Electrospinning Machine| Enhanced spin polarization in GQDs/TiO2 fibers via magnetic field and oxygen vacancies for photocatalysis

With the increasing severity of water pollution, photocatalytic technology has demonstrated significant potential in environmental remediation due to its low cost, simple operation, and absence of secondary pollution. However, its practical application is severely limited by low quantum efficiency caused by rapid recombination of photogenerated carriers. Although researchers have employed strategies such as defect engineering, element doping, and heterostructure construction to suppress carrier recombination, the role of spin electronic processes in enhancing photocatalytic performance has long been overlooked. Particularly, the mechanism of strong magnetic fields' influence on photocatalytic processes remains unclear and even controversial, necessitating in-depth research.

Recently, Professor Long Yunze and Associate Professor Zhang Jun's team from Qingdao University published their latest research titled "Enhanced spin polarization in GQDs/TiO₂ fibers via magnetic field and oxygen vacancies for photocatalysis" in npj Clean Water, a journal under Nature. The researchers prepared graphene quantum dots/titanium dioxide (GQDs/TiO₂) fiber membranes containing oxygen vacancies using electrospinning and solvothermal methods. The study found that oxygen vacancies, as key active sites, can induce spin polarization during photocatalytic reactions, a phenomenon confirmed by X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations.

More importantly, this research systematically investigated, for the first time, the effects of moderate (500 mT) and ultra-strong magnetic fields (1000–5000 mT) on photocatalytic performance. The results showed that magnetic fields significantly enhance spin polarization in the GQDs/TiO₂ system, improve carrier separation, and accelerate methylene blue (MB) degradation. Under a 500 mT magnetic field, the GQDs/TiO₂ fiber membrane achieved a degradation efficiency of 95.35% for MB, with a rate constant (*k*) of 17.79574 mg g⁻¹ min⁻¹/²—a 25.82% increase in efficiency and a 52.44% increase in rate constant compared to zero-field conditions. At 3000 mT, the degradation efficiency further improved to 98.35% (*k* = 17.86387 mg g⁻¹ min⁻¹/²).

The exceptional performance is attributed to the magnetic field-enhanced spin polarization, which promotes the synergistic effect between oxygen vacancies and photogenerated electrons while significantly suppressing carrier recombination. Additionally, the study proposed a novel concept of "magnetic field-induced progressive energy level modulation effect," revealing how defect-state energy levels gradually adjust under magnetic fields until reaching a stable state.

This work provides critical insights into the mechanism of magnetic field and oxygen vacancy synergy in driving radical generation, opening new pathways for designing advanced photocatalysts for water pollution treatment and sustainable energy solutions.

Fig. 1: Synthesis and characterization of GQDs/TiO₂ fiber membranes.

The GQDs/TiO₂ fiber membranes were prepared via a two-step method combining electrospinning and solvothermal synthesis (Fig. 1a). Characterization techniques such as XRD (Fig. 1b), FTIR (Fig. 1c), Raman spectroscopy (Fig. 1d), TEM/HRTEM (Fig. 1e), and SEM/EDS mapping (Fig. 1f) confirmed the material's structure, phase purity, presence of oxygen vacancies (Vₒ; e.g., broadening/attenuation of Ti-O-Ti peaks in Raman, *g* = 2.004 signal in EPR), and uniform distribution of C, O, and Ti elements. XPS analysis (Fig. 2a–c) indicated successful integration of GQDs into TiO₂, with increased electron density around oxygen vacancies (531.14 eV in O 1s spectra). EPR (Fig. 2d) and Ti L-edge XAS (Fig. 2e) directly confirmed oxygen vacancy-induced spin polarization (increased spin-up electrons), while DFT calculations (Fig. 2f) revealed spin-polarized density of states.

Fig. 2: Electronic structure, magnetic, and optical properties of the catalyst.

Fig. 3: Photocatalytic degradation performance of GQDs/TiO₂ fiber membranes.

MB photocatalytic degradation experiments (Fig. 3) were conducted using a custom in-situ magneto-photocatalytic setup integrated with a Physical Property Measurement System (PPMS). Results clearly demonstrated that applying external magnetic fields (0–5000 mT) improved the performance of GQDs/TiO₂ fiber membranes compared to zero-field (GQDs/TiO₂-0 mT), with optimal performance at 3000 mT (98.35% degradation). Even at an industrially feasible 500 mT (achievable with permanent magnets), degradation efficiency reached 95.35%, surpassing many reported catalysts. Magnetic fields suppressed carrier recombination via two pathways: Lorentz force acting on mobile charge carriers and, more critically, magnetic field-enhanced spin polarization. In-situ PL (Fig. 4a), transient photocurrent (Fig. 4b), and electrochemical impedance (Fig. 4c) confirmed the central role of spin polarization in promoting charge separation. Radical trapping experiments (Fig. 4f) and EPR detection (Fig. 4g) identified hydroxyl radicals (•OH) as the primary active species.

Interestingly, the system exhibited a "lagged response" to magnetic fields, leading to the proposal of the "magnetic field-induced progressive energy level modulation effect", where magnetic fields gradually adjust catalyst energy levels until stabilization. Theoretical calculations (Fig. 4i) showed that magnetic fields significantly reduced reaction energy barriers. Finally, the study elucidated the photocatalytic reaction pathway of GQDs/TiO₂ fiber membranes under magnetic fields (Fig. 4j): spin polarization extends carrier lifetime, valence band holes drive water molecule activation to generate •OH (Pathway 1), and conduction band electrons participate in •OH formation via adsorbed oxygen at oxygen vacancies (Pathway 2), collectively accelerating MB oxidation.

Fig. 4: Mechanistic analysis of magnetic field-enhanced photocatalysis.

In summary, this work successfully prepared GQDs/TiO₂ fiber membranes with oxygen vacancy-induced spin polarization, systematically revealed the mechanism by which magnetic fields (especially strong ones) enhance photocatalytic performance via spin polarization, and innovatively proposed the "magnetic field-induced progressive energy level modulation" effect. The magnetic field-enhanced spin polarization strategy offers new perspectives for understanding photocatalytic mechanisms, highlighting the vast potential of magnetic field regulation in advancing photocatalytic technology for wastewater treatment and sustainable energy conversion.

Paper link::https://doi.org/10.1038/s41545-025-00492-0