Microwave Gas Sensors Based on Electrodeposited Polypyrrole-Nickel Phthalocyanine Hybrid Films

Abstract

Previous studies have shown that the incorporation of sulfonated metallophthalocyanines into sensitive sensor materials can improve electron transfer and thus species detection. Herein, we propose a simple and easy alternative to the use of generally expensive sulfonated phthalocyanines by electropolymerizing polypyrrole together with nickel phthalocyanine in the presence of an anionic surfactant. The addition of the surfactant not only helps the incorporation of the water-insoluble pigment into the polypyrrole film, but the obtained structure has increased hydrophobicity, which is a key property for developing efficient gas sensors with low sensitivity to water. The obtained results show the effectiveness of the materials tested for the detection of ammonia in the range of 100 to 400 ppm. It is shown by comparing the microwave sensor responses that the film without nickel phthalocyanine (hydrophilic) produces greater variations than the film with nickel phthalocyanine (hydrophobic). These results are consistent with the expected results since the hydrophobic film is not very sensitive to residual ambient water and therefore does not interfere with the microwave response. However, although this excess response is usually a handicap, as it is a source of drift, in these experiments the microwave response shows great stability in both cases.

Keywords: ammonia; conducting polymers; gas sensor; microwave transduction; phthalocyanine.

Scheme 1

Schematic representation of polypyrrole with nickel phthalocyanine in the presence of an SDS surfactant.

Figure 1

(A) Cyclic voltammograms for the electropolymerization of PPy films on a Pt electrode in the presence of SDS. (B) Cyclic voltammograms for the electropolymerization of PPy films on a Pt electrode in the presence of SDS and NiPc.

Figure 2

Amperometric polymerization of PPy-SDS and PPy/NiPc-SDS films on an FTO electrode.

Figure 3

IRRAS spectra of PPy−SDS and PPy/NiPc−SDS films electrodeposited on an FTO electrode using chronoamperometry at +1 V/SCE.

Figure 4

Energy-dispersive spectroscopy of PPy−SDS and PPy/NiPc−SDS films electrodeposited on an FTO electrode using chronoamperometry at +1 V/SCE.

Figure 5

(A) SEM micrograph of PPy-SDS film electrodeposited on an FTO electrode using chronoamperometry at +1 V/SCE. (B) SEM micrograph of PPy/NiPc-SDS film electrodeposited on an FTO electrode using chronoamperometry at +1 V/SCE.

Figure 6

(A) Contact angle of PPy-SDS film electrodeposited on an FTO electrode using chronoamperometry at +1 V/SCE. (B) Contact angle of PPy/NiPc-SDS film electrodeposited on an FTO electrode using chronoamperometry at +1 V/SCE.

Figure 7

Setup that was used: VNA = Vector Network Analyzer, CG = carrier gas, MFC = mass flow controller, RH = relative humidity sensor, T = temperature sensor, FC = Faraday cage, MS = mass spectrometer [45].

Figure 8

(A) Resonator structure and its dimensions (A): Wf = 2 mm, Wf2 = 0.150 mm, Lf = 30 mm, Lf2 = 6 mm, A = 3 mm, B = 6 mm, C = 5 mm, number of coils = 7, coils width = 0.150 mm, coil spacing = 0.150 mm [40]. (B) Gas sensor without deposited sensitive material. (C) Gas sensor coated with a PPy/NiPc-SDS sensitive layer (this layer was electrodeposited using chronoamperometry by applying a potential of +1.0 V/SCE for 5 min).

Figure 9

Variations in the magnitude of the PPy/NiPc−SDS microwave sensor (in blue) and the ammonia concentration in air (in orange) over time at 6.12 GHz.

Figure 10

Variations in the magnitude of the PPy−SDS microwave sensor (in blue) and the ammonia concentration in air (in orange) over time at 6.75 GHz.

Figure 11

Variations in the magnitude of the microwave responses for the PPy−SDS sensor and the PPy/NiPc−SDS sensor as a function of the ammonia concentration in air.