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Nitrate-processing and characterization of a cobalt-doped barium tin oxide perovskite: Magnetic, transport and photoelectrochemical properties
Materials Science in Semiconductor Processing 30 (2015) 571–577
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Nitrate-processing and characterization of a cobalt-doped barium tin oxide perovskite: Magnetic, transport and photoelectrochemical properties K. Cherifi, N. Allalou, G. Rekhila, M. Trari n, Y. Bessekhouad Laboratory of Storage and Valorization of Renewable Energies, Faculty of Chemistry (USTHB), BP 32 16111, Algiers, Algeria
a r t i c l e i n f o
Keywords: Perovskite BaSn0.95Co0.05O3 δ Thermo-power Variable range hopping Photo-electrochemical
abstract The perovskite BaSn0.95Co0.05O3 δ, prepared by nitrate route, crystallizes in a cubic symmetry with a slight isotropic contraction of the host lattice. The oxygen crystal field splits the Co3 þ : 3d orbital by a value of 2.02 eV with a low spin configuration of Co3 þ (t 2g 6 eg 0 ). The average magnetic susceptibility ( 2 10 5 emu cgs mol 1) is temperature independent, in conformity with itinerant electrons behavior. The sample is nominally non-stochiometric and the conductivity follows a thermally activated hopping of lattice polaron with an activation energy of 19 meV. The thermal variation of the thermo-power suggests a finite density of states at the Fermi level. The zero temperature conductivity is finite and the charge carriers in localized states are thermally excited into extended states above the mobility edge. A variable range hopping is predicted at low temperature from the non-linearity of ln(σ) versus T 1 plot while the hump observed near 25 K is ascribed to the phonon drag. The photo-electrochemical characterization in KOH medium indicates p type behavior with a flat band of 0.21VSCE and a donor density of 6.95 1020 cm 3. The electrochemical impedance spectroscopy is undertaken and the response is modeled by an equivalent circuit with the predominance of the bulk contribution. The depressed semicircle is attributed to the faradic charge transfer with a constant phase element (CPE) while the straight line at low frequencies is due to the Warburg diffusion. & 2014 Elsevier Ltd. All rights reserved.
1. Introduction The stannate ASnO3 in which A is an alkaline earth crystallizes in the perovskite structure typified by the mineral CaTiO3. They have found many applications in electronic [1], optoelectronic [2], dielectric [3] and photoelectrochemical conversion [4] and are actively employed as high frequency devices [5]. Continued interest in materials suitable in low temperature electrical components has led to extensive investigations on the hopping
n
Corresponding author. Tel.: þ213 21 24 79 55; fax: þ213 21 24 80 08. E-mail address: [email protected] (M. Trari).
http://dx.doi.org/10.1016/j.mssp.2014.10.018 1369-8001/& 2014 Elsevier Ltd. All rights reserved.
conduction in doped oxides [6] and our interest for the stannates is due to their insulating metal transition [7]. The orbital degeneracy of 3d metals in the doped stannates is lifted by the octahedral crystal field that splits the five orbitals into a set of three fold degenerate t2g separated by doubly degenerate eg states by an amount 10Dq ( 2 eV). Within a simple rigid band diagram, the oxygen deficit produces electrons to the conduction band. This would be expected to apply to system in which the hopping is involved with paired electrons where the charged defects are small polaronic CoO6/SnO6 octahedra [8]. The charge transfer is fast enough to lead to delocalization of electrons with formally mixed valences Co3 þ 2 þ and/or Sn4 þ /2 þ and electrons hopping through octahedra sharing common
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K. Cherifi et al. / Materials Science in Semiconductor Processing 30 (2015) 571–577
corners. In such a case, the carriers concentration increases and the Fermi level (EF) moves above the mobility edge [9]. The mean free path (l) is expected to be short and decreases with increasing temperature but remains large compared to the inter-cation distance (Ioffe Regel criterion) [10]. The deviation from the linearity in the thermal dependence of the conductivity at low temperatures indicates that the conduction occurs via variable range hopping (VRH) as evidenced from the plot ln(σ) versus T0.25. VRH involves hopping over large distances and the partial localization is due to cations disorder [11]. The present work is devoted to the synthesis of the perovskite BaSn0.95Co0.05O3 by chemical route and the study of magnetic and transport properties at low temperatures. Moreover, the main physical parameters (gap, donor density, flat band potential etc.) are studied on the base of the optical properties and photo-electrochemical (PEC) characterizations. 2. Experimental Ba(CO3)2 (Merck, 99%), Co(NO3)2 6H2O (Fluka, 98%) and Sn (Merck, 99%) are used as starting reagents. Sn is chemically polished in HCl (0.5 N), washed with water and dried at 100 1C under dynamic vacuum ( 1 mbar). Stochiometric amounts are dissolved in a minimum of concentrated HNO3 and the solution is evaporated on a hot plate. At this level, the thermal analysis (TG) is examined by means of a commercial thermo-analyzer (Netzsch STA409C/CD). The amorphous powder is ground in an agate mortar, pressed into circular pellets (∅¼13 mm, thickness 1 mm) under 5 kbar and heated at 900 1C. The density is determined by the hydrostatic method, the comparison between theoretical and experimental densities gives a compactness of 95% while the diameter shrinkage (∅init ∅fin)/∅init is 10%. The phase is identified by X-ray diffractometry (XRD, PANalytical X'Pert PRO), using Cu Kα anticathode (λ¼0.154178 nm). The lattice constants are refined by the least-squares method using Si as standard (a¼0.54305 nm at 25 1C). The magnetic susceptibility (χ) is determined down to liquid helium temperature in an automatic DSM8 Manics type susceptometer under a field of 18,000 G, diamagnetic contributions
are taken from Ref. [12]. The equipment is standardized with Gd2(SO4)3 8H2O. The UV–vis spectrum is recorded with a double beam spectrophotometer (Specord 200 Plus) equipped with an integrating sphere, PPFE is used as reference. Electrical contact on the back pellet is made by hot soldering copper wires with silver cement. The conductivity measurements are performed over the range (4.2–300 K) using the four probe technique. The thermo-power is measured with homemade equipment; the temperature gradient through the pellet is determined by two pairs of chromel alumel thermocouples. The small heat conductivity of the stannate makes it possible to use large temperature gradient down to 4 K. The electrochemical characterization is carried out in a standard cell transparent to visible light. The back pellet is isolated with epoxy resin so that the outside surface (1.32 cm2) is exposed to the solution. The emergency electrode is a Pt electrode (Tacussel, 1 cm2) and all potentials are given with respect to a saturated calomel electrode (SCE). The intensity–potential J(V) curves are plotted with a PGZ301 potentiostat (radiometer analytical) and the capacitance measurements are measured at a frequency of 10 kHz. The data of the electrochemical impedance spectroscopy (EIS) are collected at the open circuit potential over the range (1 mHz–10 kHz) with an AC sin wave amplitude of 10 mV. 3. Results and discussion 3.1. Physical characterization The chemical route has succeeded in the preparation of small polaron oxides [13]. The essential weight losses in the TG plot (Fig. 1) start at 180 1C and end at 750 1C, they are attributed to water departure and nitrates decomposition of cobalt, barium and tin. The plateau region above 750 1C indicates the phase formation. The XRD pattern of BaSn0.95Co0.05O3 is characteristic of single phase with noticeably narrow peaks and a well crystallized oxide (Fig. 2). All peaks are indexed in a cubic symmetry (SG: Pm3m, No. 221) in agreement with the JCPDS card no. 15-0780. In the
100.0
TG (%)
99.5
99.0
98.5
98.0
0
200
400
600
800
Temperature (°C) Fig. 1. TG plot of the aqueous mixture of Ba(NO3)2, Sn(NO3)4 and Co(NO3)2.
K. Cherifi et al. / Materials Science in Semiconductor Processing 30 (2015) 571–577
573
Fig. 2. XRD pattern of the perovskite BaSn0.95Co0.05O3 elaborated by nitrate route. Inset: the pattern of the parent oxide BaSnO3.
Fig. 3. Thermal variation of the magnetic susceptibility of BaSn0.95Co0.05O3.
perovskite BaSnO3, Sn4 þ occupies 6 fold coordination while Ba2 þ is accommodated in center of 12-coordinate cavity, the refined lattice constant is found to be 0.41164 nm. The Cosubstitution in the Sn-sub-lattice leads to the contraction of the crystal lattice; the peaks slightly shift to larger angles with respect to BaSnO3 (Fig. 2, inset) because the ionic radius of Co3 þ (0.0525 nm, LS) is smaller than that of Sn4 þ (0.069 nm) in octahedral site [14], the slight lattice contraction of the substituted oxide (a¼0.41160 nm) is due to the small rate substitution. The pattern does not reveal any extra peaks that could result from the ordering between Sn and Co atoms. The ideal ASnO3 consists of a three-dimensional lattice of regular SnO6 octahedra sharing corners with 1801 Sn–O–Sn angle where the A-ion is surrounded by the nearest 12 oxygen. The tolerance factor is a dimensionless number which indicates the distortion from the ideal perovskite. From a crystallochemical point of view, BaSn0.95Co0.05O3 adopts undistorted structure with a tolerance factor of 0.923. BaSn0.95Co0.05O3 exhibits a weak paramagnetism ( 2 10 5 emu cgs mol 1) over a wide temperature range (Fig. 3).
Therefore, the electronic configuration of Co3 þ is low spin (t 2g 6 eg 0 ), consistent with itinerant electrons behavior. Taking into account the diamagnetic orbital contribution, the paramagnetic term (χ) of collective electrons, assuming a parabolic band is given by [15] 4mnμB 2 mo 2 χ¼ NA 2=3 1 ð1Þ 2 1=3 2 3mn h 3 where mn is the effective mass, mo is the electron rest mass, mB is the Bohr magneton (9.28 10 21 emu cgs), h is the Planck constant and NA is the holes density.The sharp upturn below 60 K is due to localized electrons paramagnetic impurity. The covalent character of Co–O bond makes a relatively strong ligand field. Hence, the stabilization of the field splitting overcomes the mutual repulsion of 3d electrons, thus permitting electrons pairing with a low spin configuration. The fundamental absorption in the UV–vis spectrum is used to determine the optical transition [16]. The intercept of the linear plot of (αhν)m with the hν-axis (Fig. 4) yields an Eg value of 2.02 eV and the transition is directly allowed
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Fig. 4. Direct optical transition of BaSn0.95Co0.05O3. Inset: the indirect allowed transition.
(Fig. 5). σ(T) is finite as T tends to zero σ ðT Þ ¼ σ o þf ðT Þ
Fig. 5. Thermal variation of the electrical conductivity of BaSn0.95Co0.05O3. Inset: the VRH at low temperatures.
(m¼2), α being the optical adsorption coefficient. The optical transition is of d–d characteristic and involves lower and upper bands of Co3 þ : 3d: orbital which splits in the octahedral environment into a filled valence band (VB) and empty level forming the conduction band (CB), 2 eV above the O2 : 2p level. A further transition is observed at 1.30 eV (Fig. 4, inset). The shorter Co–O length should increase both the covalency and the charge transfer integral. Furthermore, the small energetic difference between Co3 þ : 3d and O2 : 2p orbitals makes better mixing leading to a broadening of the conduction band and a narrowing of the gap. The electrical conductivity (σ) of BaSnO3 is low and could not be measured, it can be assumed to be less than 10 8 (Ω cm) 1. The enhanced conductivity is due to the increased holes concentration since the substitution of Sn4 þ by Co3 þ in BaSnO3 introduces acceptors levels within the gap region. Hence, the charge neutrality requires the generation of extra holes whose associated energy levels lie above VB. The charge delocalization leads to a thermal independence of the conductivity with degenerate behavior
ð2Þ
where f(T) is determined by the electron–electron interaction and localization contribution to the conductivity and σo is the function of the disorder. The question to be settled is whether the conductivity enhancement is driven by the holes density or the mobility. Over the temperatures range (50–300 K), σ obeys to an exponential law with an activation energy (Ea) of 19 meV, smaller than the thermal energy kT ( 26 meV). The small energy Ea, i.e. the separation between the Fermi level and VB, rules out any intrinsic conductivity, in agreement with a conduction mechanism by adiabatic polaron hopping, with a shift of the Fermi level toward VB. The holes coming from the shallow acceptors are ionized for T4220 K even for low effective mass. Therefore, the activated temperature dependence of the conductivity arises mainly from the enhanced mobility σ ¼ N A emh αT 1 exp Ea =kT ð3Þ e is the electron charge. The low mobility mh300 K {1.54 10 3 cm2 V 1 s 1},1 defined as the average drift velocity in an electric field of unit force, is due to the obstruction of O2 ions to electrons hopping between mixed cobalt valences. The energy EF lies above the mobility edge since the materials remain metallic. The attraction of one electron by cations is screened by other electrons and the critical concentration (nc) for the metallic state is given by the Mott criterion [17] Ënc 1=3 0:25; ao C
ao ¼ 0:053mo =mn
ð4Þ
ao being the effective Bohr radius. Assuming that we are dealing with hydrogen-like acceptor, the radius ao is 0.18 nm and this should yield an effective mass mn of 0.3mo. Taking into account the activation energy (19 meV) and assuming that each cobalt yields one hole, a nominal concentration (NA,n) of 6.95 1020 cm 3 is determined. The minimum conductivity given by the relation {σmin ¼72.9/d (nm)}, is equal to 65 (Ω 1 cm 1), a value by 1
Calculated from the relation σ300 K ¼(mheNA).
K. Cherifi et al. / Materials Science in Semiconductor Processing 30 (2015) 571–577
575
400
S (µV/K)
300
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
200
100
0
0
50
100 150 200 Temperature (K)
250
300
Fig. 6. Thermal variation of the thermo-power of BaSn0.95Co0.05O3.
one order of magnitude greater than that observed at saturation (12 Ω 1 cm-1). This is due to the apparent mean free path (l) of itinerant electrons which is large compared to the inter-atomic distance (the Ioffe Regel criteria). We evaluate {l¼m(3kTmn)1/2q 1} to 10 nm which is larger than the average separation d between Co3þ ions (1.1 nm¼ NA1/3), assuming a regular repartition. So, one can conclude that the slight temperature dependence of the conductivity is attributed to electrons scattering by Co3 þ ions. This can also be assigned to localized t2g-electron hopping of low spin Co3 þ states (Co3 þ þe )þCo3 þ -Co3 þ þ(Co3 þ þe ) [8]. The change of the slope in the conductivity plot is caused by a transition from small polaron hopping (SPH) to VRH according to Mott's model [11]. The conductivity versus T 0.25 at low temperatures (Fig. 5, inset) is related to VRH and involves jump over large distances rather than between nearest neighbors. The transport occurs below θD/4 by VRH in three dimensions among localized sites with non-interacting carriers, θD being the Debye temperature. The polaron is due to a lattice distortion which occurs between two neighboring sites. The change of the slope in the conductivity plot is due to the passage from SPH to VRH occurring below the band mobility edge in deep sub-band gap localized at the Fermi level according to Mott's model [11]: γ σ ¼ σ o exp T o =T with γ ¼ 0:25 ð5Þ where To is the Mott characteristic temperature, N(EF) the density of states at the Fermi level and k the Boltzmann's constant. Fig. 6 (inset) gives the logarithm of the conductivity versus T 0.25 at low temperatures; it is related to VRH which involves hopping over large lengths (5). The metal-insulating transition can be considered of Anderson type because of the potential, originating from the random distribution of Co3 þ . As the electrons concentration increases, the band width of the impurity band within the gap region generated by the acceptor like states is broadened till it becomes of the order of the fluctuation amplitude of the random potential leading to charge localization [18].
Fig. 7. J(V) profile of BaSn0.95Co0.05O3 in KOH solution (0.1 M) at 298 K; scan rate 10 mV/s.
The thermo-power is less sensitive to the grain boundaries than the conductivity and is fairly rare for stannates. The sign of S is correlated with that of the dominant carriers. The positive value confirms that the holes are the majority carriers. S increases linearly with temperature (Fig. 7), indicating mobility is thermally activated as in 2
S ¼ π 2 k T=3e f∂ ln σ=∂EgEF
ð6Þ
Above 240 K, the thermo-power is nearly constant in agreement with the model SPH [19]. This typically happens when the energy EF drops above the mobility edge and for systems where the carriers concentration, located in a wide band (B»kT), is temperature independent. The energy EF is evaluated at 0.4 eV from the relation 2
S¼
ðπ 2 k TÞ ð3eEf Þ
ð7Þ
One can anticipate that when the Co3 þ content increases in the solid solution BaSn1 xCoxO3, the impurity band splits into a filled lower band and empty upper band due to the correlation effect and intra-atomic repulsion; in such a case the electron–electron interaction becomes strong compared to the band width. An attractive energy is then required to overcome the electrostatic repulsion and for hopping between states in the Coulomb gap, S should remain constant with temperature. The phonon drag accounts for the deviation of S(T) from the linear behavior at 25 K. 3.2. Photo-electrochemical characterization BaSn0.95Co0.05O3 is an interesting material for PEC characterization. It is quite stable and shows an excellent electrochemical stability in alkaline medium (Fig. 8). The dark current (Jd) is less than 50 mA cm 2 while the oxygen evolution is not observed up to 1.5 V. Additional support of the electron delocalization is given by the weak peak of the electrochemical couple Co3 þ /2 þ which should appear at 1.4 V [12]. A fast electron transfer is a characteristic by peaks separation of 0.059 V/n (reversible system). The peak at 0 V is attributed to the internal process i.e. electrochemical couple Sn4 þ /2 þ while that at 0.5 V is due to the reduction of dissolved oxygen into H2O2 since
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shows that there are fewer holes than that would follow the nominal composition (NA,n ¼6.95 1019 cm 3) and this can be associated with the concomitant formation of oxygen vacancies:
this peak disappears in N2 saturated solution. Below 0.8 V, the current decreases considerably, due to water reduction. On the reverse scan, the peak at 0.77 V is assigned to the reversible oxidation. The differential capacitance at the interface is measured over a voltage region where the current flow is zero as C 2 ¼ 0:5 fεεo NA g V fb –V–kT=e ð8Þ
Oo 20:5 O2 þ Vo þ2 e
ð9Þ
The species are written according to the Kroger–Vink notation. Therefore the oxide is more accurately formulated as BaSn0.95Co0.05O3 δ and the electroneutrality is achieved with the reduction of Sn4 þ in Sn2 þ ; the δ value works out to be 0.04. The potential Vfb outlines the energetic position of the valence band (BaSn0.95Co0.05O3 VB) with respect to vacuum
where the symbols have their usual significations. Extrapolating the line to infinite capacitance (C 2 ¼0) yields a flat band potential Vfb of 0.22 V (Fig. 9) whereas the negative slope confirms the p type behavior. The density NA (6.05 1020 cm 3) calculated from [2/(eεεo) slope] 1,
P ¼ 4:75 þ eV fb Ea
ð10Þ
The P value ( 0.20 V/4.55 eV) is typical of oxides in which VB is made up of metal–3d orbital. EIS is measured at the open circuit potential (OCP ¼ 0.261 V) to determine the contributions of the electrolyte, bulk and grain boundaries of the electrode. The arc at high frequencies is due to the faradaic charge transfer (intrinsic behavior) of the junction BaSn0.95Co0.05O3/solution. The center is localized below the real axis with a depletion angle of 151, suggesting a constant phase element (CPE). Hence, the impedance deviates from the capacitive behavior and this is due to several factors among which are the surface roughness and the nonhomogeneity of the electrode. Recent theories suggest the substitution of the interfacial capacitance by CPE: Q(iω) n where n is the homogeneity factor (0 on r1) related to the phase angle φ (¼nπ/2). The slight offset near the origin is attributed to the series resistance of the electrolyte (Rel). The data (Table 1) are modeled to an electrical equivalent circuit using the software Zview, the bulk resistance (Rb) is in parallel with the capacitance, connected in series with the resistance Rel. The absence of arc at medium frequency indicates a negligible contribution of the grain boundaries to the electrical conductivity because of the high compactness of the pellet. The circuit is completed by the Warburg impedance which characterizes the diffusion processes in the low frequency domain, evidenced by the straight line at 451. The oxide has been tested successfully for the iodide oxidation under sunlight; the results are satisfactory and will be reported consecutively.
Fig. 8. Mott–Schottky characteristic of BaSn0.95Co0.05O3 in KOH (0.1 M) solution.
4. Conclusion The perovskite BaSn0.95Co0.05O3 is prepared by chemical route. The magnetic and transport properties are studied over the temperature range (4.2–300 K). The lattice constant changes little with respect to BaSnO3 owing to the
Fig. 9. Nyquist plots of BaSn0.95Co0.05O3 in KOH (0.1 M) solution in the dark (b) and under illumination (a).
Table 1 The electrochemical parameters of the perovskite BaSn0.95Co0.05O3 in KOH (0.1 M) solution both in the dark and under illumination.
Dark Light
Rel (Ω cm2)
Rb (Ω cm2)
β1 (1)
CPE (μF cm–2) 10–5
n
Ws (Ω cm2)
βw (1)
ωmin (kHz)
τ (ms)
450 240
460 255
–15 –13
4.9 15
0.67 0.72
38.51 1.93
47.36 60.42
13.82 6.55
0.072 0.152
K. Cherifi et al. / Materials Science in Semiconductor Processing 30 (2015) 571–577
small substitution rate and closeness of ionic radii of Sn4 þ and Co3 þ . The temperature-independent susceptibility is consistent with itinerant electrons behavior and a low spin configuration of Co3 þ . The optical gap is a characteristic of d–d transition. The transport properties are significantly enhanced by cobalt substitution; the weak dependence of the conductivity on the temperature is in conformity with degenerate behavior. The conduction mechanism changes from an Arrhenius type law to a variable range hopping at low temperatures. The random potential produced by the cobalt disorder leads to a Anderson type localization. The capacitance measurements confirmed the p type behavior. The electrochemical impedance in alkaline medium is characteristic of single relaxation time and a faradaic charge transfer.
Acknowledgments The authors would like to thank Dr. R. Brahimi for her helpful discussions. The financial support is provided by the Faculty of Chemistry (Algiers). They also wish to express their appreciation to M.L. Chabane Chaouch, Teacher of English at ALC Algiers, for proofreading and editing the present manuscript.
577
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Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Nitrate-processing and characterization of a cobalt-doped barium tin oxide perovskite: Magnetic, transport and photoelectrochemical properties K. Cherifi, N. Allalou, G. Rekhila, M. Trari n, Y. Bessekhouad Laboratory of Storage and Valorization of Renewable Energies, Faculty of Chemistry (USTHB), BP 32 16111, Algiers, Algeria
a r t i c l e i n f o
Keywords: Perovskite BaSn0.95Co0.05O3 δ Thermo-power Variable range hopping Photo-electrochemical
abstract The perovskite BaSn0.95Co0.05O3 δ, prepared by nitrate route, crystallizes in a cubic symmetry with a slight isotropic contraction of the host lattice. The oxygen crystal field splits the Co3 þ : 3d orbital by a value of 2.02 eV with a low spin configuration of Co3 þ (t 2g 6 eg 0 ). The average magnetic susceptibility ( 2 10 5 emu cgs mol 1) is temperature independent, in conformity with itinerant electrons behavior. The sample is nominally non-stochiometric and the conductivity follows a thermally activated hopping of lattice polaron with an activation energy of 19 meV. The thermal variation of the thermo-power suggests a finite density of states at the Fermi level. The zero temperature conductivity is finite and the charge carriers in localized states are thermally excited into extended states above the mobility edge. A variable range hopping is predicted at low temperature from the non-linearity of ln(σ) versus T 1 plot while the hump observed near 25 K is ascribed to the phonon drag. The photo-electrochemical characterization in KOH medium indicates p type behavior with a flat band of 0.21VSCE and a donor density of 6.95 1020 cm 3. The electrochemical impedance spectroscopy is undertaken and the response is modeled by an equivalent circuit with the predominance of the bulk contribution. The depressed semicircle is attributed to the faradic charge transfer with a constant phase element (CPE) while the straight line at low frequencies is due to the Warburg diffusion. & 2014 Elsevier Ltd. All rights reserved.
1. Introduction The stannate ASnO3 in which A is an alkaline earth crystallizes in the perovskite structure typified by the mineral CaTiO3. They have found many applications in electronic [1], optoelectronic [2], dielectric [3] and photoelectrochemical conversion [4] and are actively employed as high frequency devices [5]. Continued interest in materials suitable in low temperature electrical components has led to extensive investigations on the hopping
n
Corresponding author. Tel.: þ213 21 24 79 55; fax: þ213 21 24 80 08. E-mail address: [email protected] (M. Trari).
http://dx.doi.org/10.1016/j.mssp.2014.10.018 1369-8001/& 2014 Elsevier Ltd. All rights reserved.
conduction in doped oxides [6] and our interest for the stannates is due to their insulating metal transition [7]. The orbital degeneracy of 3d metals in the doped stannates is lifted by the octahedral crystal field that splits the five orbitals into a set of three fold degenerate t2g separated by doubly degenerate eg states by an amount 10Dq ( 2 eV). Within a simple rigid band diagram, the oxygen deficit produces electrons to the conduction band. This would be expected to apply to system in which the hopping is involved with paired electrons where the charged defects are small polaronic CoO6/SnO6 octahedra [8]. The charge transfer is fast enough to lead to delocalization of electrons with formally mixed valences Co3 þ 2 þ and/or Sn4 þ /2 þ and electrons hopping through octahedra sharing common
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corners. In such a case, the carriers concentration increases and the Fermi level (EF) moves above the mobility edge [9]. The mean free path (l) is expected to be short and decreases with increasing temperature but remains large compared to the inter-cation distance (Ioffe Regel criterion) [10]. The deviation from the linearity in the thermal dependence of the conductivity at low temperatures indicates that the conduction occurs via variable range hopping (VRH) as evidenced from the plot ln(σ) versus T0.25. VRH involves hopping over large distances and the partial localization is due to cations disorder [11]. The present work is devoted to the synthesis of the perovskite BaSn0.95Co0.05O3 by chemical route and the study of magnetic and transport properties at low temperatures. Moreover, the main physical parameters (gap, donor density, flat band potential etc.) are studied on the base of the optical properties and photo-electrochemical (PEC) characterizations. 2. Experimental Ba(CO3)2 (Merck, 99%), Co(NO3)2 6H2O (Fluka, 98%) and Sn (Merck, 99%) are used as starting reagents. Sn is chemically polished in HCl (0.5 N), washed with water and dried at 100 1C under dynamic vacuum ( 1 mbar). Stochiometric amounts are dissolved in a minimum of concentrated HNO3 and the solution is evaporated on a hot plate. At this level, the thermal analysis (TG) is examined by means of a commercial thermo-analyzer (Netzsch STA409C/CD). The amorphous powder is ground in an agate mortar, pressed into circular pellets (∅¼13 mm, thickness 1 mm) under 5 kbar and heated at 900 1C. The density is determined by the hydrostatic method, the comparison between theoretical and experimental densities gives a compactness of 95% while the diameter shrinkage (∅init ∅fin)/∅init is 10%. The phase is identified by X-ray diffractometry (XRD, PANalytical X'Pert PRO), using Cu Kα anticathode (λ¼0.154178 nm). The lattice constants are refined by the least-squares method using Si as standard (a¼0.54305 nm at 25 1C). The magnetic susceptibility (χ) is determined down to liquid helium temperature in an automatic DSM8 Manics type susceptometer under a field of 18,000 G, diamagnetic contributions
are taken from Ref. [12]. The equipment is standardized with Gd2(SO4)3 8H2O. The UV–vis spectrum is recorded with a double beam spectrophotometer (Specord 200 Plus) equipped with an integrating sphere, PPFE is used as reference. Electrical contact on the back pellet is made by hot soldering copper wires with silver cement. The conductivity measurements are performed over the range (4.2–300 K) using the four probe technique. The thermo-power is measured with homemade equipment; the temperature gradient through the pellet is determined by two pairs of chromel alumel thermocouples. The small heat conductivity of the stannate makes it possible to use large temperature gradient down to 4 K. The electrochemical characterization is carried out in a standard cell transparent to visible light. The back pellet is isolated with epoxy resin so that the outside surface (1.32 cm2) is exposed to the solution. The emergency electrode is a Pt electrode (Tacussel, 1 cm2) and all potentials are given with respect to a saturated calomel electrode (SCE). The intensity–potential J(V) curves are plotted with a PGZ301 potentiostat (radiometer analytical) and the capacitance measurements are measured at a frequency of 10 kHz. The data of the electrochemical impedance spectroscopy (EIS) are collected at the open circuit potential over the range (1 mHz–10 kHz) with an AC sin wave amplitude of 10 mV. 3. Results and discussion 3.1. Physical characterization The chemical route has succeeded in the preparation of small polaron oxides [13]. The essential weight losses in the TG plot (Fig. 1) start at 180 1C and end at 750 1C, they are attributed to water departure and nitrates decomposition of cobalt, barium and tin. The plateau region above 750 1C indicates the phase formation. The XRD pattern of BaSn0.95Co0.05O3 is characteristic of single phase with noticeably narrow peaks and a well crystallized oxide (Fig. 2). All peaks are indexed in a cubic symmetry (SG: Pm3m, No. 221) in agreement with the JCPDS card no. 15-0780. In the
100.0
TG (%)
99.5
99.0
98.5
98.0
0
200
400
600
800
Temperature (°C) Fig. 1. TG plot of the aqueous mixture of Ba(NO3)2, Sn(NO3)4 and Co(NO3)2.
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Fig. 2. XRD pattern of the perovskite BaSn0.95Co0.05O3 elaborated by nitrate route. Inset: the pattern of the parent oxide BaSnO3.
Fig. 3. Thermal variation of the magnetic susceptibility of BaSn0.95Co0.05O3.
perovskite BaSnO3, Sn4 þ occupies 6 fold coordination while Ba2 þ is accommodated in center of 12-coordinate cavity, the refined lattice constant is found to be 0.41164 nm. The Cosubstitution in the Sn-sub-lattice leads to the contraction of the crystal lattice; the peaks slightly shift to larger angles with respect to BaSnO3 (Fig. 2, inset) because the ionic radius of Co3 þ (0.0525 nm, LS) is smaller than that of Sn4 þ (0.069 nm) in octahedral site [14], the slight lattice contraction of the substituted oxide (a¼0.41160 nm) is due to the small rate substitution. The pattern does not reveal any extra peaks that could result from the ordering between Sn and Co atoms. The ideal ASnO3 consists of a three-dimensional lattice of regular SnO6 octahedra sharing corners with 1801 Sn–O–Sn angle where the A-ion is surrounded by the nearest 12 oxygen. The tolerance factor is a dimensionless number which indicates the distortion from the ideal perovskite. From a crystallochemical point of view, BaSn0.95Co0.05O3 adopts undistorted structure with a tolerance factor of 0.923. BaSn0.95Co0.05O3 exhibits a weak paramagnetism ( 2 10 5 emu cgs mol 1) over a wide temperature range (Fig. 3).
Therefore, the electronic configuration of Co3 þ is low spin (t 2g 6 eg 0 ), consistent with itinerant electrons behavior. Taking into account the diamagnetic orbital contribution, the paramagnetic term (χ) of collective electrons, assuming a parabolic band is given by [15] 4mnμB 2 mo 2 χ¼ NA 2=3 1 ð1Þ 2 1=3 2 3mn h 3 where mn is the effective mass, mo is the electron rest mass, mB is the Bohr magneton (9.28 10 21 emu cgs), h is the Planck constant and NA is the holes density.The sharp upturn below 60 K is due to localized electrons paramagnetic impurity. The covalent character of Co–O bond makes a relatively strong ligand field. Hence, the stabilization of the field splitting overcomes the mutual repulsion of 3d electrons, thus permitting electrons pairing with a low spin configuration. The fundamental absorption in the UV–vis spectrum is used to determine the optical transition [16]. The intercept of the linear plot of (αhν)m with the hν-axis (Fig. 4) yields an Eg value of 2.02 eV and the transition is directly allowed
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Fig. 4. Direct optical transition of BaSn0.95Co0.05O3. Inset: the indirect allowed transition.
(Fig. 5). σ(T) is finite as T tends to zero σ ðT Þ ¼ σ o þf ðT Þ
Fig. 5. Thermal variation of the electrical conductivity of BaSn0.95Co0.05O3. Inset: the VRH at low temperatures.
(m¼2), α being the optical adsorption coefficient. The optical transition is of d–d characteristic and involves lower and upper bands of Co3 þ : 3d: orbital which splits in the octahedral environment into a filled valence band (VB) and empty level forming the conduction band (CB), 2 eV above the O2 : 2p level. A further transition is observed at 1.30 eV (Fig. 4, inset). The shorter Co–O length should increase both the covalency and the charge transfer integral. Furthermore, the small energetic difference between Co3 þ : 3d and O2 : 2p orbitals makes better mixing leading to a broadening of the conduction band and a narrowing of the gap. The electrical conductivity (σ) of BaSnO3 is low and could not be measured, it can be assumed to be less than 10 8 (Ω cm) 1. The enhanced conductivity is due to the increased holes concentration since the substitution of Sn4 þ by Co3 þ in BaSnO3 introduces acceptors levels within the gap region. Hence, the charge neutrality requires the generation of extra holes whose associated energy levels lie above VB. The charge delocalization leads to a thermal independence of the conductivity with degenerate behavior
ð2Þ
where f(T) is determined by the electron–electron interaction and localization contribution to the conductivity and σo is the function of the disorder. The question to be settled is whether the conductivity enhancement is driven by the holes density or the mobility. Over the temperatures range (50–300 K), σ obeys to an exponential law with an activation energy (Ea) of 19 meV, smaller than the thermal energy kT ( 26 meV). The small energy Ea, i.e. the separation between the Fermi level and VB, rules out any intrinsic conductivity, in agreement with a conduction mechanism by adiabatic polaron hopping, with a shift of the Fermi level toward VB. The holes coming from the shallow acceptors are ionized for T4220 K even for low effective mass. Therefore, the activated temperature dependence of the conductivity arises mainly from the enhanced mobility σ ¼ N A emh αT 1 exp Ea =kT ð3Þ e is the electron charge. The low mobility mh300 K {1.54 10 3 cm2 V 1 s 1},1 defined as the average drift velocity in an electric field of unit force, is due to the obstruction of O2 ions to electrons hopping between mixed cobalt valences. The energy EF lies above the mobility edge since the materials remain metallic. The attraction of one electron by cations is screened by other electrons and the critical concentration (nc) for the metallic state is given by the Mott criterion [17] Ënc 1=3 0:25; ao C
ao ¼ 0:053mo =mn
ð4Þ
ao being the effective Bohr radius. Assuming that we are dealing with hydrogen-like acceptor, the radius ao is 0.18 nm and this should yield an effective mass mn of 0.3mo. Taking into account the activation energy (19 meV) and assuming that each cobalt yields one hole, a nominal concentration (NA,n) of 6.95 1020 cm 3 is determined. The minimum conductivity given by the relation {σmin ¼72.9/d (nm)}, is equal to 65 (Ω 1 cm 1), a value by 1
Calculated from the relation σ300 K ¼(mheNA).
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575
400
S (µV/K)
300
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
200
100
0
0
50
100 150 200 Temperature (K)
250
300
Fig. 6. Thermal variation of the thermo-power of BaSn0.95Co0.05O3.
one order of magnitude greater than that observed at saturation (12 Ω 1 cm-1). This is due to the apparent mean free path (l) of itinerant electrons which is large compared to the inter-atomic distance (the Ioffe Regel criteria). We evaluate {l¼m(3kTmn)1/2q 1} to 10 nm which is larger than the average separation d between Co3þ ions (1.1 nm¼ NA1/3), assuming a regular repartition. So, one can conclude that the slight temperature dependence of the conductivity is attributed to electrons scattering by Co3 þ ions. This can also be assigned to localized t2g-electron hopping of low spin Co3 þ states (Co3 þ þe )þCo3 þ -Co3 þ þ(Co3 þ þe ) [8]. The change of the slope in the conductivity plot is caused by a transition from small polaron hopping (SPH) to VRH according to Mott's model [11]. The conductivity versus T 0.25 at low temperatures (Fig. 5, inset) is related to VRH and involves jump over large distances rather than between nearest neighbors. The transport occurs below θD/4 by VRH in three dimensions among localized sites with non-interacting carriers, θD being the Debye temperature. The polaron is due to a lattice distortion which occurs between two neighboring sites. The change of the slope in the conductivity plot is due to the passage from SPH to VRH occurring below the band mobility edge in deep sub-band gap localized at the Fermi level according to Mott's model [11]: γ σ ¼ σ o exp T o =T with γ ¼ 0:25 ð5Þ where To is the Mott characteristic temperature, N(EF) the density of states at the Fermi level and k the Boltzmann's constant. Fig. 6 (inset) gives the logarithm of the conductivity versus T 0.25 at low temperatures; it is related to VRH which involves hopping over large lengths (5). The metal-insulating transition can be considered of Anderson type because of the potential, originating from the random distribution of Co3 þ . As the electrons concentration increases, the band width of the impurity band within the gap region generated by the acceptor like states is broadened till it becomes of the order of the fluctuation amplitude of the random potential leading to charge localization [18].
Fig. 7. J(V) profile of BaSn0.95Co0.05O3 in KOH solution (0.1 M) at 298 K; scan rate 10 mV/s.
The thermo-power is less sensitive to the grain boundaries than the conductivity and is fairly rare for stannates. The sign of S is correlated with that of the dominant carriers. The positive value confirms that the holes are the majority carriers. S increases linearly with temperature (Fig. 7), indicating mobility is thermally activated as in 2
S ¼ π 2 k T=3e f∂ ln σ=∂EgEF
ð6Þ
Above 240 K, the thermo-power is nearly constant in agreement with the model SPH [19]. This typically happens when the energy EF drops above the mobility edge and for systems where the carriers concentration, located in a wide band (B»kT), is temperature independent. The energy EF is evaluated at 0.4 eV from the relation 2
S¼
ðπ 2 k TÞ ð3eEf Þ
ð7Þ
One can anticipate that when the Co3 þ content increases in the solid solution BaSn1 xCoxO3, the impurity band splits into a filled lower band and empty upper band due to the correlation effect and intra-atomic repulsion; in such a case the electron–electron interaction becomes strong compared to the band width. An attractive energy is then required to overcome the electrostatic repulsion and for hopping between states in the Coulomb gap, S should remain constant with temperature. The phonon drag accounts for the deviation of S(T) from the linear behavior at 25 K. 3.2. Photo-electrochemical characterization BaSn0.95Co0.05O3 is an interesting material for PEC characterization. It is quite stable and shows an excellent electrochemical stability in alkaline medium (Fig. 8). The dark current (Jd) is less than 50 mA cm 2 while the oxygen evolution is not observed up to 1.5 V. Additional support of the electron delocalization is given by the weak peak of the electrochemical couple Co3 þ /2 þ which should appear at 1.4 V [12]. A fast electron transfer is a characteristic by peaks separation of 0.059 V/n (reversible system). The peak at 0 V is attributed to the internal process i.e. electrochemical couple Sn4 þ /2 þ while that at 0.5 V is due to the reduction of dissolved oxygen into H2O2 since
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shows that there are fewer holes than that would follow the nominal composition (NA,n ¼6.95 1019 cm 3) and this can be associated with the concomitant formation of oxygen vacancies:
this peak disappears in N2 saturated solution. Below 0.8 V, the current decreases considerably, due to water reduction. On the reverse scan, the peak at 0.77 V is assigned to the reversible oxidation. The differential capacitance at the interface is measured over a voltage region where the current flow is zero as C 2 ¼ 0:5 fεεo NA g V fb –V–kT=e ð8Þ
Oo 20:5 O2 þ Vo þ2 e
ð9Þ
The species are written according to the Kroger–Vink notation. Therefore the oxide is more accurately formulated as BaSn0.95Co0.05O3 δ and the electroneutrality is achieved with the reduction of Sn4 þ in Sn2 þ ; the δ value works out to be 0.04. The potential Vfb outlines the energetic position of the valence band (BaSn0.95Co0.05O3 VB) with respect to vacuum
where the symbols have their usual significations. Extrapolating the line to infinite capacitance (C 2 ¼0) yields a flat band potential Vfb of 0.22 V (Fig. 9) whereas the negative slope confirms the p type behavior. The density NA (6.05 1020 cm 3) calculated from [2/(eεεo) slope] 1,
P ¼ 4:75 þ eV fb Ea
ð10Þ
The P value ( 0.20 V/4.55 eV) is typical of oxides in which VB is made up of metal–3d orbital. EIS is measured at the open circuit potential (OCP ¼ 0.261 V) to determine the contributions of the electrolyte, bulk and grain boundaries of the electrode. The arc at high frequencies is due to the faradaic charge transfer (intrinsic behavior) of the junction BaSn0.95Co0.05O3/solution. The center is localized below the real axis with a depletion angle of 151, suggesting a constant phase element (CPE). Hence, the impedance deviates from the capacitive behavior and this is due to several factors among which are the surface roughness and the nonhomogeneity of the electrode. Recent theories suggest the substitution of the interfacial capacitance by CPE: Q(iω) n where n is the homogeneity factor (0 on r1) related to the phase angle φ (¼nπ/2). The slight offset near the origin is attributed to the series resistance of the electrolyte (Rel). The data (Table 1) are modeled to an electrical equivalent circuit using the software Zview, the bulk resistance (Rb) is in parallel with the capacitance, connected in series with the resistance Rel. The absence of arc at medium frequency indicates a negligible contribution of the grain boundaries to the electrical conductivity because of the high compactness of the pellet. The circuit is completed by the Warburg impedance which characterizes the diffusion processes in the low frequency domain, evidenced by the straight line at 451. The oxide has been tested successfully for the iodide oxidation under sunlight; the results are satisfactory and will be reported consecutively.
Fig. 8. Mott–Schottky characteristic of BaSn0.95Co0.05O3 in KOH (0.1 M) solution.
4. Conclusion The perovskite BaSn0.95Co0.05O3 is prepared by chemical route. The magnetic and transport properties are studied over the temperature range (4.2–300 K). The lattice constant changes little with respect to BaSnO3 owing to the
Fig. 9. Nyquist plots of BaSn0.95Co0.05O3 in KOH (0.1 M) solution in the dark (b) and under illumination (a).
Table 1 The electrochemical parameters of the perovskite BaSn0.95Co0.05O3 in KOH (0.1 M) solution both in the dark and under illumination.
Dark Light
Rel (Ω cm2)
Rb (Ω cm2)
β1 (1)
CPE (μF cm–2) 10–5
n
Ws (Ω cm2)
βw (1)
ωmin (kHz)
τ (ms)
450 240
460 255
–15 –13
4.9 15
0.67 0.72
38.51 1.93
47.36 60.42
13.82 6.55
0.072 0.152
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small substitution rate and closeness of ionic radii of Sn4 þ and Co3 þ . The temperature-independent susceptibility is consistent with itinerant electrons behavior and a low spin configuration of Co3 þ . The optical gap is a characteristic of d–d transition. The transport properties are significantly enhanced by cobalt substitution; the weak dependence of the conductivity on the temperature is in conformity with degenerate behavior. The conduction mechanism changes from an Arrhenius type law to a variable range hopping at low temperatures. The random potential produced by the cobalt disorder leads to a Anderson type localization. The capacitance measurements confirmed the p type behavior. The electrochemical impedance in alkaline medium is characteristic of single relaxation time and a faradaic charge transfer.
Acknowledgments The authors would like to thank Dr. R. Brahimi for her helpful discussions. The financial support is provided by the Faculty of Chemistry (Algiers). They also wish to express their appreciation to M.L. Chabane Chaouch, Teacher of English at ALC Algiers, for proofreading and editing the present manuscript.
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