Constructing Ordered Three-Dimensional TiO2 Channels for Enhanced Visible-Light Photocatalytic Performance in CO2 Conversion Induced by Au Nanoparticles

Article abstract of DOI:10.1002/asia.201701807

As a typical photocatalyst for CO₂ reduction, practical applications of TiO₂ still suffer from low photocatalytic efficiency and limited visible-light absorption. Herein, a novel Au-nanoparticle (NP)-decorated ordered mesoporous TiO<

Full text of DOI:10.1002/asia.201701807

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Constructing ordered three-dimensional channels of TiO2 for
enhanced visible-light photo-catalytic performance of CO2
conversion induced by Au nanoparticles
Authors: Hairong Xue, Tao Wang, Hao Gong, Hu Guo, Xiaoli Fan,
Bin Gao, Yaya Feng, Xianguang Meng, Xianli Huang, and
Jianping He
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Constructing ordered three-dimensional channels of TiO2 for
enhanced visible-light photo-catalytic performance of CO2
conversion induced by Au nanoparticles
Hairong Xue,[a,b] Tao Wang,*[b] Hao Gong,[b] Hu Guo,[b] Xiaoli Fan,[b] Bin Gao,[b] Yaya Feng,[b]
Xianguang Meng,[c] and Jianping He*[b]
Abstract: As a typical photo-catalyst for CO2 reduction, the practical
application of TiO2 still suffers from low photo-catalytic efficiency and
limited visible light absorption. Here, a novel Au-nanoparticle (NP)-
decorated ordered mesoporous TiO2 (OMT) composite (OMT-Au)
was successfully fabricated, in which Au NPs uniformly disperse on
the OMT. Due to the surface plasmon resonance (SPR) effect
derived from the excited Au NPs, TiO2 possesses high photo-
catalytic performance for CO2 reduction under visible light. The
ordered mesoporous exhibits the superiority of material and
structure. This ordered mesoporous structure of TiO2 with a high
surface area offers more catalytic activity sites. More importantly, the
three-dimensional transport channels ensures smooth flow of gas
molecules, high-efficient adsorption ability of CO2, and the fast and
steady transmission of hot electrons excitation on Au NPs, thus
leading to further improving photo-catalytic performance. These
results highlight the possibility of improving visible light photo-
catalysis for CO2 reduction by constructing OMT-based Au-SPR-
induced photo-catalysts.
constraints, including significant photo-catalytic efficiency loss
and limited absorption of visible light photos.[15-18] Recently,
because of high specific surface area and plentifully
interconnected porous network, some mesoporous photo-
catalysts have been explored for photo-catalysing CO2
reduction.[19-22] In addition, compared with the photo-catalysts
with disordered mesoporous structure, intensive research efforts
shown that ordered mesoporous materials can effectively
separate photo-induced electrons and facilitate the diffusion of
reactants, thus leading to enhanced photo-catalytic activity.[23-30]
Therefore, benefiting from ordered mesoporous channels and
uniformly distributed porosity, ordered mesoporous TiO2 shows
the high potential as a promising candidate for photo-catalysis in
conversion of CO2.
Although the photo-catalytic activity of TiO2 for CO2 reduction
can be effectively improved by constructing ordered mesoporous
structure, the utilization of solar energy still should be increased.
As
a cutting-edge research field, it was discovered that
plasmonic photo-catalysis originated from the surface plasmon
resonance (SPR) has been applied in CO2 reduction, organic
transformation, and environmental, served as a visible-light
photo-catalysis.[31-39] Based on the SPR of plasmonic-metal NPs
(e.g., Au), their surface electrons can collectively oscillate
induced through the resonant photon, simultaneously energetic
hot electrons are produced by non-radiative excitation[33,40]. In
the SPR process, because of the short lifetime of the hot
electrons (~ 10−3 ns), a semiconductor are often used in
conjunction with plasmonic-metal NPs.[40-41] Due to the hot
electron injection into a semiconductor, lifetime of the hot
electrons can be effectively extended (~100 ns).[40,42-43] For this
plasmonic-metal NPs/semiconductor composite, it is shown that
the conduction band (CB) of the semiconductor can be injected
with the hot electrons of plasmonic-metal NPs over the Schottky
barrier, which can intrinsically drive the visible-light photo-
catalysis[40]. Over the past few years, some researches have
shown that the semiconductors incorporated with plasmonic-
metal nanoparticles (NPs) shown enhanced visible-light
absorption, which was beneficial to overcome the limitation of
photo-absorption.[44-46] For example, plasmonic Au/TiO2 photo-
catalyst has been reported that H2 generation rate can be
significantly increased under visible light irradiation in alkaline
reaction environment, benefiting from the effectively suppressive
recombination of holes and hot electrons through fast
consumption of the holes in Au NPs.[44] Moreover, when BiOCl
nanosheets are modified with Au NPs to construct plasmonic
photo-catalyst, the Au-induced high aerobic oxidation activity for
2-propanol under visible-light can be obtained due to the more
efficient hot-electron transfer.[45] In both water oxidation and iso-
Introduction
Photo-catalytic CO2 reduction has attracted increasing research
attention due to its tremendous potential in solving
environmental and energy crisis, which provides a renewable
approach for producing the valuable hydrocarbon compounds.[1-
7]
Normally, CO2, as an oxidized form of carbon, is very stable
and difficult to be activated in the reduction process.[8] For this
challenge of CO2 conversion, various semiconductor photo-
catalysis have been explored to accelerate the photo-catalytic
CO2 conversion.[9-13] Thereinto, TiO2 is a typical wide-band gap
semiconductor, which has served as the most common photo-
catalysts in photo-catalytic CO2 reduction.[14] However, the
practical application of TiO2 still suffers from some major
[a]
[b]
Dr. H. Xue
College of Chemical Engineering, Zhejiang University of Technology
Hangzhou, Zhejiang 310014, P.R. China.
Prof. Dr. T. Wang,* H. Gong, H. Guo, X. Fan, B. Gao, Y. Feng, Prof.
Dr. J. He*
College of Materials Science and Technology, Jiangsu Key
Laboratory of Materials and Technology for Energy Conversion,
Nanjing University of Aeronautics and Astronautics, Nanjing 210016,
P.R. China.
E-mail: wangtao0729@nuaa.edu.cn; jianph@nuaa.edu.cn.
Prof. Dr. X. Meng
[c]
Photo-functional Materials Research Platform, College of Materials
Science and Engineering, North China University of Science and
Technology, Tangshan 063210, P.R. China.
Supporting information for this article is given via a link at the end of
the document.
propanol
degradation,
Au–SrTiO3–TiO2
photo-catalyst
possesses multicomponent junction nanostructure, which
exhibits the effectively enhanced Au-induced photo-catalytic
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10.1002/asia.201701807
Chemistry - An Asian Journal
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performance under visible light.[46] Obviously, both plasmonic-
metal NPs and semiconductor are two indispensable
counterparts of the plasmonic photo-catalysts. During a long
period in the past, the intense investigation of these photo-
catalysts focus on the function of the plasmonic-metal NPs, such
as sizes, shapes and so on.[31-32,47-48] In contrast, there is much
less attention for the semiconductor, which is used as a hot
electron acceptor. Until recently, for the plasmonic photo-
catalysts, the essential role of semiconductor materials was
preliminarily recognized, some researchers began to explore the
relationship between the photo-catalytic activity and structure of
the semiconductor.[44-46] Compared with the common TiO2 NPs,
mesocrystal TiO2 can accumulate more hot electrons within a
much shorter time in the SPR process, thus leading to the longer
lifetime and enhanced photo-catalytic activity of the Au-SPR-
induced photo-catalysts. In order to develop and design efficient
Au-induced visible-light photo-catalysts, the modification of the
semiconductor should be one of great importance.
part can cover on the surface of OMT. Finally, Au-incorporated
OMT composite can be successfully synthesized by the
reducing action of ascorbic acid, in which the Au NPs uniformly
distribute into the channels or on the surface of OMT.
Figure 1. Schematics showing preparation of OMT-Au composite.
Herein, we report the Au-SPR-induced photo-catalytic
performance under visible-light in CO2 reduction can be
effectively improved by constructing an ordered mesoporous
TiO2-Au (OMT-Au) plasmonic photo-catalyst. Compared with
commercial TiO2 nanopowders-Au (P25-Au) and disordered
mesoporous TiO2-Au (MT-Au), the photo-catalytic activity for
CO2 reduction of OMT-Au is much higher due to the high
specific surface area and ordered channel structure of OMT. In
addition, charge separation of OMT-Au is more efficient and
electron transport across the OMT-Au junction is also much fast.
As a result, for OMT-Au, the total reduced rate (CH4 + CO) is 10
and 7 times higher than those obtained over P25-Au and MT-Au,
respectively. More importantly, based on 13CO2 detection
technique, the C atoms of CH4 have been confirmed that they
are from CO2. These results indicate that rational design and
fabrication of OMT–Au can be an attractive method to develop
efficient Au-SPR-induced photo-catalysts for CO2 reduction
under visible-light.
The XRD patterns of OMT-Au, MT-Au and P25-Au composites
are shown in Figure 2(a), characterized their crystalline structure.
As shown in Figure 2(a), there are several intensity diffraction
peaks of the three samples in the XRD patterns, which are all in
accord with the standard card (JCPDS no. 21-1272) of anatase
phase TiO2. In addition, it also can be found some well-defined
characteristic XRD peaks of Au (JCPDS no. 01-089-3697). This
result shows the successful synthesis of the TiO2 based Au-
SPR-induced photo-catalysts. By using N2 adsorption-desorption
isotherms, we can analyze the pore structure of the three
samples. According to the IUPAC classification, type IV isotherm
can be observed in all samples shown in Figure 1(b), suggesting
that all samples have the typical mesoporous structure.[49-50]
Furthermore, it is noteworthy that OMT-Au composite shows a
H1-type hysteresis loop and an obvious N2 uptake occurring at
P/PO of 0.4-0.7, indicating the one-dimensional cylindrical
channels existed in the mesoporous materials. For MT-Au and
P25-Au composites, they all show H3-type hysteresis loops,
which are associated with the slit-shaped pores derived from the
aggregation of plate-like NPs. Figure 2(c) describes the pore-
size distribution curves of the samples. The curve illustrates that
OMT-Au composite possesses a narrow pore-size distribution.
Moreover, as shown in Table 1, the OMT-Au composite exhibits
the highest specific surface area, desirable pore volume and the
largest pore size, calculated by using BET method and BJH
model. Therefore, we infer that structural features of the OMT-
Au composite provide many advantages for improving the photo-
catalytic performance under visible-light in conversion of CO2.
The higher specific surface area can offer more catalytic activity
sites for photo-catalysis. At the same time, desirable pore
volume and large pore size are beneficial to sufficient infiltration
of electrolyte to obtain facile ion transportation.
Results and Discussion
Figure 1 displays the designed route for the preparation of OMT-
Au composite, which is synthesized by an EISA strategy
combined with impregnation-reduction method. Before the EISA
process, soft-template agent (F127), “binder” (phenolic resol
precursor) and metal salts (tetrabutyl titanateare) are uniformly
dissolved into ethanolic solution, which can co-assemble into
nanorod-like micelles. Subsequently, the obtained orange colour
solution is transferred into culture dish, followed by evaporation
of the ethanol solvent (room temperature) and thermal
polymerization (100 ºC). After thermosetting, the obtained dark
green film is further treated by thermal treatment (350 ºC) in N2
atmosphere, then the F127 template will be remove to form a
stable puce polymer-Co-TiO2. Then, well-crystallized TiO2 with
ordered mesoporous structure can be formed after removing
organics, heated at 400 º C in air. During the impregnation
process, a part of gold salt precursor can impregnate into the
channels of OMT through capillary action, meanwhile, another
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10.1002/asia.201701807
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enhancing the visible-light photo-catalytic performance for CO2
reduction.
Figure 2. (a) The XRD patterns; (b) N2 adsorption/desorption isotherms; (c)
pore size distribution curves of OMT-Au, MT-Au and P25-Au composites.
Table 1. Pore-structure parameters of OMT-Au, MT-Au and P25-Au
composites.
Sample
SBET/m2·g
Vtotal/cm3·g
D/nm
OMT-Au
MT-Au
181
148
72.9
0.26
0.21
0.11
5.8
5.5
6.2
P25-Au
Figure 3. (a,b) FESEM images; (c,d) TEM images; (e,f) STEM and EDX
mapping images of OMT-Au and MT-Au composite. (a,c,e): OMT-Au; (b,d,f):
MT-Au.
The particle size, dispersibility and crystallinity of OMT-Au,
MT-Au and P25-Au composites can be characterized by SEM
and TEM measurement. As shown in Figure 3(a), the OMT-Au
composite exhibits an obviously ordered mesoporous structure
of TiO2, and the Au NPs uniformly distribute on the surface of
OMT. Moreover, the ordered parallel mesoporous channel of the
OMT-Au composite can be clearly seen in Figure 3(c), whose
pore-size is estimated to be ∼5 nm. The HRTEM images (I)
(inset of Figure 3(c)) and Figure S1 further confirms the ordered
mesoporous structure of TiO2, which is made up of the abundant
high crystalline TiO2 NPs with a diameter of 5~10 nm. It also can
be found that Au NPs with the particle size of 20~25 nm
possesses the high crystallization, evidenced by their clear
lattice fringes calculated to be ~0.235 nm, corresponding to the
interlayer spacing of Au (111) facets. The evenly distribution of
Ti, O and Au elements in the OMT-Au NPs was described by the
EDX elemental mappings (Figure 3(e)), indicating the successful
combination between OMT and Au NPs. Contrastively, Although
MT-Au composite also can successfully combine with Au NPs, it
shows the disordered mesoporous structure of TiO2. For P25-Au
composite, it consists of TiO2 NPs and the particle size of TiO2 is
around 20~30 nm (Figure S2, Supporting Information).
Therefore, these results show OMT-Au composite has an
ordered mesoporous structure of TiO2, high crystallization of
NPs and uniform distribution of Au NPs, which are beneficial to
Figure 4(a) shows the UV-visible spectra of the OMT-Au, MT-
Au and P25-Au photo-catalysts. It can be seen that all photo-
catalysts exhibits typical absorption peaks in the 520-560 nm
wavelength range. As we all know, TiO2 served as a photo-
catalyst shows no response under visible-light. Therefore, these
above mentioned absorption peaks are attributed to the
existence of the Au NPs, which suggests that the hot electrons
can accumulate in TiO2 during the SPR process. We carried out
the CO2 photo-reduction reaction under visible light in a gas-
closed circulation system, in the presence of water vapor, CO2,
and photocatalysts. As shown in Figure 4(b) and (c), similar to a
lot of reports,[4,51] the CO2 is reduced to CH4 and CO, and the
amounts of products for OMT-Au are larger than the amounts of
products for MT-Au and P25-Au. The generated rates of CH4
and CO over the photocatalysts were calcuted according to the
amounts evolved in 6 h (Figure 4(d)). The generated rates of
CH4 over OMT-Au, MT-Au and P25-Au are 0.33, 0.28, and 0.21
μmol g-1 h-1, respectively. The generated rate of CO over OMT-
Au is 1.83 μmol g-1 h-1, while the amount of CO generated on
P25-Au and MT-Au was negligible. For CO2 reduction, the
overpotential of CH4 evolution is much smaller than that of CO
evolution. So, compared with CH4 evolution, the generated rate
of CO over OMT-Au is much higher than those of MT-Au and
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can contribute to reduction of CO2. Meanwhile, the holes in the
Au NPs can contribute to the oxidation of H2O. Therefore, the
combination of TiO2 and Au NPs can potentially improve the
photo-catalytic performance under visible-light in conversion of
CO2. Notably, CO2 reduction occurs at the semiconductor, so
the photo-catalytic activity and stability of the TiO2 have an
important influence on the photo-catalytic performance for CO2
reduction. In this work, OMT-Au composite as a plasmonic
photo-catalyst shows excellent photo-catalytic performance
mainly benefits by structural and material superiorities. The
structural features and photo-catalytic CO2 reduction mechanism
of OMT-Au composite is shown in Figure 5. As a photo-catalyst,
this ordered mesoporous TiO2 with the high specific surface area
provides more reaction sites, which exist both on the TiO2 NPs
surface and in the pores. Furthermore, this highly ordered
mesoporous structure of TiO2 can harvest more light due to the
multiple scattering. Their continuous pore channels ensure
smooth flow of gas molecules and high-efficient adsorption
ability of CO2. Moreover, three-dimensional network formed from
the porous TiO2 framework is beneficial to the fast and steady
transmission of hot electrons, thus leading to the enhanced
separation/transport of electron-hole. For MT-Au and P25-Au,
because of the presence of Au NPs, hot electrons also can be
produced by the excitation of SPR on Au NPs. However, either
P25 or MT shows the lower the photo-catalytic activity and
stability, due to the absence of the structural features of OMT.
Therefore, the photo-catalytic activity for CO2 reduction of OMT-
Au is much higher than that of MT-Au and P25-Au. As a new Au-
P25-Au, due to its higher photo-catalytic activity. The total
reduced rate (CH4 + CO) of CO2 over OMT-Au is 7 times higher
than the MT-Au, and 10 times higher than the P25-Au.
Furthermore, as shown in Figure 4(b), after 10 h, both of the
generated rates of CH4 on P25-Au and MT-Au rapidly reduce,
but the generated rates of CH4 on OMT-Au is still stable. In our
previous studies, we demonstrated that the better ordered
mesostructure of TiO2 is, the higher photo-catalytic activity and
stability for reduction of CO2.[21-22] The isotopic analysis of
carbon was necessary and conducted by using 13CO2 to replace
12CO2. The products were analyzed in the GC-MS spectrum. As
shown in Figure S3(a) and (b) (Supporting Information), the
peaks with the retention time 02:10 and 02:22 are confirmed to
be the products 13CO (named X) and 13CH4 (named Y) for OMT-
Au. The intensity ration of X+ and (X-13)+ of 13CO (m/z=29, 16)
matched that of CO (m/z=28, 16), and the intensity ration of Y+,
(Y-1)+ and (Y-2)+ of 13CH4 (m/z=17, 16, 15) matched that of
12CH4 (m/z=16, 15, 14). The result of the GC-MS spectrum for
12CO2 reduction is similar to that of 13CO2 reduction (Figure
S4(a,b)). That is to say, the products CO and CH4 are proved to
be changed from the reduction of CO2. In consequence, the
OMT-Au composite is a promising choice, served as an efficient
Au-SPR-induced photo-catalyst for CO2 reduction under visible-
light.
SPR-induced photo-catalyst, OMT-Au shows
a remarkable
photo-catalytic performance for CO2 reduction under visible light.
Figure 5. Possible mechanism of OMT-Au photo-catalysts in CO2 reduction.
Figure 4. (a) UV-vis absorption spectra and differential spectra (inset); (b) CH4
and (c) CO evolutions under visible light irradiation over OMT-Au, MT-Au and
P25-Au photo-catalysts; (d) Comparison of photo-catalytic activity of OMT-Au,
MT-Au and P25-Au photo-catalysts.
Conclusions
We have successfully fabricated a novel Au-nanoparticle-
decorated ordered mesoporous TiO2 composite (OMT-Au) by
using a multi-constituents co-assembly strategy combined with
impregnation-reduction method. For OMT-Au composite, TiO2
possesses high photo-catalytic performance for CO2 reduction
under visible light, due to the Au-induced SPR effect. Moreover,
photo-catalytic performance of OMT-Au composite can be
further improved, profiting from the ordered mesoporous
structure of TiO2. The smooth flow of gas molecules, high-
efficient adsorption ability of CO2, and the fast and steady
transmission of hot electrons can be obtained by the high
surface area and three-dimensional transport channels of OMT.
Figure 5 shows a schematic illustration of CO2 photo-reduction
through the SPR of Au NPs under visible light. Based on the
results of UV-visible spectra, this reaction process involves two
stages. On the one hand, under visible light illumination,
collective oscillation of electrons can produce hot electrons by
the excitation of SPR on Au NPs. Compared with Schottky
barrier energy φSB, the energies of hot electrons is higher, so
these hot electrons can be injected into the conduction band of
semiconductor (TiO2). Simultaneously, the holes can leave in the
Au NPs. On the other hand, the hot electrons injected into TiO2
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airtight circulation system was used to test the photo-reduction of CO2, in
which the light source is a 300 W xenon arc lamp equipped with a glass
filters (L-42). After uniformly dispersing the sample (0.1 g) on a porous
quartz fiber filter film in the reaction cell, the deionized H2O (3 mL) was
injected into the airtight system and then the pure CO2 gas (80 kPa) was
also introduced. A gas chromatograph (GC-14B, Shimadzu Co., Japan)
was used to measure the products. The isotope experiments of 13C were
performed by using a gas chromatographmass spectrum (JEOL-GCQMS,
JMS-K9 and 6890N Network GC system, Agilent Technologies). The
isotopic analysis of 13C was conducted by using 13CO2 to replace 12CO2,
and its process is the same as 12CO2.
Finally, for, the total reduced rate (CH4 + CO) of OMT-Au
composite is 10 and 7 times higher than those obtained over
P25-Au and MT-Au, respectively. More importantly, based on
13CO2 detection technique, the C atoms of CH4 have been
confirmed that they are from CO2. Therefore, design and
preparation of OMT-based Au-SPR-induced photo-catalysts is a
promising pathway to improving photo-catalysis for CO2
reduction under visible light.
Experimental Section
Acknowledgements
Preparation of OMT and MT composites:
The well-crystallized ordered mesoporous TiO2 was by evaporation
induced self-assembly process, following the two-step calcination
processes. Typically, of triblock copolymer Pluronic (2.0 g) was poured
into ethanol (20.0 mL) at 40 °C. After vigorous stirring for 1 h, soluble
resol (2.5 g) and tetrabutyl titanate (3.4 g) were added into above
solution under vigorous stirring for 2 h. The solution was poured into Petri
dishes and kept at 100 °C for 24 h in an oven. The obtained film was
then carbonized in the N2 atmosphere at 350 °C for 4 h, by using a
heating rate of 1 °C min-1. Finally, the ordered mesoporous TiO2 was
calcined at 400 °C for 5 h under air atmosphere with a heating rate of
1 °C min-1. The disordered mesoporous TiO2 can be obtained by using
the similar synthesis of OMT without adding soluble resol, named as MT.
This research was sponsored by the Natural Science
Foundation of Jiangsu Province (BK20160795), the National
Natural Science Foundation of China (51602153, 11575084, and
21703065), the Natural Science Foundation of Zhejiang
Province (LQ18B010005), the Research Start-Up Fund of NUAA
(90YAH16008), and a project funded by the Priority Academic
Program Development of Jiangsu Higher Education Institutions
(PAPD). The authors express their appreciations for the above
financial support.
Keywords: surface plasmon resonance • ordered mesoporous
structure • Au nanoparticles • visible-light photo-catalytic
performance • CO2 reduction
Preparation of OMT-Au, MT-Au and P25-Au composites:
Typically, potassium chloroaurate was dissolved into distilled water. This
homogeneous transparent solution was dropwise added into the as-
prepared OMT powder. Subsequently, excessive ascorbic acid aqueous
solution was further dropwise added into above OMT powder. During
drying in the vacuum oven, almost all of gold salt precursor was
converted into Au NPs, due to the reducibility of excessive ascorbic acid.
After drying, Au NPs-ordered mesoporous TiO2 composite with around 5
wt.% of Au content was formed, marked as OMT-Au. The preparation of
disordered mesoporous TiO2-Au NPs and commercial P25-Au NPs was
similar to the OMT-Au, marked as MT-Au and P25-Au, respectively.
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10.1002/asia.201701807
Chemistry - An Asian Journal
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Hairong Xue,[a,b] Tao Wang,*[b] Hao
Gong,[b] Hu Guo,[b] Xiaoli Fan,[b] Bin
Gao,[b] Yaya Feng,[b] Xianguang Meng,[c]
and Jianping He*[b]
Page No. – Page No.
Constructing ordered three-
A novel Au-nanoparticle (NP)-decorated ordered mesoporous TiO2 (OMT)
composite (OMT-Au) was successfully fabricated, in which Au NPs uniformly
disperse on the OMT. Due to the surface plasmon resonance (SPR) effect derived
from the excited Au NPs, TiO2 possesses high photo-catalytic performance for CO2
reduction under visible light.
dimensional channels of TiO2 for
enhanced visible-light photo-catalytic
performance of CO2 conversion
induced by Au nanoparticles
This article is protected by copyright. All rights reserved.