Advanced Nanomaterials for Inexpensive Gas Microsensors

Synthesis, Integration and Applications

Eduard Llobet Valero

• Publisher: Elsevier
• Year: 2019
• Print ISBN: 9780128148273
• eBook ISBN: 9780128148280
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Advanced Nanomaterials for Inexpensive Gas Microsensors: Synthesis, Integration and Applications presents full coverage in the area of gas sensing nanomaterials, from materials, transducers and applications, to the latest results and future direction. Experts present work on metal oxides, carbon-based and hybrid materials, fabrication and application. The book brings together three major themes, including synthesis, functionalization and the characterization of advanced nanomaterials, all emphasizing synthesis techniques that ease the integration of nanomaterials in transducers. Chapters encompass a wide spectrum of sensing technologies, including advanced nanomaterials (metal oxides, carbon materials and graphene) and organic molecular materials and atomic layers (MoS2).The book's authors examine the coupling of sensitive nanomaterials to different types of transducer elements and their applications, including direct growth and additive fabrication techniques as a way to obtain inexpensive gas microsensors, principal transduction schemes, and advanced operating methods.- Presents technological solutions and applications of gas sensors in varied areas of chemistry, physics, material science and engineering- Examines advanced operating methods (e.g., temperature modulation, self-heating, light-activated response, noise methods) to enhance stability, sensitivity, selectivity and reduce power consumption- Provides a critical review of current applications and their expected future evolution, demonstrating the most promising approaches and future expectations in the development of inexpensive gas micro- and nanosensors

Chapter 1

Introduction

Eduard Llobet MINOS-EMaS, Universitat Rovira i Virgili, Tarragona, Spain

Abstract

This is an introductory chapter for the book Advanced Nanomaterials for Inexpensive Gas Microsensors. It starts by giving a short and focused minireview on the most successful and more researched gas-sensitive nanomaterials. It continues by introducing some of the methods that are currently used for synthesizing these and, for their integration in transducing platforms, identifying those methods that hold more promise for the mass production of nanomaterials and gas-sensitive devices. Finally, the chapter ends by giving a general overview of the content of the different sections and chapters that integrate the book.

Keywords

Gas sensing; Low-dimensional nanomaterial; Chemical vapor deposition; Solution processing; Bottom-up synthesis; Additive manufacturing

1.1 Introduction

The last years have seen a sustained and ever-increasing interest in the development of ubiquitous sensing. Sensors are present nowadays in different platforms such as portable devices (e.g., tablets or smartphones), domestic appliances (e.g., washing machines, dishwashers or ovens), and cars, only to cite a few. New developments in consumer electronics, especially in the rapidly evolving field of wearable electronics require the use of different types of sensor devices. Most of the microsensors that are currently routinely integrated into commercially available platforms as the ones mentioned before are physical sensors (accelerometers, gyroscopes, magnetic sensors, temperature sensors, optical sensors for measuring water turbidity, etc.). In contrast, the use of gas microsensors within the ubiquitous sensing paradigm, if any, remains incipient. However, should gas microsensors meet the stringent performance and cost requirements of this new paradigm, then a wide spectrum of applications and enormous associated markets would develop. These would include, but are not limited to, indoor and outdoor air quality control, security in key infrastructures, intelligent cooking, food quality and safety monitoring, remote monitoring of the elderly, point-of-care diagnostics, and personal healthcare via breath analysis or perspiration. This explains why intense research efforts have been devoted to the continuous amelioration of gas microsensors.

Many of these research efforts are oriented toward the quest for new nanomaterials that can meet both the specifications of widespread, continuous gas detection and the industrial demands for device integration. Nanomaterials comprise low-dimensional (i.e., 0D, 1D, 2D, or 3D) inorganic, carbon, and molecular materials and their hybrids. Gas-sensitive nanomaterials will find commercial application provided they are produced employing scalable techniques that enable the mass production of high-quality materials at affordable costs. In that sense, solution processing methods and additive fabrication techniques are seen as interesting enabling technologies for coupling nanomaterials to their application substrates. Additionally, nanomaterials can often be operated at low temperatures above the ambient or even at room temperature, and their response and recovery dynamics can be ameliorated by using light irradiation. Achieving ultralow power consumption gas sensors is a key aspect for realizing their enormous potential for being integrated in a new generation of ubiquitous, portable, or personal sensor systems.

The organization of this introductory chapter is as follows. It will start by giving a focused minireview on gas-sensitive nanomaterials, will continue by reviewing some of the methods for synthesizing these and their integration in different transducer platforms, and will end by giving a general overview of the content of the different chapters that integrate this book.

1.2 Gas-sensitive nanomaterials

The research in gas-sensitive nanomaterials is mostly targeted at achieving high sensitivity and stability and improved (i.e., reduced) limit of detection and selectivity, in such a way that traces of target gas molecules (e.g., pollutants) can be detected even when in the presence of significantly higher concentrations of other gases or vapors (interfering species).

Nanomaterials possess unique morphological properties that make them very attractive for achieving very high sensitivity. In some cases, virtually all their atoms are exposed to the surrounding chemical environment. Some of the most researched gas-sensitive nanomaterials and nanostructures consist of the following:

• • Nanoparticles (NPs) with few nanometers in diameter, which are often referred to as zero-dimensional nanomaterials. In either simple or core-shell structures, metal oxide NPs can become fully depleted of charge carriers when exposed to clean air, and NP films may experience dramatic changes in electrical conductivity upon the adsorption of gas molecules on their surface. Few-nanometer NPs experience quantum confinement effects (they are also referred to as quantum dots) and present localized plasmon resonance, which can be quenched or enhanced upon the adsorption of gas molecules. Therefore, the use of NPs has been reported in conductometric, optical, and plasmonic gas sensing [1].
• • Nanowires (NWs), nanofibers (NFs), nanotubes (NTs), nanoneedles (NNs), or nanobelts (NBs) are nanomaterials with diameters ranging between a few to 100 nm and lengths easily reaching two or three orders of magnitude higher than their diameters (from tens of microns to few millimeters). This is why these are often referred to as one-dimensional nanomaterials. NWs, NNs, or NBs are often single-crystalline metal oxides in which the width of their central conduction channel is modulated by the adsorption of gas molecules. Similarly to NPs, low-diameter NWs can become fully depleted of charge carriers when in clean air, experiencing dramatic changes in electrical conductivity when exposed to traces of pollutant gases [2–8]. Besides inorganic NWs, the use of conducting polymer NWs in which electrical conductivity results from the existence of charge carriers, due to doping, and from the ability of those charge carriers to move along the π bonds of the polymer chains has been reported. Polyaniline (PANI) NWs exhibit a p-type semiconductor behavior, and their sensitivity was significantly higher than that of traditional PANI thin films, due to their higher surface-to-volume ratio [9]. NFs generally consist either of polycrystalline metal oxide nanoparticles arranged in fiber form [10] or of a calcined polymer strand that results in a carbon NF (CNF) [11,12]. NTs are long cylindrical structures related to the fullerenes. Carbon NTs (CNTs) are by far the most researched nanotube structure for gas sensing. While single-walled CNTs consist of a two-dimensional hexagonal lattice of carbon atoms bent and joined in one direction to form a hollow cylinder, multiwalled CNTS comprise two or more nested single-walled CNTs. In polycrystalline metal oxide NFs, the electrical conductivity is modulated by changes in the height of potential barriers that develop at the grain interfaces. These changes are caused by gas adsorption. The electrical conductivity of CNFs and CNTs is also affected by charge transfer and associated p or n doping resulting from the adsorption of gas molecules on their surface or outer wall, respectively. All these mechanisms explain that these nanomaterials have been exploited in conductometric sensors [13–14]. However, the remarkable gas adsorption capacity of CNFs and CNTs explains why these nanomaterials have been employed as well in resonant, gravimetric gas sensors such as bulk acoustic wave (BAW), surface acoustic wave (SAW), or cantilever beams [14].
• • Nanoplatelets, nanoflakes, and few-layer and single-layer materials are two-dimensional materials. The most researched two-dimensional material in gas sensing has been graphene and its derivatives [14,15,16]. Graphene is an allotrope of carbon in the form of a two-dimensional, atomic-scale, hexagonal lattice in which one atom forms each vertex. Graphene-related materials are graphene oxide (GO) and reduced graphene oxide (rGO). GO is a chemically modified graphene containing oxygen functional groups such as epoxides, alcohols, and carboxylic acids. The carbon-to-oxygen ratio in GO is approximately three to one. GO can be a semiconductor or insulator, depending on the degree of oxidation. GO results from the chemical exfoliation of graphite. When oxidizing agents react with graphite, the interplanar spacing between the layers of graphite is increased. The completely oxidized compound can then be dispersed in a base solution such as water, and GO is then produced. rGO is a product of the reduction of GO, and different techniques may be used for achieving this; however, the quality and properties of rGO always differ from those of pristine, mechanically exfoliated graphene. By controlling the degree of remnant oxidation in rGO, its electronic and optical properties can be tuned in large scope. Besides graphene nanomaterials, transition metal dichalcogenides (TMDs) such as WS₂ or MoS₂ have been explored more recently for gas sensing. TMD monolayers are atomically thin semiconductors of the type MX₂, where M is a transition metal atom (e.g., Mo and W) and X is a chalcogen atom (S, Se, or Te). One layer of M atoms is sandwiched between two layers of X atoms. A MoS₂ monolayer is only 6.5 Å thick. These materials show a nanoplatelet or nanoflake morphology when they are about 10 monolayer thick (~ 6.5–10 nm). While TMD stacks show indirect bandgap, TMD monolayers show direct bandgap, making them suitable semiconductors for transistors. The electronic properties of these two-dimensional nanomaterials are affected when gases form their environment get adsorbed on their surface. Edges have been found to have increased amount of adsorption sites [17].
• • Zeolites, metal-organic frameworks, and hierarchical nanomaterials are known as three-dimensional materials. Zeolites are the aluminosilicate members of the family of microporous solids and mainly consist of Si, Al, O, and metals including Ti, Sn, or Zn. They are often referred to as molecular sieves because they possess the ability to selectively sort molecules based primarily on a size exclusion process. This is due to a very regular pore structure of molecular dimensions. The maximum size of the molecular or ionic species that can enter the pores of a zeolite is controlled by the dimensions of the channels, and this can be controlled by the amount Si/Al ratio and the metal employed in a particular zeolite formul...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributors
• Chapter 1: Introduction
• Chapter 2: Inorganic nanomaterials
• Chapter 3: Molecular materials for gas sensors and sensor arrays
• Chapter 4: Carbon nanomaterials
• Chapter 5: Hybrid and 2D nanomaterials
• Chapter 6: Fabrication techniques for coupling advanced nanomaterials to transducers
• Chapter 7: CMOS-based resistive and FET devices for smart gas sensors
• Chapter 8: Optical devices
• Chapter 9: Resonant microcantilever devices for gas sensing
• Chapter 10: Advanced operating methods
• Chapter 11: Indoor air quality monitoring
• Chapter 12: Low-cost sensors for outdoor air quality monitoring
• Chapter 13: Monitoring perishable food
• Chapter 14: Point of care breath analysis systems
• Chapter 15: Concluding remarks and outlook
• Index