UV irradiation effect on Al‐doped anatase titanium dioxide

Abstract

Electron paramagnetic resonance (EPR) measurement under uv light irradiation was performed on Al‐doped anatase TiO2 crystals grown by a chemical vapor transport method. In an as‐grown crystal, characteristic signals of EPR sextuplets are observed at 100–40 K. The intensity of the sextuplets decreases gradually with decreasing temperature. The sextuplets arise from a photogenerated hole of S =1/2 interacting with an Al nuclear spin of I =5/2. The angular dependence of the sextuplets is successfully explained by a model where four symmetrically equivalent paramagnetic species giving the sextuplets are present in the crystal and the principal z‐axis of a diagonal g‐matrix for the paramagnetic species has a small deviation from the crystalline c‐axis. The analysis of the angular dependence of the sextuplets gives anisotropic g ‐values of gxx=2.0254, gyy=2.0016, and gcomplexes=2.0129, and hyperfine coupling constants of Axx=4.93, Ayy=6.04, and Acomplexes=5.98 G. An EPR signal from bulk Ti3+ is also observed at temperatures below 40 K regardless of uv irradiation. In an annealed crystal, the sextuplets are observed at temperatures below 70 K under uv irradiation. The EPR signal corresponding to bulk Ti3+ is absent in the entire temperature range. These experimental results differ greatly from those for the as‐grown crystal (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photogenerated electron- and hole-trap distributions in Al-doped anatase TiO2 crystals measured by cryogenic EPR imaging

“…The inset of figure 3 shows the EPR spectra of sample A at 118 K and 47 K under UV-light irradiation with a magnetic field nearly parallel to the [100] axis (H~// [100]). By aluminium doping, trap sites for both electrons and holes are created [23,24]. EPR signal of trapped holes shows the sextuplet spectrum resulting from the hyperfine interaction (HFI) with a nuclear spin of Al (I = 5/2), because holes are trapped at the trivalent Al site, which is substitutional to tetravalent Ti (See figure 1(c)).…”

“…The EPR spectrum changes depending on the direction of the applied magnetic field owing to the anisotropy of the HFI and g-values [23,24]. The angular dependence of the EPR signals in the ac-plane of sample B is shown in figure S3 in the Supplementary Information.…”

“…The angular dependence of the EPR signals in the ac-plane of sample B is shown in figure S3 in the Supplementary Information. We figured out the crystal orientations indicated in figure 2 and S1 and S2 from EPR signal's angular dependence in each sample compared to the reported ones [23,24]. The analysis of visible edges of crystals in microscopic photographs helps further determination, for example, we measured the angle between the edges of the surface plane in sample A to be 42°and identified it as the (101) plane with edges of the [111̅ ] and [1 1 1…”

“…̅ ̅ ], the EPR spectra of electrons and holes (figure 7(d)) were sufficiently separated owing to the anisotropy of HFI and g-values [23,24] to obtain an individual image of electrons or holes by post-filtering processes. Figures 7(b ̅ ) plane on the top left, and the (0 1 1 ̅ ) plane on the bottom.…”