Relaxor ferroelectric and photocatalytic behaviour of Ba0.785Bi0.127Y0.017TiO3 composition

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Solar Energy 85 (2011) 443–449 www.elsevier.com/locate/solener

Relaxor ferroelectric and photocatalytic behaviour of Ba0.785Bi0.127Y0.017TiO3 composition A. Kerfah a, K. Taı¨bi a, S. Omeiri b, M. Trari c,⇑ a

Crystallography-Thermodynamics Laboratory, Faculty of Chemistry, USTHB, P.O. Box 32, Al Alia, 16111 Algiers, Algeria b Technical and Scientific Research Centre of Physical Analysis (CRAPC), P.O. Box 248, RP 16004 Algiers, Algeria c Laboratory of Storage and Valorization of Renewable Energies, Faculty of Chemistry, P.O. Box 32, Al Alia, 16111 Algiers, Algeria Received 3 June 2010; received in revised form 11 November 2010; accepted 8 December 2010 Available online 22 January 2011 Communicated by: Gion Calzaferri

Abstract The composition of Ba0.785Bi0.127Y0.017TiO3 (BaBiYTiO) belongs to the BaTiO3–Bi2O3–Y2O3 system. A dielectric study on ceramics performed at temperatures in the range (77 K–500 K) and frequency (102–2  105 Hz) revealed ferroelectric relaxor behaviour with a phase transition close to room temperature. The conductivity of BaBiYTiO obeys to an Arrhenius-type law with activation energy equals to 0.21 eV, in conformity with a small polaron hopping where most electrons are localized. The oxide is lightly doped leading to a wide space charge region (480 nm) in which photoeffect occurs. The presence of domains promotes the separation of the charge carriers through conversion of light into chemical energy. The electrode acquired n-type behaviour, evidenced from the negative thermopower and anodic photocurrent. The flat band potential Vfb (0.46 VSCE) and the electron density ND (2.82  1015 cm3) were determined in KOH solutions (0.5 M). The Nyquist plot exhibits two well defined time constants characteristic of bulk and grains boundaries contributions and an equivalent electrical circuit has been proposed according to the Randles model. The energy band diagram shows the potentiality of the oxide for the solar-energy conversion. BaBiYTiO has been tested successfully for H2 production upon visible light when combined to the delafossite CuFeO2 as sensitizer. An evolution rate of 24 lmol mn1 and a quantum yield of 0.4% under polychromatic light were obtained. Ó 2010 Elsevier Ltd. All rights reserved. Keywords: Ferroelectric; Relaxor; Lead-free; Perovskite; Hydrogen, Solar energy

1. Introduction Investigations of ferroelectric properties in complex perovskites (AA0 BB0 O3) by various substitutions in the A and/or B sites were widely studied in recent years, where much attention has been given to the perovskite with disorder cations. The latter leads to a new kind of ferroelectric materials called relaxor (Cross, 1994; Uchino, 1994). However, the current ferroelectric relaxor materials are leadbased ceramics and derived compounds. Unfortunately, these materials have a drawback due to the volatility and ⇑ Corresponding author.

E-mail address: [email protected] (M. Trari). 0038-092X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2010.12.009

the toxicity of lead oxide (PbO) throughout their synthesis process. Recently, research works are oriented toward environmental friendly materials. In this way, several lead-free compositions derived from BaTiO3 were investigated by homo- and/or hetero-valent substitutions (Ravez and Simon, 1997, 2000). Moreover, the relaxors present large dielectric constants and wide space charge region. These characteristic are interesting for solar applications, especially in photocatalytic hydrogen production (Yang et al., 2006; Li et al., 2009). Furthermore, current interest in alkaline earth perovkites (AA0 BB0 O3) as photoelectrochemical (PEC) functional materials is based in part on aspects like water splitting and environmental protection (Boumaza et al., 2010; Liu et al.,

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2006). The behaviour related to the ferroelectric properties of the catalyst was early reported (Van Damme and Keith Halls, 1981). The authors assert that the influence of ferroelectricity is thought to arise from the effect of polarized surface layers on the shape of the energy bands. The solar energy is very attractive from ecological viewpoint and as far as we know, there has not been any work on the solar conversion over perovskite based hetero-systems much except our previous work (Bellal et al., 2009; Helaı¨li, 2010). An efficient n type electrode requires not only a small forbidden band (Eg) but also a flat band potential (Vfb) as cathodic as possible to increase the band bending at the interface. The last requirement implies a low electron affinity. It is quite possible to substitute hetero-valent atoms like yttrium and bismuth which introduce extra electrons into the perovskite lattice. Moreover, the transport properties of BaTiO3 related compounds change significantly when exposed to appropriate light (hm > Eg). The photo-conductivity is due to the presence of donor-type point defects which at room temperature are partially ionized giving rise to itinerant electrons in the conduction band. They do, however, suffer from the disadvantage of low quantum efficiencies resulting from the large gap absorbing the UV radiations which entails only 5% of the sunlight. In order to contribute to a clean and sustainable energy, we present in this work, the ferroelectric properties of new lead-free compound of BaBiYTiO composition. The second part is devoted to PEC characterization of BaBiYTiO to assess its photocatalytic properties. We present the role of this material in the PEC hydrogen production. In neutral solutions, the conduction band is located below the potential of water reduction, where BaBiYTiO has been tested successfully for hydrogen evolution in conjunction with a sensitizer. The delafossite CuFeO2 was reported to have a high reducing ability (Younsi et al., 2005; Saadi et al., 2006) 2. Experimental The chemical substances were BaCO3 (Merck 99.9%), Bi2O3, Y2O3, and TiO2 (Aldrich 99.9%). The Ba0.785Bi0.127Y0.017TiO3 composition was obtained by the chemical reaction: ½ð1  aÞð1  xÞ þ að1  yÞBaCO3 þ ð1  aÞx=3Bi2 O3

gen atmosphere at 1100 °C. The weight loss was determined for each heat treatment, and was found to be less than 1%. The diameter shrinkage D/// = / (/init  /inal)//init and the compactness (experimental density/theoretical density) were 0.14 and 0.94 respectively. The purity of the single phase was verified by powder Xray diffraction (XRD). The XRD patterns were recorded at room temperature using a Philips diffractometer using ˚ ) in the angular range CuKa radiation (k = 1.5406 A 5 < 2h < 80°. For the dielectric measurements, the sample (into disk form) was polished and gold electrodes were sputtered on both sides. The real and imaginary relative permittivities e0r and e00r were determined under helium atmosphere using an Agilent Meter (model 4263B LCR), in the temperature range (77 K–500 K) and frequencies (102 Hz–105 Hz). The thermopower (S) was measured by a digital Tacussel microvoltmeter (model: Aris 20,000) with an impedance of 1012 X. The temperature gradient was determined with a thermo couple (type K). The pellet was maintained between two stainless steels in fabricated sample holders. An auxiliary heater was placed on one face of the pellet to have a temperature gradient of 10 K through the pellet. PEC characterization was done in a standard three electrode cell. Electrical contacts of less than 10 X resistance were established by soldering copper wires onto the back pellets with conductive silver paint. The working electrode was encapsulated in a glass holder and isolated with epoxy resin. The auxiliary electrode was a Pt foil (Tacussel, 1 cm2), all the potentials were referenced with respect to a saturated calomel electrode (SCE) and monitored with a Voltalab PGZ301 potentiostat (Radiometer analytical). The electrolyte KOH (0.5 M), used to maintain high electro-conductivity, was continually flushed with nitrogen, and the electrode was irradiated through a flat window with a 650 W halogen lamp (Dyr, General Electric). The interfacial capacitance was measured as a function of the potential with a 10 mV step1. The complex impedance data, recorded at the open circuit potential (OCP), were acquired using small amplitude wave signals through a frequency response analyser in the frequency range (103–105 Hz). Details of procedure and apparatus for H2-evolution are given elsewhere (Djellal et al., 2008). All the solutions used for the photocatalytic study were prepared from reagents of analytical-grade quality and doubly distilled water.

þ ay=3Y2 O3 þ TiO2 ! Bað1aÞð1xÞþað1yÞ Bi2ð1aÞx=3 Y2ay=3 TiO3 þ ½ð1  aÞð1  xÞ þ að1  yÞCO2 ; where a = 0.05, x = 0.20, and y = 0.50. Prior to heat treatments, disks (8 mm diameter and 1 mm thickness) were prepared (1 h grinding to powder then compression to 100 MPa). Calcination was carried out at 850 °C for 15 h, followed by 2 h sintering under oxy-

3. Results and discussion 3.1. Ferroelectric properties of BaBiYTiO The XRD pattern of BaBiYTiO, depicted in Fig. 1 along with that of CuFeO2, shows single phase characteristic of cubic symmetry. The thermal variation of the real part of permittivity (e0r ) shows one broad peak at various frequencies (Fig. 2). Moreover, the temperature Tm of e0r maximum

A. Kerfah et al. / Solar Energy 85 (2011) 443–449

445

Fig. 1. Powder X-ray diffraction of BaBiYTiO. Inset: the pattern of the delafossite CuFeO2.

150 0.1 kHz 1 kHz 10 kHz 100 kHz

1200

0.1 kHz 1 kHz 10 kHz 100 kHz

120

1000 90

ε' r

800

ε'' r

600

60

400

30

200 100

150

200

250

300

350

400

450

0

500

150

200

Temperaure (K)

350

400

450

500

Fig. 3. Thermal variation of e00r for the ceramic BaBiYTiO.

25

20 4

shifts to higher frequencies and a frequency dispersion occurs for T < Tm: e0r decreases when the frequency increases. The temperature and frequency variations of the imaginary part of the permittivity (e00r ) were also specific (Fig. 3): the temperature Tm of e00r maximum shifts to larger values at high frequencies, but unlike the evolution of e0r , e00r increases with increasing the frequency. In addition, there is a small deviation from the Curie–Weiss law (Fig. 4) whose temperature transition (To) from the para- to ferroelectric behaviour is greater than Tm. Such behaviour is characteristic of dipole interactions responsible of certain type of short range order. A strong dielectric dispersion appears according to a Vo¨gel–Fulcher (VF) relationship (Viehland et al., 1990; Glazounov and Tagantsev, 1998). The data log f vs. Tm (Fig. 5) fit with a VF law which leads to the temperature TVF (228 ± 10 K) and the activation

300

Temperature (K)

1/ε' r × 10

Fig. 2. Thermal variation of e0r for the ceramic with composition BaBiYTiO.

250

15

10

5

0 100

150

200

250

300

350

400

450

500

Temperature (K) Fig. 4. Temperature dependences of 1/e0r for the ceramic BaBiYTiO.

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6 Log f Exp Log f Calc

5

100

(αhν)

2

4

Logf 3

E g =2.43eV 50

2

1 0

0 300

310

320

330

2.40

340

2.44

2.48

3.2. Photoelectrochemical characterization In BaTiO3, the conduction band (CB), originating from r anti bonding mixture of hybridized Ti: 3d–O2: 2p orbital is separated from the valence band (VB) of anionic O2: 2p parentage by a forbidden band exceeding 3 eV and the oxide is of little practical use for the solar conversion. If we assume that VB lies approximately at the same energy regardless of the Bi content, the less electropositive character of bismuth in BaBiYTiO leads to a weaker covalency of Ti–O bond and thus to a destabilization of pTi–O CB. This results in a largest electro affinity and a lowest gap Eg. The optical data (Fig. 6) provides a clear cut off absorption which is expected for direct transitions. The plots (ahm)1/n vs. the incident photon hm should be linear with intercept equal to the interband transition energies. The exponent n depends on the type of transition; n = 2 for indirect allowed and n = ½ for direct allowed transition. The extrapolation of (ahm)2 to hm-axis gives a direct transition at 2.43 eV. Considering the potential of BaBiYTiO for the solarenergy conversion, it was of interest to study its physical and PEC properties. As already mentioned, the electrical conductivity of BaTiO3 can be tailored by doping on both Ba- and Ti-sublattices with hetero-valent ions (Yoon and

2.60

Ur, 2008). Frequency dependence of AC conductivity of BaBiYTiO shows that the conduction occurs predominantly by electron hopping between localized sites. The conductivity (r) increases with increasing temperature and follows an Arrhenius-type law with semi conductinglike behaviour (Fig. 7): r ¼ r0 expfEr =RT g

ð1Þ

The pre-exponential term is temperature independent. The well defined activation energy Er (0.21 eV), determined from the slope, is due to the formation of barriers at grains boundaries and is in perfect agreement with that obtained from the VF relation (0.22 eV). The thermal variation of the thermopower (S) is in conformity with such mechanism. S is negative over the entire temperature range indicating that electrons are the major charge carriers. S increases with increasing temperature up to 100 °C and then remains almost constant implying a thermally activated density. The large S value predicts a low electrons concentration (see below). The oxide exhibits an excellent chemical stability over the whole pH range. The electro kinetic parameters i.e. 0

-20

-1

energy Ea (0.22 eV). All these dielectric characteristics are typical of relaxor behaviour (Cross, 1994). In our case, the relaxor effect is certainly due to a cationic disorder in the A site. In fact, the substitution of Ba2+ by Bi3+ and Y3+ gives rise to non stoechiometric oxides and a composition heterogeneity which promotes the relaxor effect. From a structural viewpoint, the relaxor behaviour is generated by the presence of polar nanodomains with electric charge unbalance and leads to a local polarization responsible of frequency dispersion. Such polarization should promote the separation of photogenerated electron/hole (e/h+) pairs, an attractive property for the PEC conversion.

2.56

Fig. 6. Determination of the direct energy gap of BaBiYTiO.

S (mV K )

Fig. 5. The thermal variation of log (frequency) with Tm for the composition BaBiYTiO.

2.52

hν (eV)

Temperature (K)

-40

-60 320

360

400

440

480

Temperature (K) Fig. 7. The thermal variation of the thermopower of BaBiYTiO.

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447

200 90

Cgb

60

Rb

CPE

-Z i ( Ω.cm²)

100

J (µA/cm²)

0 -100

Rc

Rel

30 0

1.5 log J (µA/cm²)

-200 -300

-30

1.0 0.5

-60

0.0 -0.5

-400

0.4

-1.5

-1.0

-0.5

0.0

0.5

0.5 0.6 0.7 0.8 Potential (mV)

1.0

0.9

0

Fig. 8. The Mott Shottky plot of n-BaBiYTiO in 0.5 M KOH electrolyte. The frequency was set at 103 Hz.

the exchange current density (4.9 lA cm2) and the corrosion potential (+0.65 V), determined from the semi logarithmic plot (Fig. 8, Inset), predict long lived material. The voltammogram recorded in the dark in alkaline electrolyte (KOH 0.5 M, pH  13) shows a small current density over a large potential range (Fig. 8). Additional support of the electrons localization is brought by the existence of the peak O (0.56 V) corresponding to the electrochemical couple Ti3+/4+. By contrast, there is no anodic current indicating a low electrocatalytic oxygen evolution. On the reverse scan, the peak R (0.30 V) is attributed to the reversible reduction of Ti4+. Below 0.6 V, the current shoots up considerably due to hydrogen evolution (gas bubbles are noticeable on the electrode). The flat band potential Vfb is accurately determined from the interfacial capacitance using the Mott Schottky relation:

C



2 eeeo N D

¼

  kT V  V fb  e

ð2Þ

6

4

-2

-2

4

C (nF cm )×10

5

8

2

Vfb=-0.46 Volt

0 -0.6

-0.3

0.0

0.3

0.6

200

Zr (Ω

1.5

Potential (V)

2

100

0.9

Potential (V)

Fig. 9. Cyclic voltametric curve in the dark of BaBiYTiO in KOH solution (0.5 M), scan rate 10 mV s1. Inset: the corresponding Tafel plot.

300

400

cm²)

Fig. 10. Complex plane impedance plots in KOH solution (0.5 M). Inset: the equivalent electrical circuit.

where all the symbols have their usual significations. The slope is inversely proportional to the effective concentration ND. The linear plot C2(V) indicates a constant density ND (Fig. 9) whose value (2.82  1015 cm3) characterizes a lightly doped semiconductor with a broad depletion width d (480 nm)1:  0:5 2eeo ðV  V fb Þ d¼ ð3Þ eN D The difference (V  Vfb) represents the band bending at the interface. The positive slope lends a further support of n type conductivity. The transition to the plateau region in the C2(V) curve indicates a change from an accumulation state to a depletion one where the conduction band is flattened out. The ionization of the substituted atoms introduces extra electrons into the perovskite lattice and the number of electrons (more or less trapped) is increased. So, for lightly doped specimen, the Fermi level Ef lies far from CB whose energy is given by: P ¼ E  eV fb  DE

ð4Þ

where E° is the free energy of the electron vs. the saturated calomel electrode (=4.75 eV). The high energetic position P (4.08 eV/0.67 V) is typical of materials in which CB is made up of 3d orbital with a strong reducing power able to reduce water into hydrogen even in strongly basic electrolytes. The electrochemical impedance spectroscopy (EIS) is a powerful tool for studying the electrical properties and gives information of the oxide/solution junction as well as mechanisms of electro analytical reactions at the junction BaBiYTiO/electrolyte. It was measured at OCP (0.461 V) and is useful to distinguish between the various mechanisms. In the Nyquist representation, the resistances contribution is generally illustrated by three parallel circuits (R–C) connected in series and corresponding to the bulk, the grains boundary and the diffusion. Fig. 10 gives 1

Calculated for a band bending (V  Vfb) of 0.5 V.

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the real (Zr) and imaginary components (Zi) in the complex plan in KOH electrolyte. The low frequency semicircle is due to the grain boundaries and the intersection with the real axis gives the corresponding resistance Rc (82 X cm2) whereas the high frequency semicircle is attributed to the bulk resistance Rb (142 X cm2) and represents the most important contribution. The centre is localized below the real axis with an angle of 8° ascribed to a constant phase element (CPE), due mainly to the roughness of the electrode and the inhomogeneity of the surface which contribute significantly to the frequency dispersion. The intercept near the origin corresponds to the electrolyte solution whose small value Rel (7.5 X cm) is due to KOH ionic conductor. Due to the polycrystalline nature of the oxide, the barrier is modeled as double layer over the grains region and the data are analyzed by fitting them to an equivalent electrical circuit thanks to a Randles model (Fig. 10, Inset). 3.3. Photocatalytic hydrogen production Hydrogen production continues to receive much attention and the above results are relevant of potential applications of BaBiYTiO for the solar to chemical energy. The spontaneous polarization in relaxor ferroelectric materials represents an attractive property for the water photo splitting and constitutes the originality of the present work. As noticed above, the presence of domains favours the separation of (e/h+) pairs; the charges carriers migrate in opposite directions under the electric field of the polar nanoregions and promotes photoredox reactions. The ionization of the substituted atoms onto Ba-sites introduces extra electrons in the lattice, the consequence being a decrease of the space charge region. Therefore, in order to limit the electrons concentration the substitution is compensated by the cations vacancies. BaBiYTiO is a good photo catalyst but does not match the sunlight adequately. With a gap of 2.43 eV, most of the sun spectrum is sub band gap and therefore weakly converted although its beneficial features like the chemical stability, the non toxicity and the cathodic potential Vfb. A straightforward and elegant solution to improve its performance would be the utilization of hetero-systems where little systematic work has been done because of the difficulty of adjusting the electronic bands of SCs. Therefore it is necessary to find a suitable activator to shift the photoresponse of BaBiYTiO toward longer wavelengths. On the other hand, CuFeO2–CB (1 V) lies far below the H2O/ H2 level, the electrons exchange should occur iso energetically and the photoactivity is expected to be low (Derbal et al., 2008). However, we have taken the advantage of the pH insensitivity of CuFeO2–CB and at pH 13, the band bending at the interface BaBiYTiO/CuFeO2 (0.67 V) is close to that claimed previously by Gerisher (1978). Electrons excited from the delafossite CuFeO2, acting as electrons pump, are injected into BaBiYTiO–CB and subsequently transferred to adsorbed water, resulting in the hydrogen evolution.

80

volume of evolved Hydrogen (μmol)

448

60

CuFeO 2 40

CuFeO 2 /BaBiYTiO BaBiYTiO

20

0 0

10

20

30

40

50

Time (min) Fig. 11. Volume of evolved hydrogen as a function of illumination time at pH 13.

The diffuse nature of the solar energy allows low capital investment and hydrogen can be produced renewably in large amounts. However, the solar-energy conversion into hydrogen is temperature dependent, thus the experiment were performed at 50 °C above which vaporization losses become an increasing problem. The shiftless of CuFeO2– CB with pH permits an adequate location of BaBiYTiO– CB whose potential varies as 0.059 V pH1. The best performance for H2 formation is achieved in (KOH, 0.5 M) solution with an evolution rate of 24 lmol g1 min1 (Fig. 11). The quantum yield (g) is defined as twice the number of hydrogen molecules produced per incident photon: 2 ðmolecules of H2 Þ=ðinput number of photonsÞ The factor 2 arises because the hydrogen liberation is two electrons event. The hydrogen evolution drops after less than 1 h of continued irradiation, due to the saturation of catalytic sites by molecular hydrogen. This was evident from the fact that with bubbling the solution by nitrogen, the initial performance of the system was nearly restored. However, because of the low electron mobility in perovskites which averages 0.1 cm2 V1 s1 (Hadjarab et al., 2006), the diffusion length is small. The ideal case was to elaborate the oxide which nanosized dimension with a diffusion length of the same order of magnitude than the domains. These issues are presently under way and will be considered in ongoing issues. 4. Conclusion According to the dielectric investigations, the introduction of Bi3+ and Y3+ in the BaTiO3 lattice leads to a new ferroelectric relaxor material. The value of Tm, near to room temperature, allows the use of relaxor properties in normal conditions. The new composition Ba0.785Bi0.127Y0.017TiO3 is very interesting for ferroelectric applications and can replace lead-based ferroelectric ceramics to prevent the environmental pollutions during the preparation step. The transport properties were slightly enhanced by deviation

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from the stochiometry, inducing titanium mixed valences. The thermopower measurement indicated n type semi conductivity with a low electron density and a wide depletion width. The originality for the solar applications resides in the existence of polar domains which favours the charge separation upon illumination. The oxide emerged as having a great promise as photo electrode. Its conjunction with the delafossite CuFeO2 extends the spectral photoresponse toward the visible region and the hetero-system has been tested successfully for the hydrogen formation under visible light. Acknowledgements The authors are indebted to Dr. S. Habi Benhariz for the dielectric measurements. This work was supported by the Faculty of Chemistry (Algiers). References Bellal, B., Hadjarab, B., Bouguelia, A., Trari, M., 2009. Visible light photocatalytic reduction of water using SrSnO3 sensitized by CuFeO2. Theor. Exper. Chem. 45, 172–179. Boumaza, S., Boudjemaa, A., Omeiri, S., Bouarab, R., Bouguelia, A., Trari, M., 2010. Physical and photoelectrochemical characterizations of hematite a-Fe2O3: application to photocatalytic oxygen evolution. Solar Energy 84, 715–721. Cross, L.E., 1994. Relaxor ferroelectrics: an overview. Ferroelectrics 151, 305–310. Derbal, A., Omeiri, S., Bouguelia, A., Trari, M., 2008. Characterisation of new hetero-system CuFeO2/SnO2 application to visible-light induced hydrogen evolution, Int. J. Hydrogen Energy 33, 4274–4282. Djellal, L., Bouguelia, A., Kadi Hanifi, M., Trari, M., 2008. Bulk pCuInSe2 photo-electrochemical solar cells, Sol. Energy Mater. Sol. Cells 92, 594–600.

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