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Chemical and electrochemical intercalation of a viologen into V2O5 xerogel films
Colloids and Surfaces,
241
49 (1990) 241-245
Elsevier Science Publishers
B.V., Amsterdam
Chemical and Electrochemical Intercalation of a Viologen into V205 Xerogel Films IKUO KATO, TERUYUKI Department
of Applied
NAKATO,
Chemistry,
KAZUYUKI
KURODA and CHUZO KATO*
Waseda University,
Ohkubo-3,
Shinjuku-ku,
Tokyo 169
(Japan) (Received 23 August 1989; accepted
14 December
1989)
ABSTRACT Heptylviologen was intercalated into V,05 xerogel films by chemical and electrochemical methods. Chemical intercalation occurred by the reaction of a V,Os xerogel film with a propylene carbonate (PC ) solution of heptylviologen ( HV ) . The viologen was present as a solvated form in the interlayer of V,05 gel. A reversible intercalation-deintercalation of HV was carried out by electrochemical reduction-oxidation of the V205 xerogel film in a PC solution of HV. The electrochemical intercalation took place only when the interlayer region of V205 gel was expanded by PC.
INTRODUCTION
Vanadium pentoxide gel (V20,.nH,0) is one of the layered transition metal oxides and has a higher intercalating ability than other layered transition metal oxides, which is based on its flexible layered structure [l-5]. Its specific physical and chemical properties have attracted much attention; V,O, gel has been investigated as an electrochromic material [ 61, and the anisotropic semiconducting properties of V,O, gel can be used for electrical switching devices [ 71. On the other hand, viologens (1,l‘ -disubstituted-4,4’ -bipyridinium salts) have been investigated as electrochromic materials based on their one electron reduction [ 81. We have already synthesized V,O, gel-viologen intercalation compounds by the reaction of viologen powders with undried gels [ 91. However, the utilization of the intercalation compounds may be limited because the products were obtained as flocculated precipitates. Since practical materials are often used in films, fibers, and single crystals, the use of V,O, xerogel films as the host substance for the inclusion of viologens is worthy of investigation. Vanadium pentoxide films can easily be prepared from V,O, gel by drying on substrates. The films are composed of V205 xerogel (V205. 1.6H20) [lo]. *To whom all correspondence
0166-6622/90/$03.50
should be addressed.
0 1990- Elsevier Science Publishers
B.V.
242
In the present study, the reactions of V,O, xerogel films with propylene carbonate (PC ) solutions of heptylviologen were carried out in order to obtain intercalation compounds in the form of films. Since the viologen were allowed to react with the gel as a PC solution, some effects of solvents on intercalation were also discussed. In addition, we investigated the electrochemical intercalation of heptylviologen into xerogel films because electrochemical techniques have been found to be effective for intercalation in V205 gel [ 111. EXPERIMENTAL
V205 gel was prepared from a sodium metavanadate aqueous solution by ion exchange with H+ and by being allowed to stand for over 4 weeks. A V205 xerogel film was obtained by drying V205 gel coated on a glass plate under reduced pressure. Chemical intercalation of heptylviologen was carried out by immersing the xerogel film in a PC solution of heptylviologen (1 mmol dm-” ) for 24 h. The film thus prepared was dried at 100°C under reduced pressure. All samples were analyzed by X-ray diffraction (XRD ) with Ni filtered CuK, radiation. For the electrochemical experiment, a V205 xerogel film was prepared on an SnO, electrode (1 cm*). Cyclic voltammetry measurements were performed in a 1 mmol dmw3 heptylviologen PC solution. The counter electrode was a platinum wire, and an. Ag/Ag+ reference electrode was used. RESULTS AND DISCUSSION
The products obtained by the reaction of V,O, xerogel with heptylviologen were CharacterSeed by XRD. Figure 1(b) shows the XRD pattern of the V,O, xerogel film immediately after being immersed in a PC solution of heptylviologen. The basal spacing of the film increased to 2.52 nm and was larger than that of the V205 xerogel [Fig. 1(a) ] by 1.37 nm. When a V205 gel film was immersed in PC, the basal spacing increased to ca 2.2 nm [Fig. 1 (d) 1, corresponding to bilayer adsorption of PC. The bilayer adsorption of PC in V,O, gel has been reported by Aldebert et al. [ 21. However, this basal reflection peak was unstable, and shifted to a higher 2 8 angle with broadening when the film was exposed in air. On the contrary, the basal reflection peak of the film treated with heptylviologen was stable. These observations confirmed intercalation of heptylviologen to V,O, xerogel films. Figures 1 (c) and (e) show the XRD patterns of the films dried at 100°C under reduced pressure. The basal spacing of the PC-adsorbed film decreased to 1.16 nm, the value of which was close to that of V205 xerogel [Fig. 1 (e) 1, whereas the film which was treated with heptylviologen showed a basal spacing of 1.79 nm after drying [Fig. 1 (c) 1. This fact also evidenced the intercalation of the viologen. The decrease in the interlayer space of the viologen interca-
243
Cd)
(e) I
2
I 20
40
60
2 e / deg
Fig. 1. XRD patterns of VzO, xerogel films: (a) untreated; (b) immersed in PC solution of heptylviologen (undried); (c) dried (b) at 100°C under reduced pressure; (d) immersed in PC (undried); and (e) dried (d) at 100” C under reduced pressure. (*) indicates a change of scale.
lated film is probably due to the elimination of PC in the interlayer of Vz05 gel. PC may solvate heptylviologen ions in the interlayer region and the stability of the intercalation compound before drying can also be explained by the solvation. The basal spacing of the V,O, xerogel-heptylviologen intercalation compound prepared here was different from those of the intercalation compounds which were synthesized by the reaction of undried gel with heptylviologen powders [9]. Those basal spacings were 1.35 nm (dd=0.47 nm) when heptylviologen was arranged as a flat monolayer and 1.96 nm (dCt= 1.08 nm) when the arrangement was a transversal monolayer. In the present system, the amount of intercalated viologen could not be determined since the viologen adsorbed on the outer surface of the film could not be eliminated by washing, because the film decomposed on washing, and the orientation of guest species was not simple because of solvation effects. However, the basal spacing of 1.79 nm suggests an inclined monolayer arrangement of heptylviologen. Figure 2 shows cyclic voltammograms of the V,O, xerogel films in PC solutions of heptylviologen. When the potential was scanned between -2.25 and -0.75 V relative to the Ag/Ag+ reference electrode, two redox couples ap-
244
I
I
-3
-2 Potential
-1
0 /
V
vs.
1
2
J
Ag/Agl
Fig. 2. Cyclic voltammograms of V,O, xerogel on an SnO, electrode in 1 mmol drnm3 PC solution of heptylviologen; scanning range was: (a) between - 2.25 and -0.75 V versus Ag/Ag+; and (b) between - 2.25 and 1.75 V.
peared [Fig. 2 (a) 1. The color of V205 gel on the electrode was red without any color change during the scanning, although the solution around the electrode turned blue, indicating the formation of heptylviologen radical cations. The basal spacing of the V,O, gel, which was removed when the potential was - 0.75 V, was 1.20 nm corresponding to that of untreated Vz05 xerogel. Hence, it was concluded that the redox reactions were reduction-oxidation of heptylviologen in two steps as shown in scheme 1 [ 121 and that Vz05 itself did not react.
When the potential range was extended to + 1.75 V, new redox peaks appeared ( - 0.8 and 1.3 V) and the peaks due to the reduction-oxidation of heptylviologen disappeared [Fig. 2 (b) 1. The blue color of viologen radical cations was not observed. However, cathodic scanning turned the color of V205 gel to green under the potential of - 0.8 V, and the color turned red again on anodic scanning with the potential more positive than 1.3 V. The basal spacings of the V205 gel were 2.16 nm at a potential of 1.75 V and 2.45 nm at -2.25 V. The former value corresponded to an intercalation of PC and the latter indicated that of heptylviologen (see Fig. 1). These results indicated that reductionoxidation of Vz05 gel occurred, followed by electrochemical intercalationdeintercalation of heptylviologen without reduction of the viologen dication. The difference in the electrochemical behavior with the potential range may
245
be associated with the difference in the interlayer condition of V,O, gel. The anodic scanning of the potential beyond 1.3 V caused oxidation of V4+ to V5+ in the V,O, gel, thereby decreasing the negative charge of the V205 layer (V,O, gel naturally contains some V4+, which is the origin of the intercalating property ) . Under this condition, PC can easily be intercalated in V,O, gel, because the electrostatic attraction between the V,O, layers is reduced and the interlayer region is expanded. When the PC-intercalated V205 gel is reduced, the amount of V4+ increases. Hence, heptylviologen can be taken into the expanded interlayer region with the reduction of V,O, gel to compensate for the negative charge of the layer caused by V4+ formation.
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12
J. Livage, M.C. Leroy and M. Michand, Mater. Res. Bull., 13 (1978) 1117. P. Aldebert, N. Baffier, N. Gharbi and J. Livage, Mater. Res. Bull., 16 (1981) 949. A. Bouhaouss and P. Aldebert, Mater. Res. Bull., 18 (1983) 1247. P. Aldebert and V. Paul-Boncour, Mater. Res. Bull., 18 (1983) 1263. H. van Damme, M. Leteller, D. Tinet, B. Kihal and R. Erre, Mater. Res. Bull., 19 (1984) 1635. T. Yoshino, N. Baba and Y. Kouda, Jpn. J. Appl. Phys., 26 (1987) 782. J. Livage, Mater. Res. Sot. Symp. Proc., 32 (1984) 125. M. Yamana and T. Kawata, Nippon Kagaku Kaishi, 1977,941. T. Nakato, I. Kato, K. Kuroda and C. Kato, J. Colloid Interface Sci., 133 (1989) 4517. P. Aldebert, N. Baffier, N. Gharbi and J. Livage, Mater. Res. Bull., 16 (1981) 669. B. Araki, C. Mailhe, N. Baffier, J. Livage and J. Vedel, Solid State Ionics, 9/10 (1983) 439. CL. Bird and A.T. Kuhn, Chem. Sot. Rev., 10 (1981) 49.
241
49 (1990) 241-245
Elsevier Science Publishers
B.V., Amsterdam
Chemical and Electrochemical Intercalation of a Viologen into V205 Xerogel Films IKUO KATO, TERUYUKI Department
of Applied
NAKATO,
Chemistry,
KAZUYUKI
KURODA and CHUZO KATO*
Waseda University,
Ohkubo-3,
Shinjuku-ku,
Tokyo 169
(Japan) (Received 23 August 1989; accepted
14 December
1989)
ABSTRACT Heptylviologen was intercalated into V,05 xerogel films by chemical and electrochemical methods. Chemical intercalation occurred by the reaction of a V,Os xerogel film with a propylene carbonate (PC ) solution of heptylviologen ( HV ) . The viologen was present as a solvated form in the interlayer of V,05 gel. A reversible intercalation-deintercalation of HV was carried out by electrochemical reduction-oxidation of the V205 xerogel film in a PC solution of HV. The electrochemical intercalation took place only when the interlayer region of V205 gel was expanded by PC.
INTRODUCTION
Vanadium pentoxide gel (V20,.nH,0) is one of the layered transition metal oxides and has a higher intercalating ability than other layered transition metal oxides, which is based on its flexible layered structure [l-5]. Its specific physical and chemical properties have attracted much attention; V,O, gel has been investigated as an electrochromic material [ 61, and the anisotropic semiconducting properties of V,O, gel can be used for electrical switching devices [ 71. On the other hand, viologens (1,l‘ -disubstituted-4,4’ -bipyridinium salts) have been investigated as electrochromic materials based on their one electron reduction [ 81. We have already synthesized V,O, gel-viologen intercalation compounds by the reaction of viologen powders with undried gels [ 91. However, the utilization of the intercalation compounds may be limited because the products were obtained as flocculated precipitates. Since practical materials are often used in films, fibers, and single crystals, the use of V,O, xerogel films as the host substance for the inclusion of viologens is worthy of investigation. Vanadium pentoxide films can easily be prepared from V,O, gel by drying on substrates. The films are composed of V205 xerogel (V205. 1.6H20) [lo]. *To whom all correspondence
0166-6622/90/$03.50
should be addressed.
0 1990- Elsevier Science Publishers
B.V.
242
In the present study, the reactions of V,O, xerogel films with propylene carbonate (PC ) solutions of heptylviologen were carried out in order to obtain intercalation compounds in the form of films. Since the viologen were allowed to react with the gel as a PC solution, some effects of solvents on intercalation were also discussed. In addition, we investigated the electrochemical intercalation of heptylviologen into xerogel films because electrochemical techniques have been found to be effective for intercalation in V205 gel [ 111. EXPERIMENTAL
V205 gel was prepared from a sodium metavanadate aqueous solution by ion exchange with H+ and by being allowed to stand for over 4 weeks. A V205 xerogel film was obtained by drying V205 gel coated on a glass plate under reduced pressure. Chemical intercalation of heptylviologen was carried out by immersing the xerogel film in a PC solution of heptylviologen (1 mmol dm-” ) for 24 h. The film thus prepared was dried at 100°C under reduced pressure. All samples were analyzed by X-ray diffraction (XRD ) with Ni filtered CuK, radiation. For the electrochemical experiment, a V205 xerogel film was prepared on an SnO, electrode (1 cm*). Cyclic voltammetry measurements were performed in a 1 mmol dmw3 heptylviologen PC solution. The counter electrode was a platinum wire, and an. Ag/Ag+ reference electrode was used. RESULTS AND DISCUSSION
The products obtained by the reaction of V,O, xerogel with heptylviologen were CharacterSeed by XRD. Figure 1(b) shows the XRD pattern of the V,O, xerogel film immediately after being immersed in a PC solution of heptylviologen. The basal spacing of the film increased to 2.52 nm and was larger than that of the V205 xerogel [Fig. 1(a) ] by 1.37 nm. When a V205 gel film was immersed in PC, the basal spacing increased to ca 2.2 nm [Fig. 1 (d) 1, corresponding to bilayer adsorption of PC. The bilayer adsorption of PC in V,O, gel has been reported by Aldebert et al. [ 21. However, this basal reflection peak was unstable, and shifted to a higher 2 8 angle with broadening when the film was exposed in air. On the contrary, the basal reflection peak of the film treated with heptylviologen was stable. These observations confirmed intercalation of heptylviologen to V,O, xerogel films. Figures 1 (c) and (e) show the XRD patterns of the films dried at 100°C under reduced pressure. The basal spacing of the PC-adsorbed film decreased to 1.16 nm, the value of which was close to that of V205 xerogel [Fig. 1 (e) 1, whereas the film which was treated with heptylviologen showed a basal spacing of 1.79 nm after drying [Fig. 1 (c) 1. This fact also evidenced the intercalation of the viologen. The decrease in the interlayer space of the viologen interca-
243
Cd)
(e) I
2
I 20
40
60
2 e / deg
Fig. 1. XRD patterns of VzO, xerogel films: (a) untreated; (b) immersed in PC solution of heptylviologen (undried); (c) dried (b) at 100°C under reduced pressure; (d) immersed in PC (undried); and (e) dried (d) at 100” C under reduced pressure. (*) indicates a change of scale.
lated film is probably due to the elimination of PC in the interlayer of Vz05 gel. PC may solvate heptylviologen ions in the interlayer region and the stability of the intercalation compound before drying can also be explained by the solvation. The basal spacing of the V,O, xerogel-heptylviologen intercalation compound prepared here was different from those of the intercalation compounds which were synthesized by the reaction of undried gel with heptylviologen powders [9]. Those basal spacings were 1.35 nm (dd=0.47 nm) when heptylviologen was arranged as a flat monolayer and 1.96 nm (dCt= 1.08 nm) when the arrangement was a transversal monolayer. In the present system, the amount of intercalated viologen could not be determined since the viologen adsorbed on the outer surface of the film could not be eliminated by washing, because the film decomposed on washing, and the orientation of guest species was not simple because of solvation effects. However, the basal spacing of 1.79 nm suggests an inclined monolayer arrangement of heptylviologen. Figure 2 shows cyclic voltammograms of the V,O, xerogel films in PC solutions of heptylviologen. When the potential was scanned between -2.25 and -0.75 V relative to the Ag/Ag+ reference electrode, two redox couples ap-
244
I
I
-3
-2 Potential
-1
0 /
V
vs.
1
2
J
Ag/Agl
Fig. 2. Cyclic voltammograms of V,O, xerogel on an SnO, electrode in 1 mmol drnm3 PC solution of heptylviologen; scanning range was: (a) between - 2.25 and -0.75 V versus Ag/Ag+; and (b) between - 2.25 and 1.75 V.
peared [Fig. 2 (a) 1. The color of V205 gel on the electrode was red without any color change during the scanning, although the solution around the electrode turned blue, indicating the formation of heptylviologen radical cations. The basal spacing of the V,O, gel, which was removed when the potential was - 0.75 V, was 1.20 nm corresponding to that of untreated Vz05 xerogel. Hence, it was concluded that the redox reactions were reduction-oxidation of heptylviologen in two steps as shown in scheme 1 [ 121 and that Vz05 itself did not react.
When the potential range was extended to + 1.75 V, new redox peaks appeared ( - 0.8 and 1.3 V) and the peaks due to the reduction-oxidation of heptylviologen disappeared [Fig. 2 (b) 1. The blue color of viologen radical cations was not observed. However, cathodic scanning turned the color of V205 gel to green under the potential of - 0.8 V, and the color turned red again on anodic scanning with the potential more positive than 1.3 V. The basal spacings of the V205 gel were 2.16 nm at a potential of 1.75 V and 2.45 nm at -2.25 V. The former value corresponded to an intercalation of PC and the latter indicated that of heptylviologen (see Fig. 1). These results indicated that reductionoxidation of Vz05 gel occurred, followed by electrochemical intercalationdeintercalation of heptylviologen without reduction of the viologen dication. The difference in the electrochemical behavior with the potential range may
245
be associated with the difference in the interlayer condition of V,O, gel. The anodic scanning of the potential beyond 1.3 V caused oxidation of V4+ to V5+ in the V,O, gel, thereby decreasing the negative charge of the V205 layer (V,O, gel naturally contains some V4+, which is the origin of the intercalating property ) . Under this condition, PC can easily be intercalated in V,O, gel, because the electrostatic attraction between the V,O, layers is reduced and the interlayer region is expanded. When the PC-intercalated V205 gel is reduced, the amount of V4+ increases. Hence, heptylviologen can be taken into the expanded interlayer region with the reduction of V,O, gel to compensate for the negative charge of the layer caused by V4+ formation.
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12
J. Livage, M.C. Leroy and M. Michand, Mater. Res. Bull., 13 (1978) 1117. P. Aldebert, N. Baffier, N. Gharbi and J. Livage, Mater. Res. Bull., 16 (1981) 949. A. Bouhaouss and P. Aldebert, Mater. Res. Bull., 18 (1983) 1247. P. Aldebert and V. Paul-Boncour, Mater. Res. Bull., 18 (1983) 1263. H. van Damme, M. Leteller, D. Tinet, B. Kihal and R. Erre, Mater. Res. Bull., 19 (1984) 1635. T. Yoshino, N. Baba and Y. Kouda, Jpn. J. Appl. Phys., 26 (1987) 782. J. Livage, Mater. Res. Sot. Symp. Proc., 32 (1984) 125. M. Yamana and T. Kawata, Nippon Kagaku Kaishi, 1977,941. T. Nakato, I. Kato, K. Kuroda and C. Kato, J. Colloid Interface Sci., 133 (1989) 4517. P. Aldebert, N. Baffier, N. Gharbi and J. Livage, Mater. Res. Bull., 16 (1981) 669. B. Araki, C. Mailhe, N. Baffier, J. Livage and J. Vedel, Solid State Ionics, 9/10 (1983) 439. CL. Bird and A.T. Kuhn, Chem. Sot. Rev., 10 (1981) 49.