- Email: [email protected]
Microstructure study of amorphous vanadium oxide films
Applied Surface Science 143 Ž1999. 6–10
Microstructure study of amorphous vanadium oxide films Joachim P. Schreckenbach a
a,)
, Peter Strauch
b
Department of Chemistry, Technische UniÕersitat ¨ Chemnitz, D-09107 Chemnitz, Germany b Institute of Inorganic Chemistry, UniÕersitat ¨ Leipzig, D-04103 Leipzig, Germany Received 15 September 1998; accepted 9 January 1999
Abstract Conversion films of vanadium oxides are potentiodynamically generated on vanadium in acetate electrolyte systems at high voltages. The microstructure of the about 5 mm thin anodic films is investigated. X-ray diffraction and transmission electron microscopy indicate the films are complete amorphous. X-ray photoelectron spectroscopy ŽXPS. measurements show V 2p 3r2 binding energies of mixed valance vanadium sub oxides. Electron spin resonance ŽESR. experiments on isolated films at 130 K point to paramagnetic V 4q centers in a disordered octahedral oxygen surrounding. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Anodic films; Vanadium oxide; X-ray photoelectron spectroscopy ŽXPS.; Electron spin resonance ŽESR.
1. Introduction Thin films of vanadium oxides have potential use for a wide variety of applications such as high energy density lithium microbatteries, electrochromic devises or catalysts and vanadium oxides are even an constituent of the surface of medical Ti–Al–V implants w1–3x. Various deposition and coating techniques results in vanadium oxide films, for example flash evaporation w1x, sol–gel technique w4x, vacuum evaporation w5x or electron beam evaporation w6x have been reported. Electrochemical processing, increasingly used for the fabrication of coatings and conversion films by surface modification w7,8x of transition metals has been more rarely investigated in case of the generation of anodic vanadium oxide films. One of the )
Corresponding author. Tel.: q49-371-531-1478; Fax: q49371-531-1371; E-mail: [email protected]
reasons is the difficulty to stabilize the prepared film and to avoid its dissolution and chemical decomposition by side reactions during the anodization. A suitable electrolyte to stabilize the anodic oxide film is the system glacial acetic acidrsodium boraterwater w9–11x. In this case the generation of the anodic oxide film results from conversion reactions of the anode material with the oxygen compounds of the electrolyte system according to the brutto reaction: xV q yO 2y™ VxO y q 2 yey Due to the high complexity of the VrO systems mixed valence vanadium oxides and multiphase formation are expected. The possible oxidation states of vanadium varies from V 2q to V 5q and the stoichiometric composition of the corresponding oxides is given by VO, V2 O 3 , VO 2 and V2 O5 , respectively. Furthermore a homologous series of Magneli phases VnO 2 ny1 Ž n s 3–8. and the complex oxides
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 0 8 4 - 7
J.P. Schreckenbach, P. Strauchr Applied Surface Science 143 (1999) 6–10
VnO 2 nq1 Ž n s 3,4,6. are described for bulk vanadium oxides w12x. The present study was undertaken to prepare vanadium oxide conversion films by high electric field anodization, to characterize the microstructure oft these generated anodic films and to determine the vanadium valence state by means of scanning electron microscopy ŽSEM., X-ray photoelectron spectroscopy ŽXPS. and electron spin resonance ŽESR. experiments.
2. Experimental The vanadium oxide films were prepared in an electrochemical cell by potentiodynamic anodic conversion at potentials up to 600 V. The power supply was a computer-controlled DC Heinzinger PHN. Solutions consisting of AR-grade acetic acid, 0.02 M sodium borate ŽNa 2 B 4 O 7 P 10H 2 O., 1 M water and additional 0.1 M barium acetate were used as electrolyte. The experiments were carried out with flag-shaped vanadium foil anodes 10 = 10 mm2 , 0.127 mm thickness and 99.7% purity ŽAldrich. while platinum wire constituted as cathode. The electrolyte temperature was thermostatically controlled to 258C. The vanadium samples were cleaned in acetone and electrochemically polished Ž15 V. for 60 s in a mixture of 80% methanol and 20% sulfuric acid. After the coating process the samples were rinsed in acetone and dried and stored in an argon atmosphere. For characterization, specimens were examined by scanning electron microscopy using a JEOL 840 A with EDS KEVEX DELTA. X-ray photoelectron spectra were recorded on a SAGE 100 ŽSPECS. using Mg-K a radiation and an electron detector pass energy of 50 eV for survey and 14 eV for high-resolution scans. To take into account some shift caused by charging of the sample surface, all spectra were adjusted taking the carbon 1s peak at 285.0 eV as reference. The electron paramagnetic resonance experiments were carried out on oxide powder, mechanically separated from the coated samples. The spectra were recorded in the X-band Ž n s 9.5 GHz. with a Bruker ESP300E spectrometer at 130 K and at room temper-
7
ature. The simulation of the spectrum was carried out with the program package WINEPR w13x using the experimental data as input.
3. Results and discussion In the selected electrolyte system, under anodic polarization, vanadium generates a passivating vanadium oxide film. During the anodization experiments the growth of an orange brown conversion film was observed. Fig. 1 shows an SEM image of a cross crack of the about 5 mm thick vanadium oxide film. The films exhibit a homogeneous structure, grain boundaries or polycrystalline structures are not visible. Neither X-ray diffraction nor selected area diffraction carried out by transmission electron microscopy on electron transparent specimens show any microcrystalline phases, which points the glassy amorphous nature of these vanadium oxide conversion films. High resolution X-ray photoelectron spectra were acquired for the O 1s and V 2p binding energy range of 500–540 eV ŽFig. 2.. The recorded spectra show three intensive peaks corresponding to the core level binding energies of O 1s, V 2p1r2 and V 2p 3r2 , respectively. The main signal of the O 1s spectrum has a binding energy of 530.6 eV and a full wide half
Fig. 1. SEM image of a cross crack of the anodic vanadium oxide film.
8
J.P. Schreckenbach, P. Strauchr Applied Surface Science 143 (1999) 6–10
Fig. 2. XPS spectrum of the O 1s and V 2p binding energies.
maximum ŽFWHM. of 1.78 eV which corresponds to the values for electron-beam evaporated vanadium ŽV. oxide films w6x. A small peak shape asymmetry appears in the high energy range at about 531.5 eV and can be attributed to the presence of chemisorbed water. For polycrystalline V2 O5 films the V2p 3r2 core level binding energy was reported at 516.4 eV w6x. The observed binding energy maximum for the electrochemical conversion films of 515.5 eV is too low to assign this value exclusively for V 5q. The calculated binding energy difference Ž D BE. between O 1s and V 2p 3r2 Ž D BE s O1s–V 2p 3r2 s 15.1 eV. displays also a divergence to the expected values of 13 eV for V2 O5 w4,6x. This difference and the high FWHM of 3.4 eV of the V2p 3r2 signal indicate the presence of VrO sub stoichiometric oxides in the anodic conversion film. Recently published results describe the determination of V 4q and V 5q in sol–gel films by XPS difference spectra based on the displacement between V 4q 2p 3r2 and V 5q 2p 3r2 peak positions w4x. The reported binding energies of 514.97 eV and 516.95 eV, respectively, correspond to our results and the presence of mixed valence vanadium oxides is responsible for the broadened V2p 3r2 signal. Isolated, paramagnetic V 4q centers have been doubtless detected by EPR spectroscopy. Fig. 3 shows the experimental EPR spectrum recorded at 130 K together with a simulation of the spectrum with the experimental data. The spectrum is axially
symmetric and can be described by the following spin Hamiltonian Eq. Ž1.: HSp s g 5 m B Bz P S z q g H m B Ž B x P S x q B y P S y . q AV5 P S z P IzV q AVH Ž S x P I xV q S y P I yV .
Ž 1.
where all symbols have their usual meaning. The z axis corresponds to the main axis of the axially symmetric g and A tensors. The quadrupole term and the nuclear Zeeman term can be neglected. In the typical spectrum of isolated V 4q centers with the expected hyperfine octets Ž S s 2, I V s 7r2, natural abundance of 51 V s 99.8%.. The parallel and perpendicular features are well resolved but overlapping. Because of the nonequivalence of the hfs-line distances the usual second-order expressions of per-
Fig. 3. EPR spectrum of a powder sample of this anodic vanadium oxide film Žrecorded at 130 K..
J.P. Schreckenbach, P. Strauchr Applied Surface Science 143 (1999) 6–10
9
Table 1 EPR-parameters of selected V 4q species Compoundrhost lattice
g 5a
a gH
a,b g av
AV,a 5
AV,a H
Ref.
Our sample at 130 K Our sample at 295 K VO 2qrin ZnŽNH 4 . 2 ŽSO4 . 2 P 6H 2 O VO 2qrin CsAlŽSO4 . 2 P 12H 2 O VO 2qrin RbAlŽSO4 . 2 P 12H 2 O V2 O5 Žamorphous, hydrated. VO 2qrin GeO 2 Žamorphous. V 4qrin GeO 2 Žtetragonal. V 4qrin GeO 2 Žrhombic. VO 2qrin KNO 3 VOSO4 P 2H 2 O V 4qrin a-Al 2 O 3 VO 32y
1.936
1.981 1.981 1.979 1.975 1.977 1.976 1.976 1.963 1.974
183.5 not resolved 182.8 183 " 1 182.2 178.6 175.5 175.5 134.4 179.8
71.5
1.933 1.932 1.923 1.932 1.929 1.929 1.921 1.935
1.966 1.976 1.965 1.963 1.958 1.962 1.960 1.960 1.949 1.966 1.96 " 0.02
this paper this paper w15x w16x w16x w17,18x w19x w19x w19x w20x w21x w22x w23x
a b
g 5 f g H 1.97 " 0.02
72.0 65.7 66.6 68.8 68.2 68.2 36.7r37.5 71.9
AV5 s AVH s 1.3 " 0.2 2.02 " 0.02
Exp. errors Žif not otherwise stated. g: "0.003; A: "0.5. g av s Ž g 5 q 2 g H .r3.
turbation theory w14x were used to determine the spectrum parameters. The obtained parameter set Žsee Table 1. is very close to those reported for VO 2q doped ZnŽNH 4 . 2 ŽSO4 . 2 P 6H 2 O w15x and CsrAl alum, CsAlŽSO4 . 2 P 12H 2 O w16x were the V 4q ŽVO 2q . occupies the octahedral Zn2q- or Al 3q-sites in the host lattice, respectively. The parameters are also interestingly closed to hydrated amorphous V2 O5 w17,18x, to VO 2q doped amorphous GeO 2 with identical parameters for V 4q doped tetragonal GeO 2 but different from those for rhombic GeO 2 host lattice w19x. Also the parameters reported for VO 2q doped single crystals of KNO 3 w20x with aragonite structure Žcoordination number 9. are very close and ‘free’ solvated vanadyl sulphate gives a related g value w21x. A value of g s 1.97 but a very small coupling
constant is reported for a-Al 2 O 3 doped with V 4q, where the vanadium is replacing the nearly ideal octahedral surrounded aluminum w22x. Contrastingly, the g value for VO 32y with an expected tetrahedral coordination sphere is found to be a significantly higher g value of 2.02 w23x. These parameters are summarized for comparison together with our results in Table 1. These results point towards a slightly distorted octahedral oxygen surrounding of the detected V 4q centers. The EPR spectrum of the same sample recorded at room temperature shows only a broad signal at g s 1.976 Žsee Fig. 4.. Due to line broadening effects no hyperfine structure is resolved. The comparison of this behavior with the results of studies of the temperature dependencies of amorphous and crystalline samples w24x gives an argument for the existence of very small but possibly nanocrystalline surrounding of the detected V 4q centers in this paper.
4. Conclusions
Fig. 4. Room temperature Ž295 K. EPR spectrum of the same sample as shown in Fig. 3.
This investigation shows the possibility of the anodic conversion film generation on vanadium at potentials up to 600 V. X-ray diffraction and transmission electron microscopy indicate the complete glassy amorphous structure of these films. X-ray photoelectron spectroscopy measurements show
10
J.P. Schreckenbach, P. Strauchr Applied Surface Science 143 (1999) 6–10
V 2p 3r2 binding energies at 515.5 eV characteristic for mixed valance vanadium sub oxides. The typical spectrum of isolated V 4q centers with the expected hyperfine octets Ž S s 2, I V s 7r2. could be detected by means of EPR measurement. The resulting parameter set of the g and A values points toward a distorted octahedral wVO6 x coordination geometry of these centers. Because of the temperature dependency of the EPR spectra a nanocrystalline surrounding of these V 4q centers can be expected.
Acknowledgements The authors thank Prof. Dr. R. Kirmse and Prof. G. Marx ŽChemnitz. for helpful discussions. The support of this work by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.
References w1x C. Julien, A. Gorenstein, A. Khelfa, J.P. Guesdon, I. Ivanov, Mater. Res. Soc. Symp. Proc. 369 Ž1995. 639. w2x M. Ask, U. Rolander, J. Lausmaa, B. Kasemo, J. Mater. Res. 5 Ž8. Ž1990. 1662. w3x G.C. Granqvist, Solid State Ionics 70 Ž1994. 678.
w4x Y. Chen, K. Xie, Z.X. Liu, Appl. Surf. Sci. 133 Ž1998. 221. w5x G.A. Khan, C.A. Hogarth, J. Mater. Sci. 2 Ž1991. 1087. w6x C.V. Ramana, O.M. Hussain, B.S. Naidu, C. Julien, M. Balkanski, Mater. Sci. Eng. B 52 Ž1998. 32. w7x P.C. Searson, T.P. Moffat, Crit. Rev. Surf. Chem. 3 Ž1994. 171. w8x J. Schreckenbach, F. Schlottig, D. Dietrich, A. Hofmann, G. Marx, J. Mater. Sci. Lett. 14 Ž1995. 1344. w9x R.G. Keil, R.E. Salomon, J. Electrochem. Soc. 115 Ž1968. 628. w10x M.R. Arora, R. Kelly, J. Mater. Sci. 12 Ž1977. 1673. w11x J. Schreckenbach, K. Witke, D. Butte, G. Marx, Fresenius J. Anal. Chem. 363 Ž1999. 211. w12x H. Oppermann, Vanadiumoxide, Akad. Verl. Berlin, 1983. w13x R.T. Weber, Win-EPR-Symfonia, EPR Division, Bruker Instruments, version 1.2, 1995. w14x B. Bleaney, Philos. Mag. 42 Ž1951. 441. w15x R.H. Borcherts, C. Kikuchi, J. Chem. Phys. 40 Ž1964. 2270. w16x A. Manoongian, J.A. McKinnon, Can. J. Phys. 45 Ž1967. 2769. w17x P. Tougne, P. Legrand, C. Sanchez, J. Livage, J. Phys. Chem. Solids 42 Ž1981. 101. w18x M. Nabavi, C. Sanchez, J. Livage, Philos. Mag. 63 Ž1991. 941. w19x I. Siegel, Phys. Rev. 134 Ž1964. A193. w20x K.V.S. Rao, M. Dattatreya Sastry, P. Venkateswarlu, J. Chem. Phys. 49 Ž1968. 1714. w21x C.A. Hutchinson, L.S. Singer, Phys. Rev. 89 Ž1951. 256. w22x J. Lambe, C. Kikuchi, Phys. Rev. 118 Ž1960. 71. w23x F.M. Lanchaster, W. Gordy, J. Chem. Phys. 19 Ž1951. 1181. w24x M. Henri, C. Sanchez, C. R’Kha, J. Livage, J. Phys. Chem. 14 Ž1981. 829.
Microstructure study of amorphous vanadium oxide films Joachim P. Schreckenbach a
a,)
, Peter Strauch
b
Department of Chemistry, Technische UniÕersitat ¨ Chemnitz, D-09107 Chemnitz, Germany b Institute of Inorganic Chemistry, UniÕersitat ¨ Leipzig, D-04103 Leipzig, Germany Received 15 September 1998; accepted 9 January 1999
Abstract Conversion films of vanadium oxides are potentiodynamically generated on vanadium in acetate electrolyte systems at high voltages. The microstructure of the about 5 mm thin anodic films is investigated. X-ray diffraction and transmission electron microscopy indicate the films are complete amorphous. X-ray photoelectron spectroscopy ŽXPS. measurements show V 2p 3r2 binding energies of mixed valance vanadium sub oxides. Electron spin resonance ŽESR. experiments on isolated films at 130 K point to paramagnetic V 4q centers in a disordered octahedral oxygen surrounding. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Anodic films; Vanadium oxide; X-ray photoelectron spectroscopy ŽXPS.; Electron spin resonance ŽESR.
1. Introduction Thin films of vanadium oxides have potential use for a wide variety of applications such as high energy density lithium microbatteries, electrochromic devises or catalysts and vanadium oxides are even an constituent of the surface of medical Ti–Al–V implants w1–3x. Various deposition and coating techniques results in vanadium oxide films, for example flash evaporation w1x, sol–gel technique w4x, vacuum evaporation w5x or electron beam evaporation w6x have been reported. Electrochemical processing, increasingly used for the fabrication of coatings and conversion films by surface modification w7,8x of transition metals has been more rarely investigated in case of the generation of anodic vanadium oxide films. One of the )
Corresponding author. Tel.: q49-371-531-1478; Fax: q49371-531-1371; E-mail: [email protected]
reasons is the difficulty to stabilize the prepared film and to avoid its dissolution and chemical decomposition by side reactions during the anodization. A suitable electrolyte to stabilize the anodic oxide film is the system glacial acetic acidrsodium boraterwater w9–11x. In this case the generation of the anodic oxide film results from conversion reactions of the anode material with the oxygen compounds of the electrolyte system according to the brutto reaction: xV q yO 2y™ VxO y q 2 yey Due to the high complexity of the VrO systems mixed valence vanadium oxides and multiphase formation are expected. The possible oxidation states of vanadium varies from V 2q to V 5q and the stoichiometric composition of the corresponding oxides is given by VO, V2 O 3 , VO 2 and V2 O5 , respectively. Furthermore a homologous series of Magneli phases VnO 2 ny1 Ž n s 3–8. and the complex oxides
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 0 8 4 - 7
J.P. Schreckenbach, P. Strauchr Applied Surface Science 143 (1999) 6–10
VnO 2 nq1 Ž n s 3,4,6. are described for bulk vanadium oxides w12x. The present study was undertaken to prepare vanadium oxide conversion films by high electric field anodization, to characterize the microstructure oft these generated anodic films and to determine the vanadium valence state by means of scanning electron microscopy ŽSEM., X-ray photoelectron spectroscopy ŽXPS. and electron spin resonance ŽESR. experiments.
2. Experimental The vanadium oxide films were prepared in an electrochemical cell by potentiodynamic anodic conversion at potentials up to 600 V. The power supply was a computer-controlled DC Heinzinger PHN. Solutions consisting of AR-grade acetic acid, 0.02 M sodium borate ŽNa 2 B 4 O 7 P 10H 2 O., 1 M water and additional 0.1 M barium acetate were used as electrolyte. The experiments were carried out with flag-shaped vanadium foil anodes 10 = 10 mm2 , 0.127 mm thickness and 99.7% purity ŽAldrich. while platinum wire constituted as cathode. The electrolyte temperature was thermostatically controlled to 258C. The vanadium samples were cleaned in acetone and electrochemically polished Ž15 V. for 60 s in a mixture of 80% methanol and 20% sulfuric acid. After the coating process the samples were rinsed in acetone and dried and stored in an argon atmosphere. For characterization, specimens were examined by scanning electron microscopy using a JEOL 840 A with EDS KEVEX DELTA. X-ray photoelectron spectra were recorded on a SAGE 100 ŽSPECS. using Mg-K a radiation and an electron detector pass energy of 50 eV for survey and 14 eV for high-resolution scans. To take into account some shift caused by charging of the sample surface, all spectra were adjusted taking the carbon 1s peak at 285.0 eV as reference. The electron paramagnetic resonance experiments were carried out on oxide powder, mechanically separated from the coated samples. The spectra were recorded in the X-band Ž n s 9.5 GHz. with a Bruker ESP300E spectrometer at 130 K and at room temper-
7
ature. The simulation of the spectrum was carried out with the program package WINEPR w13x using the experimental data as input.
3. Results and discussion In the selected electrolyte system, under anodic polarization, vanadium generates a passivating vanadium oxide film. During the anodization experiments the growth of an orange brown conversion film was observed. Fig. 1 shows an SEM image of a cross crack of the about 5 mm thick vanadium oxide film. The films exhibit a homogeneous structure, grain boundaries or polycrystalline structures are not visible. Neither X-ray diffraction nor selected area diffraction carried out by transmission electron microscopy on electron transparent specimens show any microcrystalline phases, which points the glassy amorphous nature of these vanadium oxide conversion films. High resolution X-ray photoelectron spectra were acquired for the O 1s and V 2p binding energy range of 500–540 eV ŽFig. 2.. The recorded spectra show three intensive peaks corresponding to the core level binding energies of O 1s, V 2p1r2 and V 2p 3r2 , respectively. The main signal of the O 1s spectrum has a binding energy of 530.6 eV and a full wide half
Fig. 1. SEM image of a cross crack of the anodic vanadium oxide film.
8
J.P. Schreckenbach, P. Strauchr Applied Surface Science 143 (1999) 6–10
Fig. 2. XPS spectrum of the O 1s and V 2p binding energies.
maximum ŽFWHM. of 1.78 eV which corresponds to the values for electron-beam evaporated vanadium ŽV. oxide films w6x. A small peak shape asymmetry appears in the high energy range at about 531.5 eV and can be attributed to the presence of chemisorbed water. For polycrystalline V2 O5 films the V2p 3r2 core level binding energy was reported at 516.4 eV w6x. The observed binding energy maximum for the electrochemical conversion films of 515.5 eV is too low to assign this value exclusively for V 5q. The calculated binding energy difference Ž D BE. between O 1s and V 2p 3r2 Ž D BE s O1s–V 2p 3r2 s 15.1 eV. displays also a divergence to the expected values of 13 eV for V2 O5 w4,6x. This difference and the high FWHM of 3.4 eV of the V2p 3r2 signal indicate the presence of VrO sub stoichiometric oxides in the anodic conversion film. Recently published results describe the determination of V 4q and V 5q in sol–gel films by XPS difference spectra based on the displacement between V 4q 2p 3r2 and V 5q 2p 3r2 peak positions w4x. The reported binding energies of 514.97 eV and 516.95 eV, respectively, correspond to our results and the presence of mixed valence vanadium oxides is responsible for the broadened V2p 3r2 signal. Isolated, paramagnetic V 4q centers have been doubtless detected by EPR spectroscopy. Fig. 3 shows the experimental EPR spectrum recorded at 130 K together with a simulation of the spectrum with the experimental data. The spectrum is axially
symmetric and can be described by the following spin Hamiltonian Eq. Ž1.: HSp s g 5 m B Bz P S z q g H m B Ž B x P S x q B y P S y . q AV5 P S z P IzV q AVH Ž S x P I xV q S y P I yV .
Ž 1.
where all symbols have their usual meaning. The z axis corresponds to the main axis of the axially symmetric g and A tensors. The quadrupole term and the nuclear Zeeman term can be neglected. In the typical spectrum of isolated V 4q centers with the expected hyperfine octets Ž S s 2, I V s 7r2, natural abundance of 51 V s 99.8%.. The parallel and perpendicular features are well resolved but overlapping. Because of the nonequivalence of the hfs-line distances the usual second-order expressions of per-
Fig. 3. EPR spectrum of a powder sample of this anodic vanadium oxide film Žrecorded at 130 K..
J.P. Schreckenbach, P. Strauchr Applied Surface Science 143 (1999) 6–10
9
Table 1 EPR-parameters of selected V 4q species Compoundrhost lattice
g 5a
a gH
a,b g av
AV,a 5
AV,a H
Ref.
Our sample at 130 K Our sample at 295 K VO 2qrin ZnŽNH 4 . 2 ŽSO4 . 2 P 6H 2 O VO 2qrin CsAlŽSO4 . 2 P 12H 2 O VO 2qrin RbAlŽSO4 . 2 P 12H 2 O V2 O5 Žamorphous, hydrated. VO 2qrin GeO 2 Žamorphous. V 4qrin GeO 2 Žtetragonal. V 4qrin GeO 2 Žrhombic. VO 2qrin KNO 3 VOSO4 P 2H 2 O V 4qrin a-Al 2 O 3 VO 32y
1.936
1.981 1.981 1.979 1.975 1.977 1.976 1.976 1.963 1.974
183.5 not resolved 182.8 183 " 1 182.2 178.6 175.5 175.5 134.4 179.8
71.5
1.933 1.932 1.923 1.932 1.929 1.929 1.921 1.935
1.966 1.976 1.965 1.963 1.958 1.962 1.960 1.960 1.949 1.966 1.96 " 0.02
this paper this paper w15x w16x w16x w17,18x w19x w19x w19x w20x w21x w22x w23x
a b
g 5 f g H 1.97 " 0.02
72.0 65.7 66.6 68.8 68.2 68.2 36.7r37.5 71.9
AV5 s AVH s 1.3 " 0.2 2.02 " 0.02
Exp. errors Žif not otherwise stated. g: "0.003; A: "0.5. g av s Ž g 5 q 2 g H .r3.
turbation theory w14x were used to determine the spectrum parameters. The obtained parameter set Žsee Table 1. is very close to those reported for VO 2q doped ZnŽNH 4 . 2 ŽSO4 . 2 P 6H 2 O w15x and CsrAl alum, CsAlŽSO4 . 2 P 12H 2 O w16x were the V 4q ŽVO 2q . occupies the octahedral Zn2q- or Al 3q-sites in the host lattice, respectively. The parameters are also interestingly closed to hydrated amorphous V2 O5 w17,18x, to VO 2q doped amorphous GeO 2 with identical parameters for V 4q doped tetragonal GeO 2 but different from those for rhombic GeO 2 host lattice w19x. Also the parameters reported for VO 2q doped single crystals of KNO 3 w20x with aragonite structure Žcoordination number 9. are very close and ‘free’ solvated vanadyl sulphate gives a related g value w21x. A value of g s 1.97 but a very small coupling
constant is reported for a-Al 2 O 3 doped with V 4q, where the vanadium is replacing the nearly ideal octahedral surrounded aluminum w22x. Contrastingly, the g value for VO 32y with an expected tetrahedral coordination sphere is found to be a significantly higher g value of 2.02 w23x. These parameters are summarized for comparison together with our results in Table 1. These results point towards a slightly distorted octahedral oxygen surrounding of the detected V 4q centers. The EPR spectrum of the same sample recorded at room temperature shows only a broad signal at g s 1.976 Žsee Fig. 4.. Due to line broadening effects no hyperfine structure is resolved. The comparison of this behavior with the results of studies of the temperature dependencies of amorphous and crystalline samples w24x gives an argument for the existence of very small but possibly nanocrystalline surrounding of the detected V 4q centers in this paper.
4. Conclusions
Fig. 4. Room temperature Ž295 K. EPR spectrum of the same sample as shown in Fig. 3.
This investigation shows the possibility of the anodic conversion film generation on vanadium at potentials up to 600 V. X-ray diffraction and transmission electron microscopy indicate the complete glassy amorphous structure of these films. X-ray photoelectron spectroscopy measurements show
10
J.P. Schreckenbach, P. Strauchr Applied Surface Science 143 (1999) 6–10
V 2p 3r2 binding energies at 515.5 eV characteristic for mixed valance vanadium sub oxides. The typical spectrum of isolated V 4q centers with the expected hyperfine octets Ž S s 2, I V s 7r2. could be detected by means of EPR measurement. The resulting parameter set of the g and A values points toward a distorted octahedral wVO6 x coordination geometry of these centers. Because of the temperature dependency of the EPR spectra a nanocrystalline surrounding of these V 4q centers can be expected.
Acknowledgements The authors thank Prof. Dr. R. Kirmse and Prof. G. Marx ŽChemnitz. for helpful discussions. The support of this work by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.
References w1x C. Julien, A. Gorenstein, A. Khelfa, J.P. Guesdon, I. Ivanov, Mater. Res. Soc. Symp. Proc. 369 Ž1995. 639. w2x M. Ask, U. Rolander, J. Lausmaa, B. Kasemo, J. Mater. Res. 5 Ž8. Ž1990. 1662. w3x G.C. Granqvist, Solid State Ionics 70 Ž1994. 678.
w4x Y. Chen, K. Xie, Z.X. Liu, Appl. Surf. Sci. 133 Ž1998. 221. w5x G.A. Khan, C.A. Hogarth, J. Mater. Sci. 2 Ž1991. 1087. w6x C.V. Ramana, O.M. Hussain, B.S. Naidu, C. Julien, M. Balkanski, Mater. Sci. Eng. B 52 Ž1998. 32. w7x P.C. Searson, T.P. Moffat, Crit. Rev. Surf. Chem. 3 Ž1994. 171. w8x J. Schreckenbach, F. Schlottig, D. Dietrich, A. Hofmann, G. Marx, J. Mater. Sci. Lett. 14 Ž1995. 1344. w9x R.G. Keil, R.E. Salomon, J. Electrochem. Soc. 115 Ž1968. 628. w10x M.R. Arora, R. Kelly, J. Mater. Sci. 12 Ž1977. 1673. w11x J. Schreckenbach, K. Witke, D. Butte, G. Marx, Fresenius J. Anal. Chem. 363 Ž1999. 211. w12x H. Oppermann, Vanadiumoxide, Akad. Verl. Berlin, 1983. w13x R.T. Weber, Win-EPR-Symfonia, EPR Division, Bruker Instruments, version 1.2, 1995. w14x B. Bleaney, Philos. Mag. 42 Ž1951. 441. w15x R.H. Borcherts, C. Kikuchi, J. Chem. Phys. 40 Ž1964. 2270. w16x A. Manoongian, J.A. McKinnon, Can. J. Phys. 45 Ž1967. 2769. w17x P. Tougne, P. Legrand, C. Sanchez, J. Livage, J. Phys. Chem. Solids 42 Ž1981. 101. w18x M. Nabavi, C. Sanchez, J. Livage, Philos. Mag. 63 Ž1991. 941. w19x I. Siegel, Phys. Rev. 134 Ž1964. A193. w20x K.V.S. Rao, M. Dattatreya Sastry, P. Venkateswarlu, J. Chem. Phys. 49 Ž1968. 1714. w21x C.A. Hutchinson, L.S. Singer, Phys. Rev. 89 Ž1951. 256. w22x J. Lambe, C. Kikuchi, Phys. Rev. 118 Ž1960. 71. w23x F.M. Lanchaster, W. Gordy, J. Chem. Phys. 19 Ž1951. 1181. w24x M. Henri, C. Sanchez, C. R’Kha, J. Livage, J. Phys. Chem. 14 Ž1981. 829.