Band-gap measurements of bulk and nanoscale hematite by soft x-ray spectroscopy

Chemical and photochemical processes at semiconductor surfaces are highly influenced by the size of the band gap, and ability to control the band gap by particle size in nanomaterials is part of their promise. The combination of soft x-ray absorption and emission spectroscopies provides band-gap determination in bulk and nanoscale itinerant electron semiconductors such as CdS and ZnO, but this approach has not been established for materials such as iron oxides that possess band-edge electronic structure dominated by electron correlations. We performed soft x-ray spectroscopy at the oxygen $K$-edge to reveal band-edge electronic structure of bulk and nanoscale hematite. Good agreement is found between the hematite band gap derived from optical spectroscopy and the energy separation of the first inflection points in the x-ray absorption and emission onset regions. By applying this method to two sizes of phase-pure hematite nanoparticles, we find that there is no evidence for size-driven change in the band gap of hematite nanoparticles down to around 8 nm.

Figure 1

Synchrotron powder XRD. (a) Comparison of all XRD data. The inset shows a detectable lattice expansion in the smallest particles that is quantified by unit-cell refinement. (b) Experimental and simulated XRD patterns for the best-fit results of unit-cell refinement to the 8 nm nanoparticle data.

Figure 2

PDF analysis of the real-space structures of bulk and nanoscale hematite. (a) Stacked plot of the short-range region of the PDF curves. The vertical bar at approximately $3.4\text{Å}$ highlights the third shell (Fe-O) correlations that are slightly broadened in the 8 nm nanoparticles. (b) Experimental (markers) and best-fit (dark line) PDF for all samples. Residuals (dashed line) are given below each pattern.

Figure 3

The Kebulka-Munk function, $F\left(r\right)$, obtained from UV-visible diffuse reflectance spectra of powders of bulk and nanoparticulate hematite. The vertical line indicates the inflection point in the nanoparticle absorption spectra determined from the first derivative plotted below, which lies at 562 nm (2.2 eV).

Figure 4

Soft XAS and XES of bulk hematite at the oxygen $K$ edge. (a) The bulk hematite band gap, $E_g$, is determined from the XAS and nonresonant XES spectra by taking the derivative to locate the inflection points in the valence-band and conduction-band onsets. Using this approach, $E_g=2.2\text{eV}$. (b) O $K$-edge XAS and resonant and nonresonant XES.

Figure 5

Oxygen $K$-edge XES and XAS of bulk hematite and ${\text{Fe}}_2{\text{O}}_3$ nanoparticles. (a) Comparison of XAS and XES data stacked vertically for clarity. (b) Comparison of the XAS pre-edge region. (c) Example peak fit to the pre-edge for the 8 nm nanoparticle sample.