Facile Synthesis and Tensile Behavior of TiO2 One-Dimensional Nanostructures

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 [210]. 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.


Introduction
Titanium dioxide (TiO 2 ) 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][2][3]. Both ''wet-chemistry'' and ''dry'' synthetic methods have been used to prepare TiO 2 1D nanostructures. The ''wet-chemistry'' methods such as solgel 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 TiO 2 nanostructures on Ti-coated substrates [4][5][6]. This method requires precise control of source temperatures and reaction temperatures, which can be experimentally challenging. The MOCVD method can grow well-aligned TiO 2 1D nanostructures [7,8]. However, the MOCVD system setup is complicated and expensive. The annealing method grows TiO 2 1D nanostructures by direct oxidation of Ti foils using acetone, ethanol, or dibutyltin dilaurate (DBTDL) vapor as oxygen (O 2 ) sources [9][10][11]. While this method is relatively simple, the use of organic vapor could introduce carbon contamination and result in the growth of TiO 2 coreamorphous carbon shell structures [10]. Thus, it is necessary to seek simpler and more reliable ''dry'' synthetic methods to synthesize high quality TiO 2 1D nanostructures. In addition, since mechanical stability is a crucial factor for structural integrity for the intended applications of TiO 2 nanostructures, it is important to study the mechanical properties of individual TiO 2 1D nanostructures.
In our previous work, a facile approach to synthesize TiO 2 1D nanostructures by direct heating of nickel (Ni)coated TiO powders was demonstrated [12]. In this work, an even simpler one-step ''dry'' synthetic approach is reported, which produces single crystalline rutile TiO 2 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 TiO 2 1D nanostructures is reported.

Materials Synthesis and Characterization
Single crystalline rutile TiO 2 1D 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 Ò IV TSC). Ni-coated Ti grids were then loaded into a quartz boat and placed in the desired position inside a quartz tube (/: 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 backscattering configuration; 514.5 nm excitation green laser). Figure 1a, b is low and high magnification SEM images of as-synthesized 1D nanostructures grown on a 400 mesh Ti grid, respectively. Uniformly distributed nanostructures consisting of both wire-and belt-like morphologies can be found all over the grid. These nanostructures are 20-80 nm in width and 5-20 lm in length. Figure 1c is the micro-Raman spectrum revealing three major peaks at *224, 444, and 607 cm -1 . These peaks match closely to the reference values for rutile TiO 2 [13]. Figure 1d is the XRD spectrum whose most diffraction peaks can be indexed to the rutile TiO 2 according to the JCPDS card No. 21-1276 [14]. TEM/EELS/diffraction pattern analyses revealed that the nanostructures are single crystalline, and most of them have the catalytic material Ni on their tips. Figure 1e is a low magnification TEM image, showing a 1D nanostructure with a catalytic particle on its tip. Figure 1f is a high magnification TEM image of a part of a 1D nanostructure. The corresponding fast Fourier transform (FFT) pattern indicates the single crystalline nature of the nanostructure. The inset in a shows a bare Ti grid before reaction. c A micro-Raman spectrum shows three peaks at 224, 444, and 607 cm -1 , corresponding to the Raman active modes B 1g , E g and A 1g of rutile TiO 2 , respectively. d A XRD spectrum shows diffraction peaks, most of which could be indexed to rutile TiO 2 . The higher intensity background recorded before the (110) peak was contributed from the glass slide used to hold the samples. e A low magnification TEM image shows a catalytic material on the tip of a nanostructure. The inset shows lattice fringes clearly. The distance between the neighboring fringes is 0.358 nm, which is close to the d-spacing of (110) plane of rutile TiO 2 (d (110) = 0.325 nm) [14]. The origin of observed larger interplanar spacing is unclear. Similar phenomena were reported by other researchers [15,16]. Factors such as measurement errors, existence of possible impurities, surface relaxation [17], and the nature of substrate materials could all play a role. The growth direction of the nanostructure is around 17°away from [110], which is close to the [210] direction. In short, the as-synthesized nanostructures were characterized to be single crystalline rutile TiO 2 with the preferred growth direction close to the [210]. Several growth controlling factors, including catalytic materials, growth temperature and growth duration, were investigated systematically.
(i) Catalytic Materials. Figure 2a, b shows the nanostructures synthesized without and with the catalytic material Ni at 850°C for 60 min, respectively. It is obvious that the growth of TiO 2 nanostructures can be greatly enhanced by using the catalytic material. The optimum thickness of catalytic material film is *2 nm. Thicker or thinner films produced less 1D nanostructures. When employing different catalytic materials in control experiments, the effectiveness of them was found to be in the order of Ni [ (Au, Ag) [ (Pd, Pt). While a catalytic material was used in syntheses, it can be detected from the tips of most of nanostructures by the TEM/EELS observation.
(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, d 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, f shows the nanostructures synthesized in 15 and 120 min, respectively. Prolonged reaction time produced longer and slightly thicker TiO 2 nanostructures. In short, the growth of TiO 2 1D 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 TiO 2 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 [18] 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 [19]. Despite the various growth mechanisms, it is believed that the growth is governed by the chemical reaction: Ti (g or s) ? O 2 (g) ? TiO 2 (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 [12]. It was observed that the amount of O 2 plays a critical role in the formation of TiO 2 1D nanostructures. Deliberate introduction of 1 sccm O 2 into the reaction chamber suppressed the growth of TiO 2 nanostructures, but enhanced the formation of polycrystalline TiO 2 film. Similar results have been seen from growth of TiO 2 nanostructures directly from Ti foils using small organic molecules (e.g., acetone, water) as the O 2 source [9]. In order to quantify the exact amount of O 2 needed for growth of TiO 2 1D nanostructures from Nicoated Ti grids, a new O 2 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 Behavior of As-Synthesized TiO 2 1D Nanostructures
Nanoscale tensile loading [20][21][22] of individual TiO 2 1D nanostructures was performed with a custom-made nanomanipulator inside the vacuum chamber of a scanning electron microscope (JEOL JSM-7400F). In short, two Atomic Force Microscopy (AFM) chips were mounted on the two opposing linear positioning stages of the nanomanipulator. An AFM chip with long (compliant) cantilevers (MikroMasch, Inc.; Chip NSC 12, lengths 350 and 300 lm, nominal force constants 0.3 and 0.5 N/m, respectively) was mounted on the X-Y linear stage, and an AFM chip with short (stiff) cantilevers (MikroMasch, Inc.; Chip NSC 12, lengths 90 and 110 lm, nominal force constants 14.0 and 7.5 N/m, respectively) was mounted on the opposing Z linear stage together with the TiO 2 1D nanostructures source (i.e., a Ti grid with 1D nanostructures on it) (Fig. 3a). Through nanomanipulation, an individual TiO 2 1D nanostructure was picked up from the source and clamped between the two opposing AFM tips with the electron beam induced deposition method (Fig. 3b). The long (compliant) cantilever, served as the force-sensing element, was then gradually moved away from the short (stiff) cantilever by actuating a piezoelectric bender (Noliac A/S.; CMBP 05) with a dc voltage. An increasing tensile load was thus applied to the nanostructure until it fractured. In our current experimental approach, the applied tensile load and strain in the nanostructure were not directly obtained during the loading process. During the test, the tensile load was increased in discrete steps and SEM images at each loading step were acquired. The applied tensile load and strain in nanostructure at each loading step were obtained later based on the corresponding force-sensing cantilever deflection and nanostructure elongation from image analysis [20,21]. The bending stiffness of the forcesensing AFM cantilever was calibrated with a resonance method in vacuum right before the test [23].
Six nanoscale tensile tests were successfully performed on four individual TiO 2 1D nanostructures, with the sample #2 being repeatedly tested three times. The experimental results are summarized in Table 1. Based on the stressstrain relationships obtained, all these nanostructures appeared to fracture in a brittle manner, and the failure strain ranged from 0.6 to 4.7%. SEM observation of the nanostructure fragments did not reveal any visible necking.
The fracture strength of the TiO 2 nanostructure ranged from 0.3 to 4.2 GPa with an average value of *1.4 GPa. The corresponding Young's modulus obtained from linear data fitting of the stress-strain curve ranged from 47 to 89 GPa, with an average value of *56 GPa. Sample #3 was noticed to have a smallest value of diameter but a highest value of Young's modulus, indicating a possible size effect [24].
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 [21]. 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. For a tetragonal crystal system of class 4/mnm, the Young's modulus (E) along a unit vector [l 1 l 2 l 3 ] can be expressed as [25] 1 where S ij (i, j run from 1 to 6) are stiffnesses and can be converted from compliances (i.e., elastic constants, C ij ) [25]. Using the available elastic constants for rutile TiO 2 [26], the Young's modulus of [210] direction was calculated to be *239 GPa, which is higher than the experimental value (*56 GPa  [32], mainly due to the polycrystalline nature of the nanofibers and inherent error associated with the testing method [30]. While the causes of our measured lower modulus of TiO 2 1D nanostructures need further investigation, the observed larger interplanar spacing might be one reason.

Conclusions
In summary, a simple synthetic process to produce TiO 2 1D nanostructures by heating Ni-coated Ti grids has been described. The as-synthesized 1D nanostructures were characterized to be single crystalline rutile TiO 2 , with the preferred growth direction close to [210]. 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 TiO 2 . The reported synthetic technique could facilitate the in situ growth study of 1D nanostructures by TEM. The mechanical characterization of TiO 2 1D nanostructures provides useful information for future device integration of these nanoscale building blocks.