Metal Oxide Nanomaterials for Chemical Sensors

Since the development of the first models of gas detection on metal-oxide-based sensors much effort has been made to describe the mechanism responsible for gas sensing. Despite progress in recent years, a number of key issues remain the subject of controversy; for example, the disagreement between the results of electrophysical and spectroscopic characterization, as well as the lack of proven mechanistic description of surface reactions involved in gas sensing. In the present chapter the basics as well as the main problems and unresolved issues associated with the chemical aspects of gas sensing mechanism in chemiresistors based on semiconducting metal oxides are addressed.

In this chapter we present recent advances in the study of metal oxide surfaces and put them in relation to gas sensing properties. A reoccurring scheme is the dependence of chemical surface properties on the crystallographic orientation of the surface. This dependence will become more important in gas sensing applications as nanomaterials with controlled crystal shapes are being designed. In particular we focus on differences of the surface properties of the two polar surfaces of ZnO and the two most abundant bulk terminations of rutile TiO₂, i.e. the (110) and (011) crystallographic orientations. On the example of these metal oxides, we describe the use of vacuum based surface science techniques, especially scanning tunneling microscopy and photoemission spectroscopy, to obtain structural, chemical, and electronic information.

The chemical approaches to improvement of selectivity of semiconductor metal oxide gas sensors are the main subject of this chapter. Current concepts of interrelationships between metal oxide chemical composition, crystal and surface structure and its activity in the reaction with gas phase components are considered. Application of such concepts to the design of sensor materials based on nanocrystalline SnO₂ is discussed thoroughly. Experimental data concerning chemical composition, solid–gas chemical interaction activity and sensor properties is given and critically analysed. The possibility of utilization of solid–gas chemical reaction activity concepts for directed synthesis of new metal oxide semiconductor sensor materials with selective response to given gases is highlighted.

This chapter gives an overview about the application of metal oxides in chemiresistors. A generalized model of working principle and the influence of particle size, microstructure, volume and surface doping are discussed. The quality factors of sensor performance and the necessity of high-throughput experimentation and combinatorial techniques for the development of new sensor materials are explained. In this context high-throughput impedance spectroscopy is presented as a rapid characterization method of a large number of samples. The complete workflow is introduced involving material synthesis and analysis, layer preparation by a laboratory robot, impedometric characterization and automated data evaluation. As examples two series of surface and volume doping demonstrate the systematic identification of new sensor materials.

Chemo-resistive sensors utilizing meal oxides form a very important type of sensors for gas detection. They are based on the interaction between gas molecules and surface ionosorbed oxygen species accompanied by electron transfer, which eventually leads to the change of material resistance. This process is controlled by a few external parameters (working temperature) and internal parameters (microstructure, chemical composition and crystal structure). While most parameters have been paid sufficient attention to, the influence of crystal structures is still largely unexplored. On the other hand, metal oxides exist in more than one crystalline form. The structural and property difference between different structures is expected to affect the sensing behavior of the material. Taking TiO₂ and WO₃ as examples, this chapter reviews how to selectively synthesize desired crystal structures and how they are related to the performance as agas sensor. TiO₂ exists in two major polymorphs, with rutile being the thermodynamically stable phase and anatase being the metastable one. Compared to rutile, anatase is more open-structured and more chemically active and has lower surface energy. The hydrothermal method has been proved to be very effective in anatase synthesis as long as particle size is well controlled (normally under 20 nm) and dopants could stabilize this phase. Studies have found that anatase shows higher sensitivity as a gas sensor which is believed to be attributed to its higher chemical activity.WO₃ undergoes a series of phase transition when it is cooled down and γ-WO₃ is usually the room-temperature (RT) stable phase. The low-temperature stable phase, ε-WO₃, is the least symmetric among all the phases and is the only one with a ferroelectric feature. By a rapid solidification method called flame spray pyrolysis, ε-WO₃ is able to be synthesized in high purity at RT. Doping with silicon and chromium could effectively stabilize this phase up to 500 °C by forming boundary domains or surface layers. The dopant-stabilized ε-WO₃ shows high sensitivity and unique selectivity to polar gas molecules, esp. acetone, which may be due to the strong interaction between the ε-WO₃ surface dipole and polar molecules.

In order to develop next generation chemical sensors using nano-scale materials, we need to understand the sensing mechanisms at atomic level. This requires synthesizing chemical sensing materials with controlled structure, chemical composition and surface morphology. Although the commonly used wet chemical synthesis methods provide quality materials for large-scale production of materials, alternative thin film deposition techniques such as sputtering, chemical vapor deposition (CVD), and molecular beam epitaxy (MBE) can also be useful to achieve atomic-scale control over the structure and composition over a large fabrication area for potential device fabrication as well as to gain an understanding of the chemical sensing properties of nano-scale materials. Especially, MBE has been used to synthesize metal oxide thin films with ultra-pure, well-ordered surfaces, which can be used to understand the effect of surface morphology, structure, and composition on the gas sensing properties. In this chapter, we provide a detailed discussion of thin film growth using MBE along with some in situ characterization capabilities such as reflection high energy electron diffraction (RHEED) and low energy electron diffraction (LEED). In addition, this chapter focuses on the discussion of the growth, characterization and gas sensing properties of metal oxide thin films such as doped CeO₂ and SnO₂. The chapter also emphasizes the significance of various in situ and ex situ characterization techniques to understand the material properties there by developing methodologies to synthesize better materials with tunable characteristics for sensing applications.

Solid state gas sensors based on semiconducting metal oxides have been widely investigated and utilized in environmental monitoring, chemical process controls and personal safety. In recent years, one dimensional nanostructures, such as nanowires, nanorods, nanotubes and nanobelts, have attracted much attention due to their great potential application in gas sensing, and for overcoming fundamental limitations due to their ultra high surface-to-volume ratio. A variety of methods have been developed to fabricate these nanostructures. The nanostructure based gas sensors demonstrated excellent response and recovery characteristics. However, the developed methods are not convenient for mass production and improvements on sensitivity, selectivity and long term stability are still needed. Atomic layer deposition (ALD) is a film deposition technique based on the sequential use of self-terminating surface reactions. Due to the unique nature of the reaction process, ALD becomes an ideal deposition technique to form atomic thin films and nanolaminate structures. ALD is finding ever more applications for emerging nanodevices. The potential to control thickness at the sub-nm level, and the ability to deposit thin films over highly corrugated substrates with high aspect ratio topography makes ALD of great interest in fabrication of one dimensional nanomaterial. Utilizing fabrication through nanotechnology, ALD has found new opportunities in gas sensors based on metal oxide semiconductors. In this chapter, the general characteristics of atomic layer deposition, the sensing performance enhancements by quasi-1 dimensional nanostructures and nanomaterials, the method to fabricate such nanostructures and the recent exploration of ALD in gas sensing studies are reviewed.

This chapter summarizes microwave irradiation methods for the preparation of metal oxide nanoparticles and their catalytic and sensing properties and applications. Microwave irradiation provides rapid decomposition of metal precursors and can be extended for synthesis of a wide range of metal oxide nanoparticles with various compositions, sizes and shapes. This chapter introduces the microwave method and describes the nucleation and growth process for the formation nanocrystals. We offer a broad overview of metal oxide nanostructures synthesized by microwave irradiation including: ZnO, TiO₂, CeO₂, other rare earth metal oxides, transitional metal oxides and metal ferrite nanostructures. Finally, we describe the application of metal oxides in the photocatalytic degradation of organic dyes and gas sensing devices.

Detection of chemicals species such as industrial toxic and inflammable gasses, chemical warfare agents, disease related chemicals, is of paramount importance to public safety and health. The driving force is to develop highly sensitive, selective, and stable sensors with rapid detection and recovery time. Metal oxide thin films have long been used as chemical sensors due to the rich oxygen vacancies on the surface that are electrically and chemically active. The chemical adsorption induces redox reactions and consequently alters the electrical conductance. However, there have been a number of limitations: relatively high operation temperature, indirect and inefficient refreshing method, and lack of chemical selectivity. In the surge of quasi-one-dimensional (Q1D) metal oxide nanowire research, it is demonstrated that the unique shape anisotropy significantly enhances the sensor performances due to the large surface-to-volume ratio and size comparable to the Debye screening length. This not only enhances the sensitivity at room temperature, but provides efficient modulation of the surface detection and desorption processes. More importantly, it opens a pathway for developing wireless low-power sensor network to transmit information from a remote site. This chapter provides a review of the state-of-the-art research covering the synthesis and fundamental properties of Q1D metal oxide systems, and focusing on their applications as chemical sensors.

There has been significant recent interest in the use of surface-functionalized thin film and nanowire ZnO for sensing of gases, heavy metals, UV photons and biological molecules. For the detection of gases such as hydrogen, the ZnO is typically coated with a catalyst metal such as Pd or Pt to increase the detection sensitivity at room temperature. Functionalizing the surface with oxides, polymers and nitrides is also useful in enhancing the detection sensitivity for gases and ionic solutions. The use of enzymes or adsorbed antibody layers on the ZnO surface leads to highly specific detection of a broad range of antigens of interest in the medical and homeland security fields. We give examples of recent work showing sensitive detection of glucose and lactic acid and the integration of the sensors with wireless data transmission systems to achieve robust, portable sensors.

Metal oxide nanowire sensors with complex morphologies and compositions have shown promising properties due to their high surface-to-volume ratio and stable structures against agglomeration. In this chapter, a series of metal oxide nanostructures modified via surface coating, morphology variation, doping and appropriate energy band engineering have been investigated, and the sensing mechanism is discussed. By using nanostructures with complex morphologies and compositions in simple material synthesis routes, the structure of the sensitive material is modified, the electronic transport of the sensor is regulated and the sensing properties can be greatly improved, including enhancing the sensitivity and selectivity, lowering the working temperatures, reducing the response time and achieving long-term stability.

Optical analysis of metal oxides as a means of transduction in sensing devices provides an alternative method for the detection of target analytes. The optical properties of metal oxides are rich with opportunity as luminescence, dielectric function changes and optically active dopants are all sensitive to environmental changes and can be utilized in a sensing device. A detailed description of the latest work on the photoluminescence of metal oxides will be provided, with a focus on ZnO, SnO₂ and TiO₂ nanoscale thin films as well as nanorods. Changes in the dielectric function of perovskite metal oxide coated long period fiber gratings has been shown to be a convenient sensing device and results pertaining to these metal oxides containing alkaline earth metals and their use in the detection of CO₂ will be highlighted. Optically active dopants, pertaining specifically to metal nanoparticles such as Au, Cu and Ag, can serve as beacons for reactions and environmental changes within metal oxide nanocomposite thin films through the interrogation of changes in their plasmonic properties. In particular sensing applications within harsh conditions, including elevated temperature and either oxidizing or reduction gas environments will be detailed. Each of these optical techniques will be reviewed and any key environmental, film deposition methods or reaction conditions will be highlighted in light of their potential effects on sensor detection limits, sensitivity or selectivity characteristics.

Metal oxide nanostructures with hetero-contacts and phase boundaries offer a unique platform for designing materials architectures for sensing applications. Besides the size and surface effects, the modulation of electronic behaviour due to junction properties leads to modified surface states that promote selective detection of analytes. The growing possibilities of engineering nanostructures in various compositions (pure, doped, composites, heterostructures) and forms (particles, tubes, wires, films) has intensified the research on the integration of different functional material units in a single architecture to obtain new sensing materials. In addition, new concepts of enhancing charge transduction by surface functionalization and use of pre-concentrator systems are promising strategies to promote specific chemical interactions, however the challenge related to reproducible synthesis and device integration of nanomaterials persist.

Nanomaterials are becoming increasingly important for next-generation chemical sensing devices. In particular, quasi-one-dimensional materials, such as nanowires, are attracting a great deal of interest. While early examples have demonstrated the promise offered by these nanoscale materials, challenges still remain for integration, systematic characterization and evaluation of such materials in operational devices. Here, a means to assess the performance of nanowire-based materials as chemical microsensors is illustrated with two examples. Polycrystalline nanowire sensing materials are integrated with microsensor substrates that feature an embedded heater, facilitating the use of temperature to interrogate the response characteristics of sensing materials. By changing the operating temperature, different effects are observed as a function of nanowire loading density (aligned tin oxide nanowires) or overall material morphology (tungsten oxide materials, including a thin film). Further, by using conventional signal processing and data analysis approaches, the sensitivity and selectivity of these materials as a function of material scale and morphology are characterized.

During the last decade, quasi-1D metal oxide nanostructures were proven to be a promising material platform to design new gas sensing elements. This chapter surveys the recent developments of the analytical devices based on multisensor arrays made of metal oxide nanowires. We briefly discuss the advantages and challenges of electronic noses and the major milestones of their development. We show that evolution of the nanowire based electronic noses follows the same trends: from fabrication of the devices based on discrete nanowires to creation of integrated systems made of nanowire mats and finally realization of a monolithic sensor array made from a single nanowire. The parameters and performance of such analytical systems is reviewed and fabrication protocols are discussed.

The current scenario of metal-oxide gas sensing shows, on one side, highly innovative silicon-based platforms, as outcomes of microelectronic and micromachining manufacturing processes, while on the other side, several techniques and methods for the synthesis of the metal-oxide active layers in the form of nanoporous-nanostructured coatings. The high specific surface area of nanoporous coatings improves the interaction with the atmosphere, while the nanostructure offers characteristic surface-dependent electrical properties. Changes in these electrical properties upon gas exposure and interfacial chemical reactions allow for the development of novel, nano-enhanced gas sensors. The base element of innovative micromachined platforms for gas sensing is the microhotplate. Although microhotplates have the same functional parts of traditional devices (integrated heater, electrodes for resistance readout), micromachining provides considerable improvements. These include, for example, the 2–3 orders of magnitude reduction in power consumption for heating: a feature that may disclose the possibility for remote powering through batteries or photovoltaic cells. Moreover, microhotplates originate from the manufacturing track of microelectronics, hence the concept of “system integration” turns out straightforwardly. Within this perspective, the microhotplate may be considered as just an individual component of a many-element sensing platform, including for example, other transducers, or even on-board front-end electronics. Integration concepts are also needed for optimizing the functionalization of the microhotplate with the metal-oxide nanostructured sensing layer, whose batch deposition should become one step of a device production pipeline. As two beautiful countries separated by the sea, with just few bridges in between, difficulties still exist from the point of view of the integration of metal-oxide nanomaterials on microhotplates and micromachined platforms in general. In fact, although many different techniques for the production of metal-oxide nanomaterials have been developed so far, each one of them suffers difficulties, at various degrees, with respect to the fundamental step of microhotplate functionalization. For example, the high temperature step required by certain techniques for stoichiometric oxide synthesis, may be incompatible with microhotplate safety, while the mechanical stress during deposition may result in microhotplate destruction and a subsequent low production yield. The chapter will describe the concepts and the technologies behind microhotplates manufacturing with respect to drawings adopted, chosen materials, and system integration approaches. Techniques and methods for metal-oxide nanomaterial production will be reviewed, highlighting weaknesses and strength points, once they would be employed for microhotplate functionalization. Recent developments on the use of nanoparticle beams to directly deposit nanoporous coatings on microhotplate batches will be included: besides providing thermal and mechanical compatibility with microhotplates, these methods also offer the possibility to synthesize a wide group of different metal-oxides, which is beneficial for an array approach to gas sensing. Relevant examples of sensing performances of microhotplates-based devices will be reported as well.