4 Laser-Induced Synthesis and Processing of Nanoparticles …
157
An enhancement of the hydrogen production is also obtained by irradiating the
sample using visible light, shown also in Fig. 4.14b for P25. In this case, however, it
is possible to note that the total amount of hydrogen production with visible light is
considerably lower than that found with UV irradiation. This behavior is reasonable,
considering the energy band-gap measurements. The hydrogen production yield for
irradiated nanoparticles was compared with that obtained after a standard reduction
procedure at 500 °C for 1 h in hydrogen atmosphere [30]. It can be seen that the
photocatalytic performances in the H 2 reduced sample is much lower compared to
laser irradiation. This confirms the laser irradiation process, not only as a lower cost
and easier process with respect to standard reduction, but also as a more efficient
approach in generating higher photocatalytic active samples.
In this regard, highly visible light photoactive titania colloids have been prepared
by UV laser irradiation of anatase titania colloids in water or ethanol. The nature
of solvent/titania interaction affects the induced modifications by laser processing
and consequently the photocatalytic performances. In particular, theoretical calculations showed that water molecules only weakly interact with the titania surface
resulting in the coexistence of surface oxygen vacancy defects, as confirmed by
photoluminescence (PL) spectra, and OH groups on titania surfaces as shown by
Fourier transform infrared spectra (FT-IR) spectrum. In this scenario, laser irradiation leads to the passivation of oxygen vacancies due to surface hydroxylation as
confirmed both by PL and FT-IR spectra. These laser-induced modifications resulted
in an increased photocatalytic activity under UV irradiation. A similar result was
observed for samples laser-treated in ethanol; furthermore, a photocatalytic activity
under solar irradiation was also conferred to anatase sample by laser processing in
this solvent. Ethanol molecules completely passivate defects in anatase surface as
confirmed by FT-IR spectra, theoretical calculations, and the quenching of visible
emission band in PL spectra of colloids before the laser processing. In this case, the
UV laser irradiation of anatase surface covered by ethanol moieties lead to higher
degree of titania reduction due to the holes scavenger effect of ethanol itself. In addition, ethanol moieties adsorbed on titania surfaces are photooxidezed during the laser
processing, as confirmed by FT-IR spectra resulting in the introduction of vacancies
not susceptible to ethanol absorption and an indirect C doping of titania surface by the
incorporation of carboxylated species obtained as by-products of ethanol moieties
photoconversion. Under artificial solar light, this sample showed comparable performance respect to other TiO 2 composites tested in the same conditions and higher
activity with respect to C doped titania samples.
These results point out the light on another advantage of the ‘in liquid’ laser
irradiation process: a proper evaluation of the dispersing medium-semiconductor
interaction allows to tune the process, in order to improve the photocatalytic activity
of the semiconductor itself and extend it to visible light wavelength range. This is a
very interesting aspect from a practical point of view.
Furthermore, in comparison to the other TiO 2 -based samples [94] synthetized
with structural and chemical modifications, for the photocatalytic hydrogen production the laser-treated TiO 2 was the best sample under UV irradiation, whereas under
solar/visible light irradiation the TiO 2 chemically modified with other metals showed
157
An enhancement of the hydrogen production is also obtained by irradiating the
sample using visible light, shown also in Fig. 4.14b for P25. In this case, however, it
is possible to note that the total amount of hydrogen production with visible light is
considerably lower than that found with UV irradiation. This behavior is reasonable,
considering the energy band-gap measurements. The hydrogen production yield for
irradiated nanoparticles was compared with that obtained after a standard reduction
procedure at 500 °C for 1 h in hydrogen atmosphere [30]. It can be seen that the
photocatalytic performances in the H 2 reduced sample is much lower compared to
laser irradiation. This confirms the laser irradiation process, not only as a lower cost
and easier process with respect to standard reduction, but also as a more efficient
approach in generating higher photocatalytic active samples.
In this regard, highly visible light photoactive titania colloids have been prepared
by UV laser irradiation of anatase titania colloids in water or ethanol. The nature
of solvent/titania interaction affects the induced modifications by laser processing
and consequently the photocatalytic performances. In particular, theoretical calculations showed that water molecules only weakly interact with the titania surface
resulting in the coexistence of surface oxygen vacancy defects, as confirmed by
photoluminescence (PL) spectra, and OH groups on titania surfaces as shown by
Fourier transform infrared spectra (FT-IR) spectrum. In this scenario, laser irradiation leads to the passivation of oxygen vacancies due to surface hydroxylation as
confirmed both by PL and FT-IR spectra. These laser-induced modifications resulted
in an increased photocatalytic activity under UV irradiation. A similar result was
observed for samples laser-treated in ethanol; furthermore, a photocatalytic activity
under solar irradiation was also conferred to anatase sample by laser processing in
this solvent. Ethanol molecules completely passivate defects in anatase surface as
confirmed by FT-IR spectra, theoretical calculations, and the quenching of visible
emission band in PL spectra of colloids before the laser processing. In this case, the
UV laser irradiation of anatase surface covered by ethanol moieties lead to higher
degree of titania reduction due to the holes scavenger effect of ethanol itself. In addition, ethanol moieties adsorbed on titania surfaces are photooxidezed during the laser
processing, as confirmed by FT-IR spectra resulting in the introduction of vacancies
not susceptible to ethanol absorption and an indirect C doping of titania surface by the
incorporation of carboxylated species obtained as by-products of ethanol moieties
photoconversion. Under artificial solar light, this sample showed comparable performance respect to other TiO 2 composites tested in the same conditions and higher
activity with respect to C doped titania samples.
These results point out the light on another advantage of the ‘in liquid’ laser
irradiation process: a proper evaluation of the dispersing medium-semiconductor
interaction allows to tune the process, in order to improve the photocatalytic activity
of the semiconductor itself and extend it to visible light wavelength range. This is a
very interesting aspect from a practical point of view.
Furthermore, in comparison to the other TiO 2 -based samples [94] synthetized
with structural and chemical modifications, for the photocatalytic hydrogen production the laser-treated TiO 2 was the best sample under UV irradiation, whereas under
solar/visible light irradiation the TiO 2 chemically modified with other metals showed