Band Gap Narrowing of Titanium Oxide Semiconductors by Noncompensated Anion-Cation Codoping for Enhanced Visible-Light Photoactivity

“Noncompensated $n\mathrm{\text{−}}p$ codoping” is established as an enabling concept for enhancing the visible-light photoactivity of ${\mathrm{TiO}}_2$ by narrowing its band gap. The concept embodies two crucial ingredients: The electrostatic attraction within the $n\mathrm{\text{−}}p$ dopant pair enhances both the thermodynamic and kinetic solubilities, and the noncompensated nature ensures the creation of tunable intermediate bands that effectively narrow the band gap. The concept is demonstrated using first-principles calculations, and is validated by direct measurements of band gap narrowing using scanning tunneling spectroscopy, dramatically redshifted optical absorbance, and enhanced photoactivity manifested by efficient electron-hole separation in the visible-light region. This concept is broadly applicable to the synthesis of other advanced functional materials that demand optimal dopant control.

Figure 1

Density of states (DOS) of anatase ${\mathrm{TiO}}_2$ for different $n\mathrm{\text{−}}p$ codoping pairs: (a) Cr-N resulting in net $n$-type doping; (b) V-C resulting in net $p$-type doping; (c) V-N resulting in compensated codoping; and (d) Cr-C resulting in compensated codoping. The dashed lines designate pure anatase ${\mathrm{TiO}}_2$.

Figure 2

(a) and (b) Calculated relative formation energy for all possible combinations of interstitial and substitutional configurations of Cr-N and V-C in anatase ${\mathrm{TiO}}_2$ as a function of the chemical potential of Ti and O. (c) Calculated kinetic barriers for a single Cr (the blue dashed line), a single N (the red dashed line), and a Cr-N pair (the black solid line) going from interstitial to substitutional sites in anatase ${\mathrm{TiO}}_2$. (d) and (e) X-ray photoelectron spectroscopy measurements of the Cr and N concentrations in anatase ${\mathrm{TiO}}_2$ nanocrystals as a function of the annealing temperature. The numbers in the legend designate the nominal at.% of Cr in the precursor.

Figure 3

(a) UV-vis diffuse reflectance spectra of pure and doped anatase ${\mathrm{TiO}}_2$ nanocrystals annealed at $500°\mathrm{C}$. The numbers in the legend designate the nominal at.% of Cr in the precursor. (b) STS spectra of pure and doped anatase ${\mathrm{TiO}}_2$. The $I\mathrm{\text{−}}V$ curves are shifted vertically for better visualization. The inset displays a STM image of undoped ${\mathrm{TiO}}_2$ nanocrystal surface. (c) The red line shows the effect of charge generation on the EPR signal from 1 at.% Cr-N codoped anatase ${\mathrm{TiO}}_2$ nanocrystals induced by $\le 2.5\mathrm{eV}$ light illumination. The green line shows the recovery of the EPR signal after the light is turned off. (d) The narrower EPR linewidth $\Delta$ for 1, and 2.5 at.% Cr-N codoping than for Cr only doping is indication of enhanced photocatalytic efficiency in Cr-N codoping.