Page 51 - Multifunctional Photocatalytic Materials for Energy

40 Multifunctional Photocatalytic Materials for Energy

proved to be the possibility of controlling, at the nanoscale level, material spatial or-
ganization and interface electronic structure, of great interest for the production of
systems and devices with improved efficiency. The development of composites and/or
heterostructured photoelectrodes has also been recognized as a key issue to achieve an
enhanced system activity in PEC H 2 O splitting, even from sunlight and seawater, key
issues for a sustainable hydrogen generation.
Overall, the present findings will help future efforts aimed at developing more
efficient photoanodes exceeding the current state-of-the-art performances. Indeed,
the complexity and variety of examples reported and discussed herein demonstrate
that the choice of adequate fabrication and processing conditions is indeed crucial
in order to move the target processes “from the lab to the fab”, i.e., toward func-
tional utilization under real-world conditions. Nevertheless, in spite of various re-
search studies, activities in these fields still have a long way to go in terms of both
fundamental and applied research. In fact, the obtained photocurrents (often be-
−2
low 5 mA × cm ) are far from the industrial requirements of high efficiencies, and
driving up the photoelectrode activity still requires a better understanding of the
physical and chemical processes that occur at the solid/liquid interface [22]. In this
regard, the use of ex-situ pre-treatment of pristine electrodes has been proposed as a
successful way to boost their PEC performances in water splitting. A recent example
concerns the exposure of WO 3 photoanodes, prepared by a spin-coating process, to
sustained UV illumination in air [22]. This process has been shown to result in a 30%
enhancement of the system photoactivity, an effect traced back to an increase in the
corresponding surface area. Whereas illumination did not generate any change in the
WO 3 onset potential, the application of a similar treatment to BiVO 4 photoanodes
resulted in a cathodic shift of ≈230 mV, as well as in the obtainment of a less porous
surface and in a reduced recombination of photogenerated charge carriers due to the
suppression of surface defects [12]. These differences yield useful insights into how
UV irradiation affects the properties of semiconductor materials used in PEC appli-
cations [22]. Additional freedom is offered by ex-situ plasma treatments to control
the density of oxygen vacancies and attain improved photoactivity. Recent attempts
in this direction have already been successfully carried out on ultra-thin hematite
nanoflakes, obtained by annealing of iron foils in air (Fig. 3.12). The increased
number of oxygen vacancies after plasma treatment, resulting in an increased car-
rier density, was interpreted as the main cause for the registered enhancement of the
system PEC activity [122].
Nevertheless, it is worth highlighting that, even after reaching the goal of effi-
cient water photosplitting by stable and low cost materials, only the first part of the
problem will be solved. In fact, key open challenges to be solved afterwards include
the storage, transportation, and utilization of H 2 . To circumvent these problems, an
amenable option to be pursued will concern the use of solar energy to generate other
chemicals different from hydrogen [6,14,64]. The development and implementation
of these strategies, which are subjects of current intensive research, may open doors
to the development of energy technologies based on unexplored light-powered chem-
ical reactions, of great interest for the production of advanced functional devices that
might revolutionize current well-established technologies.