Electronic structure and photocatalytic properties of Ag-La codoped CaTiO3

Article abstract of DOI:10.1016/j.jallcom.2011.11.142

Recently, ion-codoped semiconductor systems have been employed as photocatalysts with the objective of improving their photocatalytic activities under visible-light irradiation. In this paper, the effects of monovalence silver ion and trivalence lanthanum codoping into the photocatalytic activity of CaTiO₃ powder for overall water splitting were studied experimentally and theoretically. Pure and Ag-La codoped CaTiO₃ powder, prepared by sol-gel method which is assisted with ultrasonic technique for the first time, is further characterized by ultraviolet-visible (UV-vis) absorption spectroscopy. The UV-vis spectra indicate that the Ag ⁺-La³⁺ ions doping not only enhanced the photocatalytic activity under ultraviolet-visible (λ > 300 nm) light irradiation but also made the photocatalysts have visible light (λ > 400 nm) response. Photocatalytic activity of codoped CaTiO₃ powder for hydrogen evolution under UV light is increased dramatically than that of pure CaTiO ₃ powder when the doping amount is 3 mol%. The electronic structures of pure and codoped CaTiO₃ were investigated using density functional theory (DFT). The results of DFT calculation illuminate that the visible-light absorption bands in the Ag-La codoped CaTiO₃ catalyst are attributed to the band transition from the Ag 4d5s to the O 2p + Ti 3d hybrid orbital.

Full text of DOI:10.1016/j.jallcom.2011.11.142

Journal of Alloys and Compounds 516 (2012) 91–95
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Electronic structure and photocatalytic properties of Ag–La codoped CaTiO3
Hongjie Zhanga,∗, Gang Chena,∗, Xiaodong Heb, Jing Xuc
a Department of Chemistry, Harbin Institute of Technology, Harbin 150001, China
b School of Astronautics, Harbin Institute of Technology, Harbin 150001, China
c Heilongjiang Institute of Technology, Department of Materials and Chemistry Engineering, Harbin 150001, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Recently, ion-codoped semiconductor systems have been employed as photocatalysts with the objective
of improving their photocatalytic activities under visible-light irradiation. In this paper, the effects of
monovalence silver ion and trivalence lanthanum codoping into the photocatalytic activity of CaTiO3
powder for overall water splitting were studied experimentally and theoretically. Pure and Ag–La
codoped CaTiO3 powder, prepared by sol–gel method which is assisted with ultrasonic technique for
the first time, is further characterized by ultraviolet–visible (UV–vis) absorption spectroscopy. The
UV–vis spectra indicate that the Ag+–La3+ ions doping not only enhanced the photocatalytic activity
under ultraviolet–visible (ꢀ > 300 nm) light irradiation but also made the photocatalysts have visible
light (ꢀ > 400 nm) response. Photocatalytic activity of codoped CaTiO3 powder for hydrogen evolution
under UV light is increased dramatically than that of pure CaTiO3 powder when the doping amount is
3 mol%. The electronic structures of pure and codoped CaTiO3 were investigated using density functional
theory (DFT). The results of DFT calculation illuminate that the visible-light absorption bands in the Ag–La
codoped CaTiO3 catalyst are attributed to the band transition from the Ag 4d5s to the O 2p + Ti 3d hybrid
orbital.
Received 6 September 2011
Received in revised form
28 November 2011
Accepted 28 November 2011
Available online 6 December 2011
Keywords:
Codoped
Photocatalytic activity
Hydrogen evolution
Visible light
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
the design of visible-light driven materials. There are many reports
that the doped photocatalysts and semiconductor electrodes, such
The photocatalytic water splitting reaction driven by solar
energy is one of the attractive targets due to the urgent demand of
clean and renewable sources. Since TiO2 electrode was first stud-
ied for water decomposition under UV-light in 1972 [1], several
efforts have been made to improve the catalytic activity. Almost all
of the photocatalysts developed in the past two decades have been
composed of transition metal oxides involving octahedrally coor-
dinated metal ions such as Ti4+, Zr4+, Nb5+, and Ta5+ [2–4]. Among
the vast majority of metal oxide photocatalysts, perovskite-type
oxides are prominent for their broad diversity of properties. In an
ABO3 perovskite, varying the stoichiometry or doping with a cation
of a different valence state can, in principle, change the electronic
properties [5]. Therefore, one can control the composition of the
constituent cations and can potentially alter the electronic struc-
ture originated from a donor or an acceptor level occurring in the
forbidden band of a perovskite oxide by doping a foreign element.
On the other hand, the doping of foreign elements into active pho-
tocatalysts with wide band gaps in order to make a donor or an
acceptor level in the forbidden band is one of the effective ways in
as TiO2 [6–8] and SrTiO3 [9–11] respond to visible light. CaTiO3
(CTO), whose mineral name is perovskite, is one of the best-known
wide gap oxides. The Eg of CTO is about 3.5 eV and the material is an
insulator. However, careful donor-doping makes it conductive [12].
The low cost, the ease of synthesis of the material, and the extraor-
dinarily high chemical stability against acids led us to consider its
application as a photocatalyst in a wet environment [13].
The focus of this work was on the effects of sliver and lanthanum
codoped perovskite CaTiO3 on photocatalytic properties. CaTiO3
alone is active only under UV radiation, but the small amount dop-
ing of Ag and La ions can make CTO response to visible light. In the
present study, the band structures and the photocatalytic proper-
ties of the perovskite-type materials CTO and Ag–La codoped CTO
were investigated experimentally and theoretically, and the roles
of Ag+ and La3+ in the band structure were also discussed.
2. Experimental details
2.1. Preparation of materials
Ag–La codoped CaTiO3 powder was prepared by a sol–gel method. In order to
enhance the properties of the material, ultrasonic dispersing technique was used in
this process. Tetrabutyl titanate was dissolved in 20 mL anhydrous ethanol under
stirring, citric acid as chelating agent was added to tetrabutyl titanate solution under
a constantly magnetic stirring or an ultrasonic dispersing. After stirring vigorously
∗
Corresponding authors. Fax: +86 451 86413753.
E-mail addresses: mis-hongjie@163.com (HJ. Zhang),
gchen@hit.edu.cn (G. Chen).
0925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2011.11.142

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HJ. Zhang et al. / Journal of Alloys and Compounds 516 (2012) 91–95
Fig. 1. The supercell crystal structure of CaTiO3: (a) pure CTO and (b) Ag–La codoped CTO.
or dispersing for about 30 min at room temperature, the mixed stoichiometrically
solution of Ca(NO3)2, Ag(NO3) and La(NO3)3 was added drop-wise to the above tita-
nium organic solution slowly to avoid tetrabutyl titanate hydrolyzing. The solution
was kept under continuously stirring or dispersing at a temperature of 50 ◦C until it
became viscous, then the sol was transferred to an oven and dried at 120 ◦C for 12 h
to obtain the xerogel. When a dried gel was formed, it was burned by self-spread
process in order to remove organic compounds and nitric acid. The burning remains
were calcined at 850 ◦C for 7–10 h for decarbonization and homogenization of the
final samples.
2.2. Characterization of catalysts
The crystal structures of the sample powders were characterized by X-ray
diffractometer (XRD) with CuK␣ radiation. The surface areas were determined
by BET measurements (BF, ST2000). Diffuse reflection spectra were measured by
UV–vis spectrometry (PG, TU-1900) to estimate band gaps of photocatalysts. The
photocatalytic reactions proceed as follows: The photocatalyst powder (0.1 g) was
dispersed in aqueous methanol solutions (volume ratio 1/20) (420 mL) by a mag-
netic stirrer in an outer irradiation quartz cell, which was connected to a closed
gas-circulating system. The reactant solution was evacuated several times to ensure
complete air removal, followed by irradiation under a 350 W high-pressure Hg lamp
or 350 W high-pressure Xe lamp (>400 nm) via a Pyrex glass tube filled with NaNO2
aqueous solution to block ultraviolet (UV) light [14]. Gas evolution was analyzed by
gas chromatography (Agilent, GC-6820, TCD, Ar carrier).
The electronic band-structure calculation was based on the full-potential linear-
muffin-tin-orbital (FP-LMTO) method [15], which uses the generalized gradient
approximation (GGA) PW91 [16], an improvement of the local spin-density approx-
imation (LSDA) within DFT [17] that is known to be an efficient and accurate
scheme for solving the many-electron problem of a crystal. The density of the
Monkhorst–Pack k-point mesh is 6 × 6 × 6. The investigation is done using a super-
cell model based on the so-called large unit cell (LUC) approach [18], which has
very high reliability for studies of doping in crystals. The two calcium atoms are
replaced by impurity atoms of Ag and La, respectively, for CaTiO3 crystals as shown
in Fig. 1. A full geometry optimization is associated with the electronic structure
calculation.
3. Results and discussion
3.1. Characterization of materials
Fig. 2. X-ray diffraction patterns of (a) Ca1−2xAgxLaxTiO3 (x = 0.00–0.04) and (b) X-
ray diffraction peaks around 33.1◦ of Ca1−2xAgxLaxTiO3 (x = 0.00–0.04).
XRD patterns of pure CaTiO3 and Ag–La codoped calcium
titanate are shown in Fig. 2. The pattern can be indexed to the
CaTiO3 orthorhombic host lattice phase only, and no impurities
are detected. X-ray diffraction patterns of Ag–La codoped CTO
powders are very similar to each other. A careful comparison
of the (1 1 0) diffraction peaks in the range of 2ꢁ 33.1◦ (Fig. 2b)
showed that the peak positions are slightly shifted to higher angles
with the increase in the amounts of doped Ag–La. The shift indi-
cates that a part of Ag–La at least is homogeneously doped into
small shifts to higher angles observed in the diffraction patterns of
Ca1−2xAgxLaxTiO3 (x = 0.00–0.04) suggest the substitution of Ag and
La ions for Ca ions in the bulk which occupy A sites in perovskite
structures.
3.2. UV–vis diffuse reflection spectra
´
´
the CaTiO3 lattice. Ionic radii of La3+ (1.18 A) and Ag (1.15 A) are
+
˚
˚
Diffuse reflection spectra of the photocatalysts at room temper-
ature are shown in Fig. 3. The reflectivity spectrum was transformed
to absorbance intensity through Kubelka–Munk method. The onset
of absorbance spectra of these photocatalysts show a shift to
´
´
4+
closer to Ca2+ ion (1.00 A) than that of Ti (0.68 A) which is at
the position of B sites in perovskite structures. If Ti4+ is replaced
by Ag+ and La3+, a large shift should be observed. Therefore, the
˚
˚

HJ. Zhang et al. / Journal of Alloys and Compounds 516 (2012) 91–95
93
shows irregular ball morphology. Compared with Fig. 4a–e, Ag–La
codoped CaTiO3 (see Fig. 4b–e) have significant influence on the
morphologies of samples. The typical changes after Ag–La codoping
are that the grain size becomes smaller. The small particle is advan-
tageous for the photocatalytic activity. The decrease in the particle
size probably cause a decrease in the migration distance of pho-
togenerated electrons and holes to reach the reaction site on the
surface, which probably contributes to the highly photocatalytic
activities.
3.4. Band structure and photoabsorption properties
The band structure of Ag–La codoped CaTiO3 with Ag and La at
the Ca site (Fig. 1), which is the energetically preferable configu-
ration, is displayed in Fig. 5c. For a comparison, those of undoped
and Ag-doped CaTiO3 are also plotted (Fig. 5a and b, respectively).
The band structure of undoped CaTiO3 displays a direct band gap
about 2.45 eV at G (gamma point). Although the calculated band
gap of the undoped CaTiO3 is underestimated compared with the
experimental value (about 3.5 eV) due to the well-known limita-
tion of GGA, the characteristics of the band structure as well as
the relative variations of the band gap are expected to be quali-
tatively reasonable and reliable. A comparison of panels a and b
in Fig. 5 shows that some unoccupied acceptor states are intro-
duced above the valence band resulting in a band gap narrowing
of about 0.23 eV, while the conduction band shows no significant
change. The narrowed band gap may lead to the reduction of pho-
ton transition energy and thus redshifts of the optical absorption
edge into the visible light region in Ag-doped CaTiO3. Furthermore,
we must acknowledge that merely inducing visible light absorp-
tion does not guarantee satisfactory photocatalytic activity. It is
pointed out that the Ag-doped CaTiO3 displays a p-type conduc-
tivity character to keep charge balance (shown in Fig. 2b), whereas
p-type conductivity in the oxide semiconductor with wide band
gap is difficult to obtain, and these acceptor states are thermo-
dynamically unstable [10]. Consequently, sliver-doping inevitably
Fig. 3. UV–vis diffuse spectra of Ca1−2xAgxLaxTiO3 (x = 0.00–0.04).
longer wavelength with Ca2+ being substituted by Ag+ and La3+
,
the charge-transfer transition between Ag ion 4d5s electrons to
the O 2p + Ti 3d hybrid orbital may explain the appearance of the
red-shift. For the samples of Ag–La codoped CaTiO3, two types
of absorption are generated in the visible light region, namely, a
strong absorption in the UV region shorter than 360 nm, and a broad
absorption extending into the visible region. Ag–La codoping in cal-
cium titanate introduces a band in the visible region at <550 nm
along with the main CTO peak at 360 nm. The absorption band at
550 nm is due to the transition of Ag 4d5s electrons to conduction
band (CB).
3.3. Morphology
The typical SEM images of nondoped-CaTiO3 and Ag–La codoped
CaTiO3 powders prepared by the same conditions are illus-
trated in Fig. 4. The images demonstrate that the material
Fig. 4. SEM images of Ca1−2xAgxLaxTiO3: (a) x = 0.00, (b) x = 0.01, (c) x = 0.02, (d) x = 0.03, and (e) x = 0.04.

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HJ. Zhang et al. / Journal of Alloys and Compounds 516 (2012) 91–95
Fig. 5. Band structures of (a) CaTiO3, (b) CaTiO3 doped with Ag, and (c) CaTiO3
codoped with Ag–La.
Fig. 6. DOS of (a) CaTiO3, (b) CaTiO3 doped with Ag, and (c) CaTiO3 codoped with
Ag–La.
introduces oxygen vacancies in the CaTiO3 structure as a conse-
quence of electron transfer from the Ti3+ states derived from oxygen
vacancies to the partially occupied Ag 5s states during the sample
preparation process. Nevertheless, oxygen vacancies always play
an undesirable role as the recombination center of photogener-
ated electron-hole pairs during the photocatalytic reaction, which
badly decreases the photocatalytic activity. Therefore, the sliver (p-
type dopant) related defect bands must be passivated by another
n-type dopant for the intent of effective photocatalytic activity. La
is one of the promising candidates chosen for codoping with Ag.
The band structure character of Ag–La codoped CaTiO3 shown in
Fig. 5c indicates that the bandwidth of the valence band is fur-
ther expanded. The band gap narrows by about 0.55 eV compared
with that of the undoped CaTiO3, which thus may lead to visible
light responding. In this case, we can see that the valence band
maximum increases greatly compared to the pure CaTiO3, whereas
the change of conduction band is small indicating that the reducing
ability is not reduced significantly. Furthermore, Fig. 5c also shows
that the highest occupied level is pinned at the top of the valence
band, which means that Ag–La codoping can keep the charge
balance without the formation of undesired oxygen vacancies
and eliminate the disadvantages aforementioned in the Ag-doped
CaTiO3.
To examine the origin of the electronic structure modification,
the projected DOS (density of states) plots of the Ag–La codoped
CaTiO3 are presented in Fig. 6c. The projected DOS plots of undoped
CaTiO3 and Ag-doped CaTiO3 are also plotted in Fig. 5, panels a and
b, respectively, as reference. For undoped CaTiO3, the upper valence
band is dominantly composed of O 2p states and the conduction
band Ti 3d states. Some Ti 3d states sufficiently disperse within

HJ. Zhang et al. / Journal of Alloys and Compounds 516 (2012) 91–95
95
Table 1
BET surface area and H2 evolution of Ca1−2xAgxLaxTiO3.
Type of catalyst
Surface area
Activity
(m2 g−1
)
(␮mol h−1 gcat−1
)
H2/UV
H2/vis
CaTiO3
9.64
10.61
11.39
12.17
11.44
325.4
549.6
1053.8
1064.2
436.9
0.0
2.6
2.9
10.1
4.1
Ca0.98Ag0.01La0.01TiO3
Ca0.96Ag0.02La0.02TiO3
Ca0.94Ag0.03La0.03TiO3
Ca0.92Ag0.04La0.04TiO3
the O 2p states illuminating the covalent property of the Ti–O
bond. In addition, only a few Ca related electronic states appear
in the O 2p valence band, which demonstrates ionic interaction
between Ca and TiO6. It is noted that the bandwidth of O 2p states
in the upper valence band is about 4.25 eV. After the introduction of
substitutional Ag for Ca (Fig. 6b), some Ag 5s partially occupied
states are introduced above the valence band and overlap with O
2p states with the O 2p valence band expanded to be about 4.85 eV,
which results in the band gap narrowing. Fig. 6c shows that the
introduction of a La atom results in the O 2p valence band suffering
a further expansion to be about 5.12 eV, whereas La related states
have few contributions neither to the valence band nor to the con-
duction band due to the ionic interaction between La and TiO6. In
this situation, partially occupied Ag 5s states are present as totally
occupied. To keep charge balance, this Ag atom requires one more
electron from the CaTiO3 lattice than a Ca atom does, so the Ag may
act as a single acceptor. Meanwhile, the La atom releases one more
electron than a Ca atom to the CaTiO3 lattice and may act as sin-
gle donor. Spontaneously, the extra electron brought by La dopant
pairs up with the unpaired Ag 5 s electron in CaTiO3 lattice, namely,
the electron on the donor level passivates the same amount of hole
on the acceptor level, so that, this codoped system can still be of
semiconductor character. Neither acceptor levels nor donor levels
appearing within the band gap also confirm that the charge bal-
ance is maintained owing to the codoping of Ag and La, and thus
the photocatalytic activity under visible light can be improved to
an extensive degree. The charge compensation process, Ti3+ (d1)
change into Ti4+ (d0) and partially occupied Ag 5s states change into
completely occupied, indicates a stabilizing effect. Subsequently, it
can be deduced that there presents a donor-acceptor pair (DAP)
recombination [19] in the Ag–La codoped CaTiO3, in which both
acceptor levels and donor levels do not appear within the band gap
resulting in the codoped CaTiO3 a tempting structure with a nar-
rowed band gap and the recombination center of photogenerated
carriers is suppressed. So the Ag–La codoped CaTiO3 system pos-
sesses high photostability and high photocatalytic activity under
visible light like reported in Refs. [9] and [10].
rate will be increased with the average distance between capture
traps is shorter, at the same time, because the solubility of doped
ions in the CaTiO3 is limited, the higher doped concentration can
lead to enrichment of doped ions on the surface of catalyst, which
makes the photocatalytic activity reduce. Photocatalytic activities
of Ag–La codoped CaTiO3 powder for hydrogen evolution under UV
and visible light are increased dramatically than that of pure CaTiO3
powder.
4. Conclusions
CaTiO3 codoped with Ag–La, prepared by sol–gel method cou-
pled with ultrasonic technique, was shown to be an effective
photocatalyst for water splitting. Theoretical calculations and
experimental studies have demonstrated that increase of photocat-
alytic activities both under UV and visible light are caused by the
Ag–La codoping. Neither acceptor levels nor donor levels appear-
ing within the band gap also confirm that the charge balance is
maintained owing to the codoping of Ag and La, and thus the photo-
catalytic activity under visible light can be improved to an extensive
degree.
Acknowledgements
This work was supported by the National Nature Science Foun-
dation of China (20871036, 21071036), Natural Science Foundation
of Heilongjiang Province (ZD201011), and China Postdoctoral Sci-
ence Foundation (20100480982).
References
[1] A. Fujishima, K. Honda, Nature 238 (1972) 37–38.
[2] M.Y. Guo, M.K. Fung, F. Fang, X.Y. Chen, A.M.C. Ng, A.B. Djurisˇic, W.K. Chan, J.
Alloys Compd. 509 (2011) 1328–1332.
[3] J.X. Sun, G. Chen, Y.X. Li, C. Zhou, H.J. Zhang, J. Alloys Compd. 509 (2011)
1133–1137.
[4] G.K. Zhang, J. Gong, H.H. Gan, F. Lü, J.Alloys Compd. 509 (2011) 9791–9797.
[5] L.S. Ji, J.J. Li, W.P. Fang, J. Alloys Compd. 489 (2010) L13–L16.
[6] G.Q. Wang, W. Lan, G.J. Han, Y. Wang, Q. Su, X.Q. Liu, J. Alloys Compd. 509 (2011)
4150–4153.
[7] L.C. Ji, C.C. Wu, S. Han, N. Yao, Y.Y. Li, Z.B. Li, B. Chi, J. Pu, J. Li, J. Alloys Compd.
509 (2011) 6067–6071.
3.5. Photocatalytic activity for water splitting
[8] X. Liu, Z.Q. Liu, J. Zheng, X. Yan, D.D. Li, S. Chen, W. Chu, J. Alloys Compd. 509
(2011) 9970–9976.
[9] T. Puangpetch, T. Sreethawong, S. Chavadej, Int. J. Hydrogen Energy 35 (2010)
6531–6540.
[10] W.J. Dong, X.Y. Li, J. Yu, W.C. Guo, B.J. Li, L. Tan, C.R. Li, J.J. Shi, G. Wang, Mater.
Lett. 67 (2012) 131–134.
[11] T. Puangtch, P. Sommakettarin, S. Chavadej, T. Sreethawong, Int. J. Hydrogen
Energy 35 (2010) 12428–12442.
[12] H.J. Zhang, G. Chen, Y.X. Li, Y.J. Teng, Int. J. Hydrogen Energy 35 (2010)
2713–2716.
[13] K.A. Shimura, H. Miyanaga, H. Yoshid, Stud. Surf. Sci. Catal. 175 (2010) 85–92.
[14] Y.X. Li, G. Chen, Q. Wang, X. Wang, A.K. Zhou, Z.Y. Shen, Adv. Funct. Mater. 20
(2010) 3390–3398.
[15] O. Jepsen, O.K. Andersen, A.R. Mackintosh, Phys. Rev. B 13 (1975) 3060–3083.
[16] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865–3868.
[17] P. Hohenberg, W. Kohn, Phys. Rev. 136 (1964) B864–B871.
[18] A. Shluger, V. Stefanovich, Phys. Rev. B 42 (1990) 9664–9673.
[19] W. Wei, Y. Dai, M. Guo, L. Yu, B.B. Huang, J. Phys. Chem. C 113 (2009)
15046–15050.
The specific surface area (BET) and dependence of the photocat-
alytic activity of un-doped CaTiO3 and Ag–La codoped CaTiO3 under
UV light and visible light on the doping amount are listed in Table 1.
It can be seen from the table that the specific surface areas raise
monotonously with the continuous increments of codoped Ag–La
amounts, which agrees with the SEM results. The photocatalytic
activity increased with increasing doping amount to a maximum
at 3 mol%, above which the activity dropped gradually. It is well
known that the concentration of the doped ions exists an opti-
mal value. When the doped concentration is lower than optimal
doped concentration, there are not enough capture traps of charge
carriers in the semiconductor, so photocatalytic activity increased
with increasing of doped concentration; When the doped concen-
tration is higher than optimal doped concentration, recombination