Structural investigations on semiconductor nanostructures: wet chemical approaches for the synthesis of novel functional structures

Recently nanotechnology is experiencing a flourishing progress in a variety of arenas from science to engineering and to biology. The fabrication of nanoscale building blocks, understanding their properties, and organizing these building blocks in to devices for various applications are the main objectives of nanotechnology. As an active field in nanotechnology, the work presented in this thesis is mostly focused on the fundamental study about the fabrication of functional semiconductor nanostructures by wet chemical approaches. In particular, the effects of structure-directing agents, which is the core objective of this research are discussed in detail. Zinc oxide is intrinsically an n-type semiconducting materials that finds applications in electronics, optics, and catalysis. ZnO nanowires have been emerged as a potential candidate in various modern devices with enhanced efficiency. Hydrothermal growth is a widely used technique for the synthesis of ZnO nanowires owing to cost effectiveness, simplicity, and easy upscaling. The use of a polymer additive, polyethylene glycol in the reaction medium has a huge effect on the final morphology of the prepared structures. This method is used to grow hexabranched ZnO nanostructures with increased total and polar surface area. The structures have shown enhanced photocatalytic activity than the conventional nanowires. ZnO based heterostructures have attracted research attention in recent years for their novel interface properties. Controlled secondary growth approaches using organic capping agents, in combination with doping, have been intrigued for the preparation of ZnO core-shell nanowires. Thus Sb-doped and Co-doped ZnO core-shell nanowires have been prepared by this method. The Sb-doped ZnO core-shell nanowires have been synthesized using polyethylene glycol-assisted process in aqueous medium. The Co-doped ZnO core-shell nanowires have been prepared in ethylene glycol-assisted process where ethylene glycol acts both as the growth medium and a structure-directing agent. Since both the routes use a secondary growth method, the shell is grown on an already prepared nanowire core; the dopants are selectively distributed in the shell. CuI is intrinsically a p-type semiconductor that is mainly studied for its applications in the fields of solar energy conversion, catalysis, and solid electrolytes. In the later chapters, antisolvent crystallization of CuI nanoparticles and their assembly to form superstructures are discussed. This process also has been tuned using polymer additives to obtain specific morphologies with high porosity. Owing to the chemical flexibility of CuI to convert to other materials, the superstructures have been used as sacrificial templates to fabricate highly convoluted and porous CuO and TiO2 structures. When the CuI superstructures prepared by polymer assisted assembly has been used as adsorbents, they showed a high adsorption capability. As mentioned above, the works presented in this thesis are fundamental study on the synthesis of functional semiconductor nanostructures. In general, this study gives an account on the effect of specific structure- directing agents on the morphology and optimized synthetic routes for the preparation of specific semiconductor nanostructures with desired structures

Enantio- and Z-Selective d-Hydroarylation of Aryldienes via Rh-Catalyzed Conjugate Addition

Metal-catalyzed enantioselective conjugate arylations of electron-poor alkenes are highly selective processes for C(sp2)–C(sp3) bond formation. d-Selective hydroarylations of electron-poor dienes are less well developed and reactions that deliver high enantioselectivity while giving single alkene isomer products are elusive. Here we report the Rh-catalyzed d-arylation of aryldienes that gives nearly exclusive Z-1,4-addition products (generally with >95:5 positional and geometrical selectivity). This remote functionalization provides access to chiral diarylated butenes from readily available precursors poised for further functionalization, including in the synthesis of bioactive molecules. Mechanistic studies suggest that protonolysis of a Rh-allyl intermediate generated by diene insertion into a Rh-aryl is the rate determining step and occurs by an inner-sphere proton transfer pathway

Fabrication of micron-sized tetrahedra by Si〈1 1 1〉 micromachining and retraction edge lithography

A new method is proposed to prepare micron-sized anisotropic-shaped particles: tetrahedral structures bounded by 〈1 1 1〉 faces. It is based on the micromachining of 〈1 1 1〉-oriented silicon wafers and retraction edge lithography (REL). The size of these Si structures is tunable but limited: roughly from 20 to 2000 nm. The importance of this method is that the fabricated structure resembles almost perfectly the mathematical tetrahedron. Furthermore, the technique offers room to change the anisotropic property of the particle by selective modification of the faces using self-aligned lithography

Homoepitaxial branching: an unusual polymorph of zinc oxide derived from seeded solution growth

The development of hydrothermal synthesis has greatly promoted bottomup nanoscience for the rational growth of diverse zinc oxide (ZnO) nanostructures. In comparison with normal ZnO nanowires,ZnO nanostructures with a larger surface area, for instance, branched nanowires, are more attractive in the application fields of catalysis, sensing, dye-sensitized solar cells etc. So far the ZnO branched nanowires achieved by either one-step or multistep growth always present a boundary-governed nonepitaxial branch/stem interface. In this report, seeded growth of single-crystalline ZnO hexabranched nanostructures was achieved by selecting polyethylene glycol (PEG) as capping agent based on a low-temperature, laterally epitaxial solution growth strategy. We investigated the generality of this PEG-assisted growth process using different ZnO seed layers including continuous film, patterned dots, and vertically aligned nanowire arrays. It was revealed that PEG is a distinctive c-direction inhibitor responsible for the lateral growth and subsequent branching of ZnO due to its nonionic and nonacidic feature and weak reactivity in the solution system. All the obtained branched nanostructures are of single crystallinity in nature, which is methodologically determined by the homoepitaxial growth mode. This PEGassisted process is versatile for diameter tuning and branch formation of ZnO nanowires by secondary growth. Our proof-of-concept experiments demonstrated that the ZnO hexabranched nanostructures presented superior photocatalytic efficiency for dye degradation relative to the normal ZnO nanowires

Photocatalytic carbon dioxide reduction at p-type copper(I) iodide

A p-type semiconductor, CuI, has been synthesized, characterized, and tested as a photocatalyst for CO2 reduction under UV/Vis irradiation in presence of isopropanol as a hole scavenger. Formation of CO, CH4, and/or HCOOH was observed. The photocatalytic activity of CuI was attributed to the very low potential of the conduction band edge (i.e., −2.28 V vs. NHE). Photocurrents generated by the studied material confirm a high efficiency of the photoinduced interfacial electrontransfer processes. Our studies show that p-type semiconductors may be effective photocatalysts for CO2 reduction, even better than extensively studied n-type titanium dioxide, owing to the low potential of the conduction band edge

Synthesis, characterization and cytotoxicity of polyethylene glycol coupled zinc oxide-chemically converted graphene nanocomposite on human OAW42 ovarian cancer cells

Present work reports for the first time on successful synthesis of polyethylene glycol (PEG) coupled ZnO-chemically converted graphene (CCG) nanocomposites (ZGP) as well as pristine nano ZnO (ZO), ZnO-CCG (ZG) and ZnO-PEG (ZP) by adopting facile solvothermal method using zinc acetate dihydrate, graphene oxide and PEG as precursor materials. X-ray diffraction measurement of samples showed nanocrystalline hexagonal ZnO. Agglomeration of ZnO nanoparticles formed microspheres in ZG, and the agglomeration was found to be decreased in ZGP as revealed from field emission scanning and transmission electron microscopes. Raman and FTIR spectral studies evidenced the presence of chemically interacted CCG and polyethylene glycol in the nanocomposites. Content of the organics in ZGP was determined by thermogravimetric analysis. A mechanism was proposed on the formation of ZGP nanocomposite. From the measurement of in vitro cytotoxicity, quantitative cell viability (CV) of human ovarian cancer cell line, OAW42, was obtained from control to a maximum of 200 mu g/ml of sample concentrations. An excellent CV of the cancer cells was observed (nearly similar to 80% of viable cells at 50 mu g/ml dose with respect to the control) for ZGP compared to ZO, ZG and ZP samples. The effective role of CCG and PEG in ZGP nanocomposite for enhancing the cell viability was explained. This simple strategy could be beneficial for synthesis of other metal oxide towards biomedical applications. Copyright (c) 2015 John Wiley & Sons, Ltd