(Invited) True Particle-Size Dependence of Photocatalytic Activity of Octahedral-Shaped Anatase Titania

Abstract

Here, attempts to prepare octahedral-shaped anatase titanium(IV) oxide (titania) particles (OAPs) of uniform bulk structure (anatase with negligible non-crystalline components), surface structure (only exposing {101} facets) and particle size are reported as the first and sole example of a "pure" metal-oxide powder. The OAP samples were prepared by sonication-hydrothermal (HT) reaction of potassium-titanate nanowires through the process modified from the previous report [1]. Characterization of the samples by Rietveld analysis of XRD patterns (including non-crystalline phases using an internal standard) [2], reversed double-beam photoacoustic spectroscopy (RDB-PAS; a method measuring energy-resolved distribution of electron traps mainly located on the surface of metal-oxide and the other semiconducting samples) [3,4] and SEM observations revealed the homogeneousness of crystalline bulk, surface and particle size, respectively, to indicate the samples "pure". By controlling the preparation conditions, several OAP samples with different average size ranging of ca. 60–260 nm keeping the purity of the OAP shape. Roughly speaking, the particle-size distribution of those samples was the size of ca. 80% particles in the range of +20% of the average. Photocatalytic activities of those samples in (a) methanol dehydrogenation with in-situ deposited platinum to yield hydrogen, (b) oxidative decomposition of acetic acid under aerobic conditions to yield carbon dioxide and (c) oxygen evolution from deaerated aqueous silver sulfate were examined under ultraviolet-light (> 290 nm) irradiation to find that there seemed no particle-size dependence for the systems (a) hydrogen evolution and (b) carbon-dioxide evolution, while the activity in system (c) to liberate oxygen was almost linearly increased with the particle size [5]. These particle-size dependences of photocatalytic activity seem strange but could be "true" considering the purity of those samples, i.e., almost pure bulk crystal component exposing only anatase{101} facets. The previously reported particle-size dependence might be incorrectly derived due to the unknown effects by the other structural parameters.[1] Molecules 19, 19573 (2014). [2] Catal. Today 313, 218 (2018). [3] Chem. Commun. 52, 12096 (2016). [4] Electrochim. Acta 264, 83 (2018). [5] To be submitted.