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*,† †
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, People’s Republic of China ‡ School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459, Singapore S Supporting Information *
ABSTRACT: High-crystalline g-C3N4 nanosheets (HC−CN) with reduced structural defects have been constructed through Ni-foam-induced thermal condensation because Ni-foam not only serves as a template for deposition of the 2D g-C3N4 nanosheets with high surface area to prevent stacking of g-C3N4 nanosheets but also acts as a catalyst to promote the polymerization and crystallization of g-C3N4 via effective dehydrogenation of the −NH2 group. The obtained HC−CN exhibits superior photocatalytic performance for H2 evolution under visible light irradiation (λ > 400 nm), which significantly benefits from the prolonged lifetime of photogenerated charge carriers and the increase of the transfer path within 2D structures of high-crystalline g-C3N4 nanosheets. polymerization or condensation with the −NH2 groups left.13 The unreacted −NH2 groups, often defined as structure defects, act as charge trap sites in photocatalytic reactions, resulting in low photocatalytic activity.14,15 To solve this problem, the high-crystalline g-C3N4 has been prepared by an alkali-metal salt medium in the process of thermal condensation.16−18 Nonetheless, the obtained g-C3N4 was not as good as we expected, seemingly aroused by the secondary change of the g-C3N4 structure in the molten salt system, during which alkali metals would coordinate into the C−N plane of g-C3N4 and change the 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 to some extent.16 Thus, exploration of an alternative route is urgently needed to obtain high-crystalline gC3N4 nanosheets with reduced structural defects. As we know, 2D g-C3N4 with sp2 bonded aromatic C−N rings possess an analogous structure to graphene with sp2 bonded aromatic C− C rings. High-quality and uniform graphene could be synthesized directly on copper foils using chemical vapor deposition, during which the copper foil substrates serve as both the catalyst and template to decompose the carbon source, such as CH4, via a dehydrogenation reaction and deposit graphene on their exposed surfaces.19,20 Similarly, a thin nickel
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he importance of energy conversion and environmental protection in the 21st century constitutes worldwide challenges. To resolve this problem, the use of solar energy to cleave water into hydrogen over semiconductor photocatalysts has aroused enormous interest.1 Large amounts 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 a low-cost, sustainable, and highly 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 and good thermal and chemical stability of g-C3N4 allowed it to be developed rapidly as one of the star materials in photocatalysis.5 However, the small specific surface area and high electron−hole recombination rate that could limit its 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 semiconductors,10,11 and so on. In particular, the construction of g-C3N4 nanosheets with a two-dimensional (2D) structure has attracted considerable attention because of their fascinating properties that 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 are often amorphous with low crystallinity due to the incomplete © 2018 American Chemical Society
Received: December 24, 2017 Accepted: January 30, 2018 Published: January 30, 2018 514
DOI: 10.1021/acsenergylett.7b01328 ACS Energy Lett. 2018, 3, 514−519
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Cite This: ACS Energy Lett. 2018, 3, 514−519
Letter
ACS Energy Letters 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 highcrystalline g-C3N4 nanosheets with sp2 bonded aromatic C−N rings could be prepared by using a 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 gC3N4 nanosheets, resulting in extraordinary performance for photocatalytic H2 evolution. The scheme for synthesis of the high-crystalline g-C3N4 nanosheets (HC−CN) is as follows (Scheme 1): First, Ni-foam is immersed in the aqueous solution
were not observed, owing to the fact that the Ni-foam has been completely removed and no impurity remained after acid etching. Importantly, the (002) diffraction peak for HC−CN becomes narrower than that of CN, indicating the increased degree of condensation and the product of high-crystalline gC3N4. In addition, the (002) diffraction peak of HC−CN shifts slightly toward high angles, indicating that the interlayer distance between the basic sheets of HC−CN became smaller.25 The high-crystalline structure of 2D HC−CN could be further verified by TEM and HRTEM. In comparison with CN (Figure S1a), the HC−CN appears as a 2D sheet-like structure with wrinkles and folds (Figure 1b), declaring the formation of a 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 a catalyst to effectively optimize the crystal structure of g-C3N4. In this case, a significantly lower EPR spectral intensity was observed from the HC−CN photocatalytst (Figure S2), indicating a decreased unpaired electron density attributed 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 the nanosheet. As shown in Figure 1d, the two samples show similar type IV adsorption−desorption characteristics with type H3 hysteresis loops.26 The shape of the H3 hysteresis loop for HC−CN becomes obvious, which is associated with slit-like pores originating from 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 further confirm the formation of 2D nanosheet structures in HC−CN. Therefore, it is confirmed that the high-crystalline gC3N4 nanosheets are successfully obtained by using Ni-foam as the template and catalyst. 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 Analogous to that phenomenon, it is considered that the Ni-foam played an important role in the formation of high-crystalline g-C3N4 nanosheets with graphitic structure. As shown in Figure S3, after the thermal condensation reaction of the dicyandiamide-modified Ni-foam at 550 °C, the obtained intermediate product was first analyzed by XRD patterns. It can be clearly seen that these peaks belong to g-C3N4, Ni3N, and Ni, suggesting that the Ni-foam has participated in the reaction for HC−CN formation.27 From the TEM image of the HC−CN photocatalyst collected by ultrasonic treatment (Figure S4a), some nanoparticles are detected on the surface of g-C3N4 nanosheets, and the lattice fringes of nanoparticles correspond to the plane of Ni3N28 and Ni.29 From combination XRD and HRTEM characterization, the intermediate product of HC−CN is composed of a mixture of g-C3N4, Ni3N, and Ni, suggesting that 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 °C31 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-
Scheme 1. Schematic for the Formation of HC−CN Photocatalyst
of dicyandiamide and then heated at 80 °C for 12 h. The dicyandiamide will recrystallize and anchor on the Ni-foam or also fill up the pores of Ni-foam. Subsequently, the dicyandiamide-modified Ni-foam is treated at 550 °C to fabricate high-crystalline g-C3N4 nanosheets through a thermal condensation reaction. In this process, dicyandiamide easily becomes the vapor gas when the temperature exceeds the boiling point at 252 °C, and then g-C3N4 nanosheets form and precipitate on the Ni-foam at higher temperature. Therefore, the Ni-foam could act as the template to prevent stacking of gC3N4 nanosheets and generate 2D g-C3N4 nanosheets with high surface area and may serve as a 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 are obtained after removing the Ni-foam by acid etching. Bulk CN was prepared without Ni-foam for comparison. 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 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 a similar diffraction peak for CN, suggesting the retention of the basic crystal structure of g-C3N4. Meanwhile, the topic peaks of Ni-foam 515
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Figure 1. XRD patterns of CN and HC−CN photocatalysts (a); TEM and HRTEM of CN and HC−CN photocatalysts (b,c); nitrogen adsorption−desorption isotherms with corresponding pore size distribution curves (inset) of CN and HC−CN photocatalysts (d).
Thus, a higher ratio of sp2 C−NC 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, respectively. Therefore, the HC−CN photocatalyst has a higher degree of condensation, which is in accordance with the results 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 gaps of HC−CN and CN are calculated through the Tauc approach and estimated to be 2.92 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 photoinduced charge carrier excitation, separation, and migration are closely related to the structural defects of semiconductor materials.14 The photoluminescence (PL) spectra and time-resolved fluorescence decay spectroscopy are usually used to explore the behavior of charge carriers 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 that 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 is a benefit for the transfer of charge carriers, which can be further 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
foam. Simultaneously, the Ni-foam may accelerate the dehydrogenation of −NH2 groups to reduce the structural defects and improve the 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, X-ray photoelectron spectroscopy (XPS) is performed. As shown in Figure S6, there are only three peaks attributed to C, N, and O for HC−CN, suggesting that the Nifoam has been completely removed after acid etching. The C 1s spectrum (Figure S6b) can be deconvoluted into three peaks, located at 284.6, 286.1, and 288.0 eV. The peak at 284.6 eV is attributed to the graphite sp2 C−C bonds. The peak 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 belongs to the sp2 hybridized carbon bonded to N in an aromatic ring (N−CN).34 For N 1s (Figure S6c), there are four peaks located at 398.4, 399.6, 400.8, and 404.2 eV, which are related to the sp2 N atoms in the triazine rings (C−NC), sp3 N atoms in H−N−(C)3, NH groups, and the charging effects.35 There are 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 shown in Table S1. The sp2 N atoms play a key role in band gap absorption and therefore as an important part in the structure. Meanwhile, the sp3 H−N−(C)3 and C− NHx bonds are an outstanding measure of the structure defects. 516
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photocatalyst was carried out after the photocatalytic reaction. From TEM and HRTEM analyses (Figure S8), lattice fringes of both the Pt nanoparticle and g-C3N4 could be observed, suggesting that the HC−CN is highly stable under visible light irradiation. Furthermore, the wavelength-dependent apparent quantum efficiency (AQE) of H2 evolution over HC−CN is shown in Figure.S7. The tendency of AQE is similar to the UV−vis diffuse reflectance spectra, indicating that the H2 production is primarily induced by the light-excited electrons. The AQE of HC−CN is determined to be about 6.17% at 420 nm, which is better than that of other g-C3N4-based photocatalysts reported previously (Table S3). At last, the photocatalytic stability of HC−CN was examined for prolonged irradiation time. The photocatalytic activity without noticeable deactivation was observed after four cycles, which confirms that HC−CN is a stable photocatalyst and has potential for longterm photocatalytic applications. Photocurrent measurements and electrochemical impedance spectroscopy (EIS) further verify the high-crystallinity and 2D nanosheet structure upon separation and transportation of photogenerated electron−hole pairs. The higher photocurrent intensity for HC−CN suggests more efficient separation of photogenerated charge carriers (Figure 3c). Meanwhile, semicircular Nyquist plots of HC−CN show obviously reduced diameter (Figure 3d), further confirming the smaller charge transfer resistance and higher separation efficiency of photogenerated electron−hole pairs for photocatalytic H2 evolution in HC−CN compared to that in CN. On the basis of the above experimental and characterization results, we may conclude that the enhancement of the photocatalytic H2 evolution is attributed to three aspects: (i) The enlarged band gap that is aroused by the quantum size effect: From a combination of the XPS valence band spectra (Figure S9) and band gap energy (Figure 2b), the band potentials are shown in Figure S10. It can be clearly seen that the negative shift of the conduction band (CB) occurred on
Figure 2. UV−vis diffuse reflectance spectra (a) and (ahν)2 versus hν plot (b) of CN and HC−CN photocatalysts, PL spectra (c), and time-resolved fluorescence decay spectra (d) of CN and HC−CN photocatalysts.
of the transfer path within 2D structures of g-C3N4 in the photocatalytic reaction, which is beneficial for improvement of photocatalytic activity. It is expected that the HC−CN could be a promising visible light photocatalyst for H2 evolution from water. As shown in Figure 3a, the HC−CN exhibits superior H2 evolution compared to CN under 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). Enhancement of the photocatalytic H2 evolution results from the highcrystallinity, 2D nanosheet structures, larger surface area, and efficient separation of photoexcited charge carriers in HC−CN. Simultaneously, morphology characterization of HC−CN
Figure 3. Photocatalytic H2 evolution rate of CN and HC−CN photocatalysts under visible-light irradiation (a), recycling of HC−CN photocatalyst for H2 evolution (b), photocurrent responses (c), and (d) EIS spectra of CN and HC−CN photocatalysts. 517
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ACKNOWLEDGMENTS The authors acknowledge financial support from the National Nature Science Foundation of China (21471040 and 21303030) and the China Scholarship Council (CSC) program.
HC−CN. The more negative potential of CB made the electrons have stronger reduction capability, which is accelerated for the photocatalytic H2 evolution. (ii) The higher separation efficiency of photogenerated carriers: The HC−CN photocatalyst possesses high-crystallinity with low structural defects, which could accelerate the rate of charge transfer. Meanwhile, the 2D structure decreases the transfer distance of photogenerated carriers from the bulk phase to the surface of HC−CN, reducing the efficiency of charge recombination and facilitating the photocatalytic H2 evolution. (iii) The larger specific surface area of HC−CN provides more photocatalytic reaction centers for H2 evolution. In conclusion, high-crystalline g-C3N4 nanosheets with reduced structural defects have been fabricated by the Nifoam-induced thermal condensation reaction. The HC−CN possesses high-crystallinity, lower defects, large surface area, and 2D nanosheet structures. The prolonged lifetime and efficient separation of photogenerated charge carriers in HC− CN significantly contribute to the enhanced H2 evolution. This work may open a new view for the design and synthesis of highcrystalline g-C3N4 photocatalysts. 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.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b01328. Experimental section, characterization, photocatalytic and photoelectrochemical measurements; TEM and HRTEM of CN photocatalyst; XRD pattern of Nifoam−HC−CN; TEM and HRTEM images of Nifoam−HC−CN; FT-IR spectra of CN and HC−CN photocatalysts; XPS full spectra, high-resolution XPS spectra of C 1s, N 1s, and Ni 2p of CN and HC−CN photocatalysts; wavelength-dependent apparent quantum efficiency of H2 evolution over HC−CN photocatalysts; TEM and HRTEM images of HC−CN after photocatalytic reaction; XPS valence band spectra of CN and HC−CN photocatalysts; schematic illustration of the band structure over CN and HC−CN photocatalysts; contribution ratio of different peaks in N 1s for CN and HC−CN photocatalysts according to XPS analysis; decay lifetimes and the average life time of photoexcited charge carriers in CN and HC−CN photocatalysts; and comparison of photocatalytic H2 evolution of photocatalyst from recent publications (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Weinan Xing: 0000-0002-6198-0837 Wenguang Tu: 0000-0002-0800-9777 Gang Chen: 0000-0003-1502-0330 Notes
The authors declare no competing financial interest. 518
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DOI: 10.1021/acsenergylett.7b01328 ACS Energy Lett. 2018, 3, 514−519
