Metal oxide single-component light-powered micromotors for photocatalytic degradation of nitroaromatic pollutants

Nitroaromatic compounds, widely used in explosives, pesticides, dyes, and pharmaceuticals, are persistent and poorly biodegradable, leading to groundwater and soil contamination and risks to human health. Traditional purification based on biological oxidation and physical methods (adsorption, nanofiltration) is limited because these methods are either ineffective or only transfer pollutants without destroying them. Advanced oxidation processes (AOPs), especially photocatalysis that generates reactive oxygen species (ROS), have been explored using materials such as TiO2, ZnO, Fe2O3, and various heterostructures. However, conventional photocatalysts suffer from passive diffusion and often require agitation and multicomponent architectures. Light-powered micromotors can convert light into motion, enhancing local mixing and contact with pollutants to overcome mass-transfer limitations. Prior micromotor systems frequently require noble-metal coatings (e.g., Pt, Au) to achieve propulsion, increasing cost and fabrication complexity. This study investigates single-component, noble-metal-free WO3 micromotors that self-propel under asymmetric UV illumination and evaluate their ability to photocatalytically degrade nitroaromatic pollutants (picric acid and 4-nitrophenol) in water, examining motion behavior, diffusion enhancement, degradation efficiency, reusability, and the roles of ROS.

The paper surveys photocatalytic AOPs for degrading organic pollutants and emphasizes the roles of ROS such as hydroxyl and superoxide radicals. Prior works include nanostructured TiO2, ZnO, Fe2O3, and Z-scheme heterostructures (e.g., MoS2/g-C3N4, ZnWO4/NiFe2O4) for dye and pollutant degradation, as well as doped/modified materials (e.g., Cu-doped ZnO). For micromotors, TiO2/Fe3O4/Pt tubular micromotors and ZnO/Pt or hematite/Pt Janus micromotors showed effective pollutant degradation but relied on noble metals to enable propulsion. Other light-driven microrobots (e.g., BiOI-based, MXene-derived, Au–WO3@C Janus, ZnO brush-shaped) demonstrate the promise of light-powered motion for environmental remediation but often use multicomponent designs and/or Pt/Au coatings. This context motivates a simpler, single-component, precious-metal-free micromotor architecture to reduce cost and complexity while retaining active motion and photocatalytic efficacy.

Synthesis of WO3 micromotors: 50 mL deionized water in an 80 mL beaker; dissolve 1 mmol Na2WO4 with magnetic stirring; add 25 mmol glucose; stir to homogeneity. Transfer to an autoclave and heat at 200 °C for 20 h. Cool to room temperature, wash precipitate with water and ethanol, dry overnight, then calcine in air at 550 °C (samples covered with aluminum foil). The resulting green WO3 microspheres were collected for experiments.

Characterization: Morphology by SEM (Tescan MIRA 3 XMU) with EDX mapping (Oxford Instruments). XPS (Kratos Analytical Axis Supra) to assess chemical states; XRD to identify crystalline phase (monoclinic WO3). UV–Vis absorption spectra used to estimate optical bandgap via Tauc plot.

Motion experiments: Prepare a 5 µL aqueous suspension of WO3 micromotors; optionally add H2O2 to final concentrations of 0, 0.1, or 1% (v/v). Record motion with an inverted microscope (Nikon ECLIPSE Ts2R) and camera (Basler acA1920-155uc) at 25 fps. UV illumination at 365 nm (CoolLED PE-100, 1.6 W cm−2) applied with on/off cycles (~5 s) to test light-responsive propulsion. Trajectories and velocities were tracked using NIS Elements software. Mean squared displacement (MSD) was analyzed to determine diffusion coefficients using MSD = 4DAt (Brownian) and MSD = 4DAt + v^2 t^2 (self-propulsion), where DA is the apparent translational diffusion coefficient.

Photodegradation experiments: For PA, mix 2 mg mL−1 micromotors with 50 µM picric acid and 1% H2O2 in UV-transparent cuvettes; irradiate with 356 nm UV (9 W) for up to 120 min in a closed box. For 4-NP, use 150 µM 4-nitrophenol with the same procedure. At various times, centrifuge (3 min) to remove micromotors; measure UV–Vis spectra (Jasco V-750). Use the absorbance at 354 nm (PA) to compute degradation efficiency: (C0 − Ct)/C0 × 100%. Controls included PA/4-NP with UV only, H2O2 only, UV + H2O2, and combinations with WO3 to isolate the contributions of motion and photocatalysis. Radical trapping experiments employed EDTA (10 mg L−1, hole scavenger) and isopropanol (0.25 µL mL−1, •OH scavenger) under UV with 1% H2O2 to probe active species. Reusability was assessed over five consecutive PA degradation cycles.

• WO3 micromotors are monoclinic-phase microspheres (1–2 µm) with rough, hierarchical surfaces; XPS indicates W6+ and surface OH groups; optical bandgap ~2.72 eV.
• Under UV illumination, WO3 micromotors self-propel in pure water (fuel-free) via asymmetric photochemical gradients (self-phoresis). Motion is enhanced by H2O2. Diffusion coefficient D from MSD: 2.0 ± 0.1 µm^2 s−1 (pure water, UV) and 20 ± 1 µm^2 s−1 (1% H2O2, UV), a ~10× increase. Average speed increases with H2O2 from 5 ± 1 µm s−1 (0%) to 26 ± 2 µm s−1 (1%).
• PA degradation (50 µM, 2 mg mL−1 WO3, 1% H2O2, 356 nm UV): 72% removal after 2 h with micromotors under UV + H2O2. Controls showed negligible removal without micromotors; with WO3 + UV (fuel-free motion) only 28% degradation; with WO3 + H2O2 (no UV) only 5%.
• Reusability: After five PA degradation cycles, efficiency remained up to ~55%, indicating retained photocatalytic activity.
• Radical trapping indicates •OH as the dominant reactive species driving PA oxidation: isopropanol (•OH scavenger) markedly reduced degradation; EDTA (h+ scavenger) increased photoactivity. Based on band positions (ECB ~0.77 VNHE), O2 reduction to O2•− is unfavorable, while holes (h+) and H2O2 reduction pathways generate •OH.
• 4-NP degradation: With H2O2-driven motion under UV, 40% removal after 2 h; fuel-free motion in water achieved 11%, consistent with lower propulsion and mass transfer.
• Light-controlled on/off propulsion was demonstrated with rapid response to UV switching and remote control.

The study addresses the mass-transfer limitations of heterogeneous photocatalysis by employing self-propelled, single-component WO3 micromotors that actively mix and increase contact with pollutants under light. Autonomous motion was confirmed by MSD analysis, showing Brownian motion in the dark and superdiffusive behavior under asymmetric UV illumination; propulsion and diffusion were further enhanced by H2O2, which also promotes ROS generation. Photodegradation tests on nitroaromatics (PA and 4-NP) showed substantially higher removal with powered micromotors than with fuel-free motion or static conditions, demonstrating that active motion improves degradation efficiency. Radical trapping identified •OH as the key ROS, consistent with WO3 band energetics that favor water oxidation and H2O2 reduction to •OH while disfavoring O2 reduction. The results validate that low-cost, noble-metal-free, single-component micromotors can couple propulsion with photocatalysis to remediate persistent nitroaromatic pollutants, reducing complexity compared to Pt/Au-based Janus systems and highlighting their relevance for scalable water treatment.

This work demonstrates a scalable, low-cost synthesis of precious-metal-free, single-component WO3 micromotors that self-propel under UV light and effectively degrade nitroaromatic pollutants in water. The micromotors enhance mass transfer, achieving up to 72% PA removal and 40% 4-NP removal in 2 h without external agitation, with motion and degradation performance further improved by H2O2. Mechanistic studies confirm •OH as the dominant ROS. These findings establish WO3 micromotors as promising moving catalysts for environmental remediation. Future efforts could focus on enhancing activity under lower-energy/visible light, optimizing reusability and recovery, and validating performance in complex real wastewater matrices and against broader contaminant classes.

• Reliance on UV illumination; visible-light activity was not demonstrated.
• Highest degradation performance required the presence of H2O2; fuel-free operation was less effective.
• Degradation was incomplete within 2 h (e.g., 72% for PA, 40% for 4-NP).
• Reusability showed a decrease in efficiency over cycles (to ~55% after five cycles).
• Experiments were conducted in controlled lab solutions; performance in real wastewater matrices and long-term stability were not evaluated within this study.