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

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 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.

Full text

Photogenerated Carriers Transfer in Dye−Graphene−SnO2
Composites for Highly Efficient 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 graphene−SnO2aerosol nano-
composites 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.
KEYWORDS: graphene, dye, cophotosensitizer, tin oxide, visible-light-driven photocatalysis
1. INTRODUCTION
Since the photoelectrochemical water splitting over a titanium
oxide (TiO2) electrode was first reported by Fujishima and
Honda in 1972,
1
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
crises.
2
To date, various active semiconductor photocatalysts
have been rapidly developed as well, such as TiO2, zinc oxide
(ZnO) and tin oxide (SnO2), and tin disulfide (SnS2), etc.
3−13
Especially, SnO2has also been extensively used in other fields,
such as sensors,
14
solar cells,
15
and Li-ion batteries,
16
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.
17,18
SnO2catalysts could well operate under ultraviolet (UV)
irradiation,
19
but they generally perform poorly under visible
light due to the wide band gap (Eg= 3.6 eV),
20
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 effect on improving the
photocatalytic efficiency.
21
For the SnO2-based catalysts,
broadening the light-absorption band and minimizing the
recombination of photogenerated electron−hole pairs as two
significant issues are being widely explored by various design
strategies, such as the element doping
22
and the heterojunction
constructing,
23
and so on. In addition, small-size SnO2quantum
dots (QDs) with large specific surface area have been
demonstrated to stimulate surface reactions for achieving
more superior photocatalytic performance.
3,24
Many prepara-
tion methods including biomolecule-assisted hydrothermal
approach
19
and surfactant-assisted solvothermal method
25
have been tried; however, it also remains difficult to obtain
stable and monodispersed SnO2QDs because of the massive
surface free energy.
25
Thus the appropriate surface treatment is
usually necessary to keep QDs stable for obtaining large specific
surface area and efficient photocatalytic activity. In a word, the
visible-light-driven photocatalytic efficiency of SnO2QDs
catalysts depends to a great extent on their dispersity, and
capabilities of absorbing visible light and of preventing
recombination of photogenerated carriers.
26
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
structure.
27,28
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.
29,30
Addi-
tionally, owing to special π-conjugation structure, large specific
surface area,
31
and high conductivity,
32
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
www.acsami.org
© 2013 American Chemical Society 613 dx.doi.org/10.1021/am4047014 |ACS Appl. Mater. Interfaces 2014, 6, 613−621

adsorption of organic contaminants and the separation of
photogenerated carriers.
33
Hence, by integrating with graphene,
the wide band gap semiconductors are promising to realize
efficient visible-light-driven photocatalysis, such as TiO2-
graphene,
34
ZnO-graphene
35
and ZnS-graphene,
36
etc. At
present, very limited literatures have reported the photo-
catalytic performance of SnO2-graphene nanocomposites for
environmental cleanup,
37,38
although they have been widely
studied for Li-ion battery,
39
gas sensor,
40
and supercapacitor.
41
Moreover, the mechanism of visible-light-drivien photocatalysis
has not been completely clarified yet as well, which is
imperative for advancing the application of visible-light-
activated semiconductor−graphene composite photocatalysts.
Herein, we synthesized the SnO2−graphene 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 effective
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 significant 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. EXPERIMENTAL SECTION
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 diffraction (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 UV−vis 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 purification.
2.3. Synthesis of SnO2QDs. SnO2QDs were obtained by a
modified hydrothermal synthesis based on our previous report.
17
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 Teflon-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 Hummer’s method.
42,43
Graphite, NaNO3, KMnO4,H
2SO4,andaTeflon reactor placed into
a stainless steel autoclave were completely cooled in a refrigerator at
0−4°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 Teflon
reactor. As soon as the H2SO4was added, the reactor was sealed in the
stainless steel autoclave and transferred in the refrigerator maintaining
at 0−4°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 SnO2−Graphene Aerosol (SGA). The
SGAs were obtained by simple self-assembly of GO and SnO2QDs
under hydrothermal condition,
44,45
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 Teflon-
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
field 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 diffraction
(SAED), respectively. The specific 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−
Emmett−Teller (BET) method. XRD patterns were obtained by an X-
ray diffractometer (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 diffuse reflectance absorption spectra (DRS)
of the samples were recorded by a UV−vis 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 440−800
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 10−5mol/L (about 4.79 ×10−5g/L) of RhB aqueous
solution, respectively. Then the resulted solutions were magnetically
stirred in the dark for 60 min at room temperature to establish
adsorption−desorption 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
dx.doi.org/10.1021/am4047014 |ACS Appl. Mater. Interfaces 2014, 6, 613−621614

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 ×10−5
mol/L (4.79 ×10−5g/L). After the adsorption−desorption
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 influence of the amount of the SnO2QDs in the SGA
samples, the dose−effect of the SGA photocatalysts on degrading RhB,
and the recyclability of the SGA with the best photocatalytic efficiency.
3. RESULTS AND DISCUSSION
3.1. Formation of SnO2−Graphene 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.
29,30
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,
44
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
dx.doi.org/10.1021/am4047014 |ACS Appl. Mater. Interfaces 2014, 6, 613−621615

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.
44
Therefore, in spite of the same addition amount of the GO
for preparing the GA and the SGA, the obvious difference 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.
29,30
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 specific 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
efficiency. 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,
18
and the SAED pattern
in the inset of Figure 1b shows the bright diffraction 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),
46
directly
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 diffraction peak at around 2θ= 9.9°
corresponds to the (002) phase of the stacked GO sheets.
44
For
the hydrothermally reduced GA, a broad characteristic peak of
graphene appears at about 2θ= 24.5°instead of the original
diffraction peak, indicating that the GO have been effectively
reduced to graphene in the GA.
44
For the SGA, the two
obvious diffraction 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 significant 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.
45
Obviously, this
combination between graphene and SnO2QDs facilitates the
increase in the specific surface area, as confirmed 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 CC skeletal vibration of the sp2
domains (1616 cm−1), the FT-IR spectrum of GO shows the
presence of oxygenated functional groups near 847 cm−1(O−
CO), 1051 cm−1(alkoxy C−O−C), 1220 cm−1(carboxyl
C−OH), 1726 cm−1(carbonyl CO in carbonyl, and carboxyl
moieties), 3161 cm−1(O−H in water) and 3382 cm−1
(structural O−H groups on the graphene sheets).
47−49
Compared with GO, the vibration of O−CO and structural
O−H disappear and the intensity of the C−O−C and CO
peaks significantly 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 effectively reduced.
Especially, the unsuppressed peaks of C−OH vibration (1220
cm−1) and the residual CO vibration may root from
carboxylic acid groups (−COOH)
44,50
that usually can not be
completely removed by chemical reduction.
51,52
However,
these groups are believed to be able to facilitate nanoparticles
dispersion by interacting with surface hydroxyl groups (−OH)
of SnO2,
29,53
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 Sn−O−Sn symmetric
stretching (667 cm−1)
18
inside the SnO2remains unchanged
after combining with graphene, but the Sn−O asymmetric
stretching (539 cm−1) on SnO2surface decreases to 501 cm−1,
and an additional Sn−O vibration peak
54
at 2359 cm−1that
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 confirm the effective 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 Eg≈3.95 eV (see
Figure S8 in the Supporting Information), whereas all the SGA
samples exhibit continuous absorption in the range of 400−700
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.
36
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
dx.doi.org/10.1021/am4047014 |ACS Appl. Mater. Interfaces 2014, 6, 613−621616

with increasing graphene content can be ascribed to the
increase in surface electric charge of SnO2in the composite.
55
Therefore, the combination of the graphene and SnO2QDs
realize effectively 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 confirmed
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 efficiency of the photocatalyst. Hence, the
adsorption processes of different samples to RhB dye were
recorded and shown in Figure S4 (see the Supporting
Information). All suspensions of catalyst and dye achieve the
adsorption−desorption 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 specific surface
area of the 3D porous layered structure,
28,31
as surpported by
the BET areas of SnO2QDs, P25 TiO2(around 50 m2/g),
56
GA, and SGA3 in Figure S6 (see the Supporting Information).
The excellent adsorptivity of the SGA can be also intuitively
reflected by the absorption spectra and corresponding photos
of the RhB aqueous solutions adsorbed by different samples, as
displayed in Figure 4a.
The influence 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
first 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 specific 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 efficiently the aggregation of SnO2QDs, thus resulting
in the reductions of specific surface area and dye adsorption
efficiency. 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-
ents.
37,57
This proves that the SGA is promising for fabricating
high-performance adsorbent for pratical pollution cleaning.
Importantly, the observed superior adsorptivity would be
beneficial 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
different 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 efficiency is better than SnO2QDs, P25 TiO2,
and GA. This superior photocatalytic activity of the SGA can be
Figure 3. UV−vis 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 ×10−3g/L to 2 g/L,
respectively, operating temperature is 298 K).
ACS Applied Materials & Interfaces Research Article
dx.doi.org/10.1021/am4047014 |ACS Appl. Mater. Interfaces 2014, 6, 613−621617

directly manifested by the absorption spectra and the
corresponding photos of RhB aqueous solutions catalyzed by
different catalysts for 40 min, as shown in Figure S11 (see the
Supporting Information). To quantitatively evaluate the
photocatalytic efficiencies 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-first-order kinetic model.
58,59
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 ×10−4min−1. For
other catalysts (SnO2QDs, P25 TiO2, GA, and SGA), the
corresponding degradation rates with k= 4.30 ×10−4min−1,
5.12 ×10−3min−1, 2.28 ×10−2min−1, and 9.13 ×10−2min−1
were respectively obtained, and the SGA exhibits significantly
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-
sites.
29,60
This variation trend in photocatalytic efficiency of the
SGA is in line with that in its adsorption efficiency, confirming
the dependence of photocatalytic activity on the adsorptivity. In
addition, the dose−effect 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 efficiency.
61
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 efficiency and
photocatalytic efficiency 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
confirmed by XRD, SEM and the photon efficiency (ξ) for the
recycled SGA after five 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 efficiency ξof the SGA is recorded in Note S1 (see the
Supporting Information), showing an insignificant recession for
the five cycles. These results demonstrate unambiguously that
the SGA is stable, efficient, 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.
33,53,62−68
How-
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)
53,62−65
and
dye-sensitizer-like graphene photosensitizer (electron
donor).
36,69
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.
38,66
Kamat and co-workers first
demonstrated the feasibility of using graphene as an electron
reservoir (electron-transfermedium)forphotogenerated
electrons from TiO2in the TiO2-graphene composite photo-
catalysts.
53,62−65
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
29,38
and ZnO-
graphene
67,70
etc. In contrast, the concept of graphene
photosensitizer (electron donor) was recently first proposed
by Du et al.,
69
and was verified experimentally by Xu et al.
36
Figure 5. (a) Degradation curves of the RhB aqueous solutions containing different photocatalysts after 40 min of the visible light irradiation: Blank
sample (without catalyst), SnO2QDs, P25 TiO2, GA, and SGA. (b) Dye adsorption efficiency and photocatalytic efficiency of the recycled SGA to
RhB of separate 5 cycles of the continuous dark-adsorption and photocatalytic processes.
ACS Applied Materials & Interfaces Research Article
dx.doi.org/10.1021/am4047014 |ACS Appl. Mater. Interfaces 2014, 6, 613−621618

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 significant for understanding
the photocatalytic mechanism and designing similar visible-
light-responsive semiconductor−graphene photocatalysts at the
level of carrier transfer pathway. In fact, the work function of
graphene (−4.42 eV)
38
is more positive than that of the ZnS
(−7.0 eV).
71,72
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:
73
(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 1−11. Because the
work function of graphene (−4.42 eV) is more positive than
that of SnO2(−4.5 eV),
38
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.
36,74
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
significantly 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,
38,75
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 specifically, the
excited RhB*creates a hole-like RhB•+(RhB with a hole+) (eqs
1−3).
76
The recombination between the injected electrons and
holes in the RhB•+(dotted line in Figure 6) would retard the
degradation of RhB.
75
Fortunately, after loading SnO2on
graphene sheets, because of the suitable work function and high
conductivity of graphene,
32,33
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 6−11).
77,78
Notably, in the CB of SnO2,
because of the larger specific 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 6−8). 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)
+→
*
v
g
raphene h graphene (4)
*→ +
−
g
raphene e graphen
e
(5)
+→
−•
−
eOO
22
(6)
+→
•− + •
OHHO
22 (7)
+→
−•
eHOHO
222
(8)
+→•
−
eHO OH
22 (9)
+→•+
+−
+
h
ole H O/OH OH H
2
(10)
+• → + + + +
−+
−
RhB OH CO H O NO NH Cl
22 3 4
(11)
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 6−9.
For the ground-state RhB, the photogenerated holes (hole+)
would diffuse to the surface and react with adsorbed water and
OH−to 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.
19,77
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
RhB.
4. CONCLUSIONS
In summary, we have synthesized the SnO2−graphene 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
dx.doi.org/10.1021/am4047014 |ACS Appl. Mater. Interfaces 2014, 6, 613−621619

degradation. Maximum adsorption efficiency, dye degradation
rate constant and the optimized dosage for degrading RhB dye
are about 126 mg/g, 9.13 ×10−2min−1, and 0.5 g/L,
respectively. This superior dye adsorptivity is attributed to the
special π-conjugation and larger specific surface area of the 3D
porous layered structure. The excellent photocatalytic activity
should be due to the combination of strong adsorptivity and
effective separation of photogenerated carriers in the SGA.
Significantly, 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
significant to exploit graphene-based solar energy conversion
devices.
■ASSOCIATED CONTENT
*
SSupporting Information
XRD pattern and TEM image of the SnO2QDs, TEM image of
GO, scheme illustration of synthesis of 3D GA and SGA,
adsorption−desorption equilibrium curves of the catalysts to
RhB dye, Schematic illustration of the photocatalytic reactor,
Photonic efficiencies 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 efficiency and photo-
catalytic efficiency of the SGA with different mass ratio of SnO2
to graphene, photocatalytic efficiency of the SGA3 after 40 min
visible-light irradiation with different dosages, SEM and TEM
images of the SGA3 and the SGA1, absorption spectra and
corresponding photograph (inset) of the RhB aqueous
solutions photocatalyzed by different catalysts for 40 min,
apparent reaction rate constants (k) of the RhB photodegraded
by different 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
http://pubs.acs.org/.
■AUTHOR INFORMATION
Corresponding Authors
*Tel.: +86 0514 87970587. Fax: +86 0514 87975467. E-mail:
xxy@yzu.edu.cn.
*E-mail: jghu@yzu.edu.cn
Notes
The authors declare no competing financial interest.
■ACKNOWLEDGMENTS
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.
■REFERENCES
(1) Fujishima, A.; Honda, K. Nature 1972,238,37−38.
(2) Zhang, J.; Yu, J. G.; Jaroniec, M.; Gong, J. R. Nano Lett. 2012,12,
4584−4589.
(3) Nakata, K.; Fujishima, A. J. Photochem. Photobiol. C 2012,13,
169−189.
(4) Zhang, Y. C.; Yang, M.; Zhang, G.; Dionysiou, D. D. Appl. Catal.,
B2013,142−143, 249−258.
(5) Zhang, Y. C.; Du, Z. N.; Li, S. Y.; Zhang, M. Appl. Catal., B 2010,
95, 153−159.
(6) Zhang, Y. C.; Li, J.; Zhang, M.; Dionysiou, D. D. Environ. Sci.
Technol. 2011,45, 9324−9331.
(7) Zhang, Y. C.; Li, J.; Xu, H. Y. Appl. Catal., B 2012,123−124,18−
26.
(8) Zhang, Y. C.; Yao, L.; Zhang, G.; Dionysiou, D. D.; Li, J.; Du, X.
Appl. Catal., B 2014,144, 730−738.
(9) Yang, M. Q.; Zhang, N.; Xu, Y. J. ACS Appl. Mater. Interfaces
2013,5, 1156−1164.
(10) Tang, Z. R.; Zhang, Y.; Xu, Y. J. ACS Appl. Mater. Interfaces
2012,4, 1512−1520.
(11) Liu, S.; Zhang, N.; Tang, Z. R.; Xu, Y. J. ACS Appl. Mater.
Interfaces 2012,4, 6378−6385.
(12) Hu, K.; Robson, K. C. D.; Johansson, P. G.; Berlinguette, C. P.;
Meyer, G. J. J. Am. Chem. Soc. 2012,134, 8352−8355.
(13) Ardo, S.; Meyer, G. J. J. Am. Chem. Soc. 2010,132, 9283−9285.
(14) Song, F.; Su, H. L.; Han, J.; Lau, W. M.; Moon, W. J.; Zhang, D.
J. Phys. Chem. C 2012,116, 10274−10281.
(15) Gubbala, S.; Chakrapani, V.; Kumar, V.; Sunkara, M. K. Adv.
Funct. Mater. 2008,18, 2411−2418.
(16) Huang, J. Y.; Zhong, L.; Wang, C. M.; Sullivan, J. P.; Xu, W.;
Zhang, L. Q.; Mao, S. X.; Hudak, N. S.; Liu, X. H.; Subramanian, A.;
Fan, H. Y.; Qi, L.; Kushima, A.; Li, J. Science 2012,330, 1515−1520.
(17) Miyauchi, M.; Nakajima, A.; Fujishima, A.; Watanabe, T.;
Hashimoto, K. Chem. Mater. 2002,14, 2812−2816.
(18) Zhuang, S. D.; Xu, X. Y.; Pang, Y. R.; Hu, J. G.; Yang, C.; Tong,
L.; Zhou, Y. X. RSC Adv. 2013,3, 20422−20428.
(19) Wu, S. S.; Cao, H. Q.; Yin, S. F.; Liu, X. W.; Zhang, X. R. J. Phys.
Chem. C 2009,113, 17893−17898.
(20) Brovelli, S.; Chiodini, N.; Lorenzi, R.; Lauria, A.; Romagnoli, M.;
Paleari, A. Nat. Commun. 2012,3, 690.
(21) Xie, G. C.; Zhang, K.; Guo, B. D.; Liu, Q.; Fang, L.; Gong, J. R.
Adv. Mater. 2013,25, 3820−3839.
(22) Pan, S. S.; Shen, Y. D.; Teng, X. M.; Zhang, Y. X.; Li, L.; Chu, Z.
Q.; Zhang, J. P.; Li, G. H.; Hu, X. Mater. Res. Bull. 2009,44, 2092−
2098.
(23) Zhang, Y. C.; Du, Z. N.; Li, K. W.; Zhang, M.; Dionysiou, D. D.
ACS Appl. Mater. Interfaces 2011,3, 1528−1537.
(24) Zhu, H. L.; Yang, D. R.; Yu, G. X.; Zhang, H.; Yao, K. H.
Nanotechnology 2006,17, 2386.
(25) Chu, D. R.; Mo, J. H.; Peng, Q.; Zhang, Y. P.; Wei, Y. G.;
Zhuang, Z. B.; Li, Y. D. ChemCatChem 2011,3, 371−377.
(26) Chen, X. B.; Liu, L.; Yu, P. Y.; Mao, S. S. Science 2011,331,
746−750.
(27) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang,
Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004,306,
666−669.
(28) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007,6, 183−191.
(29) Zhang, H.; Lv, X. J.; Li, Y. M.; Wang, Y.; Li, J. H. ACS Nano
2010,4, 380−386.
(30) Zhang, Y.; Tang, Z. R.; Fu, X.; Xu, Y. J. ACS Nano 2010,4,
7303−7314.
(31) Geim, A. K. Science 2009,324, 1530−1534.
(32) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K.
M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S.
Nature 2006,442, 282−286.
(33) Huang, X.; Qi, X. Y.; Boey, F.; Zhang, H. Chem. Soc. Rev. 2012,
41, 666−686.
(34) Chen, C.; Cai, W. M.; Long, M.; Zhou, B. X.; Wu, Y. H.; Wu, D.
Y.; Feng, Y. J. ACS Nano 2010,4, 6425−6432.
(35) Li, B. J.; Cao, H. Q. J. Mater. Chem. 2011,21, 3346−3349.
(36) Zhang, Y. H.; Zhang, N.; Tang, Z. R.; Xu, Y. J. ACS Nano 2012,
11, 9777−9789.
(37) Seema, H.; Kemp, K. C.; Chandra, V.; Kim, K. S. Nanotechnology
2012,23, 355705.
ACS Applied Materials & Interfaces Research Article
dx.doi.org/10.1021/am4047014 |ACS Appl. Mater. Interfaces 2014, 6, 613−621620

(38) Zhang, J. T.; Xiong, Z. G.; Zhao, X. S. J. Mater. Chem. 2011,21,
3634−3640.
(39) Wang, L.; Wang, D.; Dong, Z. H.; Zhang, F. X.; Jin, J. Nano Lett.
2013,13, 1711−1716.
(40) Zhang, Z. Y.; Zou, R. J.; Song, G. S.; Yu, L.; Chen, Z. G.; Hu, J.
Q. J. Mater. Chem. 2011,21, 17360−17365.
(41) Li, F. H.; Song, J. F.; Yang, H. F.; Gan, S. Y.; Zhang, Q. X.; Han,
D. X.; Ivaska, A.; Niu, L. Nanotechnology 2009,20, 455602.
(42) Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958,80,
1339.
(43) Bao, C. L.; Song, L.; Xing, W. Y.; Yuan, B. H.; Wilkie, C. A.;
Huang, J. L.; Guo, Y. Q.; Hu, Y. J. Mater. Chem. 2012,22, 6088−6096.
(44) Zhang, Z. Y.; Xiao, F.; Guo, Y. L.; Wang, S.; Liu, Y. Q. ACS
Appl. Mater. Interfaces 2013,5, 2227−2233.
(45) Xu, Y. X.; Sheng, K. X.; Li, C.; Shi, G. Q. ACS Nano 2010,4,
4324−4330.
(46) Wang, G. X.; Wang, B.; Wang, X. L.; Park, J.; Dou, S. X.; Ahn,
H.; Kim, K. J. Mater. Chem. 2009,19, 8378−8384.
(47) Acik, M.; Lee, G.; Mattevi, C.; Pirkle, A.; Wallace, R. M.;
Chhowalla, M.; Cho, K.; Chabal, Y. J. Phys. Chem. C 2011,115,
19761−19781.
(48) Szabó, T.; Berkesi, O.; Forgó, P.; Josepovits, K.; Sanakis, Y.;
Petridis, D.; Dé
ká
ny, I. Chem. Mater. 2006,18, 2740−2749.
(49) Tiwari, J. N.; Mahesh, K.; Le, N. H.; Kemp, K. C.; Timilsina, R.;
Tiwari, R. N.; Kim, K. S. Carbon 2013,56, 173−182.
(50) Xu, J.; Wang, L.; Zhu, Y. F. Langmuir 2012,28, 8418−8425.
(51) Li, D.; Muller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Nat.
Nanotechnol. 2008,3, 101−105.
(52) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.;
Kleinhammes, A.; Jia, Y. Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Carbon
2007,45, 1558−1565.
(53) Williams, G.; Seger, B.; Kamat, P. V. ACS Nano 2008,2, 1487−
1491.
(54) Li, Z. J.; Qin, Z.; Zhou, Z. H.; Zhang, L. Y.; Zhang, Y. F.
Nanoscale Res. Lett. 2009,4, 1434−1438.
(55) Luo, Q. P.; Yu, X. Y.; Lei, B. X.; Chen, H. Y.; Kuang, D. B.; Su,
C. Y. J. Phys. Chem. C 2012,116, 8111−8117.
(56) Cheng, W. Y.; Deka, J. R.; Chiang, Y. C.; Rogeau, A.; Lu, S. Y.
Chem. Mater. 2012,24, 3255−3262.
(57) Girgis, B. S.; Soliman, A. M.; Fathy, N. A. Microporous
Mesoporous Mater. 2011,142, 518−525.
(58) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W.
Chem. Rev. 1995,95,69−96.
(59) Zhou, X. M.; Lan, J. Y.; Liu, G.; Deng, K.; Yang, Y. L.; Nie, G. J.;
Yu, J. G.; Zhi, L. J. Angew. Chem., Int. Ed. 2012,51, 178−182.
(60) Xu, Y. J.; Zhuang, Y. B.; Fu, X. Z. J. Phys. Chem. C 2010,114,
2669−2676.
(61) Reddy, M. P.; Venugopal, A.; Subrahmanyam, M. Appl. Catal., B
2007,69, 164−170.
(62) Xiang, Q. J.; Yu, J. G.; Jaroniec, M. Chem. Soc. Rev. 2012,41,
782−796.
(63) Lightcap, I. V.; Kosel, T. H.; Kamat, P. V. Nano Lett. 2010,10,
577−583.
(64) Prezhdo, O. V.; Kamat, P. V.; Schatz, G. C. J. Phys. Chem. C
2011,115, 3195−3197.
(65) Kamat, P. V. J. Phys. Chem. Lett. 2011,2, 242−251.
(66) Czerw, R.; Foley, B.; Tekleab, D.; Rubio, A.; Ajayan, P. M.;
Carroll, D. L. Phys. Rev. B 2002,66, 033408.
(67) Williams, G.; Kamat, P. V. Langmuir 2009,25, 13869−13873.
(68) Li, Q.; Guo, B. D.; Yu, J. G.; Ran, J. R.; Zhang, B. H.; Yan, H. J.;
Gong, J. R. J. Am. Chem. Soc. 2011,133, 10878−10884.
(69) Du, A.; Ng, Y. H.; Bell, N. J.; Zhu, Z. H.; Amal, R.; Smith, S. C.
J. Phys. Chem. Lett. 2011,2, 894−899.
(70) Akhavan, O. ACS Nano 2010,4, 4174−4180.
(71) Fang, X. S.; Gautam, U. K.; Bando, Y.; Dierre, B.; Sekiguchi, T.;
Golberg, D. J. Phys. Chem. C 2008,112, 4735−4742.
(72) Ruyven, L. J. V.; Williams, F. E. Phys. Rev. Lett. 1966,16, 889−
890.
(73) Chen, C. C.; Ma, W. H.; Zhao, J. C. Chem. Soc. Rev. 2010,39,
4206−4219.
(74) Kamat, P. V.; Gevaert, M.; Vinodgopal, K. J. Phys. Chem. B 1997,
101, 4422−4427.
(75) Xiong, Z.; Zhang, L. L.; Ma, J.; Zhao, X. S. Chem. Commun.
2010,46, 6099−6101.
(76) Kamat, P. V. Chem. Rev. 1993,93, 267−300.
(77) Zhang, H. L.; Hu, C. G. Catal. Commun. 2011,14,32−36.
(78) Sajjad, A. K. L.; Shamaila, S.; Tian, B. Z.; Chen, F.; Zhang, J. L. J.
Hazard. Mater. 2010,177, 781−791.
ACS Applied Materials & Interfaces Research Article
dx.doi.org/10.1021/am4047014 |ACS Appl. Mater. Interfaces 2014, 6, 613−621621