In this work, ultrasonic irradiation and destabilizer solvent were used for destabilizing colloidal platinumdispersions. The stabilized platinum nanoparticles were prepared in w/o microemulsion systems com-posed of sodium bis-(2-ethylhexyl) sulfosuccinate (AOT) and four different solvents, namely, cyclohexane,n-hexane, n-heptane, and n-nonane. The recovery process of Pt nanoparticles from the colloidal systemswas performed by exposing the colloidal samples to ultrasonic irradiation and applying various desta-bilizing solvents. Analysis of UV–visible spectra confirms that the quantity of Pt nanoparticles removedfrom the suspension depends on the length of time of the ultrasonic irradiation and the nature of themicroemulsion oil phase. A critical time for the ultrasonic irradiation has been introduced for the phaseseparation of colloidal systems. To perform the solvent study, four destabilizer solvents, namely, diox-
ane, ethyl acetate, diethyl ether, and tetrahydrofuran, were used for breaking the colloidal suspension ofplatinum nanoparticles. Based on the ‘good solvent’ and ‘poor solvent’ idea, it is verified that the effect ofthe destabilizer solvents on the aggregation process follows the following order: tetrahydrofuran > ethylacetate > dioxane > diethyl ether.
Microemulsion systems present a flexible reaction mediumor the synthesis of uniform and size-controlled nanoparticlesNPs). Several examples of this micellar synthetic route for theroduction of metal and semiconductor nanoparticles have beenescribed in several review articles (Adair et al., 1998; Eastoe
Warne, 1996; Eastoe, Hollamby, & Hudson, 2006; Ganguli,anguli, & Vaidya, 2010). A significant limitation of this syn-
hetic method is the difficulty of isolating products from theighly stable colloids. Traditional separation techniques, such asltracentrifugation, solvent evaporation, addition of anti-solvents,emperature-induced phase separation (Chen & Wu, 2000; Dutta,akupca, Reddy, & Salvatl, 1995; Liu et al., 2003; Steigerwald et al.,988; Zhang et al., 2002, 2003) and several other specializedechniques, have been applied to recover nanoparticles from col-
Please cite this article in press as: Salabat, A., & Soleimani, S. Ultrasosuspensions of platinum nanoparticles. Particuology (2014), http://dx.
ow-energy methods of phase separation in colloidal dispersionsnd microemulsions in a review paper. In this review, polymer-
solvent-, and UV-induced approaches to phase separation haveeen explored. Winkelmann and Schuchmann (2011) described aethod to precipitate nanoparticles using a miniemulsion tech-
ique. In this technique, the stable miniemulsion was produced by high-pressure homogenization process. In our previous works,he effects of tailored solvent mixtures and a photo-destabilization
ethod on w/o microemulsions containing silica nanoparticlesSalabat, Eastoe, Mutch, & Tabor, 2008; Salabat, Eastoe, Vesperinas,abor, & Mutch, 2008) have been investigated in detail. Thesepproaches to phase separation may also find applications in cata-yst preparation by releasing specific nanoparticles onto a support.his method opens up the doors for the preparation of well-definedodel catalysts in a way that can be scaled up for the preparation
f real catalysts.Continuing our research into methods for the recovery of
anoparticles from colloidal systems, we introduce in this paperwo different routes to destabilize colloidal systems contain-
nic irradiation and solvent effects on destabilization of colloidaldoi.org/10.1016/j.partic.2014.02.002
ng platinum NPs. In the first method, we investigate the effectf ultrasonic irradiation on the recovery of platinum NPs fromicroemulsion systems composed of AOT as the surfactant and
yclohexane, n-hexane, n-heptane, or n-nonane as the solvent. In
and Institute of Process Engineering, Chinese Academy of Sciences.
he second route, the solvent was changed to affect the harvestingf Pt NPs from the colloidal systems. For this purpose, several novelestabilizer solvents, such as dioxane, ethyl acetate, and diethylther were added to a stabile microemulsion system that containedOT as the surfactant, n-hexane as the oil phase, and Pt NPs as
dispersed phase. Colloidal dispersions of Pt exhibit absorptionands in the UV-Vis region due to the resonant excitation of surfacelasmons (Ji, Chen, Wai, & Fulton, 1999). Therefore, UV–vis spec-roscopy was used in all of the experiments to follow the recoveryrocess of Pt NPs from the colloidal systems at 298 K.
. Experimental
.1. Materials
Hexachloroplatinic acid (H2PtCl6) (>99%), hydrazine hydrateN2H4·H2O), and various solvents, including cyclohexane, n-eptane, n-hexane, n-nonane, tetrahydrofuran (THF), dioxane,thyl acetate, and diethyl ether were obtained from Merck-chuchardt company. The purity of the solvents was more than9%. The anionic surfactant sodium bis-(2-ethylhexyl) sulfosucci-ate (AOT) with a purity of 96% was purchased from Acros Companynd used as received.
.2. Preparation of Pt colloidal suspensions
Colloidal suspensions of platinum nanoparticles were pre-ared in w/o microemulsions at a fixed w value of 5, where
= [H2O]/[AOT]. For the oil phase, four solvents were used: cyclo-exane, n-hexane, n-heptane, and n-nonane. Guidance on suitableicroemulsion compositions was obtained from the work of Nave,
astoe, Heenan, Steytler, and Grillo (2000). First, individual w/oicroemulsions, one containing a 0.01 M aqueous solution of2PtCl6 and another containing N2H4·H2O were prepared. All of therepared microemulsion systems were transparent and thermo-ynamically stable at 298 K. When each mixture was equilibratedfter 3 h of stirring, the hydrazine microemulsion was added drop-ise to the Pt complex solution under vigorous stirring, and the
ystem was left to mix for 4 h. The prepared colloidal systems weretable without precipitation for more than one month.
.3. Ultrasonic irradiation test
To test the effects of ultrasonic irradiation, stable samples com-rising nanoparticles dispersed in microemulsions were sonicatedsing a Pulse 570s POT Riscaldamento W.400 POT.U/S PICO, operat-
ng at a frequency of 38 kHz and a maximum power of 465 W. A vialontaining 10 mL of particle dispersion was placed in the reactionontainer. The temperature of the reaction container was kept con-tant at 298 K during the ultrasonic irradiation test. The progressf the destabilization process with time at a fixed ultrasonic poweras monitored by obtaining UV–vis spectra (Specord S600 JENAG Model).
. Results and discussion
.1. Destabilization of Pt colloidal suspensions by ultrasonicrradiation
The effect of ultrasound on dispersion stability was studied by
Please cite this article in press as: Salabat, A., & Soleimani, S. Ultrasosuspensions of platinum nanoparticles. Particuology (2014), http://dx.
xposing stabilized Pt NPs to ultrasonic irradiation. The progressf the Pt removal process was followed over time at a fixed ultra-ound power by obtaining UV–vis spectra after 15, 60, 120, 180,nd 240 min. It was observed that the concentration (or UV–vis
osti
ig. 1. Effect of ultrasonic irradiation time on the phase stability of Pt colloidalystems for various oil phases; tC = 60 min.
bsorbance) of the Pt colloidal suspensions remained constant after40 min of sonication. The results showed that for all solvents,he absorbance of the colloidal suspensions increased with timep to 60 min (Fig. 1), indicating an increase in the concentrationf Pt nanoparticles. The ultrasonic irradiation of a colloidal sys-em produces bubbles with unusual physical conditions (Leighton,994; Povey, 2013). These bubbles oscillate, grow in size, and moveapidly through the liquid with speeds of up to 1.6 m/s. Upon col-apse, they produce transient hot spots with temperatures peakingt several thousand degrees, capable of inducing many chemi-al transformations. Thus, the increase in particle concentration isationalized as originating from the enhanced mixing and reactionate offered by the high local shear field and hotspots created by theavitation bubbles that occur during ultrasonic irradiation. This haseen shown to increase the reaction rate for the synthesis of nano-aterials (Enterazi & Ghows, 2011). Afterward, the absorbance of
he systems decreased with time, also shown in Fig. 1. This suggestshat the particles start to aggregate and phase separate after thisime, most likely because the microemulsion undergoes a phaseransition. As microemulsions are inherently thermodynamicallytable, this would most likely occur due to growth of the createdubbles. This process is composed of the creation of a surface, evap-ration of gases to fill vacuoles, and the adsorption of impuritiessurfactant molecules) from the bulk phase (Cho, Kim, Chun, & Kim,005). It means that the surfactant molecules leave the stabilizedanoparticles and transfer to the bubble surface.
Based on this observation, a critical time (tC) for the phaseeparation of colloidal systems by ultrasonic irradiation can bentroduced. Before the critical time, the ultrasonic irradiation,
ith the frequency and power conditions stated above, causes anncrease in the UV–vis absorbance, interpreted as an increasingarticle concentration, and at t < tC, the colloidal systems are com-letely dispersed. Increasing the irradiation time past the criticalime (t > tC) induces aggregation of the nanoparticles, resulting in
acroscopic phase separation.It is interesting to note that the quantity of Pt NPs that aggregate
ith time during ultrasonic irradiation is also strongly dependent
nic irradiation and solvent effects on destabilization of colloidaldoi.org/10.1016/j.partic.2014.02.002
n the oil phase (solvent type) of the microemulsion system. Ashown in Fig. 2, when the oil phase is cyclohexane, the removal ofhe Pt NPs from the colloidal dispersion by ultrasonic irradiations the least effective (Fig. 2(a)). For other solvents, the ultrasonic
A. Salabat, S. Soleimani / Particuology xxx (2014) xxx–xxx 3
us oil
irodoaRiftva
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separate the remaining nanoparticles. To monitor the progress ofthe nanoparticle-removal process, the absorption peak of the col-loidal systems was determined, as shown in Fig. 3. In all of theexperiments, the volume of the destabilizing solvent was three
Fig. 2. Removal of Pt NPs with time under ultrasonic irradiation for vario
rradiation can destabilize the colloidal dispersions much moreapidly (Figs. 2(b)–(d)). This result can be understood in termsf the chemical nature of the solvents, and in particular of theiriffering gas solubilities. The most rapid and effective cavitationccurs for the solvents in which air is most soluble, as a greatermount of dissolved gas exists to form cavitation bubbles (Battino,ettich, & Tominaga, 1984). UV–visible absorption data show that
n all cases, 20 min of centrifugation with a speed of 1000 r/minor the irradiated systems can remove the remaining portion ofhe nanoparticles. It is observed that the centrifugation process isery effective for the system containing n-heptane. These resultsre presented in Table 1.
.2. Destabilization of Pt colloidal suspensions by changing theolvent
As a low-energy alternative to the ultrasonic collection of par-icles, using appropriate solvents was also explored. Specially, aange of various destabilizer solvents, dioxane, diethylether, ethylcetate, and THF, were applied to remove Pt NPs stabilized in
Please cite this article in press as: Salabat, A., & Soleimani, S. Ultrasosuspensions of platinum nanoparticles. Particuology (2014), http://dx.
/o microemulsion systems comprising water/AOT/cyclohexanet 298 K. To capture the precious nanoparticles from the colloidaluspension, the solvents were added dropwise under vigoroustirring for 24 h. Then, the colloidal systems were centrifuged to
FaA
phases: (a) cyclohexane, (b) n-heptane, (c) n-hexane, and (d) n-nonane.
nic irradiation and solvent effects on destabilization of colloidaldoi.org/10.1016/j.partic.2014.02.002
ig. 3. The effect of the destabilizing solvents dioxane, diethylether, ethyl acetate,nd THF on removing Pt NPs stabilized in a w/o microemulsion system containingOT/cyclohexane at 298 K.
imes the volume of the colloidal system. As seen from Fig. 3,he absorption peak decreased from 0.706 for the initial colloidaluspension to 0.595, 0.402, 0.335, and 0.219 after the addition ofiethyl ether, dioxane, ethyl acetate, and THF, respectively.
In our previous work, the concept of a ‘good’ solvent and a ‘poor’olvent for aggregation in microemulsions and silica colloidal sys-ems was introduced (Salabat et al., 2008a), similar to the conceptf a theta solvent and an anti-solvent for a polymeric system. Aood solvent destabilizes, and a poor solvent stabilizes, the col-oidal systems due to the differing aggregation characteristics ofhe surfactant in each. We posit that the phase separation of thet nanocolloid systems can also be explained by this concept. Theesults obtained show that phase separation is minimized with theddition of diethyl ether and maximized with the addition of THF.his suggests that diethyl ether is a somewhat poor solvent for AOTnd does not significantly disrupt aggregation of the surfactant,hereas THF is a good solvent for AOT, causing it to dissolve and
xist as a monomer rather than aggregate into micelles (Hollambyt al., 2008). In other words, the effect of the destabilizing solventsn the attractive interactions between Pt NPs and their separationan be arranged as follows: THF > ethyl acetate > dioxane > diethylther, where the control parameter is the aggregation state of AOTn the various solvent compositions.
. Conclusions
Two different methods, ultrasonic irradiation and appropriateolvents, for the destabilization of platinum colloidal suspensionsere experimentally evaluated. It was observed that the duration
f the ultrasonic irradiation is a critical factor for the stabiliza-ion/destabilization process. A critical time of ultrasonic irradiationtC) was introduced for the phase separation of colloidal systems.efore the critical time, the colloidal systems have a tendency toompletely disperse; after tC, aggregation of nanoparticles occurs,nd phase separation is dominant.
To investigate the destabilizer solvent method for the stabi-ization/destabilization process of the colloidal platinum system,he solvents dioxane, diethylether, ethyl acetate, and THF werepplied to water/cyclohexane microemulsions. It was verified thathe aggregation of the Pt NPs and then phase separation are
inimized with the addition of diethyl ether and maximizedith the addition of THF. Based on the ‘good solvent’ and ‘poor
olvent’ concept, the effect of the destabilizing solvents on theggregation process could be arranged as follows: THF > ethylcetate > dioxane > diethyl ether, where THF causes the greatestnstability and thus the fastest recovery of nanoparticles.
cknowledgment
Please cite this article in press as: Salabat, A., & Soleimani, S. Ultrasosuspensions of platinum nanoparticles. Particuology (2014), http://dx.
The authors would like to express their appreciation to the Arakniversity research fund for providing financial support for this
dair, J. H., Li, T., Kido, T., Havey, K., Moon, J., Mecholsky, J., et al. (1998). Recentdevelopments in the preparation and properties of nanometer-size sphericaland platelet-shaped particles and composite particles. Materials Science & Engi-neering, 23, 139–242.
attino, R., Rettich, T. R., & Tominaga, T. (1984). The solubility of nitrogen and air inliquids. Journal of Physical and Chemical Reference Data, 13, 567–600.
hen, D. H., & Wu, S. H. (2000). Synthesis of nickel nanoparticles in water-in-oilmicroemulsions. Chemistry of Materials, 12, 1354–1360.
ho, S. H., Kim, J. Y., Chun, J. H., & Kim, J. D. (2005). Ultrasonic formation of nanobub-bles and their zeta-potentials in aqueous electrolyte and surfactant solutions.Colloids and Surfaces A: Physicochemical and Engineering Aspects, 269, 28–34.
utta, P. K., Jakupca, M., Reddy, K. S. N., & Salvatl, L. (1995). Controlled growth ofmicroporous crystals nucleated in reverse micelles. Nature, 374, 44–46.
astoe, J., Hollamby, M., & Hudson, L. (2006). Recent advances in nanoparticle syn-thesis with reversed micelles. Advances in Colloid and Interface Science, 128,5–15.
astoe, J., & Warne, B. (1996). Nanoparticles and polymer synthesis in microemul-sions. Current Opinion in Colloid & Interface Science, 1, 800–805.
nterazi, M. H., & Ghows, N. (2011). Micro-emulsion under ultrasound facilitatesthe fast synthesis of quantum dots of CdS at low temperature. Ultrasonics Sono-chemistry, 18, 127–134.
anguli, A. K., Ganguli, A., & Vaidya, S. (2010). Microemulsion-based synthesis ofnanocrystalline materials. Chemical Society Reviews, 39, 474–485.
icheva, G., & Yordanov, G. (2013). Removal of citrate-coated silver nanoparticlesfrom aqueous dispersions by using activated carbon. Colloids and Surfaces A:Physicochemical and Engineering Aspects, 431, 51–59.
ollamby, M. J., Tabor, R., Much, K. J., Trickett, K., Eastoe, J., Heenan, R. K., et al.(2008). Effect of solvent quality on aggregate structures of common surfactants.Langmuir, 24, 12235–12240.
i, M., Chen, X., Wai, C. M., & Fulton, J. L. (1999). Synthesizing and dispersing silvernanoparticles in a water-in-supercritical carbon dioxide microemulsion. Journalof The American Chemical Society, 121, 2631–2632.
eighton, T. G. (1994). The acoustic bubble. London: Academic Press.iu, D., Zhang, J., Han, B., Chen, J., Li, Z., Shen, D., et al. (2003). Recovery of TiO2
nanoparticles synthesized in reverse micelles by antisolvent CO2. Colloids andSurfaces A: Physicochemical and Engineering Aspects, 227, 45–48.
yakonkaya, O., & Eastoe, J. (2009). Low energy methods of phase separation incolloid dispersions and microemulsions. Advances in Colloid and Interface Science,149, 39–46.
ave, S., Eastoe, J., Heenan, R. K., Steytler, D., & Grillo, I. (2000). What is so specialabout aerosol-OT? 2. Microemulsion systems. Langmuir, 16, 8741–8748.
ovey, M. J. W. (2013). Ultrasound particle sizing: A review. Particuology, 11,135–147.
alabat, A., Eastoe, J., Mutch, K. J., & Tabor, R. F. (2008). Tuning aggregation ofmicroemulsion droplets and silica nanoparticles using solvent mixtures. Journalof Colloid and Interface Science, 318, 244–251.
alabat, A., Eastoe, J., Vesperinas, A., Tabor, R. F., & Mutch, K. J. (2008). Photorecoveryof nanoparticles from an organic solvent. Langmuir, 24, 1829–1832.
teigerwald, M. L., Alivisatos, A. P., Gibson, J. M., Harris, T. D., Kortan, R., Muller, A.J., et al. (1988). Surface derivatization and isolation of semiconductors clustermolecules. Journal of The American Chemical Society, 110, 3046–3050.
zilagyi, I., Polomska, A., Citheriet, D., Sadeghpour, A., & Borkovrc, M. (2013). Charg-ing and aggregation of negatively charged colloid latex particles in the presenceof multivalent oligoamine cations. Colloids and Surfaces A: Physicochemical andEngineering Aspects, 392, 34–41.
zilagyi, I., Sadeghpour, A., & Borkovec, M. (2012). Destabilization of colloidalsuspensions by multivalent ions and polyelectrolytes: From screening to over-charging. Langmuir, 28, 6211–6215.
inkelmann, M., & Schuchmann, H. P. (2011). Precipitation of metal oxide nanopar-ticles using a miniemulsion technique. Particulogy, 9, 502–505.
hang, J., Han, B., Liu, J., Zhang, X., He, J., Liu, Z., et al. (2002). Recovery of silver
nic irradiation and solvent effects on destabilization of colloidaldoi.org/10.1016/j.partic.2014.02.002
nanoparticles synthesized in AOT/C12E4 mixed reverse micelles by antisolventCO2. Chemistry – A European Journal, 8, 3879–3883.
hang, R., Liu, J., Han, B., He, J., Liu, Z., & Zhang, J. (2003). Recovery of nanoparticlesfrom (EO)8(PO)50(EO)8/p-xylene/H2O microemulsions by tuning the tempera-ture. Langmuir, 19, 8611–8614.
