CAVITATION DEPOSITION OF ZnS NANOPARTICLES INTO COMPOSITE WITH MONTMORILLONITE PARTICLES Richard DVORSKÝ, Petr PRAUS, Jana TROJKOVA, Soňa ŠTUDENTOVÁ, Jiří LUŇÁČEK VSB-Technical University of Ostrava, Ostrava, Czech Republic, richard.dvorsky@vsb.cz Abstract In this paper we report on ZnS nanoparticles produced by precipitation of zinc acetate and sodium sulphide and stabilized by cetyltrimethylammonium bromide (CTAB) in aqueous media and deposited on montmorillonite (MMT). For the deposition a new method based on intensive cavitation was used. The montmorillonite carrier was first disintegrated by intense turbulent mixing in an ultrasonic field into a fine aqueous nanodispersion and then vacuum-freeze dried at -24 C. The dry montmorillonite aggregates were then added into the stable pure aqueous nanodispersion of ZnS and CTAB. In the regime of intensive microcavitation flow in the ultrasonic field, the ZnS nanoparticles were deposited on montmorillonite particles to form the ZnS-CTA-MMT nanocomposites. Subsequently, the system was dried by vacuum freeze-drying at -24 C. Repeating the procedure several times, we can produce the stable ZnS-CTA-MMT nanocomposite powder with a significantly higher content of the ZnS nanoparticles. Keywords: ZnS nanoparticles, nanocomposite, montmorillonite, cavitation, deposition 1. INTRODUCTION In our previous experiments, the ZnS nanoparticles of size 3-5 nm were produced through the chemical reaction of sodium sulphide and zinc acetate in aqueous dispersions. They were stabilized by adsorption of CTAB on their surface [1-3]. A transparent dispersion with a slight blue coloration was originated. Their fabrication was mainly motivated by perspective use of their semiconducting properties in photocatalytic degradation of various substances. Due to the requirement of long-term stability, the dispersion of ZnS nanoparticles was then shaken with MMT particles for 24 h and thus they were deposited into larger nanoaggregates of montmorillonite ZnS-CTA-MMT. The resulting ZnS-CTA-MMT nanocomposite was filtered, washed a few times with deionized water and dried at 60 70 C. The obtained powder is also much more suitable for practical handling. In this paper we report on the alternative method of preparation of the ZnS-CTA-MMT microparticles by the cavitation deposition of the ZnS nanoparticles on the MMT carrier particles in aqueous dispersions. By applying this method we expect increase of the number of ZnS nanoparticles in the ZnS-CTA-MMT nanocomposite which would be useful for further applications, e.g. photocatylasis. 2. METHODS The method for precipitation of ZnS nanoparticles in the presence of CTAB was described in our earlier papers, e.g. [1,2]. The molar ration Zn 2+ :S 2- was kept at 1:1.5. Preparation of the ZnS-CTA-MMT nanocomposite was also described in [1]: 300 ml of the ZnS and CTAB dispersion was shaken with sodium montmorillonite (Wyoming) with the particles size <5 μm for 24 hours. The solid phase was centrifuged, filtered, washed with demineralized water and finally dried at 70 C. The alternative method investigated in this paper was based on using a cavitation implosion [4] which intensifies deposition of ZnS nanoparticles on the MMT surface, especially into the pores of MMT particles. The MMT particles were desintegrated by intensive turbulent stirring in an ultrasonic field into the fine aqueous microdispersion that was consequently dried by freezing at -24 C. The dried product was
desintegrated by vibration in a closed vessel and added to the aqueous nanodispersion of ZnS and CTAB. Then the dispersion was cavitated in the ultrasonic filed and the ZnS nanoparticles were deposited into the MMT particles as demonstrated in Fig. 1. Fig. 1 Scheme of cavitation implosion and penetration of ZnS nanoparticles into pores created among MMT crystallites forming ZnS-CTA-MMT aggregates. Then, the system was dried by freezing at -24 C. The resulting nanocomposite agglomerates contained the higher amount of ZnS nanoparticles. 3. EXPERIMENTAL RESULTS The precipitated ZnS nanoparticles were deposited on MMT particles forming the ZnS-CTA-MMT nanocomposite by the already used method based on shaking the mixed dispersion for 24 h [1] and by the new method of the cavitation deposition. These nanocomposites are shown in Fig. 2.
Fig. 2 The SEM micrographs of ZnS-CTA-MMT nanocomposite prepared by shaking method (left) and by cavitation deposition (Field Emission SEM - QUANTA 450 FEG (EDAX)) Unlike the shaking method, the nanomaterial prepared by the cavitation deposition (right) has obvious lamellar structure which is also shown in more detail in Fig. 3. Fig. 3 SEM micrograph of isolated nanocomposite particle with lamellar microstructure after cavitation deposition of ZnS nanoparticles on MMT (Large Field Detector - Field Emission SEM - QUANTA 450 FEG (EDAX)) For comparison of the deposition efficiency, the Energy-Dispersive X-ray spectroscopy microanalysis was performed during the SEM analysis as demonstrated in Fig. 2. Six surface layers with the dimensions of 100 x 100 x 5 μm were repeatedly analysed and the results indicated 1.5 times higher content of the ZnS nanoparticles deposited by the cavitation than in case of the deposition by shaking for 24 h. The analysis was performed with the uncertainty of 9.6 %.
Fig. 4 TEM micrographs of ZnS nanoparticles deposited on MMT by shaking (left) and cavitation (right). (TEM JEOL JEM-2100) The expected higher content of the ZnS nanoparticles in the cavitated nanomaterial was also confirmed by comparing TEM micrographs shown in Fig. 4. For intended next e.g. photocatalytic applications the high content of well accessible ZnS nanoparticles placed on high surface area is required. Therefore, their fixation on the maximal area of MMT pores is important. The specific surface area (SSA) and cumulative pore volume ( ) the ZnS-CTA-MMT nanocomposites prepared by both methods were measured. The nanocomposite prepared by the shaking method had SSA = 37.2 m 2 g -1, = 0.214 cm 3 g -1 and the cavitated one had SSA = 79.5 m 2 g -1, = 0.983 cm 3 g -1. These data indicate that the cavitation methods increases the specific surface area of about 53 %. The content of ZnS in ZnS-CTA-MMT obtained by the shaking method was about 7 wt. % [1]. 4. CONCLUSIONS Preliminary experiences with the cavitation deposition of the ZnS nanoparticles into the ZnS-CTA-MMT nanocomposite particles confirmed our basic presumptions resulting from the theoretical model. The measurements of material parameters confirmed expected increase of number of the deposited ZnS nanoparticles and also enlargement of the specific surface area of the nanocomposites prepared by the cavitation method. The nanocomposites will be consequently used for some photocatalytic reactions. AKNOWLEDGEMENTS This research was performed at VŠB-Technical University of Ostrava, sponsored by the Grant Agency of the Czech Republic under the project P107/11/1918 and the Regional Material Technology Research Centre (RMTVC) under the project CZ.1.05/2.1.00/01.0040. LITERATURE [1] Kozák, O., Praus, P., Kočí, K., Klementová, M.: Preparation and characterization of ZnS nanoparticles deposited on montmorillonite, Advanced Science, Journal of Colloid and Interface Science 352 (2010) 244 251 [2] Praus, P., Dvorský. R., Horínková, P., Pospíšil, M., Kovář, P.: Precipitation, stabilization and molecular modeling of ZnS nanoparticles in the presence of cetyltrimetylamonium bromide, Journal of Colloid and Interface Science, 377 (2012) 58 63 [3] Praus, P., Dvorský, R., Kozák, O.: Precipitation of ZnS nanoparticles and their deposition on montmorilonite, Advanced Science, Engineering and Medicine, 3 (2011) 113 118.
[4] Dvorský, R., Luňáček, J., Slíva, A.: Dynamics Analysis of Microparticles Cavitation Disintegration During Nanopowder Preparation in New Water Jet Mill (WJM), Advanced Powder Technology, Volume 22, Issue 5, September 2011, Pages 639-643