X‑Site Dependency of Optical and Electronic Properties in Ti 3 (C2– y N y )T x Carbonitride MXenes

TL;DR

This paper explores how changing the nitrogen content in a type of material called MXene affects its optical and electronic properties.

Contribution

The study demonstrates independent control of optical and electronic behaviors in carbonitride MXenes through X-site chemistry.

Findings

Higher nitrogen content increases halogen termination and light-matter interaction.

Nitrogen content causes blue-shifted optical absorbance and reduced electrical conductivity.

Work function remains nearly constant, indicating it is mainly influenced by M and T_x chemistries.

Abstract

MXenes, a family
of two-dimensional transition metal
carbides,
nitrides, and carbonitrides with the general formula of M
n+1X
n
T
x
(where M represents an early transition metal, X is C and/or
N, and T
x
is the surface functional groups),
offer exceptional tailorability in structure, composition, and surface
chemistry. Among them, nitrogen-containing MXenes have enhanced electronic
and optical properties compared to their carbon analogs. Yet, challenges
in synthesizing them have made nitride and carbonitride MXenes the
least explored class. Herein, we report the synthesis of Ti3Al­(C2–y
N
y
) MAX phases using a high-aluminum method to minimize oxygen
impurities as well as other competing binary and ternary phases. Therein,
subsequent etching and delamination of Ti3Al­(C2–y
N
y
) MAX phases into Ti3(C2–y
N
y
)­T
x
MXenes were done using a coupled
HF/HCl/LiCl method. Systematic variation of X-site chemistry (Ti3C2T
x
, Ti3C1.75N0.25T
x
, Ti3C1.5N0.5T
x
, Ti3C1.25N0.75T
x,
and Ti3CNT
x
) enabled
direct correlations between chemistry and optoelectronic properties.
Increased nitrogen content leads to increased preference for halogen
terminations, stronger light-matter interaction, blue-shifted optical
absorbance, and decreased electrical conductivity. Despite these variations,
the work function remains nearly constant across all compositions,
indicating that it is primarily dictated by M and T
x
chemistries. These findings demonstrate that solid-solution
carbonitride MXenes provide a platform to independently control optical
and electronic behaviors, offering opportunities for MXene-based optoelectronic
and energy applications.

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