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A Facile Surface Passivation of Hematite Photoanodes with TiO2 Overlayers for Efficient Solar Water Splitting Mahmoud G. Ahmed,† Imme E. Kretschmer,‡ Tarek A. Kandiel,*,† Amira Y. Ahmed,† Farouk A. Rashwan,† and Detlef W. Bahnemann‡,§ †
Department of Chemistry, Faculty of Science, Sohag University, Sohag 82524, Egypt Photocatalysis and Nanotechnology Research Unit, Institut für Technische Chemie, Leibniz Universität Hannover, Callinstrasse 3, D-30167 Hannover, Germany § Laboratory for Nanocomposite Materials, Department of Photonics, Faculty of Physics, Saint-Petersburg State University, Ulianovskaia street 3, Peterhof, Saint Petersburg 198504, Russia ‡
S Supporting Information *
ABSTRACT: The surface modification of semiconductor photoelectrodes with passivation overlayers has recently attracted great attention as an effective strategy to improve the charge-separation and charge-transfer processes across semiconductor−liquid interfaces. It is usually carried out by employing the sophisticated atomic layer deposition technique, which relies on reactive and expensive metalorganic compounds and vacuum processing, both of which are significant obstacles toward large-scale applications. In this paper, a facile water-based solution method has been developed for the modification of nanostructured hematite photoanode with TiO2 overlayers using a water-soluble titanium complex (i.e., titanium bis(ammonium lactate) dihydroxide, TALH). The thus-fabricated nanostructured hematite photoanodes have been characterized by X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy. Photoelectrochemical measurements indicated that a nanostructured hematite photoanodes modified with a TiO2 overlayer exhibited a photocurrent response ca. 4.5 times higher (i.e., 1.2 mA cm−2 vs RHE) than that obtained on the bare hematite photoanode (i.e., 0.27 mA cm−2 vs RHE) measured under standard illumination conditions. Moreover, a cathodic shift of ca. 190 mV in the water oxidation onset potential was achieved. These results are discussed and explored on the basis of steady-state polarization, transient photocurrent response, open-circuit potential, intensity-modulated photocurrent spectroscopy, and impedance spectroscopy measurements. It is concluded that the TiO2 overlayer passivates the surface states and suppresses the surface electron−hole recombination, thus increasing the generated photovoltage and the band bending. The present method for the hematite electrode modification with a TiO2 overlayer is effective and simple and might find broad applications in the development of stable and high-performance photoelectrodes. KEYWORDS: nanostructured hematite photoanodes, water splitting, passivation overlayers, TiO2, hydrogen production, photoelectrochemistry
1. INTRODUCTION Securing clean and renewable energy supply is a key challenge that the scientific community presently faces. With growing energy demands arising from industrialization and population growth, along with environmental concerns related to increasing CO2 emissions, the development of alternative energy sources to replace the burning of fossil fuels is both imperative and vital. No doubt that solar energy-driven systems are promising ways to provide the world energy needs. For instance, photovoltaic cells are effective at producing electricity from the sun; however, storage difficulties and high costs render it economically not preferable.1 An efficient route is to convert solar energy into storable chemical fuels (e.g., molecular hydrogen, H2) via photoelectrochemical water splitting. However, one of the barriers that needs to be overcome © XXXX American Chemical Society
prior to a widespread use of this technology will be the discovery and development of stable and efficient photoelectrodes. The first example for electrochemical water photolysis that has been discovered was using rutile TiO2 under UV light irradiation,2 with the solar to hydrogen conversion efficiency over TiO2 being limited by its large band gap (i.e., 3.0 eV). In fact, effective materials for solar hydrogen production should fulfill many requirements such as (1) efficient capture and conversion of photons into electrons and holes, (2) effective utilization of photogenerated holes and electrons for fuel production, and (3) stable solar-to-fuel Received: August 1, 2015 Accepted: October 9, 2015
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DOI: 10.1021/acsami.5b07065 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
ultrathin TiO2 deposits prepared using the ALD technique on the performance of hematite photoanodes.26 Although a cathodic shift of ca. 100 mV was achieved, the photocurrent response was not significantly enhanced. However, although ALD has emerged into a powerful technique for thin-layer deposition (e.g., alumina),25,27 it seems on the basis of the aforementioned reports25,26 that the ALD technique does not produce high-performance TiO2-modified hematite photoanodes. Moreover, ALD relies also on reactive and expensive metalorganic compounds and on vacuum processing, both of which are significant obstacles toward large-scale applications.28 Moreover, ALD is an expensive technique and hence limited to only a few research groups.29 Thus, it is desirable to develop an alternative cheap, simple, and water-based solution method for the surface modification of hematite photoanodes with TiO 2 overlayers. Recently, Hisatomi et al.28 have developed a chemical bath deposition method for the modification of hematite with group 3 oxide overlayers (i.e., alumina, Ga2O3, and In2O3), but to the best of our knowledge, a simple water-based solution method for the modification of hematite with a TiO2 overlayer has not been reported yet. In fact, most of the titanium precursors (e.g., alkoxides and tetrachloride) are water-sensitive and hydrolyze rapidly, rendering control of the hydrolysis process quite difficult. In this work, an aqueous solution of commercially available titanium bis(ammonium lactate) dihydroxide (TALH) has been employed for hematite surface modification with TiO 2 overlayers. The advantages of the use of TALH as a TiO2 precursor are that it is a water-soluble precursor and thus does not require an alcohol-based solution and that it is stable at ambient temperature in air hence eliminating the need of an inert atmosphere during the adsorption process. The TALH complex exhibits a negative charge and can be readily adsorbed onto the hematite photoanode surface from an aqueous solution under conditions where the solution’s pH is lower than the point of zero charge (pHPZC) of α-Fe2O3. Afterwards, gentle heat treatment of the hematite electrodes leads to the formation of amorphous thin TiO2 layers. Interestingly, the modification of hematite photoanodes employing this simple and low-cost fabrication strategy dramatically improved the photocurrent density (i.e., by a factor of 4.5 as compared with that of bare hematite at 1.23 V vs RHE). Moreover, a significant cathodic shift of the onset potential of O2 generation of approximately 190 mV under standard illumination conditions was also achieved. The role of TiO2 as a passivation overlayer is discussed and explored on the basis of steady-state polarization, transient photocurrent response, open-circuit potential, intensity-modulated photocurrent spectroscopy, and impedance spectroscopy measurements.
production that is economically sensible compared to the capital cost of installation.1,3 However, to date, no material fulfills all these requirements, with hematite (α-Fe2O3) still appearing to be a standing candidate. For instance, hematite has good optical properties due to its favorable bandgap (2.2 eV), allowing it to absorb a rather large portion of the solar spectrum (i.e., UV light as well as the majority of the visible light). This makes hematite theoretically able to convert 16.8% of the total solar irradiance into H2.4 Other merits of hematite include its excellent stability in basic medium both in the dark and under illumination, its ample abundance, low cost, nontoxicity, and suitable valence band energy for water oxidation.5,6 However, hematite has significant challenges limiting its practical performance for the water oxidation process.7 These include short hole-diffusion length (2−4 nm), poor carrier conductivity, and slow kinetics of water oxidation.7,8 Moreover, the bottom of the conduction band is energetically positioned below the water reduction potential; thus, a bias potential is required to drive the photoelectrochemical water splitting reaction.4,9 This bias potential can, for example, be obtained from a photovoltaic cell. Recently, it was reported that a tandem device containing a single perovskite solar cell and a doped hematite photoanode can achieve a solar-to-hydrogen conversion efficiency of 2.4%.10 The further enhancement of the efficiency of such devices requires the improvement of the efficiency of both the perovskite solar cell and the hematite photoanode. To date, enormous efforts have been exerted to enhance the efficiency of hematite. For example, to alleviate the high recombination rate of the photogenerated carriers and to accommodate the short hole diffusion distance, nanostructured hematite electrodes (nanorods,11 nanowires,12 nanoflakes,13 etc.) have been fabricated.14 The slow kinetics of water oxidation causing the high overpotential can be overcome by modification of the surface with cocatalysts such as Co-Pi15 or IrO2.16 Also, the conductivity of hematite has been improved by doping with different metals (e.g. Ti,17 Mn,18 Ta,19 Sn,20 etc.). In particular, titanium as a dopant element has received great attention because of its different roles in enhancing the practical performance of hematite. For example, Wang et al. noticed that titanium acts as n-type dopant causing an increase of the electron concentration.17 Glasscock et al. observed a cathodic shift in the potential onset as a result of the improved electrical conductivity and the enhanced charge carrier density.21 Furthermore, Hahn et al. attributed the high efficiency of titanium-doped hematite films to both the improved electron transport within the bulk of the film and the suppression of the recombination at the film−electrolyte interface because of a stronger electric field near the surface.22 Herein, rather than the doping of hematite with titanium cations, we have investigated the effect of surface passivation of hematite photoanodes with a TiO2 overlayer. In fact, the surface modification of semiconductor photoelectrodes with passivation overlayers has recently attracted great attention as an effective strategy to improve the charge-separation and charge-transfer processes across the semiconductor−liquid interfaces.23 For instance, it was reported that the stability and the efficiency for water oxidation of Si, GaAs, and GaP photoanodes can be greatly enhanced by coating their surfaces with a TiO2 overlayer using atomic layer deposition (ALD).24 Le Formal et al. found that an Al2O3 overlayer deposited using ALD reduces the overpotential required for water oxidation on hematite photoanodes by as much as 100 mV, whereas TiO2 overlayers showed no beneficial effect.25 Yang et al. reinvestigated the effect of
2. EXPERIMENTAL SECTION Hematite Films Preparation. The hematite films have been prepared on fluorine-doped tin oxide (FTO) glass substrates by a simple template-less film-processing technique as previously reported.30 Briefly, the FTO glass substrates were cleaned in an ultrasonic bath with acetone, followed by cleaning with ethanol and water. Subsequently, the FTO substrates were vertically placed in a glass vessel containing 5 mL of aqueous ferric chloride solution (0.15 mol L−1, Sigma-Aldrich) and heated for 6 h at 95 °C. The thus-grown uniform yellow films of β-FeOOH on the FTO substrates were then rinsed by water to remove the inorganic salts. After two-step annealing (i.e., 550 °C for 1 h and 800 °C for 20 min), reddish-brown films of hematite were obtained. B
DOI: 10.1021/acsami.5b07065 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Modification of Hematite Films with TiO2 Overlayers. The asprepared hematite films were immersed in different concentrations of aqueous TALH solutions (i.e., 0.5, 2.5, and 5 mmol L−1 at pH 5.5 for 1 h). Afterward, the films were rinsed with bidistilled water and then heat treated at 350 °C for 1 h. Hematite Photoanodes Fabrication. The hematite photoanodes were fabricated by connecting copper wires to the uncoated area of the conductive FTO surface using silver paste. After drying of the paste, the connections between the wires and the FTO glass substrates except for 0.5 cm2 of the coated area were sealed with nonconductive epoxy resin and connected to a plastic tube for easy handling. Photoelectrochemical Measurements. All photoelectrochemical measurements were recorded for bare and TiO2-modified hematite photoanodes using an Autolab PGSTAT302N potentiostat in a threeelectrode configuration with 1.0 mol L−1 NaOH aqueous solution as an electrolyte, a platinum wire as counter electrode, and Hg/HgO (1.0 mol L−1 NaOH) as a reference electrode. All three electrodes were placed in a three-electrode Teflon-made photoelectrochemical cell adapted with a quartz window on one side for illumination. The photocurrent−potential curves were measured under simulated solar light generated by an Osram XBO 70 W xenon lamp in a Müller LXH 100 lamp housing coupled with an air mass 1.5 global filter (Sciencetech Inc.). The light intensity of the simulated light was adjusted to 1 sun (100 mW cm−2) using a Thorlab digital hand-held energy meter console (PM100D) connected with a calibrated highsensitivity thermal sensor (S401C, Thorlabs). Polarization data for Tafel plots were obtained by changing the potential, stepwise (5 mV steps) in the positive direction, with sufficiently long delays to achieve steady-state conditions (5.0 s). The electrolyte resistance was quantified using the impedance method, which permitted all polarization plots to be corrected for the iR drop. The modification of hematite photoanodes with a Co-Pi cocatalyst was carried out using the photoassisted electrodeposition technique as previously reported.15 The electrodeposition was carried out at a potential of 0.2 V vs Ag/AgCl for 10 s from an aqueous solution of Co(NO3)2 (0.5 mmol L−1) in 0.1 mol L−1 sodium phosphate buffer (pH 7). The incident photon-to-current conversion efficiencies (IPCE) were measured under a bias potential at 1.23 V vs RHE. The photoanode was irradiated by a collimated light beam using high-power LEDs (Thorlabs). Intensity Modulated Photocurrent Spectroscopy. The intensity modulated photocurrent spectroscopy (IMPS) response was measured using the same photoelectrochemical workstation except that the light source was a triple LED array (470 nm) driven by the output current of the Autolab LED Driver. The dc output of the LED Driver is controlled by the DAC164 of the Autolab, and the ac output of the LED Driver is controlled by the FRA32 M module. The ac amplitude was set to 10% of the dc output. All measurements were recorded with the NOVA software. The IMPS response was examined at a frequency ranging from 0.1 Hz to 10 kHz at different applied potentials (i.e., 0.7−1.2 V vs RHE at 0.1 intervals). The light intensity of the LED array at the electrode surface was measured using the PM100D energy meter console connected with the S401C calibrated high-sensitivity thermal sensor (Thorlab) and found to be 52.0 mW cm−2. Impedance Spectroscopy Measurements. Nyquist plots were measured at different applied potentials with the frequency range being modulated between 100 kHz to 0.1 Hz at an amplitude frequency of 10 mV under standard illumination conditions. Mott− Schottky plots were examined at a frequency of 100 Hz ranging from −0.4 to 1.4 V vs RHE in the dark. Characterization. XRD patterns of the bare and of TiO2-modified films were collected by using a Bruker D8 diffractometer operating with a Cu Kα1,2 energy source at 40 kV and 40 mA. Field-emission scanning electron microscopy (FE-SEM) measurements were recorded on a JEOL JSM-6700F field-emission instrument using a secondary electron detector (SE) at an accelerating voltage of 2.0 kV. X-ray photoelectron spectroscopy (XPS) analysis was carried out by using a physical electronics PHI 5800 ESCA system employing
monochromatized Al Kα radiation (13 kV and 250 W). A prominent maximum peak of C 1s at 284.6 eV was taken as the reference to calibrate the XPS spectra.
3. RESULTS AND DISCUSSION The poor conductivity of the charge carriers in hematite can be improved by doping and by employing nanostructuring strategies,20,31,32 but the slow charge transfer to solution remains the performance bottleneck, especially when driving complex reactions such as water oxidation.33 The recombination of the photogenerated charge carriers at surface defects is apparently too fast compared with the speed of hole transfer to the solution, thus decreasing the light-to-current conversion efficiency.34 These surface defects at the hematite/liquid interface where holes accumulate can, for example, be blocked by depositing a passivation layer. Mostly, the sophisticated ALD technique is employed for the deposition of such layers.26 Herein, we have developed a novel green and simple waterbased solution method for the modification of nanostructured hematite photoanodes with a TiO2 overlayer. Bare hematite films have been prepared by the template-less film-processing technique resulting in the growth of highly anisotropic βFeOOH akaganeite films on conductive FTO glass from aqueous ferric chloride solutions as previously reported.30 The two-step thermal treatment of the obtained films at high temperature leads to the conversion of the grown β-FeOOH nanorods into hematite as evidenced from its (110) and (300) diffractions located at 35.7 and 64.05° (2θ), respectively (Figure 1). The other diffraction peaks shown in Figure 1 can be readily indexed to the characteristic SnO2 peaks resulting from the conductive fluorine-doped SnO2 layer.
Figure 1. XRD diffraction patterns of bare and TiO2-modified hematite films. The star symbols indicate Bragg positions for hematite, according to the Powder Diffraction File (PDF) No. 033-0664 (International Centre for Diffraction Data (ICDD), 1996). The other peaks indicate Bragg positions for FTO according to PDF No. 0411445 (ICDD, 1996).
The obtained nanostructured hematite films were modified with TiO2 overlayers using the TALH complex. The TALH complex is a water-stable and commercially available titanium dioxide precursor and has a negative charge. Hematite has surface hydroxyl groups that can be either deprotonated (negatively charged) or protonated (positively charged) depending on the pH of the solution. It has been reported C
DOI: 10.1021/acsami.5b07065 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 2b. No aggregation or new features have been observed, indicating that indeed only a thin TiO2 overlayer is deposited on the surface of hematite. Attempts have been made to investigate the cross section, but unfortunately, it was not possible to distinguish the TiO2 layer, which might indicate that the layer of TiO2 deposited on top of the hematite film has a thickness less than the resolution limit of the employed SEM microscope (i.e.,
