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Photogenerated Carriers Transfer in Dye-Graphene-SnO2 Composites for Highly Efficient Visible-Light Photocatalysis


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The visible-light-driven photocatalytic activities of graphene-semiconductor catalysts have recently been demonstrated, however, the transfer pathway of photogenerated carriers especially where the role of graphene still remains controversial. Here we report graphene–SnO2 aerosol nanocomposites that exhibit more superior dye adsorption capacity and photocatalytic efficiency compared with pure SnO2 quantum dots, P25 TiO2, and pure graphene aerosol under the visible light. This study examines the origin of the visible-light-driven photocatalysis, which for the first time links to the synergistic effect of the cophotosensitization of the dye and graphene to SnO2. We hope this concept and corresponding mechanism of cophotosensitization could provide an original understanding for the photocatalytic reaction process at the level of carrier transfer pathway as well as a brand new approach to design novel and versatile graphene-based composites for solar energy conversion.
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Photogenerated Carriers Transfer in DyeGrapheneSnO2
Composites for Highly Ecient Visible-Light Photocatalysis
Shendong Zhuang, Xiaoyong Xu,*Bing Feng, Jingguo Hu,*Yaru Pang, Gang Zhou, Ling Tong,
and Yuxue Zhou
School of Physics Science and Technology, Yangzhou University, Yangzhou 225002, P.R. China
SSupporting Information
ABSTRACT: The visible-light-driven photocatalytic activities
of graphene-semiconductor catalysts have recently been
demonstrated, however, the transfer pathway of photogenerated
carriers especially where the role of graphene still remains
controversial. Here we report grapheneSnO2aerosol nano-
composites that exhibit more superior dye adsorption capacity
and photocatalytic eciency compared with pure SnO2
quantum dots, P25 TiO2, and pure graphene aerosol under
the visible light. This study examines the origin of the visible-
light-driven photocatalysis, which for the rst time links to the
synergistic eect of the cophotosensitization of the dye and
graphene to SnO2. We hope this concept and corresponding mechanism of cophotosensitization could provide an original
understanding for the photocatalytic reaction process at the level of carrier transfer pathway as well as a brand new approach to
design novel and versatile graphene-based composites for solar energy conversion.
KEYWORDS: graphene, dye, cophotosensitizer, tin oxide, visible-light-driven photocatalysis
Since the photoelectrochemical water splitting over a titanium
oxide (TiO2) electrode was rst reported by Fujishima and
Honda in 1972,
semiconductor photocatalytic technology has
attracted wide attention because of its potential for hydrogen
generation and pollutants degradation, which is a well-accepted
strategy to simultaneously solve the energy and environmental
To date, various active semiconductor photocatalysts
have been rapidly developed as well, such as TiO2, zinc oxide
(ZnO) and tin oxide (SnO2), and tin disulde (SnS2), etc.
Especially, SnO2has also been extensively used in other elds,
such as sensors,
solar cells,
and Li-ion batteries,
because of
its excellent gas sensitivity, photoelectrical properties and
chemical stability. And because of the high photochemical
stability and catalytic activity, SnO2has the potential to be an
alternative candidate to the commercial TiO2photocatalyts.
SnO2catalysts could well operate under ultraviolet (UV)
but they generally perform poorly under visible
light due to the wide band gap (Eg= 3.6 eV),
like most wide
band gap semicondutor catalysts. As is well-known, UV light
accounts for only a small fraction (4%) of the solar energy in
comparison with visible light (43%); therefore, any shift in the
optical response of a photocatalyst from UV to visible spectral
range would produce a positive eect on improving the
photocatalytic eciency.
For the SnO2-based catalysts,
broadening the light-absorption band and minimizing the
recombination of photogenerated electronhole pairs as two
signicant issues are being widely explored by various design
strategies, such as the element doping
and the heterojunction
and so on. In addition, small-size SnO2quantum
dots (QDs) with large specic surface area have been
demonstrated to stimulate surface reactions for achieving
more superior photocatalytic performance.
Many prepara-
tion methods including biomolecule-assisted hydrothermal
and surfactant-assisted solvothermal method
have been tried; however, it also remains dicult to obtain
stable and monodispersed SnO2QDs because of the massive
surface free energy.
Thus the appropriate surface treatment is
usually necessary to keep QDs stable for obtaining large specic
surface area and ecient photocatalytic activity. In a word, the
visible-light-driven photocatalytic eciency of SnO2QDs
catalysts depends to a great extent on their dispersity, and
capabilities of absorbing visible light and of preventing
recombination of photogenerated carriers.
Recently, with the rise of graphene, it has been found that
graphene has a considerable absorption of visible light, in
addition to UV light, because of the unique electronic
Moreover, because of the large-size two-dimen-
sional surface, the graphene sheet could behave as a giant solid
Supporting Information (stabilizer) of nanoparticles through
interfacial interaction to avoid particle aggregation.
tionally, owing to special π-conjugation structure, large specic
surface area,
and high conductivity,
graphene could enhance
the photocatalytic activity of catalyst by facilitating the
Received: October 24, 2013
Accepted: December 3, 2013
Published: December 3, 2013
Research Article
© 2013 American Chemical Society 613 |ACS Appl. Mater. Interfaces 2014, 6, 613621
adsorption of organic contaminants and the separation of
photogenerated carriers.
Hence, by integrating with graphene,
the wide band gap semiconductors are promising to realize
ecient visible-light-driven photocatalysis, such as TiO2-
and ZnS-graphene,
etc. At
present, very limited literatures have reported the photo-
catalytic performance of SnO2-graphene nanocomposites for
environmental cleanup,
although they have been widely
studied for Li-ion battery,
gas sensor,
and supercapacitor.
Moreover, the mechanism of visible-light-drivien photocatalysis
has not been completely claried yet as well, which is
imperative for advancing the application of visible-light-
activated semiconductorgraphene composite photocatalysts.
Herein, we synthesized the SnO2graphene aerosol nano-
composite (SGA) via a simple self-assembled hydrothermal
reduction method. Such a SGA performs more excellent dye
adsorptivity and visible-light-driven photocatalytic activity
relative to pure SnO2QDs, P25 TiO2, and pure graphene
aerosol (GA). The superior photocatalytic activity is due to the
combination of strong dye adsorption capacity and eective
separation of photogenerated carriers. Notably, such a SGA
photocatalyst with the stable recyclability is promising to be
applied to environmental remediation. Interestingly, the
possible photocatalytic mechanism that Rhodamine B (RhB)
dye and graphene serving as visible-light cophotosensitizers for
SnO2driving the degradation of RhB dye has been proposed
based on the work function-engineered carrier-transfer route.
We hope this concept of cophotosensitizer and corresponding
mechanism could deepen further the understanding of the
photocatalytic reaction process in graphene-based composites,
which is signicant to exploit graphene-based solar energy
conversion devices, such as dye-sensitized solar cell, photo-
chemical water splitting and photocatalytic pollutant cleaning-
up, etc.
2.1. Method Summary. The SGA composites were prepared by a
simple hydrothermal reduction with self-assembly of GO and SnO2
QDs. The reduction degree of GO and the combination of graphene
and SnO2were evaluated by high resolution transmission electron
microscope (HRTEM), X-ray diraction (XRD) and Fourier trans-
form infrared spectra (FT-IR). The absorptive capacity of visible light
and the sources of visible-light photogenerated carriers in the SGA
were revealed by UVvis absorption spectra and photoluminescence
(PL) spectra, respectively.
2.2. Materials. Expandable graphite (60 meshes) was supplied by
Qingdao Jinrilai Graphite Co., Ltd.. Tin(IV) Chloride Pentahydrate
(SnCl4·5H2O, 99%), potassium permanganate (KMnO4, 99.5%),
sodium nitrate (NaNO3, 99%), hydrogen peroxide (H2O2, 30%),
hydrochloric acid (HCl, 36.0%-38.0%), and sulphuric acid (H2SO4,
98%) were purchased from Sinopharm Chemical Reagent Co., Ltd.
(Shanghai, China). Deionized water was obtained from local sources.
All materials were used without further purication.
2.3. Synthesis of SnO2QDs. SnO2QDs were obtained by a
modied hydrothermal synthesis based on our previous report.
Typically, 50 mL of deionized water was added to 8.7645 g of SnCl4·
5H2O followed by magnetic stirring for 30 min at room temperature.
The resulting SnCl4solution was transfer to a 50 mL of Teon-lined
autoclave and hydrolyzed at 180 °C for 2 h. After the autoclave cooling
down naturally to room temperature, the supernatant was discarded.
The obtained precipitate was washed 3 times by centrifugation (8000
rpm, 5 min) with deionized water to remove the soluble impurities and
free reactants and dried at 70 °C for 24 h to obtain SnO2QDs (see
Figure S1 in the Supporting Information).
2.4. Preparation of Graphene Oxide (GO). GO was synthesized
from graphite powder by an improved Hummers method.
Graphite, NaNO3, KMnO4,H
2SO4,andaTeon reactor placed into
a stainless steel autoclave were completely cooled in a refrigerator at
04°C for 4 h. Then cooled graphite (0.5 g), NaNO3(0.5 g), KMnO4
(3 g), and H2SO4(600 mL) were successively added into the Teon
reactor. As soon as the H2SO4was added, the reactor was sealed in the
stainless steel autoclave and transferred in the refrigerator maintaining
at 04°C for 1.5 h and then heated at 90 °C in an oven for 2 h. After
the autoclave was cooled naturally to room temperature, the
supernatant was removed. The obtained mud was diluted with 60
mL of deionized water followed by mechanical stirring for 2 h. With
mechanical stirring, 5 mL of H2O2was dripped into the suspension
until the slurry turned golden yellow. After let stand for 3 h, the
supernatant was poured out. The obtained golden yellow slurry was
washed by centrifugation with HCl and deionized water until the pH
of the supernatant is larger than 5 to obtain graphene oxide (GO)
suspension (see Figures S2 and S3 in the Supporting Information).
After a drying process at 50 °C for 48 h, the GO was obtained.
2.5. Preparation of SnO2Graphene Aerosol (SGA). The
SGAs were obtained by simple self-assembly of GO and SnO2QDs
under hydrothermal condition,
as illustrated in Figure S3 (see the
Supporting Information). In a typical synthesis, GO was added into 30
mL of deionized water with magnetic stirring for 30 min to produce 2
mg/mL of homogeneous GO solution. Then the prepared SnO2QDs
were added to the GO solution by the mass ratio of SnO2to GO of
4:1, 2:1, 1:1, 1:3, and 1:5, respectively. After a 15 min magnetic
stirring, the mixtures were sonicated for 1 h and magnetically stirred
for 5 min. Then the mixtures were then sealed in 50 mL of Teon-
lined autoclaves, respectively, and hydrothermally treated 180 °C for 2
h. After the autoclaves were cooled naturally to room temperature, the
black cylinders (SnO2-graphene hydrogel, SGH) were obtained. The
3D graphene hydrogel (GH) was prepared under the same conditions
without adding SnO2QDs. The as-synthesized GH and SGH samples
were frozen for 24 h and dried at 60 °C for 24 h in the vacuum to
obtain the 3D graphene aerosol (GA) and the SnO2-graphene aerosols
(SGAs). At last, these SGA samples are labeled as SGA1, SGA2, SGA3,
SGA4, and SGA5, respectively.
2.6. Characterization. The fragments were scratched from the 3D
cylinder for sample testing. The morphologies were observed using a
eld emission scanning electron microscopy (FESEM, Hitachi S-4800
II), a transmission electron microscopy (TEM, Philips Tecnai 12) and
HRTEM (FEI Tecnai G2 F30 S-TWIN) equipped with energy-
dispersive X-ray spectrum (EDS) and selected area electron diraction
(SAED), respectively. The specic surface area was measured using a
nitrogen gas sorption surface area tester (3H-2000PS2, BeiShiDe
Instrument S&T (Beijing) Co., Ltd.) and calculated by the Brunauer
EmmettTeller (BET) method. XRD patterns were obtained by an X-
ray diractometer (Shimadzu XRD-7000) equipped with a Cu Kα
radiation source, λ= 0.154 nm. FT-IR spectra were recorded on a FT-
IR Microscope (Varian Cary 670) by the samples being loaded in KBr
pellets, respectively. The diuse reectance absorption spectra (DRS)
of the samples were recorded by a UVvis spectrophotometer (Varian
Cary 5000) in the range from 200 to 800 nm equipped with an
integrated sphere attachment and with BaSO4as a reference. PL
spectra were drew at room temperature with a luminescence
spectrophotometer (Edinburgh EPL-375) using a Xenon laser with a
420-nm excitation light and recorded in the spectral range of 440800
nm in order to escape the impact of duplicate wavelength light from
the laser. In the PL measurement, the dosage of the samples is
constant 100 mg, respectively.
2.7. Measurements of Dye Adsorptivity and Photocatalytic
Activity. For comparison of dye adsorptivity of the catalysts, 100 mg
of pure SnO2QDs, Degussa P25 TiO2, GA, and SGA were dispersed
in 200 mL of 105mol/L (about 4.79 ×105g/L) of RhB aqueous
solution, respectively. Then the resulted solutions were magnetically
stirred in the dark for 60 min at room temperature to establish
adsorptiondesorption equilibrium between the catalyst and RhB dye.
During the adsorption process in the dark, about 5 mL of suspension
was taken out from the reactor at an interval of 15 min and centrifuged
to separate the catalyst. Then the concentration of RhB in the
supernatant liquid was monitored at its maximum absorption
ACS Applied Materials & Interfaces Research Article |ACS Appl. Mater. Interfaces 2014, 6, 613621614
wavelength (553 nm), using a Vis spectrophotometer (JH 722S,
Shanghai Jinghua Technological Instrument Co., Ltd.). Moreover, the
optimal mass ratio of SnO2to graphene in the SGA and the maximum
adsorptivity of the SGA were explored as well.
The photocatalytic activities of the catalysts were evaluated in terms
of the degradation rate of RhB with the concentration of 1 ×105
mol/L (4.79 ×105g/L). After the adsorptiondesorption
equilibrium, the suspensions of catalyst and RhB dye were irradiated
by a 300 W xenon lamp with a 420 nm cutofffilter (λ> 420 nm, GHX-
2 Photochemical Reactions Instrument, Yangzhou University City
Science and Technology Co., Ltd.). The schematic of the photo-
catalytic reactor was shown in Figure S5 (see the Supporting
Information). During the illumination process, about 5 mL of
suspension was taken out at an interval of 10 min and centrifuged
to separate the photocatalysts. The dye degradation process was
monitored by concentration changes of the RhB at its typical
absorption wavelength (553 nm), using the JH 722S Vis
spectrophotometer. The photocatalysts separated were washed by
centrifugation with ethanol and deionized water for 3 times to remove
fully the residual organic species, and reused for the next run. We also
studied the inuence of the amount of the SnO2QDs in the SGA
samples, the doseeect of the SGA photocatalysts on degrading RhB,
and the recyclability of the SGA with the best photocatalytic eciency.
3.1. Formation of SnO2Graphene Composite. Images
a and e in Figure 1 show the SEM images of the SGA (SGA5,
the mass ratio of SnO2to graphene at 1:5) and GA,
respectively. The SGA exhibits well-established 3D layered
porous structure with submicrometer-size pores. And the
aggregation of graphene sheets is slighter in the SGA than the
GA, thereby this may be attributed to the decoration of SnO2
QDs on nearest graphene sheets.
Although GO suspension
can also self-assembled into 3D hydrogel, graphene sheets tend
to stack due to losing oxygen-containing groups in the
hydrothermal reduction process,
as shown in images e and f
Figure 1. (a) SEM, (b, c) TEM, and (d) HRTEM images of the SGA, the inset in b is the SAED pattern of the SGA. (e) SEM and (f) TEM images
of the GA. Scale bars: (a, e) 1 μm, (b, f) 50 nm, (c) 10 nm, (d) 2 nm. Here, the SGA refers to the SGA5 sample (the mass ratio of SnO2to graphene
in the SGA5 is 1:5).
ACS Applied Materials & Interfaces Research Article |ACS Appl. Mater. Interfaces 2014, 6, 613621615
in Figure 1. Compared with the SGA and the pristine GA, SnO2
QDs in the SGA could act as spacers between graphene sheets,
preventing the sheets from stacking to some extent.
Therefore, in spite of the same addition amount of the GO
for preparing the GA and the SGA, the obvious dierence in
the volume of these two kinds of 3D cylinders presents, as
shown in Figure S3 (see the Supporting Information). On the
other hand, the graphene sheet could be as a large-size solid
stabilizer or dispersant for SnO2QDs through interfacial
interaction to avoid interparticle aggregation.
This could be
supported by the uniform deposition of SnO2QDs on
graphene sheets in the SGA, as displayed in TEM images b
and c in Figure 1, where the average size of QDs is of about 4
nm similar with pure SnO2QDs in Figure S1 (see the
Supporting Information). Therefore, the present combination
between SnO2QDs and graphene sheets can not only stabilize
SnO2QDs but also create the layered porous structure to
increase the specic surface area of the SGA, as supported by
the BET areas of the SnO2QDs, GA, and SGA5 in Figure S6
(see the Supporting Information), which is conducive to
enhancing the contaminant adsorptivity and photocatalytic
eciency. In addition, the lattice-resolved HRTEM image of
the SGA in Figure 1d shows a interplanar spacing of 0.33 nm
for the adjacent (110) planes of SnO2,
and the SAED pattern
in the inset of Figure 1b shows the bright diraction rings of
SnO2, indicating the formation of the crystalline phase of SnO2.
Moreover, the SnO2QDs are surrounded by wavy strips of
graphene nanosheet (d-spacing = 0.38 nm),
indicating the combination of SnO2and graphene. The EDS
spectrum of the SGA (Figure S6f, see the Supporting
Information) exhibits the presence of C, O, and Sn elements,
further supporting the fomation of the SGA composite.
Reduction degrees of GO in the GA and the SGA were
revealed by the XRD patterns and FT-IR spectra. Figure 2a
plots the XRD patterns of the as-synthesized GO, GA and SGA
samples. For GO, the sharp diraction peak at around 2θ= 9.9°
corresponds to the (002) phase of the stacked GO sheets.
the hydrothermally reduced GA, a broad characteristic peak of
graphene appears at about 2θ= 24.5°instead of the original
diraction peak, indicating that the GO have been eectively
reduced to graphene in the GA.
For the SGA, the two
obvious diraction peaks located at 26.3 and 33.9°can be
indexed respectively to the (100) and (101) phases of
tetragonal rutile-type SnO2, whereas the peak of graphene is
almost invisible. It suggests a signicant decrease in the layer-
stacking regularity of graphene sheets after implanting SnO2
QDs, and indicates that the SnO2QDs can act as spacers to
separate graphene sheets to some extent.
Obviously, this
combination between graphene and SnO2QDs facilitates the
increase in the specic surface area, as conrmed by the
increased BET area of the SGA5 in Figure S6 (see the
Supporting Information).
Because the oxygen-containing functional groups are active
in the IR region, FT-IR spectra were used to qualitatively
evaluate the deoxygenating degree. As depicted in Figure 2b,
apart from the aromatic CC skeletal vibration of the sp2
domains (1616 cm1), the FT-IR spectrum of GO shows the
presence of oxygenated functional groups near 847 cm1(O
CO), 1051 cm1(alkoxy COC), 1220 cm1(carboxyl
COH), 1726 cm1(carbonyl CO in carbonyl, and carboxyl
moieties), 3161 cm1(OH in water) and 3382 cm1
(structural OH groups on the graphene sheets).
Compared with GO, the vibration of OCO and structural
OH disappear and the intensity of the COC and CO
peaks signicantly decrease in the FT-IR spectra of the GA and
the SGA. These changes suggest that most oxygen-containing
functional groups in GO have been eectively reduced.
Especially, the unsuppressed peaks of COH vibration (1220
cm1) and the residual CO vibration may root from
carboxylic acid groups (COOH)
that usually can not be
completely removed by chemical reduction.
these groups are believed to be able to facilitate nanoparticles
dispersion by interacting with surface hydroxyl groups (OH)
of SnO2,
as shown in Figure 1c, d. Furthermore, compared
with the FT-IR spectra of the SGA and SnO2QDs in Figure S7
(see the Supporting Information), the SnOSn symmetric
stretching (667 cm1)
inside the SnO2remains unchanged
after combining with graphene, but the SnO asymmetric
stretching (539 cm1) on SnO2surface decreases to 501 cm1,
and an additional SnO vibration peak
at 2359 cm1that
does not appear in those of GO, GA and SnO2QDs appears in
the FT-IR of the SGA. This indicates a new interfacial
interaction between SnO2and graphene sheet. Hence, the FT-
IR results can further conrm the eective reduction of GO and
combination of graphene and SnO2.
3.2. Absorption to Visible Light. To evaluate the capacity
of absorpting visible light, we recorded the absorption spectra
of the as-synthesized SnO2QDs, GA, and SGA samples and
shown in Figure 3. The SnO2QDs almost do not absorb any
visible light due to the wide band gap of Eg3.95 eV (see
Figure S8 in the Supporting Information), whereas all the SGA
samples exhibit continuous absorption in the range of 400700
nm. This may be attributed to the presence of graphene, since
the black GA sample can well harvest the visible light and
provide the possibility of enhancing the visible light absorption
for the SGA.
The observed increase in visible light absorption
Figure 2. (a) XRD patterns and (b) FT-IR spectra of the GO, GA, and SGA (SGA5, the mass ratio of SnO2to graphene in the SGA5 is 1:5).
ACS Applied Materials & Interfaces Research Article |ACS Appl. Mater. Interfaces 2014, 6, 613621616
with increasing graphene content can be ascribed to the
increase in surface electric charge of SnO2in the composite.
Therefore, the combination of the graphene and SnO2QDs
realize eectively the optical response shifting of SnO2-based
catalysts from the UV to the visible spectral range, which
reveals the potential of the SGA composites for the visible-light-
driven photocatalysis. This inference would be well conrmed
by the RhB dye degradation over the SGA under the visible
light irradiation as below.
3.3. Dye Adsorptivity and Catalytic Activity. 3.3.1. Dye
Adsorptivity of The SGA. Pollutants adsorptivity is allimportant
for catalytic eciency of the photocatalyst. Hence, the
adsorption processes of dierent samples to RhB dye were
recorded and shown in Figure S4 (see the Supporting
Information). All suspensions of catalyst and dye achieve the
adsorptiondesorption equilibrationaftera60-mindark
adsorption process. For RhB, almost no self-degradation occurs
in the dark. In the cases of SnO2QDs, P25 TiO2, GA, and SGA
(SGA3, the mass ratio of SnO2to graphene at 1:1),
approximate 42.8, 49.2, 79.4, and 98.4% of the RhB can be
absorbed after the dark adsorption process, respectively.
Compared with SnO2QDs, P25 TiO2, and GA, the stronger
dye adsorption capacity of the SGA may be due to the special
π-conjugation structure of graphene and larger specic surface
area of the 3D porous layered structure,
as surpported by
the BET areas of SnO2QDs, P25 TiO2(around 50 m2/g),
GA, and SGA3 in Figure S6 (see the Supporting Information).
The excellent adsorptivity of the SGA can be also intuitively
reected by the absorption spectra and corresponding photos
of the RhB aqueous solutions adsorbed by dierent samples, as
displayed in Figure 4a.
The inuence of mass ratio of SnO2to graphene in the SGA
on the adsorptivity is explored further. With increasing relative
content of SnO2, the adsorption capacity of the SGA increases
rst then declines, and reaches the maximum at an optimal ratio
of 1:1 (SGA3), as shown in Figure S9a (see the Supporting
Information). This may be resulting from that at lower ratio
range (SGA5-SGA3), the decoration of more SnO2QDs on
nearest graphene sheets leads to larger specic surface area of
the SGA, however, at higher ratio range (SGA3-SGA1), the
excess SnO2QDs occupy spaces that belong to RhB molecules
and the low amount of graphene into the matrix can no longer
prevent eciently the aggregation of SnO2QDs, thus resulting
in the reductions of specic surface area and dye adsorption
eciency. This hypothesis can be demonstrated by the results
of SEM and TEM images (Figure 1 and Figure S10, see the
Supporting Information) and BET areas (Figure S6, Supporting
Information). Furthermore, to ascertain the maximum
adsorptivity (Qmax) of the SGA, the adsorption isotherm of
RhB dye as a function of its concentration over the SGA was
depicted in Figure 4b. The Qmax of the SGA is approximately
126 mg/g, outperforming many currently available adsorb-
This proves that the SGA is promising for fabricating
high-performance adsorbent for pratical pollution cleaning.
Importantly, the observed superior adsorptivity would be
benecial to improving the photocatalytic performance of the
SGA composite.
3.3.2. Photocatalytic Activity. The photocatalytic activity of
the as-synthesized SGA was further investigated by monitoring
the photodegradation of RhB dye after the dark adsorption
process. Figure 5a shows the time-dependent degradation
curves of RhB dye in the presence of blank sample (without
catalyst) and SnO2QDs, P25 TiO2, GA, and SGA (SGA3, the
mass ratio of SnO2to graphene at 1:1) catalysts under visible
light irradiation (λ> 420 nm). Herein, C0and Care
respectively the initial concentration after the adsorption
desorption equilibrium and the actual concentration of RhB at
dierent irradiation time, thus the lower C/C0denotes the
higher photodegradation degree of the RhB dye. As shown in
Figure 5a, RhB hardly exhibits self-degradation under visible
light (only about 1%), whereas in the presence of the SGA,
RhB can be almost completely degraded within 40 min and the
photocatalytic eciency is better than SnO2QDs, P25 TiO2,
and GA. This superior photocatalytic activity of the SGA can be
Figure 3. UVvis absorption spectra of the as-synthesized SnO2QDs,
GA, and SGA samples.
Figure 4. (a) Absorption spectra and the corresponding photograph (inset) of the adsorptive RhB aqueous solutions without adsorbent as well as in
the presence of the SnO2QDs, P25 TiO2, GA and SGA (SGA3) adsorbent. (b) Dye adsorption isotherms of RhB on the SGA3 (the mass ratio of
SnO2to graphene is 1:1, concentration of the SGA is 0.5 mg/mL (100 mg/200 mL), initial concentration of RhB is from 4.79 ×103g/L to 2 g/L,
respectively, operating temperature is 298 K).
ACS Applied Materials & Interfaces Research Article |ACS Appl. Mater. Interfaces 2014, 6, 613621617
directly manifested by the absorption spectra and the
corresponding photos of RhB aqueous solutions catalyzed by
dierent catalysts for 40 min, as shown in Figure S11 (see the
Supporting Information). To quantitatively evaluate the
photocatalytic eciencies of these catalysts, the corresponding
apparent reaction rate constants (k) of the RhB degradation
were calculated by using an equation ln(C/C0)=kt based on
the pseudo-rst-order kinetic model.
The results are
summarized in Figure S12 (see the Supporting Information).
For the blank experiment (without catalysts), the RhB degrades
at relatively slow reaction rate with k= 2.05 ×104min1. For
other catalysts (SnO2QDs, P25 TiO2, GA, and SGA), the
corresponding degradation rates with k= 4.30 ×104min1,
5.12 ×103min1, 2.28 ×102min1, and 9.13 ×102min1
were respectively obtained, and the SGA exhibits signicantly
the prominent catalytic activity. The mechanism of visible-light-
driven photocatalysis of the SGA would be expounded in the
following sections.
The optimal mass ratio of SnO2to graphene for the SGA
catalyst was also ascertained in Figure S9a (see the Supporting
Information). The excessively high relative content, whether
SnO2or graphene, lowers the photocatalytic activity of the SGA
nanocomposite for degrading RhB and the photocatalytic
activity reaches the maximum at the mass ratio of 1:1 (SGA3).
Similar phenomena have also been observed in the gas-phase
degradation of volatile organic contaminants and liquid-phase
degradation of dyes over P25 TiO2-graphene nanocompo-
This variation trend in photocatalytic eciency of the
SGA is in line with that in its adsorption eciency, conrming
the dependence of photocatalytic activity on the adsorptivity. In
addition, the doseeect of the SGA catalyst was studied as
well. The optimal dose of the SGA3 photocatalyst may be 0.5
g/L (100 mg of catalyst per 200 mL of RhB aqueous solution),
as shown in Figure S9b (see the Supporting Information). For
higher doses of the SGA, excess suspended catalysts may hinder
the penetration and absorption of the incident light, thereby
reducing the photocatalytic eciency.
3.3.3. Recyclability of SGA. To evaluate the reusability of the
SGA as adsorbent and photocatalyst, we performed another
four cycles of sequential dark-adsorption and photocatalytic
processes toward RhB dye using the recycled SGA, as shown in
Figure 5b. No obvious changes in adsorption eciency and
photocatalytic eciency indicate the SGA can keep stable
recyclability in acting as the pollutant adsorbent and photo-
catalyst. The refreshable adsorptivity should be attributed to the
subsequent photodegradation to the adsorbed dye and the
stability of porous structure. This structural stability is further
conrmed by XRD, SEM and the photon eciency (ξ) for the
recycled SGA after ve cycles. As shown in Figure S13 (see the
Supporting Information), no noticeable changes (such as
graphene being afresh oxidized into GO) occur both in the
XRD pattern and in the porous layered morphology before and
after the dark-adsorption and photocatalytic process. And the
photon eciency ξof the SGA is recorded in Note S1 (see the
Supporting Information), showing an insignicant recession for
the ve cycles. These results demonstrate unambiguously that
the SGA is stable, ecient, and recyclable pollutant adsorbent
and photocatalyst.
3.3.4. Mechanism of Visible-Light-Driven Photocatalytic
Activity of SGA Catalyst. The most reports on graphene
semiconductor composite catalysts seem to reach a consensus
that, because of special π-conjugation structure and high
conductivity, graphene acts as electron reservoir accepting the
photogenerated electrons from semiconductor.
ever, when the work functions of semiconductor and graphene
are taken into account for analyzing the electron transport
pathway, the roles of graphene should indeed divide into two
cases, i.e., graphene electron reservoir (acceptor)
dye-sensitizer-like graphene photosensitizer (electron
When the work function of graphene is more
negative than that of the semiconductor, graphene usually
behaves as an electron reservoir accepting photogenerated
electrons from the conduction band (CB) of the semi-
conductor, for example the TiO2-graphene photocatalyst, in
which the work function of TiO2and graphene is 4.4 and
4.42 eV, respectively.
Kamat and co-workers rst
demonstrated the feasibility of using graphene as an electron
reservoir (electron-transfermedium)forphotogenerated
electrons from TiO2in the TiO2-graphene composite photo-
These pioneering works have stimulated wide
research of semiconductor-graphene nanocomposites, and their
proposed concept of graphene electron reservoir (acceptor or
mediator) is now extensively utilized in explaining the
photocatalytic mechanism in various semiconductor-graphene
composite photocatalysts like TiO2-graphene
and ZnO-
etc. In contrast, the concept of graphene
photosensitizer (electron donor) was recently rst proposed
by Du et al.,
and was veried experimentally by Xu et al.
Figure 5. (a) Degradation curves of the RhB aqueous solutions containing dierent photocatalysts after 40 min of the visible light irradiation: Blank
sample (without catalyst), SnO2QDs, P25 TiO2, GA, and SGA. (b) Dye adsorption eciency and photocatalytic eciency of the recycled SGA to
RhB of separate 5 cycles of the continuous dark-adsorption and photocatalytic processes.
ACS Applied Materials & Interfaces Research Article |ACS Appl. Mater. Interfaces 2014, 6, 613621618
based on the visible-light-activated photocatalysis of the
graphene-photosensitized ZnS. Lamentedly, the work functions
of the graphene and ZnS were not taken into account in their
discussion, which are extremely signicant for understanding
the photocatalytic mechanism and designing similar visible-
light-responsive semiconductorgraphene photocatalysts at the
level of carrier transfer pathway. In fact, the work function of
graphene (4.42 eV)
is more positive than that of the ZnS
(7.0 eV).
Therefore, it is reasonable to believe that when
the work function of graphene is more positive than that of the
semiconductor, graphene in principle acts as a photosensitizer
providing photogenerated electrons to the CB of the
semiconductor. Interestingly, this photocatalytic process led
by graphene-sensitized semiconductor under visible light is just
similar to the dye-photosensitization process, which is generally
composed of following parts:
(a) dye/graphene is excited by
the visible light, (b) injects photogenerated electrons onto the
CB of the semiconductor, (c) the injected electrons are trapped
by surface sites of the semiconductor, (d) electron acceptors
such as oxygen (O2) are reduced by trapped electrons, and (e)
subsequent radical reactions. In the present case of the SGA
photocatalysts degrading RhB dye, the possible photocatalytic
process is illuminated in Figure 6 and eqs 111. Because the
work function of graphene (4.42 eV) is more positive than
that of SnO2(4.5 eV),
graphene can act as a visible-light-
driven photosensitizer for SnO2, being photoexcited from
ground-state (graphene) to excited-state (graphene*) and
providing photogenerated electrons (e) to the CB of SnO2,
as marked in red in Figure 6.
Such an electron-transfer
route is further suggested by the photoluminescence (PL)
comparison between the dried SnO2and the SGA samples in
Figure S14 (see the Supporting Information). For the SGA, a
signicantly enhanced broad PL band ranging from 450 to 700
nm is observed, and it should be ascribed to the electron
contribution of graphene to SnO2, indicating that graphene
may be a photosensitizer for SnO2. In addition, because of the
more positive work function of excited RhB*than that of
graphene (the corresponding work functions of RhB, excited
RhB*, and graphene are 5.45, 3.08, and 4.42 eV,
respectively), the excited RhB*can also inject favorably
photogenerated electrons into the graphene plane via a dye-
sensitized process, thus a downstream channel of electrons is
formed as illustrated in black in Figure 6. More specically, the
excited RhB*creates a hole-like RhB+(RhB with a hole+) (eqs
The recombination between the injected electrons and
holes in the RhB+(dotted line in Figure 6) would retard the
degradation of RhB.
Fortunately, after loading SnO2on
graphene sheets, because of the suitable work function and high
conductivity of graphene,
graphene could act as an electron
mediator facilitating electron transferring from the excited
RhB*and excited graphene*migrating toward the CB of SnO2
(Figure 6), and enhancing the separation of electrons and holes
in the RhB. The reduction and oxidation reactions would then
happen on the CB of SnO2and the ground-state RhB,
respectively (eqs 611).
Notably, in the CB of SnO2,
because of the larger specic surface area of the SGA than that
of the SnO2, more dissolved oxygen (O2) in water acting as the
electron scavenger could be reduced by photogenerated
electrons (e) to produce superoxide radical anions (O2
and hydrogen peroxide (H2O2) (eqs 68). The recombination
of photogenerated electrons and holes thereby could be
avoided to the maximum extent, which facilitates the
photocatalytic activity of the RhB-SGA composites. Therefore,
we believe that RhB dye and graphene act as the
cophotosensitizer to stimulate the dye photodegradation
under visible light as follows.
hvRhB(C H ClN O ) RhB
28 31 2 3 (1)
*→ +
RhB e RhB (2)
RhB RhB hole (3)
raphene h graphene (4)
*→ +
raphene e graphen
•− +
22 (7)
22 (9)
ole H O/OH OH H
+• → + + + +
22 3 4
Detailed free radical reaction process: the newly formed
intermediates (O2
and H2O2) from the CB of SnO2could
further create the reactive hydroxyl radicals (OH). By this
process, electron acceptors (O2) could restrain the electron
hole recombination in the RhB and SnO2according to eqs 69.
For the ground-state RhB, the photogenerated holes (hole+)
would diuse to the surface and react with adsorbed water and
OHto produce OH species (eq 10). The RhB could then be
degraded by the formed strongly oxidizing OH species into
small molecules, such as CO2,H
2O, etc., following eq 11.
Moreover, RhB*adsorbed directly onto SnO2surface could
inject directly electrons into the CB of SnO2, as marked in
green in Figure 6, which also in turn results in degradation of
In summary, we have synthesized the SnO2graphene aerosols
(SGA) via a simple self-assembled hydrothermal reduction
method with SnO2QDs and GO sheets. Such SGA exhibits
more superior dye adsorption capacity and visible-light-driven
photocatalytic activity relative to pure SnO2QDs, P25 TiO2,
and pure GA. The optimal mass ratio of SnO2to graphene in
the SGA may be 1:1 for the dye adsorption and photo-
Figure 6. Schematic illustration of photosensitized degradation of the
RhB dyes over the SGA photocatalyst under the visible irradiation (λ>
420 nm).
ACS Applied Materials & Interfaces Research Article |ACS Appl. Mater. Interfaces 2014, 6, 613621619
degradation. Maximum adsorption eciency, dye degradation
rate constant and the optimized dosage for degrading RhB dye
are about 126 mg/g, 9.13 ×102min1, and 0.5 g/L,
respectively. This superior dye adsorptivity is attributed to the
special π-conjugation and larger specic surface area of the 3D
porous layered structure. The excellent photocatalytic activity
should be due to the combination of strong adsorptivity and
eective separation of photogenerated carriers in the SGA.
Signicantly, the synergistic photosensitization of the dye and
graphene as a novel concept was proposed to explain well the
photocatalytic mechanism in terms of carrier transfer, which is
signicant to exploit graphene-based solar energy conversion
SSupporting Information
XRD pattern and TEM image of the SnO2QDs, TEM image of
GO, scheme illustration of synthesis of 3D GA and SGA,
adsorptiondesorption equilibrium curves of the catalysts to
RhB dye, Schematic illustration of the photocatalytic reactor,
Photonic eciencies of the catalysts, Nitrogen adsorption
desorption curves of samples, EDS of the SGA5, FT-IR of as-
prepared SnO2QDs, Variation of (αhν)2versus the photon
energy (hν) of SnO2QDs, adsorption eciency and photo-
catalytic eciency of the SGA with dierent mass ratio of SnO2
to graphene, photocatalytic eciency of the SGA3 after 40 min
visible-light irradiation with dierent dosages, SEM and TEM
images of the SGA3 and the SGA1, absorption spectra and
corresponding photograph (inset) of the RhB aqueous
solutions photocatalyzed by dierent catalysts for 40 min,
apparent reaction rate constants (k) of the RhB photodegraded
by dierent catalysts, XRD pattern and SEM image of the
recycled SGA3, PL spectra of SnO2in the dried SnO2QDs and
the dried SGA3 excited at 420 nm, as well as molecular
structural formula and possible degradation mechanism of the
RhB. This material is available free of charge via the Internet at
Corresponding Authors
*Tel.: +86 0514 87970587. Fax: +86 0514 87975467. E-mail:
The authors declare no competing nancial interest.
This work was supported by the National Natural Science
Foundation of China (Grants 11104240, 21101135, and
11374253), the Jiangsu Government Scholarship for Oversea
Studies in 2012, the Innovation Project (Grants
CXZZ12_0892, 201211117034, and 201211117040) and the
Natural Science Foundation of China (Grant 11KJB150020) of
Jiangsu Province. And we thank Mr. Jun Zhu, Haitao Chen,
Chuan Hu, Hao Wang, and Long Yao, as well as the Testing
Center of Yangzhou University for technical support.
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ACS Applied Materials & Interfaces Research Article |ACS Appl. Mater. Interfaces 2014, 6, 613621621
... Development of visible light photocatalyst has been most driven since visible light accounts for nearly 45%, whereas only 5% accounts for UV irradiation. Hence, when taken towards industrial application, it is highly desirable to develop graphene-based metal oxide nanocomposite, which degrades organic contaminants even under visible light [13]. ...
... The SnO2/rGO composites exhibit significant degradation. The catalytic degradation efficiency was found to be better than previous reports [12][28] [13]. It can be seen from the Fig. 8b. ...
Hydrothermally-synthesized, size and shape-controlled SnO2 nanospheres were electrostatically self-assembled over the surface of reduced graphene oxide (rGO). The control over decoration of SnO2 nanospheres on rGO was studied by microscopic analysis and the crystallographic structure of SnO2 hybrid nanocomposites (SnO2: rGO) was determined from X-Ray diffraction analysis (XRD). Hybrid nanocomposite in 1:3 ratio exhibited (i) excellent methylene blue degradation capability within 3 min under visible light condition and (ii) high specific capacitance of 337.52 F/g at 0.5 A/g in 1 M H2SO4 electrolyte with 89% of retention after 5000 cycles.
... The peaks at 26.56°, 33.82°, 51.50°, and 64.6°are assigned to the (100), (101), (211), and (112) planes of the tetragonal rutile tin dioxide structure, respectively, based on the JCPDS 41-1445 library card (Fig. 1b). The lack of GO crystallographic peaks in the SnO 2 @rGO XRD pattern (Fig. 1c) may result from the low amount of graphene oxide in the hybrid, the disruption of the order of GO planes by SnO 2 nanoparticles (NPs), or the insertion of the GO sheets within the SnO 2 lattice [48][49][50]. The broad widths of the peaks are attributed to the small size of the crystallites; the intersheet spacing calculated by the Scherrer equation is 9.500 ± 0.009 nm. ...
... These nanospheres, with an average diameter of 8 nm, are stuck to the rGO sheets (Fig. 4c). Within the nanohybrid structure, the layout of tin dioxide nanospheres on the rGO sheets prevents restacking of the rGO sheets, and agglomeration of the nanospheres is prevented by rGO sheets [48,49]. Fig. 3c also shows that the surface area of the nanohybrid is increased, in agreement with the BET results. ...
Conference Paper
SnO2 and SnO2/reduced graphene oxide (rGO) nano hybrid were synthesized and used for fabrication of chemiresistive gas sensors for methane (CH4) detection at relatively low temperatures. SnO2 nano particles were dispersed in rGO sheets through one pot hydrothermal method. The nano structured materials were characterized by X-ray diffractometer (XRD) and Field Emission Scanning Electron Microscope (FESEM). The sensors responses to 1000ppm CH4 were measured at temperature range of 50-250oC. Our results indicate that rGO significantly increases the response of SnO2-based sensor to methane gas from 10% to 46.7%. Presence of rGO sheets in the hybrid, hindered the agglomeration of SnO2 nano particles and on the other hand, SnO2 nano partilces inhibited restacking of rGO sheets, so nano hybrid with high specific surface area for adsorption of oxygen molecules and sensing reaction with methane gas was prepared. In addition, p-n junctions between SnO2 nano particles and reduced graphene oxide sheets, causes a new depletion layer and enhanced the sensing response. Moreover, sensor optimum operating temperature for methane detection, reduced to 150oC for SnO2-rGO, since addition of reduced graphene oxide, lowered the electrical resistance of the sensing material and makes it work at lower temperatures.
... Bentonit juga memiliki sifat adsorpsi yang sangat baik sehingga penggunaan bentonit sebagai padatan pengemban akan memudahkan dan mempercepat proses transfer massa adsorbat sehingga kontak antara..oksida logam fotokatalis dengan senyawa organik lebih mudah terjadi dan reaksi akan lebih cepat berlangsung. Beberapa penelitian yang telah dilakukan menunjukkan peningkatan kemampuan komposit bentonit dengan beberapa jenis semikonduktor seperti ZnO (Sitepu et al., 2016;Diantariani et al., 2016), TiO2 (Leksono, 2012), Fe2O3 (Riskiani et al., 2019), SnO2 (Zhuang et al., 2014), ZnS (Aruna et al., 2002), CuS (Nurdani, 2009), CeO2 (Futikhaningtyas et al., 2013, ZrO2 (Zhang et al., 2001) ...
Bentonit adalah salah satu jenis bahan yang umum.digunakan sebagai penyerap limbah zat warna. Namun, terdapat kesulitan dalam pemisahan dan pengumpulan kembali..adsorben setelah proses penyerapan. Oleh karena itu, dalam penelitian ini bentonit dijadikan komposit dengan oksida besi yang bersifat magnetik untuk mengatasi masalah tersebut. Penelitian ini bertujuan untuk mendegradasi zat warna Napthol blue black menggunakan bentonit-Fe3O4 bukan sekedar adsorben melainkan sebagai fotokatalis.. Penelitian ini.meliputi penentuan.massa bentonit-Fe3O4 optimum, pH optimum, waktu irradiasi optimum, serta..efektivitas.fotodegradasi..pada kondisi optimum...Hasil penelitian menunjukkan bahwa..kondisi..optimum..yang diperoleh yaitu massa bentonit-Fe3O4 sebesar 50 mg, pH. 5 dan waktu irradiasi dengan sinar UV adalah 30 menit. Persentase fotodegradasi pada kondisi optimum sebesar 98,95±0,01% yang menunjukkan fotodegradasi menggunakan fotokatalis bentonit-Fe3O4 sehingga fotodegradasi dengan fotokatalis yang disintesis ini sangat efektif... Kata kunci: fotodegradasi, katalis bentonit-Fe3O4, Napthol blue black
... These nanospheres, with an average diameter of 8 nm, are stuck to the rGO sheets (Fig. 4c). Within the nanohybrid structure, the layout of tin dioxide nanospheres on the rGO sheets prevents restacking of the rGO sheets, and agglomeration of the nanospheres is prevented by rGO sheets [48,49]. Fig. 3c also shows that the surface area of the nanohybrid is increased, in agreement with the BET results. ...
Stannic oxide nanoparticles and various compositions of SnO2@rGO (reduced graphene oxide) nanohybrids were synthesized by a facile hydrothermal method and utilized as chemiresistive methane gas sensors. To characterize the synthesized nanohybrids, BET (Brunauer-Emmett-Teller), XRD, FESEM, TEM, FTIR, and Raman techniques were used. Sensing elements were tested using a U-tube flow chamber with temperature control. To obtain the best sensor performance, i.e., the highest signal and the fastest response and recovery times, the sensing element composition, operating temperature, and gas flow rate were optimized. The highest response (change in resistance) of 47.6% for 1000 ± 5 ppm methane was obtained with the SnO2@rGO1% nanohybrid at 150 °C and a flow rate of 160 sccm; the response and recovery times were 61 s and 5 min, respectively. A sensing mechanism was suggested, based on the experiments.
... SnO 2 nanostructures have received enormous interest as a major family of functional materials in recent years due to their unique electrical and catalytic properties [1,2]. SnO 2 nanostructures have been widely used for various electrochemical and catalytic applications, such as gas sensors [3,4], photocatalysts [5][6][7], and anode materials for lithium-ion batteries [8][9][10]. In real applications, a small size and a large specific area are essential for the high performance of SnO 2 . ...
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Uniform SnO2 fibers were prepared from the electrospinning method in this paper. The mechanical properties of a single SnO2 fiber were characterized by three-point bending experiments with atomic force microscopy. Finite element method was employed to simulate the shape of the SnO2 fiber during the bending process. The elastic modulus of SnO2 fibers increased with the calcined temperature. A high elastic modulus of 72.59 GPa was obtained with a diameter of 160 nm. The results indicate that atomic force microscopy tips penetrated the surfaces under maximum loading. Graphical abstract The force-displacement curve from simulation is in accordance with the results from experiments by using AFM and theoretical calculation. The equivalent strain nephogram indicates that the maximal strain is about 0.0143 at the midpoint of the fiber. Open image in new window
MoS2 nanomaterial with the micro-pompon structure was synthesized by a surfactant-assisted hydrothermal method. The morphologies and structures of as-prepared MoS2 micro-pompon were investigated by adding different types of surfactants such as cetyltrimethyl ammonium bromide (CTAB), sodium dodecylbenzene sulphonate (SDBS), and polyvinyl pyrrolidone (PVP). The results indicated that the morphology of MoS2 could be controlled and changed effectively by the cationic surfactant of CTAB. A reasonable growth mechanism for hollow structured MoS2 micro-pompon by hydrothermal processes was proposed. Further, photocatalytic degradation properties of MoS2 micro-pompon under visible light were evaluated by degradation of common organic dyes, which include rhodamine B (RhB), congo red, methyl orange, and methylene blue. The results indicated that MoS2 micro-pompon owned the highly selective catalytic ability to RhB with degradation efficiency of 95% in 60 min and 68% in 30 min. With the additive of the surfactant, the MoS2-CTAB sample exhibited an enhanced ability of photocatalytic activity where degradation efficiency was improved to 92% in 30 min. The method employed in this work could be expanded to fabricate other sulfides with the controllable morphology and structure to further regulate the photocatalytic performance.
Uniform SnO 2 fibers were prepared from electrospinning method in this paper. The mechanical properties of a single SnO 2 fiber were characterized by three-point bending experiments with atomic force microscopy (AFM). Finite element method (FEM) was employed to simulate the shape of SnO 2 fiber during the bending process. Results indicate that portions of the AFM tips penetrated the surfaces under the maximum loading were a truth that had to be taken into account.
Graphene, the mother of all carbon materials has unlocked a new era of biomedical nanomaterials due to its exceptional biocompatibility, physicochemical and mechanical properties. It is a single atom thick, nanosized, 2-dimensional structure and provides high surface area with adjustable surface chemistry to form hybrids. The present article provides a comprehensive review of ever-expanding application of graphene nanomaterials with different inorganic and organic materials in drug delivery and theranostics. Methods of preparation of nanomaterials are elaborated and biological and physicochemical characteristics of biomedical relevance are also discussed. Graphene form nanomaterials with metallic nanoparticles with multiscale application. First, graphene act as a platform to attach nanoparticles and provide excellent mechanical strength. Second, graphene improves efficacy of metallic nanoparticles in diagnostic, biosensing, therapeutic and drug delivery application. Graphene based polymeric nanocomposites find wider application in drug delivery with flexibility to incorporate hydrophilic, hydrophobic, sensitive and macromolecules. In addition, grapheme quantum dots and graphene hybrids with inorganic nanocrystal and carbon nanotubes hybrids have shown interesting properties for diagnosis and therapy. Finally, we have pointed out research trends that may be more common in future for graphene based nanomaterials. Keywords: Graphene, Graphene oxide, Nanomaterials, Biosensors, Theranostics
A composite of Ni-ZnO/rGO/nylon-6 electrospun nanofiber decorated by Ag nanoparticles was successfully introduced as a novel hybrid material with good visible light-driven photocatalytic and antibacterial properties having potential for repeated use. The Fourier transform infrared (FT-IR) spectroscopy, X-ray powder diffraction (XRD), scanning electron microscopy (SEM), UV–vis diffuse reflectance spectroscopy (DRS) and thermogravimetric analysis (TGA) measurements were employed to characterize the phase structure, morphology, optical properties, and thermal stability of the as-synthesized composite. The results confirmed the successful incorporation of Ni-ZnO/rGO nanocomposite into the electrospun nylon-6 fibers as well as deposition of Ag nanoparticles on the surface of fibers. The effective photodegradation of methyl orange (MO) dye under visible light was demonstrated under the visible-light irradiation. Owing to the rational design and special structure of Ni-ZnO/rGO/nylon-6/Ag nanofibers, this new hybrid provides an efficient electron-hole separation to further react with adsorbed dye molecules and degrade them into non-toxic products. Furthermore, the antibacterial properties of the nanofibers were investigated against Escherichia coli (Gram-negative) and Bacillus Subtilis (Gram-positive) bacteria and excellent antibacterial activities against both studied bacteria were found. This work offers a new class of inorganic/organic nanocomposites with promising efficiency as good economically and environmentally friendly candidates for visible light-driven photocatalysis and water filter media.
The integration of plasmonic metal with wide-bandgap semiconductor is a promising approach to utilize the visible light without compromise of the redox ability of photogenerated charge carriers. However, a larger work function of metal than that of semiconductor is indispensable to enable the injection of hot electrons from plasmonic metal to semiconductor. In this paper, we demonstrated that reduced graphene oxide (rGO) nanosheets as conductive “bridge” can breakthrough the restriction and transfer hot electrons from Ag of smaller work function to TiO2 of larger work function. In the design, both of the Ag nanocubes and TiO2 nanosheets are co-deposited on the surface of rGO nanosheets to form Ag-rGO-TiO2 structure, which was characterized by XRD, TEM, Raman and XPS spectra. On one hand, the Ag-rGO interface facilitates the transfer of hot electrons from Ag to rGO through conductor–conductor contact. On the other hand, the new formed Schottky junction on the rGO-TiO2 interface further pumps the transferred electrons to the surface of TiO2 for photocatalytic reduction reaction resulted from the larger work function of rGO than that of TiO2. Enabled by this unique design, the hydrogen production activity achieved under visible light irradiation is dramatically enhanced in comparison with that of Ag-TiO2 counterpart with the direct contact between the same Ag nanocubes and TiO2 nanosheets. This work represents a step toward the rational interfacial design of plasmonic metal-semiconductor hybrid structures for broad-spectrum photocatalysis.
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The ultrafine SnO2 quantum dots (QDs) modified with poly(ethylene glycol methyl ether) (PEGME) (PEGME-SnO2 QDs) were synthesized via hydrothermal method. X-ray diffraction and high-resolution transmission electron microscopy were employed to illustrate that the PEGME-SnO2 QDs are uniform, monodispersed and about 4 nm in diameter. Then infrared spectrum and thermogravimetric analysis were used to prove that PEGME groups are bound tightly to SnO2 surfaces. The as-synthesized PEGME-SnO2 QDs excellently achieved photocatalytic degradation to Rhodamine B dye (RhB). The photon efficiency of the PEGME-SnO2 QDs catalyst and corresponding RhB dye degradation rate constant could reach 0.0058% and 9.98 × 10−2 min−1, respectively. This outstanding photocatalytic performance could be attributed to not only large surface-to-volume ratio and high crystallinity of the ultrafine and monodispersed QDs, but also good hydrophilicity and conductivity of the PEGME surface modifier. Remarkably, such PEGME-SnO2 QDs with outstanding photocatalytic efficiency and stable recyclability are promising to be applied to environmental purification.
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The visible-light-driven photocatalytic activities of graphene-semiconductor catalysts have recently been demonstrated, however, the transfer pathway of photogenerated carriers especially where the role of graphene still remains controversial. Here we report graphene-SnO2 aerosol nanocomposites that exhibit more superior dye adsorption capacity and photocatalytic efficiency compared with pure SnO2 quantum dots, P25 TiO2 and pure graphene aerosol under the visible light. This study examines the origin of the visible-light-driven photocatalysis, which for the first time links to the synergistic effect of the co-photosensitization of the dye and graphene to SnO2. We hope this concept and corresponding mechanism of co-photosensitization could provide an original understanding for the photocatalytic reaction process at the level of carrier transfer pathway as well as a brand new approach to design novel and versatile graphene-based composites for solar energy conversion.
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A facile, one-step, template-less, surfactant-free hydrothermal process, using a metal salt as the precursor, is developed to prepare submicrometer sized mesoporous TiO 2 nanoparticle aggregates (NPGs). The as-prepared TiO 2 NPGs are crystalline of the anatase phase, with a high specific surface area of 166 m 2 /g, an average pore size of 8.9 nm, and an average NPG size of 840 nm. With these NPGs, a new form of composite photoanode, consisting of the mesoporous TiO 2 NPGs and xerogels, is proposed for high efficiency dye-sensitized solar cells (DSSCs). TiO 2 xerogels are incorporated into the TiO 2 NPGs layer with an impregnation process to form the TiO 2 NPGs/xerogels composite. A high power conversion efficiency of 8.41% is achieved for DSSCs based on the TiO 2 NPGs/xerogels composite photoanode, representing a 38% efficiency boost over the efficiency of 6.11% achieved with a P25 TiO 2 based cell. The success of the present composite TiO 2 nanostructure can be attributed to the effective utilization of the inter-NPG space with the infiltration of the TiO 2 xerogels, the excellent structural connectivity within and across the NPG and xerogel domains for fast electron transport, the high specific surface areas of both the NPGs and xerogels for providing abundant dye adsorption for generation of photoinduced electrons, the formation of a TiO 2 xerogel blocking layer on top of the photoanode substrate, and the submicrometer size of the NPGs for much improved light harvesting efficiency. This new type of composite photoanode, different from the 0D/1D nanostructure based ones, proves effective by taking structural advantages from both constituent nanostructures, the mesoprous NPGs and xerogels, and opens up a new way of thinking in the structural design of the photoanodes.
Nature is greatly capable of providing inspiration for the novel design of functional materials. Herein, the efficient bioreactors’ construction of pollen grains inspires us to mimic them for superior gas sensing application. By developing a facile two-step soakage process and subsequent calcinations, the bioreactors are mimicked fully: (I) biosensitive pollen coats on pollen grains are replaced by gas sensitive tin oxide (SnO2) coats, and (II) the fine hierarchical scaffolds are maintained by the self-support of newly formed SnO2 coats. For gas sensing application, as-fabricated SnO2 microreactors exhibit high and fast responses to nitrogen dioxide (219.5 to NO2 of 50 ppm) and other gases. The good sensing properties should be indeed ascribed to the specific construction of microreactors, which shows elaborate hierarchical porous structures and large accessible space/surface area favorable for both gas molecule transports and sensing reactions. This present strategy provides us with new insight on the exploring of effective and low-cost gas sensors, and it could further extend to other pollen grains of numerous different morphologies and other types of bioreactors that are abundant in nature.
Photocatalytic removal of rhodamine B (RhB) and methyl orange (MO) has been studied using the hierarchical SnO2 nanoflowers and SnO2 nanorods under the simulated sunlight irradiation to investigate the influence of morphology on the photocatalytic activity. The hierarchical SnO2 nanoflower catalyst shows higher photocatalytic activity compared with SnO2 nanorod catalyst owing to its larger specific surface area and hierarchical structure, which can promote sunlight absorption and provide radial microchannels for reactant diffusion. The experiment also demonstrates a good photostability and reusability of the SnO2 nanoflower catalyst in photocatalystic degradation.
SnS2/TiO2 nanocomposites with adjustable TiO2 contents were synthesized directly via the solvothermal reactions of SnCl4·5H2O, thioacetamide and different amounts of tetrabutyl titanate in the mixed solvents of ethanol and acetic acid at 180 °C for 12 h. The structures, compositions, Brunauer–Emmett–Teller (BET) specific surface areas and optical properties of the as-synthesized products were characterized by X-ray diffraction, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, high resolution transmission electron microscopy, N2 adsorption and UV–vis diffuse reflectance spectra, and their photocatalytic properties were tested for the reduction of aqueous Cr(VI) under visible light (λ > 420 nm) irradiation. Furthermore, contrast photocatalytic experiments were also conducted for different doses of the as-synthesized SnS2/TiO2 nanocomposite, SnS2 and physical mixture of SnS2 and TiO2. It was found that the as-synthesized SnS2/TiO2 nanocomposite with a suitable TiO2 content (e.g., 44.5 mass% TiO2) not only exhibited extraordinary superior photocatalytic activity to SnS2, TiO2 and physical mixture of SnS2 and TiO2 (44.5 mass%) at different catalyst doses, but also had good photocatalytic stability. Moreover, Cr(VI) can be reduced to Cr(III) by SnS2/TiO2-mediated photocatalysis. The tight heterojunction structure of the as-synthesized SnS2/TiO2 nanocomposite, which can facilitate interfacial electron transfer and reduce the separation and self-agglomeration of two components, was considered to play an important role in achieving its greatly improved photocatalytic performance.
The photochemical behavior of C60 adsorbed on TiO2 particles has been investigated using diffuse reflectance laser flash photolysis. At submonolayer coverages, irreversible oxidation of C60 is observed on titanium dioxide particles. A photochemical transient with a difference absorption maxima at ∼390 nm and at wavelengths greater than 700 is observed following the 532 nm laser pulse excitation of the C60-coated TiO2 particles. This transient is not sensitive to the presence of oxygen. The bleaching in the 600 nm region confirms the depletion of C60 during the surface photochemical oxidation of C60- Formation of both triplet excited state and the oxidation product are observed at higher coverages. This suggests that direct interaction with the oxide surface is crucial for observing the photochemical oxidation of C60. A biphotonic electron ejection from excited C60 is followed by the formation of fullerene epoxide on the TiO2 surface. The diffuse reflectance laser flash photolysis experiments which highlight the surface photochemistry process of C60 are presented.
The hydroxyapatite (HAP) is prepared by precipitation method and examined for the photocatalytic degradation of calmagite, a toxic and non-biodegradable azo-dye compound. The physicochemical properties of hydroxyapatite material were characterized using BET surface area, XRD, FT-IR, and SEM analysis. The FT-IR analysis of the hydroxyapatite revealed that the peak intensity due to absorbance of surface PO43− group centered at wave number 1030cm−1 is drastically decreased upon exposure to UV for 1h. The study includes dark adsorption experiments at different pH conditions, influence of the amount of catalyst, and effect of pH on photocatalytic degradation of dye, chemical oxygen demand (COD) removal, biological oxygen demand (BOD5) increase and SO42− and NO3− ions evolution during the degradation. At optimum photocatalytic experimental conditions the same is compared with commercial degussa P-25 TiO2. The photocatalytic treatment significantly reduced the COD (92% removal) and increased the BOD5/COD ratio to 0.78. Considerable evolution of SO42− (8.5mgL−1) and NO3− (12.2mgL−1) ions are achieved during the degradation process, thus reflecting the usefulness of the hydroxyapatite photocatalytic treatment in calmagite removal in wastewater.