The oxide semiconductor with different band gap energy i.e. manganese dioxide (MnO2) (Eg: 1.30eV) and zinc oxide (ZnO) (Eg: 3.37 eV) nanoparticles were used to degrade RhodamineB (RhB) under irradiation of UV light (254 nm).The MnO2nanoparticles were synthesized by hydrothermal method (160 °C, 4 h) using analytical grade manganese sulfate hydrate and potassium permanganate (VII) as precursors. The XRD analysis reveals that the nanoparticles were d-MnO2.The photocatalytic study shows that degradation of RhB solution by ZnO nanoparticles (rate constant: 0.02749 min-1) was approximately four times faster than the d-MnO2nanoparticles (rate constant:0.0067 min-1). This observation could be attributed to the higher reducing and oxidizing power of ZnO in producing free radicals for photodegradation of RhB solution.

An ultraviolet (UV) shielding agent based on Fe-doped zinc oxide nanorods (ZnONRs) was synthesized by ex-situdoping using spray pyrolysis technique. These Fe-doped ZnO NRs could reduce the inherent photocatalytic activity of zinc oxide while still maintaining their ultraviolet filtering capability. In this work, the effect of doping duration by spray pyrolysis technique on the optical property and photocatalytic efficiency of ZnO NRs was studied. The room temperature photoluminescence (PL) analysis on the Fe-doped ZnO NRs indicates the red-shift of violet emission peak, i.e. from 378.97 nm (undoped) to 381.86 nm (60 mins.doping). Besides, the reduction of IUV/Vis ratio of PL reveals that the ex-situ Fe doping deteriorated the crystal quality of ZnO NRs. The photocatalytic study shows that the rate constant of Fe-doped ZnO NRs was smaller than the undoped ZnO NRs. It means that the Fe-doped ZnO NRs were less effective in degrading the RhB solution.

Heterogeneous photocatalysis particularly using semiconductor material to degrade organic compound is intensively studied in recent years. Semiconductors are good photocatalysts attributed to theirs light absorption properties, carrier charge transport characteristics and excited-state lifetimes. In this work, mixture of ZnO (3.23 eV) and V2O5 (2.38 eV) nanoparticles were used to degrade RhB solution under UV and visible light irradiation, respectively. The rod-like ZnO nanoparticles have an average diameter of 244.1 ± 94.5 nm whereas spherical-like V2O5 nanoparticles have an average diameter of 231.9 ±14.0 nm. Under UV light irradiation, pure ZnO nanoparticles have the highest photodegradation efficiency with rate constant of 0.034 ± 0.004 min-1 . This was attributed to its high reducing power of electrons and oxidation power of holes for producing free radicals. On the contrary, pure V2O5 nanoparticles demonstrated the highest photodegradation efficiency with rate constant of 0.013± 0.001 min-1 under visible light irradiation since it has profusion absorption in visible light region. The result provides useful guideline for designing a waste water purifier driven by sunlight for organic pollutants removal based on a mixture of wide bandgap and narrow bandgap semiconductor nanomaterials.

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