Raman Spectra in Vanadate Nanotubes Revisited - Nano Letters

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NANO LETTERS

Raman Spectra in Vanadate Nanotubes Revisited

2004 Vol. 4, No. 11 2099-2104

A. G. Souza Filho,*,† O. P. Ferreira,‡ E. J. G. Santos,† J. Mendes Filho,† and O. L. Alves*,‡ Departamento de Fı´sica, UniVersidade Federal do Ceara´ , P.O. Box 6030, Fortaleza-CE, 60455-900, Brazil, and LQES - Laborato´ rio de Quı´mica do Estado So´ lido, Instituto de Qui´mica, UniVersidade Estadual de Campinas-UNICAMP, P. O. Box 6154, 13083-970, Campinas-SP, Brazil Received July 20, 2004; Revised Manuscript Received September 23, 2004

ABSTRACT In this letter we report the Raman spectra of vanadate nanotubes (VONTs). The spectra present a clear signature that can be used for probing the tubular structure. The temperature effects on the structure of dodecylamine- and Cu-intercalated VONTs were studied by changing the laser power density during the Raman measurements. We have found that low laser power densities promote the decomposition of VONTs, leading to the collapse of the tubular structure and converting the nanotubes into V2O5 oxide. The decomposition occurs through an intermediate compound that is isostructural to V2O5 xerogel. The Raman experiments in VONT-based systems should be performed at extremely low laser power densities.

1. Introduction. Nanomaterials are attractive because they exhibit physical and chemical properties that are very different from their bulk counterparts. Among these nanostructured forms, those with tubular structure are very promising for exploiting the size-induced properties and for building complex architetures at the nanoscale. The pioneer discovery of carbon nanotubes by Iijima1 has impulsed the search for nanotubes of other layered materials. Nowadays, there are a considerable number of nanotubes from inorganic materials such as MoS2, WS2, BN, NiCl2, VOx and NbS2, among others.2,3 The list of tubular materials is constantly increasing. The vanadate nanotubes (VONTs) are set apart from the inorganic nanotubes because of their tunable physicochemical properties.4-7 VONTs have many of the useful properties of the bulk counterpart V2O5, however, significantly enhanced. The high specific surface area of VONTs makes them more attractive as electrodes in Li batteries8 and considerably improves the efficiency in redox reactions.9,10 This property comes from the fact that being vanadium, a transition metal, it can presents a number of oxidation states thus being able to form several compounds with both singleand mixed-valence. The tubular structure has been attributed to the mixed-valent character of the vanadium atom where half of the atoms are in V4+ state and the another half in the V5+ state. The chemical formula of VONT is VOx with x ≈ 0.24.5 * Corresponding authors. Souza Filho: e-mail [email protected]; FAX +55(85)288-9912. Alves: e-mail [email protected]. † Universidade Federal do Ceara ´. ‡ UNICAMP. 10.1021/nl0488477 CCC: $27.50 Published on Web 10/23/2004

© 2004 American Chemical Society

Transmission electron microscopy (TEM) images have unveiled that VOx layers are warped forming a scroll structure for which the interlayer space is intercalated with organic molecules3-6,8,11 that are used as templates for growing these nanostructures. What makes VONTs interesting is that its structure is quite flexible and several exchange reactions (the substitution of these organic chains by metal cations) can be performed preserving the tubular structure.11 Recently, some studies have been devoted to investigate the optical properties of VONTs including optical absorption and infrared and Raman spectroscopy.12,13 Cao et al.12 have reported that the optical gap is dependent on the sheet distance, presenting a red shift when this parameter increases. Those authors also attributed an infrared mode located at 113 cm-1 as being the radial breathing mode of the VONTs. Conversely, the reported Raman spectra of VONTs is similar to those of bulk V2O5.13 The authors of ref 13 have proposed a method for removing the residual organic template by using a proper laser irradiation technique without damaging the tubular structure. Despite these studies, there are no systematic reports in the literature of heating effects on the vibrational properties of a VONTs system. In this letter, we report the effects of temperature on the stability of the VONT tubular structure by using Raman spectroscopy. The study was performed in samples intercalated with dodecylamine and Cu. The heating was provided by changing the laser power density. We have found that the collapse of the tubular structure is not reversible and that very low laser power density is enough to decompose the VONTs into V2O5. We have identified the spectral signature

of the VONTs and established a protocol that should be used for making appropriate use of Raman spectroscopy for studying these vanadate-based tubular structures. 2. Experimental Section. 2.1. Sample Preparation. The VONTs were prepared based on the method proposed by Nesper et al.6 We have studied two kinds of samples. One of them has dodecylamine intercalated between the sheets (Do-VONTs) and the another has copper (Cu-VONTs) obtained through the exchange reaction. The Do-VONTs were prepared as follows. A 2.73 g (15 mmol) amount of vanadium oxide (Sigma) and 2.85 g (15 mmol) of dodecylamine (Aldrich, 98%) were added to 25 mL of ethanol and submitted to stirring for 2 h. Deionized water (30 mL) was added the suspension. This mixture was stirred during 48 h at room temperature. A small part of the resulting suspension was isolated through filtration, and the remaining mixture was transferred to an autoclave and left for 8 days at constant temperature of 165 °C. The obtained solid was isolated through filtration and washed with ethanol and hexane. Finally, the solid was dried in vacuum for 6 h at 80 °C. The Cu-VONTs were obtained by taking 200 mg of DoVONTs in contact with 100 mL of Cu(NO3)2‚3H2O (Synth, P. A.) solution with 5.0 × 10-2 mol L-1 and submitted to a continuous stirring for 24 h at room temperature. The obtained products were isolated through filtration, being washed with ethanol and dried in vacuum. Thermal annealing was performed in the 150-500 °C temperature range by using a tubular furnace (Barnstead/Thermolyne model 21130) in static air atmosphere during 30 min. 2.2 Experimental Techniques. (a) Electron Microscope Images. Transmission electron microscope (TEM) images were obtained using a Carl Zeiss CEM-902. (b) X-ray Diffraction. X-ray powder diffraction patterns were obtained with a Shimadzu XRD6000 diffractometer, using Cu KR (λ ) 1.5406 Å) radiation operating with 30 mA and 40 kV. A scan rate of 1° min-1 was employed. (c) Infrared Spectroscopy. The Fourier transform infrared (FTIR) spectra were recorded using the KBr pellet technique on a Bomem MB spectrometer in the 4000-400 cm-1 frequency range. A total of 16 scans and a resolution of 4 cm-1 were employed in getting the spectra. (d) Raman Spectroscopy. The spectral excitation was here provided by an Ar+ ion laser, using the 514.5 nm laser line (2.41 eV) and with a variable power laser density. The scattered light was analyzed with a Jobin Yvon T64000 spectrometer, equipped with a cooled charge coupled device (CCD) detector. The slits were set for a resolution of 2 cm-1. 3. Results and Discussion. The transmission electron microscope images of the obtained Do-VONTs are shown in Figure 1. The nanotubes present several layers and are several microns in length with an average inner (outer) diameter of 45 nm (100 nm). In Figure 2 we shown the X-ray diffraction patterns for both Do-VONTs (upper trace) and Cu-VONTs (lower trace). The diffractograms are typical of the tubular structure.4,14 The diffraction peaks of CuVONTs are more broad than those of Do-VONTs, thus indicating that the exchange of the dodecylamine template with Cu leads to more disordered walls. The interlayer space 2100

Figure 1. Transmission electron microscope images of DoVONTs at low (left panel) and high (right panel) magnification.

Figure 2. X-ray diffraction patterns of Cu-VONTs (lower trace) and Do-VONTs (upper trace). The inset to the figure stands for the TEM image of the Cu-VONTs.

has been evaluated from X-ray diffraction data. The DoVONTs have an interlayer distance of 2.58 nm. It is clear the interlayer contraction for the Cu intercalated system where the (001) diffraction peak has shifted toward higher angles. The interlayer distance has been evaluated as being 1.13 nm in agreement with a previous report.14 Thus, the exchange reaction has been efficient in replacing the dodecylamine chains by Cu, keeping the tubular structure intact as can be seen in the TEM image showed in the inset to Figure 2. The exchange reaction efficiency can be further checked by analyzing the FTIR spectra shown in Figure 3. It should be noted that the C-H stretching modes (in the 2800-3000 cm-1 frequency range) of the dodecylamine chains are absent from the Cu-VONTs spectra. A similar behavior is observed for the C-H bending mode located at 1467 cm-1 and for the N-H bending mode (at 1595 cm-1) of the amine group. The band at 1618 cm-1 (marked with a down arrow) is related to the H-O-H bending mode of water molecules.14 The remaining modes below 1250 cm-1 are related to the vanadium oxide layers and the spectra do not exhibit dramatic changes except that the peaks for the Cu-VONTs are broader than for the Do-VONTs. This is related to the less ordered structure of the layers in agreement with the X-ray results. Nano Lett., Vol. 4, No. 11, 2004

Figure 3. Fourier transform infrared spectra of Cu-VONTs (upper trace) and Do-VONTs (lower trace).

Figure 4. (a) Raman spectra taken with four well-defined laser power density pointed by horizontal arrows in the panel (b). The upper trace in (a) is the Raman spectrum of the V2O5 bulk material used as precursor for the nanotube synthesis. (b) Two-dimensional plot of the laser power density dependence of the Raman spectra of Cu-VONTs in the low-frequency region. The orange (blue) regions stand for high (low) intensity.

Given the structural and morphological characterization of both Do- and Cu-VONTs, we now move to analyze the Raman spectra of these nanostructured materials. We have performed systematic experiments by changing laser power density in small steps, thus allowing us to get a detailed understanding on how the VONT structure evolves with temperature. In Figures 4b and 5b we show the twodimensional plot for the laser power density dependence of the Raman spectra for Cu-VONTs in the low- and highfrequency region, respectively. The orange areas stand for high intensity and blue areas for low intensity. It should be pointed out that this plot was constructed by normalizing the spectra with its maximum intensity value for showing purpose. At low-density power (up to 870 µW/µm2) the Nano Lett., Vol. 4, No. 11, 2004

Figure 5. The same as in Figure 4 except that the data are for the high-frequency region. The peaks marked with an asterisk (*) are experimental artifacts.

Raman spectrum of Cu-VONTs in the low-frequency range is characterized by broad features located at 162 and 250 cm-1 (see lower trace in Figure 4a) and 910 cm-1 (see lower trace in Figure 5a). Hardcastle and Wachs15 have proposed an empirical equation to relate the highest frequency observed in the spectra with the V-O bond length in vanadium-based oxides. The relation R ) - ln(ω/21349)/1.9176, where R is the bond length in Å and ω is the frequency in cm-1. Following this equation, a frequency of 910 cm-1 observed for Cu-VONTs corresponds to an average V-O bond length of 1.64 Å. The broad peak indicates a large range of bond lengths compatible with the disorder degree that the sample presents. This bond length is larger that what is observed for V2O5, which is 1.60 Å. V4+ has an ionic radius higher than V5+, which implies that the former would exhibit higher bond lengths thus decreasing the frequency. Since the tubes contain V4+ and V5+, the mode observed at 910 cm-1 confirms that the spectra come from the VONT. By incrementing the laser power density some peaks get more defined, and it is also possible to observe new defined peaks at 70, 841, 880, 929, 981, and 1027 cm-1, indicated by down arrows in Figures 4a and 5a. When the laser power density reaches a certain level (1592 µW/µm2), we can observe dramatic changes in the Raman spectrum. It is clear the appearance of sharp modes at 111 and 142 cm-1 which are marked with up arrows in Figure 4a. These peaks along with those located at 282, 296, 395, 694 and 994 cm-1 (also marked with up arrows) are the clear signature of the V2O5 oxide.16 By further increasing the laser power densities, these features gain intensity and the spectrum taken at the maximum power density reached in the experiment is identical to the Raman spectrum of V2O5 oxide used as precursor (see upper trace in Figure 4a). Therefore, the local heating caused by the laser promoted the decomposition of the Cu-VONTs into V2O5 oxide. Thus, we attribute the broad modes observed at 162, 250, and 910 cm-1 as being the spectral signature of the tubular structure. At this point a question that naturally arises is: Do the VONTs decompose 2101

Figure 6. (a) Fourier transform infrared spectra of Cu-VONTs decomposed at different temperatures. The upper and lower traces stand for the V2O5 precursor and as prepared Cu-VONTs, respectively. (b), (c) and (d) stand for TEM image of the samples decomposed at 300 °C.

directly into the V2O5 oxide? The laser power dependence shown in Figures 4 and 5 suggested that there is an intermediate structure between the VONTs and bulk V2O5. The sharp peaks observed at 70, 267, 880, 981, and 1027 cm-1 indicate the presence of an ordered structure. Those peaks have frequencies very close to those observed for V2O5 xerogels.17 The structure of V2O5 xerogels is characterized by the presence of V2O5 bilayers stacked along the c-axis of a monoclinic unit cell.18 The lower symmetry, compared with the orthorhombic bulk V2O5, explains why the spectra of xerogels present more modes. The typical Raman modes for V2O5‚0.6H2O are observed at 55, 267, 890, 985 and 1020 cm-1. These modes are measured at room temperature and they are very close to the modes we have observed for CuVONTs with intermediated laser power densities. The discrepancies in frequency are natural since the samples are in two different environments and submitted to different temperatures. We have estimated the temperature by using the anti-Stokes-to-Stokes ratio, and we found that the peaks typical of the xerogel structure appear when the temperature in the laser spot is about 300 °C. Based on this, we propose that the intermediate compound between the nanotubes and V2O5 is a vanadate compound with the same structure as the xerogel. To further support our interpretation we have performed FTIR experiments in the samples obtained after the thermal annealing at different temperatures. The results are shown in Figure 6a along with the spectra of as-prepared CuVONTs and V2O5. We can observe that the FTIR spectra are strongly dependent on the decomposition temperature. The first remarkable feature is the softening of the VdO vanadyl stretching. It changes from 1002 to 1022 cm-1 as the decomposition temperature change from 30 to 500 °C. The VONT wall has both V5+ and V4+. When the decomposition temperature increases, the oxidation V4+ f V5+ takes place (the decomposition was performed in static air atmosphere) and the ionic radius decreases, thus decreasing 2102

Figure 7. Raman spectra of Do-VONTs for several laser power densities. The upper trace stands for V2O5 bulk material used as precursor for the nanotube synthesis. The inset to the figure stands for the peaks coming from the dodecylamine.

the VdO bond length. This phenomenon, called the secondorder Jahn-Teller effect, is responsible for stiffening the Vd O frequency as observed in Figure 6a. The modes observed in the FTIR spectra at 530, 765, and 1010 cm-1 for samples treated at 300 °C are typical of the xerogel structure, thus corroborating the Raman scattering results. We also have analyzed the morphology changes during the thermal decomposition. In Figure 6b-d we show the TEM images of the samples treated at 300 °C. We can observe that samples are still present the tubular morphology. However, we can also observed the presence of nanoplates (Figure 6b). We can observe that the nanotube walls are getting disordered as shown in Figure 6c. We also show the morphology of the nanoplates in Figure 6d. The TEM images suggest that the morphology changes occur as follows. The tubular structure collapses into the nanoplates. By further heating the samples, the temperature will induce the segregation of V2O5 and a Cu-based vanadate phase. This is confirmed by analyzing the FTIR spectra of the sample treated at 400 °C where we can already observe the modes typical (marked with up arrows in Figure 6a) of V2O5. Based on the results we have presented, we can propose a model for the thermal decomposition of the Cu-VONTs. Following the works of Petkov et al.,18,19 the structures of the VONTs and xerogel are triclinic and monoclinic, respectively. Both structures present lower symmetry than the bulk V2O5 which is orthorhombic. The Raman measurements are consistent with the symmetry change sequence because when the intermediate compound (isostructural to xerogel) decompose into V2O5 we observed a decrease in the number of Raman modes. In contrast, the situation is not clear when the Cu-VONTs changes for the xerogel structure because we could only observe broad bands in the spectra of the tubes. The disorder in the VONT wall certainly would prevent the observation of well resolved modes in the spectra. The possible structural transformation path during the thermal decomposition process is: Cu-VONTs (triclinic) Nano Lett., Vol. 4, No. 11, 2004

f xerogel structure (monoclinic) f bulk V2O5 (orthorhombic). Thermal and in situ structural studies are under way in order to get details of the decomposition process, but the main picture is already stated from our spectroscopic studies. A similar laser power density dependence was observed for the Do-VONTs, which data are shown in Figure 7. Before the conversion of Do-VONTs into V2O5, the Raman spectrum is very weak and only for a laser power density of 2730 µW/µm2 does the spectrum (up to 1000 cm-1) resemble that of Cu-VONTs recorded at low laser power densities where the presence of the mode at 162 cm-1 is observed. The Raman spectra of Do-VONTs also present features between 1200 and 1400 cm-1 (see inset to Figure 7) that are related to the dodecylamine chains. The spectra recorded at higher laser power density do not present the modes of the organic chain, thus confirming that Do-VONTs have been decomposed into V2O5 oxide similar to Cu-VONTs. We also observed for intermediate laser power densities the modes related to the xerogel structure (see the mode located at 70 cm-1 marked with an up arrow in Figure 7). By comparing with Cu-VONTs, the laser power density needed for decomposing the Do-VONTs into the xerogel structure and V2O5 oxide is always higher for different experiments we have performed. This can be attributed to a better heating flow in the Do-VONT samples because they have a good crystallinity in contrast to the Cu-VONTs, which are more disordered. It should be pointed out that the laser power density where the VONTs start to convert into V2O5 oxide changes from experiment to experiment. This is reasonable since the texture of the powdered sample changes from spot to spot and it is not possible to get the same focus condition from one spot to another. To support our interpretation we have checked the reversibility of the process. In Figure 8a we show the Raman spectra taken at the lowest power density before (lower trace) and after (upper trace) the laser heating for the Cu-VONT samples. It is clear that the spectra are completely different, thus indicating that an irreversible process took place. In fact, the spectrum recorded after the heating has the signature of the V2O5 oxide, thereby confirming that the laser heating has decomposed the VONTs into V2O5 oxide. The same behavior is observed for Do-VONTs as can be observed in Figure 8b. In this case, we can monitor the modes from the organic chain to further validate the laser-induced decomposition process. It is clear that after heating the modes from the organic chain are not present in the spectra (see inset to the Figure 8b). This indicates that the tubular structure has been collapsed and V2O5 has been formed. Finally, a simple/direct check for the formation of the V2O5 oxide is gathered by observing the optical image of the sample surface. Cu-VONT (Do-VONT) samples are darkgreen (dark), colored and after being illuminated with the laser a yellowish circle with 10 µm in diameter was observed as shown in the insets to Figure 8a,b. This is the typical color of the V2O5 compound. This color change occurs exactly when the peak at 162 cm-1 disappears, thereby indicating that the sample is basically V2O5. Nano Lett., Vol. 4, No. 11, 2004

Figure 8. Raman spectra of (a) Cu-VONTs (obtained with 1370 µW/µm2) and (b) Do-VONTs (obtained with 2730 µW/µm2) before (lower traces) and after (middle traces) heating. The upper trace stand for V2O5 bulk material used as precursor for the nanotube synthesis.

4. Conclusions. In summary, we have reported a detailed study of the Raman spectra of vanadate nanotubes (VONTs). We have identified the spectral signature of the tubular structure being the same for both dodecylamine- and Cuintercalated VONTs. The spectra are characterized by peaks at 162, 250, and 910 cm-1. We also have investigated the temperature effects on the vibrational and structural properties of VONTs by changing the laser power density during the Raman measurements and measuring the FTIR spectra of the samples decomposed at different temperatures. We have found that the tubular structure is sensitive to the temperature effects and that the decomposition of the tubes into V2O5 oxide occurs through an intermediate compound that is isostructural to V2O5 xerogel. The irreversibility of the decomposition process is confirmed by observing the color changes at the laser spot. The laser power density threshold needed for decomposing the VONTs into V2O5 oxide depends on the intercalated species. Therefore, one 2103

should be very careful in getting the Raman spectra of VONTs because the local heating effect due to the laser easily converts the nanotubes into V2O5 oxide. Finally, our study allowed to establish the Raman signature of the VONTs and that this technique is powerful and useful as an easy/ quick tool for probing VONTs samples. Acknowledgment. The authors A.G.S.F. and O.P.F. acknowledge financial support from the Brazilian agencies CAPES (PRODOC grant) and CNPq, respectively. The authors are indebted to Dr. Carlos A. P. Leite for assistance with TEM images. This is a contribution of Millenium Institute of Complex Materials. References (1) Iijima, S. Nature 1991, 354, 56. (2) Rao, C. N. R.; Nath, M. Dalton Trans. 2003, 1. (3) Spahr, M. E.; Bitterli, P.; Nesper, R.; Muller, M.; Krumeich, F.; Nissen, H. U. Angew. Chem., Int. Ed. 1998, 37, 1263. (4) Krumeich, F.; Muhr, H. J.; Niederberger, M.; Bieri, F.; Schnyder, B.; Nesper, R. J. Am. Chem. Soc. 1999, 121, 8324. (5) Muhr, H. J.; Krumeich, F.; Schonholzer, U. P.; Bieri, F.; Niederberger, M.; Gauckler, L. J.; Nesper, R. AdV. Mater. 2000, 12, 231.

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Nano Lett., Vol. 4, No. 11, 2004