10.1 Ti(0001) Skin and TiO 2 Nanocrystals
209
10.1.4 Defect Enhanced Photocatalytic Ability
When a TiO 2 surface is illuminated by light with energy equaling to that of the
band gap, the radiation excites electrons in the ground VB to the upper conduction
band (CB, e
− ), leaving a hole (h
+ ) behind the VB. The excited carriers are highly
reactive to radicals with robust reducing and oxidizing capacity. The carriers may
recombine or be trapped by metastable surface states, or react with suitable electron
acceptors/donors pre-adsorbed on the catalyst surface.
Major concerns in improving the photocatalytic ability of TiO 2 include:
(i) reducing the band gap to match the wavelength of visible light for electron
excitation;
(ii) lowering the work function to ease migration of the excited electrons; and,
(iii) prolonging the lifetime of the carriers for slowing electron-hole recombination.
Currently, the band gap matches only UV range that accounts only 4% of solar
light. During photocatalytic reaction, when the reduction and oxidation do not proceed simultaneously, there is an electron accumulation in the CB, thereby causing a
fast recombination of the e–h pairs. Therefore, improve the utilization rate of sunlight by modulating band gap and work function and by raising carrier lifetime and
electroaffinity via locally pinning the polarized electrons is a feasible means.
Atomic undercoordination and hetero-coordination are useful in this situation. At
first, the entrapment of the valence electrons deepens the energy states and enlarges
the electroaffinity, which polarizes the conduction electrons shifting up in energy
to lower the work function. Meanwhile, the polarization screens and splits the local
potential to create the polarized states to raise the upper edge of the VB. This process narrows the band gap. The strong localization of the defect dipoles by pinning
prolongs the carrier life effectively. Therefore, undercoordinated defects modulate
the band gap, carrier life, and work function is a promising means.
It is also clear now why metal atoms added to a defected TiO 2 and graphene
surface could improve the efficiency of CO oxidation at room temperature compared
with the otherwise fully covered surface, like Au [25]. TiO 2 defect dipoles polarize
the undercoordinated metal adatoms further to lowers the work function and hence
improves the reactivity of the supported metallic adatom dipoles [26].
10.2 ZnO Nanocrystals Passivated with H, N, and O
10.2.1 ZPS: Size-Induced Entrapment-Polarization
Transition
Figure 10.3a shows the joint under- and hetero-coordination effects on the Zn 2p
energy. XPS revealed that a transition from the positive to the negative CLS happens
at 8.5 nm [27]. When the ZnO crystal reduces its size from 200 to 8.5 nm, entrapment
209
10.1.4 Defect Enhanced Photocatalytic Ability
When a TiO 2 surface is illuminated by light with energy equaling to that of the
band gap, the radiation excites electrons in the ground VB to the upper conduction
band (CB, e
− ), leaving a hole (h
+ ) behind the VB. The excited carriers are highly
reactive to radicals with robust reducing and oxidizing capacity. The carriers may
recombine or be trapped by metastable surface states, or react with suitable electron
acceptors/donors pre-adsorbed on the catalyst surface.
Major concerns in improving the photocatalytic ability of TiO 2 include:
(i) reducing the band gap to match the wavelength of visible light for electron
excitation;
(ii) lowering the work function to ease migration of the excited electrons; and,
(iii) prolonging the lifetime of the carriers for slowing electron-hole recombination.
Currently, the band gap matches only UV range that accounts only 4% of solar
light. During photocatalytic reaction, when the reduction and oxidation do not proceed simultaneously, there is an electron accumulation in the CB, thereby causing a
fast recombination of the e–h pairs. Therefore, improve the utilization rate of sunlight by modulating band gap and work function and by raising carrier lifetime and
electroaffinity via locally pinning the polarized electrons is a feasible means.
Atomic undercoordination and hetero-coordination are useful in this situation. At
first, the entrapment of the valence electrons deepens the energy states and enlarges
the electroaffinity, which polarizes the conduction electrons shifting up in energy
to lower the work function. Meanwhile, the polarization screens and splits the local
potential to create the polarized states to raise the upper edge of the VB. This process narrows the band gap. The strong localization of the defect dipoles by pinning
prolongs the carrier life effectively. Therefore, undercoordinated defects modulate
the band gap, carrier life, and work function is a promising means.
It is also clear now why metal atoms added to a defected TiO 2 and graphene
surface could improve the efficiency of CO oxidation at room temperature compared
with the otherwise fully covered surface, like Au [25]. TiO 2 defect dipoles polarize
the undercoordinated metal adatoms further to lowers the work function and hence
improves the reactivity of the supported metallic adatom dipoles [26].
10.2 ZnO Nanocrystals Passivated with H, N, and O
10.2.1 ZPS: Size-Induced Entrapment-Polarization
Transition
Figure 10.3a shows the joint under- and hetero-coordination effects on the Zn 2p
energy. XPS revealed that a transition from the positive to the negative CLS happens
at 8.5 nm [27]. When the ZnO crystal reduces its size from 200 to 8.5 nm, entrapment