Journal of Colloid and Interface Science 417 (2014) 115–120
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Highly efficient heterojunction photocatalyst based on nanoporous g-C3N4 sheets modified by Ag3PO4 nanoparticles: Synthesis and enhanced photocatalytic activity Deli Jiang, Jianjun Zhu, Min Chen, Jimin Xie ⇑ School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China
a r t i c l e
i n f o
Article history: Received 22 July 2013 Accepted 15 November 2013 Available online 23 November 2013 Keywords: Nanoporous g-C3N4 Ag3PO4 nanoparticles Photocatalysis Rhodamine B Degradation
a b s t r a c t Novel visible-light-driven heterojunction photocatalyst composed by Ag3PO4 nanoparticles and nanoporous graphitic carbon nitride sheets (Ag3PO4/p-g-C3N4) was synthesized by a facile and green method. The results showed that photocatalytic activity of Ag3PO4/p-g-C3N4 was much higher than that of pure p-g-C3N4 in the photodegradation of Rhodamine B under visible light irradiation. The kinetic constant of Rhodamine B degradation over Ag3PO4 (33.3 mol%)/p-g-C3N4 was about 5 and 2 times higher than that over pure p-g-C3N4 and Ag3PO4, respectively. The enhanced photocatalytic performance is attributed to the stronger visible light absorption and the heterojunction between Ag3PO4 nanoparticles and p-g-C3N4, which could induce the low recombination rate of photoinduced electron–hole pairs. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction In recent years, the photocatalytic technology through semiconductors for the degradation of the organic pollutants in wastewater and air has been extensively studied [1,2]. However, the semiconductor materials available to date are generally limited by either poor photocatalytic efficiency in the visible light range or insufficient charge separation ability. To address this issue, much effort in recent years has been focused on the exploration and fabrication of novel semiconductor catalysts to improve photocatalytic properties [3,4]. Recently, graphite-like carbon nitride (g-C3N4) has been explored as a promising candidate for hydrogen evolution and environment purification under visible light irradiation [5]. However, there are still some inherent drawbacks existed in the g-C3N4 photocatalysis, such as the high recombination rate of its photogenerated electron–hole pairs, pure g-C3N4 adsorbs only blue light up to 450 nm, and low specific surface area [6–8]. To address these problems, several strategies, such as nanostructuring [9,10], doping [11–13], and copolymerization [14,15], have been exploited to improve the photocatalytic activity of g-C3N4. Among these methods, constructing heterojunction between g-C3N4 and other materials, such as deposited noble metals [16], semiconductor (e.g. TiO2, ZnO, CdS, Bi2WO6, SmVO4, BiOBr, AgX (Br, I)) [17–23], P [23], or graphene [24] could improve photocatalytic efficiency of g-C3N4. ⇑ Corresponding author. Fax: +86 511 88791800. E-mail addresses: [email protected], [email protected] (J. Xie). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.11.042
It has been recently found that the photocatalytic efficiency of g-C3N4 was strongly dependent on its porosity and surface area [25,26]. Dong et al. reported that the porousification could enhance the photooxidation ability of g-C3N4 by providing more photogenerated holes and active sites [27]. Chen and co-worker also found that the porous g-C3N4 exhibited much higher photocatalytic efficiency than the pristine g-C3N4 [28]. Han et al. synthesized porous g-C3N4 without any template according to Le Chatelier’s principle, and the resulting porous g-C3N4 material with high BET surface area (201–209 m2 g1) shows a ten times enhanced photocatalytic activity for methyl orange photodegradation under visible light irradiation [29]. Recently, our group found that porous g-C3N4-based BiOI/g-C3N4 heterostructures exhibited enhanced visible-light-driven photocatalytic activity in the degradation of methylene blue in aqueous solution [30]. Although porous g-C3N4 (donated as p-g-C3N4) have been found to show enhanced photocatalytic activity compared to bulk ones, few studies have been focused on the synthesis of p-g-C3N4-based heterostructured photocatalyst. Currently, it has been reported that Ag3PO4 has extremely high photooxidative capabilities for both water splitting and degradation organic dyes under visible light irradiation [31–34]. A number of Ag3PO4-based composite photocatalysts were also intensively studied to further enhance the photocatalytic efficiency of Ag3PO4 [35–38]. Considering the excellent photooxidation activity of Ag3PO4 and large specific surface of the nanoporous g-C3N4, here we for the first time report the facile synthesis of novel Ag3 PO4 nanoparticles/p-g-C3N 4 (named as Ag3 PO4 /p-g-C3 N4 )
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heterostructured photocatalysts. The resultant Ag3PO4/p-g-C3N4 photocatalyst shows significantly higher photocatalytic activity toward the degradation of Rhodamine B under visible light irradiation than the pure p-g-C3N4. This work may provide some insight into solving the unsatisfactory catalytic activity and low efficiency covering solar radiation for practical applications of g-C3N4.
and centrifuged to remove the particles. The filtrates were analyzed by a Shimadzu UV-2450 spectrophotometer and the characteristic absorption peak of RhB at 554 nm was used to determine the extent of its degradation.
2. Experimental section
The examination experiment process of reactive oxygen species is similar to the photodegradation experiment. Different quantity of scavengers was added into the RhB solution prior to addition of the catalyst. The dosages of these scavengers were referred to the previous studies [18,39].
2.1. Synthesis of the photocatalyst All chemicals were reagent grade and used without further purification. Porous graphitic carbon nitride (p-g-C3N4) was synthesized by thermal treatment of 10 g of urea in a crucible with a cover under ambient pressure in air, similar with reported methods [27,28]. After dried at 80 °C for 24 h, the precursor was heated to 550 °C at a heating rate of 2.3 °C/min in a tube furnace for 4 h in air. The resulted final light yellow powder were washed with nitric acid (0.1 mol/L) and distilled water to remove any residual alkaline species (e.g. ammonia) adsorbed on the sample surface, and then dried at 80 °C for 24 h. Ag3PO4/p-g-C3N4 composite photocatalysts were prepared by in situ growth of Ag3PO4 on g-C3N4 surface. Typically, 0.092 g of g-C3N4 powders and different amounts of AgNO3 were dissolved in 200 mL of deionized water; after ultrasonication for 20 min, a certain amount of Na2HPO412H2O was dropped into the solution under vigorous stirring. After stirring for 4 h, the product was collected by centrifugation, washed with distilled water, and dried in an oven at 60 °C for 24 h. According to this method, different molar ratios of the Ag3PO4 to p-g-C3N4 samples were obtained and labeled as 20% Ag3 PO4 /p-g-C 3N4 , 25% Ag3PO 4/p-g-C3N 4, 33% Ag3PO4/p-g-C3N4, and 50% Ag3PO4/p-g-C3N4, respectively. 2.2. Characterization The crystal structure of the samples was investigated using Xray diffraction (XRD; Bruker D8 Advance X-ray diffractometer) with Cu Ka radiation. The acceleration voltage and the applied current were 40 kV and 40 mA, respectively. The morphology of the samples was examined by transmission electron microscopy (TEM; FEI JEM-2100 and FEI Tecnai G2 F20) operated at 200 kV and scanning electron micrograph (SEM) using a field emission SEM (FESEM) instrument (Hitachi S-4800 II, Japan). The Brunauer–Emmett–Teller (BET) specific surface area and pore size distribution of the samples were characterized by nitrogen adsorption with TristarII3020 instrument. UV–vis diffuse reflection spectroscopy (DRS) was performed on a Shimadzu UV-3100 spectrophotometer using BaSO4 as the reference. Infrared spectra were obtained on KBr pellets on an Equinox 55 spectrometer (Bruker) in the range of 4000–500 cm1. The photoluminescence (PL) spectra of the photocatalyst were obtained by a Varian Cary Eclipse spectrometer with an excitation wavelength of 325 nm. 2.3. Photocatalytic activity study The photocatalytic activity of the Ag3PO4/p-g-C3N4 photocatalysts was tested in the degradation reaction of RhB aqueous solution (10 mg/L) under irradiation of a 300 W tungsten light lamp. For a typical photocatalytic experiment, 0.05 g of catalyst powder was added into 50 mL of the above RhB solution in quartz tube. Prior to irradiation, the suspensions were magnetically stirred in dark for 30 min to ensure the establishment of an adsorption/ desorption equilibrium. The above suspensions were kept under constant air-equilibrated conditions before and during the irradiation. At given time intervals, about 4 mL of aliquots were sampled
2.4. Detection of reactive oxygen species
3. Results and discussion 3.1. Morphology and structure In the present work, the p-g-C3N4 sheets were fabricated facilely by one-step polymerization of low cost and environmentally benign urea. The SEM image in Fig. S1a shows that the as-prepared pure p-g-C3N4 is consisted of thin sheet-like with wrinkles and irregular shape. The TEM image (Fig. S1b) shows that a number of mesopores of several tens of nanometres in size can be observed in the p-g-C3N4 sheets, which was highly analogous to previously reported result [27]. The digital image in the inset of Fig. 1a clearly shows the large-scale synthesis of p-g-C3N4 can be achieved by via the current method. Fig. 1shows the TEM images of the as-prepared Ag3PO4/ p-g-C3N4 photocatalysts. It can be found that the size of supported Ag3PO4 nanoparticles can be controlled by changing the initial concentration of AgNO3 employed, which was distinctive from the result reported by Kumar et al [40]. The TEM image of 20% Ag3PO4/p-g-C3N4 reveals that the Ag3PO4 nanoparticles with very small size of around 5 nm are uniformly deposited on the p-g-C3N4 surface (Fig. 1a), revealing the successful formation of Ag3PO4/p-g-C3N4 composite. The defined interconnection (or heterojunction) between Ag3PO4 nanoparticles and p-g-C3N4 is believed to favor the transfer of photogenerated electrons from p-g-C3N4 to Ag3PO4, thus enhancing the charge separation and photocatalytic efficiency. TEM image taken on the 25% Ag3PO4/p-g-C3N4 sample indicates that the size of Ag3PO4 nanoparticles increases greatly and reaches approximately 10 nm (Fig. 1b). Further increasing the AgNO3 precursor resulted in a gradual increase in the size of Ag3PO4 nanoparticles. As indicated in Fig. 1c, the p-g-C3N4 sheets are densely decorated with a large amount of rather large Ag3PO4 nanoparticles with a wide size distribution ranging from 10 to 40 nm. However, the TEM image of 50% Ag3PO4/ p-g-C3N4 displays that the sample is consisted by p-g-C3N4 and isolated Ag3PO4 nanoparticle aggregates (Fig. 1d). The average size of the individual Ag3PO4 nanoparticle increases significantly and reaches near 200 nm. It is expected that there is no strong interfacial charge transfer taking place between p-g-C3N4 and isolated Ag3PO4 aggregates, and as a consequence 50% Ag3PO4/p-g-C3N4 will show relatively poor photocatalytic activity in comparison with Ag3PO4/p-g-C3N4 sample with low Ag3PO4 content. Fig. 2 shows the XRD patterns of as-prepared samples. As shown in Fig. 2a, the p-g-C3N4 sample reveals two distinct diffraction peaks at 27.40° and 13.04°, which can be indexed to the (0 0 2) and (1 0 0) diffraction planes of the graphite-like carbon nitride and correspond to the characteristic interplanar staking peaks of aromatic systems and the interlayer structural packing, respectively. These two peaks decreased in intensity gradually with the increase in introduced Ag3PO4 and finally disappeared in the Ag3PO4/p-gC3N4 (50%) sample, whereas the diffraction peaks of Ag3PO4 intensified gradually (Fig. 2b–e). Fig. 2f shows that all the diffraction
D. Jiang et al. / Journal of Colloid and Interface Science 417 (2014) 115–120
a
b
50 nm
50 nm
c
d
50 nm
200 nm
117
•
(a)
and 549 cm1, which are all ascribed to PAO stretching vibrations of PO 4 [41]. This result indicates that a small amount of Ag3PO4 nanoparticles were formed in the resultant samples.
p-g-C3N4
(b)
•
20% Ag3PO4/p-g-C3N4
(c)
•
25% Ag3PO4/p-g-C3N4
(d)
33% Ag3PO4/p-g-C3N4
(e)
50% Ag3PO4/p-g-C3N4
3.2. N2-Sorption studies
210
Intensity (a.u.)
•
002
100
Fig. 1. TEM images of 20% Ag3PO4/p-g-C3N4 (a), 25% Ag3PO4/p-g-C3N4 (b), 33% Ag3PO4/p-g-C3N4 (c), and 50% Ag3PO4/p-g-C3N4 (d).
10
20
30
40
50
60
420 421
400
222 320 321
310
220
211
200
(f)
110
Ag3PO4
70
80
2 Theta (degree) Fig. 2. XRD patterns for the as-prepared samples: (a) pure p-g-C3N4, (b) 20% Ag3PO4/p-g-C3N4, (c) 25% Ag3PO4/p-g-C3N4, (d) 33% Ag3PO4/p-g-C3N4, (e) 50% Ag3PO4/p-g-C3N4, and (f) pure Ag3PO4.
peaks of Ag3PO4 sample could be well indexed to the body-centered cubic structure of Ag3PO4 (JCPDS No. 06-0505). Thus, the Ag3PO4/p-g-C3N4 composite samples exhibit diffraction peaks corresponding to both p-g-C3N4 and Ag3PO4, reflecting the presence of two phases. In addition, it should be noted that no diffraction peak ascribed to Ag nanoparticles can be observed in the XRD pattern of all the composite samples, and it imply in a sense that the current composite system could prevent the reduction of Ag3PO4 into metallic Ag nanoparticles. To further confirm the formation of Ag3PO4/p-g-C3N4 composites, the FT-IR spectra of p-g-C3N4, and a series of Ag3PO4/p-gC3N4 photocatalysts with various Ag3PO4 contents were shown in the Fig. S2. In the FT-IR spectrum of p-g-C3N4, the peaks at 1640 cm1 were attributable to C@N stretching vibration modes, while the 1242, 1320 and 1403 cm1 to aromatic CAN stretching vibration modes. The peak at 808 cm1 was related to characteristic breathing mode of triazine units. The Ag3PO4/p-g-C3N4 composite sample has a similar spectrum but two new peaks at 1014 cm1
To know porous nature and specific surface area of p-g-C3N4 and Ag3PO4/p-g-C3N4 composites, a nitrogen adsorption–desorption isotherm is measured. Fig. 3a shows the isotherm for different samples, which exhibit a type IV with a H3 hysteresis loop according to the IUPAC classification, reflecting the presence of a mesoporous structure of the composites. It can be found that the synthesized p-g-C3N4 shows a relatively large specific surface area (23.19 m2/g). Compared with p-g-C3N4, the Ag3PO4/p-g-C3N4 composites exhibit decreased BET surface areas due to the incorporation of Ag3PO4 which has very low BET surface area (0.21 m2/g). We think low BET specific surface areas for the 25% Ag3PO4/p-gC3N4 and 50% Ag3PO4/p-g-C3N4 samples are may be related to the somewhat p-g-C3N4 particle aggregation. The pore-size distribution of the samples are also estimated using the Barrett–Joyner–Halenda (BJH) method from the desorption branch of the isotherm, as shown in Fig. 3b. The calculated pore size distribution using BJH method indicated that the size of mesopores is not uniform ranging from 8 to 50 nm, which is consistent with the TEM observation. The specific surface area and pore size distribution of different samples are summarized in Table 1. It is interesting found that the pore size of the samples seems to increase with the increase in Ag3PO4 loading content. This increase trend may be related to the formation of new nanostructures (such as Ag3PO4 nanoparticles aggregation) with an even larger pore size (see Fig. 1c and d), which results in an enhanced (total) pore size in the sample. 3.3. UV–vis diffuse reflectance spectra and band gap energy The optical properties of as-prepared Ag3PO4/p-g-C3N4 composites, pure Ag3PO4, and g-C3N4 samples were examined using DRS
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a
a p-g-C3N4 25% Ag3PO4/p-g-C3N4 33% Ag3PO4/p-g-C3N4
30
1.2
p-g-C3N4 20% Ag3PO4/p-g-C3N4
1.0
20% Ag3PO4/p-g-C3N4
40
Absorbance (a.u.)
Vol Adsorbed (cm3 STP g-1)
50
50% Ag3PO4/p-g-C3N4 20
10
25% Ag3PO4/p-g-C3N4 33% Ag3PO4/p-g-C3N4
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50% Ag3PO4/p-g-C3N4 Ag3PO4
0.6 0.4 0.2 0.0 200
0 0.0
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Relative Pressure (p/p0)
b
b
0.0040 0.0035
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2.5
0.0030
p-g-C3N4
0.0025
20% Ag3PO4/p-g-C3N4
1/2
2.0
F(R) (hv)
dV/dD (cm3g-1nm-1)
500
Wavelength (nm)
25% Ag3PO4/p-g-C3N4
0.0020
33% Ag3PO4/p-g-C3N4 50% Ag3PO4/p-g-C3N4
0.0015
1.5 p-g-C3N4 20% Ag3PO4/p-g-C3N4
1.0
25% Ag3PO4/p-g-C3N4 33% Ag3PO4/p-g-C3N4
0.0010
0.5
50% Ag3PO4/p-g-C3N4 Ag3PO4
0.0005 0.0
0.0000
2
0
50
100
150
Pore Diameter (nm) Fig. 3. (a) Nitrogen sorption isotherm and (b) Barrett–Joyner–Halenda (BJH) pore size distribution plot (inset) of the as-prepared samples.
Table 1 Parameters obtained from N2 desorption isotherm measurements. Samples
Surface area (m2 g1)
Pore volume (cm3 g1)
Average pore size (nm)
p-g-C3N4 20% Ag3PO4/p-g-C3N4 25% Ag3PO4/p-g-C3N4 33% Ag3PO4/p-g-C3N4 50% Ag3PO4/p-g-C3N4 Ag3PO4
23.19 20.84 6.31 12.84 5.30 0.21
0.079 0.083 0.024 0.069 0.024 /
13.64 15.87 15.79 21.52 18.49 /
4
5
6
Fig. 4. (a) UV–vis DRS and plots of (a h m)1/2 vs hm of pure p-g-C3N4, Ag3PO4 and (b) as-prepared Ag3PO4/p-g-C3N4 samples.
Among them, n is determined from the type of optical transition of a semiconductor (n = 1 for direct transition and n = 4 for indirect transition). Thus, as shown in Fig. 4b, Eg of Ag3PO4 and p-g-C3N4 was determined to be 2.45 eV and 2.70 eV, respectively, according to a plot of (a h m)2 versus energy (hm). The Eg of Ag3PO4/p-g-C3N4 composite is less than that of p-g-C3N4, indicating that the Ag3PO4 can be acted as an efficient visible-light sensitizer for the p-g-C3N4. 3.4. Photocatalytic activity
technique (Fig. 4a). As expected, the bare Ag3PO4 could absorb solar energy with a wavelength shorter than 530 nm, while the g-C3N4 holds an onset of absorption at 460 nm. In Ag3PO4/ p-g-C3N4 composite sample, the absorption toward the visible light region is remarkably enhanced compared to pure p-g-C3N4 particles. Moreover, the absorption intensity of the Ag3PO4/p-g-C3N4 composites increases with increasing amounts of Ag3PO4, which should be attributed to the increase in the Ag3PO4 concentration. Thus, it is clear that all these Ag3PO4/p-g-C3N4 samples are responsive to visible light, implying that they are able to work with visible light. The energy level and band gap of the semiconductors play a crucial role in determining its physical properties. The band gap energy of a semiconductor can be calculated by the following formula [42]:
ahm ¼ Aðhm Eg Þn=2
3
hv (eV)
200
ð1Þ
where a, h, m, Eg and A are absorption coefficient, Planck constant, light frequency, band gap energy, and a constant, respectively.
To compare the photocatalytic activity of pure p-g-C3N4 and Ag3PO4/p-g-C3N4 over different reaction times, a series of photodegradation experiments were carried out by using RhB as a model pollutant under visible light irradiation. Fig. 5 shows the variation in concentration of RhB (C/C0) with irradiation time over different photocatalysts, where C0 is the initial concentration of RhB and C is its concentration at time t. The blank test confirms that RhB is only slightly degraded in the absence of catalysts, indicating that the photolysis can be negligible. Pure p-g-C3N4 exhibited a weak photocatalytic activity that could degrade RhB by only 60.2% after 1 h of visible light irradiation, while pure Ag3PO4 have a moderate photocatalytic activity that could degrade RhB by only 84.6% after the same period of time. Decorating with Ag3PO4 nanoparticles, all the Ag3PO4/p-g-C3N4 composite photocatalysts show enhanced abilities for the degradation of RhB in comparison with the pure p-g-C3N4 and even Ag3PO4. Furthermore, the Ag3PO4 NPs content is pivotal for optimal photocatalytic activity. When the Ag3PO4 NPs content is increased beyond 33.3 mol%, a decrease in the photocatalytic activity was observed. The origin of this effect can be explained as follows: the suitable Ag3PO4 nanoparticles content causes their good dispersion on the g-C3N4 surface, which favors
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1.0 0.9
RhB self-degradation p-g-C3N4
0.8
Ag3PO4
C/C0
0.7
20% Ag3PO4/p-g-C3N4
0.6
25% Ag3PO4/p-g-C3N4
0.5
33% Ag3PO4/p-g-C3N4 50% Ag3PO4/p-g-C3N4
0.4
EVB ¼ X Ee þ 0:5Eg
0.3 0.2 0.1 0.0 0
10
20
30
40
50
60
70
80
90
100
Time (min) Fig. 5. Photocatalytic degradation of RhB as a function of irradiation time over pure p-g-C3N4, Ag3PO4 and Ag3PO4/p-g-C3N4 samples.
the transfer and separation of the charge carriers. However, at a higher Ag3PO4 nanoparticles content, there is no strong interfacial charge transfer taking place between p-g-C3N4 and isolated Ag3PO4 aggregates, leading to a relative low charge separation efficiency, which as a consequence will decrease the photocatalytic activity. The decomposition processes have been modeled as a pseudo first order reaction with the kinetics expressed by the equation ln(C0/C) = kt, where k is the reaction rate constant. The pseudofirst-order rate constants (k) reflect the rate of degradation of RhB over the photocatalysts under visible-light irradiation, as displayed in Fig. 6. It can be clearly seen that the apparent rate constants (k, min1), determined from the slopes of ln (C0/C) versus irradiation time are 0.0149 min1 for pure p-g-C3N4. By combing with Ag3PO4 nanoparticles, Ag3PO4/p-g-C3N4 composites apparently exhibit higher photocatalytic activities compared with pure p-g-C3N4. In particular, the sample with 33.3 mol% Ag3PO4 shows the highest catalytic activity with a k of 0.0739 min1, about 5 and 2 times higher than pure p-g-C3N4 and Ag3PO4, respectively. 3.5. Plausible photocatalytic mechanism of Ag3PO4/p-g-C3N4 Basically, the fundamental process of semiconductor photocatalysis involves electron and hole pairs after light irradiation. The electron and hole pairs could migrate to surfaces of the semiconductor to react with the adsorbed O2 and H2O, respectively, in pro ducing the redox sources of O 2 and OH radical species, which lead
7
ð2Þ
where EVB is the valence band edge potential, X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms, and the value of X for Ag3PO4 is ca. 5.96 eV, Ee is the energy of free electrons on the hydrogen scale (about 4.5 eV), Eg is the band gap energy of the semiconductor. Based on the band gap positions, the CB and VB edge potentials of p-g-C3N4 are determined at 1.13 and 1.57 eV, respectively. The CB and VB edge potentials of Ag3PO4 are determined at 0.24 and 2.69 eV, respectively. The CB and VB edge potentials of p-g-C3N4 were more negative than that of Ag3PO4. Therefore, such difference between the CB potentials of p-g-C3N4 and Ag3PO4 allowed the electron transfer from the CB of p-g-C3N4 to that of Ag3PO4. As shown in Fig. 7, when the Ag3PO4/p-g-C3N4 system is irradiated with visible light, both Ag3PO4 and p-g-C3N4 can be activated. The excited electrons at the conduction band of p-g-C3N4 are transferred to the conduction band of coupled Ag3PO4 crystallites. At the same time, simultaneous holes on the VB of Ag3PO4 could migrate to that of p-g-C3N4 because of the enjoined electric field. Therefore, the recombination process of the electron–hole is inhibited, leading to an enhanced photoactivity of the Ag3PO4/p-g-C3N4 composite. Photoluminescence (PL) spectra originate from the migration, transfer, and separation efficiency of the photogenerated charge carriers in a semiconducting material. There is a strong correlation between PL intensity and the photocatalytic performances. In general, higher PL intensity indicates the higher recombination of the charge carriers. Fig. S3 shows the PL spectra of the p-g-C3N4 and 33% Ag3PO4/p-g-C3N4 samples. We can see that after the formation of Ag3PO4/p-g-C3N4 composite, the PL intensity of the composite decreased rapidly, demonstrating the efficient charge separation of the composite. Therefore, it is reasonable to conclude that the enhanced photocatalytic activity could be attributed to the effective interfacial charge transfer between Ag3PO4 and p-g-C3N4. To further understand the nature of the primary active species involved in visible light degradation of RhB over Ag3PO4/p-g-C3N4 composite photocatalysts, we have carried out the control
-1
RhB self-degradation, k=0.0011 min -1 p-g-C3N4, k=0.0149min
6
NHE (eV)
-1
-1.5
Ag3PO4, k=0.0340 min 5
-1
20% Ag3PO4/p-g-C3N4, k=0.0433 min
-1
ln (C0/C)
to the efficient destruction of organic pollutants. In order to fully understand photocatalytic mechanism for the as-prepared Ag3PO4/ p-g-C3N4, the band edge positions of the valence band and conduction band of Ag3PO4 and p-g-C3N4, should be determined because they are also strongly related to the process of photocatalytic oxidation of organic compounds. The valence band potentials of Ag3PO4 and p-g-C3N4, were estimated in this study according to the following empirical equation [43]:
-0.5
25% Ag3PO4/p-g-C3N4, k=0.0512 min
4
RhB
-1
33% Ag3PO4/p-g-C3N4, k=0.0739 min
-1
3
50% Ag3PO4/p-g-C3N4, k=0.0583min
0.5
2
O2 O2•–
Visible light
RhB
Eg=2.45 eV
1.5
1
Eg=2.70 eV
Degradation product
Visible light
p-g-C3N4
2.5
Degradation product
0 0
10
20
30
40
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60
Time (min) Fig. 6. Rhodamine B degradation curves of ln(C0/C) versus time for pure p-g-C3N4, Ag3PO4 and Ag3PO4/p-g-C3N4 samples, and reaction rate constant, k, obtained from linear fitting.
3.5
Ag3PO4
Fig. 7. Schematic diagram of electron–hole pairs separation and the possible reaction mechanism over Ag3PO4/p-g-C3N4 photocatalyst under visible light irradiation.
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experiments with addition of scavenger for holes (h+) and OH radical species. As shown in Fig. S4, with 0.1 g of ammonium oxalate (AO) as a hole scavenger added to the solution, the rate for degradation of RhB over 33% Ag3PO4/p-g-C3N4 sample is remarkably decreased. When 2 mL of tertbutyl alcohol (TBA), as a scavenger for OH radical species, is added, no obvious decrease in degradation rate is observed, indicating the absence of OH radical species. Meanwhile, with the addition of 1 mmol of O 2 scavenger BQ, the degradation of RhB was slightly depressed, suggesting the existence of O 2 only with very low concentration in the reaction. These results suggest that, the holes play a dominant role toward the degradation of RhB over 33% Ag3PO4/p-g-C3N4 under visible light irradiation. 3.6. Stability of the photocatalyst The stability of Ag3PO4/p-g-C3N4 was tested for the degradation of RhB dye under identical reaction conditions. After complete degradation, the catalyst was separated and washed with a large amount of deionized water. The recovered catalyst was dried in an oven at 80 °C for 4 h and used for the next run. As shown in Fig. S5, the experimental results about RhB dye removals demonstrated that after 5 times, the RhB dye removals were maintained between 90% and 80% and that no significant drop was found. It indicates that the photocatalytic activity of our Ag3PO4/p-g-C3N4 was maintained well. We supposed that the little mass loss in the progress of recovery is the main reason for the slight decrease in photocatalytic activity of reused photocatalysts in five repeated cycles. 4. Conclusions In summary, novel visible light-driven Ag3PO4/p-g-C3N4 heterojunction photocatalysts were synthesized by in situ precipitation of Ag3PO4 nanoparticles on the surface of p-g-C3N4 sheets. The heterojunction photocatalysts showed obviously superior photocatalytic activity for the photodegradation of RhB under the irradiation of visible light. The kinetic constant of RhB degradation over Ag3PO4 (33.3 mol%)/p-g-C3N4 was 5 and 2 times than that over pure p-g-C3N4 and Ag3PO4. The stronger visible light absorption and the defined interfaces (or heterojunction) between Ag3PO4 nanoparticles and p-g-C3N4 could contribute to the enhanced photocatalytic performance. The photocatalytic mechanism study revealed that the degradation of RhB under visible light irradiation over the as-prepared Ag3PO4/p-g-C3N4 is mainly via the direct hole oxidation mechanism. This work may provide some insight into solving the unsatisfactory catalytic activity and low efficiency covering solar radiation for practical applications of g-C3N4. Acknowledgments This work was supported by the financial supports of National Natural Science Foundation of China (21003065), Jiangsu Planned Projects for Postdoctoral Research Funds (1202040C), College Natural Science Research Program of Jiangsu Province (13KJB610003) and Research Foundation for Talented Scholars of Jiangsu University (11JDG149 and 10JDG133).
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Highly efficient heterojunction photocatalyst based on nanoporous g-C3N4 sheets modified by Ag3PO4 nanoparticles: Synthesis and enhanced photocatalytic activity Deli Jiang, Jianjun Zhu, Min Chen, Jimin Xie ⇑ School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China
a r t i c l e
i n f o
Article history: Received 22 July 2013 Accepted 15 November 2013 Available online 23 November 2013 Keywords: Nanoporous g-C3N4 Ag3PO4 nanoparticles Photocatalysis Rhodamine B Degradation
a b s t r a c t Novel visible-light-driven heterojunction photocatalyst composed by Ag3PO4 nanoparticles and nanoporous graphitic carbon nitride sheets (Ag3PO4/p-g-C3N4) was synthesized by a facile and green method. The results showed that photocatalytic activity of Ag3PO4/p-g-C3N4 was much higher than that of pure p-g-C3N4 in the photodegradation of Rhodamine B under visible light irradiation. The kinetic constant of Rhodamine B degradation over Ag3PO4 (33.3 mol%)/p-g-C3N4 was about 5 and 2 times higher than that over pure p-g-C3N4 and Ag3PO4, respectively. The enhanced photocatalytic performance is attributed to the stronger visible light absorption and the heterojunction between Ag3PO4 nanoparticles and p-g-C3N4, which could induce the low recombination rate of photoinduced electron–hole pairs. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction In recent years, the photocatalytic technology through semiconductors for the degradation of the organic pollutants in wastewater and air has been extensively studied [1,2]. However, the semiconductor materials available to date are generally limited by either poor photocatalytic efficiency in the visible light range or insufficient charge separation ability. To address this issue, much effort in recent years has been focused on the exploration and fabrication of novel semiconductor catalysts to improve photocatalytic properties [3,4]. Recently, graphite-like carbon nitride (g-C3N4) has been explored as a promising candidate for hydrogen evolution and environment purification under visible light irradiation [5]. However, there are still some inherent drawbacks existed in the g-C3N4 photocatalysis, such as the high recombination rate of its photogenerated electron–hole pairs, pure g-C3N4 adsorbs only blue light up to 450 nm, and low specific surface area [6–8]. To address these problems, several strategies, such as nanostructuring [9,10], doping [11–13], and copolymerization [14,15], have been exploited to improve the photocatalytic activity of g-C3N4. Among these methods, constructing heterojunction between g-C3N4 and other materials, such as deposited noble metals [16], semiconductor (e.g. TiO2, ZnO, CdS, Bi2WO6, SmVO4, BiOBr, AgX (Br, I)) [17–23], P [23], or graphene [24] could improve photocatalytic efficiency of g-C3N4. ⇑ Corresponding author. Fax: +86 511 88791800. E-mail addresses: [email protected], [email protected] (J. Xie). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.11.042
It has been recently found that the photocatalytic efficiency of g-C3N4 was strongly dependent on its porosity and surface area [25,26]. Dong et al. reported that the porousification could enhance the photooxidation ability of g-C3N4 by providing more photogenerated holes and active sites [27]. Chen and co-worker also found that the porous g-C3N4 exhibited much higher photocatalytic efficiency than the pristine g-C3N4 [28]. Han et al. synthesized porous g-C3N4 without any template according to Le Chatelier’s principle, and the resulting porous g-C3N4 material with high BET surface area (201–209 m2 g1) shows a ten times enhanced photocatalytic activity for methyl orange photodegradation under visible light irradiation [29]. Recently, our group found that porous g-C3N4-based BiOI/g-C3N4 heterostructures exhibited enhanced visible-light-driven photocatalytic activity in the degradation of methylene blue in aqueous solution [30]. Although porous g-C3N4 (donated as p-g-C3N4) have been found to show enhanced photocatalytic activity compared to bulk ones, few studies have been focused on the synthesis of p-g-C3N4-based heterostructured photocatalyst. Currently, it has been reported that Ag3PO4 has extremely high photooxidative capabilities for both water splitting and degradation organic dyes under visible light irradiation [31–34]. A number of Ag3PO4-based composite photocatalysts were also intensively studied to further enhance the photocatalytic efficiency of Ag3PO4 [35–38]. Considering the excellent photooxidation activity of Ag3PO4 and large specific surface of the nanoporous g-C3N4, here we for the first time report the facile synthesis of novel Ag3 PO4 nanoparticles/p-g-C3N 4 (named as Ag3 PO4 /p-g-C3 N4 )
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heterostructured photocatalysts. The resultant Ag3PO4/p-g-C3N4 photocatalyst shows significantly higher photocatalytic activity toward the degradation of Rhodamine B under visible light irradiation than the pure p-g-C3N4. This work may provide some insight into solving the unsatisfactory catalytic activity and low efficiency covering solar radiation for practical applications of g-C3N4.
and centrifuged to remove the particles. The filtrates were analyzed by a Shimadzu UV-2450 spectrophotometer and the characteristic absorption peak of RhB at 554 nm was used to determine the extent of its degradation.
2. Experimental section
The examination experiment process of reactive oxygen species is similar to the photodegradation experiment. Different quantity of scavengers was added into the RhB solution prior to addition of the catalyst. The dosages of these scavengers were referred to the previous studies [18,39].
2.1. Synthesis of the photocatalyst All chemicals were reagent grade and used without further purification. Porous graphitic carbon nitride (p-g-C3N4) was synthesized by thermal treatment of 10 g of urea in a crucible with a cover under ambient pressure in air, similar with reported methods [27,28]. After dried at 80 °C for 24 h, the precursor was heated to 550 °C at a heating rate of 2.3 °C/min in a tube furnace for 4 h in air. The resulted final light yellow powder were washed with nitric acid (0.1 mol/L) and distilled water to remove any residual alkaline species (e.g. ammonia) adsorbed on the sample surface, and then dried at 80 °C for 24 h. Ag3PO4/p-g-C3N4 composite photocatalysts were prepared by in situ growth of Ag3PO4 on g-C3N4 surface. Typically, 0.092 g of g-C3N4 powders and different amounts of AgNO3 were dissolved in 200 mL of deionized water; after ultrasonication for 20 min, a certain amount of Na2HPO412H2O was dropped into the solution under vigorous stirring. After stirring for 4 h, the product was collected by centrifugation, washed with distilled water, and dried in an oven at 60 °C for 24 h. According to this method, different molar ratios of the Ag3PO4 to p-g-C3N4 samples were obtained and labeled as 20% Ag3 PO4 /p-g-C 3N4 , 25% Ag3PO 4/p-g-C3N 4, 33% Ag3PO4/p-g-C3N4, and 50% Ag3PO4/p-g-C3N4, respectively. 2.2. Characterization The crystal structure of the samples was investigated using Xray diffraction (XRD; Bruker D8 Advance X-ray diffractometer) with Cu Ka radiation. The acceleration voltage and the applied current were 40 kV and 40 mA, respectively. The morphology of the samples was examined by transmission electron microscopy (TEM; FEI JEM-2100 and FEI Tecnai G2 F20) operated at 200 kV and scanning electron micrograph (SEM) using a field emission SEM (FESEM) instrument (Hitachi S-4800 II, Japan). The Brunauer–Emmett–Teller (BET) specific surface area and pore size distribution of the samples were characterized by nitrogen adsorption with TristarII3020 instrument. UV–vis diffuse reflection spectroscopy (DRS) was performed on a Shimadzu UV-3100 spectrophotometer using BaSO4 as the reference. Infrared spectra were obtained on KBr pellets on an Equinox 55 spectrometer (Bruker) in the range of 4000–500 cm1. The photoluminescence (PL) spectra of the photocatalyst were obtained by a Varian Cary Eclipse spectrometer with an excitation wavelength of 325 nm. 2.3. Photocatalytic activity study The photocatalytic activity of the Ag3PO4/p-g-C3N4 photocatalysts was tested in the degradation reaction of RhB aqueous solution (10 mg/L) under irradiation of a 300 W tungsten light lamp. For a typical photocatalytic experiment, 0.05 g of catalyst powder was added into 50 mL of the above RhB solution in quartz tube. Prior to irradiation, the suspensions were magnetically stirred in dark for 30 min to ensure the establishment of an adsorption/ desorption equilibrium. The above suspensions were kept under constant air-equilibrated conditions before and during the irradiation. At given time intervals, about 4 mL of aliquots were sampled
2.4. Detection of reactive oxygen species
3. Results and discussion 3.1. Morphology and structure In the present work, the p-g-C3N4 sheets were fabricated facilely by one-step polymerization of low cost and environmentally benign urea. The SEM image in Fig. S1a shows that the as-prepared pure p-g-C3N4 is consisted of thin sheet-like with wrinkles and irregular shape. The TEM image (Fig. S1b) shows that a number of mesopores of several tens of nanometres in size can be observed in the p-g-C3N4 sheets, which was highly analogous to previously reported result [27]. The digital image in the inset of Fig. 1a clearly shows the large-scale synthesis of p-g-C3N4 can be achieved by via the current method. Fig. 1shows the TEM images of the as-prepared Ag3PO4/ p-g-C3N4 photocatalysts. It can be found that the size of supported Ag3PO4 nanoparticles can be controlled by changing the initial concentration of AgNO3 employed, which was distinctive from the result reported by Kumar et al [40]. The TEM image of 20% Ag3PO4/p-g-C3N4 reveals that the Ag3PO4 nanoparticles with very small size of around 5 nm are uniformly deposited on the p-g-C3N4 surface (Fig. 1a), revealing the successful formation of Ag3PO4/p-g-C3N4 composite. The defined interconnection (or heterojunction) between Ag3PO4 nanoparticles and p-g-C3N4 is believed to favor the transfer of photogenerated electrons from p-g-C3N4 to Ag3PO4, thus enhancing the charge separation and photocatalytic efficiency. TEM image taken on the 25% Ag3PO4/p-g-C3N4 sample indicates that the size of Ag3PO4 nanoparticles increases greatly and reaches approximately 10 nm (Fig. 1b). Further increasing the AgNO3 precursor resulted in a gradual increase in the size of Ag3PO4 nanoparticles. As indicated in Fig. 1c, the p-g-C3N4 sheets are densely decorated with a large amount of rather large Ag3PO4 nanoparticles with a wide size distribution ranging from 10 to 40 nm. However, the TEM image of 50% Ag3PO4/ p-g-C3N4 displays that the sample is consisted by p-g-C3N4 and isolated Ag3PO4 nanoparticle aggregates (Fig. 1d). The average size of the individual Ag3PO4 nanoparticle increases significantly and reaches near 200 nm. It is expected that there is no strong interfacial charge transfer taking place between p-g-C3N4 and isolated Ag3PO4 aggregates, and as a consequence 50% Ag3PO4/p-g-C3N4 will show relatively poor photocatalytic activity in comparison with Ag3PO4/p-g-C3N4 sample with low Ag3PO4 content. Fig. 2 shows the XRD patterns of as-prepared samples. As shown in Fig. 2a, the p-g-C3N4 sample reveals two distinct diffraction peaks at 27.40° and 13.04°, which can be indexed to the (0 0 2) and (1 0 0) diffraction planes of the graphite-like carbon nitride and correspond to the characteristic interplanar staking peaks of aromatic systems and the interlayer structural packing, respectively. These two peaks decreased in intensity gradually with the increase in introduced Ag3PO4 and finally disappeared in the Ag3PO4/p-gC3N4 (50%) sample, whereas the diffraction peaks of Ag3PO4 intensified gradually (Fig. 2b–e). Fig. 2f shows that all the diffraction
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and 549 cm1, which are all ascribed to PAO stretching vibrations of PO 4 [41]. This result indicates that a small amount of Ag3PO4 nanoparticles were formed in the resultant samples.
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Fig. 1. TEM images of 20% Ag3PO4/p-g-C3N4 (a), 25% Ag3PO4/p-g-C3N4 (b), 33% Ag3PO4/p-g-C3N4 (c), and 50% Ag3PO4/p-g-C3N4 (d).
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peaks of Ag3PO4 sample could be well indexed to the body-centered cubic structure of Ag3PO4 (JCPDS No. 06-0505). Thus, the Ag3PO4/p-g-C3N4 composite samples exhibit diffraction peaks corresponding to both p-g-C3N4 and Ag3PO4, reflecting the presence of two phases. In addition, it should be noted that no diffraction peak ascribed to Ag nanoparticles can be observed in the XRD pattern of all the composite samples, and it imply in a sense that the current composite system could prevent the reduction of Ag3PO4 into metallic Ag nanoparticles. To further confirm the formation of Ag3PO4/p-g-C3N4 composites, the FT-IR spectra of p-g-C3N4, and a series of Ag3PO4/p-gC3N4 photocatalysts with various Ag3PO4 contents were shown in the Fig. S2. In the FT-IR spectrum of p-g-C3N4, the peaks at 1640 cm1 were attributable to C@N stretching vibration modes, while the 1242, 1320 and 1403 cm1 to aromatic CAN stretching vibration modes. The peak at 808 cm1 was related to characteristic breathing mode of triazine units. The Ag3PO4/p-g-C3N4 composite sample has a similar spectrum but two new peaks at 1014 cm1
To know porous nature and specific surface area of p-g-C3N4 and Ag3PO4/p-g-C3N4 composites, a nitrogen adsorption–desorption isotherm is measured. Fig. 3a shows the isotherm for different samples, which exhibit a type IV with a H3 hysteresis loop according to the IUPAC classification, reflecting the presence of a mesoporous structure of the composites. It can be found that the synthesized p-g-C3N4 shows a relatively large specific surface area (23.19 m2/g). Compared with p-g-C3N4, the Ag3PO4/p-g-C3N4 composites exhibit decreased BET surface areas due to the incorporation of Ag3PO4 which has very low BET surface area (0.21 m2/g). We think low BET specific surface areas for the 25% Ag3PO4/p-gC3N4 and 50% Ag3PO4/p-g-C3N4 samples are may be related to the somewhat p-g-C3N4 particle aggregation. The pore-size distribution of the samples are also estimated using the Barrett–Joyner–Halenda (BJH) method from the desorption branch of the isotherm, as shown in Fig. 3b. The calculated pore size distribution using BJH method indicated that the size of mesopores is not uniform ranging from 8 to 50 nm, which is consistent with the TEM observation. The specific surface area and pore size distribution of different samples are summarized in Table 1. It is interesting found that the pore size of the samples seems to increase with the increase in Ag3PO4 loading content. This increase trend may be related to the formation of new nanostructures (such as Ag3PO4 nanoparticles aggregation) with an even larger pore size (see Fig. 1c and d), which results in an enhanced (total) pore size in the sample. 3.3. UV–vis diffuse reflectance spectra and band gap energy The optical properties of as-prepared Ag3PO4/p-g-C3N4 composites, pure Ag3PO4, and g-C3N4 samples were examined using DRS
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Pore Diameter (nm) Fig. 3. (a) Nitrogen sorption isotherm and (b) Barrett–Joyner–Halenda (BJH) pore size distribution plot (inset) of the as-prepared samples.
Table 1 Parameters obtained from N2 desorption isotherm measurements. Samples
Surface area (m2 g1)
Pore volume (cm3 g1)
Average pore size (nm)
p-g-C3N4 20% Ag3PO4/p-g-C3N4 25% Ag3PO4/p-g-C3N4 33% Ag3PO4/p-g-C3N4 50% Ag3PO4/p-g-C3N4 Ag3PO4
23.19 20.84 6.31 12.84 5.30 0.21
0.079 0.083 0.024 0.069 0.024 /
13.64 15.87 15.79 21.52 18.49 /
4
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6
Fig. 4. (a) UV–vis DRS and plots of (a h m)1/2 vs hm of pure p-g-C3N4, Ag3PO4 and (b) as-prepared Ag3PO4/p-g-C3N4 samples.
Among them, n is determined from the type of optical transition of a semiconductor (n = 1 for direct transition and n = 4 for indirect transition). Thus, as shown in Fig. 4b, Eg of Ag3PO4 and p-g-C3N4 was determined to be 2.45 eV and 2.70 eV, respectively, according to a plot of (a h m)2 versus energy (hm). The Eg of Ag3PO4/p-g-C3N4 composite is less than that of p-g-C3N4, indicating that the Ag3PO4 can be acted as an efficient visible-light sensitizer for the p-g-C3N4. 3.4. Photocatalytic activity
technique (Fig. 4a). As expected, the bare Ag3PO4 could absorb solar energy with a wavelength shorter than 530 nm, while the g-C3N4 holds an onset of absorption at 460 nm. In Ag3PO4/ p-g-C3N4 composite sample, the absorption toward the visible light region is remarkably enhanced compared to pure p-g-C3N4 particles. Moreover, the absorption intensity of the Ag3PO4/p-g-C3N4 composites increases with increasing amounts of Ag3PO4, which should be attributed to the increase in the Ag3PO4 concentration. Thus, it is clear that all these Ag3PO4/p-g-C3N4 samples are responsive to visible light, implying that they are able to work with visible light. The energy level and band gap of the semiconductors play a crucial role in determining its physical properties. The band gap energy of a semiconductor can be calculated by the following formula [42]:
ahm ¼ Aðhm Eg Þn=2
3
hv (eV)
200
ð1Þ
where a, h, m, Eg and A are absorption coefficient, Planck constant, light frequency, band gap energy, and a constant, respectively.
To compare the photocatalytic activity of pure p-g-C3N4 and Ag3PO4/p-g-C3N4 over different reaction times, a series of photodegradation experiments were carried out by using RhB as a model pollutant under visible light irradiation. Fig. 5 shows the variation in concentration of RhB (C/C0) with irradiation time over different photocatalysts, where C0 is the initial concentration of RhB and C is its concentration at time t. The blank test confirms that RhB is only slightly degraded in the absence of catalysts, indicating that the photolysis can be negligible. Pure p-g-C3N4 exhibited a weak photocatalytic activity that could degrade RhB by only 60.2% after 1 h of visible light irradiation, while pure Ag3PO4 have a moderate photocatalytic activity that could degrade RhB by only 84.6% after the same period of time. Decorating with Ag3PO4 nanoparticles, all the Ag3PO4/p-g-C3N4 composite photocatalysts show enhanced abilities for the degradation of RhB in comparison with the pure p-g-C3N4 and even Ag3PO4. Furthermore, the Ag3PO4 NPs content is pivotal for optimal photocatalytic activity. When the Ag3PO4 NPs content is increased beyond 33.3 mol%, a decrease in the photocatalytic activity was observed. The origin of this effect can be explained as follows: the suitable Ag3PO4 nanoparticles content causes their good dispersion on the g-C3N4 surface, which favors
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1.0 0.9
RhB self-degradation p-g-C3N4
0.8
Ag3PO4
C/C0
0.7
20% Ag3PO4/p-g-C3N4
0.6
25% Ag3PO4/p-g-C3N4
0.5
33% Ag3PO4/p-g-C3N4 50% Ag3PO4/p-g-C3N4
0.4
EVB ¼ X Ee þ 0:5Eg
0.3 0.2 0.1 0.0 0
10
20
30
40
50
60
70
80
90
100
Time (min) Fig. 5. Photocatalytic degradation of RhB as a function of irradiation time over pure p-g-C3N4, Ag3PO4 and Ag3PO4/p-g-C3N4 samples.
the transfer and separation of the charge carriers. However, at a higher Ag3PO4 nanoparticles content, there is no strong interfacial charge transfer taking place between p-g-C3N4 and isolated Ag3PO4 aggregates, leading to a relative low charge separation efficiency, which as a consequence will decrease the photocatalytic activity. The decomposition processes have been modeled as a pseudo first order reaction with the kinetics expressed by the equation ln(C0/C) = kt, where k is the reaction rate constant. The pseudofirst-order rate constants (k) reflect the rate of degradation of RhB over the photocatalysts under visible-light irradiation, as displayed in Fig. 6. It can be clearly seen that the apparent rate constants (k, min1), determined from the slopes of ln (C0/C) versus irradiation time are 0.0149 min1 for pure p-g-C3N4. By combing with Ag3PO4 nanoparticles, Ag3PO4/p-g-C3N4 composites apparently exhibit higher photocatalytic activities compared with pure p-g-C3N4. In particular, the sample with 33.3 mol% Ag3PO4 shows the highest catalytic activity with a k of 0.0739 min1, about 5 and 2 times higher than pure p-g-C3N4 and Ag3PO4, respectively. 3.5. Plausible photocatalytic mechanism of Ag3PO4/p-g-C3N4 Basically, the fundamental process of semiconductor photocatalysis involves electron and hole pairs after light irradiation. The electron and hole pairs could migrate to surfaces of the semiconductor to react with the adsorbed O2 and H2O, respectively, in pro ducing the redox sources of O 2 and OH radical species, which lead
7
ð2Þ
where EVB is the valence band edge potential, X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms, and the value of X for Ag3PO4 is ca. 5.96 eV, Ee is the energy of free electrons on the hydrogen scale (about 4.5 eV), Eg is the band gap energy of the semiconductor. Based on the band gap positions, the CB and VB edge potentials of p-g-C3N4 are determined at 1.13 and 1.57 eV, respectively. The CB and VB edge potentials of Ag3PO4 are determined at 0.24 and 2.69 eV, respectively. The CB and VB edge potentials of p-g-C3N4 were more negative than that of Ag3PO4. Therefore, such difference between the CB potentials of p-g-C3N4 and Ag3PO4 allowed the electron transfer from the CB of p-g-C3N4 to that of Ag3PO4. As shown in Fig. 7, when the Ag3PO4/p-g-C3N4 system is irradiated with visible light, both Ag3PO4 and p-g-C3N4 can be activated. The excited electrons at the conduction band of p-g-C3N4 are transferred to the conduction band of coupled Ag3PO4 crystallites. At the same time, simultaneous holes on the VB of Ag3PO4 could migrate to that of p-g-C3N4 because of the enjoined electric field. Therefore, the recombination process of the electron–hole is inhibited, leading to an enhanced photoactivity of the Ag3PO4/p-g-C3N4 composite. Photoluminescence (PL) spectra originate from the migration, transfer, and separation efficiency of the photogenerated charge carriers in a semiconducting material. There is a strong correlation between PL intensity and the photocatalytic performances. In general, higher PL intensity indicates the higher recombination of the charge carriers. Fig. S3 shows the PL spectra of the p-g-C3N4 and 33% Ag3PO4/p-g-C3N4 samples. We can see that after the formation of Ag3PO4/p-g-C3N4 composite, the PL intensity of the composite decreased rapidly, demonstrating the efficient charge separation of the composite. Therefore, it is reasonable to conclude that the enhanced photocatalytic activity could be attributed to the effective interfacial charge transfer between Ag3PO4 and p-g-C3N4. To further understand the nature of the primary active species involved in visible light degradation of RhB over Ag3PO4/p-g-C3N4 composite photocatalysts, we have carried out the control
-1
RhB self-degradation, k=0.0011 min -1 p-g-C3N4, k=0.0149min
6
NHE (eV)
-1
-1.5
Ag3PO4, k=0.0340 min 5
-1
20% Ag3PO4/p-g-C3N4, k=0.0433 min
-1
ln (C0/C)
to the efficient destruction of organic pollutants. In order to fully understand photocatalytic mechanism for the as-prepared Ag3PO4/ p-g-C3N4, the band edge positions of the valence band and conduction band of Ag3PO4 and p-g-C3N4, should be determined because they are also strongly related to the process of photocatalytic oxidation of organic compounds. The valence band potentials of Ag3PO4 and p-g-C3N4, were estimated in this study according to the following empirical equation [43]:
-0.5
25% Ag3PO4/p-g-C3N4, k=0.0512 min
4
RhB
-1
33% Ag3PO4/p-g-C3N4, k=0.0739 min
-1
3
50% Ag3PO4/p-g-C3N4, k=0.0583min
0.5
2
O2 O2•–
Visible light
RhB
Eg=2.45 eV
1.5
1
Eg=2.70 eV
Degradation product
Visible light
p-g-C3N4
2.5
Degradation product
0 0
10
20
30
40
50
60
Time (min) Fig. 6. Rhodamine B degradation curves of ln(C0/C) versus time for pure p-g-C3N4, Ag3PO4 and Ag3PO4/p-g-C3N4 samples, and reaction rate constant, k, obtained from linear fitting.
3.5
Ag3PO4
Fig. 7. Schematic diagram of electron–hole pairs separation and the possible reaction mechanism over Ag3PO4/p-g-C3N4 photocatalyst under visible light irradiation.
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experiments with addition of scavenger for holes (h+) and OH radical species. As shown in Fig. S4, with 0.1 g of ammonium oxalate (AO) as a hole scavenger added to the solution, the rate for degradation of RhB over 33% Ag3PO4/p-g-C3N4 sample is remarkably decreased. When 2 mL of tertbutyl alcohol (TBA), as a scavenger for OH radical species, is added, no obvious decrease in degradation rate is observed, indicating the absence of OH radical species. Meanwhile, with the addition of 1 mmol of O 2 scavenger BQ, the degradation of RhB was slightly depressed, suggesting the existence of O 2 only with very low concentration in the reaction. These results suggest that, the holes play a dominant role toward the degradation of RhB over 33% Ag3PO4/p-g-C3N4 under visible light irradiation. 3.6. Stability of the photocatalyst The stability of Ag3PO4/p-g-C3N4 was tested for the degradation of RhB dye under identical reaction conditions. After complete degradation, the catalyst was separated and washed with a large amount of deionized water. The recovered catalyst was dried in an oven at 80 °C for 4 h and used for the next run. As shown in Fig. S5, the experimental results about RhB dye removals demonstrated that after 5 times, the RhB dye removals were maintained between 90% and 80% and that no significant drop was found. It indicates that the photocatalytic activity of our Ag3PO4/p-g-C3N4 was maintained well. We supposed that the little mass loss in the progress of recovery is the main reason for the slight decrease in photocatalytic activity of reused photocatalysts in five repeated cycles. 4. Conclusions In summary, novel visible light-driven Ag3PO4/p-g-C3N4 heterojunction photocatalysts were synthesized by in situ precipitation of Ag3PO4 nanoparticles on the surface of p-g-C3N4 sheets. The heterojunction photocatalysts showed obviously superior photocatalytic activity for the photodegradation of RhB under the irradiation of visible light. The kinetic constant of RhB degradation over Ag3PO4 (33.3 mol%)/p-g-C3N4 was 5 and 2 times than that over pure p-g-C3N4 and Ag3PO4. The stronger visible light absorption and the defined interfaces (or heterojunction) between Ag3PO4 nanoparticles and p-g-C3N4 could contribute to the enhanced photocatalytic performance. The photocatalytic mechanism study revealed that the degradation of RhB under visible light irradiation over the as-prepared Ag3PO4/p-g-C3N4 is mainly via the direct hole oxidation mechanism. This work may provide some insight into solving the unsatisfactory catalytic activity and low efficiency covering solar radiation for practical applications of g-C3N4. Acknowledgments This work was supported by the financial supports of National Natural Science Foundation of China (21003065), Jiangsu Planned Projects for Postdoctoral Research Funds (1202040C), College Natural Science Research Program of Jiangsu Province (13KJB610003) and Research Foundation for Talented Scholars of Jiangsu University (11JDG149 and 10JDG133).
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