Influence of Morphology of Facetted Anatase Titania Particles on Their Photocatalytic Activity

Introduction  Decahedral anatase titania particles (DAPs) have been prepared and classified as a highly active photocatalyst [1-2].  DAPs are characterized by the presence of {001} and {101} facets.  The effect of particle morphology on their photocatalytic activity is still under discussion.  In the present study, DAPs were prepared by the gas-phase oxidation of titanium(IV) chloride (TiCl₄).  The purpose of this study is to clarify the influence of particle morphology on photocatalytic activity using various DAP samples prepared under different synthesis conditions.

Experimental  The preparation of DAPs was performed by rapid heating (around 1373 K) and quenching of mixture of TiCl₄ and oxygen [3].  Particle aspect ratio d₀₀₁/d₁₀₁ (PAR) was estimated by X-ray diffraction measurements.  Through the analysis of images of scanning electron microscopy (SEM), DAPs were classified into the following groups.  DAP: particles with decahedral shape and DAP-like: particles with decahedral shape with noticeable defects and/or mounds.  Three reaction systems, (a) oxidative decomposition of acetic acid (CO₂ system), (b) dehydrogenation of methanol with in situ photodeposition of  platinum (H₂ system), and (c) oxygen evolution from an aqueous solution of silver salt (O₂ system) were chosen.  Samples were irradiated at > 290 nm by a 400-W high-pressure mercury lamp.

Results and discussion  Among DAP samples of various structural properties prepared under regulated reaction conditions, e.g., line speed ratio of TiCl₄ and O₂ flows in the reactor, four samples of similar specific surface area (SSA; 14.8–16.0 m² g^-1) and particle size (64–67 nm) were chosen to make the following discussion on the effect of morphology simpler.

               PAR dependence of relative photocatalytic activity with reference to that of Evonik P25 was studied.  It is rather unclear, but the activities in CO₂, H₂ and O₂ systems seemed decreasing, increasing and almost constant, respectively, with increasing PAR.  On the other hand, those seemed increasing, decreasing and almost constant, respectively, with increasing DAP content as the DAP-content dependence of photocatalytic activity.  These tendencies suggest that the higher the PAR, the lower the DAP content and the photocatalytic activities of DAPs depends on the particle morphology since the other structural properties such as SSA and particle size are almost the same for these four samples.

Conclusions  Checking the dependences of photocatalytic activities of DAPs of almost same specific surface area and particle size on particle aspect ratio and DAP content, it is suggested that particle morphology itself gives influence on the activity.

References  [1] Chem. Mater. 2009, 21, 2601-2603.  [2] Top. Catal. 2010, 53, 455-461.  [3] Catal. Today 2011, 164, 391.Introduction  Decahedral anatase titania particles (DAPs) have been prepared and classified as a highly active photocatalyst [1-2].  DAPs are characterized by the presence of {001} and {101} facets.  The effect of particle morphology on their photocatalytic activity is still under discussion.  In the present study, DAPs were prepared by the gas-phase oxidation of titanium(IV) chloride (TiCl₄).  The purpose of this study is to clarify the influence of particle morphology on photocatalytic activity using various DAP samples prepared under different synthesis conditions.

Experimental  The preparation of DAPs was performed by rapid heating (around 1373 K) and quenching of mixture of TiCl₄ and oxygen [3].  Particle aspect ratio d₀₀₁/d₁₀₁ (PAR) was estimated by X-ray diffraction measurements.  Through the analysis of images of scanning electron microscopy (SEM), DAPs were classified into the following groups.  DAP: particles with decahedral shape and DAP-like: particles with decahedral shape with noticeable defects and/or mounds.  Three reaction systems, (a) oxidative decomposition of acetic acid (CO₂ system), (b) dehydrogenation of methanol with in situ photodeposition of  platinum (H₂ system), and (c) oxygen evolution from an aqueous solution of silver salt (O₂ system) were chosen.  Samples were irradiated at > 290 nm by a 400-W high-pressure mercury lamp.

Results and discussion  Among DAP samples of various structural properties prepared under regulated reaction conditions, e.g., line speed ratio of TiCl₄ and O₂ flows in the reactor, four samples of similar specific surface area (SSA; 14.8–16.0 m² g^-1) and particle size (64–67 nm) were chosen to make the following discussion on the effect of morphology simpler.

PAR dependence of relative photocatalytic activity with reference to that of Evonik P25 was studied.  It is rather unclear, but the activities in CO₂, H₂ and O₂ systems seemed decreasing, increasing and almost constant, respectively, with increasing PAR.  On the other hand, those seemed increasing, decreasing and almost constant, respectively, with increasing DAP content as the DAP-content dependence of photocatalytic activity.  These tendencies suggest that the higher the PAR, the lower the DAP content and the photocatalytic activities of DAPs depends on the particle morphology since the other structural properties such as SSA and particle size are almost the same for these four samples.

Conclusions  Checking the dependences of photocatalytic activities of DAPs of almost same specific surface area and particle size on particle aspect ratio and DAP content, it is suggested that particle morphology itself gives influence on the activity.

References  [1] Chem. Mater. 2009, 21, 2601-2603.  [2] Top. Catal. 2010, 53, 455-461.  [3] Catal. Today 2011, 164, 391.