MXenes dots as photocatalysts

Ruben Ramirez Grau¹, María Cabrero-Antonino¹, Hermenegildo García¹*, Ana Primo¹*¹Instituto de Tecnología Química, Consejo Superior de Investigaciones científicas-Universitat Politècnica de Valencia, Avda. de los Naranjos s/n, 46022, Valencia, Spain

Introduction

MXenes are 2D nanomaterials constituted by alternating one-atom thick layers of an early transition metal of the 3, 4, 5 or 6 groups of the Periodic Table (denoted as “M”) and a carbide, nitride or carbonitride layer (denoted as “X”) and have a general formula of M_n+1X_n.¹ We recently reported that MXene dots (MDs) prepared by liquid-phase laser ablation of the corresponding MAX phase exhibit intrinsic photocatalytic hydrogen generation activity in the absence of any other photoresponsive component.² Decrease of lateral size of MXenes to a few nanometers results in the operation of quantum confinement effects reflected in optical absorption and photoluminescence of MDs, these MDs exhibiting photocatalytic activity. Theoretical DFT calculations for Ti₃C₂ models indicates that the bandgap depends on the diameter of the MDs and on the nature of the surface functional groups. The compositional and structural versatility of MXenes will allow band engineering to align the energy values of the valence and conduction band to the values required to promote a given chemical reaction.

Continuing with this line of research, it is of interest to further explore the photocatalytic activity of these MDs. Specifically, it would be important to determine whether or not MDs promote the photocatalytic CO₂ reduction, the efficiency of the process and the products formed. Herein it is reported that MDs are active in the photocatalytic CO₂ reduction using H₂ as electron and proton donor.

Materials and Methods

3 mg of MAX phase were suspended in 3 ml of MilliQ water and the suspension was submitted to ultrasounds in order to prepare a homogenous dispersion. Then, the system was irradiated for 3 h using the second harmonic of a Q switched Nd:YAG laser (Quantel Brilliant, 532 nm, 5 mJ/pulse, 5 ns fwhm) operating at 1 Hz. Finally, the resultant material was centrifuged for 4 h at 4000 rpm and, then, 1 h more at 13 000 rpm to isolate from the bulk MAX and other phases that sediment in the process the small MXene QD that remains in the supernatant.

Results and Discussion

Four MDs, namely Ti₃C₂, Ti₂C, V₂C and Nb₂C were prepared from the corresponding Al MAX phases by 522 nm laser ablation in aqueous suspension, as previously reported.² After preparation, the four MD samples were characterized by different techniques, including AFM and transmission optical microscopy. Table 1 lists the corresponding average thickness and average particle lateral size values for each sample. The activity of the four MD samples was evaluated for the photocatalytic CO₂ hydrogenation under UV-Vis irradiation at 200 ^oC at 1 bar pressure, using a H₂/CO₂ ratio of 1/3. For the four samples, evolution of CO and methane as the only products was observed in different proportions. Fig. 1 shows the temporal product evolution upon irradiation of each MD, while Table 1 indicates the CO and methane production rates determined from these plots.

Significance

Photocatalytic CO₂ reduction is a clean technology that converts CO₂ into high-value-added products. The interest of the present study is to show that, MXenes by themselves, can exhibit an intrinsic photocatalytic activity that can be modulated depending on the composition. Since MXenes allow a wide range of compositions, including bimetallic solid solutions and a wide range of surface functional groups, there is still much room for improvement in the intrinsic photocatalytic efficiency by adjusting these parameters and also to achieve photocatalytic stability.