Electrochemical solid–solid conversion of bismuth oxide to bismuth metal Oliver Pyper, Brigitte Hahn and Robert Scho� llhorn Institut fu� r Anorganische und Analytische Chemie, T echnische Universita�t Berlin, Strasse des 17. Juli 135, D-10623 Berlin, Germany It is shown that binary and ternary bismuth oxides can be reduced quantitatively in aqueous electrolytes at ambient temperature via a two-phase solid–solid transition by electron/proton transfer to metastable porous metal and alloy systems.Single-crystal studies demonstrate the pseudomorphous character of the transition; topochemical correlations between educt and product phase could not be established, however. The solid-state reaction may proceed even at rather low temperatures of -40°C. In a recent publication a concept has been discussed that For structural investigations by powder X-ray diffractometry concerns the controlled formation of metastable porous homo- the Guinier technique and a powder diffractometer (Siemens geneous and heterogeneous systems by the electrochemical D5000, linear counter) were used with Cu-Ka radiation.reduction of transition-metal oxides, chalcogenides and halides Further characterization was achieved by transmission electron to the corresponding metal systems via electron/proton transfer microscopy (JEOL JSEM 200 B) and scanning electron at ambient temperature in a solid–solid conversion.1 The basic microscopy with an EDX analytical probe (Hitachi Svalidity of the reaction scheme has been demonstrated with 2700/Kevex).Magnetic susceptibility data were obtained using copper compounds as the example.Prerequisites for this type a Faraday balance. of reaction are a low enthalpy of formation of the metal compound and a thermodynamically or kinetically accessible redox potential of the metal cation–metal couple. Many oxides Results and Discussion of the p-block main-group metals are known to exhibit moder- Reduction of bismuth oxide ate enthalpies of formation, e.g.the oxides of the heavier elements thallium, lead and bismuth. We have performed a Preliminary investigations on the cathodic reduction of sintered study on the reactivity of bismuth(III ) oxide, which has been Bi2O3 pellets in aqueous neutral electrolyte showed the forma- proposed in earlier investigations as electrode material in tion of a metallic grey zone around the platinum contact site primary batteries with aprotic electrolytes.2–4 We report here which spread rapidly across the entire pellet surface; the end on the solid–solid conversion of Bi2O3 in aqueous electrolytes point of the reduction was indicated by the sudden formation and on similar reactions of some related binary bismuth of hydrogen gas.Analytical investigations confirmed the quan- compounds. Since the potential formation of new metastable titative conversion of the oxide to bismuth metal according alloys is a principal aspect in our investigations of these solid– to eqn. (1). solid reactions we also included studies on appropriate ternary bismuth phases. Bi2O3+6H++6e-�2Bi0+3H2O (1) The potential vs.charge transfer diagram for the galvanostatic Experimental cathodic reduction of Bi2O3 working electrodes in 0.1 mol dm-3 aqueous KOH in air atmosphere is given in Fig. 1. The Bismuth oxide, Bi2O3, was prepared by sintering pressed pellets initial strong overpotential is due to the small original reaction (diameter 12 mm, thickness ca. 1 mm) of the carbonate zone at the platinum point contact.Bi2O3 is a wide bandgap (BiO)2CO3 for 20 h at 750 °C. Ternary bismuth phases with semiconductor and the reaction can start only at the small Cu, Pb, W, Yb were obtained by sintering of the binary oxides metalliclead (Pt)/Bi2O3/electrolyte triphase boundary. Further in the appropriate stoichiometric ratio according to the progress of the reaction occurs subsequently at the bismuth literature: Bi2CuO4,5 Pb2Bi6O11,6 (Bi2O3)0.75(WO3)0.25,7 metal/Bi2O3 moving boundary zone.Since the Bi2O3/Bi Bi1.714Yb0.286O3.8,9 The oxyhalides BiOCl,10 BiOl11 and the interface area increases rapidly in the initial reaction period oxyacetate BiO(CH3CO2)12 were prepared by crystallization the overvoltage decreases strongly and is followed by a poten- from aqueous solutions.Large natural single crystals were tial plateau. Towards the end of the reaction the interface used in the case of bismuth sulfide, Bi2S3. region diminishes again which results in an increasing overpot- Electrochemical reactions were performed in three-electrode ential. The end of the solid–solid conversion is indicated by a cells with commercial potentiostat equipment, a Hg/HgO potential step and the formation of molecular hydrogen at the reference electrode and 0.1 mol dm-3 KOH as the standard working electrode.If the reduction is carried out in contact electrolyte. Working electrodes were sintered pellets, contacted with air the experimental charge transfer observed (as deter- by platinum clamps, or, in the case of oxyhalides and bismuth mined by the potential step) is usually somewhat higher oxyacetate, in platinum grid-pressed powder electrodes with 1 compared to the calculated value of 6 e- per Bi2O3.Under mass% PTFE. Bismuth sulfide crystals were contacted by platinum clamps. an inert gas atmosphere (nitrogen, argon) charge-transfer data 465 J. Mater. Chem., 1997, 7(3), 465–469466 Fig. 1 Potential vs. charge transfer diagram for the cathodic reduction Fig. 3 Low-temperature galvanostatic reduction of Bi2O3 at T= of Bi2O3 working electrodes in 0.1 mol dm-3 KOH at different current -40°C; (electrolyte 7 mol dm-3 aqueous KOH; sintered pellet values (300 K); #, air; &, N2 atmosphere working electrode, mass 392 mg, current 5 mA) close to the theoretical value are found owing to the absence The potential plateau observed in the galvanostatic of oxygen.The influence of the current density is illustrated in reduction of Bi2O3 to Bi suggests a two-phase process, in Fig. 2. In contact with air low current densities (i.e. long agreement with the reaction assumed by eqn. (1). This is reaction times) lead to strongly increased values for the charge confirmed by the X-ray diffraction data given in Fig. 4, which transfer due to the partial oxidation of Bi at the cathode by confirm the coexistence of the oxide educt Bi2O3 and the metal O2. High current densities result in a strong overpotential product Bi; no intermediate phase can be detected. leading to slow discharge of hydrogen as a competing process. In order to determine the reduction potential in potentio- No significant influence of the variation of the electrolyte static mode a step-screening experiment was performed (poten- concentration (0.1–10 mol dm-3) and of the electrolyte cation tial step chronocoulometry).At a potential of -750 mV vs. (Li, Na, K) on the charge transfer could be observed. the Hg/HgO reference electrode, a charge transfer of 5.9 e- We were interested to find out whether this solid-state f.u.-1 could be measured.The current vs. time diagram is given reaction would proceed also at temperatures below 25°C with reasonable kinetics. The experiment was carried out in a cryostat at -40 °C with concentrated aqueous KOH (7 mol dm-3) as the electrolyte in order to avoid solidification. The galvanostatic curve is given in Fig. 3. The initial region is characterized by a strong overpotential followed by a potential plateau.The sharp potential step indicates a charge transfer of 6 e- f.u.-1 (f.u.=formula unit) in very good agreement with the calculated value. It is surprising that this non-topotactic solid-state reaction can proceed with reasonable reaction rates and quantitative conversion at rather low temperatures.Fig. 4 Galvanostatic reduction of Bi2O3. Upper section: change of Fig. 2 Cathodic reduction of Bi2O3 in aqueous KOH: dependence relative intensities of selected educt and product reflections with the charge transfer; lower section: observed lattice parameters of the of the nominal charge-transfer values upon the cell current; #, air; &, N2 atmosphere coexisting phases vs.charge transfer.J . Mate r . Chem., 1997, 7(3), 465&ndash the reduced samples were determined by dc susceptibility measurements in the temperature range 200–300 K; in agreement with literature data, diamagnetism was observed. Reduction of single-crystal material For a demonstration of the pseudomorphous character of the solid-state transformation under investigation and for the control of potential structural correlations between educt and product, single-crystal material is required.Bi2O3 crystals were not available but single crystals of bismuth oxychloride, BiOCl, and of bismuth sulfide, Bi2S3, could be obtained. We were able to show that both phases can be reduced quantitatively to bismuth metal according to Fig. 6 X-Ray diffractogram of bismuth metal obtained by cathodic BiOCl+H2O+3e-�Bi0+2OH-+Cl- (2) reduction of Bi2O3 at -40 °C (cf.data given in Fig. 3) Bi2S3+3H2O+6e-�2Bi0+3OH-+3SH- (3) in Fig. 5. The shape of the curve can be explained by the BiOCl single crystals were obtained as platelets with quad- Bi2O3/Bi interface area change as discussed above; irregularit- ratic shape. The crystals were rather small (edge length ies are assumed to be due to crack regions in the sintered 5–10 mm) and very thin, which turned out to be most favourable sample.The quantitative charge transfer at a defined potential for TEM investigations. Fig. 7 shows a TEM image, Fig. 8 an suggests again the conclusion that no potential intermediate electron diffraction image of the single crystal used. The phases appear.electrochemical reduction was performed in aqueous suspen- The product obtained by cathodic reduction is brittle and sion in platinum vessels. TEMimages after reductionconfirmed exhibits a metallic grey colour. After washing in water and that the morphology of the platelets was retained. Dark-field acetone and drying in vacuum the electrode mass was identical images displayed a system of interconnected metal clusters to the calculated value within <1%; i.e.no residual electrolyte with an average particle diameter of 500–1000 A° . Electron or water was retained in the pore system. The lattice parameters diffraction studies showed that all lines of the pattern obtained of the products prepared at 20°C and -40°C were close to could be attributed to crystalline bismuth metal (Fig. 9). There those reported in the literature. For samples prepared at was no indication, however, for a preferential orientation of ambient temperature the increase in Bragg linewidth was the metal particles identified. rather small as compared to well ordered polycrystalline In the case of the sulfide, Bi2S3, large metallic grey crystal bismuth metal. From particle size calculations13 it can be assumed that the primary particles have a nominal diameter >500 A° .The linewidth broading increases with increasing diffraction angle, which suggests specificlattice defects.Bismuth metal prepared from Bi2O3 at -40°C showed, as expected, rather broad reflections, however, indicating strong lattice disorder (Fig. 6). A calculation of the nominal particle size (domains with coherent diffraction) yielded a value of<400 A° .SEM images showed crack regions at the surface (10 mm range); the resolution limit of the instrument used did not allow detection of the pore structure. Magnetic properties of Fig. 5 Potential step chronocoulometry of a Bi2O3 electrode: current Fig. 7 TEM image of two BiOCl crystals. Dimensions of the smaller vs. time diagram for the reaction at-750 mV (0.1 mol dm-3 aqueous KOH, pressed sintered pellets of mass 100 mg).Area=-37.2 mA h crystal ca. 5×5 mm.468 Fig. 9 Electron diffraction pattern of bismuth metal obtained by Fig. 8 Electron diffraction patternof the BiOCl crystals shown in Fig. 7 cathodic reduction of BiOCl. Beam diameter ca. 0.5 mm. agglomerates from a natural source (bismuthinite, Namibia, transfer curve for the reduction of sintered Bi2CuO4 working Helikon II, East) were available.It was possible to isolate electrodes in 0.1 mol dm-3 KOH under an inert gas atmos- needles with 0.3×0.3×0.8 mm3 size by cleaving. Galvanostatic phere is given in Fig. 10. The electrochemical charge-transfer reduction yielded a charge transfer equivalent to 5.9 e- f.u.-1; value of 8.2 e- f.u.-1 correspond reasonably well with the X-ray data confirmed that the product was bismuth metal.value calculated according to eqn. (4). EDX–SEM studies demonstrated that, independent of the location of the current lead contact point, the reaction started Bi2CuO4+4H2O+8e-�(Bi2/Cu)+8(OH)- (4) synchronously at all crystal edges and at local crystal defects, The X-ray diagram of the product exhibits a strong back- which is a consequence of the high conductivity of the narrow ground with broadened reflections corresponding to bismuth bandgap semiconductor sulfide.X-Ray rotation photographs metal while no other lines, in particular no reflections that can of Bi2S3 crystal needles (c axis perpendicular to X-ray beam, be attributed to copper metal, can be detected. This may be observation of 0k0 reflections) were made before and after understood in terms of a mixture of an amorphous metastable reduction.In the latter case only faintly structured closed copper–bismuth alloy along with a fraction of crystalline diffraction cone cross-sections could be observed. It must be bismuth metal. An alternative interpretation of the X-ray data concluded again that there is no correlation between the initial could be based, however, on a model that involves microdo- orientation of the Bi2S3 crystal and the orientation of the metal mains of copper metal, too small to give appreciable diffraction crystallites present after reduction, although the transition is intensity, in a bismuth matrix.EXAFS measurements are in clearly pseudomorphous. progress to decide between the two models. If this material is heated to 650 °C for 12 h in closed evacuated quartz ampoules Reduction of ternary bismuth oxides then X-ray data reveal the presence of the diffraction lines of bismuth as well as of copper metal (along with some weak The principal idea here was to prove the possibility of the preparation of a metastable porous alloy and heterogeneous reflections belonging to Bi2O3, indicating surface oxidation of the product).Similar results have been obtained in our earlier phases via low-temperature electrochemical conversion of appropriate oxide systems;1 an overview is given in Table 1. investigations on the electrochemical reduction of superconducting Bi2Sr2CaCu2O8+x.15 Bismuth and copper are not miscible in the solid state,14 the reduction of bismuth oxocuprate, Bi2CuO4, at 300 K could Yellow–orange sintered pellets of the ternary bismuth–lead oxide Pb2Bi6O11 could be reduced electrochemically to a grey therefore be expected to lead to the formation of a metastable amorphous alloy since the related binary oxides Bi2O3 and product with a metallic appearance; the charge transfer value of 21.9 e- f.u.-1 found experimentally is in agreement with the CuO can both be reduced to metal.The potential vs. chargeJ . Mate r . Chem., 1997, 7(3), 465–469 469 Table 1 List of bismuth compounds investigated material prep.a symmetry ncalc.b nobs.b EDc/mV Bi2O3 (BiO)2CO3 (750°C) monoclinic 6 6.2 (20°C) -755 6.0 (-40 °C) Bi2S3 mineral (xx) orthorhombic 6 5.9 -1050 BiOCl ref. 10 (xx) tetragonal 3 3.15 -750 BiOI ref. 11 tetragonal 3 3.15 -660 BiO(ac)d ref. 12 tetragonal 3 3.15 -720 Bi2CuO4 ref. 5 tetragonal 6 (Bi)+2 (Pb) 8.2 -750 Pb2Bi6O11 ref. 6 monoclinic 4 (Pb)+18 (Bi) 21.9 -750/-1050 Bi14W2O27 ref. 7, 18 tetragonal 42 (Bi) 43 -1100 Bi1.72Yb0.28O3 ref. 9 cubic 5.14 (Bi) 5.3 -720 axx=Single crystal. bn=Charge transfer (e- f.u.-1).cED=Potential for reduction vs. Hg/HgO reference electrode measured under current. dac=CH3CO2. ytterbium oxide should remain as dispersed phases in the metal matrix. X-Ray investigations of sintered pellets of the yellow–green bismuth–tungsten oxide found mainly single-phase Bi14W2O27 (tetragonal).18 They could be reduced galvanostatically with a charge-transfer value of 43 e- f.u.-1 as compared with the calculated value of 42 e- f.u.ed on eqn. 6. Bi14W2O27+42H2O+42e-�14Bi+2WO3+42OH- (6) The X-ray diffractogram of the product exhibit only the lines for bismuth metal except for a few additional reflections that could not be indexed reliably owing to the low line intensities. Obviously tungsten oxide remains as an amorphous oxide–hydrate fraction in the product.Bi1.72Yb0.28O3 can be considered as a rare-earth-metal stabilized d-Bi2O3. The X-ray diagram of the reduction product shows the lines for bismuth metal along with a strong back- Fig. 10 Potential vs. charge transfer diagram for the cathodic reduction ground in the lower 2h range. Amorphous ytterbium oxide– of Bi2CuO4 (sintered pellet contacted with platinium clamp, mass hydroxide is assumed to be dispersed in the metal matrix. 650 mg, current 3 mA, 0.1 mol dm-3 aqueous KOH as electrolyte, N2 An attempt was made to reduce a bismuth compound with atmosphere) rather large anions (i.e. a strong spatial ‘dilution’ of Bi3+) in order to find out whether the line width and particle size of calculated value of 22 e- if both lead and bismuth are reduced the product would increase and decrease respectively.The to the metallic state [eqn (5)]. compound selected was bismuth oxyacetate; the X-ray data of the reduction product did not however exhibit line broadening Pb2Bi6O11+22H2O+22e-�(Pb2/Bi6)+22OH- (5) superior to that found for the cathodic reduction of bismuth Upon exposure to air the grey material changes to grey– oxide.black over a few hours owing to oxidation processes. The Pb/Bi phase diagram exhibits only one intermediate e- Reoxidation reactions phase around the composition Pb7Bi3.16 X-Ray diagrams of Samples prepared by reduction of Bi2O3 undergo slow visible the reduction product indicate the presence of bismuth metal oxidation upon storage in air at 300 K: after several days and Pb7Bi317 as the major crystalline components (along with changes from metallic grey to golden yellow and blue–black traces of Pb3O4, probably due to oxidation).Only two weak could be observed while no measurable change in mass or X- lines corresponding to Pb metal could be detected. After ray diffraction pattern was detected. Upon electrochemical exposure of reduced samples to air for 3 weeks at 300 K oxidation of bismuth metal (prepared from Bi2O3) similar continued oxidation takes place: besides the reflection of changes in colour are observed.If samples are first oxidized bismuth metal and Bi2O3 a series of lines corresponding to a- until the potential of O2 gas formation is reached and sub- PbO were found, while the lines associated with Pb7Bi3 and sequently reduced, then a charge transfer of ca. 0.3 e- f.u.-1 Pb had virtually disappeared. We conclude that the freshly at -740 mV can be measured. Quantitative reoxidation is reduced product does contain also a component of amorphous thus not possible; the high stability of thin oxide layers on lead–bismuth alloy. bulk bismuth metal upon electrochemical oxidation has been For studies on the preparation of heterogeneous metal–non- described earlier.19 reducible oxide porous systems the two ternary phases (Bi2O3)0.75(WO3)0.25 (described as cubic)7 and Bi1.72Yb0.28O38,9 (cubic) were selected (Table 1).We assumed that in both cases Conclusions only Bi3+ could be reduced to the metal while tungsten and The processes investigated belong to an interesting specific type of low-temperature solid-state reaction. The exact mor-470 6 JCPDS, International Centre for Diffraction Data, Newtown phology of the microscopic pore structure has not been estab- Square, PA, 1990, PDF card no. 41-0404. lished so far, it may depend on several parameters.1 The data 7 Y. J. Lee, C. O. Park, H. D. Baek and J. S. Hwang, Solid State of the present study on bismuth compounds favour the model Ionics, 1995, 76, 1.of a matrix system built up from approximately isometric 8 H. Iwahara, T. Esaka, T. Sato and T. Takahashi, J. Solid State metal cluster units rather than a sponge-like structure similar Chem., 1981, 39, 173. 9 JCPDS, International Centre For Diffraction Data, Newtown to, e.g., reticulated glassy carbon materials. The results of a Square, PA, 1990, PDF card no. 41-0288. preliminary scanning tunnelling electron microscopy study on 10 G. Brauer, Handbuch der pra� parativen anorganischen Chemie, the reduction of a copper oxide single crystal also appear to Ferdinand Enke Verlag, Stuttgart, 1975, p. 596. support the cluster model;20 it seems problematic, however, to 11 Gmelin, Handbuch der anorganischen Chemie, vol. 19 (Wismut), conclude from structures found in the surface region on the VCH, Weinheim, 1964, pp. 722ff. 12 B. Aurivillius, Acta Chem. 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