J. Brunet

Evaluation of atmospheric pollution by two semiconductor gas sensors

Abstract After a brief introduction about: the different gaseous pollutants present in the atmosphere; the cause of their concentration rise; their consequences on human health; and the different devices used to monitor each gas, two types of semiconductor gas sensors are presented. The first of them is a simple InP-based resistive sensor. The sensitive layer is a thin n-type InP epitaxial layer grown on a semi-insulating InP substrate. The electrical resistance, increasing in the presence of oxidising gases, is measured between two ohmic contacts. The second sensor is a thin film of copper phthalocyanine deposited by sublimation on an alumina substrate. The conductivity of the phthalocyanine layer increases in the presence of oxidising gases. Influence of NO 2 and O 3 , especially at low concentration, is investigated. Influence of other parameters like operating temperature or interfering gases is also studied. In conclusion, optimisation of sensor characteristics to improve the evaluation of oxidising air pollutants concentrations is discussed.

Air quality evaluation by monolithic InP-based resistive sensors

Abstract After a brief summary on environmental pollution and commonly used materials for NO 2 and O 3 sensing, simple InP-based resistive sensors are studied. The gas sensitive device is a thin n-type InP epitaxial layer grown on a semi-insulating InP substrate. The electrical resistance of the layer, measured between ohmic contacts, increases in the presence of oxidising gases, the most important variations occurring in NO 2 and ozone-containing atmospheres. A complete study performed in laboratory at a moderate operating temperature of 80°C on the sensor exposed to nitrogen dioxide enabled to obtain a precise NO 2 calibration curve. In an outdoor application, only NO 2 , ozone and humidity seem to act on the sensor resistance, confirming laboratory experiments. One of these devices was used to monitor air pollution in an urban atmosphere. The resistance changes were compared with the readings of a commercial analysing equipment. The results clearly show that the sensor resistance variation follow the total O 3 and NO 2 concentrations variation in air. Different operating methods are discussed for these type of sensors, depending on what of these two pollutants has to be measured.

Nanomaterials for the Selective Detection of Hydrogen Sulfide in Air

This paper presents a focused review on the nanomaterials and associated transduction schemes that have been developed for the selective detection of hydrogen sulfide. It presents a quite comprehensive overview of the latest developments, briefly discusses the hydrogen sulfide detection mechanisms, identifying the reasons for the selectivity (or lack of) observed experimentally. It critically reviews performance, shortcomings, and identifies missing or overlooked important aspects. It identifies the most mature/promising materials and approaches for achieving inexpensive hydrogen sulfide sensors that could be employed in widespread, miniaturized, and inexpensive detectors and, suggests what research should be undertaken for ensuring that requirements are met.

Comparisons between NO/sub 2/- and O/sub 3/-sensing properties of phthalocyanines and n-InP thin films in controlled and noncontrolled atmospheres

This paper describes two different semiconductor gas sensors devoted to the detection of oxidizing pollutants in the atmosphere. The first sensor consists of thin films of phthalocyanines as sensing layers (CuPc, ZnF/sub 16/Pc, and LuPc/sub 2/) evaporated onto alumina substrate fitted with interdigitated electrodes. The second sensor is realized with a mineral monocrystalline semiconductor: n-doped epitaxial layer grown on a semi-insulating substrate of indium phosphide. Each sensor has been submitted to low-controlled concentrations of ozone and nitrogen dioxide, and their detection characteristics, such as response time, stability, and sensitivity, are described. Comparison of these two sensors shows their complementary sensing characteristics, and NO/sub 2/ and O/sub 3/ act in the same way. Measurements under noncontrolled atmosphere (urban air) have been realized and have demonstrated the potentialities of these structures to be used as oxidizing pollutant detectors. Proposed methods to improve the detection of oxidizing species in urban air are discussed.

Electrochemical Sensors Based on Screen-Printed Electrodes: The Use of Phthalocyanine Derivatives for Application in VFA Detection.

Here, we report on the use of electrochemical methods for the detection of volatiles fatty acids (VFAs), namely acetic acid. We used tetra-tert-butyl phthalocyanine (PcH2-tBu) as the sensing material and investigated its electroanalytical properties by means of cyclic voltammetry (CV) and square wave voltammetry (SWV). To realize the electrochemical sensing system, the PcH2-tBu has been dropcast-deposited on carbon (C) orgold (Au)screen-printed electrodes (SPEs) and characterized by cyclic voltammetry and scanning electron microscopy (SEM). The SEM analysis reveals that the PcH2-tBu forms mainly aggregates on the SPEs. The modified electrodes are used for the detection of acetic acid and present a linear current increase when the acetic acid concentration increases. The Cmodified electrode presents a limit of detection (LOD) of 25.77 mM in the range of 100 mM–400 mM, while the Aumodified electrode presents an LOD averaging 40.89 mM in the range of 50 mM–300 mM. When the experiment is realized in a buffered condition, theCmodified electrode presents a lower LOD, which averagesthe 7.76 mM. A pronounced signal decay attributed to an electrode alteration is observed in the case of the gold electrode. This electrode alteration severely affects the coating stability. This alteration is less perceptible in the case of the carbon electrode.

Improved selectivity towards NO2 of phthalocyanine-based chemosensors by means of original indigo/nanocarbons hybrid material

A new and original gas sensor-system dedicated to the selective monitoring of nitrogen dioxide in air and in the presence of ozone, has been successfully achieved. Because of its high sensitivity and its partial selectivity towards oxidizing pollutants (nitrogen dioxide and ozone), copper phthalocyanine-based chemoresistors are relevant. The selectivity towards nitrogen dioxide results from the implementation of a high efficient and selective ozone filter upstream the sensing device. Thus, a powdered indigo/nanocarbons hybrid material has been developed and investigated for such an application. If nanocarbonaceous material acts as a highly permeable matrix with a high specific surface area, immobilized indigo nanoparticles are involved into an ozonolysis reaction with ozone leading to the selective removal of this analytes from air sample. The filtering yields towards each gas have been experimentally quantified and establish the complete removal of ozone while having the concentration of nitrogen dioxide unchanged. Long-term gas exposures reveal the higher durability of hybrid material as compared to nanocarbons and indigo separately. Synthesis, characterizations by many complementary techniques and tests of hybrid filters are detailed. Results on sensor-system including CuPc-based chemoresistors and indigo/carbon nanotubes hybrid material as in-line filter are illustrated. Sensing performances will be especially discussed.

Indigo molecules adsorbed on carbonaceous nanomaterials as chemical filter for the selective detection of NO2 in the environment

In order to enhance the durability of chemical filters for ozone molecules, devoted to microsystem for the selective detection of NO2 in the environment, the adsorption of indigo molecules onto the surface of carbonaceous nanomaterials (multi-walled carbon nanotubes, a mixture of nanodisks/nanocones, nanofibres) was investigated. The surface of the multi-walled carbon nanotubes was coated by π-stacking with adsorbed indigo molecules. An excess of indigo has resulted in a biphasic sample where nanotubes covered with indigo coexist with free indigo particles. Although similar filtering yields toward O3 (close to 100%) and NO2 (around 0%) were obtained as compared to individual materials, the indigo/MWCNTs samples exhibit enhanced durability as chemical filter at high ozone concentration (1 ppm).

Electrodes Modification Based on Metal-Free Phthalocyanine: Example of Electrochemical Sensors for the Detection of Acetic Acid

Electroanalytical properties of tetra-tert-butyl phthalocyanine (PcH 2-tBu) modified electrodes are studied by cyclic voltammetry (CV). The modified electrodes are obtained by CV deposition techniques on gold (Au) and glassy carbon (C) screen-printed electrodes (SPEs) and used for the electrochemical detection of acetic acid (AA). Based on the CV experiments, the electrodeposition mechanism is detailed. The modified PcH 2-tBu electrodes reveal one oxidation and one reduction peak within the potential window of the working electrodes. In the presence of the analyte (acetic acid), the modified electrodes show sensitivity in the range of 10 mM to 400 mM. For the PcH 2-tBu modified Au electrode, a limit of detection (LOD) of 5.89 mM (based on the +0.06 V peak) was obtained while for the PcH 2-tBu modified C electrode a LOD of 17.76 mM (based on the +0.07 V peak) was achieved. A signal decay of 17%, based on 20 experiments, is obtained when gold is used as working electrode. If carbon is used as working electrode a value of 7% is attained. A signal decay is observed after more than 50 cycles of experiments and is more pronounced when higher concentrations of acetic acid are used. A mechanism of sensing is proposed at the end.

Functionalized Carbon Nanotubes-Based Gas Sensors for Pollutants Detection: Investigation on the Use of a Double Transduction Mode

With an objective to fabricate Carbon nanotubes (CNTs) based sensors, the solution route is investigated. The dispersion routes are chosen here to avoid the CNTs to form bundles which can reduce their surface area. The results show that SWNTs-based gas sensors made by the surfactant method is possible if the annealing temperature is correctly chosen. The use of a surfactant allows preparing sensing layers which present responses to NO2 exposure in the 50-200 ppb Range. In a second procedure the CNTs are noncovalently functionalized and used as sensing material for BTX (Benzene, Toluene and Xylenes) detection. The noncovalent functionalisation occurs through p-p stacking between the SWNTs framework and the highly delocalized π-system of the macrocycle which are phthalocyanines and porphyrines derivatives. The SWNTs materials are characterized by standard techniques (UV-Vis spectroscopy, TGA, TEM, Raman analysis). For BTX detection, we used a double transduction mode: IDEs (Interdigitated electrodes) and QCM (Quartz Crystal Microbalance) in order to get insight into the sensing mechanism.

New Indigo/Nanocarbons Hybrid Material as Chemical Filter for the Enhancement of Gas Sensor Selectivity towards Nitrogen Dioxide

A new hybrid material performed by the immobilization of indigo particles on nanocarbonaceous matrix has been developed, characterized and implemented with chemoresistors. If phthalocyanine-based chemoresistors provide a high sensitivity, a low threshold of detection and a partial selectivity towards oxidizing pollutants, indigo/nanocarbons hybrid material acts as a selective ozone filter from air sample and so strongly enhances the sensor selectivity towards nitrogen dioxide. The functionalization, highlighted by scanning electron microscopy, X-ray diffraction and thermogravimetric analysis, occurs in a non-covalent way and proceeds through pi-stacking interactions. With filtering yields higher than 99.5% for ozone and less than 5% for nitrogen dioxide, indigo adsorbed onto multi-wall carbon nanotubes has been identified as the most selective filtering material while exhibiting a much greater durability than indigo or nanotubes separately. Associated to phthalocyanine-based gas sensor, the sustainable, selective and real-time monitoring of NO2 in ppb range has been successfully achieved.