The facile preparation of Ag decorated TiO2/ZnO nanotubes and their potent photocatalytic degradation efficiency

Article abstract of DOI:10.1039/c7ra09466c

Hollow TiO₂ and TiO₂/ZnO nanotubes were produced using a simple and novel approach combining electrospinning with an impregnation calcination method. Using a photoreduction method, silver nanoparticles (AgNps) were successfully decorated on the surface of nanotubes. During the synthesis process, the electrospinning procedure provided sacrificial templates for the fabrication of hollow nanotubes by producing PAN nanofibers, which can be immobilized on the TiO₂ surface through a soaking method. Later, the composite nanofibers of TiO₂/PAN were calcined into pure TiO₂ nanotubes after complete combustion of the sacrificial templates. The morphology of the samples was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The structure and components of the samples were characterized by X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX) and X-ray photoelectron spectroscopy (XPS). Upon blending ZnO into TiO₂, a new crystallite ZnTiO₃ was formed in addition to the ZnO and TiO₂ crystallites, and the ultra-violet light absorption efficiency was detected using UV-vis diffuse reflectance spectroscopy (DRS). The photocatalytic properties were evaluated using the degradation of methyl orange (MO) dye. As observed from the results, the Ag/TiO₂/ZnO nanotubes could degrade MO efficiently under UV irradiation, which showed excellent photocatalytic activity compared to pure TiO₂. The synthetic mechanism of Ag/TiO₂/ZnO nanotubes and the mechanism for the enhanced photocatalytic activity have been proposed.

Full text of DOI:10.1039/c7ra09466c

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PAPER
The facile preparation of Ag decorated TiO /ZnO
2
nanotubes and their potent photocatalytic
Cite this: RSC Adv., 2017, 7, 50064
degradation efficiency
Shengzhe Zhao,c †a Zhiqiang Cheng,†*ab Lijuan Kang, Mingtang Li
a
a
and Zhiling Gao
2 2
Hollow TiO and TiO /ZnO nanotubes were produced using a simple and novel approach combining
electrospinning with an impregnation calcination method. Using a photoreduction method, silver
nanoparticles (AgNps) were successfully decorated on the surface of nanotubes. During the synthesis
process, the electrospinning procedure provided sacrificial templates for the fabrication of hollow
nanotubes by producing PAN nanofibers, which can be immobilized on the TiO
2
surface through
nanotubes
a soaking method. Later, the composite nanofibers of TiO /PAN were calcined into pure TiO
2
2
after complete combustion of the sacrificial templates. The morphology of the samples was
characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The
structure and components of the samples were characterized by X-ray diffraction (XRD), energy
dispersive X-ray analysis (EDX) and X-ray photoelectron spectroscopy (XPS). Upon blending ZnO into
2 3 2
TiO , a new crystallite ZnTiO was formed in addition to the ZnO and TiO crystallites, and the ultra-
violet light absorption efficiency was detected using UV-vis diffuse reflectance spectroscopy (DRS). The
Received 26th August 2017
Accepted 2nd October 2017
photocatalytic properties were evaluated using the degradation of methyl orange (MO) dye. As observed
from the results, the Ag/TiO
showed excellent photocatalytic activity compared to pure TiO
ZnO nanotubes and the mechanism for the enhanced photocatalytic activity have been proposed.
2
/ZnO nanotubes could degrade MO efficiently under UV irradiation, which
DOI: 10.1039/c7ra09466c
2
. The synthetic mechanism of Ag/TiO
2
/
rsc.li/rsc-advances
study the fabrication of photocatalysts with unique nano-
Introduction
13
structures. Compared with traditional TiO
materials show enhanced photocatalytic activity
been utilized in the elds of sensing, solar energy conversion, due to their surface active sites, crystalline structure, specic
2
crystals, nano-
One-dimensional (1-D) semiconductor nanostructures have structured TiO
2
1
4–19
catalysis, biomedicine and in various electronic components surface area, crystal morphology and crystal size.
With the
development of nanotechnology, structures with special shapes
Among the copious inorganic nanostructures reported, tita- have been prepared for nanocrystalline TiO in the form of
nium dioxide (TiO
), a multifunctional semiconductor metal nanorods, nanoowers, nanotubes, nanobers, and nano-
oxide, has attracted an extensive amount of research attention particles.
because of its unique optical, electronic, and antibacterial pursued as a quintessential photocatalyst by virtue of their high
1
–4
because of their excellent physical and chemical properties.
2
2
20–24
2
To date, TiO nanotubes have been extensively
5–7
properties. Recently, the application of TiO photocatalysts surface-to-volume ratio and superior physicochemical proper-
2
25
has mainly been focused on decomposing organic pollutants in ties. Moreover, TiO
2
nanotubes have been shown to manifest
contaminated air or water, which is of importance for envi- intrinsic advantages over their TiO
ronmental protection. Although notable advances have been in terms of their size, shape, structure and properties. Moreover
nanoparticle counterparts
2
8,9
made, the high recombination rate of the photogenerated heterogeneous photocatalysts have also made signicant prog-
2
6–28
electron/hole pairs and the low utilization rate of ultra-violet ress in the eld of photocatalysis.
light hinders their further application in industry.
Coupling TiO
2
with other
10–12
There- semiconductors and noble metal-based heterostructures or
fore, a number of research groups have made great efforts to composites provides a benecial solution for their photo-
5
catalytic applications.
In addition, zinc oxide (ZnO) is also an excellent n-type
a
College of Resources and Environment, Jilin Agriculture University, Changchun
semiconductor exhibiting comparable efficiency in various
1
30118, China. E-mail: czq5974@163.com
29
b
photocatalytic reactions to TiO2. Semiconductors comprising
ZnO or noble metal silver modied TiO have been studied to
School of Aerospace Engineering, Tsinghua University, Beijing 100000, China
College of Chemistry, Jilin University, Changchun 130118, China
c
2
30–32
improve the photocatalytic effect,
which can reduce the
†
Zhiqiang Cheng and Shengzhe Zhao equally contributed to this work.
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recombination of charge carriers and increase the rate of the dissolved in 10 mL of the TBOT ethanol solution. The composite
28
electron-transfer process. The most commonly used modi- bers in which the precursor solution contained 0.01, 0.05 and
cation methods include the microemulsion hydrothermal 0.10 g of zinc acetate were named P1, P2 and P3, respectively.
process, metal–organic chemical vapor deposition, template
The last step was calcination. The P0 to P3 precursors were
ꢀ
induced method and wet chemical method, which have been calcined in a vacuum atmosphere at 550 C for 2 h at a heating
33–36
ꢀ
ꢁ1
used to successfully synthesize the desired heterostructures.
rate of 2 C min to stabilize the morphology of the precursor
To date, the synthesis methods used to prepare hollow nano- bers. Meanwhile, vacuum calcining could carbonize the PAN
tubes have included anodic oxidation to produce metal oxide bers so that the sacricial templates were decomposed
nanotubes, hydrothermal treatment of ne TiO
2
particles with completely in the subsequent aerobic calcination conditions.
ꢀ
an alkaline solution and chemical or physical deposition on The precursor carbide bers were calcined at 550 C for 2 h to
37,38
sacricial colloidal templates.
These methods are oen completely burn the carbide sacricial templates at a heating
ꢀ
ꢁ1
confronted with problems such as template removal, tedious rate of 2 C min . Aer annealing, the hollow TiO and TiO /
2
2
2
operation procedures, and poor adhesion of nanotube to the ZnO nanotubes were obtained. The TiO /ZnO samples were
37
underlying substrate. A simple and efficient synthesis method named TZ1, TZ2, and TZ3. As the nal step, decoration with the
for hollow heterostructured nanotubes is constantly being AgNps was carried out using the TZ2 sample, which showed the
looked for.
In this study, hollow TiO
best catalytic effect for further investigation. TZ2 was immersed
/ZnO nanotubes in a 0.01 M AgNO solution with magnetic stirring in the dark
2
, ZnO and TiO
2
3
were produced using a simple and novel approach combining for 1 h. Subsequently, the mixture was subjected to UV irradi-
electrospinning with an impregnation calcination method. ation for 5 min. Aer cleaning and drying, the Ag decorated
Silver nanoparticles (AgNps) were successfully and efficiently TiO /ZnO nanotubes were obtained, and the Ag/TiO /ZnO
2
2
decorated on the surface and inner walls of the TiO
2
/ZnO samples were named as ATZ.
nanotubes. The surface morphology and dimensions of the
samples were observed by SEM and TEM. XRD, XPS and EDX
Characterization
2
analyses identied that the sample was pure TiO with an
Information on the phases and purity of the samples was ob-
tained by X-ray powder diffraction (XRD) measured on a D/MAX
anatase structure and the AgNps and ZnO were modied
successfully. Because the hollow nanotube structure can
provide a larger specic surface area and more active sites, the
hybrid nanoparticles can increase the rate of electron-transfer
process and reduce the recombination of charge carriers that
2250 V diffractometer (Rigaku, Japan) using Cu Ka radiation
(
l ¼ 0.15418 nm) under 40 kV and 30 mA, and the scanning
ꢀ
ꢀ
range was from 20 to 80 . The morphologies of the TiO
nanotubes were observed by scanning electron microscopy
SEM, SHIMADZU X-550) equipped with energy dispersive X-ray
2
/ZnO
2
the Ag/TiO /ZnO photocatalyst have a much higher photo-
catalytic activity than pure TiO .
2
(
analysis (EDX) and transmission electron microscopy (TEM,
Tecnai G2, operating voltage: 15 kV). X-ray photoelectron spec-
troscopy (XPS) was performed on a VG ESCALAB LKII instru-
ment with a Mg-Ka-ADES source (hn ¼ 1253.6 eV) at a residual
gas pressure below 1028 Pa. Photoluminescence (PL) spectra
were measured on a uorescence spectrometer (Hitachi F-4500)
at an excitation wavelength of 370 nm. UV-vis spectra were
recorded on a UV-vis spectrophotometer (Shimadzu, UV-2550)
over a scanning range from 300 to 800 nm. The degradation
process was monitored using UV-vis spectrophotometry
measuring the absorption of MO at 465 nm.
Experimental
Materials
Polyacrylonitrile (PAN, M
w
z 80 000), tetrabutyl titanate (TBOT,
9
7%), AgNO and methyl orange (MO) were purchased from
3
Aladdin; N,N-dimethylformamide (DMF, 99%), absolute
ethanol and zinc acetate (99%) were obtained from Beijing
chemical works. All of the chemicals used in our experiments
were of analytical grade and used without further purication.
Methods
Photocatalytic experiments for MO
The solution was prepared by adding 0.62 g of PAN to 5.0 mL of Methyl orange (MO) has been vastly used as a model compound
ꢀ
DMF with magnetic stirring at 50 C. Aer PAN was completely to indicate photocatalytic activity, which is primarily attributed
dissolved in DMF, the prepared precursor solution was loaded to its frequent occurrence in industrial waste discharge. The
into a syringe, which was connected to a 16 kV high voltage. The photocatalytic activity of the as-prepared samples was evaluated
ꢁ
1

ow rate of the solution was controlled at 0.5 mL h and the using the decomposition of MO under ultraviolet light irradia-
ꢁ
1
distance from the needle to the rotate acceptor was 14 cm. The tion. An aqueous solution of MO (40 mL, 10 mg L ) was placed
composite polymer lms were prepared in a fume hood with the in a vessel and the samples (40 mg) were added. Then, the
humidity kept below 30%. Then, the PAN bers were soaked in solution was magnetically stirred in the dark for 2 h to ensure
the TBOT/ethanol precursor solution for 5 min in which the the establishment of an adsorption–desorption equilibrium
volume ratio of TBOT to ethanol solution was 1 : 10. Aer between the MO dye and the as-prepared samples. Later the
complete inltration, the composite precursor bers were mixture was exposed to an ultra-violet radiation lamp (Philips,
placed in the air to dry naturally and were named P0. To obtain TL/05, 125 W, distance 5 cm, spot size 365 nm), and the
2
the TiO /ZnO nanotubes, a certain amount of zinc acetate was suspensions were magnetically stirred at ambient temperature
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to ensure that all of the catalysts were in contact with the dye
solution. At given time intervals, a few millilitres of the solution
were drawn from the mixture and centrifuged to separate the
samples. Then, the solution was loaded into a UV-vis spectro-
photometer (Shimadzu, UV-2550). The degradation efficiency of
MO was monitored by comparing the characteristic absorption
intensity of MO at 465 nm at different times with that of the
original MO.
Results and discussion
Fig. 1 shows the XRD patterns of the samples. The strong and
sharp diffraction peaks indicate that the products were well
crystallized. The gure shows that the main diffraction peaks of
ꢀ ꢀ ꢀ ꢀ ꢀ
TiO located at 25.4 , 37.9 , 48.2 , 55.2 and 62.8 can be
2
indexed to the (101), (004), (200), (211) and (204) directions,
which characterize the anatase structure of TiO2 (JCPDS 21-
1272) and no by-product peaks are found, illustrating that the
purity of the sample was high. Compared with pure TiO
2
, the
ꢀ
ꢀ
ꢀ
TZ2 and TZ3 samples display weak peaks at 32.8 , 35.3 , 48.9
ꢀ
and 53.5 , conrming the formation of the ZnTiO
3
phase
(
JCPDS 26-1500), and there was also a certain amount of rutile
ꢀ
2
phase at 27.4 in all of the TiO /ZnO samples. Compared with
the diffraction pattern of the three TiO /ZnO samples, the peaks
2
Fig. 2 The SEM and the EDX (inset) images of the samples: (A) PAN
nanofibers, (B) ZnO porous nanotubes, (C) TiO2 nanotubes, (D) TZ1
for ZnTiO3 and rutile show an enhanced trend due to the
2
+
2+
addition of metal ions (Zn ), which indicates the Zn could nanotubes, (E) TZ2 nanotubes and (F) TZ3 nanotubes. The XRD
patterns are shown in the insets.
38,39
impede the TiO
with pure TiO /ZnO, the pattern observed for the ATZ sample
showed weak peaks at 44.58 and 64.68 , which can be indexed
2
transition from anatase to rutile.
Compared
2
ꢀ
ꢀ
to the (200) and (220) directions of Ag (JCPDS 03-0921), produced by the electrospinning technique and used as the
respectively and this phenomenon may due to the modied Ag templates for fabricating the hollow nanotubes. Fig. 2B presents
being very small. The d-spacing of anatase at the (101) plane the porous ZnO hollow nanotubes, which were obtained by
˚
replacing the TBOT solution with a zinc acetate ethanol solution
observed for ATZ was calculated to be 3.53 A. This is in good
agreement with the measured value observed from the HRTEM (mass fraction: 10%) keeping the other operations the same.
image (Fig. 3D).
The XRD pattern displayed in the inset demonstrates that the
ZnO crystallites have a hexagonal crystalline phase. Fig. 2C
2
demonstrates that the TiO samples are mainly comprise rela-
tively regular and uniform smooth nanotubes. The mean
Morphology and formation mechanism
The morphology of the samples was characterized by SEM and
TEM. Fig. 2A shows that PAN nanobers with high porosity were
diameter of the TiO nanotubes was 300 nm and the uniformity
2
was above 80%, which indicate that the TBOT/ethanol solution
permeates into the surface of the PAN template bers success-
fully and without any aggregation in the calcination process.
The SEM images of the three types of TiO
shown in Fig. 2D–F. Comparing the SEM images of TiO
2
/ZnO samples are
, TZ1
2
and TZ2, we found that there were no differences in the
morphology on the surface aer doping with ZnO, which indi-
cated that there was no obvious aggregation observed for the
ZnO nanoparticles, and the ZnAc/TBOT/ethanol solution was
evenly distributed on the surface of the bers. Because of the
existence of Zn ions, the TiO
acetate to form the lattice type of ZnTiO
Fig. 2F has a relatively rough surface, which may be due to too
2
particles would react with zinc
3
. Notably, TZ3 shown in
2
+
much Zn doping leading to excessive internal stress in the
crystallization process.
Fig. 3B shows the SEM images of the ATZ nanotubes, which
demonstrate no difference from the TiO nanotubes, which may
2
Fig. 1 The XRD patterns of the samples.
be because the AgNps are very small. For proof of the existence
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2
Fig. 3 The TEM image of the TiO nanotubes (A). SEM image of the
Fig. 4 The XPS spectra of the TiO
2
and ATZ nanotubes: (A) full
ATZ nanotubes (B). The TEM image of the ATZ nanotubes (C) and the
HRTEM of the ATZ nanotubes (D). A high magnification SEM image of
a ATZ nanotube (E) and the EDX spectra of the ATZ nanotubes (F).
spectrum of the sample and (B) the Ti 2p spectrum.
formation of Ag–Ti–O–Zn, which causes a change in the elec-
trons surrounding Ti and was in accordance with the formation
of ZnTiO suggested in the XRD spectrum.
3
of the AgNps, TEM was used to further examine the morphology
of the samples. As shown in Fig. 3C, the TEM image of the ATZ
sample clearly shows that the ZnO and AgNps are uniformly
The photocatalytic efficiency of the catalyst is highly related
to the light absorption including the amount, wavelength range,
and absorption peak position. To prove the light absorption
properties of the samples at different wavelengths, the UV-vis
diffuse reectance spectra (DRS) were recorded. From the DRS
results (Fig. 5A), the reection spectra of the samples are shown.
distributed on the surface compared with the TiO nanotubes
2
(Fig. 3A). The diameter of the AgNps were in the range 5–30 nm,
which demonstrate that the silver decoration was successful.
Fig. 3D is the HRTEM image of ATZ, which was obtained under
the same conditions as those for ATZ nanotubes displayed in
Fig. 3C. The crystal lattice can be observed directly with
d spacings of 0.285 nm and 0.237 nm corresponding to ZnO
2
Compared with the pure TiO nanotubes, the TZ2 sample has
a lower reectivity in the range of 300–600 nm, particularly in
2+
the ultra-violet light region. With the addition of Zn , the
(100) and Ag (111), which are in good agreement with the XRD
energy absorbance for UV and visible light was increased.
Therefore, the results indicate that the hollow tubes of TiO2/
ZnO can absorb light in the ultra-violet region and the light
could be utilized more efficiently for photocatalytic purposes.
results and strong evidence for the existence of ZnO and the
AgNps.
X-ray photoelectron spectroscopy (XPS) was introduced to
clarify the elemental and chemical states in the samples. As
shown in Fig. 4A, the Ti, Zn, Ag and O elements were visible in
Moreover, as mentioned in the XRD results, the ZnTiO
tallites were formed in the hybrids and thus, this may induce
defects in the crystal lattice structure of TiO or ZnO, leading to
3
crys-
2
the fully scanned spectra of pure TiO and the ATZ sample,
2
respectively. However, the spectra show not only Ti, Zn, Ag and
O elements but also some C elements. In the EDX of the
samples, we can only observe Ti, Zn, Ag and O. The XPS peak for
C 1s can be ascribed to the hydrocarbons from the XPS instru-
ment itself. The high resolution XPS spectra of Ti 2p are
exhibited in Fig. 4B. The Ti 2p spectra show peaks at 458.2 eV
an increase in the absorption amount. Compared with TZ2, the
absorption of TZ3 was slightly lower, which may due to such
a large amount of ZnO doping in the ZnTiO crystallites that the
3
overall structure was unstable. The ATZ nanotubes showed the
lowest reectivity in the ultra-violet and visible light region. The
phenomenon of reectance dramatic decline may due to the
unique hollow structure, by the photoreduction method the
AgNps were attach to the surface and inner wall of the TZ2
nanotubes form Ag–semiconductor–Ag sandwich structure,
which means the receiving area of the light is increased and
could be accepted many times that under the same intensity of
and 463.8 eV, which correspond to Ti 2p3/2 and Ti 2p1/2
respectively. The distance between the two peaks was 5.6 eV,
which is identical to that of neat TiO , indicating that Ti was in
,
2
4+
33
the form of Ti . However, comparing to the Ti 2p of TiO , the
2
Ti 2p binding energy of ATZ was increased for the lower Fermi
39
2
level than that of TiO . These phenomena are attributed to the
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Fig. 5 The UV-vis DRS of the samples and the deduced band gap
energy (inset) (A). The calculated band edge (VB, CB) positions of the
TiO and TZ2 nanotubes (B).
2
light, all of this make the ATZ tubes could absorbance more
energy.
Fig. 6 The photocatalytic degradation rate of MO using the as-
prepared samples (A). The UV-vis absorption spectra recorded for the
The band gap energy of the TiO , TZ2 and ATZ nanotubes
2
was obtained from DRS spectra by applying the Kubelka–Munk photocatalytic degradation of MO in the presence of the ATZ nano-
tubes (B) and the repeatability tests studied using the ATZ nanotubes
for five cycles (inset).
function as shown in Fig. 5A (inset). The band gap energies were
2.95 eV, 2.83 eV and 1.25 eV. It was realized that the modica-
tion of the materials through doping or controlling the
morphology on the nanoscale will help tune the band energy of
the materials, which may cause a change in the electronic
transportation by shiing the band edge positions and thereby
lead to the change in their band gap energies.
From the band gap energies obtained, the locations of the
conduction band (CB) and valence band (VB) in the samples
concentration and the concentration of MO at time t, respec-
tively). Fig. 6A shows the concentration change curves for MO in
the photodegradation process aer being irradiated with UV
light for 42 min. The photocatalytic degradation efficiency of
the ATZ sample was 96.7%, which was slightly better than that
found for TZ2 (89.2%) and much higher than that of the TZ1
40
were calculated using the empirical equation: ECB ¼ X ꢁ E
e
ꢁ E
g
/
2
(71.8%), TZ3 (79.1%) and pure TiO nanotubes (56.6%). The
2
, where ECB is the CB edge potential and X is the electronega-
degradation rate of MO without the catalyst was only 0.95%.
tivity of the semiconductor, which is the arithmetic mean of the
electronegativity of the constituent atoms and the rst ioniza-
tion energy. Ee is the energy of the free electrons on the
Compared with the pure TiO tubes, the ATZ tubes showed an
2
amazing improvement of the degradation effect, which was
consistent with the UV-vis DRS results. Fig. 6B shows the UV-vis
absorption spectra of MO in the presence of the ATZ nanotubes.
In addition, we investigated the photocatalytic stability of the
ATZ heterostructure. The results indicate that the hybrids
exhibit good stability aer being reused for ve cycles (Fig. 6B
inset).
hydrogen scale (approximately 4.5 eV), and E is the band gap
g
energy of the semiconductor. Similarly, E can be calculated
VB
4
1
from the equation: EVB ¼ ECB + E
g
. The determined band edge
positions of the TiO and TZ2 nanotubes and their comparison
2
with the potential of the standard hydrogen electrode (SHE) are
depicted in Fig. 6B.
Synthesis of the Ag/TiO /ZnO nanotubes and mechanism of
2
Photocatalytic activities of the samples
the enhanced photocatalytic activity
To investigate their photocatalytic activity, the as-prepared When the PAN/DMF nanobers are soaked in the TBOT ethanol
samples were used for the degradation of an MO solution solution, the DMF and precursor solution dissolve each other
under UV irradiation (Fig. 7). The photocatalytic degradation and allow the TBOT to permeate into the surface of the PAN
efficiency was evaluated by C
0
/C
t
(where C
0
and C
t
are the initial nanobers. Upon increasing the temperature in the vacuum
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Fig. 8 The photoluminescence spectra of the TiO
2
and AZT nano-
tubes, and the concept of band gap engineering in the AgNps modified
TZ2 nanotubes (inset).
the photo-excited electrons and thus, inhibit the recombination
+
40
of the e/h pairs.
It was found that the band gap of TZ2 dramatically declined
aer being modied by the AgNps (Fig. 5). The observed
reduction in the band gap may be due to charge transfer from
Fig. 7 An illustration of the formation mechanism of the Ag/TiO
nanotubes.
2
/ZnO the electron donor levels formed by the 5s orbital of the Ag ions
to the conduction band of the TZ2 host, which is schematically
represented in Fig. 8 (inset). It is known that the band gap of
2
TZ2 is formed by the orbital overlap between TiO and ZnO. In
calcination process, the PAN nanobers gradually form into
PAN carbide bers to provide a template for nanoseed growth
and benet the subsequent calcination step. Under the condi-
tions of aerobic calcination, the surface concentration of TBOT
the AgNps modied TZ2, the Ag 5s level lies below the
conduction band of the TZ2 host. This newly formed Ag 5s
energy level effectively interacts with the valence O 2p orbital
and leads to a charge-transfer transition from the Ag energy
increases leading to the TiO
2
nanoseeds to be more likely to
42
levels to the conduction band of TZ2. Therefore, the band gap
form nanoparticles. These nanoparticles grown on the surface
of the PAN carbide bers form a tubular structure and aer the
was found to be reduced in Ag modied TZ2 compared to pure
TZ2, which is consistent with the XPS analysis.
carbide bers are completely decomposed, the hollow TiO2
Through the photoreduction method, AgNps were success-
fully modied on the surface of the nanotubes, and the surface
and inner walls of the hollow TZ2 nanotubes formed the Ag–
semiconductor–Ag sandwich structure, which could provide
a larger specic surface area and more active sites. Fig. 9 shows
a schematic of the photocatalytic mechanism. When the ATZ
nanotubes are obtained. The ZnO and TiO2 nanotubes had
different morphology, because the titanium element was exist-
ing in the form of TBOT molecules, which could form a Ti–O–Ti
stable structure on the surface prevent titanium from reuniting.
Therefore, smooth hollow TiO
aer calcination. The zinc element was in the form of Zn in
the immersion solution and during the heating process, Zn
2
nanotubes could be obtained
2
+
2
+
was reunited to form the nucleus; the nanoparticles formed on
the surface of the sacricial template, eventually forming the
porous tube structure. The formation mechanism of the TiO
ZnO nanotubes was similar to the process of TiO with the
difference being that the Zn ions combined with TiO to form
a semiconductor–semiconductor (S–S) heterojunction.
2
/
2
2
Photoluminescence (PL) spectroscopy is related to the
separation and recombination of photogenerated electrons and
holes. As shown in Fig. 8, the PL spectra of the TiO nanotubes
2
have a strong emission peak at 390 nm at an excitation wave-
length of 370 nm. However, the PL intensity of the ATZ hybrid
was signicantly decreased with the addition of ZnO and Ag.
The most important reason for this phenomenon is the defects
2
in the crystal structure of TiO , which act as traps for capturing
Fig. 9 A schematic of the electron–hole separation and energy band
matching of the ATZ nanotubes under UV illumination.
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tubes are illuminated by UV light with photon energy, the
electrons (e ) in the valence band (VB) can be excited to the
6 H. Y. Zhu, R. Jiang, Y. Q. Fu, R. R. Li, J. Yao and S. T. Jiang,
Appl. Surf. Sci., 2016, 369, 1.
ꢁ
conduction band (CB) with the simultaneous generation of the
same amount of holes (h ) le behind. The deexed energy
7 T. P. Xie, L. J. Xu, C. L. Liu and Y. Wang, Appl. Surf. Sci., 2013,
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+
band in the space charge region facilitates electron transfer
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from the CB of ZnO to the CB of TiO
lifetime of the photogenerated pairs. The electrons accumu-
lated on the CB of TiO and ZnO can be transferred to O
adsorbed on the surface to form oxygen radicals such as cO
and cOH. Moreover, the photoinduction holes on the VB can
2
, which increases the
2
2
ꢁ
2
induce the surface bound H O to produce the hydroxyl radical 11 Y. M. Liang, N. Guo, L. L. Li, R. Q. Li, G. J. Ji and S. C. Gan,
2
(cOH), which is a strong oxidant that can effectively degrade
Appl. Surf. Sci., 2015, 332, 32.
organic chemicals. The role of the AgNps and ZnO in the ATZ 12 J. Li, Z. Liu and Z. Zhu, RSC Adv., 2014, 4, 51302.
heterostructure was to increase the electron transfer rate and 13 L. Tan, M. Kumar, W. An and H. Gao, ACS Appl. Mater.
suppress charge recombination by transferring the photo-
Interfaces, 2010, 2, 498.
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28
2
33
being signicantly enhanced.
16 M. Wang, J. Ioccozia, L. Sun, C. Lin and Z. Lin, Energy
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1
7 J. Goldberger, D. Sirbuly, M. Law and P. Yang, J. Phys. Chem.
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Conclusions
In summary, hollow TiO
tained using electrospinning combined with an impregnation
calcination method. Using a photoreduction method, AgNps 19 H. Yang, G. Liu, S. Qiao, C. Sun, Y. Jin, S. Smith and G. Lu, J.
were successfully decorated on the surface of the nanotubes.
Am. Chem. Soc., 2009, 131, 4078.
2
and TiO
2
/ZnO nanotubes were ob- 18 L. Xu, D. Gao, J. Song, L. Shen, W. Chen, Y. Chen and
S. Zhang, New J. Chem., 2015, 39, 5553.
The synthesis mechanism of the hollow TiO nanotubes and the 20 S. Das and W. Daud, RSC Adv., 2014, 4, 20856.
2
ATZ hybrids, which enhanced their photocatalytic activity, was 21 W. Ong, L. Tan, S. Chai, S. Yong and A. Mohamed, Nanoscale,
also proposed. Moreover, the hollow ATZ tubes were synthe-
sized using a simple and environmentally friendly method and 22 P. Kar, S. Sardar, S. Ghosh, M. Parida, B. Liu, O. Mohammed,
may have good potential applications in solving environmental P. Lemmens and S. Pal, J. Mater. Chem. C, 2015, 3, 8200.
problems and provide a simple and efficient method for 23 A. Kar and A. Patra, J. Mater. Chem. C, 2014, 2, 6706.
2014, 6, 1946.
manufacturing hollow nanotubes.
24 Y. Liu, J. Goebl and Y. Yin, Chem. Soc. Rev., 2013, 42, 2610.
2
2
2
2
5 M. Zhang, C. Shao, J. Mu, Z. Zhang, Z. Guo, P. Zhang and
Y. Liu, CrystEngComm, 2012, 14, 605.
Conflicts of interest
6 L. Zhang, D. Jing, X. She, H. Liu, D. Yang, Y. Lu, J. Li,
Z. Zheng and L. Guo, J. Mater. Chem. A, 2014, 2, 2071.
7 J. Tian, Y. Leng, Z. Zhao, Y. Xia, Y. Sang, P. Hao, J. Zhan,
M. Li and H. Liu, Nano Energy, 2015, 11, 419.
8 S. Zhao, Z. Cheng, L. Kang, Y. Zhang and X. Zhao, Mater.
Lett., 2016, 182, 305.
There are no conicts to declare.
Acknowledgements
This study was nancially supported by the Jilin province
science
and
technology
development
projects 29 F. Lu, W. Cai and Y. Zhang, Adv. Funct. Mater., 2008, 18, 1047.
30 S. Malato, P. Fern ´a ndez-Ib ´a n˜ ez, M. Maldonado, J. Blanco
and W. Gernjak, Catal. Today, 2009, 147, 1.
(20150411005XH).
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1 S. Kumar and K. Rao, Appl. Surf. Sci., 2015, 355, 939.
2 G. Col ´o n, M. Hidalgo, J. Navio, E. Meli ´a n, O. D ´ı az and
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