Combining Mesoporosity and Ti-Doping in Hematite Films for Water

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Combining Mesoporosity and Ti-doping in Hematite Films for Water Splitting Catherine Henrist J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5091476 • Publication Date (Web): 27 Dec 2014 Downloaded from http://pubs.acs.org on January 4, 2015

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Combining Mesoporosity and Ti-doping in Hematite Films for Water Splitting

Journal: Manuscript ID: Manuscript Type: Date Submitted by the Author: Complete List of Authors:

The Journal of Physical Chemistry jp-2014-091476.R1 Article 23-Dec-2014 Toussaint, Caroline; University of Liege, Chemistry, GrEEnMat Le Tran, Hoang; University of Liege, Solid State Physics, Interfaces and Nanostructures Colson, Pierre; University of Liege - Chemistry Department, Group of Research in Energy and Environment from Materials (GreenMAT) Dewalque, Jennifer; University of Liege, Chemistry, GrEEnMat Vertruyen, Bénédicte; University of Liege, Chemistry Institute Gilbert, Bernard; Univ. of LIEGE, Chemistry Nguyen, Ngoc; University of Liege, Solid State Physics, Interfaces and Nanostructures Cloots, Rudi; University of Liege, Chemistry, GrEEnMat Henrist, Catherine; University of Liege, Chemistry, GrEEnMat

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Combining Mesoporosity and Ti-doping in Hematite Films for Water Splitting Caroline Toussaint1, Hoang Long Le Tran2, Pierre Colson1, Jennifer Dewalque1, Bénédicte Vertruyen1, Bernard Gilbert3, Ngoc Duy Nguyen2, Rudi Cloots1 and Catherine Henrist1* 1

GREEnMat-LCIS, Department of Chemistry, University of Liège, Belgium

2

Solid-state Physics - Interfaces and Nanostructures (SPIN), Department of Physics, University

of Liège, Belgium 3

Laboratoire de chimie analytique et d’électrochimie, Department of chemistry, University of

Liège, Belgium


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Keywords:

Fe2O3, soft-templating, nanostructuration, silica scaffold, water photelectrolysis

Abstract: In this study, we report the synthesis of Ti-doped mesoporous hematite films by soft-templating for application as photoanodes in the photoelectrolysis of water (water splitting). Because the activation of the dopant requires a heat treatment at high temperature (≥ 800°C), it usually results in the collapse of the mesostructure. We have overcome this obstacle by using a temporary SiO2 scaffold to hinder crystallite growth and thereby maintain the mesoporosity. The beneficial effect of the activated dopant has been confirmed by comparing the photocurrent of doped and undoped films treated at different temperatures. The role of the mesostructure was investigated by comparing dense, collapsed and mesoporous films heated at different temperatures and characterized under front and back illumination. It turns out that the preservation of the mesotructure enables a better penetration of the electrolyte into the film and therefore, reduces the distance that the photogenerated holes have to travel to reach the electrolyte. As a result, we found that mesoporous films with dopant activation at 850°C perform better than comparable dense and collapsed films.


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Introduction Hydrogen production through photoelectrolysis of water (also called water splitting) is one way to harvest solar energy into an energy form that can be stored and transported. One of the main advantages of H2 as a fuel is that its combustion only produces water. The photoelectrolysis of water is therefore able to produce a clean fuel (H2) from an inexhaustible and pollution-free energy source (sunlight).1 In 1972, Fujishima and Honda reported the photoelectrolysis of water using UV radiation, a TiO2 photoanode and a platinum cathode.2 Since then, there has been a lot of research to find suitable electrode materials in order to use a broader range of the solar spectrum. 1-3 Hematite (α-Fe2O3) is investigated as a photoanode for water splitting because of its high stability in water, low cost, abundance and its band gap enabling the absorption of visible light (Eg between 1.9 eV and 2.2 eV depending on the synthesis method).4 However, the position of its conduction band does not enable the direct reduction of water5 and an additional voltage bias has to be applied, possibly provided by a coupled photovoltaic system.6-9 In theory, hematite might reach a maximum photocurrent density of 12.6 mA/cm2 (based on its absorption and the AM 1.5G solar spectrum).4 However, the experimental values are well below this figure, because of poor charge carrier transport and slow kinetics of water oxidation. The best values are currently around 3 mA/cm2 (at 1.23 V vs reference hydrogen electrode)5, 10 for nanostructured films with an additional dopant to improve conductivity and a catalyst to reduce the overpotential.4-5, 11-18 Nanostructuration of the photoanode is used as a strategy to address the short diffusion length of holes in hematite by reducing the distance that photogenerated holes have to travel to reach the electrolyte.4-5


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Nanostructuration of the hematite photoanode can be achieved by various methods, such as colloidal solution deposition19-22 , electrochemical deposition23-26 , spray pyrolysis11, 27-28 , hydrothermal synthesis13, 29-30, atmospheric pressure chemical vapor deposition5, 31 or copolymerbased soft templating.16, 32-33 In this latter technique, copolymer micelles act as a template for the condensation of a sol-gel precursor of hematite. The removal of the templating agent, usually by heat treatment, produces a mesoporous film with a continuous inorganic network and interconnected porosity. There have been a few reports about hematite mesoporous photoanodes prepared with this technique16, 32 but Hamd et al.16 found that there is a collapse of the mesoporosity beyond 500°C which is far below the temperature requested for the dopant activation (typically around 800°C).21 Therefore, the first objective of the present study was to reinforce the mesoporosity obtained by the soft templating method so that the hematite films could sustain the high temperature treatment needed for dopant activation. The first part of the paper describes how this could be achieved thanks to a SiO2 scaffolding strategy adapted from Brillet et al. and Ogawa et al.19, 34 The influence of the dopant on the photocurrent is then studied by comparison between Ti-doped and Ti-free films. Finally, mesoporous films are compared in term of photocurrent with dense and collapsed films in order to investigate whether the pore accessibility offered by the mesoporous structure leads to a performance improvement or not. The photoelectrochemical curves are discussed in relation with the (micro)structural characteristics of the films.

Experimental All films were synthesized by spin coating, as schematized in the left-hand side of Figure 1. The details of composition and preparation of the precursor solutions are provided as a flowchart in


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supporting information (Figure S1). The structuring agent used to create mesoporosity was a poly(isobutylene)-block-poly(ethylene oxide) copolymer (PIB5000-PEO11500 from Polymer Source). Dense films were prepared without structuring agent for comparison. For doped films, 4.5 mol% of Ti (relative to Fe) were added to the precursors solution as titanium tetraisopropoxide (TTIP). In all cases, 50 µL of solution were spin-coated (500 rpm for 5s followed by 3500 rpm for 30 s) on a 1 mm-thick aluminoborosilicate glass/FTO substrate (10Ω/sq, Solaronix) under a relative humidity maintained between 15% and 20%. Each layer was stabilized at 250°C for 15 min on a hot plate and calcined at 470°C for 10 min in a preheated tubular furnace. Multilayer films were obtained by repeating the same procedure for each layer as described in Figure 1. Some films were submitted to an additional heat treatment at higher temperature as described in the 'results' section. Prior to this final heat treatment, some mesoporous films were reinforced by a SiO2 scaffold34: these mesoporous films were immersed in a solution containing 195 µL tetraethylorthosilicate (TEOS), 0.211 g hexadecyltrimethylammonium chloride (C16TAC), 17.7 mL distilled water, 100 mL methanol and 8 mL NH3 25%. After 1h in the solution kept at 0°C under stirring, the samples were washed with water and dried before the final heat treatment at 800°C or 850°C. In order to remove SiO2 after the final heat treatment, the samples were soaked in NaOH 5 mol/L for 10 min, washed with distilled water and dried.


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Figure 1: Flowchart for the preparation of a dense film, a collapsed film and a mesoporous film. The hydrodynamic diameter of micelles was determined by dynamic light scattering with a viscotek 802 DLS. The films were characterized by X-ray diffraction with a Bruker D8 grazing incidence diffractometer (CuKα radiation, 0.5° incidence angle). Transmission electron micrographs of fragments scratched from single-layer films were obtained from a JEOL JEM1400 (W cathode, 100 kV). The surface morphology of the films was examined by atomic force microscopy using a Digital Instruments Nanoscope III microscope (Veeco). The image collection was performed in tapping mode (resonance frequency: 75 kHz; normal spring: C = 3 N/m) with a super sharp tip (Team-Nanotec). The thickness of the films was measured on polished crosssections by scanning electron microscopy (FEG-SEM XL30, FEI). The UV-visible measurements were carried out with a Perkin Elmer 9 from 350 nm to 800 nm. The photoelectrochemical performances of the films were measured in a three electrode electrochemical cell with a quartz window. In this cell, the working electrode was the hematite film on a FTO glass substrate, the counter electrode was a platinum wire and the reference was a saturated calomel electrode (SCE). The electrolyte was NaOH 1 mol/L (pH=13.6). Calculation of the potential versus the reversible hydrogen electrode (RHE) is: VRHE= VSCE + 0.241 volts+ 0.059 volts . pH = VSCE + 1.044 volts . The hematite electrode was scanned from -400 mV to 700 mV vs. calomel at 10 mV/s with a potentiostat/galvanostat (BioLogic SP200). In the graphs, the potential is reported relative to the RHE potential. The hematite films were illuminated with an intensity of 1 sun by a 450W xenon lamp (Oriel, ozone free) with a KG3 filter (3mm, Schott). The illuminated surface of the film was 0.24 cm². The samples were illuminated from the back (substrate) or front (film) side (see Figure 6).


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Results and discussion From hybrid film to crystallized mesoporous film The composition of the precursor solution corresponds to a 1:3 molar ratio between iron ions and the ethylene oxide units in the PIB-b-PEO structuring agent. The H2O:metal molar ratio is 6.8, as adopted by Brezesinski et al.35 The Ti:Fe molar ratio is about 4.5 %. In such conditions, diffusion light scattering measurements revealed the presence of micelles with an average hydrodynamic diameter of ~32 nm in the precursor solution. In order to convert the hybrid films obtained after spin-coating into mesoporous films, a heat treatment was optimized to decompose the structuring agent without leading to a collapse of the inorganic network. The first step of the heat treatment was a 15-min dwell at 250°C, i.e., a temperature below the decomposition temperature of PIB-b-PEO (~400°C 36). The objective of this first step is to complete the evaporation of the solvents and to promote the condensation of the inorganic species into a mechanically strong network, able to sustain the elimination of the structuring agent without collapsing. The temperature of the second step of the heat treatment was chosen to ensure not only the decomposition of the structuring agent but also the beginning of the crystallization of hematite phase. A serie of tests was run by placing films for different durations in a furnace preheated at different temperatures. It was found that a treatment at 470°C for 10 min reproducibly produces a mesoporous film (Figure 2b) with incipient crystallization of the hematite phase, as shown in the grazing incidence X-ray diffractogram (Figure 2a).


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Figure 2: Single-layer mesoporous film after heat treatment at 470°C for 10 min: (a) X-ray diffractogram; (b) TEM micrograph highlighting the mesostructure. Increasing the temperature and/or the duration of the heat treatment in an attempt to improve crystallization always resulted in the collapse of the mesoporous structure. Therefore it was decided to strengthen the mesoporosity of the films obtained at 470°C by coating the pore walls with a transitional SiO2 layer, which was removed after the final heat treatment by dissolution in NaOH. This strategy is adapted from a work of Brillet et al.19, where the authors used such a SiO2 layer to prevent the crystallite growth in nanoparticles films submitted to high temperature treatment. Since the collapse of mesoporous films during crystallization is usually considered to be related to a crystallite growth effect, it seemed reasonable to expect that the SiO2 scaffolding approach could be applied to our case. An overview of the different further heat treatments is shown in Figure 1. In summary, three microstructures are obtained (dense, collapsed and mesoporous) and two different final heat treatments were applied: 800°C 10 min and 850°C 30 min. In the following, the samples will be named D800, D850, C800, C850, M800 and M850. Figure 3 shows TEM micrographs collected after removal of the SiO2 scaffold for mesoporous films treated at 800°C for 10 min (M800) or 850°C for 30 min (M850). The mesoporosity is retained despite the high temperature of the heat treatment. By comparison, the micrographs of


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films submitted to the same heat treatments without SiO2 scaffold (Figure 3 C800 and C850) reveal the collapse of the mesoporous network. The collapsed film heated at 800°C (C800) still shows a significant porosity while grain growth is especially obvious for the film heated at 850°C (C850). The micrographs of the dense films (Figure 3 D800 and D850) prepared without structuring agent (see Figure S1) also reveal a significant grain growth and sintering during the heat treatment at 850°C.

Figure 3: TEM micrographs of single-layer mesoporous (M), collapsed (C) and dense (D) films calcined at 800°C for 10 min or at 850°C for 30 min.

The tuning of the film properties (crystallization and microstructure) was done on single layer films. However, thicker films are needed in order to obtain higher photoelectrochemical efficiencies (see absorption as a function of the thickness in Figure S2). Therefore, the further analyses were performed on six-layer films (~600 nm). In all cases, each of the six successive layers was heated at 250°C and 470°C. As described above for the single-layer samples, some six-layer films were then heated at 800°C or 850°C, with or without a SiO2 scaffold.


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The AFM images in Figure 4 show the different surface morphologies of the films of six-layer films and their evolution as a function of the heat treatment In the case of the mesoporous film, the scale of the images (Y and X scales of 1 µm and Z-scale of 50 nm) can not reveal the nanostructure of the surface (Figures 4a and 4c) therefore inset images (Y and X scales of 0.5 µm and Z-scale of 10 nm) are also provided (Figures 4b and 4d). First, we compare the three films calcined at 800°C (Figures 4a, 4e and 4g). The dense film shows a very smooth surface composed of relatively small grains. The collapsed film shows a more porous structure with grains of similar size as the dense film. The mesoporous film shows a nanostructure at a smaller Z-scale than the two other films and presents the smallest grains, which are responsible of a high specific surface area. The comparison of the three films calcined at 850°C (Figures 4c, 4f and 4h) shows that the mesoporous film retains a very fine nanostructure, the collapsed film displays the biggest grains and the dense film presents sintered grains. Indeed, the increase of the temperature and the duration of the heat treatment results in grain sintering in the case of the dense films. Concerning the collapsed films, we can observe a high increase of the grain size from 800°C to 850°C. This will lead to a decrease of the specific surface area. In the case of mesoporous films, the fine nanostructure is preserved compared to the collapsed film.


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Figure 4: AFM 3D images of mesoporous, collapsed and dense films calcined at 800°C (a, e, g) or 850°C (c, f, h). Images a, c, e, f, g and h have a Z-scale of 50 nm and the two inset images (b, d) have a Z-scale of 10 nm.

The XRD patterns of the six-layer mesoporous films heated at 470°C (M470 film), 800°C (M800 film) or 850°C (M850 film) are presented in Figure 5a. The increase in diffracted intensity (area under peak) confirms the improvement in crystallization driven by the heat treatment at high


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temperature (800°C or 850°C). The size of the coherent crystal domains (hereafter "crystallite size") of the six-layer films was estimated from the full width at half maximum of the (104) reflection of hematite using the Scherrer equation. The evolution of the crystallite size for the mesoporous (M), collapsed (C) or dense (D) films heated at 470°C, 800°C or 850°C is shown in Figure 5b. The most interesting observation is that the crystallite size in the mesoporous films remains almost constant, confirming that the SiO2 scaffold effectively hinders crystallite growth. In the case of the dense and collapsed films, the crystallite size increases with the temperature. Although crystallite size (from XRD) and grain size (from TEM or AFM) are different concepts, it is worth noting that the crystallite size trends are in good agreement with the grain size evolution observed by TEM for the single-layer films (see Figure 3).

Figure 5: (a) X-ray diffractograms of six layer mesoporous films after heat treatment at 470°C for 10 min, at 800°C for 10 min or at 850°C for 30 min. (b) Crystallite size for six-layer mesoporous(M), collapsed(C) and dense (D) films calcined at 470°C for 10 min, at 800°C for 10 min or at 850°C for 30 min.

Photoelectrochemical performances


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The performances of the six-layer hematite films as photoanode for water photoelectrolysis were characterized by measuring the current (J in mA/cm²) generated in the electrochemical cell as a function of the applied bias voltage (E in V vs. RHE). As mentioned in the introduction, the need for a bias voltage is due to the fact that the position of the hematite conduction band is too low for the direct reduction of water. The contribution of the photocurrent is then determined by comparing the J(E) curves acquired in dark and under illumination. The oxidation of water into O2 at a (photo)anode is a complex 4-electrons process. The large overpotential commonly observed in hematite photoanodes has been attributed to a combination of slow kinetics and possible trapping surface states.37-40 To reach high performances, a suitable catalyst is usually adsorbed on the photoanode surface.3, 5, 41-43 However, no catalyst has been used in the present work. Since the specific surface area and topography of the mesoporous, collapsed or dense films are different, adding a catalyst would introduce an uncontrolled variable in the study. This deliberate absence of catalyst will result in larger voltage onsets and lower photocurrents, which should not be compared to catalyst-assisted performances in the literature. Measurements of photocurrent can be carried out under back (Figure 6a) or front (Figure 6b) illumination. The following discussion will frequently draw on the fact that the zone where the majority of electron-hole pairs are photogenerated is not the same under front or back illumination, because of light absorption by the film and by the substrate. In the case of front illumination, most electron-hole pairs are created in the top layer of the hematite film, at the semiconductor-liquid junction (SCLJ).18, 33 In the case of back illumination, most electron-hole pairs are created near the hematite-FTO glass substrate interface. These trends are schematized in Figure 6 where the intensity curves were built from UV-visible measurements described in supporting information (Figure S2).


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Figure 6: Absorption in the six-layer, 600 nm thick hematite film at two wavelengths: 400 nm (purple) and 530 nm (green) under back illumination (a) and under front illumination (b). In the case of back-side illumination, absorption by the FTO-glass substrate is taken into account. Another point to consider is the photon absorption by the FTO glass substrate in the case of back-side illumination (see Figure S2). This typically results in a decrease of 20% to 30% of the photogenerated current, as estimated by comparing front-illumination measurements carried out with or without a FTO-glass substrate between the light source and the electrochemical cell (see Figure 10). The influence of this effect will be detailed later in the text. All the (micro)structural analyses in the previous section where done on Ti-doped films. However, the titanium ions may be inactive in some of the films. Before comparing the electrochemical performances of Ti-doped films with different microstructures, it is therefore necessary to investigate the influence of the titanium dopant by comparing the Ti-doped films with Ti-free films obtained from Ti-free precursor solutions.


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Figure 7 presents the J(E) curves of Ti-free and Ti-doped mesoporous films heated at 470°C, 800°C or 850°C. Data collected in the dark are shown as dashed curves; data under front or back illumination are shown as plain curves. In the case of Ti-free and Ti-doped mesoporous films heated at 470°C, there is no significant difference between the dark current and the current under front illumination (see Figures 7a and 7b). This means that there is no photocurrent. Under back illumination (see Figures 7c and 7d), the Ti-free film shows no photocurrent either, contrary to the Ti-doped film where there is a significant difference between the dark current and the current under back illumination. This means that the presence of titanium has a positive effect on the mobility of the electrons and holes photogenerated near the film–substrate interface. The fact that the same effect is not observed under front illumination suggests that the dopant activation is more effective close to the substrate, probably because the first layers of the multilayer film were submitted to several treatments at 470°C during the deposition of the next layers.


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Figure 7: Comparison of the photocurrents obtained under front (a,b) and under back (c,d) illumination between doped (b,d) and undoped (a,c) six-layer mesoporous films. The dark currents correspond to the dashed curves and the photocurrents correspond to the solid curves. In the case of mesoropous Ti-free films (see Figures 7a and 7c), heating at 800°C or 850°C results in the appearance of a significant photocurrent only in the case of back illumination. The absence of photocurrent under front illumination suggests that the electrons, which are photogenerated mainly near the film surface, do not reach the FTO current collector before recombination. In the case of back illumination, there might be a contribution of Sn doping by diffusion from the FTO layer, as reported by Frydrych et al. and Ling et al. 13, 44 In the case of mesoporous Ti-doped films (Figures 7b and 7d), heating at 800°C or 850°C results in the appearance (front illumination) or the enhancement (back illumination) of the photocurrent. Under front illumination, there is a marked difference in photocurrent between the


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films heated at 800°C for 10 min or at 850°C for 30 min. Figure 8 shows additional data for intermediate annealing conditions (800°C for 30 min and 850°C for 10 min). In the case of the short heat treatments (10 minutes), increasing the temperature weakly affects the photocurrent. However, extending the heat treatment duration is more effective, especially at 850°C. Higher temperatures or durations were not tested to limit the resistance increase of the FTO glass.45

Figure 8: Evolution of the photocurrent of a six-layer mesoporous film as a function of the duration and the temperature of the final heat treatment. Amongst the possible hypotheses to explain this improvement of photocurrent are (i) an increase in the charge carrier density due to better dopant activation and/or (ii) the passivation of surface states. The charge carrier density of the mesoporous films heated at 800°C for 10 min or at 850°C for 30 min was estimated by electrochemical impedance spectroscopy via Mott-Schottky plots (see details in supporting information, Figure S4). Similar charge carrier density was found for the two films from the slopes of the Mott-Schottky plots. However, the obtained values (1.2 1022 cm-3 for the M800 and 1.1 1022 cm-3 for the M850) are overestimated because it is expected that the real film surfaces, which could not be measured, are actually much larger. Regarding the possible passivation of surface states, we have used a cyclic voltammetry (CV) technique


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described by Bertoluzi et al.46 and Zandi and Hamann47. The absence of a second peak in the CV curves (shown in supporting information S5) indicates that the surface states are already passivated in the mesoporous films whatever the temperature and duration of the heat treatment. The main conclusion from Figure 7 is that, by comparison with the Ti-free films at the same temperatures, the positive influence of Ti-doping in films is confirmed for films heated at temperatures high enough to activate the dopant21 (here: 800°C and 850°C). There appears to be no strong effect of the increase of the temperature and the duration of the heat treatment on the charge carrier density, the surface state passivation and the crystallite size (see Figure 5b). Therefore the easier electron transfer is probably related to lower barriers to inter-grain transport.48 In view of the above results, the next step is to study the influence of the microstructure of the hematite Ti-doped films heated at 800°C or 850°C. Figure 9 shows the J(E) curves of the mesoporous (M), collapsed (C) and dense (D) films under front or back illumination. The comparison between the films with different microstructures is possible because all films contain a similar mass of hematite per unit area, as ascertained by measurement of the X-ray emission of iron (FeKα) under a 15kV standardized electron beam in SEM. However, the comparison between front and back illumination is made more difficult by the fact that the FTO-glass substrate acts as a filter for some of the radiation (see Figure S2). In order to estimate the influence of this effect and include it in the following discussion, Figure 10 shows plots comparing data under front illumination (full triangles), under front illumination with a FTO glass substrate placed between the light source and the electrochemical cell (empty triangles) and under back illumination (full circles).


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In the case of the films heated at 800°C measured under front illumination (see Figure 9a), the dense and the collapsed films deliver more photocurrent than the mesoporous film. This suggests that the transfer of the electrons photogenerated near the SCLJ toward the current collector is more difficult in the case of the mesoporous film. Possible reasons are a longer conduction path due to the tortuosity of the mesoporous structure and/or the smaller crystallite size (see Figure 5b) with crystallite boundary defects acting as recombination centers.12, 48-50 Comparing Figures 9a and 9b shows the influence of an increase of the temperature to 850°C, under front illumination. The case of the mesoporous film was already discussed above and the strong improvement of the mesoporous film performance under front illumination was attributed to an easier electron transfer probably related to lower barriers to inter-grain transport.48 The charge transfer being no longer an issue, the mesoporous film then performs better than the dense film by taking advantage of the higher surface area in contact with the electrolyte. The performances of the dense film are also improved but less than the mesoporous film, possibly due to the sintering suggested by the AFM images (see Figures 4g and 4h). In the case of the collapsed film, there is a deterioration of the performance in the film heated at 850°C compared to 800°C, possibly due to the individual grain growth (see Figure 3b and Figures 4e vs 4f) resulting in a loss of specific surface.


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Figure 9: Comparison of the photocurrents obtained for six-layer Ti-doped films with different microstructures (dense, collapsed and mesoporous) as a function of the temperature of the final heat treatment. Films were calcined at 800°C for 10 min (a,c) or 850°C for 30 min (b,d) and are analyzed under front (a,b) and back (c,d) illumination. Under back illumination at 800°C, the dense and collapsed films display lower photocurrent (Figure 9c) than under front illumination (Figure 9a). This is still true if the effect of the FTO glass substrate is taken into account (Figures 10a and 10b). These observations are in agreement with the fact that under front illumination, the travel of holes to electrolyte is short because the majority of the electron-hole pairs are photogenerated near the hematite film surface, while under back illumination, the travel of holes to the electrolyte is long because the majority of the electron/hole pairs are photogenerated near the FTO glass substrate (see Figure 6) and the electrolyte does not penetrate deeply into the dense and the collapsed films.


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On the contrary, the performance of the mesoporous film heated at 800°C is almost as good under back illumination as under front illumination, or even slightly better if the effect of the FTO glass substrate is taken into account (Figure 10c). Besides, under back illumination the mesoporous films perform better than the dense and collapsed films for both temperatures (Figures 9c and 9d). These observations confirm that the connected mesoporosity favors electrolyte infiltration so that the diffusion pathway of photogenerated holes to the electrolyte is shorter than for the dense or the collapsed films whatever the electron/hole pair generation position.20, 33 The comparison of Figures 9c and 9d shows that the performance of the mesoporous films under back illumination does not improve much by increasing the temperature from 800°C to 850°C. This confirms that the marked improvement observed under front illumination is due to an easier electron transfer from the film surface to the electrode (see Figures 9b and 10d).


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Figure 10: Comparison of the photocurrent obtained under front illumination (full triangles), under front illumination with a FTO glass placed between the light source and the electrochemical cell (hollow triangles) and under back illumination (circles) for dense, collapsed and mesoporous six-layer Ti-doped films calcined at 800°C for 10 min (respectively a, b, c) and for the mesoporous film calcined at 850°C for 30 min (d). Conclusions We have successfully prepared mesoporous hematite films by soft-templating. Our results highlight the role of the dopant to enhance the performances of the hematite electrodes by the comparison between doped and undoped films. A heat treatment at high temperature (800°C or 850°C) is necessary to activate the dopant and improve the crystallization of the films. The challenge of preserving the mesoporosity up to these high temperatures was achieved by using a SiO2 scaffold that limits the crystallite growth responsible for the collapse of the mesostructure at high temperature. The preservation of this open porosity is beneficial for the penetration of the electrolyte deeper into the film. This was confirmed by the highest photocurrent obtained with the mesoporous film under back illumination compared to the collapsed film and the dense film. We have also showed an improvement of the performances of the mesoporous film under front illumination when the temperature and the duration of the final heat treatment are increased (850°C for 30 min instead of 800°C for 10 min). This leads to a higher photocurrent for the mesoporous film compared to the dense and the collapsed film in the same conditions. Acknowledgements The authors would like to acknowledge the Laboratory of Photonics and Interfaces of the Ecole Polytechnique Fédérale de Lausanne for fruitful discussion.


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This work was supported by the Department of Chemistry of the University of Liège. Supporting Information Flowchart of solution preparations UV-visible: data processing EIS measurements and data processing Cyclic voltammetry: measurements and results This information is available free of charge via the Internet at http://pubs.acs.org Corresponding Author *E-mail address: [email protected] Phone number: +3243663438 References 1. Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chemical Reviews 2010, 110, 6446-6473. 2. Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37-8. 3. Cho, S.; Jang, J.-W.; Lee, K.-H.; Lee, J. S. Research Update: Strategies for Efficient Photoelectrochemical Water Splitting Using Metal Oxide Photoanodes. APL Materials 2014, 2, 010703. 4. Sivula, K.; Le Formal, F.; Graetzel, M. Solar Water Splitting: Progress Using Hematite (Α-Fe2o3) Photoelectrodes. ChemSusChem 2011, 4, 432-449. 5. Tilley, S. D.; Cornuz, M.; Sivula, K.; Graetzel, M. Light-Induced Water Splitting with Hematite: Improved Nanostructure and Iridium Oxide Catalysis. Angew. Chem., Int. Ed. 2010, 49, 6405-6408, S6405/1-S6405/3. 6. Brillet, J.; Cornuz, M.; Le Formal, F.; Yum, J.-H.; Gratzel, M.; Sivula, K. Examining Architectures of Photoanode-Photovoltaic Tandem Cells for Solar Water Splitting. J. Mater. Res. 2010, 25, 17-24.


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Graphical abstract:


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. 50x30mm (300 x 300 DPI)


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Figure 1: Flowchart for the preparation of a dense film, a collapsed film and a mesoporous film. 177x43mm (300 x 300 DPI)



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Figure 3: TEM micrographs of single-layer mesoporous (M), collapsed (C) and dense (D) films calcined at 800°C for 10 min or at 850°C for 30 min. 40x20mm (300 x 300 DPI)


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Figure 3: TEM micrographs of single-layer mesoporous (M), collapsed (C) and dense (D) films calcined at 800°C for 10 min or at 850°C for 30 min. 79x83mm (300 x 300 DPI)



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Figure 4: AFM 3D images of mesoporous, collapsed and dense films calcined at 800°C (a, e, g) or 850°C (c, f, h). Images a, c, e, f, g and h have a Z-scale of 50 nm and the two inset images (b, d) have a Z-scale of 10 nm. 79x156mm (300 x 300 DPI)


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Figure 5: (a) X-ray diffractograms of six layer mesoporous films after heat treatment at 470°C for 10 min, at 800°C for 10 min or at 850°C for 30 min. (b) Crystallite size for six-layer mesoporous(M), collapsed(C) and dense (D) films calcined at 470°C for 10 min, at 800°C for 10 min or at 850°C for 30 min. 78x70mm (300 x 300 DPI)



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Figure 6: Absorption in the six-layer, 600 nm thick hematite film at two wavelengths: 400 nm (purple) and 530 nm (green) under back illumination (a) and under front illumination (b). In the case of back-side illumination, absorption by the FTO-glass substrate is taken into account. 82x86mm (300 x 300 DPI)


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Figure 7: Comparison of the photocurrents obtained under front (a,b) and under back (c,d) illumination between doped (b,d) and undoped (a,c) six-layer mesoporous films. The dark currents correspond to the dashed curves and the photocurrents correspond to the solid curves. 82x110mm (300 x 300 DPI)



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Figure 8: Evolution of the photocurrent of a six-layer mesoporous film as a function of the duration and the temperature of the final heat treatment. 82x59mm (300 x 300 DPI)


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Figure 9: Comparison of the photocurrents obtained for six-layer Ti-doped films with different microstructures (dense, collapsed and mesoporous) as a function of the temperature of the final heat treatment. Films were calcined at 800°C for 10 min (a,c) or 850°C for 30 min (b,d) and are analyzed under front (a,b) and back (c,d) illumination. 82x112mm (300 x 300 DPI)



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Figure 10: Comparison of the photocurrent obtained under front illumination (full triangles), under front illumination with a FTO glass placed between the light source and the electrochemical cell (hollow triangles) and under back illumination (circles) for dense, collapsed and mesoporous six-layer Ti-doped films calcined at 800°C for 10 min (respectively a, b, c) and for the mesoporous film calcined at 850°C for 30 min (d). 173x96mm (300 x 300 DPI)


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Figure S1: Flowchart for the preparation of the solutions. 83x85mm (600 x 600 DPI)



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Figure S2: Transmission (%) of the FTO glass and mesoporous films of 1, 3 or 6 layers treated at 800°C for 10min. 81x59mm (300 x 300 DPI)


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Figure S3: Equivalent circuit for the system of an hematite film deposited on a FTO glass in contact with the electrolyte. 64x31mm (300 x 300 DPI)



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Figure S4: Mott-Schottky plots for 6 layer mesoporous hematite films treated at 800°C for 10 min (a) or 850°C for 30 min (b). 65x24mm (300 x 300 DPI)


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Figure S5: CV curves for dense, collapsed and mesoporous films treated at different temperatures. 162x148mm (300 x 300 DPI)


Combining Mesoporosity and Ti-Doping in Hematite Films for Water

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