Rare earth co-doped ZnO photocatalysts: Solution combustion synthesis and environmental applications

A novel europium (Eu) and terbium (Tb) co-doped ZnO nanoparticles had been synthesized by a facile, cost efficient and rapid combustion method. The physical and chemical properties of the as-synthesized nanoparticles were determined by XRD, SEM, EDS, BET, FTIR, XPS, UV-vis DRS, PL, EIS and photocurrent density. XRD patterns confirm that Eu and Tb ions are stably inserted into the framework of ZnO, and the crystallite size decreases to 21 nm compared to that of pure ZnO (32 nm) as the amount of co-dopants increases to optimal value (3 mol% of each). With the insertion of Eu and Tb ions into ZnO, the recombination rate of photo generated charge carriers is found to be low. It is observed that doped Eu and Tb ions can enter the lattice structure of ZnO with a suitable doping concentration, and the obtained doped ZnO have ordered hexagonal wurtzite structures and nearly spherical morphology with high specific surface area and high porosity. Meanwhile, the introduction of Eu and Tb ions can effectively extend the spectral response from UV to visible region for the catalysts, thus reducing the band gap from 3.25 to 2.91 eV. Further analysis by means of XPS measurement showed that the existence of mixture of Eu2+/Eu3+ and Tb3+/Tb4+ oxidation states and high content of the surface chemisorbed oxygen species also contributed to the high photocatalytic activity. It was found that Eu and Tb co-doped ZnO shows highly efficient and stable visible light activity and provides a 100% MB degradation within 15 min, while the degradation time for Eu doped ZnO and Tb doped ZnO is reduced gradually to 50 and 42 min, respectively to obtain the 100% MB degradation. It was also found that the photocatalytic hydrogen evolution activity over Eu and Tb co-doped ZnO can be significantly increased to 533.8 and 792 mu mol with 0.2 wt% catalyst dose and initial solution pH 9, respectively under similar conditions with good stability. Moreover, simultaneous introduction of Eu and Tb effectively promoted the yield of CH4 to 4.59 mu mol, which was 3.4 fold higher in comparison to pure ZnO. Thus the present approach could provide a versatile strategy for the synthesis of novel and efficient visible light activated photocatalysts.