Metal oxide-based resistive-type gas sensors are solid-state devices which are widely used in a number of applications from health and safety to energy efficiency and emission control. Nanomaterials such as nanowires, nanorods, and nanoparticles have dominated the research focus in this field due to their large number of surface sites facilitating surface reactions. Previous studies have shown that incorporating two or more metal oxides to form a heterojunction interface can have drastic effects on gas sensor performance, especially the selectivity. Recently, these effects have been amplified by designing heterojunctions on the nano-scale. These designs have evolved from mixed commercial powders and bi-layer films to finely-tuned core–shell and hierarchical brush-like nanocomposites. This review details the various morphological classes currently available for nanostructured metal-oxide based heterojunctions and then presents the dominant electronic and chemical mechanisms that influence the performance of these materials as resistive-type gas sensors. Mechanisms explored include p–n and n–n potential barrier manipulation, n–p–n response type inversions, spill-over effects, synergistic catalytic behavior, and microstructure enhancement. Tables are presented summarizing these works specifically for SnO2, ZnO, TiO2, In2O3, Fe2O3, MoO3, Co3O4, and CdO-based nanocomposites. Recent developments are highlighted and likely future trends are explored.

Nanoscale metal oxide-based heterojunctions for gas sensing: A review

▪ Abstract Metal-oxide nanowires can function as sensitive and selective chemical or biological sensors, which could potentially be massively multiplexed in devices of small size. The active nanowire sensor element in such devices can be configured either as resistors whose conductance is altered by charge-transfer processes occurring at their surfaces or as field-effect transistors whose properties can be controlled by applying an appropriate potential onto its gate. Functionalizing the surface of these entities offers yet another avenue for expanding their sensing capability. In turn, because charge exchange between an adsorbate and the nanowire can change the electron density in the nanowire, modifying the nanowire's carrier density by external means, such as applying a potential to the gate, could modify its surface chemical properties and perhaps change the rate and selectivity of catalytic processes occurring at its surface. Although research on the use of metal-oxide nanowires as sensors is still i...

Chemical Sensing and Catalysis by One-Dimensional Metal-Oxide Nanostructures

SnO2 nanowire gas sensors have been fabricated on Cd−Au comb-shaped interdigitating electrodes using thermal evaporation of the mixed powders of SnO2 and active carbon. The self-assembly grown sensors have excellent performance in sensor response to hydrogen concentration in the range of 10 to 1000 ppm. This high response is attributed to the large portion of undercoordinated atoms on the surface of the SnO2 nanowires. The influence of the Debye length of the nanowires and the gap between electrodes in the gas sensor response is examined and discussed.

Fabrication of a SnO2 Nanowire Gas Sensor and Sensor Performance for Hydrogen

6.2. Orthogonality versus Independence 584 6.3. Cross-sensitivity and Diversity 585 6.4. Multiple Roles of Redundancy 585 7. Data Preprocessing 586 7.1. Baseline Correction 586 7.2. Scaling 587 7.2.1. Global Techniques 588 7.2.2. Local Techniques 588 7.2.3. Nonlinear Transforms 588 8. Drift Compensation 588 8.1. Univariate Drift Compensation 589 8.2. Multivariate Drift Compensation 589 9. Feature Extraction from Sensor Dynamics 591 9.1. Transient Analysis 591 9.1.1. Oversampling Procedures 592 9.1.2. Ad hoc Transient Parameters 593 9.1.3. Model-Based Parameters 593 9.1.4. Comparative Studies 595 9.2. Temperature-Modulation Analysis 596 10. Multivariate Calibration 599 10.1. Multiway Analysis 599 10.2. Dynamical Models 602 11. Array Optimization 604 11.1. Sensor Selection 604 11.2. Feature Selection 605 11.3. Optimization of Excitation Profiles 607 12. Conclusion and Outlook 608 13. References 609

Higher-order chemical sensing.

A review of some papers published in the last fifty years that focus on the semiconducting metal oxide (SMO) based sensors for the selective and sensitive detection of various environmental pollutants is presented.

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Semiconducting Metal Oxide Based Sensors for Selective Gas Pollutant Detection

This paper describes the development and use of microdevices and microarrays in chemical sensor research. The surface-micromachined “microhotplate” structure common within the various platforms included here was originally designed for fabricating conductometric gas microsensor prototypes. Microhotplate elements include functionality for measuring and controlling temperature, and measuring the electrical properties of deposited films. As their name implies, they are particularly well-suited for examining temperature-dependent phenomena on a micro-scale, and their rapid heating/cooling characteristics has led to the development of low power sensors that can be operated in dynamic temperature programmed modes. Tens or hundreds of the microhotplates can be integrated within arrays that serve as platforms for efficiently producing processing/performance correlations for sensor materials. The microdevices also provide a basis for developing new types of sensing prototypes and can be used in investigations of proximity effects and surface transient phenomena.

Microhotplate Platforms for Chemical Sensor Research

Gas microsensor arrays often have closely-spaced elements, typically separated by hundreds of microns. For such devices, crosstalk between elements operated within a gaseous environment is a concern because sensing materials held at elevated temperatures have an increased probability for disrupting gas flows and activating gas-film interactions that can consume analytes or evolve reaction products. To explore such effects in a microarray, microhotplate array platforms were used to sense carbon monoxide. Carbon monoxide sensing was chosen as a model system because the oscillatory kinetics of CO oxidation on Pt films are known to exhibit sensitive gas-phase coupling. Under proper conditions, a Pt/SnO2 microsensor was observed to oscillate between two stable CO/O2 coverage ratios. The high CO coverage state results in higher film conductance, and the oscillation frequency is extremely sensitive to gas-phase CO concentration. Crosstalk was observed between adjacent microsensors with Pt particles supported on SnO2 films, as evidenced by synchronization of sensor response and mixed-mode oscillations (similar to those observed in CO oxidation on Pt films). The range of crosstalk effects was studied by operating a single device in an oscillatory CO sensing mode and heating neighboring elements in an array. The operation of nearby sensors is believed to produce reactions that effectively lower the CO partial pressure at the monitored device, thereby reducing the period of oscillation. The magnitude of the effect was calculated from the frequency change using a CO concentration calibration, and the effect is greater than 10% when operating 15 neighboring sensors. The effects of heating neighboring microsensors have also been studied for hydrogen and methanol sensing on both Pt/SnO2 and bare SnO2 microsensor arrays. While crosstalk effects are observed for these gases, the effects we observe on Pt/SnO2 sensors are less pronounced than in the case of CO sensing. A less than 1% effect occurs for methanol sensing and the largest effect for H2 sensing was an apparent concentration decrease of ≈4% when heating 15 neighboring devices in 10 μmol/mol (10 ppm) of the analyte. On the bare SnO2 devices, we observe an apparent increase in concentration of methanol and H2 of approximately 5 and 15%, respectively. While crosstalk is only measured for the case of conductometric sensing in this work, similar phenomena are likely to occur for sensors that utilize other detection principles.

Chemical crosstalk between heated gas microsensor elements operating in close proximity

A CMOS compatible micro-gas-sensor system was designed and fabricated in a standard CMOS process through MOSIS (1). The chip was post- processed to create microhotplates using bulk micro- machining techniques. Tin-oxide-sensing films were grown over post-patterned gold sensing electrodes using a low-pressure chemical vapor deposition (CVD) technique. The thermal properties of the microhotplates include a one-millisecond thermal time constant and a 10°C/mW thermal efficiency. The microhotplate operates at temperatures between 20°C and 450°C. Tin- oxide-sensing films were isothermally characterized using different gas molecules and concentrations. This paper describes the fabrication, characterization, and design of interface circuitry of a novel CMOS integrated micro-gas-sensor system. .

http://lib.tkk.fi/Books/2001/isbn9512263378/papers/1414.pdf

Implementation of CMOS Compatible Conductance-Based Micro-Gas-Sensor System

Conductometric gas microsensors offer the benefits of ppm-level sensitivity, real-time data, simple interfacing to electronics hardware, and low power consumption. The type of device we have been exploring consists of a sensor film deposited on a "microhotplate"- a 100 micron platform with built-in heating (to activate reactions on the sensing surface) and thermometry. We have been using combinatorial studies of 36-element arrays to characterize the relationship between sensor film composition, operating temperature, and response, as measured by the device’s sensitivity and selectivity. Gases that have been tested on these arrays include methanol, ethanol, dichloromethane, propane, methane, acetone, benzene, hydrogen, and carbon monoxide, and are of interest in the management of environmental waste sites. These experiments compare tin oxide films modified by catalyst overlayers, and ultrathin metal seed layers. The seed layers are used as part of a chemical vapor deposition process that uses each array element’s microheater to activate the deposition of SnO2, and control its microstructure. Low coverage (2 nm) catalytic metals (Pd, Cu, Cr, In, Au) are deposited on the oxides by masked evaporation or sputtering. This presentation demonstrates the value of an array-based approach for developing film processing methods, measuring performance characteristics, and establishing reproducibility. It also illustrates how temperature-dependent response data for varied metal/oxide compositions can be used to tailor a microsensor array for a given application.

J E. Tiffany

Papers

Microhotplate Platforms for Chemical Sensor Research

Chemical crosstalk between heated gas microsensor elements operating in close proximity

Implementation of CMOS Compatible Conductance-Based Micro-Gas-Sensor System

Microarray study of temperature-dependent sensitivity and selectivity of metal/oxide sensing interfaces