Dispelling the Myth of Passivated Codoping in TiO₂

Abstract

Modification of TiO2 to increase its visible light activity and promote higher performance photocatalytic ability has become a key research goal for materials scientists in the past 2 decades. One of the most popular approaches proposed this as “passivated codoping”, whereby an equal number of donor and acceptor dopants are introduced into the lattice, producing a charge neutral system with a reduced band gap. Using the archetypal codoping pairs of [Nb + N]- and [Ta + N]-doped anatase, we demonstrate using hybrid density functional theory that passivated codoping is not achievable in TiO2. Our results indicate that the natural defect chemistry of the host system (in this case n-type anatase TiO2) is dominant, and so concentration parity of dopant types is not achievable under any thermodynamic growth conditions. The implications of passivated codoping for band gap manipulation in general are discussed.

Photocatalytic, structural and optical properties of mixed anion solid solutions Ba3Sc2−xInxO5Cu2S2 and Ba3In2O5Cu2S2−ySey

“…† The majority of the O 2p states lie around À6 eV to À2 eV for each compound, owing to the electronegativity of oxygen and the reason why typically in metal oxide photocatalysts such as TiO 2 the VBM lies deep compared to the redox potentials of water. 51 It is expected that a better bulk band alignment will exist for each of these compounds due to the VBM moving towards the vacuum due to the overlap between Cu 3d and Ch p. This has the added effect of increasing dispersion at the VBM giving rise to lower effective masses. For Ba 3 Sc 2 O 5 Cu 2 S 2 , Ba 3 -In 2 O 5 Cu 2 S 2 and Ba 3 In 2 O 5 Cu 2 Se 2 , the direct fundamental band gaps are modelled as 3.04 eV, 1.37 eV and 1.09 eV respectively and are direct (at G) with the exception of Ba 3 Sc 2 O 5 Cu 2 S 2 where the VBM occurs at G and the CBM at the X point.…”

Computational prediction of the thermoelectric performance of LaZnOPn (Pn = P, As)

“…Conversely, n-type materials tend to possess high electron affinities and lower-lying conduction band maxima. 97,112 In the case of these systems, the ionisation potentials and electron affinities for both materials are relatively small, indicating p-type dopability, providing there are no compensating low formation energy donor defects present. A study of the defect properties will indicate the doping potential of LaZnOP and LaZnOAs, and also provide guidance on optimised experimental growth conditions.…”

“…3) calculated relative to the O 1s core level within the bulk using HSE06 + SOC reveals that both materials have high-lying conduction band minima and valence band maxima, relative to the vacuum level. These features are characteristic of native p-type semiconducting materials, such as Cu 2 O and the isostructural LaCuOSe, which is a degenerate p-type semiconductor when doped with Mg. 71,[95][96][97] High-lying valence band maxima, and the resulting low ionisation potentials drives the formation of holes in semiconducting materials, while lower electron affinities limit n-type dopability due to charge compensation from the higher concentration p-type defects. From these results we propose that LaZnOP and LaZnOAs show promise as p-type thermoelectric materials, although careful assessment of the defect chemistry must be performed to identify a suitable dopant.…”

A Review of Inorganic Photoelectrode Developments and Reactor Scale‐Up Challenges for Solar Hydrogen Production

“…[ 93,100,119,268 ] For reasons not fully understood, this strategy appears to be less viable in TiO 2 , possibly due to the reduced number of sites available for co‐doping, which may be connected to a reduced tolerance of the anatase/rutile lattice to doping in comparison to perovskites, [ 265,268 ] as well as possible resistance of the natural defect chemistry of TiO 2 to co‐doping. [ 269 ] An additional factor which may distinguish Rh‐doped and La,Rh‐co‐doped SrTiO 3 from a host of inefficacious analogs is the strong p‐type character observed in both materials, [ 93,100,265,270 ] the origin of which has not yet been satisfactorily explained for the materials, given the electronic structures predicted by predicted by density functional theory (DFT). [ 264,271 ] Of note, more narrow bandgap photoelectrode materials have also been subject to doping to further reduce the bandgap and therefore improve light harvesting, including BiVO 4 .…”