DOI: 10.1002/cctc.201501072
Full Papers
Core–Shell Composites Based on Multiwalled Carbon
Nanotubes and Cesium Tungsten Bronze to Realize Charge
Transport Balance for Photocatalytic Water Oxidation
Yingqiang Zhao, Fengyun Han, Qian Wang, Guan-Wei Cui,* Xi-Feng Shi, Xin-Yuan Xia,
Junfeng Xie, Yong Li, and Bo Tang*[a]
Core–shell composite photocatalysts based on multiwalled
carbon nanotubes (MWCNTs) and Cs2W3O10 are first synthesized
by an ion-directing method. Cs+ is preadsorbed on the surface
of MWCNTs by electrostatic forces and directs the crystallization of Cs2W3O10 to form a uniform layer on the surface of the
MWCNTs. This core–shell arrangement of a MWCNT@Cs2W3O10
composite photocatalyst with a 15 nm thick Cs2W3O10 layer
allows the charge transport balance to be achieved in the pho-
tocatalytic water oxidation process and confers a high photoactivity. Other reference composite photocatalysts that deviate
from this balance result in deteriorated photoactivity. The established synthetic method provides a new method for the
design and synthesis of composite photocatalysts, and the realization of charge transport balance is helpful to understand
the nature of the enhancement of photoactivity in photocatalytic processes.
Introduction
Tungsten oxide (WO3), as an important n-type semiconductor[1, 2] and water oxidation photocatalyst,[3–5] has attracted intense attention because of its high incident-photon-to-current
efficiency and visible-light absorbance (Eg … 2.7 eV).[6, 7] However, its low electronic conductivity causes a low charge separation efficiency, which hinders WO3 to become a commercial
photocatalyst greatly. The incorporation of other building
blocks to construct composite photocatalysts can enhance the
photocatalytic performance of WO3. Carbon-based materials,
such as graphene, carbon nanotubes (CNTs), carbon quantum
dots, and activated carbon (AC), are beneficial building blocks
for the preparation of various composites.[8–10] They usually
serve as electron conductors and electron reservoirs in the
photocatalytic process, which can remedy the limitations of
WO3.
CNTs are good building blocks to meet the requirements for
the construction of a WO3-C composite photocatalyst.[11–13] The
ideal arrangement of the WO3-multiwalled carbon nanotubes
(MWCNT) composite photocatalyst is C@WO3 ; that is, MWCNTs
localized inside a WO3 phase. In this arrangement, WO3 can
come in to contact with the reactants well, and the electronstorage ability of the carbon materials can be preserved fully.
However, as a result of the intrinsic character of the WO3 nucleation mechanism, it is difficult to cover the CNT surface uniformly with a WO3 crystal layer. Therefore, previous studies on
this C@WO3-type composite only realized relatively low
amounts of WO3 particles dispersed randomly on the
CNTs.[14–16] Although excellent charge separation can be realized, the low density of the photoactive sites results in a limited
enhancement of the photocatalytic efficiency. In addition to
charge separation, another important factor, the charge transport balance between building blocks, has been ignored for
years. This balance requires the match of the charge generation and charge transport rates, which is similar to the concept
of “current matching” in the “Z-scheme” system. Clearly, this
balance can be realized by adjusting the mass ratio of WO3 to
the carbon material. However, as far as we know, no reported
composite photocatalyst with dispersed WO3 particles supported on MWCNTs is close to reaching this balance. Therefore, it is
of great importance to construct a core–shell-type C@W composite photocatalyst to illustrate the effects of charge transport
balance in the photocatalytic process.
Although the MWCNT@metal oxide arrangement has been
realized with metal oxides, such as TiO2, ZrO2, CeO2,[17] and
SiO2,[18] no such arrangement has been reported for WO3. The
most likely reason is that, compared with the metal oxides
mentioned above, the rates of hydrolysis/alcoholysis for tungstate or tungsten chloride are too fast to control. As a result,
homogeneous nucleation rather than heterogeneous nucleation is the main path for the growth of WO3 crystals. To solve
this problem during the loading of WO3 on CNTs, a new strategy is proposed in this research. Tungsten bronze is a type of
WO3-based material that has been reported to exhibit an excellent photocatalytic activity.[19] The formation mechanism of
tungsten bronzes requires the participation of ions (usually
alkali ions) that remain in the tunnel formed by WO6 octahedra. Inspired by this formation mechanism, we hypothesized
that if the alkali ions can be preadsorbed on the surface of the
[a] Y. Zhao, F. Han, Q. Wang, G.-W. Cui, X.-F. Shi, X.-Y. Xia, J. Xie, Y. Li, B. Tang
College of Chemistry, Chemical Engineering and Materials Science
Shandong Normal University, Jinan, 88 Wenhua East Road
Shandong 250014 (P.R. China)
E-mail: [email protected]
[email protected]
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201501072.
ChemCatChem 2016, 8, 624 – 630
624
Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Papers
CNTs, they will induce the formation of tungsten bronzes on
the surface of CNTs and finally form a CNT@WO3 core–shell
structure.
Herein, a new strategy is provided by applying Cs+ as a directing ion to construct core–shell-structured C@WO3 composite photocatalysts. The results show that a significant enhancement of photoactivity can be realized by establishing
a charge transport balance.
Before the reaction, the absorption peaks are centered at ñ =
3440 and 1440 cm¢1 (Figure S1), which indicates that abundant
carboxylate groups exist on the surface of the MWCNTs.[20] The
existence of carboxylate groups can be attributed to the oxidation of the MWCNTs before use. If Cs2W3O10 particles are
formed on the surface of the MWCNTs, the intensity of the two
peaks weakens, and a strong absorption appears in the fingerprint region, which can be ascribed to the formation of
a Cs2W3O10 phase. With the increasing concentration of the W
source, a Cs2W3O10 layer is formed, and the intensity of the two
peaks at ñ = 3440 and 1440 cm¢1 weakens continuously.
The thickness of the Cs2W3O10 layer of the composites can
be controlled by adjusting the concentration of WCl6 in the
starting materials during the synthesis. Here, the core–shell
composite with Cs2W3O10 shell thicknesses of 20 nm (initial
WCl6 concentration 0.015 m), 15 nm (initial WCl6 concentration
0.010 m), 10 nm (initial WCl6 concentration 0.005 m), are denoted as C@W-20, C@W-15, and C@W-10, respectively. Additionally, if the initial concentration of WCl6 was decreased to 0.001 m,
dispersed Cs2W3O10 particles rather than a layer were formed
on the surface of the MWCNTs. This composite (C/W) provides
access to meaningful and contrasting experiments in the characterization of the photoactivity.
Results and Discussion
Synthesis mechanism
Detailed synthesis processes are shown in Scheme 1. In the
first step, Cs ions were preadsorbed on the surface of the
MWCNTs during a long stirring time. Then, during the controlled hydrolysis of the W salts, the Cs ions anchored on the
surface of the MWCNTs act as nucleation centers to direct the
formation of the Cs2W3O10 layer.
Material characterization
The as-prepared composite photocatalyst was analyzed by
XRD. The diffraction peaks coincide well with those of
Cs2W3O10 (PCPDF 87-2010; Figure 1). The structure of Cs2W3O10
is illustrated in Figure S2 and is similar to that of a sodium
tungsten bronze reported previously,[19] which reveals the importance of Cs+ in the formation of the layer. As a result of the
high crystallinity of Cs2W3O10, diffraction peaks of oxidized
MWCNTs are too low to be detected. We checked the diffraction peaks of the pure Cs2W3O10 particles carefully and found
that its crystal structure is the same as that of the layer in the
core–shell composite. However, the peak intensity is not as
sharp as that of the core–shell composite, which indicates that
the MWCNTs may exhibit a template effect that induces the al-
Scheme 1. Schematic illustration of the synthesis of MWCNT@Cs2W3O10 composites.
The addition of Cs salts is necessary because of the directing
effect of the ions for the formation of a Cs2W3O10 layer. Control
experiments without the addition of Cs salts or with the addition of other alkali salts (including ammonium) were conducted, and no core–shell-structured composite was obtained. Interestingly, although other alkali salts such as Na+, K+, and Rb+
can also form tungsten bronze-type materials, these ions
cannot direct the formation of a corresponding layer on the
MWCNTs, as Cs+ does. Therefore, it is speculated that the
radius of the ion is critical for the formation of tungsten oxides
on the surface of the MWCNTs. Cs+, which is one of the largest
alkali ions, is essential to act as the nucleation center and
direct the alcoholysis of WCl6 on the surface of the MWCNTs.
Another important factor in the formation of the core–shell
structure is the presence of carboxylate groups on the surface
of the MWCNTs, which are produced by an oxidation process.
No core–shell structure was formed with unoxidized MWCNTs.
It is understood that the adsorption of Cs+ needs the electrostatic attraction from the carboxylate group on the surface of
the oxidized MWCNTs. FTIR spectroscopy revealed the template effect functionalized by the surface carboxylate groups.
ChemCatChem 2016, 8, 624 – 630
www.chemcatchem.org
Figure 1. Powder XRD patterns of MWCNTs, Cs2W3O10, and the core–shell
MWCNT@Cs2W3O10 composite.
625
Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Papers
coholysis of the precursors and confines condensation to the
Cs2W3O10 crystallites around the walls of the MWCNTs. In addition, compared to the standard card, the sharpest peak of
Cs2W3O10 is located at 2 q = 24.78 rather than 2 q = 29.08, which
indicates that the preferred lattice is [1 3 2] rather than [3 4 1].
This again proved the effect of the Cs ion to direct the formation of Cs2W3O10 crystals.
The core–shell structure of the desired MWCNT@Cs2W3O10
composite photocatalyst was determined by TEM. Compared
with a C/W composite photocatalyst reported previously (Figure 2 a), the method used in this research could form a uniform
Figure 3. a) XPS survey spectrum of the core–shell composite and b) highresolution W 4f core-level spectrum.
X-ray photoelectron spectroscopy (XPS) further confirmed
the presence of Cs+ in the core–shell composite (Figure 3 a). In
addition, the two peaks of W 4f centered at binding energies
of 35.7 and 37.6 eV indicate that the valence of W is six, which
means that there is no direct W¢C covalent bonding between
Cs2W3O10 and the MWCNTs (Figure 3 b). Instead, it is most probable that the Cs2W3O10 and MWCNTs are connected by W¢O¢C
bonding, which is consistent with the above-mentioned synthesis mechanism.
UV/Vis spectroscopy was used to investigate the light-absorption properties of Cs2W3O10 and the composites. Compared
with the pure Cs2W3O10 samples, the core–shell-structured
composites show strong absorption from the UV to visible
light region (Figure S4), which is attributed to the presence of
MWCNTs. Comparatively, a clear absorption edge can be observed for pure Cs2W3O10, and the corresponding bandgap was
calculated to be 2.88 eV, which shows a clear blueshift compared with that of WO3 (2.7 eV) that could be ascribed to the
well-known small size effect[21] or to a band energy change
caused by Cs ions.[22]
Figure 2. TEM images of a) MWCNT/Cs2W3O10 composite and b, c) core–
shell-structured MWCNT@Cs2W3O10 composite. d) HRTEM image of core–
shell-structured MWCNT@Cs2W3O10, and a typical selected area electron diffraction (SAED) pattern is shown inset.
Cs2W3O10 layer on the surface of the MWCNTs (Figure 2 b). The
Cs2W3O10 layer thickness of the as-prepared composite photocatalyst is approximately 15 nm, and other core–shell composites with different layer thicknesses are shown in Figure S3.
The thickness of the shell can be controlled easily by adjusting
the initial concentration of the W source. High-resolution transmission electron microscopy (HRTEM; Figure 2 c and d) of the
samples revealed that the shell is composed of Cs2W3O10 clusters with a diameter of approximately 4 nm. The lattice space
between adjacent planes is 0.36 nm, which corresponds to the
[1 3 2] planes of Cs2W3O10 and coincides with the main peak of
the XRD results. The images also demonstrate that the structural characteristics of the MWCNTs are maintained after the
preparation of the composites, which indicates that the integrity of the MWCNTs is not altered during the formation of the
composites. This is particularly important to retain the electronic and mechanical properties of the MWCNTs in the final
composites.
ChemCatChem 2016, 8, 624 – 630
www.chemcatchem.org
Photoactivity
To evaluate the ability of the composite photocatalysts to
photo-oxidize water, samples of C@W-20, C@W-15, C@W-10,
and C/W were suspended in aqueous AgNO3 and illuminated
626
Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Papers
by using a 300 W Xe arc lamp using its full spectrum. In addition, 0.2 g of La2O3 was added as a buffer to keep the pH of
the solution neutral. The amount of O2 evolved over the
course of irradiation is shown in Figure 4. It is generally accepted that MWCNTs are excellent electron-collecting materials and
that the application of MWCNTs in composite photocatalysts
will improve the charge separation efficiency significantly.[8]
From this point of view, the order of the photoactivity of the
as-prepared samples should be C@W-20 > C@W-15 > C@W10 > C/W > Cs2W3O10 according to the amount of Cs2W3O10 on
the surface of the MWCNTs (Table S1). However, the actual
order is C@W-15 > C@W-10 > C/W > C@W-20 > Cs2W3O10, and
the highest amount of Cs2W3O10 resulted in the worst photoactivity. Clearly, there must be some underlying reasons aside
from charge separation.
The
mechanism
of
water
oxidation
on
the
MWCNT@Cs2W3O10 composite photocatalyst is shown in
Scheme S1. Under light irradiation, the electron–hole pair photogenerated by Cs2W3O10 will separate according to the energy
level of the MWCNTs and Cs2W3O10. Photoelectrons will transfer
to the surface of the MWCNTs, where they are collected and
react with sacrificial agents. Holes move to the surface of the
Cs2W3O10 crystals and oxidize water to O2. Thus, we hypothesize that the charge transport balance plays a significant role
to determine the photoactivity of the composite photocatalysts. The essential aspect of this hypothesis is that the charge
generation and charge transport rates should match, that is,
the rate of electron generation by Cs2W3O10 should equal the
rate of electron transport to the MWCNTs. Fortunately, the successful synthesis of the MWCNT/Cs2W3O10 core–shell composite
provided an appropriate model to discuss the effect of this
balance by controlling the ratio of Cs2W3O10 to MWCNTs. According to this hypothesis, the highest amount of O2 produced
by C@W-15 (22.6 mmol g¢1 h¢1) indicated that this composition
is the best to establish a charge transport balance. A higher
(C@W-20) or lower (C@W-10) ratio would deviate from the balance and result in deteriorated photocatalytic efficiency.
The concept of the charge transport balance can be proven
by the transient current response. Transient current response
profiles under light irradiation with applied potentials of 1 V
versus the reversible hydrogen electrode (RHE) are shown in
Figure 5. The photocurrent transients observed with the C@W-
10 and C@W-15 electrodes (Figure 5 a and b, respectively)
show nearly square profiles with only small initial peaks, which
demonstrate the almost complete suppression of surface electron–hole recombination according to a previous report.[23]
Comparatively, C@W-20 shows relatively rough profiles (Figure 5 c). We consider that the difference between these samples comes from the different extents of charge transport. For
C@W-15, the rate of photoelectron generation nearly equals
the electron-collecting ability of the MWCNTs, and the charge
transport equilibrium can be established quickly. For C@W-10,
the rate of photoelectron generation is lower than the electron-collecting ability of the MWCNTs because of the relatively
low amount of Cs2W3O10 particles. Under this circumstance, although the charge transport equilibrium can still be established quickly, the electron-collecting ability of the MWCNTs
cannot be used fully. This is why the photocurrent density of
C@W-10 is lower than that of C@W-15. For C@W-20, which has
a relatively high amount of Cs2W3O10, the rate of photoelectron
generation is beyond the electron-collecting ability of the
MWCNTs and results in excess electrons on the surface of the
Cs2W3O10. Most of the excess electrons will recombine with
holes and make the curve of the transient current rough. This
phenomenon can also be observed for pure Cs2W3O10. It is
strange that the C/W sample shows the highest photocurrent
density of all samples. We consider that the increased photocurrent density may be brought about by the MWCNTs because the amount of Cs2W3O10 is quite low and most of the
MWCNT surfaces are exposed. MWCNTs alone showed a similar
photocurrent response to C/W, which is not presented. The
charges photogenerated by MWCNTs cannot initiate water oxidation, which is why the photoefficiency of C/W is lower than
that of the core–shell composites in the water oxidation process.
Interestingly, the photocatalytic efficiency of C@W-20 is even
lower than that of C/W. The reason for this behavior may be
that the layer of Cs2W3O10 is too thick for the sacrificial agent
to tunnel through. As a result, only those electrons transferred
to the surface of Cs2W3O10 can be captured by Ag+. Under the
extreme circumstance that the layer of the Cs2W3O10 shell is
sufficiently thick, it will hinder the electron-storage ability of
the MWCNTs, which is most similar to the use of pure
Cs2W3O10. This behavior is why pure Cs2W3O10 shows a similar
Figure 4. Left: time courses of O2 evolution on as-prepared photocatalysts. Right: calculated rates of O2 production of the as-prepared photocatalysts.
ChemCatChem 2016, 8, 624 – 630
www.chemcatchem.org
627
Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Papers
Figure 5. Transient current response profiles of the as-prepared composite photocatalysts in chopping mode with applied potentials of 1 V vs. RHE. a) C@W10, b) C@W-15, c) C@W-20, d) C/W.
photoactivity to C@W-20. HRTEM images of the samples after
the photocatalytic reactions support the above statement
clearly. For C@W-15, Ag particles can be found on the surface
of the MWCNTs close to the Cs2W3O10 clusters (Figure S5). However, for C@W-20, most of the Ag particles are located on the
surface of the Cs2W3O10, and hardly any Ag can be found on
the surface of the MWCNTs.
One way to verify this theory is to conduct photocatalytic
water splitting experiments by altering the incident light intensity. Under a lower light intensity irradiation (< 10 mW cm¢2),
the rate of O2 evolution of all samples increases sharply with
the increase in light intensity (Figure 6). It is considered that
only a small amount of the charge separation should occur;
therefore, the photogenerated photoelectrons will be handled
easily. Under this circumstance, the charge recombination is
minor, and the increase in the rate depends mainly on the increase in light intensity. Therefore, the slopes of all samples
were similar to each other. However, with a higher incident
light intensity (> 10 mW cm¢2), the difference became clear. For
samples C@W-10 and C@W-15, the rate of O2 evolution was
maintained at a relatively high value, which indicates good
charge separation efficiency caused by the presence of
MWCNTs. Comparatively, the rates of sample C@W-20 and pure
Cs2W3O10 decreased greatly. This can be ascribed to the quick
increase in the number of electrons, which causes an imbalance in the charge transport and accumulated on the surface
of the Cs2W3O10. As a result, electron–hole recombination
became clear.
It is acknowledged that although WO3 was able to oxidize
water to oxygen, the severe photocorrosion will decrease its
ChemCatChem 2016, 8, 624 – 630
www.chemcatchem.org
Figure 6. O2 evolution rates of as-prepared photocatalysts under a varying
incident light intensity.
photocatalytic efficiency greatly, even in the presence of a sacrificial agent. Therefore, it is important to examine the stability
of the composite photocatalysts. The stability results of the
composite photocatalysts are shown in Figure S6. For C@W-15
and C/W, because of the excellent electron-collecting ability of
the MWCNTs, the fast and efficient electron–hole separation
ensures the steady photocatalytic reaction, even after a long irradiation time. Comparatively, C@W-20 showed poor stability.
This can be easily understood if we consider the imbalance of
charge transport, as mentioned above. The accumulated electrons of C@W-20 will undoubtedly cause severe photocorrosion and result in a deteriorated photoactivity.
628
Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Papers
We also explored the photocatalytic behavior of the as-prepared composite photocatalysts for the photocatalytic degradation of organic dyes (Rhodamine B; RhB). The photocatalytic
reaction obeys a pseudo-first-order kinetic equation and can
be described simply by dC/dt = kC, in which C is the concentration of RhB and k [min¢1] denotes the overall degradation rate
constant.[24] Apparently, a higher k value means a higher photoactivity. All of the prepared MWCNT-Cs2W3O10 composites
showed higher photoactivity than the pure Cs2W3O10 samples
(Figure 7), which indicates a significant synergistic effect be-
Figure 8. Fluorescence intensity of the supernatant liquid for Cs2W3O10 and
MWCNT@Cs2W3O10 composites. All supernatant liquid was irradiated under
visible light (> 420 nm) for 30 min.
charge transport balance was studied by adjusting the thickness of the Cs2W3O10 layer, and the results show that the
MWCNT@Cs2W3O10 composite photocatalyst with a 15 nm thick
Cs2W3O10 layer could achieve the best charge transport balance
and thereby the highest photoactivity. This model composite
photocatalyst provides a new insight into the improvement of
photoactivity by establishing a balance of charge transport.
With the other conditions constant, the composite photocatalyst with the best match of the charge transport will result in
the best photoactivity. The established method in this study
provides new insights into the understanding of the photocatalytic process, which is helpful for the design and synthesis of
other photocatalysts with potential commercial value.
Figure 7. Fitted lines of the photocatalytic degradation of RhB by Cs2W3O10
and MWCNT@Cs2W3O10 composites under visible light (> 420 nm).
tween the MWCNTs and Cs2W3O10. The unintuitive photoactivity order for water oxidation and dye degradation between
C@W-20 and C/W can be explained by the different mechanisms for these two photocatalytic processes (Scheme S1). Hydroxyl radicals (COH) are the active species in the photodegradation process and can be produced by both electrons and
holes.[12] Although the chance of electron–hole recombination
increases for C@W-20 because of its thick Cs2W3O10 layer, the
high amount of Cs2W3O10 could still produce more photocharges than C/W, which has too little Cs2W3O10 on the MWCNT surface. The proposed active oxygen intermediate species COH
can be determined by a terephthalic acid photoluminescence
probing assay (TA-PL).[25] C@W-15 gave the strongest signal
(Figure 8), which indicates that the highest amount of hydroxyl
radicals was produced. Although excellent charge separation
can be obtained for C/W, the limited amount of Cs2W3O10 on
the surface of the MWCNTs results in the lowest hydroxyl radical production. The overall tendency of the hydroxyl radical
concentration coincides with the photocatalytic degradation
sequence of the as-prepared samples.
Experimental Section
Pretreatment of MWCNTs
First, raw MWCNTs (500 mg) were added to a 100 mL round-bottomed flask and dispersed into water by sonication for 10 min.
Nitric acid (50.0 mL) was added, and the final mixture was stirred
continuously at 120 8C for 24 h to introduce oxygen groups onto
the MWCNT surface. After cooling to RT, the mixture was filtered
through a 0.45 mm polytetrafluoroethylene (PTFE) membrane. The
oxidized MWCNTs were collected and washed with deionized
water until the pH was neutral before drying under vacuum at
70 8C for 24 h.
Synthesis of composite photocatalysts
Typically, 0.1 m WCl6/C2H5OH solution was prepared by dissolving
WCl6 powder (8 g) in absolute ethanol (200 mL). The calculated
amount of MWCNTs was dispersed in absolute ethanol (15 mL)
with the addition of Cs2SO4 under vigorous magnetic stirring for
1 h. The 0.1 m WCl6/C2H5OH solution and the MWCNT/Cs2SO4 solution were mixed and stirred for 5 min and then transferred into
a 20 mL Teflon-lined stainless-steel autoclave for a hydrothermal reaction at 100 8C for 48 h. Finally, the products were cooled to RT,
washed sequentially with distilled water and absolute ethanol
three times, and then dried in air at 60 8C for 20 h.
Conclusions
Core–shell-nanostructured composite photocatalysts based on
multiwalled carbon nanotubes (MWCNTs) and Cs2W3O10 were
synthesized by a Cs-ion-directing method for the first time.
The presence of carboxylate on the surface of the MWCNTs
and an appropriate ion radius are the key factors in this
method. The extent that the photoactivity was affected by the
ChemCatChem 2016, 8, 624 – 630
www.chemcatchem.org
629
Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Papers
Pure Cs2W3O10 particles were synthesized by the same method
without the addition of MWCNTs.
Acknowledgements
Characterization
This work was supported by the 973 Program (2013CB933800)
and the National Natural Science Foundation of China
(21227005, 21390411, 91313302, 21405096).
XRD patterns of the products were recorded by using a Rigaku
(Japan, D/max-gA) X-ray diffractometer equipped with graphite
monochromatized CuKa radiation (l = 1.54178 æ). The optical properties were analyzed by UV/Vis diffuse reflectance spectroscopy
(DRS) by using a Shimadzu UV-2500 spectrophotometer with
BaSO4 powder used as a reference (100 % reflectance). TEM and
HRTEM were performed by using a TECNAI G2 high-resolution
transmission electron microscope operated at 200 kV. FTIR spectra
were recorded by using a Bruker VERTEX-70. XPS spectra were recorded by using a Kratos AXIS ULTRA spectrometer with a monochromated AlKa X-ray source (1486.7 eV) operated at an emission
current of 15 mA and an anode potential of 15 kV.
Keywords: cesium · nanotubes · oxidation · photochemistry ·
tungsten
[1] W. Cheng, Y. Ju, P. Payamyar, D. Primc, J. Rao, C. Willa, D. Koziej, M. Niederberger, Angew. Chem. Int. Ed. 2015, 54, 340 – 344; Angew. Chem.
2015, 127, 347 – 351.
[2] W. Sun, M. T. Yeung, A. T. Lech, C.-W. Lin, C. Lee, T. Li, X. Duan, J. Zhou,
R. B. Kaner, Nano Lett. 2015, 15, 4834 – 4838.
[3] Y. Miseki, K. Sayama, RSC Adv. 2014, 4, 8308 – 8316.
[4] D. Chandra, K. Saito, T. Yui, M. Yagi, Angew. Chem. Int. Ed. 2013, 52,
12606 – 10609; Angew. Chem. 2013, 125, 12838 – 12841.
[5] D. J. Martin, P. J. T. Reardon, S. J. A. Moniz, J. Tang, J. Am. Chem. Soc.
2014, 136, 12568 – 12571.
[6] G. Liu, J. F. Han, X. Zhou, L. Huang, F. X. Zhang, X. L. Wang, C. M. Ding,
X. J. Zheng, H. X. Han, C. Li, J. Catal. 2013, 307, 148 – 152.
[7] K. Villa, S. Murcia-Lopez, T. Andreu, J. R. Morante, Appl. Catal. B 2015,
163, 150 – 155.
[8] W. J. Lee, J. M. Lee, S. T. Kochuveedu, T. H. Han, H. Y. Jeong, M. Park,
J. M. Yun, J. Kwon, K. No, D. H. Kim, S. O. Kim, ACS Nano 2012, 6, 935 –
943.
[9] M.-Q. Yang, B. Weng, Y.-J. Xu, J. Mater. Chem. A 2014, 2, 1710 – 1720.
[10] R. K. Yadav, G. H. Oh, N. J. Park, A. Kumar, K. J. Kong, J. O. Baeg, J. Am.
Chem. Soc. 2014, 136, 16728 – 16731.
[11] A. A. Taha, F. T. Li, Catal. Sci. Technol. 2014, 4, 3601 – 3605.
[12] X. H. Zhang, X. H. , Lu, Y. Q. Shen, J. B. Han, L. Y. Yuan, L. Gong, Z. Xu,
X. D. Bai, M. Wei, Y. X. Tong, Chem. Commun. 2011, 47, 5804 – 5806.
[13] J. J. Guo, Y. Li, S. M. Zhu, Z. X. Chen, Q. L. Liu, D. Zhang, W. J. Moon,
D. M. Song, RSC Adv. 2012, 2, 1356 – 1363.
[14] X. An, J. C. Yu, Y. Wang, Y. Hu, X. Yu, G. Zhang, J. Mater. Chem. 2012, 22,
8525 – 8531.
[15] C. A. Ma, Y. X. Jin, M. Q. Shi, Y. Q. Chu, Y. H. Xu, W. P. Jia, Q. H. Yuan, J. B.
Chen, D. K. Chen, S. M. Chen, J. Electrochem. Soc. 2014, 161, F246 – F251.
[16] Y. Yin, C. A. Ma, Z. Wu, M. Zhao, L. Chen, Y. Chu, J. Mater. Sci. Technol.
2015, 31, 888 – 894.
[17] M. Cargnello, M. Grzelczak, B. Rodr†guez-Gonz‚lez, Z. Syrgiannis, K.
Bakhmutsky, V. La Parola, L. M. Liz-Marz‚n, R. J. Gorte, M. Prato, P. Fornasiero, J. Am. Chem. Soc. 2012, 134, 11760 – 11766.
[18] T. Sainsbury, D. Fitzmaurice, Chem. Mater. 2004, 16, 3780 – 3790.
[19] L. Wang, J. Zhan, W. Fan, G. Cui, H. Sun, L. Zhuo, X. Zhao, B. Tang,
Chem. Commun. 2010, 46, 8833 – 8835.
[20] B. Derkus, K. C. Emregul, E. Emregul, Mater. Sci. Eng. C 2015, 56, 132 –
140.
[21] N. Haldolaarachchige, Q. Gibson, J. Krizan, R. J. Cava, Phys. Rev. B 2014,
89, 104520.
[22] T. M. Mattox, A. Bergerud, A. Agrawal, D. J. Milliron, Chem. Mater. 2014,
26, 1779 – 1784.
[23] D. K. Zhong, S. Choi, D. R. Gamelin, J. Am. Chem. Soc. 2011, 133, 18370 –
18377.
[24] J. Liu, O. Margeat, W. Dachraoui, X. Liu, M. Fahlman, J. Ackermann, Adv.
Funct. Mater. 2014, 24, 6029 – 6037.
[25] L. Zhao, X. Chen, X. Wang, Y. Zhang, W. Wei, Y. Sun, M. Antonietti, M.-M.
Titirici, Adv. Mater. 2010, 22, 3317 – 3321.
Photocurrent measurements were performed by using an electrochemical analyzer (CHI660D Instruments) with a standard threeelectrode system. The as-prepared electrodes (C@W-20, C@W-15,
C@W-10, C/W), Pt sheet, and Ag/AgCl electrode were used as the
working electrode, counter electrode, and reference electrode, respectively. The working electrode had an active area of … 2.0 cm2
on an indium tin oxide (ITO) glass substrate. Electrodes were calcined at 350 8C for 30 min with a 2 8C min¢1 heating rate under Ar
flow. The electrolyte was 0.1 m NaClO4 aqueous solution, which
was purged with Ar for 30 min before tests.
Photocatalytic experiments
Oxygen evolution was determined by online GC equipped with
a thermal conductivity detector, which was connected to a closed
gas-evolution system. Photocatalyst powder (0.2 g) was dispersed
in AgNO3 solution (200 mL, 0.03 mol L¢1), and La2O3 (0.2 g) was
added as a buffer. The system was degassed thoroughly and irradiated by using a 300 W Xe lamp equipped with a cut-off filter (>
420 nm). The visible-light intensity was 30 mW cm¢2. The amount
of oxygen was sampled every hour. The incident light intensity
was controlled by altering the current of the Xe lamp and determined by an optical power meter (OPHIR, NOVA II).
Stability characterization was conducted as follows. To exclude the
interference of Ag particle accumulation, after 10 h of light irradiation, all photocatalysts were collected and treated with nitric acid
to remove the Ag particles on the surface of the photocatalysts.
The recycled photocatalysts were washed with distilled water until
the pH of the centrifugate was neutral before another 10 h light irradiation was conducted.
The photocatalytic degradation of RhB was conducted in an aqueous solution under visible light by using a 300 W Xe lamp
equipped with a cut-off filter (> 420 nm). The visible-light intensity
was 30 mW cm¢2. Photocatalyst (20 mg) was added to RhB solution
(20 mL, 60 mg L¢1) in a quartz tube at RT under air. The solution
was stirred continuously for 3 h in the dark to ensure the establishment of an adsorption–desorption equilibrium before the light was
turned on. The concentration of RhB during the degradation was
monitored by colorimetric measurement by using a UV/Vis spectrometer (TU-1900, Beijing Purkinje General Instrument Co., Ltd.).
ChemCatChem 2016, 8, 624 – 630
www.chemcatchem.org
Received: September 30, 2015
Revised: November 14, 2015
Published online on December 9, 2015
630
Ó 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim