Research Article Cite This: ACS Catal. 2018, 8, 8376−8385
pubs.acs.org/acscatalysis
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. 2018.8:8376-8385. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 11/23/18. For personal use only.
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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 S Supporting Information *
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 Waals heterostructures (IVDWHs) within gC3N4, 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 with 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 attracted 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, photoreforming, and environmental remediation.6−10 However, because of the intrinsic properties of graphitic sp2 hybridized arrangement of tri-striazine 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 Waals (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. © 2018 American Chemical Society
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 transferring oriented charges in each component’s Received: June 25, 2018 Revised: August 2, 2018 Published: August 3, 2018 8376
DOI: 10.1021/acscatal.8b02459 ACS Catal. 2018, 8, 8376−8385
Research Article
ACS Catalysis
(Scheme 1b), leading to a favorable band-offset 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,b, that the potential energies of the OCN layer and CN sublayer in OCNK-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,d) 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 to the O adjuster atom. Moreover, the interlayer electron transfer channel (Figure 1e,f) 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 electron−hole pairs. 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 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). 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, Kfunctionalized 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 toward the lower 2θ after the O and K modifications (Figure S2b), consistent with an expansion of the layered structure due to the 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, K-functionalized CN (OCN-K-CN3). High-resolution 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 to Figure S3 for the original O 1s scan spectra). Compared with CN, a new peak emerges at the lower binding energy, located 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-K-CN IVDWHs are successfully prepared in our O, K-functionalized CN without any contamination from S or other elements.
multilayer structure because of the weakness of vdW forces between adjacent layers.28−31 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 However, 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 model” (Scheme 1a). Guided by this theoretical design, 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
we, for the first time, fabricate O, K-modified 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 with 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.
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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 8377
DOI: 10.1021/acscatal.8b02459 ACS Catal. 2018, 8, 8376−8385
Research Article
ACS Catalysis
Figure 1. (a,b) 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 labeled in blue and depletion in yellow, and the isosurfaces were both set to 0.005 eV Å−3 for e and f.
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.
order to directly observe the SO42− dissociation process at 823 K, the ab initio molecular dynamics (AIMD) calculations (Figure 2e) were performed in a time evaluation of 10 ps. At the initial stage (
