Abstract
Well-aligned ZnO nanorod arrays were assembled on activated carbon fibers by a stepwise sequence of sol-gel and hydrothermal synthesis methods. These ZnO nanorod arrays on activated carbon fibers having different characteristics such as surface area, rod concentration, aspect ratio and defect level, were applied as catalysts for the photodegradation of an aqueous methylene blue solution. They showed very promising methylene blue adsorbility in the dark (ca. 0.025–0.031 mg methylene blue m−2 catalyst, vs. 0.072 mg methylene blue m−2 activated carbon fibers). Significantly, the defect level of ZnO nanorod arrays has a major effect on the turnover frequency compared to other characteristics. A synergistic effect between activated carbon fibers and ZnO nanocrystals on enhancing turnover frequency was more significant for the well-assembled ZnO nanorod arrays on activated carbon fibers catalysts compared to the mechanically mixed ZnO powder with activated carbon fibers catalyst. Further, turnover frequency for the ZnO nanorod arrays on activated carbon fibers (0.00312 molmethylene blue molZnO−1 h−1) was twice higher than that for the corresponding bare ZnO nanorod arrays, and 3 times higher than that for a commercial ZnO powder. In addition, ZnO nanorod arrays on activated carbon fibers show high degradation (77.5%) and mineralization (55.0%) levels for methylene blue, and also good reusability (or stability) as demonstrated by a sequential 5-time recycle routine. These outstanding features indicate that activated carbon fibers supported ZnO nanorod arrays have significant potential to be used as catalysts for photodegradation.
Highlights
ZnO is regarded as an alternative photocatalyst (Chakrabarti and Dutta, 2004) for photocatalytic treatment of wastewater, in which TiO2 is commonly employed (Feng et al, 2014)
ZnO NRAs/activated carbon fibers (ACFs) catalysts were prepared by a stepwise approach developed earlier by the authors (Luo et al, 2020), which involves coating of ACFs with ZnO seed layers (ZnO SLs) via a sol-gel synthesis followed by growth of the well-aligned ZnO nanorod arrays (ZnO NRAs) via a hydrothermal synthesis step (Experimental Section 2.2)
Pre-coating ZnO SLs on ACFs was of prime importance as evidenced by the formation of randomly assembled ZnO nanorods (ZnO NRs) and tufted spheres on the ACFs surface (Fig. 1-b) when only hydrothermal synthesis protocol using pristine ACFs (Fig. 1-a) was applied
Summary
ZnO is regarded as an alternative photocatalyst (Chakrabarti and Dutta, 2004) for photocatalytic treatment of wastewater, in which TiO2 is commonly employed (Feng et al, 2014). One of the practical solutions is to immobilize or anchor ZnO to substrates, such as glass (Vaiano and Iervolino, 2018), reduced graphene oxide (Nguyen et al, 2019; Fan et al, 2015), Nylon 6 (Ummartyotin and Pechyen, 2016), carbonized oak (Tafreshi et al, 2019) and activated carbon (Melian et al, 2009; Raizada et al, 2014) Among these reported substrates, activated carbon materials are attractive and synergistic effects have been observed on photocatalytic degradation efficiency of supported or mixed ZnO catalysts. ZnO NRAs/ ACFs catalysts with different characteristics such as surface area, aspect ratio, rod concentration and defect level, were studied for photocatalytic degradation of aqueous methylene blue solution. The results provide insights in the effects of catalyst structure as well as synergistic effects on photocatalytic degradation when using well-aligned ZnO nanorod arrays on activated carbon fibers
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