Enhanced photo-induced charge separation and solar-driven photocatalytic activity of g-C3N4 decorated by SO42−

Materials Science in Semiconductor Processing 40 (2015) 508–515

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Enhanced photo-induced charge separation and solar-driven photocatalytic activity of g-C3N4 decorated by SO24
Junbo Zhong a,n, Jianzhang Li a, Xinlu Liu a, Qizhao Wang b, Hao Yang c, Wei Hu a, Chaozhu cheng a, Jiabo Song a, Minjiao Li a, Tian Jin a a Key Laboratory of Green Catalysis of Higher Education Institutes of Sichuan, College of Chemistry and Pharmaceutical Engineering, Sichuan University of Science and Engineering, Zigong 643000, PR China b College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, PR China c Sichuan Tianren Chemical Engineering Co. Ltd., Chengdu 610031, PR China

a r t i c l e in f o

abstract

Article history: Received 10 April 2015 Received in revised form 1 June 2015 Accepted 10 June 2015

SO4 decorated g-C3N4 with enhanced photocatalytic performance was prepared by a facile pore impregnating method using (NH4)2S2O8 solution. The photocatalysts were characterized by the Brunauer–Emmett–Teller (BET) method, X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–vis diffuse reflectance spectroscopy (DRS), transmission electron microscopy (TEM), Fourier-transform infrared (FT-IR), X-ray photoelectron spectroscopy (XPS) and surface photovoltage (SPV) spectroscopy, respectively. The separation efficiency of photo-generated charge was investigated using benzoquinone as scavenger. The results demonstrate that sulfating of g-C3N4 increases the adsorption of rhodamine B on g-C3N4, the hydroxyl content and the separation efficiency of photo2
generated charge. The photocatalytic activity of SO4 /g-C3N4 for decolorization of rhodamine B and methyl orange (MO) aqueous solution was evaluated. The result shows 2
that loading of 6.0 wt% SO4 results in the best photocatalytic activity under simulated 2
solar irradiation and SO4 play an important role in boosting the photocatalytic activity. & 2015 Elsevier Ltd. All rights reserved.

Keywords: g-C3N4 Sulfating Photo-generated charge separation Photocatalytic performance

2


1. Introduction Photocatalysis has attracted increasing attention to settle the global energy crisis and environmental pollution utilizing sun light [1,2]. To use solar energy directly, various semiconductor photocatalysts have been developed and studied, such as BiVO4, Bi2O3, α-Fe2O3, Bi2WO6, BiOI, Ag3PO4 and so on. Among these solar-driven photocatalysts, graphitic carbon nitride (g-C3N4) has recently emerged as a novel and

n

Corresponding author. Tel./fax: þ 86 813 5505601. E-mail addresses: [email protected] (J. Zhong), [email protected] (J. Li), [email protected] (X. Liu), [email protected] (Q. Wang), [email protected] (H. Yang), [email protected] (W. Hu), [email protected] (C. cheng), [email protected] (J. Song), [email protected] (M. Li), [email protected] (T. Jin). http://dx.doi.org/10.1016/j.mssp.2015.06.025 1369-8001/& 2015 Elsevier Ltd. All rights reserved.

promising photocatalyst due to its good electronic properties, high thermal and chemical stability, and easy preparation [2– 5]. It is well-known that the band gap of g-C3N4 is about 2.7 eV, thus g-C3N4 can absorb visible light up to 460 nm [3,5]. Furthermore, the conduction band of g-C3N4 is extremely negative, so photo-generated electrons have high reduction ability [6]. Continuous efforts have been taken to investigate g-C3N4 in the potential applications in purification of contaminated water, H2 evolution, CO2 reduction, solar fuels and so on [7–18]. However, there are some inherent drawbacks which restrict the application of g-C3N4 in photocatalysis, such as the high recombination rate of photo-generated electron–hole pairs and weak oxidative ability of photogenerated hole [19–21]. To promote the photocatalytic performance of g-C3N4, tremendous attempts have been made to improve the quantum yield of g-C3N4, for example, designing

J. Zhong et al. / Materials Science in Semiconductor Processing 40 (2015) 508–515

nanoporous structures [22], doping [23–26], constructing heterojunctions [27–30] and so on. Among these approaches, modification of g-C3N4 with sulfate is an effective and simple strategy to promote the photocatalytic performance. Yan and coworkers prepared graphitic carbon nitride by directly heating sulfuric acid treated melamine precursor at 873 K [31]. The photocatalytic performance of the as prepared carbon nitride was evaluated under visible light irradiation. The results demonstrate that catalyst fabricated from sulfuric acid treated melamine shows 2 times higher photocatalytic H2 production rate than that on sample synthesized from untreated 2
melamine. Zhong et al. prepared SO4 /SiO2–TiO2 [32]. The results reveal that sulfating of SiO2–TiO2 increases the formation rate of OH radicals produced during the photocatalytic reaction process, resulting in high photocatalytic activity. However, in photocatalysis field, the effects of photo2
induced charge separation and the loading of SO4 on the 2
photocatalytic performance of SO4 /g-C3N4 prepared by a pore impregnating method using ammonium persulfate solution have been seldom concerned. The primary objective of 2
this work is to investigate the effect of SO4 modification on the photo-induced charge separation and the relation with 2
the photocatalytic activity of SO4 /g-C3N4. The separation of photo-generated charge was studied by surface photovoltage spectroscopy and using benzoquinone as scavenger. The photocatalytic performance was studied by de-colorization of rhodamine B and MO aqueous solution under simulated solar illumination. 2. Experimental section 2.1. Preparation of photocatalysts All chemicals (analytical grade reagents) were supplied from Chengdu Kelong Chemical Reagent Factory and used as received. All the studies were done using deionized water and reagents of A.R grade. g-C3N4 was prepared by heating urea in a muffle furnace. 20.0 g of urea was put into an alumina crucible with a cover, and then baked in air at 823 K for 4 h. After cooled to room temperature, the resulted yellow product was collected and ground into powder. 2
2
SO4 /g-C3N4 photocatalysts with different SO4 loadings were prepared by a pore impregnating method using (NH4)2S2O8 aqueous solution. Briefly, 2 g as-prepared g-C3N4 was put into a beaker, water was added and made g-C3N4 to be humidified (no water was observed on the surface of gC3N4) assisted by ultrasonication, the volume of the water is the water pore volume of 2 g g-C3N4. Desired (NH4)2S2O8 was dissolved in water (the water pore volume of 2 g g-C3N4), then 2 g g-C3N4 was added into the (NH4)2S2O8 solution mentioned above and ultrasonic dispersed for 20 min. The mixture was kept in a static condition for 1 h, dried in 353 K for 1 h, and then dried in 373 K overnight. Finally, the powders were baked in air at 573 K for 2 h. In this way, 2
SO4 modified g-C3N4 photocatalysts with different wt% of 2
2
SO4 were prepared. The samples with different wt% of SO4 (0%, 0.8%, 2.5%, 4.2%, 6.0% and 7.7%) were named 0%, 0.8%, 2.5%, 4.2%, 6.0% and 7.7%, respectively. g-C3N4 was also dealt with the same procedure as mentioned above without the

509 2


presence of (NH4)2S2O8. To study the stability of SO4 on the surface of g-C3N4, 1 g 6.0% sample was added into 100 mL water, and then was stirred for 24 h. Finally, the sample was washed several times with 2000 mL distilled water and absolute ethanol, and then dispersed in absolute ethanol and dried at 60 1C in air overnight, the sample as named 6.0%-washed. 2.2. Characterization of the photocatalysts The specific surface area and pore size measurements were performed on an SSA-4200 automatic surface analyzer (Builder, China) using BET function. The solid samples were evacuated at 523 K for 1 h, and then cooled to 77 K using liquid N2 at which point N2 adsorption was measured. X-ray diffraction (XRD) patterns were recorded on a DX-2600 X-ray diffractometer using Cu Kα (λ¼0.15406 nm) radiation and equipped with a graphite monochromator. The X-ray tube was operated at 40 kV and 25 mA. Samples were scanned from 2θ equal to 101 up to 901 and the X-ray diffraction line positions were determined with a step size of 0.031. The UV– vis diffuse reflectance spectroscopy (DRS) was recorded using a TU-1950 UV–vis spectrophotometer equipped with an integrating sphere, using BaSO4 as the reference. Scanning electron microscope (SEM) images were taken with a JSM7500F scanning electron microscope (JEOL, Japan), using an accelerating voltage of 5 kV. Before SEM measurement, the sample was dispersed in water assisted by ultrasonication for 10 min and then gold plating was performed. Fouriertransform infrared (FT-IR) was characterized using an FT-IR spectrophotometer (NICOLET 6700, America) in KBr pellet. Transmission electron microscopy (TEM) was recorded on a Philips Fei Tecnai Sprit electron microscope at an accelerating voltage of 120 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed on a XSAM 800. A Mg Kα Xray source was used at 12 kV and 12 mA. The measurement error of XPS is 70.1 eV. The measurements of surface photovoltage spectroscopy (SPV) were carried out according to the procedure described in Ref. [33]. The powder samples were sandwiched between two ITO glass electrodes, and the change of surface potential barrier between in the presence of light and in the dark is SPV signal. 2.3. Evaluation of photocatalytic activity In this work, rhodamine B was chosen as model dye. In a typical photocatalytic experiment, photocatalyst was dispersed into 50 mL rhodamine B/MO aqueous solution (10 mg/L). The measurements of photocatalytic activity were carried out in a Phchem III photochemical reactor (Beijing NBET Technology Co., Ltd., China) under intense stirring. The illumination source was a 500 W Xe lamp (simulated sun light), the lamps were encapsulated in a cooling quartz jacket and positioned in the middle of the reactor. Quartz test tubes were located around the lamp and the distance from the lamp to the quartz test tubes was 10 cm. The decolorization reaction was performed at room temperature. The pH value of the rhodamine B and MO was 7.0 and 2.5, respectively. After regular intervals, the samples were centrifuged (9000 rpm) to separate the photocatalyst for analysis. The concentration of

J. Zhong et al. / Materials Science in Semiconductor Processing 40 (2015) 508–515

3. Results and discussion 3.1. Characterization of photocatalysts Table 1 shows the surface parameters of photocatalysts 2
with different SO4 loadings. It is clear that the specific surface area tends to deteriorate upon modifying g-C3N4 2
with SO4 . However, considering the measurement error of specific surface area (72 m2 g
1), these photocatalysts 2
have no obvious difference; this result indicates that SO4 within an amount cannot change effectively specific sur2
face area of SO4 /g-C3N4. The results in our case are not the same as the results reported by Yan and coworkers [31]. The BET surface area determined by N2 adsorption is 15.6 and 8.6 m2/g for C3N4 synthesized from sulfuric acid treated melamine and untreated melamine, respectively. The different results in BET surface area may due to the different precursor and treatment process. Combined with the photocatalytic activity, the specific surface area is not the decisive factor that influences the photocatalytic performance. The XRD patterns of the photocatalysts are shown in Fig. 1. As shown in Fig. 1, the peaks at 27.401of g-C3N4 corresponds to the (002) planes of the tetragonal phase g-C3N4 (JCPDS 871526). For pure g-C3N4, no other peaks were detected, indicating high purity of sample. However, peaks of (NH4)2SO4 2
(JCPDS 76-0579) were observed as the content of SO4 increasing. (NH4)2SO4 is acid in nature, thus the presence of (NH4)2SO4 is beneficial to form solid super acid, improving the photocatalytic activity. Moreover, no obvious shift of diffrac2
tion angles of g-C3N4 was observed for SO4 /g-C3N4 samples, 2
which demonstrates that SO4 is on the surface of g-C3N4. To 2
study the stability of SO4 on the surface of g-C3N4, 6.0% sample was washed and detected by XRD, the results show that no (NH4)2SO4 was observed on the surface of g-C3N4. 2
The optical properties of SO4 /g-C3N4 were investigated by UV–vis diffuse reflectance spectroscopy. Due to the partial overlap of the diffuse reflectance spectra of 2
SO4 /g-C3N4 samples, thus only the diffuse reflectance Table 1 Specific surface parameters of photocatalysts.

spectra of the 0% and the 6.0% samples were presented and the results are shown in Fig. 2. The UV–vis diffuse reflectance spectroscopy appears the same features. The absorption edge is 460 nm, corresponding to a band gap of ca. 2.70 eV, which matches the value reported by Wang 2
and coworkers [3]. The results show that SO4 modification cannot introduce new impurity energy levels to the valence band (VB) energy level of g-C3N4. Combined with the photocatalytic activity, it is clear that the difference in photocatalytic activity is not caused by optical properties of photocatalysts. The SEM images of the 0% and 6.0% samples are shown in Fig. 3. As shown in Fig. 3, two photocatalysts both exhibit irregular lump and bar-like morphology, which indicates that the morphology of the photocatalysts cannot be changed 2
appreciably by the SO4 modifying. No abundant pores were observed in the particles, this result accords with the low surface area as shown in Table 1. Fig. 3c and d shows the 2
segments of the irregular lump and bar-like SO4 /g-C3N4, no 2
SO4 was observed on the surface on the g-C3N4. FT-IR spectra of photocatalysts are presented in Fig. 4, the band located at 1630 cm
1 is assigned to the hydroxyl groups. Compared with pure g-C3N4 sample, the peaks located at 2
1630 cm
1 of g-C3N4 modified by SO4 are stronger, suggest2
ing that SO4 /g-C3N4 photocatalysts have more hydroxyl groups than the pure g-C3N4. Usually, the increase of surface hydroxyl content on the surface of photocatalyst has a beneficial effect on the enhancement of photocatalytic activity 36000

(002)

*:ONH4P2SO4

6.0%-washed

30000

Intensity (a.u.)

rhodamine B/MO was measured by a 756 PC spectrophotometer by the Lambert–Beer law. To detect the related reactive species during photocatalytic process, scavengers were added into the photocatalytic reaction system. The scavengers used in this study are isopropanol (IPA) for OH [34], ammonium oxalate
(AO) for h þ [35], and benzoquinone (BQ) for O2 [36]. The experiment was similar to the photocatalytic decolorization experiment. The concentration of scavengers in the photocatalytic reaction system was 0.2 mmol/L.

7.7%

*

**

24000

6.0%-fresh 18000

4.2%

12000

2.5%

6000

0.8% 0%

0 15

20

25

30

35

40

45

2 Theta (degree) Fig. 1. XRD patterns of photocatalysts.

0.8

0.6

Absorbance

510

0.4 0%

0.2 Catalysts (%) SBET (m2/g) Pore volumes (cm3/g) Pore size (nm) 0 0.8 2.5 4.2 6.0 7.7

10.7 9.7 9.1 8.4 7.5 6.0

0.0061 0.0056 0.0054 0.0052 0.0046 0.0035

11.1 11.4 11.6 11.7 12.4 12.7

460

6.0% 0%

0.0

300

400

500

600

700

800

Wavelength (nm)

Fig. 2. UV–vis diffuse reflection spectra of photocatalysts.

J. Zhong et al. / Materials Science in Semiconductor Processing 40 (2015) 508–515

511

Fig. 3. SEM and TEM images of photocatalysts; (A) 0%; (B) 6.0%; (C) 0%; and (D) 7.7%;

7.7%

1415 OH

617

6.0%

Intensity (a.u.)

[37]. The surface hydroxyl groups can trap the photo-induced holes to enhance the separation efficiency of photo-induced electron–hole pairs [38], promoting the photocatalytic performance. Compared with the pure g-C3N4 sample, some new 2
peaks appeared on the SO4 modified photocatalysts from
1 between 500 cm and 1500 cm
1, the peak at 1415 cm
1 is þ the characteristic peak of NH4 . The peaks located at 617 and 1240 cm
1 are the characteristic peaks of SQO bond or S–O bond [39], it is widely acknowledge that the strong covalence SQO bond has strong ability of inducing electron for the formation of solid super acid. The results of FT-IR accord well with the results of XRD. The X-ray photoelectron spectroscopy (XPS) was carried out to determine the surface chemical composition of 2
g-C3N4 and SO4 /g-C3N4 as well as the valence states of various species. Fine XPS spectra of C 1s, N 1s and S2p of photocatalysts are present in Fig. 5. The two peaks at the binding energies of 284.6 eV and 288.9 eV are observed in the C 1s fine XPS spectrum for phototocatalysts. The former corresponds the sp2 C–C bonds in adventitious carbon species, while the latter to is ascribed to the sp2hybridized C ([N–C]N) in the aromatic skeleton rings of gC3N4 [40]. As shown in Fig. 5b, the main peak at 398.6 eV of the N1s spectra originates from the sp2-bonded N involved in the triazine rings, which dominates in g-C3N4 [41], no others peaks were observed. In addition, considering the measurement error, no obvious shifts of C1s and N1s of photocatalysts were observed. Fig. 5c shows the high-resolution XPS spectrum of the S2p region on the 2
surfaces of the SO4 /g-C3N4. Researchers have reported that S 2p peaks are found for high oxidation states at BE 4168 eV [42]. So 168.1 eV is assigned to the binding

4.2% 2.5% 0.8% 1240 0% 3500

3000

2500

2000

1500

1000

500

-1

Wavenumber Ocm P Fig. 4. FT-IR spectra of photocatalysts. 2


energy of S6 þ (such as SO4 ) [43], which accords well with the results of XRD and FT-IR. Combined the results of 2
XRD, it is difficult to determine whether SO4 is physical adsorption or weak chemical bonding on the surface of gC3N4, remaining for further investigation. 3.2. Photo-induced charge separation 2


The SPV responses of g-C3N4 and SO4 /g-C3N4 composites are shown in Fig. 6. It can be seen that g-C3N4 displays obvious SPV response from 300–500 nm, which is attributed to the electronic transitions from the valence band to conduction band according to the DRS spectra and energy band structure of g-C3N4. As shown in Fig. 6, the 2
SPV response of SO4 /g-C3N4 in UV and visible light region

512

J. Zhong et al. / Materials Science in Semiconductor Processing 40 (2015) 508–515

60000 287.9

Photovoltage (mV)

Intensity(a.u.)

7.7%

40000 6.0% 30000 20000

6.0%

0.12

284.6

50000

2.5%

0.08

2.5% 7.7%

0.04

0.8% 0%

4.2%

7.7% 10000

0.00

0%

300

0 296

294

292

290

288

286

284

282

280

6.0%

350

278

400

450

500

Wavelength (nm)

Binding energy (eV) Fig. 6. SPV responses of photocatalysts. 100000

398.6

40

39.70

60000

Decolorization (%)

7.7%

80000

Intensity (a.u.)

39.76

6.0%

40000 2.5% 20000 0% 0 408

406

404

402

400

398

396

394

32 27.53 24 15.21

16 8

392

0

Binding energy(eV)

Blank

IPA

AO

BQ

Scavenger Fig. 7. Effects of a series of scavengers on rhodamine B decolorization over g-C3N4; the concentration of rhodamine B was 10 mg L
1, the concentration of photocatalysts was 1 g L
1, illumination time ¼ 1.5 h, Scavenger dosage¼0.2 mmol/L.

3600 3200 7.7%

Intensity (a.u.)

2800 168.1

2400 2000

6.0%

1600 1200 2.5% 800 174

172

170

168

166

164

162

160

Binding energy (eV) Fig. 5. Fine XPS spectra of C 1s, N 1s and S2p of photocatalysts. 2


gradually increases as the loading of SO4 increasing, and reaches the maximum at 6.0%, then drops dramatically at 7.7%. According to the principle of SPV, the strong SPV response corresponds to the high separation rate of photoinduced charge [44,45], thus, it is clear that the 6.0% sample exhibits the highest charge separation rate among the experimented compositions, while 0% sample has the lowest charge separation rate. Furthermore, obvious SPV response is observed in the visible-light region for all the decorated samples, which indicates that the photoinduced electron–hole pairs after modifying g-C3N4 with 2
SO4 can be separated effectively under the irradiation of visible light, which is beneficial to the photocatalytic performance under simulated solar irradiation. It is interesting to note that the SPV signal of 6.0% sample is stronger and complex than other samples, which shows that 6.0% sample has many trapping states, that is, photo-

excited 6.0% sample has many surface net charges. Surface net charge promotes the flow of electrons and holes in different directions, enhancing greatly the charge separation as shown in Fig. 6. It is commonly accepted that among all the factors that affect the photocatalytic performance, the number of photo-generated charge carriers plays an important role in influencing the photocatalytic activity: the more the number of carriers, the better the photocatalyst [46]. High photocatalytic activity can be benefited from the high charge separation rate; this result is in good consistent with the results of photocatalytic performance. In order to detect the reactive species during the photocatalytic process and separation efficiency of photogenerated charges, the effects of three scavengers on the photocatalytic decolorization of rhodamine B over pure gC3N4 were studied. The effects of three scavengers on the photocatalytic decolorization of rhodamine B are shown in Fig. 7. The photocatalytic decolorization of rhodamine B drops dramatically from 39.76% to 15.21% after adding BQ,
indicating that O2 is the main active species in the photocatalytic decolorization process. When AO is added, the photocatalytic decolorization efficiency of rhodamine B drops to 25.73%. However, when IPA is added, no change of decolorization of rhodamine B is observed, which suggests that no OH exists during the photocatalytic process. The results fit well with the results reported in

J. Zhong et al. / Materials Science in Semiconductor Processing 40 (2015) 508–515

40

30

20 15.21 10

12

8

0.8%

2.5%

4.2%

6.0%

7.45

4

0 0%

14.59

32.86

Decolroization (%)

Decolorization(%)

15.4

16 34.50

0

513

7.7%

0%

0.8%

3.3. Measurements of photocatalytic activity The photolysis of rhodamine B solution (10 mg/L) under simulated sun light irradiation without photocatalyst after 1.5 h is so small that can be ignored, the adsorption of rhodamine B on different photocatalysts after 1.5 h in dark is shown in Fig. 9. The adsorption of rhodamine B over different photocatalysts tends to increase upon sulfating, the enhanced adsorption of rhodamine B has a beneficial effect on the photocatalytic activity of catalyst. Among these photocatalysts, 6.0% has highest adsorption toward rhodamine B. Sulfating can form solid super acid, which will increase the acidic sites on the g-C3N4 surface; however, excessive amount 2
of the SO4 is detrimental to the adsorption of rhodamine B possibly due to the coverage of too many g-C3N4 active sites 2
by the SO4 , illustrating by the results of XRD. The results of

6.0%

7.7%

Fig. 9. Adsorption of rhodamine B on the different photocatalysts after 1.5 h in dark, the concentration of rhodamine B was 10 mg L
1, the concentration of photocatalysts was 1 g L
1.

90

83.16 78.13

Decolorization(%)

75 60 45

39.76

30 15 0

0%

0.8%

2.5%

4.2%

6.0%

7.7%

Photocatalyst

74.6

75 62.6

Decolorization(%)

Ref. [47]. Compared with the standard redox potential of OH
/OH (1.99 V vs. NHE), the VB potential of g-C3N4 (1.4 V vs. NHE) is more negative than the standard redox þ potential of OH
/OH, thus hVB cannot oxidize OH
to generate OH radicals directly in photocatalytic reaction system [48]. The results in our case demonstrate that h þ play the secondary important role in the decolorization of rhodamine B. To further investigate the separation efficiency of photoinduced charges of all photocatalysts, the effect of BQ (0.2 mmol/L) on the photocatalytic decolorization of rhodamine B over different photocatalysts was evaluated. The results are presented in Fig. 8. As shown in Fig. 8, decolorization of rhodamine B over 6.0% is the highest, which illustrates that 6.0% sample holds the highest separation efficiency of photoinduced charges to some extent. The separation efficiency of 2
photo-induced charges increases gradually with the SO4 2
content increasing, and reaches a maximum when the SO4 content is 6.0% and then decreases at 7.7%. Compared with other samples, the 6.0% sample has more reactive species to participate in the photocatalytic reaction, which favors the photocatalytic decolorozation of rhodamine B. The results of photo-induced charges separation accord well with the results of photocatalytic performance.

4.2%

Photocatalyst

Photocatalyst Fig. 8. Effects of BQ on rhodamine B decolorization over photocatalysts, the concentration of rhodamine B was 10 mg L
1, the concentration of photocatalysts was 1 g L
1, illumination time ¼ 1.5 min, BQ dosage¼ 0.2 mmol/L.

2.5%

73.1 60.4

60 45 30 15 0

0%

0.8%

2.5%

4.2% 6.0%(A) 7.7% 6.0%(B)

Photocatalyst Fig. 10. Catalytic activity of photocatalysts; (A) the concentration of rhodamine B was 10 mg L
1, the initial pH of rhodamine B is 7.0, the concentration of photocatalysts was 1 g L
1; the irradiation time was 1.5 h; (B) the concentration of MO was 10 mg L
1, the initial pH of MO is 2.5, the concentration of photocatalysts was 0.4 g L
1; the irradiation time was 15 min, 6.0%(A)-fresh, 6.0%(B)-washed; the pH value of rhodamine B and MO solution was adjusted by HClO4 and sodium hydroxide solution.

adsorption of rhodamine B on different photocatalysts is in good consistent with the activities of photocatalysts. 2
The photocatalytic activities of SO4 /g-C3N4 are shown 2
in Fig. 10. As shown in Fig. 10, all SO4 /g-C3N4 photocatalysts possess better photocatalytic activity than the

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J. Zhong et al. / Materials Science in Semiconductor Processing 40 (2015) 508–515

pure g-C3N4 and the 6.0% sample exhibits the best photocatalytic activity among the experimented compositions. Moreover, the results indicate that a very large amount of 2
the SO4 is harmful for the activity possibly due to the 2
coverage of g-C3N4 active sites by the SO4 [13], thus activity of 7.7% is worse than that of 6.0%. This study demonstrates that sulfating can enhance photocatalytic activity of g-C3N4, the results in our case are the same as the results reported by Yan and coworkers [31]. Moreover, the decolrozation of MO is faster than that of rhodamine B, which is due to the nature of pollutants and different pHs of solution. It is interesting to note that after washing, the photocatalytic activity of 6.0% decreases, which indicates 2
that SO4 play an important role in boosting the photocatalytic activity, however, the detail mechanism needs further study. Based on all the results in this paper, the enhanced phot2
ocatalytic performance of SO4 /g-C3N4 could attributed to the increased adsorption of rhodamine B on photocatalysts, the hydroxyl content and high photo-induced charges separation 2
of SO4 /g-C3N4. 4. Conclusions 2
SO4 decorated g-C3N4 photocatalyst with different wt% of 2
SO4 were prepared by a facile pore impregnating method 2using (NH4)2S2O8 solution. Decorating of g-C3N4 by SO4

increases the adsorption of rhodamine B on photocatalysts, the hydroxyl content and the photo-induced charges separa2
2
tion. When the wt% of SO4 is 6.0%, SO4 /g-C3N4 demonstrates the best photocatalytic activity under simulated sun light irradiation. Sulfating is an effective and simple approach to enhance the photocatalytic activity of g-C3N4.

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16] [17]

[18]

Acknowledgments

[19]

This project was supported financially by the program of Science and Technology Department of Sichuan province (No. 2015JY0081), Students Innovation Project of Sichuan Province (No. 201410622003), Research Fund Projects of Sichuan University of Science and Engineering (No. 2013PY03), the Project of Zigong city (No. 2014HX14), Construct Program of the Discipline in Sichuan University of Science and Engineering and the Opening Project of Key Laboratory of Green Catalysis of Sichuan Institutes of High Education (No. LZJ1302).

[20]

[21]

[22]

[23]

References [1] A. Kudo, Y. Miseki, Heterogeneous photocatalyst materials for water splitting, Chem. Soc. Rev. 38 (2009) 253–278. [2] X.B. Chen, S.H. Shen, L.J. Guo, S.S. Mao, Semiconductor-based photocatalytic hydrogen generation, Chem. Rev. 110 (2010) 6503–6570. [3] X.C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, A metal-free polymeric photocatalyst for hydrogen production from water under visible light, Nat. Mater. 8 (2009) 76–80. [4] K. Schwinghammer, M.B. Mesch, V. Duppel, C. Ziegler, J. Senker, B. V. Lotsch, Crystalline carbon nitride nanosheets for improved visible-light hydrogen evolution, J. Am. Chem. Soc. 136 (2014) 1730–1733. [5] Y. Wang, X.C. Wang, M. Antonietti, Polymeric graphitic carbon nitride as a heterogeneous organocatalyst: from photochemistry to

[24]

[25]

[26]

[27]

multipurpose catalysis to sustainable chemistry, Angew. Chem. Int. Ed. 51 (2012) 68–89. H. Katsumata, T. Sakai, T. Suzuki, S Kaneco, Highly efficient photocatalytic activity of g-C3N4/Ag3PO4 hybrid photocatalysts through Zscheme photocatalytic mechanism under visible light, Ind. Eng. Chem. Res. 53 (2014) 8018–8025. X.J. Bai, S.C. Yan, J.J. Wang, L. Wang, W.J. Jiang, S.L. Wu, C.P. Sun, Y.F. Zhu, A simple and efficient strategy for the synthesis of a chemically tailored g-C3N4 material, J. Mater. Chem. A 2 (2014) 17521–17529. M.L. Lu, Z.X. Pei, S.X. Weng, W.H. Feng, Z.B. Fang, Z.Y. Zheng, M.L. Huang, P. Liu, Constructing atomic layer g-C3N4–CdS nanoheterojunctions with efficiently enhanced visible light photocatalytic activity, Phys. Chem. Chem. Phys. 16 (2014) 21280–21288. Y.T. Gong, M.M. Li, H.R. Li, Y. Wang, Graphitic carbon nitride polymers: promising catalysts or catalyst supports for heterogeneous oxidation and hydrogenation, Green Chem. 17 (2015) 715–736. Y. Zheng, J. Liu, J. Liang, M. Jaroniecc, S.Z. Qiao, Graphitic carbon nitride materials: controllable synthesis and applications in fuel cells and photocatalysis, Energy Environ. Sci. 5 (2012) 6717–6731. L.F. Ming, H. Yue, L.M. Xu, F. Chen, Hydrothermal synthesis of oxidized g-C3N4 and its regulation of photocatalytic activity, J. Mater. Chem. A 2 (2014) 19145–19149. J.J. Zhu, P. Xiao, H.L. Li, S.A.C. Carabineiro, Graphitic carbon nitride: synthesis, properties, and applications in catalysis, ACS Appl. Mater. Interfaces 6 (2014) 16449–16465. X.T. Wu, C.G. Liu, X.F. Li, X.G. Zhang, C. Wang, Y.K. Liu, Effect of morphology on the photocatalytic activity of g-C3N4 photocatalysts under visible-light irradiation, Mater. Sci. Semicond. Process. 32 (2015) 76–81. Y.P. Yuan, L.S. Yin, S.W. Cao, L.N. Gu, G.S. Xu, P.W. Du, H. Chai, Y.S. Liao, C. Xue, Microwave-assisted heating synthesis: a general and rapid strategy for large-scale production of highly crystalline g-C3N4 with enhanced photocatalytic H2 production, Green Chem. 16 (2014) 4663–4668. S.W. Cao, Y.P. Yuan, J. Barber, S.C.J. Loo, C. Xu, Noble-metal-free gC3N4/Ni(dmgH)2 composite for efficientphotocatalytic hydrogen evolution under visible light irradiation, Appl. Surf. Sci. 319 (2014) 344–349. S.W. Cao, J.G. Yu, g-C3N4-based photocatalysts for hydrogen generation, J. Phys. Chem. Lett. 5 (2014) 2101–2107. K. Wang, Q. Li, B.S. Liu, B. Cheng, Wingkei Ho, Jiaguo Yu, Sulfurdoped g-C3N4 with enhanced photocatalytic CO2-reductionperformance, Appl. Catal. B: Environ. 176 (2015) 44–52. S.W. Cao, J.X. Low, J.G. Yu, M. Jaroniec, Polymeric photocatalysts based on graphitic carbon nitride, Adv. Mater. 27 (2015) 2150–2176. S.C. Yan, Z.S. Li, Z.G. Zou, Photodegradation performance of g-C3N4 fabricated by directly heating melamine, Langmuir 25 (2009) 10397–10401. Z.H. Chen, P. Sun, B. Fan, Z.G. Zhang, X.M. Fang, in situ template-free ion-exchange process to prepare visible-light-active g-C3N4/NiS hybrid photo-catalysts with enhanced hydrogen evolution activity, J. Phys. Chem. C 118 (2014) 7801–7807. Z.A. Huang, Q. Sun, K.L. Lv, Z.H. Zhang, M. Li, B. Li, Effect of contact interface between TiO2 and g-C3N4 on the photoreactivity of g-C3N4/ TiO2 photocatalyst: (001) vs (101) facets of TiO2, Appl. Catal. B: Environ. 164 (2015) 420–427. F. He, G. Chen, Y.G. Yu, Y. S Zhou, Y. Zheng, S. Hao, The sulfur-bubble template-mediated synthesis of uniform porous g-C3N4 with superior photocatalytic performance, Chem. Commun. 51 (2015) 425–427. L.G. Zhang, X.F. Chen, J. Guan, Y.J. Jiang, T.G. Hou, X.D. Mu, Facile synthesis of phosphorus doped graphitic carbon nitride polymers with enhanced visible-light photocatalytic activity, Mater. Res. Bull. 48 (2013) 3485–3491. S.C. Yan, Z.S. Li, Z.G. Zou, Photodegradation of rhodamine B and methyl orange over boron-doped g-C3N4 under visible light irradiation, Langmuir 26 (2010) 3894–3901. S.Z. Hu, L. Ma, J.G. You, F.Y. Li, Z.P. Fan, G. Lu, D. Liu, J.Z. Gui, Enhanced visible light photocatalytic performance of g-C3N4 photocatalysts co-doped with iron and phosphorus, Appl. Surf. Sci. 311 (2014) 164–171. Y.Y. Bu, Z.Y. Chen, Effect of oxygen-doped C3N4 on the separation capability of the photoinduced electron–hole pairs generated by O– C3N4@TiO2 with quasi-shell–core nanostructure, Electrochim. Acta 144 (2014) 42–49. C. Chang, L.Y. Zhu, S.F. Wang, X.L. Chu, L.F. Yue, Novel mesoporous graphite carbon nitride/BiOI heterojunction for enhancing photocatalytic performance under visible-light irradiation, ACS Appl. Mater. Interfaces 6 (2014) 5083–5093.

J. Zhong et al. / Materials Science in Semiconductor Processing 40 (2015) 508–515

[28] S. Kumar, S. Tonda, A. Baruah, B. Kumar, V. Shanker, Synthesis of novel and stable g-C3N4/N-doped SrTiO3 hybrid nanocomposites with improved photocurrent and photocatalytic activity under visible light irradiation, Dalton Trans. 43 (2014) 16105–16114. [29] Y.F. Li, L. Fang, R.X. Jin, Y. Yang, X. Fang, Y. Xing, S.Y. Song, Preparation and enhanced visible light photocatalytic activity of novel g-C3N4 nanosheets loaded with Ag2CO3 nanoparticles, Nanoscale 7 (2015) 758–764. [30] Y.P. Yuan, S.W. Cao, Y.S. Liao, L.S. Yin, C. Xue, Red phosphor/g-C3N4 heterojunction with enhanced photocatalytic activities for solar fuels production, Appl. Catal. B: Environ.140–141 (2013) 164–168. [31] H.J. Yan, Y. Chen, S.M. Xu, Synthesis of graphitic carbon nitride by directly heating sulfuric acid treated melamine for enhanced photocatalytic H2 production from water under visible light, Int. J. Hydrog. Energy 37 (2012) 125–133. [32] J.B. Zhong, J.Z. Li, J. Zeng, S.T. Huang, W. Hu, J.F. Chen, M.J. Li, J. Wang, S.L. Zhang, Enhanced photocatalytic activity of sulfated silica-titania composites prepared by impregnation using ammonium persulfate solution, Mater. Sci. Semicond. Process. 26 (2014) 62–68. [33] J.B. Zhong, J.Z. Li, F.M. Feng, Y. Lu, J. Zeng, W. Hu, Z. Tang, Improved photocatalytic performance of SiO2–TiO2 prepared with the assistance of SDBS, J. Mol. Catal. A: Chem. 357 (2012) 101–105. [34] L.S. Zhang, K.H. Wong, H.Y. Yip, C. Hu, J.C. Yu, C.Y. Chan, P.K. Wong, Effective photocatalytic disinfection of E. coli K-12 using AgBr–Ag– Bi2WO6 nanojunction system irradiated by visible light: the role of diffusing hydroxyl radicals, Environ. Sci. Technol. 44 (2010) 1392–1398. [35] S.G. Meng, D.Z. Li, M. Sun, W.J. Li, J.X. Wang, J. Chen, X.Z. Fu, G.C. Xiao, Sonochemical synthesis, characterization and photocatalytic properties of a novel cube-shaped CaSn(OH)6, Catal. Commun. 12 (2011) 972–975. [36] M.C. Yin, Z.S. Li, J.H. Kou, Z.G. Zou, Mechanism investigation of visible light-induced degradation in a heterogeneous TiO2/Eosin Y/ rhodamine B system, Environ. Sci. Technol. 43 (2009) 8361–8366. [37] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69–96. [38] T. Ohno, K Tokieda, S. Higashida, M. Matsumura, Synergism between rutile and anatase TiO2 particles in photocatalytic oxidation of naphthalene, Appl. Catal. A 244 (2003) 383–391.

515 2


[39] H.L. Yin, Z.Y. Tan, Y.T. Liao, Y.J. Feng, Application of SO4 /TiO2 solid super acid in decontaminating radioactive pollutants, J. Environ. Radioact. 87 (2006) 227–235. [40] M. Wu, Q. Wang, Q. Sun, P. Jena, Functionalized graphitic carbon nitride for efficient energy storage, J. Phys. Chem. C 117 (2013) 6055–6059. [41] M. Tahir, C. Cao, N. Mahmood, F.K. Butt, A. Mahmood, F. Idrees, S. Hussain, M. Tanveer, Z. Ali, I. Aslam, Multifunctional g-C(3)N(4) nanofibers: a template-free fabrication and enhanced optical, electrochemical, and photocatalyst properties, ACS Appl. Mater. Interfaces 6 (2013) 1258–1265. [42] N. Yao, C.C. Wu, L.C. Jia, S Han, B Chi, J. Pu, L Jian, Simple synthesis and characterization of mesoporous (N, S)-codoped TiO2 with enhanced visible-light photocatalytic activity, Ceram. Int. 38 (2012) 1671–1675. [43] J.A. Rengifo-Herrera, K. Pierzchala, A. Sienkiewicz, L. Forró, J. Kiwi, C. Pulgarin, Abatement of organics and Escherichia coli by N, S codoped TiO2 under UV and visible light. Implications of the formation of singlet oxygen (1O2) under visible light, Appl. Catal. B 88 (2009) 398–406. [44] L. Kronik, Y. Shapira, Surface photovoltage phenomena: theory, experiment and application, Surf. Sci. Rep. 254 (1999) 1–205. [45] L.Q. Jing, J. Wang, Y. Qu, Y. Luan, Effects of surface-modification with Bi2O3 on the thermal stability and photoinduced charge property of nanocrystalline anatase TiO2 and its enhanced photocatalytic activity, Appl. Surf. Sci. 256 (2009) 657–663. [46] Y. Lu, Y. Lin, D. Wang, L. Wang, T. Xie, T. Jiang, A high performance cobalt-doped ZnO visible light photocatalyst and its photogenerated charge transfer properties, Nano Res. 4 (2011) 1144–1152. [47] Y.X. Yang, Y.N. Guo, F.Y. Liu, X. Yuan, Y.H. Guo, S.Q. Zhang, W. Guo, M. X. Huo, Preparation and enhanced visible-light photocatalytic activity of silver deposited graphitic carbon nitride plasmonic photocatalyst, Appl. Catal. B: Environ. 142–143 (2013) 828–837. [48] F.Z. Su, S.C. Mathew, G. Lipner, X.Z. Fu, M. Antonietti, S. Blechert, X.C. Wang, mpg-C3N4-catalyzed selective oxidation of alcohols using O2 and visible light, J. Am. Chem. Soc. 132 (2010) 16299–16301.