Nano-MgO

Nanoscale metal oxides also exhibit biocidal properties due to their abrasive nature, alkaline surfaces, oxidizing power (when elemental halogens are preadsorbed), and the fact that their average particle charge (positive) attracts bacteria (which generally carry overall negative charge).11 In fact, nano-MgO is... 

FIGURE 8.7 Transmission emission microscope (TEM) image of a single polyethylene oxide nanofiber embedded with nano MgO at regular intervals. (Hussain, M.M. and Ramkumar, S.S., 2006, Functionalized nanofibers for advanced applications, Indian J. Fiber Text. Res.. 31, 41-51.)... 

Wang, L. J. Yang. Transesterification of soybean oil with nano-mgo or not in supercritical and subcritical methanol. FuellOQl, 86, 328-333. 

Figure 5.1. Representation of the multiple uses of nano-MgO in heterogeneous catalysis.

In this ehapter, reeent appheations of nano-MgO either as a catalyst or as support for asymmetric organic reactions are described. The work described here mostly involves the use of commercial MgO, CM-MgO [specific surface area (SSA) 30m /g], conventionally prepared MgO, CP-MgO (NA-MgO) (SSA 250 m /g), aerogel prepared nano active plus MgO, AP-MgO (NAP-MgO) (SSA 590 m /g). 

All these results rule out the formation of bimetallic species and indicate retention of the coordination geometries of the specific divalent anions anchored to nano-MgO in their monomeric form upon counterionic stabilization and use. 

Fig. 2.11 Photographs of representative as-annealed MgO ceramics fabricated through hot pressing of the nano-MgO powder containing 4 % LiF at 1100 °C for 1 h in Ar, with sizes of 12.7 mm in diameter and 0.5 mm in thickness. Reproduced with permission from [4]. Copyright 2004, Elsevier...

The above equations are deduced assuming the existence of dislocation activity. Although dislocation activity has been shovm in YTZP [17, 63, 64], this is likely to be an artifact (for details, see Section 15.3.1). Recently, superplastically deformed nano-MgO with grain size of 37 nm has been reported to deform at temperatures between 700 and 800 °C, and the stress exponent results in a value of 2. Dislocation activation in this system with this grain size requires an applied compressive stress in excess of 3 GPa. Such calculated stresses are far higher than the yield stresses measured experimentally (i.e., 190 to 640 MPa) [79]. 

Hoseini-Sarvari M and Parhizgar G. Regioselective Friedel-Crafts alkylation of indoles with epoxides using nano MgO. Green Chem. Lett. Rev. 2012 5(3) 439-449. 

This section will compare the interactions between electrolyte and the LiCoO surfaces under various conditions, pristine or nano-MgO modified, charged or uncharged, and find out the reasons for the improved electrochemical performances of MgO/LiCoO, cathode materials. 

Commercial LiCoOj, nano-MgO, MgO/LiCoOj powders and the electrode material scratched from the aluminum current collector were thoroughly mixed with KBr and pressed into pellets respectively. Liquid EC and fresh electrolyte were cast on KBr pellets. A droplet of the binder dissolved in dimethyl formamide (DMF) was also cast onto KBr peUets and then heated at 150°C for over 24 hours in air. All these samples were separately stored in hermetically sealed containers, ready for the Fourier transform infrared (FTIR) measurements. Charged electrode samples for the X-ray photoelectron spectroscopic (XPS) study were fixed on cleaned sample holders (copper) with a piece of conductive tape and stored in a sealed container. All the above operations were carried out in argon atmosphere unless specified. When everything was ready for the XPS and FTIR instruments, the containers were opened and the samples were transferred into the vacuum chambers of the instruments and the chambers were vacuumed immediately. The exposure time of the sample to air was less than 10 seconds. The FTIR spectra were the average of 200 scans on a BIO-RAD FTS-60 spectrometer. XPS spectra were collected on an ESCALAB5 (VG Scientific energy resolution 0.1 eV) with a non-monochromatic Mg Ka radiation (1253.6 eV). Before measurements, the XPS samples were sputtered with Ar beam (2 KeV, 40 pA) for 10 minutes to remove the SEI layer. 

The IR absorption spectra of commercial LiCoOj, nano-MgO and nano-MgO coated LiCoO have been shown in Figure 22. It is seen that LiCoOj has two strong absorption peaks at 522 and 610 cm while nano-MgO has a broad hump at around 1483 cm . Another two strong bands are observed at 640 cm and 420 cm in nano-MgO. These broad peaks are characteristic of nanometer sized MgO. However, when the surface of LiCoOj particle is coated with nano-MgO, no obvious spectral variation is observed. The two peaks of pristine LiCoO are still there and their relative intensities remain unchanged. The reason for the spectral features is that the content of nano-MgO on LiCoOj is very low (about 1.5 mol% ). Therefore some surface-sensitive characterization techniques are necessary for the identification of the MgO/LiCoOj interlayer properties. The other peaks observed in Figure 22 are attributed to the instrumental error (the sharp peak at around 1400 cm" , for example) or some contamination to the KBr pellets because these weak peaks reappear in all these samples. 

The above electrolyte residue on nano-MgO and commercial LiCoO is in fact a mixture of EC, LiPF and their decomposition products. Figure 24 shows the CH stretching bands of the fresh electrolyte and the electrolyte residue on different surfaces. Clearly the spectrum of electrolyte residue on MgO/LiCoO is somewhat similar to that of the electrolyte, but the spectra of the electrolyte... 

Figure 22 FTIR spectra of commercial LiCoOa, nano-MgO and nano-MgO coated liCo02 powders. Reproduced from [122] with permission of The Electrochemical Society Inc.

Figure 23 Comparison of the IR spectra of pure EC (a), the electrolyte (b), and electrolyte residue on different substrates (c) commercial liCoOa (d) nano-MgO (e) nano-MgO coated LiCoOa (f) the binder fa- cathode preparation (the strong bands around 600 cm in c are characteristic bands of commercial liCoOa). Reproduced from [122] with permission of The Electrochemical Society Inc.

Figure 26 shows the IR spectra of some ring stretching modes of EC in the electrolyte residue. As marked with asterisk (at 1020 cm ), DMC and LiPF are almost undetectable in the electrolyte residue on commercial LiCoO, and nano-MgO. Compared with the spectrum of pure EC and the electrolyte, a new component is observed at approx. 1197 cm in the electrolyte residue on the three substrates. Another two new components are detected at 1210 and 1088 cm in the residue on MgO/LiCoOj. As the content of the decomposed products on nano-MgO coated LiCoOj is very low, the appearance of these intense bands is believed to be due the variation of configuration (the molecular orientation and the bonding strain) of the EC molecules on the substrate. 

In the current experiment, as DMC is removed during sample washing and drying, the impact of the nano-MgO coating on the interaction of LiCoOj with DMC is still not clear (not recognizable). Improved experimental design is performed and further investigation results will be published elsewhere. 

---