Design of gold nanoparticles-decorated SiO2@TiO2 core/shell nanostructures for visible light-activated photocatalysis

Article abstract of DOI:10.1039/c6ra27591e

In this study, we designed core/shell nanostructures (CSNs) of silicon dioxide (SiO₂)/titanium dioxide (TiO₂), which were decorated with gold (Au) nanoparticles (NPs), to activate the visible light-driven photocatalytic reaction that is not feasible with only the SiO₂@TiO₂ CSN because there is no absorption in the visible wavelengths. The CSNs were simply synthesized by hydrothermal method. In the core/shell structure, TiO₂ covered the SiO₂ core surface, which acts as a template. The attachment of additional Au NPs inside or outside the core/shell led to absorbance in visible wavelengths of light from the Sun, from which most radiation comes with a spectrum peak at yellow wavelength, owing to the localized surface plasmon resonance (LSPR) peak at approximately 550?nm. The SiO₂@Au@TiO₂@Au CSN (0.01 g) decomposed 1% of methyl orange (MO) solution in 15 mL deionized water in 1 h under the visible light. The visible light-driven photocatalytic reaction was ascribed to the LSPR around the Au NPs that were deposited inner or outer the mesoporous TiO₂ shell.

Full text of DOI:10.1039/c6ra27591e

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Design of gold nanoparticles-decorated SiO2@TiO2
core/shell nanostructures for visible light-activated
photocatalysis
Cite this: RSC Adv., 2017, 7, 7469
a
a
b
c
d
*
Ryeri Lee, Yogeenth Kumaresan, Sei Young Yoon, Soong Ho Um, Il Keun Kwon
a
*
and Gun Young Jung
In this study, we designed core/shell nanostructures (CSNs) of silicon dioxide (SiO2)/titanium dioxide (TiO2),
which were decorated with gold (Au) nanoparticles (NPs), to activate the visible light-driven photocatalytic
reaction that is not feasible with only the SiO2@TiO2 CSN because there is no absorption in the visible
wavelengths. The CSNs were simply synthesized by hydrothermal method. In the core/shell structure,
TiO2 covered the SiO2 core surface, which acts as a template. The attachment of additional Au NPs
inside or outside the core/shell led to absorbance in visible wavelengths of light from the Sun, from
which most radiation comes with a spectrum peak at yellow wavelength, owing to the localized surface
plasmon resonance (LSPR) peak at approximately 550 nm. The SiO2@Au@TiO2@Au CSN (0.01 g)
decomposed 1% of methyl orange (MO) solution in 15 mL deionized water in 1 h under the visible light.
The visible light-driven photocatalytic reaction was ascribed to the LSPR around the Au NPs that were
deposited inner or outer the mesoporous TiO2 shell.
Received 30th November 2016
Accepted 13th January 2017
DOI: 10.1039/c6ra27591e
www.rsc.org/advances
of the sun-light. To use the abundant visible light as an energy
source for photocatalytic activity, many different approaches
1 Introduction
have been made to narrow the TiO2 bandgap for absorbing the
light at visible wavelengths by doping the TiO2 with transition
metal dopants (tungsten, copper and iron)11–13 and anionic
nonmetal dopants (nitrogen and iodine).14,15 However, these
doping methods have many drawbacks such as a low doping
concentration, reduced electron–hole lifetime, and low corro-
sion resistance.16,17 Another method to generate the visible light
responsive TiO2 was achieved by using noble metals, e.g., gold
(Au), silver (Ag), and platinum (Pt).18–20 Noble metals are
successfully used because of their chemical stability and visible-
light absorption due to the localized surface plasmon resonance
(LSPR),21,22 in which the conducting electrons on the noble
metal surface undergo a collective oscillation induced by the
light at a certain wavelength. Thus, at a plasmon resonant
frequency the light absorption of noble metal NPs can be made.
Because of this unique effect, noble metals assist the photo-
catalytic activity of semiconductors by absorbing the light at
visible wavelengths and injecting the photo-generated electrons
from the noble metal into the conduction band of the
semiconductor.
TiO2 is a well-known mineral with many attractive characteris-
tics such as nontoxicity, chemical stability to acidic and/or basic
conditions, long-term stability, and good oxidizing ability.1,2
Thus, many researchers have studied the usage of TiO2 to
extend its applications in different elds such as UV blocking
pigments, self-cleaning materials, dye-sensitized solar cells,
humidity sensors, lithium-ion storage, and photocatalysts.3–9
TiO2 NPs have been used as an effective photocatalyst to solve
environmental problems by decomposing organic pollutants in
water into CO2 and H2O.9,10 When the sun light illuminates TiO2
in water, TiO2 oxidizes H2O molecules into hydroxyl radicals
(cOH), which can decompose the organic pollutants in water.10
However, TiO2 has a limited photocatalytic activity only in
the UV-light region (<400 nm) because of its wide bandgap
(ꢀ3.2 eV [388 nm] for the anatase crystalline phase) and fast
recombination of photo-generated electrons with holes.1 UV
light comprises a small fraction (approximately 4%) of the sun-
light spectrum, whereas visible light comprises 44.4% intensity
aSchool of Materials Science and Engineering, Gwangju Institute of Science and
Technology (GIST), Gwangju 61005, Republic of Korea. E-mail: gyjung@gist.ac.kr
bDepartment of Cosmetic Science, Daejeon Health Institute of Technology, Daejeon,
34504, Republic of Korea
Guo et al. reported that the Au NPs deposited at both sides of
5 nm thick TiO2 sheet showed an enhanced near-eld ampli-
tude of localized surface plasmon resonance between the nearly
touching Au NPs with a spacing of 5 nm, which could boost the
photocatalytic activity of TiO2 sheet.23 Recently, Kim et al.
synthesized Au NPs-decorated SiO2@amorphous TiO2 core/
shell structures and incorporated those into the photoanodes
cDepartment of Chemical Engineering, SKKU Advanced Institute of Nanotechnology
(SAINT), Sungkyunkwan University, Suwon, 16419, Republic of Korea
dDepartment of Maxillofacial Biomedical Engineering, Institute of Oral Biology, School
of Dentistry, Kyung Hee University, Seoul, 02447, Republic of Korea. E-mail: kwoni@
khu.ac.kr
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in dye-sensitized solar cells. Because of the plasmonic effect at was added into the boiling Au precursor solution for 15 min
the Au NPs, short-circuit current increased and power conver- until the solution changed from colourless to red violet.
sion efficiency was accordingly enhanced by 14%.24 In addition,
Hong et al. introduced 60 nm-sized Au NPs-attached TiO2
nanobers into an electron-transporting layer in perovskite
solar cells and improved the conversion efficiency by 15%
because of the LSPR effect.25
2.3 Synthesis of SiO2@Au@TiO2@Au CSN
To obtain the SiO2@Au@TiO2 CSN, 15 mM TBT in ethanol was
injected using a syringe pump into 50 mL ethanol solution
having the dispersed SiO2@Au CSN under mild stirring (300
rpm). Aer adding 4 mL DI water, the solution was transferred
Until now, the location of Au NPs (inner or outer of the TiO2
shell) and the number of Au NP layers within the CSNs, which
can play a signicant role in enhancing the optical properties in
the visible-light region, have not been deeply studied. In this
study, we designed various visible light-driven core/shell
nanostructured (CSN) photocatalysts, in which the SiO2 core
was decorated with various arrangements of both Au NPs layer
and TiO2 shell. To investigate the LSPR effect of various CSNs on
the photocatalytic activity, their optical properties and methyl
orange (MO) decomposition abilities were compared among
them, and with those of the commercially available mixed-
phase of anatase and rutile TiO2 (Degussa, P25).
ꢁ
to a Teon autoclave and placed in an oven at 130 C for 12 h.
Aer cooling to room temperate, the precipitate was washed
with ethanol 3 times using a centrifuge. The washed precipitate
of SiO2@Au@TiO2 CSN was dispersed in ethanol and mixed
with APTMS for further amine functionalization on the TiO2
shell surface as previously mentioned. Aer the amine func-
tionalization, the Au NP solution was added into the SiO2@-
Au@TiO2 CSN-dispersed DI solution for the nal Au NP
attachment to the outer shell of TiO2.
2.4 Photocatalytic experiments
Three different CSNs (SiO2@Au@TiO2, SiO2@TiO2@Au, and
SiO2@Au@TiO2@Au) were prepared to measure the photo-
catalytic activity, and to compare their activity with that of the
widely used P25 (TiO2, Degussa). The photocatalytic activity was
measured in ambient atmosphere at room temperature by
degrading the methyl orange (MO) solution with illumination
with an AM 1.5 simulated sunlight source (SANNEI solar
simulator, Class A). The UV light (l < 400 nm) was cut off using
a UV shield lm (SK-2, Sunnano) to observe the photocatalytic
activity under only visible light at a power density of 80 ꢂ 2.5
mW cmꢃ2. Each CSN powder (0.01 g) was added into 20 mL DI
water with 1 vol% of MO. The CSN suspensions were vigorously
stirred in the dark for 30 min to reach adsorption/desorption
equilibrium of MO molecules on the CSN surface. At every
15 min, 1 mL suspension was sampled and ltered to measure
the resident MO concentration using a UV-visible spectrometer
(AvaSpec-ULS2048L-USB2 Spectrometer, Jinyoung tech Inc.).
2 Experimental
2.1 Materials and reagents
All chemical reagents were of analytical grade and used without
further purication: ethanol (CH3CH2OH, anhydrous, Samchun
Chem.), tetraethyl orthosilicate (TEOS, Si(OC2H5)4, 99%, Sigma-
Aldrich Co., Ltd), ammonia hydroxide solution (NH4OH, 28–
30%, Sigma-Aldrich Co., Ltd), titanium butoxide (TBT,
Ti(OCH2CH2CH2CH3)4, 97%, Sigma-Aldrich Co., Ltd), (3-ami-
nopropyl)trimethoxysilane (APTMS, H2N(CH2)3Si(OCH3)3, 97%,
Sigma-Aldrich Co., Ltd), gold(III) chloride hydrate (HAuCl4-
$xH2O, 99.999%, Sigma-Aldrich Co., Ltd), sodium citrate
tribasic dehydrate (HOC(COONa)(CH2COONa)2$2H2O, 99.9%,
Sigma-Aldrich Co., Ltd), and P25. Commercial MO (C14H14N3-
NaO3S, 0.1%, Sigma-Aldrich Co., Ltd) was used as a target dye.
All aqueous solutions were prepared with deionized (DI) water,
ltered using a Milli-Q plus system (r $ 18 MU cm).
3 Result and discussion
2.2 Synthesis of SiO2@Au CSN
3.1 Structure observation
¨
SiO2 spheres were produced using the well-known Stober
method.26 0.3 mM TEOS in 80 mL ethanol as a SiO2 precursor We synthesized the SiO2@Au@TiO2@Au CSN by coating the Au
was mixed with 3 mL ammonia hydroxide and 5 mL DI water, NPs and TiO2 on the SiO2 core in sequence as depicted in Fig. 1.
and stirred at room temperature for 12 h. Amine functionali- The amine (NH3+) functionalization facilitated the attachment
zation at the SiO2 sphere surface is necessary for the attachment of Au NPs onto the both SiO2 core and TiO2 shell surface.
of Au NPs by electrostatic interaction. 1 mL APTMS was slowly Therefore, uniform amine functionalization was the most
added into the prepared SiO2 spheres-dispersed DI solution important step to achieve the dense coverage of Au NPs. To
under mild stirring (400 rpm), and the solution was placed in obtain the uniform amine functionalization, dispersion of the
a conventional oven at 70 ꢁC for 4 h to initiate the amine SiO2 spheres in ethanol at a low concentration was necessary.
functionalization at the Au NP surface. Aer the reaction, the The Au NPs prepared using the Turkevich method were sur-
amine functionalized SiO2 spheres were thoroughly washed rounded with negatively charged citrate ions (C3H5O(COO)33ꢃ),
three times using a centrifuge (5000 rpm for 30 min). The which improved the dispersion and stabilization of Au NPs in
collected amine-functionalized SiO2 spheres were mixed with the solution owing to the electrostatic repulsion between them.
the Au NP solution, which was prepared using the Turkevich Meanwhile, those negatively charged citrate ions electrostati-
method;27 0.5 mM gold(III) chloride hydrate was added into 30 cally attracted the positively charged NH3+ functionalized SiO2
mL boiling DI water (hot-plate temperature: 300 ꢁC) with spheres to form the Au NPs coating. Scanning electron micro-
vigorous stirring (800 rpm). 100 mM sodium citrate DI solution scope (FE-SEM, JEOL 2010F) images of the SiO2, SiO2@Au,
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Fig.
1 Schematic representation of the SiO2@Au@TiO2@Au CSN
synthesis.
SiO2@Au@TiO2, and SiO2@Au@TiO2@Au CSNs are shown in
Fig. 2a–d. Fig. 2a shows the 200 nm-sized SiO2 core with
a smooth surface. In contrast, the SiO2@Au has a rough surface
with the Au NPs (Fig. 2b). The high-resolution image of it (inset
of Fig. 2b) shows the average size of Au NPs of 15 nm. Fig. 2c
shows two SiO2@Au@TiO2 CSNs aer complete coverage with
the TiO2 shell. In this case, the Au NPs are invisible aer coating
the 30 nm-thick TiO2 shell, which was composed of a few
nanometer-sized TiO2 NPs with a relatively smooth SiO2@-
Au@TiO2 CSN surface.
Fig. 2 SEM images of (a) bare SiO2, (b) SiO2@Au, (c) SiO2@Au@TiO2,
and (d) SiO2@Au@TiO2@Au. (e) HAADF-STEM image and the corre-
sponding EDS element mapping images of silicon (Si, purple), gold (Au,
yellow) and titanium (Ti, skyblue) for the SiO2@Au@TiO2 CSN. The inset
of (c) is the high-resolution TEM image of Au NPs on the SiO2 sphere
surface.
The TiO2 shell was formed by two different mechanisms
above 130 ꢁC: intermediate titanium hydroxide (Ti(OH)4) was
formed in the hydrolysis reaction (i) and converted into TiO2 by
condensation reaction (ii) or TBT decomposition by reaction
with the intermediate Ti(OH)4 into TiO2 and by-product of
butanol (iii):28
corresponding elemental mappings of Si, Au and Ti. It is shown
that the Au NPs coated the SiO2 core surface and were then
thoroughly covered by the TiO2 shell.
Fig. 3 shows the X-ray diffraction (XRD) patterns from three
different samples using Cu K-alpha radiation (40 kV, 100 mA,
Rigaku D/max-2400). The broad XRD pattern between 20ꢁ and
30ꢁ comes from the amorphous SiO2 sphere (JCPDS 29-0085,
ICSD data).29 The SiO2@Au powder revealed two sharp peaks at
38.18ꢁ and 44.38ꢁ, which correspond to the Au cubic crystal
(111) and (200) peaks, respectively (JCPDS 65-2870).30 The
diffraction peaks of the anatase TiO2 crystal planes of (101),
(200), (105) and (211) were observed from the SiO2@-
Au@TiO2@Au powder (JCPDS-521-1272, ICSD data),31 which
were not shown in the SiO2@Au diffraction pattern. These
peaks indicate that the synthesized TiO2 shell had anatase
crystallinity, which has been reported to show the best photo-
catalytic activity among various TiO2 crystallinities (rutile,
brookite, and anatase).32 Only Au, TiO2, and SiO2 diffraction
Ti(OCH2CH2CH2CH3)4 + 4H2O /
Ti(OH)4 + 4CH3CH2CH2CH2OH
(i)
Ti(OH)4 / TiO2 + 2H2O
(ii)
Ti(OCH2CH2CH2CH3)4 + Ti(OH)4 /
2TiO2 + 4CH3CH2CH2CH2OH
(iii)
Fig. 2d shows the Au NPs that were coated onto the entire
surface of a SiO2@Au@TiO2 CSN. For the Au NP attachment, the
NH3+ functionalization treatment was also performed to the
TiO2 shell. Fig. 2e shows a high-angle annular dark-eld
(HAADF) image of the SiO2@Au@TiO2 CSN and the
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absorption peak from the Au NPs is broadened and shied to
the red.35 In our case, when the Au NPs were located at both the
inner and outer TiO2 shell, the LSPR absorption band was
interestingly broadened through visible wavelengths and even
extended to the infrared region, which could be caused by the
plasmonic coupling between the two Au NPs layers with a gap of
30 nm-thick TiO2 dielectric shell. We made three identical
sample sets, showing the similar absorption spectra tendency
among the samples.
3.3 Photocatalytic activity of core/shell materials under
visible light
In order to test the visible light-driven photocatalytic ability of
the synthesized CSNs, the MO solution was photocatalytically
degraded by illuminating the MO solution containing the
dispersed CSN powders with visible light. MO is widely used by
Fig. 3 X-ray diffraction patterns of as-prepared SiO2, SiO2@Au and chemists to indicate the titration of acid by observing its colour
SiO2@Au@TiO2@Au CSNs.
change below pH 3.1. Because of its mutagenic properties and
acidity (pH 3.47), direct contact should be avoided for health
concern.36 Therefore, the decomposition of MO is required for
peaks were observed, demonstrating that no impurities
remained in the synthesized CSNs.
health concern and environment safety.
At rst, the MO solution without any photocatalyst was
irradiated to check the photolysis of MO itself under visible
wavelengths with a simulated sunlight source at an intensity of
80 ꢂ 2.5 mW cmꢃ2. But, no photolytic reaction of the only MO
solution to the visible light was observed. Fig. 5a–d compare the
absorbance variations of MO aer photodegradation under
visible light. Compared to the commercially available P25, all
Au-decorated CSN samples obviously showed enhanced visible
light-driven photocatalytic activities. The intensity of MO
absorption peak (l ¼ 485 nm) decreased with the irradiation
time. Meanwhile, in the case of P25 sample, the MO was not
decomposed at all with the irradiation time because the visible
light cannot provide sufficient energy to excite the electrons
over its large bandgap to generate electron–hole pairs. However,
in the case of Au NPs-coated CSN samples (SiO2@Au@TiO2,
SiO2@TiO2@Au, and SiO2@Au@TiO2@Au), electrons are
generated under visible light because of the LSPR at the Au NPs
and transferred to the conduction band of TiO2 within femto
seconds as shown in Fig. 6.37 The transferred electrons can
generate hydroxyl radicals (cOH) at the TiO2/water interface,38,39
which eventually decompose the MO in the solution into non-
hazardous by-products. Meanwhile, aer electrons' transfer to
the TiO2, the holes remained in the Au NPs oxidized the inter-
facing water to produce the hydroxyl radicals, which addition-
ally decomposed the MO molecules.40–44
3.2 Optical characteristics of core/shell materials
The UV-Vis. absorption spectra of ethanol solutions (4 mL) that
contained 0.001 g of each synthesized SiO2@TiO2, SiO2@-
Au@TiO2, SiO2@TiO2@Au, and SiO2@Au@TiO2@Au CSN are
shown in Fig. 4. The four SiO2@TiO2-based CSNs have an
absorption in the UV range and absorption edge at 313 nm.
Aer the Au NPs attachment, the SiO2@TiO2@Au sample had
an absorption peak at 550 nm, which is ascribed to the collec-
tive oscillation of metal conduction electrons driven by
a specic wavelength (LSPR). This peak was red-shied to
572 nm in the SiO2@Au@TiO2 sample. Because the LSPR is
sensitive to the complex dielectric environments,33 the number
of neighboring Au NPs,34 and interparticle spacing of Au NPs,35
the LSPR peak from the Au NPs can shi depending on their
arrangement and location. It is reported that when the inter-
particle spacing becomes smaller lithographically, the
The SiO2@Au@TiO2@Au sample had the best photocatalytic
performance among the CSN samples. In this case, both sides of
the TiO2 shell were coated by the Au NPs, which generated more
visible light-driven electrons and consequently hydroxyl radi-
cals at the TiO2/water interface.
Fig. 5e shows the degradation efficiency of the photocatalyst
by measuring the change in MO concentration (C/C0) with
irradiation time (C0 is the initial concentration and C is the MO
concentration aer a certain irradiation time). The degradation
efficiency is dened as follows:
Fig. 4 UV-visible absorption spectra of various SiO2@TiO2 CSNs.
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Fig. 5 UV-vis absorption spectra showing the degradation of MO solution with varying irradiation time under visible light by (a) P25, (b)
SiO2@TiO2@Au, (c) SiO2@Au@TiO2, and (d) SiO2@Au@TiO2@Au. (e) A comparison of the photocatalytic activity of reference, SiO2, SiO2@TiO2, the
Au NPs-decorated CSN photocatalysts and P25 under the visible light.
shell around the 200 nm diameter SiO2, the amount of TiO2
material is minimal (roughly only 0.0017 g in each suspension
sampling, which was simply calculated from the volume ratio
and density of the SiO2 and TiO2 with ignorance of the weight of
Au NPs). However, in comparison to the P25 sample, the Au-
decorated CSN samples (SiO2@Au@TiO2, SiO2@TiO2@Au,
and SiO2@Au@TiO2@Au CSN) demonstrated an active photo-
catalytic ability under the visible light.
3.4 Porosity study of SiO2@TiO2 core/shell
To evidence the inltration of MO solution to the inner Au NPs
Fig. 6 Photocatalytic mechanism of the Au NPs-decorated SiO2@-
layer, the porosity of TiO2 shell was checked by the Brunauer–
Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH)
methods. A TEM image of Fig. 7a depicts the SiO2 spheres
coated with the mesoporous TiO2 shell, which is composed of
a few nm-sized nanoparticles as shown in inset gure.
Adsorption/desorption of nitrogen (N2) analysis was performed
with a ASAP 2020 (Micromeritics, USA) at 77.30 K. Prior to
performing the BET experiment, the SiO2 and SiO2@TiO2
powder samples were degassed at 150 ꢁC. In Fig. 7b, the N2 gas
adsorption/desorption isotherm curves for SiO2 and SiO2@TiO2
samples follow the type (IV) isotherm curve with a hysteresis
loop, representing the mesoporosity of both samples.46 The BET
surface area of SiO2@TiO2 and SiO2 were 12.83 m2 gꢃ1 and
134.88 m2 gꢃ1, respectively. Hence, the SiO2@TiO2 powder
Au@TiO2 CSN under the visible light irradiation.
Degradation (%) ¼ (1 ꢃ C/C0) ꢄ 100%
There was no photocatalytic activity with the SiO2, SiO2@-
TiO2 and P25 samples, the Au-decorated CSNs decomposed
nearly the entire MO quantity in an hour under visible light,
illustrating that the visible light was absorbed at the Au NPs due
to LSPR. The photodegradation rate is expressed as follows:45
ln(C/C0) ¼ ꢃkt
where, k (minꢃ1) is the degradation rate constant and is calcu- absorbed much larger amount of N2 gas than that of the SiO2
lated by tting the experimental data curve with an exponential powder owing to the mesoporous TiO2 shell. In comparison, the
function. The k values of the SiO2@TiO2@Au, SiO2@Au@TiO2, previously synthesized SiO2@TiO2 core/shell had a surface area
and SiO2@Au@TiO2@Au CSNs are 4.74 ꢄ 10ꢃ2, 4.45 ꢄ 10ꢃ2
,
of only 14.6 m2 gꢃ1
.
Owing to the higher TiO2 shell meso-
47
and 5.69 ꢄ 10ꢃ2, respectively. Considering the 30 nm-thick TiO2 porosity and closely packed Au NPs of our core/shell structure, it
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revealed light absorption in visible wavelengths because of the
localized surface plasmonic resonance at the Au NPs and even
in infrared wavelengths in the case of SiO2@Au@TiO2@Au CSN.
Therefore, the Au NPs-decorated CSNs decomposed MO solu-
tion under the visible light. The plasmonic effect of Au NPs led
to electron generation under the visible light, which could not
occur for P25. In 1 h, 0.01 g Au NPs-decorated CSNs (SiO2@-
Au@TiO2, SiO2@TiO2@Au, SiO2@Au@TiO2@Au) decomposed
1 vol% MO in 20 mL DI water.
Acknowledgements
This work was supported by the Pioneer Research Center
Program (NRF-2015M3C1A3022548) and by the “GRI (GIST
Research Institute)” Project through a grant provided by GIST in
2016.
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