US EPA

1589174 

Journal Article 

Electrons in the Band Gap: Spectroscopic Characterization of Anatase TiO2 Nanocrystal Electrodes under Fermi Level Control 

Berger, T; Anta, JA; Morales-Florez, V 

2012 

Yes 

Journal of Physical Chemistry C
ISSN: 1932-7447
EISSN: 1932-7455 

116 

21 

11444-11455 

10.1021/p212436b 

WOS:000304574500004 

Macroscopic properties of semiconductor nanoparticle
networks in functional devices strongly depend on the electronic structure of the material.
Analytical methods allowing for the characterization of the electronic structure in situ, i.e.,
in the presence of an application-relevant medium, are therefore highly desirable. Here, we
present the first spectral data obtained under Fermi level control of electrons accumulated in
anatase TiO2 electrodes in the energy range from the MIR to the UV (0.1-3.3 eV). Band gap states
were electrochemically populated in mesoporous TiO2 films in contact with an aqueous electrolyte.
The combination of electrochemical and spectroscopic measurements allows us for the first time to
determine both the energetic location of the electronic ground states as well as the energies of
the associated optical transitions in the energetic range between the fundamental absorption
threshold and the onset of lattice absorption. On the basis of our observations, we attribute
spectral contributions in the vis/NIR to d-d transitions of Ti3+ species and a broad MIR
absorption, monotonically increasing toward lower wavenumbers, to a quasi-delocalization of
electrons. Importantly, signal intensities in the vis/NIR and MIR. are linearly correlated.
Absorbance and extractable charge show the same exponential dependence on electrode potential.
Our results demonstrate that signals in the vis/NIR and MIR are associated with an exponential
distribution of band gap states.