Effective ways to enhance the photocatalytic activity of ZnO nanopowders: High crystalline degree, more oxygen vacancies, and preferential growth

Article abstract of DOI:10.1039/c9nj04767k

The practical application of ZnO nanopowders (NPs) in purifying wastewater could be determined by some issues, such as low-cost and scalable preparation, high photocatalytic activity, antiphotocorrosion, and convenient recycling. In this contribution, a series of ZnO NPs were prepared in a scalable way in the 220 °C, 250 °C and 280 °C autoclaves, respectively. The structural, morphological and surface details of the samples were systematically examined by XRD, SEM, FT-IR, TEM, Raman, PL, and UV-visible spectroscopy. Along with the increase of the reaction temperature, the mean diameter of ZnO NPs increased in the range of 110-130 nm, and the color of the products was lighter, resulting in a smaller surface area but a stronger UV absorbance in the region of 200-400 nm. Moreover, the preferential growth along the (002) plane, the crystalline degree and the green emission intensity of ZnO 280 are more overt than those of ZnO 250 and ZnO 220. Some other parameters including stretching vibration and E₂(high) mode of Zn-O bonding or diffraction peaks of ZnO 280 also shifted to lower wavenumbers or lower angles when compared to those of ZnO 220, revealing the existence of more oxygen vacancies in ZnO 280. As expected, ZnO 280 showed the best photocatalytic activity and antiphotocorrosion among the three samples in the degradation of rhodamine B due to the availability of abundant oxygen vacancies, preferential growth along the (002) direction and the high crystalline degree. This suggests that, along with the increase of the preparation temperature, the obtained ZnO nanopowders exhibit better photocatalytic activity. Therefore this work could provide a route to synthesize well crystalline ZnO nanopowders and open up an avenue for commercial applications of ZnO NPs in the treatment of organic wastewater.

Full text of DOI:10.1039/c9nj04767k

NJC
View Article Online
View Journal | View Issue
PAPER
Effective ways to enhance the photocatalytic
activity of ZnO nanopowders: high crystalline
degree, more oxygen vacancies, and preferential
growth†
Cite this: New J. Chem., 2019,
43, 19223
Yingying Lv, Jiayu Lin, Siyan Peng, Laijun Zhang and Leshu Yu
*
The practical application of ZnO nanopowders (NPs) in purifying wastewater could be determined by
some issues, such as low-cost and scalable preparation, high photocatalytic activity, antiphotocorrosion,
and convenient recycling. In this contribution, a series of ZnO NPs were prepared in a scalable way in
the 220 1C, 250 1C and 280 1C autoclaves, respectively. The structural, morphological and surface
details of the samples were systematically examined by XRD, SEM, FT-IR, TEM, Raman, PL, and
UV-visible spectroscopy. Along with the increase of the reaction temperature, the mean diameter of
ZnO NPs increased in the range of 110–130 nm, and the color of the products was lighter, resulting in
a smaller surface area but a stronger UV absorbance in the region of 200–400 nm. Moreover, the
preferential growth along the (002) plane, the crystalline degree and the green emission intensity of ZnO
280 are more overt than those of ZnO 250 and ZnO 220. Some other parameters including stretching
vibration and E2(high) mode of Zn–O bonding or diffraction peaks of ZnO 280 also shifted to lower
wavenumbers or lower angles when compared to those of ZnO 220, revealing the existence of more
oxygen vacancies in ZnO 280. As expected, ZnO 280 showed the best photocatalytic activity and anti-
photocorrosion among the three samples in the degradation of rhodamine B due to the availability of
abundant oxygen vacancies, preferential growth along the (002) direction and the high crystalline
degree. This suggests that, along with the increase of the preparation temperature, the obtained ZnO
nanopowders exhibit better photocatalytic activity. Therefore this work could provide a route to synthe-
size well crystalline ZnO nanopowders and open up an avenue for commercial applications of ZnO NPs
in the treatment of organic wastewater.
Received 18th September 2019,
Accepted 1st November 2019
DOI: 10.1039/c9nj04767k
rsc.li/njc
such as metallic oxides (sulfides or nitrides), heterojunction
nanocomposites, oxynitrides, oxysulfides, graphitic carbon
1 Introduction
Photocatalytic degradation is a typical environmental remediation nitrides, and so on.2–4 In particular, intensive studies have
method and widely used to treat organic wastewater, in which been devoted to the synthesis, properties and prototypes of
photons and a catalyst are usually involved.1 Under light metallic oxide photocatalysts, especially for their typically
irradiation, the electrons from the valence band in the semi- representative TiO2- and ZnO-based nanomaterials due to
conductor are excited to the conduction band in some semi- their physical stability, nontoxicity and high catalytic
conductor materials, and electron–hole separations are created. activity.5
Thereafter, the generated electron–hole pairs on the surface of
Besides having some common choices listed above, ZnO has
the catalyst trigger the desired redox reaction between organic some other evident advantages in the sense that it absorbs over
molecules and water or oxygen. Such a semiconductor material is a larger fraction of the UV spectrum and has a lower prepara-
commonly called the photocatalyst. To date, many photocatalytic tion cost in contrast to TiO2. Unfortunately, ZnO possesses the
nanomaterials have been exploited to possibly actualize the inherent drawback of the photocorrosion effect in photo-
photocatalytic degradation of organic pollutants in wastewater, catalysis, which causes fast electron–hole recombination and
the decrease in photocatalytic efficiency and stability, finally
limiting its practical applications.6 Recently, some new techni-
ques such as coating, hierarchical structures or doping with
School of Chemistry and Environmental Science, Key Laboratory of Polymer
Preparation and Processing, Shangrao Normal University, Jiangxi, 334001, China.
transition metal ions have been developed to delay photocorro-
E-mail: yuleshu2008@126.com; Fax: +86-793-8152865; Tel: +86-793-8152865
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nj04767k sion and prevent fast electron–hole recombination.7–9 But these
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019 New J. Chem., 2019, 43, 19223--19231 | 19223

View Article Online
Paper
NJC
additional techniques would undoubtedly increase the cost of was obtained. The yield of the product was ca. 95–97%, based
ZnO-based photocatalysts and ultimately hinder their practical on the amount of Zn powder used. The slight loss of the
applications in wastewater. Hence the practical application of product is from the transfer process.
ZnO nanopowders (NPs) in purifying wastewater needs to be
Note: (1). Para polyphenol-doped Teflon-lined autoclaves
considered by some issues, such as low-cost and scalable must be used when the reaction temperature is higher than
preparation, high photocatalytic activity, antiphotocorrosion, 250 1C; (2). The preparation temperature cannot be higher than
and convenient recycling.
300 1C; (3). The products synthesized at 220, 250 and 280 1C
In the past, ZnO NPs were usually prepared via wet chemical correspond to ZnO 220, ZnO 250 and ZnO 280, respectively.
routes due to them being simple, less expensive and scalable
processes, in which some Zn-containing salts were decomposed
in liquid-phase reactions.10 However, the obtained products
suffered from either low photocatalytic activity or difficulty in
recycling, probably owing to the small size, porous structure
and low crystalline degree of ZnO NPs. In our previous study,
ZnO NPs were cheaply prepared in a scalable way by a soluble
salt-assisted route via the simple oxidation of the Zn–Na2SO4
mixture. Nevertheless, the as-synthesized product was inferior
to Degussa P25 in the degradation of rhodamine B (Rh-B), most
probably due to the low crystalline degree of the product.11,12
In our very recent work, ZnO NPs were synthesized via a simple
reaction of zinc powder to H2O vapor in a 230 1C autoclave.
And the as-prepared products showed good photocatalytic
activity in the degradation of Rh-B under solar light irradiation,
even better than that of Degussa P25.13 In this contribution, a
series of ZnO NPs were prepared in a scalable way in carbon-
doped Teflon-lined autoclaves at 220 1C, 250 1C and 280 1C,
respectively. It was found that the increasing preparation
temperature led to a higher crystalline degree, more preferen-
tial growth along the (002) direction and more oxygen vacancies
of the product. And these factors are greatly helpful for delaying
2.2 Materials characterization
The product was systematically characterized by scanning
electron microscopy (SEM; Hitachi SU8010 apparatus working
at 15 kV, Japan), X-ray diffraction (XRD; Cu-Ka radiation,
l = 0.15418 nm, Rigaku MinFlex II, Japan), Fourier transform
infrared spectroscopy (FT-IR, Nicolet 6700 FT-IR spectrometer,
USA, at a resolution of 2 cmÀ1), transmission electron micro-
scopy (TEM; JEM-2010 apparatus with an acceleration voltage
of 200 kV, Japan), micro-Raman spectroscopy (LRS-5, excited
with an Ar+ laser at 532 nm, China), photoluminescence
spectroscopy (PL, Hitachi F-7000, excited with a Xe+ Lamp
at 365 nm, Japan) ultraviolet-visible (UV-Vis) diffuse reflec-
tance spectroscopy (PerkinElmer Lambda 950 spectrophoto-
meter equipped with a diffuse reflectance accessory, USA),
Brunauer–Emmett–Teller (BET) surface area analysis (Micro-
meritics ASAP 2460, USA), and electron balance (Metter Toledo
AL104, China).
2.3 Photocatalytic activity of the synthesized materials
2.3.1 Degradation of Rh-B over various ZnO samples. The
photocorrosion and suppressing electron–hole recombination, photocatalytic activity of ZnO samples was evaluated by the
thereby ultimately enhancing the photocatalytic activity of ZnO photodegradation of Rh-B aqueous solution at an initial
NPs in the process of degrading organic molecules.14–16 Based concentration of 12 mg LÀ1 under irradiation with a 36 W
on the obtained results, it is fully expected that, a higher mercury lamp (l = 365 nm). Unless noted otherwise, 50 mg of
preparation temperature up to 300 1C or even 400 1C, if catalyst powder was added to 35 mL of Rh-B solution in a quartz
the preparation conditions are permitted, might lead to the cup, corresponding to a catalyst dosage of 1.4 g LÀ1. Before UV
formation of better ZnO NPs in the degradation of organic irradiation, the whole system was placed in the dark under
wastewater. Therefore this work would provide an avenue for magnetic stirring for 30 min to ensure adsorption/desorption
commercial applications of ZnO NPs in the treatment of equilibrium. During UV irradiation, about 2 mL aliquots were
organic wastewater.
sampled at given time intervals and centrifuged. Then the top
clear solution was analyzed by recording the variations of the
absorption in the UV-Vis absorbance spectra of Rh-B. According
to the standard curve between concentration and absorption,
the value of (1 À C/C0) was calculated, which is denoted as the
degradation ratio.
2 Experimental
2.1 Materials preparation
The preparation procedure was similar to our recent report
2.3.2 Photocorrosion test of various ZnO samples. 1.000 g
except for the variations of reaction temperatures in the range of ZnO samples was added to a 35 mL quartz cup filled with
of 220–280 1C, and the replacement of Teflon-lined autoclaves water, and then was magnetically stirred under irradiation
with para polyphenol-doped Teflon-lined ones.13 In detail, at 365 nm UV (36 W). After every light irradiation for 5 h,
5.000 g of zinc powder (steel gray, 99.99%, 200 mesh, pur- the ZnO NP turbid liquid was stood for 10 min, and dried in
chased from Aladdin, seen in Fig. S1, ESI†) and a weighing an oven at 100 1C after pouring away the top clear solution.
bottle filled with 10 mL H2O were put into a 100 mL carbon The residual ZnO was weighed using an electron balance. The
doped Teflon-lined autoclave. The autoclave was sealed and photocorrosion rate of ZnO samples was calculated by
maintained at a given temperature for 10 h in an oven and then (1 À Wt)/1, where 1 is the initial dosage of the ZnO samples,
cooled to room temperature naturally. Finally, the product was and Wt is the residual weight of ZnO samples at every given
dried in an oven at 80 1C for 3 h and a colored powder (Fig. 1d) time after photocorrosion.
19224 | New J. Chem., 2019, 43, 19223--19231 This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019

View Article Online
NJC
Paper
Fig. 1 XRD patterns in the diffraction angle ranges of (a) 28–801 and (b) 30–381, FT-IR spectra (c), and photography (d) of the as-prepared ZnO samples.
3. Results and discussion
3.1 Structure, surface and optical analysis of the photocatalyst
nanoparticles along the direction of the (002) crystal plane are
112 nm, 119 nm and 130 nm, respectively. More interestingly,
3.1.1 XRD patterns, FT-IR spectra and photographs of ZnO the diffraction peaks of ZnO 250 and ZnO 280 shift to low
samples. In order to eliminate instrument error, Si powder was angles by 0.141 as compared to those of ZnO 220, showing that
added into the ZnO NPs for XRD examination, and all the there is a great difference in the surface among them. The
diffraction angles of the examined samples were calibrated by surface of the various ZnO samples was further analyzed by
using the Si(111) crystal plane as a reference. Fig. 1a and b show FT-IR spectroscopy (Fig. 1c). There is only one hump peak in
the XRD patterns of the products, in which the peaks reveal the the range of 300–1500 cmÀ1, associated with the characteristic
wurtzite bulk ZnO (JCPDS-36-1451) and the high crystalline stretching vibration mode of Zn–O bonding.19 But as far as the
degree for ZnO NPs. Moreover the I002/I100 ratio and diffraction Zn–O bonding stretching vibration is concerned, ZnO 250 and
peak intensity of the sample increase with the preparation ZnO 280 show a lower wavenumber shift by 22 cmÀ1 than ZnO
temperature, suggesting the more preferential growth along 220. It goes without saying that the position of the Zn–O
the (002) plane and a higher crystalline degree, since the high bonding stretching vibration is determined by the bond force
temperature helps in the high energy plane (002) growth of the constant of Zn–O bonding. It was reported that the existence of
hexagonal phase structure.17 And the high activity of the (002) plenty of oxygen defects would weaken the Zn–O strength,
Zn2+ face could be preferentially adsorbed by a large amount of thereby decreasing the bond force constant, and ultimately
OHÀ because of its positive charge. This would lead to a greater leading to a shift of the IR absorption peak toward low
rate of production of OHꢀ radicals, and hence enhance wavenumbers or of the diffraction peak toward low angles.20
the photocatalytic activity of ZnO.15,18 The broadening of the The absence of the absorption peak at ca. 3400 cmÀ1 (Fig. S2,
diffraction peaks is attributed to the small size effect of the ESI†), usually attributed to the normal polymeric O–H stretching
products. And the half peak width of the samples shows a little vibration of H2O, discloses that ZnO NPs absorb less water vapor
decrease along with the increase of the reaction temperature, and have a less mesoporous structure and a small surface
indicating that the size of the ZnO NPs increases with the area.12,14 Besides, their colors also change greatly (Fig. 1d).
reaction temperature. According to the Debye–Scherrer formula Along with the increase of the reaction temperature, the color of
d(hkl) = kl/(b  cos y), the mean diameters of the ZnO the products is lighter, which is useful for permeating UV light.
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019 New J. Chem., 2019, 43, 19223--19231 | 19225

View Article Online
Paper
NJC
patterns and the FT-IR spectra could be also corroborated in
the micro-Raman (Fig. 2a), PL (Fig. 2b) and UV-Vis diffuse
reflectance spectra (Fig. 3).
As shown in Fig. 2a, the peaks of the ZnO nonpolar optical
phonon high E2 mode and low E2 vibration are located at
ca. 432–434 cmÀ1 and 98 cmÀ1, respectively. And the second-
order Raman peak at 325 cmÀ1 is also seen, arising from
the Brillouin zone boundary (M point) phonon 2-E2(M).
Additionally, the 198 cmÀ1 peak is assigned to the double low
E2 vibration. The E2 mode is usually used to analyze the stress
state in films due to its high sensitivity to stress.21 It is noted
that the E2(high) mode of Zn–O bonding shifted to a lower
wavenumber along with the increase of the preparation tem-
perature, indicating that more defects such as oxygen vacancy
(V+O) exist in ZnO 250 or ZnO 280 than in ZnO 220.22 V+O in ZnO
not only led to the increased intra-tensile stress of the
products,11 but also acted as potential wells to trap electrons,
and aided in electron–hole pair separation, ultimately enhancing
the photocatalytic activity.15,23,24 Furthermore, the luminescence
properties of the products could be influenced by the oxygen
vacancy in ZnO.
In Fig. 2b, the UV emission at B383 nm corresponds to the
near band edge (NBE) emission (3.2 eV, shown in Fig. 3). The
green emission centered at 492 nm in the range of 450–550 nm
originated from the recombination of electrons with holes
trapped in singly ionized V+O, and is commonly seen in ZnO
materials synthesized under oxygen-deficient conditions.25
Moreover, the ratio of green PL intensity to NBE PL intensity
increases proportionally with the preparation temperature,
revealing more V+O content in ZnO 280 than in ZnO 250 or
ZnO 220, and higher photocatalytic activity of ZnO 280.26 This
Fig. 2 Raman (a) and room-temperature photoluminescence (b) spectra
for the as-synthesized ZnO NPs.
The color change of the products might be closely related to the is in line with the results obtained above by XRD, FT-IR and
crystalline degree of the samples. The high crystalline degree, micro-Raman techniques.
less porous structure, and small surface area of the products
are also helpful for inhibiting photocorrosion.14–16
Fig. 3 shows the UV-Vis diffuse reflectance spectra of ZnO
NPs. It is seen that, there is higher UV absorption in ZnO 280
3.1.2 Micro-Raman, PL and UV-Vis diffuse reflectance than in ZnO 250 or ZnO 220. So, the prepared ZnO 280 can fully
spectra of ZnO NPs. Undoubtedly, the surface of the products use most of the UV light among the three samples, suggesting
could have a direct effect on their optical properties. Hence that ZnO 280 could be a more ideal photocatalyst under UV
many changes in the series of the products reflected in the XRD irradiation. The absorbance ranging from 400 nm to 700 nm in
Fig. 3 UV-Vis diffuse reflectance (a) spectra, and plots of (ahg)2 vs. the energy of absorbed light (b) for the as-synthesized ZnO NPs.
19226 | New J. Chem., 2019, 43, 19223--19231 This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019

View Article Online
NJC
Paper
ZnO NPs is because of the colored product (from darker color to Furthermore, the size of the individual particulate is increasing
lighter color, see photographs of ZnO NPs in Fig. 1d). The plot along with the preparation temperature, and the hexagonal-
of (ahg)2 vs. the energy of absorbed light shows that the energy cross section pencil-like morphology of the products is easy to
of the direct allowed band gap of various ZnO NPs is 3.2 eV, observe, in good accordance with the diffraction peak broadening
which is responsible for the UV emission at B383 nm (Fig. 2b). and the preferential growth along the (002) plane in the XRD
The absorption edges of ZnO are red-shifted (bulk ZnO, 3.36 eV patterns.17 The surface of the ZnO nanoparticles is very smooth
at room temperature) probably due to the increased intra- and has less mesopores, which is beneficial for the antiphoto-
tensile stress in the particles (also proved using the Raman corrosion and quick sediment of ZnO, and thereby for the
spectrum in Fig. 2a).11
photocatalytic activity and recycling of the photocatalyst.13–16
Overall, based on the information obtained from XRD,
3.2.2 TEM images and SAED patterns. The accurate size of
FT-IR, sample colors, micro-Raman, PL and UV-Vis diffuse the ZnO NPs was further confirmed by TEM (Fig. 5). It is seen
reflectance, a conclusion was made: with the increasing that, the size of the ZnO nanoparticles (Fig. 5a, c and e) is in the
preparation temperature, the synthesized ZnO NPs exhibit a range of several tens to hundreds of nanometers, and increases
better crystalline degree, more V+O and preferential growth with the reaction temperature, which is consistent with the
along the (002) plane, which are well known to be helpful for XRD (Fig. 1a and b) and SEM analysis (Fig. 4). The ripple-like
antiphotocorrosion and the enhancement of photocatalytic contrast observed in the TEM images is due to the tensile-
activity.14–16,18,23,24,26 In this contribution, ZnO 280 is the best stress in ZnO NPs (also proved by the micro-Raman spectra in
photocatalyst.
Fig. 2a and UV-Vis diffuse reflectance spectra in Fig. 3).27 Fig. 5b,
d and f show the corresponding selected area electron diffraction
(SAED) patterns, and the diffraction spots are brighter and
brighter along with the increase of the preparation temperature,
further indicating the higher crystalline degree for ZnO 280
than ZnO 250 or ZnO 220, also in agreement with the results of
3.2 Morphological and structural observations
3.2.1 SEM images of ZnO NPs. Fig. 4 shows the SEM
images of the products without sputtering Au or Pt films onto
the surface of ZnO NPs. It is seen that many nanoparticles with
a mean size of several hundred nanometers in diameter
were aggregated to form a large particulate. Also, the sizes of
the ZnO NP particulates correspond to the used Zn powder,
indicating that most of the original Zn particulates did not
break up thoroughly into many individuals after undergoing
the strong reaction with water in the autoclave (Fig. S1, ESI†).
Fig. 4 SEM images of ZnO 220 (a and b), ZnO 250 (c and d) and ZnO
Fig. 5 TEM images of ZnO 220 (a and b), ZnO 250 (c and d) and ZnO 280
280 (e and f).
(e and f).
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019 New J. Chem., 2019, 43, 19223--19231 | 19227

View Article Online
Paper
NJC
the XRD patterns (Fig. 1a). The bright diffraction spots in the 3.3 BET surface area of the photocatalyst
SAED patterns are irregular because of the location of the electron
The surface area and pore structure of the photocatalyst are
directly related to its photocatalytic activity. Fig. 6 shows the
corresponding nitrogen adsorption/desorption isotherms and
Barrett–Joyner–Halenda (BJH) pore-size distribution curve of
the ZnO NPs. These isotherms belong to Type IV H3 loops,
revealing that the ZnO NPs are comprised of aggregates
forming slit-like mesopores.29 With the increase of the reaction
temperature, the corresponding loop and BET surface area of
the sample decrease, and the number of slit-like mesopores
of the sample increases, both revealing that the ability of
absorbing N2 becomes weaker and weaker for the sample
prepared at high temperature. Moreover, the adsorption and
desorption isotherms overlap more with increasing reaction
temperature, which shows that the multimolecular and mono-
molecular adsorptions are more reversible. This is due to the
larger size of the sample when increasing the reaction tem-
perature, though having been disclosed by XRD, SEM and TEM.
On the other hand, the surface area of the product can be
theoretically expressed using the formula SBET = 6000/dr (where
r is the density of ZnO with a value of 5.8, and d is its average
diameter of ca. 110–130 nm calculated using the Debye–
Scherrer formula and also shown in Fig. 5), since there is no
beam at the grain boundaries among several ZnO nanoparticles,28
and indicate the single crystal structure of ZnO nanoparticles.
Besides, there are no mesopores in each ZnO nanoparticle,
agreeing with the results of the FT-IR spectra (Fig. 1c) and SEM
images (Fig. 4).
Fig. 6 Nitrogen adsorption–desorption isotherm of the as-prepared ZnO
Fig. 7 Degradation ratio (a) of Rh-B over ZnO NPs under UV irradiation
NPs; the insets show the corresponding BJH pore-size distribution plots.
for different times and the corresponding kinetic study (b).
19228 | New J. Chem., 2019, 43, 19223--19231 This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019

View Article Online
NJC
Paper
porous hole in the approximately ball-shaped ZnO nanoparticles temperature (Fig. 7a), and its order corresponds to the changing
(reflected by Fig. 1c and 5). The calculated surface area of the values of the XRD diffraction peak intensity, FT-IR wavenumber
prepared product might be in the range of 7.95–9.23 m2 gÀ1
.
shift, PL spectra green light intensity, and UV absorbance. The
Therefore, the actual surface area (6.04–6.98 m2
g
À1) of the Rh-B degradation was attributed to the help of ZnO samples since
products might suggest a little agglomeration of the ZnO NP itself degradation was almost negligible under UV irradiation.11,13
powder (seen in Fig. 5a, c and e), but the agglomeration degree is Additionally, the absorbance wavelength of the degraded solution
less with the higher preparation temperature. This indicates that (Fig. S3, ESI†) did not shift during 40 min of the photocatalytic
ZnO 280 is a better dispersing agent than others in a liquid process, probably indicating that the molecule of Rh-B was
reaction system and contributes to the photocatalytic reaction.
decomposed directly into some small molecules such as CO2
and H2O, not leading to the formation of the intermediates.
The reaction kinetics was further depicted by the curve of
ln(C0/C) vs. reaction time t, and the pseudo-first-order reaction
could be inferred (Fig. 7b). The reaction rate constant (denoted
as k) over ZnO 280 (0.117) is higher than that over ZnO 250
or ZnO 220 (0.096 and 0.076), also much higher than that of
P25 (0.079).13 Furthermore, the catalytic activity of ZnO 280
increases with the catalyst loading (Fig. S4, ESI†). The optimal
catalyst dosage of ZnO 280 is as high as 14 g LÀ1, much larger
than that of P25, since the prepared ZnO in this work has a
small surface area and a high density.12 The photocatalytic
degradation efficiency was decreased when the catalyst dosage
is over 14 g LÀ1. This could be attributed to the shadowing
effect, since the high turbidity from the high concentration of
ZnO 280 scatters UV light, leading to the decrease of the
penetration depth of the UV light.12,30 Beyond these good traits
of ZnO 280, it still has other distinguishing features such as
low-cost and scalable preparation (Zn powder is used), and
convenient recycling (due to quick sedimentation). Therefore,
ZnO 280 would also have a wide application prospect in
purifying wastewater like P25.
3.4 Photocatalytic activity of various ZnO NPs
3.4.1 Photocatalytic degradation of RhB over various ZnO
NPs under UV irradiation. According to the characterization
above, ZnO 280 has some striking traits, such as plenty of
oxygen vacancies, preferential growth along the (002) direction,
and a high crystalline degree. These good characteristics make
ZnO 280 more promising in the treatment of organic waste-
water than ZnO 250 and ZnO 220. As expected, ZnO 280, though
having the smallest surface area among the three samples,
showed excellent photocatalytic activity on the degradation of
Rh-B, and the obtained result is shown in Fig. 7. It is seen that,
the photocatalytic activity was enhanced with the preparation
3.4.2 Photocorrosion test of ZnO NPs and their corres-
ponding photocatalytic activity. Fig. 8 shows the photocorro-
sion test of ZnO NPs, indicating that ZnO 280 has the lowest
photocorrosion rate among the three samples. After long-time
UV irradiation for 20 h (equal to 30 times ZnO recycles,
provided one recycle take 40 min), the diffraction peak intensity
of Zn 220 decreased more than the other two samples (not
shown), suggesting the high crystalline degree of the products
helps to enhance antiphotocorrosion. Therefore, it is possible
Fig. 8 Photocorrosion test of ZnO NPs.
Fig. 9 Degradation ratio (a) of Rh-B over photocorrosion-suffered ZnO NPs under UV irradiation for different times and the corresponding kinetic
study (b).
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019 New J. Chem., 2019, 43, 19223--19231 | 19229

View Article Online
Paper
NJC
4 D. Q. Zhang, G. S. Li and J. C. Yu, Advanced photocatalytic
nanomaterials for degrading pollutants and generating
fuels by sunlight, in Energy Efficiency and Renewable Energy
Through Nanotechnology, ed. L. Zhang, Springer, London,
2011. pp. 679–716.
5 M. A. Fox and M. T. Dulay, Heterogeneous photocatalysis,
Chem. Rev., 1993, 93, 341–357.
6 H. B. Fu, T. G. Xu, S. B. Zhu and Y. F. Zhu, Photocorrosion
inhibition and enhancement of photocatalytic activity for
ZnO via hybridization with C60, Environ. Sci. Technol., 2008,
42, 8064–8069.
7 B. Subash, B. Krishnakumar, M. Swaminathan and M. Shanthi,
Highly effcient, solar active, and reusable photocatalyst:
Zr-loaded Ag–ZnO for reactive red 120 dye degradation with
synergistic effect and dye-sensitized mechanism, Langmuir,
2013, 29, 939–949.
8 S. G. Meng, D. Z. Li, X. Z. Zheng, J. X. Wang, J. Chen,
J. L. Fang, Y. Shao and X. Z. Fu, ZnO photonic crystals with
enhanced photocatalytic activity and photostability, J. Mater.
Chem. A, 2013, 1, 2744–2747.
to synthesize well crystallized ZnO by overcoming its inherent
drawback of the photocorrosion effect.
Fig. 9 shows the degradation ratio of Rh-B over photocorrosion-
suffered ZnO NPs under UV irradiation for different times, and
the corresponding kinetic study. After suffering from 20 h of
photocorrosion, nearly equal to 30 times ZnO recycles (40 min
for one recycle, seen in Fig. 7a), ZnO 280 showed a little
decreased degradation efficiency of Rh-B when compared with
the one first used, while ZnO 250 and ZnO 220 showed a
notable decreased degradation efficiency of Rh-B (Fig. 7). This
is consistent with the decreased order of the XRD diffraction
peak intensity (Fig. S5, ESI†). In a word, ZnO 280 has more
repeated photocatalytic activity than ZnO 250 or ZnO 220
probably due to the better antiphotocorrosion.
4 Conclusions
A series of ZnO NPs were prepared in a scalable way in the
220 1C, 250 1C and 280 1C autoclaves, respectively. Along with
the increase of the preparation temperature, the ZnO NPs
showed better photocatalytic activity and antiphotocorrosion
in the degradation of rhodamine B under UV irradiation, due to
the existence of plenty of oxygen vacancies, preferential growth
of the (002) plane, and the high crystalline degree. On account
of the obtained results, it is fully speculated that, a higher
synthesis temperature up to 300 1C or even 400 1C, if permitted,
might lead to the formation of better ZnO NPs for the degrada-
tion of organic substances. Therefore this work would pave the
way to commercial applications of ZnO NPs in the treatment of
organic wastewater.
´
´
´
´
9 T. V. Tan, L. D. Rıo, T. Valdes-Solıs and G. Marban, Fabrica-
tion of wire mesh-supported ZnO photocatalysts protected
against photocorrosion, Appl. Catal., B, 2013, 140–141,
189–198.
10 L. P. Xu, Y.-L. Hu, C. Pelligra, C.-H. Chen, L. Jin, H. Huang,
S. Sithambaram, M. Aindow, R. Joesten and S. L. Sui, ZnO
with different morphologies synthesized by solvothermal
methods for enhanced photocatalytic activity, Chem. Mater.,
2009, 21, 2875–2885, and references therein.
11 Y. Y. Lv, L. S. Yu, H. Y. Huang, Y. Y. Feng, D. Z. Chen and
X. Xie, Application of soluble salt-assisted route to scalable
synthesis of ZnO nanopowders with repeated photocatalytic
activity, Nanotechnology, 2012, 23, 065402.
Conflicts of interest
12 Y. Y. Lv, L. S. Yu, H. Y. Huang, H. L. Liu and Y. Y. Feng,
Preparation, characterization of P-doped TiO2 nanoparticles
and their excellent photocatalystic properties under the
solar light irradiation, J. Alloys Compd., 2009, 488, 314–319.
13 Y. Y. Lv, L. S. Yu, C. G. Li and L. S. Yang, ZnO nanopowders
and their excellent solar light/UV photocatalytic activity on
degradation of dye in wastewater, Sci. China: Chem., 2016,
59, 142–149.
14 M. Pudukudy, A. Hetieqa and Z. Yaakob, Synthesis, char-
acterization and photocatalytic activity of annealing depen-
dent quasi spherical and capsule like ZnO nanostructures,
Appl. Surf. Sci., 2014, 319, 221–229.
There are no conflicts to declare.
Acknowledgements
This work was supported by the NSFC (21663021 and 21561028),
NSFJXP (2017ACB20040), the JX Education Department Science
Fund (GJJ 180867) and the National College Students’ Innovation
& Entrepreneurship Training Program (2019-CX-05).
References
1 M. M. Khin, A. S. Nair, V. Babu, R. Murugan and 15 A. Mclaren, T. Valdes-Solis, G. Q. Li and S. C. Tsang, Shape
S. Ramakrishna, A review on nanomaterials for environmental
and size effects of ZnO nanocrystals on photocatalytic
remediation, Energy Environ. Sci., 2012, 5, 8075–8109.
activity, J. Am. Chem. Soc., 2009, 131, 12540–12541.
2 S. H. S. Chan, T. Y. Wu, J. C. Juan and C. Y. Teh, Recent 16 G. R. Li, T. Hu, G. L. Pan, T. Y. Yan, X. P. Gao and H. Y. Zhu,
developments of metal oxide semiconductors as photocata-
lysts in advanced oxidation processes (AOPs) for treatment
of dye waste-water, J. Chem. Technol. Biotechnol., 2011, 86,
1130–1158.
3 I. Ullah, S. Ali, M. A. Hanif and S. A. Shahid, Nanoscience
for environmental remediation: a review, Intern. J. Chem.
Biochem. Sci., 2012, 2, 60–77.
Morphology-function relationship of ZnO: polar planes,
oxygen vacancies, and activity, J. Phys. Chem. C, 2008, 112,
11859–11864.
17 C. Liu, Z. Hu, Q. Wu, X. Z. Wang, Y. Chen and J. M. Zhu,
Controllable synthesis of one-dimensional aluminum
nitride nanostructures through vapor-solid epitaxial growth,
J. Nanoelectron. Optoelectron., 2006, 1, 114–118.
19230 | New J. Chem., 2019, 43, 19223--19231 This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019

View Article Online
NJC
Paper
18 A. Dodd, A. Mckinley, T. Tsuzuki and M. Saunders, Tailoring the 24 L. Guo, Q. Zhao, H. D. Shen, X. X. Han, K. L. Zhang,
photocatalytic activity of nanoparticulate zinc oxide by transition
metal oxide doping, Mater. Chem. Phys., 2009, 114, 382–386.
19 Q. Perales-Percz, A. Parra-Palomino, R. Singhal, Y. Voyles,
Y. Zhu, W. Jia and M. S. Tomar, Evidence of ferromagnetism
D. J. Wang, F. Fu and B. Xu, Ultrafine Au nanoparticles
anchored on Bi2MoO6 with abundant surface oxygen
vacancies for efficient oxygen molecule activation, Catal.
Sci. Technol., 2019, 9, 3193–3202.
in Zn1ÀxMxO (M = Ni, Cu) nanocrystals for spintronics, 25 L. E. Greene, M. Law, J. Goldberger, F. Kim, J. C. Johnson,
Nanotechnology, 2007, 18, 315606.
20 Y. H. Zheng, C. Q. Chen, Y. Y. Zhan, X. Y. Lin, Q. Zheng,
K. M. Wei, J. F. Zhu and Y. J. Zhu, Luminescence and
Y. F. Zhang, J. R. Saykally and P. D. Yang, Low-temperature
wafer-scale production of ZnO nanowire arrays, Angew.
Chem., Int. Ed., 2003, 42, 3031–3034.
photocatalytic activity of ZnO nnaocrystals: correlation between 26 M. Pudukudy and Z. Yaakob, Simple chemical synthesis of
structure and property, Inorg. Chem., 2007, 46, 6675–6682.
21 F. Decremps, J. P. Porres, A. M. Saitta, J. C. Chervin and
novel ZnO nanostructures: Role of counter ions, Solid State
Sci., 2014, 30, 78–88.
A. Polian, High-pressure Raman spectroscopy study of 27 Z. W. Pan, Z. R. Dai and Z. L. Wang, Nanobelts of semi-
wurtzite ZnO, Phys. Rev. B: Condens. Matter Mater. Phys.,
2002, 65, 092101.
22 Y. Zhang, H. B. Jia, R. M. Wang, C. P. Chen, X. P. Luo,
D. P. Yu and C. Lee, Low-temperature growth and Raman
conducting oxides, Science, 2001, 291, 1947–1949.
28 K. Kim, Z. Lee, W. Regan, C. Kisielowski, M. F. Crommie
and A. Zettl, Grain boundary mapping in polycrystalline
graphene, ACS Nano, 2011, 5, 2142–2146.
scattering study of vertically aligned ZnO nanowires on Si 29 M. Kruk and M. Jaroniec, Gas adsorption characterization
substrate, Appl. Phys. Lett., 2003, 83, 4631–4633.
of ordered organic-inorganic nanocomposite Materials,
23 D. J. Wang, H. D. Shen, L. Guo, C. Wang, F. Fu and Y. C.
Chem. Mater., 2001, 13, 3169–3183.
Liang, Ag/Bi2MoO6Àx with enhanced visible-light-responsive 30 Q. Xiao, J. Zhang, C. Xiao, Z. C. Si and X. K. Tan, Solar
photocatalytic activities via the synergistic effect of surface
oxygen vacancies and surface plasmon, Appl. Surf. Sci., 2018,
436, 536–547.
photocatalytic degradation of methylene blue in carbon-
doped TiO2 nanoparticles suspension, Sol. Energy, 2008, 82,
706–713.
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019 New J. Chem., 2019, 43, 19223--19231 | 19231