Solar Hydrogen Generation: Photocatalytic and Photoelectrochemical Methods - Hydrogen Generation, Storage, and Utilization - page 37

FIGURE 3.4 Schematic energy diagram of PEC water splitting with (a) photoanode, (b) photocathode,
and (c) n- type photoanode and p -type photocathode. Source : Reproduced with permission from Liu et
al. [31]. (See color insert.)
reducing water. Recently, the photo-instability of Cu 2 O in water was
addressed by deposition of a protective layer on the electrode surface by
atomic layer deposition (ALD) method (Fig. 3.5a) [32]. The protective
oxide layer forms a staggered type II band offset with Cu 2 O, so photogen-
erated electrons can flow from the Cu 2 O conduction band through the pro-
tective layer to the electrolyte for water reduction. Additionally, the n -type
oxide layer should have a conduction band negative to the hydrogen evolu-
tion potential, without reductive degradation reaction at the potentials within
the bandgap. Moreover, the oxide should have low overpotential for water
reduction. The Cu 2 O films used for this study were synthesized by electro-
deposition method, with a thickness of 1.3 μm [32]. Individual Cu 2 O grains
of the film were about 1 μm in size with a cubic morphology, and had a
predominant (111) orientation (Fig. 3.5b). Various metal oxide coatings
were tested and Cu 2 O can be stabilized by coating with 5 × (4 nm
ZnO/0.17 nm Al 2 O 3 )/11 nm TiO 2 and Pt nanoparticles (Fig. 3.5a). Under
AM 1.5 illumination, the as-deposited bare Cu 2 O produced a photocurrent
of −2.4 mA cm −2 at 0.25 V versus the reversible hydrogen electrode (RHE)
in a nitrogen purged 1 M Na 2 SO 4 electrolyte buffered at a pH of 4.9 (Fig.
3.5c). However, the cathodic current quickly decreased to zero, indicating
that bare Cu 2 O is not stable for PEC water reduction (Fig. 3.5c, inset). On
the contrary, the surface-protected Cu 2 O with 5 × (4 nm ZnO/0.17 nm
Al 2 O 3 )/11 nm TiO 2 /Pt shows substantially enhanced photoactivity and