ARTICLE pubs.acs.org/JPCC
Nanosized Rutile (TiO2) Thin Film upon Ion Irradiation and Thermal Annealing Jiaming Zhang,† Jie Lian,*,‡ Fereydoon Namavar,§ Jianwei Wang,† Hani Haider,§ Kevin Garvin,§ and Rodney C. Ewing*,† †
Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan 48109-1005, United States Department of Mechanical, Aerospace & Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, United States § Department of Orthopaedic Surgery and Rehabilitation, University of Nebraska Medical Center, Omaha, Nebraska 68198-5360, United States ‡
ABSTRACT: Among three TiO2 polymorphs, rutile is thermodynamically unstable as compared with anatase and brookite when the crystal size is ∼10 nm. In this study, nanocrystalline rutile with an average grain size of ∼6 nm was synthesized in a thin film geometry by ion beam-assisted deposition (IBAD) with an amorphous TiO2 interlayer between the rutile film and Si-substrate. Nonstoichiometry produced by high-intensity ion bombardment during deposition may have stabilized the metastable rutile phase on the nanoscale. The phase stability of nanosized rutile was investigated by irradiation using 1 MeV Kr2+ combined with thermal annealing. In situ transmission electron microscopy (TEM) results indicate that partial amorphization occurred in nanocrystalline rutile when irradiated at room temperature, whereas the nanocrystals remained stable upon irradiation at 573 K. Ion beam-induced recrystallization occurred in the amorphous TiO2 at a 573 K, significantly lower than the temperature for thermally induced recrystallization, which occurs at 673 K. This is an example of radiation-enhanced kinetics of a phase transformation. With a further increase in the irradiation temperature, to 1073 K, the tetragonal rutile transformed to triclinic Ti5O9.
1. INTRODUCTION Titanium dioxide has a variety of technological applications, such as photocatalysis, gas sensors, and thin film electronic devices.1 5 Recently, there has been extensive interest in the use of thin films coated with TiO2 as a self-cleaning transparent surface exhibiting superhydrophilicity.6 8 These potential applications are very sensitive to the surface properties of nanocrystalline TiO2 particles and the interface properties between TiO2 film and the substrate.9 In addition, the chemical and physical properties of the nanocrystalline TiO2 are highly sizedependent.10 13 Among three naturally occurred polymorphs of TiO2, rutile is stable as the bulk phase and at high temperature, whereas anatase and brookite are relatively stable on the nanoscale.14 17 Theoretical calculations suggested that the surface enthalpies of the three polymorphs are sufficiently different such that a crossover in the thermodynamic stability occurs, with anatase, brookite, or both stable in smaller grains.16 This energy crossover has been confirmed by a high-temperature oxide melt solution calorimetric study.17 Upon thermal annealing, concurrent with coarsening, anatase and brookite usually transform to rutile, and the transformation process depends on parameters such as initial particle size, impurity content, the initial phase, reaction atmosphere, and so on.17 21 Recently, both experiments and theoretical calculations have show that rutile exists in nonstoichiometric form that contains complex defects including titanium interstitials and oxygen vacancies.22,23 Long-range, ordered defects exist r 2011 American Chemical Society
in titanium oxides and oxygen deficient phases, so-called Magneli phases, such as TinO2n 1 with 3 e n e 6, exhibit interesting optical and electrical properties.24,25 Therefore, it is important to study the stability of the polymorphs of titanium oxides on the nanoscale and to understand the interplay between phase transformations among the different polymorphs and the associated properties for technological applications. In this study, thin films of nanocrystalline rutile, with an average grain size of 6 nm, were synthesized using an ion-beam assisted deposition (IBAD) technique and characterized by TEM. The intense Ar+ bombardment during the IBAD process may lead to nonstoichiometry of the thin film and help to stabilize rutile phase on a length scale in which anatase and brookite are more thermodynamically stable. The microstructural evolution and the phase stability of nanocrystalline rutile and amorphous TiO2 were further investigated upon intense ion beam irradiation and thermal annealing.
2. EXPERIMENTAL DETAILS The nanocrystalline TiO2 thin films and an amorphous interfacial TiO2 layer on Si substrates were prepared by IBAD at the Nanotechnology Laboratory of the University of Nebraska Medical Center.32,33 The IBAD combines an electron beam Received: June 15, 2011 Revised: September 2, 2011 Published: October 10, 2011 22755
dx.doi.org/10.1021/jp2056283 | J. Phys. Chem. C 2011, 115, 22755–22760
The Journal of Physical Chemistry C
ARTICLE
Figure 1. (a) Cross-sectional bright-field TEM image of an as-deposited rutile thin film with an amorphous interface on a Si-substrate. (SAED pattern with tetragonal rutile shown in the inset.) (b) Dark-field and (c) high-resolution TEM images showing crystal size: ∼6 nm in the thin film.
evaporation system with an ion beam bombardment in a high vacuum environment with a base pressure of 10 8 Torr. The crystal size, thin film morphology and crystallinity were controlled by optimizing ion beam conditions (varying the ion-toatom arrival ratio) with the energy from 0 1500 eV, ion currents (0 500 μA/cm2), and well-controlled deposition temperatures. For nanocrystalline rutile, TiO2 was deposited onto the silicon substrate with an ion beam consisting of a mixture of argon and oxygen at a constant current density of 200 μA/cm2 and energy of 500 eV. The source material was 99.95% pure rutile TiO2 from Alfa Aesar. The deposition rates were 0.5 Å/s for amorphous region, with an argon-to-oxygen ratio of 1:1, and 2.5 Å/s for nanocrystalline region, with an argon-to-oxygen ratio of 1:5. During the deposition process, the vacuum pressure was at 1.8 10 4 Torr, and the temperature of the target reached ∼100 °C due to beam heating. The low-energy bombardment during IBAD was important for controlling the grain size, microstructure, crystallinity, and stoichiometry. Upon the synthesis of nanocrystalline rutile thin film, the microstructure, composition, and phase were characterized by TEM. The phase stability of the nanocrystalline rutile and the structural evolution were further investigated upon intensive ion beam irradiation and thermal annealing. The ion beam irradiation experiments using 1 MeV Kr2+ were performed on prethinned cross-sectional TEM foils prepared by mechanical polishing, followed by ion milling. IVEM-tandem facility at Argonne National Laboratory provides the feasibility of in situ TEM characterization of the phase stability and microstructure evolution upon ion bombardment as a function of ion fluence and temperature. The ion flux was set to be 6.25 1010 ions/ cm2. In situ TEM observations were performed when the samples were irradiated over the temperature range of 300 1073 K. Ex situ high-resolution TEM and scanning TEM elemental mapping were completed using JEOL 2010F.
3. RESULTS AND DISCUSSION 3.1. Microstructure of As-Deposited TiO2 Film. Figure 1a shows a cross-sectional TEM image of the as-deposited TiO2 thin film (∼400 nm thick) on a Si wafer. The surface of the TiO2 thin film is composed of condensed, nanocrystalline TiO2. The interfacial amorphous region (in the bilayer) was created by a high current intensity argon beam irradiation (i.e., with an ion-toatom arrival ratio five times greater than that used to produce nanocrystalline surface layer). Application of a high current for the ion beam generally enhances the adhesion of the crystalline thin film and substrate. The selected area electron diffraction (SAED) pattern (inset of Figure 1) shows that the polycrystalline
rings can be identified as the tetragonal rutile structure (P4/mnm, a0 = 0.459 nm, c0 = 0.296 nm). There are no other diffraction rings corresponding to anatase or brookite in the SAED pattern, suggesting that the nanocrystalline TiO2 is entirely rutile. A darkfield image (Figure 1b) and high-resolution TEM (HRTEM) image (Figure 1c) show that the average grain size is 6 ( 2 nm based on the measurement of 40 particles assuming a spherical geometry. The broadening of (110) diffraction maximum in the SAED pattern can be explained by strain induced during the deposition process. The thin film is continuous and dense, and no obvious porosity was observed based on the low magnification and HRTEM images. The structure and properties of the thin film are affected by the deposition conditions, such as the ratio of oxygen to titanium and ion beam currents. Previous studies showed that nanocrystalline anatase formed with ion bombardment at lower currents (
