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The Spatially Oriented Charge Flow and Photocatalysis Mechanism on Internal van der Waals Heterostructures Enhanced g-C3N4 Jieyuan Li, Zhiyong Zhang, Wen Cui, Hong Wang, Wanglai Cen, Grayson Johnson, Guangming Jiang, Sen Zhang, and Fan Dong ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02459 • Publication Date (Web): 03 Aug 2018 Downloaded from http://pubs.acs.org on August 3, 2018
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The Spatially Oriented Charge Flow and Photocatalysis Mechanism on Internal van der Waals Heterostructures Enhanced g-C3N4 Jieyuan Li†, ‡, Zhiyong Zhang§, Wen Cui†, Hong Wang†, Wanglai Cen‡, #, Grayson Johnson§, Guangming Jiang†, Sen Zhang*,§ and Fan Dong*,† †
Chongqing Key Laboratory of Catalysis and New Environmental Materials, College of
Environment and Resources, Chongqing Technology and Business University, Chongqing 400067, P. R. China ‡
College of Architecture and Environment, Sichuan University, Chengdu, Sichuan 610065, P. R.
China §
Department of Chemistry, University of Virginia, Charlottesville, Virginia, 22904, United
States #
Department of Chemistry, Institute for Computational Engineering and Sciences, University of
Texas at Austin, Austin, Texas, 78712-0165, United States
Corresponding Authors: * E-mail:
[email protected] (Dr. Sen Zhang) * E-mail:
[email protected] (Dr. Fan Dong). Tel.: +86 23 62769785 605. Fax: +86 23 62769785 605.
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ABSTRACT: Undirected charge transfer and inhibited interlayer electron migration largely limit the photocatalytic efficiency of two-dimensional (2D) layered graphitic carbon nitride (gC3N4). Herein, we present a facile chemical approach to forming internal van der Walls heterostructures (IVDWHs) within g-C3N4, which enhance the interlayer coulomb interaction and facilitate the spatially oriented charge separation. Such a structure, generated through simultaneous g-C3N4 intralayer modification by O and interlayer intercalation by K, enables the oriented charge flow between the layers, enhancing the accumulation of the localized electrons and promoting the production of active radicals for the activation of reactants as suggested by density functional theory calculations. The resultant O, K-functionalized g-C3N4 with IVDWHs shows an enhanced photocatalytic activity, with nearly 100% enhancement of NO purification efficiency compared to pristine g-C3N4. The reaction mechanism for NO purification is also provided, in which the functionality of IVDWHs could effectively restrain the production of toxic intermediates and promote the selectivity for final products. This work provides a strategy to engineer heterostructures within 2D materials for tunable charge carrier separations and migrations for advanced energy and environmental catalysis.
KEYWORDS van der Waals heterostructures • g-C3N4 • photocatalysis •oriented charge flow • reaction mechanism
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INTRODUCTION Two-dimensional (2D) materials have raised enormous and ever-growing attention because of their unique physicochemical properties and the associated new opportunities for vast applications.1-5 Among 2D materials with rapidly expanding ranges of compositions and structures, g-C3N4 (labeled as CN), which is composed of earth abundant elements and possesses a favorable band-gap structure for visible-light absorption, has been recognized as a promising photocatalytic material for water splitting, photo-reforming, and environmental remediation.6-10 However, due to the intrinsic properties of graphitic sp2 hybridized arrangement of tri-s-triazine units and the chemically inert stacking of CN multilayers,11-13 CN usually shows an undirected in-plane electron migration and a weak van der Walls (vdW) interlayer interaction,14-16 making its catalytic efficiency heavily restrained by the rapid charge carrier recombination. It is highly desirable, but technically challenging, to develop a simple and effective strategy to enable the oriented charge flow and separation within g-C3N4 interlayer structures. Recent advances in constructing van der Waals heterostructures (VDWHs) have provided unprecedented opportunities to precisely manipulate the electronic structure of 2D materials.17-19 Such VDWHs are made by assembling two or more different 2D crystals (monolayer or multilayers) with the controlled stacking sequences. Since chemical composition and electronic structure of each component can be rationally designed, one can tune the charge displacement and flow orientation at the heterostructure interface.20-23 The possibility of making multilayer VDWHs has been demonstrated experimentally, using graphene, MoS2 and hexagonal boron nitride (h-BN) as building blocks.24-27 Although they exhibit oriented charge transfer crossing the heterostructure interface, traditional VDWHs still suffer from low efficiency in separating and
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transferring oriented charges in each component’s multilayer structure due to the weakness of vdW forces between adjacent layers.28-31
Scheme 1. Schematic illustration of the internal van der Waals heterostructure (IVDWH): (a) “Cake model” and structure of OCN-K-CN; (b) Calculated total density of states (TDOS) of CN and OCN layers; (c) Band sketch of the OCN-K-CN IVDWH. Herein, we design and synthesize an O, K-functionalized CN containing abundant heterostructures within CN, called internal VDWHs (IVDWHs), which can direct favorable interlayer charge flows for enhanced photocatalysis. Our previous studies have demonstrated that intercalating alkali metals between CN layers can create an internal electric field (IEF), which functions as an interlayer electron transfer channel for the charge carrier diffusion.32-34 But such an enhanced charge flow crossing layers has not been directed yet. In this work, based on density functional theory (DFT) calculations, we find that once O “adjuster” atoms are introduced into a layer of CN, a VDWH is established between the O-modified CN layer (OCN) and adjacent CN sublayer. Meanwhile, interlayer charge flow can be expedited by intercalated elemental K, which serves as a “mediator” to strengthen the interlayer vdW interaction and is shown as the “cake
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model” (Scheme 1a). Guided by this theoretical design, we, for the first time, fabricate O, Kmodified CN with IVDWHs through a facile one-step pyrolysis reaction. As a result, this photocatalyst with spatially oriented charge flow, referred to as OCN-K-CN, exhibits much enhanced photocatalytic efficiency and selectivity for NO purification compared to the pristine CN. The present work highlights a robust approach to construct novel IVDWHs within 2D materials for the desirable comprehensive charge flow control, which could be generalized to other 2D materials for advanced energy and environmental photocatalytic applications. RESULTS AND DISCUSSION DFT calculations are used to understand the electronic structure details and property advantages of OCN-K-CN as depicted in Scheme 1. Upon the addition of O on CN, OCN band structure is evidently adjusted by the O adjustor atoms (Scheme 1b), leading to a favorable bandoffset between the OCN layer and the CN sublayer and therefore a spatial charge carrier separation (light-generated hole migration to OCN and electron transfer to CN, Scheme 1c). Furthermore, the effect of band-offset between CN and OCN layers became intensified with the increase in O concentration (Figure S1). By comparing the electronic structures of pristine CN and OCN-K-CN, we find, as shown in Figure 1a and 1b, that the potential energies of the OCN layer and CN sublayer in OCN-K-CN are significantly increased after the incorporation of O and K. The difference of potential difference between layers provides the driving force for electron transfer from the OCN layer to the CN sublayer through the interlayer K channel, which effectively realizes the spatial charge separation. We further considered the Bader effective charge35 (Figure 1c and 1d) to understand the role of O adjuster atoms. It is found that the local charge distribution in the OCN layer is altered with the incorporation of O, resulting in increased electron depletion from the adjacent C and N atoms
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to the O adjuster atom. Moreover, the interlayer electron transfer channel (Figure 1e and 1f) is strengthened with the introduction of O adjuster atoms. These accumulated electrons around the O adjuster atoms, plus the mediation of intercalated K, can reinforce the internal vdW force to effectively facilitate the interlayer charge flow and accelerate the spatial separation of electronhole pairs.
Figure 1. (a, b) The layered electrostatic potential energy for pristine CN (a) and OCN-K-CN (b); (c, d) Calculated Bader effective charge for pristine CN (c) and OCN-K-CN (d); (e, f) Charge density difference of K-CN (e) and OCN-K-CN (f). Blue, green, red and gold spheres depict N, C, K and O atoms. Charge accumulation is labelled in blue and depletion in yellow, and the isosurfaces were both set to 0.005 eV Å-3 for e and f. In combination with the theoretical simulations, we prepared the O, K-functionalized CN by a one-step pyrolysis of thiourea in the presence of K2SO4. K2SO4 is chosen as a K and O source for CN modification. Since it contains no OH- or NO3- anions, the formations of detrimental N vacancies in CN36 and undesirable nitric-like species can be prevented in the as-obtained
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product. By controlling the molar ratio of thiourea and K2SO4, O and K composition can be readily controlled in the functionalized CN products. The obtained samples with different O and K compositions are labeled as OCN-K-CN1, OCN-K-CN3 and OCN-K-CN5 (see Experimental section).
Figure 2. (a-c) Elementary component analysis: XPS survey spectra (a), high resolved XPS devolution of O 1s (b) and deconvolution of K 2p and S 2p (c) of pristine CN and OCN-K-CN; (d) Optimized local structure and electronic localization function (ELF) of SO42- dissociation on CN; (e) Time evolution over 10 ps for SO42- dissociation at the temperature of 823 K in the course of reaction. All lengths are given in Å. The X-ray diffraction (XRD) patterns (Figure S2a in the Supporting Information) indicate that the layered graphitic structures of pristine CN are well-maintained in O, K-functionalized CN, with two characteristic {100} and {002} diffraction peaks exhibiting at 13.1° and 27.4°, respectively. It is noted that the {002} peaks downshift towards the lower 2θ after the O and K modifications (Figure S2b), consistent with an expansion of the layered structure due to the
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intercalation of K. The X-ray photoelectron spectroscopy (XPS) survey spectra (Figure 2a) indicate that the O atomic concentration increased from 1.7% in CN to 5.7% in O, Kfunctionalized CN (OCN-K-CN3). High resolved XPS devolution of O 1s spectra reveals that O in pure CN presents only one peak at 532.1 eV (α), which can be ascribed to the surface adsorbed water (Figure 2b, refer Figure S3 for the original O 1s scan spectra). Compared with CN, a new peak emerges at the lower binding energy, locating at 530.8 eV (β) in the O, Kfunctionalized CN, which is attributed to C-O species in OCN layer.37-38 It also confirms the existence of K (Figure 2c) with an atomic percentage of 1.4% in OCN-K-CN3, while no obvious S 2p signal is observed in O, K-functionalized CN. These analyses clearly suggest that OCN-KCN IVDWHs are successfully prepared in our O, K-functionalized CN without any contamination from S or other elements. Theoretical calculations are further conducted to rationalize the formation of C-O species in the presence of K2SO4 while minimizing the contamination of S. As shown in Figure 2d, S-O bond (dS-O, 1.77 Å) is more extended than its typical value (ca. 1.45 Å in SO42-) at the standard DFT condition (1 atm, 0 K) when interacting with CN. The electronic localization function (ELF) also shows that the covalence in S-O bond is weakened. It suggests that the S-O bond in SO42- has a tendency towards cleavage, allowing possible SO42- dissociation on CN to generate C-O species, which is consistent with the XPS result. Moreover, the reaction pathway for the SO42- dissociation was calculated with CI-NEB method (Figure S4). It is found that the reaction requires only 0.19 eV of the activation energy to proceed, implying that the SO42- is facilely dissociated under the co-pyrolysis temperature (823 K). In order to directly observe the SO42dissociation process at 823 K, the ab initio molecular dynamics (AIMD) calculations (Figure 2e) was performed in a time evaluation of 10 ps. At the initial stage (
