Ag3PO4 Hybrid

Apr 18, 2014 - Photocatalysts through Z‑Scheme Photocatalytic Mechanism under ... C3N4 photocatalyst is combined with Ag3PO4, the obtained g-...
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Highly Efficient Photocatalytic Activity of g‑C3N4/Ag3PO4 Hybrid Photocatalysts through Z‑Scheme Photocatalytic Mechanism under Visible Light Hideyuki Katsumata,*,†,§ Tsubasa Sakai,†,§ Tohru Suzuki,‡ and Satoshi Kaneco† †

Department of Chemistry for Materials, Graduate School of Engineering, Mie University, Tsu, Mie 514-8507, Japan Environmental Preservation Center, Mie University, Tsu, Mie 514-8507, Japan

‡

S Supporting Information *

ABSTRACT: Highly efficient visible-light-driven g-C3N4/Ag3PO4 hybrid photocatalysts with different weight ratios of g-C3N4 were prepared by a facile in situ precipitation method and characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectrometry and UV−vis diffuse reflectance spectroscopy. Under visible-light irradiation (>440 nm), g-C3N4/Ag3PO4 photocatalysts displayed the higher photocatalytic activity than pure g-C3N4 and Ag3PO4 for the decolorization of methyl orange (MO). Among the hybrid photocatalysts, g-C3N4/Ag3PO4 with 25 wt % of g-C3N4 exhibited the highest photocatalytic activity for the decolorization of MO. The complete decolorization of MO was achieved for only 5 min of visible-light irradiation. X-ray photoelectron spectroscopy results revealed that metallic Ag particles on the surface of g-C3N4/Ag3PO4 hybrid were formed during the catalysts preparation. In addition, the quenching effects of different scavengers displayed that the reactive h+ and O2•− play the major role in the MO decolorization. The photocatalytic activity enhancement of g-C3N4/Ag3PO4 hybrid photocatalysts could be ascribed to the efficient separation of electron−hole pairs through a Z-scheme system composed of Ag3PO4, Ag and g-C3N4, in which Ag particles act as the charge separation center. The evidence of the Z-scheme photocatalytic mechanism of the hybrid photocatalysts could be obtained from a photoluminescence technique.

1. INTRODUCTION Semiconductor photocatalysis is one of the most important technologies for environmental remediation because it can utilize solar energy to decompose various organic compounds in open air and aqueous conditions.1 Recently, it has been shown that silver orthophosphate (Ag3PO4) possesses excellent photocatalytic properties, owing to the efficient separation of photoexcited electrons and holes. Interestingly, Ag 3 PO4 exhibited extremely high oxidative capabilities, under visiblelight irradiation, for the evolution of O2 from water, as well as for the decolorization of organic dyes.2 Moreover, the photocatalytic activity of Ag3PO4 can be further improved through the preparation on the shape, morphology and crystal face of Ag3PO4 crystals.3−11 In addition, several kinds of compounds have been used as composite materials with Ag3PO4, which include wide-band gap semiconductors (TiO2, ZnO, SnO2, CeO2, etc.),12−16 narrow-band gap semiconductors (BiMoO4, Bi2WO6, CdS and BiOI)17−20 and carbon materials (oxidized graphene, graphene and carbon quantum dots).21−25 The Ag3PO4-based composite with a wide-band gap semiconductor results from the low visible-light activity due to the wide-band gap component. The Ag3PO4 mixed with carbon materials can facilitate electron transport and inhibit the electron−hole recombination because of the excellent electrical conductivity of carbon materials. However, the carbon materials generally have a more negative conduction band potential than that of Ag3PO4, which prevents the photogenerated electrons transfer from Ag3PO4 to the carbon materials. Among them, the hybrids with narrow-band gap semiconductors are considered © 2014 American Chemical Society

to be useful photocatalysts because they can improve the optical absorption property and enhance the photogenerated charge separation. As a typical metal free inorganic semiconductor, graphitic C3N4 (g-C3N4) has attracted intensive attention for H2 generation,26−28 pollutant degradation29 and CO2 reduction.30,31 It is well-known that the narrow-band gap of gC3N4 is about 2.7 eV, which can absorb visible light up to 460 nm.32,33 Furthermore, the CB minimum of g-C3N4 is extremely negative, so photogenerated electrons should have a high reduction ability. However, the photocatalytic efficiency of the pure g-C3N4 is limited by the high recombination rate of its photogenerated electron−hole pairs.34 As described above, one of the techniques for increasing the separation efficiency of photogenerated electron−hole pairs is to form a composite photocatalyst using two kinds of semiconductors. Suitable matching of the band levels of the conduction and valence bands in the two semiconductors offers appropriate driving forces to separate and transfer photogenerated electron−hole pairs.35 Many studies have confirmed that g-C3N4 is a good candidate for synthesizing semiconductor heterojunctions with higher photocatalytic activity.33 As Ag3PO4 and g-C3N4 are both visible-light-driven photocatalysts, after the polymeric gC3N4 photocatalyst is combined with Ag3PO4, the obtained gReceived: Revised: Accepted: Published: 8018

March 21, 2014 April 18, 2014 April 18, 2014 April 18, 2014 dx.doi.org/10.1021/ie5012036 | Ind. Eng. Chem. Res. 2014, 53, 8018−8025

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C3N4/Ag3PO4 composite may be a promising candidate for efficient photocatalytic activity under visible-light irradiation. However, there are a only few reports on the photocatalytic activity evaluation of g-C3N4/Ag3PO4 hybrid photocatalyst.36−40 Furthermore, no attention has been paid to the photocatalytic mechanism of g-C3N4/Ag3PO4-catalyzed photodegradation under visible light, which has remained unclear to date. In this paper, novel g-C3N4/Ag3PO4 hybrid photocatalysts with different weight ratios of g-C3N4 were synthesized through an in situ precipitation method and were characterized. Methyl orange (MO, C14H14N3NaO3S) was used as a model pollutant to evaluate the photocatalytic activity of the g-C3N4/Ag3PO4 hybrid under visible-light irradiation (>440 nm), and the optimal weight fraction of g-C3N4 was determined. In addition, the possible mechanism of g-C3N4/Ag3PO4 photocatalysis is discussed based on radical trapping and hydroxyl radical detection experiments, and a photoluminescence (PL) technique.

were measured at room temperature using a Shimadzu RF5300PC system equipped with solid sample holder. 2.3. Photocatalytic Activity and Detection of Reactive Oxygen Species. The photocatalytic activities of the sample photocatalysts were evaluated by the decolorization of MO under visible-light irradiation. Typically, 30 mL of MO solution and 30 mg of photocatalyst were added to a 50 mL Pyrex glass cell. The initial concentration of MO in all experiments was 10 mg/L and the MO solution containing the photocatalyst powder was magnetically stirred before and during irradiation. The original pH of the dye solution was ca. 6.4. Before irradiation, the photocatalyst suspension containing MO was allowed to equilibrate for 30 min in the dark. The sample solution was irradiated with a 300 W Xe lamp (MAX-303, Asahi Spectra) in conjunction with a UV cut filter (Y-44, HOYA), which was positioned on the side of the reaction cell. After the desired irradiation time, the sample solution was filtered through a 0.20 μm PTFE membrane filter. The decolorization of MO was determined by measuring the absorbance of the solution at 464 nm using a UV−visible spectrophotometer (Shimadzu UV-1650PC). All experiments were conducted in triplicates and the results showed at the mean values. The relative standard deviations for the activities of the photocatalysts were range of 2.8−7.4%. The procedures of the scavenging experiments of reactive oxygen species and the detection of ·OH are described elsewhere.41

2. EXPERIMENTAL SECTION 2.1. Preparation of Photocatalysts. All chemicals used in this study were of analytical grade and were used without further purification. g-C3N4 powder was prepared by direct heating of urea (Nacalai tesque) at 500 °C in a muffle furnace for 2 h in an alumina crucible with a cover at a heating rate of 20 °C/min; the further heat treatment was performed at 520 °C for 2 h. After the product cooled to room temperature, it was then collected and ground into a powder. Ag3PO4 was synthesized by a simple precipitation reaction in a dark condition at room temperature. In a typical synthesis, 50 mL of 0.05 M AgNO3 (Nacalai tesque) aqueous solution was dropped into 0.1 M Na3PO4 (50 mL, Wako Pure Chemicals) aqueous solution under mild stirring. After the solution was stirred for 3 h, the obtained precipitate was collected, washed with water for 3 times and dried at 60 °C for 24 h. The synthesis of g-C3N4/ Ag3PO4 hybrid photocatalysts was as follows: 0.42 g of AgNO3 was dissolved into 50 mL of water, and then an appropriate amount of g-C3N4 powder was suspended into this solution and stirred at room temperature for 1 h. Next, 50 mL of 0.1 M Na3PO4 aqueous solution was dropped into the suspended solution and stirred for 3 h. The obtained product was collected, washed with water and dried. g-C3N4/Ag3PO4 hybrid photocatalysts with different weight ratios of g-C3N4 were prepared. The weight ratios of g-C3N4 to Ag3PO4 were 0.1:1, 0.25:1, 0.50:1 and 0.70:1 and were denoted as 10, 25, 50, and 70 wt % g-C3N4/Ag3PO4, respectively. 2.2. Characterization. The powder X-ray diffraction (XRD, RIGAKU Ultima IV, sample horizontal type) was used in order to record the diffraction patterns of photocatalysts employing Cu Kα radiation. A Hitachi S-4000 scanning electron microscope and a JEOL JEM-1011 transmission electron microscope were employed to observe the morphologies of the samples. Fourier transform infrared spectra of the samples were recorded using a SPECTRUM 100 FTIR spectrometer (Perkin Elmer) equipped with an ATR assembly. X-ray photoelectron spectroscopy (XPS) measurements were carried out with a PHI Quantera SXM photoelectron spectrometer using Al Kα radiation. The UV−visible diffuse reflectance spectra of the photocatalysts were recorded using a Shimadzu UV-2450 spectrophotometer equipped with an integral sphere assembly. PL spectra of photocatalyst powders

3. RESULTS AND DISCUSSION 3.1. Characterization. Figure 1 displays the XRD patterns of g-C3N4/Ag3PO4 hybrid photocatalysts with different weight

Figure 1. XRD patterns of (a) g-C3N4, (b) Ag3PO4, (c) 10 wt %, (d) 25 wt %, (e) 50 wt %, (f) 70 wt % g-C3N4/Ag3PO4 hybrid photocatalysts.

ratios of g-C3N4, together with those of pure g-C3N4 and Ag3PO4. It was observed that Ag3PO4 was indexed as bodycentered cubic phase (JCPDS No. 06-0505), while two broad peaks around 27.4 and 13.0° were observed in the XRD pattern of g-C3N4, corresponding to the (002) and (100) diffraction planes, respectively. The former, which corresponds to the interlayer distance of 0.326 nm, is attributed to the long-range interplanar stacking of aromatic units; the latter, with a much weaker intensity, which corresponds to a distance d = 0.681 nm, is associated with interlayer stacking.26 The g-C3N4/ Ag3PO4 hybrids exhibited a coexistence of both g-C3N4 and Ag3PO4 phases. Although the characteristic peak of g-C3N4 was 8019

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Figure 2. SEM and TEM images of (a) (d) g-C3N4, (b) (e) Ag3PO4, (c) (f) 25 wt % g-C3N4/Ag3PO4 hybrid photocatalyst.

not observed from the 10 wt % g-C3N4/Ag3PO4 hybrid, the intensity of the (002) diffraction peak of g-C3N4 gradually increased with the weight ratio of g-C3N4 in the hybrids. No other crystal phases were found in the XRD patterns. Figure 2 shows scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of g-C3N4, Ag3PO4 and 25 wt % g-C3N4/Ag3PO4 composite samples. It is clearly seen in Figure 2a,d that the morphology of g-C3N4 was smooth, thin and flat sheets. In addition, a typical porous morphology of g-C3N4 powders was exhibited.42 From Figure 2b,e, as-prepared Ag3PO4 particles have an irregular spherical morphology with about 0.1−2.0 μm and a tendency to aggregate. Ag3PO4 particles were well anchored on the surface of g-C3N4 in g-C3N4/Ag3PO4 hybrid, as shown in Figure 2c,f. The diameter of Ag3PO4 in the hybrid was almost the same as the pure Ag3PO4. In addition to relatively large size Ag3PO4, a large number of Ag3PO4 particles with about 6−16 nm were clearly found on the surface of g-C3N4 in a higher magnification TEM image of the hybrid (Figure S1, Supporting Information). The morphology studies revealed that g-C3N4 could serve as a support to bound Ag3PO4 particles in the hybrid system, which leads to the formation of interfaces between g-C3N4 and Ag3PO4 and to favorable photogenerated charge transfer at the interface. Fourier transform infrared spectrometry (FTIR) spectra of gC3N4, Ag3PO4 and g-C3N4/Ag3PO4 hybrid photocatalysts are shown in Figure 3. For the pure g-C3N4, the peaks at 1655 cm−1 could be attributed to C−N stretching vibration modes, whereas those at 1255, 1336, 1430 and 1592 cm−1 could be assigned to aromatic C−N breathing modes.43 The sharp band at 812 cm−1 and the broad bands at around 3200 cm−1 were indicative of the breathing mode of the triazine units and the N−H stretching vibration modes, respectively.34,44 For the pure Ag3PO4, The two peaks at 972 and 554 cm−1 are assigned to the P−O stretching vibration modes of PO4.45 All the characteristic peaks of g-C3N4 and Ag3PO4 were observed in the g-C3N4/Ag3PO4 hybrid composite photocatalyst. It was clearly seen that only Ag, P, O, C and N elements were detected in the XPS survey spectrum (Figure 4a). No peaks for other elements were found, indicating that the g-C3N4/Ag3PO4 hybrid photocatalyst is primarily composed of Ag, P, O, C and N elements. It can be seen from the C 1s spectrum (Figure 4c)

Figure 3. FTIR of (a) g-C3N4, (b) 25 wt % g-C3N4/Ag3PO4 hybrid and (c) Ag3PO4.

that the g-C3N4/Ag3PO4 composite showed the two C 1s peaks located at 284.6 and 287.8 eV. The former is ascribed to the adventitious hydrocarbon from the XPS instrument itself and defect-containing sp2-hybridized carbon atoms present in graphitic domains, whereas the latter one is assigned to C− N−C coordination.46 In the N 1s spectrum (Figure 4e), several binding energies could be separated including triazine rings (C−N−C, 398.4 eV), tertiary nitrogen (N−(C)3, 400.1 eV), amino functions (N−H, 401.6 eV) and the charge effects (403.8 eV).46 As shown in Figure 4d, the Ag 3d peaks of the gC3N4/Ag3PO4 sample could be separated as the Ag+ and Ag0 peaks. The weak peaks at 369.4 and 375.2 eV were attributed to Ag0, whereas the strong peaks at 367.8 and 373.8 eV were assigned to Ag+ of Ag3PO4,47 indicating that Ag0 was formed on the surface of g-C3N4/Ag3PO4 photocatalysts and has contact with both Ag3PO4 and C3N4, which can act as a charge transmission bridge in the hybrid catalyst.48 This indicates that a little amount of Ag0 was formed during the hybrid compound preparation because the electron-rich structure of g-C3N4 provides electrons to Ag+ on the surface of Ag3PO4 to form Ag0 particles. The O 1s peak centered at 530.6 eV is associated with the O2− in Ag3PO4. The other O 1s peak at 532.4 eV is associated with the presence of an −OH group or a water molecule on the surface of the g-C3N4/Ag3PO4 composite photocatalyst (Figure 4f). A broad peak in the range of 131 to 135 eV of the P 2p spectrum is observed for the hybrid sample, 8020

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According to the plot of (αhν)n/2 vs hν, the band gaps (Eg) of Ag3PO4 and g-C3N4 were estimated to be 2.45 and 2.75 eV, respectively (Figure S2, Supporting Information).37,38 The band structure of g-C3N4/Ag3PO4 can be estimated according to the following empirical equations: E VB = χ − E e + 0.5Eg ECB = E VB − Eg

where EVB and ECB are the valence and conduction band edge potentials, respectively; χ is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms; Ee is the energy of free electrons on the hydrogen scale (about 4.5 eV vs NHE). The χ values for g-C3N4 and Ag3PO4 are 4.7249 and 5.96 eV,20 respectively. Thus, the EVB’s of g-C3N4 and Ag3PO4 are calculated to be 1.60 and 2.69 V vs NHE and their corresponding ECB ’s are −1.15 and 0.24 V vs NHE, respectively. 3.2. Photocatalytic Activity of g-C3N4/Ag3PO4 Hybrid Photocatalysts. Figure 6 displays the photocatalytic decolor-

Figure 4. XPS spectra of 25 wt % g-C3N4/Ag3PO4 hybrid photocatalyst.

which is corresponding to the phosphorus came from PO43− (Figure 4b). Figure 5 displays the UV−vis diffuse reflectance spectra of the g-C3N4/Ag3PO4 hybrid composites with different weight Figure 6. Photocatalytic activities of g-C3N4, Ag3PO4 and 10, 25, 50, and 70 wt % g-C3N4/Ag3PO4 hybrid photocatalysts on the decolorization of MO under visible-light irradiation (>440 nm).

ization of MO as a function of irradiation time over different photocatalysts. On the basis of the experiment without any photocatalyst (blank), direct photolysis of MO under visible light could be neglected. It can be seen from Figure 6 that all the g-C3N4/Ag3PO4 hybrid photocatalysts showed higher photocatalytic activities than pure g-C3N4 and Ag3PO4. For pure g-C3N4, the photocatalytic activity was the lowest and the decolorization of MO was 8.3% and 44.7% for 15 min and 3 h (Figure S3, Supporting Information) of visible-light irradiation, respectively. Among the hybrid composites, 25 wt % g-C3N4/ Ag3PO4 exhibited the highest photocatalytic activity for the decolorization of MO. Surprisingly, it took only 5 min of visible-light irradiation for the complete decolorization of MO over 25 wt % g-C3N4/Ag3PO4 hybrid. However, the complete decolorization of MO over pure Ag3PO4 spent about 15 min. In addition, the mechanical mixture of g-C3N4 and Ag3PO4 with a 0.25:1 weight ratio, in which the weight ratio of g-C3N4 and Ag3PO4 in the sample was the same as that of the 25 wt % gC3N4/Ag3PO4 hybrid, showed lower photocatalytic performance than the 25 wt % g-C3N4/Ag3PO4 hybrid, even lower than that of pure Ag3PO4, suggesting that the efficient charge

Figure 5. UV−vis diffuse reflectance spectra of g-C3N4, Ag3PO4 and 10, 25, 50 and 70 wt % g-C3N4/Ag3PO4 hybrid photocatalysts.

ratios of g-C3N4, together with those of Ag3PO4 and g-C3N4. Pure g-C3N4 has an absorption edge at about 460 nm, whereas pure Ag3PO4 has a broader absorption in the visible region with an absorption edge at about 530 nm. The g-C3N4/Ag3PO4 hybrids exhibit the two absorption edges at both 460 and 530 nm, implying a combination of the optical absorption property of g-C3N4 with that of Ag3PO3. For the g-C3N4/Ag3PO4 hybrids, as the weight ratio of g-C3N4 increased, the absorption edge at about 460 nm for g-C3N4 increased whereas that at 530 nm for Ag3PO4 simultaneously decreased. 8021

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transfer at their interface, which could be attributed to the heterojunction between g-C3N4 and Ag3PO4 formed in the gC3N4/Ag3PO4 hybrid photocatalyst, occurred in g-C3N4/ Ag3PO4 rather than in the mixture. The stability of the as-synthesized photocatalyst was studied by several recycle test experiments. The recycle test was performed five times on 25 wt % g-C3N4/Ag3PO4 and pure Ag3PO4 samples. Both the photocatalytic stabilities of Ag3PO4 and g-C3N4/Ag3PO4 greatly deteriorated (Figure S4, Supporting Information) because of an increment to the Ag0 phase during photocatalysis.50 In fact, the diffraction peak at 38.2° (JSPDS 04-0783) assigned to Ag0 was found in the used 25 wt % g-C3N4/Ag3PO4 after one recycling run, whereas the crystal structures of both g-C3N4 and Ag3PO4 were well maintained (Figure S5, Supporting Information). To further confirm the increase of metallic Ag in the used g-C3N4/Ag3PO4 hybrid photocatalyst, the used sample was examined by XPS (Figure S6, Supporting Information). The atom contents of metallic Ag in Ag species on fresh and used 25 wt % g-C3N4/Ag3PO4 hybrids were 4.0% and 11%, respectively, indicating that the photogeneration of Ag0 particles from the hybrid sample gradually occurs during the photocatalytic reaction. For a large number of metallic Ag particles, it is unlikely to transfer electrons to O2, although Ag0 particles could work as an electron trapping center;51 therefore, the photocatalytic activity of the hybrid photocatalyst gradually decreased. However, the g-C3N4/Ag3PO4 hybrid photocatalyst exhibited much higher photocatalytic activities than pure Ag3PO4 all along in the recycling photocatalytic process. Further, the decolorization efficiency of pure Ag3PO4 rapidly decreased with the number of recycles whereas that of the composite was maintained during three to five recycling runs. After five recycling runs, the photocatalytic activity of the hybrid sample was still as high as that of the fresh Ag3PO4 photocatalyst. Because MO can absorb visible light, the sensitization for the g-C3N4/Ag3PO4 hybrid should be considered. To ensure the photocatalytic activity of g-C3N4/Ag3PO4 and exclude the dye sensitization under visible light, the activity test was investigated for the degradation of colorless organic compound under visible-light irradiation. 2-Chlorophenol (2-CP) was chosen as a model compound to evaluate the photocatalytic performance of the g-C3N4/Ag3PO4 hybrid. The degradation of 2-CP increased with increasing the irradiation time, indicating that the g-C3N4/ Ag3PO4 hybrid could effectively degrade 2-CP under visible light (Figure S7, Supporting Information). This result is powerful evidence to ensure the photocatalytic performance of the g-C3N4/Ag3PO4 hybrid because colorless 2-CP cannot sensitize g-C3N4/Ag3PO4 hybrid photocatalysts. 3.3. Photocatalytic Mechanism of g-C3N4/Ag3PO4 Hybrid Photocatalysts. To understand the higher photocatalytic activity of the g-C3N4/Ag3PO4 hybrid relative to gC3N4 and Ag3PO4, the PL spectra of the photocatalysts excited at 265 nm were recorded (Figure 7). It is well-known that the recombination of electron−hole pairs can release energy in the form of PL emission. In general, a lower PL intensity indicates lower recombination of charge carriers, leading to higher photocatalytic activity. It is clear that the PL spectra of pure gC3N4 photocatalysts have a strong emission peak at around 460 nm, which could be related to the recombination of the photoexcited electron−hole of g-C3N4.52 From Figure 7, it can be clearly seen that the PL intensities of g-C3N4/Ag3PO4 hybrid photocatalysts were lower than that of pure g-C3N4, which means that the recombination of the photoexcited electron−

Figure 7. PL spectra of g-C3N4, Ag3PO4 and 10, 25, 50, and 70 wt % gC3N4/Ag3PO4 hybrid photocatalysts.

hole of the composite photocatalysts were lower than that for pure g-C3N4. It indicates that after the formation of a heterojunction between g-C3N4 and Ag3PO4, the recombination of the photoexcited electron−hole on the g-C3N4 surface is suppressed. However, g-C3N4/Ag3PO4 hybrid photocatalysts showed higher PL intensities than that of pure Ag3PO4 (inset of Figure 7). Therefore, the recombination of the charge carriers at the interface of g-C3N4/Ag3PO4 hybrids would be higher than that of pure Ag3PO4. The PL results are in agreement with the results of photocatalytic activity. It means that higher and lower PL intensities than those of pure Ag3PO4 and g-C3N4, respectively, indicate higher photocatalytic activity in the experimental conditions. The higher PL intensities of the hybrids than those of pure Ag3PO4 would be attributed to the higher recombination rate between photoexcited electrons in the CB of Ag3PO4 and photogenerated holes in the VB of gC3N4 on the interface (probably, Ag0 formed on the surface of Ag3PO4, which can act as a charge transmission bridge) of the hybrids, suggesting that rich electrons in the CB of g-C3N4 and holes in the VB of Ag3PO4 participate in the reduction reaction of dissolved O2 and the oxidation of MO, respectively. As a result, the charge separation could be promoted on the gC3N4/Ag3PO4 hybrids, leading to the higher photocatalytic activity of the hybrid photocatalysts. On the basis of these results, it concluded that the g-C3N4/Ag3PO4 system is a typical Z-scheme photocatalyst. To investigate the photocatalytic mechanism of the g-C3N4/ Ag3PO4 hybrid in more detail, the effect of scavengers on the decolorization of MO was examined to shed light on the predominant reactive oxygen species in the photocatalytic process (Figure 8). In this study, t-butyl alcohol (TBA), ammonium oxalate (AO) and benzoquinone (BQ) acted as the scavengers for ·OH, h+ and O2•− introduced into the photocatalytic process, respectively. As is clear from Figure 8, the addition of TBA did not affect the decolorization rate of MO over 25 wt % g-C3N4/Ag3PO4, suggesting that ·OH was not main reactive species in the photocatalytic process. On the contrary, the photocatalytic decolorization of MO was obviously suppressed after the addition of AO and BQ. According to these results, it can be clearly seen that h+ and O2•− were main reactive species for 25 wt % g-C3N4/Ag3PO4 in the photocatalytic decolorization process of MO under visiblelight irradiation. Because the CB potential of g-C3N4 is −1.15 V as described above, photogenerated electrons from g-C3N4 can 8022

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higher than the band gap in g-C3N4 and Ag3PO4, band gap excitation occurred, which leads to the formation of photoexcited holes in the VB and electrons in the CB. The photoelectrons in the CB of Ag3PO4 could easily shift into Ag nanoparticles through the Schottky barrier because of the more positive Fermi energy of Ag than the CB level of Ag3PO4. Simultaneously, the holes in the VB of g-C3N4 can also shift into Ag nanoparticles due to the more negative Fermi energy of Ag than the VB level of g-C3N4. In the photocatalytic process, Ag nanoparticles at the interface of g-C3N4/Ag3PO4 can act as a recombination center for the electrons from the CB of Ag3PO4 and holes from the VB of g-C3N4, inhibiting the electron−hole pairs recombination in both Ag3PO4 and g-C3N4, and enhancing the interfacial charge transfer. On the other hand, the electrons in the CB of g-C3N4, with more negative potentials, have a strong reduction power whereas the holes in the VB of Ag3PO4 display a strong oxidation ability. The high reducing electron located on the CB of g-C3N4 would react with O2 to form O2•−, which can further oxidize MO. The holes located on the VB of Ag3PO4 would photocatalytic oxidize MO directly, due to the high positive potential of the EVB of Ag3PO4 and the inductive effect of PO43−, respectively. The above active species trapping experiments and PL analysis would support the Z-scheme photocatalytic mechanism. If the charge carriers of gC3N4/Ag3PO4 photocatalysts transfer according to the conventional electron−hole separation process for a great number of hybrid photocatalysts, the electrons in the CB of g-C3N4 would migrate to the CB of Ag3PO4, and holes in the VB of Ag3PO4 would transfer to the VB of g-C3N4.36−40 This can result from the efficient charge separation of the photoinduced charge carriers. However, the electrons in the CB of Ag3PO4 cannot reduce dissolved O2 to O2•− through one-electron reduction because the CB potential of Ag3PO4 is more positive than that of the O2/O2•− couple. In addition, the oxidation power of the hybrid photocatalysts decreases because the VB potential of gC3N4 is relatively low. This would mean that g-C3N4/Ag3PO4 hybrid has a lower reduction/oxidation ability and photocatalytic activity than pure g-C3N4 and Ag3PO4. Therefore, it can be concluded that the photocatalytic mechanism of gC3N4/Ag3PO4 hybrids is not in accordance with the traditional charge separation process. Namely, a typical Z-scheme photocatalyst is favorable for the degradation of organic compounds. A similar Z-scheme mechanism for AgI/Ag3PO4 composite photocatalysts has been reported by other researchers.48

Figure 8. Photocatalytic activities of 25 wt % g-C3N4/Ag3PO4 hybrid photocatalyst on the decolorization of MO in the presence of different scavengers under visible-light irradiation (>440 nm).

reduce O2 to O2•− (E°(O2/O2•−) = −0.33 V vs NHE) through a one-electron reduction reaction. On the other hand, it has been reported that the oxidation mechanism of Ag3PO4 proceeds through direct h+ attack to target organic compounds.41,53 To further study whether ·OH was formed on the surface of g-C3N4/Ag3PO4 under visible light, a PL technique with coumarin as a probe molecule was carried out (Figure S8, Supporting Information). It can be seen that no obvious PL signal at 460 nm was observed, indicating that no ·OH was formed in the photocatalytic process and was further confirmed to be a main active species. On the basis of the above results, the photocatalytic mechanism for g-C3N4/Ag3PO4 hybrid samples is tentatively proposed and schematically illustrated in Figure 9. For pure g-

4. CONCLUSIONS High-efficiency visible-light-driven g-C3N4/Ag3PO4 hybrid photocatalysts were synthesized by a facile in situ precipitation process. The as-prepared g-C3N4/Ag3PO4 hybrids exhibited the excellent photocatalytic activity on the decolorization of MO, which was superior to those of pure g-C3N4 and Ag3PO4 under visible-light irradiation (>440 nm). The formation of metallic Ag particles was found on the surface of the hybrid photocatalyst during catalyst preparation. Ag particles formed played an important role on the photocatalytic activity of gC3N4/Ag3PO4, that is, they act as the charge separation center in the photocatalytic process. The excellent photocatalytic activity of the hybrid photocatalysts may originate from the efficient separation of photogenerated electron−hole pairs through the Z-scheme system composed of Ag, Ag3PO4 and gC3N4. The Z-scheme photocatalytic process of the hybrid photocatalysts was also supported by PL analysis. On the basis

Figure 9. Z-scheme photocatalytic mechanism of g-C3N4/Ag3PO4 hybrid photocatalyst under visible-light irradiation (>440 nm).

C3N4, the photogenerated electrons and holes in g-C3N4 tend to recombine and only a fraction of them participates in the photocatalytic reaction, resulting in low activity, as shown in Figure 6. For g-C3N4/Ag3PO4 hybrid photocatalysts, Ag nanoparticles at the interface of g-C3N4/Ag3PO4 hybrids act as the charge separation center to form the visible-light-driven g-C3N4/Ag3PO4 Z-scheme system. Furthermore, PO43− ions with large negative charges in Ag3PO4 prefer to attract holes and repel electrons,54 which is in favor of the formation visiblelight-driven g-C3N4/Ag3PO4 Z-scheme system. With energy 8023

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of the results of this study, the g-C3N4/Ag3PO4 hybrid photocatalytic system is expected to be effective as a useful visible-light photocatalyst for practical applications.

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ASSOCIATED CONTENT

S Supporting Information *

Additional experimental procedure; TEM image of the hybrid photocatalyst; Tauc plots; photocatalytic activity of g-C3N4; stability of the photocatalysts; XRD pattern of used photocatalyst; XPS spectra of used photocatalyst; photocatalytic degradation of 2-CP; hydroxyl radicals detection. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], Tel: +81-59231-9425, Fax: +81-59231-9425. Author Contributions §

H. Katsumata and T. Sakai contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by Scientific Research (C) No. 24510095 from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.

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