Synthesis of hierarchical dendritic micro-nano structure ZnFe₂O₄ and photocatalytic activities for water splitting

Hierarchical dendritic micro-nano structure ZnFe₂O₄ have been prepared by electrochemical reduction and thermal oxidation method in this work. X-ray diffractometry, Raman spectra and field-emission scanning electron microscopy were used to characterize the crystal structure, size and morphology. The results show that the sample (S-2) is composed of pure ZnFe₂O₄ when the molar ratio of Zn^{2 +}/Fe^{2 +} in the electrolyte is 0.35. Decreasing the molar ratio of Zn^{2 +}/Fe^{2 +}, the sample (S-1) is composed of ZnFe₂O₄ and α-Fe₂O₃, whereas increasing the molar ratio of Zn^{2 +}/Fe^{2 +}, the sample (S-3) is composed of ZnFe₂O₄ and ZnO. The lattice parameters of ZnFe₂O₄ are influenced by the molar ratio of Zn^{2 +}/Fe^{2 +}: Zn at excess decreases the cell volume whereas Fe at excess increases the cell volume of ZnFe₂O₄. All the samples have the dendritic structure, of which S-2 has micron-sized lush branches with nano-sized leaves. UV-Vis diffuse reflectance spectra were acquired by a spectrophotometer. The absorption edges gradually blue shift with the increase of the molar ratio of Zn^{2 +}/Fe^{2 +}. Photocatalytic activities for water splitting were investigated under Xe light irradiation in an aqueous olution containing 0.1 mol·L^{− 1} Na₂S/0.02 mol·L^{− 1} Na₂SO₃ in a glass reactor. The relatively highest photocatalytic activity with 1.41 μmol·h^{− 1} · 0.02 g^{− 1} was achieved by pure ZnFe₂O₄ sample (S-2). The photocatalytic activity of the mixture phase of ZnFe₂O₄ and α-Fe₂O₃ (S-1) is better than ZnFe₂O₄ and ZnO (S-3).