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Visible-light driven Gd
2
Ti
2
O
7
/GdCrO
3
composite for hydrogen evolution
K. M. Parida,* Amtul Nashim and Saroj Ku. Mahanta
Received 11th August 2011, Accepted 12th September 2011
DOI: 10.1039/c1dt11517k
A series of Gd
2
Ti
2
O
7
/GdCrO
3
composites are prepared by solid state combustion method using
Gd(NO
3
)
3
, TiO
2
, Cr
2
O
3
as metal source and urea as a fuel. The composites are characterized by X-ray
diffraction (XRD), scanning electron microscopy (SEM), UV-visible diffuse reflectance spectra
(DRUV–vis), Brunauer–Emmett–Teller (BET) surface area measurements, photoluminescence spectra
(PL), X-ray photoelectron spectroscopic (XPS) studies, photocurrent measurements etc. The
photocatalytic activity of the composites is examined towards hydrogen production without using any
co-catalyst under visible light illumination. The rate of formation of hydrogen is measured by the
photocatalytic activity measurement device and gas chromatography (GC). The highest efficiency is
observed over the composite GTC (Cr:Gd:Ti = 1 : 1 : 1). On the basis of photocurrent measurements
and PL, a mechanism for the enhanced photocatalytic activity has been discussed.
1. Introduction
Over the last few years, methods have been consistently developed
for photocatalytic water splitting under visible light irradiation.
This method can be considered promising for the production of the
alternative energy.
1
In 1972 Fujishima and Honda discovered TiO
2
as a UV active photocatalyst for H
2
production.
2
Since then, much
attention has been paid to the development of the oxide based
photocatalyst. It has been seen that almost all oxide semiconductor
photocatalysts are stable but active under UV light irradiation
such as LnTaO
4
,
3
ATaO
3
(Li, Na, K),
4
PbWO
4
5
etc. and very few
visible active oxide photocatalysts are known such as NiM
2
O
6
(M = Nb, Ta),
6
ZnFe
2
O
7
,
7
Bi
2
WO
6
.
8
In a recent review, several new
photocatalyst oxide materials were described.
9
Binary metal oxides like perovskite, pyrochlore, spinels and
delafossites show excellent photocatalytic activity due to the
orientation of atoms in a layer structure and the presence of
vacancies in the crystal structure. From the photocatalytic point
of view pyrochlores have the properties of an ideal photocatalyst.
Almost all pyrochlores are UV active like Sr
2
M
2
O
7
(M = Nb
and Ta),
10
Ca
2
Ta
2
O
7
and A
2
Ta
2
O
6
(A = Na, K).
11
Recently
some UV active pyrochlores are turned into visible active by
doping such as Cr–Fe–La
2
Ti
2
O
7
,
12
Co–La
2
Ti
2
O
7
,
13
Sm
2
Ti
2
O
5
S
2
14
and Y
2
Ta
2
O
5
N
2
.
15
Generally recombination of electron and hole
pairs, leads to the decrease in the photocatalytic activity. Thus
preventing the recombination is another key strategy to improve
photocatalytic activity besides expanding the light absorption
edge of the photocatalyst. By collecting photogenerated electrons
and holes on the different semiconductor surfaces like combining
suitable conduction and valence bands or loading noble metal
Colloids and Materials Chemistry Department, Institute of Minerals and
Materials Technology (CSIR), Bhubaneswar, 751 013, Orissa, India.
E-mail: paridakulamani@yahoo.com; Fax: +91-6742581637; Tel: +91-674-
2581636-425
particles on the surface of the photocatalyst enhance the redox
reactions of the electrons and holes, respectively. Au/KTiNbO
5
,
16
Cr doped Ba
2
In
2
O
5
/In
2
O
3
,
17
M–SrTiO
3
,
18
WOx–TiO
2
19
and CuO–
TiO
2
20
was reported to be this kind of photocatalyst.
RETO
3
perovskite-type oxides, with RE = rare earth and T =
transition metals play an important role in solid oxide fuel cells,
catalysts, materials for electrodes and chemical sensors.
21
The
photocatalytic activities of LaMO
3
(M = Cr, Mn, Fe, Co) in
aqueous media for degradation of various dyes are also known.
Recently LaFeO
3
was found to be an efficient photocatalyst
towards water splitting.
22
Gd
2
Ti
2
O
7
is a UV active photocatalyst
which shows overall water splitting under UV light.
23
The band gap
of Gd
2
Ti
2
O
7
is 3.2 eV, this band gap is too high to show any activity
under visible light. So incorporating or doping with transition
metals or colored materials is one of the previously used methods
to reduce the band gap. In our present investigation, we have
studied the photocatalytic activity of Cr
2
O
3
induced Gd
2
Ti
2
O
7
. In
this case a separate phase of GdCrO
3
(P-type semiconductor)
24
is formed due to which the photocatalytic activity of Gd
2
Ti
2
O
7
is extended towards visible light. A tentative mechanism for
the enhanced photocatalytic activity of the Gd
2
Ti
2
O
7
/GdCrO
3
composite is also discussed in detail.
2. Experimental
2.1 Preparation of the catalysts
Gd
2
Ti
2
O
7
/GdCrO
3
(abbreviated as X GTC) composites were
synthesized by solid state combustion method. In this synthesis
Gd(NO
3
)
3
, TiO
2
, Cr
2
O
3
were taken as metal source and urea
was used as a fuel
25,26
to lower the temperature of the reaction.
Above were mixed in the stoichiometric amount (Cr:Gd:Ti:urea =
X:1 : 1 : 4) by keeping Gd:Ti:urea ratio fixed and varying the Cr
content. The mixture was grounded mechanically in the presence
This journal is © The Royal Society of Chemistry 2011 Dalton Trans., 2011, 40, 12839–12845 | 12839
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Published on 24 October 2011 on http://pubs.rsc.org | doi:10.1039/C1DT11517K
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