Facile synthesis of monodisperse porous ZnO nanospheres for organic pollutant degradation under simulated sunlight irradiation: The effect of operational parameters

Article abstract of DOI:10.1016/j.materresbull.2016.11.024

Monodisperse porous ZnO nanospheres were successfully fabricated through a facile and cost-effective polyol-mediated preparation approach, followed by annealing at 400 °C for 2 h. The photocatalytic activity of the ZnO products was evaluated by the decolorization of rhodamine B (RhB) solution under simulated sunlight irradiation. The effects of operational parameters on the photo-degradation reaction progress, including catalyst dosage, initial dye concentration, reaction temperature, initial pH value, and the addition of inorganic oxidants, transition-metal ions and inorganic anions were investigated in detail. In addition, reactive species in the oxidation of RhB solution were pinpointed by adding a series of scavengers into the photocatalytic reaction system and O₂^[rad]? were determined to be the main reactive species. A potential mechanism of photocatalysis of RhB under simulated sunlight irradiation is proposed. The catalyst is found to be reusable.

Full text of DOI:10.1016/j.materresbull.2016.11.024

Accepted Manuscript
Title: Facile synthesis of monodisperse porous ZnO
nanospheres for organic pollutant degradation under simulated
sunlight irradiation: The effect of operational parameters
Author: Chong Yang Jisheng Yu Qingsong Li Yingmin Yu
PII:
S0025-5408(16)31125-4
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.materresbull.2016.11.024
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23-9-2016
6-11-2016
6-11-2016
Please cite this article as: Chong Yang, Jisheng Yu, Qingsong Li, Yingmin Yu, Facile
synthesis of monodisperse porous ZnO nanospheres for organic pollutant degradation
under simulated sunlight irradiation: The effect of operational parameters, Materials
Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2016.11.024
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Facile synthesis of monodisperse porous ZnO nanospheres for
organic pollutant degradation under simulated sunlight
irradiation: The effect of operational parameters
Chong Yang, Jisheng Yu, Qingsong Li, Yingmin Yu
State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering,
China University of Petroleum, Qingdao 266580, China
*Corresponding author. Tel.:+86 532 8698 1853; E-mail addresses:
licup01@163.com.
1

Graphical Abstract
Highlights


Monodisperse porous ZnO nanospheres were succesfully synthesized.
The material featured excellent photocatalytic activity towards degradation of
various organic pollutants.


The operational parameters affecting dye degradation were studied in detail.
The photodegradation of RhB followed the pseudo-first-order kinetics.
•-
 O2 are the main reactive species during the photocatalytic process.
ABSTRACT
Monodisperse porous ZnO nanospheres were successfully fabricated through a facile
and cost-effective polyol-mediated preparation approach, followed by annealing at
400 °C for 2 h. The photocatalytic activity of the ZnO products was evaluated by the
2

decolorization of rhodamine B (RhB) solution under simulated sunlight irradiation.
The effects of operational parameters on the photo-degradation reaction progress,
including catalyst dosage, initial dye concentration, reaction temperature, initial pH
value, and the addition of inorganic oxidants, transition-metal ions and inorganic
anions were investigated in detail. In addition, reactive species in the oxidation of
RhB solution were pinpointed by adding a series of scavengers into the photocatalytic
•-
reaction system and O2 were determined to be the main reactive species. A potential
mechanism of photocatalysis of RhB under simulated sunlight irradiation is proposed.
The catalyst is found to be reusable.
Keywords: A. Nanostructures; A. Semiconductors; B. Chemical synthesis; D.
Catalytic properties
1. Introduction
Discharge of large amount of wastewater containing various kinds of organic
contaminants, such as synthetic dyestuffs, halocarbons, and phenol derivatives, is
extremely harmful to environment and human health owing to their
non-biodegradability, toxicity and potential carcinogenic nature [1, 2]. It is estimated
that approximately 10,000 different types of commercial available textile dyes with an
annual production rate of over 7 × 105 metric tonnes are produced; additionally, 30 %
3

of these dyes are consumed over 1,000 tonnes and 90 % of the textile products are
utilized at the level of 100 tonnes annually. As reported [3, 4], approximately 2–20 %
of textile dyes are directly released as aqueous effluents and another 10–25 % are lost
during the dyeing process, which cause serious pollution to the environment. In
general, the complex molecular structure and persistence in nature of synthetic dyes
render them difficult to be efficiently degraded by natural means. Up to now, many
remedial strategies have been developed as approaches for eliminating recalcitrant
pollutants from industrial effluents, including adsorption [5], biological treatment [6],
membrane filtration [7], ultrasound irradiation [8] and heterogeneous photocatalytic
oxidation [9, 10]. Among the aforementioned wastewater treatment technologies,
semiconductor-assisted photocatalytic technology has drawn considerable attention,
due to its potential application in the removal of recalcitrant organic and inorganic
pollutants of contaminated effluents by utilizing solar energy to completely convert
them into innocuous or less harmful compounds [11–13].
Zinc oxide (ZnO) is one of the most important semiconductor photocatalysts used
extensively in the field of environmental remediation because of its high catalytic
activity, superior photochemical stability, non-toxicity, low cost, and ability to make
use of ample sunlight [11, 14, 15]. ZnO has approximately the same band gap energy
as TiO2, and presents photocatalytic capability similar to or better than that of TiO2.
Moreover, it is suggested that ZnO semiconductor exhibits a higher efficiency than
TiO2 for photocatalytic oxidation of pulp mill bleaching wastewater [16], phenol [17]
and 2-phenylphenol [18]. Over the last few decades, the photocatalytic degradation of
4

organic contaminants under artificial solar light (or direct solar light) irradiation with
ZnO has been pervasively used in environmental photocatalysis [19–24]. For instance,
Jia et al. [19] investigated the influencing parameters of cibacron brilliant yellow
3G-P dye degradation using nanosized ZnO as a photocatalyst and found the dye
solutions with concentration of 50 ppm were 98 % decolorized after 60 min of UV-vis
light irradiation. In another development, Meng et al. [25] reported that an ordered
porous ZnO inverse opal structure exhibited promising photocatalytic activity for the
degradation of methyl orange under UV-vis light illumination, due to the synergistic
effect of the inverse opal structure and the photonic crystal effect.
In previous works [26–28], we have stated the formation mechanism of the
monodisperse spherical ZnO nanomaterials via the developed two-stage solution
approach and examined the effects of calcination temperature and particle size on the
photocatalytic decolorization of aqueous MB dye over as-synthesized porous ZnO
products. However, to the best of our knowledge, few efforts have been devoted to
explore the effects of operational parameters, especially the detailed photocatalytic
mechanism, over the as-synthesized porous ZnO nanospheres for the
photodegradation of organic azo dyes and 4-nitrophenol under UV-vis light
illumination, and thus, this area is necessary for further study.
In the present study, monodisperse porous ZnO nanospheres were firstly synthesized
via a facile and cost-effective polyol-mediated preparation route using zinc acetate
dihydrate and diethylene glycol as starting materials. The structure, morphology and
optical property of the as-made ZnO products were investigated using various
5

analytical techniques. Furthermore, the effects of operational parameters, such as the
catalyst dosage, initial dye concentration, reaction temperature, initial pH value and in
the presence of inorganic oxidants, metal ions and inorganic anions on photocatalytic
activity of as-synthesized ZnO as photocatalyst in treating organic pollutants under
simulated sunlight irradiation have been examined. It was found that RhB dye could
be efficiently bleached by the as-synthesized ZnO products under the irradiation of
UV-vis light, 97.30 % of RhB decolorization was achieved after 140-min irradiation
under certain experimental conditions. The possible roles of the additives on the
photocatalytic decolorization of aqueous RhB dye solution were analyzed carefully. In
addition, a mechanism by which RhB dye was decolorized over the as-obtained ZnO
products under UV-vis light illumination was proposed.
2. Experimental
2.1. Materials
Zinc acetate dihydrate (Zn(CH3COO)2·2H2O), diethylene glycol (DEG), absolute
ethanol, methylene blue (MB), 4-nitrophenol, coumarin (C9H6O2), p-benzoquinone,
triethanolamine, isopropyl alcohol, potassium periodate (KIO4), potassium
peroxydisulfate (K2S2O8), hydrogen peroxide (H2O2), potassium bromate (KBrO3),
potassium chlorate (KClO3), manganese (II) chloride (MnCl2), cobalt chloride
(CoCl2), nickel chloride (NiCl2), zinc chloride (ZnCl2), sodium carbonate (Na2CO3),
sodium dihydrogen phosphate (NaH2PO4), sodium chloride (NaCl), and sodium
nitrate (NaNO3) were purchased from Sinopharm Chemical Reagent Co., Ltd. Methyl
orange (MO) and rhodamine B (RhB) were supplied by Aladdin Industrial
6

Corporation. Sodium sulfate (Na2SO4) was obtained from Xilong Chemical Co., Ltd.
Copper chloride (CuCl2) was purchased from Tianjin Basf Chemical Co., Ltd. Iron
chloride (FeCl2) was purchased from Xiya Chemical Industry Co., Ltd. All chemicals
were of analytical reagent grade and used as such without further purification.
Double-distilled water was employed throughout for all aqueous solution, as a
component of the mixed water and organic pollutant solutions. TiO2-P25 was received
from Degussa (Germany). It has a mean particle size of 30 nm and BET specific
surface area of 50 m2/g. The characteristics and chemical structure of RhB, MB, MO
and 4-nitrophenol were given in Table S1 of supporting information.
2.2. Synthesis of porous ZnO nanospheres
Monodisperse porous ZnO nanospheres were prepared through a facile and
cost-effective polyol-mediated preparation approach as described earlier [26].
In the first stage, Zn(CH3COO)2·2H2O (10 mmol) was dissolved to DEG (100 mL) in
a 250 mL three-necked flask. The above reaction mixtures were stirred thoroughly
using a magnetic stir-plate while it was heated to 160 °C and maintained at this
temperature for 1 h. Reflux with water cooling was utilized to prevent solvent
evaporation. The resultant milk-like suspension was centrifuged (Xiangyi Centrifuge
Instrument Co., Ltd., China, TG16-WS) at 7000 rpm for 90 min, after which the
solution was separated into two gradations. The supernatant was decanted off and
washed with ethanol for several times by a repeated sonication-centrifugation process.
The solid phase formed from the supernatant was redispersed in 20 mL of ethanol by
sonication to form a stable colloidal suspension, which was stored for use in the next
7

stage. The reaction in the second stage was started in a similar way as the first. One
difference was that some volume of the suspension was added to the solution at
150 °C. The remaining steps were the same as the above mentioned in the first stage.
After the reaction was completed, a large amount of milky slurry was formed. The
resulting solution was cooled naturally to room temperature, after which it was
separated by centrifugation and subjected to successive washing with doubly distilled
water and ethanol for several times, and then dried in an oven at 60 °C for 12 h. Lastly,
the solid was annealed at 400 °C for 2 h in a muffle furnace to obtain the
nanostructured ZnO samples.
2.3. Characterization
The morphology and particle size of the as-synthesized products were collected by
transmission electron microscopy (TEM) and high resolution TEM using a JEM-2100
system with selected area electron diffraction (SAED). Moreover, the crystal phase
and crystal size were determined using powder X-ray diffraction (XRD, D/max-2500,
Rigaku, Japan) with CuKα monochromatized radiation (λ = 1.54 Å) in the 2θ range
from 20° to 80°. Fourier transform infrared spectrometer (FT-IR) was carried out on a
Nexus spectrometer (Nicolet, USA) from 4000 to 400 cm-1 at room temperature. The
sample was prepared as KBr pellet for FT-IR spectroscopy. The specific surface area
was obtained through nitrogen adsorption at 77 K on the basis of BET
(Brunauer-Emmett-Teller) equation with a Micrometrics ASAP 2010 instrument.
Diffuse reflectance UV-vis spectroscopy (DRS) was recorded on a Shimadzu
UV-2600 spectrophotometer (Japan) equipped with an integrated sphere diffuse
8

reflectance accessory, and BaSO4 was used as a reference.
2.4. Photocatalytic evaluation
The photocatalytic activities of the prepared porous ZnO products were investigated
by photooxidation of the organic azo dyes and 4-nitrophenol in aqueous solution. The
photocatalytic oxidation reaction was carried out using a 300 W xenon lamp
(Perfectlight Scientific Pty Ltd., Beijing, China) equipped with a 100-mL jacketed
beaker reactor (Pyrex) of 4.5 cm i.d. 7.0 cm o.d. and 9.0 cm length. The distance
between the light and solution was about 10 cm. In a typical test, a certain amount of
as-prepared ZnO nanoparticle was placed in 50 mL of aqueous RhB solution with
known initial concentration. The pH of the solution was adjusted by 0.1 mol/L of HCl
or 0.1 mol/L of NaOH to the required pH. The reactor vessel was surrounded by
circulation of water through the external jacket from a thermostatic bath, which
maintained the temperature of the aqueous solution at 25 °C during the photoreaction.
Before the light illumination, the reaction mixture was magnetically stirred for 30 min
in the dark to attain adsorption-desorption equilibrium between dye and ZnO products.
The decolorization rate of RhB was assessed by measuring the absorbance of the
reaction liquid at 20 min interval. Approximately 3 mL of solution were syringed out
from the reaction suspension, centrifuged at 7000 rpm for 20 min, and subsequently
filtered through a Millipore filter (pore size, 0.45 µm) to separate the solid particles,
and the corresponding UV-vis spectra were recorded to monitor the progress of the
oxidation of the organic dyes by a UV-vis spectrophotometer (Shanghai Lengguang
Technology Co., Ltd., China, 759S) with doubly distilled water as a reference.
9

To explore the universality of the obtained products on the photocatalytic oxidation of
organic pollutants, identical operations were performed except that the RhB solution
was replaced by 50 mL of MB (10 mg/L), MO (10 mg/L) and 4-nitrophenol (5 mg/L)
aqueous solutions, respectively. The absorbance of RhB, MB, MO, and 4-nitrophenol
was monitored at the wavelengths of 553, 664, 462 and 400 nm, respectively.
2.5. Estimation of hydroxyl radicals
Coumarin can readily react with •OH radicals to generate highly fluorescent hydroxyl
products, 7-hydroxycoumarin, as displayed in Eq. (1). As a result, the generation of
•OH radicals at the surface of catalyst under the UV-vis light irradiation can be
detected by a fluorescence technique reported by Ishibashi et al [29]. The detailed
experimental procedure was similar to that of the photocatalytic oxidation experiment,
except that the aqueous solution of dye was replaced by a solution of coumarin (50
mL, 0.1 g/L). After the reaction, the solution was centrifuged, subsequently filtered
with a 0.45 µm Millipore filter to separate the ZnO particles, and the PL spectra of
generated 7-hydroxycoumarin were determined in the intensity of peak at about 453
nm by a fluorescence spectrophotometer (Shanghai Lengguang Technology Co., Ltd.,
China, F97Pro) with an excitation wavelength of 340 nm.
,
(1)
3. Results and discussion
3.1. Characterization of as-synthesized porous ZnO nanospheres
10

Fig. S1(a) illustrates the representative XRD pattern for the ZnO products. All of the
diffraction peaks can be well indexed as the hexagonal wurtzite crystal structure of
ZnO (JCPDS card No. 36-1451). The absence of characteristic peaks corresponding to
impurities indicated high purity of the products [10, 30, 31]. Again, the sharp and
strong diffraction peaks further revealed the ZnO products were of good crystallinity.
Moreover, the average crystallite size was calculated using the Debye-Scherrer
equation [32], D = kλ/βcosθ, where D is the average crystallite size, k is a
dimensionless shape factor, with a typical value of around 0.89, λ is wavelength of
X-ray radiation, β is full width at half maximum (FWHM) of the (101) peak and θ is
Bragg’s diffraction angle. The average crystallite size obtained from the XRD data
was estimated to be 12.6 nm. The FT-IR spectrum of the sample is shown in Fig.
S1(b). The well resolved strong and sharp band centered at around 451 cm-1 was
ascribed to characteristic vibration mode of Zn-O bond, which further corroborated
the formation of ZnO. A relatively weak absorption band at 3442 cm-1 corresponded
to the O-H stretching vibration of water adsorbed at the surface of ZnO photocatalyst.
The structure and morphology of the ZnO nanoparticles were investigated by TEM
and HRTEM. As shown in Fig. 1(a), the as-synthesized samples were mainly
constituted of uniform sphere-like nanoparticles. Fig. 1(b) shows a histogram of the
particle diameters, which clearly illustrates that the average diameter of the
as-prepared ZnO nanoparticles was approximately 168 nm. Fig. 1(c) shows the typical
TEM image of the ZnO products, and it can be seen that the ZnO particles consisted
of many small ZnO grains, and the surface of the ZnO nanoparticles was rough.
11

Moreover, a careful observation under higher magnification (Fig. 1(d)) confirms that
each ZnO nanosphere was assembled by many small primary nanocrystals with an
average diameter of approximately 14 nm, which was in accordance with the above
XRD analysis. Noticeably, it can be found that there were a large number of irregular
cavities inside the nanoparticle (marked with green circles), which resulted from the
release of carbon dioxide and water during annealing process. This feature endows the
porous ZnO nanospheres with a high surface area. The BET surface area of porous
ZnO products was 43.9 m2/g, which is higher than that of the reported nanostructured
ZnO [22, 33]. Furthermore, the lattice fringes of ZnO nanoparticles are clearly
observed in Fig. 1(e) and its insets. The lattice spacings were about 0.250 and 0.281
nm, corresponding to the (101) and (100) planes of hexagonal wurtzite-structure ZnO
nanoparticles, respectively [14, 30]. The bright rings in the corresponding SAED
pattern (Fig. 1(f)) revealed the polycrystalline nature of the synthesized ZnO
architectures.
The as-synthesized ZnO was subjected to DRS analysis to provide information on the
optical quality of reaction products. As displayed in Fig. S2(a), the as-synthesized
ZnO products exhibited a strong absorption wavelength shorter than 400 nm, mainly
concentrated in UV region, with a tail extending to the visible light region, being
attributed to the intrinsic band gap absorption of ZnO [14, 30]. The band gap energy
(Eg) was estimated from Fig. S2(b) by extrapolation of linear part of the spectrum to
the energy axis [10, 32]. The Eg value for the synthesized ZnO products was estimated
to be 3.13 eV, which was approximately located at 396 nm. Thus, compared with the
12

absorption edge (386 nm) of pure ZnO [19], the as-synthesized ZnO products in our
work presented a red shift, suggesting that it might result in a higher photocatalytic
activity under visible light.
3.2. Photocatalysis under simulated sunlight irradiation
3.2.1. Evolution of UV-vis spectra during photocatalysis process
We also investigated the potential application of the obtained porous ZnO products to
be used as a photocatalyst in wastewater treatment. The photooxidation behavior of
rhodamine B (RhB), a hazardous dye as well as a common model to test the
photooxidation capability, over porous ZnO products as function of irradiation time
under simulated sunlight illumination is illustrated in Fig. 2. The spectrum of RhB in
visible region displayed a main peak with a maximum at 553 nm. It can be also found
that the absorption bands of RhB showed two types of spectral changes. One was a
slight hypsochromic shift (Δλ = 34 nm) in the absorbance maximum, and the other
was a decrease in the absorbance. The gradual hypsochromic shifts of the absorption
maximum were caused by deethylation of RhB during irradiation because of the
attack by one of the active oxygen species on the N-ethyl group, reminiscent of
similar phenomena observed by Zhao’s group on a RhB/TiO2 system illuminated
under visible irradiation [34] and Zhu’s group for visible-light-induced
photooxidation of RhB over nanosized Bi2WO6 [35], while the reduction of the
characteristic absorption band indicated that a rapid cleavage of the conjugated
chromophore structure of RhB molecules occurred simultaneously, and the color of
the solution changed from pink to colorless after 140 min of irradiation (inset of Fig.
13

2), indicating the gradual decomposition of RhB dye molecules during UV-vis light
irradiation [36].
3.2.2. Effect of light irradiation and porous ZnO products
The profiles of the photocatalytic oxidation of 10 mg/L RhB dye aqueous solution
containing 10 mg ZnO as function of irradiation time under varied conditions are
displayed in Fig. 3. It is evident from the figure that approximately 97.30 % of RhB
dye was bleached at the irradiation time of 140 min in the presence of both ZnO and
light (curve d). This was contrasted with only 10.49 % decolorization for the identical
experiment in the presence of light illumination without the ZnO products (curve b),
and the negligible RhB decolorization (4.28 %) in the presence of ZnO products, but
without light illumination (curve a). These experiments demonstrated synergistic
degradation of RhB dye by the simulated solar light and a photocatalyst. Using a UV
cut-off filter (λ > 420 nm) caused the reduction of the RhB loss from 97.30 % to
82.67 % (curve c). Hence, it can be concluded that simulated solar light is more
effective than visible light for ZnO particle to activate the RhB dye decolorization. It
is well known that ZnO is a wide-gap semiconductor, which theoretically cannot be
excited under visible light illumination. The reason for the high removal ratio (~
82.67 % RhB loss) over ZnO catalysts in visible light region may be due to the
presence of dye sensitization mechanism along with the usual ZnO sensitization [33,
36–38]. Furthermore, the degradation kinetics (curve e) revealed a similar trend as
discussed above. The function of ln(Co/C) versus reaction time (t) showed a strong
linear relationship (regression coefficient of 0.9976), indicating the pseudo-first-order
14

reaction kinetics of RhB decolorization catalyzed by the as-synthesized porous ZnO
products under UV-vis light irradiation:
l nC(o C/ ) k
,
(2)
where Co is the initial concentration of RhB before irradiation, C is the concentration
of RhB at any reaction time, and k is the pseudo-first-order reaction rate constant. Via
a linear fit, the degradation apparent rate constant k value for the as-synthesized ZnO
products can be determined to be 0.025 min-1.
The photocatalytic activity of the as-prepared ZnO products was compared with the
commercial TiO2-P25 under simulated sunlight irradiation at constant dye
concentration (10 mg/L, pH = 6.5) and catalyst dosage (10 mg). As depicted in Fig.
S3, TiO2-P25 exhibited a slightly high reaction rate of RhB decolorization compared
to the synthesized ZnO products within 140 min, whereas the values of percent RhB
removal over TiO2-P25 and synthesized ZnO products are 82.74 % and 85.36 %,
respectively, at the time of 80 min, indicating that the obtained ZnO products and
commercial TiO2-P25 have almost the same decolorization reaction rate. For practical
application, the sedimentation properties of the photocatalysts are extensively
concerned. For the tested ZnO products, the sedimentation properties were
preliminarily compared by suspending 3 mg of the catalyst in 3 mL double-distilled
water under ultrasonic process, followed by static sedimentation for 80 min. As
presented in Fig. S4, compared with TiO2-P25 powders, ZnO products possessed
faster sedimentation velocity, and nearly all ZnO products were precipitated from the
suspension, implying its easy separation for recycle usage. Thus subsequent
15

experiments were performed with the as-synthesized ZnO as a photocatalyst due to its
high photocatalytic activity, superior sedimentation property and low cost.
Fig. S5 presents the comparison of porous ZnO products on the photocatalytic
oxidation of RhB, MO, MB and 4-nitrophenol aqueous solutions under simulated
sunlight irradiation, where the data starts from the initial concentration of pollutants
after the adsorption-desorption equilibrium. As can be seen, the obtained ZnO
products exhibited better removal performance on the photocatalytic oxidation of
these organic pollutants. In addition, it can be found from Figs. S6, S7 and S8 that the
changes of UV-vis spectra and colors of aqueous MO, MB and 4-nitrophenol
solutions demonstrated similar trends when compared to that of RhB dye. The above
results suggested that the porous ZnO photocatalysts had no selectivity and could
effectively degrade different kinds of organic contaminants. Consequently, it can be a
prospective alternative applied in environmental purification.
3.3. Effect of various factors on photocatalytic process
3.3.1. Effect of photocatalyst amount
To investigate the influence of photocatalyst amount, photocatalytic oxidation
experiments were carried out by employing different amounts of ZnO (5, 10, 20 or 30
mg) under constant process conditions: dye concentration = 10 mg/L, reaction
temperature = 25 °C and initial pH = 6.5. The percentage of RhB loss as a function of
photocatalyst amount is shown in Fig. 4. The catalytic efficiency was studied in terms
of reaction rates, turnover number (mole reacted RhB per mole catalyst), and quantum
yield (reacted dye molecules per incident photon). It can be seen that there was
16

appreciable increase in the reaction rate of RhB decolorization with increasing
catalyst amount. The reason of this observation is thought to be the fact that
increasing photocatalyst dosage results in an increase in the number of active sites
that are available at the surface of the catalyst [39]. Another acceptable explanation is
that the density of catalyst particles in the area of illumination is improved after the
ZnO dosage is increased [11, 40, 41]. However, turnover number values, measured
after 140 min, relatively decreased as catalyst amount was increased. This suggested
that the relative efficiency of the catalyst was lowered by increasing catalyst amount.
Similar phenomena were observed in earlier reports and this was attributed to
screening the radiations away from the ZnO particle surfaces [24, 42]. The quantum
yield value, measured after 140 min, showed an increase with increasing catalyst
amount at the beginning, but then decreased. The reduction in quantum yield was
another evidence of light screening effect associated with higher catalyst amount. The
above results suggested that the photocatalyst concentration is one of the most
important factors in the photocatalytic oxidation of RhB dye.
3.3.2. Effect of initial dye concentration
In order to examine the impact of various dye concentrations on the photocatalytic
degradation, experiments were conducted by varying the concentration of RhB from 5
to 30 mg/L. The values of ZnO dosage, reaction temperature and initial pH were
constant at 10 mg, 25 °C and 6.5. As it appears in Fig. 5, the RhB loss percentage was
significantly lowered with an increase in the initial concentration of RhB. The
decolorization rate of the RhB dye aqueous solution with 5 mg/L concentration
17

increased sharply in the initial 60 min (93.31 %). In contrast, for the more highly
concentrated solutions, such as the solution with 30 mg/L concentration, only 27.82 %
RhB loss was observed after the same reaction time. This negative effect can be
interpreted by the following reasons: (1) The number of RhB molecules that adsorbed
on the active sites of the catalyst surface, increases with rise in the initial
concentration of RhB dye. Therefore, competitive adsorption of O2 and OH- on the
same sites decreases, meaning a lower generation rate of O2•- and •OH radicals, which
are the principal oxidants necessary for the high degradation rate [11, 42]. (2) On the
other hand, by increasing the dye concentration, a large number of dye molecules
along with the intermediates generated may compete for the constant total active sites
available for adsorption at a fixed concentration of ZnO [22, 35]. (3) Additionally,
with increase in the initial dye concentration, the solution becomes more intensely
colored and the path length of photons entering into the solution decreases, so the
absorption of photos by the catalysts decreases, and consequently the decolorization
rate is reduced [39, 43, 44]. Despite the lowering in removal percentage of RhB with
increased initial concentration, the values of turnover number and quantum yield
calculated after 140 min increased. Lower values were observed at the concentration
of 5 mg/L, which is ascribed to the fact that all added RhB molecules were degraded
with no more remaining.
3.3.3. Effect of reaction temperature
The oxidation of aqueous RhB solution by ZnO under UV-vis light illumination was
carried out under different temperature (15, 25, 35 and 45 °C). The values of ZnO
18

dosage, initial dye concentration and initial pH were constant at 10 mg, 10 mg/L and
6.5, respectively. As displayed in Fig. 6, the RhB loss percentage was found gradually
improved with an increase in temperature from 15 to 35 °C. It is well known that the
mass transfer of different species increases as the reaction temperature rises, thus
improving the reaction rate [45]. Nevertheless, it should be pointed out that the rate of
RhB decolorization decreased slightly with the reaction temperature further increased
to 45 °C, which can be attributed to possible escape of the oxygen molecules
dissolved inside oxygen [24]. The oxygen molecules are involved with the mechanism
of photocatalytic degradation reaction. Further, the activation energy (Ea) of the
photocatalytic degradation reaction is calculated according to the Arrhenius equation
followed as:
Ea
RT
k = ko exp(
)
,
(3)
Ea
L n k = Ln k
o
RT
,
(4)
where k and ko are the reaction rate constant and frequency factor, respectively. T and
R represent the absolute temperature and ideal gas constant (8.314 J/(mol K)),
respectively. The function plot is depicted in Fig. S9 and the value of activation
energy for the degradation of RhB was 6.81 KJ/mol, which was small and close to the
value (~6.1 KJ/mol) reported by Hilal’s group for the degradation of phenazopyridine
over naked ZnO catalyst [46]. This is contrary to thermally induced catalytic reactions
where the rates progressively increase with higher temperature. Photochemical
degradation reaction rates, with wide band gap semiconductor catalysts, are known to
19

be insensitive to temperature [46, 47]. Energy provided by heating at ambient
temperatures (~ 45 °C) is far less than 3.0 eV, viz. heat energy is only in IR region,
not UV. Furthermore, higher temperatures are responsible for removal of oxygen from
the reaction solution, which is necessary for pollutant oxidation.
3.3.4. Effect of solution pH
The solution pH is an important parameter in photocatalytic degradation reactions for
industrial applications. It has been reported that the pH of zeropoint charge for ZnO is
approximately 9.0 (denoted as pHzpc) [48]. At pH lower than the pHzpc of the ZnO, the
stable suspension forms because their net positive charge prevents agglomeration.
Similar phenomena are observed when pH is higher than the pHzpc of ZnO, where
ZnO particles are surface negatively charged by adsorbing OH- ions. The effect of pH
values on the percentage of RhB loss was studied in the pH range of 3–11 at a 10
mg/L dye concentration and 10 mg catalyst dosage and the obtained results are
depicted in Fig. 7. The pH of the reaction solution was adjusted before light
irradiation and it was not controlled during the course of the photocatalytic reaction. It
was apparently found that alkaline condition was very beneficial to the photocatalytic
oxidation of RhB dye. The rate of RhB decolorization was significantly enhanced
with increasing pH value of the dye solution, which may be due to the increase in
adsorption of dye molecules. RhB is an aromatic amino acid with amphoteric
characteristics because of the presence of both amino and carboxyl groups and it
exists in three distinct monomeric molecular forms (cation, zwitterions and lactone)
depending on the solvent environment [49]. Thus, the charge state of RhB is
20

determined by solution pH. The pKa value for the aromatic carboxyl group presented
on RhB molecule is approximately 4.0. When the solution pH is less than 4.0, the
RhB ion takes on a positive charge on one of the nitrogens while the carboxyl group is
unionized. ZnO has a point of zero charge at pH 9.0 [24, 48], below which the ZnO
surface is predominantly positively charged. The electrostatic repulsion between
cationic RhB and positively charged ZnO leads to the reduced percentage of RhB
adsorption. When solution pH is higher than 4.0, the carboxyl group gets ionized and
the zwitterions form of RhB is generated. The zwitterions form of RhB in water may
increase dimerization of RhB [50], which makes the molecule difficult to enter most
of the pore structure of ZnO nanoparticles. When the solution pH increases above 9.0,
excessive OH- compete with COO- in binding with -N+ and the aggregation of RhB
reduced. Hence, an increase in the adsorption of RhB dye on the ZnO photocatalyst
can be inferred at pH > 9. The results obtained are in accordance with previous report,
and a similar increase in degradation rate was observed by Kaur et al. [51] for
photocatalytic oxidation of safranin O (SO) using highly robust ZnO as catalyst. An
experiment to verify the dark adsorption of RhB onto ZnO nanoparticles with
different pH value was carried out, as shown in Fig. S10. The percentage of
adsorption at pH 3, 5, 6.5, 9 and 11 were found to be 7.95, 11.93, 13.97, 21.10 and
34.25 %, respectively, after the attainment of adsorption equilibrium (30 min). As a
result, the improvement in the photocatalytic decolorization rate in the basic region is
possibly due to the high adsorption of RhB molecules on the catalyst surface. On the
other hand, ZnO is an amphoteric oxide able to react as a base at strong acidic
21

aqueous solution according to the Eq. 5 [22].
ZnO  2H(aq) =Zn2 +H2O
（S)
(aq)
,
(5)
So, part of ZnO dissolved at pH 3, resulting in the relatively lower decolorization rate
[33]. Because of the amphoteric property of ZnO semiconductor, water molecules in
alkaline solution adsorbed on its excited surface and decomposed by oxidative
potential of the hole, generating OH-. The formed OH- could be oxidized further by
the hole to form more •OH radicals. The •OH radicals lead to partial or complete dye
decomposition. Therefore, alkaline conditions were beneficial to photocatalytic
oxidation.
3.3.5. Influence of inorganic oxidants on photocatalytic oxidation of RhB dye
In the irradiated aqueous ZnO suspensions, molecular oxygen on the ZnO surface
provides a natural sink for the photogenerated electrons to prevent the electron-hole
-
-
-
2-
pair recombination process. The inorganic oxidants such as IO4 、BrO3 、ClO3 、S2O8
and H2O2, can compensate for the lack of oxygen either because of oxygen
consumption or slow oxygen mass transfer to improve the oxidation rates of organic
substrates via quenching the conduction band electrons and via formation of reactive
radicals intermediates. The influence of addition of these inorganic oxidants on
photocatalytic decolorization rate of RhB dye was studied at the different
concentrations and illustrated in Fig. 8. We can observe that all of the added inorganic
oxidants had a promotional effect on the photocatalytic process and enhanced the
photocatalytic decolorization rate of RhB dye. The positive effect of these inorganic
-
oxidants involved in photocatalytic system was found to be in the order of IO4 >
22

-
-
S2O82- > H2O2 > BrO3 > ClO3 .
The enhancement in photocatalytic oxidation of RhB dye may be ascribed to the
following three factors:
(1) The enhancement of RhB dye removal by the addition of oxidants may be due to
the trapping of the photogenerated conduction-band electrons, thus increasing the
electron-hole separation time [13, 52, 53, 54].
(2) On the other hand, the oxidant with higher UV absorption is more effective
toward degradation of organic compounds in industry effluent, which is due to high
absorption of UV light resulting in the generation of reactive radical species. As
reported by Swaminathan’s group [43], the UV absorption of the above oxidants
-
-
-
followed an order of IO4 > S2O82- > H2O2 > BrO3 > ClO3 . It can be inferred that the
-
higher photocatalytic decolorization rate of IO4 than other oxidants may be attributed
-
to its high absorption capacity toward UV light, while ClO3 , with the least reaction
rate, has no UV absorption.
(3) In addition, the combination of some inorganic oxidants and simulated sunlight
irradiation provides a strong oxidant system via formation of other oxidizing species.
-
•
•
IO4 can produce several powerfully reactive intermediate radicals (IO3 , •OH, IO4
and O•-) in aqueous solution through the following reactions depicted in Eqs. 6–8,
resulting in the increase of photocatalytic decolorization rate of the RhB dye [52].
IO4 +hv  IO3 +O-
-
,
(6)


O- + H 
O
,
(7)
-

OH+IO4 OH- +IO4
,
(8)
23

When S2O82- undergoes thermolysis or photolysis in an aqueous solution, it can
•-
produce sulfate radical anion (SO4 ) (Eq. 9) [55]. The radical, as a highly reactive
oxidant with a standard redox potential of 2.6 V vs. NHE (normal hydrogen electrode)
[1, 43, 53, 56], could react with H2O to generate •OH radicals (Eq. 10).
-
S2O82- +heat / hv  2SO4
,
(9)

2-
SO4 +H2O  OH+SO4 +H+
,
(10)
As is well known, H2O2 is a strong oxidizing agent with high reactive oxygen content
which is widely used in advanced oxidation technique for degradation of organic
pollutants. Some literatures reported that H2O2 may split photolytically to generate
hydroxyl radicals directly under UV light irradiation via the following reaction (Eq.
11) [52, 57]:
H2O2 +hv  2OH
,
(11)
However, H2O2 has extremely low absorption of solar light. Thus, the oxidation of the
dye by photolytic H2O2 will be insignificant. Under the solar light irradiation, H2O2
can also react with superoxide radical anions to produce •OH radicals shown in the
equation (Eq. 12).
H2O2 +O2  OH+H+ +O2
-
,
(12)
-
-
In the cases of BrO3 and ClO3 , both oxidants cannot produce other oxidizing species,
since they have lower solar light absorption [43].
To further explore the effects of these inorganic oxidant concentrations on the
photocatalytic activity over porous ZnO photocatalysts, the experiments were
conducted with oxidant concentration ranging from 50 to 400 mg/L, as presented in
24

Fig. 8. It is noticeable that as the concentration of inorganic oxidant was increased,
the photocatalytic decolorization rate of RhB dye was improved, which can be
explained by the above three reasons. But when the concentration of these oxidants
-
-
-
(i.e., IO4 , BrO3 and ClO3 ) was higher, the photocatalytic removal rate decreased.
This observation may be due to the generation of excessive •OH radicals in the
presence of higher concentration of some oxidants, which recombines excessive •OH
radicals to produce less-reactive substances. On the other hand, employing of higher
-
-
-
concentration of IO4 , BrO3 and ClO3 oxidants may result in adsorption effect of I-,
Br- and Cl- ions at the surface of ZnO in the breakdown solution, respectively, thereby
inhibiting surface photochemistry from occurring to some extent [53].
3.3.6. Influence of transition-metal ions on photocatalytic oxidation of RhB dye
The impacts of different transition-metal ions on the photocatalytic activity of
obtained porous ZnO products were investigated using RhB as a model dye under
UV-vis light irradiation. As shown in Fig. 9, it is clearly seen that all of the added
transition-metal ions had a beneficial influence on the photocatalytic decolorization
rate of RhB dye. The enhancement effect of these metal ions was ranked as follows:
Zn2+ > Mn2+ > Ni2+ > Cu2+ > Fe2+ > Co2+. The common anion of all of the added metal
ions is Cl-; therefore, only metal ions are involved in the promotional effect. As some
authors stated that transition-metal ions could behave as a scavenger of electron from
ZnO surface, thus inhibiting the recombination of electron-hole pairs according to the
equation below [43, 53, 58].
Mn+ +eCB-  M(n-1)+
(13)
,
25

Where Mn+ represents Zn2+, Mn2+, Ni2+, Cu2+, Fe2+, Co2+ metal ions.
To examine the influence of transition-metal ion concentration on the photocatalytic
oxidation of RhB dye, a series of experiments were performed with metal ion
concentration ranging from 50 to 400 mg/L. As the concentration of added
transition-metal ions was increased, the photocatalytic decolorization rate of RhB dye
was enhanced, but further increase in transition-metal ion concentration in reaction
solution resulted in a reduction in the decolorization rate of RhB dye. The retardation
influence of high transition-metal ion concentration may be attributed to the following
two factors: (1) The high concentration of transition-metal ions has a filter effect
because of UV light absorption and decreases the light intensity in the reaction
solution, thus reducing the decolorization rate of RhB dye [32]. (2) On the other hand,
when the concentration of added metal ions is higher, the M+ cations generated via the
electrons existed as the dominating ions in the reaction solution, and the M+ cations
can be oxidized to M2+ ions through the photoinduced holes at the surface of
photocatalyst or by •OH radicals based on the following equations (Eqs. 14 and 15),
thus leading to the decrease in photocatalytic decolorization rate of the RhB dye [53,
59].
+
2
M+ + h  M
,
(14)
M+ +OH+H+  M2+ H2O
,
(15)
3.3.7. Influence of inorganic anions on photocatalytic oxidation of RhB dye
Dye-containing wastewater usually contains not only organic contaminants but also
abundant inorganic anions such as sulfate, carbonate, nitrate and so on [60]. Hence, it
26

is necessary to investigate the impact of these ions on the photocatalytic process. The
effects of added inorganic anions on the removal of RhB dye was studied at different
concentrations varying in the range of 0.2–1.0 g/L and displayed in Fig. 10. It can be
found that all of added inorganic anions had an inhibition effect on the photocatalytic
process and lowered the photocatalytic decolorization rate of RhB dye. The
-
detrimental effect of these inorganic anions was found to rank in the order of H2PO4 >
-
CO32- > Cl- > SO42- > NO3 . Since the common cation of all the added inorganic
anions (Na+) is inert; hence, only anions are involved in the retardation effect. In the
meanwhile, it can be seen that as the concentration of inorganic anions was increased,
and the photocatalytic decolorization of RhB dye became slower.
The inhibitive influence of these inorganic anions on the photocatalytic reaction could
be attributed to the following possible reasons: (1) The retardation effect of inorganic
anions may be due to a competitive adsorption of these anions and the target organic
substance at the surface of catalyst, leading to blockage of the active sites at the
surface of photocatalyst, thus inhibiting the photooxidation of organic substance by
reactive species [61, 62]. (2) Another possible reason to explain the inhibition effect
in the presence of inorganic anions is that an inorganic salt layer forms at the ZnO
nanoparticle surface, which obstructs the approach of RhB molecules, thus resulting
in the reduction of RhB dye molecules adsorption [13, 60]. (3) Additionally, these
negatively charged inorganic anions can react with holes (h+) and hydroxyl radicals
(•OH) to produce less-reactive species due to their lower oxidation potentials, and
those inorganic anions behaved similar to h+ and •OH scavengers, which is a great
27

impediment to the progress of the reaction, thus leading to a reduction in the
photocatalytic decolorization rate [56].
As has been widely reported [32], dihydrogen phosphate ions could react with h+ and
•
•OH to generate less reactive H2PO4 radicals through the following equations (Eqs.
16 and 17), leading to a reduction in the decolorization rate of RhB dye.
-

H2PO4 +OH  H2PO4 +OH-
(16)
,
-

H2PO4 +h+  H2PO4
(17)
,
•-
Carbonate ions can act as •OH radical scavenger to produce CO3 radicals which has
less oxidizing potential than the hydroxyl radicals (Eq. 18), thus resulting in the
reduction of decolorization rate of RhB dye [8, 39, 44].
CO32- +OH CO3- +OH-
(18)
,
Chloride ions can also act as scavengers of h+ and •OH according to the following
equations (Eqs. 19 and 20). Moreover, the recombination of two Cl• would terminate
the radical generation (Eq. 21) [8, 13, 54, 61].
Cl- +OH Cl +OH-
(19)
,
Cl- +h+ Cl
(20)
(21)
,
Cl +Cl  Cl2
,
Sulfate anions can also retard the photocatalytic decolorization rate of the RhB dye
•-
•-
via scavenging •OH and h+ to generate SO4 radicals (Eqs. 22 and 23). Since SO4 is
less reactive than •OH radicals and h+, the addition of SO42- inhibits the photocatalytic
oxidation of the RhB dye [8, 41, 61].
2-
-
SO4 +OH SO4 +OH-
(22)
,
28

2-
-
SO4 +h+ SO4
(23)
,
-
In case of the nitrate ions, we can observe that the addition of NO3 did not affect the
-
photocatalytic decolorization rate of RhB dye appreciably. The role of NO3 in the
photooxidation process can be interpreted through two factors:
(1) Nitrate ions are known to generate •OH radicals under UV light irradiation
through the following equations (Eqs. 24 and 25) [63].
-
NO3- +h  NO3  NO2 +O
(24)
,
O-  H2O  OH+OH
(25)
,
-
(2) On the other hand, NO3 , especially at high concentrations, would exert an inner
filter effect and reduce the UV light intensity acting on the target molecules, thus
inhibiting the photocatalytic oxidation of RhB dye [13, 57]. It is obvious that the
-
detrimental effect of NO3 was dominant on its promotional effect.
3.4. Discussion of photocatalysis mechanism
3.4.1. Influences of different scavengers
It is important to ascertain the main active species in the photocatalytic process for
elucidating the photocatalytic mechanism. The reactive species in photocatalytic
process could be detected trough trapping experiments of hydroxyl radicals (•OH),
•-
holes (h+) and superoxide radical anions (O2 ) by introducing isopropanol (IPA),
triethanolamine (TEOA) and p-benzoquinone (BQ) [64, 65], respectively, in the
system at the beginning of the photocatalytic reaction. Effects of a series of
scavengers on the photocatalytic oxidation toward RhB dye over ZnO photocatalysts
are shown in Fig. 11. Under simulated sunlight illumination, the percentage of RhB
loss decreased the most rapidly from 97.30 % to 33.71 % after the addition of BQ
29