Facile synthesis of rGO/Ag@AgCl core-shells nanocomposite and their multifunctional efficacy as a photocatalyst and antimicrobial agent for decontamination of water

Article abstract of DOI:10.1016/j.jallcom.2020.157988

An environmentally friendly process for synthesis of rGO/Ag@AgCl Core-shells nanocomposite was developed by Bitter gourd leaf extract. The core-shell morphology was confirmed by STEM-EDX line profile and elemental mapping images. As-synthesized nanocomposite was utilized as photocatalyst for degradation of Clofibric acid (CA) and Naphthol Green B (NGB) from aqueous phase under solar irradiation. As-synthesized nanocomposites showed photodegradation up to 98% of CA and 98.7% of NGB within 40 and 30 min, respectively. Here, we also investigated the optimum amount of nanocomposites for the photodegradation process. ESR analyses confirmed that the active species such as hydroxyl and superoxide anion radicals were mainly accountable for the degradation of organic pollutants. The presence of rGO enhanced the catalytic activity of Ag@AgCl NPs. The as-prepared nanocomposite was also utilized for the removal of biological impurities against bacterial strains. Hence form the present study, it was confirmed that the synthesized rGO-Ag@AgCl nanocomposites act as a potential material for wastewater treatment.

Full text of DOI:10.1016/j.jallcom.2020.157988

Journal Pre-proof
Facile synthesis of rGO/Ag@AgCl core-shells nanocomposite and their
multifunctional efficacy as a photocatalyst and antimicrobial agent for
decontamination of water
Th.Babita Devi, M. Ahmaruzzaman
PII:
S0925-8388(20)34352-8
DOI:
https://doi.org/10.1016/j.jallcom.2020.157988
JALCOM 157988
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 26 September 2020
Revised Date: 14 November 2020
Accepted Date: 16 November 2020
Please cite this article as: T.B. Devi, M. Ahmaruzzaman, Facile synthesis of rGO/Ag@AgCl core-
shells nanocomposite and their multifunctional efficacy as a photocatalyst and antimicrobial
agent for decontamination of water, Journal of Alloys and Compounds, https://doi.org/10.1016/
j.jallcom.2020.157988.
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Credit Author Statement:
Th. Babita Devi: Conceptualization, Methodology, Data curation, Writing- Original draft
preparation. M. Ahmaruzzaman: Supervision, Writing- Reviewing and Editing.

Graphical abstract

Facile synthesis of rGO/Ag@AgCl core-shells nanocomposite and their
multifunctional efficacy as a photocatalyst and antimicrobial agent for
decontamination of water
Th. Babita Devi and M. Ahmaruzzaman*
Department of Chemistry National Institute of Technology Silchar, Assam -788010
Abstract
An environmentally friendly process for synthesis of rGO/Ag@AgCl Core-shells nanocomposite
was developed by Bitter gourd leaf extract. The core-shell morphology was confirmed by
STEM-EDX line profile and elemental mapping images. As-synthesized nanocomposite was
utilized as photocatalyst for degradation of Clofibric acid (CA) and Naphthol Green B (NGB)
from aqueous phase under solar irradiation. As-synthesized nanocomposites showed
photodegradation up to 98% of CA and 98.7% of NGB within 40 and 30 min, respectively. Here,
we also investigated the optimum amount of nanocomposites for the photodegradation process.
ESR analyses confirmed that the active species such as hydroxyl and superoxide anion radicals
were mainly accountable for the degradation of organic pollutants. The presence of rGO
enhanced the catalytic activity of Ag@AgCl NPs. The as-prepared nanocomposite was also
utilized for the removal of biological impurities against bacterial strains. Hence form the present
study, it was confirmed that the synthesized rGO-Ag@AgCl nanocomposites act as a potential
material for wastewater treatment.
Keywords: Reduced graphene oxide; Clofibric acid ; Naphthol Green B; photocatalyst.
*Corresponding author: (M. Ahmaruzzaman: Email ID -mda@2002@gmail.com)
1

1. Introduction
Recently, the treatment of industrially polluted waste-water which is hard to remove from
aqueous phase biochemically is a significant zone of research worldwide [1-2]. The paper,
textile, and wood industries are mainly responsible for the pollution of water. Such types of
industry are users of toxic elements, dyes, and solvents for dyeing. It was consumed above 60%
of toxic dyes and many of them, containing one or more toxic and explosive chemicals for such
type of purpose. Recently it was observed that Naphthol Green B dye used for the dyeing
purpose. Naphthol Green B (NGB) dye, is a radical configuration consisting of conjugated
double bonds containing delocalized electrons, which containing chromophore groups, such as
azo (-N=N-), nitroso (-NO or N-OH), nitro (-NO2 or =NO-OH), sulfur (C=S) [3]. Such azo dyes
are very highly toxic, and their by-products (degraded) are carcinogenic and cause significant
environmental and water harm.
Nowadays, the psychotropic drug is highly consumed for the treat symptoms of mental disorders
such as depression, schizophrenia, bipolar disorder has a disability angiocardiopathy problem,
such as coronary heart disease, high blood pressure, arrhythmia, and cardiac function failure [4-
11]. Among those pharmaceutical compounds, Clofibric acid (CA) is utilized for the treatment of
diseases, such as coronary heart disease, high blood pressure, arrhythmia, and cardiac function,
and epilepsy. It is highly motile and very persistent in the environment (lakes and rivers) because
it is non-biodegradable [12-13]. Thus, it is very important to remove such organic and
pharmaceutical pollutants from the environment and water purification.
Adsorption process is commonly utilized for removal of such type of toxic drug from aqueous
phase because of its simplicity and general applicability [14-16]. However, such type of method
created some problems during separation efficiency. The formation of secondary by-products is
2

the main disadvantages of the adsorption process. Nowadays, Sunlight-induced degradation
process is one of the most simple and green approaches for removal, such type of toxic elements
from aqueous phase [17].
From the literature, it was observed that TiO2 photocatalyst has been widely considered as a
traditional photocatalyst because of its non-toxicity as compared with other metal nanoparticles
(such as silver and gold) and long-term stability [18-20]. However, some drawbacks are
associated with the practical application of TiO2 photocatalyst because of its high band gap.
Therefore, several researchers carried out experiments to prepare very effective photocatalytic
materials for removal of various contaminants from aqueous phase [21-22]. Recently, it was
observed that hybrid NPs and nanocomposites have been used in different application [23-28].
But X-hybrid/nanocomposite (X=Ag/Au) were showed an excellent photocatalytic material for
elimination of such noxious compounds. Recently, reduced graphene oxide is utilized as a phase
matrix for the development of nanocomposites and its subsequent application for photocatalytic
removal of various organic contaminants from aqueous phase [29-31]. The polluted water
contains various types of unwanted pollutants, such as toxic elements, microbes, fungi, etc. Due
to the high surface area of graphene, it can sufficiently absorb such toxic particles into the
surface pores of rGO. The high specific surface area allows more metal ion (attached on the
surface of rGO) to come into full contact with its inner surface and damages bacterial cell wall
[32-35].
Hence, main objectives of this present investigation are summarized below:
(i) Fabrication and characterization of plasmon-induced reduced graphene oxide nanocomposites
using various techniques.
3

(ii) CA (drug) and NGB dye was selected as target compounds and degraded these toxic
compounds using as-synthesized nanocomposites.
(iv) To examine the antimicrobial activities of the synthesized rGO-Ag@AgCl nanocomposites.
Herein, we report, for the first time, the preparation of rGO-Ag@AgCl nanocomposites using
bitter gourd (medicinal plant) leaf extract.
The chemical structure of Clofibric acid and Naphthol Green B dye were given below:
Fe3+
N
O
HO
O
Cl
O
NaO3S
Napthol Green B
3
(Clofibric acid)
2. Materials and Method
2.1. Materials
Reduced graphene oxide was purchased from Nanoshel LCC Punjab, India and Clofibric acid [2-
(4-chlorophenoxy)-isobutyric acid] and Naphthol Green B (trisodium, iron (III), 5-nitroso-6-
oxidonaphthalene-2-sulfonate) were purchased from Merck Ltd. India. All solutions were
prepared in double-distilled water.
2
.2. Synthesis of reduced graphene oxide loaded Ag@AgCl nanocomposites (rGO-
Ag@AgCl)
Ag@AgCl nanoparticles were synthesized using Bitter gourd leaf extract. The method for the
preparation of Ag@AgCl NPs was modified, as mentioned in previous work [36]. The MC leaf
extract was added dropwise in 20 mL of AgNO3 (0.001M) solution under stirred condition and
4

reaction mixture was changed into brown color within 10 min. About 30 mg of rGO was
dispersed in 300 mL colloidal solution of Ag@AgCl NPs and stirred at room temperature for 24
h. After 24 h, brown color of nanoparticles was changed into a black and indicated formation
of rGO-Ag@AgCl nanocomposites. The growth mechanism of rGO-Ag@AgCl nanocomposites
was shown schematically (Scheme 1).
2.3. Photodegradation of hazardous dye and pharmaceutical compound rGO-Ag@AgCl
nanocomposites as photocatalyst
The optimization study of catalysts also reported in this present work. The experiment was
carried out by keeping a solution concentration of 10 ppm for both NGB and CA. To evaluate
optimum amount of catalyst, the experiment was carried out by changing the amount of catalyst
from (0.01 to 0.05) gm in 250 mL of NGB and CA solution. Here the catalysts were added in
each 250 mL solution of NGB and CA solution and checked the catalytic activities. It was found
that photodegradation efficiency increased with increase in catalyst amount from 0.01g to 0.025
g. However, the efficiency of photodegradation is decreased with increase in catalyst amount
from 0.03 g to 0.05 g. The optimum amount of the catalyst was observed to be 0.025 g in the
degradation of both NBG and CA from aqueous phase [37-39].
In a photocatlytic experiments, specific amount of photocatalyst (0.025 gm) suspended in 250
mL of 10 mg/L aqueous solutions of CA and NGB. CA and NGB solutions were kept in a dark
environment under agitation for 30 min to achieve the adsorption-desorption equilibrium
followed by exposure to sunlight (average solar radiation ~4.29 kWh/m2/day) for
photodecomposition [37]. A total of 4 mL of NGB and CA aqueous suspensions were
withdrawn, centrifuged and recorded by noting the absorbance at their respective
10 min.
λmax after every
5

2.4. Recycling of photocatalysts
The life span of a catalyst for a longer period leads to a considerable decrease of cost. It is very
important to check the reusability of rGO-Ag@AgCl. The details of the recovery of the
photocatalyst and their subsequent use in photodegradation were reported in our previous study
[36]. The recovered catalysts were reutilized for photodegradation of NGB and CA upto fourth
cycles, and their catalytic losses were also calculated.
2.5. Antimicrobial activities of rGO-Ag@AgCl nanocomposites
The biological activities of nanocomposites were also investigated by Kirby Bauer or Disc
standard disc diffusion method. Here we used two different gram-negative and gram-positive
bacteria like Pseudomonas aeruginosa
(P. aeruginosa), Escherichia coli (E. coli),
Staphylococcus aureus S. aureus), and Staphylococcus epiderminis (S. epiderminis). The
(
details of experiments for antimicrobial activities were reported in our previous study [40]. After
24 h of incubation, the zones of inhibition (mm) were calculated using standard scale. Here, code
B1, B2, B3 and B4 were presented the Staphylococcus aureus
(S. aureus), Escherichia coli (E.
coli)), Pseudomonas aeruginosa P. aeruginosa), and Staphylococcus epiderminis (S.
(
epiderminis). The Code 1 represents rGO-Ag@AgCl, Code 2 presents rGO and Code 3 AgNO3
solution.
2.6. Characterization
The synthesized rGO-Ag@AgCl nanocomposites were well characterized by various analyses.
Here we investigated the absorption pattern (UV-visible), Powder X-ray diffraction (XRD)
patterns, morphology and diffracted ring pattern (TEM, AFM and SEM), the functional groups
(FT-IR), element mapping (S-TEM, EDX and EDX line profile analysis), elemental composition
and chemical status (XPS), radical determination (ESP), and the surface area and pore size of
6

nanocomposites (BET). We adopted the same method and instrument as mentioned in our
previous work [36].
3 Results and Discussion
3.1. Ultraviolet-visible spectroscopy
The optical absorption properties of the synthesized Ag@AgCl/rGO nanocomposite were
investigated using UV-Vis spectrophotometry and represented in Fig. 1 (a). The broad band
centered at 460 nm was attributed to the surface plasmon resonance (SPR) of Ag nanoparticles.
The UV cut-off in the range 280 – 320 nm can be ascribed to the photo-excitation of electrons
from valence band (VB) to conduction band (CB) of AgCl and the sharp peak centered at ~230
nm can be assigned to the π-π* transition of aromatic C=C bonds of rGO. The surface plasmon
resonance of Ag nanoparticles can induce enhanced light absorption capacity to the Ag@AgCl
photocatalyst and the hybridization with rGO may increase the life span of photogenerated
excitons by delocalizing electrons to the C=C backbone [40]. Therefore, Ag@AgCl/rGO is
believed to have high light absorption capacity and low charge recombination rate which can
synergistically enhance the photocatalytic property of the engineered nanohybrid.
Further, to determine the direct band gap energies of the Ag@AgCl/rGO nanocomposite, Tauc’s
theorem can be employed using the following equation [41-42],
(α
hv)2 = A(hv-Eg)
where h is Planck’s constant,
α
is the absorption coefficient, is the photon frequency, Eg is the
band gap energy and A is a proportionality constant. The X-axis intersection point of the linear
fit of Tauc’s plot gives an estimate of the band gap energy. The Tauc’ plot showed two possible
tangents corresponding to surface plasmon resonance of Ag nanoparticle and electronic
7

excitation of AgCl semiconductor nanoparticle (Fig. 1(b)). The calculated band gap energies for
Ag and AgCl using Tauc’s method were found to be 1.8 eV, and 3.7 eV respectively.
The conduction band (CB) and valence band (VB) positions of AgCl can be calculated from the
following equation [43],
ECB = X-Ec-0.5Eg
EVB= ECB + Eg
where X is the absolute electronegativity of the semiconductor (XAgCl = 6.07 eV [43]), Ec is the
energy of free electrons in hydrogen scale (4.5 eV), Eg is the band gap energy, ECB and EVB are
the conduction band and valence band potentials, respectively. Accordingly, the ECB and EVB of
AgCl were calculated to be -0.28 eV and 3.42 eV, respectively.
3.2. Morphology studies
3.2.1. Scanning and Transmittance electron microscope
Surface morphologies of inner and outer surfaces of the nanocomposites were investigated by
scanning and transmission electron microscope. Fig. 2 (a-b) demonstrates scanning electron
microscope (SEM) and transmission electron microscope (TEM) images of rGO-Ag@AgCl
nanocomposites. From SEM and TEM images, shape and average particles size of
nanocomposites can be calculated. From the SEM image, spherical shape of nanocomposites was
observed and calculated average particles size was found to be ~40 nm. TEM images of rGO-
Ag@AgCl nanocomposites showed that a core-shell shape was formed. The core and shell
structure was clearly identified by brightness differences between the silver and silver chloride of
the particles. This was due to the difference of densities between Ag and AgCl atoms [44-45].
The core and shell NPs Ag@AgCl were further attached on the surface of the reduced graphene.
8

The calculated shell thickness and average particles size, from TEM image were 3 and 10 nm,
respectively. Here the shell thickness was found to be 3 nm, therefore Ag core was also marked
in plasmonic contribution [44-46]. Fig. 2 (c-d) displayed HR-TEM and SAED pattern of rGO-
Ag@AgCl nanocomposites. The calculated d-spacing value from HR-TEM images corresponds
to the (111) plane of Ag crystal and (222) plane of AgCl crystal respectively. The SAED pattern
revealed the polycrystalline nature of the nanocomposites. The d-spacing values were calculated
from the SAED pattern and observed to be 0.138, 0.195 and 0.0876 nm and corresponds to the
lattice plane (400), (220), (620) of fcc structure of AgCl, respectively and 0.116 nm corresponds
to lattice plane (222) of Ag (fcc) structure.
3.2.2. Atomic force microscope microscopy
Further, the core shell morphology was confirmed by AFM (Atomic force microscope) analysis.
The AFM was performed in contact mode and image was returned in two forms: 3-dimenssion
(3D) topography and 2-dimension (2D) scan as seen in Fig. 2 (e-d). In the 2D image of AFM the
formation of core-shell morphology was clearly observed. The nanocomposites showed
monodispersed and well separated from each other.
3.3. X-ray Diffraction Pattern analysis
Fig. 3 (a-b) displays XRD spectra of reduced graphene oxide (rGO) and rGO-Ag@AgCl
nanocomposites. The XRD pattern of rGO showed a broader peak at 23.74° and 43.8o and appeared
as the peak of graphite (amorphous) (JCPDS Card no.75 -1621). All peaks were corresponded to
(002) and (101) planes of graphite with d-spacing values of 0.339 and 0.204 nm [47-48]. The peaks at
32.2°, 46.2°, 57.6° and 67o corresponded to the lattice planes of AgCl (planes (200), (220), (222)
and (400)) and reflected the face centered cubic (fcc) structure of crystal (JCPDS File No: 131-
1238). The remaining peak at 44.2o was due to the lattice planes (200) of Ag and reflected the
9

face centered cubic (fcc) structure of Ag (JCPDS file: 65-2871). The board peak at 24.74°
corresponds to (002) plane of graphite. The JCPDS data number 225, demonstrated that NPs have a
space group Fm3m (for Ag and AgCl) and edge lengths were found to be 4.079 and 5.549 for Ag
and AgCl, respectively. The crystallite particle size of Ag and AgCl atom were calculated from
the XRD spectra. The calculated average crystallite particle sizes were 12 and 25.5 nm for Ag
and AgCl, respectively. The crystallite particle size of rGO was not determined because of
amorphous structure.
3.4. X-ray photoelectron (XPS) and energy dispersive x-ray (EDX) spectroscopy
analysis
The elemental composition and chemical status of the nanocomposites were investigated by XPS
and EDX analyses. Fig. 4 (a-d) displays XPS spectra of Ag (3d), Cl (2p), C (1s) and O (1s)
respectively. Fig. 4 (a) shows the Ag 3d spectra and further Ag 3d region fragmented into the
two components and coexist in Ag3d5/2 and Ag 3d3/2 signals, respectively. These two peaks may
be divided into two peaks at 366.55, 367.58 eV and 372.58, 373.58 eV, where the peaks at 366.55 and
372.58 eV for the Ag+, and those at 367.58 and 373.58 eV are attributed to metallic Ag0. Hence from
the XPS spectra of Ag 3d, it was confirmed the presence of Ag0 and Ag+ in rGO-Ag@AgCl
nanocomposites. The binding energies of Cl (2p1/2 and 2p3/2
196.24 eV. In XPS spectra of C1s, five deconvolution peaks correspond to the C=C
(283.15 eV), C–C (284.7 eV), C–O (hydroxyl, 285.76 eV), C=O (carbonyl, 287 eV), and O–
C=O (carboxyl, 288.2 eV) groups, respectively. The binding energies of O (1s) 531.8 and 532.3
) are approximately 197.76 and
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
eV are associated with oxygen in the states of C O and C–OH/C–O–C, respectively. The survey
spectra of rGO-Ag@AgCl were demonstrated in Fig. 4 (e).
Elemental compositions of rGO-Ag@AgCl nanocomposites were also investigated by EDX (Fig. 4 f).
10

The EDX spectra of rGO-Ag@AgCl nanocomposites show the presence of Ag, Cl, C, and O.
The elements, such as C and O were from rGO reduced graphene oxide and Ag, Cl from
precursor materials and MC leaf. Hence, from the EDX and XPS analyses, it was confirmed the
formation of rGO-Ag@AgCl nanocomposites.
3.5. Functional group determination by FT-IR spectroscopy
Fig. 5 (a) shows the FT-IR spectrum of reduced graphene oxide and indicated the presence of
peaks at 3280, 1696, 1550 and 1034 cm-1. These peaks can be assigned to the stretching vibration
of C-OH (hydroxyl), stretching vibration of C=C, vibration of C=C (may be because of skeletal
vibration of unoxidized graphite domains), and C-O stretching, respectively [49-50]. The peak
assigned to C-OH was changed to 3289 cm-1 in the spectrum of rGO-Ag@AgCl nanocomposites
(Fig. 5 b). The peaks at 2923, 2359, were due to C–C stretch (in-ring) and C-O bond. The
remaining peaks at 1640, and 1536 cm-1 were assigned to -C=C asymmetric stretching of the
aromatic ring, and C=N stretching, respectively. The appearance of peaks 1020 cm-1 was due to
the alkoxy C-C [51-52]. The presence of Cl (peak at 520 cm-1) was also observed in the FT-IR
spectra of the. The appearance of peak at 520 cm-1 was assigned for the presence of Cl in the as-
synthesized nanocomposites.
3.6. STEM-HAADF imaging and EDX line analysis
Fig. 6 (a) depicted the STEM-HAADF micrograph of the rGO-Ag@AgCl nanocomposites. It
was observed from Fig. 6 that the core was comparatively brighter than the shell. The
arrangement of the core-shell morphology was further confirmed by elemental mapping executed
using the Ag–La1 and Cl–Ka1 energies with the STEM-HAADF-EDX technique (Fig. 6 (b-c)).
Major elements, such as carbon and oxygen of reduced graphene oxide were investigated by C–
Ka1 and O-Ka1 energies (Fig. 6 (d-e)). The overlapped X-ray elemental mapping image (red
11

dots- represented-Ag and white dots- represented-Cl, green dots-carbon, and blue dots-oxygen)
was represented in Fig. 6 (f). It was apparent from the figure (Fig. 6 (f)) that Ag was at the center
and Cl (from AgCl) at the periphery. Further, it was surrounded by C and O. Hence, it was
perceptible from all the micrographs that Ag is situated in core and was bounded by the AgCl
shell. Fig. 6 (g) (EDX line profile) showed that the core was made of Ag (red line) and
completely surrounded by the shell of Cl (from AgCl green line). The core-shell morphologies of
Ag@AgCl were attached on the surface of rGO and also observed in EDX line profile (aqua
color –carbon and violet color- oxygen).
3.7. BET analysis
The porosity and surface area of rGO-Ag@AgCl nanocomposites were also investigated in this
present work by BET analysis. The activity of adsorbent in the adsorption process depended
porosity and surface area of rGO-Ag@AgCl nanocomposites. High values of porosity and
surface indicate good adsorption characteristics for elimination of hazardous dyes and
pharmaceutical compounds. The surface area and pore size distribution (PSD) were calculated
from N2 adsorption data. The surface area of nanocomposites was found to be 630.67 m2g-1 as
evident from the BET analysis. The value of surface area indicates single point surface area of
the nanocomposites. Fig. 7a displays the hysteresis loop of N2 adsorption-desorption isotherm
and follows Type-IV isotherm and demonstrated that as-synthesized nanocomposites are
mesoporous (d=2-50 nm) in nature, with no pores opened at both ends. The calculated pore
diameter was found to be 4.25 nm (Fig. 7b). From the BET analyses, it was confirmed that dye
and pharmaceutical molecules were quickly accommodated on the mesoporous surface of the
rGO-Ag@AgCl nanocomposites. Hence, surface area and PSD data of rGO-Ag@AgCl
nanocomposites showed superior values for elimination of dye and pharmaceutical compounds.
12

3.8. Removal of hazardous dye and pharmaceutical compound from aqueous phase using
synthesized rGO-Ag@AgCl nanocomposites
The BET results (N2 adsorption/desorption isotherms) showed that nanocomposites followed
typical IV isotherm [53], and it was confirmed that rGO has a strong adsorption capacity. Here
we also investigated adsorption ability of nanocomposites for exclusion of NGB dye and CA
drug before sunlight irradiation. The reaction mixture was kept under magnetic stirring in dark
conditions for the adsorption/desorption equilibrium study. Approximately, 35-40% NGB (dye)
and 25-30% CA (drug) adsorption were detected following 1 h treatment in dark condition [54].
It shows that as-synthesized nanocomposites possess a higher adsorption capacity of dye and
pharmaceutical molecules from aqueous phase. The adsorption process occurred on
nanocomposites via a
graphene.
π−π conjugation between CA and NGB molecules and aromatic regions of
The photocatalytic activities of synthesized nanocomposites were evaluated for the degradation
of NGB dye and CA drug under solar irradiation. Fig. 8(a-b) showed photodegradation spectra of
NGB and CA using rGO-Ag@AgCl nanocomposites as a photocatalyst under sunlight. The
absorption spectra of NGB showed a prominent absorption band at around 715 nm and band
intensity decreases with the addition of photocatalyst. From the absorption spectra, it was also
found that color of the dye solution faded away with increase in radiation time and indicated the
destruction of chromophore structure of the dye. The absorption spectra depicted that NGB dye
was completely degraded completely within 30 min using rGO-Ag@AgCl.
Similarly, the photodegradation of CA was also investigated. Clofibric acid showed prominent
absorption band at around 227 nm and characteristic band intensity decreases with addition of
13

the nanocomposites. The absorption spectra showed that CA was completely degraded within 40
min utilizing rGO-Ag@AgCl nanocomposites.
The photodegradation of NGB and CA obeyed the pseudo-first-order kinetics and shown in
equation (1).
ln (C0/Ct) = kt…………………………………(1)
All the parameters were mentioned in our previous work [33]. Fig. 8(c-d) showed kinetics plot of
ln [C0/Ct] versus radiation time (t) for degradation of NGB dye and CA using rGO-Ag@AgCl
nanocomposites as a photocatalyst. The plot gave a linear relationship, and hence, slope of the
line represented the rate constant (k) for photodegradation. The rate constant for the degradation
of NGB and CA were 9.16×10-2 and 4.57×10-2 min-1 respectively. Moreover, the percentage
degradation was calculated using the equation given below:
Percentage degradation = [(C0 – C)/C0] x 100………….. (2)
Here, parameter C0 and C presents concentration before and after degradation process. The
percentage efficiency for degradation by solar irradiation was found to be 98.7 % for NGB and
98% of CA. Table 1 presents the result obtained from the photodegradation of NGB and CA.
3.9. Recycle studies rGO-Ag@AgCl nanocomposites (photocatalyst)
The photocatalysts, rGO-Ag@AgCl were reused for the degradation of NGB and CA upto fourth
cycles and the loss of their catalytic activity was also determined. Fig. 9 shows the investigation
recycle properties of rGO-Ag@AgCl photocatalyst during photodegradation of NGB and CA
under solar irradiation.
3.10. Probable degradation products of NGB and CA during photodegradation
The probable by-products during photodegradation of NGB dye were naphthalene (m/z 128), 2-
formylcinnamaldehyde (160.17), naphthoquinone (m/z 158.15), formaldehyde (m/z 30), O-
14

phthalaldehyde (m/z 134), E-penta-2,4-diene (m/z 82.1) and ethane (m/z 28) [55-57]. The
possible intermediates or by-products obtained from degradation of Clofibric acid are 2-(4-
hydroxyphenoxy)-isobutyric acid (m/z 195), 2-or 3 hydroxyisobutyric acid (m/z 104),
hydroquinone (m/z 110), isobutyric acid (m/z 88), 4-chlorophenol (m/z 125), 4-chlorocatechol
(m/z 143), Hydroxybenzoquinone (m/z 124), p-Benzoquinone (m/z 108) etc. CO2 and H2O were
formed as a final product after complete degradation of CA and NGB [58-60]. Fig.10 (a-b)
displayed the probable by-products formation during the photodegradation of CA and NGB
using nanocomposites as a photocatalyst.
3.11. Probable mechanism of nanocomposites during the photodegradation of CA and
NGB under solar irradiation
The plausible mechanism of enhanced photocatalytic degradation using rGO-Ag@AgCl
nanocomposite can be proposed on the basis of heterojunction structure of rGO-Ag@AgCl. In
the first step, when nanocomposites were dispersed in NGB and CA solution, molecules of dyes
and drug were absorbed on rGO surface via a π−π conjugation between NGB and CA molecules
and aromatic regions of graphene. In second step, photon (due to the sunlight irradiation) with
adequate energy falls on the surface of the catalyst. The presence of Ag nanoparticles in
Ag@AgCl/rGO enables it to absorb visible radiation as indicated by the UV-Vis spectral
analysis (Fig. 1 (a)). Most of the photoexcited electrons due to SPR of Ag cluster can transfer to
conduction band of AgCl [43]. Also, under UV irradiation, electronic excitation can occur within
AgCl semiconductor to generate electrons at CB and holes at VB.
A photogenerated electron–hole (e-- h+) pairs are formed on AgNPs surface under visible region
because of SPR effect of Ag. AgCl nanoparticles cannot absorb visible light because of its high
band gap (3.25 eV). Therefore, the photocatalytic characteristic of Ag@AgCl is ascribed due to
15

the plasmonic absorption of metallic silver (Ag) nanoparticles. Then, plasmon-induced electrons
of silver (Ag) nanoparticles transfer to the rGO nanosheets via the conduction band of AgCl and
reduce oxygen. But plasmon-induced holes (h+) are retained on Ag NPs surface and react with
water molecules to generate hydroxyl radicals [61-63]. Thereafter, these holes (h+) are
transferred to the surface of AgCl surface and leads to the oxidation of Cl− ions to form Clo
atoms (radicals). This transfer takes place due to the high oxidation ability of the chloride ions
-
present in AgCl. Meanwhile, electrons on rGO sheets react with O2 and produces •O2 radical.
Finally, the NGB and CA molecules adsorbed on active sites of rGO-Ag@AgCl and oxidized by
active species, such as •O2 radical. In the entire system, rGO sheet acts as an electron mediator,
and enhance the photocatalytic activity for separation and transfer of electrons [64-65]. This can
be explained by the fact that rGO pieces may decrease the resistance of separation and
transportation charge produced from Ag nanocrystal (whose surface plasmon resonance is
susceptible to visible light) [65]. The probable mechanistic pathway of rGO-Ag@AgCl
nanocomposites during the photo degradation of CA and NGB were displayed below and in
Scheme 2.
-
AgNPs (e-) +AgNPs+(h+)
AgNPs + hv
H2O + h+
AgNPs*
-
OH + H+
-
OH + h+
.
OH
-
-
AgNPs (e-)
.O2
O
O
+
+
AgNPs
2
+
or
rGO (e-)
-
.O2
2
.
Degradation product
Degradation product
(CO2+H2O)
(CO2+H2O)
NGB or CA
+
OH
-
.O2
NGB or CA
+
To detect the photogenerated radicals, such as super oxide anion radical in the photodegradation
process, spin-trapping ESR method was utilized. Fig. 11 (a-b) displays the ESR spectra of
16

DMPO-superoxide anion radicals during the photodegradation of Naphthol Green B and
Clofibric acid. The reagent like DMPO (5,5-dimethyl-1-pyrroline-N-oxide) is usually used for
trapping radicals because of its capability of generating stable radicals such as DMPO-
superoxide anion. Fig. 11 clearly showed the formation of superoxide anion radicals (-•O2) [66].
-
It was reported that the active species like •O2 was mainly responsible for NGB and CA
degradation. Therefore, ESR study evidently indicates the production of both the radicals. These
radicals are accountable for NGB and CA degradation.
3.12. Antimicrobial Test
The inhibition zones were calculated after 24 h of incubation and displayed in Table 2. From the
antimicrobial test, it was observed that rGO-Ag@AgCl nanocomposites showed greater
inhibition zones than that of rGO and AgNO3 solution. Fig. 12 presents the photographic images
of antimicrobial activities of rGO-Ag@AgCl nanocomposites.
3.13. Antibacterial mode of actions of rGO-Ag@AgCl nanocomposites
The probable mechanism action of rGO-Ag@AgCl nanocomposites was cited as “insertion mode
of action”. Here, the sharp edges of graphene nanosheets (act as knives) cut through cell
membranes of bacteria, hence, the leakage of intracellular substrates causing eventually cell
death [67-69]. Then hydrocarbon tails of the lipids and the lipophilic flat surface of graphene
were attracted each other and then allow the graphene to sink in between the lipid tails,
embedding the graphene nano sheets in cell membrane [70-71]. In next step, metal NPs (Ag+)
were enter in the bacterial cell membranes, as a result their structural compositions were changed
because of the interaction with the sulfate group of bacteria (active site of enzymes). As a result,
few of the bacterial enzymes were blocked which is required for metabolism and electrolyte
transport system for bacterial mechanism and finally weakens bacterial survival and deteriorates
17

its activity. The metallic nanoparticles affect the DNA backbone of the bacteria hindering its
replication process, reduce its immunity by binding itself to the bacterial wall rendering
structural imperfections within its cells. Therefore, metal ions also serve as defensive tool for
minimizing bacteria proliferation and weakening bacterial development [72-74].
4. Conclusion
In summary, rGO-Ag@AgCl nanocomposites were successfully synthesized by loading reduced
graphene oxide (rGO) on Ag@AgCl nanoparticles. TEM image showed core and shell
morphology of as-synthesized nanocomposites and further, it was also confirmed by STEM and
elemental mapping analyses. The synthesized nanocomposites were successfully used as a
photocatalyst and antimicrobial agents. As-synthesized nanocomposites showed 98 % of CA and
98.7 % of NGB within 40 and 30 min, respectively. This indicates efficient catalytic activities of
rGO-Ag@AgCl nanocomposites for elimination of hazardous organic pollutants from water and
wastewater. As-prepared nanocomposite showed good antimicrobial activities against
Staphylococcus aureus (S. aureus), Escherichia coli (E. coli)), Pseudomonas aeruginosa (P.
aeruginosa), and Staphylococcus epiderminis (S. epiderminis) strains. The rGO-Ag@AgCl
nanocomposites showed superior antimicrobial properties than that of rGO and AgNO3. The
above studies established that the synthesized rGO-Ag@AgCl nanocomposites could be utilized
as a potential material for wastewater treatment.
Acknowledgements
Authors, would like to acknowledge SAIF-NEHU Shillong, SAIF–IIT Bombay, IISc Bangalore,
and CSMCRI Gujarat for providing the SEM, TEM, EDX, XRD, and XPS facilities.
18

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List of Schemes:
1. SCHEME 1 displays schematic diagram of the growth of rGO-Ag@AgCl
nanocomposites
2. SCHEME 2 displays probable mechanistic pathway of rGO-Ag@AgCl nanocomposites
during the photo degradation of CA and NGB
List of Table:
1. TABLE 1 presented comparative analyses of the results obtained for the
photodegradation of NGB and CA under solar irradiation in presence rGO-Ag@AgCl,
Ag@AgCl-AC and Ag-AC of as photocatalyst.
2. TABLE 2 presented displays the results obtained from the antimicrobial test of rGO-
Ag@AgCl, nanocomposites.
25

List of Figures:
1. FIGURE 1 (a-b) displays the UV-visible spectra and Tauc’s plot of rGO-Ag@AgCl
nanocomposites.
2. FIGURE 2 (a-f) displays the SEM, TEM, HR-TEM and SAED pattern, AFM image and
3D image of rGO-Ag@AgCl nanocomposites.
3. FIGURE 3 (a-b) displays the XRD spectra of rGO and rGO-Ag@AgCl nanocomposites.
4. FIGURE 4 (a-d) displays XPS spectra of Ag (3d), Cl (2p), C (1s) and O (1s)
respectively.
5. FIGURE 4 (e-f) displays XPS survey spectra and EDX spectra of rGO-Ag@AgCl
6. FIGURE 5 (a-b) presents FT-IR spectrum of reduced graphene oxide and rGO-
Ag@AgCl nanocomposites.
7. FIGURE 6 (a-e) depicted the STEM-HAADF micrograph image, elemental mapping
energies of silver (Ag–La1) chlorine (Cl–Ka1), Carbon (C kal1-2) and Oxygen (O Ka1)
with the STEM-HAADF-EDX technique.
8. FIGURE 6 (f-g) depicted combined elemental mapping and STEM-EDX line profile
images of rGO-Ag@AgCl nanocomposites.
9. FIGURE 7 (a-b) depicted Nitrogen adsorption − desorption isotherm (BET) and Pore
volume vs pore radius (using the BJH method) of rGO-Ag@AgCl nanocomposites.
10. FIGURE 8(a-b) represented the absorption spectra of photodegradation of NGB and CA
by solar irradiation using rGO-Ag@AgCl, nanocomposites as photocatalyst.
11. FIGURE 8 (c-d) represented the plot of ln [C0/Ct] versus irradiation time (t) for the
degradation of NGB dye and CA drug using rGO-Ag@AgCl, as photocatalyst,
respectively.
12. FIGURE 8 (e-f) showed the percentage degradation of NGB and CA with time using
rGO-Ag@AgCl as photocatalyst.
13. FIGURE 9 displays the recycle studies of rGO-Ag@AgCl photocatalyst during the
photodegradation of NGB and CA under solar irradiation.
14. FIGURE 10 (a-b) displayed the probable by-product formation during the
photodegradation of CA and NGB using nanocomposites as a photocatalyst.
26

15. FIGURE11 (a-b) displays the ESR spectra of DMPO-hydroxyl and DMPO- superoxide
anion radicals during the photodegradation of Naphthol Green B and Clofibric acid .
16. FIGURE 12 (a-d) displays the antimicrobial photographic images of Staphylococcus
aureus (code B1), Escherichia coli (code B2), Pseudomonas aeruginosa (code B3), and
Staphylococcus epiderminis (code B4) microbes using rGO-Ag@AgCl
300 mL colloidal solution
of Ag@AgCl (MC) NPs
rGO (30 mg)
Stirred of 24 h at room temperature
Core shell NPs
rGO-sheet
rGO-Ag@AgCl nanocomposite
SCHEME 1 displays a schematic diagram of the growth of rGO-Ag@AgCl nanocomposites
27