(Invited) A Fingerprint of Metal Oxide Powders: Characterization and Identification with Energy-Resolved Distribution of Electron Traps
Authors: Ohtani Bunsho , Nitta Akio , Takashima Mai , Takase Mai , Murakami Naoya 
Published: February 2020

Who can identify a powder, e.g., in an unlabeled bottle found in a laboratory? How can fluctuation in the quality of powder prepared by a routine procedure be evaluated? The X-ray diffraction (XRD) pattern may indicate the crystalline structure and composition, and nitrogen-adsorption and particle-size measurements may provide information on some physical/structural properties, but they are not sufficient for identification. This is in contrast to practical identification in the field of organic chemistry, i.e., showing a reasonable fit of elemental composition and the NMR pattern with theoretical/authentic ones. Such a difference in identification for molecules and particles must be due to the existence of surface on particles. For example, it is well known that numerous kinds of titania (titanium(IV) oxide) powders all assigned to anatase crystalline by XRD have different properties and reactivities in chemical reactions occurring on the surface.

Here we propose a method for identification of metal-oxide powders with energy-resolved distribution of electron traps (ERDT), as a fingerprint, measured using newly developed reversed double-beam photoacoustic spectroscopy (RDB-PAS) [1]. RDB-PAS covers a sufficient energy range of ETs with higher resolution than that of the previously reported photochemical method [2]. As a brief explanation of the principle of RDB-PAS, the procedure involves (i) detecting a photoacoustic signal, by modulated LED light (625 nm), induced by accumulation of electrons filling ETs from a deep level to a shallow level under irradiation of scanned (from longer wavelengths to shorter wavelengths) continuous monochromatic light for excitation of valence-band electrons directly to ETs, (ii) differentiating the resultant spectra from the longer wavelength side and converting the signal intensity to absolute density of ETs with reference to results obtained by the photochemical method, (iii) plotting ERDT as a function of energy difference from the top of the VB (VBT) and (iv) comparing with the energy of the CB bottom (CBB), in reference to the VBT, which is estimated by conventional PA spectra (corresponding to photoabsorption spectra) . As a result, both ERDT and CBB are plotted as a function of energy difference from the VBT. The ERDT patterns and conduction-band bottom (CBB) positions, measured by ordinary photoacoustic spectroscopy, for more than 20 commercially available or non-profitably provided titanium(IV) oxide (titania) powders were different from each other depending on the kind of sample. Degrees of coincidence (ζ) of ERDT/CBB were evaluated for a given pair of samples as a product of coincidence of (a) ERDT-pattern matching, (b) total ET density and (c) CBB position. Samples collected from closed position in a bottle gave high ζ, but samples with different code names showed low ζ except for pairs of samples prepared in the same way but coded differently. Furthermore, it was shown that the higher the ζ, the higher the degree of coincidence of photocatalytic activity (ζpc) of titania samples, i.e., photocatalytic activity of titania samples may be governed by ERDT/CBB.

Thus, ERDT/CBB data provide solely and exclusively the way of identification and characterization as well as the way of prediction of performance and/or reactivity of metal-oxide powders as e.g., catalysts or photocatalysts.

[1] Nitta, A., Takase, M., Takashima, M., Murakami, N. and Ohtani, B. A fingerprint of metal oxide powders--Characterization and identification with energy-resolved distribution of electron traps., submitted .

[2] Ikeda, S., Sugiyama, N., Murakami, S.-y., Kominami, H., Kera, Y., Noguchi, H., Uosaki, K., Torimoto, T. and Ohtani, B. Quantitative Analysis of Defective Sites in Titanium(IV) Oxide Photocatalyst Powders. Phys. Chem. Chem. Phys., 5, 778-783 (2003).

Figure 1