Probing C and N doped titanium dioxide with hard x-ray photoelectron spectroscopy

Titanium dioxide (TiO₂) is a widely used band-gap semiconductor with vast applications in photocatalysis, photovoltaic, and spintronics.    Recent advances by Japanese researchers has sparked interest in TiO₂ for electro-photo-catalytic splitting of water, since this mineral is a widely used white pigment with UV absorbance characteristics.  The Achilles heal of this application is its large band-gap (3.20 and 3.00 eV for anatase and rutile crystal phases, respectively).  Hence the push toward doping for lower band-gap.  Among the several anionic dopants attempted, N was the most effective for lower band-gap and thus enhance photocatalysis under visible light.  This is made possible by the overlap of the N 2p orbitals with the O 2p orbitals with respect to the energy it would take for a photon to knock an electron out of those orbitals and into the so-called conduction band, where current can be generated.  This makes intuitive sense since N and O are neighbors on the periodic table.  Doping with N can be further enhanced with C.  Doping with only carbon is bad because its 2p orbitals do not overlap with the O 2p orbitals, which would create separate quantum levels for conducting electrons to occupy and not do anything.  However, the C 2p orbital overlaps with the N 2p orbital.  This means we'd have 2p orbitals from two dopant atoms overlap with the oxygen 2p, which gives engineers more flexibility in lowering the TiO₂ band-gap.

Up to then, little data has been collected on these C- and N-doped TiO₂. The paper by Ruzybayev et al attempts to fill those gaps.   Experimental data was acquired via hard x-rays from the National Synchrotron Light Source (NSLS)  in Brookhaven National Laboratory (now closed because NSLS II is opening now).  Hard x-rays have an energy range of 5-15 keV (about the same as medical x-rays), and penetrate into the sample bulk; soft x-rays range 1-5 keV and only penetrate the surface of a sample .  Both types of x-rays are useful, but it depends on the application.  This makes for an interesting paper because my master's thesis was a theoretical study of hard x-rays inducing photoemission from a magnetic multilayer structure.  The technique is known as hard x-ray photoelectron spectroscopy (HXPS).

The experimental section will be skipped for brevity, but the specimens were TiO₂ films that were only 500 nm thick.  That's close to the wavelength of blue light.  The optical band-gap of pure TiO₂ and co-doped TiO₂ is shown in UV-Vis diffuse reflectance spectroscopy data on Figure 1.

Figure 1.  Approximated band-gaps for pure TiO2 and C and N doped TiO2.

Pure TiO₂ has a band-gap of 3.30 eV, as evidenced by the upward slope of the solid curve in Figure 1.  Codoping lowered the band-gap to 2.39 eV, (deeply slanted slope of the dashed curve).  The analysis from HXPS data is displayed in Figure 2.  This can be considered the "meat" of the paper because it explicitly shows how the C and N dopants affect the electronic structure of TiO₂.  "Electronic structure" has many contexts, but the important one here is the oxidation state of titanium.

Figure 2.  HXPS of titanium 2p orbital in pure form and doped with C and N.

The two peaks in the pure TiO₂ spectrum show the characteristic 2p_3/2 and 2p_1/2 spin-orbit split of Ti⁴⁺ (marked A and B in the upper curve, respectively).  The superscript 4+ represents its oxidation state, i.e., the charge experienced by the titanium atom after giving away four electrons to two oxygen atoms (each oxygen atom having two additional electrons).  Doping induced two additional peaks adjacent to the the orbitals.  The 'C' and 'D' peaks represent Ti³⁺ due to the extra net electron contributed by the dopants, which then produces oxygen vacancies.  The vacancies produce an occupied Ti 3d orbital just above the valance band maximum, which is the last quantum state for an electron to occupy before surpassing the band-gap to reach the conduction band.  The jump to the conduction band is the key to generating electricity from light.  The smaller the jump for the electron, the easier it is to generate electricity from a hypothetical TiO₂ cell.

Photoelectron spectroscopy can be measured at ultra-violet wavelengths.  That is useful for measuring the valance band maximum, which the authors have done here.  More specifically, they measured the change in photoelectron kinetic energy relative to the O 2p orbital.

Figure 3.  Valence orbital data of pure, C doped, N doped, and both C and N co-doped TiO₂.  These spectra are measured relative to the O 2p orbital.

Spectra from TiO₂ doped only with N or only with C are included to ascertain the contributions to the altered band-gap by the individual dopants. The key feature is the tailing of the large peak at 5 eV.  The curve for pure TiO₂ is flat at this kinetic energy, but doped TiO₂ curves are still still slanting downward here, with the C and N co-doped curve being the highest one; in jargon, the C and N co-doped curve has the highest valence band maximum.

Experimental data was compared with data calculated from computational models that varied the locations of dopant atoms in a TiO₂ unit cell.  According to Figure 4, what matters is which atoms are the dopant atoms bonded to.  That in turn affects the photoelectron spectra due to the so-called density of states (DOS), which I won't go into because that is too advanced for this blog.  What I can say is the DOS allows you to calculate photoelectron spectra that you then compare to experiment.  If theory  and experiment don't match, try a different model.

Figure 4.  Density of states for co-doped TiO₂ unit cells.  In the insets, blue, light blue, red, and orange spheres represent Ti, O, C, and N atoms, respectively.

Trial-and-error led the authors to conclude that the model in the lower left of Figure 4 was the closest match to the spectra from the experiment, as seen in Figure 5.

Figure 5.  DOS and experimental photoelectron valence band for C and N co-doped TiO₂.  The experimental curve is the green curve from Figure 3.  The red curves are the theoretical photoelectron spectra, and the dark curves (excluding experiment) are the DOS.

That unit cell is represented in the middle spectrum of Figure 5.  It is a fairly good match despite the sloping background of the experimental spectrum.  This has led the authors to conclude that carbon preferentially sticks to titanium, while nitrogen prefers oxygen.  The results show that electronic structure of TiO₂ can be manipulated to decrease the band-gap for photocatalysis.