g-C3N4

Subscriber access provided by LUNDS UNIV

Letter

Sulfur-Mediated Self-Templating Synthesis of Tapered C-PAN/g-C3N4 Composite Nanotubes towards Efficient Photocatalytic H2 Evolution Fang He, Gang Chen, Jianwei Miao, Zhen Xing Wang, Dongmeng Su, Song Liu, Weizheng Cai, Liping Zhang, Sue Hao, and Bin Liu ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00398 • Publication Date (Web): 12 Oct 2016 Downloaded from http://pubs.acs.org on October 16, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Energy Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 18

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

ACS Energy Letters

Sulfur-Mediated Self-Templating Synthesis of Tapered C-PAN/g-C3N4 Composite Nanotubes towards Efficient Photocatalytic H2 Evolution Fang He,†,‡ Gang Chen,*,† Jianwei Miao,‡ Zhenxing Wang,†,§ Dongmeng Su,§ Song Liu,‡ Weizheng Cai,‡ Liping Zhang,‡ Sue Hao,† and Bin Liu*,‡ †

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and

Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, P.R. China. ‡

School of Chemical and Biomedical Engineering, Nanyang Technological University,

Singapore 637459, Singapore. §

Division of Chemistry and Biological Chemistry, Nanyang Technological University, Singapore

637371, Singapore. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (G. C.); *E-mail: [email protected] (B. L.)

ACS Paragon Plus Environment

1

ACS Energy Letters

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

Page 2 of 18

ABSTRACT: Hollow one-dimensional (1-D) nanostructures have drawn great attention in heterogeneous photocatalysis. Herein, we report that tapered polyacrylonitrile-derived carbon (C-PAN)/g-C3N4 composite nanotubes can be synthesized through a facile sulfur-mediated selftemplating method via thermal condensation of polyacrylonitrile (PAN), melamine and sulfur. The hollow tapered C-PAN/g-C3N4 composite nanotubes exhibit superior photocatalytic H2 evolution performance under visible light irradiation. The 5 wt% C-PAN/g-C3N4 composite nanotubes show 16.7 times higher photocatalytic H2 evolution rate than that of pure g-C3N4, which is even 4.7 times higher than that of 5 wt% C-PAN/g-C3N4 nanosheet composite obtained without sulfur. The hollow nanotubular composite structure provides g-C3N4 with higher specific surface area, enhanced light absorption and better charge carrier separation and transfer, which synergistically contribute to the superior photocatalytic activity. Our work provides a new strategy to develop carbon-based architected photocatalysts.

TOC GRAPHICS

Tapered

polyacrylonitrile-derived

carbon

(C-PAN)/g-C3N4

composite

nanotubes

were

synthesized through a facile sulfur-mediated self-templating method. The as-obtained C-PAN/g-


2

Page 3 of 18

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

ACS Energy Letters

C3N4 composites exhibit exceptional photocatalytic H2 evolution performance under visible light irradiation. Graphitic carbon nitride (g-C3N4) — a metal-free polymeric photocatalyst has attracted fascinating interest in photocatalysis owing to its proper electronic structure as well as high thermal and chemical stability.1-4 Unfortunately, bulk g-C3N4 often presents low surface area and fast charge recombination, which greatly restricts its photocatalytic activity.5-7 Over the past few years, numerous strategies have been adopted to optimizing the photoactivity of g-C3N4 including modulation of electronic structure, design of nanostructure, as well as construction of heterostructures.8-13 Doping14 or copolymerization15 is efficient to modulate the electronic structure of g-C3N4 and thus the photocatalytic performance. Designing nanostructures with various types of morphologies such as nanopores,16 nanosheets,17 nanorods,18 and nanospheres,19 can benefit mass transfer in catalysis, surface area and charge carrier separation. Construction of heterostructures by coupling with other semiconductors will greatly suppress the recombination of photoinduced charge carriers.20-22 Among which, design of hollow one-dimensional (1-D) nanostructures can offer a great advantage to improving both the specific surface area and charge carrier mobility.23-26 However, it is very challenging to fabricate hollow g-C3N4 nanostructures due to the fact that synthesis of g-C3N4 usually requires high temperature. Currently, hollow gC3N4 nanostructures can be synthesized by hard-templating method27, 28 or from self-assembled supermolecular precursors of cyanuric acid-melamine complex.29-31 But these methods are very cumbersome and suffer from poor structural and morphological control. Herein, we developed a novel sulfur-mediated self-templating method to synthesize tapered hollow

C-PAN/g-C3N4

composite

nanotubes

via

one-step

thermal

condensation

of

polyacrylonitrile (PAN), melamine and sulfur. Sulfur acted as an artificial template to guide the


3

ACS Energy Letters

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

Page 4 of 18

formation of 1-D composites, which was eventually evaporated and removed during the synthesis. The as-obtained C-PAN/g-C3N4 composites exhibit exceptional photocatalytic H2 evolution performance under visible light irradiation, among which, the 5 wt% C-PAN/g-C3N4 composite nanotubes show 16.7 times higher photocatalytic H2 evolution activity than that of pure g-C3N4. The significant enhancement can be attributed to the enlarged surface area, enhanced light absorption and greatly improved charge carrier separation and transfer, stem from the hollow nanotubular composite structure of C-PAN/g-C3N4. The crystal structure of as-synthesized C-PAN/g-C3N4 composite nanotubes was investigated by X-ray diffraction (XRD). Figure 1a compares the XRD patterns of g-C3N4 and C-PAN/gC3N4 composite nanotubes with different C-PAN contents. The XRD pattern of g-C3N4 exhibits two typical diffraction peaks at 13.0°and 27.4°, which can be indexed as the (100) peak of inplane structural packing motif and the (002) peak of interlayer stacking of aromatic segments, respectively.32, 33 Compared with g-C3N4, the C-PAN/g-C3N4 composite nanotubes also display two characteristic diffraction peaks of g-C3N4, indicating that the C-PAN/g-C3N4 composite nanotubes retain the original crystal structure of g-C3N4. The FT-IR spectra in Figure 1b demonstrate that all C-PAN/g-C3N4 composite nanotubes show similar FT-IR spectra with gC3N4, further verifying the formation of g-C3N4 phase in the C-PAN/g-C3N4 composite nanotubes. Figure 1c displays the Raman spectra of g-C3N4, C-PAN and 5 wt% C-PAN/g-C3N4 composite nanotubes. The characteristic peaks at 1362 and 1613 cm-1 of C-PAN can be assigned to the D- and G-bands, respectively of graphitized carbon,34 indicating that PAN has been successfully carbonized during the heating process. Both characteristic peaks from C-PAN and g-C3N4 appear in C-PAN/g-C3N4 composite nanotubes, suggesting the formation of the composite structure. Elemental analysis indicates that the C/N molar ratio of pure g-C3N4 and C-


4

Page 5 of 18

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

ACS Energy Letters

PAN/g-C3N4 composite nanotubes with different C-PAN contents is 0.689, 0.739, 0.937 and 1.106, respectively, further supporting the formation of C-PAN in the C-PAN/g-C3N4 composites. Although sulfur is critical in promoting the formation of C-PAN/g-C3N4 nanotubes, no signal of sulfur can be detected in the XPS spectrum (Figure 1d). Furthermore, both of the backbone C and N in C-PAN/g-C3N4 composite nanotubes contain sp2-hybridized carbon structure,35 suggesting the formation of the basic substructure units of g-C3N4 (Figure S1).

Figure 1. (a) XRD patterns and (b) FT-IR spectra of 1) g-C3N4, 2) 3 wt% C-PAN/g-C3N4, 3) 5 wt% C-PAN/g-C3N4 and 4) 10 wt% C-PAN/g-C3N4 composite nanotubes. (c) Raman spectra of 1) g-C3N4, 2) C-PAN and 3) 5 wt% C-PAN/g-C3N4 composite nanotubes. (d) XPS survey spectrum of 5 wt% C-PAN/g-C3N4 composite nanotubes. The microstructures of sulfur-mediated C-PAN/g-C3N4 composites were revealed by FESEM and TEM, which displays hollow tapered tubular nanostructure with some surface stripes (Figure 2a-d, Figure S2). The HRTEM image shows that the surface stripe in the external rim of the


5

ACS Energy Letters

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

Page 6 of 18

nanotube has lattice distance about 0.34 nm, which is the typical one between graphite layers, suggesting the existence of C-PAN; while no visible lattice spacing of g-C3N4 can be detected in C-PAN/g-C3N4 composite nanotubes due to the low crystallinity (Figure S3). Additionally, TEM mapping further confirms the absence of sulfur in the nanotubes (Figure S4).

Figure 2. Typical (a, b) FESEM and (c, d) TEM images of 5 wt% C-PAN/g-C3N4 composite nanotubes. Scale bar: 200 nm. To explore the formation mechanism of tapered C-PAN/g-C3N4 composite nanotubes, a series of control experiments were performed. If melamine or PAN was used as the only precursor, the as-synthesized product contains pure g-C3N4 or C-PAN, which displays large layered sheet (Figure S5a) or small flake structure (Figure S5b), respectively. Our previous study suggest that calcining the mixture of PAN and melamine produces large C-PAN/g-C3N4 composite nanosheets (Figure S5c).36 Addition of sulfur in the melamine precursor system facilitates the


6

Page 7 of 18

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

ACS Energy Letters

formation of hollow structures (Figure S5d).37 Whereas, 1-D nanotubes can only be produced by calcining melamine precursors together with both PAN and sulfur. To go a step further, we also investigated the morphological evolution of C-PAN and 5 wt% C-PAN/g-C3N4 composite obtained in the presence of sulfur as a function of calcination temperature. As displayed in Figure S6, C-PAN that was obtained at 400 °C in presence of sulfur during calcination displays rod-like structure, indicating that sulfur can act as an artificial template to guide the formation of 1-D nanostructures. Further increase in calcination temperature to 650 °C results in sulfur evaporation and formation of 1-D nanotubes. Similarly, the C-PAN/g-C3N4 composite synthesized in presence of sulfur at 400 °C also shows nanorod structure (Figure S7), with uniform distribution of C, N and S elements across the entire nanorods (Figure 3a to d). By slowly increasing the calcination temperature from 400 to 650 °C, the hollowing process could be captured as displayed in Figure 3e to h. The sulfur, which was mainly retained in the core of nanorods (Figure 3i to l), was slowly evaporated and released to the environment, leading to the formation of hollow nanotubes. Figure 3m summarizes the formation steps of tapered C-PAN/gC3N4 composite nanotubes. During heating, sulfur was first melt to form sulfur droplets. Meanwhile, PAN is carbonized (C-PAN) and then adsorb on the surface of sulfur droplets. The adsorbed C-PAN diffuses and penetrates into the sulfur droplets to form the 1-D sulfur/C-PAN solution. Further heating at higher temperatures condenses melamine, which grows on the 1-D sulfur/C-PAN solution to form the composite. The unsteady state dynamics of the sulfur evaporation eventually causes the tapering of nanotubes and formation of stripes on the outer nanotube surface.


7

ACS Energy Letters

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

Page 8 of 18

Figure 3. FESEM image (a), and (b-d) the corresponding elemental mapping images of C, N and S for 5 wt% C-PAN/g-C3N4 composite obtained at 400°C in presence of sulfur. TEM images of 5 wt% C-PAN/g-C3N4 composite obtained at (e) 400 °C, (f) 500 °C, (g) 550 °C, (h) 650 °C in presence of sulfur. HAADF-STEM image (i) and (j-l) the corresponding elemental mapping images of C, N and S for 5 wt% C-PAN/g-C3N4 composite obtained at 500°C in presence of sulfur. Scale bar: 200 nm. (m) Schematic illustration for the formation of C-PAN/g-C3N4 composite nanotubes.


8

Page 9 of 18

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

ACS Energy Letters

The specific surface area of C-PAN/g-C3N4 composite nanotubes with different C-PAN contents estimated from N2 adsorption-desorption isotherm (Figure S8) is 33.5, 37.8, and 39.8 m² g-1, respectively, which is much higher than that of pure g-C3N4 (21.6 m² g-1). And the specific surface area of C-PAN was determined to be 7.1 m2 g-1. The corresponding pore size and pore volume of C-PAN/g-C3N4 composite nanotubes were displayed in Table S1. Compared with pure g-C3N4, C-PAN/g-C3N4 composite nanotubes exhibit higher specific surface area, while maintaining nearly the same pore size and pore volume. The higher specific surface area of the C-PAN/g-C3N4 composite nanotubes is likely to contribute to the enhanced photocatalytic activity. Pure g-C3N4, 3 wt% C-PAN/g-C3N4, 5 wt% C-PAN/g-C3N4, and 10 wt% C-PAN/gC3N4 composite nanotubes exhibit an absorption edge at ca. 456, 456, 470, 484 nm, respectively, corresponding to the estimated band gap of 2.72, 2.72, 2.64, and 2.56 eV (Figure S9). The absorption edge of C-PAN is at 898 nm, corresponding to the band gap of 1.38 eV (Figure S10). As compared with pure g-C3N4, the C-PAN/g-C3N4 composite nanotubes also show much enhanced light absorption, owing to the multiple scattering of incident light from the micronanostructure.38, 39 The band structure of C-PAN and g-C3N4 were investigated by Mott-Schottky method and valence band (VB) XPS. The flat-band potential derived from Mott-Schottky plots for C-PAN, pure g-C3N4, 3 wt% C-PAN/g-C3N4, 5 wt% C-PAN/g-C3N4, and 10 wt% C-PAN/g-C3N4 composite nanotubes are -0.55, -0.70, -0.70, -0.70, -0.69 V vs. Ag/AgCl, respectively (Figure 4ac). The conduction band (CB) edge of g-C3N4 is more negative than that of C-PAN, indicating that photogenerated electrons can transfer from g-C3N4 to C-PAN to improve the separation of photogenerated charge carriers. Combined with the band gap energy, the VB position of C-PAN and g-C3N4 can be calculated, which is at 0.83 and 2.02 V vs. Ag/AgCl, respectively. Based on


9

ACS Energy Letters

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

Page 10 of 18

the relative VB and CB positions of C-PAN and g-C3N4, a schematic illustration of the energy band diagram for C-PAN and g-C3N4 is constructed in Figure 4d. The band structure of C-PAN and g-C3N4 were further examined by VB XPS (Figure S11). The VB maxima of C-PAN and gC3N4 are revealed to be 0.80 and 1.72 eV respectively. The VB potential of g-C3N4 is more positive than that of C-PAN, which is consistent with the results of Mott-Schottky plots.

Figure 4. Mott-Schottky plot of (a) g-C3N4, (b) C-PAN, and (c) for 1) g-C3N4, 2) 3 wt% CPAN/g-C3N4, 3) 5 wt% C-PAN/g-C3N4 and 4) 10 wt% C-PAN/g-C3N4 composite nanotubes, respectively, and (d) schematic diagram of electronic band structure of the g-C3N4 and C-PAN composite. The charge-carrier dynamics of g-C3N4 and C-PAN/g-C3N4 composite nanotubes was investigated by photoluminescence (PL) emission.40,

41

As shown in Figure 4a, pure g-C3N4

shows a strong PL emission peak, while the PL intensity for C-PAN/g-C3N4 composite nanotubes decreases significantly with increase of C-PAN contents (Figure 5a), indicating much improved charge carrier separation efficiency of C-PAN/g-C3N4 composite nanotubes. The lifetime of charge carriers for g-C3N4 and C-PAN/g-C3N4 composite nanotubes was further


10

Page 11 of 18

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

ACS Energy Letters

investigated by time-resolved fluorescence decay spectra. The fluorescent intensity of g-C3N4 and 5 wt% C-PAN/g-C3N4 composite nanotubes decays exponentially (Figure 5b). Fitting the decay spectra gives two radiative time constants with corresponding percentages listed in Table S2. Both the short and long lifetimes of charge carriers are greatly improved for 5 wt% CPAN/g-C3N4 composite nanotubes. The average lifetime for g-C3N4 and 5 wt% C-PAN/g-C3N4 composite nanotubes based on radiative time constants as well as their corresponding percentages were calculated to be 7.06 ns and 8.08 ns, respectively, indicating that the average lifetime of C-PAN/g-C3N4 composite nanotubes has been effectively prolonged. The photocatalytic performance was evaluated by photocatalytic H2 evolution reaction in TEOA aqueous solution under visible light irradiation (λ > 400 nm). Figure 5c compares the photocatalytic H2 evolution rate of different samples. Pure g-C3N4 produces H2 at a rate of 10.6 µmol h−1, while no H2 could be detected if g-C3N4 was replaced by C-PAN prepared in presence of sulfur calcined at 400 °C or 650 °C. All C-PAN/g-C3N4 composite nanotubes show much enhanced photocatalytic H2 evolution activity. Among which, the 5 wt% C-PAN/g-C3N4 composite nanotubes reach the optimum H2 production rate of 177.5 µmol h−1, which is about 16.7 times higher than that of pure g-C3N4 and even 4.7 times higher than that of C-PAN/g-C3N4 composite nanosheets synthesized without sulfur. We further investigated the H2 evolution activities of 5 wt % C-PAN/g-C3N4 composite nanotubes under irradiation using different longpass cut-off optical filters. The H2 evolution activity was strongly dependent on the incident wavelength (Figure S12), indicating a photocatalytic reaction. The measured photocatalytic H2 evolution rate of 5 wt% C-PAN/g-C3N4 composite nanotubes irradiated at 420 nm monochromatic light reaches 46.2 µmol h-1, with an apparent quantum efficiency of 5.6 %. After being normalized with surface area, the photocatalytic enhancement factor for 5 wt% C-PAN/g-


11

ACS Energy Letters

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

Page 12 of 18

C3N4 composite nanotubes decreased from 16.7 to 9.6 as compared to the bulk g-C3N4, suggesting that both more photogenerated charge carriers and higher surface area contribute to the enhancement of photocatalytic H2 evolution. As reported in our previous work, C-PAN with the aromatic structure could act as both effective electron transfer channel as well as the hydrogen H2 reaction co-catalyst to promote the charge separation to enhance the photocatalytic activity of g-C3N4.36 Additionally, we also investigated the photocatalytic H2 evolution performance of 5 wt% C-PAN/g-C3N4 composite nanorods prepared in the presence of sulfur calcined at 500 ºC with solid sulfur core, which displayed a hydrogen evolution rate of 78 µmol h-1. This value is much lower than that of 5 wt% C-PAN/g-C3N4 composite nanotubes (177.5 µmol h-1) prepared at 650 ºC. Thus the observed superior photocatalytic activity of C-PAN/gC3N4 composite nanotubes can be attributed to the hollow tubular composite structure, which provides larger specific surface area, enhanced light absorption, as well as greatly improved charge carrier separation and transfer. Furthermore, after 15 hour of continuous irradiation, the 5 wt% C-PAN/g-C3N4 composite nanotubes show excellent stability with no obvious deactivation in photocatalytic activity as shown in Figure 5d.


12

Page 13 of 18

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

ACS Energy Letters

Figure 5. (a) Photoluminescence spectra of 1) g-C3N4, 2) 3 wt% C-PAN/g-C3N4, 3) 5 wt% CPAN/g-C3N4 and 4) 10 wt% C-PAN/g-C3N4 composite nanotubes. (b) Time-resolved fluorescence decay spectra of 1) pure g-C3N4 and 2) 5 wt% C-PAN/g-C3N4 composite nanotubes. (c) Photocatalytic H2 evolution rate of 1) pure g-C3N4, 2) 5 wt% C-PAN/g-C3N4 obtained without sulfur in the precursor, 3) sulfur-mediated g-C3N4, and 4) 3 wt% C-PAN/g-C3N4, 5) 5 wt% C-PAN/g-C3N4 and 6) 10 wt% C-PAN/g-C3N4 composite nanotubes. (d) Photocatalytic H2 evolution as a function of reaction time for 1) pure g-C3N4 and 2) 5 wt% C-PAN/g-C3N4 composite nanotubes in 10 vol% TEOA aqueous solution under visible light irradiation (λ > 400 nm). To further demonstrate improved charge carrier transfer in the C-PAN/g-C3N4 composite nanotubes, linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and chopped photocurrent measurements were conducted. The 5 wt% C-PAN/g-C3N4 composite nanotubes exhibit a much higher cathodic current than that of pure g-C3N4 in the LSV scan (Figure S13a). Figure S13b reveals that the diameter of the Nyquist plot for 5 wt% C-PAN/gC3N4 composite nanotubes is much smaller than that of g-C3N4, suggesting a lower charge transfer resistance for the C-PAN/g-C3N4 composite nanotubes. The photocurrent response with light on and off for g-C3N4 and 5 wt% C-PAN/g-C3N4 composite nanotubes were measured. The 5 wt % C-PAN/g-C3N4 composite nanotubes show 4.2 times enhanced photocurrent density up to 1.1 µA cm-2 over g-C3N4 (0.26 µA cm-2, Figure S13c). In summary, we have developed a novel sulfur-mediated self-templating method to make hollow tapered C-PAN/g-C3N4 composite nanotubes. The obtained C-PAN/g-C3N4 composite nanotubes display exceptional photocatalytic H2 evolution activities. Specifically, the 5 wt% CPAN/g-C3N4 composite nanotubes exhibit the optimum H2 evolution rate up to 177.5 µmol h−1,


13

ACS Energy Letters

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

Page 14 of 18

which is about 16.7 times higher than that of pure g-C3N4. The hollow tubular composite structure with larger specific surface area, enhanced light absorption, and much improved charge carrier separation and transfer contributes synergistically to the superior photocatalytic H2 evolution performance. Our work provides a new facile strategy to develop carbon-based architected photocatalysts with superior photocatalytic activity. ASSOCIATED CONTENT Supporting Information. Experimental methods, XPS spectra of 5 wt% C-PAN/g-C3N4 composite nanotubes, TEM images for 3 wt% C-PAN/g-C3N4 and 10 wt% C-PAN/g-C3N4 composite nanotubes, g-C3N4, C-PAN, 5wt% C-PAN/g-C3N4 composite sheets, and sulfurmediated g-C3N4 (1:1), TEM and HRTEM images of 5 wt% C-PAN/g-C3N4 composite nanotubes, TEM image and the corresponding elemental mapping of C, N and S for 5 wt% CPAN/g-C3N4 composite nanotubes, TEM images for C-PAN obtained in the presence of sulfur at 400 °C and 650 °C, FESEM images for 5 wt% C-PAN/g-C3N4 composite obtained in presence of sulfur at 400 °C, Nitrogen adsorption-desorption isotherms and the corresponding pore size distribution curves of g-C3N4 and C-PAN/g-C3N4 composite nanotubes, Effects of C-PAN content on the textural properties of the C-PAN/g-C3N4 composite nanotubes, UV-vis absorption spectra and the corresponding plots of (αhν)1/2 vs. photon energy (hν) for the determination of band gap energy for g-C3N4, C-PAN and C-PAN/g-C3N4 composite nanotubes, VB XPS spectra of g-C3N4 and C-PAN, Wavelength dependent photocatalytic H2 evolution rate for 5 wt% CPAN/g-C3N4 composite nanotubes, HER polarization curves, EIS Nyquist plots, and Chopped photocurrent of g-C3N4 and C-PAN/g-C3N4 composite nanotubes, Radiative fluorescence lifetimes and their relative percentages of photoexcited charge carriers for g-C3N4 and 5 wt% CPAN/g-C3N4 composite nanotubes.


14

Page 15 of 18

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

ACS Energy Letters

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (G. C.); *E-mail: [email protected] (B. L.) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the projects of Natural Science Foundation of China (21271055, 21471040), the Fundamental Research Funds for the Central Universities (HIT. IBRSEM. A. 201410), Nanyang Technological University startup grant: M4080977.120, and Singapore Ministry of Education Academic Research Fund (AcRF) Tier 1: 2015-T1-002-108. Fang He thanks the support from China Scholarship Council (CSC) program. REFERENCES 1. Wang, X.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M. A Metal-Free Polymeric Photocatalyst for Hydrogen Production from Water under Visible Light. Nat. Mater. 2009, 8, 76-80. 2. Ong, W. J.; Tan, L. L.; Ng, Y. H.; Yong, S. T.; Chai, S. P. Graphitic Carbon Nitride-Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer To Achieving Sustainability? Chem. Rev. 2016, 116, 7159-7329. 3. Liu, J.; Liu, Y.; Liu, N.; Han, Y.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S. T.; Zhong, J.; Kang, Z. Metal-Free Efficient Photocatalyst for Stable Visible Water Splitting via a TwoElectron Pathway. Science 2015, 347, 970-974. 4. Gao, G.; Jiao, Y.; Waclawik, E. R.; Du, A. Single Atom (Pd/Pt) Supported on Graphitic Carbon Nitride as an Efficient Photocatalyst for Visible-Light Reduction of Carbon Dioxide. J. Am. Chem. Soc. 2016, 138, 6292-6297. 5. Lau, V. W.; Mesch, M. B.; Duppel, V.; Blum, V.; Senker, J.; Lotsch, B. V. Low-MolecularWeight Carbon Nitrides for Solar Hydrogen Evolution. J. Am. Chem. Soc. 2015, 137, 1064-1072. 6. Cao, S.; Low, J.; Yu, J.; Jaroniec, M. Polymeric Photocatalysts Based on Graphitic Carbon Nitride. Adv. Mater. 2015, 27, 2150-2176. 7. Zheng, Y.; Lin, L.; Wang, B.; Wang, X. Graphitic Carbon Nitride Polymers toward Sustainable Photoredox Catalysis. Angew. Chem. Int. Ed. 2015, 54, 12868-12884.


15

ACS Energy Letters

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

Page 16 of 18

8. Zheng, D.; zhang, G.; Wang, X. Integrating CdS Quantum Dots on Hollow Graphitic Carbon Nitride Nanospheres for Hydrogen Evolution Photocatalysis. Appl. Catal. B: Environ. 2015, 179, 479-488. 9. Ong, W.-J.; Putri, L. K.; Tan, L.-L.; Chai, S.-P.; Yong, S.-T. Heterostructured AgX/g-C3N4 (X=Cl and Br) Nanocomposites via a Sonication-Assisted Deposition-Precipitation Approach: Emerging Role of Halide Ions in the Synergistic Photocatalytic Reduction of Carbon Dioxide. Appl. Catal. B: Environ. 2016, 180, 530-543. 10. Li, H.; Liu, Y.; Cui, Y.; Zhang, W.; Fu, C.; Wang, X. Facile Synthesis and Enhanced VisibleLight Photoactivity of DyVO4/g-C3N4I Composite Semiconductors. Appl. Catal. B: Environ. 2016, 183, 426-432. 11. Ong, W. J.; Tan, L. L.; Chai, S. P.; Yong, S. T. Graphene Oxide As a Structure-Directing Agent for the Two-Dimensional Interface Engineering of Sandwich-Like Graphene-g-C3N4 Hybrid Nanostructures with Enhanced Visible-Light Photoreduction of CO2 to Methane. Chem. commun. 2015, 51, 858-861. 12. Ong, W. J.; Tan, L. L.; Chai, S. P.; Yong, S. T. Heterojunction Engineering of Graphitic Carbon Nitride (g-C3N4) via Pt Loading with Improved Daylight-Induced Photocatalytic Reduction of Carbon Dioxide to Methane. Dalton trans. 2015, 44, 1249-57. 13. Pan, Z.; Zheng, Y.; Guo, F.; Niu, P.; Wang, X. Decorating CoP and Pt Nanoparticles on Graphitic Carbon Nitride Nanosheets to Promote Overall Water Splitting by Conjugated Polymers. ChemSusChem 2016, DOI: 10.1002/cssc.201600850. 14. Liu, G.; Niu, P.; Sun, C.; Smith, S. C.; Chen, Z.; Lu, G. Q.; Cheng, H. M. Unique Electronic Structure Induced High Photoreactivity of Sulfur-doped Graphitic C3N4. J. Am. Chem. Soc. 2010, 132, 11642-11648. 15. Zhang, J.; Chen, X.; Takanabe, K.; Maeda, K.; Domen, K.; Epping, J. D.; Fu, X.; Antonietti, M.; Wang, X. Synthesis of a Carbon Nitride Structure for Visible-Light Catalysis by Copolymerization. Angew. Chem. Int. Ed. 2010, 49, 441-444. 16. Duan, J.; Chen, S.; Jaroniec, M.; Qiao, S. Z. Porous C3N4 Nanolayers@N-graphene films as Catalyst Electrodes for Highly Efficient Hydrogen Evolution. ACS Nano 2015, 9, 931-940. 17. Yang, S.; Gong, Y.; Zhang, J.; Zhan, L.; Ma, L.; Fang, Z.; Vajtai, R.; Wang, X.; Ajayan, P. M. Exfoliated Graphitic Carbon Nitride Nanosheets as Efficient Catalysts for Hydrogen Evolution under Visible Light. Adv. Mater. 2013, 25, 2452-2456. 18. Li, X.-H.; Zhang, J.; Chen, X.; Fischer, A.; Thomas, A.; Antonietti, M.; Wang, X. Condensed Graphitic Carbon Nitride Nanorods by Nanoconfinement: Promotion of Crystallinity on Photocatalytic Conversion. Chem. Mater. 2011, 23, 4344-4348. 19. Zhang, J.; Zhang, M.; Yang, C.; Wang, X. Nanospherical Carbon Nitride Frameworks with Sharp Edges Accelerating Charge Collection and Separation at a Soft Photocatalytic Interface. Adv. Mater. 2014, 26, 4121-4126. 20. He, F.; Chen, G.; Yu, Y.; Zhou, Y.; Zheng, Y.; Hao, S. The Synthesis of Condensed C-PDAg-C3N4 Composites with Superior Photocatalytic Performance. Chem. Commun. 2015, 51, 68246827. 21. He, F.; Chen, G.; Zhou, Y. S.; Yu, Y. G.; Li, L. Q.; Hao, S.; Liu, B. ZIF-8 Derived Carbon (C-ZIF) as a Bifunctional Electron Acceptor and HER Cocatalyst for g-C3N4: Construction of a Metal-Free, All Carbon-Based Photocatalytic System for Efficient Hydrogen Evolution. J. Mater. Chem. A 2016, 4, 3822-3827.


16

Page 17 of 18

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

ACS Energy Letters

22. Hou, Y.; Wen, Z.; Cui, S.; Guo, X.; Chen, J. Constructing 2D Porous Graphitic C3N4 Nanosheets/Nitrogen-doped Graphene/Layered MoS2 Ternary Nanojunction with Enhanced Photoelectrochemical Activity. Adv. Mater. 2013, 25, 6291-6297. 23. Chen, J.; Yang, H. B.; Miao, J.; Wang, H. Y.; Liu, B. Thermodynamically Driven OneDimensional Evolution of Anatase TiO2 Nanorods: One-step Hydrothermal Synthesis for Emerging Intrinsic Superiority of Dimensionality. J. Am. Chem. Soc. 2014, 136, 15310-15318. 24. Xiao, F. X.; Miao, J.; Tao, H. B.; Hung, S. F.; Wang, H. Y.; Yang, H. B.; Chen, J.; Chen, R.; Liu, B. One-dimensional Hybrid Nanostructures for Heterogeneous Photocatalysis and Photoelectrocatalysis. Small 2015, 11, 2115-2131. 25. Liu, B.; Aydil, E. S. Growth of Oriented Single-Crystalline Rutile TiO2 Nanorods on Transparent Conducting Substrates for Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2009, 131, 3985-3990. 26. Liu, B.; Chen, H. M.; Liu, C.; Andrews, S. C.; Hahn, C.; Yang, P. Large-Scale Synthesis of Transition-Metal-Doped TiO2 nanowires with Controllable Overpotential. J. Am. Chem. Soc. 2013, 135, 9995-9998. 27. Wang, X.; Maeda, K.; Chen, X.; Takanabe, K.; Domen, K.; Hou, Y.; Fu, X.; Antonietti, M. Polymer Semiconductors for Artificial Photosynthesis: Hydrogen Evolution by Mesoporous Graphitic Carbon Nitride with Visible Light. J. Am. Chem. Soc. 2009, 131, 1680-1681. 28. Jun, Y.-S.; Hong, W. H.; Antonietti, M.; Thomas, A. Mesoporous, 2D Hexagonal Carbon Nitride and Titanium Nitride/Carbon Composites. Adv. Mater. 2009, 21, 4270-4274. 29. Tong, Z.; Yang, D.; Sun, Y.; Nan, Y.; Jiang, Z. Tubular g-C3N4 Isotype Heterojunction: Enhanced Visible-Light Photocatalytic Activity through Cooperative Manipulation of Oriented Electron and Hole Transfer. Small 2016, 12, 4093-4101. 30. Shalom, M.; Inal, S.; Fettkenhauer, C.; Neher, D.; Antonietti, M. Improving Carbon Nitride Photocatalysis by Supramolecular Preorganization of Monomers. J. Am. Chem. Soc. 2013, 135, 7118-7121. 31. Jun, Y.-S.; Lee, E. Z.; Wang, X.; Hong, W. H.; Stucky, G. D.; Thomas, A. From MelamineCyanuric Acid Supramolecular Aggregates to Carbon Nitride Hollow Spheres. Adv. Funct. Mater. 2013, 23, 3661-3667. 32. Liang, Q.; Li, Z.; Yu, X.; Huang, Z. H.; Kang, F.; Yang, Q. H. Macroscopic 3D Porous Graphitic Carbon Nitride Monolith for Enhanced Photocatalytic Hydrogen Evolution. Adv. Mater. 2015, 27, 4634-4639. 33. Zhang, J. S.; Guo, F. S.; Wang, X. C. An Optimized and General Synthetic Strategy for Fabrication of Polymeric Carbon Nitride Nanoarchitectures. Adv. Funct. Mater. 2013, 23, 30083014. 34. Ji, L.; Lin, Z.; Medford, A. J.; Zhang, X. Porous Carbon Nanofibers from Electrospun Polyacrylonitrile/SiO2 Composites as an Energy Storage Material. Carbon 2009, 47, 3346-3354. 35. Lin, Z.; Wang, X. Nanostructure Engineering and Doping of Conjugated Carbon Nitride Semiconductors for Hydrogen Photosynthesis. Angew. Chem. Int. Ed. 2013, 52, 1735-1738. 36. He, F.; Chen, G.; Yu, Y.; Hao, S.; Zhou, Y.; Zheng, Y. Facile Approach to Synthesize gPAN/g-C3N4 Composites with Enhanced Photocatalytic H2 Evolution Activity. ACS Appl. Mater. Interfaces 2014, 6, 7171-7179. 37. He, F.; Chen, G.; Yu, Y.; Zhou, Y.; Zheng, Y.; Hao, S. The Sulfur-Bubble TemplateMediated Synthesis of Uniform Porous g-C3N4 with Superior Photocatalytic Performance. Chem. Commun. 2015, 51, 425-427.


17

ACS Energy Letters

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

Page 18 of 18

38. Guo, S.; Deng, Z.; Li, M.; Jiang, B.; Tian, C.; Pan, Q.; Fu, H. Phosphorus-Doped Carbon Nitride Tubes with a Layered Micro-nanostructure for Enhanced Visible-Light Photocatalytic Hydrogen Evolution. Angew. Chem. Int. Ed. 2016, 55, 1830-1834. 39. Han, Q.; Wang, B.; Gao, J.; Cheng, Z.; Zhao, Y.; Zhang, Z.; Qu, L. Atomically Thin Mesoporous Nanomesh of Graphitic C3N4 for High-Efficiency Photocatalytic Hydrogen Evolution. ACS Nano 2016, 10, 2745-2751. 40. Zhang, G.; Zhang, M.; Ye, X.; Qiu, X.; Lin, S.; Wang, X. Iodine Modified Carbon Nitride Semiconductors as Visible Light Photocatalysts for Hydrogen Evolution. Adv. Mater. 2014, 26, 805-809. 41. Zhang, M.; Wang, X. Two Dimensional Conjugated Polymers with Enhanced Optical Absorption and Charge Separation for Photocatalytic Hydrogen Evolution. Energy Environ. Sci. 2014, 7, 1902-1906.


18

g-C3N4

Recommend Documents