The Preparation And Properties Of Co-doped TiO₂ And VO_x Nanomaterials

Recently, materials with nanosized structures have been the subject of especially intense research in the field of material research. Based on the preparation and application of one-dimensional nanomaterials, in this paper, we have reported on the latest progress in research of nanomaterials and our research achievements. The main contents are as follows.1. The preparation and properties of Co-doped TiO₂ nanomaterials.Ferromagnetic diluted magnetic semiconductors (DMSs) consist of non-magnetic semiconducting materials doped with a few at.% of impurity magnetic cations, which is greatly demanding for the development of spintronic devices. Co-doped TiO₂ is a promising candidate as transparent DMSs due to properties like a high Tc above room temperature, excellent optical transmission in the visible and near-infrared regions, and high n-type carrier mobility. Thus Co-doped TiO₂ material appears promising for potential spintronics applications. Until now, many efforts have been made to explore the synthesis and magnetic properties of Co-doped TiO₂ films ; however, only a few research groups reports the direct synthesis of Co-doped TiO₂ 1D nanostructures . In order to comprehend the role of dimensionality and to identify the origin of the ferromagnetism in Co-doped TiO₂, it seems interesting to elaborate Co-doped TiO₂ 1D nanostructures.In this paper, we have detailly investigated the structural and magnetic properties of Co-doped TiO₂ nanomaterials. The main contents are as follows: (1) Ti_1-xCo_xO₂ (x=0-0.07) films were prepared by sol-gel spin coating; (2) The Co-doped TiO₂ nanotube arrays have been synthesized by a sol-gel template method; (3) Co-doped titanate nanotubes were prepared from hydrothermal treatment on Co-doped anatase TiO₂ powders in a concentrated NaOH aqueous solution.It was found that the Co was incorporated in the TiO₂ lattice as Co²⁺ and substituted for the Ti site with no evidence of metallic Co. Also, it showed oxygen vacancies played a major role in ferromagnetism of the Co-doped TiO₂ system. In our case, the room temperature ferromagnetism of the Co-doped TiO₂ system is originated from the exchange interaction between Co²⁺ mediated by oxygen vacancies, not being caused by metallic Co. Furthermore, the results suggested that ferromagnetism of the Co-doped TiO₂ system could be suppressed by their quantum size.2. The preparation and properties of VO_x nanomaterials.Rechargeable magnesium battery may be a candidate of high energy density battery due to its natural abundance, a relatively low price of its raw materials, and an expected higher safety of batteries based on metallic magnesium compared to lithium. Vanadium oxide VO_x (2.0≤x≤2.5) has an anisotropic layered structure, which favors ionic intercalations between the layers to form M_xV₂O₅ phase without far-reaching restructuring. On the other hand, one-dimensional VO_x nanomaterials have the large surface area and short diffusion distance. Thus, one-dimensional VO_x nanomaterials are the attractive cathode materials for rechargeable magnesium battery.In this study, we have detailly investigated the structural properties of VO_x nanomaterials. The main contents are as follows.(1) V₂O₅ thin films were deposited on single-crystal Si and glass substrates by sol-gel spin coating. It showed that annealing to 300℃was sufficient to produce crystallineα-V₂O₅ (orthorhombic) on single-crystal Si (111) orβ-V₂O₅ (tetragonal) on glass. Thus,β-V₂O₅ does not necessarily exist at high pressure and can be formed at atmospheric pressure by sol-gel method. Also, it was found that it was found thatβ-V₂O₅ was metastable.(2) Vanadium oxide nanotubes (VONTs) have been synthesized by hydrothermal treatment from V₂O₅.nH₂O sols as precursor and dodecylamine as structure-directing template. It was found that the synthesis of VONTs from V₂O₅.nH₂O sols needs shorter duration of hydrothermal treatment (1-5 days), which may give a real advantage in the synthesis of VONTs. Raman spectroscopy studies showed that the VONTs could be decomposed even at low laser power irradiation. It showed that the thermal stability of VONTs was relatively low and the tubular morphology was destroyed at temperatures higher than 300℃. Also, we have found that a large proportion of the amines can be exchanged by Co²⁺ and the tubular morphology is not affected by this reaction.(3) VO₂(B) nanomaterials were prepared by prolonging the duration of the hydrothermal treatment and the hydrothermal aftertreatment. It showed that during the hydrothermal treatment the thermal decomposition of the protonated amines promotes the reduction of V⁵⁺ species to V⁴⁺ species, which leads to the collapse of VONTs structure resulting in formation of VO₂(B).(4) The layered V₂O₅.nH₂O and VO_x nanobelts have been synthesized by hydrothermal treatment from NH₄VO₃ as precursor and CTAB as structure-directing template under varying pH conditions. For the VO_x nanobelts, they exhibit a layered structure in which positively charged CTAB are inserted between negatively charged vanadium oxide planes. It is revealed that CTAB not only serves as the template directing the assembly and 1D growth of the nanobelts, but also act as a reducing agent to make some V⁵⁺ reduce to V⁴⁺, leading to the formation of VO_x nanostructures.We expect that these VO_x nanomaterials could be an attractive cathode material for rechargeable magnesium battery. More research is in progress to study the electrochemical performance of these VO_x nanomaterials.