Organic–inorganic hybrid photocatalyst g

Colloids and Surfaces A: Physicochem. Eng. Aspects 467 (2015) 188–194
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Colloids and Surfaces A: Physicochemical and
Engineering Aspects
journal homepage: www.elsevier.com/locate/colsurfa
Organic–inorganic hybrid photocatalyst g-C3 N4 /Ag2 CO3 with highly
efficient visible-light-active photocatalytic activity
Na Tian, Hongwei Huang ∗ , Ying He, Yuxi Guo, Yihe Zhang ∗
Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials,
School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China
h i g h l i g h t s
g r a p h i c a l
• g-C3 N4 /Ag2 CO3
composites were
synthesized by a facile in situ
precipitation route.
• The 1:12 g-C3 N4 /Ag2 CO3 sample shows the most improved
photocatalytic performance.
• The enhanced photocatalytic activity was attributed to a heterojunction
mechanism.
• The active species h+ and • OH were
detected by species trapping experiment.
The efficient charge transfer at the interface of g-C3 N4 /Ag2 CO3 heterojunction leads to an effective photoexcited electron–hole separation and promote the photocatalytic activity.
a r t i c l e
a b s t r a c t
i n f o
Article history:
Received 7 September 2014
Received in revised form
20 November 2014
Accepted 28 November 2014
Available online 9 December 2014
Keywords:
g-C3 N4
Ag2 CO3
Visible-light
Photocatalyst
Methyl orange (MO)
Novel g-C3 N4 /Ag2 CO3 organic–inorganic hybrid photocatalysts have been prepared by a facile in situ
precipitation route. The crystal structure and optical property of the as-prepared samples have been
characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron
microscopy (SEM), and diffuse reflection spectroscopy (DRS). The photocatalytic experiments indicated
that the as-prepared g-C3 N4 /Ag2 CO3 photocatalyst exhibited significantly enhanced photocatalytic activity than the pure g-C3 N4 and Ag2 CO3 samples toward degrading methyl orange (MO) under visible light
irradiation ( > 420 nm). A possible photocatalytic mechanism was proposed based on the photoluminescence (PL) spectra, photocurrent spectra and a series of radical trapping experiments. The remarkably
improved photocatalytic performance should be ascribed to the heterostructure between Ag2 CO3 and
g-C3 N4 , which greatly promoted the photoinduced charge transfer and inhibited the recombination of
electrons and holes.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
Recently, the exploration of photocatalysts with high photocatalytic efficiency under visible-light irradiation has drawn
considerable attention for their potential value to solve the
∗ Corresponding authors. Tel.: +86 10 82332247; fax: +86 10 82332247.
E-mail addresses: [email protected] (H. Huang), [email protected] (Y. Zhang).
http://dx.doi.org/10.1016/j.colsurfa.2014.11.049
0927-7757/© 2014 Elsevier B.V. All rights reserved.
a b s t r a c t
environmental pollution problems [1–7]. Great efforts have been
expended toward the development of new visible-light-responsive
photocatalysts in order to utilize the solar light in the visible region
( > 420 nm), which covers the largest proportion of the solar spectrum. Until now, varieties of visible-light-driven photocatalysts
have been reported, such as g-C3 N4 [8,9], Bi-based photocatalysts
[10–12], Ag-based compounds [13–18], plasmonic noble metal
(gold, silver) nanoparticles [19], and so on. Among them, Ag-based
photocatalysts have been believed to be promising photocatalysts
N. Tian et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 467 (2015) 188–194
with high efficiency and identified to be well-known photosensitive
materials in recent years, such as Ag3 PO4 [13], Ag3 VO4 [14], Ag2 O
[15], and AgX (X = Cl, Br, I) [16–18]. It is well known that Ag2 CO3 is
a highly visible-light-responsive photocatalyst with efficient photodegradation ability on methyl orange (MO), rhodamine B (RhB)
and methylene blue (MB) dyes [20,21].
As a new photocatalyst, graphite-like carbon nitride (g-C3 N4 )
has been explored as a promising candidate for hydrogen evolution and environment purification under visible-light irradiation
[22,23]. However, there are still some inherent drawbacks which
exist for the use of g-C3 N4 in photocatalysis, such as the high recombination rate of its photogenerated electron–hole pairs and limited
visible-light absorption below 450 nm [22,24]. Recently, continuous attempts have been made to improve the quantum yield of
g-C3 N4 , for example, designing nanoporous structures, nanotube
structures, doping with nonmetal, or metal elements, coupling with
grapheme, building heterostructures [25,26], etc. Among them,
construction of heterostructures by means of combining g-C3 N4
with other appropriate semiconductors is an effective way to
improve the photocatalytic activity.
In this work, the g-C3 N4 /Ag2 CO3 composites were synthesized by a facile in situ precipitation method and characterized
by XRD, SEM, XPS, DRS and PL. The photocatalytic activities of
the as-prepared samples were investigated by degradation of
methyl orange (MO) under visible light irradiation ( > 420 nm). The
g-C3 N4 /Ag2 CO3 photocatalyst exhibited a much higher photoreactivity than the pure samples, which should be attributed to the
heterostructure between g-C3 N4 and Ag2 CO3 , thus resulting in the
efficient separation of photoinduced charge carriers. Accordingly,
the possible photocatalytic mechanism was proposed.
2. Experimental
2.1. Preparation of g-C3 N4
All of the chemical reagents used in the experiments were analytical grade without further purification. Graphitic carbon nitride
(g-C3 N4 ) was synthesized through a pyrolysis process, as described
in previous work [22,25,26].
2.2. Preparation of g-C3 N4 /Ag2 CO3 photocatalyst
189
Fig. 1. XRD patterns of g-C3 N4 , Ag2 CO3 and g-C3 N4 /Ag2 CO3 composites with molar
ratios of 1:10, 1:12 and 1:14.
obtained using a fluorescence spectrometer (Hitachi F-4600) at
room temperature. The transient photocurrent responses experiments were detected by a CHI660C electrochemical workstation
(Shanghai, China).
2.4. Photocatalytic evaluation
The photocatalytic activities of the prepared photocatalysts
were evaluated by degradation of methyl orange (MO) under visible light irradiation of a 500 W xenon lamp ( > 420 nm). The
as-prepared photocatalyst (50 mg) was dissolved in an aqueous
solution of MO (50 mL, 0.01 mM). Then, the solution was vigorously stirred in the dark for 1 h to achieve an adsorption/desorption
equilibrium between the dye and the photocatalyst. After that, the
light was turned on, and at regular time intervals, certain liquid
was sampled and analyzed by UV-vis spectrophotometer at the
characteristic band of 464 nm to determine the concentration of
MO.
The g-C3 N4 /Ag2 CO3 composite photocatalysts were prepared
via a facile in situ precipitation method. In a typical synthesis of
1:12 g-C3 N4 /Ag2 CO3 , 0.005 mol of Na2 CO3 ·10H2 O were dissolved
in 20 mL deionized water. And then the above solution was added
drop-wise to 20 mL mixed solution of AgNO3 (0.012 mol) and gC3 N4 (0.001 mol). After stirring for 10 min, the precipitates were
collected and washed for several times with deionized water.
Finally, the products were dried in vacuum at 60 ◦ C for 10 h, and the
1:12 g-C3 N4 /Ag2 CO3 composite was obtained. Other C3 N4 /Ag2 CO3
samples (1:10 and 1:14) were prepared using the same method,
and the pure Ag2 CO3 was obtained without the addition of g-C3 N4 .
The active species trapping experiment process is similar to the
RhB photodegradation experiment. Before adding the photocatalyst, scavengers for hydroxyl radicals (• OH), superoxide radical
(• O2 − ) and holes (h+ ) were introduced into the MO solution. The
species and dosages of these scavengers were reported in the previous studies [22,26,27].
2.3. Characterization
3.1. Characterization of composite photocatalysts
X-ray diffraction (XRD) patterns of the as-prepared samples
were carried out on a Bruker D8 instrument using Cu-K␣ radiation
(40 kV/40 mA). Scanning electron microscopy (SEM) was performed on a field emission scanning electron microscope (Hitachi
S-4800) with an acceleration voltage of 10 kV. The UV–vis diffuse
reflectance spectra (DRS) of photocatalysts were recorded on a Cary
5000 (America Varian) UV-vis spectrometer. The surface properties
of the samples were analyzed by X-ray photoelectron spectroscopy
(XPS) on Thermo ESCALAB 250 (USA) recorded at room temperature in air. Photoluminescence spectra (PL) of the samples were
Fig. 1 shows XRD patterns of g-C3 N4 , Ag2 CO3 , and gC3 N4 /Ag2 CO3 composites with molar ratios of 1:10, 1:12 and 1:14.
The two peaks at 13.04◦ and 27.40◦ of g-C3 N4 correspond to the
(1 0 0) and (0 0 2) planes of the tetragonal phase g-C3 N4 (JCPDS 871526), respectively [27]. For Ag2 CO3 , the peaks in its XRD pattern
could be well indexed to the monoclinic phase of Ag2 CO3 (JCPDS
26-0339) [28]. The narrow and sharp peaks observed in the spectra
of Ag2 CO3 and g-C3 N4 /Ag2 CO3 composites suggest that they are all
of high crystalline. There are no obvious g-C3 N4 peaks that can be
detected in g-C3 N4 /Ag2 CO3 composites due to the low intensity of
2.5. Active species trapping experiment
3. Results and discussion
190
N. Tian et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 467 (2015) 188–194
Fig. 2. High resolution XPS spectra of 1:12 g-C3 N4 /Ag2 CO3 (a) Ag 4d, (b) C 1s, (c) O 1s and (d) N 1s.
the g-C3 N4 . And no impurity peaks were observed, which implies
that the final products of g-C3 N4 and Ag2 CO3 were of pure phases.
The XPS was employed to analyze the oxidation state and the
surface chemical composition of g-C3 N4 /Ag2 CO3 composites and to
further study the interaction between g-C3 N4 and Ag2 CO3 . The Ag
3d5/2 and Ag 3d3/2 peaks appear at the binding energies of 364.0 eV
and 368.0 eV, respectively (as shown in Fig. 2a). According to the
literature [29], the peaks of C 1s located at 288.7, 286.4 and 284.8 eV
are attributed to the sp2 hybridized C (C (N)3 ) as shown in Fig. 2b.
In Fig. 2c, the O 1s peak at 530.9 eV is ascribed to the O in Ag2 CO3
[30]. The N 1s XPS spectra of 1:12 g-C3 N4 /Ag2 CO3 nanocomposite
are shown in Fig. 2d. The broad peak can be fitted to three peaks
at 400.9 eV, 399.6 eV and 398.3 eV, suggesting three independent
environments for N in g-C3 N4 /Ag2 CO3 . The peaks at 399.6 eV and
400.9 eV are attributed to tertiary nitrogen (N (C3 )) groups and
the charging effects [31]. The peak at 398.3 eV, which is the main N
1s peak, can be assigned to the sp2 -hybridized nitrogen (C N C),
thus confirming the presence of sp2 -bonded g-C3 N4 [32]. According
to the results of XPS, the 1:12 g-C3 N4 /Ag2 CO3 composite has been
successfully prepared.
The size and morphology of the samples are observed by SEM
as shown in Fig. 3. In Fig. 3a, g-C3 N4 products were in agglomeration, but the stacking layers could be clearly seen, which was in line
with the literature report [11]. The particle size of as-synthesized
cuboid-like Ag2 CO3 with smooth surfaces was about 200–500 nm
in length (Fig. 3b). Fig. 3c revealed that the two types of materials were all found in the sample of 1:12 g-C3 N4 /Ag2 CO3 , and
the Ag2 CO3 nanocuboids are assembled on the surface of the gC3 N4 sheets. Obviously, the coexistence of Ag2 CO3 and g-C3 N4
does not change their respective morphology. This result further
demonstrated the successful synthesis of g-C3 N4 /Ag2 CO3 composite photocatalyst.
UV–vis DRS of the samples were shown in Fig. 4. The absorption
edge of monoclinic Ag2 CO3 locates at about 540 nm, whereas gC3 N4 exhibits an absorption edge at 460 nm. Compared to Ag2 CO3 ,
the g-C3 N4 /Ag2 CO3 composites all exhibited a mixed absorption
property of g-C3 N4 and Ag2 CO3 and red shift in the visible region.
These observations are attributed to the interaction between gC3 N4 and Ag2 CO3 in the composites. The covered spectral range was
increased with the increase of the g-C3 N4 content. As a result, the
combination between g-C3 N4 and Ag2 CO3 enhanced the absorption
in the visible light region, which may lead to a higher photocatalytic
activity of the g-C3 N4 /Ag2 CO3 composites. The band gap energy
of the prepared photocatalysts can be calculated by the formula:
˛hv = A (hv − Eg )n/2 . Due to the indirect-band-gap transition properties of the g-C3 N4 and Ag2 CO3 [33,34], the band gap values of
g-C3 N4 and Ag2 CO3 could be estimated by the plots of (˛h)1/2
versus photon energy (hv), which was calculated to be 2.70 and
2.30 eV, respectively. The conduction band (CB) and valence band
(VB) potential of g-C3 N4 and Ag2 CO3 can be calculated by the following equations [22]:
EVB = X − Ee + 0.5Eg
(1)
ECB = EVB − Eg
(2)
where EVB is the VB edge potential; X is the electronegativity of
the semiconductor, which is the geometric average of the absolute
electronegativity of the constituent atoms, and X values of g-C3 N4
and Ag2 CO3 are 4.67 eV and 6.13 eV, respectively. Ee is the energy of
free electrons on the hydrogen scale (Ee = 4.5 eV) [24]; and Eg is the
band gap energy of the semiconductor. The band gap energies of gC3 N4 and Ag2 CO3 are adopted as 2.70 eV and 2.30 eV, respectively.
Calculated by Eqs. (1) and (2), the positions of the CB and VB of gC3 N4 are −1.13 and 1.57 eV, while those values for Ag2 CO3 are 0.37
and 2.67 eV, respectively.
3.2. Photocatalytic activity
To evaluate the photocatalytic properties of pure g-C3 N4 ,
Ag2 CO3 and g-C3 N4 /Ag2 CO3 , methyl orange (MO) serving as a
model dye was degraded over these photocatalysts under visible
light irradiation ( > 420 nm) at room temperature. Fig. 5a shows
the degradation curves of MO solution over g-C3 N4 /Ag2 CO3 . It
can be found that, for 1:12 g-C3 N4 /Ag2 CO3 , approximately 81%
N. Tian et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 467 (2015) 188–194
191
Fig. 4. UV–vis diffuse reflectance spectra of g-C3 N4 , Ag2 CO3 and g-C3 N4 /Ag2 CO3
photocatalysts with molar ratios of 1:10, 1:12 and 1:14.
the decrease of the characteristic absorption band of MO at 464 nm
can be obviously observed.
3.3. Mechanism investigation on the enhanced photocatalytic
activity
Fig. 3. SEM images of g-C3 N4 , Ag2 CO3 and g-C3 N4 /Ag2 CO3 photocatalysts with
molar ratios of 1:10, 1:12 and 1:14.
MO dye was removed within 60 min. Whereas, only 19% and
31% MO were removed in the same time period in the case of
pure g-C3 N4 and Ag2 CO3 , respectively. It indicates that the gC3 N4 /Ag2 CO3 composites exhibited much higher photocatalytic
activity than the two single components. Fig. 5b presents the
degradation rates of MO over g-C3 N4 , Ag2 CO3 and g-C3 N4 /Ag2 CO3
photocatalysts. The 1:12 g-C3 N4 /Ag2 CO3 photocatalyst displayed
the highest apparent rate constant 0.024 min−1 , which is about
8 and 3.7 times as high as those of g-C3 N4 (k = 0.002 min−1 )
and Ag2 CO3 (k = 0.0063 min−1 ), respectively. These results clearly
demonstrate that the g-C3 N4 /Ag2 CO3 photocatalyst system can significantly enhance their photocatalytic activity. Fig. 5c shows the
time-dependent absorption spectra of MO solution in the presence
of 1:12 g-C3 N4 /Ag2 CO3 heterostructured sample. From this figure,
PL spectrum analysis was carried out to investigate the
separation efficiency of photogenerated electrons and holes in gC3 N4 /Ag2 CO3 photocatalyst. Fig. 6 shows the PL spectra of the
pure Ag2 CO3 , pure g-C3 N4 and g-C3 N4 /Ag2 CO3 composite. The
main emission peak was centered at about 460 nm for the pure
g-C3 N4 sample, which was similar to the literatures [29–32]. The
g-C3 N4 /Ag2 CO3 composite photocatalysts exhibit the emission
peaks locating at almost the same position with the pure Ag2 CO3 ;
but decreased emission intensities, which suggested that the gC3 N4 /Ag2 CO3 composites had much lower recombination rate of
photogenerated charge carriers. According to the above results,
the recombination of photogenerated charge carriers was greatly
inhibited in the heterostructured g-C3 N4 /Ag2 CO3 , demonstrating
that the g-C3 N4 /Ag2 CO3 photocatalyst composites possess higher
separation efficiency than the pure Ag2 CO3 and g-C3 N4 .
It is well known that the photocatalytic activity was determined by the band gap, separation efficiency of photogenerated
electrons and holes, and oxidation potential of photogenerated
holes [35]. And the crucial factor for photocatalytic activity was the
interface charge separation efficiency of photogenerated charges.
In order to find the migration path of the photogenerated electrons and holes, and reveal the photocatalytic mechanism of the
greatly improved photocatalytic activities of g-C3 N4 /Ag2 CO3 composites, the active pieces trapping experiments were conducted.
Some sacrificial agents, such as iso-propyl alcohol (IPA), disodium
ethylenediaminetetraacetate (EDTA) and 1,4-benzoquinone (BQ)
were used as the scavengers of hydroxyl radical (• OH), hole (h+ )
and superoxide radical (• O2 − ), respectively. Fig. 7 shows that the
photodegradation efficiency of MO over 1:12 g-C3 N4 /Ag2 CO3 is not
affected by the addition of BQ, indicating that almost no superoxide radicals (• O2 − ) were involved in the degradation of MO. When
IPA was added, the photodegradation efficiency of MO obviously
decreased, which revealed that the existence of • OH radical species.
Besides, the photocatalytic activity was thoroughly suppressed by
addition of EDTA, suggesting that the h+ pathways play a crucial role
in the process of MO oxidation. These results indicated that the h+
and • OH were the main active species in the MO photodegradation
process.
192
N. Tian et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 467 (2015) 188–194
Fig. 6. Photoluminescence spectra of g-C3 N4 , Ag2 CO3 , and g-C3 N4 /Ag2 CO3 photocatalysts with molar ratios of 1:10, 1:12 and 1:14.
Fig. 7. Photocatalytic degradation of MO over 1:12 g-C3 N4 /Ag2 CO3 photocatalyst
alone and with the addition of IPA, EDTA and BQ.
Fig. 5. (a) Photocatalytic degradation curves of MO over as-prepared samples under
the irradiation of visible-light ( > 420 nm); (b) UV–visible spectra of MO at different
visible irradiation times in the presence of 1:12 g-C3 N4 /Ag2 CO3 ; (c) Apparent rate
constants for the photodegradation of MO over g-C3 N4 /Ag2 CO3 under the irradiation
of visible-light ( > 420 nm).
To further reveal the mechanism of the interaction between
g-C3 N4 and Ag2 CO3 , the photocurrent responses of the 1:12 gC3 N4 /Ag2 CO3 and pure g-C3 N4 and Ag2 CO3 photocatalyst film
electrodes were measured and shown in the following Fig. 8. It
can be seen from Fig. 8 that prompt photocurrent responses were
observed over all the three electrodes as the light was turned
on. In contrast to the cases of pristine samples, the photocurrent
density generated by the 1:12 g-C3 N4 /Ag2 CO3 photocatalyst is
obviously improved, and the intensity is about 3.5 and 2.1 times
as those of pure g-C3 N4 and Ag2 CO3 , respectively. It is believed
that the stronger photocurrent intensity reveals the higher electrons and holes separation efficiency. The results disclosed that
the 1:12 g-C3 N4 /Ag2 CO3 composites have higher separation rate
of electron–hole pairs than pristine g-C3 N4 and Ag2 CO3 .
Fig. 8. Comparison of transient photocurrent responses of the g-C3 N4 , Ag2 CO3 and
1:12 g-C3 N4 /Ag2 CO3 .
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193
References
Fig. 9. Band structure diagram and electron–hole separation of g-C3 N4 /Ag2 CO3
composite.
Based on the above analysis, the possible mechanism for the
photocatalytic activity enhancement of g-C3 N4 /Ag2 CO3 composites was proposed in Fig. 9. Under visible-light-irradiation, the
electron–hole pairs in the two individual semiconductors were
separated; then, the photogenerated electrons at the VB were
excited to transfer to the CB, leaving the holes in the VB of gC3 N4 and Ag2 CO3 . Because of the more negative potential, the
electrons in the CB of g-C3 N4 were injected to that of Ag2 CO3 ,
and subsequently transferred to the surface of the solid photocatalyst. Meanwhile, the holes (h+ ) in the VB of Ag2 CO3 would
migrate to that of g-C3 N4 owing to the strong interfacial interaction. Besides, Due to the more positive VB level of Ag2 CO3
than that of • OH/OH− , some remained h+ could be transformed
into • OH by oxidizing OH− that absorbed on the surface of
the photocatalysts. Thus, the h+ and • OH were the two active
species and play the crucial roles for photodecomposition of
MO.
4. Conclusions
In this work, we successfully synthesized g-C3 N4 /Ag2 CO3
composite photocatalysts by a simple in situ precipitation
method. The photocatalytic experiments demonstrated that the
g-C3 N4 /Ag2 CO3 composites exhibit the much better photocatalytic performance than the individuals in degradation of MO
under visible light irradiation ( > 420 nm). The enhanced photocatalytic activity should be attributed to the efficient separation
of photoinduced charge carriers resulted from effective fabrication of g-C3 N4 /Ag2 CO3 heterojunction, as confirmed by the PL
spectra. The radicals trapping experiment demonstrated that h+
and • OH play critical roles in the photocatalytic process. Our
results indicated that g-C3 N4 /Ag2 CO3 is a very efficient composite photocatalyst to remove organic pollutants under visible
light irradiation. It will be a generalized strategy to design
new hybrid heterostructured photocatalysts with high performance.
Acknowledgements
This work was supported by the National Natural Science
Foundations of China (Grant No. 51302251), the Fundamental Research Funds for the Central Universities (2652013052),
the special co-construction project of Beijing city education
committee, Key Project of Chinese Ministry of Education (No.
107023).
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