Magnetic field effect on heterogeneous photocatalysis

Abstract

Magnetic field effect (MFE) on heterogeneous photocatalysis is investigated, using mostly ZnO powder and methylene blue (MB) solution. Reproducibility with errors less than 2% is obtained with our newly-developed in-situ measurement system. Multiple parameters are involved in MFE phenomena. MFE magnitude depends on magnetic field intensity, while other parameters are also important, including: initial MB concentration, settling time of solution, and level/state of dissolved oxygen (DO). Short-range-order, or very-short-range-order diffusion in the Helmholtz layer just outside powder appears responsible for MFE. Accordingly, powder surface conditions, e.g. H2O, CO2 adsorption, significantly influence MFE, and magnetic adsorption appears correlated. Temperature-dependent MFE and magneto-hydrodynamic effect on photocatalytic silver reduction/deposition are also observed for the first time.A model is proposed, whereby, in the volume in close proximate to the powder, electrostatic potential and the corresponding magnetic flux would be perturbed, resulting in the appearance of a net Lorentz force on paramagnetic DO, which is essentially responsible for various MFE phenomena (OANS model). DO could be involved with (i) formation of DO-dye complex, (ii) modifying powder adsorption layer, (iii) scavenging rate of excited electrons, (iv) spin chemistry involving singlet–triplet conversion, and (v) manifestation of magneto-hydrodynamic effect. Simultaneous multiple mechanisms are suggested for MFE.