4 Laser-Induced Synthesis and Processing of Nanoparticles …
153
The photocatalyst is the substance which can modify the rate of chemical reaction
using light irradiation. Photocatalysis finds a large number of applications in many
fields such as the removal of pollutants from water or air and hydrogen production. In
particular, water treatment technology by photocatalysis is low cost, environmentally
friendly, and sustainable, while photocatalytic water splitting for the production of
hydrogen may become more competitive with respect to the conventional production
method (hydrocarbon steam reforming) as the cost continues to decrease with the
technology advancement.
Since the first report by Fujishima and Honda [75], photocatalytic water-splitting
using titanium dioxide TiO 2 for hydrogen production offers a promising way for
clean, low-cost, and environmentally friendly production of hydrogen by solar
energy. Presently, the solar-to-hydrogen energy conversion efficiency is too low for
the technology to be economically sound. The main barriers are the rapid recombination of photo-generated electron/hole pairs as well as backward reaction and the poor
activation of TiO 2 by visible light. Some investigators studied the effects of addition
of sacrificial reagents and carbonate salts to prohibit rapid recombination of electron/hole pairs and backward reactions. Other research focused on the enhancement
of photocatalysis by modification of TiO 2 by means of doping of pure titania with
metals [76–78], non-metals [79, 80] or the fabrication of TiO 2 -based nanohybrids
[81, 82].
In addition to above-mentioned methods, recent investigations have also demonstrated that the presence of defect-containing (blue) TiO 2 significantly improves the
photocatalytic activity in the overall solar spectrum, including visible and UV [83,
84]. Blue titania surface defect states are composed by under-coordinated Ti
4+ sites
and oxygen vacancies which act as anchoring and charge injection/recombination
sites, playing a crucial role in the electron injection and recombination dynamics.
Literature presents several methods to dope TiO 2 with metals, non-metals, or
combining it with carbon nanostructures that involve chemical processes with the
possibility of introducing impurities inside the chemical structure of titania. In addition, treatment under vacuum or under reducing conditions and treatments by using
hydrogen plasmas have been used for reducing titania and introducing defects. These
methods unavoidably involve high processing temperatures (400–700 °C), vacuum
systems, long processing times, and multistep operations. For these reasons, it is
exceptionally desirable to explore simple and economic strategies to modify titanium
dioxide with increased photocatalytic activity.
Pulsed lasers are powerful tools for the time-efficient preparation and/or modification of functional materials [85] and these are recently investigated for defects
introduction in metal oxides semiconductors. Yang et al. obtained defective anatase
by laser ablation of a titanium target in water by using a nanosecond 532 nm laser
[86] or by using a near-IR laser [87]. Bulk defects such as oxygen vacancies of the
LAL-synthesized titania samples were observed and these induced a clear blueshift
and broadening of the band gap band in the Raman spectra. Besides nanoparticles
formation by laser ablation, laser irradiation methods make it possible to modify the
chemical/physical properties of nanoparticles directly dispersed in solution such as
153
The photocatalyst is the substance which can modify the rate of chemical reaction
using light irradiation. Photocatalysis finds a large number of applications in many
fields such as the removal of pollutants from water or air and hydrogen production. In
particular, water treatment technology by photocatalysis is low cost, environmentally
friendly, and sustainable, while photocatalytic water splitting for the production of
hydrogen may become more competitive with respect to the conventional production
method (hydrocarbon steam reforming) as the cost continues to decrease with the
technology advancement.
Since the first report by Fujishima and Honda [75], photocatalytic water-splitting
using titanium dioxide TiO 2 for hydrogen production offers a promising way for
clean, low-cost, and environmentally friendly production of hydrogen by solar
energy. Presently, the solar-to-hydrogen energy conversion efficiency is too low for
the technology to be economically sound. The main barriers are the rapid recombination of photo-generated electron/hole pairs as well as backward reaction and the poor
activation of TiO 2 by visible light. Some investigators studied the effects of addition
of sacrificial reagents and carbonate salts to prohibit rapid recombination of electron/hole pairs and backward reactions. Other research focused on the enhancement
of photocatalysis by modification of TiO 2 by means of doping of pure titania with
metals [76–78], non-metals [79, 80] or the fabrication of TiO 2 -based nanohybrids
[81, 82].
In addition to above-mentioned methods, recent investigations have also demonstrated that the presence of defect-containing (blue) TiO 2 significantly improves the
photocatalytic activity in the overall solar spectrum, including visible and UV [83,
84]. Blue titania surface defect states are composed by under-coordinated Ti
4+ sites
and oxygen vacancies which act as anchoring and charge injection/recombination
sites, playing a crucial role in the electron injection and recombination dynamics.
Literature presents several methods to dope TiO 2 with metals, non-metals, or
combining it with carbon nanostructures that involve chemical processes with the
possibility of introducing impurities inside the chemical structure of titania. In addition, treatment under vacuum or under reducing conditions and treatments by using
hydrogen plasmas have been used for reducing titania and introducing defects. These
methods unavoidably involve high processing temperatures (400–700 °C), vacuum
systems, long processing times, and multistep operations. For these reasons, it is
exceptionally desirable to explore simple and economic strategies to modify titanium
dioxide with increased photocatalytic activity.
Pulsed lasers are powerful tools for the time-efficient preparation and/or modification of functional materials [85] and these are recently investigated for defects
introduction in metal oxides semiconductors. Yang et al. obtained defective anatase
by laser ablation of a titanium target in water by using a nanosecond 532 nm laser
[86] or by using a near-IR laser [87]. Bulk defects such as oxygen vacancies of the
LAL-synthesized titania samples were observed and these induced a clear blueshift
and broadening of the band gap band in the Raman spectra. Besides nanoparticles
formation by laser ablation, laser irradiation methods make it possible to modify the
chemical/physical properties of nanoparticles directly dispersed in solution such as