Nanotechnostructured Catalysts TiO Nanoparticles for Water Purification - Nanotechnologies for Water Environment Applications - page 53

Figure 3.6 Energy diagram of the interface between an n-type semiconductor surface
and electrolyte (Lin, 2008). The energy scale is presented in both absolute vacuum
energy scale (AVS) and its corresponding normal hydrogen electrode scale (NHE). A
is the electron affinity, χ is the electron negativity, I is the ionization energy, E F is the
Fermi level energy of the system, E C and E V are the energy level at conduction
(LUMO) and valance band edge (HOMO),

E F is the increased potential at interface
due to band bending, V H is the potential drop of Helmholtz layer, E o unoc and E o oc are
the energy levels of the unoccupied and occupied redox couple. E F o unoc is the Fermi
level energy at system equilibrium. φ R is the reduction potential of the acceptor redox
couple. φ O is the oxidation potential of the donor redox couple.
3.3.2 Particle Size Effect

Δ

The bandgap of the semiconductor is governed by its electronic structure. In
addition to the crystalline structure of the semiconductor, the bandgap energy also
depends on its particle size. Decreasing the particle size leads to the increase in specific
surface area. When the particle size is reduced to a few tenths of a nanometer, the
surface atomic structure begins to lose its stoichiometry due to reduction in periodical
arrangement in crystal lattice. The oxide particles can have excess oxygen with
dangling bonds due to the loss of its stoichiometry which, in turn, populate the shallow
electronic states (energy traps) along the band edges (E V and E C ) within the forbidden