Extending the Ï-Conjugation of g-C3N4 by Incorporating Aromatic

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Article 3

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Extending the #-Conjugation of g-CN by Incorporating Aromatic Carbon for Photocatalytic H Evolution from Aqueous Solution 2

Po-Kai Chuang, Kwun-Han Wu, Te-Fu Yeh, and Hsisheng Teng ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01266 • Publication Date (Web): 09 Aug 2016 Downloaded from http://pubs.acs.org on August 16, 2016

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ACS Sustainable Chemistry & Engineering

Extending the π-Conjugation of g-C3N4 by Incorporating Aromatic Carbon for Photocatalytic H2 Evolution from Aqueous Solution

Po-Kai Chuang,a Kwun-Han Wu,a Te-Fu Yeh,a and Hsisheng Teng*,a,b

a

Department of Chemical Engineering and Research Center for Energy Technology and Strategy, National Cheng Kung University, No. 1 University Road, Tainan City 70101, Taiwan

b

Center for Micro/Nano Science and Technology, National Cheng Kung University, No. 1 University Road, Tainan City 70101, Taiwan

AUTHOR INFORMATION Corresponding Author *Email address: [email protected].

1

ACS Paragon Plus Environment


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ABSTRACT This study details the synthesis of high-activity g-C3N4 catalysts for H2 generation from a triethanolamine aqueous solution under visible light. We anneal a mixture of urea and NH4Cl to obtain g-C3N4 nanosheets, which are subsequently solvated with ethanol molecules and annealed to form aromatic carbon-doped g-C3N4. The results of analyses conducted using X-ray photoelectron, Fourier-transform infrared, and

13

carbon-13 nuclear magnetic resonance spectroscopies demonstrated

that annealing the ethanol molecules leads to the grafting of aromatic heterocycles on the g-C3N4 nanosheets and substitution of nitrogen with carbon. The grafted aromatic heterocycles and doped carbon atoms extend the π-conjugation system in g-C3N4 to reduce the band gap and facilitate the separation of photogenerated charges. The carbon-incorporating

also

preserve

the

crystallinity

of

g-C3N4

during

high-temperature annealing, which facilitates the suppression of the recombination of photogenerated charges at defect sites. The developed aromatic carbon-doped g-C3N4 effectively catalyzes H2 generation from the aqueous solution, achieving apparent quantum yields of 14% and 2.2% under 420- and 550-nm monochromatic irradiation, respectively, whereas urea-derived g-C3N4 reached only 3.4% and 0.1%. The proposed strategy of extending the π-conjugation system is promising for promoting the activity of carbon-nitride photocatalysts.

Keywords: graphitic carbon nitride, water splitting, photocatalysis, hydrogen production, charge separation

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INTRODUCTION Fossil fuels will soon be depleted, and their combustion is accompanied by the release of a considerable amount of CO2, which results in global warming and environmental pollution.1,2 Therefore, a critical energy crisis and climate transition compel us to develop renewable, ecofriendly, and economical energy technologies, especially the promising approach of photocatalytic water splitting with solar energy.3,4 Metal-free organic semiconductors such as graphitic carbon nitride (g-C3N4)5 and graphene oxide (GO)6 have recently received worldwide attention for solar water splitting or H2 generation because of their abundance, nontoxicity, suitable band potentials, and high thermal and chemical stability. For g-C3N4, certain detrimental factors have resulted in poor photoactivity, including a low specific surface area, the extreme charge recombination of photogenerated carriers, and limited light absorption in the visible region.7,8 To address these disadvantages, previous researchers have developed strategies such as nanostructure engineering,9,10 heteroatom doping,11,12 and coupling with other materials.13,14 These strategies have modified the texture and electronic structure of g-C3N4, but they are either cumbersome or exhibit limited promotion in photocatalysis. Hence, developing a simple strategy for modifying the morphology and electronic structure has become an urgent concern for enhancing the photocatalytic H2-evolution efficiency of g-C3N4. Urea is one of the most commonly used precursors for the synthesis of g-C3N4 sheets.14,15

Under

thermal

treatment,

urea

decomposes

and

produces

oxygen-containing gas bubbles, inevitably resulting in porosity formation on g-C3N4 sheets. Urea-derived g-C3N4 photocatalysts have exhibited specific surface areas larger than those of bulk g-C3N4 samples produced from precursors such as 3



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cyanamide,10 dicyandiamide,16 and melamine,11–13 but these urea-derived catalysts lacked visible-light absorption ability and exhibited a high recombination probability of photogenerated charges.15 To enhance photocatalytic activity, extending the absorption spectrum to long wavelengths and promoting the charge separation rate are essential. Chemical copolymerization has been an effective approach for modifying the band structure and optoelectronic properties of g-C3N4,17–19 thus extending the delocalization of π electrons and improve visible-light photocatalytic performance and stability. Although these modifications could promote the photocatalytic activity of g-C3N4, the employed organic compounds contained complex aromatic heterocycles that were not ecofriendly. To overcome the disadvantages reported in previous studies, we devised a sustainable strategy for the synthesis of high-activity g-C3N4-based photocatalysts (Figure 1). The raw materials for catalysts are urea and NH4Cl, which decompose and condensate in thermal annealing, yielding g-C3N4 nanosheets (CNSs) with the simultaneous evolution of gaseous NH3 and HCl. The next step involves segregating the stacked CNSs in ethanol, which solvates CNSs by forming hydrogen bonds. Finally, the solvated CNSs are subjected to annealing in argon to form carbon-doped annealed g-C3N4 nanosheet (C-aCNS) specimens, in which aromatic heterocycles produced from ethanol annealing are incorporated with CNSs and nitrogen vacancies in the triazine units produced in annealing are repaired by ethanol-derived carbon atoms. Attaching aromatic heterocycles and substituting nitrogen in triazine units with carbon may extend the delocalization of electrons in the π-conjugation system, thus enlarging the absorption of the solar spectrum and promoting charge separation and transportation.17,20,21 The C-aCNSs, which possess a high surface area (130 m2 g-1) and small band gap (2.44 eV), exhibit high photocatalytic activity in producing H2 4


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from a triethanolamine (TEOA) aqueous solution under visible irradiation, establishing an AQY of 14% under 420-nm monochromatic irradiation.

Figure 1.

Schematic representation of the synthesis process for C-aCNS. First,

CNS was derived from the urea and NH4Cl mixture in thermal annealing condensation. The next step involved solvating CNS sheets in ethanol by forming hydrogen bonding. Finally, the solvated CNS sheets were subjected to thermal annealing in argon to graft aromatic heterocycles and substitute nitrogen in triazine units with carbon and form C-aCNS.

EXPERIMENTAL SECTION Material Synthesis The urea-derived g-C3N4 specimen (CN) was prepared by directly heating 20 g of urea (J. T. Baker, USA) at 550 ºC for 3 h, at a ramp rate of 1 ºC min−1, in a quartz crucible with a cover (in static air). For CNS preparation, we mixed 20 g of urea and 3 g of NH4Cl (Macron, USA) with 5 mL of water, and then evaporated the water at 80 ºC, followed by heating at 550 ºC for 3 h at a rate of 1 ºC min−1 in static air. The C-aCNS specimen was synthesized by blending 0.5 g of a CNS with 50 mL of 99.5% ethanol (Echo, Taiwan) stirred for 6 h, followed by filtration and washing with 5



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ethanol several times. Afterward, the muddy mixture was annealed at 550 ºC for 3 h in a tube furnace flowed with argon. In addition, 0.5 g of a CNS was annealed, but without blending with ethanol, in the same manner at 600 ºC to produce an aCNS specimen. All products were collected after they were naturally cooled to room temperature.

Characterization of the Specimens The structures and morphology of the specimens were inspected using field-emission scanning electron microscopy (SEM; Hitachi SU8010, Japan) and transmission

electron

microscopy

(TEM;

Jeol

2100F,

Japan).

Moreover,

energy-dispersive X-ray spectroscopy (EDS) profiles were recorded. The specific surface area and pore volume were determined according to N2 adsorption at 77 K by using an adsorption apparatus (Micromeritics ASAP 2020, USA). The crystalline structure of the specimens was characterized by powder X-ray diffraction (XRD) by using a Rigaku RINT-2000 (Japan) diffractometer with Cu Ka radiation excited at 40 kV and 40 mA. X-Ray photoelectron spectroscopy (XPS; Kratos AXIS Ultra DLD, UK) with Al Ka radiation was used to quantitatively analyze the chemical composition of the specimens. Fourier-transform infrared (FTIR) spectroscopic analysis was conducted in diffuse reflectance mode on a Jasco FTIR-4100 (Japan) spectrometer. Solid-state 13C-NMR spectra were collected using a Bruker Avance III HD Spectrometer. Electron paramagnetic resonance (EPR) spectra were recorded on a Bruker model E-580 spectrometer (Germany), with a 400-W mercury lamp (Oriel Instrument, USA) as a light source. The optical absorption spectra of the specimen powders were obtained using a Hitachi U-4100 (Japan) spectrophotometer. The photoluminescence (PL) spectra of the powders were measured at an ambient 6


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temperature by using a fluorescence spectrophotometer (Hitachi F-700, Japan) equipped with a Xenon lamp for excitation and 900-mm−1 grating. The slit widths for excitation and emission were 10 and 5 nm, respectively.

Electrochemical Analysis of the Specimens The electronic structure of the specimens was analyzed with an electrochemical impedance spectroscope (Zahner IM6e, Germany) and Thales software. The specimens were deposited on a glassy carbon electrode by drop-casting, and electrochemical analysis was performed in a 0.5 M H2SO4 solution by using a Pt-foil counter and a Ag/AgCl reference electrode. Measurements involved the application of sinusoidal potential perturbation with a small amplitude (10 mV) superimposed on a fixed DC potential, which varied within a potential range of −1 to 1 V (vs. Ag/AgCl).

Photocatalytic Activity Measurements Photocatalytic reactions were conducted at approximately 25 ºC in a gas-enclosed system with side irradiation from a 300-W Xenon lamp (Oriel Instruments, model 66901, USA). The prepared catalysts (0.05 g) were suspended in a 10 vol% TEOA aqueous solution (275 mL) containing H2PtCl6·6H2O (Alfa Aesar, USA) stored in a Pyrex vessel to synthesize a 3 wt% Pt-deposited catalyst in situ during the photocatalytic reaction. The wavelength of the incident light was limited to 420−800 nm with a UV-cutoff filter (Asahi Spectra, XUL0422, USA) and an IR-cutoff filter (Asahi Spectra, XIS0810, USA). For AQY measurements, 420-, 450-, 500-, and 550-nm band-pass filters (Newport, 20BPF10 series, USA) were used to obtain monochromatic irradiations. The intensity of the light irradiated on the reacting system was determined using a photodetector (Oriel Instrument, model 71964, USA). 7



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The

amount

of

H2

produced

was

determined

by

gas

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chromatography

(Hewlett-Packard 6890, USA; molecular sieve 5A column, thermal conductivity detector, argon carrier gas).

RESULTS AND DISCUSSION Morphology and Microstructure of the Specimens For this study, the CN specimen was synthesized through the thermal condensation of urea at 550 ºC for 3 h for a comparison with the CNS specimen that was obtained through the thermal condensation of a mixture of urea and NH4Cl under the same conditions. Figure 2 (a and b) shows that both the CN and CNS specimens appeared to be white-yellow powders. We also annealed a CNS under argon flow at 600 ºC for 3 h to produce the aCNS specimen for a comparison with the C-aCNS specimen that was obtained by annealing an ethanol-solvated CNS. The aCNS specimen, which was obtained from 600 ºC annealing, exhibited a higher photocatalytic activity than did the specimen derived from 550 ºC annealing. Figure 2 (c and d) shows that the aCNS and C-aCNS are orange and brown, respectively. The colors of the specimens indicate that the visible-light absorption ability is ranked as follows: C-aCNS > aCNS > CNS ≈ CN.

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Figure 2.

Photographs of (a) CN, (b) CNS, (c) aCNS, and (d) C-aCNS powders,

displaying their various colors and appearances.

Figure 3 (a and b) displays the TEM images of the CN and CNS, both of which consisted of stacked sheets spanning several micrometers. The sheets are crinkled and curled because of surface energy minimization.22 Compared with CN, the CNSs had less dense stacking and contained hollows on the surface, likely caused by the bubbling of NH3 and HCl gases generated from NH4Cl decomposition during CNS synthesis.23 Figure 3 (c and d) displays the TEM images of the aCNS and C-aCNS, which consisted of sheets with fewer wrinkles than those of the CN and CNS. CNS annealing may have ruptured the connections among the CNS building blocks (i.e., polymeric melon units7,8,19,24) to reduce the size of the continuous 2D domain and therefore avoid wrinkle formation. This change in morphology indicates an increase 9



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in the specific surface area of the CNS powder from the annealing treatments. The C-aCNSs were smaller than the aCNS, indicating that annealing on the ethanol molecules that adhered to the CNS may have modified polymeric melon units to substantially reduce the sheet size.

Figure 3.

TEM images of (a) CN, (b) CNS, (c) aCNS, and (d) C-aCNS.

Figure S1 displays the SEM images of the specimens. The CN specimen principally consisted of thick lamellas (Figure S1a). Compared with the CN, the CNS had a less dense structure (Figure S1b), a finding that was in agreement with TEM analysis. The structures of the aCNS and C-aCNS were flaky (Figure S1 (c and d)), which was in agreement with TEM analysis, showing that both the aCNS and C-aCNS consisted of flat sheets. Nevertheless, the flakes of the C-aCNS were thinner than those of the aCNS. Both TEM and SEM analyses revealed that the specific 10


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surface area of the specimens were ranked in the order of C-aCNS > aCNS > CNS > CN, which is consistent with the surface areas determined by N2 adsorption (130, 98, 86, and 66 m2g-1, respectively).

Crystalline Structure and Chemical Composition of the Specimens Figure 4 displays the XRD patterns of the specimens. All of the specimens exhibited two principal peaks: a dominant peak at 27.6°, resulting from the interplanar spacing of the (002) planes; and a minor peak at 12.8°, resulting from the (100) in-plane structural repeating motifs of tri-s-triazine units.12,25 The appearance of the characteristic peaks indicated that the basic g-C3N4 structure was adequately preserved in all specimens. The CN and CNS specimens exhibited similar XRD patterns even when they displayed different textures and morphologies. The aCNS specimen exhibited substantially lower peak intensities compared to those of the CN and CNS specimens. The annealing treatment in argon may have introduced nitrogen vacancies into the g-C3N4 layers and broken in-plane hydrogen bonding, thus resulting in structural twisting and crystallinity loss.25 The C-aCNS specimen had stronger XRD peak intensities compared with the aCNS. The presence of the adhered ethanol must have restricted structural distortions during annealing by filling the nitrogen vacancies in the triazine units with ethanol-derived carbon atoms. The (002) peak of the C-aCNS exhibited a slight shift to a larger 2θ value of 27.8°, indicating a decrease in the interlayer stacking distance. Smaller interlayer spacing is advantageous for the transport of photogenerated charges.26

11



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Figure 4.

Powder XRD patterns of CN, CNS, aCNS, and C-aCNS.

We further employed XPS to elucidate alterations in the chemical structure resulting from the annealing treatments. Figure 5 displays the XPS survey spectra, which exhibited C 1s (288 eV), N 1s (399 eV), and O 1s (533 eV) peaks. Quantitative analysis on the XPS spectra yielded C/N atomic ratios of 0.66 (CN), 0.67 (CNS), 0.71 (aCNS), and 0.73 (C-aCNS). EDS and TEM analyses also revealed similar C/N ratios (Figure S2). The aCNS specimen had a lower nitrogen content than did the CN and CNS, indicating that the annealing treatment introduced nitrogen vacancies and thus distorted the crystalline structure. The C-aCNS specimen had the highest C/N ratio, which may have resulted from carbon deposition caused by ethanol annealing. Some ethanol-derived carbon atoms may have repaired some nitrogen vacancies in the C-aCNS, because XRD analysis reveal the high crystallinity the C-aCNS.

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Figure 5.

Full-range XPS spectrum of CN, CNS, aCNS, and C-aCNS.

Figure 6a displays the C 1s XPS spectra and the constituent peaks from deconvolution (i.e., graphitic carbon [C-C/C=C]) at 284.9 eV and sp2-hybridized carbon (N-C=N) at 288 eV.20,27 Table 1 lists the carbon bonding compositions of all the specimens. In a manner similar to the results obtained from the XPS survey spectra, Table 1 shows the high content of graphitic carbon in the C-aCNS. Figure 6b shows the N 1s spectra and the constituent peaks, which were located at 398.6, 400.4, and 401.2 eV for sp2-hybridized nitrogen (N-C=N), tertiary nitrogen (N-(C)3), and terminal amino groups (C-NH), respectively.20,28 The high content in N-(C)3 and low content in C-NH for C-aCNS implied that graphitic carbon (or pyridine-like) species had replaced the terminal amino groups on the g-C3N4 domain.

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Figure 6.

(a) C 1s XPS spectra of specimens (solid lines) and the constituent peaks

from curve deconvolution (dashed lines). (b) N 1s XPS spectra of specimens (solid lines) and the constituent peaks from curve deconvolution (dashed lines). Peak deconvolution was conducted using a Gaussian function.

Table 1.

Atomic ratios (C 1s)/(N 1s) and carbon and nitrogen bonding

compositions as determined with XPS analysis (Figures 5 and 6) for the CN, CNS, 14


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aCNS, and C-aCNS specimens.

Specimens

Atomic

Carbon Bonding

Nitrogen Bonding

Ratio

Composition (%)

Composition (%)

C 1s/N 1s

C-C/C=C

N-C=N

N-C=N

N-(C)3

C-NH

CN

0.66

16.9

83.1

81.8

13.5

4.7

CNS

0.67

17.7

82.3

81.1

14.7

4.2

aCNS

0.71

28.4

71.6

79.3

16.6

4.1

C-aCNS

0.73

34.8

65.2

78.7

18.3

3.0

Figure 7a displays the FTIR spectra of all the specimens. These spectra consisted of three characteristic absorption bands: a sharp peak at 812 cm−1 corresponding to the breathing mode of the tri-s-triazine units; the band at 1130−1670 cm−1 associated with the stretching vibration mode of C-N/C=N in the tri-s-triazine heterocyclic rings; and the broad band at 3000−3300 cm−1 attributed to the terminal amino groups (-NH2 or -NH groups) and the hydroxyl group of the adsorbed H2O.12,29 The characteristic peaks of the aCNS specimen exhibited substantially reduced intensities, because the annealing treatment deteriorated the chemical structure of g-C3N4 to a degree. The C-aCNS specimen exhibited higher peak intensities compared with the aCNS because of the presence of ethanol during annealing, a finding that is in agreement with the results of XRD analysis. Figure 7b displays the focused spectra within 1400−1700 cm−1. Compared with the other specimens, C-aCNS exhibited additional peaks at 1510, 1525, and 1557 cm−1, which corresponded to the C=C skeletal vibration of aromatic rings.30 This FTIR analysis confirmed the schematic shown in Figure 1, in which annealing with ethanol resulted in the incorporation of additional carbon species into the g-C3N4 structure while maintaining the basic architecture of g-C3N4. The proposed structure for the C-aCNS was further verified by solid-state

13

C

NMR analysis (Figure 7c). The NMR spectra of the g-C3N4-based species exhibited 15



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two distinct peaks at approximately 169 and 161 ppm, which are characteristic assignments of sp2-hybridized carbon atoms in the tri-s-triazine units of g-C3N4.31 The C-aCNS spectrum exhibited an additional broad peak at 118 ppm, which is representative of aromatic rings like benzene, pyridine, or pyrimidine.17,18,21 This result supports the interpretation from FTIR analysis regarding C-aCNS being produced with the incorporation of aromatic heterocycles into g-C3N4 matrices and substitution of nitrogen in the triazine units with carbon.

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Figure 7.

(a) FTIR absorption spectra of the specimens. (b) Focused FTIR spectra

within 1400−1700 cm−1. (c) Solid-state 13C NMR spectra of the specimens.

We conducted EPR measurements to determine how the annealing treatment affected the behavior of photogenerated charges. Figure 8 displays the EPR spectra of the specimens under visible-light irradiation. The spectra exhibit a single Lorentzian line with a g-value of 2.0034, which was attributed to the unpaired electrons of the 17



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g-C3N4 networks within π-bonded clusters.18 The intensity of the EPR signal of the aCNS was higher than that of the CN specimen, indicating that the annealing treatments

promoted

the

separation

of

photogenerated

charges

through

charge-capturing at the defect sites.9,17,32 The C-aCNS specimen had a substantially stronger EPR signal compared with the aCNS, showing that the incorporated aromatic heterocycles or substitution of triazine nitrogen with carbon in the C-aCNS extended the π-conjugated system to delocalize electrons and efficiently limit the recombination of photogenerated e−/h+ pairs.9,18,21 The results of EPR analysis implied that the use of this simple synthesis process for the C-aCNS (the schematic displayed in Figure 1) produced a regulated g-C3N4 structure that tended to facilitate the separation of photogenerated exitons. This structural feature of the C-aCNS is advantageous for application in photocatalytic and photosynthetic reactions.

Figure 8.

Solid-state EPR spectra of CN, aCNS, and C-aCNS, obtained under

visible light irradiation.

Optical and Electronic Properties of the Specimens 18


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Figure 9a shows the diffuse reflectance spectra of the specimens. The CN and CNS specimens exhibited similar absorption behaviors, displaying an absorption edge at approximately 470 nm. The spectra of the aCNS and C-aCNS exhibited an absorption extension to the long-wavelength regime, and the C-aCNS exhibited a long absorption tail toward 800 nm. The annealing treatments effectively extended the absorption spectrum of g-C3N4 to the long-wavelength regime. The results of diffuse reflectance analysis were consistent with the colors of the specimens observed under daylight illumination (Figure 2). The direct band gaps of the specimens were determined to be 2.76, 2.75, 2.39, and 2.44 eV for the CN, CNS, aCNS, and C-aCNS, respectively, on the basis of the Tauc plots determined by converting the diffuse reflectance spectra (Figure S3).10,11 XRD and FTIR analyses indicated that the small band gap of the aCNS mainly resulted from the creation of interband defect states in g-C3N4,25 whereas the extension of the π-conjugation system in g-C3N4 caused the small gap of the C-aCNS. Figure 9b displays the PL spectra of the specimens under excitation at 280 nm. The CN specimen exhibited the highest PL intensity, indicating an effective radiative recombination of photogenerated excitons in the g-C3N4 crystal. The PL intensity of the specimens decreased correspondingly with the structural crystallinity, and the aCNS exhibited the lowest intensity. Structural imperfections can result in nonradiative charge recombination at defect sites to lower PL emissions.32 Even when exhibiting low PL intensity, the C-aCNS specimen was found to exhibit the strongest EPR signal, indicating that the charges in the C-aCNS were effectively separated and transported, rather than lost to nonradiative recombination. In addition, the PL emissions of the C-aCNS had the longest wavelength, centered at 475 nm, which was 25 nm longer than that of the CN specimen. This red shift in PL emissions confirmed 19



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that the incorporated aromatic heterocycles and substitution of triazine nitrogen with carbon in the C-aCNS extended the π-conjugation system in g-C3N4.

Figure 9.

Optical properties of the specimens. (a) Diffuse reflectance spectra of the

CN, CNS, aCNS, and C-aCNS powders. (b) PL spectra of CN, CNS, aCNS, and C-aCNS powders under excitation at 280 nm.

We further elucidated the electronic structure of the specimens through electrochemical analysis. We used the Mott–Schottky equation to analyze the data 20


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obtained from electrochemical impedance spectroscopy.33 Figure S4 displays the capacitance variation of the space charge region in the specimens with the applied potential. The positive slopes of the linear relationships implied that all the specimens were n-type semiconductors. The flat-band potentials (VF) of the specimens were the intercepts of the extrapolated linear relationships on the abscissa. Figure S4 shows that the CN, CNS, aCNS, and C-aCNS specimens had similar VF values of approximately −1.1 eV (vs. Ag/AgCl). Assuming a negligible difference between VF and the conduction band minimum (CBM) level for the n-type specimens, we determined the valence band maximum (VBM) levels by incorporating the band-gap values obtained from diffuse reflectance spectra. Figure 10 displays the schematic energy-level diagrams of the specimens compared with the levels for water reduction and oxidation. The annealing treatments resulted in an upshift of the VBM levels. The formation of defect states (mainly nitrogen vacancies) should have caused a VBM shift for the aCNS,34 because Figure S4 displays high capacitance values for the aCNS (Figure S4). The capacitance is associated with the number of defect states. Regarding the C-aCNS, which exhibited capacitance values similar to those of the CN and CNS, the upshift of VBM was therefore ascribed to the extension of the π-conjugated system that resulted from aromatic carbon incorporation and carbon replacing nitrogen in triazine.17,21 Figure 10 shows that all of the specimens could donate charges for H2 or O2 generation from water decomposition. The effectiveness of the separation and interfacial transfer of photogenerated charges governs the photocatalytic activity of the specimens.

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Figure 10. Schematic energy-level diagrams of CN, CNS, aCNS, and C-aCNS compared with the levels for H2 and O2 generation from water (at pH = 0).

Photocatalytic Activity and Stability of the Specimens The photocatalytic activities of the specimens were evaluated on the basis of H2 generation from a 10 vol% TEOA aqueous solution. The photocatalytic reaction was performed using a gas-enclosed system with external visible-light irradiation (420−800 nm at an intensity of 42 mW cm−2). Figure 11 displays the time course of H2 production for the specimens deposited with 3 wt% Pt as the cocatalyst. The activity of the specimens was ranked in the order of C-aCNS > aCNS > CNS > CN. The C-aCNS specimen exhibited an activity more than four times that of CN. The CN specimen was derived from urea, and was considered more active compared with g-C3N4 materials derived from other raw materials.11,12,16 The C-aCNS was more active than aCNS, even if the aCNS had a smaller band gap for higher light absorption. The incorporated aromatic heterocycles and replacement of triazine nitrogen with carbon must have facilitated charge separation, as supported by EPR analysis (Figure 8), to promote the H2 evolution rate. The red shift in the PL emissions of the C-aCNS validated the extension of the π-conjugation system to 22


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facilitate charge separation, whereas such a red shift was not observed for the aCNS. Figure S5 displays the long-term H2 evolution data for a reaction for which the C-aCNS specimen was used as the photocatalyst. The C-aCNS specimen was stable in the reaction, exhibiting only 14% of activity decay over 120 h of the reaction.

Figure 11. Time course of H2 production from 10 vol% TEOA aqueous solution containing 0.05 g of 3 wt% Pt-deposited CN, CNS, aCNS, and C-aCNS. The system was irradiated with visible light (420−800 nm) at an intensity of 42 mW cm−2.

This study evaluated the AQY values for H2 production under monochromatic irradiation by using the following equation (1): AQY =

number of H 2 molecules produced × 2 × 100(% ) number of incident photons

(1)

We used various irradiation wavelengths to evaluate the AQY values, and the data are shown in summary form in Table S1. The superiority of the C-aCNS to the other specimens was more evident for irradiations with longer wavelengths, indicating that the extended π-conjugation effectively reduced the gap energy for charge excitation. The AQY values of the C-aCNS reached 14% at 420 nm and 2.2% at 550 23



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nm, which are among the highest values reported in previous studies.10,19,35 Figure 12 displays a comparison between the diffuse reflectance spectrum and AQY values for the C-aCNS. The trend of the wavelength-dependent AQY was in agreement with its optical absorption spectrum, indicating that the rate-governing step for H2 generation was light harvesting, and that the incorporated aromatic heterocycles and replacement of triazine nitrogen with carbon effectively facilitated the separation and transport of photogenerated charges.

Figure 12. Comparison between the optical absorption spectra of C-aCNS and the apparent quantum yields (AQYs) for H2 production from the 10 vol% TEOA aqueous solution over 3 wt% Pt-deposited C-aCNS under 420-, 450-, 500-, and 550-nm monochromatic light irradiation.

SUMMARY AND CONCLUSION For this study, we developed the simple method of grafting aromatic carbon and replacing triazine nitrogen with carbon for g-C3N4 to enhance photocatalytic H2 evolution from an aqueous solution. Ethanol played a key role in this aromatic carbon 24


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grafting process, in which ethanol molecules were anchored on CNSs through hydrogen bonding to segregate the sheets and became the carbon source for forming aromatic heterocycles and substituting triazine nitrogen with carbon on g-C3N4 in the annealing treatment. The use of the devised method substantially increased the surface area of g-C3N4 and resulted in the retention of its crystallinity during high-temperature annealing. The incorporation of aromatic heterocycles and substitution of triazine nitrogen with carbon extended the π-conjugation system of g-C3N4, which facilitated the separation of photogenerated charges for subsequent chemical reactions at the interface. The red shift of PL emissions and a strong EPR signal under irradiation confirmed the extension of π-conjugation in improving charge separation. This modification increased the photocatalytic activity of urea-derived g-C3N4 by more than four times. By using TEOA as the hole scavenger, the resultant photocatalyst provided an AQY of 14% for H2 generation under 420-nm monochromatic illumination. This catalyst also exhibited high stability during a reaction under days of continuous illumination. These findings show the high potential of grafting aromatic heterocycles and substituting triazine nitrogen with carbon through ethanol annealing for extending the π-conjugation systems of carbon nitrides, and offer an ecofriendly path for the preparation of photosensitive materials applicable in photocatalysis and other photoenergy conversion systems.

ASSOCIATED CONTENT Supporting Information This information is available free of charge on the ACS Publications website at DOI: 25



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SEM images of the specimens; C/N atomic ratios as determined by EDS; Tauc plots of the specimens; electrochemical Mott–Schottky plots of the specimens; long-term H2 evolution data for photocatalysis with C-aCNS; and AQYs for specimens under irradiation at various wavelengths.

ACKNOWLEDGMENTS This research was supported by the Ministry of Science and Technology, Taiwan (104-2221-E-006-231-MY3, 104-2221-E-006-234-MY3, 104-3113-E-006-005, and 104-3113-E-006-011-CC2), and the Ministry of Education, Taiwan, to the Aim for the Top University Project at National Cheng Kung University.

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Extending the π-Conjugation of g-C3N4 by Incorporating Aromatic Carbon for Photocatalytic H2 Evolution from Aqueous Solution

Po-Kai Chuang,a Kwun-Han Wu,a Te-Fu Yeh,a and Hsisheng Teng*,a,b

TOC

Synopsis

Aromatic carbon-incorporated g-C3N4 synthesized by annealing ethanol-solvated g-C3N4 nanosheets for extending the π-conjugated system exhibits remarkable photocatalytic water-splitting performance.

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Extending the Ï-Conjugation of g-C3N4 by Incorporating Aromatic

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