Degradation of methylene blue in aqueous dispersion of hollow titania photocatalyst: Study of reaction enhancement by various electron scavengers

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Abstract

Submicrometer-sized titania hollow spheres have been synthesized by employing sulfonated polystyrene latex particles (averaged diameter: 200 nm) as a template in conjunction with the sol–gel method. Utilization of 0.20 mol/L titanium tetrabutoxide and 25 g/L latex led to the formation of anatase particles having shell thickness of about 15 nm and void diameter of about 150 nm. Photocatalytic activity of the titania hollow spheres was examined by focusing on its enhancement by electron scavengers in the photocatalytic decomposition of methylene blue (MB). The electron scavengers employed were inorganic oxidants such as ClO3, BrO3, IO4, H2O2, and S2O2−8. Differences in electronegativity, atomic radius of the halogens, and the number of highly reactive radical and nonradical intermediates were proven to be important criteria for an electron scavenger to yield high efficiency in the MB photodecomposition. Based on the results, the effect of the oxidants used were found to be in the order of S2O82>IO4>BrO3>H2O2>ClO3.

Graphical abstract

Submicrometer-sized titania hollow spheres have been synthesized by employing sulfonated polystyrene latex particles as a template in sol–gel method. Photocatalytic activity of the hollow spheres was investigated by focusing on its enhancement by utilizing various electron scavengers. The effect of all oxidants used in UV/TiO2 system on enhancing photodecomposition of MB was found to be in the order of S2O82>IO4>BrO3>H2O2>ClO3.

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Introduction

Reactive dyes are widely used in textile industries because of their simple dyeing procedure and stability during washing process [1]. But, the main drawback of these kinds of reactive dyes is their low fixation rate. Therefore, textile wastewaters introduce intensive color and toxicity to aquatic systems. Thus, much attention has been paid to the techniques to remove or to degrade such reactive dyes [1].

In recent years, research in new non-biological methods has led to processes which actually destroy these pollutants instead of simply extracting them from water (e.g., adsorption by active carbon, air stripping, etc.). It has been shown that the use of TiO2, O3, H2O2, and Fenton (a mixture of ferrous ion with H2O2) are more efficient in the photodegradation of organic pollutants in comparison to that of direct photolysis [2], [3]. Among them, one of the common observations is that the enhancement of organic decomposition is due to the generation of powerful non-selective hydroxyl radical (OH) produced in the process of photodegradation.

Photocatalytic process, which utilizes TiO2 semiconductor photocatalyst, has received increasing attention because of its low cost, relatively high chemical stability of the catalyst, and the possibility of using sunlight as a source of irradiation [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. However, it has a limitation that the quantity of OH radicals cannot be increased infinitely because overdosing of TiO2 scatters the light in the solution [15]. Therefore, new developments of these technologies have focused on searching for better oxidants to increase the generation of radicals or to optimize the photodegradation process.

It was reported that the use of inorganic oxidants, such as H2O2, ClO3, BrO3, and S2O8, in TiO2 system increased the quantum efficiencies either by inhibiting electron–hole pair recombination through scavenging conduction band electrons at the surface of TiO2 or by offering additional oxygen atom as an electron acceptor to form the superoxide radical ion (O2) [16], [17], [18]. According to the investigation on H2O2, adequate dose of H2O2 led to a faster degradation of organic compounds in the TiO2 photocatalytic system [19]. However, the degradation was suppressed if excess H2O2 was used. This is due to the undesirable consumption of OH radical that was previously formed in the solution by H2O2, leading to generation of less-reactive HO2 radicals instead [2].

Enhancement of the TiO2-catalyzed photodegradation of organic compounds by several inorganic oxidants was mainly attributed to the increased electron scavenging from the extra oxidant sources. A mechanism of periodate (IO4) during UV illumination has also been proposed, where several reactive radicals and non-radical intermediates such as IO3, OH, and IO4 assist the photodegradation process [3], [20].

In our previous works, we reported synthesis of submicrometer-sized titania hollow spheres with tunable shell thickness and void volume [21] and their photocatalytic activity by employing methylene blue (MB) [22], which is widely used as a standard target compound in a test of photocatalysts. MB can also be regarded as a model compound of organic pollutants because it is mixed into some fertilizers as a dye. We also have optimized the enhanced-photocatalytic activity of titania hollow spheres in the photodegradation of MB by utilizing peroxydisulfate (S2O2−8) as an electron scavenger [23]. In the present works, we have expanded our investigations to enhancing the photocatalytic activity of titania hollow spheres photocatalyst by employing various electron scavengers such as oxyhalogens (KClO3, KBrO3, KIO4), H2O2, and K2S2O8. The effects of the electron scavengers on photocatalytic decomposition of MB by the titania hollow spheres have been elucidated in detail in this study.

Section snippets

Chemical

Styrene (99.0%, Sigma Aldrich) was distilled under reduced pressure after being washed with NaOH solutions. Sodium p-styrenesulfonate (NaSS, Sigma Aldrich), titanium tetrabutoxide (TBOT, 97.0%, Sigma Aldrich), ethanol (99.5%, Wako), MB (Sigma Aldrich, reagent grade), hydrogen peroxide (H2O2, Wako), potassium persulfate (KPS, 98.0%, Sigma Aldrich), potassium chlorate (KClO3, Wako), potassium bromate (KBrO3, Wako), potassium periodate (KIO4, Sigma Aldrich), and commercially available TiO2

Results and discussion

Fig. 1 shows examples of SEM and TEM images of titania hollow spheres prepared using sulfonated PS latex template (25 g/L) and 0.20 M TBOT–ethanol precursor. This sample of titania hollow sphere has about 15 nm shell thickness and about 150 nm void diameter. As shown in a previous work [21], the utilization of sulfonated PS latex template particles has resulted in the formation of smooth spherical shells and well-defined titanium dioxide layers. Their larger surface area and lower density give

Conclusions

Submicrometer-sized titania hollow spheres have been synthesized by employing sulfonated polystyrene latex particles as a template in conjunction with the sol–gel method. The use of 0.20 M TBOT–ethanol precursor has lead to the formation of rigid and smooth shell of titania hollow spheres with the shell thickness of about 15 nm and the void diameter of about 150 nm. The X-ray data shows that the resulted titania hollow spheres contains mostly of anatase form. The UV absorption spectra revealed

Acknowledgements

We express our thanks to Profesor Mitsunori Yada and Miss Yuko Inoue in the Faculty of Science and Engineering, Saga University for their help in SEM, TEM, and XRD measurements. The present study was partly supported by Grants-in-Aid for Scientific Research (17510088) from Japan Society for the Promotion of Science (JSPS).

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