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Electrodeposited Cr-Doped α-Fe2O3 thin films active for photoelectrochemical water splitting
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Electrodeposited Cr-Doped a-Fe2O3 thin films active for photoelectrochemical water splitting Feriel Bouhjar a,b,*, Lotfi Derbali c, Bernabe Marı´ a, Brahim Bessaı¨s b Institut de Disseny I Fabricacio, Universitat Politecnica de Valencia, Camı´ de Vera S/n, 46022, Valencia, Spain Laboratoire Photovoltaı¨ques, Centre de Recherches et des Technologies de, L’Energie Technopole H.lif, 2050, Tunisia c Laboratory of Semiconductors, Nanostructures and Advanced Technology, Research and Technology Centre of Energy, Borj-Cedria Science and Technology Park, BP 95,2050, Hammam-Lif, Tunisia a
b
highlights Hematite (a-Fe2O3) films doped with Chromium were obtained by electrodeposition. As Cr substitutes Fe, Cr doping shifts XRD lines to lower angles. Cr doping improves the photoelectrochemical activity of a-Fe2O3 films. Photocurrent is 6 times greater for 8 at.% Cr-doping than for undoped a-Fe2O3.
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abstract
Article history:
Polycrystalline hematite (a-Fe2O3) Chromium (Cr)-doped thin films were electrodeposited
Received 26 January 2018
on fluorine-doped tin oxide-coated glass substrates. The electrodeposition bath comprised
Received in revised form
an aqueous solution containing FeCl3$6H2O, NaCl, and H2O2.Chromium was added to the
21 October 2019
electrolyte at such a proportion that the Cr/(Cr þ Fe) ratio remained within the 2e8 at. %
Accepted 26 October 2019
range. The as-deposited films were subsequently annealed in air at 650 C for 2 h. The
Available online xxx
structure and morphological characteristics of the undoped and Cr-doped a-Fe2O3 thin films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM)
Keywords:
and UVeVis spectroscopy. Cr doping led the main XRD lines to shift to lower angles, which
Hematite
mostly resulted from substituting Fe3þ for Cr4þ ions that leads to a-Fe2O3 lattice contrac-
Chromium
tion. The SEM observations showed that the roughness and aspect of surfaces changed
XRD analysis
with the Cr doping level. The photoelectrochemical (PEC) performance of the a-Fe2O3 films
FESEM analysis
was examined by chronoamperometry and linear sweep voltammetry techniques. The Cr-
Optical properties
doped films exhibited greater photoelectrochemical activity than the undoped a-Fe2O3 thin
Photoelectrochemical properties
films. The highest photocurrent density was obtained for the 8% Cr-doped a-Fe2O3 films in 1 M NaOH electrolyte. All the samples achieved their best IPCE at 400 nm. The IPCE values for the 8 at.% Cr-doped hematite films were 20-fold higher than that of the undoped sample.This Cr-doped hematite films ‘excellent photoelectrochemical performance was mainly attributed to improved charge carrier properties. Such high photoactivity was attributed to the large active surface area and increased donor density caused by increasing the Cr doping in the a-Fe2O3 films. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. Laboratoire Photovoltaı¨ques, Centre de Recherches et des Technologies de, L’Energie Technopole H.lif, 2050, Tunisia. E-mail address: [email protected] (F. Bouhjar). https://doi.org/10.1016/j.ijhydene.2019.10.215 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Bouhjar F et al., Electrodeposited Cr-Doped a-Fe2O3 thin films active for photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.215
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Introduction Over the last decade, the scientific community has paid much attention to iron oxides due to their potential applications, such as magnetic devices [1], gas sensors [2e4], solid state lithium ion batteries [5], supercapacitors [6], photocatalysis [7,8], dye-sensitized solar cells [9], solar filters [10], etc. In addition to these uses, iron oxide has potential applications in medical science, such as a drug delivery system, cancer therapy and magnetic resonance imaging given its biocompatibility and low toxicity [11,12]. In this work, we studied in particular the influence of chromium doping on the photoelectrochemical (PEC) performance of a-Fe2O3 (hematite) thin films. It should be noted that metal oxide semiconductors fulfill the material requirements of the PEC water-splitting process, namely the need for long-term stability at potentials beyond the stability window of water; otherwise materials will self-oxidize rather than oxidize water. Among the various metal oxide semiconductors used in PEC applications, such as zinc oxide (ZnO) [13], tungsten trioxide (WO3) [14] and bismuth van date (BiVO4) [15], hematite has stirred much interest thanks to its enviro-friendly aspects, abundance, low cost, chemical and electrochemical stability in aqueous environments under illumination, and also for its high absorbance of most of the solar spectrum (e.g. ¼ 2.2 eV, l ¼ 564 nm). Furthermore, it possesses a proper valence band edge located at a lower energy level than the chemical potential of the water oxidation reaction (H2O/O2). However, hematite has three main critical drawbacks: first, it’s extremely poor electrical conducting property with a short hole diffusion length of 2e4 nm compared to light penetration depth (300e500 nm for visible light), which results in the rapid non radiative electronhole recombination inside the semiconductor [16,17]. Second, as hematite possesses a conduction band edge at an energy level that is below the reversible hydrogen potential, an external electrical bias is necessary to permit water reduction by photogenerated electrons. Finally, a high overpotential is needed to overcome the poor oxygen evolution kinetics at the hematite semi-conductor-liquid junction (SCLJ) [18]. Therefore, improving the performance of hematite photoanodes is necessary to approach the theoretical predictions for viable PEC technology. Several strategies are being adopted to enhance the performance of hematite photoanodes. Some are nanostructuring of hematite layers [19,20] which use underlayers and overlayers [21e26], and doping [27]. Doping is considered a powerful route for tuning the charge carrier concentration and to, thereby, modifies the conductivity and catalytic properties of semiconductor metal-oxide photoelectrodes [28,29]. A wide range of elements, which mostly belong to transition metals such as Sn [30], Cu [31], Pt [32], Si [33e36], Ti [37], Al [38], Cd [39], Mo [40], Cr [41] and Ta [42] has been used for doping purposes in hematite films. To dope a-Fe2O3, diverse methods have been attempted with a range of synthesizing methods, including sol-gel, hydrothermal, magnetron sputtering, atomic layer deposition, spray pyrolysis, atmospheric pressure chemical vapor deposition (APCVD) and electrodeposition. These dopants influence the conductivity of
hematite as well as band gap width, the Fermi level, and charge-transfer processes. However according to the literature, some researchers have reported contradictory effects for doping on photoelectrochemical performance. This apparent discrepancy in the results has been mainly attributed and endorsed to the doping concentration and processing methods. Recently, highly photoactive a-Fe2O3 electrodes doped with Pt, Mo, and Cr were prepared by McFarland and Kleiman by the electrodeposition method [35]. They found that the photoactivity of iron oxide improved by codeposition with Mo or Cr. The best performing samples were 5% Cr and 15% Mo doped, which had IPCEs at 400 nm of 6% and 12%, respectively, with an applied potential of 0.4 V vs. Ag/AgCl. These IPCE values were 2.2 and 4 higher than the undoped sample for the 5% Cr and 15% Mo samples, respectively. No evidence was found that improved performance was due to the electrocatalytic effects of the dopant on the surface of the hematite thin film. The main effect of the Mo and Cr dopants is to improve the charge transport properties of hematite so that a bigger fraction of the photon-generated electron/hole pairs is available for surface redox chemistry. This study reports the synthesis of chromium- (III) doped hematite films with varying at% of Cr (III) (0e8 at%) prepared by the electrodeposition method on conductive glass substrates. As far as we are aware, no significant data have been published on the optical, structural and electrical properties of a-Fe2O3 films doped with chrome.
Experimental details Nanostructured a-Fe2O3 thin films were obtained after electrochemically annealing deposited iron films on fluorinedoped tin oxide glass substrates (FTO, TEC 8, Pilkington glass). The FTO substrate was ultrasonically pre-cleaned by being sequentially rinsed in acetone, distilled water and ethanol. The electrodeposition bath consisted of an aqueous solution that contained FeCl3$6H2O (5 mM), KF (5 mM), NaCl (0.1 M), and H2O2 (1 M). Potassium fluoride was added to the solution to shift the reduction potential from Fe3þ to Fe2þ, which made it more negative. KCl was used as a supporting electrolyte. The role of H2O2 was to produce OH during the reduction process by increasing pH in the vicinity of the working electrode surface and to lead to the deposition of iron hydroxides (FeOOH). Furthermore for doping purposes, different amounts of Cr (ClO4)3 were used. Cr (ClO4)3was added to the electrolyte at such a proportion that the Cr/(Cr þ Fe) ratio fell within the 2e8 at.% range. All the doping percentages reported herein were those of the electrodeposition solution rather than those that exist in electrodeposited and annealed films, unless otherwise indicated. A typical three-electrode electrochemical cell, which contained the FTO substrates (working electrode, 1 cm 1.5 cm), a platinum counter electrode (2 cm 2 cm) and an Ag/AgCl (3 M) reference electrode, was used for the electrodeposition process of a-Fe2O3 (Fig. 1a). Iron films were deposited on the FTO substrate from an iron precursor solution (FeCl3$6H2O) by applying a constant potential of 0.15 V (versus Ag/AgCl electrode) for 10 min. A yellow-colored FeOOH compound
Please cite this article as: Bouhjar F et al., Electrodeposited Cr-Doped a-Fe2O3 thin films active for photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.215
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formed during the deposition process according to the following reactions: H2 O2 þ 2e /2OH
(1)
FeF2þ þ3OH / FeOOH þ F þ H2 O
(2)
3H2 O2 þ2FeF2þ þ6e /2 FeOOH þ 2F þ2H2 O
(3)
After each deposition, films were washed with deionized water. The as-deposited samples were then annealed in air at 650 C for 2 h (heated at a rate of 10 C/min to reach the required temperature) to obtain highly activated a-Fe2O3nanostructures.After annealing, films were found to be well homogeneous and transparent in one part of the visible spectrum (Fig. 1b). The crystal structure of the a-Fe2O3and the Cr-doped aFe2O3thin films were investigated by X-ray diffraction (XRD) using a Rigaku Ultima IV diffractometer in the Bragg-Bentano configuration with CuKa radiation (l ¼ 1.54060 A). Chemical composition, surface morphology and topography were characterized by energy dispersive spectroscopy (EDS) coupled to field emission scanning electron microscopy (FESEM, a Zeiss ULTRA 55 model equipped with an In-Lens SE detector). To determine the band gap energy, optical absorption was determined by recording the transmission spectra using a UVeVisible spectrophotometer (Shimadzu, UV-2450). The samples’ photo electrochemical properties were measured with a high-throughput photo electrochemical setup described elsewhere [43,44]. Briefly, PEC measurements were taken in a quartz cell to facilitate light to reach the photoelectrode surface. The surface area of the working electrode was 0.25 cm2. The electrolyte consisted of 1 M solution of NaOH (pH ¼ 13.6) with a pure nitrogen stream bubbling before and during the test to remove dissolved oxygen. Films were illuminated with a 300 W Xenon lamp (PLSSXE300/300UV) equipped with a UV cut-off filter (420 nm). The luminous intensity of the Xenon lamp was estimated to be about 150 mW/cm2. For the J-V curve, the experiment was performed at 100 mV s1 by linear sweep voltammetry (LSV) within a range of - 0.2 V to þ 0.5 V (vs. Ag/
3
AgCl). The chronoamperometry curves of the thin films were also obtained at þ 0.1 V both in the dark and under illumination. For the wavelength-dependent photocurrent measurements, a monochromator that gave a ~20 nm bandpass from 360 to 680 nm was used together with cut-off filters to eliminate secondary armonics. To detect low photocurrent intensity, the setup was completed with a photo-chopper and a lock-in amplifier (signal recovery). The absolute intensity of the incident excitation light was measured by a radiometer/ photometer (international light). The incident photon to electron conversion efficiency (IPCE) of the samples was calculated as follows: IPCEð%Þ ¼
1240$iphotocurrent ðmA=cm2 Þ *100% lðnmÞ$jphotons ðmW=cm2 Þ
(4)
where iphotocurrent is the photocurrent densities, lðnmÞis the incident light wavelength, and jphotons is measured irradiance.
Results and discussion Structural characterization Fig. 2 shows the X-ray diffraction patterns for the undoped and Cr-doped Fe2O3 annealed samples. The XRD peaks for the doped and undoped samples corresponded to the (012), (104), (110), (113), (024), (116) and (300) planes of the Fe2O3 phase, which peaked at 2q ¼ 24.2 , 33.1 , 35.6 , 40.9 , 49.5 , 54.1 , and 64 , respectively. No peak corresponding to the mixed oxides or the impurity phase was detected in any sample. The heA, matite (a-Fe2O3, space group: R-3c (167), a ¼ 5.036 b ¼ 5.036 A, and c ¼ 13.74 A) reference pattern used to index peaks was JCPDS 33e0664. Hence the undoped and Cr-doped samples had the same crystal structure and only the a-Fe2O3 phase existed. Samples’crystallite size was calculated by the Debye-Scherrer formula: D¼
Kl b cosq
(5)
where l ¼ 1.5405 nm is the wavelength of Cu Ka radiation, b is
Fig. 1 e (a) Schematic representations of a typical three-electrode electrochemical cell, indicating the steps followed for the synthesis of the Fe2O3 thin films, (b) Image of the a-Fe2O3 film electrodeposited on the FTO substrate. Please cite this article as: Bouhjar F et al., Electrodeposited Cr-Doped a-Fe2O3 thin films active for photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.215
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undoped a-Fe2O3 nanoparticles. Fig. 3(bee) illustrates the SEM views of the Cr-doped a-Fe2O3 films: the morphology of the Crdoped a-Fe2O3 corresponds to 2%, while 4% Cr shows no significant variation compared to the undoped a-Fe2O3 film. However beyond 4% Cr (6% and 8%), the surface morphology of a-Fe2O3 changes and the film appear dense and more uniform than the undoped film. These results predict that Cr doping not only controls the growth mechanism during the cathodic electrodeposition of the a-Fe2O3 thin films, but affects the size and overall morphology of nanostructures.
The EDS analyses
Fig. 2 e XRD patterns of the a-Fe2O3and Cr-doped a-Fe2O3 films at different Cr doping percentages.
the full width at the half maximum (FWHM) of the main diffraction peak in radian, q is the Bragg angle and k is Scherrer’s constant (equaled 0.90 according to the wide spread practical). The (012) orientation was used to calculate crystallite size as it corresponded to the lowest angle (and gave the best accuracy) (Table 1). The average crystallite size decreased from 44 nm to 11.7 nm as Cr doping varied from 0% to 8%.This led to a gradual decline in the intensity of the (012) and (104) XRD lines (Fig. 2). It is worth noting the shift of the XRD line position value to higher angles as regards the XRD reference powder. This was probably due to the replacement of Fe3þ ions (crystal A) radius ¼ 0.74 A) with smaller Cr4þ ions (crystal radius ¼ 0.69 as Cr content increased (See Table 1) which, in turn, led to aFe2O3 crystal lattice contraction. This result suggests that Crdoping affects the crystallite structure of a-Fe2O3, as further confirmed by the EDS and optical analyses. On the other hand, no Cr-related phases, particularly Cr oxides, appeared on the XRD pattern, which indicates that at first glance Cr was incorporated into the a-Fe2O3 lattice. Nevertheless, we should be careful as small amounts of amorphous Cr oxides (CrO2 or Cr2O3) may exist beyond the major a-Fe2O3 phase.
Optical characterization of the Cr-doped Fe2O3 thin films
Morphological characterization Fig. 3 shows the top-view SEM images of the undoped and Crdoped a-Fe2O3 thin films. Fig. 3a depicts the vertically grown segregated nanostructured islands made up of the small
Table 1 e Variation of the position of (012) the diffraction peak, FWHM and crystallite size for the undoped and Crdoped a-Fe2O3 versus the Cr concentration. Sample Fe2O3 Fe2O3:Cr 2% Fe2O3:Cr 4% Fe2O3:Cr 6% Fe2O3:Cr 8%
An elemental analysis of the a-Fe2O3 thin film was achieved from the Energy Dispersive X-ray analysis (EDS) spectra. The EDS spectra for the undoped and Cr-doped a-Fe2O3 are displayed in Fig. 4-A. The K line of the Fe element peaked at 6.398 keV, while the K line of oxygen peaked at 0.525 keV. The atomic percentages of Fe, Cr and O in the undoped and Crdoped a-Fe2O3 are shown in Table 2. As no lines related to Si were detected (Fig. 4-A (a)), we could guess that the oxygen line that peaked at 0.525 keV came solely from a-Fe2O3and SnO2. Therefore, it is feasible to guess that oxygen did not arise from glass (SiO2), but solely from the a-Fe2O3 films and from the SnO2 substrate (Fig. 4-A), which makes obtaining an exact Fe/O ratio (2/3) rather difficult. Notwithstanding, there appeared to be a stoichiometric composition (Fe/O atomic ratio ¼ 2:3) for the undoped films (see Table 2). As Cr doping increased, the Cr-doped hematite seemed to deviate from stoichiometry. However, the introduction of Cr atoms into the a-Fe2O3 lattice conserved the same hematite structure with a slight lattice contraction, as discussed in the XRD section (Fig. 2). Excess oxygen (detected by EDS) mainly arose from SnO2.So as all the Cr-doped films conserved the hematite structure; we assumed that the films did not deviate very much from the stoichiometric composition. Furthermore, the elemental mapping images (Fig. 4-B) of the Fe (Ka), O (Ka), Cr(Ka) reveal that the Cr dopants are distributed over all the nanostructures without any segregation on the surface or inside the crystals.
2q( ) (012) XRD line
FWHM
Crystallite size [nm]
24.18 24.21 24.28 24.30 24.32
0.193 0.274 0.300 0.374 0.591
44.0 31.6 23.2 17.7 11.7
The effect of Cr doping on the band gap energy of the synthesized films was determined from the optical transmission spectra within the wavelength range of 400e1000 nm. Fig. 5a shows the transmittance spectra for the undoped and Crdoped hematite thin films. Films’ transparency went beyond 60% at the600 nm wavelength and no spectra showed interference phenomena, probably due to the roughness and thickness in homogeneity, which could lead to rather high diffuse reflectivity. With increasing Cr content, the absorption edge (500e650 nm) shifted toward the longer wavelength region, as seen in Fig. 5b. It would appear that a-Fe2O3 had high absorbance in the blue region, which indicates its applicability as an absorbing material within this wavelength range. As films were rough enough, the band gap energy (Eg ) could be estimated from the optical transmission spectra by
Please cite this article as: Bouhjar F et al., Electrodeposited Cr-Doped a-Fe2O3 thin films active for photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.215
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Fig. 3 e SEM of the electrodeposited a-Fe2O3 (a) Undoped, (b) 2% Cr, (c) 4% Cr, (d) 6% Cr, and (e) 8% Cr.
Fig. 4 e A: The EDS spectrum for the undoped and Cr-doped a-Fe2O3. (a) Undopeda-Fe2O3, (b) Cr-doped 2 at.%, (c) Cr-doped 4 at.% (d) Cr-doped 6 at.% and (e) Cr-doped 8 at.%. B: Element Mapping images of the CreFe2O3 photoanode. Please cite this article as: Bouhjar F et al., Electrodeposited Cr-Doped a-Fe2O3 thin films active for photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.215
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Table 2 e The EDS analysis (atomic percent of elements) in the undoped and Cr-doped a-Fe2O3 films. Sample
O (at %)
Fe (at %)
Cr (at %)
60.00 64.72 65.01 65.10 73.99
40.00 31.92 31.47 29.30 19.94
0 3.36 3.52 5.60 6.07
Fe2O3 Cr 2% Cr 4% Cr 6% Cr 8%
Fe2O3was estimated to be 2.13 eV, which is slightly less than that of bulk a-Fe2O3 (2.3 eV). We can see that the optical band gap of the Cr-doped a-Fe2O3 remains within the 2.11 eVe2.18 eV range. Therefore, the Cr-doping within the 2%e8% range does not seem to evidently affect the optical band gap of the Cr-doped a-Fe2O3.
Photoelectrochemical properties of the Cr-doped a-Fe2O3 electrode The photocurrent response was performed under visible light irradiation. In order to improve the photocurrent response of the a-Fe2O3 films, it was necessary to enhance the charge carrier transport in the bulk and at the surface to reduce carrier recombination at both sites. The photocurrent density was extracted from the J-V curves by taking the difference between the current measured under illumination and in the dark per square centimeter of exposed working electrode area. All the measurements were taken in the 1M NaOH [47,48] electrolyte with a potential bias of 0.1 V vs. Ag/AgCl. Fig. 7 depicts the J-V characteristics in both the dark and illumination for the undoped and 8% Cr-doped a-Fe2O3. Fig. 7a shows the current-voltage characteristics of the undoped aFe2O3 measured in the dark and visible light. Current density is quite constant between 0.1 V and þ0.35 V, but then increases with the applied potential. An alternating top-down current density can be obtained from switching illumination On/Off. Fig. 7 (b) shows the J-V characteristics of the electrochemical device, where an 8% Cr-doped a-Fe2O3 was taken as a working electrode. We can observe that the photocurrent is about two orders of magnitude higher than that observed for the undoped a-Fe2O3 samples, and it increases linearly with the potential applied within the studied polarization range (0.2 V- þ0.5 V). Fig. 8 (a) shows the variation in the photocurrent intensity of the a-Fe2O3 and Cr-doped a-Fe2O3 electrodes, while the Cr doping level varies. It is worth noting the increase of the photocurrent from 1.102 mA/cm2 vs. Ag/AgCl to about
Fig. 5 e The transmittance (a) and absorbance (b) spectra of the undoped and Cr doped a-Fe2O3.
calculating the absorption coefficient, which can be estimated as follows [45]. a¼
1 1 ln d ðTÞ
(6)
The relation between the absorption coefficient and incident light energy hn was approached as so [46]. n
ahv ¼ Aðhv EgÞ
(7)
where a is the absorption coefficient, A is a constant, h is Planck’s constant, n is the photon frequency, Eg is the optical band gap, and n equals 1/2 for the direct band-gap transitions and 2 for the indirect ones. Fig. 6 shows the Tauc plot for the direct band gap transitions for the undoped and Cr-doped aFe2O3 films. The optical band gap energy of the undoped a-
Fig. 6 e The Tauc plot of the undoped and Cr-doped aFe2O3.
Please cite this article as: Bouhjar F et al., Electrodeposited Cr-Doped a-Fe2O3 thin films active for photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.215
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Fig. 7 e (a) The IeV characteristics of the undoped a-Fe2O3 electrode and (b) the J-V characteristics of the 8% Cr-doped a-Fe2O3 electrode in the dark and with illumination at a scan rate of 0.1 mV s¡1. 6.102 mA/cm2 vs. Ag/AgCl as the Cr doping level rises from 2% to 8%, respectively. Fig. 8 (b) shows the photocurrent density for the undoped and Cr-doped a-Fe2O3 electrodes after several On/Off light cycles.The quasi-stability and reproducibility of the photocurrent signal are observed in the figure. No overshoot was observed at the beginning or the end of the On/Off cycle, which indicates that the direction of electron diffusion was apparently free of grain boundaries, which can create traps that hinder electron movement and slow down photocurrent generation [49]. The above results clearly demonstrated that the photocurrent density generated from the Cr-doped a-Fe2O3 electrode was significantly higher than that of the undoped electrodes due to the presence of an easier electron transport mechanism in the Cr-doped a-Fe2O3 electrode. In fact from structural studies we showed that the XRD lines exhibited a slight shift toward higher diffraction angles as Cr doping increased. This allowed us to assume that Fe3þ ions were replaced with smaller Cr4þ ions. Accordingly, Cr (Cr4þ) acts as an electron donor in the a-Fe2O3 matrix, which apparently confirmed the substitution of Fe3þ for Cr4þ ions which, at first glance was in accordance with the XRD results.In fact Cr doping increased the donor concentration and enhanced charge carriers transportation by increasing the electric field
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Fig. 8 e (a) The photocurrent intensity for the Cr-doped Fe2O3 electrodes under on/off illumination conditions, measured in the 1M NaOH electrolyte with a bias potential of þ0.1 V vs. Ag/AgCl, (b) The time dependence of the photocurrent intensities for the Cr-doped a-Fe2O3 electrodes in successive illumination cycles.
across the space charge layer. The increase in the donor concentration would reduce the space charge layer width; hence the charge carriers in the region would be efficiently separated before recombination [50e55]. Moreover, a higher concentration of dopant would provide more defectscattering/recombination properties by inhibiting increased separation efficiency. This could also explain the variation in the photocurrent density with doping levels. Fig. 9 shows the performance of the doped samples compared to the undoped sample. Significant performance enhancements were observed upon doping throughout the illumination wavelengths. The performance of the 2 at.% Crdoped films was 4-fold better than that of the undoped sample. The IPCE of the 8 at.% Cr-doped films measured at 400 nm with an applied bias of þ0.1 V (vs. Ag/AgCl) was 9%, which corresponded to a 20-fold improvement over the undoped hematite. The higher photon energy was absorbed on the outmost layers of hematite and, therefore, the photogenerated holes had a shorter diffusion path to reach the surface, where they would contribute to the H2O oxidation reaction. An anodic applied bias would increase the collection efficiency of electrons, and IPCE improvement is seen in Fig. 9.
Please cite this article as: Bouhjar F et al., Electrodeposited Cr-Doped a-Fe2O3 thin films active for photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.215
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references
Fig. 9 e The IPCE for the undoped and Cr-doped a-Fe2O3 films at an applied potential of þ0.1 V (vs. Ag/AgCl) in 1 M NaOH. Furthermore, the applied bias would enable H2O reduction at the Pt counter-electrode by overcoming the mismatch (0.1 V (vs. Ag/AgCl)) between the hematite conduction band edge level and the reversible hydrogen potential. Improved IPCE performance is not likely to be related to doped samples’ increasing absorption because no significant change was obtained in the absorption spectra of the different Cr-doped samples (see Fig. 7).
Conclusion The Cr-doped a-Fe2O3 films were successfully deposited on the FTO-coated glass substrates using the electrodeposition technique and a subsequent annealing process. The concentration of the incorporated Cr atoms (Cr4þ ions) was controlled by varying the Cr (ClO4)3 concentration in the precursor solution. The Cr dopant acts as an ionized donor and it increased the carrier density in the a-Fe2O3 films. The major effect of Cr doping is likely to improve the conductivity and the charge transport properties of the a-Fe2O3 films. Hence a bigger fraction of photons that generate electronehole pairs were available for surface redox chemistry. The best performing samples had a doping rate of 8 at% Cr which, in turn, had an IPCE of 9% at 400 nm, with an applied potential of þ0.1 V (vs. Ag/AgCl). These IPCE values were 20-fold higher than that of the undoped sample. The apparent optimum Cr-doping at 8 at.% could effectively balance these competing effects and yield the best PEC performance for this material. The Cr-doped a-Fe2O3 thin films provided potential applications in photocatalysis for water splitting or in photoelectrical devices.
Acknowledgements This work was supported by the Ministry of High Education and Scientific Research (Tunisia), the Ministry of Economy and Competitiveness (Spain) (ENE2016-77798-C4-2-R) and the Generalitat Valenciana (Prometeus 2014/044).
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Electrodeposited Cr-Doped a-Fe2O3 thin films active for photoelectrochemical water splitting Feriel Bouhjar a,b,*, Lotfi Derbali c, Bernabe Marı´ a, Brahim Bessaı¨s b Institut de Disseny I Fabricacio, Universitat Politecnica de Valencia, Camı´ de Vera S/n, 46022, Valencia, Spain Laboratoire Photovoltaı¨ques, Centre de Recherches et des Technologies de, L’Energie Technopole H.lif, 2050, Tunisia c Laboratory of Semiconductors, Nanostructures and Advanced Technology, Research and Technology Centre of Energy, Borj-Cedria Science and Technology Park, BP 95,2050, Hammam-Lif, Tunisia a
b
highlights Hematite (a-Fe2O3) films doped with Chromium were obtained by electrodeposition. As Cr substitutes Fe, Cr doping shifts XRD lines to lower angles. Cr doping improves the photoelectrochemical activity of a-Fe2O3 films. Photocurrent is 6 times greater for 8 at.% Cr-doping than for undoped a-Fe2O3.
article info
abstract
Article history:
Polycrystalline hematite (a-Fe2O3) Chromium (Cr)-doped thin films were electrodeposited
Received 26 January 2018
on fluorine-doped tin oxide-coated glass substrates. The electrodeposition bath comprised
Received in revised form
an aqueous solution containing FeCl3$6H2O, NaCl, and H2O2.Chromium was added to the
21 October 2019
electrolyte at such a proportion that the Cr/(Cr þ Fe) ratio remained within the 2e8 at. %
Accepted 26 October 2019
range. The as-deposited films were subsequently annealed in air at 650 C for 2 h. The
Available online xxx
structure and morphological characteristics of the undoped and Cr-doped a-Fe2O3 thin films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM)
Keywords:
and UVeVis spectroscopy. Cr doping led the main XRD lines to shift to lower angles, which
Hematite
mostly resulted from substituting Fe3þ for Cr4þ ions that leads to a-Fe2O3 lattice contrac-
Chromium
tion. The SEM observations showed that the roughness and aspect of surfaces changed
XRD analysis
with the Cr doping level. The photoelectrochemical (PEC) performance of the a-Fe2O3 films
FESEM analysis
was examined by chronoamperometry and linear sweep voltammetry techniques. The Cr-
Optical properties
doped films exhibited greater photoelectrochemical activity than the undoped a-Fe2O3 thin
Photoelectrochemical properties
films. The highest photocurrent density was obtained for the 8% Cr-doped a-Fe2O3 films in 1 M NaOH electrolyte. All the samples achieved their best IPCE at 400 nm. The IPCE values for the 8 at.% Cr-doped hematite films were 20-fold higher than that of the undoped sample.This Cr-doped hematite films ‘excellent photoelectrochemical performance was mainly attributed to improved charge carrier properties. Such high photoactivity was attributed to the large active surface area and increased donor density caused by increasing the Cr doping in the a-Fe2O3 films. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. Laboratoire Photovoltaı¨ques, Centre de Recherches et des Technologies de, L’Energie Technopole H.lif, 2050, Tunisia. E-mail address: [email protected] (F. Bouhjar). https://doi.org/10.1016/j.ijhydene.2019.10.215 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Bouhjar F et al., Electrodeposited Cr-Doped a-Fe2O3 thin films active for photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.215
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Introduction Over the last decade, the scientific community has paid much attention to iron oxides due to their potential applications, such as magnetic devices [1], gas sensors [2e4], solid state lithium ion batteries [5], supercapacitors [6], photocatalysis [7,8], dye-sensitized solar cells [9], solar filters [10], etc. In addition to these uses, iron oxide has potential applications in medical science, such as a drug delivery system, cancer therapy and magnetic resonance imaging given its biocompatibility and low toxicity [11,12]. In this work, we studied in particular the influence of chromium doping on the photoelectrochemical (PEC) performance of a-Fe2O3 (hematite) thin films. It should be noted that metal oxide semiconductors fulfill the material requirements of the PEC water-splitting process, namely the need for long-term stability at potentials beyond the stability window of water; otherwise materials will self-oxidize rather than oxidize water. Among the various metal oxide semiconductors used in PEC applications, such as zinc oxide (ZnO) [13], tungsten trioxide (WO3) [14] and bismuth van date (BiVO4) [15], hematite has stirred much interest thanks to its enviro-friendly aspects, abundance, low cost, chemical and electrochemical stability in aqueous environments under illumination, and also for its high absorbance of most of the solar spectrum (e.g. ¼ 2.2 eV, l ¼ 564 nm). Furthermore, it possesses a proper valence band edge located at a lower energy level than the chemical potential of the water oxidation reaction (H2O/O2). However, hematite has three main critical drawbacks: first, it’s extremely poor electrical conducting property with a short hole diffusion length of 2e4 nm compared to light penetration depth (300e500 nm for visible light), which results in the rapid non radiative electronhole recombination inside the semiconductor [16,17]. Second, as hematite possesses a conduction band edge at an energy level that is below the reversible hydrogen potential, an external electrical bias is necessary to permit water reduction by photogenerated electrons. Finally, a high overpotential is needed to overcome the poor oxygen evolution kinetics at the hematite semi-conductor-liquid junction (SCLJ) [18]. Therefore, improving the performance of hematite photoanodes is necessary to approach the theoretical predictions for viable PEC technology. Several strategies are being adopted to enhance the performance of hematite photoanodes. Some are nanostructuring of hematite layers [19,20] which use underlayers and overlayers [21e26], and doping [27]. Doping is considered a powerful route for tuning the charge carrier concentration and to, thereby, modifies the conductivity and catalytic properties of semiconductor metal-oxide photoelectrodes [28,29]. A wide range of elements, which mostly belong to transition metals such as Sn [30], Cu [31], Pt [32], Si [33e36], Ti [37], Al [38], Cd [39], Mo [40], Cr [41] and Ta [42] has been used for doping purposes in hematite films. To dope a-Fe2O3, diverse methods have been attempted with a range of synthesizing methods, including sol-gel, hydrothermal, magnetron sputtering, atomic layer deposition, spray pyrolysis, atmospheric pressure chemical vapor deposition (APCVD) and electrodeposition. These dopants influence the conductivity of
hematite as well as band gap width, the Fermi level, and charge-transfer processes. However according to the literature, some researchers have reported contradictory effects for doping on photoelectrochemical performance. This apparent discrepancy in the results has been mainly attributed and endorsed to the doping concentration and processing methods. Recently, highly photoactive a-Fe2O3 electrodes doped with Pt, Mo, and Cr were prepared by McFarland and Kleiman by the electrodeposition method [35]. They found that the photoactivity of iron oxide improved by codeposition with Mo or Cr. The best performing samples were 5% Cr and 15% Mo doped, which had IPCEs at 400 nm of 6% and 12%, respectively, with an applied potential of 0.4 V vs. Ag/AgCl. These IPCE values were 2.2 and 4 higher than the undoped sample for the 5% Cr and 15% Mo samples, respectively. No evidence was found that improved performance was due to the electrocatalytic effects of the dopant on the surface of the hematite thin film. The main effect of the Mo and Cr dopants is to improve the charge transport properties of hematite so that a bigger fraction of the photon-generated electron/hole pairs is available for surface redox chemistry. This study reports the synthesis of chromium- (III) doped hematite films with varying at% of Cr (III) (0e8 at%) prepared by the electrodeposition method on conductive glass substrates. As far as we are aware, no significant data have been published on the optical, structural and electrical properties of a-Fe2O3 films doped with chrome.
Experimental details Nanostructured a-Fe2O3 thin films were obtained after electrochemically annealing deposited iron films on fluorinedoped tin oxide glass substrates (FTO, TEC 8, Pilkington glass). The FTO substrate was ultrasonically pre-cleaned by being sequentially rinsed in acetone, distilled water and ethanol. The electrodeposition bath consisted of an aqueous solution that contained FeCl3$6H2O (5 mM), KF (5 mM), NaCl (0.1 M), and H2O2 (1 M). Potassium fluoride was added to the solution to shift the reduction potential from Fe3þ to Fe2þ, which made it more negative. KCl was used as a supporting electrolyte. The role of H2O2 was to produce OH during the reduction process by increasing pH in the vicinity of the working electrode surface and to lead to the deposition of iron hydroxides (FeOOH). Furthermore for doping purposes, different amounts of Cr (ClO4)3 were used. Cr (ClO4)3was added to the electrolyte at such a proportion that the Cr/(Cr þ Fe) ratio fell within the 2e8 at.% range. All the doping percentages reported herein were those of the electrodeposition solution rather than those that exist in electrodeposited and annealed films, unless otherwise indicated. A typical three-electrode electrochemical cell, which contained the FTO substrates (working electrode, 1 cm 1.5 cm), a platinum counter electrode (2 cm 2 cm) and an Ag/AgCl (3 M) reference electrode, was used for the electrodeposition process of a-Fe2O3 (Fig. 1a). Iron films were deposited on the FTO substrate from an iron precursor solution (FeCl3$6H2O) by applying a constant potential of 0.15 V (versus Ag/AgCl electrode) for 10 min. A yellow-colored FeOOH compound
Please cite this article as: Bouhjar F et al., Electrodeposited Cr-Doped a-Fe2O3 thin films active for photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.215
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formed during the deposition process according to the following reactions: H2 O2 þ 2e /2OH
(1)
FeF2þ þ3OH / FeOOH þ F þ H2 O
(2)
3H2 O2 þ2FeF2þ þ6e /2 FeOOH þ 2F þ2H2 O
(3)
After each deposition, films were washed with deionized water. The as-deposited samples were then annealed in air at 650 C for 2 h (heated at a rate of 10 C/min to reach the required temperature) to obtain highly activated a-Fe2O3nanostructures.After annealing, films were found to be well homogeneous and transparent in one part of the visible spectrum (Fig. 1b). The crystal structure of the a-Fe2O3and the Cr-doped aFe2O3thin films were investigated by X-ray diffraction (XRD) using a Rigaku Ultima IV diffractometer in the Bragg-Bentano configuration with CuKa radiation (l ¼ 1.54060 A). Chemical composition, surface morphology and topography were characterized by energy dispersive spectroscopy (EDS) coupled to field emission scanning electron microscopy (FESEM, a Zeiss ULTRA 55 model equipped with an In-Lens SE detector). To determine the band gap energy, optical absorption was determined by recording the transmission spectra using a UVeVisible spectrophotometer (Shimadzu, UV-2450). The samples’ photo electrochemical properties were measured with a high-throughput photo electrochemical setup described elsewhere [43,44]. Briefly, PEC measurements were taken in a quartz cell to facilitate light to reach the photoelectrode surface. The surface area of the working electrode was 0.25 cm2. The electrolyte consisted of 1 M solution of NaOH (pH ¼ 13.6) with a pure nitrogen stream bubbling before and during the test to remove dissolved oxygen. Films were illuminated with a 300 W Xenon lamp (PLSSXE300/300UV) equipped with a UV cut-off filter (420 nm). The luminous intensity of the Xenon lamp was estimated to be about 150 mW/cm2. For the J-V curve, the experiment was performed at 100 mV s1 by linear sweep voltammetry (LSV) within a range of - 0.2 V to þ 0.5 V (vs. Ag/
3
AgCl). The chronoamperometry curves of the thin films were also obtained at þ 0.1 V both in the dark and under illumination. For the wavelength-dependent photocurrent measurements, a monochromator that gave a ~20 nm bandpass from 360 to 680 nm was used together with cut-off filters to eliminate secondary armonics. To detect low photocurrent intensity, the setup was completed with a photo-chopper and a lock-in amplifier (signal recovery). The absolute intensity of the incident excitation light was measured by a radiometer/ photometer (international light). The incident photon to electron conversion efficiency (IPCE) of the samples was calculated as follows: IPCEð%Þ ¼
1240$iphotocurrent ðmA=cm2 Þ *100% lðnmÞ$jphotons ðmW=cm2 Þ
(4)
where iphotocurrent is the photocurrent densities, lðnmÞis the incident light wavelength, and jphotons is measured irradiance.
Results and discussion Structural characterization Fig. 2 shows the X-ray diffraction patterns for the undoped and Cr-doped Fe2O3 annealed samples. The XRD peaks for the doped and undoped samples corresponded to the (012), (104), (110), (113), (024), (116) and (300) planes of the Fe2O3 phase, which peaked at 2q ¼ 24.2 , 33.1 , 35.6 , 40.9 , 49.5 , 54.1 , and 64 , respectively. No peak corresponding to the mixed oxides or the impurity phase was detected in any sample. The heA, matite (a-Fe2O3, space group: R-3c (167), a ¼ 5.036 b ¼ 5.036 A, and c ¼ 13.74 A) reference pattern used to index peaks was JCPDS 33e0664. Hence the undoped and Cr-doped samples had the same crystal structure and only the a-Fe2O3 phase existed. Samples’crystallite size was calculated by the Debye-Scherrer formula: D¼
Kl b cosq
(5)
where l ¼ 1.5405 nm is the wavelength of Cu Ka radiation, b is
Fig. 1 e (a) Schematic representations of a typical three-electrode electrochemical cell, indicating the steps followed for the synthesis of the Fe2O3 thin films, (b) Image of the a-Fe2O3 film electrodeposited on the FTO substrate. Please cite this article as: Bouhjar F et al., Electrodeposited Cr-Doped a-Fe2O3 thin films active for photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.215
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undoped a-Fe2O3 nanoparticles. Fig. 3(bee) illustrates the SEM views of the Cr-doped a-Fe2O3 films: the morphology of the Crdoped a-Fe2O3 corresponds to 2%, while 4% Cr shows no significant variation compared to the undoped a-Fe2O3 film. However beyond 4% Cr (6% and 8%), the surface morphology of a-Fe2O3 changes and the film appear dense and more uniform than the undoped film. These results predict that Cr doping not only controls the growth mechanism during the cathodic electrodeposition of the a-Fe2O3 thin films, but affects the size and overall morphology of nanostructures.
The EDS analyses
Fig. 2 e XRD patterns of the a-Fe2O3and Cr-doped a-Fe2O3 films at different Cr doping percentages.
the full width at the half maximum (FWHM) of the main diffraction peak in radian, q is the Bragg angle and k is Scherrer’s constant (equaled 0.90 according to the wide spread practical). The (012) orientation was used to calculate crystallite size as it corresponded to the lowest angle (and gave the best accuracy) (Table 1). The average crystallite size decreased from 44 nm to 11.7 nm as Cr doping varied from 0% to 8%.This led to a gradual decline in the intensity of the (012) and (104) XRD lines (Fig. 2). It is worth noting the shift of the XRD line position value to higher angles as regards the XRD reference powder. This was probably due to the replacement of Fe3þ ions (crystal A) radius ¼ 0.74 A) with smaller Cr4þ ions (crystal radius ¼ 0.69 as Cr content increased (See Table 1) which, in turn, led to aFe2O3 crystal lattice contraction. This result suggests that Crdoping affects the crystallite structure of a-Fe2O3, as further confirmed by the EDS and optical analyses. On the other hand, no Cr-related phases, particularly Cr oxides, appeared on the XRD pattern, which indicates that at first glance Cr was incorporated into the a-Fe2O3 lattice. Nevertheless, we should be careful as small amounts of amorphous Cr oxides (CrO2 or Cr2O3) may exist beyond the major a-Fe2O3 phase.
Optical characterization of the Cr-doped Fe2O3 thin films
Morphological characterization Fig. 3 shows the top-view SEM images of the undoped and Crdoped a-Fe2O3 thin films. Fig. 3a depicts the vertically grown segregated nanostructured islands made up of the small
Table 1 e Variation of the position of (012) the diffraction peak, FWHM and crystallite size for the undoped and Crdoped a-Fe2O3 versus the Cr concentration. Sample Fe2O3 Fe2O3:Cr 2% Fe2O3:Cr 4% Fe2O3:Cr 6% Fe2O3:Cr 8%
An elemental analysis of the a-Fe2O3 thin film was achieved from the Energy Dispersive X-ray analysis (EDS) spectra. The EDS spectra for the undoped and Cr-doped a-Fe2O3 are displayed in Fig. 4-A. The K line of the Fe element peaked at 6.398 keV, while the K line of oxygen peaked at 0.525 keV. The atomic percentages of Fe, Cr and O in the undoped and Crdoped a-Fe2O3 are shown in Table 2. As no lines related to Si were detected (Fig. 4-A (a)), we could guess that the oxygen line that peaked at 0.525 keV came solely from a-Fe2O3and SnO2. Therefore, it is feasible to guess that oxygen did not arise from glass (SiO2), but solely from the a-Fe2O3 films and from the SnO2 substrate (Fig. 4-A), which makes obtaining an exact Fe/O ratio (2/3) rather difficult. Notwithstanding, there appeared to be a stoichiometric composition (Fe/O atomic ratio ¼ 2:3) for the undoped films (see Table 2). As Cr doping increased, the Cr-doped hematite seemed to deviate from stoichiometry. However, the introduction of Cr atoms into the a-Fe2O3 lattice conserved the same hematite structure with a slight lattice contraction, as discussed in the XRD section (Fig. 2). Excess oxygen (detected by EDS) mainly arose from SnO2.So as all the Cr-doped films conserved the hematite structure; we assumed that the films did not deviate very much from the stoichiometric composition. Furthermore, the elemental mapping images (Fig. 4-B) of the Fe (Ka), O (Ka), Cr(Ka) reveal that the Cr dopants are distributed over all the nanostructures without any segregation on the surface or inside the crystals.
2q( ) (012) XRD line
FWHM
Crystallite size [nm]
24.18 24.21 24.28 24.30 24.32
0.193 0.274 0.300 0.374 0.591
44.0 31.6 23.2 17.7 11.7
The effect of Cr doping on the band gap energy of the synthesized films was determined from the optical transmission spectra within the wavelength range of 400e1000 nm. Fig. 5a shows the transmittance spectra for the undoped and Crdoped hematite thin films. Films’ transparency went beyond 60% at the600 nm wavelength and no spectra showed interference phenomena, probably due to the roughness and thickness in homogeneity, which could lead to rather high diffuse reflectivity. With increasing Cr content, the absorption edge (500e650 nm) shifted toward the longer wavelength region, as seen in Fig. 5b. It would appear that a-Fe2O3 had high absorbance in the blue region, which indicates its applicability as an absorbing material within this wavelength range. As films were rough enough, the band gap energy (Eg ) could be estimated from the optical transmission spectra by
Please cite this article as: Bouhjar F et al., Electrodeposited Cr-Doped a-Fe2O3 thin films active for photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.215
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Fig. 3 e SEM of the electrodeposited a-Fe2O3 (a) Undoped, (b) 2% Cr, (c) 4% Cr, (d) 6% Cr, and (e) 8% Cr.
Fig. 4 e A: The EDS spectrum for the undoped and Cr-doped a-Fe2O3. (a) Undopeda-Fe2O3, (b) Cr-doped 2 at.%, (c) Cr-doped 4 at.% (d) Cr-doped 6 at.% and (e) Cr-doped 8 at.%. B: Element Mapping images of the CreFe2O3 photoanode. Please cite this article as: Bouhjar F et al., Electrodeposited Cr-Doped a-Fe2O3 thin films active for photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.215
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Table 2 e The EDS analysis (atomic percent of elements) in the undoped and Cr-doped a-Fe2O3 films. Sample
O (at %)
Fe (at %)
Cr (at %)
60.00 64.72 65.01 65.10 73.99
40.00 31.92 31.47 29.30 19.94
0 3.36 3.52 5.60 6.07
Fe2O3 Cr 2% Cr 4% Cr 6% Cr 8%
Fe2O3was estimated to be 2.13 eV, which is slightly less than that of bulk a-Fe2O3 (2.3 eV). We can see that the optical band gap of the Cr-doped a-Fe2O3 remains within the 2.11 eVe2.18 eV range. Therefore, the Cr-doping within the 2%e8% range does not seem to evidently affect the optical band gap of the Cr-doped a-Fe2O3.
Photoelectrochemical properties of the Cr-doped a-Fe2O3 electrode The photocurrent response was performed under visible light irradiation. In order to improve the photocurrent response of the a-Fe2O3 films, it was necessary to enhance the charge carrier transport in the bulk and at the surface to reduce carrier recombination at both sites. The photocurrent density was extracted from the J-V curves by taking the difference between the current measured under illumination and in the dark per square centimeter of exposed working electrode area. All the measurements were taken in the 1M NaOH [47,48] electrolyte with a potential bias of 0.1 V vs. Ag/AgCl. Fig. 7 depicts the J-V characteristics in both the dark and illumination for the undoped and 8% Cr-doped a-Fe2O3. Fig. 7a shows the current-voltage characteristics of the undoped aFe2O3 measured in the dark and visible light. Current density is quite constant between 0.1 V and þ0.35 V, but then increases with the applied potential. An alternating top-down current density can be obtained from switching illumination On/Off. Fig. 7 (b) shows the J-V characteristics of the electrochemical device, where an 8% Cr-doped a-Fe2O3 was taken as a working electrode. We can observe that the photocurrent is about two orders of magnitude higher than that observed for the undoped a-Fe2O3 samples, and it increases linearly with the potential applied within the studied polarization range (0.2 V- þ0.5 V). Fig. 8 (a) shows the variation in the photocurrent intensity of the a-Fe2O3 and Cr-doped a-Fe2O3 electrodes, while the Cr doping level varies. It is worth noting the increase of the photocurrent from 1.102 mA/cm2 vs. Ag/AgCl to about
Fig. 5 e The transmittance (a) and absorbance (b) spectra of the undoped and Cr doped a-Fe2O3.
calculating the absorption coefficient, which can be estimated as follows [45]. a¼
1 1 ln d ðTÞ
(6)
The relation between the absorption coefficient and incident light energy hn was approached as so [46]. n
ahv ¼ Aðhv EgÞ
(7)
where a is the absorption coefficient, A is a constant, h is Planck’s constant, n is the photon frequency, Eg is the optical band gap, and n equals 1/2 for the direct band-gap transitions and 2 for the indirect ones. Fig. 6 shows the Tauc plot for the direct band gap transitions for the undoped and Cr-doped aFe2O3 films. The optical band gap energy of the undoped a-
Fig. 6 e The Tauc plot of the undoped and Cr-doped aFe2O3.
Please cite this article as: Bouhjar F et al., Electrodeposited Cr-Doped a-Fe2O3 thin films active for photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.215
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Fig. 7 e (a) The IeV characteristics of the undoped a-Fe2O3 electrode and (b) the J-V characteristics of the 8% Cr-doped a-Fe2O3 electrode in the dark and with illumination at a scan rate of 0.1 mV s¡1. 6.102 mA/cm2 vs. Ag/AgCl as the Cr doping level rises from 2% to 8%, respectively. Fig. 8 (b) shows the photocurrent density for the undoped and Cr-doped a-Fe2O3 electrodes after several On/Off light cycles.The quasi-stability and reproducibility of the photocurrent signal are observed in the figure. No overshoot was observed at the beginning or the end of the On/Off cycle, which indicates that the direction of electron diffusion was apparently free of grain boundaries, which can create traps that hinder electron movement and slow down photocurrent generation [49]. The above results clearly demonstrated that the photocurrent density generated from the Cr-doped a-Fe2O3 electrode was significantly higher than that of the undoped electrodes due to the presence of an easier electron transport mechanism in the Cr-doped a-Fe2O3 electrode. In fact from structural studies we showed that the XRD lines exhibited a slight shift toward higher diffraction angles as Cr doping increased. This allowed us to assume that Fe3þ ions were replaced with smaller Cr4þ ions. Accordingly, Cr (Cr4þ) acts as an electron donor in the a-Fe2O3 matrix, which apparently confirmed the substitution of Fe3þ for Cr4þ ions which, at first glance was in accordance with the XRD results.In fact Cr doping increased the donor concentration and enhanced charge carriers transportation by increasing the electric field
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Fig. 8 e (a) The photocurrent intensity for the Cr-doped Fe2O3 electrodes under on/off illumination conditions, measured in the 1M NaOH electrolyte with a bias potential of þ0.1 V vs. Ag/AgCl, (b) The time dependence of the photocurrent intensities for the Cr-doped a-Fe2O3 electrodes in successive illumination cycles.
across the space charge layer. The increase in the donor concentration would reduce the space charge layer width; hence the charge carriers in the region would be efficiently separated before recombination [50e55]. Moreover, a higher concentration of dopant would provide more defectscattering/recombination properties by inhibiting increased separation efficiency. This could also explain the variation in the photocurrent density with doping levels. Fig. 9 shows the performance of the doped samples compared to the undoped sample. Significant performance enhancements were observed upon doping throughout the illumination wavelengths. The performance of the 2 at.% Crdoped films was 4-fold better than that of the undoped sample. The IPCE of the 8 at.% Cr-doped films measured at 400 nm with an applied bias of þ0.1 V (vs. Ag/AgCl) was 9%, which corresponded to a 20-fold improvement over the undoped hematite. The higher photon energy was absorbed on the outmost layers of hematite and, therefore, the photogenerated holes had a shorter diffusion path to reach the surface, where they would contribute to the H2O oxidation reaction. An anodic applied bias would increase the collection efficiency of electrons, and IPCE improvement is seen in Fig. 9.
Please cite this article as: Bouhjar F et al., Electrodeposited Cr-Doped a-Fe2O3 thin films active for photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.215
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references
Fig. 9 e The IPCE for the undoped and Cr-doped a-Fe2O3 films at an applied potential of þ0.1 V (vs. Ag/AgCl) in 1 M NaOH. Furthermore, the applied bias would enable H2O reduction at the Pt counter-electrode by overcoming the mismatch (0.1 V (vs. Ag/AgCl)) between the hematite conduction band edge level and the reversible hydrogen potential. Improved IPCE performance is not likely to be related to doped samples’ increasing absorption because no significant change was obtained in the absorption spectra of the different Cr-doped samples (see Fig. 7).
Conclusion The Cr-doped a-Fe2O3 films were successfully deposited on the FTO-coated glass substrates using the electrodeposition technique and a subsequent annealing process. The concentration of the incorporated Cr atoms (Cr4þ ions) was controlled by varying the Cr (ClO4)3 concentration in the precursor solution. The Cr dopant acts as an ionized donor and it increased the carrier density in the a-Fe2O3 films. The major effect of Cr doping is likely to improve the conductivity and the charge transport properties of the a-Fe2O3 films. Hence a bigger fraction of photons that generate electronehole pairs were available for surface redox chemistry. The best performing samples had a doping rate of 8 at% Cr which, in turn, had an IPCE of 9% at 400 nm, with an applied potential of þ0.1 V (vs. Ag/AgCl). These IPCE values were 20-fold higher than that of the undoped sample. The apparent optimum Cr-doping at 8 at.% could effectively balance these competing effects and yield the best PEC performance for this material. The Cr-doped a-Fe2O3 thin films provided potential applications in photocatalysis for water splitting or in photoelectrical devices.
Acknowledgements This work was supported by the Ministry of High Education and Scientific Research (Tunisia), the Ministry of Economy and Competitiveness (Spain) (ENE2016-77798-C4-2-R) and the Generalitat Valenciana (Prometeus 2014/044).
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Please cite this article as: Bouhjar F et al., Electrodeposited Cr-Doped a-Fe2O3 thin films active for photoelectrochemical water splitting, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.215