- Email: [email protected]
TiO2 under visible light
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 7 5 3 3 e1 7 5 3 9
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Photocatalytic hydrogen evolution on the hetero-system polypyrrol/TiO2 under visible light C. Belabed a,*, N. Haine a, Z. Benabdelghani b, B. Bellal b, M. Trari b a
Laboratory of Materials Physic, Faculty of Physic (USTHB) BP 32, 16111, Algiers, Algeria Laboratory of Storage and Valorization of Renewable Energies, Faculty of Chemistry, U.S.T.H.B., BP 32, 16111, Algiers, Algeria
b
article info
abstract
Article history:
The semiconducting properties of the hetero-system polypyrrol (PPy)/TiO2 are investigated
Received 11 June 2014
for the first time to assess its photo catalytic hydrogen evolution under visible light. The
Received in revised form
hetero-system PPy/TiO2 is stable up to ~345 K, with a weight loss accounting for ~1.2%. The X-
18 August 2014
ray diffraction shows mixed phases with the rutile TiO2 variety. Optical transitions at 0.42
Accepted 21 August 2014
and 3.01 eV, directly allowed, are determined for PPy and TiO2 respectively. The p-type
Available online 17 September 2014
conductivity of PPy is evidenced from the MotteSchottky plot; In neutral solution, a flat band potential of 0.11 VSCE and a holes density of 4.57 1021 cm-3 are obtained. The electro-
Keywords:
chemical impedance spectroscopy, measured over a wide frequency range (1 mHze105 Hz),
Polypyrrol (PPy)
reveals the contribution of the bulk and grain boundaries with a constant phase element
TiO2
(CPE). The energetic diagram predicts the electron injection from PPy into TiO2 and the best
Hetero-system
photoactivity is achieved at pH ~7. A hydrogen liberation rate of 1.15 mmol h1 (g catalyst)1
Photo-electrochemical
and a quantum efficiency of 0.17% under full light (29 mW cm2) are determined in presence
Electrochemical impedance
S2 O2 3 . The photoactivity is completely restored during the second cycle, indicating a zero
spectroscopy
deactivation effect.
Hydrogen
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Fossil energy resources will soon start declining and the solar energy is being considered as attractive which meets a growing demand in many fields like the photovoltaic [1] thermal conversion [2] photocatalysis [3] and environmental protection [4,5]. With an average insulation of 1250 W m2 and 3500 h year1 (South Algeria), the solar energy is an inexhaustible energetic source [6]. However, this energy is intermittent and can be useful only if convenient methods of storage are developed. In this respect, hydrogen is a clean fuel,
suitable for the chemical storage [7,8] and possesses a high energy capacity per unit masse (28,900 cal g1). It can be stored during the off hours and released on demand. The water splitting into hydrogen and oxygen can be achieved either thermally [9] or photocatalytically [10,11]. In the former case, only a small amount of water vapor can be dissociated at temperature exceeding 2700 K by using solar concentrators. In addition, the gases separation renders the process more complex and the mixture must be quenched to preclude their recombination. By contrast, the water photoelectrolysis works under mild operating conditions with no special set up. However, the semiconductor (SC) oxides like
* Corresponding author. Tel.: þ213 21 24 79 55; fax: þ213 21 24 80 08. E-mail address: [email protected] (C. Belabed). http://dx.doi.org/10.1016/j.ijhydene.2014.08.107 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 7 5 3 3 e1 7 5 3 9
SnO2, ZnO and TiO2 used in the photo-electrochemical (PEC) conversion, are chemically stable against photo-corrosion but possess a large gap (Eg > 3 eV) and are consequently unattractive for the exploitation of the solar energy which contains only 5% of UV light [12,13]. Accordingly, active research is oriented toward the organic semiconductors (SCs) [14,15]. In this respect, the conducting polymers have attracted a great interest from both academic and industrial points of views over the last decade [16]. Such materials not only work in a similar way to inorganic SCs but have further advantages like environmental friendliness, easy synthesis and light weight. The use of polymers as photoelectrodes is of increasing interest and polypyrrole (PPy) is promising for the solar energy conversion [17]. The chemical stability over a fair pH range and the position of the conduction band (CB) below the level of hydrogen generation make PPy attractive for the water reduction. Moreover, PPy is relatively low cost and has a small optical gap (Eg) allowing the capture of a large part of the solar spectrum. Unlike physical properties, the PEC characterization of PPy has been little investigated [18] and to our knowledge, the hydrogen photoevolution has not been reported before now. Conducting PPy is sensitive to the pH of the solution and the PPy chain undergoes a deprotonation in basic solutions (pH ~ 9e11); it converts in acids (pH ~ 2e4) into insulator quinoid accompanied by the deintercalation of counter anions where the chain becomes protonated with however a small increase of the electrical conductivity. On the other hand, the photocatalysis can be significantly enhanced on hetero-systems, by coupling narrow and wide band gap SCs [19e21]. The synergy between the properties of each material results in enhanced performance of composite SCs under visible illumination. TiO2 absorbs UV light (l < 380 nm) and shows little activity under sunlight but can be used as support for PPy. The present work is devoted to the PEC characterization of the hetero-system PPy/TiO2 prepared by chemical route and its application for the hydrogen evolution upon visible illumination.
nanopowder. The UVeVisible spectra are recorded with a double beam spectrophotometer (Specord Plus 2000), equipped with an integrating sphere, PTFE is used as standard. The powder (~200 mg) is cold pressed into pellets under a pressure of 5 102 MPa and the mechanical properties are good. In order to minimize the contact resistance, silver paint is deposited on the back pellets which are mounted in glass holders to give a projected surface area of 1.32 cm2. The PEC characterization is performed in aqueous electrolyte under potentiostatic conditions using a standard cell with a Pt counter electrode. The electrode potential is monitored by a computer controlled PGZ301 potentiostat (Radiometer analytical) and given with respect to a saturated calomel electrode (SCE). An inert atmosphere is maintained by passing nitrogen during the electrochemical measurements. The variation of the interfacial capacitance is measured at 10 kHz. The complex impedance spectroscopy is acquired from small amplitude wave signals with a frequency response analyzer over the range (102e105 Hz). The photocatalytic experiments are performed in a Pyrex reactor equipped with a cooling system; the temperature is regulated at 323 ± 1 K thanks to a thermo-stated bath (Julabo); above 323 K, the loss by vaporization becomes important. 200 mg of PPy/TiO2 powder are suspended in 250 mL of 2 or S2 O2 M) by electrolyte containing X2 (¼S2 O2 3 3 , 10 magnetic agitation (210 rpm). The pH is adjusted by addition of KOH or HCl. Prior each test, nitrogen is bubbled over the solution for 35 min with a constant rate of 10 mL mL1. Visible light is produced by three tungsten lamps (200 W) disposed symmetrically around the reactor, providing a total intensity of 29 mW cm2 (2 1019 photons s1). Hydrogen is identified by gas chromatography (Shimadzu IGC 121 ML); traces of water are also detected. The volume is collected with a water manometer in an inverted burette through water displacement. Blank tests are performed and no hydrogen is liberated in the dark. All solutions are made up of reagents grade chemicals in distilled water (0.8 MU-cm). The experiments are repeated three times with very reproducible results (±2%).
Experimental The hetero-system PPy/TiO2 is prepared by chemical oxidation: 0.2 mol of pyrrole is dissolved in 150 mL of water containing 0.25 g of TiO2 (Sigma Aldrich, 99.5%). The required amount of H2SO4 (1 M) is slowly added to the solution under stirring for 30 min. Then, 50 mL of (NH4)2S2O8, (0.2 M), are added drop wise to the pyrrole solution. The reaction occurs under magnetic agitation during 3 h at 292 K; the temperature is controlled by a thermostated bath. The precipitate PPy/TiO2 is filtered, washed with water till the test with barium becomes negative and dried at 323 K for several days under vacuum. Thermal analysis (TG) is carried out with a Q 500 thermoanalyzer at a heating rate of 10 K min1. The phases are identified by X-ray diffraction using monochromatized Cu Ka radiation (l ¼ 0.15418 nm) at a scan rate of 2 (2q) min1; the patterns are compared to the powder database. The crystallite size of TiO2 (50.9 nm) is evaluated from the full width at half maximum of the diffraction peak (110), yielding a
Fig. 1 e XRD patterns of the hetero-system PPy/TiO2 prepared by chemical route.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 7 5 3 3 e1 7 5 3 9
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Results and discussion The XRD pattern shows two phases system (Fig. 1); the broad band at low angles (12.4 ) is characteristic of PPy while all other peaks are assigned to TiO2 (rutile phase) in agreement with the JCPDS Card N 71-0650. The peaks of TiO2 are narrow indicating a well crystallized oxide. The TG plot (Fig. 2) is carried out in order to delimit the thermal stability of the hetero-system PPy/TiO2. The decomposition, accounting for a small weight loss (~1.2%), occurs over a large temperature range; it starts at ~ 340 K and ends at ~ 600 K with an inflexion point at ~ 410 K. By contrast, the TG plot of PPY, exhibits a drastic weight loss (~60%) at 450 K and the weight levels off beyond 540 K, due the PPY thermal degradation as reported in our previous work [22]. The optical properties of the hetero-system are important in photocatalysis and are determined from the diffuse reflectance spectra. The relation between the absorption coefficient (a) and incident photon energy (hn) is given by the Tauc relation: m
ðahvÞ ¼ Cðhv Eg
(1)
C is a constant, the exponent m depends on the transition type, m ¼ 2 and 1/2 respectively for direct or indirect transitions. The intercept of the linear plot (ahn)2 of the heterosystem with the hn-axis (Fig. 3) yields two direct optical transitions at 0.42 and 3.01 eV respectively for PPy and TiO2, in conformity with the mixed phases. The band structure of PPy consists of a lowest unoccupied molecular orbital (LUMO) which constitutes the valence band (VB) separated by a forbidden band (Eg) from the highest occupied molecular orbital (HUMO: conduction band, CB), thus referring to p / p* transition [23,24]. The direct transition of TiO2 is due to the charge transfer O2: 2p / Ti4þ: 3d. Frequency dependence of AC conductivity (s) of the heterosystem shows a degenerate behavior with a moderate conductivity (~0.1 U cm) over the narrow stability range and is
Fig. 2 e TG plot and the derivate curve of the hetero-system PPy/TiO2.
Fig. 3 e The direct optical transitions of the hetero-system PPy/TiO2.
slightly frequency dependent (Fig. 4). The curve indicates a conduction mechanism by electrons jump among localized sites with a very low activation energy. The increase of the conductivity above 360 K corresponds to the decomposition of PPy in agreement with the TG results. The knowledge of the energetic position of the bands LUMO and HOMO is a prerequisite for the photocatalytic study and the photoelectrochemistry is widely used for providing the semiconducting properties to build the energy band diagram. The intensity potential J(V) curve of the hetero-system is plotted to elucidate the electrochemical behavior and to confirm the semi conducting properties of PPy. The cyclic J(V) plot (Fig. 5) indicates an irreversible redox process (sluggish system) since the oxidation (O1, ~ 0.7 V) and reduction (R1, ~ 0 V) peaks are separated by a large value. On a more quantitative point of view, a fast electron transfer is
Fig. 4 e Logarithm of the electrical conductivity of the hetero-system PPy/TiO2 vs. the reciprocal temperature at different frequencies. Inset: the permittivity of PPy vs. temperature at 10 kHz.
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Fig. 5 e The cyclic J(V) characteristic of the hetero-system PPy/TiO2 in Na2S2O3 solution (pH 6.8) under N2 bubbling, scan rate 10 mV s¡1.
characterized by a peaks separation of 0.059 V/n, n being the number of exchanged electrons; this means that the kinetic is dependent on the electrode material. Below ~ 0.25 V, the current shoots up considerably, due to the hydrogen evolution reaction (HER); it can also be observed, that HER is pH independent. One can mention in passage that the over-voltages are minimized on PPy/TiO2 and the hetero-system can be attractive in electro catalysis. The photo-electrochemistry is of great help for drawing the energy band diagram of the hetero-junction PPy/TiO2/electrolyte. The flat band potential of PPy (Vfb ¼ þ0.108 V) is provided from the intercept of the linear part at C2 ¼ 0 of the Mott-Schottky plot (Fig. 6): C2 ¼ ð2=eεεo NA Þ V Vfb
(2)
where εo is the permittivity of vacuum and e the electron charge. The negative slope confirms the semiconducting properties and is characteristic of p type behavior with a holes density (NA) of 4.57 1021 cm3; the determination of the permittivity of PPy (~30) at ambient temperature has been determined at 10 kHz from the dielectric measurements [22]. The flat region above 0.05 V indicates a charge accumulation at the interface PPy/electrolyte. The energetic position of the conduction band is given by: P ¼ 4:75 þ eVfb þ Ea Eg
Fig. 6 e The MotteSchottky characteristic of PPy in neutral solution (pH 6.8).
The impedance of the constant phase element (CPE) expresses the deviation from a pure capacitive behavior: p 1 Z ¼ QðjuÞ
(4)
where Q is a frequency independent constant, j the imaginary number (j2 ¼ 1) and u the angular frequency. The homogeneity factor (p) is associated with the roughness of the electrode, the surface sates within the gap region and the non homogeneity of the current distribution. The p value of the first semicircle (¼0.80) is readily obtained from the relation {q ¼ p/2 (1p)} indicating a deviation from a capacitive behavior with a high depletion angle (20 ). The second semicircle (p ¼ 0.92), close to unity is close to a capacitive behavior. The depletion indicates that the electron hopping occurs by crossing a low potential barrier (intrinsic behavior) which prevails over the grains boundaries, in conformity with the electrical conductivity of the hybrid material (s300Ke0.07 U cm). The intercepts with the abscissa axis give the resistances of the electrolyte (Rel ¼ 100 U cm2), bulk (Rb ¼ 400 U cm2) and grains boundaries (Rgb ¼ 3390 U cm2). The
(3)
The activation energy (Ea) of PPy can be neglected owing to its degenerate conductivity. The P value (0.312 V/4.44 eV) is more cathodic than the HER potential and should lead a spontaneous hydrogen evolution under illumination. Hence the valence band is located at 0.108 V i.e. at 4.86 eV below vacuum. The EIS measurements of PPy/TiO2, plotted at the open circuit potential (OCP), permit to quantify the various contributions (bulk, grains boundaries and diffusion) at the interface SC/electrolyte. The first semi circle obtained in the Nyquist representation (Zim against Zreal, Fig. 7) is slightly depressed indicating a predominant capacity (C1 ¼ 7.75 108 F cm2).
Fig. 7 e The Nyquist representation of the hetero-system PPy/TiO2 in neutral electrolyte (pH 6.8).
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 7 5 3 3 e1 7 5 3 9
EIS data are suitably adjusted to the proposed equivalent electrical circuit by using the software Zview (Fig. 7, Inset).
SCs; the hydrogen is evolved according to the following mechanism:
Photocatalysis
PPy þ hn / PPy-CB (e) þ PPy-VB (hþ)
PPy is applied for the solar energy conversion, so the chemical stability is of crucial importance and the first factor to take into account for the long term applications. For this purpose, we have examined the stability by storing PPy in various electrolytes over three months period. There was no observable degradation in acid and neutral solutions. On the contrary, in alkaline electrolyte the color turns to brown, such behavior is due to dedoping phenomenon, resulting from the neutralization of bi-polaron sites. On the other hand, the water photo electrolysis requires a gap of at least 1.23 eV and oxygen is not expected to be evolved on PPy because of i) its small gap ii and the position of the valence band, less anodic than the potential of O2/H2O level. Hence, a reducing agent must be added to scavenge the minority carriers, thus resulting in an enhanced photoactivity. The number of candidates is very limited because of the small gap of PPy (0.42 eV) and S2 O2 3 seems appropriate owing to its potential (Reaction 10); its oxidation should proceed fast enough to keep the holes concentration below the critical threshold required to trigger the photo corrosion is also tested for a comparative purpose of PPy. S2 O2 3 because of its wider chemical stability (pH ~ 3e12). PPy in solution exhibits a lower activity: 5.20 mL against S2 O2 3 8.60 mL over PPy/TiO2 after 100 min illumination i.e. an enhancement of 65% (Fig. 8). The electrons exchange between PPy-CB and H2O/H2 couple should occur isoenergetically. However, the relatively large energetic difference makes the electrons transfer weak. TiO2, whatever the crystallographic form, absorbs only in the UV region and exhibits no activity under visible illumination; it is used as support for PPy and mediates the electrons transfer from PPy-CB to H2O molecules. The further advantage is that PPy/ TiO2 is prepared by chemical oxidation giving a homogeneous dispersion and an intimate contact between the two
Fig. 8 e The volume of evolved hydrogen on PPy and PPy/ TiO2as function of illumination time at pH 6.8 in presence of S2 O2 3 .
(5)
Cathodic pole PPy-CB (2 e) þ 2H2O / PPy þ H2 þ 2 OH
(6)
2 S2 O2 6 þ 2e /2SO3
(7)
Anodic pole þ
2SO2 3 þ PPy VB 2h
þ
S2 O2 6 þPPy VBð2 h
/S2 O2 6
E 0:2V
þ þ 2H2 O/2SO2 4 þ 4H
(8) E 1V
(9)
or. þ
S2 O2 3 þ PPy VBð8h
E 0:54V þ 10OH /2SO2 4 þ 5H2 O (10)
þ
S2 O2 3 þ PPy VB 4h
þ þ 3H2 O/2SO2 3 þ 6H
E 0:82V (11)
The best activity occurs at pH 6.8 in presence of S2 O2 3 which favors the charges separation and protects the polymer against photocorrrosion. The rate constants of the PEC reactions increase with increasing temperature and the tests are performed at 323 K. The volume of hydrogen increases monotonically over illumination time with an average evolution rate of 1.15 mmol h1 (g catalyst)1. The end product SO2 4 is indifferent (Reaction 10) and does not compete with water for the photoelectrons; its redox potential is far above the potential of O2/H2O level and should maintain a more or less linear behavior. We have also used S2 O2 3 in alkaline medium (pH ~ 12) for a comparative purpose and the results are given in Fig. 9. It is worthwhile to mention that S2 O2 3 is chemically instable in acidic medium (pH < 4) and converts to elemental þ sulfur fS2 O2 3 þ 2H3 O /S þ SO2 þ 3H2 Og.
Fig. 9 e The volume of evolved hydrogen as a function of illumination time at pH 12 and pH 6.8 in presence of S2 O2 3 .
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 7 5 3 3 e1 7 5 3 9
Conclusion
Fig. 10 e The volume of evolved hydrogen as a function of illumination time at various pHs in presence of SO2¡ 3 .
When a reaction product is not removed from the solution, it may react at the opposite pole of the crystallite and a cyclic process appears leading to redox equilibrium and a regression of the photoactivity. This occurs with SO2 3 , the rate is high at the beginning and decelerates gradually over illumination time and this clearly indicates that both the reaction (8) and (9) occur simultaneously (Fig. 10). PPy absorbs over the whole solar spectrum, thus permitting the determination of the quantum efficiency (h) of light-to-chemical energy (hydrogen): h ¼ 2 fnumber of H2 mol:=photons fluxg
(12)
The number 2 enters in the relation (12) because the hydrogen formation requires two electrons; a value of 0.17% is 2 determined with S2 O2 3 against 0.08% for SO3 . Interestingly, the initial performance of the catalyst during the second cycle is completely restored and no deactivation is observed (Fig. 11). The hetero-system is applied for nickel reduction upon solar light. The preliminary results are satisfactory and the detailed study will be published soon.
Fig. 11 e The volume of evolved hydrogen as a function of illumination time at pH 6.8 in S2 O2 3 solution during two successive cycles.
Because of its chemical stability, low cost and non-toxicity, polypyrrol in conjunction with TiO2 (rutile variety) is attractive for the water photoreduction. The optical properties indicate absorption over the whole solar spectrum. The photoelectrochemical characterization showed p type behavior with a conduction band more cathodic than the hydrogen evolution level. The electrochemical impedance is characteristic of the bulk and grain boundaries behavior. The homogeneous dispersion of polypyrol in TiO2 makes the hetero-system PPy/ TiO2 efficient for the hydrogen photo evolution; the activity is compared favorably with respect to PPy and an improvement of 65% is registered. The regression in the performance in SO2 3 medium is attributed to the accumulation of S2 O2 which 6 competes with water for the photoelectrons. The deactivation of the catalyst is a priory excluded and the initial activity is entirely restored for further cycles.
Acknowledgments The authors thank B. Allouche for his assistance in the X-ray diffraction. This work was financially supported by both the Faculties of Chemistry and Physic.
references
[1] Do Sacramento EM, de Lima LC, Oliveira CJ, Nejat Veziroglu T. Hydrogen energy system and prospects for reducing emissions of fossil fuels pollutants in the Ceara state Brazil. Int J Hydrogen Energy 2008;33:2132e7. [2] Midilli A, Dincer I. Hydrogen as a renewable and sustainable solution in reducing global fossil fuel consumption. Int J Hydrogen Energy 2008;33:4209e22. [3] Plantard G, Goetz V, Correia F, Cambon JP. Importance of a medium's structure on Photocatalysis using TiO2-coated foams. Sol Energy Mater Sol Cells 2011;95:2437e42. [4] Aklalouch M, Calleja A, Granados X, Ricart S, Boffa V, Ricci F, et al. Hybrid sol-gel layers containing CeO2 nanoparticules as UV-protection of plastic lenses for concentrated photovoltaics. Sol Energy Mater Sol Cells 2014;120:175e82. [5] Han J, Ryu J, Lee I. A preliminary infrastructure design to use fossil fuels with carbon capture and storage and renewable energy systems. Int J Hydrogen Energy 2012;37:17321e35. [6] Younsi M, Saadi S, Bouguelia A, Aider A, Trari M. Synthesis and characterization of oxygen-rich delafossite CuYO2þxapplication to H2-photo production. Sol Energy Mater Sol Cells 2007;91:1102e9. [7] Sarı A, Alkan C, Bic¸er A, Karaipekli A. Synthesis and thermal energy storage characteristics of polystyrene-graft-palmitic acid copolymers as solid-solid phase change materials. Sol Energy Mater Sol Cells 2011;95:3195e201. [8] Bagtache R, Rekhila G, Abdmeziem K, Trari M. Characterization of a copper phosphate triazole metal organic framework material (Cu3PO4(C2N3H2)2OH) and oxygen evolution studies. Mater Sci Semicond Process 2014;23:144e50. [9] Rekhila G, Bessekhouad Y, Trari M. Visible light hydrogen production on the novel ferrite NiFe2O4. Int J Hydrogen Energy 2013;38:6335e43.
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[10] Bessekhouad Y, Trari M, Doumerc JP. CuMnO2, a novel hydrogen photoevolution catalyst. Int J Hydrogen Energy 2003;28:43e8. [11] Bessekhouad Y, Trari M. Photocatalytic hydrogen from suspension of spinel powders AMn2O4 (A¼ Cu and Zn). Int J Hydrogen Energy 2002;27:357e62. [12] Khataee AR, Zarei M. Photocatalysis of a dye solution using immobilized ZnO nanoparticles combined with photoelectrochemical process. Desalination 2011;273:453e60. [13] Wang R, Xu D, Liu J, Li K, Wang H. Preparation and photocatalytic proprieties of CdS/La2Ti2O7 nanocomposites under visible light. Chem Engin J 2011;68:455e60. [14] Shirakawa H. Synthesis and characterization of highly conducting polyacetylene. Synth Met 1995;69:3e8. ^ a S, Barra G. [15] Merlini C, Bruna S, Mu¨ller RD, Ecco L, Ramo Polypyrrole nanoparticles coated amorphous short silica fibers: synthesis and characterization. Polym Test 2012;31:971e7. [16] Belabed CH, Abdi A, Benabdelghani Z, Rekhila G, Etxeberria A, Trari M. Photoelectrochemical properties of doped polyaniline: application to hydrogen photoproduction. Int J Hydrogen Energy 2013;38:6593e9. [17] Wang Y, Wei W, Liu X, Gu Y. Research progress on polymer heterojunction. Sol Energy Mater Sol Cells 2012;98:129e45.
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[18] Deng F, Li Y, Luo X, Yang L, Tu X. Preparation of conductive polypyrrole/TiO2 nanocomposite via surface molecular imprinting technique and its photocatalytic activity under simulated solar light irradiation. Colloids surfaces A 2012;395:183e9. [19] Brahimi R, Bellal B, Bessekhouad Y, Bouguelia A, Trari M. Physical properties of CuAlO2 single crystal. J Cryst Growth 2008;310:4325e9. [20] Jing D, Guo L. Hydrogen production over Fe-doped tantalum oxide from an aqueous methanol solution under the light irradiation. J Phys Chem Solids 2007;68:2363e9. [21] Helaili N, Bessekhouad Y, Bouguelia A, Trari M. p-Cu2O/nZnO heterojunction applied to visible light orange II degradation. Sol Energy 2010;84:1187e92. [22] Belabed Ch, Rekhila G, Doulache M, Zitouni B, Trari M. Photoelectrochemical characterization of polypyrrol: application to visible light induced hydrogen production. Sol Energy Mater Solar Cells 2013;114:199e204. res conjugue s et polyme res conducteurs [23] Attias AJ. Polyme lectroniques. Techniques de l'inge nieur E; 2002. p. 1862. e [24] J. Cornil, J.L. Bredas. Nature of the optical transitions in charged oligothiophenes.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Photocatalytic hydrogen evolution on the hetero-system polypyrrol/TiO2 under visible light C. Belabed a,*, N. Haine a, Z. Benabdelghani b, B. Bellal b, M. Trari b a
Laboratory of Materials Physic, Faculty of Physic (USTHB) BP 32, 16111, Algiers, Algeria Laboratory of Storage and Valorization of Renewable Energies, Faculty of Chemistry, U.S.T.H.B., BP 32, 16111, Algiers, Algeria
b
article info
abstract
Article history:
The semiconducting properties of the hetero-system polypyrrol (PPy)/TiO2 are investigated
Received 11 June 2014
for the first time to assess its photo catalytic hydrogen evolution under visible light. The
Received in revised form
hetero-system PPy/TiO2 is stable up to ~345 K, with a weight loss accounting for ~1.2%. The X-
18 August 2014
ray diffraction shows mixed phases with the rutile TiO2 variety. Optical transitions at 0.42
Accepted 21 August 2014
and 3.01 eV, directly allowed, are determined for PPy and TiO2 respectively. The p-type
Available online 17 September 2014
conductivity of PPy is evidenced from the MotteSchottky plot; In neutral solution, a flat band potential of 0.11 VSCE and a holes density of 4.57 1021 cm-3 are obtained. The electro-
Keywords:
chemical impedance spectroscopy, measured over a wide frequency range (1 mHze105 Hz),
Polypyrrol (PPy)
reveals the contribution of the bulk and grain boundaries with a constant phase element
TiO2
(CPE). The energetic diagram predicts the electron injection from PPy into TiO2 and the best
Hetero-system
photoactivity is achieved at pH ~7. A hydrogen liberation rate of 1.15 mmol h1 (g catalyst)1
Photo-electrochemical
and a quantum efficiency of 0.17% under full light (29 mW cm2) are determined in presence
Electrochemical impedance
S2 O2 3 . The photoactivity is completely restored during the second cycle, indicating a zero
spectroscopy
deactivation effect.
Hydrogen
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Fossil energy resources will soon start declining and the solar energy is being considered as attractive which meets a growing demand in many fields like the photovoltaic [1] thermal conversion [2] photocatalysis [3] and environmental protection [4,5]. With an average insulation of 1250 W m2 and 3500 h year1 (South Algeria), the solar energy is an inexhaustible energetic source [6]. However, this energy is intermittent and can be useful only if convenient methods of storage are developed. In this respect, hydrogen is a clean fuel,
suitable for the chemical storage [7,8] and possesses a high energy capacity per unit masse (28,900 cal g1). It can be stored during the off hours and released on demand. The water splitting into hydrogen and oxygen can be achieved either thermally [9] or photocatalytically [10,11]. In the former case, only a small amount of water vapor can be dissociated at temperature exceeding 2700 K by using solar concentrators. In addition, the gases separation renders the process more complex and the mixture must be quenched to preclude their recombination. By contrast, the water photoelectrolysis works under mild operating conditions with no special set up. However, the semiconductor (SC) oxides like
* Corresponding author. Tel.: þ213 21 24 79 55; fax: þ213 21 24 80 08. E-mail address: [email protected] (C. Belabed). http://dx.doi.org/10.1016/j.ijhydene.2014.08.107 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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SnO2, ZnO and TiO2 used in the photo-electrochemical (PEC) conversion, are chemically stable against photo-corrosion but possess a large gap (Eg > 3 eV) and are consequently unattractive for the exploitation of the solar energy which contains only 5% of UV light [12,13]. Accordingly, active research is oriented toward the organic semiconductors (SCs) [14,15]. In this respect, the conducting polymers have attracted a great interest from both academic and industrial points of views over the last decade [16]. Such materials not only work in a similar way to inorganic SCs but have further advantages like environmental friendliness, easy synthesis and light weight. The use of polymers as photoelectrodes is of increasing interest and polypyrrole (PPy) is promising for the solar energy conversion [17]. The chemical stability over a fair pH range and the position of the conduction band (CB) below the level of hydrogen generation make PPy attractive for the water reduction. Moreover, PPy is relatively low cost and has a small optical gap (Eg) allowing the capture of a large part of the solar spectrum. Unlike physical properties, the PEC characterization of PPy has been little investigated [18] and to our knowledge, the hydrogen photoevolution has not been reported before now. Conducting PPy is sensitive to the pH of the solution and the PPy chain undergoes a deprotonation in basic solutions (pH ~ 9e11); it converts in acids (pH ~ 2e4) into insulator quinoid accompanied by the deintercalation of counter anions where the chain becomes protonated with however a small increase of the electrical conductivity. On the other hand, the photocatalysis can be significantly enhanced on hetero-systems, by coupling narrow and wide band gap SCs [19e21]. The synergy between the properties of each material results in enhanced performance of composite SCs under visible illumination. TiO2 absorbs UV light (l < 380 nm) and shows little activity under sunlight but can be used as support for PPy. The present work is devoted to the PEC characterization of the hetero-system PPy/TiO2 prepared by chemical route and its application for the hydrogen evolution upon visible illumination.
nanopowder. The UVeVisible spectra are recorded with a double beam spectrophotometer (Specord Plus 2000), equipped with an integrating sphere, PTFE is used as standard. The powder (~200 mg) is cold pressed into pellets under a pressure of 5 102 MPa and the mechanical properties are good. In order to minimize the contact resistance, silver paint is deposited on the back pellets which are mounted in glass holders to give a projected surface area of 1.32 cm2. The PEC characterization is performed in aqueous electrolyte under potentiostatic conditions using a standard cell with a Pt counter electrode. The electrode potential is monitored by a computer controlled PGZ301 potentiostat (Radiometer analytical) and given with respect to a saturated calomel electrode (SCE). An inert atmosphere is maintained by passing nitrogen during the electrochemical measurements. The variation of the interfacial capacitance is measured at 10 kHz. The complex impedance spectroscopy is acquired from small amplitude wave signals with a frequency response analyzer over the range (102e105 Hz). The photocatalytic experiments are performed in a Pyrex reactor equipped with a cooling system; the temperature is regulated at 323 ± 1 K thanks to a thermo-stated bath (Julabo); above 323 K, the loss by vaporization becomes important. 200 mg of PPy/TiO2 powder are suspended in 250 mL of 2 or S2 O2 M) by electrolyte containing X2 (¼S2 O2 3 3 , 10 magnetic agitation (210 rpm). The pH is adjusted by addition of KOH or HCl. Prior each test, nitrogen is bubbled over the solution for 35 min with a constant rate of 10 mL mL1. Visible light is produced by three tungsten lamps (200 W) disposed symmetrically around the reactor, providing a total intensity of 29 mW cm2 (2 1019 photons s1). Hydrogen is identified by gas chromatography (Shimadzu IGC 121 ML); traces of water are also detected. The volume is collected with a water manometer in an inverted burette through water displacement. Blank tests are performed and no hydrogen is liberated in the dark. All solutions are made up of reagents grade chemicals in distilled water (0.8 MU-cm). The experiments are repeated three times with very reproducible results (±2%).
Experimental The hetero-system PPy/TiO2 is prepared by chemical oxidation: 0.2 mol of pyrrole is dissolved in 150 mL of water containing 0.25 g of TiO2 (Sigma Aldrich, 99.5%). The required amount of H2SO4 (1 M) is slowly added to the solution under stirring for 30 min. Then, 50 mL of (NH4)2S2O8, (0.2 M), are added drop wise to the pyrrole solution. The reaction occurs under magnetic agitation during 3 h at 292 K; the temperature is controlled by a thermostated bath. The precipitate PPy/TiO2 is filtered, washed with water till the test with barium becomes negative and dried at 323 K for several days under vacuum. Thermal analysis (TG) is carried out with a Q 500 thermoanalyzer at a heating rate of 10 K min1. The phases are identified by X-ray diffraction using monochromatized Cu Ka radiation (l ¼ 0.15418 nm) at a scan rate of 2 (2q) min1; the patterns are compared to the powder database. The crystallite size of TiO2 (50.9 nm) is evaluated from the full width at half maximum of the diffraction peak (110), yielding a
Fig. 1 e XRD patterns of the hetero-system PPy/TiO2 prepared by chemical route.
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Results and discussion The XRD pattern shows two phases system (Fig. 1); the broad band at low angles (12.4 ) is characteristic of PPy while all other peaks are assigned to TiO2 (rutile phase) in agreement with the JCPDS Card N 71-0650. The peaks of TiO2 are narrow indicating a well crystallized oxide. The TG plot (Fig. 2) is carried out in order to delimit the thermal stability of the hetero-system PPy/TiO2. The decomposition, accounting for a small weight loss (~1.2%), occurs over a large temperature range; it starts at ~ 340 K and ends at ~ 600 K with an inflexion point at ~ 410 K. By contrast, the TG plot of PPY, exhibits a drastic weight loss (~60%) at 450 K and the weight levels off beyond 540 K, due the PPY thermal degradation as reported in our previous work [22]. The optical properties of the hetero-system are important in photocatalysis and are determined from the diffuse reflectance spectra. The relation between the absorption coefficient (a) and incident photon energy (hn) is given by the Tauc relation: m
ðahvÞ ¼ Cðhv Eg
(1)
C is a constant, the exponent m depends on the transition type, m ¼ 2 and 1/2 respectively for direct or indirect transitions. The intercept of the linear plot (ahn)2 of the heterosystem with the hn-axis (Fig. 3) yields two direct optical transitions at 0.42 and 3.01 eV respectively for PPy and TiO2, in conformity with the mixed phases. The band structure of PPy consists of a lowest unoccupied molecular orbital (LUMO) which constitutes the valence band (VB) separated by a forbidden band (Eg) from the highest occupied molecular orbital (HUMO: conduction band, CB), thus referring to p / p* transition [23,24]. The direct transition of TiO2 is due to the charge transfer O2: 2p / Ti4þ: 3d. Frequency dependence of AC conductivity (s) of the heterosystem shows a degenerate behavior with a moderate conductivity (~0.1 U cm) over the narrow stability range and is
Fig. 2 e TG plot and the derivate curve of the hetero-system PPy/TiO2.
Fig. 3 e The direct optical transitions of the hetero-system PPy/TiO2.
slightly frequency dependent (Fig. 4). The curve indicates a conduction mechanism by electrons jump among localized sites with a very low activation energy. The increase of the conductivity above 360 K corresponds to the decomposition of PPy in agreement with the TG results. The knowledge of the energetic position of the bands LUMO and HOMO is a prerequisite for the photocatalytic study and the photoelectrochemistry is widely used for providing the semiconducting properties to build the energy band diagram. The intensity potential J(V) curve of the hetero-system is plotted to elucidate the electrochemical behavior and to confirm the semi conducting properties of PPy. The cyclic J(V) plot (Fig. 5) indicates an irreversible redox process (sluggish system) since the oxidation (O1, ~ 0.7 V) and reduction (R1, ~ 0 V) peaks are separated by a large value. On a more quantitative point of view, a fast electron transfer is
Fig. 4 e Logarithm of the electrical conductivity of the hetero-system PPy/TiO2 vs. the reciprocal temperature at different frequencies. Inset: the permittivity of PPy vs. temperature at 10 kHz.
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Fig. 5 e The cyclic J(V) characteristic of the hetero-system PPy/TiO2 in Na2S2O3 solution (pH 6.8) under N2 bubbling, scan rate 10 mV s¡1.
characterized by a peaks separation of 0.059 V/n, n being the number of exchanged electrons; this means that the kinetic is dependent on the electrode material. Below ~ 0.25 V, the current shoots up considerably, due to the hydrogen evolution reaction (HER); it can also be observed, that HER is pH independent. One can mention in passage that the over-voltages are minimized on PPy/TiO2 and the hetero-system can be attractive in electro catalysis. The photo-electrochemistry is of great help for drawing the energy band diagram of the hetero-junction PPy/TiO2/electrolyte. The flat band potential of PPy (Vfb ¼ þ0.108 V) is provided from the intercept of the linear part at C2 ¼ 0 of the Mott-Schottky plot (Fig. 6): C2 ¼ ð2=eεεo NA Þ V Vfb
(2)
where εo is the permittivity of vacuum and e the electron charge. The negative slope confirms the semiconducting properties and is characteristic of p type behavior with a holes density (NA) of 4.57 1021 cm3; the determination of the permittivity of PPy (~30) at ambient temperature has been determined at 10 kHz from the dielectric measurements [22]. The flat region above 0.05 V indicates a charge accumulation at the interface PPy/electrolyte. The energetic position of the conduction band is given by: P ¼ 4:75 þ eVfb þ Ea Eg
Fig. 6 e The MotteSchottky characteristic of PPy in neutral solution (pH 6.8).
The impedance of the constant phase element (CPE) expresses the deviation from a pure capacitive behavior: p 1 Z ¼ QðjuÞ
(4)
where Q is a frequency independent constant, j the imaginary number (j2 ¼ 1) and u the angular frequency. The homogeneity factor (p) is associated with the roughness of the electrode, the surface sates within the gap region and the non homogeneity of the current distribution. The p value of the first semicircle (¼0.80) is readily obtained from the relation {q ¼ p/2 (1p)} indicating a deviation from a capacitive behavior with a high depletion angle (20 ). The second semicircle (p ¼ 0.92), close to unity is close to a capacitive behavior. The depletion indicates that the electron hopping occurs by crossing a low potential barrier (intrinsic behavior) which prevails over the grains boundaries, in conformity with the electrical conductivity of the hybrid material (s300Ke0.07 U cm). The intercepts with the abscissa axis give the resistances of the electrolyte (Rel ¼ 100 U cm2), bulk (Rb ¼ 400 U cm2) and grains boundaries (Rgb ¼ 3390 U cm2). The
(3)
The activation energy (Ea) of PPy can be neglected owing to its degenerate conductivity. The P value (0.312 V/4.44 eV) is more cathodic than the HER potential and should lead a spontaneous hydrogen evolution under illumination. Hence the valence band is located at 0.108 V i.e. at 4.86 eV below vacuum. The EIS measurements of PPy/TiO2, plotted at the open circuit potential (OCP), permit to quantify the various contributions (bulk, grains boundaries and diffusion) at the interface SC/electrolyte. The first semi circle obtained in the Nyquist representation (Zim against Zreal, Fig. 7) is slightly depressed indicating a predominant capacity (C1 ¼ 7.75 108 F cm2).
Fig. 7 e The Nyquist representation of the hetero-system PPy/TiO2 in neutral electrolyte (pH 6.8).
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EIS data are suitably adjusted to the proposed equivalent electrical circuit by using the software Zview (Fig. 7, Inset).
SCs; the hydrogen is evolved according to the following mechanism:
Photocatalysis
PPy þ hn / PPy-CB (e) þ PPy-VB (hþ)
PPy is applied for the solar energy conversion, so the chemical stability is of crucial importance and the first factor to take into account for the long term applications. For this purpose, we have examined the stability by storing PPy in various electrolytes over three months period. There was no observable degradation in acid and neutral solutions. On the contrary, in alkaline electrolyte the color turns to brown, such behavior is due to dedoping phenomenon, resulting from the neutralization of bi-polaron sites. On the other hand, the water photo electrolysis requires a gap of at least 1.23 eV and oxygen is not expected to be evolved on PPy because of i) its small gap ii and the position of the valence band, less anodic than the potential of O2/H2O level. Hence, a reducing agent must be added to scavenge the minority carriers, thus resulting in an enhanced photoactivity. The number of candidates is very limited because of the small gap of PPy (0.42 eV) and S2 O2 3 seems appropriate owing to its potential (Reaction 10); its oxidation should proceed fast enough to keep the holes concentration below the critical threshold required to trigger the photo corrosion is also tested for a comparative purpose of PPy. S2 O2 3 because of its wider chemical stability (pH ~ 3e12). PPy in solution exhibits a lower activity: 5.20 mL against S2 O2 3 8.60 mL over PPy/TiO2 after 100 min illumination i.e. an enhancement of 65% (Fig. 8). The electrons exchange between PPy-CB and H2O/H2 couple should occur isoenergetically. However, the relatively large energetic difference makes the electrons transfer weak. TiO2, whatever the crystallographic form, absorbs only in the UV region and exhibits no activity under visible illumination; it is used as support for PPy and mediates the electrons transfer from PPy-CB to H2O molecules. The further advantage is that PPy/ TiO2 is prepared by chemical oxidation giving a homogeneous dispersion and an intimate contact between the two
Fig. 8 e The volume of evolved hydrogen on PPy and PPy/ TiO2as function of illumination time at pH 6.8 in presence of S2 O2 3 .
(5)
Cathodic pole PPy-CB (2 e) þ 2H2O / PPy þ H2 þ 2 OH
(6)
2 S2 O2 6 þ 2e /2SO3
(7)
Anodic pole þ
2SO2 3 þ PPy VB 2h
þ
S2 O2 6 þPPy VBð2 h
/S2 O2 6
E 0:2V
þ þ 2H2 O/2SO2 4 þ 4H
(8) E 1V
(9)
or. þ
S2 O2 3 þ PPy VBð8h
E 0:54V þ 10OH /2SO2 4 þ 5H2 O (10)
þ
S2 O2 3 þ PPy VB 4h
þ þ 3H2 O/2SO2 3 þ 6H
E 0:82V (11)
The best activity occurs at pH 6.8 in presence of S2 O2 3 which favors the charges separation and protects the polymer against photocorrrosion. The rate constants of the PEC reactions increase with increasing temperature and the tests are performed at 323 K. The volume of hydrogen increases monotonically over illumination time with an average evolution rate of 1.15 mmol h1 (g catalyst)1. The end product SO2 4 is indifferent (Reaction 10) and does not compete with water for the photoelectrons; its redox potential is far above the potential of O2/H2O level and should maintain a more or less linear behavior. We have also used S2 O2 3 in alkaline medium (pH ~ 12) for a comparative purpose and the results are given in Fig. 9. It is worthwhile to mention that S2 O2 3 is chemically instable in acidic medium (pH < 4) and converts to elemental þ sulfur fS2 O2 3 þ 2H3 O /S þ SO2 þ 3H2 Og.
Fig. 9 e The volume of evolved hydrogen as a function of illumination time at pH 12 and pH 6.8 in presence of S2 O2 3 .
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Conclusion
Fig. 10 e The volume of evolved hydrogen as a function of illumination time at various pHs in presence of SO2¡ 3 .
When a reaction product is not removed from the solution, it may react at the opposite pole of the crystallite and a cyclic process appears leading to redox equilibrium and a regression of the photoactivity. This occurs with SO2 3 , the rate is high at the beginning and decelerates gradually over illumination time and this clearly indicates that both the reaction (8) and (9) occur simultaneously (Fig. 10). PPy absorbs over the whole solar spectrum, thus permitting the determination of the quantum efficiency (h) of light-to-chemical energy (hydrogen): h ¼ 2 fnumber of H2 mol:=photons fluxg
(12)
The number 2 enters in the relation (12) because the hydrogen formation requires two electrons; a value of 0.17% is 2 determined with S2 O2 3 against 0.08% for SO3 . Interestingly, the initial performance of the catalyst during the second cycle is completely restored and no deactivation is observed (Fig. 11). The hetero-system is applied for nickel reduction upon solar light. The preliminary results are satisfactory and the detailed study will be published soon.
Fig. 11 e The volume of evolved hydrogen as a function of illumination time at pH 6.8 in S2 O2 3 solution during two successive cycles.
Because of its chemical stability, low cost and non-toxicity, polypyrrol in conjunction with TiO2 (rutile variety) is attractive for the water photoreduction. The optical properties indicate absorption over the whole solar spectrum. The photoelectrochemical characterization showed p type behavior with a conduction band more cathodic than the hydrogen evolution level. The electrochemical impedance is characteristic of the bulk and grain boundaries behavior. The homogeneous dispersion of polypyrol in TiO2 makes the hetero-system PPy/ TiO2 efficient for the hydrogen photo evolution; the activity is compared favorably with respect to PPy and an improvement of 65% is registered. The regression in the performance in SO2 3 medium is attributed to the accumulation of S2 O2 which 6 competes with water for the photoelectrons. The deactivation of the catalyst is a priory excluded and the initial activity is entirely restored for further cycles.
Acknowledgments The authors thank B. Allouche for his assistance in the X-ray diffraction. This work was financially supported by both the Faculties of Chemistry and Physic.
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