Page 41 - Multifunctional Photocatalytic Materials for Energy

30 Multifunctional Photocatalytic Materials for Energy

improve WO 3 stability is offered by the production of heterostructured photoanodes
with external protective overlayers (see also Section 3.3.1). For instance, the develop-
ment of BiVO 4 /WO 3 electrodes enabled to obtain good stability and enhanced photo-
current values [87,88], thanks also to an improved charge carrier separation (compare
Section 3.3.4; [43]). In this regard, coupling of WO 3 even with other semiconductors
(CdS, TiO 2 , Cu 2 O, …) has attracted considerable attention. Among these, WO 3 /TiO 2
thin films prepared by pulsed electrodeposition [89] yielded an improved and very
favorable performance with respect to the single oxides, which was traced back to a
suppression of recombination phenomena [33]. In addition, the analysis of core-shell
WO 3 /TiO 2 and TiO 2 /WO 3 nanorod arrays fabricated by evaporation [21] demonstrated
that TiO 2 -core/WO 3 -shell structures had a net photoresponse in the UV spectral region
(λ ≤ 400 nm), whereas WO 3 -core/TiO 2 -shell structures showed stronger Vis light ab-
sorption. In another study, Wei et al. prepared WO 3 /p-Cu 2 O multilayers by sequential
electrodeposition, obtaining a higher photoactivity than pure WO 3 and p-Cu 2 O films
[90]. This improvement was related to the formation of p–n junctions at the interface
between the two oxides, and, in particular, to the accumulation of electrons and holes
in Cu 2 O conduction band and WO 3 valence band, respectively.
Other ongoing research activities have been addressed to investigate composite
photoanodes widely used to physically separate the light absorption and catalyst func-
tion [22]. In this context, recent efforts have been dedicated to plasmonic metal/semi-
conductor systems [18,41,44,91,92] in which the formation of metal-oxide junctions
[92–95] played a key role in the overall process [66,96]. In fact, the localized surface
plasmon resonance (SPR), arising from the collective oscillation of free electrons in
metal particles upon interaction with resonant photons, enables the excitation energy
transfer from metal NPs to the adjacent semiconductor, with an ultimate enhanced
formation of electron-hole pairs. In addition, plasmonic metal NPs can improve the
semiconductor absorbance by scattering of incident light [18,34,44]. Enhancing light
absorption by the SPR of metal NPs is particularly important in the case of indirect
band gap systems, such as WO 3 , for which the 400–500 nm wavelength range criti-
cally influences the absorbed solar light amount. In this regard, it has been demon-
strated that the dispersion of silver NPs on FTO substrates prior to WO 3 deposition
(Fig. 3.7A) is a successful mean to enhance performances [66,97]. Nevertheless, the
use of silver NPs in photoelectrochemical devices is only possible in such a kind of
“embedded” configuration, in which the particles are protected from corrosion phe-
nomena by the semiconductor overlayer [41]. As a consequence, attention was de-
voted to the use of Au NPs, and an interesting work involved the capping of Au NPs
dispersed on WO 3 photoanode surface with Keggin-type Mo polyoxometalate (POM)
3−
(i.e., PMo 12 O 40 ) ions [44], which have also been investigated as highly effective
OECs [13]. An inherent advantage brought about by their use is that the interaction
between POMs and Au NPs effectively prevents the agglomeration of capped particles
on the WO 3 surface, thanks to electrostatic repulsion effects. In addition, the negative
charge of POM-capped Au NPs promotes their interaction with WO 3 surfaces, which
are positively charged in acidic media. In this regard, Fig. 3.7B shows the arrangement
of the polyoxomolybdate-capped Au NPs around the larger WO 3 particles.