Surface electronic states in three-dimensional ${\mathrm{SnO}}_2$ nanostructures

The electronic structure of three-dimensional nanostructures (aerogels) is studied by soft x-ray absorption near-edge structure (XANES) spectroscopy. High-resolution O -edge and Sn - and -edge XANES spectra of monolithic nanocrystalline rutile aerogels with different surface areas (i.e., different surface-to-volume atom fractions) are compared with spectra of full-density rutile and tetragonal SnO. Spectra are interpreted based on the electronic densities of states in calculated with both cluster (self-consistent real-space multiple scattering) and band-structure (linear muffin-tin orbital) methods. Results show that, in contrast to the currently widely accepted picture, the presence of undercoordinated surface atoms not only affects the Fermi level position but also changes the structure of the conduction band by introducing additional Sn-related electronic states close to the conduction band minimum. These additional states are due to oxygen deficiency and are attributed to a surface reconstruction of nanoparticles forming the aerogel skeleton. Results of this study are important for understanding the physical processes underlying the performance of gas sensors based on nanostructures.

Figure 1

Bright-field TEM images of ${\mathrm{SnO}}_2$ aerogels annealed at temperatures of (a) 20, (b) 250, (c) 400, and (d) 550 °C. All four images are of the same magnification.

Figure 2

Oxygen $K$-edge and Sn $M_{4,5}$-edge XANES spectra of ${\mathrm{SnO}}_2$ aerogels annealed at different temperatures as indicated (in °C). Spectra taken from full-density rutile ${\mathrm{SnO}}_2$ (labeled as “bulk”) and tetragonal SnO are also shown for comparison.

Figure 3

(a) Tin $M_{4,5}$-edge XANES spectra for ${\mathrm{SnO}}_2$ aerogels annealed at different temperatures as indicated (in °C). Spectra taken from full-density rutile ${\mathrm{SnO}}_2$ (labeled as “bulk”) and tetrag- onal SnO are also shown for comparison. Angular momentum resolved PDOS of $p$ and $f$ character for Sn atoms in bulk rutile ${\mathrm{SnO}}_2$ calculated by RSMS and LMTO methods are shown in (b) and (c), respectively. The features of $M_5$-edge XANES spectra and PDOS were brought to the same energy scale with the conduction band minimum at zero energy.

Figure 4

(a) Tin $M_3$-edge XANES spectra for ${\mathrm{SnO}}_2$ aerogels annealed at different temperatures as indicated (in °C). A spectrum taken from bulk rutile ${\mathrm{SnO}}_2$ is also shown for comparison. Angular momentum resolved PDOS of $s$ and $d$ character for Sn atoms in bulk rutile ${\mathrm{SnO}}_2$ calculated by RSMS and LMTO methods are shown in (b) and (c), respectively. The XANES spectra and PDOS were brought to the same energy scale with the conduction band minimum at zero energy. The strong peak centered on $\sim 11\mathrm{eV}$ in the figure was at $\sim 719\mathrm{eV}$ in the as-measured spectra.