Novel VO2(M)-ZnO heterostructured dandelions with combined thermochromic and photocatalytic properties for application in smart coatings

Article abstract of DOI:10.1039/c5nj02875b

Hierarchical VO₂(M)-ZnO dandelions with ZnO nanorods grown radially on VO₂(M) nanoparticle cores have been successfully fabricated for the first time. In these dandelions, the VO₂(M) NPs were prepared by a TiO₂ seed-assisted hydrothermal method and the dandelion-like ZnO nanorods were formed over two steps: heteroseed-induced nucleation and the subsequent heteroepitaxial growth processes. The coupled ZnO could increase the chemical stability of VO₂(M) at relatively high temperatures. In addition, the VO₂(M)-ZnO composite film with decreased phase transition temperature (T_c = 62.6 °C) simultaneously displayed an enhanced visible transmission (T_vis-l = 52.2%) and solar modulating ability (ΔT_sol = 9.3%) as compared with the pure VO₂(M) film. Besides, the VO₂(M)-ZnO dandelions also exhibited improved photocatalytic performance, likely due to the synergistic effect of the VO₂(M)-ZnO heterojunction, unique dandelion-like hierarchical structure and high specific surface area. This is the first report of such a single VO₂(M)-ZnO dandelions structure with energy saving and environmental protection effects that offer significant potential for creating a multifunctional smart coating.

Full text of DOI:10.1039/c5nj02875b

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Novel VO2(M)–ZnO heterostructured dandelions
with combined thermochromic and photocatalytic
properties for application in smart coatings†
Cite this:DOI: 10.1039/c5nj02875b
Wenjing Li,ab Shidong Ji,*a Guangyao Sun,ab Yining Ma,ab Hehe Guoab and
Ping Jin*ac
Hierarchical VO2(M)–ZnO dandelions with ZnO nanorods grown radially on VO2(M) nanoparticle cores
have been successfully fabricated for the first time. In these dandelions, the VO2(M) NPs were prepared
by a TiO2 seed-assisted hydrothermal method and the dandelion-like ZnO nanorods were formed over
two steps: heteroseed-induced nucleation and the subsequent heteroepitaxial growth processes. The
coupled ZnO could increase the chemical stability of VO2(M) at relatively high temperatures. In addition,
the VO2(M)–ZnO composite film with decreased phase transition temperature (Tc = 62.6 1C) simultaneously
displayed an enhanced visible transmission (Tvis-l = 52.2%) and solar modulating ability (DTsol = 9.3%) as
compared with the pure VO2(M) film. Besides, the VO2(M)–ZnO dandelions also exhibited improved
photocatalytic performance, likely due to the synergistic effect of the VO2(M)–ZnO heterojunction,
unique dandelion-like hierarchical structure and high specific surface area. This is the first report of such
a single VO2(M)–ZnO dandelions structure with energy saving and environmental protection effects that
offer significant potential for creating a multifunctional smart coating.
Received (in Montpellier, France)
17th October 2015,
Accepted 7th January 2016
DOI: 10.1039/c5nj02875b
www.rsc.org/njc
and the environment. It is known that photocatalysis is effective
for the degradation of organic waste because it is economic,
1. Introduction
Vanadium dioxide (VO2) is a well-known inorganic thermochromic simple and can completely eliminate toxic chemicals from
material, which exhibits an abrupt reversible semiconductor– wastewater.4,5 Zinc oxide (ZnO) nanomaterials with a wide band
metal transition (SMT) at around 68 1C.1 The low-temperature gap energy (B3.37 eV) are potential semiconductor photocatalysts6–8
monoclinic VO2(M) is a semiconductor and infrared (IR) trans- and have attracted intense research attention in recent years.9–12
parent, while the high-temperature rutile VO2(R) is metallic and It is interesting that a composite material with a combination
exhibits a high IR reflectance.2 This distinctive phase transition of VO2 and ZnO would present both energy saving and environ-
occurs with an abrupt change in the optical properties, which mental protection properties.
enables the VO2 films to automatically switch between a trans-
For the practical application of VO2-based smart windows,
parent state and a dark state in response to environmental low visible transmission (Tvis), poor solar modulating ability
temperature without the use of any external switching device. (DTsol) and weak chemical stability are the major drawbacks.13,14
The special characteristics of VO2 make it more promising in Single VO2 films have a high reflectance and strong absorption
applications as smart windows as compared to other inorganic due to their larger refractive index (RI) (n(VO2(M)) = 2.7–2.8 and
and polymeric thermochromic materials.3
n(VO2(R)) = 2.0–2.5)15,16 and large absorption coefficient, resulting
Furthermore, organic pollutants as a by-product of rapid in an insufficient optical properties. According to the previous
industrial development are a serious threat to human health reports and optical calculations,17 involving an additional anti-
reflection layer or dispersing the VO2(M) nanoparticles in a
dielectric matrix could improve the optical properties; e.g.,
TiO2/VO2/glass multilayer structure,18,19 ZnO:Al/VO2 double-
layered film,20 ZnO/VO2/ZnS sandwiched structure21 and VO2–
ZrV2O7 composite film.22 However, it is difficult to incorporate
dielectric layers with a certain refractive index and appropriate
thickness into complicated stack structures13 and the above-
mentioned structures are either expensive or they barely demon-
a State Key Laboratory of High Performance Ceramics and Superfine Microstructure,
Shanghai Institute of Ceramics, Chinese Academy of Sciences, Dingxi 1295,
Changning, Shanghai, 200050, China. E-mail: p-jin@mail.sic.ac.cn,
sdki@mail.sic.ac.cn; Fax: +86-21-6990-6208; Tel: +86-21-6990-6208
b Graduate School of Chinese Academy of Sciences, Beijing 100049, China
c Materials Research Institute for Sustainable Development, National Institute of
Advanced Industrial Science and Technology(AIST), 2266-98 Shimoshidami,
Moriyama-ku, Nagoya 463-8560, Japan
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nj02875b strate photocatalytic performance. Besides, VO2 is gradually
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oxidized into toxic V2O5 in a wet ambient atmosphere, which The final VO2–ZnO dandelions were washed, collected and
leads to a considerable deterioration of its thermochromic dried for characterization.
properties. Enhancing the chemical stability of VO2 against
2.3. Characterization
oxidation is also urgently required. Although a series of studies
reported by our group have demonstrated that VO2@SiO2
23,24
The crystalline structures of the as-prepared crystals were
investigated using powder X-ray diffraction (XRD, Model D/max
2550 V, Rigaku, Japan) using CuKa1 (l = 1.5418 Å). The morphology
was observed by field-emission scanning electron microscopy
(FE-SEM, HITACHI S-3400, Japan) and transmission electron
microscopy (TEM, JEM-2010, JEOL, Japan) equipped with energy
dispersive spectroscopy (EDS). The attachments of high-
resolution transmission electron microscopy (HRTEM) images
and selected area electron diffraction (SAED) of JEM-2010 were
used to obtain the crystallographic information. The compositions
of the powders were detected by X-ray photoelectron spectroscopy
(XPS, Axis Ultra DLD, Kratos, England). Zeta potential measure-
ments were recorded on a ZetaPALS Zeta potential analyzer.
Specific surface area was measured on an ASAP2010 (Micro-
meritics, USA) instrument using the N2 adsorption method
with BET analysis. The optical transmission and absorption
spectra were recorded on a UV-Vis spectrophotometer (HITA-
CHI U-3010, Japan). The photoluminescence (PL) spectra of the
photocatalysts were detected using a Perkin Elmer LS-50B
spectrometer.
and VO2@TiO225 core–shell structures can modulate the optical
properties and enhance the chemical stability of VO2(M), there
are no reports on the production of the hierarchical VO2(M)–
ZnO dandelions with enhanced thermochromic and photo-
catalytic performance, along with a high chemical stability.
In this study, we have reported a successful attempt to fabricate
the hierarchical VO2(M)–ZnO dandelions for the first time. The
VO2(M)–ZnO nanoparticles exhibited multiple functions: thermo-
chromism from the VO2 for solar energy modulation, photo-
catalytic properties from the ZnO for environmental purification,
and highly enhanced chemical stability of VO2(M). The enhanced
thermochromic properties may arise from the anti-reflection effect
and good dispersibility introduced by the transparent ZnO, and
the higher photocatalytic activity could be ascribed to the unique
VO2–ZnO heterojunction and high specific surface area. The
abovementioned technique breakthroughs will play a role in
obtaining the composite materials with integrated energy-saving
and environmental protection functions for building applications.
2. Experimental section
2.4. Photocatalytic measurements
2.1. Synthesis of VO2(M) nanoparticles
The photodegradation reactions were carried out under atmo-
spheric conditions using a 300 W Xe lamp (240 nm–2 mm) as the
irradiation source. The as-fabricated photocatalyst (0.03 g)
was added to a cylindrical container containing 100 ml of
rhodamine B (RhB) aqueous solution (10ꢁ5 M). Before irradiation,
the solution was stirred in the dark for 20 min to obtain a good
dispersion and to reach an adsorption–desorption equilibrium
between the organic molecules and the catalyst surface. To
minimize the effects of heating, the photoreactor was fixed in a
glass jacket and cooled using flowing water. For every 10 min
irradiation intervals, about 3 ml of the suspension was collected
and separated by centrifugation. The residual concentration of
RhB in the supernatant was measured using a UV-vis spectro-
meter at a maximum absorption wavelength of around 554 nm.
All the reagents were of analytical grade and used without
further purification. VO2(M) nanoparticles were prepared via
a rutile TiO2 seed-assisted hydrothermal synthesis as reported
in the literature.26 In a typical procedure, V2O5 and H2C2O4ꢀ
2H2O powders (1 : (1–3) molar ratio) were directly added to
50 ml deionized water with continuous stirring. Then, an
appropriate amount of rutile TiO2 was added to the above-
mentioned solution. After adequate ultrasonication of the
reaction mixture, the resulting solution was transferred into a
100 ml Teflon-lined autoclave and maintained at 240 1C for
24 h. The final products were washed, collected and dried for
further use.
2.2. Synthesis of VO2(M)–ZnO dandelions
Briefly, 0.05 g of the obtained VO2(M) particles was dispersed in
100 ml of deionized water with adequate ultrasonication. Then,
20 ml of a 0.1 M C6H5Na3O7ꢀ2H2O (sodium citrate) solution was
3. Results and discussion
3.1. Preparation of VO2(M)–ZnO dandelions
added into the VO2(M) suspension for further modification. The X-ray diffraction (XRD) patterns of the as-prepared samples
After stirring for 10 h, the obtained NaCA-capped VO2(M) are shown in Fig. 1. All the diffraction peaks in Fig. 1a could be
particles were centrifuged and washed, and then dispersed in perfectly indexed as the monoclinic phase of VO2 synthesized
100 ml of Zn(CH3COO)2 ethanol solution (0.02 M). Under by a TiO2(R) seed-assistant hydrothermal synthesis. Rutile TiO2
stirring for 1 h, 50 ml of a KOH–ethanol solution (0.04 M) has the same space group and similar lattice parameters as
was introduced. The mixed solution was transferred to a water found for VO2(R), thus the rutile TiO2 can be used as seeds to
bath and heated to 60 1C. After allowing it to stand at 60 1C promote the formation, control the morphology and reduce the
for 2 h, the obtained products were rinsed and redispersed size of VO2(M).26 As observed in Fig. 1b, two sets of diffraction
in 200 ml of deionized water containing Zn(CH3COO)2 and peaks exist for the VO2(M)–ZnO sample, which were corres-
HMT (C6H16N4) (1 : 2 molar ratio). The mixed solution was pondingly ascribed to the hexagonal wurtzite ZnO (JCPDS
heated to 90 1C and maintained at this temperature for 2 h. 36-1451) and VO2(M) (JCPDS 43-1051). The diffraction peaks
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we found that the radially arrayed ZnO nanorods grew around a
core to form the ‘‘dandelion’’ structures, and that each nanorod
was 20–50 nm in diameter and about 200 nm in length. The
typical TEM image shown in Fig. 2d further reveals that the
average diameter of the core was about 100 nm, which is in
accordance with the size of the initially prepared VO2(M) NPs.
Moreover, a high-resolution TEM (HRTEM) of the ZnO nanorod
is shown in Fig. 2e. The fringe spacing of 0.52 nm agreed
well with the spacing of the (001) lattice planes of ZnO,27 and
the diffraction spots of the corresponding selected area electron
diffraction (SAED) pattern could be indexed as the (001),
(101) and (100) reflections (inset in Fig. 2e). This demonstrated
that the ZnO nanorods were structurally uniform and can be
described as single crystals with a [100] growth direction. In
addition, the EDS measured in one signal dandelion (the
red oval in Fig. 2d) showed that the VO2(M)–ZnO dandelions
were composed of Zn, V, Ti and O, which is consistent with the
XRD results.
Fig. 1 XRD patterns of the as-prepared products: (a) VO2(M) and (b)
VO2(M)–ZnO.
Different amounts of the VO2(M) NPs were introduced
were sharp and intense, indicating the highly crystalline character. to obtain the optimal VO2(M)–ZnO dandelion structure. As seen
Furthermore, no characteristic peaks for impurities, such as in Fig. S1b (ESI†), when 0.1 g of VO2(M) NPs were used in the
Zn2V2O7 and Zn3(OH)2V2O7ꢀ2H2O, were observed, suggesting experiment, the products consisted of inhomogeneous nanosheets
that the compositions of the abovementioned samples were and dandelions. The intensity of the ZnO diffraction peaks was
ZnO and VO2.
weaker. Upon decreasing the amount of VO2(M) NPs, the
Fig. 2a–e present the representative SEM and TEM images nanosheets were gradually decreased and uniform VO2(M)–ZnO
of the abovementioned two samples. According to Fig. 2a, the dandelions were formed. When the usage of the VO2(M) NPs was
synthesized VO2(M) nanoparticles were grown into quasi-spherical set as 0.05 g, products with good morphology and high crystallinity
shape with a size of around 50–100 nm. Fig. 2b and c are the were obtained, as shown in Fig. S1d (ESI†).
low- and high-magnification SEM images of the VO2(M)–ZnO
To investigate the chemical state of the as-synthesized
crystals, respectively. It could be seen that the product consisted VO2(M)–ZnO dandelions, X-ray photoelectron spectroscopy
of relatively uniform dandelion-like assemblies that were about (XPS) was carried out, and the corresponding results are shown
400 nm in size. From the enlarged SEM image shown in Fig. 2c, in Fig. 3. The full-scan spectrum (Fig. 3a) demonstrates that V,
Fig. 2 (a) TEM image of the VO2(M) NPs, (b and c) Low- and high-magnification SEM images, (d) TEM image, (e) HRTEM image and SAED patterns (inset),
and (f) EDS patterns of the VO2(M)–ZnO dandelions.
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Fig. 3 XPS patterns of the VO2(M)–ZnO dandelions: (a) XPS full spectrum, (b) V2p spectra, (c) Zn2p spectra, and (d) O1s spectra.
Zn, O, and C exist in the products. The C element could be As seen in Fig. S2b (ESI†), the pure ZnO NRs had a size of around
attributed to adventitious carbon-based contaminants, and the 150 nm in diameter and 1–2 mm in length. If the VO2(M) were not
binding energy for the C1s peak was 284.6 eV, which was used modified with sodium citrate, the final product consisted of much
as the reference for calibration. The Ti element from the more single ZnO NRs and the dandelion-like NPs with an
TiO2(R) seeds was unobserved due to the deep level and weak uneven morphology that were seriously aggregated (Fig. S2c,
signal. As shown in Fig. 3b, the peak located at 516.3 eV ESI†). Furthermore, when the experiment was conducted in the
corresponded to V2p3/2, which is in agreement with a previous absence of ZnO seeds, the ZnO mainly grew into hexahedron
study28 and indicates that the vanadium was in +4 state. Fig. 3c shape with a size of around 1 mm and was partially formed into
shows the Zn2p spectrum of the VO2(M)–ZnO dandelions, which nanosheets coated on the surface of the VO2(M) NPs, as shown
contains two representative peaks located at 1021.4 and 1044.5 eV, in Fig. S2d (ESI†). The surface preparation and subsequently
corresponding to Zn2p3/2 and Zn2p1/2 of Zn2+, respectively. The introduced ZnO seeds are important for the generation of the
shape of the O1s spectrum shown in Fig. 3d was asymmetric, which hierarchical VO2(M)–ZnO dandelions. The surface preparation
indicates that there is more than one chemical state according might be beneficial for modifying the Zeta potential of VO2(M)
to the binding energy. Thus, the region of O1s is divided into two (from ꢁ8.76 to ꢁ49.21 mV, Table 1), which could promote the
peaks. The main peak centered at 530.2 eV is attributed to O2ꢁ electrostatic attractions between the Zn2+ ions and the VO2(M)
ions on the crystal lattice of the Zn2+ and V4+ ion array (OZn–O NPs. The introduced ZnO seeds were beneficial for epitaxial
and OV–O).29 The shoulder at 531.5 eV is associated with hydroxide growth and the formation of rod-like ZnO.
interactions such as the adsorption of OH groups.30
Therefore, based on the abovementioned results and discussion,
we proposed a tentative mechanism for the formation of the
3.2. The formation mechanism of the VO2(M)–ZnO
dandelions
Table 1 The Zeta potential data of the products at different synthesis
stages
To investigate the formation mechanism of the VO2(M)–ZnO
dandelions, three other samples were prepared for comparison:
ZnO NRs, VO2(M)–ZnO without Na-CA and VO2(M)–ZnO with-
out ZnO seeds. The XRD patterns (Fig. S2a, ESI†) indicated that
Na-CA
capped
VO2 NPs VO2 NPs
ZnO seed
VO2
NPs
Zn2+ adsorbed coated VO2
Products/ethanol
NPs
the samples consisted of ZnO and VO2(M) without other impurities. Zeta potential (mV) ꢁ8.76 ꢁ49.21
5.56
ꢁ6.15
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It was found that the VO2(M) in our study remained unchanged
at 290 1C and then oxidized into V2O5 at 320 1C (Fig. 4a), while
the pure VO2(M) reported in the literature25 partly changed into
V3O7 and V2O5 at 290 1C. The increased stability may due to the
existence of the TiO2(R) seeds. On the contrary, VO2(M)–ZnO
still remained in its original phase compositions after being
annealed at 380 1C (Fig. 4b), indicating its good chemical
stability. Upon further increasing the annealing temperature
to 430 1C, VO2(M) just reacted with the ZnO at the interface to
form Zn2V2O7 without the existence of V2O5. The abovementioned
phenomenon indicated that the VO2(M)–ZnO composite exhibited
enhanced stability at high temperatures. The enhancement may
be ascribed to the two aspects: one was the existence of the TiO2(R)
seeds and the other was the protection from the coupled ZnO.
TiO2 and ZnO are more stable than VO2(M) at a relatively high
temperature. The TiO2(R) seeds could increase the structural
stability and the dense ZnO could act as an effective barrier
layer to prevent the diffusion of oxygen into the VO2 lattice.
This result was superior to the previously reported VO2@SiO2
and VO2@TiO2 structures.24,25
The reversible phase transition temperature of the pure
VO2(M) and VO2(M)–ZnO composite powders were measured
using differential scanning calorimetry (DSC) and the curves
are shown in Fig. 5a. It was found that the endothermic peak in
the heating process of pure VO2(M) was located at 67.8 1C.
However, the phase transition temperature of the heterostructured
VO2(M)–ZnO dandelions was decreased to 62.6 1C. This result
was lower than the previously reported VO2@SiO2 nanorod-
based powders32 and VO2(M)@C belt-like structure.33 After
coating the powder onto a float glass substrate, the films were
prepared and then inserted into the UV-Vis spectrophotometer
to measure the transmittance at a low (20 1C) and a high tempera-
ture (90 1C). The optical transmittance spectra are shown in Fig. 5b.
It can be clearly seen that the VO2(M)–ZnO composite film exhibited
perfect optical properties. It reached a sufficiently high visible
transmission (Tvis-l = 52.2%) and still retained an excellent solar
modulating ability (DTsol = 9.3%), which are better than that
found for a pure VO2(M) film (Tvis-l = 35.0% and DTsol = 8.5%),
as shown in Table 2.
Scheme 1 Schematic illustration of the heteroseed assembly induced
formation of VO2(M)–ZnO dandelions.
VO2(M)–ZnO dandelions, which was similar to that reported
previously.31 As illustrated in Scheme 1, first, when the modified
VO2(M) NPs were dispersed in the ZnAC2 solution, Zn2+ ions
were captured by the negatively charged VO2(M) NPs due to
electrostatic attractions. Then, the Zn2+ ions loaded VO2(M) NPs
with positively charged surface (5.56 mV, seen in Table 1) acted as
the cores for the formation of the ZnO seeds after the addition of
KOH. Besides, the excess KOH added would further switch the
surface charge from positive to negative (from 5.56 to ꢁ6.15 mV,
Table 1), which actively led to the spontaneous self-assembly of the
primary VO2(M)–ZnO heteroseeds to the VO2(M)–ZnO core–shell
heteroseeds. Finally, the VO2(M)–ZnO heteroseeds served as non-
planar substrates for the growth of ZnO nanorod arrays and
eventually lead to the formation of VO2(M)–ZnO dandelions.
3.3 Antioxygenic and thermochromic properties
The VO2(M) and VO2(M)–ZnO were annealed in air at different
temperatures for 15 min at a heating rate of 10 1C minꢁ1 to
study the antioxygenic properties. The phase evolution after
being annealed at different temperatures is shown in Fig. 4.
Fig. 4 XRD patterns of the samples after heat treatment at different temperatures for 15 min: (a) VO2(M) and (b) VO2(M)–ZnO.
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Fig. 5 (a) DSC curves and (b) optical transmittance spectra at a low (20 1C) and a high temperature (90 1C) for the VO2(M) NPs and VO2(M)–ZnO
dandelions.
3.4 Photocatalytic activity
Table 2 The optical properties, binding energy and specific surface area
of the samples
To demonstrate the photocatalytic activities of the as-synthesized
VO2(M)–ZnO dandelions, the photocatalytic decomposition of RhB
as a test reaction was carried out under simulated solar light
produced using irradiation from a Xe-lamp. As observed in Fig. 6a,
the major absorption peaks of RhB at around 550 nm diminished
gradually as the irradiation proceeded and almost vanished after
50 min. Correspondingly, the color of the suspension completely
Sample Tc (1C) Tvis-l (%) DTsol (%) lc-l (nm) Eg (eV) SBET (m2 gꢁ1
)
VO2
ZnO
67.8
—
35.0
—
52.2
8.5
—
9.3
498
—
475
2.27
2.95
2.67
—
34.5
69.5
VO2–ZnO 62.6
The solar transmittance was calculated by the formula Tsol
¼
Ð
ꢀÐ
jðlÞTrðlÞdl jðlÞdl and the solar transmittance modulation efficiency
was calculated using DTsol = Tsol_201C ꢁ Tsol_901C. j(l) is the solar irradiance transformed from deep pink to translucent at the end (insert in
spectrum for air mass 1.5 corresponding to the sun standing 371 above the
Fig. 6a). These observations revealed that the chromophoric
structure of RhB was gradually decomposed during the reaction
horizon and Tr(l) is the transmittance of our sample.36
process. Furthermore, for comparison, pure ZnO NRs were also
In addition, the existence of ZnO could widen the optical
investigated as a photocatalytic reference. As observed in Fig. 6b,
band gap from 2.27 eV in a pristine VO2 film to 2.67 eV in the
the VO2(M)–ZnO dandelions exhibited the highest photocatalytic
VO2(M)–ZnO heterostructured film, as shown in Fig. S3 (ESI†).
For a direct semiconductor, the band gap energy, Eg, can be
determined as follows:
activity. After being irradiated for 20 min, the degradation
efficiency of the VO2(M)–ZnO dandelions was of about 80%,
whereas the value obtained for the ZnO NRs was about 46%.
(ahn)2 = A(hn ꢁ Eg)
(1)
The degradation process approximately complied with the
pseudo-first-order kinetics,37 as shown in formula (2).
where a, n, Eg and A are the absorption coefficient, light
frequency, band gap energy and constant, respectively.34 By
linear extrapolation of (ahn)2 against hn, the value of the Eg was
obtained as the intercept at the a = 0 axis.
ln C/C0 = ꢁkt
(2)
where C and C0 refer to the remnants and initial concentration
The change in Eg reflected the nature and the concentration of RhB, respectively. k is the degradation constant and the
of the defects. The incorporation of Zn and the interfacial stress values for VO2(M)–ZnO, ZnO NRs and Blank are 0.0782, 0.03
induced a distortion in the VO2 materials and increased the and 0.005 minꢁ1, respectively, which were calculated from
concentration of the defects, resulting in a wider optical band Fig. 6c. Accordingly, the photocatalytic efficiency displayed
gap and a lower phase transition temperature. The widened the following order: VO2(M)–ZnO 4 ZnO NRs 4 Blank.
optical gap could lead to a blue shift in the absorption edge
In addition, to evaluate the stability of the catalyst during
from 498 to 475 nm, which, in turn, resulted in a significant the photo-degradation of RhB in an aqueous solution, the reacted
increase in the visible transmittance (as seen in Table 2). This VO2(M)–ZnO catalyst was collected and washed several times with
positive effect of the coupled ZnO on the optical properties and deionized water to remove the residual dye impurities and then
the phase transition temperature of the VO2(M) film was dried at 60 1C for 12 h to measure the mass. The mass remained
similar to that previously reported on Zn-doped VO2(M) films.35 unchanged, which indicated that the VO2(M)–ZnO catalyst was
In addition, the refractive index of the ZnO was lower than that stable without any dissolution.
found for VO2(M), the existence of the ZnO can act as an anti-
reflection layer to increase the transmittance.
The surface structures of the as-synthesized ZnO NRs and
VO2(M)–ZnO dandelions were analysed by nitrogen sorption
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Fig. 6 (a) Absorption spectra and photographs of RhB solutions in the presence of the VO2(M)–ZnO dandelions under UV-vis light at different periods of
time. (b and c) Degradation profiles and kinetic linear simulation curves for RhB over different samples: Blank, ZnO NRs and VO2(M)–ZnO dandelions.
isotherm technique, and the curves are presented in Fig. 7a.
Brunauer–Emmett–Teller (BET) analysis revealed that the surface it can be inferred that the ZnO NRs were uniformly grafted on
area of the VO2(M)–ZnO dandelions was 69.5 m2 ꢁ1, which the surface of the VO2(M) NPs to form the VO2(M)–ZnO hetero-
was much larger than that found for pure ZnO (34.5 m2 gꢁ1
junction due to intimate interfacial contacts. Under simulated
Based on the results of the abovementioned measurements,
g
)
(Table 2). The special hierarchical structure of the VO2(M)–ZnO solar light irradiation, the photogenerated electrons and holes
dandelions could both increase the surface area and prevent the could be stored separately, as shown in Scheme 2. The electronꢁs
ꢂ
aggregation of the catalyst nanoparticles, which are beneficial could reduce the dissolved molecular oxygen to produce O2
to increase the active sites and improve the photocatalytic superoxide radical anions (O2 + e - O2 ꢁ), while the holes were
ꢂ
performance. In addition, the composite with narrowed band ultimately trapped by the hydroxyl groups (or H2O) on the
gap could increase the utilization efficiency of the simulated surface to yield OHꢂ radicals (h+ + HO2 - OHꢂ + H+). The
ꢁ
ꢂ
solar light. Furthermore, the charge separation and recombination O2 and OHꢂ radicals are strong oxidizing agents that can
properties of the as-prepared samples were investigated using decompose the organic dye. Furthermore, because of the better
photoluminescence (PL) emission spectroscopy. In Fig. 7b, it separation of photogenerated electrons and holes attributed to
ꢁ
ꢂ
indicates that the VO2(M)–ZnO dandelions exhibited a much the 3D heterojunction structure, more O2 and OHꢂ radicals
lower emission intensity than the pure ZnO NRs. A previous were produced, improving the photocatalytic activity.
work reported that the PL emission below 413 nm occurred by
In addition, the photo-induced holes have strong oxidability.
the recombination of the free excited electrons and holes.38 The They could also react with the surface O2ꢁ ions on ZnO to
lower emission intensity indicated that the recombination of release O2 (ZnO + 2h+ - Zn2+ + 1/2O2), leading to the dissolution of
the photogenerated charge carrier was greatly inhibited in the ZnO.39 Thus, the pure ZnO nanostructures display significant
VO2(M)–ZnO heterostructures.
photocorrosion phenomena. For the VO2(M)–ZnO heterojunction,
Fig. 7 (a) Nitrogen adsorption–desorption isotherm and (b) PL emission spectra of ZnO NRs and VO2(M)–ZnO dandelions.
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Scheme 2 Schematic illustration of the photocatalytic mechanism of the
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41
ture42 are beneficial to prohibit the photocorrosion of the ZnO.
´
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4. Conclusions
In summary, the 3D VO2(M)–ZnO dandelions were prepared for
the first time. They exhibited unique features: enhanced photo-
catalytic ability, improved thermochromic properties and
chemical stability. The enhanced photocatalytic ability could
be primarily attributed to the formation of a heterojunction
structure at the interface between ZnO and VO2(M), which
greatly promoted the efficient separation of photogenerated
electron–hole charge carriers. The improved thermochromic
properties and chemical stability were ascribed to the ZnO. This
synthetic process has advantages including simplicity, speed
and cost-effectiveness. It is expected to have potential application
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Acknowledgements
This study was financially supported by the high-tech project of
MOST (2014AA032802), the national sci-tech support plan, the
National Natural Science Foundation of China (NSFC, No.:
51372264), and the Science and Technology Commission of
Shanghai Municipality (STCSM, No.: 13NM1402200).
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