Spectroscopic signature of negative electronic compressibility from the Ti core-level of titanium carbonitride MXene

Abstract

Two-dimensional transition metal carbides, carbonitrides, and nitrides, called MXenes, exhibit high metallic conductivity, ion intercalation capability, and reversible redox activity, prompting their applications in energy storage and conversion, electromagnetic interference (EMI) shielding, and electronics, among many other fields. It has been shown that replacement of ∼50% of carbon atoms in the most popular MXene family member, titanium carbide (Ti3C2Tx), by nitrogen atoms, forming titanium carbonitride (Ti3CNTx), leads to drastically different properties. Such properties include very high negative charge in solution and extreme EMI shielding effectiveness, exceeding all known materials, even metals at comparable thicknesses. Here, by using ultraviolet photoemission spectroscopy (UPS), the electronic structures of Ti3CNTx and Ti3C2Tx are systematically investigated and compared as a function of charge carrier density. We observe that, in contrast to Ti3C2Tx, the Ti 3p core-level of Ti3CNTx exhibits a counterintuitive shift to a lower binding energy of up to ∼250 meV upon increasing the electron density, which is a spectroscopic signature of negative electronic compressibility (NEC). These experimentally measured chemical potential shifts are well captured by the density functional theory (DFT) calculation. The DFT results also further suggest that the hybridization of titanium–nitrogen bonding in Ti3CNTx helps to promote the available states of Ti atoms for receiving more electrons above the fermi level and leads to the observed NEC. Our findings explain the differences in electronic properties between the two very important and widely studied MXenes and also suggest a new strategy to apply the NEC effect of Ti3CNTx in energy and charge storage applications.

2D non-carbide MXenes: an emerging material class for energy storage and conversion

“…Carbonitride MXenes prepared through this method have been used in sensing, energy storage, and biomedical applications. [56][57][58][59] 3.1.3 KF-HCl etching method. Liang and co-workers reported the synthesis of Ti 2 C 0.5 N 0.5 T x MXene from its MAX phase Ti 2 AlC 0.5 N 0.5 using the KF-HCl (in situ HF) etching approach similar to that of the LiF-HCl mild etching method.…”

“…The carbonitride MXene Ti 3 CN shows remarkable EMI shielding effectiveness (SE), superb charge storage capacity, high negative charge in solution, excellent stability, and good optical and transport properties. 58,60 The exact role of nitrogen atoms and their inuence on the properties of the carbonitride MXene are yet to be explored and understood. However, a few reports have elucidated the origin of properties of carbonitride MXenes through experimental and theoretical studies.…”

Intercalation and surface modification of two-dimensional transition metal carbonitride Ti3CNTx for ultrafast supercapacitors

“…Recent experimental studies have revealed some unique properties of Ti 3 CNT x , such as a higher EMI shielding effect, superior charge storage capacity, 13,14 and more negative surface charge than Ti 3 C 2 T x MXenes. 15 In addition, it is speculated that the additional Ti-N bond in Ti 3 CNT x has a signicant effect on the charge storage capacity. 15 Therefore, Ti 3 CNT x MXene is an excellent candidate for the electrode material of supercapacitors, and it is a signicant study to construct twodimensional capacitive materials with Ti 3 CNT x MXenes.…”

“…15 In addition, it is speculated that the additional Ti-N bond in Ti 3 CNT x has a signicant effect on the charge storage capacity. 15 Therefore, Ti 3 CNT x MXene is an excellent candidate for the electrode material of supercapacitors, and it is a signicant study to construct twodimensional capacitive materials with Ti 3 CNT x MXenes. Ti 3 CNT x nanosheets also suffer from severe restacking like other MXenes, thereby reducing the accessibility of 2D nanochannels.…”

MXene/Carbon Nanotube Hybrids: Synthesis, Structures, Properties, and Applications

“…[6] The general chemical formular for MXenes is M n + 1 X n T x (n = 1-4). Specifically, M stands for early transition metals (including Ti, [7] V, [8] Nb, [9] Mo, [10] etc. ); X represents C and/or N; T x refers to surface terminating groups (e. g., =O, À OH, and À F).…”