18 research outputs found

    Wearable Nano-Based Gas Sensors for Environmental Monitoring and Encountered Challenges in Optimization

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    With a rising emphasis on public safety and quality of life, there is an urgent need to ensure optimal air quality, both indoors and outdoors. Detecting toxic gaseous compounds plays a pivotal role in shaping our sustainable future. This review aims to elucidate the advancements in smart wearable (nano)sensors for monitoring harmful gaseous pollutants, such as ammonia (NH3), nitric oxide (NO), nitrous oxide (N2O), nitrogen dioxide (NO2), carbon monoxide (CO), carbon dioxide (CO2), hydrogen sulfide (H2S), sulfur dioxide (SO2), ozone (O3), hydrocarbons (CxHy), and hydrogen fluoride (HF). Differentiating this review from its predecessors, we shed light on the challenges faced in enhancing sensor performance and offer a deep dive into the evolution of sensing materials, wearable substrates, electrodes, and types of sensors. Noteworthy materials for robust detection systems encompass 2D nanostructures, carbon nanomaterials, conducting polymers, nanohybrids, and metal oxide semiconductors. A dedicated section dissects the significance of circuit integration, miniaturization, real-time sensing, repeatability, reusability, power efficiency, gas-sensitive material deposition, selectivity, sensitivity, stability, and response/recovery time, pinpointing gaps in the current knowledge and offering avenues for further research. To conclude, we provide insights and suggestions for the prospective trajectory of smart wearable nanosensors in addressing the extant challenges

    Innovative ozone sensors for environmental monitoring working at low temperature

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    L'abstract è presente nell'allegato / the abstract is in the attachmen

    Synthesis, structural transformation and surface properties of two dimensional metal oxides and their compounds

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    This PhD project explores the synthesis and fundamental properties of two dimensional (2D) metal oxides, and investigates their feasibilities in applications such as catalysis and electronics. The field of 2D materials research is mostly focused on layered crystals, out of which ultra-thin planes (few monolayers) can be readily extracted. The high surface area of 2D nanosheets allows for efficient interaction with the molecular entities in the vicinity of the oxide, which can be bound to or electrostatically adsorb onto the basal surface. In the first part of this PhD, layered α-MoO3 is exfoliated into 2D nanosheets forming defect-rich 2D α-MoO3-x. The synthesized compound is found to display superior activity for the electrocatalytic hydrogen evolution reaction (HER) with a low overpotential and fast electron transfer for hydrogen production. The combination of oxygen deficient structure and large surface area of the 2D nanosheets with structural defects and steps played as key factors to achieve high HER activity. Thus far, the field of 2D materials research is predominantly focused on naturally stratified or layered materials due to the ease of their exfoliation. However, there are many important types of metal oxide compounds, which do not occur in the form of natural layered crystals, and hence cannot be readily transformed into 2D nanosheets. If these crystals are synthesized in 2D morphologies, they can offer the required large surface-area for various catalytic reactions. In the second part of this PhD, a two-step synthesis approach is explored to synthesize 2D nanosheets of non-layered crystals. First, layered α-MoO3 is exfoliated into 2D α-MoO3-x. Subsequently, these defect rich 2D α-MoO3-x nanosheets are transformed into stable 2D PbMoO4 nanosheets using a solution phase topotactic reaction. The transformed 2D PbMoO4 nanosheets display trap states within their bandgap, enabling their efficient performance as a photocatalyst under visible light irradiation. The transformed 2D nanosheets act as an excellent photocatalyst to degrade the organic pollutants in water. The presented method in this work can be likely extended to establish a variety of highly stable defect rich 2D metal molybdates for potential applications in visible light based photocatalysis, which are otherwise challenging to synthesize. Finally, synthesis of a non-layered transparent conductive oxide (TCOs) as 2D nanosheets is also explored using a liquid metal reaction approach. This highly scalable technique produces ultra-large 2D indium tin oxide (ITO) nanosheets, which can potentially be utilized in the future industrial production of TCOs. ITO is a widely used transparent conductor which finds application in every-day electronics such as touch screens and flat panel displays. One key limitation of ITO is the brittle nature of this ceramic, prohibiting its use in flexible electronics. Additionally, the commercial deposition of this material mostly relies on vacuum based techniques. Herein, the wafer scale printing synthesis of highly flexible 2D ITO nanosheets with a typical thickness of 1 nm is presented. A low temperature liquid metal printing approach is utilized to directly deposit 2D ITO onto a variety of substrates including polymers. The final nanosheets feature two orders of magnitude lower light absorption when compared with graphene, while maintaining suitable electrical conductance for device fabrication. The developed technique will enable low cost, printable and flexible optoelectronics, while also providing an alternative to graphene with superior transparency for the creation of van der Waals heterostructures

    Functionalization of two-dimensional transition metal oxides for the sensing applications

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    Preparation and application of 0D-2D nanomaterial hybrid heterostructures for energy applications

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    As research efforts into the two-dimensional (2D) materials continue to mature, finding applications in which they can be productively used has become of greater interest. Applications in the energy sector are of particular importance, with the pressing need to decarbonize our economy and to live more sustainably. For a material to be optimal for an application we typically tailor their specific properties and characteristics to best fit with the desired parameters. In the past, this has included the forging of metal alloys or the doping of semiconductors, allowing us to controllably adjust the material properties. For two-dimensional materials, one of the best routes for such controlled manipulation is via forming a heterostructure, or hybrid, with another nanomaterial. In this review, we will explore the emergence of 0D-2D hybrid materials, where a 2D layered material is combined with a zero-dimensional (0D) nanoparticle or fullerene to adjust and enhance overall performance. We will cover the basics of their structure and modes of interaction, the different synthetic methods used for their assembly and preparation and review several energy applications in which promising results have already been achieved

    Indium-free transparent conductive oxides for improved solar cell performance and reliability

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    The rising adoption of solar cells worldwide necessitates reducing solar cell costs, enhancing cell efficiency, and long-term module reliability. New solar cell architectures such as silicon heterojunction (HJT) and thin-film technology like an organic solar cell, perovskite, III-V, copper indium gallium selenide (CIGS), etc. are actively being investigated. The majority of them implement a transparent conductive oxide (TCO), predominantly indium tin oxide (ITO), in their device structure. With the rising cost and depleting indium reserves, it is essential to find alternatives. This thesis focuses on developing and analysing indium-free TCOs fabricated using atomic layer deposition (ALD) and explores various applications of TCOs for solar cells. It begins with a detailed examination of the evolving relevant literature, followed by a detailed description of the techniques used throughout the thesis. ALD grown ZnO based TCO is studied. Firstly, a DFT analysis of various dopants of ZnO is presented. Subsequently, Zr doped ZnO is fabricated, characterized, and implemented as an electron selective layer for organic photovoltaic cells (OPV). The introduction of Zr as a dopant increased electron mobility and a reduction of sheet resistance. This was translated into an OPV device which demonstrated an increase in 1% abs efficiency due to increased carrier collection. Graphene is a promising TCO due to its high conductivity and transparency. Unfortunately, the transfer process hinders its implementation in a solar cell as a top or bottom contact. In this work, the first transfer-free method was developed by growing graphene directly onto an ALD-grown functional layer (NiOx). It will be shown that the NiOx layer gets partially reduced to Ni by carbon, which subsequently catalysis the graphene growth. Potential induced degradation (PID) has once again become a major reliability issue for solar manufacturers. But the determination and identification of PID before module fabrication is still a challenge. This work presents a novel method to provide accelerated lamination free PID testing at a solar cell level. This method was validated by implementing it on cells from different manufacturers. In addition, this novel method was used to test the effectiveness of ALD films, mainly ZnO and Al-doped ZnO (AZO), to prevent PID at a solar cell device level. It will be shown that the addition of a 5 nm TCO thin film can prevent the Na diffusion into the solar cell, thus protecting the cell from PID. Finally, the techno-economic analysis of ALD TCOs for solar cells is presented. It was shown that with the current tools in the market and indium costs, ALD based TCOs are an economical alternative. Furthermore, a Levelized costs of electricity (LCOE) study on the ALD capping layer demonstrated a >1% improvement in LCOE, suggesting that PID prevention using ALD capping layer is technologically and economically advantageous

    Green synthetic fuels: Renewable routes for the conversion of non-fossil feedstocks into gaseous fuels and their end uses

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    Innovative renewable routes are potentially able to sustain the transition to a decarbonized energy economy. Green synthetic fuels, including hydrogen and natural gas, are considered viable alternatives to fossil fuels. Indeed, they play a fundamental role in those sectors that are di cult to electrify (e.g., road mobility or high-heat industrial processes), are capable of mitigating problems related to flexibility and instantaneous balance of the electric grid, are suitable for large-size and long-term storage and can be transported through the gas network. This article is an overview of the overall supply chain, including production, transport, storage and end uses. Available fuel conversion technologies use renewable energy for the catalytic conversion of non-fossil feedstocks into hydrogen and syngas. We will show how relevant technologies involve thermochemical, electrochemical and photochemical processes. The syngas quality can be improved by catalytic CO and CO2 methanation reactions for the generation of synthetic natural gas. Finally, the produced gaseous fuels could follow several pathways for transport and lead to different final uses. Therefore, storage alternatives and gas interchangeability requirements for the safe injection of green fuels in the natural gas network and fuel cells are outlined. Nevertheless, the effects of gas quality on combustion emissions and safety are considered

    Stable and efficient photoelectrodes for solar fuels production

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    [eng] The excessive consumption of non-renewable energy sources such as fossil fuels has lead the world to a global climate change, urging for new energy consumption habits together with developing cost- effective alternative renewable technologies. Photoelectrochemical (PEC) water splitting allows for direct conversion of solar light and water into hydrogen and oxygen, storing energy into chemical bonds, solving the storage problem of photovoltaic technology. It has demonstrated to produce pure hydrogen and oxygen in significant efficiencies, although this technology is not ready for market implementation due to lack of efficient, stable and scalable photoelectrodes. In this work, we undertake a journey from improving the efficiency of stable metal-oxide-based photoanodes to stabilizing efficient photovoltaic materials by the introduction of protective, transparent, conductive and catalytic layers. Efforts have focused on using cost-effective and scalable materials and techniques. Metal oxide candidate TiO2 is reported stable in alkaline electrolytes and at anodic potentials, but they present low photon to current conversion efficiencies. This is due to excessively large band gap, absorbing small part of the visible spectra, and small electron and hole mobility. Its efficiency is increased both by microstructuring the substrate and nanostructuring the thin film into nanorods, and by modifying the electronic structure with a reductive H2 treatment, enhancing potential drop inside the nanorods. The strategy is shifted into stabilizing highly efficient short band gap semiconductor materials used by the photovoltaic industry. Silicon based photocathodes are protected from acidic electrolyte corrosion by TiO2 overlayers grown by atomic layer deposition (ALD). Temperature is found to play a key role for both efficient film conductivity and stability, being this caused by polycrystalline films formation. ALD enabled high thickness control and pinhole-free layers, together with lower crystallization temperatures than other techniques. Copper-indium-gallium-selenide (CIGS) solar cells fabricated on flexible stainless steel substrates are also protected from corrosion by TiO2 ALD protective layers. The transparent conductive oxide (TCO) already used in solar cells is found necessary for efficient p-n junction formation and charge transport to the hydrogen evolution reaction. Copper-zinc-tin- sulfide/selenide (CZTS/Se) solar cells, where scarce indium and gallium are substituted by tin and zinc, are implemented for PEC devices with TiO2 overlayers too. By modifying the S/Se ratio, band gap can be tuned, an especially interesting characteristic to design tandem PEC devices. ALD deposited protective layers are also studied in anodic polarizations and alkaline electrolytes. By varying the deposition temperature of TiO2, completely amorphous, mixed amorphous and crystalline and fully crystalline films are deposited, and a clear conductivity increase is observed correlated to crystallization. Preferential conductivity paths are observed inside crystalline grains, proposed to be related to crystalline defects and grain boundaries. Few hundred hours stability tests reveals significant photocurrent decrease, with no observed dissolution of the Si photoabsorber. This is attributed to oxidative potentials and electrolyte hydroxides diminishing the n-type semiconductor behavior of TiO2 and forming a barrier to charge injection into the oxygen evolution reaction. UV superimposed illumination partially recovered conductivity. NiO films are ALD-deposited on Si photoanodes and conductivity is found to decrease when temperature is increased from 100 to 300 ºC, simultaneous to a change in preferential crystal growth direction. Higher stoichiometric film, being formed when increasing temperature, decreases Ni2+ vacancies, responsible of the p-type semiconductor behavior. Impressive 1000 hours stability measurements are obtained. Although, this is only attained under periodic cyclic voltammetries, avoiding partial deactivation of the photoanodes. This is attributed to chemical modifications at the surface in such highly oxidative conditions
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