Physical and photoelectrochemical characterizations of hematite α-Fe2O3: Application to photocatalytic oxygen evolution

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Solar Energy 84 (2010) 715–721 www.elsevier.com/locate/solener

Physical and photoelectrochemical characterizations of hematite a-Fe2O3: Application to photocatalytic oxygen evolution S. Boumaza a,b, A. Boudjemaa a,b, S. Omeiri a,c, R. Bouarab a, A. Bouguelia c, M. Trari c,* a

Technical and Scientific Research Centre of Physical Analysis (CRAPC), BP 248, RP 16004 Algiers, Algeria b Laboratory of Chemistry of Natural Gas, Faculty of Chemistry (USTHB), BP 32, 16111 Algiers, Algeria c Laboratory of Storage and Valorization of Renewable Energies, Faculty of Chemistry (USTHB), BP 32, 16111 Algiers, Algeria Received 28 August 2009; received in revised form 20 December 2009; accepted 27 January 2010 Available online 23 February 2010 Communicated by: Associate Editor Gion Calzaferri

Abstract The physical properties and photoelectrochemical characterization of a-Fe2O3, synthesized by co-precipitation, have been investigated in regard to solar energy conversion. The optical gap is found to be 1.94 eV and the transition is indirectly allowed. The chemical analysis reveals an oxygen deficiency and the oxide exhibits n-type conductivity, confirmed by a negative thermopower. The plot log r vs 1/T shows linearity in the range (400–670 K) with the donor levels at 0.14 eV below the conduction band and a break at 590 K, attributed to the ionization of the donors. The conduction occurs by small polaron hopping through mixed valences Fe2+/3+ with an electron mobility l400 K of 103 V cm2 s1. a-Fe2O3 exhibits long term chemical stability in neutral solution and has been characterized photoelectrochemically to assess its activity as bias-free O2-photoanode. The flat band potential Vfb (0.45VSCE) and the electron density ND (1.63  1018 cm3) were determined, respectively, by extrapolating the linear part to C2 = 0 and the slope of the Mott Schottky plot. At pH 6.5, the valence band (+1.35VSCE) is suitably positioned with respect to the O2/H2O level (+0.62 V) and a-Fe2O3 has been evaluated 2 1 þ for the chemical energy storage through the photocatalytic reaction: (2SO2 3 þ 2H ! S2 O3 þ O2 þ H2 O, DG = 213.36 kJ mol ). The 2 3 1 1 best photoactivity occurs in SO3 solution (0.025 M, pH 8) with an oxygen rate evolution of 7.8 cm (g catalyst) h . Ó 2010 Elsevier Ltd. All rights reserved. Keywords: Photoelectrochemical; Hematite; Co-precipitation; Photoanode

1. Introduction The photocatalytic water oxidation is a challenging goal for the electrolysis by lowering the oxygen over-potential (Lanz et al., 1999). Indeed, The O2 evolution occurs at high over-voltage resulting in energy losses. Binary oxides M2O3, where M is a trivalent metal, crystallize in the corundum structure and occur in n- as well as p-types (Aroutiounian et al., 2006). In this category, the hematite a-Fe2O3 was selected as prototype due to its technological usage as catalysts (Ferretto and Glisenti, 2002; Liu and Sun, 2007) and photocatalysts (Fu et al., 2005). Moreover, *

Corresponding author. Tel.: +213 21 24 79 50; fax: +213 21 24 73 11. E-mail address: [email protected] (M. Trari).

0038-092X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2010.01.025

it has found increasing applications because of its dielectric properties and its breakthrough having unexpectedly large thermopower (Kulkarni and Lokhande, 2003). The hematite is predicted to be an insulator with localized Fe3+: 3d electron (Goodenough, 1971). However, the slight deviation from the stoichiometry induces an enhancement of the electrical properties allowing photoelectrochemical (PEC) characterization. On the other hand, there is an increasing interest in the photoactive functional oxide materials. As one of the main fields of the solar energy research concerns the development of so-called solar fuels (Younsi et al., 2005) and considerable attention has been focused on developing new semiconductors (SC) for the PEC conversion (Saadi et al., 2006; Boumaza et al., 2009). Practical advantages pertinent to solar energy have

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been reported early (Gerisher, 1979). Fe2O3 has a gap Eg which averages 2 eV, absorbing 40% of the sunlight. Additionally, it is low cost, non-toxic and exhibits a chemical stability over a broad pH range (Lindgren et al., 2002); these characteristics make it attractive for photocatalytic applications. The crystal lattice contains octahedral FeO6 units, which determine that both the valence band (VB) and the conduction band (CB) are associated with the same cation namely Fe3+. Hence, VB is pH-independent and for a given pH, can be appropriately positioned with respect to O2/H2O redox level in solution. However, whereas the physical properties of the hematite have been investigated (Goodlet et al., 2004; Glasscok et al., 2008), the transport properties so far reported have been limited to the electrical conductivity, although some papers were devoted to the PEC characterization (Valenzuela et al., 2002 references therein– Aroutiounian et al., 2007). The PEC behavior of hematite is dominated by a relatively slow charge transfer and a high rate of electron–hole (e/h+) recombination resulting in low conversion efficiencies. This occurs because the diffusion length (intrinsic parameter) is small compared to the crystallite size. Therefore, it is of interest to study the photoelectric properties of Fe2O3, elaborated by chemical route. The holes must be captured by reducing species present in nanopores and the distance they have to diffuse before reaching the interface is reduced below the diffusion length. Sun represents potentially an enormous energetic resource if convenient ways could be found to transform it into suitable forms. Photocatalytic oxygen evolution has drawn interest in the water electrolysis in order to reduce the over-voltage and many materials have been tested in various electrolytes (Tanaka et al., 1985). As a-Fe2O3 is chemically stable, it has been used for storage through the up hill reaction 2 þ ð2SO2 3 þ 2H ! S2 O3 þ O2 þ H2 OÞ, and light could be used for this purpose. Oxygen production is combined with whose potential is above CB to the reduction of SO2 3 achieve light induced charge separation thus improving photostability. 2. Experimental a-Fe2O3 was synthesized by co-precipitation according to previous works (Beydoun et al., 2000). The high purity reagents FeSO47H2O (99.0%) and Fe2(SO4)35H2O (99.5%) with a molar ratio (1/1) were dissolved in water to which 100 ml of NH4OH (8.5 M) was added. The mixture was maintained for 30 min under vigorous agitation. The solution was filtrated; the precipitate was washed several times with water and dried at 110 °C. Then, the powder (labeled A) was heated at 400 °C (5 °C min1) for 150 min. The crystal phase has been identified by X-ray diffraction (XRD) using Cu Ka radiation (k = 0.154178 nm). Thermal analysis (TG) was carried out in air (5 °C min1) with a computer-controlled thermoanalyser (Setaram Setsys 16/18). The surface area was determined by the BET method using nitrogen gas

as adsorbate at liquid nitrogen temperature on ASAP 2010 micromeritics apparatus. The IR spectrum was recorded in the range 4000–400 cm1 with a spectrometer Brucker Vector 22 using dried spectroscopic KBr. The Fe2+ content (oxygen deficiency) has been determined by chemical analysis, the sample (8.2 mg) was dissolved in HCl (8 N) under nitrogen atmosphere and Fe2+ was titrated by KMnO4. The corrosion process involves immersion of the oxide in neutral solution (KCl, 0.5 M) for two months period; Fe3+ thus generated was titrated by a standard Sn2+ solution. The pellets were cold pressed under 3 kbar and sintered at 400 °C; the compactness approximates 0.6. Silver paint was deposited on both sides of the pellet and the electrical conductivity was measured by the standard two probe technique. The thermoelectric power was performed by the differential technique. The thermo emf was measured by a digital microvoltmeter (Tacussel, Aris 20000) with an impedance of 1012 X while the temperature gradient was determined with a thermo couple (type K). A copper wire was soldered with silver paint to the contacting face of the pellet which was mounted in a glass holder using epoxy resin. The electrochemical measurements were carried out in a three electrode cell under nitrogen blanket. The aqueous solution contains Na2SO4 (0.5 M) as supporting salt. A Pt electrode (Tacussel, 1 cm2) served as auxiliary electrode; the electrode potential was monitored by a potentiostat Voltalab PGZ301 and scaled with respect to a saturated calomel electrode (SCE). The interfacial capacitance has been measured as a function of the potential, AC voltage signal of 1 kHz in frequency and 10 mV peak to peak in magnitude was applied to the system. The electrode was biased at 0.4 V and the photocurrent spectrum has been measured with the output of 650 W Xenon lamp (Dyr) using series of filters, the light path in the solution was approximately 2 mm. The quantum efficiency (g) was calculated by dividing the electron flow (Jph  Jd) in the external circuit by the incident photon flux at each wavelength measured with a digital light meter (Testo 545). The photocatalytic activity was determined by measuring the volume of evolved oxygen. The tests were carried out in a double walled reactor whose temperature was regulated at 50 ± 1 °C by a circulating water bath (Julabo). The powder (250 mg) was dispersed under constant magnetic agitation in 200 ml of freshly prepared solution (0.025 M, SO2 3 ) deoxygenated in advanced for 35 mn. The light source consists of three tungsten lamps (200, Osram) providing a flux intensity of 29 mW cm2. Oxygen was identified by gas chromatograph (TCD) IGC 121 ML with a catharometer detector and argon as a carrier gas. The amount was collected volumetrically in a water manometer, owing to its low solubility (2.3  105 cm3/L) at 25 °C (Gevantman, 1997–1998), and corrected from the blank test. The solutions were prepared from reagents of analytical grade quality and twice distilled water.

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3. Results and discussion TG of the powder (A) was recorded up to 800 °C to measure the effect of heating on the synthesis conditions (Fig. 1), two weight losses are easily seen in the plot. The first one (3.72%) up to 200 °C is originated from the removal of adsorbed water whereas the second one (3.01%) extending up to 600 °C is due to the progressive elimination of sulfate. Similar effect has been observed by others (Bocangra-Diaz et al., 2003). The XRD pattern (Fig. 2) is characteristic of a single phase where all the peaks index in the corundum structure with the space group ðR3cÞ according to the JCPDS Card No. 89-0598. The refined lattice constants {a = 0.500(5) nm, c = 1.341(9) nm} agree with those previously reported (Barrero et al., 2004). The experimental density qexp (5.04 g cm3), measured by pycnometry, is close to that calculated on the base of six formula weights by unit cell (5.46 g cm3). The pattern indicates a medium crystallinity with a particle size L of 25 nm. L was estimated from the full width at half maximum (FWHM) through the empirical (0.94k/ b cos h) where b is the broadening of the most intense XRD peak (1 1 0) and h is the diffraction angle. The specific surface area (47 m2 g1), deduced from the relation (6/qexpL), is in excellent agreement with that determined from BET (46 m2 g1) and this indicates a small porosity

Fig. 1. TG curve of the powder (A) dried at 110 °C.

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of the powder. The lack of peak broadening indicates that the crystallites agglomerate in grains. The IR spectrum (Fig. 3) is quite similar to that given in the literature (Chung and Lee, 1991) and is essentially featureless down to 2400 cm1. Each Fe3+ ion is octahedrally oxygen coordinated and the bands 2362, 1384 and 534 cm1 are associated with FeO6 modes (Nyquist and Kagel, 1971). The peaks centred at 3400 and 1600 cm1 are attributed, respectively, to adsorbed water and CO2 when the sample was handled in air. The small intensity bands located at 2924 and 1125 cm1 are due to unreacted sulfate. Dense pellets (60%) were obtained by sintering pressed pellets at 400 °C. The Fe–Fe distance in a-Fe2O3 is larger than the critical length for collective behavior, equal to 0.289 nm in metal iron, and the stoichiometric oxide is expected to be insulator with a conductivity r300 K < 10 6 1 X cm1 (Goodenough, 1971). However, the transport properties depend on the deviation from the stoichiometry; the chemical analysis gave an oxygen deficiency and the 2 2þ oxide is accurately formulated Fe3þ 0:995 Fe0:005 O2:995 leading to iron mixed valences, which in turn induces enhanced electrical conductivity. The defect structure has been the subject of numerous investigations (Saric et al., 1998) and discussion of the transfer in oxides is based on the model of thermally activated hopping through local electron states. The outer 3d electrons in Fe2O3 can be considered as localized and the conduction occurs predominantly by electron hopping between mixed states Fe2+/3+ located in crystallographically equivalent sites. We should bare in mind that the ionic radii of Fe2+ and Fe3+ are close to each other in sixfold coordination. The small polaron is based on strong electron lattice interaction and the transfer involves large activation energy DE. The thermal evolution log r(1/T), illustrated in (Fig. 4, inset), follows an Arrhenius-type law with DE equal to 0.14 eV; such value indicates an extrinsic conductivity where only a small portion of the total number of the carriers is delocalized. The plot deviates from the linearity at 590 K, which may be attributed to the increasing carriers concentration brought by surface oxidation. The increase of the concentration is evidenced by the fact that the conductivity is larger on the reverse scan compared to the initial oxide. The low electron

Fig. 2. Powder X ray diffraction pattern of Fe2O3 prepared by co-precipitation.

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Fig. 3. Infra red spectrum of Fe2O3 after annealing at 400 °C in air.

50

-6.0

-50 -6.5 -1

Log σ (Ω cm )

-1

S ( mV K )

0

-100 -150

-7.0 -7.5 -8.0 -8.5

-200

1.4

1.6

1.8 3

2.0

2.2

2.4

2.6

-1

10 /T ( K )

300

400

500 T (K)

600

700

Fig. 4. The thermal variation of the thermopower of Fe2O3. Inset: The logarithm of the electrical conductivity (r) vs reciprocal temperature.

tured by Fe3+ giving Fe2+ which can be considered as neg). The levels are degenerated atively charged defects (Fe2þ Fe3þ and the donors are rather Fe2+ more or less trapped in the vicinity of oxygen vacancies. The magnitude of S and r are strongly correlated and when r is in the insulating range (<103 X1 cm1), S is generally large (>1 mV K1). S increases first moderately up to 360 K before reaching a constant value in conformity with the small conductivity. The temperature dependence of S(T) (Fig. 4) indicates a mobility le thermally activated. The large S values confirm that most charge carriers originating from the donors are bonded in surface polaron states. The negative thermopower can also be ascribed to an electron mobility greater than the hole one. The general expression of the thermopower for a two type carrier sample is given by the Chambers formula (Dordor et al., 1986): S¼

mobility le407 K (103 cm2 V1 s1), calculated from the relation (r = NDele) is typical of oxides in which the electrons must overcome a further potential barrier due to oxygen octahedra and move in a narrow cationic CB not exceeding 2 eV. So, one can anticipate that the localized energy levels of Fe2+ ions are responsible of the weak photocurrent (see below). The thermopower S is less sensitive to grain boundaries since it is measured at zero current. The negative sign confirms that the majority mobile carriers are electrons; oxygen leaves the crystal lattice and the reaction can be written according to Kroger–Vink notation: 1 OO ! O2 þ V O þ 2e 2

ð1Þ

The electrons concentration ND is based on a charge balance. Formally, each extracted oxygen donates two electrons to CB. Therefore, the charge neutrality is maintained by the fact that the electrons released by oxygen are cap-

ðS n rn þ S p rp Þ ðrn þ rp Þ

where S n ¼ enln and S p ¼ enlp     k EC  E F k EF  E v þ A ; Sp ¼  þA Sn ¼  kT kT e e

ð2Þ

ð3Þ

where rn, Sn and rp, Sp are the respective contributions of electrons and holes to conductivity and thermoelectric power. It has been reported (Warnes et al., 1984), that with electrons and holes having different mobilities, the electron is the most mobile and confers to Fe2O3 n-type character up to 850 K, above which a transition to p-type conductivity occurs. Because of our interest in using Fe2O3 for the energy storage, the resistance to corrosion is a crucial parameter to determine for the practical application. The oxide exhibits a good chemical stability in both neutral and alkaline electrolytes. The rate dissolution, measured in KCl (0.5 M) solution over two month period through the analysis of dissolved iron, was found to be only 0.7 lmol m2 month1.

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0.4

14

-2

-0.6 -0.8

-5.5 -6.0 -6.5

-1.2

-1.0

-0.8

-0.6

-0.4

-1.5

0.4

Vfb = - 0.22 V

0.2 0.0 -0.4 -0.2 0.0

4

-1.0 -0.5 Potential (V)

0.0

0.5

0.2 0.4 0.6 potential (V)

0.8

1.0

Vfb= - 0.45V

-0.2

Potential (V)

-2.0

6

0.6

2

-7.0

-1.0 -2.5

-5.0

8

2

4

-4.5

-0.4

10

+7

+7 -2 4 -2 C 10 (F cm )

R1 R2 -4.0 log Jd (mA. cm )

Jd (μA. cm-2)

0.0

0.8

2

12

Jph 10

O1 O2

-0.2

1.0 ( mA cm )

O2 0.2

719

0 -2.0

1.0

Fig. 5. The cyclic J(V) characteristic of Fe2O3 plotted in the dark under N2 bubbling. Inset: The corresponding semi-logarithmic plot. Electrolyte: NaOH (0.5 M, pH 13.1), scan rate 10 mV s1, 25 °C.

The electrokinetic parameters i.e. the exchange current density (20 lA cm2), the corrosion potential (0.995 V) and the polarization resistance (3.27 kX cm2) determined from the semi-logarithmic plot (Fig. 5, inset), corroborate such result. However, Fe2O3 suffers from the disadvantage of low quantum efficiency (g) because of the low mobility, and the small faradic rate constant at the surface (Chang et al., 1988). Thus, our strategy to improve PEC efficiency was to prepare the oxide by chemical route in order to shorten the traveled carriers’ paths below the diffusion length. Fe2O3 is an interesting material for PEC characterization and the suitable position of the electronic bands make it important to explore. The voltammogram recorded in the dark in basic solution (NaOH 0.5 M, pH  13) shows a small current density over a large potential range (Fig. 5). Additional support of the localization of Fe: 3d electrons is brought by the existence of peaks corresponding to the electrochemical couple Fe2+/3+. Two distinct peaks O1 (0.77 V) and O2 (0.64 V) are easily seen on the cyclic J(V) characteristic (inset a, Fig. 6). This indicates that Fe2+ comes from two different kinds in non-equivalent sites. FeO6 unit in a kink site has five bonds to the lattice and one “dandgling bond” which interact more easily with the solution since Fe2+ is more exposed and require smaller activation energy. It becomes less coordinate and easy to oxidize (peak O1) compared to Fe2+ octahedrally coordinated inside the crystal (peak O2). On the reverse scan, the peaks R1 (0.79 V) and R2 (1.1 V) are attributed to the reversible reduction of Fe3+ while the large current below 1.5 V is due to H2-liberation. As expected for n-type SC, the photocurrent (Jph) appears for potentials anodic of Vfb, the separation of (e/h+) pairs is achieved by the junction electric field developed across the space charge region whose length d (13.8 nm)1 is in conformity with a low density ND: 1 Calculated for a band bending of 0.5 V. The permittivity e (=50) was taken from Goodenough (1971).

-1.5

-1.0

-0.5

0.0

Potential (V) Fig. 6. The Mott Schottky characteristic of Fe2O3 in Na2SO4 (0.5 M) solution plotted at a frequency of 10 kHz. Inset: The plot of J 2ph vs applied potential.

 d¼

2ee0 ðV  V fb Þ eN D

12 ð4Þ

e and e0 are, respectively, the permittivity of the material and free space. According to the Gartner model, the photocurrent is given by (Jiang et al., 1994): J 2ph ¼ const: a2 d2 V  V fb

ð5Þ

where a is the absorption coefficient; the potential Vfb (0.22 V) is determined from the cross point of the extended plot to potential axis in the ðJ 2ph  V Þ curve (Fig. 6, inset). However, Vfb has been accurately determined from the Mott Schottky equation:   1 2 kT ð6Þ ¼  V  V  fb eee0 AN A e C 2SC where A is the surface area and the other symbols have their usual meanings. The negative slope confirms the ntype behavior of the hematite. The potential Vfb (0.45 V) and the electron density ND (1.63  1018 cm3) were determined, respectively, from the extrapolation to C2 = 0 and the slope (Fig. 6). Vfb outlines the energetic position P of a-Fe2O3-CB with respect to vacuum: P ¼ 4:75 þ eV fb  DE

ð7Þ

The P value (4.16 eV/0.59 V) is typical of materials in which CB is made up from 3d orbital. This study underlines once more the crucial role played by the band structure which determines the position of the electronic bands. The corundum structure contains layers of hexagonal close packed oxygen atoms with Fe3+ ions in two thirds of the octahedral sites. The energy of VB is at 6 eV below vacuum whereas CB, consisting of empty Fe levels, of unknown but probably narrow width, is at 4 eV. The hematite exhibits a red color and both electronic bands HOMO (VB) and LUMO (CB) are predominantly Fe-3d orbital in character with a separation characterized by the strength of the octahedral ligand field. The photocurrent efficiency (g), measured with monochromatic light, was determined by

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dividing the electron flow in the external circuit (photocurrent Jph subtracted from the dark current Jd) by the incident photon flux (u) determined with a calibration photodiode at each wavelength:

2.0

2

- Zi (kΩ cm )

ðJ ph  J d Þ eu

ð8Þ

where e is the elementary charge. The photocurrent-wavelength spectrum has been analyzed to get both the energy and type of interband transitions. The electrode was biased at +0.4 V, a value belonging to the plateau region in the Jph(V) curve. The g(k) characteristic clearly shows the existence of one wavelength range which allows the determination of Eg, in the same manner as for a in the optical transitions: ðghmÞ

2=n

¼ const: ðhm  Eg Þ

1.5

1.0

0.5

0.0

- 10.7 °

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

2

Zr (kΩ cm ) Fig. 8. Complex impedance of Fe2O3.

ð9Þ

where n equals 1 or 4, respectively, for direct or indirect transitions. Fig. 7 shows that the plot with n = 4 is linear and the intercept with hm axis yields Eg value of 1.94 eV, close to that determined previously (Glasscok et al., 2008). Due to the polycrystalline nature of Fe2O3, the barrier is modeled as double layer over the grains region. The resistance contributions are generally represented by three parallel circuits (R-C) connected en series and corresponding to the bulk, the grains boundary and the diffusion. Fig. 8 represents the real (Zr) and imaginary component (Zi) in the complex plan. At high frequency, the data show a semicircle indicating that the relaxation times of the bulk and the grains boundaries are close to each other with an impedance of 3.71 kX cm2. An overlapping is seen. The centre of the semicircle is localized below the real axis with an angle of 10.7° ascribed to a constant phase element (CPE), a single barrier of the junction Fe2O3/electrolyte confirming one relaxation time of the electrical equivalent circuit. We also note a slight offset from the origin (Rel = 17.7 X cm2) due to the ionic electrolyte (NaOH). As mentioned in the introduction, the oxygen evolution during the water photosplitting is of special interest in the

water electrolysis and there is a direct relation between the specific surface area and over-potential. Various preparations of a-Fe2O3 resulted in differences in the roughness factor (porosity) and hence in active surfaces. The smaller the over-potential, the rougher the surface and the larger the specific surface area. The oxide, in our case, has been elaborated at 400 °C by chemical way producing relatively a large active surface of 46 m2 g1. The potential of Fe2O3-VB (1.35 V) is pH insensitive owing to its cationic parentage. Hence, one would have the further advantage of being able, by changing the pH, to shift the level of O2/H2O couple over a range of 0.6 V, the latter varies as (1.23–0.06pH). At pH  8, Fe2O3-VB is located above the level of water oxidation. This hypothesis has found experimental support and the hematite has been tested positively for oxygen evolution in alkaline solution (Fig. 9). The PEC process may be kinetically enhanced in presence of electron scavenger which competes with the photodriven dissolution. However, only redox couples with Eredox > Edecom can be used; sulfite (SO2 3 ) being particularly favorable. It is chemically stable and its potential (0.82 V) is lower than the 0.5 1.4

1.2

0.4

Volume of O (cm3) 2

1.2

(ηhv)

0,5

0.8

0.4

0.3 0.8

0.2

0.6 0.4

0.1

Eg = 1.94 eV

0.0

1.0

0.2

0.0

0.0

2.0

2.4 2.8 hv (eV)

τ (%)

g¼

2.5

3.2

3.6

Fig. 7. Determination of the indirect band gap of Fe2O3.

0

10

20

30

40

50

60

Fig. 9. Time course of oxygen evolution for Fe2O3 in SO2 (pH 8) 3 electrolyte.

S. Boumaza et al. / Solar Energy 84 (2010) 715–721

photodecomposition potential Ed (1 V), calculated from the reaction (Fe2O3 + 6H2O + 6h+ ? 2+ Fe(OH)3 + 6H + 3/2O2). For direct storage, the two half electrochemical reactions have to proceed in opposite directions and the stored energy (213.36 kJ mol1) is the difference between the free energies of involved couples 2 and O2/H2O. The oxygen evolves namely SO2 3 =S2 O3 mostly on Fe2O3 with the yield s (%). Over time, the volume tends toward saturation. The competitive oxidation with oxygen formation is therefore thought to of S2 O2 3 be the main reason for the curvature V(O2)-time. The present work suggests an attractive way of chemical storage of solar energy in the system. 2 þ ð2SO2 3 þ 2H ! S2 O3 þ O2 þ H2 OÞ

4. Conclusion The hematite a-Fe2O3 emerged as a promising cheap photoanode. The electrical properties were enhanced by a deviation from the stoichiometry inducing variable iron valences. The chemical analysis revealed an oxygen deficiency and the oxide is a low mobility, hopping n-type semiconductor. The thermopower and the capacitance measurement confirmed the n-type conductivity. The optical gap and the chemical stability make a-Fe2O3 attractive for the solar conversion. It has been characterized photoelectrochemically with an optical absorption at 1.94 eV. The difference in oxygen over-potential is a consequence of a roughness of the electrode surface. The electronic bands are pH insensitive and this property has been exploited. At basic pH, the valence band is suitably positioned and the oxide has been successfully tested for oxygen evolution under visible light and ensures chemical storage. Acknowledgments The authors are grateful to Dr. S. Omeiri for his technical assistance and Dr. T. Bourezgue for critically reading the manuscript. This work was supported financially by the Faculty of Chemistry (Algiers). References Aroutiounian, V.M., Arakelyan, V.M., Shahnazaryan, G.E., Stepanyan, G.M., Khachaturyan, E.A., Wang, H., Turner, J.A., 2006. Photoelectrochemistry of semiconductor electrodes made of solid solutions in the system Fe2O3–Nb2O5. Sol. Energy 80 (9), 1098–1111. Aroutiounian, V.M., Arakelyan, V.M., Shahnazaryan, G.E., Hovhannisyan, H.R., Wang, H., Turner, J.A., 2007. Photoelectrochemistry of tindoped iron oxide electrodes. Sol. Energy 81 (11), 1369–1376. Barrero, C.A., Arpe, J., Sileo, E., Sa´nchez, L.C., Zysler, R., Saragovi, C., 2004. Ni- and Zn-doped hematite obtained by combustion of mixed metal oxinates. Physica B 354, 27–34. Beydoun, D., Amal, R., Low, G.K.-C., Mc Evoy, S., 2000. Novel photocatalyst: titania-coated magnetite: activity and photodissolution. J. Phys. Chem. B 104 (18), 4387–4396.

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