  A systematic study of electronic structure and band gap states is conducted
to analyze the mono doping and charge compensated codoping of rutile TiO2 with
Rh and Nb, using the DFT+U approach. Doping of rutile TiO2 with Rh atom induces
hybridized O 2p and Rh 4d band gap states leading to a red shift of the optical
absorption edge, consistent with previous experimental studies. Since Rh
mono-doping may induce recombination centers, charge-compensated codoping with
Rh and Nb is also explored. This codoping induces an electron transfer from Nb
induced states to Rh 4d states which suppresses the formation of Rh4+, thereby
leading to a reduction in recombination centers and to the formation of more
stable Rh3+. A combination of band gap reduction by 0.5 eV and the elimination
of band gap states that account for recombination centers makes (Rh,Nb)-
codoped TiO2 a more efficient and stable photocatalyst.
