172
these metal ions can create for highly selective CO 2 photoreduction. Recently,
Fujishima and co-workers reported a good selectivity of products between HCHO
and CH 3 OH (4.5:1) via CO 2 reduction using copper-doped platinum (PtCu/SiO 2 )
(Fujishima et al. 2013).
Secondly, forming the structural defectiveness allows improving the visiblelight- driven photocatalytic systems by forming the vacancies for the material surface, resulting in altering the electronic properties (e.g., light absorbability
intensification, charge transportation and separation, valence band/conduction band
activation), thus enhancing the CO 2 reduction into fuel products (e.g., HCHO). For
example, oxygen vacancies of the semiconductor surface can offer a high degree of
photoactivity towards CO 2 reduction. Formation of oxygen vacancies can progress
if the number of oxygen molecules is de facto less than those necessary for forming
a normal structure. Therefore, instead of annealing under normal conditions, this
process is mostly conducted in an oxygen-reducing atmosphere. A recent work
attempted to introduce a novel positron annihilation lifetime spectroscopy (PALS)
in the characterization of the oxygen vacancies for modified TiO 2 photocatalyst (Ao
and Lee 2004). Surface oxygen vacancies content in the TiO 2 occupied the percentage of 3% only, boosting the photocatalytic activity. Similarly, Co 3 O 4 single-unitcell layers support rich in surface oxygen vacancies was investigated by Zhang and
co-workers (Zhang et al. 2017). This system showed an outstanding activity for
electroreduction of CO 2 with current density of 2.7 mA cm
−2
and high formate
selectivity (85%) after 40 h. In addition, Zhang and co-workers provided an insight
into a clear atomic-level correlation between CO 2 reduction and oxygen vacancies.
Thirdly, to improve the sensitization-based photocatalytic activity, using dye
sensitizers is considered as a remarkable approach to widen the bandgap of photocatalytic systems. The role of sensitizers relies on intensifying the absorbability
towards visible-light (e.g., sunlight) source and intercepts the recombination
between electrons and holes. For example, C 1 - and C 2 -products formation, including
HCOOH, HCHO, CH 3 OH, and C 2 H 5 OH from the photocatalytic reduction of CO 2
using dye-sensitized TiO 2 film under visible-light irradiation, was reported with
high performance and efficiency (mostly 1.5 mmol cm
−2
for HCHO yield reaching
after 30 h illumination) (Qin et al. 2013). The design of functionalized TiO 2 film
with a high sensitizer (bis(tetrabutylammonium)-cis-bis(isothiocyanato)bis(2,2″bipyridyl-4,4″-dicarboxylato)-ruthenium(II) (N719) restricted the electrons and
holes recombination in dye-sensitized zone. Especially, the formation rate of
HCOOH, CH 3 OH, and HCHO on zinc phthalocyanine (ZnPc)- or CoPc-sensitized
TiO 2 was reported to be so far higher compared with TiO 2 catalyst only (Indrakanti
et al. 2009). Nevertheless, optimization of catalyst dosage and selection of sensitizers for the effectiveness and chemical stability of dye-sensitized photocatalytic systems may become a challenge to reach their applications.
Fourthly, the SPR effect implies the collective oscillation of the conduction electrons in nanomaterials under illumination (Ingram et al. 2011). Noble metals
nanoparticles, namely, Au, Ag, and Pt, firmly absorb in the ultraviolet-visible region
because of their SPR effect. This mechanism is interpreted due to the collective
oscillations of conduction band electrons in metals nanoparticles, stimulated by
T. D. Nguyen et al.
these metal ions can create for highly selective CO 2 photoreduction. Recently,
Fujishima and co-workers reported a good selectivity of products between HCHO
and CH 3 OH (4.5:1) via CO 2 reduction using copper-doped platinum (PtCu/SiO 2 )
(Fujishima et al. 2013).
Secondly, forming the structural defectiveness allows improving the visiblelight- driven photocatalytic systems by forming the vacancies for the material surface, resulting in altering the electronic properties (e.g., light absorbability
intensification, charge transportation and separation, valence band/conduction band
activation), thus enhancing the CO 2 reduction into fuel products (e.g., HCHO). For
example, oxygen vacancies of the semiconductor surface can offer a high degree of
photoactivity towards CO 2 reduction. Formation of oxygen vacancies can progress
if the number of oxygen molecules is de facto less than those necessary for forming
a normal structure. Therefore, instead of annealing under normal conditions, this
process is mostly conducted in an oxygen-reducing atmosphere. A recent work
attempted to introduce a novel positron annihilation lifetime spectroscopy (PALS)
in the characterization of the oxygen vacancies for modified TiO 2 photocatalyst (Ao
and Lee 2004). Surface oxygen vacancies content in the TiO 2 occupied the percentage of 3% only, boosting the photocatalytic activity. Similarly, Co 3 O 4 single-unitcell layers support rich in surface oxygen vacancies was investigated by Zhang and
co-workers (Zhang et al. 2017). This system showed an outstanding activity for
electroreduction of CO 2 with current density of 2.7 mA cm
−2
and high formate
selectivity (85%) after 40 h. In addition, Zhang and co-workers provided an insight
into a clear atomic-level correlation between CO 2 reduction and oxygen vacancies.
Thirdly, to improve the sensitization-based photocatalytic activity, using dye
sensitizers is considered as a remarkable approach to widen the bandgap of photocatalytic systems. The role of sensitizers relies on intensifying the absorbability
towards visible-light (e.g., sunlight) source and intercepts the recombination
between electrons and holes. For example, C 1 - and C 2 -products formation, including
HCOOH, HCHO, CH 3 OH, and C 2 H 5 OH from the photocatalytic reduction of CO 2
using dye-sensitized TiO 2 film under visible-light irradiation, was reported with
high performance and efficiency (mostly 1.5 mmol cm
−2
for HCHO yield reaching
after 30 h illumination) (Qin et al. 2013). The design of functionalized TiO 2 film
with a high sensitizer (bis(tetrabutylammonium)-cis-bis(isothiocyanato)bis(2,2″bipyridyl-4,4″-dicarboxylato)-ruthenium(II) (N719) restricted the electrons and
holes recombination in dye-sensitized zone. Especially, the formation rate of
HCOOH, CH 3 OH, and HCHO on zinc phthalocyanine (ZnPc)- or CoPc-sensitized
TiO 2 was reported to be so far higher compared with TiO 2 catalyst only (Indrakanti
et al. 2009). Nevertheless, optimization of catalyst dosage and selection of sensitizers for the effectiveness and chemical stability of dye-sensitized photocatalytic systems may become a challenge to reach their applications.
Fourthly, the SPR effect implies the collective oscillation of the conduction electrons in nanomaterials under illumination (Ingram et al. 2011). Noble metals
nanoparticles, namely, Au, Ag, and Pt, firmly absorb in the ultraviolet-visible region
because of their SPR effect. This mechanism is interpreted due to the collective
oscillations of conduction band electrons in metals nanoparticles, stimulated by
T. D. Nguyen et al.