Synthesis and catalytic activity of new Gd2BiSbO7 and Gd2YSbO7 nanocatalysts

Article abstract of DOI:10.1016/j.molcata.2010.02.003

Gd₂BiSbO₇ and Gd₂YSbO₇ were prepared for the first time and the structural and photocatalytic properties of Gd₂BiSbO₇ and Gd₂YSbO₇ were investigated. The results showed that Gd₂BiSbO₇ and Gd₂YSbO₇ crystallized with the pyrochlore-type structure, cubic crystal system by space group Fd3m. The lattice parameters a for Gd₂BiSbO₇ and Gd₂YSbO₇ were 10.70352(7) and 10.65365(1) ?, respectively. The band gaps of Gd₂BiSbO₇ and Gd₂YSbO₇ were estimated to be 2.081 and 2.396 eV, respectively. The photocatalytic degradation of rhodamine B (RhB) over Gd₂BiSbO₇ and Gd₂YSbO₇ was investigated under visible light irradiation. The results showed that Gd₂YSbO₇ and Gd₂BiSbO₇ owned higher catalytic activity compared with Bi₂InTaO₇. Moreover, Gd₂YSbO₇ showed higher catalytic activity compared with Gd₂BiSbO₇ for RhB photocatalytic degradation. The photocatalytic RhB degradation followed the first-order reaction kinetics, the apparent first-order rate constant k being 0.01766, 0.01906 and 0.00318 min^-1 with Gd₂BiSbO₇, Gd₂YSbO₇ and Bi₂InTaO₇, respectively. Complete removal of RhB was realized after visible light irradiation for 230 and 240 min with Gd₂YSbO₇ and Gd₂BiSbO₇. The reduction of the total organic carbon and the evolution of CO₂ revealed complete removal of RhB during the photocatalytic process by Gd₂YSbO₇ and Gd₂BiSbO₇. The possible photocatalytic degradation pathway of RhB was revealed under visible light irradiation. Crown Copyright

Full text of DOI:10.1016/j.molcata.2010.02.003

Journal of Molecular Catalysis A: Chemical 321 (2010) 1–9
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Editor’s Choice paper
Synthesis and catalytic activity of new Gd BiSbO and Gd YSbO nanocatalysts
2
7
2
7
a,∗
a
a
b
c
d
Jingfei Luan , Kun Ma , Bingcai Pan , Yongmei Li , Xiaoshan Wu , Zhigang Zou
a
State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Hankou Road 22, Jiangsu, Nanjing 210093, People’s Republic of China
State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Tongji University, Shanghai 200092, People’s Republic of China
National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, People’s Republic of China
b
c
d
Eco-Materials and Renewable Energy Research Center, Nanjing University, Nanjing 210093, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Gd BiSbO and Gd YSbO were prepared for the first time and the structural and photocatalytic proper-
2
7
2
7
Received 26 September 2009
Received in revised form 31 January 2010
Accepted 1 February 2010
ties of Gd2BiSbO7 and Gd2YSbO7 were investigated. The results showed that Gd2BiSbO7 and Gd2YSbO7
crystallized with the pyrochlore-type structure, cubic crystal system by space group Fd3m. The lattice
parameters a for Gd2BiSbO7 and Gd2YSbO7 were 10.70352(7) and 10.65365(1) Å, respectively. The band
gaps of Gd2BiSbO7 and Gd2YSbO7 were estimated to be 2.081 and 2.396 eV, respectively. The photocat-
alytic degradation of rhodamine B (RhB) over Gd2BiSbO7 and Gd2YSbO7 was investigated under visible
light irradiation. The results showed that Gd2YSbO7 and Gd2BiSbO7 owned higher catalytic activity com-
pared with Bi2InTaO7. Moreover, Gd2YSbO7 showed higher catalytic activity compared with Gd2BiSbO7
for RhB photocatalytic degradation. The photocatalytic RhB degradation followed the first-order reac-
Available online 6 February 2010
Keywords:
Gd2BiSbO7
Gd2YSbO7
Structural property
Photocatalytic degradation
Visible light irradiation
−
1
tion kinetics, the apparent first-order rate constant k being 0.01766, 0.01906 and 0.00318 min with
Gd2BiSbO7, Gd2YSbO7 and Bi2InTaO7, respectively. Complete removal of RhB was realized after visible
light irradiation for 230 and 240 min with Gd2YSbO7 and Gd2BiSbO7. The reduction of the total organic
carbon and the evolution of CO2 revealed complete removal of RhB during the photocatalytic process by
Gd2YSbO7 and Gd2BiSbO7. The possible photocatalytic degradation pathway of RhB was revealed under
visible light irradiation.
Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1
. Introduction
Novel photocatalysts have attracted extensive attention from
light [66–70] and under visible light [27,71–75]. Within the context
of visible light photodegradation of rhodamine B we may outline
the work of Zhu et al. [27] utilizing Bi WO , the work of Li and Ye
2
6
both academic and industrial organizations [1–26] since the first
manuscript of Honda and Fujishima in 1972, where electrochemi-
cal photolysis of water at a semiconductor electrode was reported
[71] utilizing Pb Nb O13/fumed SiO2 composite and the work of
Zhao et al. [72] who used TiO2 nanostripe.
3 4
Visible light photodegradation phenomena are not limited to
titanium dioxide. In fact other oxides, and in particular mixed
oxides such as A B O7 compounds are often considered to have
[
1]. Recently, some photocatalysts with different structures have
been prepared to investigate the effective utilization of solar energy
12,17,27–35]. In particular, many scientific investigations on the
2
2
[
photocatalytic properties. In our previous work [45], we have found
photocatalytic degradation of aqueous organic contaminants have
been reported [15,36–62]. Within all categories of dyestuffs, rho-
damine B is one of the most important representatives of xanthene
dyes, and it is widely used as a photosensitizer, a quantum counter
and an active medium in dye lasers, etc. [63,64]. However, rho-
damine B is resistant to biodegradation and direct photolysis, and as
a N-containing dye, it undergoes natural reductive anaerobic degra-
dation, yielding potentially carcinogenic aromatic amines [27,65].
Therefore, it is urgent and important to investigate the degradation
of rhodamine B. Rhodamine B is often used as a probe contaminant
to evaluate the activity of a photocatalyst both under ultraviolet
that Bi InTaO7 crystallizes with the pyrochlore-type structure, acts
2
as a photocatalyst under visible light irradiation and seems to
have potential for activity improvement upon modification of its
structure. Along this line, it can be postulated that substitution of
5
+
5+
3+
3+
3+
Ta by Sb , substitution of In by Bi or substitution of In by
Y
3
+
, and substitution of Bi by Gd3+ in this compound may lead
3+
to an increase about carriers concentration, which may result in
improved photocatalytic properties.
Gd YSbO7 and Gd BiSbO7 are semiconductor compounds that
2
2
were never synthesized before. The similarity between the molec-
ular composition of these two compounds and other A B O7
2
2
compounds suggests that these two compounds may possess
photocatalytic properties under visible light irradiation and may
be to those of other members of the family. This contribution
reports the preparation and characterization of Gd2YSbO7 and
∗ Corresponding author. Tel.: +86 0 13585206718; fax: +86 0 2583707304.
E-mail address: jfluan@nju.edu.cn (J. Luan).
1
381-1169/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.02.003

2
J. Luan et al. / Journal of Molecular Catalysis A: Chemical 321 (2010) 1–9
Gd BiSbO7. The structural, photophysical and photocatalytic prop-
ment Factory Beijing Normal University, China) was determined
2
−
6
−1 −1
erties of Gd YSbO7 and Gd BiSbO7 were investigated in detail.
to be 4.76 × 10 Einstein L
s under visible light irradiation
2
2
A comparison among the photocatalytic properties of Gd YSbO7,
(wavelength range of 400–700 nm). The incident photon flux on
the photoreactor was varied by adjusting the distance between
the photoreactor and the Xe arc lamp. No pH adjustment was
done and the initial pH value was 7.0. The concentration of RhB
was determined based on the absorption at 553.5 nm as mea-
sured by a UV–vis spectrophotometer (Lambda 40, PerkinElmer
Corporation, USA). The inorganic products obtained from RhB
degradation were analyzed by ion chromatograph (DX-300, Dionex
Corporation, USA). The identification of RhB and its degrada-
2
Gd BiSbO7 and Bi InTaO7 was completed in order to elucidate the
2
2
structure–photocatalytic activity relationship in these newly syn-
thesized compounds.
2
. Experimental
The novel photocatalysts were synthesized by a solid-state reac-
tion method. Sb O5, Gd O , Bi O , Y O , In O , and Ta O5 with
2
2
3
2
3
2
3
2
3
2
purity of 99.99% (Sinopharm Group Chemical Reagent Co., Ltd.,
Shanghai, China) were used as starting materials. All powders
were dried at 200 C for 4 h before synthesis was performed. In
tion intermediate products were performed by GC–MS (HP 6890
TM
Series Gas Chromatograph (AT
column, 20.3 m × 0.32 mm, ID of
◦
◦
0.25 ␮m)) operating at 320 C, which was connected to HP 5973
mass selective detector, and to a flame ionization detector with
H2 as the carried gas. Intermediate products were measured by
LC–MS (Thermo Quest LCQ Duo, USA, Beta Basic-C18 HPLC column:
150 mm × 2.1 mm, ID of 5 ␮m, Finnigan, Thermo, USA). Here, 20 ␮L
of post-photocatalysis solution was injected automatically into the
LC–MS system. The eluent contained 60% methanol and 40% water,
order to synthesize Gd YSbO7, the precursors were stoichiomet-
2
rically mixed, then pressed into small columns and put into an
alumina crucible (Shenyang Crucible Co., Ltd., China). Finally, calci-
◦
nation was carried out at 1320 C for 50 h in an electric furnace
(
KSL 1700X, Hefei Kejing Materials Technology Co., Ltd., China).
◦
Similarly, Gd BiSbO7 was prepared by calcination at 1040 C for
3
4
2
◦
−1
0 h, and Bi InTaO7 was prepared by calcination at 1050 C for
and the flow rate was 0.2 mL min . MS conditions included an
2
◦
6 h. The crystal structures of Gd YSbO7 and Gd BiSbO7 were ana-
electrospray ionization interface, a capillary temperature of 27 C
2
2
lyzed by the powder X-ray diffraction method (D/MAX-RB, Rigaku
Corporation, Japan) with CuK␣ radiation (ꢀ = 1.54056). The data
were collected at 295 K with a step-scan procedure in the range
with a voltage of 19.00 V, a spray voltage of 5000 V and a constant
sheath gas flow rate. The spectrum was acquired in the negative ion
−
1
scan mode, sweeping the m z range from 50 to 600. Evolution of
◦
◦
TM
of 2ꢁ = 10–100 . The step interval was 0.02 and the time per step
was 1.2 s. The chemical composition of the compound was deter-
mined by scanning electron microscope-X-ray energy dispersion
spectrum (SEM-EDS, LEO 1530VP, LEO Corporation, Germany) and
X-ray fluorescence spectrometer (XFS, ARL-9800, ARL Corporation,
CO2 was analyzed with an intersmat IGC120-MB gas chromato-
graph equipped with a porapack Q column (3 m in length and an
inner diameter of 0.25 in.), which was connected to a catharometer
detector.
The total organic carbon (TOC) concentration was determined
with a TOC analyzer (TOC-5000, Shimadzu Corporation, Japan). The
photonic efficiency was calculated according to the following equa-
tion [76,77]:
3
+
3+
3+
Switzerland). The oxygen content, Bi content, Gd content, Y
5
+
content and Sb content of Gd YSbO7 and Gd BiSbO7 were deter-
2
2
mined by X-ray photoelectron spectroscopy (XPS, ESCALABMK-2,
VG Scientific Ltd., UK). The chemical composition within the depth
profile of Gd YSbO7 or Gd BiSbO7 was examined by the argon
R
ϕ =
Io
2
2
ion denudation method when X-ray photoelectron spectroscopy
was used. The optical absorption of Gd YSbO7 and Gd BiSbO7 was
where ϕ is the photonic efficiency (%), and R is the rate of RhB
2
2
−
1
−1
s ), and Io is the incident photon flux (Ein-
analyzed with an UV–vis spectrophotometer (Lambda 40, Perkin-
Elmer Corporation, USA). The surface areas were measured by the
Brunauer–Emmett–Teller (BET) method (MS-21, Quantachrome
Instruments Corporation, USA) with N2 adsorption at liquid nitro-
gen temperature. The particle sizes of the photocatalysts were
measured by malvern’s mastersize-2000 particle size analyzer
degradation (mol L
stein L
−
1
−1
s ).
3. Results and discussion
3.1. Characterization
(
Malvern Instruments Ltd., UK). The particle morphology was mea-
sured by transmission electron microscope (Tecnal F20 S-Twin, FEI
Corporation, USA).
Fig. 1 presents TEM images of Gd YSbO7 and Gd BiSbO7, reveal-
ing nanosized particles and regular round shapes, having 20–40 nm
2
2
The photocatalytic degradation of rhodamine B (RhB) (Tian-
jin Kermel Chemical Reagent Co., Ltd.) was performed with 0.8 g
Gd YSbO7, Gd BiSbO7 or Bi InTaO7 powder suspended in 300 mL
in diameter for Gd YSbO7 particle size and having 40–60 nm in
2
diameter for Gd BiSbO7 particle size. It could be seen from Fig. 1
2
that the mean particle size of Gd YSbO7 was smaller than that of
2
2
2
2
0.0293 mM RhB solution in a pyrex glass cell (Jiangsu Yancheng
Gd BiSbO7. SEM-EDS spectrum taken from the prepared Gd YSbO7
2
2
Huaou Industry, China). Before irradiation, the suspensions were
magnetically stirred in the dark for 45 min to ensure establish-
indicated the presence of gadolinium, oxygen, yttrium and anti-
mony. SEM-EDS spectrum taken from the prepared Gd BiSbO7 also
2
ment of an adsorption/desorption equilibrium among Gd YSbO7,
indicated the presence of bismuth, oxygen, gadolinium and anti-
mony. Other elements could not be identified.
2
Gd BiSbO7, Bi InTaO7, the RhB dye and atmospheric oxygen. The
2
2
photocatalytic reaction system consisted of a 300 W Xe arc lamp
with the main emission wavelength at 436 nm (Nanjing JYZCPST
Co., Ltd.), a magnetic stirrer and a cut-off filter (ꢀ > 400 nm, Jiangsu
Nantong JSOL Corporation, China). The Xe arc lamp was sur-
rounded by a quartz jacket and was positioned within the inner
part of a photoreactor quartz vessel (5.8 cm in diameter and
Figs. 2 and 3 present the powder X-ray diffraction patterns of
Gd YSbO7 and Gd BiSbO7, respectively together with full-profile
2
2
structure refinements of the collected data as obtained by the
TM
RIETAN
[78] program, which is based on Rietveld analysis. The
results of the final refinement for Gd YSbO7 and Gd BiSbO7 indi-
2
2
cated a good agreement between the observed and calculated
intensities for the pyrochlore-type structure and a cubic crystal
system having a space group Fd3m (O atoms were included in the
model). The lattice parameters a for Gd BiSbO7 and Gd YSbO7 were
6
8 cm in length), through which a suspension of RhB and pho-
tocatalyst was circulated. An outer recycling water glass jacket
◦
maintained a near constant reaction temperature (22 C), and
2
2
the solution was continuously stirred and aerated. 2 mL aliquots
were sampled at various time intervals. The incident photon flux
Io measured by a radiometer (Model FZ-A, Photoelectric Instru-
10.70352(7) and 10.65365(1) Å, respectively. All the diffraction
peaks for Gd YSbO7 and Gd BiSbO7 could be successfully indexed
based on the lattice constant and above space group. The atomic
2
2

J. Luan et al. / Journal of Molecular Catalysis A: Chemical 321 (2010) 1–9
3
Fig. 2. X-ray powder diffraction patterns and Rietveld refinements of
◦
Gd2YSbO7 prepared by a solid-state reaction method at 1320 C. A difference
(
observed − calculated) profile is shown beneath. The tic marks represent reflection
positions.
Fig. 3. X-ray powder diffraction patterns and Rietveld refinements of
◦
Gd2BiSbO7 prepared by
a
solid-state reaction method at 1040 C.
A
differ-
ence (observed − calculated) profile is shown beneath. The tic marks represent
Fig. 1. TEM images of (A) Gd2YSbO7 and (B) Gd2BiSbO7.
reflection positions.
catalyst with decrease of the M ionic radii, Y3+ (1.019 Å) < Bi3+
(1.17 Å).
coordinates and structural parameters of Gd YSbO7 and Gd BiSbO7
are listed in Tables 1 and 2, respectively. It could be seen from
2
2
Figs. 2 and 3 that not only Gd YSbO7 was a single phase but also
Our X-ray diffraction results showed that Gd2YSbO7, Gd2BiSbO7
and Bi2InTaO7 crystallized with a same pyrochlore-type structure.
The cubic system structure with space group Fd3m for Bi2InTaO7
2
Gd BiSbO7 was a single phase. In addition, Our XRD results showed
2
that Gd YSbO7 and Gd BiSbO7 crystallized with the same struc-
2
2
5+
5+
3+
ture, and 2ꢁ angles of each reflection of Gd BiSbO7 changed with
kept unchanged using Ta being substituted by Sb , In being
2
3+
3+
3+
3+
3+
Bi being substituted by Y . The lattice parameter decreased from
substituted by Bi and Bi being substituted by Gd . The cubic
a = 10.70352(7) Å for Gd BiSbO7 to a = 10.65365(1) Å for Gd YSbO7,
which indicated a decrease for lattice parameter of the photo-
system structure with space group Fd3m for Bi2InTaO7 also kept
2
2
unchanged using Ta5+ being substituted by Sb , In being sub-
5+
3+
Table 1
Table 2
Structural parameters of Gd2YSbO7 prepared by solid-state reaction method.
Structural parameters of Gd2BiSbO7 prepared by solid-state reaction method.
Atom
x
y
z
Occupation factor
Atom
x
y
z
Occupation factor
Gd
Y
Sb
O(1)
O(2)
0.00000
0.50000
0.50000
0.00000
0.50000
0.50000
0.12500
0.12500
0.00000
0.50000
0.50000
0.12500
0.12500
1.0
0.5
0.5
1.0
1.0
Gd
Bi
Sb
O(1)
O(2)
0.00000
0.50000
0.50000
0.00000
0.50000
0.50000
0.12500
0.12500
0.00000
0.50000
0.50000
0.12500
0.12500
1.0
0.5
0.5
1.0
1.0
−0.16519
−0.14538
0.12500
0.12500

4
J. Luan et al. / Journal of Molecular Catalysis A: Chemical 321 (2010) 1–9
Table 3
Binding energies (BE) for key elements.
Compounds
Bi4f7/2 BE (eV)
Sb3d5/2 BE (eV)
Y3d5/2 BE (eV)
Gd3d5/2 BE (eV)
O1s BE (eV)
Gd2YSbO7
Gd2BiSbO7
530.90
530.50
156.80
1189.20
1188.80
531.20
530.60
159.40
stituted by Y3+ and Bi3+ being substituted by Gd3+. The outcome
of refinements for Gd YSbO7 generated the unweighted R fac-
2
tors, RP = 12.16% with space group Fd3m. Similarly, the outcome
of refinements for Gd BiSbO7 generated the unweighted R fac-
2
tors, RP = 12.55% with space group Fd3m. Zou et al. [14] refined
the crystal structure of Bi InNbO7 and obtained a large R factor
2
for Bi InNbO7, which was owing to a slightly modified structure
2
model for Bi InNbO7. Based on the high purity of the precursors
2
that were used in this study and the EDS results that did not trace
any other elements, it was unlikely that the observed space groups
originated from the presence of impurities. Therefore, it was sug-
gested that the slightly high R factors for Gd YSbO7 or Gd BiSbO7
2
2
were due to a slightly modified structure model for Gd YSbO7 or
2
Gd BiSbO7. It should be emphasized that the defects or the dis-
2
order/order of a fraction of the atoms could result in the change of
structures, including different bond-distance distributions, thermal
displacement parameters and/or occupation factors for some of the
atoms.
The XPS spectra of Gd YSbO7 and Gd BiSbO7 were measured.
2
2
The various elemental peaks, corresponding to specific binding
energies are given in Table 3. The results further suggested that the
oxidation state of Gd, Y, Sb and O ions from Gd YSbO7 was +3, +3,
2
+
5 and −2, respectively. For Gd YSbO7, the average atomic ratios of
2
Gd:Y:Sb:O, based on averaging our XPS, SEM-EDS and XFS results
gave values of 2.00:0.97:1.02:6.98, respectively. Similarly, the oxi-
Fig. 4. Upper trace: action spectra of rhodamine B degradation with Gd2YSbO7
under visible light irradiation. Lower trace: absorption spectra of Gd2YSbO7.
dation state of Gd, Bi, Sb and O ions from Gd BiSbO7 was +3, +3, +5
2
and −2, respectively. For Gd BiSbO7, the average atomic ratios of
respectively, indicating that Gd2BiSbO7 possessed narrower band
gap compared with that of Gd2YSbO7 and the optical transition for
these oxides was directly allowed.
2
Gd:Bi:Sb:O, based on averaging our XPS, SEM-EDS and XFS results
gave values of 2.00:0.98:0.99:6.97, respectively. Hence, it could be
deduced that the resulting material was of high purity under our
preparation conditions. It was noteworthy that neither shoulders
3.2. Photocatalytic activity
nor widening of any XPS peaks of the Gd YSbO7 or Gd BiSbO7 were
2
2
observed, suggesting (albeit not proving) the absence of any other
phases.
In general, the process for photocatalysis by semiconductors
begins with the direct absorption of supra-band gap photons
and the generation of electron–hole pairs in the semiconduc-
Figs. 4 and 5 present the absorption spectra of Gd YSbO7
2
and Gd BiSbO7, respectively. In contrast to the well-known TiO2
2
whose absorption edge was at less than 380 nm, the newly synthe-
sized absorption edges of Gd YSbO7 and Gd BiSbO7 were found
2
2
to be at 479 and 499 nm, respectively, which were at the visi-
ble region of the spectrum. It was noteworthy that the apparent
absorption (defined hereby as 1-transmission) could not take into
consideration reflection and scattering. As a consequence, the
apparent absorbance at sub-band gap wavelengths (490–700 nm
for Gd YSbO7, and 520–700 nm for Gd BiSbO7) was higher than
2
2
zero.
For a crystalline semiconductor, the optical absorption near the
n
band edge followed the equation: [79,80] ˛hꢂ = A(hꢂ − Eg) . Here, A,
˛
, Eg and ꢂ are proportional constant, absorption coefficient, band
gap and light frequency, respectively. Within this equation, n deter-
mines the character of the transition in a semiconductor. Eg and n
can be calculated by the following steps: (i) plotting ln(˛hꢂ) versus
ln(hꢂ − Eg) assuming an approximate value of Eg and (ii) deduc-
ing the value of n based on the slope in this graph. (iii) Refining
1/n
the value of Eg by plotting (˛hꢂ) versus hꢂ and extrapolating the
1
/n
plot to (˛hꢂ) = 0. Based on this method, Fig. 6 shows the plot of
˛hꢂ)1 versus hꢂ for Gd YSbO7 and Gd BiSbO7. It was obviously
/n
(
2
2
from Fig. 6 that the value of Eg for Gd YSbO7 and Gd BiSbO7 was
2
2
Fig. 5. Upper trace: action spectra of rhodamine B degradation with Gd2BiSbO7
under visible light irradiation. Lower trace: absorption spectra of Gd2BiSbO7 and
rhodamine B.
calculated to be 2.396 and 2.081 eV, respectively, while the n values
of Gd YSbO7 and Gd BiSbO7 were calculated to be 0.42 and 0.48,
2
2

J. Luan et al. / Journal of Molecular Catalysis A: Chemical 321 (2010) 1–9
5
Fig. 6. Plot of (˛hꢂ)2 versus hꢂ for Gd2YSbO7 and Gd2BiSbO7.
Fig. 7. Photocatalytic degradation of rhodamine B under visible light irradiation
in the presence of Gd2YSbO7, Gd2BiSbO7, Bi2InTaO7 as well as in the absence of a
photocatalyst.
tor particles. This is followed by diffusion of the charge carriers
to the surface of the particle. Reduction changes in the UV–vis
spectrum of rhodamine B (RhB) upon exposure to visible light
2
in Fig. 8, which presents a linear correlation (R > 0.99) between
ln(C/Co) (or ln(TOC/TOCo)) and the irradiation time for the visible
light photocatalytic RhB degradation at the presence of Gd2YSbO7,
Gd2BiSbO7 or Bi2InTaO7. Here, C represents the RhB concentra-
tion at time t, and Co represents the initial RhB concentration, and
TOC represents the total organic carbon concentration at time t
and TOCo represents the initial total organic carbon concentration.
According to the relationship between ln(C/Co) and the irradia-
(
ꢀ > 400 nm) in the presence of Gd YSbO7 or Gd BiSbO7 were
2 2
realized, respectively. The measurements were performed under
−
3
oxygen-saturation conditions ([O ]sat = 1.02 × 10 M). The degra-
2
dation of RhB did not occur in the dark within Gd YSbO7/RhB
2
suspension or Gd BiSbO7/RhB suspension or Bi InTaO7/RhB sus-
2
2
pension or RhB suspension. As presented in Fig. 5 typical RhB
peaks at 553.5 and 525 nm are clearly noticed. A complete dis-
appearance of the absorption signal, indicating a complete color
change from deep pink into colorless solution was obtained
with Gd YSbO7 within 230 min and with Gd BiSbO7 within
2
2
2
2
40 min. Here, the initial rate of RhB degradation was about
−9
−1 −1
.123 × 10 mol L
s and the initial photonic efficiency was
estimated to be 0.04460% (ꢀ = 420 nm) for Gd YSbO7. Similarly, the
2
−
9
−1 −1
s
initial rate of RhB degradation was about 2.035 × 10 mol L
and the initial photonic efficiency was estimated to be 0.04275%
ꢀ = 420 nm) for Gd BiSbO7. In contrast, the photocatalytic effi-
(
2
ciency with Bi InTaO7 was inferior to that with Gd YSbO7 or
2
2
Gd BiSbO7. For example, within 220 min of exposure to the visi-
2
ble light, the RhB concentration decreased only from 0.0293 mM
to 0.0157 mM and the initial rate of RhB degradation was only
−
9
−1 −1
1
.030 × 10 mol L
s . The initial photonic efficiency was esti-
mated to be 0.02164% (ꢀ = 420 nm) for Bi InTaO7.
2
The kinetics of RhB degradation under visible light irradia-
tion was deduced based on the spectral changes and is presented
in Fig. 7, which depicts the kinetics with Gd YSbO7, Gd BiSbO7
2
2
and Bi InTaO7 as well as with the absence of a photocatalyst. As
2
expected, reduction of RhB signal in the control measurements,
taken in the absence of a photocatalyst, was pimping. In addi-
tion, the photodegradation conversion of RhB was 79.3%, 74.7% and
3
1.4% after visible light irradiation for 100 min with Gd YSbO7,
2
Gd BiSbO7 and Bi InTaO7 as catalysts, respectively. For compar-
2
2
ison, RhB photodegradation measured by us with fluorinated
titanium dioxide made according to the experimental scheme pub-
lished by Ref. [56] yielded, under the same experimental conditions,
no more than 59.4% of removal.
Based on above results, fast degradation rate was observed
with Gd YSbO7 and Gd BiSbO7, and the photocatalytic degrada-
2
2
tion activity of Gd YSbO7 or Gd BiSbO7 was higher than that of
2
2
Bi InTaO7, moreover, the photocatalytic degradation activity of
2
Gd YSbO7 was a little higher than that of Gd BiSbO7.
2
2
The first order nature of the photocatalytic degradation kinetics
with Gd YSbO7, Gd BiSbO7 and Bi InTaO7 is clearly demonstrated
Fig. 8. Observed first-order kinetic plots for the photocatalytic rhodamine B degra-
dation with Gd2YSbO7, Gd2BiSbO7 and Bi2InTaO7 under visible light irradiation.
2
2
2

6
J. Luan et al. / Journal of Molecular Catalysis A: Chemical 321 (2010) 1–9
tion time, the apparent first-order rate constant k was estimated to
ation. According to the relationship between ln(TOC/TOCo) and
the irradiation time, the apparent first-order rate constant k was
−1
−1
be 0.01906 min with Gd YSbO7, 0.01766 min with Gd BiSbO7
2
2
−1
−1 −1
with Gd YSbO7, 0.01532 min
2
and 0.00318 min with Bi InTaO7, indicating that Gd YSbO7 and
estimated to be 0.01695 min
with Gd BiSbO7 and 0.00307 min with Bi InTaO7, indicating that
2
2
−
1
Gd BiSbO7 were more suitable than Bi InTaO7 for the photocat-
2
2
2
2
alytic degradation of RhB under visible light irradiation, at the
same time, Gd YSbO7 was more suitable than Gd BiSbO7 for
the photocatalytic degradation of RhB under visible light irradi-
the photodegradation intermediates of RhB probably appeared
during the photocatalytic degradation of RhB under visible light
irradiation.
2
2
Fig. 9. Suggested photocatalytic degradation pathway scheme for rhodamine B under visible light irradiation in the presence of Gd2YSbO7 and Gd2BiSbO7.

J. Luan et al. / Journal of Molecular Catalysis A: Chemical 321 (2010) 1–9
7
Fig. 10. CO2 production kinetics during the photocatalytic degradation of rho-
damine B with Gd2YSbO7, Gd2BiSbO7 and Bi2InTaO7 under visible light irradiation.
Fig. 11. Disappearance of total organic carbon (TOC) during the visible light photo-
catalytic degradation of rhodamine B with Gd2YSbO7, Gd2BiSbO7 and Bi2InTaO7.
The photodegradation intermediates of RhB in our experi-
ment were identified as 1,2-benzenedicarboxylic acid, benzoic acid,
adipic acid, oxalic acid, malonic acid and pentanedioic acid. Based
on the intermediate products found in this work, a possible photo-
catalytic degradation pathway for RhB is proposed in Fig. 9. This
pathway was similar, but not identical to the one proposed by
Horikoshi et al. [81] for the photodegradation of RhB under ultra-
violet light and visible light illumination assisted by microwave
radiation using TiO2 as the photocatalyst. According to Zhang et al.
The photocatalytic performance of the new compounds
Gd YSbO7 and Gd BiSbO7, under visible light irradiation was
2
2
remarkable. This superior quality can be even more appreciated
if one considers the fact that the specific surface area of these
compounds is by far smaller than that of titanium dioxide. Here,
BET isotherm measurements of the three compounds gave a spe-
2
−1
2
−1
2
−1
cific surface area of 1.28 m g , 1.17 m g and 1.26 m g for
Gd YSbO7, Gd BiSbO7 and Bi InTaO7, respectively, which was
2
2
2
almost 36 times smaller than that of TiO which was measured
2
2
−1
.
[
75], the RhB photodegradation occurred via two competitive pro-
to be 46.24 m g
cesses: one process was N-demethylation, and the other process
was the destruction of the conjugated structure. Thus we consid-
ered that chromophore cleavage, opening-ring and mineralization
would be the main photocatalytic degradation pathway of RhB in
our experiment. RhB was converted to smaller organic species and
ultimately was mineralized together with other organic groups to
inorganic products such as CO2 and water. Fig. 10 shows the CO2
As depicted in Fig. 7, some decrease in the RhB UV–vis
absorbance signal was obtained under visible light irradiation
even in the absence of a photocatalyst. Here, the initial rate of
−
9
−1 −1
RhB degradation was estimated to be 0.074 × 10 mol L
s and
the photonic efficiency averaged after 220 min of exposure was
0.00155% (ꢀ = 420 nm). It was suggested that the observed disap-
pearance of RhB in the absence of a photocatalyst was due to direct
dye-sensitization, which was similar to the observation of Zhao et
al. [82] regarding alizarin red and X3B dyes.
yield during the photocatalytic degradation of RhB with Gd YSbO7,
2
Gd BiSbO7 or Bi InTaO7 under visible light irradiation. The results
2
2
showed that the CO yield increased gradually with increasing reac-
Figs. 4 and 5 show the action spectra of RhB degradation in the
2
tion time with these three photocatalysts. The CO production rate
presence of Gd YSbO7 and Gd BiSbO7 under visible light irradia-
2
2 2
with Gd YSbO7 or Gd BiSbO7 was higher than the CO yield with
tion. A clear photonic efficiency (0.02855% at its maximal point) at
2
2
2
Bi InTaO7, in line with the absorption curves (Figs. 4 and 5) of
2
wavelengths which corresponded to sub-Eg energies of the photo-
Gd YSbO7 and Gd BiSbO7. For example, the CO production fol-
catalysts (ꢀ from 479 nm to 700 nm) was observed for Gd YSbO7
2
2
2
2
lowing visible light irradiation for 200 min was 0.2407 mmol with
Gd YSbO7, 0.2359 mmol with Gd BiSbO7 and 0.1080 mmol with
and Gd BiSbO7. The existence of photonic efficiency at energies
2
2
2
where no photons were absorbed by the photocatalysts, and in
particular the correlation between the low-energy action spec-
trum and the absorption spectrum of RhB, clearly demonstrated
that any photodegradation at wavelengths above 479 nm should be
attributed to photosensitization by the dye RhB itself (Scheme I):
Bi InTaO7.
2
Total organic carbon (TOC) measurements (Fig. 11) revealed
segmental disappearance of organic carbon within 220 min of
exposure of a solution containing Gd YSbO7 or Gd BiSbO7 or
2
2
Bi InTaO7. The results showed that 73.16% or 68.60% or 30.47% of
TOC decrease was obtained after visible light irradiation for 100 min
2
Visible light
−→ RhB∗
(1)
Scheme I : RhB(ads)
(ads)
with Gd YSbO7 or Gd BiSbO7 or Bi InTaO7 as a photocatalyst. In
2
2
2
∗
RhB
+ Gd YSbO7 (or Gd BiSbO7)
succession, the complete mineralization of RhB with Gd YSbO7 or
(ads)
2
2
2
Gd BiSbO7 as the photocatalyst was achieved after 245 min irradi-
2
+
→
Gd YSbO7 (or Gd BiSbO7) (e) + RhB
(2)
2
2
(ads)
ation or 260 min irradiation due to the decrease of the TOC (100%).
The turnover numbers (the ratio between total amount of evolved
gas and catalyst that was used) after 60 h of reaction time under
visible light irradiation were calculated to be (with addition of
Gd2YSbO7 (or Gd2BiSbO7) (e) + O2 → Gd2YSbO7
•
−
(
or Gd BiSbO7) + O2
(3)
2
0.00879 mmol additional RhB in the solution after every 6 h irra-
diation) more than 1.12 and 1.01 for Gd YSbO7 and Gd BiSbO7,
According to this mechanism, RhB adsorbed on Gd YSbO7 or
2
2
2
respectively. These turnover numbers were enough to prove that
the reactions occurred catalytically. The reactions stopped when
the light was turned off in this experiment, showing the obvious
light response.
Gd BiSbO7 is excited by visible light irradiation. An electron is then
2
injected from the excited RhB to the conduction band of Gd YSbO7
2
or Gd BiSbO7 where the electron is scavenged by molecular oxy-
2
gen. Scheme I serves to explain the results obtained with Gd YSbO7
2

8
J. Luan et al. / Journal of Molecular Catalysis A: Chemical 321 (2010) 1–9
or Gd BiSbO7 under visible light irradiation, where Gd YSbO7 or
2
2
Gd BiSbO7 may serve at most to reduce recombination of electrons
2
and holes via the scavenging of electrons [83].
The situation was different below 479 nm, where the photonic
efficiency correlated well with the absorption spectra of Gd YSbO7
2
and Gd BiSbO7. This evidently showed that the mechanism which
2
was responsible for the photodegradation of RhB went through
band gap excitation of Gd YSbO7 and Gd BiSbO7. Although detailed
2
2
experiments about the effect of oxygen and water on the degrada-
tion scheme were not performed, it was sensible to assume that the
mechanism in the first steps was similar to the one observed for
Gd YSbO7 or Gd BiSbO7 under supra-band gap irradiation, namely
2
2
(
Scheme II):
Visible light
+
+ e−
Scheme II : Gd YSbO7 (or Gd BiSbO7) −→
h
(4)
(5)
(6)
2
2
−
•
−
e
+ O → O2
2
+
+ OH− → •OH
h
Previous luminescence studies have shown that the closer the
Fig. 12. Suggested band structures of Gd2YSbO7 and Gd2BiSbO7.
◦
M–O–M bond angle is to 180 , the more delocalized is the excited
state [84], and the charge carriers can move easily in the matrix.
The mobility of the photoinduced electrons and holes influences
the photocatalytic activity because high diffusivity increases the
probability that the photogenerated electrons and holes will reach
reactive sites on the catalyst surface. Based on above results, the
The presented results indicated that Gd YSbO7 (or Gd BiSbO7)/
2
2
(
visible light) photocatalysis might be regarded as a method for
practical treatment of diluted colored wastewater. Our Gd YSbO7
2
(
or Gd BiSbO7)/(visible light) photocatalysis system could be
lattice parameter a = 10.65365(1) Å for Gd YSbO7 was smaller than
2
2
utilized for decolorization, purification and detoxification in tex-
tile industries and printing and dyeing industries in semi-arid
the lattice parameter a = 10.70352(7) Å for Gd BiSbO7, thus the
2
photoinduced electrons and holes inside Gd YSbO7 were easier and
2
countries. We designed Gd YSbO7 (or Gd BiSbO7)/(visible light)
faster to reach the reactive sites on the catalyst surface compared
2
2
photocatalysis system without demanding chemical reagents or
using high pressure of oxygen or heating. The decolorized and
detoxified water were submitted to our new system for treatment
and the results showed that the Gd YSbO7 (or Gd BiSbO )/(visible
2 2
7
light) photocatalysis system might provide a valuable treatment for
purifying and reusing colored aqueous effluents.
with those of Gd BiSbO7, as a result, the photocatalytic degrada-
2
tion activity of Gd YSbO7 was higher than that of Gd BiSbO7. In this
2
2
◦
experiment, the Y–O–Sb bond angle was 136.795 and the Bi–O–Sb
bond angle was 118.764 , indicating that the Y–O–Sb or Bi–O–Sb
bond angle was close to 180 . Thus the photocatalytic activity of
◦
◦
Gd YSbO7 or Gd BiSbO7 was accordingly higher. The crystal struc-
2
2
tures of Gd YSbO7, Gd BiSbO7 and Bi InTaO7 were the same, but
2
2
2
their electronic structures were considered to be a little different.
For Gd YSbO7 or Gd BiSbO7, Sb was 5p-block metal element, and
4. Conclusions
2
2
Gd was 5d-block rare earth metal element, and Y was 4d-block
metal element, but for Bi InTaO7, Ta was 5d-block metal element,
Gd2YSbO7 and Gd2BiSbO7 were prepared by solid-state reaction
method for the first time. The structural, optical absorption and vis-
ible light photocatalytic properties of Gd2YSbO7 and Gd2BiSbO7
were investigated and compared with that of Bi2InTaO7. XRD
results indicated that Gd2YSbO7 and Gd2BiSbO7 crystallized with
the pyrochlore-type structure and cubic crystal system (space
group Fd3m). The lattice parameters of Gd2YSbO7 and Gd2BiSbO7
were found to be a = 10.65365(1) Å and a = 10.70352(7) Å. The band
gaps of Gd2YSbO7 and Gd2BiSbO7 were estimated to be about 2.396
and 2.081 eV such that Gd2YSbO7 or Gd2BiSbO7 showed a strong
optical absorption in the visible light region (ꢀ > 400 nm). Photo-
catalytic decomposition of aqueous solutions of RhB were observed
under visible light irradiation in the presence of Gd2YSbO7 or
Gd2BiSbO7 accompanied with the formation of end products such
as carbon dioxide and water. Complete removal of carbon was
obtained as indicated from TOC measurements with Gd2YSbO7
or Gd2BiSbO7 as a catalyst. Hence it could be concluded that
Gd2YSbO7 (or Gd2BiSbO7)/vis system might be regarded as an effec-
tive way for treating of textile industry wastewater. Gd2YSbO7 or
Gd2BiSbO7 also showed higher photocatalytic activity compared
with Bi2InTaO7 for RhB photocatalytic degradation under visible
light irradiation. The photocatalytic RhB degradation followed the
first-order reaction kinetics. The apparent first-order rate con-
2
indicating that the photocatalytic activity might be affected by not
only the crystal structure but also the electronic structure of the
photocatalysts. Based on above analysis, the difference of pho-
tocatalytic degradation of RhB among Gd YSbO7, Gd BiSbO7 and
2
2
Bi InTaO7 could be attributed mainly to the difference in their
2
crystalline and electronic structure.
Fig. 12 shows the suggested band structures of Gd YSbO7 and
2
Gd BiSbO7. Recently, the electronic structures of InMO4 (M = V, Nb
2
and Ta) and BiVO were reported by Oshikiri et al. based on the first
4
principle calculations [85]. The conduction bands of InMO4 (M = V,
Nb and Ta) were mainly composed of a dominant d orbital compo-
nent from V 3d, Nb 4d and Ta 5d orbitals, respectively. The valence
bands of the BiVO4 photocatalyst were composed of a small Bi 6s
orbital component and a dominant O 2p orbital component. The
band structures of Gd YSbO7 and Gd BiSbO7 should be similar to
2
2
InMO4 (M = V, Nb and Ta) and BiVO . Therefore, we concluded that
4
the conduction band of Gd YSbO7 was composed of Gd 5d, Y 4d
2
and Sb 5p orbital component and the valence band of Gd YSbO7
2
was composed of a small dominant O 2p orbital component. Sim-
ilarly, the conduction band of Gd BiSbO7 was composed of Gd 5d
2
and Sb 5p orbital component. The valence band of Gd BiSbO7 was
2
composed of a small Bi 6s orbital component and a dominant O 2p
−
1
orbital component. Direct absorption of photons by Gd YSbO7 or
stant k was 0.01906, 0.01766 or 0.00318 min
with Gd2YSbO7,
2
Gd BiSbO7 could produce electron–hole pairs in the catalyst, indi-
cating that the larger energy than the band gap was necessary for
decomposing RhB by photocatalysis.
Gd2BiSbO7 or Bi2InTaO7 as a catalyst. The possible photocatalytic
degradation pathway of RhB was revealed under visible light irra-
diation.
2

J. Luan et al. / Journal of Molecular Catalysis A: Chemical 321 (2010) 1–9
9
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