Download 2. Experimental

PHYSICS, CHEMISTRY AND APPLICATION OF NANOSTRUCTURES, 2011
STATE OF Au CLUSTERS IN Au−In2O3 NANOCOMPOSITE
AND THE NATURE OF INTERACTION BETWEEN THE
COMPONENTS
E. OVODOK, M. IVANOVSKAYA, D. KOTSIKAU,
Research Institute for Physical Chemical Problems, Belarusian State University
14 Leningradskaya, Minsk, 220030, Belarus. E-mail: [email protected]
I. ASARKO
Physical department of the Belarusian State University
5 Bobruiskaya, Minsk, 220030, Belarus.
Mutual influence of gold nanoparticles and indium oxide on the formation of Au-In2O3
nanocomposite prepared by the sol-gel method has been studied in the paper. It was
shown by EPR, DSC/TG and optical spectroscopy that the chemical interaction between
In2O3 and Au is accompanying by the transfer of electron density and the formation of
(Au0n)Au(3−δ)+ clusters under heating of the nanocomposite.
1. Introduction
Gold nanoparticles on the surface of metal oxides are known to be effective
catalysts for heterogeneous processes of low temperature oxidation of CO [1].
However, the activity of gold nanoparticles depends on their size. It was found
that the gold particles with a size ranging from 3 to 5 nm are only active in this
process. High catalytic and photocatalytic activity of gold nanoclusters in the
reaction of CO oxidation is observed when gold is deposited on various metal
oxides like TiO2, SnO2, Fe2O3, Al2O3, MgO [2]. It was shown in [3] that both
particle size and the method of introduction of gold into indium oxide affect the
magnitude of electrical resistance drop of semiconducting Au−In2O3 layers when
exposed to CO. However, the origin of high activity of gold nanoclusters still
remains unclear.
Previous papers show that low-temperature activity of Au/MOx (where M ≡
metal) materials depends not only on the size of gold particles, but also on the
ratio of metallic gold (Au0) concentration to the concentration of oxidized forms
(AuII, AuIII) in them [4]. The stabilization of the oxidized states of gold is
controlled by the manner of its interaction with a metal oxide.
The objective of the present paper is to establish the nature of the interaction
between gold and indium oxide under the formation of Au−In2O3
nanocomposites, which are characterized by high activity in heterogeneous
processes of low-temperature CO oxidation.
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2
2. Experimental
In2O3 and Au−In2O3 samples were obtained by the sol-gel approach. To prepare
the Au−In2O3 composite, HAuCl4 solution was added to the sol of In(OH)3. The
composite samples with 0.01−1.0 wt. % of Au was prepared. The In(OH)3 sol
was synthesized by In(NO3)3 hydrolysis with ammonia solution to form indium
hydroxide precipitate. Then it was washed 5 times by centrifugation, peptized by
adding some drops of concentrated HNO3 together with simultaneous ultrasonic
treatment for 2 min (22 kHz). The In2O3 and Au−In2O3 samples were studied in
a form of xerogels (obtained by drying the corresponding sols at 30 °C), powders
and films (obtained by annealing the xerogels at 700 °C).
The samples were characterized by means of electron paramagnetic
resonance (EPR), optical spectroscopy and thermal analysis (DSC/TG). EPRspectra were recorded on a VARIAN E 112 spectrometer with a frequency of
9.35 GHz at 77 K. Thermal analysis was performed on powdered samples dried
at 50 °C for 24 h with a NETZCH STA 449 C instrument in the temperature
range of 20−1000 °C in air with 5 °/min heating rate. Optical absorption spectra
of the films were recorded on a SPECORD M40 spectrometer using two sources
of radiation − for visible region (320−750 nm) and for UV region
(185−360 nm).
3. Results and Discussion
Indium hydroxide sol remained stable when the aqueous HAuCl4 solution was
introduced to form a sol assigned here as Au−In2O3. In the sample dried at
30 °C, chemical interaction between gold and indium hydroxide is absent
according to the acquired IR-spectroscopy data. The presence of gold in the
indium hydroxide sol has an influence on the formation of indium oxide under
heating. The dehydration of the xerogel and removal of nitrate ions are slightly
suppressed in the Au−In2O3 sample as compared to the In2O3 sample
(endothermic effect at 259 and 250 °C, respectively). The crystallization of
indium oxide in the composite proceeds at higher temperature (314 °C) than in
the individual indium hydroxide (308 °C). Suppressing the process of amorphous
hydroxide transformation into crystalline oxide phase, as a rule, promotes the
formation of oxygen vacancies [5].
Optical spectra of the Au−In2O3 films with different gold content are shown
in Figure 1. The absorption band at 530 nm in the spectrum of the Au−In2O3
sample with 0.5 wt. % of Au could indicate the presence of nanosized gold
particles. It was estimated earlier that the diameter of Au particles in the films is
3.5−6 nm. With increasing gold concentration, the absorption band shifts to
3
560 nm and its intensity increases that is caused by Au particle growth. A weak
band at 300−305 nm is also observed in the optical spectra of the Au−In2O3
films with 0.01−0.5 wt. % of Au. Some authors [6] connect the appearance of
the absorption in this spectral region with the formation of Aun clusters, where
n < 10. They can be stabilized in indium oxide as a result of the interaction
between AuIII and nonstoichiometric indium oxide. Oxygen vacancies are active
adsorption sites in In2O3. There is evidence that the absorption at 304 nm in
optical spectra is typical of Inn clusters [7]. Singly charged oxygen vacancies (Fcentres) and In2+ are the most typical structural defects of In2O3. Therefore, the
appearance of the absorption band around 300 nm in the spectrum of the
Au−In2O3 film may indicate the formation of In−Au bond. In/Au contacts are
probably playing the role of active centres in catalysis, on the analogy with the
In/Pt contacts as reported by [8]. Au3+ ions are able to occupy both oxygen
vacancies and oxygen interstices. The latter are typical of the fluorite-type
crystal lattice of In2O3. The presence of 5-fold coordinated cations of In3+ and
In2+, and oxygen vacancies favors the stabilization of oxidized forms of gold and
the formation of nanosized gold particles. One can expect a partial transfer of
electron density from In2+ to Au3+ cations and the formation of clusters by the
following equation: [In(3−)+(O2−)5 Au3+(Au0)n]  [In3+(O2−)5 Au(3−)+(Au0)n].
Figure 1 Optical spectra of the Au−In2O3 films
with different Au content (wt. %):
(1) − 1.0; (2) – 0.5; (3) – 0.2; (4) – 0.01.
Annealing temperature is 700 °C.
Figure 2 EPR spectra of the In2O3 powder (1),
and Au−In2O3 powder with 0.5 wt. % of Au (2)
and 1.0 wt. % of Au (3) recorded at 77 K.
Annealing temperature is 700 °C.
Absorption lines at 220 nm and 265 nm indicate the change in the symmetry
of coordination surrounding of some indium ions in the Au−In2O3
nanocomposites. The mentioned bands are attributed to the transitions
accompanying by О2− → In3+ charge transfer. The absorption band at 265 nm
may be originated from the presence of indium ions coordinated with five atoms
of oxygen − [InO5], which is very typical of In2O3 oxide. The band at 220 nm is
caused by [InO5Au] centres.
4
EPR spectroscopy data confirm the mentioned above chemical interaction
between the components in the Au−In2O3 nanocomposite. A broad resonance
signal with unresolved hyperfine structure from 113In and 115In nuclei (J = 9/2), is
observed in the EPR spectrum of the In2O3 sample, recorded at 77 K (Figure 2).
It is attributed to the In2+ paramagnetic centres. The broadening of the lines
found in the EPR spectra is evoked by a high symmetry of the crystal field,
which is close to octahedral one. In the spectrum of Au−In2O3 sample, this
signal broadens and the magnitude of the g-factor increases (Table 1).
Table 1. Parameters of EPR spectra of the In2O3 and Au−In2O3 powders with different Au content
g-factor
В, mT
Irel
In2O3
2.052
6.25
1.0
Au−In2O3 (0.5 wt. % Au)
2.056
7.25
0.9
Au−In2O3 (1.0 wt. % Au)
2.057
8.25
0.8
Sample
The indicated changes in the EPR spectra are evidently connected with the
presence of In2+ ions surrounded by gold ions, which distort the symmetry of the
coordination surrounding of the paramagnetic centre. The interaction of the
unpaired electron localized on In2+ ions with a nuclear spin of 197Au (100 %,
J = 3/2) can also contribute to the broadening of the resonance signals.
4. Conclusion
Thermal treatment at 700 °C of the Au−In2O3 nanocomposite (0.5 wt. % of Au),
prepared by mixing of In(OH)3 sol and HAuCl4 solution, evokes chemical
interaction between the components accompanied by electron density transfer
from indium to gold that promotes the stabilization of oxidized states of gold
(AuII, AuIII) in the nanoparticles.
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