Physical and photo-electrochemical characterizations of α-Fe2O3. Application for hydrogen production

international journal of hydrogen energy 34 (2009) 4268–4274

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Physical and photo-electrochemical characterizations of a-Fe2O3. Application for hydrogen production A. Boudjemaaa,b, S. Boumazaa,b, M. Traric, R. Bouarabb, A. Bougueliac,* a

Technical and Scientific Research Centre of Physical Analysis (CRAPC), BP 248, RP 16004 Algiers, Algeria 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 b

article info

abstract

Article history:

The optical, electrical and photo-electrochemical properties of dense hematite a-Fe2O3

Received 2 February 2009

have been studied for the photo-catalytic hydrogen production. The band gap was evalu-

Received in revised form

ated at 1.96 eV from the diffuse reflectance spectrum and the transition is directly allowed;

23 March 2009

further indirect transition occurs at 2.04 eV. The oxygen deficiency permits the altering of

Accepted 25 March 2009

the transport properties and the oxide exhibits n type behavior with activation energy of

Available online 18 April 2009

0.11 eV. a-Fe2O3 is found to be photo-electrochemically active. The flat band potential Vfb (0.51 VSCE) and the density ND (19.12  1019 cm3) were obtained respectively by extrap-

Keywords:

olating the linear part to C2 ¼ 0 and the slope of the Mott–Schottky plot. The complex

Hematite a-Fe2O3

impedance pattern is circular in the high frequency region followed by a straight line in the

Photo catalytic

low frequency one, a behavior attributed to the Warburg ionic diffusion. The conduction

Hydrogen

band edge (0.62 VSCE) lies below the H2O/H2 level (0.50 VSCE) and Fe2O3 offers the

Mott–Schottky plot

possibility to be used as hydrogen photocathode. The best activity was obtained in SO2 3

Complex independence

(0.5 M, pH 13.8) solution with a rate evolution of 6 ml (g catalyst)1 min1. ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Our research program aims to study the properties of potentially useful materials for the solar energy conversion and particularly for the water photo-splitting [1,2]. Accordingly, considerable interest is focused on developing low cost SC for the PEC devices [3]. Generally, the stable oxides are high energy band SC that absorb only in the UV region, ruling out the exploitation of direct sunlight. By contrast, narrow band gap SC are either chemically instable or have a low energetic position of CB and would therefore require external bias assistance for the water photo-reduction [4]. This result from the deep lying VB deriving mainly from O2: 2p orbital which is constant in energy, regardless of the nature of the metal

whereas CB is from cationic character. For n type SC to be used in photo-electrochemistry, a large band bending at the interface is needed and this imposes a cathodic potential Vfb. Some general principles appeared for the choice of satisfactory materials in photoelectrochemistry and for oxides with partially filled d-levels, the potential Vfb and the gap Eg are two contradictory features which obey the empirical relation: Vfb ðVSCE Þ ¼ 2:70  ðEg ðeVÞÞ=e [5,6]. Hence, more negative is the potential Vfb, the larger is the gap and up to now, no oxide having the required characteristics has been found. To overcome this drawback, our strategy is to use materials with weak electro negativity and whose electronic bands derive from cationic parentage [7,8]. So, our investigation has been oriented toward oxides with new crystalline

* Corresponding author. Tel.: þ213 21 24 79 50; fax: þ213 21 24 80 08. E-mail address: [email protected] (A. Bouguelia). 0360-3199/$ – see front matter ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.03.044

international journal of hydrogen energy 34 (2009) 4268–4274

List of symbols A BET CB C e Eg EIS h Jph k L ND OCP PEC S SC SCE

the surface of the electrode Brunauer, Emmet et Teller conduction band capacitance the electron charge band gap electrochemical impedance spectroscopy Planck constant photocurrent Boltzmann constant (1.38  1023 J K1) diffusion length electron density open circuit potential photo-electrochemical thermopower semiconductor saturated calomel electrode

structure. Although this strategy has not been fully explored in photo-electrochemistry, a less electronegative cation with high spin d5 configuration may be used for this approach. The hematite a-Fe2O3 is one of the number of so called Al2O3 corundum structure and is an active catalyst for many important reactions like the dehydrogenation of ethyl benzene to styrene [9]. Recently, it has been considered as low cost-effective semiconductor for photocatalysis [10]. It is known that the transport properties of binary oxides are modified by deviation from the stoichiometry and hematite occurs in n- as well as p-types [11]. In choosing Fe2O3 as basic material, we have primarily taken into consideration the existence of mixed iron valences in octahedral sites. The oxygen deficiency decreases the formal charge of iron and the oxide possesses interesting properties many of which are associated to mixed states Fe2þ/3þ. The treatise of p type will be deferred to a next paper on the photo-catalytic oxygen evolution. One of the most interesting aspects of the corundum is the band structure. Both bands VB and CB are made up of mainly Fe3þ: 3d wave function separated by a gap of w2 eV [12]. They are pH insensitive and one of them presents the further advantage of being adequately positioned with respect to redox levels in solution. The hematite has received considerable attention for the solar energy conversion due to its high resistivity to corrosion and moderate gap; it can capture w40% of the solar light. In addition, Fe is by far most attractive from an ecological aspect as well as an economical standpoint. On the other hand, it is worth noting that although the physical properties were actively studied, the photo-electrochemistry has not yet been fully explored. Hydrogen is a clean energy carrier and provides an ideal way of storing solar energy. Nowadays, the most common production is based on the methane reforming process which releases large amount of CO2 in the atmosphere. Its production by a photo-catalytic process has attracted world wide attention and it is taking a mounting position due to the

T V VB Vfb Von W XRD DE b me a l rexp q s s0 30 3

4269

absolute temperature applied potential valence band flat band potential photocurrent onset potential depletion width X-ray diffraction activation energy the broadening of the XRD line electron mobility optical absorption coefficient the wavelength of CuKa radiation experimental density diffraction angle (radians) conductivity pre-exponential term permittivity of vacuum permittivity of the material

energy resources. The present paper deals with the transport properties and the PEC characterization of Fe2O3 and its application for photo-catalytic hydrogen production.

2.

Experimental

Fe2O3 (Merck, purity 99%) was cold pressed into pellets and annealed at 673 K in air for 12 h; the compactness approximates 60%. The crystalline phase was confirmed by XRD using a Phillips PW 1710 diffractometer with a copper anticathode. The lattice constants were refined by the least square method from corrected d values. The density rexp (5.04 g cm3), measured picnometrically, is close to the theoretical one rtheo (5.14). The porous structure (specific surface area) was determined via nitrogen adsorption isotherm at 77 K. The diffuse reflectance spectrum, which goes from 200 to 800 nm, was obtained on a Jasco V-530 spectrophotometer equipped with MgO coated integrating-sphere. The conductivity data were collected by 5 K-step measurements in the temperature range of 300–700 K using the two probe technique. The thermopower S (¼DV/DT ) was measured on thick sintered pellets; the emf DV was determined by a digital micro voltmeter (Tacussel, Aris 20,000) with an impedance of 1012 U while the temperature gradient DT was determined with a thermo couple (type K). PEC characterization was carried out at room temperature after standing overnight in the working electrode (A ¼ 0.12 cm2) under zero current flow. A platinum auxiliary electrode (Tacussel, 1 cm2) was used as auxiliary electrode and all potential were scaled against SCE. The electrolyte KOH (0.5 M) and Na2SO4 (0.33 M), with the same ionic strength 3.5 mol/L, were continually flushed with nitrogen. The J(V) characteristics were plotted with a potentiostat/galvanostat (PGZ301 Voltalab) interfaced to a computer controlled data acquisition system. The Mott–Schottky analysis was made with a voltage of 10 mV (peak-to-peak) at a frequency of 10 kHz. EIS has been performed on the frequency range

international journal of hydrogen energy 34 (2009) 4268–4274

(45 Hz–5 MHz). Monochromatic light was obtained by passing light from 650 W Halogen lamp through a series of filters (PHYWE). The (Jph  Uph) plots were measured in a two electrode cell with external resistance boxes and two multimeters. The pzzp has been accurately determined by measuring the equilibrium pH of aqueous solution containing a dispersion of powdered sample. Metal oxide is added to aqueous solutions of known pH with no other absorbable species. The pH of the solution will drift on addition of the powder toward pzzp. At this point, the pH of the solution becomes independent of the amount of the powder added. For the corrosion test, 10 mg of Fe2O3 was soaked under nitrogen atmosphere in 30 mL of neutral solution (Na2SO4, 0.1 M) for three weeks; the generated Fe2þ has been titrated by KMnO4 in acidic medium. The photocatalytic tests were carried out in a double walled laboratory scale cylindrical Pyrex reactor whose temperature was regulated at 50  0.1  C by a circulating water bath (Polystat– Fisher). The solution (KOH 0.5 M, Na2SO3 0.025 M) was flushed free of oxygen with ultrapure nitrogen for 35 min. After temperature stabilization, the powder suspension (250 mg catalyst/200 ml solution) was exposed under constant magnetic stirring to three tungsten lamps (200 W, Osram), providing an intensity of 29 mW cm2. Evolved hydrogen has been identified by gas chromatography and the amount was determined volumetrically with a water manometer. All the chemicals were of analytical grade and the solutions prepared in doubly distilled water.

3.

Results and discussion

3.1.

Structural properties

The XRD pattern of our oxide, depicted in Fig. 1, shows that all the peaks belong to the hematite phase according to the JCPDS card No 89-0598. However, the diffraction peaks shifted slightly to higher 2q angles with respect to Fe2O3. The oxygen

400 350

(αhν)2 (eV2 cm-2)

4270

300 250 200 150 100 Eg = 1.96 eV

50 0 1.8

2.0

2.4

2.6

2.8

3.0

3.2

hν (eV) Fig. 2 – Determination of the direct allowed band gap of Fe2O3.

departure leads to expansion of the unit cell volume (0.018%). This simply can be attributed to larger ions Fe2þ in octahedral environment generated by a charge compensating process compared to Fe3þ. The parameters deduced from the pattern, a ¼ 503.0 and c ¼ 136.9 pm are in good agreement with those given in the JCPDS card. The mean crystallite size D (25 nm) was determined from the width b at half maximum of the most XRD intense line (104) through the Scherrer formula: D ¼ 0.94 l(b cos q)1. The specific surface area (43 m2 g1), deduced from the relation (6/rexpD), is in perfect agreement with that obtained from BET (46 m2 g1).

3.2.

Optical and electrical properties

The optical gap and the band edges position play a crucial role in the photo-catalysis. The diffuse reflectance spectrum of Fe2O3 exhibits a broad absorption in the visible region, extending up the UV region, the shoulder in the spectrum accounts for the red color of Fe2O3. Fig. 2 clearly shows the existence of one wavelength range which allows the determination of the inter band transitions in a crystallized SC, close to the band energy gap varies as: n

ðahnÞ ¼ const: hn  Eg

Fig. 1 – XRD pattern of a-Fe2O3 annealed in air at 673 K.

2.2




(1)

where n equals 2 or 0.5 respectively for direct or indirect transitions. The plot with n ¼ 2 is linear and the intercept with hn axis yields a direct optical gap Eg of 1.96 eV, close to that determined previously [13]. a-Fe2O3 crystallizes in hcp array of oxygen atoms in which Fe3þ ions occupy two-thirds of the octahedral sites. The defect structure of Fe2O3 has been the subject of numerous studies [14]. The Fe–Fe separation (0.289 nm), is larger than the critical interatomic length (0.225 nm) below which collective behavior is expected and the oxide exhibits a semi-conducting behavior. However, the properties are sensitive to the deviation from the stoichiometry and the correspondence between the electrical data from various samples is poor. In our case, the conduction in Fe2O3 is likely to take place by electrons hopping from Fe2þ to higher valent state Fe3þ through octahedra sharing common corners. Such transfer involves high activation energy DE which could be possibly be made only at

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international journal of hydrogen energy 34 (2009) 4268–4274

Fe2þ/3þ and confirms the localization of outer Fe: 3d electrons in conformity with the low conductivity. On the reverse scan, the reduction process is not observed until 0.5 V:

T (K) 350

400

450

500

550

0.0 -0.4

Fe2O3 þ 2deþdH2O / Fe2O3d þ 2dOH

0,5

-1.2 0,0 -1.6

-0,5 -2.0

EA = 0,11 eV

J2ph ¼ const: a2 W2 ðV  Von Þ

-1,0 1,5

2,0

2,5

3,0

3,5

The plot of intercepts the x-axis at the potential Von equal to 0.72 V (Fig. 4, Inset).

Fig. 3 – Logarithm of the electrical conductivity vs. reciprocal temperature and thermal variation of the thermopower of Fe2O3.

3.3.1.

(2)

s0 is temperature-independent. The large DE value (0.11 eV) is due to the formation of barriers at grains boundaries. The system is electrically neutral and the free electrons mainly arise from oxygen vacancies whose associated electronic energy level lie below the bottom of CB: 

(3)

DE is very close to that obtained in Ref. [15] although the latter was more compact (synthesis temperature at 1300  C). The mobility me (¼0.46 cm2 V1 s1), calculated from the relation s ¼ meND is typical of oxides in which the conduction band derives from narrow cationic 3d-orbital. The thermal variation of S is given in Fig. 3. S is negative indicating that the majority carriers are electrons and increases up 350 K before reaching saturation. In such a case, the conduction mechanism occurs predominantly by small polaron hopping through mixed valence Fe3þ/2þ. The constancy of S with T indicates that the density ND is thermally activated rather than the mobility [16].

3.3.

Determination of Vfb, L and ND

The potential Vfb can be accurately determined from the capacitance measurement [18]:

high temperature. s increases with increasing temperature (Fig. 3) indicating semi conducting-like behavior and follows an Arrhenius type law:

Oo 4 1⁄2 O2 þ Vo þ 2e

(5)

J2ph

1000/T (K-1)

s ¼ s0 expð  DE=kTÞ

(4)

The current increases steeply below 1 V due to H2 evolution (gas bubbles are noticeable on the electrode). The PEC behavior was elucidated by plotting the J(V) characteristic under illumination. The increase of the photocurrent Jph along the anodic polarization confirms n type conductivity. The potential Von has been determined from the relation [17]:

Photoelectrochemistry

Fe2O3 is chemically stable over a broad pH range and the measurements were done in both KOH (0.5 M, pH 13.8) and Na2SO4 (0.33 M, pH 6.8) electrolytes in order to keep the same ionic force. The overpotential-current characterization in Na2SO4 enabled the corrosion rate to be calculated from the Stern–Geary equation. The value of the exchange current density (0.21 mA cm2), the corrosion potential (0.82 V) and the polarization resistance (155 U cm2) indicate a long lived material. In the anodic region, the oxidation peak at þ0.5 V in the J(V) curve, is attributed to the electrochemical couple

  1 2 kT V  Vfb  ¼ 2 C e330 AND e

(6)

The permittivity 3 (¼50) was taken from Ref. [19]. The potential Vfb (0.51 V) and the density ND (1.91  1019 cm3) are provided from the x-axis intercept and the slope of the straight line (Fig. 4). Vfb is about 0.2 V less negative than reported by Aroutiounian et al. [15]. The positive slope lends a further support of n type character. The potential Vfb is not or only slightly dependent on pH and lies below the H2O/H2 level (0.54 V); some results, concerning the flat band potential, are contrary to those of previous reports [20] where the authors claim that the potential Vfb varies by 0.06 V pH1. Vfb outlines the energetic position of CB in the electrochemical scale:   P ¼ 4:75 þ eVfb þ 0:056 pH  pHpzzp þ DE

(7)

12 0.4

10

8

Jph2 (mA2)

-0.8

C-2x10-22 (F-2 cm-4)

log σ (Ω-1 cm-1)

1,0

S (mV K-1)

1,5

6

0.3

0.2 Von = -0.72 mV

0.1 -0.80

4

-0.75

-0.70

-0.65

-0.60

V (mV)

2

Vfb = -0.51V

0 -1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

Potential (V) Fig. 4 – The Mott–Schottky plot of Fe2O3 electrode in KOH (0.5 M). Inset: relationship of J2ph vs. applied potential under full light.

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international journal of hydrogen energy 34 (2009) 4268–4274

0:5  230 3ðV  Vfb Þ eND

Jph  Jd expðaW0 Þ ¼1 ef0 1 þ aL

(9)

both the length L and the coefficient a are function of l. The hl  V characteristics are given for four wavelengths in the range of 0.2 V < V < þ 1.5 V. The combination of Eqs. (8) and (9) gives:  lnð1  hÞ ¼ a

230 3 eND

0:5

 0:5 V  Vfb þlnð1 þ aLÞ

(10)

eq. 10 is valid further away from Vfb with a zero (e  hþ) recombination. The values of a and L (Table 1) were obtained respectively from the intercept of linear plots of ln(1  h) vs. (V  Vfb)1/2 and the slope (Fig. 5). The shape of the curve, and particularly the smooth increase of Jph above 0.5 V can be described by the Gartner model [22]. At high potential, the plot is linear but when approaching Vfb, it deviates from the straight line suggesting the inapplicability of the model at low band bending because of the easy recombination process. The highest value of a is obtained at lmax (480 nm) (Fig. 6), the corresponding penetration depth a1 (71 mm) is higher than W (12 nm) and L (2 nm). This indicates that most of the incident light (99.7%) falls in the bulk of Fe2O3 and only 0.25% in the

Table 1 – Various semi-conducing parameters of anodic oxide. l (nm) 346 420 441 485

346 nm

0.10

441 nm 420 nm 0.05

(8)

The large W value (12 nm), for a band bending (V–Vfb) of 0.5 V, is due to the low density ND. The width W can be increased by biasing the electrode with a redox couple having a high potential close to VB so that more light could be absorbed being within the depletion layer, the hole scavenger SO2 3 particularly suitable. The recombination of (e  hþ) pairs must be kept at a low level and this can be achieved only if the thickness W is smaller than the diffusion length L and the size D. In the absence of recombination process, the quantum yield (hl) is given by Ref. [21]: hl ¼

485 nm

a (cm1)

L (nm)

317.09 865.35 6.64 139.45

42 4 105 2

l is the wavelength of incident light; a is the absorption coefficient of the oxide; L is the diffusion length of the minority carrier.

0.00 0.50

0.52

0.54

0.56

0.58

0.60

0.62

(V- Vfb)1/2 (V1/2) Fig. 5 – Plot of Lln(1 L h) vs (V L Vfb) for determination of various semi-conducting parameters of Fe2O3 in KOH (0.5 M). depletion layer and explains the lost by recombination of (e/hþ) pairs before reaching the electric field developed across the space charge region. This situation looks similar to that obtained on PbO [22] EIS of the junction Fe2O3/solution, measured at OCP (0.283 V) in the frequency range (0.1–105 Hz), allows to distinguish between the various conduction mechanisms. The exchange of majority carriers through the interface is represented by the Nyquist plot (Fig. 7) whose nature confirms the predominance of the bulk contribution; such behavior is similar to that observed by Aroutiounian et al. [23,24] on Fe2O3-Ta doped ceramic. The arc at high frequencies is attributed to a faradic charge transfer and the extrapolation with the x-axis leads to the resistance Rdl (2.89 kU cm2). We also noted a slight offset near the origin indicating a low series resistance attributed to the electrolyte Rel (24 U cm2). The experimental data were refined by the least square method thanks to the Randles model. The centre of the arc is localized below the real axis with an angle of 6 . This slight depletion, expressed by CPE, is due to the inhomogeneity of the electrode 0.8 0.7

Quantum yield η ( )

W¼

0.15

-ln (1-η)

The P value (6.1 eV) indicates that CB is made up primarily from Fe: 3d-orbital. Fe2O3 would appear to be a promising SC for photo-catalytic applications. However, the relatively low fill factor (0.28) evaluated from the power characteristic (Uph  Jph) indicates that the pellet remains highly resistive owing to the small grain size and grain boundaries. The depletion length (W ) occurs when SC is brought in contact with the electrolyte. A Schottky barrier is formed in the space charge region by the adjustment to equilibrium with the redox couple in solution. The amount of band bending occurring within the length W is given by

0.6 0.5 0.4 0.3 0.2 0.1 0.0 350

400

450

500

550

600

650

Wavelength (nm) Fig. 6 – Quantum yield of Fe2O3 electrode polarized at 0.42 V in KOH (0.5 M) electrolyte.

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international journal of hydrogen energy 34 (2009) 4268–4274

which is at the origin of the dispersion of time constants of the circuit RC. Such behavior can be due to various phenomena like surface states, the porosity of the electrode, the distribution of current and potential field as well as slow adsorption [25]. The straight line at low frequencies is attributed to the diffusion of electro active species in the solution known as the Warburg impedance (Zw), which indicates that the anionic diffusion prevails over the electron transfer. Since the ionic diffusion and the heterogeneous charge transfer occur as successive mechanisms, the equivalent electrical circuit (Fig. 7, Inset) is composed of the impedance ZW in series with the resistance Rdl which in turn are connected in parallel with CPE.

3.3.2.

Energy (eV) 0

CB (-0.62 V) ΔE = 0.11 eV - 4.75

Eg = 1.96 eV

Hydrogen

VB (1.34 V)

We have taken advantage of the pH-insensitivity of the potential Vfb and at pH 6.5, Fe2O3-CB is energetically higher than the reduction potential of water (0.50 V), determined from the J(V) plot. Hence, the hematite has been successfully tested for the H2 production under visible light. The environmentally friendly production of hydrogen from water using solar energy offers a great potential for reducing green house gas emission such as CO2. Taking into account the large over potentials which exceed 0.5 V, the gap of Fe2O3 does not allow the water photoelectrolysis. Therefore, hydrogen evolution is combined with SO2 3 oxidation in order to evaluate the PEC performance. The energy band diagram of the junction (Fig. 8), predicts that Fe2O3 is able to produce hydrogen in neutral solution under visible irradiation using Na2SO3 as supporting electrolyte. The choice of SO2 3 , with potential of 0.51 V, allows an extension of the width W by increasing the band bending and consequently a decrease of (e  hþ) pairs. Evolved hydrogen increases monotonically with irradiation 2 time. SO2 3 oxidation to S2O6 takes place via hole injection in VB located at þ1.34 V:

Fe2O3 þhn / eCB þ hþVB

(11)

2 þ 2 SO2 3 þ 2 h / S2O6

(12)

Potential (V/SCE) n-Fe2O3

KOH electrolyte

Fig. 8 – The energy band diagram of Fe2O3/electrolyte junction.

2  þ SO2 3 þ 2OH þ 2 h / SO4 þ H2O

(13)

2H2O þ 2e / H2 þ 2OH

(14)

We emphasis that VB is located above the potential of 2 couple (1.17 V) and consequently the reactions SO2 4 /SO3 (12) and (13) proceed successively and not in parallel. The evolution rate, 6 ml (g catalyst)1 min1, evaluated from the linear plot progressively decreases and the volume reaches saturation after w1 h probably because of the competitive reduction of S2O2 6 . The quantum efficiency (s) of the light conversion into hydrogen is given by:  s ¼ 2 number of H2 mol: s1 =photons flux s1

6

0,8

CPE

2.8 Rel

- Zi (kΩ cm2)

2.0 Rel

1.6

ZW

1.2 0.8

0,6 4 0,4

2

τ( )

Volume of H2 (cm3)

2.4

0,2

0.4 0,0 0

0.0

6° 0.0

0.5

1.0

1.5

0 2.0

2.5

3.0

Zr (kΩ cm2) Fig. 7 – Complex impedance of Fe2O3. Inset: the corresponding equivalent circuit.

3.5

4.0

20

40

60

80

100

120

Time (min) Fig. 9 – Rate of H2 evolution and the corresponding yield (s) (0.025 M)/KOH (0.5 M) electrolyte under on Fe2O3 in SO2L 3 nitrogen atmosphere, 302 K.

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The factor 2 enters in the relation because H2 formation is two electron events. Fig. 9 shows the H2 evolution from water with photocatalyst suspension. The depletion width W and the low hole diffusion length L compared to the very large length penetration depth are responsible of the low conversion.

4.

Conclusion

Fe2O3 has been investigated as low cost photoanode material. The presence of iron mixed valences is responsible of enhanced transport properties and this work aims at investigating the electrical properties and PEC characterization of a-Fe2O3d. The oxide is a low mobility polaron semiconductor, which exhibits n type conductivity originating from oxygen deficiency. The conductivity is thermally activated and the electron transport occurs predominantly by electron hopping. An approach of ascertaining the intrinsic photoactivity is to make use of reducing ability of the conduction band deriving from cationic character. It is for this property that Fe2O3 has been exploited for the water reduction. Fe2O3-CB is more cathodic than the level of H2O/H2 and the energy band diagram should lead to a spontaneous H2 reduction. This prediction has found an experimental support and a-Fe2O3 has been tested successfully for H2 evolution.

Acknowledgments The authors would like to acknowledge Dr S. Omeiri for his help and suggestions regarding this work. They are also pleased to show their deep thanks to T. Bourezgue for his carefully reading the manuscript.

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