A DFT+U study of Rh, Nb codoped rutile TiO2

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.

💡 Research Summary

This paper presents a comprehensive first‑principles investigation of the electronic structure modifications induced by rhodium (Rh) mono‑doping and rhodium‑niobium (Rh,Nb) charge‑compensated co‑doping in rutile TiO₂, using the DFT+U methodology. The authors first validate their computational setup by reproducing the experimental band gap of pristine rutile TiO₂ (~3.0 eV) with Hubbard‑U corrections of 4.5 eV for Ti 3d and 6.0 eV for O 2p states, thereby ensuring an accurate description of the strongly correlated d‑electrons.

In the Rh‑doped system, Rh substitutes Ti⁴⁺ sites, leading to a mixed oxidation state (Rh³⁺/Rh⁴⁺). The calculations reveal that Rh 4d orbitals strongly hybridize with O 2p states, generating occupied defect levels within the band gap at approximately 1.5–2.0 eV above the valence band maximum. These mid‑gap states cause a red‑shift of the optical absorption edge, consistent with experimental observations of enhanced visible‑light activity. However, the same states act as recombination centers, facilitating non‑radiative electron‑hole recombination and potentially limiting photocatalytic efficiency.

Niobium doping alone introduces Nb⁵⁺ in place of Ti⁴⁺, supplying an extra electron that creates shallow donor states just below the conduction band minimum. This modestly raises the carrier concentration and slightly improves conductivity but does not significantly alter the band gap.

The core of the study focuses on Rh,Nb co‑doping. Two structural motifs are examined: (i) Rh and Nb occupying adjacent Ti sites and (ii) Rh and Nb separated by several lattice spacings. Both configurations are energetically favorable, with the adjacent arrangement being marginally more stable. Bader charge analysis demonstrates a clear electron transfer from Nb‑derived donor states to Rh‑related acceptor states. Consequently, Rh is stabilized in the +3 oxidation state, and the Rh‑induced mid‑gap states disappear. The overall band gap contracts by about 0.5 eV, moving the absorption edge into the visible region (~500 nm). Importantly, the elimination of recombination centers is predicted to enhance charge separation and prolong carrier lifetimes.

Structural relaxation shows that the lattice distortion caused by the Rh‑Nb pair is minimal (energy penalty < 0.2 eV), indicating that the co‑doped material can be synthesized without severe crystallographic disruption. The authors discuss practical synthesis routes such as sol‑gel processing, hydrothermal methods, and plasma‑assisted doping, and they propose that the co‑doped material should be tested in standard photocatalytic reactions (e.g., degradation of organic dyes, hydrogen evolution from water splitting) to validate the theoretical predictions.

In conclusion, the DFT+U results provide a mechanistic explanation for the superior photocatalytic performance of Rh,Nb co‑doped rutile TiO₂: (1) a modest band‑gap narrowing that extends light absorption into the visible spectrum, (2) removal of Rh‑related recombination centers through charge compensation, and (3) preservation of the rutile lattice integrity. The study highlights charge‑compensated co‑doping as a general strategy for engineering transition‑metal‑oxide photocatalysts with enhanced efficiency and stability.