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Electrons in the Band Gap - Spectroscopic Characterization of Anatase TiO2 Nanocrystal Electrodes under Fermi Level Control Thomas Berger, Juan Antonio Anta, and Victor Morales-Florez J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp212436b • Publication Date (Web): 13 Feb 2012 Downloaded from http://pubs.acs.org on May 7, 2012

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The Journal of Physical Chemistry

Electrons in the band gap - Spectroscopic characterization of anatase TiO2 nanocrystal electrodes under Fermi level control Thomas Bergera*, Juan A. Antaa, and Víctor Morales-Flórezb a

Departamento de Sistemas Físicos, Químicos y Naturales, Área de Química Física, Universidad

Pablo de Olavide, Ctra. Utrera, km 1, E-41013 Sevilla, Spain b

Instituto de Ciencia de Materiales de Sevilla, CSIC-Universidad de Sevilla, Av. Américo

Vespucio, 49, E-41092 Sevilla, Spain

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 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. Based on 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.

Keywords: Titanium dioxide, mesoporous thin films, electronic properties, spectroelectrochemistry, UV/Vis spectroscopy, IR spectroscopy.

* Corresponding author: Tel.: +34 95434 9315; Fax: +34 95434 9814; E-mail: [email protected] 1

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1. Introduction Electronic properties strongly influence the performance of mesoporous semiconductor nanocrystal films in sensoric, photovoltaic, photocatalytic and optoelectronic applications. A fundamental knowledge of these properties is therefore important both for the analysis and for the optimization of functional devices.1-6

One of the most studied semiconductors is TiO2 and its properties have been found to depend strongly on morphology. The electronic properties of nanosized particle assemblies can significantly differ from the respective bulk properties. A blueshift of the absorption threshold in TiO2 nanocrystals has been reported at particle sizes below 2 nm and was attributed to quantum confinement,7-9 or increased lattice strain in the particles arising from nanoscaling.10,11 On the other hand, nanosized semiconductors are characterized by a high specific surface area. As a consequence high concentrations of electronic states exist in the band gap originating from the truncation of the crystal lattice at the particle surface as well as from intrinsic and extrinsic point defects. In addition, sintering of nanosized particles results in the formation of a network of crystallographically misaligned crystallites with a high density of particle-particle interfaces. These interfaces are considered regions of high defect concentrations.12

On rutile TiO2 (110) single crystal surfaces, oxygen vacancies and hydroxyl groups are the most common point defects and their electron-trapping nature has been confirmed by quantumchemical calculations.13,14 However, in general the exact nature of defect states in TiO2 is not yet clear. For defects in reduced or n-type doped TiO2, which are often referred to as Ti3+ centers (forming upon the localization of an excess electron at a Ti site), theoretical and experimental efforts have been made to identify the degree of localization as well as the location 2


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of the excess electron in the surface, subsurface or bulk region. Different types of Ti3+ centers in the bulk of anatase and rutile TiO2 have been studied by quantumchemical calculations and specific states in the band gap have been observed.15-18 For both polymorphs, several Ti sites have been found to be close in energy indicating that the occupation of several sites is possible at room temperature. Such occupation could appear as a “delocalization” to experimental techniques.17

Electron transport in TiO2 takes place via small-polaron hopping in a complex potential energy landscape with local minima originating from stoichiometric self-trapping sites19 as well as from intrinsic and extrinsic defects.20-22 As a consequence, transport properties of nanoparticulate films differ fundamentally from those of single crystals with electron diffusion coefficients being orders of magnitude smaller in the former case.23,24 This slow transport is attributed to a high concentration of localized states, which act as electron-trapping sites. The diffusion coefficient is strongly affected by both the number and the energy distribution of these trap states25,26 as well as by their location.27 Electron transport in mesoporous semiconductor electrodes of dye-sensitized solar cells (DSCs) is found to strongly depend on light intensity and/or bias voltage. The observed transport dynamics can be modeled by trap-limited transport involving an exponential distribution of localized states in the band gap. Such a distribution has been proofed experimentally for anatase TiO2 and ZnO electrodes by voltammetry,28,29 charge extraction,30-32 impedance spectroscopy33-35 and intensity modulated photovoltage spectroscopy.36 However, neither nature and localization of the band gap states nor the origin of their distribution are well understood. Trapping of electrons at heterogeneous defects in the bulk or surface regions of the mesoporous oxide, at particle-particle interfaces as well as coulombic trapping due to local field

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effects, which may originate from the interaction of electrons with the polar TiO2 crystal or with cations of the electrolyte, could contribute to the observed distribution.2,37

Electrons can be accumulated in nanosized semiconductors in contact with an electrolyte e.g. by photoinduced charge carrier separation or by electrochemical polarization. The accumulation of the negative charge brings about charge compensation by counter ions from solution via cation adsorption at the oxide surface or in some cases via cation intercalation into the oxide bulk.38-40 Negative polarization of nanocrystalline TiO2 electrodes in aqueous electrolytes was found to enhance in some cases the photocatalytic performance of the semiconductor.41,42 This beneficial effect was attributed to the electrochemical doping of the films by generation of Ti3+ species and concomitant intercalation of H+ 41,42 or Li+ 42. However, the degree of such an improvement was found to strongly depend on the nature of the electrode.41 TiO2 nanotube electrodes previously intercalated with protons via electrochemical doping showed limited performance in DSCs. The detrimental effect was attributed to a pinning of the Fermi level and to an increase of recombination.43 On the other hand, an enhancement of the DSC performance was observed after visible light soaking and was attributed to the formation of shallow transport levels originating from photoinduced H+ intercalation.44 These shallow trapping states are supposed to accelerate charge carrier transport within the nanocrystalline films without deteriorating the open circuit photovoltage. In addition an increase of the density of localized states as well as a broadening of their distribution have been claimed upon H+ intercalation. The above-mentioned observations clearly evidence the necessity of knowledge on the interplay between microscopic characteristics like electronic properties and cation intercalation behavior, on the one hand, and the macroscopic performance of nanoparticle assemblies in a functional device, on the other hand.

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Experimental approaches combining spectroscopic with electrochemical methods have been proven useful for the elucidation of processes occurring at the semiconductor/solution interface.45-51 Electron accumulation in TiO2 films has been extensively studied by spectroelectrochemical measurements in the UV/Vis/NIR region.52 The challenge in this type of experiments is the correlation of the broad spectral signatures typically observed with specific sites in or on the oxide semiconductor.4 Furthermore, identification of the excited state level of an optical transition is challenging, as excitation of a trapped electron could be via a polaronic excited state or into a delocalized conduction band (CB) state.4 In this regard optical spectroscopy can yield the transition energies between the initial and final electronic states, but it does not provide information on their energetic positions. On the other hand, deposition of nanocrystal films on conducting substrates and their application in an electrochemical cell allows under a certain set of conditions to control the Fermi level53 in the semiconductor.26,28 The density of states (DOS) of the semiconductor film in contact with an electrolyte can be estimated by electrochemical methods provided that the energies of the electronic states are stationary upon a variation of the electrode potential.26,54 A combination of spectroscopic and electrochemical methods may therefore yield information on the concentration and energetic position of electronic states as well as on the transition energies between ground and excited states.

Here a spectroscopic study of the electrochemical population of electronic states in anatase TiO2 nanocrystal electrodes in contact with an aqueous electrolyte is presented. For the first time the whole energy range from the UV to the MIR is covered, allowing for the detection of electronic transitions in the region between the fundamental absorption threshold and the onset of lattice absorption under Fermi level control. Data in the MIR region have been obtained using a new experimental set-up allowing for the combination of attenuated total refection (ATR) 5



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spectroscopy and electrochemical measurements. The electrodes have been prepared using a state-of-the-art method based on a commercially available particle suspension and can be considered a standard material as evidenced by numerous applications in photocatalytic and photovoltaic studies as reported in literature.2

2. Experimental Chemicals and materials. Ti-Nanoxide T paste was purchased by Solaronix. Methanol (SigmaAldrich, puriss. p.a.), HClO4 (Sigma-Aldrich, ACS reagent, 70%), DClO4 (Aldrich, 68 wt. % in D2O, 99 atom % D) and D2O (Aldrich, 99 atom % D) were used as received. All H2O solutions were prepared using water with a conductivity of 18 MΩ⋅cm (Millipore, Milli-Q). Thin film preparation. The TiO2 paste was spread with a glass rod onto a F-doped SnO2 (FTO) transparent glass substrate (Pilkington, TEC 8) or, alternatively, onto a Ti foil (Goodfellow, 99.6+%, 25 µm) using Scotch tape as spacer. Afterwards, the electrode was sintered for 1 h at 450 °C in air. The resulting sintered layers are transparent in the Vis/NIR (Figure S1a) and have a thickness of 2.3 ± 0.7 µm (Figure S1b). The films are formed by nanocrystals with a mean diameter of 20 nm and show pure anatase structure (Figure S1c). A copper wire was attached to the conducting substrate with silver epoxy. The contact area and the uncovered parts of the substrate were finally sealed by epoxy resin. Electrochemical and photoelectrochemical measurements. Measurements were performed at room temperature in a standard three-electrode electrochemical cell. All potentials were measured against and are referred to a Ag/AgCl/KCl(3M) electrode (BASi, RE-5B), whereas a Pt wire was used as a counter electrode. Measurements were performed with a computer-controlled Autolab PGSTAT302N potentiostat. The current and charge densities are given on the basis of the geometric area. A 450 W Xe arc lamp (Oriel) equipped with a water filter was used for 6


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UV/Vis irradiation of the electrode from the electrolyte side (EE illumination). The applied light irradiance was measured with an optical power meter (Gentec TUNER) equipped with a bolometer (Gentec XLP12-1S-H2) being 500 mW·cm-2. UV/Vis/NIR spectroelectrochemical measurements. For UV/Vis/NIR measurements a TiO2 film deposited on FTO was used as the electrode. Electrochemical measurements were performed with a computer-controlled Autolab PGSTAT101 potentiostat. A schematic representation of the UV/Vis/NIR spectroelectrochemical cell is shown in Figure S2a and consists of an electrochemical cell as described above connected to a suprasil cuvette. The cuvette was placed in the light beam of a fiber optic system (Ocean Optics, source: DT-MINI-2-GS, spectrometer: Maya2000). The spectra were obtained by averaging 400 scans at an integration time of 25 ms and a boxcar smoothing width of 3. MIR spectroelectrochemical measurements. For MIR measurements a TiO2 film deposited on a Ti foil was used as the electrode. Electrochemical measurements were performed with a computer-controlled Autolab PGSTAT101 potentiostat. The MIR-spectroelectrochemical cell consists of an electrochemical cell as described above connected in the lower part to a hemispheric ZnSe prism (Figure S2b). Figure S3 shows the single beam spectrum of the cell. The electrode is pressed mechanically against the ATR prism for MIR measurements. Electrodes were polarized in the electrolyte bulk or, alternatively, when pressed against the prism. The ATR prism was placed in a reflection unit (PIKE Technologies, Veemax II) attached to a Bruker IFS 66/S FTIR spectrometer equipped with a MCT detector. Measurements were performed at an incident angle of 55° using unpolarized light. The spectra were obtained by averaging 50 (time-resolved spectra) or 500 scans at a resolution of 4 cm-1.

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Figure 1. Cyclic voltammograms (a,b) and photocurrent transients at different electrode potentials (c) of an anatase TiO2 nanocrystal electrode in the absence (a) and presence (b,c) of 1 M methanol. Electrolyte: N2-saturated 0.1 M HClO4 aqueous solution; Scan rate: 20 mV⋅s-1; Irradiance: 500 mW⋅cm-2.

3. Results and discussion 3.1 Electrochemical and photoelectrochemical measurements The immobilization of anatase TiO2 nanocrystals in the form of thin mesoporous films on a conducting substrate allows us to control in electrochemical measurements the Fermi level in the particle network. The possibility of Fermi level control is based on the specific electrode characteristics as the small size of the building blocks, a low level of doping, good electronic 8


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connectivity (within the particle network as well as between the film and the conducting substrate) and the presence of a surrounding equipotential surface.26,28 Beyond control (as exploited in potentiostatic experiments – Figures 1c, 2, 4 and 5) the Fermi level can be tracked even at open circuit (during open circuit potential decay – Figure 3).

A cyclic voltammogram (CV) of an anatase TiO2 nanocrystal electrode in N2-saturated 0.1 M HClO4 aqueous solution contains two characteristic signals (Figure 1a). A very weak capacitive peak at EAg/AgCl = 0.1 V and a high capacitive current monotonically increasing toward lower electrode potentials. The onset potential of this dark current lies at about –0.3 V. From the CVs recorded in the presence of 1 M methanol in the dark and during exposure to UV light (Figure 1b) it can be seen that the photocurrent onset is located at a potential, which is significantly lower than the onset potential of the capacitive dark current. Photocurrent transients allow for a more accurate determination of the photocurrent onset (Figure 1c) and yield a value of –0.7 V. The presence of methanol leads to higher photocurrents thus facilitating the determination of the photocurrent onset, however, the same value has been obtained in the pure electrolyte (Figure S4). The photocurrent onset gives an estimate of the CB edge as at this potential the Fermi level of the substrate coincides with the quasi-Fermi level of the illuminated TiO2 film, which lies typically right below the CB. It has to be mentioned that the value of the CB edge, as estimated here from the photocurrent onset, lies about 0.3 V more negative than the value determined previously from the analysis of spectroelectrochemical data.55 In that case the determination was based on the measurement of the Vis/NIR signal as a function of applied potential and for the analysis of the data it was assumed that the Vis/NIR signal is proportional to the density of electrons in the CB and that the bands are pinned as the Fermi level is displaced toward more negative potentials. At least concerning the first assumption, no definitive conclusion has been 9



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reached in literature up to now and observations made in the present study rather point to an exponential distribution of band gap states as the origin of the Vis/NIR signal (see below). Difficulties with determining the CB position of nanostructured semiconductors have been highlighted only recently by Ardo et al..52

The CV in Figure 1a is in agreement with previous studies.28 The monoenergetic state at EAg/AgCl = 0.1 V has previously been attributed to band gap states at the particle surface56,57,58 or at the particle-particle interface.12,41 The reversible capacitive current at potentials below -0.3 V is connected to electron accumulation within the mesoporous film coupled to H+ uptake (adsorption/intercalation) from the electrolyte40

TiIVO2 + e- + H+ ↔ TiIII(O)(OH)

(1)

Such an intrinsic film capacitance can be calculated according to28,59

C = Ca exp[-α·e·E/(kB·T)] + const

(2)

where Ca is a preexponential factor, e the elementary charge, kB the Boltzmann constant, T the absolute temperature and E the electrode potential. A value of α between 0.2 and 0.4 can be estimated from the voltammogram in Figure 1a, indicating that the capacitive current results from electrons injected into exponentially distributed band gap states.28 From the shape of the voltammogram it may be deduced that significant band edge unpinning does not occur in the potential range studied.6,28 Additional support for the assignment of the cathodic current to the filling of states below the CB edge comes from Figure 1b and 1c. The onset of the reversible 10


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capacitve current in the dark voltammogram (EAg/AgCl ~ -0.30 V) is clearly located at a more positive potential than the photocurrent onset (EAg/AgCl = -0.70 V). At the zero-photocurrent potential all photogenerated charge carriers are lost by recombination and its value gives an estimate of the CB edge position.60

Figure 2. Temporal evolution of UV/Vis/NIR- (a,b) and MIR-spectra (d,e) of anatase TiO2 nanocrystal electrodes at EAg/AgCl = -0.6 V. The reference spectra were taken at EAg/AgCl = 0.4 V. (c,f) Chronocoulometric profiles recorded simultaneously. Electrolyte: N2-saturated 0.1 M HClO4 aqueous solution; Charge extracted upon back polarization from -0.6 V to 0.4 V: (c) Qextracted = 2.8 mC⋅cm-2, (f) Qextracted = 2.3 mC⋅cm-2.

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3.2 UV/Vis/NIR spectroelectrochemical measurements Upon cathodic polarization at EAg/AgCl = -0.6 V an absorption signal appears in the UV/Vis/NIR (Figure 2a). The signal is completely reversible with respect to an anodic back polarization to 0.4 V (Figure 2 b). The UV/Vis/NIR signal contains a negative contribution at wavelengths below 380 nm. Above 380 nm a broad absorption leveling off toward longer wavelengths is observed. From the chronocoulometric profiles (Figure 2c) it can be seen that not all of the current injected to the electrode during cathodic polarization is extracted upon anodic back polarization. This clearly indicates that the currents are not purely capacitive, but that significant faradaic losses due to electron transfer to solution species (as residual oxygen) take place. These faradaic currents can also be seen in the CV in Figure 1a where they cause a deviation from symmetry of cathodic and anodic currents.

From the charge extracted upon back polarization from –0.6 V to 0.4 V (Qextracted = 2.8 mC·cm-2, Figure 2c) the number of accumulated electrons per particle can be estimated. Assuming spherical particles with a diameter of 20 nm, a particle volume fraction of 0.5 and a film thickness of 2.3 µm a value of ~650 electrons/particle is obtained. The Lambert-Beer law is valid in the Vis range for the potentials (more precisely, the number of accumulated charges) studied (Figure S5a). Using the absorbance value from Figure 2a, a decadic extinction coefficient at 700 nm of 1200 M-1·cm-1 is calculated. This value is in perfect agreement with previous studies.47,61 The charge extraction method62 allows for determining the number of electrons lying above a socalled demarcation limit. Electrons, which due to their energetic or local position can not communicate with the conducting substrate on the time scale of the extraction step, will however not be accessible in these experiments. The fact that the absorbance change in the UV/Vis/NIR associated with charge accumulation is fully reversible upon charge extraction indicates, 12


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however, that this contribution can be neglected in the present case. The extracted charge thus may give an estimate of the DOS provided that the energies of the states are stationary upon a variation of the electrode potential (i.e. in the case of band pinning).

Figure 3. Absorbance decay at 600 nm (2.1 eV) (a) and at 2000 cm-1 (0.25 eV) (d) and open circuit potential decay (b,e) of anatase TiO2 nanocrystal electrodes after initial polarization for 240 s at EAg/AgCl = -0.6 V. The absorbance values are normalized to the respective values measured at EAg/AgCl = -0.6 V. The electrolyte (0.1 M HClO4 aqueous solution) was purged with N2 or O2, alternatively. After the initial cathodic polarization the electrode was switched to open circuit (OC) or stepped back to 0.4 V, alternatively. (c,f) Semi-logarithmic plot of absorbance vs. open circuit potential as extracted from the data recorded in a N2-saturated electrolyte. Arrows indicate the slopes corresponding to different α-values (T = 298 K).

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The absorbance and potential decay at open circuit following cathodic polarization at EAg/AgCl = 0.6 V was studied in N2- and O2-saturated electrolyte (Figure 3 and Figure S6). The increase of the electron acceptor concentration in solution significantly accelerates the decay rate of both the absorbance and the electrode potential. The simultaneous tracking of both quantities allows for determining the absorbance vs. electrode potential relationship in situ. Figure 3c shows that the absorbance decreases approximately exponentially with increasing potential. Assuming that the absorbance is proportional to the integrated capacitance of Equation 2 an α value between 0.20 and 0.25 can be extracted from the semilogarithmic plot (T = 298 K). The absorbance decay following accumulation is much faster (Figure 3a and Figure 2c) if the electrode potential is externally stepped back to a potential, which is sufficiently positive to extract the accumulated electrons (extraction step: EAg/AgCl = 0.4 V, Figure 1a).

Variation of the electrode potential during the accumulation step (Figure 4a) yields an AVis/NIR vs. E relationship (Figure 4b) that strongly resembles the one extracted from the open circuit potential decay (Figure 3c), indicating that good potentiostatic control has been achieved in the experiments. Subsequent charge extraction upon anodic back polarization yields the number of accumulated electrons as a function of the electrode potential applied during cathodic polarization. Interestingly, the extracted charge (Qextracted) decreases exponentially with increasing potential (Figure 4b). More importantly, the slope in the semilogarithmic plot of Qextracted resembles the one of AVis/NIR (equivalent to α = 0.23 in Equation 2). Thus AVis/NIR and Qextracted are linearly correlated (Figure 4c). The UV/Vis/NIR signal envelope (Figure S5b) does not depend significantly on electrode potential. Figure 5 contains different representations of the UV/Vis/NIR spectrum, concretely A vs. photon energy (Figure 5a), log A vs. log λ (Figure 5b) and ln A vs. photon energy (Figure 5c). 14


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Figure 4. UV/Vis/NIR- (a) and MIR-spectra (d) of anatase TiO2 nanocrystal electrodes at different electrode potentials (polarization time: 240 s). The reference spectra were taken at EAg/AgCl = 0.4 V. (b,e) Semi-logarithmic plot of absorbance and extracted charge vs. electrode potential. Arrows indicate the slopes corresponding to different α-values (T = 298 K). (c,f) Correlation plots of absorbance vs. extracted charge. Electrolyte: N2-saturated 0.1 M HClO4 aqueous solution; Charge extracted upon back polarization from -0.6 V to 0.4 V: (b) Qextracted = 2.8 mC⋅cm-2, (e) Qextracted = 2.4 mC⋅cm-2.

The onset of the Vis/NIR signal (Figure 4a) occurs at potentials, which are considerably more positive than the CB edge (EAg/AgCl = -0.7 V). This is in line with previous studies, which considered the presence of a CB tail as a possible reason for such a behavior.63 The spectra in Figures 2a, 4a and 5a are in perfect agreement with former reports.64-66 A decrease of the

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absorbance at energies near the fundamental absorption threshold has typically been attributed to the Burstein-Moss shift, which is associated with the filling of CB states by accumulated electrons and a bleaching of transitions to these states.60,64,66 In addition, the absorbance decrease can originate from the filling of band gap states near the CB edge.65 There exists some controversy concerning the absorbance increase at energies below the fundamental absorption threshold in literature, reflecting the difficulties in the assignment of the very broad signals typically observed. The broad absorption has been attributed to electrons localized in band gap states (Ti3+ centers), to electrons in the CB, or to a superposition of both. The leveling off of the absorbance at longer wavelengths in the visible and NIR range, which in some cases culminates in a pronounced maximum, has been interpreted in terms of d-d transitions of localized Ti3+ states.65,67 The d-d transition is symmetry forbidden, however, symmetry breaking by asymmetric ligand field splitting or vibronic coupling makes weak absorption possible. The monotonic increase towards longer wavelengths, on the other hand, was attributed to the Drude absorption of free CB electrons.66 Free electron transitions in the CB require an additional interaction to conserve momentum. The change in momentum can be provided by coupling with phonons or by scattering at impurities. Free CB electrons will give rise to a broad IR signal with an absorbance A developing as68,69

A(λ) = C·λp

(3)

Here λ is the wavelength (in µm) and C is a proportionality constant. The scattering constant p can range from 1.5 to 3.5 depending on the statistical weight of the processes, which provide the momentum change. Though linear regions were found in some cases upon electrochemical polarization in the double logarithmic presentation of the visible spectrum,70 these spectra were 16


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characterized by slopes p < 1.5 in line with our findings (Figure 5b). Figure 5a-c shows that the Vis/NIR spectrum can be fitted neither by a power law (Equation 3), nor by a single exponential function. Clearly, a definitive assignment of the signals in the Vis/NIR region requires an extension of the spectral range towards the MIR.

Figure 5. UV/Vis/NIR- (a) and MIR-spectra (d) of anatase TiO2 nanocrystal electrodes at EAg/AgCl = -0.6 V as taken from Figure 4. (b,e) Double-logarithmic plot of absorbance vs. wavelength. (c,f) Semilogarithmic plot of absorbance vs. photon energy.

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In addition to a broad absorption in the Vis/NIR range, electron paramagnetic resonance (EPR) spectra have previously evidenced the presence of Ti3+ species in TiO2 particles after negative polarization in acidic aqueous solution.65 On the other hand, it was observed that the modification of the TiO2 particle surface by enediol ligands induces significant changes in the Vis/NIR spectra after negative polarization in an aqueous electrolyte.48 These spectral changes were attributed to the modification of surface trapping sites by the adsorbates. Spectral signatures in the Vis/NIR range similar to those reported in the present study after negative polarization (Figures 2a and 4a) have been previously observed in anatase TiO2 after UV light induced charge carrier separation,71-75 thermal reduction76 and injection of radiolytically generated hydrated electrons.77 EPR spectra recorded after UV light induced charge carrier separation78,79 and thermal reduction79 have evidenced the presence of Ti3+ species.

3.3 MIR spectroelectrochemical measurements Apart from the signal in the UV/Vis/NIR an additional signal in the MIR is observed upon cathodic polarization (Figure 2d). There is no significant difference either in intensity or in the envelope of the stationary MIR signal between experiments where the cathodic polarization was performed in the electrolyte bulk or with the electrode pressed against the prism surface (Figure S7). In order to minimize polarization time, cathodic polarization was performed in the following experiments in the electrolyte bulk prior to moving the electrode to the surface of the ATR prism. However, anodic back polarization was always performed with the electrode pressed against the prism. The MIR spectrum is composed of a broad signal monotonically increasing toward lower wavenumbers with a sharp cut-off near 900 cm-1 and of negative going bands at 3240 cm-1, 1655 cm-1 and 1120 cm-1. Importantly, the signal envelope does not change significantly with polarization time (Figure S8a) or potential (Figure S8b). Only at intermediate times or potentials 18


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an additional transient contribution at 890 cm-1 can be observed. A stationary absorption is reached in the UV/Vis/NIR (Figure 2b) much faster than in the MIR (Figure 2e), which may readily be explained by the different geometry of the two spectroelectrochemical cells (Figure S2). In the MIR-cell diffusion is significantly slowed down by the thin electrolyte layer, which forms between the electrode and the surface of the ATR prism, when the electrode is pressed against the prism (Figure S2b, inset). In addition, the faradaic losses as observed in the chronocoulometric profiles are more pronounced in the UV/Vis/NIR-cell (Figure 2c and 2f), once again reflecting the slow diffusion of acceptor species (as residual oxygen) to the TiO2 surface in the MIR-cell (Figure 2f).

The structureless signal, which increases monotonically toward lower wavenumbers and falls off at 900 cm-1 can be associated with the accumulation of electrons in the TiO2 film. The negative bands on the other hand originate from electrolyte species (Figure S9). The intensity decrease of electrolyte bands probably results from a modification of the electrolyte structure in the mesopores upon polarization. The fact that the relative intensity of the ClO4- band at 1120 cm-1 does not change with respect to the monotonous signal (Figure S8) indicates the absence of significant changes in the double layer upon polarization as anticipated from the shape of the cyclic voltammogram (Figure 1a). If experiments are performed using 0.1 M DClO4 in D2O as the electrolyte, water bands shift to lower wavenumbers whereas the ClO4- band and the monotonic signal stay unchanged (Figures 6 and S9). Importantly, the signal cut-off at 900 cm-1 is still observed in the deuterated electrolyte indicating that it is not due to a negative contribution of the librational mode of water, which is shifted toward lower wavenumbers upon deuteration (Figure S9).

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Figure 6. MIR-spectra of anatase TiO2 nanocrystal electrodes at different electrode potentials (polarization time: 240 s). The reference spectra were taken at EAg/AgCl = 0.4 V. Electrolyte: N2-saturated 0.1 M HClO4 in H2O (dotted lines) and N2-saturated 0.1 M DClO4 in D2O (solid lines).

As in the case of the Vis/NIR signal, transfer of electrons, associated with the MIR signal, to solution gets accelerated if the electrolyte is purged with O2 (Figure 3d and e). More importantly, as observed previously in the Vis/NIR, AMIR decreases exponentially with increasing potential (EOC, Figure 3f). Again, assuming that AMIR is proportional to the integrated capacitance of Equation 2 an α value between 0.20 and 0.25 can be extracted.

The absorbance decay is immediate (Figure 3d and Figure 2f) if the electrode potential is externally stepped back to EAg/AgCl = 0.4 V. The AMIR vs. E relationship (Figure 4e) obtained by variation of the electrode potential during the accumulation step (Figure 4d) again strongly resembles the one extracted from the open circuit potential decay (Figure 3f). More importantly, as reported above for the Vis/NIR range, the slope in the semilogarithmic plot of Qextracted resembles the one of AMIR (equivalent to α = 0.23 in Equation 2). Thus not only AMIR and Qextracted (Figure 4f), but also AVis/NIR and AMIR are linearly correlated.

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A double logarithmic plot of AMIR vs. wavelength λ (in µm) is represented in Figure 5e. Apart from the negative contributions of the electrolyte (around 0.5 µm and 0.8 µm), linear regions with a slope between 1.5 and 2.5 are observed. Alternatively, the monotonic signal in the MIR spectrum can be approximated by an exponential function (Figure 5f).

Several studies on electronic transitions in TiO2 in the IR range have been reported, though there did not exist up to now spectral data in the MIR under Fermi level control. Szczepankiewicz et al.80 observed a monotonic IR signal upon UV light induced charge carrier separation in vacuum. The signal was attributed to free CB electrons and was successfully fitted by Equation 3 yielding a scattering constant p = 1.7. The lifetime of states associated with the IR signal varied from seconds to hours depending on surface hydration.80 In contrast, lifetimes in the µs range were observed by transient absorption spectroscopy.81,82 Interestingly, the monotonic IR signal was formed upon band gap excitation of TiO2 not only under high vacuum conditions,79,80 but also in contact with an aqueous phase in the presence of hole acceptors.83-85 Furthermore, a stationary signal was observed during ethanol photooxidation even in the presence of small amounts of O2 in the gas phase.86 Tamaki et al.87,88 and Yoshihara et al.83 have measured transient absorption spectra of TiO2 nanocrystal films in different environments in the Vis to IR region following band gap excitation. The absorption spectrum of electrons was composed of a broad band in the Vis/NIR and an IR signal increasing monotonically toward lower wavenumbers. Interestingly, both signals were found to decay with identical time constants in the ps and µs time range. When TiO2 films were irradiated in methanol, a decrease of the relative intensity in the Vis with respect to the IR was observed upon O2 saturation. Based on this observation the Vis signal was attributed in these studies to trapped electrons at the surface and the IR signal to free electrons in

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the bulk. The monotonic IR signal was furthermore observed after thermal reduction of TiO2 powders under high vacuum conditions.79

Panayotov and Yates89,90 reported that TiO2 nanoparticles can be n-type doped by atomic hydrogen thereby producing electrons located in shallow traps in the TiO2 lattice. A monotonic IR signal extending from 4000 cm-1 to a sharp cut-off near 1000 cm-1 was observed and successfully fitted by Equation 3 yielding a scattering constant p = 1.5. Recent calculations have shown, that H-doping produces Ti3+ species as a result of H dissociation into a proton, bound to a lattice oxygen, and an extra electron.15 A localized solution with the extra electron in a 3d state of a Ti ion and the proton bound to a neighboring O anion (Ti-OH species) was found to be about 0.18 eV more stable than a delocalized solution with the extra electron fully delocalized on a 3dxy CB state. This indicates that the formation of localized states may be facilitated in the presence of hydroxyl groups. Indeed, Ti3+ species have been detected by EPR spectroscopy in anatase nanocrystals doped with atomic hydrogen.91 The electrochemical accumulation of electrons in TiO2 in aqueous electrolyte is related to H atom doping in the gas phase insofar as electron localization and proton adsorption/intercalation occur in parallel in both cases (Equation 1).

Different types of optical transitions have been proposed up to now to explain the IR signature of electrons in TiO2: intra-CB transitions of free electrons,81 excitation of shallowly trapped electrons to the CB81 and polaron excitations by coupling with two-dimensional surface phonons.92 As discussed below, an additional description associated with intra-band gap transitions of electrons in an exponential distribution of states below the CB edge could be envisaged. In Figure 7 these different models are adapted to the case of an exponential

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distribution of band gap states. In the following we will discuss the different possibilities in the light of our experimental observations.

Figure 7. Possible transitions in the MIR of accumulated electrons in anatase TiO2 nanocrystal electrodes: (a) Intra-conduction-band transitions associated with phonon absorption, (b) excitation of shallowly trapped electrons to the conduction band (c) polaron excitations by coupling with two-dimensional surface phonons and (d) intra-band-gap transitions.

Intra-CB transitions (Figure 7a): Free CB electrons give rise to a broad IR signal, which can be described by Equation 3. Indeed, MIR signals similar to the one observed here have been successfully fitted by this equation.80-84,89 Warren et al.84 outlined that there exists some ambiguity of how to account in the intra-CB model for the sharp cut-off near 900 cm-1, however, others attributed it to the onset of TiO2 lattice vibrations. On the other hand, the capacitance associated with the occupancy of the CB is described by Equation 2 with α = 1, if Boltzmann statistics (ideal behavior) is assumed for free electrons.93 However, an α-value between 0.20 and 0.25 was extracted from Figures 3f and 4e indicating that, in the absence of significant band edge unpinning, the accumulated charge and the concomitant absorbance increase may alternatively be 23



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associated with the population of band gap states. This assignment would explain the observation of the onset of charge accumulation at potentials significantly more positive than the photocurrent onset (Figure 1b and c).

Excitation of shallowly trapped electrons to the CB (Figure 7b): To account for the observation of a sharp absorption cut-off below 1000 cm-1 it was suggested that shallowly trapped electrons in the band gap could be excited by IR radiation into the CB.81,84,89,94 In such a case the absorption coefficient for ionization of the band gap state would decrease with the transition energy and give rise to a monotonic IR signal decreasing toward higher wavenumbers.81 In this model only photons with an energy exceeding the ionization energy of the trap can be absorbed.94,95 An absorption cut-off at around 1000 cm-1 would therefore be associated with an ionization energy ∆Etrap-CB of 0.12 eV. However, semiconductor nanocrystals are typically characterized by a broad distribution of band gap states rather than by a single monoenergetic state. The absorption cut-off would therefore depend on the Fermi level (EF) in the semiconductor film shifting to lower wavenumbers as EF moves toward the CB edge (ECB). However, such a shift is not supported by our results (Figure 4d) even though the EF – ECB distance has been varied (under supposition of band pinning) from 0.4 to 0.1 V, thereby sampling trap states with an ionization energy ∆Etrap-CB between 0.4 and 0.1 eV, equivalent to photon wavenumbers of 3200 cm-1 and 800 cm-1, respectively.

Polaron excitation and intra-band gap transitions (Figure 7c and 7d): The photocurrent onset was used to estimate the CB edge of the anatase TiO2 nanocrystal electrode in 0.1 M HClO4 aqueous solution, yielding a value of EAg/AgCl = -0.7 V (Figure 1c). Electrode polarization in the potential range from –0.2 V to –0.6 V allows therefore, in the absence of significant band 24


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unpinning, for a displacement of the Fermi level within the band gap toward the conduction band edge. Thus, electron accumulation under Fermi level control renders possible the sampling of band gap states as a function of their energy. From the energy-dependence of Qextracted (Figure 4 b and e) an exponential distribution of band gap states is deduced. These band gap states give rise to a capacitive current in the voltammogram at potentials below EAg/AgCl = -0.3 V (Figure 1a). The linear correlation of Qextracted and the absorbance in the Vis/NIR (AVis/NIR) and in the MIR (AMIR) indicates that both signals originate from electronic states located in the band gap.

In line with a previous assignment by Szczepankiewicz et al.,92 the MIR signal could be associated with polaron excitation (Figure 7c). In this case transitions of electrons in shallow surface traps would be coupled with two-dimensional surface phonons. These authors have shown that the physical motion of the lattice (phonons) required to translate such polarons gives rise to MIR spectral signatures similar to those observed in the present study. The absorbance cut-off could then be attributed to the onset of TiO2 lattice vibrations.

The monotonic IR signal could alternatively be assigned to direct optical transitions of electrons accumulated in an exponential DOS in the band gap below the Fermi level to states in the exponential tail and in the CB (Figure 7d). This mechanism would be similar to the one described recently by Panayotov et al.94 with the difference that in addition to excitations into the CB, transitions into other states in the exponential CB tail seem to be possible as well. Energetically close-lying states within the exponential DOS, which is possibly built up by Ti3+ species in different local environments, might form the basis for a quasi-delocalization of the electrons. As mentioned above, electron accumulation in the mesoporous TiO2 film is coupled to H+ uptake from the electrolyte. In general, electronic species forming upon charge accumulation can 25



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therefore be considered (e-)(H+) centers. More specifically, such a center could be associated with an electron in a 3d state of a Ti ion and a proton bound to a neighboring O anion as predicted by theory.15 The sharp absorption cut-off at around 900 cm-1 could then be interpreted as the minimum energy necessary for overcoming the local potential well of the (e-)(H+) center, which may not depend on the energetic position of the band gap state with respect to the CB edge, but only on the nature of the corresponding point defect. In this case electron transfer could be connected to the diffusion of a neutral H atom at the surface or in the lattice and to its subsequent ionization under the formation of a new (e-)(H+) center.

At the moment, a distinction between the two mechanisms, i.e. polaron excitation vs. direct optical transition, is not possible. However, experiments based on a modification of the surface or the bulk properties of the samples, which are planned for the future, may help to give a decisive assignment.

The absorbance in the Vis/NIR can be attributed to d-d transitions of Ti3+ species in accordance with former reports. Furthermore, the Vis/NIR spectrum will contain some contribution from the monotonic signal observed in the MIR, which probably extends into the Vis/NIR. As mentioned above, signal intensities in the Vis/NIR and in the MIR are linearly correlated. The signals may originate from a single type of electronic species or, alternatively, from two different species in equilibrium, one connected to the signal in the Vis/NIR and the other to the monotonic MIR signal. In the latter case the corresponding equilibrium constant would be independent of the Fermi level position thus giving rise to identical slopes in the semilogarithmic plots of AUV/Vis and AMIR vs. E (Figure 4b and e). An association of one of the signals with band gap states and

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the other with CB electrons would fail as in this case a linear correlation between the respective absorbance values would not be observed.

Due to the high reversibility of the accumulation process – electrons can be extracted on a time scale of seconds (Figures 1a, 2 and 3) – we suppose that charge accumulation in anatase TiO2 nanocrystal electrodes is mainly associated with surface and/or subsurface states.

According to our above interpretation optical transitions in the MIR are associated with a quasidelocalization of charges. If the respective transitions originate from surface and/or subsurface states it might be possible to tune this property by semiconductor modification. In this regard, the adsorption of molecules or ions on the semiconductor surface and the modification of the morphology or the crystal structure of the crystallites within the film will yield useful information. Recently, by comparing TiO2 electrodes with comparable nanowire morphology, an exponential trap distribution below the CB was evidenced for the anatase crystal structure, while it was absent in rutile.29 Upon cathodic polarization in acidic solution a broad absorption in the Vis/NIR was observed in both cases. On the other hand, studies of photoinduced charge carrier separation in anatase and rutile TiO2 point to the possibility that the monotonic IR signal might be characteristic for the anatase crystal structure only.85,96 Indeed, a preliminary spectroscopic characterization of rutile TiO2 nanocrystal electrodes under Fermi level control performed in our laboratory corroborate this assumption.

The quasi-delocalization of electrons at the surface or subsurface as proposed above, may be the result of a high density of energetically close-lying energy levels in the semiconductor film15,17 in conjunction with the specific structure of the semiconductor/electrolyte interface. The presence of 27



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2-dimensional delocalized surface states might be of high importance for the functioning of nanostructured semiconductor films in photovoltaic or photocatalytic applications. Gregg and al.97 attributed the performance increase of DSCs upon UV illumination to the formation of specific surface states. It was argued that at a high enough density electrical communication between surface states could take place, thus leading to an improved conducting pathway to the substrate. Surface electron transport was claimed recently also by Thompson and Yates98 to describe the kinetics of O2 photodesorption from rutile TiO2(110) surfaces. Finally, Wang et al.44 claimed the formation of subsurface states upon photoinduced proton intercalation in TiO2 nanoparticles. When reaching the percolation threshold, these states may provide an additional transport pathway for electrons without promoting recombination with the redox shuttle.

Finally, we want to emphasize that a combined spectroscopic and electrochemical approach as reported above is not limited only to processes at the electrode/solution interface in the absence of light. In addition, this method may by useful for the in situ study of photoinduced processes. Especially the extension of spectroelectrochemical measurements to the MIR by the new experimental set-up presented here opens up the possibility of tracking simultaneously electronic states in the semiconductor as well as molecules at the surface or in the electrolyte (e.g. reaction educts or products during a photocatalytic reaction). Furthermore, the present approach can provide interesting information about recombination losses in nanoscopic solar cells, due to electron transfer to the redox shuttle in the electrolyte, or to the ionized sensitizer. The MIRspectroelectrochemical approach presented here is therefore expected to become a valuable tool for studying photocatalytic or photovoltaic processes in situ. First results of photoinduced charge carrier separation under Fermi level control will be reported elsewhere.

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4. Conclusions We have studied charge accumulation in anatase TiO2 electrodes in contact with an aqueous electrolyte by a combined electrochemical and spectroscopic approach. The following conclusions have been reached in this work: (1) Accumulation at electrode potentials significantly more positive than the photocurrent onset potential gives rise to capacitive currents in the voltammogram. (2) Upon charge accumulation a broad absorption in the Vis/NIR, leveling off toward longer wavelengths (lower energies), and a broad MIR-signal, monotonically increasing toward lower wavenumbers (lower energies), are observed. (3) Both signals are fully reversible upon positive electrode polarization as well as upon electron transfer to O2 at open circuit. (4) The signals in the Vis/NIR and the MIR are linearly correlated with each other and with the number of extracted charges. (5) We attribute the Vis/NIR absorption to d-d transitions of Ti3+ species and the MIR absorption to transitions of electrons accumulated below the Fermi level in an exponential DOS in the band gap. (6) 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.

Acknowledgments We are grateful to R. Gómez and T. Lana-Villarreal for valuable discussions. This work was financially supported by the Spanish Ministry of Science and Innovation (MICINN) through the project HOPE CSD2007-00007 (Consolider-Ingenio 2010) and the Ramón y Cajal program as well as by the Junta de Andalucía through projects P07-FQM-02595, P07-FQM-02600 and P09FQM-04938. V.M.F. thanks the CSIC for financial support through the JAE program.

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Supporting Information Schematic representations of the spectroelectrochemical cells, data on thin film characterization and supporting experimental data can be found in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

Table of contents graphic:

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