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E C
Δ E C
E C
2.5 eV
2.1 eV
E V
Δ E V
E V
Cu 2 O
Cu 2 O:N
figure 3.3 energy level diagram of Cu 2 O reference film and Cu 2 O : N film with N-doping concentration of 2.9%. Reproduced by
permission from Ref. [57]. © 2009, American Institute of Physics.
Band structure and electric properties of doped Cu 2 O are extensively investigated based on the first principles of dFT. Nolan
et al. [52] proposed two conditions related to the dopant that are key for increasing the band gap. The first condition is that
dopants with ionic radii (La 3+ , Sr 2+ , and Ba 2+ dopants) larger than Cu + will result in a larger band gap over stoichiometric Cu 2 O,
while other dopants with ionic radii smaller than Cu + will show no enhancement of the band gap. The second condition is avoid-
ing misalignment of dopant electronic states with Cu 2 O bands, which turns out to be the most important aspect of doping. If
dopant electronic states interact with the VB or CB of Cu 2 O, for example, In 3+ or Cd 2+ , or produce defect states in the Cu 2 O band
gap, for example, Ce 4+ , the band gap is reduced. Nolan et al. [49] also reported that substitutional cation doping with Al and Au/
Ag decreases the band gap of Cu 2 O. Martínez-Ruiz [48] have drawn the conclusion that doping with Ag atoms decreases the
band gap, while doping with Ni results in a p-type semiconductor with impurity levels above the allowed VB maximum and
doping with Zn results in an n-type semiconductor with impurity levels above the CB minimum.
N [57-59] and Cl [60-62] elements are mostly experimentally studied as dopants for Cu 2 O. Nitrogen is an impurity that can
be easily incorporated in Cu 2 O at high concentrations [57] and is a very effective p-type dopant [58], in agreement with the
hypothesis that nitrogen acts as a substitutional impurity for oxygen atoms. Nakano et al. [57] investigated the effect of N
doping for Cu 2 O films deposited by reactive magnetron sputtering. N-doped Cu 2 O films have a positive shift in the position of
the VB edge. Here, the CB shift Δ E C of the Cu 2 O : N films is estimated from the optical band gap Δ E g and VB shift Δ E V . More
interestingly, the CB edge is found to be negatively shifted, with a value a little more than the Δ E V . Band-gap diagrams of Cu 2 O
and Cu 2 O : N with N-doping concentration of 2.9% are typically shown in Figure  3.3. In this case, the Δ E V and Δ E C are
estimated to be 0.17 and 0.23 eV, respectively. Both band edge shifts tend to enlarge with increasing N-doping concentration up
to 3%. However, Malerba et al. [63] excluded the band-gap widening observed by Nakano et al. [57]. They studied the optical
properties of sputtered Cu 2 O thin films doped with nitrogen concentrations between 1 and 2.5 at.%. All of the doped samples
exhibited two clearly defined absorption bands at energies below the gap, with an intensity well correlated with the N
concentration. This result show that both the two-subband absorption processes are related to optical transitions toward two
impurity levels introduced by nitrogen or by more complex defects formed by nitrogen. The current main reason for the studies
on Cl-doped Cu 2 O is that Cu 2 O has potential for application in solar cells [60, 61]. Han et al. [61, 62] found a remarkable improve-
ment in electric conductivity and no significant change in the band gap for Cu 2 O. Other nonmetal-doped (such as Si [64, 65])
and transition metal (Cd [66], Ni [67], Co [68], Mo [69])-doped Cu 2 O were also investigated. Additionally, Tseng et al. [70]
reported that the absorption edges shift to the right (red shift) with an increase in Ag content, which implies shortening of optical
band gap due to Ag doping.
3.2.2.2 Hybridization The semiconductor hybridization approach has been shown to be another effective method for
improving photocatalytic activity through good photogenerated charge separation with the formation of heterojunction structure.
In this configuration, several advantages can be obtained: (i) an improvement of charge separation; (ii) an increase of the charge
carrier lifetime; and (iii) an enhancement of the interfacial charge transfer efficiency to adsorbed substance on the surface. As
is well known, Cu 2 O used as a photocatalyst has low energy-conversion efficiency (<1% [71, 72]), which is due to the fact that
the light-generated charge carriers in Cu 2 O cannot be efficiently transferred to the surface and are lost during recombination
[73]. Moreover, semiconductor materials with band gaps suitable for capturing a significant fraction of the incident solar
spectrum energy (≈0.8-2.4 eV) typically suffer from UV-induced photocorrosion [74], as does Cu 2 O. Combining Cu 2 O com-
posites with other semiconductors or conductive materials may be a good solution to these problems. Cu 2 O-based cocatalysts
such as Cu 2 O/TiO 2 [75, 76], Cu 2 O/ZnO [77], Cu 2 O/chitosan [78], Cu 2 O/multiwalled carbon nanotubes [79], and Au/Cu 2 O [80,
81] show enhanced charge separation efficiency. Cu 2 O-based heterojunction devices such as Cu 2 O/ZnO [82] and Cu 2 O/NiO
[83] show better electric and photo properties or higher quantum efficiency than pure Cu 2 O. Among these composites, the
Cu 2 O/TiO 2 system has attracted the most extensive research. The band-gap energies of Cu 2 O and TiO 2 are around 2.0 and 3.2 eV,
respectively. As shown in Figure 3.4, the CB minimum and the VB maximum of Cu 2 O lie above those of TiO 2 . Therefore, the
 
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