Page 49 - Multifunctional Photocatalytic Materials for Energy

38 Multifunctional Photocatalytic Materials for Energy

the idea of exploiting ZnO nanorods as an electronic transmission channel and BiVO 4
as a light absorber [23,112]. The composite photoanode exhibited a better activity in
−2
PEC water splitting, with a photocurrent density up to 1.7 mA × cm , higher than
that of bare ZnO, thanks mainly to (i) the enlarged spectral region response related
to BiVO 4 and (ii) the formation of ZnO/BiVO 4 heterojunctions, promoting a higher
electron-hole separation.
Another possibility of preparing low cost and stable heterostructured systems con-
cerns the joint use of Fe 2 O 3 , BiVO 4 , and ZrO 2 . In this context, Shaddad et al. [9] devel-
oped an electrodeposition process which enables to improve H 2 O oxidation kinetics
for BiVO 4 photoanodes through the sequential addition of Zr and Fe precursors. Upon
annealing, these precursors are converted into monoclinic ZrO 2 and α-Fe 2 O 3 nanopar-
ticles. Fig. 3.11A and B show representative TEM micrographs for optimized BiVO 4 -
ZrO 2 and BiVO 4 -ZrO 2 -Fe 2 O 3 photoelectrodes. The former comprise highly crystalline
NPs (mean diameter ≈5–10 nm), with interplanar distances corresponding to the (111)
(Fig. 3.11A) and (1-11) reflections of monoclinic ZrO 2 . Chemical mapping indicated
that an important zirconium fraction is present at the BiVO 4 surface. Nevertheless, it
is also worthwhile noting that Zr can indeed substitute Bi in the monoclinic BiVO 4
lattice, as shown by XRD analyses, indicating a peak shift toward higher angular val-
ues upon Bi(III) substitution by the smaller Zr centers. As regards BiVO 4 -ZrO 2 -Fe 2 O 3
samples, they additionally contained homogeneously distributed hematite nanoparti-
cles, whose (104) planes are clearly shown in Fig. 3.11B.
Fig. 3.11C illustrates the photoelectrochemical behavior of reference Fe 2 O 3
and BiVO 4 photoanodes, along with the best performing composite electrodes.
Photocurrents densities at 1.23 V versus RHE for different Zr and Fe contents
(Fig. 3.11D) were used to identify the optimal preparative conditions pertaining to the
systems reported in Fig. 3.11C. The best performance was obtained for 2.5 mol % Zr
−2
and, on the other hand, to 2 mC × cm of Fe charge deposition. In Fig. 3.11C, a re-
markable increase in the photocurrent is shown for the optimized BiVO 4 -ZrO 2 -Fe 2 O 3
photoanode, which was explained by the cooperative catalytic role of monoclinic ZrO 2
and α-Fe 2 O 3 nanoparticles on the BiVO 4 surface. Although J values in Fig. 3.11C are
lower than previous reports on BiVO 4 photoanodes, the enhanced efficiency achieved
with Zr and Fe additions and the method simplicity appear to be powerful tools for the
engineering of more efficient systems [9].

3.4 Conclusions and future trends

The use of an abundant form of energy, sunlight, and a readily available chemical,
H 2 O, in natural photosynthesis, offer an elegant example in order to efficiently harvest
and utilize solar light for energy production. To this regard, a PEC system to split
water and yield H 2 can be considered as an ideal platform where the various concepts
regarding different photosynthesis aspects can be tested on a single material. In fact, in
semiconductor-based PEC reactions, a photoelectrode is expected to simultaneously
perform different functions, i.e., light absorption, photogenerated charge separation,
and catalysis of the target process.