Synthesis of SnO2 nanoparticles through the controlled precipitation route
Introduction
The synthesis of nanoparticles has become a highly developed field owing to the scientific and technological interest due to the structural peculiarities and unusual physical and chemical properties they may lead to [1], [2]. Nano-scaled particles are of great importance if the conformation of ceramics is considered; they have been found to enhance the mechanical, electrical, thermal, catalytic and optical properties of diverse ceramic materials [3], [4]. The specific advantages of these nanocrystallised materials lie in the high homogeneity attained and in the possibility of sintering at lower temperatures. The crystalline structure, the size and shape of the particles and the superficial characteristics are highly dependent on the followed route of synthesis. Then, it is necessary to adjust the synthesis methods of nanoparticles to assure the high quality of the ceramic powder; properties related to size uniformity, particle morphology, purity and chemical homogeneity are critical during the processing and sintering steps of the compacted powder and determine the microstructure of the ceramic and the final properties of the resulting piece. A brief description of the synthesis route is no longer satisfactory; there exists an urge to know the basic physicochemical processes that occur during the formation of the nanoparticles. This would provide an aid in establishing the mechanisms of particle formation and would also be of importance in controlling the final properties of the ceramic powder obtained [5], [6].
Tin dioxide (SnO2), with a rutile-type crystalline 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 [7]. Due to its physical properties, such as transparency and semiconductivity, it is an oxide of great interest from the technological point of view for gas sensors [8], white pigments for conducting coatings, transparent conducting coatings for furnaces and electrodes [9], opto-conducting coatings for solar cells [10], catalysts [11], [12] and surge arrestors (varistors) [13], [14]; an increasing interest in the use of anodes of SnO2 in lithium batteries has been recently noticed [15].
Nanoparticles of tin dioxide have been synthesized through different chemical routes, such as precipitation [15], [16], hydrothermal [8], [17], sol–gel [18], [19], hydrolytic [20], carbothermal reduction [21] and polymeric precursor [22] methods among others. Even though the development of agglomerates is to be avoided, their growing is somehow inevitable due to the small diameter of the oxide particles and to the presence of the compounds involved in the mentioned procedures, mainly solvents. On the other hand, the most used tin precursors are tin chlorides (SnCl2 or SnCl4) provided their low cost and their easy handling characteristics; however, the chlorine ion is difficult to remove from the system and it seriously alters the superficial and electrical properties of the material modifying, for instance, the sensibility of the gas sensor [23], favouring the agglomeration of particles [24] and leading to higher sintering temperatures [25]. The chloride problem can be avoided through the usage of organic tin compounds, such as alkoxides. However, these reagents are rather costly which makes the industrial synthesis implementation hardly attainable.
In this work, the synthesis of tin dioxide nanoparticles through the controlled precipitation method is described. On the basis of potentiometric and conductimetric titrations, the reaction of the system to the precipitant (ammonium hydroxide) addition and the evolution of the process in different stages are discussed. The main physicochemical phenomena that take place are addressed and a detailed description of the critical steps to eliminate chloride ions and to reduce the nanoparticle agglomeration is also given in this paper.
Section snippets
Pontentiometric titration
Potentiometric titration curves of aqueous dissolutions of SnCl2·2H2O (Mallinckrodt) 0.01, 0.1 and 0.3 M were obtained through the addition of 0.5 ml every 15 s of a 28 wt% solution of ammonium hydroxide (NH4OH, Merck) with a Metrohm Dosimat 685 dosimeter. The pH evolution was recorded with a Metrohm 744 pH-meter working with a glass electrode previously calibrated with buffer solutions of pH 4 and 7. Distilled and deionised water was used for the SnCl2·2H2O dissolution, which was kept in
Titration curves: tin hydrolysis and condensation reactions
Potentiometric and conductimetric titration curves of the 0.3 M SnCl2 dissolution are shown in Fig. 1. Four regions can be clearly distinguished in the potentiometric titration curve; two steps (pH AB and CD) and two plateaus (pH BC and DE).
During the dissolution of the tin(II) chloride, before the addition of the ammonium hydroxide solution, the pH drops below 2 indicating that the concentration of H+ species is increasing. This could be originated by the difficult dissociation of SnCl2 in the
Conclusions
Through the controlled precipitation method the synthesis of SnO2 with defined properties, such as particle size and shape and with a high reproducibility was achieved. The optimum pH value that yielded the desired product, cassiterite, as the major phase was determined to be 6.25 on the basis of XRD analyses.
The tin complexes and compounds that aroused in the system were described in detail through hydrolysis and condensation reactions. At low pH values, the formation of basic chlorides or
Acknowledgements
The authors express their thanks to the Project VIII.13 PROALERTA-CyTED, to Program CIAM, to CONICET (Argentina) and to COLCIENCIAS No. 1103-14-17900 (Colombia) for the financial support. Thanks are also given to Patricia Mosquera from the Laboratory of Microscopy of the UNICAUCA, Popayán.
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