Scholarly article on topic 'One-chip integrated dual amperometric/field-effect sensor for the detection of dissolved hydrogen'

One-chip integrated dual amperometric/field-effect sensor for the detection of dissolved hydrogen Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — C. Huck, P. Jolly, P. Wagner, A. Poghossian, M.J. Schöning

Abstract A new Si-based one-chip combined amperometric/field-effect sensor for monitoring the concentration of dissolved hydrogen (H2) has been developed. The specific feature of the realized H2-sensor chip is the implementation of a field-effect pH sensor for indirect detection of the dissolved H2, in addition to amperometric measurements. The feasibility of the proposed approach has been demonstrated by simultaneous amperometric/field-effect detection of dissolved H2 in electrolyte solutions.

Academic research paper on topic "One-chip integrated dual amperometric/field-effect sensor for the detection of dissolved hydrogen"

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Procedia Engineering 25 (2011) 1161 - 1164

Proc. Eurosensors XXV, September 4-7, 2011, Athens, Greece

One-chip integrated dual amperometric/field-effect sensor for the detection of dissolved hydrogen

C. Huck, P. Jollya, P. Wagnerc, A. Poghossiana'b, M.J. Schöning

aInstitute of Nano- and Biotechnologies, Aachen University of Applied Sciences, 52428 Jülich, Germany bPeter Grünberg Institute (PGI-8), Research Centre Jülich GmbH, 52525 Jülich, Germany cInstitute for Materials Research, Hasselt University, 3590 Diepenbeek, Belgium

Abstract

A new Si-based one-chip combined amperometric/field-effect sensor for monitoring the concentration of dissolved hydrogen (H2) has been developed. The specific feature of the realized H2-sensor chip is the implementation of a field-effect pH sensor for indirect detection of the dissolved H2, in addition to amperometric measurements. The feasibility of the proposed approach has been demonstrated by simultaneous amperometric/field-effect detection of dissolved H2 in electrolyte solutions.

© 2011 Published by Elsevier Ltd.

Keywords: dissolved hydrogen; amperometric sensor; field-effect sensor

1. Introduction

Climate change concerns and high oil prices are the driving force for renewable energy sources such as biogas. Biogas production has the potential to replace some of the limited fossil fuels [1]. In order to realize its full potential, real-time and reliable monitoring of the biogas process is crucial for a stable and efficient operation of the biogas production. One of the most important parameters for an early warning of process disturbances is the concentration of dissolved hydrogen, because it is a key factor in the intricate balance between microbial species involved in the multistep degradation during anaerobic digestion [2,3].

In this work, a new one-chip integrated dual amperometric/field-effect sensor for the detection of dissolved H2 has been developed. The chip combines two thin-film Pt electrodes and a field-effect

* Corresponding author. Tel.: +49-241-6009-53144; fax: +49-241-6009-53235. E-mail address: m.j.schoening@fz-juelich.de.

1877-7058 © 2011 Published by Elsevier Ltd. doi: 10.1016/j.proeng.2011.12.286

capacitive electrolyte-insulator-semiconductor (EIS) pH sensor for amperometric and field-effect measurements, respectively. For the first time, an EIS pH sensor has been applied for the indirect measurement of dissolved H2. The sensor detects the product of H2 oxidation, i.e. H+ ions generated at the polarized working electrode of the amperometric part of the combined sensor.

2. Experimental

2.1. Sensor fabrication

Fig. 1 shows the schematic structure of the developed one-chip integrated dual amperometric/field-effect H2 sensor. The capacitive field-effect EIS sensor consists of an Al-Si-SiO2-Ta2O5 structure (p-Si with resistivity of 1-10 Qcm; 30 nm thermally grown SiO2) with a Ta2O5 layer as pH-sensitive gate insulator material. Ta2O5 is well known for its outstanding properties for field-effect pH sensing, having a nearly Nerstian pH sensitivity as well as a high corrosion resistance in a wide pH range [4,5]. The Ta2O5 film was prepared by thermal oxidation of an electron-beam deposited 30 nm Ta layer in a dry oxygen atmosphere at 517 °C for about 30 min, yielding a ~60 nm thick Ta2O5 layer. A 300 nm Al film was deposited on the rear-side of the wafer as a contact layer for the field-effect sensor.

For amperometric measurements, two 200 nm thick platinum electrodes (working and counter electrode) were prepared onto a 20 nm thick titanium adhesion layer by means of electron-beam evaporation and patterned by photolithography and lift-off technique. Finally, the wafer was then cut into chips of 14 mm x 14 mm.

For electrical connection of the EIS sensor the Al rear side contact was glued with electrically conductive adhesive onto a printed circuit board (PCB). The front side contacts to the thin film Pt electrodes were provided by means of an ultrasonic wedge bonder. The bonds were encapsulated with silicon rubber.

Ta205 Si02 p-Si Al

Fig. 1. Schematic layer structure of the dual amperometric/field-effect sensor integrated on one chip. 2.2. Gas-sensing set-up and testing methodology

For experiments, the PCB substrate with the dual amperometric/field-effect sensor chip was mounted into a home-made measuring cell and tested under a simulated biomass reactor atmosphere. The test

chamber was provided with a gas inlet and outlet as well as an inlet for the liquid-junction Ag/AgCl reference electrode. Hydrogen gas was dissolved in the electrolyte (0.25 mM Polymix buffer solution) through a perforated tube by mixing with nitrogen in various ratios using commercial mass flow controllers (Bronkhorst High-Tech). A custom Lab VIEW "virtual instrument" program was developed for experimental control and data acquisition. The measuring set-up for the electrochemical characterization of the one-chip integrated dual amperometric/field-effect sensor is presented in Fig. 2.

The field-effect pH sensor has been characterized by means of constant-capacitance method using an impedance analyzer (Zahner Elektrik). For operation, a DC polarization voltage is applied between the Al rear-side contact and a Ag/AgCl reference electrode to set the working point of the sensor, and a small AC voltage (20 mV, 20 Hz) is applied to the system in order to measure the capacitance of the sensor.

Electrochemical characterization of the amperometric dissolved H2 sensor was performed using a standard three-electrode configuration under potential control using a commercial potentiostat (PalmSens). The potential of the working electrode was set at +0.55 V versus the Ag/AgCl reference electrode and the current was detected between the working and the counter electrode.

Fig. 2. Measuring set-up for the electrochemical characterization of the one-chip sensor in terms of dissolved H2.

3. Results and discussion

Fig. 3 demonstrates an example of a simultaneous amperometric/field-effect detection of dissolved H2 with the combined sensor chip. With the 1% hydrogen dosage, almost identical maximum output currents were observed for the amperometric sensor over a substantial number of measuring cycles, showing the stability and reproducibility of the developed sensor. At the same time, potential changes of about 30 mV in the direction corresponding to lower pH values have been observed for the pH-sensitive field-effect EIS sensor. Taking into account that the pH sensitivity of the EIS structure with the Ta2O5 film as pH-sensitive gate material is usually in the range of 55-58 mV/pH [6,7], these signal changes correspond to a local pH lowering at the EIS surface by ApH~0.5

The obtained results demonstrate the feasibility of the proposed approach for a simultaneous amperometric/field-effect detection of dissolved H2 in electrolyte solutions. Such a combination of two transducer principles, namely, the amperometric and field-effect, might allow in future more accurate,

selective and reliable measurements of dissolved H2 in biogas reactors as an early warning indicator of digester failures.

Fig. 3. Simultaneous amperometric/field-effect measurement of dissolved H2 with the combined sensor chip.

Acknowledgements

The authors thank the federal ministry of education and research (BMBF, Germany) for financial support of the project "EMSiG".

References

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