2
I. Romero Ocaña et al. / Inorganica Chimica Acta xxx (2015) xxx–xxx
Table 1
todehydrogenation [46,47]. In this context, the use of composite
materials comprising brookite is particularly interesting, since this
polymorph is much less investigated with respect to anatase and
rutile, although it demonstrated promising performances in photo-
catalytic applications [36,48,49]. Nowadays, a careful control of the
material morphology at nanoscale level can significantly con-
tribute to the understanding of the reactivity and bust the reac-
tivity by exposing preferentially more reactive facets [50–54].
This approach has been widely applied to the anatase phase allow-
ing its preparation in many different shapes [50–53,55–58]. Exam-
ples of the preparation of brookite and rutile with a controlled
morphology are significantly less frequent in the literature
[49,59–65]. Generally speaking, the production of preferential
facets can be achieved by the addition of adequate controlling
agents that adsorb on specific facets during the hydrothermal
treatments employed in the preparation of anatase TiO2
[51,52,58]. The adsorption of different controlling agents, such as
Fꢀ or Clꢀ, modify the relative stability of the different facets and
their growth rate, finally altering the ratio of the exposed facets
and the shape of the obtained nanocrystals [52]. The obtainment
of TiO2 nanocrystals with controlled morphology favors the elec-
tron–hole separation since electrons and holes are accumulated
on different facets [56,62,66]. Inspired from the synthesis of ana-
tase nanorods reported by Li et al. [57], we prepared anatase/
brookite nanocomposites by hydrothermal treatment performed
under different conditions, i.e. by changing the precursor/water
ratio, the heating method and the reaction time. The characterization
of the obtained materials allowed us to correlate the parameters
employed during the synthesis with the morphological and struc-
tural characteristics and the photocatalytic performances in hydro-
gen production.
Synthesis conditions of prepared samples.
Sample
Na-titanate/water
mass-to-volume ratio (mg/mL)
Time (h)
Heating method
A1
A2
A3
B1
B2*
B3
C1
C2
C3*
C4
C5
C6
C7
3.8
7.7
13.9
3.8
7.7
13.9
7.7
7.7
7.7
7.7
7.7
7.7
7.7
24
24
24
24
24
24
12
18
24
30
36
42
48
Convection
Convection
Convection
Irradiation
Irradiation
Irradiation
Irradiation
Irradiation
Irradiation
Irradiation
Irradiation
Irradiation
Irradiation
*
B2 and C3 are the same sample.
A reference sol–gel TiO2 was prepared accordingly to Gombac
et al. [24]. Briefly, TiO2-SG was prepared by hydrolysis of titanium
butoxide in ethanol using HNO3 as catalyst [24]. The gel was aged
at room temperature for 24 h, dried at 80 °C overnight and finally
calcined in air at 450 °C for 6 h.
0.2 wt% of Pt was loaded over these materials by photodeposi-
tion during the first photocatalytic catalytic test (see Section 2.3 for
details).
2.2. Characterization techniques
X-ray diffraction (XRD) patterns were collected by a Philips
X’Pert diffractometer using a Cu K
a (k = 0.154 nm) X-ray source
in the range 10° < 2h < 100° and data were analyzed by using the
PowderCell 2.0 software. Mean crystallite sizes were calculated
applying the Scherrer’s equation to the principal reflection of each
phase [(101) for anatase and (210) for brookite].
2. Experimental
2.1. Synthesis of the photocatalysts
The morphology of the composite materials and the distribution
of the supported Pt nanoparticles were evaluated by High Resolu-
tion Transmission Electron Microscopy (HR-TEM) and High Angle
Annular Dark Field-Scanning Transmission Electron Microscopy
(HAADF-STEM) images recorded by a JEOL 2010-FEG microscope
operating at the acceleration voltage of 200 kV. The microscope
has 0.19 nm spatial resolution at Scherzer defocus conditions in
HR-TEM mode and a probe of 0.5 nm was used in HAADF-STEM
mode.
In this work, anatase/brookite nanocomposites were synthe-
sized from Na-titanate adapting the procedure reported by Li
et al. for the preparation of anatase nanorods [57].
Briefly, the Na-titanate precursor was synthesized by
hydrothermal treatment of the commercial Degussa P25 TiO2
(750 mg) at 120 °C for 24 h in a 45 mL Teflon-lined autoclave con-
taining a NaOH 10 M solution (30 mL). The white precipitate was
collected by centrifugation, washed several times with bidistilled
water until the solution pH decreases below 11 and subsequently
dried at 80 °C overnight.
Anatase/brookite nanocomposites were then synthesized by
hydrothermal transformation of the dry Na-titanate precursor.
Specifically, a 45 mL Teflon-lined autoclave was charged with
30 mL of bidistilled water and a selected amount of Na-titanate,
magnetically stirring for 1 h. The hydrothermal process was per-
formed putting the autoclave into a pre-heated oven at 200 °C
for the desired time. The effect of the following parameter on the
synthesis was studied: (i) the Na-titanate/water mass-to-volume
ratio employed (adjusting the amount of dry Na-titanate), (ii) the
heating method and (iii) the reaction time. In order to change
the heating method, two different ovens were employed: a labora-
tory oven heating the autoclave by convection or a furnace heating
the autoclave by irradiation. The conditions employed to synthe-
size the different samples are summarized in Table 1. After the
hydrothermal treatment, the white precipitate was collected by
centrifugation, washed several times with deionized water to
remove the Na+ and finally dried at 60 °C overnight. These materi-
als were then used for the characterization and the photocatalytic
tests.
Raman spectra were recorded with
a Renishaw inVia
microspectrometer equipped with a Nd:YAG laser using an excita-
tion wavelength of 532 nm. The samples were dispersed onto a Si
wafer and analyzed using a laser power of 1 mW in the range
100 < cmꢀ1 < 1500.
Kr physisorption at the liquid nitrogen temperature using a
Micromeritics ASAP 2020 automatic analyser. The samples were
previously degassed under vacuum at 120 °C for 12 h. The surface
area was calculated applying the Brunauer-Emmett-Teller (BET)
equation to the physisorption isotherm in the range 0.1 < p/
p0 < 0.34.
2.3. Catalytic activity
Photocatalytic hydrogen production was studied using a Teflon-
lined flow-photoreactor irradiated with a Lot-Oriel Solar Simulator
equipped with a 150 W Xe lamp and an Atmospheric Edge Filter
with a cut-off at 300 nm [47]. 50 mg of dried TiO2 powder were
suspended into EtOH 96% (80 mL) and 20
lL of an aqueous solution
of Pt(NO3)2 (5 mg Pt mLꢀ1) were added in order to obtain a final