No4

3.4

Inorganic and inorganic compounds nanomaterials

Cadmium selenide (CdSe), an important II–VI semiconductor material, has been one of the most extensively studied semiconductor systems in nanoscale form over the last 20 years due to the specific photoelectric properties and potential use in optoelectronics, luminescent materials, lasing materials, and biomedical imaging [98].

Scheme 9 Mechanism for the pH-responsive self-assembly of MCP nanoparticles [97].

Scheme 10 Mechanism for the temperature-responsive self-assembly of MCP nanoparticles [97].

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In ref. [99], CdSe nanostructures with urchin-like shape were successfully synthesized in two types of microemulsion solutions (water content w=13.5), which were prepared by adding equivalent volume of Na2SeSO3 or Cd[(NH3)4]2+ solution drop by drop to an n-octane/cetyl trimethyl ammonium bromide (CTAB)/1-butanol system. The X-ray pow- der diffraction pattern of the product showed that it was pure CdSe in zinc blende structure rather than thermody- namically favored wurtzite structure. It was found that nu- merous onedimensional CdSe nanorods radiated from the center of the agglomerate to form urchin-like nanostructures and grow along the (111) crystal planes. Figure 33 showed the TEM images of the CdSe nanoparticle. The photolumi- nescence spectrum of the urchin-like nanostructures indi- cated that there was a blue-shift as compared with that of the bulk CdSe. Additionally, these interesting urchin-like nanostructures showed an increased specific surface area. This study provides a simple method to prepare urchin-like CdSe nanostructures on large scale, which may broad their practical applications. Hollow spheres have received considerable attention recently because of diverse applications, including catalysis [100], drug delivery [101], electric magnetic absorption [102], chemical sensors[103], structural materials [104]. As

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an important semiconductor material, CuS has attracted much attention due to not only its excellent electrical, opti- cal properties but also its potential applications in optical recording material [105], sensors [106], solar cell [107], and catalysts [108] and so on. CuS hollow spheres have been synthesized using microemulsion templates. In ref. [109], CuS hollow spheres have been successfully synthesized through a facile microemulsion-template-interfacial-reaction route using copper naphthenate as metal precursor and thioacetamide as the source of S2. In this way, hollow spheres could be obtained directly since the reaction of two reactants respectively dissolved in two different phases of an O/W microemulsion only occurs at the oil/ water interface. Therefore, it is a key for forming hol- low spheres to optimize the interfacial reaction rate by controlling reaction conditions. Furthermore, the size of the hollow spheres can be tailored by changing the content of oil phase. In this study, the average diameter of the CuS hollow spheres can be adjusted from 110 to 280 nm by changing the content of oil phase from 0.5 mL to 1.5 mL. In addition, the reaction temperature is a very important factor for forming CuS hollow spheres and the appropriate reac- tion temperature is about 50°C. Figure 34 showed the TEM images of the samples prepared at 50°C. A schematic illustration of the formation mechanism of the CuS is shown in Scheme 11.

Figure 32 TEM images of (a) magnetic chitosan oligosaccharide pluronic (MCP) nanoparticles and (b) chitosan-coated magnetic nanoparticles [97]. Figure 34 TEM images of the samples prepared at 50°C with different oil phase contents: (a) 0.5 mL, and (b) 1.5 mL [109].

Figure 33 (a) Low magnification FE-SEM image of the product prepared at 120°C for 12 h (w=13.5); (b) FE-SEM image of an individual urchin-like CdSe nanostructure; (c) FE-SEM image of a spherical structure; and (d) EDS spectrum of the spherical structure [99].

Scheme 11 Schematic illustration of the formation of the CuS hollow spheres [109].

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Semiconductor nanocrystals are of great interests for both fundamental research and industrial development. This is due to their unique size-dependent optical and electronic properties and their exciting utilization in the field of lightemitting diode [110], electrochemical cells, laser [111], hydrogen producing catalyst [112], biological label [113]. They are usually prepared in the liquid reaction system at high temperatures for several hours. These methods are ei- ther expensive, explosive, moisture sensitive or extreme toxic, energy consuming. In most cases, the particles are formed by arresting precipitation of reaction precursors of the reverse W/Omicroemulsions under certain conditions [114]. It is worth mentioning that microemulsions and ul- trasound irradiation are two synthetic methods for na- nosized sulfides that have been studied intensively and have aroused more and more attention [115]. Ghows et al. [116] reported that cadmium sulfide nanoparticles with a hexagonal phase (~10 nm) were prepared at a relatively low temperature (70°C). This synthesis was carried out shortly (30 min) through a new microemulsion (O/W) induced by ultrasound without surfactant. Ultrasound can provide an excess energy for new interface formation and obtain emulsions even in the absence of surfactants. This technique avoids some problems that normally exist in conventional microemulsion synthesis such as the presence of different additives and calcinations. In addition, it was possible to tune the particle size, the band gap, and the phases of CdS nanoparticles by changing the vari- ables such as ultrasonic irradiation time, intensity, precursor, and ratio of the components. It was also found that the synthesized nanoparticles had a bandedge emission at about 460 nm with a blue-shift to a higher energy which is due to the typical quantum confinement effects. Figure 35 showed

the TEM, HRTEM image, and SAED of CdS nanoparticles. The results confirmed that both ethylenediamine and ultrasound played important roles in the formation of the final nanostructure. Scheme 12 illustrated the proposed model for the formation of CdS nanoparticle. Middle-phase microemulsions (MPMs) are multi-phase equilibrium systems comprising the microemulsions and residual water or residual oil, i.e., a lower-phase oil in water (O/W, Winsor I) microemulsion with excess oil upper-phase or an upper-phase water in oil (W/O, Winsor II) microemulsion with excess water lower-phase or MPMs with bicontinuous water and oil are called BC (Winsor III) microemulsion. In ref. [118], middle-phase microemulsions (MPMs) in two systems of a cationic surfactant, tetradecyltrimethylammonium bromide (TTABr)/n-butanol/isooctane/ Na2CO3 or CaCl2 and an anionic surfactant, sodium dodecyl sulfate (SDS)/n-butanol/ iso-octane/Na2CO3 or CaCl2, were used to synthesize nanostructured calcium carbonates. MPMs provide a simple and versatile reaction medium, i.e., up- perphase W/O, BC, and O/W structured equilibrium microemulsions to be used for synthesizing hierarchically structured CaCO3 on the nanometer scale. On the basis of the investigations on the phase behavior of the MPMs, the hierarchically structured calcium carbonates with dendrites, ellipsoids, square-schistose cubes, and spheres were synthesized through the MPM-based routes (Figure 36). This work was to develop a method to synthesize nanometer-scale inorganic particles, open alternative pathways to synthesize complex superstructures of inorganic materials, and construct the correlation between the morphologies of CaCO3 and the structures of microreactor media. Particles with well-defined pore morphology are essential for many areas of modern technology. Potential applications include catalysis [119] and electrocatalysis [120], chromatography [121], and drug delivery [122]. Precise control over the pore size and shape is crucial for the

Figure 35 (a) TEM, (b) HRTEM image, and (c) SAED of CdS nanoparticles [117].

Figure 36 SEM images of CaCO3 synthesized from the upper-phase equilibrium W/O microemulsions of the TTABr system [118].

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Scheme 12 Proposed model for the formation of CdS nanoparticle [117].

successful performance of the particles. It allows for optimization of fluid transport in a catalyst, determines the molecular release of solute by a drug delivery vehicle, or defines the size selectivity in chromatography. Ref. [123] showed that hierarchically bimodal porous structures could be obtained by templating silica microparticles with a specially designed surfactant micelle/microemulsion mixture. Tuning the phase state by adjusting the surfactant composition and concentration allows for the controlled design of a system where microemulsion droplets coexist with smaller surfactant micellar structures. The microemulsion droplet and micellar dimensions determine the two types of pore sizes. They also demonstrate the fabrication of carbon and carbon/platinum replicas of the silica microspheres using a “lost-wax” approach. Such particles have great potential for the design of electrocatalysts for fuel cells, chromatography separations, and other applications.

Scheme 13 showed the sketch of liquid silica precursor emulsion system. Figure 37 showed the TEM of the particles. Zhang et al. [124] investigated the effect of ultrasound on

Scheme 13 Sketch of liquid silica precursor emulsion system. (a) Aqueous silica precursor emulsion drops (light gray) in hexadecane oil (dark gray). Microemulsion droplets form and occupy the internal drop volume (small dark gray circles). (b) Single aqueous silica precursor drop. CTAB is above the CMC, forming micelles (red) in addition to microemulsion droplets (dark gray). (c) Oil/water interface with adsorbing surfactants from the two immiscible phases [123].

Figure 37 Characterization of templated carbon particles and templated carbon particles decorated with platinum nanoparticles. (a) SEM image of carbon particle surface; (b) TEM image of the carbon particle cross section showing the internal structure; (c) TEM of the templated carbon particle decorated with platinum nanoparticles; (d) TEM of the cross section of templated carbon particle decorated with platinum nanoparticles showing the internal structure and dispersion of the platinum nanoparticles; (e) TEM of the templated carbon particle decorated with platinum nanoparticles at higher magnification [123].

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the microenvironment of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles in isooctane. It showed that the micellar shape transformed from spherical to ellipsoidal with ultrasound. On the basis of these investigations, ZnS nanorods and nanofibers were synthesized in the reverse micelles by the ultrasound-induced method (Figure 38). A possible mechanism for ultrasound-induced formation of nanorods and nanofibers in reverse micelles was discussed (Scheme 14). It revealed that ultrasound resulted in reaggregation of the reverse micelles and thus enlarged the water core of the micelles. Spherical ZnS nanoparticles can also transform into nanorods and nanofibers in the reverse micelles with the aid of ultrasound, and their length can be controlled by ultrasound time. Xiao et al. [125] demonstrated a facile, controllable, and universal route for the synthesis of hierarchical mesoporous

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zeolites template from a mixture of small organic ammonium salts and mesoscale cationic polymers. The route involves a one-step hydrothermal synthesis, and the templated mixture is homogeneously dispersed in the synthetic gel. In their work, hierarchical mesoporous Beta zeolite (Beta-H) was crystallized in the presence of TEAOH and a mesoscale cationic polymer, polydiallyldimethylammonium chloride (PDADMAC). Low-magnification scanning electron microscopy (SEM) images of the calcined sample of Beta-H are shown in Figures 39(a) and 39(b). The presence of hierarchical mesoporosity in the Beta-H sample is attributed to the use of the molecular and aggregated cationic polymer PDADMAC. The molecular weight of the cationic polymer lies in the range 1 × 105–1 × 106, and its size is estimated to be 5–40 nm, which is in good agreement with the dimen- sions of the mesopores obtained from high-resolution (HR) TEM studies (Figure 39(d)). The cationic polymers could effectively interact with negatively charged inorganic silica species in alkaline media, resulting in the hierarchical mesoporosity. The addition of a greater amount of cationic polymer to the synthetic gel yields Beta zeolite with larger mesoporosity, indicating the controllable mesoporosity of the zeolite sample. These novel zeolites exhibit excellent catalytic properties compared with conventional zeolites. After that, Liu et al. [126] reported that ordered hexagonal mesoporous silica materials with additional disordered large-mesopore networks (DL-SBA-15s) had been prepared by one-pot process using urea as a producer of gas. The result showed that DL-SBA-15s not only had ordered 2D hexagonal mesopores (about 10 nm), but also had another disordered large-mesopore network (about 20 nm) interconnected with hexagonal mesoporous channels, which were confirmed by nitrogen isotherms (Figure 40). The ordered hexagonal mesopores were templated by

Figure 38 TEM photographs of ZnS particles with different ultrasound times (t). (a) t = 0 h; (b) t = 0.5 h; (c) t = 2 h; (d) t = 4 h [124].

Scheme 14 Proposed mechanisms for the ultrasound-induced formation of nanorods and nanofibers in AOT microemulsion [124].

Figure 39 Electron microscopy images of calcined Beta-H: (a), (b) SEM images at low and high magnification, respectively (the separation between each marker represents 5 mm and 100 nm, respectively); (c), (d) TEM images at low and high magnification, respectively [125].

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Scheme 15 Proposed mechanism on synthesis of DLSBA-15 samples [ 1 2 6 ] .

Figure 40 SEM images of DL-SBA-15-85 in (a) low magnification, (b) high magnification of area A and (c) high magnification, and (d) schematic drawing of the interconnected pore channels in area B [126].

polymer surfactant micelle, and disordered largemeso- pores were formed by gaseous expansion due to the de- composition of urea added in the silica gel. The proposed mechanism on synthesis of DL-SBA-15 samples was il- lustrated in Scheme 15.