Fabrication of a Highly Sensitive Single Aligned TiO2 and Gold

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Fabrication of a Highly Sensitive Single Aligned TiO2 and Gold Nanoparticle Embedded TiO2 Nano-Fiber Gas Sensor Alireza Nikfarjam,* Seyedsina Hosseini, and Nahideh Salehifar MEMS&NEMS Laboratory, Faculty of New Sciences & Technologies, University of Tehran, P.O. Box 14395-1561, Tehran 33316-196363, Iran ABSTRACT: In this research, a single-aligned nanofiber of pure TiO2 and gold nanoparticle (GNP)-TiO2 were fabricated using a novel electro-spinning procedure equipped with secondary electrostatic fields on highly sharp triangular and rectangular electrodes provided for gas sensing applications. The sol used for spinning nanofiber consisted of titanium tetraisopropoxide (C12H28O4Ti), acetic acid (CH3COOH), ethanol (C2H5OH), polyvinylpyrrolidone (PVP), and gold nanoparticle solution. FE-SEM, TEM, and XRD were used to characterize the single nanofiber. In triangular electrodes, the electrostatic voltage for aligning single nanofiber between electrodes depends on the angle tip of the electrode, which was around 1.4−2.1, 2−2.9, and 3.2−4.1 kV for 30°, 45°, and 60°, respectively. However, by changing the shape of the electrodes to rectangular samples and by increasing distance between electrodes from 100 to 200 μm, electro-spinning applied voltage decreased. Response of pure TiO2 single nanofiber sensor was measured for 30−200 ppb carbon monoxide gas. The triangular sample revealed better response and lower threshold than the rectangular sample. Adding appropriate amounts of GNP decreased the operating temperature and increased the responses. CO concentration threshold for the pure TiO2 and GNP-TiO2 triangular samples was about 5 ppb and 700 ppt, respectively. KEYWORDS: single-aligned nanofiber, pure TiO2, gold nanoparticle (GNP)-TiO2, electro-spinning, secondary electrostatic field, gas sensing

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forms ionic oxygen species [O2−,O−]. By introducing reducing gases, they react with ionic oxygen atoms. Then electrons will be injected into TiO2 subsequently followed by increase in electrical conductivity. On the contrary, if oxidizing gas molecules are absorbed on the surface, electrons will be depleted, which will decrease electrical conductivity. The resulting conductivity change will be transformed to electrical signal.29 To fabricate nanofibers, distinct procedures have been suggested, among them electrospinning is highly regarded.30,31 This technique is a facile, versatile, and cost-effective approach that offers production of long continuous nanofibers with a variety of materials including polymers, metal oxides, composites, etc.,32−35 as well as the ability to modify fiber diameter within a wide range from micro to nanometers. Single nanofiber-based electronics could be employed for ultrasensitive gas sensors because of rapid capture, diffuse, and release of gas molecules, which creates a gas sensor with expeditious response and recovery times. Recently, fabrication of single nanofiber has been investigated by several research groups. Guo et al. fabricated single TiOTPyP nanofiber by

INTRODUCTION One dimensional (1D) nanostructures such as nanofibers, nanowires, nanorods, and nanobelts have been used in advanced devices.1−7 Among them, in recent years nanofibers and nanowires have received considerable attention because of their inherent properties like high surface to volume ratio and high aspect ratio.8,9 Compared with nanowires, nanofibers have advantages of ease of fabrication and low processing cost.10−12 Nanofibers have special mechanical, electrical, and optical properties and could be used in various applications, such as tissue engineering, protective clothing, and sensors.13−15 Metal oxide semiconductor nanofibers are mainly used in photocatalysts, photo/electrochromics, solar cells, and sensors.16−19 Metal oxide semiconductors such as TiO2, ZnO, In2O3, SnO2, and WO3 with different nanostructures play an important role in producing gas sensors with significant sensitivity to toxic gases, reasonable cost, and small size.20−24 Among various types of semiconductor sensors, the TiO2 sensor’s high sensitivity, low cost, fast response, and longterm stability makes it an ideal candidate.25 The TiO2 gas sensor not only detects oxidizing gases (O2, NO2), but also responds to reducing gases (CO, H2, NH3, H2S, VOCs). The former lead to increase resistance, but the latter lead to resistance decrease.26−28 TiO2 as an n-type semiconductor absorbs oxygen from air, which acts as an electron receptor and © 2017 American Chemical Society

Received: December 4, 2016 Accepted: April 19, 2017 Published: April 19, 2017 15662

DOI: 10.1021/acsami.6b15554 ACS Appl. Mater. Interfaces 2017, 9, 15662−15671

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ACS Applied Materials & Interfaces dispersing nanofibers on the substrate and patterning single nanofiber using a micromanipulator and an individual organic ribbon mask. They could detect 250 ppb vapor-phase H2O2.36 Sun and colleagues, for the first time, reported controllable electrospinning based on near-field electrospinning (NFES). They also reported that single nanofiber can be achieved when relative moving speed between spinneret and collector is comparable to electrospinning speed.37 Li et al. synthesized two conductive Si strips and reported that nanofibers could be transferred from strips to other substrates as single nanofiber.38−40 Zheng and co-workers deposited a U-shaped single nanofiber by controlling the movement of the collector.41 To the best of our knowledge, focus on fabrication of singlealigned nanofibers by employing the secondary field has not been reported in any literature. In this novel technique, we utilize a secondary field to obtain single-aligned nanofiber (Figure 1). In the aforementioned methods, achieving singlenanofiber depends on the movement speed of the microstage.42,43

Figure 2. XRD pattern of pure TiO2 and GNP/TiO2 single nanofiber.

formula, the percentage of rutile and anatase phases was estimated about 28% and 72% for the pure TiO2 and 22% and 78% for the GNP-TiO2 nanofibers, respectively. content of anatase (%) = IA /(IA + 1.265IR ) × 100%

(1)

The XRD of the GNP-TiO2 nanofiber demonstrated no considerable diffraction peak for gold, probably because of the small particle size and low value of the GNPs. Figures 3a and 3b exhibit the FESEM images of singlealigned TiO2 nanofiber calcined at 500 °C on triangular (angle

Figure 1. Schematic representation of different fields during the electrospinning process.

Here, we present a novel method for fabricating singlealigned nanofibers. A secondary field with patterned electrodes can precisely control position of the single nanofiber. Besides, the obtained single-aligned nanofiber based device was used as a gas sensor with ultrahigh sensitivity and fast response and recovery times. The limit of detection (LOD) was as low as 700 ppt, which is 1 order of magnitude higher than nanowire based gas sensors.44,45 This article is the first experimental evidence of a highly sensitive single-aligned TiO2 nanofiber gas sensor based on the secondary field. Besides, gold as a catalyst, in form of nanoparticle was added to the solution used in the electrospinning setup. Gold is one of the best catalysts for detecting CO; in addition its nanoparticle structure with specific size and shape has photocatalytic activity under visible light illumination. GNP-TiO2 single-aligned nanofiber with triangular shaped electrode CO sensors showed best response, and response and recovery times. Based on our observation, we achieved the lowest LOD not yet reported for CO sensors.

Figure 3. FE-SEM images of (a) aligned pure TiO2 single nanofiber on a triangular type electrode, (b) aligned pure TiO2 single nanofiber on a rectangular type electrode, (c) aligned fiber (before thermal treatment) on the tip of a triangular electrode, and (d) TEM image of a pure TiO2 nanofiber.11

of tip 30° and gap 30 μm) and rectangular (width and gap 200 μm) type electrodes, respectively. Figure 3c exhibits the nanofiber before calcination. In Figure 3d, the TEM image of calcined TiO2 nanofiber (prepared using 12 wt % PVP concentration) also shows the nanofiber construction details. Obtained results indicate smooth and uniform surface for the nanofiber with a diameter of about 100 nm. Study of Electrospinning Procedure. In this process, the electrostatic field between electrodes was considered constant at 25 V to evaluate the effect of other parameters, including the shape of electrodes and distance between them, which have important effects on threshold voltage for achieving an aligned single nanofiber. The general results of fabrication are demonstrated in Tables 1 and 2. In triangular samples,

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RESULTS AND DISCUSSION Structural Analyses. Figure 2 shows the XRD patterns of the prepared pure TiO2 and GNP/TiO2 nanofibers after calcination. Both anatase and rutile phases can be observed in the pure TiO2 and GNP/TiO2 samples. However, adding GNPs changed the contents of anatase and rutile phases in comparison to the pure sample. According to the following 15663


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ACS Applied Materials & Interfaces

Simulation. Electric field simulation was carried out using COMSOL Multiphysics 5.0 software, which is depicted in Figure 4. Using COMSOL software, electric field distribution between electrodes with and without secondary field were analyzed. Figure 4a represents the contour distribution of the electric field between nozzle and collector and between the electrodes. This simulation was done at 1.4 kV and 25 V between nozzle and collector and between electrodes, respectively. It is clear that electric field intensity is extreme at the nozzle tip and the electric field intensity diminishes by increasing the distance from nozzle to collector. Besides, Figure 4b and c show the effect of secondary field between electrodes. Figure 4b confirms that the secondary field between electrodes leads to ease of aligning nanofibers. This means that aligning nanofibers is more difficult in the absence of the secondary field (Figure 4c). Sensing Measurement. In the first step, I−V curves for rectangular and triangular samples are presented in Figures 5a and 5b. The diameter of the fibers in both types of electrodes are about 100 nm. As shown in the Figure 5a, the diagram slope has been raised by increase in temperature. This means that the amount of current has direct relationship with temperature. Regarding the rectangular sample, at 350, 300, and 250 °C, the current is 450, 165, and 15 pA for 2 V, respectively. However, regarding the triangular samples in this situation (Figure 5b), the amount of current is about 750, 300, and 25 pA. The responses of the triangular and rectangular pure TiO2 single nanofiber-based sensor were measured at 300 °C and in the constant voltage of 3.5 V. The response was defined as the ratio of the sensor resistance in dry air to the sensor resistance in the presence of detected gas (Rair/Rgas). Results obtained for triangular and rectangular samples were compared and are shown in Figure 6a. The values obtained for 30−200 ppb carbon monoxide gas are approximately 1.7−161 and 7−190 for rectangular and triangular samples, respectively. Moreover, the triangular sample changed the threshold of carbon monoxide gas concentration from 30 to 10 ppb. Generally, triangular samples revealed better response than rectangular samples, and thus, the triangular samples has been used for further tests in the study. Figure 6b and 6c revealed and compared the outcomes gained for pure TiO2 and GNP-TiO2 samples with triangular type electrodes as a function of CO concentrations (1−200 ppb) at different operating temperatures (200−350 °C). According to Figure 6b, the response of the pure TiO2 single nanofiber to 5−200 ppb CO is 3−158, 4−170, 7−190, and 5− 178 at 200, 250, 300, and 350 °C, respectively. These results demonstrated that the best operating temperature for the pure TiO2 sample is 300 °C. Correspondingly, for the GNP-TiO2 single nanofiber sensor, the response is 13−490, 18−597, 15− 561, and 11−521 at 200, 250, 300, and 350 °C, respectively (Figure 6c). Generally, doping GNP caused considerable changes in operating temperature of about 50 °C toward lower values from about 300 to 250 °C. Besides, the GNP-TiO2 single nanofiber sensor has the ability to detect CO in concentrations greater than 150 ppb at low operating temperatures in the range of 50−150 °C. According to Figure 6d, the response of this sample for 200− 350 ppb at 50, 100, and 150 °C is 8−41, 17−67, and 42−121, respectively. Although the amount of responses are lower in comparison to the responses in Figure 6c, the sample can be used at lower temperatures even at room temperature without using extra power for increasing the temperature of the sensor.

Table 1. Design of Experiment for Triangular Samples electric field (V/cm)

angle the tip of electrode (deg)

distance between electrodes (μm)

700 800 900 1050 1000 1450 1600 2050

30 30 30 30 45 45 60 60

30 100 150 200 30 200 30 200

Table 2. Design of Experiment for Rectangular Samples electric field (V/cm)

widths of electrode (μm)

distance between electrodes (μm)

2600 1950 1700

30 100 150 200

30 100 150 200

corresponding to the distance between electrodes, the usage voltage between electrodes for aligning nanofibers was around 1.4−2.1, 2−2.9, and 3.2−4.1 kV for 30°, 45°, and 60°, respectively as shown in Table 1. On the contrary (as shown in Table 2), by changing the shape of the electrodes to the rectangular ones, the increased width and distance between electrodes has an indirect effect on the voltage needed for the aligned nanofiber. It means that by increasing distance between electrodes from 100 to 200 μm, the applied voltage decreased from 8.9 kV to 3.4 kV. In both tables, the distance between syringe tip and collector was constant (20 mm). Moreover, the amounts of electric field were used instead of applied voltage in these tables. It was found that the orientation of fibers on both the insulating area and the edge of the conductors is independent of the composition and diameter of the fibers, and they are collected straighter on the edge of the conductor with a larger insulating area. The electrostatic forces acting on a fiber comprises two components:38 one in the plane of the collector (FH) and the other perpendicular to the plane (FV). The latter is inversely proportional to the area of insulating region (A).

FH ∝ A 1 A As a result, in rectangular samples, if we employ a collector with a larger insulating area, the fiber will experience a stronger FH, adjusting its orientation to the preferred direction. Furthermore, a decrease in FV causes the fiber to have more time to remain in motion and adjust its orientation before it contacts the electrode. As shown above, decreasing the tip angles of electrodes in the triangular samples plays an important role in changing field intensity. In low voltages, a high electric field can be achieved by sharp tips.46 The local field, F, is equal to F = βE, where β is the field-enhancement factor, and E is the macroscopic field. The field-enhancement factor is proportional to the reverse electrode tip angle FV ∝

β∝f

( α1 ),

where α is the angle of the electrode tip.47 In

other words, the diameter of tip curvature has a high impact on the local electric field created between the electrodes. 15664


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Figure 4. (a) Representation of contour distribution of electric field between nozzle tip and collector and between electrodes, and representation of electric field between electrodes (b) with and (c) without the secondary field.

Figure 5. I−V curve for pure TiO2 single nanofiber at different temperatures for (a) rectangular samples and (b) triangular samples.

In addition, in Figure 6e, we can see that the GNP−TiO2 single nanofiber sensor can be used to detect CO at concentrations as low as the ppt range. The response of this sample at 250 °C for 700−1000 ppt of CO is 2.1 to 7.4.

Time-dependent responses of TiO2 and GNP-TiO2 upon exposure to various concentrations of CO gas are shown in Figure 7a and 7b. The response of these samples at CO concentrations of 30, 40, 50, 60, and 70 ppb is 13, 26, 34, 48 for 15665


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Figure 6. (a) Response of pure TiO2 nanofiber triangular and rectangular samples in the presence of different concentrations of CO gas at 300 °C. Response of (b) pure TiO2 single nanofiber triangular sample and (c) GNP-TiO2 triangular sample in the presence of CO gas as a function of concentration at 200−350 °C. The response of GNP-TiO2 triangular samples in the presence of CO gas (d) at lower temperatures and (e) as a function of concentration (ppt) at 250 °C.

imately 150) suggesting that it can be employed for various related applications. As the figure displays, by adding GNP, the response to all gases increased significantly, although sensor exhibits the lowest response to NO2 and NH3 in comparison to CO and H2. The performance of our GNP-TiO2 nanofiber CO sensor was compared with recently developed CO sensors based on different morphologies of TiO2 with various additives (Table 3). Discussion. The detection mechanism of TiO2 single nanofiber sensors in the presence of CO as a reducing gas involves the partial chemical reduction of sensitive single nanofiber.22 At higher temperatures, the O2 molecules are ionized on the TiO2 nanograins to form active ionic oxygen species like [O−]. Because of the high electron affinity of oxygen and since TiO2 is an n-type semiconductor, they extract electrons from the semiconductor conduction band and create a depletion region into the grain surfaces, decreasing the effective radius of the grains for electron transport. By introducing CO gas, a reaction between ionic oxygen atoms [O−] and CO molecules occurs [CO + O− → CO2 + e−]. This interaction releases electrons and reinjects them into the

Pure TiO2, and 75, 116, 195, 245, and 310 for GNP-TiO2 single nanofiber samples, respectively (Figures 7a and 7b). The pure TiO2 sample did not display any detectable response in the range of below 20 ppb compared with the GNP-TiO2 samples. Response and recovery times at 30−70 ppb CO are 3−5s and 4−6s for GNP-TiO2 at 250 °C and 5−7s and 6−9s for pure TiO2 samples at 300 °C, respectively. Continuous test was performed for certain concentrations of CO (50 ppb) to investigate repeatability and stability of the pure TiO2 and GNP-TiO2 single nanofiber triangular sensors. Figures 7c and 7d clearly demonstrate that by exposing 50 ppb CO, sensors respond rapidly and after several periodical tests, the responses of pure TiO2 (Figure 7c) and GNP-TiO2 (Figure 7d) single nanofiber sensors returned to their initial values without any noticeable diversion. Responses of several gases were tested to investigate the effect of GNP additive on sensing properties. Figure 7e shows the responses of pure TiO2 at 300 °C and GNP-TiO2 at 250 °C single nanofiber sensors in the presence of 50 ppb of several gases. The response of pure TiO2 for H2 is larger than other gases (approximately 36). However, the GNP-TiO2 single nanofiber sensor expresses great response to CO (approx15666


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Figure 7. Time-dependent response of (a) pure TiO2 at 300 °C, and (b) GNP-TiO2 samples at 250 °C in different CO concentrations. Repeatability of (c) pure TiO2 at 300 °C, and (d) GNP-TiO2 triangular sensors at 250 °C in sequential exposure (5 cycles) to 50 ppb of CO. (e) Response of pure and GNP-TiO2 single nanofiber triangular sensor in the presence of 50 ppb of several gases.

Table 3. Comparison Investigation on the Sensing Performance of TiO2-Based CO Gas Sensors between This Work and Reported Results nanostructure

temperature (°C)

response

concentration

tres/trec (s)

Cu/TiO2 nanofiber TiO2 nanofiber TiO2 hollow hemisphere TiO2 nanoporous TiO2 xerogel CNT/TiO2 MWCNT/TiO2 Au@TiO2 single aligned GNP-TiO2 nanofiber

300 200 250 500 350 350 400 325 250

3 1.1−2.6 4220 5 6.81 15.8 89.2 17 75

5 ppm 1−15 ppm 500 ppm 280 ppm 50 ppm 50 ppm 100 ppm 500 ppm 30 ppb

4/8 32−86/84−109 9/11

LOD 1 ppm 1 ppm 5 ppm

∼5/92 4/16 5/8 3/4

10 ppm 700 ppt

ref 48 49 50 51 52 53 54 55 this work

Besides, when GNPs make contact with TiO2 nanograins, a Schottky barrier forms between them and the electrons flowing from the TiO2 nanograins to the GNPs. This occurs because of the work function difference between gold and TiO2 nanograins. This widens the depletion region into the TiO2 nanograins, which tends to further reduce sensor conductivity. Thus, the changes of the sensor conductivity will be improved when CO molecules interact with preadsorbed oxygen ionic atoms to release and reinject electrons into the depletion region

depletion region created into the TiO2 nanograins’ surfaces. This, significantly reduces the height and width of the barriers created between neighboring grains, resulting in an increase in sensor conductance. GNPs also cover TiO2 nanograins to some extent, but CO molecules can penetrate through the free spaces between the grains and react with ionic oxygen species (O2−, O2−, O−).22 GNPs as catalysts, increase sensor response by reducing the activation energy required for the mentioned interactions. 15667


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Figure 8. Cross-section of a typical TiO2 nanofiber consisting of several nanograins and the effect of CO molecules absorbing in electronic mode (a) before and (b) after CO exposure.

created into the TiO2 nanograins surfaces.11 GNPs affect the rate of attraction and repulsion of gas molecules and improve response and recovery times. These improvements are dependent on the interaction of the CO molecules and ionic oxygen species (O2−, O2−, and O−) under the catalytic effect of the GNPs.11 Figure 8 schematically shows the cross-section of a typical TiO2 nanofiber consisting of several nanograins as depicted in the TEM image (Figure 3d) before and after CO exposure. In the right-hand of these figures, the barrier height created between neighboring grains and the absorbing effect of the CO molecule on narrowing and lowering this electric barrier height are shown. The GNPs used as catalyst with specific size and shape have additional photo catalysis effects under visible light.22 The GNPs supported on the TiO2 nanograins exhibited higher catalytic activity when illuminated with visible light. Consequently, CO molecules could be oxidized easier by lowering the activation energy of the following reactions, which tends to release the trapped electrons to the body and increase conductivity. Visible light can enhance local electromagnetic fields and heat, and activate GNPs by Local Surface Plasmon Resonance (LSPR) effect. This lowers activation energy for the reactions. The GNPs have high absorption in UV spectrum and have a local maximum at 550 nm (visible light).

investigated. According to results, triangular electrodes have better ability to detect CO gas in lower gas concentrations and showed higher response and lower response and recovery times in comparison to rectangular electrodes. The response of these samples at CO concentrations of 30−200 ppb is 7−190 and 18−597 for pure TiO2 (300 °C) and GNP-TiO2 single nanofiber triangular samples (250 °C), respectively. Moreover, the response and recovery times were 5−7s and 6−9s for pure TiO2 triangular sample at 300 °C, and 3−5s and 4−6s for GNP-TiO2 triangular sample at 250 °C, ordinary. The threshold of rectangular pure TiO2, triangular pure TiO2 and triangular GNP-TiO2 samples was approximately 30 ppb, 5 ppb, and 700 ppt, respectively. The results obviously exhibited that adding GNP and using sharp triangular shape electrodes highly impact sensitivity improvement of the sensor in the range of ppt. Furthermore, the obtained results for testing different gases demonstrated that the response of pure TiO2 for H2 was larger than other gases; however, the GNP-TiO2 sensor showed the best response for CO, suggesting that it can be employed for different related applications. In addition, during the measurement process the sensor properties were stable with excellent repeatability.

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METHODS

Materials. Titanium tetraisopropoxide (C12H28O4Ti, 97%) was supplied from Sigma-Aldrich. Acetic Acid (CH3COOH, 100%), ethanol (C2H5OH, 99.9%), and polyvinylpyrrolidone (PVP) were purchased from Merck. Preparation of Electrodes. The device consists of a silicon substrate that hosts a microheater and a stack gas sensor. The schematic of different layers (Figure 9a), final representation (Figure 9b), and photograph of the sensor (Figure 9c) are shown. Single side polished Si wafer (⟨1 0 0⟩, p-type, 4 in in diameter, 460 μm in thickness) was used for the fabrication process of the device.

CONCLUSION In the current study, we fabricated a single-aligned nanofiber of TiO2 and GNP-TiO2 as an active material for gas sensing applications by secondary field assisted electrospinning method. Besides, nanolithography was employed to fabricate very sharp rectangular and triangular electrodes. The effect of shape and distance between electrodes on sensor properties were also 15668


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Figure 9. (a) Schematic representation of different layers of gas sensor on closed membrane; (b) final representation of the sensor, (c) the final photograph of sensor, and (d) photograph of microheater. Figure 10. (a) FE-SEM image of triangular type electrode and its tip curvature. SEM images of (b) rectangular and (c) triangular electrode configurations. (d) Schematic representation of electrospinning of single aligned nanofiber.

The wafer was thoroughly cleaned by RCA 1 and 2 cleaning process.56 First, 1.8 μm thick SiO2 was thermally grown on a double-side substrate by means of wet oxidation process. Approximately 2 μm positive photoresist (Shipley 1813) was spun-coated onto the backside of the wafer and patterned as an etch mask for SiO2. A rectangular window was opened on the backside of the wafer by Buffered Oxide Etch (BOE), completely removing the SiO2 layer. Subsequently, in order to fabricate closed-membrane, silicon substrate was wet-etched by TMAH (25%, Merck, Germany).57−59 Double side lithography was used to pattern the microheater on the closed membrane onto the front side of the wafer. The microheater was fabricated by depositing nickel over a chromium adhesion layer. Before metal deposition, a photoresist was spun-coated and patterned in reversal mode to form microheater and thermistor. Without removing the photoresist, 15 nm/70 nm Cr/Ni stack were evaporated on the patterned photoresist. Afterward, lift-off process was carried out in acetone in an ultrasonic bath (Figure 9d). Thereafter, the fabrication process of the gas sensor comprising gold electrodes on the microheater began by evaporating 200 nm SiO2 as an electrical barrier layer through electron beam evaporation. Electron beam lithography was employed to fabricate sharp tip electrodes. A 7 nm/80 nm thick Ti/Au stack were evaporated on the SiO2 layer. SU-8 2050 was spun-coated as a mask for electron beam lithography. Electron beam lithography capability is achieved by incorporating a purpose built pattern exposure system with a TESCAN VEGA II Scanning Electron Microscope (SEM). Next, without removing the photoresist, Ti/Au were etched by argon plasma cleaner. The residual photoresist was cleaned in oxygen plasma. Figure 10a shows the sharp fabricated tip and its tip curvature. Two different configuration of electrodes were fabricated on silicon substrate. The first series were rectangular type, which included width and gap of 30, 100, 150, and 200 μm between tips of electrodes. Another type of electrode was the triangular electrode that comprised electrodes with angle tips of 30°, 45°, 60° and gap of 30, 100, 150, and 200 μm. Figure 10b shows an SEM image of the rectangular configuration of an electrode with width and gap of 200 μm, and 10c shows the triangular type with an angle tip of 30° and 30 μm gap between the electrodes. Description of Electrospinning Setup to Produce SingleAligned Nanofiber between Electrodes. Figure 10d demonstrates the schematic setup used for electrospinning single nanofiber. First of all, to achieve the best controllable deposition, the distance between syringe tip to collector was changed from 1 to 20 mm. Further, for wetting the tip, the needle of the syringe was immersed and pulled out of the polymer solution (the solution described in the next part). In the next step, to obtain nanofibers with 90 ± 10 nm in diameter, a 25 μm diameter syringe tip was connected to a high voltage supply. The

electrospinning voltage used between the anode and cathode was variable to obtain single aligned nanofiber. The applied voltage between the syringe tip as anode and electrode as cathode was in the range of 1−6 kV. This value is lower than conventional applied voltages in electrospinning setups. The voltage depends on the type of electrodes, tip angle of electrodes, width of electrodes, and distance between electrodes that will be used to estimate the voltage for each electrode. In addition, an electrostatic field (25 V) was used between the electrodes to align single nanofiber. All experiments were conducted under room temperature and 1 atm pressure. Finally, the nanofiber was calcined at 500 °C for 1 h to eliminate polymers and obtain metal oxide single nanofiber between the electrodes. Preparation of Gold Nanoparticle (GNP). The sodium citrate reduction of gold chloride in an aqueous solution was used to synthesize the GNP solution. First, 10 mL of 0.0288 mol HAuCl4· 3H2O aqueous solution and 20.6 mL of 0.0358 mol N(C8H17)4Br (toluene) were added to a flask and mixed for 15 min. In the second step, 23.8 mL of 0.0139 mol 4-methylbenzenethiol (HS−C6H4−CH3) (toluene) and 8.25 mL of 0.3836 mol NaBH4 (aqueous) were added to the first solution. In the end, the final solution was stirred for 3 h to separate between the black/brown organic phase and aqueous phase. The final product was GNPs encapsulated in 4-methylbenzenethiol. Using ethanol and freezing overnight increased the volume of the solution that caused separation of the GNPs from the solution. A 0.2 μm PTFE filter was utilized to filter the black precipitates. The size of the GNP was estimated using Gaussian size distribution which showed about 20 nm. Preparation of GNP-TiO2 Solution for Electrospinning. The first step to synthesis of the GNP-TiO2 solution was performed through mixing of 1 mL of ethanol (C2H5OH, 99.9%) with 0.5 g of titanium tetraisopropoxide (C12H28O4Ti, 97%) and 1 mL of acetic acid (CH3COOH, 100%) for 20 min. In the second step, 2.5 mL of ethanol with 12 wt % of polyvinylpyrrolidone (PVP) and 7% molar ratios of GNP:TiO2 was mixed for 40 min. This molar ratio of GNP:TiO2 selected, since samples with this ratio of GNP:TiO2 have the best detection for CO gas.11 The obtained solution was added to the first solution and mixed again under vigorous stirring for 2 h. Eventually, the final solution was used for electrospinning single nanofiber as described in the following. Gas Sensing Setup. A dynamic test setup was used to measure the gas sensing properties of the sensor that contains several parts including glass chamber, gas inlet and outlet, gas mixer, several MFCs 15669


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(precision Mass Flow Controller from Alicat Company), and some controller parts. The microheater and microthermistor that were fabricated backside of the sensing material were used to estimate the exact temperature of the sensor. To monitor the sensor characteristics, we used an I−V source measure unit (Keithley 6487). Characterization Techniques. Morphological investigation of electrospun nanofibers were carried out by field emission scanning electron microscope (FE-SEM HITACHI S41-60) at an accelerating voltage of 10 kV. XRD operations were carried out using a Philips X’Pert MPD appliance with Cu−Ka monochromatized radiation (λ = 1.54178 Å) at 40 kV/30 mA. Performance of nanofibers was measured using an I−V source measure unit (Keithley 6487).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Alireza Nikfarjam: 0000-0002-6839-9041 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS There is no financial support for this study. REFERENCES

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