Porous Zinc Oxide Thin Films: Synthesis Approaches and Applications

Zinc oxide (ZnO) thin films have been widely investigated due to their multifunctional properties, i.e., catalytic, semiconducting and optical. They have found practical use in a wide number of application fields. However, the presence of a compact micro/nanostructure has often limited the resulting material properties. Moreover, with the advent of low-dimensional ZnO nanostructures featuring unique physical and chemical properties, the interest in studying ZnO thin films diminished more and more. Therefore, the possibility to combine at the same time the advantages of thin-film based synthesis technologies togetherwith a high surface area and a porous structuremight represent a powerful solution to prepare ZnO thin films with unprecedented physical and chemical characteristics that may find use in novel application fields. Within this scope, this review offers an overview on the most successful synthesis methods that are able to produce ZnO thin films with both framework and textural porosities. Moreover, we discuss the related applications, mainly focused on photocatalytic degradation of dyes, gas sensor fabrication and photoanodes for dye-sensitized solar cells

Nanostructured Metal Oxide Semiconductors towards Greenhouse Gas Detection

Climate change and global warming are two huge current threats due to continuous anthropogenic emissions of greenhouse gases (GHGs) in the Earth’s atmosphere. Accurate measurements and reliable quantifications of GHG emissions in air are thus of primary importance to the study of climate change and for taking mitigation actions. Therefore, the detection of GHGs should be the first step when trying to reduce their concentration in the environment. Throughout recent decades, nanostructured metal oxide semiconductors have been found to be reliable and accurate for the detection of many different toxic gases in air. Thus, the aim of this article is to present a comprehensive review of the development of various metal oxide semiconductors, as well as to discuss their strong and weak points for GHG detection

Development of carbon nanostructures from non-conventional resources

Carbon nanostructures (CNSs) perpetuate the scientific interest over decades due to their remarkable properties and emerging technological applications. The development of sustainable technologies for the synthesis of CNSs from natural resources grabbed immense research attention aiming to implement these high-end materials in wide range of nano electronic devices through safe and environmentally friendly routes. Even though a number of top down and bottom up approaches have been developed for the production of CNSs, most of them either aided by catalysts or involved solvent assisted multi-step process that considerably increase the cost of production and hinders the realization of low cost CNSs based commercial devices. In addition, vast majority of these techniques use high pure petroleum derived hydrocarbon gas precursors that are non-renewable and expensive. Hence, it is imperative to develop scalable techniques that can derive high quality CNSs directly on arbitrary substrates from naturally derived carbon feed stocks. This work aims to develop an environmentally benign plasma enhanced chemical vapor deposition technique for fabricating CNSs from Citrus sinensis essential oil, a bio renewable precursor, and explored the potential of these nanostructures for gas sensing application. C. sinensis essential oil, obtained through cold extraction of orange peels is a rich source of non-synthetic hydrocarbon compounds principally limonene. Inherently volatile in nature, C. sinensis essential oil can serve as an ideal candidate material compatible to plasma enhanced chemical vapor deposition. This thesis investigated the fabrication of vertically-oriented graphene nanostructures from C.sinensis essential oil through radio frequency plasma enhanced chemical vapor deposition process, the fundamental properties, extend to which the process parameters influenced the structure and morphological features, and the suitability of the developed vertical graphene arrays for gas sensing applications. Special attention is paid to probe deep into the morphological evolution with the help of a set of advanced analytical characterization methods and multi-parameter model simulations. In the first phase, C.sinensis vapors were subjected to low RF power glow discharge that resulted in the formation of plasma polymer thin films, and the material properties were studied as a function of input RF energy. The fundamental properties of plasma polymer thin films fabricated at different RF power level (10−75 W) were characterized with variable angle spectroscopic ellipsometry, UV-visible spectroscopy, Fourier transform infrared spectroscopy X-ray photoelectron spectroscopy and atomic force microscopy. Optical characterization showed that independent of deposition power films exhibited good transparency (~90 %) in the visible region and a refractive index of 1.55 at 500nm. The optical band gap measured around 3.60 eV and falls within the insulating region. The atomic force microscopic (AFM) images revealed that the surface is pinhole-free and smooth at nanoscale, with average surface roughness dependent on the deposition power. Film hardness increased from 0.50 GPa to 0.78 GPa as applied power increased from 10 to 75 W. In the second phase, experiments were modified by redesigning the experimental set up in order to eliminate hydrogen from the deposits leaving only crystalline carbon. The RF power deliberately kept high, substrate temperature was raised and hydrogen gas fed into the reactor in controlled manner. A sequence of experiments were carried out by systematically changing the process parameters such as in put RF power (300-500W), hydrogen flow rate (10-50 sccm) and deposition duration (2-8 min) and analysed the structural and morphological evolution of the resulted vertical graphene nanostructure. The structure-property correlation of vertical graphene arrays with the plasma process parameters was performed. The Raman spectra ascertained the formation of less defected multilayered graphene nanostructures and scanning electron microscopic images provided the primary evidences of morphological evolution. The potential of the novel analytical techniques such as Hough transformations, fractal dimension distributions and Minkowski connectivity for the analysis of graphene array morphology was then successfully demonstrated. Worth noting that, these advanced techniques displayed significant changes and revealed the complex morphological transformation of C. sinensis derived vertical graphene subjected to change in process conditions. Precisely, vertical graphene nanowalls obtained at 300 and 500W presented a narrow height distribution profile but much wider array formed at 400 W. Fourier and Hough transformation spectra showed a prominent change with an increase in power, thus highlighted change in the morphology with the density of nanoflakes. Similarly, 2D FFT transform spectra of vertical graphene samples also presented notable changes with increasing hydrogen flux. The most narrow height distributions, well-shaped Hough transformation spectra and distribution of fractal dimensions obtained for structures formed at 20 and 50 sccm of hydrogen flow rate. In addition to this, the principal characteristics of thus fabricated vertical graphene such as flake length (Lvg) and flake half width (Wvg) are theoretically modelled by an ad hoc model based on a large number of interaction elemental processes and correlated with the experimental results. The combination of the experimental and simulation results ensured important insights and deeper understanding of the processes that govern formation of the vertical graphene morphology.Vertical graphene nanostructures having superior structural and morphological properties were successfully fabricated at an input RF energy of 500W, hydrogen flow rate of 30 sccm and deposition duration of 6 minutes. The third phase presented an in-depth study of the properties of C.sinensis oil derived graphene over a set of conducting (copper and nickel) and insulating substrates (silicon and quartz). The SEM images of thus fabricated graphene patterns showed the unique feature of vertically interconnected and non-agglomerated carbon nanowall structures having maze-like and petal-like networks. Moreover, the normalized height distribution function and 2-D FFT spectra analysis ascertained that vertical graphene formed on silicon substrates displayed the most uniform distribution. X-ray photoelectron spectroscopy spotted only the presence of carbon and the transmission electron microscopic studies revealed the formation of unique onion-like closed loops. The 3-D nanoporous structure of C.sinensis oil derived graphene showed high hydrophobicity and measured a water contact angle of 129°. The surface energy studies were performed using Neumann model, Owens-Wendt-Kaelble approach and van Oss- Chaudhury-Good relation and estimated within the range 35‒41 mJ/m². Finally, plasma reformed vertical graphene from C. sinensis was integrated into a sensor device prototype to evaluate the performance in gas sensing. The chemiresistive type sensor exhibited sensing activity towards acetone. In summary, this thesis has identified a viable renewable resource and successfully developed a process that transform them into vertical graphene nanostructures. Furthermore, the fabricated graphene was integrated to real world devices and evaluated the performance. The outcomes of this investigation add knowledge base to the state-of-the-art of green chemistry approach for the synthesis of vertical graphene carbon nanostructures

Sustainable synthesis of graphene-based materials and applications

Adeel Zafar manufactured graphene and graphene-composites at ambient conditions from plant extracts, and for the first time nitrogen-doped graphene-oxide using a single step is fabricated. He developed electrochemical sensors for the detection of various insecticides and pesticides. This work is significant to achieve sustainability in the field of nanotechnology

Palladium (II) Oxide Nanostructures as Promising Materials for Gas Sensors

One of the most important environment monitoring problems is the detection of oxidizing gases in the ambient air. Negative impact of noxious oxidizing gases (ozone and nitrogen oxides) on human health, sensitive vegetation, and ecosystems is very serious. For this reason, palladium (II) oxide nanostructures have been employed for oxidizing gas detection. Thin and ultrathin films of palladium (II) oxide were prepared by thermal oxidation at dry oxygen of previously formed pure palladium layers on polished poly-Al2O3, SiO2/Si (100), optical quality quartz, and amorphous carbon/KCl substrates. At ozone and nitrogen dioxide detection, PdO films prepared by oxidation at T = 870 K have demonstrated good values of sensitivity, signal stability, operation speed, and reproducibility of sensor response. In comparison with other materials, palladium (II) oxide thin and ultrathin films have some advantages at gas sensor fabrication. Firstly, for oxidizing gas detection, PdO films with p-type conductivity are more perspective than the material with n-type conductivity. Secondly, at ambient conditions, palladium (II) oxide is insoluble in water and does not react with it. These facts are favorable for the fabrication of gas detectors because they make possible to minimize the air humidity influence on PdO sensor response values. Thirdly, the synthesis procedure of PdO films is rather simple and is compatible with planar processes of microelectronic industry

Morphology Control of Tin Oxide Nanostructures and Sensing Performances for Acetylene Detection

Morphology Control plays an important role in gas sensing properties of metal oxide semiconductor based gas sensors. In this study, various morphologies of SnO2 nanostructures including nanobulks, nanospheres, nanorods, and nanowires were successfully synthesized via a simple hydrothermal method assisted with different surfactants. X-ray powder diffraction and scanning electron microscopy were employed to characterize the prepared products. Gas sensors were fabricated by screen-printing the as-prepared SnO2 nanostructures onto planar ceramic substrates. Moreover, their gas sensing properties were systematically investigated towards acetylene gas (C2H2), an important fault hydrocarbon dissolved in power transformer oil. Experiments indicate that the SnO2 nanowires based sensor exhibits excellent gas sensing properties, such as lower operating temperature, higher gas response, quicker response-recovery time and good stability than those of SnO2 nanobulks, nanospheres and nanorods. These results imply SnO2 nanowires a promising sensing morphology for C2H2 detection and provide us a feasible way to develop high-performance gas sensor by tailoring the microstructures and morphologies of the materials in further

Quantitative hydrogen and methane gas sensing via implementing AI based spectral analysis of plasma discharge

In this report we explore the feasibility of a quantitative gas detection system concept based on alternations in spectral emissions of a radio frequency power generated plasma in presence of a target gas. We then proceed with training a deep learning residual network computer vison model with the spectral data obtained from the plasma to be able to perform regressive calculation of the target gas content in the plasma. We explore this concept with hydrogen and methane gas present in the plasma at know quantities to evaluate the applicability of the concept as hydrogen or methane detection system. We will demonstrate that the system is well capable of quantitatively detecting either of the gases efficiently while it is challenging to estimate hydrogen content in presence of methane

Development of Green Synthetic Approaches for the Potential Application of Carbon and Semiconductor Nanomaterials for Emerging Applications

The increasing interest towards the synthesis and modification of different nanomaterials is attributed to their outstanding mechanical, physical and electrical properties that allow their use in different fields. In the last decades, novel nanomaterials have been successfully synthesized in order to provide materials with improved performances to be employed for water treatment, photocatalysis, to replace silicon–based devices in electronics and so on. For example, carbon-based materials are promising candidates for the fabrication of conductive inks and future non-volatile memory devices. However, the absence of an eco-sustainable, straightforward and time effective process for their production has hindered their large-scale application in electronics. The aim of this thesis is to explore alternative synthetic approaches for the synthesis of different materials and their structural modification in order to gain a better understanding how the processes could be controlled to have desired structure and hence materials with improved performances. In particular, laser ablation in liquids (PLA) and electrochemical processes will be the focus of this study. It has been shown that pulsed laser ablation of carbon materials and TiO2¬ nanoparticles can be used for the synthesis of new materials and/or modification of their structure. The laser ablation compared to other common synthetic approaches has many advantages. One of which is the eco-sustainability of the process, since the synthesis is performed in water without the use or production of products harmful for the environment. The second advantage is the versatility of the technique that allows the synthesis and modification of different nanomaterials depending on the target material employed. In this thesis it will be demonstrated that laser ablation of a dispersion of graphene oxide can be employed as a straightforward technique to induce structural modifications of the material, i.e. reduction of the graphene oxide sheets and synthesis of graphene quantum dots varying laser ablation time and ablation power. The nanomaterials obtained can be mixed with silver nanoparticles for the fabrication of hybrid conductive inks, which have a resistivity lower than inks made with only silver nanoparticles. The versatility of the laser ablation is demonstrated by extending the study to titanium dioxide powders. It will be discussed that the laser ablation of TiO2 nanoparticles leads to nanoparticles with different crystalline structures. Indeed, with a proper control over the laser ablation parameters, such as ablation time and laser power, it is possible to induce a phase transformation of TiO¬2 nanoparticles whether they are dispersed in water or deposited onto a substrate. Similar to the laser ablation, the electrochemical processes such as the electrophoretic deposition (EPD) allows the synthesis and deposition of different type of materials. In particular, in this thesis this technique will be employed for the straightforward synthesis of carbon nanowalls (CNWs). These carbon-based materials are usually synthesized by chemical vapor deposition, which requires the use of precursor gases and high temperatures and pressures. Whereas, the method developed during my research allows a time-effective synthesis of these nanomaterials; moreover, the deposition of the CNWs directly onto conductive substrate permits for the first time the fabrication of carbon-based resistive switching memory devices. This technique could be used for the development on a large scale of this type of devices, whose broad fabrication has been hindered due to the complex production mechanisms. Another advantage of the electrochemical processes is the possibility of modifying the chemical composition of the materials. In this thesis, the anodic oxidation has been used for the first time to oxidize the carbon structures obtained by EPD in order to engineer their electrical performances. In literature, the anodic oxidation has been used to study the redox processes in electronic devices or to increase the electrochemical capacitance of carbon materials, but never as a specific technique to tailor the materials properties. As aforementioned EPD, like PLA, is a versatile technique and in this study it has been used for the growth of ZnO rods. ZnO rods are usually grown by hydrothermal processes, which can be time consuming. In this thesis, the growth of the rods has been conducted directly on conductive substrates, which were then patterned for the fabrication of electronic devices

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

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

Synthesis Of Graphene Nanomaterials And Their Application In Electrochemical Energy Storage

The need to store and use energy on diverse scales in a modern technological society necessitates the design of large and small energy systems, among which electrical energy storage systems such as batteries and capacitors have attracted much interest in the past several decades. Supercapacitors, also known as ultracapacitors, or electrochemical capacitors, with fast power delivery and long cycle life are complementing or even replacing batteries in many applications. The rapid development of miniaturized electronic devices has led to a growing need for rechargeable micro-power sources with high performance. Among different sources, electrochemical micro-capacitors or micro-supercapacitors provide higher power density than their counterparts and are gaining increased interest from the research and engineering communities. Rechargeable Li ion batteries with high energy and power density, long cycling life, high charge-discharge rate (1C - 3C) and safe operation are in high demand as power sources and power backup for hybrid electric vehicles and other applications. In the present work, graphene-based graphene materials have been designed and synthesized for electrochemical energy storage applications, e.g., conventional supercapacitors (macro-supercapacitors), microsupercapacitors and lithium ion batteries. Factors influencing the formation and structure of graphitic petals grown by microwave plasma-enhanced chemical vapor deposition on oxidized silicon substrates were investigated through process variation and materials analysis. Insights gained into the growth mechanism of these graphitic petals suggest a simple scribing method can be used to control both the location and formation of petals on flat Si substrates. Transitional metal oxides and conducting polymers have been coated on the graphitic petal-based electrodes by facile chemical methods for multifunctional energy storage applications. Detailed electrochemical characterization (e.g., cyclic voltammetry and constant galvanostatic charge/discharge) has been carried out to evaluate the performance of electrodes