Surface decoration of commercial micro-sized TiO2 by means of high energy ultrasound: A way to enhance its photocatalytic activity under visible light

doi:10.1016/j.apcatb.2014.10.004

Applied Catalysis B: Environmental

Volume 178, November 2015, Pages 124-132

https://doi.org/10.1016/j.apcatb.2014.10.004 Get rights and content

Highlights

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Surface decoration of micro-sized TiO₂ by means of high-energy ultrasound.
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Decorated micro-sized TiO₂ powders were tested as photocatalysts for VOC abatement.
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Toluene and acetone degradation were monitored under UV and visible light.
•
W, Re, Cu and Mo nanoparticles onto TiO₂ surface.

Abstract

Although the TiO₂ is one of the most promising materials for its photocatalytic potential related to the pollution abatement, it strongly suffers from the low photocatalytic activity if it is used under visible light and not under UV light. Among the various possible modifications, the surface decoration with metal or metal oxides NPs can be a good strategy to increase the potential of TiO₂ in the visible range. In this paper, a sonochemical method that exploits the use of high-energy ultrasounds is suggested to obtain this surface decoration. The support is a commercial and micrometric TiO₂, cheaper and easier to handling than the nanometric P25. Samples were tested on the photodegradation of toluene and acetone in a gas phase system, using both the UV and LED lamp.

Graphical abstract

Introduction

TiO₂ is the most widely investigated photocatalyst due to its high photo-activity, low cost, low toxicity and good chemical and thermal stability. It was first used for the remediation of environmental pollutants in 1977 and since then there was a dramatic increase of the research in this area because of the potential for water and air purification [1].

Titanium dioxide is typically an n-type semiconductor due to the oxygen deficiency. The band gap is 3.2 eV for Anatase: this is the main TiO₂ polymorph and it is the most active phase in terms of photocatalytic activity.

In photocatalysis, light of energy greater than the band gap of the semiconductor excites an electron from the valence band to the conduction band (e_cb⁻) generating a positive hole in the valence band (h_VB⁺): in the case of titanium dioxide, because of the 3.2 eV band gap, UV light is required. Positive holes can oxidize OH⁻ or water at the surface to produce radical dot OH radicals, which are extremely powerful oxidants.

A lot of studies describe the negative effects on health related to the exposition to chemical pollutants, found in particular in the indoor environment [2], [3], [4]. The latter are classified in the VOC's (Volatile Organic Compounds) category, and most of them are toxic or carcinogenic. VOC's are chemical substances of different nature: it is important to underline that more than 300 species were detected in the indoor atmosphere, with a concentration from 2 to 10 times higher than in the outdoor one.

As modern people spend the most part of their time in the indoor environment, it is clear that the air quality of these places is a crucial point of priority interest [2], [5].

Among many AOPs (Advanced Oxidation Processes), TiO₂ photocatalysis is one of the most viable environmental cleanup technology: even if TiO₂ needs to have a higher activity to be economically competitive, from a practical point of view alternative materials that are as advantageous as TiO₂ are hard to be found [6].

The recombination of photogenerated charge carriers is one of the main limitations in semiconductor photocatalysis, and the crucial problem related to the practical use of the TiO₂ is its inability to be active under the visible light. This problem becomes harder when one thinks about TiO₂ used in the indoor areas where the lighting system is moving towards the total use of LED lamps, which are UV-radiation free.

Therefore, the most important demand is the introduction of the visible light activity that is absent with pure TiO₂ [7]. In this sense, several modification methods were developed in order to accelerate the photoconversion, enable the absorption of visible light, and alter the reaction mechanism or control products and intermediates [8]. The surface deposition of metal or metal oxide nano-particles (NPs) can be useful because of many factors: metals onto the TiO₂ surface can enhance the electron transfer or the charge separation and improve the formation of the free hydroxyl radicals; it was shown that also metal oxide particles can have a positive effect because they support the charge separation and prevent their recombination [9], [10].

The reason why the surface metal NPs affect the photochemical properties of TiO₂ is related to three main concepts: firstly, the UV radiation leads to a Fermi level shift, indicating an electron transfer at the interface that promote the photocatalytic reactions; another important aspect to take into account is the presence of free electrons in the metal particles that can be excited by light and finally the possibility that metals act as an electron sink promoting also in this case the charge separation.

Decoration of M- or MO-NPs is commonly implemented by means of ultra-sounds (US) in aqueous or organic solutions where ceramics or polymer substrate powders are dispersed [11]. The idea in this instance is to apply the same method using the micrometric TiO₂ as substrate, depositing on its surface species such as tungsten or rhenium oxide, molybdenum or copper. Because it was widely demonstrated that the TiO₂ surface modification can be useful to improve its photocatalytic activity, sonochemistry is a novel and interesting way to obtain the surface decoration of TiO₂ powder with metal nanoparticles. The sonochemical method described in this paper has been used for instance in the decoration of anode materials for Solid Oxide Fuel Cells (SOFC) [12].

Finally, it is important to underline that the substrate is micrometric TiO₂ and not the nanometric one: the upgrade of the micrometric TiO₂ is crucial because of the important drawbacks of the nanometer powders. The latter could be inhaled and come into direct contact with the cells of the organism; although the negative TiO₂ effects on human health were not fully demonstrated yet, some animals test have reported that TiO₂ nanoparticles are more dangerous than the micro ones and they have an higher influence to cancer, lung cancer in particular [13]. Based on the above mentioned, the use of micro-TiO₂ in order to take advantage from its photocatalytic properties is a challenging study to develop [14], [15], [16].

Section snippets

Materials and characterization methods

TiO₂ 1077 by Kronos is a micro-sized photocatalyst classified as pigment [15]. The main properties are summarized in Table 1.

For the sonochemical method, the precursor materials are purchased and used without further purification; they are Mo(CO)₆ (≥99.9% Sigma Aldrich), Re₂(CO)₁₀ (98% Aldrich), W(CO)₆ (99.99% Sigma Aldrich), and CuCl₂·2H₂O (≥99% Sigma Aldrich).

A Bandelin SONOPLUS HD 3200 utilizing a 200 W U/S generator and a sonication extension horn of 13 mm diameter generating US are employed.

Sample characterization

As reported in Table 2, surface decoration has almost no effect on the surface area of the samples, but for the Cu-decorating sample for which a marked reduction of surface area is observed.

As evidenced in the XRD patterns reported in Fig. 1, surface decoration does not affect the structural properties of Kronos 1077. All the XRD patterns exhibit the presence of the peaks characteristic of the anatase phase [ICDD anatase file no. 21-1272]. In particular, for both W- and Re-containing samples

Conclusions

Through the innovations introduced by sonochemistry, it was possible to obtain this new type of surface decoration of a pigmentary micro-TiO₂, proving that this modification method can improve the photocatalytic activity of the material, in particular under the visible light, where pure TiO₂ is not an effective photocatalyst.

The most interesting species loaded on the TiO₂ surface are the tungsten oxide and the rhenium oxide because they have the best performances in terms of degradation of

Acknowledgements

This research was supported by LIFE + Environment Policy and Governance project DIGITALIFE-LIFE13 ENV/IT/000140. The authors would like to thank Dr. D.G. Kanellopoulou, with Lab of Inorganic Materials Technology, NTUA, for all helpful regarding the samples preparation as well as Dr. V. Oldani and Dr. B. Sacchi for all the XPS measurements.

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Its photocatalytic properties are derived from the formation of photogenerated charge carriers (hole and electron) which occurs upon the absorption of UV light corresponding to the bandgap. The photogenerated holes in the valence band diffuse to the TiO2 surface and react with adsorbed water molecules, forming hydroxyl radicals (OH)[21,22,23,24,25]. The 'Fenton reagent' is a mixture of hydrogen peroxide and divalent iron salts (Fe2+/H2O2), which is an efficient oxidant for a wide variety of organic compounds.
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Surface decoration of commercial micro-sized TiO2 by means of high energy ultrasound: A way to enhance its photocatalytic activity under visible light

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials and characterization methods

Sample characterization

Conclusions

Acknowledgements

Appl. Catal. B: Environ.

Environ. Int.

Building Environ.

Int. J. Hygiene Environ. Health

J. Photochem. Photobiol. C: Photochem. Rev.

Solar Energy

Ceram. Int.

Ultrason. Sonochem.

Ultrason. Sonochem.

Cement Concrete Comp.

Appl. Surf. Sci.

Appl. Catal. B: Environ.

Chem. Phys. Lett.

J. Photochem. Photobiol. A: Chem.

J. Catal.

Surface decoration of commercial micro-sized TiO₂ by means of high energy ultrasound: A way to enhance its photocatalytic activity under visible light