Theoretical Study of the Deposition of Pt Clusters on Defective

Article pubs.acs.org/JPCC

Theoretical Study of the Deposition of Pt Clusters on Defective Hexagonal Boron Nitride (h‑BN) Sheets: Morphologies, Electronic Structures, and Interactions with O Duo Xu, Yue-jie Liu, Jing-xiang Zhao,* Qing-hai Cai, and Xuan-zhang Wang* Key Laboratory of Photonic and Electronic Bandgap Materials, Ministry of Education, Harbin Normal University, Harbin, 150025, China S Supporting Information *

ABSTRACT: Recent studies have shown that two-dimensional nanomaterials can serve as excellent candidates for the support of metal nanoparticles, thus rendering them to have potential applications for developing novel electrocatalysts in the oxygen reduction reaction (ORR). In this work, by performing density functional theory calculations, we investigate the structural and electronic properties of Pt clusters deposited on several defective hexagonal boron nitride (h-BN) sheets as well as their interaction with O that is closely related to the electrocatalytic activity of these composites in ORR. As compared with the pristine h-BN sheet, we find that the binding strength of Pt clusters on defective h-BN sheets is significantly enhanced, which is attributed to their strong hybridization with the sp2 dangling bonds at the vacancy site. Thus, the point defect on h-BN sheets plays a key role on anchoring Pt clusters, ensuring their high stability. More importantly, the interfacial interaction can effectively modify the averaged d-band center of the deposited Pt clusters, greatly influencing their interaction with O. The adsorption energies of O on Pt13 clusters deposited on h-BN sheets with VB, VN, and VB+N vacancies are −2.32, −1.77, and −1.86 eV, respectively, which are weaker than that on a free Pt13 cluster (−3.70 eV), suggesting that the kinetics over these composites in ORR will be promoted. Thus, we expect that Pt nanoclusters deposited on defective h-BN sheets would exhibit good catalytic performance in ORR.

1. INTRODUCTION Graphene, a one planar sheet of sp2-bonded carbon atoms arranged in a hexagonal lattice, has recently attracted considerable attention in diverse fields owing to its exciting properties, such as high ratio aspect, large area, high carrier mobility, high-integer quantum, Hall effect at room temperature, spin transport, high elasticity, and electromechanical modulation.1−3 In particular, graphene has been shown to be an excellent substrate material for dispersion of the transitionmetal (TM) nanoparticle, due to its large surface, outstanding electronic and thermal conductivity, as well as the high mechanical strength and potential low production cost.2,4,5 However, the sintering of TM nanoparticles on pristine graphene has imposed great limitations for its application for fabricating graphene-based electrocatalysts. Luckily, the presence of point defects, such as vacancies, on the as-synthesized graphene (such as reduced graphene oxide) provides a powerful tool to enhance the binding strength of TM nanoparticles on graphenes.6,7 More interestingly, the catalytic reactivity of the supported TM nanoparticles on defective graphenes can also be greatly modified.8−17 For example, Liu et al. have found that the defective or doped graphene substrate not only can stabilize various TM nanoparticles (such as Pd,9 Ru,10,11 Fe,12 and alloy13) but also can enhance their catalytic performance in the oxygen reduction reaction (ORR),9,13 arene hydrogenation,11 and NH3 decomposition.12 Lim and Fampiou have independ© 2014 American Chemical Society

ently reported that the deposited Fe, Al, and Pt nanoparticles on defective graphene exhibit both high stability and superior catalytic activity in ORR.14−17 On the other hand, the great advance in graphene research has encouraged scientists to explore other two-dimension-based materials. Among them, the hexagonal boron nitride (h-BN) sheet has been a recent hot topic, as it shares the same honeycomb lattice structure as graphene. However, the ionic nature of the B−N bond in an h-BN sheet makes it a polar semiconductor with a wide band gap (∼6.00 eV), which is different from graphene with a zero band gap. Moreover, an hBN sheet possesses more important advantages over graphene.18,19 For example, (1) the h-BN sheet exhibits high thermal stability up to 1000 K, and (2) the h-BN sheet is more resistant to oxidation. The above properties render h-BN sheets to have amazing prospects for fabricating novel nanodevices to operate in a harsh environment.18,20 As in graphene, various point defects can inevitably exist during the preparation of a single-layer h-BN sheet or be deliberately introduced into an hBN sheet in a controllable manner by electron beam irradiation.21,22 These vacancies endow the h-BN sheet with exciting electronic and magnetic properties23,24 and thus have Received: September 2, 2013 Revised: April 2, 2014 Published: April 11, 2014 8868

dx.doi.org/10.1021/jp4087943 | J. Phys. Chem. C 2014, 118, 8868−8876

The Journal of Physical Chemistry C

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the convergence in RMS force was set as 0.002 Ha/Å. To achieve sufficient accuracy, 20 image structures were inserted between initial and final states. The binding energy (Eb) of Ptn clusters on an h-BN sheet was defined as Eb = (Eh‑BN sheet + EPtn) − EPtn/h‑BN sheet, where Eh‑BN sheet, EPtn, and EPtn/h‑BN sheet stand for the total energies of the h-BN sheet, Ptn clusters in vacuum, and the adsorbed h-BN sheet by Ptn clusters, respectively. According to such a definition, a more positive value of Eb indicates stronger binding of a cluster to the substrate. For the study on O adsorption, the adsorption energy (Ead) relative to 1/2O2(g) was defined as Ead = Etotal(O + Ptn/h-BN sheet) − Etotal(Ptn/h-BN sheet) − Etotal(O2)/2, where Etotal is the total electronic energies of the systems in parentheses.

greatly widened its potential application for the development of gas sensors and spintronic devices.25 In light of the obvious advantages of h-BN sheets over graphene and the effectiveness of TM deposition on defective graphene, we feel that it is highly desirable to study the deposition of TM nanoclusters on various defective h-BN sheets, which is useful to understand and evaluate their potential for designing novel h-BN sheet-based electrocatalysts in ORR. The adsorption of some metal atoms on pristine and defected h-BN surfaces has been systematically reported very recently, using density functional theory (DFT) methods.26−33 However, theoretical studies on the deposition of metal clusters with various sizes on various defective h-BN sheets are very lacking, especially for their catalytic performance in ORR. Considering that Pt nanoparticles have been represented as one of the best electrocatalysts for ORR,15 in the present work, using the first-principle calculations, we study the adsorption of Ptn (n = 1−4, 13) clusters on defect-free and defective h-BN sheets, including monovacancies (removing a B or N atom from a pristine h-BN sheet, labeled as VB or VN defect) and divacancies (removing a pair of neighboring B and N atoms from a pristine h-BN sheet, labeled as VB+N defect). Two key issues would be mainly focused on as follows: (1) the binding strength of Pt clusters on various defective h-BN sheets and (2) the interaction of these composites with O. The former determines the stability of Pt clusters on defective h-BN sheets, whereas the latter is directly related to the catalytic performance of these deposited Pt composites in ORR. Notably, although there has been no experimental evidence for the synthesis of Ptdeposited h-BN sheets until now, the deposition of a single metal atom or a cluster of several atoms (such as Fe, Co, Mo, and Pt) on defective graphene has been synthesized in experiment.34,35 Hence, it is quite possible to deposit a Pt atom or cluster onto an h-BN sheet by adopting a similar technique.

3. RESULTS AND DISCUSSION 3.1. Adsorption of a Single Pt Atom. In Figure 1, we present the geometric structures and electronic properties of

2. COMPUTATIONAL DETAILS Calculations were based on spin-polarized DFT using the generalized gradient approximation (GGA) for the exchange− correlation potential prescribed by Perdew−Burke−Ernzerhof (PBE),36 implemented in the DMol3 package.37All-electron calculations were employed with the double numerical basis sets plus the polarization functional (DNP), which are comparable to the Gaussian 6-31G(d,p) basis set in size and quality. For the 5d transition-metal atoms Pt, the scalar relativistic effects (DSSP) were considered for its core electrons. A (6 × 6) supercell with periodic boundary conditions in the x−y plane was employed to model the deposition of Ptn clusters on infinite defective h-BN sheets. The vacuum space was set to 20 Å in the z direction to avoid the interaction between periodic images. During the geometric optimization, we used a 2 × 2 × 1 mesh of k-points38 according to the convergence test, as shown in Table S1 in the Supporting Information. All structures were fully relaxed without any symmetry constraints. The convergences in energy, force, and displacement were set as 2 × 10−5 Ha, 0.004 Ha/Å, and 0.005 Å, respectively. Density of states (DOSs) and electron density distributions based on the equilibrium structures were computed with a 5 × 5 × 3 mesh of k-points. The Hirshfeld method39 was used to calculate the charge transfer. The linear synchronous transit (LST/QST) and nudged elastic band (NEB) methods40 in the DMol3 module were performed in order to obtain the minimum energy pathway (MEP), in which

Figure 1. Optimized structures of defective h-BN sheets with (a) VB, (b) VN, and (c) VB+N defects, along with the corresponding projected DOSs. (B: pink; N: blue. The blue lines correspond to p-DOS, and the red lines correspond to s-DOS. The green dotted lines correspond to the Fermi level.)

defective h-BN sheets with VB, VN, and VB+N vacancies. Considering the adsorption of a single Pt atom on these h-BN sheets, there are several stable adsorption sites (hollow/top/ bridge sites) that can be rendered inequivalent through substrate reconstructions. For brevity, the obtained most stable configurations are listed in Figure 2 and the corresponding structural parameters and binding energies are summarized in Table 1. 8869



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lowest-energy configurations, structural parameters, and binding energies are shown in Figure 3 and Table 2. For a pristine

Figure 3. Optimized structures of Pt2 cluster on (a) defect-free and defective h-BN sheets with (b) VB, (c) VN, and (d) VB+N defects.

Table 2. Structural Parameters, Binding Energies, and Charge Transfer of Pt2 Cluster on Various Substrates Eb (eV) defect-free VB defect VN defect VB+N defect

Figure 2. Optimized structures of a single Pt atom on (a) defect-free and defective h-BN sheets with (b) VB, (c) VN, and (d) VB+N defects.

defect-free VB defect VN defect VB+N defect

1.81 7.53 7.65 8.44

dPt−B (Å)

1.99 2.07

dPt−N (Å)

Q (e)a

2.02 1.96

0.09 0.44 0.15 0.29

2.07

dPt−B (Å)

1.99 2.26

dPt−N (Å)

dPt−Pt (Å)

Q (e)

2.19 1.97

2.41 2.38 2.53 2.43

0.03 0.28 0.10 0.42

1.98

h-BN sheet, the most stable configuration is a vertically oriented dimer bound at a bridge site. The Pt−Pt bond length in the adsorbed form is elongated to 2.41 Å as compared to the gas-phase bond length of 2.33 Å. When the Pt2 cluster is adsorbed at a monovacancy, one Pt atom is consistently adsorbed on an on-top site, while the other Pt atom is out of touch with the substrate. The distances between the adsorbed Pt2 cluster and the VB and VN defects are 1.97 and 1.99 Å, respectively. At a divacancy, one of the Pt atoms binds with two unsaturated B atoms at the defect site with a length of 2.26 Å, while the other Pt atom locates on two unsaturated N atoms at the defect site with a length of 1.98 Å. Furthermore, the Pt2 adsorption induces a locally structural deformation to both the Pt2 cluster and the defective h-BN sheets. The bond lengths of Pt−Pt of the adsorbed Pt2 cluster on VB, VN, and VB+N defects are elongated to 2.38, 2.53, and 2.43 Å, respectively, and the N−N, B−B, B−B/N−N bond distances in the three defective h-BN sheets are increased to 2.80, 2.73, and 2.76/2.77 Å, respectively. As before, such deformation is attributed to the strong hybridization of the Pt2 cluster and the dangling bonds of the defective h-BN sheets. 3.3. Adsorption of Pt Trimers. The Pt3 cluster has two isomers, including the linear and triangular configurations. Upon adsorption on h-BN sheets, we considered adsorption of the two isomers both parallel and perpendicular to the h-BN sheet substrate. Figure 4 and Table 3 list the most stable configurations, structural parameters, and binding energies. For a pristine h-BN sheet, the most stable configuration is that two Pt atoms of the Pt3 cluster are attached to its two N atoms, while the third Pt atom binds directly with the above Pt atoms. The length of the newly formed Pt−N bond is 2.24 Å. At the VB defect, one Pt atom at the vertex of the triangle occupies the site of the missing B atom with an out-of-plane displacement as before, while the second and third Pt atoms are directly bonded only to the first Pt atom. At the VN defect, the first Pt atom is used to saturate the three dangling B atoms, the second Pt atom is located on the neighboring B−N bond, and the third Pt

Table 1. Structural Parameters, Binding Energies, and Charge Transfer of a Single Pt Atom on Various Substrates Eb (eV)

1.49 7.42 8.06 8.90

a

The positive value indicates that the charge is transferred from the Pt atom to the substrate.

On a pristine h-BN sheet, the Pt atom is adsorbed most strongly on the N site and is located at 2.02 Å above this substrate. At VB and VN defects, the Pt atom essentially binds as a substitutional atom that is displaced by 0.98 and 1.22 Å, respectively, due to the greater lengths of the newly formed N− Pt (1.97 Å) and B−Pt (1.99 Å) as compared to a B−N bond (1.45 Å). At a VB+N defect, the Pt atom occupies a “cross” configuration and is slightly displaced out of the h-BN sheet plane by 0.49 Å. Understandably, the strong interaction of the Pt atom with a defective h-BN sheet is responsible for the structural distortion of substrates, especially at the vacancy site. For example, the N−N distance of the VB defect is increased from 2.64 to 2.94 Å and the B−B bond of the VN defect is increased from 2.28 to 2.72 Å, while the B−B and N−N bonds in the VB+N defect are elongated by 0.82 and 0.93 Å, respectively. The significant elongation of these bonds suggests that they have been broken upon the Pt deposition, which can be further testified by their charge densities in Figure S1 (Supporting Information): the charge overlaps between B−B and N−N bonds around vacancy in the three defective h-BN sheets have vanished. Therefore, the strong binding between Pt and the defective h-BN sheet can be attributed to two processes: (1) the breaking of the bonds at the vacancy site and (2) the formation of the B/N−Pt interaction. 3.2. Adsorption of Pt Dimers. Next, we study the adsorption of a Pt2 cluster on the four substrates. The obtained 8870



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atom binds with the two Pt atoms. For a divacancy, one Pt atom occupies a “cross” configuration and is slightly displaced out of the defective h-BN sheet, while the other two Pt atoms locate on the top of the dangling B atoms at a vacancy site with the distance of 2.09 Å, forming an inverted triangle. Similarly, the Pt3 cluster adsorption also leads to the elongation of the Pt−Pt bond (Table 3) and the breaking of the B−B and/or N− N bond(s) at the vacancy site to varied degrees. 3.4. Adsorption of Pt Tetramers. For the Pt4 cluster, we consider its adsorption of the planar and tetrahedral pyramidal configurations to various h-BN sheets. In Figure 5 and Table 4, we list the most stable configurations of the Pt4 cluster and the corresponding binding energies. When the Pt4 cluster is attached to the pristine h-BN sheet, the tetrahedral geometry is more stable by 46 meV/atom, although the planar one is energetically preferred in the gas phase by 13 meV/atom.40 Moreover, in this configuration, three Pt atoms occupy the B− N bonds of the substrate, while the fourth Pt atom directly binds with the above Pt atoms (Figure 5a). At VB and VN defects, the cluster is adsorbed as an inverted tetrahedron with one vertex passivating the missing N and B atoms, accompanying with an out-of-plane displacement (Figure 5b,c). When adsorbed at the divacancy, the cluster is converted to a twisted rhombus, as shown in Figure 5d. In particular, one of the Pt atoms is used to saturate the dangling B and N atoms of the VB+N defect, two other Pt atoms are adsorbed on the B− N bonds, and the fourth Pt atom binds with the above three Pt atoms. Once again, the B−B and/or N−N bonds at VB, VN, and VB+N defects are broken due to Pt4 adsorption. Interestingly, the morphology of a Pt4 cluster is dependent on the substrate: a planar rhombus on the VB+N defect is found, while a tetrahedron is obtained on VB and VN defects. 3.5. Adsorption of Pt13 Cluster. Finally, we explore the adsorption of a Pt13 cluster, which represents the smallest magic cluster according to the geometric shell model.41,42 In light of the extremely expensive cost for calculating the stable configuration of the Pt13 cluster with various shapes on these h-BN sheets, only the distorted cuboctahedron (D 4h ) configuration is considered because it is much more stable

than other symmetry configurations, such as icosahedron (Ih) and regular cuboctahedron (Oh), by ∼1 eV.15 Various initial adsorption configurations are considered for the Pt13 cluster on these substrates, including a vertex, bond, triangular, or square face of the Pt13 cluster is attached to the active sites of these substrates. After the carefully geometrical optimization for each initial configuration, the obtained most stable configurations are given in Figure 6, and the corresponding binding energies, structural parameters, and charge transfer are presented in Table 5. The results indicate that the Pt13 cluster prefers to be adsorbed on the defect-free h-BN sheet with one triangular face parallel to this substrate (Figure 6a), forming three Pt−N bonds with the length of 2.27 Å (Table 5). On VB and VN defects, this cluster uses one vertex Pt atom to saturate the dangling N and B atoms around defects, forming a nearsubstitutional configuration (Figure 6b,c). Meanwhile, two other Pt atoms are bound with the basal N atoms, leaving some undercoordinated Pt atoms (Figure 6b,c). When the Pt13 cluster is attached to the VB+N defect, one apex atom interacts with the dangling B and N atoms at a divacancy site, forming a cross configuration as before. As shown in Table 5, the shortest Pt−B and Pt−N distances range from 1.98 to 2.40 Å and from 2.03 to 2.34 Å, leading to the formation of interfacial Pt−B and Pt−N bonds. Because the rest of the cluster is adsorbed on the B and N atoms of the VB+N defect, four new chemical bonds are also formed, including two Pt−B bonds and Pt−N bonds, with fewer undercoordinated Pt atoms than those of the VB or VN defect. Considering that the number of active sites of a catalyst is greatly dependent on the number of its undercoordinated surface atoms, the above results give us a hint: one can tune the potential catalytic activity of the deposited Pt cluster by controlling the extent of undercoordination of Pt atoms. As before, the structures of adsorbate and substrates are distorted in various ways. For example, the Pt−Pt distance at the interface of the Pt13/defect-free h-BN sheet is elongated from 2.81 to 3.63 Å. In Figure 7, we display the binding energies of various Pt clusters on these h-BN sheets to compare their relative

Table 3. Structural Parameters, Binding Energies, and Charge Transfer of Pt3 Cluster on Various Substrates Eb (eV) defect-free VB defect VN defect VB+N defect

2.06 7.64 8.06 9.72

min dPt−B (Å)

min dPt−N (Å)

min/max dPt−Pt (Å)

Q (e)

1.99 2.08

2.24 1.96 2.29 2.01

2.50/2.54 2.47/2.58 2.50/2.67 2.53/2.58

0.09 0.42 0.11 0.54

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Table 4. Structural Parameters, Binding Energies, and Charge Transfer of Pt4 Cluster on Various Substrates defect-free VB defect VN defect VB+N defect

Eb (eV)

min dPt−B (Å)

min dPt−N (Å)


Q (e)

2.29 7.95 8.16 9.63

2.33

2.28 2.00

2.57/2.76 2.58/2.65 2.57/2.80 2.49/3.54

0.12 0.58 0.17 0.43

1.99 2.08

2.03

Figure 7. Binding energies of Ptn clusters on various h-BN sheets.

From the thermodynamic point view, the binding energies of a Pt atom on pristine and defective h-BN sheets with VB, VN, and VB+N defects are 1.81, 7.53, 7.65, and 8.44 eV, respectively. The binding strength between a Pt atom and the h-BN sheet substrate is increased by 4.2 times at least when the substrate varies from a pristine h-BN sheet to a defective one, which is also larger than the cohesive energy of bulk Pt (5.84 eV). Dynamically, the diffusion of the adsorbed Pt atom on a pristine h-BN sheet along the N−B−N direction is likely to occur with a barrier of 0.74 eV. However, the diffusion barrier of a single Pt atom from a vacancy to the nearest active site is greatly increased. For example, the calculated diffusion barrier of a Pt atom on the h-BN sheet from the VB vacancy site to its neighboring N site is 6.08 eV and is endothermic by 5.11 eV. This fact vigorously excludes the clustering problem of deposited Pt clusters. Therefore, the free Pt atoms on the defect-free region of the h-BN sheet easily diffuse to gain additional stability, and the most favorable sites for Pt atoms to nucleate are the defect sites. 3.6. Electronic Structure of Adsorbed Pt13 Clusters. Having obtained the most stable configurations of Ptn clusters on various h-BN sheets, we now turn our attention to exploring the effects of substrates on the electronic structures of deposited clusters. For h-BN sheets with VB, VN, and VB+N defects, their density of states (DOSs) are characterized with a sharp peak in the vicinity of the Fermi level (Figure 1), originating from the states of B or N atoms at a vacancy site. Clearly, the unpaired


stabilities. The results suggest the following: (1) The formation of all of these Ptn/h-BN sheet composites is thermodynamically favored. Because of the existence of the dangling bonds, the binding energies of the Ptn clusters on defective h-BN sheets are much larger than those on the pristine one, confirming the superior role of the defective h-BN sheet as an effective substrate to anchor Pt nanoparticles. Among the four substrates, the VB+N defect on the h-BN sheet exhibits the highest chemical reactivity toward these Pt clusters, followed by the VN defect, as shown in Figure 7. (2) The adsorption of various Pt13 clusters on these h-BN sheets is the strongest, independent of substrates. This indicates that the deposited Ptn clusters with “magic numbers” n have a higher stability than other cluster sizes. In addition, because Pt clusters are much more active than the flat Pt(111) surface,43 it is understandable that their interaction with h-BN sheets (even a pristine one) is much stronger than with Pt(111).30 Another important problem of Pt nanocluster-based catalysts is their sintering during the reactions. The adsorption of a single Pt atom is taken as an example to address this issue.

Table 5. Structural Parameters, Binding Energies, and Charge Transfer of Pt13 Cluster on Various Substrates

defect-free VB defect VN defect VB+N defect

Eb (eV)

min dPt−B (Å)

min dPt−N (Å)


Q (e)

2.35 9.70 9.42 11.83

2.40 1.98 2.22

2.26 2.03 2.34 2.10

2.58/3.63 2.54/3.98 2.58/4.08 2.61/4.00

0.10 0.54 0.16 0.60

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Figure 8. DOSs of interfacial B/N and Pt atoms and the corresponding contour plots of differential charge density of (a) Pt13/pristine h-BN sheet, (b) Pt13/VB defect, (c) Pt13/VN defect, and (d) Pt13/VB+N defect. The Fermi level is set as zero in red dotted lines. The charge accumulation region is rendered in red, and the charge depletion region is in blue. The isovalue is ±0.003 au.

Table 6. Change of d-Band Center (εd) and Charge before and after Pt13 Depositiona Pt1 Pt2 Pt3 Pt4 Pt5 Pt6 Pt7 Pt8 Pt9 Pt10 Pt11 Pt12 Pt13 average a

Pt13

Pt13/defect-free

Pt13/VB defect

Pt13/VN defect

Pt13/VB+N defect

−1.79(−0.01) −1.79(−0.01) −1.79(−0.01) −1.79(−0.01) −1.79(−0.01) −1.79(−0.01) −1.79(−0.01) −1.79(−0.01) −1.79(−0.01) −1.79(−0.01) −1.79(0.16) −1.79(−0.01) −3.94(−0.01) −1.96

−1.86(−0.02) −1.91(−0.03) −1.90(−0.03) −1.83(−0.02) −2.22(0.05) −1.90(−0.03) −2.06(−0.02) −2.25(0.06) −2.24(0.06) −2.05(−0.02) −3.60(0.13) −2.07(−0.02) −1.86(−0.01) −2.14

−2.40(0.10) −2.08(−0.01) −1.97(−0.03) −2.12(0.00) −2.26(0.08) −2.48(0.01) −1.82(−0.01) −4.09(0.35) −1.61(−0.03) −2.18(−0.01) −3.61(0.08) −2.50(0.02) −2.17(−0.01) −2.41

−2.31(−0.05) −2.76(0.01) −2.18(−0.04) −2.32(−0.03) −1.81(−0.07) −2.32(−0.02) −1.80(−0.08) −4.41(0.19) −2.78(0.09) −2.60(0.00) −3.79(0.09) −2.68(0.01) −2.58(0.07) −2.64

−3.84(0.26) −2.35(0.09) −1.71(−0.07) −2.07(−0.03) −2.31(0.06) −1.98(−0.03) −2.66(0.13) −2.80(0.06) −1.83(−0.05) −2.66(0.05) −3.69(0.11) −1.95(−0.03) −2.04(0.05) −2.45

See Figure 6 for the notation of the Pt atoms, and the values of d-band and charge are listed.

sp2 dangling-bond states of the undercoordinated B or N atoms contribute to the DOS intensity around the EF, leading to the high reactivity of defective h-BN sheets. In Figure 8, we list the DOSs of the deposited Pt13 cluster on different substrates. It is apparent that the sharp peaks in the vicinity of the Fermi level (Figure 1) disappear after the Pt13 cluster adsorption. Meanwhile, the strong hybridization between d states of Pt

and p states of defective h-BN sheets can be observed, indicating that the Pt13 cluster uses its valence electrons to saturate the dangling-bond states of the vacancy in h-BN sheets. For all Pt13/defective h-BN sheet composites, the defect states around the EF of substrates are all shifted to low energy levels due to their strong interaction with Pt13 clusters by comparing Figures 1 and 8 for various cases. Instead of maintaining the 8873



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highest DOS intensity right below the EF on the pristine h-BN sheet, the main peaks of Pt d states are also shifted downward to overlap with the sp states of the h-BN sheet with VB, VN, and VB+N defects. For the Pt13/pristine h-BN sheet composite, the main peak of the Pt d state is crossing the EF. However, on VB, VN, and VB+N defects, the main peaks are shifted to −0.69, −0.87, and −0.38 eV, respectively. The above changes in DOS of the interfacial Pt atoms suggest that the interfacial interaction has obvious effects on the electronic structures of the deposited clusters. Since the proposed “d-band center (εd)” by Nørskov and Hammer41 has been widely used in various transition metals and alloy systems for reaction mechanism studies and design,9−13,44,45 we also calculate the εd of the deposited Pt13 clusters on various h-BN sheets. For the freestanding Pt13 cluster, its averaged εd is −1.96 eV, in which the εd values of the Pt atoms in the center and vertex are −3.94 and −1.79 eV, respectively. In comparison, the εd of the deposited Pt clusters on h-BN sheets has been modified in various ways (Table 6), which is generally down-shifted from the EF. In detail, the averaged εd values of the Pt clusters on defect-free and defective h-BN sheets with VB, VN, and VB+N defects are −2.14, −2.41, −2.64, and −2.45 eV, respectively. This indicates that substrates have obvious effects on the electronic structures of the deposited Pt13 clusters. According to the d-band model, the lowering of εd makes the deposited Pt13 nanoparticles low reactivity and low possibility for O-poisoning to take place. Aiming at evaluating the catalytic performance of these deposited Pt13 clusters on h-BN sheets in ORR, the adsorption of O on these composites was further investigated. This is because the reduction of oxygen to water on a catalyst involves many steps, and it is well-known that the first electron transfer or oxygen adsorption together with a proton and electron transfer to form superoxide is the rate-determining step.46,47 Hence, a more reactive catalyst, such as Pt nanoparticles whose d-band center is adjacent to the Fermi level, can bind the adsorbates more strongly and make the electron transfer and O−O bond break more easily. However, the desorption kinetics of the subsequent reactions of the intermediates (including O, OH, and OOH) may be slow owing to their strong adsorption. Therefore, a catalyst with good performance in ORR is that it can moderately bind with the adsorbates to balance the kinetics of O−O bond breaking and desorption of the oxygen-containing intermediates produced from the former step. One effective method to improve the ORR kinetics of a catalyst is to reduce the oxygen adsorption energy by lowering its d-band center, which has been testified by the recent experimental results and theoretical calculations.48−51 Therefore, the binding strength of O-containing intermediates (such as atomic O) is a key indicator to evaluate the catalytic activity trends across different catalysts. When a single O atom is attached to these deposited Pt13 clusters, we sample between 18 and 27 different adsorption sites on every cluster, as shown in Table S2 (Supporting Information). In Table 7, we report the average adsorption energies (Ead) of atomic O on the h-BN sheet-supported and free Pt13 clusters relative to 1/2 O2(g). Statistical errors are estimated using a 95% confidence interval of the Student’s tdistribution, which has been employed in Fampiou’s study.52 The fully relaxed configurations for one selected case (the most stable) each of O adsorption on supported clusters are presented in Figure 9.

Table 7. Average Adsorption Energy (Ead) for O Adsorption on Pt13 Supported on Pristine and Defective h-BN Sheets, as Well as on Freestanding Pt13 Clusters h-BN sheet substrate pristine VB VN VB+N free clusters

Ead (eV) −2.47 −1.58 −1.40 −1.44 −3.40

± ± ± ± ±

0.15 0.09 0.05 0.08 0.15

Figure 9. Side view of selected lowest-energy DFT configurations for adsorption of an O on the Pt13 clusters on deposited (a) pristine and defective h-BN sheets with (b) VB, (c) VN, and (d) VB+N defects.

The O adsorption energy results reported in Table 7 indicate that the presence of a point defect in the h-BN sheet substrate weakens the interaction of the atomic O with the Pt13 clusters. In particular, for the three different defective h-BN sheet-based substrates (VB, VN, and VB+N), the O adsorption energy on the Pt13 cluster is lower by 0.89, 1.07, and 1.03 eV, respectively, compared with the average adsorption energy on Pt13 clusters supported on the pristine h-BN sheet; the difference in average adsorption energies relative to freestanding Pt13 is even larger (1.82, 2.00, and 1.96 eV, respectively). This suggests that the ORR kinetics over these composites will be promoted because the poisoning of the ORR active sites is effectively hindered. In this sense, these deposited Pt13 clusters by these defective h-BN sheets would exhibit good catalytic performance in ORR. In Figure 10, we correlate this d-band shift to the average O adsorption energy. We find that the Ead is directly correlated with the shift of the εd, which originates from the strong interaction between the Pt clusters and the h-BN sheet substrates. We note that the average adsorption energies of O 8874



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AUTHOR INFORMATION

Corresponding Authors

*Phone: 86-451-88060580. E-mail: [email protected] (J.x.Z.). *E-mail: [email protected] (X.-z.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Nos. 21203048, 11074061) and the Scientific Research Fund of Heilongjiang Provincial Education Department (No. 12531195). The authors would like to show great gratitude to the reviewers for raising invaluable comments and suggestions.

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Figure 10. Calculated average adsorption energies (Ead) of atomic O on these deposited Pt13 clusters versus εd. Error bars indicate 95% confidence intervals obtained from sampling over multiple adsorption sites on the cluster.

on the Pt13 clusters deposited on the three defective h-BN sheet substrates are even lower than that on Pt(111) (Ead = −1.93 eV),53 suggesting their possible superior catalytic activity in ORR. Thus, one can improve the catalytic activity of Pt-based catalysts by careful selection of the h-BN sheet substrates.

4. CONCLUSIONS In summary, we have comprehensively studied the deposition of a series of Pt clusters on several defective h-BN sheets by means of DFT methods. Because of the introduction of various point defects, the binding energies of these Pt clusters on h-BN sheets are increased by several electronvolts, which is mainly ascribed to the strong orbital hybridization of the d states of the Pt clusters with the sp2 dangling bonds of the defect sites in substrates. In this sense, these defective h-BN sheets can be used as templates for the Pt clusters assembly. Moreover, a certain amount of electrons is transferred from the Pt clusters to the substrates, resulting in the redistribution of the charge in the deposited Pt clusters. As a result, the values of the averaged d-band center of the supported Pt clusters are shifted from −1.96 eV of the freestanding Pt13 cluster to the present −2.14 eV (on defect-free h-BN sheet), −2.41 eV (on VB defect), −2.64 eV (on VN defect), and −2.45 eV (on VB+N defect), respectively. The downshift of the d-band center of the Pt clusters interferes with the O adsorption: the average adsorption energy of atomic O on deposited Pt13 clusters is reduced in various ways, as compared with on the freestanding cluster or supported on the defect-free pristine h-BN sheet. This suggests that these defective h-BN sheets not only stabilize the Pt clusters but also enhance their catalytic performance in the oxygen reduction reaction, which is very useful to develop h-BN sheet-based electrocatalysts.

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ASSOCIATED CONTENT

S Supporting Information *

The convergence test of k-points, adsorption of the Pt13 on various h-BN sheet substrates, and charge densities of a single Pt atom adsorbed on various defective h-BN sheets. This 8875



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Theoretical Study of the Deposition of Pt Clusters on Defective

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