Enhanced visible light photocatalytic activity of novel polymeric g-C3N4 loaded with Ag nanoparticles

Applied Catalysis A: General 409–410 (2011) 215–222

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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata

Enhanced visible light photocatalytic activity of novel polymeric g-C3 N4 loaded with Ag nanoparticles Lei Ge ∗ , Changcun Han, Jing Liu, Yunfeng Li Department of Materials Science and Engineering, College of Science, China University of Petroleum Beijing, Beijing 102249, PR China

a r t i c l e

i n f o

Article history: Received 30 August 2011 Received in revised form 1 October 2011 Accepted 4 October 2011 Available online 8 October 2011 Keywords: g-C3 N4 Photocatalysis Functional Semiconductors

a b s t r a c t Novel polymeric g-C3 N4 photocatalysts loaded with noble metal Ag nanoparticles were prepared via a facile heating method. The obtained Ag/g-C3 N4 composite products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), UV–vis diffuse reflection spectra (DRS) and photoluminescence spectra (PL). The photocatalytic activities of Ag/g-C3 N4 samples were investigated based on the decomposition of methyl orange and hydrogen evolution under visible light irradiation. The XPS results revealed that it was the metallic Ag0 deposited on polymeric g-C3 N4 samples. The Ag/g-C3 N4 photocatalysts exhibited significantly enhanced photocatalytic performance for the degradation of methyl orange and hydrogen production compared with pure g-C3 N4 . The optimal Ag content was determined to be 1.0 wt%, and the corresponding hydrogen evolution rate was 10.105 ␮mol h−1 , which exceeded that of pure g-C3 N4 by more than 11.7 times. The enhanced photocatalytic performance could be attributed to the synergic effect between Ag and g-C3 N4 , which promoted the migration efficiency of photo-generated carriers. The proposed mechanism for the enhanced visible light photocatalytic activity of g-C3 N4 modified by a small amount of Ag was further confirmed by photoluminescence spectroscopy. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Semiconductor-based photocatalysis has attracted extensive interest since the photo-induced splitting of water on TiO2 electrodes was discovered [1–3]. Photocatalysis is one of the most promising technologies for pollutants decomposition and hydrogen evolution by the generation of • OH radicals and other oxidative radicals [4–6]. In the past decades, various inorganic materials, such as oxides, sulfides, and oxynitride have been explored as photocatalysts for hydrogen production and environmental purification under UV or visible light irradiation [7–10]. One of the main goals in materials science fields is to find photocatalysts with high quantum efficiency and catalytic performance [11,12]. However, the lowusage of visible light has restrained the photocatalytic activity of photocatalysts in the environmental remediation [13]. Therefore, it is urgent to develop the novel photocatalysts for pollutants degradation, which have appropriate band gap, strong oxidative ability and high stability in water solution system.

∗ Corresponding author. Tel.: +86 010 89733200; fax: +86 010 89733200. E-mail address: [email protected] (L. Ge). 0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2011.10.006

Recently, some semiconductors have been found to be active for degradation of organic contaminants or splitting of water under visible light irradiation, such as BiVO4 [14], Bi2 WO6 [15], BiFeO3 [16], K10 [Nb2 O2 (H2 O)2 ][SiNb12 O40 ] [17], CMPs [18], etc. They all show certain absorption ability in the visible light range. However, these novel photocatalysts have some disadvantages. Especially, the photo-generated charge carriers may recombine during the migration process, leading to the decrease of the photocatalytic activity. In order to increase the utilization rates of visible light, numerous methods have been employed to modify the photocatalysts, such as doping of metal or nonmetal elements [19], quantum dots sensitization [20], and coupling with other semiconductors [21]. Therefore, designing more efficient visible light induced and high stability photocatalysts, which can meet the requirement of practical environmental purification, represents the central challenge of photocatalysis research. As the most stable allotrope of carbon nitride, graphitic carbon nitride (g-C3 N4 ) has the smallest direct band gap due to the sp2 hybridization of carbon and nitrogen forming the ␲conjugated graphitic planes [22]. The polymeric g-C3 N4 has been successfully used as metal-free heterogeneous catalysts for FriedelCrafts reactions and the chemical binding and reduction of CO2

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[23]. Recently, Wang et al. first reported that the g-C3 N4 with a band gap of 2.7 eV achieved functionally as a stable photocatalyst for H2 production from water containing electron donor under visible light irradiation [24]. Several following literatures have demonstrated the photocatalytic performance for H2 and O2 evolution via water splitting [25–28]. Very recently, Yan et al. synthesized the g-C3 N4 and applied as photocatalysts to degrade organic dyes (methyl orange) under visible light irradiation. However, the photocatalytic activity of pure g-C3 N4 is still low due to the fast recombination of photo-generated charge carriers [29]. One of the promising methods is to design metal–semiconductor composites by combing noble metal nanoparticles with semiconductors to facilitate charge separation [30,31]. The Ag@AgCl [32], Ag–Bi2 WO6 [33], Ag/AgBr [34] have been demonstrated to be efficient photocatalysts under visible light irradiation. The possible reason is that the noble metals can strongly absorb visible light owing to their localized surface plasmon resonance (SPR) and the SPR can significantly enhance the absorption of visible light [35,36]. Maeda et al. reported that the noble metals such as Pt, Au and Pd deposited on g-C3 N4 exhibited enhanced activity for H2 evolution [27]. Yan et al. deposited Ag onto g-C3 N4 surface and improve the photocatalytic activity [29]. However, the effect of noble metal Ag loading amount on the optical absorption, photoluminescence property and photocatalytic activity of g-C3 N4 was not discussed, and the photocatalytic mechanism was needed to further investigate. In this work, noble metal silver was introduced to g-C3 N4 photocatalyst with different weight ratios for the first time. The effect of Ag loading amount on the optical absorption, photoluminescence property and photocatalytic performance were investigated in detail. The photocatalytic mechanism of Ag/g-C3 N4 photocatalyst was proposed based on photocatalytic results and PL spectra, and the enhanced photo-activity came from the promotion of charge separation efficiency caused by the synergy between Ag and g-C3 N4 . Our results demonstrated that Ag/gC3 N4 exhibited significantly enhanced photocatalytic activity in degradation of methyl orange and hydrogen evolution, and the composite photocatalysts was stable after cycling photocatalytic experiments.

2. Experimental

3.0wt% Ag/g-C3N4 2.0wt% Ag/g-C3N4 1.5wt% Ag/g-C3N4 1.0wt% Ag/g-C3N4 0.5wt% Ag/g-C3N4

Pure g-C3N4

30

60

2

90

Deg

Fig. 1. XRD patterns of pure g-C3 N4 and Ag/g-C3 N4 composite photocatalysts with different Ag contents.

2.2. Characterization The crystal structure of the Ag/g-C3 N4 composite photocatalysts was investigated by using X-ray diffraction (Rigaka D/max 2500v/pc X-ray diffractometer) with Cu K␣ radiation at a scan rate of 0.1◦ 2 s−1 . The accelerating voltage and the applied current were 40 kV and 40 mA, respectively. The morphology of the samples was detected by using a FEI Quanta 200F field emission scanning microscopy (FE-SEM) with an accelerating voltage of 10 kV, and transmission electron microscopy (TEM; JEOL JEM2100; accelerating voltage, 200 kV). High-resolution transmission electron microscopy (HR-TEM, FEI Tecnai G2 F20) was operated at 200 kV to observe the crystallinity and arrangement of Ag and gC3 N4 . X-ray photoelectron spectroscopy (XPS) measurements were done on a PHI Quantum 1600 XPS instrument with a monochromatic Mg K␣ source. The binding energy scale was calibrated with respect to C1s peak of hydrocarbon contamination of 284.6 eV. UV–vis diffuse reflection spectra were recorded on a Shimadzu UV-3100 spectrophotometer, using BaSO4 as reference. The photoluminescence (PL) spectra of photocatalysts were detected on a Varian Cary Eclipse spectrometer with excitation wavelength of 325 nm.

2.1. Synthesis of photocatalysts 2.3. Photocatalytic activity All chemicals are reagent grade and used without further purification. The metal-free g-C3 N4 powders were synthesized by heating melamine in a muffle furnace. In a typical synthesis run, 5 g of melamine was placed in a semi-closed alumina crucible with a cover. The crucible was heated to 500 ◦ C and held for 2 h at a heating rate of 10 ◦ C min−1 . Further deammoniation treatment was performed at 520 ◦ C for 2 h. After the reaction, the alumina crucible was cooled to room temperature. The products were collected and ground into powders. The preparation of Ag/g-C3 N4 composite photocatalysts is described as follows: the as-prepared g-C3 N4 sample was added to 5 ml of distilled water containing an appropriate amount of AgNO3 in a ceramic dish. The suspension was stirred using a glass rod during evaporation of water under the irradiation of an infrared light. The resulting powder was collected and calcined in air at 300 ◦ C for 1 h. Then, Ag/g-C3 N4 composite photocatalysts with different amounts of Ag were obtained. The weight percentages of Ag in the initial photocatalyst precursors were 0, 0.5, 1.0, 1.5, 2.0, 3.0, 4.0 and 5.0 wt%, respectively.

The photocatalytic activities of Ag/g-C3 N4 composite samples were evaluated by the photocatalytic degradation of methyl orange in an aqueous solution under visible light irradiation. A 500 W Xe lamp with a 420 nm cutoff filter was used as the light source to provide visible light irradiation. The irradiating intensity is 1.07 mW/cm2 . In each experiment, a 0.15 g amount of photocatalyst was added into 50 ml methyl orange solution with a concentration of 10 mg l−1 . Prior to irradiation, the suspensions were magnetically stirred in the dark for 30 min to obtain the saturated absorption of methyl orange onto the catalysts. At irradiation time intervals of every 0.5 h, the suspensions were collected, and then centrifuged (5000 rpm, 10 min) to remove the photocatalyst particles. The concentrations of the methyl orange were monitored using a UV–vis spectrophotometer (Japan Shimadzu UV-vis 1700) by checking the absorbance at 505 nm during the photodegradation process. The photocatalytic H2 evolution experiments were performed in a 300 ml quartz reactor at ambient temperature, and two openings of the reactor were sealed using high vacuum grease. The PLS-SXE

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300UV Xe arc lamp through a UV-cutoff filter (>420 nm) with light intensity of 0.78 mW/cm2 was used as the light source. In a typical photocatalytic experiment, 0.1 g of photocatalyst powder was suspended in 270 ml of aqueous solution containing 25% methanol

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scavenger in volume. Before photocatalytic experiments, the vacuum pump was used for 30 min to remove the dissolved oxygen and to ensure the anaerobic conditions in the reaction vessel. The 300 W Xe lamp with a 420 nm cutoff filter was applied to execute

Fig. 2. TEM, HR-TEM and SEM images of the as-prepared samples. (a) SEM micrographs of pure g-C3 N4 ; (b) 1.0 wt% Ag/g-C3 N4 ; (c) 3.0 wt% Ag/g-C3 N4 ; (d) 5.0 wt% Ag/g-C3 N4 ; (e) TEM micrographs of 1.0 wt% Ag/g-C3 N4 ; (f) HR-TEM images of 1.0 wt% Ag/g-C3 N4 showing the arrangement of g-C3 N4 and Ag crystallites.

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Fig. 3. XPS spectra of 1.0 wt% Ag/g-C3 N4 composite photocatalysts: (a) survey XPS spectrum; (b) C1s; (c) N1s; (d) Ag3d5; (e) Schematic diagram of the chemical structure of polymeric g-C3 N4 .

the photocatalytic reaction, and the products were analyzed by gas chromatograph (Shimadzu GC-8A, high purity Argon as a carrier gas) equipped with a thermal conductivity detector. 3. Results and discussion 3.1. Characterization of the Ag/g-C3 N4 samples XRD is used to investigate the phase structures of the samples, and the typical diffraction patterns are shown in Fig. 1. All of the

peaks in the XRD patterns of the samples could be indexed to the hexagonal phase of g-C3 N4 (JCPDS 87-1526). The peak at 27.4◦ is due to the stacking of the conjugated aromatic system, which is indexed for graphitic materials as the (0 0 2) peak of the g-C3 N4 . No significant diffraction peaks of any other phases or impurities can be detected in the composite samples, which indicate the introducing of Ag species does not affect the crystal structure of g-C3 N4 photocatalysts. However, the diffraction intensities of the peak at 27.4◦ become weaker with increasing Ag contents, which indicate that the introducing of Ag species restrain the growth of crystal

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(h) 5.0wt% Ag-g-C 3N4 (g) 4.0wt% Ag-g-C 3N4 (f) 3.0wt% Ag-g-C 3N4

Intensity (a.u.)

structure of g-C3 N4 . No diffraction peaks of Ag species are detected, which may be explained by the small amounts of Ag species introducing (maximal 5.0 wt%) and high dispersion in the composite samples. The morphology and microstructure of the Ag/g-C3 N4 samples were revealed by SEM, TEM and HRTEM. Fig. 2(a)–(d) shows the SEM micrographs of the as-prepared samples with different Ag doping amount. The products exhibit aggregated morphologies. The pure g-C3 N4 samples appear to have aggregated particles, which contain many smaller crystals. After introducing Ag, the Ag/g-C3 N4 composite samples show agglomeration structures, which are similar with pure g-C3 N4 . The EDS was performed to analyze the Ag contents in the composite samples. The results indicate that the actual Ag contents are 0.53 wt%, 1.06 wt%, 2.91 wt%, respectively, for the Ag/g-C3 N4 samples of 0.5 wt%, 1.0 wt% and 3.0 wt%. Therefore, there is only a little difference of Ag contents between the initial precursors and final products. The morphology of the 1.0 wt% Ag/g-C3 N4 sample was explored by TEM, and showed in Fig. 2(e). The Ag/g-C3 N4 composite sample is found to consist of 10–20 nm primary particles. Fig. 2(f) shows the arrangement of the g-C3 N4 and Ag nanoparticles via HRTEM, where g-C3 N4 and Ag crystallites are seen as having different orientations and lattice spacing. By carefully measuring the lattice parameters using a DigitalMicrograph and comparing with the data in JCPDS, two different kinds of lattice fringes are clearly observed. One fringe with d = 0.336 nm matches the (0 0 2) crystallographic plane of gC3 N4 (JCPDS 87-1526). The lattice spaces of the Ag crystallites are determined as 0.236 nm, belonging to the (1 1 1) crystallographic planes of cubic Ag (JCPDS 89-3722). Therefore, the obvious interface between g-C3 N4 and Ag nanoparticles is observed in Fig. 2(f). This finding suggests that the metal-semiconductor heterojunction structure indeed forms from these two materials, and is confirmed by the HRTEM analysis. The X-ray photoelectron spectroscopy (XPS) was carried out to determine the chemical composition of the 1.0 wt% Ag/g-C3 N4 sample and the valence states of various species present therein. The binding energies obtained in the XPS analysis were corrected for specimen charging by referencing C1s to 284.60 eV. Fig. 3(a) presents the survey scan XPS spectrum of the composite sample. The results indicated the presence of C, N, Ag and a small amount of O, which may be due to the surface absorption and oxidation. Therefore, it may be concluded that the as-prepared sample is Ag/g-C3 N4 phase. Fig. 3(b) shows the high-resolution XPS spectra of C1s. Two peaks can be distinguished to be centered at 284.6 and 288.2 eV. The major peak at 284.6 eV is exclusively assigned to carbon atoms (C–C bonding) in a pure carbon environment, i.e., graphitic or amorphous carbons either in our sample or adsorbed to the surface [37]. The peak at 288.3 eV is identified as originating from carbon atoms bonded with three N neighbors in its chemical structure as shown in Fig. 3(f). Fig. 3(c) presents the XPS spectrum of N1s. It could be deconvoluted into two peaks with binding energies of 398.3 and 399.2 eV. The peak at 399.2 eV corresponds to N atoms bonded to three sp2 carbon atoms in the C–N network. The peak at 398.2 eV is attributed to N atoms sp2 -bonded to two carbon atoms [37]. The XPS data gives an evidence for the existence of graphite-like sp2 bonded structure in graphitic carbon nitride. Fig. 3(f) shows the characteristic Ag3d5 peak that has a 6.0 eV splitting of the 3d doublet, which is corresponding to the metallic Ag0 species [6]. This result confirms the presence of metallic silver doped in the g-C3 N4 photocatalysts. Optical absorption of the as-prepared pure g-C3 N4 and Ag/gC3 N4 samples was investigated using an UV–vis spectrometer. As shown in Fig. 4, the g-C3 N4 sample has photo-absorption from UV light to visible light, and the wavelength of the absorption edge is 460 nm, which could be responsible for the visible-light

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(e) 2.0wt% Ag-g-C 3N4 (d) 1.5wt% Ag-g-C 3N4 (c) 1.0wt% Ag-g-C 3N4 (b) 0.5wt% Ag-g-C 3N4 (a) Pure g-C3N4

300

400

500

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700

800

Wavelength (nm) Fig. 4. UV–vis diffuse reflectance spectra of the as-prepared samples: (a) pure gC3 N4 ; (b) 0.5 wt% Ag/g-C3 N4 ; (c) 1.0 wt% Ag/g-C3 N4 ; (d) 1.5 wt% Ag/g-C3 N4 ; (e) 2.0 wt% Ag/g-C3 N4 ; (f) 3.0 wt% Ag/g-C3 N4 ; (g) 4.0 wt% Ag/g-C3 N4 ; (h) 5.0 wt% Ag/gC3 N4 .

induced photocatalytic activity. However, when Ag species is loaded, the absorption intensity in the visible light region is significantly improved, and the absorption commences to enhance along with the increase of Ag species content, while the powder colors shift from yellowish to light grey with increasing Ag loading. This phenomenon could be attributed to a charge-transfer transition between the Ag species and the g-C3 N4 conduction or valence band. The wavelength threshold is determined by elongating the baseline and the steepest tangent of the UV–vis spectra and the wavelength of the intersection was g . The wavelength threshold of the pure g-C3 N4 samples is 463 nm, corresponding to the band gaps from 2.67 eV. As presented, the wavelength threshold of Ag/g-C3 N4 composite photocatalysts with 1.0 wt% Ag species is estimated as to be 532 nm, corresponding to the band gap of 2.33 eV. The DRS result indicates that the visible light absorptions are enhanced and the band-gaps tend to be narrower with increasing of Ag species content. Therefore, the visible light responses of these Ag/g-C3 N4 series samples are significantly improved by the Ag doping, and thus it should have enhanced photoactivities than the pure g-C3 N4 samples. 3.2. Photocatalytic activity of the samples The photocatalytic activities of as-prepared samples were evaluated by the degradation of organic dye and hydrogen evolution under visible light irradiation. Methyl orange was chosen as a representative hazardous dye to evaluate the photocatalytic performance, which showed a major absorption band at 505 nm. The photodegradation process of methyl orange was recorded by the temporal evolution of the spectrum and all of the samples processed in the similar procedure. Fig. 5 shows the photocatalytic activities of the Ag/g-C3 N4 composite samples with different Ag contents under visible light irradiation ( > 420 nm). The blank test confirms that methyl orange is only slightly degraded in the absence of catalysts, indicating that the photolysis can be ignored. Methyl orange concentration very gradually decreases in the presence of pure g-C3 N4 under visible light. Only 28.9% MO is photodegraded after irradiated for 3 h. However, the photocatalytic performance is significantly enhanced after modification by noble metal Ag, which indicates that introducing Ag species on the surface of g-C3 N4 play primary role in the

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Fig. 5. Degradation rate of methyl orange under visible light irradiation in the presence of Ag/g-C3 N4 samples: (a) blank; (b) pure g-C3 N4 ; (c) 0.5 wt% Ag/g-C3 N4 ; (d) 1.0 wt% Ag/g-C3 N4 ; (e) 1.5 wt% Ag/g-C3 N4 ; (f) 2.0 wt% Ag/g-C3 N4 ; (g) 3.0 wt% Ag/g-C3 N4 ; (h) 4.0 wt% Ag/g-C3 N4 ; (i) 5.0 wt% Ag/g-C3 N4 .

enhancement of methyl orange photodegradation. Among them, the 1.0 wt% Ag/g-C3 N4 photocatalyst exhibits the highest activity; almost 100% of methyl orange can be photodegraded under this condition. The inset in Fig. 5 illustrates the variations in methyl orange absorbance around 505 nm over 1.0 wt% Ag/g-C3 N4 . The absorbance of methyl orange obviously decreases with increase of irradiation time. No absorbance peak is observed after irradiated for 3 h, which indicates complete methyl orange decomposition. Photocatalytic results demonstrated that the doping amount of Ag has a great influence on the photocatalytic activity. As shown in Fig. 5, the degradation rate is 0.933, 1.00, 0.831, 0.727, 0.613, 0.333 and 0.430, respectively, for Ag/g-C3 N4 ratio of 0.5 wt%, 1.0 wt%, 1.5 wt%, 2.0 wt%, 3.0 wt%, 4.0 wt% and 5.0 wt%. Thus, the optimal content of Ag on g-C3 N4 is approximate to 1.0 wt% from the experimental results and the photocatalytic activity is decreased with higher Ag content. This can be explained from that the excess Ag species may act as a recombination center, or cover the active sites on the g-C3 N4 surface and thereby reduce the efficiency of charge separation. Therefore, it is important to make a balance between the active trapping sites favoring the inhibition of the recombination of electron–hole pairs and fewer trapped parts leading to a lower capacity for the separation of interfacial charge transfer. According to our results, the photocatalytic activity of g-C3 N4 modified by Ag is effectively enhanced compared with that of pure g-C3 N4 sample. The reason should be attributed to the interaction between g-C3 N4 and Ag, which takes an important role in the enhancement of photocatalytic performance. To quantitatively investigate the reaction kinetics of the methyl orange degradation, the experimental data can be fitted by applying a first-order model as expressed by Eq. (1) [3]. This equation is well established for photocatalytic experiments when the pollutant is within the millimolar concentration range. −ln

C C0

= kt,

(1)

where C0 and C are the dye concentrations in solution at times 0 and t, respectively, and k is the apparent first-order rate constant.

Fig. S2 shows the effect of Ag content on the methyl orange photodegradation rate using Ag/g-C3 N4 . Upon varying the Ag content within 0.5–5.0 wt%, the plot of the irradiation time (t) against ln(C0 /C) is nearly straight line. Fig. S3 shows that the Ag content greatly influences the photo-degrading rate (k) of the composite samples. The 1.0 wt% Ag/g-C3 N4 sample exhibits the highest photodegrading efficiency, which is about 23-folds higher than that of pure g-C3 N4 . Therefore, the 1.0 wt% Ag/g-C3 N4 is the best performing sample and is selected for the cycling study. The colored dyes such as methyl orange and rhodamine B usually absorb visible light and could be degraded by dye sensitization effect in the presence of photocatalysts [38]. The photocatalytic H2 production from colorless methanol aqueous solution is a typical photocatalytic reaction without dye sensitization effect [28,39]. Therefore, in this study, the photocatalytic H2 evolution via water reduction was performed to exclude the dye sensitizing effect. Fig. 6 shows the photocatalytic H2 evolution over the Ag/g-C3 N4 composite samples with different Ag contents under visible light irradiation. As shown in Fig. 6, the Ag content has a significant influence on the photocatalytic activity of g-C3 N4 . The pure g-C3 N4 sample without Ag loading shows visible light photocatalytic activity and the H2 production rate reaches 0.862 ␮mol h−1 . The photocatalytic performance is significantly enhanced after introduction of silver. The photocatalytic activity of the composite samples is further improved with increasing Ag content from 0.5 wt% to 5.0 wt%. The 1.0 wt% Ag/g-C3 N4 shows the highest H2 evolution rate of 10.105 ␮mol h−1 , which is about 11.7 folds higher than that of pure g-C3 N4 . The Ag content is pivotal for achieving the high photocatalytic activity of the Ag/g-C3 N4 composite. The suitable Ag content causes its well dispersion on the g-C3 N4 surface, which favors the transfer and separation of the charge carriers [40]. As shown in Fig. 6, when the Ag content is higher than 1.0 wt%, a further increase in Ag content causes a decrease in the photocatalytic H2 evolution. The stability of a photocatalyst is important for its assessment and application. To evaluate the stability of the composite catalysts, we carried out recycling photocatalytic H2 evolution tests on 1.0 wt% Ag/g-C3 N4 samples under the same conditions. Fig. 7

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pure g-C3N4

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0.5wt% Ag/g-C3N4

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Rate of H 2 evolution ( mol h )

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1.5wt% Ag/g-C3N4 2.0wt% Ag/g-C3N4 3.0wt% Ag/g-C3N4 4.0wt% Ag/g-C3N4 5.0wt% Ag/g-C3N4

400

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Wavelength (nm)

Samples Fig. 6. Comparison of the photocatalytic activity of the Ag/g-C3 N4 composite samples for the H2 production from methanol aqueous solution under visible light irradiation.

displays the H2 evolution curve in every photocatalytic run. No obvious decrease of H2 evolution was observed after four cycles. Therefore, the Ag/g-C3 N4 photocatalyst can be concluded as stable during the photocatalytic H2 evolution process. Notably, the photocatalytic activities of Ag/g-C3 N4 composite samples are significantly enhanced in the presence of small amounts of Ag species distributed over pure g-C3 N4 photocatalysts, which can be explained from the photoluminescence (PL) analysis. The photoluminescence spectra are widely used to investigate the migration, transfer, and recombination processes of the photogenerated electron–hole pairs in a semiconductor, since PL emission arises from the recombination of free carriers. Fig. 8 shows the PL spectra of pure g-C3 N4 and Ag/g-C3 N4 samples excited by 325 nm. From the figure, it can be observed that there is a significant decrease in the PL intensity of Ag/g-C3 N4 compared to that of pure g-C3 N4 . A weaker intensity of the peak represents a lower recombination probability of photogenerated charge carriers. Therefore, the Ag dispersed on the surface of g-C3 N4 could effectively inhibit the recombination of photogenerated charge carriers, which is helpful for the separation of photogenerated electron–hole pairs in g-C3 N4 . The significant PL quenching is observed in the Ag/g-C3 N4

Fig. 8. Photoluminescence (PL) spectra of pure g-C3 N4 and Ag/gC3 N4 samples.

composite photocatalysts as the content of Ag increases. The quenching is due to the photogenerated charge transfer between the Ag and g-C3 N4 . It can be established that the Ag/g-C3 N4 composite photocatalysts are very promising for the photocatalysis with satisfying efficiency. 3.3. Photocatalytic mechanisms The photocatalytic results have shown the excellent photoactivity of the Ag/g-C3 N4 composite samples on degradation of methyl orange and hydrogen evolution. It follows that the Ag/g-C3 N4 composite photocatalysts may have highly potential applications in the conservation of the environment. Not only limited to the experimental results, the photocatalytic mechanism was necessary to investigate and guide the further improvement of its photocatalytic performance. The specific schematic diagrams of methyl orange degradation and hydrogen evolution over Ag/g-C3 N4 composite photocatalyst are illustrated in Figs. 9 and 10. The doped Ag particles could act as electron traps to facilitate the separation of photogenerated electron–hole pairs and promote interfacial electron transfer process. In the photocatalytic reaction process, it is well established that conduction band electrons (e− ) and valence band holes (h+ ) are generated when aqueous Ag/g-C3 N4 composite suspension is

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Rate of H2 evolution ( mol h )

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Irradiation time (h) Fig. 7. Cycling runs for the photocatalytic H2 evolution in the presence of 1.0 wt% Ag/g-C3 N4 composite under visible light irradiation.

Fig. 9. The schematic diagram of methyl orange degradation over Ag/g-C3 N4 composite photocatalyst under visible light irradiation.

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Acknowledgements This work was financially supported by the National Science Foundation of China (Grant No. 21003157), Beijing Nova Program (Grant No. 2008B76) and Doctor Foundation of Chinese Ministry of Education (Grant No. 200804251014). We thank the Microstructure Laboratory for Energy Materials and Prof. Lishan Cui for the support of TEM and SEM testing. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.apcata.2011.10.006. References

Fig. 10. The photocatalytic hydrogen evolution mechanism for the enhanced electron transfer in the Ag/g-C3 N4 composites.

irradiated with visible light. The recombination of the electrons and the holes must be prevented as much as possible if a photocatalyzed reaction want to be favored. In this photo-degradation system, the photogenerated electrons could react with electron acceptors such as O2 existed in the system, reducing it to superoxide radical anion O2 •− . The photogenerated electrons excited to the conduction band of g-C3 N4 under visible light irradiation are entrapped by Ag nanoparticles due to its high Schottky barriers at the metal–semiconductor interface (Ag/g-C3 N4 ), which was confirmed by the photoluminescence (PL) spectra. Thus, the transfer of charge carriers has been improved and the recombination of the electrons and holes has been inhibited. The electrons could be scavenged by present molecular oxygen absorbed on the surface and produce the reactive oxygen radicals, while the valence holes photogenerated on the surface of g-C3 N4 could not react with OH− or H2 O molecules to form • OH radicals, which can ascribed to the reason that the standard redox potential of the g-C3 N4 valence band (+1.57 eV) is more negative than that of • OH/OH− (+1.99 eV). The reactive oxygen radicals and the photogenerated holes are responsible for the degradation of methyl orange. As shown in Fig. 10, for the hydrogen evolution system, the injected electrons accumulate on the Ag nanoparticles and can effectively reduce H2 O to produce H2 , while the holes at the valence band of g-C3 N4 can react with methanol as a sacrificial reagent. Therefore, the recombination of the electron–hole pairs can be inhabited; the Ag/g-C3 N4 composite photocatalysts show the excellent photocatalytic performance and have very promising applications in the field of environmental purification. 4. Conclusions Novel Ag/g-C3 N4 composite photocatalysts were prepared via a facile heating method. The doping of Ag species did not affect the morphology and the crystal structure of g-C3 N4 photocatalysts. The Ag species was determined to be metallic Ag0 . The Ag/g-C3 N4 displayed strong light absorption and a red-shift in the visible light region. The composite photocatalysts exhibited enhanced photocatalytic activity in the presence of small amounts of Ag species distributed over pure g-C3 N4 photocatalysts under visible light irradiation, and the highest efficiency was observed at 1.0 wt% Ag/g-C3 N4 sample. The enhanced photocatalytic activity can be attributed to the reason that electrons can be trapped by Ag0 on the surface of catalyst, and the presence of Ag0 could increase the interfacial charge transfer and inhibit the recombination of electron–hole pairs. The Ag/g-C3 N4 composite photocatalyst is a promising photocatalytic material which has good potential for application to pollutants purification.

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