Facile Synthesis and Tensile Behavior of TiO2 One-Dimensional Nanostructures
© to the authors 2009
Received: 23 August 2009
Accepted: 28 October 2009
Published: 18 November 2009
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© to the authors 2009
Received: 23 August 2009
Accepted: 28 October 2009
Published: 18 November 2009
High-yield synthesis of TiO2 one-dimensional (1D) nanostructures was realized by a simple annealing of Ni-coated Ti grids in an argon atmosphere at 950 °C and 760 torr. The as-synthesized 1D nanostructures were single crystalline rutile TiO2 with the preferred growth direction close to . The growth of these nanostructures was enhanced by using catalytic materials, higher reaction temperature, and longer reaction time. Nanoscale tensile testing performed on individual 1D nanostructures showed that the nanostructures appeared to fracture in a brittle manner. The measured Young’s modulus and fracture strength are ~56.3 and 1.4 GPa, respectively.
Titanium dioxide (TiO2) one-dimensional (1D) nanostructures have received extensive research attention recently because of their promising applications in photo-catalysis, gas and humidity sensing, solar water splitting, bio-scaffolds, and others [1–3]. Both “wet-chemistry” and “dry” synthetic methods have been used to prepare TiO2 1D nanostructures. The “wet-chemistry” methods such as sol–gel process and anodic oxidation require further heat treatment to improve the crystallinity of as-synthesized nanostructures, which adds to the complexity of the processes. A few “dry” synthetic methods including vapor transport, metal–organic chemical vapor deposition (MOCVD), and annealing have been reported. The vapor transport method involves thermal evaporation of titanium (Ti) sources (e.g., Ti or TiO powders), transport of Ti-containing vapors, and final growth of TiO2 nanostructures on Ti-coated substrates [4–6]. This method requires precise control of source temperatures and reaction temperatures, which can be experimentally challenging. The MOCVD method can grow well-aligned TiO2 1D nanostructures [7, 8]. However, the MOCVD system setup is complicated and expensive. The annealing method grows TiO2 1D nanostructures by direct oxidation of Ti foils using acetone, ethanol, or dibutyltin dilaurate (DBTDL) vapor as oxygen (O2) sources [9–11]. While this method is relatively simple, the use of organic vapor could introduce carbon contamination and result in the growth of TiO2 core-amorphous carbon shell structures . Thus, it is necessary to seek simpler and more reliable “dry” synthetic methods to synthesize high quality TiO2 1D nanostructures. In addition, since mechanical stability is a crucial factor for structural integrity for the intended applications of TiO2 nanostructures, it is important to study the mechanical properties of individual TiO2 1D nanostructures.
In our previous work, a facile approach to synthesize TiO21D nanostructures by direct heating of nickel (Ni)-coated TiO powders was demonstrated . In this work, an even simpler one-step “dry” synthetic approach is reported, which produces single crystalline rutile TiO2 1D nanostructures by direct heating of Ni-coated Ti grids in an argon (Ar) environment at the atmospheric pressure. The mechanical properties of individual nanostructures were studied by a nanoscale tensile testing method using a custom-made nanomanipulator inside the vacuum chamber of a scanning electron microscope. According to the knowledge of the authors, this is the first time that the tensile behavior of rutile TiO2 1D nanostructures is reported.
Single crystalline rutile TiO21D nanostructures were synthesized by annealing catalytic material-coated Ti grids in Ar at the atmospheric pressure. Typical synthetic conditions are described in this paragraph, whereas conditions used in control experiments (e.g., variation of reaction temperatures) will be described later. Briefly, commercial Ti grids (Structure Probe Inc; mesh size varies from 100 to 400 mesh) were used as the starting material without any further cleaning procedures. A thin film of Ni (~2 nm) was deposited on Ti grids by magnetron sputtering (Denton Vacuum: Desk ®IV TSC). Ni-coated Ti grids were then loaded into a quartz boat and placed in the desired position inside a quartz tube (ϕ: 1 in. diameter) of a home-built horizontal tube furnace system. The system was first evacuated to ~10 mTorr and then brought back to the atmospheric pressure (~760 Torr) with Ar (Linde: 99.999% UHP). A continuous flow of 10 sccm (standard cubic centimeter per minute) Ar was then introduced and maintained for the rest of experiment. The quartz tube was ramped up to 950 °C (center position temperature measured outside the quartz tube by a thermocouple) in 60 min and soaked at that temperature for 30 min, followed by cooling down to room temperature in ~4 h. The Ti grids were then taken out and characterized by scanning electron microscopy (SEM) (JEOL JSM-6480), transmission electron microscopy (TEM; JEOL JEM-2100F) including electron energy loss spectroscopy (EELS) and selected area electron diffraction (SAED), X-ray diffraction (XRD; PANalytical X’Pert Pro diffractometer), and micro-Raman spectroscopy (Reinshaw RM 2000 confocal micro-Raman system in the back-scattering configuration; 514.5 nm excitation green laser).
Several growth controlling factors, including catalytic materials, growth temperature and growth duration, were investigated systematically.
(ii)Growth Temperature. The center position temperature of the tube furnace was varied from 750 to 1050 °C with an interval of 100 °C while the reaction time was kept as 60 min. Figure 2c, 2d shows the nanostructures synthesized at 750 and 1050 °C, respectively. At higher temperatures, longer, thicker, straighter, and more heavily populated nanowires can be grown.
(iii)Growth Duration. Reaction time was varied from 15 to 120 min while the center position temperature of the tube furnace was kept at 950 °C. Figure 2e, 2f shows the nanostructures synthesized in 15 and 120 min, respectively. Prolonged reaction time produced longer and slightly thicker TiO2 nanostructures. In short, the growth of TiO21D nanostructures can be enhanced by using catalytic materials, higher reaction temperature and longer reaction time.
The aforementioned experimental results raise a question: how many growth mechanisms are involved in the growth of TiO2 nanostructures from Ni-coated Ti grids? The observation of Ni existing on the tips of most nanostructures suggests that the Vapor–Liquid–Solid (VLS) growth  might be the dominating mechanism. However, for the small amount of nanostructures without Ni on their tips and even structures directly grown from bare Ti grids, other growth mechanisms such as Vapor–Solid (VS) and solid state oxidation growth could be involved . Despite the various growth mechanisms, it is believed that the growth is governed by the chemical reaction: Ti (g or s) + O2 (g) → TiO2 (s). Although our experiments were done in the Ar atmosphere, the oxygen could come from the leakage of air into the reaction chamber and other possible sources . It was observed that the amount of O2 plays a critical role in the formation of TiO2 1D nanostructures. Deliberate introduction of 1 sccm O2 into the reaction chamber suppressed the growth of TiO2 nanostructures, but enhanced the formation of polycrystalline TiO2 film. Similar results have been seen from growth of TiO2 nanostructures directly from Ti foils using small organic molecules (e.g., acetone, water) as the O2 source . In order to quantify the exact amount of O2 needed for growth of TiO2 1D nanostructures from Ni-coated Ti grids, a new O2 mass flow controller capable of controlling gas at 0.2 sccm level has been integrated into the tube furnace system recently. The results of these additional studies will be presented elsewhere.
Tensile testing results on four TiO21D nanostructures with sample #2 repeatedly tested three times
Breaking force (μN)
Tensile strength (MPa)
Failure strain (%)
Young’s modulus (GPa)
The sample #2 and its fragments were repeatedly loaded three times, with higher breaking force required for each successive test as well as increased failure strain. Such trend has been observed in our previous multiple tensile loading studies on individual multi-wall carbon nanotubes . Considering that a nanostructure under uniaxial tension should fail at the “critical flaw” along its length, the resulting nanostructure fragments should contain less significant defects than the original one, and should thus possess a higher fracture strength. The Young’s modulus values for the sample #2 obtained from linear fit of the three stress–strain curves are very close, as expected.
where S ij (i j run from 1 to 6) are stiffnesses and can be converted from compliances (i.e., elastic constants, C ij ) . Using the available elastic constants for rutile TiO2, the Young’s modulus of  direction was calculated to be ~239 GPa, which is higher than the experimental value (~56 GPa). Literature search shows that lower Young’s moduli for 1D nanostructures have been reported [27–30]. For example, the Young’s moduli of ZnO 1D nanostructures were measured to be 29 ± 8 GPa  and 31.1 ± 1.3 GPa , which are significantly lower than the calculated Young’s modulus of bulk ZnO (Ebulk ZnO  = 140 GPa ). Despite of measurement errors, surface stress might be the key reason causing the lower modulus . Lee et al. reported the three-point bending of anatase polycrystalline TiO2 nanofibers, the average elastic modulus of these fibers (~75.6 GPa) was found to be incomparable with the calculated value for bulk anatase TiO2 (e.g., Ebulk anatase  = 192 GPa) , mainly due to the polycrystalline nature of the nanofibers and inherent error associated with the testing method . While the causes of our measured lower modulus of TiO2 1D nanostructures need further investigation, the observed larger interplanar spacing might be one reason.
In summary, a simple synthetic process to produce TiO21D nanostructures by heating Ni-coated Ti grids has been described. The as-synthesized 1D nanostructures were characterized to be single crystalline rutile TiO2, with the preferred growth direction close to . Tensile behavior of individual 1D nanostructures was studied by nanoscale tensile testing with a nanomanipulator in an scanning electron microscope. The measured Young’s modulus was ~56 GPa, lower than the value for bulk TiO2. The reported synthetic technique could facilitate the in situ growth study of 1D nanostructures by TEM. The mechanical characterization of TiO21D nanostructures provides useful information for future device integration of these nanoscale building blocks.
T. Xu appreciates the support of the start-up fund and junior research grant at the University of North Carolina at Charlotte (UNC Charlotte). W. Ding appreciates the support of the start-up fund at Clarkson University. We are grateful to the Center for Optoelectronics and Optical Communications at UNC Charlotte, the Center for Advanced Materials Processing at Clarkson, and NUANCE center at Northwestern University for supplying multi-user facilities used for this work.