1 3
Topics in Current Chemistry (2019) 377:27
decomposition, depended on a limited amount of Cd
2+
. Therefore, different strategies based on CdS have been explored to enhance the photocatalytic activity in
the FA decomposition. Chen et al. [88] reported that CdS particles were embedded
on titanate nanotubes (TNTs) using a hydrothermal synthesis. From TEM images,
confined CdS particles as well as CdS particles suported on the surface of TNTs
were observed. Moreover, XRD patterns and UV spectra confirmed the presence
of CdS particles. The photocatalytic activity towards H 2 production was evaluated
in a 10 vol% FA solution. CdS/TNTs produced 179.4 µmol of H 2 after 3 h while
TNTs generated only 0.09 µmol. This enhancement could be related to the transfer
of photogenerated electrons from the valence band to the conduction band. These
electrons could be transferred to TNTs because both valence band and conduction
band positions of CdS are higher compared to TNTs. Recently, Liou et al. [89] published a composite that consisted of Pt, CdS, and TNTs synthesized by microwave
hydrothermal method. The H 2 evolution rate for different catalysts were assessed
in an aqueous FA solution (10 vol%) under visible light. Neither TiO 2 nor TNTs
catalysts displayed activity towards the FA decomposition. However, the addition
of CdS into titania structures promoted photocatalytic activity, being more active in
the case of Cds/TNTs (295.0 µmol h
−1
) than CdS/TiO 2 (118.7 µmol h
−1
). This effect
was attributted to a higher active surface area for TNTs compared to TiO 2 , which
boosted the adsorption of FA onto the active sites and reduced recombination of
the electron–hole pair. After this, Pt was loaded on titania-based photocatalysts in
order to trap the photogenerated electrons, resulting in photocatalysts that were more
active towards FA decomposition. To do that, impregnation and photodeposition
methods were carried out, leading to Pt-CdS/TNT samples prepared by the thermal
impregnation method with different Pt loadings, which exhibited poorer photocatalytic activities compared to that of CdS/TNT photocatalyst. This decline in H 2 production was related to agglomeration after thermal impregnation. However, Pt-CdS/
TNT synthesized by photodeposition of Pt displayed a remarkable photocatalytic
activity for H 2 production (661.1 µmol h
−1
). The enhancement of H 2 production was
due to a smaller Pt nanoparticle size and better distribution of nanoparticles onto
CdS/TNT, resulting in better contact between Pt and TNTs, which improved separation of the electron–hole pair. Concerning the mechanism, it was suggested that the
photogenerated electrons were transferred from the valence band to the conduction
band of CdS, and therefore the concentration of holes increased in the valence band.
According to the band gap of TNTs, the photoelectrons in CdS jumped quickly into
TNTs. Moreover, it was indicated that Ti
3+
species present in TNTs (due to the oxygen vacancy) may capture electrons, and that Pt nanoparticles act as active sites
for H 2 production. Additionally, Ti
4+
species were suggested as hole co-catalysts
because the photogenerated holes in the CdS surface can be trapped by Ti
4+
species,
avoiding the CdS photocorrosion.
A different strategy was tackled by Li et al. [90], who reported the use of visible
light-driven photocatalysts based on a CdS–ZnS composite with heterogeneous structure. In this system, CdS, with a narrow band gap and high photo-sensitivity, showed
great reactivity for the production of H 2 , while ZnS, with a wider band gap, reduced
the recombination of electron–hole pairs. The nanoparticles of the CdS–ZnS photocatalyst were prepared by a microemulsion technique in a system composed of water/
207
Reprinted from the journal
Topics in Current Chemistry (2019) 377:27
decomposition, depended on a limited amount of Cd
2+
. Therefore, different strategies based on CdS have been explored to enhance the photocatalytic activity in
the FA decomposition. Chen et al. [88] reported that CdS particles were embedded
on titanate nanotubes (TNTs) using a hydrothermal synthesis. From TEM images,
confined CdS particles as well as CdS particles suported on the surface of TNTs
were observed. Moreover, XRD patterns and UV spectra confirmed the presence
of CdS particles. The photocatalytic activity towards H 2 production was evaluated
in a 10 vol% FA solution. CdS/TNTs produced 179.4 µmol of H 2 after 3 h while
TNTs generated only 0.09 µmol. This enhancement could be related to the transfer
of photogenerated electrons from the valence band to the conduction band. These
electrons could be transferred to TNTs because both valence band and conduction
band positions of CdS are higher compared to TNTs. Recently, Liou et al. [89] published a composite that consisted of Pt, CdS, and TNTs synthesized by microwave
hydrothermal method. The H 2 evolution rate for different catalysts were assessed
in an aqueous FA solution (10 vol%) under visible light. Neither TiO 2 nor TNTs
catalysts displayed activity towards the FA decomposition. However, the addition
of CdS into titania structures promoted photocatalytic activity, being more active in
the case of Cds/TNTs (295.0 µmol h
−1
) than CdS/TiO 2 (118.7 µmol h
−1
). This effect
was attributted to a higher active surface area for TNTs compared to TiO 2 , which
boosted the adsorption of FA onto the active sites and reduced recombination of
the electron–hole pair. After this, Pt was loaded on titania-based photocatalysts in
order to trap the photogenerated electrons, resulting in photocatalysts that were more
active towards FA decomposition. To do that, impregnation and photodeposition
methods were carried out, leading to Pt-CdS/TNT samples prepared by the thermal
impregnation method with different Pt loadings, which exhibited poorer photocatalytic activities compared to that of CdS/TNT photocatalyst. This decline in H 2 production was related to agglomeration after thermal impregnation. However, Pt-CdS/
TNT synthesized by photodeposition of Pt displayed a remarkable photocatalytic
activity for H 2 production (661.1 µmol h
−1
). The enhancement of H 2 production was
due to a smaller Pt nanoparticle size and better distribution of nanoparticles onto
CdS/TNT, resulting in better contact between Pt and TNTs, which improved separation of the electron–hole pair. Concerning the mechanism, it was suggested that the
photogenerated electrons were transferred from the valence band to the conduction
band of CdS, and therefore the concentration of holes increased in the valence band.
According to the band gap of TNTs, the photoelectrons in CdS jumped quickly into
TNTs. Moreover, it was indicated that Ti
3+
species present in TNTs (due to the oxygen vacancy) may capture electrons, and that Pt nanoparticles act as active sites
for H 2 production. Additionally, Ti
4+
species were suggested as hole co-catalysts
because the photogenerated holes in the CdS surface can be trapped by Ti
4+
species,
avoiding the CdS photocorrosion.
A different strategy was tackled by Li et al. [90], who reported the use of visible
light-driven photocatalysts based on a CdS–ZnS composite with heterogeneous structure. In this system, CdS, with a narrow band gap and high photo-sensitivity, showed
great reactivity for the production of H 2 , while ZnS, with a wider band gap, reduced
the recombination of electron–hole pairs. The nanoparticles of the CdS–ZnS photocatalyst were prepared by a microemulsion technique in a system composed of water/
207
Reprinted from the journal