Monitoring Ti3C2Tx MXene Degradation Pathways Using Raman Spectroscopy

Extending applications of Ti3C2Tx MXene in nanocomposites and across fields of electronics, energy storage, energy conversion, and sensor technologies necessitates simple and efficient analytical methods. Raman spectroscopy is a critical tool for assessing MXene composites; however, high laser powers and temperatures can lead to the materials’ deterioration during the analysis. Therefore, an in-depth understanding of MXene photothermal degradation and changes in its oxidation state is required, but no systematic studies have been reported. The primary aim of this study was to investigate the degradation of the MXene lattice through Raman spectroscopic analysis. Distinct spectral markers were related to structural alterations within the Ti3C2Tx material after subjecting it to thermal- and laser-induced degradation. During the degradation processes, spectral markers were revealed for several specific steps: a decrease in the number of interlayer water molecules, a decrease in the number of −OH groups, formation of C–C bonds, oxidation of the lattice, and formation of TiO2 nanoparticles (first anatase, followed by rutile). By tracking of position shifts and intensity changes for Ti3C2Tx, the spectral markers that signify the initiation of each step were found. This spectroscopic approach enhances our understanding of the degradation pathways of MXene, and facilitating enhanced and dependable integration of these materials into devices for diverse applications, from energy storage to sensors.

To study the stability of MXene, individual MXene films were stored for one week in a gas cuvette (1) with oxygen or (2) with nitrogen.The MXene film samples were then characterized by measuring Vis-NIR extinction and Raman spectra.The extinction spectra of the samples stored in different gas environments were compared with the extinction spectrum of MXene film annealed at 500 ºC.
For a comprehensive study of the quality of MXene flakes, MXene films were kept in an oxygen or nitrogen environment and investigated by Vis-NIR spectroscopy.The observed spectral differences between fresh and oxygenated films are thought to arise from changes in the surface groups with more Ti 3 C 2 O 2 and TiO 2 for the sample stored in an oxygen environment.The nitrogen environment was expected to slow down oxidation and lattice deterioration.In the spectrum of the oxygenated MXene film, a shift of the extinction band from ≈750 nm to ≈775 nm and an apparent decrease in optical density in the entire spectral range are observed.Recently, the increased conductivity due to the removal of MXene surface groups was reported. 1The lower extinction of the oxidized MXene film may indicate more of =O surface groups.The film annealed at 500 °C (heated) reveals no extinction band since the organic material was incinerated, and only the TiO 2 remained on the sample, as evidenced by Raman spectroscopy performed later.
Vis-NIR extinction spectroscopy allows distinguishing between multilayered MXene and single-layered MXene flakes.In addition, the transmittance of MXene increased by 59% (from 11% to 27%) at 750 nm and by 72% (from 11% to 27%) at 2200 nm during oxidation.This suggests that Vis-NIR absorption spectroscopy seems to be a convenient and efficient method for determining the quality of the synthesized MXene.Raman spectroscopy can supply comprehensive information on the evolution of the MXene lattice structure.MXene films aged in oxygen and nitrogen environments differ from each other.It is assumed that the number of terminal groups that become oxygenated in the film aged in an oxygen medium increases.The previously determined spectral changes in the MXene lattice can be adapted to MXene film aging in this case.Based on previously observed changes, one can see the recurring differences between the fresh MXene sample and MXene kept in different gas environments (Figure S3).A decrease in oxidation rate is observed for MXene kept in nitrogen.In contrast, the main differences are observed for the oxygenated MXene.Major markers include the ratios I ω2 /I ω4(O) and I ω4(O) /I ω4(OH) , which are 1.09 and 0.89 for the fresh sample, respectively.These ratios spike to 1.27 and 1 for the oxygenated sample.For sample kept in nitrogen environment these ratios are I ω2 /I ω4(O) -1.16 and I ω4(O) /I ω4(OH) -0.9.
It is worth mentioning that the blueshift was observed for the band at 372 cm -1 and the redshift at 620 cm -1 .3][4][5] We can conclude that the formation of TiO 2 anatase has not yet advanced in this sample, so the blueshift at 372 cm -1 is still apparent.Furthermore, the terminal groups have been changed to =O.This can be traced from the I ω2 /I ω4(O) and I ω4(O) /I ω4(OH) ratios.When comparing the Raman spectra of single-layered MXene collected using different excitation wavelengths, significant changes were observed primarily under pre-resonance conditions (785 nm laser, as evident from Vis-NIR spectra).Additionally, pre-resonance conditions were observed for 457 nm (as evidenced by the increase in absorption towards the UV range in Vis-NIR spectra).These observations were reflected in the Raman spectrum, showing an increased resonance band at 120 cm -1 and shifts of the complex 370 and 620 cm -1 bands, which served as oxidation markers, as in our paper and the study by Berger et al. 6 Interestingly, the increased intensity in the 200 cm -1 band is observed for 532 nm excitation wavelength together with the redshift of the 620 cm -1 band.The minor differences between spectra can be attributed to variations in the MXene absorption capacity for other excitation wavelengths and differences in excitation power.
Excitation with a 785 nm laser provides photon energy close to the resonance Raman condition.The resonance Raman condition for MXene samples in our study was excitation with a 750 nm wavelength (Fig. 2).The pre-resonance excitation yields Raman spectra with a high intensity of several resonant spectral bands (Figure S4).These bands include 122 cm -1 (associated with the in-plane ω 1 vibrational mode of all atomic groups), 513 cm -1 (associated with the ω 6 mode of Ti 3 C 2 (OH) 2 ) and 722 cm -1 (associated mainly with the ω 3 mode of Ti 3 C 2 O 2 ) band.The complex band at approx.372 cm -1 changes shape due to increased intensity in the spectral range at approx.366 cm -1 and decreased intensity of the bands in the 380-460 cm -1 spectral range (compare with Figure S4 A and Figure S4 B).Overall, the most significant changes in intensity are observed in the spectral region for the surface groups.Similar spectral changes are observed for multi-layered MXene with pre-resonant (785 nm) and nonresonant (633 nm) excitations.The intensity of the resonant bands at 123 cm -1 , 512 cm -1 , and 737 cm -1 increases prominently.However, the spectral range of the surface groups undergoes less enhancement (fewer surface groups overall), while the ω 2 mode undergoes relatively strong enhancement compared to single-layered MXene flakes.Obtaining the Raman spectrum with infrared excitation (1064 nm) leads to strong absorption and heating of the sample.This behavior of MXene is of particular interest.Non-linear absorption was observed in Ti 3 C 2 T x MXene for this wavelength leading to saturable and enhanced optical absorbance.[10]    Similar changes in lattice deterioration are observed with 457 nm excitation.The anatase phase of TiO 2 appears from 310 -390 kW/cm 2 .A blueshift is observed for the complex band at approx.380 cm -1 as a sign of MXene lattice oxidation, but it remains until the significant formation of TiO 2 is observable.The rutile phase becomes more prominent from 2.4 MW/cm 2 .The evolution from the anatase phase to rutile is presented in Fig. S9, while the laser power density increases from 1.5 to 4.9 MW/cm 2 .

Figure S2 .
Figure S2.Vis-NIR extinction spectra of fresh MXene flakes (black line) and MXene flakes stored in cuvettes for one week with different gases: nitrogen (yellow line), oxygen (blue line), or annealed at 500°C (red line).

Figure S3 .
Figure S3.Raman spectrum of Ti 3 C 2 T x MXene in different environments.Fresh sample (A), after two weeks in nitrogen (B), and after two weeks in oxygen (C) (spectra are offset for clarity).The excitation wavelength was 633 nm.

Figure S6 .
Figure S6.Raman spectrum of deteriorating Ti 3 C 2 T x MXene due to 633 nm laser illumination.The laser power density was varied from 30 to 580 kW/cm 2 .The spectra are normalized to the spectral band at approximately 200 cm -1 .

Figure S7 .
Figure S7.Raman spectrum of deteriorating Ti 3 C 2 T x MXene due to 457 nm laser illumination.The laser power density was varied from 240 kW/cm 2 to 1.9 MW/cm 2 .The laser power density applied to the sample increases from bottom to top.

Figure S8 .
Figure S8.Raman spectrum of deteriorating Ti 3 C 2 T x MXene due to 457 nm laser illumination.The laser power density was varied from 1.5 to 4.9 MW/cm 2 .The laser power density applied to the sample increases from bottom to top.

Figure S9 .
Figure S9.Raman spectrum of deteriorating Ti 3 C 2 T x MXene due to 532 nm laser illumination.The laser power density was varied from 26 kW/cm 2 to 2 MW/cm 2 .The laser power density applied to the sample increases from bottom to top.

Figure S10 .
Figure S10.Raman spectrum of deteriorating Ti 3 C 2 T x MXene due to 532 nm laser illumination.The laser power density was varied from 2 MW/cm 2 to 26 MW/cm 2 .The laser power density applied to the sample increases from bottom to top.