Intrinsic point defects and the n -type dopability of Bi2MoO6 with higher photocatalytic performance: A hybrid functional study

Abstract

Intrinsic point defects play a vital role in regulating the electronic structure and electron carrier concentration (${n}_{0}$) of ${\mathrm{Bi}}_{2}\mathrm{Mo}{\mathrm{O}}_{6}$, which has been demonstrated as a promising photocatalyst. Unfortunately, the source of $n$-type conductivity for its intrinsic defects has proved challenging. Using first-principles hybrid density functional calculations, we investigate the formation energies and electronic structure of intrinsic point defects in ${\mathrm{Bi}}_{2}\mathrm{Mo}{\mathrm{O}}_{6}$. The self-consistent Fermi energies and equilibrium carrier and defect concentrations are calculated using the sc-fermi code based on thermodynamic simulation. It is found that the easy formation of the +2 charged O vacancy (${V}_{\mathrm{O}}^{2+}$) is responsible for its intrinsic $n$-type conductivity under Bi-rich/O-poor conditions, and thus, it can have a very high density (reaching up to ${10}^{19} \mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$), making it the dominant defect in ${\mathrm{Bi}}_{2}\mathrm{Mo}{\mathrm{O}}_{6}$. Under O-rich conditions, only adopting both ${\mathrm{D}}^{+}$ doping and quenching methods in ${\mathrm{Bi}}_{2}\mathrm{Mo}{\mathrm{O}}_{6}$ can reach a higher carrier density $(>{10}^{17}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3})$. Moreover, under the broad range of chemical potentials of $\ensuremath{-}2.50\phantom{\rule{0.16em}{0ex}}\mathrm{eV}<\mathrm{\ensuremath{\Delta}}{\ensuremath{\mu}}_{\mathrm{Bi}}<0, \ensuremath{-}6.32\phantom{\rule{0.16em}{0ex}}\mathrm{eV}<\mathrm{\ensuremath{\Delta}}{\ensuremath{\mu}}_{\mathrm{Mo}}<\ensuremath{-}1.48\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$, and $\ensuremath{-}2.19\phantom{\rule{0.16em}{0ex}}\mathrm{eV}<\mathrm{\ensuremath{\Delta}}{\ensuremath{\mu}}_{\mathrm{O}}<\ensuremath{-}0.65\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$, after quenching from 700 to 300 K, ${\mathrm{Bi}}_{2}\mathrm{Mo}{\mathrm{O}}_{6}$ can be effectively donor doped without significant compensation, and ${n}_{0}$ increases as $\mathrm{\ensuremath{\Delta}}{\ensuremath{\mu}}_{\mathrm{O}}$ decreases or temperature increases. Our results can give a guide in selecting optimum growth and quenching conditions to achieve high $n$-type doping and suppress compensation.