Room temperature ferromagnetic Zn0.98Co0.02O powders with improved visible-light photocatalysis

The realization of ferromagnetic diluted magnetic semiconductors requires the fine tuning of the energy levels of the band structure, which might have important applications in photocatalysis. The magnetic properties and visible-light photocatalytic properties of Zn0.98Co0.02O nanoparticles post-annealed under different ambient conditions (vacuum, hydrogen, vacuum followed by hydrogen) have been studied. The X-ray diffraction patterns show that all samples have a hexagonal wurtzite structure without any impurity phase. However, the Raman spectra indicate a small amount of Co3O4 impurities, which is significantly suppressed during the hydrogenation process. The as-prepared and vacuum-annealed Zn0.98Co0.02O nanoparticles show dominant paramagnetic properties, while strong room temperature ferromagnetism is observed after the hydrogenation process. This is explained by the ferromagnetic mediation between doped Co ions by the interstitial H ions. The photocatalytic activity of the resulting samples was evaluated by the degradation of Rhodamine B under visible light irradiation. It was found that the Zn0.98Co0.02O annealed in vacuum exhibited the best photodegradation properties. The enhanced performance is mainly attributed to the existence of oxygen vacancies which might decrease the surface recombination centers and improve the charge separation efficiency. The photocatalysis of hydrogenated samples is deteriorated significantly due to the passivation of oxygen vacancies by hydrogen, which confirms the ferromagnetic mediation by H+ ions instead of oxygen vacancies. Interestingly, the vacuum-annealed Zn0.98Co0.02O samples further annealed under hydrogen ambient conditions exhibit both room temperature ferromagnetism and sizable photocatalytic ability.