Fabrication of ZnO, ZnS, Ag-ZnS, and Au-ZnS microspheres for photocatalytic activities, CO oxidation and 2-hydroxyterephthalic acid synthesis

Article abstract of DOI:10.1016/j.jallcom.2016.03.070

This paper reports the synthesis of ZnS microspheres with cubic and hexagonal crystal phases using a solvothermal method. These materials were characterized by a range of techniques. The effects of Au and Ag-doping on ZnS were also examined. Hexagonal phase ZnO microspheres could be obtained by a CO oxidation process over the ZnS microspheres. The onset of CO oxidation occurred above 350 °C, which was increased by Au and Ag doping. The synthesized materials were used for the photocatalytic degradation of mixed dyes {methylene blue (MB) + methyl orange (MO) + rhodamine B (RhB)}. The observed photocatalytic degradation rate over the photocatalysts was in the order, RhB < MB 蠐 MO. The photocatalytic degradation activity of the synthesized materials was observed in the order, Au-ZnS ≈ Ag-ZnS 蠐 ZnS < ZnO. ZnO was ～6 times more efficient in the photocatalytic synthesis of 2-hydroxyterephthalic acid than the ZnS and metal-doped ZnS. These results further support the importance and wide applications of the synthesized ZnS, Ag-ZnS, Au-ZnS and ZnO microspheres.

Full text of DOI:10.1016/j.jallcom.2016.03.070

Accepted Manuscript
Fabrication of ZnO, ZnS, Ag-ZnS, and Au-ZnS microspheres for photocatalytic
activities, CO oxidation and 2-hydroxyterephthalic acid synthesis
Young In Choi, Seungwon Lee, Seog K. Kim, Young-Il Kim, Dae Won Cho,
Mohammad Mansoob Khan, Youngku Sohn
PII:
S0925-8388(16)30639-9
10.1016/j.jallcom.2016.03.070
JALCOM 36958
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 4 January 2016
Revised Date: 21 February 2016
Accepted Date: 11 March 2016
Please cite this article as: Y.I. Choi, S. Lee, S.K. Kim, Y.-I. Kim, D.W. Cho, M.M. Khan, Y. Sohn,
Fabrication of ZnO, ZnS, Ag-ZnS, and Au-ZnS microspheres for photocatalytic activities, CO oxidation
and 2-hydroxyterephthalic acid synthesis, Journal of Alloys and Compounds (2016), doi: 10.1016/
j.jallcom.2016.03.070.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Fabrication of ZnO, ZnS, Ag-ZnS, and Au-ZnS microspheres for photocatalytic
activities, CO oxidation and 2-hydroxyterephthalic acid synthesis
1
,†
1,†
1
1
1
Young In Choi, Seungwon Lee, Seog K. Kim, Young-Il Kim, Dae Won Cho,
2
,
1,
Mohammad Mansoob Khan, * and Youngku Sohn *
1
2
Department of Chemistry, Yeungnam University, Gyeongsan 38541, Republic of Korea.
Chemical Sciences, Faculty of Science, Universiti Brunei Darussalam, Jalan Tungku Link,
Gadong, BE 1410, Brunei Darussalam.
†
These authors equally contributed to this work.
*
Corresponding authors e-mail: youngkusohn@ynu.ac.kr; mmansoobkhan@yahoo.com
Abstract
This paper reports the synthesis of ZnS microspheres with cubic and hexagonal crystal
phases using a solvothermal method. These materials were characterized by a range of
techniques. The effects of Au and Ag-doping on ZnS were also examined. Hexagonal
phase ZnO microspheres could be obtained by a CO oxidation process over the ZnS
microspheres. The onset of CO oxidation occurred above 350°C, which was increased by
Au and Ag doping. The synthesized materials were used for the photocatalytic
degradation of mixed dyes {methylene blue (MB) + methyl orange (MO) + rhodamine B
(RhB)}. The observed photocatalytic degradation rate over the photocatalysts was in the
order, RhB < MB << MO. The photocatalytic degradation activity of the synthesized
materials was observed in the order, Au-ZnS ≈Ag-ZnS << ZnS < ZnO. ZnO was ~6 times
more efficient in the photocatalytic synthesis of 2-hydroxyterephthalic acid than the ZnS
and metal-doped ZnS. These results further support the importance and wide applications
of the synthesized ZnS, Ag-ZnS, Au-ZnS and ZnO microspheres.
Key words: ZnO; ZnS; cubic and hexagonal; CO oxidation; photocatalytic dye
degradation; 2-hydroxyterephthalic acid
1

ACCEPTED MANUSCRIPT
1
. Introduction
Metal oxides, such as TiO , ZnO, SnO and CeO , are well known
2
2
2
semiconductors for novel applications [1-4]. Among these, ZnO is one of the most
studied metal oxide materials for catalysis, photocatalysis, photoelectrochemical, sensors,
solar cells, and biological applications [1-20]. ZnO nanostructures are prepared by
hydrothermal, solvothermal, electrodeposition, and chemical as well as physical
deposition methods [1-20]. ZnO has a wide direct band gap (~3.37 eV), which restricts its
activity to the UV region. In addition, a large exciton binding energy of 60 meV (or
longer-lived electron-hole pair) limits the photodegradation reaction under normal
conditions [12,14]. ZnS is a type of semiconductor (chalcogenides) that is also well
known for its photocatalytic applications [21-34].
The advances in hybrid photoactive materials (metal-semiconductor) have
resulted in significant progress in heterogeneous visible light-induced photocatalysis by
suppressing the recombination of photogenerated electron-hole pairs. The catalytic and
photocatalytic activities of ZnO and ZnS can be enhanced by hybridization with noble
metals, such as silver (Ag) and gold (Au). Metals anchored/loaded to ZnO and ZnS
nanocomposites have been prepared using a range of approaches [5-14,21-41], such as
biogenic synthesis using biofilms [14], flame spray pyrolysis [36], photo-
reduction/chemical reduction [37], and ball-milling [41]. Ansari et al. synthesized Ag-
ZnO nanocomposites by an electrochemical biogenic synthesis method using an active
biofilm electrode, and observed enhanced photocatalytic properties for the degradation of
methyl orange, methylene blue and 4-nitrophenol [14]. Manna et al. prepared Ag@ZnO-
coated cotton fabrics by polyamine-mediated mineralization and a reduction method and
revealed their photocatalytic activities for the photodegradation of rhodamine B (RhB)
and the disinfection of bacteria under visible light [25]. Misra et al. prepared Au@ZnS
core-shell nanoparticles and reported an increase in photocatalytic activity for methyl
orange and an increase in photocurrent induced by surface plasmon resonance [25].
2 2
Photocatalytic CO reduction was also tested over Ru-functionalized ZnS, and CO was
found to be major reduction product [26]. Zhang et al. decorated ZnO nanorods with Ag
nanoprisms and Ag nanoparticles, and observed that Ag nanoprisms produced a much
higher photocurrent intensity owing to the shape-dependent localized surface plasmon
2

ACCEPTED MANUSCRIPT
resonance (SPR) effect [35]. The SPR of Ag (or Au) is believed to enhance visible light
harvesting and increase the photocatalytic activity [14,35,36,38]. In addition, the band
gap and defects play important roles in the photocatalytic activity of a material [1-
4
,14,39]. Oxygen vacancies formed by impurity doping are also known to play a
significant role in improving the photocatalytic activity [1-4,18]. Fang et al. controlled
the surface defects of ZnO nanorods by varying the quenching temperature and cooling
rate. The concentration of defects was found to be responsible for suppression electron-
hole recombination and increasing the photocatalytic dye degradation rates [18].
This paper reports a very simple and facile method for the synthesis of cubic and
hexagonal ZnS, Ag-ZnS and Au-ZnS microspheres using a solvothermal method.
Hexagonal phase ZnO microspheres were synthesized using a unique CO oxidation
treatment. Most studies reported an improvement in the photocatalytic activity upon
testing with a pure dye at the optimal amount of Ag on ZnO. The photocatalytic activity
also dependents on the nature of the dye and reaction conditions. In this study, a more
complicated mixed dye {methyl orange (MO) + methylene blue (MB) + RhB} was used
as a model system. The photocatalytic dye degradation rate was expected to be different
because the three different dyes will show various UV-visible absorption bands and
different surface charges. To the best of the authors’ knowledge, this is the first report of
the synthesis of novel photocatalysts and the treatment of mixed dyes using ZnS, Ag-ZnS,
Au-ZnS, and ZnO microspheres, on CO oxidation, and the visible light-assisted synthesis
of 2-hydroxyterephthalic acid. The syntheses of ZnO microspheres and 2-
hydroxyterephthalic acid are the exceptional findings of this study, which is efficient and
facile.
2
. Experimental Section
2
.1. Chemicals
Zn(NO ) ·6H O (98%), ZnSO ·7H O (≥ 99.0%), Au(III) chloride trihydrate (99.9%),
3
2
2
4
2
and thiourea (≥99.0%) were purchased from Sigma-Aldrich. Ethylene glycol (EG, 99%,
Daejung Co., South Korea), AgNO (Duksan Co., South Korea), MO (TCI, Japan), MB
3
3

ACCEPTED MANUSCRIPT
(Merck, Germany), RhB (Junsei Chem., Japan), and terephthalic acid (TCI, Japan) were
used as received.
2
.2. ZnS synthesis, and Au and Ag-loading
ZnS with two different crystal phases were synthesized using a solvothermal method
as described below. The two different ZnS nanostructures were designated ZnS(h) and
ZnS(c). For the synthesis of ZnS(h), solutions of 0.1 M zinc nitrate and 0.1 M thiourea in
ethylene glycol (EG) were prepared. 20.0 mL of the two solutions were mixed thoroughly,
transferred to a Teflon-lined stainless autoclave and placed in an oven at 200°C for 12 hrs.
After the reaction, the final product was naturally cooled, collected by centrifugation, and
washed several times with deionized water and ethanol. The white powder was finally
dried in an oven at 80°C. For the synthesis of ZnS(c), solutions of 0.1 M zinc sulfate and
0
.1 M thiourea were prepared in EG. A 20.0 mL volume of each of the two solutions
were mixed and the same experimental procedures used for ZnS(h) were followed. For
Au and Ag doping, 1.0 mol% of a 0.1 M Au chloride solution or 0.1 M Ag nitrate
solution were mixed with the abovementioned solutions before being placed in an oven.
For Au and Ag-doped ZnS(h) and ZnS(c) samples, the samples were designated as Au-
ZnS(h), Ag-ZnS(h), Au-ZnS(c), and Ag-ZnS(c) microspheres.
2
.3. Photocatalysts characterization
To examine the crystal structures of the synthesized powder materials, powder X-ray
diffraction (XRD, PANalytical X’Pert Pro MPD) was performed using Cu Kα radiation
40 kV and 30 mA). The morphologies were examined by field-emission scanning
electron microscopy (FE-SEM, Hitachi SE-4800) and transmission electron microscopy
TEM, Hitachi H-7600) operated at 100 kV. For the SEM and TEM observations, the
(
(
samples were mounted on a Si substrate and a lacey carbon Cu grid, respectively. A
double beam UV-Vis spectrophotometer (SCINCO NeoSys-2000) was used to measure
the UV-Vis diffuse reflectance/absorption spectra of the powdered samples. The
photoluminescence (PL, SCINCO FluoroMate FS-2) spectra of the powder samples were
4

ACCEPTED MANUSCRIPT
obtained at an excitation wavelength of 250 nm. X-ray photoelectron spectroscopy (XPS,
Thermo-VG Scientific K-Alpha) was performed using a monochromated Al Kα X-ray
source (1486.6 eV) and a hemispherical energy analyzer. An electron flood gun was used
to neutralize the surface charge. For the Brunauer-Emmett-Teller (BET) surface area
measurements, a ChemBET TPR/TPD analyzer (Quantachrome Instruments, USA)
equipped with a thermal conductivity detector was used. Before the BET measurements,
the samples were degassed at 200°C for 2 hrs.
2
.4. CO oxidation and ZnO preparation
ZnS conversion to ZnO was performed by a CO oxidation reaction over the ZnS
samples. The CO oxidation experiment was conducted under CO(1.0%)/O /(2.5%)/N2
2
mixed gas flow conditions. The gas flow rate and heating rate were fixed at 40 mL/min
and 10 K/min, respectively. The powder sample (10 mg) was mounted in a U-shape
quartz tube (an inner diameter of 4.0 mm). The reaction product was monitored using a
quadrupole mass spectrometer (RGA200, Stanford Research System, USA). The CO
conversion percentage (%) was calculated using {([CO] -[CO] )/[CO] × 100%}.
in
out
in
2
.5. Adsorption and photocatalytic dye degradation tests
To prepare the mixed dye solution, the same amounts of methyl orange (MO),
rhodamine B (RhB) and methylene blue (MB) solutions was mixed to have the same
concentration of 10 mg/L. For the adsorption (in dark) and photocatalytic dye
degradation tests (under visible light), 10 mg of the photocatalyst was dispersed fully into
a 50 mL mixed dye solution and stirred in the dark. Upon achieving the adsorption-
desorption equilibrium, the mixed dye solution and photocatalyst mixture was irradiated
with visible light with magnetic stirring. The photodegradation experiment was observed
for 6 hrs and the rate of dye photodegradation was monitored by taking 2.0 mL of the
sample every 1 hr, centrifuging it to remove the catalyst and recording the UV-vis
spectrum. The dye concentrations after adsorption and visible light irradiation (with time)
5

ACCEPTED MANUSCRIPT
were obtained by measuring the level of UV-visible absorption using a UV–Vis
spectrophotometer (Jasco V-530).
2
.6. Synthesis of 2-hydroxyterephthalic acid
The synthesis of 2-hydroxyterephthalic acid (HTPA) was performed to examine
•
the formation of OH radicals over the photocatalyst in a solution during visible light
-5
irradiation. The photocatalyst was dispersed in 50 mL of a 0.002 M NaOH (with 2.4×10
mol terephthalic acid) solution. After irradiating visible light for 6 hrs, the solution was
centrifuged and the photoluminescence (PL) spectrum was measured. HTPA was formed
•
by a reaction of terephthalic acid (TPA) with OH radicals. The amount of HTPA could
be examined by PL because TPA is non-luminescent, whereas HTPA is. The PL
measurements were conducted at an excitation wavelength of 320 nm using a SCINCO
FluoroMate FS-2.
3
. Results and Discussion
3
.1. Morphology and XRD of ZnS and Au and Ag-doped ZnS
Figure 1 shows SEM images and XRD patterns of the as-synthesized ZnS(h), ZnS(c),
and Ag and Au-doped ZnS(h) and ZnS(c) microspheres. All SEM images show spherical
shapes but different sizes. The diameters of the ZnS(h) microspheres prepared with
Zn(NO
increased to 300-500 nm. For Au-doped ZnS(h), the diameters were increased further to
00-1000 nm. The diameters of the ZnS(c) sample prepared with ZnSO and thiourea in
3 2
) and thiourea in EG were 200-300 nm. The diameters of the Ag-doped ZnS(h)
8
4
EG were ~10 times larger than that of ZnS(h) and was found to be 1 - 3 µm. For the Ag-
doped ZnS(c) sample, the particle size was much smaller. On the other hand, for Au-
doped ZnS(c), larger particles were also observed, compared to undoped ZnS(c).
The XRD patterns of ZnS(h) and ZnS(c) revealed two different sizes. Interestingly,
both the XRD patterns were entirely different. For ZnS(h), the XRD peaks were well
matched to the reference patterns of wurtzite hexagonal (P63mc) ZnS (JCPDS 1-080-
6

ACCEPTED MANUSCRIPT
0
007) [33]. On the other hand, for ZnS(c), the XRD patterns appeared to match the
reference patterns of cubic (F-43m) ZnS (JCPDS 1-080-0020). All crystal planes were
assigned to the corresponding XRD peaks. Upon Ag-doping to ZnS(h), the XRD patterns
became sharper, particularly for the (002), (110) and (112) planes. For the Ag-doped
ZnS(c), the (111), (220) and (311) planes became shaper. No XRD peaks for Ag were
found, indicating the amorphous nature and/or small amount of Ag. Upon Au-doping of
ZnS(h) and ZnS(c), the XRD patterns of hexagonal ZnS(h) and cubic ZnS(c) showed no
significant change. On the other hand, for the Au-doped samples, new XRD peaks were
observed at 2θ = 38.2°, 44.3°, 64.6°, and 77.5°, which were assigned to the (111), (002),
(022), and (113) planes, respectively, of cubic crystal phase (Fm-3m, JCPDS 98-004-
4
362) Au. The Au XRD peaks of Au-ZnS(c) were more intense than those of Au-ZnS(h).
This confirms the synthesis of ZnS(h), ZnS(c), Ag-ZnS(h), Ag-ZnS(c), Au-ZnS(h), and
Au-ZnS(c) microspheres and their purity.
Figure 1. SEM images (left) of (a) ZnS(h), (b) ZnS(c), (c) Ag-ZnS(h), (d) Ag-ZnS(c), (e)
Au-ZnS(h), and (f) Au-ZnS(c) samples, and the corresponding XRD profiles (g and h).
7

ACCEPTED MANUSCRIPT
3
.2. UV-Vis absorption and PL of ZnS and Au and Ag-doped ZnS
Figure 2 shows the UV-Vis diffuse reflectance absorption (Fig. 2a) and PL spectra
(
Fig. 2c and 2d) of ZnS(h), ZnS(c), Ag-ZnS(h), Ag-ZnS(c), Au-ZnS(h), and Au-ZnS(c)
2
samples. The [αhν] vs. photon energy (E, eV) plots are also shown (Fig. 2b), where α
and h are the absorbance and Planck’s constant, respectively. The Tauc equation, αhν =
n/2
A(hν – E ) , was used to estimate the band gap, where A is an empirical value and n is 4
g
for an indirect band gap (e.g., TiO ) and n = 1 for a direct band gap (e.g., ZnS and ZnO)
2
[42,43]. For undoped ZnS(h) and ZnS(c), the band gaps were estimated to be 3.6 and 3.45
eV, respectively, which is in good agreement with the literature [33]. Zeng et al. reported
wurtzite ZnS films (band gaps of 3.50−3.64 eV) prepared by spray pyrolysis at 310°C
2
using a solution of ZnCl and thioacetamide [33]. For the Au and Ag-doped samples, the
absorption in the visible region were strongly enhanced. On the other hand, for Au-doped
ZnS(c), the band gap edge showed a slight change, compared to that of undoped ZnS(c).
This was attributed to the quantum confinement effect and surface plasmon resonance
effects of the synthesized materials [14,35,36,38].
The PL spectra (Fig. 2c) of ZnS(h) and ZnS(c) showed broad peaks near 380 nm and
4
70 nm, which were attributed to band gap emission and defects, respectively [33]. For
the Au and Ag-doped samples (Fig. 2d), the PL peaks decreased significantly, which is
due to a metal-dipole quenching effect [42,43]. For Au-ZnS(c), the PL intensity showed a
small decrease compared to the other metal-doped samples. This was attributed to the
decrease in charge carrier recombination [42-46].
8

ACCEPTED MANUSCRIPT
Figure 2. UV-visible diffuse reflectance absorption spectra (a, and b), and PL (c, and d)
spectra of the ZnS(h), ZnS(c), Ag-ZnS(h), Ag-ZnS(c), Au-ZnS(h), and Au-ZnS(c)
samples. An excitation wavelength of 250 nm was used to obtain the PL spectra.
3
.3. XPS of cubic and hexagonal phase ZnS
XPS was used to examine the surface electronic structures of cubic and hexagonal
phase ZnS. Figure 3 shows the survey and high resolution Zn 2p and O 1s XP spectra of
hexagonal phase ZnS(h) and cubic phase ZnS(c). The survey scan only showed Zn and S,
with C as a surface impurity. The high resolution Zn 2p peaks were normalized to the
same intensity. The S 2p peaks were rescaled with the same normalization factor for Zn
2
p. For ZnS(h), the Zn 2p3/2 (Zn 2p ) XPS peak was observed at a binding energy (BE)
1/2
of 1021.5 (1044.1) eV, with a spin-orbit splitting of 22.6 eV, which is in good agreement
9

ACCEPTED MANUSCRIPT
with the literature [46-48]. For ZnS(c), the Zn 2p XPS peak was observed at a higher BE
(by 0.5 eV) with the same spin-orbit splitting energy. The S 2p XPS peaks of ZnS(h) and
ZnS(c) were observed at 161.6 and 161.9 eV, respectively. Interestingly, the S 2p XPS
peak area for ZnS(h) was ~10% larger than that for ZnS(c). This indicates that the facets
of ZnS(h) have more exposed S [46].
Figure 3. Survey and high resolution Zn 2p and O 1s XP spectra of hexagonal phase
ZnS(h) and cubic phase ZnS(c).
3
.4. Photocatalytic dye degradation over bare ZnS and Au and Ag-doped ZnS
The photocatalytic dye degradation of mixed dyes (MO + RhB + MB) was
performed under visible light irradiation. First, adsorption-desorption was examined and
the photocatalytic dye degradation of mixed dyes over ZnS(h), ZnS(c), Ag-ZnS(h), Ag-
ZnS(c), Au-ZnS(h), and Au-ZnS(c) was then performed [41,49,50]. Each sample was
dispersed in 50 mL of the mixed dye solution, which were a mixture of MO, RhB and
MB with the same concentration of 10 mg/L. Figure 4 shows the UV-Vis absorption
1
0

ACCEPTED MANUSCRIPT
spectra of each sample after adsorption in the dark and after photoirradiation. Because the
mixed dye contains three different dyes, the corresponding absorption spectrum shows
three absorption regions of MO (450 nm), RhB (550 nm) and MB (650 nm). The three
peaks positions were used to analyze the extent of photocatalytic dye degradation. For
ZnS(c), upon adsorption in the dark, the three UV-Vis absorption intensities decreased by
0 0
less than 10% (or Cad/C = 90%), where C and Cad are the UV-Vis absorption intensities
before (initial) and after adsorption, respectively. Upon Ag and Au-doping of ZnS(c), the
adsorption performance was decreased. The UV-Vis absorption intensities in the MO and
RhB regions showed a negligible decrease, and that in the MB region showed only a 2-
3
% decrease. For ZnS(h), MO and RhB showed 12% adsorption, whereas MB showed
only 2% adsorption over the photocatalyst. Upon Ag and Au-doping into ZnS(h), the
adsorption performance for MO and RhB decreased, whereas that of MB increased. For
the photocatalytic dye degradation over ZnS(c), the UV-Vis intensity was decreased
gradually with increasing visible light irradiation. After 6 h, the UV-Vis absorption in the
case of MO was decreased drastically. In the case of RhB, the UV-Vis absorption was
decreased by 47% (or C6h/Cad = 53%), where C6h and Cad are the UV-Vis absorption
intensities after 6 h photoirradiation and adsorption, respectively. For MB, the UV-Vis
absorption was decreased by 79% (or C /C = 21%). The photocatalytic performance
6
h
ad
was observed in the order, RhB < MB << MO over ZnS(c). Upon Ag and Au-doping, the
photocatalytic performance for RhB and MB decreased significantly but the performance
for MO was unaffected. For Ag-doped ZnS(c), the RhB and MB absorption spectra
decreased in intensity by 12% (or C /C = 88%) and 51% (or C /C = 49%),
6
h
ad
6h ad
respectively. For Au-doped ZnS(c), the catalytic performance was somewhat better than
the Ag-doped ZnS(c). The UV-Vis absorption spectra of RhB and MB were decreased by
1
7% (or C /C = 83%) and 59% (or C /C = 41%), respectively. For ZnS(h), the UV-
6h ad 6h ad
Vis absorption spectra of RhB and MB were decreased by 47% (or C6h/Cad = 53%) and
1% (or C /C = 19%), respectively. Upon Ag and Au-doping, the catalytic activity also
8
6
h
ad
decreased. For Ag-ZnS(h), the UV-Vis absorption spectra of RhB and MB were
decreased by 10% (or C /C = 90%) and 62% (or C /C = 48%), respectively. For Au-
6
h
ad
6h ad
ZnS(h), the catalytic performance was somewhat poor compared to that for Ag-doped
ZnS(h). The UV-Vis absorption spectra of RhB and MB showed decrease in intensity of
1
1

ACCEPTED MANUSCRIPT
3
% (or C6h/Cad = 97%) and 44% (or C6h/Cad = 56%), respectively. Although the
absorption in the visible light was increased after Ag and Au-doping, the catalytic activity
was poorer. This suggests that the photocatalytic mechanism is likely governed more by a
dye-sensitized mechanism, which will be discussed below.
Figure 4. UV-Vis absorption spectra for the adsorption (C /C ) and photocatalytic dye
ad
0
degradation (C/Cad) under visible light irradiation over bare (left column), Ag (middle
column) and Au-doped (right column) cubic (top panel) and hexagonal (bottom panel)
crystal phase ZnS microspheres.
3
.5. CO oxidation of ZnS and Ag and Au-doped ZnS
The conversion of ZnS to ZnO is very important industrially. Figure 5 shows the
first and second run CO oxidation profiles of ZnS(c), ZnS(h), Ag-ZnS(c), Ag-ZnS(h),
Au-ZnS(c), and Au-ZnS(h) samples. The CO conversion % was calculated using
1
2

ACCEPTED MANUSCRIPT
{
([CO] -[CO] )/[CO] ×100%} [51-57]. For the first runs of the ZnS(c) and ZnS(h)
in
out
in
microspheres, CO oxidation began to occur at approximately 370 and 350°C, respectively.
In the second runs, the onset of CO oxidation was observed at 400 and 350°C,
respectively. With increasing temperature, the CO oxidation conversion (%) in the
second run was higher than that in the first run. The increase in CO conversion in the
second run was attributed to the change in crystal phase [51,55] from ZnS to ZnO, as
discussed below. For Ag and Au-doped ZnS(c), the CO oxidation profiles were similar.
In addition, a difference in the onset of CO oxidation between the first and the second
runs was also observed at a similar temperature of 420°C. The level of CO conversion
increased sharply at the above temperature. For Ag and Au-doped ZnS(h), the CO
oxidation profiles were somewhat different from those of the bare ZnS(c) and ZnS(h),
and Ag and Au-doped ZnS(c). For the first runs of the Ag-ZnS(h) and Au-ZnS(h)
microspheres, the CO oxidation onsets were observed at ~390 and 400°C, respectively. In
the second runs, the CO oxidation onsets were observed at ~430 and 400°C, respectively.
The Ag and Au-doped samples showed poor CO oxidation activity compared to the other
samples.
1
3

ACCEPTED MANUSCRIPT
st
nd
Figure 5. 1 and 2 run CO oxidation profiles of the ZnS(c), ZnS(h), Ag-ZnS(c), Ag-
ZnS(h), Au-ZnS(c), and Au-ZnS(h) samples.
3
.6. Morphology and XRD of ZnO by CO oxidation of ZnS
The morphologies and XRD patterns of the ZnS(c) and ZnS(h) were examined after
the CO oxidation reactions. Figure 6 shows SEM and TEM images and XRD patterns of
the samples. The SEM and TEM images revealed microspheres with two different sizes;
the samples from ZnS(h) and ZnS(c) were 200-300 nm and 1 - 3 µm in size, respectively.
Although the sizes showed no critical changes after CO oxidation, the surface became
1
4

ACCEPTED MANUSCRIPT
rough. TEM revealed the slight aggregation of smaller particles. Finally, the crystal
structures of ZnS(c) and ZnS(h) were examined after the CO oxidation reaction. The two
samples showed the same XRD patterns that matched well with those of hexagonal
(P63mc) ZnO (JCPDS 1-089-7102). The three major peaks were observed at 2θ = 31.7°,
3
4.4° and 36.2°, corresponding to the (100), (002) and (101) planes, respectively. The
other peaks were assigned to the corresponding planes. The samples obtained from CO
oxidation over ZnS(h) and ZnS(c) were designated ZnO-h(h) and ZnO-c(h), respectively.
Based on the XRD results (Figure 6) and the CO oxidation profiles (Figure 5), ZnO was
formed from ZnS via 2ZnS + 3O
2 2
→ 2ZnO + 2SO at above 400°C. Once ZnO is formed,
CO oxidation begins to occur via ZnO + CO + 1/2O → ZnO + CO . ZnS(h) showed a
2
2
lower CO oxidation onset compared to ZnS(c). The lower CO oxidation onset could be
attributed to the smaller size (SEM images in Figure 1 and 6) and/or more exposed sulfur
(S 2p XP spectra in Figure 3) results for ZnS(h). The BET surface area of a selected
ZnS(h) was examined before and after CO oxidation. The measured BET surface area of
2
2
ZnS(h) was 65.8 m /g, which decreased substantially to 17.0 m /g after CO oxidation,
forming ZnO-h(h).
Figure 6. SEM (top left), TEM (bottom left) images and powder XRD profiles (right) of
the ZnO-c(h) and ZnS-h(h) samples prepared by the CO oxidation of ZnS(c) and ZnS(h),
respectively.
1
5

ACCEPTED MANUSCRIPT
3
.7. UV-visible absorption and PL of ZnO
Figure 7 shows UV-visible diffuse reflectance/absorption and PL spectra of the ZnO-
h(h) and ZnO-c(h) samples. The reference ZnO (Sigma-Aldrich) sample showed a sharp
absorption edge at approximately 387 nm (or 3.2 eV). The absorption edges of the ZnO
samples extended to the visible region (red shifted) and showed smooth arising edges.
Both the ZnO samples showed enhanced visible light absorption. The absorption band
2
gaps were calculated from the plots (inset of Fig. 7) of [αhν] versus hν [42,43]. The band
gaps of the ZnO-h(h) and ZnO-c(h) samples were calculated to be 2.97 and 2.85 eV,
respectively, which is in the visible light range.
The PL spectra of ZnO commonly showed a broad peak at 470 nm, which could be
attributed to defects [2,46,51]. The PL intensity of ZnO-h(h) was somewhat lower than
that of ZnO-c(h) because of high charge separation and low recombination of the charge
carriers [2,46,51]. Therefore, more effective electron–hole pair separation in the ZnO-
h(h) microspheres will be expected to improve the photocatalytic activity.
Figure 7. UV-visible diffuse reflectance/absorption (left) and PL (right) spectra of the
ZnO-h(h) and ZnO-c(h) samples. The UV-Vis absorption spectrum of a commercial ZnO
is shown as a reference. The excitation wavelength for PL was 250 nm.
1
6

ACCEPTED MANUSCRIPT
3
.8. Photocatalytic dye degradation using ZnO, ZnS(h), ZnS(c), ZnO-h(h), ZnO-c(h), Ag-
ZnS(h), Ag-ZnS(c), Au-ZnS(h), and Au-ZnS(c) microspheres
The adsorption and photocatalytic performance of the ZnO-h(h) and ZnO-c(h)
samples were examined for the mixed dyes. Figure 8 shows the UV-Vis absorption
spectra of the two samples after adsorption in the dark and under increasing
photoirradiation times. Similar to the ZnS samples, the adsorption performances of ZnO-
h(h) and ZnO-c(h) were quite low. The UV-Vis absorption intensities were decreased by
0
less than 10% (or Cad/C = 90%). Upon irradiating with visible light for 1 hr, the UV-Vis
absorption for MO was decreased abruptly, whereas that for RhB showed a negligible
change. The UV-Vis absorption for MB was decreased by 15% (or C1h/Cad = 85%) and
1
8% (or C /C = 82%), where C and C are the UV-Vis absorption intensities after 1
6h ad 1h ad
hr photoirradiation and adsorption, respectively. With increasing irradiation time, the
UV-Vis absorption intensities for RhB and MB decreased gradually. For MB, the UV-Vis
absorption intensity decreased more rapidly than that for RhB. The photocatalytic
performance over the ZnO-h(h) and ZnO-c(h) samples was in the order, RhB < MB <<
MO. After photoirradiation over ZnO-h(h) for 6 hrs, the UV-Vis absorption intensities for
RhB and MB decreased by 61% (or C /C = 39%) and 83% (or C /C = 17%),
6
h
ad
6h ad
respectively. For ZnO-c(h), the UV-Vis absorption intensities of RhB and MB were
decreased by 51% (or C /C = 49%) and 86% (or C /C = 14%), respectively. In the
6
h
ad
6h ad
UV-Vis absorption spectra with increasing irradiation time, no change in peak position
was observed, but the peak intensity decreased gradually. This suggests that the dye
molecules were oxidized/decomposed completely without forming any intermediate. The
•
−
de-ethylation products from RhB were reported to be produced from active O
2
radical
species [42,49,50]. As a result, the UV-Vis absorption peak of RhB is found to be blue-
shifted [42,49,50]. In the present study, however, no change in peak position was
•
−
observed. Therefore, O
2
radicals are not the major active species for dye degradation.
Figure 8 shows the plots of photocatalytic degradation (C /C ) of RhB and MB in
6h
ad
the mixed dye solution over the ZnS(h), ZnS(c), ZnO-h(h), ZnO-c(h), Ag-ZnS(h), Ag-
ZnS(c), Au-ZnS(h), and Au-ZnS(c) microspheres after visible light irradiation for 6 hrs.
For RhB and MB in the mixed dye, the photocatalytic activity was observed in the order,
1
7

ACCEPTED MANUSCRIPT
Au-ZnS ≈ Ag-ZnS << ZnS < ZnO. The undoped ZnS(c) and ZnS(h) (and Ag-doped
samples) showed no critical differences in activity for both RhB and MB. On the other
hand, ZnO-h(h) showed somewhat higher activity than ZnO-c(h) for RhB, but lower
activity for MB. The Au-ZnS(c) showed higher activity than Au-ZnS(h) for both RhB
and MB.
Figure 8. UV-Vis absorption spectra for the photocatalytic dye degradation (top panel) of
mixed dyes under visible light over the ZnO-h(h) and ZnO-c(h) samples obtained by CO
oxidation. The inset shows the corresponding degraded dye solutions. Photocatalytic
degradation (C /C ) of RhB (left) and MB (right) regions for the mixed dye solution
6
h
ad
over ZnS(h), ZnS(c), ZnO-h(h), ZnO-c(h), Ag-ZnS(h), Ag-ZnS(c), Au-ZnS(h), and Au-
ZnS(c) microspheres after visible light irradiation for 6 h. C and C are the UV-Vis
6h
ad
absorption intensities after 6 hrs visible irradiation and adsorption in dark, respectively.
1
8

ACCEPTED MANUSCRIPT
3
.9. Synthesis of 2-hydroxyterephthalic acid
Finally, this study examined the photocatalytic activity of ZnS(c), ZnS(h), ZnS-c(h),
•
and ZnS-h(h) microspheres by probing the hydroxyl radical ( OH) formation. For this, 2-
hydroxyterephthalic acid (HTPA) synthesis was performed, which is formed by a
•
reaction of terephthalic acid (TPA) with OH radicals [41-44]. TPA is non-luminescent,
whereas HTPA is luminescent, showing a broad PL band at 425 nm under the excitation
of UV light. For this reason, the PL intensity is related directly to the amount of HTPA
•
formed (or OH radicals) [41-44]. Figure 9 shows the PL spectra of the TPA solutions
reacted with ZnS(c), ZnS(h), ZnS-c(h), and ZnS-h(h) microspheres under visible light
irradiation for 6 hrs. A broad peak was commonly observed at 425 nm, which was
assigned to HTPA synthesized from TPA. The PL intensity was observed in the order,
ZnS(h) < ZnS(c) << ZnO-h(h) < ZnO-c(h). ZnO showed a significantly stronger PL
intensity than ZnS. The Ag and Au-doped ZnS samples also showed weak PL intensities.
In addition, the larger microspheres (related with cubic phase) commonly showed a
somewhat stronger PL intensity than smaller microspheres (related to hexagonal phase).
This might be due to the lower band gap of ZnO-c(h) (or more absorption of visible light).
Figure 9. PL spectra (left) of the TPA solutions reacted with ZnS(c), Zn(h), ZnS-c(h),
and ZnS-h(h) microspheres under visible light for 6 h. An excitation wavelength of 320
nm was used to collect the PL. PL intensities at 425 nm for ZnS(c), ZnS(h), ZnO-c(h),
1
9

ACCEPTED MANUSCRIPT
ZnO-h(h), Ag/ZnS-c(h), Ag/ZnS-h(h), Au/ZnS-c(h), and Au/ZnS-h(h) microspheres. The
•
left inset shows that terephthalic acid reacts with OH to form luminescent 2-
hydroxyterephthalic acid.
Based on the present photocatalytic dye degradation experiments, the following
simplified mechanism can be proposed [41-44,49,50]:
−
+
Mixed dyes + visible light → Dye (e CB + h VB
)
−
CB
+
ZnO + visible light → ZnO (e
+ h VB)
−
CB
+
−
+
Dye (e
+ h VB) + ZnO (or ZnS) → ZnO or ZnS (e
) + Dye (h VB)
CB
−
CB
•
−
ZnO or ZnS (e
) + adsorbed O → O + ZnO or ZnS
2 2
•
−
+
•
•
O2 + H → O H (or OH)
2
+
+
•
2
ZnO or ZnS (h VB) + H O → H + OH
+
-
•
ZnO or ZnS (h ) + OH → OH
VB
•
-
+
•
O , h or OH species + Dye → dye degradation and mineralization
2
Compared to ZnO (or ZnS), the dyes absorb more visible light due to conjugation.
Therefore, the initial dye degradation mechanism is started mainly by formation of a hole
in the valence band (VB) and an electron in the conduction band (CB) of a dye. The
electrons in the CB are transferred to the CB of ZnO (or ZnS). The electron transfer
efficiency is determined mainly by the aligned energy level [14,42,43]. ZnO could
directly absorb visible light. This is because the calculated band gap (2.85 eV) of ZnO-
c(h) is lower than that (2.97 eV) of ZnO-h(h) and ZnO-c(h), which absorbs more visible
•
−
light to form electrons and holes. The active O
2
species are formed by electron capture
by adsorbed oxygen, which depends on the oxygen dissolved in solution. On the other
•
−
•
hand, based on the present study, O2 is not a major species. Active OH radicals are
•
formed by the mechanisms described above. The formation of OH radicals was much
greater for ZnO than for ZnS. Therefore, mineralization/degradation of the dyes is
+
•
facilitated mainly by the formation of h and OH species.
2
0

ACCEPTED MANUSCRIPT
4
. Summary
This paper reported the synthesis of different crystal phases (cubic and hexagonal)
and sizes (1-3 µm and 200-300 nm) of ZnS, Ag-ZnS, and Au-ZnS microspheres, which
were synthesized using a solvothermal method. Hexagonal ZnO microspheres were
synthesized by a CO oxidation reaction of ZnS. The physicochemical properties of the
synthesized materials were examined by UV-visible absorption/reflectance spectroscopy,
PL spectroscopy, SEM, TEM/HRTEM, and XPS spectroscopy. Their photocatalytic
activities and characteristics were first tested by degrading a mixed dye (MO + MB +
RhB) under visible light irradiation. The dye degradation rate of the mixed dye over the
photocatalysts was observed in the order, RhB < MB << MO, whereas the photocatalytic
activity was in the order, Au-ZnS ≈ Ag-ZnS << ZnS < ZnO. The slight photo-degraded
MO was sensitized by the dyes, resulting in rapid degradation. Finally, ZnS and ZnO
were also used for the successful synthesis of 2-hydroxyterephthalic acid from
•
terephthalic acid under visible light irradiation. ZnO showed ~6 times more OH radical
formation activity (probed from the synthesis of 2-hydroxyterephthalic acid) than the ZnS
and metal-doped ZnS. The CO oxidation activity was observed at above 350°C. The
activity was higher for smaller size ZnO and became lower after Au and Ag doping. This
novel method can offer new strategies for the synthesis of chalcogenides (such as ZnS),
metal oxides (such as ZnO), and nanocomposites (such as Ag-ZnS and Au-ZnS) for
visible light-induced photocatalysis as well as a facile route to synthesis of novel
chemicals.
Acknowledgements. This study was supported financially by a National Research
Foundation of Korea (NRF) grant funded by the Korean government (MEST) (NRF-
2
014R1A1A2055923).
References
. M. M. Khan, S. A. Ansari, D. Pradhan, M. O. Ansari, D. H. Han, J. Lee and M. H. Cho,
Band gap engineered TiO nanoparticles for visible light induced
1
2
2
1

ACCEPTED MANUSCRIPT
photoelectrochemical and photocatalytic studies, J. Mater. Chem. A 2014, 2 (2014)
6
37–644.
2
3
4
5
. S. A. Ansari, M. M. Khan, S. Kalathil, A. Nisar, J. Lee and M. H. Cho, Oxygen
Vacancy Induced Band Gap Narrowing of ZnO Nanostructure by Electrochemically
Active Biofilm, Nanoscale 5 (2013) 9238–9246.
. S. A. Ansari, M. Mansoob Khan, M. O. Ansari, J. Lee and M. H. Cho, Highly
photoactive SnO nanostructures engineered by electrochemically active biofilm, New
2
J. Chem. 38 (2014) 2462−2469.
. M. M. Khan, S. A. Ansari, D. Pradhan, D. H. Han, J. Lee and M. H. Cho, Defect-
Induced Band Gap Narrowed CeO Nanostructures for Visible Light Activities, Ind.
2
Eng. Chem. Res. 53 (2014) 9754–9763.
. S. Satter, M. Hoque, M. Rahman, M. Mollah and M. Susan, An approach towards the
synthesis and characterization of ZnO@Ag core@shell nanoparticles in water-in-oil
microemulsion, RSC Adv. 4 (2014) 20612-20615.
6
7
. H. Barani, Surface activation of cotton fiber by seeding silver nanoparticles and in situ
synthesizing ZnO nanoparticles, New J. Chem. 2014, 38 (2014) 4365-4370.
. Z. Li, F. Zhang, A. Meng, C. Xie and J. Xing, ZnO/Ag micro/nanospheres with
enhanced photocatalytic and antibacterial properties synthesized by a novel continuous
synthesis method, RSC Adv. 5 (2015) 612-620.
8
9
1
1
. B. Divbanda, M. Khatamiana, G. Eslamianb and M. Darbandi, Synthesis of Ag/ZnO
nanostructures by different methods and investigation of their photocatalytic efficiency
for 4-nitrophenol degradation, Appl. Surf. Sci., 284 (2013) 80–86.
. S. Motshekga, S. Ray, M. Onyango and M. Momba, Microwave-assisted synthesis,
characterization and antibacterial activity of Ag/ZnO nanoparticles supported
bentonite clay, J. Hazard. Mater. 262 (2013) 439–446.
0. Y. Zuo, Y. Qin, C. Jin, Y. Li, D. Shi, Q. Wu and J. Yang, Double-sided ZnO
nanorods arrays on single-crystal Ag holed microdisks with enhanced photocataltytic
efficiency, Nanoscale 5 (2013) 4388-4394.
1. G. Tang, S. Tian, Z. Zhou, Y. Wen, A. Pang, Y. Zhang, D. Zeng, H. Li, B. Shan and
C. Xie, ZnO Micro/Nanocrystals with Tunable Exposed (0001) Facets for Enhanced
2
2

ACCEPTED MANUSCRIPT
Catalytic Activity on the Thermal Decomposition of Ammonium Perchlorate, J. Phys.
Chem. C 118 (2014) 11833–11841.
1
1
2. P. G. Li, W. H. Tang, X. Wang, Synthesis of ZnO nanowire arrays and their
photoluminescence property, J. Alloys Compd. 479 (2009) 634-637.
3. Y. Zheng, C. Chen, Y. Zhan, X. Lin, Q. Zheng, K. Wei and J. Zhu, Photocatalytic
activity of Ag/ZnO heterostructure nanocatalyst: correlation between structure and
property, J. Phys. Chem. C. 112 (2008) 10773–10777.
1
1
1
4. S. Ansari, M. Khan, M. Ansari, J. Lee and M. Cho, Biogenic Synthesis,
Photocatalytic, and Photoelectrochemical Performance of Ag–ZnO Nanocomposite, J.
Phys. Chem. C. 117 (2013) 27023-2703.
5. M. Alenezi, S. Henley, N. Emerson and S. Silva, From 1D and 2D ZnO
nanostructures to 3D hierarchical structures with enhanced gas sensing properties,
Nanoscale 6 (2014) 235-247.
6. D. Su, H. Fu, X. Jiang and G. Wang, ZnO nanocrystals with a high percentage of
exposed {42¯ 2¯ 3¯ } reactive facets for enhanced gas sensing performance, Sens.
Actuators. B 186 (2013) 286–292.
1
1
7. H. Ma, P. Williams and S. Diamond, Ecotoxicity of manufactured ZnO nanoparticles
-
A review, Environ. Pollut. 172 (2013) 76-85.
8. J. Fang, H. Fan, Y. Ma, Z. Wang and Q. Chang, Surface defects control for ZnO
nanorods synthesized by quenching and their anti-recombination in photocatalysis,
Appl. Surf. Sci. 332 (2015) 47-54.
1
2
2
9. S. Kundu, S. Sain, B. Satpati, S. Bhattacharyyab and S. Pradhan, Structural
interpretation, growth mechanism and optical properties of ZnO nanorods synthesized
by a simple wet chemical route, RSC Adv. 5 (2015) 23101–23113.
0. D. Pradhan, Z. Su, S. Sindhwani, J. Honek and K. T. Leung, Electrochemical growth
of ZnO nanobelt-like structures at 0°C: synthesis, characterization, and in-situ
glucose oxidase embedment, J. Phys. Chem. C. 115 (2011) 18149-18156.
1. C. Wanga, Y. Ao, P. Wang, S. Zhang, J. Qian, J. Hou, A simple method for large-
scale preparation of ZnS nanoribbon film and its photocatalytic activity for dye
degradation, Appl. Surf. Sci. 256 (2010) 4125–4128.
2
3

ACCEPTED MANUSCRIPT
2
2
2. N. Soltania, E. Saiona, W. M. M. Yunusa, M. Erfania, M. Navaserya, G.
Bahmanrokha, K. Rezaee, Enhancement of visible light photocatalytic activity of ZnS
and CdS nanoparticles based on organic and inorganic coating, Appl. Surf. Sci. 290
(2014) 440–447.
3. A. L. Stroyuk, A. E. Raevskaya, A. V. Korzhak and S. Y. Kuchmii, Zinc sulfide
nanoparticles: Spectral properties and photocatalytic activity in metals reduction
reactions, J. Nanopar. Res. 9 (2007) 1027–1039.
2
2
4. X. Fang, T. Zhai, U. K. Gautam, L. Li, L. Wua, Y. Bando and D. Golberg, ZnS
nanostructures: From synthesis to applications, Prog. Mater. Sci. 56 (2011) 175–287.
5. M. Misra, R. K. Gupta, A. K. Paul and M. Singla, Influence of gold core
concentration on visible photocatalytic activity of gold-zinc sulfide core-shell
nanoparticle, J. Power Sources 294 (2015) 580-587
2
2
6. T. Baran, S. Wojtyla, A. Dibenedetto, M. Arest and W. Macyk, Zinc sulfide
functionalized with ruthenium nanoparticles for photocatalytic reduction of CO
Appl. Catal. B 178 (2015) 170–176
2
,
7. X. Xiao, X.-B. Ma, H.Yuan, P.-C. Liu, Y.-B. Lei, H. Xu, D.-L. Du, J.-F. Sun and Y.-J.
Feng, Photocatalytic properties of zinc sulfide nanocrystals biofabricated by metal-
reducing bacterium Shewanella oneidensis MR-1, J. Hazard. Mater. 288 (2015) 134–
1
39
2
2
8. A. Roychowdhury, S. P. Pati, S. Kumar and D. Das, Effects of magnetite
nanoparticles on optical properties of zinc sulfide in fluorescent-magnetic Fe3O4/ZnS
nanocomposites, Powder Technol. 254 (2014) 583–590.
9. D. Zhu, W. Li, H.-M. Wen, Y. Hu, J. Wang, J.-M. Zhu, W.-T. Ni and C.-Q. Gu,
Development of polyacrylic acid-functionalized porous zinc sulfide nanospheres for a
non-aqueous solid phase extraction procedure toward alkaloids, RSC Adv. 5 (2015)
2
9820-29827.
3
0. S. K. Dash, T. Ghosh, S. Roy, S. Chattopadhyay and D. Das, Zinc sulfide
nanoparticles selectively induce cytotoxic and genotoxic effects on leukemic cells:
involvement of reactive oxygen species and tumor necrosis factor alpha, J. Appl.
Toxicol. 34 (2014) 1130-1144.
2
4

ACCEPTED MANUSCRIPT
3
3
3
3
3
1. T. Baran, A. Dibenedetto, M. Aresta, K. Kruczala and W. Macyk, Photocatalytic
Carboxylation of Organic Substrates with Carbon Dioxide at Zinc Sulfide with
Deposited Ruthenium Nanoparticles, ChemPlusChem 79 (2014) 708-715.
2. E. Baniasadi, I. Dincer and G. F. Naterer, Hybrid photocatalytic water splitting for an
expanded range of the solar spectrum with cadmium sulfide and zinc sulfide catalysts,
Appl. Catal. A 455 (2013) 25-32.
3. X. Zeng, S. S. Pramana, S. K. Batabyal, S. G. Mhaisalkar, X. Chen and K. B. Jinesh,
Low temperature synthesis of wurtzite zinc sulfide(ZnS) thin films by chemical spray
pyrolysis, Phys. Chem. Chem. Phys. 15 (2013) 6763-6768
4. E. Baniasadi, I. Dincer and G. F. Naterer, Measured effects of light intensity and
catalyst concentration on photocatalytic hydrogen and oxygen production with zinc
sulfide suspensions, Int. J. Hydrogen Energy 38 (2013) 9158-9168.
5. J. Manna, S. Goswami, N. Shilpa, N. Sahu, and R. K. Rana, Biomimetic Method To
Assemble Nanostructured Ag@ZnO on Cotton Fabrics: Application as Self-Cleaning
Flexible Materials with Visible-Light Photocatalysis and Antibacterial Activities,
ACS Appl. Mater. Interfaces 7 (2015) 8076–8082.
3
3
3
6. M. Height, S. Pratsinis, O. Mekasuwandumrong and P. Praserthdam, Ag-ZnO
catalysts for UV-photodegradation of methylene blue, Appl. Catal. B 63 (2006) 305-
3
12.
7. M. Khan, C. Wei, M. Chen, J. Tao, N. Huang, Z. Qi and L. Li, CTAB-mediated
synthesis and characterization of ZnO/Ag core–shell nanocomposites, J. Alloys
Compd. 612 (2014) 306–314.
8. Y. Zang, J. Yin, X. He, C. Yue, Z. Wu, J. Li and J. Kang, Plasmonic-enhanced self-
cleaning activity on asymmetric Ag/ZnO surface-enhanced Raman scattering
substrates under UV and visible light irradiation, J. Mater. Chem. A 2 (2014) 7747-
7
753.
3
9. S. Ansari, M. Khan, J. Lee and M. Cho, Highly visible light active Ag@ZnO
nanocomposites synthesized by gel-combustion route, J. Ind. Eng. Chem. 20 (2014)
1
602-1607.
2
5

ACCEPTED MANUSCRIPT
4
4
4
4
4
0. M. Hsu and C. Chang, Ag-doped ZnO nanorods coated metal wire meshes as
hierarchical photocatalysts with high visible-light driven photoactivity and
photostability, J. Hazard. Mater. 278 (2014) 444–453.
1. Y. I. Choi, H. J. Jung, W. G. Shin and Y. Sohn, Band gap-engineered ZnO and
Ag/ZnO by ball-milling method and their photocatalytic and Fenton-like
photocatalytic activities, Appl. Surf. Sci. 356 (2015) 615-625.
2. Y. Park, Y. Na, D. Pradhan, B. Min and Y. Sohn, Adsorption and UV/Visible
photocatalytic performance of BiOI for methyl orange, Rhodamine B and methylene
blue: Ag and Ti-loading effects, CrystEngComm 16 (2014) 3155–3167.
3. W. Kim, D. Pradhan, B. Min and Y. Sohn, Adsorption/photocatalytic activity and
x
fundamental natures of BiOCl and BiOCl I1−x prepared in water and ethylene glycol
environments, and Ag and Au-doping effects, Appl. Catal. B. 147 (2014) 711-725.
4. Y. Na, Y. Kim, D. Cho, D. Pradhan and Y. Sohn, Adsorption/photocatalytic
performances of hierarchical flowerlike BiOBr
orange, Rhodamine B and methylene blue, Mater. Sci. Semicond. Process. 27 (2014)
81-190.
5. A. Nayak, S. Lee, Y. Sohn and D. Pradhan, Synthesis of In S microspheres using a
x
Cl1−x nanostructures for methyl
1
4
4
4
4
4
2
3
template-free and surfactant-less hydrothermal process and their visible light
photocatalysis, CrystEngComm16 (2014) 8064-8072.
6. G. Wang, B. Huang, Z. Li, Z. Lou, Z. Wang, Y. Dai and M.-H. Whangbo, Synthesis
and characterization of ZnS with controlled amount of S vacancies for photocatalytic
H production under visible light, Sci. Rep. 5 (2015) 8544.
2
7. L. Yu, W. Chen, D. Li, J. Wang, Y. Shao, M. He, P. Wang and X. Zheng, Inhibition
of photocorrosion and photoactivity enhancement for ZnO via specific hollow ZnO
core/ZnS shell structure, Appl. Catal. B 164 (2015) 453–461.
8. Y. -P. Zhu, J. Li, T. -Y. Ma, Y. -P. Liu, G. Du and Z. -Y. Yuan, Sonochemistry-
assisted synthesis and optical properties of mesoporous ZnS nanomaterials, J. Mater.
Chem. A 2 (2014) 1093-1101.
9. Y. I. Choi, Y. I. Kim, D. W. Cho, J. S. Kang, K. T. Leung and Y. Sohn, Recyclable
magnetic CoFe O /BiOX (X= Cl, Br and I) microflowers for photocatalytic treatment
2
4
2
6

ACCEPTED MANUSCRIPT
of water contaminated with methyl orange, rhodamine B, methylene blue, and a mixed
dye, RSC Adv. 5 (2015) 79624-79634.
5
5
5
5
5
5
0. H. J. Yoon, Y. I. Choi, E. S. Jang and Y. Sohn, Graphene, charcoal, ZnO, and
ZnS/BiOX (X= Cl, Br, and I) hybrid microspheres for photocatalytic simulated real
mixed dye treatments, J. Ind. Eng. Chem. 32 (2015) 137-152.
1. W. Kim, D. Pradhan and Y. Sohn, Fundamental nature and CO oxidation activities of
indium oxide nanostructures: 1D-wires, 2D-plates, and 3D-cubes and donuts, J. Mater.
Chem. A, 2013, 1, 10193-10202.
2
2. W. Kim, S. Lee and Y. Sohn, Metallic Sn spheres and SnO @C core-shells by
anaerobic and aerobic catalytic ethanol and CO oxidation reactions over SnO2
nanoparticles, Sci. Rep. 5 (2015) 13448.
3. Y. Park, S. Kim, D. Pradhan and Y. Sohn, Surface treatment effects on CO oxidation
reactions over Co, Cu, and Ni-doped and codoped CeO catalysts, Chem. Eng. J. 250
2
(
2014) 25-34.
4. S. Lee, S. Park, B. Min, J. Kang and Y. Sohn, Structural/spectroscopic analyses and
/O /CO responses of thulium (III) oxide nanosquare sheets, Appl. Surf. Sci. 307
2014) 736-743.
H
2
2
(
5. Y. Na, S.W. Lee, N. Roy, D. Pradhan and Y. Sohn, Room temperature light-induced
recrystallization of Cu O cubes to CuO nanostructures in water, CrystEngComm 16
2
(2014) 8546-8554.
5
5
6. Y. Sohn, SiO nanospheres modified by Ag nanoparticles: Surface charging and CO
2
oxidation activity, J. Mol. Catal. A. 379 (2013) 59-67.
7. Y. Park, S. Kim, D. Pradhan and Y. Sohn, Thermal H -treatment effects on CO/CO
2
2
conversion over Pd-doped CeO comparison with Au and Ag-doped CeO , React.
2
2
Kinet. Mech.Catal. 113 (2014) 85-100.
2
7

ACCEPTED MANUSCRIPT
Highlights
►
Undoped, Au and Ag-doped cubic and hexagonal ZnS
microspheres were prepared.
►
Hexagonal ZnO microspheres were prepared by post-CO
oxidation of the ZnS.
►
Photocatalytic dye degradation was tested for mixed dye (MO +
RhB + MB) under visible light.
►
Photocatalytic synthesis of 2-hydroxyterephthalic acid was
examined.