The impact of ZnO load, stability and morphology on the kinetics of the photocatalytic degradation of caffeine and resazurin

Zinc oxide (ZnO) powders with diverse morphologies were synthesized with various solvothermal techniques. Water, 1-butanole and ethylene glycol were used as the solvents, while the temperature of the solvothermal synthesis was varied from 90 °C to 120 °C. Resazurin and caffeine were used for the evaluation of the photocatalytic activity of all the prepared samples, and in particular to compare ZnO with commercial titanium dioxide photocatalytic material (P25 Evonik Degussa) using UV–vis spectroscopy. The composition, crystallinity, and morphology of the prepared materials were investigated with FTIR, XRD, TEM and FEG-SEM techniques. The band gaps of the obtained semiconductors were measured because the band gap of hydrozincite, determined in this study to be 4.1 eV, has not been reported previously. The specific surface area (BET) and the porosity of the prepared particles were estimated. The crystal size in one dimension was estimated and was found to play an important role in the photocatalytic activity, which increased with a smaller size. A higher degree of aggregation caused the opposite effect. Thus, a more aggregated material with a larger surface area than a less aggregated one exhibited a lower photocatalytic activity. The particle morphology strongly influenced the photocatalytic process, while the degree of crystallinity was not comparably significant. The external mass-transfer (diffusion to the catalytic surface) resistance was negligible, and consequently the first-order, second-order, and Langmuir–Hinshelwood mechanism kinetics of the photocatalysis were examined.

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► The photocatalytic activity of ZnO varied with its morphology; ZnHC was not photocatalytic.
► Rod-like crystals showed a better photocatalytic effect than flower- and hedgehog-like crystals.
► More profound photocatalytic effect was achieved with smaller crystals.
► The band gap of hydrozincite was 4.1 eV, determined for the first time.