Co Embedded in Two-Dimensional Carbon-Nitride for Z

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Bimetallic Pd/Co Embedded in Two-Dimensional CarbonNitride for Z-Scheme Photocatalytic Water Splitting Liyan Xie, Xiyu Li, Xijun Wang, Wanying Ge, Jinxiao Zhang, Jun Jiang, and Guozhen Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10521 • Publication Date (Web): 28 Dec 2018 Downloaded from http://pubs.acs.org on December 31, 2018

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The Journal of Physical Chemistry

Bimetallic Pd/Co Embedded in Two-Dimensional Carbon-Nitride for Z-Scheme Photocatalytic Water Splitting Liyan Xie, Xiyu Li, Xijun Wang, Wanying Ge, Jinxiao Zhang, Jun Jiang, Guozhen Zhang*

Hefei National Laboratory for Physical Sciences at the Microscale, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), CAS Center for Excellence in Nanoscience, School of Chemistry and Materials Science, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China..

E-mail: [email protected]

ABSTRACT: Two-dimensional s-triazine-based graphitic carbon nitride material with a band gap of 3.18 eV has emerged as a promising photocatalyst for water splitting. Here we propose a Zscheme photocatalyst by embedding Pd(OH)2− and Co(OH)2− groups in different parts of g-CN simultaneously. Density functional theory calculations show that it extends solar light absorption of g-CN into visible and infrared regions, realizes efficient charge separation, and can catalyse ACS Paragon Plus Environment

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water splitting to produce O2 and H2, respectively. The staggered band alignment meets the requirement of Z-scheme. Importantly, ab initio non-adiabatic molecular dynamics simulations suggest that the photo-generated holes evolution accords with anticipation for subsequent water-splitting reactions. The computed Gibbs free energy and over-potentials confirm the feasibility of hydrogen evolution reactions on Co-centered fragment and oxygen evolution reactions on Pd-centered fragment, respectively. It is expected that this new Z-scheme model designed on a single material platform may provide an alternative way for achieving efficient photo-driven water splitting.

INTRODUCTION Two-dimensional (2D) materials have aroused extensive research interest in photocatalysis because of their unique structural and electronic properties.1-5 s-Triazine-based graphitic carbon nitride (g-CN) is a new promising 2D material for photocatalytic water splitting.6-9 It possesses unique electronic structure, a high surface to volume ratio, and abundant sites for anchoring cocatalysts.1, 9 However, the ability of harvesting solar energy with pristine g-CN is severely limited due to its intrinsically large band gap which falls in the ultraviolet range.7 Doping atomically dispersed metal groups into the matrix of g-CN can not only tune the band gap, but also provide highly active sites for catalytic reactions because of coordinatively unsaturated metal atoms.10 ACS Paragon Plus Environment

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It is well established that transition metals (such as Fe, Co, Pt, Pd) can be incorporated into the g-CN by strong interaction between the isolated metal atoms and the lone-pair electrons of nitrogen atoms.10-12 We have also found in a previous study that the incorporation of transition metal groups into g-CN can improve visible light absorption.13 Further, g-CN supported with Pd(OH)2− (denoted as Pd(OH)2@CN) and Co(OH)2− (denoted as Co(OH)2@CN) molecular fragments meet the requirement of band alignment for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively.13 However, it is difficult for a single metal doped g-CN to achieve full water splitting reactions as well as efficient charge separation. A Z-scheme strategy, consisting of two semiconductors (one for HER, the other for OER) with staggered band alignment and a mediator between them for charge transportation14, is a promising approach to address the above mentioned technical challenges. It keeps the photoexcited holes (h+) and electrons (e−) carriers (with stronger reduction/oxidation capabilities) separated on different moieties, while neutralizing the photo-excited e−/h+ pairs (that have weaker reduction/oxidation potentials) through redox mediators15. Generally, a Z-scheme possesses the following characteristics: 1) effective utilization of visible light with longer wavelengths in a system; and 2) physical separation of two active sites helps to restrain reverse reactions and charge separation.16 As for all-solid state Z-scheme systems, it is evitable to ACS Paragon Plus Environment

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encounter the retardation of charge mobility between two semiconductors because of the resistance of ohmic contact at the interface.17 Therefore, many efforts have been made to improve charge mobility, such as increasing the contact areas, improving the tightness of interfaces and providing highly-conductive interfacial mediator.18 However, interfaces of heterojunctions still retain and most widely used conductive mediators employ precious metals like Ag or Au17. Aiming at completely removing the interface of heterojunctions and reducing the cost of materials, we designed a Z-scheme model system using a single piece of g-CN as the support. By simultaneously embedding both Pd(OH)2− and Co(OH)2− groups into g-CN (denoted as PdCo@CN), we seamlessly stitch two semiconductors in Z-scheme in a single material. Efficient charge mobility and charge separation in a plane can be achieved thanks to the grand πconjugation of g-CN, according to a recent study on a composite of graphitic carbon ring and gC3N4.19 In the Pd-Co@CN system, the Pd-centered fragment possesses photo-excited holes with a stronger oxidation capability while the Co-centered fragment holds photo-excited electrons with a stronger reduction potential. Meanwhile, the photo-excited e−/h+ pairs with weaker reduction/oxidation potentials will neutralize rapidly as suggested by an ab initio nonadiabatic molecular dynamics simulation. Water molecule can be effectively adsorbed on both ACS Paragon Plus Environment

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active sites. The computed Gibbs free energies and over-potentials for producing H2 and O2 confirm the feasibility of both reactions on their respective active sites. These results validate our proposed Z-scheme model as a photocatalyst for water splitting.

COMPUTATIONAL METHODS The isolated Pd(OH)2@CN and Co(OH)2@CN models were built respectively by loading Pd(OH)2− or Co(OH)2− into vacancies formed by CN atoms in a 2×2×1 unit of g-CN, with OHs located at the most stable configuration (Figure S1). To form the integrated system (PdCo@CN), they were simply merged within the same g-CN platform (Figure S2). First-principles calculations were performed at the spin-polarized level of density functional theory (DFT) by using Perdew−Burke−Ernzerhof (PBE)20 functionals and the plane-wave projector augmented wave (PAW)21 method as implemented in the Vienna Ab Initio Simulation Package (VASP)22-23. The GGA+U method was applied to describe partially filled d-orbitals, with the correlation energy (U) and exchange energy (J) as 4 eV and 1 eV, respectively20, 24. The energy cutoff for the planewave basis set is 450 eV, and the convergence criteria for the residual force and energy on each atom during structure relaxation were set to be 0.01 eV/Å and 10−5 eV, respectively. The vacuum space was set to 20 Å in the c direction, which was sufficient to avoid interactions between the ACS Paragon Plus Environment

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system and its most adjacent periodic images. The Brillouin zone was sampled with a Monkhorst−Pack mesh with a 6×6×1 k-point grids. The hybrid Heyd−Scuseria−Ernzerhof (HSE06)25-26 functional was adopted in the calculation of the density of state (DOS) and charge distributions. Time dependent ab initio non-adiabatic molecular dynamics (AI-NAMD) simulations27 were used to map out the hole (or electron) localization in Pd-Co@CN after light irradiation. We carried out the ab initio molecular dynamics in VASP. A 2.5 ps microcanonical trajectory was simulated with a time step of 1 fs at a temperature of 300 K.

RESULTS AND DISCUSSION Our previous study found that a single layer of g-CN has a band gap of 3.18 eV by using HSE06 functional.13 The band gap can be effectively reduced by embedding transition metal groups in g-CN. As Figure 1a shows, individual Pd(OH)2@CN and Co(OH)2@CN have a band gap of 1.56 and 0.80 eV,13 respectively, extending the optical absorption range into the visible and infrared regions. Further, the valence band maximum (VBM) of Pd(OH)2@CN is ~ 0.70 eV lower than the oxidation potential of H2O/O2, allowing for the oxidation of water by the photogenerated holes in the valence band of Pd(OH)2@CN. Meantime, the conduction band minimum ACS Paragon Plus Environment

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(CBM) of Co(OH)2@CN is ~ 0.20 eV higher than the reduction potential of H+/H2, enabling for the reduction of water with photo-excited electrons in the conduction band of Co(OH)2@CN. More importantly, Pd(OH)2@CN and Co(OH)2@CN display a staggered band alignment, which meets the requirement of a Z-scheme. Based on these facts, we designed a Z-scheme system as illustrated in Figure 1b. Upon light irradiation, photo-excited electron-hole pairs with stronger reduction/oxidation capabilities will be generated in separated fragments, while the photoexcited electron from the Pd-fragment will rapidly fill the photo-excited hole of the Co-fragment through the charge mobility of CN network.

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Figure 1 (a) The energy band alignment of CN, Pd(OH)2@CN and Co(OH)2@CN. The Vacuum level is set at 0 eV, and the chemical reaction potentials for H+/H2 and O2/H2O are plotted as dotted lines; (b) Schematic diagram of the Z-scheme to split water.

Before investigating the whole Z-scheme system, we studied the individual Pd(OH)2@CN and Co(OH)2@CN moieties using the model shown in Figure S1. We calculated charge density differences to investigate the charge redistribution before and after loading Pd(OH)2−/Co(OH)2− on the CN network of Pd(OH)2@CN and Co(OH)2@CN, respectively. As displayed in Figure 2a, charge redistribution mainly occurred around the Pd(OH)2/CN interface. By contrast, for Co(OH)2@CN, charge redistribution takes place inside the CN network and charge transfer occurring at the interface of Co(OH)2/CN is negligible (Figure 2b). Next we analyzed the VBM and CBM of Pd(OH)2@CN and Co(OH)2@CN from their corresponding DOS diagrams. In Figure 2c, the black line is the total density of states (TDOS) of Pd(OH)2@CN, and the red and green lines represent the partial density of states (PDOS) of CN and of the Pd(OH)2 fragment, respectively. For Pd(OH)2@CN, the VBM is mainly determined by the Pd(OH)2 fragment while its CBM is mainly determined by CN. Therefore, upon light exposure, photo-excited holes in the Pd(OH)2 part and photo-excited electrons in the CN ACS Paragon Plus Environment

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network would be well separated. For Co(OH)2@CN, as exhibited in Figure 2d, both the Co(OH)2 moiety and CN contribute to the VBM and the CBM, suggesting a lesser extent of charge separation. Despite the insufficient separation of e-/h+ pairs in Co(OH)2@CN, if we combine Pd(OH)2@CN and Co(OH)2@CN, photo-excited holes in the Co-centered fragment can be neutralized by photo-excited electrons in the Pd-centered fragment thanks to the charge mobility of CN network.

Figure 2 (a) The calculated charge differences between g-CN without and with a Pd(OH)2motif. The iso-surface is 0.005 e/Å3. (b) Simulated charge differences between g-CN without ACS Paragon Plus Environment

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and with a Co(OH)2- motif. The iso-surface is 0.005 e/Å3 (yellow: charge accumulation; blue: charge depletion). (c) DOS of Pd(OH)2@CN. Black line: TDOS of Pd(OH)2@CN, red line: PDOS of CN atoms, green line: PDOS of Pd(OH)2. (d) DOS of Co(OH)2@CN. Black line: TDOS of Co(OH)2@CN, red line: PDOS of CN atoms, green line: PDOS of Co(OH)2.

We then examine the integrated Pd-Co@CN system. The impact of model size and positions of anchoring sites of the transition metals is found to be negligible, as suggested by the comparison of DOS of different model systems (Figure S3). Therefore, we use the model shown in Figure S2 to make further calculations. Figure 3b displays the TDOS of the integrated system and PDOS of both individual Pd-fragment (blue dotted polygon in Figure 3a) and Co-fragment (red dotted polygon in Figure 3a). The PDOS of the Pd-fragment is similar to TDOS of Pd(OH)2@CN (Figure 2c), while Co-fragment is similar to Co(OH)2@CN (Figure 2d). The similarity between the DOS of both isolated fragment and their respective counterpart in the integrated model, suggests that the combination of Pd- and Co-fragments does not significantly alter the electronic structures of either fragment. As expected, their VBM and CBM form a staggered band alignment, which meets the essential requirement of a Z-scheme. Moreover, the energy gap of the composite system is around 0.41 eV, which enables transfer of a photoACS Paragon Plus Environment

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excited e- from the vicinity of a Pd-fragment to a photo-excited h+ near a Co-fragment through CN network. To verify that photo-induced charges truly evolve in the fashion of a Z-scheme, the transfer process of photo-excited holes in the Co-fragment is simulated by AI-NAMD. Initially, Cofragment absorbs light and produces a photo-excited hole in the VBM. However, a majority of holes are rapidly depleted in the Co-fragment and within 2.5 ps accumulate in the Pd-fragment as shown in Figure 3c. Besides, transfer process of photo-excited electrons in the Pd-fragment is simulated by the same way. As show in Figure S4, the photo-excited electron in the CBM of Pd-fragment can transfer from the Pd-fragment to the Co-fragment within 500 fs. Such an ultrafast charge transfer results in separated photo-excited electrons (at the CBM of the Cofragment) and holes (at the VBM of the Pd-fragment). This is further confirmed by the band decomposed charge densities of the VBM and CBM of the whole system (Figure 3d). The great degree of charge separation validates our Z-scheme design based on metal-embedded g-CN materials.


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Figure 3 (a) The optimized atomic structure of the integrated system consisting of both Pdand Co-fragments. Red polygon zone: Co-fragment, blue polygon zone: Pd-fragment. (b) DOS of Pd-Co@CN. Black line: TDOS of Pd-Co@CN; red line: PDOS of the Co-fragment; blue line: PDOS of the Pd-fragment. (c) Spatial evolution of holes originating from the Co-fragment as a


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function of time at T=300K. (d) The band decomposed charge density diagrams of the VBM (green bubble) and CBM (yellow bubble) after charge transfer in the Pd-Co@CN. The isosurface is 0.01 electrons per Å3.

Next, we estimated the OER and HER performance of the integrated system. Band alignment indicates that the Pd-fragment is the active site for OER. We first calculated the adsorption energy of one H2O molecule in the vicinity of a Pd(OH)2 fragment; calculation details are presented in the Supporting Information. When the water molecule is loaded on the same side of the CN network as the Pd(OH)2 fragment (See Figure S5), the adsorption energy is -0.53 eV. When they are on opposite sides (See Figure S5), it is -0.62 eV. Thus, the latter is more favorable for the adsorption of H2O. To study the overall water splitting on Pd-Co@CN, we used the model proposed by Nørskov, in which the OER occurs via the following four elementary steps28-29: * + H2O  *OH + (H+ + e-)

(1)

*OH  *O + (H+ + e-)

(2)

*O + H2O  *OOH + (H+ + e-) *OOH  O2 + (H+ + e-)

(3)

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Where * represents the Pd-Co@CN integrated system. The optimized structures of *OH, *O and *OOH formed by OH, O and OOH adsorbed on the integrated system * are shown in Figure 4a (See details of computing free energy changes for the four elementary reaction steps in the Supporting Information). The free energy profiles of OER and HER were computed at the condition of pH = 7 because neutral condition is a reasonable assumption that ensures the stability of OH group in both Pd and Co fragments. Meantime, free energy profiles at acidic condition (pH = 0) are given in Figure S6 in Supporting Information. It can be seen from the free energies change in Figure 4b, steps (1)-(3) are endergonic and step (4) is exergonic when the electrode potential (U) is 0 V. Notably, step (1) is the rate-determining step for the OER, which requires a Gibbs free energy of +1.82 eV (black line in Figure 4b). When U=0.82 V (green line in Figure 4b), the ideal potential for OER at pH=7, step (2) becomes exergonic while steps (1) and (3) remain endergonic; step (1) still requires a Gibbs free energy of 1.00 eV. When the U increases to 1.82 eV (red line in Figure 4b), all four elementary reaction steps become exergonic. The over-potential for the reaction can be calculated with the following equation: η=max[∆G1, ∆G2, ∆G3, ∆G4] - 0.82, where 0.82 V is the potential for an ideal OER catalyst at pH=7. Therefore, the over-potential is 1.00 eV (Figure 4b),


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which is 0.04 eV smaller than that of Pd(OH)2@CN for OER reported in our previous calculations.13

Figure 4 (a) The optimized atomic structure of Pd-Co@CN showing positions of adsorbed OH, O and OOH; (b) Schematic diagram of the Gibbs free energy changes for the four elementary OER steps on Pd-Co@CN, where pH = 7. (c) Schematic diagram of the Gibbs free energy


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changes for HER on Pd-Co@CN with H positioned at site 1, site 2 or site 6, red dot line: ideal value for HER, where pH = 7, U = 0V. (U vs Reversible Hydrogen Electrode).

In addition to the OER reaction, we also explored the HER process. The adsorption energy of an H atom at several possible sites on the Pd-Co@CN surface were calculated (See Figure S7), including the top and side C/N atom sites around the Co-fragment. After relaxing to minimum energy, three adsorption sites (sites 1, 2 and 6) are observed (See Table S1). Among those possible adsorption configurations, bonding of H atoms near the sides of N atoms is found to be the most stable (sites 1 and 2). The adsorption energies for sites 1 and 2 are -0.12 eV and -0.22 eV, respectively, suggesting that either of them can be a favorable reaction site for HER. Whereas for site 6, the adsorption (+1.27 eV) is unstable. We also investigated the mechanism of the HER reaction at sites 1, 2 and 6 according to the following two elementary steps:28, 30-31 * + (H+ + e-)  *H *H + (H+ + e-)  H2 + *

(5) (6)

where * represents Pd-Co@CN. The Gibbs energies for rate determining steps of site 1 and site2 are -0.33 eV and -0.39 eV, respectively. When pH=7，η=max[ΔG5, ∆G6] – (-0.41), where -0.41 V is the potential for an ACS Paragon Plus Environment

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ideal HER catalyst at pH=7. Therefore, over-potentials at sites 1 and 2 are 0.08 eV and 0.02 eV, respectively, indicating that the sites near an N atom have high HER activities. In contrast, the over-potential at site 6 is 1.06 eV, a value unfavorable for HER. The over-potential of HER is merely 0.02 eV thanks to the presence of the Co(OH)2 group, which is a value much smaller than that of the pure g-CN network as reported previously (0.82 eV).7

CONCLUSION In summary, we have successfully designed a Pd-Co@CN Z-scheme model system that can produce H2 and O2 simultaneously via photocatalytic water splitting. The new structure effectively extends sunlight absorption into visible region compared with pristine g-CN. The spatial separation of active sites for production of H2 and O2 and the staggered band alignment not only improves charge separation, but also suppresses reversal of the photocatalytic water splitting reaction. The CN network ensures rapid charge transfer between the Pd-fragment and the Co-fragment after photo-excitation. The over-potential of OER by Pd-fragment is 1.00 eV. Noticeably, the over-potential for HER by Co-fragment is as low as 0.02 eV, which suggests a significant boost of H2 production compared with pristine g-CN (η=0.82 eV). By seamlessly integrating two semiconductors with different band gaps into a single system, our design ACS Paragon Plus Environment

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provides a new way to endow 2D material-based Z-scheme photocatalyst with good visible light absorption, efficient charge separation and excellent catalytic performance. Importantly, our research provides a way to achieve high charge mobility at the interface of the Z-scheme system by building the system in the same 2D platform. A material can be employed in this strategy if it meets the following requirements: (1) chemical modifications (e.g. doping) on this material can create a proper staggered band alignment as required by Z-scheme; (2) the photo-excited e−/h+ pairs that have weaker reduction/oxidation potentials can efficiently transfer between two parts; (3) the over-potentials of HER and OER are sufficiently small, which is beneficial to the process of water splitting reactions. It is expected that other materials consisting of π-conjugated triazine or tri-s-triazine motifs can also be employed in this strategy as long as: (1) they are semiconductors; (2) their band gaps can be tuned by metal atoms; (3) they have π-conjugated plane for efficient charge transportation.


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Supporting Information

Optimized geometry of Pd(OH)2@CN, Co(OH)2@CN (Figure S1), Pd-Co@CN (Figure S2), H2O adsorbed on the Pd-Co@CN surface (Figure S5) and H atom adsorbed on the Pd-Co@CN surface (Figure S7); DOS by PBE functional shown in Figure S3; Spatial evolution of photoexcited electrons originating from the Pd-fragment (Figure 4); Gibbs free energies of OER and HER at pH = 0 (Figure S6); Calculation details about adsorption energy and Gibbs free energy.

ACKNOWLEDGMENT: This work was financially supported by the MOST (No. 2014CB848900, 2018YFA0208702), NSFC (No. 21790351, 21703221, 21633006), the Fundamental Research Funds for the Central Universities (WK2060030027). Supercomputing Center of University of Science and Technology of China is acknowledged for the computing resource.

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TOC Graphic


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Co Embedded in Two-Dimensional Carbon-Nitride for Z

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