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Se-modified polymeric carbon nitride nanosheets with improved photocatalytic activities
Journal of Catalysis 375 (2019) 104–112
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Se-modified polymeric carbon nitride nanosheets with improved photocatalytic activities Honghui Ou a, Chao Tang a, Yongfan Zhang a, Abdullah M. Asiri b, Maria-Magdalena Titirici c,d, Xinchen Wang a,⇑ a
State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou 350116, PR China Center of Excellence for Advanced Materials Research (CEAMR), Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia School of Engineering and Materials Science, Queen Mary University of London, London E1 4NS, UK d Department of Chemical Engineering, Imperial College London, South Kensington Campus, SE7 2AZ London, UK b c
a r t i c l e
i n f o
Article history: Received 17 March 2019 Revised 12 May 2019 Accepted 23 May 2019
Keywords: Conjugated polymers Carbon nitride Nanosheets Se Photocatalysis
a b s t r a c t The carbon nitride (CN) with selenide (Se) modification and porous thin nanosheet structure (m-CNNSs) has been successfully presented via an effective two-step continuous thermal treatment method. The asprepared m-CNNSs show a photocatalytic H2 generation and CO2 reduction performance under visible light, the apparent quantum yield of H2 generation (k = 420 nm) reached 8.1%. This improved photocatalytic performance originates from the large surface areas and porous nanostructure that accelerated separation of photoexcited charge carriers and promoted mass-transfer process. Particularly, the formation of the Se modified in the thin CN sheets further endow it with reduced band gap, more exposed active edges, and extended visible light absorption range. This work not only presents a simple strategy to enhance the photocatalytic performance for CN, but also opens ups a new pathway for the rational preparation of effective polymeric photocatalysts by harnessing the synergistic effects to simultaneously optimize the electronic and frame structure of polymeric photocatalysts. Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction Photocatalytic technology is a method to convert the lowdensity but abundant solar energy to the high-density and storable chemical energy, which is one of the idealized approaches to solve the current worldwide environmental and energy issues [1,2]. Over the past years, the large-scale development of photocatalytic technology has been limited by the lack of efficient photocatalysts. Therefore, the development of photocatalyst is the focus of the development of photocatalytic technology. However, in the near 40 years, much progress has been made for this purpose, mostly around inorganic semiconductors with engineered nanostructure and electronic structure like TiO2 [3,4]. Polymer or supermolecular systems have been rarely explored [5,6]. Conjugated polymer semiconductors also have drawn much attention in artificial photosynthesis owing to some advantages in terms of their properties, such as tunable electronic structure, metal-free composition and processability [7–9]. However, the electrical conductivity and exciton splitting are widely concerned ⇑ Corresponding author. E-mail address: [email protected] (X. Wang). URL: http://wanglab.fzu.edu.cn (X. Wang). https://doi.org/10.1016/j.jcat.2019.05.029 0021-9517/Ó 2019 Elsevier Inc. All rights reserved.
in conjugated polymers semiconductors [10,11]. Nevertheless, lately, the use of a conjugated triazine-based polymers (CTPs) with different electron-donating units chain lengths in the backbone for photocatalytic water splitting show excellent photocatalytic activity for oxygen evolution [12]. In addition, Zhang and co-workers synthesized a series of conjugated polymer to be used as semiconductor photocatalyst [13,14]. These materials also exhibit exceptional electrical conductivity and exciton splitting under photochemical reaction conditions. However, these conjugated polymers mainly feature one-dimensional (1D) linear skeleton structures and have a high exciton binding energy of the Frenkeltype excitons [15]. Recently, one of the most promising and representative twodimensional (2D) conjugated polymeric semiconductor is covalently-bonded graphitic carbon nitrides (CN), which are also metal-free and composed of nitrogen and carbon mostly [16]. CN as a new kind of organic polymeric semiconductor photocatalyst has attracted worldwide research, owing to many advantages for artificial photosynthesis; including extraordinary chemically and thermally stable, visible light response and easy preparation [17– 19]. In comparison with conventional bulk counterparts, fewlayer CN nanosheets (2D) have presented significant advantages to elevate the photocatalytic activity; for instance, they have large
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surface areas, more exposed active sites, and shortened transport distances of charge carriers from the bulk to the surface active site [20–22]. In addition, making porous structure in 2D CN nanosheets can further greatly improve the charge and mass transport across the 2D CN nanosheets during the photocatalytic process [23,24]. This is not surprising that the modification of CN into thin porous nanosheets is an effective path way to improve its photocatalytic performance. However, the photocatalytic properties of 2D thin porous CN nanosheets for solar energy conversion is restricted by the strong quantum confinement effect (QCE). An important approach to tuning the band gap structure, increasing electron carriers, as well as extending the light absorption is doping CN semiconductor photocatalyst with an electron donor impurity. Compared to N and C element, it can be seen that Se element with the lower electronegativity and the largest atom radius, and the interaction between the lone pair electrons and the nucleus is weak. To date, only a few studies have focused on the Se modified CN with enhanced photocatalytic performance. For instance, Baeg and co-workers have synthesized the Se-doped CN (Se-CN) photocatalyst by a facile single step methodology [25]. This Se-CN achieves near 11 times higher photocatalytic rate of formic acid production from CO2 than pristine CN. Very recently, we have reported that carbon nitride is modified with Se and cyano as donor and acceptor groups (DA-HM), respectively [26]. Compared with the pure CN, the DA-HM has the characteristics of p-n type homostructure and wider visible spectrum absorption range. Under the irradiation of simulated sunlight, it has high activity of photocatalytic H2 evolution and dye degradation. However, porous CN nanosheets with both few-layered thickness and Se doping has not yet been reported to the best of our knowledge. In addition, the theoretical insight of the effect of Se doping on the electronic band structure of CN is still missing. Herein, we described the preparation of Se modified thin CN porous nanosheets (m-CNNSs) via a facile annealing process. Compared to previous jobs of Se modified CN, except for the Se modified, the m-CNNSs also with the porous structure, ultra-thin two-dimensional structure and large surface areas. Benefiting from the feature of thin porous nanosheets and preferable conductive property, m-CNNSs exhibit significantly boosted excition dissociation and an improved photocatalytic activity for the H2 production and CO2 reduction reaction. To further study the origin of such a high photocatalytic performance, both experimental and theoretical studies were carried out. 2. Experimental section 2.1. Materials Urea and diphenyl diselenide were purchased from Alfa Aesar Chemicals Co. Ltd. (China) and used without further purification. 2.2. Preparation of CN CN was prepared by heating urea (8 g) at 550 °C for 2 h with a ramping rate of 4 °C min1. 2.3. Preparation of m-CNNSs The m-CNNSs samples were prepared by directly mixing CN powders (0.2 g) with different amounts (varied from 0, 0.04, 0.08, 0.16 and 0.32 g) of diphenyl diselenide (DDS) in ethanol (20 mL). Then, it was evaporated with ethanol vapor to generate a solid mixture. After grinded into powders, they were placed in a crucible with a cover to polymerize the precursors in a muffle furnace at 550 °C for 1 h. After being naturally cooled down, the samples were
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took out from the crucible and ground again. In order to remove SeOx, the products were dispersed in water and sonicated for 1 h. The suspensions were centrifuged at 10000 rpm 5 min and washed with deionized ethanol/water for three times, and then the precipitations were vacuum dried at 60 °C overnight. The final samples were denoted as m-CNNSsx, where x represents to the initial amount of DDS (x = 0.04, 0.08, 0.16 and 0.32 g). 2.4. Photocatalytic H2 evolution experiments 50 mg catalyst powder, 100 mL of H2O, and 10 mL of triethanolamine and 3 wt% Pt were added into a Pyrex top-irradiation reaction vessel. Before the 300 W xenon lamp irradiates, the reaction vessel is repeatedly vacuumed to completely remove air. A cutoff filter is used to control the light wavelength of the incident light for H2 evolution. During the reaction, the temperature of the reaction solution is kept at 12 °C through the flow of cooling water. The H2 were analyzed by gas chromatography, using argon as the carrier gas. The apparent quantum yield (AQY) for H2 evolution was measured as follow:
AQY ¼
Ne 2 M NA h c 100% 100% ¼ SPtk Np
Conditions: LED lamps with k = 420 nm. The irradiation area was controlled as 3 3 cm2. Where, Ne, M, Np and NA is the amount of reaction electrons, the amount of H2 molecules, the incident photons and Avogadro constant, respectively. In addition, c is the speed of light, h is the Planck constant, S is the irradiation area, t is the photoreaction time, P is the intensity of the irradiation and k is the wavelength of the light. 2.5. Photocatalytic CO2 reduction experiments The photocatalytic reactions of CO2 reduction were carried out in the Schlenk flask reactor (80 mL). In the Schlenk flask, dispersing 50 mg catalyst , 15 mg 2, 2-bipyridine, 1 lmol CoCl2 and 1 mL triethanolamine in 5 mL solvent (acetonitrile: 3 mL; H2O, 2 mL). Before the 300 W xenon lamp irradiates, the reaction vessel is repeatedly vacuumed to completely remove air and filled with CO2 gas (1 atm). In the process of photocatalysis reaction, the reaction system was energetically stirred and the temperature of the reaction solution is kept at 40 °C through the flow of cooling water. The photocatalyst was directly recovered and then dispersed into fresh solution for circulation experiment. After reaction, the products (CO and H2) were analyzed by gas chromatography. 2.6. Theoretical calculations The pure PBE [27] XC functional and the plane wave basis sets [28] as implemented in the Vienna ab initio simulation package (VASP) are used to performed the DFT calculations [29–31]. The C s2p2, N s2p3 were treated as valence electrons. 550 eV is set as the cutoff energy for the plane-wave basis. The geometry optimization was performed, until the the Hellmann-Feynman forces on the atoms were less than 0.01 eV Å1 and total energies converged to 104 eV, using the conjugate gradient technique. The high symmetry k-paths of the band calculations were obtained from the literature [32]. In this paper, the theoretical calculation are made using the model of tri-s-triazine-based CN. Although some programs have been developed, the theoretical calculation of random structures are still challenging. Therefore, we only constructed the single layer CN based on tri-s-triazine-based as the preliminary model. In this regard, the effects of those atoms on the electronic
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properties and band structure of tri-s-triazine-based CN need to be further studied. 2.7. Characterization XRD measurements, DRS, FTIR spectra, PL spectra and EPR measurements were performed on a Bruker D8 Advance diffractometer with Cu-Ka1 radiation (k = 1.5406 Å), a Varian Cary 500 Scan UV– visible system, a BioRad FTS 6000 spectrometer, an Edinburgh FI/ FSTCSPC 920 spectrophotometer and a Bruker model A300 spectrometer, respectively. SEM was performed on Nova Nano 230 microscope. TEM was performed on a FEI Tencai 20 microscope. AFM, nitrogen adsorption-desorption isotherms and XPS data were recorded by a Veeco Nanoscope IVa Multimode system, a Micromeritics ASAP 2020 surface area and porosity analyzer and a Thermo ESCALAB250 instrument with a mono-chromatized Al Ka line source (200 W). The photocurrent performance analysis was performed using a BioLogic VSP-300 electrochemical system. 3. Results and discussion The surface morphology and detailed microstructure of the mCNNSs were studied by transmission electron microscopy (TEM) (Figs. 1 and S8). The typical TEM images of CN and m-CNNSs0.08 samples are shown in Fig. 1a and b, respectively. Obviously, the stacked sheets are observed for the CN sample. However, apparent
porous nanosheets with some wrinkles are observed for m-CNNSs0.08. SEM images of m-CNNSs0.08 are shown in Fig. 1c, and it is obvious that m-CNNSs0.08 has a large number of inplane holes, ranging in the size from a few nanometers to about 100 nm. In addition, the surface of m-CNNSs0.08 is no longer smooth. The thickness of the m-CNNSs0.08 was measured by atomic force microscopy (AFM). As shown in Figs. 1d and S10, the thickness of the m-CNNSs0.08 was about 0.7–2 nm, indicating that the m-CNNSs0.08 is composed of only about 2–5 layers of CN (the distance between the two CN layers is about 0.33 nm) [33]. The pore size distribution of the m-CNNSs was studied through the Brunauer-Emmett-Teller (BET) method. The pore size distributions (Fig. 2c) manifest that m-CNNSs0.08 has mesopores and micropores. Larger pores may exist, but they can’t be measured by this method. Furthermore, the specific surface area of the m-CNNSs was studied by the Barret-Joyner-Halenda (BJH) model method. The BET surface area of m-CNNSs0.08 is determined to be 133 m2 g1, which is larger than that of CN (72 m2 g1). Obviously, the high surface area of m-CNNSs0.08 can be mainly due to its rich pores and unique ultrathin morphology. The structure of m-CNNSs0.08 was then studied by powder X-ray diffraction (XRD). As shown in Fig. 2a, the peaks centering at 13.0° is corresponding to the in-plane structural repeating units of tri-striazine [34]. In addition, the peaks centering at 27.4° is due to the stacking of the CN-conjugated layers of CN [35,36]. m-CNNSs exhibits a XRD curve similar to that of CN and no new crystallization
Fig. 1. (a) TEM image of CN. (b) TEM image, (c) SEM image and d) AFM image and the corresponding thickness curves determined along the line (inset) of m-CNNSs0.08.
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Fig. 2. Characterization of the samples. (a) XRD pattern of CN and m-CNNSs0.08. The insets are their possible structures (Carbon, nitrogen and selenium atoms are respectively indicated by grey, blue and red spheres in the atomic model). (b) High-resolution XPS spectra of m-CNNSs0.08. (c) N2-sorption isotherms (inset: the corresponding pore-size distribution) and (d) UV–vis DRS spectra of CN and m-CNNSsx.
peaks were appeared for the m-CNNSs samples, indicating that this thermal modification process does not change the phase structure of CN. In addition, the (0 0 2) peak of m-CNNSs0.08 became weaker, clearly demonstrating the reduced layer thickness of the layered carbon nitride crystals. Furthermore, the intensity of the (1 0 0) peak at 13.0°of m-CNNSs0.08 also became weaker, which was ascribed to the in-plane structural deformation. The chemical structure of m-CNNSs is verified by the combination of FTIR spectroscopy, and Raman spectra. In the Fourier transform infrared (FTIR) spectra (Fig. S1), all samples reveal a sharp peak at 812 cm1, which is related to the characteristic breathing mode of the heptazine heterocyclic ring [37]. The band between 1637 and 1240 cm1 are also attributed to the tri-s-triazine main structural unit [38,39]. In addition, a band in the range of 3200– 3600 cm1 is attributed to the stretching mode of N-H [40]. The Raman spectra of the m-CNNSs0.08 are shown in Fig. S2. Several strong characteristic peaks at 720, 980 and 1200–1700 cm1 were observed. Similar to CN, these peaks of m-CNNSs0.08 are attributed to heptazine heterocyclic ring (C6N7) units stretching vibrations. The virtually identical FTIR spectroscopy and Raman spectra, again indicating that the chemical structure of the m-CNNSs0.08 is remained basically the same as that of CN. The state of Se of m-CNNSs0.08 was detected by X-ray photoelectron spectroscopy (XPS) characterization (Fig. 2b). A clear binding energy (BE) of 56.3 eV was seen for the m-CNNSs0.08, which is corresponding to the XPS peak of Se 3d. According to the XPS analysis, the selenium content in m-CNNSs0.08 was about 0.2 at %. These
results show that some Se atoms are introduced into the CN matrix and have strong interaction with CN matrix. In addition, compared to the binding energy of Se-N (52.3 eV) and Se-O (59.0–58.1 eV), this state of Se (56.44 eV) may be attributed to SeAC bond [41]. Although no exact location of substitutional nitrogen to give rise to Se-C bond has been reported, sulfur (S) [42] and oxygen (O) [43,44] doped CN obtained by substitutional nitrogen to form SAC/OAC bond have been widely reported recently. On the basis of previous reports and this experimental observations, it is reasonable to infer its representative chemical structure of SeAC (Fig. 2a inset), where Se replaces N in CN to form SeAC bonds. In addition, the XPS spectra of N 1s and C 1s were further examined. As shown in Fig. S3b, three bonding states of carbon species are observed. The peak of 286.9 eV and the peak of 288.0 eV can be assigned to the major aromatic carbon species in m-CNNSs0.08, resulting from the sp2-hybridized carbon in N-containing aromatic rings [45,46]. The other peak at 284.6 eV was identified as carbon impurities. As shown in Fig. S3c, three bonding states of nitride species are also appeared. The peak at 398.6 eV, 400.7 eV and 404.5 eV are identified as the sp2 hybridized nitrogen involved in aromatic rings, the amino groups (NAH) and the charging effects about the heterocycles, respectively [47]. The light absorption properties of the synthesized samples are examined by the diffused reflectance spectra (DRS). As show in Fig. 2d, very few difference in the light absorption curve have been observed for CN and m-CNNSs samples. However, as displayed in the inset of Fig. 2d, when the Se content increased, the optical
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edges of m-CNNSs sample (500–600 nm) slightly red-shifted compared with the original CN sample (425 nm). The observations disclose that the CN nanosheets decoration with Se species is superior in enhancing the light adsorption ability. The color of the pristine CN is pale yellow, while the color changes to browntong, and then brown for the samples with increased concentrations of decorations from m-CNNSs0 to m-CNNSs0.16 (Fig. S11), which is consistent with absorption spectra of the samples. The photocatalytic properties of the m-CNNSsx were examined in the visible-light-induced CO2 reduction reaction. As shown in Fig. 3a, the photocatalytic activity for pristine CN is nearly zero under visible light irradiation (k > 420 nm) for 1 h, whereas the m-CNNSsx exhibit obviously enhanced photocatalytic activity for the reduction of CO2 to CO. Furthermore, the m-CNNSs0.08 presents optimal photocatalytic performance for CO evolution (10 lmol h1). However, it also revealed that the activity decreased when further increasing or decreasing the dopant. The stability of catalyst was examined under the same experimental conditions for five cycles. As show in Fig. 3b, no obvious deactivation of CO evolution takes place after five cycles. The control experiments show that no CO evolution is observed in the absence of photocatalyst (m-CNNSs0.08) and visible light (Table S1). In addition, no liquid products of CO2 were found in the liquid phase. We then performed the hydrogen evolution reaction under visible light irradiation (k > 420 nm) (using Pt as a co-catalyst and triethanolamine as an electron donor). As shown in Figs. 3c and S13, an optimum hydrogen evolution rate (HER) of 130 lmol h1 is obtained for the m-CNNSs0.08 sample, which is much higher than that of CN (17 lmol h1) and m-CNNSs0 (42 lmol h1). It is also
revealed that the HER decreased when further increasing or decreasing the dopant. To demonstrate that the H2 evolution in photocatalytic process is initiated by light absorption by photocatalyst, wavelength-dependent H2 evolution of m-CNNSs0.08 was carried out by using band-pass optical filters. As show in Fig. 3d, the HER corresponded well to the UV–Vis reflectance spectrum of mCNNSs0.08. When the light wavelength is extended to k > 550 nm, m-CNNSs0.08 still exhibit a considerable HER about 6.5 lmol h1, whereas the photocatalytic activity for CN (0.1 lmol h1) is very low. It was also confirmed that no gas evolution was observed under dark conditions. To get insight into the mechanism for the improved photocatalytic performance, spectroscopic and photoelectrochemical studies were conducted. The charge-carriers separation/ recombination was investigated by photoluminescence spectra (PL). m-CNNSsx samples give a PL spectra that are not similar to the pristine CN (Fig. 4a). With the increasing Se content, the PL intensity has obvious quenching phenomenon. The PL quenching in principle suggesting its faster interface charge transport, which may be attributed to the optimization in the structure and morphology, and the more metallic nature of Se species facilitates electron relocalization to hinder the charge recombination. To further investigate the charge-carriers separation/recombination in mCNNSs0.08, we then investigated the time-resolved PL decay spectra. As shown in Fig. 4c, the average PL lifetimes were calculated for the m-CNNSs0.08. It was found that the average PL lifetime of m-CNNSs0.08 (5.10 ns) was significantly lower than that of mCNNSs0 (5.81 ns) and CN (7.18 ns). This result further confirmed the improvement of singlet exciton dissociation in m-CNNSs0.08.
Fig. 3. Photocatalytic activity test of the samples. (a) CO2 reduction performance of different samples under visible light irradiation (k > 420 nm). (b) Production of CO/H2 in the stability tests of m-CNNSs0.08 sample. (c) Photocatalytic H2 evolution activity of CN (i), m-CNNSs0 (ii), and m-CCNNSs0.08 (iii) under visible light irradiation (k > 420 nm). (d) Wavelength dependence of the HER with m-CCNNSs0.08.
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Fig. 4. (a) Room temperature (298 K) steady-state PL spectra of CN and m-CNNSsx samples. (b) photocurrent-time dependence, (c) time resolved fluorescence kinetics monitored at the corresponding emission peaks, and (d) EPR spectra of samples.
In addition, the photocurrent density was also carried out to evaluate the charge-transfer properties of m-CNNSs0.08. As shown in Fig. 4b, it can be seen that the photocurrent density increases significantly and is higher than that of m-CNNSs0 and CN, indicating that the charge carrier separation efficiency in m-CNNSs 0.08 is higher than that of the m-CNNSs0 and CN. The improved the charge-carriers separation of m-CNNSs0.08 over CN can be attributed to the presence of Se as well as the porosity. The extension of the covalent system for m-CNNSs0.08 was studied by solid-state electron paramagnetic resonance (EPR) characterizations. As show in Fig. 4d, a signal line centering at g = 2.0030 is observed for m-CNNSsx either in the dark or with light irradiation, indicating the generation of unpaired electrons in the carbon atoms of m-CNNSsx aromatic rings [48]. Obviously, the EPR intensity of m-CNNSs0.08 was stronger than that of CN in the dark, indicative of the promoting the delocalization of the electrons and optimizes the surface structure for charge separation for mCNNSs0.08. In addition, upon visible light irradiation (k > 420 nm) for 5 min, the EPR intensity of CN and m-CNNSs0.08 has increased. This is because the samples can promote the generation of radical pairs under visible light irradiation. To get more insights, the effect of Se atom doping on the electronic structure of CN (CN-Se) was theoretically investigated. Here, by taking the atomic structure models in Fig. S2 as an example, the total density of states (DOS) and partial density of states for CN-Se and CN were calculated. As show in Fig. 5, the VB of both CN-Se and CN was mainly consisted of N p states, whereas the CB of each one was composed by C p and N p states. However, CN-Se show
narrowed band gaps in comparison to that of CN. It should be pointed out that, although Se atom do not change the composition of CB and VB, they influence the band edge position and the band gap. In addition, it doesn’t have any electrons distribution around the Fermi level for CN, because its Fermi level is in the middle of the band gap [23,49]. Interestingly, the band Fermi energy for CN-Se is shifted to the conduction band, indicating that electron density is distributed around the Fermi level. Therefore, the interfacial Schottky barrier of CN-Se would be reduced during photocatalytic process, because there are a large number of electrons around the Fermi level. On basis of the above analysis, the reasons of the enhanced photocatalytic properties of m-CNNSs0.08 can be summarized briefly. First, both the surface and morphology structures of m-CNNSs0.08 are modified to supply more porous and surface area, which facilitate the rapid transfer of reactants and products (Fig. 6). Second, both the electronic structure and texture of m-CNNSs0.08 has been modified to harvest more visible photon. It’s worth noting that the porous structure of m-CNNSs0.08 can also enhance visible light capture by increasing the optical path through the multiple scattering effects. Third, the implanted Se in m-CNNSs0.08 could extend the conjugated system and optimize the electronic structure of m-CNNSs0.08, leading to a reduced photocarrier transfer barrier, as verified by PL, EPR analysis and DFT calculations results. Therefore, the combinative method simultaneously modified the crystal, surface, electronic and textural structures of polymer carbon nitrides to optimize the photocatalytic activity in photocatalytic H2 production and CO2 reduction.
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Fig. 5. Theoretical calculations. Calculated DOS for (a) CN and (c) Se atom doped CN. Calculated band structure for (b) CN and (d) Se atom doped CN. Fermi level is shifted to 0 eV. DOS = density of states.
Fig. 6. Graphical illustration of the structural advantages of the atomically-thin CN porous nanosheets structure compared to bulk CN structures for photocatalytic reduction. (excitons and H2O/CO2 are respectively indicated by red and blue spheres).
4. Conclusions In conclusion, we adopted an effective method to construct fewlayered thin porous and selenide-doped CN nanosheets (mCNNSs). Benefiting from the abundant in-plane holes, the strong
visible light harvesting ability and the efficient photoexcited charge utilization, the m-CNNSs0.08 exhibited enhanced photocatalytic activity for the H2 production and CO2 reduction reaction. The hydrogen evolution rate of m-CNNSs0.08 under visible light reached 130 lmol h1 with an apparent quantum yield of 8.1%
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(k = 420 nm) and is nearly 7 times faster than that of CN (17 lmol h1). This well demonstrates the advantage of thin porous and Se-containing nanosheets in optimizing photocatalytic activity of the CN photocatalyst. Benefiting from a friendly interface and the inherent 2D porous structure, the ultrathin mCNNSs could also provide application prospects in the fields of bioimaging, environmental remediation, energy conversion and energy storage. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (21425309, 21761132002 and 21861130353), the National Key R&D Program of China (2018YFA0209301), the National Basic Research Program of China (2013CB632405), the 111 Project (D16008) and Chang Jiang Scholars Program of China (T2016147). Xinchen Wang would like to thank the Royal Society for an Advanced Newton Fellowship (NAF\R1\180198) to enable the collaboration with Prof. Titirici. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2019.05.029. References [1] S. Fukuzumi, T. Kishi, H. Kotani, Y.-M. Lee, W. Nam, Highly efficient photocatalytic oxygenation reactions using water as an oxygen source, Nat. Chem. 3 (2011) 38–41. [2] X.-H. Li, X. Wang, M. Antonietti, Solvent-free and metal-free oxidation of toluene using O2 and g-C3N4 with nanopores: nanostructure boosts the catalytic selectivity, ACS Catal. 2 (2012) 2082–2086. [3] M.A. Mahadadalkar, S.W. Gosavi, B.B. Kale, Interstitial charge transfer pathways in a TiO2/CdIn2S4 heterojunction photocatalyst for direct conversion of sunlight into fuel, J. Mater. Chem. A 6 (2018) 16064–16073. [4] L. Martínez, M. Benito, I. Mata, L. Soler, E. Molins, J. Llorca, Preparation and photocatalytic activity of Au/TiO2 lyogels for hydrogen production, Sustain. Energy Fuels (2018), https://doi.org/10.1039/C8SE00293B. [5] W. Kewei, Y. Li-Ming, W. Xi, G. Liping, C. Guang, Z. Chun, J. Shangbin, T. Bien, C. Andrew, Covalent triazine frameworks via a low-temperature polycondensation approach, Angew. Chem. Int. Ed. 56 (2017) 14149–14153. [6] M. Liu, L. Chen, S. Lewis, S.Y. Chong, M.A. Little, T. Hasell, I.M. Aldous, C.M. Brown, M.W. Smith, C.A. Morrison, L.J. Hardwick, A.I. Cooper, Threedimensional protonic conductivity in porous organic cage solids, Nat. Commun. 7 (2016) 12750. [7] G. Zhang, Z.-A. Lan, X. Wang, Conjugated polymers: catalysts for photocatalytic hydrogen evolution, Angew. Chem. Int. Ed. 55 (2016) 15712–15727. [8] K. Wang, L.-M. Yang, X. Wang, L. Guo, G. Cheng, C. Zhang, S. Jin, B. Tan, A. Cooper, Covalent triazine frameworks via a low-temperature polycondensation approach, Angew. Chem. Int. Ed. 56 (2017) 14149–14153. [9] Z.-A. Lan, W. Ren, X. Chen, Y. Zhang, X. Wang, Conjugated donor-acceptor polymer photocatalysts with electron-output ‘‘tentacles” for efficient hydrogen evolution, Appl. Catal. B-Environ. 245 (2019) 596–603. [10] J. Xu, F. Xu, M. Qian, F. Xu, Z. Hong, F. Huang, Conductive carbon nitride for excellent energy storage, Adv. Mater. 29 (2017) 1701674. [11] G. Liu, G. Zhao, W. Zhou, Y. Liu, H. Pang, H. Zhang, D. Hao, X. Meng, P. Li, T. Kako, J. Ye, In situ bond modulation of graphitic carbon nitride to construct p– n Homojunctions for enhanced photocatalytic hydrogen production, Adv. Funct. Mater. 26 (2016) 6822–6829. [12] Z.-A. Lan, Y. Fang, Y. Zhang, X. Wang, Photocatalytic oxygen evolution from functional triazine-based polymers with tunable band structures, Angew. Chem. Int. Ed. 57 (2018) 470–474. [13] L. Wang, W. Huang, R. Li, D. Gehrig, P.W. Blom, K. Landfester, K.A. Zhang, Structural design principle of small-molecule organic semiconductors for metal-free, visible-light-promoted photocatalysis, Angew. Chem. Int. Ed. 55 (2016) 9783–9787. [14] C. Yang, B.C. Ma, L. Zhang, S. Lin, S. Ghasimi, K. Landfester, K.A. Zhang, X. Wang, Molecular engineering of conjugated polybenzothiadiazoles for enhanced hydrogen production by photosynthesis, Angew. Chem. Int. Ed. 55 (2016) 9202–9206. [15] G. Zhang, M. Zhang, X. Ye, X. Qiu, S. Li, X. Wang, Iodine modified carbon nitride semiconductors as visible light photocatalysts for hydrogen evolution, Adv. Mater. 26 (2014) 805–809. [16] X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, A metal-free polymeric photocatalyst for hydrogen production from water under visible light, Nat. Mater. 8 (2009) 76–80.
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Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat
Se-modified polymeric carbon nitride nanosheets with improved photocatalytic activities Honghui Ou a, Chao Tang a, Yongfan Zhang a, Abdullah M. Asiri b, Maria-Magdalena Titirici c,d, Xinchen Wang a,⇑ a
State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou 350116, PR China Center of Excellence for Advanced Materials Research (CEAMR), Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia School of Engineering and Materials Science, Queen Mary University of London, London E1 4NS, UK d Department of Chemical Engineering, Imperial College London, South Kensington Campus, SE7 2AZ London, UK b c
a r t i c l e
i n f o
Article history: Received 17 March 2019 Revised 12 May 2019 Accepted 23 May 2019
Keywords: Conjugated polymers Carbon nitride Nanosheets Se Photocatalysis
a b s t r a c t The carbon nitride (CN) with selenide (Se) modification and porous thin nanosheet structure (m-CNNSs) has been successfully presented via an effective two-step continuous thermal treatment method. The asprepared m-CNNSs show a photocatalytic H2 generation and CO2 reduction performance under visible light, the apparent quantum yield of H2 generation (k = 420 nm) reached 8.1%. This improved photocatalytic performance originates from the large surface areas and porous nanostructure that accelerated separation of photoexcited charge carriers and promoted mass-transfer process. Particularly, the formation of the Se modified in the thin CN sheets further endow it with reduced band gap, more exposed active edges, and extended visible light absorption range. This work not only presents a simple strategy to enhance the photocatalytic performance for CN, but also opens ups a new pathway for the rational preparation of effective polymeric photocatalysts by harnessing the synergistic effects to simultaneously optimize the electronic and frame structure of polymeric photocatalysts. Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction Photocatalytic technology is a method to convert the lowdensity but abundant solar energy to the high-density and storable chemical energy, which is one of the idealized approaches to solve the current worldwide environmental and energy issues [1,2]. Over the past years, the large-scale development of photocatalytic technology has been limited by the lack of efficient photocatalysts. Therefore, the development of photocatalyst is the focus of the development of photocatalytic technology. However, in the near 40 years, much progress has been made for this purpose, mostly around inorganic semiconductors with engineered nanostructure and electronic structure like TiO2 [3,4]. Polymer or supermolecular systems have been rarely explored [5,6]. Conjugated polymer semiconductors also have drawn much attention in artificial photosynthesis owing to some advantages in terms of their properties, such as tunable electronic structure, metal-free composition and processability [7–9]. However, the electrical conductivity and exciton splitting are widely concerned ⇑ Corresponding author. E-mail address: [email protected] (X. Wang). URL: http://wanglab.fzu.edu.cn (X. Wang). https://doi.org/10.1016/j.jcat.2019.05.029 0021-9517/Ó 2019 Elsevier Inc. All rights reserved.
in conjugated polymers semiconductors [10,11]. Nevertheless, lately, the use of a conjugated triazine-based polymers (CTPs) with different electron-donating units chain lengths in the backbone for photocatalytic water splitting show excellent photocatalytic activity for oxygen evolution [12]. In addition, Zhang and co-workers synthesized a series of conjugated polymer to be used as semiconductor photocatalyst [13,14]. These materials also exhibit exceptional electrical conductivity and exciton splitting under photochemical reaction conditions. However, these conjugated polymers mainly feature one-dimensional (1D) linear skeleton structures and have a high exciton binding energy of the Frenkeltype excitons [15]. Recently, one of the most promising and representative twodimensional (2D) conjugated polymeric semiconductor is covalently-bonded graphitic carbon nitrides (CN), which are also metal-free and composed of nitrogen and carbon mostly [16]. CN as a new kind of organic polymeric semiconductor photocatalyst has attracted worldwide research, owing to many advantages for artificial photosynthesis; including extraordinary chemically and thermally stable, visible light response and easy preparation [17– 19]. In comparison with conventional bulk counterparts, fewlayer CN nanosheets (2D) have presented significant advantages to elevate the photocatalytic activity; for instance, they have large
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surface areas, more exposed active sites, and shortened transport distances of charge carriers from the bulk to the surface active site [20–22]. In addition, making porous structure in 2D CN nanosheets can further greatly improve the charge and mass transport across the 2D CN nanosheets during the photocatalytic process [23,24]. This is not surprising that the modification of CN into thin porous nanosheets is an effective path way to improve its photocatalytic performance. However, the photocatalytic properties of 2D thin porous CN nanosheets for solar energy conversion is restricted by the strong quantum confinement effect (QCE). An important approach to tuning the band gap structure, increasing electron carriers, as well as extending the light absorption is doping CN semiconductor photocatalyst with an electron donor impurity. Compared to N and C element, it can be seen that Se element with the lower electronegativity and the largest atom radius, and the interaction between the lone pair electrons and the nucleus is weak. To date, only a few studies have focused on the Se modified CN with enhanced photocatalytic performance. For instance, Baeg and co-workers have synthesized the Se-doped CN (Se-CN) photocatalyst by a facile single step methodology [25]. This Se-CN achieves near 11 times higher photocatalytic rate of formic acid production from CO2 than pristine CN. Very recently, we have reported that carbon nitride is modified with Se and cyano as donor and acceptor groups (DA-HM), respectively [26]. Compared with the pure CN, the DA-HM has the characteristics of p-n type homostructure and wider visible spectrum absorption range. Under the irradiation of simulated sunlight, it has high activity of photocatalytic H2 evolution and dye degradation. However, porous CN nanosheets with both few-layered thickness and Se doping has not yet been reported to the best of our knowledge. In addition, the theoretical insight of the effect of Se doping on the electronic band structure of CN is still missing. Herein, we described the preparation of Se modified thin CN porous nanosheets (m-CNNSs) via a facile annealing process. Compared to previous jobs of Se modified CN, except for the Se modified, the m-CNNSs also with the porous structure, ultra-thin two-dimensional structure and large surface areas. Benefiting from the feature of thin porous nanosheets and preferable conductive property, m-CNNSs exhibit significantly boosted excition dissociation and an improved photocatalytic activity for the H2 production and CO2 reduction reaction. To further study the origin of such a high photocatalytic performance, both experimental and theoretical studies were carried out. 2. Experimental section 2.1. Materials Urea and diphenyl diselenide were purchased from Alfa Aesar Chemicals Co. Ltd. (China) and used without further purification. 2.2. Preparation of CN CN was prepared by heating urea (8 g) at 550 °C for 2 h with a ramping rate of 4 °C min1. 2.3. Preparation of m-CNNSs The m-CNNSs samples were prepared by directly mixing CN powders (0.2 g) with different amounts (varied from 0, 0.04, 0.08, 0.16 and 0.32 g) of diphenyl diselenide (DDS) in ethanol (20 mL). Then, it was evaporated with ethanol vapor to generate a solid mixture. After grinded into powders, they were placed in a crucible with a cover to polymerize the precursors in a muffle furnace at 550 °C for 1 h. After being naturally cooled down, the samples were
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took out from the crucible and ground again. In order to remove SeOx, the products were dispersed in water and sonicated for 1 h. The suspensions were centrifuged at 10000 rpm 5 min and washed with deionized ethanol/water for three times, and then the precipitations were vacuum dried at 60 °C overnight. The final samples were denoted as m-CNNSsx, where x represents to the initial amount of DDS (x = 0.04, 0.08, 0.16 and 0.32 g). 2.4. Photocatalytic H2 evolution experiments 50 mg catalyst powder, 100 mL of H2O, and 10 mL of triethanolamine and 3 wt% Pt were added into a Pyrex top-irradiation reaction vessel. Before the 300 W xenon lamp irradiates, the reaction vessel is repeatedly vacuumed to completely remove air. A cutoff filter is used to control the light wavelength of the incident light for H2 evolution. During the reaction, the temperature of the reaction solution is kept at 12 °C through the flow of cooling water. The H2 were analyzed by gas chromatography, using argon as the carrier gas. The apparent quantum yield (AQY) for H2 evolution was measured as follow:
AQY ¼
Ne 2 M NA h c 100% 100% ¼ SPtk Np
Conditions: LED lamps with k = 420 nm. The irradiation area was controlled as 3 3 cm2. Where, Ne, M, Np and NA is the amount of reaction electrons, the amount of H2 molecules, the incident photons and Avogadro constant, respectively. In addition, c is the speed of light, h is the Planck constant, S is the irradiation area, t is the photoreaction time, P is the intensity of the irradiation and k is the wavelength of the light. 2.5. Photocatalytic CO2 reduction experiments The photocatalytic reactions of CO2 reduction were carried out in the Schlenk flask reactor (80 mL). In the Schlenk flask, dispersing 50 mg catalyst , 15 mg 2, 2-bipyridine, 1 lmol CoCl2 and 1 mL triethanolamine in 5 mL solvent (acetonitrile: 3 mL; H2O, 2 mL). Before the 300 W xenon lamp irradiates, the reaction vessel is repeatedly vacuumed to completely remove air and filled with CO2 gas (1 atm). In the process of photocatalysis reaction, the reaction system was energetically stirred and the temperature of the reaction solution is kept at 40 °C through the flow of cooling water. The photocatalyst was directly recovered and then dispersed into fresh solution for circulation experiment. After reaction, the products (CO and H2) were analyzed by gas chromatography. 2.6. Theoretical calculations The pure PBE [27] XC functional and the plane wave basis sets [28] as implemented in the Vienna ab initio simulation package (VASP) are used to performed the DFT calculations [29–31]. The C s2p2, N s2p3 were treated as valence electrons. 550 eV is set as the cutoff energy for the plane-wave basis. The geometry optimization was performed, until the the Hellmann-Feynman forces on the atoms were less than 0.01 eV Å1 and total energies converged to 104 eV, using the conjugate gradient technique. The high symmetry k-paths of the band calculations were obtained from the literature [32]. In this paper, the theoretical calculation are made using the model of tri-s-triazine-based CN. Although some programs have been developed, the theoretical calculation of random structures are still challenging. Therefore, we only constructed the single layer CN based on tri-s-triazine-based as the preliminary model. In this regard, the effects of those atoms on the electronic
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properties and band structure of tri-s-triazine-based CN need to be further studied. 2.7. Characterization XRD measurements, DRS, FTIR spectra, PL spectra and EPR measurements were performed on a Bruker D8 Advance diffractometer with Cu-Ka1 radiation (k = 1.5406 Å), a Varian Cary 500 Scan UV– visible system, a BioRad FTS 6000 spectrometer, an Edinburgh FI/ FSTCSPC 920 spectrophotometer and a Bruker model A300 spectrometer, respectively. SEM was performed on Nova Nano 230 microscope. TEM was performed on a FEI Tencai 20 microscope. AFM, nitrogen adsorption-desorption isotherms and XPS data were recorded by a Veeco Nanoscope IVa Multimode system, a Micromeritics ASAP 2020 surface area and porosity analyzer and a Thermo ESCALAB250 instrument with a mono-chromatized Al Ka line source (200 W). The photocurrent performance analysis was performed using a BioLogic VSP-300 electrochemical system. 3. Results and discussion The surface morphology and detailed microstructure of the mCNNSs were studied by transmission electron microscopy (TEM) (Figs. 1 and S8). The typical TEM images of CN and m-CNNSs0.08 samples are shown in Fig. 1a and b, respectively. Obviously, the stacked sheets are observed for the CN sample. However, apparent
porous nanosheets with some wrinkles are observed for m-CNNSs0.08. SEM images of m-CNNSs0.08 are shown in Fig. 1c, and it is obvious that m-CNNSs0.08 has a large number of inplane holes, ranging in the size from a few nanometers to about 100 nm. In addition, the surface of m-CNNSs0.08 is no longer smooth. The thickness of the m-CNNSs0.08 was measured by atomic force microscopy (AFM). As shown in Figs. 1d and S10, the thickness of the m-CNNSs0.08 was about 0.7–2 nm, indicating that the m-CNNSs0.08 is composed of only about 2–5 layers of CN (the distance between the two CN layers is about 0.33 nm) [33]. The pore size distribution of the m-CNNSs was studied through the Brunauer-Emmett-Teller (BET) method. The pore size distributions (Fig. 2c) manifest that m-CNNSs0.08 has mesopores and micropores. Larger pores may exist, but they can’t be measured by this method. Furthermore, the specific surface area of the m-CNNSs was studied by the Barret-Joyner-Halenda (BJH) model method. The BET surface area of m-CNNSs0.08 is determined to be 133 m2 g1, which is larger than that of CN (72 m2 g1). Obviously, the high surface area of m-CNNSs0.08 can be mainly due to its rich pores and unique ultrathin morphology. The structure of m-CNNSs0.08 was then studied by powder X-ray diffraction (XRD). As shown in Fig. 2a, the peaks centering at 13.0° is corresponding to the in-plane structural repeating units of tri-striazine [34]. In addition, the peaks centering at 27.4° is due to the stacking of the CN-conjugated layers of CN [35,36]. m-CNNSs exhibits a XRD curve similar to that of CN and no new crystallization
Fig. 1. (a) TEM image of CN. (b) TEM image, (c) SEM image and d) AFM image and the corresponding thickness curves determined along the line (inset) of m-CNNSs0.08.
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Fig. 2. Characterization of the samples. (a) XRD pattern of CN and m-CNNSs0.08. The insets are their possible structures (Carbon, nitrogen and selenium atoms are respectively indicated by grey, blue and red spheres in the atomic model). (b) High-resolution XPS spectra of m-CNNSs0.08. (c) N2-sorption isotherms (inset: the corresponding pore-size distribution) and (d) UV–vis DRS spectra of CN and m-CNNSsx.
peaks were appeared for the m-CNNSs samples, indicating that this thermal modification process does not change the phase structure of CN. In addition, the (0 0 2) peak of m-CNNSs0.08 became weaker, clearly demonstrating the reduced layer thickness of the layered carbon nitride crystals. Furthermore, the intensity of the (1 0 0) peak at 13.0°of m-CNNSs0.08 also became weaker, which was ascribed to the in-plane structural deformation. The chemical structure of m-CNNSs is verified by the combination of FTIR spectroscopy, and Raman spectra. In the Fourier transform infrared (FTIR) spectra (Fig. S1), all samples reveal a sharp peak at 812 cm1, which is related to the characteristic breathing mode of the heptazine heterocyclic ring [37]. The band between 1637 and 1240 cm1 are also attributed to the tri-s-triazine main structural unit [38,39]. In addition, a band in the range of 3200– 3600 cm1 is attributed to the stretching mode of N-H [40]. The Raman spectra of the m-CNNSs0.08 are shown in Fig. S2. Several strong characteristic peaks at 720, 980 and 1200–1700 cm1 were observed. Similar to CN, these peaks of m-CNNSs0.08 are attributed to heptazine heterocyclic ring (C6N7) units stretching vibrations. The virtually identical FTIR spectroscopy and Raman spectra, again indicating that the chemical structure of the m-CNNSs0.08 is remained basically the same as that of CN. The state of Se of m-CNNSs0.08 was detected by X-ray photoelectron spectroscopy (XPS) characterization (Fig. 2b). A clear binding energy (BE) of 56.3 eV was seen for the m-CNNSs0.08, which is corresponding to the XPS peak of Se 3d. According to the XPS analysis, the selenium content in m-CNNSs0.08 was about 0.2 at %. These
results show that some Se atoms are introduced into the CN matrix and have strong interaction with CN matrix. In addition, compared to the binding energy of Se-N (52.3 eV) and Se-O (59.0–58.1 eV), this state of Se (56.44 eV) may be attributed to SeAC bond [41]. Although no exact location of substitutional nitrogen to give rise to Se-C bond has been reported, sulfur (S) [42] and oxygen (O) [43,44] doped CN obtained by substitutional nitrogen to form SAC/OAC bond have been widely reported recently. On the basis of previous reports and this experimental observations, it is reasonable to infer its representative chemical structure of SeAC (Fig. 2a inset), where Se replaces N in CN to form SeAC bonds. In addition, the XPS spectra of N 1s and C 1s were further examined. As shown in Fig. S3b, three bonding states of carbon species are observed. The peak of 286.9 eV and the peak of 288.0 eV can be assigned to the major aromatic carbon species in m-CNNSs0.08, resulting from the sp2-hybridized carbon in N-containing aromatic rings [45,46]. The other peak at 284.6 eV was identified as carbon impurities. As shown in Fig. S3c, three bonding states of nitride species are also appeared. The peak at 398.6 eV, 400.7 eV and 404.5 eV are identified as the sp2 hybridized nitrogen involved in aromatic rings, the amino groups (NAH) and the charging effects about the heterocycles, respectively [47]. The light absorption properties of the synthesized samples are examined by the diffused reflectance spectra (DRS). As show in Fig. 2d, very few difference in the light absorption curve have been observed for CN and m-CNNSs samples. However, as displayed in the inset of Fig. 2d, when the Se content increased, the optical
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edges of m-CNNSs sample (500–600 nm) slightly red-shifted compared with the original CN sample (425 nm). The observations disclose that the CN nanosheets decoration with Se species is superior in enhancing the light adsorption ability. The color of the pristine CN is pale yellow, while the color changes to browntong, and then brown for the samples with increased concentrations of decorations from m-CNNSs0 to m-CNNSs0.16 (Fig. S11), which is consistent with absorption spectra of the samples. The photocatalytic properties of the m-CNNSsx were examined in the visible-light-induced CO2 reduction reaction. As shown in Fig. 3a, the photocatalytic activity for pristine CN is nearly zero under visible light irradiation (k > 420 nm) for 1 h, whereas the m-CNNSsx exhibit obviously enhanced photocatalytic activity for the reduction of CO2 to CO. Furthermore, the m-CNNSs0.08 presents optimal photocatalytic performance for CO evolution (10 lmol h1). However, it also revealed that the activity decreased when further increasing or decreasing the dopant. The stability of catalyst was examined under the same experimental conditions for five cycles. As show in Fig. 3b, no obvious deactivation of CO evolution takes place after five cycles. The control experiments show that no CO evolution is observed in the absence of photocatalyst (m-CNNSs0.08) and visible light (Table S1). In addition, no liquid products of CO2 were found in the liquid phase. We then performed the hydrogen evolution reaction under visible light irradiation (k > 420 nm) (using Pt as a co-catalyst and triethanolamine as an electron donor). As shown in Figs. 3c and S13, an optimum hydrogen evolution rate (HER) of 130 lmol h1 is obtained for the m-CNNSs0.08 sample, which is much higher than that of CN (17 lmol h1) and m-CNNSs0 (42 lmol h1). It is also
revealed that the HER decreased when further increasing or decreasing the dopant. To demonstrate that the H2 evolution in photocatalytic process is initiated by light absorption by photocatalyst, wavelength-dependent H2 evolution of m-CNNSs0.08 was carried out by using band-pass optical filters. As show in Fig. 3d, the HER corresponded well to the UV–Vis reflectance spectrum of mCNNSs0.08. When the light wavelength is extended to k > 550 nm, m-CNNSs0.08 still exhibit a considerable HER about 6.5 lmol h1, whereas the photocatalytic activity for CN (0.1 lmol h1) is very low. It was also confirmed that no gas evolution was observed under dark conditions. To get insight into the mechanism for the improved photocatalytic performance, spectroscopic and photoelectrochemical studies were conducted. The charge-carriers separation/ recombination was investigated by photoluminescence spectra (PL). m-CNNSsx samples give a PL spectra that are not similar to the pristine CN (Fig. 4a). With the increasing Se content, the PL intensity has obvious quenching phenomenon. The PL quenching in principle suggesting its faster interface charge transport, which may be attributed to the optimization in the structure and morphology, and the more metallic nature of Se species facilitates electron relocalization to hinder the charge recombination. To further investigate the charge-carriers separation/recombination in mCNNSs0.08, we then investigated the time-resolved PL decay spectra. As shown in Fig. 4c, the average PL lifetimes were calculated for the m-CNNSs0.08. It was found that the average PL lifetime of m-CNNSs0.08 (5.10 ns) was significantly lower than that of mCNNSs0 (5.81 ns) and CN (7.18 ns). This result further confirmed the improvement of singlet exciton dissociation in m-CNNSs0.08.
Fig. 3. Photocatalytic activity test of the samples. (a) CO2 reduction performance of different samples under visible light irradiation (k > 420 nm). (b) Production of CO/H2 in the stability tests of m-CNNSs0.08 sample. (c) Photocatalytic H2 evolution activity of CN (i), m-CNNSs0 (ii), and m-CCNNSs0.08 (iii) under visible light irradiation (k > 420 nm). (d) Wavelength dependence of the HER with m-CCNNSs0.08.
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Fig. 4. (a) Room temperature (298 K) steady-state PL spectra of CN and m-CNNSsx samples. (b) photocurrent-time dependence, (c) time resolved fluorescence kinetics monitored at the corresponding emission peaks, and (d) EPR spectra of samples.
In addition, the photocurrent density was also carried out to evaluate the charge-transfer properties of m-CNNSs0.08. As shown in Fig. 4b, it can be seen that the photocurrent density increases significantly and is higher than that of m-CNNSs0 and CN, indicating that the charge carrier separation efficiency in m-CNNSs 0.08 is higher than that of the m-CNNSs0 and CN. The improved the charge-carriers separation of m-CNNSs0.08 over CN can be attributed to the presence of Se as well as the porosity. The extension of the covalent system for m-CNNSs0.08 was studied by solid-state electron paramagnetic resonance (EPR) characterizations. As show in Fig. 4d, a signal line centering at g = 2.0030 is observed for m-CNNSsx either in the dark or with light irradiation, indicating the generation of unpaired electrons in the carbon atoms of m-CNNSsx aromatic rings [48]. Obviously, the EPR intensity of m-CNNSs0.08 was stronger than that of CN in the dark, indicative of the promoting the delocalization of the electrons and optimizes the surface structure for charge separation for mCNNSs0.08. In addition, upon visible light irradiation (k > 420 nm) for 5 min, the EPR intensity of CN and m-CNNSs0.08 has increased. This is because the samples can promote the generation of radical pairs under visible light irradiation. To get more insights, the effect of Se atom doping on the electronic structure of CN (CN-Se) was theoretically investigated. Here, by taking the atomic structure models in Fig. S2 as an example, the total density of states (DOS) and partial density of states for CN-Se and CN were calculated. As show in Fig. 5, the VB of both CN-Se and CN was mainly consisted of N p states, whereas the CB of each one was composed by C p and N p states. However, CN-Se show
narrowed band gaps in comparison to that of CN. It should be pointed out that, although Se atom do not change the composition of CB and VB, they influence the band edge position and the band gap. In addition, it doesn’t have any electrons distribution around the Fermi level for CN, because its Fermi level is in the middle of the band gap [23,49]. Interestingly, the band Fermi energy for CN-Se is shifted to the conduction band, indicating that electron density is distributed around the Fermi level. Therefore, the interfacial Schottky barrier of CN-Se would be reduced during photocatalytic process, because there are a large number of electrons around the Fermi level. On basis of the above analysis, the reasons of the enhanced photocatalytic properties of m-CNNSs0.08 can be summarized briefly. First, both the surface and morphology structures of m-CNNSs0.08 are modified to supply more porous and surface area, which facilitate the rapid transfer of reactants and products (Fig. 6). Second, both the electronic structure and texture of m-CNNSs0.08 has been modified to harvest more visible photon. It’s worth noting that the porous structure of m-CNNSs0.08 can also enhance visible light capture by increasing the optical path through the multiple scattering effects. Third, the implanted Se in m-CNNSs0.08 could extend the conjugated system and optimize the electronic structure of m-CNNSs0.08, leading to a reduced photocarrier transfer barrier, as verified by PL, EPR analysis and DFT calculations results. Therefore, the combinative method simultaneously modified the crystal, surface, electronic and textural structures of polymer carbon nitrides to optimize the photocatalytic activity in photocatalytic H2 production and CO2 reduction.
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Fig. 5. Theoretical calculations. Calculated DOS for (a) CN and (c) Se atom doped CN. Calculated band structure for (b) CN and (d) Se atom doped CN. Fermi level is shifted to 0 eV. DOS = density of states.
Fig. 6. Graphical illustration of the structural advantages of the atomically-thin CN porous nanosheets structure compared to bulk CN structures for photocatalytic reduction. (excitons and H2O/CO2 are respectively indicated by red and blue spheres).
4. Conclusions In conclusion, we adopted an effective method to construct fewlayered thin porous and selenide-doped CN nanosheets (mCNNSs). Benefiting from the abundant in-plane holes, the strong
visible light harvesting ability and the efficient photoexcited charge utilization, the m-CNNSs0.08 exhibited enhanced photocatalytic activity for the H2 production and CO2 reduction reaction. The hydrogen evolution rate of m-CNNSs0.08 under visible light reached 130 lmol h1 with an apparent quantum yield of 8.1%
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(k = 420 nm) and is nearly 7 times faster than that of CN (17 lmol h1). This well demonstrates the advantage of thin porous and Se-containing nanosheets in optimizing photocatalytic activity of the CN photocatalyst. Benefiting from a friendly interface and the inherent 2D porous structure, the ultrathin mCNNSs could also provide application prospects in the fields of bioimaging, environmental remediation, energy conversion and energy storage. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (21425309, 21761132002 and 21861130353), the National Key R&D Program of China (2018YFA0209301), the National Basic Research Program of China (2013CB632405), the 111 Project (D16008) and Chang Jiang Scholars Program of China (T2016147). Xinchen Wang would like to thank the Royal Society for an Advanced Newton Fellowship (NAF\R1\180198) to enable the collaboration with Prof. Titirici. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2019.05.029. References [1] S. Fukuzumi, T. Kishi, H. Kotani, Y.-M. Lee, W. Nam, Highly efficient photocatalytic oxygenation reactions using water as an oxygen source, Nat. Chem. 3 (2011) 38–41. [2] X.-H. Li, X. Wang, M. Antonietti, Solvent-free and metal-free oxidation of toluene using O2 and g-C3N4 with nanopores: nanostructure boosts the catalytic selectivity, ACS Catal. 2 (2012) 2082–2086. [3] M.A. Mahadadalkar, S.W. Gosavi, B.B. Kale, Interstitial charge transfer pathways in a TiO2/CdIn2S4 heterojunction photocatalyst for direct conversion of sunlight into fuel, J. Mater. Chem. A 6 (2018) 16064–16073. [4] L. Martínez, M. Benito, I. Mata, L. Soler, E. Molins, J. Llorca, Preparation and photocatalytic activity of Au/TiO2 lyogels for hydrogen production, Sustain. Energy Fuels (2018), https://doi.org/10.1039/C8SE00293B. [5] W. Kewei, Y. Li-Ming, W. Xi, G. Liping, C. Guang, Z. Chun, J. Shangbin, T. Bien, C. Andrew, Covalent triazine frameworks via a low-temperature polycondensation approach, Angew. Chem. Int. Ed. 56 (2017) 14149–14153. [6] M. Liu, L. Chen, S. Lewis, S.Y. Chong, M.A. Little, T. Hasell, I.M. Aldous, C.M. Brown, M.W. Smith, C.A. Morrison, L.J. Hardwick, A.I. Cooper, Threedimensional protonic conductivity in porous organic cage solids, Nat. Commun. 7 (2016) 12750. [7] G. Zhang, Z.-A. Lan, X. Wang, Conjugated polymers: catalysts for photocatalytic hydrogen evolution, Angew. Chem. Int. Ed. 55 (2016) 15712–15727. [8] K. Wang, L.-M. Yang, X. Wang, L. Guo, G. Cheng, C. Zhang, S. Jin, B. Tan, A. Cooper, Covalent triazine frameworks via a low-temperature polycondensation approach, Angew. Chem. Int. Ed. 56 (2017) 14149–14153. [9] Z.-A. Lan, W. Ren, X. Chen, Y. Zhang, X. Wang, Conjugated donor-acceptor polymer photocatalysts with electron-output ‘‘tentacles” for efficient hydrogen evolution, Appl. Catal. B-Environ. 245 (2019) 596–603. [10] J. Xu, F. Xu, M. Qian, F. Xu, Z. Hong, F. Huang, Conductive carbon nitride for excellent energy storage, Adv. Mater. 29 (2017) 1701674. [11] G. Liu, G. Zhao, W. Zhou, Y. Liu, H. Pang, H. Zhang, D. Hao, X. Meng, P. Li, T. Kako, J. Ye, In situ bond modulation of graphitic carbon nitride to construct p– n Homojunctions for enhanced photocatalytic hydrogen production, Adv. Funct. Mater. 26 (2016) 6822–6829. [12] Z.-A. Lan, Y. Fang, Y. Zhang, X. Wang, Photocatalytic oxygen evolution from functional triazine-based polymers with tunable band structures, Angew. Chem. Int. Ed. 57 (2018) 470–474. [13] L. Wang, W. Huang, R. Li, D. Gehrig, P.W. Blom, K. Landfester, K.A. Zhang, Structural design principle of small-molecule organic semiconductors for metal-free, visible-light-promoted photocatalysis, Angew. Chem. Int. Ed. 55 (2016) 9783–9787. [14] C. Yang, B.C. Ma, L. Zhang, S. Lin, S. Ghasimi, K. Landfester, K.A. Zhang, X. Wang, Molecular engineering of conjugated polybenzothiadiazoles for enhanced hydrogen production by photosynthesis, Angew. Chem. Int. Ed. 55 (2016) 9202–9206. [15] G. Zhang, M. Zhang, X. Ye, X. Qiu, S. Li, X. Wang, Iodine modified carbon nitride semiconductors as visible light photocatalysts for hydrogen evolution, Adv. Mater. 26 (2014) 805–809. [16] X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, A metal-free polymeric photocatalyst for hydrogen production from water under visible light, Nat. Mater. 8 (2009) 76–80.
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