Highly covalent FeIII–O bonding in photo-Fenton active Sn-doped goethite nanoparticles

The relationship between local structure and photo-Fenton catalytic ability of tin-doped goethite with the composition of α-SnxFe1-xOOH (x = 0, 0.05, 0.10, 0.15 and 0.20, abbreviated as Sn100x) was investigated by X-ray diffractometry (XRD), 57Fe-and 119Sn-Mössbauer spectroscopies, X-ray absorption near-edge spectroscopy (XANES), transmission electron microscopy (TEM), Brunauer–Emmett–Teller surface area measurement (BET), diffuse reflectance spectroscopy (DRS), and ultraviolet–visible absorption spectroscopy (UV–vis). Room temperature 57Fe-Mössbauer spectra of Sn0 and Sn5 showed a sextet with isomer shift (δ) of 0.37 mm s−1, quadrupole splitting (Δ) of −0.27 mm s−1, and internal magnetic field (Hint) of 33.2 T due to goethite. In contrast, those of Sn10, Sn15 and Sn20 showed a doublet with increasing δ from 0.34 to 0.37 mm s−1 and Δ from 0.62 to 0.63 mm s−1 due to the superparamagnetic phase. On the other hand, increasing δ from −0.01 to 0.13 mm s−1 and decreasing Δ from 1.15 to 0.53 mm s−1 was observed from the 119Sn-Mössbauer spectra of Sn100x with ‘x’ from 0.05 to 0.20. These results indicate that the SnIV–O chemical bond becomes shorter, while that of FeIII–O longer by introducing tin into goethite, which causes a decrease in the bandgap energy (Eg). TEM image of Sn100x showed that needle-like particles with an average length of 181 and 61 nm were respectively observed in Sn0 and Sn5, whereas nanoparticles with the average size of 4.1, 6.4 and 10.2 nm were for Sn10, Sn15 and Sn20, respectively. From the BET measurement, the specific surface area (SSA) of Sn100x increased from 53 to 178 m2 g−1 with Sn concentration. Photo-Fenton catalytic ability of Sn100x using 20 μmol L−1 methylene blue aqueous solution and 0.4 mol L−1 H2O2 under visible-light irradiation revealed that Sn15 showed the largest apparent-first-order rate constant (k) of 35•10−3 min−1. We conclude that Sn15 is the best catalyst because of its large SSA of 187 m2 g−1 and the smallest bandgap energy of 1.77 eV.