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Jan. 2008, Volume 2, No.1 (Serial No.2)
Journal of Environmental Science and Engineering, ISSN1934-8932, USA
Decomposition of formaldehyde based on gold-coated TiO2 nanoparticles
YU Chung-chin1, LIU Yu-chuan2, CHIU Wen-hui2, WANG Cheng-cai1
(1. Department of Environmental Engineering, Nano Materials Applications R & D Center, Vanung University, Taiwan;
2. Department of Chemical and Materials Engineering, Nano Materials Applications R & D Center, Vanung University, Taiwan)
Abstract: The synthesis of gold nanoparticles caped with
visible light-responsible TiO2 nanoparticles was prepared by
using electrochemical Oxidation-Reduction Cycles (ORC) in
0.1 M HCl aqueous solution containing 60 mM visible
light-responsible TiO2 nanoparticles. Firstly, an Au substrate
was cycled in a deoxygenated aqueous solution containing 0.1
M HCl and 60 mM anatase TiO2 nanoparticles from -0.28 to
+1.22 V vs Ag/AgCl at 500 mV/s with 25 scans. The durations at
the cathodic and anodic vertexes are 10 and 5 s, respectively.
After this process, Au-and TiO2-containing complexes were left
in the solution. Then a Pt electrode immediately replaced the Au
working electrode, and a cathodic overpotential of 0.6 V from
the Open Circuit Potential (OCP) was applied under
sonification to synthesize Au nanoparticles. Encouragingly, the
prepared Au nanoparticles caped with visible light-responsible
TiO2 nanoparticles are more active for the decomposition of
formaldehyde than pure visible light-responsible TiO2
nanoparticles are in the same condition. After 5 days testing, the
formaldehyde was decomposed ca. 35% in containing Au
nanoparticles caped with visible light-responsible TiO2
nanoparticles, but the formaldehyde was decomposed only ca.
25% in containing pure visible light-responsible TiO2
nanoparticles.
Key
words:
formaldehyde;
TiO2
nanoparticles;

Oxidation-Reduction Cycles (ORC)
1. Introduction
The nanoparticles exhibit their unique electronic
structure and extremely large surface areas, so there
widely applications have been researched, especially in
and catalysts modification [1]. There are several
developed methods for nanoparticles fabrication
including chemical reduction [2], sonochemical
*
Acknowledgements: This work was supported by the fund of
the Vanung University.
YU Chung-chin (1963- ), male, assistant professor; main
research
field:
environmental
material.
E-mail:
[email protected].
36
reduction[3], annealing from high-temperature
solutions[4], metal evaporation [5], and laser ablation [6]
etc.. It is useful to develop effective methods for sizeand shape-controlled synthesis of metal nanoparticles
due to these properties can significantly effect their
corresponding characterization [7,8]. Kobayashi et al [9]
reported a sol-gel processing of silica-coated gold
nanoparticles for preparation of gels with different
particle sizes and shapes. WANG et al. [10] reported
polyelectrolyte multi-layers for preparing silver
nanoparticles composites with controlled metal
concentrations and particle sizes by adjusting the pH
value of solutions. Generally, the advantages of
electrochemical methods over the chemical ones are
the high purity of the particles and the control of
particle size by adjusting applied potentials or current
densities [7,11].
In the nanoscale field, titania is one of the most
investigated oxide materials recently owning to its
important applications in environmental cleanup [12],
photocatalysts [13], solar cells [14] and polarimetric
interference sensor [15]. With the development of
visible light-responsible TiO2 nanoparticles [16,17], it is
now possible to combine the effects of TiO2 and metal
nanoparticles on photoreactions to improve the
environmental cleanup effects.
In the studies of surface-enhanced Raman
scattering
(SERS),
the
electrochemical
oxidation-reduction cycles (ORC) procedure [18,19] is a
better way to produce SERS-active metal substrates
because a controllable and reproduced surface
roughness can be easily generated [20,21]. In this work,
we use an electrochemical ORC roughening procedure
Decomposition of formaldehyde based on gold-coated TiO2 nanoparticles
to obtain Au-containing complexes in a 0.1 M HCl (US
pharmacopoeia grade for following applications)
aqueous solution from an Au substrate. Then Au caped
with visible light-responsible TiO2 nanoparticles are
immediately synthesized in the same solution with
visible light-responsible TiO2 nanoparticles by using a
sonoelectrochemical reduction method. The prepared
Au nanoparticles caped with visible light-responsible
TiO2 nanoparticles are examined for the catalytic
decomposition of formaldehyde in reactor.
Formaldehyde will cause damage to lung of people in
indoor air. The improved environmental cleanup
effects and the reduced formaldehyde were
investigated.
2. Experimental section
2.1 Chemical reagents
The electrolytes reagents (p. a. grade) purchased
from Acros organics were used as received without
further purification. Visible light-responsible TiO2
nanoparticles with particle sizes from 20 to 30 nm were
purchased from QF-NANO TECH. Co., Ltd, Taiwan.
All of the solutions were prepared using deionized 18.2
M cm water provided from a MilliQ system.
2.2
Preparation
of
containing
Au
nanoparticles caped with visible light-responsible
TiO2 nanoparticles
All of the electrochemical experiments were
performed in a three-compartment cell at room
temperature, 24℃, and were controlled by a
potentiostat (model PGSTAT30, Eco Chemie). A sheet
of silver foil with a bare surface area of 0.238 cm2, a 2
× 2 cm2 platinum sheet, and a silver-silver chloride
(Ag/AgCl) electrode were employed as the working,
counter, and reference electrodes, respectively. Before
the Oxidation-Reduction Cycles (ORC) treatment, the
silver electrode was mechanically polished (model
Minimet 1000, Buehler) successively with 1 and 0.05
m of alumina slurry to a mirror finish. Then the
electrode was cycled in a deoxygenated 0.1 M HCl
aqueous solution containing 60 mM TiO2 nanoparticles
from -0.28 to +1.22 V vs Ag/AgCl at 500 mV/s for
three scans without any duration at the cathodic and
anodic vertexes (called modified roughen Au substrate
prepared with this procedure). Finally, the potential
was hold at the cathodic vertex before the roughened
Au electrode was took from the solution and rinsed
throughout with deionized water. For comparison,
roughened Au substrates without the additive of TiO2
were also prepared by using the same roughening
condition (called unmodified roughen Au substrate
prepared with this procedure).
The ultrasonic irradiation was performed by using
an ultrasonic generator (model XL2000, Microson) and
operated at 20 kHz with a barium titanate oscillator of
3.2 mm diameter to deliver a power of 100 W.
2.3 Characteristics of gold nanoparticles and
decomposition of formaldehyde
A single drop of the sample-containing solution
was placed on a 300 mesh Cu/carbon film transmission
electron microscopy (TEM) sample grid and was
allowed to be dried in a vacuum oven. Then the sample
was examined by using a Philips Tecnai G2 F20
transmission electron microscopy (TEM).
Continuous concentrations of prepared 2 ppmv
formaldehyde were added in the reactor (30 ㎝×30 ㎝
×20 ㎝ ) and kept temperature at 22℃. The gas
detector
tube
(KITAGAWA, NO710A)
of
formaldehyde analyzed concentration of formaldehyde
gas after 5 days from irradiating visible light (>400
nm) to visible light-responsible TiO2 nanoparticles for
photoreaction. The HCHO will react with (NH2OH)3,
H3PO4 in the tube and then produce H3PO4 and
HCH=NOH in which the color change yellowish
orange for reddish orange. The measuring range of the
detector tube is 0.05-2.00 ppm and the detecting limit is
0.005 ppm.
3. Results and discussion
37
Decomposition of formaldehyde based on gold-coated TiO2 nanoparticles
Because the controllable and reproduced surface
roughness can be easily generated, the electrochemical
Oxidation-Reduction Cycles (ORC) procedure is a
better way to produce SERS-active metal substrates.
After the ORC procedure of roughening the Au
substrate, it would leave some unreduced species,
possibly positively charged Au clusters, on the Au
surface, which was shown in previous studies [18,22].
Fig. 1 shows the Cyclic Voltammograms (CV) for the
dissolution and redeposition of Au substrates in 0.1 M
HCl.
0.04
Current/mA
0.02
maximum band is observed. As shown in spectrum b of
Fig. 2, the absorbance maximum of Au-containing
complexes after the ORC treatment and before the
sonoelectrochemical reduction appears approximately
at 310 nm, which is markedly different from that of
zero-valent Au located at ca. 520 nm [23,24].
Correspondingly, the appearance of the characteristic
absorbance maximum at ca. 550 nm, as shown in
spectrum c of Fig. 1, after the sonoelectrochemical
reduction at an overpotential of 0.6 V reveals that the
elemental Au (0) caped with visible light-responsible
TiO2 nanoparticles can be readily synthesized by the
electrochemical reduction at room temperature under
ultrasonication in the solution containing Au
complexes, which is prepared in this study.
0
Absorbance/ a.u.
b
-0.02
-0.04
-0.4
0
0.4
0.8
1.2
c
1.6
E/ V vs. Ag/ AgCl
Fig. 1
In preparing well-dispersed metal nanoparticles in
aqueous solutions by electrochemical reduction
methods, one necessary requirement is to accelerate the
transfer of gold clusters formed from the cathodic
vicinity to the bulk solution. The sonification and the
mechanical stirring used in this study can satisfy this
requirement. Further experiments indicate that only the
mechanical stirring would result in serious deposition
of Au nanoparticles on the substrate. Similarly, only
sonification would cause the diffusion issue for the
reactant of Au-containing complexes. Fig. 2(a) shows
the UV-vis absorption spectrum of a blank solution of
0.1 M HCl, which was used as electrolytes in the ORC
treatment. As expected, no marked absorption
38
a
I-E curve for roughening gold substrates
200
400
600
800
Wavelength/nm
Fig. 2 UV-vis spectra of various solutions: (a) 0.1 M HCl
blank solution; (b) Au complexes-containing solution after
roughing the Au substrate; (c) Au nanoparticles-containing
solution after a sonoelectrochemical reduction treatment to
the Au complexes-containing solution
Fig. 3 illustrates the sonoelectrochemically
cathodic reduction process at an overpotential of 0.6 V
from positively charged Au-containing complexes to
elemental Au nanoparticles caped with visible
light-responsible TiO2 nanoparticles. The dispersion
and the particle size of the gold nanoparticles caped
Decomposition of formaldehyde based on gold-coated TiO2 nanoparticles
with visible light-responsible TiO2 nanoparticles
prepared at a cathodic overpotential of 0.6 V with
sonification in an aqueous solution are examined by
using the TEM micrograph, as shown in Fig. 2. The
nanoparticles with a diameter of ca. 50 nm demonstrate
gold nanoparticles (shadow in the figure) caped with
visible light-responsible TiO2 nanoparticles.
Fig. 3
TEM micrograph of Au nanoparticles, prepared at
a cathodic overpotential of 0.6 V from the OCP of 0.78 V vs
Ag/AgCl with sonification, showing size and dispersion
Unsupported Au nanoparticles exhibit activity for
the CO oxidation at room temperature [25]. However, as
shown in the literature [26,27], Au nanoparticles
supported on some oxidize supports can active for the
low-temperature oxidation of CO, too. By further
investigating their applications in other fields, it is
found that the electrochemically prepared Au
nanoparticles also demonstrate a catalytic activity for
the decomposition of aldehyde in alcohol solution [28].
It is known formaldehyde in air can cause damage to
lung of people. Removal of formaldehyde from air
would be contributive to human healthy. In this work,
the reactor containing 2 ppmv formaldehyde in air was
prepared to evaluate the catalytic activity of the
prepared Au nanoparticles caped with visible
light-responsible TiO2 nanoparticles for the
decomposition of formaldehyde. The gas detector tube
(KITAGAWA, NO710A) of formaldehyde analyzed
concentration of formaldehyde gas after 5 days from
irradiating visible light (>400 nm) to visible
light-responsible TiO2 nanoparticles for photoreaction.
The HCHO will react with (NH2OH)3, H3PO4 in the
tube and then produce H3PO4 and HCH=NOH in which
the color change yellowish orange for reddish orange.
The measuring range of the detector tube is 0.05-2.00
ppm and the detecting limit is 0.005 ppm. The content
of formaldehyde in the reactor is reduced from original
2 to 1.5 ppmv by adding visible light-responsible TiO2
nanoparticles in reactor after 5 days, so the
formaldehyde was decomposed ca. 25%. Varying
visible light-responsible TiO2 nanoparticles with the
prepared Au nanoparticles, the contents of
formaldehyde in air can be further reduced to 2 and 1.3
ppmv, so the formaldehyde was decomposed ca. 35%,
respectively. This result indicates that the
electrochemically prepared Au nanoparticles caped
with visible light-responsible TiO2 nanoparticles in this
work are higher catalysts (about 40%) for the
decomposition of formaldehyde than pure visible
light-responsible TiO2 nanoparticles.
4. Conclusions
In this work, the prepared Au nanoparticles caped
with visible light-responsible TiO2 nanoparticles were
prepared based on sonoelectrochemical methods in 0.1
M HCl of US pharmacopoeia grade from gold
substrates. The prepared Au nanoparticles caped with
visible light-responsible TiO2 nanoparticles are active
catalysts for the decomposition of 2 ppmv
formaldehyde in reactor. After 5 days testing, the
formaldehyde was decomposed by ca. 35% under
visible light irritation. In the same condition, the
formaldehyde was decomposed by ca. 25% under
visible
light
irritation
with
pure
visible
light-responsible TiO2 nanoparticles. This effect can be
extended to commercial visible light-responsible TiO2
39
Decomposition of formaldehyde based on gold-coated TiO2 nanoparticles
nanoparticles for completely removing formaldehyde
in indoor air.
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(Edited by Roy, Rita and Candice)