Key Takeaways & Executive Findings
- •• Deionized water is identified as the optimal dispersing solvent for achieving tightly packed and highly oriented Ti3C2Tx nanosheets, enhancing film quality. • Increasing centrifugal speed of MXene aqueous suspensions simultaneously boosts both Seebeck coefficient and electrical conductivity, yielding an ultrahigh power factor of ~156 μW m−1 K−2. • Constructing MXene-based nanocomposites significantly enhances the Seebeck coefficient but disrupts nanosheet stacking, leading to reduced electrical conductivity. • The study establishes a clear correlation between the thermoelectric performance of neat Ti3C2Tx films and the stacking density of nanosheets, guiding future MXene TE material design.
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 Ti3C2Tx 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 Ti3C2Tx 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 Ti3C2Tx films and the stacking of nanosheets is elucidated, which would stimulate further exploration of MXene TEs.
1. Introduction
Since the initial discovery in 2011 [1], transition metal carbides and nitrides (MXenes), as an emerging class of two-dimensional (2D) materials, have become increasingly hotspots thanks to their superior electrical, optical and hydrophilic attributes as well as excellent chemical stability and mechanical flexibility [2–4]. MXenes are nominally represented by a formula of Mn+1XnTx (n = 1, 2 or 3) where M is an early transition metal (such as Sc, Ti, V, Mo or Nb), X represents carbon or nitrogen, and T denotes surface terminal groups such as −O, −OH and −F [5, 6]. In principle, MXenes were synthesized by selective etching that removed the A element (such as Al, Si, Ge or Sn) from the parent carbide or nitride compounds [7]. Hitherto, over 30 kinds of MXenes have been successfully synthesized, among which Ti3C2Tx MXene remains the best studied, holding great prospects for energy storage [8], electromagnetic interference shielding [9, 10], sensor [11, 12], catalysis [13] and fire warning [14], all benefiting from the diverse merits in physical, chemical and mechanical properties [15]. Electrical conductivity (σ) of Ti3C2Tx MXene films can be readily tuned; for instance, an outstanding σ as high as ~15,000 S cm−1 was realized in MXene films containing highly aligned large Ti3C2Tx flakes by blade-coating, whereas the σ of MXene films obtained by HF etching was only ~1,500 S cm–1 [16, 17]. Moreover, the surface functional groups also affected the σ of Ti3C2Tx sheets, with −O-terminated Ti3C2Tx usually exhibiting higher σ than that of −F or −OH-terminated ones [18].
Thermoelectric (TE) materials, as environmentally friendly materials that can directly convert waste heat into electricity, have received increasing attention in the framework of sustainable deployment [19–21]. The TE performance of materials is evaluated by the dimensionless figure of merit (zT), defined as zT = S2σT/κ whereby S, T and κ are the Seebeck coefficient, temperature and thermal conductivity, respectively. A desirable zT value is ideally attained by a synergy of high σ, high S and low κ, in spite of their intimate couplings [22, 23]. It should be stressed that the S is closely correlated to the electronic band structure of the materials, which is more chemically tailorable in Ti3C2Tx than those massively studied 2D materials—MoS2 [24], SnSe [25] and black phosphorus [26], while presenting excellent σ comparable to graphene [27]. These merits have endowed Ti3C2Tx MXene with a promising candidate for TE applications.
Most reported studies were focused on TE composites with Ti3C2Tx MXene that was exploited as one single component in conjunction with carb...
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Junhui Tang, Renyang Zhu, Ya-Hsin Pai, Yan Zhao, Chen Xu, Ziqi Liang (2024). Thermoelectric Modulation of Neat Ti3C2Tx MXenes by Finely Regulating the Stacking of Nanosheets. Nano-Micro Letters. https://doi.org/10.1007/s40820-024-01594-z
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Frequently Asked Questions
What is the main finding of this study on Ti3C2Tx MXene thermoelectrics?
The study demonstrates that the thermoelectric performance of neat Ti3C2Tx films can be finely tuned by controlling nanosheet stacking. Using deionized water as a dispersing solvent and increasing centrifugal speed to obtain small nanosheets simultaneously enhances both the Seebeck coefficient and electrical conductivity, achieving an ultrahigh power factor of ~156 μW m−1 K−2.
How does the choice of dispersing solvent affect Ti3C2Tx film properties?
Deionized water is found to be superior to other polar solvents for achieving tightly packed and highly oriented MXene nanosheets, which is crucial for optimizing electrical conductivity and thermoelectric performance.
What is the impact of constructing MXene-based nanocomposites on thermoelectric properties?
Constructing nanocomposites significantly enhances the Seebeck coefficient but disrupts the intimate stacking of MXene nanosheets, leading to a reduction in electrical conductivity. This trade-off highlights the importance of maintaining nanosheet stacking for balanced thermoelectric performance.
What is the significance of the power factor achieved in this study?
The achieved power factor of ~156 μW m−1 K−2 is exceptionally high for neat MXene films, indicating that Ti3C2Tx MXenes hold great promise as thermoelectric materials when their nanosheet stacking is finely regulated.
What are the potential applications of this research?
The findings provide a fundamental understanding of how nanosheet stacking influences thermoelectric properties, which can guide the design of high-performance MXene-based thermoelectric materials for waste heat recovery and energy harvesting applications.
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