Synthesis of TiO2 coated ZnO nanorod arrays and their stability in photocatalytic flow reactors

• TiO2 layer on the top surfaces of ZnO increases its photocatalytic activity.
• TiO2 coating on ZnO nanorods decreases surface dissolution of Zn2 + ions.
• TiO2 gel in between ZnO nanorods improves the verticality and strength of the nanorods.
• Point defects in the TiO2 coatings lead to ZnO dissolution by pit formation.
• TiO2 coated ZnO nanorods can be immobilized photocatalysts in flow reactors.

Immobilized photocatalysts are preferred over suspended catalyst particles in slurry reactors due to elimination of filtration process. The requisite condition for their use is the mechanical and chemical stability under flow conditions. The stability of TiO2 coated ZnO nanorods (TiO2/ZnO) immobilized at the bottom of a prototype continuous-flow reactor designed in the form of a rectangular channel is investigated under continuous flow conditions in this work. Morphology and structure of the nanorod arrays are characterized by field emission scanning electron microscopy and X-ray diffraction. Optical properties of the rods are determined by UV–Vis spectroscopy measurements. Dissolution trends of zinc ions from the TiO2/ZnO rods are determined under dark, UV radiation and photodegradation conditions, and compared with the results of uncoated ZnO. The effect of the number of TiO2 film coatings and the calcination time of the spin-coated films are investigated as parameters. Coating of the ZnO nanorods with TiO2 has greatly reduced its dissolution under UV radiation to a level less than the dissolution of uncoated ZnO rods in complete darkness. Photocatalysis under UV radiation was found to increase the dissolution rate of zinc ions. This increase was attributed to cavity or pit formation on the upper surfaces of nanorods. TiO2 coating was found to decrease the dissolution rate significantly by reducing the extent of pit formation. TiO2 was also found to increase the catalytic effectiveness of ZnO nanorod film in the photodegradation of methyl orange dye selected as a model organic pollutant.