Mg-doped WO
3
as a novel photocatalyst for visible light-induced
water splitting
Dong Won Hwang
a
, Jindo Kim
a
, Tae Jin Park
b
Y and Jae Sung Lee
aY
*
a
Department of Chemical Engineering and School of Environmental Science and Engineering, Pohang University of Science and Technology
(POSTECH), San 31Hyoja-dong, Pohang 790-784, Republic of Korea
b
Clean Technology Research Center, Korea Institute of Science and Technology, 39-1 Hawolkok-dong, Sungbuk-gu, Seoul 136-791, Republic of Korea
Received 30 October 2001; accepted 23 January 2002
Mg-doped WO
3
with band gap energy of about 2.6 eV is a viable photocatalyst for visible light-induced water splitting in the presence of
hole scavengers. The conduction band edge position of p-type Mg-doped WO
3
was ÿ2.7 V versus SCE at pH 12. By doping Mg on WO
3
, the
conduction and valence band positions were shifted by 2.25 V negatively, leading to a conduction band edge position which was negative
enough for H
ions to be reduced thermodynamically, with little change in band gap energy. The negative shift in band position might
be ascribed to lowering of the eective electron anity of WO
3
by doping Mg with a very low electron anity.
KEY WORDS: Mg-doped WO
3
; photocatalyst; visible light; water splitting; band position shift.
1. Introduction
Production of hydrogen from decomposition of water
using solar energy has been considered an ultimate
technology to solve both energy and environmental
problems resulting from current use of fossil fuels. An
ecient photocatalyst is needed for the technology to
work. Although there has been remarkable progress in
the last decades for photocatalysts working under
ultra-violet light [1±4], this progress has rarely been
extended to visible light. The ®rst requirement for visible
light-induced photocatalyst is the proper band gap
energy. However, only a few materials of photocatalytic
activity satisfy this condition, such as CdS, WO
3
and
Fe
2
O
3
. Although CdS is a good candidate for photo-
catalytic water reduction, it has a fatal photocorrosion
problem in that CdS itself is oxidized by the photogener-
ated hole. Kudo and Sezikawa have reported some CdS-
based photocatalysts modi®ed by Cu-ZnS or Ni-ZnS
[5,6], over which H
2
was produced from H
2
O in the pre-
sence of sacri®cial agents such as Na
2
S and methanol.
Although WO
3
and Fe
2
O
3
have proper band gaps for
visible light absorption, these materials could not be
used in photocatalytic water reduction due to the
improper conduction band edge position relative to the
reduction potential of water [7]. There were few reports
on non-sul®de-type photocatalysts that produce hydro-
gen from water under visible light. In search of an
oxide photocatalyst working under visible light, we
discovered that modi®cation of WO
3
by doping with
MgO gave interesting photocatalytic properties. In this
paper, we report Mg-doped WO
3
as a novel non-sul®de,
visible light-induced photocatalyst, over which H
2
evolves photocatalytically in the presence of a sacri®cial
agent.
2. Experimental
Mg-doped WO
3
was prepared by an impregnation
method: WO
3
(Aldrich 99.99%) was added to an
aqueous solution containing a required amount (5±
20 wt% of powder) of MgNO
3
X
6H
2
O (Aldrich) and
then water was evaporated in a rotary evaporator. Mg-
doped WO
3
was calcined at 773 K in air for 3 h,
ground in a mortar, pelletized and then sintered in a
platinum crucible at a temperature between 1173 K and
1523 K for 17 h. The Pt deposition on Mg-doped WO
3
was performed by photoplatinization; 0.05 g of H
2
PtCl
6
was introduced into the reaction system (EtOH
50 ml distilled water 300 ml) containing 1 g of catalyst,
and then 2 h of UV irradiation through a Pyrex ®lter was
conducted. Photocatalytic reaction was carried out at
room temperature in a closed gas circulation system
using a high-pressure Hg lamp (Ace Glass Inc., 450 W)
placed in an inner irradiation-type Pyrex reaction cell
[1,2]. Ultra-violet light (`400 nm) was removed with a
solution ®lter (1 M NaNO
2
). The catalyst (0.2 g) was
suspended in distilled water (350 ml) by magnetic
stirring. The rates of H
2
evolution were determined
from analysis of the gas phase by gas chromatography
(TCD, molecular sieve 5 A
Ê
column and Ar carrier).
The crystal structure of the sintered powder was deter-
mined by X-ray diraction (XRD, Mac Science Co.,
M18XHF) and the band gap energy was measured by
Catalysis Letters Vol. 80, Nos. 1±2, May 2002 (# 2002) 53
1011-372X/02/0500-0053/0 # 2002 Plenum Publishing Corporation
*
To whom correspondence should be addressed.
E-mail: jlee@postech.ac.kr