Synthesis microemulsion methods

Over the past few years, a large number of experimental approaches have been successfully used as routes to synthesize nanorods or nanowires based on titania, such as combining sol-gel processing with electrophoretic deposition,152 spin-on process,153 sol-gel template method,154-157 metalorganic chemical vapor deposition,158-159 anodic oxidative hydrolysis,160 sonochemical synthesis,161 inverse microemulsion method,162 molten salt-assisted and pyrolysis routes163 and hydrothermal synthesis.163-171 We will discuss more in detail the latter preparation, because the advantage of this technique is that nanorods can be obtained in relatively large amounts. 

Generally, two common methods, the Stober method and the reverse microemulsion method are used for synthesis of silica nanoparticles. As derivatives of a sol-gel process, both methods involve hydrolysis of a silicon alkoxide precursor to form a hydroxysilicate followed by polycondensation of the hydroxysilicate to form a silica nanoparticle [44]. 

In summary, a suitable association between dye molecules and the silica matrix is necessary for synthesis of DDSNs. Without the presence of chemical bonds or electronic interactions, the dye molecules will leak out from silica nanoparticles through the silica pores [22], Such DDSNs will provide unstable florescence signals and cannot be used as a labeling agent in bioanalysis. Meanwhile, water solubility is critical for a dye molecule when using a reverse microemulsion method to make the DDSNs. 

A great number of works on the aqueous synthesis of ceria NPs have been reported from very early years, since the ultrafine ceria powders are re-examined from fhe view point of nanotechnology for example, the simple precipitation methods (Zhang et al., 2002a), the microemulsion methods (Masui et al., 1997 Zarur and Ying, 2000), urea assisted hydro-thermal methods (Hirano and Inagaki, 2000 Hirano and Kato, 1999), hydrothermal methods with supercritical conditions (Adschiri et al., 2001). 

CH4 oxidation has been experienced for ceria supported on a barium hexaaluminate, an heat resistant support. Preparation by a new reverse microemulsion method leads to ceria nanoparticles deposited on support and having a BET area close to 100 mVg after calcination at 1000 0 [72]. Such ultrahigh disperse nanoparticles show exceptional thermal resistance the authors mentioned that ceria particles prepared with a size of 6 nm sinters only to 18 nm after a calcination at 1IOO°C under a water containing atmosphere. Of course excellent activity in methane combustion has been observed. According to their experimental conditions calculated specific activity expressed as mol(CH4).h. m was estimated to 6.4x10 at 500°C whereas Bozo [44J reported a value of 1.5x1 O at the same temperature both values look similar. Thus the difference in methane conversion may be related to BET area only which is spectacularly preserved using the reverse micro-emulsion method for synthesis. 

We will describe the current state-of-the-art of the microemulsion method for the preparation of metal-based catalysts. First, some general considerations concerning the nature of a microemulsion and its relation to the preparation of particles will be given. Then, both the preparation of solid oxides and metal-supported catalysts by microemulsion will be detailed. When possible, the properties of the solids prepared by microemulsion will be compared with those of their counterparts prepared by traditional techniques. Particular attention will be paid to the description of the catalytic properties of these solids. There is a large body of work in the field of organic synthesis, and enzyme catalysis in which microemulsion techniques play an important role. However this topic is not included in this paper, for that purpose several reviews are available, see for example those by Holmberg and Lawrence Rees... 

The preparation of sulphated zirconia designed for catalyst supports was studied by Boutonnet et al. . Zirconia prepared in microemulsion showed a pure tetragonal structure compared with zirconia prepared by an impregnation -precipitation procedure which also contained monoclinic phase. Platinum-promoted sulphated zirconia catalysts were prepared both in anionic and non-ionic microemulsions. Furthermore, the catalytic activity and selectivity for the isomerization of hexanes were tested. The catalysts produced by the microemulsion method showed a higher selectivity towards isomers but a lower activity when compared to catalysts prepared by impregnation technique. More recently, a study of zirconia synthesis from micro and macroemulsion systems has been conducted . Spherical ZrOa particles ranging from tens of nanometers to a few micrometers were produced. 

S. Rojas, S. Eriksson, and M. Boutonnet (KTH, Stockholm, Sweden) focus on the use of microemulsion techniques for catalyst synthesis. They discuss this as an alternative to traditional methods such as impregnation, ion exchange, and use of organometallic complexes. One specific advantage of the microemulsion method is that it results in a typically narrow particle size distribution. This is true because the metal particle is formed without being influenced by the support. They describe the specific processes used to prepare catalysts with this technique. 

In the late eighties, the new field of nano-technology, began to develop [284]. Considering the data presented in [284 - 287] and [41, pp. 227 - 265], the following main conclusions can be made. Nano-particles are small atomic clusters of 10 - 1000 atoms, which exhibit properties between the molecular and micro-heterogeneous size limits, and may contain crystalline, amorphous and quasi-crystalline phases. The microemulsion method is one of the novel techniques for the synthesis of nano-particles due to the ability of microemulsions to solubilise substances which are insoluble in both polar and apolar media [284]. 

Figure 3 Microemulsion-mediated synthesis protocols, (a) Microemulsion-plus-trigger method (b) two-microemulsion (or microemulsion-plus-microemulsion) method (c) microemulsion-plus-reactant method. (From Ref. 17.)...

M. Eslamian, M. Shekarriz Recent advances in nanoparticle synthesis by spray and microemulsion methods, Recent Patents in Nanotechnology, 3(2), 99-115 (2009). 

A two-microemulsion method was employed by Chhabra et al. [266] for synthesis of titania particles. A surfactant, Triton X-100 and a co-surfactant, n-hexanol were mixed with cyclohexane, the oil phase, to obtain reverse micelles. In one of these, an aqueous solution of titanium tetrachloride (0.3M) formed the microemulsion core in the other, an ammonia solution was added. The two microemulsions thus obtained were mixed under vigorous stirring at room temperature to start a precipitation reaction leading to the formation of hydroxide precursor particles. The particles were collected by centrifugation and washed with chloroform and methanol. The anatase particles obtained at 700°C had a size... 

Hingorani et al [276] developed a two-microemulsion method for the synthesis of zinc oxide. The surfactant used was cetyltrimethylammonium bromide (CTAB), along with 1-butanol as a co-surfactant the continuous phase was n-octane. The two microemulsions, one containing an aqueous solution of 0.1 M Zn(N03)2 and the other, an aqueous solution of 0.1 M (NH4)2C03 (both in the core of the W/O microemulsion), were separately prepared but finally mixed. Ammonium carbonate acted as the precipitating agent. The microemulsion composition was (wt%) surfactant 10, co-surfactant 10, oil phase 44 and aqueous phase 36. The uniform particles of zinc carbonate had a minimum size of 5 nm the corresponding ZnO particles had a size of 40 nm and a surface area of 26.4 mVg. 

Song and Kim [287] used a two-microemulsion method for the synthesis of Sn02 particles the combinations were ... 

A comparison between conventional precipitation and microemulsion-mediated synthesis of PbTi03 has been reported by Fang etaL [299]. The basic system in the two-microemulsion method was 24.0 wt% (NP-5+NP-9) and 56.0 wt% cyclohexane. The water phase (20.0 wt% in both cases) was either a solution of Pb(N03)2 and Ti0(N03)2 or an ammonia solution. Mixed under vigorous stirring, the... 

Mn-doped CdS nanoparticle formation (and fixation in xerogels) has been reported by Counio et ai [355]. As in several other cases, the two-microemulsion method was used for sulfide formation. The basic system was AOT/heptane and the two microemulsions contained aqueous solutions of (i) Cd(N03)2 + Mn(N03)2 and (ii) Na2S the concentrations (mol/1) were [Cd ] = 0.20, [Mn T = 0.12, [S ] = 0.38. Microemulsion (i) was added to (ii) of equal volume for synthesis and pyridine added to cap the particles and avoid agglomeration. The capped particles were washed with petroleum ether to remove AOT, dispersed in pyridine and the dispersion mixed with a sol obtained from CH3Si(OC2H5)3 for fabrication of bulk disks or thin films with entrapped CdS Mn (1-2 nm). 

In a recent work [417], the two-microemulsion method was used for synthesis. In microemulsion I, the continuous phase was prepared by mixing n-heptane with a surfactant or surfactant mixture from C12E4, C,2E5, i.e. penta(ethylene glycol)dodecyl ether, i.e. hexa(ethylene glycol)dodecyl ether or AOT. An... 

G. L. Li and G. H. Wang, Synthesis of nanometer-sized Ti02 particles by a microemulsion method,... 

Zhang F, Yang Q, Pan B, Xu R, Wang H and Ma G (2007), Proton conduction in Lao,9Sro,iGao,8Mgo,203 ceramic prepared via microemulsion method and its application in ammonia synthesis at atmospheric pressure , Materials Lett, 61, 4144 148. 

Fig. 11 Schematic representation of the reverse microemulsion method for the synthesis of dye doped silica nanoparticles...

Several methods are found in the literature for the synthesis of Ce02-based mixed oxides, including the coprecipitation method [7-10], the hydrothermal method [11], the microemulsion method [3,12], the glycothermal method [13,14], the citrate method [6], and the Pechini method [5]. 

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