CRYSTAL GROWTH & DESIGN
Thermal Deposition Growth and Luminescence Properties of Single-Crystalline V2O5 Elongated Nanostructures C. Díaz-Guerra* and J. Piqueras Departamento de Física de Materiales, Facultad de Físicas, UniVersidad Complutense de Madrid, Ciudad UniVersitaria s/n, E-28040 Madrid, Spain
2008 VOL. 8, NO. 3 1031–1034
ReceiVed July 3, 2007; ReVised Manuscript ReceiVed December 4, 2007
ABSTRACT: Vanadium pentoxide (V2O5) nanorods and nanotips have been grown without a catalyst by a thermal deposition method using compacted V2O5 powder as the starting material. The tips, with diameters of about 50–100 nm, grow on the top of rods of larger size. Transmission electron microscopy shows that the obtained structures are single crystalline. The luminescence behavior of the nanostructures has been investigated by cathodoluminescence (CL) in the scanning electron microscope. CL reveals that the presence of nanostructures leads to changes in the spectral distribution of the emission. The obtained results are discussed by considering the band structure of V2O5. Introduction Vanadium pentoxide (V2O5) is a material with structural, optical, and electrical properties that are considered of interest for different applications, such as rechargeable lithium batteries, electrochromic devices, and spintronic devices.1,2 V2O5 has a layered structure that enables intercalation of Li ions between adjacent layers, involving storage of energy which is released during deintercalation, a process that can be applied in Li batteries. As an electrochromic material, it shows reversible changes of optical absorption upon charging and discharging processes via electrochemical reactions. V2O5 is also used in chemical sensors and as a catalyst. A large surface area favors fast intercalation and high charge and discharge rate and is also important in sensing, catalysis, and field emission applications. In the last few years, different authors have reported on the synthesis and properties of V2O5 nanorods, nanotubes and other nano- and microstructures with large surface to volume ratios. Precisely, vanadium pentoxide nanowires or nanorods have been fabricated by sol–gel,3 reverse micelle,4 or template-based electrodeposition5,6 techniques. Optical absorption and transmittance of V2O5 have been sometimes investigated7 in connection with its electrochromic properties, while reports on the luminescence properties are scarce,8 which is probably due to the low quantum efficiency of this material. In the present work, V2O5 nanostructures have been grown by a thermal deposition method during sintering of compacted powder under Ar flow. The nanostructures grow on the sample surface without the use of a foreign substrate or a catalyst. This method has been previously applied to grow elongated microand nanostructures of different semiconductor oxides,9–13 but it has not been applied to V2O5. The samples have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) in the cathodoluminescence (CL) and secondary electron modes, and by transmission electron microscopy (TEM). Experimental Section The starting material was commercially available V2O5 powder (99.999% nominal purity) with a range of particle size up to about 400 µm. The samples were prepared by compacting the powder under * To whom correspondence should be addressed. Phone: + 34 91 3944548. Fax: + 34 91 3944547. E-mail:
[email protected].
compressive load to form disks of about 7 mm in diameter and 2 mm thickness. The disks were then placed inside a tubular furnace and annealed under Ar flow at temperatures between 680 and 700 °C for 10 to 24 h. XRD measurements were performed in a Philips X’Pert PRO MPD diffractometer. Secondary electron and CL observations were carried out in a Leica 440 SEM and a Hitachi 2500 SEM. The CL measurements were carried out at 85 K with a Hamamatsu R928 photomultiplier working in photon counting mode and a Hamamatsu PMA-11 charge coupled device camera. CL spectra were corrected for the response of the collection system. Structures grown on the V2O5 disks were gently scratched, deposited on a microscope grid, and then observed in a Jeol JEM-2000 FX TEM operating at 200 kV.
Results and Discussion All the peaks found in the XRD patterns from both the starting material and the thermally treated samples can be unambiguously indexed to orthorhombic R-V2O5 (JCPDS card 85-0601). Figure 1 shows the XRD pattern of the as-received V2O5 powder and that of a sample treated at 700 °C for 15 h, showing a preferential [001] orientation. Such preferential orientation was actually observed in all the XRD patterns from treated samples investigated in the present work. No impurities were detected by SEM energy dispersive X-ray microanalysis measurements carried out in the grown nanostructures. Both temperature and annealing time were found to be critical parameters. Treatments at 680 or 700 °C for 10 h led to the growth of rods with lengths up to 10 µm and cross-sectional dimensions in the range 0.2–1 µm on the sample surface. Figure 2a shows the grain distribution in the untreated disk, while Figure 2b show the rods, or elongated platelets, which appeared after the mentioned treatment at 700 °C. Increasing the annealing time to 15 h in the same temperature range led to the formation of thinner and longer rods, which sometimes agglomerate to form urchin-like structures (Figure 3a), to the appearance of terraces on some rods (Figure 3b), as well as to the formation of nanoneedles and nanotips at the top of the rods (Figure 3c,d). Other extended treatments at lower temperatures, for example, 675 °C, did not lead to the growth of rods or other elongated nanostructures but produced a terraced structure on the grain surface (Figure 4). The height of the observed terraces varies between 100 and 400 nm approximately, as estimated from SEM micrographs. Selected area electron diffraction (SAED) patterns obtained in the TEM reveal the single-crystalline nature of the nanotips and nanorods grown on the oxide disks. Figure 5 shows TEM images of a rod-shaped crystal and a nanotip about 650 nm long and
10.1021/cg070612w CCC: $40.75 2008 American Chemical Society Published on Web 02/08/2008
1032 Crystal Growth & Design, Vol. 8, No. 3, 2008
Díaz-Guerra and Piqueras
Figure 1. XRD patterns of as-received V2O5 powder (a) and a sample treated at 700 °C for 15 h (b).
Figure 3. SEM images of structures in a sample annealed at 700 °C for 15 h. (a) Urchin-like structures, (b) rod with terraced surfaces, (c) and (d) rods ending in nanotips and nanoneedles.
Figure 2. SEM images showing the grain structure of the as-received V2O5 powder (a) and the nanorods grown after annealing at 700 °C for 10 h (b).
65 nm wide. The corresponding SAED patterns, as that shown in one inset of this figure, are identified as the (001) pattern of orthorhombic V2O5, in agreement with XRD measurements.
The present results demonstrate the growth of vanadium pentoxide micro- and nanostructures by thermal deposition methods. In particular, it has been found, as in the case of other oxides,9–13 that the structures grow directly on the surface of the pressed powder without the use of catalyst or a foreign substrate. The process leads to structures, as the above-described nanotips or dense arrays of micro- and nanorods, which are not normally grown when solution methods are used. It has been very recently reported14 that V2O5 nanotubes with 100 nm tip radius exhibit excellent field emission properties. This suggests that V2O5 nanotips, as those grown in this work (Figure 3), have potential applications as field emitters.
Single-Crystalline V2O5 Elongated Nanostructures
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Figure 4. SEM image of a sample treated at 675 °C for 15 h showing a terraced surface.
Figure 6. Representative CL spectra (85 K, 20 kV) of the untreated V2O5 powder and the nanotips (dotted line) and nanorods (dashed line) obtained after annealing at 700 °C for 15 h.
Figure 7. Schematic representation of the V2O5 band structure showing the filled O 2p and empty V 3d states.
Figure 5. TEM micrographs of a nanorod (a) and a nanotip (b) from the sample shown in Figure 2. The inset shows the SAED pattern of the nanotip.
The luminescence properties of individual nanostructures and the starting material were investigated by CL spectroscopy in a scanning electron microscope. The CL spectrum of the untreated V2O5 powder shows a band peaked at about 1.7 eV and much weaker emission at higher energies (Figure 6). In the structures grown on the compacted V2O5 disks, the CL spectra change as a function of the time of treatment and on the particular nanostructure considered. As a general result, broad emission bands in the ranges 2.1–2.4 eV and 3.2–3.3 eV are observed in the regions containing the nanostructures. Figure 6 shows a CL spectrum recorded in one of the rods, as those shown in Figure 3, and a CL spectrum recorded in one of the nanotips located
at the end of a rod. Generally, CL spectra of the rods show a main emission band peaked near 1.7 eV and a band at 3.1–3.3 eV, while spectra of the nanotips show the main band at 3.3 eV as well as a band centered at about 2.2 eV. Nishio and Kakihana8 reported on a broad photoluminescence (PL) band centered at 1.85 eV at room temperature in a study on visible light photochromism in sintered V2O5. The PL intensity decreased after strong laser irradiation, but the origin of this luminescence was not addressed. The optical properties of V2O5, mainly absorption, have been previously discussed by considering the band structure of this oxide.7,15 Absorption measurements indicate that the absorption edge of V2O5 single crystals varies between 2.15 and 2.22 eV and slightly depends on the orientation of the sample.16 The reported values for thin films at room temperature vary between 1.95 and 2.40 eV depending on the stoichiometry of the samples investigated, which in turn is influenced by the growth method and deposition conditions.17,18 The layer structure of this material leads to the splitting of the V3d conduction band. The Fermi energy lies in the gap between the O2p and the V3d bands. Although the direct gap is about 3.3 eV, the split-off localized band lies at about 0.6 eV below the main V3d band, so that it is located within the main energy gap, as illustrated in the schematic representation shown in Figure 7. The optical band gap for near stoichiometric V2O5 corresponds to the energy between the top of the O2p valence band and the split-off part of the V3d band.7,19 However, if the split-off band is filled, the optical band gap would correspond to the energy between the top of the O2p band and the bottom of the main part of the V3d band, since transitions between the two parts of the V3d band are parity forbidden. As described above, the CL spectra of the nanostructures, especially the nanotips, show two emission bands peaked at about 3.3 eV and 2.2–2.4 eV. The 3.3 eV emission can be explained by radiative
1034 Crystal Growth & Design, Vol. 8, No. 3, 2008
transitions between the main V3d conduction band and the O2p valence band, while the 2.2–2.4 luminescence can be attributed to transitions between the V3d split-off band and the same O2p band (see Figure 7). In the first case, the transition energy (∼3.3 eV) would be the sum of the two band gap energies shown in Figure 7 plus the width of the split-off V3d band. Possible transitions from higher energy states located inside the main V3d band would result in higher energy photons. Since the 3.3 eV luminescence is not observed in the starting material, it appears that the 3.3 eV transitions are more favorable in the case of nanostructures grown during the thermal treatments. This could be due to a different electronic structure leading to partial filling of the V3d split-off band, which would favor radiative transitions between the main V3d band and the O2p valence band. The exact origin of sub band gap optical transitions at about 1.8 eV observed in the CL spectra of the starting powder and of some of the treated samples is, to our knowledge, not known but could be possibly related to defect states present in higher density in the untreated material, since compositional and structural measurements did not reveal the existence of impurities or other oxides different from V2O5. Oxygen vacancies are the most commonly found defects in vanadium pentoxide.18,20 The structure of V2O5 consists of alternating layers of V + O atoms and O alone, which can be seen by the translation along the c-axis. Oxygen vacancies can be easily formed in the O layer between two V-O layers in (001) type planes. Empty 3d orbitals of vanadium atoms adjacent to a vacancy are able to localize excess electrons. This leads to the formation of localized states in the band gap,20 which may be involved in the emission observed peaked at 1.70 eV. Actually, enhanced absorption below 2.4 eV has been reported in vacuum annealed V2O5 films and attributed to an oxygen deficiency.21,22 Conclusions In summary, thermal treatments of compacted V2O5 powder at 680–700 °C, under argon flow, leads to the growth of nanorods and nanowires on the sample surface with sizes and morphology which depend on the growth parameters. Some of the treatments lead to the growth of nanotips with diameters of several tens of nanometers on top of the nanorods. Near band-
Díaz-Guerra and Piqueras
gap luminescence, involving valence band and at least two conduction bands, is favored in the nanowires as compared with bulk material. An emission band peaked at about 1.70 eV in all the investigated structures is tentatively attributed to defect centers involving oxygen vacancies. Acknowledgment. This work was supported by MEC through project MAT2006-01259.
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