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

Nitroaromatic compounds, widely used in various industries, pose significant environmental and health risks due to their persistence and resistance to biodegradation. Traditional wastewater treatment methods, such as biological oxidation and physical processes (activated carbon adsorption, nano-filtration), are often inadequate. Advanced oxidation processes (AOPs), particularly photocatalysis, offer a promising alternative. While nanostructured photocatalysts like TiO₂, ZnO, and Fe₂O₃ show potential, their passive diffusion limits efficiency. Light-powered micromotors, which convert light energy into motion, offer a solution by enhancing mass transfer through active movement. However, most micromotor designs incorporate expensive noble metals (Au, Pt). This research introduces a cost-effective and scalable method to produce single-component WO₃ micromotors for the efficient photocatalytic degradation of nitroaromatic pollutants. The use of WO₃, a readily available and less expensive material, eliminates the need for noble metal coatings, making this a more sustainable and practical approach for water remediation.

Existing literature highlights the challenges in removing nitroaromatic pollutants from water using traditional methods. AOPs, especially photocatalysis using various nanomaterials (TiO₂, ZnO, Fe₂O₃), have shown promise but suffer from slow diffusion rates. The use of light-powered micromotors to overcome this limitation has been explored, with studies demonstrating improved degradation of pollutants. However, many reported micromotors rely on noble metal components (Pt, Au), increasing fabrication costs and complexity. This work addresses this limitation by exploring a single-component WO₃ micromotor design, offering a potential cost-effective and sustainable alternative.

WO₃ micromotors were synthesized using a two-step process: a hydrothermal reaction followed by calcination. A homogeneous solution of tungsten precursor and glucose was subjected to hydrothermal conditions (200°C, 20 h). The resulting product was then calcined at 550°C in air to form WO₃ micromotors. The micromotors were characterized using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) to confirm their morphology, composition, and crystal structure. The micromotor motion was studied under UV light illumination in pure water and in the presence of varying concentrations of H₂O₂. The motion was recorded using an inverted microscope and analyzed to determine velocity and mean squared displacement (MSD). Photocatalytic degradation experiments were conducted using picric acid (PA) and 4-nitrophenol (4-NP) as model nitroaromatic pollutants. The degradation efficiency was determined using UV-Vis spectroscopy, and radical trapping experiments were performed to elucidate the degradation mechanism. Reusability tests were conducted to assess the long-term performance of the micromotors.

SEM images revealed WO₃ microspheres with sizes ranging from 1 to 2 µm, exhibiting a rough surface due to a hierarchical structure of assembled nanoparticles. EDX mapping confirmed the even distribution of W and O elements. XRD analysis matched the standard monoclinic phase of WO₃. XPS analysis confirmed the W⁶⁺ oxidation state. The WO₃ micromotors displayed autonomous motion under UV-light illumination, both in pure water and in the presence of H₂O₂. The addition of H₂O₂ significantly enhanced micromotor speed and diffusion coefficient. An optical bandgap of 2.72 eV was determined from the absorption spectrum. Photocatalytic degradation experiments showed that the WO₃ micromotors effectively degraded PA (72% efficiency after 2 h in 1% H₂O₂), with the degradation efficiency reduced to 28% in fuel free conditions. Reusability tests demonstrated that the micromotors retained high photocatalytic activity over multiple cycles (55% efficiency after five cycles). Radical trapping experiments indicated that ·OH radicals were the primary reactive oxygen species responsible for PA degradation. The degradation of 4-NP was also demonstrated, with higher efficiency observed in the presence of H₂O₂ (40% vs 11% in fuel-free conditions).

The results demonstrate the successful fabrication of highly efficient, low-cost, and sustainable single-component WO₃ micromotors for photocatalytic degradation of nitroaromatic pollutants. The enhanced mass transfer provided by the self-propulsion of the micromotors significantly improves the degradation efficiency compared to stationary photocatalysts. The findings address the limitations of existing noble metal-based micromotors by offering a more economical and environmentally friendly alternative. The identification of ·OH radicals as the key species responsible for degradation provides valuable insights into the mechanism, paving the way for further optimization of the process.

This study successfully demonstrates the synthesis and application of single-component WO₃ micromotors for efficient photocatalytic degradation of nitroaromatic pollutants. The simple, scalable, and cost-effective synthesis method, combined with the enhanced mass transfer due to self-propulsion, provides a promising solution for water remediation. Future research could focus on exploring other metal oxide materials and optimizing micromotor design for even greater efficiency and broader application in environmental cleanup.

The study focuses on two model pollutants, PA and 4-NP, and further investigation is needed to evaluate the effectiveness of the micromotors against a wider range of nitroaromatic compounds. The influence of other water constituents on micromotor performance and degradation efficiency should also be investigated. While reusability was demonstrated, a more comprehensive lifespan study would be beneficial.