Rapid synthesis of tin (IV) oxide nanoparticles by microwave induced thermohydrolysis

doi:10.1016/j.jssc.2008.02.040

Journal of Solid State Chemistry

Volume 181, Issue 6, June 2008, Pages 1439-1444

https://doi.org/10.1016/j.jssc.2008.02.040 Get rights and content

Abstract

Tin oxide nanopowders, with an average size of 5 nm, were prepared by microwave flash synthesis. Flash synthesis was performed in aqueous solutions of tin tetrachloride and hydrochloric acid using a microwave autoclave (RAMO system) specially designed by the authors. Energy dispersive X-ray analysis (EDX), X-ray powder diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area analysis, nitrogen adsorption isotherm analysis, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, and transmission electron microscopy (TEM), were used to characterize these nanoparticles. Compared with conventional synthesis, nanopowders can be produced in a short period (e.g. 60 s). In addition, high purity and high specific surface area are obtained. These characteristics are fundamental for gas sensing applications.

Graphical abstract

Tin oxide-based gas sensors have been extensively studied in recent years in order to understand and improve their sensing properties to a large variety of gaseous species. As is well known, high specific surface area increase the responses of gas sensors. Microwave-induced thermohydrolysis appears as an efficient way to produce nanoparticles in a very short time, with controlled size (4–5 nm) and high-specific area (160–190 m² g⁻¹). Pictogram represents our original microwave reactor, the RAMO (French acronym of Réacteur Autoclave Micro-Onde), containing the reactants and submitted to the microwave irradiation (multicolour candy represent obtained material), and a typical TEM image of the as-prepared SnO₂ nanoparticles.

Introduction

Tin(IV) oxide (SnO₂, rutile-type structure) is an n-type wide band gap (3.5 eV) semiconductor that presents a proper combination of chemical, electronic and optical properties that make it advantageous in several applications, such as catalysts [1], [2], gas sensors [3], [4], heat mirrors [5], varistors [6], [7], transparent electrodes for solar cells [8], glass melting electrodes and optoelectronic devices [9]. Especially, SnO₂ nanoparticles have been intensively studied for gas sensing applications not only because of their relatively low operating temperature, but also due to the fact that they can be used to detect both reducing and oxidizing gases.

Nanoparticles of tin dioxide have been synthesized by various synthesis methods such as sol–gel [10], microemulsion [11], spray pyrolysis [12], gel combustion technique (i.e., Pechini method) [13], decomposition of an organometallic precursor [14], hydrothermal synthesis [15]. Among these methods, a conventionally accepted method is the synthesis from precursor hydroxides precipitated by the direct addition of bases (as NH₄OH) to tin chloride aqueous solutions (SnCl₄). Although this technique rapidly yields a large amount of powder but thermal annealing of powder is necessary to obtain high crystallinity. Consequently, surface area significantly decreases in relation to magnitude of thermal treatment (temperature and duration). Generally, powders with high surface area allow strong increase of gas sensors sensitivity.

Microwave-assisted synthesis is an emerging technology using the ability of microwave heating to accelerate chemical reactions. Some liquids and solids are able to transform electromagnetic energy into heating. Since the mid-90s more than 90 publications relating to inorganic synthesis have been published. Most of the reports of microwave-induced chemistry describe the use of commercial laboratory systems deriving from household microwaves. Numerous information about microwave–material interactions, dielectric properties, key ingredients for mastery of chemical microwave process and laboratory or pilot scale reactors should be found in [16] and [17] respectively.

Among various other inorganic compounds, SnO₂ powder has already been synthesized using microwave heating by Cirera et al. [18] and by Michel et al. [19]. These synthesis have been carried out by means of the RAMO system (French acronym of Reacteur Autoclave MicroOnde), which is an original microwave device designed by our research team [17]. Our microwave oven allows high electric field strength within heated samples compared to those of domestic oven or commercial device (CEM or Millestone). The heating system consists of a microwave generator, a waveguide and a resonant cavity loaded with the RAMO system. The microwave generator used is a continuous wave system (2.45 GHz) with microwave power up to 2 kW. The autoclave is made with polymer materials which are microwave transparent, chemically inert and sufficiently strong to accommodate the pressure induced. A fiber-optic thermometry system, a pressure transducer and a manometer allow to measure simultaneously the temperature and the pressure within the reactor. Temperature measurements under microwave heating are very difficult and non-perturbing temperature sensor could be used. The system is controlled by pressure. The microwave power is adjusted in order to allow constant pressure within the vessel. A pressure release valve incorporated permits to use this experimental device routinely and safely. This experimental device is able to raise the temperature from ambient to 200 °C in less than 20 s (the pressure is close to 1.2 MPa and the heating rate is close to 5 °C s⁻¹). This device is able to produce rapid bulk heating. Due to strong thermal gradients induced by microwave heating, strong stirring occurs for liquids leading to thermal uniformity of heated medium. Hence, it combines advantages of forced hydrolysis (homogeneous precipitation) and very fast heating rate. Our device system has allowed production of various nanomaterials such as iron oxides [20], zirconia [21], [22], titanium oxide [23], tin oxide [19], nanocomposites [24], [25], manganese oxide [26] and more recently nickel ferrite [27]. The RAMO system associates advantages of solution nucleation processes with core heating and heating rate induced by microwave heating.

Contrary to our previous work restricted to colloidal tin oxide [28], the aim of the present work was to prepare SnO₂ nanocrystalline powder with high surface area by microwave-induced thermohydrolysis. Conventional operating conditions are generally based on salts aqueous solutions using hydroxide precipitation induced by pH increase. The aim of this work is the study of microwave induced thermohydrolysis within acidic solutions (hydrochloric acid). Obviously, the products will be compared with those prepared with the others microwave conditions [24], [26], [29], [30]. The various nanopowders produced were characterized by energy dispersive X-ray analysis spectrometer (EDXS), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area analysis, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy and transmission electron microscopy (TEM).

Our simplest method for the generation of tin (IV) oxide nanoparticules is based on microwave thermohydrolysis of metal salt solutions [17], [21], [22], [23], [24]. It is well known that most polyvalent cations as tin (IV) readily hydrolyze, and that deprotonation of coordinated water molecules is greatly accelerated with increasing temperature. Since hydrolysis products are intermediates to precipitation of metal oxides, these species can be generated at the proper rate to eventually yield nanoparticles by the adjustment of heating rate, temperature and pH. The mechanism of hydroxylation or olation reaction involves a scheme given by $[Sn ({OH}_{2})_{6}]^{4 +} + h H_{2} O \to [Sn (OH)_{h} ({OH}_{2})_{6 - h}]^{(4 - h) +} + h H_{3} O^{+}$ whereas the oxolation reaction involves a scheme given by $n [Sn (OH)_{h} ({OH}_{2})_{6 - h}]^{(4 - h) +} + q H_{2} O \to [{Sn}_{n} (OH)_{nh + q} ({OH}_{2})_{n (6 - h) - q}]^{(n 4 - nh - q) +} + q H_{3} O^{+}$

Obviously, these complexes act as precursors to nucleation and they affect the particle growth. The composition and the rate of generation of these species, controlled by heating rate, will determine the chemical and physical natures of the resulting precipitate. The authors have defined operating conditions able to facilitate concomitant olation and oxolation reactions for tin (IV) precursor. Hence, the general balance of the reactional scheme is given by ${SnCl}_{4} + 6 H_{2} O \to {SnO}_{2} + 4 H_{2} O + 4 HCl$

According to this reaction scheme, side product is hydrochloric acid leading to more acidic solutions after microwave treatment.

Section snippets

Preparation

The reaction media were constituted by tin tetrachloride aqueous solutions with hydrochloric acid. Tin (IV) chloride (99.995%, Aldrich) and hydrochloric acid (Prolabo, RP Normapur) were used without further purification. Addition of tin (IV) chloride has been done under vigorous stirring, in previously acidified water. The hydrochloric acid concentration is fixed at 0.1 mol L⁻¹ and the tin salt concentrations are 0.1 and 0.08 mol L⁻¹. Table 1 summarizes samples data (name, concentration of tin

Structural analysis

The structure and purity of the nanoparticles were firstly determined by the XRD. Tin oxide has cassiterite structure with tetragonal framework lattice (rutile type structure) with a=4.7382(4) Å, c=3.1871(1) Å and the space group P42/mnm. Sn⁴⁺ and O²⁻ ions are in 2a and 4f positions, respectively. Fig. 3 shows the XRD patterns of samples before (raw powder) and after annealing treatment under different temperatures. The XRD pattern of raw powders (sample A) showed the presence of very broad peaks

Conclusion

SnO₂ nanopowders were synthesized by microwave flash synthesis performed in tin tetrachloride aqueous acidic solutions. Raw powders obtained have specific surface area from 166 to 191 m² g⁻¹ with average particle sizes close to 5 nm, whatever the holding time of synthesis used in this work. Crystallites sizes increased in relation to annealing temperature 6, 9, 21 and 33 nm for 200, 400, 600 and 800 °C, respectively. Moreover, average size is slightly small. These sizes, estimated by XRD, are

Acknowledgments

The authors wish to acknowledge the Regional Council of Burgundy for his financial support. They are also grateful to Farouk Azzaz, Remy Chassagnon and Marie-Laure Leonard for their help in BET, MET/EDX and TGA/DSC experiments respectively, from the Institut Carnot Bourgogne.

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Rapid synthesis of tin (IV) oxide nanoparticles by microwave induced thermohydrolysis

Abstract

Graphical abstract

Introduction

Section snippets

Preparation

Structural analysis

Conclusion

Acknowledgments

Mater. Lett.

Sensors Actuators B

Sensors Actuators B

Sensors Actuators B

Thin Solid Films

J. Eur. Ceram. Soc.

Mater. Chem. Phys.

Electrochem. Acta

J. Phys. Chem. Solids

Mater. Lett.

J. Colloid Interf. Sci.

Mater. Lett.

Appl. Surf. Sci.

Inorg. Chem. Commun.

Sensors Actuators B

Mater. Res. Bull.

J. Solid State Chem.

J. Solid State Chem.