Page 48 - Multifunctional Photocatalytic Materials for Energy

Metal oxide electrodes for photo-activated water splitting 37

(A) (B)

500 nm 200 nm
(C) (D)

500 nm 200 nm
(E) 0.8 (F) 1.8
0.8 Dense BiVO 4 4 1.6 1.23 V RHE BiVO 4 4
Porous BiVO
BiVO -CoPi
0.6
1.4
J (mA¥ cm –2 ) 0.6 J (mA¥ cm –2 ) 0.4 0.4 0.6 0.8 1.0 1.2 1.4 J (mA¥ cm –2 ) 1.2
0.2
1.0
0.4
0.0
0.8
0.6
Voltage (V) vs. RHE
0.2
0.4
0.0 0.2
0.0
0.4 0.6 0.8 1.0 1.2 1.4 0 100 200 300 400 500 600
Voltage (V) vs. RHE
Time (s)
Fig. 3.10 Plane-view SEM micrographs of dense (A and B) and porous (C and D) BiVO 4
thin films deposited by spin-coating onto FTO. After drying at 150°C, specimens were
annealed at 400°C for 10 min and then cooled at room temperature for six times, and finally
treated in air at 400°C for 2 h. In order to obtain a dense film (A and B), the specimen was
−1
introduced into the oven at 150°C and heated up to 400°C at a rate of 2.5°C × min [65].
In a different way, to produce a porous morphology (C and D), the sample was introduced
into an oven already at 400°C. (E) Photocurrent density/voltage curves for BiVO 4 thin films
with dense and porous morphologies recorded under chopped simulated sunlight irradiation
−2
[AM 1.5G, 100 mW × cm ; 0.1 M sodium phosphate (NaPi) buffer, pH ≈ 7]. Inset: responses
under continuous illumination; the dark current curve is reported in black for comparison.
(F) Chrono-amperometry (CA) measurements under chopped illumination (1.23 V vs. RHE;
0.1 M NaPi electrolyte) for dense BiVO 4 and BiVO 4 -CoPi photoanodes, obtained by CoPi
photo-electrodeposition on the pristine system.
Adapted with permission from S. Hernández, G. Gerardi, K. Bejtka, A. Fina, N. Russo,
Evaluation of the charge transfer kinetics of spin-coated BiVO 4 thin films for sun-driven water
photoelectrolysis, Appl. Catal. B 190 (2016) 66–74. Copyright Elsevier B.V., 2016.
works have explored the coupling of BiVO 4 with various metal oxides. In this respect,
the formation of WO 3 /BiVO 4 heterojunctions is effective in enhancing BiVO 4 pho-
toanode performances, since the mutual band positions of the two oxides favor the
transfer of photogenerated electrons from BiVO 4 to WO 3 [43]. In another study [24],
PEC performances of a W-doped BiVO 4 /FTO photoanode toward H 2 O oxidation were
boosted by the electrodeposition of an amorphous TiO 2 layer, passivating surface de-
fects and thus blocking recombination phenomena. In a different way, Yan et al. devel-
oped a hybrid liquid phase route to 1D ZnO/BiVO 4 heterojunction photoanodes, with