Graphene oxide/polystyrene composite nanofibers on quartz crystal microbalance electrode for the ammonia detection

Abstract

A nanostructured complex, graphene oxide (G-COOH)/polystyrene (PS) nanofiber, was fabricated as a novel sensing material coated on a quartz crystal microbalance (QCM) to realize ammonia detection in this study. Nanoporous G-COOH/PS nanofibers with an average diameter of 569 nm were fabricated via electrospinning; the fibers were composed of ultrathin nanowires with a primary diameter of 37 nm. This unique structure presented a large surface active region, making the fibers an optimal candidate for gas-sensing applications. The physical and chemical properties of the G-COOH/PS nanofibers were characterized by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), N₂ physical adsorption, Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The results indicated that the G-COOH/PS composite nanofibers had a mesoporous structure and that G-COOH sheets were randomly dispersed in the nanofibers. A gas-sensing test showed that the G-COOH/PS nanofibers, when incorporated into a QCM sensor system, exhibited good and desirable sensing behavior, including high sensitivity, fast response and good reversibility, making them a promising candidate as an ammonia detector.

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1 Introduction

Ammonia, a precursor to fertilizers, contributes significantly to the nutritional needs of terrestrial organisms.¹ Ammonia is also an important raw material for synthesizing many chemical products, such as nitric acid and ammonium chloride.² However, ammonia is a hazardous air pollutant with a distinct pungent odor. Exposure to high concentrations (500 ppm or more) of ammonia gas can lead to severe burning of the skin, eyes, throat, or lungs, causing permanent blindness and lung disease.³ The US Occupational Safety and Health Administration (OSHA) has set a standard for safe exposure of 35 ppm for less than 15 min. The National Institute for Occupational Safety and Health recommends that the level in workroom air be limited to 50 ppm for 5 min of exposure. China has set a safety exposure limit of 0.2 mg m⁻³ (∼1.72 ppm). Therefore, the monitoring of ammonia is of great importance in the relevant fields.

Over the past decades, many types of ammonia sensors with different sensing platforms have been investigated, including electrical,^4,5 mass,^6,7 or optical-based sensors.^8,9 However, semiconductor sensors, as the main type of electrical sensors, suffer from several critical limitations, such as a limited maximum sensitivity and insufficient specificity. Moreover, this type of sensor requires high operating temperatures, which may increase operation complexity and power consumption. On the other hand, most optical systems are expensive and complex, and thus not suitable for miniaturization and integration. Therefore, there remains an increasing demand for a facile, sensitive, and reliable ammonia detection method. The quartz crystal microbalance (QCM), a mass sensitive technique, is considered a good candidate in these respects. When the surface of a quartz crystal electrode is coated with a sensitive material, the device measures frequency changes that are related to the amount of mass being absorbed by the sensing coating according to the Sauerbrey equation.¹⁰ The device can detect trace mass changes in the nanogram range on the surface of quartz crystal electrode and can be operated at room temperature. In recent years, QCM sensors coated with various sensing films, such as polymers,^11,12 metal oxides,^13,14 and nanotubes,^15,16 have been used extensively for gas/vapor detection and analysis. Among these various sensing materials, electrospun nanofibers have been received great attention because electrospinning is a relatively simple and versatile method for fabricating three-dimensional (3D) nanofibrous membranes with high specific surface area (SSA), which could significantly improve the sensitivity of QCM sensors. Ding et al.¹⁷ first applied electrospun nanofibrous membranes on QCM electrodes as highly sensitive gas sensors in 2004. Since then, various polymer nanofibers with high SSA and high porosity have been prepared via electrospinning as sensing materials or nanostructure substrates coated on QCM electrodes. Wang et al.¹⁸ fabricated a highly sensitive humidity sensor by electrospinning nanofibrous polyacrylic acid (PAA)/poly(vinyl alcohol) (PVA) membranes on a QCM. The sensitivity of fibrous composite PAA/PVA membranes is twice that of corresponding flat films at 95% relative humidity (RH). To further extend the range of application of sensing materials incorporated into QCMs, a two-step strategy for fabricating sensing coatings on QCMs has been developed. The strategy involves the electro-spinning deposition of nanofiber nets on the surface of a QCM resonator, followed by surface modification. For instance, a PEI-functionalized PA-6 (PEI/PA-6) nanofiber-net-based QCM sensor exhibited high sensitivity and a fast response/recovery time (120 s/50 s) to humidity.¹⁹ The maximum frequency shifts changed by approximately three orders of magnitude as the RH varied from 2% to 95%. QCM sensors of this type have been successfully applied for the detection of noxious gases, such as formaldehyde^20–22 and HCl,²³ and experimental results indicate that the electrospinning nanofibers/QCM combined sensor system exhibits higher sensitivity and a faster response to target gases due to its high SSA.

Graphene (G), a two-dimensional sheet of carbon atoms bonded through sp² hybridization, has been studied for gas sensors due to its remarkable properties, such as large SSA (∼2630 m² g⁻¹), high electrical and thermal conductivity and superior mechanical strength. Z. Wu et al.²⁴ fabricated a conductometric ammonia sensor based on graphene/PANI nanocomposites. The graphene/PANI sensor exhibits much higher sensitivity (ca. 5 times) than that of PANI, which indicated that sensitivity of graphene/PANI sensor is enhanced by the graphene added into PANI. Another ammonia gas sensor based on reduced graphene oxide (RGO)–polyaniline (PANI) hybrids exhibited excellent sensing performance to NH₃ gas.²⁵ The sensitivity of RGO–PANI hybrid device is triple those of bare PANI nanofiber sensor and bare graphene device at 50 ppm ammonia gas. The high SSA of graphene has also made it very attractive in the hydrogen storage and supercapacitor fields. G. Srinivas et al.²⁶ synthesized a graphene-based powder with a BET surface area of 640 m² g⁻¹ and studied its hydrogen adsorption capacity. The molecular hydrogen adsorption capacities of graphene were determined to be approximately 1.2 wt% and 0.1 wt% at 77 K and 298 K, respectively, significantly better than those exhibited by a range of other carbon and nanoporous materials. It was observed that the adsorption capacity at 77 K and 1 atm was largely governed by BET surface area, and revealed a linear relationship between BET surface area and H₂ uptake capacity. Meryl D. Stoller et al.²⁷ developed a graphene-based ultracapacitors. Due to graphene's high SSA and excellent electrical conductivity, the specific capacitances of the devices were 135 and 99 F g⁻¹ in aqueous and organic electrolytes, demonstrating the potential in high-performance, electrical energy storage applications.

The aim of this work was to fabricate 3D nanofibrous membranes with high SSA as a sensing material coated on a QCM to realize rapid and accurate ammonia detection. In this study, polystyrene (PS) nanofibers doped with carboxyl graphene (G-COOH) were fabricated via electrospinning. PS was chosen as an ideal template material to prepare 3D structured fibrous membranes with high SSA. G-COOH was used as the sensing material because of its high SSA, which exhibits high sensitivity toward ammonia gas due to the selective interaction between ammonia molecules and carboxyl groups. The highly nanoporous G-COOH/PS fibers were developed via electrospinning to expose more carboxyl groups on the surface of the nanofibers. The fibers were then deposited on a QCM to fabricate an ammonia gas sensor. To the best of our knowledge, this is the first study of a QCM sensor fabricated using electrospun G-COOH/PS composite nanofibers. The ammonia-sensing properties of the sensor were examined, and the operating mechanism of the sensor was also investigated in this study.

2 Experimental

2.1 Materials

Polystyrene (PS) (350 kDa) was purchased from Sigma-Aldrich Corporation. A dispersion of carboxyl graphene (concentration: 10 mg ml⁻¹, carboxyl ratio: 5.0 wt%, dispersion solvent: water) was provided by Nanjing XFNANO Materials Tech Co., Ltd (China). Analytical-grade N,N-dimethylformamide (DMF) and tetrahydrofuran (THF) were obtained from Sinopharm Chemical Regent Corporation in Shanghai (China). Volatile organic compounds (VOCs), including ammonia and nitrogen (Analytical grade, Degree of purity: 99.999%), were supplied by Shengtong Gas Corporation in Guangzhou (China).

2.2 Preparation of spinning solutions

To prepare a spinning solution of G-COOH/PS nanofibers, a dispersion of carboxyl graphene (280 μl) was added to a mixture of the solvents THF and DMF (1/3, w/w) and then sonicated for 30 min. PS (0.5 g) was dissolved in this dispersion, and the mixture was stirred for 6 h to obtain a homogeneous suspension. The final suspension was then sonicated for an additional 30 min. The electrospinning solution of pure PS nanofibers was fabricated according to the same procedure, except that the carboxyl graphene solution was replaced with distilled water.

2.3 Preparation of G-COOH/PS nanofibrous membranes on QCM electrode

A schematic illustrating the deposition of G-COOH/PS membranes on a QCM electrode is depicted in Fig. 1. Briefly, the spinning solution was loaded into a syringe and injected through a metal needle connected to a high-voltage power supply (Dalian Teslaman High Voltage Co., China). A solution feed rate of 1 ml h⁻¹ was maintained using a syringe pump (LSP01-2A, Baoding Longer Precision Pump Co., Ltd.). In our experiment, a high voltage of 25 kV was applied, and the distance between the pinhead and the QCM electrode was fixed at 19 cm. The temperature and RH of the electrospinning environment were maintained at 25 °C and 40%, respectively. The fibrous membranes were deposited on the grounded electrode of QCM (5 MHz, AT-cut quartz crystal with Au electrodes) and then dried at 80 °C in vacuum for 1 h to remove the trace solvent after the electrospinning process.

Fig. 1 Schematic diagram for preparation of sensing G-COOH/PS membranes on QCM electrode surface.

2.4 The sensor apparatus for ammonia detection

A schematic diagram of the ammonia-sensing system is presented in Fig. 2. The QCM sensor was installed in the testing chamber (10 L), which was kept at a constant temperature of 25 °C and a relative humidity of 45–55%. The operating temperature and RH in the chamber were monitored in real time using a Thermo-Hygrometer (Testo605-H1, Testo Ltd., Germany). The nitrogen was used as a carrier gas to remove residual ammonia and regulate the humidity in the testing chamber. Ammonia was injected by a modified gas injection system according to Quy's work.²⁸ During the experimental process, the concentration of ammonia was increased successively from 1 to 40 ppm. The sensing performance of the QCM sensor toward ammonia was determined by the shifts in resonance frequency, which was related to the amount of mass being absorbed by the sensing coating according to the Sauerbrey equation. The resonance frequencies were measured via a QCM digital controller (QCM-1000, Nanosensing Research System), and the data output by the sensors were recorded by a computer. The entire process was carried out at constant temperature and humidity in a laboratory.

Fig. 2 Schematic diagram of QCM sensing system for ammonia monitoring.

2.5 The characterization of G-COOH/PS membrane

The morphologies of the PS and G-COOH/PS nanofibers were characterized by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). Image analysis software (Image-Pro Plus) was used to measure the fiber diameters. Brunauer–Emmett–Teller (BET) analyses of pure PS and G-COOH/PS fibrous membranes were performed using a surface area analyzer (ASAP2020, Micromeritics Co., USA). The chemical structure of G-COOH powder, PS and G-COOH/PS were characterized by Fourier transform infrared spectroscopy (FT-IR) and Raman spectroscopy. Elemental analyses of the G-COOH powder, pure PS and G-COOH/PS membranes were carried out by X-ray photoelectron spectrometry (XPS).

3 Results and discussion

3.1 The morphologies of pure PS and G-COOH/PS fibers

Fig. 3 showed FE-SEM images and the fiber diameter distributions of pure PS and G-COOH/PS composite nanofibers, respectively. With respect to the pure PS fibers, Fig. 3a showed a beads-on-string morphology with an average fiber diameter of approximately 1 μm; moreover, the fibers exhibited a porous surface. In Fig. 3b, the G-COOH/PS composite nanofibers displayed a random-fiber morphology with an average fiber diameter of 569 nm. Close inspection revealed that each nanofiber had a rough porous surface composed of agglomerated ultrathin nanowires (approximately 37 nm). The diameter of the nanowires was one order of magnitude smaller than that of the original electrospun nanofibers, making the G-COOH/PS membranes more attractive for use in the gas-sensing field. Compared with that of the pure PS nanofibers, the diameter of the G-COOH/PS nanofibers decreased after adding G-COOH. This phenomenon should be attributed to the high conductivity of G-COOH, which increased the conductivity of the electrospinning solution and decreased the fiber diameter as a result.²⁹ TEM was performed to further confirm the morphologies of the as-prepared nanofibers (Fig. 4). The images showed that the pure PS and G-COOH/PS composite nanofibers exhibited a porous interior structure packed with ultrathin nanowires. This unique structure indicated a large surface active region for the enhancement of the material's gas-sensing properties. As indicated by the arrows in Fig. 4b, the G-COOH was randomly dispersed in the nanofibers with some aggregation.

Fig. 3 FE-SEM images of electrospun (a) pure porous PS nanofibers and (b) G-COOH/PS composite nanofibers. Inset of (a and b) showed the corresponding fiber diameter distributions.

Fig. 4 TEM images of electrospun (a) pure porous PS nanofibers and (b) G-COOH/PS composite nanofibers.

The porous structure of the pure PS and G-COOH/PS fibers were also confirmed by nitrogen adsorption–desorption isotherm and BET surface area analyses, as shown in Fig. 5. According to the International Union and Applied Chemistry (IUPAC) classification,³⁰ the isotherms of PS and G-COOH/PS belong to type II with a H₃ hysteresis loops characteristic, demonstrating that the mesoporous system is predominant. As shown in the inset curves, the pore widths of the PS and G-COOH/PS fibers were mainly distributed over the range of 20–90 nm, with average pore diameters of 39 nm and 38 nm, respectively. The SSAs of the PS and G-COOH/PS fibers were 29.46 and 24.64 m² g⁻¹, respectively. These figures confirmed the hierarchical mesoporous nanostructure of the as-prepared G-COOH/PS fibers, suggesting their potential in gas-sensing applications.

Fig. 5 Nitrogen adsorption–desorption isotherm of (a) pure porous PS fibers and (b) G-COOH/PS composite nanofibers. The inset of (a and b) showed the pore size distributions.

3.2 FT-IR and Raman results

The loading of G-COOH sheets into and on PS fibers was confirmed by FT-IR and Raman spectroscopy. Fig. 6 showed the FT-IR spectra of the G-COOH powder, pure PS and G-COOH/PS membranes. The IR spectrum of G-COOH (Fig. 6a) displayed characteristic peaks at 3436 cm⁻¹ (hydroxyl stretching), 1735 cm⁻¹ (carboxyl C=O stretching). In the G-COOH/PS spectrum (Fig. 6b), the representative C=O peak at 1718 cm⁻¹ appeared, whereas the characteristic O–H peak disappeared, which should be attributed to the cationic-π bonding between OH groups and the π system of the phenyl ring.³¹ Other characteristic peaks were observed for the blend fibers, which indicated that G-COOH sheets embedded in the nanofibers after electrospinning.

Fig. 6 FT-IR spectra of (a) G-COOH powder, (b) G-COOH/PS composite membrane and (c) pure porous PS membrane.

The Raman spectra (from 1080 to 2000 cm⁻¹) of the G-COOH powder, PS and G-COOH/PS membranes were shown in Fig. 7. The Raman spectrum of G-COOH was characterized by two main features, a G band at 1600 cm⁻¹ and a D band at 1359 cm⁻¹. The G band arose from the in-plane bond-stretching motion of pairs of sp²-bonded carbon atoms. The D band in the G-COOH was derived from the reduction in size of the in-plane sp² domains during oxidation.³² The Raman spectrum of PS between 1080 and 2000 cm⁻¹ displayed characteristic peaks at 1600 cm⁻¹ (aromatic C=C stretching vibration) and 1457 cm⁻¹ (C–H deformation of CH₂).³³ In the G-COOH/PS spectrum, the representative G peak of G-COOH appeared, and the peak at 1600 cm⁻¹ was higher than that of pure PS, which indicated the presence of the representative D peak of G-COOH. Other characteristic peaks were observed for the blend fibers, demonstrating that G-COOH sheets were well dispersed in the PS nanofibers.

Fig. 7 Raman spectra of (a) G-COOH powder, (b) G-COOH/PS composite membrane and (c) pure porous PS membrane.

3.3 XPS spectra of pure PS and G-COOH/PS membranes

To further examine the surface composition, the XPS spectra of the G-COOH/PS composite nanofibers, pure PS and G-COOH powders were studied. Fig. 8a showed the wide-survey XPS spectrum of pure PS, which indicated that the sample contained C as the main component. Fig. 8c showed the wide-survey XPS spectrum of the G-COOH powder, which suggested the presence of C and O. The surface oxygen level of the G-COOH powder was 35.84%, indicating the abundant presence of oxygen-containing groups. Fig. 8b showed the wide-survey XPS spectrum of the G-COOH/PS composite nanofibers, which confirmed the presence of C and O. The surface oxygen level of the G-COOH powder was 3.83%, indicating that some G-COOH sheets remained and exposed on the surface of the G-COOH/PS nanofibers after electrospinning.

Fig. 8 XPS survey scan spectra of (a) pure porous PS membrane, (b) G-COOH/PS composite membrane and (c) G-COOH powder.

3.4 Ammonia-sensing properties

Fig. 9 showed the dynamic response of the G-COOH/PS nanofibrous membrane-based QCM sensor toward different concentrations of ammonia against a background of ambient air with ∼40% RH at room temperature. For comparison, we prepared PS porous fibers with a BET surface area similar to that of the G-COOH/PS membrane and investigated the response of the PS-based QCM sensor toward ammonia of the same concentration under the same test conditions (Fig. 9, inset top right). The QCM sensor coated with the porous PS nanofibrous membrane did not show distinct frequency shift until the ammonia concentration reached as high as 30 ppm. Moreover, the maximum frequency shifts were only 0.5 and 0.3 Hz at 30 and 40 ppm, respectively. As shown, the G-COOH/PS nanostructure possessed a higher sensitivity compared with the PS structure due to its G-COOH content, whose carboxyl groups could interact with ammonia molecules. The frequency of the G-COOH/PS-based QCM sensor decreased upon exposure to increasing ammonia concentrations ranging from 1 to 40 ppm. Upon each injection of ammonia into the chamber, the response showed a sharp drop and reached a steady value after several minutes. The frequency shifts of the G-COOH/PS nanofibrous film-coated QCM sensor exposed to 1, 3, 10, 20, 30 and 40 ppm of ammonia were 0.3, 0.5, 1.8, 4.8, 4 and 3.5 Hz, respectively. We observed that the response amplitude of the sensor gradually increased with the gas concentration. However, high concentrations of ammonia results in less variation of the responses due to the saturated adsorption of ammonia molecules.

Fig. 9 The frequency shifts of QCM sensors coated with G-COOH/PS membranes upon exposure to increasing ammonia concentrations. The inset shows the sensing property of PS porous fibers under the same conditions.

Compared with semiconductor ammonia sensor, the G-COOH/PS-coated QCM sensor exhibited high sensitivity, good reversibility and sufficient specificity. The device can be operated at room temperature, which is greatly better than that of semiconductor ammonia sensor. When compared with optical ammonia sensor, the as-prepared G-COOH/PS QCM system is low-cost and easy operation, which is suitable for miniaturization and integration. However, the as-prepared G-COOH/PS QCM sensor suffers from a limited maximum sensitivity. To further enhance the sensing properties of the device, the aim of our next study was to improve the dispersion of G-COOH and expose more G-COOH on the surface of G-COOH/PS nanofibers. As observed in this study, the G-COOH used as a sensitive material tended to aggregate due to strong interplanar interactions. Thus, the performance of the composites was reduced because G-COOH was poorly dispersed in the composites. The poor dispersion of G-COOH in the spinning solution could cause re-agglomeration or re-stacking problems in fabricating composite materials, which limited its use in sensor applications. Moreover, G-COOH was generally trapped inside the nanofibers, which had no effect on sensing performance. Future work will focus on improving the dispersion of G-COOH embedded into nanofibers whose surface was exposed after electrospinning, enhancing the performance of this promising platform for sensor applications.

3.5 The reversibility of QCM sensor

Fig. 10 showed reversibility test for G-COOH/PS-based QCM sensor exposed to different concentrations of ammonia. When frequency curve reached equilibrium after the injection process, the nitrogen gas was used to remove the ammonia until a complete desorption was achieved. Then the sensor was exposed to previously used ammonia concentrations. By exposing the sensor to repeated adsorption–desorption cycles, good reversibility of the as-prepared sensor was observed. As shown in Fig. 10, the frequency of the sensor was back to its original value after drying the electrode with nitrogen. And the frequency shifts were nearly similar under the same ammonia concentrations in three injections. These results indicated that the G-COOH/PS-based QCM sensor presented an excellent reversibility.

Fig. 10 Reversibility testing for QCM sensors coated with G-COOH/PS membranes upon exposure to increasing ammonia concentrations.

3.6 Gas-sensing mechanism

The literatures showed that a unique structure and high porosity were advantageous in achieving strong and rapid responses via the effective and quick diffusion of detective vapor.^34,35 The results of the present work were in good agreement with these observations. The highly porous G-COOH/PS nanofibrous film can offer large contact area and allow for the access of ammonia with minimal diffusion resistance, enhancing vapor diffusion and mass transport in the sensing material.

As shown in Fig. 2, when highly purified ammonia gas was added to the sealed sensing chamber through the gas injection system, the frequency shift of the QCM sensor was related to the additional mass loading of adsorbed ammonia molecules on the QCM (Δm), which was primarily ascribed to the reversible interaction between the ammonia molecules and the functional groups (–COOH) of the G-COOH nanofibers, as illustrated in Fig. 11.³⁶ This sensing process was regarded as physical adsorption via molecular force, and the resonance frequency can be described by the Sauerbrey equation as follows:¹⁰

Δf = −2f₀²Δm/A(μρ)^1/2 (1)

where Δf represents the frequency shift of the QCM sensor (Hz); f₀ is the intrinsic frequency of an empty QCM platform; Δm is the mass variation per unit area (g cm⁻¹⁻²); A is the electrode surface area of the QCM (1 cm²); and μ and ρ are the shear modulus and density of quartz crystal, 2.947 × 10¹¹ dyne cm⁻² and 2.648 g cm⁻³, respectively. In this study, the inherent frequency of the QCM chip was 5 MHz, and the sensitivity of the sensor reached 17.67 ng Hz⁻¹.

Fig. 11 Schematic representation of the interaction mechanism between ammonia and G-COOH/PS.

4 Conclusions

As a new sensing material, mesoporous G-COOH/PS nanofibers with a porous interior and exterior were successfully deposited on the electrode of a QCM via electrospinning. These nanofibers, agglomerated with ultrathin nanowires (approximately 37 nm), possessed a high SSA of 24.64 m² g⁻¹. This unique structure offered a much higher contact area and allowed for access to ammonia molecules through its carboxyl groups. We confirmed that G-COOH existed in G-COOH/PS membranes based on FT-IR and Raman spectroscopy. XPS analysis demonstrated that the surface oxygen level of the G-COOH/PS composite membrane was 3.83%. A gas-sensing test showed that the as-prepared G-COOH/PS-coated QCM sensor exhibited high sensitivity and good reversibility at room temperature. This work provides a new direction for the fabrication of efficient GO-based nanofibers for gas-sensing applications. Indeed, the ideal dispersion of G-COOH, which was embedded into nanofibers whose surface was exposed after electrospinning, would enhance the performance of this promising platform for sensor applications.

Acknowledgements

This work was supported by the Science Foundation for Young Teachers of Wuyi University (no. 2013zk12), Foundation for distinguished young talents in higher education of Guangdong, China (no. 2013LYM_0092) and the Project of Department of Education of Guangdong Province (no. 2013KJCX0184).

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