Dimension-matched plasmonic Au/TiO2/BiVO4 nanocomposites as efficient wide-visible-light photocatalysts to convert CO2 and mechanistic insights

Article abstract of DOI:10.1039/c8ta02889c

Dimension (D)-matched plasmonic Au/TiO₂/BiVO₄ nanocomposites have been successfully constructed by first preparing BiVO₄ nanoflakes via an ion exchange process from BiOCl nanosheets, followed by coupling (001) facet-exposed anatase TiO₂ nanosheets and then modifying plasmonic Au nanorods. The well-designed 1D/2D/2D nanocomposites exhibit exceptional visible-light photocatalytic activities for CO₂ conversion, and these nanocomposites indicate ～30-fold and ～60-fold enhancements compared to the resulting BiVO₄ nanoflakes and the previously-reported BiVO₄ nanoparticles, respectively. Based on steady-state surface photovoltage spectroscopy, transient-state surface photovoltage responses, wavelength-dependent photocurrent action spectra and fluorescence spectra related to the amount of produced OH species, it is confirmed that the exceptional photocatalytic activity can be comprehensively ascribed to the following phenomena which result from the fabrication of 2D nanoflakes: increased specific surface area of BiVO₄, coupling of TiO₂ nanosheets to form a new proper-energy platform, which can accept photogenerated electrons from BiVO₄ and plasmonic Au so as to greatly enhance the charge separation, and modified plasmonic Au with the extended wide-visible-light absorption to 660 nm, which can act as the co-catalyst for the transferred electrons to induce reduction reactions. Moreover, dimension matching in the fabricated nanocomposite is very favorable for photogenerated charge transfer and separation. Furthermore, isotope experiments of ¹³CO₂ and D₂O along with electrochemical measurements suggest that the produced H atoms are dominant active radicals, which can initiate the conversion of CO₂.

Full text of DOI:10.1039/c8ta02889c

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
rsc.li/materials-a

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Dimension-matched plasmonic Au/TiO /BiVO nanocomposites as
2
4
efficient wide-visible-light photocatalysts to convert CO and
mechanism insights
2
Received 00th January 20xx,
Accepted 00th January 20xx
a,b
a
a
a
b
a
DOI: 10.1039/x0xx00000x
Ji Bian, Yang Qu,* Xuliang Zhang, Ning Sun, Dongyan Tang and Liqiang Jing*
www.rsc.org/
Dimension (D)-matched plasmonic Au/TiO
BiVO nanoflakes via an ion exchange process from BiOCl nanosheets, followed by coupling (001) facet-exposed anatase
TiO nanosheets and then modifying plasmonic Au nanorods. The well-designed 1D/2D/2D nanocomposites exhibit
exceptional visible-light photocatalytic activities for CO conversion, by ~30-fold and ~60-fold enhancement compared to
the resulting BiVO nanoflakes and the previously-reported BiVO nanoparticles, respectively. Based on the steady-state
2 4
/BiVO nanocomposites have been successfully constructed by firstly preparing
4
2
2
4
4
surface photovoltage spectroscopy, transient-state surface photovoltage responses, wavelength-dependent photocurrent
action spectra and fluorescence spectra related to the produced •OH amounts, it is confirmed that the exceptional
photocatalytic activity is comprehensively attributed to the fabricated 2D nanoflakes to increase the specific surface area
4 2 4
of BiVO , to the coupled TiO nanosheets as a new proper-energy platform to accept photogenerated electrons from BiVO
and plasmonic Au so as to greatly enhance the charge separation, and to the modified plasmonic Au with the extended
wide-visible-light absorption to 660 nm to take as the co-catalyst for the transferred electrons to induce reduction
reactions. Moreover, the dimension matching in the fabricated nanocomposite is much favorable for the photogenerated
1
3
charge transfer and separation. Furthermore, it is suggested by isotope experiments of CO
2 2
and D O, along with
electrochemical measurements, the produced H atoms are dominant active radicals to initiate the conversion of CO
2
.
techniques have been intensively investigated toward the
objective, however, these techniques are always accompanied
Introduction
with low mechanical strength, less stability, catalyst poisoning,
[5-6]
The increasing depletion of world-wide energy caused by the
rapid consumption of fossil fuels raises an event of concerns
and electrode corrosion.
Hence, it is necessary to develop
alternative techniques to overcome the disadvantages
mentioned above. Semiconductor photocatalytic technology is
a promising approach to solve the energy and environmental
challenges mainly due to its mild reaction conditions, good
[
1]
about the energy shortage and global warming. Nowadays,
1% of the energy comes from the burning of mineral
resources. This huge usage will make it speedy dried up, and
also result in excessive emission of carbon dioxide (CO ) into
the atmosphere, leading to the environmental detrimentally
8
[7-8]
2
duration and no collateral contamination. Therefore, it is a
feasible way to address the issues of excessive emission of CO
with clean energy production.
2
[2]
greenhouse effect. In this regard, developing renewable
energy and converting greenhouse gas CO into hydrocarbon
fuel is a desirable pathway to address both energy crisis and
2
Since there are ca. 46% visible light in the solar spectrum, it
is essential to develop visible-light responsive photocatalysts
[3-4]
environmental issues.
Although some traditional
[9-11]
with narrow bandgap, such as Fe
Hereinto, BiVO
nontoxicity, high chemical stability and appropriate bandgap
2 3 3 4
O , WO and BiVO .
4
is a promising photocatalyst owing to its
[12-13]
structure.
4
However, the photocatalytic activity of BiVO is
still unsatisfactory mainly due to the rather small specific
surface area, sluggish charge separation and limited visible-
light absorption. Therefore, it is of great significance to
improve the photocatalytic activities of CO
2
reduction
synergistically by increasing specific surface area, enhancing
charge separation and extending visible-light absorption.
In general, the specific surface area of BiVO
4
is
comparatively small, attributed to the resulting large particle
size. This is because of its small solubility product constant in
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the aqueous solution which leads to the rapid nucleation and different parts in the fabricated heterojunctions for efficient
[14]
crystallite growth. Although various strategies like creating photocatalysis.
pores and decreasing particle size have been developed to It is noticed that the visible-light absorption of BiVO
increase the surface area of BiVO with some success, the limited with the threshold of ~ 570 nm. In fact, it is feasible to
resulting surface area is still unsatisfactory and/or begin from ~530 nm when TiO
4
is
4
[20]
2
is used to modify it. Thus, it
In addition, the is highly desired to extent the visible-light utilization range of
porous structure may be collapsed during the photocatalytic BiVO . Recently, plasmonic metals like Au and Ag have driven
reactions, consequently leading to reduced surface area and much attention because of their strong and wide visible-light
[15-16]
accompanied by poor structure stability.
4
[24-25]
declined activity. These factors lead to the limited responses, especially for Au.
improvement of photocatalytic activities of BiVO . In recent demonstrated that the photocatalytic oxidation of
years, two-dimensional (2D) nanostructured materials have formaldehyde could be achieved over plasmonic Au/TiO
drawn much attentions due to their relative large surface area under visible-light irradiation, in which the hot electrons
Zhu and coworkers
4
2
[
17-18]
and good charge transportation capacity.
spurs us to obtain 2D nanostructured BiVO
This naturally excited by surface plasmonic resonance (SPR) Au could
for efficient overcome the Schottky barrier to transfer to the conduction
4
photocatalysis. However, it is difficult to directly prepare 2D band of TiO
BiVO via traditional methods due to its too small solubility reaction on TiO
product constant. Thus, it is highly desired to develop a new much feasible to couple with plasmonic Au to the pre-
method to prepare 2D BiVO . Notably, BiOCl, another bismuth- prepared TiO /BiVO nanocomposite for wide visible-light
based material is sheet-like and unstable. In this case, it is responsive photocatalysis. Simultaneously, the coupled Au
expected that 2D BiVO with large specific surface area would would still exhibits good catalytic functions to the transferred
2
and subsequently initiate the photocatalytic
[26]
4
2
surfaces. Naturally, it is expected that it is
4
2
4
4
be obtained by a controllable ion exchange process from BiOCl electrons for inducing reduction reactions. For SPR Au, its
nanosheets as precursors. Unfortunately for this, it has seldom visible-light response depends on its morphology, and
been reported up to date, which is of great significance for generally its nanorods display much wide visible-light
[
27]
efficient photocatalysis on BiVO
4
.
absorption compared to its spherical nanoparticles.
Moreover, it is anticipated that the 1D nanorods are much well
usually displays a sluggish charge separation. This is matching to the fabricated 2D/2D heterojunctions relative to
mainly because its conduction band bottom position is the 0D nanoparticles. Therefore, dimension-matched
positive, indicating that its photogenerated electrons are not 1D/2D/2D plasmonic Au/TiO /BiVO nanocomposites are
energetic enough to induce reduction reactions newly proposed for efficient visible-light driven CO conversion
Similar to other visible-light responsive oxides like Fe
2 3
O ,
[19]
BiVO
4
2
4
2
[20]
thermodynamically. As for this, it is much meaningful to by photocatalysis. Unfortunately, it has been not reported to
prolong the lifetime of excited proper-energy-level electrons the best of our knowledge, and it is obviously of significance to
under visible-light irradiation for efficient photocatalysis on efficient photocatalysis for producing solar fuels.
BiVO
that coupling with wide bandgap oxide nanoparticles such as CO
TiO , ZnO and SnO as new platforms for accepting the speaking, CO
2
4
. Fortunately, we have confirmed in our previous work
In addition, the mechanism and pathway of photocatalytic
conversion to produce fuels are still ambiguous. Generally
reduction could take place directly by
2
2
2
electrons at high-level potential could effectively prolong the photogenerated electrons, or indirectly by the produced H
charge lifetime and enhance the charge separation, radicals with photogenerated electrons. As for the direct
consequently leading to the improved photocatalytic activities reduction process, it requires the photogenerated electrons
[
21-22]
for degrading pollutants.
TiO is a suitable energy platform for BiVO
chemically stable and low cost with a proper conduction band electrons possibly decreases to a certain degree if CO
It is clearly demonstrated that possess enough thermodynamic energy (about -1.9 V vs
[28]
2
4
since it is a NHE).
However, the required energy of photogenerated
easily
Therefore, it is
to be reduced by the
transfer and separation to a certain degree. In other words, low-energy photogenerated electrons below -1.9 V. Relatively,
the dimension matching between different parts for it is much easier for photogenerated electrons to reduce H
constructing effective heterojunction should be considered. to produce H (about 0 V vs NHE). In this case, the produced H
However, it is often neglected. Thus, to fabricate an efficient radicals would initiate the conversion of CO . However, there
D BiVO -based photocatalyst, a 2D anatase TiO nanosheet is still lacking of evidences to support the above two aspects.
should be designed to couple with. In recent years, (001) facet- Therefore, it is meaningful to clarify the related process
exposed anatase TiO 2D nanosheet (001T) has attracted great mechanism.
attention because of its high photocatalytic activity. This
stimulates us to prepare 2D/2D TiO /BiVO heterojunctions for
2
[29]
bottom level. However, the interfacial contacts between zero- adsorbs onto the photocatalyst surfaces.
dimension nanoparticles are weak, affecting the charge possible for the chemically-adsorbed CO
2
2
O
2
2
4
2
2
[23]
2
4
Experimental section
improving the interfacial connection to promote the charge
transfer and separation. Noticeably, related works have
seldom been reported to date. Therefore, it is much
interesting to emphasize the dimensional match importance of
Synthesis of BiVO
of Bi(NO ·5H O was dissolved in 100 mL of HNO
solution with a concentration of 2 mol/L under vigorous
magnetic stirring. Then, 1 g of PEG and 1.17 g of NH VO were
4
nanoparticles. In a typical synthesis, 4.85 g
3
)
3
2
3
aqueous
4
3
2
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4
added to the above mixture, and pH was adjusted to 7 with with BaSO as a reference. The morphology of the samples was
NH ·H O. Then, the mixture solution was ultrasonicated for 30 analyzed by scanning electron microscopy (SEM) on a Hitachi
3
2
min followed by continuous stirring for 1 h. Finally, the S-4800 instrument (Tokyo, Japan). Transmission electron
solution was centrifuged and washed with deionized water microscopy (TEM) images of the samples were performed by a
and absolute ethyl alcohol by turns and kept at 60 °C in an JEOL JEM-2010 electron microscope with an acceleration
oven. The sample was named as BV-NP.
Synthesis of BiVO nanoflakes. The BiVO
voltage of 200 kV. The N
2
adsorption-desorption isotherm
4
4
nanoflakes were were carried out by Micromeritics Tristar II 3020 system
prepared by the conversion of bismuth oxychloride (U.S.A). The SS-SPS and TS-SPV measurement for samples were
nanosheets. In a typical procedure, 0.26 g of BiOCl nanosheets implemented on home-built equipment, which has been
[
20,21]
were dispersed in 30 mL water, after stirring for 10 min, described in detail elsewhere.
-1
sonicated for 5 min. Then 20 mL of 6.09 g·L NaVO
dropwise to the above solution and stirred for 30 min. Finally, related to the formed hydroxyl radicals, CO
the mixture solution was transferred into a 100 mL Teflon- DRIFTS, electrochemical reduction, PEC measurement and
lined autoclave, sealed and heated at 150 °C for 8 h. After the photocatalytic activities for CO conversion are described in
3
was added
Other experimental sections including fluorescence spectra
2
-TPD, in-situ
2
system was cooled down to room temperature, the resulting detail in Electronic Supplementary Information.
product was washed with deionized water for several times,
and then dried at 60 °C in an oven. The sample was named as
Results and Discussion
BV-NF.
2
Synthesis of (001) facet-exposed anatase TiO nanosheets. In
2
D flake-like structured BiVO
exchange approach from BiOCl nanosheets.
BiVO nanoparticles (BV-NP) and BiVO nanoflakes (BV-NF) are
both identical to that of pure monoclinic BiVO (JCPDC no. 14-
88) confirmed by XRD patterns (Figure S1a). No obvious
4
was prepared via an ion
[30]
4
a typical procedure, 5 mL of Ti(OBu) was mixed with 20 mL of
The obtained
absolute ethanol under vigorous stirring. After that,
hydrofluoric acid solution (0.9 mL, 40%) was added. The
resulting solution was stirred for 1 h, transferred in a Teflon
lined stainless-steel autoclave, and then kept at 160 °C for 24
h. After the system was cooled to room temperature, the
white precipitate was collected, washed with ethanol and
deionized water for several times by turns and named as 001T.
Synthesis of Au nanorods. There are two primary steps in this
procedure. The first step is to prepare Au seeds. 364 mg CTAB
4
4
4
6
difference in optical absorption is observed for BV-NP and BV-
NF, which corresponds to a band gap of 2.16 eV characterized
by the DRS spectra (Figure S1b). The morphology of BV-NF was
investigated by the SEM images. It can be seen that the flake-
4
like BiVO is uniform with a mean width of 200-500 nm, and
average thickness of 20 nm is clearly observed from the
outward side (Figure S1c). Nitrogen adsorption-desorption
isotherm was employed to examine the surface area, and it
turns out that the surface areas of BV-NP and BV-NF are 1.1
and 1 mg HAuCl
4
·3H
2
O were dissolved in 10 mL of deionized
was added to the
water. 10 min later, 0.6 mL (10 mM) NaBH
4
above solution under vigorous magnetic stirring. The as-
obtained Au seeds were used in the growth of nanorods. The
second step is to prepare Au nanorods. 364 mg CTAB, 2 mg
2
-1
and 11.6 m g , respectively (Figure S1d). Apparently, BV-NF
exhibits large surface area compared with the conventional
nanoparticles. To explore the properties of charge carriers,
fluorescence spectra related to the formed hydroxyl radicals
4 2 3
HAuCl ·3H O and 100 μL (4 mM) AgNO solution were mixed in
10 mL deionized water. After stirring for 10 min, 70 μL (78.8
mM) of ascorbic acid was added to the solution and immersed
in water bath at 27-30 °C, then 12 μL of Au seed was added
and the reaction was allowed to proceed for 1 h. Finally, the
as-synthesized Au nanorods were purified by centrifugation to
remove free CTAB, and named as AuNR.
(
•OH) was employed by means of the coumarin fluorescent
method, in which the coumarin easily reacts with •OH to
produce luminescent 7-hydroxycoumarin. In general, the
stronger is the fluorescent signal, the larger is the produced
[31]
•
OH amounts, and the higher is the photogenerated charge.
Synthesis of AuNR-loaded 001T/BV-NF nanocomposite. The
nanocomposites were prepared by the wet chemical method.
In the typical synthesis, a proper amount of (001) facet-
It can be seen that the produced •OH amounts of BV-NF are
much higher than those of BV-NP (Figure S1e), indicating that
the flake-like structured BiVO
The visible-light photocatalytic activities of BV-NP and BV-NF
for CO reduction were evaluated with H O as the reducing
agent (Figure S1f), in which CH as the predominant product
4
exhibits high charge separation.
exposed anatase TiO
dispersed in 30 mL of absolute ethyl alcohol (the mole ratio of
TiO to BiVO is 5%), and a certain ratio of AuNR (the mole
ratio of AuNR to BiVO is 2%) were mixed to the above solution
2 4
nanosheets and BiVO nanoflakes were
2
2
2
4
4
4
was detected. It clearly shows that BV-NF exhibits much high
photoactivities compared with BV-NP, which is in good
agreement with the produced •OH amounts.
under magnetic stirring for about 1 h. Finally, the mixture was
dried in water bath at 80 °C for 6 h and calcined at 450 °C in air
for 2 h, and the sample was represented by AuNR/001T/BV-
NF.
To further enhance the charge separation of BV-NF, 2D (001)
facet-exposed
TiO
2
coupled
BV-NF
(001T/BV-NF)
Characterization. The X-ray powder diffraction (XRD)
patterns of the samples were recorded by using a Bruker D8
advance diffractometer with CuKα radiation. The UV-Vis
diffuse reflectance spectra (UV-Vis DRS) of the samples were
acquired on a Model Shimadzu UV2550 spectrophotometer
nanocomposite was prepared, along with general anatase TiO
2
nanoparticle coupled one (T/BV-NF) as the reference. The XRD
patterns for BV-NF, T/BV-NF and 001T/BV-NF are displayed in
Figure S2a. One can see that there are very-weak characteristic
peaks of anatase TiO
2
detected on T/BV-NF and 001T/BV-NF
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nanocomposites, which is due to its tiny amount. expected, T/BV-NF electrode shows an increased photocurrent
Simultaneously, the bandgap and optical properties of BV-NF density compared with BV-NF one, and the photocurrent
doesn’t change after coupling with TiO
2
(Figure S2b). signal of 001T/BV-NF electrode is much larger than that of
Moreover, the TEM images of 001T/BV-NF (Figure S3) indicate T/BV-NF. This is further supported by the electrochemical
that the surfaces of BV-NF are covered by sheet-like 001T, impedance spectra (Figure 1c). T/BV-NF exhibits a decreased
forming an intimate heterojunction, since the lattice fringes of capacitive arc radius compared with BV-NF, and 001T/BV-NF
(
200) plane with the inter-planar distance of 0.26 nm and the shows the smallest arc radius, implying that the charge
spacing of the (001) lattice plane with 0.24 nm are attributed transfer from BiVO to TiO is promoted. As a result, the
to BiVO and TiO , respectively. Definitely, the intimate photocatalytic activities for CO conversion of T/BV-NF and
connection of 2D heterojunction is stable and plays a vital role 001T/BV-NF nanocomposites achieve about 3-fold and 9-fold
4
2
4
2
2
in the charge transfer and separation.
enhancement compared with that of BV-NF, respectively
Figure 1d). Notably, it shows 3-fold increase in photoactivity
when TiO morphology is changed from 0D particles to 2D
(
2
nanosheets. In addition, according to the nitrogen adsorption-
desorption isotherm, 001T/BV-NF exhibits nearly the same
surface area compared with the T/BV-NF one (Figure S4). This
implies that the improved photocatalytic performance of
0
01T/BV-NF is mainly due to the matched 2D/2D structure of
TiO and BiVO , rather than the surface area increase.
4
2
Figure 2 HRTEM image of AuNR/001T/BV-NF (a) and DRS spectra of 001T/BV-NF,
AuNP/001T/BV-NF and AuNR/001T/BV-NF (b).
Figure 1 SS-SPS responses (a), Photoelectrochemical of linear sweep voltammetric (LSV)
scan responses under visible light illumination and dark (b), electrochemical impedance
spectra (EIS) (c) and photocatalytic activities for CO
irradiation for hours (d) of BV-NF, T/BV-NF and 001T/BV-NF. PEC and EIS
measurements were carried out with a three-electrode system in 0.5M Na SO
2
conversion under visible light
4
In order to investigate the effects of multi-dimensional
nanostructured gold on the visible-light absorption and
photocatalytic activities, plasmonic Au nanoparticles (AuNP)-
and Au nanorods (AuNR)-loaded 2D 001T/BV-NF
nanocomposites were prepared. As shown in Figure S5, there
2
4
electrolyte, with the tested film as the working electrode, Ag/AgCl as the reference
electrode, and Pt plate as the counter electrode.
The steady-state surface photovoltage spectroscopy (SS-
SPS) is a well-accepted photophysical technique, which reveals
the surface structural characteristics, surface states, along with
the generation, separation and recombination of
photogenerated charge carriers by means of the changes in
the surface potential of a semiconductor before and after
illumination. In general, the strong SPS response usually
indicates the high charge separation. As seen from Figure 1a,
BV-NF shows a rather low SPS response. However, T/BV-NF
nanocomposite exhibits an enhanced SPS response. This is
is a diffraction peak at 38.2° in XRD patterns, which is assigned
to gold. SEM image of AuNR modified 001T/BV-NF shows a
uniform dispersion of AuNR on 001T/BV-NF surface (Figure
S6a). In addition, the elemental composition of
AuNR/001T/BV-NF nanocomposites was analyzed by EDS,
indicating the presence of Bi, V, O, Ti, Au elements in the
nanocomposite, as shown in Figure S6b, implying the
successful preparation of Au/TiO /BiVO nanocomposite. TEM
2
4
and HRTEM images were employed to investigate the
morphological and structural features of Au modified 001T/BV-
NF nanocomposites. It is confirmed from Figure 2a and Figure
S7, both AuNP and AuNR form intimate ternary
2
attributed to the introduced TiO as a proper-energy platform
to accept electrons. In particular, the 2D 001T/BV-NF
heterojunction displays much higher SPS response, indicating
high charge separation. As for the constructed 2D/2D
nanocomposite, it is much favorable for the charge
transportation, transfer and separation owing to the intimate
connection of the geometrical dimensional matching.
Photoelectrochemical experiments of linear sweep
voltammetric (LSV) scans for BV-NF, T/BV-NF and 001T/BV-NF
heterojunctions with BiVO nanoflakes and TiO nanosheets
4
2
since the lattice spacing with 0.204 nm is attributed to the
200) facet of Au. It is clear that AuNP is spherical with ~10 nm
(
in diameter, while the AuNR with a length of ca. 20 nm and a
width of 8 nm. Figure 2b shows the DRS spectra of 001T/BV-NF
nanocomposites with Au modification. It is clear that the
optical absorption behavior of BiVO4 doesn’t change after
modifying Au. However, there are obvious long-wavelength
electrodes in 0.5M Na
2 4
SO electrolyte were carried out under
visible-light illumination from the back side (Figure 1b). As
4
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absorption peaks in the spectra, resulting from the surface arc radii compared with that of 001T/BV-NF, especially for
plasmon resonance (SPR) of Au. It can be seen that AuNP AuNR-loaded one. This implies that the constructed 1D/2D/2D
shows a distinct SPR peak at 600 nm, while the SPR one of nanocomposite exhibits much better charge transportation,
AuNR shifts to 660 nm. Thus, it is confirmed that the SPR transfer and separation, which is agreeable with the above
effects of Au could extend the visible-light absorption of BiVO
especially for the AuNR.
4
,
photophysical (above-mentioned SS-SPS and TS-SPV) results. It
is attributed to the dimensional matching of different
The photogenerated charge separation was explored by the constituents in the fabricated nanocomposites. Thus, it is
SPS technique (Figure 3a). One can notice that Au-loaded anticipated that the fabricated dimension-matching 1D/2D/2D
0
01T/BV-NF nanocomposites exhibit strong SPS signals nanocomposites would exhibit much high photocatalytic
compared with 001T/BV-NF one. It is worth noting that there activities.
are two types of SPS signals for Au modified ones. The peaks The photocatalytic activities for CO
2
reduction were
located at ~450 and ~600 nm are attributed to the inherent evaluated under visible-light irradiation as shown in Figure 3d.
charge separation of BiVO and injection of hot electrons from The reduction products like CH and CO, along with a certain
4
4
SPR Au, respectively. In addition, the SPS response of AuNR- amount of O2 as the oxidation product, are detected.
loaded one is much stronger than that of AuNP-loaded one, Apparently, the photocatalytic activities for 001T/BV-NF are
indicating the AuNR is much favorable to modify the resulting improved after loading Au, and the AuNR-loaded
001T/BV-NF for visible-light absorption and charge separation. nanocomposite displays higher photoactivity than the AuNP-
The TS-SPV method is an advanced technique to reveal the loaded one. Interestingly, the AuNP-loaded and AuNR-loaded
dynamic behaviours of photoinduced charge carriers. nanocomposites achieve 2-fold and 3-fold photoactivity
Generally speaking, for a nanosized semiconductor, its built-in enhancements compared with 001T/BV-NF one, respectively.
electric field could be neglectable so that its effect on the It is noted that the AuNR/001T/BV-NF one exhibits 30-time
carrier separation is much small. Thus, the charge separation is photoactivity enhancement compared to BV-NF. By
mainly affected by the diffusion process, always with the comparison with some other published works (Table S1), it is
lifetime within the microsecond timescale. As shown in Figure clear that the photocatalytic activities of AuNR-loaded
3
b, the slow-time SPV signal of 001T/BV-NF is enhanced after 001T/BV-NF in this work exhibit excellent performance for CO
2
[32-35]
Au modification, especially for the AuNR-modified one. conversion.
In order to evaluate the stability of
Moreover, it is confirmed that the charge carrier lifetime of AuNR/001T/BV-NF, photocatalytic CO reduction experiments
2
001T/BV-NF is prolonged after AuNR modification by 300 μs.
were carried out under 8 hours continuous irradiation, as
shown in Fig. S9. One can see that the amounts of evolved CH4
and CO linearly become large with increasing the irradiation
time indicating that the fabricated AuNR/001T/BV-NF
nanocomposite is much stable.
Discussion
4
As described above, the flake-like BiVO with large specific
surface area exhibits much high photocatalytic activities
compared with general nanoparticles. Remarkably, the
photoactivities could be further improved by coupling with
TiO
is mainly attributed to the matched 2D/2D structures between
TiO and BiVO for the promoted charge transfer and
separation. Moreover, the photocatalytic activities of
01T/BV-NF nanocomposites could be greatly improved by
2
, and it is much obvious with (001) facet-exposed one. This
2
4
0
modifying with plasmonic Au, especially for the AuNR-loaded
one. Thus, it is of great significance to reveal the mechanism of
the enhanced charge separation on dimension-matched
Figure 3 SS-SPS responses (a), TR-SPV responses (b), Fluorescence spectra related to
the formed hydroxyl radicals under visible light excitation (c) and Photocatalytic
activities for CO
2
conversion under visible light irradiation (d) of 001T/BV-NF, 1D/2D/2D AuNR-modified 001T/BV-NF nanocomposite,
AuNP/001T/BV-NF and AuNR/001T/BV-NF. Inset Fig. 3a shows enlarged SPS responses especially for the SPR effects.
from 500 to 800 nm.
Monochromatic photocurrent action spectra were thus
performed, as shown in Figure 4a. One can learn that the
To further investigate the photogenerated charge photocurrent density of 001T/BV-NF gradually increases as the
properties, photochemical experiments, including fluorescence excitation wavelength decreases from 550 to 400 nm.
spectra related to the amounts of produced hydroxyl radicals Noticeably, it begins to sharply increase under 530 nm
and EIS spectra for exploring interface resistance were carried excitation, which is the threshold wavelength to facilitate the
out. As shown in Figure 3c and Figure S8, it is clear that Au- transfer of visible-light excited proper-energy electrons of BV-
[20]
modified ones exhibit large •OH amounts and small capacitive NF to 001T.
In particular, the photocurrent density of
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Journal of Materials Chemistry A
2
001T/BV-NF greatly increases after loading Au, especially for (Figure 5a), it is much favorable for H O reduction on the Au-
the AuNR-loaded one. This indicates that it is much favorable loaded 001T/BV-NF, especially for the AuNR one. However, it
for the photogenerated charge separation of 001T/BV-NF to should be pointed out that the onset potentials of different
load Au nanorods. Moreover, the obvious photocurrent BV-NF-based electrodes for H
response of AuNR-loaded one is observed even under 660 nm in the CO -bubbled system (Figure 5b), implying that the
excitation, which is attributed to the hot electrons transfer to reduction behavior of CO is not preferred. This is attributed to
TiO from SPR AuNR under visible-light irradiation. To further the un-promoted adsorption of CO after loading Au by means
confirm this, the FS spectra related to the amounts of of the CO temperature-programmed desorption curves
produced hydroxyl radicals was carried out under different- (Figure 5b inset). Thus, it is deduced that the Au-loaded
2
O reduction are similar to those
2
2
2
2
2
wavelength monochromatic light excitation (Figure 4b).
001T/BV-NF prefers H
suggested that the produced H radicals from H
dominant active radicals to initiate the conversion of CO
2
O reduction to CO
2
. Therefore, it is
2
O reduction are
2
.
To further disclose the produced H atoms from the
+
photocatalytic reduction of H O other than of H and their
2
roles in the subsequent conversion of CO , isotopic D O and
2
2
1
3
CO2 labeling experiments were employed under identical
photocatalytic reaction conditions. The photocatalytic
products were identified and quantified with GC-MS
technique. When H O is substituted with D O (Figure S10), C,
2
2
CD, CD , CD , and CD are detected, implying the important
2
3
4
Figure 4 Normalized photocurrent action spectra (a) and Fluorescence spectra related
to the formed hydroxyl radicals under 660 nm excitation (b) of 001T/BV-NF,
AuNP/001T/BV-NF and AuNR/001T/BV-NF. Inset Fig. 4b shows FS signals under 590 nm
excitation.
role of H in H O. In addition, a certain amount of CH OH as an
2
3
effective hole capturer is deliberately added in the
photocatalytic system, in which the photocatalytic activities for
CO2 conversion would be improved, simultaneously to
+
effectively provide the source of H for being favorable to
[36-37]
produce H radicals.
Based on the photoelectrochemical results, two typical
monochromatic wavelengths of 590 and 660 nm were chosen.
One can see that 001T/BV-NF shows a neglectable fluorescent
response under 590 nm excitation owing to the threshold
4
wavelength of 570 nm for exciting BiVO . Notably, the amount
of produced hydroxyl radical is obviously increased after
loading Au, and it is much larger for the AuNR-modified one
even at 660 nm excitation. This is in good agreement with the
above photoelectrochemical results, indicating that 1D AuNR is
much beneficial for the extended visible-light response and the
subsequent charge separation compared to 0D AuNP. This
further confirms that the geometrical dimension matching of
different constituents in the fabricated 1D/2D/2D
nanocomposite is crucial for the effective charge transfer and
separation.
Figure 6 Amounts of CH
by GC-Mass spectrometry for AuNR/001T/BV-NF nanocomposite. The amount of CH
evolved in pure H O and methanol containing H O solution are denoted as a and b,
2
in pure D O and methanol containing D O
4 4
detected by GC analysis and ion peak intensity of CD detected
4
2
2
respectively. The ion peak intensity of CD
solution are denoted as a’ and b’.
4
2
As expected (Figure 6), the amount of produced CH
presence of CH OH on AuNR/001T/BV-NF is much larger than
that in H O by the GC test, and the amount of produced CD in
the presence of CH OH on AuNR/001T/BV-NF is also much
larger than that in D O by the GC-MS test. Interestingly, it is
noted that the activity-enhancement times after adding CH OH
O and in D O are similar. The results reasonably indicate
4
in the
3
2
4
3
2
3
Figure 5 Electrochemical reduction curves in N
2
-bubbled (a) and CO
2
-bubbled systems in H
2
2
(
b) of 001T/BV-NF, AuNP/001T/BV-NF and AuNR/001T/BV-NF nanocomposites. The that the H atoms in the formed CH are mainly originated from
4
inset in (b) shows CO
performance was measured in 0.5 M Na
electrode was used as the reference electrode.
2
-TPD curves of the three samples as mentioned. Electrochemical
2 3
H O and CH OH and could capture the holes to promote
2 4
SO
solution, and Hg/Hg
2
Cl
2
(saturated KCl)
charge separation. This is further supported by the in-situ
DRIFTS (Figure S11a). One can see that the adsorption of
methanol reach the equilibrium state in 30 min. Interestingly,
under visible-light irradiation, as shown in Figure S11b, the
To explore the mechanism of CO
2
conversion,
electrochemical reduction measurements were carried out in
different gas-bubbled systems. In the N -bubbled system
2
intensity of methanol decreases while that of CO increases
2
significantly, indicating that methanol mainly acts as the
6
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ARTICLE
sacrificial agent to capture photo-generated holes, which is AuNR/001T/BV-NF nanocomposite displays a wide visible-light
consistent well with the isotope results. In addition, signals response range from 400 nm to 660 nm, which is 34.8% of the
1
3
13
13
resulting from C, CH, CH
13
13
2
,
CH
3 4
and CH by replacing solar spectrum and 78.9% of the visible light. This indicates
1
3
2 2
CO with CO could be detected by the GC-Mass test (Figure that there is a great potential for solar fuel production by
S12). This further confirms that the produced CH
resource.
4
is from CO
2
designing plasmonic-assisted BiVO
This work provides a feasible route to fabricate high-activity
-based nanophotocatalysts for solar fuel production.
4
-based nanophotocatalysts.
On the basis of the above results and discussion, a BiVO
schematic for the photogenerated charge transfer and
separation and its induced reaction on the designed
4
Conflicts of interest
dimension-matched Au-loaded 001T/BV-NF nanocomposite
under different visible-light wavelength ranges has been
proposed (Figure 7). When the excitation wavelength range is
from 400 to 530 nm, the produced proper-energy-level
There are no conflicts to declare
electrons of BiVO would transfer to TiO , and then to Au as Acknowledgements
4
2
the co-catalyst for H production. When it is larger than 530
nm, the plasmonic Au-induced hot electrons with high
thermodynamic energy will inject to the adjacent TiO2 to
produce H. The formed H by visible-light irradiation would
The authors are grateful for financial support from the NSFC
project (U1401245, 21501052 and 91622119), the Program for
Innovative Research Team in Chinese Universities (IRT1237),
the China Postdoctoral Science Foundation (2015M570304),
Special Funding for Postdoctoral of Heilongjiang Province (LBH-
TZ06019) and UNPYSCT-2016173.
continuously initiate the conversion of CO . Obviously, the
2
synergistic mechanism in the SPR Au-loaded 001T/BV-NF
results in the excellent photocatalytic activities for CO2
conversion.
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