Development of new gas sensors based on oxidized galinstan

For the first time, we have fabricated and tested conductometric sensors based on oxidized liquid galinstan towards NO2 and NH3 gases at low operating temperatures. Galinstan based films on silicon substrates have been studied with two different loadings. Surface morphology of the films was investigated by means of field emission scanning electron microscopy (FESEM). The sensor with higher galinstan loading showed a better sensitivity, which can be attributed to a higher surface area, as confirmed by SEM. At 100°C, a detection limit as low as 1 and 20 ppm was measured for NO2 and NH3, respectively

Tailor synthesis of 0D, 1D and 2D transition metal dichalcogenide nanostructures

The work presented in this doctoral thesis is a compilation of different approaches on the synthesis of transition metal dichalcogenides (TMDs) and the discussion on their probable formation mechanism. As the first step, a combination of metal organic chemical vapor deposition and chemical vapor transport approaches was taken to synthesize MoS2 nanotube bundles and iodine was used as a mineralizing agent. The reaction was carried out in a horizontal tube furnace. In the absence of iodine, no nanotubes were formed. Therefore, it is demonstrated that the iodine content of the precursors led to the enhanced mobility of the Mo and S constituents, the formation of point defects within the MoS2 layers and finally scrolling of MoS2 nanosheets. rnThis synthetic method was further extended to synthesize WS2 nested fullerenes and the formation of core-shell 2H-WS2@IF-WS2. Intermediate nanoparticles were studied using different electron microscopy techniques. The internal volume of the nested fullerenes was studied using a combined scanning electron microscopy/focused ion beam technique by cutting the cross sections of the core-shell nanoparticles. The lamellar reaction intermediates were found occluded in the fullerene particles. The role of the reaction and annealing temperature on the composition and morphology of the final product were also investigated. The stiffness of the WS2 shell was measured using intermittent contact-mode AFM.rnIn addition, a facile route for the synthesis of WS2 nanotubes starting from solvothermally derived tungsten oxide nanowires was demonstrated. Defect-rich multiwalled WS2 nanotubes were made by reductive sulfidization of W18O49 nanowires that were obtained solvothermally from WCl6 in different alcohols. W18O49 nanowires were also synthesized using a hot injection method, but these nanowires failed to form WS2 nanotubes. The defect-rich nanotubes were highly dispersible in organic solvents and were easily functionalized by Au, MnO and Pt@Fe3O4 Janus nanoparticles on the basis of Pearson’s HSAB principle which proved the direct transfer of defects from the precursor to the end product. rnIn order to investigate whether the preservation of the morphology applies for any other structures rather than 0D and 1D precursors, the oxide to sulfide conversion method was utilized to convert WO3 low aspect ratio nanorods to corresponding sulfides. In this case, nested tungsten sulfide geometrical nanoparticles with 90° apex described as “nanocoffins” were obtained and the automated diffraction tomography was used to investigate the effect of the oxide precursor crystal structure on the final morphology of the sulfide product. The box-like morphology was shown to originate through topotactical dehydration reaction of the precursor, i.e. a WO3•1/3H2O crust on WO3, followed by epitactic induction of intermediate hexagonal WO3 which serves as a template to maintain the particle shape in final product. In fact, a cascade of topotactic reaction leading to epitactic induction leads to the formation of closed rectangular boxes made from hexagonal layers.rnAs a step further, a lithiation/exfoliation approach was taken to synthesize a new class of TMDs in 2D form. Here, the restriction of convenient layered chalcogenide nanoparticles toward the intercalation was overwhelmed by selecting Nb1-xWxS2 coin-roll nanowires (CRNWs) as appropriate intercalation host. CRNWs were intercalated using n-BuLi in an inert atmosphere and the exfoliation of the Li-intercalated CRNWs using H2O led to the formation of graphene-type sheets of Nb1-xWxS2. It was demonstrated that the in situ functionalization of the graphene-type sheets using gold nanoparticles enhanced the stabilization of these sheets in the aquatic solution.Die vorliegende Arbeit befasst sich mit verschiedenen Syntheseansätzen für die Darstellung von Übergangsmetall-Dichalkogeniden (transition metal dichalcogenides, TMDs) und deren möglichen Bildungsmechanismen. Im ersten Schritt wurde eine Kombination aus Metallorganischer Chemischer Gasphasenabscheidung und chemischer Gasphasenabscheidung genutzt um Bündel von MoS2-Nanoröhren herzustellen, wobei Jod als mineralisierendes Agenz genutzt wurde. Die Reaktion wurde in einem horizontalen Röhrenofen gefahren. In Abwesenheit von Jod wurden keine Nanoröhren gebildet. Es konnte gezeigt werden, dass der Jodgehalt im Präkursor zu einer gesteigerten Mobilität der Mo- und S-Bestandteile, einer Entstehung von Punktdefekten in der MoS2-Schicht und letztendlich zum Zusammenrollen zu MoS2-Nanoblättern führt. rnDiese Synthesemethode wurde erweitert um ineinander verschachtelte WS2-Fullerene und Kern-Schale 2H-WS2@IF-WS2 zu synthetisieren. Intermediär auftretende Nanopartikel wurden mittels verschiedener Elektronenmikroskopietechniken untersucht. Das Innere der verschachtelten Fullerene wurde anhand einer kombinierten Rasterelektronenmikroskopie/fokussierter Ionenstrahl-Technik untersucht, indem die Kern-Schale-Partikel aufgeschnitten wurden. Die lamellenartigen Reaktionsintermediate waren in den Fullerenpartikeln eingeschlossen. Zusätzlich wurde der Einfluss der Reaktion- und Heiztemperatur auf die Komposition und Morphologie des finalen Produkts studiert. Die Steifheit der WS2-Schalen wurde mit der Hilfe des intermittierenden Modus der Raskterkraftmikroskopie bestimmt.rnWeiterhin wurde eine einfach Syntheseroute für die Synthese von WS2-Nanoröhren, ausgehend von solvothermal dargestelltem Wolframoxid-Nanodrähten, demonstriert. Defektreiche, mehrschichtige WS2-Nanoröhren wurden durch reduktive Sulfidierung von W18O49-Nanodrähten dargestellt, welche solvothermal aus WCl6 in verschiedenen Alkoholen erhalten wurden. Durch ein Heiß-Einspritz-Verfahren konnten ebenfalls W18O49-Nanodrähte hergestellt werden, allerdings eigneten sich diese nicht als Präkursor für die Darstellung von WS2-Nanoröhren. Die defektreichen Nanoröhren waren leicht in organischen Lösungsmitteln dispergierbar und konnten einfach mit Au, MnO und Pt@Fe3O4-Janusnanopartikeln auf Basis von Pearsons HSAB-Konzeptes funktionalisiert werden. rnUm der Fragestellung nachzugehen, ob die Erhaltung der Morphologie auch für höherdimensionale Strukturen und nicht nur für 0D und 1D Präkursoren gilt, wurde die Oxid-zu-Sulfid-Umwandlungsmethode verwendet, um die WO3-Nanoröhren mit niedrigem Aspektverhältnis in die entsprechenden Sulfide zu transformieren. In diesem Fall, wurden verschachtelte, geometrische Nanopartikel von Wolframsulfid mit einem 90° Scheitel erhalten, welche als „Nanocoffins“ beschrieben wurden. Die automatische Beugungstomographie wurde genutzt um den Effekt der Kristallstruktur des oxydischen Präkursors auf die finale Morphologie des sulfidischen Produktes zu untersuchen. Es konnte gezeigt werden, dass die kistenförmige Morphologie aus der topotaktischen Dehydrierung des Präkursors herrührt, d.h. einer WO3•1/3H2O Kruste auf WO3, gefolgt von epitaktischem Einbau von intermediärem hexagonalem WO3, welches als Templat zur Erhaltung der Partikelform im Endprodukt dient. Tatsächlich führt eine Kaskade von topotaktischen Reaktionen zum epitaktischen Einbau, was wiederum zu der Bildung von geschlossenen rechtwinkligen Kisten aus hexagonalen Ebenen führt. Der nächste Schritt bestand darin mittels eines Lithiierung/Ablätterung-Ansatzes eine neue Klasse von TMDs in 2D Form zu synthetisieren. Die Beschränkung auf geeignet geschichteten Chalgogenid-Nanopartikeln für die Interkalation wurde durch die Auswahl von münzrollartigen Nb1-xWxS2-Nanodrähten (coin-roll nanowires, CRNWs) als zweckmäßiger Einlagerungsverbindung überwunden. CRNWs wurden mittels Verwendung von n-BuLi in einer inerten Atmosphäre interkaliert. Das Abblättern der Li-interkalierten CRNWs durch H2O führte zur Bildung von graphenartigen Blättern von Nb1-xWxS2. Es konnte gezeigt werden, dass die in situ Funktionalisierung der graphenartigen Blätter mit Goldnanopartikeln eine gesteigerte Stabilität dieser Blätter in wässrigen Lösungen zur Folge hatte

Tetrathiafulvalene-7,7,8,8-tetracyanoquinodimethane and tetrathiafulvalene-2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane organic charge-transfer complexes: Reusable catalysts for electron-transfer reactions

The application of organic charge-transfer complexes such as TTF-TCNQ (TTF=tetrathiafulvalene, TCNQ=7,7,8,8-tetracyanoquinodimethane) is well known in the area of organic electronics. However, the applicability of this material and its derivatives has not been explored for catalytic reactions. Herein, we report on the catalytic properties of both TTF-TCNQ and the significantly less-known fluorinated analogue TTF-TCNQF4 (TCNQF4=2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane). The model reaction of ferricyanide ion reduction by thiosulfate ions was chosen, for which it was found that both materials were indeed catalytically active. Significantly, the fluorinated TCNQF4 analogue showed considerably higher catalytic activity than TTF-TCNQ. In addition, TTF-TCNQF4 was found to be highly stable under the catalytic conditions and could be recovered and reused without any loss in performance for at least 10 catalytic cycles. This work opens up new avenues for investigating these types of materials for catalytic reactions

Directing nanostructure formation of gold via the in situ under potential deposition of a secondary metal for the detection of nitrite ions

In this work the underpotential deposition of metals such as copper and lead during the electrochemical deposition of gold is investigated to understand the influence that the incorporation of a second metal has on the morphology of gold nanostructures. The incorporation of Pb or Cu, even at concentrations as low as 0.2 %, significantly influence the morphology of the deposit where the formation of elongated structures from a central gold structure is favoured. These nanostructures are characterised by cyclic voltammetry, X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) and tested for their suitability as a sensing layer for the electrochemical detection of nitrite ions in aqueous solution. A limit of detection of 0.3 µM is achieved with a linear range up to 1 mM which is adequate for the determination of nitrite contained within food products

Utilizing p-type native oxide on liquid metal microdroplets for low temperature gas sensing

Highlights • Development of an amorphous p-type oxidised galinstan film confirmed by XRD and Hall effect measurements; • Sensing performance of oxidised galinstan based conductometric devices towards NO2, NH3 and CH4 at low operating temperatures up to 150 °C; • Optimal response with a more stable baseline at an operating temperature of 100 °C; Abstract Liquid metals based on gallium oxidize under ambient conditions to form a native oxide on the surface. Here we take advantage of the semiconducting properties of this oxide layer for gas sensing applications. In particular, the development of gas sensors that operate at low temperatures is an ongoing challenge. Therefore, to address this problem, we fabricated conductometric sensors based on an oxidized liquid metal galinstan layer, and investigated their sensitivity towards NO2, NH3 and CH4 gases. The fabrication of the sensing layer was achieved via a simple approach, involving the sonication of the liquid metal in acetonitrile to produce a solution of micro/nanodroplets and dropcasting it onto a non-conducting alumina substrate at different loadings. The material properties of the developed film were extensively investigated by means of field emission scanning electron microscopy, Raman spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The results confirmed the presence of an amorphous oxide on the surface of the droplets. Hall effect measurements indicated that the oxide film was p-type, which influenced the sensing response towards the different gases. We demonstrated that a physisorption process occurs at 100 °C, leading to a detection limit as low as 1 and 20 ppm for NO2 and NH3, respectively

A new study on binder performance and formulation modification of anti-corrosive primer based on ethyl silicate resin

Zinc-rich ethyl silicate coatings are quite successful in protecting steel against corrosion under severe exposing conditions. In spite of providing excellent cathodic protection to steel structure after film curing, two-component zinc-rich ethyl silicate coatings have some limitations, one of which is inadequate shelf life as a result of in-can binder gelation. In this work, the preparation steps of ethyl silicate such as pre-hydrolysis, dehydration and organometallic reactions were surveyed and herein an approach towards understanding the cause and effect relationship of the use of ingredients is presented. The effects of water and catalytic acid dosages on gel time under accelerated conditions and the effect of alcoholic solvent order on the rate of the hydrolysis and dehydration reactions were studied via Karl-Fischer test determining the water content of hydrolysate. A thriving optimization in shelf life without any loss in physical–mechanical characteristics of the final film (e.g. hardness, adhesion, solvent and salt spray resistance) was obtained

Galvanic replacement of the liquid metal galinstan

The galvanic replacement reaction is a highly versatile approach for the creation of a variety of nanostructured materials. However, the majority of reports are limited to the replacement of metallic nanoparticles or metal surfaces. Here we extend this elegant approach and describe the galvanic replacement of the liquid metal alloy galinstan with Ag and Au. This is achieved at a macrosized droplet to create a liquid metal marble that comprises a liquid metal core and a solid metal shell, whereby the morphology of the outer shell is determined by the concentration of metallic ions used in the solution during the galvanic replacement process. In principle, this allows one to recover precious metal ions from solution in their metallic form, which are immobilized on the liquid metal and therefore easy to recover. The reaction is also undertaken at liquid metal microdroplets created via sonication to produce Ag- and Au-based galinstan nanorice particles. These materials are characterized with SEM, XRD, TEM, SAED, EDX, XPS, UV–visible spectroscopy, and open-circuit potential versus time experiments to understand the galvanic replacement process. Finally, the nanosized materials are investigated for their catalytic activity toward the reduction of methylene blue in the presence of sodium borohydride. This approach illustrates a new avenue of research for the galvanic replacement process and, in principle, could be applied to many more systems

Diffusion-driven formation of MoS2 nanotube bundles containing MoS2 nanopods

MoS2 nanotube bundles along with embedded nested fullerenes were formed in a gas phase reaction of molybdenum carbonyl and H2S gas with the assistance of I2. The amorphous Mo-S-I intermediates obtained through quenching a modified MOCVD reaction in a large temperature gradient were annealed at elevated temperature in an inert atmosphere. Under the influence of the iodine the amorphous precursor formed a surface film with an enhanced mobility of the molybdenum and sulfur components. Point defects within the MoS2 layers combined with the enhanced surface diffusion lead to a scrolling of the inherently instable MoS2 lamellae

Generation of catalytically active materials from a liquid metal precursor

A facile route to prepare catalystically active materials from a galinstan liquid metal alloy is introduced. Sonicating liquid galinstan in alkaline solution or treating it in reducing media results in the creation of solid In/Sn rich microspheres that show catalytic activity toward both potassium ferricyanide and 4-nitrophenol reduction

Investigation of room temperature gas sensing properties of metal-organic charge transfer complex CuTCNQF4

The ability to detect and monitor toxic and greenhouse gases is highly important, however to achieve this at room temperature and allow for remote sensing applications is a significant challenge. Here, we tackle this issue and investigate conductometric gas sensors that operate at room temperature using a microstructured CuTCNQF<small>₄</small> (TCNQF<small>₄</small> = 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) sensing layer formed via simply immersing an e-beam evaporated Cu film in an acetonitrile solution of TCNQF<small>₄</small>. These films were characterized by scanning electron microscopy, Fourier transform infra-red, Raman and X-ray photoelectron spectroscopy. The sensors show a significant response to NO<small>₂</small> and NH<small>₃</small> gases and a negligible response to ethanol at 25°C. Significantly the sensors were tolerant to humidity and a low detection limit of 100 ppb is achieved for NO<small>₂</small> gas