  We demonstrate experimentally a chemical codoping approach that would
simultaneously narrow the band gap and control the band edge positions of oxide
semiconductors. Using TiO2 as an example, we show that a sequential doping
scheme with nitrogen (N) leading the way, followed by phosphorous (P), is
crucial for the incorporation of both N and P into the anion sites. Various
characterization techniques confirm the formation of the N-P bonds, and as a
consequence of the chemical codoping, the band gap of the TiO2 is reduced from
3.0 eV to 1.8 eV. The realization of chemical codoping could be an important
step forward in improving the general performance of electronic and
optoelectronic materials and devices.
