Michael Noyong

Enhancement of capacitive deionization capacity of hierarchical porous carbon

Capacitive deionization (CDI) has attracted huge interest as an energy-efficient and eco-friendly desalination strategy. Its development is presently limited due to the relatively low CDI capacitances of carbon materials. Herein, hierarchical porous carbon materials (HPCs) derived from ethylenediaminetetraacetic acid (EDTA) upon annealing were used, which showed impressive CDI performance with a maximum desalination capacity of 34.27 mg g−1 in 40 mg L−1 NaCl aqueous solution. Such capability was attributed to the appropriate hierarchical pore structure, high specific surface area (2185.71 m2 g−1), large pore volume (1.368 cm3 g−1) and reasonable graphitization degree, which were also confirmed by the high specific capacitances of 182 F g−1 in 1 mol L−1 NaCl and 260 F g−1 in 6 mol L−1 KOH. Since the physisorption capacity was nearly 0, and the regeneration process was facile and complete, such economical HPCs materials show potential for practical desalination applications in the future. Moreover, the HPCs electrodes presented ion selectivity in competitive multi-ionic solutions by kinetic behavior difference or static capacitance difference.

In situ nanomanipulation system for electrical measurements in SEM

A versatile in situ measuring system in a SEM with four independently moveable tips was developed. The system allows manipulation as well as electrical contacting of objects on the micro- and nanometer scale. The SEM provides a high vacuum (HV) chamber, but also a variable pressure (VP) mode which allows imaging of conducting or nonconducting objects and surfaces. In this work, we show the experimental setup and capabilities of this system while measuring a platinum surface.

Multivalency of PEG-thiol ligands affects the stability of NIR-absorbing hollow gold nanospheres and gold nanorods

In this work the effect of multivalency on the stability of NIR-absorbing HAuNSs and AuNRs functionalized by mono-, bi- and tridentate polyethyleneglycol (PEG) thiol ligands is reported. Comparison of commercially-available monodentate and self-synthesized bi- and tridentate methoxy terminated thiol-polyethyleneglycol ligands having molecular weights of around 5000 Da shows the stability increase of HAuNSs and AuNRs for bi- and tridentate ligands, attributed to the multivalency of the ligands. The stability was explored according to three different aspects: (1) stability towards competition reactions with the strong binding ligand dithiothreitol, (2) resistance towards oxidative Au dissolution with potassium cyanide, and (3) colloidal stability, tested by the addition of NaCl. Our PEGylation approach leads to AuNRs where the CTAB concentration is below the detection limit of the performed analytical methods, which is vital for any clinical applications. Furthermore, we found strikingly high biocompatibility after PEGylation for both particle types whereby we observed no significant difference in cytotoxicity comparing the mono-, bi- and tridentate PEGylated species.

Electrically conducting nanopatterns formed by chemical e-beam lithography via gold nanoparticle seeds.

We report the formation of thiol nanopatterns on SAM covered silicon wafers by converting sulfonic acid head groups via e-beam lithography. These thiol groups act as binding sites for gold nanoparticles, which can be enhanced to form electrically conducting nanostructures. This approach serves as a proof-of-concept for the combination of top-down and bottom-up processes for the generation of electrical devices on silicon.

Microstructured Hydrogel Templates for the Formation of Conductive Gold Nanowire Arrays

Microstructured hydrogel allows for a new template-guided method to obtain conductive nanowire arrays on a large scale. To generate the template, an imprinting process is used in order to synthesize the hydrogel directly into the grooves of wrinkled polydimethylsiloxane (PDMS). The resulting poly(N-vinylimidazole)-based hydrogel is defined by the PDMS stamp in pattern and size. Subsequently, tetrachloroaurate(III) ions from aqueous solution are coordinated within the humps of the N-vinylimidazole-containing polymer template and reduced by air plasma. After reduction and development of the gold, to achieve conductive wires, the extension perpendicular to the long axis (width) of the gold strings is considerably reduced compared to the dimension of the parental hydrogel wrinkles (from ≈1 μm down to 200-300 nm). At the same time, the wire-to-wire distance and the overall length of the wires is preserved. The PDMS templates and hydrogel structures are analyzed with scanning force microscopy (SFM) and the gold structures via scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy. The conductivity measurements of the gold nanowires are performed in situ in the SEM, showing highly conductive gold leads. Hence, this method can be regarded as a facile nonlithographic top-down approach from micrometer-sized structures to nanometer-sized features.

Resistive Switching of Sub-10 nm TiO2 Nanoparticle Self-Assembled Monolayers

Resistively switching devices are promising candidates for the next generation of non-volatile data memories. Such devices are up to now fabricated mainly by means of top-down approaches that apply thin films sandwiched between electrodes. Recent works have demonstrated that resistive switching (RS) is also feasible on chemically synthesized nanoparticles (NPs) in the 50 nm range. Following this concept, we developed this approach further to the sub-10 nm range. In this work, we report RS of sub-10 nm TiO2 NPs that were self-assembled into monolayers and transferred onto metallic substrates. We electrically characterized these monolayers in regard to their RS properties by means of a nanorobotics system in a scanning electron microscope, and found features typical of bipolar resistive switching.

Competing strain relaxation mechanisms in epitaxially grown Pr0.48Ca0.52MnO3 on SrTiO3

We investigated the impact of strain relaxation on the current transport of Pr0.48Ca0.52MnO3 (PCMO) thin films grown epitaxially on SrTiO3 single crystals by pulsed laser deposition. The incorporation of misfit dislocations and the formation of cracks are identified as competing mechanisms for the relaxation of the biaxial tensile strain. Crack formation leads to a higher crystal quality within the domains but the cracks disable the macroscopic charge transport through the PCMO layer. Progressive strain relaxation by the incorporation of misfit dislocations, on the other hand, results in a significant decrease of the activation energy for polaron hopping with increasing film thickness.

Easy-Preparable Butyrylcholinesterase/Microgel Construct for Facilitated Organophosphate Biosensing

A versatile guest matrix was fabricated from a temperature- and pH-sensitive poly(N-isopropylacrylamide)-co-(3-(N,N-dimethylamino)propylmethacrylamide) microgel (poly(NIPAM-co-DMAPMA), MG) for the gentle incorporation of butyrylcholinesterase (BChE). The microgel/BChE films were built up on a surface of graphite-based screen-printed electrodes (SPEs) premodified with MnO2 nanoparticles via a two-step sequential adsorption under careful temperature and pH control. On this basis, a rather simple amperometric biosensor construct was formed, which uses butyrylthiocholine as BChE substrate with subsequent MnO2-mediated thiocholine oxidation at a graphite-based SPE. The complexation of BChE with the microgel was found to be safe and effective, as confirmed by a high operational and rather good long-term storage stability of the resultant SPE-MnO2/MG/BChE biosensors. The small mesh size of the microgel with respect to the size of BChE results in a predominant outer complexation of BChE within the dangling chains of the microgel rather than a deep penetration of the enzyme into the microgels. Given such surface localization, BChE is easily accessible both for the substrate and for cholinesterase inhibitors. This was supported by the analytical characteristics of the SPE-MnO2/MG/BChE biosensor that were examined and optimized both for the substrate and for the enzyme detection. The SPE-MnO2/MG/BChE biosensor enabled precision detection of organophosphorus pesticides (diazinon(oxon), chlorpyrifos(oxon)) in aqueous samples with minimized interference from extraneous (nonanalyte) substances (e.g., ions of heavy metals). The detection limits for diazinon(oxon) and chlorpyrifos(oxon) were estimated to be as low as 6 × 10-12 M and 8 × 10-12 M, respectively, after 20 min of preincubation with these irreversible inhibitors of BChE.

Electrochemical and Electronic Charge Transport Properties of Ni-Doped LiMn2O4 Spinel Obtained from Polyol-Mediated Synthesis

LiNi0.5Mn1.5O4 (LNMO) spinel has been extensively investigated as one of the most promising high-voltage cathode candidates for lithium-ion batteries. The electrochemical performance of LNMO, especially its rate performance, seems to be governed by its crystallographic structure, which is strongly influenced by the preparation methods. Conventionally, LNMO materials are prepared via solid-state reactions, which typically lead to microscaled particles with only limited control over the particle size and morphology. In this work, we prepared Ni-doped LiMn2O4 (LMO) spinel via the polyol method. The cycling stability and rate capability of the synthesized material are found to be comparable to the ones reported in literature. Furthermore, its electronic charge transport properties were investigated by local electrical transport measurements on individual particles by means of a nanorobotics setup in a scanning electron microscope, as well as by performing DFT calculations. We found that the scarcity of Mn3+ in the LNMO leads to a significant decrease in electronic conductivity as compared to undoped LMO, which had no obvious effect on the rate capability of the two materials. Our results suggest that the rate capability of LNMO and LMO materials is not limited by the electronic conductivity of the fully lithiated materials.

Polyol mediated synthesis and electrochemical performance of nanostructured LiMn2O4 cathodes

Nanoparticulate single phase LiMn2O4 spinel was prepared via polyol method and applied as a cathode in a lithium ion battery. The effects of calcination temperature (250 °C 800 °C) as well as of postsynthetic treatment by ball milling on the physiochemical and electrochemical properties of LiMn2O4 were studied by means of powder XRD, SEM, cyclic voltammetry and charge/discharge cycling. With increasing calcination temperature, the electrochemical activity and discharge capacity increased. The measurements revealed that the electrochemical performance of LiMn2O4 can be further improved by ball milling before calcination. Furthermore, the ball milling process allowed reducing the calcination temperature needed to obtain electrochemically active material.