Cesium Salts as Mild Chemical Scissors To Trim Carbon Nitride for

Jun 14, 2019 - Carbon nitride (CN) is considered to be one of the most promising materials for solar photocatalytic hydrogen evolution. However, its l...
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Research Article Cite This: ACS Sustainable Chem. Eng. 2019, 7, 12351−12357

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Cesium Salts as Mild Chemical Scissors To Trim Carbon Nitride for Photocatalytic H2 Evolution Wenming Xu,† Xianghui An,† Qinggang Zhang,† Zhen Li,† Qinhua Zhang,† Zheng Yao,† Xiaokai Wang,† Sha Wang,‡ Jingtang Zheng,† Jing Zhang,§ Wenting Wu,*,† and Mingbo Wu*,†

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†

State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of Petroleum (East China), No.66 Changjiang West Road, Qingdao 266580, P.R. China ‡ Jiangsu Provincial Key Laboratory of Pulp and Paper Science and Technology, Nanjing Forestry University, No.159 Longpan Road, Nanjing 210037, China § State Key Laboratory of Safety and Control for Chemicals, SINOPEC Research Institute of Safety Engineering, No.339 Songling Road, Qingdao 266071, China S Supporting Information *

ABSTRACT: Carbon nitride (CN) is considered to be one of the most promising materials for solar photocatalytic hydrogen evolution. However, its low crystallinity degree, to some extent, limited its photocatalytic activity. Unlike previous reports, we developed a feasible method to improve the crystallinity of carbon nitride using unmelted CsCl as mild chemical scissors to trim pristine carbon nitride instead of its precursor or the intermediates with incomplete structure. As a result, the regular poly(heptazine imides) structures with higher π conjugation were exposed to the surface, wherein the bonded Cs+ ions on the surface of carbon nitride changed the charge distribution. The high regular structure can wipe out the defect sites, reducing recombined sites of electron and hole, and poly(heptazine imides) structure can greatly reserve the visible light absorption ability from pristine carbon nitride. Benefiting from these excellent features, the resultant product exhibits excellent photocatalytic hydrogen evolution activity, which is about 23 times higher than that of bulk carbon nitride. KEYWORDS: Carbon nitride, Unmelted salt, Crystallinity, Electron−hole separation, Photocatalytic hydrogen evolution

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INTRODUCTION

efficiency of electron−hole pairs trapping, migration, and transfer for carbon nitride in photocatalysis.13−16 Generally, pristine carbon nitride has a common structure, like heptazine and/or triazine. Using a chemical scissor, to trim and expose the regular structure of carbon nitride to the surface, can provide a new vision for fabricating carbon nitride for photocatalysis. Previously, eutectic salt mixtures, like LiCl and KCl,17,18 LiBr and KBr,19 LiCl, NaCl, and KCl,20 and MCl/SnCl2 (M = Na, K, Cs),21 reacting with precursors or condensation intermediates to improve the crystallinity of carbon nitride have been explored. However, most of them

A large number of scientists concentrate on the use of solar energy,1,2 especially on solar-light-driven water splitting to produce hydrogen gas using photocatalysts, which is one of the noble strategies to solve global energy and environment issues.3−5 Carbon nitride, as a representative polymer, is considered to be one of the most promising materials for photocatalytic hydrogen evolution because of its suitable band gap, visible absorption capacity, abundant and cheap precursors, and so on.6−10 The photocatalysis always happened at the surface of the photocatalyst. However, it is difficult to form a regular structure on the surface of carbon nitride, mainly because of the defects, fold structure, incomplete polymerization, and so forth.11,12 These particularly limit the © 2019 American Chemical Society

Received: March 27, 2019 Revised: May 22, 2019 Published: June 14, 2019 12351

DOI: 10.1021/acssuschemeng.9b01717 ACS Sustainable Chem. Eng. 2019, 7, 12351−12357

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Schematic diagram for the preparation process of modified carbon nitride using CsCl as a chemical scissor and its application in photocatalytic H2 evolution.

Figure 2. (a) SEM image of Cs-CN-2h. (b) TEM mapping images of Cs-CN-2h. (c) High-resolution TEM image shows the interlayer lattice images of Cs-CN-2h. (d) In-plane lattice and the enlarged scale image with the superimposed structural model (inset) from image (e). (e) Highresolution TEM images of Cs-CN-2h. (f) In-plane mesocrystalline lattice and the enlarged scale image with the superimposed structural model (inset) from image (e).

activity, which is about 23 times higher than that of bulk carbon nitride.

bring about poly(triazine imide) structures instead of poly(heptazine imide) structures; the lower π conjugation of poly(triazine imides) may reduce the visible light absorption ability in photoctalysis.22−26 It indicates that, to some extent, the molten salt can prevent the polymerization process, and destroy the carbon nitride structure, especially LiCl.27 From another perspective, the salt can be a good candidate as a chemical scissor under relative mild reaction conditions. In this work, we would like to challenge the common method from the precursors or intermediates to obtain the regular structure of carbon nitride. Cesium ions are less active than lithium ions, and CsCl cannot break the carbon nitride structure such as poly(heptazine imides). We demonstrate that using a single salt with common carbon nitride product can improve its crystallinity, even though the subsequent modifying temperature is below the melting point of salt (645 °C). Carbon nitride and cesium chloride are first ground and then heated in a tubular furnace for 2 h at 550 °C under a nitrogen atmosphere. Finally, the samples were achieved by centrifugation with water and drying overnight (Figure 1). With clipping of cesium chloride, carbon nitride was trimmed with crystallinity structure for the catalysis system, and its surface area was improved remarkably. These excellent properties enable the resultant product to exhibit effective electron−hole pairs trapping, migration, transfer properties, and subsequent excellent photocatalytic hydrogen evolution

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EXPERIMENTAL SECTION

Chemicals. Dicyandiamide (99%) and CsCl (99%) were purchased from Aladdin Industrial Corporation and used without further purification. Preparation of Photocatalysts. Bulk g-C3N4 was prepared by heating 10 g of dicyandiamide up to 550 °C with a ramp rate of 2.3 °C/min and kept for 4 h in a tube furnace covered by aluminum foil under a N2 atmosphere. The resultant product was denoted as CN. After that, 500 mg of CN and 1000 mg of CsCl were mixed and fully ground. Then the mixture was put into a tube furnace and heated to 550 °C for 2 h with a ramp rate of 5 °C/min in a nitrogen atmosphere. After cooling to room temperature, the product was further ground and ultrasonically dispersed in deionized water. Finally, it was washed with deionized water by centrifugation several times and dried at 60 °C overnight. This sample was denoted as CsCN-2h. For comparison, bulk g-C3N4 was ground fully and heated further to 550 °C for 2 h under the same conditions, the product of which was referred to as CN-2h. Dicyandiamide (1000 mg) was ground with 1000 mg of CsCl and then heated to 550 °C for 4 h with a ramp rate of 2.3 °C/min. After cooling to room temperature, the product was further ground and ultrasonically dispersed in deionized water. Finally, it was washed with deionized water by centrifugation several times and dried at 60 °C overnight.The sample was denoted as D-CsCN. 12352

DOI: 10.1021/acssuschemeng.9b01717 ACS Sustainable Chem. Eng. 2019, 7, 12351−12357

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Figure 3. (a) XRD patterns of the samples. (b) Solid-state 13C CP-MAS NMR of the samples. Inset: the structure of triazine (left) and heptazine (right). (c) FT-IR spectra of the samples. The high-resolution XPS of C 1s (d) and N 1s (e) of the samples. (f) Raman spectra (325 nm excitation) of CN and Cs-CN-2h. Image and Spectroscopic Characterization. The samples were characterized by X-ray diffraction (XRD) (X’Pert PRO MPD, Holland), scanning electron microscopy (SEM) (Hitachi SU8010, Japan), Fourier transform infrared spectrometry (FT-IR) (Thermo Nicolet NEXUS670, USA), and X-ray photoelectron spectroscopy (XPS) using a Kratos AXIS Ultra spectrometer equipped with a prereduction chamber, Elementar Vario EL III instrument (Elementar, Germany). The UV−vis diffused reflectance spectra were obtained from the dry-pressed disk samples using a Scan UV−vis spectrophotometer (UV−vis DRS UV-2700, Shimadzu, Japan) equipped with an integrating sphere assembly, using BaSO4 as a reflectance sample. Brunauer−Emmett−Teller (BET) surface areas were obtained on a nitrogen adsorption apparatus (Micromeritics ASAP 2020M) with all samples degassed at 423 K for 12 h prior to measurements. Time-resolved fluorescence decay spectra were obtained with an Edinburgh FLS980 spectrophotometer with the excitation wavelength at 375 nm and the emission wavelength at 450 nm. Photoluminescence spectra (PL) were measured on a fluorospectrophotometer (F97pro, Lengguang Tech, China). Photocatalytic Hydrogen Activity Test. Photocatalytic hydrogen evolution was performed as follows. Four milligrams of the catalyst powder was dispersed in 4 mL of aqueous solution containing 10 vol % triethanolamine (TEOA) scavengers flushed with Ar gas. Pt (2 wt %) was loaded on the surface of the photocatalyst as a cocatalyst using an in situ photodeposition method with K2PtCl6. The solution was then irradiated with a 300 W xenon lamp equipped with a 420 nm cutoff filter at room temperature. The concentration of hydrogen gas in a headspace was quantified by a Shimadzu GC-2014 gas chromatograph (Ar carrier, a capillary column with molecular sieves 5A) equipped with a thermal conductivity detector.

uniformly distributed. All these indicate that CsCl can gently and uniformly control the morphology of carbon nitride. From the high-resolution TEM images of Cs-CN-2h (Figures 2c−f), there are obvious lattice structures on both the interlayer and in-plane . In Figure 2c, the lattice spacing is 0.32 nm, which corresponds to the interlayer structure.28 Figure 2d shows that it has a clear hexagonal lattice structure, and its lattice spacing is 1.05 nm, which is likely from the in-plane periodicity. Actually, the preparation of carbon nitride is hardly obtained with completely perfect structure; to some extent, it may result in the formation of mesocrystalline structure (Figure 2f). Therefore, similar lattice structure with average lattice spacing (≈1.05 nm) can be seen, suggesting that it is also from the inplane periodicity. These results are consistent with XRD tests (vide infra). These improvements on the lattice structure may efficiently enhance the charge transfer and reduce detrimental recombination defect sites.28−30 The chemical unit structure of this photocatalyst was carefully studied by XRD, solid-state 13C CP-MAS NMR, FT-IR, XPS, and elemental analysis. The XRD of CN-2h exhibits a similar pattern to that of CN, indicating that only postannealing has little influence on improving the crystal phase structure and crystallinity (Figure 3a). But there is a noticeable decrease in the peak intensities of Cs-CN-2h after dealing with CsCl, which may be due to the scattering effect by the position of cesium ions lacking a strict regularity in the lattice.30 The main pattern of Cs-CN-2h resembles that of a poly(heptazine imide) phase.31 The solid-state 13C CP-MAS NMR were used to carefully analyze the structure of Cs-CN-2h as shown in Figure 3b. Two obvious resonance peaks were found at 165.2 and 159.5 ppm for Cs-CN-2h. The former peak is due to the C(e) atoms (N2−CN or terminal CN2(NHx)) from heptazine or triazine. Compared to that of CN (156.8 ppm), the latter peak (159.5 ppm) has a little shift, which may be ascribed to the presence of Cs ion changing the electron density.32 For the latter peak, it can be attributed to the C(i) atoms (C−N3) from heptazine.30 Therefore, Cs-CN-2h may be the mixture of heptazine/triazine or only heptazine in the

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RESULTS AND DISCUSSION The morphology was first studied by SEM and TEM. The pristine carbon nitride is bulk material but nonuniform. With the help of CsCl, it shows that the pristine carbon nitride was trimmed into small particles (Cs-CN-2h) with the average range of 50−300 nm in diameter (Figure 2a and Figure S1 in the Supporting Information). The surface area of Cs-CN-2h (40.82 m2 g−1) is 4 times higher than that of CN (10.10 m2 g−1) (Table S1). In addition, TEM mapping images (Figure 2b) show that the elements (e.g., Cs, C, and N) were 12353

DOI: 10.1021/acssuschemeng.9b01717 ACS Sustainable Chem. Eng. 2019, 7, 12351−12357

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ACS Sustainable Chemistry & Engineering Table 1. Peak Analysis from High-Resolution XPS N 1sa

N1, N2, and N3 come from Table S2; ∞ means infinite.

a

Cs-CN-2h framework. In FT-IR spectra, the peak at 670 cm−1 is the typical peak for poly(triazine imides). But it is worth noting that there is no obvious peak for Cs-CN-2h, indicating that Cs-CN-2h mainly contains heptazine unit structures (Figure 3c).30 For the photocatalysis, it always happened at the surface of catalysis materials, so the XPS spectra can be used to analyze the surface structure in the resultant products (Figure 3d,e and Figure S2). Signals for Cs-CN-2h and D-Cs-CN corresponding to the elements C, N, Cs, and O were observed in the wide spectrum survey. The C 1s spectra of CN in Figure 3d can be resolved into two peaks; however, the C 1s spectra of Cs-CN2h and D-Cs-CN can be resolved into three peaks centered at 284.6, 286.4, and 288.3 eV. The peaks at 284.6 and 288.3 eV correspond to sp2 C−C bonding in the standard reference carbon and CN3 carbon atoms in the heterocycle ring, respectively. And the additional peak at 286.4 eV arises from surface C−OH, wherein the oxygen in the resultant products comes from the tiny amounts (3 ppm V) of O2 in the N2 flow and/or from the water adsorbed on the surface of the raw material. The N 1s signal of Cs-CN-2h and D-Cs-CN (Figure 3e) consists of five peaks at 397.6, 398.7, 400.5, 401.3, and 404.0 eV. The first signal at 397.6 eV is due to the deprotonated nitrogen atoms N⊖,30 which more easily absorb Cs+ ions and trim the carbon nitride. The last four peaks are similar to those of CN, and can correspond to the sp2hybridized nitrogen (C−NC) and the tertiary nitrogen N− (C)3 groups involved in the heptazine rings, the amino functions (C−N−H), and the charging effects, respectively.33 The area and scale of the peaks in N 1s spectrum survey are shown in Table S2 and Table 1, which could further illustrate the heptazine structure of then samples. N1, N2, and N3 represent the peaks of C−NC, N−C3, and C−N−H, respectively. The ratio of N1 to N2 is partly indicative of the polymerization degree of the samples. The ratio of ideal C3N4 is 3.00, and the smaller ratio means the higher degree of polymerization. The smallest ratio, 5.72 for Cs-CN-2h, indicates the highest polymerization degree. The percentage of N3 indicates the amount of terminal amino on the surface of the samples. The percentage of N3 for CN is the smallest (4.30%) among the prepared materials, probably because some of the small molecules that are not polymerized are encased in the polymerized carbon nitride. The percentage for Cs-CN-2h is 5.98%, which is smaller than that for D-Cs-CN (8.27%). It indicates Cs-CN-2h has a higher degree of polymerization and less terminal amino groups. The ratio of N2 to N3, to some extent, could explain the structure of the samples. The higher the ratio, the larger the polymerized carbon nitride. The ratio

for Cs-CN-2h is 2.16, which is larger than that for D-Cs-CN (1.35). That is to say, the size of polymerization for Cs-CN-2h is larger than that of D-CN-2h, which is consistent with the TEM images. The ratio for D-Cs-CN is very near 1.00, which is the value for the melon polymer, suggesting that the structure of D-Cs-CN is on the verge of being a melon polymer. The molar ratio of C/N can be used to determine the structural perfection of carbon nitride. The C/N molar ratio of Cs-CN-2h modified by CsCl is about 0.705 (Table S3), which is close to the expected poly(heptazine imide) (0.71), and is between the melon polymer (0.68) and the ideal C3N4 (0.75).30 The value for D-Cs-CN (0.700) is also in agreement with the above analysis that D-Cs-CN is on the verge of being a melon polymer. This phenomenon may be because CsCl trimmed the carbon nitride into small fragments. CsCl plays an important role in this modification of carbon nitride. Interestingly, few signals corresponding to Cl were detected (Figure S2a), even in the higher-resolution Cl 2p spectra of XPS (Figure S2b). On the basis of XPS results (Table S4), the surface Cl content of resultant products exhibits 0.27 at. % for Cs-CN-2h and 0.25 at. % for D-Cs-CN, which can be ignored. Figure S3 is the XRD pattern of the byproduct of Cs-CN-2h deposited on the inside surface of the quartz tube, indicating that NH4Cl was present in the byproduct. These results can further explain the decrease of N and Cl elements formed NH4Cl and then got away with N2, just as found in our previous study.32 Figure S2c shows Cs 3d spectrum with the Cs+ signals contribution. The surface Cs contents are 4.80 at. % for Cs-CN-2h and 4.11 at. % for D-CsCN, respectively (Table S4). The zeta potential of pristine CN is −25.2 mV (Figure S4), while the zeta potential of Cs-CN-2h is −35.8 mV, indicating that Cs-CN-2h is more likely to coordinate with positively charged ions. Besides, the FT-IR (Figure 3c) absorption bands at 996 and 1156 cm−1 for CsCN-2h are due to the symmetric and asymmetric vibrations of NC2 bonds and of metal−NC2 groups, respectively.30 As shown in Raman spectra (Figure 3f), there is further charge transfer between carbon nitride and Cs atoms. Three obvious peaks were observed at 707, 764, and 978 cm−1 in Figure 3f. The peak at 978 cm−1 is due to the breathing modes of heptazine units.18,34 The other two peaks at 707 and 764 cm−1 are a doubly degenerate mode associated with in-plane bending vibrations of the C−NC linked heptazine linkages.18,34 The densities of the peak decrease after CsCl modification, and the peak of Cs-CN-2h shifts from 707 to 726 cm−1. All these indicate that charge transfer is occurring between carbon nitride and Cs atoms, and Cs ion with positive charge coordinated with the negatively charged nitrogen in the 12354

DOI: 10.1021/acssuschemeng.9b01717 ACS Sustainable Chem. Eng. 2019, 7, 12351−12357

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charge transfer of electrons and holes with high mobility to new localized states.40 These results indicate that Cs-CN-2h greatly represses the electron−hole pair recombination, which is advantageous to photocatalytic hydrogen evolution. Photocurrent response and electrochemical impedance spectroscopy (EIS) are common methods to reflect the transfers of photoexcited charges.41−43 The photocurrent signal (Figure 4b) produced in Cs-CN-2h was large as compared to that in other samples, distinctly proving the efficient separation of photogenerated charge carriers and a reformative radiative charge mobility in Cs-CN-2h.44 The phenomenon was further corroborated by EIS in Figure 4c. CsCN-2h with a decreased semicircular arc radius suggests a smaller impedance.45 It is reasonable to expect that Cs-CN-2h is a promising photocatalyst because of its larger surface area and efficient charge separation. To this end, the photocatalytic properties were evaluated by photocatalytic hydrogen evolution (PHE), which was measured in water using visible light irradiation (λ > 420 nm), triethanolamine (10 v%) as sacrificial agent, and Pt (2 wt %) as cocatalyst. Figure 4d shows the PHE activities of the resultant products in the first 4 h. The cycle stability test of Cs-CN-2h was also tested and the results are shown in Figure S10. The resultant products modified with CsCl have significantly increased H2 production, especially Cs-CN-2h. Cs-CN-2h exhibits about 23 times higher hydrogen evolution (6.17 μmol) than that of the bulk CN (0.27 μmol). The significantly enhanced hydrogen production for Cs-CN-2h is primarily attributed to the significant improvement in the mesocrystalline and relatively perfect heptazine structure, which can efficiently enhance the charge transfer and reduce detrimental recombination defect sites. In addition, the larger surface area and Cs+ dopant are also favorable conditions. Moreover, when the PHE test was carried out with CN by adding CsCl (5 wt % Cs of CN, just like the Cs content of CsCN-2h) to the reaction solution, no obvious difference was observed (Figure S11), which indicates that Cs+ ion in the solution does not influence the activity of PHE. However, the structure of the Cs+ ion on the surface may change the surface properties to speed up the transfer of the photogenerated carriers. Figure S12 shows the H2 production of Cs-CN-2h in the first 4 h under dark conditions, indicating the catalyst is a photocatalyst. Moreover, the H2 evolution rate of Cs-CN-2h was strongly dependent on the wavelength of the incident light (Figure S13).

heptazine unit in the form of metal−NC2 group, as depicted in Figure S5. On the basis of the structure modification, the optical properties of the resultant products have been improved for photocatalysis. As shown in the diffuse reflectance spectra (DRS, Figure S6), it is obvious that the absorption edge of CsCN-2h shows a red shift in comparison to the other samples, mainly because of higher π conjugation and charge transfer between Cs+ and heptazine. The color of Cs-CN-2h is yellow green (Figure S6b). The band gaps were further determined from the transformed Kubelka−Munk function (Figure S7). Combined with the valence band (VB) calculated from ultraviolet photoelectron spectrometry (UPS) (Figure S8), the VB, CB, and band gaps are summarized in Figure S9. The band gaps narrowed from 2.76 eV (CN) to 2.67 eV (Cs-CN2h). The suitable CB (−1.04 eV) to standard potential of H+/ H2 of Cs-CN-2h portend the higher activity of photocatalytic hydrogen evolution when compared to other resultant products. These are beneficial for enhancing visible absorption ability and band structures of photocatalysts. The photoluminescence (PL) spectra and time-resolved florescence decay spectra could reveal the efficiency of electron−hole pairs trapping, migration, and transfer, which are the key parameters in determining the photocatalytic performance.35−39 As shown in Figure 4a inset, Cs-CN-2h has

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Figure 4. (a) Time-resolved photoluminescence spectra and photoluminescence spectra (inset) under 350 nm excitation. (b) Transient photocurrent response under λ > 420 nm light irradiation of samples. (c) Electrical impedance spectra (EIS) Nyquist plots of samples. (d) Product H2 for samples in first 4 h using 4 mg samples, visible light irradiation (λ > 420 nm), triethanolamine (10 vol %) as sacrificial agent, and Pt (2 wt %) as cocatalyst.

CONCLUSION In summary, we have reported a feasible method to prepare poly(heptazine imides) materials possessing high crystallinity structure using unmelted CsCl and stable carbon nitride. Unlike previous report, the heptazine structure mainly reserved and processed well visible light absorption ability. By modification of cesium chloride, the resultant product has bonded Cs+, and the crystallinity and surface area have also been improved remarkably. The Cs+ ions on the surface of carbon nitride changed the charge distribution, and the high crystallinity wiped out the defect sites, which may be the recombined site for electron and hole. Because of higher crystallinity, a larger surface area, and fewer defect sites, the resultant product shows effective electron−hole pairs trapping, migration, and transfer properties. Benefiting from these excellent features, the resultant product exhibits excellent

very weak PL intensity compared to the others. This PL quenching could be due to the mesocrystalline structure and lower surface defect density (surface defects acting as recombination centers for the separated electron−hole).15 These results were further confirmed by time-resolved florescence decay spectra. As shown in Figure 4a, the florescent lifetime of Cs-CN-2h (4.7 ns) shows quick decay kinetics as compared to that of the CN (9.00 ns) counterpart. The lifetime implies that the relaxation of a small fraction of CsCN-2h excitons occurs via nonradiative paths, presumably by 12355

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photocatalytic hydrogen evolution activity, which is about 23 times higher than that of bulk carbon nitride.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b01717.

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SEM and TEM images, BET surface areas, XPS, elements contents, the XRD of byproduct, zeta potential, diffuse reflectance spectra, electronic band structure of samples, cycle stability test of Cs-CN-2h, and discussion of some experimental data (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.W.). *E-mail:[email protected] (M.W.). ORCID

Wenting Wu: 0000-0002-8380-7904 Mingbo Wu: 0000-0003-0048-778X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by NSFC (51672309, 21503279, and 51372277) and the Fundamental Research Funds for Central Universities (18CX07009A). We also acknowledge the State Key Laboratory of Molecular Engineering of Polymers (Fudan University, K2109-29), the Young Taishan Scholars Program of Shandong Province (tsqn20182027), Initiative Funds of Scientific Research for Metasequoia Talent (163105049), and Technological Leading Scholar of 10000 Talent Project (W03020508).

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DOI: 10.1021/acssuschemeng.9b01717 ACS Sustainable Chem. Eng. 2019, 7, 12351−12357