J. Phys. Chem. 1988, 92, 6157-6160
6157
Concentration Dependence of Extended X-ray Absorption Fine Structure and X-ray Absorption Near-Edge Structure of Copper( I I ) Perchlorate Aqueous Solution: Comparison of Solute Structure in Liquid and Glassy States Masaharu Nomura**+and Toshio Yamaguchit Photon Factory, National Laboratory for High Energy Physics, Oho, Tsukuba 305, Japan, and Department of Chemistry, Faculty of Science, Fukuoka University, Nanakuma, Jonan- ku. Fukuoka 81 4-01, Japan (Received: December 23, 1987; In Final Form: April 21, 1988)
EXAFS and XANES have been measured on aqueous solutions of copper(I1) perchlorate hexahydrate between 0.3 mol dm-3 and its saturation in both liquid and glassy states. No structural or electron configurational change by vitrification was found. The structure of hexaaquacopper(I1) complex does not change with concentration (res = 1.96 8, and Tax = 2.3 A), but the outer-sphere coordination of perchlorate ions occurs and the axial Debye-Waller factor increases in the concentrated solutions. The XANES spectra change with concentration, which is also due to the outer-sphere coordination of perchlorate ion. The slowly cooled solution produces crystalline units, but there is very weak long-range order in it. It is indicated that XANES is strongly affected by the environment of a few angstroms distant from an X-ray absorbing atom in this system.
Introduction Copper(I1) perchlorate is well-known to take the distorted octahedral structure of [CU(OH,)~]*' in its aqueous solution owing to the Jahn-Teller effect. There have been some reports on its structure in aqueous solutions by means of X-ray ~catteringl-~ and EXAFS (extended X-ray absorption fine stru~ture)!.~ However, significant distribution on the axial Cu-0 bond length (2.28-2.60 A) was found among these reports. Structural change of solute as a function of concentration seems to be one of the reasons of the above described distribution. The reported axial bond length increases and the equatorial one decreases with the increase of concentration except those reported by Sham et a1.4 (see Table 11). This tendency may be the reflection of the outer-sphere coordination of perchlorate ions which was indicated by a study of the osmotic coefficient and electronic spectra at concentrations exceeding 1 mol dm-3.6 The lack of X-ray scattering data on dilute solutions is due to the difficulty in separating the copper-oxygen interactions from the oxygenoxygen interactions in bulk water and free perchlorate ion.' EXAFS is superior in this point because only the interaction between a central atom and ligands can be observed without any interference from solvent and counterion, which thus makes it possible to elucidate the structural parameters in dilute solutions. However, there is no EXAFS study on highly concentrated solutions nor on the structural change of the complex with the change of solute concentration. If direct contact of complexes and/or outer-sphere associate of copper(I1) and perchlorate ion cannot be neglected in the solution, XANES (X-ray absorption near-edge structure) and structural parameters derived from EXAFS will change with increasing solute concentration. The distribution of copper-axial oxygen bond lengths is also due to the large thermal vibration or distribution of the bond lengths. In order to decrease the thermal fluctuation and examine the property of Cu-0 bonds, we have compared EXAFS spectra of both liquid and glassy solutions. In connection with the study on glassy state, it is important to know whether the solute in glassy state has a structure similar to that in liquid state, which has been believed with little evidence.' EXAFS and XANES are suited to investigate the structure and electronic state of complexes in glassy solutions, because they have no restrictions on the state of sample and because especially the latter is very sensitive to the change of symmetry of the complex around an X-ray absorbing atom. If the solute crystallizes and changes its structure during vitrification, XANES will change. However, there is no EXAFS and XANES study on solute structure in glassy solutions except 'National Laboratory for High Energy Physics. t Fukuoka
University.
0022-3654/88 /2092-6157$01.50/0
TABLE I: Compositions of the Aqueous Solutions'
abbrevb [Cu2+] CUPC-0.3 0.29 CUPC-0.3G 0.29 CUPC-1.OG 0.92 CUPC-2.3 2.3 CUPC-2.5 2.48 CUPC-s 3.44 CUAC-S 0.30 0.08 CUAC-SG
ICl0,l I
'The concentrations are in mol d ~ n - ~H20-gly . means the equivolume mixture of water and glycerol is used as the solvent. *G means glycerol was added and S means saturated solution. our report on copper(I1) acetate solution^.^ The comparison of EXAFS and XANES data on solutions and crystalline sample will also clarify the significance of the medium-range order to them. In order to solve the inconsistency in the reported structures of copper(I1) aqua complex, we have measured the concentration dependence of EXAFS and XANES on aqueous solutions and their glasses of copper(I1) perchlorate between 0.3 mol dm-3 and saturation. Those of copper(I1) acetate solutions have been measured in order to determine the condition of vitrification. Experimental Section Sample solutions were made by dissolving weighed amounts of reagent grade copper(I1) perchlorate hexahydrate, Cu(C104)2.6H20, or copper(I1) acetate monohydrate, Cu2(CH3CO0)4.2H20,in distilled water or an equivolume mixture of distilled water and glycerol. Glycerol was added in order to prevent crystallization of sample solutions during vitrification when their concentration was less than 1 mol dm-3. The copper content was determined by direct titration with a standard EDTA solution using PAN (a-pyridyl-P-azonaphthol)as an indicator, and the results are listed in Table I with their abbreviations. Powdery C ~ ( C 1 0 ~ ) ~ . 6was H ~mixed 0 with Apiezon grease at 5 OC, and (1) Ohtaki, H.; Maeda, M. Bull. Chem. SOC.Jpn. 1974, 47, 2197. (2) Ohtaki, H.; Yamaguchi, T.; Maeda, M. Bull. Chern. SOC.Jpn. 1976, 49, 701. (3) Magini, M. Inorg. Chern. 1982, 21, 1535. (4) Sham, T. K.; Hastings, J. B.; Perlman, M. L. Chem. Phys. Lett. 1981,
83,391.
(5) Tajiri, Y . ; Wakita, H. Bull. Chem. SOC.Jpn. 1986, 59, 2285. (6) Libus, Z.; Sadowska, T. J . Phys. Chem. 1969, 73, 3229. (7) Nomura, M.; Yamaguchi, T. J . Phys. (Paris) 1986, C8, 619.
Q 1988 American Chemical Societv
6158 The Journal of Physical Chemistry, Vol. 92, No. 21, 1988 C
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-
8960
9000
9040
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BC
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1
8960
Nomura and Yamaguchi
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BC
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EIeV EIeV Figure 1. XANES spectra and their energy derivatives of the samples at room temperature. Aqueous copper(I1) perchlorate solutions: (a) 0.29 M (CUPC-0.3G); (b) 0.92 M (CUPC-1.OG); (c) 2.48 M (CUPC-2.5); (d) 3.44 M (CUPC-S). (e) C U ( C ~ O ~ ) ~powder; - ~ H ~(f) O aqueous 0.08 M copper(I1) acetate solution (CUAC-SG); (g) Cu2(CH3C00),.2H20 powder.
its spectra were measured in vacuo to prevent deliquescence. Sample solutions were sealed in cells of appropriate thickness or treated as liquid film. Special care was taken with regard to the sample thickness in order to prevent the degradation of spectra by higher order harmonics.8 Glass samples were prepared by immersing their original solutions into liquid nitrogen, and then they were kept at this or lower temperature in a vacuum cryostat. The vitrification of the samples was visually checked to see that they were transparent. Sometimes cracks were observed in the glass, but they did not degrade the spectra. EXAFS and XANES were measured in transmission mode at the BLlOB station of Photon Factory (typical operating conditions were 2.5 GeV and 160 mA). X-rays were monochromatized by an Si(311) channel-cut monochromator. The absolute X-ray photon energy was not calibrated, but the energy of a small peak in the edge of metallic copper was assumed to be 8980.6 eV.8 The energy resolution of the optical system was ca. 0.8 eV and the energy reproduced within f0.2 eV. Reproducibility of the EXAFS spectra was checked carefully.
Results XANES at Room Temperature. XANES spectra and their derivatives of solution and powdery samples are shown in Figure 1. Five prominent structures marked as A-E are found in the spectra of solutions. Structures A, C, and E do not change their shape with the increase of concentration. Peak A is assigned to 1s 3d electronic transition, indicating that the copper is di~ a l e n t . However, ~ the height of peak C normalized to atomic absorption decreases and shoulders B and D become clearer with increasing solute concentration. Two factors were expected to be responsible for this spectral change. One of them was the change of solute medium by adding glycerol to water. However, t h e XANES of CUPC-0.3 was t h e same as t h a t of CUPC-0.3G, indicating no significant change in the environment of Cu(I1) by the addition of glycerol in the solution. The other was hydrolysis of Cu(II), but this was found to be negligible. As the logarithm of equilibrium constant ( p ) of Cu2+ + OH- = CuOH+ is 6.41° and the pH value of the solutions is less than 3, the molar ratio of [CuOH'] to total copper in the solutions becomes less than 0.1%. Moreover, there was no precipitate in the solutions. When
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(8) Nomura, M.; Kazusaka, A.; Kakuta, N.; Ukisu, Y.; Miyahara, K. J . Chem. Soc., Faraday Trans. 1 1987, 83, 1227. (9) Nomura, M.; Kazusaka, A.; Kakuta, N.; Ukisu, Y.; Miyahara, K. Chem. Phys. Lett. 1985, 122, 538. (10) Hogfeldt, E. Stability Constants of Metal-lon Complexes Part A: Inorganic Ligands; Pergamon: Oxford, 1982; p 44.
$0
9000
9040
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9000
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E/eV E/eV Figure 2. XANES spectra and their energy derivatives of the samples at liquid nitrogen temperature. Aqueous copper(I1) perchlorate solutions: (a) 0.29 M (CUPC-0.3G); (b) 0.92 M (CUPC-1.OG); (c) 2.48 M (CUPC-2.5); (d) 3.44 M (CUPC-S); (e) slowly cooled 0.29 M aqueous solution (CUPC-0.3). (f) C ~ ( C 1 0 ~ ) ~ . 6powder. H~0
0.1 volume of concentrated perchloric acid solution (70%) was added to CUPC-0.3G, no change was found in XANES, which thus supports the small contribution of the hydrolysis in the solution. A similar result was reported by LibuS and Sadowska using electronic spectroscopy.6 Powdery copper(I1) perchlorate hexahydrate shows a different XANES spectrum from those of solutions, though a quite similar spectrum is expected from the presence of the [ C U ( O H ~ ) ~unit J~+ in both states." In the crystalline sample, shoulders D and E become clearer and shoulder B changes its shape, which is more apparent in their derivatives rather than in the raw spectra. The XANES spectrum and its derivative of CUAC-S are similar to those of CUPC-0.3G except shoulder D (see the derivatives of Figure l ) , which indicates the presence of a fairly similar octahedral monomer structure in both solutions. The different shoulder D of the CUAC-S may be caused by the coordination of acetate groups to copper(II).12 The powdery copper( 11) acetate monohydrate shows a quite different XANES spectrum from those of other solutions, which is due to its characteristic dimeric structure in the crystalline state.13 Thus when the solute crystallizes during vitrification, a drastic change in XANES takes place, which was verified when CUAC-S was cooled. XANES at Liquid Nitrogen Temperature. When the solutions have been vitrified, structures B and D become more enhanced than those in liquid state (Figure 2). Other structures do not change significantly by vitrification. XANES of glass samples changes similarly as that of liquid samples as a function of concentration. Even rapidly cooled CUAC-S was opaque and showed a XANES spectrum similar to that of its crystalline state. However, rapidly cooled CUAC-SG was optically transparent and its XANES spectrum resembles that in aqueous solution, indicating little of the solute crystallizes during vitrification. This fact confirms the importance of the vitrification condition and the effectiveness of visual inspection. It is expected that slowly cooled CUPC-0.3 produces crystalline copper(I1) perchlorate hexahydrate. However, the XANES of the frozen sample is different from that of the crystalline sample, though it was opaque, Le., less sharp shoulders D and E and the (11) Mani, N. V.; Ramaseshan, S . Z . Kristallogr. 1961, 115, 97. (12) Martell, A. E.; Smith, R. M. Critical Stability Constants; Plenum: New York, 1979; Vol. 3. According to the equilibrium constants the composition of 0.1 mol dm-' solution is at least 45% monoacetato, 40% diacetato, and the remaining aqua complexes. We have tentatively assigned the species as hexaaqua complex in ref 7, which must be corrected. (13) Brown, G. M.; Chidambaram, R. Acta Crystallogr., Sect. B S t r u t . Crystallogr. Cryst. Chem. 1973, B29, 2393.
Structure of Copper(I1) Perchlorate Aqueous Solution 15:O 1
I
The Journal of Physical Chemistry, Vol. 92, No. 21, 1988 6159 TABLE 11: Structural Parameters of the Aqueous Solutions"
CUPC-0.3G CUPC-1.OG CUPC-2.5 CUPC-s
Solution 1.96 0.07 2.29 0.12 1.96 1.96 0.06 2.29 0.16 1.96 0.06 2.27 0.16
CUPC-0.3G CUPC-1.OG CUPC-2.5 CUPC-s
Glass 1.97 0.06 2.33 0.10 1.97 1.97 0.05 2.27 0.13 1.96 0.05 2.31 0.14
Crystal Cu(C104)2*6H20RT 1.95 0.06 2.34 0.10 Cu(C104)z.6Hz0 LT 1.98 0.05 2.35 0.11 frozen CUPC-0.3 1.95 0.05 2.32 0.14 1 mol dm-' aq
1.0 mol dm-) aq 1.9 mol dm-' aq 2.9 mol dm-' aq 3.6 mol dm-3 aq 00 00
10
20
30
40
50
60
r/A Figure 3. Fourier transforms of copper(I1) perchlorate crystal and its aqueous solution. Aqueous solutions of different concentrations show transforms similar to that of part a. (a) 2.48 mol dm-3 Cu(I1) aqueous solution (CUPC-2.5) at room temperature; (b) CUPC-2.5 in glassy state; ~ Hliquid ~O (c) C ~ ( C 1 0 ~ ) ~ . 6atHroom ~ 0 temperature; (d) C U ( C I O ~ ) ~ - at
nitrogen temperature. different shape of shoulder B as shown in their derivatives. EXAFS. The pre-edge background of EXAFS spectra was fitted with quadratic equations, and atomic absorption ( p o ) was determined by use of cubic spline functions.8 Fourier transforms of k 3 x ( k )over the wave-vector range of around 3.4 I k1A-I 5 15 are shown in Figure 3 without the phase shift correction. Thus the apparent peak position instead of true interatomic distance is used in this section. All the Fourier transforms except that on crystalline copper(I1) acetate9 show a similar feature, so only four spectra are shown in the figure. One main peak with a shoulder in the shorter interatomic distance side is observed at around 1.6 A, the shoulder being due to the k dependence of the phase shift. The peak is assigned to equatorial Cu-0 bonds. Long axial Cu-0 bonds are generally weak, so that the corresponding peak is difficult to be observed in the Fourier transforms, as is well-known in the lite r a t ~ r e . ~Furthermore, .~ no appreciable medium-range order is found in the transforms of copper(I1) perchlorate powder at room temperature. When it is cooled to liquid nitrogen temperature, however, peaks appear around 4-5 8,. They can be ascribed to the medium-range order in the crystals, Cu.-C1,O(C1O4). The magnitude of these peaks in the Fourier transform of slowly cooled CUPC-0.3 or CUPC-2.3 was too low to be separated from the background level.
Discussion The curve fittings of EXAFS wiggles were performed as described in a previous paper by using a plane wave, single scattering formula,8 and the best fit values are listed in Table 11. The ratio of the coordination number of the equatorial and axial ligands was fixed as 4:2 during the fitting process. Errors in interatomic distances and Debye-Waller factors are estimated as f 0 . 0 2 and f0.005 8, for equatorial Cu-0 and f0.05 and fO.O1 8, for axial ones. A large Debye-Waller factor of axial bonds (uax),little difference between axial and equatorial interatomic distances, and small backscattering factor of oxygen atom result in a rather large error of axial interaction. The interatomic distances used in this section and in the table have been corrected for the phase shift. Before discussing the results on the solute structure of this system, we summarize the structural parameters already reported
1.96 2.00 1.98 1.98 1.94
0.04 0.06 (0.04) (0.05) (0.04)
2.60 2.28 2.34 2.39 2.43
0.12 0.10
1
EXAFS this work
EXAFS EXAFS (0.07) XS (0.09) XS (0.06) XS
4
5 3
3 1
'Values in parentheses are the square root of 6. b X S , X-ray scattering. in Table 11. The equatorial Cu-0 bond lengths previously reported are similar (1.94-2.00 A), but they seem to decrease with the increase of solute concentration. On the other hand, reproted axial bond lengths scatter between 2.28 and 2.60 8, and seem to increase with increasing concentration. The value of 2.6 8, reported by Sham et al.* is unusually long compared with those of other r e p ~ r t s l - ~and . ~ one exception of the tendency described above. In the present study, we could not reproduce such a high spurious peak as shown in Figure l a of ref 4. Furthermore, their k 2 x ( k ) curve dumps very slowly and is similar to our k3x(k)curve rather than our k 2 x ( k ) one. A structural parameter determined in this work is 1.96 8, for four equatorial Cu-O bonds and 2.3 8, for two axial Cu-O bonds, independent of concentration. Thus the tendency of the C u ~ 0 bond lengths with different solute concentrations described above might be an accidental one though the difference exceeded their estimated errors. Although no concentration dependence of bond lengths nor of the equatorial Debye-Waller factor (aq) was found, the axial Debye-Wa!Jer factor (urn)does increase with the increase of concentration. This increase in uaxis also found in glassy samples, indicating it is not due to thermal vibration but due to distribution of the bond lengths. This tendency is found in a paper by Magini3 though he did not mention it. No essential structural change occurs when the solutions are vitrified as seen from their spectra. The apparent increase in the axial bond length by the vitrification is not certain, since it is within the experimental error. An assumption of the symmetric distribution of interatomic distances may be inappropriate. However, the decrease of Debye-Waller factor on the axial interaction with decreasing temperature is significant, which is ascribed to smaller thermal vibration at lower temperature. This fact suggests the weak axial bonds. XANES of the [ C U ( O H ~ ) ~ unit ] ~ ' has been calculated by Garcia et al. using a single-electron multiple-scattering theoryI4 and was compared with the experimental data of 50 mmol dm-3 copper(I1) perchlorate aqueous solution. Their experimental spectrum shows a curious shoulder that is not observed in our spectrum at the lower energy side of the main peak (Figure 6 in ref 14). Similar curious steps are found in their spectrum of crystalline CuO. Thus, these structures will be artificial ones. According to their theory, shoulder B does not appear in the edge when the difference between axial Cu-0 and equatorial Cu-0 bond lengths is smaller than 0.3 A, but it is clearly seen when the (14) Garcia, J.; Bianconi, A.; Benfatto,
1986, C8, 49.
M.; Natoli, C. R. J . Phys. (Paris)
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The Journal ofPhysica1 Chemistry, Vol. 92, No. 21, 198‘8
Nomura and Yamaguchi
the Cu-0 bond lengths of 2.08 (X2), 2.16 (XZ), and 2.28 (X2) difference is 0.6 A, which coincides with our experimental XANES 8,.11 We tried curve fittings on k 3 x ( k )according to this model, spectra of dilute solutions. With the increase of concentration, however, shoulders B and D become clear and the height of peak Le., the ratio of coordination number was fixed as 2:2:2, but they C decreases without any significant change of the axial bond were not successful. However, the fitting based on a tetragonally length, which cannot be explained by the theory. distorted octahedron, [ C U ( O H ~ ) ~ ( O H ~was ) ~ ]fairly ~ + , good. Thus the rhombic distortion must be the averaged structure of tetThe change of XANES as a function of concentration may be ragonally distorted units directed in various directions. In the ascribed to the change of environment around copper(I1). Sigcrystal, every water molecule in [ C U ( O H ~ ) ~ ]is*bound + to oxygen nificant environmental change may occur if perchlorate ion coatoms of perchlorate ions in the outer shell, and they build up a ordinates to copper directly instead of water. However, the direct network in the crystal. The rather small a,, compared with that contact of perchlorate ion and copper(I1) is not plausible because of its very low coordinating ability.I5 Futhermore, this model of concentrated solutions may be due to this uniform outer-sphere around a copper atom. is excluded by a UV-vis study6 of the C U ( C ~ O ~ ) ~ - M ~ ( C ~ O ~ ) ~ -environment H~O The difference of spectra between crystalline copper( 11) persystem owing to a blue shift of the d-d band with increasing solute chlorate hexahydrate and slowly cooled solutions must be clarified. concentration and also excluded by Raman16 and NMRI7 specAccording to visual inspection, CUPC-0.3 and CUPC-2.3 are no troscopic studies of aqueous copper(I1) perchlorate solution that longer in a glassy state but in a polycrystalline state when they showed spectra corresponding to free perchlorate ion only. The are slowly cooled down. However, the XANES of the frozen change of environment around copper(I1) will also occur when dimer or polymer is formed in the solution, but it must not be a solutions is different from that of the crystalline sample. The small peak at 4-5 8, in the Fourier transform of the crystal at low major species because there found no peaks in the Fourier temperature is an indicator of the medium-range order as seen transforms corresponding to Cu-Cu interaction. The composition in Figure 3. In the frozen solutions, the corresponding peak is = 1:2:7 of CUPC-S is calculated as [CU(OH~)~~+]:[C~~~]:[H~~] from its measured density of 1.68 g ~ m - At ~ . this condition, the not observed, indicating no medium-range order in the solutions. outer-sphere coordination of perchlorate ions to copper(I1) is most The u,, of frozen solution is larger than that of the crystalline plausible; Le., perchlorate ions exist just outside of CU(OH2)6’+ sample, which reflects a nonuniform environment around copin high probability. This model has been proposed by the above per(I1) in the case of the frozen solutions. Compared with the described UV-vis absorption spectroscopic study. According to results on copper(I1) acetate solutions, the complex itself is no more the complex in solution; however, the size of the crystal is this model, the Jahn-Teller distortion may be partially relaxed owing to the asymmetry of the ligand field around a copper atom. very small and long-range ordering is not perfect. Therefore the Consequently the axial Debye-Waller factor increases because difference of the XANES may be due to the change of mediumthere are two different axial Cu-OH2 bonds: one is outer-sphere range order and redistribution of charge owing to it, which incoordinated by water and the other is by perchlorate ion. dicates that the outer-sphere coordination and charge distribution Futhermore, as the bond between an axial water molecule and should be included in the calculation of XANES. In summary, the structure of hexaaquacopper(I1) does not Cuz+ is weak, the water molecule behaves as if it is rather free change with concentration, but an increase in the axial Debyeand it may make a weak bond with a perchlorate ion in the outer Waller factor and the change of the XANES spectra were obsphere. Once the weak bond is formed, the oxygen atom becomes served with increasing solute Concentration, indicating the outslightly negative. This change of charge in the axial position will er-sphere coordination of perchlorate ion to the hexaaquacopexplain the change of structures B-E in XANES and changes in UV-vis spectra.6 Theoretical calculation will be necessary for per(I1) in solution. XANES spectrum changes with concentration, which also indicates the outer-sphere coordination of perchlorate further discussion. The complex unit in copper(I1) perchlorate hexahydrate crystal ion. The slowly cooled solution produces crystalline units, but must be discussed. According to the literature, six oxygen atoms there is little medium-range order such as in crystalline copper(I1) perchlorate hexahydrate. form a rhombic distorted octahedron around a copper atom with (15) Gowda, N. M. N.; Naikar, S. B.; Reddy, G. K. N. Adu. Inorg. Chem. Radiochem. 1984, 28, 2 5 5 . (16) Hester, R. E.; Plane, R. A. Inorg. Chem. 1964, 3, 769. (17) Klanberg, F.; Hunt,J. P.; Dodgen, H. W. Inorg. Chem. 1963,2, 139.
Acknowledgment. This work has been performed under the approval of the Photon Factory Advisory Committee (Proposal NO. 85-021). Registry No. Cu(C104)2,13770-18-8.
