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Iodometric Determination of Electrons Incorporated into Cages in 12CaO · 7Al2O3 Crystals Toshihiro Yoshizumi,†,‡ Satoru Matsuishi,‡ Sung-Wng Kim,§ Hideo Hosono,‡,§ and Katsuro Hayashi*,†,‡ Secure Materials Center, Materials and Structures Laboratory, and Frontier Research Center, Tokyo Institute of Technology, 4259 Nagatsuta, Yokohama 226-8503, Japan ReceiVed: June 14, 2010
The accurate determination of the concentration of electrons incorporated into cages in 12CaO · 7Al2O3 (C12A7) has been examined using iodometry. Preliminary experiments confirmed that iodine is reduced in preference to protons by electron-enriched C12A7, even in aqueous solution. The electron concentrations in several C12A7 samples are measured using a procedure where the iodine molecules in an I2-HCl solution are partially reduced by electrons released from the samples, followed by titration of the residual iodine molecules. The accuracy of this procedure is confirmed by the excellent agreement of the results with those from thermogravimetric analysis. The reevaluated maximum electron mobility is nearly twice as high as that reported previously, and the critical electron concentration for the metal-insulator transition is revised slightly because of the accuracy of the iodometry results. 1. Introduction Nonstoichiometry and/or the presence of ions with unusual valence states play crucial roles in the chemical and physical properties of metal oxides. For example, oxygen deficiency in cuprate superconductors is connected to the hole concentration, which affects the superconducting transition temperature. Iodometric titration (iodometry) has been successfully applied to accurately determine the extent of oxygen deficiency in superconductors1 and other transition-metal oxides.2 The advantage of iodometry over other quantification methods, such as thermogravimetric analysis and evolved gas volumetry,3 is that a smaller amount of sample is required to achieve a given accuracy. In the case of superconductors, a sample of ∼10 mg allows the oxygen nonstoichiometry to be determined with an accuracy of better than 0.15%.1 This study uses iodometry to determine the concentration of electrons incorporated in cages of 12CaO · 7Al2O3 (C12A7, mayenite). The crystal lattice of C12A7 is composed of positively charged subnanometer-sized cages, in which various anions, such as OH-,4,5 F-,6 Cl-,6 H-,7,8 Au-,9 O-,10 O2-,10,11 S2-, and O22-, are incorporated to compensate for the positive charges. The stoichiometric unit cell is expressed as [Ca24Al28O64]4+(O2-)2, where two extraframework O2- ions occupy two of the twelve cages in the unit cell. The extraframework O2- ions can be exchanged for electrons by chemical reduction, giving a unit cell that is represented as [Ca24Al28O64]4+(O2-)2-x(e-)2x, where x ranges from 0 to 2. The theoretical maximum of the exchange, that is, x ) 2, corresponds to an electron concentration of 2.3 × 1021 cm-3. Because the electron occupies the empty space of the inner cage in a similar manner to the extraframework anions, the fully or sometimes partially electron-exchanged C12A7 are termed “electrides”12 according to their analogy to ionic salt electrides composed of organic clathrates of saline cations and electrons.13 Although the oxygen nonstoichiometry * To whom correspondence should be addressed. Phone: +81-45-9245337. Fax: +81-45-924-5365. E-mail:
[email protected]. † Secure Materials Center. ‡ Materials and Structures Laboratory. § Frontier Research Center.
directly relates to the electron concentration in C12A7, it does not cause a valence change in the framework species. This is in contrast to typical oxygen nonstoichiometry that is compensated by a valence change in the transition-metal cations. Although a redox reaction involving altervalent cations has been utilized in conventional iodometry to determine the oxygen nonstoichiometry, this study will demonstrate that electrons can also act alone as a redox species in iodometry even in aqueous solution. Stoichiometric C12A7 is a good electrical insulator similar to typical light metal oxides. Electron doping affords C12A7 with electronic conductivity. When the electron concentration, Ne, is below ∼1 × 1019 cm-3, each electron is well localized in a cage and intercage migration is mediated by thermally activated hopping; that is, the material behaves as an insulator. The localized electron can be quantified readily by electron paramagnetic resonance (EPR) spectroscopy.7,12 As Ne increases above ∼5 × 1020 cm-3, the conductivity mechanism shifts from hopping to metallic conduction. In this Ne range, EPR is not available due to a skin effect in highly conductive media. Thus, high concentrations of Ne have been evaluated by analysis of reflection spectra.14 However, this analysis is complex and the sample preparation requires very careful surface treatment. In this study, Ne in the range from ∼1 × 1021 to 2 × 1021 cm-3 and the critical Ne value for the metal-insulator transition are evaluated accurately and easily using iodometry. 2. Experimental Section Electron-doped samples were prepared by the metallic Ti reduction process as described in ref 18. In brief, a Czochralskigrown C12A7 single crystal was cut into slices with approximate dimensions of 1 × 5 × 10 mm. The crystal slices were sealed with Ti pellets in evacuated silica glass capsules and then heated under the conditions listed in Table 1. After heating, the surface of the samples was covered with a reacted layer consisting of a series of TiO2-δ phases, which was then removed carefully by mechanical grinding. Parts of the samples were further ground into a powder using an alumina mortar for iodometry and thermogravimetric analysis (TGA).
10.1021/jp1054364 2010 American Chemical Society Published on Web 08/24/2010
Iodometric Determination of Electrons in 12CaO · 7Al2O3
J. Phys. Chem. C, Vol. 114, No. 36, 2010 15355
TABLE 1: Sample Annealing Conditions and Electron Concentration (Ne) Determined by Iodometry, TGA, Reflectance Analysis, and Electrical Conductivity Ne (cm-3)
annealing conditions sample no. 1 2 3 4 5
temperature (°C) 1100 900 900 800 800
time (h)
iodometry
24 24 12 24 12
2.10(2) × 10 6.9 × 1020 4.8 × 1020 3.6 × 1020 3.5 × 1020
To confirm that the electron in C12A7 is the exclusive reductant of I2 into I- ions, the following preliminary experiment was performed. All chemicals used in this experiment were reagent grade and received from Kanto Chemicals Co., Inc., Japan. About 100 mg of powdered sample 1 was placed in a Pyrex glass beaker. An aqueous solution of I2 (1.0 × 10-3 M, 30 mL) was added. The mixture was magnetically stirred, and its temperature was maintained at 20 °C. An aqueous solution of HCl with a pH of ∼0.1 (20 mL) was gradually added to dissolve the C12A7 powder. Changes in the pH and potential of the solution over time were measured using a pH-potential meter (F-53, Horiba, Japan) equipped with a Ag/AgCl glass electrolyte pH probe and a Ag/AgCl potential probe. The iodometric titration was performed with the following procedure. Approximately 10 mg of each powered sample was placed in Pyrex glass vessels. Aqueous I2 solution (1.0 × 10-3 or 5.0 × 10-3 M, ∼2 mL) was then poured into each vessel. The pH was adjusted to ∼0.5 with HCl. The vessel was sealed to prevent the reoxidation of I- ions by ambient oxygen, and the stirred solution was maintained at a temperature of 20 °C. The sample dissolved, and the encaged electrons were consumed by the reduction of I2 into I- ions. After confirming complete dissolution of the sample, the amount of residual I2 was titrated using sodium thiosulfate solution (1.0 × 10-3 or 5.0 × 10-3 M). Observation of the endpoint was enhanced by adding a few drops of starch solution, which induces a violet coloration. To validate the results from iodometry, Ne was also evaluated using TGA, optical reflectance spectra,14 and the conductivity-Ne relationship reported previously.18 Approximately 30 mg of each sample was used for TGA. The change in the weight of the sample during heating from room temperature to 1350 °C at a rate of 10 °C min-1 in a dry N2 atmosphere was measured using a thermogravimetricdifferential thermal analyzer (TG-DTA, Rigaku Thermoplus). The oxygen partial pressure, p(O2), of the exhaust gas from the TG-DTA was monitored using a zirconia oxygen sensor. Measurements were repeated a few times for each sample to ensure reproducibility and enhance statistical accuracy. For the reflection spectroscopy, the Ti-reduced samples were polished to a mirror surface using slurries of diamond and alumina. Fresh C12A7 surfaces were oxidized or hydrated simply by exposure to air. The samples were dipped into a solution of H2PO4 (1 M), followed by rinsing with distilled water. Removal of the hydrated surface layer and formation of an ultrathin phosphate protection layer should hinder the deterioration of the crystal surface. Reflectance spectra were measured from 200 to 2500 nm with a Hitachi U-4000 spectrophotometer. The value of Ne was estimated according to the analytical procedure described in ref 14. For the dc conductivity measurement, platinum electrodes were formed on the phosphate-stabilized surface by sputtering. The conductivities were measured using a four-probe configuration over the temperature range from 2 to 300 K. Ne was estimated from the conductivity-Ne relationship measured at 300 K and that extrapolated at 0 K as reported in ref 18.
TGA 21
2.2(2) × 10 7(2) × 1020 5(2) × 1020 4(2) × 1020 4(2) × 1020
21
reflectance
conductivity
2.0 × 10 9 × 1020 8 × 1020 8 × 1020 7 × 1020
2.2 × 1021 1.1 × 1021 7 × 1020 4 × 1020 2 × 1020
21
3. Results Figure 1 shows the potential change during the dissolution of sample 1 in the solution containing HCl and I2. The inset shows the change in potential as a function of pH. Because C12A7 is basic, the increase in pH is a measure of the amount of sample dissolved in the solution containing HCl and I2. Initially, the potential of the solution was fixed at 0.62 V (vs NHE). When a certain amount of the sample had dissolved, the potential dropped abruptly and subsequently settled at around 0 V. The upper and lower dashed lines in the inset of Figure 1 indicate the redox potential of I2/I- in aqueous solution:16
I2(aq) + 2e- T 2IE(I2 /I-) ) 0.621 V and that for H+/H2
2H+ + 2e- T H2 E(H+ /H2) ) 0.000 -
RT ln 10 pH V F
(1)
where R is the gas constant, T is the temperature, and F is the Faraday constant. The coincidence of the measured change in potential of the two redox potentials can be interpreted as follows: encaged electrons exclusively reduce I2 to form I- ions until all of the I2 molecules are reduced; after that, the reduction of protons begins. These observations indicate that (1) encaged electrons have high enough potential to reduce water to evolve hydrogen gas; however, (2) if I2 is present, it is reduced in preference to water. Hence, determining the concentration of caged electrons by iodometry is possible in aqueous solution.
Figure 1. Changes in potential of the solution containing I2 and HCl during dissolution of sample 1. The inset shows the change in potential versus pH with a measurement resolution of 0.01. Dashed lines indicate the redox potentials of I2(aq)/I- and H+/H2.
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Figure 3. Electron mobility versus Ne estimated by iodometry (solid lines) and reported in ref 18 (dashed and dotted lines). The mobility at 0 K was obtained by extrapolation to the low temperature.
Figure 2. Correlation between Ne determined by iodometry and TGA (a) and comparison with Ne obtained from analysis of reflectance spectra (circles) and the conductivity-Ne relationship in ref 18 (squares). (b). Filled squares were determined using optical transmission spectroscopy.
The Ne values for five samples determined by both iodometric titration and TGA are indicated in Table 1. The Ne value (2.1 × 1021 cm-3) obtained by iodometry for the sample that was reduced the most (sample 1) nearly reached the theoretical maximum, validating iodometry as an electron determination procedure. The Ne values were also estimated by TGA because this is the most accepted reference method for confirming the accuracy of iodometry.1 In all of the measurements, a small weight loss due to physisorbed and chemisorbed water was observed in the temperature range of ∼100 to 200 °C. The effective weight change caused by oxidation was taken from the difference in weight between the unoxidized “dry” region at around 200 °C and the fully oxidized plateau region at around 1300 °C. For example, full oxidation of C12A7 with x ) 2, [Ca24Al28O64]4+(4e-) + O2(g) f [Ca24Al28O64]4+(O2-)2, corresponds to a weight gain of 1.17%. The typical p(O2) value in the exhaust gas from the TG-DTA furnace was 1 × 10-3 atm, which is suitable for fully oxidizing the sample and preventing formation of excess oxygen species, such as O-, O2-, and O22-.17 Figure 2a compares the results from iodometry and TGA. The coincidence of the two sets of data within the measurement error confirms the accuracy of the iodometry results. The Ne values obtained from analysis of infrared reflectance spectra and the conductivity-Ne relationship are also indicated in Table 1. These values are compared with those obtained from iodometry in Figure 2b. In the data of the conductivity-Ne relationship, the Ne values greater than 0.5 × 1021 cm-3 were obtained in the same manner as that in ref 14 using reflectance
analysis, whereas Ne values of less than 0.5 × 1021 cm-3 were obtained from the absorption of the F+ band in transmission spectra.18 Namely, all data indicated by open symbols in Figure 2b are based on the analysis of reflectance spectra. Around 2 × 1021 cm-3, these data agree well with the Ne values determined by iodometry. However, the discrepancy becomes larger as Ne decreases. This dependency will be examined further in section 4. The Ne dependence of the electron mobility was reevaluated using the conductivity and iodometry-based data and is plotted in Figure 3 together with the previous conductivity-Ne relationship,18 which was principally determined from analysis of reflectance spectra. The mobility at 300 K for Ne values around 1 × 1021 cm-3 is found to be nearly twice as high as that determined previously. The mobility at 0 K was obtained from extrapolation of the conductivity at low temperature to 0 K. The threshold Ne value for the metal-insulator transition is 7 × 1020 cm-3, which is slightly lower than that reported previously (1.0 × 1021 cm-3).18 4. Discussion The simultaneous increase in Ne and electron mobility, as shown in Figure 3, has been regarded as a property unique to C12A7. When the electron concentration is low, each isolated electron is localized in a cage because of lattice relaxation caused both by itself and by the presence of extraframework O2- ions in neighboring cages. The localized electron migrates to neighboring empty cages by hopping, resulting in some electrical conductivity in the “insulating” state. As the electron concentration increases, the lattice relaxation ceases, and the electrons become delocalized over many cages, occupying the cage conduction band and inducing the metal-insulator transition. The increase in the ratio of delocalized electrons is the main contributor to the rapid increase in the total electron mobility. The separation of such delocalized and localized electrons is advantageous for reflectance analysis.14 The former is ascribed to a Drude-type component in the optical response near the zeroenergy limit, whereas the latter is estimated mainly from Lorentz-type components for the intercage transition band at ∼0.4 eV, intracage transition at ∼2.8 eV, and higher transition bands. The good agreement between the Ne values obtained by iodometry and reflectance analysis for higher Ne (Figure 2b) suggests that the values obtained for delocalized electrons from the Drude component are correct, but the Ne of the localized electrons have been overestimated. The reason for this may be
Iodometric Determination of Electrons in 12CaO · 7Al2O3 attributed partially to the superposition of the 0.4 eV intercage band with the Drude component and limited resolution of the higher-energy Lorentz components. Thus, we suggest that the combination of iodometry for determining the total electron concentration and reflectance analysis for assessing the delocalized electron concentration will provide a greater understanding of electron-transport properties. Finally, reaction selectivity in aqueous solution was observed. The present study demonstrates that the selectivity is 100% for the combination of the C12A7 electride with I2. Such a high selectivity was also demonstrated for the pinacol coupling reaction of benzaldehydes using C12A7 electride in aqueous solution.19 However, a direct relationship was not observed between the electron-withdrawing properties of the substituents of the benzaldehydes and the product yield. Thus, the redox potential is most likely not the sole factor determining the selectivity. In these reactions,19 a gel layer formed between C12A7 and the aqueous solution is considered to play a central role in the reaction as a reaction field. However, its contribution to the enhancement of selectivity is unclear because no gel layer is formed in our experiment due to the high acidity of the solution. Thus, further experiments are needed to resolve the key factor affecting selectivity. For example, various combinations of species with different valences in C12A7 (such as e-, H-, Au-, O-, O2-, and O22-) and redox species in aqueous media are possible. The pH may also be important because it affects the formation of a gel layer as well as the activity of proton-related species. In addition, expansion of iodometry to other inorganic electride systems20 is a promising possibility. 5. Conclusions Iodometry was used to quantify the electron concentration in C12A7, and the following findings were made. (1) In the presence of I2 in aqueous solution, the encaged electrons reduce I2 molecules in preference to protons, enabling the accurate determination of the concentration of encaged electrons by iodometry. Furthermore, the accuracy of the iodometry results was ascertained by their agreement with those obtained using TGA. (2) From the reevaluated Ne value determined by iodometry, the electron mobility is found to be nearly 2 times higher than the maximum reported previously. The critical Ne for the
J. Phys. Chem. C, Vol. 114, No. 36, 2010 15357 metal-insulator transition is revised slightly from 1.0 × 1021 to 0.7 × 1021 cm-3. Acknowledgment. This work was supported by Grants-inAid for Elements Strategy Project (No. 08055013) and Grantin-Aid for Young Researchers A (No. 19685019) from MEXT, Japanese Government. References and Notes (1) Vlaeminck, H.; Goossenes, H. H.; Mouton, R.; Hoste, S.; Van der Kelen, G. J. Mater. Chem. 1991, 1, 863–866. (2) Mizusaki, J.; Tagawa, H.; Naraya, K.; Sasamoto, T. Solid State Ionics 1991, 49, 111–118. (3) Conder, K.; Rusiecki, S.; Kaldis, E. Mater. Res. Bull. 1989, 24, 581–587. (4) Imlach, J. A.; Glasser, L. S. D.; Glasser, P. F. Cem. Concr. Res. 1971, 1, 57–61. (5) Lee, D.-K.; Kogel, L.; Ebbinghaus, S. G.; Valov, I.; Wiemhoefer, H.-D.; Lerch, M.; Janek, J. Phys. Chem. Chem. Phys. 2009, 11, 3105– 3114. (6) Jeevaratnam, J.; Glasser, F. P.; Glasser, L. S. D. J. Am. Ceram. Soc. 1964, 47, 105–106. (7) Hayashi, K.; Matsuishi, S.; Kamiya, T.; Hirano, M.; Hosono, H. Nature 2002, 419, 462–465. (8) Medvedeva, J. E.; Freeman, A. J.; Bertoni, M. I.; Mason, T. O. Phys. ReV. Lett. 2004, 93, 016408. (9) Miyakawa, M.; Kamioka, H.; Hirano, M.; Kamiya, T.; Sushko, P. V.; Shluger, A. L.; Matsunami, N.; Hosono, H. Phys. ReV. B 2006, 73, 205108. (10) Hayashi, K.; Hirano, M.; Matsuishi, S.; Hosono, H. J. Am. Chem. Soc. 2002, 124, 738–739. (11) Hosono, H.; Abe, Y. Inorg. Chem. 1987, 26, 1192–1195. (12) Matsuishi, S.; Toda, Y.; Miyakawa, M.; Hayashi, K.; Kamiya, T.; Hirano, M.; Tanaka, I.; Hosono, H. Science 2003, 301, 626–629. (13) (a) Dye, J. L. Inorg. Chem. 1997, 36, 3816–3826. (b) Dye, J. L.; Wagner, M. J.; Overny, G.; Huang, R. H.; Nagy, T. F.; Tomanek, D. J. Am. Chem. Soc. 1996, 118, 7329–7336. (14) Matsuishi, S.; Kim, S. W.; Kamiya, T.; Hirano, M.; Hosono, H. J. Phys. Chem. C 2008, 112, 4753–4760. (15) Kurashige, K.; Toda, Y.; Matsuishi, S.; Hayashi, K.; Hirano, M.; Hosono, H. Cryst. Growth Des. 2006, 6, 1602–1605. (16) Allen, J. B., Ed. Encyclopedia of Electrochemistry of the Elements; M. Dekker: New York, 1973; Vol. 1, p 92. (17) (a) Hayashi, K.; Matsuishi, S.; Hirano, M.; Hosono, H. J. Phys. Chem. B 2004, 108, 8920–8925. (b) Hayashi, K.; Hirano, M.; Hosono, H. Chem. Lett. 2005, 34, 586–587. (18) Kim, S. W.; Matsuishi, S.; Nomura, T.; Kubota, Y.; Takata, M.; Hayashi, K.; Kamiya, T.; Hirano, M.; Hosono, H. Nano Lett. 2007, 7, 1138– 1143. (19) Haritha, B.; Toda, Y.; Hirano, M.; Hosono, H.; Takeuchi, D.; Osakada, K. Org. Lett. 2007, 9, 4287–4289. (20) Ichimura, A. S.; Dye, J. L. J. Am. Chem. Soc. 2002, 124, 1170–1171.
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