Visible light driven reduction of carbon dioxide with water on modified Sr3Ti2O7 catalysts

Article abstract of DOI:10.1039/c4ra11985a

Layered perovskite type Sr₃Ti₂O₇ catalysts, doped with N, S and Fe have been prepared by modified polymer complex method, characterized and evaluated for photo reduction of CO₂ in aqueous alkaline medium using U

Full text of DOI:10.1039/c4ra11985a

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Visible light driven reduction of carbon dioxide with
water on modified Sr Ti O catalysts
3
2 7
Cite this: RSC Adv., 2015, 5, 5958
ac
a
b
b
Velu Jeyalakshmi, Rajaram Mahalakshmy, Kanaparthi Ramesh, Peddy V. C. Rao,
b
b
c
Nettem V. Choudary, Gandham Sri Ganesh, Kandasamy Thirunavukkarasu,
Konda Ramasamy Krishnamurthy and Balasubramanian Viswanathan*
c
c
Layered perovskite type Sr
complex method, characterized and evaluated for photo reduction of CO
using UV-visible radiation. EDXA and XRD data reveal the incorporation of the dopants into the titanate
Ti O
3 2 7
catalysts, doped with N, S and Fe have been prepared by modified polymer
2
in aqueous alkaline medium
6+
3+
matrix. The presence of N in substitutional and interstitial locations, S as S and Fe as Fe species are
indicated by XPS analysis. DRS and photo luminescence studies show that the dopants form additional
energy levels within the band gap, promoting visible light absorption and retarding recombination of the
charge carriers. Morphological changes and smaller crystallites also minimize recombination. Layered
Received 8th October 2014
Accepted 8th December 2014
structure of Sr
reaction centres. Structural, morphological and photo physical characteristics of doped catalysts improve
the activity significantly. Sr Ti co-doped with N, S and Fe together, displays maximum apparent
quantum yield for CO reduction products.
3 2 7
Ti O facilitates easy transport of charge carriers and separation of oxidation/reduction
DOI: 10.1039/c4ra11985a
3
2 7
O
www.rsc.org/advances
2
promoters/co-catalysts have been explored for this application.
Mixed metal oxides that belong to the family of perovskites,
Introduction
9
10
The photocatalytic reduction of CO (PCRC) with water to yield ABO
hydrocarbons or articial photo synthesis is a topic for intense Au, CuO, NiO and RuO
investigations due to its scientic as well as technological for photo reduction of CO
importance. The process is considered as one of the options for layered structure like, ALa
moderation of global warming due to the rising levels of catalyst exhibit better performance, since the layered structure
atmospheric CO and the possible use of CO
as an alternative facilitates faster transport of charge carriers and the interlayer
source for energy. A wide range of binary/ternary and multi space could be used as oxidation/reduction reaction sites,
3
, like SrTiO
3
and NaTaO
display signicant and stable activity
. In addition, perovskites with
15 (A ¼ Ca, Sr, Ba) with Ag as co-
3
with various co-catalysts Pt, Ag,
2
2
2
1
Ti
O
4
4
11
2
2
2
3
12
component semi-conducting oxides have been explored for leading to the separation of charge carriers.
CO photo reduction. The most essential characteristics of a
viable catalyst for this application are
In this report, we present our results on CO
2
photo reduc-
2
4
tion on another layered perovskite Sr Ti , which in combi-
3
2 7
O
(
a) The valence band top energy level has to be more positive nation with NiO as co-catalyst, is known to be an efficient
13
with respect to the oxidation potential for water.
catalyst for photo catalytic splitting of water. Though the
9
(
b) The conduction band bottom energy level has to be parent perovskite SrTiO
more negative with respect to the reduction potential for best of our knowledge, there are no such reports on Sr
CO
As shown in Fig. 1, the conduction band bottom energy level is
TiO
3
, has been explored for PCRC, to the
Ti O .
2 7
3
2
.
are some of the oxides that suitable for the subsequent reduction of CO
2
aer the initial
is an ideal
2
, ZnO, CdS, GaP, SiC, SrTiO
3
ꢀ
satisfy the above criteria. While majority of the PCRC studies activation to form CO
2
c and hence Sr
3
Ti
2
O
7
5
are concerned with titania and its modied versions as cata- candidate for investigation, but for the wider band gap.
3 2 7
lysts, several mixed metal oxide semi-conductors like, Incorporation of suitable dopants could render Sr Ti O active
6
7
8
NaTaO
3
,
ZnGa
2
O
4
and Zn
2
GeO
4
along with a range of in the visible region. Increasing the life time of the photo
generated charge carriers is another important factor respon-
sible for improving the efficiency of the photo catalytic reduc-
a
Department of Chemistry, Thiagarajar College, Madurai Kamaraj University,
Madurai-625021, India
tion. In the present work, the effect of doping/co-doping
Sr Ti with N, S and Fe has been explored in order to
achieve reduction in the band gap energy and minimize the
b
3
2 7
O
Corporate R&D Centre, Hindustan Petroleum Corporation Ltd, Bangalore-560066,
India
c
National Centre for Catalysis Research (NCCR), Indian Institute of Technology charge carrier recombination.
Madras, Chennai-600036, India. E-mail: bvnathan@iitm.ac.in; Fax: +91 44
2
2574202; Tel: +91 44 22574241
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Evolution 600 spectrophotometer equipped with a Praying
Mantis diffuse reectance accessory.
Photo luminescence spectra were recorded under excitation
with a 450 W xenon lamp and the data were collected using
Jobin Yvon Fluorolog 3-11 spectrouorometer.
Surface area of the catalysts was measured using Micro-
meritics ASAP 2020. Samples were degassed at 373 K for 2 h and
at 423 K for 3 h. Pure nitrogen at liquid nitrogen temperature
(77 K) was used as the adsorbate.
Scanning electron micrographs and elemental analysis by
EDXA were recorded using FEI, Quanta 200 SEM unit with EDXA
attachment. The samples in powder form were taken on the
carbon tape and mounted on the SEM sample holder.
The X-ray photoelectron spectra of the catalysts were
recorded using Omicron Nanotechnology instrument with Mg
a
K radiation. The base pressure of the analysis chamber during
ꢀ
10
the scan was 2 ꢂ 10
millibar. The pass energies for indi-
vidual scan and survey scan are 20 and 100 eV, respectively. The
spectra were recorded with step width of 0.05 eV. The data were
processed with the Casa XPS program (Casa Soware Ltd, UK),
and calibrated with reference to the adventitious carbon peak
3 2 7
Fig. 1 VB & CB energy levels of Sr Ti O with respect to the potential
for reduction of CO and oxidation of water.
2
(284.9 eV) in the sample. Peak areas were determined by inte-
gration employing a Shirley-type background. Peaks were
considered to be a 70 : 30 mix of Gaussian and Lorentzian
functions. The relative sensitivity factors (RSF) were obtained
from literature.
Experimental
Preparation of catalysts
3 2 7
Neat Sr Ti O was prepared by adopting polymer complex
14
method, reported by Yoshino et al. aer some modications at
our end. Ethylene glycol and methanol ratio and pH of the
medium were further optimized to get phase pure Sr Ti O .
Photo catalytic reduction
3
2 7
Titanium tetra butoxide (2 moles) was added to a mixture of Activity of the catalysts in UV visible region (300–700 nm) was
ethylene glycol and methanol (1 : 2 mole ratio) with vigorous evaluated in batch mode using jacketed all glass reactor
15
stirring. To this mixture, citric acid (to get 1 : 0.5 mole ratio of (620 ml) tted with quartz window (5 cm dia) and lled with
glycol:citric acid) and strontium nitrate (3 moles as per the 400 ml of aqueous 0.2 N NaOH solution to increase the solu-
ꢁ
stoichiometry) were added. Heating the mixture at 130 C for bility of CO and act as hole scavenger. 0.4 g of catalyst was
2
2
0 h resulted in a polymer complex gel, which was pyrolized in dispersed in the alkaline solution with vigorous stirring
ꢁ
ꢁ
air at 350 C, followed by calcination at 900 C for 2 h. Urea or (400 rpm). Increasing the stirring rate beyond 400 rpm did not
thio-urea (2 moles in each case) as precursors for N doping and affect the conversion, indicating that under the present exper-
NS co-doping and Fe
Sr Ti ) as such, were introduced along with strontium nitrate, at this rate. Aqueous alkaline solution (pH-13.0) was saturated
prior to polyester formation to obtain doped Sr Ti . Doped with CO by continuous bubbling for 30 minutes aer which pH
catalysts are represented by the general formulae-Sr
2 3
O powder (3 wt% with respect to imental conditions, the mass transfer limitations are overcome
3
2 7
O
3
2 7
O
2
3
Ti
2
O
7
turned 8.0. Reactor in-let and out-let valves were then closed
and irradiation with Hg lamp with 77 W power (from WACOM
(
neat), Sr
3
Ti
2
O
(7ꢀx)
N
N ,
y
x
,
Sr
Sr Ti
3
3
Ti(2ꢀx)
(2ꢀxꢀy)
S
x
O
(7ꢀy)
N
y
,
Sr
Nz
3
x 7
Ti(2ꢀx)Fe O ,
Sr Ti Fe O
Fe S O
(7ꢀz)
signifying HX-500 lamp house) was started. Gas and liquid phase samples
at periodic intervals were taken out with gas-tight/liquid
syringes and analysed by GC. Liquid phase products (hydro-
carbons) were analysed on PoroPlot Q capillary column with FID
and gas phase products on Molecular Sieve 13ꢂ column with
3
(2ꢀx)
x
(7ꢀy)
x
y
different dopant compositions.
Characterization
The crystal phase of the catalysts was analysed by X-ray
TCD. Potassium ferric oxalate was used as standard for acti-
nometry. Apparent quantum yield (AQY) was calculated based
on the quantities of different products formed per hour and
using the formula
diffractometer (Rigaku-MiniFlex-II) using Cu Ka radiation (l ¼
˚
ꢀ1
ꢁ
1
3
.54056 A) with the scan range of 2q ¼ 5–90 at a speed of
ꢁ
min . The crystallite sizes were calculated by the Scherrer's
formula, t ¼ Kl/b cos q, where t is the crystallite size, K is the
number of reacted electrons
constant dependent on crystallite shape (0.9 for this case) and
AQYð%Þ ¼
ꢂ 100
number of incident photons
˚
l ¼ 1.54056 A, b is the FWHM (full width at half maximum) and
q is the Bragg's angle.
Diffuse reectance absorption spectra of the catalysts in the
Blank experiments (reaction with irradiation without catalyst
UV-visible region were recorded using a Thermo Scientic and reaction in dark with catalyst) were conducted to ensure
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2
that the products formed were due to photo reduction of CO .
When the solution with dispersed catalyst was purged, satu-
rated with nitrogen and irradiated, very small quantities of
hydrocarbons, possibly due to the conversion of residual carbon
on catalyst surface, was observed up to six hours, aer which no
product could be detected. However, on purging and saturation
2
with CO , hydrocarbons in increasing amounts up to 20 h and
beyond could be observed, thus establishing that the products
are actually due to PCRC. Based on these observations, with
each catalyst composite, the solution saturated with nitrogen
was rst irradiated for 12 h to remove hydrocarbons formed
Fig. 3 Shift in d-lines for doped Sr
3 2 7 3 2 7 3
Ti O for (a) Sr Ti O , (b) Sr Ti(2ꢀx)
x
O
7
3 x y (7ꢀz) z
Fe , (c) Sr Ti(2ꢀxꢀy)Fe S O N .
from carbon residues and then saturated with CO so that PCRC
2
on clean catalyst could be followed further for 20 h.
Addition of the dopants prior to polyester/gel formation has
ensured effective incorporation of the dopant elements N, S and
Fe into the titanate matrix. This aspect is conrmed by the
qualitative EDXA spectral data presented in Fig. 5. DRS proles
for neat and doped/co-doped (separately with N, N–S, Fe–N,
Results and discussions
Characterization of the catalysts
13
All the characteristic d-lines for Sr Ti O phase are observed in Fe–N–S and Fe) Sr
3
Ti
energy of 3.14 eV observed for neat Sr
The amount of dopants being small, no major changes in the is close to the value of 3.2 eV reported earlier.
XRD patterns for the doped samples are observed, except for a
When doped with N or co-doped with N and S (proles (b) &
small shi in d-lines for Fe doped samples, as shown in Fig. 3. (c) in Fig. 6) a distinct shi in light absorption edge, tending
2
O
7
catalysts are shown in Fig. 6. Band gap
3
2 7
the XRD patterns for neat and doped samples (Fig. 2).
3
Ti in the present case
2 7
O
13
3
+
This shi in d-line could be due to the location of doped Fe
towards visible region, is evident. With Fe, either alone or by co-
doping with N or N–S, the adsorption edge turns into a near
4
+
˚
˚
ions with ionic radius of 0.64 A in Ti ion (0.61 A) sites.
SE micrograph for the neat Sr Ti O as shown in Fig. 4 continuum, extending deeper into the visible region and indi-
3
2 7
reveals a distinct plate like morphology. Doping with N, S and cating excitations from two different energy levels within the
Fe brings out perceptible changes in the morphology. A gradual band gap. Reduced band gap energy values, observed for the
decrease in crystallite size, as measured by X-ray line broad- doped/co-doped catalysts are given in Table 1.
ening analysis of doped Sr
3
Ti
2
O
7
catalysts, is observed (Table 1).
Photo luminescence spectra of the catalysts (Fig. 7) bring out
Preparation by polymer complex method has resulted in additional features of doped catalysts.
2
ꢀ1
moderate BET surface area of 25 m g for the neat Sr
sample.
3
Ti
2
O
7
Undoped Sr Ti O shows two photo luminescence (PL)
3 2 7
emission lines at 470 nm and 482 nm with signicant inten-
sity, arising due to the recombination of charge carriers. With
N doping and co-doping of N and S, the intensity of the PL
lines is reduced to some extent. However, on doping with Fe
Fig. 2 XRD patterns for neat and doped Sr
, (c) Sr (d) Sr
, (g) Sr
3
Ti
2
O
7
: (a) Sr
3 2 7 3 2
Ti O , (b) Sr Ti -
O
(7ꢀx)
3
N
x
3
Ti(2ꢀx)
S
x
O
N
(7ꢀy)
z
N
y
3
Ti(2ꢀx)Fe
Ti(2ꢀxꢀy)Fe
x
O
7
, (e) Sr
3
Ti(2ꢀx)Fe
x
(7ꢀy) y
O N ,
Fig. 4 Changes in the morphology of doped Sr
micrographs. (a) Sr Ti , (b) Sr Ti , (c) Sr
d) Sr
3 2 7
Ti O catalysts-SE
(f) Sr
Ti(2ꢀxꢀy)Fe
x
S
y
O
(7ꢀz)
3
x
S
y
O
(7ꢀz)
N
z
-used.
3
2
O
7
3
2
O
N
(7ꢀx) x
3 x (7ꢀy) y
Ti(2ꢀx)S O N ,
(
3
x y (7ꢀz) z
Ti(2ꢀxꢀy)Fe S O N .
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Table 1 Crystallite size and band gap values for neat and doped
Sr Ti O catalysts
3 2 7
Crystalline
size (nm)
Band gap
(eV)
Photo catalysts
Sr
Sr
Sr
Sr
Sr
Sr
3
3
3
3
3
3
Ti
Ti
2
2
O
O
7
46.3
37.2
34.3
33.5
24.9
21.9
3.14
2.99
2.85
2.73
2.57
2.39
(7ꢀx)
N
x
x (7ꢀy) y
Ti(2ꢀx)S O N
Ti(2ꢀx)Fe
Ti(2ꢀx)Fe
x
O
7
x
O
(7ꢀy) y
N
x y (7ꢀz) z
Ti(2ꢀxꢀy)Fe S O N
and co-doping of Fe with N and N–S, sharp reduction in
intensity of the PL lines is observed. Decrease in the intensity
of PL lines indicates that the recombination of charge carriers
is retarded in presence of the dopants. Such an effect would
lead to an increase in the life time of the photo generated
electrons and holes and hence, an increase in PCRC activity.
These modications brought out by the dopants in the elec-
tronic structure of Sr Ti O have profound inuence on the
Fig. 6 Diffuse Reflectance spectra for neat and doped Sr
3
Ti
2
O
7
catalysts. (a) Sr
d) Sr
3
Ti
, (e) Sr
2
O
7
,
(b) Sr
Ti(2ꢀx)Fe
3
Ti
2
O
(7ꢀx)
N
x
,
(c) Sr
3
Ti(2ꢀx)
Ti(2ꢀxꢀy)Fe
S
x
O
(7ꢀy)
O
(7ꢀz)
N
N
y
,
3
2 7
(
3
Ti(2ꢀx)Fe
x
O
7
3
x
O
(7ꢀy)
N
y
, (f) Sr
3
x
S
y
z
.
PCRC activity.
Fig.
d) Sr
5
EDAX spectra and data for neat and doped Sr
3 2
Ti O
7
catalysts. (a) Sr
Ti O
3 2 7
,
3 2
(b) Sr Ti O
(7ꢀx)
N
x
,
3
(c) Sr Ti(2ꢀx)
S
x
O
(7ꢀy)
N ,
y
(
3 x y (7ꢀz) z
Ti(2ꢀxꢀy)Fe S O N .
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presents a broad prole, which could be resolved into a main
peak at 398.5 eV and another of low intensity at 400.6 eV. For Fe,
N and S co-doped sample N1s core level is observed at 400.1 eV.
N and S co-doped and Fe, N and S co-doped samples show S2p
1
/2
6
+
lines at 168.9 and 168.4 eV respectively, due to S species.
3
+
Fe2p3/2 lines at 709.4 & 714.0 eV (Fig. 9), are attributed to Fe
species.
Photo catalytic reduction of CO on neat and doped Sr Ti O
2 7
2
3
Typical trends in product distribution on the neat Sr Ti O and
3
2 7
Fe, N and S co-doped Sr Ti O , for photo reduction during 20 h
3
2 7
on stream period are presented in Fig. 10a and b, these exper-
iments were carried out up to 20 hours. Similar trends are
observed on other doped catalysts as well, with variations in the
rate and the quantities of products formed. Methanol is the
major product, followed by ethanol and acetaldehyde. Methane,
ethane and ethylene are formed in trace quantities. Hydrogen
and oxygen were also detected in the gas phase. All the catalysts
exhibit activity up to 20 h. Rates of formation of products are
high during initial 6–8 h beyond which the rate of product
formation tends to slow down. Based on the initial rates (upto
Fig. 7 Photo luminescence spectra for neat and doped Sr
3
Ti
2
O
7
catalysts. (a) Sr
d) Sr
3
Ti
2
O
7
,
(b) Sr
3
Ti
2
O
(7ꢀx)
x
N
x
,
(c) Sr
3
Ti(2ꢀx)
S
x
O
(7ꢀy)
N
O
y
,
(
3
Ti(2ꢀx)Fe
x
O
(7ꢀy)
N
y
, (e) Sr
3
Ti(2ꢀxꢀy)Fe
S
y
O
(7ꢀz)
z
N , (f) Sr
3
Ti(2ꢀx)Fe
x
7
.
ꢀ
1
ꢀ1
10 hours in micro moles g
h ) for the formation of different
products, and the number of photo electrons involved in each
case, apparent quantum yields (AQY) for all the catalysts have
been calculated and presented in Table 2 and Fig. 11.
Chemical states of the dopants have been analysed by XPS
technique (Fig. 8 and 9 and table therein). N1s core level for N
doped sample is observed at 398.4 eV. N and S co-doped sample
It is clear that doping/co-doping of Sr Ti O with Fe, N and S
3
2 7
has brought out signicant improvements in the photo catalytic
Fig. 8 XPS profiles for N1s core level for doped Sr
3 2
Ti O
7
catalysts (a) Sr Ti O
3 2 (7ꢀx)
x 3
N , (b) Sr Ti(2ꢀx)
S
O
x (7ꢀy)
N
y
, (c) Sr
3
x y (7ꢀz) z
Ti(2ꢀxꢀy)Fe S O N .
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Fig. 9 XPS profiles for S2p & Fe2p levels for Sr
3
x y (7ꢀz) z
Ti(2ꢀxꢀy)Fe S O N catalyst.
reduction of CO
XRD patterns for the fresh Fe, N and S doped Sr
2
, as indicated by the increase in AQY values. with N and S leads to further reduction to 2.85 eV. This is due to
Ti (Fig. 2f) the synergistic effect of co-doping, wherein, the 2p-states of
3
2 7
O
and for the same catalyst aer 20 h of use (Fig. 2g), do not reveal both N and S overlap with that of O2p, leading to narrowing of
any signicant changes, indicating structural stability of the the band gap. Such effects due to co-doping of N and S have
17,21,22
catalyst.
been observed in SrTiO and TiO .
3 2
N1s core level binding energy value of 398.4 eV observed for
N doped Sr Ti O , which is close to the value of 398.5 eV
reported for nitrogen doped SrTiO , indicating the presence of
3
3
2 7
Discussions-photo physical characteristics and activity
23
While the effect of doping SrTiO
ture) with anions like, N, S and metal oxides (of Fe, Co, Ni
and Mn) has been studied in detail,
knowledge, similar investigations on doped Sr Ti O phases
3
(with cubic perovskite struc-
anionic nitrogen in substitutional locations of oxygen. Co-
doping with N and S shows a main peak at 398.5 eV besides a
small intensity peak at 400.6 eV, while with Fe, N and S
co-doping, a broad major peak at 400.1 eV is observed, due to
nitrogen species in the interstitial locations involving N in
16–19
to the best of our
3
2
7
have not been reported so far. Structurally, Sr
related to SrTiO
Ruddlesden Popper-RP phases) formed from SrTiO
general formula SrO (SrTiO
with n ¼ 2. The structural models
of the possible RP phases are given in Fig. 12. In Sr Ti every
3
Ti
2
O
7
is closely
24,25
3
and is one of the intergrowth phases (known as
with a
different environments like, N–O–Ti–O or O–N–Ti–O.
observations are in line with the literature reports
nitrogen doping in TiO
These
on
26–28
3
3 n
)
, wherein two energy levels corre-
2
20
3
2 7
O
sponding to substitutional and interstitial nitrogen within the
band gap is proposed.
two cubic perovskite (SrTiO ) layer is separated by a single SrO
3
layer.
S2p1/2 lines observed for N, S and Fe, NS doped Sr
3 2 7
Ti O
The band gap energy values for both SrTiO and Sr Ti O are
3
3
2 7
samples at 168.9 and 168.4 eV (table in Fig. 8) respectively shows
that S is present as S species in the lattice. Presence of
6
+
nearly the same (ꢃ3.2 eV). While doping Sr
3 2 7
Ti O with N
reduces the band gap from 3.14 to 2.99 eV (Table 1), co-doping
Fig. 10 Trends in products distribution during CO
2
photo reduction (a) Sr
Ti O
3 2 7
3 x y (7ꢀz) z
and (b) Sr Ti(2ꢀxꢀy)Fe S O N catalysts.
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3 2 7
Table 2 Products distribution and quantum yield data for neat and doped Sr Ti O catalysts
ꢀ
1
ꢀ1
Products obtained from CO
CH
2
reduction (mmol g
h )
Photo catalysts
4
2
C H
4
C
2
H
6
CH OH
3
C
2
H
4
O
C
2
H
5
OH
C
3
H
6
O
3
C H
6
H
2
AQY (%)
Sr
Sr
Sr
Sr
Sr
Sr
3
3
3
3
3
3
Ti
Ti
Ti(2ꢀx)
Ti(2ꢀx)Fe
Ti(2ꢀx)Fe
x y (7ꢀz) z
Ti(2ꢀxꢀy)Fe S O N
2
2
O
O
7
0.16
0.07
0.01
0.23
0.22
0.13
0.08
0.01
0.04
0.55
0.23
1.1
0.03
0.02
0.14
0.12
0.19
0.3
19.9
21.2
41.3
29.9
48.9
60.1
1.5
3.1
0.1
0.7
0.35
3.99
1.9
2.1
3.4
6.1
7.8
9.9
—
—
0.8
2.7
3.1
3.5
0.04
0.08
0.01
0.01
0.08
0.04
1.7
1.0
0.6
0.3
0.5
0.7
0.003
0.005
0.006
0.006
0.009
0.011
(7ꢀx)
N
x
S
x
O
(7ꢀy)
N
y
x
O
7
x
O
(7ꢀy)
N
y
2
+
Fe states are created within the band gap. Excitation of elec-
trons from these levels to the conduction band of SrTiO
corresponds to light absorption in the visible region. Recently,
3
18
Zhou et al., based on DFT calculations on the electronic level
3
+
3
characteristics Fe doped SrTiO , have shown that Fe ions
4
+
could take up substitutional sites of Ti ions and some Fe 3d
states are located just above the top of the O2p valence band.
Such a conguration results in narrowing of the band gap.
Similar changes occurring in Fe doped Sr
the reduction in band gap values. Shi in the XRD d-lines
observed for Fe, N and S co-doped Sr Ti is indicative of the
location of doped Fe ions in the titania matrix (Fig. 3).
3 2 7
Ti O could explain
3
2 7
O
3
+
3
+
29
30
2
Incorporation of Fe in Sr
have been reported earlier.
3
Ti
2
O
7
and TiO
lattice networks
Apart from enabling the visible light absorption, Fe doping
plays a prominent role amongst the dopants, in increasing the
life time of charge carriers. Amongst several metal ions explored
Fig. 11 Apparent quantum yields for CO
doped Sr Ti catalysts. (a) Sr
, (d) Sr
2
photo reduction on neat and
3
+
as electron traps for titania, Fe is considered to be the most
3
2
O
7
3 2
Ti O
7
,
(b) Sr
3
Ti
2
O
O
(7ꢀx)
(7ꢀy)
N
N
x
,
,
3
1
3+
5
effective one as it can easily transform from stable Fe (d )
(
(
c) Sr
3
Ti(2ꢀx)
S
x
O
(7ꢀy)
N
y
3
Ti(2ꢀx)Fe
x
O
7
, (e) Sr
3
Ti(2ꢀx)Fe
x
y
2
+
6
f) Sr Ti(2ꢀxꢀy)Fe S
3 x y
O
(7ꢀz)
N
z
.
conguration to relatively unstable Fe (d ), which can again
3
+
transfer electron to form Fe . In this manner, Fe could effec-
tively trap electrons and release it to enable charge migration
and interfacial charge transfer. This aspect is very well reected
in the photoluminescence spectra of Fe doped samples (Fig. 7).
All the elementary steps in a typical photo chemical reaction
like, charge pair generation, charge trapping, charge release
3
2,33
and migration and interfacial charge transfer
facilitated by doping with Fe.
could be
34
2
As suggested by Cong et al. for TiO , N doping could help in
quenching photo luminescence by trapping of electrons in
oxygen vacancies and holes by the doped anionic nitrogen
species in the following steps:
0
V
CBꢀ
/ O
O
+ e
V
0
(1)
(2)
ꢀ
+
N
+ hVB / N
It is reported that sulfation of titania surface increases the
number and strength of the acid sites, which in turn, retard
2
0
Fig. 12 Structural models of Ruddlesden Popper phases.
35,36
recombination of charge carriers.
sulfur doping would be applicable to sulfated Sr
Such an effect due to
Ti as well,
3
2 7
O
2
ꢀ
resulting in the minimization of recombination. All the three
dopants play a dual role, of reducing the band gap and mini-
mizing the recombination of charge carriers.
S
species is ruled out, as its ionic size is too large (0.184 nm) to
2
ꢀ
be located in the place of O (0.14 nm).
2
1,22
Studies on the effect of doping of SrTiO with Fe
revealed that additional energy levels identied with Fe and
have
3
3
+
5964 | RSC Adv., 2015, 5, 5958–5966
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Paper
The extension of light absorption edge into visible region
and increased life time of charge carriers are the two major
factors responsible for the observed increase in PCRC activity
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doped samples, is the other contributing factor, since smaller
size could shorten the path length for the diffusion of charge
carriers from the bulk to the surface and hence reduce the
37
probability for recombination. Additionally, the layered
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and separation of oxidation/reduction reaction centres within
3
2 7
O
12
the interlayer space. The formulation, with co-doping of N, S
and Fe together, wherein the inuence of these factors is
maximum, displays maximum activity (Table 2). Apparent
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with N and S, under identical experimental conditions, is less,
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could be a better alternative
2
conversion on titania P-25, when doped
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3
2 7
O .
When modied suitably, Sr
3
Ti
2
O
7
for PCRC application.
6
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7
S. C. Yan, S. X. Ouyang, J. Gao, M. Yang, J. Y. Feng, X. X. Fan,
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Neat and doped Sr Ti O samples have been prepared by
3
2 7
modied polymer complex method. The inuence of doping
has been investigated by detailed characterization of the cata-
lysts with XRD, EDXA, SEM, DRS, photo luminescence and X-ray
photo electron spectroscopic techniques. Doping/co-doping
with anions N, S and metals like Fe, results in the creation of
additional energy levels within the band gap, leading to the
absorption of visible light and also minimization of the
recombination of charge carriers. Formation of smaller crys-
tallites also reduces the probability for recombination. Layered
structure of Sr Ti O facilitates easy transport of charge carriers
3 2 7
and separation of oxidation/reduction reaction centers. These
factors contribute towards the signicant improvement in
activity for CO photo reduction on Sr Ti O co-doped with N, S
2 3 2 7
8
9
M. Halmann, M. Ulman and B. A. Blajeni, Sol. Energy, 1983,
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1
1
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1
1
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NCCR gratefully acknowledges the continuous and generous
support provided to the project, by M/s Hindustan Petroleum
Corpn Ltd, Mumbai, the support rendered by the Department of
Science & Technology, Govt. of India, New Delhi, for establish-
ing NCCR with all laboratory facilities and Indian Institute of
Technology Madras, Chennai, for all administrative and infra-
structure support.
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5966 | RSC Adv., 2015, 5, 5958–5966
This journal is © The Royal Society of Chemistry 2015