167
tion of H 2 O by photoexcited holes is trapped in the surface active sites of the CO 2
reduction co-catalyst (CRC)/water oxidation co-catalyst (WOC), respectively. Steps
3 and 6 are the corresponding recombining and releasing of the energy in the form
of photons or heat.
Normally, step 4 is a multistep process since it involves a series of reactions,
namely, cleaving C–O bonds and forming C–C/C–H bonds. Mostly, a specific CRC
should be employed in order to effectively and selectively reduce CO 2 to the desired
product. During the process, at least two electrons are required to form stable products, which are easier to detect and quantify than the unstable ones or intermediates,
as can be seen in Table 6.1. When a photoexcited hole reaches the semiconductor
surface, step 5 also occurs. Separation of photoexcited photon-hole pairs on the
surface can be promoted by enhancing water oxidation, resulting in an improvement
of CO 2 photoreduction. However, lack of active sites makes recombination of surface charge (step 6) to happen, causing negative effect on the photocatalytic CO 2
reduction. Therefore, enhancement on trapping of the surface should be considered
by improving the surface properties, such as surface active sites and its morphology,
of the semiconductors.
Meanwhile, step 3 that is a competitive process of step 2 plays an essential role
in deactivation process due to the significantly decreasing number of photogenerated charge carriers by forming photons or heat. The overall efficiency can be
enhanced by simultaneously improving the transfer of photoexcited electrons and
holes to semiconductor surface and inhibiting their recombination. It can be seen
that steps 2 and 3 are obviously affected by the nature of a photocatalyst. Therefore,
strategies that can enhance separation and transport of the charge carriers such as
designing nanostructured semiconductors and their heterojunctions deserve to be
investigated.
Fig. 6.3 The overall photocatalytic CO 2 reduction process over a semiconductor. Adapted from Li
et al. (2014b) with permission. (Copyright 2014, Springer Nature. (ads adsorption))
6 Conversion of Carbon Dioxide into Formaldehyde
tion of H 2 O by photoexcited holes is trapped in the surface active sites of the CO 2
reduction co-catalyst (CRC)/water oxidation co-catalyst (WOC), respectively. Steps
3 and 6 are the corresponding recombining and releasing of the energy in the form
of photons or heat.
Normally, step 4 is a multistep process since it involves a series of reactions,
namely, cleaving C–O bonds and forming C–C/C–H bonds. Mostly, a specific CRC
should be employed in order to effectively and selectively reduce CO 2 to the desired
product. During the process, at least two electrons are required to form stable products, which are easier to detect and quantify than the unstable ones or intermediates,
as can be seen in Table 6.1. When a photoexcited hole reaches the semiconductor
surface, step 5 also occurs. Separation of photoexcited photon-hole pairs on the
surface can be promoted by enhancing water oxidation, resulting in an improvement
of CO 2 photoreduction. However, lack of active sites makes recombination of surface charge (step 6) to happen, causing negative effect on the photocatalytic CO 2
reduction. Therefore, enhancement on trapping of the surface should be considered
by improving the surface properties, such as surface active sites and its morphology,
of the semiconductors.
Meanwhile, step 3 that is a competitive process of step 2 plays an essential role
in deactivation process due to the significantly decreasing number of photogenerated charge carriers by forming photons or heat. The overall efficiency can be
enhanced by simultaneously improving the transfer of photoexcited electrons and
holes to semiconductor surface and inhibiting their recombination. It can be seen
that steps 2 and 3 are obviously affected by the nature of a photocatalyst. Therefore,
strategies that can enhance separation and transport of the charge carriers such as
designing nanostructured semiconductors and their heterojunctions deserve to be
investigated.
Fig. 6.3 The overall photocatalytic CO 2 reduction process over a semiconductor. Adapted from Li
et al. (2014b) with permission. (Copyright 2014, Springer Nature. (ads adsorption))
6 Conversion of Carbon Dioxide into Formaldehyde