Nanoarchitecture of semiconductor titania nanosheets revealed by polarization-dependent total reflection fluorescence X-ray absorption fine structure

The nanoarchitecture of unilamellar titania nanosheets was studied by X-ray absorption fine structure (XAFS) analysis utilizing two different geometrical arrangements, s- and p-polarization. In total reflection fluorescence mode using synchrotron radiation X-ray, an XAFS signal was detected in high yield for monolayer films of nanosheets having a thickness of only similar to10 Angstrom, which were deposited via electrostatic self-assembly. Obtained extended X-ray absorption fine structure oscillations were dependent on the polarization due to the two-dimensional anisotropy of the nanosheets. This dependence was helpful in analysis of the nanosheet architecture, which was conducted on the basis of free energy force field (FEFF) simulations of the parent layered titanate. Satisfactory curve-fitting of the EXAFS data of the unilamellar nanosheet was achieved. The obtained results revealed the elongation of some interatomic distances, particularly those having a component along the nanosheet normal, when compared with the crystal structure data for the parent layered titanate. This structural modification led to 4% expansion in sheet thickness upon exfoliation, which can be understood by mitigation of electrostatic repulsion between titanium ions in the negatively charged nanosheet and its countercations.