Biomimetic synthesis of tin dioxide nanoparticles by emulsion liquid membrane

Liu Lu, Wu Qingsheng, Ding Yaping#, Liu Huajie
(Department of Chemistry, Tongji University, Shanghai, 200092;   #Department of Chemistry, Shanghai University, Shanghai 200436, China)


Abstract A novel biomimetic emulsion liquid membrane system with carrier has been developed to the synthesis of SnO2 nanoparticles. This membrane system consists of kerosene as solvent, L152 as surfactant, TBP as carrier, SnCl4 solution as external aqueous phase and NH3·H2O-NH4Cl (pH=10) buffer solution as internal aqueous phase. The precursor Sn(OH)4 nanoparticles with diameter of 3-4 nm are initially synthesized through ion-transport between this membrane. The final product SnO2 nanoparticles with diameter of 2-3 nm are prepared after treating precursor at 400°C for 6h. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) are used to characterize the product. Ultraviolet-visible (UV-Vis) spectrum shows a blueshift of 20 nm while comparing with that of bulk materials, which can be attributed to the quantum size effect of nanomaterials.

    Tin dioxide is one of the most important metal oxide semiconductor materials, and is extensively applied to ceramic, gas sensor and catalyst [1-3], etc. Especially, scientists are interested in SnO2 nanomaterials for their evident surface effect and pay much attention to their synthesis methods[4-6]. There are several methods to be used for obtaining SnO2 nanomaterials such as surfactant-mediated method[7,8], Sol-Gel[9], chemical precipitation method[10], hydrolytic method[11,12], hydrothermal method[13-14], etc. However, in order to avoid agglomeration and obtain smaller SnO2 nanoparticles in size, many chemists and material chemists have made efforts to research on more efficient synthesis methods.
    It is well known that cell has the functions of ATP-driven active transport[15] and forming nanosize biomineralization products using collagen as template[16]. For example, many biomaterials like tooth and bone consist of nanocrystals assembled through bio-macromolecule as template[17]. So it is feasible that nanomaterials can be synthesized by simulating the bio-process. In this paper, a novel biomimetic emulsion liquid membrane system with carrier is developed to the synthesis of SnO2 nanoparticles.
    The water-in-oil (w/o) emulsion was prepared in emulsification step by initially dissolving the surfactant dialkylene succinimide (L152) and tributyl phosphate(TBP) in the kerosene and then adding NH3·H2O-NH4Cl (pH=10) buffer solution, The emulsification was carried out for 10-15 minutes when the agitator was run at the rate of 3000 rpm. The volume ratio of membrane phase to internal aqueous phase (Roi) was 1.0.
    According to certain Rew (volume ratio of external aqueous phase to w/o emulsion), 25ml w/o emulsion prepared above was added to 50ml external aqueous phase (0.1M SnCl4 solution). Then the w/o/w emulsion was agitated at 500rpm for 10min. The w/o emulsion and SnCl4 external aqueous phase solution were separated by funnel, w/o emulsion was centrifugated, and the precipitate was washed with petroleum ether and alcohol for 3-4 times. The precipitate was treated at 400°C for 6 hours, then the sample was immersed into alcohol.
    Figure 1(a) shows the morphology and structure of SnO2 sample (Philips EM400ST transmission electron microscopy, accelerating voltage of 100 kV), TEM image reveals both smooth solid particles structure with diameters of 2-3nm. TEM image of precursor Sn(OH)4 shows in Figure 1(c) and reveals both smooth and amorphous solid particles structure with diameters of 3-4nm. On the other hand, the SnO2 particle size was not changed while increasing the temperature of precursor treatment, Figure 1(b) reveals TEM image of SnO2 sample with diameter of 2-3nm which comes from precursor treated at 300°C.

Figure 1 Morphology of SnO2 sample and Sn(OH)4 precursor
(a) SnO2 products at 400°C; (b) SnO2 products at 300°C; (c) Sn(OH)4 precursor

A XRD pattern of the as-prepared SnO2 sample is shown in Figure 2 (XRD, Philips Pw1700 X-ray diffractometer, employing Cu-Kα radiation, λ=1.54056Å), all peaks in the pattern can be identified to the known cubic structure of SnO2 (JCPDS 21-340). The measured lattice parameter is a=6.068Å, consistent with the reported value (a=6.077Å).

Figure 2 XRD pattern of SnO2 nanoparticles

In the synthesis process, the choice of reagent in internal-aqueous phase is important to obtain pure SnO2, alkali with Sn4+ form precursor Sn(OH)4, however, only using base NH3·H2O as reagent in internal-aqueous phase does not bring about impurity(salt) after the precursor is treated at high temperature. When NH3·H2O concentration is too high, the emulsion liquid membrane system in SnCl4 external-aqueous phase solution(pH<1) unsteadily exist, on the other hand the forming precursor is influenced if NH3·H2O concentration is low, so NH3·H2O-NH4Cl buffer solution (pH10) is chosen as reagent in internal-aqueous phase which have pH value steady and small size precursor formed. At the same time, formed NH4Cl is decomposed in the form of NH3 and HCl which evaporate, so pure SnO2 nanoproducts are obtained. Tributyl phosphate is the carrier in the system and transportation process is shown in Figure 3.

Figure 3
Transportation process of SnCl4

Precursor Sn(OH)4 could be formed as follows: Combine Sn4+ in external-aqueous phase with TBP in membrane phase to form associating compounds on interface between external-aqueous phase and membrane phase. According to concentration gradient in membrane phase, diffusion of concentration happens. Then these associating compounds arrive at interface between membrane phase and internal-aqueous phase and react with OH in internal-aqueous phase to form Sn(OH)4 nucleus. Because there are carriers(TBP concentration 10%), transportation speed is speeded up; and because Ksp[Sn(OH)4] is small(10-56), congregate precipitate rate is far larger than that of orientation. So a great many of Sn(OH)4 nucleus are formed and rapidly precipitated in the form of amorphous Sn(OH)4 nanoparticles.
Figure 4 shows the absorption peak of ultraviolet-visible absorption, the graph reveals there is an absorption peak in 324 nm to the SnO2 nanomaterials. It is well known that band gap energy of SnO2 bulk materials is 3.6eV, the most peak of SnO2 nanoparticles(λ=324nm) has a large deviation(20 nm), which is compared with the most peak of SnO2 (λ=344nm). The reason is that as the particle size decreases, the blueshift occurs from the bulk band gap of 3.6eV (344nm) due to the well-known quantum size effect.