Higher Yield Urea-Derived Polymeric Graphitic Carbon Nitride with

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Higher yield urea-derived polymeric graphitic carbon nitride with mesoporous structure and superior visible-light-responsive activity Lei Shi, Lin Liang, Fangxiao Wang, Mengshuai Liu, Kunlong Chen, Kening Sun, Naiqing Zhang, and Jianmin Sun ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01139 • Publication Date (Web): 05 Nov 2015 Downloaded from http://pubs.acs.org on November 9, 2015

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ACS Sustainable Chemistry & Engineering

Higher yield urea-derived polymeric graphitic carbon nitride with mesoporous structure and superior visible-light-responsive activity Lei Shi‡, Lin Liang§, Fangxiao Wang‡, Mengshuai Liu‡, Kunlong Chen‡, Kening Sun‡, Naiqing Zhang‡ *and Jianmin Sun†, ‡ * †: State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of

Technology, Harbin 150080, China ‡: The Academy of Fundamental and Interdisciplinary Science, Harbin Institute of

Technology, Harbin 150080, China §: School of Life Science and Technology, Harbin Institute of Technology, Harbin

150080, China *Corresponding

authors:

Naiqing

Zhang,

[email protected];

[email protected] Tel.:+86 451 86403715.

1

ACS Paragon Plus Environment

Jianmin

Sun,


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ABSTRACT A series of mesoporous graphitic carbon nitride (mg-C3N4) materials have been prepared with urea and tetraethylorthosilicate (TEOS) as the precursors, which were thermally polycondensed to obtain the g-C3N4/silica composites, after silica was removed, mg-C3N4 with large surface area (170 m2 g-1) was successfully prepared. Excitingly, TEOS did not only act as mesoporous–directing agent, but also as the promoter for the urea polycondensation to g-C3N4, which made the urea polycondensation proceed at relatively low temperature. Thus, volatilization or/and decomposition of urea in process of thermal treatment were reduced, resulting in the product yield of g-C3N4 from 0.3-0.4 g/10 g urea remarkably increasing to 1.2 g/10 g urea. Moreover, superior photocatalytic activities were observed for degrading methyl orange (MO) and H2 generation from water splitting over mg-C3N4 photocatalyst. The facilely developed method for high-yield mesoporous g-C3N4 from cost-effective urea was more attractive for its wide applications in environmental treatment and energy development fields.

KEYWORDS: Mesoporous g-C3N4; Urea; TEOS; High yield; Visible-light catalysis

2


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INTRODUCTION In recent years, visible-light-driven photocatalysts have drawn considerable attentions due to their efficient utilization of solar energy, thus solving the present crisis

of energy

and environmental troubles.

As a

promising metal-free

visible-light-responsive photocatalyst, polymeric graphitic carbon nitride (g-C3N4) has shown excellent photocatalytic performance for H2 and O2 evolutions via water splitting and degradations of organic pollutants.

1-5

Besides, the simple preparation for

g-C3N4 was also attractive to its practical applications. Generally, pyrolysis of nitrogen-rich precursors such as cyanamide, dicyandiamide, urea, thiourea, melamine, guanidine hydrochloride et al could easily obtain g-C3N4.

6-11

However, the high

recombination rates of the photogenerated electron-hole pairs and low surface areas confined its photocatalytic performance. Hence, many strategies including nonmetal doping,

12

noble metal deposition,

13, 14

coupling with semiconductor composites,

15, 16

and introducing mesoporous structures, 17-19 have been employed to modify g-C3N4 and exhibited the encouraging photocatalytic activities. In general, a photocatalyst with a large surface area is desirable to the improved photocatalytic activity, owing to the more active sites provided for adsorption and reaction. Among the precursors for g-C3N4, urea is more appealing in that its cheapness, nontoxicity and abundance. Additionally, the prepared g-C3N4 via urea pyrolysis with nanostructural frameworks also presents relatively large surface area due to the presence of oxygen species as leaving groups, which could modify the polymerization process through the peeling effect.

20

Nevertheless, the yield of urea-derived g-C3N4 3



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was unsatisfactory at low about 0.3-0.4 g C3N4/10 g urea because of huge volatilization and/or decomposition of urea.

21, 22

Thus, the low yield for urea-derived g-C3N4 would

undoubtedly limit its widely practical applications. In comparison with the huge works focused on improving photocatalytic activity of g-C3N4, however, the research on increasing the yield of g-C3N4 was relatively scarce. Therefore, it was more attractive to prepare g-C3N4 material together with high yield and superior photocatalytic activity for extensive application fileds. In order to develop a simple and economic strategy for the fabrication of mesoporous g-C3N4 photocatalyst together with high yield, in this paper we introduced TEOS during the process of urea polymerization. Theoretically, TEOS hydrolysis can in-situ generate nanosized SiO2, which could act as mesostructure-directing agent for the mesoporous g-C3N4 formation. Moreover, urea could interact with silicic acid produced from TEOS hydrolysis through acid-base interaction to reduce the volatilization and decomposition of urea realizing the highly productive g-C3N4. As anticipated excitingly, the resultant mg-C3N4 with large surface area of 170 m2 g-1 was successfully obtained, meanwhile, the yield of mg-C3N4 from urea was dramatically increased. Moreover, the photocatalytic activities over mg-C3N4 for photodegradation of methyl orange (MO) and splitting water to H2 were examined under visible light and displayed much more superior activities than the reference bulk g-C3N4. The developed present method opened the facile and cost-economic preparation of g-C3N4 with high surface and high yield, which is widely applied in energy-related topics, environmental remediation and other new emerging fields. 4


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EXPERIMENTAL Synthesis of mg-C3N4: 10 g urea was dissolved into the mixture of ethanol (10 g) and 0.2 M HCl (15 g), then 8 mL TEOS was slowly dropped into the above solution, stirred vigorously for 2 h and transferred into Petri dish for evaporation the solvents. The obtained solid material was stayed in the crucible with a cover, and heated at 550 °C for 2 h then cooled to room temperature. Subsequently, the as-prepared composites were stirred in hydrofluoric acid for 24 h for SiO2 removal, then the product denoted as mg-C3N4-2 was collected by filtration, washed by water and ethanol, finally dried at 80 °C for 12 h. Similarly, mg-C3N4-1 and mg-C3N4-3 were obtained respectively with 4 mL and 12 mL TEOS. For comparison, bulk g-C3N4 was prepared by directly heating 10 g urea at 550 °C for 2 h without TEOS addition. Denoted urea (250), urea (300), urea (350) and urea (400) indicated the treatment of 10 g urea at 250, 300, 350 and 400 °C for 2 h. The prepared processes for urea-TEOS (250), urea-TEOS (300), urea-TEOS (350) and urea-TEOS (400) were similar to mg-C3N4-2 at the different calcination temperatures labeled in bracket.

Material characterizations: N2 adsorption-desorption isotherms were collected at 77 K using a Quantachrome NOVA 2000 surface area and porosity analyzer, samples were outgassed at 150 °C for 12 h prior to measurements. The morphology of the sample was examined by transmission electron microscopy (TEM, Tecnai G2 Spirit). The patterns of X-ray diffraction were carried out on Bruker D8 Advance X-ray powder diffractometer with Cu Kα radiation (40 kV, 30 mA) for phase identification. 5



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Thermal gravimetric analysis (TG) and differential scanning calorimetry (DSC) measurements were conducted using a STA449F3 (NETZSCH Germany) instrument. Fourier transform infrared spectroscopy (FTIR) was recorded in transmission mode in 4000-400 cm-1 on Perkin Elmer spectrum 100 FTIR spectrometer using KBr discs. X-ray photoelectron spectroscopy (XPS) measurements were recorded on a Thermo Fisher Scientific Escalab 250Xi. The shift of the binding energy was calibrated using an internal standard of C1s level at 284.6 eV. Elemental analyses (EA) for the carbon and nitrogen contents were performed on Vario Microcube CHN analyzer. The UV-vis diffuse reflectance spectra (DRS) were measured by Shimadzu 2550 UV-vis spectrometer. The photoluminescence spectra (PL) were obtained on Perkin Elmer LS55 spectrometer with excitation wavelength of 325 nm. The solid-state

13

C NMR

measurements were carried out on Bruker Ultrashield 400 spectrometer.

Photocatalytic testing: The photocatalytic performance of the as-made samples was evaluated through degrading MO under visible light. A 300 W Xe lamp with a 400 nm cut-off ﬁlter was used as the light source to provide visible-light irradiation. 50 mg photocatalyst was dispersed into 100 mL 10 mg·L-1 MO solution for photocatalytic examination under magnetic stirring. Prior to the light irradiation, the dispersion was kept in dark for 60 min under magnetic stirring to reach the adsorption-desorption equilibrium. Then the solution was irradiated and collected every 20 min, after centrifugation of the catalyst, the remained solution was analyzed on UV-vis spectrometer. For comparison, the photodegradation reactions were also carried out in 6


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the presence of bulk g-C3N4 and in the absence of any catalyst. The efficiency of degradation was calculated by C/C0, wherein C is the concentration of remaining dye solution at each irradiated time, and C0 is the initial concentration. The visible-light-induced catalytic H2 evolution was carried out in a Pyrex top-irradiation reaction vessel connected to a closed glass gas-circulation system. H2 production was performed on a dispersion of 50 mg photocatalyst in an aqueous solution containing distill water (90 mL) and triethanolamine (10 mL). 3 wt% Pt was loaded on the surface of photcatalyst by in-situ photodeposition method using H2PtCl6 as the starting material. The reactant solution was evacuated several times to remove the air prior to the irradiation under visible light. The evolved gases were analyzed by gas chromatography (SP7800) using N2 as the carrier gas.

RESULTS AND DISCUSSION The synthesis process of mg-C3N4 was described in Figure 1A. Firstly, urea and TEOS were mixed in an acidified solution of water and ethanol. After a period of stirring, a clear homogeneous solution was formed, which indicated that urea, TEOS and subsequently the formed silicic acid were completely dissolved in the mixture. Then the solution was volatilized to remove the solvent, the obtained materials were thermally treated at 550 °C for urea monomer polymerization to obtain g-C3N4/SiO2 composites, in which TEOS generated SiO2 and urea formed g-C3N4. After the silica was removed, mesoporous g-C3N4 was produced. TEM image (Figure 1B) clearly displayed small and abundant mesopores (bright areas) with the average sizes of 7



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approximately 3-4 nm. A

B

Figure 1 (A) Proposed synthesis procedure for mg-C3N4 and (B) TEM image of mg-C3N4-2.

As shown in Table 1, it was found that the surface areas of resultant mg-C3N4 were much higher than bulk g-C3N4, and BET increased with TEOS amount adding, which was resulted from more mesopores formed with more TEOS addition. The surface area of mg-C3N4-2 was improved to 170 m2g-1, however, with further increasing TEOS to 12 mL in the mg-C3N4-3 sample, the surface area was relatively decreased to 127 m2g-1, which was possibly the result of part pores collapse during the process of silica washing.

6

More interesting, the yield of as-prepared mg-C3N4 was 8


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remarkably increased to 1.25 g/10g urea due to the introduced TEOS compared with 0.34 g bulk g-C3N4/10g urea. However, what reasons led to the enhanced yield? In the formation process of mg-C3N4, via acid-base interaction, the amine groups of urea interacted with the silanol groups of silicic acid produced from acidic hydrolysis of TEOS, 23 which might reduce the volatilization and/or decomposition of urea. To verify the above deduction, the TG-DCS of urea and urea-TEOS (the adding amount of TEOS was 8 mL) were investigated. TG curves in Figure 2 exhibited that the decomposition rate for bare urea system was much faster than urea-TEOS system, and the weight loss for treatment of urea was higher than that in urea-TEOS system. Below 260 °C, the weight loss was about 79% for the pristine urea, however, 64% loss was occurred in the urea-TEOS treatment process, suggesting that TEOS hindered the decomposition and/or volatilization of urea to some extent during the polycondensation process. Table 1 Physicochemical properties of various samples BET Sample

a

(m2g-1)

Pore

Pore

Yieldc

C

N

C/N atomic

volume

width b

(g)

(wt%)

(wt%)

ratio

(cm3g-1)

(nm)

bulk g-C3N4

65

0.17

35

0.34

33.17

59.96

0.64

mg-C3N4-1

74

0.18

3.8

1.20

29.60

53.66

0.64

mg-C3N4-2

170

0.34

3.7

1.24

31.65

57.60

0.64

mg-C3N4-3

127

0.31

3.7

1.25

31.33

56.86

0.64

a: BET surface area, b: Average pore width was determined by the BJH method, c: The yield was obtained from 10 g urea. 9



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Figure 2 TG curves for (a) urea and (b) urea-TEOS (Inset is the corresponding DSC thermograms).

In addition, we compared the XRD patterns of polymerization products from urea and urea-TEOS at different temperatures. As shown in Figure 3A, urea (250) and urea (350) exhibited the same XRD patterns, which agreed well with cyanuric acid.

24

The

XRD patterns of urea (400) at 10.67, 18.48, and 21.41° suggested the existence of complex polycondensed from melamine and cyanuric acid (PMCA).

25

Hence, we

could infer that during the urea polymerization, urea firstly formed cyanuric acid by primary polycondensation, then parts of cyanuric acid reacted with ammonia from the decomposition of urea to further synthesize melamine, subsequently, the remaining cyanuric acid polymerized with melamine to form PMCA, and finally to obtain g-C3N4 with increasing thermal treatment temperature. However, for urea-TEOS system, we could clearly observe that the XRD patterns of urea-TEOS (300) were the same to urea (400), with treatment temperature increasing, the peak intensities ascribed to PMCA became weaker and gradually disappeared, simultaneously, the peak at 27.8° ascribed 10


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to g-C3N4 became stronger.

26

Obviously, the introduction of TEOS helped the

intermediate formations of g-C3N4 product under the relatively milder temperature. With the temperature increase to 550 °C, the peaks of PMCA totally disappeared and the diffractions at 27.6° and 13.0° were obvious, respectively corresponding to (002) and (100) plane resulted from the graphite-like stacking of the conjugated aromatic units of CN and in-plane structural packing motif of hexagonal g-C3N4 (JCPDS card no. 87-1526).

Figure 3 X-ray diffraction patterns for products polymerized from (A) urea and (B) urea-TEOS at different temperatures.

The FTIR spectra of mg-C3N4-2 and bulk g-C3N4 were displayed in Figure 4A. mg-C3N4-2 and bulk g-C3N4 exhibited the same patterns, the absorption peak at 810 cm-1 was attributed to the out-of-plane skeletal bending modes of the triazine cycles, 27 and the absorption bands in the range of 1200-1700 cm-1 were assigned to the typical stretching modes of CN heterocycles.

11, 28

In the range of the 3000-3500 cm-1 region,

the broad band was attributed to N-H and O-H groups. 29 In addition, the solid-state 13C NMR spectrum verified the structure of mg-C3N4-2 (Figure 4B). Two distinct signals at 11



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156.5 and 164.1 ppm could be respectively attributed to the C atoms in melem (CN3) and CN2(NH2), indicative of characteristic poly(tri-s-triazine) structures in the prepared sample. 30

Figure 4 (A) FTIR spectra of (a) bulk g-C3N4 and (b) mg-C3N4-2, (B) Solid-state 13C NMR spectrum of mg-C3N4-2.

XPS data further supported the results obtained from FTIR spectra. In C1s spectrum (Figure 5A), 284.6 and 288.2 eV were respectively assigned to carbon atoms in a pure carbon environment and sp2-bonded carbon (C=N).

31

For N 1s spectrum in

Figure 5B, the small peak located at 404.4 eV was caused by the positive charge localization in heterocycles. 32 The big peak was further deconvoluted into three peaks at 398.8, 399.6 and 400.8 eV, which were respectively attributed to N atom sp2-bonded to two carbon atoms (C−N=C),

33

tertiary nitrogen (N−(C)3),

34

and amino functional

groups with hydrogen atom (NH2 or NH groups). 35 And the C/N ratios of bulk g-C3N4 and as-prepared mg-C3N4 samples were all about at 0.64 in Table 1, lower than 0.75 for the ideal crystal g-C3N4, which was owing to the incomplete condensations of urea thus led to the existence of hydrogen atoms in g-C3N4 products. Elemental analyses 12


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results were consistent with FTIR and XPS results.

Figure 5 XPS spectra of (A) C 1s and (B) N 1s of mg-C3N4-2 sample. The UV-visible diffuse reflectance spectra of the resultant samples were exhibited in Figure 6A. Bulk g-C3N4 and mg-C3N4-2 both displayed similar photoabsorptions from ultraviolet to visible light, and their band gap absorption edges were around 445 and 459 nm, corresponding to the band gaps at 2.78 and 2.70 eV. The phenomena indicated that TEOS introduction hardly had an effect on the band gap. Since photoluminescence spectrum emission arises from the recombination of excited electrons and holes, thus, PL technique is useful for disclosing the migration, transfer and recombination processes of the photogenerated electron-hole pairs in the semiconductors.

36

In Figure 6B, the strong photoluminescence was quenched in the

case of mg-C3N4, suggesting that the energy-wasteful charge recombinations happening in the polymeric matrix was greatly suppressed. The improved charge separation efficiency was attributed to the pore structure of mg-C3N4, the existence of pores in the mg-C3N4 facilitated the transfer and separation of charge carriers, prolonged the lifetimes of photoinduced charge carriers and thus led to the lowed PL intensity.

37, 38

Moreover, the residual silica as well as the presence of OH groups on 13



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the surface of the as-prepared material had some effects on the enhancement of the charge separations. 9, 39, 40 The improved separation efficiency for electron-hole pairs would be in favor of the acceleration of photocatalytic quantum efficiency.41

Figure 6 (A) UV-visible diffuse reflectance spectra and (B) Photoluminescence spectra of (a) bulk g-C3N4 and (b) mg-C3N4-2.

The photocatalytic capability of as-made samples was evaluated through the degradation efficiency for MO under visible light. Prior to illumination, the adsorption of MO over the photocatalyst was carried out in dark, as shown in Figure 7A, adsorption equilibriums were reached within 60 min for all the samples. The amount of MO adsorption on bulk g-C3N4 was low at 13.4 %. For as-made mg-C3N4 samples, adsorption amounts of MO were improved with surface area increasing, and reached 18.1%, 28.5% and 21.0% for mg-C3N4-1, mg-C3N4-2 and mg-C3N4-3. For the photodegradation process, the degradation efficiency of bulk g-C3N4 was also low at 41%. It was noticeable that the resultant mg-C3N4 samples displayed significantly enhanced degradation efficiencies, especially for mg-C3N4-2, almost 90% MO was photodegraded within 120 min. Simultaneously, the reaction rate constant k was also 14


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evaluated, the linear relationships between ln(C0/C) and reaction time in Figure 7B matched well with the pseudo-first-order reaction kinetics (R≥0.99). The reaction rate constants k for bulk g-C3N4, mg-C3N4-1, mg-C3N4-2 and mg-C3N4-3 were respectively calculated at 0.00317, 0.01104, 0.01513, and 0.01217 min-1. Obviously, mg-C3N4-2 possessed the highest rate constant and reached 4.77 times larger than that of bulk g-C3N4.

Figure 7 (A) The adsorption and degradation curves of MO; (B) The first-order kinetic plots for degradation of MO; (C) The curves of producing H2 and (D) Hydrogen evolution rates over various samples. (a) Bulk g-C3N4, (b) mg-C3N4-1, (c) mg-C3N4-2, (d) mg-C3N4-3 and (e) blank. Moreover, the photocatalytic H2 evolution from water splitting over bulk g-C3N4 and as-prepared mg-C3N4 samples were also investigated. As shown from Figure 7C, 15



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H2 was steadily produced with irradiation time prolonging. Similarly, the bulk g-C3N4 showed low hydrogen evolution rate at 64 µmol h-1 under visible-light irradiation (Figure 7D). In comparison, mg-C3N4 samples exhibited much higher hydrogen evolution rates (HER=180, 272, 240 µmol h-1). The photocatalytic H2 evolution rate of mg-C3N4-2 was about 4.25 times higher than that of bulk g-C3N4. Obviously, mg-C3N4 exhibited the remarkably enhanced photocatalytic activities for degrading MO and hydrogen evolution from water splitting. Generally, for a photocatalyst, the absorption to light, BET surface area and the charge separation efficiency are the main factors that influence its photocatalytic activity. The absorption intensities of bulk g-C3N4 and mg-C3N4 in the visible-light region were almost the same, which meant that the effects of light absorption might be neglected. Nevertheless, the photocatalytic activities of mg-C3N4 were more superior to bulk g-C3N4, which was ascribed to larger surface area and mesoporous structure in the former samples. The high surface area made the resultant mg-C3N4 improve the adsorption capacity to the reactants and could supply more catalytic active sites, thus, leading to the acceleration in photocatalysis rates. Furthermore, the migration and separation of charge carriers were facilitated in the mg-C3N4. Consequently, attractive photocatalytic efficiency of mg-C3N4 was obtained through these above integrated positive effects. Moreover, the stability of a photocatalyst is important for the practical applications. Hence, the photocatalytic recycling experiments for degrading MO and splitting water over mg-C3N4-2 were repeated up to three times under the same conditions and the results were demonstrated in Figure 8. It was evident that the 16


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photodegradation activity for MO did not change obviously and still remained 90% of the first activity even after three runs, which indicated that mg-C3N4 was not photocorroded during the catalytic process and kept excellent stability. Simultaneously, the durability of mg-C3N4-2 for H2 evolution from water splitting was carried out by three consecutive operations in Figure 8B. As irradiation time prolonging, the produced H2 increased stably without evident deactivation, further revealing the unexceptionable stability of mg-C3N4 sample. In addition, the structure of the spent catalyst was further measured by XRD after three runs. The XRD pattern of the spent mg-C3N4 was similar to that of the fresh catalyst (Figure 9), suggesting the structure of mg-C3N4 was stable during the photoreaction process.

Figure 8 Recycling runs for (A) photodegradation of MO and (B) H2 evolution by water splitting over mg-C3N4-2 under visible-light irradiation. 17



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Figure 9 The XRD patterns of the fresh and spent mg-C3N4-2.

CONCLUSIONS In the development of mesoprous g-C3N4 synthesis, we proposed the introduction of TEOS as mesoporous-directing agent to fabricate mg-C3N4 through the in-situ hydrolysis TEOS to SiO2. Simultaneously, TEOS acted as the promoter for urea thermal polymerization to g-C3N4 and made the polycondensation proceed under relatively low temperature, which significantly reduced volatilization and/or decomposition of urea in the process of thermal treatment, so that high yield urea-derived mg-C3N4 material was successfully obtained. The prepared mg-C3N4 presented larger surface area, lower recombination rates for photoinduced electrons and holes, and more suprerior photocatalytic activity for degrading MO and H2 evolution by splitting water under visible light. The present developed synthesis method for g-C3N4 provided a simple and economic strategy for the fabrication of mesoporous g-C3N4 material with high surface area and high yield, which was promisingly applied in environmental treatment and energy development fields. 18


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ACKNOWLEDGEMENT We sincerely acknowledge the financial supports from National Natural Science Foundation of China (21373069), Science Foundation of Harbin City (NJ20140037), State Key Lab of Urban Water Resource and Environment of Harbin Institute of Technology (HIT2015DX08) and the Fundamental Research Funds for the Central Universities (HIT IBRSEM. 201327).

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For Table of Contents Use Only Title: Higher yield urea-derived polymeric graphitic carbon nitride with mesoporous structure and superior visible-light-responsive activity Authors: Lei Shi, Lin Liang, Fangxiao Wang, Mengshuai Liu, Kunlong Chen, Kening Sun, Naiqing Zhang and Jianmin Sun

High

yield

urea-derived

mesoporous

g-C3N4

with

superior

and

stable

visible-light-responsive activity for degrading pollutants and producing H2 was facilely prepared.

25


Higher Yield Urea-Derived Polymeric Graphitic Carbon Nitride with

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