882
Chemistry Letters 2002
H2 Evolution from Aqueous Potassium Sulfite Solutions under Visible Light Irradiation
over a Novel Sulfide Photocatalyst NaInS2 with a Layered Structure
Akihiko Kudo,ꢀ Akira Nagane, Issei Tsuji, and Hideki Kato
Department of Applied Chemistry, Faculty of Science, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601
(Received May 15, 2002;CL-020416)
ꢁ
NaInS2 which consisted of anion layers of InS2 with a
2.3 eV band gap showed the photocatalytic activity for H2
evolution from an aqueous K2SO3 solution under visible light
irradiation (ꢀ > 420 nm).
visible light (ꢀ > 420 nm) through a cut-off filter (HOYA, L42)
from a 300 W Xe lamp (ILC technology;CERMAX LX-300). A
Pt cocatalyst was photodeposited on the NaInS2 powder in situ
using H2PtCl6ꢂ6H2O (Tanaka Kikinzoku;37.55% as Pt). The
amount of H2 evolved was determined using an on-line gas
chromatography (Shimadzu;GC-8A, MS-5A column, TCD, Ar
carrier). A quantum yield was measured at 440 nm using filers
combined with a band-pass filter (Kenko;BP44, the half width:
8.9 nm) and a cut-off filter (HOYA;L42), and a thermopile
(OPHIR;a 3A-P-SH head and a NOVA energy monitor).
Figure 1 shows the crystal structure of NaInS2.6 Anion layers
consist of edge-shared InS6 octahedra and Naþ cations exist
between the layers. Transition metal chalcogenides with layered
structure such as MoS2 have been studied as semiconductor
photoelectrodes.7 The layers of the photoactive transition metal
chalcogenides are stacked with the van der Waals force while that
of NaInS2 is stacked with a Coulomb force. In this point, NaInS2
is a new type of a photoactive layered sulfide.
Development of photocatalysts with a visible light response
has been urged for hydrogen production from water using a solar
light energy. In general, oxide semiconductor photocatalysts
which possess sufficient conduction band levels for H2 evolution
by the reduction of water have band gaps wider than ca. 3 eV.1
Although these materials are stable toward photocatalytic
reactions in aqueous media they respond to only UV light. On
the other hand, although metal chalcogenide semiconductors such
as CdS are generally not stable because of photocorrosion, many
of them possess absorption bands in a visible light region.2
Therefore, the metal chalcogenide semiconductor is a promising
material group for surveying new visible light respondent
photocatalysts for H2 evolution from water. Well-known sulfide
photocatalysts such as CdS and ZnS possess ordinary three-
dimensional bulk structures. On the other hand, layered oxides
such as K4Nb6O173;4 and K2La2Ti3O103;5 show the high activities
for water splitting into H2 and O2 in a stoichiometric amount
under UV irradiation. The two dimensional structure contributes
to separation of photogenerated electrons and holes, and active
sites for H2 and O2 evolution resulting in showing the high
activities. Therefore, studying photocatalytic properties of
sulfides with such a layered structure will be interesting. The
present paper reports the photocatalytic activity of NaInS2 with a
layered structure for H2 evolution from an aqueous K2SO3
solution under visible light irradiation.
White precipitation of a Na–In sulfide precursor was
prepared by adding an aqueous Na2S solution (1.25 mol dmꢁ3
,
120 ml) into an aqueous solution (0.25 mol dmꢁ3 each, 80 ml) of
In(NO3)3 (Kojunndo Chemical, 99.99%) and NaNO3 (Kanto
Chemical, 99.0%), and stirring the mixed solution for 20 h at
300 K. X-ray diffraction measurements (Rigaku;MiniFlex) were
carried out for the product materials. In2S3 was not obtained in the
present preparation process. The precursor precipitation was
heated at 423 K for 0.5 h for drying and subsequently heated at
573 K for 2 h in an N2 gas flow. The heat treatment gave
crystalline NaInS2 powder. The BET surface area of the
crystalline NaInS2 powder was 14 m2gꢁ1. This crystalline NaInS2
powder was further treated with water for 10 h at 300 K. Diffuse
reflection spectra were obtained using a UV-vis-NIR spectro-
meter (Jasco;Ubest V-570) and were converted from reflection to
absorbance by the Kubelka–Munk method. Photocatalytic
reactions were conducted in a gas-closed circulation system.
The NaInS2 powder was dispersed in an aqueous K2SO3 solution
(0.5 mol dmꢁ3, 320 ml). The photocatalyst was irradiated with
Figure 1. Crystal structure of NaInS2.6
Figures 2 and 3 show diffuse reflection spectra and X-ray
diffraction of NaInS2 and the related compounds, respectively.
The amorphous precursor as a precipitate was white. Heat-treated
crystalline NaInS2 was pale yellow. Water-treated NaInS2 was
orange-yellow and possessed intensive absorption band in the
visible light region. The X-ray diffraction patterns did not change
with the water treatment as shown in Figure 3. No difference in
XRF measurement between nontreated and water-treated NaInS2
was observed. The reason why the color changed by the water
treatment is not clear at the present stage;it might be caused by the
layered structure. The band gap of the water-treated NaInS2 was
Copyright Ó 2002 The Chemical Society of Japan
Kudo, Akihiko
