Checking the Synergetic Effect between Anatase and Rutile

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Checking the Synergetic Effect between Anatase and Rutile Rui Ma, and Tao Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b03381 • Publication Date (Web): 17 Jul 2019 Downloaded from pubs.acs.org on July 22, 2019

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The Journal of Physical Chemistry

Checking the Synergetic Effect between Anatase and Rutile Rui Ma and Tao Chen*

School of Marine Science and Environment Engineering, Dalian Ocean University, 52 Heishijiao Street, Dalian 116023, China

ABSTRACT

To check the synergetic effect between anatase and rutile, a series of anatase-rutile coupled TiO2 photocatalysts were prepared by calcination of the physical mixtures of amorphous TiO2 and rutile. These catalysts were characterized by SEM, XRD, and UVVis diffuse reflectance spectroscopy. The photocatalytic activities of these mixed-phase photocatalysts were evaluated with photocatalytic degradation of methylene blue and photocatalytic reforming of methanol for H2 production. The dependence of the photocatalytic activities on anatase content, different sources of irradiation light and

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anatase-rutile interacting strength was investigated in detail. All of these results show that the photocatalytic activity increases with the increasing of anatase content in the mixedphase photocatalysts, which indicates that the synergetic effect between anatase and rutile doesn’t work in these TiO2 photocatalytic systems.

INTRODUCTION

In the past decades, TiO2 photocatalyst has received considerable attention because of its potential applications in treatment of environmental pollutants1 and utilization of solar energy.2 Two major crystalline phases of TiO2, anatase and rutile, have been widely studied. Anatase usually shows higher photocatalytic activity than rutile,3-5 but rutile shows higher activity than anatase for a few reactions exemplified by photocatalytic water oxidation.6-8 More interesting is the phenomenon that the mixed-phase of anatase and rutile TiO2 photocatalysts show higher photocatalytic activities than either pure anatase or pure rutile,9-15 which is called the synergetic effect between anatase and rutile. The synergetic effect between anatase and rutile was also observed in TiO2 dye sensitized solar cells.16


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Due to the natural research interests and the motivations of further understanding the mechanism of photocatalysis on TiO2 so as to develop more effective photocatalysts, many researchers have devoted to studying the synergetic effect between anatase and rutile. Charge transfer between anatase and rutile which can promote the separation of photogenerated charge carriers has been considered to be the intrinsic mechanism for the synergetic effect.9-11, 17-20 The intimate contact between anatase and rutile particles is a prerequisite for the efficient charge transfer. Because of the more negative potential for the conduction band of anatase,21 many researchers proposed that the synergetic effect is owing to the electron transfer from anatase to rutile.10, 11, 17, 19, 22, 23 Kawahara et al. 24 and Shen el al.

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gave direct experimental evidences for the electron transfer from

anatase to rutile. However, the opposite direction of the electron transfer (from rutile to anatase) has been observed by other researchers.16, 26, 27 Yang et al. reported that both directions of the electron transfer between anatase and rutile are possible.28 Hole transfer between anatase and rutile was also reported.29 Besides the charge transfer models, other mechanistic models have also been proposed to explain the synergetic effect between anatase and rutile. Ohno et al. reported


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that the rutile TiO2 photocatalyst has poor activity for the reduction of O2, and the presence of a small amount of an anatase component is crucial for an efficient photocatalytic reaction on TiO2 particles using O2 as the electron acceptor.30 Xu’s group reported that the intrinsic activity of rutile is masked by its poor ability for O2 adsorption31and the O2 transfer from anatase to rutile14 rather than the interparticle charge transfer is the main reason for the high activity of mixed-phase TiO2 in an aerated aqueous solution. Although a large number of efforts have been devoted to unravelling the mechanism of the synergetic effect between anatase and rutile, it should be noted that the opinions on the most fundamental question in this topic of whether does this synergetic effect really exist are still in debates. Bickley et al. first proposed the synergetic effect between anatase and rutile to explain the outstanding activity of Degussa P25, a standard TiO2 photocatalyst consisting of anatse, rutile and a small amount of amorphous titania.32 However, Ohtani et al.33and Apopei et al.34suggested a less-probable synergetic effect in P25 after investigating the pure anatase and rutile isolated from P25. Ohno et al. observed an obvious synergetic effect by physically mixing of small size anatase particles (1:1 A-R-T photocatalysts. This order does not change for the photocatalysts calcined at 750 oC in which a portion of anatase already


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transformed into rutile. All of these results indicate that the synergetic effect between anatase and rutile doesn’t exist at all in the common TiO2 photocatalytic systems. It is noteworthy that this conclusion excludes the contribution of the charge transfer between anatase and rutile to the activity of anatase-rutile mixed phase photocataysts in the common TiO2 photocatalytic systems but not the charge transfer itself. In another word, the interparticle charge transfer between anatase and rutile which is widely proposed in literature may proceed efficiently, but its contribution to the activity improvement is quite limited. Our finding seems to be less important because it excludes rather than provides a strategy for developing improved photocatalysts, but we believe that it is the truth we have to face and it gives a clear answer to the basic question on the synergetic effect between anatase and rutile that has been in debates for a long time. CONCLUSIONS A series of anatase-rutile coupled TiO2 photocatalysts were successfully prepared by calcination of the physical mixtures of amorphous TiO2 and rutile, which makes it possible for the fair activity comparison of the anatase-rutile mixed phase TiO2 with pure phase TiO2 photocatalysts. The photocatalytic activities of these photocatalysts always


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increase with the increasing of anatase content in the mixed-phase photocatalysts, which suggests that the synergetic effect between anatase and rutile doesn’t exist in the common TiO2 photocatalytic AUTHOR INFORMATION

Corresponding Author * E-mail address: [email protected], Tel : 86-15841130650 ACKNOWLEDGMENT

This work was supported by the Guidance Program of Natural Science Foundation of Liaoning Province (201602097) and the Open Project Program of the State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics (N-14-03). .

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TOC Graphic

improve activity?

Charge Transfer

eh+

eh+ Anatase

high activity

h+

h+ e-

e-

h+ Rutile

e-

low activity


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Checking the Synergetic Effect between Anatase and Rutile

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