Sulfur Dioxide (SO2) Detection Using Composite of Nickel Benzene Carboxylic (Ni3BTC2) and OH-Functionalized Single Walled Carbon Nanotubes (OH-SWNTs)

Introduction

During the last two decades, Metal-Organic frameworks (MOFs) have attracted considerable attention among research communities due to their unique tunable physical and chemical properties which build on the selecting organic linker and central metal (Campbell et al., 2015; Avery et al., 2016; Bodkhe et al., 2019). MOFs have large surface areas with high porosity. MOFs have been used for various applications like gas adsorption, storage, separation, sensors, and catalysis (Koo et al., 2016; Ullman et al., 2016; Dmello et al., 2018; Li H. et al., 2018). MOFs are newly introduced materials in electronics and optoelectronic devices (Dolgopolova and Shustova, 2016; Campbell and Dincă, 2017; Stassen et al., 2017). Campbell and Dincă (2017) reported successful synthesis of Cu₃(HITP)₂ and Ni₃(HITP)₂ MOFs with high electrical conductivity and reported that it was able to be used for chemiresistive ammonia vapor sensing. The results obtained for Cu₃(HITP)₂ show a good response to ammonia, whereas Ni₃(HITP)₂ did not show any observable response to ammonia vapor under identical experimental conditions.

SWNTs are well known for their unique electrical, mechanical, and thermal properties (Patil et al., 2017; Deshmukh et al., 2018). These properties make them a unique material for sensing applications. Pristine CNTs are not very active in a chemical process. Therefore, activation/functionalization of CNTs need to be performed through various methods viz. wet chemical methods, photo-oxidation, and oxygen-plasma or gas phase treatment, which shows great chemical reactivity of carbon nanotubes (Patil et al., 2017; Gurova et al., 2019; Hoang et al., 2019). The CNTs are familiarized with a graphite surface which contains oxygen-containing groups, mainly carboxyl and hydroxyl (Zhang et al., 2003; Yook et al., 2010; Liang et al., 2016). This introduces the possibility of further modification which will affect the solubility and reactivity of OH-SWNTs.

There are many research groups focusing on SO₂ gas sensor due to its toxicity (Farmanzadeh and Ardehjani, 2018; Li Q. et al., 2018; Nagarajan and Chandiramouli, 2018; Khan et al., 2019; Zhou et al., 2019). The Occupational Safety and Health Administration (OSHA) permissible exposure limit (PEL) for SO₂ is 5 ppm. Advancement in civilization has had severe impacts on the environment. Researchers across the globe are taking crucial efforts to scale new materials for the detection hazardous gases. Some of them are using functionalized or composites of OH-SWNTs for SO₂ detection (Li Q. et al., 2018; Nagarajan and Chandiramouli, 2018).

SWNTs have walls of graphene sheet which leads to π– π electron conjugation to interact with functional group. The functional groups on the SWNTs outer side and tips make them more active channels in a chemical reaction, which is why they have better sensitivity and repeatability. Importantly, Wen et al., 2015 reported that the SWNTs act as an excellent backbone and conductive surface with Ni-MOF. In the present investigation, the composite of Ni₃BTC₂/OH-SWNTs was synthesized through a solvothermal method and it was used for the detection of Sulfur dioxide (SO₂) in ChemFET modality. To the best of our knowledge, no one has yet reported reversible SO₂ gas sensing on a Ni₃BTC₂/OH-SWNTs composite.

Experimental

The Gold Microelectrodes Pattern

The gold micro electrodes on Si/SiO₂ substrate were prepared as reported earlier (Shirsat et al., 2009). The gold microelectrodes had a 3 μm gap between the two electrode tips, which had a length of 200 μm, and these microelectrodes were deposited on Si (thickness 525 μm) /SiO₂ (100 nm thickness) using a standard photolithography process. The gold (Au) had a thickness of 180 nm and the underneath layer of chromium (Cr) had a thickness of 20 nm, and these were deposited on Si/SiO₂ substrate. Finally, the desired micro-pattern was developed using Lift-off technique.

Functionalization of SWNTs

The SWNTs oxidation process was carried out by using HNO₃ oxidizing agent (Hu et al., 2003). The purified 0.1 g of SWNTs (Sigma Aldrich) was mixed with 20 ml of HNO₃. Then the mixture was continuously stirred for 20 h. The prepared suspension releases bubbles which indicates oxygen formation. Then, resultant suspension was ultrasonicated for 60 min at medium power level (VWR 100C ultrasonic bath) followed by centrifugation (REMI R-24) at 12,000 rpm for 60 min. The decanted suspension was sucked using a syringe and used for further process.

Synthesis of Ni₃BTC₂/OH-SWNTs Composite

0.1 mol of oxidized OH-SWNTs mixed with nickel (II) acetate tetrahydrate (molychem, 98%) and 3 mmol Trimesic acid (Sigma-Aldrich, 95%) mixed with 20 ml of N,N-Dimethylformamide (DMF) (Sigma-Aldrich, 99.8%) was continuously stirred for 30 min. at 1000 rpm. The solvothermal method was used for the synthesis of the desired material by using a Teflon-lined stainless-steel autoclave, sealed, and placed in a furnace. The mixture was heated for 140°C for 24 h. After the completion of the reaction, the mixture was cooled at room temperature. The resultant composite was filtered and washed with DMF. Remaining pure precipitate powder was dried at room temperature.

Finally, 0.1 mmol composite of Ni₃BTC₂/OH-SWNTs was mixed with 10 ml DMF solvent and sonicated for 60 min. at medium power. A 0.2 μl drop from the resultant suspension was used to bridge a 3 μm gap between two gold microelectrodes on Si/SiO₂ substrate, as shown in the schematic and optical image (Figure 1).

FIGURE 1

Figure 1. Schematic and optical image for a single microelectrode, drop casted with Ni₃BTC₂/OH-SWNTs material.

Results and Discussion

X-Ray Diffraction

The X-ray diffraction patterns were measured using Bruker D8 Advanced system, with a potential difference of 40 kV and a current of 40 mA with source Cu Kαλ = 1.5406 Å radiation. The XRD patterns of pristine Ni₃BTC₂ and Ni₃BTC₂/OH-SWNTs composite were shown in Figure 2. The pristine Ni₃BTC₂ signature peaks (12.54°, 18.49°, 21.29°, and 25°) of the diffraction pattern are well matched with reported data (Jin et al., 2013; Figure 2a). The percentage of crystallinity for pristine Ni₃BTC₂ was 43.3%. The reduction in crystallinity was observed (27.1%) after incorporation of OH-SWNTs in Ni₃BTC₂, calculated by DIFFRAC.EVA software. The composite of Ni₃BTC₂/OH-SWNTs diffraction pattern (Figure 2b) having the same 2θ° peaks position with a decrease in intensity compared with pristine Ni₃BTC₂ pattern confirms there was no change in the crystal structure of Ni₃BTC₂. The peak intensity decreased for Ni₃BTC₂/OH-SWNTs diffraction pattern, indicating incorporation of OH-SWNTs inside the Ni₃BTC₂ MOF. The XRD pattern of functionalized OH-SWNTs incorporated in Ni₃BTC₂ is shown in Figure 2b, whereas the extra peak at 2θ^o = 26^o confirms the existence of OH-SWNTs (Figure 3b) which is attributed to the graphite structure (002) of OH-SWNTs (Xiao and Xu, 2012).

FIGURE 2

Figure 2. XRD patterns, (a) pristine Ni₃BTC₂ and (b) composite of Ni₃BTC₂/OH-SWNTs.

FIGURE 3

Figure 3. XRD patterns (2θ° range 24° to 27°), (a) pristine Ni₃BTC₂ and (b) composite of Ni₃BTC₂/OH-SWNTs.

The crystallite size of pristine Ni₃BTC₂ and composite Ni₃BTC₂/OH-SWNTs was calculated using the Debye–Scherrer’s formula in Equation (1).

$$\text{D}=\frac{0.9\mathrm{\lambda }}{\mathrm{\beta }cos\mathrm{\theta }}\mathit{\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{0.5em}}(1)$$

where D is crystallite size, λ is the wavelength of x-ray source radiation, i.e., CuKα wavelength is 1.5406Å, β is full width at half maxima (FWHM) calculated from Gauss fitting, pristine Ni₃BTC₂ is 0.126, and composite Ni₃BTC₂/OH-SWNTs is 0.122. The θ° (12.44°) is the Braggs angle of diffraction. The calculated crystallite size for pristine Ni₃BTC₂ was 705.2 Å and for composite Ni₃BTC₂/OH-SWNTs was 730.1 Å.

Fourier-Transform Infrared Spectroscopy (FTIR)

The FTIR spectra was recorded using Bruker Alpha ATR with a ZnSe window in the range of 4000–500 cm^–1. The recorded graph shows a transmittance spectra of pristine Ni₃BTC₂ and composite of Ni₃BTC₂/OH-SWNTs as shown in Figures 4a,b, respectively). The spectra of pristine Ni₃BTC₂ and composite of Ni₃BTC₂/OH-SWNTs samples have been confirmed by the asymmetric and symmetric stretching peaks of the –C = O group at 1620 and 1416 cm^–1, respectively. The –C-OC stretching (etheric) group was found at 1080 cm^–1 (Saber-Samandari and Gazi, 2013). The peaks at 1145 cm^–1 were attributed to C-O group, whereas the C-C vibrations band presented at 1456 cm^–1. The peak at 1632 cm^–1 was attributed to C = O functional group. The aromatic C = C stretching vibration band corresponds with 1539–1658 cm^–1. The intensity of mentioned peak decreases in composite Ni₃BTC₂/OH-SWNTs spectra (Figure 4b) as compared to pristine Ni₃BTC₂ is due to the interaction between carboxyl groups of Ni₃BTC₂ and amide groups of functionalized SWNTs (Karkeh-abadi et al., 2016). The stretching peak of O-H at 3024 cm^–1 appeared in the composite material, which confirms the carboxylic group gained oxidation with HNO₃. Defect formation in sp3 C-H stretching of the benzene ring in the OH-SWNTs is shown in Figure 4b.

FIGURE 4

Figure 4. FTIR spectras, (a) pristine Ni₃BTC₂ and (b) composite Ni₃BTC₂/OH-SWNTs materials.

Current-Voltage (I/V) Characterization

The I/V studies were carried out using a semiconductor parameter analyzer (SPA, Keithley 4200A) at room temperature. West bond wire bounder 7476D was used to have electrical contact from gold microelectrodes to PCB for further measurements. Experimental measurements were carried out at a constant voltage range from −1 V to 1 V. Resistance of pristine Ni₃BTC₂ and composite of Ni₃BTC₂/OH-SWNTs was determined by using an inverse slope of I–V curve. Both curves of pristine and composite materials were passed through origin and show an ohmic nature of materials (Figure 5). The pristine material resistance was 8.85 MΩ and after incorporation of OH-SWNTs resistance decreased substantially to 25.5 KΩ. It was clearly observed that functionalized OH-SWNTs actively help in the enhancement of conduction. The micro network of OH-SWNTs into Ni₃BTC₂ has created a channel connectivity. Moreover, free electron mobility was increased due to the incorporation of OH-SWNTs into pristine Ni₃BTC₂ material. This new network has provided a conductive pathway that can charge transport though the micro network of SWNTs into Ni₃BTC₂ (Choi et al., 2012; Eletskii et al., 2015).

FIGURE 5

Figure 5. Electrical (I-V) characteristics of (a) pristine Ni₃BTC₂ and (b) composite of Ni₃BTC₂/OH-SWNTs.

Field Emission Scanning Electron Microscopy (FESEM)

The morphology of pristine Ni₃BTC₂ and composite of Ni₃BTC₂/SWNTs was studied using a Field emission scanning electron microscopy (FESEM) (using Hitachi High-Tech, S-4800) (Figure 6). Figure 6a reveals the microstructure of pristine Ni₃BTC₂ in which the OH-SWNTs were apparently encapsulated, forming a micro-network structure which is visible in Figure 6b.

FIGURE 6

Figure 6. Field emission scanning electron microscopy (FESEM) images of (a) pristine Ni₃BTC₂ and (b) composite of Ni₃BTC₂/OH-SWNTs.

Field Effect Transistor (FET) Measurements

The transfer and output characteristics of Ni₃BTC₂/OH-SWNTs were studied to understand the FET behavior using SPA-4200A (Figure 7). In this case, composite material Ni₃BTC₂/OH-SWNTs shows p-channel behavior. For FET measurement, OH-SWNTs conducting nature has effectively contributed in composite material. In output characteristics V_ds measurements were carried out between −0.5 to 0.5 V by varying V_gs −1 to −3 V with step 1V. Negative back gated voltage has created a repulsive force between the oxide layer (free charge carrier) and the silicon substrates. It has affected the depletion region populated by positive charges, which is attracted toward electrons and results in a decrease in the depletion barrier. This confirms the presence of holes as a majority charge carrier in channel. This in turn confirms the p-type behavior of Ni₃BTC₂/OH-SWNTs composite. Transfer characteristic was recorded at V_ds −0.1 to −0.4V with step 0.1 V by changing V_gs −20 to 20 V. This confirms the switching behavior of the device with threshold voltage (V_TH) 1.5 V at −0.1V V_ds.

FIGURE 7

Figure 7. FET measurements of Ni₃BTC₂ /OH-SWNTs composite material (A) Output and (B) Transfer characteristics.

The charge carrier mobility was calculated by using Equation (2),

$$\mathrm{\mu }=\pm \left(\frac{\partial \mathrm{\sigma }}{\partial \text{Vgs}}\right)\left(\frac{t}{\text{Cox}}\right)(1)$$

The $\frac{\partial \mathrm{\sigma }}{\partial \text{Vgs}}$ was calculated from linear fit slope from transfer characteristics at V_ds −0.1, where t was thickness of composite material Ni₃BTC₂/OH-SWNTs deposited on gold microelectrodes ∼50 μm and C_ox is capacitance per unit area of 100nm SiO₂ layer calculated by Equation (3)

$${\text{C}}_{\text{ox}}=\frac{3.9{\mathrm{\epsilon }}_0}{{\mathrm{t}}_0}(2)$$

where, ε₀ is the permittivity of free space constant, i.e., 8.85 × 10^–12 C²/Nm² and t₀ is the thickness of SiO₂ layer, i.e., 100 nm. The charge carrier mobility of ∼2.18 × 10^–6 cm² V^–1 s^–1 was calculated from Equation (2) at V_ds −0.1 V.

Chemfet Sensing Studies

The sensing study was carried out by using an indigenously developed dynamic gas sensing system. The V_gs (-3 V) and V_ds (-0.4 V) was kept constant. Data collection was done by Keithley SPA-4200A source measuring unit at room temperature. The gas measuring device (Ni₃BTC₂/OH-SWNTs) was kept in an ∼8 cm³ seal packed chamber. Inserting gas concentration was balanced by using a mass flow controller (Alicat) flow rate 200 SCCM. A tedlar bag was used for accumulating different concentrations of SO₂ gas. Dry air was used to inhabit initial conditions, i.e., for baseline and balancing gas concentrations to avoid any impact from humidity. Once the initial condition was achieved, the sensor was exposed to gas analytes with various concentrations ranging from 4 to 20 ppm at a constant value of V_gs (-3 V) and V_ds (-0.4 V). The sensor device having composite material Ni₃BTC₂/OH-SWNTs shows excellence response and recovery for various (higher to lower) concentrations of SO₂ (Figure 8). The response time of 4.59 s. with a recovery time of 11.04 s. was recorded at 15 ppm of SO₂ concentration (Figure 9). The calibration plot is shown in Figure 10. It shows a little more deviation at higher concentrations as compared to lower concentrations.

FIGURE 8

Figure 8. Dynamic sensor response curve of Ni₃BTC₂ /OH-SWNTs for various concentration of SO₂.

FIGURE 9

Figure 9. At 15 ppm SO₂ gas concentration, response and recovery time with baseline.

FIGURE 10

Figure 10. Standard error bar at SO₂ gas concentration varies from 20 to 4 ppm.

The FET study confirms p-type Ni₃BTC₂/OH-SWNTs composite material with holes as a majority carrier. When electron acceptor (SO₂) gas analytes (Yao et al., 2011) come in contact with p-type material, it causes a transfer of electrons from the composite material, as shown in Equation (4), and generates a number of vacancies (holes). It was responsible for decreasing the resistance of the material while interacting with SO₂ gas analytes.

$${\text{SO}}_2+{\text{e}}^{-}={\text{SO}}_2^{-}(3)$$

The sensitivity of Ni₃BTC₂/OH-SWNTs composite sensor was investigated by exposing 5 ppm concentration of various gas analytes like NO₂, CH₄, CO, and C₂H₂, as shown in Figure 11. Using the same experimental conditions, SO₂ gas showed a much higher sensitivity when compared with other tested gas analytes. These indicate that Ni₃BTC₂/OH-SWNTs composite sensor was highly sensitive toward SO₂ gas. The low response for CH₄, CO, and C₂H₂ was attributed to the nucleophilic nature of these gases (Zhang et al., 2017).

FIGURE 11

Figure 11. Selectivity of the Ni₃BTC₂/OH-SWNTs composite sensor toward 5 ppm of various gas analytes.

Conclusion

Pristine Ni₃BTC₂ and composite of Ni₃BTC₂/OH-SWNTs were successfully synthesized using the solvothermal method. The electrical, structural, surface morphological, and spectroscopic characterization confirms the successful incorporation of OH-SWNTs in Ni₃BTC₂ MOF. The FET measurement confirms p-type behavior of Ni₃BTC₂/OH-SWNTs composite material. The ChemFET sensing at constant value of V_gs (-3 V) and V_ds (−0.4 V) shows an excellent response. The incorporated OH-SWNTs in Ni₃BTC₂ MOF enhances the sensing properties of the composite material. The lower detection limit of 4ppm with a response time of 5 s. and recovery time of 10 s. was observed. The sensor shows excellent repeatability.

Data Availability Statement

All datasets generated for this study are included in the article/supplementary material.

Author Contributions

NI, SM, and PS contributed to experimental work. NI contributed to formal analysis, GB contributed to XRD and FTIR analysis, and TA-G, MM, and SS contributed to FET analysis and gas sensing. NI contributed to the investigation and writing of the original draft. MS contributed to conceptualization, writing the review, editing, and supervision.

Funding

We extend their sincere thanks to Inter University Accelerator Center (IUAC), New Delhi, India (UFR nos. 62320 and 62321), UGC-DAE CSR (RRCAT), Indore (Project No. CSR-IC BL66/CRS- 183/2016-17/847), DST-SERB, New Delhi (Project No. EEQ/2017/000645), Rashtria Uchachatar Shiksha Abhiyan (RUSA), Government of Maharashtra, UGC-SAP Programme (F.530/16/DRS-I/2016 (SAP-II) Dt.16-04-2016) and DST-FIST (Project No. SR/FST/PSI-210/2016(C) dtd. 16/12/2016) for providing financial support.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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