Band Engineering versus Catalysis: Enhancing the Self-Propulsion of Light-Powered MXene-Derived Metal-TiO2 Micromotors To Degrade Polymer Chains

Abstract

Light-powered micro- and nanomotors based on photocatalytic semiconductors convert light into mechanical energy, allowing self-propulsion and various functions. Despite recent progress, the ongoing quest to enhance their speed remains crucial, as it holds the potential for further accelerating mass transfer-limited chemical reactions and physical processes. This study focuses on multilayered MXene-derived metal-TiO₂ micromotors with different metal materials to investigate the impact of electronic properties of the metal-semiconductor junction, such as energy band bending and built-in electric field, on self-propulsion. By asymmetrically depositing Au or Ag layers on thermally annealed Ti₃C₂T_x MXene microparticles using sputtering, Janus structures are formed with Schottky junctions at the metal-semiconductor interface. Under UV light irradiation, Au-TiO₂ micromotors show higher self-propulsion velocities due to the stronger built-in electric field, enabling efficient photogenerated charge carrier separation within the semiconductor and higher hole accumulation beneath the Au layer. On the contrary, in 0.1 wt % H₂O₂, Ag-TiO₂ micromotors reach higher velocities both in the presence and absence of UV light irradiation, owing to the superior catalytic properties of Ag in H₂O₂ decomposition. Due to the widespread use of plastics and polymers, and the consequent occurrence of nano/microplastics and polymeric waste in water, Au-TiO₂ micromotors were applied in water remediation to break down polyethylene glycol (PEG) chains, which were used as a model for polymeric pollutants in water. These findings reveal the interplay between electronic properties and catalytic activity in metal-semiconductor junctions, offering insights into the future design of powerful light-driven micro- and nanomotors with promising implications for water treatment and photocatalysis applications.

Keywords: MXenes; Schottky junctions; microrobots; photocatalysis; plastics; polymers; titanium dioxide; water purification.

Scheme 1. Light-Powered Metal–Semiconductor Micromotors Are Designed by Asymmetrically Depositing Au or Ag Layers on the Surface of Ti₃C₂T_x MXene-Derived TiO₂ Microparticles

These micromotors, characterized by different metal–TiO₂ interfaces, allow the investigation of the interplay between the effects related to the built-in electric field of the Schottky junction and the metal catalytic properties in obtaining higher self-propulsion velocities in pure water and H₂O₂ solutions. E_F, Fermi level; E_C and E_V, TiO₂ conduction and valence band energy levels; hν, photon energy; Φ_B, Schottky barrier; E_bi, built-in electric field; e^–, electron; and h⁺, hole.

Figure 1

Modeling energy band bending and built-in electric field in metal–TiO₂ junctions with different types of metal materials. Scheme of the energy band diagram of the metal–TiO₂ Schottky junction, simulated TiO₂ conduction band minimum (CBM) energy as a function of depth and distance from the metal nanoparticle’s center, and simulated electric field at the TiO₂ surface under the metal nanoparticle for (a) Au–TiO₂ and (b) Ag–TiO₂ junctions formed by a metal nanoparticle (20 nm in diameter) on the surface of a TiO₂ microparticle. Φ_m and Φ_s, work functions of the metal and TiO₂; χ, TiO₂ electron affinity; E₀, vacuum energy level; E_F, Fermi level; E_C and E_V, TiO₂ conduction and valence band energy levels; E_g, TiO₂ energy bandgap; hν, photon energy; Φ_B, Schottky barrier; V_bi, built-in potential, E_bi, built-in electric field; e^–, electron; and h⁺, hole.

Figure 2

Fabrication and characterization of MXene-derived metal–TiO₂ micromotors. (a) Scheme of the fabrication steps. (b) SEM image of an MXene-derived TiO₂ microparticle. (c) Raman spectra of Ti₃C₂T_x MXene and MXene-derived TiO₂ microparticles. (d) Tauc plot for bandgap energy (E_g) determination of MXene-derived TiO₂ microparticles. (e) EDX elemental mapping images for Ti, O, Au, and Ag in MXene-derived metal–TiO₂ micromotors. Scale bars are 2 μm.

Figure 3

Motion behavior of MXene-derived metal–TiO₂ micromotors in pure water. (a) Time-lapse micrographs showing the trajectories of an Au–TiO₂ micromotor and an Ag–TiO₂ micromotor for successive on–off switching of UV light irradiation at time intervals of approximately 10 s in pure water, and the corresponding instantaneous velocities as a function of time. Scale bars are 5 μm. (b) Mean squared displacement (MSD) data fitting of several metal–TiO₂ micromotors in the absence (dark) and presence (light) of UV light irradiation in pure water. MSD data were fitted based on eqs 3 and 4. Error bars are not shown for clarity. (c) Average diffusion coefficients and velocities obtained from MSD data fitting. Error bars represent the standard deviation. (d) Scheme of Au–TiO₂ micromotor motion mechanisms in the absence and presence of UV light irradiation in pure water.

Figure 4

Motion behavior of MXene-derived metal–TiO₂ micromotors in a low concentration of 0.1 wt % H₂O₂. (a) Time-lapse micrographs showing the trajectories of an Au–TiO₂ micromotor and an Ag–TiO₂ micromotor for successive on–off switching of UV light irradiation at time intervals of approximately 10 s in 0.1 wt % H₂O₂, and the corresponding instantaneous velocities as a function of time. Scale bars are 5 μm. (b) Mean squared displacement (MSD) data fitting of several metal–TiO₂ micromotors in the absence (dark) and presence (light) of UV light irradiation in 0.1 wt % H₂O₂. MSD data were fitted based on eqs 3 and 4. Error bars are not shown for clarity. (c) Average diffusion coefficients and velocities obtained from MSD data fitting. Error bars represent the standard deviation. (d) Scheme of Ag–TiO₂ micromotors motion mechanisms in the absence and presence of UV light irradiation in 0.1 wt % H₂O₂.

Figure 5

Polymer degradation experiments.(a) From top to bottom: ESI-MS spectra of untreated PEG, PEG treated with UV-light irradiation in pure water for 8 h, and PEG treated with Au–TiO₂ micromotors under UV light irradiation in pure water for 8 h. (b) From top to bottom: ESI-MS spectra of untreated PEG, PEG treated with UV-light irradiation in 0.1 wt % H₂O₂ for 8 h, and PEG treated with Au–TiO₂ micromotors under UV light irradiation in 0.1 wt % H₂O₂ for 8 h.