porous g-C3N4 heterojunction hybrids with enhanced visible-light photocatalytic activity

Materials Chemistry and Physics 234 (2019) 75–80

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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Facile fabrication of α-Fe2O3/porous g-C3N4 heterojunction hybrids with enhanced visible-light photocatalytic activity Jirong Bai a, b, Haiyang Xu b, Gang Chen b, Wenhua Lv b, Zhijiang Ni b, Zhilei Wang b, Jiaying Yang d, Hengfei Qin c, *, Zheng Zheng b, **, Xi Li b, *** a

College of Chemical Engineering and Materials, Changzhou Institute of Technology, Changzhou, 213022, China Department of Environmental Science and Engineering, Fudan University, Shanghai, 200433, China Department of Chemistry and Environmental Engineering, Jiangsu University of Technology, Changzhou, 213001, China d Shanghai Tianjiabing Secondary School International Curriculum Center, Shanghai, 200433, China b c

H I G H L I G H T S

� α-Fe2O3/PCN shows enhanced optical absorption and charge transfer and separation. � Active specie .�O2 and hþwere found to play a key role during the photocatalytic reaction. � α-Fe2O3/PCN exhibits significant improved visible-light-driven photocatalytic activity. A R T I C L E I N F O

A B S T R A C T

Keywords: α-Fe2O3 Porous g-C3N4 Heterojunction Photocatalytic activity

α-Fe2O3/porous g-C3N4 (α-Fe2O3/PCN) photocatalysts were fabricated via a facile and green method. Structural characterization indicated α-Fe2O3 nanoparticles were successfully loaded onto PCN. The α-Fe2O3/PCN nano­ composites were proven durable with significantly enhanced photocatalytic activity for Rhodamine B photo­ degradation than the single components under visible light irradiation. The improved photoactivity was mainly attributed to the broadened visible light absorption, enriched active sites, and formation of α-Fe2O3/PCN het­ erojunction that addressed the recombination of photoinduced electrons and holes. The α-Fe2O3/PCN hybrids also showed high recyclability and chemical stability.

1. Introduction One fascinating and promising was of environment remediation is to use active semiconductor photocatalysts, which can absorb solar energy to excite electron-hole pairs and directly degrade organic contamination [1–3]. Among numerous reported photocatalysts, graphite carbon ni­ trogen (g-C3N4) is an important new-generation two-dimensional layered material with relatively narrow band gap and adequate con­ duction band edge position and has been widely used in decomposition of water pollutant, water splitting and CO2 photocatalytic conversion under visible light irradiation [4–6]. This environmental-friendly ma­ terial has many extraordinary properties, low cost, high photostability, nontoxicity and avoidance of metal element [7]. Unfortunately, its wide application is restricted by the poor quantum efficiency due to its weak

visible light response, low specific surface area and serious charge car­ rier recombination. Over the past decades, various effective modifica­ tion strategies have been employed, including nanostructure amelioration [8,9], doping with heteroatom [10,11], and construction of g–C3N4–based semiconductor heterojunction [12,13]. Among those strategies, turning g-C3N4 into a porous structure is an effective way to improve its photocatalytic activity, in which the porous structure provides more transfer channels for photoinduced carrier charge and possesses more active sites due to larger specific special area [14,15]. Previous research was focused on the template method. Wang et al. fabricated porous g-C3N4 (PCN) with ionic liquids as soft tem­ plates, dramatically improved photocatalytic activity due to the enlarged specific special area [16]. Liu et al. reported a new strategy to prepare mesoporous g-C3N4 by thermally condensing cyanamide into

* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (H. Qin), [email protected] (Z. Zheng), [email protected] (X. Li). https://doi.org/10.1016/j.matchemphys.2019.05.047 Received 19 March 2018; Received in revised form 14 October 2018; Accepted 21 May 2019 Available online 31 May 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.

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Materials Chemistry and Physics 234 (2019) 75–80

BCN -Fe2O3/PCN

20

30

40

50

2Theta / Degree

(214) (300)

(024)

(113)

(104) (110)

(012)

10

(116) (112)

PCN -Fe2O3

(100)

Intensity / a.u.

(002)

SiO2 nanotubes and significantly enhanced the photocatalytic activity in water splitting and photodegradation of Rhodamine B (RhB) [17]. Both studies illustrated the presence of porous structure could significantly enhance charge-separation efficiency and photocatalytic performance. However, the soft template method inevitably increases carbon content, and most of hard template strategies usually remove template etching by NaOH or HF solution, which is environmental-unfriendly [18]. In addition, both of them are time-consuming and less repeatable. There­ fore, it is still challenging to develop a facile, green and repeatable fabrication route. Another commonly-used method is to construct g-C3N4 with other well-matched narrow-band-gap semiconductor to form heterojunction hybrids and suppress the recombination of photoexcited electron-hole pairs, thus improving photocatalytic efficiency. To date, numerous g–C3N4–based heterostructure photocatalysts have been studied, such as MoS2/g-C3N4 [19,20], carbon dot/g-C3N4 [21,22] and α-Fe2O3/g-C3N4 [12,23]. Among them, α-Fe2O3 is a semiconductor with relative narrow band gap energy, non-toxicity, low cost and photochemical stability [24]. The unique photoelectronic properties and high visible light ab­ sorption (~43%) make it an ideal candidate for coupling with g-C3N4 to form heterojunction hybrid [24]. Therefore, coupling α-Fe2O3 with g-C3N4 obviously enhanced its visible light response and efficiently suppressed the recombination of electrons and holes through the for­ mation of heterojunctions. In the present study, we fabricated α-Fe2O3/PCN heterojunction nanocomposites via a facile and cost-effective two-step method starting from thiourea, urea and ferric nitrate. PCN was obtained simply through thermal polymerization of the mixture of thiourea and urea. The pho­ tocatalytic activity was evaluated via photocatalytic RhB degradation under visible light irradiation. Larger specific special area and faster separation efficiency should be responsible for the significantlyenhanced photocatalytic activity of α-Fe2O3/PCN. In order to investi­ gate the cause of such a high photocatalytic activity, we implemented various characterizations systemically and proposed the possible mechanism after free radical capturing experiments.

60

70

80

Fig. 1. XRD patterns of BCN, PCN, Fe2O3 and a-Fe2O3/PCN.

process at 400 � C for 4 h. 2.4. Material characterization Samples were characterized on a Bruker D8 Advance X-ray diffrac­ tometer (XRD, 40 kV, 40 mA, 2θ ¼ 10–80� at 8 � min 1, Cu-Kα radiation), a JEM-2100 transmission electron microscope (TEM, acceleration voltage ¼ 200 kV), a Thermo ESCALAB250 X-ray photoelectron spec­ troscope (XPS, Al Kα radiation), a Cary 500 ultraviolet–visible diffuse reflectance spectrometer (DRS), and an Edinburgh Shimadzu RF-5301 photoluminescence (PL) spectrophotometer (360 nm, room tempera­ ture). Nitrogen adsorption-desorption isotherms were recorded on a Micromeritics Tristar 3000 analyzer at 77.4 K.

2. Experimental

2.5. Photocatalytic activity evaluation

2.1. Chemicals

Photocatalytic activity was evaluated by RhB degradation experi­ ments (50 mL, 10 mg/L) in photoreaction vessels under visible light irradiation (500 W Xe arc lamp with 420-nm cut-off filter). The solutions were stirred in the dark for 30 min, which ensured the adsorptiondesorption balance between the photocatalyst and RhB. During the degradation reactions, the suspensions were stirred and sampled at 5 min interval (1 mL each time); the samples were each diluted to 3 mL with deionized water and centrifuged at 10000 rpm for 5 min. Then RhB concentrations were measured at 554 nm on a UV-3600 UV–Vis spec­ trophotometer (Spectrumlab 752s, Xunda, Shanghai). In each radicalcapture experiment, 50 mL of an RhB solution (10 mg/L) was added with 1 mmol Na2-EDTA, 1 mmol IPA or 0.02 mmol BQ for measurement of RhB concentration change before the standard photocatalytic tests.

Iron (III) nitrate nonahydrate (Fe(NO3)3⋅9H2O), thiourea (CH4N2S), melamine (C2H6O), isopropanol (IPA, C3H8O), p-benzoquinone (p-BQ, C6H4O2) and ethylenediaminetetraacetic acid (Na2-EDTA, C10H18N2Na2O10) (all Aladdin Industrial Corporation) were of analytical grade and used as received. Distilled water was used throughout all processes. The model pollutant was RhB from the textile industry. 2.2. Preparation of PCN PCN was fabricated using a facile calcination condensation method: 3 g of thiourea and 10 g of urea were heated at 8 � C⋅min 1 in crucibles covered at air atmosphere and 550 � C in a muffle furnace for 3 h. Bulk gC3N4 (BCN) was prepared from melamine via the same method. The samples were ground into fine powder.

3. Results and discussion 3.1. Structural and morphological characterization

2.3. Preparation of α-Fe2O3/PCN heterojunction nanocomposites

The crystal structures and phase compositions of BCN, PCN, α-Fe2O3 and α-Fe2O3/PCN were examined with the powder XRD meter (Fig. 1). BCN and PCN both show two peaks at 13.1 and 27.4� which correspond to the (100) and (002) reflection of g-C3N4 respectively (JCPDS no.87–1526) and are related to in-plane structural packing motif of tri-striazine units and interlayer stacking of conjugated aromatic system [25], respectively. Diffraction peak around 13.1� , the hole-to-hole dis­ tance can be calculated to be 0.687 nm, and another peak at 27.4� corresponds to the inter-layer distance can be calculated to be 0.322 nm. Interestingly, PCN exhibits a lower-intensity peak due to the quantum

The α-Fe2O3/PCN composites were prepared via a facile calcination method: 0.5 g of PCN powder and 0.02 g of Fe(NO3)3⋅9H2O were dis­ solved in 50 mL of deionized water and stirred at room temperature for 10 min. Then the mixed slurry was stirred at 80 � C until water evapo­ ration. The resulting solids were ground to fine powders and heated in crucibles covered under air atmosphere at 400 � C in a muffle furnace for 4 h. The final products were washed with pure water and ethanol 3 times, and dried at 60 � C. As comparison, α-Fe2O3/BCN was also pre­ pared by the same method. Pure α-Fe2O3 was obtained by the above 76

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Materials Chemistry and Physics 234 (2019) 75–80 (b)

1200

1000

800

600

400

398.6

(c)

284.6

Intensity a.u. 292

200

Binding Energy / eV

size effect. For single Fe2O3, the narrow sharp peaks at 24.2, 33.3, 35.7, 40.9 and 49.5� which match well with the (012), (104), (110), (113) and (024) crystal planes of high-crystalline hematite crystal phase α-Fe2O3 (JCPDS no. 33–0664, Hexagonal) [26,27]. The main peak of α-Fe2O3/PCN composites can be readily ascribed to body-centered PCN. After coupling with α-Fe2O3, the peak intensity at 27.4� is obviously decreased, indicating α-Fe2O3 and PCN well combined and interacted. The chemical composition and chemical status of elements of α-Fe2O3/PCN composites were investigated by XPS. The positions of the XPS peaks were corrected using the C 1s peak at 284.6 eV as the refer­ ence. The full XPS spectrum of α-Fe2O3/PCN in Fig. 2(a) reveals chem­ ical elements C, N, Fe, O and S on the surface of α-Fe2O3/PCN. The two main peaks at 288.1 and 284.7 eV in Fig. 2(b) could be distinguished for C1s and are attributed to carbon atoms with three nitrogen atoms (C(N)3) in g-C3N4 lattice and to carbon atoms (C–C bonding) in graphitic or amorphous carbons, respectively [28]. The high-resolution N 1s spec­ trum in Fig. 2(c) can be deconvoluted into four peaks with binding en­ ergy at 404.3, 401.2, 399.9 and 398.6 eV, respectively, which correspond to the charging effects or positive charge localization in heterocycles [29], tertiary nitrogen in N-(C)3, tertiary nitrogen in – N–C) in triazine rings C–N–H [30], and sp2-hybridized nitrogen (C– [31], respectively. The high-resolution XPS spectrum of Fe 2p shows the binding energies of 711.5 eV for Fe 2p5/2 and 725.4 eV for Fe 2p3/2 can be assigned to the Fe3þ of α-Fe2O3/PCN (Fig. 2(d)) [32]. Fig. 3 displays the morphology of α-Fe2O3/PCN. TEM images show a-

C 1S

288.1

S 2P

Fe 2P

O 1S

C 1S

Intensity a.u.

(a)

290

(d)

N 1S

288

286

Binding Energy / eV

284

282

Fe 2P

Fe 2P1/2

408

Intensity a.u.

Intensity a.u.

Fe 2P3/2

399.9 404.3

406

404

401.2

402

400

Binding Energy / eV

398

396

710

720

730

Binding Energy / eV

740

Fig. 2. (a) XPS spectrum of α-Fe2O3/PCN composite, (b) high-resolution XPS spectrum of C 1s, (c) N 1s and (d) Fe 2p.

Fig. 3. TEM images of (a)–(d) α-Fe2O3/PCN.

(b)

BCN PCN -Fe2O3/PCN

Absorbtance (a.u.)

(a)

-Fe2O3

( h

2

-Fe2O3

PCN -Fe2O3/PCN

2.75 2.1 400

500

600

Wavelength (nm)

700

800

1.6

1.8

2.0

2.81

2.2

h

2.4

2.6

2.8

3.0

Fig. 4. (a) UV–vis DRS spectra of α-Fe2O3, PCN, and α-Fe2O3/PCN; (b) plot of transformed Kubelka-Munk function versus light energy. 77

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Materials Chemistry and Physics 234 (2019) 75–80

Fig. 5. (a) Photocatalytic activity over different photocatalysts and (b) kinetic curves of photocatalysts and comparison of apparent rate constants (ka) in RhB degradation under visible light irradiation; (c) Cycling degradation by α-Fe2O3/PCN.

Fe2O3 is well dispersed on the surface of the layered g-C3N4 and in the particle size of 5–10 nm (Fig. 3(d)). Clearly, α-Fe2O3 has adhered to the surfaces of PCN to form α-Fe2O3/PCN heterojunction interface, which is favorable the transfer of photogenerated electrons and holes.

-Fe2O3/PCN

Intensity(a.u.)

3.2. Optical characterization The optical adsorption properties of α-Fe2O3/PCN composites asso­ ciated with electronic structures were evaluated by UV–vis DRS (Fig. 4). Clearly, α-Fe2O3 responded well to visible light, while BCN absorbed from UV to visible light with an absorption edge around 460 nm. The PCN/α-Fe2O3 heterojunction composites displayed an obvious red shift edge and higher visible light absorption intensity compared with PCN. Therefore, α-Fe2O3/PCN composites were also visible-light-driven pho­ tocatalysts that integrated the absorption properties of PCN and α-Fe2O3. The band energy gap (Eg) of α-Fe2O3 and g-C3N4, both direct semiconductors, can be estimated empirically as follows [33]: (αhν)n/2 ¼ A(hν-Eg)

PCN -Fe2O3

400

(1)

where α is the absorption coefficient, ν is light frequency, h is Planck constant (hv is proton energy), and n is 1 and 4 for direct and indirect transition, respectively (n ¼ 4 for g-C3N4). The Eg values of α-Fe2O3 and PCN were calculated to be 2.1 and 2.81 eV, respectively (Fig. 4(b)).

450

500

550

Wavelength(nm)

600

650

Fig. 6. Room temperature PL spectra of α-Fe2O3, PCN and α-Fe2O3/ PCN composites.

concentrations over BCN and PCN nanoparticles decreased by about 6.3% and 58.5% after 15 min of irradiation, respectively (Fig. 5(a)), compared with the RhB degradation efficiency of α-Fe2O3/PCN (91.1%). Results indicate the synergistic effect of heterojunction and pore struc­ ture played an important role in the photocatalytic activity of α-Fe2O3/ PCN. Fig. 5(b) shows a linear relationship between –ln(C/C0) and t (time). According to –ln(C/C0) ¼ kat, the ka of α-Fe2O3/PCN was calcu­ lated to be 0.1278, which was 1.43 and 2.27 times that of PCN and α-Fe2O3/BCN, respectively. The stability of α-Fe2O3/PCN was further evaluated by the reuse after photocatalytic RhB degradation. The pho­ tocatalytic efficiency of α-Fe2O3/PCN was not reduced significantly after five cycles (Fig. 5(c)), indicating its high photostability under visible-

3.3. Photocatalytic activity evaluation The photocatalytic performances of α-Fe2O3/PCN composites were evaluated by photocatalytic degradation of aqueous RhB under visible light irradiation (Fig. 5(a)). The degradation efficiency of photocatalysts was described by C/C0, the concentration ratio after and before a certain reaction time. No obvious photodegradation was found in the blank experiment without adding catalyst, which confirmed the stability of RhB under this test condition. Prior to visible light irradiation, the mixture of the RhB solution and a catalyst were stirred in the dark for 30 min to achieve the adsorption-desorption equilibrium. The RhB 78

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Materials Chemistry and Physics 234 (2019) 75–80

2

500

Volume absorbed(cm /g, STP)

-Fe2O3/PCN SBET= 78.92m /g

3

400

2

PCN

SBET= 82.47m /g

BCN

SBET= 25.11m /g

2

300 200 100 0 0.0

0.2

0.4

0.6

Relative pressure (p/p0)

0.8

1.0 Fig. 9. Z-scheme mechanism diagram of electron-hole pair separation and the possible reaction mechanism over α-Fe2O3/PCN photocatalyst under visible light irradiation.

Fig. 7. Nitrogen adsorption-desorption isotherms of BCN, PCN and α-Fe2O3/ PCN composites.

(82.47 vs. 25.11 m2/g), suggesting PCN could provide more active sites for photocatalytic reaction. The existence of oxygen in urea was conducive to enlarging the specific surface area of PCN because the formation of ammonia gas and CO2 during the condensation polymeri­ zation suppressed the advance of crystal boundary [34]. Moreover, the photocatalysts are characteristic of type IV (IUPAC classification) (Fig. 7), indicating the existence of a porous structure. Meanwhile, the hysteresis loop is H3-shaped and in the high relative pressure range (0.9–1.0) while the adsorption-desorption isotherms display strong ab­ sorption, which also suggest the formation of pore structure [35]. Larger specific surface area indicates more active sites, and the existence of pore structure provides more carrier charge transfer channels, both of which help to improve photocatalytic activity. Trapping experiments were conducted with the presence of BQ (scavenger of �O2 ), IPA (scavenger of �OH) or Na2-EDTA (scavenger of hþ) [35]. The RhB degradation efficiency significantly decreased from 91% to 12% and 36% under visible light irradiation in the presence of BQ and Na2-EDTA, respectively, but was not significantly affected by the addition of IPA (Fig. 8). It was inferred �O2 on the catalyst surface was the primary active species during the photocatalysis, hþ was also responsible for RhB degradation, and �OH in the solution was not a main active species. The band gap level of a semiconductor can be theoretically evaluated as follows [33]:

Photodegradation rate

1.0

0.8

0.6

0.4

0.2

0.0

NO scavenger

EDTA-2Na

BQ

IPA

Fig. 8. Photocatalytic degradation experiments with the addition of radical scavenger.

light irradiation. 3.4. Photocatalytic mechanism

EVB ¼ X-E0þ0.5 Eg

(2)

ECB ¼ EVB-Eg

(3)

where EVB is the VB edge potential, ECB is the CB edge potential, X is the electronegativity of the semiconductor, Eg is the band gap of the semi­ conductor, and E0 is the energy of free electrons on the normal hydrogen electrode (4.5 eV). X is 5.87 eV for α-Fe2O3 and 4.73 eV for g-C3N4 [36, 37]. Therefore, the ECB and EVB were calculated to be 0.32 and 2.42 eV, respectively for α-Fe2O3, and 1.58 and 1.23 eV, respectively for PCN. On basis of the above experimental results and analysis, we proposed the schematic mechanism underlying the improvement of RhB photo­ degradation over α-Fe2O3/PCN (Fig. 9). Under the visible light irradia­ tion, both α-Fe2O3 and PCN could absorb visible light to generate electrons and holes, and transferred the photoexcited electrons to the conduction band, leaving holes in the valence band. It was generally accepted the photoexcited electrons and holes recombined very easily and fast, which could be efficiently inhibited by coupling PCN with α-Fe2O3 to form heterojuction. Z-scheme mechanism was proposed to better explain the enhanced photocatalytic performance of α-Fe2O3/

The recombination of photogenerated electrons and holes as well as the lifetime of charge carriers was investigated by PL spectra. Briefly, a higher PL intensity means a faster recombination rate of photoinduced electron-hole pairs and a prolonged lifetime of charge carriers. PL spectra of α-Fe2O3, PCN and α-Fe2O3/PCN composites excited at 360 nm were shown in Fig. 6. Clearly, PCN had the main peak at about 475 nm which corresponded to its band gap energy. However, the peak intensity of PCN coupled with α-Fe2O3 decreased significantly, indicating the recombination rate of photogenerated electrons and holes was addressed effectively due to the formation of α-Fe2O3/PCN hetero­ junction. Consequently, α-Fe2O3/PCN exhibited the lower PL intensity and thereby lower recombination efficiency, which agrees well with the results of DRS and photocatalytic activity. The nitrogen adsorption-desorption isotherms of BCN, PCN, and α-Fe2O3/PCN Brunauer-Emmett-Teller (BET) surface areas are showed in Fig. 7, the specific surface area of PCN is about three times that of BCN 79

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PCN. Although the CB and VB edge potentials of PCN are more negative than those of α-Fe2O3, the CB potential of α-Fe2O3 is lower than the redox potential of O2/�O2 ( 0.33 eV); therefore, the electrons on the CB of PCN, rather than α-Fe2O3, reduced O2 to �O2 [38,39]. The VB po­ tential of α-Fe2O3 is more positive than the oxidation potential of RhB (0.95eV) and (�OH/OH )(1.99eV/NHE), indicating the photogenerated holes of α-Fe2O3 have a strong oxidative ability and can oxide RhB to H2O, CO2 or other intermediates [38]. Therefore, the addition of α-Fe2O3 could accelerate the electron-hole separation, thereby greatly inhibited the recombination and improved the photocatalytic efficiency of α-Fe2O3/PCN composites. As a consequence, the α-Fe2O3/PCN com­ posites exhibit better photocatalytic properties than PCN under visible light irradiation. In addition, the enhanced light absorption, larger specific special area and existence of pore structure are other crucial reasons underlying the enhancement of photocatalytic activity.

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4. Conclusions The stable heterojunction structure of α-Fe2O3/PCN composites was constructed via a facile calcination method. The introduction of α-Fe2O3 enhanced the photocatalytic performance of PCN. After 15 min of visible light irradiation, the photocatalytic efficiency of α-Fe2O3/PCN in RhB photodegradation was tremendously enhanced in comparison with PCN. The enhanced activity of α-Fe2O3/PCN composites was probably attributed to the synergic effect between α-Fe2O3 and PCN in hetero­ junction formation, larger BET areas and more light absorption. The existence of heterojunction accelerated the separation of electrons and holes and hindered their recombination, therefore obviously improving the photocatalytic efficiency of PCN after the addition of α-Fe2O3. Acknowledge The work was supported by the National Natural Science Foundation of China (Grant No. 31800495), Natural Science Foundation of Jiangsu Province (Grant No. BK20181040), Natural Science Foundation of the Higher Educations Institutions of Jiangsu Province (Grant No. 17KJB430014), Key University Science Research Project of Jiangsu Province (Grant No. 16KJA430007). References [1] C. Chen, W. Ma, J. Zhao, Semiconductor-mediated photodegradation of pollutants under visible-light irradiation, Chem. Soc. Rev. 39 (2010) 4206–4219. [2] K. Singh, J. Nowotny, V. Thangadurai, Amphoteric oxide semiconductors for energy conversion devices: a tutorial review, Chem. Soc. Rev. 42 (2013) 1961–1972. [3] S. Kang, L. Zhang, C. Yin, Y. Li, L. Cui, Y. Wang, Fast flash frozen synthesis of holey few-layer g-C3N4 with high enhancement of photocatalytic reactive oxygen species evolution under visible light irradiation, Appl. Catal. B Environ. 211 (2017) 266–274. [4] 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. [5] C. Tan, X. Cao, X.J. Wu, Q. He, J. Yang, X. Zhang, J. Chen, W. Zhao, S. Han, G. H. Nam, M. Sindoro, H. Zhang, Recent advances in ultrathin two-dimensional nanomaterials, Chem. Rev. 117 (2017) 6225–6331. [6] J.Q. Wen, J. Xie, X.B. Chen, X. Li, A review on g-C3N4-based photocatalysts, Appl. Surf. Sci. 391 (2017) 72–123. [7] Z. Zhao, Y. Sun, F. Dong, Graphitic carbon nitride based nanocomposites: a review, Nanoscale 7 (2015) 15–37. [8] J. Xiao, Y. Xie, C. Li, J.-H. Kim, K. Tang, H. Cao, Enhanced hole-dominated photocatalytic activity of doughnut-like porous g-C3N4 driven by down-shifted valance band maximum, Catal. Today 307 (2017) 147–153. [9] K. Kailasam, A. Fischer, G. Zhang, J. Zhang, M. Schwarze, M. Schroder, X. Wang, R. Schomacker, A. Thomas, Mesoporous carbon nitride-tungsten oxide composites for enhanced photocatalytic hydrogen evolution, ChemSusChem 8 (2015) 1404–1410. [10] S. Liu, H. Zhu, W. Yao, K. Chen, D. Chen, One step synthesis of P-doped g-C3N4 with the enhanced visible light photocatalytic activity, Appl. Surf. Sci. 430 (2018) 309–315. [11] J.C. Wang, C.X. Cui, Y. Li, L. Liu, Y.P. Zhang, W. Shi, Porous Mn doped g-C3N4 photocatalysts for enhanced synergetic degradation under visible-light illumination, J. Hazard Mater. 339 (2017) 43–53.

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