-hydroxycoumarin
2.OH
+
H2O
+
O
O
O
O
OH
coumarin
7-hydroxycoumarin
Scheme 1. Formation of 7-hydroxycoumarin in the reaction of coumarin with hydroxyl radicals.