Structure and photocatalytic performance of magnetically separable titania photocatalysts for the degradation of propachlor
Introduction
Semiconductor photocatalysts have attracted much attention because of their potential application in the removal of organic and inorganic species from aquatic environments [1], [2]. Nowadays, TiO2 is widely recognized as the most promising photocatalyst for environmental applications. A major disadvantage of semiconductor oxides is the need of an additional and expensive separation step involving the removal of the photocatalyst from the treated water. In order to overcome this problem magnetically separable photocatalysts have been developed enabling the easy recovery of the photocatalyst by an external magnetic field [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13].
The magnetic photocatalytic systems that have been developed so far contain magnetite (Fe3O4) or maghemite (γ-Fe2O3) [3], [4], [5], [6], [7], [8], [9], [10] or spinel-like ferrites (NiFe2O4, Zn0.35Ni0.65Fe2O4) [11], [12], [13] serving as magnetic cores.
It has been established that direct contact between titanium oxide and iron oxide, i.e. TiO2/Fe3O4 or TiO2/γ-Fe2O3, is responsible for low photoactivities because Fe3O4 or γ-Fe2O3 act as recombination centres for electrons and positive holes, especially in the case of magnetite [3]. For example, it has been shown that the absorption of strong light by γ-Fe2O3 nanoparticles causes low photocatalytic activity when the amount of γ-Fe2O3 exceeds that of TiO2 in the composite [5]. On the other hand, the use of a SiO2 layer between the Fe3O4 or γ-Fe2O3 core and the TiO2 shell, i.e. TiO2/SiO2/magnetic oxide, has been found to promote the photocatalytic activity of the catalyst by preventing the injection of charges from TiO2 particles to magnetic particles [7], [8]. In other words, the SiO2 layer inhibits the photodissolution of the catalyst. In order to be then potentially applicable, the insulator has to be chemically inert and electronically passive toward titanium oxide.
Regarding the TiO2/spinel-ferrite systems, the TiO2/SiO2/Zn0.35Ni0.65Fe2O4 photocatalyst shows high activity in the photoxidation of oxalate [12]. Similarly, the TiO2/SiO2/NiFe2O4 photocatalyst displays enhanced photoactivity for decomposition of methylene blue [11]. Furthermore, the magnetically separable nitrogen-doped photocatalyst TiO2−xNx/SiO2/NiFe2O4 is a good photocatalyst for the degradation of methyl orange, under UV and visible light irradiation [13]. However, the activity of the composite system was considerably lower than that of the prestine TiO2−xNx because of the non-perfect coating of SiO2 around the surface of NiFe2O4.
Undoubtedly, the two important factors that determine the photoactivity, as well as the stability and the magnetic properties of all photocatalysts, are the silica layer and the heat treatment during the preparation process. It has been demonstrated that the heat treatment step has important implications on the magnetite properties of the prepared photocatalysts due to chemical or physical changes of the magnetic core [3]. The use of calcined magnetic cores for the preparation for the preparation of Fe3O4–SiO2–TiO2 [7] or Zn0.35Ni0.65Fe2O4–SiO2–TiO2 [12] photocatalysts does not solve the problem, because the sol–gel synthesis of TiO2 requires a heating step for anatase growth.
This study is focused on overcoming the disadvantages from heating by exploiting the use of Degussa pristine TiO2 nanoparticles for the fabrication of stable magnetic photocatalysts. This novel method based on a protective lining made up of two oppositely charged polyelectrolytes avoids the heat treatment step and results in highly efficient and magnetically separable photocatalysts. The structural and magnetic nature of photocatalysts were investigated by several techniques and their photocatalytic efficiency was evaluated for degradation of propachlor, a chloroacetanilide herbicide (Fig. 1) that is commonly used.
Section snippets
Materials
Ammonium peroxydisulfate (NH4)2S2O8, ammonium iron (II) sulphate hexahydrate Fe(NH4)2(SO4)2·6H2O, potassium hydroxide KOH, poly(diallyldimethylammonium)chloride 20 wt.% with typical Mw ∼100,000–200,000 and poly(sodium 4-styrenesulfonate) average with typical Mw∼70,000 were obtained from Aldrich and used as received. Commercial titania nanoparticles of P25 (ca. 80% anatase, 20% rutile, BET area ca. 50 m2/g) were supplied by Degussa (Germany). Propachlor analytical standard was purchased from
Characterization of γ-Fe2O3/PSS−/PDD+/TiO2 nanocomposites
The composite TiO2 photocatalyst is made up of a magnetic particle (γ-Fe2O3) lined with a thin membrane from two oppositely charged polyelectrolytes PSS−/PDD+ to which the Degussa P25 TiO2 nanoparticles are attached. The XRD pattern of the positively charged TiO2 particles [TiO2/PDD]+Cl− (Fig. 2) gives the characteristic diffraction peaks of anatase and rutile because the pristine material (TiO2-Degussa P25) contains both phases (80% anatase and 20% rutile). The strongest peak at 2θ = 25.2°
Conclusions
Using the polyelectrolyte templates, we prepared a novel kind of γ-Fe2O3/TiO2 photocatalysts which can be easily separated by a magnetic filed. The key components of these photocatalysts are the insulator polyelectrolytes that constitute the corrosion protective membrane between maghemite and titanium dioxide and they inhibit the negative influence of the γ-Fe2O3 on the photocatalysis. The developed nanocomposites exhibit good catalytic activity towards organic pollutants such as
Acknowledgements
This work has been supported by the Projects of the Ministry of Education of the Czech Republic (1M6198959201 and MSM6198959218) and by the project of ACSR (KAN115600801). We also thank Philippos Pomonis and Alexandros Katsoulidis at University of Ioannina for the Energy Dispersive Spectra (EDS) measurements as well as Dr. Christos Trapalis at NCSR “Demokritos” for the ICP analysis.
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