Procedia Engineering 120 ( 2015 ) 368 – 371

1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of the organizing committee of EUROSENSORS 2015doi: 10.1016/j.proeng.2015.08.640

ScienceDirectAvailable online at www.sciencedirect.com

EUROSENSORS 2015

Detection of organophosphorus (DMMP) vapour using phthalocyanine-palladium bilayer structures

Paulina Powroźnika, Wiesław Jakubika, Anna Kaźmierczak-Bałataa

a Institute of Physics CSE, Silesian University of Technology, Konarskiego 22B, 44-100 Gliwice, Poland

Abstract

We report the use of phthalocyanine-palladium thin film bilayer structures as a sensor for chemical nerve agents. The electrical response behavior at room temperature of metal-free phthalocyanine-palladium and cobalt phthalocyanine-palladium structures, to nerve agent simulant gas, dimethyl-methylphosphonate (DMMP) was investigated. Acetone, methanol, ethanol, iso-propanol and water vapour were used as interferent vapours. Moreover, topography of samples surface was examined by atomic force microscope. Results were repeatable, showing that these materials are sensitive and selective for DMMP vapours at ppb concentrations. However, metal-free phthalocyanine-palladium structure is not selective in presence of acetone vapours. © 2015 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the organizing committee of EUROSENSORS 2015.

Keywords: gas sensor, DMMP, phthalocyanine, palladium

1. Introduction

Nowadays, in response to terroristic activity, there is an urgent need for sensors of chemical warfare agents. Due to safety requirements, in laboratory work, simulants like dimethyl methylophosphonate (DMMP) for sarin are used. In recent years many materials like carbon nanotubes [1], zeolites [2] and organic compounds [3, 4] were studied as DMMP sensors and modifications to improve sensitivity and selectivity of semiconductive metal oxides were proposed [5, 6, 7]. In this work we introduce the phthalocyanine-palladium structures as novel materials sensitive to DMMP vapors. Advantages of phthalocyanines as gas sensors are their developed surface with many

© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of the organizing committee of EUROSENSORS 2015

369 Paulina Powroźnik et al. / Procedia Engineering 120 ( 2015 ) 368 – 371

active centers and low working temperature, whereas the thin palladium films serve as catalytic agent and reduce the overall resistance of the structure. Moreover, organophosphorus vapours exhibits alkalinity, so hydrogen-bond compounds, like metal-free phthalocyanine are suitable sensing materials for DMMP detection [8].

2. Experimental

The structures under study were deposited on a glass plate by vacuum evaporation method. One sample consisted of ~100 nm metal-free phthalocyanine layer and ~10 nm palladium layer, the other one of ~100 nm cobalt phthalocyanine and ~10 nm palladium. DMMP vapor was prepared from a commercial available vapor generator, Owlstone OVG-4, with certified DMMP tube (calibrated in 70 oC). Vapours of acetone and alcohols were dosed without calibration with Owlstone OFC-1 Flow Controller. Humidity of flowing air was measured with Owlstone Water Vapour Generator OHG-4. To investigate the topography of palladium-phthalocyanine structures the standard atomic force microscopy technique was applied. The PSIA XE-70 scanning microscope worked in non-contact mode. The Budget Sensors Tap300 Al cantilevers (resonance freq. 300 kHz, spring constant 40 Nm-1) were used. The XEI®, PSIA and Gwyddion® image processing software allowed to correct sample inclination and distortions caused by z-scanning stage. No other corrections in the images were made. Similar investigations were performed in [9, 10]. Resistance of both samples was monitored with Agilent Multimeter at room temperature (Fig. 1).

Fig 1. Electric planar module with sensor structure: a) view along electrodes, b) top view

3. Results and discussion

3.1. Topography of structures surfaces

The exemplary AFM images were collected on sensing layer between electrodes for metal-free phthalocyanine (H2Pc) layer (Fig. 2 a) and cobalt phthalocyanine layer (CoPc) (Fig. 2 b). The roughness (RMS) of H2Pc surface was varying from 11 nm to 16 nm and for CoPc surface from 7 nm to 10 nm.

Fig. 2. Topography image (1x1 m2) of: a) metal-free phthalocyanine layer b) cobalt phthalocyanine layer

a b

a b

370 Paulina Powroźnik et al. / Procedia Engineering 120 ( 2015 ) 368 – 371

3.2. Sensing properties

The relationship between the resistance and DMMP vapour concentration of H2Pc-palladium structure is given in the Fig. 3a. Resistance of the sample was decreasing with increasing DMMP concentration. In the range of 10 ppb to 60 ppb a logarithmic dependence was observed. Measurements changes were carried out also for decreasing DMMP concentration to show repeatability. The dependence of resistance on pure, dry air flow is showed in the Fig. 3b. Changes were not significant, compared to those in DMMP presence and their direction followed an opposite trend. Therefore we can assume that sample response was due to the presence of DMMP vapour. H2Pc-palladium structure exhibited good sensitivity to low DMMP vapour concentrations.

The behavior of electrical response of CoPc-palladium structure with changes of DMMP concentration with 10 minutes cycles is given in the Fig. 4a. Investigation was carried out in the range of 10 ppb to 300 ppb. Similar changes of resistance were observed for two measurements. Sensitivity of this material to DMMP vapour was lower than sensitivity of H2Pc-palladium in a range of very low concentrations. This result could be related to the higher surface area of H2Pc-palladium structure and hydrogen-bond acidity of metal-free phthalocyanine. The Fig. 4b illustrates the dependence of resistance on pure, dry air flow for cycles with the same total flow rate as those with DMMP vapour. As for the H2Pc-palladium structure, changes in presence of air were not significant.

Behavior of samples in humidity varying conditions was also studied and results showed that both materials are not sensitive to this factor. Moreover, the electrical response of H2Pc-palladium sample in presence of acetone and alcohols (methanol, ethanol, propanol) was investigated. The structure was found sensitive to acetone, but it did not exhibit sensing properties for alcohols

Fig.3. Dependence of H2Pc-palladium structure resistance on: a) DMMP concentration in two directions of concentration changes; gas generator

oven temperature: 70 oC; R – resistance, c – DMMP concentration; b) pure dry (~1 % RH) air flow rate

a b

371 Paulina Powroźnik et al. / Procedia Engineering 120 ( 2015 ) 368 – 371

Fig. 4. Dependence of CoPc-palladium structure resistance on: a) DMMP concentration; gas generator oven temperature: 70 oC; b) pure air flow rate

4. Conclusion

In the present work we reported the results of investigation of phthalocyanine-palladium bilayer structures for thedetection of chemical warfare agents simulant, DMMP. This kind of structures combines good sensing properties of phthalocyanines with advantages of palladium, as catalytic properties and low resistance, enabling work at room temperature. H2Pc-palladium structure revealed better sensitivity to ppb concentrations of DMMP than CoPc-palladium. We proposed as explanation differences in the surface morphology and chemical properties of hydrogen phthalocyanine suitable for detection of organophosphorus vapours. It was shown that both samples are not sensitive to humidity and structure with metal-free phthalocyanine does not react with alcohols.

Acknowledgements

The work was financed by the Silesian University of Technology grant No: BK 219/RIF/2015

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