  The development of efficient photoelectrodes via defect engineering of
wide-band gap metal oxides has been the prime focus for many years.
Specifically, the effect of carbon defects in wide-band gap metal oxides on
their performance in photoelectrochemical (PEC) applications raised numerous
controversies and still elusive. Herein, the effect of various carbon defects
in m-ZrO2 was investigated using the density functional theory to probe the
thermodynamic, electronic, and optical properties of the defective structures
against pristine m-ZrO2. The defect formation energies revealed that elevating
the temperature promotes and facilitates the formation of carbon defects.
Moreover, the binding energies confirmed the stability of all studied complex
carbon defects. Furthermore, the band edge positions against the redox
potentials of water species revealed that all the studied defective structures
can serve as photoanodes for water splitting. Additionally, CO3c (carbon atom
substituted O3c site) was the only defective structure that exhibited slight
straddling of the redox potentials of water. Importantly, all investigated
defective structures enhanced light absorption with different optical
activities. Finally, CO3cVO3c (carbon atom substituted O3c associated with O3c
vacancy) defective m-ZrO2 enjoyed low direct band gap (1.9 eV), low defect
formation energy, low exciton binding energy, high mobility of charge carriers,
fast charge transfer, and low recombination rate. Concurrently, its optical
properties were exceptional in terms of high absorption, low reflectivity and
improved static dielectric constant. Hence, the study recommends CO3cVO3c
defective m-ZrO2 as the leading candidate to serve as a photoanode for PEC
applications.
