Simultaneously Enhanced Charge Separation and Transfer in Cocatalyst-Free Hematite Photoanode by Mo/Sn Codoping

It is well-known that sluggish surface
charge transfer on a cocatalyst-free
hematite photoanode limits its solar conversion efficiency in photoelectrochemical
(PEC) water splitting. Here, Mo is used as a codopant synergistically
with Sn in α-Fe2O3 to solve this problem.
Besides causing a slight narrowing of bandgap, morphological change,
and even sublimation loss of α-Fe2O3,
Mo doping also imports low-valent Fe due to charge compensation with
MoV/VI as indicated by X-ray photoelectron spectroscopy
and Bader charge computation, which increases the densities of donor
and surfaces states. Although rate law analysis demonstrates slight
retardation of surface reaction kinetics, strongly inhibited charge
recombination in surface states by Mo doping still contributes to
improving the photocurrent density and reducing the onset potential
of α-Fe2O3 and Sn–Fe2O3 photoanodes. An optimized Mo/Sn–Fe2O3 photoanode can realize a low onset potential of 0.68
V vs a reversible hydrogen electrode (VRHE) and a photocurrent
density of 1.97 mA cm–2 at 1.23 VRHE,
enhanced by 58% and 20 times compared to Sn–Fe2O3 and α-Fe2O3, respectively. It
is demonstrated that Mo doping promotes charge transfer which differs
from most traditional n-type dopants that facilitate charge separation
but inhibit charge transfer. This report expands the n-type dopant
family of α-Fe2O3 for efficient PEC water
splitting.