Experimental and theoretical investigation of ligand effects on the synthesis of ZnO nanoparticles

ZnO nanoparticles with highly controllable particle sizes(less than 10 nm) were synthesized using organic capping ligands in Zn(Ac)2 ethanolic solution. The molecular structure of the ligands was found to have significant influence on the particle size. The multi-functional molecule tris(hydroxymethyl)-aminomethane (THMA) favoured smaller particle distributions compared with ligands possessing long hydrocarbon chains that are more frequently employed. The adsorption of capping ligands on ZnnOn crystal nuclei (where n = 4 or 18 molecular clusters of(0001) ZnO surfaces) was modelled by ab initio methods at the density functional theory (DFT) level. For the molecules examined, chemisorption proceeded via the formation of Zn...O, Zn...N, or Zn...S chemical bonds between the ligands and active Zn2+ sites on ZnO surfaces. The DFT results indicated that THMA binds more strongly to the ZnO surface than other ligands, suggesting that this molecule is very effective at stabilizing ZnO nanoparticle surfaces. This study, therefore, provides new insight into the correlation between the molecular structure of capping ligands and the morphology of metal oxide nanostructures formed in their presence

Biomimetic bidirectional switchable adhesive inspired by the gecko

The gecko adhesive system has attracted significant attention since the discovery that van der Waals interactions, which are always present between surfaces, are predominantly responsible for their adhesion. The unique anisotropic frictional-adhesive capabilities of the gecko adhesive system originate from complex hierarchical structures and just as importantly, the anisotropic articulation of the structures. Here, by cleverly engineering asymmetric polymeric microstructures, a reusable switchable gecko-like adhesive can be fabricated yielding steady high adhesion (F⊥ ≈ 1.25 N/cm2) and friction (Fâ̂¥ ≈ 2.8 N/cm2) forces when actuated for "gripping", yet release easily with minimal adhesion (F⊥ ≈ 0.34 N/cm2) and friction (Fâ̂¥≈ 0.38 N/cm2) forces during detachment or "releasing", over multiple attachment/detachment cycles, with a relatively small normal preload of 0.16 N/cm2 to initiate the adhesion. These adhesives can also be used to reversibly suspend weights from vertical (e.g., walls), and horizontal (e.g., ceilings) surfaces by simultaneously and judiciously activating anisotropic friction and adhesion forces. This design opens the way for new gecko-like adhesive surfaces and articulation mechanisms that do not rely on intensive nanofabrication in order to recover the anisotropic tribological property of gecko adhesive pads, albeit with lower adhesive forces compared to geckos. A switchable gecko-inspired adhesive that shows anisotropic adhesion and friction properties is presented. With proper actuation, the polymer-based dry adhesive provides strong adhesion and friction forces for attachment (or gripping), or weak forces for detachment (or releasing). The reusability of the dry adhesive is also demonstrated over multiple attachment/detachment cycles. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

“Green” Aqueous Synthesis and Advanced Spectral Characterization of Size-Selected Cu2ZnSnS4 Nanocrystal Inks

Structure, composition, and optical properties of colloidal mercaptoacetate-stabilized Cu2ZnSnS4 (CZTS) nanocrystal inks produced by a “green” method directly in aqueous solutions were characterized. A size-selective precipitation procedure using 2-propanol as a non-solvent allows separating a series of fractions of CZTS nanocrystals with an average size (bandgap) varying from 3 nm (1.72 eV) to 2 nm (2.04 eV). The size-selected CZTS nanocrystals revealed also phonon confinement, with the main phonon mode frequency varying by about 4 cm−1 between 2 nm and 3 nm NCs

Photovoltage method for the research of CdS and ZnO nanoparticles and hybrid MEH-PPV/nanoparticle structures

The paper demonstrates an application of photovoltaic effect in the research of nanomaterials and hybrid structures of polymers with nanoparticles. Photovoltage (PV) measurement was utilized for estimating the size of CdS and ZnO nanoparticles and diagnostics of hybrid structures with poly[2-methoxy-5-(2- ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), namely nano-CdS/MEH-PPV and nano-ZnO/MEH-PPV. CdS average nanocrystal diameters 4.0 and 4.8 nm were calculated from the onset values of energies corresponding to the steep increase in the PV signals using the effective mass model. The range of size of ZnO particles was from about 5 up to 50 nm. Nanoparticle distribution obtained by transmission electron microscopy (TEM) measurements shows that the calculated diameters agree fairly well with the size of nanoparticles with the highest occurrence. The PV spectrum represents a sum of the spectra corresponding to the various nanoparticle sizes. The distribution of the nanoparticles was also obtained by a simple mathematical treatment of the PV spectra, and agreement with the results of TEM was found. PV and TEM measurements were performed on commercial CdS and ZnO nanoparticles and on CdS nanoparticles prepared in our laboratory. We employ triethanolamine as a protective agent to cover the surface of CdS nanoparticles. Nano-CdS/MEH-PPV and nano-ZnO/MeH-PPV hybrid structures were prepared, and influence of the particles on charge transport was shown by the PV spectra measurements. © 2014 Springer Science+Business Media

Self-Drying: A Gecko's Innate Ability to Remove Water from Wet Toe Pads

When the adhesive toe pads of geckos become wet, they become ineffective in enabling geckos to stick to substrates. This result is puzzling given that many species of gecko are endemic to tropical environments where water covered surfaces are ubiquitous. We hypothesized that geckos can recover adhesive capabilities following exposure of their toe pads to water by walking on a dry surface, similar to the active self-cleaning of dirt particles. We measured the time it took to recover maximum shear adhesion after toe pads had become wet in two groups, those that were allowed to actively walk and those that were not. Keeping in mind the importance of substrate wettability to adhesion on wet surfaces, we also tested geckos on hydrophilic glass and an intermediately wetting substrate (polymethylmethacrylate; PMMA). We found that time to maximum shear adhesion recovery did not differ in the walking groups based on substrate wettability (22.7±5.1 min on glass and 15.4±0.3 min on PMMA) but did have a significant effect in the non-walking groups (54.3±3.9 min on glass and 27.8±2.5 min on PMMA). Overall, we found that by actively walking, geckos were able to self-dry their wet toe pads and regain maximum shear adhesion significantly faster than those that did not walk. Our results highlight a unexpected property of the gecko adhesive system, the ability to actively self-dry and recover adhesive performance after being rendered dysfunctional by water