Although organic compounds are practically identified by their elemental compositions and nuclear magnetic resonance (NMR) patterns, metal-oxide powders cannot be identified by bulk composition and crystalline structure. Such a difference between molecules and powders is attributable to existence of both surface and bulk for metal-oxide powders. What has been conventionally evaluated as structural properties of metal-oxide powders is crystal phase, primary particle diameter, secondary particle diameter and specific surface area, but those properties reflect only bulk composition or size. In other words, a lack of sufficient analytical method that enables measurement of a property reflecting surface structure has prevented characterization and thereby identification of metal-oxide powders. In this study, we focus on energy-resolved density of electron traps (ERDT) located mainly on the surface of metal-oxide powders, and evaluate ERDT in various particulate samples as a function of energy from valence-band top (VBT) by reversed double-beam photoacoustic spectroscopy (RDB-PAS), in which photoabsorption of trapped electrons directly excited from valence band to electron traps by scanned continuous light is measured by PAS [1].

Representative ERDT patterns of titania powders are shown in Fig. 1. Conduction-band bottom (CBB) depending on the crystalline phase and total density of electron traps increasing with increase in the specific surface area indicate that CBB and total density of electron traps can reflect bulk composition and bulk size, respectively. On the other hand, ERDT patterns of TIO-1 and TIO-13 seem to be clearly different. This result indicates that ERDT can reflect surface structural property. By calculating degree of coincidence z, multiplied by each degree of coincidence for ERDT pattern matching, total density of electron traps and CBB position, some given pairs of two samples which are made by the same manufacturing method show higher z values than those of other pairs. Therefore, it is suggested that an ERDT/CBB pattern can be a fingerprint for metal-oxide powders. Evaluation of VBT by observing ERDT of mixture sample is also discussed.

Fig. 1 Representative ERDT/CBB patterns as energy function from VBT for titania powders.

[1] A. Nitta, M. Takase, M. Takashima, N. Murakami, B. Ohtani, Chem. Commun., 52, 12096-12099 (2016).