Facile green fabrication of nanostructural Ni-doped ZnO hollow sphere as an advanced photocatalytic material for dye degradation

Article abstract of DOI:10.1016/j.molliq.2017.07.050

Through a green protocol, hollow forms of x-Ni-ZnO (x?=?0, 2, 3, 5, and 10?wt% Ni) were prepared in ultrasonic bath at a low temperature using template fructose. For characterizing the shape, structure, crystallite phase, purity, and chemical composition of the prepared hollow spheres, the powder X-ray diffraction (PXRD), Fourier transform infra-red spectroscopy (FT-IR), field-emission scanning electron microscopy (FE-SEM), energy dispersive analysis of X-ray (EDAX), High-resolution transmission electron microscopy (HRTEM), inductively coupled plasma (ICP), and ultraviolet–visible absorption (UV–vis) spectroscopy were employed. Besides, the photo-catalytic properties of these hollow spheres for degrading Congo Red (CR) as an Azo dye were taken into account. According to the results, Ni-ZnO hollow spheres outperform the pure ZnO hollow spheres in terms of the photocatalytic activities. Furthermore, in order to show the impact of various morphologies on the CR dye degradation, the Ni-ZnO nanostructures were produced with rod-like, flower-like, and wood-like morphologies. Our synthesized Ni-ZnO hollow spheres seem to be highly promising for future applications in different commercial fields and industries. Moreover, they are applicable for photodegradation of other organic pollutants.

Full text of DOI:10.1016/j.molliq.2017.07.050

Accepted Manuscript
Facile green fabrication of nanostructural Ni-doped ZnO hollow
sphere as an advanced photocatalytic material for dye degradation
Seyed Mohsen Mousavi, Ali Reza Mahjoub, Reza Abazari
PII:
S0167-7322(17)31525-8
doi: 10.1016/j.molliq.2017.07.050
MOLLIQ 7628
DOI:
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
8 April 2017
9 July 2017
14 July 2017
Please cite this article as: Seyed Mohsen Mousavi, Ali Reza Mahjoub, Reza Abazari ,
Facile green fabrication of nanostructural Ni-doped ZnO hollow sphere as an advanced
photocatalytic material for dye degradation, Journal of Molecular Liquids (2017), doi:
1
0.1016/j.molliq.2017.07.050
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ACCEPTED MANUSCRIPT
Facile green fabrication of nanostructural Ni-doped ZnO hollow sphere as an advanced
photocatalytic material for dye degradation
a,b
a,
a
Seyed Mohsen Mousavi , Ali Reza Mahjoub *, Reza Abazari
a
Department of Chemistry, Tarbiat Modares University, P.O. Box. 14155-4383 Tehran, Iran
b
Assistant Professor, Department of Chemistry, Farhangian University, Tehran, Iran
E-mail: mahjouba@modares.ac.ir; smm4566@gmail.com; reza.abazari@modares.ac.ir; Fax:
98 21 82883455; Tel.: +98 21 82883442.
+
1

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Abstract
Through a green protocol, hollow forms of x-Ni-ZnO (x = 0, 2, 3, 5, and 10 wt % Ni) were
prepared in ultrasonic bath at a low temperature using template fructose. For characterizing the
shape, structure, crystallite phase, purity, and chemical composition of the prepared hollow
spheres, the powder X-ray diffraction (PXRD), fourier transform infra-red spectroscopy (FT-IR),
field-emission scanning electron microscopy (FE-SEM), energy dispersive analysis of X–ray
(EDAX), High-resolution transmission electron microscopy (HRTEM), inductively coupled
plasma (ICP), and ultraviolet–visible absorption (UV–Vis) spectroscopy were employed. Besides,
the photo-catalytic properties of these hollow spheres for degrading Congo Red (CR) as an Azo
dye were taken into account. According to the results, Ni-ZnO hollow spheres outperform the
pure ZnO hollow spheres in terms of the photocatalytic activities. Furthermore, in order to show
the impact of various morphologies on the CR dye degradation, the Ni-ZnO nanostructures were
produced with rod-like, flower-like, and wood-like morphologies. Our synthesized Ni-ZnO
hollow spheres seem to be highly promising for future applications in different commercial fields
and industries. Moreover, they are applicable for photodegradation of other organic pollutants.
Keywords: X-Ni-ZnO, Hollow sphere, Chemical method, Photocatalytic activity; Waste water,
Azo dye.
2

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1
.
Introduction
Nowadays, environment is becoming more and more contaminated by the emission of different
dye pollutants from the industries. In spite of the fact that different chemical and biological
degradation methods have already been proposed to remove such dye pollutants from
wastewater, the results have shown to be largely unsatisfactory [1-3]. In recent years, application
of semiconductors (e.g., ZnO and TiO ) for the photocatalytic degradation of the dye pollutants
2
in water has received much attention since these photocatalysts are highly efficient, inexpensive,
ecofriendly, and nonpoisonous [4–7]. However, since ZnO and TiO show a low solar energy
2
conversion efficiency, have a wide band gap of 3 to 3.2 eV, and demonstrate a high charge
recombination rate of the photo-induced electron/hole pairs, their applications in the
semiconductor photocatalytic oxidation for removal of the environmental contaminants is
restricted [8-11]. Furthermore, TiO and ZnO react only towards the UV illumination which is
2
less than 4 percent of the solar spectrum, their photo-catalytic yields and practical uses are
limited [12]. Consequently, a substantial challenge is to develop innovative photocatalysts with
superior stability and efficient visible light absorption [13-16]. As recorded in the literature, ZnO
has a more appropriate photocatalytic efficiency than TiO , which can be explained by its quite
2
high absorption over a greater portion of the UV light spectrum. Besides, in comparison with
TiO , ZnO is intensely luminescent and serves as a highly active photocatalyst. The quick
2
recombination of the charge carriers is the key factor that affects the ZnO photocatalytic
activities [17-19].
Some previous studies have considered the ZnO coupling with other semiconductors. The
reasons for this are to improve the light adsorption and light utilization efficiency and to increase
the separation of the photogenerated electrons and holes [20, 21]. Therefore, various ions are
3

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able to be doped into ZnO so that the recombination of the photo-generated pairs of electron-hole
2
+
can be inhibited. From the literature, Ni ion is known to be a crucial dopant related to the
2
+
2+
magnetic materials [22, 23]. Moreover, Ni can be substituted for Zn in ZnO lattice due to the
2
+
2+
fact that Ni (0.69 A) has a valence which is identical to that of Zn and has a radius which is
2
+
close to that of the Zn (0.74 A). In their study, Zhu and his colleagues concluded that Ni-
doping is likely to improve the photocatalytic oxidation of aromatic alcohols in NH -MIL-
2
1
25(Ti) [24]. Sasikala et al. suggested a route for enhancing the photocatalytic efficiency of the
Ni doped CdS nanostructures [25].
As proposed in the literature, preparation of the metal oxide nanostructures with controllable size
and morphology is of great importance for their potential applications. This can also help to
control the chemical and physical features of these materials [26-28]. Some previous works have
already reported the Ni-ZnO hetero-structured nano-crystals (e.g., as particle and nanowire).
However, in recent years, much weight has been given by different researchers to the preparation
of the nanostructural Ni-ZnO hollow spheres because the hollow spheres demonstrate a large
surficial area, low density, distinct optical properties, and good permeation [29], and are easily
applicable in various scopes like drug delivery, catalysis, photonic crystals, etc. A number of
synthesis methods have already been introduced to produce these materials from compounds.
Most of these synthetic methods have presumed the silica nanospheres to be a sacrificial
template material. While silica colloids can be observed in different commercial sizes, the
removal of the silica templates may take a long time and highly aggressive and toxic
hydrofluoric acids are inevitably used. Thus, an urgent need can be felt for introducing a
straightforward preparation route under such mild conditions as low temperature and reduced
4

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workup. In one of our previous works, through the carbon microspheres pattern, we made use of
the ZnO hallow spheres for the degradation of the Congo Red pollutant [30].
In the present study, using a facile, ecofriendly, and energy-saving sonochemical approach,
preparation and characterization of the nanostructural x-Ni-ZnO (x = 0, 2, 3, 5, 10 wt % Ni)
hollow spheres are for the first time prepared and characterized. Here, for the formation of the
hollow core, carbon microspheres are employed as a template that can be spontaneously
removed. Moreover, for investigation of the photocatalytic behavior of these hollow spheres, CR
dye degradation is taken into account.
2
. Experimental
2
.1 Materials and physical techniques
All reagents for the synthesis and analysis were commercially available from Merck Company
and used as received. Fructose, zinc(II) acetate dihydrate (Merck, 99.9%), nickel (II) acetate
tetrahydrate (Merck, 99.9%). Doubly-distilled water was used to prepare aqueous solutions. The
infrared spectra were recorded on a nicolet fourier transform IR, nicolet 100 spectrometer in the
-1
range 400-4000 cm using the KBr disk technique. Powder X-ray diffraction (PXRD)
measurements were performed using a Philips X’pert diffractometer with monochromated Cu-Kα
radiation (λ= 1.54056 Å). Also, the morphology of samples was characterized by a field
emission scanning electron microscope (FE-SEM) (Mira TESCAN) with gold coating. The UV-
Vis absorption spectra were examined on a UV-Visible spectrometer (Shimadzu UV 2100).
High-resolution transmission electron microscopy (HRTEM) images were recorded with a
Philips cm 30 instrument.
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2
.2. Synthesis of carbon spheres
The preparation route typically used for carbon spheres is as follows [31]. Fructose dissolving
10 grams) in deionized water (100 ml) was first performed to obtain a transparent solution.
(
After that, the obtained suspension was transferred into a 150-ml Teflon-lined autoclave. At a
temperature of 160 ℃ and for 4 hours, the autoclave was then placed into a blast drying oven and
heated. After cooling down the oven to room temperature, through infiltration, the resultant
carbon spheres were separated from the suspension and was subsequently washed three times
with deionized water. Finally, it was dried at 80 ℃ for 2 h in a way that excessive water was
evaporated.
2
.3. Preparation of zinc oxide hollow spherical nanostructures
In an ultrasonic bath at ambient temperature for 1 hour, 2.2 g of zinc(II) acetate dihydrate was
added to 40 ml of water and 1 g of carbon spheres as templates. The subsequently centrifuged
mixture was washed with distilled water. After that, it was calcined in a furnace having a
temperature of 500 ℃ for 6 hours so that ZnO hollow spheres could be synthesized.
2
.4. Ni-doped ZnO hollow spheres preparation
In order to prepare x-Ni-ZnO (x = 2, 3, 5, and 10 wt % Ni) hollow spheres, in 40 mL of distilled
water proper quantities of zinc(II) acetate dihydrate and nickel (II) acetate tetrahydrate of
analytical reagent grade were combined and dissolved. Moreover, 1 g of carbon microspheres
was homogeneously dispersed in 20 ml of distilled water. In the ultrasonic bath at room
temperature, the templates were put for 1 h. The mixed solution was then aged for one day under
ambient conditions. The subsequently centrifuged product was washed using distilled water.
6

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-1
After, the product was annealed from the room temperature to 500 ℃ at 1℃ min rate and
placed at 500 ℃ for 2 hours in air. Resultantly, Ni-ZnO hollow spherical nanostructures with a
white to greenish white color were produced. The produced hollow spheres were finally
employed as photo-catalyst to degrade the Congo Red pollutant.
2
.5. Investigation of the photocatalytic reactions
In order to evaluate the photocatalytic behaviors of the doped and un-doped samples, the CR dye
photo-catalytic degradation in water under visible light irradiation was measured. For the
investigation of the photocatalytic properties, in an open Pyrex reaction vessel, 50 ml of the 10-
ppm solution of the CR dye (as a suspension) and 25 mg of the suggested photocatalysts (i.e., the
pure ZnO and x-Ni-ZnO hollow spheres) were utilized. During the photolysis, the magnetic
stirring of this suspension was done. In order that the solution remained oxygen-saturated
throughout the reaction, using an air pump, air was blown into the mixture. While the
photocatalytic reaction was performed, the dispersions temperature was kept constant at 30 ℃.
The dye solution was first stirred in a dark place for 30 min. Under the visible light illumination
of the suspension, at fixed intervals (starting from zero time) 3 mL of the homogenized
dispersion aliquots were adopted and centrifuged at 4000 rpm for five minutes (The irradiation
was done with a UV lamp (30 W, UV-C, λ = 253.7 nm, 4.89 eV, Philips (The Netherlands))
and/or a 400 W high-pressure mercury-vapor lamp (providing visible light ≥ 300 nm)). Through
the measurement of the CR dye absorbance (at the wavelength of 598 nm), the reaction was thus
assumed to be a function of the illumination time.
3
. Result and discussion
7

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Through the X-ray diffraction, structural factors and crystalline phases of the obtained samples
were taken into account. XRD patterns related to the pure ZnO and x-Ni-ZnO hollow spheres are
represented in Fig 1. The high crystallinity of the prepared samples is well indicated by the their
relatively sharp and intense peaks. Furthermore, according to the XRD patterns, all of the
samples showed crystal structure with high intensity, which is in agreement with JCPDS
database of card number 36-1451. As it can be seen in the figure, with the increase in the amount
2
+
of Ni , no shift happens in the peaks related to the ZnO. This can be due to the closeness of the
+
2
2+
2+
value of the ionic radius of Zn to that of the Ni . Besides, as the value of Ni increases, its
related peak goes up.
Fig. 1, XRD diffraction pattern of the x-Ni-ZnO hollow spheres [x = 0 (a), 3 (b), 5 (c), and 10 (d)
wt % Ni].
FT-IR spectroscopy can afford to analyze the compound purity and characterize the structural
integrity of the catalysts. Furthermore, FT-IR is highly reactive against the vibrations in the
−
1
metal-oxygen bonds. In general, the region below 1000 cm shows that the intensive bands in
8

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all the spectra are related to the stretching vibration mode of the M-O bond [32]. In this regard,
formation of the metal oxide can be corroborated. In Fig. 2, FT-IR spectra of the pure ZnO and
−
1
x-Ni-ZnO hollow spheres are represented. Therefore, the broad band at 525 cm can be due to
II
−1
the stretching vibrations of ZnO -O. The observed band at 421.7 cm is associated with the
II
II
Ni –O stretching. However, perhaps because of the slight quantity of the Ni ions in our
prepared hollow spheres of Ni-ZnO, this issue cannot be observed in this figure. Furthermore,
because of the adsorption of water in the produced hollow spheres, some peaks are observed at
−
1
3
438, 1631 and 1381 cm [33, 34]. According to the Ni-ZnO hollow spheres spectra, no bands
are seen for the formation of the new compound. Besides, no organic species exist while
synthesizing the Ni-ZnO hollow spheres. It can thus be mentioned that FT-IR spectroscopy is in
agreement with the results of the XRD patterns.
Fig. 2. FTIR spectra of the X-Ni-ZnO nanostructures [x = 0 (pink), 2 (green), 3 (red), 5 (yellow),
and 10 (blue) wt % Ni].
9

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SEM images for the surfaces of the synthesized pure carbon microspheres, pure ZnO hollow
spheres, and x-Ni-ZnO hollow spheres (x = 0, 2, 3, 5, 10 wt % Ni) in the ultrasonic conditions
are shown in Fig. 3. As Figs. 3a and 3b show, carbon microspheres have granulated and rough
surfaces. The surficial granules in the present study are around 7 nm in size, which agrees with
the results from the previous studies [35]. Indeed, formation of the carbon spheres surfaces is
attained by the aggregation of a number of uniform carbonaceous particles that are 7 nm in size.
After the calcination of carbon sphere@ZnO and carbon sphere@Ni-ZnO, pure ZnO hollow
spheres and Ni-ZnO hollow spheres are obtained, which, due to the vacant space formation in
their center, demonstrate a large surficial area compared to the carbon sphere@ZnO and carbon
sphere@Ni-ZnO. However, due to the fact that these surficial ZnO and Ni-ZnO particles are
approximately 60 nm in size, their surface area is smaller than that of the pure carbon
microspheres (Figs. 3c to 3j). It should also be noted that, as it can be seen in the figures, with
the increase in the amount of Ni, the particles morphology on the hollow sphere surface has
changed from a state of more spherical to a state of more spiral. This can be caused by the
variation in the Zn-to-Ni molar ratio, which has resulted in very slight changes in the
morphology of the hollow spheres.
1
0

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Fig. 3. SEM images of pure carbon spheres (a, b) in two different scale bars and the x-Ni-ZnO
hollow spheres [x = 0 (c, d), 3 (e, f), 5 (g, h) and 10 (i, j) wt % Ni].
1
1

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In order to get a better understanding of morphology of the prepared hollow spheres, HRTEM
analysis is here used. HRTEM analyses of the pure ZnO hollow spheres and Ni-ZnO hollow
spheres calcined at 500 ℃ are shown in Figs. 4a-d. The hollow spherical microstructures of the
products in these images can be due to the existence of a bright internal layer and a dark external
layer in the spheres. In other words, in HRTEM images, a marked difference exists between the
dark edges and the light centers of the spheres. This indicates the formation of hollow spheres
and is congruent with the SEM results. These layers also show sufficient overlap with enough
transparency. As shown by these images, our prepared hollow spheres are 1.2-1.8 µm in
diameter, while the thickness of the coarse shell is approximately 40 nm.
1
2

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Fig. 4. HR-TEM images of the x-Ni-ZnO hollow spheres [x = 0 (a), 3 (b), 5 (c), and 10 (d) wt %
Ni]
2
+
Besides, in this study, in order to show the presence of Ni ions in hollow spheres, EDAX is
taken into account. Fig. 5 shows the chemical composition of Ni-ZnO hollow spheres, of which
the results are congruous with those of XRD patterns. The peaks shown in Fig. 5 are related to
2
+
the Zn, Ni and O elements. Therefore, Ni ions presence in these hollow spheres is confirmed.
Existence of the other peaks in these spectra can be ascribed to the presence of the Au element
used for the sample sputter coating at EDAX stage. Besides, as shown in the insert of the images,
1
3

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the obtained weight ratios are approximately equal to the predicted value from the initial
amounts during the synthesis.
2
+
Based on the EDX spectra of Ni-ZnO hollow spheres, atomic concentration of Ni ions for Ni-
doped ZnO hollow spheres is very small (2%-10%). For the Ni-doped samples, the
stoichiometric ratio is comparatively appropriate. Ni ions incorporation into the ZnO lattice is
also evident. A proper concord exists between the XRD and the EDX results. Also, in an effort to
determine the accurate contents of x-Ni-ZnO hollow spheres, inductively coupled plasma (ICP)
measurement was performed. The amounts of Ni are about 29.7, 44.7, 67.8, and 136.3 mg/L in
the etching solution for the samples with 2, 3, 5, and 10 % Ni, respectively. These contents are
close to the contents of raw materials, where is 28, 42, 70, and 140 mg/L, respectively. To further
confirm the composition of the 3-Ni-ZnO hollow spheres as the best photocatalyst in this study,
we performed elemental mapping analysis. The elemental maps of 3-Ni-ZnO hollow sphere
clearly show the distribution of elements of Zn, Ni, and O in this sample (Fig. 6). As are clear
from this figure, through the elemental analysis of 3-Ni-ZnO hollow spheres, it was corroborated
2
+
that the Ni ions were doped into ZnO.
1
4

ACCEPTED MANUSCRIPT
Fig. 5. EDAX analysis of the x-Ni-ZnO hollow spheres, the Ni weight content (x) is increased
with the arrow [3 (a), 5 (b), and 10 (c) wt %].
1
5

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Fig. 6. The elemental mapping of the 3-Ni-ZnO hollow spheres.
In order to get a better understanding of the morphology of the prepared hollow spheres,
analysis of the UV-Vis optical absorption was taken into account in order to obtain the values of
the optical band gap related to the superior photo-catalytic agent of the present work (3-Ni-ZnO
hollow spheres), and subsequently photocatalytic characteristics were explored. Energy band gap
will be computed by relation (1). From the previous works [27], it has been found that the energy
band gap and λonset are related. This can be expressed by the following formulation.
Eg = 1239.8 / λonset
(1)
1
6

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In the above relation, λ shows absorption edge wavelength in nm in the spectrum and E stands
g
for band gap energy (eV). Using Eq. (1), an energy band gap value of 3.02 was resulted for the
prepared hollow spheres of 3-Ni-ZnO. The said value is observed to be in conformity with the
reported values in the literature [37].
In Fig. 7, photocatalytic properties of the prepared hollow spheres for the CR dye photo-
degradation at a pH of 9.5 are compared. In one of our previous works, we estimated the optimal
value of the ZnO hollow spheres, which was equal to 9.5. Therefore, the same optimum value is
also here taken into account. In the absence of light, the dye concentration does not change even
in the presence of the catalyst. This well shows the importance of the light irradiation for the
conduction of appropriate interactions. On the other hand, in the mere presence of light, when the
synthesized hollow spheres are not used, no improvement in the photo-catalytic degradation is
observed compared to the case where light irradiation was absent. The CR dye photo-catalytic
degradation even after 80 min from the beginning of the reaction was unsatisfactory. In order to
enhance the reaction, it is therefore required to make use of light irradiation in the presence of
the catalytic quantities. In order to determine the photo-catalytic effects of the produced hollow
spheres, a comparison has been made between the degradation efficiency of pure ZnO hollow
spheres and x-Ni-ZnO hollow spheres. When pure ZnO hollow spheres were used as
photocatalysts, the results were not good. Even after eighty minutes from the reaction beginning,
the color of the solution was not transparent and a pink dye was visible to the naked eye. Using
the UV-Vis, the CR dye photocatalytic degradation percentage was approximately 45%.
However, the other 55% is not a negligible amount and can clearly cause a serious environmental
hazard. Then, x-Ni-ZnO hollow spheres with various Ni-to-Zn weight ratios (2%, 3%, 5%, and
1
0%) were utilized. As Fig. 7 shows, after 80 min from the initiation of the CR dye
1
7

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photodegradation, the degradation percentage for 5-Ni-ZnO hollow spheres was equal to 89%.
2
+
The quantity of Ni ions was then increased so that the x-Ni-ZnO hollow spheres quantity
changed from 5-Ni-ZnO to 10-Ni-ZnO. Resultantly, a reduction in the photocatalytic efficiency
was observed from 89% to 86%. In the case of 3-Ni-ZnO hollow spheres, where the quantity of
2
+
the Ni ions was reduced from 5% to 3%, the dye photocatalytic degradation was almost
completed. As such, the CR dye degradation arrived at 100% after 80 min of the reaction. When
2
+
the Ni ions quantity was further reduced from 3% to 2%, the CR dye photocatalytic
degradation efficiency went down and became 91%. It can thus be concluded that when the
weight ratio of 3% is used for Ni to Zn (namely, 3-Ni-ZnO), the superior efficiency for these two
metals is attained.
Fig. 7. Photocatalytic efficiency of the x-Ni-ZnO hollow spheres [in dark (black), without
photocatalyst (orange), x = 0 (pink), 2 (green), and 3 (red), 5 (yellow), 10 (blue) wt % Ni] for CR
dye degradation under ultraviolet light irradiation.
1
8

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One of the advantages of the investigated photocatalyst is its stability during photodegradation
reaction. Fig. 8 shows that after four repetitive usages of 3-Ni-ZnO hollow spheres each
including 80 min of irradiation, no appreciable change in its photocatalytic activity occurs. So,
this can be considered as a good photocatalyst for photodegradation of CR dye.
Fig. 8. Recycle and reuse of 3-Ni-ZnO hollow spheres for CR dye photocatalytic degradation.
From the literature, it is already known that nanostructural size and morphology are the key
factors influencing the efficiency of the photocatalysis procedure. Therefore, in the present work,
the CR dye photo-catalytic degradation has been taken into account so that the morphological
impacts can be best understood. In order to explore the photocatalytic efficiency of the Ni-ZnO
nanostructures with varied morphologies, three other nanostructures with flower-like, rod-like
and wood-like morphologies, as shown in Fig. 9, were also considered.
1
9

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Fig. 9. SEM images of the 3-Ni-ZnO nanostructures synthesized via solvothermal (a, b),
microwave (c, d), and sol-gel (e, f).
The photocatalytic properties of these nanostructures were then compared with that of the Ni-
ZnO hollow spheres. Based on the results, in the visible light region, these morphologies are not
efficient even when the reaction time is lengthened for 120 minutes. As Fig. 10 shows, under
visible irradiation, the rod-like, flower-like, and wood-like morphologies have an efficiency of
7
9%, 69%, and 55% after 120 min from the start of the reaction, respectively.
2
0

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Fig. 10. Photocatalytic efficiency of the 3-Ni-ZnO nanostructures with different morphologies
[
wood like (blue), flower like (brown), rod like (green), and hollow sphere (red)] for CR dye
degradation under visible light.
The CR dye is photo-catalytically removed under the following procedure. After the UV light
irradiation (λ = 598 nm) of the Ni-doped ZnO, the electron-hole pairs are obtained from the
excitation of a valence band electron to the band of conduction [37]. This is shown in Eq. (2):
-
+
ZnO + hν → ZnO (e + h VB)
(2)
CB
After that, both the electron and the hole are able to react with the electron acceptors (e.g., O2)
−
and the electron donors (e.g., H O and OH ) in a way that some very reactive radical species
2
such as hydroxyl radicals (E =+3.06 V) can be produced [37].
0
+
+
+
•+
h
h
+ CR → CR → oxidation of the CR
VB
VB
VB
+
•
+ H O → H + OH
2
-
•
h
+ OH → OH
•
OH + CR → degradation of the CR
2
1

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On the whole, because of metallic ion doping in a semiconductor, a shifting in Fermi level
toward more negative potentials occurs and the interfacial charge transfer procedure becomes
more efficient [38]. Consequently, in Ni/ZnO structure, the electron transfer from ZnO band of
conduction to the novel Fermi level (E ) takes place [39]. Accordingly, the CR dye
f
photodegradation is improved by the hollow spheres of Ni-ZnO since these hollow spheres yield
a greater number of hydroxyl radicals than the pure form of ZnO. Moreover, in the novel Fermi
level, the absorbed O can readily entrap the photo-generated electrons [40]. As such, superoxide
2
radicals are synthesized and subsequently HO molecules are made in the presence of water [40,
2
4
1]. After the hydroxyl radicals are produced, reduction of HO to H O happens. Note that the
2 2 2
hydroxyl radicals are the main species causing the molecular dye degradation [40, 42].
-
•
-
2
O + e → O
2
•
-
+
•
O + H → HO
2
2
•
-
+
HO + e + H → H O
2
2
2
-
•
-
H O + e → OH + OH
2
2
Acknowledgements
This work was supported by the Tarbiat Modares University. The authors are grateful for the
financial support.
4
. Conclusion
In this work, pure ZnO and Ni-ZnO hollow spheres have been produced using a green chemical
protocol. Based on the XRD and IR results, these hollow spheres are observed to be formed
without any structural disorder. Furthermore, EDX results are indicative of the existence of Zn,
2
2

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Ni and O elements in the synthesized Ni-ZnO hollow spheres. It has also been found that the Ni-
ZnO hollow spheres have a better photocatalytic performance than the single ZnO for the CR
dye degradation. Among the x-Ni-ZnO hollow spheres, the most photocatalytic activity is related
to the 3-Ni-ZnO hollow spheres. Moreover, it has been found that our synthesized Ni-ZnO
hollow spheres have superior photocatalytic characteristics, compared to other morphologies.
2
3

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