Atomic structure, comparative stability and electronic properties of hydroxylated Ti₂C and Ti₃C₂ nanotubes

Abstract

Recently, hydroxylated and fluorinated graphene-like titanium carbide TiC_x layers have been solvothermally fabricated in large amounts from so-called MAX phase Ti₃AlC₂. We assume, that a wide family of novel planar and tubular forms of titanium carbides may exist and design the atomic models for monolayers and nanotubes with nominal stoichiometry Ti₂C, Ti₃C₂ and for their hydroxylated forms Ti₂C(OH)₂, Ti₃C₂(OH)₂. The stability and electronic properties of these nanostructures are examined by means of density-functional theory tight-binding method depending on the composition and the type of OH arrangement. We reveal that the type of OH termination plays a minor role in the variation of nanotubes’ strain energies, but causes a difference in the relative stability of their parent planar phases. The electronic structure for all nanotubes studied has metallic-like character, while their precursors (planar layers) demonstrate either metallic-like or semiconducting behavior depending on the arrangement of the surface OH groups.

Introduction

Titanium carbide (TiC) exhibits an unique combination of excellent physical and chemical properties, such as high melting point, high strength, low electrical resistivity, good anticorrosion and antiwear properties, which make this material highly suitable for various technological applications: as coating, in cutting tools, as reinforcing component in oxide or non-oxide ceramics, in hard alloys, etc. [1], [2], [3], [4]. In turn, nanosized forms of TiC with specific morphologies may provide a wider and diverse ability for further tuning of physical properties and the fabrication of new materials with promising applications and unexpected technological prospects.

Nowadays, a rich set of various physical and chemical pathways for the synthesis of nanostructured TiC has been developed, see for example [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. Along with quite conventional approaches, such as carbothermal reduction, chemical vapor deposition, sol–gel method, arc discharge technology, etc., also some rather exotic routes are proposed like biotemplate methods which use natural nanoporous bamboo [15] or cotton fibers [16] as both the renewable carbon source and the template in the synthesis of TiC nanowires. As result, a large set of monolithic TiC nanostructures – such as nanoparticles, nanocrystallites, nanowires (nanorods) with different morphology has been synthesized [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16].

One of the major challenges in the fabrication of nanostructured TiC materials (as well as of carbides and nitrides of others d metals) is the synthesis of hollow quasi-one-dimensional (tube-like) forms of titanium carbide. The reason is quite obvious: a formation of tube-like nanoforms of isotropic 3D-like crystals with isotropic system of interatomic bonds is much more problematic, than that for the nanotubes of layered compounds (such as graphite, BN, MoS₂, etc.) which possess sharply anisotropic systems of interatomic bonding including strong bonds within the layers versus very weak van der Waals-type bonding between the adjacent layers. Indeed, despite a set of theoretical models for monocrystalline TiC nanotubes was proposed [11], [17], [18], [19], any convenient attempt to fabricate such tubular material was absent.

Very recently, a promising possibility to obtain the TiC_x nanotubes was demonstrated after the experiments [20], where the graphene-like titanium nanocarbides have been successfully synthesized from so-called MAX phases. These ternary phases (see reviews [21], [22], [23], [24], [25], [26]) with the general formula M_n+1AX_n (n = 1, 2, or 3) can be described as intergrowth structures consisting of hexagonal 2D-like blocks [M_n+1X_n] (where M = d metals; X = C or N) alternating with planar atomic sheets A (where A are sp elements such as Al, Ga, Si, Ge, S, etc.) The system of inter-atomic interactions in MAX phases is quite anisotropic with strong directional M–X bonds inside blocks [M_n+1X_n], whereas the bonds between elemental monoatomic sheets A and blocks [M_n+1X_n] are weaker [21], [22], [23], [24], [25], [26]. From these reasons, taking Ti₃AlC₂ as example, the experiments for the extraction of the most weakly bonded Al from this MAX phase were undertaken. New 2D graphene-like material with nominal composition Ti₃C₂ was successfully prepared by treatment of Ti₃AlC₂ powders in HF (Ti₃AlC₂ + 3HF = AlF₃ + 3/2H₂ + Ti₃C₂) and further sonication of the reaction product for exfoliation of nanoblocks Ti₃C₂ [20]. Moreover, a preliminary evidence for the formation of scrolls from graphene-like Ti₃C₂ was shown [20].

One may assume that the graphene-like titanium nano-carbides can become successful precursors for the TiC_x nanotubes (NTs). The results [20] open also other interesting prospects. A further possibility to update the properties of the proposed TiC_x nanotubes arises from their decoration by various atoms or functional groups – e.g., like in experiments [20], where the exfoliated species Ti₃C₂ were terminated by OH groups and/or by fluorine ions from aqueous environment. Moreover, since about ∼70 synthesized MAX phases are known [21], [22], [23], [24], [25], [26], the preparation of other graphene-like carbides and nitrides of Sc, Ti, V, Cr, Zr, Nb, Mo, Hf and Ta may be proposed, and, in turn, these species may be considered as precursors for rolled up nano-forms of these carbides or nitrides – such as nanoscrolls or nanotubes.

Following these circumstances, in this work we present the atomistic models and report the results of DFTB simulations of stability and electronic properties for TiC_x nanotubes, constructed from graphene-like sheets Ti₂C and Ti₃C₂ – as building blocks of Ti₂AlC and Ti₃AlC₂ MAX phases. Besides, the models of hydroxylated Ti₂C, and Ti₃C₂ nanotubes (Ti₂C(OH)₂, and Ti₃C₂(OH)₂ NTs) are examined, taking into account that the experimentally exfoliated species Ti₃C₂ were terminated by OH groups. This choice allows to examine the properties of proposed TiC_x NTs as a function of their composition (Ti₂C versus Ti₃C₂) and to consider the influence of OH termination on electronic properties and comparative stability of these novel tubular structures.