g-C3N4 in Phenol

Article pubs.acs.org/IECR

Improved Photochemical Reactivities of Ag2O/g‑C3N4 in Phenol Degradation under UV and Visible Light Hai-Tao Ren,† Shao-Yi Jia,† Yan Wu,† Song-Hai Wu,† Tian-He Zhang,† and Xu Han*,‡,∥ †

School of Chemical Engineering and Technology, ‡School of Environmental Science and Engineering, and ∥Key Laboratory of Systems Bioengineering, Ministry of Education, Tianjin University, Tianjin, P. R. China S Supporting Information *

ABSTRACT: Ag2O/g-C3N4 composites synthesized in this study were applied in the photocatalytic degradation of phenol under UV- and visible-light irradiation. X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy analysis demonstrated that Ag2O nanoparticles were well distributed on the surface of g-C3N4, and the heterostructure of Ag2O/g-C3N4 was formed. Compared with pure g-C3N4 and Ag2O, the Ag2O/gC3N4 composite (8:1) displayed much higher photocatalytic activities in phenol degradation under UV- and visible-light irradiation. The degradation rate constant of 8:1 was 0.069 min−1 under visible light, which was almost 230 and 2.1 times more than that of pure g-C3N4 and Ag2O, respectively. Moreover, the formation of a certain amount of Ag0 on the surface of Ag2O under illumination contributed to the high stability of Ag2O/g-C3N4 photocatalysts. It was also found that photogenerated holes during the photocatalytic process played the predominant role in phenol degradation. The improved photochemical reactivities were attributed to the formation of the heterostructure between Ag2O and g-C3N4, the strong visible-light absorption, and the high separation efficiency of photoinduced electron−hole pairs resulting from the highly dispersed Ag2O particles. nanoparticles.27,28 Because g-C3N4 is an n-type semiconductor, the combination of Ag2O and g-C3N4 can easily form a p−n heterojunction, which then results in a more effective separation of photogenerated electrons and holes. Phenol is a toxic and carcinogenic compound commonly found in industrial wastewater.31−33 It causes serious threats to the ecosystem and to human health. Recently, Ag2O-26 and Ag2O-based heterostructures (such as Ag2O/BiWO6,30 Ag2O/ Ag3PO4,34 and Ag2O/GO35) have been used and are effective in the photocatalytic degradation of phenol. However, to the best of our knowledge, studies on the Ag2O/g-C3N4 system are scarce,36,37 and no study has been reported on the photodegradation of phenol by Ag2O/g-C3N4 under UV and visible light. In the present study, heterostructured photocatalysts, Ag2O/ g-C3N4, were prepared by a simple and effective chemical precipitation method at room temperature. The photocatalytic performances of the catalysts were evaluated by decomposing phenol under UV- and visible-light irradiation. The reactive species involved in phenol photodegradation were determined, and a possible mechanism for the enhanced performance of Ag2O/g-C3N4 was also proposed.

1. INTRODUCTION The growing concerns about energy and environmental problems have stimulated extensive studies on heterogeneous photocatalysts. Of the well-known semiconductor photocatalysts, TiO2 has attracted more attention owing to its stability, nontoxicity, and low-cost.1,2 However, because of the relatively wide band gap (3−3.2 eV) and fast recombination of the photogenerated electron−hole pairs, TiO2 cannot use solar energy efficiently. Therefore, it is highly desirable to develop novel photocatalysts with good charge separation as well as a wide response wavelength range. Graphitic carbon nitride (g-C3N4), a polymeric semiconductor, has recently been reported with the advantages of good stability and an appealing electronic structure with a medium band gap of 2.7 eV.3 In addition, g-C3N4 can easily be synthesized via polymerization of cheap feedstocks such as cyanamide,3,4 dicyandiamide,5−8 melamine,9−12 urea,5,13−16 and thiourea.5,17 Nevertheless, the low quantum efficiency and fast recombination of photogenerated electron−hole pairs limited the photocatalytic activities of pure g-C3N4. In order to overcome these problems and improve the photochemical reactivities of g-C3N4, heterostructured composites such as gC3N4/BiOBr,18 g-C3N4/Bi2WO6,19 g-C3N4/CdS,20 g-C3N4/ CuInS 2 , 21 g-C 3 N 4 /Co 3 O 4 , 22 g-C 3 N 4 /Fe 2 O 3 , 23 g-C 3 N 4 / Ag3VO4,24 and g-C3N4/Ag3PO425 are widely investigated. Ag2O, a p-type semiconductor with a narrow band gap of 1.2 eV, is found to be a self-stable and highly efficient photocatalyst under visible light.26 It has been widely used as an efficient sensitizer to tune the light response of wide-band-gap semiconductors into the visible region and improve their photocatalytic activities.27−30 For example, Ag2O/TiO2 exhibited an enhanced photocatalytic activity under visible light in the decomposition of dyes compared with pure TiO2 and Ag2O © 2014 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Preparation of Ag2O/g-C3N4 Photocatalysts. The g-C3N4 material was synthesized according to a procedure described in the previous study.8 Dicyandiamide (DCDA, 99%) was placed in a crucible with a cover. The crucible was then Received: Revised: Accepted: Published: 17645

August 20, 2014 October 17, 2014 October 17, 2014 October 17, 2014 dx.doi.org/10.1021/ie503312x | Ind. Eng. Chem. Res. 2014, 53, 17645−17653

Industrial & Engineering Chemistry Research

Article

heated to 550 °C in a muffle furnace with a ramp rate of 2.3 °C min−1, and a temperature of 550 °C was maintained for 4 h before the crucible was cooled to room temperature. The residual yellow agglomerates were milled into powder in a mortar. Ag2O/g-C3N4 catalysts were prepared through a pHmediated chemical precipitation method. Typically, 0.1 g of g-C3N4 was dispersed in 20 mL of deionized water with stirring for 1 h, and then the pH of the suspension was adjusted to 8.45. Subsequently, a measured amount of 0.2 M AgNO3 was added dropwise to the suspension. The mixture was stirred in the dark for 0.5 h to reach Ag+ sorption equilibrium. After that, a measured amount of 0.5 M NaOH was slowly added to the Ag+ preequilibrated mixture. The mixture was stirred at room temperature for 1 h, centrifuged, and washed repeatedly with deionized water three times. The obtained pellets were then freeze-dried and denoted as x:y (x:y refers to the mass ratio of Ag2O/g-C3N4). In this study, 1:8, 1:1, and 8:1 were prepared by varying the amount of AgNO3 and NaOH. The pure Ag2O nanoparticles were prepared by the same method without the addition of g-C3N4. 2.2. Characterization. The crystalline structure of the synthetic catalysts was characterized by X-ray diffraction (XRD; Rigaku D/max 2200/PC) with Cu Kα radiation (λ = 1.5406 Å). Fourier transform infrared (FTIR) spectra were collected on a Thermo Nicolet Nexus FTIR spectrometer via a KBr pressed disk method. X-ray photoelectron spectroscopy (XPS) analysis was carried on a PHI5000 Versa Probe electron spectrometer. The morphology and particle size of the products were examined by transmission electron microscopy (TEM; Fei Tecnai G2 F20). The Brunauer−Emmett−Teller (BET) surface area (SBET) was determined by using a Quantachrome NOVA 2000 instrument. The UV−vis diffuse-reflectance spectroscopy (DRS) spectra were measured in the range of 200−800 nm using a Shimadzu UV-2550 spectrophotometer, with BaSO4 for the corrected baseline. Photoluminescence (PL) spectra of the catalysts were recorded using a Fluorolog 321 photoluminescence spectrometer (Horiba Jobin Yvon, France) at room temperature. The excitation wavelength was 325 nm, and the range of 350−600 nm was recorded. Total organic carbon (TOC) was measured with a Shimadzu TOC5000 analyzer. 2.3. Photocatalytic Testing. The photocatalytic performance was evaluated by the degradation of phenol, and experiments were carried out using a model XPA-VII photocatalytic reactor (Xujiang Electromechanical Plant, Nanjing, China). UV and visible light were provided by a 300 W high-pressure mercury lamp and a 500 W xenon lamp with a UV-cutoff filter (λ ≥ 420 nm), respectively. A mixture of 20 mg of the Ag2O/g-C3N4 composite and 20 mL of phenol (10 mg L−1) was stirred for 30 min in a quartz tube in the dark to reach sorption equilibrium of phenol on the catalyst. The results showed that the sorption percentage of phenol on the catalyst was no more than 3%. During irradiation, 0.5 mL of the suspension was withdrawn regularly and centrifuged, and the supernatant was analyzed to determine the residual concentration of phenol with the colorimetric method at 510 nm38 by a UV−vis spectrophotometer (Mapada, UV-6300). For comparison, the reactions were carried out under the same conditions in the presence of pure Ag2O or g-C3N4 or in the absence of catalyst. The percentage of degradation was calculated as C/C0, where C is the phenol concentration at time t and C0 is the initial concentration.

3. RESULTS AND DISCUSSION 3.1. Characterization of Ag2O/g-C3N4 Composites. The composition and morphology of the as-prepared catalysts were characterized by XRD, FTIR, XPS, and TEM analysis. As shown in Figure 1, XRD patterns illustrated the crystalline

Figure 1. XRD patterns of the as-prepared g-C3N4, Ag2O, and Ag2O/ g-C3N4 composites (mass ratios of Ag2O/g-C3N4 were 1:8, 1:1, and 8:1).

phase evolution of the as-prepared Ag2O/g-C3N4 composites with different mass ratios, together with the patterns of pure gC3N4 and Ag2O. Two diffraction peaks at 13.1° and 27.5° in pure g-C3N4 were indexed to the (100) and (002) planes of hexagonal g-C3N4 (JCPDS 87-1526), respectively.3,8 In pure Ag2O, the diffraction peaks at 2θ of 26.7°, 33.0°, 38.3°, 55.2°, 65.7°, and 69.0° were attributed to the respective (110), (111), (200), (220), (311), and (222) planes of the cubic crystal phase (JCPDS 41-1104).36,37 In the Ag2O/g-C3N4 composite with a mass ratio of 8:1, no diffraction peak of g-C3N4 was found owing to the relatively low content of g-C3N4 (Figure 1). With a decreasing mass ratio of Ag2O/g-C3N4 from 8:1 to 1:8, the typical diffraction peak of g-C3N4 gradually appeared and no other impurity peak was observed, indicative of the high purity of these catalysts (Figure 1). In the Ag2O/g-C3N4 composite with a mass ratio of 1:8, almost no diffraction peak of Ag2O could be found owing to the relatively low content of Ag2O (Figure 1). In order to further verify the presence of g-C3N4 and Ag2O on the catalysts, FTIR analysis was conducted (Supporting Information, Figure S1). In pure g-C3N4, the peak at 808 cm−1 was attributed to the presence of triazine units,8,36 and the characteristic peaks at 1239, 1319, 1408, 1570, and 1636 cm−1 could be assigned to the typical stretching vibrations of CN heterocycles.36 The broad peak at 3000−3500 cm−1 was ascribed to the stretching vibration of N−H and that of O−H of the physically adsorbed water.8,36 In pure Ag2O, the characteristic peak at 598 cm−1 was related to Ag−O bond vibration.36 The peak at 1381 cm−1 corresponded to the H− O−H bending vibration of the adsorbed water molecules on the surface.25 The broad peak around 3500 cm−1 and the peak at 1634 cm−1 belonged to the O−H stretching vibration.25 FTIR spectra of the 1:8 and 8:1 composites represented the overlap of the spectra of both pure g-C3N4 and Ag2O. It is worth noting that the intensity of the peak at 808 cm−1 decreased with a decrease of the g-C3N4 content (Supporting Information, Figure S1). 17646

dx.doi.org/10.1021/ie503312x | Ind. Eng. Chem. Res. 2014, 53, 17645−17653


Article

Figure 2. XPS spectra with survey scans: (a) g-C3N4, Ag2O, and the 8:1 composite; (b) Ag 3d of Ag2O and the 8:1 composite; (c) O 1s of Ag2O and the 8:1 composite; (d) N 1s of g-C3N4 and the 8:1 composite; (e) C 1 s of g-C3N4, Ag2O, and the 8:1 composite.

The surface chemical compositions of pure g-C3N4, Ag2O, and Ag2O/g-C3N4 (8:1) and the interaction of Ag2O with gC3N4 were analyzed by XPS (Figure 2). Figure 2a displays the XPS survey spectra of pure Ag2O, g-C3N4, and the 8:1 composite. Compared with pure Ag2O and g-C3N4, the 8:1 composite was composed of Ag, O, N, and C elements. The Ag 3d peaks of Ag2O were located at 367.6 and 373.6 eV (Figure 2b), which corresponded to the Ag 3d5/2 and Ag 3d3/2 binding energies.39 However, the binding energies of Ag 3d5/2 and Ag 3d3/2 in the 8:1 composite were 367.8 and 373.8 eV, which were higher than those of pure Ag2O. A similar phenomenon was also found in the XPS spectra of O 1s (Figure 2c). The binding energy of O 1s in the 8:1 composite (531.6 eV) was

higher than that of pure Ag2O (530.8 eV). The N 1s peak of gC3N4 and the 8:1 composite was observed at 398.0 and 398.8 eV in Figure 2d and originated from CN−C coordination.24 The shift can be ascribed to the strong interaction between Ag2O and g-C3N4. As shown in the XPS spectra of C 1s (Figure 2e), one peak at 284.4 eV was observed in pure Ag2O and belonged to external C contamination. In the case of g-C3N4, the peak centered at 284.2 eV could be ascribed to C−C coordination of the surface adventitious C, whereas the peak at 287.6 eV corresponded to sp3-bonded C in C−N of g-C3N4.40 The 8:1 composite also displayed two C 1s peaks at 284.6 and 288.0 eV, respectively, which were higher than the binding energies of pure g-C3N4. This was due to the fact that Ag2O 17647



Article

Figure 3f, the nanoparticles agglomerated in pure Ag2O and the size of these nanoparticles was up to 200 nm. It is likely that g-C3N4 with a layer structure provided numerous nucleation sites for the growth of Ag2O particles, leading to the homogeneous dispersion of Ag2O particles on the surface of g-C3N4 with smaller size. This was important for improvement of the charge separation efficiency and an increase of the photocatalytic activity.36 SBET of g-C3N4 was 336.1 m2 g−1, as determined by the BET method at liquid-nitrogen temperature (Supporting Information, Table S1). However, SBET of the composites (20.3, 9.2, and 4.0 m2 g−1 for the 1:8, 1:1 and 8:1 composites) decreased dramatically. This was due to the fact that the growth of Ag2O on the surface of g-C3N4 can block part of the void space of gC3N4. Compared with pure Ag2O (SBET of 2.6 m2 g−1), SBET of the 8:1 composite increased significantly, implying that g-C3N4 facilitated dispersion of the Ag2O particles. 3.2. Optical Properties of Ag2O/g-C3N4 Photocatalysts. The optical properties of the as-prepared pure g-C3N4, Ag2O, and Ag2O/g-C3N4 composites were investigated by UV− vis DRS. As shown in Figure 4, the absorption edge of pure g-

hybridized with g-C3N4 resulted in an inner shift of the C 1s orbital.24 The results of XPS distinctly confirmed the presence of chemical bonds between Ag2O and g-C3N4 rather than a simple physical mixture. Figure 3 shows the morphology and particle size of pure gC3N4, Ag2O, and Ag2O/g-C3N4 photocatalysts. The as-

Figure 4. UV−vis DRS spectra and colors of the as-prepared g-C3N4, Ag2O, and Ag2O/g-C3N4 composites (mass ratios of Ag2O/g-C3N4 were 1:8, 1:1, and 8:1).

C3N4 was at 460 nm, which originated from its band gap of 2.69 eV and was consistent with the reported data.10 As for the pure Ag2O sample, it exhibited a wide and strong light absorption in the whole UV−vis range of 200−800 nm, which confirmed its excellent photocatalytic activity.27,36 Furthermore, after doping of Ag2O on the surface of g-C3N4, the visible-light responses of the composites were significantly improved. The wavelength thresholds of composites 1:8 and 1:1 were calculated to be 471 and 529 nm, corresponding to the band gap at 2.63 and 2.34 eV, respectively. As for the 8:1 sample, it exhibited a wide light absorption in the whole UV−vis range of 200−800 nm (Figure 4). The results of UV−vis DRS showed that the heterostructured Ag2O/g-C3N4 composites could greatly improve the optical absorption property and increase the utilization efficiency of solar light. 3.3. Photocatalytic Activities. The photocatalytic activities of the as-prepared Ag2O/g-C3N4 composites with different contents of Ag2O were evaluated by the photodegradation of phenol under UV- and visible-light irradiation. The blank experiment indicated that the removal percentage of phenol was 16% by direct UV photolysis of phenol in the absence of photocatalyst. For comparison, the activities of pure Ag2O and g-C3N4 were also tested under the same conditions. The

Figure 3. TEM images of (a) pure g-C3N4, (b−d) Ag2O/g-C3N4 composites [(b) 1:8, (c) 1:1, and (d) 8:1], and (f) pure Ag2O. (e) HRTEM image of the selected area marked in part d.

prepared g-C3N4 sample showed a layer structure (Figure 3a), which was in agreement with the previous study.41 After the introduction of Ag2O, numerous dark Ag2O particles were evenly dispersed on the surface of g-C3N4 (Figure 3b). With an increase of the Ag2O loading from 1:8 to 8:1, the average size of the Ag2O nanoparticles increased from 6.8 to 10.7 nm (Figure 3b−d). A high-resolution TEM (HRTEM) image of the 8:1 composite further revealed that the interplanar spacing of 0.27 nm corresponded to the (111) plane of Ag2O, which was consistent with XRD analysis (Figures 1 and 3e). Moreover, Figure 3e also reveals a close interaction between Ag2O and gC3N4 in the as-prepared composite.42 Combining the above characterization results, we could therefore conclude the formation of Ag2O/g-C3N4 heterojunctions. As shown in 17648



Article

Figure 5. (a) First-order kinetics and rate constants of phenol degradation under UV irradiation. (b) First-order kinetics and rate constants of phenol degradation under visible-light irradiation. Reaction conditions: phenol = 10 mg L−1; photocatalyst = 1 g L−1.

Table 1. Phenol Photodegradation Performances in the Presence of Ag2O- or Ag2O-Based Heterostructures photocatalyst

photocatalyst concn (g L−1)

initial phenol concn (mg L−1)

Ag2O

5

10

Ag2O/ Bi2WO6 Ag2O/ Ag3PO4 Ag2O/GO

5

10

0.5

20

0.5

50

experimental conditions 350 W xenon lamp equipped with a UV-cutoff filter (λ ≥ 400 nm) 350 W xenon lamp equipped with a UV-cutoff filter (λ ≥ 400 nm) 300 W Xe arc lamp equipped with a cold mirror to provide visible light 300 W Xe lamp with an optical filter to provide visible light

phenol degradation rate

ref

after 40 min of irradiation: 85%

26

after 30 min of irradiation: k = 0.0135 min−1 after 30 min of irradiation: 96.2%

30 34

after 100 min of irradiation: 51%

35

much higher photocatalytic activity than that of pure g-C3N4 and Ag2O, and phenol was completely degraded after 90 min. Figure 5b shows the relationships between ln(C0/C) and the irradiation time under visible-light irradiation, and the kinetics could be described with a pseudo-first-order model (R2 shown in the Supporting Information, Table S2). The rate constants of 1:8, 1:1, and 8:1 were 0.0006, 0.0190, and 0.0690 min−1, respectively (Figure 5b). The 8:1 composite exhibited the highest rate constant, which was approximately 230 times higher than that of g-C3N4 (0.0003 min−1) and 2.1 times higher than that of pure Ag2O (0.0330 min−1) under the same conditions. The improved photocatalytic activities were attributed to the formation of a heterostructure between Ag2O and g-C3N4, and the strong visible-light absorption and high separation efficiency of photoinduced electron−hole pairs resulted from the highly dispersed Ag2O particles.36 Previous studies also mentioned phenol photodegradation in the presence of Ag2O- or Ag2O-based composites, and their results are summarized in Table 1. Among these photocatalysts, phenol removal was incomplete. However, the as-prepared Ag2O/g-C3N4 photocatalyst in our study could completely degrade phenol under UV or visible light. The reusability was another important factor to evaluate the performance of catalysts in practical application. The recycling degradation of phenol under UV- and visible-light irradiation over the 8:1 composite was investigated. After each run, the catalyst was collected and washed with deionized water before the next run. After three cycles, phenol conversion could still reach 100% and 93.7% under UV- and visible-light irradiation, respectively (Supporting Information, Figure S2). This indicated the good stability and reusability of Ag2O/g-C3N4 catalysts under UV and visible light.

removal percentage of phenol was 26% by pure g-C3N4. With respect to the Ag2O/g-C3N4 composites, it could be observed that the photocatalytic activities increased with increasing mass ratios of Ag2O and g-C3N4 from 1:8 to 8:1. When the ratio of Ag2O/g-C3N4 was 8:1, the as-prepared composite exhibited much higher photocatalytic activity than that of pure g-C3N4 or Ag2O, and phenol could be completely degraded in 20 min. To quantitatively determine the reaction kinetics of phenol photodegradation by the as-prepared samples, the experimental data were fitted with the pseudo-first-order model, ln(C0/C) = kt, where k was the apparent first-order rate constant. All plots of ln(C0/C) against t exhibited linear trends (Figure 5a), demonstrating that the phenol photodegradation process could be described by the pseudo-first-order model (R2 shown in the Supporting Information, Table S2). The rate constants of 1:8, 1:1, and 8:1 were 0.013, 0.113, and 0.237 min−1, respectively (Figure 5a). The 8:1 composite exhibited the highest rate constant, which was approximately 23.7 times larger than that of g-C3N4 (0.010 min−1) and 1.5 times larger than that of pure Ag2O (0.163 min−1) under the same conditions. To broaden its application in the whole range of sunlight, the photocatalytic performance of phenol photodegradation was also conducted under visible-light irradiation. The blank experiment demonstrated that the direct photolysis of phenol under visible-light irradiation was negligible in the absence of photocatalyst and the degradation of phenol resulted from the photocatalytic reaction. Under the same conditions, the removal percentage of phenol was only 2.3% by pure g-C3N4 after 180 min under visible-light irradiation. With respect to the Ag2O/g-C3N4 composites, it could be observed that the photocatalytic activities increased with increasing mass ratios of Ag2O and g-C3N4 from 1:8 to 8:1. When the mass ratio of Ag2O and g-C3N4 was 8:1, the as-prepared composite exhibited 17649



Article

The TOC assay for the photocatalytic degradation of phenol at different intervals over the Ag2O/g-C3N4 composite (8:1) is shown in the Supporting Information, Figure S3. After 40 min of UV irradiation, a relatively high mineralization rate of 27% TOC removal was obtained, indicating that phenol was partially mineralized (Supporting Information, Figure S3a). However, under visible-light irradiation, TOC removal was only 12% after 180 min of irradiation, demonstrating that only a small fraction of phenol was mineralized (Supporting Information, Figure S3b). The UV absorption spectrum was utilized to investigate the changes of phenol and its degradation intermediates along with the irradiation time (Figure 7). A significant decrease in the phenol absorbance at λ = 269 nm (100% in 20 min) in the presence of the 8:1 composite under UV irradiation was found in Figure 7a. In addition, the absorbance spectra showed another absorbance at λ = 249 nm as well as a tailing absorbance with a shoulder in the region of λ = 315−400 nm (Figure 7a). The new absorbance at λ = 249 nm was probably related to an intermediate degradation product (containing benzene-ring-like quinones),43 and the broad tailing absorption spectra were indicative of other aromatic degradation intermediates.44 Figure 7b shows a decrease in the phenol absorbance at λ = 269 nm in the presence of the 8:1 composite under visible light, finally disappearing after 90 min of reaction. In addition, the absorbance spectra showed another absorbance at λ = 249 nm, which was associated with an intermediate degradation product like quinones.43 This indicated that phenol was oxidized to a complex mixture of UV-absorbing intermediates, while the absence of broad tailing absorption spectra might suggest that the degradation pathways of phenol were different under UV and visible light. 3.4. Detection of Reactive Oxidative Species. Generally, photoinduced reactive species including trapped holes, • OH radicals, and O2•− are expected to be involved in the photocatalytic process. To reveal the photocatalytic mechanism, the main oxidative species in the photocatalytic process were detected through the trapping experiments of radicals using CH3OH as the hydroxyl radical scavenger,37,45 N2 as the superoxide radical scavenger,46 and formic acid (HCOOH) as the hole radical scavenger.47,48 As shown in Figure 8a, photocatalytic activities of the Ag2O/ g-C3N4 composite (8:1) under UV irradiation decreased obviously with the addition of a hole scavenger (HCOOH;

The composition of the recyclable catalyst after UV- and visible-light irradiation was also characterized by XRD analysis. As shown in Figure 6, a new diffraction peak appeared at 44.3°

Figure 6. XRD patterns of the as-prepared Ag2O/g-C3N4 composite (8:1) and after three cycles of photodegradation of phenol under UV irradiation.

(the intensity was the same as that after one cycle; data not shown), which could be indexed to the (200) plane of metallic Ag0 (JCPDS 04-0783). After visible-light irradiation, this new peak was not clearly observed because of the low content of metallic Ag0 (data not shown). This indicated Ag0 species was formed during the photocatalytic process arising from the partial photoreduction of Ag2O due to the more positive potential of Ag+/Ag (0.7991 eV vs SHE) compared with that of O2/•O2− (−0.046 eV vs SHE).36 Previous studies demonstrated that Ag/Ag2O after the formation of a certain amount of Ag0 on the surface of Ag2O could exhibit a stable structure and prevent the photoreduction of Ag2O.26,30,36 Accordingly, the photogenerated electrons could easily transfer to the metallic Ag0 sites to restrict the recombination of photogenerated electrons and holes, which subsequently increased the photocatalytic activities. Therefore, the prepared Ag2O/g-C3N4 composite could be regarded as a highly active and stable photocatalyst under UV and visible light, which had a great potential to be used in the applications of environmental remediation.

Figure 7. UV−vis absorption spectra of phenol with different irradiation times over the Ag2O/g-C3N4 composite (8:1): (a) UV-light and (b) visiblelight irradiation. 17650



Article

Figure 8. Plots of photogenerated carrier trapping during phenol degradation over the Ag2O/g-C3N4 composite (8:1): (a) UV-light and (b) visiblelight irradiation.

(+1.40 eV vs SHE).26,36 Considering the inner electric field and energy band structure, it was reasonable to conclude that electron transfer between g-C3N4 and Ag2O was partially restricted, while hole transfer could be accelerated. This caused an efficient separation of photogenerated electrons and holes and enhanced the photocatalytic activities. Subsequently, AgI in Ag2O was partially reduced to metallic Ag0 by photogenerated electrons [eq 2], which was also confirmed by XRD analysis after irradiation (Figure 6). The formed Ag0 attached to Ag2O could work as an electron pool and transfer the photogenerated electrons from Ag2O to combine with O2. Electrons were trapped by O2 to produce • O2− [eq 3], and •O2− radicals combined with H2O to be further transformed to •OH [eqs 4−6]. Moreover, the EVB (gC3N4, +1.57 eV vs SHE; Ag2O, +1.4 eV vs SHE) values were lower than the standard redox potential of •OH/H2O (+2.68 eV vs SHE),25,37 indicating that the photogenerated holes on gC3N4 and Ag2O could not oxidize H2O to •OH. Hence, h+ on the valence band (VB) of g-C3N4 and Ag2O would react with phenol directly. These reactive species (h+, •OH, •O2−, etc.) could efficiently degrade phenol into intermediate products and finally into CO2 and H2O [eq 7]. Among these reactive species, hole radicals played the predominant role in the photodegradation of phenol.

the conversion of phenol was 29.7%) and reduced slightly with the addition of a hydroxyl radical scavenger (CH3OH; the conversion of phenol was 98.7%) and a superoxide radical scavenger (N2; the conversion of phenol was 93.2%), indicating that hole radicals were the main oxidative species. Similar results were also found under visible-light irradiation (Figure 8b). The degradation rate exhibited a significant decrease in the presence of HCOOH, and the conversion of phenol was only 9.3%. However, the presence of CH3OH and N2 moderately affected the phenol degradation, and the conversions of phenol were 91% and 80.2%, respectively. These results suggested that hole radicals were also the main active species in the photodegradation of phenol under visible light. 3.5. Possible Photocatalytic Mechanism. On the basis of the above discussion, a possible photocatalytic mechanism of the Ag2O/g-C3N4 catalyst under UV- and visible-light irradiation was proposed and is illustrated in Scheme 1. It Scheme 1. Proposed Photocatalytic Mechanism over the Ag2O/g-C3N4 Composites

Ag 2O/g‐C3N4 + hv → e− + h+ −

Ag 2O + H 2O + 2e → 2Ag + 2OH O2 + e− → •O2− •

O2− + H+ → •OOH

•

was well-known that Ag2O and g-C3N4 were typical p-type and n-type semiconductors, respectively. Under UV- or visible-light irradiation, both Ag2O and g-C3N4 could be excited to generate electrons (e−) and holes (h+) [eq 1]. In the Ag2O/g-C3N4 composite, the photogenerated electrons had a tendency to transfer from Ag2O to g-C3N4 and the holes had an opposite transfer because of the inner electric field existing in the p−n junctions (inner electric field direction: g-C3N4 → Ag2O).36 Meanwhile, ECB of g-C3N4 (−1.12 eV vs SHE) was more negative than that of Ag2O (+0.20 eV vs SHE), and EVB of gC3N4 (+1.57 eV vs SHE) was more positive than that of Ag2O

(1) −

(2) (3) (4)

OOH + H+ + e− → H 2O2

(5)

H 2O2 + e− → •OH + OH−

(6)

h+, •O2− , •OH + phenol → intermediate products → CO2 + H 2O

(7)

As a result, it could be concluded that the enhanced photocatalytic activity of the Ag2O/g-C3N4 composites could be attributed to three aspects: (i) the improved dispersibility of Ag2O on the surface of g-C3N4; (ii) the enhanced optical absorption property arising from the heterostructures between 17651



Article

parameters of phenol degradation (Table S2). This material is available free of charge via the Internet at http://pubs.acs.org.

Ag2O and g-C3N4; (iii) the synergetic effects of the inner electric field and matched energy band structure that improved the separation efficiency of the photogenerated electrons and holes. The PL spectrum was useful for revealing the mitigation, transfer, and recombination processes of the photogenerated electron−hole pairs in a semiconductor. It was generally believed that a higher PL intensity always indicated a faster recombination of photogenerated electrons and holes.37 PL spectra of pure g-C3N4, Ag2O, and Ag2O/g-C3N4 composites at an excitation wavelength of 325 nm are shown in Figure 9. The

■

*Telephone: +86-15222072695. E-mail: hanxu_mail2013@ 163.com. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We greatly acknowledge financial support from the National Natural Science Foundation of China (Grants 41003040, 41373114, and 41201487) and the Open Funding Project of the Key Laboratory of Systems Bioengineering, Ministry of Education. We are also grateful for the Program of Introducing Talents of Discipline to Universities (Grant B06006).

■

REFERENCES

(1) Chen, X.; Mao, S. S. Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications. Chem. Rev. 2007, 107, 2891−2959. (2) Chen, X.; Liu, L.; Yu, P. Y.; Mao, S. S. Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals. Science 2011, 331, 746−750. (3) Wang, X. C.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domenet, K.; Antonietti, M. A Metal-Free Polymeric Photocatalyst for Hydrogen Production from Water under Visible Light. Nat. Mater. 2009, 8, 76−80. (4) Takanabe, K.; Kamata, K.; Wang, X. C.; Antonietti, M.; Kubota, J.; Domen, K. Photocatalytic Hydrogen Evolution on Dye-Sensitized Mesoporous Carbon Nitride Photocatalyst with Magnesium Phthalocyanine. Phys. Chem. Chem. Phys. 2010, 12, 13020−13025. (5) Zhang, Y. W.; Liu, J. H.; Wu, G.; Chen, W. Porous Graphitic Carbon Nitride Synthesized via Direct Polymerization of Urea for Efficient Sunlight-Driven Photocatalytic Hydrogen Production. Nanoscale 2012, 4, 5300−5303. (6) Zhang, G. G.; Zhang, J. S.; Zhang, M. W.; Wang, X. C. Polycondensation of Thiourea into Carbon Nitride Semiconductors as Visible Light Photocatalysts. J. Mater. Chem. 2012, 22, 8083−8091. (7) Zhang, J. S.; Zhang, M. W.; Sun, R. Q.; Wang, X. C. A Facile Band Alignment of Polymeric Carbon Nitride Semiconductors to Construct Isotype Heterojunctions. Angew. Chem., Int. Ed. 2012, 51, 10145−10149. (8) Niu, P.; Zhang, L. L.; Liu, G.; Cheng, H. M. Graphene-Like Carbon Nitride Nanosheets for Improved Photocatalytic Activities. Adv. Funct. Mater. 2012, 22, 4763−4770. (9) Yan, S. C.; Li, Z. S.; Zou, Z. G. Photodegradation of Rhodamine B and Methyl Orange over Boron-Doped g-C3N4 under Visible Light Irradiation. Langmuir 2010, 26, 3894−3901. (10) Yan, S. C.; Li, Z. S.; Zou, Z. G. Photodegradation Performance of g-C3N4 Fabricated by Directly Heating Melamine. Langmuir 2009, 25, 10397−10401. (11) Wang, Y. J.; Wang, Z. X.; Muhammad, S.; He, J. Graphite-Like C3N4 Hybridized ZnWO4 Nanorods: Synthesis and Its Enhanced Photocatalysis in Visible Light. CrystEngComm 2012, 14, 5065−5070. (12) Samanta, S.; Martha, S.; Parida, K. M. Facile Synthesis of Au/gC3N4 Nanocomposite: An Inorganic/Organic Hybrid Plasmonic Photocatalyst with Enhanced Hydrogen Gas Evolution Under Visible-Light Irradiation. ChemCatChem 2014, 6, 1453−1462. (13) Dong, F.; Wu, L. W.; Sun, Y. J.; Fu, M.; Wu, Z. B.; Lee, S. C. Efficient Synthesis of Polymeric g-C3N4 Layered Materials as Novel Efficient Visible Light Driven Photocatalysts. J. Mater. Chem. 2011, 21, 15171−15174. (14) Chai, B.; Peng, T. Y.; Mao, J.; Li, K.; Zan, L. Graphitic Carbon Nitride (g-C3N4)-Pt-TiO2 Nanocomposite as an Efficient Photo-

Figure 9. PL spectra of the as-prepared g-C3N4, Ag2O, and Ag2O/gC3N4 composites.

wide emission peak of pure g-C3N4 was at about 460 nm, which was ascribed to the band-gap recombination of electron−hole pairs.22,49 The presence of Ag2O could not change the emission peak position but rather reduced its relative intensity compared with pure g-C3N4. This demonstrated that the photogenerated electron−hole pairs could be effectively separated by the formed Ag2O/g-C3N4 heterojunction.

4. CONCLUSIONS A series of Ag2O/g-C3N4 heterostructured photocatalysts were synthesized by a pH-mediated chemical precipitation method. Among these photocatalysts, the 8:1 Ag2O/g-C3N4 composite exhibited the optimal photocatalytic activity for phenol degradation. Under UV- and visible-light irradiation, phenol was completely degraded in 20 and 90 min by this photocatalyst, respectively. The apparent rate constant k of the 8:1 catalyst was 0.069 min−1 under visible-light irradiation, which was almost 230 and 2.1 times of that of pure g-C3N4 and Ag2O, respectively. The improved photocatalytic activities were attributed to the formation of a heterostructure between Ag2O and g-C3N4, the strong visible-light absorption, and the high separation efficiency of photoinduced electron−hole pairs resulting from the highly dispersed Ag2O particles. Moreover, hole radicals were the main active species in the photodegradation of phenol under UV and visible light. The Ag2O/gC3N4 composite could therefore be used as a promising photocatalyst in the applications of environmental remediation.

■

AUTHOR INFORMATION

Corresponding Author

ASSOCIATED CONTENT

S Supporting Information *

FTIR analysis (Figure S1), recycle testing (Figure S2), TOC analysis (Figure S3), BET analysis (Table S1), and regression 17652



Article

catalyst for Hydrogen Production under Visible Light Irradiation. Phys. Chem. Chem. Phys. 2012, 14, 16745−16752. (15) Liu, J. H.; Zhang, Y. W.; Lu, L. H.; Wu, G.; Chen, W. SelfRegenerated Solar-Driven Photocatalytic Water-Splitting by Urea Derived Graphitic Carbon Nitride with Platinum Nanoparticles. Chem. Commun. 2012, 48, 8826−8828. (16) Martha, S.; Nashim, A.; Parida, K. M. Facile Synthesis of Highly Active g-C3N4 for Efficient Hydrogen Production Under Visible Light. J. Mater. Chem. A 2013, 1, 7816−7824. (17) Hong, J. D.; Xia, X. Y.; Wang, Y. S.; Xu, R. Mesoporous Carbon Nitride with in situ Sulfur Doping for Enhanced Photocatalytic Hydrogen Evolution from Water under Visible Light. J. Mater. Chem. 2012, 22, 15006−15012. (18) Fu, J.; Tian, Y.; Chang, B.; Xi, F.; Dong, X. BiOBr−Carbon nitride Heterojunctions: Synthesis, Enhanced Activity and Photocatalytic Mechanism. J. Mater. Chem. 2012, 22, 21159−21166. (19) Tian, Y.; Chang, B.; Lu, J.; Fu, J.; Xi, F.; Dong, X. Hydrothermal Synthesis of Graphitic Carbon Nitride−Bi2WO6 Heterojunctions with Enhanced Visible Light Photocatalytic Activities. ACS Appl. Mater. Interfaces 2013, 5, 7079−7085. (20) Fu, J.; Chang, B.; Tian, Y.; Xi, F.; Dong, X. Novel C3N4−CdS Composite Photocatalysts with Organic−Inorganic Heterojunctions: in situ Synthesis, Exceptional Activity, High Stability and Photocatalytic Mechanism. J. Mater. Chem. A 2013, 1, 3083−3090. (21) Yang, F.; Kuznietsov, V.; Lublow, M.; Merschjann, C.; Steigert, A.; Klaer, J.; Thomas, A.; Schedel-Niedrig, T. Solar Hydrogen Evolution using Metal-Free Photocatalytic Polymeric Carbon Nitride/CuInS2 Composites as Photocathodes. J. Mater. Chem. A 2013, 1, 6407−6415. (22) Han, C.; Ge, L.; Chen, C.; Li, Y.; Xiao, X.; Zhang, Y.; Guo, L. Novel Visible Light Induced Co3O4−g-C3N4 Heterojunction Photocatalysts for Efficient Degradation of Methyl Orange. Appl. Catal., B 2014, 147, 546−553. (23) Ye, S.; Qiu, L. G.; Yuan, Y. P.; Zhu, Y. J.; Xia, J.; Zhu, J. F. Facile Fabrication of Magnetically Separable Graphitic Carbon Nitride Photocatalysts with Enhanced Photocatalytic Activity under Visible Light. J. Mater. Chem. A 2013, 1, 3008−3015. (24) Wang, S. M.; Li, D. L.; Sun, C.; Yang, S. G.; Guan, Y.; He, H. Synthesis and Characterization of g-C3N4/Ag3VO4 Composites with Significantly Enhanced Visible-Light Photocatalytic Activity for Triphenylmethane Dye Degradation. Appl. Catal., B 2014, 144, 885−892. (25) Kumar, S.; Surendar, T.; Baruah, A.; Shanker, V. Synthesis of a Novel and Stable g-C3N4−Ag3PO4 Hybrid Nanocomposite Photocatalyst and Study of the Photocatalytic Activity under Visible Light Irradiation. J. Mater. Chem. A 2013, 1, 5333−5340. (26) Wang, X. F.; Li, S. F.; Yu, H. G.; Yu, J. G.; Liu, S. W. Ag2O as a New Visible-Light Photocatalyst: Self-Stability and High Photocatalytic Activity. Chem.Eur. J. 2011, 17, 7777−7780. (27) Zhou, W. J.; Liu, H.; Wang, J. Y.; Liu, D.; Du, G. J.; Cui, J. J. Ag2O/TiO2 Nanobelts Heterostructure with Enhanced Ultraviolet and Visible Photocatalytic Activity. ACS Appl. Mater. Interfaces 2010, 2, 2385−2392. (28) Sarkar, D.; Ghosh, C. K.; Mukherjee, S.; Chattopadhyay, K. K. Three Dimensional Ag2O/TiO2 Type-II (p−n) Nanoheterojunctions for Superior Photocatalytic Activity. ACS Appl. Mater. Interfaces 2013, 5, 331−337. (29) Wu, M.; Yan, J. M.; Zhao, M.; Jiang, Q. Facile Synthesis of an Ag2O−ZnO Nanohybrid and Its High Photocatalytic Activity. ChemPlusChem 2012, 77, 931−935. (30) Yu, H. G.; Liu, R.; Wang, X. F.; Wang, P.; Yu, J. G. Enhanced Visible-Light Photocatalytic Activity of Bi2WO6 Nanoparticles by Ag2O Cocatalyst. Appl. Catal., B 2012, 111, 326−333. (31) Saeed, M.; Ilyas, M. Oxidative Removal of Phenol from Water Catalyzed by Nickel Hydroxide. Appl. Catal., B 2013, 129, 247−254. (32) Silva, C. G.; Faria, J. L. Effect of Key Operational Parameters on the Photocatalytic Oxidation of Phenol by Nanocrystalline Sol−Gel TiO2 under UV Irradiation. J. Mol. Catal. A: Chem. 2009, 305, 147− 154.

(33) Silva, C. G.; Sampaio, M. J.; Carabineiro, S. A. C.; Oliveira, J. W. L.; Baptista, D. L.; Bacsa, R.; Machado, B. F.; Serp, P.; Figueiredo, J. L.; Silva, A. M. T.; Faria, J. L. Developing Highly Active Photocatalysts: Gold-Loaded ZnO for Solar Phenol Oxidation. J. Catal. 2014, 316, 182−190. (34) Wang, P. Q.; Bai, Y.; Luo, P. Y.; Liu, J. Y. Ag2O/Ag3PO4 Heterostructures: Highly Efficient and Stable Visible-Light-Induced Photocatalyst for Degradation of Methyl Orange and Phenol. Micro Nano Lett. 2013, 7, 340−344. (35) Ji, Z. Y.; Shen, X. P.; Yang, J. L.; Xu, Y. L.; Zhu, G. X.; Chen, K. M. Graphene Oxide Modified Ag2O Nanocomposites with Enhanced Photocatalytic Activity under Visible-Light Irradiation. Eur. J. Inorg. Chem. 2013, 36, 6119−6125. (36) Xu, M.; Han, L.; Dong, S. J. Facile Fabrication of Highly Efficient g-C3N4/Ag2O Heterostructured Photocatalysts with Enhanced Visible-Light Photocatalytic Activity. ACS Appl. Mater. Interfaces 2013, 5, 12533−12540. (37) Shi, L.; Liang, L.; Ma, J.; Wang, F. X.; Sun, J. M. Enhanced Photocatalytic Activity over the Ag2O−g-C3N4 Composite under Visible Light. Catal. Sci. Technol. 2014, 4, 758−765. (38) Clesceri, L.; Greenberg, A.; Eaton, A. Standard Methods for the Examination of Water and Wastewater, 20th ed; APHA: Washington, DC, 2001. (39) Meliána, E. P.; Díaza, O. G.; Rodrígueza, J. M. D.; Colónb, G.; Navíob, J. A.; Macíasb, M.; Peña, J. P. Effect of Deposition of Silver on Structural Characteristics and Photoactivity of TiO2-based Photocatalysts. Appl. Catal., B 2012, 127, 112−120. (40) Li, Y.; Zhang, H.; Liu, P.; Wang, D.; Li, Y.; Zhao, H. CrossLinked g-C3N4/rGO Nanocomposites with Tunable Band Structure and Enhanced Visible Light Photocatalytic Activity. Small 2013, 9, 3336−3344. (41) Xu, H.; Yan, J.; Xu, Y.; Song, Y.; Li, H.; Xia, J.; Huang, C.; Wan, H. Novel Visible-Light-Driven AgX/Graphite-Like C3N4 (X = Br, I) Hybrid Materials with Synergistic Photocatalytic Activity. Appl. Catal., B 2013, 129, 182−193. (42) Huang, L. Y.; Xu, H.; Li, Y. P.; Li, H. M.; Cheng, X. N.; Xia, J. X.; Xu, Y. G.; Cai, G. B. Visible-Light-Induced WO3/g-C3N4 Composites with Enhanced Photocatalytic Activity. Dalton Trans. 2013, 42, 8606−8616. (43) Wang, W. M.; Song, J.; Han, X. Schwertmannite as a New Fenton-Like Catalyst in the Oxidation of Phenol by H2O2. J. Hazard. Mater. 2013, 262, 412−419. (44) Liu, L.; Liu, H. J.; Zhao, Y. P.; Wang, Y. Q.; Duan, Y. Q.; Gao, G. D.; Ge, M.; Chen, W. Directed Synthesis of Hierarchical Nanostructured TiO2 Catalysts and Their Morphology-Dependent Photocatalysis for Phenol Degradation. Environ. Sci. Technol. 2008, 42, 2342−2348. (45) Hu, C.; Peng, T. W.; Hu, X. X.; Nie, Y. L.; Zhou, X. F.; Qu, J. H.; He, H. Plasmon-Induced Photodegradation of Toxic Pollutants with Ag-AgI/Al2O3 under Visible-Light Irradiation. J. Am. Chem. Soc. 2010, 132, 857−862. (46) Pan, C.; Zhu, Y. New Type of BiPO4 Oxy-Acid Salt Photocatalyst with High Photocatalytic Activity on Degradation of Dye. Environ. Sci. Technol. 2010, 44, 5570−5574. (47) Tan, T.; Beydoun, D.; Amal, R. Effects of Organic Hole Scavengers on the Photocatalytic Reduction of Selenium Anions. J. Photochem. Photobiol., A 2003, 159, 273−280. (48) Jiang, W. J.; Zhang, M.; Wang, J.; Liu, Y. F.; Zhu, Y. F. Dramatic Visible Activity in Phenol Degradation of TCNQ@TiO2 Photocatalyst with Core-Shell Structure. Appl. Catal., B 2014, 160, 44−50. (49) Li, H. P.; Liu, J. Y.; Hou, W. G.; Du, N.; Zhang, R. J.; Tao, X. T. Synthesis and Characterization of g-C3N4/Bi2MoO6 Heterojunctions with Enhanced Visible Light Photocatalytic Activity. Appl. Catal., B 2014, 160, 89−97.

17653


g-C3N4 in Phenol

Recommend Documents