Template-Induced High-Crystalline g-C3N4 Nanosheets for Enhanced

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Template-Induced High-Crystalline g-C3N4 Nanosheets for Enhanced Photocatalytic H2 Evolution Weinan Xing, Wenguang Tu, Zhonghui Han, Yidong Hu, Qingqiang Meng, and Gang Chen ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01328 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on January 31, 2018

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ACS Energy Letters

Template-Induced High-Crystalline g-C3N4 Nanosheets for Enhanced Photocatalytic H2 Evolution

Weinan Xing, †‡ Wenguang Tu, ‡ Zhonghui Han, †Yidong Hu, † Qingqiang Meng, †Gang Chen, *†

†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

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (G. C.)

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ABSTRACT: The high-crystalline g-C3N4 (HC-CN) nanosheets with reduced structural defects has been constructed through the Ni-foam induced thermal condensation, because Ni-foam not only serves as the template for the deposition of the 2D g-C3N4 nanosheets with high surface area to prevent the stacking of g-C3N4 nanosheets, and but also acts as catalyst to promote the polymerization and crystallization of g-C3N4 via the effective dehydrogenation of -NH2 group. The obtained HC-CN nanosheets exhibt superior photocatalytic performance for H2 evolution under visible light irradiation (λ> 400 nm), which significantly benefit from the prolonged lifetime of photogenerated charge carriers and the increase of the transfer path within 2D structures of high-crystalline g-C3N4 nanosheets.

TOC GRAPHICS

The high-crystalline g-C3N4 (HC-CN) nanosheets with reduced structural defects has been constructed through the Ni-foam induced thermal condensation. The obtained HC-CN nanosheets exhibt superior photocatalytic performance for H2 evolution under visible light irradiation.


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The importance of the energy conversion and environmental protection in the 21st century constitutes the worldwide challenges. To resolve this problem, the use of solar energy to cleave water into hydrogen over semiconductor photocatalysts aroused enormous interest.1 Large amount of materials have been explored as semiconductor photocatalysts over the recent decades. Unfortunately, the low H2 product and low utilization efficiency of sunlight greatly limit the potential practical applications.2 Therefore, the development of low-cost, sustainable, and high efficient photocatalyst became a crucial issue. Graphitic carbon nitride (g-C3N4) has recently emerged as a metal-free heterogeneous catalyst for solar energy utilization.3-4 The well-suited band position, good thermal and chemical stability of g-C3N4 made it to be developed rapidly as one of star materials in photocatalysis.5 However, the small specific surface area and high electron–hole recombination rate that could limit it catalytic efficiency are urgently needed to overcome.

6

Consequently,

various modification methods have already emerged, including element doping,7 morphological control,8-9 coupling with other semiconductor10-11 and so

on.

In

particular,

the

construction

of

g-C3N4

nanosheets

with

two-dimensional (2D) structure has attracted considerable attention, because of their fascinating properties would endow this material with optimal photocatalytic activity. For example, it has been proved that the g-C3N4 nanosheets possess higher surface area, enlarged redox potentials, and prolonged charge carrier lifetime in comparison with bulk g-C3N4.12 However, the current reported g-C3N4 nanosheets often amorphous with low crystallinity due to the incomplete polymerization or condensation with -NH2 groups left.13 The unreacted-NH2 groups often defined as the structure defects act as the charge trap sites in photocatalytic reactions, resulting in the low photocatalytic activity. 14-15 To solve this problem, the high-crystalline g-C3N4 has been prepared by alkali-melt salt medium in the process of thermal condensation.16-18 3 ACS Paragon Plus Environment


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Nonetheless, the obtained g-C3N4 was not as good as we expected, seemingly aroused by the secondary change of g-C3N4 structure in the molten salt system, during which alkali metals would coordinate into the C–N plane of g-C3N4 and change spatial charge distribution. Meanwhile, the bulk crystalline carbon nitrides prepared by this method often suffered from the higher surface defect density, which also limited the improvement of the photocatalytic activity in some extent.16 Thus, it is urgent needed to explore alternative route to obtain the high-crystalline g-C3N4 nanosheets with reduced structural defects. As we know, 2D

g-C3N4 with sp2 bonded aromatic C–N rings processes the analogous structure to graphene with sp2 bonded aromatic C–C rings. The high-quality and uniform graphene could be synthesized directly on copper foils using chemical vapor deposition, during which the copper foils substrate serve as both catalyst and template to decompose the carbon source such as CH4 via dehydrogenation reaction and deposit graphene on their exposed surfaces.19-20 Similarity, thin nickel layer film21 or Ni foam22 could also be selected as templates and catalysts to fabricate the high crystallinity few-layer graphenes or graphene foam, respectively. Encouraged by the product of high crystallinity graphene with the metal substrates as the catalysts and templates, it seems plausible that the high-crystalline g-C3N4 nanosheets with sp2 bonded aromatic C–N rings could be prepared by using the similar method. Herein, we used the Ni-foam as the template and catalyst to synthesize the high-crystalline

g-C3N4

nanosheets

with

defined

structures and reduced structural defects. The high-crystalline structures can prolong the lifetime of photogenerated charge carriers and increase the transfer path within 2D structures of g-C3N4 nanosheets, resulting in extraordinary

performance for photocatalytic H2 evolution. The scheme for synthesis the high-crystalline g-C3N4 nanosheets (HC-CN) are as follows (Scheme 1): Firstly, Ni-foam immersed in the aqueous solution of dicyandiamide and then heated at 80 °C for 12h. The dicyandiamide will be recrystallized and anchored 4 ACS Paragon Plus Environment

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on the Ni-foam or also fills up the pores of Ni-foam. Subsequently, the dicyandiamide modified Ni-foam was treated at 550 °C to fabricate high-crystalline g-C3N4 nanosheets through thermal condensation reaction. In this process, dicyandiamide easily become the vapor gas when the temperature exceeds the boiling point at 252 °C and then g-C3N4 nanosheets will form and precipitate on Ni foam at higher temperature. Therefore, the Ni-foam could act as the template to prevent the stacking of g-C3N4 nanosheets and generate the 2D g-C3N4 nanosheets with high surface area, and may serve as catalyst to promote the polymerization and crystallization of g-C3N4 via reducing structural defects (-NH2 or NH groups) from the effective dehydrogenation. Finally, the high-crystalline g-C3N4 nanosheets (HC-CN) is obtained after removing the Ni-foam by acid etching. Bulk CN were prepared without Ni foam for comparison.

Scheme 1. Schematic for the formation of HC-CN nanosheets photocatalyst.

The high-crystalline g-C3N4 nanosheets with high surface area could be confirmed by the X-ray diffraction (XRD) pattern, high-resolution transmission electron microscopy (HRTEM), and nitrogen adsorption−desorption isotherm. As illustrated from the XRD patterns in Figure 1a, the intense peaks at 13.0 ° 5 ACS Paragon Plus Environment


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and 27.5° could be attributed to the in-plane repeated units and the interlayer stacking reflection in g-C3N4, respectively.23-24

The HC-CN exhibits the

similar diffraction peak of CN, suggesting the remaining of basic crystal structure of g-C3N4. Meanwhile, the topic peaks of Ni foam were not observed, owing to that the Ni-foam has been completed removed and no impurity was remained after acid etching. Importantly, the (002) diffraction peak for HC-CN is become narrower than that of CN, indicating the increased degree of condensation and the product of high-crystalline g-C3N4. In addition, the (002) diffraction peak of HC-CN shifts slightly toward the high angels, indicating the interlayer distance between the basic sheets of HC-CN become smaller.25 The high-crystalline structure of 2D HC-CN nanosheets could be further verified by the TEM and HRTEM. In comparison with CN (Figure S1a), the HC-CN appears the existence of 2D sheet-like structure with wrinkles and folds (Figure 1b), declaring the formation of nanosheet structure in HC-CN. Importantly, the crystal lattice of HC-CN is obvious in the HRTEM of Figure 1c, which is distinctly different with subcrystalline CN in Figure S1b. The lattice distance of these lattice fringes is about 0.33 nm, corresponding to the (002) plane of g-C3N4 and also revealing the graphitic structure of the prepared HC-CN. It is revealed that Ni-foam could act as catalyst to effectively optimize the crystal structure of g-C3N4. In this case, a significantly lower EPR spectral intensity was observed from HC-CN nanosheets photocatalytst (Figure S2), indicated a decreased unpaired electrons density attribute to the reduced structural defects. The BET specific surface area and the porous structure of HC-CN are used to further confirm the nature of nanosheet. As shown in Figure 1d, the two samples show similar type IV adsorption–desorption characteristics with type H3 hysteresis loops26. The shape of H3 hysteresis loop for HC-CN become obvious, which is associated with slit-like pores originating from the stacking of the g-C3N4 nanosheet. The BET specific surface area of HC-CN is 39.24 m2 g-1, which is much larger than that of CN (7.73 m2 g-1). These results can 6 ACS Paragon Plus Environment

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furtherly confirm the formation of 2D nanosheet structures in HC-CN. Therefore, it is confirmed that the high-crystalline g-C3N4 nanosheets are successfully obtained by using Ni-foam as the template and catalyst.

Figure 1 XRD patterns of CN and HC-CN nanosheets photocatalysts (a); TEM and HRTEM of CN and HC-CN nanosheets photocatalysts (b,c); Nitrogen adsorption–desorption isotherms with the corresponding pore size distribution curves (inset) of CN and HC-CN nanosheets photocatalysts (d).

As

mentioned above, Ni-foam could act as the template and catalyst for the

nucleation of graphene growth via dehydrogenation of the starting material.22 Be analogous to that phenomenon, it is considered that the Ni-foam played the important role for the formation of high-crystalline g-C3N4 nanosheets with 7 ACS Paragon Plus Environment


graphitic structure.

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As shown in Figure S3, after the thermal condensation

reaction of the dicyandiamide modified Ni-foam at 550 °C, the obtained intermediate product was firstly analyzed by XRD pattern. It can be clearly seen that these peaks belong to g-C3N4, Ni3N and Ni, suggesting the Ni-foam has participated the reaction for the HC-CN formation.27 From the TEM image of the HC-CN samples collected by ultrasonic treatment (Figure S4a), some nanoparticles are detected on the surface of g-C3N4 nanosheets and the lattice fringes of nanoparticles are corresponding to the plane of Ni3N,28 and Ni,29 respectively. Combination the XRD and HRTEM characterization, intermediate product of HC-CN are composed by the mixture of g-C3N4, Ni3N, and Ni, suggesting the layer of Ni on the Ni foam surface may interact with the intermediate product during the g-C3N4 process to transform the nanoparticles easily peeled off from Ni foam.30 Therefore, the growth mechanism of HC-CN may be that the dicyandiamide molecules absorbed on the surface of Ni-foam could become the vapor gas after the temperature exceeds the boiling point at 252 °C

31

and then undergo a set of polymerization processes to form and

deposit g-C3N4 on Ni foam during which the release of significant amounts of ammonia can reduce the Ni into Ni3N on the Ni foam. Simultaneously, because the Ni foam may accelerate the dehydrogenation of -NH2 groups to reduce the structural defects and improve the high-crystallinity of g-C3N4. Therefore, the

high-crystalline g-C3N4 nanosheet is obtained. As shown in the Fourier transform infrared (FT-IR) spectra (Figure S5), the characteristic absorption bands of both samples, 3000−3500 cm−1 for O−H or N−H stretching, 1200−1700 cm−1 for stretching of CN heterocycles, and 807 cm−1 for stretching of heptazine units indicate the formation of g-C3N4.

32-33

Meanwhile, to understand the surface composition and chemical states, the X-ray photoelectron spectroscopy (XPS) are performed. As shown in Figure S6, there are only three peaks attribute to the C, N and O for HC-CN, suggesting the Ni-foam has been completely remove after the acid etching. The 8 ACS Paragon Plus Environment

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C 1s spectrum (Figure S6b) can be deconvoluted to three peaks, located at 284.6 eV, 286.1 eV and 288.0 eV. The peak at 284.6 eV is attributed to the graphite sp2 C-C bonds. The peaks at 286.1 eV is a result of sp2 C atoms in the s-triazine rings attached to the −NH2 or NH group (C-NHx) while the peak at 288.0 eV is belong to the sp2 hybridized carbon bonded to N in aromatic ring (N-C=N).34 For N1s (Figure S6c) , there are four peaks located at 398.4 eV, 399.6 eV, 400.8 eV and 404.2 eV, which are related to the sp2 N atoms in the triazine rings (C−N=C), sp3 N atoms in H-N-(C)3, NH groups, and the charging effects.35 No chemical shifts of N 1s and C 1s in comparison with CN, indicating the presence of basic substructure units of g-C3N4 in HC-CN. In addition, the contribution ratio of different peaks in N 1s for CN and HC-CN is calculated and showed in Table S1. The sp2 N atoms play a key role in band gap absorption and therefore as an important part in structure. Meanwhile, the sp3 H-N−(C)3 and C−NHx bonds as an outstanding measure of the structure defects. Thus, a higher ratio of sp2 C-N=C bonds to the sum of sp3 H-N-[C]3 and C-NHx bonds means a higher degree of condensation.36-37 The calculated ratios are 2.65 and 2.17 for HC-CN and CN. Therefore, the HC-CN sample has the higher degree of condensation, which is in accordance with the result of XRD and HRTEM analysis. The structure change of g-C3N4 can directly influence its light absorption properties. The optical properties of the samples are investigated by UV-vis absorption spectra. As shown in Figure 2a, the absorption edge of HC-CN exhibits an obvious blue-shift by comparison with CN. Meanwhile, the band gap of HC-CN and CN are calculated through Tauc approach and estimated to be 2.92 eV and 2.75 eV, respectively (Figure 2b). The blue-shifting of the absorption edge and enlarged band gap are due to the quantum size effect caused by the ultrathin nanosheet structures of HC-CN.38 The efficiency of photo-induced charge carrier excitation, separation and migration are closely related to the structural defects of semiconductor 9 ACS Paragon Plus Environment


materials.14

The

Photoluminescence

(PL)

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spectra

and

time-resolved

fluorescence decay spectroscopy are usually used to explore the behavior of charge carrier in semicondutors. As shown in Figure 2c, a remarkably weakened emission peak is observed for HC-CN in contrast with CN, indicating the decrease of the recombination efficiency of charge carriers in HC-CN.39 This phenomenon also supports the structural defects are reduced in HC-CN, because the defects often act as the recombination centers of photogenerated charge carriers. In addition, the high-crystalline HC-CN is benefit for the transfer of charge carriers, which can be furtherly proved by time-resolved fluorescence decay spectroscopy. The longer lifetime of the charge carriers in HC-CN is readily observed from Figure 2d and Table S2, signifying the increase the transfer path within 2D structures of g-C3N4 in the photocatalytic

reaction, which is beneficial for the improvement

photocatalytic activity.

Figure 2 UV-Vis diffuse reflectance spectra (a) and (ahν)2 versus hν plot (b) of CN

and HC-CN nanosheets photocatalysts, PL spectra (c) and time-resolved fluorescence decay spectra (d) of CN and HC-CN nanosheets photocatalysts


of

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It is expected that the high-crystalline HC-CN nanosheets could be a promising visible light photocatalyst for the H2 evolution from water. As shown in Figure 3a, the HC-CN exhibits a superior H2 evolution than CN under the visible light irradiation. The average H2 evolution rate of HC-CN is 80.85 µmol h−1, which is 20 times higher than that of CN (3.95 µmol h−1). The enhancement of the photocatalytic H2 evolution is resulted from the high crystallinity, 2D nanosheet structures, larger surface area, and efficient separation of photoexcited charge carriers in HC-CN. Simultaneously, the morphology characterization of HC-CN nanosheets photocatalyst was carried out after the photocatalytic reaction. From the TEM and HRTEM analysis (Figure S8), both of lattice fringes of Pt nanoparticle and g-C3N4 could be observed, suggested the high-crystalline HC-CN nanosheets are highly stable under visible light irradiation. Furthermore, the wavelength-dependent apparent quantum efficiency (AQE) of H2 evolution over HC-CN nanosheets is shown in Figure.S7. The tendency of AQE is similar to the UV-Vis diffuse reflectance spectra, indicating the H2 production is primarily induced by the light-excited electrons. The AQE of HC-CN nanosheets is determined to be about 6.17% at 420 nm, which is better than other g-C3N4 based photocatalyst reported previously (Table S3). At last, the photocatalytic stability of HC-CN was examined for prolonged the irradiation time. The photocatalytic activity without noticeable deactivation was observed after 4 cycles, which confirms that HC-CN is a stable photocatalyst and potential for long-term photocatalytic applications. The photocurrent measurements and electrochemical impedance spectroscopy (EIS) furtherly verify the high-crystallinity and 2D nanosheet structure on separation and transportation of photogenerated electron−hole pairs. The higher photocurrent intensity for HC-CN suggests the more efficient separation of photogenerated charge carriers (Figure 3c). Meanwhile, semicircular Nyquist plots of HC-CN show obviously reduced diameter (Figure 3d), furtherly confirm the smaller charge transfer resistance and higher separation eﬃciency 11 ACS Paragon Plus Environment


of photogenerated electron-hole pairs for photocatalytic H2 evolution in HC-CN compared to CN. Based on the above experimental and characterization results, we may conclude the enhancement of the photocatalytic H2 evolution is attributed to three aspects. (i) The enlarged band gap which aroused by the quantum size effect. Combination the XPS valence band spectra (Figure S9) with bandgap energy (Figure 2b), the band potentials are shown in Figure S10. It can be clearly seen that the negative shift of conduction band (CB) is occurred on HC-CN. The more negative potential of CB made the electrons with stronger reduction capability, which is accelerated for the photocatalytic H2 evolution. (ii) The higher separation eﬃciency of photogenerated carriers. The HC-CN nanosheets photocatalyst possess high-crystallinity with low structural defects, which could accelerate the rate of charge transfer. Meanwhile, the 2D structure decrease the transfer distance of photogenerated carriers from the bulk phase to the surface of HC-CN, reduced the efficiency of charge recombination and facilitated the photocatalytic H2 evolution. (iii) The larger specific surface area of HC-CN provides more photocatalytic reaction centers for H2 evolution.


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Figure 3 Photocatalytic H2 evolution rate of CN and HC-CN nanosheets photocatalysts under visible-light irradiation (a), Recycling of HC-CN nanosheets photocatalysts for H2 evolution (b), Photocurrent responses (c) and (d) EIS spectra of CN and HC-CN nanosheets photocatalysts. In conclusion, the high-crystalline g-C3N4 nanosheets with reduced structural defects have been fabricated by the Ni-foam induced thermal condensation reaction. The HC-CN nanosheets possess the high-crystallinity, the lower defects, the large surface area, and 2D nanosheets structures. The prolonged lifetime and efficient separation of photogenerated charge carriers in HC-CN nanosheets significantly contributes for the enhanced H2 evolution. This work may open a new view for the design and synthesis of high-crystalline g-C3N4 photocatalyst. Meanwhile, benefiting from the inherent structure of high-crystallinity, the g-C3N4 nanosheets support a broad range of potential applications in biology, sensors, energy storage and conversion.

ACKNOWLEDGMENT The authors acknowledge the financially supported by the National Nature Science Foundation of China (21471040 and 21303030), and the China Scholarship Council (CSC) program.

Supporting Information Available Experimental section, Characterization, Photocatalytic and photoelectrochemical measurements; TEM and HRTEM of CN photocatalyst; XRD pattern of Ni-foam-HC-CN; TEM and HRTEM images of Ni-foam-HC-CN; FT-IR spectra of CN and HC-CN nanosheets photocatalysts; XPS full spectra, High-resolution XPS spectra of C1s, N1s and Ni 2p of CN and HC-CN nanosheets photocatalysts; Wavelength-dependent apparent quantum efficiency (AQE) of H2 evolution over HC-CN nanosheets photocatalyst; TEM and HRTEM images of HC-CN nanosheets after the potocatalytic raction; XPS valence band spectra of CN and HC-CN nanosheets photocatalysts; Schematic illustration of the band structure over CN and 13 ACS Paragon Plus Environment


HC-CN nanosheets photocatalysts; The contribution ratio of different peaks in N 1s for CN and HC-CN nanosheets photocatalysts according to XPS analysis; The decay life times and their fractional contribution of photoexcited charge carriers in CN and

HC-CN nanosheets photocatalysts; Comparison of photocatalytic H2 evolution of photocatalyst from recent publications. NOTES There are no conflicts to declare.

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ACS Energy Letters

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Template-Induced High-Crystalline g-C3N4 Nanosheets for Enhanced

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