Electrospun hollow ZnO/NiO heterostructures with enhanced photocatalytic activity

Article abstract of DOI:10.1039/c5ra08903d

ZnO/NiO hollow nanofibers with high photocatalytic activity are successfully fabricated by impregnating electrospun polyethersulfone (PES) nanofiber webs in nickel acetate and zinc acetate solutions and subsequent thermal treatment. From scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images, it can be observed that the morphology of the ZnO/NiO products is a hollow structure successively. According to statistics, the ZnO/NiO hollow nanofibers show diameters of approximate 414 nm with inner diameters of about 261 nm. Fourier transform infrared radiation (FTIR) and X-ray diffraction (XRD) measurements demonstrate that the product is composed of ZnO/NiO hollow nanofibers with hexangular structure ZnO and cubic structure NiO. The formation mechanism is studied in detail by TGA and DSC. The photocatalytic activity of the hollow ZnO/NiO heterojunction nanofibers for the degradation of methyl orange (MO) is much higher than that of pure ZnO and NiO nanofibers, which may be ascribed to the unique hollow structure and the highly efficient separation of photogenerated electron-hole pairs. Furthermore, the photocatalytic mechanism of the hollow ZnO/NiO nanofibers is expounded.

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ZnO/NiO hollow nanofibers with excellent photocatalytic activity were fabricated via the electrospinning and
using PES as template. The formation and the photocatalytic mechanisms of ZnO/NiO hollow nanofibers
were studied.
2
54x190mm (96 x 96 DPI)

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ARTICLE
Electrospun Hollow ZnO/NiO Heterostructure with
Enhanced Photocatalytic Activity
Cite this: DOI: 10.1039/x0xx00000x
a
a
a
a
a
a
Jianing Li
,
aF, ei Zhao, Li Zhang, Mingyue Zhang, Haifeng Jiang, Shu Li, and
Junfeng Li, *
Received 00th January 2012,
Accepted 00th January 2012
ZnO/NiO hollow nanofibers with high photocatalytic activity are successfully fabricated by
impregnating electrospun Polyethersulfone (PES) nanofibers webs in nickel acetate and zinc
acetate solution and subsequent thermal treatment. From scanning electron microscope (SEM)
and transmission electron microscopy (TEM) images, it can be observed that the morphology
of ZnO/NiO products is hollow structure successively. According to statistics, the ZnO/NiO
hollow nanofibers show diameters of approximate 414 nm with the inner diameters of about
DOI: 10.1039/x0xx00000x
www.rsc.org/
2
61nm. Fourier transform–Infrared radiation (FI–IR) and X–ray diffraction (XRD)
measurements demonstrate that the product is highly ZnO/NiO hollow nanofibers with
hexangular structure ZnO and cubic structure NiO. The formation mechanism is studied in
detail by TGA and DSC. The photocatalytic activity of the hollow ZnO/NiO heterojunction
nanofibers for the degradation of methyl orange (MO) is much higher than that of pure ZnO
and NiO nanofibers, which may be ascribed to the unique hollow structure and the high
efficient separation of photogenerated electron–hole pairs. Furthermore, the photocatalytic
mechanism of the hollow ZnO/NiO nanofibers is expounded.
2
1
22
(
PVP) , polyaniline (PANI) are commonly used as polymer
Introduction
templates to fabricate the composite nanofibers by electrospining
method. Only the fibrous structure of composite can be obtained by
these precursors. Notably, polyethersulfone (PES) can be considered
as a new template for preparing inorganic composite nanofibers with
special hollow structure since it has a good performance with
Many kinds of semiconductor metal oxide–based nanocomposities,
such as TiO2–SnO2 , ZnO–SnO , ZnO–TiO , and so forth, have
attracted much attention of scientific researchers from all over the
world and are being widely utilized in the field of optoelectronics,
gas sensor and various photocatalysis due to their unique optical,
1
2
3
2
2
23
thermal stability, hydrolysis resistance and chemical resistance .
4
ꢀ6
electronic, and photoactive properties
. The fabrication of
Unfortunately, there are rarely reports about using PES as template
to prepare the composite materials, so it is worth researching further.
Besides, among some composite materials, ZnO/NiO nanomaterials
with special structure have attracted the most attention for
application in photocatalysts, considering their low cost. Some
reports point out that the ZnO/NiO composite materials have been
applied to degrade nonbiodegradable dyes through photocatalytical
composite nanostructures based on the combination of
semiconducting oxides offers a pivotal path to combine the different
physical and chemical properties of individual components into one
7
system . Zinc oxide (ZnO), as an n–type semiconductor (Eg = 3.2
eV), has high photosensitivity, high catalytic activity, suitable
bandgap, and low cost . In the past several years, the composite
semiconductors, which were formed by ZnO and other metal oxides
8
,9
8
routes . The superior functional performance of the ZnO/NiO
1
0ꢀ12
such as TiO , CuO, Cr O and so on, had been reported
. Among
2
2
3
semiconducting oxides is mainly attributed to the build–up of an
inner electric field at the P/N junction interfere
recombination of photogenerated electron–hole pairs can be
effectively suppressed, thereby improving the efficiency of net
charge transfer in the reaction process by fabricating composite
these composite materials, the ZnO/NiO composite materials have
been intensively investigated that are applied as photocatalysts, gas
sensors and electrical properties
7,24
. The
1
3,14
.
In recent years, many efforts have been made to
the fabrication of various semiconductors nanofibers, which include
25
nanostructures . However, in some case, ZnO/NiO composite
materials with special structures, especially for hollow structure,
may possess the excellent degradation efficiency of organic
pollutants in photocatalytic activity, which impel us to further
penetrate into its application.
1
5
the chemical oxidative polymerization method , in–situ deposition
16
17
18
polymerization , the template method , hydrothermal synthesis ,
1
9
metal organic chemical vapor deposition , and so on. To the best of
our knowledge, electrospinning technique, a versatile and mature
method, has been exploited to manufacture nanofibers with
controllable diameters, compositions and porosities. In terms of the
fabrication for nanofibers, this method possesses the advantage of
efficient composition control. In addition, according to recent
In this work, the hollow ZnO/NiO nanofibers which were
expected to have application in photocatalytic activity were
prepared by impregnating electrospun Polyethersulfone (PES)
nanofibers webs in nickel acetate and zinc acetate solution and
subsequent calcination. The structure and physicochemical
2
0
literatures, poly(vinyl alcohol) (PVA) , polyvinylpyrrolidone
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property of the calcined samples were elucidated by TGA, conducted from room temperature to 900 °C at a heating rate of
DSC, SEM, TEM, FT–IR, XRD, EDX and DRS, respectively. 10 °C/min under a flowing air atmosphere. The surface
In addition, the formation mechanism of the nanofibers morphology and dimension of the nanofibers were observed
structures was investigated. Whereafter, the photocatalytic under scanning electron microscope (SEM) (SHIMDZU SSX–
activities of the as–prepared samples (hollow ZnO/NiO 550, Japan) equipped with energy dispersive X–ray analysis
composite nanofibers; pure ZnO nanofibers and pure NiO (EDX) and transmission electron microscopy (TEM) (JEMꢀ
nanofibers) were carried out in detail.
2100F, Japan). Before testing, the nanofibers were sputter
coated with gold using ETD–2000 auto sputter coater
(
Elaborate Technology Development Co., Ltd., China) with a
Experimental Section
current of 4 mA for 2 min. Ground on the SEM images, the
diameter of nanofibers was measured using image visualization
software Image J. The crystalline structures of the calcined
nanofibers were identified by the X–ray diffraction (XRD).
XRD analysis was obtained using a Siemens D5005 XRD
diffractometer in 2θ region of 20–80 ° with Cu Kα radiation.
Fourier transform–Infrared radiation (FT–IR) spectrometer
Chemicals and Materials
Zinc acetate A.R. [Zn(CH COO) ꢁ 2H O] was purchased from
3
2
2
Beijing Chemical Works Fine Chemicals Reagent Company,
Beijing, China. Nickel (II) acetate tetrahydrate [Ni(CH COO)2
·
3
4
H O] was provided by Sinopharm Chemical Regent Co., Ltd.,
2
(
SHIMDZU, 1.50SU1, Japan) was used to identify the vibration
China. Polyethersulfone (PES) was supplied by Changchun Jida
plastic engineering research Co., Ltd., China. N,N–
dimethylformamide (DMF, Xilong Chemical Co., Ltd., China) and
purified water (Wahaha Group) were used as solvent directly. All of
these chemical reagents were analytical grade, without further
purification.
in functional groups presented in the samples. The spectra were
obtained with 20 scans per sample ranging from 4000 to 400
cm . UV–vis diffuse reflectance spectra (DRS) of the
nanofibers were measured using a diffuse reflectance accessory
of UV–vis spectrophotometer (DRS) (SHIMDZU UVꢀ3600,
Japan).
ꢀ1
Photocatalytic Activity Measurement
Preparation of the n-Type ZnO/p-Type NiO
Nanofibers
The photocatalytic activities of the as–prepared samples
(
hollow ZnO/NiO composite nanofibers, pure ZnO nanofibers
Based on a typical procedure, the pale yellow electrospinning and pure NiO nanofibers) were evaluated by the photocatalytic
solution was prepared by dissolving PES in N,N–
dimethylformamide (DMF) at a concentration of 34 wt. %. After
degradation of a model pollutant, methyl orange (MO), under
ultraviolet light irradiation. The reaction suspensions were
vigorous stirring at 80 °C for 24 hours, the precursor solution for prepared by adding the nanofibers photocatalyst (20 mg) into
electrospinning could be obtained. The polymer solution was charily MO aqueous solution (40 ml, 10 mg/L). The suspension was
sucked into the syringe with an internal diameter of 0.69 mm which
was connected to a high–voltage power supply (Beijing Yongkang
stirred in the dark for 1 h to obtain a fine dispersion and ensure
the absorption–desorption equilibrium between the organic
Industry Technology Development Co., Ltd., China). A piece of flat molecules and the catalyst surfaces. Then the solution was
aluminum foil as the collector was placed at 26 cm away from the tip exposed to an ultraviolet (UV) – radiation lamp (UV–8, 12 W,
of the spinning nozzle. The applied high voltage was held at 13 KV
and the feeding rate of the solution was controlled at 0.04 mm/min
3
65 nm) irradiation whilst stirring at room temperature. At
given time intervals, a few milliliters of solution were drawn
by means of a single syringe pump. Finally, a dense web of from the mixture and centrifuged to separate the nanofibers.
electrospun composite nanofibers was distributed uniformly over the Afterwards, the solution was loaded into
a
UV–vis
collector. All experiments were carried out at ambient temperature.
The fibers were subsequently exposed to the air overnight to remove
the residual organic solvent.
spectrophotometer (SHIMDZU UV–3600, Japan). In order to
demonstrate the stability and usage of the catalysts, we recycled
the used ZnO/NiO hollow nanofibers. The relative
concentration of MO was monitored by comparing the
characteristic absorption intensity at 464 nm with that of the
3
g nickel acetate [Ni(CH COO) · 4H O] and 3 g zinc acetate
3 2 2
2
2
[
Zn(CH COO) · 2H O] were mixed with 100 ml purified water (add
3 2 2
original MO solution .
a little acetic acid) in a beaker under vigorous stirring for 2 h.
Subsequently, the prepared PES nanofibers were immersed into the
above solution for 2 days. After drying, the samples were calcinated
Results and Discussion
at 800 °C in air with a heating rate of 5 °C /min for 3 h, and the n– TGA curves of pure PES nanofibers, pure Zn(CH COO) ꢁ 2H O
3
2
2
type ZnO/p–type NiO nanofibers were successfully prepared. powders, pure Ni(CH COO)2
ꢁ
4H O powders and PES/
3
2
Moreover, to investigate the photocatalytic activity of the n–type Zn(CH COO) ꢁ 2H O/Ni(CH COO) ꢁ 4H O composite nanofibers
3
2
2
3
2
2
ZnO/p–type NiO nanofibers in detail, pure ZnO and pure NiO were shown in Figure 1 A. Moreover, in order to see the weight loss
nanofibers that can be used for comparison were prepared through range obviously, the DTG curves were displayed in Figure 1 B and
26
the same experimental conditions.
the weight loss rate was calculated as below :
dW
dt
1  dW 
t
Characterization
= −
(1)


W
dt


0
Thermal gravimetric analysis (TGA) was performed on a TG–
DTG instrument (Beijing Hengjiu Instrument Ltd., Beijing,
China) to determine the temperature of possible decomposition
As observed in Figure 1 A (a) and Figure 1 B (a), the thermal
decomposition process of pure PES nanofibers began from 480 °C. It
and crystallization of the nanofibers. Measurements were was shown that the Polyethersulfone was a good thermal stability
2
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polymer. The major weight loss of pure PES nanofibers occurred at nanofibers. (B) DTG curves of (a) pure PES nanofibers; (b)
80 °C and 660 °C. This was consistent with nearly 100 % mass loss PES/Zn(CH COO) ꢁ 2H O/Ni(CH COO) ꢁ 4H O composite
5
3
2
2
3
2
2
in the range of 480–850 °C, which was corresponded to the nanofibers. (C) DSC curves of PES/Zn(CH COO)
ꢁ
3
2
decomposition of main chain of the PES. After 850 °C, no more 2H O/Ni(CH COO) ꢁ 4H O composite nanofibers.
2
3
2
2
weight loss can be seen, indicating that the PES was decomposed
completely. The TGA curve of pure Zn(CH COO) · 2H O powders
Exemplary scanning electron microscope (SEM)
3
2
2
(
Figure 1 A (b)) presented almost 85 % total weight loss from 200 to micrographs of PES nanofibers and PES/Zn(CH COO)
ꢁ
3
2
3
50 °C, which could be ascribed to the decomposition of the 2H O/Ni(CH COO)
ꢁ 4H O composite nanofibers were
2
3
2
2
2
7
CH COO group of zinc acetate . Additionally, the TGA curve of depicted in Figure 2 (a) and (b). As shown in Figure 2 (a), the
3
pure Ni(CH COO) · 4H O powders (Figure 1 A (c)) seemed to electrospun PES nanofibers formed network structure and
3
2
2
exhibit two steps of decomposition with a total weight loss of 70 %. appeared to be relatively smooth and homogeneous surface due
The first step appeared from 70 to 130 °C was attributed to part to the polymeric property. According to Gaussian fitting and
decomposition of Ni(CH COO) ꢁ 4H O and liberation of the crystal statistics, the mean diameter of randomly oriented nanofibers
3
2
2
water. The second step, which occurred in the range of 300–370 °C, was 387 nm. After immersing in a mixture of zinc acetate and
can be ascribed to the complete decomposition of Ni(CH COO) . As nickel acetate solution, it could be seen that the outer layer of
3
2
can be seen from Figure 1 A (d) and Figure 1 B (b), the PES nanofibers was evently filled with zinc acetate and nickel
PES/Zn(CH COO) 2H O/Ni(CH COO) 4H O composite acetate so that the surface of the nanofibers became rough
ꢁ
ꢁ
3
2
2
3
2
2
nanofibers had four–step thermogram. Firstly, 15 % weight loss had [Figure 2 (b)]. After statistics, the mean diameter of the
occurred below the 120 °C; Secondly, there was a 36 % weight loss nanofibers was 556 nm which was slightly increased. This
between 120 and 370 °C; Also, from 370 to 640 °C, there was a 20 might be ascribed to the expansion of PES nanofibers and the
%
weight loss; and finally, 5 % weight loss existed in the adsorption of inorganic salts around the nanofibers during the
temperature range of 740–800 °C. However, above 800 °C, there process of immersion. The SEM images of the sintered
was no evident weight loss. Referring to the analyses above, the first products were shown in Figure 2 (c)–(e). Surprisingly, the
step was mainly ascribed to part decomposition of Ni(CH COO) · ZnO/NiO heterojunction nanofibers exhibited
a
hollow
3
2
4
H O and liberation of the crystal water, which showed dramatic microstructure. From the perspective of the surface vertical
2
weight loss approximate at 90 °C. The second step was attributed to view [Figure 2 (c)], it could be seen that the surface of these
the decomposition of zinc acetate and nickel acetate. The third step randomly oriented nanofibers was rough after calcination at 800
was resulted from the complete decomposition of acetate and °C, and the mean diameter of the hollow ZnO/NiO
degradation of the main chain of PES. The fourth step was related to heterojunction nanofibers was approximately 414 nm. In Figure
the entirely removal of the organic constituents in the nanofibers. It 2 (d), the ruptured sections clearly showed the hollow structure
has been reported that the existence of inorganic salt can be of ZnO/NiO heterojunction nanofibers. In addition, Figure 2 (e)
conducive to the decomposition of the polymer. When the showed the typical SEM images of the above ZnO/NiO hollow
temperature reached about 800 °C, the TGA curve was gentle, nanofibers taken at high magnification. A large quantity of
manifesting that the composite nanofibers had transformed into pure hollow nanofibers with nearly uniform diameter could be
inorganic oxide entirely. Considering the complete decomposition of clearly seen in Figure 2 (e). According to the statistics, the
the PES nanofibers at high temperature, the increase of the residual inner diameter of the hollow nanofibers was approximate 261
mass amount after the TGA would correspond to the addition of the nm. Since this special hollow structure was formed, the contact
inorganic salt. This confirmed the presence of inorganic salt on the area of prepared ZnO/NiO heterojunction nanofibers was
PES nanofibers.
increased, which could improve the application of
photocatalytic activity. In order to further study the
DSC measurement was implemented in a range of room microstructure of the ZnO/NiO hollow nanofibers, transmission
temperature to 900 °C in order to prove the existence of various electron microscopy (TEM) observation was carried out. As
substances and evaluate their thermal behavior. As observed in shown in Figure 2 (f), the ZnO/NiO nanofibers possessed a
the DSC curve (Figure 1 C), the endothermic peak appeared at hollow structure. It can be seen clearly that the hollow
1
00 °C, which could be attributed to the loss of moisture and nanofibers were composed of nanoparticles and the rough
the melt of Ni(CH COO) . The sharp exothermic peak at 350 surface linked with ZnO and NiO nanoparticles. The energy–
3
2
°
C corresponded to the decomposition of acetate. Meanwhile, dispersive X–ray (EDX) spectra [Figure 2 (g)] confirmed that
the broad exothermic peaks at about 380 °C, 540 °C and 595 °C the heterojunction nanofibers were composed of Zn, Ni, and O,
in the DSC curve were thought to the continued decomposition which was consistent with the XRD results. In addition, the
of acetate and degradation of main chain from PES. All the weight percentage and the atomic percentage of Zn, Ni, O
above DSC analyses could be confirmed by an obvious weight elements in this structure were calculated, respectively.
loss in the TGA curve at the corresponding temperature range
from 25 °C to 900 °C.
The formation mechanism of the hollow ZnO/NiO
heterojunction nanofibers can be explained as follow. The
prepared PES nanofibers via electrospinning were impregnated
into the Zn(CH COO) · 2H O and Ni(CH COO) · 4H O
3
2
2
3
2
2
2
+
mixed solution. Due to soaking for a long time, the Zn and
2+
Ni were evenly adsorbed on the surface of PES nanofibers. In
the subsequent heating treatment, the inorganic salts were
decomposed first compared to the PES nanofibers from the
2+
2+
TGA curves (Figure 1 (A)). At the same time, the Zn and Ni
Fig 1. (A) TGA curves of thermal decomposition of (a) pure
PES nanofibers; (b) pure Zn(CH COO) ꢁ 2H O powders; (c)
were slowly oxidized into the ZnO and NiO. With the extension
of heating time and the increase of heating temperature, the
PES nanofibers were gradually degraded. Ultimately, the inner
3
2
2
pure
Ni(CH COO)2
ꢁ
4H O
powders
and
(d)
3
2
PES/Zn(CH COO) ꢁ 2H O/Ni(CH COO) ꢁ 4H O composite
3
2
2
3
2
2
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layer organic constituents were entirely removed and the metal
oxides were connected together and formed of the hollow
ZnO/NiO heterojunction nanofibers. The schematic of possible
formation process was shown in Figure 3.
Fig 3. Formation mechanism of hollow ZnO/NiO composite
nanofibers.
The X–ray diffraction (XRD) patterns of the as–obtained
nanofibers were shown in Figure 4. It could be seen from
Figure 4 (a) that all the diffraction peaks could be indexed from
the (111), (200), (220), (311), (222) planes of the cubic
structure for NiO (JCPDF–73–1523). The Figure 4 (b) with
typical characteristic peaks such as (100), (002), (101), (102),
(
110), (103), (112), (201), (202) corresponded to hexagonal
wurtzite structure of ZnO (JCPDF–75–0576). As shown in
Figure 4 (c), there are 14 main diffraction peaks corresponding
to (100), (002), (101), (111), (200), (102), (110), (220), (103),
(
112), (201), (311), (202), (222) planes at 31.898 °, 34.578 °,
3
6
6.393 °, 37.169 °, 43.168 °, 47.679 °, 56.731 °, 62.609 °,
2.988 °, 68.072 °, 69.208 °, 75.196 °, 77.082 ° and 79.158 °,
respectively. Therefore, the hollow ZnO/NiO heterojunction
nanofibers had two crystalline phases of both cubic structure
NiO and hexagonal structure ZnO simultaneously, no
characteristic peaks for impurity were observed. Notably, the
sharp and small values of the full widths at half maxima
(
FWHM) of the diffraction peaks revealed that the hollow
2
8
ZnO/NiO composite nanofibers were of high crystallinity .
The results of XRD patterns further confirmed that the
calcination temperature of 800 °C was sufficient to remove PES
completely and formed the two oxides.
Fig 2. SEM micrographs of (a) pure PES nanofibers; (b) PES
nanofibers immersed in a mixture of Zn(CH COO) ꢁ 2H O
3
2
2
and Ni(CH COO) ꢁ 4H O solutions for 2 days; ZnO/NiO
3
2
2
hollow nanofibers from different positions (c) vertical view of
the surface; (d) cross section of the surface; (e) cross section of
the partial surface at high magnification; and their
corresponding diameter distribution; (f) TEM image of the
ZnO/NiO hollow nanofibers; (g) EDX spectrum of the Fig 4. XRD patterns of (a) pure NiO nanofibers; (b) pure ZnO
ZnO/NiO hollow nanofibers;
nanofibers; (c) ZnO/NiO hollow nanofibers.
FTIR spectrum was also performed to identify the
structure of the obtained nanofibers, which was shown in
Figure 5. As expected, the prepared pure PES nanofibers
[
5
Figure 5 (a)] showed typical aromatic bands at 1580, 876, 716,
60 cm from the benzene ring with one or more substitutes.
ꢀ
1
4
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Furthermore, the absorption peaks located at about 1485, 1245,
ꢀ
1
1
150 cm were corresponding to C–C stretching vibration,
aromatic ether band and aromatic sulfone band,
2
9,30
respectively
. Figure 5 (b) depicted the spectrum of the
PES/Zn(CH COO)2 ꢁ 2H O/Ni(CH COO) ꢁ 4H O blend
3
2
3
2
2
nanofibers and it was the same as Figure 5 (a). Notably, the
ꢀ
1
broad band at about 3400 cm was assigned to the O–H
Fig 6. (A) UV–vis diffuse reflectance (DR) spectra of (a) NiO
asymmetrical stretching vibration, which came from H O in
2
nanofibers; (b) ZnO nanofibers; (c) hollow ZnO/NiO
Ni(CH COO) ꢁ 4H O and Zn(CH COO) ꢁ 2H O. When the
3
2
2
3
2
2
2
nanofibers; (B, C) the plots of the (αh
v
) vs photon energy (h
v)
nanofibers were annealed at 800 °C [Figure 5 (c)], the
representative absorption peaks, which belonged to the organic
groups of the polymer, disappeared and a new intense
for ZnO and NiO nanofibers, respectively.
ꢀ
1
To identify the photocatalytic performance of the hollow
ZnO/NiO heterojunction nanofibers for the degradation of organic
pollutions, we carried out experiments of the photocatalytic
degradation of methyl orange (MO) as a text reaction. The decrease
of the absorption band intensities of the dyes indicated that MO had
been degraded by UV light irradiation, catalyzed by ZnO/NiO
nanocomposites. The time–dependent UV–vis spectra of MO during
the irradiation were illustrated in Figure 7 (A). One can see that the
characteristic peak of MO at 464 nm that was associated with the azo
band (–N = N–) gradually disappeard after 150 min under UV light
broadband at about 430 cm came out. This band assigned to
the Zn–O vibration of hexagonal ZnO and Ni–O vibration of
3
1,32
cubic NiO appeared but overlapped
, indicating that the
nanofibers obtained at this high temperature were pure
inorganic species of ZnO/NiO. The results accorded well with
XRD and EDX analysis.
8
irradiation in the presence of ZnO/NiO hollow nanofibers .
Furthermore, in the comparative experiments, the pure ZnO
nanofibers and pure NiO nanofibers were used as a photocatalytic
reference to investigate the photocatalytic activity of the hollow
ZnO/NiO nanofibers. The degradation profiles of the same amount
of different catalysts were shown in Figure 7 (B). The degradation
efficiencies of MO after 150 min irradiation were about 40 %, 76.5
%, 91.89 % for pure NiO, pure ZnO and the ZnO/NiO hollow
nanofibers, respectively. An obvious degradation of MO was
observed under UV light in the presence of the ZnO/NiO hollow
nanofibers. Notably, the ZnO/NiO hollow nanofibers exhibited the
highest photocatalytic activity as compared with both pure ZnO and
pure NiO nanofibers, which might be attributed to the ZnO/NiO
nanofibers unique hollow structure. The result might be favorable to
the reaction with MO, leading to an excellent photocatalytic
property. For a better comparison of the photocatalytic efficiency of
the above electrospun nanofibers, the kinetics analysis of
degradation of MO under UV light irradiation was also investigated.
The kinetic linear simulation curves of the photocatalytic
degradation of MO over the above nanofibers photocatalysts showed
Fig 5. FTIR spectra of (a) pure PES nanofibers; (b)
PES/Zn(CH COO) ꢁ 2H O/Ni(CH COO) ꢁ 4H O composite
nanofibers (c) ZnO/NiO hollow nanofibers.
3
2
2
3
2
2
Figure 6 A showed the UV–vis diffuse reflectance (DR)
spectroscopy of the pure NiO, pure ZnO and hollow ZnO/NiO
nanofibers. In Figure 6 A [(a) and (c)], the strong absorption
peaks of the as–electrospun NiO and ZnO nanofibers appeared
that the photocatalytic degradation process followed the first–order
34,35
kinetics
. The explanation was described below:
2
8,33
ln(C0 ) = kt
at 315 and 365 nm, respectively
. Furthermore, the weak
(2)
absorption peak with a maximum at 720 nm was observed in
NiO nanofibers, which could be considered as intra–3d
Ct
2
+
28
transitions of Ni in the cubic crystal field of NiO . As
observed in Figure 6 A (b), all of the absorption bands existed
in the UV–vis DR spectroscopy of the hollow ZnO/NiO
nanofibers, which were ascribed to the characteristic absorption
of ZnO and NiO. It was confirmed that the hollow ZnO/NiO
nanofibers were composite materials that were consisted of
ZnO and NiO. The band–gap values of ZnO and NiO were
−1
where k (min ) is the degradation rate constant, C is the
0
concentration of MO at adsorption equilibrium and Ct is the
concentration of MO at a time different intervals. To calculate
reaction rate constant for the photodegradation of MO with different
catalysts, the linear relationship between ln(C /C ) and the reaction
0
t
time was summarized in Figure 7 (C). It was indicated that the order
of rate constant was ZnO/NiO hollow nanofibers > pure ZnO
nanofibers > pure NiO nanofibers, which was consistent with the
activity studies above. To prove the possibility of recyclability of the
ZnO/NiO nanofibers, cycle experiments were performed. As can be
seen from Figure 7 (D), the photocatalytic activity of the ZnO/NiO
nanofibers for the degradation of MO still kept above 80 percentage
of the high level even recycle five times. With the increase number
of repeated use, a small amount of the loss in process of recycle was
the main reason for the slight lower photocatalytic efficiency than
calculated based on the well–established equation α=A(hvꢀ
1/2
Eg) /hv, where α, v, Eg, and A were the absorption coefficient,
8
light frequency, band gap energy, and a constant, respectively .
As observed in Figure 6 B and C, the band gap of ZnO was
evaluated to be 3.15 eV, while the band gap of NiO was found
to be about 3.32 eV.
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1ꢀ3 | 5

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RSC Advances
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Journal Name
DOI: 10.1039/C5RA08903D
before. Therefore, the recyclability of the hollow ZnO/NiO
heterojunction nanofibers was possible.
The high photocatalytic activity of the hollow ZnO/NiO
heterojunction nanofibers for MO decolorization was primarily due
to electron hole pair recombination inhibition by charge transfer
processes. Figure 8 showed the proposed energy band structure
diagram of the hollow ZnO/NiO heterojunction photocatalysts.
When the ZnO/NiO heterojunction was formed, the electron transfer
occurred from ZnO to NiO while the holes transfer occurred from
NiO to ZnO until the system obtained equalization. When the
ZnO/NiO photocatalysts were irradiated by UV light with photon
energy higher than or equal to the band gaps of ZnO and NiO, the
ꢀ
electrons (e ) in the valence band (VB) could be excited to the
conduction band (CB) with simultaneous generation of the same
+
amount of holes (h ) in the VB (Eq.(3)). On account of the ZnO was
lower than that of NiO, upon light–activation the electron transfer
3
6
from the CB of NiO to that of ZnO . The semiconductor NiO can
act as a sink for photogenerated holes, making charge separation
8
more efficient and thus suppressing recombination processes .
Conversely, the photogenerated holes transfer could take place from
the VB of ZnO to the VB of the NiO, suggesting that the
photogenerated electrons and holes were efficiently separated
Figure 7. (A) Adsorption spectra of MO solutions in the presence of
ZnO/NiO hollow nanofibers under UV light at different periods of
time; (B) Photocatalytic degradation of MO concentration changes
as a function of irradiation times after added different photocatalysis;
(C) Kinetic linear simulation curves of MO photocatalytic
degradation with different electrospun nanofibers: (a) NiO
nanofibers; (b) ZnO nanofibers; (c) ZnO/NiO hollow nanofibers;
(Eq.(4)). The photogenerated electrons and holes in the ZnO/NiO
photocatalysts could inject into a reaction medium and participate in
7
chemical reactions .
ꢀ
+
ꢀ
+
UV
ZnO/NiO + hv
ZnO (e , h ) + NiO (e , h )
(3)
(4)
(D) The diagram of degradation ratio of MO versus recycle number
with the ZnO/NiO hollow nanofibers.
ꢀ
+
ꢀ
+
+
ꢀ
ZnO (e , h ) + NiO (e , h )
NiO (h ) + ZnO (e )
-
.
-
e + O
O2
(5)
2
.
-
.
-
O2 + H O
HO + OH
(6)
(7)
(8)
(9)
2
2
.
.
HO2 + H O
H O + OH
2
2
2
-
.
-
H O + e
OH + OH
2
2
.
-
.
-
H O + O
OH + OH + O
2
2
2
2
Figure 8. Schematic representation of the proposed energy band
(10) structure of the hollow ZnO/NiO heterojunction photocatalysts.
.
OH + MO
intermediates
CO + H O
2
2
Under the UV light irradiation, the photogenerated electrons Conclusions
moved to the ZnO side, meanwhile, the photogenerated holes moved
In this work, by using the electrospinning technology and
to the NiO side. The electronic acceptors like oxygen could easily
ꢀ
physical adsorption, the ZnO/NiO heterojunction nanofibers
with hollow structure were successfully fabricated. Initially, the
electrospun PES nanofibers had smooth surfaces with diameter
.- of 387 nm. Then the PES nanofibers webs were impregnated in
a mixture of zinc acetate and nickel acetate solutions and
subsequently calcinated in air for 3 h, which induced the
formation of ZnO/NiO hollow nanofibers. After statistics, the
ZnO/NiO hollow nanofibers showed diameters of approximate
trap the photogenerated electron (e ) to produce a superoxide anion
.
-
.-
radical (O ). The formed O were reacted with absorbed H O to
2
2
2
.
produce H O , which would provide hydroxyl radical (OH ) by
2
2
ꢀ
acting as a direct electron acceptor by reaction with e and O
Eqs.(5)–(9)). The hydroxyl radical (OH ) was an extremely strong
oxidant for decomposing the organic dyes (Eq.(10))
2
.
(
3
3,37,38
.
4
14 nm with the inner diameters of about 261 nm. The calcined
ZnO/NiO heterojunction nanofibers were characterized by
TGA, DSC, SEM and TEM techniques. Therefore, the
formation mechanism of the nanofibers was confirmed.
Moreover, the photocatalytic performance test indicated that the
6
| J. Name., 2012, 00, 1ꢀ3
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Journal Name
ARTICLE
DOI: 10.1039/C5RA08903D
ZnO/NiO hollow nanofibers exhibited higher photocatalytic 25 Hameed, A.; Montini, T.; Gombac, V.; Fornasiero, P., Photochemical
&
Photobiological Sciences. 2009, 8, 677.
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degradation of MO dye under UV light irradiation, which might
be attributed to the highly efficient separation of
photogenerated electron–hole pairs. Therefore, we believe that
the ZnO/NiO hollow nanofibers with high photocatalytic
activity can be considered as a promising photocatalyst for dyes
2
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To whom correspondence should be addressed.
Eꢀmail: jfli@jlu.edu.cn (Junfeng Li)
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