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

Abstract

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 TiO 2 electrodes in the energy range from the MIR to the UV (0.1−3.3 eV). Band gap states were electrochemically populated in mesoporous TiO 2 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 Ti 3+ 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.

IR spectroscopy applied to metal oxide surfaces: adsorbate vibrations and beyond

“…In these differences in spectra (a spectrum recorded before UV exposure is subtracted from the spectrum recorded after UV exposure), we clearly observe a main feature localized at 910 cm −1 along with two additional features at 1205 and 1375 cm −1 . The observation of such UV-induced features in TiO 2 has been reported previously for titania powders by Yates and co-workers [89][90][91][92][93][94][95]. However, in this earlier powder data, the sharper features at 1205 and 1375 cm −1 could not be identified.…”

IR–Spectrophotoelectrochemical Characterization of Mesoporous Semiconductor Films

“…The resulting sintered layers with a film thickness of ∼3 μm consist of anatase nanocrystals with a mean diameter of 20 nm and are transparent in the vis/ NIR. 10 The SPEC cell consists of a glass body, which is connected to a hemispheric ZnSe prism ( Figure 1). An Ushaped perforation of the FTO substrate has to be realized prior to thin film deposition ( Figure 1, bottom left).…”

“…The appearance of the absorption signals at potentials more positive than the photocurrent onset clearly indicates a location of the corresponding electronic states in the band gap. 10 The signals in the vis/NIR and the MIR were linearly correlated with each other and with the number of extracted charges. Furthermore, absorbance and extractable charge showed the same exponential dependence on electrode potential indicating that the signals in the vis/NIR and MIR are associated with an exponential distribution of band gap states.…”

Defects in Metal Oxide Nanoparticle Powders

“…9.3) these absorptions are associated with transitions due to (i) free charge carriers in the mid-infrared (MIR), (ii) excitations of electrons in shallow trap states (MIR-NIR), (iii) d-d transitions of Ti 3+ ions, which are located either in regular or in interstitial lattice sites (Vis) and (iv) the absorption edge due to interband transitions (UV), which may experience a shift to higher energies upon oxide reduction [60]. It is important to note that related spectral signatures have also been observed in TiO 2 after UV light induced charge carrier separation [120], treatment with [60] and on nanocrystal films after cathodic polarization (lower part) [99]. Electron transfer to O 2 and charge extraction upon anodic polarization, respectively, lead to the entire annihilation of all absorption features observed atomic hydrogen [121], injection of radiolytically generated hydrated electrons [122] or, by using a combined spectroscopic and electrochemical approach, upon negative polarization of mesoporous electrodes in aqueous electrolytes (Fig.…”

“…Electron transfer to O 2 and charge extraction upon anodic polarization, respectively, lead to the entire annihilation of all absorption features observed atomic hydrogen [121], injection of radiolytically generated hydrated electrons [122] or, by using a combined spectroscopic and electrochemical approach, upon negative polarization of mesoporous electrodes in aqueous electrolytes (Fig. 9.3b) [81,99]. Polarization at sufficiently negative potentials induces electron accumulation within the mesoporous film coupled to H + uptake (adsorption/intercalation) from the electrolyte [123] …”