Thermoelectric Modulation of Neat Ti3C2Tx MXenes by Finely Regulating the Stacking of Nanosheets

Abstract

Emerging two-dimensional MXenes have been extensively studied in a wide range of fields thanks to their superior electrical and hydrophilic attributes as well as excellent chemical stability and mechanical flexibility. Among them, the ultrahigh electrical conductivity (σ) and tunable band structures of benchmark Ti₃C₂T_x MXene demonstrate its good potential as thermoelectric (TE) materials. However, both the large variation of σ reported in the literature and the intrinsically low Seebeck coefficient (S) hinder the practical applications. Herein, this study has for the first time systematically investigated the TE properties of neat Ti₃C₂T_x films, which are finely modulated by exploiting different dispersing solvents, controlling nanosheet sizes and constructing composites. First, deionized water is found to be superior for obtaining closely packed MXene sheets relative to other polar solvents. Second, a simultaneous increase in both S and σ is realized via elevating centrifugal speed on MXene aqueous suspensions to obtain small-sized nanosheets, thus yielding an ultrahigh power factor up to ~ 156 μW m^-1 K^-2. Third, S is significantly enhanced yet accompanied by a reduction in σ when constructing MXene-based nanocomposites, the latter of which is originated from the damage to the intimate stackings of MXene nanosheets. Together, a correlation between the TE properties of neat Ti₃C₂T_x films and the stacking of nanosheets is elucidated, which would stimulate further exploration of MXene TEs.

Keywords: Electrical conductivity; MXene; Nanosheet stacking; Seebeck coefficient; Thermoelectrics.

Fig. 1

Synthesis and characterization of Ti₃C₂T_x MXene films. a Synthetic procedures of single-layered (SL)-Ti₃C₂T_x MXene from Ti₃AlC₂ MAX powders. Characterization of Ti₃AlC₂ powders, ML-Ti₃C₂T_x powders, and SL-Ti₃C₂T_x thin film: b–d FE-SEM images, e HR-TEM image, f XRD patterns, and g TP-AFM image. h Temperature-dependent thermoelectric properties of ML- and SL-Ti₃C₂T_x thin films during the heating process.

Fig. 2

Effects of dispersing solvents on Ti₃C₂T_x MXene films. a Thermoelectric properties of Ti₃C₂T_x thin films obtained by processing with different dispersing solvents of H₂O, DMF and EtOH. b GIWAXS profiles of ML- and SL-Ti₃C₂T_x thin films acquired by H₂O-based suspensions as well as Ti₃C₂T_x thin films obtained by DMF- and EtOH-based suspensions. TGA results of Ti₃C₂T_x films obtained from suspensions with c H₂O, d DMF and e EtOH as dispersing agents, respectively. XPS patterns of f Ti 2p and g O 1s for Ti₃C₂T_x thin films processed with various dispersing solvents.

Fig. 3

Effects of nanosheet size on Ti₃C₂T_x MXene films. a Distribution of nanosheet sizes of Ti₃C₂T_x suspensions obtained at different centrifugal speeds. b TE performance of σ, S and PF of single-layered Ti₃C₂T_x thin films processed at various centrifugal speeds. c Cross-sectional SEM images of Ti₃C₂T_x thin films with various nanosheet sizes. d UPS profiles of Ti₃C₂T_x thin films processed at a centrifugal speed of 10,000 and 6000 rpm, respectively. e Determination of carrier concentration and Hall mobility of Ti₃C₂T_x thin films.

Fig. 4

Thermoelectric properties of a Seebeck coefficient (S), b electrical conductivity (σ) and c power factor (PF) of single-layered Ti₃C₂T_x thin films obtained at different centrifugal speeds and its composites with NaDC and SWCNTs. Cross-sectional SEM images of d Ti₃C₂T_x/NaDC and e Ti₃C₂T_x/SWCNTs composites. f Determination of carrier concentration and Hall mobility of neat Ti₃C₂T_x thin film centrifuged at 10,000 rpm and its composite films with NaDC and SWCNTs, respectively.