Self-assembled mesoporous carbon nitride with tunable texture for

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Self-assembled mesoporous carbon nitride with tunable texture for enhanced visible-light photocatalytic hydrogen evolution Shuo Zhao, Yiwei Zhang, Jiasheng Fang, Hao Zhang, Yanyun Wang, Yuming Zhou, Wenxia Chen, and Chao Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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ACS Sustainable Chemistry & Engineering

Self-assembled mesoporous carbon nitride with tunable texture for enhanced visible-light photocatalytic hydrogen evolution Shuo Zhao,a Yiwei Zhang,*a Jiasheng Fang,a Hao Zhang,*b Yanyun Wang,a Yuming Zhou,*a Wenxia Chen,a Chao Zhang,a a

School of Chemistry and Chemical Engineering, Southeast University, Jiangsu

Optoelectronic Functional Materials and Engineering Laboratory, Nanjing 211189, P. R. China. b

Department of Chemical and Materials Engineering, University of Alberta,

Edmonton, T6G 1H9, Canada. E-mail: [email protected]; [email protected]; [email protected] Tel: +86 25 52090617; Fax: +86 25 52090617.

Abstract Mesoporous carbon nitride with novel morphology has been successfully fabricated by using ionic liquid and supramolecular precursor cyanuric acid-melamine complex as the template and precursor, respectively. The texture of carbon nitride is controlled by adjusting the ionic liquid concentration and type. The results find that a disordered hollow carbon nitride box morphology with pores in the wall can be obtained under lower IL1 concentration. With increasing of ionic liquid content, the texture turns to be a layered structure. The disordered nanosheets and hollow tube with large pores in the shell can be fabricated by using IL-2 and IL-3 as templates, respectively. Besides, as-prepared mesoporous carbon nitride samples display the excellent light harvesting

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properties and fast separation rate of photogenerated carrier. Moreover, mesoporous carbon nitride CN-0.1 exhibits a higher photocatalytic hydrogen evolution rate (120.3 µmol h-1) under visible light irradiation owing to the special structure and the presence of multiple mesopores, which is approximately 25.6, 1.7 times as high as that of bulk carbon nitride calcined by melamine and CN, respectively. Furthermore, photocatalytic RhB degradation rate is enhanced by using CN-0.1 compared with pure CN. This method provides more insights into designing novel visible light driven photocatalysts by using ionic liquids. Keywords: photocatalysis; ionic liquid; graphitic carbon nitride; water splitting; photodegradation Introduction Driven by the increasing global crisis of energy shortage and environmental pollution, semiconductor-based photocatalysis has been served as a clean, sustainable and safe technology to convert solar energy into a usable energy form[1-4]. The finding of water splitting by TiO2 electrodes in 1972 resulted in the study and design of numerous semiconductors including sulfides, oxides and oxynitrides for hydrogen generation, CO2 reduction and pollutant elimination[5-13]. Graphitic carbon nitride with excellent optical, chemical and catalytic properties has attracted widespread attention and been applied in photochemistry[14-16]. Additionally, carbon nitride possesses the suitable band gap for water splitting and can be one-step synthesized by thermally condensing precursors (melamine, urea and thiourea). However, the bulk carbon nitride is limited by the undesirable photocatalytic performance owing to the low surface area, the low


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separation rate of photogenerated carriers and the insufficient solar light absorption[17-19]. Thus, many modification methods have been developed to solve these problems, such as elemental doping and heterostructure construction. These methods benefit the separation of photogenerated charge carriers, prolong the lifetime, and finally enhance the photocatalytic activity[3, 20-25]. Guo et al.[26] synthesized the phosphorus-doped hexagonal tubular carbon nitride and they found that phosphorus doping contributed to the narrower band gap and increased electric conductivity. Adhikari[14] and co-workers successfully fabricated a new g-C3N4/SrTa2O6 heterojunction photocatalyst with enhanced photocatalytic activity. Besides, the morphology modification is also an easy and efficient method to improve the optical and electronic properties. Carbon nitride with a modified architecture indeed could improve the charge transfer rate and enhance the photocatalytic activity. Therefore, it is necessary and meaningful to develop the carbon nitride with a high surface area and controllable morphology. At present, hard-templating method is usually applied for synthesizing the porous structured carbon nitride with novel morphology[26-28]. Zhong et al.[29] reported that mesoporous carbon nitride with order channels could be synthesized by using SBA-15. Xu and co-workers[30] obtained the mesoporous carbon nitride with bimodal pore systems and high surface areas by using colloid silica as a hard template. However, toxic and expensive reagents, such as HF, ammonium bifluorides and NaOH solution, are involved in the template removal process[31, 32]. It is harmful to the environment and block the further practical application. Accordingly, it is pressing to design



environmentally friendly technology to fabricate porous carbon nitride. The soft templating method has been regarded as a facile way to fabricate the porous materials with large surface area[33-35]. Ionic liquid, which are molten salts at room temperature, has been studied in all kinds of fields, such as chemistry, catalysis, electrochemistry and organic synthesis[36, 37]. Especially, ionic liquid can be used as the template or the structure-directing agent to induce the porous material due to the presence of the hydrophobic core and the hydrophilic head group, which is similar with the surfactant[38, 39]. A series of porous materials have been fabricated by taking advantage of self-assembly behavior of ionic liquid as well as the synergistic effect between ionic liquid and surfactant in our previous works[40, 41]. It is certified that ionic liquid can form micelles which induce the construction of special morphology and texture. This method can be applied in the synthesis of mesoporous carbon nitride materials. While, the interaction between the soft templates and precursor may influence the degree of polymerization, then affect the final structure of carbon nitride. Therefore, it is necessary to choose an appropriate precursor for preparing mesoporous carbon nitride. Nowadays, it is found that the self-assembly of melamine and cyanuric acid through hydrogen bonding can induce the formation of cyanuric acid-melamine complex which can synthesize the carbon nitride materials with a controllable texture. Peer et al.[32] obtained the mesoporous carbon nitride by using cyanuric acid-melamine complex as the precursor in the water. Xu and co-workers[42] introduced a new type of hierarchically structured nanoporous carbon nitride using supramolecular aggregates cyanuric acid-melamine complex as the precursor by a


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one-step vapor-solid deposition approach. Nevertheless, these works are limited to the low surface area. Thus, development of carbon nitride with large surface area and controllable morphology has been of great significance. In our previous work, we have successfully used ionic liquid and cyanuric acid-melamine complex to synthesize the hollow mesoporous g-C3N4 spheres in DMSO[43]. In order to further analyze the influence of solvent and ionic liquid type to texture of carbon nitride, a deeper research is needed. Herein, we present the application of cyanuric acid-melamine complex and ionic liquid as the precursor and template for the synthesis of mesoporous carbon nitride with the tunable texture in water, respectively. The texture and morphology of carbon nitride is controlled by adjusting the ionic liquid concentration and type. Moreover, the photocatalytic activity is evaluated by the photocatalytic hydrogen evolution under visible light irradiation. Furthermore, the structure-activity relationship is studied in detail based on the supramolecular assembly of cyanuric acid and melamine through hydrogen bonding, the structure-directing character of ionic liquid as well as the relationship of precursor and template. Experimental section Chemicals The ionic liquid, 1-butyl-3-vinylimidazolium bromide (IL1), melamine and cyanuric acid

is

purchased

from

Sinopharm

Chemical

Reagent

Co.

Ltd.

Poly

(1-butyl-3-vinylimidazolium bromide) (IL2) is synthesized by autopolymerization of IL-1. Poly (1-butyl-3-vinylimidazolium bromide acrylamide) (IL3) are polymerized



by 1-butyl-3-vinylimidazolium bromide and acrylamide. The chemical formulas of these ionic liquids are listed below:

Synthesis of mesoporous carbon nitride Typically, a certain amount of ionic liquid (0.05, 0.08, 0.1, 0.15 g) was added into 40 mL of deionized water and then mixed with 0.5 g of cyanuric acid. Meanwhile, 0.5 g of melamine was dissolved in 40 mL of water in another beaker and kept stirring for 1 h. Then the melamine solution was added to the above cyanuric acid solution and kept stirring for another 20 h. The final white powder was centrifuged with water and dried at 50℃ overnight, then calcined at 550℃ for 4 h under the N2 atmosphere with heating rate of 2.3℃ per minute. The final yellow powder was denoted as CN-X (X=0.05, 0.08, 0.1, 0.15). For comparison, the samples synthesized with IL2, IL3 or without ionic liquid were also prepared and named as CN-A-0.1, CN-B-0.1 and CN, respectively. Characterization The morphology of samples was detected by scanning electron microscopy (JEOL JSM-5600L SEM Instrument with a working distance of 3-4 mm and an electron voltage of 3.0 kV). A JEM-2010 instrument with an accelerating voltage of 100 kV


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was used to obtain Transmission Electron Microscopy (TEM) images. The X-ray diffraction (XRD) patterns were recorded to exhibit the crystalline structure of the samples on a Bruker D8 Advance Diﬀractometer (Germany) with Cu-Kα radiation (λ = 1.5418 Å) at a scanning rate of 0.02 S-1 in the range of 5° to 70°, with an operation voltage and current maintained at 40 kV and 40 mA. X-ray photoelectron spectroscopy (XPS) was obtained on an ESCALAB MKII X-ray photo-electron spectrometer using Mg Kα radiation. Fourier transform infrared (FT-IR) spectra were obtained to study the chemical structures and bonds by using a Thermo Fisher FTIR6700 instrument. Ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS) were carried out to examine the optical absorbance of samples on a UV-vis spectrophotometer (UV-3600, Shimadzu) with an integrating sphere attachment. The photoluminescence (PL) spectra were obtained using the room temperature photoluminescence with a 400 nm He-Cd laser excitation wavelength (Shimadzu RF-5301). Photocatalytic hydrogen evolution test Photocatalytic hydrogen evolution test was performed in a 250 mL of reactor. 50 mg of photocatalysts was added into 100 mL of aqueous solution which containing 10 mL of triethanolamine as the sacrificial electron donor. Besides, 5 wt% of Pt was used as the cocatalyst and photodeposited onto the surface of carbon nitride. Before reaction, the mixture should be evacuated for 30 min to remove the air completely. The reaction was irradiated by using a 300 W Xe lamp equipped with a UV-light cutoff filter (400 nm). And the generated hydrogen was detected by a gas chromatogragh (GC 9890A)



and the pure argon gas was used as carrier gas. Photocatalytic degradation reaction For photocatalytic degradation reaction, 50 mg of samples were dispersed into 100 mL of RhB solution (10 mg/L). After stirring in dark for 30 min to ensure absorption-desorption equilibrium, the mixture was irradiated with a 500 W Xe lamp with a 400 nm cutoff filter. During the process, 3 mL of mixture was sampled, centrifuged and then analyzed by a UV-vis spectrophotometer. Photoelectrochemical measurements A standard three-electrode system (Ag/AgCl (saturated KCl) as the reference electrode, Pt wire as the counter electrode, prepared photocatalysts as the working electrodes and Na2SO4 solution (0.5 M) as electrolyte) was used to carry out the photoelectrochemical measurements. 2 mg of as-prepared carbon nitride was added into 1 mL of DMF to obtain a slurry. Then 20 µL of the slurry were dropped onto a ITO slice (0.5 cm2) and finally dried at 50℃ to obtain the working electrode. Results and discussion Structure and morphology Cyanuric acid-melamine supramolecular aggregates are formed by mixing cyanuric acid and melamine in water (Fig. S1). In spite of the low solubility of cyanuric acid and melamine in water, the supramolecular aggregates can be formed. The morphology of cyanuric acid-melamine supramolecular aggregates strongly depends on the solvent owing to different surface energies. Thus, the addition of ionic liquid may influence the formation of hydrogen bonding, which leading to the


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rearrangement of supramolecular aggregates and finally change the resulted morphology of as-prepared carbon nitride. FT-IR spectrum was carried out to certify the formation of the supramolecular aggregates and the results were shown in Fig. 1. It is found that there is a clear difference among melamine, cyanuric acid and supramolecular aggregates due to the presence of hydrogen bonding. Especially, C=O stretching vibration of cyanuric acid shifts from 1691 to a higher value 1734 cm-1, and the peak at 812 cm-1 which is attributed to the triazine ring vibration moves to a lower value 767 cm-1. Besides, the different features in the range of 1400-1800 cm-1 (C-N) and 3000-3500 cm-1 (stretching vibration of amino groups) indicate the presence of hydrogen bonding in the cyanuric acid-melamine supramolecular aggregates[32].

Fig. 1 FTIR patterns of cyanuric acid-melamine supramolecular aggregates, cyanuric acid and melamine

The crystallinity of as-prepared mesoporous carbon nitride was reflected in the XRD pattern and the results were listed in Fig. 2A. Evidently, all the samples display the similar XRD patterns, suggesting the successfully fabrication of carbon nitride by using ionic liquids as templates. A well lamellar stacking peak (002) at about



26.7°certifies the sheets-like structure. The peak at about 2θ = 13° is corresponding to the (100) plane of carbon nitride, owing to the in-plane graphene structure. FT-IR analysis was carried out to further study the structure of as-prepared mesoporous carbon nitride. As shown in Fig. 2B, the broad peaks at around 1200-1630 cm-1 are corresponding to the aromatic C-N heterocycles, which may be caused by the extended carbon nitride network. The peak at 811 cm-1 is caused by the characteristic breathing vibration modes of aromatic carbon nitride heterocycles, suggesting the presence of triazine units. Besides, the broad peak at 2800-3500 cm-1 is the stretching vibration of N-H and O-H, which is related to the uncondensed amino groups and absorbed water molecules.

Fig. 2 XRD (A), FTIR (B) and XPS (C, D) patterns of as-prepared mesoporous carbon nitride

X-ray photoelectron spectroscopy (XPS) measurement was carried out to check the


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chemical composition on the surface of as-prepared CN-0.1. In Fig. 2C, the peak at 284.9 eV is denoted as standard carbon, which belonging to the sp2 hybridized carbon in the form of C-C. While, another peak at 288.0 eV represents the N-C=N of aromatic carbon nitride[3]. As for Fig. 2D, the N 1s can be deconvoluted into four peaks, including the peak at 404.2 eV for positive charge localization in heterocycles, the peaks at 400.9 eV, 400.0 eV and 398.5 eV for C-N-H, N-(C)3 and C-N=C, respectively[44]. SEM images are shown in Fig. 3, displaying the morphology of as-prepared carbon nitride photocatalysts. It can be found that the sample CN synthesized without ionic liquid exhibits a disordered hollow carbon nitride box morphology with some pores in the wall. While, the original shape tends to be broke down with increasing the ionic liquid, certifying the fact that ionic liquid has a great influence to the formation of cyanuric acid-melamine aggregates based on the hydrogen bond and then finally affects the carbon nitride structure. Especially, the texture of sample CN-0.1 turns to be a layered structure and ultrathin layers with many pores as the building units. Besides, CN-0.15 exhibits a hierarchical porous carbon nitride nanostructure, whose pores should be related to the decomposition of ionic liquids. And the sample CN-A-0.1 synthesized by IL-2 shows the disordered sheet-like structure. Interestingly, the sample CN-B-0.1 displays a tubular structure with large pores in the shell, which may be caused by the fact that large scale ionic liquid micelles are formed due to the long chain of poly ionic liquid IL-3. Herein, it can be concluded that the content and type of ionic liquid play a decisive influence to the final morphology and texture of



carbon nitride.

Fig. 3 SEM images of as-prepared mesoporous carbon nitride: (A) CN, (B) CN-0.05, (C) CN-0.08, (D) CN-0.1, (E) CN-0.15, (F) CN-A-0.1, (G, H) CN-B-0.1

Fig. 4 TEM images of CN-0.1 with different scale


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To further investigate the structure of as-fabricated CN-0.1, TEM analysis was applied. As shown in Fig. 4, the ultrathin nanosheets with mesopores can be found from TEM images, which is consistent with the SEM result. The ionic liquid, the NH3 and CO2 gases contribute to formation of multiple pores. On one hand, ionic liquid can self-assemble into micelles due to the special structure in the aqueous solution. In the calcination process, ionic liquid micelles can be decomposed and ammonia gas releases from the materials, which leading to the formation of hierarchical porous structure. On the other hand, ionic liquid micelles can affect the self-assembly of cyanuric acid-melamine complex formed by hydrogen bonding and lead to the arrangement of supramolecular aggregates and formation of novel texture. Compared with CN, the samples CN-X exhibit large surface area and multiple pores, which contributed to the exposure of active sites and improvement of photocatalytic activity. The SEM and TEM images give evidence the significance of ionic liquid to the cyanuric acid-melamine complex. Besides, it is known that ionic liquid can form hydrogen bonds with water due to the special structure[40, 41], which can affect the self-assembly behavior of cyanuric acid-melamine complex. And the amide bond of IL-3 may also connect with the precursor by the hydrogen bond to form a new complex and then induce unique carbon nitride with multiple pores. Ionic liquid possesses C, N elements, and it can be degraded into CO2 and NH3 during the calcination process, which may greatly affect the texture and morphology of carbon nitride. Based on the self-assembly of cyanuric acid-melamine complex by using hydrogen bonding and structure-directing property of ionic liquids, the possible



formation mechanism is proposed and shown in Fig. 5.

Fig. 5 Possible formation mechanism of mesoporous carbon nitride

N2 adsorption-desorption isotherms were carried out to analyze the porous structure of as-fabricated carbon nitride samples. As depicted in Fig. S2, the type IV sorption isotherm with a pronounced hysteresis loop can be clearly found, indicating the hierarchical porous texture. Additionally, the large uptake at high pressure represents the existence of mesopores or macropores. The high intensity of CN-0.1 states the enhanced BET surface area. The calculated surface area of CN-0.1 is as large as 499 m2/g, which is about 4.7 times than that of pure CN (106 m2/g). Moreover, the pore volume of CN-0.1 increases from 0.330 cm3/g to 0.928 cm3/g with the modification of ionic liquid, further certifying the template effect of ionic liquid and the strong interaction between precursor and template. From the pore size distribution curves, it


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is found that CN-0.1 possesses main three peaks at about 3 nm, 4.8 nm and 13 nm, which is different from that of CN (only one peak at about 3 nm). This phenomenon should be related to the templating effect of ionic liquid and the release of NH3 and CO2 gases. In addition, the presence of ionic liquid leads to the rearrangement of cyanuric acid-melamine complex, then the formation of special structured carbon nitride with large pores, which may be the other reason of the presence of new peaks in pore size distribution curves.

Fig. 6 UV-vis diffuse reflectance spectra (A), estimated band gaps (B), exact appearance (C), reflection path (D), PL spectra (E), fluorescence lifetime and ESR (F) of carbon nitride samples

Optical absorption, as an important factor for photocatalytic activity, was studied and



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the results were listed in Fig. 6A. All the samples exhibit the strong absorption between 200 nm and 450 nm, which is caused by the generation of photogenerated electron-hole pairs under the light irradiation. Obviously, the samples synthesized with ionic liquid show the enhanced intensity than CN. Especially, the sample CN-0.1 displays the highest intensity among as-prepared materials, suggesting the improved light harvesting ability. This phenomenon is related to the multiple light reflection within the space of layers and the porous structure which induced by the ionic liquid and NH3 gas. The color change from light yellow to yellow, then to yellow-gray further certifies this point (Fig. 6C). In addition, the carbon residue caused by the degradation of ionic liquid may remain in the texture of carbon nitride, which may be the other reason for color change. Carbon nitride with hierarchical porous structure benefits improving the optical properties and enhancing the photocatalytic activity. The corresponding mechanism is shown in Fig. 6D. Besides, compared with CN, a little blue shift can be found for the absorption edge of CN-X. The corresponding band gaps of CN, CN-0.1, CN-A-0.1 and CN-B-0.1 are about 2.84, 2.85, 2.89 and 2.89 eV, respectively (Fig. 6B). The blue shift can be related to the famous quantum confinement effect, which is consistent with the previous works[45,

46]

. The

photoluminescence (PL) spectra of as-prepared mesoporous carbon nitride was used to investigate the recombination rate of photogenerated carrier. Moreover, the high PL intensity represents the fast recombination rate. As can be seen from Fig. 6E, the peak of CN-X is blue shifted from 460 to 450 nm, which is due to the famous quantum confinement effect[45,

46]

. This phenomenon is corresponding to the UV-vis DRS


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results. Besides, the PL intensity of samples synthesized with ionic liquid exhibits an obvious decrease than pure CN, which may be caused by the mesoporous structure induced by the ionic liquid. Furthermore, the porous structure and layered texture can enhance the charge carriers transfer rate, then improve the photocatalytic activity. ESR analysis was taken out to study the optical and electronic ability of CN and CN-0.1. As for Fig. 6F, one single Lorentzian line, which is assigned to the unpaired electrons on π-conjugated CN aromatic rings[3], can be found from both samples. After modification with ionic liquid, the intensity becomes stronger, stating the modification can enhance the electron mobility. With the light irradiation, the ESR intensity of CN and CN-0.1 becomes stronger, further suggesting the excellent photoelectric response and generation of photogenerated electron-hole pairs[47, 48]. Photocatalytic activity

Fig. 7 H2 evolution rate of carbon nitride under visible light irradiation (λ≥400 nm) (A) and



cycling runs of CN-0.1 (B) and photodegradation of RhB (C)

Hydrogen evolution reactions which performed in an aqueous solution containing 10 vol.% triethanolamine under visible light irradiation were applied to evaluate the photocatalytic activity of mesoporous carbon nitride. 5% of Pt nanoparticles as co-catalyst were photodeposited onto the surface of carbon nitride. As illustrated in Fig. 7A, both bulk CN synthesized by melamine and CN show a low photocatalytic performance (4.7, 70.6 µmol h-1, respectively), while CN samples modified by ionic liquid display an enhanced hydrogen production, which is attributed to structural advantages of mesoporous carbon nitride. In addition, the hydrogen production has an obvious upward tendency from CN-0.05 to CN-0.1. As expected, the sample CN-0.1 with layered structure shows a remarkable hydrogen evolution performance, as high as 120.3 µmol h-1, which is nearly 25.6 and 1.7 times higher than that of bulk CN and CN synthesized without ionic liquid, even far exceeds that of CN-A-0.1 (94.1 µmol h-1) and CN-B-0.1 (48.2 µmol h-1). Notably, there is a decrease when the ionic liquid increase to 0.15 g, which may be due to the raising of carbon residue in the network caused by the decomposition of ionic liquid. However, CN-0.15 remains possessing a higher hydrogen generation (77.2 µmol h-1) than CN. As we all known, it is different to compare our results with the previous work owing to the different reaction conditions and photocatalyst compositions. Therefore, it must be treated cautiously. Despite the difference, carbon nitride samples synthesized with ionic liquid show the improved photocatalytic activity. Therefore, the superiority of carbon nitride modified with ionic liquids reveals the importance of mesoporous carbon nitride for hydrogen


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generation reaction. Cyclic stability of photocatalytic hydrogen generation over CN-0.1 was analyzed by operating the reaction for four cycles. As shown in Fig. 7B, the layered structured carbon nitride with multiple pores remains a high hydrogen production and no obvious decrease can be found, evidencing the high stability of semiconductor. Besides, the XRD, FT-IR and UV-vis DRS analysis (Fig. S3) reveal that there is no obvious difference in two samples before and after reaction, further certify the excellent stability. The photocatalytic degradation of RhB (10 mg/L) is also carried out to further study the photocatalytic activity of as-prepared mesoporous carbon nitride (Fig. 7C). It is found that the sample CN-0.1 exhibits a higher photocatalytic activity than that of CN. The RhB degradation rate of CN-0.1 can reach 87% after 300 min irradiation, while just 33% of RhB can be removed by CN for the same irradiation time. Based on the pseudo-first-order kinetic model, the data is re-plotted to get the reaction kinetics of RhB degradation. ln(C0/C) = kt

(1)

C0 and C are the RhB concentration at irradiation time 0 and t, respectively. The rate constant of CN-0.1 is 0.00638 min-1, which is about 4.5 times as high as that of CN. Fan et al.[49] found that the rate constant of as-prepared porous graphitic carbon nitride npg-CN-0.3b was 0.00672 min-1 in the RhB degradation with the addition of 0.1 g of catalysts, which is similar with our results. While, only 50 mg of carbon nitride was used in our work. Thus, as-prepared carbon nitride modified by ionic liquid possessed



the enhanced photocatalytic activity. The mesoporous structure and high surface area may contribute to high photocatalytic activity of CN-0.1, which is consistent with the hydrogen generation results. Photoelectrochemical analysis As-prepared carbon nitride samples were used as electrode materials to study the photoelectrochemical activity. The measurements including transient photocurrent and electrochemical impedance spectra (EIS) were carried out in a typical three-electrode cell. From Fig. 8A, as-prepared CN-0.1 exhibits the higher photocurrent intensity than that of other samples, certifying the fast mobility of photogenerated carriers. The photocurrent intensity of samples implies an order of CN-0.1 > CN-A-0.1 > CN-0.15 > CN > CN-B-0.1, which is well agreement with the law of hydrogen generation results. Moreover, EIS in dark of samples were analyzed. As shown in Fig.8B, EIS semicircular of CN-0.1 and CN-A-0.1 is smaller than that of CN synthesized without ionic liquid, stating the modification of ionic liquids can promote the inter facile charge transfer. The sample CN-B-0.1 exhibits a large EIS semicircular, which may be related to the carbon residue in the porous structure. And the EIS result is consistent with the result of photocurrent and hydrogen evolution test, further giving evidence that the modification of ionic liquid can improve the optical property and enhance the photocatalytic activity.


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Fig. 8 Photocurrent response under visible light irradiation (A) and electrochemical impedance spectroscopy plots (B) for as-prepared carbon nitride

The proposed mechanism According to the above results and analysis, the possible mechanism for hydrogen generation by as-prepared mesoporous carbon nitride is proposed. As illustrated in Fig. S4, the carbon nitride is excited under visible light irradiation, and the photoexcited electrons from valence band (VB) can transfer to the conduction band (CB). Then, the photogenerated electrons flow into the surface of Pt nanoparticles which act as the co-catalyst in this system for reducing protons to hydrogen molecules. Besides, the photogenerated holes gathering on the VB of carbon nitride can oxidize the TEOA to TEOA+, which contributes to the charge separation and benefits the improved photocatalytic performance. Conclusions In conclusion, a facile method to fabricate the mesoporous carbon nitride nanosheets with tunable morphology and catalytic property is proposed by using ionic liquid and supramolecular precursor cyanuric acid-melamine complex as the template and precursor, respectively. The texture and morphology of carbon nitride is controlled by



adjusting the ionic liquid concentration and type. The results found that the supramolecular assembly behavior of cyanuric acid and melamine, the template effect of ionic liquid as well as the interaction between precursor and template came to a joint decision for the final structure of carbon nitride. Ionic liquid can form hydrogen bonds with water due to the special structure, which can affect the self-assembly behavior of cyanuric acid-melamine complex. The ionic liquid molecular and cyanuric acid-melamine complex can interact by the hydrogen bond to form a new complex and then induce unique carbon nitride with multiple pores. A disordered hollow carbon nitride box morphology with some pores in the wall could be obtained with lower IL1 concentration. While, the texture turned to a layered structure with the increasing of ionic liquid content. And the sample CN-A-0.1 synthesized by IL-2 showed the disordered sheet-like structure. Interestingly, a tubular structure with large pores in the shell could be obtained by using IL-3, due to the large scale ionic liquid micelles. Besides, as-prepared mesoporous carbon nitride displayed the large surface area, excellent light harvesting properties and fast separation rate of photogenerated carrier. Especially, the BET surface area of mesoporous carbon nitride CN-0.1 was as high as 499 m2/g, which was about 4.7 times than that of pure CN (106 m2/g). Moreover, CN-0.1 exhibited a higher photocatalytic hydrogen evolution rate (120.3 µmol h-1) under visible light irradiation owing to the special structure and the large surface area, which was approximately 25.6 times and 1.7 times as high as the that of bulk carbon nitride calcined by melamine (4.7 µmol h-1) and CN synthesized without ionic liquid (70.6 µmol h-1). Besides, photocatalytic degradation rate was also


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enhanced by using CN-0.1, compared with CN synthesized without ionic liquid. This method opens a pathway to design novel visible light driven photocatalysts with advanced structure and properties by using ionic liquids for addressing the increasing energy shortage and environmental pollution. Acknowledgments The authors are grateful to the financial supports of the National Natural Science Foundation of China (Grant No. 21676056, 21376051 and 51673040), ‘‘Six Talents Pinnacle Program’’ of Jiangsu Province of China (JNHB-006), Qing Lan Project of Jiangsu Province (1107040167), the Fundamental Research Funds for the Central Universities (KYCX17_0136), Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX17_0136), and A Project Funded by the Priority Academic Program

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For Table of Contents Use Only

The mesoporous carbon nitride nanosheets with tunable morphology and enhanced photocatalytic property are fabricated by using ionic liquid and cyanuric acid-melamine complex as the template and precursor.


Self-assembled mesoporous carbon nitride with tunable texture for

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