Structural, optical, and electronic properties of vanadium oxide nanotubes X. Liu, 1 C. Täschner, 1 A. Leonhardt, 1 M. H. Rümmeli, 1 T. Pichler, 1 T. Gemming, 1 B. Büchner, 1 and M. Knupfer 1 1 Leibniz-Institut für Festkörper- und Werkstoffforschung Dresden, D-01069 Dresden, Germany Received 13 June 2005; published 7 September 2005 The structure and electronic properties of vanadium oxide nanotubes have been studied using optical, photoemission, and electron energy-loss spectroscopies. Electron diffraction indicates the tubular structure with an interlayer distance of about 26.1 Å. Photoemission spectroscopy gives an atomic ratio of vanadium to oxygen of 1:2.5, which is very close to vanadium pentoxide. From V 2p core-level photoemission and vana- dium L-edge excitation spectroscopy, an averaged vanadium valency of about 4.4 + is obtained. The lowest lying interband electronic excitations reflect this mixed valency. These excitations are rather localized as revealed by essentially no dispersion as a function of momentum transfer in electron energy-loss spectroscopy. DOI: 10.1103/PhysRevB.72.115407 PACS numbers: 73.61.Wp, 71.20.Tx, 79.60.-i, 78.70.Dm I. INTRODUCTION Recently, one-dimensional nanoscale materials 1–3 have at- tracted considerable attention due to their remarkable physi- cal and chemical properties and their great potential for nanodevices. The worldwide research activities have been particularly motivated by the discovery and the successful synthesis of one-dimensional carbon nanotube materials, 4,5 which are about several nanometers in diameter with a high aspect ratio. The special properties of nanotube materials stimulated researchers to find more nanomaterials and initi- ated the production of inorganic tubular materials. Conse- quently, a number of reports have been focused on the syn- thesis of transition metal oxide e.g., ZnO, ZrO, or Co 3 O 4 , etc. nanotubes. 6 Among these one-dimensional nanostruc- ture transition metal oxides, the vanadium oxide nanotubes VO x -NTs 7–10 consisting of scroll-like layers of vanadium oxide are especially interesting because vanadium displays a number of different oxidation states and can thus form single-valent and mixed-valent compounds with various properties and can result in structural versatility. 11,12 The di- ameter of these VO x -NTs is in the range of 100 nm and the interlayer distance between the VO x layers depends on the size of spacer molecules which control the sandwiching dur- ing the formation process. 7–10 In addition, the tubular VO x materials have considerable scientific and technological sig- nificance owing to their interesting electrochemical and cata- lytic properties; 13–15 in particular, they are considered as promising nanoscale electrodes for lithium-ion batteries. 16 In this contribution, we present a detailed investigation of hydrothermally 7,8 synthesized VO x -NTs with different spec- troscopic methods. The tubular layer structure was analyzed with electron diffraction and transmission electron micros- copy TEM. X-ray photoemission spectroscopy gives a ratio of V to O of about 1 to 2.5, i.e., close to V 2 O 5 . The average valence of vanadium is about 4.4 + deduced from the V 2p photoemission peak position and the V L-excitation edge. The electronic interband excitations, as observed in the optical absorption and electron energy-loss spectra, can be assigned to different transitions due to the mixed-valent nature. II. EXPERIMENTAL In order to synthesize VO x -NTs, we have used the hydrothermal procedure described in Refs. 7 and 8. The starting material consisted of a mixture of vanadium V 5+  tri-isopropoxide VOiOPr 3  and dodecylamine CH 3 -CH 2  11 -NH 2 . The final product is a black powder. Thin films of VO x -NTs with a thickness of about 100 nm were produced for all measurements. They were prepared by drop coating the VO x -NTs from ethanol suspension onto KBr single crystals. After subsequent drying and dissolution of the crystal, the films were mounted on standard electron mi- croscopy grids and transferred into the corresponding spec- trometer or microscope. Prior to the EELS electron energy- loss spectroscopy measurements, the VO x -NT films were heated up to 150 °C in an ultrahigh vacuum UHV to re- move potential contaminations. The structure and chemical composition of the VO x -NTs was verified by high-resolution and analytical TEM in an FEI Tecnai F30 using x-ray spectroscopy and EELS. The XPS x-ray photoemission spectroscopy measurements were performed in a Physical Electronics’ PHI 5600 commercial spectrometer equipped with a monochromatic Al-K  x-ray source with an energy resolution of about 0.35 eV. The wide energy XPS spectrum of the VO x -NT films shows that there is no contribution from the substrate, remaining solvent, or any other contaminations. The bulk-sensitive electron energy-loss spectroscopy measurements were carried out in a purposed-built EELS spectrometer. 17 The energy and mo- mentum resolution are 0.18 eV and 0.03 Å -1 for the low- energy loss function and electron diffraction, respectively. For the core-level excitation measurements, a slightly re- duced momentum and energy resolution of 0.1 Å -1 and 0.3 eV was chosen. The loss function Im-1/ q , , which is proportional to the dynamic structure factor Sq , , has been measured for various momentum transfers q. Such momentum-dependent measurements can provide informa- tion on the dispersion and/or localization or the spatial dis- tribution of electronic excitations, 18,19 or they can allow the identification of optically forbidden excitations. 20 The optical absorption measurements were performed using a Bruker PHYSICAL REVIEW B 72, 115407 2005 1098-0121/2005/7211/1154075/$23.00 ©2005 The American Physical Society 115407-1