Jisun Im

Encapsulation of Zinc Oxide Nanorods and Nanoparticles

A simple method for encapsulating zinc oxide nanoparticles within an organic matrix is described that consists of dispersing them in an ethanolic solution, adding an organothiol, and stirring while heating. Electron microscopy, photoemission, Raman spectroscopy, and thermal gravimetric analyses demonstrate that partial dissolution of the oxide occurs, accompanied by encapsulation within a matrix consisting of a 1:2 zinc/thiol complex. Using this methodology, it is possible to surround ZnO within diverse matrices, including fluorescent ones. The process is demonstrated for 1-dodecanethiol (DDT) and fluorescent 2-naphthalenethiol (NPT). For DDT, ZnO nanorods become surrounded by a layer of the zinc-thiol complex that is greater than 100 A thick. In the case of NPT, significantly greater dissolution of the ZnO occurs, with the encapsulated rods taking on a spherical geometry, as evidenced by electron microscopy.

Anomalous vapor sensor response of a fluorinated alkylthiol-protected gold nanoparticle film.

Monolayer-protected gold nanoparticle films generally swell and increase their electrical resistance when exposed to organic vapors. Films of gold nanoparticles protected by 1H,1H,2H,2H-perfluorodecanethiol (PFDT) exhibit an anomalous response in which the resistance decreases for all vapors investigated. Electron microscopy illustrates that the PFDT-functionalized gold nanoparticles are hexagonally ordered with an interparticle separation of 3 nm. Quartz crystal microbalance measurements confirm substantial mass uptake, but the relatively large interparticle separation and insulating properties of the gold particles lead to a porous film whose electrical resistance is strongly influenced by changes in the relative permittivity and reversible, vapor-induced changes in film morphology.

Thiol Adsorption on and Reduction of Copper Oxide Particles and Surfaces

The adsorption of 1-dodecanethiol at room temperature and at 75 °C on submicron cuprous and cupric oxide particles suspended in ethanol has been investigated by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and transmission electron microscopy. Thiol adsorption occurs in all cases via Cu-S bond formation, with partial dissolution of CuO at 75 °C and formation of a copper-thiolate complex replacement layer. Regardless of temperature, the surface of the CuO particles is essentially completely reduced to either Cu2O or metallic copper, as evidenced by loss of the characteristic Cu(2+) XPS features of dried powder samples. Companion ultrahigh-vacuum studies have been performed by dosing clean, oxygen-dosed, and ozone-treated single crystal Cu(111) with methanethiol (MT) gas at room temperature. In the latter case, the surface corresponds to CuO/Cu(111). XPS confirms MT adsorption in all cases, with an S 2p peak binding energy of 162.9 ± 0.1 eV, consistent with methanethiolate adsorption. Heating of MT-covered Cu(111) and oxygen-dosed Cu(111) leads to decomposition/desorption of the MT by 100 °C and formation of copper sulfide with an S 2p binding energy of 161.8 eV. Dosing CuO/Cu(111) with 50-200 L of MT leads to only partial reduction/removal of the CuO surface layers prior to methanethiolate adsorption. This is confirmed by ultraviolet photoelectron spectroscopy (UPS), which measures the occupied states near the Fermi level. For both the colloidal CuO and single crystal CuO/Cu(111) studies, the reduction of the Cu(2+) surface is believed to occur by formation and desorption of the corresponding dithiol prior to thiolate adsorption.

Adsorption of mercaptosilanes on nanocrystalline and single crystal zinc oxide surfaces

The wide band gap and unique photoluminescence (PL) spectrum of nanocrystalline zinc oxide (nano-ZnO) make it attractive for a variety of photonics and sensor applications. Toward the goal of modifying the electronic structure and optical properties of nano-ZnO, the adsorption of 3-mercaptopropyltriethoxysilane (MPTES) has been investigated. Nano-ZnO rods having widths of 10-20 nm and lengths of 100-300 nm were functionalized by ultrasonicating them in a hot ethanol/water solution and adding MPTES. FTIR and X-ray photoelectron spectroscopy (XPS) of the modified nano- ZnO confirm silane functionalization. The presence of hydroxyl groups prior to functionalization suggests that adsorption to ZnO occurs primarily via a condensation reaction and the formation of Zn-O-Si bonds. Comparison has been made to 3-mercaptopropyltrimethoxysilane (MPTMS) adsorbed in ultrahigh vacuum onto sputter-cleaned single crystal ZnO(0001) in which MPTMS vapor is leaked into the vacuum chamber. In this case, bonding occurs via the thiol groups, as indicated by angle-resolved XPS studies. Similar experiments in which sputter-cleaned ZnO(0001) is dosed with dodecanethiol (DDT) confirm adsorption via S-Zn bond formation. Photoluminescence measurements of MPTES-functionalized nano-ZnO show an increase in intensity of the UV emission peak and a decrease in the visible peak relative to the unfunctionalized particles. The reduction of the visible emission peak is believed to be due to passivation of surface defects.

Thiol adsorption on metal oxides: an approach for selective deposition on zinc oxide nanoparticles

We have previously discovered a novel, facile approach to encapsulate ZnO nanorods within thiol complexes. This approach results in a thiol uptake of 30-40% and a 400-500 nm thick thiol-Zn-thiol complex encapsulation layer surrounding ZnO nanorods. By controlling experimental parameters, it is possible to control the thiol deposition, enabling less uptake, which results in a surface monolayer instead of encapsulation. Through this approach, thiol modification of other metal oxide materials, namely TiO2, Al2O3, and MgO, has been attempted. FTIR analysis indicates that thiol adsorption occurs only on ZnO; chemisorption of thiols on other nanoparticles is not evident. Ultrahigh vacuum single crystal adsorption studies demonstrate that ZnO(0001) is also more susceptible to thiol monolayer formation, as evidenced by lack of methanethiol adsorption on TiO2(110) and MgO(0001). These results indicate that the facile thiol modification approach opens a new avenue for surface modification of multi-component metal oxide materials by enabling selective thiol modification of ZnO. This work has potential applicability for creating multiple ligand-functionalized materials, which could be useful for the design of novel multiplexing sensors and photovoltaics.

Effect of surface modification on the optical properties of nanocrystalline zinc oxide materials

The wide band gap and unique photoluminescence (PL) spectrum of nanocrystalline zinc oxide (nano-ZnO) make it useful for a variety of photonics and sensor applications. Toward the goal of modifying the electronic structure and optical properties of nano-ZnO, nanorods were functionalized with electron withdrawing organosilanes, 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDS) and pentafluorophenyltriethoxysilane (PFS), and a partially conjugated heterobifunctional molecule, p-maleimidophenyl isothiocyanate (PMPI). Fourier transform infrared (FTIR) spectroscopy and x-ray photoelectron spectroscopy (XPS) confirmed the presence of the modifiers on the nano-ZnO surface and verified covalent attachment. PL spectroscopy was performed to evaluate the influence of the modifiers on the nano-ZnO inherent optical behavior. An increase in the nano-ZnO near-band edge emission (UV) was evident for the organosilane modifiers, despite their differing electronic structures, while the defect emission (visible) remained unchanged. However, surface modification with the non-silane modifier PMPI resulted in unaltered UV and visible emission intensity. The varying influence of the modifiers may be due to the absence of a silane group in the PMPI, allowing for more efficient electron transport to the modifier. The influence of size/shape of the nanocrystalline ZnO was also examined by reacting spherical nanoparticles with PFDS. Preliminary results indicate that PFDS modification of the nanospheres resulted in similar PL behavior as the nanorods; although, the inherent PL of the spheres differs from the nanorods. These studies will elucidate the role of modifier structure on surface-modified nano-ZnO optical behavior, so that optical tailoring of the nano-ZnO inherent PL can be realized.