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Intercalation of alkylviologen dications into the layered vanadium pentoxide
Inorganica Chimica Acta 359 (2006) 1050–1054 www.elsevier.com/locate/ica
Intercalation of alkylviologen dications into the layered vanadium pentoxide Keqiang Lai, Aiguo Kong, Fan Yang, Bo Chen, Hanming Ding, Yongkui Shan *, Songping Huang Department of Chemistry, East China Normal University, Shanghai 200062, China Received 22 March 2005; received in revised form 5 December 2005; accepted 6 December 2005 Available online 18 January 2006
Abstract A novel series of inorganic–organic intercalation compounds were prepared by intercalating of alkylviologen dications into the layered vanadium pentoxide in the system of two phases of the liquid and the solid, and characterized by the techniques of X-ray diffraction (XRD), FT infrared (FTIR), X-ray photoelectron spectroscopy (XPS) and UV/vis diffuse reflectance spectroscopy (DRS). Ó 2005 Elsevier B.V. All rights reserved. Keywords: Intercalation; Alkylviologen; Vanadium pentoxide; Photocatalytic
1. Introduction In the past decades, layered intercalation compounds have been receiving significant attention owing to their applications in various fields such as energy-storage applications, electrocatalysis, novel energy-conversion systems, proton-pump electrodes, sensors, chemiresistive detectors, heterogeneous catalytic processes and photochemical redox reactions [1,2]. In particular, interest is in the insertion of organic molecules into layered hosts for the purpose of synthesizing organic–inorganic composite materials with welldefined stoichiometries and organized structures, for such resulting intercalation compounds may lead to a material with properties superior to the sum of the properties of their components. The synergic effect resulting from the combined properties of organic–inorganic components can give rise to very interesting functional properties, especially in what concerns electrochemical, electrochromic and conduction properties [3–5].
*
Corresponding author. Tel./fax: +86 21 6223 3503. E-mail address: [email protected] (Y. Shan).
0020-1693/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2005.12.009
Layered V2O5 are suitable host materials for intercalation chemistry because their interlayer spaces in conjunction with their semiconducting properties enable them to act as cathodic materials for advanced lithium batteries, etc. Indeed, V2O5 as the inorganic host has been extensively studied with a variety of species, such as, polyaniline [6], polypyrrole [7], polythiophene [8], alkali-metal ions [9], melanin [10], pyridine [11], methyl yellow [12], benzidine [13], alkylamines [14]. Alkylviologen, due to its cation radical having a strong absorption band in the visible region of the spectrum, is an appropriate candidate for the investigation of photoelectrochemical processes. However, alkylviologen exhibits relatively poor photocatalytic activity when it is used alone, since the majority of the photogenerated e– h pairs simply undergo recombination [15]. Thus, delaying the recombination of photoinduced e–h pairs or improving charge separation certainly becomes crucial factor in enhancing its photocatalytic activity. In the present work, intercalation of alkylviologen cations into the layered vanadium pentoxide has been performed in the system of two phases of the liquid and the solid. The structure and the properties of these inorganic–organic intercalation compounds are investigated by XRD, FT-IR, XPS and DRS.
K. Lai et al. / Inorganica Chimica Acta 359 (2006) 1050–1054
2. Experimental 2.1. Synthesis The new inorganic–organic intercalation compounds were synthesized using the improved method of literature [16]. The mixture of V2O5 and the iodide salt of alkylviologen (RVI2, R = methyl, ethyl, propyl, nonyl, dodecyl abbreviated as MV2, EV2, PrV2, NV2, DV2 hereafter) in the molar ratio of 1–3.8 in 100 ml of water or acetone was refluxed in a flask equipped with a condenser for 2– 8 h, iodine crystals were sublimed onto the condenser, and dark green compounds were formed in the aqueous solution. The products were separated and purified by washing with deionized water and acetone for several times. After being dried in air under ambient temperature, we obtained the powdered intercalation compounds, and the molecular formula of the products were presented as (RV)0.25V2O5, which were determined by elemental analysis. The yield is essentially quantitative based on V2O5. 2.2. Characterization The XRD patterns were recorded on a D8-Adnance diffractometer using Cu Ka (filtered) radiation at 40 kV and 40 mA. The FTIR spectra were measured on a Nicolet NEXUS 670 FT-IR spectrometer in the range of 400– 4000 cm1 at room temperature, with the samples dispersed in KBr and pressed into pellets. The XPS spectra were made with a PHI 5000c ESCA System working by using Mg Ka radiation as the excitation source. Charging of samples was corrected by setting the binding energy of adventitious carbon (C1s 284.6 eV). The XPS analysis was done at ambient temperature and pressure typically on the order of less than 107 Torr. The XPS data analysis was performed with the XPS-PEAK 4.1 program [17]. The Shirley function was used to subtract the background. The V2p XPS signals was fitted with mixed Lorentzian– Gaussian curves. The DRS spectra were recorded in the range of 200–900 nm at room temperature using a JASO V-550 spectrophotometer (JASCO, Japan) equipped with an integration sphere.
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the guest with the host lattice or its xerogels [18]. The organic–inorganic intercalation of V2O5 in a crystal form is typically carried out under hydrothermal conditions in the presence of a strong organic Lewis base or ammonia as the molecular template [19]. However, in present investigation, the use of the iodide salt of alkylviologen can cause V2O5 to undergo reductive layer reconstruction in aqueous solution, forming a novel inorganic–organic intercalation compound containing the RV2+ as the cations template depending on the static interaction of the cations and anions. The schematic scheme of the layer reduction– exfoliation–restacking procedure was shown as following in Fig. 1. Fig. 2 shows the typical powder X-ray diffraction patterns for the inorganic–organic intercalation compounds prepared and the vanadium pentoxide. It was observed that a great of the change have occurred in the crystal structure of V2O5 while alkylviologen cations was intercalated into between the layers of vanadium pentoxide. The (0 0 1) reflections lying at 15.51°(2h) in the powder X-ray diffraction patterns of V2O5 indicates it has a basal layer spacing of 0.57 nm. In comparison with the diffractograms of V2O5, the (0 0 1) reflections shift to the lower 2h value in the diffractograms of the intercalation compounds, which account for the increase of the layered spacing for these inorganic–organic intercalation materials after alkylviologen cations was intercalated into V2O5. The different size of the alkylviologen cations lead to the different layered spacing in the intercalation of host and guest. For example, the intercalation product of the iodide salt of methylviologen has a basal layered spacing of 0.81 nm, which increased to 2.49 nm when was treated with iodide salt of dodecylviologen. This shift of 1.68 nm
d=2.49nm 001 d=2.26nm 001
DV2
d=1.29nm 001
NV2
d=0.88nm d=0.81nm
PrV2
001
EV2
001
MV2
001
d=0.57nm
V2O5
3. Results and discussion 0
The conventional synthetic method for intercalating molecules into layered V2O5 hosts is a direct reaction of
10
20
30 40 2 Theta
50
60
Fig. 2. XRD patterns of V2O5 and (RV)0.25V2O5.
Fig. 1. Schematic image of the reduction–exfoliation–restacking procedure.
K. Lai et al. / Inorganica Chimica Acta 359 (2006) 1050–1054
of the basal spacing is not enough to be related to the change in size of the corresponding cations, but maybe related to a change in the tilt angle of the cations. In the higher angles region, some (h k l) diffraction lines subjected to V2O5 disappear and some wider (h k l) reflections with the lower intensity appear in the XRD patterns of the inorganic–organic intercalation compounds, and which is a characteristic of a turbostratic structure where the lamellar structure of the host is preserved but mutually shifted in the directions of the x and/or y axes. In addition, the number of the (h k l) diffraction lines decreases in the X-ray diffractograms of the intercalation compounds prepared with the increase of the size of the alkylviologen cations. This phenomenon testifies that the structural regularity of the intercalation compounds is decreased with the increase of the alkyl chain in the alkylviologen cations. The FTIR absorption spectra of the (RV)0.25V2O5 are presented in Fig. 3. The bands due to characteristic vibrations of the alkylviologen cations at 1165 cm1 (d CH); 1430 cm1 (d CH2); 1480 cm1 (d CH3); 1620 cm1 (mC@C); 2840–3100 cm1 (mCH) related to pyridine rings and alkyl chains; 3440 cm1 (mOH, assigned to absorbed water); are clearly observed in the samples prepared, which further illustrate that presence of the organic phases in the new inorganic–organic hybrid compounds. The band at 1000 cm1 is assigned to the V@O stretching vibrations and the bands at 810 and 560 cm1 are attributed to the in-plane and out-of-plane V–O–V vibrations in the V2O5 inorganic phases, respectively. Comparing to the characteristic bands of bulk V2O5 (1020, 820 and 595 cm1), these three corresponding bands in the intercalation compounds have shifted to lower wave-numbers (red shift), which may be caused by the static interaction between the partial reduced vanadium in the vanadium pentoxide and alkylviologen cations, leading to V@O bonds and V–O–V bonds weakened [20]. XPS spectra of V 2p and O1s in the inorganic–organic intercalation compounds are displayed in Fig. 4. The binding energy and FWHW of the peaks in each compound XPS spectra are listed in Table 1, and the V2p3/2 binding energies and FWHM in the vanadium oxides reported
O1s
EV2
NV2 PrV2 EV2 MV2
520 525 Binding energy (eV)
530
Fig. 4. XPS spectra of V2p and O1s in (RV)0.25V2O5.
Table 1 V2p3/2 binding energies and FWHM of (RV)0.25V2O5 Compounds
MV2
EV2
PrV2
NV2
DV2
V2p3/2
515.9 2.91
515.9 2.71
515.9 2.68
516.0 2.97
516.0 2.82
BE (eV) FWHM (eV)
Table 2 V2p3/2 binding energies and FWHM of vanadium oxides reported Compound V2p3/2
BE (eV) FWHM (eV)
V2O4
V2O5
515.6 4.0
517.0 1.4
are given in Table 2 [21]. It is observed that the peaks of V2p3/2 in the XPS spectra are broad and asymmetrical, which hint that the vanadium atoms in these compounds are in different chemical environments. By using the XPS-PEAK 4.1 program on a Shirley function, the V2p XPS signals in MV2 can be deconvoluted into individual spectral lines as shown in Fig. 5 [22,23]. The two peaks located at the binding energy of 516.8 and 515.4 eV in the V2p3/2, which is attributed to V5+ and V4+, respectively. Comparison with that of V2O5, slight red shift of the binding energy of V2p3/2 can be observed, but comparison with that of V2O4, slight blue shift appears.
30000 V2p3/2 In te n si t y (a .u .)
Transmittance (%)
PrV2
V2p1/2
515
DV2 NV2
DV2
V2p3/2
Intensity (a.u.)
1052
25000 20000
V2p1/2
15000
MV2 10000
4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm-1) Fig. 3. FT-IR spectra of (RV)0.25V2O5.
512 514 516 518 520 522 524 526 Binding energy (eV) Fig. 5. Curve-fitting XPS spectra of V2p in MV2.
K. Lai et al. / Inorganica Chimica Acta 359 (2006) 1050–1054
Absorbance
215
DV2 NV2 PrV2 EV2 MV2
1053
behaviors may provide potential applications in the utilization of solar energy when those inorganic–organic intercalation compounds act as catalysts. Acknowledgement We thank National Natural Science Foundation of China (Grants Nos. 20173017 and 20273021) for financial support.
240
V2O5 200 300 400 500 600 700 800 900 1000 Wavelength (nm) Fig. 6. UV–Vis DRS spectra of (RV)0.25V2O5, V2O5.
These phenomena may be attributed to the interaction between V4+ and V5+ in vanadium oxide phase. The UV/vis diffuse reflectance spectra of (RV)0.25V2O5 are shown in Fig. 6. It is interesting to observe there is a strong band around 205–235 nm with a maximum of 215 nm, which can be assigned to the lower-energy charge-transfer associated with O to V electron transfer for square-pyramidal VIV species [24,25]. Comparing to the bulk V2O5 (240 nm), this absorption band appear in the higher wave-number region. The blue shift may be interpreted as the reduction of V5+ to V4+, which has been confirmed by XPS. The weak absorption band around 235–270 nm can be attributed to p(t)2 ! d(e) oxygen to square-pyramidal VV charge-transfer transition. There is a wide and strong absorption band in the range from 300 to 500 nm, which is composed of some absorption bands caused by many factors, such as, a2(p),b1(p) ! b2(xy) transitions owing to bridging oxygen in square-pyramidal coordination, p ! p* transitions and the charge transfer of the V@O double bond structure from O to V. Additionally, there is a medium intensity absorption in the 600–900 nm range, which can be related to the d–d transition of VO2+ ions and the absorption of the radical cation RV+ [24,25]. In summary, DRS confirms the inorganic–organic intercalation compounds have been prepared, and which possess special optical properties and may be suitable to be investigated in the photocatalytic reactions. 4. Conclusions The new inorganic–organic intercalation compounds were obtained by intercalating of alkylviologen dications into layered vanadium pentoxide in the system of two phases of the liquid and the solid. XRD and FT-IR analysis suggests the lamellar structure of V2O5 still exist in the new compounds. The investigation of XPS indicates that the phase of vanadium oxides is composed of mixedvalence vanadium oxides. DRS spectra show that the inorganic–organic hybrid materials give birth to the strong absorption within ultraviolet and visible light region. These
References [1] J. Muster, G.T. Kim, J.G. Park, Y.W. Park, S. Roth, M. Burghard, Adv. Mater. 12 (2000) 420. [2] P. Gomez-Romero, Adv. Mater. 13 (2001) 163. [3] F.R. Gramble, F.J. Disalvo, R.A. Klemm, T.H. Geballe, Science 168 (1970) 568. [4] B.A. Averill, S.M. Kauzlarich, Mol. Cryst. Liq. Cryst. 107 (1984) 55. [5] M.G. Kanatzidis, R. Bissessur, D.C. DeGroot, J.L. Schindler, C.R. Kannewurf, Chem. Mater. 5 (1993) 595. [6] Z.F. Li, E. Ruckenstein, Langmuir 18 (2002) 6956. [7] G.R. Goward, F. Leroux, L.F. Nazar, Electrochim. Acta 43 (1998) 1307. [8] M.G. Kanatzidis, C.G. Wu, H.O. Marcy, D.C. DeGroot, C.R. Kannewurf, Chem. Mater. 2 (1990) 222. [9] Y.-J. Liu, J.A. Cowen, T.A. Kaplan, D.C. DeGroot, J. Schindler, C.R. Kannewurf, M.G. Kanatzidis, Chem. Mater. 7 (1995) 1616. [10] E. Arashiro, E.C. Zampronio, C.A. Brunello, T.A.F. Lassali, H.P. Oliveria, C.F.O. Graeff, Int. J. Inorg. Mater. 3 (2001) 727. [11] E. Ruiz-Hitzky, B. Casal, J. Chem. Soc., Faraday Trans. 82 (1986) 1597. [12] T. Kuwahara, H. Tagaya, J. Kadokawa, Inorg. Chem. Commun. 4 (2001) 63. [13] H. Hasbah, D. Tinet, M.M. Crespin, R. Erre, R. Setton, H. VanDamme, J. Chem. Soc., Chem. Commun. (1985) 935. [14] Y.-P. Zhang, R.C. Haushalter, A. Clearfield, Inorg. Chem. 35 (1996) 4950. [15] C.L. Bird, A.T. Kuhn, Chem. Soc. Rev. 10 (1981) 49. [16] A. Bose, P.-G. He, C. Liu, B.D. Ellman, R.J. Twieg, S.-P. Huang, J. Am. Chem. Soc. 124 (2002) 4. [17] (a) C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, in: G.E. Muilenberg (Ed.), Handbook of X-Ray Photoelectron Spectroscopy, Perkin–Elmer Corporation, MN, 1978; (b) Available from:
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K. Lai et al. / Inorganica Chimica Acta 359 (2006) 1050–1054
[24] (a) T. Blasco, P. Concepcio´n, J.M. Lo´pez Nieto, J. Pe´rez-Pariente, J. Catal. 152 (1995) 1; ¨ ssi, M. (b) G. Genti, S. Perathoner, F. Trifiro´, A. Aboukais, C.F. Aı Guelton, J. Phys. Chem. 96 (1992) 2617.
[25] (a) H. Ronde, J.G. Snijders, Chem. Phys. Lett. 50 (1977) 282; (b) J. Hanuza, B. Jezowska-Trzebiatowska, W. Oganowski, J. Mol. Catal. 29 (1985) 109; (c) H. So, M.T. Pope, Inorg. Chem. 11 (1972) 1441.
Intercalation of alkylviologen dications into the layered vanadium pentoxide Keqiang Lai, Aiguo Kong, Fan Yang, Bo Chen, Hanming Ding, Yongkui Shan *, Songping Huang Department of Chemistry, East China Normal University, Shanghai 200062, China Received 22 March 2005; received in revised form 5 December 2005; accepted 6 December 2005 Available online 18 January 2006
Abstract A novel series of inorganic–organic intercalation compounds were prepared by intercalating of alkylviologen dications into the layered vanadium pentoxide in the system of two phases of the liquid and the solid, and characterized by the techniques of X-ray diffraction (XRD), FT infrared (FTIR), X-ray photoelectron spectroscopy (XPS) and UV/vis diffuse reflectance spectroscopy (DRS). Ó 2005 Elsevier B.V. All rights reserved. Keywords: Intercalation; Alkylviologen; Vanadium pentoxide; Photocatalytic
1. Introduction In the past decades, layered intercalation compounds have been receiving significant attention owing to their applications in various fields such as energy-storage applications, electrocatalysis, novel energy-conversion systems, proton-pump electrodes, sensors, chemiresistive detectors, heterogeneous catalytic processes and photochemical redox reactions [1,2]. In particular, interest is in the insertion of organic molecules into layered hosts for the purpose of synthesizing organic–inorganic composite materials with welldefined stoichiometries and organized structures, for such resulting intercalation compounds may lead to a material with properties superior to the sum of the properties of their components. The synergic effect resulting from the combined properties of organic–inorganic components can give rise to very interesting functional properties, especially in what concerns electrochemical, electrochromic and conduction properties [3–5].
*
Corresponding author. Tel./fax: +86 21 6223 3503. E-mail address: [email protected] (Y. Shan).
0020-1693/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2005.12.009
Layered V2O5 are suitable host materials for intercalation chemistry because their interlayer spaces in conjunction with their semiconducting properties enable them to act as cathodic materials for advanced lithium batteries, etc. Indeed, V2O5 as the inorganic host has been extensively studied with a variety of species, such as, polyaniline [6], polypyrrole [7], polythiophene [8], alkali-metal ions [9], melanin [10], pyridine [11], methyl yellow [12], benzidine [13], alkylamines [14]. Alkylviologen, due to its cation radical having a strong absorption band in the visible region of the spectrum, is an appropriate candidate for the investigation of photoelectrochemical processes. However, alkylviologen exhibits relatively poor photocatalytic activity when it is used alone, since the majority of the photogenerated e– h pairs simply undergo recombination [15]. Thus, delaying the recombination of photoinduced e–h pairs or improving charge separation certainly becomes crucial factor in enhancing its photocatalytic activity. In the present work, intercalation of alkylviologen cations into the layered vanadium pentoxide has been performed in the system of two phases of the liquid and the solid. The structure and the properties of these inorganic–organic intercalation compounds are investigated by XRD, FT-IR, XPS and DRS.
K. Lai et al. / Inorganica Chimica Acta 359 (2006) 1050–1054
2. Experimental 2.1. Synthesis The new inorganic–organic intercalation compounds were synthesized using the improved method of literature [16]. The mixture of V2O5 and the iodide salt of alkylviologen (RVI2, R = methyl, ethyl, propyl, nonyl, dodecyl abbreviated as MV2, EV2, PrV2, NV2, DV2 hereafter) in the molar ratio of 1–3.8 in 100 ml of water or acetone was refluxed in a flask equipped with a condenser for 2– 8 h, iodine crystals were sublimed onto the condenser, and dark green compounds were formed in the aqueous solution. The products were separated and purified by washing with deionized water and acetone for several times. After being dried in air under ambient temperature, we obtained the powdered intercalation compounds, and the molecular formula of the products were presented as (RV)0.25V2O5, which were determined by elemental analysis. The yield is essentially quantitative based on V2O5. 2.2. Characterization The XRD patterns were recorded on a D8-Adnance diffractometer using Cu Ka (filtered) radiation at 40 kV and 40 mA. The FTIR spectra were measured on a Nicolet NEXUS 670 FT-IR spectrometer in the range of 400– 4000 cm1 at room temperature, with the samples dispersed in KBr and pressed into pellets. The XPS spectra were made with a PHI 5000c ESCA System working by using Mg Ka radiation as the excitation source. Charging of samples was corrected by setting the binding energy of adventitious carbon (C1s 284.6 eV). The XPS analysis was done at ambient temperature and pressure typically on the order of less than 107 Torr. The XPS data analysis was performed with the XPS-PEAK 4.1 program [17]. The Shirley function was used to subtract the background. The V2p XPS signals was fitted with mixed Lorentzian– Gaussian curves. The DRS spectra were recorded in the range of 200–900 nm at room temperature using a JASO V-550 spectrophotometer (JASCO, Japan) equipped with an integration sphere.
1051
the guest with the host lattice or its xerogels [18]. The organic–inorganic intercalation of V2O5 in a crystal form is typically carried out under hydrothermal conditions in the presence of a strong organic Lewis base or ammonia as the molecular template [19]. However, in present investigation, the use of the iodide salt of alkylviologen can cause V2O5 to undergo reductive layer reconstruction in aqueous solution, forming a novel inorganic–organic intercalation compound containing the RV2+ as the cations template depending on the static interaction of the cations and anions. The schematic scheme of the layer reduction– exfoliation–restacking procedure was shown as following in Fig. 1. Fig. 2 shows the typical powder X-ray diffraction patterns for the inorganic–organic intercalation compounds prepared and the vanadium pentoxide. It was observed that a great of the change have occurred in the crystal structure of V2O5 while alkylviologen cations was intercalated into between the layers of vanadium pentoxide. The (0 0 1) reflections lying at 15.51°(2h) in the powder X-ray diffraction patterns of V2O5 indicates it has a basal layer spacing of 0.57 nm. In comparison with the diffractograms of V2O5, the (0 0 1) reflections shift to the lower 2h value in the diffractograms of the intercalation compounds, which account for the increase of the layered spacing for these inorganic–organic intercalation materials after alkylviologen cations was intercalated into V2O5. The different size of the alkylviologen cations lead to the different layered spacing in the intercalation of host and guest. For example, the intercalation product of the iodide salt of methylviologen has a basal layered spacing of 0.81 nm, which increased to 2.49 nm when was treated with iodide salt of dodecylviologen. This shift of 1.68 nm
d=2.49nm 001 d=2.26nm 001
DV2
d=1.29nm 001
NV2
d=0.88nm d=0.81nm
PrV2
001
EV2
001
MV2
001
d=0.57nm
V2O5
3. Results and discussion 0
The conventional synthetic method for intercalating molecules into layered V2O5 hosts is a direct reaction of
10
20
30 40 2 Theta
50
60
Fig. 2. XRD patterns of V2O5 and (RV)0.25V2O5.
Fig. 1. Schematic image of the reduction–exfoliation–restacking procedure.
K. Lai et al. / Inorganica Chimica Acta 359 (2006) 1050–1054
of the basal spacing is not enough to be related to the change in size of the corresponding cations, but maybe related to a change in the tilt angle of the cations. In the higher angles region, some (h k l) diffraction lines subjected to V2O5 disappear and some wider (h k l) reflections with the lower intensity appear in the XRD patterns of the inorganic–organic intercalation compounds, and which is a characteristic of a turbostratic structure where the lamellar structure of the host is preserved but mutually shifted in the directions of the x and/or y axes. In addition, the number of the (h k l) diffraction lines decreases in the X-ray diffractograms of the intercalation compounds prepared with the increase of the size of the alkylviologen cations. This phenomenon testifies that the structural regularity of the intercalation compounds is decreased with the increase of the alkyl chain in the alkylviologen cations. The FTIR absorption spectra of the (RV)0.25V2O5 are presented in Fig. 3. The bands due to characteristic vibrations of the alkylviologen cations at 1165 cm1 (d CH); 1430 cm1 (d CH2); 1480 cm1 (d CH3); 1620 cm1 (mC@C); 2840–3100 cm1 (mCH) related to pyridine rings and alkyl chains; 3440 cm1 (mOH, assigned to absorbed water); are clearly observed in the samples prepared, which further illustrate that presence of the organic phases in the new inorganic–organic hybrid compounds. The band at 1000 cm1 is assigned to the V@O stretching vibrations and the bands at 810 and 560 cm1 are attributed to the in-plane and out-of-plane V–O–V vibrations in the V2O5 inorganic phases, respectively. Comparing to the characteristic bands of bulk V2O5 (1020, 820 and 595 cm1), these three corresponding bands in the intercalation compounds have shifted to lower wave-numbers (red shift), which may be caused by the static interaction between the partial reduced vanadium in the vanadium pentoxide and alkylviologen cations, leading to V@O bonds and V–O–V bonds weakened [20]. XPS spectra of V 2p and O1s in the inorganic–organic intercalation compounds are displayed in Fig. 4. The binding energy and FWHW of the peaks in each compound XPS spectra are listed in Table 1, and the V2p3/2 binding energies and FWHM in the vanadium oxides reported
O1s
EV2
NV2 PrV2 EV2 MV2
520 525 Binding energy (eV)
530
Fig. 4. XPS spectra of V2p and O1s in (RV)0.25V2O5.
Table 1 V2p3/2 binding energies and FWHM of (RV)0.25V2O5 Compounds
MV2
EV2
PrV2
NV2
DV2
V2p3/2
515.9 2.91
515.9 2.71
515.9 2.68
516.0 2.97
516.0 2.82
BE (eV) FWHM (eV)
Table 2 V2p3/2 binding energies and FWHM of vanadium oxides reported Compound V2p3/2
BE (eV) FWHM (eV)
V2O4
V2O5
515.6 4.0
517.0 1.4
are given in Table 2 [21]. It is observed that the peaks of V2p3/2 in the XPS spectra are broad and asymmetrical, which hint that the vanadium atoms in these compounds are in different chemical environments. By using the XPS-PEAK 4.1 program on a Shirley function, the V2p XPS signals in MV2 can be deconvoluted into individual spectral lines as shown in Fig. 5 [22,23]. The two peaks located at the binding energy of 516.8 and 515.4 eV in the V2p3/2, which is attributed to V5+ and V4+, respectively. Comparison with that of V2O5, slight red shift of the binding energy of V2p3/2 can be observed, but comparison with that of V2O4, slight blue shift appears.
30000 V2p3/2 In te n si t y (a .u .)
Transmittance (%)
PrV2
V2p1/2
515
DV2 NV2
DV2
V2p3/2
Intensity (a.u.)
1052
25000 20000
V2p1/2
15000
MV2 10000
4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm-1) Fig. 3. FT-IR spectra of (RV)0.25V2O5.
512 514 516 518 520 522 524 526 Binding energy (eV) Fig. 5. Curve-fitting XPS spectra of V2p in MV2.
K. Lai et al. / Inorganica Chimica Acta 359 (2006) 1050–1054
Absorbance
215
DV2 NV2 PrV2 EV2 MV2
1053
behaviors may provide potential applications in the utilization of solar energy when those inorganic–organic intercalation compounds act as catalysts. Acknowledgement We thank National Natural Science Foundation of China (Grants Nos. 20173017 and 20273021) for financial support.
240
V2O5 200 300 400 500 600 700 800 900 1000 Wavelength (nm) Fig. 6. UV–Vis DRS spectra of (RV)0.25V2O5, V2O5.
These phenomena may be attributed to the interaction between V4+ and V5+ in vanadium oxide phase. The UV/vis diffuse reflectance spectra of (RV)0.25V2O5 are shown in Fig. 6. It is interesting to observe there is a strong band around 205–235 nm with a maximum of 215 nm, which can be assigned to the lower-energy charge-transfer associated with O to V electron transfer for square-pyramidal VIV species [24,25]. Comparing to the bulk V2O5 (240 nm), this absorption band appear in the higher wave-number region. The blue shift may be interpreted as the reduction of V5+ to V4+, which has been confirmed by XPS. The weak absorption band around 235–270 nm can be attributed to p(t)2 ! d(e) oxygen to square-pyramidal VV charge-transfer transition. There is a wide and strong absorption band in the range from 300 to 500 nm, which is composed of some absorption bands caused by many factors, such as, a2(p),b1(p) ! b2(xy) transitions owing to bridging oxygen in square-pyramidal coordination, p ! p* transitions and the charge transfer of the V@O double bond structure from O to V. Additionally, there is a medium intensity absorption in the 600–900 nm range, which can be related to the d–d transition of VO2+ ions and the absorption of the radical cation RV+ [24,25]. In summary, DRS confirms the inorganic–organic intercalation compounds have been prepared, and which possess special optical properties and may be suitable to be investigated in the photocatalytic reactions. 4. Conclusions The new inorganic–organic intercalation compounds were obtained by intercalating of alkylviologen dications into layered vanadium pentoxide in the system of two phases of the liquid and the solid. XRD and FT-IR analysis suggests the lamellar structure of V2O5 still exist in the new compounds. The investigation of XPS indicates that the phase of vanadium oxides is composed of mixedvalence vanadium oxides. DRS spectra show that the inorganic–organic hybrid materials give birth to the strong absorption within ultraviolet and visible light region. These
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