Article pubs.acs.org/JPCC
Electrochemical Doping of Compact TiO2 Thin Layers Marketa Zukalova, Milan Bousa, Zdenek Bastl, Ivan Jirka, and Ladislav Kavan* J. Heyrovský Institute of Physical Chemistry, v.v.i. Academy of Sciences of the Czech Republic, Dolejškova 3, CZ-182 23 Prague 8, Czech Republic S Supporting Information *
ABSTRACT: Electrochemical n-doping of dense thin films of TiO2 (anatase) occurs upon proton insertion from acidic aqueous electrolyte solution. Details of the reaction are investigated by cyclic voltammetry and by electrochemical impedance spectroscopy. The good-quality films are ideally compact, mimicking the properties of a macroscopic single-crystal electrode. The n-doping is stable for at least weeks of electrode storage in air at room temperature. Doping manifests itself by characteristic morphological differences of the surface. Electrochemical Li+ insertion indicates that there is a competition between Li+ and H+ ions in the lattice, detectable by cyclic voltammetry.
1. INTRODUCTION Compact TiO2 thin films are used as blocking layers in solidstate dye-sensitized solar cells (Graetzel cells) and in the methylammonium−lead−iodide (perovskite) based solar cell,1−4 achieving impressive solar conversion efficiencies.4,5 Here, the titania film serves as electron collector and simultaneously as a buffer layer, preventing recombination of photoexcited electrons from the substrate, typically F-doped SnO2 conducting glass (FTO) with the hole conductor, typically 2,2′-7,7′-tetrakis(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene (spiro-OMeTAD). Perovskite is usually deposited on a mesoporous oxide serving as a scaffold to increase the surface area for light harvesting. However, recent studies showed that the scaffold was not essential for proper function of the perovskite cell. In the so-called planar cell, the perovskite is deposited directly on the compact TiO2 film on FTO, without compromising the cell’s performance.4,6 Photoelectrochemical water reduction to hydrogen presents another application of compact titania. Here, the titania layer protects the active electrode materials such as cuprous oxide7,8 or silicon.9−11 Titania is attractive for solar water splitting on Cu2O or Si by its favorable energy band positions, that is, by easy electron transfer to the electrolyte solution. Second, even the ultrathin (several nanometers) titania layer works surprisingly well in preventing corrosion of these semiconductor light absorbers.7−11 The compact titania film is grown on top of FTO, usually by spray pyrolysis.12 This technique is popular both in the traditional dye-sensitized solar cells13 and in the perovskite solar cells.1−3,14−17 Alternative pathways toward blocking underlayers include DC-magnetron sputtering,18 electrochemical deposition,19,20 atomic layer deposition (ALD),20−22 spincoating,6 and sol−gel dip-coating.23,24 The latter technique is © 2014 American Chemical Society
attractive for its simplicity and easy scale-up. The as-deposited films almost always exhibit some n-doping, which arises from substoichiometric oxygen deficiency and the Ti3+ impurities. For instance, films made by spray pyrolysis, ALD, or electrodeposition can have donor concentrations as high as 1020 cm−3, found by electrochemical impedance.23,24 Deliberate and controlled n-doping of TiO2 can be conveniently carried out by electrochemical reduction. In the absence of any other redox couple in an aqueous electrolyte solution, the proton insertion is the only faradaic process occurring on the titania electrode at sufficiently negative potentials in acidic electrolyte solutions. It is described schematically by the brutto reaction:25,26 TiO2 + e− + H+ → TiOOH
(1)
This concept was pioneered on the single-crystal anatase electrodes by Pelouchova et al.27 and further extended by Berger et al.25,28 for nanocrystalline electrodes. The nanocrystalline titania exhibited temporary electron/proton uptake lasting for hours. Later on, the same group reported on a more stable doping that persisted for days.26 Although the electrochemistry of titania has been investigated thoroughly,28−30 a systematic study of reaction 1 is still missing. The doped titania showed enhanced photocatalytic and photoelectrochemical performance.26,27,31,32 For instance, the electrochemically doped single-crystal anatase provided larger photocurrents upon UV excitations and interestingly enhanced photocurrents arising from sub-bandgap excitations.27 Similarly, the oxygen-vacancy/Ti3+ doped titania Received: May 6, 2014 Revised: October 20, 2014 Published: October 20, 2014 25970
dx.doi.org/10.1021/jp504457v | J. Phys. Chem. C 2014, 118, 25970−25977
The Journal of Physical Chemistry C
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macroscopic surface normal. Survey scan spectra and highresolution spectra of Ti 2p and O 1s photoelectrons were measured. The spectra were curve-fitted after subtraction of Shirley background using the Gaussian−Lorentzian line shape and the damped nonlinear least-squares algorithms (software XPSPEAK 4.1). The sample was FTO-supported TiO2 film (2.5 × 0.8 cm2), of which one-half was electrochemically doped and the other half was nondoped. The sample was fixed in tantalum boat, and the spectra were measured in one experiment from the ∼1 cm2 spot, which was focused either at the doped part or at the nondoped part of the sample. The used experimental arrangement enabled direct comparison of the raw spectra of both parts of the sample. Optical (UV−vis) spectra were measured on a PerkinElmer Lambda 1050 spectrophotometer. Electrochemical experiments were carried out in a onecompartment cell using Autolab PGstat-302N equipped with the FRA module (Ecochemie) controlled by the GPES-4 software. The reference electrode was Ag/AgCl (sat. KCl). The electrolyte solution for doping was aqueous 0.5 M KCl; pH was adjusted by HCl. The electrolyte solution was purged with Ar, and the measurement was carried out under Ar atmosphere in the hermetically closed electrochemical cell. Impedance spectra were measured at varying potentials that were scanned from positive to negative values. Spectra were acquired and evaluated using Zplot/Zview (Scribner) software. The Li-insertion experiments were carried out in 1 M LiPF6 + ethylene carbonate (EC)/dimethylcarbonate (DMC) (1/1 by volume) in a glovebox under argon. In this case, the reference and auxiliary electrodes were Li metal; hence, the potentials are referred to the Li/Li+ (1M) reference electrode. Electrolytes, solvents, and redox-active molecules were of standard quality (p.a. or electrochemical grade) purchased from Aldrich or Merck and used as received.
exhibited enhanced visible light absorption and H2 production from water splitting.32 The efficiency of a classical (liquid-junction) dye-sensitized solar cell was improved too, if the TiO2 photoanode was fabricated from the electrochemically doped TiO2.26 The doped photoanode delivered larger photocurrent and larger photovoltage, which were attributed to the accelerated electron transport, improved electron injection, and reduced recombination.26 However, there are also opposite claims, that the oxygen-vacancy/Ti3+ enhances recombination in the dyesensitized solar cell.33 The efficiency of classical solid-state dye-sensitized solar cell with spiro-OMeTAD hole conductor was not significantly changed as a result of n-doping, but the electron mobility increased.34 To the best of our knowledge, the effect of electrochemical doping was not yet tested in perovskite cells nor in dense polycrystalline layers. Here, we report on our initial study, which is concentrated on the detailed investigation of electrochemical doping of sol−gelmade compact layers.
2. EXPERIMENTAL SECTION Poly(hexafluorobutyl methacrylate), PHFM (as in ref 35), was purchased from Advanced Materials-JTJ, Czech Republic (AMJTJ). The corresponding monomer, i.e., 2,2,3,4,4,4hexafluorobutyl methacrylate (HFM), was obtained from Aldrich or from AMJTJ. FTO glass (TEC 15 from LibbeyOwens-Ford, 15 Ohm/sq) was cleaned ultrasonically in acetone and ethanol. The solution for dip-coating was prepared as follows: 10.2 mL HCl (37%, Aldrich) was mixed with 14.4 g of titanium ethoxide (Aldrich). Separately, 5 g of HFM (or alternatively 5 g of PHFM) was dissolved in 56 mL of 1-butanol (Aldrich). These two solutions were then mixed under vigorous stirring; 1 mL of acetone was added, and the solution was stirred for 2 h at 40 °C. The films were deposited on FTO at room temperature via dip-coating at a withdrawal rate of 0.8 or 1.6 mm/s and finally calcined for 2 h at 350 °C in air.35 Sometimes the second layer was deposited by repeating the dip-coating protocol. The layer thickness was measured by profilometer (Dektak 150, Veeco) or by scanning electron spectroscopy (SEM) (Hitachi FE SEM S-4800 microscope) with an EDX (energydispersive X-ray) detector attached. Raman spectra were excited by He−Ne laser using the 633 nm (1.96 eV) or the Ar+/Kr+ laser using the 532 nm (2.33 eV) excitation. Spectra were recorded by a Labram HR spectrometer (Horiba Jobin-Yvon) interfaced to an Olympus microscope (objective 50×). The spectrometer was calibrated by the F1g mode of Si at 520.2 cm−1. For Raman mapping, the 532 nm laser was used with 100× objective and a numerical aperture of 0.9. The Eg(1) Raman mode, fitted with a Lorentzian curve, was used for mapping. The sampled spot was ca. 1 μm2. The X-ray photoelectron spectra (XPS) were measured using multitechnique spectrometer ESCA 310 (Gammadata Scienta, Sweden) equipped with a hemispherical electron analyzer operated at a constant pass energy of 20 eV and giving an energy resolution, expressed by the full width at half-maximum (fwhm) of the Au 4f7/2 line, of 1.1 eV. Mg Kα radiation was used for electron excitation. The binding energy scale was calibrated using the Au 4f7/2 (84.0 eV) and Cu 2p3/2 (932.6 eV) photoemission lines. The pressure of residual gases in the analysis chamber during spectra acquisition was 6 × 10−10 mbar. The spectra were measured at room temperature and collected at a detection angle of 45° with respect to the
3. RESULTS AND DISCUSSION The dense titania layers were deposited on FTO by dip-coating as detailed in the Experimental Section. Synthetic conditions were systematically varied both by screening of various structure-directing agents in the precursor solutions and by modification of the withdrawal rate and number of layers. The synthetic parameters for particular samples are listed in Table S1 (Supporting Information). The films were typically ca. 60 nm thick for one dip-coating cycle at 0.8 mm/s withdrawal rate in accord with the earlier works.23,24,35 Proportionally thicker films are fabricated during repeated layer-by-layer deposition or by faster withdrawal. The quality of layers was tested by aqueous solution of K3Fe(CN)6 as the model pH-independent redox probe.23,24 Although the band-edge positions in titania vary depending on the determination technique and on the source reference,28,29,36−38 the redox potential of Fe(CN)63−/4− (0.24 V vs Ag/AgCl) is well positive to the flatband potential (φFB) of TiO2 (anatase) at all the accessible pHs in aqueous media:39 φFB = −0.36 − 0.059· pH
(in V vs Ag/AgCl)
(2)
Consequently, the titania dense layer acts as a rectifying interface, at which no anodic currents of Fe(CN)64− oxidation flow. Casual pinholes, if any, in the layer manifest themselves by the anodic current of ferrocyanide oxidation, which occurs at naked FTO in the pinhole.20 The onset of cathodic current (corresponding to the ferricyanide reduction) appears at 25971
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potentials negative to the φFB, when the titania electrode is in the accumulation (metal-like) regime. Figure S1 (Supporting Information) shows example voltammograms of Fe(CN)63−/4− on dense layers of different synthetic history. This technique is particularly suitable for analyzing pinholes of various types as detailed elsewhere.23,24 It is obvious that the occurrence of pinholes depends on the preparative protocol, but both HFM and PHFM containing precursors may provide good-quality films at certain conditions. Excellent and reproducible blocking is found on the one-layer samples prepared with the aid of Aldrich’s HFM (sample M10; see Supporting Information, Table S1 and Figure S1). This particular sample was grown at a slower rate (0.8 mm/s) and had a thickness of ca. 60 nm. It was chosen for further detailed studies reported below. Figure 1 shows cyclic voltammograms of this dense film in 0.5 M KCl, pH 2.4. For comparison, also displayed is the
Electrochemical doping was carried out simply by keeping the potential for a selected time at −1 V vs Ag/AgCl, which is well below the φFB (eq 2). The proton insertion occurs with the concomitant accumulation of electrons (eq 1). In accord with the study of Berger et al.,25 we observe reversible prewaves at ca. −0.1 V that shift positively when the doping progresses. This effect further develops with prolonged doping, but the proton uptake is stable, if the electrode is removed from the electrolyte solution, washed, dried, and stored in air. For instance, after 6 days of storage, the cyclic voltammogram did not show any dramatic dedoping, i.e., return to the voltammogram of the virgin electrode (Figure 1). Stirring at 250 rpm causes stronger electrochemical doping, as compared to doping in unstirred solution: the currents are expectedly larger, and we observe massive hydrogen evolution with the concomitant H2 oxidation as on pure FTO. Similarly to the previous case, the doping persists upon storage in air. Even after 6 weeks in air, the electrode does not return to the stage of symmetrical cyclic voltammogram, which is characteristic for a virgin sample or for the weakly doped25 one. Electrochemical doping leads to considerable asymmetry of the voltammogram at negative potentials with pronounced cathodic current (Figure 1). Exactly the same effect was observed for dense layers made by ALD or electrodeposition23 but not for porous nanocrystalline layers made by sintering of commercial nanoparticles.25 We propose that this difference is a signature of the dense nature of our films as well as those made by ALD and electrodeposition.23 Nanoparticles, even if they are agglomerated or sintered, still behave electrochemically like individual nanocrystals, because they are surrounded by the electrolyte solution penetrating into pores. Hence, the space charge layer (band bending) cannot develop in such small sizes at the low doping levels assumed in nanocrystals.25 More specifically, the thickness of space charge region (depletion layer), LD, equals: 1/2 ⎛ 2ε ε ⎞1/2 ⎛ kT ⎞ ⎟ L D = ⎜ 0 r ⎟ ⎜φ − φFB − e ⎠ ⎝ eND ⎠ ⎝
(3)
where e is the electron charge, ε0 is the permittivity of free space, εr is the dielectric constant of the semiconductor, ND the number of donors per unit volume, φ is the applied voltage, φFB is the flatband potential, k is Boltzmann’s constant, and T is the temperature. On the other hand, the dense layers of 60 nm thickness and large doping levels (ND ≈ 1020 cm−3, see below)24 do support the built-in electrical field (eq 3), i.e., they behave electrochemically like macroscopic single crystals. (A practical consequence of this similarity is the experimental observation of the depletion-layer capacitance by Mott−Schottky analysis,23,24 vide infra.) In our dense layers, the inserted protons (eq 1) diffuse from the surface to the bulk and are not recovered in the subsequent anodic scan. However, if the doping is carried out for a long time and is boosted by stirring (that is, with larger currents; cf. Figure 1), the layer is saturated with protons and the anodic counterpeak sets in. We note that an asymmetric voltammogram with enhanced cathodic charge also is observed on the Li-insertion into anatase single-crystal electrode but not in polycrystalline porous electrodes.41 The reason for such a similarity is obvious: the Li insertion is formally equivalent to proton insertion (eq 1). The unprecedented stability of doping, which persists for at least weeks in air (Figure 1), is a consequence of the dense nature of
Figure 1. Cyclic voltammograms of dense TiO2 layers (equivalent to the sample M10). Electrolyte solution, 0.5 M KCl; scan rate = 50 mV/ s, pH = 2.4. Doping was carried out by keeping the electrode at −1 V vs Ag/AgCl for the time indicated. Unless stated otherwise, the electrolyte solution was not stirred. Traces are offset for clarity, but the current scale is identical for all voltammograms. The cyclic voltammogram of bare FTO is shown for reference.
voltammogram of the bare FTO electrode. We trace hydrogen evolution on FTO at negative potentials with the corresponding anodic counterpeak, corresponding to the oxidation of H2 dissolved in the electrolyte solution near the FTO surface. Our FTO turned out to be quite stable at fast voltammetric scanning (Figure 1), with no apparent reductive breakdown of the electrode material. The virgin TiO2 electrode (labeled “no doping”; Figure 1) shows the typical reversible capacitive current, which is rising exponentially as is expected for the chemical capacitance of TiO2.40 25972
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Figure 2. Scanning electron microscopy images of representative samples. From top-left to bottom-right: (a) bare FTO, (b) FTO covered by dense titania layer (equivalent to sample M10), (c) as previous sample, but with selected area on the film surface showing a causal crack in the dense layer, (d) the same layer after electrochemical doping at −1 V vs Ag/AgCl for 20 min, (e) ditto after electrochemical doping at −1 V vs Ag/AgCl for 40 min with stirring at 250 rpm, (f) as sample (d) but selected area on the sample showing causal crack in the doped layer.
our films, mimicking the properties of a large single-crystal electrode. This is illustrated also by an earlier study35 showing that these films have no measurable porosity and a very small roughness factor (
