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Abstract

This work presents a fabrication of room-temperature gas sensor for hydrogen sulfide (H₂S) adsorption. Pristine titanium dioxide (TiO₂) nanofibers, reduced graphene oxide (rGO) sheets, and reduced graphene oxide-wrapped titanium dioxide (rGO-wrapped TiO₂) nanofibers were presented in the form of integrated suspension used for a gas-sensing layer. The TiO₂ nanofibers were firstly synthesized by using an electrospinning method with a polyvinylpyrrolidone (PVP) polymer. The rGO sheets were then wrapped around TiO₂ nanofibers by a hydrothermal method. Scanning electron microscope, transmission electron microscope, X-ray diffractometer, and Raman spectrometer confirmed the presence of rGO sheets onto the surface of TiO₂ nanofibers. Ultraviolet-visible spectrophotometer was also considered and displayed to calculate the band gap of TiO₂ and rGO-wrapped TiO₂ nanofibers. After preparing the gas-sensing suspensions, they were dropped onto the polyethylene terephthalate substrates with silver-interdigitated electrodes. The gas-sensing properties of sensors were evaluated for H₂S adsorption at room temperature. Based on the results, the rGO-wrapped TiO₂ nanofiber gas sensor exhibited higher H₂S sensitivity and selectivity than pristine TiO₂ nanofiber and pure rGO gas sensors. The H₂S-sensing mechanism of rGO-wrapped TiO₂ nanofiber gas sensor was discussed based on a formation of p-n heterojunctions between p-type rGO sheets and n-type TiO₂ nanofibers. Furthermore, a direct charge-transfer process by physisorption was also highlighted as a second H₂S-sensing mechanism.

1. Introduction

Hydrogen sulfide (H₂S) is an extremely harmful and flammable gas. It smells like rotten eggs at low concentration in the air. The harm of H₂S is dependent on its concentration and exposure time. A short-term exposure to over 500-1000 ppm of H₂S is immediately fatal [1]. Repeated exposure to H₂S in concentrations even 10-500 ppm can cause serious damage to organs and central nervous system [1–3]. Therefore, the sensor for the detection of H₂S is required to be developed with high sensitivity and fast response at low concentration. In the past several decades, the metal oxide semiconductor (MOS) nanostructures have become one of the popular materials in gas-sensing applications. The MOS gas sensors have been led to the adsorption in toxic gases [4–6]. However, the adsorption of most H₂S needs to operate at high temperatures [7, 8], although some types of MOS gas sensors can be operated at room temperature under the influences of humidity [9–13]. The MOS provides a large number of free electrons in the conduction band and oxygen vacancies on the surface of the metal semiconductors, resulting in strong adsorption characteristics and high reactivity on the surface of gas molecules [14–17]. Among various MOS materials, titanium dioxide (TiO₂) and its composite have been reported as a popular material for applications in lithium-ion storage [18, 19] and photoelectrocatalysis [20–22]. Due to its strong oxidizing power, abundant existence in nature, nontoxicity, and long-term physical and chemical stabilities, TiO₂-based gas sensor has been widely used as an efficient gas absorber [23–29]. However, its application has been restricted because of issues such as a low sensitivity, poor selectivity, large band gap, and high operation temperature. Recently, graphene, graphene oxide (GO), and reduced graphene oxide (rGO) have been accepted as a good candidate for improving the functional properties of metal oxide at room temperature [30–36]. It has been very sensitive to chemicals owing to its monolayer structure such as high thermal conductivity, high specific surface area, and high electron mobility at room temperature. However, it has still low sensitivity, poor selectivity, and slow recovery capability which is not suitable for gas-sensing applications. The hybrid nanostructures based on p-type rGO and n-type TiO₂ semiconductors have been mentioned to the formation of p-n heterojunctions, resulting in the enhancement of gas-sensing properties for the nanocomposite materials [37, 38]. The hybrid nanostructures of graphene and semiconductor metal oxide have mainly prepared by electrospinning and hydrothermal processes [38–40]. The electrospun nanofibers have been reported in the special properties such as high specific surface area and porosity [40–43]. Therefore, they are beneficial for application in gas sensor or gas absorber. More importantly, there have not been many publications for the use of rGO-wrapped TiO₂ nanofibers as a room-temperature H₂S gas sensor although some works on fabrication methods of TiO₂ nanofibers followed by TiO₂ wrapped with rGO have been reported [19, 44].

In this work, we have synthesized rGO-wrapped TiO₂ nanofibers by using a combination of electrospinning and hydrothermal methods. The samples were characterized by a scanning electron microscope (SEM), a transmission electron microscope (TEM), an X-ray diffractometer (XRD), a Raman spectrometer, and an ultraviolet-visible (UV-Vis) spectrophotometer. The sensing performance of rGO-wrapped TiO₂ nanofiber gas sensors was systematically evaluated, relating to the sensitivity, selectivity, and stability properties for H₂S adsorption at room temperature. The pristine TiO₂ and pure rGO gas sensors were also fabricated and evaluated as comparisons. The H₂S-sensing mechanism of rGO-wrapped TiO₂ nanofibers is discussed based on the formation of p-n heterojunctions. Furthermore, a direct charge-transfer process by physisorption is also highlighted as a second possible H₂S-sensing mechanism.

2. Materials and Methods

2.1. Preparation of rGO-Wrapped TiO₂ Nanofibers

The preparation process of TiO₂ solution is shown in Figure 1(a). Firstly, 0.9 g polyvinylpyrrolidone (PVP) polymer was dissolved and stirred in 10 ml ethanol for 1 h. During stirring, 2 ml tetrabutyl titanate (TBT) and 1 ml acetic acid (CH₃COOH) were added into the solution. The mixed solution was continuously stirred for 1 h to obtain a homogeneous precursor solution. To prepare the electrospinning TiO₂ nanofibers, the TiO₂ solution was then loaded into a plastic syringe with a small metallic nozzle. It was fixed on a syringe pump and connected to a high voltage power supply of 18 kV. The distance between the nozzle and an aluminium rolling collector was 15 cm. The collector was controlled by a DC motor controller with a fixed angular speed of 0.5 rpm. A sheet of TiO₂ nanofibers was continuously accumulated on a rolling collector for 40 min. The schematic illustration for preparing the TiO₂ nanofiber by using electrospinning method is shown in Figure 1(b). Inset shows a photograph of nanofiber sheet on a rolling collector. After the electrospinning process, the as-spun TiO₂ nanofibers were heated at a fixed temperature of 500°C for 2 h to decompose PVP and crystallize TiO₂.

Figure 1

Schematic illustration of preparation for (a) TiO₂-based solution, (b) electrospun-TiO₂ nanofibers (inset shows a photograph of nanofiber sheet on a rolling collector), and (c) rGO-wrapped TiO₂ nanofibers.

(b)

(c)

For preparation of rGO-wrapped TiO₂ nanofibers, a sheet of accumulated TiO₂ nanofibers was crushed with a clean mortar and pestle. Then, 50 mg of GO was dissolved in a solution containing 20 ml of dimethyl sulfoxide (DMSO) and 20 ml of deionized (DI) water under ultrasonication for 2 h. At a same time, 70 mg of TiO₂ nanofibers was then added into the solution to obtain a homogeneous suspension. Then, it was transferred to a 40 ml Teflon autoclave and heated up to 180°C for 10 h. After that, it was then rinsed several times with DI water and dried at 70°C for 12 h. Finally, the rGO-wrapped TiO₂ nanofibers were obtained. For preparation of rGO sheets, the GO powder was treated at the same condition without TiO₂ nanofibers. The schematic illustration of rGO-wrapped TiO₂ nanofibers in preparation process is shown in Figure 1(c). The samples were examined by SEM (Quanta 450 FEI), TEM (Hitachi HT7700), XRD (Bruker D8 Advance), and Raman spectrometer (Perkin Elmer Spectrum GX). The UV-Vis spectrophotometer (Perkin Elmer Lambda 950) was conducted and displayed to calculate the band gap of samples.

2.2. Fabrication of rGO-Wrapped TiO₂ Nanofiber Gas Sensor

To prepare the rGO-wrapped TiO₂ nanofiber gas sensor, the dried powder of rGO-wrapped TiO₂ nanofiber (20 mg) was dispersed in 1 ml DI water and 1 ml ethanol under ultrasonication for 5 min. Then, the suspension was dropped onto a polyethylene terephthalate (PET) substrate with silver-interdigitated electrode (Ag-IDE) and dried at room temperature. Figure 2 shows the photograph of dimensional Ag-IDE before and after drop-casting with 20 μl rGO-wrapped TiO₂ nanofiber-sensing suspension. It is seen in Figure 2(a) that the width and the interdigit spacing of the electrode are found to be ~1.0 mm. The size of a drop-casted sensing suspension is approximately $5.0 \times 10.0$ mm², as shown in Figure 2(b).

Figure 2

Photograph of dimensional Ag-IDE (a) before and (b) after drop-casting with a sensing suspension on PET substrate.

(b)

2.3. Gas-Sensing Measurement

The gas-sensing properties of pristine TiO₂ nanofiber, rGO, and rGO-wrapped TiO₂ nanofiber gas sensors were evaluated on their performances by using a gas-measurement system. The system consists of a tank of H₂S, a tank of pure air, two mass flow controllers, a mixer, a test chamber, a circuit board, a power supply, a digital multimeter, and a ventilator. The fabricated gas sensor was loaded into a test chamber which is connected to a voltage divider circuit. The H₂S and pure air were used as target and background gases, respectively. The sensing signals were determined by the measurement of sensor resistance in every second when they were exposed to target and background gases. The signals were monitored by a laptop computer operated with a program in LabVIEW software. After the measurement, the remaining gas was removed out of the chamber by the ventilator. The schematic illustration of gas-measurement system is displayed in Figure 3.

Figure 3

Schematic illustration of gas-measurement system (inset shows a photograph of stainless-steel chamber with electrical signal wires).

3. Results and Discussion

3.1. Morphological, Structural, and Electronic Properties of rGO-Wrapped TiO₂ Nanofibers

The morphologies of a graphene oxide (GO) sheet, TiO₂ nanofibers, and rGO-wrapped TiO₂ nanofibers are displayed by TEM and SEM images, as shown in Figure 4. Figure 4(a) demonstrates TEM image of a GO sheet in DMSO solvent. It is seen that the GO sheets in DMSO solvent are well spreadable. Therefore, the DMSO solvent is a good candidate for GO spread and dispersion without having an effect on the quality of GO structure. Moreover, the surfaces of TiO₂ nanofibers examined by SEM (Figures 4(b) and 4(c)) seem to be smooth due to the low viscosity of the solutions during the electrospinning process. After the hydrothermal process, the rGO sheets were wrapped around the surface of TiO₂ nanofibers with nonuniform distribution due to the continuous sonication. The morphology of rGO-wrapped TiO₂ nanofibers is displayed by TEM image, as shown in Figure 4(d). Moreover, the rough surface of rGO-wrapped TiO₂ nanofibers can be observed in Figures 4(e) and 4(f). It may be due to the deformation of PVP polymer after the heat-treatment process. The diameter distributions of TiO₂ and rGO-wrapped TiO₂ nanofibers were analyzed from different areas of various SEM images by using ImageJ software. As shown by histogram graphs in inset of Figures 4(b) and 4(e), the diameter distributions of TiO₂ and rGO-wrapped TiO₂ nanofibers can be found in the range of $57 \pm 19$ nm and $81 \pm 15$ nm, respectively.

Figure 4

(a) TEM image of GO sheet. SEM images of (b) pristine TiO₂ nanofiber bundle and (c) pristine TiO₂ nanofiber individual with high magnification. (d) TEM image of rGO-wrapped TiO₂ nanofibers. SEM images of (e) rGO-wrapped TiO₂ nanofiber bundle and (f) rGO-wrapped TiO₂ nanofiber individual with high magnification.

(b)

(c)

(d)

(e)

(f)

Figure 5 shows TEM image of rGO-wrapped TiO₂ nanofiber and its elemental map. It provides information to support the above statement about the existence of titanium (Ti), oxygen (O), and carbon (C) elements on the surface of sample. The energy-dispersive X-ray spectroscopy (EDS) mapping affirms uniform distribution of Ti, O, and C elements for the rGO-wrapped TiO₂ nanofibers, as shown in Figure 5. The atomic percentages of rGO/TiO₂ nanocomposite are 58.83% (Ti), 29.94% (O), and 11.23% (C).

Figure 5

(a) TEM image of rGO-wrapped TiO₂ nanofiber and its elemental map of (b) titanium, (c) oxygen, and (d) carbon.

(b)

(c)

(d)

Figure 6 shows the XRD patterns of pristine TiO₂ nanofibers and rGO-wrapped TiO₂ nanofibers. The 2θ peaks of 25.2° (101), 37.8° (004), 48.1° (200), 55.2° (221), and 62.7° (118), according to JCPDS file number 21-1272, corresponding to the anatase TiO₂ are observed (see Figure 6(a)). This pattern presents a reference in the comparison between pristine TiO₂ and rGO-wrapped TiO₂ nanofibers. As can be seen in Figure 6(b), the more 2θ peaks of 27° (110), 41° (111), 44° (210), and 69° (301), according to JCPDS file number 88-1175, corresponding to the rutile TiO₂ can be also observed. The percentages of rutile ( $W_{R}$ ) and anatase ( $W_{A}$ ) phases for the TiO₂ can be calculated from the intensities of the rutile ( $I_{R}$ ) and anatase ( $I_{A}$ ) peaks in XRD [45], as defined by $\begin{matrix} (1) & W_{R} (%) = \frac{1}{1 + (I_{R} / 0.8 I_{A})} \times 100, \\ (2) & W_{A} (%) = 100 - W_{R} . \end{matrix}$

Figure 6

XRD patterns of (a) pristine TiO₂ nanofibers and (b) rGO-wrapped TiO₂ nanofibers.

It is observed that the rutile percentage of pristine TiO₂ nanofibers is 0%, while the anatase percentage is 100%. For the rGO-wrapped TiO₂ nanofiber sample, it contains 29.03% rutile and 70.97% anatase. Therefore, the calculated ratio between anatase and rutile phases of the rGO-wrapped TiO₂ nanofibers is found to be 2.4. Furthermore, the XRD pattern of TiO₂ nanofibers reveals only peaks for the anatase phase, while the XRD pattern of rGO-wrapped TiO₂ nanofibers reveals different peaks for rGO, anatase, and rutile phases of TiO₂. However, the diffraction peaks of rGO at 24.5° cannot be observed separately.

The existence of rGO sheets on TiO₂ nanofibers was confirmed by Raman spectra, as displayed in Figure 7. It is seen in Figure 7(a) that the Raman spectrum of rGO-wrapped TiO₂ nanofibers presents the four peaks of $E_{g} (1)$ , $B_{1 g} (1)$ , $A_{1 g} (1)$ , and $E_{g} (2)$ around 140 to 640 cm^-1 attributed to the vibration modes of anatase and rutile TiO₂ [46, 47]. For the pristine TiO₂ nanofibers, the peak of Raman spectrum from the rutile TiO₂ is not observed. However, the Raman spectrum of rGO-wrapped TiO₂ nanofibers refers to the anatase phase but two small peaks at 446 cm^-1 and 610 cm^-1, corresponding to the $B_{1 g} (R)$ and $E_{g} (2) (R)$ modes, respectively. This shows the presence of rutile phase in the rGO-wrapped TiO₂ nanofibers, consistent with the XRD results. In addition, the sp³ vibration mode of carbon defect peak (D-peak) and sp² vibration mode of carbon nature peak (G-peak) can be also observed. However, the D-peak and G-peak were shifted from 1349 to 1350 cm^-1 and 1608 to 1610 cm^-1 after the rGO sheets were wrapped around TiO₂ nanofibers. This may be due to the presence of the carbon atoms with rGO defects. Moreover, the $I_{D} / I_{G}$ ratio was considered to measure the structural disorder of samples. It can be seen that the $I_{D} / I_{G}$ ratio increases from 1.10 to 1.25 because of the increase of sp³ to sp² carbon bonds during the hydrothermal process. The references of Raman spectra for the pure rGO sheets and pristine TiO₂ nanofibers are also displayed in Figures 7(b) and 7(c), respectively.

Figure 7

Raman spectra of the synthesized (a) rGO-wrapped TiO₂ nanofibers, (b) pure rGO sheets, and (c) TiO₂ nanofibers.

The UV-Vis spectra of the Kubelka-Munk transformed reflectance are demonstrated in Figure 8. The Tauc plots are displayed to calculate the band gap of pristine TiO₂ and rGO-wrapped TiO₂ nanofibers, according to the following: $\begin{matrix} (3) & {(α hv)}^{2} = B_{i} (hv - E_{g}), \end{matrix}$ where $α$ represents the absorption coefficient, $B_{i}$ refers to the absorption constant, $E_{g}$ is the band gap, and $hv$ is the photon energy. The $E_{g}$ was obtained by the point value of fitted line and $x$ axis from the curve of ${(α hv)}^{2}$ and $hv$ energy. The band gap of pristine TiO₂ and rGO-wrapped TiO₂ nanofibers is found to be 3.20 eV (Figure 8(a)) and 2.95 eV (Figure 8(b)), respectively. The decrease of $E_{g}$ for the rGO-wrapped TiO₂ nanofibers may be due to the shift of the Fermi level by adding the rGO sheets [48]. It may refer to the enhancement of migration for free electrons and repression of electron-hole recombination after the p-n heterojunctions are formed.

Figure 8

Variation of ${(α hv)}^{2}$ vs. $hv$ of (a) pristine TiO₂ nanofibers and (b) rGO-wrapped TiO₂ nanofibers.

(b)

3.2. Gas-Sensing Properties

The gas-sensing properties of pristine TiO₂ nanofibers, pure rGO sheets, and rGO-wrapped TiO₂ nanofibers were evaluated by using the gas-measurement system. The changes in resistance of gas sensors were conducted to identify the performances of the sensors. Figure 9 shows the resistance changes of pristine TiO₂ nanofiber gas sensor exposed to H₂S, C₂H₂, H₂, CH₄, and CO₂ with a concentration of 10, 20, 50, 70, and 100 ppm at room temperature. It can be seen that the baseline resistance of pristine TiO₂ nanofiber gas sensor falls within the order of high resistance (MΩ). Moreover, the sensor resistances do not change obviously under all gases. Therefore, the pristine TiO₂ nanofiber gas sensor does not response or low response to all gases at room temperature. These results are consistent with the previous publications [41, 49, 50].

Figure 9

Resistance changes of pristine TiO₂ nanofiber gas sensor exposed to various gases with different concentrations at room temperature.

Since the response of pristine TiO₂ nanofiber gas sensor for H₂S sensing is very low at room temperature, only the results of rGO and rGO-wrapped TiO₂ nanofiber gas sensors will be more reported. The changes in resistance of rGO-wrapped TiO₂ nanofiber and rGO gas sensors upon exposure to H₂S with different concentrations from 10 to 100 ppm at room temperature are shown in Figures 10(a) and 10(b), respectively. It can be seen that the rGO-wrapped TiO₂ nanofiber gas sensor shows a high response to H₂S, while the response of rGO gas sensor is very low. In addition, both gas sensors exhibit increments of resistance upon exposure to H₂S before recovering to their baseline lines in dry air. This represents the p-type semiconductor behaviour to a reducing gas. The calculation of gas response is defined by $\begin{matrix} (4) & S (%) = \frac{R_{g} - R_{a}}{R_{a}} \times 100, \end{matrix}$ where $R_{g}$ and $R_{a}$ are the resistances of the gas sensor under target gas and dry air, respectively. The calculated values of gas response for the rGO gas sensor exposed to H₂S in the concentrations of 10, 20, 50, 70, and 100 ppm are found to be 0.2, 0.5, 1.61, 2.2, and 2.7%, respectively. In the same concentrations, the values of gas response for the rGO-wrapped TiO₂ nanofiber gas sensors are further found to be 13.3, 16.7, 26.7, 32.1, and 44.6%, respectively. The results affirm the advantage of heterojunction state for rGO-wrapped TiO₂ nanofiber gas sensor in the better H₂S response than pure semiconducting state of rGO gas sensor.

Figure 10

Resistance changes of (a) rGO-wrapped TiO₂ nanofiber and (b) rGO gas sensors exposed to H₂S with various concentrations at room temperature.

The sensitivity was defined by the slope of linear graph in the relation of gas response versus gas concentration [51–53]. Figure 11(a) shows linear graphs of gas response as a function of H₂S concentration for the rGO-wrapped TiO₂ nanofiber and rGO gas sensors. It can be found that the calculated values of slopes for rGO-wrapped TiO₂ nanofiber and rGO gas sensors are 0.341 and 0.029 ppm^-1, respectively. The coefficients of determination ( $R^{2}$ ) for rGO-wrapped TiO₂ nanofiber and rGO gas sensors fitted and calculated by OriginLab software are also found to be 0.994 and 0.973, respectively. Therefore, the wrapping of rGO sheets around TiO₂ nanofibers leads to a better sensitivity of the H₂S gas sensor at room temperature. The response time was defined as the time to reach 90% of the change for the sensor resistance after exposure to a target gas [51–53]. For the recovery time, it was further defined as the time for recovering the sensor resistance to its baseline after the target gas was suddenly cut off [51–53]. The trend lines of response and recovery times for rGO-wrapped TiO₂ nanofiber gas sensor exposed to H₂S with various concentrations at room temperature are shown in Figure 11(b). It is seen that the recovery time of sensor is faster than the response time. Moreover, the response time increases from 13.0 s to 20.0 s when the concentration of H₂S increases from 10 to 100 sccm, whereas the recovery time of sensor also increases from 5.0 to 8.0 s. Therefore, the response/recovery times of sensor increase with increasing H₂S concentrations.

Figure 11

(a) Gas responses of rGO-wrapped TiO₂ nanofiber and rGO gas sensors and (b) response/recovery times of rGO-wrapped TiO₂ nanofiber gas sensor as a function of H₂S concentration at room temperature.

(b)

The changes in resistance of rGO-wrapped TiO₂ nanofiber sensors upon exposure to H₂S with concentrations of 70 and 10 ppm are displayed in Figures 12(a) and 12(b), respectively. It can be seen that the resistance of sensor exposed to 70 ppm H₂S could not return back to its baseline resistance. This is due to the fact that the H₂S molecules with high concentration cannot fully purge off from the sensor surface by the air under ambient condition. This cause leads to the drift of baseline resistance for the gas sensor after testing in every cycle. For the rGO-wrapped TiO₂ nanofiber sensor under 10 ppm H₂S, it can be observed that the resistance of sensor immediately decreases and nearly recovers to its baseline resistance over 4 cycles. This is due to the almost complete desorption of H₂S molecules after the purging process.

Figure 12

Resistance changes of rGO-wrapped TiO₂ nanofiber gas sensor exposed to H₂S with concentrations of (a) 70 ppm and (b) 10 ppm at room temperature.

As can be found from the preliminary results, the thickness of rGO wrapped around the TiO₂ nanofibers is dependent on the GO content. The SEM image of rGO-wrapped TiO₂ nanofiber prepared by 100 mg of GO powder is shown in Figure 13(a). It is clearly seen that there are some thick rGO sheets around the TiO₂ nanofiber. The nearby rGO sheets could be merged and formed into larger sheets under a condition of high GO content. Moreover, we have already investigated the effect of GO content on the H₂S response of rGO-wrapped TiO₂ nanofiber gas sensor at room temperature. It should be noted that the GO powder was dissolved in 20 ml DMSO and 20 ml DI water followed by the addition of 70 mg TiO₂ nanofibers before the rGO-wrapped TiO₂ nanofibers were formed by hydrothermal process. As shown in Figure 13(b), the values of H₂S response for the sensor under different GO contents of 20, 50, 75, and 100 mg are found to be 22.4, 44.6, 10.2, and 7.9%, respectively. These results show that the optimal content of GO powder for wrapping the rGO around the TiO₂ nanofiber to obtain an effective room-temperature H₂S gas sensor is 50 mg. In the case of rGO-wrapped TiO₂ nanofiber gas sensor with high content of GO (75 and 100 mg), the response of sensor is low because the number of charge carriers in rGO-wrapped TiO₂ nanofibers is much higher than the charge transfer. This explanation is consistent with a previous work of another group. They have reported about the low response of three-dimensional graphene-carbon nanotube nanostructures to toluene under the condition of high carbon content [54].

Figure 13

(a) SEM image of rGO-wrapped TiO₂ nanofiber prepared by 100 mg of GO powder. (b) Gas responses of rGO-wrapped TiO₂ nanofiber gas sensor as a function of prepared GO content.

(b)

The generated p-n heterojunctions of nanocomposite materials have been affirmed by baseline resistance of gas sensor [54, 55]. For the n-type semiconducting TiO₂ gas sensor, it is well known that electrons are the most common type of charge carrier. The decrease in baseline resistance of n-type metal oxide and p-type carbon materials can be attributed to the increased electron concentration due to the formation of p-n heterojunctions. In this work, the baseline resistance of pristine TiO₂ nanofiber gas sensor falls within ~2.5 MΩ while the baseline resistance of rGO-wrapped TiO₂ nanofiber is found to be ~3.7 kΩ. This could be affirmed to the formation of p-n heterojunctions between TiO₂ nanofibers and rGO sheets.

Figure 14 shows the selectivity histogram of rGO-wrapped TiO₂ nanofiber and rGO gas sensors toward different gases, including 100 ppm H₂S, 500 ppm C₂H₂, 500 ppm H₂, 1000 ppm CH₄, and 1000 ppm CO₂ at room temperature. It is seen that the rGO-wrapped TiO₂ nanofiber gas sensor exhibits a high response to H₂S and low responses to C₂H₂, H₂, CH₄, and CO₂, while the rGO gas sensor shows the low responses to all gases. It is well known that the gas response of sensor increases with increasing gas concentration. Although the concentrations of the C₂H₂, H₂, CH₄, and CO₂ are added more to over 500 ppm, the very low responses to the gases for the both sensors are still observed. These results confirm the good selectivity of rGO-wrapped TiO₂ nanofiber gas sensor to H₂S at room temperature.

Figure 14

Selectivity histogram of rGO-wrapped TiO₂ nanofiber and rGO gas sensors exposed to various gases with different concentrations at room temperature.

The H₂S-sensing measurement of rGO-wrapped TiO₂ nanofiber gas sensor was further repeated for 70 days, as shown in Figure 15. The long-term stability with only ∼5% of reduction from its initial response under room-temperature storage indicates that the gas responses are almost stable under ambient conditions. Moreover, the H₂S-sensing performance of the room-temperature rGO-wrapped TiO₂ nanofiber gas sensor in this work is superior to previous works, as listed in Table 1. Although there have been some works revealing the sensor performance in higher H₂S response, the high operation temperature of them is still an obstacle [41, 56].

Figure 15

Long-term stability measurement of rGO-wrapped TiO₂ nanofiber gas sensor in a period of 70 days upon repetitive exposure to 100 ppm H₂S at room temperature.

Table 1

Comparison of H₂S gas sensors from previous works and this work.

Materials	Operating temperature (°C)	Response (%)	Response definition	H₂S concentration (ppm)	Ref.
Fe₂O₃ decorated CuO nanorods	150	4.0	$R_{g} / R_{a}$	5	[8]
Ag-doped Fe₂O₃ hollow nanofibers	RT	19.4	$R_{a} / R_{g}$	100	[11]
Cu-doped ZnO/rGO	RT	1.3	$(R_{g} - R_{a}) / R_{a} \times 100$	250	[12]
SnO₂-Fe₂O₃ nanoparticles	55	14.9	$(I_{g} - I_{a}) / I_{a}$	2.5	[16]
Ag-doped TiO₂	350	240	$(R_{g} - R_{a}) / R_{a} \times 100$	10	[41]
2D MoO₃ nanoflakes	300	75	$(R_{g} - R_{a}) / R_{a} \times 100$	10	[56]
rGO-wrapped TiO₂ nanofibers	RT	44.6	$(R_{g} - R_{a}) / R_{a} \times 100$	100	This work

The H₂S-sensing mechanisms of rGO-wrapped TiO₂ nanofiber gas sensor have been discussed based on two possible mechanisms such as the formation of p-n heterojunctions and the direct charge-transfer process. The free electrons in rGO regions will transfer to TiO₂ nanofibers when the heterojunctions are formed. This cause results in a depletion of electrons on the rGO surfaces [37]. It leads to the bending of energy band in n-type semiconducting TiO₂ nanofiber, causing an increase of baseline resistance from ~0.93 kΩ to ~3.54 kΩ after wrapping the rGO on the TiO₂ nanostructures (see Figure 10). In air, the chemisorbed oxygen groups will trap free electrons from the conduction band of TiO₂ and the Fermi level of rGO, according to the reaction $: O_{2} (ads) + e^{-} ⟶ {O_{2}}^{-} (ads)$ [37]. It should be noted that H₂S represents a reducing gas property. When rGO-wrapped TiO₂ nanofiber gas sensor is exposed to H₂S, the gas molecules will react with adsorbed oxygen groups. The free electrons trapped by O₂ ^- groups will return back to conduction band of rGO-wrapped TiO₂ nanofibers, resulting in the increase of resistance of p-type semiconducting gas sensor. The reaction can be labeled as follows: $2 H_{2} S (g) + 3 {O_{2}}^{-} (ads) ⟶ 2 H_{2} O (g) + 2 S O_{2} (g) + 3 e^{-}$ [28, 57]. The schematic illustration of the proposed H₂S-sensing mechanism is illustrated in Figure 16(a). The increase of resistance for the gas sensor upon exposure to H₂S can be explained by the increase of the height for the potential barrier at p-n heterojunction, according to the following: $\begin{matrix} (5) & R = R_{o} e^{Δ \emptyset q / k T} . \end{matrix}$

Figure 16

(a) Schematic illustration of H₂S-sensing mechanism and (b) p-n heterojunction of rGO-wrapped TiO₂ nanofiber gas sensor.

(b)

where $R$ is the sensor resistance, $R_{o}$ is a constant, $q$ is the charge of electron, $Δ ϕ$ is the height of potential barrier at p-n heterojunction, $k$ is Boltzmann’s constant, and $T$ is the sensing-layer temperature [37, 54]. The p-n heterojunction of rGO-wrapped TiO₂ nanofiber gas sensor is illustrated in Figure 16(b). The nonrecovery of resistance for the sensor under high H₂S concentration was presented after the stream of H₂S gas was cut off. This is due to the highly strong bonding between H₂S molecules and oxygen containing groups on rGO-wrapped TiO₂ nanofiber surface. This may lead to a creation of baseline drift for the resistance of the rGO-wrapped TiO₂ nanofibers at the higher concentration of H₂S (70 ppm).

For the second possible mechanism, the rGO-wrapped TiO₂ nanofiber gas sensor is exposed to lower concentration of H₂S (10 ppm). It has been discussed based on the direct charge-transfer process between H₂S molecules and rGO-wrapped TiO₂ nanofiber surfaces by physisorption. The Van der Waals interaction has been considered as a dominant action in the presentation of this possible mechanism. The increments in the active areas and π-π interactions can be improved by the wrapping of rGO sheets around TiO₂ nanofibers. When the H₂S molecules are adsorbed onto active areas of rGO-wrapped TiO₂ nanofiber, the holes of rGO-wrapped TiO₂ nanofiber will react to free electrons from H₂S molecules. Then, the resistance of the p-type semiconducting rGO-wrapped TiO₂ nanofiber gas sensor increases. Moreover, the H₂S molecules can be easily purged under dry air at room temperature due to the weak interactions of physisorption. This cause may lead to the near complete recovery of the baseline resistance for the gas sensor under lower concentration of H₂S (10 ppm).

4. Conclusions

The rGO-wrapped TiO₂ nanofibers were successfully synthesized by the combination of electrospinning and hydrothermal methods. The confirmation of existence for rGO sheets on the surface of TiO₂ nanofibers was characterized by SEM, TEM, XRD, and Raman spectroscopy. The UV-Vis spectra were employed to investigate the band gaps of nanofibers before and after wrapping the rGO sheets on the surface of TiO₂ nanofibers. The nanofibers were evaluated for H₂S adsorption at room temperature. As the results, the rGO-wrapped TiO₂ nanofiber gas sensor presented a p-type semiconducting behaviour and good response upon exposure to H₂S at room temperature. The hydrothermal wrapping of rGO sheets around the TiO₂ nanofibers with an optimal GO content of 50 mg led to the optimal enhancement of H₂S response at room temperature. Moreover, the rGO-wrapped TiO₂ nanofiber gas sensor exhibited a good selectivity and long-term stability toward H₂S as well. The H₂S-sensing mechanism of rGO-wrapped TiO₂ nanofibers gas sensor was proposed based on the formation of p-n heterojunctions of p-type rGO sheets and n-type TiO₂ nanofibers. Finally, the direct charge-transfer process between H₂S molecules and rGO-wrapped TiO₂ nanofiber surfaces by physisorption was mentioned as a second possible H₂S-sensing mechanism. All the results proved that the rGO-wrapped TiO₂ nanofibers can be considered as a good selection for applications in H₂S adsorption at room temperature.

Footnotes

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

The authors would like to thank for gas measurements and facilities provided by Dr. Anurat Wisitsoraat from Carbon-based Devices and Nanoelectronics Laboratory, National Electronics and Computer Technology Center, Thailand. This work was financially supported by the Research and Development Institute, Rajamangala University of Technology Krungthep, Thailand (Grant No. ST-1045).

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Adsorption of Hydrogen Sulfide on Reduced Graphene Oxide-Wrapped Titanium Dioxide Nanofibers

Abstract

1. Introduction

2. Materials and Methods

2.1. Preparation of rGO-Wrapped TiO2 Nanofibers

2.2. Fabrication of rGO-Wrapped TiO2 Nanofiber Gas Sensor

2.3. Gas-Sensing Measurement

3. Results and Discussion

3.1. Morphological, Structural, and Electronic Properties of rGO-Wrapped TiO2 Nanofibers

3.2. Gas-Sensing Properties

4. Conclusions

Footnotes

Data Availability

Conflicts of Interest

Acknowledgments

References

2.1. Preparation of rGO-Wrapped TiO₂ Nanofibers

2.2. Fabrication of rGO-Wrapped TiO₂ Nanofiber Gas Sensor

3.1. Morphological, Structural, and Electronic Properties of rGO-Wrapped TiO₂ Nanofibers