Nanotechnology

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Facile synthesis of graphitic C3N4 nanoporous-tube with highly enhancement of visible-light photocatalytic activity To cite this article before publication: Ruiru Zhao et al 2017 Nanotechnology in press https://doi.org/10.1088/1361-6528/aa929a

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Facile synthesis of graphitic C3N4 nanoporous-tube with highly

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enhancement of visible-light photocatalytic activity

Ruiru Zhao, Jianping Gao, Shunkang Mei, Yongli Wu, Xiaoxue Wang, Xiangang Zhai, Jiangbing Yang, Chaoyue Hao, Jing Yan *

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China.

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Department of Chemistry, School of Science, Tianjin University, Tianjin 300350,

* Corresponding Author. Tel.: +8602227403475; Fax: (+86)22-2740-3475.

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E-mail address: [email protected]

AUTHOR SUBMITTED MANUSCRIPT - NANO-114785.R1

Abstract

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A simple and convenient method was used to synthesize graphitic carbon nitride (g-C3N4) nanoporous-tube by using SiO2 nanoparticles as pore formers. The structure of the g-C3N4 nanoporous-tube was characterized by the SEM and TEM images.

Taking photodegradation of RhB as an example, the photocatalytic activity of the asprepared g-C3N4 nanoporous-tube was investigated. It can photodegrade 90％ RhB in

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40 minutes under visible-light irradiation and obtain a k value of 0.04491 min−1 , which is 8.16 times that of bulk g-C3N4, 3.09 times that of tubular g-C3N4 and 1.48 times that of tubular g-C3N4-SiO2. The significant enhancement in photocatalytic

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efficiency is due to the edge effect of the pores and the special structure of the tubes. In addition, the possible mechanism of photocatalytic degradation of RhB was also proposed based on the trapping experiment of active species, which indicated that the

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superoxide radicals (O2•−) and the holes (h+) were the main reactive species in this photocatalyst. This work may open up a new idea of innovation in g-C3N4 structure and inspire its follow-up study. Keywords:

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g-C3N4; Nanoporous-tube; Photocatalyst; Organic pollutant degradation; 1. Introduction

Due to the increasing severity of the environmental pollution in the human

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development, so how to use green methods to degrade these environmental pollutants has already become a worldwide issue [1-5]. Photodegradation technology, as a simple and environmentally-friendly approach [6], has aroused great attention [7]. In

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recent years, many efforts have been focused on exploring high effective and alternative visible light photocatalysts [8] because of its appreciated solar energy

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utilization [9-12]. Graphitic carbon nitride (g-C3N4) has become a promising candidate of visible light photocatalyst for its fruitful use of sunlight, excellent

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thermal and chemical stability, non-toxic and low-cost characteristics [13-18]. And gC3N4 has remarkable application in the photodegradation of organic dyes such as RhB

[19-20]. Although, g-C3N4 has an available band gap of about 2.7 eV as a visible light photocatalyst, it still suffers from formidable challenges for negligible activity in solar

energy utilization without co-catalyst [21], mainly due to its narrow visible-light

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response range, low surface area, fast recombination of photogenerated electron-hole

pairs and poor quantum yield [22-25]. Therefore, researchers have been seeking new

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techniques to overcome these drawbacks through various improvement methods, including coupling it with other materials, both of metal and nonmetal ions, or joining with other semiconductor, fabricating nanocomposites [26] or microporous

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nanomaterials [27, 28] of g-C3N4 [29, 30]. But all of these methods need to use other substances, and how to increase its photocatalytic performance without adding other materials is desirable.

Since the discovery of tubular nanomaterials such as carbon nanotubes [31-35] in photocatalytic fields, much attention has been paid to them, because of their unique

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physicochemical properties, electronic and optical properties [33]. Tubular g-C3N4 is attractive for its less recombination of electron-holes pairs, high surface active sites and increasing transfer of photogenerated electrons [2, 33-34]. Some groups have

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attempted to prepare g-C3N4 nanotubes using catalytic self-assembly method [36] or using other nanotube as template (for example, titanate nanotubes [37]). However, these methods often consist of complex steps or need poisonous solvents to remove

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the template which may cause environmental pollution. Using soft templates may lead to heteroatoms in the matrix, such as carbon atoms, and result in lattice disorder of g-

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C3N4 [38]. Nanoporous materials have also been extensively studied in the past decade,

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because porous structure can increase the specific surface area and active site of

photocatalysts [37]. Different pore-forming materials have been applied in making porous materials, such as Ni, CaCO3, SiO2 [38, 39] and so on. Orlando groups synthesized N, P-doped porous carbon that showed high electrocatalytic performance

[40]. Ye groups heated g-C3N4 in an oxygen atmosphere to form porous structure,

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which was beneficial for increasing the photocatalytic activity [41]. Ni groups found

that the proportions of the surface nitrogen atoms and carbon atoms affected the

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adsorption capacity of the porous carbonaceous materials to hydrophobic organic compounds [42].

In this article, inspired by the previous reports, we tried a strategy to combine the

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nanoporous and tubular structure together to form g-C3N4 nanoporous-tube in order to take advantage both of nanopores and tubes. The large surface area of the porous structure can not only absorb organic pollutants more efficiently and offer more active sites for electron transfer, but also prevent further aggregation of particles and

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accelerate the separation of pollutants and catalysts [38, 43]. Moreover, a tubular structure can offer internal and external surface active sites for photocatalytic reactions and reduce the mass transfer resistance for photocatalysts, thereby promoting transfer of photogenerated electrons [44]. The g-C3N4 nanoporous-tubes

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were prepared by a simple template method via burning a blend of melamine and SiO2 in a crucible. The current synthesis procedure is simple, environmentally-friendly, and

does not need to join other templates to achieve the purpose in the tubular synthesis

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process. The optical properties and structure of the g-C3N4 nanoporous-tubes were studied [45, 46]. In order to verify the photocatalytic activities of the as-prepared g-

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C3N4 nanoporous-tubes, the photodegradation of Rhodamine B (RhB) under visiblelight irradiation was used as a model and investigated [47, 48]. In addition, the

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mechanism of photocatalytic degradation of RhB was also proposed. 2. Experimental 2. 1 Materials

Melamine (C3H6N6) and ammonia solution (NH3·H2O) were purchased from

Acros Chemical Companies. Ethanol absolute (C2H6O), RhB and isopropyl alcohol

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(C3H8O) were purchased from Sigma-Aldrich Chemistry Corporation. Ethyl silicate (TEOS, C8H20O4Si) was supplied by DuPont. Sodium oxalate (Na2C2O4) and p-

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Quinone (C6H4O2) were purchased from Aladdin Chemical Reagent. All of the above reagents were analytical grade and were used without further purification.

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2. 2 Sample preparation 2.2.1 Synthesis of SiO2 nanoparticles

85.6 ml ethanol and 4 ml ammonia were put into the three-necked flask, then 6.4 ml TEOS /ethanol solution was successively dropwised into the above solution under stirring. After stirred for 20 h at 30 °C, the mixture was centrifuged under 9000 r/min,

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washed with deionized water and ethanol for several times and dried at 80 °C for 8 h in an oven.

2.2.2 Preparation of tubular g-C3N4

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A certain amount of melamine was put into the tube furnace keeping at a heating

rate of 4 °C/min at 600 °C for 4 h. After natural cooling, the resulting sample was ground into powder for subsequent use. For comparison, the bulk g-C3N4 was

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obtained by calcining of melamine at a heating rate of 10 °C /min at 550 °C for 4 h, just as previous work [2].

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2.2.3 Preparation of g-C3N4 nanoporous-tube Firstly, melamine was dried at 80 °C for 48 hours, and then a certain amount of

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melamine and prepared SiO2 were dissolved into the deionized water. After stirring the resulting mixture for 2 h at room temperature, the product was centrifuged with 9000 r/min, dried at 80 °C for 8 h in an oven. Then the dried precipitate was put into

the tube furnace, heated to 600 °C at a heating rate of 4 °C/min and kept for 4h. The

resulting product was ground into the power in an agate mortar. Subsequently, the

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product was dispersed in deionized water and then an excess of HF was added to dissolve SiO2. Finally, the product was stirred for 8 h at room temperature,

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centrifuged at 9000 r/min and dried at 80 °C for 8 h in an oven to obtain the g-C3N4 nanoporous-tube. For comparison, other porous g-C3N4s were also prepared at temperature at 450 °C, 500 °C, 550 °C, respectively. The preparation process was the

different.

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same as the g-C3N4 nanoporous-tube except that the keeping temperature was

The mass ratios of SiO2 in the mixed system for SiO2 and melamine were controlled at 3.0%, 6.0%, 10.0%, 20.0%, 30.0%, 40.0% and 60.0%. For the sake of

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simplicity, the bulk g-C3N4 prepared at 550 °C is named as BCN and the tubular gC3N4 prepared at 600 °C with a heating rate of 4 °C/min as TCN. Besides, g-C3N4 nanoporous-tubes with different SiO2 contents were named as PTCN3, PTCN6, PTCN10, PTCN20, PTCN30, PTCN40 and PTCN60.

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2.2.4 Synthesis of PTCNs

The preparation of PTCNs is shown in Scheme 1. The PTCNs were prepared by

a simple template method via burning a blend of melamine with SiO2 in a crucible and

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then an excess of HF was added to remove SiO2.

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2.3. Material characterization

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Scheme 1. Schematic illustration of preparation of the PTCNs

The crystal structure of all samples was measured on X-ray diffraction (XRD, D

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/ MAX-2500, Japan) with a Cu Kα radiation source, voltage: 30 kV and current: 30 mA in the range of 10-70° (2θ) with steps of 10° min−1. The surface elemental compositions were further tested using X-ray photoelectron spectrometer with a Mg Ka anode (PHI1600 ESCA System, PERKIN ELMER, US). The morphology and microstructure of the prepared samples were performed by transmission electron

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microscopy (TEM, Philips Tecnai G2 F20) at 200 kV and scanning electron micrograph (SEM, JEOL-6700FESEM, Japan) by a field emission SEM (FE-SEM) instrument. Specific surface areas were tested by a nitrogen adsorption instrument

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(Quadrasorb SI-3MP) at 77 K after degassed at 300°C for 5 h. The pore size distributions were tested by the Barrett-Joyner-Halenda (BJH) method. Raman spectra were measured by a Thermo Scientific DXR Raman microscope at the room temperature with an excitation wavelength of 532 nm. The UV-vis diffuse reflectance

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spectra (DRS) of heterogeneous were recorded from a UV-vis spectrophotometer

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(UV2450, Shimadzu, Japan), using BaSO4 as reflectance. The Fourier transform infrared (FT-IR) spectra of samples were analyzed on a Bruker ALPHA FT-IR in the

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region of 500-4000 cm−1. Photoluminescence spectra (PL) of photocatalysts were recorded on a VARIAN CARY Eclipse Fluorescence Spectrometer at the room temperature with an excitation wavelength at λ = 310 nm.

Electrochemical measurement of the electrochemical impedance spectroscopy (EIS) was conducted on a computer controlled work station (Shanghai Chenhua CHI

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660D) in 1 M KOH at room temperature. It was conducted in the frequency range from 10 mHz to 100 kHz at open circuit potential with a potential amplitude of 5 mV.

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2.4. Photocatalytic activity test

The photocatalytic activities of the prepared samples were evaluated by photodegrading

of

RhB

under

visible

light

radiation

using

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light source of Hg lamp (450 W) by a cutoff filter (λ > 380 nm) to remove the UV light in the light reaction process. Briefly, 5 mg as-prepared photocatalysts was added to 20 ml, 70mg/L RhB solution to form a suspension. Before illumination, the resulting final samples were ultrasound for several minutes, then the mixture was

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stirred in the dark for 30 minutes to form the balance of adsorption/desorption between the photocatalysts and RhB. In the light reaction process, the suspension should also constant vigorous stirring. The reaction temperature was maintained at room temperature to prevent some thermal catalysis [1]. Every half an hour, a UV-vis

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spectrophotometer was used to measure the adsorption diagram of the solution. The concentration of RhB was obtained by measuring the absorption intensity at its maximum absorption wavelength (554 nm). The degradation rate was calculated by

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the following equation: Degradation rate= (C0 – Ct) / C0 × 100%

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= (A0 – At) / A0 ×100% Where, C0 and Ct are the concentrations of RhB before and after reaction,

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respectively. A0 and At present the absorbance intensities of RhB aqueous solution before illumination and after degradation for t minutes, respectively.

In order to determine the stability of the as-prepared photocatalyst, cyclic experiment was carried out. Briefly, the used PTCN20 photocatalyst (5 mg) was

removed by centrifugation, rinsed with water, dried in a vacuum oven and then

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subjected to another cycle of RhB photodegradation.

To explore the active species of photocatalytic degradation of RhB, a series of

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active species capture tests were carried out. The trapping test was similar to those of the photocatalytic process except that the quenching chemical was added to the RhB solution. In the trapping tests, 1,4-benzoquinone (BQ, 2 mM), Na2C2O4 (2 mM) and

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isopropanol (IPA, 2 mM) were used as the scavengers of O2•−, h+ and •OH to investigate the active species in photocatalytic experiments, respectively. 3. Results and discussion

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3.1. Materials and characterization

Figure S1 shows the XRD patterns of PTCN, BCN and TCN. It can be seen that the characteristic peaks of all the samples are concentrated at 13.1° and 27.2°. This shows that adding SiO2 into the melamine for forming a nanoporous-tube structure

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does not change the crystalline structure of g-C3N4. The strong peaks at 27.2° is corresponding to the (002) plane, which is associated with interlayer stacking of the conjugated aromatic with a distance of d = 0.323 nm [49]. The weak peaks at 13.1° is

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corresponding to the (100) plane, which is attributed to the in-plane repeated units with an inter planar distance of d = 0.645 nm [50].

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PTCN

TCN 4000 3500 3000 2500 2000 1500 1000 Wavenumber (nm)

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Intensity (a.u.)

BCN

500

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Fig. 1. FTIR spectra of BCN, TCN, PTCN

In order to demonstrate the molecular structure of BCN, TCN and PTCN, all the photocatalysts were characterized by FT-IR spectroscopy. From the spectra of Fig. 1,

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it can be obviously observed that there is no obvious difference in these spectra. They all have peaks at 800 cm−1 (the vibration of triazine) [51]. The broad band of 3000-

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3500 cm−1 is the characteristic absorption of the non-condensed amino group and the water molecules on the catalyst surface [52]. Other absorption peaks include those at 1630 cm−1 (C=N), 1541 cm−1 (C=N), 1450 cm−1 (C-N), 1398 cm−1 (C-N), 1313 cm−1 (C-N), 1235 cm−1 (C-N), 1206 cm−1 (C-N) [53].

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Fig. 2 shows the Raman spectroscopy of the PTCN. The broad peaks at 1000– 2000 cm−1 are ascribed to the vibrations of various N bending modes. The characteristic peaks at 1453 cm−1 and 1582 cm−1 belong to the D and G bands [54]. Others characteristic peaks are the typical vibration modes of CN heterocycles that are

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consistent with other reports [54, 55].

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1453

1629 1695 1582

Intensity (a.u.)

1295 1254 1149

1000

1200

1400

1600

1800

Raman Shift (cm-1)

2000

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Fig. 2. Raman spectra of PTCN

The typical SEM images of BCN, TCN and PTCN and TEM images of SiO2, BCN and PTCN are presented in Fig. 3, respectively. Fig. 3a shows the SiO2

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nanoparticles with diameters about 50-70 nm. As shown in Fig. 3b and c, the BCN has obvious layered structure, indicating that BCN is composed of g-C3N4 sheets. The

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SEM image of TCN in Figure 3d shows a tubular structure owing to the scrolling of the g-C3N4 sheets. PTCN in Fig. 3e also shows a tubular structure like TCN. The PTCN tubes with a diameter of 200-400 nm are orderly arranged [54]. Fig. 3f is the TEM image of PTCN. It shows porous structure and the pore size is almost equal to the size of SiO2 nanoparticles. This indicates that the template SiO2 nanoparticles have

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been removed and left pores in g-C3N4 tubes. So, g-C3N4 nanoporous-tubes could be prepared by using SiO2 nanoparticles as templates. Moreover, Fig. 3g displays highangle annular darkfield (HAADF) image of the PTCN and the corresponding EDS elemental mappings (Fig 3h, i and j) demonstrate that C atoms, N atoms and O atoms

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are uniformly distributed in the PTCN. Further, Fig. 3k shows the SAED pattern of PTCN, and the diffraction ring can be indexed as (002) diffraction (d = 0.323 nm) [49].

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And one possible reason for the formation of a tubular structure may be the

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release of NH3 from the pyrolysis of melamine. The produced NH3 passes vertically through the medium-density melamine layers to form a slightly curled g-C3N4

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nanosheet. Therefore, with the help of NH3, the slightly curled g-C3N4 nanosheets assemble to tubes so that the total surface of free energy is minimized [54, 56].

What is more, the Fig. S2 (a, b and c) present the SEM images of PCN450, PCN500 and PCN550, respectively. They are not tubes. Their preparation conditions

are almost the same as those of PTCN (600 °C) and the only difference is the keeping

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temperature. It indicates that keeping temperature is a key factor which decides the shape of g-C3N4. And the TEM images of PCN450, PCN500 and PCN550 are shown

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in Fig. S2 (d, e and f), which can present the corresponding pore structure.

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Fig. 3. SEM images of: BCN (b), TCN (d), PTCN (e), TEM images of: SiO2 (a) BCN (c), PTCN (f). High angle annular dark field (HAADF) STEM image of PTCN (g), and the corresponding EDS elemental mappings of: C (h), N (i) and O (j) and SAED pattern of PTCN (k) . The optical properties of BCN and PTCN were studied with UV–vis DRS (diffuse reflectance spectrum) analyses. As shown in Fig. S3a, they are all photoresponsive to visible light regions with an absorb edge at about 450 nm, which is consistent with previous reports [2]. And it is clear that absorption intensities of

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PTCN are relatively stronger than those of BCN in both visible and ultraviolet ranges [3]. This may be due to the nano-size effect of nanoporous and tubular structures [53]. In addition, the band gap energy (Eg) of the semiconductor materials can be

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calculated by the following formula: ahv = A (hv − Eg) n/2

Where a, hv, A and Eg represent the absorption coefficient, Planck constant, a

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Page 13 of 34

constant and band gap energy, respectively [49]. According to the UV-vis data in Fig. S3b, BCN and PTCN have band gaps about 2.64 eV and 2.63 eV respectively, which

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is consistent with previous reports [57]. It indicates that the porous-tube structure does not change the band gap of g-C3N4.

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The specific surface area and pore size distributions of the prepared samples

were measured by nitrogen adsorption desorption isotherm analysis. As shown in Fig. 4a, all of the as-prepared materials exhibit type IV isotherms with H3 hysteresis

loops showing the mesoporosity of the samples regardless of their differences in

structure [2]. The Brunauer–Emmett–Teller (BET) surface areas of BCN, TCN,

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TCN+SiO2 and PTCN20 are 8.5, 16.2, 42.7 and 107.4 m2/g, respectively. It can be

seen that surface area of TCN is larger than the BCN because of the special structure

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of tubes with both exposed internal and external surfaces. Obviously, PTCN20 has the largest specific surface area, because PTCN20 takes advantage of both nanopores and tubes. The high surface area of PTCN20 will be beneficial to the adsorption of

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RhB molecules and offers more active sites in photocatalytic reactions. Fig. 4b reveals the BJH (Barrett–Joyner–Halenda) of pore-size distribution curves. They all exhibit a narrow peak centered at about 10 nm, which is ascribed to the release of NH3 from the pyrolysis of melamine act as soft-templates [2]. PTCN20 also has a broad pore size distribution at about 50-70 nm, which is consistent with the TEM

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result.

(a)

0.6

150

SBET= 8.5 m2 g-1

TCN

SBET= 16.2 m2 g-1

dv/dlog(D) Pore volume (cm3 g-1)

200

BCN

TCN+SiO2 SBET= 42.7 m2 g-1

PTCN20 SBET= 107.4 m2 g-1

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Volume adsorbed (cm3 g-1)

250

100

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50 0

0.0

0.2

0.4 0.6 0.8 Relative Pressure ( P/PO)

1.0

BCN TCN TCN+SiO2 PTCN20

0.5 0.4 0.3 0.2 0.1 0.0 0

20

40 60 80 Pore diameter (nm)

100

Fig. 4. N2 adsorption-desorption isotherms (a) and corresponding

Page 15 of 34

pore-size distribution curves (b) To investigate the chemical valence state, XPS analysis is performed. The XPS

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spectra in Fig. 5 revealed that both TCN and PTCN contain carbon, nitrogen as well

as a small amount of oxygen. Since the process of preparing g-C3N4 is carried out in an inert atmosphere, the presence of oxygen may be attributed to the absorption from

the atmosphere during the post-treatment process or sample preparation [58]. The C1s spectra in Fig. 5 a-b can be deconvoluted into two peaks. The peaks for TCN at

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284.8 eV and 288.2 eV (a) are slightly moved to 284.6 eV and 287.8 eV (b) for PTCN. These two peaks represent the presence of two different environment carbons

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in these samples. The peaks at 284.8 eV and 284.6 eV are identified as sp2-bonded carbon in the C-C bond, and the peaks at 287.6 eV and 288.2 eV are assigned to sp2bonded carbon in the N-C=N group [56]. The N1s spectra in Fig. 5 c-d can be

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divided to three peaks. Similarly, the peaks at 398.5 eV and 398.3 eV represent sp2hybridized nitrogen in triazine rings (C=N-C), the peaks at 400.6 eV and 400.3 eV can be attributed to the tertiary nitrogen N-(C)3 groups, and the minor peaks centered at 403.6 eV and 403.4 eV belong to the positive charge of localization in

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heterocyclic and charging effect, which is due to effect of π-excitations [59].

N-C=N

a

280 282 284 286 288 290 292 294 296 Binding Energy (ev)

N-C=N

C1s Intensity (a.u.)

C-C

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Intensity (a.u)

C1s

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b

C-C

280 282 284 286 288 290 292 294 296 Binding Energy (ev)

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C=N-C

Intensity (a.u.)

Intensity (a.u.)

N-(C)3

N-(C)3

π-exciations 394

396

398

400

402

404

406

d

N1s

c

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C=N-C

N1s

π−excitations

394

408

Binding Energy (ev)

396

398 400 402 404 Binding Energy (ev)

406

408

Fig. 5. XPS spectra of C1s of BCN (a) and PTCN (b); and N1s of BCN (c) and PTCN

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(d). 3.2. Photocatalytic activities

The photocatalytic activities of all the prepared samples were estimated by the

an

degradation organic dyes [60, 61] of RhB in visible light irradiation (λ> 420 nm). Before the irradiation, the mixture was stirred in the dark environment for 30 minutes

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to reach adsorption/desorption balance between the photocatalysts and RhB, and the RhB concentrations after adsorption/desorption equilibrium [62] were taken as the initial concentrations (Co). As shown in the Fig. 6a, in the absence of photocatalysts or visible light irradiation, the degradation of RhB was almost negligible, indicating that RhB was remarkably stable under experimental conditions [63]. The results in Fig.6b

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demonstrate that P25 (a benchmark photocatalyst) has the lowest photocatalytic activity compared with other prepared photocatalysts. RhB is degraded 40％ in the 120 minutes by BCN, which reveals that BCN has lower photocatalytic activity in the

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tested conditions. This may be due to the fast recombination of photogenerated electron-hole pairs and poor quantum yield, which is consistent with previous reports [64]. However, RhB are almost fully degraded within 120 minutes by other three

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Page 16 of 34

photocatalysts. The relatively high photocatalytic activity of TCN may be attributed to the

Page 17 of 34

special structure of tubes of reducing mass transfer resistance for photocatalysts [37], enhancing the ability of photodegradation of RhB. The photocatalytic activity of

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TCN+SiO2 is higher than that of TCN. This may be because the impurity energy level

of SiO2 could assist accelerating the separation and transfer of electron–hole pairs of

TCN [65]. Obviously, the PTCN has the enormously highest photocatalytic activity and it can photodegrade 90％ RhB in the 40 minutes, which is remarkably higher than the reported. For example, the holey few-layer g-C3N4 composited by Wang groups

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could photodegrade only 80％ RhB after 80 minutes [66]; the tubular g-C3N4 required 180 min for completely degradation of RhB [67]; the GO/Ag2CrO4/g-C3N4

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composites synthesized by Zeng groups could merely photodegrade 82.23％in the 60 min of irradiation [68]. The high photocatalytic activity may be due to the edge effect of pores which makes more easily attachment for the RhB pollutant, prevents further

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aggregation of particles [42]. Additionally, PTCN can reduce recombination of electron-holes pairs [43] and therefore effectively improve the photocatalytic activities.

The reaction kinetics of photodegradation RhB was investigated by fitting

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pseudo-first-order model and obtained the corresponding apparent rate constants (k). As shown in Fig. 6b, the k values of the BCN, TCN and TCN +SiO2 and PTCN are 0.0055 min−1, 0.01453 min−1 and 0.03032 min−1, 0.04491 min−1, respectively. And PTCN has the highest k value of 0.04491 min−1, which is abnormally higher than the

ce

reported articles. For instance, the 3D porous thermally exfoliated g-C3N4 with a k value of 0.0358 min−1 [52]. So ， their photocatalytic activities are in the order: PTCN > TCN+SiO2 > TCN.

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AUTHOR SUBMITTED MANUSCRIPT - NANO-114785.R1

As shown in Fig. 6c, with the increasing of SiO2 content, the photodegradation

AUTHOR SUBMITTED MANUSCRIPT - NANO-114785.R1

activity of PTCN enhances correspondingly, and the maximum degradation activity is achieved at PTCN20. Further increasing SiO2 content decreases the photocatalytic

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activity. The exact reason is not clear. As shown in Fig. 6d, the k values of PTCN3, PTCN6, PTCN10, PTCN20, PTCN30, PTCN40 and PTCN60 are 0.0264 min−1,

0.0273 min−1, 0.03987 min−1, 0.04491 min−1, 0.0411 min−1, 0.04089 min−1and 0.0405 min−1, respectively. The photocatalytic activity of the PTCN20 is the highest, which is

(b)

(a) 1.0

PTCN

0.04 0.8

0.03

Blank SiO2 P25 BCN TCN TCN+SiO2 PTCN

0.2 0.0

0.02

TCN+SiO2

an

0.4

k min-1

C/CO

0.6

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8.16 times of BCN, 3.09 times of TCN and 1.48 times of TCN +SiO2.

TCN

0.01

P25

BCN

0.00

(c)

20

40 60 80 100 Irradiation time / min

(d)

BCN PTCN3 PTCN6 PTCN10 PTCN20 PTCN30 PTCN40 PTCN60

1.0 0.8

0.4 0.2

0.03

pte 0

20

40 60 80 100 Irradiation time (min)

PTCN3

PTCN30 PTCN40 PTCN60

PTCN6

0.02

0.01

0.0

PTCN20 PTCN10

0.04

k min-1

0.6 C/Co

120

dM

0

BCN

0.00

120

ce

Fig.6. Photocatalytic degradation of RhB solution over the as-prepared samples (a) and (c), kinetic linear simulation of RhB photocatalytic degradation over the different photocatalysts (b) and (d).

For comparison, the photocatalytic degradation of RhB by other porous g-C3N4

is shown in Fig. 7. The photocatalytic activity of PTCN is the highest, indicating that

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Page 18 of 34

the tubular structure has a great effect on the photocatalytic efficiency.

Page 19 of 34

1.0

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0.8

C/CO

0.6 0.4 BCN PCN450 PCN500 PCN550 PTCN

0.2 0.0 0

20 40 60 Irradiation time (min)

80

100

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Fig. 7. Photocatalytic activities of BCN, PCN450, PCN500, PCN550 and PTCN 1.0

PH=2 PH=4 PH=6 PH=8 PH=10 PH=12

0.8

0.4 0.2 0.0 0

20

an

C/Co

0.6

40 60 80 100 Irradiation time / min

120

dM

Fig. 8. The effect of solution pH on the degradation of RhB Besides, in order to study the effect of pH on the degradation efficiency of RhB, a series of experiments in different pH were conducted. The results are shown in the Fig. 8. The degradation efficiency was reduced with the increase of pH. Obviously,

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the acidic condition is more suitable for the photodegradation than alkaline condition. The reasons can be explained below. In the photodegradation process, there is interaction between the–N= in PTCN and –COOH in RhB (The PTCN and RhB molecular structures can be seen in Fig. S4) [69], resulting in adsorption of RhB

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adsorbed on g-C3N4 and growing of the degradation efficiency. At high pH, –COOH turns into –COO–, resulting in a significant reduction in the amount of adsorption during degradation and slowing down of the photodegradation rate [70].

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3.3. Reactive species trapping test

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In order to investigate the active species of photocatalytic degradation of RhB, a series of active species capture tests were carried out. 1,4-benzoquinone (BQ, 2 mM),

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Na2C2O4 (2 mM) and isopropanol (IPA, 2 mM) were used as the scavengers of O2•−,

h+ and •OH to investigate the active species in photocatalytic experiments, respectively. As shown in Fig. 9, after the addition of IPA, the degradation activity of

PTCN20 is almost invariable, indicating that •OH does not have much impact on the photodegradation of RhB. However, BQ and Na2C2O4 have a significant effect on the

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photocatalytic activity of PTCN20, demonstrating that O2•− and h+ are the main active

an

species during the photodegradation process.

PTCN20

80 60

dM

Degradation Percentage %

100

40 20 0

No scavenger

IPA

Na2C2O4

BQ

pte

Fig. 9. Photocatalytic activities of the PTCN20 for the degradation of RhB solution in the presence of different scavengers under visible-light irradiation 3.4 Possible photocatalytic mechanism

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Based on the above results, a possible chemical mechanism for the degradation of RhB for PTCN is proposed below, and meanwhile a possible photocatalytic mechanism is shown in Scheme 2.

PTCN + hv → PTCN (h+ + e−)

(1)

O2+ e−→O2•−

(2)

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Page 20 of 34

Page 21 of 34

O2•− + RhB → degradation product

(3)

h+ + RhB → degradation product

(4)

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Under irradiation, PTCN produces h+ and e−. The dissolved oxygen in the solution acts as an electron scavenger to capture the photo-generated electrons and produce

active species O2•− and •OH [71]. The previous studies have demonstrated that the

O2•− and h+ are the main reactive species that photocatalytically degrade RhB molecules, while the •OH can be negligible. PTCN possesses a nanoporous and

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special tubular structure, which means higher specific surface area and more reactive

dM

an

sites which are very beneficial for photodegradation of RhB.

pte

Scheme 2. The possible photocatalytic mechanism in PTCN

3.4. Fluorescence analysis and electrochemical impedance spectra (EIS) Fluorescence test is a good method for analyzing the separation efficiency of photogenerated electron-holes for photocatalysts. As can be seen from the Fig. 10a, all

ce

the emission peaks for the photocatalysts are concentrated at about 460 nm that determined by the band gap energy of g-C3N4 (about 2.7 eV). BCN has the strongest photoluminescence, and PTCN has the weakest photoluminescence. It indicates that

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AUTHOR SUBMITTED MANUSCRIPT - NANO-114785.R1

photogenerated electron-holes recombination efficiency of PTCN is the lowest. Therefore, PTCN has the highest photochemical degradation activity. The

AUTHOR SUBMITTED MANUSCRIPT - NANO-114785.R1

fluorescence efficiency of TCN+SiO2 is also relatively low, which may be due to the impurity energy level of the SiO2 can increase the transfer of photogenerated electrons

cri pt

[65], but this effect is weaker than the porous edge effect. Furthermore, the same

result can be proved by the electrochemical impedance spectra (EIS). The resulting Nyquist plots are shown in Fig. 10b. The Nyquist plots clearly show that BCN has

the bigger radius of curvature between the BCN and PTCN, indicating a higher charge transfer resistance [65]. And the PTCN has much smaller radius of curvature on the

us

EIS Nyquist plot, which implies that it exhibits a decreased interfacial charge transfer resistance, indicating a remarkably enhanced activity in separation of photogenerated

an

electron–hole pairs [39].

(b)

400

450

500

550

30

BCN PTCN

25 20

dM

Intensity (a.u)

PTCN BCN TCN TCN+SiO2

600

Wavelength (nm)

-Z,,/ohm

(a)

15 10 5 0 -5

0

2

4

6

8 10 12 14 16 18 Z,/ ohm

pte

Fig. 10. Photoluminescence spectra of PTCN, BCN, TCN and TCN+SiO2(a); EIS Nyquist plots of BCN, TCN and PTCN photocatalysts under irradiation of visible light (λ > 420 nm) (KOH = 1 M) (b).

3.5. Cyclic stability

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The capability of reuse is an important indicator with the catalytic ability for a

photocatalyst, so repeated experiments of PTCN20 were conducted and the results are shown in Fig. 11. It can be seen the PTCN20 does not exhibit a significant loss of

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Page 22 of 34

photocatalytic activity in the four recycles. So, the PTCN20 showed remarkable stability and reproducibility after four cycles.

Page 23 of 34

1.0 2nd run

C/CO

3rd run

4th run

0.6 0.4 0.2 0.0 0

50

100

150 200 250 Time (min)

300

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1st run 0.8

350

irradiation

an

4. Conclusion

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Fig. 11. Cycle runs of the PTCN20 for the degradation of RhB under visible-light

In this paper, a series of g-C3N4 nanoporous-tube (PTCN) photocatalysts were prepared by a facile and convenient method. The as-prepared PTCNs exhibit

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considerably higher photocatalytic activities than BCN with respect to RhB degradation under visible light irradiation. When the content of SiO2 in the precursor is 20%, the photocatalytic activity of the PTCNs increases up to the maximum, which is 8.16 times higher than that of BCN for RhB degradation. The PTCNs also exhibit

pte

considerable stability toward RhB degradation. The remarkable photocatalytic performance is attributed to their unique tubular structure and the edge effect of the pores. The results demonstrate that the structural innovation is a promising technology for increasing photocatalytic activity of g-C3N4 photocatalysts.

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The method for preparing the nanoporous-tube structure is simple and

inexpensive and does not introduce other impurities, so it has wide applicability. Metal and/or non-metallic nanomaterials can be loaded on to PTCN to fabricate

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AUTHOR SUBMITTED MANUSCRIPT - NANO-114785.R1

various functional materials such as catalytic materials for applications in hydrogen generation, adsorption materials for heavy metal ions and organic pollutants, and

AUTHOR SUBMITTED MANUSCRIPT - NANO-114785.R1

biomaterials for drug delivery. The PTCN can be further modified for the use of air purification, gas storage and solar energy, etc.

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Acknowledgments

This work was supported by the National Science Foundation of China (51573126).

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