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Charge Transfer Reductive in Situ Doping of Mesoporous TiO Photoelectrodes – Impact of Electrolyte Composition and Film Morphology Jesus Idigoras, Juan Antonio Anta, and Thomas Berger J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09926 • Publication Date (Web): 22 Nov 2016 Downloaded from http://pubs.acs.org on November 23, 2016

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Charge Transfer Reductive in situ Doping of Mesoporous TiO2 Photoelectrodes – Impact of Electrolyte Composition and Film Morphology

Jesús Idígoras,a Juan A. Anta,a Thomas Bergera,b*

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

Department of Chemistry and Physics of Materials, University of Salzburg, Hellbrunnerstraße

34/III, A-5020 Salzburg, Austria

Corresponding author: [email protected], Department of Chemistry and Physics of Materials, University of Salzburg, Hellbrunnerstraße 34/III, A-5020 Salzburg, Austria, Phone: +43-662-8044-5931

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Abstract Some material properties depend not only on synthesis and processing parameters, but may furthermore significantly change during operation. This is particularly true for high surface area materials. We used a combined electrochemical and spectroscopic approach to follow the changes of the photoelectrocatalytic activity and of the electronic semiconductor properties of mesoporous TiO2 films upon charge transfer reductive doping. Shallow donors (i.e. electron/proton pairs) were introduced into the semiconductor by the application of an external potential or, alternatively, by band gap excitation at open circuit conditions. In the latter case the effective open circuit doping potential depends critically on electrolyte composition (e.g. the presence of electron or hole acceptors). Transient charge accumulation (electrons and protons) in nanoparticle electrodes results in a photocurrent enhancement which is attributed to the deactivation of recombination centers. In nanotube electrodes the formation of a space charge layer results in an additional decrease of charge recombination at positive potentials. Doping is transient in nanoparticle films, but turns out to be stable for nanotube arrays.


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Introduction Semiconductor oxides constitute a versatile class of materials for electronic, optical, environmental and energy applications.1–6 The importance of these materials results, on the one hand, from their suitable intrinsic properties and, on the other hand, from the possibility to systematically alter their performance by bulk or surface modification. Semiconductor oxides such as ZnO, SnO2 and TiO2 amongst others show typically n-type conductivity due to a stoichiometric imbalance resulting from oxygen deficiency.7 Consequently, these materials are characterized by the presence of reduced cation sites a situation which is commonly referred to as self-doping. An impressive performance of self-doped semiconductor nanocrystals has previously

been

demonstrated

in

different

fields

including

the

synthesis

of

novel

nanocomposites,8 sensors,9 supercapacitors,10 batteries,11 synthetic chemistry12 or solar fuel generation.13 Semiconductor self-doping, however, not necessarily implies a deviation from the stoichiometric metal-to-oxygen ratio, but may result furthermore from the presence of hydrogen impurities acting as shallow donors.14 Importantly, hydrogen sources are ubiquitous during the whole process chain (from synthesis, to processing and application) of a semiconductor material.15 In general, doping comprises the inclusion or substitution of metal or non-metal impurities into the semiconductor lattice and has proved to be a useful strategy to manipulate the properties of the material by introducing new electronic states and new optical transitions.2,3 By these means, doping may affect charge transport, charge separation and charge generation, processes which influence the material performance in several applications. In this context the electron accumulation in a semiconductor electrode compensated by counter ion uptake from solution is referred to as electrochemical or charge transfer reductive doping.16,17 In the case of TiO2 the proton-assisted electrochemical reduction can be described by



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TiIVO2 + e- + H+  TiIII(O)(OH)

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(Eq. 1)

Protons may adsorb at the surface or may get inserted into the bulk thus acting as counter ions compensating the negative charge build-up upon formation of Ti(III) donor centers.18 This process may thus formally be considered as hydrogen-doping of the semiconductor. A beneficial effect of electrochemical doping on the photoelectrochemical performance, which was first evidenced for anatase single crystals16 and later corroborated for nanostructured TiO2 films of adequate morphology,17 was attributed to a temporarily persisting increase of the carrier density in the semiconductor. Recently, electrochemical doping was exploited to improve the materials performance in different applications ranging from dye-sensitized solar cells19–21 to photocatalysis21–26 and supercapacitors.27 An important difference of electrochemical doping as compared to metal-doping is the nature of electron charge compensation. In the latter case, electrons are persistently compensated by lattice-bound metal ions, whereas in the former case compensation is established by reversible adsorption of protons at the surface or by insertion in the lattice, respectively. Consequently, electrons can be transferred from the semiconductor to acceptors a process that can be accompanied by concomitant proton stabilization (proton-coupled electron transfer).28 While the reversibility of electrochemical doping opens up the possibility of driving reduction reactions at charged semiconductors29,30 it implicates on the other hand a limited persistence of those effects, which are connected to the accumulation of the charges in the material. For mesoporous TiO2 films, the persistence of charge accumulation and, thus, of performance enhancement is expected to depend critically on film morphology.16,17,21,31 In spite of the vast literature, there is still a lack of understanding about the influence of the morphology on the performance enhancement induced by a reductive doping treatment. Additionally, the equivalence between doping produced by an external voltage and by optical excitation has not been properly discussed. With this purpose, and using a combined 4 ACS Paragon Plus Environment

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spectroscopic and electrochemical approach, we have studied the charge transfer reductive doping of nanostructured TiO2 electrodes of different morphology. Whereas IR spectroscopy is used to track the doping persistence, photocurrent measurements were performed to elucidate the impact of doping on the photoelectrocatalytic activity of the films. We show that charge transfer reductive doping is a real in situ process which may take place not only upon the application of an external potential, but also upon band gap excitation at open circuit conditions. Special attention is paid to the impact of electrolyte composition and film morphology on photocurrent enhancement and doping persistence. For this purpose results from mesoporous films consisting of random networks of differently sized nanoparticles and oriented arrays of anodically grown nanotubes will be compared. Experimental Chemicals and materials. Anatase TiO2 particle powders were obtained from Solaronix (TiNanoxide T), Sigma Aldrich (SIGMA T-8141) and Sachtleben (E3-692-39-001), respectively. A mixed-phase anatase/rutile TiO2 powder was purchased from PI-KEM (TiO2 99.9+%). Formic acid (Sigma-Aldrich, puriss. p.a.), NH4F (Fluka, purum, ~40 %), ethylene glycol (Sigma Aldrich, anhydrous, 99.8%) and HClO4 (Sigma-Aldrich, ACS reagent, 70%) were used as received. All H2O solutions were prepared using water with a conductivity of 18 M·cm (Millipore, Milli-Q). Preparation of nanoparticle films. Slurries of the commercial TiO2 nanoparticles were prepared by grinding 1 g TiO2 powder with 3.2 mL H2O, 60 µL acetylacetone (99+%, Aldrich) and 60 µL Triton X (Aldrich) and were spread with a glass rod onto fluorine-doped tin oxide (FTO) conducting glass (Pilkington, TEC 15, resistance 15 Ω/□) using Scotch tape as a spacer. Then the films were annealed and sintered for 1 h at 450 °C in air. 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. The thickness of nanoparticle films was determined by scanning electron microscopy (Hitachi S4800 SEM-FEG). 5 ACS Paragon Plus Environment


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Preparation of anatase TiO2 nanotube (NT) arrays. NT electrodes were prepared by anodization of a Ti foil (Goodfellow, 99.6+%, 25 µm) in ethylene glycol containing 0.5% NH4F and 10% H2O.32,33 Electrochemical oxidation was performed at room temperature in a two electrode cell (Pt counter electrode, electrode separation: 2 cm) at a DC voltage of 40 V (anodization time: 4 h). After anodization, the samples were rinsed with water and annealed for 1 h in air at 500 °C. Electrochemical and photoelectrochemical measurements. Measurements were performed in a standard three-electrode electrochemical cell. Electrolytes were purged from O2 by bubbling N2 through the solution. Alternatively, O2 was bubbled through the electrolyte to maximize the concentration of dissolved oxygen. In some experiments formic acid was used as a hole scavenger. All potentials were measured against and are referred to a Ag/AgCl/KCl(3M) electrode (CRISON). A Pt wire was used as a counter electrode. Measurements were performed with a computer-controlled Autolab PGSTAT302N potentiostat. The current 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 UV/Vis irradiation of the electrode from the electrolyte side. 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. Vis/NIR

and

MIR

spectroelectrochemical

measurements.

For

spectroelectrochemical

measurements TiO2 films deposited on a Ti foil were used as the electrode. The spectroelectrochemical cells consist of an electrochemical cell as described above connected in the lower part to a Suprasil window (Diffuse reflectance (DR) Vis/NIR-spectroelectrochemical experiments) or to a hemispheric ZnSe prism (Attenuated total reflection (ATR) IRspectroelectrochemical experiments). The Suprasil window of the Vis/NIR – cell was connected to the integrating sphere of a fiber optic system (Ocean Optics, source: DT-MINI-2-GS, spectrometer: Maya2000). DR-Vis/NIR-spectra represent the difference of the Kubelka-Munk function (KM) of spectra taken at the reference potential and at defined potentials in the charge 6 ACS Paragon Plus Environment

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accumulation region of the electrodes. Details of the MIR-spectroelectrochemical set-up can be found in Ref.21. The spectra were obtained by averaging 500 scans at a resolution of 4 cm -1 and are represented as –log(R/R0), where R0 and R are the reflectance values corresponding to the single beam spectra recorded for the reference and the sample, respectively. Electrochemical measurements were performed with a computer-controlled Autolab PGSTAT101 potentiostat.

Results and Discussion Charge transfer reductive doping of random particle networks Electrodes prepared from commercial TiO2 samples consist of random particle networks and show significant differences with respect to primary particle size (Figure S1a-d). The properties of the investigated TiO2 nanoparticle films are summarized in Table 1. Table 1: Properties of TiO2 nanoparticle films Sample name

Particle size/nm

Film thickness/µm

Crystal structure

Supplier

TiO2-20nm

20 ± 5

1.5 ± 0.5

anatase

Solaronix

TiO2-25nm

25 ± 10

1.5 ± 0.5

anatase

Sachtleben

TiO2-45nm

45 ± 20

6±2

anatase/rutile (1:1) [Ref.17]

PI-KEM

TiO2-160nm

160 ± 80

5±1

anatase

Sigma Aldrich

Cyclic voltammograms (CVs) of TiO2 particle electrodes in 0.1 M HClO4 aqueous electrolyte are characterized by reversible capacitive currents at potentials EAg/AgCl < 0 V (Figure 1a-d). For anatase TiO2 films these currents have previously been associated with the reversible electron injection from the conducting substrate into an exponential distribution of band gap trap states.6,34 The accumulated charge was shown to scale linearly with the internal area of the semiconductor/electrolyte interface.35 CVs of TiO2-20nm and TiO2-25nm electrodes as well as the CV of the TiO2-45nm electrode feature comparable current densities. Capacitive current 7 ACS Paragon Plus Environment


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densities are lowest for the TiO2-160nm electrode reflecting a low electrochemically active surface area.

Figure 1: Cyclic voltammograms of TiO2-20nm, TiO2-25nm, TiO2-45nm and TiO2-160nm nanoparticle electrodes in the dark (a-d) and upon light exposure (e-h) before (pristine films, continuous blue line) and after electrochemical doping by application of an external bias 8 ACS Paragon Plus Environment

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(discontinuous red line): Edop = -0.6 V, tdop = 1000 s;. Electrolyte: N2-flushed 0.1 M HClO4 aqueous solution. Irradiance: 500 mW·cm−2. At the onset of the accumulation region CVs furthermore feature a pair of capacitive peaks (Figure 1a-d). These capacitive peaks are characteristic of mesoporous electrodes consisting of particle networks36 being absent on dense polycrystalline37 and single crystal electrodes35 and oriented arrays of elongated, quasi-one-dimensional structures featuring a low number of crystallite interconnections (such as nanowires, nanocolumns and nanotubes).38 However, their intensity depends not only on morphology, but to some extent also on electrolyte composition. As shown in a previous work, the corresponding accumulated charge can not be associated with an absolute number of states in the porous films, but corresponds rather to an apparent density of states, which varies with the pH value of the electrolyte or upon adsorption of small molecules.38 These voltammetric features have been attributed to the population/depopulation of deep trap states39 located at the particle surface or at particle/particle interfaces.38,40,41 Importantly it was observed that the population of these states critically influences the photocurrent response of the electrode.38,42 In the following we will focus in detail on the intensity and the position of this pair of capacitive peaks upon a reductive electrochemical modification of the mesoporous films. Long lasting electrode polarization at EAg/AgCl = -0.6 V leads to an intensity increase of the pair of peaks for all the electrodes (Figure 1a-d). Furthermore, for the TiO2-45nm and TiO2-160nm electrodes a progressive shift of these capacitive features towards more positive potentials can be observed in the CVs (Figure 1c,d).17 Both the intensity increase and the shift are reversible with respect to prolonged electrode polarization at EAg/AgCl = 0.8 V and have previously been associated with a transient accumulation of electron/proton pairs in the oxide.17 In addition to the modification of the pair of capacitive peaks an increase of the capacitive current is furthermore observed at -0.1 V < EAg/AgCl < -0.4 V. This increase is clearly seen for the TiO2-25nm electrode (Figure 1b), whereas it is less pronounced for the other electrodes. Such a behavior has been 9 ACS Paragon Plus Environment


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attributed to the generation of new subsurface states below the conduction band edge19 or, for semiconductor structures sustaining significant internal electric fields,

to the formation of a

depletion region at the semiconductor surface upon charge accumulation.20 Upon UV exposure photocurrents appear at EAg/AgCl > -0.3 V in the CVs due to water oxidation (Figure 1e-h). For pristine electrodes the photocurrent (which saturates for all electrodes at EAg/AgCl > 0.1 V) does not scale with the electrochemically active surface area of the electrodes and is highest for electrodes TiO2-20nm and TiO2-160nm (j0.8V = 0.60 and 0.52 mA·cm-2, respectively, Figure 1e,h) and significantly lower for electrodes TiO2-25nm and TiO2-45nm (j0.8V =0.15 and 0.25 mA·cm-2, respectively, Figure 1f,g). Electrochemical doping leads to significant changes in the CVs. We observe for all electrodes a significantly increased photocurrent (Figure 1e-h). The photocurrent enhancement is highest for TiO2-45nm and TiO2-160nm electrodes and significantly lower for TiO2-20nm and TiO2-25nm electrodes. There is no significant shift of the photocurrent onset potential upon electrochemical doping neither in the pure electrolyte nor in the presence of 1M HCOOH acting as a hole scavenger (Figure S2). Consequently, a significant displacement of the band edges upon doping can be excluded. The shape of the photocurrent profile does not change upon doping. From the voltammetric response of the pristine and the electrochemically doped electrodes (Figure 1a-d) we can conclude that the electron accumulation at negative potentials apparently contains two contributions. The symmetry of the cyclic voltammograms (Figure 1a-d) indicates a high reversibility of charge accumulation/extraction for short residence times at negative potentials (scan velocity: 20 mV·s-1) reflecting the typical capacitive behavior of metal oxide electrodes.39 This highly reversible behavior points to the involvement of electronic states located at the semiconductor/electrolyte interface. However, the long-lasting effect of electrochemical doping observed after extended cathodic polarization points to the population of electronic states in the subsurface regions of the semiconductor nanocrystals with a concomitant insertion of charge compensating protons into the bulk region of the particles. 10 ACS Paragon Plus Environment

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Figure 2: Schemes highlighting Fermi level (EF) displacement in mesoporous semiconductor oxide films upon (I) voltage-induced and (II) light-induced charge transfer reductive doping in acidic aqueous solution. (a-d) Voltage-induced and (e-j) light-induced doping of TiO2-45nm 11 ACS Paragon Plus Environment


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electrodes. Electrode potential profiles upon doping: (a,c) externally applied bias and (e,g,i) open circuit photopotential. (b,d,f,h,j) Photocurrent transients at EAg/AgCl = 0.8 V of electrodes before (pristine film) and after doping. Electrolyte: (a,b,e,f) N2-flushed 0.1 M HClO4 aqueous solution, (c,d,g,h) N2-flushed 1 M HCOOH/0.1 M HClO4 aqueous solution, (i,j) O2-flushed 1 M HCOOH/0.1 M HClO4 aqueous solution. Irradiance: 500 mW·cm−2. Three consecutive doping cycles have been performed in the case of light-induced doping in N2-flushed 0.1 M HClO4 aqueous solution (e,f), as indicated by the arrow. To

accurately

quantify

the

photocurrent

enhancement

upon

electrochemical

doping

chronoamperometric measurements upon UV exposure at EAg/AgCl = 0.8 V, i.e. at a potential in the saturation region of the photocurrent were performed (Figure 2). After the sintering of particle films deposited from suspension, pristine TiO2-45nm electrodes were immersed in the electrolyte, polarized at EAg/AgCl = 0.8 V and then exposed to UV light. A 0.1 M HClO4 aqueous solution purged from O2 (by bubbling with N2) without any additional hole scavenger (Figure 2a,b) or in the presence of 1 M formic acid was used as the electrolyte (Figure 2c,d). Upon light exposure a constant photocurrent is rapidly established in both cases (Figure 2b,d). After measuring the photocurrent transients of the pristine films in the respective electrolyte, electrodes were polarized in the absence of UV light at EAg/AgCl = - 0.6 V for 1000 s to accumulate electron/proton pairs in the oxide (electrochemical doping, Figure 2I). Finally, to sample the effect of doping on the photocurrent, we again recorded a photocurrent transient of the doped films. The ratio of the stationary photocurrent of pristine electrodes and the photocurrent determined after the doping step is defined as the (time-dependent) photocurrent enhancement factor (PCEF). After electrochemical doping we observe for both electrolytes that the photocurrent is significantly higher than for the pristine electrode, however, the photocurrent decreases continuously with time (Figure 2b,d).1 This highlights that electrochemical doping exerts a transient effect on the photoelectrochemical properties of the films. The persistence of

1

To confirm that the increased current measured upon UV exposure of doped electrodes corresponds indeed to a faradaic photocurrent and does not simply result from a light-induced extraction of charges accumulated during the doping step, an additional control experiment was performed as discussed in the Supporting Information (Figure S3).


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this effect depends on the morphology of the porous film as will be discussed in more detail below. The photocurrent enhancement is much more pronounced in the pure electrolyte (PCEF > 20) than in the presence of formic acid (PCEF ~ 3). By acting as a hole scavenger HCOOH clearly decreases electron/hole recombination even on a pristine electrode as reflected by a ~10 fold higher photocurrent (Figure 2b,d). This highlights that electrochemical doping effectively decreases recombination in the film and is most effective for those systems suffering from strong recombination. Charge accumulation in the semiconductor film, and thus electrochemical doping, may take place not only upon the application of an external potential (Figure 2I), but also at open circuit potential upon band gap excitation (Figure 2II).26 We complemented our study therefore by experiments featuring a charge transfer reductive doping step that consists of light induced charge accumulation at open circuit conditions. These conditions resemble the situation relevant for conventional photocatalysis, where the photoactive material is operated – in particle suspension or immobilized as a porous film – at open circuit conditions. For this purpose, the photocurrent transient of a pristine electrode was measured at 0.8 V for different electrolyte compositions (Figure 2f,h,j). In the next step electrodes where exposed to UV light at open circuit conditions. A negative shift of the open circuit potential upon light exposure (Figure 2e,g,i) reflects the displacement of the Fermi level in the semiconductor to more negative potentials as a consequence of hole transfer to electrolyte species and electron accumulation in the thin film (Figure 2II). In the pure electrolyte (i.e. 0.1 M HClO4 purged from O2, Figure 2e,f) an open circuit potential of EOCAg/AgCl = -0.33 V was measured after 1000 s of UV/Vis exposure. Following UV exposure at open circuit, a significant photocurrent enhancement was observed once the electrode was polarized again at EAg/AgCl = 0.80 V (Figure 2f). The enhancement increases with increasing UV exposure time at open circuit as demonstrated by repeating the doping step at



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open circuit. The light-induced beneficial in situ doping of the TiO2 film can thus be tracked by the photocurrent increase (Figure 2f). Obviously, electron accumulation upon light exposure (Figure 2II) beneficially influences the photoelectrochemical properties of the TiO2 electrode as does external charge accumulation by the application of a cathodic potential (Figure 2I). Light induced charge accumulation constitutes thus a real in situ doping process of the mesoporous semiconductor films. However, the photocurrent enhancement in the pure electrolyte (Figure 2f) is much weaker (PCEF ~ 3) than upon external polarization at EAg/AgCl = -0.60 V (PCEF > 20, Figure 2b). The number of charges accumulated in band gap states of anatase TiO2 films is known to increase exponentially with decreasing potential.39,43–46 The effect of electrochemical doping will thus significantly depend on the Fermi level position during doping. The presence of hole and electron acceptors in the electrolyte critically influences the efficiency of charge separation during photocatalyst operation and thus the degree of charge accumulation. The corresponding open circuit potential constitutes the effective doping potential and results from the kinetic balance between the transfer of photogenerated electrons and holes, respectively. We have therefore studied the impact of the electrolyte composition (presence/absence of electron or hole scavengers) on the photocurrent enhancement following a light-induced charge transfer reductive doping step. In the presence of 1 M HCOOH the open circuit potential upon light exposure (and thus the effective doping potential) shifts towards more negative potentials (EOCAg/AgCl = - 0.60 V) as compared to the pure electrolyte. The presence of the hole scavenger has an important impact on the recombination characteristics of the undoped semiconductor/electrolyte system, on the one hand, and on the light-induced doping process, on the other hand. As recombination is suppressed by hole scavenging, higher open-circuit photopotentials can be generated. This effect is further enhanced in the case of formic acid as a consequence of photocurrent multiplication, i.e. the generation of more than one electron per transferred hole due to charge injection from the highly reducing primary oxidation product (HCOO) to the semiconductor.47 14 ACS Paragon Plus Environment

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Higher doping levels (more negative potentials) may therefore be obtained in the presence of formic acid. The photocurrent enhancement following light-induced doping at EOCAg/AgCl = - 0.60 V (PCEF ~ 3, Figure 2h) is almost identical to the enhancement induced by external polarization at the same doping potential in the dark (Figure 2d). This coincidence clearly demonstrates that the improvement of the photoelectrocatalytic performance of the films depends on the Fermi level position during electrochemical doping, being however independent of the perturbation mode. This conclusion is perfectly in line with our previous observation that the occupancy of electronic states does not depend on the type of external perturbation, if, alternatively, an external bias voltage or band gap excitation at open circuit are used to set the Fermi level position in a mesoporous TiO2 electrode.48 The results presented here show, furthermore, that the density of shallow donors (i.e. electron/proton pairs) in mesoporous TiO2 electrodes in contact with an aqueous electrolyte has to be considered a dynamic thin film property, which may significantly change during operation. This highlights the need for determining materials’ properties under working conditions. From an analytical point of view we demonstrate the value of electrochemical methods for the characterization of photocatalytic systems, as processes taking place at open circuit can not only be tracked by open circuit potential measurements, but can even be simulated to some extent by external Fermi level control. Electron accumulation in photoexcited TiO2 catalysts is less pronounced in the presence of dissolved oxygen, the most common electron acceptor in photocatalysis. We performed experiments in oxygen-saturated electrolyte solutions in order to study the influence of O2 on the light-induced doping of the photocatalyst under working conditions. In the absence of a hole acceptor the open circuit potential measured on a TiO2-45nm electrode under UV exposure changes from -0.35 V in a N2-flushed electrolyte (Figure 2e) to -0.15 V in the O2-flushed electrolyte (not shown). Concomitantly, only a small and short-lasting increase of ~20 % of the photocurrent is observed at anodic polarization. Under these conditions photogenerated electrons are efficiently transferred to the acceptor in solution and electron accumulation in the 15 ACS Paragon Plus Environment


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semiconductor is too weak to give rise to a significant doping. In the presence of formic acid (Figure 2I), on the other hand, an open circuit potential of -0.48 V is observed even in the presence of dissolved oxygen, which gives rise to a photocurrent enhancement by a factor of ~2 as sampled upon anodic polarization (Figure 2j). However, the presence of oxygen causes a decrease of the photocurrent enhancement by ~30% as compared to the N2-flushed solution (Figure 2h). The stationary doping degree of a working photocatalyst will consequently be linked to the photopotential that can be generated under given experimental conditions. The photopotential results from the competition between charge (electron and proton) accumulation in the semiconductor and the charge (electron and hole) transfer to acceptors in solution. It will depend thus on parameters such as catalyst activity in general, light irradiance or electrolyte composition (e.g. presence of electron or hole acceptors). At steady state a constant doping level in the photocatalyst will be reached and maintained as long as a photopotential is generated in the semiconductor. The question is how the doping level (and thus the displacement of the Fermi level with respect to the pristine electrode) will develop once the perturbation by light exposure terminates. A possible relaxation back to the initial charging state will be governed by the propensity of electron/proton pairs to leave the oxide i.e. on the driving force for electrons to be transferred to the conducting substrate or to acceptors in solution and for protons to diffuse out of the oxide into the electrolyte. As electronic conductivity in TiO2 is much higher than proton conductivity49 we expect the latter to be the rate determining process. The diffusion coefficient of small cations in TiO2 is anisotropic.50,51 Furthermore, the average distance for inserted protons to be travelled in order to reach the oxide/electrolyte interface will depend on crystallite size. The film morphology is thus expected to influence critically the persistence of electrochemical doping via both size and shape of the particles forming the mesoporous electrode.


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Figure 3: (a) Time dependence of the photocurrent enhancement factors (PCEF) of nanoparticle electrodes after electrochemical doping as determined from the photocurrent transients measured at EAg/AgCl = 0.8 V. The inset shows the photocurrent transients of the pristine electrodes; Irradiance: 500 mW·cm−2. (b) Temporal variation at EAg/AgCl = 0.4 V of the normalized IR absorbance at 1300 cm-1 resulting from shallow trapped electrons in nanoparticle electrodes after electrochemical doping; Electrochemical doping by application of an external bias: Edop = 0.6 V, tdop = 1000 s; Electrolyte: N2-flushed 0.1 M HClO4 aqueous solution.

Both the photocurrent enhancement and its persistence turn out to depend significantly on film morphology (Figure 3a). All electrodes were electrochemically doped for 1000 s at EAg/AgCl = -0.6 V in pure 0.1 M HClO4 purged from O2. Longer doping times did not further affect the photoelectrochemical film properties. The persistence of the photocurrent enhancement was then sampled upon UV exposure at EAg/AgCl = 0.8 V. It is found to be highest for the TiO2-160nm electrode with a PCEF of 6.5 after 3600 s of polarization at 0.8 V followed by the TiO2-45nm electrode (PCEF = 5.5) and the TiO2-25nm electrode (PCEF = 2.1). For the TiO2-20nm electrode in contrast the photocurrent reaches its initial value (PCEF = 1) after just 1500 s. Ignoring the probable impact of particle morphology as an influencing parameter, these results would suggest faster proton depletion for smaller particles.



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Aiming at a direct tracking of the doping persistence (Figure 3b) we used in the following UV/Vis and IR spectroscopy. Only recently we have shown that spectroelectrochemical methods are a valuable tool to detect localized Ti3+ centers absorbing in the Vis/NIR and shallow (e-)(H+) traps absorbing in the MIR upon electron accumulation in TiO2 electrodes.46,52 The Vis/NIR spectrum of a TiO2-45nm electrode after 1000 s of external polarization at EAg/AgCl = -0.6 V is shown in Figure 4a. Charge accumulation in TiO2 electrodes gives rise to a broad absorption,40,52,53 which has been attributed to d-d transitions of Ti3+ species (electrons localized in band gap states)54 possibly containing a contribution from electrons in the CB.40,53 The signal is completely reversible with respect to charge extraction at EAg/AgCl = 0.4 V for 600 s (Figure 4b). Thus, Vis/NIR spectroscopy does not indicate any persistent doping of the electrode.


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Figure 4: (a) Diffuse reflectance UV/Vis and (c) ATR-IR difference spectra of a TiO2-45nm electrode upon polarization to EAg/AgCl = Edop = -0.6 V (t-0.6V = 1000s) and back polarization to 0.4 V. Temporal variation of the normalized KM function (b) and of the normalized IR absorbance at 1300 cm-1 upon back polarization to 0.4 V. Background spectra were taken for the pristine films at EAg/AgCl = 0.4 V. Electrolyte: N2-flushed 0.1 M HClO4 aqueous solution. (e) Scheme highlighting charge (electron and proton) accumulation at surface and subsurface regions of the TiO2 nanoparticles. 19 ACS Paragon Plus Environment


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Upon electrochemical charge accumulation a broad signal monotonically increasing toward lower wavenumbers with a sharp cut-off near 900 cm-1 appears in the MIR spectrum of a TiO245nm electrode and has been attributed to shallowly trapped electrons in the band gap of anatase TiO2 (Figure 4c).46,55,56 Negative going bands at around 3250 cm-1 and 1660 cm-1 can be attributed to water stretching and bending modes, respectively. Upon charge extraction at EAg/AgCl = 0.4 V the signal decreases, however, a residual portion of ~20 % of the stabilized IR absorption persists even after prolonged polarization for 3600 s (Figure 4c,d). MIR spectroscopy thus allows to track the doping and dedoping of the electrode in situ. These experiments allow furthermore attributing the signals in the Vis/NIR and in the MIR to different electron centers. Tentatively we assign the Vis/NIR active centers showing a high reversibility with respect to back-polarization to localized Ti3+ species at the TiO2 particle surface and MIR active centers to shallow (e-)(H+) traps, which are located at least partially in subsurface regions thus allowing for a persistent electrochemical doping of the electrode (Figure 4e). Importantly, we find that electrodes showing a long lasting photocurrent enhancement (Figure 3a) are those, which are characterized also by a high persistence of charge accumulation as sampled by MIR spectroscopy (Figures 3b and S4). Charge accumulation is most persistent for TiO2-160nm and TiO2-45nm electrodes, whereas a lower persistence is observed for the TiO2-25nm electrode. In the case of the TiO2-20nm electrode the MIR signal is depleted immediately after backpolarization pointing to an efficient electron and proton depletion for small crystallites. Charge transfer reductive doping of oriented nanotube films The microstructure of anatase TiO2 nanotube arrays (NT electrode) is shown in Figure 5a-d. Nanotubes have a diameter of 150 – 200 nm and a wall thickness, which decreases from ~ 70 nm at the bottom to ~ 30 nm at the top of the tubes. The CVs of NT electrodes are characterized by monotonically increasing/decreasing capacitive currents at cathodic potentials (Figure 5e). The additional pair of capacitive peaks at more positive potentials as observed for nanoparticle electrodes (Figure 1) is absent for NT electrodes in line with a previous study. 38 The currents 20 ACS Paragon Plus Environment

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show a partial irreversibility at the scan rate used (20 mV·s-1). Long lasting cathodic polarization leads to significant changes of the voltammetric response. Concretely, a progressive shift of the current onset towards more positive potentials is observed upon increasing polarization time at EAg/AgCl = -0.6 V. Importantly, these changes are irreversible with respect to prolonged anodic polarization or electrode storage at open circuit conditions (Figure S5). These results are a first indication of persistent, electrochemically induced charge accumulation in the NT electrodes and thus a charge transfer reductive doping of the semiconductor.

Figure 5: Scanning electron micrographs of anatase TiO2 nanotube films (a-d). Cyclic voltammograms of nanotube electrodes in the dark (e) and upon light exposure (f) before (pristine films) and after electrochemical doping by application of an external bias (Edop = -0.6 V) for different doping times. The first four cycles measured for a pristine electrode upon light exposure are also shown (f, 1-4). Electrolyte: N2-flushed 0.1 M HClO4 aqueous solution. Irradiance: 500 mW·cm−2.



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Bisquert and coworkers20 have previously studied the dynamic change of the capacitance in TiO2 NT electrodes upon electrochemical doping. Importantly, it was observed that pristine NT films display a chemical capacitance, which increases exponentially with applied potential thus resembling the behavior of mesoporous nanoparticle films. However, upon electrochemical doping (i.e. insertion of protons from the electrolyte into the NTs) an increase of the capacitance was observed and attributed to the formation of a depletion region within the NTs. Our observations are in line with such an interpretation. The formation of a space charge layer, however, should also be reflected in the photocurrent response of the NT electrode. We, therefore, measured CVs of pristine and electrochemically doped NT electrodes upon UV exposure. Indeed, electrochemical doping influences not only the dark response of the NT electrode, but also the photoelectrochemical properties (Figure 5f). To determine the photocurrent generated at a pristine NT film, electrodes were immersed in the electrolyte immediately after thermal annealing, polarized at EAg/AgCl = 1.8 V and then exposed to UV light. Four consecutive CVs were then recorded (Figure 5f, scans 1 – 4). Importantly, a progressive increase of the photocurrent is observed with increasing scan number. The photocurrent profile is characterized by a steep slope that culminates in an almost constant saturation photocurrent at EAg/AgCl > 0.4 V resembling the photocurrent profile observed for nanoparticle electrodes (Figure 1). Long lasting cathodic polarization for 60 – 660 s has an additional impact on the voltammetric response. Concretely, a further increase in photocurrent is observed after 60 s of polarization, whereas the onset and the shape of the photocurrent remain almost unchanged. Upon prolonged polarization for 360 s, however, a significant change of the shape of the photocurrent profile is observed in addition to a displacement of the photocurrent onset towards more positive potentials. Such a change was not observed for NP electrodes. The photocurrent profile attains approximately an inverted parabolic shape. This characteristic shape indicates the development of a space charge region.57 The corresponding internal electric field within the NTs counteracts recombination of photoinduced electron−hole pairs resulting in an increasing 22 ACS Paragon Plus Environment

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photocurrent. Further increase of the polarization time does not induce major changes. Electrochemical measurements thus clearly evidence a significant modification of film properties for NT electrodes upon electrode characterization and, more specifically, as a consequence of electrochemical charge accumulation. Photocurrent transients were recorded at EAg/AgCl = 1.8 V, i.e. at a potential in the saturation region of the photocurrent (Figure 5). Again, electrodes were immersed in the electrolyte immediately after thermal annealing, polarized anodically and then exposed to UV light. Upon light exposure an initial photocurrent spike followed by the rapid establishment of a constant photocurrent is observed (Figure 6). Following the measurement of photocurrent transients of pristine films, electrodes were cathodically polarized in the absence of UV light at EAg/AgCl = - 0.6 V for different times. After every charge accumulation (i.e. electrochemical doping) step, photocurrent transients were recorded at 1.8 V. Even very short polarization times (t ≤ 20 s) at 0.6 V induce significant changes in the photocurrent profiles. Concretely, a transient increase in photocurrent is observed that slowly relaxes back to the initial value before electrochemical doping. Longer polarization times lead to further photocurrent enhancement, however, for polarization times t ≥ 500 s photocurrent transients do not change any more. Reaching this situation, the photocurrent remains virtually unchanged even after 1500 s at 1.8 V (Figure 6a) in contrast to nanoparticle electrodes, where a significant decrease of the photocurrent enhancement factor is observed immediately after back-polarization (Figure 3a). The high stability of the photocurrent enhancement on NT electrodes is reflected by the invariance of the CVs measured immediately after electrochemical doping and after electrode storage in the electrolyte for 12 h at open circuit conditions (Figure S5).



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Figure 6: (a) Photocurrent transients at EAg/AgCl = 1.8 V for an anatase TiO2 nanotube electrode before (pristine film) and after electrochemical doping by application of an external bias (Edop = 0.6 V, different doping times); Irradiance: 500 mW·cm−2. (b) ATR-IR difference spectra of a nanotube electrode upon polarization to EAg/AgCl = Edop = -0.6 V for different times (t-0.6V = 800 s and 6000 s, respectively) and upon back polarization to 1.8 V (t1.8V = 1500 s). Temporal variation of the normalized IR absorbance at 1300 cm-1 upon back polarization to 1.8 V. Background spectra were taken for the pristine films at EAg/AgCl = 1.8 V. Electrolyte: N2-flushed 0.1 M HClO4 aqueous solution. 24 ACS Paragon Plus Environment

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The observation that increasing doping times lead to more persistent photocurrent enhancements suggests a doping time-dependent doping persistence as demonstrated by IR spectroscopy (Figure 6b). Whereas for short doping times (t-0.6V = 800 s at EAg/AgCl = - 0.6 V) charge accumulation is completely reversible with respect to back-polarization (t1.8V = 1500 s at EAg/AgCl = 1.8 V), increased doping times lead to a long lasting doping as tracked by the IR signal of shallow trapped electrons (Figure 6b,c).2 As in the case of nanoparticle electrodes, charge transfer reductive doping takes place also at open circuit upon light exposure (Figure S6). To exclude doping induced changes of the TiO2 crystal structure we performed an ex situ Raman study of those electrodes showing the highest doping persistence i.e. TiO2-160nm nanoparticle films and NT arrays. The Raman spectra of the pristine films contain exclusively anatase TiO2 specific bands (Figure S7). Importantly, no change in the spectra is observed after electrochemical doping at Edop = -0.6 V (tdop = 1000 s). Clearly, morphological properties determine not only the capacitive behavior of the porous electrodes studied here (i.e. NT arrays or films consisting of differently sized nanoparticles), but also the persistence of the electrochemically accumulated charges and thus the charge separation efficiency of photogenerated electron-hole pairs. Due to the small particle size, a low level of doping and the presence of a surrounding equipotential surface, polarization of nanoparticle electrodes at sufficiently negative potentials induces a homogeneous shift of the Fermi level within the porous film.39 For these films reversible capacitive currents increasing exponentially with decreasing potential are associated with a chemical capacitance of TiO 2.34 An additional contribution to the total capacitance of the semiconductor can come into play in dense polycrystalline films,31 nanotube arrays (characterized by dense polycrystalline tube walls)20 or single crystal electrodes58 (Figure 7). In contrast to nanoparticles these structures can sustain a

2

The absolute values of the doping time necessary to reach a stable photocurrent on the one hand (t-0.6V = 500 s, Figure 6a), and the maximum IR intensity of shallow trapped electrons on the other hand (t-0.6V = 6000 s, Figure 6b) can not directly be compared due to the difference in the geometry of 21 photoelectrochemical and spectroelectrochemical cells.



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space charge layer (band bending) especially at high doping levels. The depletion layer formation in NT electrodes upon electrochemical doping is clearly reflected by the modification of the CVs both in the absence and in the presence of UV light (Figure 5e,f). The presence of the internal electrical field will facilitate the separation of photogenerated electrons and holes thus contributing to a photocurrent enhancement. However, even in the initial stages of electrochemical doping, where the film capacitance and the shape of the photocurrent profile remain almost unchanged (Figure 5f, 1-4) the photocurrent increases significantly. In addition, photocurrent enhancement upon electrochemical doping is also observed for NP electrodes, which are not expected to sustain a significant band bending. Obviously, there is an additional mechanism contributing to the observed improvement of the photoelectrocatalytic activity of the porous films. Indeed, Kamat and coworkers22 observed the deactivation of recombination centers in TiO2 electrodes upon electrochemical doping and attributed it to trap filling and the generation of Ti3+/H+ centers. Apart from differences concerning the presence or absence of internal electric fields, significant differences of the doping persistence were found for nanoparticle films, NT arrays and dense layers.31 Whereas the doping is very stable in NT electrodes (Figure 6a) and in dense layers,31 it is less persistent in nanoparticle films (Figure 3). In the latter case our results point, furthermore, to a faster dedoping for smaller crystallites (Figure 3). This can be explained by a smaller average distance inserted protons need to travel in order to reach the oxide/electrolyte interface and thus to diffuse out of the oxide bulk into the electrolyte. Nanocrystals forming the random particle network, even if they are aggregated and sintered, expose at the oxide/electrolyte interface a variety of different facets offering thus different diffusion channels for protons to escape to the electrolyte. Although not explicitly considered in the present study, we expect therefore particle shape in addition to particle size to significantly impact on doping persistence. In dense layers and in nanotubes (characterized by dense tube walls) proton diffusion through the solid and transfer to the electrolyte may be limited by the compact polycrystalline structure. 26 ACS Paragon Plus Environment

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This may be one reason for the higher persistence of electrochemical doping in NT electrodes as observed in this and in previous studies. Morphological properties of nanotube arrays can nowadays be controlled in a very reproducible way.59 These materials constitute, therefore, a valuable model system, which will allow to address in future studies the impact of film properties such as the nanotubes’ wall thickness or aspect ratio on electrochemical doping.

Figure 7: Scheme highlighting charge (electron and proton) extraction from TiO 2 films of different morphology: nanoparticle films, nanotube films and polycrystalline films. Straight arrows indicate directions associated with low activation energies for ion diffusion. For crystallites in nanotubes and dense films ion diffusion along low energy directions may not end at the oxide/electrolyte interface thus slowing down proton diffusion through the solid and transfer to the electrolyte. In the present study we focus mainly on phase-pure anatase TiO2 nanostructures, however, a beneficial effect of electrochemical charge accumulation on photoelectrocatalytic properties has been observed recently also for phase-pure rutile TiO2 films.60 Whereas the macroscopic photoelectrochemical response to electrochemical doping (i.e. the appearance of a transient photocurrent enhancement) was found to be qualitatively comparable for both crystal structures,60 microscopic details determining such an improvement may be crystal structuredependent. Indeed, spectroscopic fingerprints of electrons electrochemically accumulated in anatase and rutile TiO2 films are different. As shown above (Figure 4), localized Ti3+ centers absorbing in the Vis/NIR and shallow (e-)(H+) traps absorbing in the MIR are generated upon electron accumulation in nanostructured anatase electrodes.46,52 On rutile electrodes, in contrast, 27 ACS Paragon Plus Environment


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such MIR-active (e-)(H+) traps are absent.52 Significant differences concerning the spectroscopic properties of excess electrons have also been observed in anatase and rutile TiO2 powders following electron injection from atomic hydrogen, a process which to some extent resembles the electrochemical hydrogen doping discussed above (Equation 1).61,62 Electrochemical doping of nanoparticulate films is related to hydrogen atom ionization at particle powders insofar as electron localization and proton adsorption/insertion occur in parallel in both cases.63 A characteristic electron center was detected by electron paramagnetic resonance (EPR) spectroscopy upon electron injection from atomic hydrogen into anatase TiO2 particles.61,62 It was show that for this electron center the excess electron is delocalized in the d orbitals of several structurally analogous lattice cations in the oxide bulk resembling a large polaron. We postulate that the population of large polarons upon electrochemical doping contributes to the improvement of the photoelectrocatalytic properties of anatase TiO2 films by accelerating charge transport21 and gives rise to the appearance of the MIR signal (Figures 4c and 6b). 46 In line with such an assignment, Giamello and coworkers64 correlated the polaron-like electron center in hydrogen-doped anatase particles with a broad absorption in the NIR monotonically increasing toward higher wavelengths. For rutile TiO2 excess electrons are known to be localized in nature.61,65 In this case the beneficial effect of electrochemical doping has been attributed to the energetic modification and passivation of trapping sites at particle/particle interfaces.60 Certainly, such a mechanism may also contribute to the overall activity increase observed for anatase electrodes.22 Conclusions A combined spectroscopic and electrochemical approach was used to study charge accumulation in porous TiO2 films upon electrode polarization in an acidic aqueous electrolyte. For nanoparticle films charge transfer reductive doping results in a transient photocurrent enhancement which is attributed to the deactivation of recombination centers upon concerted 28 ACS Paragon Plus Environment

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uptake of electrons and protons. The persistence of the photocurrent enhancement and charge accumulation depends on film morphology. Concretely, doping persistence scales with particle size for the electrodes investigated. For nanotube electrodes an additional mechanism (band bending) is found to contribute to the photocurrent enhancement. CV measurements performed in the dark and upon UV exposure point to the formation of a space charge layer in doped electrodes resulting in a further decrease of charge recombination. The results found for the different particle sizes and nanostructures investigated demonstrate that proton diffusion in the mesoporous electrode is a determining factor in both the doping and dedoping processes. Finally, charge transfer reductive doping may take place not only upon the application of an external potential, but also upon band gap excitation at open circuit and constitutes thus a real in situ process. This finding is relevant for photocatalysis, where the photoactive material is operated – in particle suspension or immobilized as a porous film – at open circuit conditions. Supporting Information Further electrochemical and spectroelectrochemical data as well as Raman spectra of pristine and electrochemically doped TiO2 films. ACKNOWLEDGEMENTS We thank Manuel Macias Montero for SEM measurements. B. Proft from Sachtleben Chemie GmbH (Duisburg, Germany) is acknowledged for kindly providing us with Sachtleben anatase TiO2 powder. This work was financially supported by the Spanish Ministry of Science and Innovation (MICINN) through the Ramón y Cajal program. J.I. and J.A.A. thank Junta de Andalucía for financial support via grant FQM 1851 and FQM 2310 and Ministerio de Economía y Competitividad of Spain under grant MAT2013-47192-C3-3-R. J.I. acknowledges MICINN for a FPU grant. T.B. gratefully acknowledges support from the Austrian Science Fund (FWF): [P28211-N36].



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