Graphene based nanoassembly for simultaneous detection and degradation of harmful organic contaminants from aqueous solution

Article abstract of DOI:10.1039/c6ra01784c

Graphene based nanoassemblies that can simultaneously detect and degrade harmful organic contaminants from water are important for conquering the risk of hazardous chemicals. A unique semiconductor-metal-graphene nanosheet assembly design has been successfully fabricated by a wet chemical method. Such a nanoassembly arrangement is made up of ZnO-Ag core-shell nanostructures and integrates on graphene nanosheets (GNSs). High-resolution scanning transmission electron microscopy reveals that ZnO-Ag core-shell nanostructures consist of a core ZnO nanosphere covered by a Ag shell and are chemically decorated on the GNS surface. This ZnO-Ag-GNS nanoassembly is SERS active and hence able to sense organic contaminants such as acridine orange dye. The sunlight-driven photocatalytic activity of the ZnO-Ag-GNS nanoassembly design demonstrates the tremendous enhancement in the detection and degradation of organic contaminants. The results in the present study show that the enhancement in the photocatalytic activity is attributed to the interfacial charge transfer effect in ZnO-Ag-GNS interfaces. This new design exhibits an efficient photocatalytic degradation of organic dye under sunlight irradiation. This could have great potential for a high sensing, stable and reusable material for energy and environmental cleaning applications.

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Graphene based Nanoassembly for simultaneous Detection and
Degradation of Harmful Organic Contaminants from Aqueous
Solution
Received 00th January 20xx,
Accepted 00th January 20xx
R. Ajay Rakkesha, D. Durgalakshmia,b and S. Balakumara*
DOI: 10.1039/x0xx00000x
Graphene based nanoassemblies that can simultaneously detect and degrade the harmful organic contaminants from
water are important for conquering risk of hazardous chemicals. A unique semiconductor-metal-graphene nanosheet
assembly design, have been successfully fabricated by a wet chemical method. Such nanoassembly arrangement is made
up of ZnO-Ag core-shell nanostructures, integrates on the graphene nanosheets (GNS). High-resolution scanning
transmission electron microscopy reveals ZnO-Ag core-shell nanostructures consist of (core ZnO nanosphere covered by
Ag shell) are chemically decorated on the GNS surface. This ZnO-Ag-GNS nanoassembly is SERS active and hence able to
sense organic contaminant such as acridine orange dye. The sunlight-driven photocatalytic activity of the ZnO-Ag-GNS
nanoassembly design demonstrates the tremendous enhancement in the detection and degradation of organic
contaminant. Results in the present study show that the enhancement in the photocatalytic activity that is attributed to
the interfacial charge transfer effect in ZnO-Ag-GNS interfaces. This new design exhibits an efficient photocatalytic
degradation of organic dye under sunlight irradiation. This could be a great potential for highly sensing, stable and
reusable material for energy and environmental cleaning applications.
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recombined due to wide band gap energy and instability in the
1. Introduction
higher state, which is a crucial factor that limits the
photocatalytic activity by low quantum efficiency.3a-c In order
to avoid the rapid recombination rate of electron-hole pair, a
widely reported approach is to incorporate metal
Sensing and destroying of harmful organic contaminants in
water bodies remains a major problem in the ecological
system. The detection of toxic organic dye effluents released
from various industries like tanneries, textiles, etc., containing
nitroaromatic dyes used as a colouring agent and finds their
way into rivers. These hazardous contaminants will destroy the
ecosystem in the near future. The photocatalytic performances
of semiconductor based nanoassemblies are helpful in the
remediation of hazardous organic contaminants from water
bodies.1a-c
nanomaterials on
a
n-type semiconductor.4-8 The
photogenerated electrons transfer to the metal nanomaterial
across a junction and become trapped owing to the Schottky
barrier whereas the holes generously spread to the
semiconductor surface.
There are two factors that subjected to reducing the
electron-hole pair recombination rate: (i) the appearance of
defects during the formation of interface engineering in the
hybrid nanostructures7a-c and (ii) some electron rich co-
catalysts with semiconducting metal oxides are incorporated
to form a hetero-junction between the host semiconductor
and the co-catalyst that generates a local electric field, which
separates the electron–hole pairs completely.11-12 However,
there are certain countable limitations are occur owing to the
lack of bulk-to-surface sites in the created interfaces that
reduce the exposed metal-semiconductor surfaces and thus
makes it complicated for the reaction molecules to lift up the
photo-induced charge carriers for reduction reactions on the
surface.9-10
Under excitation with an energized photons need to
separate electrons and holes in a semiconductor and allowing
them to initiate degradation of organic dyes at the interface.
ZnO, a wide band gap n-type semiconducting nanomaterial, is
a widely studied system for photocatalytic applications.2a-b For
a photocatalytic reaction process, the reaction molecules
receive electrons and holes from the surface sites for the
further reduction reactions. In competition with the charge
carriers (i.e., electrons and holes), most of them were
aNational Centre for Nanoscience and Nanotechnology, University of Madras,
Guindy Campus, Chennai 600 025, India. *E-mail: balasuga@yahoo.com; Fax: +91-
044-22352494/22353309; Tel: +91-044-2202749.
Subsequently, we decided to adopt a new materials design
to conquer this undesirable condition. This new nanoassembly
made by integrating a graphene nanosheet, which is a well
identified two-dimensional material with excellent physio-
chemical properties and very high electron mobility, into
bDepartment of Physics, Women`s Christian College, Chennai 600 006, India.
†Electronic Supplementary Informaꢀon (ESI) available: [UV-Vis DRS spectra of ZnO,
ZnO-Ag core-shells
DOI: 10.1039/x0xx00000x
and
ZnO-Ag-GNS
hybrid
nanoassemblies].
See
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fabricated n-type semiconductor (ZnO) – metal (Ag) core–shell (200 mL) at ~5 °C and stirred for 30 minutes. 1.5 M of Oxalic
nanostructure. Interestingly, Ag nanoparticles showing acid (H2C2O4.2H2O) dissolved in double-distilled water (200 mL)
Localized Surface Plasmon resonance effect (LSPR) improves at ~5 °C was slowly added to the cold solution of Zinc acetate.
Raman scattering of organic contaminants and thus facilitates The mixture was further stirred for 2 hours at low temperature
the sensing through surface enhanced Raman spectroscopy till the solution gradually turned into white precipitate. The
(SERS).11-13 As the graphene nanosheet is in close contact with precipitate was then filtered and washed with ethanol and
the metal nanomaterial in this assembly, it is anticipated that allowed to dry at 80 °C for 2 hours and that end up in Zinc
the electrons gathered on the conduction band of oxalate formation. Zinc oxalate was further calcined at 400 °C
semiconductor can transfer to the graphene nanosheets for 2 hours to obtain pure ZnO nanomaterial.
through a metal-graphene pathway and entirely taken up by 2.3 Preparation of ZnO-Ag core-shell nanostructures
oxidized molecules and simultaneously the holes reside on the
semiconductor for reduction reactions.14-20 The adaptability of
this new nanoassembly is superior than other recent reported
Experimentally, a facile wet chemical approach has been
developed to fabricate the ZnOꢀAg coreꢀshell nanostructures
with different Ag shell thickness. In detailed experimental
procedure, a given amount of silver nitrate (1 mM, 4 mM and
7mM) and 100 mg of zinc oxide were dispersed in 100 mL of
ethanol under vigorous magnetic stirring. Subsequently, the
suspension was aged at room temperature for 2 hours and
finally, it was dried isothermally at 80˚C for 12 hours and then
the residue was transferred into the furnace at various
temperatures for 2 hours to crystallize the materials. The
thickness of the Ag shell could be varied by changing the
concentration of silver nitrate precursor as 1 mM, 4 mM and 7
mM and named as ZnOꢀAg – 1, ZnOꢀAg – 2 and ZnOꢀAg – 3
respectively.
2.4 Synthesis of ZnO-Ag-GNS Nanoassembly
literatures, because of its capability of both sensing and rapid
photodegradation of organic contaminants in aqueous
medium.21a-e The significance of the each component in the
ZnO-Ag-GNS nanoassembly is portrayed in fig 1.
In this work, we demonstrate a unique semiconductor–
metal–graphene nanoassembly design, which can perfectly
overcomes the above mentioned limitations and thus
enhances the photocatalytic activity by LSPR and interfacial
charge transfer effects. The versatility of using this unique
nanoassembly can simultaneously detect and degrade the
organic contaminants dispersed in the aqueous solution under
natural sunlight irradiation.
The synthesis procedure for ZnO-Ag-GNS nanoassembly was
schematically explained in the fig. 2. The as-synthesized GNS
(100 mg) was dispersed in double distilled water (100 mL)
using ultrasonicator to form a colloidal suspension. The GNS
solution was added to a higher silver nitrate concentration of
ZnO-Ag core-shell (ZnO-Ag – 3) suspension (100 mg of material
is dispersed in 100 mL double distilled water). The obtained
precipitate was stirred vigorously and separated by
centrifugation at 4000 rpm. The resulting product was washed
with ethanol and dried in a vacuum oven at 100°C for 24 hours
to get a ZnO-Ag-GNS nanoassembly.
Fig.1 (a) GNS and Ag Nps works to enable detection of organic contaminants in the
aqueous solution and (b) Photoinduced charge carriers from the semiconductor can
transfer to the graphene nanosheets through a metal-graphene pathway and entirely
diffused the target molecules.
2. Experimental Section
2.1 Materials
All the chemicals used in this work were analytical grade and
were used without any further purification. Graphite (325
mesh, Alfa Aesar), potassium permanganate (Sigma-Aldrich),
hydrogen peroxide (30 wt%, Sigma-Aldrich), sulfuric acid
(Rankem Chemicals), hydrochloric acid (Rankem Chemicals),
Fig.2 Schematic illustration of ZnO-Ag-GNS nanoassembly was synthesized by a wet
zinc acetate dihydrate (Fischer Chemicals), oxalic acid (Fischer
chemical processes.
Chemicals), silver nitrate (SRL Ltd.), ethanol (Hayman limited),
and acridine orange dye (Fischer Chemicals).
2.5 Characterization
2.2 Synthesis of ZnO nanomaterials
X-ray diffraction (XRD) data were collected with a PANalytical
In
a
typical experiment, 1.0
M
of zinc acetate
X-Ray diffractometer using Cu Kα (λ = 1.54 Å) radiation. The
[Zn(CH3COO)2.2H2O] was dissolved in double-distilled water
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samples were scanned in the 2θ range of 10° to 80°. All of the The crystalline structures, phase purity and textural features of
peaks were assigned and compared with data from Joint the as-synthesized nanostructures were studied by XRD
Committee on Powder Diffraction Standards (JCPDS) database. analysis. The XRD patterns of the pure ZnO, ZnO-Ag core-shells
XPS measurements were obtained using ESCA+, Omicron and ZnO-Ag-GNS stack design were shown in fig. 3. All of the
Nanotechnology
ESCA
probe
spectrometer
with samples were crystalline in nature and of hexagonal wurtzite
monochromatized Al Kα X-rays (energy: 1486.6 eV). The X-ray phase ZnO (JCPDS No. 36-1451). However, the ZnO-Ag core-
power applied was 300 W. The pass energy was 50 eV for shells exhibited additional Ag diffraction peaks located in the
survey scans and 20 eV for specific regions. Sample solution fig. 3, which could be indexed to the metallic silver with face-
was spotted on a molybdenum sample plate and dried in centered cubic system (JCPDS No. 01-1167), signifying that the
vacuum. Spectra in the required binding energy range were Ag nanomaterials were covered on the ZnO surface. Increase
collected, and an average spectrum was taken. Each spectrum in the Ag concentration, the Ag peak intensity also increases
was confirmed after scanning eight times for confirmation accordingly.22-23 The XRD patterns of the ZnO-Ag-GNS stack
(except survey scan, which was scanned only once). While show a broad peak at 25.7 and 41.16 degrees, corresponding
taking the spectra, the scan steps per second was fixed for all to the (002) and (100) reflections. It is evident that the
of the narrow scans. Beam-induced damage of the sample was graphene oxide is significantly reduced to graphene
reduced by adjusting the X-ray flux. The base pressure of the nanomaterials.24a-c Very low quantity of ZnO-Ag-3 core-shell
instrument was 5.0 × 10-10 mB. The binding energy was nanostructure were integrates onto the GNS surfaces that can
calibrated with respect to the adventitious C 1s feature at be evidently compared and analyzed with individual
284.6 eV. Most of the spectra were deconvoluted to their nanomaterials. However, the existence of metallic Ag in ZnO-
component peaks, using the software CASA-XPS. High- Ag-GNS can be obviously elucidated by XPS analysis, as
resolution scanning transmission electron microscopy discussed in the next section.
(HRSTEM) of the samples was carried out using a FEI Tecnai G2
S-twin instrument with a UHR pole piece. HRSTEM samples
were prepared by drop-casting two or three drops of sample-
dispersed ethanol solution to carbon-coated copper grids and
allowed to dry at room temperature overnight. The
photoluminescence (PL) spectrum was recorded by Perkin
Elmer MPF-44B equipment under (325 nm) excitation.
2.6 Photocatalytic activity
The Photocatalytic activity of ZnO-Ag-GNS nanoassembly was
evaluated by the degradation of Acridine orange (AO) under
solar light. In a typical process, 150 mL of (3.0X10-5 M, 3.5X10-5
M and 4.0 X10-5 M) concentrations of AO dye solution and 50
mg of ZnO-Ag-GNS nanostructures was stirred for about 1
hour. Before exposure to illumination, the suspensions were
stirred in the dark for 30 minutes to ensure the establishment
of adsorption/desorption equilibrium of AO on the sample
surfaces. Consequently, the suspension was irradiated with
Fig. 3 XRD patterns of the pure ZnO, different shell thickness of ZnO-Ag-1, ZnO-Ag-2,
direct sunlight and this was carried out for all the experiments
ZnO-Ag-3 and ZnO-Ag-GNS hybrid nanostructures
under similar conditions on sunny days in Chennai city
(geographical location 13.04° N and 80.17° E on the southern-
3.2 Chemical state analysis
east coast of India), between 12:00 p.m. and 3:00 p.m.
(outside temperature, 29°C to 31°C). The measured
illuminance power of the direct sunlight at the given interval of
time was around 110000 lux to 120000 lux obtained by using
Digital illuminance meter (TES Electrical Electronic Corp.
Taiwan). At a given time interval of irradiation, 5 mL of the
suspension was withdrawn and subsequently centrifuged at a
rate of 3000 rpm for 5 minutes. UV–Vis absorption spectra of
the supernatant were then measured using a UV–visible
spectrophotometer.
To obtain further essential information on surface structure,
chemical analysis and oxidation states of ions of the as-
prepared nanostructures, XPS analysis was performed on the
ZnO-Ag-GNS hybrid nanostructures. Figure 4 (a) exhibits the
survey spectrum, which indicates that the sample contains Zn,
O, Ag and C elements and evidently confirms that no other
elements were indentified. The peaks located at 1020.4 eV and
1043.4 eV correspond to Zn 2p3/2 and Zn 2p1/2, respectively,
representing the existence of Zn2+ state (in fig. 4 b).25 Figure 4
(c) shows the Ag 3d XPS spectra with two broad band’s at
367.3 eV and 373.3 eV binding energies, which are ascribed to
3. Results and discussion
Ag 3d5/2 and Ag 3d3/2
, respectively. No other peak
3.1 Structural analysis
corresponding to Ag2O or AgO was observed in the XPS
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spectra, which indicates that the Ag species coated on ZnO are
in form of metallic state, not in Ag2O or AgO.26
Fig. 5 Electron micrographs of (a) ZnO, (b) 1 mM Ag concentration of ZnO-Ag-1, (c) 4
mM concentration of ZnO-Ag-2 and (d) 7 mM concentration of ZnO-Ag-3 core-shell
nanostructures, respective insets shows its higher magnification.
Fig. 4 (a) XPS survey spectra and (b) high resolution spectra of Zn 2p, (c) Ag 3d, and (d)
C 1s regions of ZnO-Ag-GNS hybrid nanostructures.
The C 1s spectrum was deconvoluted into three peaks at 284.6
eV, 286.6 eV and 289.2 eV binding energies (in fig. 4 d). The
peak at 284.6 eV is due to the C-C bond of graphene. The peak
at 286.6 eV is attributed to the C-O bond, whereas the peak at
289.2 eV may possibly assign to the C=C bond.26
3.3 Electron microscopic studies
Fig. 6 Electron micrographs of (a) GNS and (b) ZnO-Ag-GNS hybrid nanostructures,
The electron microscopic images of the as-synthesized
nanomaterials ZnO, ZnO-Ag-1, ZnO-Ag-2, ZnO-Ag-3 core-shell
nanostructures and ZnO-Ag-GNS hybrid nanostructures, are
given in Figs. 5 and 6. The surface morphology of the ZnO
nanomaterial exhibits uniform spherical structure of diameter
ranging from 20 nm to 25 nm that is exhibited in fig. 5 (a). To
fabricate core-shell nanostructures with different shell
thickness, by increasing the silver nitrate concentration (1 mM,
4 mM and 7 mM) and coated onto the ZnO under stimulated
reaction medium (in figures 5 b, c & d). The thickness of the
outer shell is in the range of ~3 nm to 4 nm for (ZnO-Ag-1), ~4
nm to 6 nm for (ZnO-Ag-2) and ~6 nm to 8 nm for (ZnO-Ag-3)
respectively. The respective insets clearly show the surfaces of
the ZnO were completely coated with metallic Ag nanoshell of
different thickness, which were obtained by increasing the
concentration of silver nitrate precursor, which are in good
agreement with the XRD results.
respective insets shows its higher magnification images.
3.4 Detection of Organic contaminant
SERS is a versatile technique for the detection of organic
molecules in the aqueous solution; it employs the interaction
between the organic molecule (target) and the LSPR of Ag
NPs, to intensify the Raman signal of the organic molecule.
Moreover, the GNS in ZnOꢀAgꢀGNS nanoassembly can
improve SERS signals due to quenching in the fluorescence
effect along with its Ramanꢀenhancing properties.27 Herein, we
investigated the capability of this ZnOꢀAgꢀGNS nanoassembly
to perform as chemical sensors by employing acridine orange
as the target molecule. ZnOꢀAgꢀGNS nanoassembly was
disperses in aqueous solution of various concentrations of AO
and SERS studies were carried out by using 532 nm laser
excitation source. Figure 7 shows SERS spectrum of ZnO,
ZnOꢀAg coreꢀshells (7mM concentration of Ag shell material)
and ZnOꢀAgꢀGNS nanoassembly are in contact with 1mL of
3.0 X 10ꢀ5 M concentration of AO in aqueous solutions. Control
experiment was carried out with AO in aqueous solution and
with ZnO nanorods show no detectable Raman signal.
However, ZnOꢀAgꢀGNS nanoassembly under the same
conditions delivers significant enhancement of the Raman
Figure 6 evidently shows the formation of GNS with
ultrathin wrinkled silk wave like structure. This is apparent that
the GNS definitely acts as a two-dimensional support for the
incorporation and growth of homogeneous core-shell
nanostructures. After integrated with larger shell thickness of
ZnO-Ag core-shell nanostructure, the GNS was completely
decorated by crystalline ZnO-Ag core-shell nanostructures.
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signal of AO as Ag shell layer close in contact with GNS serve ZnOꢀAgꢀGNS nanoassembly by varying AO concentration is
as SERS active sites. The prominent bands in the region shown in Fig. 8. GNS not only gives a 2ꢀD platform for the
between 500 cmꢀ1 ꢀ 700 cmꢀ1 were assigned to out of plane ring fabrication of a chemical sensor but also adsorbs the organic
and CH deformation modes. The weak band around 1171 cmꢀ1 molecules with higher chemical concentrations. Furthermore,
mainly arises due to the CH deformation and CN stretching the organic molecules interact with GNS through van der Waals
vibration. Additionally, the bands between 1350 cmꢀ1 ꢀ 1650 forces, which assist in the concentration of organic molecules
cmꢀ1 corresponding to the C=C and C=N stretching vibrations onto the surface of the GNS. These interactions provide fasten
are also seen.28
to attract and immobilize the organic molecules (pollutants) at
the material interface and simultaneously degrades the
pollutants under the exposure of natural sunlight.29ꢀ30 However,
the degradation mechanism of organic pollutants using this
material can be obviously elucidated by photocatalytic
experiments, as discussed in the following sections.
3.5 Photocatalytic degradation of AO dye under direct sunlight
irradiation
The photocatalytic activity of the asꢀprepared ZnO, ZnOꢀAg
coreꢀshell, and ZnOꢀAgꢀGNS hybrid nanostructures were
studied by monitoring the degradation of mutagenic organic
pollutant (AO) under direct sunlight irradiation. The change in
the absorption spectra of AO aqueous solutions showed the
changes of its concentration. The initial concentration (C0), the
residual concentration (
illustrated by the mathematical expression as follows:
D% = (C0 C/C0) × 100% (1)
C), and the degradation rate (D%), is
−
Fig. 7 SERS spectra of 3.0X10-5 M concentration of AO in the presence of ZnO, ZnO-Ag
core-shells and ZnO-Ag-GNS nanoassembly. All the samples were kept in 1 mL of
3.0X10-5 M concentration of AO in aqueous solution and allowed to immobilize for 30
min prior to SERS experiment.
Figure 9 showed the AO degradation plot versus time for
ZnO, ZnOꢀAgꢀ1, ZnOꢀAgꢀ2, ZnOꢀAgꢀ3 coreꢀshells, and ZnOꢀ
AgꢀGNS nanoassembly photocatalysts, respectively. C/C0
spectra indicate that the 3.0 × 10−5 M concentration of AO dye
was decomposed of about 48% for pure ZnO nanomaterial
within 70 minutes, 51% for ZnOꢀAgꢀ1 coreꢀshell within 100
minutes, 57% for ZnOꢀAgꢀ2 coreꢀshell within 70 minutes, 89%
for ZnOꢀAgꢀ3 coreꢀshell within 70 minutes and 99% for ZnOꢀ
AgꢀGNS hybrid nanostructures within 20 minutes (fig. 10 c&d)
under direct sunlight irradiation. ZnOꢀAgꢀGNS nanoassembly
showed higher photocatalytic efficiency than that of the pure
and coreꢀshell nanomaterials. This result attributed that GNS
enhances the photocatalytic performance of ZnOꢀAg coreꢀshell
nanomaterial under direct sunlight irradiation.
Figures 10 (a) and (b) illustrate that the photocatalytic
degradation process of AO over ZnOꢀAgꢀGNS nanoassembly
photocatalyst under sunlight irradiation was effectual. More
importantly, by increasing the AO dye concentrations like 3.0 ×
10−5 M, 3.5 × 10−5 M, and 4.0 × 10−5 M, and it was indicated
that the ZnOꢀAgꢀGNS nanoassembly photocatalyst could easily
degrade the AO dye within 20 minutes of irradiation under
direct sunlight and can be separated and reused by
centrifugation. The stability of the nanoassembly photocatalysts
was examined by three consecutive experimental cycles, which
are very essential for the photocatalyst to be applied in
environmental technology.
After three cycles of experiment, the ZnOꢀAgꢀGNS
photocatalyst still provides high photodegradation rate under
direct sunlight irradiation. It shows that the ZnOꢀAg coreꢀshells
were decorated onto GNS by electrostatic interaction which
offers higher stability to the photocatalysts for multipurpose
remediation process. Hence, it could be significantly promote
Fig.8 SERS spectra of AO in the presence of ZnO-Ag-GNS nanoassembly at AO
concentrations of 3.0X10-5 M, 3.5 X10-5 M and 4.0 X10-5 M in aqueous solution.
ZnOꢀAgꢀGNS nanoassembly sensor consent the flexibility
of detecting wide range of organic molecules since GNS is
interfaced with Ag shell layer, that can detect a low level of
organic molecules from aqueous solution. Thus, the material
construction of ZnOꢀAgꢀGNS nanoassembly provides a flexible
route for detection of a wide range of organic molecules at
lower concentrations. For comparison, the SERS response of
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their practical application to remove various harmful mutagenic 3.6 Mechanism of enhanced photocatalytic activity under sunlight
organic pollutants from wastewater under direct sunlight
irradiation.
From the above studies, the proposed mechanism of the
efficient photocatalytic performance towards the degradation of
organic pollutant under sunlight could be credited mainly due
to LSPR effect and interfacial charge transfer process in this
unique ZnOꢀAgꢀGNS nanoassembly. We anticipate that the
electrons gathered on the conduction band of semiconductor
can transfer to the graphene nanosheets through a metalꢀ
graphene pathway and entirely taken up by oxidized molecules
in the mean time, whereas the holes reside on the
semiconductor for reduction reactions.10, 26, 31
ZnOꢀAg + hν → eꢀ (Ag) + h+ (VB, ZnO) (2)
GNS + eꢀ (Ag) → eꢀ (GNS) + ZnO
GNS + eꢀ (AO) → eꢀ (GNS) + AO
H2O + h+ (VB, ZnO) → ●OH + H+
AO (ad) + ●OH →CO2 + H2O
(3)
(4)
(5)
(6)
Fig. 11 Schematic illustration of a plausible mechanism involved in the charge transfer
process from the different energy levels of the ZnO-Ag-GNS for the photodegradation
of acridine orange dye.
Fig.
9 Time-dependent photocatalytic degradation of AO (a) dye under sunlight
irradiation, ZnO (b), ZnO-Ag-1 (c), ZnO-Ag-2 (d), ZnO-Ag-3 (e) core-shells and ZnO-Ag-
GNS hybrid nanostructures (f).
In the ZnOꢀAgꢀGNS nanoassembly, the photogenerated
electrons in the ZnO can be moved to the conduction band and
separate the holes in the valance band by fascinating solar
spectrum due to the suitable narrow band gap of ZnOꢀAg coreꢀ
shell structure. These photogenerated electrons can be
transferred to GNS and metallic Ag nanomaterials. On the other
hand, photogenerated electrons cannot flow directly from AO
to ZnOꢀAg coreꢀshell nanostructures (in fig. 11), since there
was a mismatch in their energy levels. A photoꢀexcited electron
from AO flows into CB of ZnO core in the ZnOꢀAg coreꢀshell
nanostructures via GNS, where the photogenerated electrons
react with oxygen molecules in the aqueous solution producing
oxygen peroxide radicals. The positively charged holes breaks
the hydroxide ion derived from aqueous solution to form
hydroxyl radicals.32ꢀ35 These oxygen peroxide radicals and
hydroxyl radicals generated from ZnOꢀAgꢀGNS nanoassembly
can mineralize and oxidative decomposition of acridine orange
dye to CO2, H2O, and other byꢀproducts. Surprisingly, GNS has
excellent electron mobility due to its two dimensional honey
comb structure, the interfacial charge transport of
photogenerated carriers could be achieved, and an effective
charge separation is consequently proficient. Therefore, the
photodegradation is enhanced by excellent interfacial charge
transport in ZnOꢀAgꢀGNS nanoassembly. Moreover, the
Fig. 10 Concentration dependent AO dye degradation under sunlight irradiation (a),
stability and reusability studies (b) and C/C0 & degradation percentage of AO in the
presence of as-prepared nanostructures (c&d).
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influence of interface engineering could enhance the the ability to target organic molecules with SERS active and
photocatalytic activity of ZnOꢀAgꢀGNS hybrid nanostructures.
photocatalytic activity provides
a
robust platform for
3.6 Photoluminescence studies
multipurpose sensing and destroying applications. Moreover,
this nanoassembly structure facilitates efficient charge transfer
process and the separation of electronꢀhole. In consequence, the
separation of electronꢀhole pairs is a significant process for all
the applications concerning semiconductor materials. It seems
that, this strategy may also be applicable to synthesis various
hybrid nanostructures and also it recovers all the limitations and
solutions in the photocatalytic field. We envisage that it will
create a new development in designing hybrid nanomaterials
for simultaneous sensing and photocatalytic environmental
cleaning applications.
The charge separation property and recombination rate of
photogenerated electronsꢀholes can be identified by
photoluminescence spectroscopic studies. The photogenerated
electrons in the excited state can endure recombination with the
holes, and results in irradiative release of energy in the form of
fluorescence. Based upon the fluorescence emission
characteristics of ZnO, ZnOꢀAg and ZnOꢀAgꢀGNS can be
monitored
by
measuring
the
room
temperature
photoluminescence spectra (in fig. 12). It is manifest that the
peak at 385 nm shows strong UV band and the weak green
emission peak at 535 nm have been observed for pure ZnO
nanomaterials due to its free excitonic emission near the band
edge and local defect states respectively. The PL spectra of the
ZnOꢀAg coreꢀshell nanostructure compared with the pure ZnO
nanomaterials, the intensity of the ZnO peaks decreases and
shifted significantly due to the presence of Ag layer.36ꢀ37 It is
Acknowledgements
R. Ajay Rakkesh gratefully acknowledge the University of
Madras for providing NCNSNT fellowship (C2/AL/2011/388)
supported by MHRD, India to carry out the research work.
obvious that there is
a
heterojunction between the
semiconductors that certainly reduces the defect during coreꢀ
shell formation process. Decrease in the PL emission intensity
indicates the better charge separation efficiency. Furthermore,
the emission intensity of the ZnOꢀAgꢀGNS nanostructure peak
decreased significantly, implying that the recombination rate of
photogenerated charge carriers was effectively suppressed.9 As
a result, GNS is an excellent candidate for enhance the
photocatalytic activity in terms of prolonging the electron–hole
pair lifetime, which drastically reduced the recombination rate
of electronꢀhole pair and accelerates the interfacial charge
transfer process.
Notes and references
1
(a) M. Faisal, M.A. Tariq and M. Muneer, Dyes Pig. 2007, 72,
233; (b) R. Arshad, S. Farooq, N. Iqbal and S. S. Ali, Letters in
Applied Microbiology, 2006, 42, 94; (c) R. Ajay Rakkesh and S.
Balakumar, J. Nanosci. Nanotechnol. 2015, 15, 4316.
2
3
(a) L. Linsebigler, G. Lu, and J. T. Yates, Chem. Rev., 1995, 95
735; (b) Z. S. Wua, G. Zhoua, L. C.Yina, W. Rena, F. Lia, and H.
M. Chenga, Nano Energy, 2012, , 107.
(a) Z. Yin, S. Wu, X. Zhou, X. Huang, Q. Zhang, F. Boey, and
Hua Zhang, Small, 2010, , 307; (b) R. Ajay Rakkesh and S.
Balakumar, Processing and Applications of Ceramics, 2014,
,
1
6
8
,
7; (c) R. Ajay Rakkesh and S. Balakumar, J. Nanosci.
Nanotechnol., 2013, 13, 370.
R. K. Upadhyay, N. Soinb and S. S. Roy, RSC Adv., 2014, 4,
3823.
F. Perreault, A. Fonseca de Faria and M. Elimelech, Chem.
Soc. Rev., 2015, 44, 5861.
4
5
6
7
E. Maggio, and A. Triosi, J. Phys. Chem. C, 2013, 117, 196.
(a) Y. T. Liang, B. K. Vijayan, K. A. Gray, and M. C. Hersam,
Nano Lett., 2011, 11
Durgalakshmi and S. Balakumar, J. Mater. Chem. C, 2014,
6827, (c) R. Ajay Rakkesh, D. Durgalakshmi and S. Balakumar,
RSC Adv., 2015, , 18633.
, 2865, (b) R. Ajay Rakkesh, D.
2,
5
8
9
X. Zhang, Y. L. Chen, R. S. Liu, and D. P. Tsai, Rep. Prog. Phys.,
2013, 76, 401.
M. Ahmad, E. Ahmed, Z. L. Hong, N. R. Khalid, W. Ahmed,
and A. Elhissi, J. Alloys and Compounds, 2013, 577, 717.
10 P. Dou, F. Tan, W. Wang, A. Sarreshteh, X. Qiao, X. Qiu, and J.
Chen, J. Photochem. Photobio. A: Chem., 2015, 302, 17.
11 I. L. Medintz, H. T. Uyeda, E. R. Goldman and H. Mattoussi,
Nat. Mater., 2005, 4, 435.
12 M. G. Blaber, A. I. Hendry, J. M. Bingham, G. C. Schatz and R.
P. Van Duyne, J. Phys. Chem. C, 2012, 116, 393.
13 P. V. Kamat and D. Meisel, Curr. Opin. Colloid Interface Sci.,
Fig. 12 Room temperature PL spectra of ZnO, ZnO-Ag core-shell and ZnO-Ag-GNS hybrid
nanostructures
2002, 7, 282.
14 W. Tu, Y. Zhou, Q. Liu, S. Yan, S. Bao, X. Wang, M. Xiao, and Z.
Zou, Adv. Funct. Mater., 2013, 23, 1743.
15 S. Park, J. An, R.D. Piner, I. Jung, D. Yang, A. Velamakanni, S.T.
Nguyen, and R.S. Ruoff, Chem. Mater., 2008, 20, 6592.
16 Li, and H. Cao, J. Mater. Chem., 2011, 21, 3346.
4. Conclusions
In summary, the simultaneous detection and degradation of
organic contaminant by using a unique designed ZnOꢀAgꢀGNS
nanoassembly is demonstrated. Such nanoassembly combining
17 M. Ahmad, E. Ahmed, Z.L. Hong, J.F. Xu, N.R. Khalid, A.
Elhissi, and W. Ahmed, Appl. Surf. Sci., 2013, 274, 273.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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Please do not adjust margins
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Page 8 of 9
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DOI: 10.1039/C6RA01784C
ARTICLE
Journal Name
18 J. Liu, Y. Xue, Y. Gao, D. Yu, M. Durstock, and L. Dai, Adv.
Mater., 2012, 24, 2228.
19 I. V. Lightcap, T. H. Kosel, and P. V. Kamat, Nano Lett., 2010,
10, 577.
20 L. Zhang, L. Du, X. Yu, S. Tan, X. Cai, P. Yang, Y. Gu, and W.
Mai, ACS Appl. Mater. Interfaces, 2014,
21 (a) Y. Wen, H. Ding, and Y. Shan, Nanoscale, 2011,
(b) Y. Bu, Z. Chen and W. Li, RSC Adv., 2013, , 24118; (c) F.
6, 3623.
3, 4411;
3
Xu, Y. Yuan, D. Wu, M.Zhao, Z. Gao and K. Jiang, Materials
Research Bulletin, 2013, 48, 2066; (d) D.H. Yoo, T. V. Cuong,
V. H. Luan, N. T. Khoa, E. J. Kim, S. H. Hur and S. H. Hahn, J.
Phys. Chem. C, 2012, 116, 7180; (e) J. Liu, L. Liu, X. Zhang and
T. Li, New J. Chem., 2015, 39, 5272.
22 R. S. Zeferino, M. B. Flores, and U. Pal, J. Appl. Phys., 2011,
109, 014308.
23 H. S. Kang, B. D. Ahn, J. H. Kim, G. H. Kim, S. H. Lim, H. W.
Chang, and S.Y. Lee, Appl.Phys. Lett., 2006, 88, 202108.
24 (a) L.Y. Meng, and S. J. Park, Bull. Kor. Chem. Soc., 2012, 33
,
209; (b) R. Aladpoosh and M. Montazer, Carbohydrate
Polymers, 2016, 141, 116; (c) A.S.M.I. Uddin, K. W. Lee and
G. S. Chung, Sensors and Actuators B, 2015, 216, 33.
25 L. Sun, R. Shao, L. Tang, and Z. Chen, J. Alloys Comp. 2013,
564, 55.
26 L. Sun, H. Yu, and B. Fugetsu, J. Hazard. Mater., 2012, 203
101.
,
27 R. Alam, I. V. Lightcap, C. J. Karwacki and P. V. Kamat, ACS
Nano, 2014, , 7272.
8
28 F. Zimmermann, B. Hossenfelder, J. C. Panitz and A. Wokaun,
J. Phys. Chem, 1994, 98, 12796.
29 H. Y. He, J. Klinowski, M.Forster and A. Lerf, Chem. Phys.
Lett., 1998, 287, 53.
30 N. Jung, A. C. Crowther, N. Kim, P. Kim and L. Brus, ACS Nano,
2010, 4, 7005.
31 E. Lee, J. Hong, H. Kang, and J. Jang, J. Hazard. Mater., 2012,
219, 13.
32 D. Meisel, and M. S. Matheson, Radiat. Res., 1983, 94, 604.
33 C. Harris, and P. V. Kamat, ACS Nano, 2010,
34 A. Takai, and P. V. Kamat, ACS Nano, 2011,
4, 7321.
5, 7369.
35 M. S Matheson, P. C. Lee, D. Meisel, and E. Pelizzetti, J. Phys.
Chem., 1983, 87, 394.
36 F. Xu, Y. Yuan, D. Wu, M. Zhao, Z. Gao, and K. Jiang, Mater.
Res. Bull., 2013, 48, 2066.
37 W. Xie, Y. Li, W. Sun, J. Huang, H. Xie, and X. Zhao, J.
Photochem. Photobio. A: Chem. 2010, 216, 149.
8 | J. Name., 2012, 00, 1-3
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