Ternary-component reduced graphene oxide aerogel constructed by g-C3N4/BiOBr heterojunction and graphene oxide with enhanced photocatalytic performance

Article abstract of DOI:10.1016/j.jallcom.2017.09.175

Tremendous demands for photocatalysts with high chemical activity and stability, low cost and capability of response to visible-light have stimulated intensive research on developing versatile materials with special photocatalytic applications. Here we report a novel g-C₃N₄/BiOBr-RGO aerogel ternary-component heterojunction photocatalyst for degradation of RhB under visible light irradiation. As a result of synergistic effect of the g-C₃N₄, BiOBr and RGO, the hybrid photocatalysts exhibit better activity than the single component catalysts (pure g-C₃N₄, BiOBr) and bi-component catalysts (g-C₃N₄/BiOBr) in the same time period (60 min). The enhanced photocatalytic performance originates from a novel nanoarchitecture in which BiOBr exposed more active plane and RGO worked as a conductive support further improved the photogenerated charge separation. Moreover, due to the 3D structure of RGO after reaction the photocatalysts can be easily removed from the treated solution and reused, which opens up a way for the recycling applications of catalysts in aqueous reaction systems.

Full text of DOI:10.1016/j.jallcom.2017.09.175

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Journal of Alloys and Compounds 729 (2017) 162e170
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Ternary-component reduced graphene oxide aerogel constructed by
g-C3N4/BiOBr heterojunction and graphene oxide with enhanced
photocatalytic performance
,
,
,
Xue Yu a 1, Peiwen Wu a 1, Caixia Qi b, Junjie Shi b **, Lijuan Feng a, Chunhu Li a,
Liang Wang a
,
*
a Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, College of Chemistry and Chemical Engineering, Ocean University of
China, Qingdao, China
b Shandong Applied Research Centre of Gold Nanotechnology, School of Chemistry & Chemical Engineering, Yantai University, Yantai, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 July 2017
Received in revised form
21 August 2017
Accepted 16 September 2017
Available online 19 September 2017
Tremendous demands for photocatalysts with high chemical activity and stability, low cost and capability
of response to visible-light have stimulated intensive research on developing versatile materials with
special photocatalytic applications. Here we report
a novel g-C3N4/BiOBr-RGO aerogel ternary-
component heterojunction photocatalyst for degradation of RhB under visible light irradiation. As a
result of synergistic effect of the g-C3N4, BiOBr and RGO, the hybrid photocatalysts exhibit better activity
than the single component catalysts (pure g-C3N4, BiOBr) and bi-component catalysts (g-C3N4/BiOBr) in
the same time period (60 min). The enhanced photocatalytic performance originates from a novel
nanoarchitecture in which BiOBr exposed more active plane and RGO worked as a conductive support
further improved the photogenerated charge separation. Moreover, due to the 3D structure of RGO after
reaction the photocatalysts can be easily removed from the treated solution and reused, which opens up
a way for the recycling applications of catalysts in aqueous reaction systems.
Keywords:
RGO aerogel
g-C3N4/BiOBr
Heterojunction
Visible light
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
heterojunction. By far the most commonly employed combination
is based on a semiconductoresemiconductor (SeS) architecture.
In the past decades, great efforts have been devoted to improve
the efficiencies of photocatalysts. Strategies include doping, regu-
lating the morphology and forming heterojunction in semi-
conductor photocatalysts have been employed [1,2]. Among the
proposed strategies, construction of heterojunction has been
proved to be one of the most promising ways for the preparation of
efficient photocatalysts, due to its effectiveness in the spatial sep-
arations of electronehole pairs. In general, three different types
heterojunction have been proposed which are (1) semi-
conductoresemiconductor SeS heterojunction, (2) semi-
Due to the close contact of two different semiconductors, the
charge carriers will diffused between them, which leads to a space-
charge region and induces an electric field at the interface, finally
resulting in more efficient separation, longer charge carrier life-
times and higher reaction rates [3].
Recently the two-dimensional (2D) layered graphitic carbon
nitride (g-C3N4) as an unconventional semiconductor has attracted
wide spread attention owing to its favorable visible light harvesting
capability, tunable electronic structure, high chemical and thermal
stability, and environmental friendliness [4,5]. It is characterized
with a band gap of ~2.7 eV, corresponding to an optical wavelength
of ~460 nm [6,7]. In addition, the minimum conduction band of g-
C3N4 is extremely negative (ꢀ1.12 eV vs. NHE) compared to tradi-
tional semiconductors such as TiO2 and ZnO [8], therefore, the light
excited electrons should have a higher reduction ability. However,
the photocatalytic activity of pristine g-C3N4 is usually limited due
to the fast recombination of photoinduced electronꢀhole pairs [9].
To suppress the recombination rate of photogenerated carriers,
conductoremetal
SeM
heterojunction,
and
(3)
semiconductorecarbon SeC (carbon nanotubes, graphene, etc.)
* Corresponding author.
** Corresponding author.
E-mail addresses: junjieshiding@gmail.com (J. Shi), wangliangouc@ouc.edu.cn
(L. Wang).
1
X Yu and P Wu are the first authors.
http://dx.doi.org/10.1016/j.jallcom.2017.09.175
0925-8388/© 2017 Elsevier B.V. All rights reserved.

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X. Yu et al. / Journal of Alloys and Compounds 729 (2017) 162e170
163
one of the strategies is to design appropriate heterojunction, for
example, coupling g-C3N4 with other semiconductor materials.
Cheng et al. reported that by an electrostatic re-assembly of g-C3N4
with CdS or BiOBr the photocatalytic performance for degradation
of methyl orange was greatly improved [10]. A series of other
composites were also reported such as TiO2/geC3N4, AgeAgBr/
geC3N4, Bi2WO6/geC3N4, Ag3PO4/geC3N4 [6,7].
Co., Ltd.), KBr (AR, Sinopharm Chemical Reagent Co., Ltd.). Deion-
ized water was used throughout this study. Rhodamine B (RhB) was
analytical pure and used without further purification.
2.2. Fabrication of g-C3N4/BiOBr and g-C3N4/BiOBr/RGO aerogel
2.2.1. Synthesis of g-C3N4
Among them, those involving BiOBr nanocrystals are of partic-
ular interest. As reported by Yang et al. BiOBr and g-C3N4 would
form a staggered band-gap type heterojunction [11]. Owning to the
indirect-transition band-gap and layered structure of BiOBr, the
excited electron has to travel a certain k-space distance to the
valence band (VB) and an internal static electric fields perpendic-
ular to the [Bi2O2] slabs and halogen anionic slabs will present in
BiOBr. This will lead to the efficient separation of the electrons and
holes during the photoexcitation, thus an enhanced photocatalytic
activity. To be mentioned, the facet of semiconductors also play a
key role in determining the catalytic properties [12]. For example,
Zhang et al. reported that BiOBr crystals with high ratio of exposed
(001) facets showed an increased efficiency over rhodamine B
(RhB) degradation [13]. Due to the presence of both carboxyl and
amino groups on g-C3N4, most of the work are focused on an electro
static method for the reassembly of the g-C3N4 based photo-
catalysts. However, to date, there is still no report on the prepara-
tion of BiOBr/g-C3N4 heterojunction with particular active facet of
BiOBr.
Furthermore, most of the reported heterojunction photo-
catalysts are usually in a powder form, which have a tendency to
agglomerate and the recovery of the catalysts would requires a
complex centrifugal operations. Recently, the loading of semi-
conductor and metal nanoparticles onto RGO aerogel spurred great
interest due to their high surface area could provide a large surface/
interface for heterogeneous reactions. In addition, it is known
graphene have a good conductivity, therefore could promote the
charge separation and restrain the holeeelectron recombination.
More importantly, due to the immobilization of the powder catalyst
on the three dimensional (3D) RGO aerogel surface, the separation
and recycling process of the catalyst during practical applications
could be greatly simplified.
In this work, we describe a facile two-step hydrothermal as-
sembly route to synthesize a ternary-component g-C3N4/BiOBr/
RGO heterojunction aerogel. The g-C3N4/BiOBr is synthesized
through a hydrothermal method, aiming to regulate the orienta-
tions and increasing the crystallinities of BiOBr. Followed by a
second hydrothermal step the micro-structured g-C3N4/BiOBr
could be anchored on the surface of GO, the addition of dopamine
(DA) worked as a reducing agent and cross-linker could result in a
reduced GO. After a freeze-drying process a 3D g-C3N4/BiOBr/RGO
hybrid aerogel were synthesized, the micro-structure morphology,
specific surface area and spectral properties were investigated. The
photocatalytic degradation of RhB under visible-light on the RGO
supported g-C3N4/BiOBr heterojunction are fully studied.
The g-C3N4 were prepared using a simple thermal polymeriza-
tion method as reported by Jiang et al. [14]. In a typical synthesis, a
designate amount of melamine were added into an alumina cru-
cible, then heated to 823 K (heating rate 2.3 K minꢀ1) in a muffle
furnace and maintained for 3 h. The obtained yellow powder, after
cooling down, could be collected for further application.
2.2.2. Synthesis of g-C3N4/BiOBr heterojunction
The g-C3N4/BiOBr heterojunction were fabricated through a
one-step hydrothermal approach (as shown in Scheme 1). Typi-
cally, 1 mmol of Bi(NO3)3$5H2O and 1.2 mmol of KBr were dissolved
in 10 mL HNO3 solution (2 M) and 10 mL deionized water,
respectively. Afterwards, mixing the two solution with desired
amount of g-C3N4 under ultrasonication for 1 h. Transfer the ho-
mogeneous suspension into an autoclave, followed by hydrother-
mal treatment at 160 ꢁC for 12 h. The resulting precipitates were
separated by centrifugation and subsequently washed with
deionized water for three times. The final products were obtained
after drying at 60 ꢁC for 12 h. The pure BiOBr was prepared
following the same steps just without the addition of g-C3N4.
2.2.3. Synthesis of g-C3N4/BiOBr/RGO aerogel
Graphene oxide (GO) was prepared by an improved Hummer's
method as reported by Marcano et al. [15] The g-C3N4/BiOBr/RGO
ternary composites aerogels were synthesized via a hydrothermal
growth route. The g-C3N4/BiOBr and GO were used as the colloidal
precursor, dopamine (DA) as a reducing agent, as illustrated by
Scheme 2. After adding 60 mg dopamine, 0.3 g of g-C3N4/BiOBr in a
30 ml graphene oxide (GO) dispersion (1 mg/mLꢀ1) under magnetic
stirring for 15 min, the solution was transferred into a 50 mL Teflon-
sealed autoclave and maintained at 100ꢁCfor 12 h. The weight ratio
of GO to DA was kept consistent at 1:2 to ensure the preparation of
fully reduced graphene. The resulting hydrogel was collected and
washed with deionized water for several times, and finally treated
by freeze-drying to obtain the g-C3N4/BiOBr/RGO aerogel. The ob-
tained g-C3N4/BiOBr/RGO aerogel with RGO weight ratios of 5 wt%,
10 wt% and 20 wt%, were denoted as g-C3N4/BiOBr-G5, g-C3N4/
BiOBr-G10, and g-C3N4/BiOBr-G20.
2.3. Characterization
The sizes and morphologies of as-prepared samples were
determined by transmission electron microscopy (TEM, JEM-2100).
The specific surface area was measured by a Micromeritics
ASAP2020 Surface Area and Porosity Analyzer and calculated by
using the BET equation. The phase composition of the as-prepared
samples was characterized by means of X-ray diffractometer (XRD,
Bruker D8 Advance with CuKa1 radiation at 40 kV and 30 mA).
Raman spectrum was acquired with a Dilor XY microspectrometer
using a 532 nm excitation wavelength. X-ray photoelectron spec-
troscopy (XPS) measurements were performed on a PHI Quantum
2000 XPS system (PHI, USA) with a monochromatic Al Ka source
and a chargeneutralizer. The photoluminescence (PL) spectrum was
measured using a Hitachi F-7000 fluorescence spectrophotometer
at room temperature. UVevis diffusion reflectance spectrum of the
samples were obtained on a UVeVis spectrophotometer (UV-3600,
Shimadzu, Japan).
2. Experimental
2.1. Materials
All chemical regents were of analytical grade reagents and used
without further purification. Graphite powder (325 mesh, 99.8%
purity) was purchased from Sinopharm Chemical Reagent Co., Ltd
(Shanghai, China). Dopamine was provided by Shanghai Aladdin
biochemical Polytron Technology Co., Ltd. Sulphuric acid (H2SO4,
98%), nitric acid (HNO3), potassium permanganate (KMnO4),
hydrogen peroxide (H2O2, 30 wt %) and urea were purchased from
Tianjin Bo Di Ltd. Bi(NO3)3$5H2O(AR, Sinopharm Chemical Reagent

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164
X. Yu et al. / Journal of Alloys and Compounds 729 (2017) 162e170
Scheme 1. Schematic illustration of the synthesis process of g-C3N4/BiOBr.
Scheme 2. Schematic illustration of the synthesis process of g-C3N4/BiOBr/RGO aerogel.
2.4. Photocatalytic activity
attributed to the (002) interlayer stacking of graphite-like struc-
tures in g-C3N4, which indicates the BiOBr could couple with g-C3N4
form a composite catalyst [16]. Fig. 1a also shows the XRD patterns
of g-C3N4/BiOBr/RGO photocatalyst with different RGO content.
Notably, no diffraction peak of RGO is observed in the hybrid which
can be ascribed to the low content and weak diffraction intensity of
RGO. Upon combining the g-C3N4/BiOBr with RGO, however, an
obvious change in the intensity of the (102) and (110) peak for
BiOBr can be observed. This result suggests that the g-C3N4/BiOBr
can be selectively loaded on RGO sheets with a preferential crystal
orientation. By zoom in the features between 31ꢁ and 33ꢁ in Fig. 1b,
we can find when the loading of RGO is around 5 wt% the intensity
ratio of (102)/(110) is around 3, suggesting more (102) plane could
be exposed. With the increase of the RGO loading the ratio of (102)/
(110) decreased indicates the amount of the exposed (110) plane
increased. In addition, these results could also demonstrate that the
g-C3N4/BiOBr are successfully coated on RGO support [17].
The Raman spectrum (Fig. 2) shows the characteristic peaks of
the D and G bands of graphene at 1347 and 1597 cmꢀ1. It is known
both the D and G bands are the result of vibrations of SP2 bonded
carbon atoms, whereas G band is due to in-plane vibrations of SP2
carbon atoms and D band is related with the presence of structural
defects. As shown in Fig. 2, after loading g-C3N4/BiOBr on the sur-
face of RGO the intensity ratio of the D band and G band (ID/IG ¼ 1.1)
shows slightly decrease compared with that of pure GO (ID/
IG ¼ 0.9), indicating more defect sites was formed on the RGO
support.
Photocatalytic performance in the degradation of Rhodamine B
(RhB, 10 mg/L) was investigated under visible light irradiation. A
300 W Xe lamp with a 420 nm cut off filter was used as a visible
light source. The experiments were carried out at 25 ꢁC as follows:
0.1 g of sample was added into 100 mL of RhB solution (10 mg/L).
The distance between the bottom of the lamp and the top of the
solution was 20 cm. Before irradiation, the suspension was stirred
for 30 min in dark to establish of adsorptionedesorption equilib-
rium. At given time intervals, the samples were analyzed by a
UVevis spectra of RhB using a Cary 500 ultraviolet visible spec-
trometer. Recycling experiments were conducted over the g-C3N4/
BiOBr-G10 at the same condition. After finishing a cycle, the g-C3N4/
BiOBr-G10 was separated from the reaction solution by centrifu-
gation, followed by washing and drying.
3. Results and discussion
3.1. Chemical structure, crystal structure, and components of
obtained samples
XRD was used to study the crystallographic structures and
preferential crystal orientation of the as-prepared samples. The
intense and sharp diffraction peaks imply a good crystallinity of all
the samples. As shown in Fig. 1a, the BiOBr show peaks of 2q values
at 13.3ꢁ, 24.3ꢁ, 34.3ꢁ, 35.7ꢁ and 47.3ꢁ corresponds to the (001),
(002), (012), (110) and (020) crystallographic planes of BiOBr, which
are also in good agreement with the tetragonal phase of BiOBr
(JCPDS 09-0393) [12]. For g-C3N4/BiOBr, the main diffraction peaks
are attributed to BiOBr, a further small peak at around 27ꢁ can be
The chemical states and elemental composition of g-C3N4/BiO-
Br-G10 were examined by XPS spectra. An XPS survey in Fig. 3a
confirms the presence of Bi, C, O, N, and Br in the g-C3N4/BiOBr-G10
sample. The C 1s spectrum can be deconvoluted into three peaks

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X. Yu et al. / Journal of Alloys and Compounds 729 (2017) 162e170
165
mainly composed of three peaks centered at 400.8 eV, 399.7 eV and
399.0 eV originate from the sp2-hybridized nitrogen involved in the
triazine rings (C]NeC), CeNeH and the bridged nitrogen atoms N-
(C)3, respectively [18]. In addition, the O1s spectra at 530.5 eV were
assigned to BieO [19], Moreover, the Bi 4f curve is mainly composed
of two peaks centered at 159.6 eV and 164.9 eV corresponding to
the Bi4f7/2 and Bi4f5/2, respectively, indicating the presence of Bi3þ
[20].(Support Information Fig. S1). The Br 3d curve can be decon-
voluted into two peaks centered at 68.5 eV and 69.4 eV, which are
related to the Br3d5/2 and Br3d3/2(Support Information Fig. S2).
3.2. Surface morphology of obtained samples
The density of the UNGAS is about 3.81 mg/cm3, according to the
equation
r
(mg/cm3) ¼ m (39 mg)/V (10.24 cm3). The morphology
and microstructure of the g-C3N4/BiOBr-G10 were depicted by TEM
and HRTEM. Both Fig. 4a and b indicate the flake-like BiOBr are
covered by a thin layer of g-C3N4 film. The formation of hetero-
junction structure could be proved by the high-resolution TEM
image of Fig. 4c, it shows two different orientated phases are closely
contacted with an amorphous phase in the middle, forming two
intimate interfaces. The interplanar distances are determined to be
0.28 nm in Fig. 4d, which are in line with the (012) planes from
BiOBr [21,22].
Commonly, higher specific surface area of photocatalysts was
contributed to an enhanced photocatalytic activity owing to the
presence of a large amount of surface active sites. The specific
surface areas and microstructures of the g-C3N4 samples were
characterized using N2 adsorption-desorption isotherms. Fig. 5a
shows the typical N2 adsorption-desorption isotherm of g-C3N4/
BiOBr-G10, which show a similar shape of type IV for all samples. It
should be noted that the adsorption-desorption isotherm of g-
C3N4/BiOBr-G10 is not close at low relative pressure, which may
result from the formation of poly-dopamine during the reducing
reaction [8]. As shown in Fig. 5b, the specific surface areas calcu-
lated by the BET method are 23.5, 45.8, 67.5, and 36.2 m2 gꢀ1, for g-
C3N4/BiOBr, g-C3N4/BiOBr-G5, g-C3N4/BiOBr-G10 and g-C3N4/BiO-
Br-G20 [23]. Therefore, the GO content have great influence on the
specific surface area of g-C3N4/BiOBr/RGO aerogel, it is hence
possible to fabricate g-C3N4/BiOBr/RGO aerogel that is different in
morphology by regulating the amount of GO addition.
Fig. 1. XRD patterns of obtained samples.
3.3. Optical characterization
The UVevis diffuse reflectance spectra have been used to study
the optical properties of different samples. Fig. 6a shows pure g-
C3N4 have a broad adsorption range cover both UV and visible light
regions, whereas pure BiOBr have a strong adsorption mostly in the
UV region. After forming g-C3N4/BiOBr composite the adsorption in
UV region obviously increased. Moreover, after further loading g-
C3N4/BiOBr on the RGO surpport, the absorption center and edge
undergo an obvious red shift. This can be attribute to the black
nature of the RGO, as we know the black colored body will absorb
more visible light [24]. Compared to pure g-C3N4 and BiOBr, the
increased spectra in UV region from g-C3N4/BiOBr and g-C3N4/
BiOBr/RGO suggest that the grafted BiOBr and graphene cycles
extend the p-conjugation system in g-C3N4 to reduce the band gap
and facilitate the separation of photogenerated charges [25]. As
shown in the figure, the band gap energies of samples were was
obtained by drawing a tangent line on the edge of the UV spectra to
determine the cut off wavelength at y ¼ 0, accordingly, the bad gap
of yellow BiOBr was decreased from 2.76 to 2.08eV by decorating
with RGO and g-C3N4.
Fig. 2. Raman spectra of GO (curve a), g-C3N4/BiOBr-G5 (curve b), g-C3N4/BiOBr-G10
(curve c) and g-C3N4/BiOBr-G20 (curve d).
centered 284.6 eV, 285.6 eV, 286.5 eV, 288.3 eV and 288.6 eV, which
corresponding to the sp2 CeC bonding, CeN bonding, CeO bonding
and sp2 NeC]N bonding and C]O bonding. The curve of N 1s is
To investigate the separation capacity of photogenerated car-
riers in as-prepared samples, photoluminescence (PL) spectra of g-