Surface water dependent properties of sulphur-rich molybdenum sulfides: electrolyteless gas phase water splitting

Sulfur-rich molybdenum sulfides are an emerging class of inorganic coordination polymers that are predominantly utilized for their superior catalytic properties. Here we investigate surface water dependent properties of sulfur-rich MoSx (x = 32/3) and its interaction with water vapor. We report that MoSx is a highly hygroscopic semiconductor, which can reversibly bind up to 0.9 H2O molecule per Mo. The presence of surface water is found to have a profound influence on the semiconductor's properties, modulating the material's photoluminescence by over 1 order of magnitude, in transition from dry to moist ambient. Furthermore, the conductivity of a MoSx-based moisture sensor is modulated in excess of 2 orders of magnitude for 30% increase in humidity. As the core application, we utilize the discovered properties of MoSx to develop an electrolyteless water splitting photocatalyst that relies entirely on the hygroscopic nature of MoSx as the water source. The catalyst is formulated as an ink that can be coated onto insulating substrates, such as glass, leading to efficient hydrogen and oxygen evolution from water vapor. The concept has the potential to be widely adopted for future solar fuel production

Microcavités à modes de galerie en polymère pour la détection de gaz

L’objectif principal de ce projet de maîtrise est de faire la preuve de concept d’un senseur de gaz à partir de cavités à modes de galerie en polymère. Il est considéré possible d’exploiter les propriétés absorbantes d’un polymère afin d’atteindre des sensibilités élevées pour un senseur optique microfabriqué. Le principe de détection utilisé avec ces microstructures se base sur le décalage des résonances causé par une variation du parcours optique dans la cavité. Cette variation peut être générée par la combinaison d’une variation de la l’indice de réfraction et d’une variation de la géométrie du résonateur. La configuration proposée est un microrésonateur en disque sur pilier de silicium de sorte que le polymère puisse se dilater plus facilement que dans les cavités Fabry-Pérot développées au laboratoire. Dans le cas des Fabry-Pérot, le polymère est restreint dans sa dilatation par les miroirs de silicium. Le SU-8 a été choisi comme polymère pour former le microdisque pour cette étude en raison de ses excellentes propriétés optiques et des récents travaux démontrant son utilisation pour la détection de gaz. De plus, puisqu’il s’agit d’une photorésine, le procédé de microfabrication est simplifié à une étape de photolithographie puis une gravure isotrope du silicium. Il a ainsi été possible de produire des microcavités en SU-8 pouvant atteindre un facteur de qualité de 1.58 × 105 pour une résonance se situant près de 1577nm. Les cavités ont été caractérisées pour déterminer les paramètres géométriques pouvant influencer la sensibilité à la concentration d’un gaz. Une étude effectuée sur des cavités avec des géométries différentes a permis de déterminer que le rapport de la sous-gravure du résonateur sur son rayon avait une influence sur sa sensibilité. La sensibilité en température et le temps de réponse ont également été étudiés. La sensibilité des cavités en SU-8 a, par la suite, été évaluée pour une variété de composés sous phase vapeur dont l’eau, l’isopropanol, le toluène, le limonène, le 1-butanol et l’acide pentanoïque. La limite de détection a également été calculée pour ces six composés. L’acide pentanoïque a ainsi une sensibilité d’environ 23pm/ppm et une limite de détection de 0.6 ppm comparé à l’isopropanol qui a une sensibilité de 0.07pm/ppm et une limite de détection de 64 ppm pour des modes de résonances se situant entre 1500 et 1600nm.----------Abstract The main objective of this Master’s project is to demonstrate the concept of a gas sensor using whispering gallery mode resonators in polymer. It is deemed possible to exploit the absorbent properties of a polymer in order to achieve high sensitivities for a microfabricated optical device. The detection principle used with these microstructures is based on the resonance shift caused by a variation of the optical path in the cavity. This variation can be generated by a combination of change in the refractive index and change in geometry of the resonator. The proposed configuration is a microdisk resonator on a silicon pillar. Here, the polymer can expand more easily than in Fabry-Perot microcavities previously developed in the laboratory. For these cavities, the polymer’s expansion is restricted by the silicon mirrors. The photoresist SU-8 was chosen as the polymer to form the microdisk for this study because of its excellent optical properties and recent work demonstrating its use in gas detection. Furthermore, considering the photoresist nature of SU-8, the microfabrication process is simplified at one single photolithography step and one isotropic etch of silicon. It was possible to produce microcavities in SU-8 with a quality factor up to 1.58 × 105 for a resonance near 1577nm. The cavities were characterized to determine the geometric parameters that could influence the sensitivity to the concentration of the analyte gas. The study of cavities with different geometries indicates that the ratio of the resonator undercut to its radius influenced its sensitivity. Temperature sensitivity and response time were also studied. The sensitivity of SU-8 cavities was subsequently assessed for a variety of vapour-phase compounds including water, isopropanol, toluene, limonene, 1-butanol and pentanoic acid. The detection limit was calculated for these six compounds. The sensitivity for pentanoic acid is approximately 23pm/ppm and the detection limit is 0.6 ppm, compared to isopropanol where corresponding values are 0.07pm/ppm and 64 ppm for resonance modes between 1500 and 1600nm

Role of Morphological Structure, Doping, and Coating of Different Materials in the Sensing Characteristics of Humidity Sensors

The humidity sensing characteristics of different sensing materials are important properties in order to monitor different products or events in a wide range of industrial sectors, research and development laboratories as well as daily life. The primary aim of this study is to compare the sensing characteristics, including impedance or resistance, capacitance, hysteresis, recovery and response times, and stability with respect to relative humidity, frequency, and temperature, of different materials. Various materials, including ceramics, semiconductors, and polymers, used for sensing relative humidity have been reviewed. Correlations of the different electrical characteristics of different doped sensor materials as the most unique feature of a material have been noted. The electrical properties of different sensor materials are found to change significantly with the morphological changes, doping concentration of different materials and film thickness of the substrate. Various applications and scopes are pointed out in the review article. We extensively reviewed almost all main kinds of relative humidity sensors and how their electrical characteristics vary with different doping concentrations, film thickness and basic sensing materials. Based on statistical tests, the zinc oxide-based sensing material is best for humidity sensor design since it shows extremely low hysteresis loss, minimum response and recovery times and excellent stability

Development, physicochemical characterization, and optimization of self-powered electrochemical humidity sensor based on thin-film aluminum

Циљ дисертације је развој новог типа сензора влажности ваздуха. Развијени сензор има структуру интердигиталног кондензатора, израђеног од алуминијума, димензија реда величине 1 mm2 . Сензор је израђен фотолитографским процесом. Приказан је нови начин за активирање алуминијума, применом једносмерне струје. Оптичком и микроскопијом атомских сила су одређене морфолошке карактеристике структуре. Скенирајућом електронском микроскопијом са енергијски дисперзивном спектроскопијом верификована је геометрија структуре и извршена је елементна анализа. Рендгеноструктурном анализом је утврђено да алуминијум има поликристалну структуру. Инфрацрвеном спектроскопијом са Фуријеовом трансформацијом је утврђено да се приликом реакције формирају хидроксиди алуминијума. Извршен је и прорачун енергије адсорпције воде на алуминијуму. Електрохемијском импедансном спектроскопијом је одређено еквивалентно коло сензора. Показано је да се рад сензора заснива на корозији алуминијума. Електричним мерењима одређена је временска резолуција одзива сензора (време одзива је 10 ms, време опоравка је 55 ms). Показано је да је сигнал пропорционалан локалној влажности, у опсегу од 30 % - 70 %, уз температурску зависност. Закључено је да је зависност напона од температуре последица промене адсорпционе равнотеже. Показано је да је мерење релативне влажности ваздуха највероватније омогућено спрезањем радиофреквентних и електрохемијских сигнала. Приказани су начини мерења са бољим односом сигнал шум. Практична примена сензора приказана је у експерименту у којем је коришћен као уређај за праћење дисања. Најзначајнији допринос дисертације је развој самонапајајућег сензора влаге, односно уређаја који функционише без спољашњег напајања. Резултат дисертације је иновативан уређај, први пут публикован на светском нивоу.The aim of this dissertation is the development of a new type of humidity sensor. The sensor is designed as an interdigitated capacitor, made out of aluminum, dimensions are of the order of 1 mm2 . It was fabricated using a photolithographic process. A novel procedure for aluminum activation by applying direct current is presented. Morphological characteristics of the structure were revealed by optical and atomic force microscopy. By employing scanning electron microscopy with energy dispersive microscopy, geometry of the structure was verified and elemental analysis was performed. X-ray diffraction showed that aluminum has a polycrystalline structure. Fourier transform Infrared spectroscopy showed that aluminum hydroxides are formed during reaction. Adsorption energy was also calculated. The equivalent circuit of the sensor was revealed by electrochemical impedance spectroscopy. It was shown that working principle is based on corrosion of aluminum. The time response of the sensor was obtained by electrical measurements (response and recovery time were found to be 10 ms and 55 ms, respectively). It was shown that the response is proportional to the local humidity, in the range of 30 % to 70 % with temperature dependence. This dependence is a consequence of changes in the adsorption equilibrium. It was concluded that humidity measurement was provided by coupling RF and electrochemical signals. Measurement configurations for signal with a higher signal-to-noise ratio were presented. Practical application of sensor was demonstrated through its usage as human breath detection device. The most significant contribution is the development of a full self-powered humidity sensor. The result of the dissertation is a completely innovative device, published for the first time worldwide

Structure-material property relationships and applications of low dimensional metal-chalcogenides nanostructures

The separation and optimization of graphene in 2004 by Geim and Novoselov boosted the interest in low dimensional materials within the world of electronics. One specific class of low dimensional materials is the metal chalcogenides (MCs) (i.e MoSx, Bi2S3, Bi2Se3), which has drawn notable attention owing to their tuneable exotic properties and natural abundance. This PhD thesis emphasises on several major features of low dimensional metal chalcogenides: their exfoliation techniques, growth behaviour and ultimately augmenting their fundamental properties. Due to the dimension dependent properties of metal chalcogenides, it is essential to attain full control over their lateral shape and thickness during the exfoliation process. Regardless of many rcent improvements, there are still plenty of opportunities to develop enhanced exfoliation processes, which is one of the primary focuses of this work. The main goals of this Ph.D. research are divided into two parts. First, developing a facile synthesis process for amorphous 1D inorganic polymeric materials such as amorphous molybdenum sulphide (a-MoSx) and investigate the fundamental properties of as synthesised nanostructures. The preparation route and synthesis of low dimensional materials play a significant role, that ultimately defines the material's properties. To date, many efforts have been invested in order to device an appropriate exfoliation procedure, resulting in the development of various techniques and processes which feature their respective benefits and flaws. In this work, a template acidification process was used to synthesize a-MoSx. The resulted synthesized nanostructures were found to be polymeric and one dimensional in nature. Sulphur rich molybdenum sulphides are an emerging class of inorganic coordination polymers which are predominantly utilised for their superior catalytic properties. Here the surface water dependent properties of sulphur rich a-MoSx and its interaction with water vapour were investigated. It has been observed that a-MoSx is a highly hygroscopic semiconductor. The presence of surface water is found to have profound influence on the semiconductor's properties, modulating the material's photoluminescence by over one order of magnitude, in transition from dry to moist ambient. Additionally, the conductivity of the synthesized film increases by several magnitudes as the surrounding humidity increases. As the core application, the newly discovered properties of a-MoSx has been utilized to develop an electrolyteless water splitting photocatalyst that relies entirely on the hygroscopic nature of MoSx as the water source, leading to efficient gas phase water splitting. Moreover, taking advantage of the hygroscopic nature of synthesized a-MoSx, a low energy dehumidification device has been designed that exhibits a-MoSx can be a prominent candidate for moisture sensing devices. The second goal of the work was to develop as facile synthesis process for crystalline low dimensional material such as Bi2S3 and Bi2Se3. Here the focus of this work was a family of crystals constituting covalently bound strings, held together by van der Waals forces, which can be exfoliated into smaller entities, similar to crystals made of van der Waals sheets. Depending on the anisotropy of such crystals, and the spacing between their strings in each direction, van der Waals sheets or ribbons can be obtained after the exfoliation process. In this work, it has been demonstrated that ultra-thin nanoribbons of bismuth sulfide (Bi2S3) can be synthesized via a high power sonication process. As stated earlier, preparation and exfoliation process of low dimensional materials are very crucial as the quality and individual properties of the nanostructure depends on it. Here, N-methyl-pyrrolidine was used as a solvent that was proven to be the most effective solvent for this process. The thickness and width of these ribbons are governed by the van der Waals spacings around the strings within the parent crystal. The lengths of the nanoribbons are initially limited by the dimensions of the starting bulk particles. Interestingly, these nanoribbons change stoichiometry, composition and are elongated when the duration of agitation increases, due to Ostwald ripening. An application of the exfoliated van der Waals strings is presented for optical biosensing using photoluminescence of Bi2S3 nanoribbons where bovine serum albumin has been used a model protein. The concept of exfoliating van der Waals strings could be extended to a large class of crystals for creating bodies ranging from sheets to strings, with optoelectronic properties different from that of their bulk counterparts. Finally, crystalline bismuth selenide (Bi2Se3) was exfoliated using liquid phase exfoliation technique. Generally, Bi2Se3 crystallizes in two polymorphs which are 1D van der Waals string (similar to Bi2S3) and another is two dimensional nanosheets. Depending upon the provided energy during the exfoliation process a phase transition from 2D to 1D might take place. In this work, the exfoliated films were found to be 2D nanosheets with various lateral dimensions. An expected opening in the bandgap due to quantum confinement effect has been observed. Moreover, a well-defined photoluminescence effect has been observed concluding that Bi2Se3 can be notable candidate for sensors and optoelectronic devices. In summary, the author has demonstrated several significant findings during this Ph.D. research, exploring facile synthesis processes while revealing some exotic properties of low dimensional materials. It is expected that the findings of this Ph.D. research will have an ongoing impact on future electronics and optoelectronics industries