Page 44 - Multifunctional Photocatalytic Materials for Energy

Metal oxide electrodes for photo-activated water splitting 33

(B)
(A)

300 nm

2 µm

0.5 0.10
(C) NWs/NDs (L) (D) NWs/NDs
NWs/NDs (D) NWs
0.4 NWs (L) 0.08
NWs (D)
J (mA ¥ cm –2 ) 0.3 J (mA ¥ cm –2 ) 0.06
0.04
0.2
0.1 0.02

0.0 0.00
–0.3 0.0 0.3 0.6 0.9 1.2 –0.4 –0.3 –0.2 –0.1 0.0
E (V vs. Ag/ AgCI) E (V vs. Ag/AgCI)
Fig. 3.8 (A) Large scale and (B) magnified cross-sectional SEM micrographs of as-prepared
ZnO nanowire (NW)/nanodisk (ND) arrays, fabricated through the epitaxial growth of ZnO
NDs on ZnO NWs via a hydrothermal process [25]. (C) Current density/potential curves
recorded in the dark (D) and under illumination (L; simulated solar radiation; AM1.5G,
−2
100 mW × cm ) conditions. (D) Magnified curves in the low potential region.
Adapted with permission from H. Chen, Z. Wei, K. Yan, Y. Bai, Z. Zhu, T. Zhang, S. Yang,
Epitaxial growth of ZnO nanodisks with large exposed polar facets on nanowire arrays for
promoting photoelectrochemical water splitting, Small 10 (2014) 4760–4769. Copyright
Wiley, 2014.

throughout the investigated potential range. The main reasons for this improvement
were the increased surface area, responsible for a more efficient contact with the elec-
trolyte, and the unique system hierarchical morphology, which promoted an enhanced
radiation harvesting. In addition, upon increasing the applied potential, the J/V curve
for NWs/NDs showed a sharper increase, achieving saturation at ≈0.7 V versus Ag/
AgCl, at variance with the NW sample. This phenomenon, scarcely reported for ZnO
photoanodes even in the presence of catalysts, suggested the occurrence of a more
effective charge separation and collection in NWs/NDs. In this regard, a key role is
played by the presence of NDs with much lower sizes, which enable an efficient trans-
port of photogenerated holes up to the interface with the electrolyte, thus suppressing
detrimental recombination losses. It is also worth noting that NWs/NDs showed a
net negative shift of the onset potential (−0.11 V) with respect to the pristine NWs
(Fig. 3.8D), indicating that a lower bias is needed for PEC water splitting [25].