Copper(II) phthalocyanine supported on a three-dimensional nitrogen-doped graphene/PEDOT-PSS nanocomposite as a highly selective and sensitive sensor for ammonia detection at room temperature

Abstract

Here we present a highly efficient ammonia (NH₃) gas sensor made of copper(II) tetrasulfophthalocyanine supported on a three-dimensional nitrogen-doped graphene based framework (CuTSPc@3D-(N)GF)/poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) nanocomposite sensing film with high uniformity over a large surface area. The NH₃ gas sensing performance of the nanocomposite was compared with those of sensors based on pure PEDOT-PSS and a pristine CuTSPc@3D-(N)GF. It was revealed that the synergistic behavior between both of these candidates allowed excellent sensitivity and selectivity for NH₃ gas in a low concentration range of 1–1000 ppm at room temperature. The CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite gas sensor exhibited a much better response (∼5 and 53 times, with a concentration of NH₃ gas at 200 ppm) to NH₃ gas than those of the pure PEDOT-PSS and pristine CuTSPc@3D-(N)GF gas sensors, respectively. The combination of the CuTSPc@3D-(N)GF and PEDOT-PSS facilitated the enhancement in the sensing properties of the final nanocomposite and paved a new avenue for the application of CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposites in the gas sensing field.

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Introduction

Graphene, as a two-dimensional sp² bonded carbon sheet, has attracted much attention in many diverse applications, especially chemical sensors,^1–8 due to its excellent electronics, high mechanical stiffness and specific surface-to-volume ratio, as well as its superior conductivity.^9–12 Three-dimensional (3D) graphene-based frameworks (3D-GFs) such as sponges, foams and aerogels are an important class of new-generation porous carbon materials, which exhibit high porosities, large surface areas and high electrical conductivities.^13–17 These materials can serve as strong matrices for functionalizing metals, metal oxides and electrochemically active polymers for various applications in electrochemical capacitors,^18–20 batteries^21,22 and catalysis.^23–25 Dong et al.²⁶ have demonstrated that a 3D graphene electrode, as an electrochemical sensor for detection of dopamine, exhibits remarkable sensitivity (619.6 μA mM⁻¹ cm⁻²) and a low detection limit (25 nM at a signal-to-noise ratio of 5.6), with a linear response of up to ∼25 μM. Therefore, 3D-GFs can provide a promising platform for the development of high performance electrochemical sensors for dangerous volatile organic compounds (VOCs).

Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) as a conjugated polymer (a mixture of two ionomers) has been extensively studied as the active material in sensing applications because of its good electrical conductivity, high transparency, low redox potential and good processability.^27–29 Nevertheless, its limited chemical and structural properties prevent its use in various practical applications, especially electrochemical sensing.³⁰ Its composite with carbon nanostructures could be a promising solution to the shortcomings. Jian et al.³¹ have used a PEDOT-PSS composite film with O₂ plasma-treated single-walled carbon nanotubes for the detection of ammonia (NH₃) and trimethylamine gases. Seekaew et al.²⁷ have reported the NH₃ sensing behavior of a graphene–PEDOT-PSS composite film at room temperature. This evidence indicates that PEDOT-PSS composite films show potential as useful sensing materials, but their low sensitivities restrict their application in practical VOC sensors.

As a result of the above-mentioned reasons, and also our interest in the synthesis of graphene based materials for various applications,^25,32–34 especially as new sensors,^35–37 herein, we demonstrate the use of a copper(II) tetrasulfophthalocyanine supported on a three-dimensional nitrogen-doped graphene-based framework (CuTSPc@3D-(N)GF) and PEDOT-PSS nanocomposite as a novel gas sensor. This new architecture holds great appeal as a chemical sensor due to its large surface area, 3D multiplexed network of highly conductive pathways and continuously interconnected macroporous structures, as well as its modified active surface. To demonstrate its potential, we used it here for the detection of NH₃, a highly toxic gas, which leads to irritation of the skin, eyes and respiratory tract of humans.

Experimental

Preparation of CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite

The CuTSPc@3D-(N)GF was synthesized based on our previous work.²⁵ The obtained CuTSPc@3D-(N)GF was dispersed in DI water (∼1 mg ml⁻¹) and mildly sonicated for 30 min in a bath sonicator (EUROSONIC® 4D, 50 kHz). The PEDOT-PSS aqueous solution (weight ratio = 1–6, Clevios™ P VP AI 4083, solid content 1.3–1.7%) was first dissolved in DI water with a weight concentration of 89.82%. The CuTSPc@3D-(N)GF dispersion with 6 wt% dimethyl sulfoxide (DMSO, Sigma-Aldrich Co) was added to the PEDOT-PSS solution and a homogeneous aqueous dispersion was obtained after 2 h of stirring and sonication for 30 min.

Fabrication of the CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite gas sensor

The nanocomposite gas sensor was prepared from the PEDOT-PSS and CuTSPc@3D-(N)GF materials with chemical structures schematically illustrated in Fig. 1. For the fabrication of the CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite gas sensor, interdigitated Au electrodes with 100 nm thickness were deposited on a SiO₂/Si substrate (10 × 4 mm²) using a physical vapor deposition method. The prepared CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite solution was then drop casted over the interdigitated electrode (Fig. 2a). The width and inter-spacing of the electrodes were 200 μm and 400 μm, respectively. Then, the nanocomposite gas sensor was heated for 1 h in a furnace (Exciton, EX1200-4L) at 80 °C under a nitrogen atmosphere (Fig. 2b). The pristine CuTSPc@3D-(N)GF and PEDOT-PSS gas sensors were also fabricated and tested for comparison. The fabricated CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite gas sensor is displayed in Fig. 2c and d.

Fig. 1 Schematic structures of (a) PEDOT-PSS and (b) the CuTSPc@3D-(N)GF.

Fig. 2 Schematic steps of the gas sensor fabrication process.

Characterization methods

Transmission electron microscopy (TEM) was performed using a LEO 912AB electron microscope operated at an accelerating voltage of 120 kV. Scanning electron microscopy (SEM) was performed using a S-4160 electron microscope. Atomic force microscopy (AFM) images were obtained using a tapping mode with an ARA AFM (0201/A, Ara research Co, Iran). Brunauer–Emmett–Teller (BET) surface area measurements were obtained through nitrogen adsorption at 77 K using an ASAP2020 instrument. The conductivities of the sensing films were measured using a 4-point technique probe with 10 nA of applied current. The resistances of the sensors were measured in a closed steel chamber (lab-made) with a LCR meter (Pintek-LCR900) and vapor gas flows were injected into the closed steel chamber by a mass flow meter (Alicat scientific, Tucson, USA). The reference humidity and sensor temperature were monitored by PT100 and HIH4000, respectively. The responses and selectivities of the gas sensors towards NH₃, methanol, ethanol, acetone, toluene, chlorobenzene, and water, with gas concentrations ranging from 1 ppm to 1000 ppm at room temperature, were then assessed using a standard flow-through method. A constant flux of synthetic air of about 50 cm³ min⁻¹ was mixed with the NH₃ gas source at different flow rate ratios to desired concentrations using mass flow controllers. All experiments were performed at room temperature (25 ± 2 °C) and a relative humidity of 10 ± 2%. The sensitivity was defined by the following equation:with

ΔR = R_gas − R₀ (2)

where R₀ and R_gas are the resistances of the sensor in synthetic air and the test gas, respectively.

Results and discussion

The surface morphology of the CuTSPc@3D-(N)GF is presented in the SEM and TEM images of Fig. 3a and b, respectively. Fig. 3a shows the 3D morphology and the interconnected porous structure of the ultrathin graphene nanosheets. Moreover, the pore sizes range from a few hundred nanometers to several micrometers. Fig. 3b not only clearly shows the presence of the mesopores in the carbon walls, but also reveals the wrinkled paper-like texture of the sheets, consistent with previous reports.³⁸ In addition, based on the BET method, the synthesized support (3D-(N)GFs) from our previous work has a high surface area (up to 266.0 m² g⁻¹) for the incorporation of CuTSPc as active sensing sites. The chemical compositions of the 3D-(N)GF and CuTSPc@3D-(N)GF were confirmed by several characterization methods, such as elemental analysis, Fourier transform infrared spectroscopy, thermogravimetric analysis, X-ray powder diffraction and X-ray photoelectron spectroscopy.²⁵

Fig. 3 Microstructure of the as-prepared CuTSPc@3D-(N)GF: (a) SEM and (b) TEM images.

Fig. 4a and b show the surface morphologies of the drop-coated pure PEDOT-PSS and CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite film, respectively. As can be seen, the pure PEDOT-PSS has a relatively smooth surface (Fig. 4a). On the other hand, Fig. 4b shows that the CuTSPc@3D-(N)GF is uniformly dispersed through the PEDOT-PSS matrix without obvious agglomeration. This was ascribed to the hydrophilic nature of CuTSPc on the surface of the 3D-(N)GF which not only ensured the strong bonding with PEDOT-PSS but also improved the dispersity and stability of the CuTSPc@3D-(N)GF in aqueous solution (Fig. S1†). Further roughness evaluations by AFM show that the average surface roughness (R_a) of the pure PEDOT-PSS films is 1.98 nm, while the R_a of the CuTSPc@3D-(N)GF/PEDOT-PSS is 7.52 nm (Fig. S2†). The much larger R_a of the CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite film compared to the pure PEDOT-PSS film suggests a significant enhancement in the active surface-area for gas adsorption by the CuTSPc@3D-(N)GF.^25,27

Fig. 4 SEM images of the pure PEDOT-PSS (a) and CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite (b) films.

Fig. 5a shows the dynamic response of the CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite gas sensor towards 1000 ppm NH₃ at room temperature in air. It shows that the sensor has good repeatability in terms of its response towards repeated NH₃-sensing cycles at room temperature. The sensitivity of the nanocomposite gas sensor increases upon exposure to NH₃ and recovers to the initial value upon the removal of NH₃ in air. As shown in Fig. 5b, the sensitivity of the CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite gas sensor increases dramatically when exposed to various concentrations of NH₃ ranging from 200 ppm to 800 ppm, and recovers to the original values when the NH₃ is replaced by air. The changing sensitivity behavior may be ascribed to the adsorption and desorption of NH₃ molecules on the nanocomposite sensing film.²⁷ The details of the sensing mechanism for the CuTSPc@3D-(N)GF/PEDOT:PSS nanocomposite gas sensor will be discussed in further. In addition, the conductivities of the pure PEDOT-PSS and CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite sensing films are 650 and 1430 S cm⁻¹, respectively. This indicates that the conductivity of the pure PEDOT-PSS is increased by more than double, due to a significant increase in the charge carrier concentration as a result of CuTSPc@3D-(N)GF incorporation. Therefore, CuTSPc@3D-(N)GF exhibits a dominant effect in the charge transport through the PEDOT-PSS matrix.

Fig. 5 (a) Dynamic responses of the CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite gas sensor to 1000 ppm NH₃ and (b) the sensitivity of the CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite gas sensor when exposed to different concentrations of NH₃ at room temperature.

PEDOT-PSS based gas sensors usually operate at rather low temperatures with respect to gas sensors based on metal oxides.^27,35,39 It can be seen that the CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite gas sensor shows lower sensitivity to NH₃ with an increase of temperature (Fig. 6). Therefore, the optimal temperature of the CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite gas sensor for NH₃ detection was found to be room temperature. Since the interactions between the CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite sensing film and gaseous volatile organic compounds (VOCs) are exothermic, the activation energy for the desorption of NH₃ molecules from the nanocomposite sensing film is larger than that for the adsorption. This reveals that the decrease in sensitivity at higher temperatures is a result of the higher desorption rate for NH₃ gas.

Fig. 6 The sensitivity of the CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite gas sensor as a function of temperature towards 200 ppm NH₃.

The response times of the pure PEDOT-PSS and CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite gas sensors, when they were experiencing a 95% resistance change, were estimated to be ∼6 min and ∼2.5 min, respectively. Moreover, the recovery times of the pure PEDOT-PSS and CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite gas sensors were ∼2.5 min and ∼1 min, respectively. Therefore, the CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite gas sensor exhibits relatively short response and recovery times compared with the PEDOT-PSS one (see Fig. 7).

Fig. 7 The responses and recoveries of gas sensors based on (a) pure PEDOT-PSS and (b) the CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite with 200 ppm NH₃ at room temperature.

Fig. 8 demonstrates the sensitivities of the pure PEDOT-PSS, pristine CuTSPc@3D-(N)GF and CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite gas sensors as a function of NH₃ concentration at room temperature. At a 1000 ppm NH₃ concentration, the sensitivities of the pure PEDOT-PSS, pristine CuTSPc@3D-(N)GF and CuTSPc@3D-(N)GF/PEDOT-PSS gas sensors were 14.8%, 9% and 91%, respectively. At low concentrations (50 ppm), the sensitivities for the mentioned gas sensors were 4%, 0.35% and 9%, respectively (inset of Fig. 8). As can be seen, the room temperature sensitivity of pure PEDOT-PSS was lower than that of the CuTSPc@3D-(N)GF, but its response was substantially increased after CuTSPc@3D-(N)GF incorporation. Thus, the CuTSPc@3D-(N)GF improves the interaction with NH₃, leading to a higher charge reduction only when it is included in the PEDOT-PSS network. The detection limit of NH₃ for the CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite gas sensor was thus estimated to be 10 ppm at the room temperature.

Fig. 8 Sensitivity of the pure PEDOT-PSS, pristine CuTSPc@3D-(N)GF and CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite gas sensors towards 1–1000 ppm of NH₃ at room temperature.

Fig. 9 demonstrates the selectivity of the CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite gas sensor for various VOC vapors at a concentration of 200 ppm. The sensitivities of the CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite gas sensor to NH₃, methanol, ethanol, acetone, toluene, chlorobenzene and water were 18.7%, 9.4%, 5.5%, 4.5%, 2.3%, 2.9% and 4.4%, respectively. The CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite gas sensor exhibited a remarkably high response to NH₃ and was slightly sensitive to other VOC vapors. The performance of the as-prepared nanocomposite gas sensor in this work was better than the previous reports in the literature concerning NH₃ detection (Table 1).

Fig. 9 The selectivity of the CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite gas sensor to various VOC vapors of 200 ppm concentration.

Table 1 The sensitivities (S), response times (R₁), recovery times (R₂), studied detection ranges (D_r), materials (M) and measured temperatures (T_m) of various NH₃ gas sensors^a

Authors	S (%)	R1 (s)	R2 (s)	Dr (ppm)	M	Tm (°C)
a PANI: polyaniline; SWCNTs: single wall carbon nanotubes; MWCNT: multi wall carbon nanotube.
Our work	8 (50 ppm), 91 (1000 ppm)	138	63	1–1000	CuTSPc@3D-(N)GF/PEDOT-PSS	25
Xu et al.^40	2 (10 ppm), 12 (70 ppm)	10	10	10–70	PEDOT nanowire	25
Kwon et al.^41	2.1 (5 ppm), 24 (100 ppm)	<1	30	5–100	PEDOT nanotube	25
Seekaew et al.^27	0.9 (5 ppm), 7 (1000 ppm)	180	—	5–1000	Graphene/PEDPT-PSS	25
Jian et al.^31	0.1 (2 ppm), 33 (300 ppm)	12	18	2–300	SWCNTs/PEDOT-PSS	25
Yoo et al.^42	0.015 (20 ppm), 0.075 (100 ppm)	100	700	0–100	pf-MWCNT/PANI	25
Tai et al.^43	1.67 (23 ppm), 5.55 (117 ppm)	18	58	23–141	PANI/TiO2	25
Matsuguchi et al.^44	1.16 (500 ppm)	1500	—	—	PANI	30
Kebiche et al.^45	7.1 (92 ppm)	834	600	92–4628	PANI	25
Hong et al.^46	0.14 (20 ppm), 0.2 (100 ppm)	14	148	20–2000	Palladium/polypyrrole	25
Crowley et al.^47	0.24 (100 ppm)	90	90	1–100	Inkjet-printed PANI	80
Verma et al.^48	0.9 (200 ppm)	1	420	50–200	PANI	25
Sengupta et al.^49	2.3 (100 ppm)	120	300	100	PANI	25

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The improved NH₃ sensing properties of the CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite gas sensor are mainly ascribed to the (i) increased R_a in the CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite sensing film compared to the PEDOT-PSS one, (ii) inherent sensing properties of the CuTSPc@3D-(N)GF and (iii) π–π interactions resulting from CuTSPc@3D-(N)GF loading, including the 3D-(N)GF as a support and the Cu(II) complex as an active site in the sensing film. (i) Since the R_a of the sensing film is directly proportional to the gas sensitivity,^27,50 the much larger R_a of the CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite sensing film improves the active surface area for gas adsorption. (ii) It is well-known that graphene under ambient conditions behaves as a p-type semiconductor, like conjugated polymers because their electronic properties can be reversibly controlled by doping/dedoping at room temperature.^27,51–53 In addition, the electron-withdrawing sulfonic acid groups of the CuTSPc can be viewed as responsible for the charge delocalization of holes in the valence band. Therefore, when the CuTSPc@3D-(N)GF gas sensor is exposed to an electron donating gas like NH₃, proton transfer from the sulfonic acid groups of CuTSPc to the nitrogen atoms of NH₃ directly gives the self-doped zwitterionic form. Therefore, the depletion of holes from the valence band of the CuTSPc@3D-(N)GF occurs, leading to a significant increase in the resistance. (iii) NH₃ molecules may interact with not only the CuTSPc@3D-(N)GF and PEDOT-PSS but also with the π–π bonds formed between the CuTSPc@3D-(N)GF and PEDOT-PSS.⁹ On exposure to polar molecules like NH₃, the interactions can not only induce charge-transfer across delocalized π-electrons but can also lead to the formation of a neutral polymer backbone and a decrease in charge carries, resulting in improved sensing performances.

The NH₃ sensing mechanism of the CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite gas sensor may be explained based on three possible mechanisms: (1) according to the reversible reaction:^27,45,54 CuTSPc@3D-(N)GF/PEDOT-PSS–H⁺ + NH₃ ⇄ CuTSPc@3D-(N)GF/PEDOT-PSS + NH₄⁺, proton transfer from the sulfonic acid groups of CuTSPc to the nitrogen atoms of the NH₃ molecules directly gives ammonium ions (NH₄⁺). This process is reversible, and in fact, when the NH₃ atmosphere is removed, the NH₃ molecules decrease the doping level of the CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite sensing film by partially compensating for the influence of the initial dopants,⁵⁵ which may change the resistance. (2) The NH₃ molecules are absorbed on the surface of the CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite sensing film by physisorption and the holes of the conductive CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite sensing film will interact with the electron-donating NH₃ analyte.^27,54 The charge transfer from adsorbed NH₃ molecules not only increases the delocalization degree of the conjugated π-electrons of the nanocomposite sensing film, but also decreases the electrical conductivity of the nanocomposite sensing film.^31,41 This mechanism is widely adopted for explaining the change in conductivity of conductive polymers to acidic/basic analytes (doping/dedoping processes). (3) The swelling of the CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite sensing film can increase the PEDOT distance and decrease the CuTSPc@3D-(N)GF’s conductive pathways, leading to a significant increase in the resistance of the CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite gas sensor upon NH₃ exposure and therefore an enhanced NH₃ response.

Conclusion

In this work, a novel NH₃ gas sensor based on the CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite has been successfully fabricated and studied for the first time. The resultant CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite exhibited excellent sensitivity, dynamic behavior and selectivity to NH₃ due possibly to the increase in the specific surface area, intrinsic sensing properties of the CuTSPc@3D-(N)GF and π–π interactions in CuTSPc@3D-(N)GF/PEDOT-PSS. The nanocomposite gas sensor exhibited a much better response to NH₃ gas than those of sensors based on pure PEDOT-PSS and a pristine CuTSPc@3D-(N)GF (∼5 and 53 times respectively, with the concentration of NH₃ gas at 200 ppm). The response and recovery times of the nanocomposite gas sensor (2.5 min and 1 min, respectively) were much lower than that of pure PEDOT-PSS (6 min and 2.5 min, respectively) towards 200 ppm NH₃. This suggests that the CuTSPc@3D-(N)GF/PEDOT-PSS nanocomposite gas sensor, which offers several distinct advantages for NH₃ detection over other fabricated sensors including high sensitivity, high productivity, low temperature processing and low cost, is expected to hold great promise for real-world applications.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra13976g

‡ The first two authors contributed equally to this work.

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