Phase Composition and Charge Transport in Bismuth Molybdates

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The study of structural and electrical properties of three pure bismuth molybdate phases, α-Bi2Mo3O12, β-Bi2Mo2O9 and γ-Bi2MoO6, prepared by the spray drying technique, is described and discussed. The structure of polycrystalline, layered samples
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   1023-1935/05/4105- © 2005 åÄIä “Nauka   /Interperiodica”0455      Russian Journal of Electrochemistry, Vol. 41, No. 5, 2005, pp. 455–460. From Elektrokhimiya, Vol. 41, No. 5, 2005, pp. 523–528.Original English Text Copyright © 2005 by Hartmanová, Le, Driessche, Hoste, Kundracik.  1. INTRODUCTIONThree phases of bismuth molybdates (  α  -  Bi   2  Mo   3  O   12  ,  β  -  Bi   2  Mo   2  O   9  and γ   -  Bi   2  MoO   6  ) are known as the efficientcatalysts for the selective oxidation and ammoxidationof the lower olefins. These selective oxidations arecomplex processes in which the lattice O   2–   anions areused as the main oxidation agents [1–3]. In order toreplenish the lattice oxygen vacancies caused by theoxidation step, the gaseous oxygen atoms are trapped atactive sites on the surface of the catalysts. At the sametime, the anion vacancies concomitantly migratetowards the reoxidation sites, where the dioxygen isdissociated and incorporated into the solid and thus re-establishing the srcinal fully oxidized state of themetal oxide [4]. Therefore, the good catalysts for thesereactions should own not only the ability to attack theolefins to give a rise of the oxidized products, but alsoto exhibit a high ionic conductivity at the reaction tem-perature in order to transport the needed lattice O   2–  anions in time for the oxidation step.So that, the study of charge transport in these differ-ent phases seems to be interesting from the catalyticpoint of view. Particularly, it can help to determine theability to replenish the lattice oxygen vacancies duringthe selective oxidation process of each bismuth molyb-date phase and to clarify the role of each phase in thesynergy mixtures of several phases which are consid-ered to be the better catalysts than the pure phases [5].Moreover, such a study can contribute to the applica-tion of bismuth molybdates in the gas sensors, whoseproperties depend mainly on the electrical conductivityof material used. It has been accepted [6] that theanionic conductivity in these systems, under the not-reduced condition in these systems, is realized by theoxygen ion transport. Bismuth molybdates, the propertiesof which have been investigated in the literature [7–11] upto now, were prepared essentially by the solid statereaction and precipitation. In the present study, we havestudied the charge transport in three pure bismuthmolybdate phases α  -, β  -  and γ   prepared by the spray  Phase Composition and Charge Transport in Bismuth Molybdates* M. Hartmanová   a,   z  , M. T. Le   b  , I. Van Driessche   b  , S. Hoste   b  , and F. Kundracik   c   a   Institute of Physics, Slovak Academy of Sciences,  Bratislava, 84511 Slovakia   b  Ghent University, Ghent, 9000 Belgium   c  Comenius University,  Bratislava, 84248 Slovakia  Received June 16, 2004  Abstract  —The study of structural and electrical properties of three pure bismuth molybdate phases,  α  -  Bi   2  Mo   3  O   12  , β  -  Bi   2  Mo   2  O   9  and γ   -  Bi   2  MoO   6  , prepared by the spray drying technique, is described and discussed.The structure of polycrystalline, layered samples investigated by means of XRD and Raman spectroscopy wasfound to be monoclinic for the α  -  and β  -phases and orthorhombic for the γ   -phase. The microstructure of the as-prepared samples was not sufficiently developed under the given conditions of preparation, however, the ther-mal treatment can improve it. The high polarizability of Bi   3+  cations with their lone-pair electrons influencesthe stability of the disordered oxygen sublattice. All as-prepared phases undergo a slight structural change inthe temperature region of 280–430°ë  resulting in a decrease of the electrical conductivity probably due to anorder disorder transition in the oxygen arrangement during the sample heating. The change of electricalconductivity observed was found to be reversible in the high-temperature region and irreversible in the low-temperature one. The blocking of oxygen transport by the bismuth lone-pair electrons results in an increase of the activation energy and a decrease of the electrical conductivity in the high-temperature region. Relativelyhigh relative dielectric permittivities ε   r  , 36–52, were observed in dependence on the investigated phase.  Key words  : bismuth molybdates, spray drying, microstructure, electrical conductivity, dielectric permittivity  * This article was submitted by the authors in English.    z  Corresponding author, e-mail: maria.hartmanova@savba.sk   456  RUSSIAN JOURNAL OF ELECTROCHEMISTRY   Vol. 41   No. 5   2005  HARTMANOVÁ et al  .  drying technique [12]. The present investigation couldbe useful for our future study of the synergy effect inthe catalytic activities of bismuth molybdate mixturesas well as for the application of bismuth molybdates inthe gas sensors.2. EXPERIMENTAL  Synthesis of bismuth molybdates.  The general for-mula of bismuth molybdates reads Bi   2  O   3  · n  MoO   3  ,where n = 3, 2 or 1 corresponds to α  -, β  -  and γ   phases,respectively. The preparation using spray drying wasdescribed elsewhere [12]. The precursor solutions wereprepared from the solutions of 0.014 mole  (  NH   4  )   6  Mo   7  O   24  · 4  H   2  O (Fluka, p.a.) in the aqueous solu-tion (solution A) and 0.1 mole Bi  (  NO   3  )   3  · 5ç   2  é  (Fluka,p.a.) in the aqueous solution acidulated with concen-trated HNO   3  (pH ≤  1  ) in order to keep the Bi-salt dis-solved (solution B). In order to preserve the high acidityof the medium together with the solubility of the prod-ucts, solution A was slowly added into an equivalentamount of solution B (Bi/Mo molar atomic ratio were2/3, 1/1 and 2/1). In the following, we will identify thesample with Bi/Mo of 2/3 as “2/3” sample and so on.During this mentioned addition, the concentratedHNO   3  was continuously added in order to prevent theprecipitation of bismuth molybdates. The precursorsolutions were spray dried using Büchi  -190  laboratoryspray dryer with 0.5 mm nozzle and the feeding rate of 5 ml/min at the temperature of about 225°  C. The spraydried powders obtained were directly calcined in air at  550°  C (for γ   -phase) and 600°  C (for α  -  and β  -phases)for 10 hours. The layered bismuth molybdate pellets of 1 cm in the diameter and 1 mm thick were prepared bythe hydraulic press Paul Weber 1560 under the press of 5 kPa. As the as-prepared layered pellets were still brit-tle, they were immediately sintered at temperaturesmentioned above for the other 5 hours.  Structure and phase composition  —were studiedusing X-ray diffraction (XRD) and Raman spectroscopy.X-ray diffraction patterns were obtained on SiemensD5000 diffractometer using Cu  K    α  radiation over the 2  θ  range between 10  °  and 60  °  . Raman spectra were recordedby means of Varian Carey 82 spectrometer at a laser powerof 100 mW in the range of 4000 cm   –1  –0 cm   –1  .   Morphology and microstructure  —were investigatedby Philips 501 scanning electron microscope (no con-ductive coating was needed). In order to obtain the sur-face morphology and microstructure, the pellets weremoulted using Technovit and then grinded and polished.   AC conductivity  —was measured by an impedancetechnique in air. A Solartron SI 1260 impedance/gainphase analyzer interfaced to a computer and runthrough a Lab-view program was used. The impedancemeasurements were made in the frequency range of 10 Hz–1 MHz and at temperatures ~RT–560  °  C. Thetemperature was stabilized by a Chinoterm 10A digitaltemperature controller with the accuracy of ±  0.5°  C.The platinum paste electrodes were applied to the entirefaces of the pellet.3. RESULTS AND DISCUSSION  3.1. Crystallographic Structure and Phase Composition  The crystallographic structure and phase composi-tion of the polycrystalline, layered bismuth molybdatesamples were investigated by means of the XRD andRaman spectroscopy. Both these microscopic proper-ties are often influenced by a thermal treatment. There-fore we have investigated the as-prepared samples aswell as those which were exposed to a thermal treat-ment during the measurement of the temperaturedependence of the electrical conductivity in the temper-ature range of RT–  560°  C. The strong reflectionsobserved in XRD patterns for the as-prepared samplescan be ascribed to the present phases as follows: d    hkl  (  γ   )  =3.135, 2.734, 2.689, 1.937, 1.920, 1.650, 1.630 and1.574 Å; d    hkl  (  β  )  = 3.178, 2.794, 1.985, 1.681 and1.598 Å; d    hkl  (  α  )  = 3.246, 3.194, 3.042 and 2.867 Å. Asan example, XRD pattern of the γ   -phase Bi   2  MoO   6  (sample 2/1) is shown in Fig. 1a.The observed XRD reflections of the samples coin-cide with the reference data reported by the other 020arb. units2 Θ 4003040506010200(‡)020arb. units2 Θ 4003040506010200(b)    3 .   2   2   7   3 .   1   5   5   2 .   8   2   1   2 .   7   4   8   2 .   7   0   2   2 .   4   9   1   2 .   2   7   0  :   P   t   1 .   9   6   0   1 .   9   4   3 1 .   9   2   7 Fig. 1.  XRD patterns for γ  -phase of Bi 2 MoO 6  (sample 2/1):(a) as-prepared sample and (b) thermal-treated sample.  RUSSIAN JOURNAL OF ELECTROCHEMISTRY   Vol. 41   No. 5   2005 PHASE COMPOSITION AND CHARGE TRANSPORT457 authors, for instance [13–15] and can be ascribed tothe following structures: α - Bi 2 Mo 3 O 12  (2/3) and β - Bi 2 Mo 2 O 9  (1/1) phases to the monoclinic structureand γ  - Bi 2 MoO 6  (2/1) phase to the orthorhombic struc-ture. A little amount of the “polluting” β -phase,detected by the d  hkl  = 3.227 and 2.821 Å reflections,was found in sample 2/1 ( γ  -phase). In Raman spectra,the γ  -phase is characterized by the bands of 854, 808,784 and 717 cm  –1 ,  the β -phase exhibits the main bands of 888 and 771 cm  –1 ,  the α -phase is specified by bands of 957, 929, 902, 852, 818, 671, 652 and 514 cm  –1  [16].As an example, Raman spectra of the γ  -phase(sample 2/1) are shown in Fig. 2a, 2b. It was very diffi-cult to detect the trace of the β -phase in the Ramanspectrum of the 2/1 sample ( γ  -phase). We have notobserved any essential influence of the thermal treat-ment used on the structure and phase composition of the samples as judged from the XRD and Raman mea-surements.As an example, the XRD pattern of the thermaltreated sample 2/1 ( γ  -phase) is shown in Fig. 1b. Thereflection d  hkl  = 2.270 Å in Fig. 1b belongs to the Ptpaste coated on the surfaces of the pellet as an electrodefor the conductivity measurement. 3.2. Morphology and Microstructure The surface morphology (microstructure) was stud-ied by the scanning electron microscopy (SEM). Theobserved trends in porosity of all investigated phasesafter the thermal treatment used can be related to themicrostructural development. Under the given condi-tions of preparation of α -, β -  and γ  -phases, namely thetemperature and time of sintering, no ceramic micro-structure is visible and the photomicrographs are similarto those of the pre-sintered powders. It is possible to see,e. g., in the photomicrograph of the γ  -phase (sample 2/1)(Fig. 3a). The thermal treatment of the samples duringthe conductivity measurement has improved the micro-structural development, which is most visible in the caseof the γ  -phase (Fig. 3b). The lack of clear grain bound-aries makes it impossible to calculate the average grainsize. It seems to be possible that an enhancement of thesintering temperature can improve the microstructureand also an introduction of some additives into the puremolybdate phases can decrease the sintering temperaturenecessary to achieve a well-developed microstructure. 3.3. Electrical Conductivity and Dielectric Permittivity The bulk electrical conductivity of the α -, β -,  and γ   molybdate phases was investigated by the compleximpedance method as a function of temperature for theas-prepared as well as for the thermal treated samples.The results obtained are shown in Figs. 4a, 4b.All investigated phases undergo a slight structuralchange in the temperature region of 280–430 ° C result-ing in a decrease of conductivity by approximately oneorder of magnitude. The decrease of conductivityresults in two, the high-temperature and low-tempera-ture, regions. The main reason of this conductivitydecrease could srcinate from an order-disorder transi-tion in the oxygen arrangement during the sample heat-ing. It is known that the bismuth oxides undergo anorder disorder transition in the oxygen sublatticebelow 600 ° C, which leads to a decrease in the conduc-tivity (aging effect) [17]. A high polarizability of theBi 3+  cation with its lone pair electrons ( 6 s 2 ) influencesthe stability of the disordered oxygen sublattice. Theselone-pair electrons usually occupy a volume similar tothat of an O 2–   anion and decrease the free volume in thelattice and hence block to some degree [18] the trans-port of the oxygen ions. This is a reason why the acti-vation energies increase and conductivities decrease athigh temperatures. In both observed regions, the high- 41000 Intensity,  arb. units Raman shift , cm  –1 2800600400200006(a)    8   5   5   8   0   9   7   1   6 151000 Intensity, arb. units Raman shift , cm  –1 58006004002000020(b)    8   5   5   8   0   9   7   1   6 10 Fig. 2.  Raman spectra for γ  -phase of Bi 2 MoO 6  (sample 2/1):(a) as-prepared sample and (b) thermal-treated sample.  458 RUSSIAN JOURNAL OF ELECTROCHEMISTRY   Vol. 41   No. 5   2005 HARTMANOVÁ et al . est electrical conductivity was found to be in the case of the γ  -phase (sample 2/1) with the orthorhombic struc-ture and the highest theoretical density, ρ theor  = 8.26–8.28 g/cm 3  [13]. The lowest electrical conductivity wasobserved for the α -phase (sample 2/3) with the mono-clinic structure and the lowest theoretical density, ρ theor  = 6.19–6.26 g/cm 3  [19]. The theoretical density of the monoclinic β -phase (sample 1/1) was found to be ρ theor  = 6.5 g/cm 3  [20]. Moreover, the low-temperatureregion of α -phase (sample 2/3) seems to be composedfrom two, not very distinct, subregions (Fig. 4a). Thehighest activation energy in the high-temperature region,1.92 eV, was observed for the α -phase (sample 2/3).The transition observed at the pure phases could be,according to the comparison with the lanthanummolybdates [6], suppressed by a doping.The influence of thermal treatment on the electricalconductivity is shown in Figs. 4b, 5, and 6a, 6b. Arrhe-nius conductivity plots of all thermal-treated phases arecharacterized by the straight-line behavior with theexception of α -phase. The α -phase is now composedfrom two straight-line, high-temperature and low-tem-perature, regions (Fig. 4b). The reason of this differentbehavior of α -phase needs the next investigation. Thechanges observed on the conductivity plots of all sam-ples during the heating (RT–560 ° C) are reversible inthe high-temperature region, however, irreversible inthe low-temperature one, as it can be seen, as an exam-ple, for γ  -phase in Fig. 5. The highest and the lowestelectrical conductivities were observed, for both the as-prepared and the thermal-treated samples, for γ   (2/1)and α  (2/3) phases, respectively. As an example of impedance diagram used for the estimation of the elec-trical conductivity, the impedance diagrams of bothkinds of β -phase (sample 1/1) are shown in Figs. 6a, 6b.The impedance diagrams show the broadened arcs,which can be decomposed into two single semicircles.The high-frequency semicircle can be attributed to thebulk, the low-frequency one to the grain boundary. Thethermal treatment of samples induces the microstruc-tural changes in the samples (in Sec. 3.2), which can beseen at the comparison of decomposed single semicir-cles for the as-prepared and thermal-treated samples inFigs. 6a, 6b. 10 µ m10 µ m(‡)(b) Fig. 3.  SEM photomicrographs of γ  -phase of Bi 2 MoO 6  (2/1) calcined for 10 h and sintered for the next 5 h, both at 550°ë : (a) as-prepared pellet surface and (b) thermal–treated, grinded, and polished pellet surface. 1.2 σ , S cm –1 1000/  í  , ä  –1 10  –4 2.210  –5 10  –6 10  –7 1.41.62.01.81.15 eV0.74 eV0.57 eV1.25 eV1.92 eV1.2 σ , S cm –1 1000/  í  , ä  –1 10  –4 10  –5 10  –6 10  –9 1.41.62.01.80.92 eV0.88 eV1.92 eV 1 2 3 10  –3 10  –7 10  –8 1.15 eV 1 2 3 Fig. 4.  Arrhenius plots of the ac conductivity as a functionof reciprocal temperature for (a) as-prepared samples:( 1 ) α -phase of Bi 2 Mo 3 O 12  (2/3), ( 2 ) β -phase of Bi 2 MoO 9 (1/1) and ( 3 ) γ  -phase of Bi 2 MoO 6  (2/1); (b) thermal-treatedsamples: ( 1 ) α -phase of Bi 2 Mo 3 O 12  (2/3), ( 2 ) β -phase of Bi 2 Mo 2 O 9  (1/1) and ( 3 ) γ  -phase of Bi 2 MoO 6  (2/1). (a)(b)  RUSSIAN JOURNAL OF ELECTROCHEMISTRY   Vol. 41   No. 5   2005 PHASE COMPOSITION AND CHARGE TRANSPORT459 The results obtained in this study compare well withthe analogous literature data. For instance, the behaviorof the γ  -phase (sample 2/1) in the high temperatureregion (Fig. 5) is very similar, almost identical, with thedata reported in [11]. The good agreement was found tobe in terms of the reversibility as well as the values of electrical conductivity and activation energy. Here it isnecessary to note, that the samples in the paper [11]were prepared by solid state reaction and precipitation.The low-temperature region, including the order disorder transition, is absent in the work of theseauthors. The electrical conductivity of ZrO 2  + 4.3 mol%Y 2 O 3  [21] was included into Fig. 5 in order to comparethe order of magnitude between the γ  -phase of bismuthmolybdate and YSZ for the case of incidental applica-tion of the bismuth molybdate in the gas sensors. Theintroduction of suitable additives (dopants) into thepure bismuth molybdate phases could increase theirelectrical conductivity.It is important to know the relative dielectric permit-tivity ε r  for some applications of material. The effectiverelative dielectric permittivity ε r  of the investigatedsamples was estimated from the bulk parallel capaci-tance C  p  measured at room temperature and frequencyof 1 MHz using the usual formula ε r  = C    ×   t   /  ε 0 S  , where t   is the sample thickness, S   is the surface area and ε 0  isthe permittivity of free space, 8.845  × 10  –14  F cm  –1 . Theresults obtained for the investigated bismuth molybdatephases are shown in the table.The relatively high values of ε r  obtained for the as-prepared samples were found to be practically identicalwith those for the thermal-treated ones.4. CONCLUSIONSThe study of pure α -, β -  and γ   bismuth molybdatephases has shown that the structure of polycrystallinelayered samples is monoclinic for the α -  and β -phasesand orthorhombic, for the γ  -phase; the microstructurefor the as-prepared samples is not sufficiently devel-oped under given conditions, i.e. 10 h of calcination and5 h of sintering, both at 550°ë  for γ  -phase and 600° C for α -  and β -phases; some microstructural improvement isobserved for the thermal-treated samples; the highpolarizability of Bi 3+  cations with their lone-pair elec-trons influences the stability of the disordered oxygen 1.1 σ , S cm  –1 1000/  í  , ä  –1 10  –4 2.310  –5 10  –6 10  –8 1.31.52.11.91.15 eV0.74 eV0.57 eV10  –3 10  –7 1.71.14 eV0.94 eV 1 2 4 3 Fig. 5.  Arrhenius plots of the ac conductivity in the high-temperature region for γ  -phase of Bi 2 MoO 6  (2/1): ( 1 ) as-prepared sample and ( 2 ) thermal-treated sample;( 3 ) γ   thermal-treated phase [11] and ( 4 ) ZrO 2  + 4.31 mol %Y 2 O 3  [21]. Bulk constituents of parallel capacitance  C  p  and relative per-mittivities ε r  for α -, β -, and γ  -phases of Bi 2 MoO 6 Phase (sample) C  p (pF) ε r α (2/3)25.1636 β (1/1)31.3352 γ  (2/1)42.7441 020  –Z  '', k Ω  Z  ', k Ω 20406080100400200  –Z  '', k Ω  Z  ', k Ω 20040060080010006001200400 Fig. 6.  Impedance diagrams taken at 340 ° C in the low-tem-perature region: (a) as-prepared β -phase of Bi 2 Mo 2 O 9 (1/1): (  ) measured data, (full line) fit curve and (dottedline) deconvolution data; (b) thermal-treated β -phase of Bi 2 Mo 2 O 9  (1/1): (  ) measured data, (full line) fit curve and(dotted line) deconvolution data. (a)(b)
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