4 Laser-Induced Synthesis and Processing of Nanoparticles …
149
novel utilization of artificial nanoscale materials. To meet the major challenges in
environmental sustainability, these nanomaterials in various hierarchical fashions are
stimulating important practical applications in the environmental field sector. The
rapid development in materials and catalysis science has led to significant advances
in understanding the controlled synthesis and structure–activity relationship of the
nanomaterials. The design, synthesis, and modification of novel nanomaterials allow
for enhanced performance for environmental-related applications.
In this section, we intend to present three examples in which ‘in water’ laser
irradiation of TiO 2 and Graphene Oxide (GO) colloids is able to enhance the performances towards photocatalytic water splitting and water purification (dye removal
and antibacterial activity) applications, respectively.
Activated carbons are generally used as adsorbents for dyes removal because
of their extremely high surface areas (1000 m
2 /g on average) [66]. Several
research groups specifically studied other new strategies for the synthesis of noncarbonaceous, low-cost adsorbents as TiO 2 nanowires, zeolites, zinc ferrite nanoparticles, or mesoporous Cu 2 O submicro-spheres. Anyway, carbon-based materials
remain the most attractive thanks to their properties such as structural diversity,
chemical stability, and cost-effective synthesis. Recently, graphene and graphene
oxide (GO) are drawing much attention as alternative materials for adsorption [67]
of various substances such as dyes, heavy metals and phenols [68].
The large theoretical surface area (as high as 2630 m
2 /g) of GO and the presence of the oxygen functional groups induce a negative charge on the surface that is
responsible for the formation of stable aqueous colloids, and favors the adsorption of
positively charged species, such as metal ions or cationic dyes [68–71]. Because of
such properties graphene oxide and reduced graphene oxide (rGO) could be considered promising adsorbents for dyes like methylene blue (MB). In this contest, we
have reported the reduction of GO sheets by the pulsed laser irradiation in liquid technique [72]. The main benefit of such technique with respect to the common reduction
methods for GO, is the absence of chemical by-products during the synthesis making
this method greener than chemical reduction processes.
GO was synthetized by a modified Hummers and Offeman’s method [73]. The
obtained sheets showed (AFM analysis) a thickness of ~0.7 nm, which agrees with
standard thickness for such synthesis method reported in the literature for graphene
oxide dispersions in water. The reason why GO layers result thicker than single-layer
graphene sheets (0.34 nm) is due to the existence of the functional groups containing
oxygen in the basal plane of the structure, to the roughness ascribed to sp
3 centers
and to the presence of point defects in the carbon lattice.
GO was reduced by irradiating a GO solution with the second harmonic of a
pulsed Nd:YAG laser with a pulse duration of 5 ns with a repetition rate of 10 Hz.
The irradiation process was carried out, without any focusing lens, under strong
stirring conditions, to ensure a homogeneous irradiation of the GO solution, at a
constant fluence of 0.32 J/cm
2 for different times. With this experimental set up,
we obtained stable solutions of rGO with different degree of reduction, depending
on the time of irradiation and the degree of reduction was confirmed by several
characterization analyses. We hypothesized that the reduction process under laser
149
novel utilization of artificial nanoscale materials. To meet the major challenges in
environmental sustainability, these nanomaterials in various hierarchical fashions are
stimulating important practical applications in the environmental field sector. The
rapid development in materials and catalysis science has led to significant advances
in understanding the controlled synthesis and structure–activity relationship of the
nanomaterials. The design, synthesis, and modification of novel nanomaterials allow
for enhanced performance for environmental-related applications.
In this section, we intend to present three examples in which ‘in water’ laser
irradiation of TiO 2 and Graphene Oxide (GO) colloids is able to enhance the performances towards photocatalytic water splitting and water purification (dye removal
and antibacterial activity) applications, respectively.
Activated carbons are generally used as adsorbents for dyes removal because
of their extremely high surface areas (1000 m
2 /g on average) [66]. Several
research groups specifically studied other new strategies for the synthesis of noncarbonaceous, low-cost adsorbents as TiO 2 nanowires, zeolites, zinc ferrite nanoparticles, or mesoporous Cu 2 O submicro-spheres. Anyway, carbon-based materials
remain the most attractive thanks to their properties such as structural diversity,
chemical stability, and cost-effective synthesis. Recently, graphene and graphene
oxide (GO) are drawing much attention as alternative materials for adsorption [67]
of various substances such as dyes, heavy metals and phenols [68].
The large theoretical surface area (as high as 2630 m
2 /g) of GO and the presence of the oxygen functional groups induce a negative charge on the surface that is
responsible for the formation of stable aqueous colloids, and favors the adsorption of
positively charged species, such as metal ions or cationic dyes [68–71]. Because of
such properties graphene oxide and reduced graphene oxide (rGO) could be considered promising adsorbents for dyes like methylene blue (MB). In this contest, we
have reported the reduction of GO sheets by the pulsed laser irradiation in liquid technique [72]. The main benefit of such technique with respect to the common reduction
methods for GO, is the absence of chemical by-products during the synthesis making
this method greener than chemical reduction processes.
GO was synthetized by a modified Hummers and Offeman’s method [73]. The
obtained sheets showed (AFM analysis) a thickness of ~0.7 nm, which agrees with
standard thickness for such synthesis method reported in the literature for graphene
oxide dispersions in water. The reason why GO layers result thicker than single-layer
graphene sheets (0.34 nm) is due to the existence of the functional groups containing
oxygen in the basal plane of the structure, to the roughness ascribed to sp
3 centers
and to the presence of point defects in the carbon lattice.
GO was reduced by irradiating a GO solution with the second harmonic of a
pulsed Nd:YAG laser with a pulse duration of 5 ns with a repetition rate of 10 Hz.
The irradiation process was carried out, without any focusing lens, under strong
stirring conditions, to ensure a homogeneous irradiation of the GO solution, at a
constant fluence of 0.32 J/cm
2 for different times. With this experimental set up,
we obtained stable solutions of rGO with different degree of reduction, depending
on the time of irradiation and the degree of reduction was confirmed by several
characterization analyses. We hypothesized that the reduction process under laser