semiconductor counterpart; besides, the Schottky barrier formed at the metal–semiconductor interface thus could prevent the electrons from flowing backward. In this
way, the surface sites of metal NPs become the active sites for the CO2PR reduction,
and the performance of the metal–semiconductor composite is highly depending on
the electron trapping ability of the supported metal NPs.
Xie et al. [10] compared the activity of five noble metals (Ag, Au, Rh, Pt, Pd)
supported on TiO 2 in CO2PR. The yield of CH 4 and the rate of total electrons’
consumption in the CO2PR increase with the order of TiO 2 < Ag-TiO 2 < RhTiO 2 < Au-TiO 2 < Pd-TiO 2 < Pt–TiO 2 , which equals with the same trend of the
work function of these noble metals. This result reflects the fact that the electron
trapping ability contributes to the reductive performance of supported metal catalyst
and Pt–TiO 2 in this evaluation system is superior to the others. Since Pt is a very
efficient cocatalyst in photocatalysis, the rational designation of Pt NPs with suitable
shape (expose certain facets) and particle size (both geometric and electronic) is
important. As we mentioned before, Wang et al. [9] synthesized a series of differentsized Pt NPs ranging from 0.5 to 1.5 nm loaded on the 1D TiO 2 single crystals
through a TTS method, and the 1 nm Pt NPs show the highest CH 4 yield. The author
claims that the ultrasmall Pt NPs (less than 1 nm) could prevent the electrons
transferring from TiO 2 because of its higher energy band compared with the CB of
TiO 2 ; on the contrary, bigger Pt NPs act as electron-hole recombination center which
is also detrimental in the photocatalysis. Furthermore, the author adopted the
femtosecond time-resolved TA spectroscopy to elucidate the charge transfer dynamics. After liner fitting, Pt–TiO 2 shows a greater slope compared with its TiO 2
counterpart which directly reflects the Pt NPs suppress the charge recombination
process.
In order to replace the expensive noble metal cocatalysts into some earthabundant materials, the development of noble metal-free cocatalysts with comparable performance is essential. Among them, carbon dots and graphene also play key
role in promoting the charge separation in photocatalysis. Ong et al. [55] report
carbon nanodots (CND) supported on protonated C 3 N 4 composite. The obtained
CND/pCN shows the CND with 4.4 nm diameter dispersed well on the pCN surface,
and the CND did not affect the adsorption edge of C 3 N 4 but act as conductive
electron channel for charge separation (Fig. 12.4a, b). The author adopted steadystate PL spectroscopy and time-resolved transient PL decay to verify the charge
separation kinetics. The pure p-C 3 N 4 shows an intensive and broad PL emission
peak which means a great extent of electron-hole recombination; the CND/pCN
hybrids on the other hand show obvious decrease of peak intensity which suggests
the recombination of charge carriers were suppressed (Fig. 12.4c). The emission
lifetime of CND/pCN reduced compared with pCN which means the rapid interfacial
electrons inject from pCN to CNDs and participate in the CO2PR reaction
(Fig. 12.4d). Besides the electrons’ trapping ability, CNDs also can serve as the
photosensitizer. Yu et al. [60] reported CDs/TiO 2 composite with enhanced visible
light hydrogen production rate. The author claimed that π-conjugated CDs sensitize
TiO 2 by forming C-O-Ti bond and donate electrons under visible light irradiation.
12.4 Roles and Properties of Different Cocatalysts
291
way, the surface sites of metal NPs become the active sites for the CO2PR reduction,
and the performance of the metal–semiconductor composite is highly depending on
the electron trapping ability of the supported metal NPs.
Xie et al. [10] compared the activity of five noble metals (Ag, Au, Rh, Pt, Pd)
supported on TiO 2 in CO2PR. The yield of CH 4 and the rate of total electrons’
consumption in the CO2PR increase with the order of TiO 2 < Ag-TiO 2 < RhTiO 2 < Au-TiO 2 < Pd-TiO 2 < Pt–TiO 2 , which equals with the same trend of the
work function of these noble metals. This result reflects the fact that the electron
trapping ability contributes to the reductive performance of supported metal catalyst
and Pt–TiO 2 in this evaluation system is superior to the others. Since Pt is a very
efficient cocatalyst in photocatalysis, the rational designation of Pt NPs with suitable
shape (expose certain facets) and particle size (both geometric and electronic) is
important. As we mentioned before, Wang et al. [9] synthesized a series of differentsized Pt NPs ranging from 0.5 to 1.5 nm loaded on the 1D TiO 2 single crystals
through a TTS method, and the 1 nm Pt NPs show the highest CH 4 yield. The author
claims that the ultrasmall Pt NPs (less than 1 nm) could prevent the electrons
transferring from TiO 2 because of its higher energy band compared with the CB of
TiO 2 ; on the contrary, bigger Pt NPs act as electron-hole recombination center which
is also detrimental in the photocatalysis. Furthermore, the author adopted the
femtosecond time-resolved TA spectroscopy to elucidate the charge transfer dynamics. After liner fitting, Pt–TiO 2 shows a greater slope compared with its TiO 2
counterpart which directly reflects the Pt NPs suppress the charge recombination
process.
In order to replace the expensive noble metal cocatalysts into some earthabundant materials, the development of noble metal-free cocatalysts with comparable performance is essential. Among them, carbon dots and graphene also play key
role in promoting the charge separation in photocatalysis. Ong et al. [55] report
carbon nanodots (CND) supported on protonated C 3 N 4 composite. The obtained
CND/pCN shows the CND with 4.4 nm diameter dispersed well on the pCN surface,
and the CND did not affect the adsorption edge of C 3 N 4 but act as conductive
electron channel for charge separation (Fig. 12.4a, b). The author adopted steadystate PL spectroscopy and time-resolved transient PL decay to verify the charge
separation kinetics. The pure p-C 3 N 4 shows an intensive and broad PL emission
peak which means a great extent of electron-hole recombination; the CND/pCN
hybrids on the other hand show obvious decrease of peak intensity which suggests
the recombination of charge carriers were suppressed (Fig. 12.4c). The emission
lifetime of CND/pCN reduced compared with pCN which means the rapid interfacial
electrons inject from pCN to CNDs and participate in the CO2PR reaction
(Fig. 12.4d). Besides the electrons’ trapping ability, CNDs also can serve as the
photosensitizer. Yu et al. [60] reported CDs/TiO 2 composite with enhanced visible
light hydrogen production rate. The author claimed that π-conjugated CDs sensitize
TiO 2 by forming C-O-Ti bond and donate electrons under visible light irradiation.
12.4 Roles and Properties of Different Cocatalysts
291