A room-temperature reactive-template route to mesoporous znga 2o4 with improved photocatalytic activity in reduction of co2

Article abstract of DOI:10.1002/anie.201003270

Multimetal mesoporous materials MGa2O4 (M=Zn, Ni, Co) can be synthesized by ionexchange reactions of colloidal mesoporous NaGaO2 (see scheme with TEM images), which acts as both template and precursor, so that a template-removal process can be dispensed with. Copyright

Full text of DOI:10.1002/anie.201003270

Communications
DOI: 10.1002/anie.201003270
Mesoporous Materials
A Room-Temperature Reactive-Template Route to Mesoporous
ZnGa2O4 with Improved Photocatalytic Activity in Reduction of
CO2**
Shi Cheng Yan, Shu Xin Ouyang, Jun Gao, Ming Yang, Jian Yong Feng, Xiao Xing Fan,
Li Juan Wan, Zhao Sheng Li,* Jin Hua Ye, Yong Zhou, and Zhi Gang Zou*
Mesoporous materials are of scientific and technological
interest due to their potential applications in various areas.[1]
Over the past two decades, significant effort has been devoted
to the synthesis of mesoporous materials. For instance,
mesoporous silica and phosphate metal oxides have been
synthesized and applied widely in many industrial processes.[2]
However, little progress has been made in the synthesis of
mesoporous metal oxides containing more than one type of
metal. To date, a limited number of routes including
evaporation-induced self-assembly (EISA) and nonaqueous
solvent methods have been developed to synthesize multi-
Recently, we developed a synthetic route to mesoporous
multimetal oxides that uses the inorganic starting reactants
directly as pore makers which aid in building the mesoporous
structures of multimetal oxides and improve the thermal
stability of the resulting mesostructure.[8]
However, in these reported synthetic routes, postcrystal-
lization and introducing or removing an exotemplate are
usually needed. In recent years, a route that does not require
template removal, which was named “reactive hard templat-
ing”, was developed to synthesize porous TiN/carbon compo-
site materials.[9] In this route, the template consists of
nanostructures of porous graphitic C3N4, which thermally
decomposes completely during formation of porous TiN. This
route provides a means to overcome the problems associated
with synthesizing multimetal mesoporous materials. A sim-
plified soft-chemistry route based on a reactive template is
expected to allow synthesis to proceed at room temperature
without requiring the introduction or removal of a template.
Here we report a novel direct method for preparing
mesoporous ZnGa2O4 with a wormhole framework by an
ion-exchange reaction at room temperature involving a
mesoporous NaGaO2 colloid precursor. The method does
not require any additional processes and can be extended to
prepare other porous materials, such as CoGa2O4 and
NiGa2O4.
The X-ray diffraction (XRD) pattern of NaGaO2 powder,
which can be indexed as the orthorhombic phase (JCPDS 76-
2151), is presented in Figure 1. Scanning electron microscopy
(SEM) revealed that the powder particles are irregular in
shape with little agglomeration, and most of the particles are
larger than 500 nm in diameter (see Supporting Information).
The as-prepared NaGaO2 powder can be dispersed in water to
form a suspension. When the NaGaO2 suspension is illumi-
nated with a 532 nm laser, a Tyndall effect is observed, that is,
the suspension behaves as a colloid (see Supporting Informa-
tion). Multimodal measurements of particle size distribution
by dynamic light scattering show that the NaGaO2 colloidal
particles exhibit two peak distributions: 20% of the particles
have an average size of 70 nm, and 80% an average size of
335 nm. Most particulate or macroscopic materials in contact
with a liquid acquire an electric charge on their surfaces. The
zeta potential is an important and useful indicator of this
charge that can be used to predict the stability of colloidal
suspensions. The zeta potential of NaGaO2 colloidal particles
is ꢀ21.57 mV (pH 6). This is lower than the critical zeta
potential of ꢁ 30 mV for maintaining colloid stability in an
aqueous system,[10] that is, the colloidal particles are slightly
metallic mesoporous materials such as Pb3Nb4O13,[3]
[5]
Bi20TiO32,[4] SrTiO3,
MgTa2O6, CoxTi1ꢀxO2ꢀx
,
and
Ce1ꢀxZrxO2.[6] In these routes, introducing surfactant mole-
cules or a template is a general method used to construct the
mesostructures. Challenges in using the template method to
synthesize multimetallic mesoporous materials are uncon-
trolled phase separation in the multicomponent reactions and
poor thermal and chemical stability of the resulting meso-
porous structure.[7] Maintaining the complete mesostructure
during removal of the template by heating or chemical
treatment is a key process for obtaining the expected
mesostructures, and increases the uncertainty in a given
synthetic route. In addition, to obtain a crystalline mesopo-
rous material, high-temperature heat treatment is usually
required for crystallization of the product. However, this
process probably induces collapse of mesostructures.
[*] S. C. Yan, J. Gao, M. Yang, X. X. Fan, L. J. Wan, Prof. Y. Zhou,
Prof. Z. G. Zou
Ecomaterials and Renewable Energy Research Center
National Laboratory of Solid State Microstructures, Nanjing
University, 22 Hankou Road, Nanjing 210093 (China)
E-mail: zgzou@nju.edu.cn
J. Y. Feng, Prof. Z. S. Li
Department of Materials Science and Engineering
Nanjing University, 22 Hankou Road, Nanjing 210093 (China)
E-mail: zsli@nju.edu.cn
S. X. Ouyang, Prof. J. H. Ye
Photocatalytic Materials Center (PCMC)
National Institute for Materials Science (NIMS)
Sengen, Tsukuba, Ibaraki 305-0047 (Japan)
[**] This work is supported by the National Basic Research Program of
China (No. 2007CB613305), China-Japan cooperation project of
science and technology (2009DFA61090) and the National Natural
Science Foundation of China (Nos. 50732004 and 20773064)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201003270.
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Figure 1. a) XRD patterns of the NaGaO2 powder and gel. b) TEM and
HRTEM (inset) images of NaGaO2 colloidal particles. c) High-magnifi-
cation TEM image of a typical mesopore in the NaGaO2 colloidal
particles.
stable and can aggregate easily due to the weak repulsion
between them. Upon changing the pH of the NaGaO2
colloidal suspension, the zeta potential became + 42.36 mV
at pH 3 and ꢀ32.68 mV at pH 11, that is, the stability of
NaGaO2 colloid was improved under both acidic and basic
conditions. A gel can be obtained by freeze-drying the
NaGaO2 colloidal suspension. An XRD analysis (Figure 1a)
indicated that the gel is amorphous, and energy-dispersive X-
ray (EDX) analysis showed that the Na/Ga atomic ratio in the
gel is close to 0.8:1 (see Supporting Information), probably
due to elution of sodium ions from the surface of NaGaO2
colloidal particles. The transmission electron microscopy
(TEM) image in Figure 1b shows that the NaGaO2 colloidal
particles have a typical mesostructure with a wormhole
framework. A high-resolution TEM (HRTEM) image of the
mesostructure did not reveal the presence of a crystal lattice
(inset of Figure 1b). This implies that the NaGaO2 colloidal
particles are amorphous, consistent with the XRD analysis.
The mesopores in the colloidal particles can be observed at
high magnification (Figure 1c).
To understand the ion exchange between NaGaO2
colloidal suspension and an aqueous solution of Zn-
(CH3COO)2, in situ observations were performed with an
optical microscope. When the NaGaO2 colloidal suspension
was left undisturbed under ambient conditions for an
extended period, a mesh of aggregated colloidal particles
formed on the micrometer scale due to the low stability of the
colloid (see Supporting Information). An in situ ion-exchange
reaction on the amorphous NaGaO2 framework could be
observed when Zn2+ ions were introduced into the reaction
system. As illustrated in Figure 2a, the XRD pattern of the
product from the ion-exchange reaction can be indexed as
Figure 2. Characterization of meso-ZnGa2O4. a) XRD pattern. b) TEM
image (inset: HRTEM image of the crystal lattice). c) Nitrogen
adsorption–desorption isotherms and pore size distribution (inset).
cubic spinel ZnGa2O4 (denoted meso-ZnGa2O4) with a cell
parameter of a = 8.335 ꢀ, which is in good agreement with
JCPDS 38-1240. The composition of the as-prepared
ZnGa2O4 was further confirmed by EDX analysis (see
Supporting Information). The diffraction peaks in the XRD
pattern of meso-ZnGa2O4 are clearly broadened. From the
XRD data, the grain size was estimated by the Scherrer
formula to be about 5 nm. Usually, soft-chemical routes
performed at room-temperature, such as co-precipitation,
produce amorphous ZnGa2O4.[11] In this case, the ion-
exchange process occurs by a unique reaction between a
weakly acidic Zn(CH3COO)2 (pH 6) solution and an alka-
lescent NaGaO2 (pH 8) colloidal suspension. Aggregation of
ZnGa2O4 depends strongly on the Zn-containing precursor.
In this reaction, the strong electrolyte Zn(NO3)2 instead of the
weak electrolyte Zn(CH3COO)2 will produce amorphous
ZnGa2O4. This is probably due to the faster reaction rate
between Zn(NO3)2 and NaGaO2 than between Zn-
(CH3COO)2 and NaGaO2. This fact may support the argu-
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ment that the reaction rate between Zn(CH3COO)2 and
The route reported here is applicable to preparation of
other mesoporous spinel AB2O4-type oxides. Mesoporous
CoGa2O4 and NiGa2O4 were synthesized by this method.
Unlike ZnGa2O4, the MGa2O4 (M = Co, Ni) samples obtained
by ion exchange at room temperature are amorphous. The
specific surface areas of amorphous NiGa2O4 and CoGa2O4
are 363.3 and 202.3 m2 gꢀ1, respectively. The samples exhibited
a uniform pore size distribution. TEM observations (see
Supporting Information) showed that the wormholes in these
amorphous products are similar to those in the NaGaO2
colloid due to the amorphous phase transformation with
slight particle growth. By hydrothermal treatment of these
amorphous samples at 1808C for 5 h, crystalline mesoporous
CoGa2O4 and NiGa2O4 can be obtained. Compared with the
amorphous products, the specific surface areas of crystalline
CoGa2O4 and NiGa2O4 decreased to 82.7 and 87.1 m2 gꢀ1,
respectively. The pore sizes of crystalline mesoporous
CoGa2O4 and NiGa2O4 exhibit bimodal distributions
(Figure 4), and the average values of the pore diameter
were 2.5 and 8 nm, respectively. The grain size of crystalline
CoGa2O4 and NiGa2O4 is about 10 nm (see Supporting
Information). Therefore, the results of these experiments
indicate that amorphous MGa2O4 (M = Co, Ni) can form
rapidly at room temperature, and that subsequent hydro-
thermal treatment induces crystallization and growth of the
amorphous particles. The mesopores in CoGa2O4 and
NiGa2O4 are formed by agglomeration of nanoparticles,
similar to meso-ZnGa2O4. Compared with amorphous
CoGa2O4 and NiGa2O4, the bimodal pore size distribution
and low BET surface area of crystalline CoGa2O4 and
NiGa2O4 can be attributed to grain growth during hydro-
thermal treatment.
NaGaO2 is appropriate for nucleating and growing crystalline
ZnGa2O4. The TEM images show that meso-ZnGa2O4 also
has a wormhole framework like the NaGaO2 colloidal
particles (Figure 2b). Apparently, the mesostructure of
ZnGa2O4 resulted from agglomeration of nanoparticles. An
HRTEM analysis indicates that the lattice spacing is 0.295 nm
for the (220) plane, which provides further evidence that the
product is crystalline ZnGa2O4. Nitrogen adsorption–desorp-
tion studies on as-prepared meso-ZnGa2O4 showed a type IV
isotherm, typical of mesoporous materials (Figure 2c). The
pore diameter calculated from the nitrogen adsorption
isotherm by the Barrett–Joyner–Halenda (BJH) method is
3.5 nm, and the specific surface area calculated from the
linear region of the Brunauer–Emmett–Teller (BET) plot
ranging from P/P0 = 0.05 to P/P0 = 0.15 is 110.4 m2 gꢀ1.
It is interesting to discover a formation mechanism of
mesopores that does not require introducing a template or
surfactant molecules. As illustrated in Figure 3, it seems
reasonable to speculate that the NaGaO2 powder dispersed in
Figure 3. Illustration of the formation of a mesoporous colloidal
template and an ion-exchange reaction based on the colloidal tem-
plate. a) NaGaO2 solid powder particles. b) NaGaO2 colloidal particles
obtained by dispersing the NaGaO2 powders in deionized water.
c) Formation of a mesoporous NaGaO2 colloidal template by floccu-
lation. d) Formation of mesoporous ZnGa2O4 by ion-exchange reaction
on the NaGaO2 colloidal template.
AB2O4-type oxides with spinel structure have potential
applications in many fields such as catalysis, gas sensors, and
photoelectronics. In particular, ZnGa2O4 is a transparent and
conductive material which should be useful for UV photo-
electronic devices.[12] ZnGa2O4 also shows potential as a
photocatalyst for wastewater treatment and hydrogen pro-
duction by water splitting.[13,14] Here meso-ZnGa2O4 was used
as a photocatalyst to convert CO2 into methane (CH4) under
irradiation of UV light.
water can form amorphous colloidal particles, which tend to
form the mesoporous NaGaO2 colloid by flocculation due to
the weak repulsion between the colloidal particles. The ion-
exchange process between Zn2+ and Na+ is expected to be due
to the NaGaO2 mesoporous framework, because ZnGa2O4
crystal particles could nucleate and grow there. The meso-
porous structure of the NaGaO2 colloid is maintained in the
generated ZnGa2O4 after ion exchange, probably because the
reaction rate is moderate. For the ZnGa2O4 particles sus-
pended in water, a high zeta potential of + 52.66 mV was
measured. The opposite sign of the charge of zeta potential
for the ZnGa2O4 particles and NaGaO2 colloidal particles
suggests that the particles attract each other. This would be
beneficial for maintaining the NaGaO2 mesoporous frame-
work during the ion-exchange reaction. Moreover, changes to
the pores in NaGaO2 may take place during phase trans-
formation due to a change in crystal structure. However, the
porous structure of NaGaO2 colloid can compensate for the
volume change by tuning the pore size. This may promote the
mesostructure of NaGaO2 being inherited by ZnGa2O4.
Carbon dioxide, which is released mainly by burning of
fossil fuels, is the primary cause of global warming. Convert-
ing CO2 into valuable hydrocarbons by means of solar energy
is one of the best solutions to both global warming and energy
shortage. Generally, CO2 can be photoreduced to CH4 in the
presence of water vapor by using a wide band gap semi-
conductor such as TiO2 as photocatalyst. The photogenerated
holes in the valence band oxidize water to generate hydrogen
ions by the reaction H2O ! 1=2 O2 þ 2 Hþ þ 2 eꢀ (Eroedox
=
0.82 V vs. NHE). The photogenerated electrons in the
conduction band reduce CO2 to CH4 by the reaction
CO2 þ 8 eꢀ þ 8 Hþ ! CH4 þ 2 H2O (Eroedox = ꢀ0.24 V vs.
NHE).[15] The electronic structure of ZnGa2O4 has been
calculated by using plane-wave-based density functional
theory.[14] The valence band was mainly composed of the O
2p orbitals, whereas the conduction band was formed by
hybridization of Ga 4s4p and Zn 4s4p orbitals. The band gap
of the as-prepared ZnGa2O4 was determined from the UV/Vis
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Figure 5. CH4 generation over various ZnGa2O4 samples as a function
of irradiation time.
mesostructure and more reaction sites arising from high
specific surface area (SBET = 110.4 m2 gꢀ1 for meso-ZnGa2O4
and 4.6 m2 gꢀ1 for ss-ZnGa2O4). The generation rate of CH4
over meso-ZnGa2O4 could be significantly enhanced by
loading 1 wt% RuO2 as co-catalyst to improve separation of
the photogenerated electron–hole pairs (CH4: 50.4 ppmhꢀ1),
as demonstrated in photocatalytic water splitting.[14]
In summary, a reactive templating route to preparing
mesoporous ZnGa2O4 at room temperature has been
reported. The reaction pathway involves an ion-exchange
process with a porous NaGaO2 colloidal template as the
precursor. The route is effective for synthesizing porous
AB2O4-type materials by a simple method. By using RuO2 as
co-catalyst, the as-prepared mesoporous ZnGa2O4 shows high
photocatalytic activity for converting CO2 into CH4 under
light irradiation, because of strong gas adsorption and large
specific surface area of the mesoporous photocatalyst.
Mesoporous ZnGa2O4 is potentially an effective photocata-
lyst for the photoreduction of CO2.
Figure 4. Nitrogen absorption–desorption isotherms, pore size distri-
bution (inset), TEM images, and HRTEM images of the lattice (inset)
for a, c) CoGa2O4 and b, d) NiGa2O4 obtained by hydrothermal treat-
ment of amorphous MGa2O4 (M=Co, Ni) samples, prepared by ion
exchange at room temperature.
Experimental Section
NaGaO2 solid powders were prepared by heating a stoichiometric
mixture of Na2CO3 and Ga2O3 at 8508C for 12 h. Mesoporous
ZnGa2O4 was prepared as follows: NaGaO2 colloidal suspension
(0.2 molLꢀ1, 10 mL) was added to an aqueous solution of Zn-
(CH3COO)2 (0.05 molLꢀ1, 20 mL) and the mixture stirred for 3 h at
room temperature to form mesoporous ZnGa2O4. The sediment was
separated by centrifugation and dried at 608C for 2 h.
absorption spectrum to be 4.4 eV (see Supporting Informa-
tion). The position of the conduction and valence bands in as-
prepared ZnGa2O4 was determined by the following equa-
tion: ECB = XꢀEcꢀ0.5Eg, where Ec is the energy of free
electrons on the hydrogen scale (4.5 eV), X is the electro-
negativity of the semiconductor, and Eg is the band-gap
energy of the semiconductor. The edge of the valence band
EVB of ZnGa2O4 was determined to be 3.13 V versus NHE,
which is more positive than Eo(H2O/H+) (0.82 V vs. NHE).
The edge of the conduction band was estimated to be ꢀ1.27 V
versus NHE, which is more negative than Eo(CO2/
CH4)(ꢀ0.24 V vs. NHE). This indicates that the electrons
and holes photogenerated on irradiation of ZnGa2O4 can
react with adsorbed CO2 and H2O to produce CH4. Figure 5
shows that meso-ZnGa2O4 (CH4: 5.3 ppmhꢀ1) exhibits higher
activity than ZnGa2O4 (CH4: trace) obtained by solid-state
reaction of ZnO and Ga2O3 at 11008C for 16 h (denoted ss-
ZnGa2O4). This is related to strong gas adsorption by the
Mesoporous CoGa2O4 and NiGa2O4 were synthesized by a similar
procedure. NaGaO2 colloidal suspension (0.2 molLꢀ1, 10 mL) was
added to an aqueous solution of CoSO4 or Ni(CH3COO)2
(0.05 molLꢀ1, 20 mL) and the mixture stirred for 1 h. The mixture
was then heated in a Teflon-lined hydrothermal autoclave at 1808C
for 5 h to form mesoporous crystalline CoGa2O4 or NiGa2O4.
The crystalline phases of these as-prepared products were
determined by powder XRD (Rigaku Ultima III, CuKa radiation).
The size, size distribution, and zeta potential of NaGaO2 colloidal
particles were determined by a zeta potential analyzer (Zeta PALS,
Brookhaven Instruments Co., USA). The specific surface area of the
samples was measured by nitrogen sorption at 77 K on a surface area
and porosity analyzer (Micromeritics TriStar 3000, USA) and
calculated by the BET method. The morphology of the samples was
observed by TEM (FEI Tecnai G2 F30 S-Twin, USA).
In the photocatalytic reduction of CO2, ZnGa2O4 powder (0.1 g)
was uniformly dispersed on a glass reactor with an area of 4.2 cm2. A
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300 W Xenon arc lamp was used as the light source for the
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and then high-purity CO2 gas was introduced into the reaction to
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Received: May 30, 2010
Published online: July 22, 2010
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Keywords: ion exchange · mesoporous materials ·
photochemistry · reduction · template synthesis
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