Dependence of photocatalytic activity on particle size of a shape-controlled anatase titanium(IV) oxide nanocrystal

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Abstract

Decahedral anatase titanium(IV) oxide (TiO2) with {1 0 1} and {0 0 1} exposed crystal faces was prepared by hydrothermal treatment of peroxo titanic acid (PTA) solution with polyvinyl alcohol (PVA) as a shape-control reagent. pH of the PTA solution and amounts of PVA and amorphous titania included in the PTA solution had a large influence on size and shape of the prepared particles, and particle width of the decahedral anatase TiO2 was controllable between 25 and 60 nm. Photocatalytic activity of the decahedral anatase TiO2 was examined in terms of the relationship between particle size and photocatalytic activity. Decahedral anatase TiO2 with particle width of ca. 40 nm showed excellent activity because of the optimized balance between efficient separation of redox sites and large specific surface area.

Highlights

► Size of decahedral anatase TiO2 with exposed crystal faces was controlled. ► The relationship between particle size and photocatalytic activity was examined. ► Decahedral anatase with particle width of ca. 40 nm showed excellent activity.

Introduction

Crystal size is one of the most important properties influencing performance of material function. Nano-sized particles have been expected to exhibit excellent performance due to their large specific surface area and quantum size effect. For example, nano-sized particles having a large numbers of adsorption sites have frequently been used in photocatalytic reaction, which has been a promising function for environmental remediation [1], [2]. Photocatalytic reaction is induced by excited electrons and positive holes, followed by reduction and oxidation with surface-adsorbed species. However, back reaction easily occurs on the nanoparticle surface because reduction and oxidation proceeds in neighboring sites, and this back reaction as well as recombination between excited electrons and positive holes greatly decrease the efficiency of photocatalytic reaction. Therefore, it is important to optimize the balance between adsorption ability and utilization efficiency of photogenerated electrons and positive holes by enlargement of the specific surface area, an improvement of crystallinity by thermal treatment and combined use with other materials.

Recently, the dependence of photocatalytic activity on exposed crystal faces of an anatase titanium(IV) oxide (TiO2) nanocrystal has been intensively studied [3], [4], [5], [6], [7], [8], [9], [10]. Our group has also proposed that anatase TiO2 with specific exposed crystal faces exhibits higher photocatalytic activity because of spatial separation of redox sites induced by different kinds of exposed crystal faces [11], [12]. In our previous study, we prepared decahedral anatase TiO2 particles with {1 0 1} and {0 0 1} exposed crystal faces by hydrothermal treatment of peroxo titanic acid (PTA) with polyvinyl alcohol (PVA) as a shape-control reagent [12]. PTA species are known as a water-base precursor for TiO2 with relatively high stability against hydrolysis over a wide pH range under ambient condition [13]. Moreover, shape-control reagent, such as organic acid and hydrophilic polymer, easily coordinate on the peroxio titanic acid species [12], [14]. These properties are suitable for formation of specific exposed crystal faces. The anatase TiO2 prepared in previous study showed higher activity than that of P-25, which is a well-known commercial TiO2 [12]. This is presumably because of separation of redox sites induced by predominant reduction and oxidation over {1 0 1} and {0 0 1} exposed crystal faces, which were determined by photodeposition method [11], [12]. However, the prepared shape-controlled anatase TiO2 had a primary particle size of ca. 25 nm, which is thought to be insufficient for efficient separation of redox sites. Thus, an increase in the particle size of a shape-controlled nanocrystal will possibly enhance photocatalytic activity as a result of more effective separation of redox sites (Fig. 1).

Here, we report a method for preparation of a decahedral anatase TiO2 nanocrystal with larger particle size than that of TiO2 prepared in our previous study [12]. Precursor conditions, i.e., pH of the PTA solution (pHPTA) and amounts of PVA (MPVA) and amorphous titania (Mam) included in the PTA solution, had a large influence on size and shape of the prepared particles. Moreover, stirring during hydrothermal treatment was carried out for improvement of particle size distribution and surface structure. In the present study, photocatalytic activity of the prepared shape-controlled anatase TiO2 was examined in terms of the relationship between particle size and photocatalytic activity.

Section snippets

Sample preparation

Milli-Q water (3.0 cm3) was added to 10 cm3 of titanium(IV) ethoxide and 50 cm3 of ethanol with vigorous stirring, and the mixture was stirred for 1 h at room temperature. The resulting precipitate was centrifugally separated from the solution and dried under reduced pressure. One or three grams of obtained amorphous titania was dispersed in 100 cm3 of milli-Q water and then irradiated by ultrasonication. 32 cm3 of 30% hydrogen peroxide was added, and yellow PTA solution was obtained. After 3 h, pH of

Results and discussion

All of the prepared samples were attributed to single-phase anatase TiO2 structure from results of XRD analysis. This coincided with results of our previous study indicating that hydrothermal treatment of PTA solution under a high pHPTA condition induced preferential formation of anatase TiO2 structure [12]. However, primary particle size and shape of the prepared TiO2 strongly depended on pHPTA and MPVA. Fig. 2 shows SEM images and histograms of widths and lengths of the particles obtained

Conclusions

In this paper, we discussed the relationship between photocatalytic activity and particle size of decahedral anatase TiO2 with {1 0 1} and {0 0 1} exposed crystal faces. Anatase TiO2 with well-defined faces showed higher activity, and decahedral anatase with particle width of ca. 40 nm showed excellent activity because of the optimized balance between efficient separation of redox sites and large specific surface area. The results of present study indicate that change in particle size as well as

Acknowledgements

This work was supported by the Knowledge Cluster Initiative, Grant-in-Aid of Young Scientist (B) (22750139) implemented by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), and the JST PRESTO program.

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