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High performance visible light driven photocatalysts silver halides and graphitic carbon nitride (X = Cl, Br, I) nanocomposites
Journal of Colloid and Interface Science 395 (2013) 75–80
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
High performance visible light driven photocatalysts silver halides and graphitic carbon nitride (X = Cl, Br, I) nanocomposites Yonghuan Lan a, Xiuzhen Qian a, Chongjun Zhao a,b,⇑, Zhuomin Zhang a, Xin Chen a, Zhen Li b,⇑ a Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Key Laboratory of Advanced Polymeric Materials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, PR China b Institute of Superconducting and Electronic Materials, University of Wollongong, NSW2500, Australia
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Article history: Received 24 October 2012 Accepted 15 December 2012 Available online 28 December 2012 Keywords: g-C3N4 Silver halides Visible light Photocatalysis
a b s t r a c t Novel visible light-driven phtotocatalysts composed by silver halides and graphitic carbon nitride (i.e. AgX@g-C3N4, X = Cl, Br, I) were synthesized by in situ precipitation of AgX nanoparticles on the surface of sheet-like g-C3N4. The resultant AgX@g-C3N4 nanocomposites were characterized with state-of-theart instruments, showing significant enhancement in photocatalytic degradation of methyl orange under the irradiation of visible light. Their excellent photocatalytic performance is attributed to the efficient separation of photogenerated electron–hole pairs and their higher photostability in comparison with pure AgX. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Growing energy demands, concerns over climate change and depleting fossil fuel resources have been the key drivers for the development of renewable and sustainable technologies. One of such technologies is the use of photocatalysts which play important roles in water/air purification [1], water splitting [2], selfcleaning [3], and high-efficiency solar cells [4]. Development of high-performance photocatalysts, especially the visible-light-driven (VLD) photocatalysts is highly significant for the diverse applications of this clean technology, because of pronounced visible light in solar spectrum (43%). The conventional photocatalysts are doped and undoped TiO2, ZnO and their composites. However, most photocatalysts work only under the irradiation of shortwavelength light due to their wide bandgap. An alternative photocatalyst is derived from silver halides (AgX, X = Cl, Br and I) which have been extensively used as photosensitizers. Their photosensitivity suggests the potential application in photocatalysis despite their bandgap energy is higher than the edge of visible light (e.g. the direct and indirect bandgaps of AgCl ⇑ Corresponding authors. Addresses: Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Key Laboratory of Advanced Polymeric Materials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, PR China (C. Zhao), Institute of Superconducting and Electronic Materials, University of Wollongong, NSW2500, Australia (Z. Li). Fax: +86 21 6425 0838 (C. Zhao), +61 2 4221 5731 (Z. Li). E-mail addresses: [email protected] (C. Zhao), [email protected] (Z. Li). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.12.033
are 5.15 eV and 3.25 eV, and the edge of visible light is located at 3.1 eV). However, the strong photosensitivity of AgX leads to the continuous reduction of Ag+ into Ag0 under the exposure to light, drastically decreasing their stability and lifetime. Improving the stability of silver halides has become a significant issue in exploitation of their photocatalytic application. In recent years, some methods have been attempted to improve the photocatalytic efficiency and stability of AgX, including (1) deposition of AgX nanoparticles on the inert supports [5], semiconductor materials (e.g. TiO2, WO3, BiOX, ZnO and BiVO4) [6–10], or conducting polymers [11]; (2) coating AgX with Ag nanoparticles [12]; and (3) combination of two or three approaches together [13]. Among the above methods, deposition of AgX nanoparticles on various supports is very attractive because some resultant nanocomposites show notable improvement in photocatalytic efficiency and stability [11]. However, expensive supports could limit their practical application in a large scale. Seeking cost-effective and versatile supports for decreasing fabrication cost become necessary, in addition to maintaining their high photocatalytic efficiency and stability. A promising support is graphitic carbon nitride (g-C3N4) which is made by ‘‘earth abundant’’ elements, and can be synthesized by facile methods [14,15]. g-C3N4 is the most stable allotrope of carbon nitride and has the smallest direct band gap (2.7 eV) due to the sp2 hybridization of carbon and nitrogen that forms the p conjugated graphitic planes [16,17]. g-C3N4 has been extensively utilized as an advanced photocatalysts for production of H2 and O2 via water splitting [18,19], and degradation of organic pollutants
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[17,20]. However, the low separation efficiency of photogenerated electron–hole pairs limits the application of pure g-C3N4 [21]. The photocatalytic performance can be improved by using its mesoporous structure [22], doping with metallic or nonmetallic elements [23–26], or coupling with other semiconductors [27–29], and organic dyes [30]. Recently there are two reports on the photocatalytic enhancement of AgX nanoparticles deposited on g-C3N4 [31,32], One is about AgCl/mesoporous g-C3N4 composites in which the photocatalytic activity of pure AgCl is unclear and the authors attributed the photocatalysis enhancement to the modification of electronic properties of mesoporous g-C3N4 by AgCl. The other one is about AgX/g-C3N4 (X = Br and I) photocatalysts in which the particle size of AgX is around 10 nm. Small particle size could lead to the instability and short lifetime of photocatalysts, demonstrated by the notable decrease (30%) in photocatalytic degradation of methyl orange after five cycles. Herein, we prepare different VLD photocatalysts AgX@g-C3N4 (X = Cl, Br and I) by in situ deposition of AgX nanoparticles on the surface of sheet-like g-C3N4, and then demonstrate their excellent photocatalytic activity in degradation of methyl orange under the irradiation of visible light. Our results demonstrate that AgX is the main active component in the composites and the introduction of g-C3N4 significantly improved their photocatalysis efficiency and photostability due to the fast transfer and separation of photogenerated electrons and holes. Our large particle size (60 nm) dramatically increases the photostability and lifetime, proved by the retainment of excellent photocatalytic activity after used for 10 cycles. 2. Experimental section 2.1. Materials AgNO3, KCl, KBr, KI and melamine were purchased from Shanghai Chemical Reagent Co. Ltd. (China). All these reagents were in analytical grade (AR) and used without further purification. 2.2. Synthesis of g-C3N4 and AgX@g-C3N4 (X = Cl, Br and I) nanocomposites
2.2.1. Preparation of g-C3N4 Metal-free g-C3N4 powder was synthesized by the pyrolysis of melamine in a muffle furnace according to the previous report [17]. In a typical synthesis, 10 g melamine was loaded into a semi-closed alumina crucible with a cover, and then heated to 500 °C with a rate of 5 °Cmin1. The sample was maintained at 500 °C for 2 h, followed by a deamination treatment at 520 °C for another 2 h. After the alumina crucible was cooled to room temperature, the product was collected and ground into powder. 2.2.2. Preparation of AgX@g-C3N4 (X = Cl, Br and I) nanocomposites AgX@g-C3N4 were prepared by a modified precipitation method [33]. The as-prepared g-C3N4 powder was added into 30 mL of distilled water, and sonicated for 30 min. Then 0.5926 g of AgNO3 was added to the suspension, and the mixture was stirred magnetically for 4 h at the room temperature. Subsequently KCl (or KBr, KI) aqueous solution was dropwise added into the above mixture solution. The molar ratio of Ag+ to Cl (Br or I) was 1/1. The resulting suspension was stirred for overnight, and then filtered under reduced pressure. The solid obtained was washed with distilled water and ethanol for three times, and then dried in vacuum at 70 °C over 12 h. A series of AgX@g-C3N4 (X = Cl, Br and I) with different mass of g-C3N4 were prepared and labeled as AgX@g-C3N4w% (w% is the weight percentage of g-C3N4).
2.3. Characterizations of AgX@g-C3N4 (X = Cl, Br and I) nanocomposites The sample morphology and size were recorded on a S-4800 field emission scanning microscope (FE-SEM). Their crystal structure was determined by the Rigaku D/max 2500v/pc X-ray diffractometer using Cu Ka1 radiation (k = 1.5418 Å) with a step of 5° min1. The Fourier transform spectrophotometer (FT-IR, Nicolet) was used to obtain the sample molecular structure information in the range of 2000–500 cm1. The sample UV–vis reflectance spectra were recorded on a Cary-500 spectrophotometer using BaSO4 as a reference. Thermogravimetric analysis (TGA) was performed on a thermo gravimetric analyzer (TG-DTA, HCT-3) in the temperature range of 30–800 °C under air atmosphere and the heating rate was 10 °C min1. 2.4. Photocatalytic activity of AgX@g-C3N4 (X = Cl, Br and I) nanocomposites The photocatalytic activity of the AgX@g-C3N4 nanocomposites was evaluated via the degradation of methyl orange (MO, 20 mg/L) aqueous solution under visible light irradiation. Prior to irradiation, the mixture of MO and catalyst solution was stirred for 30 min in the dark to obtain the equilibrium adsorption. The visible light (k P 400 nm) used for the photocatalytic degradation was generated by a 350 W mercury lamp equipped with an UV cutoff filter. For AgCl@g-C3N4 and AgI@g-C3N4, 0.03 g catalyst powder was added into 25 ml MO solution (the mass ratio of dye to catalyst is 1/60). The mass ratio used in the case of AgBr@g-C3N4 catalysts is 1/25. The MO/catalyst mixture was collected and centrifuged after irradiated every 3 min. The supernatant liquor was analyzed by UV–vis absorption spectra (Spectrumlab 54T UV–vis spectrophotometer, Lengguang Tech., Shanghai, China). Each degradation cycle lasted for 15 min, and the used photocatalysts were collected and used to repeat the same degradation experiment for 10 cycles in order to determine their photostability. 3. Results and discussion Our novel VLD photocatalysts AgX@g-C3N4 (X = Cl, Br, and I) were prepared by in situ deposition of AgX nanoparticles on the surface of g-C3N4. The obtained nanocomposites were fully characterized and Fig. 1 displays the X-ray diffraction (XRD) patterns of AgCl, g-C3N4 and AgCl@g-C3N4-5% composite (5% refers to the weight percent of g-C3N4 in the composite). Compared with those of pure AgCl (curve a), diffraction peaks at 2h = 31.98°, 45.96°,
Fig. 1. XRD patterns of (a) AgCl, (b) g-C3N4 and (c) AgCl@g-C3N4-5%.
Y. Lan et al. / Journal of Colloid and Interface Science 395 (2013) 75–80
54.56°, 57.22°, 67.20° and 76.50° in AgCl@g-C3N4 composite (curve c) can be indexed as (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0) and (4 2 0) planes, which are consistent with the standard diffraction peaks of face-centered cubic AgCl (JCPDS No. 31-1238). The peak at 27.72° could be an overlap of (1 1 1) plane of AgCl (2h = 27.58°) and (2 0 0) plane of g-C3N4 (2h = 27.18°) [16,30], leading to the increase in intensity ratio between this peak and other peaks in the nanocomposites, when compared with pure AgCl. The presence of g-C3N4 in the nanocomposites can be further confirmed by FTIR spectra (Fig. S1). In the spectrum of pure g-C3N4, the characteristic peaks at 1242, 1327, 1415, 1568, and 1636 cm1 are attributed to the typical stretching vibration of CN heterocycles [17]. Moreover, there are characteristic breathing modes of triazine units at 808 and 885 cm1 [17,34]. The spectrum of AgCl@g-C3N4 nanocomposite is similar to that of g-C3N4 and all characteristic peaks of g-C3N4 can be indexed. The thermostability and component contents of the composites were determined by TGA as shown in Fig. S2. There is no apparent weight loss for pure AgCl in the entire investigated temperature range. For pure g-C3N4 and AgCl@g-C3N4 nanocomposites, their weight loss started at 500 °C and ended at 730 °C, which is consistent with literature report [17]. The content of g-C3N4 can be calculated from the net weight loss, and the slight difference between the TGA results and the used amount could be attributed to the loss of g-C3N4 during sample preparation and purification. Fig. 2a–c shows the typical SEM images of pure g-C3N4, AgCl and AgCl@g-C3N4-5% nanocomposite. The pure g-C3N4 exhibits layered structure, similar to its analogue graphite. The pure AgCl prepared in the absence of g-C3N4 are large solid particles with an average size of 1–2 lm. Due to the presence of g-C3N4, the particle size of AgCl in the AgCl@g-C3N4 composite is much smaller (<500 nm). This suggests that AgCl nanoparticles formed on the surface of gC3N4 and the surface defects in g-C3N4 could serve as nanoparticle nucleation sites. The TEM image in Fig. 2d clearly shows individual AgCl nanoparticles (60 nm) deposited on the surface of g-C3N4. The selected area diffraction pattern (SAD) further proves the formation of crystalline AgCl. An interplanar spacing of 0.27 nm was clearly observed in their high resolution TEM image (Fig. S3), corresponding to the (200) lattice fringe of AgCl nanoparticles. UV–vis diffuse reflectance spectra (DRS) were measured to determine the capability of g-C3N4, AgCl and AgCl@g-C3N4-5% harvesting light at different wavelength. As shown in Fig. 3, no distinct
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Fig. 3. UV–vis DRS of AgCl, g-C3N4 and AgCl@g-C3N4-5%.
absorption band was observed in the visible and near-IR region for pure AgCl [35–37]. Pure g-C3N4 has a band edge absorption around 450 nm (2.7 eV) and matches well with literature value [38]. After deposition of AgCl on the surface of g-C3N4, the absorption intensity of the resultant nanocomposites became stronger in the whole interested spectrum window, especially in the visible light region. Furthermore, the absorption edge is red shifted and a broad absorption ranging from 420 nm to 630 nm (the center is at 515 nm) was detected. Obviously, the introduction of g-C3N4 significantly enhanced the absorption in visible light region, which satisfy the primary requirement for efficient VLD photocatalysts. The photocatalytic performance were evaluated by the photodegradation of methyl orange (MO) aqueous solution under visible light irradiation (k P 400 nm) at room temperature. The photostability and chemical stability of MO was investigated by irradiating the MO solution under the same conditions without using photocatalysts, showing no change in color and absorption intensity. This supports the high photostability of selected MO. The necessity of light for photocatalysis is proved by the fact that no degradation was observed when the mixture of MO and AgCl@g-C3N4 was stirred in the dark. No distinct color change was observed for the MO samples irradiated in the presence g-C3N4 (insert in Fig. 4). The same phenomenon was observed in N-doped TiO2 which is usually
Fig. 2. SEM images of as-synthesized (a) AgCl, (b) g-C3N4, and (c) AgCl@g-C3N4-5%; (d) TEM image of AgCl@g-C3N4-5%.
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Fig. 4. Photodegradation of MO as a function of irradiation time over different catalysts, (a) pure AgCl, (b) pure g-C3N4, (c) Ag@g-C3N4-5%, (d) AgCl@g-C3N4-5% in the dark, (e) AgCl@g-C3N4-5%, (f) mixture of AgCl and g-C3N4 (5%), (g) N-TiO2. The inset: colors change sequence of the MO solution over AgCl, g-C3N4 and AgCl@gC3N4-5% catalysts. (Mass ratio of dye to catalyst is 1/60).
sity of this peak obtained before and after irradiation. Fig. 4 demonstrates the variation of MO concentration in the 1st cycle, obtained at different irradiation time in the presence of either g-C3N4, AgCl, N-doped TiO2, AgCl@g-C3N4 nanocomposites or a mixture of AgCl and g-C3N4. Pure g-C3N4 exhibits very weak photocatalytic activity as shown by curve b in Fig. 4. Pure AgCl and AgCl@g-C3N4 nanocomposites display excellent photocatalytic activity (curves a and e in Fig. 4). MO was entirely degraded after 15 min irradiation in the presence of AgCl, accompanied by the bleaching of color. In the case of AgCl@g-C3N4 nanocomposite, better photocatalytic performance was observed, indicated by reaction time shortening from 15 min to 6 min for the complete degradation of MO. The optimum content of g-C3N4 in AgCl@g-C3N4 was found to be 5–10% by comparing their photocatalytic efficiency in degradation of MO solution, as shown in Fig. S5. Controlled photocatalysis conducted with N-doped TiO2 under the same irradiation conditions resulted in a much lower degradation rate (curve g in Fig. 4) in comparison with our nanocomposites. The above results indicate the photodegradation of MO is a pseudo-first order kinetic process, which can be expressed by the following equation:
lnðC=C 0 Þ ¼ kt Table 1 The kinetic constants for the photocatalytic degradation processes of MO under visible light irradiation. Samples
Kinetic constant k (min1)
AgCl@g-C3N4 AgCl g-C3N4 N-TiO2 AgCl@g-C3N4(dark)
0.686 0.175 0.0136 0.002 0.014
used as a benchmark of VLD photocatalysts. However, the color of MO solution was efficiently bleached in the presence of AgCl or AgCl@g-C3N4 photocatalysts (insert in Fig. 4). This demonstrates the better photocatalytic performance of our nanocomposites than conventional VLD photocatalysts. Fig. S4 shows the evolution of MO spectra as a function of irradiation time obtained from AgCl@g-C3N4-5%. The remaining MO concentration was estimated according to the Beer–Lambert law (A = e C, where A is the absorbance, e is the extinction coefficient and C is the concentration). The disappearance of the absorption peak around 460 nm is usually considered as an indicator of MO photodegradation [17], thus the efficiency of photocatalysts can be evaluated by correlating the MO concentration with the inten-
ð1Þ
where C0 and C are the initial and measured concentrations of MO [39], t and k are degradation time and kinetic constants, respectively. The kinetic constants corresponding to different photocatalysts were calculated and listed in Table 1. The kinetic constant of MO degradation with AgCl@g-C3N4-5% is 4 and 50 times of that obtained from pure AgCl and pure g-C3N4, respectively. In addition to AgCl, AgBr@g-C3N4 and AgI@g-C3N4 nanocompsites were also synthesized and tested as photocatalysts using the similar methods. As shown in Fig. 5a, excellent photocatalytic activity was observed for AgBr@g-C3N4 nanocomposites. Similarly the optimum photocatalytic performance was observed in those catalysts with a loading of 5–10% g-C3N4. We also compare the photocatalytic performance of AgBr@g-C3N4 with AgCl@g-C3N4 and AgI@g-C3N4. The degradation of MO catalyzed by AgBr@gC3N4 is faster than AgCl@g-C3N4 and AgI@g-C3N4 by a factor of ca. 1.5 and 2.5, respectively (i.e. with the same dosage of 25 mg catalysts per mg MO, the degradation time is 6 min for AgBr@gC3N4, 9 min for AgCl@g-C3N4 and more than 15 min for AgI@gC3N4, as shown in Fig. 5a and b). For the practical application, the stability of photocatalyst is the key parameter and should be taken into consideration. We used AgCl@g-C3N4-5% as an example to demonstrate the stability of our novel photocatalysts. The experiments were carried out by continuous photodegradation of MO under visible light irradiation
Fig. 5. Photodegradation of MO as a function of irradiation time over (a) AgBr@g-C3N4 catalyst with different loadings of g-C3N4, (b) AgCl, AgCl@g-C3N4-5%, AgCl@g-C3N4-10%, AgI and AgI@g-C3N4-5%, AgI@g-C3N4-10%. (Mass ratio of dye to catalyst is 1/25).
Y. Lan et al. / Journal of Colloid and Interface Science 395 (2013) 75–80
Fig. 6. Cycling times of MO degradation for AgCl, AgCl@g-C3N4-5% under visible light irradiation. (Mass ratio of dye to catalyst is 1/60).
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shows a scheme of band energy of AgCl and g-C3N4. Upon photo excitation, both AgCl and g-C3N4 produce electron–hole pairs, and the electrons photogenerated in AgCl will flow from its CB into that of g-C3N4 because the LCB of AgCl (1.20 eV) is more negative than that of g-C3N4 (1.12 eV). The transfer of electrons efficiently inhibits the reduction of Ag+ into Ag0 by electrons. These electrons are trapped by O2 in the solution to form superoxide ion ðO 2 Þ, while holes are scavenged by Cl to form active oxidation production Cl0 [36], which then oxidize MO. It should be noted that the reduction of Ag+ was not completely inhibited, because Ag0 was detected after AgCl@g-C3N4 was used for 10 cycles. But it still exhibited excellent photocatalytic performance. In contrast, pure AgCl almost lost photocatalytic activity (only 30% was remained). These results indicate that the introduction of g-C3N4 distinctly enhanced the photostability of AgCl through fast transfer of photogenerated electrons. The fast transfer of electrons not only reduced the reduction of Ag+ into Ag0, but also prohibited their recombination. The enhanced effect of g-C3N4 in the composite was verified by another controlled experiment in which AgCl and g-C3N4 are mechanically mixed and used as photocatalyst. As shown in Fig. 4, the degradation rate of MO catalyzed by the mixture was lower than that obtained from AgCl@g-C3N4 nanocomposites and pure AgCl. The higher photocatalytic efficiency of AgBr@g-C3N4 can be well understood according to our scheme. Firstly, the LCB of AgBr is higher than that of AgCl, which means the faster transfer of photogenerated electrons from the CB of AgBr to the CB of g-C3N4 than the case of AgCl@g-C3N4. Therefore the electron–hole separation in AgBr@g-C3N4 is more efficient than in AgCl@g-C3N4. Secondly, AgBr has narrower bandgap than AgCl (i.e. 2.6 eV vs. 3.3 eV), which suggests that AgBr can harvest more photons than AgCl [41]. Thus pure AgBr also exhibits higher photocatalytic degradation efficiency than AgCl (Fig. 5a and b), which is consistent with the results reported by Wang et al. [41]. 4. Conclusions
Fig. 7. Schematic photodegradation of MO catalyzed by AgCl@g-C3N4 under visible light irradiation.
for 10 cycles, in which each cycle lasted for 15 min. As shown in Fig. 6, the photocatalytic activity of AgCl@g-C3N4 was kept well even after 10 cycles (the degradation efficiency only decreased 1.5%). In contrast, the photocatalytic activity of AgCl drastically decreased from 97.5% to 30% after 10 cycles, which demonstrates the higher stability of AgCl@g-C3N4 than pure AgCl due to the presence of g-C3N4. Our results also demonstrate the higher photostability and better performance of photocatalysts than previous reports. It is known that photocatalysis involves three basic steps: (1) photogeneration of electron–hole pairs [36,40], (2) separation and transport of electrons and holes (3) photoreduction [29] or photooxidation of species by electrons or holes [36,40]. Previous results demonstrate that AgCl (or AgBr, AgI) in the composites is the main active component for the photodegradation of MO, because g-C3N4 alone did not show notable photocatalytic activity, which is consistent with photocatalytic performance of g-C3N4 [15]. The synergetic effects of AgX and g-C3N4 can be demonstrated by using AgCl@g-C3N4 as an example. The potentials of the lowest conduction band (LCB) and the highest valence band (HVB) of gC3N4 are located at 1.12 eV and 1.57 eV, respectively [38], and the LCB and HVB of AgCl are at 1.20 eV and 2.10 eV [9]. Fig. 7
Novel AgX@g-C3N4 (X = Cl, Br and I) nanocomposites were synthesized via a facile wet-chemical method. The introduction of gC3N4 not only effectively reduced the particle size of AgX, but also efficiently captured and transferred most electrons generated in AgX (X = Cl, Br and I) upon irradiation. The presence of g-C3N4 can effectively reduce the recombination of electrons and holes in comparison with pure AgCl, AgBr and AgI. It also can partially prevent the entire reduction of Ag+ into Ag0. These effects synergistically result in higher photocatalytic efficiency and higher stability of AgX@g-C3N4 nanocomposites than pure AgX (X = Cl, Br and I). AgBr@g-C3N4 shows better photocatalytic performance than AgCl@g-C3N4 due to the narrower bandgap and higher LCB potential of AgBr than that of AgCl. The optimal g-C3N4 content in these highly efficient photocatalysts is about 5–10 wt.%. Acknowledgments We are grateful for the support of the National Natural Science Foundation of China (No. 20504026), Shanghai Nanotechnology Promotion Center (No. 11nm0507000), Shanghai Leading Academic Discipline Project (B502), Shanghai Key Laboratory Project (08DZ2230500) and Australian Research Council Discovery Project (DP130102274). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2012.12.033.
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Contents lists available at SciVerse ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
High performance visible light driven photocatalysts silver halides and graphitic carbon nitride (X = Cl, Br, I) nanocomposites Yonghuan Lan a, Xiuzhen Qian a, Chongjun Zhao a,b,⇑, Zhuomin Zhang a, Xin Chen a, Zhen Li b,⇑ a Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Key Laboratory of Advanced Polymeric Materials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, PR China b Institute of Superconducting and Electronic Materials, University of Wollongong, NSW2500, Australia
a r t i c l e
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Article history: Received 24 October 2012 Accepted 15 December 2012 Available online 28 December 2012 Keywords: g-C3N4 Silver halides Visible light Photocatalysis
a b s t r a c t Novel visible light-driven phtotocatalysts composed by silver halides and graphitic carbon nitride (i.e. AgX@g-C3N4, X = Cl, Br, I) were synthesized by in situ precipitation of AgX nanoparticles on the surface of sheet-like g-C3N4. The resultant AgX@g-C3N4 nanocomposites were characterized with state-of-theart instruments, showing significant enhancement in photocatalytic degradation of methyl orange under the irradiation of visible light. Their excellent photocatalytic performance is attributed to the efficient separation of photogenerated electron–hole pairs and their higher photostability in comparison with pure AgX. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Growing energy demands, concerns over climate change and depleting fossil fuel resources have been the key drivers for the development of renewable and sustainable technologies. One of such technologies is the use of photocatalysts which play important roles in water/air purification [1], water splitting [2], selfcleaning [3], and high-efficiency solar cells [4]. Development of high-performance photocatalysts, especially the visible-light-driven (VLD) photocatalysts is highly significant for the diverse applications of this clean technology, because of pronounced visible light in solar spectrum (43%). The conventional photocatalysts are doped and undoped TiO2, ZnO and their composites. However, most photocatalysts work only under the irradiation of shortwavelength light due to their wide bandgap. An alternative photocatalyst is derived from silver halides (AgX, X = Cl, Br and I) which have been extensively used as photosensitizers. Their photosensitivity suggests the potential application in photocatalysis despite their bandgap energy is higher than the edge of visible light (e.g. the direct and indirect bandgaps of AgCl ⇑ Corresponding authors. Addresses: Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Key Laboratory of Advanced Polymeric Materials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, PR China (C. Zhao), Institute of Superconducting and Electronic Materials, University of Wollongong, NSW2500, Australia (Z. Li). Fax: +86 21 6425 0838 (C. Zhao), +61 2 4221 5731 (Z. Li). E-mail addresses: [email protected] (C. Zhao), [email protected] (Z. Li). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.12.033
are 5.15 eV and 3.25 eV, and the edge of visible light is located at 3.1 eV). However, the strong photosensitivity of AgX leads to the continuous reduction of Ag+ into Ag0 under the exposure to light, drastically decreasing their stability and lifetime. Improving the stability of silver halides has become a significant issue in exploitation of their photocatalytic application. In recent years, some methods have been attempted to improve the photocatalytic efficiency and stability of AgX, including (1) deposition of AgX nanoparticles on the inert supports [5], semiconductor materials (e.g. TiO2, WO3, BiOX, ZnO and BiVO4) [6–10], or conducting polymers [11]; (2) coating AgX with Ag nanoparticles [12]; and (3) combination of two or three approaches together [13]. Among the above methods, deposition of AgX nanoparticles on various supports is very attractive because some resultant nanocomposites show notable improvement in photocatalytic efficiency and stability [11]. However, expensive supports could limit their practical application in a large scale. Seeking cost-effective and versatile supports for decreasing fabrication cost become necessary, in addition to maintaining their high photocatalytic efficiency and stability. A promising support is graphitic carbon nitride (g-C3N4) which is made by ‘‘earth abundant’’ elements, and can be synthesized by facile methods [14,15]. g-C3N4 is the most stable allotrope of carbon nitride and has the smallest direct band gap (2.7 eV) due to the sp2 hybridization of carbon and nitrogen that forms the p conjugated graphitic planes [16,17]. g-C3N4 has been extensively utilized as an advanced photocatalysts for production of H2 and O2 via water splitting [18,19], and degradation of organic pollutants
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[17,20]. However, the low separation efficiency of photogenerated electron–hole pairs limits the application of pure g-C3N4 [21]. The photocatalytic performance can be improved by using its mesoporous structure [22], doping with metallic or nonmetallic elements [23–26], or coupling with other semiconductors [27–29], and organic dyes [30]. Recently there are two reports on the photocatalytic enhancement of AgX nanoparticles deposited on g-C3N4 [31,32], One is about AgCl/mesoporous g-C3N4 composites in which the photocatalytic activity of pure AgCl is unclear and the authors attributed the photocatalysis enhancement to the modification of electronic properties of mesoporous g-C3N4 by AgCl. The other one is about AgX/g-C3N4 (X = Br and I) photocatalysts in which the particle size of AgX is around 10 nm. Small particle size could lead to the instability and short lifetime of photocatalysts, demonstrated by the notable decrease (30%) in photocatalytic degradation of methyl orange after five cycles. Herein, we prepare different VLD photocatalysts AgX@g-C3N4 (X = Cl, Br and I) by in situ deposition of AgX nanoparticles on the surface of sheet-like g-C3N4, and then demonstrate their excellent photocatalytic activity in degradation of methyl orange under the irradiation of visible light. Our results demonstrate that AgX is the main active component in the composites and the introduction of g-C3N4 significantly improved their photocatalysis efficiency and photostability due to the fast transfer and separation of photogenerated electrons and holes. Our large particle size (60 nm) dramatically increases the photostability and lifetime, proved by the retainment of excellent photocatalytic activity after used for 10 cycles. 2. Experimental section 2.1. Materials AgNO3, KCl, KBr, KI and melamine were purchased from Shanghai Chemical Reagent Co. Ltd. (China). All these reagents were in analytical grade (AR) and used without further purification. 2.2. Synthesis of g-C3N4 and AgX@g-C3N4 (X = Cl, Br and I) nanocomposites
2.2.1. Preparation of g-C3N4 Metal-free g-C3N4 powder was synthesized by the pyrolysis of melamine in a muffle furnace according to the previous report [17]. In a typical synthesis, 10 g melamine was loaded into a semi-closed alumina crucible with a cover, and then heated to 500 °C with a rate of 5 °Cmin1. The sample was maintained at 500 °C for 2 h, followed by a deamination treatment at 520 °C for another 2 h. After the alumina crucible was cooled to room temperature, the product was collected and ground into powder. 2.2.2. Preparation of AgX@g-C3N4 (X = Cl, Br and I) nanocomposites AgX@g-C3N4 were prepared by a modified precipitation method [33]. The as-prepared g-C3N4 powder was added into 30 mL of distilled water, and sonicated for 30 min. Then 0.5926 g of AgNO3 was added to the suspension, and the mixture was stirred magnetically for 4 h at the room temperature. Subsequently KCl (or KBr, KI) aqueous solution was dropwise added into the above mixture solution. The molar ratio of Ag+ to Cl (Br or I) was 1/1. The resulting suspension was stirred for overnight, and then filtered under reduced pressure. The solid obtained was washed with distilled water and ethanol for three times, and then dried in vacuum at 70 °C over 12 h. A series of AgX@g-C3N4 (X = Cl, Br and I) with different mass of g-C3N4 were prepared and labeled as AgX@g-C3N4w% (w% is the weight percentage of g-C3N4).
2.3. Characterizations of AgX@g-C3N4 (X = Cl, Br and I) nanocomposites The sample morphology and size were recorded on a S-4800 field emission scanning microscope (FE-SEM). Their crystal structure was determined by the Rigaku D/max 2500v/pc X-ray diffractometer using Cu Ka1 radiation (k = 1.5418 Å) with a step of 5° min1. The Fourier transform spectrophotometer (FT-IR, Nicolet) was used to obtain the sample molecular structure information in the range of 2000–500 cm1. The sample UV–vis reflectance spectra were recorded on a Cary-500 spectrophotometer using BaSO4 as a reference. Thermogravimetric analysis (TGA) was performed on a thermo gravimetric analyzer (TG-DTA, HCT-3) in the temperature range of 30–800 °C under air atmosphere and the heating rate was 10 °C min1. 2.4. Photocatalytic activity of AgX@g-C3N4 (X = Cl, Br and I) nanocomposites The photocatalytic activity of the AgX@g-C3N4 nanocomposites was evaluated via the degradation of methyl orange (MO, 20 mg/L) aqueous solution under visible light irradiation. Prior to irradiation, the mixture of MO and catalyst solution was stirred for 30 min in the dark to obtain the equilibrium adsorption. The visible light (k P 400 nm) used for the photocatalytic degradation was generated by a 350 W mercury lamp equipped with an UV cutoff filter. For AgCl@g-C3N4 and AgI@g-C3N4, 0.03 g catalyst powder was added into 25 ml MO solution (the mass ratio of dye to catalyst is 1/60). The mass ratio used in the case of AgBr@g-C3N4 catalysts is 1/25. The MO/catalyst mixture was collected and centrifuged after irradiated every 3 min. The supernatant liquor was analyzed by UV–vis absorption spectra (Spectrumlab 54T UV–vis spectrophotometer, Lengguang Tech., Shanghai, China). Each degradation cycle lasted for 15 min, and the used photocatalysts were collected and used to repeat the same degradation experiment for 10 cycles in order to determine their photostability. 3. Results and discussion Our novel VLD photocatalysts AgX@g-C3N4 (X = Cl, Br, and I) were prepared by in situ deposition of AgX nanoparticles on the surface of g-C3N4. The obtained nanocomposites were fully characterized and Fig. 1 displays the X-ray diffraction (XRD) patterns of AgCl, g-C3N4 and AgCl@g-C3N4-5% composite (5% refers to the weight percent of g-C3N4 in the composite). Compared with those of pure AgCl (curve a), diffraction peaks at 2h = 31.98°, 45.96°,
Fig. 1. XRD patterns of (a) AgCl, (b) g-C3N4 and (c) AgCl@g-C3N4-5%.
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54.56°, 57.22°, 67.20° and 76.50° in AgCl@g-C3N4 composite (curve c) can be indexed as (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0) and (4 2 0) planes, which are consistent with the standard diffraction peaks of face-centered cubic AgCl (JCPDS No. 31-1238). The peak at 27.72° could be an overlap of (1 1 1) plane of AgCl (2h = 27.58°) and (2 0 0) plane of g-C3N4 (2h = 27.18°) [16,30], leading to the increase in intensity ratio between this peak and other peaks in the nanocomposites, when compared with pure AgCl. The presence of g-C3N4 in the nanocomposites can be further confirmed by FTIR spectra (Fig. S1). In the spectrum of pure g-C3N4, the characteristic peaks at 1242, 1327, 1415, 1568, and 1636 cm1 are attributed to the typical stretching vibration of CN heterocycles [17]. Moreover, there are characteristic breathing modes of triazine units at 808 and 885 cm1 [17,34]. The spectrum of AgCl@g-C3N4 nanocomposite is similar to that of g-C3N4 and all characteristic peaks of g-C3N4 can be indexed. The thermostability and component contents of the composites were determined by TGA as shown in Fig. S2. There is no apparent weight loss for pure AgCl in the entire investigated temperature range. For pure g-C3N4 and AgCl@g-C3N4 nanocomposites, their weight loss started at 500 °C and ended at 730 °C, which is consistent with literature report [17]. The content of g-C3N4 can be calculated from the net weight loss, and the slight difference between the TGA results and the used amount could be attributed to the loss of g-C3N4 during sample preparation and purification. Fig. 2a–c shows the typical SEM images of pure g-C3N4, AgCl and AgCl@g-C3N4-5% nanocomposite. The pure g-C3N4 exhibits layered structure, similar to its analogue graphite. The pure AgCl prepared in the absence of g-C3N4 are large solid particles with an average size of 1–2 lm. Due to the presence of g-C3N4, the particle size of AgCl in the AgCl@g-C3N4 composite is much smaller (<500 nm). This suggests that AgCl nanoparticles formed on the surface of gC3N4 and the surface defects in g-C3N4 could serve as nanoparticle nucleation sites. The TEM image in Fig. 2d clearly shows individual AgCl nanoparticles (60 nm) deposited on the surface of g-C3N4. The selected area diffraction pattern (SAD) further proves the formation of crystalline AgCl. An interplanar spacing of 0.27 nm was clearly observed in their high resolution TEM image (Fig. S3), corresponding to the (200) lattice fringe of AgCl nanoparticles. UV–vis diffuse reflectance spectra (DRS) were measured to determine the capability of g-C3N4, AgCl and AgCl@g-C3N4-5% harvesting light at different wavelength. As shown in Fig. 3, no distinct
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Fig. 3. UV–vis DRS of AgCl, g-C3N4 and AgCl@g-C3N4-5%.
absorption band was observed in the visible and near-IR region for pure AgCl [35–37]. Pure g-C3N4 has a band edge absorption around 450 nm (2.7 eV) and matches well with literature value [38]. After deposition of AgCl on the surface of g-C3N4, the absorption intensity of the resultant nanocomposites became stronger in the whole interested spectrum window, especially in the visible light region. Furthermore, the absorption edge is red shifted and a broad absorption ranging from 420 nm to 630 nm (the center is at 515 nm) was detected. Obviously, the introduction of g-C3N4 significantly enhanced the absorption in visible light region, which satisfy the primary requirement for efficient VLD photocatalysts. The photocatalytic performance were evaluated by the photodegradation of methyl orange (MO) aqueous solution under visible light irradiation (k P 400 nm) at room temperature. The photostability and chemical stability of MO was investigated by irradiating the MO solution under the same conditions without using photocatalysts, showing no change in color and absorption intensity. This supports the high photostability of selected MO. The necessity of light for photocatalysis is proved by the fact that no degradation was observed when the mixture of MO and AgCl@g-C3N4 was stirred in the dark. No distinct color change was observed for the MO samples irradiated in the presence g-C3N4 (insert in Fig. 4). The same phenomenon was observed in N-doped TiO2 which is usually
Fig. 2. SEM images of as-synthesized (a) AgCl, (b) g-C3N4, and (c) AgCl@g-C3N4-5%; (d) TEM image of AgCl@g-C3N4-5%.
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Fig. 4. Photodegradation of MO as a function of irradiation time over different catalysts, (a) pure AgCl, (b) pure g-C3N4, (c) Ag@g-C3N4-5%, (d) AgCl@g-C3N4-5% in the dark, (e) AgCl@g-C3N4-5%, (f) mixture of AgCl and g-C3N4 (5%), (g) N-TiO2. The inset: colors change sequence of the MO solution over AgCl, g-C3N4 and AgCl@gC3N4-5% catalysts. (Mass ratio of dye to catalyst is 1/60).
sity of this peak obtained before and after irradiation. Fig. 4 demonstrates the variation of MO concentration in the 1st cycle, obtained at different irradiation time in the presence of either g-C3N4, AgCl, N-doped TiO2, AgCl@g-C3N4 nanocomposites or a mixture of AgCl and g-C3N4. Pure g-C3N4 exhibits very weak photocatalytic activity as shown by curve b in Fig. 4. Pure AgCl and AgCl@g-C3N4 nanocomposites display excellent photocatalytic activity (curves a and e in Fig. 4). MO was entirely degraded after 15 min irradiation in the presence of AgCl, accompanied by the bleaching of color. In the case of AgCl@g-C3N4 nanocomposite, better photocatalytic performance was observed, indicated by reaction time shortening from 15 min to 6 min for the complete degradation of MO. The optimum content of g-C3N4 in AgCl@g-C3N4 was found to be 5–10% by comparing their photocatalytic efficiency in degradation of MO solution, as shown in Fig. S5. Controlled photocatalysis conducted with N-doped TiO2 under the same irradiation conditions resulted in a much lower degradation rate (curve g in Fig. 4) in comparison with our nanocomposites. The above results indicate the photodegradation of MO is a pseudo-first order kinetic process, which can be expressed by the following equation:
lnðC=C 0 Þ ¼ kt Table 1 The kinetic constants for the photocatalytic degradation processes of MO under visible light irradiation. Samples
Kinetic constant k (min1)
AgCl@g-C3N4 AgCl g-C3N4 N-TiO2 AgCl@g-C3N4(dark)
0.686 0.175 0.0136 0.002 0.014
used as a benchmark of VLD photocatalysts. However, the color of MO solution was efficiently bleached in the presence of AgCl or AgCl@g-C3N4 photocatalysts (insert in Fig. 4). This demonstrates the better photocatalytic performance of our nanocomposites than conventional VLD photocatalysts. Fig. S4 shows the evolution of MO spectra as a function of irradiation time obtained from AgCl@g-C3N4-5%. The remaining MO concentration was estimated according to the Beer–Lambert law (A = e C, where A is the absorbance, e is the extinction coefficient and C is the concentration). The disappearance of the absorption peak around 460 nm is usually considered as an indicator of MO photodegradation [17], thus the efficiency of photocatalysts can be evaluated by correlating the MO concentration with the inten-
ð1Þ
where C0 and C are the initial and measured concentrations of MO [39], t and k are degradation time and kinetic constants, respectively. The kinetic constants corresponding to different photocatalysts were calculated and listed in Table 1. The kinetic constant of MO degradation with AgCl@g-C3N4-5% is 4 and 50 times of that obtained from pure AgCl and pure g-C3N4, respectively. In addition to AgCl, AgBr@g-C3N4 and AgI@g-C3N4 nanocompsites were also synthesized and tested as photocatalysts using the similar methods. As shown in Fig. 5a, excellent photocatalytic activity was observed for AgBr@g-C3N4 nanocomposites. Similarly the optimum photocatalytic performance was observed in those catalysts with a loading of 5–10% g-C3N4. We also compare the photocatalytic performance of AgBr@g-C3N4 with AgCl@g-C3N4 and AgI@g-C3N4. The degradation of MO catalyzed by AgBr@gC3N4 is faster than AgCl@g-C3N4 and AgI@g-C3N4 by a factor of ca. 1.5 and 2.5, respectively (i.e. with the same dosage of 25 mg catalysts per mg MO, the degradation time is 6 min for AgBr@gC3N4, 9 min for AgCl@g-C3N4 and more than 15 min for AgI@gC3N4, as shown in Fig. 5a and b). For the practical application, the stability of photocatalyst is the key parameter and should be taken into consideration. We used AgCl@g-C3N4-5% as an example to demonstrate the stability of our novel photocatalysts. The experiments were carried out by continuous photodegradation of MO under visible light irradiation
Fig. 5. Photodegradation of MO as a function of irradiation time over (a) AgBr@g-C3N4 catalyst with different loadings of g-C3N4, (b) AgCl, AgCl@g-C3N4-5%, AgCl@g-C3N4-10%, AgI and AgI@g-C3N4-5%, AgI@g-C3N4-10%. (Mass ratio of dye to catalyst is 1/25).
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Fig. 6. Cycling times of MO degradation for AgCl, AgCl@g-C3N4-5% under visible light irradiation. (Mass ratio of dye to catalyst is 1/60).
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shows a scheme of band energy of AgCl and g-C3N4. Upon photo excitation, both AgCl and g-C3N4 produce electron–hole pairs, and the electrons photogenerated in AgCl will flow from its CB into that of g-C3N4 because the LCB of AgCl (1.20 eV) is more negative than that of g-C3N4 (1.12 eV). The transfer of electrons efficiently inhibits the reduction of Ag+ into Ag0 by electrons. These electrons are trapped by O2 in the solution to form superoxide ion ðO 2 Þ, while holes are scavenged by Cl to form active oxidation production Cl0 [36], which then oxidize MO. It should be noted that the reduction of Ag+ was not completely inhibited, because Ag0 was detected after AgCl@g-C3N4 was used for 10 cycles. But it still exhibited excellent photocatalytic performance. In contrast, pure AgCl almost lost photocatalytic activity (only 30% was remained). These results indicate that the introduction of g-C3N4 distinctly enhanced the photostability of AgCl through fast transfer of photogenerated electrons. The fast transfer of electrons not only reduced the reduction of Ag+ into Ag0, but also prohibited their recombination. The enhanced effect of g-C3N4 in the composite was verified by another controlled experiment in which AgCl and g-C3N4 are mechanically mixed and used as photocatalyst. As shown in Fig. 4, the degradation rate of MO catalyzed by the mixture was lower than that obtained from AgCl@g-C3N4 nanocomposites and pure AgCl. The higher photocatalytic efficiency of AgBr@g-C3N4 can be well understood according to our scheme. Firstly, the LCB of AgBr is higher than that of AgCl, which means the faster transfer of photogenerated electrons from the CB of AgBr to the CB of g-C3N4 than the case of AgCl@g-C3N4. Therefore the electron–hole separation in AgBr@g-C3N4 is more efficient than in AgCl@g-C3N4. Secondly, AgBr has narrower bandgap than AgCl (i.e. 2.6 eV vs. 3.3 eV), which suggests that AgBr can harvest more photons than AgCl [41]. Thus pure AgBr also exhibits higher photocatalytic degradation efficiency than AgCl (Fig. 5a and b), which is consistent with the results reported by Wang et al. [41]. 4. Conclusions
Fig. 7. Schematic photodegradation of MO catalyzed by AgCl@g-C3N4 under visible light irradiation.
for 10 cycles, in which each cycle lasted for 15 min. As shown in Fig. 6, the photocatalytic activity of AgCl@g-C3N4 was kept well even after 10 cycles (the degradation efficiency only decreased 1.5%). In contrast, the photocatalytic activity of AgCl drastically decreased from 97.5% to 30% after 10 cycles, which demonstrates the higher stability of AgCl@g-C3N4 than pure AgCl due to the presence of g-C3N4. Our results also demonstrate the higher photostability and better performance of photocatalysts than previous reports. It is known that photocatalysis involves three basic steps: (1) photogeneration of electron–hole pairs [36,40], (2) separation and transport of electrons and holes (3) photoreduction [29] or photooxidation of species by electrons or holes [36,40]. Previous results demonstrate that AgCl (or AgBr, AgI) in the composites is the main active component for the photodegradation of MO, because g-C3N4 alone did not show notable photocatalytic activity, which is consistent with photocatalytic performance of g-C3N4 [15]. The synergetic effects of AgX and g-C3N4 can be demonstrated by using AgCl@g-C3N4 as an example. The potentials of the lowest conduction band (LCB) and the highest valence band (HVB) of gC3N4 are located at 1.12 eV and 1.57 eV, respectively [38], and the LCB and HVB of AgCl are at 1.20 eV and 2.10 eV [9]. Fig. 7
Novel AgX@g-C3N4 (X = Cl, Br and I) nanocomposites were synthesized via a facile wet-chemical method. The introduction of gC3N4 not only effectively reduced the particle size of AgX, but also efficiently captured and transferred most electrons generated in AgX (X = Cl, Br and I) upon irradiation. The presence of g-C3N4 can effectively reduce the recombination of electrons and holes in comparison with pure AgCl, AgBr and AgI. It also can partially prevent the entire reduction of Ag+ into Ag0. These effects synergistically result in higher photocatalytic efficiency and higher stability of AgX@g-C3N4 nanocomposites than pure AgX (X = Cl, Br and I). AgBr@g-C3N4 shows better photocatalytic performance than AgCl@g-C3N4 due to the narrower bandgap and higher LCB potential of AgBr than that of AgCl. The optimal g-C3N4 content in these highly efficient photocatalysts is about 5–10 wt.%. Acknowledgments We are grateful for the support of the National Natural Science Foundation of China (No. 20504026), Shanghai Nanotechnology Promotion Center (No. 11nm0507000), Shanghai Leading Academic Discipline Project (B502), Shanghai Key Laboratory Project (08DZ2230500) and Australian Research Council Discovery Project (DP130102274). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2012.12.033.
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