2D MXenes polar catalysts for multi-renewable energy harvesting applications

The synchronous harvesting and conversion of multiple renewable energy sources for chemical fuel production and environmental remediation in a single system is a holy grail in sustainable energy technologies. However, it is challenging to develop advanced energy harvesters that satisfy different working mechanisms. Here, we theoretically and experimentally disclose the use of MXene materials as versatile catalysts for multi-energy utilization. Ti3C2TX MXene shows remarkable catalytic performance for organic pollutant decomposition and H2 production. It outperforms most reported catalysts under the stimulation of light, thermal, and mechanical energy. Moreover, the synergistic effects of piezo-thermal and piezo-photothermal catalysis further improve the performance when using Ti3C2TX. A mechanistic study reveals that hydroxyl and superoxide radicals are produced on the Ti3C2TX under diverse energy stimulation. Furthermore, similar multi-functionality is realized in Ti2CTX, V2CTX, and Nb2CTX MXene materials. This work is anticipated to open a new avenue for multisource renewable energy harvesting using MXene materials.


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at 403 K for 12 hours. The product was washed with deionized water three times, and then dried at 333 K.

Theoretical calculation.
The calculation is carried out within the framework of density functional theory of VASP. 1 Perdew-Burke-Ernzerhof (PBE) is used to exchange correlation functional, and generalized gradient approximation (GGA) is used to deal with the exchange and correlation effects self-consistently. 2 The electron wave function is expanded in the plane wave basis group with the energy cut-off of 600 eV. For structural optimizations and static calculations, the convergences for total energy and force are set as 1 × 10 -6 eV and 0.01 eV/Å respectively. Brillouin zone is sampled on a 21 × 21 ×1 grid in the gamma center scheme. In order to prevent periodic interaction along the Z direction, a vacuum space of 20 Å is set between adjacent layers. In addition, the DFT-D3 method 3 is adopted to correct the VDW interactions. Since functional groups and strains broke the inversion symmetry of the initial structures of MXene, we call off the symmetry. The adsorption energy and Barder charge for O 2 adsorption is performed according to previous report. 4 In details, the models for monolayer and bilayer Ti 3 C 2 were built by 3×3 supercells with a 20 Å vacuum layer along the z-axis.
The adsorption energy E ads was calculated using the following formula = − substrate − 2 where , substrate and 2 stand for the total energy of the O 2 adsorbed to Ti 3 C 2 slabs, Ti 3 C 2 slabs and O 2 molecule.

Characterization.
PFM is based on the atomic force microscopy, with an AC drive voltage applied to the conductive tip. Conductive Pt/Ir-coated silicon probes (EFM, Nanoworld) were used with a nominal spring constant of ~2.8 nN/nm and a free-air resonance frequency of ~75 kHz. The typical drive frequency was in the range of 320 to 380 kHz, depending S4 on the contact resonant frequency. The Fourier transform infrared spectroscopy (FTIR) data were collected using a NICOLET IS10 (Thermo Fisher) spectrophotometer at a resolution of 4cm -1 . ζ-potential measurements were recorded at room temperature on a dynamic light scattering analyzer (BI-200SM). Inductively coupled plasma optical emission spectrometry (ICP-OES) was performed on Ultima 2 to determine the leaching of Al in the solution.    Fig. 19 Adsorption property of the Ti 3 C 2 T X . a Zeta potential of Ti 3 C 2 T X . b Remaining concentration of dye solution after 1 hour pre-adsorption in the presence of Ti 3 C 2 T X . c The adsorption curves for MB over the Ti 3 C 2 T X with or without stirring. All the data in (a-c) were collected for three times, and the error bars represent the standard deviation.   Supplementary Fig. 23 Catalytic degradation of dyes over the Ti 3 C 2 T X . a Piezo-thermal catalytic degradation of methyl orange. c The kinetic rate constant curves and e the corresponding rate constants. b Piezo-thermal catalytic degradation of rhodamine B. d The kinetic rate constant curves and f the corresponding rate constants. All the data in (a, b, e, f) were collected for three times, and the error bars represent the standard deviation.

Supplementary Note 3:
The piezo-thermal catalytic reactions for MO and RhB dyes degradation were investigated in Supplementary Fig. 23. It is clearly seen that the reaction rate increased with the increase of reaction temperature.  The kinetic rate constant curves for MB degradation over the Ti 3 C 2 T X film under NIR irradiation (700-1200 nm).

Supplementary Note 4:
The rate constants for MB degradation over the Ti 3 C 2 T X film without and with stirring are determined to be 0.051 and 0.097 min -1 , respectively.  Fluorescence spectra of TA solution over Ti 3 C 2 T X without stirring in the dark at 338 K. b The absorbance of NBT solution over Ti 3 C 2 T X without stirring in the dark at 338 K.

Supplementary Note 5:
Before reaction, TA is excited under 315 nm to produce an emission peak at ca. 350 nm. Therefore, the baseline at 0 minute is not flat (Supplementary Fig. 34a). After a period of reaction, TA reacts with •OH to produce hydroxyl terephthalic acid (HTA) with a PL peak at ca. 420 nm. When the peak intensity of the HTA is weak, the TA signal is noticeable. The interaction between the two PL peaks leads to the asymmetry of the HTA peak at 420 nm. However, when the intensity of the HTA is very strong (Supplementary Fig. 32a), the influence of the TA peak at 350 nm will be greatly weakened, thus resulting in the symmetry of the HTA peak.    Supplementary Fig. 44 XRD patterns of the V 2 CT X , Nb 2 CT X and Ti 2 CT X .

Supplementary Note 7:
These samples were prepared using the same procedure as Ti 3 C 2 T X except that the Ti 3 AlC 2 was replaced by V 2 AlC, Nb 2 AlC or Ti 2 AlC.  Table 1. Calculated dipole moments of Ti 3 C 2 and Ti 3 C 2 T X monolayers.