Addressing Reproducibility of MXene Synthesis Using Electrochemical and Chemical Treatment of Ti₃AlC₂ With...

Electrochemical etching of Ti₃AlC₂ in an aqueous electrolyte consisting of 1 M NH₄Cl and 0.2 M tetramethylammonium hydroxide (TMAOH) is widely cited as a method to produce fluorine-free MXene. However, an alternative method for producing MXene by purely chemical etching of Ti₃AlC₂ with an aqueous TMAOH has also been reported. This in situ XRD study of Ti₃AlC₂ etching revealed the absence of MXene after electrochemical treatment in NH₄Cl/TMAOH electrolyte. However, the formation of MXene was detected after etching with both 0.2 M and 25% (2.8 M) TMAOH solutions using a freshly ground Ti₃AlC₂ precursor. The interlayer distance of "pristine" MXene recorded in TMAOH solution was found to be significantly different (∼17.1Å) compared to that of water-washed and dried MXene (∼15 Å) due to the effects of swelling. Incomplete conversion of the Ti₃AlC₂ into MXene is likely related to the accumulation of reaction products. Reproducibility issues previously reported for chemical etching with TMAOH are likely related to the surface passivation layer, which can be disrupted by grinding the precursor Ti₃AlC₂ powder. Moreover, MXene found in earlier published experiments with electrochemical treatment of Ti₃AlC₂ could possibly form primarily due to a chemical reaction with TMAOH during etching and subsequent "washing" with more concentrated TMAOH.