GCN) metal-free heterojunction for photocatalysis

Accepted Manuscript Full Length Article Atomic-level insight into the mechanism of 0D/2D black phosphorus quantum dot/graphitic carbon nitride (BPQD/GCN) metal-free heterojunction for photocatalysis Zhouzhou Kong, Xingzhu Chen, Wee-Jun Ong, Xiujian Zhao, Neng Li PII: DOI: Reference:

S0169-4332(18)32441-3 https://doi.org/10.1016/j.apsusc.2018.09.026 APSUSC 40330

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

13 July 2018 3 September 2018 4 September 2018

Please cite this article as: Z. Kong, X. Chen, W-J. Ong, X. Zhao, N. Li, Atomic-level insight into the mechanism of 0D/2D black phosphorus quantum dot/graphitic carbon nitride (BPQD/GCN) metal-free heterojunction for photocatalysis, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.09.026

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Atomic-level insight into the mechanism of 0D/2D black phosphorus quantum dot/graphitic carbon nitride (BPQD/GCN) metal-free heterojunction for photocatalysis Zhouzhou Kong,a,‡ Xingzhu Chen,a,‡ Wee-Jun Ong,*b Xiujian Zhao,a and Neng Li*a a

State Key Laboratory of Silicate Materials for Architectures & Research Center for

Materials Genome Engineering, Wuhan University of Technology, Hubei, 430070, P. R. China.*E-mail: [email protected] b

Department of Chemical Engineering, Xiamen University Malaysia, Jalan Sunsuria,

Bandar

Sunsuria,

43900

Sepang,

Selangor

Darul

Ehsan,

Malaysia.

*E-mail:

[email protected]; [email protected] ‡

Zhouzhou Kong and Xingzhu Chen contribute equally to this work.

Abstract Graphitic carbon nitride (g-C3N4, GCN) shows excellent photocatalytic activity for a myriad of reactions due to its unique traits and semiconducting properties. The design of a semiconductor heterojunction by hybridizing GCN and other materials with appropriate band structures has profiled one of the most fascinating approaches to enhance the photocatalytic efficiency of GCN. In our simulation, a metal-free heterojunction was developed by incorporating zero-dimensional (0D) black phosphorus quantum dots (BPQDs) with two-dimensional (2D) GCN. The 0D/2D BPQD/GCN heterojunction was systematically investigated by using density functional theory (DFT) calculations. Various orientations of BPQD on GCN were compared. The electronic structure and charge density distribution of the BPQD/GCN composite were calculated to examine the most favorable configuration. Furthermore, the charge separation and transfer mechanism of this heterojunction structure were discussed from the perspective of computation. Our study reveals that BPQDs and GCN formed a Type II heterojunction with a high stability and robust photocatalytic efficiency. Overall, the present work not only elucidates theoretical guidance for taking the merits of BPQDs and GCN, but also paves a new frontier for engineering metal-free 0D/2D heterojunction nanocomposite systems.

Keywords: graphitic carbon nitride; black phosphorus quantum dot; heterojunction photocatalysis; charge carrier separation; vdW-heterojunction; ab initio calculations

1. Introduction Recently, semiconductor-based photocatalysis has drawn considerable attention for its vast application prospect in solving global energy shortage and environmental problems. Upon solar illumination, photocatalysis can be applied in numerous catalytic reactions such as splitting water into oxygen and hydrogen [1-4], conversion of carbon dioxide into hydrocarbon fuels [5-8], photofixation of nitrogen to ammonia [9-11], decomposition of organic pollutants [12-16], and selective organic transformation [17]. Although diverse semiconductors like metal oxides [18-21], metal sulfides [22, 23], metal phosphides [24], and bismuth oxyhalides [10, 25, 26] have been widely employed as photocatalysts under visible light or ultraviolet (UV) light since the pioneering work in 1972 [27], next-generation semiconductor photocatalysts with superior properties are still in great demand. To date, designing robust and visible-light-responsive semiconductors has become an extremely popular research area. Graphitic carbon nitride (g-C3N4, GCN), as a metal-free two-dimensional (2D) semiconductor, has arouse great research enthusiasm for the past ten years [28-31]. Although GCN shares similar geometric characteristics with graphene, the band structure differs significantly from that of graphene, whose band gap is zero. GCN possesses a band gap of 2.7-2.8 eV, which means to be visible-light-active at approximately 450−460 nm [32]. GCN also shows a high temperature resistance of 500-600 oC and corrosion resistance of acid, alkali, and other organic solvents [33]. Apart from that, GCN is also characterized by its “earth-abundant” nature. However, despite such appealing properties, there are still some disadvantages impairing the photoredox efficiency of GCN. This includes a relatively small specific surface area, fast recombination of photo-generated charge carriers, and the lack of visible light absorption above 460 nm [31, 34]. These drawbacks markedly impede further applications of GCN. On the one hand, bulk GCN can be delaminated into mono- and few-layered structures with thermal oxidation or ultrasonic processing. The specific surface area can be increased and therefore there will be more active reaction sites. As a result, the photocatalytic activity of GCN shows much improvement [35]. On the other hand, constructing a semiconductor heterojunction by combining GCN and other materials with appropriate band potentials has become a promising approach to improve the photocatalytic efficiency of GCN [36-38]. Thus far, there are mainly four types of heterojunctions: Type I, Type II, Type III, and Z-scheme heterojunction systems [39-41]. The fabrication of GCN-based heterojunctions will expand the optical absorption range. It will also accelerate the separation and migration of 2

photo-induced carriers at the same time. Over the last few years, a cornucopia of GCN-based heterojunctions has been investigated [4, 37, 38, 42-49]. As one of allotropes of phosphorus (P), 2D metal-free black phosphorus (BP) is another fascinating semiconductor with remarkable performance in the photocatalytic arena [50]. BP has received tremendous attention for its high mobility of holes, strong light absorption, and tunable bandgap since 2014 [51]. Similar with GCN, BP can be mechanically exfoliated into a monolayer nanosheet, which endows different band gaps in few-layer and bulk structures. This property allows BP to be facilely tuned to emerge as an excellent photocatalyst with a broadband solar light absorption range [52, 53]. In 2017, Zhu et al. developed a binary nanohybrid (BP/GCN) composed of BP nanoflake and a GCN nanosheet for the application in photocatalytic hydrogen evolution [54]. This was the first report on the 2D/2D metal-free BP/CN photocatalyst, which has riveted worldwide attention since then. Qiu et al. have reported the successful photofixation of nitrogen by introducing a BP nanosheet as a co-catalyst on GCN nanosheetts under visible light [55]. Interestingly, the BP nanosheet is highly stable in this hybrid structure and the BP/GCN shows wideband solar absorption in visible light and near-infrared light. It offered a new way of thinking for the solution on instability of BP, which is the biggest limiting factor of the BP application. Most recently, Ran et al. reported the BP/GCN nanocomposite to be an enhanced visible-light photocatalyst for superior H2 production activity [56]. Since 2015, BP quantum dots (BPQDs) and even ultra-small BPQDs have been successfully synthesized for multifunctional applications [57-60]. Gao’s group reported that the structures of BPQDs are strongly dependent on the interaction with substrates [61]. Very recently, Kong et al. first employed BPQD as a hole-migration co-catalyst of monolayer GCN for photo-driven hydrogen evolution reaction under visible light [62]. Other than hydrogen evolution, Han et al. also employed BPQD/GCN nanocomposites to photoreduce carbon dioxide to carbon monoxide [63]. However, the reasons behind the enhancement of photocatalytic activity of BPQD/GCN are still an open question and not fully understood at present. In this work, density functional theory (DFT) calculation was used to study the BPQD/GCN hybrid heterostructure. Angles of rotation between the BPQD and GCN monolayer, and the structure of the BPQD/GCN heterojunction were thoroughly evaluated. Meanwhile, the electronic structure, charge density difference, and partial charge distribution of BPQD/GCN were investigated in-depth. As such, the BPQD/GCN heterojunction was proven to be a Type II heterojunction, inferring the effective spatial charge migration and separation across the heterointerface. Based on our present computation results, the 3

metal-free BPQD/GCN nanocomposite demonstrates a robust photocatalyst with effective electron-hole separation for photocatalysis.

2. Computational Methods The density functional theory (DFT) based simulations were carried out using the projected augmented wave (PAW) pseudopotentials implemented in the Vienna Ab-initio Simulation Package (VASP) code [64-66]. The Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) was employed for exchange-correlation functions [67]. The DFT-D3 method of Grimme was carried out to correctly describe the van der Waals interactions between the BPQD and GCN [68]. The cut-off energy for plane wave basis was set to 520 eV, using the energy and force convergence criteria per atom of 10 -4 eV and 0.02/0.05 eV/Å for the optimization of semiconductors and their heterogeneous structure. In consideration of the size of the supercell and dimensions of the structure, we employed the Monkhorst-Pack scheme k-point mesh from gamma to the 7 × 7 × 1 point, and used finer k-point for further calculations of the electronic structure. To model the heterojunction of BPQD/GCN, a 5 × 5 large supercell of GCN was built as the substrate to ensure adequate space (more than 10 Å) between two neighboring BPQDs due to the periodic structure. Then, the BPQD with 84 P atoms was tailored from the phosphorene with vacuum of more than 15 Å in all directions. The edge of the quantum dots was then terminated by 24 H atoms. There are 458 atoms in our BOQD/GCN model totally. Finally, the BPQD was placed 3 Å above the GCN to form a heterogeneous structure with a vacuum layer of more than 15 Å in the z-direction.

Fig. 1. Six BPQD/GCN heterostructures with different rotation angles of 0°(a), 30°(b), 60°(c), 4

90°(d), 120°(e), and 150°(f).

3. Results and Discussion As we know, GCN is an ordered hexagonal mesoporous 2D material. In comparison to triazine-based g-C3N4, the tri-s-triazine-based g-C3N4 was employed in our modeling and simulation due to the energetic stability. The spacing group of GCN is Cmc21. The lattice constants of the GCN monolayer calculated in our work (a = b = 7.135 Å) were in good agreement with other experimental and theoretical results [30, 69]. In addition, the space group of BP is Cmca and the crystal structure belongs to the orthorhombic system. The lattice constants of the layered black phosphorus were optimized to a = 4.492 Å and b = 3.310 Å, which were consistent with the previous results [70]. In our simulation, the 0D/2D BPQD/GCN heterojunction consisted of a 5×5 monolayer of GCN and a BPQD with 84 P atoms. The size of BPQD (average length of two edges is 17.08 Å) agreed with data reported previously [58, 60]. Six different types of rotation angles between GCN and BPQD including 0°, 30°, 60°, 90°, 120° and 150° were compared. These stacking structures are shown in Fig. 1a-f. The calculated minimum total energy of BPQD/GCN was obtained in the heterostructure with rotation angle of 90°. It was reported in previous research that GCN preferred to be corrugated rather than planar when forming composites [71, 72]. There was an obvious distortion of the GCN substrate before and after fusing with BPQD (Fig. 2a). The heterogeneous structure was optimized by decreasing the lattice parameter of the substrate (Fig. 2b). (a)

(b)

Fig. 2. (a) Three-dimensional BPQD/GCN structures before and after optimization. (b) Total energy of heterojunction along the scaling of lattice constants and side views of BPQD/GCN. 5

The average distance between the BPQD and the GCN was 4.59 Å, which was slightly larger than the interface distance of previous GCN-based heterojunctions reported [43, 69]. The average C-N bond lengths of monolayer GCN after forming a hybrid with BPQD were 1.335 Å, 1.399 Å, and 1.442 Å, which were almost similar to the C-N bond lengths (1.330 Å, 1.395 Å, and 1.477 Å) of the monolayer GCN structure before being optimized. The thermodynamic stability of the BPQD/GCN heterojunction and the interaction between BPQD and GCN were determined by the formation energy of the combination. The total reaction energy,

was calculated using Equation 1: (1)

where

is the total energy of the BPQD/GCN heterostructure,

energy of the optimized BPQD with 84 P atoms, and

is the total

is the total energy of the planar

monolayer GCN. However, the planar GCN monolayer was folded by hybridizing with BPQD. The total formation energy calculated by using Equation (1) not only accounts for the combination of BPQD with GCN and maintaining the interfacial interaction between both of them, but also accounts for turning the surface of the GCN monolayer from a planar to corrugated surface. The total formation energy can be divided into two parts: the formation energy

of BPQD/GCN and the deformation energy of the planar GCN monolayer

.

The deformation of BPQD was relatively small compared with the GCN; therefore, we did not take this into account. The relationship was shown below: (2) (3) (4) where

represents the total energy of corrugated monolayer GCN. The folded GCN

monolayer has lower total energy than the planar GCN monolayer, thus deducing that the folded phase is a more stable phase than the planar one. The corrugated monolayer GCN was formed by performing the molecular dynamics simulation with the NPT ensemble at 300 K for 30 ps (Fig. S1), using the universal forcefield in Forcite code. The calculated were -28.45 eV and -24.33 eV, respectively. Therefore, the corrugated monolayer GCN was thermally more stable than the planar structure. On top of that, 6

was

determined to be -4.12 eV, which demonstrates that the BPQD/GCN heterostructure was energetically favorable from a thermodynamics point of view. The formation energy between BPQD and GCN per unit interfacial area was calculated to be 14.1 meV/Å2, which was characterized as van der Waals force [43]. It indicated that the BPQD/GCN heterojunction belongs to van der Waals heterostructures. The density of states (DOS) of BPQD and GCN were further examined (as shown in Fig. 3a and b). The band gaps of BPQD and GCN were 1.51 and 1.23 eV, respectively. The valence band maximum (VBM) of pure GCN was mainly occupied by the 2 p orbital of N atoms and the conduction band minimum (CBM) consisted of the 2 p orbital of both C and N atoms. Following that, we focused on the DOS of the BPQD/GCN heterostructure. The total density of states (TDOS) and partial density of states (PDOS) of BPQD/GCN heterojunction were shown in Fig. 3c. The VBM of BPQD/GCN heterojunction was mainly dominated by the BPQD, while the CBM of BPQD/GCN heterostructure was contributed by the GCN. In our work, BPQD and GCN combined with each other, and the band gap of the BPQD/GCN heterojunction was 0.64 eV. There were no localized states in the forbidden band. This was attributed to the fact that the C-N bonds and P-P bonds (whose edge is terminated with H atoms) in the GCN and BPQD surfaces were saturated bonds. The interaction between the BPQD and GCN surface was mainly due to the van der Waals force, which was too weak to introduce localized states in the system. The work function of the two different semiconductors and BPQD/GCN heterostructure were also calculated with the following formula [28]: (5) where

,

, and

denote the work function, vacuum level, and Fermi level,

respectively. The work functions of the GCN monolayer and BPQD were 4.67 eV and 4.92 eV (as displayed in Fig. 3d and e). The work function of BPQD was larger than that of GCN. This indicated that the electrons tend to flow from the GCN interface to the BPQD interface, and the holes flow from the BPQD interface to the GCN interface until attaining the same in the BPQD/GCN heterostructure. Thus, the GCN and BPQD became differently charged by the spatial transfer of photo-generated electrons and holes across the heterointerface. A built-in electric field from GCN to BPQD was formed in the heterointerface between the BPQD and GCN surfaces. The direction of this electric field was in essence akin to the direction as the migration of electrons. In Fig. 3f, the work function of 7

BPQD/GCN was calculated to be 4.87 eV. Considering the huge model that we have constructed, the results of the DOS and work function indicated the staggered energy band level of the BPQD/GCN heterojunction. As such, this is helpful for enhancing the dissociation of the photo-induced electron-hole pairs, and for improving the photocatalytic efficiency and activity.

Fig. 3. The calculated TDOS and PDOS of pure GCN (a), BPQD (b) and BPQD/GCN hybrid (c) as well as the electrostatic potential of pure GCN (d), BPQD (e) and BPQD/GCN hybrid (f) (the green dash line represents the position of fermi level). Furthermore, the partial charge distribution of the BPQD/GCN heterojunction was studied. The HOMO and LUMO of the BPQD/GCN heterostructure are displayed in Fig. 4. The HOMO and LUMO of the single planar GCN and the single BPQD were both evenly distributed on the surface of the GCN monolayer and the BPQD (Fig. 4a and b). In the BPQD/GCN heterojunction, the LUMO area was mainly occupied on the BPQD, while the HOMO area was mainly covered on the GCN monolayer (Fig. 4c). The spatial separation of the HOMO and LUMO further proved that the BPQD/GCN heterojunction was conducive for the separation of photo-induced electrons and holes, hence improving the photocatalytic quantum efficiency. Moreover, the corrugation of GCN may also influence the photocatalytic activity significantly through the uneven partial charge distribution in HOMO and LUMO (Fig. S2).

8

(a)

(b)

(c)

LUMO

HOMO

Fig. 4. Partial charge density of HOMO and LUMO for BPQD (a), GCN unit cell (b), and BPQD/GCN heterojunction (c). In addition, the charge density difference of BPQD/GCN heterojunction was calculated. The charge density difference

was calculated by Equation 6. (6)

where

,

, and

represent the charge densities of the BPQD/GCN

heterostructure, the monolayer GCN, and BPQD, respectively. The result of the charge density difference is depicted in Fig. 5a and b. The yellow area represents the gathering of electrons or the depletion of holes, whereas the blue area denotes the consumption of electrons or accumulation of holes. The value of the isosurface level of the charge density difference was fairly small, mainly due to the fact that the BPQD/GCN heterostructure was metal-free, and the van der Waals force between the two semiconductors was rather weak; therefore, the charge transfer was trivial. The planar-averaged electron density difference along z-direction for the BPQD/GCN hybrid was investigated (Fig. 5c). The GCN part was mainly colored by blue, while the BPQD was mainly occupied by yellow. It is evident that the surface of the BPQD gathered electrons, whereas the GCN surface depleted electrons or gathered holes. The photo-generated carriers are separated on the opposites of heterojunction, which is one of the merits in enhancing the photocatalytic performance of this mixed-dimensional heterostructure. The charge accumulated at the interface between the BPQD surface and the GCN surface. This indicated that there was an electric dipole moment to enhance the combination. This consequence was consistent with the conclusion drawn from the work function above. 9

(a)

(c)

GCN

BPQD

(b)

Fig. 5. The side view (a) and top view (b) of charge density difference for BPQD/GCN, the isosurface level was set to be 0.001 electrons Å-3. (c) Planar-averaged charge density difference along z-direction of BPQD/GCN (the red and green shadings indicate the position of GCN and BPQD in z-direction, respectively).

Based on the DOS and work function, the schematic energy band diagram of BPQD/GCN hybrid is presented in Scheme 1. The construction of heterojunction shifts the fermi level of GCN downwards and increases the fermi level of BPQD, which determined direction of this built-in electric field was from GCN to BPQD. Therefore, the downward band bending of BPQD’s CBM and the upward band bending of GCN’s VBM were formed. The band edge potentials between BPQD and GCN were staggered.

Scheme 1. Type II photocatalytic reaction mechanism of BPQD/GCN heterojunction. The blue and red balls denote the photo-generated electrons and holes, respectively. The direction of interfacial electric field was from GCN to BPQD, resulting in the successful migration of photo-generate carriers in the opposite directions across the heterointerface. 10

As such, the BPQD/GCN heterojunction was a typical Type II band alignment. The photo-generated electrons near the interface migrated from the BPQD to the GCN, while the holes migrated via the interface in the opposite directions, which significantly enhanced the photo-induced charge carriers separation on different sites of BPQD/GCN to reduce the electron-hole recombination rate. In this Type II heterojunction, photo-generated electrons migrated from the CBM of BPQD to the CBM of GCN, while photo-induced holes migrated from the VBM of GCN to the VBM of BPQD. The migration and separation of electrons and holes were promoted, contributing to the overall photocatalytic reaction. The formation of BPQD/GCN heterojunction can take full advantage of both the BPQD and the GCN monolayer, and eventually improve the photocatalytic efficiencies of the metal-free hybrid semiconductors.

4. Conclusion In summary, we have successfully constructed a metal-free 0D/2D heterojunction with folded GCN monolayer and BPQD. The formation energy, electronic structures, charge density difference, and partial charge distribution of BPQD/GCN heterostructure were calculated by the first-principles DFT study. The BPQD was combined with a corrugated monolayer GCN with van der Waals force. The band gaps of GCN and BPQD can form a typical staggered gap. Our results indicated that this low-dimensional heterojunction belongs to Type II heterojunctions. In this nanocomposite heterojunction, photo-generated electrons migrated via the interface from BPQD to GCN, while holes migrated from GCN to BPQD acting as a hole collector. Likewise, the reduction reaction will take place on GCN, while the oxidation reaction occurs on BPQD. Our simulation also demonstrated that the photocatalytic properties of the BPQD/GCN hybrid can be improved thanks to the formation of the heterojunction. The recombination of electron-hole pairs can be suppressed thanks to the spatial separation of charge carriers, thus facilitating the photochemistry reaction. Through the migration of electrons and holes, the monolayer GCN can be more photoactive and the BPQD can be more stable in this heterojunction. Overall, this present work opens up a new research horizon in designing a family of metal-free heterojunctions by introducing quantum dots to the surface of 2D semiconductors. Our work not only reveals a promising candidate for future applications in energy utilization and environmental protection, but also provides a new doorway for constructing novel and next-generation heterosystem photocatalysts for multifunctional applications. 11

Acknowledgements We appreciate Dr. Quan Xu and Dr. Jason Street for the help on improvement of this paper. N. Li thanks financially supported by the Fund of the National Natural Science Foundation of China (No. 11604249), the Fok Ying-Tong Education Foundation for Young Teachers in the Higher Education Institutions of China (No. 161008), and the research board of the State Key Laboratory of Silicate Materials for Architectures. Z. Kong thanks the financial support from the Excellent Dissertation Cultivation Funds of Wuhan University of Technology (No. 2016-YS-006). W.-J. Ong acknowledges financial assistance and faculty start-up supports from Xiamen University Malaysia.

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X.

Chen,

X.

Zhao,

Z.

Kong,

W.-J.

Ong,

https://doi.org/10.1039/C1038TA06497K.

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Li,

J.

Mater.

Chem.

A

(2018)

Graphical abstract

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Highlights  0D/2D heterostructure of black phosphorus quantum dot and graphitic carbon nitride.  BPQD and GCN are combined by van der Waals interaction.  BPQD/GCN hybrid belongs to Type Ⅱ heterojunctions owing to the band edge positions.  A huge BPQD/GCN system to simulate the real experimental structure.

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