Low-Cost Nanomaterials for Photoelectrochemical Water Splitting - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 268

for 20 min, the coated glass was subjected to a sintering process. The resulting
mesoporous semiconducting thin films were utilized as a photoanode in dye-
sensitized solar cells (DSSC). Furthermore, Sivula et al. synthesized mesoporous
hematite (Fe 2 O 3 ) photoanodes on FTO substrate by doctor-blading and reported
their enhanced PEC properties [ 83 ]. The Fe 2 O 3 colloid solution containing the
porogen was coated onto the FTO substrate via doctor-blading with a 40 lm
invisible tape (3 M) as a spacer. The original size of the Fe 2 O 3 nanoparticles was
about 10 nm before annealing (Fig. 4 a). After annealed at 400 C, the film became
porous with necked particles of 30 nm (Fig. 4 b). Porous films with larger feature
size of the necked particles ca. 60 and 75 nm were obtained after subsequent
sintering at 700 and 800 C (Figs. 4 c and d), respectively. These porous Fe 2 O 3
films yield water-splitting photocurrents of 0.56 mA/cm 2 under standard condi-
tions (AM 1.5G 100 mW/cm 2 , 1.23 V vs. reversible hydrogen electrode, RHE)
and over 1.0 mA/cm 2

before the dark current onset (1.55 V vs. RHE).

2.3 Electrochemical Deposition Methods

Electrochemical deposition is a powerful approach for the fabrication of nanom-
aterials on conductive substrates due to its simplicity, ease of scale-up, low cost,
and environmental friendliness [ 55 , 56 , 58 ]. Nanomaterials grown directly on
conductive substrates can facilitate the transport of electrons and electrical signals,
which will broaden their applications in sensors, solar cells, electronic and elec-
trochemical devices [ 18 ]. Moreover, the morphology and composition could be
easily tuned by adjusting the electrochemical deposition parameters such as
applied potential, current density, temperature, electrolyte, substrate, etc.
The growth of various metal oxides, with different morphologies such as
nanowires, nanorod arrays, nanodendritic structures, and thin films, using elec-
trochemical deposition have been achieved. For instance, Mao et al. have used an
electrochemical deposition method to deposit iron into AAO template channels
and subsequently removed the AAO template using a NaOH solution, followed by
thermal oxidation converting the as-prepared iron nanorod arrays into Fe 2 O 3 [ 58 ].
Lu et al. recently reported the controllable growth of ZnO nanostructures such as
nanowire arrays, hierarchical architectures on FTO glass substrates using elec-
trochemical deposition [ 55 , 56 ]. The morphology and size of ZnO nanostructures
can be readily controlled by adjusting the electrochemical deposition parameters
such as reaction temperature and the concentration of electrolyte [ 55 , 56 ]. Verti-
cally aligned and single-crystalline ZnO nanowire with an average diameter of
*200 nm and length of *2 lm were grown directly on a FTO glass substrate by
cathodic electrochemical deposition [ 56 ]. Furthermore, Gan et al. recently syn-
thesized vertically aligned In 2 O 3 nanorod arrays on FTO substrate via a template-
and surfactant-free electrochemical deposition method from aqueous solution [ 18 ].
Moreover, Mao et al. synthesized three-dimensional (3D) hierarchical Cu 2 O stars
on FTO substrates using a rapid and facile electrochemical deposition approach