Page 45 - Multifunctional Photocatalytic Materials for Energy

34 Multifunctional Photocatalytic Materials for Energy

Additional efforts to broaden the ZnO spectral response into the Vis range have
concerned elemental doping [100,103,110,111,113,115] and incorporation of plas-
monic metals, such as Ag [28] and Au [26,28]. In the latter case, the light harvesting
efficiency was enhanced by the SPR effect, and, at the same time, undesired re-
combination processes were restricted, yielding a significant photocurrent enhance-
ment. In another study, ZnO/Au plasmonic structures, prepared by a hydrothermal
and photoreduction combined approach, presented a photocurrent density as high
−2
as 9.11 mA × cm at a low potential (1.0 V versus Ag/AgCl) and an efficiency 16
times higher than the pristine ZnO nanorod array [34]. These results provide useful
insights into the design of plasmonic metal/semiconductor photoanodes for solar
harvesting.
Other investigators have explored either the integration of OECs, such as CoPi,
onto a ZnO nanosystem surface [27], or coupling with other semiconductors, which
has been shown to be one of the most effective and inexpensive methods [112]. In this
regard, various heterojunction-containing composites based on ZnO-TiO 2 [17], ZnO-
NiO [60], ZnO-M(OH) x with M = Co, Ni [61,62], ZnO-BiVO 4 (see also Section 3.3.4)
[23,112], ZnO-WO x coupled with CdSe-CdS [116], ZnO-ZnS-FeS 2 [107], and ZnO-
CdS-NiO [117] have been fabricated and tested for the target functional application.
So far, the sensitization of ZnO with quantum dots (QDs) based on CdS [118], CdSe
[101], and CdTe [113] has been used to improve PEC efficiency, but the toxicity of
these systems, along with their dissolution/degradation phenomena, has stimulated
the search for harmless green sensitizers. For instance, Guo et al. recently reported
on unique photoelectrodes obtained by covalently bonding graphene quantum dots
(GQDs), functionalized with carboxyl moieties, on amino group-modified ZnO NWs.
The use of GQDs was motivated by their peculiar optical and electronic properties,
rendering them attractive candidates to build advanced functional nanomaterials [16].
SEM investigation (Fig. 3.9A) confirmed the presence of high density ZnO NW arrays,
in which NWs (mean length ≈1 μm, mean diameter ≈50 nm) grew almost perpendic-
ularly to the FTO substrate surface. ZnO nanostructures were uniformly covered by
GQDs, which did not significantly alter the pristine morphology, as also confirmed
by TEM results. The evaluation of the system photoactivity (Fig. 3.9B) revealed a
considerable photocurrent density increase for GQDs@ZnO NWs with respect to the
original ZnO NWs, indicating the occurrence of a much more efficient photoelectro-
chemical process in the former case. In addition, the system performances were highly
stable upon prolonged cycling (Fig. 3.9B, inset). This result is of utmost importance,
taking into account that a poor ZnO photostability has a detrimental effect on material
photoefficiency [17,27].
Based on the obtained results and taking into account the mutual ZnO and
GQDs energy level positions, the PEC H 2 O splitting process promoted by GQDs@
ZnO NW photoelectrodes can be interpreted as shown in the scheme presented in
Fig. 3.9C. The lower photoresponse for bare ZnO NWs could be traced back to the
limited Vis light absorption, due to the large band gap. Conversely, GQDs@ZnO
NWs enabled a favorable enhancement of Vis light harvesting. After irradiation, the
transfer of photogenerated electrons from GQDs to ZnO is enabled by the mutual
positions of energy levels, resulting in an enhanced charge separation that further