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Visible light-driven photocatalysis of doped SrTiO3 tubular
structure
Jinwen Shi,1 Shaohua Shen,1,2,* Yubin Chen,1 Liejin Guo,1 and
Samuel S. Mao2 1International Research Center for Solar-Hydrogen
Renewable and Clean Energy, State Key Laboratory of
Multiphase Flow in Power Engineering, Xian Jiaotong University,
Shaanxi 710049, China 2Environmental Energy Technologies Division,
Lawrence Berkeley National Laboratory, and Department of
Mechanical Engineering, University of California at Berkeley,
Berkeley, CA 94720, USA * [email protected]
Abstract: SrTiO3 tubular structures co-doped with Cr and Ta were
synthesized through a combination of solvothermal-hydrothermal
processes. X-ray photoelectron spectroscopy (XPS) measurements of
the oxidation state of Cr ions reveal that the formation of Cr6+
ions, which would serve as the non-radiative recombination centers
for photogenerated electrons and holes, was suppressed without the
process of high temperature hydrogen reduction. Compared to similar
co-doped materials synthesized by solid-state reaction, (Cr, Ta)
co-doped SrTiO3 tubular structures have significantly higher
photocatalytic activity for hydrogen evolution as measured in an
aqueous methanol solution under visible light irradiation. 2012
Optical Society of America OCIS codes: (160.6990)
Transition-metal-doped materials; (260.5130) Photochemistry;
(350.6050) Solar energy.
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accepted 13 Jan 2012; published 9 Mar 2012(C) 2012 OSA 12 March
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1. Introduction Solar-driven water splitting for hydrogen
production using semiconductor-based photocatalysts has attracted a
significant amount of attention from both the fundamental science
point of view and the potential as a clean energy solution. Since
Fujishima and Hondas pioneering work in 1972 [1], different classes
of semiconductor materials, such as metal oxides as best
represented by TiO2, have been developed and evaluated as potential
candidate photocatalysts for high-efficiency solar-driven hydrogen
conversion [2,3]. Considering that UV light consists of only a
small portion (~4%) of the solar spectrum, the energy conversion
efficiency for solar-hydrogen water splitting using TiO2 as the
photocatalyst is typically
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capacity of photocatalytic hydrogen production under visible
light. The oxidation states of Cr ions and their effects on
photocatalytic H2 production were discussed, which were compared to
similar co-doped materials synthesized through solid-state
reaction.
2. Experimental 2.1 Synthesis of (Cr, Ta) co-doped SrTiO3
tubular structures In the first step, tubular (Cr, Ta) co-doped
TiO2 was prepared by a solvothermal approach [9]. Typically,
approximately 4.6 g of TiOSO4xH2SO4xH2O, as the Ti source, together
with 0.31 g of Cr(NO3)39H2O and 0.28 g of TaCl5, with an elemental
ratio of Ti: Cr: Ta = 0.96: 0.04: 0.04, were dispersed in a mixed
solution of absolute ethanol (40.0 g), ethyl ether (16.62 g) and
ethylene glycerol (27.01 g) under stirring. The resulted green
dispersion was transferred into a Teflon autoclave followed by
solvothermal treatment at 110 C for 48 h. After filtering and
washing with water, the dried precipitation was annealed in air at
550 C or 900 C with ramping rate of 1 C/min to obtain crystallized
(Cr, Ta) co-doped TiO2 that have a tubular structure, denoted as
T-550 and T-900, respectively.
In the second step, T-550 and T-900 were hydrothermally reacted
with excessive Sr(OH)2 at 250 C for 1248 h to synthesize the final
tubular (Cr, Ta) co-doped SrTiO3 products (Sr: Ti = 10: 1). Process
parameters for samples denoted as ST-01, ST-02 and ST-03 are given
in Table 1.
Table 1. Information of the Final Tubular (Cr, Ta) Co-Doped
SrTiO3 Products (ST-01, ST-02 and ST-03).
Final products
Crystallized tubular (Cr, Ta) co-doped TiO2
Second-step hydrothermal process
Temperature (C) Time (h) ST-01 T-550 250 12 ST-02 T-550 250 48
ST-03 T-900 250 24
As a reference, (Cr, Ta) co-doped SrTiO3 was also synthesized
through a solid-state reaction followed by H2 reduction [16].
Stoichiometric mixture of SrCO3, TiO2, Cr2O3 and Ta2O5 was calcined
at 1050 C for 20 h in air. The ramping rate was 1 C/min. Then H2
reduction treatment at 500 C was performed on the oxide precursor
in order to obtain (Cr, Ta) co-doped SrTiO3, which was denoted as
ST-SSR. 2.2 Characterization X-ray diffraction (XRD) patterns of
the samples were obtained from a PANalytical Xpert MPD Pro
diffractometer using Ni-filtered Cu K irradiation (1.5406 ). The
patterns of powder X-ray diffraction (PXRD) in the 2 range from
10.0 to 80.0 were collected (40 kV, 40 mA; real-time multiple strip
(RTMS) detector, X'Celerator) with a scan step size of 0.0334 and
counting time of 19.685 s. A divergence slit of 1, antiscatter slit
of 2, and a 0.04 radian Soller slit were used in the incident beam
path, whereas a 6.6 mm antiscatter slit and a 0.04 radian Soller
slit were used in the diffracted beam path. X-ray photoelectron
spectroscopy (XPS) measurements were conducted on a Kratos
spectrometer (AXIS Ultra DLD) with monochromatic Al K radiation (h
= 1486.69 eV) and with a concentric hemispherical analyzer working
at 15 kV and 8 mA, and with the pressure of sample analysis chamber
under high vacuum (< 3109 Torr). Both survey-scan (pass energy
160 eV, step size 1000 meV) and high-resolution (pass energy 40 eV,
step size 50 meV) spectra of X-ray photoelectron spectroscopy (XPS)
were obtained with an analysis area of 300700 m and with charge
corrected to C 1s line of adventitious carbon set to 284.8 eV.
UV-Vis absorption spectra were recorded by a Hitachi U-4100
UVvisnear-IR spectrometer under diffuse reflectance (DR) mode with
BaSO4 as the reference. Sample morphology was examined by a JEOL
JSM-6700FE scanning electron microscope with accelerating voltage
of 15 kV. The BrunauerEmmettTeller (BET) surface areas of samples
were deduced from N2 adsorption-
#158507 - $15.00 USD Received 22 Nov 2011; revised 13 Jan 2012;
accepted 13 Jan 2012; published 9 Mar 2012(C) 2012 OSA 12 March
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desorption isotherms at 77 K. The isotherms were determined by
using an Accelerated Surface Area and Porosimetry Analyzer (ASAP
2020, Micromeritics) after degassing the samples at 120 C for 6 h.
2.3 Evaluation of photocatalytic activity Photocatalytic hydrogen
evolution was performed in a side irradiation Pyrex cell. A 300-W
Xe lamp was used as the light source, and the UV portion of the
light was removed by a cut-off filter ( > 420 nm). Hydrogen gas
was analyzed by an online thermal conductivity detector (TCD) gas
chromatograph (NaX zeolite column, nitrogen as a carrier gas). In
all experiments, an amount of 0.30 g photocatalysts was thoroughly
suspended, using a magnetic stirrer with constant rotational
velocity, into a CH3OH aqueous solution (18.5 vol%, 270 mL) in the
Pyrex cell. Nitrogen gas was purged through the cell before
reaction in order to remove oxygen. 0.5 wt% Pt as cocatalyst for
the promotion of hydrogen evolution was photo-deposited in situ on
the photocatalyst from a H2PtCl66H2O precursor. The temperature for
all photocatalytic reactions was kept at 20 C. Control experiments
showed no appreciable H2 evolution without irradiation or
photocatalysts. All the photocatalyts cannot produce hydrogen in
the absence of methanol as sacrificial reagent.
3. Results and discussion Figure 1 shows SEM images of
crystallized tubular (Cr, Ta) co-doped TiO2, which are precursors
of the final SrTiO3 products. It is noted that annealing of (Cr,
Ta) co-doped TiO2 (T-550 and T-900) would keep the morphology of a
tubular microstructure, suggesting good thermal stability. Figure
1b revealed that the tubular structure has a rough surface
consisting of spiny complex architecture for the sample annealed at
550 C (T-550). While annealed at 900 C, T-900 sample maintained the
hollow tube structure, but with the surface spiny architecture
destructed and coalesced (Fig. 1d).
Fig. 1. SEM images of crystallized tubular (Cr, Ta)-codoped
TiO2, (a,b) T-550 and (c,d) T-900. Figure 2 shows SEM images of the
final tubular (Cr, Ta) co-doped SrTiO3 materials. As
seen in Figs. 2a-2d, ST-01 and ST-02 samples exhibit similar
morphology, which keeps the tubular structure like the tubular
T-550 template. The walls of the tubes were not characterized by
spiny surface, but self-assembled nanoparticles with diameters of
4060 nm
#158507 - $15.00 USD Received 22 Nov 2011; revised 13 Jan 2012;
accepted 13 Jan 2012; published 9 Mar 2012(C) 2012 OSA 12 March
2012 / Vol. 20, No. S2 / OPTICS EXPRESS A354
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(Figs. 2b and 2d). The ST-03 sample that was hydrothermally
templated from T-900 also keeps the tubular structure (Fig. 2e),
but with the wall of the tubes comprised of uniform cubes with side
length of approximately 150 nm (Fig. 2f).
Fig. 2. SEM images of the final tubular (Cr, Ta)-codoped SrTiO3
products, (a,b) ST-01, (c,d) ST-02, and (e,f) ST-03.
X-ray diffraction (XRD) was used to characterize the structures
of the crystallized tubular (Cr, Ta) co-doped TiO2 (T-550 and
T-900). As shown in Fig. 3, XRD patterns of T-550 and T-900 could
be indexed to anatase and rutile TiO2, respectively. The broadening
of the reflections in the XRD pattern of T-550 indicated its poor
crystallinity with small crystallites on the nanometer scale, while
the intense XRD peaks of T-900 implied its good crystallinity and
large crystallites, which are resulted from higher annealing
temperature. The crystallite sizes of T-550 and T-900 calculated
from the peak at ca. 25.2 and 27.5 using the Scherrer formula are
9.6 nm and 27.1 nm, respectively. The structure of the final
tubular (Cr, Ta) co-doped SrTiO3 were also characterized by XRD as
shown in Fig. 3. ST-01 and ST-02 have similar XRD profiles that
could be assigned to the pure SiTiO3 phase. No other phases like
unreacted anatase TiO2 were observed. By contrast, ST-03 possesses
unreacted rutile TiO2 together with the SrTiO3 product, although it
experienced a longer hydrothermal reaction process than ST-01 (24 h
vs. 12 h). By using the normalized RIR (Reference Intensity Ratio)
method, the percentages of rutile (JCPDS No. 01-076-0318, RIR =
3.51) and SrTiO3 (JCPDS No. 01-079-0174, RIR = 8.15) in ST-03 were
determined to be 18.1 and 81.9 wt%, respectively. This indicated
that anatase TiO2 should be easier to react with Sr(OH)2 to
form
#158507 - $15.00 USD Received 22 Nov 2011; revised 13 Jan 2012;
accepted 13 Jan 2012; published 9 Mar 2012(C) 2012 OSA 12 March
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SrTiO3 in hydrothermal condition as compared to rutile TiO2.
Additionally, one cannot find other oxides such as Cr2O3 and
Ta2O5.
Fig. 3. XRD patterns of crystallized tubular (Cr, Ta)-codoped
TiO2 (T-550 and T-900), and tubular (Cr, Ta)-codoped SrTiO3 (ST-01,
ST-02, and ST-03).
Optical properties of tubular (Cr, Ta) co-doped SrTiO3 products
(ST-01, ST-02, and ST-03) were characterized with UV-Vis spectra
(Fig. 4). ST-01 and ST-02 prepared from T-550 possess similar
absorption bands in the visible region, with onsets around 520 nm.
The shapes of UV-Vis spectra were characteristics of metal doping,
indicating discontinuous levels formed by the dopants in the
forbidden band [12]. The absorption bands of ST-01 and ST-02 in the
visible light region can be attributed to a Cr3+ to Ti4+
charge-transfer transition, which agrees well with the absorption
spectra of SrTiO3 doped with Cr3+ ions [20] or co-doped with Cr3+
and Ta5+ [21]. ST-03 prepared from T-900 also shows an absorption
band in the visible region due to Cr3+ to Ti4+ charge-transfer
transition, but with onset red-shifted to around 550 nm when
compared to ST-01 and ST-02. Such a change in the absorption
spectra implies the existence of Cr6+ ions in ST-03 resulted from
high temperature annealing of tubular (Cr, Ta) co-doped TiO2
precursor at 900 C. As shown in the inset of Fig. 4, the color of
ST-01 and ST-02 was gray-green whereas the color of ST-03 was
yellow. The color difference gave an additional clue to the
different oxidation states of Cr ions in ST-01, ST-02 and
ST-03.
Fig. 4. UV-Vis spectra of the final tubular (Cr, Ta)-codoped
SrTiO3 (ST-01, ST-02, and ST-03) products and (Cr, Ta)-codoped
SrTiO3 as reference (ST-SSR).
#158507 - $15.00 USD Received 22 Nov 2011; revised 13 Jan 2012;
accepted 13 Jan 2012; published 9 Mar 2012(C) 2012 OSA 12 March
2012 / Vol. 20, No. S2 / OPTICS EXPRESS A356
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Figure 4 also shows the UV-Vis spectra of ST-SSR as the
reference, which was prepared by a solid-state reaction approach
followed by H2 reduction. ST-SSR had broad absorption bands in the
region of 550700 nm, which is quite similar to the (Cr, Ta)
co-doped SrTiO3 samples reported in literature [16]. Though it was
suggested that Cr6+ involved in SrTiO3 could be suppressed by Ta5+
co-doping or reduced to Cr3+ ions by H2 reduction, there are Cr6+
ions in ST-SSR as deduced from its broader absorption bands in
visible region when compared to those of ST-01, ST-02 and ST-03.
Additionally, short wavelength absorption of ST-01, ST-02 and ST-03
ascribed to the band-to-band transition of SrTiO3 were enhanced due
to the possibility of multiple reflections of trapped light in the
tubular structure [8,9,22].
X-ray photoelectron spectroscopy (XPS) measurements were carried
out to examine the oxidation states of Cr ions in the tubular (Cr,
Ta) co-doped SrTiO3 (ST-01, ST-02, and ST-03) samples. As shown in
Fig. 5, for ST-01 and ST-02 samples the peaks for Cr 2p1/2 and Cr
2p3/2 were obtained at about 586.8 eV and 577.1 eV, respectively,
which could be assigned to the Cr3+ ions [23] in ST-01 and ST-02.
No other XPS peak for Cr6+ was observed. In contrast, in the case
of ST-03, the majority of Cr ions were Cr3+, but a small amount of
Cr6+ seems to exist, as detected by the XPS peak at about 580.9 eV.
As these tubular (Cr, Ta) co-doped SrTiO3 (ST-01, ST-02, and ST-03)
samples were synthesized from tubular (Cr, Ta) co-doped TiO2
precursor, the doped Cr3+ (or Cr6+) and Ta5+ ions should locate at
the Ti sites.
Fig. 5. XPS spectra of Cr 2p of tubular (Cr, Ta)-codoped SrTiO3
(ST-01, ST-02, and ST-03). Figure 6 shows measurements of
photocatalytic H2 evolution from an aqueous methanol
solution over the tubular (Cr, Ta) co-doped SrTiO3 (ST-01,
ST-02, and ST-03) samples and the reference (ST-SSR) under visible
light irradiation ( > 420 nm). All co-doped SrTiO3
photocatalysts, including ST-SSR, are able to produce H2.
Considering the photophysical properties revealed by the UV-Vis and
XPS spectra, it could be determined that the ability of these (Cr,
Ta) co-doped SrTiO3 catalysts for visible-light-driven
photocatalytic H2 evolution arose from the doping of Cr3+ ions.
Compared with ST-01, ST-02 showed much higher H2 evolution rate.
This could be the result of fewer defects in ST-02 synthesized via
a longer hydrothermal process (48 h vs. 12 h), as the similar
morphology (Fig. 2) and visible light absorption ability (Fig. 4)
would not lead to great difference between the photocatalytic
activities of ST-01 and ST-02. In the case of ST-03, the
photocatalytic activity for H2 production was relatively low, even
though the visible light response was better than those of ST-1 or
ST-02. This could be attributed to the existence of the rutile
phase in ST-03, of which the ability for H2 evolution was low. It
is known that rutile TiO2 possesses lower conduction band level
than SrTiO3, thus the photoexcited electrons would transfer from
the conduction band of SrTiO3 to that of rutile TiO2, leading to
weaker driving force for proton reduction.
#158507 - $15.00 USD Received 22 Nov 2011; revised 13 Jan 2012;
accepted 13 Jan 2012; published 9 Mar 2012(C) 2012 OSA 12 March
2012 / Vol. 20, No. S2 / OPTICS EXPRESS A357
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This is also the reason why (Sb, Cr) co-doped SrTiO3 was
reported to display higher activity for H2 production than (Sb, Cr)
co-doped TiO2 under visible light [15]. Moreover, both ST-01 (19.5
m2/g) and ST-02 (19.1 m2/g) had much higher surface area than ST-03
(2.8 m2/g), as obtained from BET analysis. The high surface areas
could enrich the reactive sites, and also enhance the adsorption of
reactants, thereby accelerate the photocatalytic redox reactions
for hydrogen production. This may be the other reason why ST-03
showed much lower photocatalytic activity than ST-01 and ST-02.
As the reference, ST-SSR exhibited the lowest photocatalytic
activity. Its reasonable to assume that the hydrothermal method is
able to produce photocatalysts with good crystallinity as well as
high surface areas that exhibit higher photocatalytic activity for
water splitting than those synthesized by solid-state reaction
[24,25]. The more efficient light harvesting of the tubular
structure due to multi-scattering effect may be another possible
reason for the higher photocatalytic activities of hydrothermally
(vs. solid-state reaction) synthesized materials [9]. By checking
into the initial stage (~1 h) of photocatalytic reaction, ST-03
showed very low activity for H2 evolution. After this stage, a
considerable H2 evolution rate was obtained. It was assumed that
the Cr6+ ions had been reduced to the Cr3+ ions by photogenerated
electrons during this induction period [11]. In contrast, there was
no induction period for photocatalytic H2 production over ST-01 or
ST-02, owing to the absence of Cr6+ ions as revealed by XPS
measurements. The longer induction period (~3 h) of ST-SSR
indicated the presence of larger amount of Cr6+ ions, even after H2
reduction.
Fig. 6. Time courses of visible-light-driven H2 evolution over
tubular (Cr, Ta)-codoped SrTiO3 (ST-01, ST-02, and ST-03) and (Cr,
Ta)-codoped SrTiO3 as reference (ST-SSR).
4. Conclusion (Cr, Ta) co-doped SrTiO3 tubular structures were
fabricated by a solvothermal-hydrothermal two-step process. It was
found that the tubular (Cr, Ta) co-doped SrTiO3 synthesized using
anatase tubular (Cr, Ta) co-doped TiO2 as the precursor showed
higher photocatalytic activity for hydrogen production than that
synthesized from rutile precursor, in which the unreacted rutile
TiO2 with lower conduction band level than SrTiO3 led to weaker
driving force for H2 production. XPS measurements revealed the
formation of Cr6+ ions, which would work as the charge
recombination centers, could be avoided using the
solvothermal-hydrothermal two-step process. The resulted tubular
(Cr, Ta) co-doped SrTiO3 exhibited much higher photocatalytic H2
production activities without an induction period when compared to
that synthesized by the solid-state reaction approach.
#158507 - $15.00 USD Received 22 Nov 2011; revised 13 Jan 2012;
accepted 13 Jan 2012; published 9 Mar 2012(C) 2012 OSA 12 March
2012 / Vol. 20, No. S2 / OPTICS EXPRESS A358
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Acknowledgment The authors gratefully acknowledge the financial
support of the National Natural Science Foundation of China (Nos.
51102194 and 51121092), Natural Science Foundation of Shaanxi
Province (No. 2011JQ7017), Doctoral Program of the Ministry of
Education (No. 20110201120040) and National Basic Research Program
of China (No. 2009CB220000). J. Shi thanks Prof. J. Ye from
National Institute for Materials Science (NIMS), Japan for the help
with photocatalytic activity evaluation. S. Shen was supported by
the Fundamental Research Funds for the Central University from Xian
Jiaotong University, China. Additional support was provided by the
U.S. Department of Energy, Office of Energy Efficiency and
Renewable Energy.
#158507 - $15.00 USD Received 22 Nov 2011; revised 13 Jan 2012;
accepted 13 Jan 2012; published 9 Mar 2012(C) 2012 OSA 12 March
2012 / Vol. 20, No. S2 / OPTICS EXPRESS A359