Page 40 - Multifunctional Photocatalytic Materials for Energy

Metal oxide electrodes for photo-activated water splitting 29

(A) (B) 2.0
(G) 500°C
1.5
200 nm 200 nm 475°C
J (mA¥ cm –2 ) 1.0 600°C
2 µm 2 µm
0.5
400°C
(C) (D)
0.0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
200 nm 200 nm E (V vs. Ag/AgCI)
60
H 2 (H)
50 O 2
2 µm 2 µm 40
Gas evolution (mmol) 20
(E) (F) 30

200 nm 2,3 µm 10
0
2 µm 1 µm 0 1200 2400 3600 4800 6000 7200
Time (s)
Fig. 3.6 Representative SEM images of WO 3 nanoplatelet arrays grown on FTO via a
hydrothermal process, as-prepared (A), and after annealing in air for 1 h at (B) 400°C;
(C) 475°C; (D) 500°C; (E) 600°C, respectively. (F) Cross-sectional SEM micrograph of
the specimen treated at 500°C shown in (D). (G) Photocurrent density of WO 3 photoanodes
annealed at different temperatures, recorded in 0.5 M Na 2 SO 4 solutions under AM1.5G
−2
chopped illumination (100 mW × cm ). (H) H 2 and O 2 evolution versus time (irradiation
2
area: 1.9 cm ; 0.6 V vs. Ag/AgCl) obtained under simulated sunlight irradiation of a WO 3
photoanode annealed at 500°C.
Adapted with permission from X. Feng, Y. Chen, Z. Qin, M. Wang, L. Guo, Facile fabrication
of sandwich structured WO3 nanoplate arrays for efficient photoelectrochemical water
splitting, ACS Appl. Mater. Interfaces 8 (2016) 18089–18096. Copyright American Chemical
Society, 2016.

The interplay between morphology and photoelectrochemical activity was studied
by J/V measurements (Fig. 3.6G). The recorded J values were progressively enhanced
−2
by raising the annealing temperature to 500°C (highest J of 1.88 mA × cm at 1.3 V
versus Ag/AgCl), and then decreased upon treatment at 600°C. Detailed analyses re-
vealed that the PEC activity of the 500°C-treated sample, among the best reported for
WO 3 photoanodes [2], was due to a combination of the highest surface area and the
presence of monoclinic WO 3 , the most active phase for PEC water splitting.
For the best performing system, hydrogen and oxygen evolution with a molar ratio
of ca. 2:1 was demonstrated (Fig. 3.6H). Nevertheless, the data suggested the occur-
rence of side alterations related to electrode dissolution. To circumvent this problem,
acidic electrolytes were utilized [13], and PEC tests in H 2 SO 4 electrolytes on WO 3
photoanodes prepared by a liquid phase route yielded photocurrents comparable to
−2
the state-of-the-art (≈2.7 mA × cm at 1.3 V versus RHE) [19]. An alternative route to