Letter pubs.acs.org/NanoLett
Catalytic Directional Cutting of Hexagonal Boron Nitride: The Roles of Interface and Etching Agents Liang Ma† and Xiao Cheng Zeng*,†,‡ †
Department of Chemistry and Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, Nebraska 68588, United States ‡ Collaborative Innovation Center of Chemistry for Energy Materials, University of Science and Technology of China, Hefei, Anhui 230026, China S Supporting Information *
ABSTRACT: Transition-metal (TM) nanoparticle catalyzed cutting has been proven to be an efficient approach to carve out straight channels in graphene to produce high-quality nanoribbons. However, the applicability of such a catalytic approach to hexagonal boron nitride (h-BN) is still an open question due to binary element compositions. Here, our ab initio study indicates that long and straight channels along either the zigzag or the armchair direction of the BN sheet can be carved out, driven by the energetically favored TM−zigzag or TM− armchair BN interface, regardless of roughness of the TM particle surface. Optimal experimental conditions for the catalytic cutting of either BN or BN/graphene hybrid sheet across the domain boundary are proposed via the analysis of the competition between TM−BN (or TM−graphene) interface and H-terminated BN (or graphene) edge. The computation results can serve to guide the experimental design for the production of highly uniform BN (or hybrid BN/graphene) nanoribbons with atomically smooth edges. KEYWORDS: Boron nitride, catalytic cutting, nanoribbons, transition metal nanoparticle, hybrid BN/graphene domain
A
The TM nanoparticles, including iron (Fe),28 nickel (Ni),29−32 and cobalt (Co)33,34 nanoparticles, catalyzed graphene cutting is one of the most promising methods for producing high-quality graphene nanoribbons (GNRs) due to the capability in carving long and straight channels with smooth edges.26,27 Our previous study has elucidated molecular-level mechanisms underlying the directional and catalyst-dependent graphene cutting behavior based on the proper modeling of the TM−graphene interface with consideration of the role of the etching agent. The proposed mechanism is consistent with most experimental observations.35 In addition, the nanoparticle size effect on the graphene cutting is also analyzed theoretically by Qiu et al.36 The binary composition of BN results in more-complex edge structures (B-rich or N-rich) and edge chemistry than graphene and, consequently, more-complex metal−BN interfaces. Whether the TM catalytic cutting recipes are applicable to the BN sheet is still unclear. Several experimental attempts have been made to etch BN by the H2 gas on TM substrate,13,14 by the silver (Ag) catalyst particles in air (oxygen),37 or by the electron beam and plasma.38,39 The etching of BN on copper (Cu) substrate induced by H2 gas can produce various patterns,
tomically thin two-dimensional (2D) hexagonal boron nitride (h-BN) has attracted considerable interest owing to its novel properties for applications, such as its high modulus and high tensile strength, excellent thermal conductivity, and superior chemical and thermal stability.1,2 Theoretical calculations also suggest that BN nanoribbons (BNNRs), obtained from cutting a BN sheet, can exhibit tunable electronic and magnetic properties that depend on the ribbon width, edge configuration and decoration, and functionalization.3−7 Moreover, the theoretically proposed hybrid BN/graphene nanoribbon segments can stimulate half-metallicity with 100% spin polarization near the Fermi level.8 Whereas the scalable synthesis of high-quality BNNRs and hybrid BN/graphene nanoribbons with smooth edge, uniform width, and controllable BN-to-G ratios is prerequisite for the desired electronic and spintronic applications, the synthesis is still a great challenge. Unzipping BN nanotubes via plasma or oxidation etching and ion intercalation can produce BNNRs or BN flakes with large aspect ratios, but the quality of BNNRs is mixed.9−12 Recent advances in the chemical vapor deposition (CVD) growth of large area h-BN sheets13−18 and BN/graphene hybrid domains,19−25 as well as the success of graphene cutting techniques,26,27 motivate this computational study on transition-metal (TM) catalyzed cutting BN (or hybrid BN/ graphene) sheet into BNNRs (or hybrid BN/graphene nanoribbons). © 2017 American Chemical Society
Received: February 21, 2017 Revised: April 25, 2017 Published: April 25, 2017 3208
DOI: 10.1021/acs.nanolett.7b00771 Nano Lett. 2017, 17, 3208−3214
Letter
Nano Letters such as straight channels and triangle BN flakes with smooth edges, complex fractal patterns, or the quasi-hexagonal patterns, depending on the Ar-to-H2 ratio and pressure.14 The hydrogen exposure of the BN on ruthenium (Ru) (0001) substrate can cause continuously shrinking and, ultimately, the disappearance of the BN domain.13 Irregular holes can be created in the Ag catalytic etching of BN in air.37 Thus the anisotropic cutting and etching of BN sheet is TM-catalyst- and etching-agentdependent. A comprehensive mechanistic study of the TMnanoparticle-catalyzed BN cutting is needed. Here, we explore the mechanism of TM-nanoparticle-catalyzed BN cutting via atomistic modeling the interface between the TM nanoparticle and BN sheet, with an account of the role of etching agent, e.g., H2 gas. Our ab initio computation indicates that long and straight channels along either the zigzag (ZZ) or the armchair (AC) direction can be realized via the energetically preferred interfaces between the TM and the ZZ/AC edges, regardless of TM surface roughness. A previous study indicates that catalytic graphene cutting behaviors are closely related with the TM−graphene interaction strength.35 In view of the strong interaction between BN and Ni,17,40 we use the Ni(111) surface as a representative TM surface for the catalytic BN cutting. The Ni−BN interfaces are modeled by using BNNR (with different edge orientations) attached to the Ni(111) surface, while the other edge of the BNNR is passivated by H atoms. The Ni(111) surface is approximated by a three-layer Ni(111) slab model with the bottom layer fixed. The supercells of achiral/chiral BN edges and Ni(111) surface are carefully constructed to make lattice mismatch
