Enhanced photocatalytic activity of La-doped AgNbO3 under visible light irradiation

Article abstract of DOI:10.1039/b810126d

Using a solid-state-reaction method, La-doped AgNbO₃ samples of Ag_1-xLa_xNbO₃ were synthesized and their photocatalytic activities were evaluated from gaseous 2-propanol decomposition under visible light irradiat

Full text of DOI:10.1039/b810126d

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Enhanced photocatalytic activity of La-doped AgNbO under visible
3
light irradiation†
Guoqiang Li,a,b,c Tetsuya Kako, Defa Wang, Zhigang Zou and Jinhua Ye*a,b
b
b
a,c
Received 16th June 2008, Accepted 24th November 2008
First published as an Advance Article on the web 12th February 2009
DOI: 10.1039/b810126d
Using a solid-state-reaction method, La-doped AgNbO
3
samples of Ag1-xLa
x
NbO were synthesized
3
and their photocatalytic activities were evaluated from gaseous 2-propanol decomposition under visible
light irradiation. The photocatalytic activity and the surface area were increased by La doping, but the
crystal structure and light absorption property did not change. The photocatalytic activity of
Ag1-xLa
x
NbO
3
was significantly improved over that of AgNbO
3
. The Ag0.88La0.12NbO sample displayed
3
the highest photocatalytic activity: it was more than 12-fold higher than that of AgNbO
3
.
1
Introduction
visible light irradiation. The La-doped AgNbO samples showed
3
enhanced photocatalytic activities compared to that of AgNbO .
3
Since the discovery of the Honda–Fujishima effect on the TiO
electrode, photocatalysis using TiO has attracted much attention
because of its extensive application to self-cleaning, sterilization
2
2
2 Experimental
1–4
and environmental purification.
However, TiO is difficult to be utilized for indoor application
2.1 Sample preparation
2
because it is sensitive only to UV light. The intensity of UV light
in indoor spaces is very low. Many attempts have been made to
The Ag1-xLa
x
NbO
3
(0 ≤ x ≤ 0.2) samples were prepared using
a solid-state-reaction (SSR) method. Powders of Ag O (99.0%,
Wako Pure Chemical Industries Ltd., Japan), La (99.99%,
Wako), and Nb
in a mortar. The mixture was pre-heated at 800 C for 4 h and then
calcined at 970 C for 7 h in air. Then we obtained the samples.
2
overcome the disadvantages of TiO
the development of novel visible-light-sensitive photocatalysts
2
photocatalyst. One attempt is
2
O
3
5–7
2
O
5
(99.9%, Wako) were well mixed with ethanol
because the intensity of visible light in indoor artificial illumination
is much higher than that of UV light. Among visible-light-
sensitive photocatalysts, Pb and Nb- or Ag and Nb-containing
◦
◦
For comparison, Ag0.97NbO
3
and Ag1-3xLa
x
NbO
3
(x = 0.01 and
complex oxides, such as PbBi
2
Nb
2
O
9
and AgNbO , are promising
3
0
.04) were synthesized using the same SSR method.
8–11
materials.
Recently, we found that AgNbO
ing AgNbO -NaNbO and AgNbO
activities than AgNbO
solid solutions was decreased. We expected that modified AgNbO
would show high activity. Furthermore, we sought AgNbO -based
materials that showed no decrease in the visible light-absorption
property of AgNbO
Metal-ion doping usually improves photocatalytic activity.
3
-based solid solutions includ-
-SrTiO showed much higher
although the visible light sensitivity of
2
.2 Characterization
3
3
3
3
1
2,13
3
,
Crystal structures of the samples were determined with an X-ray
diffractometer (XRD, RINT 2000, Rigaku Corp., Japan) operated
3
3
˚
at 30 kV and 40 mA using Cu Ka radiation (l = 1.54178 A).
◦
◦
The scanned range was 2q = 20–70 with a step of 2q = 0.02
3
.
and 0.5 s per step. The X-ray fluorescence spectrum (XRF) was
performed using an X-ray fluorescence spectrometer (APL-9800,
Switzerland). The diffuse reflectance spectrum was recorded using
a UV-vis spectrophotometer (UV-2500, Shimadzu Corp., Japan)
and transformed to the absorption spectrum according to the
14,15
3
+
Especially, doping of La reportedly improves the activity and
surface area of a photocatalyst, but only slightly affects its band
1
6–18
3+
gap.
Moreover, the ionic radius of La (0.116 nm; CN = 8)
3
+
resembles that of Bi (0.116 nm), which can be doped into the Ag
site of AgNbO
the Ag site of AgNbO
2
Kubelka–Munk relationship: K/S = (1 - R) /2R, where R, K,
19,20
3+
3
,
suggesting the possibility of doping La at
. In this study, several La-doped AgNbO
and S represent the value of reflectance measurements (relative
3
3
value to the reference of BaSO
4
) and the absorption and scattering
samples were prepared and their photocatalytic activities were
evaluated using gaseous 2-propanol (IPA) decomposition under
21
coefficients of a sample, respectively. The morphology was ob-
served by using a scanning electron microscope (SEM, JSM-7000,
JEOL, Japan) and transmission electron microscope (TEM, JEM-
a
Department of Materials Science and Engineering, Nanjing University,
2
00CX, JEOL, Japan). The X-ray photoelectron spectroscopy
Nanjing, 210093, PR China
b
(XPS) was obtained using a Thermo ESCALAB 250 physical
electronics photoelectron spectrometer with monochromatized Al
Ka X-ray radiation (1486.6 eV). The energy resolution of the
electrons analyzed by the hemi-spherical mirror analyzer is about
Photocatalytic Materials Center (PCMC), National Institute for Mate-
rials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047, Japan.
E-mail: jinhua.ye@nims.go.jp; Fax: +81 29 859 2301; Tel: +81 29 859 2646
c
Ecomaterials and Renewable Energy Research Center (ERERC), Depart-
ment of Physics, Nanjing University, Nanjing, 210093, PR China
0
1
.2 eV. The binding energy was determined by reference to the C
s line at 284.8 eV. The surface area was evaluated from nitrogen
†
Electronic supplementary information (ESI) available: Additional
XRD patterns, XPS spectra and SEM and TEM images. See DOI:
0.1039/b810126d
1
adsorption at 77 K using the Brunauer–Emmett–Teller (BET)
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method with a surface area analyzer (Micrometrics Co., USA).
The BET surface areas for Ag1-xLa NbO
(x = 0, 0.03 and 0.12)
were 0.6, 0.7 and 0.9 m g , respectively.
x
3
2
-1
2
.3 Evaluation of photocatalytic activity
The photocatalytic activity was evaluated from decomposition of
2
0
-propanol over the photocatalyst under visible light irradiation:
.4 g of the powder sample was spread evenly on the bottom of
2
a dish with a 8.1 cm area; the dish was placed on the base of a
5
00 mL reactor. After the reactor was sealed with a quartz cover
and the inside atmosphere was replaced by synthetic air, gaseous
IPA was introduced into the reactor. Through this operation, the
concentration of IPA in the reactor became about 370 ppm. The
reactor was stored in the dark until the concentration of IPA ceased
changing and the system reached the adsorption equilibrium.
Then the reactor was irradiated with visible light (400 nm < l <
5
50 nm), which was emitted from a Xe lamp using a set of
cutoff filters (L-42, B390 and HA-30, Hoya Corp., Japan) and
a water filter. The concentrations of IPA, acetone and CO were
2
measured on a gas chromatograph (GC-14B, Shimadzu Corp.,
Japan) equipped with a flame ionization detector (FID) and a
-
2
methanizer. The visible light intensity was set to 1.6 mW cm ,
as estimated using a spectroradiometer (USR-40D, Ushio Inc.,
Japan). The apparent quantum yields for the typical samples
were calculated using the following equation: Apparent quantum
yield (%) = {(numbers of generated acetone per hour)/(numbers
of the incidence photon per hour)} ¥ 100. As a reference, the
photocatalytic activity of P25, which is a commercially available
photocatalyst with high activity under UV light irradiation was
measured under the same irradiation conditions. Taking the large
surface area of P25 and screening effect of an excess catalyst into
consideration, the mass of P25 is 0.02 g.
Fig. 1 (a) X-Ray diffraction patterns of Ag1-xLa
the references, the standard patterns for AgNbO
x
NbO
3
(0 ≤ x ≤ 0.2). As
3
and Ag are also plotted.
The peaks of AgNbO
3
of which the intensity is over 5% are assigned.
◦
(
b) The enlarged view between 24–40 . The marks (᭹) and (ꢀ) assign the
4 2 3
peaks of LaNbO and La O , respectively. The standard pattern of Ag
appears as the dotted line.
3
Results and discussion
3
.1 Catalyst characterization
Fig. 1a shows the X-ray diffraction (XRD) patterns of
Ag1-xLa
x
NbO (0 ≤ x ≤ 0.2). As references, the standard patterns
for AgNbO (PDF# 01-070-4738) and Ag (PDF# 01-87-0597)
are also shown. The samples showed a similar structure to that
of AgNbO . However, with an increase in the La content, the
3
3
3
amount of metallic silver as the impurity increased, judging from
◦
changes in the peak height (2q = 38.02 ). The amount of Ag
in Ag0.97La0.03NbO
3
and Ag0.88La0.12NbO were estimated to be
3
0
.9 atm% and 4.1 atm%, respectively, from the XRD results using
the outer standard method. Moreover, very small amounts of
LaNbO and La impurities were found when the La content
4
2
O
3
was larger than 0.03 (see Fig. 1b).
As described above, the XRD patterns of the La-doped samples
Fig. 2 Lattice parameters of Ag1-xLa
results (0 ≤ x ≤ 0.15).
x 3
NbO estimated from the XRD
resembled that of AgNbO
3
with orthorhombic perovskite struc-
ture. According to the splitting numbers of the main diffracted
To identify the chemical component, X-ray fluorescence spectra
(XRF) analysis of Ag1-xLa NbO
were carried out, as shown in Table 1. The La concentration in the
20,22
lines (see ESI, Fig. S1 and Table S1†),
the symmetry of the
x
3
(x = 0, 0.03 and 0.12) samples
La-doped samples Ag1-xLa NbO (0 < x < 0.2) was considered
x
3
to be orthorhombic, and the lattice parameters were calculated as
shown in Fig. 2. When the La content was over 0.05, the a and c
values increased slightly, suggesting that the lattice was distorted
because of higher doping concentrations.
samples increases with the rise in the initial addition amount of
La.
Fig. 3 portrays the optical absorption spectra of Ag1-xLa
x
NbO
3
(x = 0, 0.03 and 0.12). All samples can absorb visible light.
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2
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Table 1 Nominal and analytical chemical composition of Ag1-xLa
x = 0, 0.03 and 0.12)
x
NbO
3
at the initial reaction time, acetone is evolved selectively in this
(
25–27
reaction.
Fig. 5 shows the La-doping-content dependence
samples
) and P25 under visible light
irradiation. All the La-doped AgNbO samples show higher
activities than that of P25. For x less than 0.12, with an increase
in x, the acetone-evolution rates increased and Ag0.88La0.12NbO
showed the highest photocatalytic activity. The apparent quantum
yield of AgNbO are about
.4, 1.4 and 4.1%, respectively. From the inset of Fig. 5, it is
apparent that the acetone-evolution rates of Ag1-xLa NbO
(x =
.01, 0.02, 0.03 and 0.04) are higher than those of Ag1–3xLa NbO
x = 0.01 and 0.04). This superiority in photocatalytic activities
of the Ag1-xLa NbO probably derives from reduction of the
A site (Ag site) vacancies.
activities of La-doped AgNbO
NbO ) were higher than that of non-doped AgNbO
of the acetone-evolution rates over La-doped AgNbO
NbO NbO
3
Ag/Nb
La/Nb
(
Ag1-xLa
x
3
and Ag1-3xLa
x
3
Samples
Nominal
Analytical
Nominal
Analytical
3
AgNbO
Ag0.97La0.03NbO
Ag0.88La0.12NbO
3
1
0.97
0.88
0.90
0.88
0.74
0
0.03
0.12
0
0.05
0.22
3
3
3
3
, Ag0.97La0.03NbO
3
and Ag0.88La0.12NbO
3
0
x
3
0
x
3
(
x
3
11,28
In addition, the photocatalytic
3
photocatalysts (Ag1-xLa
x
NbO
3
and Ag1-3xLa
x
3
3
,
indicating that the La doping enhanced the photocatalytic activity.
Fig. 3 UV-vis diffuse reflectance spectra of Ag1-xLa
and 0.12).
x
NbO
3
(x = 0, 0.03
Furthermore, their band gaps, as estimated from the onset of
absorption edges, were 2.8 eV and independent of the La content.
However, the absorption of visible light over 450 nm increased
with the amount of La because the formation of metallic silver
increased. The other impurities, La
2
O
3
and LaNbO , did not affect
4
the visible light absorption properties of the sample because of
23
24
their large band gaps (La
2
O
3
: 5.4–5.6 eV, LaNbO 4.8 eV ).
4
Fig. 5 Acetone evolution rates over Ag1-xLa
P25 under visible light irradiation. The inset is acetone evolution rates
NbO
(x = 0, 0.01, 0.02, 0.03 and 0.04) and
(x = 0, 0.01 and 0.04) under visible light irradiation.
x
NbO
3
(0 ≤ x ≤0.2) and
The surface morphologies of the prepared samples were ob-
served by SEM, as shown in Fig. 4. The particles show some
facets, possibly due to the break-up of the aggregations along the
boundary between the particles when the sample was ground.
Also, the La-doped sample shows slightly irregularly shaped
possibly because the impurities inhibit the growth of particles. The
over Ag0.97NbO
Ag1–3xLa
3
, Ag1-xLa
x
3
x
NbO
3
The reason that Ag1-xLa
activity than AgNbO remains unclear, but in the present case,
the following three reasons are most likely. The first is related
x
NbO
3
showed a higher photocatalytic
3
average particle sizes of Ag1-xLa
x
NbO
3
(x = 0, 0.03 and 0.12) were
estimated to be 2.5, 2.1 and 1.4 mm, respectively. With an increase
in the La content (x), the particle sizes obviously decreased.
17,18
to the increased surface area resulting from La doping.
With
the decreased particle sizes, the diffusion distances of the photo-
generated holes from the bulk to the surface reaction sites shorten,
engendering an increase in the number of holes reaching the
reaction sites. Therefore, the activity often improves concomitantly
3
.2 Photocatalytic activity
The photocatalytic activity was evaluated from the average
acetone-evolution rate in decomposition of gaseous IPA because,
29,30
with increased surface area.
Fig. 4 SEM images of AgNbO
3
(a), Ag0.99La0.03NbO
3
(b) and Ag0.88La0.12NbO
3
(c).
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The second possible reason is related to the transport property
of electrons because the transport property of electrons on a
3
1,32
semiconductor is usually improved by electron doping.
The
XPS results imply that small amount of Nb are reduced (as
shown in Fig. S2†), which indicated that Ag1-xLa NbO samples
are electron-doping oxides. Therefore, the photo-generated elec-
trons on the Ag1-xLa NbO could quickly move to the surface.
5
+
x
3
x
3
This improved charge transfer of photo-generated electrons is
expected to decrease the electron-hole recombination and enhance
the photocatalytic activity. Similar results were also reported in
33
34
metal ion-doped TiO
2
and TiO2-x photocatalysts.
The third reason is related to the formation of Ag. Deposition
of a precious metal often improves the photocatalytic activity due
35–37
to the promotion of electron-hole separation,
but few studies
of the effects of Ag on the photocatalytic activity of AgNbO
3
have been carried out. To achieve this purpose, the samples with
different initial ratio of Ag to Nb were prepared by SSR method.
The back scatter SEM images and TEM images indicate that some
amounts of silver really exist on the surface randomly (see Fig. S3
38
and S4†). The relationship between the amount of Ag and the
photocatalytic activity was investigated, as portrayed in Fig. 6.
Results showed that an excess or lack of Ag in the preparation
reduced the photocatalytic activity; the optimal amount of Ag
was estimated as ca. 1 atm%, which was much smaller than the
amount of La-doping (12%) of the La-doped AgNbO
highest activity. Moreover, the activity of Ag1.01NbO was less than
one-fifth that of Ag0.88La0.12NbO . Therefore, we infer that metallic
silver’s effect on improvement in the activity of Ag0.88La0.12NbO
3
with the
3
3
3
was less likely, or that it had less effect, than the other
reasons.
Fig. 7 Variations in IPA, acetone and CO
irradiation time over Ag0.88La0.12NbO
2
concentrations with the
(a) and AgNbO (b).
3
3
4
Conclusions
Several La-doped AgNbO photocatalysts were synthesized using
3
a solid-state-reaction method; their photocatalytic activities were
evaluated from gaseous IPA decomposition under visible light ir-
radiation. La-doping did not change the band gaps of the samples,
which were remained at a constant value of 2.8 eV. Additionally,
La-doping enhanced the photocatalytic activity of AgNbO
NbO showed higher activity than Ag1-3xLa NbO
Possible reasons why Ag1-xLa
either AgNbO
3
and Ag1-xLa
x
3
x
3
.
x
x
NbO
NbO
3
showed higher activity than
are differences in (1) the
3
or Ag1-3xLa
3
surface area, (2) the mobility of the photo-generated electrons,
3) the amount of metallic silver formed or (4) defects of the A
site in the perovskite structures. Among the La-doped samples,
showed the highest photocatalytic activity, which
(
Fig. 6 Variations in acetone evolution rates over Ag
x
NbO
3
(x = 0.97, 1,
Ag0.88La0.12NbO
3
1
.01, 1.03 and 1.06) under visible light irradiation. The inset shows the
was over 12 times higher than that of AgNbO .
3
variation in the height of 106 peak (H%) with the initial ratio of Ag to
Nb (x).
Acknowledgements
The mineralization processes of IPA under visible light irra-
diation were carried out to investigate the oxidation property
The present research was partially supported by the Global
Environment Research Fund of the Japanese Government and
it was supported in part by a Grant-in-Aid for Young Scientists
from the Ministry of Education, Culture, Sports, and Technology
of Japan (MEXT) (no. 20750124) and Japan Society for the
Promotion of Science (JSPS). It was also supported by the
National Natural Science Foundation of China (no. 20528302),
the National Basic Research Program of China (973 program
well. Samples of Ag0.88La0.12NbO
3
and AgNbO
a typical sample and a reference, respectively. From Fig. 7, it is
showed a much higher activity
in mineralizing IPA to CO via acetone. These
3
were selected as
apparent that Ag0.88La0.12NbO
3
than AgNbO
3
2
findings further imply that the oxidation property of AgNbO
was improved by La doping.
3
2
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2
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Natural Science Foundation (no. BK2006718 and BK2006127).
Prof. Z. G. Zou would like to thank the Jiangsu Provincial Talented
Scholars Program.
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