Phosphorus-Doped Graphitic Carbon Nitride Nanotubes with Amino

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Phosphorus-doped graphitic carbon nitride nanotubes with amino-rich surface for efficient CO2 capture, enhanced photocatalytic activity and product selectivity Bing Liu, Liqun Ye, Ran Wang, Jingfeng Yang, Yuexing Zhang, Rong Guan, Lihong Tian, and Xiaobo Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17503 • Publication Date (Web): 26 Dec 2017 Downloaded from http://pubs.acs.org on December 28, 2017

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ACS Applied Materials & Interfaces

Phosphorus-doped

graphitic

carbon

nitride

nanotubes with amino-rich surface for efficient CO2 capture,

enhanced

photocatalytic

activity

and

product selectivity Bing Liu†, Liqun Ye‡, Ran Wang†, Jingfeng Yang†, Yuexing Zhang†, Rong Guan†, Lihong Tian†*, Xiaobo Chen§* †

Hubei Collaborative Innovation Center for Advanced Organochemical Materials, Ministry-of-

Education Key Laboratory for the Synthesis and Applications of Organic Functional Molecules, Hubei University, Wuhan 430062, P.R. China. ‡

Engineering Technology Research Center of Henan Province for Solar Catalysis, Collaborative

Innovation Center of Water Security for Water Source Region of Mid-line of South-to-North Diversion Project of Henan Province, College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Nanyang 473061, P. R. China. §

Department of Chemistry, University of Missouri – Kansas City, Kansas City, MO 64110,

USA.

ABSTRACT: Phosphorus-doped graphitic carbon nitrides (P-g-C3N4) have recently emerged as promising visible-light photocatalysts for both hydrogen generation and clean environment applications, due to a fast charge carriers transfer and an increased light absorption. However,

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their photocatalytic performances on CO2 reduction gain little attention. In this work, phosphorus-doped g-C3N4 nanotubes are synthesized through one-step thermal reaction of melamine and sodium hypophosphite monohydrate (NaH2PO2⋅H2O). The phosphine gas from the thermal-decomposition of NaH2PO2⋅H2O induces the formation of P-g-C3N4 nanotubes from gC3N4 nanosheets, leads to an enlarged BET surface area and a unique mesoporous structure, and creates an amino-rich surface. The interstitial doping phosphorus also down-shifts the conduction and valence band positions and narrows the bandgap of g-C3N4. The photocatalytic activities are dramatically enhanced in reduction of both CO2 to produce CO and CH4, and also water to produce H2, due to the efficient suppression of the recombination of electrons and holes. The CO2 adsorption capacity is improved to 3.14 times and the production of CO and CH4 from CO2 increases to 3.10 and 13.92 times that on g-C3N4 respectively. The total evolution ratio of CO/CH4 dramatically decreases to 1.30 from 6.02 for g-C3N4, indicating a higher selectivity of CH4 product on P-g-C3N4, which is likely ascribed to the unique nanotubes structure and aminorich surface.

KEYWORDS: Phosphorus-doped, carbon nitride, CO2 reduction, photocatalytic, amino-rich surface 1. INTRODUCTION The massive utilization of non-renewable fossil fuels to advance industrial developments and mankind’s living requirements has unfortunately caused increasing energy crisis and global warming problems from carbon dioxide (CO2) emission.1 CO2 is the so-called greenhouse gas in atmosphere that causes the global climate change as believed by many scientists. Therefore, it is highly desirable to reduce the CO2 concentration in the atmosphere by either sequestrating CO2


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through adsorption, transferring CO2 to other sustainable energies, or introducing new clean energy sources.2,3 Inspired by the natural photosynthesis in plants that converts CO2 and H2O to carbonhydrate and O2 using sunlight energy, artificial photosynthesis which photocatalytically converts CO2 into to reusable hydrocarbon fuels has attracted world-wide and intensive interests.4,5 However, the photoconversion or photo-reduction of CO2 is a complicated multielectrons process and various products, such as HCOOH, CO, HCHO, CH3OH and CH4, are obtained in sequence by multiple-electrons (two, four, six or eight) steps, respectively. The photocatalytic conversion activity and the product selectivity are usually very low. Therefore, it is very necessary to improve our understanding in photocatalysis in order to design photocatalysts with excellent activity and selectivity. Graphitic carbon nitride (g-C3N4) is a polymeric semiconductor consisting of earth-abundant carbon and nitrogen elements, and has a moderate bandgap of around 2.7 eV, lying nicely in the visible-light region of the solar spectrum. Since the innovative report on its photocatalytic activity on H2 production in 2009 by Wang’s group,6 tremendous efforts including doping metal / non-metal elements, tailoring the nanostructure and surface properties, and compositing with other semiconductors have been devoted to develop efficient g-C3N4 based photocatlysts for H2 evolution,6-10 photocatalytic pollution removal,11-13 and photocatalytic CO2 reduction.14 In order to improve the photocatalytic activity of g-C3N4 on CO2 reduction, most researches focused on the formation of composite catalysts to hinder the recombination of photo-generated electrons and holes, such as with inorganic semiconductors (NaNbO3,15 Ag3PO4,16 BiOI17), carbon materials,18-20

ruthenium catalysts,21 or metal-organic frameworks and porous materials to

promote the gas adsorption on catalysts.22-24 However, for the complicated CO2 reduction with multi-electrons process, developing g-C3N4 based photocatalytic materials with unique structures


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and surface properties are still imperative for efficient CO2 reduction and high product selectivity in terms of thermodynamics and kinetics. All phosphorus-doped graphitic carbon nitrides, synthesized with organophosphorus or inorganic H3PO4 being precursors, have shown enhanced electrical conductivity and high photoreactivity on reducing water to H2 or degrading environmental contaminants.25-29 Nonetheless, there are less researches of the photocatalytic CO2 reduction reaction on Pg-C3N4. Herein, we report a one-step, simple and large-scaled synthesis of P-g-C3N4 nanotubes by heating a mixture of sodium hypophosphite monohydrate and melamine at elevated temperatures. In this process, phosphine gas from the thermal decomposition of NaH2PO2⋅H2O induces the morphological transformation from two-dimensional (2D) nanosheets to onedimensional (1D) nanotubes, provides an increased BET surface area and mesoporous structures, and creates an amino-rich surface, as illustrated in Scheme 1. Phosphorus is interstitially doped into g-C3N4 in situ during its formation from melamine copolymerization30 and narrows the band gap, enhances the optical absorption in visible light and promotes the separation of charge carriers. The P-g-C3N4 nanotubes display improved photocatalytic activities on both CO2 reduction and H2 evolution.

2. RESULTS AND DISCUSSION 2.1 Morphological structure. The morphology and microstructure of the g-C3N4 and phosphorus-doped carbon nitride (P-g-C3N4) are displayed in Fig. 1. As can be seen, the g-C3N4 consists of large (about 500 nm in diameter) and stacked sheets decorated with some irregular small nanoparticles (Fig. 1A). However, P-g-C3N4 shows a completely different morphology: some short, multilayered nanotubes are seen with a diameter of 200 nm and a thickness of 20-50


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nm (Fig. 1B), likely from the rolling up of ultra-thin layers to decrease the surface energy (Fig. S1). The P-g-C3N4 is mainly composed of three elements: C, N and P, based on the EDS analysis and element mappings of selected area (Fig. S2). The P content of P-g-C3N4 is 0.02wt%. From transmission electron microscope (TEM) image shown in Fig. 1C, compactly aligned nanotubes are observed along with some small nanoparticles on their surface. The clear lattice fringes show a good crystallinity of the P-g-C3N4 nanotubes (Fig. 1D). 2.2 XRD patterns and chemical interactions. The X-ray diffraction (XRD) patterns of gC3N4 and P-g-C3N4 are assigned to the hexagonal carbon nitride (JCPDS card no. 87-1526).15 The low-angle peak at 13.1° of g-C3N4 shown in Fig. 2A is from the (100) in-plane tri-s-triazine packing motif.31 After doping, its intensity decreases and its position shifts to 13.6°, indicating phosphorus doping likely increases the disorder in the packing of tri-s-triazine motifs in P-gC3N4.31 The strong diffraction peak at 27.4° corresponds to the characteristic (002) interlayerstacking reflection of conjugated aromatic systems.31 After doping, its intensity decreases and position shifts to 28.2°. This suggests a decrease of crystallinity and a shrink of the (002) interlayer distance (from 0.330 to 0.321 nm). The decreased (002) interlayer distance in P-gC3N4 is different from those reports with organophosphorus compounds as phosphorus source.21, 22

The elements analysis (Table S1) shows the C/N molar ratios of both g-C3N4 and P-g-C3N4 are

about 0.65, slightly lower than the theoretic value of 0.75 in g-C3N4. This indicates both are nitrogen enriched. The surface composition was analyzed with the X-ray photoelectron spectroscopy (XPS). The survey spectra (Fig. S3, supporting information) indicate the existence of O, C, N, and P in P-gC3N4. The core-level XPS spectra of all elements are shown in Fig. 2B-D and Fig. S4. In C 1s core-level spectra (Fig. 2B), the peak near 284.6 eV is assigned to the graphitic carbon (C-C /


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C=C), and the peak near 287.8 eV is from the sp2-hybridized carbon in the CN heterocycles (NC=N).32 The N 1s spectra are decomposed into three different peaks (Fig. 2C). The strong peak near 398.4 eV is ascribed to sp2-hybridized nitrogen in triazine rings (C-N=C), the one at 399.1 eV derives from the tertiary nitrogen (e.g. N-(C)3),26, 27 and the small peak at 400.9 eV is from the surface amino groups (C-N- H).32,33 After the doping, the peak area ratio of C-N=C: N(C)3:C-N-H changes from 1: 0.413: 0.192 (g-C3N4) to 1: 0.305: 0.268 (P-g-C3N4). The decrease of N-(C)3 and the increase of C-N-H indicate that less amino groups are converted into tertiary nitrogen, or in other words, there are less copolymerization and more surface amino groups after doping. Meanwhile, the binding energy of tertiary N decreases from 399.3 to 399.1 eV and that of C-N-H groups changes from 400.7 to 400.5 eV after doping. In Fig. 2D, the P 2p peak centered at 133.5 eV corresponds to the P with high oxidation state (+5).34 This together with the same C / N ratio for g-C3N4 and P-g-C3N4 (elements analysis) suggests that phosphorus is interstitially doping into the carbon nitride likely in the form of PO43from instead of substituting C and / or N atoms in CN heterocycles. The down-shifts of 0.2 eV for N-(C)3 and C-N-H groups in P-g-C3N4 are likely ascribed to the strong interaction of N atoms with the interstitially doped PO43-. In Fig. S4, two peaks at 531.9 eV and 533.4 eV in the O1s XPS spectrum of g-C3N4 may be due to surface absorbed oxygen and the surface hydroxyl group.35 The stronger O1s peaks (532.1eV) in P-g-C3N4 may verify the interstitial doped PO43- in P-g-C3N4, and the larger peak at 533.6 eV indicates more hydroxyl groups. The 0.2 eV shift indicates the interaction of the doped PO43- and g-C3N4. 31

P NMR spectrum of P-g-C3N4 in Fig.S5 (supporting information) mainly shows four well-

resolved signals between 10 and -20 ppm (85% H3PO4 in aqueous solution at d = 0 ppm). Those correspond to four different chemical environments of phosphate radicals in P-g-C3N4, which are


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probably attributed to the strong interaction of interstitially doped PO43- with different types of N-containing species (XPS results). Based on the above results, we propose a possible formation mechanism of P-g-C3N4 nanotubes. In the copolymerization of melamine, the phosphine gas from the thermaldecomposition of NaH2PO2⋅H2O36-38 plays an important role in controlling the morphology of Pg-C3N4. On the one hand, it weakens the Van der Waals’ force of interlayers of graphitic C3N4 and exfoliates the bulk g-C3N4 to ultra-thin layers, which curl into nanotubes in order to decrease the surface energy. On another hand, phosphorus, interstitially doped into the g-C3N4 likely in the form of PO43- , decreases the uniplanar copolymerization and results in more amino groups on Pg-C3N4 surface. 2.3 FT-IR, Raman and UV-Vis DRS spectra. The Fourier transform infrared (FT-IR) spectra are in Fig. 3A. The sharp peak at 810 cm-1 is from the breathing of tri-s-triazine heterocycles.39 The strong peaks at 1200-1650 cm-1 are from the stretching mode of CN heterocyclic units.39,40 These characteristic vibrations indicate phosphorus doping reserves the structure of g-C3N4. The board band centered at 3160 cm-1 is due to various stretching of N-H bond.39, 40 The band at 1564 cm-1 represents the N-H stretching. The increase of its intensity after doping indicates more surface amino groups, consistent with the XPS results. A larger luminescent background in the Raman spectra is observed under 800 cm-1 for P-gC3N4 than that for g-C3N4 (Fig. S6), indicating more disorders in P-g-C3N4. The fitted curves in Fig. 3B show two peaks at 1300 cm-1 (the D band) and 1580 cm-1 (the G band). The D band comes from the surface defect and disorder on the layer basal plane and the G band is from the first-order scattering of the E2g mode.41 The intensity ratio of ID / IG, in general, is used to


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estimate the surface disorder or average size of the sp2-C domains in carbon materials.42 The higher ID / IG ratio of P-g-C3N4 (0.97) over g-C3N4 (0.89) suggests that phosphorus doping creates more defects and disorders, likely due to the decrease of tri-s-triazine units and less copolymerization. The pristine g-C3N4 mainly absorbs light with the wavelength low than 450 nm in Fig. 3C. Pg-C3N4 shows the enhanced absorption in 400-550 nm, along with an apparent color change from pale yellow (g-C3N4) to orange yellow (P-g-C3N4). Phosphorus doping in g-C3N4 results in a redshift of band absorption edge from 445 to 463 nm with a tail long in the visible light region.43 According to the Kubelka–Munk function, (αhv)1/2 - hv, the optical band gap is 2.70 and 2.58 eV for g-C3N4 and P-g-C3N4 (Fig. S7, supporting information), respectively. Therefore, a narrowed band gap is observed after phosphorus doping. Nitrogen adsorption-desorption isotherms and the curves of pore-size distributions are shown in Fig. 3D. The isotherms correspond to the classical type IV curves with a H3 hysteresis loop at P/P0 = 0.4, indicating the presence of mesoporous framework and macropores in both g-C3N4 and P-g-C3N4.44 A small hysteresis loop of g-C3N4 comes from the aggregation of nanosheets. Comparatively, a large hysteresis loop is observed in the desorption part of P-g-C3N4, contributed to the presence of slit-like structures formed by the stacking of P-g-C3N4 nanotubes. The inset in Fig. 3D shows P-g-C3N4 has mesopores below 25 nm, but g-C3N4 does not show obvious mesopores. The P-g-C3N4 has a much larger SBET of 13.38 m2/g than g-C3N4 (3.02 m2/g). This is likely due to the formation of the nanotubes and more disorders after the phosphorus doping.


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2.4 Photocatalytic activity. The photocatalytic activity was evaluated by photo-driven CO2 reduction to solar fuels and hydrogen evolution. As displayed in Fig. 4A & 4B, the generation of both intermediate CO and end product CH4 increased linearly with irradiation time on P-g-C3N4. The gas yield reaches 9.48 and 7.24 µmol⋅g-1 for CO and CH4, respectively, 3.12 and 13.9 times of those on g- C3N4 (3.03 and 0.52 µmol⋅g-1 for CO and CH4) under 4 h visible-light irradiation. The photocatalytic activity is much better than that of previously reported g-C3N4-Pt nanocomposite,45,

46

AgX-g-C3N4 (X=Cl and Br),47 amine-functionalized g-C3N4,48 and

protonated g-C3N4 / carbon dots.19 Moreover, it can be seen that the total evolution ratio of CO / CH4 is 6.02 for g-C3N4, but that on P-g-C3N4 decreases to 1.30 in Fig. 4C. This indicates that more CH4 is produced after the interstitial phosphorus doping. P-g-C3N4 also exhibits a better photocatalytic performance on hydrogen evolution as shown in Fig. 4D. The average hydrogen evolution rate on P-g-C3N4 is 303.97 µmol. g-1. h-1, and 2.69 times of that on g-C3N4 (113.08 µmol. g-1. h-1). 2.5 Band structure and photocatalytic mechanism. The photocatalytic activity depends not only on the optical absorption and the surface area, but also on the band structure, charge transfer, and the adsorption of substrates, e.g. H+ and CO2. In order to understand the underlying mechanism for the enhanced activity of P-g-C3N4, we investigate the valence band (VB) position by XPS technique. The VB band edge is determined to be 1.68 and 1.63 eV for P-g-C3N4 and gC3N4 as shown in Fig. 5A. The work function of XPS instrument used is about 4.62 eV, and the VB potential for P-g-C3N4 and g-C3N4 is calculated to be 1.80 and 1.75 V against standard hydrogen electrode (SHE, given as 4.5 eV for 0 V), respectively. Combined the optical band gap of P-g-C3N4 and g-C3N4 in UV-Vis results and the formula Eg = EVB – ECB, the calculated


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conduction band (CB) potential was -0.90 and -1.07 V for P-g-C3N4 and g-C3N4, respectively. This is accordance to the measurement result of flat potentials in the Mott-Schottky curves of gC3N4 and P-g-C3N4 (Fig. 5B). Thus, thermodynamically, the conduction potential of both g-C3N4 and P-g-C3N4 meets the reduction of H+ to H2 and CO2 to CO and CH4, according to the Fig. 5C. In addition, the photoluminescence (PL) emission, chopped photocurrent response and electrochemical impedance spectra (EIS) spectra, are employed to study the charge transfer on Pg-C3N4 and g-C3N4.49 In Fig 5D, a strong 445 nm emission peak is from the electron-hole recombination. After phosphorus doping, the PL intensity decreased sharply, indicating efficient quenching of the recombination of the electron-hole pairs. This is likely ascribed to the increased dispersion of the contour distributions of HOMO and LUMO and the carrier mobility due to the phosphorus doping.50 For each on-off, the photocurrent in Fig. 5E show a stable photocurrent increases about 2 times after phosphorus doping. The produced photocurrent intensity of the semiconductor under irradiation depends on the excited electron-hole separation rate to the electrode from the semiconductor and the electron-hole recombination rate at the interface.51 Therefore, the strong photocurrent suggests that the interstitially doping of phosphorus in carbon nitrides lead to an efficient electron-hole separation on catalyst. EIS spectra in Fig. 5F provide information of excited charge transfer on the interfaces. The well-defined semicircles in mid-frequency region are attributed to the electrons transfer on the catalyst/electrolyte interface. The lifetime of electrons can be estimated by the formula τ = 1/ωmax, where ωmax is maximum angular frequency at middle frequencies of the impedance semicircle arc.52 Obviously, P-g-C3N4 has a smaller semicircle arc than g-C3N4, indicating a faster electron transfer on the surface. Therefore, phosphorus doping increases optical


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absorption, surface area, charge separation and charge transfer on the surface, achieving higher photocatalytic efficiencies in both CO2 reduction and H2 evolution. In the process of both CO2 reduction and hydrogen evolution, the prerequisite is protons produced from the photocatalytic water splitting and their adsorption on catalysts surface. The detected zeta potential of g-C3N4 and P-g-C3N4 is -30.9 mV and -50.2 mV in water at pH = 7. The more negative potential of P-g-C3N4 favors the adsorption of protons on P-g-C3N4 than on gC3N4. We also detected the CO2 adsorption capacities of P-g-C3N4 and g-C3N4 in Fig. 6. The CO2 uptake for P-g-C3N4 is 0.39 mmol / g and 3.14 times that of g-C3N4 (0.12 mmol / g) at P/P0 =1 and room temperature, respectively. Moreover, a large hysteresis loop occurs at desorption branch of P-g-C3N4. These indicate an obvious mesoporous structure and the much stronger hostguest interaction on P-g-C3N4 than that on g-C3N4, likely related to the abundant amino groups on the surface in capturing CO2 through acid-base interaction.53, 54 Upon above discussions, the enhanced activities of photocatalytic CO2 reduction and H2 evolution on P-g-C3N4 can be understood as follows. Photoexcited charges are produced upon visible-light irradiation. The electrons generated on the CB band with suitable potentials can reduce the protons to H2 or H+/CO2 to CO and CH4 according to Fig. 5C. Phosphorus doping in g-C3N4 brings in better light absorption, better charge separation and migration, larger adsorption capacity and larger surface area for improved activities. Especially, the more negative surface and surface amino-rich groups of P-g-C3N4 enhance the H+ and CO2 adsorption, and the unique nanotubular structure benefits the gas transfer, facilitating the selectivity of CH4 formation over CO.

3. CONCLUSIONS


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In a summary, interstitially phosphorus-doped g-C3N4 nanotubes have been successfully synthesized with one-step thermal reaction between sodium hypophosphite monohydrate and melamine. The phosphorus doping leads to the formation of nanotubes instead of micro-sheets, increases the surface area, and creates more amino groups on the surface with lower zeta potential for better CO2 adsorption due to the possible acid-base interaction. The optical absorption is enhanced due to the shift of the electronic band structure. The more disorders are created due to less polymerization in the graphitic framework and less ternary nitrogen in triazine rings. Photogenerated charge carriers (electrons & holes) are better separated and migrated to the photoreactions for CO2 and water reductions with much higher activities. Thus, phosphorus doping leads to many modifications on g-C3N4, ranging from morphology change, to better optical absorption and charge separation, enlarged surface area and adsorption capability, in realizing higher activities in both photocatalytic reductions of CO2 to CO and CH4, and H2O producing H2.


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Scheme 1. The proposed formation process of P-g-C3N4.

Fig. 1. FESEM image of g-C3N4 (A) and P-g-C3N4 (B), the TEM (C) and HRTEM (D) images of P-g-C3N4.


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(002)

A

B

Intensity (a.u.)

287.8 eV

Intensity (a.u.)

P-g-C3N4

(100)

10

20

30

40

50

60

P-g-C3N4

g-C3N4 282

70

284

288

290

292

C-N-H P-g-C3N4

133.5eV

Intensity (a.u.)

C-N=C

P 2p

D

N1s N-(C)3

286

Binding Energy (eV)

2 Theta (degree)

C

C 1s

284.6 eV

g-C3N4

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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g-C3N4 397

398

399

400

401

402

124

128

Binding Energy (eV)

132

136

140

144

Binding Energy (eV)

Fig. 2. (A) XRD patterns, and (B) C 1s, (C) N 1s, and (D) P 2p core-level XPS spectra for gC3N4 and P-g-C3N4.


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g-C3N4

A

B

G band D band

Intensity (a.u.)

Transmittance (%)

P-g-C3N4

P-g-C3N4

g-C3N4

N-H 500

1000

1500

2000

2500

3000

3500

800

4000

1000

1200

-1

Wavenumber (cm

1400

1600

Raman Shift (cm

)

1800 -1

2000

)

40

D

g-C3N4 P-g-C3N4

30

3

Volume adsorbed (cm / g, STP)

C Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


g-C3N4 P-g-C3N4

300

400

500

600

700

800

20 5

10

15

20

25

30

35

40

10

0 0.0

Wavelength (nm)

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/P0)

Fig. 3. FT-IR spectra (A), Raman spectra (B), UV-Vis DRS spectra (C) and nitrogen adsorptiondesorption isotherm curves (the inset is the distribution of average pore size ) (D) of g-C3N4 and P-g-C3N4.


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8

A

B

8

Yied of CH4 (µ mol/g)

Yeild of CO (µmol / g)

10

6

P-g-C3N4 g-C3N4

4

2

0

6

4

P-g-C3N4 g-C3N4 2

0

0

1

2

3

4

0

1

2

Time (h) 2.5

C

3

4

Time (h) 20

D

P-g-C3N4

2.37

g-C3N4

2.0

1.81

1.5

1.0 0.76 0.5

Produced H2 / µ µmol

Gas production rate (µmol g-1 h-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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16

12

P-g-C3N4 8

g-C3N4

4

0.13 0.0

0

CO

20

CH4

40

60

80

100

120

Time (min)

Fig. 4. Photocatalytic reduction activity for CO2 conversion to produce CO (A) and CH4 (B), the comparison of gas production rate (C) and hydrogen generation (D) over g-C3N4 and P-g-C3N4 under visible-light irradiation.


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7

A

Valence band

1.3 KHz 2.0 KHz 3.1 KHz

B

6

g-C3N4

-2

)

Intensity (a.u.)

5

C /(10

-12

F

4

-2

3

1.68 eV

-1.07 1

-0.92

g-C3N4

1.63 eV 0

P-g-C3N4

2

P-g-C3N4 2

4

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0 -1.2

8

-1.0

-0.8

-0.6

-0.4

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Potential/(V vs. NHE at pH=7)

Binding energy (eV)

Intensity (a.u.)

D g-C3N4 P-g-C3N4

400

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Wavelength (nm) 40

100

)

E

P-g-C3N4

on

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g-C3N4

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F

80

Z'' / ohm

Photocurrent (µA / cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

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Z' / ohm

Time (s)

Fig. 5. (A) XPS valence band spectra, (B) Mott-Schottky curves, (C) the electronic structure and all reaction reduction potentials of hydrogen evolution and CO2 conversion into CO and CH4,


17


(D) photoluminescence spectra with excited wavelength of 350 nm, (E) chopped photocurrents response (E), and (F) electrochemical impedance spectra of g-C3N4 and P-g-C3N4 12

10

CO2 volume (cc/g, STP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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8

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P-g-C3N4

4

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0.0

g-C3N4 0.2

0.4

0.6

0.8

1.0

Relative pressure (P / P0) Fig. 6. CO2 adsorption-desorption isotherms of g-C3N4 and P-g-C3N4. ASSOCIATED CONTENT Supporting Information The experimental section. The observed thin layer structure in the TEM graph of P-g-C3N4. The SEM image of P-g-C3N4 (A), and EDS analysis and element mapping of the selected area in figure A (B). The XPS survey spectra of g-C3N4 and P-g-C3N4. O1s XPS spectra of g-C3N4 and P-g-C3N4. Solid state 31P NMR spectrum of P-g-C3N4. Raman spectra of g-C3N4 and P-g-C3N4. Plots of (αhv)1/2 vs. energy of the exciting energy of g-C3N4 and P-g-C3N4. The elements analysis of the as-prepared samples.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (L. Tian); [email protected] Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

Acknowledgements L. Tian thanks the National Natural Science Foundation of China (no. 51302072). X. Chen appreciates the support from the U.S. National Science Foundation (DMR1609061), and the College of Arts and Sciences, University of Missouri−Kansas City. REFERENCES (1) Forkel, M.; Carvalhais, N.; Rödenbeck, C.; Keeling, R.; Heimann, M.; Thonicke, K.; Zaehle, S.; Reichstein, M., Enhanced seasonal CO2 exchange caused by amplified plant productivity in northern ecosystems. Science 2016, 351, 696-699. (2) Seneviratne, S. I.; Donat, M. G.; Pitman, A. J.; Knutti, R.; Wilby, R. L., Allowable CO2 emissions based on regional and impact-related climate targets. Nature 2016, 529, 477-483. (3) Varghese, O. K.; Paulose, M.; LaTempa, T. J.; Grimes, C. A., High-rate solar photocatalytic conversion of CO2 and water vapor to hydrocarbon fuels. Nano Lett. 2009, 9, 731-737.


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Table of content (TOC)

Interstitially phosphorus doped g-C3N4 nanotubes show higher activity and selectivity on photocatalytic reductions of CO2 to produce CO, CH4 for relieving our energy and environmental concerns.


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Phosphorus-Doped Graphitic Carbon Nitride Nanotubes with Amino

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