Synthesis and characterization of novel Ag/ZnO nanoparticles for photocatalytic degradation of methylene blue under UV and solar irradiation

Article abstract of DOI:10.1016/j.jpcs.2019.109086

ZnO containing various proportions of metallic silver nanoparticles (0–5 wt% Ag) were synthesized by a combined sol-gel and chemical reduction method in which chitosan acts as a steric stabilizer and environmentally benign reducing agent for silver ions.

Full text of DOI:10.1016/j.jpcs.2019.109086

Accepted Manuscript
Synthesis and characterization of novel Ag/ZnO nanoparticles for photocatalytic
degradation of methylene blue under UV and solar irradiation
M.F. Abdel Messih, M.A. Ahmed, Ayman Soltan, Samy Sobhy Anis
PII:
S0022-3697(19)30210-0
DOI:
https://doi.org/10.1016/j.jpcs.2019.109086
Article Number: 109086
Reference:
PCS 109086
To appear in: Journal of Physics and Chemistry of Solids
Received Date: 27 February 2019
Revised Date: 30 June 2019
Accepted Date: 1 July 2019
Please cite this article as: M.F.A. Messih, M.A. Ahmed, A. Soltan, S.S. Anis, Synthesis and
characterization of novel Ag/ZnO nanoparticles for photocatalytic degradation of methylene blue under
UV and solar irradiation, Journal of Physics and Chemistry of Solids (2019), doi: https://doi.org/10.1016/
j.jpcs.2019.109086.
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.

Ag nanopartAicClCeEsPTED MANUSCRIPT
ZnO nanoparticles
ZnO conduction band
Surface Plasmon resonance
induced electron transfer
Ag fermi level
e-
Increasing absorbance
of samples for visible
light was attributed to
surface Plasmon
resonance of Ag

ACCEPTED MANUSCRIPT
Synthesis and characterization of novel Ag/ZnO nanoparticles for photocatalytic
degradation of methylene blue under UV and solar irradiation
M.F. Abdel Messih, M.A. Ahmed*, Ayman Soltan, Samy Sobhy Anis
Chemistry Department, Faculty of Science, Ain Shams University, Egypt
Abstract
ZnO containing various proportions of metallic silver nanoparticles (0–5 wt% Ag)
were synthesized by a combined sol-gel and chemical reduction method in which
chitosan acts as a steric stabilizer and environmentally benign reducing agent for
silver ions. Structural, morphological, textural, and optical properties of the prepared
nanoparticles were investigated by X-ray diffraction, high-resolution transmission
electron microscopy, nitrogen physisorption, diffuse reflectance spectroscopy, and
photoluminescence spectroscopy. X-ray diffraction and X-ray photoelectron
spectroscopy analyses revealed the successful transformation of silver ions into
metallic silver nanoparticles (Ag) and suggest a strong interaction between ZnO and
silver metal. Photoluminescence analysis indicated the positive influence of metallic
silver in limiting electron-hole recombination. The photocatalytic performance of
Ag/ZnO nanoparticles was studied by our following the photodegradation of the dye
methylene blue under UV and solar irradiation. The incorporation of an appropriate
amount of Ag (3 wt%) enhanced the photocatalytic activity under UV and solar
irradiation. The enhancement of photocatalytic reactivity under UV irradiation was
attributed to the possible transfer of photogenerated electrons from the ZnO
conduction band to the Ag Fermi level, which limits electron-hole recombination and
increases the degradation efficiency. However, under solar irradiation, the higher
photocatalytic reactivity was attributed to surface plasmon resonance of metallic Ag
nanoparticles. Photoluminescence analysis with use of terephthalic acid revealed that
incorporation of silver nanoparticles on the ZnO surface was accompanied by an
increase in the concentration of hydroxyl radicals, which are considered a main
reactive species in dye degradation.
Keywords
Chitosan, Ag-doped ZnO, Surface Plasmon resonance, Methylene blue photocatalytic
degradation

ACCEPTED MANUSCRIPT
1. Introduction
Synthetic dyes are used in various industries, such as the textile industry, the leather
tanning industry, and the paper industry. These toxic synthetic dyes are released in
significant amounts into the environment, causing water pollution [1]. Photocatalysis
is a promising technology for complete destruction of organic pollutants without the
formation of secondary pollutants that require further treatment [2]. Various metal
oxide semiconductors, such as TiO2 [3], ZnO [4], and SnO2 [5], are considered to be
promising photocatalysts for complete degradation of organic pollutants to
environmentally friendly species [6]. ZnO is a promising semiconductor because of
the low cost of its metal precursors, its nontoxicity, and its high photosensitivity [7–
10]. However, the high recombination rate of photogenerated electron-hole pairs [11]
and the wide band gap [12, 13] are the main drawbacks that limit the industrial use of
ZnO nanoparticles. ZnO exhibits low efficiency under solar irradiation because of its
wide band gap and rapid electron-hole recombination [9]. Various recent research
studies indicate that the introduction of noble metal nanoparticles, such as Pd [14], Pt
[15], Au [16], and Ag [17–27], on the ZnO surface enhances the photocatalytic
reactivity in degradation of various organic dyes. Among noble metals, metallic silver
nanoparticles exhibit high efficiency in enhancing the photocatalytic activity of ZnO
because of their appropriate price in comparison with other noble metals such as Au,
Pt, and Pd [28]. The enhanced UV-light-driven photocatalytic efficiency was
attributed to efficient separation of the photogenerated electrons and holes in the
presence of Ag [11, 21, 24,]; however, the higher visible-light-driven photocatalytic
efficiency was attributed to surface plasmon resonance of Ag [17, 23, 26, 27]. It is
interesting to mention that the existence of an appropriate amount of metallic silver
nanoparticles is a major factor in optimizing the photocatalytic reactivity. A small
amount of Ag0 is not sufficient to enhance the reactivity; however, a large amount of
silver metal acts as recombination centers that prevent the production of a large
amount of radicals and reduce the lifetime of the photocatalyst. The homogeneous
dispersion of metallic silver nanoparticles on the ZnO surface requires a mild
reduction process; however, most of the previous research work usually used a
vigorous reducing agent such as NaBH4, ascorbic acid, or LiAlH4. These strong
reducing agents lead to agglomeration of silver nanoparticles on the ZnO surface,

ACCEPTED MANUSCRIPT
which inhibits light penetration and results in the silver nanoparticles acting as
recombination centers for the charge carriers. In this work, Ag/ZnO nanoparticles
with various amounts of silver (1, 3, and 5 wt%) were prepared by an innovative
combined sol-gel and chemical reduction method in which chitosan plays a dual role
as a template and a reducing agent. Wei and Qian [28] attributed the reduction of
silver ions by chitosan to oxidation of chitosan OH. as shown in Fig. 1. The effect of
Ag on the photocatalytic efficiency of ZnO was investigated with methylene blue
(MB) as a cationic pollutant model. The results indicate that addition of Ag to the
ZnO matrix increases its photocatalytic efficiency. Moreover, a possible mechanism is
discussed and suggested.
2. Experimental
2.1. Materials
All the reagents, including zinc nitrate hexahydrate (Zn(NO3)2·6H2O; purity greater
than 99%), silver nitrate (AgNO3; purity greater than 98%), 2-propanol
((CH3)2CHOH; purity 99.7% or greater), low molecular weight chitosan, glacial
acetic acid (CH3COOH; purity greater than 99%), ammonia solution (NH4OH; purity
32%), and MB (C16H18ClN3S), were purchased from Sigma-Aldrich.
2.2. Preparation of pure and Ag-doped ZnO
A series of Ag/ZnO samples were prepared by our dissolving 18.22, 18.095, 17.729,
and 17.36 g of zinc nitrate in distilled water with 7.87, 23.62, and 39.37 ml of 1%
(w/v) AgNO3 to obtain 0, 1, 3, and 5 wt% metallic silver nanoparticles on the ZnO
surface (ZnAg1, ZnAg3, and ZnAg5, respectively). Each solution was subjected to
vigorous stirring for 1 h, followed by addition of 5 ml of 2% (w/v) chitosan dissolved
in glacial acetic acid with constant stirring for another 1 h. Then, drops of ammonia
solution were added until a turbid sol formed, which was subsequently converted to a
viscous gel. The gel was left to age for 3 days. Finally, the gel was filtered, washed
with distilled water, dried at 100 °C, and calcined at 500 °C for 3 h to obtain the
desired materials. Pure ZnO was prepared by the same method but without AgNO3.

ACCEPTED MANUSCRIPT
2.3. Physicochemical characterization of the photocatalysts
X-ray diffractograms of the prepared samples over the 2θ range from 20o to 100o were
obtained with a Philips X’PERT MPD diffractometer with Ni-filtered Cu Kα radiation
(λ = 0.154 nm). The crystallite size of the prepared samples was estimated from the
line broadening of the wurtzite (101) peak (2θ = 36.2°) with use of Scherrer’s
equation.
Micrographs of the samples, from which the size and shape of the particles composing
the samples were obtained, were captured by a JEOL 2100 high-resolution
transmission electron microscope. The samples for microscopic analysis were
dispersed in water by an ultrasonicator and dropped on a carbon-coated copper grid.
Nitrogen adsorption-desorption isotherms (at ̶196.6 °C) were obtained with a NOVA
3200 surface area and pore size analyzer. Before analysis, all samples were degassed
overnight at room temperature.
UV-visible diffuse absorption spectra were recorded with a Shimadzu UV-2600 UV-
vis spectrophotometer, from which the band gap energies of the samples were
determined by the Tauc method with the following equation [29]: f(R)hν = A(hν –
Eg)n, where f(R) is the so-called Kubelka-Munk function, hν is the incident photon
energy, A is an energy-independent constant, Eg is the optical band gap, and n is a
constant that determines the type of optical transitions. For a direct allowed transition,
n = 1/2, while for an indirect allowed transition, n = 2.
Photoluminescence spectra were recorded with a Lumina fluorescence spectrometer
(Thermo Fisher Scientific), from which the electronic band structures of the samples
were revealed.
2.4. Photocatalytic activity test
In the photocatalytic reactor shown in Fig. 2, the photoactivity of the samples was
investigated through photodegradation of 2 × 10-5 M MB aqueous solution. The
photocatalytic degradation of MB over each sample surface was followed by
spectrophotometric measurements using the following procedure: In the
photocatalytic reactor, 0.1 g of sample was mixed with 100 ml of as aqueous solution

ACCEPTED MANUSCRIPT
of the dye, and the mixture was stirred for 30 min in the dark to obtain adsorption-
desorption equilibrium of the dye on the sample surface. After equilibration, the
concentration of dye in terms of absorbance was measured with a Thermo Scientific
Evolution 300 UV-vis spectrophotometer, and the value was taken as the initial
concentration (C0) of the dye. Then, a mercury UV lamp operating at 365 nm was
switched on over the suspension, and at different irradiation time intervals 5 ml of dye
solution was withdrawn and analyzed with the UV-vis spectrophotometer to
determine the residual concentration (Ct) of the dye. Finally, the percent
photocatalytic activity of each sample was calculated by the following equation:
Photocatalytic activity (%) = [1 − (Ct/C0)] × 100.
The photocatalytic reactivity of ZnO and ZnAg3 under visible light was estimated
with use of a xenon lamp with an intensity of 100 W, and a cutoff filter was used to
remove UV radiation (λ ≤ 420 nm).
3. Results and discussion
3.1. Physicochemical characterization
3.1.1. Structural analysis
3.1.1.1. X-ray diffraction
X-ray diffraction (XRD) patterns of ZnO containing various proportions of metallic
silver nanoparticles are shown in Fig. 3. As shown in Fig. 3, all samples exhibited
diffraction peaks at 2θ = 31.7°, 34.4°, 36.2°, 47.5°, 56.6°, 62.8°, 66.3°, 67.9°, 69.1°,
72.6°, and 76.9°, corresponding, respectively, to the (100), (002), (101), (102), (110),
(103), (200), (112), (201), (004), and (202) crystal planes of ZnO wurtzite phase with
lattice parameters a = 3.249 Å and c = 5.206 Å (JCPDS card no. 03-065-3411). On
careful examination of the XRD patterns, one can notice the existence of three
prevailing diffraction peaks, at 2θ = 38.1°, 44.2°, and 64.4°, which correspond to the
(111), (200), and (220) crystal planes of Ag metal (JCPDS card no. 00-004-0783).
The existence of metallic silver nanoparticles confirms the successful role of chitosan
in reducing silver ions to silver nanoparticles. The crystallite sizes of pure and Ag-
doped ZnO nanocrystalline powders were determined from the broadening of the

ACCEPTED MANUSCRIPT
Wurtzite (101) peak (2θ = 36.2°) with use of Scherrer’s equation and are given in
Table 1. The average crystallite size for all the samples was 55.7 nm, with a standard
deviation of 6.5 nm, giving a range of 47.5–61 nm.
3.1.1.2. High-resolution transmission electron microscop
Fig. 4a–d shows high-resolution transmission electron microscope images of ZnO,
ZnAg1, ZnAg3, and ZnAg5. The images reveal the existence of hexagonal ZnO
nanoparticles with size ranging between 32 and 71 nm. It is interesting to notice the
homogeneous dispersion of metallic silver nanospheres on the ZnO surface for ZnAg1
and ZnAg3. However, the silver nanoparticles prefer to segregate as a single phase
when the silver content is increased to 5 wt%, as the silver nanoparticles seem to form
an outer layer coating ZnO particles. For ZnAg3, Fig. 4e shows a d spacing of 0.256,
corresponding to the (101) plane of ZnO, and of 0.235, corresponding to the (111)
plane of metallic silver nanoparticles,4 and illustrates the hybrid interaction between
Ag0 on ZnO nanoparticles
3.1.2. Textural analysis by the nitrogen physisorption method
Fig. 5 shows N2 adsorption–desorption isotherms of ZnO, ZnAg1, ZnAg3, and
ZnAg5. According to IUPAC [30], the adsorption isotherms are classified as type III,
exhibiting an H3 hysteresis loop that arises from the existence of slit-like mesopores.
A type III isotherm is a characteristic of weak gas-solid interactions that cause the
uptake at low relative pressures to be small; but once a gas molecule has become
adsorbed, the adsorbate-adsorbate forces will promote the adsorption of further gas
molecules—a cooperative process—so the isotherms will become convex to the
pressure axis. Thus in systems that give rise to a type III isotherm, the multilayer is
being built up on some parts of the surface, while the monolayer is still incomplete on
other parts. The incorporation of 1 and 3 wt% Ag is accompanied by a reduction in
sample porosity due to deposition of metallic silver nanoparticles inside the pore
structure, and the two samples seems to be completely nonporous. However, an
increase in silver content is accompanied by separation of metallic silver
nanoparticles as a separate phase, and the ZnO nanoparticles retain their porosity. The

ACCEPTED MANUSCRIPT
surface area calculated by the Brunauer-Emmett-Teller equation and the pore volume
and pore diameter estimated by the Barrett-Joyner-Halenda method are given in Table
1. The specific surface area ranges between 4 and 7.2 m2/g, influenced by the amount
of silver in each sample. Consequently, use of the Brunauer-Emmett-Teller equation
is not the most suitable method for evaluation of the monolayer capacity and specific
surface area of solids with a type III isotherm.
3.1.3. X-ray photoelectron spectroscopy analysis
The surface elemental composition and the surface oxidation state of Zn, O, and Ag of
ZnAg5 nanoparticles were analyzed by X-ray photoelectron spectroscopy (XPS). It is
evident from the full survey spectrum presented in Fig. 6 that the sample contains
three elements—Zn, O, and Ag—confirming the sample purity. In the spectra
obtained, Zn was identified through its characteristic two fitted peaks at binding
energies of 1020 and 1043 eV, corresponding to Zn 2p3/2 and Zn 2p1/2, respectively,
representing the existence of the Zn2+ oxidation state. Lattice oxygen in ZnO was
detected through its specific Gaussian-fitted peak located at a binding energy of 529.5
± 0.2 eV. In Fig. 6c the peaks at 367 and 373 eV are attributed to Ag 3d5/2 and Ag
3d3/2, respectively, corresponding to metallic silver (Ag0) [1–3]. The peaks for bulk
silver slightly shifted to lower binding energy reported in the literature (368.2 for Ag
3d5/2 and 374.2 for Ag 3d3/2) are attributed to the strong interaction between ZnO and
metallic silver nanoparticles and electron transfer between the ZnO conduction band
(CB) and the metallic silver nanoparticles [31–33].
3.1.4. Optical analysis
3.1.4.1. Diffuse reflectance spectra
The absorption spectra for ZnO containing various proportions of metallic silver
nanoparticles are illustrated in Fig. 7, showing a strong absorption edge in the UV
region (200–400 nm) due to charge-transfer absorption from the valence band (VB) to
the CB of ZnO. Incorporation of metallic silver nanoparticles was associated with a
shift of the photocatalytic response to the visible region. In particular, ZnAg5
exhibited the highest absorption edge in the visible region because of the surface

ACCEPTED MANUSCRIPT
plasmon absorption of Ag [34]. As seen in Fig. 8, the band gap energy (Eg) of the
samples was determined from the (Ahν)2 versus photon energy (hν) plot through the
intercept of its linear region with the x-axis (Table 2). Eg for pure ZnO was 3.22 eV,
and on doping with Ag metal, the band gap energy was reduced to 3.18 eV, which
causes a shift of the absorption edge to longer wavelength.
3.1.4.2. Photoluminescence spectra
Fig. 9 shows the photoluminescence spectra of ZnO and ZnAg3 for an excitation
wavelength of 325 nm. For ZnO, the presence of two bands, at 333 and 390 nm, due
to band edge emission and defect-related emission, respectively [35], is clearly
observed. The pronounced reduction in photoluminescence intensity on introduction
of metallic silver nanoparticles was attributed to a decrease in the electron-hole
recombination rate [36-37].
3.2. Photocatalytic degradation of MB over pure and Ag-doped ZnO
3.2.1. Kinetics of MB degradation
Fig. 10 illustrates the photocatalytic degradation of MB over ZnO photocatalysts
containing various proportions of metallic silver nanoparticles after 240 min of UV
irradiation with a catalyst loading of 0.1 g/100 ml. The degradation of MB was
evaluated by our following the gradual reduction of intensity of the maximum
absorption peak at 664 nm with time. To estimate the photocatalytic reactivity of the
samples with regard to degradation of of MB, catalytic parameters such as the percent
photocatalytic activity and turnover rate (number of moles converted or produced per
gram of photocatalyst per unit time) were calculated (Table 3). Two facts can be
extracted from the results in Table 3: the first is the low photocatalytic activity and
turnover rate of ZnO, which can be attributed to electron-hole recombination; the
second is the increase in ZnO photocatalytic activity and turnover rate on silver
doping, which can be attributed to the transfer of photogenerated electrons from the
ZnO CB to silver nanoparticles, which limits electron-hole recombination [38–42].
Fig. 11 illustrates the first-order kinetics for MB degradation [43–45]:

ACCEPTED MANUSCRIPT
ln Ct = ln C0 ̶ kt.
The rate constant, k, was estimated from the slope of the straight line of the ln Ct
,
versus time plot. Moreover, the half-life, t1/2 can be calculated from the following
equation:
t1/2 = ln 2/k.
The kinetic parameters are listed in Table 4. The rate constants follow the sequence
ZnAg3 > ZnAg5 > ZnAg1 > ZnO, in agreement with the results for percent
photocatalytic activity.
The results reported in Section 3.1.3.1 indicated that incorporation of metallic silver
nanoparticles shifted the photocatalytic response to visible light. Consequently, to
ascertain the capability of Ag/ZnO nanocomposites for photodegradation of organic
pollutants in the visible region, photocatalytic degradation of MB over ZnAg3 under
solar irradiation was studied. From Fig. 12, 96% of MB was removed, and by our
taking into account the 40% photolysis of MB, ZnAg3 was found to remove 56% of
MB by photodegradation under solar irradiation.
3.2.2. The proposed mechanism for Ag/ZnO photocatalytic activity
3.2.2.1. Under UV irradiation
The photocatalytic process under UV irradiation for Ag/ZnO can be investigated by
the following suggested mechanism (as shown in Fig. 13). On exposure of ZnO
nanoparticles to UV light with energy higher than or equal to the band gap energy, an
electron (e-) from the VB is transferred to the CB, creating a hole (h+) in the VB (Eq.
1) [39, 41, 46–49]. Then, the photogenerated electrons in the ZnO CB are transferred
to the Fermi level of silver nanoparticles (Eq. 2), which is energetically favorable and
suppresses electron-hole recombination. Then Ag trapped electrons and ZnO trapped
holes react with the preadsorbed oxygen (Eq. 3) and surface-bound hydroxyl groups
(–OH) (Eq. 4), respectively, to generate hydroxyl radicals. Finally the hydroxyl
radicals degrade MB by hydrogen abstraction and subsequent oxidation processes
(Eq. 5) [35].

ACCEPTED MANUSCRIPT
−
ZnO + hv (UV) → hVB+ + eCB
(1)
(2)
(3)
(4)
(5)
ZnO (eCB−) + Ag → ZnO Ag (e−)
̶
•−
•−
•
•
−
Ag (e−) + O2 →ZnO
O2
̶ ̶ Ag + O → 2HO → H O → OH + OH + O
+
ZnO
H
2
2
2
2
2
ZnO (hVB+) + OH − → ZnO + •OH
MB + •OH → H2O + MB+• → degradation products
3.2.2.2. Under solar irradiation
The photocatalytic process under solar irradiation for Ag/ZnO can be understood by
the following suggested mechanism (as shown in Fig. 14). On exposure of silver
nanoparticles to solar irradiation, electron-hole pairs are generated as a result of
absorption of visible light through surface plasmon resonance (Eq. 6). Then the
plasmon-induced electrons of silver nanoparticles are quickly injected into the ZnO
CB (Eq. 7). Then ZnO trapped electrons and Ag trapped holes react with the
preadsorbed oxygen (Eq. 8) and surface-bound hydroxyl groups (–OH) (Eq. 9),
respectively, to generate hydroxyl radicals. Finally, the hydroxyl radicals degrade MB
by hydrogen abstraction and subsequent oxidation processes (Eq. 5) [11, 17].
Consequently, Ag/ZnO nanocomposite samples appear to have different electron
transfer directions under UV irradiation and solar irradiation.
Ag + hv (visible light) → h+ + e−
(6)
(7)
(8)
(9)
Ag (e−) + ZnO → ZnO (eCB−) + Ag (h+)
−
•−
H
•
•
•−
−
+
ZnO (eCB ) + O →ZnO + O → 2HO → H O → OH + OH + O
2O2
2
2
2
2
2
Ag (h+) + OH − → Ag + •OH
3.2.2.3 The concentration of hydroxyl radicals
The concentration of hydroxyl radicals, which are considered a major reactive species
in dye degradation, was monitored by the terephthalic acid photoluminescence
probing technique. Terephthalic acid is non-fluorescent; however, it can react with

ACCEPTED MANUSCRIPT
hydroxyl radicals to give fluorescent 2-hydroxyterephthalic acid. On careful
examination of Fig. 15, one can notice that terephthalic acid, being non-fluorescent,
does not give any peak, while a pronounced peak appears at 423 nm for ZnAg3.
3.2.2.4 Photocatalytic degradation of MB under visible light:
The photocatalytic reactivity of the samples under a xenon lamp of 100 W intensity
and with a cutoff filter to remove UV radiation (λ ≤ 420 nm) was studied to
investigate the role of metallic silver nanoparticles in shifting the photocatalytic
response to the visible region. The results reveal that MB degradation increased from
28% to 76% on incorporation of 3 wt% Ag0 on the ZnO surface during 2 h of
irradiation (Fig. 16).
4. Conclusion
ZnO nanoparticles containing various proportions of metallic silver were prepared by
an innovative combined sol-gel and chemical reduction method. Chitosan was used as
an environmentally benign reducing agent to avoid the toxicity of traditionally used
reducing agents. XPS and XRD analyses revealed the existence of metallic silver
nanoparticles on the ZnO surface. Photoluminescence and XPS studies provided
evidence for strong interaction between ZnO and Ag0. The photocatalytic degradation
of MB increases with increasing silver content up to 3 wt%, followed by reduction for
the sample containing 5 wt% Ag. The ease of electron transfer between CB electrons
of ZnO and the Fermi level of metallic silver nanoparticles, which in turn limits
electron-hole recombination, is the main cause of enhancement of the
photodegradation of MB under UV and solar radiation.
References
1. E. Forgacs, T. Cserhati, G. Oros, Removal of synthetic dyes from wastewaters: a
review. Environ. Int. 30(7) (2004) 953.
2. M.A. Ahmed⁎, Z.M. Abou-Gamra, A.M. Salem, Photocatalytic degradation of
methylene blue dye over novel spherical mesoporous Cr2O3/TiO2 nanoparticles
prepared by sol-gel using octadecylamine template, J. Environ. Chem. Engin. 5
(2017) 4251–4261

ACCEPTED MANUSCRIPT
3. M.A. Messih, M.A. Ahmed, A. Soltan, S.S. Anis, Facile approach for
homogeneous dispersion of metallic silver nanoparticles on the surface of
mesoporous titania for photocatalytic degradation of methylene blue and indigo
carmine dyes. J. Photochem. Photobiol. A 335 (2017) 40.
4. A. Azam, F. Ahmed, N. Arshi, M. Chaman, A.H. Naqvi, Formation and
characterization of ZnO nanopowder synthesized by sol–gel method, J. Alloys
Compd. 496 (2010) 399.
5. M.M. Khan, S.A. Ansari, M.E. Khan, M.O. Ansari, B.K. Min, M.H. Cho, Visible
light-induced enhanced photoelectrochemical and photocatalytic studies of gold
decorated SnO2 nanostructures. New J. Chem. 39(4) (2015) 2758.
6. J. Zhao, C. Chen, W. Ma, Photocatalytic degradation of organic pollutants
under visible light irradiation, Top. Catal. 35(3–4) (2005) 269.
7. X. Zhang, Y. Wang, F. Hou, H. Li, Y. Yang, X. Zhang, Y. Wang, Effects of Ag
loading on structural and photocatalytic properties of flower-like ZnO
microspheres, Appl. Surf. Sci. 391(2017) 476.
8. N. Güy, M. Özacar, The influence of noble metals on photocatalytic activity of
ZnO for Congo red degradation, Int. J. Hydrogen Energy, 41(44) (2016) 20100.
9. B. Pant, M. Park, H.Y. Kim, S.J. Park, Ag-ZnO photocatalyst anchored on
carbon nanofibers: synthesis, characterization, and photocatalytic activities,
Synth. Met. 220 (2016) 533.
10. O. Bechambi, M. Chalbi, W. Najjar, S. Sayadi, Photocatalytic activity of ZnO
doped with Ag on the degradation of endocrine disrupting under UV irradiation
and the investigation of its antibacterial activity, Appl. Surf. Sci. 347 (2015) 414.
11. S. Sohrabnezhad, A. Seifi, The green synthesis of Ag/ZnO in montmorillonite
with enhanced photocatalytic activity, Appl. Surf. Sci. 386 (2016) 33.
12. S. Rehman, R. Ullah, A.M. Butt, N.D. Gohar, Strategies of making TiO2 and
ZnO visible light active, J. Hazard. Mater. 170(2) (2009) 560.
13. M.A. Ahmed, M.F. Abdel Messih, E.F. El-Sherbeny, Suzan F. El-Hafez, Aliaa
M.M. Khalifa, Synthesis of metallic silver nanoparticles decorated mesoporous

ACCEPTED MANUSCRIPT
SnO2 for removal of methylene blue dye by coupling adsorption and
photocatalytic processes, J. Photochem. Photobiol.. A. 346 (2017) 77-88
14. Y. Zhang, Q. Wang, J. Xu, S. Ma, Synthesis of Pd/ZnO nanocomposites with
high photocatalytic performance by a solvothermal method. Appl. Surf. Sci.
258(24) (2012) 10104.
15. J. Yuan, E.S.G. Choo, X. Tang, Y. Sheng, J. Ding, J. Xue. Synthesis of ZnO–Pt
nanoflowers and their photocatalytic applications, Nanotechnology 21(18) (2010)
185606.
16. M.N. Tahir, F. Natalio, M.A. Cambaz, M. Panthöfer, R. Branscheid, U. Kolb, W.
Tremel, Controlled synthesis of linear and branched Au@ZnO hybrid
nanocrystals and their photocatalytic properties, Nanoscale 5(20) (2013) 9944.
17. Z. Han, L. Ren, Z. Cui, C. Chen, H. Pan, J. Chen. Ag/ZnO flower
heterostructures as a visible-light driven photocatalyst via surface plasmon
resonance, Appl. Catal. B 126 (2012) 298.
18. T. Cai, L. Wang, Y. Liu, S. Zhang, W. Dong, H. Chen, X. Yi, J. Yuan, X. Xia, C.
Liu, S. Luo, Ag3PO4/Ti3C2 MXene interface materials as a Schottky catalyst with
enhanced photocatalytic activities and anti-photocorrosion performance, Appl.
Catal. B 239 (2018) 545–554 19. T. Cai, Y. Liu, L. Wang, S. Zhang, W. Dong, H.
Chen, J. Ma, C. Liu, S. Luo, Ultrafine Ag@AgI nanoparticles on cube single-
crystal Ag3PO4 (100): an all-day-active Z-Scheme photocatalyst for
environmental purification, J. Colloid Interface Sci. 533 (2019) 95–105.
20. T. Cai, Y. Liu, L. Wang, S. Zhang, Y. Zeng, J. Yuan, J. Ma, W. Dong, C. Liu, S.
Luo, Silver phosphate-based Z-Scheme photocatalytic system with superior
sunlight photocatalytic activities and anti-photocorrosion performance, Appl.
Catal. B 208 (2017) 1–13.
21. C. Jaramillo-Páez, J. A. Navío, M. C. Hidalgo, Silver-modified ZnO highly UV-
photoactive, J. Photochem. Photobiol. A 356 (2018) 112–122.
22. S.M. Lama, J.A. Queka, J.C. Sin, Mechanistic investigation of visible light
responsive Ag/ZnO micro/ nanoflowers for enhanced photocatalytic
performance and antibacterial activity, J. Photochem. Photobiol. A 353 (2018)
171–184.

ACCEPTED MANUSCRIPT
23. S.A. Ansari, M.M. Khan, J. Lee, M.H. Cho. Highly visible light active Ag@
ZnO nanocomposites synthesized by gel-combustion route. Ind. Eng. Chem. Res.
20(4) (2014) 1602.
24. B. Chai, X. Wang, S. Cheng, H. Zhou, F. Zhang. One-pot triethanolamine-
assisted hydrothermal synthesis of Ag/ZnO heterostructure microspheres with
enhanced photocatalytic activity. Ceram. Int. 40(1) (2014) 429.
25.S.M. Hosseini, I. Abdolhosseini Sarsari, P. Kameli, H. Salamati, Effect of Ag
doping on structural, optical, and photocatalytic properties of ZnO
nanoparticles, J. Alloys Compd. 640 (2015) 408-415.
26. B.M. Rajbongshi, A. Ramchiary, B.M. Jha, S.K. Samdarshi, Synthesis and
characterization of plasmonic visible active Ag/ZnO photocatalyst. J. Mater. Sci.
Mater. Electron. 25(7) (2014) 2969.
27. Y. Zhang, J. Xu, Q. Wang, Z. Pan, S. Ma, Q. Chen, Preparation and properties
of ZnO loaded with uniform Ag nanoparticles. Mater. Res. Bull. 73 (2016) 119.
28. D. Wei, W. Qian. Facile synthesis of Ag and Au nanoparticles utilizing
chitosan as a mediator agent. Colloids Surf. B 62(1) (2008) 136.
29. A. Dolgonos, T.O. Mason, K.R. Poeppelmeier, Direct optical band gap
measurement in polycrystalline semiconductors: a critical look at the Tauc
method. J. Solid State Chem. 240 (2016) 43.
30. K.S. Sing. Reporting physisorption data for gas/solid systems with special
reference to the determination of surface area and porosity (recommendations
1984). Pure Appl. Chem. 57(4) (1985) 603.
31. V. Vaiano, M. Matarangolo, J.J. Murcia, H. Rojas, J.A. Navío, M.C. Hidalgo,
Enhanced photocatalytic removal of phenol from aqueous solutions using ZnO
modified with Ag. Appl. Catal. B 225 (2018) 197–206.
32. A. Phuruangrat, S. Siri, P. Wadbua, S. Thongtem, Microwave-assisted synthesis,
photocatalysis and antibacterial activity of Ag nanoparticles supported on ZnO
flowers. J. Phys. Chem. Solids 126 (2019) 170–177.

ACCEPTED MANUSCRIPT
33. H. Liu, L. Zhong, S. Govindaraju, K. Yun, ZnO rod decorated with Ag
nanoparticles for enhanced photocatalytic degradation of methylene blue. J.
Phys. Chem. Solids,129 (2019) 46–53.
34. M.J. Sampaio, M.J. Lima, D.L. Baptista, A.M. Silva, C.G. Silva, J.L. Faria, Ag-
loaded ZnO materials for photocatalytic water treatment. Chem. Eng. J. 318
(2017) 95.
35. J.C. Bian, F. Yang, Z. Li, J.L. Zeng, X.W. Zhang, Z.D. Chen, J. Wang,
Mechanisms in photoluminescence enhancement of ZnO nanorod arrays by the
localized surface plasmons of Ag nanoparticles. Appl. Surf. Sci. 258(22) (2012)
8548.
36. J. Kaur, K. Gupta, V. Kumar, S. Bansal, S. Singhal, Synergic effect of Ag
decoration onto ZnO nanoparticles for the remediation of synthetic dye
wastewater. Ceram. Int. 42(2) (2016) 2378.
37. L. Chen, J. Li, L. Yuan, Q. Cai. Synthesis and photocatalytic application of
Au/Ag nanoparticle-sensitized ZnO films. Appl. Surf. Sci. 273 (2013) 82.
38. E. Mendoza-Mendoza, A.G. Nuñez-Briones, L.A. García-Cerda, R.D. Peralta-
Rodríguez, A.J. Montes-Luna, One-step synthesis of ZnO and Ag/ZnO
heterostructures and their photocatalytic activity, Ceram. Int. 44 (2018) 6176–
6180.
39. A.N. Kadam, D.P. Bhopate, V.V. Kondalkar, S.M. Majhi, S.-W. Lee, Facile
synthesis of Ag-ZnO core–shell nanostructures with enhanced photocatalytic
activity, J. Ind. Eng. Chem. 61 (2018) 78–86.
40. M. Wang, J. Xu, T. Sun, Y. Tang, G. Jiang, Y. Shi, Facile photochemical
synthesis of hierarchical cake-like ZnO/Ag composites with enhanced visible-
light photocatalytic activities, Mater. Lett. 219 (2018) 236–239.
41. S.M. Lama, J.A. Queka, J.C. Sin, Mechanistic investigation of visible light
responsive Ag/ZnO micro/nanoflowers for enhanced photocatalytic performance
and antibacterial activity, J. Photochem. Photobiol. A 353 (2018) 171–184.
42. S. Rajaboopathi, S. Thambidura, Enhanced photocatalytic activity of Ag-ZnO
nanoparticles synthesized by using Padina gymnospora seaweed extract, J. Mol.
Liq. 262 (2018) 148–160.

ACCEPTED MANUSCRIPT
43. M. Kang, S.J. Choung, J.Y. Park, Photocatalytic performance of nanometer-
sized FexOy/TiO2 particle synthesized by hydrothermal method, Catal. Today 87
(2003) 87–97.
44. Karakitsou, K. E., & Verykios, X. E. (1993). Effects of altervalent cation doping
of titania on its performance as a photocatalyst for water cleavage. The Journal of
Physical Chemistry, 97(6), 1184-1189.
45. Dung, N. T., Van Khoa, N., & Herrmann, J. M. (2005). Photocatalytic
degradation of reactive dye RED-3BA in aqueous TiO2 suspension under UV-
visible light. International Journal of photoenergy, 7(1), 11-15.
46. X. Chen, Z. Wu, D. Liu, Z. Gao, Preparation of ZnO Photocatalyst for the
efficient and rapid photocatalytic degradation of azo dye, Nanoscale Res. Lett. 12
(2017) 143.
47. D. Zhang, X. Pu, H. Li, Y. Moon Yu, J.J Shim, P. Cai, S. Kim, H.J. Seo,
Microwave-assisted combustion synthesis of Ag/ZnO nanocomposites and their
photocatalytic activities under ultraviolet and visible-light irradiation, Mater.
Res. Bull. 61 (2015) 321–325.
48. Y.W. Tseng, F.Y. Hung, T. S. Lui, S.J. Chang, Structural and Raman
properties of silver-doped ZnO nanorod arrays using electrically induced
crystallization process, Mater. Res. Bull. 64 (2015) 274–278.
49. M.H. Sarvari, T.H. Kachouei, F. Moeini, A novel and active catalyst Ag/ZnO
for oxidant-free dehydrogenation of alcohols, Mater. Res. Bull. 72 (2015) 98–105.
Fig. 1. The proposed mechanism for reduction of silver ions by chitosan.
Fig. 2. The photocatalytic reactor used in the study of the photocatalytic activity of
the samples.
Fig. 3. X-ray diffraction patterns of ZnO and various Ag/ZnO powders.
Fig. 4. High-resolution transmission electron microscopy images of ZnO (a), ZnAg1
(b), ZnAg3 (c), and ZnAg5 (d), and lattice spacing of ZnAg3 (e).

ACCEPTED MANUSCRIPT
Fig. 5. N2 adsorption–desorption isotherms of ZnO (a), ZnAg1 (b), ZnAg3 (c), and
ZnAg5 (d).
Fig. 6. X-ray photoelectron spectroscopy core level spectra of Zn 2p (a), O 1s (b), and
Ag 3d (c) regions for ZnAg5.
Fig. 7. Absorption spectra of pure and Ag-doped ZnO powdered samples.
Fig. 8. (Ahν)2 versus photon energy (hν) for pure and Ag-doped ZnO samples.
Fig. 9. Photoluminescence spectra of pure ZnO and ZnAg3.
Fig. 10. Photocatalytic degradation of methylene blue over ZnO (a), ZnAg1 (b),
ZnAg3 (c), and ZnAg5 (d).
Fig. 11. Pseudo-first-order kinetics of methylene blue degradation over ZnO, ZnAg1,
ZnAg3, and ZnAg5.
Fig. 12. Photocatalytic degradation of methylene blue over ZnAg3 under solar
irradiation.
Fig. 13. Ag/ZnO photocatalytic mechanism under UV light. CB, conduction band;
VB valence band.
Fig. 14. Ag/ZnO photocatalytic mechanism under sunlight. CB, conduction band; VB
valence band.
Fig. 15. Fluorescence spectral changes observed during illumination of ZnAg3 in 5 ×
10-5 M NaOH solution of terephthalic acid (excitation at 315 nm).
Fig. 16. Photocatalytic degradation of methylene blue over ZnO and ZnAg3 under a
halogen lamp.
Table 1. The crystallite size, surface area, pore volume, and pore diameter of pure and
Ag-doped ZnO nanocrystalline powders.
Ag (wt%)
Crystallite size
Surface
Pore
Pore diameter

ACCEPTED MANUSCRIPT
(nm)
area(m2/g)
volume(ml/g)
0
1
3
5
61
61
7.8
0.039
0.036
0.034
0.033
17.6
16.3
15.8
15.4
6.5
53.6
47.5
5.2
4.7
Table 2. Band gap energy and the corresponding absorption edge of the samples.
Ag (wt%)
Eg (eV)
3.22
Absorption edge (nm)
0
1
3
5
385
385
3.22
3.18
389.9
389.9
3.18
Table 3. Catalytic parameters for each photocatalyst.
Sample Ag (wt%) Photocatalytic activity
of sample for MB
Moles of MB
TOR (removed mol MB/g
converted
catalyst min)
degradation (%)
ZnO
ZnAg1
ZnAg3
ZnAg5
0
1
3
5
65.6
79.5
82.6
81.2
1.312 × 10-5
1.59 × 10-5
1.652 × 10-5
1.624 × 10-5
5.46 × 10-7
6.62 × 10-7
6.88 × 10-7
6.76 × 10-7
MB, methylene blue; TOR turnover rate.
Table 4. Kinetic parameters for methylene blue photocatalytic degradation by each
photocatalyst.
Sample
ZnO
k (min-1)
0.0043
0.0067
0.007
t1/2 (min)
161.2
103.5
99
ZnAg1
ZnAg3

ACCEPTED MANUSCRIPT
ZnAg5
0.0068
102

ACCEPTED MANUSCRIPT
Caption to figures
Fig. 1 The proposed mechanism for reducing silver ions by chitosan
Fig. 2 Schematic illustration of the photocatalytic reactor used in testing the prepared
samples photocatalytic activity.
Fig. 3 XRD patterns of ZnO and various Ag/ZnO powders.
Fig. 4 HRTEM images of ZnO (a), ZnAg1 (b), ZnAg3 (c), and ZnAg5 (d).
Fig. 5 N2 adsorption–desorption isotherms of ZnO (a), ZnAg1 (b), ZnAg3 (c), and
ZnAg5 (d).
Fig. 6 XPS patterns of ZnAg5
Fig. 7 The absorption spectra for pure and Ag doped ZnO powdered samples
Fig. 8 (Ahν)2 vs. photon energy (hν) plots for pure and silver doped ZnO samples.
Fig. 9 PL spectra for pure and 3 wt. % Ag doped ZnO.
Fig. 10 Photocatalytic degradation of methylene blue over ZnO (a), ZnAg1 (b),
ZnAg3 (c), and ZnAg5 (d).
Fig. 11 The pseudo first order kinetics of methylene blue degradation over ZnO,
ZnAg1, ZnAg3 and ZnAg5.
Fig. 12 Photocatalytic degradation of methylene blue over ZnAg3 under solar
irradiation.
Fig. 13 Fluorescence spectral changes observed during illumination of ZnAg3 in 5 x
10-5 M NaOH solution of terephthalic acid (excitation at 315 nm).
Fig. 14 Photocatalytic degradation of MB dye over ZnO and ZnAg3 under
halogen lamp
Fig. 15 Schematic of Ag/ZnO photocatalytic mechanism under UV light.

ACCEPTED MANUSCRIPT
Fig.16 Schematic of Ag/ZnO photocatalytic mechanism under sun light.
Fig. 1
CH2OH
CH2OH
O
O
O
O
OH
OH
O
NHCCH3
O
NH2
+
2Ag+
+
2Ag+
CH2OH
CH2OH
O
O
O
O
O
O
NHCCH3
O
O
NH2
+
2Ag
+
2Ag
+
+
2H+
2H+

ACCEPTED MANUSCRIPT
Fig.2

ACCEPTED MANUSCRIPT
Fig.3
ZnO
3500
3000
2500
2000
1500
1000
500
ZnO (101)
ZnAg1
ZnAg3
ZnAg5
ZnO (100)
ZnO (002)
ZnO (110)
ZnO (103)
ZnO (102)
Ag (111)
Ag (200)
0
20
30
40
50
2Theta
60
70
80

ACCEPTED MANUSCRIPT
Fig. 4

ACCEPTED MANUSCRIPT
Fig. 5

ACCEPTED MANUSCRIPT
Fig. 6
(a )
Z n 2 p
6 0 0 0 0
5 0 0 0 0
4 0 0 0 0
3 0 0 0 0
2 0 0 0 0
1 0 0 0 0
Z n 2 p 3/2
Z n 2 p 1/2
1 0 1 5
1 0 2 0
1 0 2 5
1 0 3 0
1 0 3 5
1 0 4 0
1 0 4 5
1 0 5 0
B in d in g E n e rg y (E ) (e V )
O 1 s
(b )
1 8 0 0 0
1 6 0 0 0
1 4 0 0 0
1 2 0 0 0
1 0 0 0 0
8 0 0 0
6 0 0 0
5 2 5
5 3 0
5 3 5
5 4 0
B in d in g E n e rg y (E ) (e V )

ACCEPTED MANUSCRIPT
A g3d
(c)
2000
1900
1800
1700
1600
1500
1400
A g 3d5/2
A g 3d3/2
360
365
370
375
380
B inding E nergy (E ) (eV )
Fig. XPS core level spectra of Zn 2p (a), O 1s (b) and Ag 3d (c) regions for ZnAg5 sample.
Fig. 7
ZnO
ZnAg1
1.6
ZnAg3
ZnAg5
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
200
300
400
500
600
700
800
"Wavelength nm."

ACCEPTED MANUSCRIPT
Fig. 8
ZnO
80
70
60
50
40
30
20
10
0
ZnAg1
ZnAg3
ZnAg5
1
2
3
4
5
6
7
energy (ev)