Enhanced Photoelectrochemical Water Oxidation on Nanostructured

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Enhanced Photoelectrochemical Water Oxidation on Hematite Photoanode via p-CaFeO/n-FeO Heterojunction Formation 2

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Mahmoud G. Ahmed, Tarek A. Kandiel, Amira Y Ahmed, Imme Kretschmer, Farouk Abdallah Rashwan, and Detlef W Bahnemann J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp512804p • Publication Date (Web): 12 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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The Journal of Physical Chemistry

Enhanced Photoelectrochemical Water Oxidation on Hematite Photoanode via p-CaFe2O4/n-Fe2O3 Heterojunction Formation Mahmoud G. Ahmed,a Tarek A. Kandiel,a* Amira Y. Ahmed,a Imme Kretschmer,b Farouk Rashwan,a and Detlef Bahnemannb,c a

b

Department of Chemistry, Faculty of Science, Sohag University, Sohag 82524, Egypt.

Photocatalysis and Nanotechnology Research Unit, Institut fuer Technische Chemie, Leibniz Universitaet Hannover, Callinstrasse 3, D-30167 Hannover, Germany.

c

Laboratory for Nanocomposite Materials, Department of Photonics, Faculty of Physics, SaintPetersburg State University, Ulianovskaia str. 3, Peterhof, Saint-Petersburg, 198504, Russia

ACS Paragon Plus Environment

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ABSTRACT: In this paper, hematite p-CaFe2O4/n-Fe2O3 heterojunction photoanode has been fabricated employing a facile template-less film processing technique by controlling the chemical bath. Anisotropic growth of β-FeOOH akaganeite film on FTO conductive glass from an aqueous FeCl3 solution containing CaCl2 followed by two-step thermal annealing at 550 and 800 °C, induces the formation of p-CaFe2O4/n-Fe2O3 heterojunction. The structural, morphological, electronic states and electrochemical characteristics of the films have been investigated by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and impedance spectroscopy, respectively. The heterojunction photoanode showed 100 % higher photocurrent response than that is obtained using bare hematite electrode under simulated 1-sun (100 mW/cm2). The photocurrent enhancement is attributed to the enhanced charge carrier separation and the reduced resistance in the charge transfer across the electrode and electrolyte as revealed by electrochemical impedance

spectroscopy

analysis.

The

modification

of

the

p-CaFe2O4/n-Fe2O3

heterojunction photoanode with CoPi cocatalyst further facilitate the electron transfer at the electrode/electrolyte interface and thus enhance the photoelectrochemical water oxidation. Since cheap and abundant materials have been employed for the synthesis of the heterojunction photoanode via a simple route, the current results have great importance from scientific and economical point of view.


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1. Introduction Due to the continuous increase in the fossil fuel consumption and thus the increase of the carbon dioxide (CO2) emission, in addition to the expected depletion of the world reserves of crude oil after few decades, the search for green technologies for renewable and carbon-free energy production becomes urgent. Conversion of sunlight as a source of renewable energy into clean chemical fuel such as hydrogen via photoelectrochemical (PEC) cell will meet the desired people’s demand and decrease the global warming.1, 2 In 1972, Fujishima and Honda succeeded to drive water splitting and produce molecular hydrogen by using TiO2 rutile single crystal photoanode under ultraviolet (UV) light illumination and under applied bias potential.3 However, the efficiency of TiO2 was low because of its high bandgap, i.e., 3.0 eV, and thus it can absorb only the UV part of the solar irradiance.4 After this finding, many metal oxide semiconductors have also been investigated as a photoanode or a photocathode for photoelectrochemical water splitting, e.g., WO35, 6, α-Fe2O37, 8, ZnO9, BiVO4,10, Bi2WO611, CaFe2O412, and Cu2O.13 Among these metal oxide semiconductors, hematite has several merits which makes it a good candidate as a photoanode for solar water splitting. For examples, it exhibits narrow bandgap, i.e., 2.2 eV, and thus it absorbs the light up to 560 nm. It is inexpensive material and has a good chemical stability against corrosion in alkaline solutions in dark and under illumination.14 Hematite drives the oxygen evolution reaction as it has a suitable valence band energy,15 but energetically, the conduction band position does not match the water reduction, and thus, a bias potential is required to overcome this obstacle.16 Theoretically, hematite photoanode can convert 16.8 % of solar energy into hydrogen.4, 7 however, the best reported efficiency, i.e., 0.6%,14 of hematite is too low to be compared to the


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theoretical value due to its limitations, for example: short diffusion length of photogenerated holes (2-4 nm), short life time of photoexcited charge carriers (˂ 10ps), and low oxidation evolution kinetics.7, 16 To overcome these limitations and enhance the efficiency of hematite photoanode, great efforts have been adopted.17,

18

For instance,

fabrication of nanostructured hematite (nanorods, nanowires, nanotubes…)14,

19, 20

with

the proper size has been proven to be an effective route to compensate diffusion length problem and to reduce the high recombination rate of photogenerated electrons and holes.1,

21

Different techniques have been examined to synthesize and control the

morphology of hematite nanostructures such as spray pyrolysis, hydrothermal method, electrodeposition, and atmospheric pressure chemical vapor deposition (APCDV).7, 16, 22, 23

Incorporation or doping hematite with different metals such a Ti,24, 25 Si,25 Cr,26 Mn,16

Sn,18 Ni,22 and Mg27 have also been investigated and showed improved electrical and optical properties.7 Another approach to enhance the efficiency of hematite photoanode is to create heterojunction. The internal electric field induced from band bending facilitate the

photogenerated

charge

carriers

separation.

Commonly,

the

heterojunction

photoanodes between two n-type semiconductors have been studied, e.g., WO3/BiVO428 and ZnFe2O4/Fe2O3.29 Because the p-type semiconductor oxides are rather rare, the p-n heterojunction photoanode have been rarely studied. Recently, the modification of the tantalum oxynitride photoanode with calcium ferrite as a p-type semiconductor significantly enhances the photoelectrochemical water splitting.30 Inspired by this work, in this paper, we have developed a facile hydrothermal method for the fabrication of a p-n heterojunction photoanode between n-type hematite (n-Fe2O3) and p-type calcium ferrite (CF, p-CaFe2O4). Firstly, β-FeOOH nanorods were grown directly on the fluorine-doped


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tin oxide (FTO) coated glass from chemical bath containing ferric chloride and calcium chloride mixed with the desired ratio. After heat treatment of the grown film, a pCaFe2O4/n-Fe2O3 heterojunction is formed. In comparison with the bare hematite photoanode, an enhancement of the photocurrent density by a factor of 2 times at 1.23 V vs. reversible hydrogen electrode (RHE) has been observed on the p-CaFe2O4/n-Fe2O3 heterojunction photoanode. The origin of the remarkable activity enhancement is investigated and discussed based on the impedance spectroscopy analysis.

2. Experimental Bare and CF-modified hematite films preparation. Bare hematite and p-CaFe2O4/nFe2O3 heterojunction (denoted later as CF-modified hematite for simplicity) films were prepared on fluorine doped tin oxide (FTO) coated glass by hydrothermal method as reported by Vayssieres et al.31 with a little modification followed by two-step thermal annealing. Briefly, the FTO glass substrates were cleaned in an ultrasonic bath with acetone followed by ethanol and water. A glass vessel was filled with 5 ml of ferric chloride aqueous solution (0.15 M, Sigma-Aldrich) that contains the desired amount of CaCl2 (i.e., 0, 0.5, 1.0 or 2.0 at.% Ca). Afterwards, the FTO substrate was vertically put in the vessel and heated for 6 h at 95 °C. The grown uniform yellow layer of β-FeOOH on the FTO substrate was then rinsed by water to remove the inorganic salts. After two-step annealing, i.e., at 550 °C for 1 h and at 800 °C for 20 min, a reddish-brown film of either bare or CF-modified hematite films were obtained. Characterization. XRD patterns of the bare and CF-modified films were collected by using a Bruker D8 Advance diffractometer (DMAX 2500) operating with a CuKa1,2


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energy source at 40 kV and 40 mA. SEM measurements were performed by using a Nova Nano 630 scanning electron microscope from FEI Company using a TLD detector at an accelerating voltage of 5 kV. X-ray photoelectron spectroscopy (XPS) analysis was performed by using an AMICUS/ESCA 3400 KRATOS instrument equipped with Mg anodes at 12 kV and 10 mA. A prominent maximum peak of C1s at 284.6 eV was taken as the reference to calibrate the XPS spectra. Photoanode fabrication. Electrodes were fabricated by connecting copper wires to the uncoated area of the FTO conductive surface using silver paste. After drying of the paste, the connections between wires and the FTO glass substrate except 0.5 cm2 of the coated area were sealed with non-conductive epoxy resin and connected to a plastic tube to be easily handled. Electrochemical measurements. Electrochemical measurements were carried out in 1.0 M NaOH aqueous solution using a three-electrode Teflon-made photoelectrochemical cell (PEC) adapted with a quartz window, with a Pt counter electrode, and with Ag/AgCl/KCl (3.0 M) reference electrode. An EG&G 273A potentiostat was used for all photoelectrochemical measurements. The photoanode was put in the cell as a working electrode to observe its PEC water oxidation response. The photocurrent-potential curve was measured under simulated solar light generated by a Osram XBO 70 W Xenon lamp in a Müller LXH 100 lamp housing coupled with an air mass 1.5 global filter (Sciencetech Inc.). Light intensity of the simulated light was adjusted to 1 sun (100 mW/cm2) using a Thorlab digital handheld energy meter console (PM100D) connected with a calibrated high-sensitivity thermal sensor (S401C, Thorlabs). The preliminary results indicate that the CF-modified hematite electrode prepared from a chemical bath


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contains 1.0 at.% Ca exhibits slightly higher photocurrent response than those prepared employing a chemical bath contains 0.5 or 2.0 at.% Ca as shown in Figure S1. Thus, this electrode has been used in this study. To measure the incident photon conversion efficiency (IPCE), the photoanode was irradiated by a collimated light using a high-power LEDs (Thorlabs). Measurement of stable constant photocurrent and IPCE were carried out at 1.23 V vs. RHE. The photoanode was modified by CoPi cocatalyst using photo-assisted electrochemical deposition as previously reported (conditions: potential 1.2 V vs. Ag/AgCl, time 15 s, 0.5 mM cobalt nitrate in sodium phosphate buffer (0.1M), pH = 7).32 The impedance spectroscopy measurements were performed employing the same electrochemical setup except that the VersaSTAT 4 potentiostat was used. The Mott– Schottky plots were measured in the dark and under white light illumination at fixed frequency, i.e., 100 Hz. The experiment was performed in aqueous 1.0 M NaOH solution at pH ca. 13.9. The Nyquist plots were measured at 1.23 V vs. RHE with the frequency range being modulated between 100 kHz to 0.1 Hz at amplitude frequency 10 mV under while light irradiation (λ > 420 nm).

3. Results and discussion Hematite photoanode, however, it absorbs light up to ca. 560 nm, it exhibits low efficiency due to its poor electrical conductivity which leads to get charge carriers trapped in the bulk of hematite and recombined. One of the routes to enhance the electrical conductivity of hematite is to prepare hematite single-crystalline nanostructure with minimal charge trapping defects, and highly non-isotropic morphology.31 For example, 1-


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D nanorod of a high aspect ratio showed enhanced photocurrent response as its lateral dimension matches the short hole diffusion length of hematite and electrons could flow through a direct pathway along the axial direction of the rods.33 Thermal treatment of the hematite nanorods at high temperature to enhance its crystallinity leads, however, to destroy the non-isotropic morphology. Thus a new approach should be developed to enhance the separation of the photogenerated charge carriers. In this work, beside the utilization of the template-less film processing technique developed by Vayssieres et al. to grow a highly anisotropic β-FeOOH akaganeite film on FTO conductive glass from an aqueous ferric chloride solution, the solution bath has been modified by adding calcium chloride to allow the co-deposition of calcium, more probably in hydroxide form, while the β-FeOOH nanorods are grown. The thermal treatment of the obtained film at high temperature leads to the conversion of the grown β-FeOOH nanorods into hematite, moreover, it can induce a reaction between the co-deposited calcium hydroxide and βFeOOH to form CaFe2O4. The formation of hematite in the resultant film is readily discernible from its (110) and (300) diffractions located at 35.7° and 64.05° (2θ), respectively, in the XRD pattern (see Figure 1, JCPDS no. 033-0664). The strong (110) diffraction peak at 2θ = 35.7° in comparison with that at 2θ = 64.05° implies that the hematite nanorods grew preferentially in the [110] direction. Since the conductivity in the (001) basal plane is four orders of magnitude higher than the orthogonal plane, this preferential growth is preferred and it enhances the extent of the photo-oxidation process.34 Due to the low concentration of the calcium (1.0 at. %), it was not possible to detect the CaFe2O4 formation by the XRD measurements due to the technique limitations.


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The other diffraction peaks shown in Figure 1 can be readily indexed to the characteristic SnO2 peaks (JCPDS no. 041-1445) comes from the conductive fluorine doped SnO2 layer.

(300)

(110)

CF-modified hematite

Intensity / a.u

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


•

• •

•

∗

Bare hematite •

20

30

40

•

50 60 2θ / degree

•

∗

70

80

Figure 1. XRD diffraction patterns of thermally annealed bare and CF-modified hematite films. The solid and star symbols indicate Bragg positions for FTO substrate and hematite film, respectively, according to the JCPDS no. 041-1445 and 033-0664, respectively. The existence of Ca and the formation of CaFe2O4 in the CF-modified hematite film has been proved by XPS measurements as shown in Figure 2. Figure 2a showed that Ca2p core is split into 2p3/2 (BE=346.8 eV) and 2p1/2 (BE=350 eV) peaks, which are consistent with a valence of Ca +2 in the CaFe2O4 compound as previously reported.35 Figure 2b shows the typical XPS spectra of the Fe2P core of Fe3+ valence state. It exhibits a binding energy of 710.9 eV and 724.5 correspond to the Fe2p3/2 and Fe2p1/2 lines, respectively. The satellite peaks of the Fe2p observed at 719 eV indicates that the Fe3+ is dominant. Figure 2d shows the O1s signal, there are two peaks located at 529.8 and 531.8 eV. The former corresponds to lattice oxygen and the latter corresponds to hydroxyl group at the surface.36


9


(a) Intensity / a.u.

Ca2p3/2

Ca2p1/2

354

352

350

348

346

344

Binding energy / eV

(b)

Intensity / a.u.

Fe2p1/2 Fe2p3/2

730

725

720

715

710

705

528

526

Binding energy / eV

(c) O1s

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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536

534

532

530

Binding energy / eV

Figure 2. XPS spectra of CF-modified hematite film thermally treated. (a) Ca2p, (b) Fe2p, (c) O1s.

Figure 3 shows the scanning electron microscopy (SEM) images of β-FeOOH film asprepared and after two-step thermal annealing prepared in absence (a, b) and in presence (c, d) of calcium, respectively. The top view SEM image (Figure 3a) of the as-prepared βFeOOH indicated the growth of β-FeOOH nanorods roughly perpendicular to the FTO glass substrate and they exhibit average diameter of ca. 60 nm. SEM image presented in Figure 3b showed that however the hematite particles still have a rod-like morphology,


10

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the annealing of the as-prepared β-FeOOH nanorods increase the particle diameters to an average ca. 90 nm, which is probably due to aggregation at high annealing temperature. The same observations have been seen for the films prepared in presence of calcium (Figure 3c and d). Fortunately, due to the different in the atomic number of calcium and iron, it was possible to observe the contrast difference between the calcium-rich area (darker area) and β-FeOOH (lighter area). It can be readily concluded from the lowmagnification SEM image presented in Figure S2 that ca. 40 % of the hematite film is covered with calcium hydroxide. In fact the Ca inclusion in akaganeite nanorods grown on the FTO glass substrate is highly unfavorable rather the preferential situation of Ca at the surface is anticipated as a result of the different ionic radii of Ca2+ (crystal radius, 6coordination, 1.14 Ǻ) and Fe3+ (crystal radius, 6-coordination, 0.785 Ǻ), as well as their preferred coordination modes, i.e., high spin Fe3+ prefers 6-coordination, while Ca2+ is commonly found in 8-coordination environments.37 This is particularly significant for akaganeite nanorods grown on the FTO substrate as the crystal growth takes place selectively on the nuclei formed onto the FTO substrate and a condensed phase of singlecrystalline nanorods are generated as a result of the high concentration of Fe3+.38 Thermal annealing at high temperature induces reaction between the surface deposited calcium hydroxide and β-FeOOH leading to the formation of CaFe2O4 on the top of the hematite film as it can obviously be seen in Figure 3e. The creation of heterojunction at the interface between the p-CaFe2O4 and the n-Fe2O3 is thus more likely to occur. The existence of calcium has also been confirmed by the energy-dispersive X-ray spectroscopy (EDX) as shown in Figure 3f.


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Figure 3. SEM images of β-FeOOH film as prepared and after two steps thermal annealing in absence (a, b) and in presence (c, d ) of calcium, respectively; (e) high magnification of image (d), and (f) EDX analysis of CF-modified film. To investigate the performance of bare and CF-modified hematite photoanodes towards photoelectrochemical water oxidation, the photocurrent response under simulated solar light illumination has been measured employing a three-electrode photoelectrochemical workstation. Figure 4 shows the photocurrent−potential curves of bare and CF-modified hematite photoanode. The CF-modified hematite photoanode exhibits a photocurrent density of 0.53 mA/cm2 at 1.23 V vs. RHE, i.e., 100 % higher than that is obtain on bare hematite photoanode. This photocurrent enhancement can be attributed to the enhanced charge carrier separation as a result of the p-n junction creation between p-CaFe2O4/nFe2O3.39 The loading of CF-modified heterojunction photoanode with CoPi cocatalyst


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further facilitate the electron transfer at the electrode/electrolyte interface and thus further enhance the photoelectrochemical water oxidation. 2.0

-2

Bare hematite CF-modified hematite CoPi/CF-modified hematite

Current density / mAcm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.5

1.0

0.5

0.0

0.4

0.6

0.8 1.0 1.2 Potential / V vs. RHE

1.4

1.6

Figure 4. I-V plots of bare and modified hematite in 1.0 M NaOH under chopped simulated solar light irradiation (air mass 1.5 global, 1 sun (100 mW cm-2)). To

clarify

the

origin

of

the

different

photoelectrochemical

responses,

the

electrochemical impedance spectroscopy (EIS) of the bare and modified electrode have been measured and analyzed. Figure 5a and b shows the Mott–Schottky (MS) plots of bare and CF-modified hematite photoanodes measured in the dark and under light irradiation, respectively. The donor densities of the bare and the CF-modified electrodes have been obtained from the slope of the plot based on the Mott-Schottky equation40, 41 1 C

2

=

2 εε 0 qN D A

2

( E − E FB − k B T/q )

where C is the capacitance of the space charge layer, ε the dielectric constant of the semiconductor, ε0 the vacuum permittivity, A the actual area of the electrode exposed to the electrolyte, ND the donor density, E the applied potential, EFB the flatband potential, q the


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elementary charge, kB the Boltzmann´s constant and T the temperature. The bare and CFmodified hematite electrode showed a positive slope in the dark (see Figure 5a) indicating that hematite is an n-type semiconductor with electrons as the majority carriers. The donor densities of the CF-modified hematite electrode (i.e., 3.0×1020 cm˗3) in the dark was found to be lower than that of bare hematite (i.e, 4.0×1020 cm˗3). This can be explained by the fact the electrons and holes were trapped in the depletion layer of the p–n junction as previously observed for p-NiO/nhematite heterojunction electrode.42 Under illumination, slight increase in the donor density of the bare hematite electrode (i.e, 4.4×1020 cm˗3) was observed evincing that serious recombination between the photogenerated charge carriers is exist indeed. For CF-modified hematite electrode, MS plot exhibits mainly two parts. A linear part with a positive slope which is a typical feature for an n-type semiconductor. In part 2, the semiconductor capacitance is stabilized, which is associated with the surface redox states charging as discussed in previous reports for α-Fe2O3 and TiO2.43, 44 Thus, this deviation might be attributed to the charge transfer as a result of the p-n junction between p-CaFe2O4 and n-Fe2O3 and storage of the photogenerated hole in the pCaFe2O4 layer. The donor density of the CF-modified heterojunction photoanode under illumination was found to be 8.6×1020 cm˗3 which is ca. 3 times higher than that is obtained in the dark. This is another indication of the enhanced charge carriers separation via p-CaFe2O4 modification.


14

0.5

10

Bare hematite CF-modified hematite

4

-2

1.0

(b) 1.0

0.5

-2

4

1.5

-2

2.0

Bare hematite CF-modified hematite

-2

10

(a) 2.5 C / F cm x 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C / F cm x 10

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0.0 0.0 -0.8 -0.6 -0.4 -0.2 0.0 -0.8 -0.6 -0.4 -0.2 0.0 Potential / V vs. Ag/AgCl Potential / V vs. Ag/AgCl Figure 5. Mott–Schottky plots of bare and CF-modified hematite electrodes measured in the dark (a) and and under light (b). To further gain more insight, the Nyquist plots of bare, CF-, and CoPi/CF-modified hematite photoanodes were measured at 0.2 V vs. Ag/AgCl (1.23 V vs RHE) (see Figure 6a). In general, the impedance of the bare was larger than that of the CF- and CF/CoPimodified hematite photoanodes which implies that the conductivity and the electron transport in the modified electrodes were increased which explain the higher photocurrent response shown in Figure 4. For more detailed analysis, the experimental EIS data were fitted into an equivalent circuit model includes two RC circuits (see Figure 6b) using the EIS spectrum analyzer software. The fitted impedance parameter values of the resistances (R) and constant phase elements (CPE) are listed in Table 1. The two RC circuits can be assigned to electrode||electrolyte interface (R2/CPE2) and to the electron transport inside the electrode (R1/CPE1) as assigned in previous studies.14, 45 Suggesting that the junction between p-CaFe2O4 and n-Fe2O3 is well formed, one circuit represents the electron movement between particles in the CF-modified electrode, as in bare hematite, has been used. The fitting results indicate that the CF-modified electrode exhibits decreased resistance values (R1 and R2) in accordance with the increase of photocurrent generation. It seems that the creation of the p-CaFe2O4/n-Fe2O3 junction facilitate the charge carrier


15


separation and reduce the resistance on the movement of charge carriers, therefore, improve the charge carrier characteristics. It was also noted that the CPE increases from bared to CF-modified hematite electrode. The decrease in resistance is accompanied with an increase in the capacitance as a result of the fast movement of the electrons because of the p-n junction. The modification of the CF-hematite electrode with CoPi cocatalyst further reduces the R2 value and increases the CPE2 while the R1 and CPE1 remains almost constant indicating that the CoPi cocatalyst effectively facilitate the electron transfer at the surface and thus enhance the water oxidation at the electrode/electrolyte interface.

(a) 2.0

Bare hematite CF-modified hematite CoPi /CF-modified hematite

1.6

1.2

Zim / k Ω

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.8

0.4

0.0 0

1

2

3

4

5

Zre / k Ω

(b)

Figure 6. (a) Nyquist plots of bare hematite, CF- and CoPi/CF-modified hematite photoanode and (b) Equivalent circuit model. The EIS spectra were measured in 1.0 M NaOH at 1.23 V vs. RHE under white light irradiation (λ > 420 nm).


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Table 1. Resistance and CPE values obtained by fitting the EIS spectra. R1 CPE1 601 3.67×-5

R2 CPE2 3621 7.00×10-5

37

285 5.00×10˗5

1200 8.16×10-5

32

292 5.90×10-5

981 4.84×10-4

R/CPE (Ω / F ) Bare hematite

RS

CF-modified hematite CoPi/CFmodified hematite

47

Since the down-edge of the CaFe2O4 and α-Fe2O3 conduction bands are located at -0.6 and 0.2 V vs. RHE or NHE, respectively, and they exhibit 1.9 eV and 2.1 eV bandgap, respectively,12,

46

the alignment of the p–n junction between CaFe2O4 and α-Fe2O3 is

schematically illustrated in Figure 7. When p-CaFe2O4 and n-type hematite contact each other to form a p–n junction, band bending will occur at the interface to reach an equal Fermi level (Ef p–n) in CaFe2O4 and hematite. Under illumination, the photogenerated holes in hematite are extracted into the valence band of CaFe2O4 while the photogenerated electrons transfer from CaFe2O4’s CB to the hematite’s CB and finally to the back contact. Therefore, the photogenerated electron–hole pairs are effectively separated by the p–n junction. The further modification of the CF-modified hematite photoanode with a layer of CoPi on its surface leads to enhancing the photocurrent. When the photoanode exposed to the light, the photogenerated holes travel to the semiconductor-liquid junction and accumulate on the surface as the slow water oxidation kinetics, causing the recombination of holes and electrons. The presence of CoPi capture the holes that accumulate on the surface and reduce the recombination of charge carriers and thus improve the water oxidation kinetics.32


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Page 18 of 23

Figure 7. Schematic diagram represents the band alignment and the expected charge flow of the heterojunction photoanode. The conduction band (CB) and valence band (VB) of p-CaFe2O4 and the n-Fe2O3 are presented with the redox potential of H+/H2 and O2/H2O. The incident photon-to-current conversion efficiency (IPCE) of bare hematite, CF- and CF/CoPi-modified hematite heterojunction photoanodes were measured at 1.23 vs. RHE as a function of wavelength and the results are presented in Figure 8. The IPCE values at different monochromatic light have been calculated using the following equation:16

IPCE(%) =

1240 × J ph

× 100

λ×P light

where Jph is the photocurrent density (mA/cm2), λ is the wavelength of the incident light (nm) and Plight is the light power density (mW/cm2). It can be seen from Figure 8 that the IPCE of CF-modified hematite heterojunction photoanode is approximately 2-fold higher than that of bare hematite at all wavelengths examined. This significant enhancement is due to the formed heterojuction at the interface. The modification of the heterojunction photoanode with CoPi further enhances the IPCE in consistent with the higher photocurrent response observed in Figure 4. Since cheap and abundant materials have been employed for the fabrication of the


18

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heterojunction photoanode via a simple route, the current results have great importance from scientific and economical point of view. 35 Bare Hematite

30

CF-modified Hematite CoPi/CF-modified Hematite

25 IPCE %

20 15 10 5

53 0

47 0

42 0

0

36 5

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Wavelength / nm

Figure 8. Incident photon-to-current conversion efficiency (IPCE%) of bare hematite and CF- and CF/CoPi-modified hematite heterojunction photoanode. IPCE was measured in 1.0 M NaOH at 1.23 V vs. RHE under monochromatic light irradiation.

4. Conclusion In conclusion, a facile template-less film processing technique has been used for the fabrication of p-CaFe2O4/n-Fe2O3 heterojunction photoanode by control the chemical bath. Anisotropic growth of β-FeOOH akaganeite film on FTO conductive glass from an aqueous FeCl3 solution containing CaCl2 followed by two-step thermal annealing at high temperature, i.e., 550 and 800 °C, respectively, converts the grown β-FeOOH nanorods into hematite. Moreover, it induces a reaction between the co-deposited calcium hydroxide

and

β-FeOOH

to

create

a

p-CaFe2O4/n-Fe2O3

heterojunction.

The

heterojunction photoanode showed 100 % higher photocurrent response than that is obtained using bare hematite electrode. The photocurrent enhancement is attributed to the


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Page 20 of 23

enhanced charge carrier separation and the reduced resistance in the charge transfer across the electrode and electrolyte as revealed by electrochemical impedance spectroscopy analysis. The modification of the p-CaFe2O4/n-Fe2O3 heterojunction photoanode with CoPi cocatalyst further facilitate the electron transfer at the electrode/electrolyte interface and thus enhance the photoelectrochemical water oxidation.

Acknowledgements Financial support from the Science and Technology Development Fund (STDF) of the Arab Republic of Egypt and the Federal Ministry of Education and Research (BMBF) of the Federal Republic of Germany is gratefully acknowledged (Egyptian-German Research Fund (GERF III), Project ID 5064).

Author Information Corresponding Author *Tel: +20 93 457 0000 ext. 2342; e-mail: [email protected]

Notes The authors declare no competing financial interest.

References 1. Rioult, M.; Magnan, H.; Stanescu, D.; Barbier, A., Single Crystalline Hematite Films for Solar Water Splitting: Ti-Doping and Thickness Effects. J Phys Chem C 2014, 118 (6), 3007-3014. 2. Gratzel, M., Photoelectrochemical cells. Nature 2001, 414 (6861), 338-344. 3. Fujishima, A.; Honda, K., Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238 (5358), 37-38. 4. Sivula, K.; Le Formal, F.; Gratzel, M., Solar Water Splitting: progress using Hematite (αFe2O3) photoelectrodes. ChemSusChem 2011, 4 (4), 432-49. 5. Kim, W.; Tachikawa, T.; Monllor-Satoca, D.; Kim, H.-i.; Majima, T.; Choi, W., Promoting Water Photooxidation on Transparent WO3 Thin Films using an Alumina Overlayer. Energy Environ. Sci. 2013, 6 (12), 3732-3739.


20

Page 21 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6. Hill, J. C.; Choi, K.-S., Effect of Electrolytes on the Selectivity and Stability of n-type WO3 Photoelectrodes for Use in Solar Water Oxidation. J. Physical Chem. C 2012, 116 (14), 76127620. 7. Wang, G.; Ling, Y.; Wheeler, D. A.; George, K. E.; Horsley, K.; Heske, C.; Zhang, J. Z.; Li, Y., Facile synthesis of highly photoactive α-Fe2O3-based films for water oxidation. Nano Lett. 2011, 11 (8), 3503-9. 8. Deng, J. J.; Lv, X. X.; Gao, J.; Pu, A. W.; Li, M.; Sun, X. H.; Zhong, J., Facile synthesis of Carbon-coated Hematite Nanostructures for Solar Water Splitting. Energy Environ. Sci. 2013, 6 (12), 3790-3790. 9. Xiong, F. Q.; Shi, J. Y.; Wang, D. G.; Zhu, J.; Zhang, W. H.; Li, C., Enhanced Photocatalytic Water Oxidation on ZnO photoanodes in a Borate buffer electrolyte. Catalysis Sci. Technol. 2013, 3 (7), 1699-1702. 10. Kim, T. W.; Choi, K. S., Nanoporous BiVO4 Photoanodes with Dual-Layer Oxygen Evolution Catalysts for Solar Water Splitting. Science 2014, 343, 990-994. 11. Zhang, L. W.; Baumanis, C.; Robben, L.; Kandiel, T.; Bahnemann, D., Bi2WO6 Inverse Opals: Facile Fabrication and Efficient Visible-Light-Driven Photocatalytic and Photoelectrochemical Water-Splitting Activity. Small 2011, 7 (19), 2714-2720. 12. Ida, S.; Yamada, K.; Matsunaga, T.; Hagiwara, H.; Matsumoto, Y.; Ishihara, T., Preparation of p-type CaFe2O4 photocathodes for producing hydrogen from water. J. Am. Chem. Soci. 2010, 132 (49), 17343-17345. 13. Zhang, Z.; Dua, R.; Zhang, L.; Zhu, H.; Zhang, H.; Wang, P., Carbon-layer-protected cuprous oxide nanowire arrays for efficient water reduction. ACS Nano 2013, 7 (2), 1709-1717. 14. Kim, J. Y.; Magesh, G.; Youn, D. H.; Jang, J. W.; Kubota, J.; Domen, K.; Lee, J. S., Singlecrystalline, wormlike hematite photoanodes for efficient solar water splitting. Sci Rep 2013, 3, 2681. 15. Hensel, J.; Wang, G.; Li, Y.; Zhang, J. Z., Synergistic Effect of CdSe Quantum Dot Sensitization and Nitrogen Doping of TiO2 Nanostructures for Photoelectrochemical Solar Hydrogen Generation. Nano Lett. 2010, 10 (2), 478-483. 16. Gurudayal; Chiam, S. Y.; Kumar, M. H.; Bassi, P. S.; Seng, H. L.; Barber, J.; Wong, L. H., Improving the Efficiency of Hematite Nanorods for Photoelectrochemical Water Splitting by Doping with Manganese. ACS Appl. Mater. Interfaces 2014, 6 (8), 5852-5859. 17. Bora, D. K.; Braun, A.; Erni, R.; Muller, U.; Dobeli, M.; Constable, E. C., Hematite-NiO/αNi(OH)2 heterostructure photoanodes with high electrocatalytic current density and charge storage capacity. Phys. Chem. Chem. Phys. 2013, 15 (30), 12648-12659. 18. Ling, Y.; Wang, G.; Wheeler, D. A.; Zhang, J. Z.; Li, Y., Sn-Doped Hematite Nanostructures for Photoelectrochemical Water Splitting. Nano Lett. 2011, 11 (5), 2119-2125. 19. Franking, R.; Li, L. S.; Lukowski, M. A.; Meng, F.; Tan, Y. Z.; Hamers, R. J.; Jin, S., Facile post-growth doping of nanostructured hematite photoanodes for enhanced photoelectrochemical water oxidation. Energy Environ. Sci. 2013, 6 (2), 500-512. 20. Qi, X.; She, G.; Wang, M.; Mu, L.; Shi, W., Electrochemical synthesis of p-type Zn-doped αFe2O3 nanotube arrays for photoelectrochemical water splitting. Chem. Commun. 2013, 49 (51), 5742-5744. 21. Deng, J.; Zhong, J.; Pu, A.; Zhang, D.; Li, M.; Sun, X.; Lee, S.-T., Ti-doped hematite nanostructures for solar water splitting with high efficiency. J. Appl. Phys. 2012, 112 (8), 084312. 22. Liu, Y.; Yu, Y.-X.; Zhang, W.-D., Photoelectrochemical properties of Ni-doped Fe2O3 thin films prepared by electrodeposition. Electrochim. Acta 2012, 59 (1), 121-127. 23. Zhang, P.; Kleiman-Shwarsctein, A.; Hu, Y.-S.; Lefton, J.; Sharma, S.; Forman, A. J.; McFarland, E., Oriented Ti doped hematite thin film as active photoanodes synthesized by facile APCVD. Energy Environ. Sci. 2011, 4 (3), 1020-1028.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 23

24. Kumari, S.; Singh, A. P.; Sonal; Deva, D.; Shrivastav, R.; Dass, S.; Satsangi, V. R., Spray pyrolytically deposited nanoporous Ti4+ doped hematite thin films for efficient photoelectrochemical splitting of water. Int. J. Hydrogen Energy 2010, 35 (9), 3985-3990. 25. Glasscock, J. A.; Barnes, P. R. F.; Plumb, I. C.; Savvides, N., Enhancement of Photoelectrochemical Hydrogen Production from Hematite Thin Films by the Introduction of Ti and Si. J. Phys. Chem. C 2007, 111 (44), 16477-16488. 26. Kleiman-Shwarsctein, A.; Hu, Y.-S.; Forman, A. J.; Stucky, G. D.; McFarland, E. W., Electrodeposition of α-Fe2O3 Doped with Mo or Cr as Photoanodes for Photocatalytic Water Splitting. J. Phys. Chem. C 2008, 112 (40), 15900-15907. 27. Lin, Y.; Xu, Y.; Mayer, M. T.; Simpson, Z. I.; McMahon, G.; Zhou, S.; Wang, D., Growth of p-type hematite by atomic layer deposition and its utilization for improved solar water splitting. J Am Chem Soc 2012, 134 (12), 5508-11. 28. Su, J.; Guo, L.; Bao, N.; Grimes, C. A., Nanostructured WO3/BiVO4 Heterojunction Films for Efficient Photoelectrochemical Water Splitting. Nano Lett. 2011, 11 (5), 1928-1933. 29. McDonald, K. J.; Choi, K. S., Synthesis and Photoelectrochemical Properties of Fe2O3/ZnFe2O4 Composite Photoanodes for Use in Solar Water Oxidation. Chem. Mater. 2011, 23 (21), 4863-4869. 30. Kim, E. S.; Nishimura, N.; Magesh, G.; Kim, J. Y.; Jang, J. W.; Jun, H.; Kubota, J.; Domen, K.; Lee, J. S., Fabrication of CaFe2O4/TaON heterojunction photoanode for photoelectrochemical water oxidation. J. Am. Chem. Soc. 2013, 135 (14), 5375-5383. 31. Vayssieres, L.; Beermann, N.; Lindquist, S.-E.; Hagfeldt, A., Controlled aqueous chemical growth of oriented three-dimensional crystalline nanorod arrays: application to iron (III) oxides. Chemistry of Materials 2001, 13 (2), 233-235. 32. Zhong, D. K.; Cornuz, M.; Sivula, K.; Graetzel, M.; Gamelin, D. R., Photo-assisted electrodeposition of cobalt-phosphate (Co-Pi) catalyst on hematite photoanodes for solar water oxidation. Energy Environ. Sci. 2011, 4 (5), 1759-1764. 33. Lindgren, T. r.; Wang, H.; Beermann, N.; Vayssieres, L.; Hagfeldt, A.; Lindquist, S.-E., Aqueous photoelectrochemistry of hematite nanorod array. Solar Energy Mater. Solar Cells 2002, 71 (2), 231-243. 34. Xi, L.; Tran, P. D.; Chiam, S. Y.; Bassi, P. S.; Mak, W. F.; Mulmudi, H. K.; Batabyal, S. K.; Barber, J.; Loo, J. S. C.; Wong, L. H., Co3O4-Decorated Hematite Nanorods As an Effective Photoanode for Solar Water Oxidation. J. Phys. Chem. C 2012, 116 (26), 13884-13889. 35. Liu, Z.; Zhao, Z.-G.; Miyauchi, M., Efficient Visible Light Active CaFe2O4/WO3 Based Composite Photocatalysts: Effect of Interfacial Modification. J. Phys. Chem. C 2009, 113 (39), 17132-17137. 36. Fu, L.; Yu, H.; Zhang, C.; Shao, Z.; Yi, B., Cobalt Phosphate Group Modified Hematite Nanorod Array as Photoanode for Efficient Solar Water Splitting. Electrochimica Acta 2014. 37. Baltpurvins, K. A.; Burns, R. C.; Lawrance, G. A.; Stuart, A. D., Effect of Ca2+, Mg2+, and Anion Type on the Aging of Iron(III) Hydroxide Precipitates. Environ. Sci. Technol. 1997, 31 (4), 1024-1032. 38. Vayssieres, L.; Beermann, N.; Lindquist, S. E.; Hagfeldt, A., Controlled aqueous chemical growth of oriented three-dimensional crystalline nanorod arrays: Application to iron(III) oxides. Chem. Mater. 2001, 13 (2), 233-235. 39. Li, J.; Meng, F.; Suri, S.; Ding, W.; Huang, F.; Wu, N., Photoelectrochemical performance enhanced by a nickel oxide-hematite p-n junction photoanode. Chem. Commun. 2012, 48 (66), 8213-8215. 40. Degryse, R.; Gomes, W. P.; Cardon, F.; Vennik, J., Interpretation of Mott-Schottky Plots Determined at semiconductor-electrolyte Systems. J. Electrochem. Soc. 1975, 122 (5), 711-712. 41. Salvador, P., hole Diffusion Length in n-TiO2 Single Crystal and Sintered ElectrodesPhotoelectrochemical Determination and Comparative Analysis. J. Appl. Phys. 1984, 55 (8), 2977-2985.


22

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


42. Li, J.; Meng, F.; Suri, S.; Ding, W.; Huang, F.; Wu, N., Photoelectrochemical performance enhanced by a nickel oxide-hematite p-n junction photoanode. Chem. Commun. 2012, 48 (66), 8213-8215. 43. Tomkiewicz, M., Potential Distribution at The TiO2 Aqueous-Electrolyte Interface. J. Electrochem. Soc. 1979, 126 (9), 1505-1510. 44. Klahr, B.; Gimenez, S.; Fabregat-Santiago, F.; Bisquert, J.; Hamann, T. W., Electrochemical and photoelectrochemical investigation of water oxidation with hematite electrodes. Energy Environ. Sci. 2012, 5 (6), 7626-7636. 45. Kim, J. Y.; Jang, J. W.; Youn, D. H.; Kim, J. Y.; Kim, E. S.; Lee, J. S., Graphene-carbon nanotube composite as an effective conducting scaffold to enhance the photoelectrochemical water oxidation activity of a hematite film. Rsc Advances 2012, 2 (25), 9415-9422. 46. Hellman, A.; Pala, R. G. S., First-Principles Study of Photoinduced Water-Splitting on Fe2O3. J. Phys. Chem. C 2011, 115 (26), 12901-12907.

Table of Contents Graphic and Synopsis


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Enhanced Photoelectrochemical Water Oxidation on Nanostructured

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