Efficient photocatalytic degradation of dyes using photo-deposited Ag nanoparticles on ZnO structures: simple morphological control of ZnO

Article abstract of DOI:10.1039/d0ra10945b

In this work, morphology-controlled ZnO structures were preparedviaa hydrothermal method by simple adjustments in the NaOH concentration. The NaOH concentration variation from 0.2 to 1.2 M resulted in the formation of ZnO structures in shapes such as walnut, spherical flower, flower, rod, and urchin-like. The extent of OH^?ions is the main factor influencing the growth of ZnO structures. Well-defined morphologies, good crystallinity, and optical properties were obtained for all ZnO structures. Among these ZnO structures, ZnOsf (spherical flower-like) structure showed a greater percentage of photodegradation of methyl orange and rhodamine B dyes. Surface plasmon resonance was achieved by modifying the surface of ZnO with Ag nanoparticles. ZnOsf was loaded with Ag nanoparticles by a facile photo-deposition method. Ag-ZnOsf showed superior photoactivity and recyclability for the degradation of methyl orange and rhodamine B. Therefore, modification of different ZnO structures can help realize potential catalysts for future environmental applications.

Full text of DOI:10.1039/d0ra10945b

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Efficient photocatalytic degradation of dyes using
photo-deposited Ag nanoparticles on ZnO
Cite this: RSC Adv., 2021, 11, 8709
structures: simple morphological control of ZnO
Hyeonhan Lim,†a Mohammad Yusuf, †a Sehwan Song,b Sungkyun Parkb
a
*
and Kang Hyun Park
In this work, morphology-controlled ZnO structures were prepared via a hydrothermal method by simple
adjustments in the NaOH concentration. The NaOH concentration variation from 0.2 to 1.2 M resulted in the
formation of ZnO structures in shapes such as walnut, spherical flower, flower, rod, and urchin-like. The
extent of OHꢀ ions is the main factor influencing the growth of ZnO structures. Well-defined
morphologies, good crystallinity, and optical properties were obtained for all ZnO structures. Among
these ZnO structures, ZnOsf (spherical flower-like) structure showed
a greater percentage of
photodegradation of methyl orange and rhodamine B dyes. Surface plasmon resonance was achieved by
modifying the surface of ZnO with Ag nanoparticles. ZnOsf was loaded with Ag nanoparticles by a facile
photo-deposition method. Ag–ZnOsf showed superior photoactivity and recyclability for the degradation
of methyl orange and rhodamine B. Therefore, modification of different ZnO structures can help realize
potential catalysts for future environmental applications.
Received 31st December 2020
Accepted 5th February 2021
DOI: 10.1039/d0ra10945b
rsc.li/rsc-advances
heterogeneous photocatalysts, such as TiO2, ZnS, and ZnO, have
received signicant interest for the renewable and sustainable
Introduction
harnessing of solar energy in order to solve energy issues and
environmental problems.7–12 Among these photocatalysts, ZnO
is an excellent semiconductor owing to its unique properties
such as high photosensitivity, high electron mobility, and good
physical and chemical stabilities.13,14 The synthesis and photo-
chemical reactions of ZnO have been actively studied. However,
the efficiency of ZnO is still limited due to the rapid recombi-
nation of electron–hole pairs,12,13 and the activity in the visible
region is considerably lower than that in the ultraviolet
region.15,16
In photocatalytic reactions, the morphology of the catalyst
can dramatically affect the properties and performance of the
catalyst. A considerable number of morphology-controlled TiO2
photocatalysts have been developed with various designs,
including spheres, nanosheets, nanotubes, 3-D sea urchin-like,
and 3-D hierarchically porous structures; these photocatalysts
have been proposed and utilized in photocatalytic air purica-
tion. In addition, various morphologies of CuO obtained using
the sonochemical method have been applied for studying the
photocatalytic activity of MB. However, the control of the
morphology was performed in the presence of different
surfactants. Mesoporous Cu2O prepared through a morphology-
control approach by the addition of reductants and surfactants
was also studied in the photocatalytic degradation of
MO.8,11,17–19 In addition, the presence of a base is one of the
factors triggering the formation of the nal structure. The
diverse morphologies of ZnO were obtained by controlling
Currently, excessive water contamination due to the discharge
of organic dyes through industrial wastewater, primarily from
the processing of products such as fabrics, pharmaceuticals,
petrochemicals, foodstuffs, cosmetics, and leather, has become
a severe problem given the importance of clean water in
preserving life on Earth. The exposure of dyes in a small level of
less than 1 mg Lꢀ1 can seriously affect the water quality of the
aquatic environment. Methyl orange (MO), rhodamine B (RhB),
and methylene blue (MB) are commonly used toxic, mutagenic,
and non-biodegradable azo dyes hazardous to aquatic
environments.1–4
As an eco-friendly and sustainable treatment process, pho-
tocatalysis has been demonstrated to have considerable
potential for the removal of dyes from wastewater.3,5,6 Further-
more, the development of photocatalysts is considered a prom-
ising prospect for future methods for generating renewable
energy using abundantly-available and inexpensive natural
sunlight radiation.
Photocatalysis is related to the reactions induced by
ultraviolet-visible (UV-Vis) light incident on the surface of
semiconductors. This reaction generates electrons and holes,
which act as charge carriers. Semiconductor-based
aDepartment of Chemistry, Chemistry Institute for Functional Materials, Pusan
National University, Busan, 46241, Republic of Korea. E-mail: chemistry@pusan.ac.kr
bDepartment of Physics, Pusan National University, Busan, 46241, Republic of Korea
† These authors contributed equally to this work.
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different bases in the presence of surfactant or capping agent by 12 h. The reaction mixture was naturally cooled, and the ZnO
various methods.20,21 For instance, ZnO nanorods and nano- powder was washed with deionized water and ethanol. The
ꢁ
ower morphologies were synthesized by the inuence of product was then dried in an oven at 60 C for 12 h.
ammonia. ZnO (owerlike, spindle-like, sword-like, and
umbrella-like) morphologies are formed by controlling NaOH
and ethanolamine. ZnO (egg noodle and 2D plate-like) struc-
tures were obtained by sodium citrate and urea.22–24 NaOH is the
most popular base since NaOH is easily ionized to Na+ and OHꢀ
in an aqueous solution and increases the solubility of Zn(OH)2
related to the formation [Zn(OH4)]2ꢀ complex leading to a high
yield of ZnO product.21,25 Aer all, dening the correlation
between the morphology and photocatalytic behavior of cata-
lysts remains a challenge. The same variables mentioned above
can be used to study the effect of morphology on the photo-
catalytic efficiency of ZnO structures.
Synthesis of Ag–ZnOsf composite
Ag–ZnOsf composite was synthesized using the photoreduction
method. First, 0.2 g of ZnO was dispersed in 35 mL of water and
20 mL of methanol solution. Thereaer, 5 mL of AgNO3 solution
(7.65 mM) was added. The mixture was initially stirred with
a magnetic bar (400 rpm, 25 ꢁC, 30 min) to allow the absorption
of Ag+ on the surface of ZnO. Then, the suspension was irradi-
ated with a Xe lamp for 2 h. The Xe lamp was equipped with
a liquid lter to prevent temperature rise by light. The experi-
ments were conducted at pH ¼ 7.8, 25 ꢁC. The precipitates were
centrifuged and washed with deionized water and ethanol. The
Another method to improve photocatalytic performance is by
loading the surface of the catalyst with noble metals such as Pd,
Pt, Au, and Ag.26–31 Comparative experiments using Ag/TiO2, Au/
TiO2, and Pt/TiO2 showed similar good photocatalytic activity
between Ag/TiO2 and Pt/TiO2. However, Ag is more abundant
and economical than Au and Pt. Therefore, using Ag would be
more eco-friendly and easier to apply in the industry.32–38 The
deposited Ag nanoparticles (Ag NPs) on the photocatalyst
surface can effectively suppress the recombination of photo-
generated electron–hole pairs due to the Schottky barrier
formed at the interface between the metal and semiconductor.
Moreover, surface plasmon resonance (SPR) of Ag NPs signi-
cantly improves the absorption of visible light.39–46 Thus,
modifying ZnO with Ag can improve the photocatalytic activity
of ZnO by altering its optical and electronic properties.
In this work, we present a simple method for the fabrication
of ZnO via morphology control without the addition of
a surfactant or capping agent. Furthermore, ZnO was loaded
with Ag NPs through a convenient photo-deposition method
under light irradiation. The photo-deposition reduction
approach only involves irradiation with a light source, unlike
most other techniques that require high temperatures, external
redox agent, electrical potential, or multi-step operation.47,48
The composite Ag–ZnO structure was implemented as a photo-
catalyst for the degradation of various dyes such as MO and
RhB.
ꢁ
product was then dried in an oven at 60 C for 12 h.
Photocatalytic activity
The photocatalytic activity of the as-synthesized catalyst was
estimated based on the photocatalytic degradation of MO and
RhB. First, 40 mg of the photocatalyst was dispersed in 80 mL of
an aqueous organic dye solution (10 mg Lꢀ1). The solution was
stirred in dark for 30 min to reach the adsorption–desorption
equilibrium. The mixture was then exposed to UV-Vis irradia-
tion using a Xe lamp. The degradation of the organic dye was
monitored using UV-Vis spectroscopy aer withdrawing and
centrifuging the suspension. The pH of dyes was 8.8 for RhB
and 8.7 for MO. The initial concentration of the dye used was
maintained at 10 mg Lꢀ1 based on previous work, which indi-
cated that ZnO catalysts were suitable at a relatively wide
concentration of pollutants and the best efficiency was shown at
10 mg Lꢀ1 of MO concentration.49 The efficiency of MO and RhB
degradation was determined by applying eqn (1).
ꢀ
ꢁ
C0 ꢀ C
The degradation efficiency ð%Þ ¼
ꢂ 100% (1)
C0
where C is the concentration at the irradiation time t, and C0
represents the original concentration.
Material characterization
The morphologies of the samples were analyzed using high-
resolution scanning electron microscopy (SEM) on a ZEISS
SUPRA 40VP instrument (Carl ZEISS, Germany) and eld-
emission transmission electron microscopy (FE-TEM) on
a TALOS F200X (200 keV) instrument (FEI, Netherlands). The
TEM instrument was combined with energy-dispersive X-ray
spectroscopy (EDX) elemental mapping analysis. Crystal struc-
tures in the synthesized samples were studied using X-ray
diffraction (XRD) on an Xpert3 (Malvern Panalytical Ltd.,
Netherlands) diffractometer with Cu-Ka radiation at l ¼
0.15406 nm. N2 sorption isotherms were constructed from the
Experimental
Chemicals
Zinc nitrate hexahydrate (Zn(NO3)2$6H2O, 99.0%), sodium
hydroxide (NaOH, 99.0%), silver nitrate(ii) (AgNO3, 99.9%), MO,
and RhB were purchased from Sigma-Aldrich. Ethanol (99.9%)
and methanol were procured from Samchun Company.
Synthesis of ZnO structures
Zn(NO3)2$6H2O (5 mmol) was dissolved in 20 mL of deionized data acquired at ꢀ196 ꢁC using a Micromeritics ASAP 2020
water. Thereaer, 50 mL of NaOH solution (0.2, 0.6, 0.8, 0.9, or surface area analyzer (Micromeritics Instrument Corp., US).
1.2 M) was added to the Zn(NO3)2 solution. Aer stirring for 1 h Before analysis, samples were degassed under N2 ow at 150 ꢁC
(400 rpm, 25 ꢁC), the mixture was transferred to a 120 mL for 12 h. X-ray photoelectron spectroscopy (XPS) was utilized to
ꢁ
Teon-lined stainless-steel autoclave and heated at 100 C for investigate the surface chemical properties using an
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ESCALab250-AXIS SUPRA instrument (Kratos Analytical Ltd.,
UK). The optical properties were studied using UV-Vis diffuse
reectance spectroscopy (UV-Vis DRS) with V-770 (JASCO Corp.,
Japan) and photoluminescence (PL) spectroscopy with F-7000
spectrouorometer (Hitachi, Japan). The photodegradation of
MO and RhB was monitored using acquired UV-Vis absorption
spectra UV-1800 instrument (Shimadzu, Japan).
Results and discussion
Scheme 2 Synthesis of Ag–ZnO composite by photo-deposition
method.
Scheme
1 shows the method used for controlling the
morphology of ZnO. ZnO catalysts with different shapes and
sizes were prepared via a simple hydrothermal method, without
the addition of any stabilizers, such as surfactants, polymers,
and other functional molecules. Several parameters, including
time, temperature, the concentration of precursors, the addi-
tion of surfactants, and solvents, can affect the morphology of
materials. Here, the NaOH at the concentration varied from 0.2
to 1.2 M was only used to control the growth of ZnO particles.
Ag–ZnOsf was synthesized using a photo-deposition process
with AgNO3 solution as the source of Ag NPs (Scheme 2). The
reduction of Ag was realized using the incident light from Xe
lamp, which is a one-step process under mild conditions.
The morphologies of the as-prepared ZnO samples were
initially analyzed using SEM. Fig. 1 displays SEM images of ZnO
with different shapes and sizes, formed by varying the NaOH
concentration in the aqueous solution. As shown in Fig. 1(a)
and (b), ZnO walnuts were formed when using 0.2 M NaOH. The
diameter of these ZnO walnuts was approximately 360 nm.
Fig. 1(c) and (d) shows the spherical ower-like ZnO structure
produced with 0.6 M NaOH. The diameter of the spherical
ower-like ZnO structure was approximately 3.1 mm.
Fig. 1 SEM images of various ZnO structures (a and b) walnut, (c and d)
spherical flower, (e and f) flower, (g and h) rod, (i and j) urchin.
Scheme 1 Synthetic procedure for various ZnO structures.
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Furthermore, when the concentration of NaOH was increased to
0.8 M, the morphology of ZnO changed to a ower-like struc-
ture, with a diameter of approximately 4.6 mm, as shown in
Fig. 1(e) and (f). Rod-like (380 nm) and urchin-like ZnO (4.8 mm)
structures (Fig. 1(g) and (j)) were obtained with 0.9 M and 1.2 M
NaOH, respectively.
The rate of the chemical reaction and the concentration of
Zn2+ and OHꢀ ions are the main factors inuencing the growth
of ZnO structures. The reactions for the synthesis of standard
ZnO nanostructures can be described as follows:50–52
Zn(NO3)2$6H2O + 2NaOH / Zn(OH)2 + 2NaNO3 + 6H2O (2)
Zn(OH)2 + 2H2O / [Zn(OH)4]2ꢀ + 2H+
[Zn(OH)4]2ꢀ / ZnO + H2O + 2OHꢀ
(3)
(4)
As sodium hydroxide is added to the zinc nitrate solution in
the initial reaction, zinc hydroxide (Zn(OH)2), sodium nitrate
(NaNO3), and water molecules are formed. The number of OHꢀ
ions in the solution (from NaOH) is a factor governing the
length and width of the structures, as shown in Fig. 1. As molar
concentration increases, the density of OHꢀ ions increases,
which leads to a signicant growth rate and the generation of
petal-like structures from walnut to spherical ower-like,
ower-like, and rods. ZnO structure is a hexagonal unit cell
ꢀ
consisting of polar (0001) low-index face with negative charge as
oxygen-terminated and polar 0001 faces with positive charge as
zinc-terminated sites, that stimulates a net dipole moment
through the (0001) axis (c-axis).53,54 At low NaOH (0.2 M)
concentration, ZnO walnuts were formed as the OHꢀ ion
concentration was too low to allow anisotropic growth. There-
fore, ZnO nuclei-twining aggregate particles were formed since
the [Zn(OH4)]2ꢀ was still inadequate and there was a lack of
active sites. Increasing the amount of OHꢀ ions leads to
increasing [Zn(OH)4]2ꢀ. According to the reaction process (eqn
(2)–(4)), ZnO nuclei were obtained by the dehydration of
[Zn(OH)4]2ꢀ ions. During the crystal growth, the oriented
attachment of nanoclusters was formed to minimize the total
surface energy. Then, along with the aggregation of ZnO
nanoclusters, the growth of nanosheets was generated, result-
ing in the formation of spherical ower-like morphology. The
aggregated nanoclusters could rearrange themselves along to
the c-axis due to ZnO's intrinsic anisotropic character and
decrease the nanosheet concentration to form rod or ower-like
morphology. Further increase of OHꢀ ions, causes the
[Zn(OH)4]2ꢀ ion to absorb by an electrostatic force in the posi-
tive polar plane of (0001), which enables the crystal to grow
anisotropically in (0001) direction to obtain the rod
morphology. However, with a further increase in NaOH
concentration, the solution becomes saturated, leading to
agglomeration, which indicates that rods begin to accumulate
and form an urchin-like morphology.53,55,56
Fig. 2 XRD patterns of (a) various ZnO structures and (b) Ag–ZnOsf
composite.
were observed at 2q ¼ 32.1ꢁ, 34.8ꢁ, 36.6ꢁ, 47.8ꢁ, 56.9ꢁ, 63.1ꢁ,
68.2ꢁ, and 69.3ꢁ, which were attributed to the (100), (002), (101),
(102), (110), (103), (112), and (201), crystalline planes, respec-
tively, which corresponds to the hexagonal phase of ZnO (PDF#
36-1451). The XRD patterns of other ZnO materials are pre-
sented in the upper panel in Fig. 2, and the patterns were
similar to that of ZnOsf, with slight differences in the peak
intensity. Moreover, the diffraction peaks for Ag–ZnOsf at 2q ¼
38.1ꢁ, 44.3ꢁ, and 64.5ꢁ were attributed to the (111), (200), and
(220) crystalline planes, respectively, which correspond to the
face-centered cubic (fcc) structure of Ag (PDF# 04-0783). Peaks
corresponding to other impurities were not detected. The Ag
crystallite size calculated from (111) faces by Scherrer equation
was 14.51 nm. Furthermore, to collect additional information
regarding the morphology, TEM studies were performed. Fig. 3
shows the structure of Ag NPs photo-deposited on ZnOsf, reveal
that Ag was loaded on the surface of ZnO. EDX-mapping was
also used to conrm the presence and distribution of Zn, O, and
Ag (Fig. 3(c)–(f)). The Fourier-transform (FT) patterns of Ag–
ZnOsf are shown in Fig. 3(g) and (h) (inset). The line spacing of
approximately 0.242 nm, as shown in Fig. 3(g) inset, corre-
sponds to the (101) lattice plane of ZnO, whereas the line
spacing of approximately 0.240 nm, as shown in Fig. 3(h) inset,
corresponds to the (111) lattice plane of the face-centered cubic
The crystalline structures of the synthesized materials were
characterized using XRD. Fig. 2 shows the XRD patterns of the
ve types of ZnO morphologies (Fig. 2(a)) and Ag–ZnOsf
(Fig. 2(b)) with a specic Ag particle. Diffraction peaks of ZnOsf
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Fig. 4 (a) XPS spectra of Ag–ZnOsf composite (a) survey, (b) Zn 2p
lines, (c) O 1s lines and (d) Ag 2p lines.
The surface structures were determined based on N2 sorp-
tion isotherms shown in Fig. 5(a). N2 adsorption/desorption
isotherms of the as-prepared ZnO materials exhibited a typical
type IV pattern for ZnOsf, ZnOw, ZnOr, ZnOf, indicative of
mesoporous characteristics59 and typical type III pattern for
ZnOu indicating a microporous material.59,60 The BET surface
areas of various ZnO types were in the following order: ZnOsf
(13.45 m2 gꢀ1) > ZnOw (10.46 m2 gꢀ1) > ZnOr (3.44 m2 gꢀ1) >
ZnOf (1.59 m2 gꢀ1) > ZnOu (0.93 m2 gꢀ1).
UV-Vis DRS was used to evaluate the optical properties and
determine light absorption characteristics of various ZnO
materials and aer loading Ag NPs, as shown in Fig. 5(b). All the
ZnO structures absorbed light in the ultraviolet to visible
Fig. 3 (a) SEM, (b) TEM, (c–f) EDX mapping, (g and h) HR-TEM images
of Ag–ZnOsf composite. (Inset) FT patterns of ZnO and Ag.
Ag structure. These observations are in good agreement with the
XRD pattern shown in Fig. 2(b).
Furthermore, XPS analysis was performed to analyze the
chemical composition of the Ag–ZnOsf surface. Fig. 4(a) pres-
ents the survey spectrum, showing signals of Ag, Zn, O, and C.
The atomic ratio of Zn, O, and Ag were 58.43, 38.61 and 2.96%,
respectively. The Zn 2p spectrum exhibits two main peaks of the
Zn 2p1/2 and Zn 2p3/2 states (Fig. 4(b)). The binding energy (BE)
peak of Zn 2p3/2 at 1020.85 eV is attributed to Zn–O bonds in
ZnO. The O 1s spectrum (Fig. 4(c)) indicates peaks of Zn–O
bonds (529.6 eV), and Zn–OH bonds (531.2 eV).57 The Ag 3d
spectrum (Fig. 4(d)) shows a BE peak of Ag 3d5/2 at 366.92 eV,
which is at a lower BE than that of the Ag metal (368.20 eV) and
Ag2O (367.2 eV). This peak is attributed to the interaction
between Ag NPs and ZnO.58 In addition, the XRD pattern only
indicates the signals of Ag, which conrms the presence of
metallic Ag.
Fig. 5 (a) N2 adsorption/desorption isotherms of various ZnO struc-
tures, (b) UV-Vis DRS of (c) the calculated band gap energy of various
ZnO structures and Ag–ZnOsf (d) PL spectra for ZnOsf and Ag–ZnOsf
composite.
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regions. Loading ZnOsf with Ag NPs gives an additional
absorption peak in the visible region (410–550 nm), indicating
the presence of Ag SPR. The calculated band-gap energies
(Fig. 5(c)) of the as-prepared ZnO samples, i.e., ZnOw, ZnOsf,
ZnOf, ZnOr, and ZnOu are 3.19, 3.09, 3.08, 3.17, and 3.05 eV,
respectively. The measured band-gap of as-synthesized ZnO is
less than that of the commercial ZnO (3.37 eV).61 The loading of
Ag as SPR on ZnOsf shied the band gap energy to low energy
(3.07 eV) indicating the suppression of the recombination of
photogenerated electron–hole pairs due to the improvement of
the absorption of the visible light.39–41
PL spectra were also acquired to determine the charge
recombination in Ag–ZnOsf relative to the ZnOsf. The PL
spectra of ZnOsf and Ag–ZnOsf (Fig. 5(d)) were acquired at an
excitation wavelength of 320 nm. The emission peaks were
observed at 527, 470, and 417 nm. For Ag–ZnOsf, the intensity of
the absorption peak declined signicantly, indicating lower
recombination of the electron–hole pairs, as compared to that
in bare ZnOsf. Thus, the lifetime of the photogenerated charge
carriers is improved with low photorecombination, which is
expected to improve the photocatalytic activity of Ag–ZnOsf.
The photocatalytic activity of various ZnO structures was
studied based on the degradation of MO and RhB, which were
used as model dyes. The reaction under UV-Vis light irradiation
using a Xe lamp at room temperature was investigated, and the
decrease in the concentration of the dye was monitored using
a UV-Vis spectrometer. The intensity of the maximum absorp-
tion peak of MO (464 nm) and RhB (554 nm) was monitored
over time. Fig. 6(a) and (b) shows that the absorbance of MO
and RhB decreased by approximately 99.7% and 94.2%,
respectively, in the presence of ZnOsf aer 50 min. The
absorption without catalysts (Fig. 6(c) and (d)) was also explored
to understand the role of ZnO during the photodegradation of
the dyes.
Fig. 7 The degradation rate by time of various ZnO structure for (a and
b) MO, (c and d) RhB.
photodegradation of MO and RhB were constructed, as shown
in Fig. 7(a) and (c). The percentage degradation afforded by
ZnOsf, ZnOf, ZnOr, ZnOw, and ZnOu was estimated to be 99.7,
88.0, 80.8, 74.0, and 59.3% for MO and 94.2, 92.1, 90.8, 88.6, and
69.7% for RhB, respectively. The overall activity for MO and RhB
degradation was in the following order: ZnOsf > ZnOf > ZnOr >
ZnOw > ZnOu (Fig. 7). The kinetics of the photodegradation of
MO and RhB can be dened by the following equation: ln(C0/C)
¼ kt, where k is the rst-order kinetics constant, and t is the
duration of irradiation (Fig. 7(b) and (d)). The R2 value shows
more than 0.9 for MO and RhB degradation for ve types of the
as-synthesized ZnO structures. The
k values for ZnOsf
(0.1104 minꢀ1 (MO) and 0.0542 minꢀ1 (RhB)) were the largest,
Moreover, to determine which ZnO structure afforded the
optimal photocatalytic performance, C/C0 plots for the
Fig. 6 UV-Vis spectra degradation of (a) MO, (b) RhB using ZnOsf and Fig. 8 The degradation rate comparison between Ag–ZnOsf and
(c and d) comparison without the catalyst.
ZnOsf for degradation of (a and b) MO and (c and d) RhB.
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degradation percentages (MO: Ag–ZnOsf ¼ 99.3% > ZnOsf ¼
93.0%; RhB: Ag–ZnOsf ¼ 99.7% > ZnOf ¼ 80.4%). The higher
percentage degradation of the dyes is proportional to the k
value.
The ideal photocatalytic performance of Ag–ZnOsf under
incident UV-Vis light is due to the Schottky barrier between Ag
and ZnO, which facilitates charge separation (Fig. 9). When Ag–
ZnOsf undergoes induction under the incident light, electrons
are excited from the valence band (VB) to the conduction band
(CB). The excited electrons are quickly transferred to the Ag
nanoparticles to prevent the recombination of electrons and
holes; thus, the activity and lifetime of the catalyst increase. The
electrons transferred to VB adsorb oxygen to form superoxide
radical anions (O2cꢀ), which can react with H2O to supply
hydroxyl radicals (cOH). Simultaneously, VB, as a source of
holes, interacts with H2O to produce cOH radicals. These
generated O2cꢀ and cOH species attack the dyes (MO and RhB)
and produce H2O and CO2 to complete the degradation
process.8,63,64 The recyclability of the Ag–ZnOsf composite for
the decomposition of dyes was determined based on the 10-
cycle degradation of MO. The photocatalyst was used aer
separation from the medium and was washed with ethanol for
each cycle. The photocatalytic efficiency of Ag–ZnOsf slightly
decreased for the 5th run and continued to 80% aer the 10th
run (Fig. 10). The comparison for different catalysts based on
ZnO and Ag using various methods for several dyes is displayed
Fig. 9 Probable mechanism for the reaction of Ag–ZnOsf composite
for the degradation of dyes.
as compared to those of the other structures, indicating that
ZnOsf achieves better photodegradation performance. The
higher photocatalytic activity of ZnOsf, as compared to those of
ZnOf, ZnOr, ZnOw, and ZnOu, can be attributed to the higher
surface area of ZnOsf. On the other hand, the surface
morphology of the ZnOsf structure might also affect the cata-
lytic activity because of the improved light absorption efficiency.
Based on the structural tuning of MoS2 for the photo-
degradation of MB, it was found that MoS2 ower-like struc-
tures possess additional light paths that improve the light
absorption efficiency, as compared to spherical and coil-like
structures.62 The ower-like structure (ZnOsf and ZnOf) have
a small band gap compared with the other structures indicating
that they have more light absorption efficiency. However, even
though ZnOu has a small band gap that is close to that of the
ower-like structure, the photocatalytic activity is low due to the
lower surface area. Besides, despite ZnOw having the second
highest surface area, the photocatalytic activity is relatively low
due to the highest bandgap energy. Therefore, the combination
of surface area and light absorption efficiency produces the
synergetic effect to impact the photodegradation performance.
Furthermore, the SPR of Ag NPs loaded on ZnOf was also
studied, indicating that the composite exhibited a signicantly
improved absorption of visible light. Fig. 8 shows the photo-
catalytic efficiency and kinetics of dye degradation by ZnOsf
compared to those of Ag–ZnOsf. The Ag–ZnOsf composite
degraded both MO and RhB within 35 min, with higher
Fig. 10 Recyclability test of Ag–ZnOsf composite with 10th cycles.
Table 1 Comparison of the photocatalyst performance with previous worka
Dyes/conc.
(mg Lꢀ1
Irra. time
(min)
Deg. rate
(%)
Photocatalyst
Ag–ZnOsf
Synthesis method
Cat. (mg)
40
)
Ref.
Hydrothermal, photodeposition
Microwave
MO (10)
RhB (10)
RhB (10)
4-NP (20)
MB (10)
RhB (8)
35
99.7
99.3
99
95
96
100
99.3
81.6
This work
42
Ag–ZnO ower like
150
20
80
120
180
80
Ag–ZnO heterostructure
Ag–ZnO 3D ower like
Ag–TiO2 NPs
Ball mill
10
40
500
50
43
44
45
46
Precipitation (PVP template)
Sol–gel, vapor evaporation
Microwave, borohydride reduction
MO (0.5)
CR (16)
Ag–ZnO rod
60
a
Cat.: catalyst, Conc.: concentration, Irra.: irradiation, Deg.: degradation, MB: methylene blue, 4-NP: 4-nitrophenol, and CR: congo red.
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´
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Conflicts of interest
20 S. Baruah and J. Dutta, Sci. Technol. Adv. Mater., 2009, 10(1),
18.
There are no conicts to declare.
21 J. Wojnarowicz, T. Chudoba and W. Lojkowski,
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22 X. Wang, Q. Zhang, Q. Wan, G. Dai, C. Zhou and B. Zou, J.
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Acknowledgements
This research was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF) grant 23 V. Gerbreders, M. Krasovska, E. Sledevskis, A. Gerbreders,
funded by the Korea Government (MSIP) (NRF-
2020R1I1A3067208, 2018R1D1A1B07045663, and NRF-
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