Low-Cost Nanomaterials for Photoelectrochemical Water Splitting - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 279

same as the absorbed photon conversion efficiency (APCE), indicating the benefits
of the nanostructure in the charge collection and transport [ 46 ].
Although the ultrathin nanostructure can improve the charge separation, hematite
is an indirect band-gap material that requires a thick layer of active material for
increasing light absorption. Nevertheless, thick hematite films will lead to signifi-
cant electron-hole recombination loss. Therefore, it is also important to modify the
intrinsic electronic property of hematite, with a goal of increasing charge transport
and suppressing the electron-hole recombination. Element doping is the most
common method to modify the electronic property of hematite. A number of ele-
ment dopants including titanium [ 100 , 119 ], silicon [ 5 , 36 ], and tin [ 41 , 48 ] have
been studied for hematite. These dopants function as electron donors and can sig-
nificantly enhance the electrical conductivity of hematite by increasing its donor
density. Gratzel and co-workers developed silicon-doped hematite by spray pyro-
lysis and atmospheric pressure chemical vapor deposition (APCVD) [ 5 , 36 ]. These
silicon-doped hematite achieved the best photocurrent density of 2.3 mA/cm 2 at
1.23 V versus Ag/AgCl, without the need of catalyst modification [ 36 ]. Recently, Sn
doping has also been demonstrated to be an effective dopant for hematite. Upon high
temperature annealing, Sn can diffuse into hematite film from fluorine-doped tin
oxide (FTO) glass substrate. Sn-doped hematite can also be achieved by inten-
tionally mixing Sn precursor with Fe precursor during hydrothermal synthesis of
hematite [ 48 ]. Sn doping treatment could activate hematite nanowires for PEC water
splitting. Moreover, Wang et al. also reported that Ti-doped hematite can be
achieved by a drop-annealing method using titanium butoxide as dopant precursor
[ 100 ]. Electrochemical impedance studies showed that the donor density of Ti-
doped hematite was significantly enhanced by orders of magnitude, indicating the
element doping is a good strategy to increase the electrical conductivity of hematite.
Alternative to the incorporation of extrinsic dopants into hematite, Ling et al.
also demonstrated a new method to improve the electrical conductivity of hematite
by creating intrinsic defects such as oxygen vacancies [ 47 ]. Oxygen vacancies
(Fe 2+ ) function as a shallow donor for hematite, playing a similar role as extrinsic
dopants such as silicon, titanium, and tin. Ling et al. annealed FeOOH nanowires
in an oxygen-deficient condition to create the oxygen vacancies [ 47 ]. They found
that this method could activate hematite nanowires at relatively low activation
treatment of 550 C[ 48 ]. A maximum photocurrent density of 3.37 mA/cm 2 at
1.5 V versus RHE, was obtained for the oxygen-deficient hematite nanowire
photoanode. Mott-Schottky studies showed that the carrier density was improved
by an order of magnitude, as a result of the creation of oxygen vacancies (Fig. 12 ).
This work demonstrated a simple and effective method to prepare highly photo-
active hematite nanowire for PEC water splitting application, at relatively low
activation temperature without the need of extrinsic dopants.
Furthermore, the large overpotential for water oxidation is another major issue
limiting the performance of hematite for PEC water splitting. Water oxidation
catalyst modification has been proved to be an effective method to suppress water
oxidation overpotential. A variety of catalysts such as Co-Pi [ 3 , 122 - 124 ], CoO x
[ 36 ], NiO x [ 98 ] and IrO x [ 90 ] have been previously reported. Co-Pi is the most