Plasma-Sprayed Photocatalytic Zinc Oxide Coatings
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Plasma-Sprayed Photocatalytic Zinc Oxide Coatings A. H. Navidpour1 • Y. Kalantari1 • M. Salehi1 • H. R. Salimijazi1 • M. Amirnasr2 M. Rismanchian3 • M. Azarpour Siahkali1
•
Submitted: 10 August 2016 / in revised form: 17 December 2016 Ó ASM International 2017
Abstract Fabrication of semiconductor coatings with photocatalytic action for photodegradation of organic pollutants is highly desirable. In this research, pure zinc oxide, which is well known for its promising photocatalytic activity, was deposited on stainless-steel plates by plasma spraying. The phase composition and microstructure of the deposited films were studied by x-ray diffraction analysis and scanning electron microscopy, respectively. Despite the low-energy conditions of the plasma spraying process, the zinc oxide coatings showed good mechanical integrity on the substrate. Their photocatalytic activity was evaluated using aqueous solution of methylene blue at concentration of 5 mg L-1. The results showed the potential of the plasma spraying technique to deposit zinc oxide coatings with photocatalytic action under ultraviolet illumination. Ultraviolet–visible (UV–Vis) diffuse reflectance spectroscopy confirmed that the plasma spraying method could deposit zinc oxide films with higher photoabsorption ability relative to the initial powder. Keywords coating photocatalytic activity plasma spraying semiconductor zinc oxide
Introduction Photocatalytic technologies for water splitting, degradation of chemical pollutants/dyes, and reduction of carbon dioxide have received great attention (Ref 1-4). Heterogeneous photocatalysts absorb photons with energy equal to or larger than their bandgap, resulting in production of electrons in the conduction band (CB) and holes in the valence band (VB), which accelerates oxidation-reduction reactions (Ref 5, 6). Photoexcited electrons can initiate photoreactions by forming reactive radicals such as hydroxyl radicals (HO) and superoxide radical ions (O2 ) (Ref 7). The process of heterogeneous photocatalysis using a semiconductor (such as ZnO) is illustrated by the following equations (Ref 8-10): þ ZnO þ ht ! ZnO e ðEq 1Þ CB þ hVB ; þ e CB þ hVB ! heat;
H2 O þ
hþ VB
þ
ðEq 2Þ
! H þ OH ;
hþ VB þ OH ! OH ;
ðEq 4Þ
e þ O2 ! O 2 ;
ðEq 5Þ
þ
e þ O2 + H !
HO2 ;
þ O 2 þ HO2 + H ! H2 O2 + O2 ;
O 2 & A. H. Navidpour [email protected] 1
ðEq 3Þ
ðEq 6Þ ðEq 7Þ
þ organic substances ! organic substances OO: ðEq 8Þ
OH þ organic substances ! intermediate ! product:
Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran
ðEq 9Þ
2
Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran
3
Department of Occupational Health Engineering, Isfahan University of Medical Sciences, Isfahan 81746-73461, Iran
Various solid materials such as oxides, nitrides, and sulfides have been used as photocatalysts (Ref 11). Due to its chemical stability, nontoxicity, and strong oxidizing power,
123
J Therm
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