Symposium J
Functional Nanomaterials for New Generation Solid State Gas Sensors

Session J-1 - New Nanocarbons (CNTs, Graphene, New 2D Materials)-based Gas Sensors; Nanosilicon-based Gas Sensors

J-1:IL01  Graphene and 2D Materials Based Gas Sensors
W. WLODARSKI, School of Electrical and Computer Engineering, RMIT University, Melbourne, Australia

The application of graphene and 2D materials for the gas sensing has become recently a new fast growing area of interest. Graphene and 2D materials have the tremendous potential for developing gas and vapour sensors due to their high surface to volume ratio. For graphene it is in part due to the fact that each atom in the structure interacts directly with the sensing environment and in part due to the ease with the electronic properties of graphene can be modified by this interaction. Graphene and 2D materials could be combined with different transducing platforms such as: conductometric, Surface Acoustic Waves (SAW), Schottky diodes, mass sensitive, field effect transistors, optical as well as based on the noise spectra measurements. Combining these transducers with graphene and 2D materialsr esults in the development of new generation of sensitive, reversible and stable gas and vapour sensors with several advantages which will be discussed. Numerous examples of recently developed gas and vapour sensors for: NO2, CO, CO2, SO2, H2, NH3, CH4,VOC and H2O will be presented.


J-1:IL02  Smell Sensors – Optical or by Electronics?
W. KNOLL, AIT Austrian Institute of Technology, Vienna, Austria, and Center for Biomimetic Sensor Science, Nanyang Technological University, Singapore

For the sensing of light, e.g., in optical communication, we have extremely powerful devices with the ability to detect even single photons. Similarly, the monitoring of sound in acoustic communication is technically no problem: microphones are available with amazing performance parameters. Only for chemical communication, for smell or taste detection on a technical level we have (nearly) nothing. Despite the fact that the monitoring of chemicals in chemotaxis, i.e., the chemicals-guided search for food of many organisms or the exchange of molecules between species as a way to communicate with each other, is the oldest of our sensory repertoire, we have essentially no technical device that offers the sensitivity and the bandwidth needed to sense and to differentiate many different odors and tastes. Earlier attempts to fill this gap by “artificial noses” failed (with the only notable exception being the “alcohol breath analyser” used by police) mostly because of lack of sufficient sensitivity. In order to develop and present during this talk concepts for smell sensors that could overcome these sensitivity limits we will firstly very briefly refer to the world of smells and give a brief introduction into how mammalians and insects smell. Using a biomimetic approach, i.e., using functional elements (proteins) from nature and combining them with synthetic (nano- and micro-) devices for hybrid transducers, we then describe how (vibrational) spectroscopic approaches (IR and Raman) – enhanced by surface plasmon fields - could help to develop sensor platforms with relevant performance parameters and refer in the end to novel schemes based on electronic (transistor) read-out concepts.


J-1:IL04  High Performance Chemoresistive Gas Sensors based on Self-activated Graphene and Functionalized Graphene
HO WON JANG, Department of Materials Science and Engineering, Seoul National University, Seoul, Korea

Graphene is composed of only surface carbon atoms, which suggests that graphene can be an ideal material for gas sensing. However, graphene-based gas sensors suffer forms very slow and low response to gas molecules, and incomplete recovery at room temperature. In this presentation, we will show our recent studies on room temperature gas sensors based on self-heated graphene and functionalized graphene such as graphene oxide and fluorinated graphene. We reveal that surface functionalization which sensitizes graphene electronically and chemically sensitization is key in improving gas sensing properties of graphene-based materials. We emphasize the promising use of room temperature flexible transparent low-power consumption gas sensors based on functionalized graphene and self-activated graphene for mobile applications and next generation electronic nose. We also show that functionalized graphene can be used for chemoresistive taste sensors, which promise the potential use of graphene-based materials for next generation electronic tongue. Finally, we show that 2-dimensional transition metal disulfides such as MoS2 and WS2 are promising materials for chemoresistive gas sensors.


J-1:IL05  Graphene-based Materials and Nanostructures for Discriminative Gas Sensing
A. SINITSKII, University of Nebraska - Lincoln, Lincoln, NE, USA

Graphene is often considered as a basic building block of a number of other carbon materials, many of which are regarded as promising candidates for gas sensing applications because of their high surface area, chemical stability, electrical conductivity and well-developed chemical functionalization strategies. I will give several examples of graphene-based materials and nanostructures that have been tested for discriminative gas sensing. In the first part of this presentation I will discuss gas sensing properties of nanostructures based on reduced graphene oxide (rGO). The electrical properties of rGO have been previously shown to be very sensitive to surface adsorbates, but poor selectivity of rGO-based gas sensors remains a major problem for their practical use. We addressed the selectivity problem by employing an array of rGO-based integrated sensors. The resulting rGO-based gas sensing systems can reliably recognize analytes of nearly the same chemical nature, such as methanol, ethanol and isopropanol, at a 100% success rate. In the second part of this talk I will discuss similar gas sensing systems based on other graphene-based materials and nanostructures, including atomically precise graphene nanoribbons, three-dimensional graphene nanostructures and pristine graphene.


J-1:L07  Adsorption Characterization of Fabricated Buckypapers (BPs) for Volatile Organic Compound (VOC) Sampling and Analysis
JONGHWA OH, C.T. LUNGU, University of Alabama at Birmingham, Birmingham, AL, USA; E.L. FLOYD, University of Oklahoma, Oklahoma City, OK, USA

This study is aimed to find the most adsorptive material through the fabrication of different types of single-walled carbon nanotubes (SWNTs) for use in VOC passive samplers. Arc discharge (AD) SWNT solution and high-pressure carbon monoxide (HiPco) SWNT powder were fabricated into BPs. Acetone and methanol were used for suspending and/or cleaning SWNTs. Three fabrication methods for AD BPs included non-cleaned BP, acetone-cleaned BP, and methanol-cleaned BP and one for HiPco. The fabricated buckypapers were examined for surface area (SA) and toluene adsorption using a physisorption analyzer and diffusive adsorption isotherm chamber (DAIC) system (lab-designed) at 30 ˚C. Additionally, annealing was performed at 300 ˚C for 1 hour in a furnace. As a result, AD BPs showed 211±61, 322±38, and 387±16 m²/g BET SA for non-cleaned, acetone-cleaned, and methanol-cleaned ones, respectively, while HiPco BPs exhibited 649±9 m²/g SA. The toluene adsorption capacities were 24, 33, 43, and 101 mg/g for non-cleaned, acetone-cleaned, methanol-cleaned AD BPs, and HiPco BP, respectively. SAs after annealing were 858±28, 852±31, and 879±3 m²/g for acetone-cleaned AD BP, methanol-cleaned AD BP, and HiPco BP, respectively (non-cleaned AD BP was excluded from analysis). HiPco BP had the highest SA and


Session J-2 -  Semiconductor/Ion Conduction Oxides-based Gas Nanosensors

J-2:IL01  Nanostructured Semiconductor Gas Sensors for Detection of Sub-ppm Concentrations
T. SAUERWALD, M. LEIDINGER, A. SCHÜTZE, Saarland University, Saarbrücken, Germany; J. HUOTARI, J. LAPPALAINEN, Microelectronics and Materials Physics Laboratories, University of Oulu, Oulu, Finland

The detection of low concentrations of reducing gases in air has many important applications, for example the monitoring of air quality, the detection and specification of odors and medical applications like breath analysis. In these applications, gas concentrations in the ppb and sometimes sub-ppb regions need to be monitored. Recent investigations showed that nanogranular gas sensors have sufficient sensitivity for this task, especially when optimized operation modes (e.g. temperature cycles) are applied. We successfully showed that doped metal oxide sensors (tin oxide and tungsten oxide) can detect and discriminate the most hazardous indoor trace gases formaldehyde, benzene, and naphthalene in low ppb concentrations. To improve selectivity and sensitivity even further, nanocrystalline layers of tin oxide and tungsten oxide have been prepared using pulsed laser deposition (PLD). PLD can be included easily in the production of microstructured sensors and it allows a good variation of granular size and film morphology to study the best composition effectively. The obtained films were able to detect formaldehyde, benzene, and naphthalene in the low ppb range even without noble metal catalysts. This proves PLD to be a promising technique for ultra-sensitive gas sensors.


J-2:IL02  Detection of Particulate Matter by using Limiting Current-type Oxygen Sensor
M. NISHIBORI, H. WAKITA, K. SHIMANOE, Kyushu University, Kasuga, Fukuoka, Japan; Y. SADAOKA, Ehime University, Matsuyama, Ehime, Japan

The development of particulate matter (PM) detection technology is highly required for the protection of the environment and human health. In this study, we propose PM detection by a limiting current-type oxygen sensor since the oxygen partial pressure (P(O₂)) of the reaction field remarkably changes due to the oxidation of PM and CO₂ evolution. The primitive experiments were carried out to detect a carbon black (CB) by using YSZ solid electrolyte whose diffusion layer was coated with CeO₂ catalyst. At 600 ^oC, the detection signal of the loosely-contacted CB (LC-CB) with catalyst decreased below that of blank due to decrease of P(O₂) caused by catalytic combustion of CB. The signal of CB and LC-CB showed the same tendency at 700 ^oC because of the spontaneous combustion. However, tightly-contacted CB with catalyst presented almost the same signal as blank, which indicated the absence of CB on the surface of catalyst due to the combustion below 600 ^oC. These results show that we can detect the CB combustion by P(O₂) change using limiting-current sensor.


J-2:IL04  Highly Selective Detection of Methylbenzenes using p-type Oxide Semiconductors
JONG-HEUN LEE, Department of Materials Science and Engineering, Korea University, Seoul, Republic of Korea

The p-type oxide semiconductors such as Co3O4, CuO, NiO, and Cr2O3 show excellent catalytic activity to oxidize less reactive molecules such as toluene and xylene owing to their multivalent characteristics and abundant oxygen adsorption. In this talk, I will present that Co3O4 nanostructures, Cr-doped NiO hierarchical spheres, Cr-doped Co3O4 nanorods, and Pd-loaded Co3O4 hierarchical hollow spheres show excellent selectivity and sensitivity to methyl benzenes (xylene and toluene). The microreactors can be also used as promising nano-architectures to design highly selective methyl benzene sensors. The Pd-loaded SnO2 and Co3O4 yolk-shell spheres prepared by one-pot spray pyrolysis reaction showed highly selective detection of methyl benzenes. This could be attributed to synergetic combination between the effective in-diffusion of methyl benzenes through thin and semi-permeable shells and their subsequent dissociation into smaller and more active species by Pd nanoparticles on yolk or inner part of shells. The use of p-type oxide semiconductors as sensing materials and the employment of micro-reactor design provide new strategies to achieve selective detection of methyl benzenes, which can be used to monitor indoor air pollution.


J-2:IL05  Nanocomposites-based Oxygen Gas Sensors
M. BREZEANU, B.C. SERBAN, V. AVRAMESCU, C. COBIANU, V. DUMITRU, O. BUIU, A. STRATULAT, Honeywell Romania SRL, Bucharest, Romania; A. DE LUCA, University of Cambridge; S.Z. Ali, Cambridge CMOS Sensors; F. UDREA, Univ. of Cambridge

Applications such as combustion optimization and emission monitoring in automotive, aerospace, domestic and industrial boilers require low cost O2 sensors, capable of reliable operation both at room temperature and in harsh environment conditions (temperatures up to 2250C, relative humidity up to 100%, condensing environments) and of top technical performance (wide detection range - 0-20%, high accuracy - 0.3%, short response time - 3 s, low power consumption – tens of mW). This paper reports on O2 detectors exhibiting such high performance. The sensors employ several types of SrTi1-xFexO3-δ (STFOx) as sensing layer. Synthesized either following a sonochemical route or starting from commercially available powders, STFOx is mixed with carbon nanotubes, yielding matrix nanocomposites O2 sensing layers, deposited by dip-pen nanolithography on fully CMOS-compatible Silicon-on-Insulator (SOI) micro-hotplates, employing tungsten heaters reaching 6500C maximum operating temperature and post-CMOS deposited gold interdigitated electrodes. X ray-diffraction (XRD) and Scanning Electron Microscopy (SEM) analysis, together with experimentally measured O2 response confirm the suitability of the proposed structure, employing nanocomposite sensing layers, for adverse environment applications.


J-2:IL06  Effective Design and Fabrication of Harsh Environment and Biomedical Gas Sensors
P.K. DUTTA, Department of Chemistry and Biochemistry, The Ohio State University, Columbus, OH, USA

Solid-state electrochemical devices composed of stabilized zirconia electrolytes (YSZ), e,g, oxygen sensor is used extensively for sensing in combustion environments. However, sensors for detecting other gases have not been as forthcoming. We will present our work in the area of NOx and NH3 sensors. We find that the heterogeneous catalytic activity of WO3, yttria-stabilized zirconia (YSZ), and Pt containing zeolite Y (PtY) have a significant influence on the performance of solid-state potentiometric gas sensors. Pt electrodes covered with PtY and WO3 are used as the reference and working electrodes because of the significant reactivity difference. Using highly catalytic active PtY to filter incoming gas mixtures can effectively remove interferences from CO, propane, NH3, as well as minimize effects of O2, CO2, and H2O. A second strategy involves the use of p and n-type semiconducting oxides to cancel contributions of oxidizing gases, while enhancing the response to NO, thus leading to sensitivity of ppb levels. The improvement in sensitivity has led to new applications, particularly monitoring human breath for diagnosis of asthma and upper airway inflammation. This strategy has been adapted to NH3. We will present MEMS-based strategies for manufacture of gas sensors.


J-2:IL07  Investigating the Selective Behaviour of CuO in Gas Sensing Applications
S. PALZER¹, J. WÖLLENSTEIN^1,2, J. KNEER¹, ¹Laboratory for Gas Sensors, Department of Microsystems Engineering, University of Freiburg, Germany; ²Fraunhofer Institute for Physical Measurement Techniques; Freiburg, Germany

The lack of selectivity in metal-oxide based gas sensors oftentimes prevents their use in advanced analytical applications. One exception is p-type semiconducting copper(II)oxide (CuO) which can be operated in a temperature modulation scheme to specifically detect hydrogen sulfide (H2S). Consequently, the fundamental understanding of underlying surface processes is necessary since it may lead to the development of further material systems with similar sensing properties. This presentation will illustrate how the thermal modulation protocol to determine the H2S concentration works and will show results obtained with other relevant trace gases underlining the selectivity of the approach. To investigate the surface reaction responsible for the gas induced changes in the electrical behaviour, near-edge x-ray absorption fine structure spectroscopy in combination with x-ray microscopy (NEXAFS-TXM) for ex-situ measurements has been performed. This technique has yielded spatially resolved information about the surface reaction between a trace gas and the gas sensitive material. Due to current experimental restrictions the analysis had to be performed ex-situ, which made the H2S-CuO reaction well suited. In the future, the technique may be used to directly visualize reactions in-situ.


J-2:IL08  Sensitivity and Selectivity of SnO2-based Sensor for CO and H2 Detections
XING-MIN GUO, JIE-TING ZHAO, XI-TAO YIN, University of Science and Technology Beijing, Beijing, China

SnO2-based sensor as an important type of SMO sensors, have many advantages like low cost, small size, high reliability, and long operating life comparing to other gas sensors, but selectivity has been a major obstacle on the application for discriminating gas species in mixture of multi-reduction gases. In this presentation, we will introduce our recent researches on SnO2-based sensor. It includes as follows: The sensing characteristics of SnO2-based materials modified, including Fe-, In- doping and Pd-, Au-, and Pt- loading respectively into SnO2 prepared by Sol-gel method, to CO and H2 detections were investigated to understand effects of material modification, sensor fabrication and working temperature on the sensitivity and the selectivity, further an array SnO2-based sensor with multi-factor detection simultaneously is fabricated and the concentration of CO and H2 gases are calculated according to the quantitative relationship of responses with material modification, sensor fabrication and working temperature, finally an analysis model when multi-reduction gases coexist is built to compensate the weakness of SnO2-based sensor on selectivity.


J-2:L09  Synthesis and Gas-sensing Properties of Nanoporous Cobalt Oxide Materials
S. VETTER, S. HAFFER, T. WAGNER, M. TIEMANN, Faculty of Science, Department of Chemistry, University of Paderborn, Germany

Nanostructured Co3O4 is well known for its interesting properties in catalytic processes, e.g. in CO oxidation). Moreover, as a p-type semiconductor, Co3O4 is also an interesting material for resistive gas sensors. This applies especially to nanoporous Co3O4 with large surface-to-volume ratio and defined nanostructure and morphology. We present nanoporous Co3O4 prepared by structure replication (nanocasting). The materials exhibit a high degree of thermal structural stability. Our synthesis route yields porous particles with defined and uniform shapes and sizes (few hundred nm). Sensors prepared from mesoporous Co3O4 show a strong response to CO gas in low ppm concentrations. The correlation between gas concentration and change in resistance turns out to be qualitatively different for measurements at 200 and 300 °C, respectively, indicating substantial differences in the sensing mechanism, i.e. in the interaction between the gas molecules and the semiconductor's surface. In general, the change in conductivity in Co3O4 is attributed to chemisorbed oxygen species, which are consumed by reaction with CO. Depending on the temperature, migration of oxygen in the crystal lattice occurs, affecting the sensors' characteristics.


J-2:L11  Enhanced Gas Sensing Properties of Different ZnO 3D Hierarchical Structures
A. FIORAVANTI^1,2, A. BONANNO¹, M. MAZZOCCHI³, M.C. CAROTTA⁴, M. SACERDOTI⁵, ¹CNR-IMAMOTER Ferrara, Ferrara, Italy; ²Dipartimento di Chimica, Università di Parma, Parma, Italy; ³CNR-ISTEC Faenza, Italy; ⁴Consorzio Futuro in Ricerca, Ferrara, Italy; ⁵Dipartimento di Fisica e Scienze della Terra, Università di Ferrara, Ferrara, Italy

Zinc oxide (ZnO) is known for its intrinsic characteristics (wide band gap n-type semiconductor, high exciton binding energy, piezoelectricity, etc) and it is widely employed in different applications. Despite of a large number of published papers about ZnO, only few authors investigated 3D hierarchical structures and their gas sensing properties. For this work, four different ZnO morphologies were successfully synthesized trough wet chemical routes starting from a water solution of zinc nitrate hexahydrate. SEM observations showed simple nano-particles and three nano-particles aggregates with bi-pyramidal, ellipsoidal and star-like shape, while XRD analysis confirmed an hexagonal wurtzite crystalline phase (space group P63mc) for all materials. A comparison between sensing layers prepared with the above materials has been carried out. Electrical characterizations showed that all films exhibited semiconductor n-type behavior, the conductivity being modulated by the inter-grain Schottky barrier. The gas sensing properties have been tested toward an extensive variety of analytes such as acetone, formaldehyde, ethanol, etc.


J-2:L11b  Enhanced Formaldehyde NiO Gas Sensing Properties by a Controlled Zn Doping using a Malonate Coprecipitation Synthesis
R. LONTIO FOMEKONG^1,4, D. LAHEM², M. DEBLIQUY³, J. LAMBI NGOLUI⁴, A. DELCORTE¹, ¹Institut de la Matière Condensée et des Nanosciences, Université Catholique de Louvain, Louvain-La-Neuve, Belgium; ²Materia Nova ASBL, Mons, Belgium; ³Service de Science des Matériaux, UMONS, Mons, Belgium; ⁴Laboratoire de Physico-chimie des Matériaux, département de Chimie Inorganique, Université de Yaoundé I, Yaounde, Cameroo

Formaldehyde (HCHO) is one of the most hazardous pollutants among the volatile organic compounds (VOCs). It is commonly released in residential and industrial occupational environments from many sources such as household products, building and indoor decorative materials. HCHO can cause serious health effects like asthma, pulmonary damage and cancer [1]. Then, the World Health Organization (WHO) has set a 30 min exposure limit of 0.08 ppm [2]. It is therefore urgent to have a high sensitive, good selective and rapid response formaldehyde sensor. Gas sensor based metal oxide and especially NiO appears as a good candidate for that [3, 4].
In this work we report the investigation of the effect of Zn dopant on the NiO formaldehyde sensor properties. NiO and Ni_1-xZn_xO (x= 0.02, 0.03, 0.04) were synthesized by malonate copricipitation, drop-coated on alumina substrate and characterized by XRD, SEM, XPS and BET. The gas sensing properties were investigated for HCHO, H₂, CO and NO₂ gases at different temperatures. While XRD show the presence of one single phase identified as a cubic NiO for all the Zn-doped NiO, XPS give the chemical state and the distribution of Ni and Zn on the surface of the materials. Ni_0.97Zn_0.03O shows the best sensing performance for HCHO. The chemical state of Zn, its distribution on the surface of material and the catalytic effect of ZnO could be responsible for this performance.
[1] P.R.  Chung, C.T. Tzeng, M.T.  Ke, C.Y.  Lee, Sensors 13 (2013) 4468–4484. [2] World Health Organisation, Regional Office for Europe, “WHO guidelines for indoor air quality: selected pollutants”, Geneva 2010, ISBN: 9789289002134. [3] C.Y. Lee, C.M. Chiang; Y.H. Wang, R.H. Ma, Sens. Actuators B Chem. 122 (2007) 503–510. [4] D. Lahem, F.R. Lontio, A. Delcorte, L. Bilteryst, M. Debliquy, IOP Conf. Series: Materials Science and Engineering 108 (2016) 012002.


J-2:IL12  UV Activated Hollow ZnO Microspheres for Selective VOCs Sensors at Low Temperatures
XIAOGAN LI, Dalian University of Technology, School of Electronic Science and Technology, Institute for Sensor Technology, Dalian, Liaoning, P.R. China

UV activated metal oxides for chemiresistive-type gas sensors have been recently studied aiming to lower the working temperature and thus lower-power consumption. In this work, hollow ZnO microspheres were prepared by template-assisted method and examined for VOCs detection with UV LED illumination at lower temperatures than 100 °C. The as-synthesized ZnO based sensor indicated an excellent response and good selectivity to different concentrations of ethanol (10-1000 ppm) with low-powered UV LED (2.5 mW) at 80 °C. The response time is only ~6s while the recovery is a little sluggish (~94s). The repeatability and long term stability of the sensor and sensor response were also investigated. However, at higher temperatures around 250 °C, the UV effect became negligible.
 

J-2:IL14  New Semiconductor Gas Sensor Based on Enhancing Oxygen Partial Pressure
K. SHIMANOE, N. MA, R. KATO, M. NISHIBORI, Kyushu University, Kasuga, Fukuoka, Japan

Semiconductor gas sensors are widely used for detection of inflammable and toxic gases. To detect such low concentration gases, we have reported materials design including important three functions i.e. receptor function, transducer function and utility factor. Receptor function concerns the ability of the oxide surface to interact with the target gas. In addition, when the surface is loaded with a foreign receptor like PdO, it acts as a receptor stronger than the adsorbed oxygen. In addition, we found that small size of PdO (less than 3nm) shows high sensor response to inflammable gases even under humid condition. Transducer function concerns that the electron transport through the contact can thus be achieved by migration or tunneling of the surface electrons, indifferent to the bulk electrons inside. The device resistance is then inversely proportional to the surface density of electrons. Therefore the sensor response enhances with increasing oxygen partial pressure. For the utility factor, the target gas molecules diffuse the inside of a sensing body while reacting with the oxide surface. In this presentation, we explain the above design and combination of three factors. In addition, the receptor function enhanced by increasing oxygen partial pressure in the sensing film will be presented for usual type and MEMS-type gas sensors.


J-2:L16  Patterned Laser-grown Nanowires for Hydrogen Isotopes Detection with SAW-sensors
A. MARCU¹, C. VIESPE¹, I. NICOLAE¹, B. BUTOI¹, D. PAUL¹, L. AVOTINA^1,2, C.P. LUNGU¹, ¹National Institute for Laser Plasma and Radiation Physics, Laser Department, Bucharest-Magurele, Romania; ²Institute of Chemical Physics, University of Latvia, Riga, Latvia

ZnO nanowires were grown on the active sensor surface of a surface acoustic wave (SAW) sensor via a vapour–liquid–solid (VLS) technique using pulsed laser deposition (PLD) as the particle source. The nanowire length and diameter were controlled by the growth time and temperature, while the nanowire disposal on the active surface area by the catalyst patterning. The sensor response at room temperature to various hydrogen (H2) concentrations was recorded for different ZnO morphologies and diameters and compared with the performance of a thin-film sensor with a comparable amount of ZnO material. The sensor response depended on the ZnO volume and the morphology of the active surface. An increase in the ZnO volume enhanced the frequency shift for the same H2 concentration, while a larger surface area of the longer nanowires enhanced the sensor response to low H2 concentrations, enabling detection of concentrations as low as 0.01%. Sensor noise proved to be controlled by the nanowire disposal geometry on the sensor active surface.


J-2:L17  Electronic Dopants in SnO2 and ZnO: Effect for Surface Acidity and Gas Sensor Behavior
A.V. MARIKUTSA, N.A. VOROBYEVA, M.N. RUMYANTSEVA, A.M. GASKOV, Chemistry Department, Moscow State University, Moscow, Russia

Doping of wide gap semiconductor oxides, which gives rise to the increase of carrier concentration, is important to reduce the resistance of sensor materials especially at conditions close to room temperature. We used Sb(V) as electronic dopant for SnO2 and Ga(III) or In(III) for ZnO. The nanostructured SnO2(Sb), ZnO(Ga) and ZnO(In) have been prepared by chemical co-deposition from water solution with different concentration of dopants. The powder XRD measurement of lattice parameters deviation on dopant concentration (from ICP-MS) allowed to establish that solubility is about 5 at.% and 1 at.% for Sb in SnO2 and Ga and In in ZnO, respectively. The materials with dopant concentration below solubility limits were selected for active centers characterization and gas sensor measurements. The Lewis surface acidity was characterized by TPD-NH3, the concentration of hydroxyl groups (Brønsted acidity) was estimated by DRIFT. The nature of spin centers was characterized by EPR spectroscopy. The gas sensor behavior of materials was tested towards CO, NH3, NO2 and H2S. The results demonstrate unequal dopant effect on the surface chemistry of nanoscalled SnO2 and ZnO.


J-2:IL18  Metal Oxide Nanocomposites and Surface Modifications for Chemical Sensing
M. EPIFANI, CNR-IMM, Lecce, Italy

The successful application of catalytic concepts in the performance improvement of chemoresistive gas-sensors stimulates to search for other possible applications of heterogeneous catalysis. In this work, the synthesis and gas-sensing application of nanocrystals of three different well-known catalytic systems will be reviewed, e.g. TiO2-V2O5, TiO2-WO3 and SnO2-V2O5. The three systems span a range of structures, from surface modification to true nanocomposites to nanocrystal wrapping by V2O5-like layers. The sol-gel/solvothermal synthesis of these systems will be first described. Then, their structural and chemical evolution with the thermal treatments will be illustrated, with focus onto the material structure used for device processing. Finally, examples of the gas-sensing properties will be shown. For TiO2-V2O5 and TiO2-WO3, a response improvement to acetone and ethanol up to two orders of magnitude with respect to pure TiO2 was observed, demonstrating the effectiveness of coupling different oxides in synergistic configurations. For SnO2-V2O5, the response enhancement to ammonia with respect to pure V2O5 will be used for demonstrating the properties change of thin, non-crystalline V2O5 layers with respect to the bulk counterpart, due to interaction with the SnO2 core material.


J-2:L19  Pulsed Laser Deposited Platinum Decorated Tin Oxide Nanotree Layers as Highly Sensitive Gas Sensing Material
J. HUOTARI¹, T. HAAPALAINEN¹, T. BAUR², C. ALÉPÉE³, J.PUUSTINEN¹, J. LAPPALAINEN¹, ¹Microelectronics and Materials Physics Laboratories, University of Oulu, Oulu, Finland; ²Laboratory for Measurement Technology, Department of Mechatronics, Saarland University, Saarbrücken, Germany; ³SGX Sensortech SA, Corcelles-Cormondrèche, Switzerland

Pulsed laser deposition with XeCl (λ = 308 nm, τ = 25 ns) laser was used to deposit SnO2 and ZnO-modified SnO2 layers with and without platinum decoration to be studied as resistive gas sensing material. Oxidized silicon substrates and commercial microheaters were used as sample platforms for structural characterization and gas sensing measurements, respectively. The layers were deposited at room temperature in high oxygen partial pressures from 0.1 mbar to 0.2 mbar during the process. The layers were either pure metal oxide structures or a layered metal oxide and platinum structures with 4 alternating layers of both. Post-annealing processes in furnace were used after the deposition. The structural characterization showed that the layers were formed of nanotree-like morphology with a fractal growth of nanoparticles. At 200 °C, the pure metal oxide layers were highly sensitive to NO gas, showing an increase in resistance of 1700 % for 50 ppm of NO. When the temperature was increased to 350 °C the layers responses to NO was strongly decreased, but the responses to reducing gases, such as H2, NH3 and especially to CO, were strongly increased. The platinum decoration was found to increase the responses to reducing gases in both measurement temperatures, with extremely high response.


J-2:L21  MgO-modified SrMoO4 and Nano-SrMoO4 Sensing Materials for H2 and SO2 Detection at High Temperatures
E. CIFTYUREK, K. SABOLSKY, E.M. SABOLSKY, Department of Mechanical & Aerospace Engineering, West Virginia University, Morgantown, WV, USA

Hydrogen (H2), hydrogen sulfide (H2S) and sulfur dioxide (SO2) sensors are required for the compositional monitoring of coal syngas and natural gas fuel streams at >600 °C, especially for applications of solid-oxide fuel cells, and other chemical and energy reactors. In this work, two high temperature stable compositions, nano-SrMoO4 and MgO/SrMoO4, were hydrothermally synthesized. A thick film of each was deposited by a screen-printing onto ceramic substrates with Pt-based electrodes to form a chemi-resistive type sensor. High-temperature gas sensing experiments for H2, H2S, and SO2 were conducted at 600-1000°C for 500-2000 ppm levels of the target gas. Sensitivity as a function of time (and specie concentration) were measured, and the activation energy for the sensing mechanism was characterized. The chemical and electronic states (work function and band-gap), and microstructure of the developed nanomaterial sensing layers were characterized by means of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS), atomic absorption spectroscopy (AAS), X-ray diffraction (XRD), Raman spectroscopy, temperature programmed reduction (TPR) and transmission electron microscopy (TEM).
 
 
Session J-3 - Nanometal-based Gas Sensors; Polymer-based Gas Sensors

J-3:IL02  Ultra-pure Organically-functionalised Gold Nanoparticles Nano-assemblies for Schottky-diode Gas Sensors
R. IONESCU, T.G. WELEAREGAY, G. PUGLIESE, Rovira i Virgili University, Tarragona, Spain; U. CINDEMIR, L. Österlund, Uppsala University, Uppsala, Sweden and Molecular Fingerprint Sweden AB, Uppsala, Sweden

Monolayer-capped metallic nanoparticles present excellent features for the detection of trace concentrations of volatile organic compounds (VOCs). These nanomaterials are generally produced employing conventional wet-chemistry methodology, which gives rise to traces of residual compounds. Employing a novel technological approach, we were able to produce ultra-pure organically-functionalised gold nanoparticles (AuNP). In the first step, ultra-pure AuNP were fabricated by the Advanced Gas Deposition (AGD) technique. The AuNP were subsequently functionalised with selected organic ligands by dip-coating in dissolutions prepared with conveniently volatile solvents used in applications that require high-purity on evaporation. Chemical sensing devices based on these ultra-pure nano-assemblies showed a Schottky-diode behaviour. Gas sensing measurements were performed by exposing the sensing devices to different VOCs. For performing the measurements, the current between sensors electrodes was monitored while the voltage applied was successively swept backward and forward between -10V and +10V. Sensors responses were extracted from the I-V characteristic curve at every specific voltage. The gas sensing experiments demonstrated the suitability of these devices for VOCs detection.

 
Session J-4 - Nanocomposite/Hybrid/Heterostructure-based Gas Sensors

J-4:IL01  Nanostructured Hybrid Thin Films for Gas Sensing
R. RIEDEL, TU Darmstadt, Darmstadt, Germany

The growing interest in the use of hydrogen as main fuel has increased the need for pure hydrogen (H2) production and purification. There are several by-products (CO, H2O, CO2) associated with the production of hydrogen which might damage the production rate. Therefore, separation of hydrogen from other gases is an important step in the hydrogenproduction process. If H2 can be selectively removed from the product side during hydrogen production in membrane reactors, then it would be possible to achieve complete CO conversion in a single-step under high temperature conditions. The main goal of the present work is the high temperature H2 purification and sensing by applying polymer-derived ceramics. To prove the concept, microporous SiBCN, Si3N4 and SiCN ceramic membranes have been synthesized by the polymer-pyrolysis route and their performance for the hydrogen separation have been evaluated in tubular membranes as well as in planar chemiresistors. The stability and sensing characteristics of SnO2 sensors coated with amorphous microporous SiBCN layers have been studied in oxygen-free atmospheres. Amorphous microporous-Si3N4 ceramic layers deposited on the top of GaN sensing layer followed by dry ammonia treatment leads to improved H2 to CO selectivity of Si3N4/GaN sensors.


J-4:L03  Plasmon Enhanced MOX Gas Sensor
N. CATTABIANI, C. BARATTO, G. FAGLIA, E. COMINI, G. SBERVEGLIERI, Sensor Lab, CNR-INO and University of Brescia, Brescia, Italy

The standard method to enhance the sensitivity of metal oxide (MOX) based gas sensors is to functionalize the MOX substrate with metallic nanoparticles (MNP) that act as a catalysers for a specific reaction with the target gas at a proper working temperature, usually in the order of hundreds of °C [1]. The high working temperature is detrimental to the long term stability of the sensor; moreover high working temperature might not be suitable in particular environments (explosive, biological). Here we present an innovative and low power consuming method to improve the sensitivity of MOX substrates functionalized with MNP by exploiting the plasmonic properties of such substrates. We developed sensors composed of bundle of MOX nanowires grown by Vapor-Liquid-Solid deposition on Silicon and fused silica. Silver nanoparticles have been anchored on the nanowires by means of magnetron sputtering at 400°C. These sensors have been tested as room temperature gas sensors in dark and visible light illumination. We simultaneously monitored the conductometric response and the optical absorption of the hybrid MOX-MNP sensor during the gas inlets and we proved that the highest sensitivity is achieved when the sensors are illuminated at the plasmon resonance frequency of the anchored MNP.


J-4:L04  Organic and Inorganic Photosensitizers for Visible Light Activated MOS Gas Sensors
M.N. RUMYANTSEVA, A.S. CHIZHOV, A.V. MARCHEVSKY, E.V. PODOLKO, E.V. LUKOVSKAYA, O.A. FEDOROVA, A.M. GASKOV, Moscow State University, Moscow, Russia

Creation of new materials with gas sensitivity at room temperature (RT) is a key direction in the development of gas sensors technology. We report the realization of a new principle of gas detection using the resistive type sensor operating at RT under a low power visible light source. Metal oxide semiconductors (MOS) are transparent in this spectral range. The solution of this problem becomes possible when sensitive materials are nanocomposites based on nanocrystalline MOS and photosensitizers. The relative position of the energy bands of MOS and sensitizer must ensure the transfer of photoexcited carriers from the sensitizer to the semiconductor matrix. Selected photosensitizers: CdSe quantum dots (QDS), Au nanoparticles with surface plasmon resonance, and organic dyes – tetrathiafulvalene (TTF) derivative, are characterized by absorption in the visible range of the spectrum and have high extinction coefficients. Nanocrystalline MOS (SnO2, ZnO, In2O3) were synthesized by wet chemical method. CdSe QDs were obtained via high temperature colloidal synthesis. Au nanoparticles were synthesized by colloidal route. Sensor measurements demonstrated that sensitized nanocrystalline MOS can be used for gas detection under visible light illumination at RT without any thermal heating.


J-4:L05  Green Synthesis of Biopolymer-silver Nanoparticles Composites for Gas Sensing
S.A. PANDE, Laxminarayan Institute of Technology, Nagpur, India

Silver nanoparticles, one of the noble metal nanoparticles, have attracted extensive attention in the past decades due to its wide application in catalysis, chemical sensing and nanotechnology. Recently synthesis of silver nanoparticles based on nature biopolymer has attracted intense attention because of their rich source and biocompatibility. In a recent research work, a very simple, low cost eco-friendly method is presented for the synthesis of silver nanoparticles to be used in colorimetric optical sensors based on localized SPR (LSPR) measurement for gas ammonia. Silver nitrate salts are reduced using gaur gum which acts as capping and reducing agent. Commonly used reducing agents such as trisodium citrate or sodium borohydride are replaced by a more environmental friendly green method using natural polysaccharide. Nanocomposite films ~ 1.5 μm thickness was fabricated using GG and Ag NPs. The uniformity of nanoparticles size was measured by SEM and TEM, while face centred cubic structure of crystalline silver nanoparticles was characterized using X-ray diffraction technique. The optical properties   of the composite film were tested by UV-Vis Spectroscopy. The formation of GG/Ag nanocomposite films were confirmed by SEM images. Also the resistivity of nanocomposite thin film was measured which could be then used for gas sensing application.


J-4:IL06  Nanoscale Metal Oxide-based Heterojunctions for Gas Sensing
D.R. MILLER, S.A. AKBAR, P.A. MORRIS, Department of Materials Science and Engineering, The Ohio State University, Columbus, OH, USA

The most promising recent advances in resistive-type gas sensors have come from semiconducting oxide nano-heterostructures which incorporate two or more sensor materials on the nano-scale in either core-shell, decorated nanowire or hierarchical structures. Rapid progress in new synthesis routes has made it possible to engineer and optimize specific types of nano-heterostructures for a given application but a lack of fundamental understanding of the mechanisms of these heterostructures limits progress in nano-heterostructure design. Several proposed methods to understand and characterize these structures will be discussed, as well as current problems and pitfalls that need overcome or avoided. One method is to fabricate single-nanowire sensors to obtain electrical measurements precisely knowing the geometry of the tested material without the confounding variables of inter-particle junctions and variations in particle size and morphology. The electronic structure of these materials can also be probed using high-spatial resolution techniques in a STEM microscope such as valence-loss EELS and STEM-cathodoluminescence. These techniques provide valuable information on the band gap and defect states of individual nanowires and nanoparticle coatings that have implications on sensing.


J-4:IL07  Sensing Properties of Diode-type Gas Sensors
Y. SHIMIZU, T. HYODO, Graduate School of Engineering, Nagasaki University, Nagasaki, Japan

Porous TiO₂ thin films could be fabricated easily on Ti metal substrates by anodic oxidation in H₂SO₄ solution.  These anodically oxidized TiO₂ thin films equipped with a pair of top Pd-based and bottom Ti metal electrodes show diode-type H₂ response properties in air, i.e. non-linear I-V characteristics in air, while almost ohmic behavior in H₂ balanced with air.  Another important feature of this type sensor is reversible H₂ response ability even in N₂ atmosphere and very high H₂ selectivity against various kinds of inflammable gases.  Thus, the H₂ sensing mechanism is suggested to arise from dissolution of H atoms into the Pd electrode and then a decrease in work function of Pd and in turn a decrease in Schottky barrier height at the interface between the Pd electrode and the TiO₂ thin film, which had been formed by the difference between the work function of Pd and the electron affinity of TiO₂.  Due to its H₂ sensing mechanism, this sensor shows O₂- and H₂O-dependent H₂ response properties.  This is because the surface of Pd electrodes is likely oxidized in air and is covered with hydroxyl groups in humid environment, both of which prevent the dissociation of H₂ and then dissolution of H atoms into the Pd-based electrode.  Therefore, several approaches were directed to eliminate the interferences from O₂ and H₂O.  Those include alloying of Pd with another metal, coating with polymer films or an Au film on the top Pd-based electrode.  Excellent H₂ sensing performance obtained by these approaches will be reviewed.


J-4:IL08  MIP-nanoparticle Composites and Core-shell Nanoparticles leading to Materials with Strongly Enhanced Sensitivity
P. LIEBERZEIT¹, G. MUSTAFA^1,2, W. CUYPERS¹, M. ZEILINGER¹, K. NAVAKUL^1,3, C. SANGMA³, ¹University of Vienna, Faculty for Chemistry, Department of Analytical Chemistry, Vienna, Austria; ²Quaid-e-Azam University, Islamabad, Pakistan; ³Kasetsart University,Faculty of Sciences, Department of Chemistry, Bangkok, Thailand

Molecularly imprinted polymers (MIP) have proven highly suitable for selectively detecting a wide range of analytes both in gas and liquid phase. Recent work showed that incorporating nanostructures into MIP leads to composites and core-shell nanoparticles exhibiting cumulative effects, e.g. strongly increased binding of the target compound. Some examples are: - Metal sulfide/MIP composites lead to triple sensitivity toward alcohol and thiol vapors, than would be expected from the responses of the two compounds. - TiO2-MIP core-shell nanoparticles allow detecting melamine in food without sample pretreatment. - Modifying the charge state of MIP films with graphene oxide results in strongly increased protein binding. The approaches combine the best from two worlds: the MIP provides steric and functional recognition, i.e. it acts both as a selector and as a pre-concentrator. The filler usually is affine towards the analyte of interest, e.g. due to Person hardness or electrostatic properties. As such it strongly increases affinity of the composite towards its target. However, this is much more effective in the MIP, than the corresponding non-imprinted polymer (NIP), because the latter contains no cavities and thus efficiently shields the affinity nanomaterial from its surroundings.

 
Poster Presentations

J:P01  Study of Al-ZnO Thin Films Deposited by RF Magnetron Sputtering for Gas Sensor Application
G.W.A. CARDOSO¹, G. LEAL¹, A.S. DA SILVA SOBRINHO², D.M.G. LEITE², M. MASSI^1,2, ¹Federal University of São Paulo – Science and Technology Institute, São José dos Campos, SP, Brazil; ²Technological Institute of Aeronautics, Plasmas and Processes Laboratory, São José dos Campos, SP, Brazil

The first gas sensor based on semiconductor was developed in 1968 by Taguchi as an alternative technology to others gas sensors due to its ability to be not consumed during the detection process. Among these materials, ZnO has been widely used mainly due to its high bandgap (3.7eV) and piezoelectric properties. In this work, Al doped ZnO thin films were deposited by RF magnetron sputtering using a ZnO target. The Al dopant was inserted by placing 1mm diameter wires on top of ZnO target. The processes were carried out at fixed values of power (100W), pressure (15mTorr) and target-substrate distance (80mm) and using a gas mixture of Ar (17.0sccm) and O2 (1.0sccm). The thin films were characterized after an annealing process (600ºC @ 10-3Torr for 60min) by mechanical profilometer, four points probe, X-ray diffraction (XRD) and Rutherford backscattering spectroscopy (RBS). The results indicated that all films presented the wurtzite ZnO structure with (0002) peak centered at 2θ=34.3º. RBS results indicated stoichiometric ZnO films with 1 to 3% of Al. Pure ZnO film presented insulating characteristics, while the Al-doped ZnO films presented a reduction in the resistivity values down to 10-1Ω.cm for the non-annealed films and 10-3 Ω.cm for the annealed ones.


J:P02  Crystalline Size Dependent Effect on the Gas Sensing Properties of ZnO Films based on Quantum Dots
J.F. DENG, QIUYUN FU, D.X. ZHOU, W. LUO, Y.X. HU, Z.P. ZHENG, School of Optical and Electronic Information, Huazhong University of Science & Technology, Hongshan District, Wuhan City, P.R. China

Good quality zinc oxide films based on ZnO QDs synthesized via a colloidal procedure were deposited on alumina substrates through a room-temperature spin-coating process in air ambient for H2S gas sensing application. Various grain sizes were obtained by changing the annealing temperature. The comparison of the response of ZnO films annealed at different temperature was made and the sensors exhibited quick response and recovery. The effect of the grain size of ZnO films on the H2S sensing properties was investigated. It was found that ZnO films showed the highest response to 68.5ppm H2S, when the grain size was comparable with twice Debye length after annealing at 400 °C. A maximum response of ZnO films annealed at 400°C to 68.5ppm H2S was 1600 and obtained at 105°C. Meanwhile, ZnO films showed good response selectivity toward H2S against SO2, NO2 and NH3.