Dependence of photocatalytic activity on aspect ratio of a brookite TiO2 nanorod and drastic improvement in visible light responsibility of a brookite TiO2 nanorod by site-selective modification of Fe3+ on exposed faces

doi:10.1016/j.molcata.2014.09.036

Journal of Molecular Catalysis A: Chemical

Volume 396, January 2015, Pages 261-267

https://doi.org/10.1016/j.molcata.2014.09.036 Get rights and content

Highlights

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Pure brookite TiO₂ was synthesized by a hydrothermal method.
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Reaction sites on brookite TiO₂ nanorods were separated by controlling of its morphology.
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Brookite TiO₂ nanorods loaded with Fe³⁺ ions showed visible light activity.

Abstract

Exposed crystal face-controlled brookite titanium(IV) oxide (TiO₂) nanorods with various aspect ratios were prepared by a hydrothermal process with or without PVA or PVP as an aspect reagent. The nanorod-shaped brookite TiO₂ had larger {2 1 0} and smaller {2 1 2} exposed crystal faces, which were assigned by TEM with the SAED technique. Their aspect ratios were greatly influenced by the addition of PVA or PVP as an aspect ratio control reagent to the reaction solution used in the hydrothermal treatment. The photocatalytic activity for decomposition of acetaldehyde increased with increase in the aspect ratio because the surface area ratio of {2 1 0} to {2 1 2} exposed crystal faces, which are attributed to reduction and oxidation sites, respectively, became more optimal.

The {2 1 2} exposed crystal faces of surface-controlled brookite TiO₂ were site-selectively modified with trivalent iron(III) (Fe³⁺) ions by utilizing the adsorption property of iron(III)/iron(II) (Fe³⁺/Fe²⁺) ions. The brookite TiO₂ nanorod with site-selective modification of Fe³⁺ ions showed much higher photocatalytic activity than that of commercial brookite TiO₂ loaded with Fe ions under visible-light irradiation because of the separation of redox sites. In other words, oxidation and reduction proceed over Fe³⁺ ion-modified {2 1 2} faces of the TiO₂ surface and on {2 1 0} faces of the TiO₂ surface without modification of Fe³⁺, respectively.

Graphical abstract

Introduction

Much interest has recently been shown in environmental purification by utilizing the self-cleaning effect of a semiconductor photocatalyst [1], [2]. Among the various kinds of semiconductor photocatalyst, titanium(IV) oxide (TiO₂) is the most suitable photocatalyst for application to environmental purification from the viewpoint of chemical stability, availability and no toxic properties. Photocatalytic activity over semiconductor particles is thought to depend on the physical and chemical properties of the photocatalyst, such as its crystal structure, specific surface area, particle size, and defect density [3]. Therefore, optimizing these properties has been the conventional strategy for enhancing photocatalytic activity, though the most suitable property to optimize for a specific reaction differs depending on the reaction substrate.

Since the report on titanium(IV) oxide (TiO₂) with specific exposed crystal faces by Ohno et al. [4], various groups have reported TiO₂ with control of exposed crystal faces [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. Our group has succeeded in synthesizing shape-controlled anatase rutile and brookite TiO₂ with photocatalytic properties depending on specific exposed crystal faces due to control of reaction sites [20], [21], [22], [23]. The results imply that the exposed crystal face of a TiO₂ photocatalyst is an important factor for improvement of its photocatalytic activity. This is reasonable since the surface energy, chemical surface state, and number and energy state of defective sites predominantly depend on the atomic arrangement of exposed crystal faces [24], [25], [26].

Decahedral TiO₂ having an anatase phase with {1 0 1} and {0 0 1} exposed crystal faces [4], [5], [8], [10], [11], [12], [13], [14], [23], shape-controlled TiO₂ having a brookite phase [27], [28], [29], [30], [31], [32], and dodecahedral rutile TiO₂ with {1 1 0} and {1 0 1} or {1 1 0} and {1 1 1} exposed crystal faces [6], [7], [20] have been synthesized as well-defined TiO₂ particles. Formation of these exposed crystal faces requires the use of inorganic or organic compounds as shape-control reagents [6], [7], [8], [11], [20], [23] specific precursors [9], [13], [14] and specific preparation conditions [12], [29], [30], [31], [32].

Ohno et al. reported that the photocatalytic activity of shape-controlled TiO₂ nanorods having a rutile phase depended on their exposed crystal faces, which could be controlled by using shape-control reagents or chemical etching [21]. The exposed crystal faces of shape-controlled rutile TiO₂ nanorods were assigned to {1 1 0} and {1 1 1} crystal faces, which were respectively attributed to reduction and oxidation sites [20]. In addition, shape-controlled rutile TiO₂ rods with various aspect ratios were successfully prepared by a two-step synthesis involving hydrolysis and hydrothermal processes, and their photocatalytic activities were shown to strongly depend on the aspect ratio [33].

In the present study, we prepared shape-controlled brookite TiO₂ nanorods with various aspect ratios by a hydrothermal process in the presence of a shape-control agent (polyvinyl alcohol (PVA) and/or polyvinyl pyrrolidone (PVP)). The exposed crystal faces of brookite TiO₂ nanorods were successfully assigned to crystal faces that are different from those of rutile TiO₂ nanorods. Reactivity on the exposed crystal faces was also evaluated. The exposed crystal faces of brookite TiO₂ nanorods were analyzed as a function of the aspect ratio of shape-controlled brookite TiO₂ nanorods, and the dependence of photocatalytic activity for acetaldehyde decomposition on the aspect ratio was examined. In addition, shape-controlled brookite TiO₂ nanorods with specific exposed crystal faces, which had quite high visible light activity, were modified with iron(III) (Fe³⁺) ions. In our previous study, modification of rutile TiO₂ with Fe³⁺ ions greatly improved photocatalytic reaction under visible-light irradiation because the Fe³⁺ ions worked as sensitizers for visible light. Moreover, characteristic adsorption properties of Fe³⁺/Fe²⁺ ions on TiO₂, which are strongly dependent on valence states of iron ions, induced site-selective adsorption on specific exposed crystal faces of brookite TiO₂ nanorods.

Section snippets

Materials

A titanium precursor (titanium ethoxide) was purchased from Sigma–Aldrich. Hydrogen peroxide (30%), ammonia (25%), glycolic acid, polyvinyl alcohol (PVA), and polyvinyl pyrolidone (PVP) were purchased from Wako Pure Chemical Industries, Ltd. Titanium(IV) bis(ammonium lactate) dihydroxide (TALH) and urea were purchased from Sigma–Aldrich. Glycolic acid was used to control the crystallinity and morphology of brookite TiO₂ particles. Commercial brookite TiO₂ powder was obtained from Kojundo

Morphology analysis of prepared brookite TiO₂ nanorods

Fig. 2 shows XRD patterns of the prepared brookite TiO₂ with and without PVA or PVP (50 mg) and also brookite TiO₂ using TALH as a starting material. All prepared samples were attributed to a single-phase brookite TiO₂ structure.

Fig. 3 shows a TEM image of brookite TiO₂ without a polymer. Rod-like morphology of brookite TiO₂ with a length of 100 nm, width of 25 nm and an aspect ratio of 2.7 was observed. Relative surface area of the prepared brookite TiO₂ nanorod was 47 m² g⁻¹. Assignment of exposed

Conclusions

Morphology-controlled brookite TiO₂ nanorods with a wide range of aspect ratios (AR ∼ 1.6–5.2) were successfully prepared. The exposed crystal surfaces of brookite TiO₂ nanorods were assigned to {2 1 0} and {2 1 2}. Oxidation reaction predominantly proceeded on the {2 1 2} facet, while the {2 1 0} facet was assigned to a reduction site, resulting in the achievement of a charge separation and an improvement of photocatalytic activity. PVP and PVA were effective reagents for controlling the aspect ratio of

Acknowledgements

This work was supported by the JST PRESTO program and the JST ACT-C program.

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Dependence of photocatalytic activity on aspect ratio of a brookite TiO2 nanorod and drastic improvement in visible light responsibility of a brookite TiO2 nanorod by site-selective modification of Fe3+ on exposed faces

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Morphology analysis of prepared brookite TiO2 nanorods

Conclusions

Acknowledgements

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

J. Cryst. Growth

J. Mol. Catal. A: Chem.

Appl. Catal. B: Environ.

Appl. Catal. A: Gen.

Catal. Commun.

Appl. Catal. B: Environ.

Appl. Catal. A: Gen.

Chem. Rev.

Chem. Lett.

New J. Chem.

Chem. Lett.

J. Am. Chem. Soc.

J. Am. Chem. Soc.

Chem. Commun.

Cryst. Growth Des.

Nano Lett.

Chem. Mater.

Chem. Commun.

Dependence of photocatalytic activity on aspect ratio of a brookite TiO₂ nanorod and drastic improvement in visible light responsibility of a brookite TiO₂ nanorod by site-selective modification of Fe³⁺ on exposed faces

Morphology analysis of prepared brookite TiO₂ nanorods