Chemically Deposited Nanocrystalline Metal Oxide Thin Films

Mixed oxides are gathering a lot of interest in research for different applications due to their promising properties, which arise due to the synergy effects when two or more than two oxides combine. This chapter provides a comprehensive study of the synthesis of mixed oxides using solution-processed approaches. The mixed oxide formation using solution-processed methods such as electrodeposition, precipitation, successive ionic layer adsorption and reaction, sol-gel, and chemical bath deposition is explored in this chapter. A comprehensive overview of mixed oxide synthesis using these methods, structural characterization, and application is provided to understand the synthesis approaches and progress in this field.

Thin films are of growing interest in vast technological fields, and particularly in nanotechnologies, because of their often more interesting physicochemical properties. Among these thin layers, the metal oxides receive a lot of attention because of their various applications, thanks to the existence of their dual property, electrical conductivity, and transparency in the visible, making them ideal candidates for optoelectronic applications, in photovoltaic cells or even in electrochromic windows. In this book chapter, we aim to provide an overview of the most widely used oxide-based thin films produced by electrochemical methods. We also provide the various applications of these thin films.

Recently, metal oxides have generated substantial interest due to their varied electronic and structural properties. These materials are potentially suitable for a wider application range. These applications highly depend on the morphology, physical properties, structure, impurity added, defects, and atomic structure. In this chapter, structural and electronic properties of various metal oxides such as titanium dioxide (TiO2), indium oxide (In2O3), tin (IV) oxide (SnO2), zinc oxide (ZnO), copper (I) oxide (Cu2O), copper (II) oxide (CuO), and copper dioxide (CuO2), etc. have been discussed for their better understanding.

This chapter elucidates the extant research on metal oxides carried out using density functional theory (DFT) simulations. Metal oxides include a wide range of materials ranging from metals, insulators and semiconductors. The complex nature of the metal-oxygen bond manipulates the properties to be used in applications in dielectrics, magnetism, ferroelectricity, superconductivity, light emission, optical spectroscopy, etc. to name a few. DFT simulations were successfully used to calculate the geometry, bonding energies, dipole moments as well as electronic structure of these materials. However, the presence of localized d or f electrons in metal oxides often renders the accurate prediction of electronic structure a difficult task using standard DFT methods. This problem has been overcome by methods involving Hubbard U parameters, hybrid functionals or many-body methods. Nevertheless, DFT methods are the most successful theoretical tools to model and predict material properties, including that of metal oxides. In this chapter, DFT calculations of the perovskite material, BaTiO3, have been taken as a candidate metal oxide and elaborated its structural and electronic properties, taking into account of doping and intrinsic defects, focusing on its photovoltaic applications.

Metal oxides have been greatly utilized as an active material for photovoltaic application due to their abundance in nature, low cost, optical features, electrical conductivity, and high photo conversion ability in photovoltaic technology. The efficiency of photovoltaic cells is determined by the effective ability of the carrier transport layers. In this chapter, we take an overview of the active metal oxides for photovoltaic systems. We start the chapter with a description of solar cell generation and parameters. Then, we proceeded to discuss the synthesis techniques and the types of metal oxides in which we emphasized more on the transport system of the charge carrier. In this regard, the electron and hole transport as well as the corelationship with HOMO and LUMO of metal oxide-based organic and hybrid cells are also discussed. Charge carrier generation is lower in organic metal oxides than their inorganic counterpart that encounters the problem of charge recombination. Solutions toward this recombination of charges and other associated setbacks on achieving high efficiency will be discussed as well.

Metal oxides are crystalline solids that comprise of a metal cation and an oxide anion held together by electrostatic force. The sustained interest in metal oxides and metal oxide-based heterojunction materials is because of their high carrier mobility, optical transparency and tolerance to mechanical stress, and relatively cheap. Metal oxide nanomaterials have properly defined crystal structures, are nonpoisonous, great stability, and surface properties that make them efficient semiconductors. Engineering their bandgaps through composite and/or heterojunction formation or doping improves its physical and electronic properties as well as improved light absorption in the visible region of the electromagnetic spectrum, which creates more opportunities for diverse application areas. As an example, the transition metal oxides/Si heterojunctions have been studied over the years due to the efficient charge transport that occurs at their interface. Silicon heterojunction solar cells are becoming an interesting area of research that substituted the p-n structure. The contacts at the heterojunctions serve as semipermeable membranes that selectively allow one carrier to permeate through while blocking the other carrier. This chapter discusses the general structural and electronic properties of metal oxides; and some specific properties of various transition metal oxides like titanium dioxide, nickel oxide, manganese oxide, cerium oxide, cobalt oxide, and molybdenum oxide; that find useful application in solar cells. These metal oxides can be synthesized via several techniques and have unique features that make them distinct and useful in solar cell applications. The features, characterizations, obtained results, diverse application areas, and performance improvement methods of metal oxides have been discussed in this chapter.

The rising consumption of nonrenewable energy sources has posed a serious threat to the global environment and human safety. Alternative energy sources have been focused upon to tackle these problems for reliably satisfying the current energy demands. In recent years, the photovoltaic industry has emerged as a promising alternative source as it is based on renewable energy of the sun that is abundant, clean, and inexhaustible. The photovoltaic industry is primarily based on the use of solar cells that converts the incident photon energy into electricity using optically active semiconducting materials. The optically active materials form the backbone of solar cells as they accomplish the desired energy conversion activity. Out of the commonly used optical materials (sulfides, selenides, tellurides, and oxides), metal oxides have garnered significant research attention due to their unique optical, electrical, chemical, and structural properties. These unique properties have enabled metal oxides to demonstrate versatile function in various types of solar cells (first-, second-, and third-generation). The metal oxides have efficiently demonstrated their performance as back contacts, absorbers, buffers layers, and transparent conductive oxide (TCO) layers. Furthermore, by strictly adhering to the photovoltaic objectives, that is, developing high-efficiency solar cells at a lower cost, the metal oxides are found to be the most suitable materials as they are cheap, possess easy processability, and exhibit high stability as compared to other optical materials. Hence, in the current chapter, a brief overview of the metal oxides in solar cells along with their versatile functions have been discussed. The key factors influencing the properties/performance of metal oxides along with their associated issues/solutions and potential future are highlighted.

The photovoltaic (PV) industry is attracting a lot of attention from researchers and industry alike. The conventional sources of energy pose a threat to the environment and are depleting at a rapid pace. Sooner or later, these sources need to be replaced with dependable and cleaner sources. The quest to develop solar cells with high efficiency and low cost is afoot. The first-generation solar cells achieved high efficiency but at a high manufacturing cost. The -generation solar cells had phenomenal success in balancing performance and cost but employed toxic materials. Third-generation solar cells such as perovskite solar cells have not only achieved high efficiency but have also overcome the shortcomings of previous generations. Perovskite solar cells (PSCs) consist of light-absorbing organic metal halides sandwiched between charge transport layers. The function of transport layers is to separate and transport the charges effectively. Metal oxides, due to their high chemical stability, easy processability and low cost, are most suitable as transport layers. Depending upon nature and band positions, the metal oxides can be used either as an electron or as a hole transport layer. Transport layers have a profound effect not only on the performance of the cell but also on the overall stability. In this chapter, we focus on the role of metal oxides in the fabrication and performance of PSCs. The different requirements and parameters essential for a metal oxide have been highlighted.

This chapter has taken a critical review of doped metal oxide thin films with specific emphasis on ZnO. A general overview of ZnO is concisely presented with highlights of its advantages and drawbacks. The basic drawbacks were identified as low electrical conductivity and low spectra absorption due to wide bandgap of ZnO. Doping with Al and In and also dye-sensitization of the photoelectrode were proposed as key techniques to enhance the electrical conductivity and the spectra absorption of the ZnO thin films. Chemical synthesis, characterization, and application of AZO and IZO in DSSC and non-dye–loaded PEC solar cells were reviewed. Two works of Tyona et al. were used to illustrate the chemical synthesis, characterization, and application of AZO and IZO electrodes in DSSC and non-dye–loaded PEC solar cells. The results of their study revealed that there was an effective modification of the bandgap of ZnO upon doping with Al and In, respectively, which gave rise to enhanced electrical conductivity due to increase in charge carrier concentration in the conduction band. Absorption edges were red-shifted up to 540 nm into the visible spectrum, which confirmed improvement in spectra absorption. The study also revealed that with the sensitization of the AZO photoelectrodes with rhodamine 6G dye, the absorption band edges were further red-shifted (650 nm) into the visible spectrum and that confirmed further improvement in spectra absorption of ZnO thin film electrodes. Structural, morphological, and other properties of ZnO such as film thickness and surface wettability were seen to improve upon doping. Analysis of the photovoltaic (PV) activities revealed an optimal photoelectric efficiency of 0.89% for the DSSC of AZO and 2.85% for the IZO electrode. These efficiencies were remarkably higher than the un-doped and non-dye–loaded electrodes. This clearly illustrates the positive impact of doping and dye-sensitization on ZnO.

Solar energy is energy from the sun and its provision is in abundance without payments. It is renewable and more promising than its counterpart energy source called fossil fuels. Fossil fuels have energy crises ranging from inadequacy to depletion, pollution, etc. This solar energy could be technically collected and utilized but there could still be an improved method of collecting solar energy for more advanced utilization called solar energy harvesting. Achieving this by the process of doping metal oxide thin films with impurities like carbon derivatives, organic synthetic dyes, etc., will harness dopant characteristics for optimal performance. The doped materials help in controlling the composition and structure of dopants, which enhance their performance. The metal oxide semiconductor thin films are synthesized via varieties of processes on working active layer materials with stable interfaces for solar energy conversion and versatile applications in several areas valuable for humankind.

Photoelectrochemical (PEC) water splitting is a process of separating water into clean hydrogen and oxygen gases using photocatalysts. The major challenge in PEC water splitting is the development of photocatalysts with appropriate properties for efficient and cost-effective practical production of hydrogen. The use of nanocrystalline metal oxides as photocatalysts is one of the promising routes of producing hydrogen fuel efficiently and cheaply. However, the transition metal oxides (TMOs) used today still have tremendous limitations of high cost, low visible light harvesting capacities, inefficient charge separation and transportation, and poor stability in water. The solution to these limitations is being sought by energy materials researchers by exploiting a variety of uncommon properties of TMOs through synthesis and combination of TMOs to form various mixed transition metal oxides (MTMOs) thin films and MTMOs architectures. Thus, this work focused on the review of various TMOs and MTMOs and the strategies attempted to improve the efficiency of two basic reactions of evolution hydrogen and oxygen evolution in PEC water splitting. The major outcomes of this survey are: (a) major research efforts in this direction focused on adjusting the chemical composition and modifying morphologies of the materials using nanotechnology processing routes that led to the creation of thin thicknesses, high surface areas and active sites, and enhanced electrocatalytic activities of the metal oxides; (b) despite advances in finding a cheaper and more efficient alternative to Pt cathode, the electrocatalytic activity of TMOs is still much lower to the state-of-the-art Pt cathode, and therefore simple and effective strategies are still required to develop effective metal oxide-based photocathode for hydrogen production.

Sunlight harvesting for energy generation and environmental remediation is an evolving research area which potentially opens a propitious avenue and beneficial to tackle energy and the environment issues. Conversion of sunlight into hydrogen or electricity through photoelectrochemical (PEC) cell is one of the most auspicious approaches for a feasible energy supply in which extensive development of photoelectrode is the key. Last few years witnessed the outstanding PEC performance flourished due to incorporating complexity in ZnO nanostructures and decorating them by metal nanoparticles. This chapter gives an overview of the current state of metal nanoparticles decorated ZnO nanostructures for PEC application. We discuss worthwhile role of ZnO and its key properties such as crystal structure, morphologies, and band potential beneficial from PEC viewpoint. A brief discussion on the basics of localized surface plasmon resonance (LSPR) shown by plasmonic metal nanoparticles followed by its capability towards increment in the PEC performance is displayed. Hybrids of metal nanoparticles decorated on ZnO have been utilized as photoelectrodes because of amazing features like enhanced visible light harvesting due to LSPR, higher conductivity, quicker charge transfer rate, increased photogenerated charge carrier separation, supporting band bending mechanism leading to long stability and high efficiency of PEC cell. All such features are explored in a systematic manner. A comparative section is dedicated to doped and decorated photoelectrodes for PEC studies. Rather being descriptive or thoroughgoing, this chapter talks about representative examples of novel and recent ideas to amplify PEC performance. Finally, conclusion point outs current challenges and gives an outlook.

Generation of controlled intrinsic defects such as oxygen deficiencies is the best method for modulating optical as well as electronic properties of metal oxide semiconductor nanostructures. The present chapter describes recent studies about different approaches for introducing oxygen vacancies in the chemically prepared metal oxide thin films and their useful contribution in the photocatalytic applications. Semiconductor metal oxide thin films with oxygen vacancies such as TiO2 and WO3 exhibit substantially improved photocatalytic activities in water splitting, CO2 photoreduction, and photodegradation of organic pollutants. The increased photo-activities are reflected by increased intrinsic properties by the production of oxygen deficiencies as shallow donor sites in the forbidden band gap of metal oxide semiconductors. Also, the discussion is extended to the current progress on the preparation of different metal oxides with oxygen deficiencies and their role for the improvement of photocatalytic activities of metal oxides.

Iron oxide photocatalysts are applicable to produce clean renewable energy resources. Iron oxide structures turn out as one of the promising candidates after the formation of oxygen vacancies on the surface and in bulk structures. Oxygen-deficient iron oxide nanostructures can solve global issues such as environmental pollution and energy supply. We have summarized oxygen-deficient α-Fe2O3 and Fe3O4 nanostructures, their preparation methods and properties on the basis of dimensions and different photocatalytic applications such as photoelectrochemical water splitting for hydrogen production, photocatalytic degradation and atmospheric CO2 reduction. In the end, we have highlighted the challenges and opportunities to provide perspective for different problems associated with α-Fe2O3 and Fe3O4 nanostructures.

The hydrothermal method is used to develop rutile-phase TiO2 rods on F/SnO2 coated glass substrates. XRD and TEM analysis reveals that each of the rod is a bundle made of crystalline TiO2 prismatic nanorods; and each of the nanorods has approximately 4 nm in width. The analysis of XRD results establishes that the lattice parameters of the rutile-phase TiO2 prepared for 20 h are a = b = 0.4620 nm and c = 0.2959 nm. The morphology of each bundle is found to be of tetragonal shape. Furthermore, these tetragonal bundles of TiO2 are highly oriented with respect to the substrate surface, when prepared using an optimised growth time of 20 h. It is also observed that the bundles of TiO2 nanorods act as single entities from the perspective of Raman scattering. Moreover, as the preferred orientation of the bundles perpendicular to the substrate surface improves with growth time, an unusual increase in the room and low (77 K) temperature Eg/A1g Raman band intensity ratios is observed. The low-temperature Raman peak position and peak width data are interpreted as supporting the hypothesis that the bundles of nanorods act as single entities from the point of view of the lattice phonons. Phonon symmetries and frequencies (in cm−1) of these bundles of rutile-phase TiO2 are measured at points Γ, X, M and Z Brillouin zone at room temperature and 77 K and were found to be consistent with rutile-phase TiO2 phonon symmetries and frequencies.

Performance of emergent energy storage devices like supercapacitor or batteries is based on morphology-dependent electrochemical properties of the electrode material. Hence, research interest toward spinel structure family of mixed transition metal oxides (MTMOs) is felicitous to achieve enhanced energy storage capacity. In general, transition metal oxides have unique morphologies that influence its use in various energy applications. Moreover, MTMOs show specific morphologies such as nanoplatelets, spherical nanoparticles, microspheres, nanorods, and tetragonal or hexagonal nanosheets. MTMOs are promising electrode materials because of their multiple oxidation states. Hybridization of MTMOs with graphene improves electrical conductivity and enhances the specific surface area for the faradaic redox reaction. In this chapter, MTMOs are described as active electrode materials for energy storage devices like supercapacitor and lithium-ion battery with a comprehensive study on their chemical synthesis approach.

Among many types of nanodimensional materials, 2D inorganic nanosheets (INs) derived from their bulk crystals by the protocol of exfoliation process can provide exceptional advantages in designing and developing novel type of electrode materials for energy storage applications. 2D INs can be secured as a macromolecular building blocks for the hybridization with variety of guest species like inorganic, organic, biological, and polymeric nanostructures. 2D INs and 2D-derived nanohybrids possess unique characteristics of expanded topographical area, tunable electronic structure, diverse chemical composition, and short ion diffusion path. These unique characteristics make them highly promising electrode materials for energy storage applications. Despite such exceptional features, the use of INs and IN-based hybrid electrodes for energy storage are in nascent stage. The present chapter deals with the properties, synthetic strategies, and wide spectrum of energy storage examples of supercapacitors and rechargeable batteries. Future prospective for the exploration of 2D INs and IN-based hybrids for energy storage applications of supercapacitors and rechargeable batteries is provided for the development of advanced energy storage systems using INs.

Carbon derivatives like graphene oxide (GO), reduced graphene oxide (rGO), carbon nanotubes (CNTs) and several other derived materials have arose as favourable solution in enhancing the challenges facing renewable energy transformation and storing devices. The problems they had to solve are due to large specific surface area (SSA), great chemical stability, high electrical conductivity as well as extraordinary mechanical flexibility and strength. This chapter is an assemblage of some properties of carbon derivatives and metal oxide composites for enhancement of energy storage devices (batteries and supercapacitors). This chapter will explicitly study the role of carbon derivatives in upgrading the cycle stability, life span, storage capacity and non-toxic nature of electrodes for energy storage device applications. This study will evaluate the easiest and cheapest technique of fabrication of affordable, portable and available electrode materials for these energy storage devices based on carbon derivatives.

Globally, a number of electronic devices have been developed to streamline the day-to-day life, but there is still a challenge for high energy storage and excellent stability. This chapter discusses the advantages and some faced challenges for the hydrothermal synthesis of metal oxide composites for high energy applications. Hydrothermal synthesis method is one of the commonest and relatively achievable method for researchers. The chapter highlights some of the parameters used for the synthesis of metal oxide nanoparticles in both a batch and a flow reaction system for cathode materials. The hydrothermal method is used in heteroatomic doping for improvement of the material properties. The chapter looks at the effect of solvent selection and performance properties at different operation conditions. The chapter includes a practical experiment and results in a discussion for the NaFe2O3-GO which was produced by batch hydrothermal method by the authors. The material was characterized by FESEM/EDX, XRD, and electrochemical testing of the material which resulted in the performance of the discharge capacity approximated to be 720 mA h/g.

The technology advancement toward Industrial Revolution 4.0 called upon electrode materials that are capable of delivering higher energy density than its predecessor. As a result, significant improvement in anode was observed, from ~300 mA h g−1 from graphite to ~2500 mA h g−1 from alloying Si anode. The escalating improvement of anode draws attention into developing cathode materials with better electrochemical properties. Composite materials had demonstrated promising results because they do not only improve the capacity but the electrochemical potential of the cathode materials. In this chapter, benefits of composite materials were discussed tentatively in two different types of materials, namely, intercalation-type as well as conversion-type materials. Composite conversion-type materials showed excellent electrochemical properties and could be the next promising cathode for high density battery.

Batteries with outstanding energy and power densities have been extensively investigated in the last decade for the application of emerging markets including stationary storage and electric vehicles. The use of rare-earth metals, such as sodium, lithium, and potassium anode, is not possible to date due to its high cost, dendrite formation nature, and safety issues. The metal oxides have prophesized itself as an auspicious material for energy storing applications due to its high gravimetric specific capacity, low cost, and long-life stability. Transition elements from the first transition metal series are more attractive than the other transition elements due to the high earth-abundant and high gravimetric specific capacities. The transition metal oxides (TMOs) addressed so far in batteries are mostly conversion-type anodes and insertion-type cathodes. This chapter provides a comprehensive overview of the synthesis, characterization, and applications of single TMOs for non-aqueous post-lithium-ion batteries.

The field of electrochemical energy conversion and storage has found favor in the sight of certain metal oxides (MOx) that are capable of varying their oxidation states. Some of such oxide electrodes store electric charges via a thermodynamically and kinetically favored faradaic reaction mechanism without altering their crystal phase and are said to be pseudocapacitive. While electrostatic ultracapacitors or supercapacitors (SCs) are basically electrochemically advanced forms of the parallel plate capacitors, with improved energy density; pseudocapacitors (another class of SCs), in addition to electrostatic means, deploy highly reversible redox means to achieve even greater energy density. Moreover, nowadays, even the non-capacitive (battery-type) transition MOx are tuned (by nanostructuring) to exhibit extrinsic pseudocapacitance, with energy density comparable to batteries. Indeed, nanoporous MOx materials are taking supercapacitive energy storage to a level where neither batteries (with high energy density) nor parallel-plate capacitors (with high power density) can contend. In this chapter, the basic features of SCs, their mechanisms of charge storage, various techniques for nanostructured electrode materials synthesis, and practical performances of some MOx electrode SCs with some notable applications of SCs were discussed. And lastly, future research trends in nanoporous MOx SCs were presented.

Nanoporous metal oxides (NMOs) have attracted a substantial research interest in energy storage applications. They have reported beneficial structural, morphological, and electronic properties along with high capacitance values for supercapacitor applications. Usually, porous materials are synthesized using traditional template methods, but they are high-cost and low-throughput methods, which cannot be scaled up for mass production. On the other side, the chemical methods like chemical bath deposition, hydrothermal, sol gel, and electrodeposition for the development of porous metal oxides emerged the field with enormous possibilities. Chemical methods provide atomic-level control for the reaction, and they can be commercialized for large-scale production. Further, to be an ideal candidate for supercapacitor applications, we need to design controlled synthesis to attain high surface area and useful porous structure. This chapter will briefly discuss traditional processing methodologies for porous materials, their limitations, and the development of various chemical methods with their advantages for developing efficient NMO for supercapacitor applications. The prospects and limitations of these methods for developing efficient porous materials will be discussed in detail.

Porous micro/nano structured materials have gained much interest since they have many excellent properties wisely as large surface area and confined pore structure for various applications in electrochemistry such as, catalysis, sensing and energy storage devices. Among various characteristics, structure of porous metal oxides plays vital role in determining electrochemical performance. Fortunately, nano-structuration and porosity conjunction for transition metal oxides ascertains a role in enhancing electrochemical storage of electrodes for charge storage devices mainly due to: (i) large surface area for charge storage; (ii) easy access over active material for electrolyte ions and (iii) fast electrolytic ion transportation through porous channels. This chapter reviews the current development of nanoporous transition metal oxide-based materials, with specific focus on advanced electrochemical supercapacitor applicatin.

Transition metal oxides qualify themselves as promising electrode material for supercapacitor applications by virtue of its large values of capacitance and energy density, facilitated by reversible faradaic redox reactions at the electrode-electrolyte interface. However, poor conductivity, lower surface area and lower power density curb their deployment in practical applications. Towards this end, establishing a synergistic effect among the transition metal oxides has been recognized as a feasible way to overcome the limitations of individual metal oxide components without compromising its pseudocapacitance. These hybrid metal oxide composite electrodes can achieve better electrical conductivity and enhance the flow of electrolytic ions into the active part of the electrode material, thereby utilizing the full potential of the device. This chapter intends to present a decent update on the synthesis methods, structural properties and electrochemical performances of various hybrid nanocomposite transition metal oxide materials for supercapacitor applications.

To fulfill the energy demand of the world, there is a need of sustainable energy sources and storage devices. Supercapacitor is one of the energy storage devices. Among the challenges of developing a good supercapacitor, the most important one is to prepare an electrode. Such electrodes are prepared using variety of physical and chemical methods. Among these, chemical methods are easier and cost-effective. Liquid phase deposition (LPD) is one of the simplest methods among the chemical methods. In this chapter, the main focus is on the electrode materials deposited by LPD method. The electrode materials deposited by LPD for supercapacitor applications are iron oxide (α-Fe2O3), copper oxide (CuO), and layered double hydroxides (LDHs). The chapter also explores the significant changes observed in the electrochemical performance due to deposition on different substrates like flat stainless steel (SS), mesh SS, graphene, nickel foam, e-MXene, and carbon nanotube paper. Such electrodes are evaluated for supercapacitive performance, and results are compared with the literature.

Sensors to detect the leakage of hazardous gases are indispensable to avoid accidental problems for human health. Sensors become part of industrial application and day-to-day life. Design and development of gas sensor need wide range of materials where the use of semiconducting metal oxide is on prime priority owing to its inherent exceptional chemical and physical properties and simple route of preparation through less expensive methods. Liquefied petroleum gas (LPG) is injurious to human health due to its explosive and inflammable properties. The extensive use of LPG generated a room for encroachment of sensitive and cost-effective gas sensors which can sense leakage before any severe calamities can occur. Research on sensing materials are currently aimed extensively on nanostructured materials of metal oxide semiconductor with numerous surface architectures in nano forms having their sizes in nano regime, namely, quantum dots, nanobelts, nanotubes, nanoparticles, nanowires, and nanorods due to abrupt change in conductivity under gas environment. High surface-to-volume ratios in addition to huge quantity of active sites associated with nanostructured architecture help to achieve maximum sensitivity when exposed to gas. Present chapter deals with chemically processed heterojunction-based metal oxide material towards room temperature LPG sensors with primary emphasis given on fundamental properties, basic mechanism with design and operation, recent progress, and future trends. Particularly, the chapter points out the state of the art towards the advancement in LPG sensors centered on semiconducting metal oxides with heterostructure properties. Furthermore, the comprehensive illustrations on heterojunction gas sensors based on metal oxides open up new directions for further exploration on room temperature LPG sensors.

Most homes and industrial places require a tool for detecting destructive and toxic gases very dangerous to production, human, animals and plants which brings about fabrication of detecting device capable of sense and detecting harmful gases as well as alerting users for suitable precautionary measures. Several on going studies indicates that on now metal oxide nanostructures application as gas sensors is flourishing worldwide. The prominence of nanostructured materials-established gas sensor has attracted interest of numerous research populations, because of their excellent reproducibility, high sensitivity, portability and cheap and non-toxic nature. Here, synthesis techniques, performance and application of p-type semiconducting metal oxides as gas sensors were evaluated. Also discussed were fundamental gases-sensing features of these nanostructured materials. The history, development and progress of nanostructured material gas sensors with emphasis on recent innovative researches towards enhancing their performances are also discussed.

Precise pH measurement is crucial in the assessment of various physical, chemical, and biological processes. While the common glass electrode is known for its exceptional pH sensing performance, it has limited applicability. As a result, H+ sensitive MOx, which can be made into miniature physically rugged sleeves, applicable in vivo, in harsh environments, and even in cases where the volume of the sample solution is highly restricted (such as sensing of exuding sweat), are gaining attention as pH sensors. In this chapter, we present the principal mode of operation of electrochemical pH sensors and the detailed mechanism of sensing with MOx. We also considered various fabrication methods for MOx sensing electrodes (SEs). Lastly, after describing several pH sensor performance measures, the practical performances of some MOx SEs were reviewed. In all, this chapter is meant to provide an insight into pH sensing using MOx-based electrodes.

Industrial and technological practices need a device to detect harmful and poisonous gases dangerous to humanity, animals and plants which resulted in fabrication of detecting apparatus that can sense and select harmful gases, identify them and alert man for appropriate action. Several researches are going on regarding p-type semiconducting metal oxides as gas sensors. The prominence of p-type semiconductor metal oxide-based gas sensor has attracted interest of numerous research populations, because of their excellent reproducibility, high sensitivity, portability, cheap and non-toxic nature. Here, synthesis techniques, performance and application of p-type semiconducting metal oxides as gas sensors are evaluated. Fundamentals of gas-sensing features of p-type semiconducting metal oxides were also discussed.

The capping of quantum dots (QDs) with GaSb layer in an attempt to protect the dots was observed to be strenuous owing to the low melting point of InSb (~527 °C). InSb QDs were observed to be irregular at high temperatures (>500 °C) and are disposed to either relaxation or dissolution within the GaSb matrix, thereby forming a relatively plane thin layer which exhibits features that are similar to a quantum well (QW). This study was motivated due to the progressive interest in the adjustment of semiconductor band structures by means of reduction of their dimensions, thereby increasing the bandgap energy of the material. Improvements in III–V antimony (Sb)-based semiconductor development have stimulated the keen desire to augment the emission/absorption wavelength range of this category of compounds for optoelectronic devices operating in the mid-infrared region of the electromagnetic spectrum. An appealing material system for mid-infrared (MIR) applications is indium antimonide (InSb) quantum dots (QDs) within a gallium antimonide (GaSb) matrix. However, the band alignment and emission wavelength of InSb QDs specifically on a GaSb substrate have been a subject area of contention during the past few years. This study focuses on the development of InSb/GaSb nanostructures by metal-organic chemical vapour deposition (MOCVD). The InSb sample was grown on different GaSb substrates using optimized growth parameters in order to control the size, density and aspect ratio of the dots. Interfacial growth interruptions while flowing the organometallic precursors through the reactor were monitored as a way of systematically regulating both the resulting strain and the chemical processes on the surface of the deposited sample. The samples were characterized using photoluminescence (PL) spectroscopy, scanning probe microscopy (SPM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Likewise, the band alignment, energy levels and carrier wave functions of the samples in this work were modelled theoretically using the nextnanomat software (version 3.1.0.0).