g-C3N4 composites with improved solar-driven photocatalytic performance deriving from remarkably efficient separation of photo-generated charge pairs

Materials Research Bulletin 84 (2016) 65–70

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Z-scheme TiO2/g-C3N4 composites with improved solar-driven photocatalytic performance deriving from remarkably efficient separation of photo-generated charge pairs Shengtian Huanga , Junbo Zhonga,* , Jianzhang Lia,* , Jiufu Chena , Zhen Xianga , Wei Hua , Minjiao Lia,b a Key Laboratory of Green Catalysis of Higher Education Institutes of Sichuan, College of Chemistry and Environmental Engineering, Sichuan University of Science and Engineering, Zigong, 643000, PR China b Sichuan Provincial Academician (Expert) Workstation, Sichuan University of Science and Engineering, Zigong, 643000, PR China

A R T I C L E I N F O

Article history: Received 13 June 2016 Received in revised form 20 July 2016 Accepted 31 July 2016 Available online 1 August 2016 Keywords: A. semiconductors A. inorganic compounds A. interfaces B. chemical synthesis D. catalytic properties

A B S T R A C T

High recombination rate of electron-hole pairs and weak oxidation ability of holes from g-C3N4 greatly limit the catalytic degradation efficiency of g-C3N4-based photocatalysts, thus it is crucial to further improve the photocatalytic efficiency of g-C3N4. In this paper, TiO2 was successfully loaded onto the g-C3N4 surface to induce the electron hole separation and enhance the photocatalytic efficiency. The results revealed the maximum photocatalytic performance toward discoloration of methyl orange (MO) aqueous solution under the simulated sunlight illumination at molar ratio of 3% TiO2 to g-C3N4 composite. The junction between TiO2 and g-C3N4 significantly accelerate the separation of the photogenerated charge pairs caused by the strong interactions between TiO2 and g-C3N4. Such highly efficient photo-generated charge separation can be explained by a Z-scheme charge separation and migration mechanism based on the results of the scavenger experiments and the energy band structures. ã 2016 Elsevier Ltd. All rights reserved.

1. Introduction As a metal-free and narrow bandgap semiconductor, graphitic carbon nitride (g-C3N4) has triggered great interests since its photocatalytic properties were reported by Wang in 2009 [1]. More importantly, g-C3N4 displays prominent thermal stability, reliable chemical inertness, non-toxicity, easy modification, and excellent electrical property [1–3], which making g-C3N4 one of the most attractive photocatalysts for the future application. Due to its unique features, the photocatalytic performance of g-C3N4 has been extensively studied, however, the photocatalytic properties of g-C3N4 is greatly hindered by its inherent drawbacks, such as the high recombination rate of photo-generated electrons and holes, lower specific surface area, and low quantum efficiency [4–6]. Moreover, the relative low valence band (VB) edge potential ( 1.4 eV vs. NHE) of g-C3N4 limits the photo-induced hole from gC3N4 from oxidizing into H2O or OH in order to generate
OH,

* Corresponding authors. E-mail addresses: [email protected] (S. Huang), [email protected] (J. Zhong), [email protected] (J. Li), [email protected] (J. Chen), [email protected] (Z. Xiang), [email protected] (W. Hu), [email protected] (M. Li). http://dx.doi.org/10.1016/j.materresbull.2016.07.036 0025-5408/ã 2016 Elsevier Ltd. All rights reserved.

which further decreases the photocatalytic performance of g-C3N4. Consequently, it is essential to promote the separation of photogenerated charge pairs and generate more
OH to improve the photocatalytic performance of g-C3N4 based photocatalysts. To solve this issue, tremendous efforts have been put into improving the photocatalytic performance of g-C3N4 by doping [7,8], fabricating novel structures [9,10], coupling g-C3N4 with graphene [11,12] and so on. Among all these strategies, construction of heterostructures is an effective method to enhance the photocatalytic activity, which can significantly promote the separation of photo-generated charge pairs due to the different energy band structures of photocatalysts, resulting in high separation efficiency of photogenerated charge pairs. To achieve this goal, the photocatalysts must have matched energy band potentials. Compare to the normal hydrogen electrode (NHE), the conduction band (CB) and the VB of g-C3N4 is 1.3 eV and 1.4 eV at pH 7.0, respectively [13]. It is evident that the electron from the CB of g-C3N4 has strong reduction ability, while the hole from the VB of g-C3N4 exhibits weak oxidation ability. To maintain the strong reduction ability of the electron from the CB of g-C3N4 and offset the weak oxidation ability of the hole from the VB of g-C3N4, the counterpart should have suitable energy band potentials, and the separation of the

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2. Experimental section

25000 27.50 TiO2 20000

Intensity (a.u.)

4% 15000

3% 2%

10000

1%

5000

0% 0 10

20

30

40

50

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

2.2. Characterization of samples Specific surface area was measured on a SSA-4200 automatic surface analyzer by N2 adsorption method at 77 K. The crystal phases of the photocatalysts were analyzed by a DX-2600 X-ray diffractometer. The morphology of photocatalysts was observed on a JSM-7500F scanning electron microscopy (SEM). The UV–Vis DRS in the wavelength range between 230 and 800 nm was conducted on a UV–Vis spectrophotometer (TU-1907). XPS spectra were performed on an XSAM 800 using Mg Ka at 12 kV and 12 mA. The X-ray photoelectron spectra were referenced to the C1 s peak (BE = 284.80 eV). To study the separation properties of the photo-

100 (A)

0%

80 3%

Reflectance (%)

charge pairs should have another routine which is different from the common one, otherwise the heterostructures should sacrifice the strong reduction ability of the electron from the CB of g-C3N4. Thus, seeking a semiconductor with matched energy band potentials is a challenging task. Among the photocatalysts used to couple with g-C3N4, TiO2 is thought to be an ideal candidate due to the CB and VB edge potential of TiO2 are more positive than that of g-C3N4. The photo-generated charge separation can be greatly promoted owing to the interfacial electric field when TiO2 is coupled with g-C3N4. Meanwhile, the photo-induced CB electrons from TiO2 can transfer to the VB of g-C3N4, resulting in the recombination of electrons and holes. Therefore, the VB holes with strong oxidation ability can be accumulated on TiO2 and CB electrons with strong reduction ability are accumulated on g-C3N4, increasing the separation efficiency of photo-induced charge pairs and oxidation/reduction of the electrons and hole. To test this assumption, TiO2/g-C3N4 composite photocatalysts have been fabricated, and the corresponding photocatalytic activities were evaluated by many groups, the results revealed that coupling TiO2 with g-C3N4 can promote the photocatalytic performance of bare TiO2 and g-C3N4 [14–20]. Based on the observations, charge separation mechanism was proposed. However, it is worth noting that there are a few scattered and contradictory results of charge separation mechanism from the literatures. Furthermore, the photo-induced charge separation behavior (rate and phase) of TiO2/g-C3N4 and the relation of charge separation behavior with the photocatalytic performance of composites remain unclear, and needs to be further studied. In fact, the information on the photoinduced charge separation behavior (charge separation rate and phase) can provide critical information on the charge separation mechanism and the nature of the photocatalytic reaction. Herein, the photo-induced charge separation and migration behavior of TiO2/g-C3N4 photocatalysts was investigated using SPV, and the corresponding photocatalytic activities of heterostructures were evaluated by the discoloration of MO aqueous solution under a simulated sunlight irradiation. Based on the results, a Z-scheme charge separation and migration mechanism was proposed to explain the enhancement of the solar-driven photocatalytic performance deviating from the promoted photogenerated charge separation properties.

60 TiO2

40

20

2.1. Preparation of catalysts

0 300

Catalysts

0%

1%

2%

3%

4%

SBET (m2/g)

20

25

26

29

28

500

600

700

800

10 TiO2

(B)

2

8

2

6 g-C3N4 4

2

0

Table 1 Specific surface area of photocatalysts.

400

Wavelength(nm)

αhv (eV)

All chemicals were of analytical purity and used as received. Deionized water was employed in photocatalytic experiments. gC3N4 was prepared as the procedure described in Ref. [21] by heating urea. TiO2 sol was prepared by a sol-gel routine according to the procedure given in Ref. [22] using tetrabutylorthotitanate, diethanolamine and ethanol as the raw materials. TiO2/g-C3N4 composite photocatalysts with different molar ratios of TiO2 and g-C3N4 were prepared by a pore impregnating method using calcinating TiO2 sol at 723 K for 2 h. The samples with different molar ratios of TiO2 and g-C3N4 (1%, 2%, 3% and 4%, respectively) were marked as 1%, 2%, 3% and 4%, respectively. TiO2 was prepared by baking the TiO2 gel at 723 K for 2 h. g-C3N4 was also treated as the method mentioned above without the presence of tetrabutylorthotitanate and named as 0%.

1.2

1.6

2.0

2.4

2.8

3.2

3.6

4.0

4.4

4.8

Energy (eV) Fig. 2. (a) UV–Vis DRS of photocatalysts; (b) relationship between (ahv)2 and energy.

S. Huang et al. / Materials Research Bulletin 84 (2016) 65–70

generated charge pairs, the SPV measurements of the samples were carried out on a home-built apparatus. 2.3. Photocatalytic activity evaluation Photocatalytic discoloration experiments were conducted on a Phchem III photochemical reactor followed the procedures as described in the Ref. [23]. The dosage of the photocatalysts was 1 g/L, the initial mass concentration of MO was 10 mg L 1, the pH value of MO solution was 7.0, the irradiation source was a 500 W Xe lamp (simulated sun light). The concentration of MO was analyzed on a V-1100 spectrometer using the Lambert-Beer law. 3. Results and discussion 3.1. Characterization of photocatalysts The specific surface area can greatly affect the photocatalytic performance and the measured specific surface areas are shown in Table 1. Considering the large errors (5 m2/g), it is obvious that the specific surface area has no significant difference, suggesting that the low loading of TiO2 onto the surface of g-C3N4 cannot effectively alter the specific surface area, therefore, it plays an unimportant role in the enhancement of photocatalytic activity. The crystal phases can also greatly influence the photocatalytic activity of the photocatalysts. The XRD patterns of the samples were depicted in Fig. 1. The bare TiO2 contains characteristic anatase peaks (JCPDS No. 21-1272). The peak located at 27.50 of g-C3N4 is the (002) planes of the tetragonal phase g-C3N4 (JCPDS 87- 1526). For TiO2/g-C3N4 composites, the diffraction peaks of

67

TiO2 were observed as the loading of TiO2 increases, especially at 3% and 4%, confirming the successful loading of TiO2 on the surface of g-C3N4. Furthermore, it is interesting to note that the full width at half maximum (FWHM) of g-C3N4 widens gradually as the loading of TiO2 increases, due to the strong interaction between TiO2 and g-C3N4. The strong interaction is beneficial to form strong interfacial electric field, resulting in high separation of photogenerated charge pairs. The optical properties play an important role in the photocatalytic activities. The UV–Vis DRS of the photocatalysts was displayed in Fig. 2. Due to the partial overlap of DRS of photocatalysts, only DRS of the 0% and 3% sample was presented in Fig. 2. It is evident that loading relative a low content of TiO2 onto the surface of g-C3N4 cannot alter the optical properties of g-C3N4. The bandgap of TiO2 and g-C3N4 can be obtained by using Kubelka-Munk function. As exhibited in Fig. 2 b, the bandgap of TiO2 and g-C3N4 is 3.20 and 2.8 eV, respectively, which agrees well with the values reported in Ref. [13]. The CB and VB potentials of TiO2 and g-C3N4 can be further estimated according to the following equations: EVB = X

ECB = EVB

Ec + 0.5Eg

Eg

Where EVB represents the VB edge potential, ECB is the CB edge potential, X stands for the electronegativity of the semiconductor (4.64 eV for g-C3N4 and 5.81 eV for TiO2 [24]), Ec represents the energy of free electrons on the hydrogen scale (about 4.5 eV), Eg is defined as the band gap energy of the semiconductor. Using this

Fig. 3. SEM of photocatalysts (A) 0%; (B) 2%; (C) 3%; (D) 4%.

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S. Huang et al. / Materials Research Bulletin 84 (2016) 65–70

N1s

Intensity (a.u.)

60000

40000 C1s

O1s Ti2p

Ti3p

20000

Ti3s

0 1200

1000

800

600

400

200

0

Binding energy (eV) Fig. 4. XPS survey spectrum for the surface of the 1% sample.

25000 458.4

Intensity (a.u.)

458.8

20000

4%

15000

3%

10000

2%

5000

1% TiO2

0 468

466

464

462

460

458

Binding ebergy (eV) Fig. 5. Ti 2p of photocatalysts.

456

454

0.25 (A)

3%

0.15 1% 4%

0.10

0.6

TiO2

0.4 0.2 0.0 300

0.05

350 400 Wavelength (nm)

2%

0.00 300

Photovoltage (mV)

Photovoltage (mV)

0.20

450

0%

350

400

450

500

550

600

Wavelength (nm) (B)

150

Phase (degree)

equation, the EVB of g-C3N4 and TiO2 were calculated to be +1.54 eV and +2.91 eV, while the ECB of g-C3N4 and TiO2 are 1.26 eV and 0.29 eV, respectively. Due to the different VB and CB edge potentials of two photocatalysts, it is expected that charge pairs of composites can be effectively separated, which was confirmed by the results of SPV. The SEM images of photocatalysts were exhibited in Fig. 3. All the photocatalysts display irregular lumps and cotton-like shapes. The results demonstrated the low loading TiO2 onto the surface of g-C3N4 did not change the shape of g-C3N4. To further confirm the coexistence of TiO2 and g-C3N4 in composites, the XPS spectrum of the 1% sample was illustrated in Fig. 4. The characteristic peaks for Ti, O, C and N were all detected in XPS, verifying the presence of TiO2 and g-C3N4 in the composites. Fig. 5 exhibits the high resolution XPS spectra of the Ti2p3/2 of TiO2/g-C3N4 composites. For the bare TiO2, the peak situated at 458.8 eV is assigned to Ti 2p3/2, suggesting a normal state of Ti4+. However, compared to the Ti 2p3/2 of the bare TiO2, the Ti 2p3/2 values of composites shift to a lower value, indicating the binding energy Ti element has been altered owing to the strong interaction between TiO2 and g-C3N4, proven by the results of XRD. This agrees with previous studies suggesting the shift of Ti2p is beneficial in enhanced photocatalytic activity of TiO2 [25]. Among the factors that influence the photocatalytic properties, the separation of electrons and hole is one of the most important factors in determining the photocatalytic performance. The results of the SPV measurements are displayed in Fig. 6, g-C3N4 exhibits SPV response from 300 to 500 nm based on the electronic transitions and energy band structure of g-C3N4 (Fig. 6a). Although TiO2 displays SPV response from 300 to 425 nm, as shown in Fig. 6a

g-C3N4

100

3% TiO2

50

0 300

325

350

375

400

425

Wavelength (nm) Fig. 6. SPV response and phase spectra of the as-prepared photocatalysts.

(inset), all the TiO2/g-C3N4 composites hold much stronger SPV responses than that of g-C3N4 from 300 to 550 nm. Such results demonstrate the photo-generated charge pairs of the composites can be effectively separated with visible light irradiation, which is conducive to solar photocatalytic activity. Moreover, the intensities of the SPV responses gradually increase at high loading of TiO2. At 3% molar ratio of TiO2/g-C3N4, the sample has the strongest SPV and decreased suddenly at 4%. However, the SPV response of the 4% sample is still higher than that of g-C3N4. After loading TiO2 onto the surface of g-C3N4, an interfacial electric field exists in the TiO2/ g-C3N4 composites, due to the strong interaction between TiO2 and g-C3N4, supported by the XRD and XPS results, which can accelerate the separation of electrons and holes [26]. When the loading of TiO2 is low, light can easily reach the interface of TiO2 and g-C3N4; therefore, the TiO2/g-C3N4 composites exhibit stronger SPV response than g-C3N4. However, at relative high loading of TiO2, the photons that could reach the interface remarkably decreased due to the agglomeration of the TiO2 particles on the surfaces of g-C3N4, confirmed by XRD, resulting in a relative long migration distance of photo-induced charge. Thus, the SPV response of 4% molar ratio of TiO2/g-C3N4 composite is weaker than that of 3%. The results revealed that the construction of composites can greatly elevate the SPV response of g-C3N4. Generally speaking, the strong SPV response originates from the high separation rate of photo-generated charge pairs. High separation rate of charge carriers is beneficial to the solar-driven photocatalytic performance, which can be further proved by the results of photocatalytic activity measurements. The phase values of TiO2, g-C3N4 and the 3% samples were exhibited in Fig. 6b. From 400 to 425 nm, the phase values of TiO2, g-C3N4 and the 3% molar ratio composite samples are both positive, indicating movement of the photo-induced electrons to the top electrode from which light travels through [27]. It is evident that the electronic transfer properties are the same as the g-C3N4 after coupling of TiO2 with

S. Huang et al. / Materials Research Bulletin 84 (2016) 65–70 Table 2 Parameter and discoloration kinetic equation for MO over different catalysts.

62.6 60 51.4

Decolorization (%)

69

45 36.2 28.3

30

Catalysts

discoloration kinetic equation

rate constant(kobs)

R2

0% 1% 2% 3% 4%

ln(C0/Ct) = 0.005t ln(C0/Ct) = 0.007t ln(C0/Ct) = 0.008t ln(C0/Ct) = 0.012t ln(C0/Ct) = 0.01t

0.005 min 1 0.007 min 1 0.008 min 1 0.012 min 1 0.01 min 1

0.9921 0.9975 0.9969 0.9859 0.9927

15

0

Blank

BQ

AO

IPA

Scavenger Fig. 7. Discoloration efficiency of MO over the 3% sample with scavengers (Illumination time = 80 min, Scavenger dosage = 0.2 mmol/L).

g-C3N4 when the composites are upon the exposure light from 300 to 425 nm. To confirm the formation of free radicals during the photocatalytic discoloration process, benzoquinone (BQ), isopropanol (IPA) and ammonium oxalate (AO) were added into the reaction system. The effects of the three scavengers on the discoloration efficiency of MO were exhibited in Fig. 7. As shown in Fig. 7, the photocatalytic discoloration of MO reduces from 62.6% to 51.4%, 36.2% and 28.3% after adding BQ, AO and IPA, respectively, indicating that OH, h+ and O2 coexist during the photocatalytic discoloration of MO.

Fig. 9. Schematic diagram of photo-excited electron-hole separation process.

3.2. Photocatalytic activity The discoloration of MO aqueous solution under the Xe lamp irradiation without photocatalyst and the adsorption ability of different photocatalysts toward MO after 80 min are negligible. The decolorization of MO over different catalysts was displayed in Fig. 8. It is evident that ln(C0/Ct) versus t is in linear form, which demonstrates that the decolorization process of MO over different catalysts match well a first-order model, ln(C0/Ct) = kobst fits the tendency, where C0 and Ct are the concentration of MO at time 0 and t, respectively, and kobs is the observed first-order rate constant. All the decolorization rate constants (kobs) and interrelated coefficients (R2) are all exhibited in Table 2. It is strongly clear that all the composites possess higher photocatalytic performance than that of the bare g-C3N4 and the composite with 3% TiO2/gC3N4 molar ratio possesses the best photocatalytic activity. Based on the results, it is reasonable to conclude that construction of TiO2/g-C3N4 composites can significantly promote the

0% 1% 2% 3% 4%

1.0 0.9

Ct/C0

0.8 0.7 0.6 0.5 0.4 0.3

photocatalytic activity of g-C3N4 originating from the promoted separation rate of photo-induced charge pairs due to the strong interaction between TiO2 and g-C3N4. To explain the enhancement of photocatalytic activity, a charge separation and migration mechanism with a typical Z- scheme was suggested and depicted in Fig. 9. The VB and CB edge potentials of TiO2 are 2.91 eV and 0.29 eV vs. NHE, the VB and CB edge potentials of g-C3N4 are 1.54 eV and 1.26 eV vs. NHE. Under a simulated sunlight irradiation, the photo-generated electrons are excites from VB to CB of the photocatalysts after exposure to sunlight. The VB holes (2.91 eV) from TiO2 can directly oxidize OH /H2O to form OH radicals (1.99 eV for OH /OH and 2.37 eV for H2O/OH), while the VB holes (1.54 eV) from g-C3N4 cannot oxidize OH /H2O to generate OH radicals due to the low oxidation potential. The standard redox potential of O2/O2 is 0.33 eV vs. NHE, thus, the CB electrons ( 0.29 eV) from TiO2 cannot reduce O2 to form O2 due to the high energy potential, but the CB electrons (-1.26 eV) of g-C3N4 can. Based on the results of free active radicals and the energy potentials of VB and CB of two photocatalysts, the common charge separation and migration mechanism of heterostructures is not applicable in this case. Under the simulated sunlight irradiation, the photo-induced CB electrons from TiO2 transfer to the VB of g-C3N4, resulting in the recombination of electrons and holes, therefore, a great amount of VB holes with strong oxidation ability are accumulated on VB of TiO2 and considerable quantities of CB electrons with strong reduction ability can be accumulated on CB of g-C3N4, resulting in high separation efficiency of photo-induced charge pairs. 4. Conclusion

0

20

40

60

Irradiation time (min) Fig. 8. Decay of MO over different catalaysts.

80

In summary, TiO2/g-C3N4 composite photocatalysts with significantly enhanced photocatalytic properties toward the discoloration of MO upon the simulated sunlight irradiation were

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successfully fabricated. At molar ratio of TiO2/g-C3N4 is 3%, the composite displayed the optimum photocatalytic efficiency. The enhancement of photocatalytic performance of TiO2/g-C3N4 photocatalysts towards discoloration of MO aqueous solution benefits from the strong interaction between TiO2 and g-C3N4 and the high separation rate of electron-hole pairs. The prepared TiO2/ g-C3N4 composites followed a typical Z-scheme charge separation and migration mechanism. This study provided important information to understand the charge separation mechanism and the nature of the surface photocatalytic reaction. Acknowledgements This project was supported financially by the program of Science and Technology Department of Sichuan province (No.2015JY0081), the Project of Zigong city (No.2014HX14, No. 2014HX09), Construct Program of the Discipline in Sichuan University of Science amd Engineering, the Opening Project of Key Laboratory of Green Catalysis of Sichuan Institutes of High Education (No. LZJ1302, No. LZJ1301) and Sichuan Provincial Academician (Expert) Workstation (No.2015YSGZZ03). References [1] X. 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. [2] A. Thomas, A. Fischer, F. Goettmann, M. Antonietti, J.O. Müller, R. Schlögl, J.M. Carlsson, Graphitic carbon nitride materials: variation of structure and morphology and their use as metal-free catalysts, J. Mater. Chem. 18 (41) (2008) 4893–4908. [3] G. Liao, S. Chen, X. Quan, H. Yu, H. Zhao, Graphene oxide modified g-C3N4 hybrid with enhanced photocatalytic capability under visible light irradiation, J. Mater. Chem. 22 (2012) 2721–2726. [4] Z.H. Chen, P. Sun, B. Fan, Z.G. Zhang, X.M. Fang, In situ template-free ionexchange process to prepare visible-light-active g-C3N4/NiS hybrid photocatalysts with enhanced hydrogen evolution activity, J. Phys. Chem. C 118 (2014) 7801–7807. [5] H. Xu, J. Yan, Y.G. Xu, Y.H. Song, H.M. Li, J.X. Xia, C.J. Huang, H.L. Wan, Novel visible-light-driven AgX/graphite-like C3N4 (X = Br, I) hybrid materials with synergistic photocatalytic activity, Appl. Catal. B 129 (2013) 182–193. [6] S. Ma, S. Zhan, Y. Jia, Q. Shi, Q. Zhou, Enhanced disinfection application of Agmodified g-C3N4 composite under visible light, Appl. Catal. B 186 (2016) 77–87. [7] Y. Zhou, L. Zhang, W. Huang, Q. Kong, X. Fan, M. Wang, J. Shi, N-doped graphitic carbon-incorporated g-C3N4 for remarkably enhanced photocatalytic H2 evolution under visible light, Carbon 99 (2016) 111–117. [8] J. Li, B. Shen, Z. Hong, B. Lin, B. Gao, Y. Chen, A facile approach to synthesize novel oxygen-doped g-C3N4 with superior visible-light photoreactivity, Chem. Commun. 48 (2012) 12017–12019.

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