X-ray photoelectron spectroscopy of select multi-layered transition metal carbides (MXenes)

Abstract

In this work, a detailed high resolution X-ray photoelectron spectroscopy (XPS) analysis is presented for select MXenesa recently discovered family of two-dimensional (2D) carbides and carbonitrides. Given their 2D nature, understanding their surface chemistry is paramount. Herein we identify and quantify the surface groups present before, and after, sputter-cleaning as well as freshly prepared vs. aged multilayered cold pressed discs. The nominal compositions of the MXenes studied here are Ti 3 C 2 T x , Ti 2 CT x , Ti 3 CNT x , Nb 2 CT x and Nb 4 C 3 T x , where T represents surface groups that this work attempts to quantify. In all the cases, the presence of three surface terminations,-O,-OH and-F, in addition to OH-terminations relatively strongly bonded to H 2 O molecules, was confirmed. From XPS peak fits, it was possible to establish the the average sum of the negative charges of the terminations for the MXenes. Based on this work, it is now possible to quantify the nature of the surface terminations. This information can, in turn, be used to better design and tailor these novel 2D materials for various applications.

Transparent, conductive solution processed spincast 2D Ti2CTx (MXene) films

“…To create MXenes, a MAX phase powder is typically placed in HF-containing solutions that selectively etch the A layers and replace them by -O, -OH and -F terminations, forming multilayers. The proper designation of these 2D materials is thus M n+1 X n T x , where T stands for one of the aforementioned surface terminations [16]. Multilayer MXene can be readily transformed into colloidal solutions using appropriate solvents, or by simply etching in a Li-ion containing solution [17].…”

Rendering Ti3C2Tx (MXene) monolayers visible

“…The MAX phases, in turn, are a large ( > 70) family of layered ternary transition metal carbides, where M is an early transition metal, A is an A-group element (mostly groups 13 and 14), and X is C or/and N [16]. Because during etching, the A-group element layers such as Al and Ga are replaced by -O, -OH, and -F terminations [17,18], the proper designation of the resulting 2D material is Figure 1. (a) AFM scan of a Ti 3 C 2 T x monolayer on a 285 nm thick SiO 2 substrate.…”

Selective Calixarene‐Directed Synthesis of MXene Plates, Crumpled Sheets, Spheres, and Scrolls

“…As with other 2D nanomaterials such as graphene, an increase in functionality is considered to be due to the larger lateral surface area, and the extent/type of functional surface termination groups. 10,11 Since the first report by Naguib et al Figure 2e) and more apparently in TEM images (Figure 3d), is a consistent circular cavity in the spherical MXene structures, averaging less than 1µm in diameter. The formation of MXene spheres appears to be unprecedented.…”

“…Selective chemical etching out the A layer from the MAX phase, typically using hydrofluoric acid (HF) (or lithium fluoride in hydrochloric acid or ammonium difluoride), 3 results in surface functionalized (OH, O or F, noted as T x ) stacks of M n+1 X n T x layers. 10,11 The resulting MX phase multi-layered plates can then undergo exfoliation to produce few or single layered 2D nanosheets of hexagonally arranged metal carbides or nitrides, which are said to resemble graphene sheets, leading to them being named MXenes. 12 MXenes exhibit promising properties for use as Li-ion battery electrodes, 2,8 energy storage supercapacitors, 1,13 and selective heavy-metal adsorption.…”