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J. Phys. Chem. C 2009, 113, 13470–13476
Experimental Investigation of the Oxidation of Tin Nanoparticles Pengxiang Song and Dongsheng Wen* School of Engineering and Materials Science, Queen Mary UniVersity of London, U.K., E1 4NS ReceiVed: March 22, 2009; ReVised Manuscript ReceiVed: June 17, 2009
Oxidation of tin nanoparticles is studied in this paper using a simultaneous TGA/DSC technique under both constant rates of heating and isothermal modes. A two-stage oxidation process is identified. XRD study shows that the only oxide product is SnO at a temperature below 400 °C, and SnO and SnO2 coexist at the temperature range between 400 and 900 °C. On the basis of the identified oxide constituent, the first-stage oxidation of tin nanoparticles is investigated by the model-free Kinssinger method. The activation energy is found to be dependent on the conversion ratio in a broad range of 0.32-1.33 eV, and the oxidation kinetics is identified to be the classical nucleation mechanism that can be modeled by the Avrami-Erofeev equation. The melting of tin and large pressure built up in a rigid oxide shell are believed to be responsible for the heterogeneous nucleation mechanism. 1. Introducation Metal tin and its oxides are the basis for many eutectic alloys and are widely used in the glass and semiconductor industry. For instance, as a p-type semiconductor, SnO is used as an anode material, coating substance, catalysis, and precursor for the production of SnO2,1 which itself is a popular wide-band gap semiconductor that has wide application in liquid crystal display (LCD), optoelectronic devices for solar energy conversion, and chemical sensors in biomedical fields.2 Due to the technological importance and the rapid development of nanoscience and nanotechnology, metallic tin nanoparticles and their oxides have received much attention recently. For instance, tin nanoparticles have recently been used in fabricating transparent antistatic films and antimicrobial, antifungal agents for medical bandages, and textiles when doped with silver, as well as being confined acoustic and optic phonons due to their unique electrical, biomedical, and bioscience properties.3,4 Nanostructured tin and tin oxides, such as nanowires, nanorods, and nanodiskettes, not only improve conventional properties related to their bulk counterparts but also promote other novel applications. Relevant development is advanced into synthesizing nano electronics and photonics materials such as microelectronic-mechanical systems (MEMS) and nanoelectronic-mechanical systems (NEMS). The knowledge about the control of preparation conditions of those attractive metallic and oxidized nanomaterials must be preconditioned with understanding of the oxidation mechanism. However, the oxidation process of tin particles in the range of micrometers to nanometers is not clear, especially on effect of the intermediate phases, which has been extensively investigated. Conventionally the oxidation of bulk tin materials is controlled by the diffusion of oxygen.5,6 In such a manner, the oxidizer, driven by the oxygen concentration gradient, diffuses toward the metal layer through preformed oxides, forming new oxide layers at the interface and proceeding steadily toward the interior of the bulk tin. At a temperature below 400 °C, the growth of tin layer can be described by the classical parabolic law. At elevated temperatures, different intermediates form and convert in between including SnO, Sn3O4, and SnO2, which have * To whom correspondence should be addressed. Phone: 0044-20 78823232. E-mail:
[email protected].
a great impact on the overall oxidation process. It is generally agreed that SnO, which forms at the initial stage of oxidation of Sn, is metastable and decomposes according to the disproportionation reaction even in the absence of oxygen at suitable temperatures:
2SnO ) SnO2 + Sn
(1)
At temperature higher than 1300 °C, gas phase of SnO emerges and the decomposition of SnO2 occurs.7 The disproportionation from SnO to SnO2 is dependent on many factors such as the deposition method, initial oxygen concentration, annealing temperature and humidity.8-10 For instance, the SnO-SnO2 transformation and completion are reported to occur at a wide temperature range, i.e., 250 °C < T < 600 °C, affected significantly by the annealing duration.2 It is therefore difficult to define a common transformation temperature for SnO-SnO2. In addition, the intermediate constitutions and consequent reaction kinetics for tin oxidation at bulk level are still debatable.11-15 Many different intermediates are reported including Sn3O4, Sn2O3, and SnOx suboxides.13-15 On the basis of these proposed intermediate constitutions, several contradictory mechanisms are proposed. The disproportionation of SnO is considered to be composed of two sequential first-order reactions,14 while an one-step Avrami-Erofeev reaction with an exponent of m ) 2 and activation energy, Ea ) 1.72 eV, is proposed by fitting the same process.13 Despite these disputes, the temperature range of this decomposition is roughly consistent, i.e., the onset beginning at 360 °C and SnO is fully converted at ∼550 or up to 600 °C.11,13,16 The oxidation of tin at the nanometer scale involves other distinct features such as the depression of melting temperature, variation of kinetics, and large pressure built up between the tin core and oxide shell.17-19 At the nanometer scale, a synergy of various factors may facilitate the diffusion-controlled oxidation mechanism by easing the ion diffusion path. In contrast, nucleation oxidation might be present, as the large surface to volume ratio of nanoparticles is beneficial to nuclei formation. Only very limited studies have been conducted on the oxidation of tin nanopartilces. Within these limited studies, the oxidation
10.1021/jp902580s CCC: $40.75 2009 American Chemical Society Published on Web 07/13/2009
Investigation of the Oxidation of Tin Nanoparticles
J. Phys. Chem. C, Vol. 113, No. 31, 2009 13471
Figure 1. TEM image of tin nanoparticles before oxidation.
of SnO nanoparticles of size range of 50-100 nm is found to proceed in a zero order process with an apparent activation energy of 0.7 eV.10 The oxidation of ordered Sn nanowires is proposed to be governed by a two-step process, i.e., an initial ion-diffusion-controlled step and a dominant kinetic-controlled step,19 which is consistent with the mechanism for RGTO-grown (rheotaxial growth and thermal oxidation) sub-micrometer tin particles and films.20,21 Compared to the bulk tin, the coexistence of SnO and SnO2 is observed in a narrow temperature range of 200-500 °C. Moreover, it is observed that an amorphous form of SnO2 is generated by oxidation at 225 °C, close to tin’s melting point (231.9 °C), and SnO2 is crystallized into a tetragonal form at a temperature of 350 °C, and then a dense orthorhombic SnO2 due to high pressure confined by oxide shells after an annealing of 20 h at 225 °C.18 However, none of these researches has established a quantitative description of the oxidation process, and the value of activation energy, the intermediates conversion, and phase transformation are seldom reported. The Sn-SnO oxidation at the micro-nanometer scale, which occurs at relatively low temperature, is neither experimentally investigated nor theoretically predicted. In this paper, a fundamental study of the oxidation of tin nanomaterials will be conducted with the focus of identifying the oxidation kinetics at low temperature and the formation and transformation of different intermediates. The experiments will be investigated by a simultaneous thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) method under the isothermal and isoconversion methods, with the morphology and crystal phases characterized by a scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD), as detailed below.
Figure 2. SEM image of tin nanoparticles after oxidation after annealing at 850 °C for 8 h.
2. Experimental Section
surrounding the particles, reflected by the image intensity in Figure 1. However, it is found from our investigation that the oxide layer is highly nonuniform by element concentration analysis on different locations obtained by an energy dispersive X-ray spectroscopy (EDS, Oxford Instruments, equipped with INCA Energy 300 System). EDS measurement on different areas shows that the oxide layer is highly nonuniform in terms of coverage and thickness. A large variation in oxygen concentrations is identified in different sample sites, i.e., from 1.4% to 10.6% by weight. The phase information of these initial oxide layers is further identified through a XRD study, which shows that they are composed of a mixture of SnO and SnO2. Such observation is different to other studies that considered SnO2 was the only oxide in the initial oxidation layer.21 Simultaneous TGA/DSC experiments are performed using a STA 1500 instrument under both isothermal and constant rate of heating conditions. Atmospheric air is supplied as the oxidizer. A small quantity of 10 mg of tin nanoparticles is used in the experiments, which is contained in an alumina crucible, to minimize the heat gradient inside the sample. Five series of experiments are performed at constant heating rates of 2, 5, 8, 15, and 20 °C/ min, with the cutoff temperature of 750 °C. Isothermal heating of tin nanoparticles are performed at 200, 300, 400, 500, 600, 700, 800, and 900 °C for 4 h. The samples are then cooled by inert gases to preserve the structure and phase information of tin nanoparticles for ex-situ XRD analysis (Siemens D5000). Only the data under constant rate of heating conditions are used for kinetic analysis due to the low weight gain for isothermal experiments.
Dried tin nanoparticles, which were made from the microemulsion technique and purchased from the Sigma-Aldrich company, are used in the experiments. The particles before and after oxidation are characterized under a TEM (JEOL JEM 2010) to determine their size and shape profile. The particles are spherical having a large particle size distribution, Figure 1. Large agglomerations are observed for oxidized particles after experiments as shown in Figure 2. The average particle size is 110 nm in diameter, which is consistent with the range, i.e.,
