Significantly enhanced visible-light photocatalytic activity of g-C3N4 via ZnO modification and the mechanism study

Journal of Molecular Catalysis A: Chemical 368–369 (2013) 9–15

Contents lists available at SciVerse ScienceDirect

Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata

Significantly enhanced visible-light photocatalytic activity of g-C3 N4 via ZnO modification and the mechanism study Wei Liu a , Mingliang Wang a,∗ , Chunxiang Xu b , Shifu Chen c,∗∗ , Xianliang Fu c a b c

School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China State Key Laboratory of Bioelectronics, Southeast University, Nanjing 210096, China School of Chemistry and Material Science, Huaibei Normal University, Huaibei 235000, China

a r t i c l e

i n f o

Article history: Received 2 September 2012 Received in revised form 8 November 2012 Accepted 12 November 2012 Available online 22 November 2012 Keywords: Photocatalytic activity g-C3 N4 Modification Visible light Mechanism

a b s t r a c t The highly effective ZnO/g-C3 N4 photocatalysts with different ZnO amount were prepared by an economic and environmentally friendly method. The photocatalysts were characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, UV–vis diffuse reflectance spectroscopy, and Brunauer–Emmett–Teller surface area. The results showed that photocatalytic activity of the photocatalyst was much higher than that of pure g-C3 N4 via photodegradation of Rhodamine B under visible light irradiation. The kinetic constant of RhB degradation over ZnO(15 wt.%)/g-C3 N4 was 3.1 times that of pure g-C3 N4 . Effect of ZnO content on the photocatalytic activity of ZnO/g-C3 N4 was studied in detail. The active species in RhB degradation were examined by adding a series of scavengers. The study on photocatalytic mechanism revealed that the electrons injected directly from the conduction band of g-C3 N4 to that of ZnO, resulting in the production of • O2 − and • OH radicals in the conduction band of ZnO. Simultaneously, the rich holes in the valence band of g-C3 N4 oxidized Rhodamine B directly to promote the photocatalytic degradation reaction. This work may provide some insight into solving the unsatisfactory catalytic activity and low efficiency converting solar radiation for practical applications of g-C3 N4 . © 2012 Elsevier B.V. All rights reserved.

1. Introduction Semiconductor based photocatalysts have received great attention due to their applications in environmental pollution mediation and solar energy conversion [1–6]. To date, various semiconductor materials including metal oxides [7], sulfides [8], nitrides [9], and their mixed solid solutions [10] have been exploited as photocatalysts for photocatalytic degradation of organic pollutants. Among them, zinc oxide (ZnO) is one of the most popular and most promising photocatalysts for photodegradation of some dyes in polluted water, owing to its high photocatalytic activity, low cost and innocuousness [11–14]. However, ZnO is a wide band gap energy semiconductor and active only in the ultraviolet region. Therefore, great efforts have been devoted to developing visible-light-responsive and stable photocatalysts with ZnO for solar energy conversion [15]. Recently, graphitic carbon nitride (g-C3 N4 , denoted as C3 N4 hereafter) has attracted considerable attention as a potential visible

∗ Corresponding author. Tel.: +86 25 85092237; fax: +86 25 85092237. ∗∗ Corresponding author. Tel.: +86 561 3806611; fax: +86 561 3090518. E-mail addresses: [email protected], [email protected] (M. Wang), [email protected] (S. Chen). 1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molcata.2012.11.007

light photocatalyst in water photo-splitting and organic pollutant degradation, which possesses very high thermal and chemical stability as well as interesting electronic properties [16–19]. However, the photocatalytic performance of C3 N4 is still limited by the low quantum efficiency for this pristine semiconductor. To resolve this problem, many attempts have been carried out to improve the photocatalytic performance of C3 N4 , such as nonmetal doping [20–25], noble metal deposition [26,27], preparation of nano-/porous C3 N4 [28–32]. Another good strategy to improve the quantum efficiency is to promote the separation efficiency of photogenerated electron–hole pairs by constructing heterojunction between C3 N4 and other materials, such as polyaniline [33], graphene or graphene oxide [34–36], CdS [37], TaON [38], TiO2 [39], Bi2 WO6 [40], etc. More recently, it has been reported that the synthesis of C3 N4 hybridized ZnO photocatalyst by a monolayer-dispersed method and its application for photodegradation of methylene blue [41]. From the reported results of these literatures, the C3 N4 is usually used to improve the photocatalytic performance of large bandgap semiconductors [38–41]. However, the investigation on C3 N4 modified with these materials, especially the semiconductors with wide band-gaps has been scarcely reported. Compared to the published C3 N4 -hybridized ZnO photocatalyst, the ZnO-modified C3 N4 system to be investigated in this paper has some merits in three aspects. First of all, ZnO is a host semiconductor in the published

10

W. Liu et al. / Journal of Molecular Catalysis A: Chemical 368–369 (2013) 9–15

paper. However, the host semiconductor in this paper is g-C3 N4 . Moreover, it is feasible that the visible-light photocatalytic activity of C3 N4 is enhanced via ZnO modification, because C3 N4 has a conjugated p structure and the combination of C3 N4 and ZnO is an ideal system to achieve an improved charge separation in electron-transfer processes [41]. Second, the photogenerated holes of C3 N4 have minor effect on dye degradation in the reported paper under visible light irradiation, whereas they probably have some contribution to the enhanced photocatalytic performance in ZnO-modified C3 N4 systems, because the amount of photogenerated holes for C3 N4 host semiconductor is large enough to increase their reaction probability. Thirdly, the preparation methods are different. With respect to the photocatalysts reported by Zhu et al. [41], they are prepared by employing methanol solvent, which is a well-known volatile and poisonous organic compound. For these reasons, we used an economic and environmentally friendly method to prepare various ZnO-modified C3 N4 photocatalysts (denoted as ZnO/C3 N4 hereafter). Remarkably, the photocatalytic performance for Rhodamine B (RhB) degradation was significantly enhanced in comparison with pure C3 N4 under visible light irradiation. And the corresponding photocatalytic reaction mechanism was systematically studied as well. This work may provide some insight into solving the unsatisfactory catalytic activity and low efficiency converting solar radiation for practical applications of C3 N4 as well as the design of novel heterojunction photocatalysts. In the present study, a series of ZnO/C3 N4 photocatalysts with different weight percents of ZnO were successfully synthesized by a simple ball milling method. The photocatalysts were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–vis diffuse reflectance spectroscopy (DRS), and Brunauer–Emmett–Teller (BET) surface area. The • OH radicals produced during the photocatalytic process were detected by a terephthalic acid photoluminescence probing technique (TA-PL). The photocatalytic activity of the ZnO/C3 N4 for photooxidation of RhB under visible light was enhanced remarkably compared to single-phase C3 N4 . The optimal ZnO content in the ZnO/C3 N4 photocatalyst was 15 wt.%. The stability of the prepared photocatalyst in the photocatalytic process was also investigated. The possible photocatalytic reaction mechanism was studied systematically. The injection of excited electrons from the conduction band (CB) of C3 N4 to that of ZnO increased the effective separation of photogenerated electron–hole pairs in the ZnO/C3 N4 systems and so the photocatalytic property of the photocatalysts was improved greatly. 2. Experimental 2.1. Materials Zinc oxide (ZnO, purity ≥ 99.8%) and melamine (C3 H6 N6 , purity > 99.0%) used in the experiments were supplied by Sinopharm Chemical Reagent Co. Ltd., China. Rhodamine B (RhB) and other chemicals used in the experiments were of analytically pure grade. They were purchased from Shanghai and other China Chemical Reagent Ltd. and used without further purification. Deionized water was used throughout this study. 2.2. Preparation of the photocatalyst The metal-free C3 N4 powders were synthesized by heating melamine in a program-controlled tubular furnace. In a typical synthesis run, 5 g melamine was placed into an alumina crucible with a cover for decreasing the volatilization of melamine, which was

first thermally treated at 500 ◦ C for 2 h at the temperature rise rate of 10 ◦ C/min. Further deammonation treatment was performed at 520 ◦ C for 2 h. After the tubular furnace was naturally cooled to room temperature, the products were collected and ground into powders. The preparation of ZnO/C3 N4 photocatalyst was carried out in a ND7-0.4L ball mill (made in Tianzun Electronics Co. Ltd., Nanjing University). In the process, 2 g C3 N4 powder, 0.105 g ZnO powder and 2 mL H2 O were added into a zirconic vessel with ten zirconic balls. After being milled for 6 h at the speed of 400 rpm, the wet powder was dried at 60 ◦ C, and subsequently thermally treated at 400 ◦ C for 1 h. Thus, the ZnO/C3 N4 photocatalyst in a ratio of 5 wt.% ZnO was obtained. In this way, different samples that met the previously designed ZnO ratio, namely 10 wt.%, 15 wt.%, 20 wt.% and 30 wt.% were prepared, respectively. 2.3. Characterization XRD measurements were carried out at room temperature using a BRUKER D8 ADVANCE X-ray powder diffractometer with Cu ˚ and a scanning speed of 10◦ /min. The K␣ radiation ( = 1.5406 A) accelerating voltage and emission current were 40 kV and 40 mA, respectively. The microcrystalline structure and surface characteristics of the photocatalysts were observed using a JEOL JSM-6610LV scanning electron microscope with 30 kV scanning voltages. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were performed with a JEOL-2010 transmission electron microscope, using an accelerating voltage of 200 kV. DRS measurements were carried out using a UV-3600 (SHIMADZU, Japan) UV-vis-NIR spectrophotometer equipped with an integrating sphere. The analysis ranged from 190 nm to 700 nm, and BaSO4 was used as a reflectance standard. Fluorescence emission spectra were recorded on a JASCO FP8300 type fluorescence spectrophotometer. BET surface areas were measured from N2 sorption–desorption isotherms at 77 K using a Quantachrome NOVA 2000e adsorption apparatus. 2.4. Photocatalytic activity test The photocatalytic activity of photocatalyst (0.1 g) was evaluated by degradation of 50 mL RhB (10 mg/L) in a photoreaction apparatus under visible light irradiation. The experimental procedure was carried out according to our earlier report [42]. Briefly, a 500 W Xenon lamp (Institute of Electric Light Source, Beijing) with a maximum emission at about 470 nm was laid in the empty chamber of an annular quartz tube. The wavelength of the visible light was controlled through a cutoff filter ( > 400 nm, Instrument Company of Nantong, China). The running water passes through an inner thimble of the annular quartz tube to immediately remove the heat released from the lamp. A 150 mL unsealed beaker of 5.5 cm diameter was used as the reaction vessel. The distance between the light source and the surface of the reaction solution is 11 cm. The temperature of the reaction solution is monitored throughout the entire experiment and maintained at approximately 20 ◦ C to avoid temperature effects in the analysis of the photocatalytic reaction. Prior to irradiation, the suspensions containing reactant and photocatalyst were put into the beaker, and ultrasonically vibrated for 20 min to reach the adsorption–desorption equilibrium. In the experiments, the suspensions were stirred by a magnetic stirring device and the initial solution was neutral. At given time intervals, a 5 mL sample was taken from the reaction suspension, centrifuged and filtered to determine spectrophotometrically the RhB concentration.

W. Liu et al. / Journal of Molecular Catalysis A: Chemical 368–369 (2013) 9–15

11

The photocatalytic activity of photocatalyst is calculated from the following expression [42]: C0 − C × 100 C0

ZnO

(1)

where  is the RhB conversion; C0 is the concentration of the reactant before illumination (mg/L); C is the concentration of the reactant after a certain illumination time period t (mg/L). 3. Results and discussion

Intensity(a.u.)

(%) =

30wt.% 20wt.% 15wt.% 10wt.% 5 wt.%

3.1. Characterization of the ZnO/C3 N4 photocatalyst 3.1.1. XRD analysis Fig. 1 shows the XRD patterns of the ZnO/C3 N4 samples with different ZnO amount, as well as pure C3 N4 and ZnO. It can be seen that the prepared C3 N4 photocatalyst have two distinct peaks at 13.1◦ and 27.4◦ , which can be indexed as (1 0 0) and (0 0 2) diffraction planes [38]. The ZnO sample is well crystallized and its diffraction peaks are in good agreement with the hexagonal wurtzite crystal phase of ZnO (JCPDS 65-3411). As shown in Fig. 1, the diffraction peaks of ZnO are obviously observed when ZnO content is 5.0 wt.%. And with further increasing ZnO content from 10 wt.% to 30 wt.%, the intensity of diffraction peaks of ZnO improve remarkably and no other new crystal phases are found in the XRD patterns. It demonstrates that the ZnO/C3 N4 sample presents a two-phase composition: C3 N4 and ZnO. 3.1.2. SEM analysis SEM was used to investigate the morphologies of the samples ball milled for 6 h. Fig. 2 illustrates the respective SEM photograph of C3 N4 , ZnO and ZnO(15.0 wt.%)/C3 N4 photocatalysts. As shown in Fig. 2a, the pure C3 N4 photocatalyst appears to be aggregated particles containing a large number of irregular smaller crystals.

(002)

(100) 10

20

C3N4

30

40

50

60

70

80

2θ (degree) Fig. 1. XRD patterns of C3 N4 , ZnO and ZnO/C3 N4 photocatalysts with different ZnO contents.

From Fig. 2b, it can be seen that the appearance of the ZnO powder is rod-like with partial sheet structure. Fig. 2c and d gives an overview of the typical SEM image of the ZnO(15 wt.%)/C3 N4 photocatalyst. It is clearly seen that the photocatalyst is composed of rod-like ZnO and C3 N4 . In addition, the ZnO displays agglomerate with the large diameter size, which is composed of a lot of small nanoparticles with mean size of about 60 nm, matching the size of pure ZnO particles shown in Fig. 2b. 3.1.3. TEM analysis In order to investigate the interface of the ZnO(15.0 wt.%)/C3 N4 photocatalyst, TEM and high resolution TEM (HRTEM)

Fig. 2. SEM images of C3 N4 (a), ZnO (b), and ZnO(15 wt.%)/C3 N4 sample (c and d).

12

W. Liu et al. / Journal of Molecular Catalysis A: Chemical 368–369 (2013) 9–15

Fig. 3. TEM image (a and b) and HRTEM image (c) of the ZnO(15 wt.%)/C3 N4 photocatalyst.

characterization were performed. Fig. 3a gives an overview of the typical TEM image of the ZnO/C3 N4 photocatalyst. It clearly exhibits the mean size of the particles is approximately 60 nm. Fig. 3b shows the TEM image of the junction for the sample and smooth interfaces are observed between the C3 N4 and ZnO. Fig. 3c shows the HRTEM image of the sample corresponding to the rectangle region of the TEM image in Fig. 3b. The upper part clearly exhibits the C3 N4 (0 0 2) facets with a lattice fringe spacing value of 0.325 nm. The lower part depicts the (0 0 2) plane of ZnO with a lattice plane separation of 0.259 nm. Based on the above results, it demonstrated that the structure of as-prepared sample was indeed formed from C3 N4 and ZnO, and they interacted with each other to form a heterojunction photocatalyst. As a result, the clear interface between the two semiconductors is advantageous for the migration of the photogenerated charge carriers and fundamental for improving photocatalytic degradation rate. 3.1.4. UV–vis analysis UV–vis diffuse reflectance spectroscopy was carried out to investigate the optical properties of the samples. Fig. 4 depicts UV–vis diffuse reflectance spectra of ZnO(15.0 wt.%)/C3 N4 , pure C3 N4 and ZnO. Compared with pure ZnO, the absorption wavelength range of the ZnO(15.0 wt.%)/C3 N4 photocatalyst red-shifts and the absorption intensity is also increased in the visible-light region, which therefore improves the utilization of the solar spectrum. From Fig. 4, it can also be seen that the absorption edge of the prepared C3 N4 and ZnO is at ca. 460 nm and 390 nm in the spectrum and the corresponding band gap of C3 N4 is about 2.70 eV

and 3.18 eV, respectively. The results agree well with the previous reports [9,34,41,43]. 3.2. Photocatalytic activity of the ZnO/C3 N4 3.2.1. Effect of ZnO content The photocatalytic activity of ZnO/C3 N4 photocatalyst was evaluated by photocatalytic degradation of RhB under visible light. The results are shown in Fig. 5. For comparison, the photocatalytic activity of pure C3 N4 and ZnO was also tested under identical experimental conditions. As shown in Fig. 5, the blank test showed that the concentration of RhB changed little under visible light irradiation, indicating that the photoinduced self-decomposition could be neglected in comparison with the photocatalysis caused by various catalyst particles. From Fig. 5, it was found that the photocatalytic activity of pure ZnO was negligible under visible light irradiation. Remarkably, the photocatalytic activity of ZnO/C3 N4 was much higher than that of pure C3 N4 and improved greatly with increasing ZnO content from 5 wt.% to 15 wt.%, although the photocatalytic activity had a slight decrease when the ZnO content reached the higher level (20 wt.% and 30 wt.%). It means that the optimal ZnO content is 15 wt.% in the ZnO/C3 N4 samples. The photocatalytic conversion of RhB is 97.6% in 150 min for ZnO(15 wt.%)/C3 N4 photocatalyst, while it is only 68.1% for pure C3 N4 . In a word, the ZnO/C3 N4 exhibits excellent visible light photocatalytic activity. It is well known that the photocatalytic activity is mainly governed by phase structure, adsorption ability, and separation 1.0

0.8

100

5wt.% 10wt.% 15wt.% 20wt.% 30wt.%

C3N4

80

0.6

ZnO(15.0wt.%)C3N4 60

C/C 0

Absorbance (%)

Blank ZnO C3N4

ZnO

0.4

40 0.2 20 0.0 0 200

300

400

500

600

700

30

60

90

120

150

Irradiation time (min)

Wavelength(nm) Fig. 4. UV–vis diffuse reflectance spectra of different photocatalysts.

Fig. 5. Photolysis of RhB and photocatalytic degradation of RhB over different samples under visible light irradiation.

W. Liu et al. / Journal of Molecular Catalysis A: Chemical 368–369 (2013) 9–15 Table 1 Summary of BET areas and RhB adsorption values of different samples.

13

0.027

Entry

Samples

BET area (m2 /g)

RhB adsorption (%)

1 2 3 4 5 6 7

C3 N4 ZnO(5 wt.%)/C3 N4 ZnO(10 wt.%)/C3 N4 ZnO(15 wt.%)/C3 N4 ZnO(20 wt.%)/C3 N4 ZnO(30 wt.%)/C3 N4 ZnO

9.6 10.9 12.4 13.2 14.5 16.7 33.2

6.3 6.5 7.8 8.6 9.1 9.9 10.3

15wt.%

0.024 0.021

20wt.%

k (min -1)

0.018

10wt.%

0.015 0.012

C3N4

0.009

30wt.%

5wt.%

0.006

C0 = kt C

ZnO

0.000

Samples Fig. 6. Kinetic constants for photocatalytic degradation of RhB under visible light irradiation.

activity was taken as an example to investigate the repeated experiments. The result is shown in Fig. 7a. The results showed that there was no obvious loss of photocatalytic activity after the 5th run. Furthermore, no evident difference was observed in XRD patterns of the photocatalyst before and after reaction, and there was no sight of impurity phases in the patterns of the used photocata-

(a)

1.0

2

1

3

4

0.6

0.4

0.2

0.0 0

150

300

450

3.2.2. Stability of the catalyst The catalyst’s lifetime is an important parameter of the photocatalytic process, so it is essential to evaluate the stability of the catalyst for practical application. In order to determine the stability of photocatalyst during the photocatalytic reaction, the ZnO(15 wt.%)/C3 N4 sample that performed the best photocatalytic

600

750

Irradiation time (min)

(b)

(2)

where C0 and C notate the same quantities as in Eq. (1), k is the firstorder rate constant (min−1 ) and t is the illumination time (min). The kinetics constants of RhB degradation with different samples were calculated by making a linear plot of ln(C0 /C) against time. The results are illustrated in Fig. 6. The results clearly demonstrate the optimal ZnO content is 15 wt.% with the maximal degradation rate constant of 0.02389 min−1 , which is 3.1 times as large as that of pure C3 N4 (0.00779 min−1 ).

5

0.8

after reaction

Intensity(a.u.)

ln

0.003

C/C0

efficiency of photogenerated electrons and holes [41]. The crystal phase structure of C3 N4 does not change after ZnO modification from the XRD analysis. The surface areas of the various photocatalysts were performed, and so did the RhB adsorption values after equilibration in the dark. As can be seen from Table 1, the surface area of the ZnO/C3 N4 sample is increased with the increase in ZnO content. The surface area of C3 N4 , ZnO(15 wt.%)/C3 N4 and ZnO(30 wt.%)/C3 N4 photocatalyst is 9.6, 13.2 and 16.7 m2 /g, respectively. Nevertheless, the RhB adsorption has not obvious change and it increases slightly from 6.3% to 9.9% when the ZnO content increase from 0 wt.% to 30 wt.%. As a result, the adsorptivity enhancement was not the major factor of the significant enhancement of the photocatalytic activity of C3 N4 . The enhanced visible photocatalytic property can be possibly attributed to the effective separation efficiency of photogenerated electrons and holes in ZnO/C3 N4 photocatalysts. Although the pure ZnO possesses the largest surface area and RhB adsorption (See Table 1), it plays no role in RhB degradation under the present experimental conditions, since ZnO cannot be excited by visible light irradiation. It is suggested that the increased ZnO content in the sample should decrease the amount of C3 N4 and lead to poor response of the photocatalyst under visible light irradiation, thus lower the electron transfer efficiency of the photoinduced electrons on C3 N4 particle surfaces to ZnO. This result also obviously reveals that both C3 N4 and ZnO play an important role in improving the photocatalytic activity, and there exists significant synergistic effect between C3 N4 and ZnO for photocatalytic degradation of RhB under visible light irradiation. In other words, there is a suitable ratio between C3 N4 substrate and ZnO, and the photocatalytic activity of ZnO/C3 N4 photocatalyst shows the optimal ZnO content in the photocatalyst is 15 wt.% in the present study, which is to be demonstrated by hydroxyl radical analysis in Section 3.3.3. It is known that the photocatalytic degradation of RhB follows the first-order kinetics in previous reports, and the kinetics equation can be expressed as follows [44–46]:

before reaction

10

20

30

40

50

60

70

80

2θ (degree) Fig. 7. Cycling run in the photocatalytic degradation of RhB under visible light irradiation (a), and XRD patterns of the ZnO(15 wt.%)/C3 N4 sample before and after reaction (b).

14

W. Liu et al. / Journal of Molecular Catalysis A: Chemical 368–369 (2013) 9–15

50

η (%)

40

30

20

10 Fig. 9. Schematic diagram of photoexcited electron–hole separation process.

Blank

BQ

IPA

AO

Fig. 8. Effects of a series of scavengers on the RhB conversion over ZnO(15 wt.%)/C3 N4 (illumination time = 30 min, scavenger dosage = 0.1 mmol/L).

lyst, as shown in Fig. 7b. Consequently, the ZnO/C3 N4 photocatalyst showed good stability during the photocatalytic reaction. 3.3. Possible photocatalytic mechanism 3.3.1. Reactive species The photocatalytic mechanism was investigated for the excellent photocatalytic property of the prepared ZnO/C3 N4 . It is generally accepted that the photo-degradation of RhB involves a large number of main reactive species including hydroxyl radicals (• OH), superoxide anion radicals (• O2 − ) and photogenerated holes (h+ ). Therefore, the effects of different scavengers on the degradation of RhB were examined in attempt to elucidate the reaction mechanism. The scavengers used in this study are ammonium oxalate (AO) for h+ [46], isopropanol (IPA) for • OH [47] and benzoquinone (BQ) for • O2 − [48]. As a consequence of quenching, photocatalytic oxidation reaction will be partly suppressed and the RhB conversion is lowered. The extent of decrease in the conversion, induced by a scavenger, indicates the importance of the corresponding oxidizing species. Effects of a series of scavengers on the RhB conversion over ZnO(15 wt.%)/C3 N4 are shown in Fig. 8. The photocatalytic conversion of RhB decreases the most rapidly from 52.8% to 13.7% after adding BQ, indicating • O2 − is the main active species in the photocatlytic process. When IPA and AO are added, the photocatalytic conversion of RhB decreases to 33.6% and 37.1%, respectively, which indicates • OH and h+ also play an important role in the oxidation of RhB. The same results were obtained when some other scavengers were employed to capture the active species, for example, anthraquinone and triethanolamine, tert-butanol were used as the scavenger of • O2 − , h+ and • OH respectively. In summary, the reactive species involved in the degradation of RhB are • O − , • OH and h+ . 2 3.3.2. Proposed mechanism On the base of the above information, a possible mechanism for the visible-light-driven photocatalyst ZnO/C3 N4 was proposed. Since the conduction band (CB) edge potential of C3 N4 (−1.12 eV) [38,40] was more negative than the CB edge of ZnO (−0.5 eV vs. NHE) [41], the photogenerated electrons in the CB of C3 N4 injected directly into that of ZnO under visible light irradiation, while the holes remained in the valence band (VB) of C3 N4 . The accumulated electrons in the CB of ZnO can be transferred to O2 adsorbed on the surface of the heterojunction photocatalysts and • O2 − yields because the CB edge potential of ZnO is more negative than the

standard redox potential Eo (O2 /• O2 − ) (−0.33 eV vs. NHE) [49,50]. At the same time, rich holes in the VB of C3 N4 have powerful potential to oxidize RhB directly. However, the holes cannot oxidize OH− or H2 O to • OH radicals, because the VB position of C3 N4 (1.57 eV vs. NHE) is more negative than the standard redox potential of Eo (• OH/OH− ) (1.99 eV vs. NHE) [49]. So, the • OH radicals are originated from the reaction of • O2 − with photogenerated electrons. • O2 − , • OH and h+ are the active species that effectively react with organics. The photoexcited electron–hole separation process is shown in Fig. 9. 3.3.3. Hydroxyl radical analysis From the mechanism discussed above, it is known that the • OH radicals are only originated from the reaction of photogenerated electrons with • O2 − . And thus the change tendency of • OH radicals is in accordance with that of the generated • O2 − species for different heterojunction photocatalysts. The • OH radicals produced during the photocatalytic reaction can be easily detected by a photoluminescence (PL) technique using terephthalic acid (TA) as a probe molecule. The method is rapid, sensitive, and specific, needing only simple standard PL instrumentation as well as further researches whether • OH radicals are formed on the surface of visible-light illuminated samples. On account of the above reasons, the • OH radicals instead of • O2 − species were detected in this study. As for the • OH radicals detected by the TA-PL technique, the detailed experimental procedures were reported in early literatures [51,52]. The PL emission spectra of different ZnO/C3 N4 photocatalytsts excited at

ZnO(15wt.%)/C3N4 ZnO(20wt.%)/C3N4

Intensity(a.u.)

0

ZnO(10wt.%)/C3N4 ZnO(30wt.%)/C3N4 ZnO(5 wt.%)/C3N4 Blank

350

400

450

500

550

600

Wavelength(nm) Fig. 10. PL spectral changes of different ZnO/C3 N4 photocatalysts in TA solution.

W. Liu et al. / Journal of Molecular Catalysis A: Chemical 368–369 (2013) 9–15

315 nm from TA solution suspension were measured after 30 min illumination. The results are shown in Fig. 10. It can be seen that an obvious PL signal at ca. 435 nm is observed, demonstrating that • OH are formed in the photocatalytic oxidation process, which agrees well with the results of IPA quenching. In addition, the PL intensity of ZnO(15 wt.%)/C3 N4 is the highest among all samples, suggesting the formation rate of • OH radical on its surface is the largest. When the amount of ZnO is smaller or larger than 15 wt.%, the surface of ZnO/C3 N4 sample inhibits the production of • OH radicals, which implies the photocatalytic activity of the photocatalyst is lower than that of ZnO(15 wt.%)/C3 N4 . The change tendency agrees well with that of the photocatalytic activity of the ZnO/C3 N4 in Section 3.2. The approach not only reflects indirectly • O2 − relative generation but also verifies the charge carrier separation efficiency in ZnO/C3 N4 systems. 4. Conclusions The ZnO/C3 N4 photocatalysts with different weight percents of ZnO were successfully prepared by a facile ball milling method. The photocatalytic activity of the prepared ZnO/C3 N4 was much higher than that of single-phase C3 N4 . The rate constant value of RhB degradation over ZnO(15 wt.%)/C3 N4 was 3.1 times as large as that of pure C3 N4 . The optimal ZnO content in the ZnO/C3 N4 phtocatalyst was 15 wt.%. The prepared ZnO/C3 N4 was well stable in the photocatalytic process. The enhanced photocatalytic activity of the ZnO/C3 N4 originated from a good interface connection between two semiconductors with appropriate band structures so that charge transfer could proceed smoothly. The • O2 − and • OH reactive radicals generated in the CB of ZnO played an important role in the photocatalytic degradation of RhB by ZnO/C3 N4 systems as well as direct h+ in the VB of C3 N4 . This work may provide some insight into solving the unsatisfactory catalytic activity and low efficiency converting solar radiation for practical applications of C3 N4 as well as the design of novel heterojunction photocatalysts. Acknowledgements This work was supported by the National Basic Research Program of China (No. 2011CB302004), the Natural Science Foundation of China (Nos. 20973071, 51172086, 51272081 and 21103060) and the Foundation for Young Talents in College of Anhui Province, China (No. 2011SQRL072ZD). References [1] D.J. Yang, C.C. Chen, Z.F. Zheng, H.W. Liu, E.R. Waclawik, Z.M. Yan, Y.N. Huang, H.J. Zhang, J.C. Zhao, H.Y. Zhu, Energy Environ. Sci. 4 (2011) 2279–2287. [2] S.W. Liu, C. Li, J.G. Yu, Q.J. Xiang, CrystEngComm 13 (2011) 2533–2541. [3] S.W. Hu, J.Z. Zhu, L. Wu, X.X. Wang, P. Liu, Y.F. Zhang, Z.H. Li, J. Phys. Chem. C 115 (2011) 460–467. [4] T.G. Xu, L.W. Zhang, H.Y. Cheng, Y.F. Zhu, Appl. Catal. B: Environ. 101 (2011) 382–387. [5] Y.C. Qiu, K.Y. Yan, H. Deng, S.H. Yang, Nano Lett. 12 (2012) 407–413. [6] Z. Zhou, M. Long, W. Cai, J. Cai, J. Mol. Catal. A: Chem. 353–354 (2012) 22–28. [7] H. Xie, Y.Z. Li, S.F. Jin, J.J. Han, X.J. Zhao, J. Phys. Chem. C 114 (2010) 9706–9712. [8] M. Tabata, K. Maeda, T. Ishihara, T. Minegishi, T. Takata, K. Domen, J. Phys. Chem. C 114 (2010) 11215–11220.

15

[9] F. Dong, L.W. Wu, Y.J. Sun, M. Fu, Z.B. Wu, S.C. Lee, J. Mater. Chem. 21 (2011) 15171–15174. [10] Q. Xiao, C. Yao, Mater. Chem. Phys. 130 (2011) 5–9. [11] M. Khatamian, A.A. Khandar, B. Divband, M. Haghighi, S. Ebrahimiasl, J. Mol. Catal. A: Chem. 365 (2012) 120–127. [12] C.Q. Chen, Y.H. Zheng, Y.Y. Zhan, X.Y. Lin, Q. Zheng, K.M. Wei, Dalton Trans. 40 (2011) 9566–9570. [13] J.B. Mu, C.L. Shao, Z.C. Guo, Z.Y. Zhang, M.Y. Zhang, P. Zhang, B. Chen, Y.C. Liu, ACS Appl. Mater. Interfaces 3 (2011) 590–596. [14] X.W. Wang, L.C. Yin, G. Liu, L.Z. Wang, R. Saito, G.Q. Lu, H.-M. Cheng, Energy Environ. Sci. 4 (2011) 3976–3979. [15] J. Jiang, X. Zhang, P.B. Sun, L.Z. Zhang, J. Phys. Chem. C 115 (2011) 20555–20564. [16] X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, Nat. Mater. 8 (2009) 76–80. [17] L. Ge, Mater. Lett. 65 (2011) 2652–2654. [18] K. Takanabe, K. Kamata, X. Wang, M. Antonietti, J. Kubota, K. Domen, Phys. Chem. Chem. Phys. 12 (2010) 13020–13025. [19] S.C. Yan, Z.S. Li, Z.G. Zou, Langmuir 25 (2009) 10397–10401. [20] Y.J. Zhang, T. Mori, J.H. Ye, M. Antonietti, J. Am. Chem. Soc. 132 (2010) 6294–6295. [21] Y. Wang, Y. Di, M. Antonietti, H. Li, X.F. Chen, X.C. Wang, Chem. Mater. 22 (2010) 5119–5121. [22] J. Liang, Y. Zheng, J. Chen, J. Liu, D. Hulicova-Jurcakova, M. Jaroniec, S.Z. Qiao, Angew. Chem. 124 (2012) 3958–3962. [23] G. Liu, P. Niu, C.H. Sun, S.C. Smith, Z.G. Chen, G.Q. Lu, H.-M. Cheng, J. Am. Chem. Soc. 132 (2010) 11642–11648. [24] Y. Zheng, Y. Jiao, J. Chen, J. Liu, J. Liang, A.J. Du, W.M. Zhang, Z.H. Zhu, S.C. Smith, M. Jaroniec, G.Q. Lu, S.Z. Qiao, J. Am. Chem. Soc. 133 (2011) 20116–20119. [25] S.C. Yan, Z.S. Li, Z.G. Zou, Langmuir 26 (2010) 3894–3901. [26] K. Maeda, X.C. Wang, Y. Nishihara, D.L. Lu, M. Antonietti, K. Domen, J. Phys. Chem. C 113 (2009) 4940–4947. [27] Y. Di, X.C. Wang, A. Thomas, M. Antonietti, ChemCatChem 2 (2010) 834–838. [28] X.C. Wang, K. Maeda, X.F. Chen, K. Takanabe, K. Domen, Y.D. Hou, X.Z. Fu, M. Antonietti, J. Am. Chem. Soc. 131 (2009) 1680–1681. [29] Y.J. Zhang, A. Thomas, M. Antonietti, X.C. Wang, J. Am. Chem. Soc. 131 (2009) 50–51. [30] X.F. Chen, J.S. Zhang, X.Z. Fu, M. Antonietti, X.C. Wang, J. Am. Chem. Soc. 131 (2009) 11658–11659. [31] X.F. Chen, Y.S. Jun, K. Takanabe, K. Maeda, K. Domen, X.Z. Fu, M. Antonietti, X.C. Wang, Chem. Mater. 21 (2009) 4093–4095. [32] S.W. Bian, Z. Ma, W.G. Song, J. Phys. Chem. C 113 (2009) 8668–8672. [33] L. Ge, C.C. Han, J. Liu, J. Mater. Chem. 22 (2012) 11843–11850. [34] Q.J. Xiang, J.G. Yu, M. Jaroniec, J. Phys. Chem. C 115 (2011) 7355–7363. [35] X.-H. Li, J.-S. Chen, X.C. Wang, J.H. Sun, M. Antonietti, J. Am. Chem. Soc. 133 (2011) 8074–8077. [36] G.Z. Liao, S. Chen, X. Quan, H.T. Yu, H.M. Zhao, J. Mater. Chem. 22 (2012) 2721–2726. [37] L. Ge, F. Zuo, J.k. Liu, Q. Ma, C. Wang, D.Z. Sun, L. Bartels, P.Y. Feng, J. Phys. Chem. C 116 (2012) 13708–13714. [38] S.C. Yan, S.B. Lv, Z.S. Li, Z.G. Zou, Dalton Trans. 39 (2010) 1488–1491. [39] H.J. Yan, H.X. Yang, J. Alloys Compd. 509 (2011) L26–L29. [40] L. Ge, C.C. Han, J. Liu, Appl. Catal. B: Environ. 108–109 (2011) 100–107. [41] Y.J. Wang, R. Shi, J. Lin, Y.F. Zhu, Energy Environ. Sci. 4 (2011) 2922–2929. [42] W. Liu, M.S. Ji, S.F. Chen, J. Hazard. Mater. 186 (2011) 2001–2008. [43] S. Suwanboon, P. Amornpitoksuk, N. Muensit, Ceram. Int. 37 (2011) 2247–2253. [44] S.F. Chen, Y.G. Yang, W. Liu, J. Hazard. Mater. 186 (2011) 1560–1567. [45] C. Wu, H. Chang, J. Chen, J. Hazard. Mater. 137 (2006) 336–343. [46] (a) S.F. Chen, W. Liu, H.Y. Zhang, X.L. Yu, J. Hazard. Mater. 186 (2011) 1687–1695; (b) S.G. Meng, D.Z. Li, M. Sun, W.J. Li, J.X. Wang, J. Chen, X.Z. Fu, G.C. Xiao, Catal. Commun. 12 (2011) 972–975. [47] L.S. Zhang, K.H. Wong, H.Y. Yip, C. Hu, J.C. Yu, C.Y. Chan, P.K. Wong, Environ. Sci. Technol. 44 (2010) 1392–1398. [48] M.C. Yin, Z.S. Li, J.H. Kou, Z.G. Zou, Environ. Sci. Technol. 43 (2009) 8361–8366. [49] X.L. Fu, W.M. Tang, L. Ji, S.F. Chen, Chem. Eng. J. 180 (2012) 170–177. [50] Z.H. Li, Z.P. Xie, Y.F. Zhang, L. Wu, X.X. Wang, X.Z. Fu, J. Phys. Chem. C 111 (2007) 18348–18352. [51] J.G. Yu, W.G. Wang, B. Cheng, B.L. Su, J. Phys. Chem. C 113 (2009) 6743–6750. [52] Y.H. He, D.Z. Li, G.C. Xiao, W. Chen, Y.B. Chen, M. Sun, H.J. Huang, X.Z. Fu, J. Phys. Chem. C 113 (2009) 5254–5262.