Epitaxial TiO2 Thin Film Photoanodes: Influence of Crystallographic

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C: Energy Conversion and Storage; Energy and Charge Transport

Epitaxial TiO2 Thin Film Photoanodes: Influence of Crystallographic Structure and Substrate Nature Helene Magnan, Dana Stanescu, Maxime Rioult, Emiliano Fonda, and Antoine Barbier J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11479 • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 19, 2019

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The Journal of Physical Chemistry

Epitaxial TiO2 Thin Film Photoanodes: Influence of Crystallographic Structure and Substrate Nature H. Magnan1*, D. Stanescu1, M. Rioult1, E. Fonda2, A. Barbier1 1

Service de Physique de l’Etat Condense (SPEC), CEA, CNRS UMR 3680, Universite Paris Saclay, Orme des Merisiers, CEA Saclay, F-91191 Gif sur Yvette, France. 2

Synchrotron SOLEIL, Saint-Aubin - BP48, F-91192 Gif-sur-Yvette Cedex, France

Abstract. Photocatalysis and photoelectrochemical (PEC) activities strongly depend on the chemical formulae of the electrodes materials but also on the local ionic environment that deserves to be addressed in details for a given compound. Specifically, within the framework of PEC water splitting, we studied the growth, crystal and electronic structures and PEC properties of thin epitaxial films of TiO2 deposited on Pt(111), Pt(001) and Nb:SrTiO3(001) substrates. We found that the crystallographic structure is respectively rutile (100), anatase (001) on interfacial TiO2-B (001), and anatase (001). We studied in details the relevant electronic properties such as band gap, position of valence and conduction bands, flat band potential and charge carrier density. We demonstrate that the largest photocurrent is obtained for rutile (100) deposited on Pt(111) owing to a higher position of the conduction band while the lowest photocurrent is obtained for anatase (001) deposited on Nb:SrTiO3(001) due to a detrimental larger band gap. This study shows that for pure phase epitaxial layers, rutile (100) is much more efficient than anatase (001) for solar

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water splitting. Our results suggest that bi-phase heterojunctions may be a fruitful route to tune the properties allowing for PEC photoanode improvement.

I.

INTRODUCTION

The transformation of solar energy into chemical energy to be stored in the form of hydrogen, through photoelectrochemical water splitting is a seductive energy harvesting/generation method with the important advantage of being environment friendly and free from carbon dioxide emission1,2. Since the pioneering discovery of water photo-assisted water electrolysis using semiconducting TiO2 in 1972 by Fujishima and Honda1 , titanium dioxide (TiO2) was extensively studied as a photocatalyst because of its chemical stability and nontoxicity3. In nature TiO2 adopts mainly one of the four following polymorphs (figure 1): (i) anatase (tetragonal, space group I41/amd), (ii) rutile (tetragonal, space group P42/mnm), (iii) brookite (orthorhombic, space group Pbca), and (iv) TiO2(B) (monoclinic, space group C2/m). It is commonly accepted that anatase is more efficient for light-energy conversion and photocatalytic reactions than rutile4,5 due to better charge transport properties. Compared to anatase, fewer studies on the photoactivities of brookite and TiO2(B) have been reported. However, different structures of TiO2 were compared for photocatalytic performance for the photodecomposition of organic compounds like cyclohexane, phenol, nitrophenol4, methyl orange6 … Notably only very few studies concerned single crystalline samples6,7. The purpose of the present paper focuses on the particular reaction of solar water splitting, where the sample is used as photoanode, that is the electrode where water oxidation occurs (4𝑂𝐻 ― +4ℎ + →𝑂2 +2𝐻2𝑂). For that purpose the sample has to be a semiconductor that should ideally fulfill the following requirements: strong visible light absorption, high chemical


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stability, efficient charge transport in the semiconductor, fast reaction kinetics at the electrolyte/semiconductor interface, valence band below the water oxidation potential and conduction band above the water reduction potential. In particular for polycrystalline titanium dioxide, anatase is believed to be better than rutile thanks to a better charge carrier transport6 and higher electron lifetime8, although rutile absorbs better the visible spectrum thanks to a more appropriate band gap for solar absorption (3.0 eV for rutile, and 3.2 eV for anatase). As a matter of fact, the properties of these different forms may vary with crystallite size, surface roughness and orientation of the surface in contact with water. Unfortunately, in polycrystalline samples all these contributions are entangled: it is thus not straightforward to correlate the modification of solar water splitting efficiency upon the change of crystallographic alone. The investigation of single crystalline samples is an elegant way to overcome this problem. To the best of our knowledge only one study concerns specifically single crystalline titanium dioxide within the framework of solar water splitting7. Indeed, L. Kavan et. al. compared anatase (101) and rutile (001) and found that the photocurrent is three times higher for rutile than for anatase and that the flat-band of anatase is shifted negatively by 0.2 V with respect to rutile.

Figure 1. Different crystal structures of TiO2 relevant to the present work: (a) anatase, (b) rutile and (c) TiO2(B). Blue spheres : titanium atoms , red spheres: oxygen atoms, drawing produced by VESTA9.


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In the present work we studied single crystalline thin films of titanium dioxides deposited on the following single crystal substrates: Pt(111), Pt(001) and Nb (1at.%):SrTiO3(001). Our TiO2 photoanodes consist of 20 nm thick single crystalline films deposited by atomic oxygen assisted molecular beam epitaxy (AO-MBE). AO-MBE allows a fine control of the growth conditions and in particular of the thin films stoichiometry and crystalline quality. Significantly different substrates and substrates orientations can be used to promote epitaxial thin film growth leading to different crystallographic phases (metastable or not) and orientations. We have validated our model samples approach in previous studies concerning hematite films grown by AO-MBE. We reported on the effects of Ti-doping10,11, film thickness10, stoichiometry12 and HCl-etching13 on the properties of hematite films within the framework of water photoelectrolysis. Let us now recall some elements of TiO2 growth: (i) The growth of TiO2 on SrTiO3(001) (STO) is well known from literature14,15: the lattice parameter misfit amounts 3% and the films adopt the anatase (001) crystalline structure with the epitaxial relationhip anatase// STO . (ii) The growth of TiO2 on Pt (111) has been widely studied in the ultra-thin regime for layers with thickness below 3nm. For such films different exotic phases were reported like hexagonal phase, lepidocrocite and TiO2 (B)16-18. Films with thickness above 3 nm have been found with a rutilelike (100) structure17,19 along with a (12) surface reconstruction as observed by LEED. The evaluation of the lattice parameter misfit between Pt (111) and rutile (100) is not obvious. The relevant low Miller indices in-plane lattice parameters for Pt (111) are 0.277 nm and 0.479 nm, while for bulk rutile (100), the relevant in-plane parameters are 0.296 nm and 0.459nm. Therefore the lattice parameter misfits are -6.4 % and +4.3% for the two high symmetry in-plane directions. These large values are likely to justify the occurrence of the surface reconstructions. (iii) The growth of TiO2 on Pt (001) has been studied only for the first monolayer (thickness below 0.5 nm)


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and only for under stoichiometric films (TiO2-x)20. To get a broader view let us point out that the lattice parameter of Platinum (0.392 nm) compares well with the lattice parameters of STO (0.3905 nm), Palladium (0.389 nm) and Silver (0.409 nm). On these different substrates the TiO2 film structure is respectively anatase (001), TiO2(B)(001) (in the 0-3nm thickness film range)21 and rutile (110)22. Unfortunately, the available literature is hardly of any help to predict the film structure of TiO2/Pt(001) in the large thickness regime (>5 nm). The present work aims at determining the relevant characteristics of thin epitaxial TiO2 photoanodes films. In order to investigate single crystalline electrodes as model samples, 20 nm thick layers were respectively deposited on Pt(111), Pt(001) and Nb:STO(001) substrates using Atomic Oxygen plasma assisted Molecular Beam Epitaxy (AO-MBE)23-25. These single crystalline samples have the genuine advantage of allowing us to investigate the modification of the intrinsic properties of titanium dioxide by changing the crystallographic structure and orientation only, without any change of thickness and/or stoichiometry, particles size etc…

II.

EXPERIMENTAL SECTION

Sample preparation. The titanium dioxide layers were deposited on single-crystalline Pt (111), Pt(001) and Nb:SrTiO3(001) substrates, using AO-MBE, a technique that makes possible the deposition of single crystalline layers of controlled stoichiometry, morphology and thickness9-11,25. High purity Ti (99.99% grade) was evaporated from a dedicated Knudsen cell in the presence of an atomic oxygen plasma (350 W power) to obtain well defined oxides under good vacuum conditions (i.e. 10-7 mbar working pressure, 10-10 mbar base pressure). The substrates were annealed at 450 °C under oxygen plasma during one hour prior deposition. During the growth, the


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samples were rotated continuously around their normal to ensure a homogeneous deposit and were radiative heated at a temperature of ca. 450°C with a filament located behind the sample. The typical oxide deposition rate was of about 0.05 nm/min. The deposition time was the same for the three films and the nominal thickness of our films was always 20 nm. Crystallographic structure and chemistry. In situ Reflexion High Energy Electron Diffraction (RHEED) patterns were observed and acquired during film growth to monitor the crystalline structure and quality of the samples. In situ XPS spectra were systematically recorded just after deposition to determine the stoichiometry and the electronic structure of the films. Ti2p, O1s core levels and the valence band region were recorded using Al Kα radiation. The binding energy (BE) scale was calibrated using the O1s line at 530.7 eV as a reference. In the case of titanium dioxide films deposited on Pt(111), Extended X-ray Absorption Fine Structure (EXAFS) spectra at the Ti K-edge (4960 eV) were acquired on the SAMBA beamline26 at synchrotron SOLEIL (Saint-Aubin, France) in the fluorescence mode. The spectra were recorded at room temperature in normal incidence (linear polarization of photons parallel to the film surface, i.e. a configuration more sensitive to the Ti in-plane neighboring) and in grazing incidence (20 ° from the surface) ( linear polarization of photons almost perpendicular to the film surface, i.e. a configuration more sensitive to the Ti out-of-plane neighboring). The kχ(k) EXAFS oscillations spectra were fitted using HORAE and FEFF 8.4 codes27-29 assuming anatase (001) or rutile (100) structures (cf. figure 2). More precisely, we fitted the two incidences together and used single scattering paths (distance and atom type) from the absorbing Ti atom of length up to 0.38 nm, corresponding to the octahedral shell of oxygen (O1 and O2) and the first neighbor shell of Ti neighbors (Ti1) and one more distant oxygen shell as illustrated on figure 2 in the case of rutile (100). We checked that multiple scattering was negligible. The fitting parameters for each path


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corresponding to a shell of neighbors are: (i) the amplitude for the path, (ii) the distance between the absorbing atom and the neighbor and (iii) the Debye-Waller (DW) factor to take into account additional structural disorder. In order to reduce the number of fitting parameters and to take into account the strain due to epitaxy, we linked the change in interatomic distances to a structural expansion by introducing two expansion factors: β parallel to the surface of the film and α perpendicular to it; with the exception of the octahedral shell of oxygen Ti-O shells for which TiO1 and Ti-O2 distances were allowed to vary.

Figure 2. Model of the crystal structure of rutile TiO2 (100) used in the FEFF calculations, detailing the terminology of the neighbors’ shells. Large blue spheres and small red spheres represent Ti and O atoms respectively.(The drawing was produced by VESTA9)

Photoelectrochemistry. The photoelectrochemical water splitting response of our films was studied using a three electrodes cell. All electrochemical measurements were performed at room temperature using a NaOH 0.1M (pH = 13) solution as electrolyte, a platinum wire as counter electrode and an Ag/AgCl electrode for the potential reference (VAg/AgCl = +0.97 V vs. RHE). The sample was mounted as anode (working electrode) using a dedicated sample holder designed to


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allow only the contact only between the titanium dioxide surface and the electrolyte. The illumination source was a Newport 1000 W Xe Arc Lamp with an infrared filter providing an incident light flux of around 100 mW/cm² (measured with a Newport 1918-R Power Meter). Potential control and current acquisition between the three electrodes were done using a Princeton Applied Research (PAR) 263A potentiostat controlled by a computer. For photocurrent – voltage curves, I(V), the potential was swept from -1 V to +0.7 V vs. Ag/AgCl at a speed of 50 mV/s. The photocurrent is defined as the difference between the current recorded under light and the without (dark). The measure of the photocurrent as a function of the wavelength was carried out under monochromatic light, at a bias voltage of 0.5 V vs. Ag/AgCl using a Cornerstone 130 model 74004 monochromator (Newport). The wavelength was varied between 200 and 1000 nm, with 10 nm steps. In order to increase the signal to noise ratio, we used modulated light. More precisely we used a PAR 5210 lock-in amplifier and an optical chopper at a reference frequency of 20 Hz. Electrochemical impedance spectroscopy (EIS) was performed in dark conditions. A dynamic signal of 10 mV amplitude with frequencies from 100 kHz down to 10 Hz was superimposed to a DC bias ranged from -1 V to 0.7 V vs. Ag/AgCl, with 0.1 V steps, using the potentiostat and the lock-in amplifier in order to generate the alternative signal V() and to measure the generated current I(). The cell impedance can be then calculated as: 𝑍 = 𝑉(𝜔) 𝐼(𝜔). Usually, the semiconductor electrolyte interface is simulated by a combination of ideal capacitors. However, in real systems porosity or surface roughness can produce local distortion of current density. The most frequently employed approximation in EIS is the introduction of a constant phase element (CPE) in replacement of ideal capacitance30. For this purpose, we modeled our system with a RSRCPE circuit of equivalent impedance Z given by:


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𝑍 = 𝑅𝑆 + (𝑖𝜔𝑛𝑄 + 𝑅 ―1)

―1

(1)

where RS and R are the series and space charge layer resistances respectively and 𝑖𝜔𝑛𝑄 the admittance of CPE, where n is between 0.9 and 1 (n =1 for perfect capacitance). Then we estimated the capacitance value of the CPE using the formula31: (𝑛 ― 1)/𝑛

𝐶 = 𝑄1/𝑛(𝑅𝑆 ―1 + 𝑅 ―1)

(2)

The Mott-Schottky plots A²/C² = f(V) were fitted using eq. (3) to determine the flat band potential (Vfb) and the carrier concentration (ND): 𝐴2 𝐶2

2

(

= 𝑒𝑁𝐷𝜀 𝜀𝑟 × 𝑉 ― 𝑉𝑓𝑏 ― 0

𝑘𝐵𝑇 𝑒

)

(3)

where A is the illuminated surface area, ε0 the dielectric constant of vacuum (8.85×10-12 F/m), εr the dielectric constant of titanium dioxide taken equal to 31 for anatase32 and to 173 for rutile33, V the potential applied to the electrochemical cell, kB the Boltzmann constant (1.38×10-23 J/K) and e the elementary charge (1.6×10-19 C). The flat band potential corresponds to the difference between the electrochemical potentials of the semiconductor (Fermi level) and of the electrolyte (redox level).

III.

RESULTS AND DISCUSSION

Structure and chemistry. a) TiO2/STO We will first describe the growth of TiO2 on Nb:SrTiO3(001). The RHEED patterns for the clean substrate and the 20 nm film are represented in figure 3, as well as the corresponding sketch of reciprocal surface lattice assuming an anatase (001) structure with an epitaxial relationship anatase// STO. We observe that these RHEED patterns are identical to the ones reported


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in previous studies13. In addition to the structural phase identification we have also derived from 𝑎𝑆𝑇𝑂

the RHEED patterns strike distances the measure of the in-plane lattice parameter: 𝑎𝑎𝑛𝑎𝑡𝑎𝑠𝑒 = 1.015 = 0.385 𝑛𝑚. This value is 1.5 % higher than the lattice parameter of bulk anatase (0.3795 nm), showing that at 20 nm the film is still not completely relaxed.

Figure 3. (Left panel, 2 columns) RHEED patterns of a 20 nm thick TiO2 film grown by AOMBE on Nb:SrTiO3(001), prior to (up) and at the end of deposition (down) over the two lowest index surface diffraction directions. (Right panel) The corresponding surface reciprocal lattices are represented: making explicit the diffraction directions, the elementary unit cell (in the reciprocal space) is also shown.

b) TiO2/Pt(111) At the beginning of the TiO2 growth on Pt(111), the intensity of the RHEED strikes decreases and a new pattern is observed after 60 minutes (at about 2.5 nm TiO2 thickness). This pattern becomes more and more visible and figure 4 represents the RHEED pattern of the 20 nm thick film. One observes large dots indicating a rough surface, and a Volmer-Weber growth mechanism.


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In both main diffraction directions the spacing between RHEED strikes is divided by ca. two with respect to the Pt(111) substrate. The expected crystallographic structure of TiO2/Pt(111) is rutile (100)17, i.e. a structure with tetragonal symmetry. Since the (111) surface of platinum has a hexagonal symmetry, one expects 3 variants, i.e. the deposited rutile should have three different in-plane orientations, corresponding to the equivalent lattice alignments between substrate and epitaxial layer. The resulting reciprocal lattice is represented in figure 4, we can see that with these assumptions the simulated diffraction pattern is similar to the experimental one. In order to have a confirmation of the rutile structure and a refinement of the crystallographic structure, we studied a similar sample (20 nm 0.5% Fe doped TiO2/Pt(111) which has exactly the same RHEED pattern) by EXAFS. EXAFS is a very well-suited technique to determine the local environment of a given ion with great sensitivity. The experimental kχ(k) EXAFS oscillations spectra for two incidences are shown in figure 5. The spectrum recorded in normal incidence is clearly different from the one recorded in grazing incidence, indicating an anisotropic structure. Since EXAFS is sensitive only to the local structure it is not obvious to identify unambiguously the different titanium oxide structures. As a matter of fact, in all crystallographic structures of TiO2 titanium atoms are surrounded by a slightly distorted octahedral shell of oxygen. Therefore, in order to test the accuracy of this technique we first tried to fit the data assuming either a rutile (100) or an anatase (001) structure with bulk lattice parameters (in these fits only Debye Waller factors are allowed to vary) (see figure 5). We observe that the two simulations are different and that the simulation with rutile is much closer to the data than the simulation with anatase (see for example the simulation in grazing incidence around 5 Å-1 and around 8 Å-1). Since rutile (100) is compatible with the observed RHEED patterns, we next fitted the EXAFS data by assuming a distorted rutile (100) structure: in this fit the distance between Ti and O atoms in the octahedral shell are allowed vary,


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and two dilatation factors ( perpendicular to the surface and  parallel to the surface) allow to take into account the strain imposed by the substrate. The result of the fit is plotted in figure 5. One observes that some discrepancies subsist between the experimental spectra and the simulation these can be due to the low number of fitting parameters and single scattering approach. The main parameters of the fit are reported in table 1. We found that the oxygen octahedron is slightly contracted and that the structure is 0.5 % in-plane dilated and 2.8 % out-of-plane contracted compared to bulk rutile. We used corresponding RHEED pattern (figure 4) to determine the inplane parameters, and found that cin-plane= 0.2917 nm and ain-plane =0.468 nm. Comparing these values with bulk rutile (c=0.296 nm, a= 0.4593 nm) and we can conclude that the film undergoes a slight in-plane dilatation of 0.25 %. Since the RHEED analysis gives the in-plane parameters of the surface layer and EXAFS gives the mean in-plane parameter of the whole film, we can conclude that the rutile structure is strained in the plane by the Pt(111) substrate, this dilatation (0.5 % on average) induces an out-of-plane contraction. For a thickness of 20 nm the strain is not completely relaxed and a 0.25 % expansion subsists at the surface.

Figure 4. (Left panel) RHEED patterns of a 20 nm thick TiO2 film grown by AO-MBE on Pt(111) over the two lowest indices surface diffraction directions (Right panel) The corresponding surface reciprocal lattices are represented, making explicit the diffraction directions. The elementary unit


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cells (in the reciprocal space) are also shown. (a) Pt(111) substrate, (b) 20 nm TiO2/Pt(111), as well as the 3 variants for rutile (100) (colored rectangles).

Figure 5. Raw (black lines) Ti K-edge kχ(k) EXAFS data 20 nm 0.5%Fe: TiO2/Pt(111) in normal incidence (Left panel) and in grazing incidence (Right panel) and fit of the data by FEFF calculation assuming anatase (001) structure with bulk lattice parameter (red line); rutile (100) with bulk lattice parameter (blue line), and strained rutile (100) with parameters indicated in table 1 (best fit, green line).

Table 1. Structural parameters (distance, dilatation factors) obtained by EXAFS for 20 nm 0.5%Fe: TiO2/Pt(111) using strained rutile (100) structure and corresponding lengths in bulk, the distance is given in nm, the experimental error bar is +/- 0.003 nm.

O1

O2

Ti1

rutile

0.195

0.198

0.296

Strained rutile

0.193

0.197

0.298





-2.8%

0.5%

c) TiO2/Pt(001)


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The RHEED patterns corresponding to the growth of 20 nm TiO2/Pt(001) are shown in figure 6. First, one observes a reconstruction of the clean substrate in the reciprocal space (10)*s direction, this reconstruction appears during the cleaning procedure (annealing at 450°C under oxygen plasma). Within the first 5 nm of deposition the reconstruction disappears and no additional RHEED line is observed anymore after. On the contrary, in the (11)*s reciprocal space direction two additional lines are observed between the main diffraction lines of Pt (001) for thicknesses below 5 nm. For higher thicknesses the RHEED patterns change again with the apparition of points indicating a rougher surface. Also the spacing between the RHEED lines is divided by two with respect to the Pt (001) substrate in the (11)*s direction. This latter diagram corresponds to the diffraction features of anatase (001) that is represented on figure 6d (green points). The cell of anatase is rotated by 45 ° with respect to the cell of Pt (001). The diffraction pattern in the case of a non-rotated anatase (001) cell is also represented on figure 6d (red points), which allows to explain the additional points seen in the (10)*s direction. We can thus conclude that for a 20 nm thickness the structure of the film is mainly anatase (001) with anatase//Pt alog with a small amount of the film is in-plane rotated with anatase//Pt. The in-plane lattice parameter deduced from these patterns is a= 0.375 nm, that is a contraction of 1% with respect to bulk anatase. The pattern with straight lines observed at low thickness (figure 6b), evidences an epitaxial 2D structure different from the final structure and deserves some discussion. We interpret this structure by the growth of a TiO2 (B) (001) structure in registry with Pt(001) (figure 6f). For convenience we have plotted the primitive and the conventional cell of TiO2 (B). TiO2 (B) is a metastable structure of titanium dioxide which has been discovered by Marchand et al.

34,

the

conventional cell in the (001) plane is a centered rectangle with lattice parameters a=1.216 nm and b= 0.374 nm. Assuming such a TiO2 (B) structure, we deduce from our RHEED pattern an


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estimation of the in-plane lattice parameters as a=1.122 nm and b= 0.3701 nm, which corresponds to an in-plane contracted structure. In order to understand the evolution of the structure with thickness, integrated profiles of RHEED pattern in the (11)*s direction are reported in figure 6e. We see that the transition between the two structures occurs at about 3 nm.

Figure 6. (Left panel) RHEED patterns of a 20 nm thick TiO2 film grown by AO-MBE on Pt(001) at different deposition time/thicknesses (steps a to d) over the two lowest surface diffraction directions (Right panel) The corresponding surface reciprocal lattices are represented, making explicit the diffraction directions for Pt(001) (a), For TiO2(B) (001) (b), and TiO2 anatase (001) (d). The elementary unit cells (in the reciprocal space) are also shown as well as the 2 variants for TiO2 (B) (001) and anatase (001) (colored rectangles). (e) Selected integrated cross sections taken from RHEED patterns oriented along the (11)*s direction for different deposition time/thicknesses. Rectangles indicate the peak relative to TiO2(B) (001) structure (pink) or the peak relative to anatase (001) structure (blue). (f) Sketch of the epitaxial relationship proposed for TiO2(B) (001) on Pt(001), for TiO2 (B) the primitive cell and the conventional cell are represented.


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As a partial conclusion, the growth of 20 nm TiO2 films on the three selected substrates produces three different structures: (i) a 1.5 % in-plane dilated 2D anatase (001) structure on Nb: SrTiO3 (001), (ii) a rough 1% in-plane contracted anatase (001) structure on top of a 2D TiO2 (B) layer on Pt(001), and (iii) a rough 0.2 % in-plane dilated rutile (100) on Pt(111). The stoichiometry and the electronic structure of our layers were determined by means of in situ XPS measurements. Ti2p core levels and valence band spectra obtained on our titanium dioxide films are shown on figure 7. The reference of binding energies (by considering the O1s line at 530.7 eV) is checked with the spectrum of valence band of TiO2/Pt(111) through the measurement of Pt Fermi level, still visible on the XPS spectra since platinum is not completely covered by the oxide film (3D structure). For all films the Ti2p3/2 line shows a narrow single feature at a binding energy of 459.32 eV (dash line on figure 7), which can be attributed to Ti4+ 35. The analysis of the position of the maximum of the valence band (VBM) is performed using valence band XPS spectra. The VBM changes with the substrate and is found at 2.19 eV for Pt(111), 2.33 eV for Pt(001) and 2.6 eV for Nb:SrTiO3(001).


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Figure 7. (left panel) valence band XPS spectra, (right panel ) Ti2p core levels XPS spectra for 20 nm TiO2 films deposited on Pt(111) (red), Pt(001) (black) and Nb:SrTiO3 (001) (green). The determination of the position of the maximum of valence band (VBM) is also shown on left panel with dotted lines.

Photoelectrochemical properties. Figure 8 presents the photocurrent density vs. voltage curves for three different TiO2 films used as photoanodes. The photocurrent density obtained for TiO2/Pt(111) is 1.4 times higher than for TiO2/Pt(001) and 5.5 times higher than TiO2/Nb:SrTiO3. The photocurrent onset potential is similar for all samples and situated around 0 V vs RHE. In a first approach, the rutile structure appears to be more efficient for solar water splitting than the anatase one as suggested in reference 7, although the two samples with anatase structure display


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very different photocurrents. To fully characterize the photoelectrochemical properties of these films we have also performed photocurrent measurements as a function of the wavelength. The photocurrent density data normalized to the incident light flux corresponding are plotted in figure 9. These normalized values are proportional to an “efficient absorption coefficient” which combines all the qualities mandatory for solar water splitting reaction: light absorption efficiency, conduction trough the photoanodes and electrochemical kinetics at the surface as a function of wavelength. Therefore their widths are necessary narrower than the curves that would convey only light absorption efficiency, more often shown in literature [see e.g. ref 6]. From figure 9 we can observe that their shapes depend on the substrate, showing that the window of “efficient absorption” is narrower for TiO2/Nb:SrTiO3 and larger for TiO2/Pt(001). In order to estimate the band gap, we draw “efficient Tauc plots”

36

given by (h)1/2 vs. h which can provide an

evaluation of the indirect “efficient gap” for the three films (figure 10), where  is the “efficient absorption coefficient”. The deduced “efficient band gaps” are respectively around 3.04 eV, 2.95 eV and 3.15 eV for TiO2/Pt(111), TiO2/Pt(001) and TiO2/Nb:SrTiO3(001). As explained before these values are necessarily larger than the real band gap, however they were found to be very closed to band gap values from literature for rutile (3.0 eV), anatase (3.2 eV) and TiO2 (B) (3.13 eV)

7,37.

Our results show that the best visible light absorption occurs for TiO2/Pt(001) (i.e. the

smallest band gap) although this sample does not yield the best photocurrent. Indeed, other parameters like kinetics at the surface, conductivity and valence band position can also influence the photocurrent. Moreover the band gap of this film is smaller than the values frequently found in literature for titanium dioxide. The smaller the band gap can hardly be attributed to the in-plane compressive stress since DFT calculations38,39 show that only in-plane tensile stress can induce a reduction of the band gap, yet without clear experimental confirmation. However the interface


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between TiO2 (B) and anatase may also play an important role favoring electronic states hybridization and thus reducing the apparent gap as well as increasing the recombination rate.

Figure 8. Photocurrent density vs. voltage (vs. Ag/AgCl lower scale and vs. RHE upper scale) curves for 20 nm TiO2 films deposited on Pt(111) (red), Pt(001) (black) and Nb:SrTiO3(001) (green) .


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Figure 9. “Efficient absorption”: photocurrent density normalized to light flux as function of the wavelength at 0.5 volts vs Ag/AgCl for 20 nm TiO2 films deposited on Pt(111) (red), Pt(001) (black) and Nb:SrTiO3 (001) (green). The data are normalized to the values at =270 nm. (inset) Light flux of the Xe arc lamp as a function of the wavelength.

Figure 10. “Efficient Tauc plot” deduced from curves of figure 9 for 20 nm TiO2 films deposited on Pt(111) (red), Pt(001) (black) and Nb:SrTiO3 (001) (green). The straight lines with open circles give an indication of the indirect “efficient gap” for the different thin films which are respectively 3.04 eV, 2.95 eV and 3.15 eV. Electrochemical impedance spectroscopy was performed on all samples. The admittance of CPE was determined by a fit using eq. (1) at each potential, and the charge capacitance was deduced from eq.(2). The results are reported in figure 11, in the form of Mott-Schottky plots ( 𝐴2 𝐶2 = 𝑓(𝑉)). From these curves we obtain the flat-band potential Vfb (the difference between the semiconductor Fermi level and the electrolyte redox level) and the carrier concentration (ND) by a Mott-Schottky curve fitting (eq. (3)). We use the permittivity of anatase (r =31) for TiO2/Pt(001) and TiO2/Nb:SrTiO3(001) and the one of rutile (r =171) for TiO2/Pt(111). The best fits (dashed lines) and the values of carrier concentration (color bar- inset) are indicated on figure 11. The values of flat band show a small variation: -1.17 V, -1.14 V and -1.25 V vs. Ag/AgCl for respectively TiO2/Pt(111), TiO2/Pt(001) and TiO2/Nb:SrTiO3(001). The carrier concentration (ND)


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of the films deserves some discussion. The value of ND for anatase on Nb:SrTiO3(001) is 6 times higher than ND for anatase on Pt(001) On the contrary the two films deposited on platinum show comparable ND values. It seems thus that the carrier concentration depends more on the substrate nature than on the crystallographic structure, STO substrate favoring a higher carrier concentration. An increase in carrier concentration increases the conductivity of the samples, since the conductivity σ can be expressed as 𝜎 = 𝑁𝐷 × 𝑒 × 𝜇 where µ is the electron mobility in the semiconductor. Our values of ND and flat band potential for TiO2/Nb:SrTiO3(001) are in agreement with the values reported by Matsumoto et al.32. However, we found is no evidence of the 0.2 V difference in flat-band between anatase and rutile often cited in literature7. Interestingly, the two samples with anatase (001) structure (TiO2/Pt(001) and TiO2/Nb:SrTiO3(001)) show a difference in flat-band potential of 0.11 V likely indicating that the surface morphology may be a more relevant parameter that influences more the flat-band value than the crystallographic structure itself. We found that the more negative flat band corresponds to the less rough surface.


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Figure 11. Mott-Schottky plots for 20 nm TiO2 films deposited on Pt(111) (red), Pt(001) (black) and Nb:SrTiO3 (001) (green). Dashed lines indicate the linear fit of each curve according to eq. (3). (inset) Carrier concentration (ND) deduced from the fit using eq.(3). Different values of r can lead to similar carrier concentration for very different 1/C2 curves (see text).

The relative position of conduction band in rutile and anatase is a long standing debate in literature, and is used to justify that rutile/anatase mixtures exhibit higher photocatalytic activities than single phase materials [see e.g. refs 40,41]. Importantly, the relevant parameter for solar water splitting is rather the position of the VBM and conduction band minimum (CBM) with respect to the water oxidation and reduction potentials. When a semiconductor is in contact with an electrolyte, the Fermi level of the semiconductor aligns with the redox potential of the electrolyte. This induces band bending and Helmholtz layer formation. Our XPS measurements allow positioning the VBM, and the measure of the band gap allows positioning the CBM. These results are plotted in scheme 1, with respect to the water oxidation and reduction potentials at pH=13 and at 0.8 V vs RHE. The sample which has the highest CBM is rutile TiO2/Pt(111), and it is around 0.3 eV higher than the CBM in anatase for both other samples. This is a determinant advantage in the framework of solar water splitting. Our experimental data unravels the properties of single crystal titanium dioxide layers which matter for solar water splitting. We will first compare the two samples with anatase structure but experiencing different strain states (TiO2/Pt(001) and TiO2/Nb:SrTiO3(001)). The first has a photocurrent 4 times higher than the second. This high photocurrent can have several explanations: a rougher surface increasing the effective contact area with the electrolyte, a lower band gap which allows a 35% more efficient absorption and finally maybe also a better transport of carriers toward the substrate thanks to the TiO2 (B) interface. A beneficial effect due to the band alignment


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between TiO2 (B) and anatase is however unclear due to some discrepancies in literature37,42. The only advantage of 2D anatase on SrTiO3 for solar water splitting is a higher carrier concentration ND and a lower (0.1V) flat-band potential but it is not competitive enough. Let us now compare the two samples deposited on platinum: anatase (001)/TiO2(B) (001) and rutile (100). The rutile layer sample has a photocurrent 1.4 times larger, its apparent advantage is a higher position of the CBM. This result is apparently surprising since anatase is usually believed to be better than rutile thanks to a better charge carrier transport and a higher electron lifetime. Moreover the (100) rutile orientation is less photoactive than the other orientations like (110), (101) (001) 6. However, as explained before, the studies which report that anatase performs better than rutile do not concern water oxidation but other redox couples with a different redox level 4-6.

Scheme 1. Illustration of the VBM and CBM (deduced from XPS and photoelectrochemical measurements) with respect of oxidation and reduction level of water at pH=13 and at 0.8 V vs RHE for the three samples: TiO2 films deposited on Pt(111) (red), Pt(001) (black) and Nb:SrTiO3 (001) (green) .

IV.

CONCLUSIONS


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We have studied the growth, the electronic and crystalline structures, and the photoelectrochemical properties of 20 nm thick single crystalline titanium dioxide layers of 20 nm deposited on three different substrates : Pt(111), Pt(001) and Nb: SrTiO3 (001) in the framework of solar assisted water splitting. AO-MBE was proven to be able to realize the corresponding high quality epitaxial growths. The crystallographic structure of the films is respectively rough (3D) rutile (100), rough (3D) in-plane compressed anatase (001) with a TiO2 (B) interface (3 nm), and 2D in-plane dilated anatase. XPS measurements, wavelength dependency and EIS revealed different electronic structures. It results that the rutile film deposited on Pt(111) provides the best photocurrent. We demonstrated that this is due to a higher conduction band minimum position more favorably located for water splitting. The anatase film deposited on Pt(001) has a smaller band gap, that maybe due to defects at the interface with the underlying TiO2(B) and/or compressive strain; it also has a photocurrent 4 times larger than the 2D anatase (001) layer deposited on Nb:SrTiO3 (001). These results show that 3D configurations and bi-phase heterojunction of titanium dioxide (TiO2(B) –anatase) are favorable for solar water splitting and may complement the already known beneficial case of rutile/anatase interfaces. The present study demonstrates that many parameters influence the photoelectrochemical behavior of active electrodes and that model surfaces/interfaces are very derirable to disentangle the role of each individual physical phenomenon. Corresponding Author (*) E-mail : [email protected]. Phone: +33 (0)1 69 08 94 04.


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ACKNOWLEDGMENT This work was supported by the ANR PHOTO-POT project, grant ANR-15-CE05-0014 of the French Agence Nationale de la Recherche.

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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

TABLE OF CONTENTS

TOC Graphic.


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Epitaxial TiO2 Thin Film Photoanodes: Influence of Crystallographic

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