Abstract
Photocatalysis is proven to be the most efficient and environmentally friendly method for the degradation of organic pollutants in water purification. To meet the requirement of large-scale water treatment, there are two important points: One is the lifetime and chemical stability of the photocatalyst material, especially in the complex and harsh aqueous conditions. The other is the ease of synthesis of such photocatalysts with specific nano-morphology. In this work, two common photocatalyst materials, zinc oxide (ZnO) and titanium dioxide (TiO2), are selected to form more sustainable photocatalysts with high chemical stability. This involves the combination of both TiO2 and ZnO in a two-step simple synthesis method. It appears advantageous to exploit the conformal deposition of atomic layer deposition (ALD) to achieve nanometer-thick TiO2 coating on ZnO nanowires (NWs) with a high aspect ratio, which are firmly anchored to a substrate and exhibit a large specific surface area. The high chemical stability of the ALD TiO2 coating has been investigated in detail and proven to be effective under both strong acid and strong alkaline aqueous solutions. In addition, photocatalysis experiments with organic dyes show that via this simple two-step synthesis method, the produced ZnO/TiO2 tandem photocatalysts does indeed exhibit improved chemical stability in a harsh environment, while allowing efficient photodegradation.
Highlights
The combined effects of the growing human population, climate change, and increasing pollution are placing significant pressures on water resources
An investigation is reported on a simple two-step synthesis method that can provide high-aspect ratio TiO2 /zinc oxide (ZnO) tandem photocatalysts, with the aim of enhancing their lifetime and chemical stability under harsh aqueous solution pH conditions, with applications mainly in water purification
It is demonstrated that the atomic layer deposition (ALD) TiO2 coating offers a sustainable protective layer thanks to its good conformity on the ZnO
Summary
The combined effects of the growing human population, climate change, and increasing pollution are placing significant pressures on water resources. Once the purification is complete, the separation of the photocatalytic particles and purified water is far from easy, and it often requires further filtration of the effluents, adding significant time and energy costs, and reducing the yield of the water treatment process To avoid those additional steps, an alternative solution is to rely on a surface substrate supporting nanowire arrays with a high surface-to-volume ratio. The targeted technical solution for effective water purification needs to simultaneously meet the following two conditions: (i) robust chemical resistance in the photocatalysts is needed to increase its lifetime; (ii) simple synthesis of the photocatalysts, which can be achieved on a solid substrate while maintaining a high surface-to-volume ratio, so as to achieve an effective chemical reaction rate To meet all these requirements, a thin protective TiO2 coating applied by atomic layer deposition onto the simple hydrothermally synthesized ZnO NWs could be straightforward, since atomic layer deposition (ALD). The photodegradation of organic dye under UV light illumination using TiO2 -coated ZnO NWs is investigated for different TiO2 coating thicknesses, with a focus on efficiency and chemical stability
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