Green synthesis of AgI nanoparticle-functionalized reduced graphene oxide aerogels with enhanced catalytic performance and facile recycling

Article abstract of DOI:10.1039/c5ra07267k

AgI nanoparticle-functionalized self-assembled reduced graphene oxide aerogels are constructed using vitamin C as the reducing agent. The obtained aerogels can be used as efficient catalysts for organic dye degradation, reduction of 4-nitrophenol, and synthesis of bis(indolyl)methane. A set of characterizations, including FESEM, TEM, XRD, XPS, Raman, FTIR, optical absorption, and photoluminescence techniques, confirm that the aerogel is formed from ultra-dispersed AgI nanocrystals and the self-assembly of reduced graphene oxide nanosheets into porous hydrogel structures. The obtained aerogels exhibit high photocatalytic degradation ability toward an organic dye (rhodamine-B) because of the high visible light-driven catalytic activity of AgI and the high specific surface area of graphene nanosheets with three-dimensional interconnected pores. The well-wrapped reduced graphene oxide nanosheets on AgI nanostructures could promote the transfer of photo-generated electrons, which not only effectively inhibits the recombination of electrons and holes but also suppresses the photocorrosion of AgI; this promotes the photocatalytic activity and stability. Moreover, these nanostructures show the best catalytic activity for the reduction of 4-nitrophenol to 4-aminophenol in the presence of NaBH₄ as a reducing agent. Furthermore, the AgI-reduced graphene oxide aerogel nanocomposites are active catalysts for the synthesis of bis(indolyl)methane under solvent-free conditions. The nanocomposites exhibit excellent catalytic activity and remarkable durability. This study brings a novel approach to the development of multi-responsive reduced graphene oxide aerogels via the co-assembly of various semiconductor nanocomponents for a variety of applications that involve sustained catalytic activity.

Full text of DOI:10.1039/c5ra07267k

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DOI: 10.1039/C5RA07267K
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ARTICLE
Green Synthesis of AgI Nanoparticle–Functionalized Reduced
Graphene Oxide Aerogels with Enhanced Catalytic Performance
and Facile Recycling
Received 00th January 20xx,
Accepted 00th January 20xx
D. Amaranatha Reddy, Jiha Choi, Seunghee Lee, Rory Ma and Tae Kyu Kim*
DOI: 10.1039/x0xx00000x
AgI nanoparticle–functionalized self‐assembled reduced graphene oxide aerogels are constructed using vitamin C as the
reducing agent. The obtained aerogels can be used as efficient catalysts for organic dye degradation, reduction of 4‐
nitrophenol, and synthesis of bis(indolyl)methane. A set of characterizations, including FESEM, TEM, XRD, XPS, Raman, FTIR,
optical absorption, and photoluminescence techniques, confirm that the aerogel is formed from ultra‐dispersed AgI
nanocrystals and the self‐assembly of reduced graphene oxide nanosheets into porous hydrogel structures. The obtained
aerogels exhibit high photocatalytic degradation ability toward an organic dye (Rhodamine‐B) because of the high visible
light–driven catalytic activity of AgI and the high specific surface area of graphene nanosheets with three‐dimensional
interconnected pores. The well‐wrapped reduced graphene oxide nanosheets on AgI nanostructures could promote the
transfer of photo‐generated electrons, which not only effectively inhibits the recombination of electrons and holes but also
suppresses the photocorrosion of AgI; this promotes the photocatalytic activity and stability. Moreover, these
www.rsc.org/
nanostructures show the best catalytic activity for the reduction of 4‐nitrophenol to 4‐aminophenol in the presence of NaBH
4
as a reducing agent. Furthermore, the AgI‐reduced graphene oxide aerogel nanocomposites are active catalysts for the
synthesis of bis(indolyl)methane under solvent‐free conditions. The nanocomposites exhibit excellent catalytic activity and
remarkable durability. This study brings a novel approach to the development of multi‐responsive reduced graphene oxide
aerogels via the co‐assembly of various semiconductor nanocomponents for a variety of applications that involve sustained
catalytic activity.
4
area and limited electron and ion transport. All these factors
Introduction
hinder the industrialization and limit the applications of
graphene in these fields. To overcome these issues, lightweight
Among various carbon nanostructures, graphene has recently
attracted wide and intense scientific interest because it
comprises one atom–thick, two‐dimensional (2D) atomic layers
self‐assembled 3D reduced graphene oxide aerogels (RGAs)
have been generated and offer new possibilities by exploiting
their high specific surface area, plethora of interconnecting
macropores and mesopores, fast electron transport kinetics
due to the continuous graphene backbone, and enhanced
2
1
of sp ‐hybridized carbon atoms. Moreover, graphene
possesses an excellent intrinsic mobility limit, good
transparency, a large specific surface area, high thermal
5
active sites. The green protocol for producing RGAs with
2
conductivity, and notable mechanical strength. These unique
properties have led to a growing interest in the application of
graphene sheets for a variety of sustainable applications
hierarchical microstructures involves generating reduced
graphene oxide hydrogels via in situ self‐assembly of graphene
oxide (GO) by mild chemical reduction under atmospheric
pressure and subsequent freeze‐ or critical point–drying to
replace the solvents in the wet gels with highly porous
3
including energy storage, catalysis, and adsorption. However,
2
D graphene layers tend to re‐aggregate and restack with other
graphene sheets in solution because of van der Waals forces;
this results in the formation of much thicker multilayer
graphene sheets, leading to poor accessibility to the surface
6
nanostructures with unique characteristics.
However, pure graphene sheets are not suitable for visible
light photocatalytic applications because they do not efficiently
collect solar light because of their weak absorption abilities.
Moreover, pure graphene nanosheets do not show any
photocatalytic activity and serve only as electron acceptors or
transport intermediates because of their zero band gap. In
addition, at high GO loadings, a much thicker graphene layer
Department of Chemistry and Chemical Institute for Functional Materials, Pusan
National University, Busan 609‐735, Republic of Korea.
(E‐mail) tkkim@pusan.ac.kr
Electronic Supplementary Information (ESI) available: Experimental and
characterizations, Schematic mechanism of the formation of the AgI‐graphene
hydrogel, EDS and EDS mapping, PL and CIE analysis, BET analysis, UV‐visible spectra forms and acts as a recombination center instead of providing
of the photocatalytic activity, 1H NMR of RhB with photodegradation time,
an electron pathway thereby deteriorating the photocatalytic
photocurrent responses and EIS analysis, FESEM, XPS and XRD patterns of the
7
1
13
NMR spectra of bis(indolyl)methane. See performance. Therefore, functionalization of photocatalytic
semiconductor nanoparticles with 3D graphene sheets is often
recycled catalysts,
H
and
C
DOI: 10.1039/x0xx00000x
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required to take full advantage of the unique properties of compared to bare AgI nanostructures. In addition,ViAewgIAraticnledOAnlginIe‐
graphene sheets; this is an ideal strategy for maximizing the RGAs are also catalytically active for DtOheI: 10r.e10d3u9c/tCio5RnA0o7f2674K‐
accessible surface area and developing high‐performance nitrophenol in the presence of NaBH . Furthermore, the as‐
4
recyclable photocatalysts. Considering this, researchers have synthesized AgI‐RGA nanocomposites are efficient catalysts for
recently made significant advances in generating various the synthesis of bis(indolyl)methane under solvent‐free
reduced graphene oxide ‐based nanohybrid systems containing conditions; in the presence of the nanocatalyst, the yields of the
8
9
a variety of immobilized nanoparticles such as TiO
2
, P25,
corresponding products were moderate to good.
1
0
5
11
12
BiOBr, ZnS, Cu O, and W18O49, ; they found that the
2
combination of semiconductor nanoparticles and graphene
nanosheets with three‐dimensional interconnected pores
resulted in excellent catalytic activity for photooxidation of
organic dyes and reduction of aromatic nitro groups to amino
groups at room temperature. The widely accepted mechanism
for the photocatalytic performance enhancement is increased
light absorption, efficient separation of the photogenerated
charge carriers, and extension of the lifetime of the electron‐
hole pairs because of the chemical interactions between the
Results and discussion
The surface morphologies of the as‐synthesized reduced
graphene oxide aerogels, AgI nanostructures, and AgI‐RGA
nanocomposites were characterized by FESEM and are shown
in Fig. 1(a–d). As shown in Fig. 1(a), the as‐synthesized graphene
aerogels exhibit well‐defined and interconnected 3D porous
network structures with continuous macro‐pores in the
micrometer size range. The pores of the 3D microstructures
comprise intercalated graphene sheets, and the walls of the
pores are thin; these factors indicate effective assembly of the
graphene sheets. These unique 3D porous network structures
can significantly prevent the aggregation of graphene sheets
caused by π–π interactions and van der Waals forces, which
may be beneficial for charge carrier transport. The as‐
8
graphene sheets and semiconductor nanoparticles.
Among these photocatalysts, silver iodide (AgI) is a novel
photocatalyst with promising efficiency for decomposition of
1
3
organic dyes by visible‐light irradiation. Moreover, the
photocatalytic efficiency of AgI nanostructures can be further
improved by modulating the shape, morphology, and crystal
1
4,15
faces of the AgI crystals.
However, pure AgI nanostructures
synthesized AgI nanostructures exhibit
a
cuboidal‐like
are prone to photoreduction and decompose to weakly active
silver during the photocatalytic reaction if no sacrificial reagent
morphology with rough surfaces. The rough surfaces indicate
that the AgI nanostructures are composed of tiny AgI
nanoparticles (Fig. 1(b)). Moreover, most of the nanoparticles
seem to be agglomerated, which is characteristic of surfactant‐
16
is supplied. This unsatisfactory durability is the main
hindrance to the practical application of AgI as a recyclable and
highly efficient photocatalyst. A variety of strategies, such as the
1
7,18
14
formation of AgI composites
and surface modification,
have been employed to reduce the recombination of charge
carriers and enhance the photocatalytic activity and stability.
Nevertheless, the new simple method that improves the
durability of AgI while facilitating its recovery is still required for
environmental applications. We believe that these issues may
be fully resolved by embedding AgI nanostructures in reduced
graphene oxide aerogels. Because this type of nanohybrid is
lightweight and hygroscopic, it floats on top of the solution
during the photodegradation process and can be easily
separated for recycling without any loss of photocatalyst.
1
9
We have earlier reported enhanced catalytic activity in 2D
AgI‐RGO nanocomposites. In that report, it was observed that
the rate of RhB degradation decreased more significantly with
each cycle; only 72 % RhB degradation was achieved on the
second cycle. It is suggesting that the AgI‐RGO nanostructures
are unstable under light illumination; this may be due to their
slight solubility in aqueous solutions and photocorrosion. This
unsatisfactory durability of the AgI‐RGO is the main hindrance
for the practical application of AgI as a recyclable and highly
efficient photocatalyst. With a view of looking for more stable
and enhanced activity for AgI nanostructures, herein, for the
first time, we demonstrate a facile green synthesis to fabricate
3
D AgI‐RGA nanocomposites using vitamin C as a reducing agent.
Remarkably, AgI‐RGA nanohybrids feature enhanced
photocatalytic activity and durability for the
photodecomposition of organic compounds (i.e., Rhodamine B
RhB)) under simulated sunlight and visible light irradiation
Fig.
1 FESEM images of the graphene aerogel (a), AgI (b), and AgI‐2
nanocomposites ((c)‐(d)). TEM (e) and HRTEM (f) images of the AgI‐2
(
nanocomposites.
2
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free precipitation reactions in aqueous media. Fig. 1(c–d)
present representative FESEM images of the AgI‐RGA
composites. The FESEM images clearly show that the AgI
nanoparticles are anchored uniformly on both sides of the
graphene sheets. It is noteworthy that no AgI particles that
were disassociated from the graphene sheets were observed,
suggesting efficient assembly of the AgI nanoparticles and
graphene nanosheets. Such geometric confinement of
semiconductor nanoparticles within the graphene layer can be
expected to suppress dissolution and agglomeration of the
nanoparticles, thereby improving the charge transport between
the nanostructures and graphene sheets resulting in enhanced
photocatalytic activity and stability of the composites.
Furthermore, in the composite, the graphene nanosheets form
interconnected 3D porous network structures with continuous
macropores in the micrometer size range similar to those of
bare graphene aerogel nanostructures; this indicates that the
unique interconnected porous structure of the graphene sheets
is not affected by functionalization with AgI nanoparticles.
Energy dispersion spectroscopy (EDS) analysis was carried
out to examine the chemical compositions of the AgI
nanostructures and AgI‐RGAs. Typical EDS spectra of AgI and
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DOI: 10.1039/C5RA07267K
Fig. 3 (a) and (c) XPS survey spectra of GO and the AgI‐2 nanocomposites; (b) and (d)
AgI‐2 are shown in Fig. S2(a) and (b). The EDS spectrum of the narrow‐scan spectra of C 1s for GO and the AgI‐2 nanocomposites; (e) and (f) narrow‐
scan spectra of Ag 3d and I 3d for the AgI‐2 nanocomposites.
bare AgI nanostructures reveals that the nanostructures contain
only Ag and I without any impurities, where as signals for carbon
and oxygen are clearly evident in the EDS spectrum of AgI‐RGA
To further investigate the structural information and
nanocomposites in addition to those for Ag and I. No traces
microscopic morphology, TEM and HRTEM analysis of the AgI‐2
of other elements are evident in the spectra, thereby
nanocomposites was performed, and the obtained results are
confirming the purity of the samples. To further investigate the
shown in Fig. 1(e) and (f). From these figures, it is clear that the
spatial homogeneity of these detected elements, X‐ray EDS
AgI nanoparticles are anchored uniformly on both sides of the
elemental mapping was carried out for the AgI‐graphene
graphene sheets and the graphene nanosheets form
nanocomposites, and the results are shown in Fig. S3(a–d). It
interconnected 3D porous network structures. The HRTEM
can be seen that C, Ag, and I are uniformly distributed over the
image of the AgI‐2 nanocomposite (Fig. 1(f)) shows lattice
entire matrix of the AgI‐RGA nanocomposites.
fringes from the (002) crystal plane of AgI, apart from the
stacked layers of reduced graphene oxide (RGO). The fringe
widths were calculated to be 0.367 nm for AgI, and the stacking
width was calculated to be 0.31 nm for the RGO layers.
The crystal structures and crystallinities of the as‐obtained
products were investigated by X‐ray diffraction (XRD)
measurements. Fig. 2 presents the XRD patterns of pure AgI and
AgI‐RGAs with different GO concentrations. The diffraction
peaks of the pure AgI nanostructures can be readily assigned to
the hexagonal phase of β‐AgI (JCPDS File Card No. 09‐0374),
with characteristic diffractions of the (100), (002), (101), (102),
(
110), (103), (200), and (112) planes of hexagonal crystalline β‐
AgI. In addition to these peaks, a small diffraction peak at
around 56.7°, which was ascribed to the (400) plane of γ‐AgI, is
evident. These results demonstrate the coexistence of β‐AgI
and γ‐AgI phases in the as‐synthesized nanocomposites.14 The
intense and sharp peaks and narrow peak width elucidate that
all the synthesized composites are well‐crystallized. Moreover,
no crystalline impurities related to reaction by‐products, such
as Ag metals or Ag clusters, were observed, which supports the
high purity of the product. The obtained AgI‐RGAs with different
GO concentrations display similar XRD patterns to the AgI
nanostructures, implying that the wrapped reduced graphene
Fig. 2 X‐Ray diffraction patterns of the AgI and AgI‐RGA nanostructures.
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nanosheets do not influence the crystal phase of AgI. Moreover,
typical diffraction peaks for graphene (002) were not observed;
this suggests that the layer‐stacking regularity of GO sheets
would be inhibited after reduction by AgI nanocrystals as the
spacers. The average nanocrystallite sizes (D) of the AgI and AgI‐
RGA composites were estimated using the Debye–Scherrer
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DOI: 10.1039/C5RA07267K
formula ( D  0.89 /  cos , where
Cu Kα irradiation, is the FWHM of the diffraction peak, and
is the diffraction angle for the (002) plane of hexagonal β‐

is the wavelength of the


AgI); the calculated D values are in the range of 25 – 30 nm.
To further investigate the surface composition, the valence
states of the elements and purity of the as‐synthesized products
were determined using XPS. Fig. 3(a) shows a typical wide‐scan
survey XPS spectrum of GO. The survey spectrum indicates that
the sample is composed of C and O elements, and no peaks for
other elements are evident. Fig. 3(b) shows the narrow‐scan C
1
s XPS spectrum of GO. The spectrum can be fitted by three
Fig. 4 (a) Raman spectra of GO, GA, and the AgI‐2 nanocomposites. (b) FT‐IR spectra of
pure GO, graphene aerogel, and the AgI‐2 nanocomposites. (c) Optical absorption
Gaussian peaks corresponding to carbon atoms in different
oxygen‐containing functional groups. The peak that appears at spectra of AgI and AgI‐RGAs with different GO concentrations. (d) Emission spectra of
a binding energy of 284.8 eV is assigned to a non‐oxygenated C– the AgI and AgI‐RGA nanocomposites.
2
C bond containing sp ‐hybridized carbon atoms. The spectral
bands at binding energies of 286.2
hexagonal graphitic lattice. The ratio between the intensities of
and 288.8 eV were assigned to the C–OH and O=C–OH oxygen‐
the D and G bands (I
expressed by the sp /sp carbon ratio. Compared with GO (0.93),
GA (1.01) and the AgI‐2 nanocomposite (1.04) have slightly
D
3
/I
G
) is a measure of the disorder, as
2
0
containing functional groups, respectively. Fig. 3(c) shows a
typical wide‐scan XPS spectrum of the AgI‐RGA nanocomposite.
In the spectrum, the photoelectron lines for Ag 3d, I 3d, Ag 3p,
C 1s, and O 1s were observed at their respective standard
binding energies, which confirm the existence of these
elements in the synthesized nanocomposites. Fig. 3(d) shows
the C 1s XPS core level spectrum of the AgI‐RGA nanocomposite.
In the spectrum, the de‐convoluted peaks located at 284.9,
2
D G
higher I /I values, which are attributed to a decrease in the size
2
of the sp domains upon reduction and the interactions
between the AgI nanostructures and graphene nanosheets.21
Furthermore, FTIR spectra of the GO, GA, and AgI‐2
nanocomposites were measured over the range of 400 – 4000
−
1
cm to corroborate the XPS and Raman results. As illustrated in
Fig. 4(b), in the FTIR spectrum of GO, the strong and broad
2
86.4, and 288.9 eV correspond to C–C, C–OH, and carboxylic
groups (O−C=O), respecꢀvely. However, the intensiꢀes of these
oxygen‐containing functional groups are very low compared to
that of the C 1s of GO, which indicates that the graphene in the
prepared composite has a high degree of reduction. Fig. 3(e)
shows the high‐resolution XPS spectra of Ag 3d. For Ag 3d, two
peaks were observed at binding energies of about 368.0 and
74.0 eV, which correspond to Ag 3d5/2 and 3d3/2, respectively.
The difference in the binding energies of Ag 3d5/2 and 3d3/2 is 6
eV, suggesting that the Ag ions exist in the +1 valence state in
the matrix of the AgI product. No peaks related to Ag are
evident at binding energies of 369.2 or 375.8 eV. The high‐
resolution XPS spectrum of the I 3d region is shown in Fig. 3(f)
and displays two peaks at around 619.4 and 631.1 eV; these
correlate with I 3d5/2 and 3d3/2, respectively. The XPS analysis
results evidence the reduction and functionalization of RGO and
confirm the components of the AgI‐graphene nanocomposites.
Raman spectroscopy was used to elucidate the degree of
reduction of GO in the AgI‐RGAs. Fig. 4(a) depicts room‐
temperature Raman spectra of GO, GA, and the AgI‐2
nanocomposites. The spectra of all the samples have two
−
1
absorption peak situated at about 3397 cm corresponds to
stretching vibrations of the hydroxyl groups of GO, and the peak
−
1
at ~1630 cm is related to aromatic –C=C. The absorption peaks
associated with oxygen‐containing groups, such as C–O,
phenolic C–OH, and carboxyl C=O stretching vibrations, are
2
0
−
1
22
located at 1738, 1069, and 1239 and 1379 cm , respectively.
3
All these absorption bands have high intensities indicating
successful oxidation of graphite to form GO sheets. In contrast,
in the FTIR spectra of RGA and the AgI‐2 nanocomposites, the
intensities of the peaks for the oxygen‐containing functional
groups dramatically decrease, and the absorption peaks for C–
O and C=O from COOH groups at 1738, 1239, and 1379 cm
completely vanish. In addition, in the AgI‐2 nanocomposites,
new absorption bands appear in the range of 400 – 600 cm
and were attributed to Ag‐I. This feature further proves the
reduction of GO, which is consistent with the XPS and Raman
spectroscopy results.
The optical absorption properties of the AgI nanostructures
and AgI‐RGA nanocomposites were examined via UV‐visible
absorption spectroscopy in the spectral range of 230 – 800 nm.
14
0
−
1
−
1
−
1
vibrations at around 1589 and 1347 cm , which are related to
the D and G bands of RGO, respectively. The G band is assigned
to the E2g phonon of symmetric stretching of the sp C–C bond,
and the D band is a breathing mode of k‐point phonons of A1g
(Fig. 4(c)) The pure AgI nanostructures show a sharp absorption
edge with an absorption maximum at around 427 nm, which
was attributed to the characteristic band of AgI induced by the
forbidden transition (4d to 4d 5s ) allowed by the tetrahedral
2
1
0
9
1
symmetry, originating from disruption of the symmetrical
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+
23
symmetry of the Ag ion site. The optical band gaps of the as‐ and holes and consequently weakens the PL intensity. With all
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synthesized AgI nanostructures were deduced from the of the information mentioned above, oDnOeI:c1a0n.10e39n/vCi5sRioAn072th67aKt
maximum absorbance using the band‐gap relation. The efficient charge separation could increase the lifetime of the
estimated optical band gap of the AgI nanostructures is 2.90 eV, charge carriers and enhance the efficiency of interfacial charge
which is slightly higher than that of the bulk AgI material (2.82 transfer to absorb the organic dyes, which would account for
2
4
eV). Moreover, the band gap was slightly higher than that of the higher photocatalytic activities of the AgI‐RGA
2
5
pure γ‐AgI and lower than that of pure β‐AgI. We believe that nanocomposites to some degree.
the change in the band gap of the synthesized nanostructure
The nitrogen adsorption desorption isotherm of the as
may be due to the coexistence of γ‐AgI and β‐AgI in the obtained AgI‐2 reduced graphene aerogels were investigated
26
nanostructures. However, for the AgI‐RGA nanocomposites, and are presented in Fig. S5. The samples exhibit type‐IV
two absorption bands were observed with absorption maxima isotherms with a distinct hysteresis loop in the relative pressure
at 280 and 429 nm. The absorption band at 280 nm was range of 0.7 – 1.0, which is a characteristic of the
assigned to the π–π* transition of the aromatic C–C and C=C inhomogeneous mesoporous material with cylindrical pore
bonds related to graphene oxide. However, this band was at a geometry present within the nanostructures, and facile
higher wavelength than that of graphene oxide (235 nm), connectivity between the pores. The Brunauer–Emmett–Teller
2
−1
indicating the reduction of GO and restoration of the C–C bonds (BET) surface area of AgI‐2 is 18.6970 m g with an average
5
in the RGA composites. The other absorption peak is located at pore diameter of 39.9055 nm. The observed BET surface area is
4
29 nm and was attributed to the characteristic band of AgI higher than that of the pure AgI nanostructures and which can
1
0
9
1
induced by the forbidden transition (4d to 4d 5s ) enabled by be attributed to the incorporation of RGO which has a large
the tetrahedral symmetry of the Ag ion site. Compared to the surface area, leading to more surface active sites and the
spectra of the AgI nanostructures, the absorbance peaks are increased adsorption of reactants .
+
23
2
1
more intense and the peak at 427 nm red‐shifts slightly with
The photocatalytic activities of the as‐synthesized AgI and
increasing graphene content; this is ascribed to chemical AgI‐RGA nanocomposites were probed by monitoring the
bonding between the semiconductor photocatalyst and degradation of organic dye, i.e., RhB, under simulated sunlight
graphene nanosheets. These results imply that the as‐ irradiation. The optical absorption spectra obtained for the
synthesized AgI and AgI‐RGA composites are suitable for use as photocatalysts, as a function of simulated sunlight irradiation
visible light–induced photocatalysts.
time, are shown in Fig. S6(a–f). The characteristic maximum
The interface charge‐separation and recombination absorbance intensity of RhB at 554 nm was used to assess the
properties of the as‐synthesized nanostructures were extent of degradation. The degradation of RhB was negligible
investigated using photoluminescence (PL) emission under simulated sunlight irradiation in the absence of
spectroscopy. Fig. 4(d) presents the room‐temperature PL photocatalyst, whereas in the presence of any of the
spectra of the AgI nanostructures and AgI‐RGA nanocomposites synthesized photocatalysts, the maximum absorbance intensity
under 330 nm excitation. The spectra of the AgI nanostructures diminished sharply with illumination time. Among the
reveal a broad emission comprising multiple overlapped peaks synthesized nanocomposites, the AgI‐2 nanocomposite
that cover the visible region from 430 to 600 nm. The exhibited the highest photocatalytic efficiency: The RhB dye
broadening of the emission peak is due to overlapping of completely degraded within 70 min under simulated sunlight
emissions from various defects. The Gaussian curve‐fitting for irradiation, whereas 130, 90, 80, and 100 min were required to
the broad emission of AgI shown in Fig. S4(a) indicates that the completely decompose the RhB using the AgI, AgI‐1, AgI‐3, and
broadening of the peak is due to overlapping of three emission AgI‐5 nanocomposites, respectively. Moreover, in the presence
peaks at 463, 502, and 575 nm; these three emissions were of the AgI nanostructures, the absorption maximum
attributed to distant pair donor‐acceptor (D‐A) recombination significantly shifts to a lower wavelength with irradiation time
mediated by the high density of deep trap states involving until it reaches zero absorbance, as shown in Fig. S6(a). These
exciton‐phonon interactions or crystalline defects or gradual hypsochromic shifts of the absorption maxima indicate
2
7,28
impurities.
Although the emission spectra of the AgI‐RGA that RhB is first de‐ethylated in a stepwise manner and then the
nanocomposites appear to be similar to those of AgI remaining conjugated structure is destroyed. The
nanostructures, strong, preferential luminescence quenching hypsochromically shifted maximum absorption wavelengths of
1
1
was evident with increasing GO content. This is in accordance RhB of 554, 539, 522, 510, and 498 nm correspond to N,N,N ,N ‐
1
with the fact that graphene nanosheets have a higher tendency tetraethylated rhodamine, N,N,N ‐triethylated rhodamine,
1
to accept electrons and the excited AgI nanostructures are good N,N ‐diethylated rhodamine, N‐ethylated rhodamine, and
2
9,30
electron donors. The synergism between the AgI rhodamine, respectively.
In the present study, five different
nanostructures and graphene sheets would effectively reduce intermediates form during the photoreaction over AgI
electron‐hole recombination leading to increased charge carrier nanostructures; the absorbances of the intermediates range
separation. This means that the photo‐excited electrons in the from 554 to 497 nm in the UV‐vis spectra. Moreover, the
AgI nanostructures preferentially transfer to the RGO sheets solution changed from pink to green and finally became almost
instead of returning back to the AgI vacancy defect centers colorless (Fig. S7). Whereas only two intermediates could be
1
9
thereby prolonging the lifetimes of the photogenerated holes ; identified as being formed in the presence of the AgI‐1
this significantly inhibits the recombination rate of electrons composite nanostructures, the absorbance maxima of these
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Journal Name
intermediates are located from 554 to 530 nm (Fig. S6(b)).
However, for composites AgI‐2 to AgI‐5 (Fig. S6(c–e)), no
obvious blue‐shift in the absorbance at 554 nm was observed
and the color of the solution changed directly from pink to
colorless (Fig. S7), which suggests that degradation of RhB
occurs mainly via direct mineralization yielding only gaseous
View Article Online
DOI: 10.1039/C5RA07267K
2 3 x
end products such as CO , NO , or NO . In graphene‐based AgI
nanocomposites, the electron injection rate is much faster than
in bare AgI, which causes the RhB to undergo direct
3
1
mineralization instead of stepwise de‐ethylation.
Proton NMR analysis was performed to observe the
intermediates of RhB during the degradation processes. Fig.S8
1
illustrates the H NMR spectra of the pure RhB and final
products of RhB photodegradation in the presence of AgI
nanostructures at the time intervals of 0, 30, 90 and 120 min.
The NMR signals from different aromatic hydrogen atoms and
N‐diethyl group were observed conspicuously at their
respective standard values for pure RhB. As the
photodegradation progresses, the intensity of the characteristic
peaks associated with aromatic hydrogen atoms and ethyl
group decreases, while new peaks appear at around δ 9.0 – 9.2
ppm, δ 3.0 – 3.1 ppm and δ 1.15 – 1.25 ppm. The decrease of
characteristic peaks intensity indicates the breakdown of
chromophore structures and simultaneous removal of ethyl
Fig. 5 (a, d) Photocatalytic efficiency (C
light irradiation time of the AgI and AgI‐RGA nanocomposites respectively. (b, d)
t
/C
o
) as a function of simulated sunlight and visible
groups during the degradation process. The new peaks in the Degradation efficiency in recycling of the AgI and AgI‐2 composites, under simulated
sunlight and visible light irradiation respectively. (c) Time profiles of the photocatalytic
degradation of RhB with different active species scavengers under simulated sunlight
irradiation (c), under visible light irradiation (f). (BQ: benzoquinone, EDTA:
disodiumethylenediaminetetraacetate, and TBA: tert‐butyl alcohol).
spectrum are associated with the intermediate products
formed during degradation. The NMR results further confirmed
that the de‐ethylation and degradative changes of RhB occurs
3
2,33
during the photo degradation processes
The time‐dependent degradation ratios (i.e., C
is the concentration of RhB during the reaction and C
.
t
/C
o
, where C
t
is the superoxide radical anions and hydroxyl radicals were
o
initial concentration of RhB) of RhB for all the synthesized responsible for decomposition of the RhB dye into non‐toxic
samples as a function of simulated sunlight irradiation were products, such as CO , H O, and mineral acids ((Eq. (6)), under
2
2
15
determined and are presented in Fig. 5(a). It is clearly evident simulated sunlight irradiation. From these mechanisms,
that photolysis of RhB without any photocatalyst under plausible degradation reactions were as follows:


simulated sunlight irradiation is negligible: The degradation AgI  hv  AgI e  AgI h
(1)
(2)
(3)




efficiency was less than 4 % after 130 min. This confirms that
RhB essentially does not degrade in the absence of a
photocatalyst with sunlight illumination. In the presence of the
AgI nanostructures, RhB completely photodecomposed after



AgI
AgI


e


 O
 H
 O
2
2
h
O  OH
2

OH  OH  H O
(4)
(5)
(6)
2
2
1
30 min of irradiation. The reaction mechanism for the


H O  O  OH  OH  O2
degradation of the organic dye under simulated sunlight
2
2
2
irradiation over AgI nanostructures is as follows: The photo‐ OH  RhB  H O  CO  Mineral Acids
2
2
−
generated electrons (e ) in the valence band (VB) were excited
to the conduction band (CB) by the AgI nanophotocatalyst,
which was irradiated by simulated sunlight with energy greater
+
than the threshold level. The recombined holes (h ) in the VB on
the semiconductor surface were partially localized on the
structurally defective centers of the crystalline lattice (Eq. (1)).
The reduction reaction occurred between the electrons in the
conduction band and electron acceptors, such as adsorbed O
2
molecules, which yielded superoxide radical anions (Eq. (2)).
The resultant photo‐induced holes either oxidized the organic
compound directly or were trapped by electron donors (Eq. (3)).
H
2
O
2
was formed by recombination of these oxidant radicals (Eq.
4)). H may have then reacted with the superoxide radical
anion to regenerate a hydroxyl radical (Eq. (5)). These
(
2 2
O
6
| J. Name., 2012, 00, 1‐3
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RSC Advances
Journal Name
ARTICLE
Compared with the as‐synthesized AgI nanostructures, all the introduction of EDTA anions, indicating that holes VaierweAtrthicele Omnalinine
AgI nanoparticle–functionalized GAs exhibit an appreciable degree of reactive oxidative species involved in thDeOIp: 1h0o.1t0o3c9a/tCa5lyRsAis0.72T6h7Ke
photocatalytic degradation ability. Among them, the AgI‐2 addition of BQ provoked partial inhibition of the RhB degradation
.
‐
nanostructures exhibit the fastest degradation rate: Only 70 min was suggesting that O
2
radicals also play an important role in the
required for complete photodegradation of RhB, whereas 130, 100, photocatalytic reaction. When TBA was added in the system as a
.
9
0, and 80 min were required to completely decompose RhB using scavenger for OH, it did not obviously affect the decomposition rate
AgI, AgI‐5, AgI‐4, and AgI‐3, respectively. The photocatalytic activity at all over the photocatalytic system. Based on the results, it is clear
‐
+
.
−
gradually increases with increasing graphene content up to 2 mg
;
∙
mL that h and O
2
are the major reactive species in the AgI‐2
1
36
beyond that, the photocatalytic performance deteriorates. This photocatalysis reaction system.
mainly occurs because of the formation of black reduced graphene
To determine the mineralization ratio of the degraded RhB, total
oxide sheets with zero band gaps in the composite, which shield light organic carbon (TOC) analysis was carried out. The obtained TOC
from the surface of the AgI nanocomposites; this is a universal values of the sample solutions for AgI and AgI‐2 after 70 min sunlight
−
1
−1
problem for graphene‐based photocatalysts. Moreover, excessive irradiation were 2.242 mg
graphene nanosheets act as recombination centers instead of initial RhB concentration was 10 mg
providing an electron pathway, resulting in an intrinsically lower RhB for AgI‐2 sample is much higher than that of pure AgI
∙
L , 8.341 mg
∙
L
respectively, where the
−
1
∙
L . The mineralization rate of
3
4
photo‐generation rate. All these factors lead to decreased nanostructures. This fact demonstrates that AgI‐2 nanocomposites
photocatalytic performance at high contents of graphene. However, can efficiently degrade and mineralize organic pollutants under light
all AgI‐RGAs proved to be far better than the AgI nanostructures irradiation, which is in agreement with the result of the
alone. The enhanced photocatalytic activity of AgI‐RGAs was photocatalytic test. However, after 70 min illumination time, no RhB
attributed to the following factors: Firstly, the AgI‐RGAs show better could be detected by UV‐Vis spectroscopy in the presence of AgI‐2,
−
1
adsorption capability towards RhB than the AgI nanostructures but the result of TOC is 8.341 mg∙L . This demonstrates that there
because of their higher surface area and sponge‐like nature. are still some organic carbons left‐likely aldehydes/carboxylic acids
Secondly, they more efficiently utilize visible light than the AgI but no aromatic rings. So the reaction time for the complete
nanostructures because they can float on the surface of the aqueous degradation is longer than for complete disappearance of RhB in UV–
3
7
reaction system. Thirdly, they feature efficient charge separation and vis spectra.
transportation due to the presence of graphene sheets, which To elucidate the significant role of 3D interconnected
effectively inhibit recombination of the photogenerated charge graphene nanosheets and AgI nanostructures in enhancing the
3
5
carriers resulting in high visible‐light photocatalytic activity. Finally, lifetime of electron‐hole pairs and boosting photocatalytic
the photo‐induced holes could either oxidize the organic compound ability, the transient photocurrent responses and
−
directly or be trapped by electron donors, such as OH , to produce electrochemical impendence spectra were recorded. Fig. S9(a)
∙
OH. The superoxide radical anions (O
responsible for decomposition of the RhB dye into non‐toxic light‐off cycles of AgI and AgI‐2 nanocomposite electrodes
products, such as CO , NO , NO , and mineral acids. The proposed under simulated sunlight irradiation. It is clear that the
mechanism for oxidation of RhB to mineralized compounds, CO , and photocurrent of the AgI‐2 nanocomposite electrode is much
O over the AgI‐RGA nanocomposites under solar irradiation is higher than that of the bare AgI electrode and the on‐off cycles
shown in Fig. 6. of the photocurrent are reproducible. The enhancement in the
2
and hydroxyl radicals are shows the transient photocurrent responses during light‐on and
2
3
x
2
H
2
To further reveal the photocatalytic mechanism, disodium photocurrent probably results from the expanded range of light
ethylenediaminetetraacetate (EDTA), tert‐butyl alcohol (TBA) and absorption and efficient production of photo‐induced electrons
+
.
.
‐
benzoquinone (BQ) were employed as scavengers of h , OH, and O
2
and hole separation because of the synergetic effect of the
to investigate the specific reactive species involved in the RhB interconnected graphene sheets and AgI nanostructures, which
degradation over AgI‐2 nanocomposite and are shown in Fig. 5 (c). is consistent with the photocatalytic activities. Further, to
The photodegradation of AgI‐2 was significantly suppressed by the evaluate the electron‐transport ability of the AgI and AgI‐2
nanocomposites, EIS was also carried out. Fig. S9(b) displays the
EIS results represented as Nyquist plots (Zim vs. Zre) for the pure
AgI and AgI‐2 nanocomposites under simulated sunlight
irradiation. The Nyquist plot of the AgI‐2 nanocomposite
exhibits a smaller arc diameter than that of the pure AgI
electrode, which suggests that the AgI‐2 nanocomposite can
efficiently transfer charge carriers and effect migration of
photogenerated electron‐hole pairs. The photocurrent and
electrochemical impedance studies are in good accordance with
the photocatalytic activities.
To quantitatively measure the photocatalytic efficiency, the
photodecomposition rate constant (k) of RhB with respect of
number of active sites on the materials over the AgI
nanostructures and AgI‐2 nanocomposites were calculated
from a pseudo–first order reaction kinetic model using BET
Fig. 6 Illustration of the proposed reaction mechanism for photocatalytic
degradation of RhB in an aqueous solution over AgI‐RGA nanocomposites under
simulated sunlight irradiation.
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Journal Name
surface areas, i.e., ln(C
o
/C
t
) = kt, where C
0
is the adsorption‐
To evaluate the structural stability of the AgI and AgI‐RGA
View Article Online
DOI: 10.1039/C5RA07267K
desorption equilibrium concentration before light irradiation nanocomposites after the photocatalytic experiments, we
−
1
(
mg∙L ), C
t
is the RhB concentration in the aqueous solution at conducted XRD and XPS studies. Fig. S11 presents the XRD
−
1
time t (mg∙L ), and k is the apparent pseudo–first order rate patterns of the AgI and AgI‐2 samples before and after five runs
constant (min ) . The calculated apparent k values for AgI and under simulated sunlight irradiation for the degradation of RhB.
−
1 38
−
1
‐2
AgI‐2 nanocomposites are 0.005 and 0.0155 min ∙m , The XRD patterns of the AgI nanostructures after five runs
respectively. The k value is about 3.1 times higher than that of consist of two sets of diffraction peaks: In addition to those
the bare AgI nanoparticles. This is clearly evidence of the assigned to AgI, sharp diffraction peaks (marked by
significant synergistic effect of graphene coupling at enhancing values of 38.1°, 44.3°, and 64.4° were observed; these were
the photocatalytic efficiency. indexed to the (111), (200), and (220) planes of metallic silver
♦) at 2θ
For efficient water remediation, in addition to high catalytic (JCPDS card No 89‐3722). These results confirm the formation
+
reactivity, the durability of the photocatalyst is important for of metallic silver through the reduction of Ag from the AgI
practical applications. To evaluate the durability of the AgI and nanostructures during the RhB degradation process under light
AgI‐RGA nanocomposites, we performed another five irradiation. The XRD patterns of AgI‐RGA show no notable
successive photocatalytic degradations of RhB under the same differences after photocatalysis treatment compared to those
experimental conditions. As shown in Fig. 5(b), with pure AgI, of the corresponding samples before the photodegradation
the rate of RhB degradation decreased more significantly with experiments, which suggests that the composites are stable
each cycle, and only 25.04 % RhB degradation was achieved on during the photocatalytic degradation process.
the fifth cycle, suggesting that the pure AgI nanostructures are
Furthermore, XPS spectra were recorded to determine the
0
unstable under sun light illumination; this may be due to their surface Ag contents in the AgI and AgI‐RGA nanocomposites
slight solubility in aqueous solutions and photocorrosion. and are shown in Fig. S12(a–d). The calculated surface Ag0
Moreover, we noticed that the yellow color of the AgI contents of the AgI and AgI‐RGA nanocomposite after five cycles
photocatalyst became darker after five successive are 19.38 % and 5.21 % respectively; this indicates that the
photocatalytic treatments; this indicates the formation of photocorrosion rate of the AgI‐graphene nanocomposites is far
0
+
metallic silver (Ag ) due to the reduction of Ag from the AgI slower than that of the pure AgI nanostructures. Moreover, the
nanostructures during the RhB degradation process under light C 1s deconvolution spectrum shown in Fig. S12(b) demonstrates
irradiation. More importantly, as shown in Fig. 5(b), the that the intensity of the peaks associated with the C–O and C=O
photocatalytic efficiency of the AgI‐RGA nanocomposites did moieties weaken during the degradation reactions, which
not decline significantly even after five successive indicates the continued occurrence of charge transfer and
photocatalytic cycles: It exhibits a nearly constant photo‐ interactions between the graphene sheets and AgI even after
decomposition rate as that of the first cycle. The reasons for the five degradation reactions. As a result, both the XRD and XPS
enhanced durability of the AgI‐RGA nanocomposites may be results confirm the favorable photostability of the AgI‐RGA
explained as follows: (i) The AgI‐RGAs float on top of the liquid nanocomposites, in which photocorrosion of AgI is significantly
phase during the photo‐degradation process, which means that suppressed.
there is no loss of photocatalyst and they can be easily
The photocatalytic activity of AgI and AgI‐RGAs
separated for recycling; (ii) in the aerogels, the AgI nanocomposites were also carried out under visible light
nanoparticles are anchored uniformly on both sides of the irradiation using λ≥430 nm long pass filter. The time‐dependent
graphene sheets, which protects the AgI nanostructures from degradation ratios (C /C ) of RhB for all the synthesized samples
t o
dissolution into aqueous solution due to the strong chemical as a function of visible light irradiation were determined and are
adsorption between the graphene nanosheets and AgI; and (iii) presented in Fig. 5(d). It is clearly evident that photolysis of RhB
the high conductivity of the graphene sheets enables the photo‐ without any photocatalyst under visible irradiation is negligible:
generated electrons formed in the conduction band of the AgI The degradation efficiency was less than 5 % after 100 min. This
nanostructures to transfer to the surfaces of the graphene confirms that RhB essentially does not degrade in the absence
nanosheets to quickly participate in the photocatalytic reaction, of a photocatalyst with visible light illumination. In the presence
so that the photogenerated carriers are efficiently separated. of the AgI and AgI‐RGA nanostructures, the RhB was completely
This efficient electron migration from AgI to the RGO sheets also photodecomposed. Among all, AgI‐2 nanostructures exhibit the
promotes the stability of AgI‐RGAs.
fastest degradation rate: Only 50 min was required for complete
Further the morphology of the catalysts after repeated photodegradation of RhB, whereas 100, 60, 70, and 80 min
photocatalytic experiments under solar simulator was evaluated by were required to completely decompose RhB using AgI, AgI‐3,
FESEM. It can be seen that compared to before the catalytic AgI‐4, and AgI‐5, respectively. Further, to verify the practical
o
reactions (Fig. 1(b)), numerous tiny Ag species are observed from applicability of the AgI‐RGA, the reusability was further
the surfaces after the photocatalytic process as shown in Fig. S 10 (a). evaluated under visible light irradiation over 5 photocatalytic
Where as in AgI‐2 nanocomposite there were no evident morphology cycles. Fig. 5(e) demonstrates that the photocatalytic activity
changes after photocatalysis reaction (Fig. S 10 (b)). These did not decline after five cycles. This indicates that stable and
observations imply that our AgI‐RGA nanocomposites are stable efficient performance of the AgI‐RGA nanocomposites was
during the photocatalytic degradation process.
maintained under visible light irradiation. Furthermore, reactive
oxidative species trapping experiments shown in Fig. 5 (f)
8
| J. Name., 2012, 00, 1‐3
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Journal Name
demonstrate that h and •O
the AgI‐2 photocatalysis reaction system under visible light
irradiation.
ARTICLE
+
−
2
are the major reactive species in
View Article Online
DOI: 10.1039/C5RA07267K
To evaluate the catalytic activity of the AgI nanostructures and
AgI‐RGA nanocomposite, reduction of 4‐nitrophenol to 4‐
aminophenol in the presence of NaBH was investigated. As shown
4
in Fig. 7 (a), 4‐nitrophenol exhibits a peak at 318 nm in neutral Scheme 1 AgI‐RGA–catalyzed synthesis of bis(indolyl)methane.
4
aqueous solution; upon addition of NaBH , the absorption peak red‐
shifts to 400 nm. Accordingly, the solution changes from pale yellow
to bright yellow because of the formation of 4‐nitrophenolate ions
3
9,40
under alkaline conditions.
spectra, that in the absence of catalyst, the absorption peak
at 400 nm remains unchanged, indicating that NaBH alone cannot
Moreover, it is evident from the
4
initiate the conversion reaction even after 30 min. In contrast, in the
presence of AgI and AgI‐2 catalysts, the characteristic absorption
peak at 400 nm gradually declines as the reduction reaction time
proceeds, while a new absorption peak appears at 295 nm and is
associated with the formation of 4‐aminophenol (Fig. 7(b) and (c)).
This was also evident from discoloration of the solution. A reaction
time of 60 s was required to achieve complete reduction of 4‐
nitrophenol using the AgI‐RGA composites as catalysts, whereas 120
s was required to completely reduce 4‐nitrophenol using the AgI
nanostructures. The enhanced reduction activity of the AgI‐RGAs was
attributed to the following factors: (i) the high adsorption rate of 4‐
nitrophenol onto the nanocatalyst via π–π stacking interactions
between the graphene sheets and AgI and 4‐nitrophenol; (ii) the
increased concentration of electrons due to efficient interfacial
electron transfer from the graphene sheets to AgI, which cause
Scheme 1 Proposed mechanism for AgI‐RGA–catalyzed synthesis of bis(indolyl)methane.
−1
nanocomposite is 0.0626 s , which is 1.75 times higher than that of
−1
the bare AgI nanostructures (0.0356 s ). In addition to the high
reduction capability, the reusability and stability of the catalyst is
very important for practical applications. To investigate the
reusability of the AgI‐graphene nanocomposites, their activities were
examined under the same conditions over five cycles, and the
resultant reduction efficiencies are shown in Fig. 7(d). The obtained
results indicate that the catalysts have good catalytic properties for
the reduction of 4‐nitrophenol and are readily reused.
efficient electron transfer from BH
acceptor) through the nanocatalyst (AgI), resulting in highly efficient
reduction activity; and (iii) graphene sheets prevent the aggregation
4
(donor) to 4‐nitrophenol
(
4
1
of AgI nanostructures and improve reduction performance. The
rate of reduction of the nanocatalysts can be described by pseudo–
Furthermore, the as‐synthesized AgI‐RGAs nanocomposite was
employed as a catalyst for the synthesis of bis(indolyl)methane under
solvent‐free conditions using indole (2 mmol) and aldehyde (1 mmol)
as substrates. Scheme 1 demonstrates the overall synthetic
procedure for the synthesis of bis(indolyl)methane using AgI‐2
catalyst. From the control experiment conditions, we noticed that no
coupled product was observed when the reaction was performed at
room temperature. Then, we screened the experimental conditions
and increased the temperature from room temperature to 40 °C and
t o
first order kinetics. The curves of –ln(C /C ) versus irradiation time
are linear, which is indicative of good correlation with first‐order
kinetics. The apparent reaction constant (k) of the AgI‐RGAs
o
afforded a 50 % yield for 1 h. In the case of reaction at 55 C, the
reaction was completed in 40 min with 68% yield; when the reaction
o
was performed at 70 C, the reaction was furnished within 25 min
with 85 % yield. Moreover, there was no difference in the conversion
with reaction temperatures of 80 °C and above. Thus it was proved
o
that 70 C is the optimized temperature required for effecting this
reaction by conventional heating. The resultant products were
1
13
quantitatively analyzed using H and C NMR, and the results are
presented in Fig. S13. The obtained spectral data match well with
4
2
those in earlier reports. Scheme 2 shows a possible mechanism for
the AgI‐RGA‐catalyzed synthesis of bis(indolyl)methane. The reaction
+
was initiated by activation of the carbonyl function group by the Ag
ion. Electrophilic substitution then followed at the 3‐position of the
4
Fig. 7 UV‐visible spectra of 4‐nitrophenol before and after adding NaBH solution
indole. Dehydration resulted in the formation of the intermediate,
and after 30 min without catalysis (a) and in the presence of AgI (b) and AgI‐2
nanocomposite (c). (d) Reusability of the AgI‐2 composite for the reduction of 4‐
nitrophenol.
+
which was activated again by the Ag ions, and finally, the
participation of a second mole of indole resulted in the final product,
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i.e., bis(indolyl)methane.43 In addition to its catalytic activity, the 12 X. Li, S. Yang, J. Sun, P. He, X. Xu, G. Ding, Carbon, 2014, 78, 38–
View Article Online
DOI: 10.1039/C5RA07267K
reusability of a photocatalyst is important for practical applications.
48.
The reusability of the AgI‐RGAs nanocomposite was successfully 13 W. Jiang, C. An, J. Liu, S. Wang, L. Zhao, W. Guo, J. Liu, Dalton
examined four times; we noticed a very slight loss of catalytic activity, Trans., 2014, 43, 300–305.
which suggests that the present catalysts have good stability and 14 B. Lei, M. Zhu, P. Chen, C. Chen, W. Ma, T. Li, M. Liu, ACS Appl.
sustainability.
Mater. Interfaces, 2014, 6, 4160−4169.
15
C. Ana, J. Liu, S. Wang, J. Zhang, Z. Wang, R. Long, Y. Sun, Nano
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Conclusions
1
1
6
7
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K. Vignesh, A. Suganthi, B.K. Min, M. Rajarajan, M. Kang, RSC
Adv., 2015, 5, 576–585.
We successfully prepared novel self‐assembled graphene/AgI
aerogels with robust interconnected 3D graphene networks
embedded with AgI nanoparticles. The resulting reduced
graphene oxide aerogel hybrids were used as catalysts for
organic dye degradation, reduction of 4‐nitrophenol, and
synthesis of bis(indolyl)methane; for these reactions, they
exhibited excellent catalytic activity and durability. The
enhanced catalytic activity is mainly due to its superior
adsorption capability and efficient charge separation and
transportation between RGO and the AgI nanostructures. The
charge transfer mechanism was supported by the
photoluminescence and photoelectrochemical studies.
Recycling results confirm the favourable photo stability of the
AgI‐RGA nanocomposites, in which photocorrosion of AgI was
significantly suppressed. These findings may open new
opportunities for the design of highly efficient and recyclable
catalytic graphene semiconducting aerogels for a variety of
applications that involve sustained catalytic activity.
18
19
20
2
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This work was financially supported by National Research
Foundation of Korea (NRF) grants, funded by the Korean
government (MEST and MSIP) (2013S1A2A2035406,
3
0
S. Liang, S. Zhu, Y. Chen, W. Wu, X. Wang, L. Wu, J. Mater. Chem.,
2
013R1A1A2009575 and 2014R1A4A1001690). This work was
2012, 22, 2670–2678.
also supported by the 2015 Post‐Doc. Development Program of
Pusan National University.
3
3
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DOI: 10.1039/C5RA07267K
A Table of Contents Entry
RA-ART-04-2015-007267: Green Synthesis of AgI Nanoparticle–Functionalized
Reduced Graphene Oxide Aerogels with Enhanced Catalytic Performance and
Facile Recycling by D. A. Reddy et al.
→
This work demonstrates the facile green synthesis of AgI-reduced graphene oxide
aerogels with superior photocatalytic performance and remarkable durability.
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