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Two-dimensional (2D) transition metal carbides, nitrides and carbonitrides, known as MXenes, are of interest as electrocatalysts. Tungsten-based MXenes are predicted to have low overpotentials in the hydrogen evolution reaction but their synthesis has proven difficult due to the calculated instability of their hypothetical MAX precursors. In this study, we present a theory-guided synthesis of a tungsten-based MXene, W2TiC2Tx, derived from a non-MAX nanolaminated ternary carbide (W,Ti)4C4−y precursor by the selective etching of one of the covalently bonded tungsten layers. Our results indicate the importance of tungsten and titanium ordering, the presence of vacancy defects in the metal layers, and the lack of oxygen impurities in the carbon layers for the successful selective etching of the precursor. We confirm the atomistic out-of-plane ordering of tungsten and titanium using computational and experimental characterizations. The tungsten-rich basal plane endows W2TiC2Tx MXene with a high electrocatalytic hydrogen evolution reaction performance (∼144 mV overpotential at 10 mA cm−2). This study reports a tungsten-based MXene synthesized from a covalently bonded non-MAX precursor, adding to the synthetic strategies for 2D materials.
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Theory-guided synthesis of a 2D tungsten titanium MXene from a covalently bonded layered carbide for electrocatalysis
Two-dimensional transition metal carbides, nitrides, and carbonitrides, known as MXenes, hold potential in electrocatalytic applications. Tungsten (W) based-MXenes are of particular interest as they are predicted to have low overpotentials in hydrogen evolution reaction (HER). However, incorporating W into the MXene structure has proven difficult due to the calculated instability of its hypothetical MAX precursors. In this study, we present a theory-guided synthesis of a W-containing MXene, W2TiC2Tx, derived from a non-MAX nanolaminated ternary carbide (W,Ti)4C4-y precursor by selective etching of one of the covalently bonded tungsten layers. Our results indicate the importance of W and Ti ordering and the presence of vacancy defects for the successful selective etching of the precursor. We confirm the atomistic out-of-plane ordering of W and Ti using density functional theory, Rietveld refinement, and electron microscopy methods. Additionally, the W-rich basal plane endows W2TiC2Tx MXene with a remarkable HER overpotential (~144 mV at 10 mA/cm2). This study adds a tungsten-containing MXene made from a covalently bonded non-MAX phase opening more ways to synthesize novel 2D materials.
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- Award ID(s):
- 2419026
- PAR ID:
- 10540254
- Publisher / Repository:
- ChemRvix
- Date Published:
- Format(s):
- Medium: X
- Institution:
- Purdue University
- Sponsoring Org:
- National Science Foundation
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Abstract MXenes are a rapidly growing family of 2D transition metal carbides and nitrides that are promising for various applications, including energy storage and conversion, electronics, and healthcare. Hydrofluoric‐acid‐based etchants are typically used for large‐scale and high‐throughput synthesis of MXenes, which also leads to a mixture of surface terminations that impede MXene properties. Herein, a computational thermodynamic model with experimental validation is presented to explore the feasibility of fluorine‐free synthesis of MXenes with uniform surface terminations by dry selective extraction (DSE) from precursor MAX phases using iodine vapors. A range of MXenes and respective precursor compositions are systematically screened using first‐principles calculations to find candidates with high phase stability and low etching energy. A thermodynamic model based on the “CALculation of PHAse Diagrams” (CALPHAD) approach is further demonstrated, using Ti3C2I2as an example, to assess the Gibbs free energy of the DSE reaction and the state of the byproducts as a function of temperature and pressure. Based on the assessment, the optimal synthesis temperature and vapor pressure are predicted and further verified by experiments. This work opens an avenue for scalable, fluorine‐free dry synthesis of MXenes with compositions and surface chemistries that are not accessible using wet chemical etching.more » « less
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Abstract To advance the MXene field, it is crucial to optimize each step of the synthesis process and create a detailed, systematic guide for synthesizing high‐quality MXene that can be consistently reproduced. In this study, a detailed guide is provided for an optimized synthesis of titanium carbide (Ti3C2Tx) MXene using a mixture of hydrofluoric and hydrochloric acids for the selective etching of the stoichimetric‐Ti3AlC2MAX phase and delamination of the etched multilayered Ti3C2TxMXene using lithium chloride at 65 °C for 1 h with argon bubbling. The effect of different synthesis variables is investigated, including the stoichiometry of the mixed powders to synthesize Ti3AlC2, pre‐etch impurity removal conditions, selective etching, storage, and drying of MXene multilayer powder, and the subsequent delamination conditions. The synthesis yield and the MXene film electrical conductivity are used as the two parameters to evaluate the MXene quality. Also the MXenes are characterized with scanning electron microscopy, x‐ray diffraction, atomic force microscopy, and ellipsometry. The Ti3C2Txfilm made via the optimized method shows electrical conductivity as high as ≈21,000 S/cm with a synthesis yield of up to 38 %. A detailed protocol is also provided for the Ti3C2TxMXene synthesis as the supporting information for this study.more » « less
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Abstract 2D early transition metal carbide and nitride MXenes have intriguing properties for electrochemical energy storage and electrocatalysis. These properties can be manipulated by modifying the basal plane chemistry. Here, mixed transition metal nitride MXenes, M‐Ti4N3Tx(M = V, Cr, Mo, or Mn; Tx= O and/or OH), are developed by modifying pristine exfoliated Ti4N3TxMXene with V, Cr, Mo, and Mn salts using a simple solution‐based method. The resulting mixed transition metal nitride MXenes contain 6–51% metal loading (cf. Ti) that exhibit rich electrochemistry including highly tunable hydrogen evolution reaction (HER) electrocatalytic activity in a 0.5mH2SO4electrolyte as follows: V‐Ti4N3Tx> Cr‐Ti4N3Tx> Mo‐Ti4N3Tx> Mn‐Ti4N3Tx> pristine Ti4N3Txwith overpotentials as low as 330 mV at −10 mA cm−2with a charge‐transfer resistance of 70 Ω. Scanning electrochemical microscopy (SECM) reveals the electrochemical activity of individual MXene flakes. The SECM data corroborate the bulk HER activity trend for M‐Ti4N3Txas well as provide the first experimental evidence that HER results from catalysis on the MXene basal plane. These electrocatalytic results demonstrate a new pathway to tune the electrochemical properties of MXenes for water splitting and related electrochemical applications.more » « less
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null (Ed.)Quaternary MAX phases, (Ta 1−x Ti x ) 3 AlC 2 ( x = 0.4, 0.62, 0.75, 0.91 or 0.95), have been synthesised via pressureless sintering of TaC, TiC, Ti and Al powders. Via chemical etching of the Al layers, (Ta 0.38 Ti 0.62 ) 3 C 2 T z – a new MXene, has also been synthesised. All materials contain an M-layer solid solution of Ta and Ti, with a variable Ta concentration, paving the way for the synthesis of a range of alloyed (Ta,Ti) 3 C 2 T z MXenes with tuneable compositions for a wide range of potential applications.more » « less
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Accepted Manuscript1.0
- Posted Content:
- https://doi.org/10.26434/chemrxiv-2024-dprbn
