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

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.


Introduction
Hydrogen sulfide (H 2 S) is an extremely harmful and flammable gas.It smells like rotten eggs at low concentration in the air.The harm of H 2 S is dependent on its concentration and exposure time.A short-term exposure to over 500-1000 ppm of H 2 S is immediately fatal [1].Repeated exposure to H 2 S in concentrations even 10-500 ppm can cause serious damage to organs and central nervous system [1][2][3].Therefore, the sensor for the detection of H 2 S 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][5][6].However, the adsorption of most H 2 S 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][10][11][12][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][15][16][17].Among various MOS materials, titanium dioxide (TiO 2 ) and its composite have been reported as a popular material for applications in lithium-ion storage [18,19] and photoelectrocatalysis [20][21][22].Due to its strong oxidizing power, abundant existence in nature, nontoxicity, and long-term physical and chemical stabilities, TiO 2 -based gas sensor has been widely used as an efficient gas absorber [23][24][25][26][27][28][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][31][32][33][34][35][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 TiO 2 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][39][40].The electrospun nanofibers have been reported in the special properties such as high specific surface area and porosity [40][41][42][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 TiO 2 nanofibers as a room-temperature H 2 S gas sensor although some works on fabrication methods of TiO 2 nanofibers followed by TiO 2 wrapped with rGO have been reported [19,44].
In this work, we have synthesized rGO-wrapped TiO 2 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 rGOwrapped TiO 2 nanofiber gas sensors was systematically evaluated, relating to the sensitivity, selectivity, and stability properties for H 2 S adsorption at room temperature.The pristine TiO 2 and pure rGO gas sensors were also fabricated and evaluated as comparisons.The H 2 S-sensing mechanism of rGO-wrapped TiO 2 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 H 2 S-sensing mechanism.

Materials and Methods
2.1.Preparation of rGO-Wrapped TiO 2 Nanofibers.The preparation process of TiO 2 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 (CH 3 COOH) were added into the solution.The mixed solution was continuously stirred for 1 h to obtain a homogeneous precursor solution.To prepare the electrospinning TiO 2 nanofibers, the TiO 2 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 sup-ply 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 TiO 2 nanofibers was continuously accumulated on a rolling collector for 40 min.The schematic illustration for preparing the TiO 2 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 TiO 2 nanofibers were heated at a fixed temperature of 500 °C for 2 h to decompose PVP and crystallize TiO 2 .
For preparation of rGO-wrapped TiO 2 nanofibers, a sheet of accumulated TiO 2 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 TiO 2 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 TiO 2 nanofibers were obtained.For preparation of rGO sheets, the GO powder was treated at the same condition without TiO 2 nanofibers.The schematic illustration of rGO-wrapped TiO 2 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.

Fabrication of rGO-Wrapped TiO 2 Nanofiber Gas
Sensor.To prepare the rGO-wrapped TiO 2 nanofiber gas sensor, the dried powder of rGO-wrapped TiO 2 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 TiO 2 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 × 10 0 mm 2 , as shown in Figure 2(b).
2.3.Gas-Sensing Measurement.The gas-sensing properties of pristine TiO 2 nanofiber, rGO, and rGO-wrapped TiO 2 nanofiber gas sensors were evaluated on their performances by using a gas-measurement system.The system consists of a tank of H 2 S, 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 H 2 S 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  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.

Results and Discussion
3.1.Morphological, Structural, and Electronic Properties of rGO-Wrapped TiO 2 Nanofibers.The morphologies of a graphene oxide (GO) sheet, TiO 2 nanofibers, and rGO-wrapped TiO 2 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 TiO 2 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 TiO 2 nanofibers with nonuniform distribution due to the continuous sonication.The morphology of rGO-wrapped TiO 2 nanofibers is displayed by TEM image, as shown in Figure 4(d).Moreover, the rough surface of rGO-wrapped TiO 2 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 TiO 2 and rGO-wrapped TiO 2 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 TiO 2 and rGO-wrapped TiO 2 nanofibers can be found in the range of 57 ± 19 nm and 81 ± 15 nm, respectively.
Figure 5 shows TEM image of rGO-wrapped TiO 2 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 TiO 2 nanofibers, as shown in Figure 6 shows the XRD patterns of pristine TiO 2 nanofibers and rGO-wrapped TiO 2 nanofibers.The 2θ peaks of 25.2 °(101), 37.8 °(004), 48.1 °(200), 55.2 °(221), and 62.7 according to JCPDS file number 21-1272, corresponding to the anatase TiO 2 are observed (see Figure 6(a)).This pattern presents a reference in the comparison between pristine TiO 2 and rGO-wrapped TiO 2 nanofibers.As can be seen in Figure 6(b), the more 2θ peaks of 27 °(110), 41 44 °(210), and 69 °(301), according to JCPDS file number 88-1175, corresponding to the rutile TiO 2 can be also observed.The percentages of rutile (W R ) and anatase (W A ) phases for the TiO 2 can be calculated from the intensities of the rutile (I R ) and anatase (I A ) peaks in XRD [45], as defined by It is observed that the rutile percentage of pristine TiO 2 nanofibers is 0%, while the anatase percentage is 100%.For the rGO-wrapped TiO 2 nanofiber sample, it contains 29.03% rutile and 70.97% anatase.Therefore, the calculated ratio between anatase and rutile phases of the rGOwrapped TiO 2 nanofibers is found to be 2.4.Furthermore, the XRD pattern of TiO 2 nanofibers reveals only peaks for the anatase phase, while the XRD pattern of rGO-wrapped TiO 2 nanofibers reveals different peaks for rGO, anatase, and rutile phases of TiO 2 .However, the diffraction peaks of rGO at 24.5 °cannot be observed separately.
The existence of rGO sheets on TiO 2 nanofibers was confirmed by Raman spectra, as displayed in Figure 7.It is seen in Figure 7(a) that the Raman spectrum of rGOwrapped TiO 2 nanofibers presents the four peaks of E g 1 , B 1g 1 , A 1g 1 , and E g 2 around 140 to 640 cm -1 attributed to the vibration modes of anatase and rutile TiO 2 [46,47].For the pristine TiO 2 nanofibers, the peak of Raman spectrum from the rutile TiO 2 is not observed.However, the  Adsorption Science & Technology Raman spectrum of rGO-wrapped TiO 2 nanofibers refers to the anatase phase but two small peaks at 446 cm -1 and 610 cm -1 , corresponding to the B 1g R and E g 2 R modes, respectively.This shows the presence of rutile phase in the rGO-wrapped TiO 2 nanofibers, consistent with the XRD results.In addition, the sp 3 vibration mode of carbon defect peak (D-peak) and sp 2 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 TiO 2 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 sp 3 to sp 2 carbon bonds during the hydrothermal process.The references of Raman spectra for the pure rGO sheets and pristine TiO 2 nanofibers are also displayed in Figures 7(b) and 7(c), respectively.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 TiO 2 and rGO-wrapped TiO 2 nanofibers, according to the following: 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 αhv 2 and hv energy.The band gap of pristine TiO 2 and rGO-wrapped TiO 2 nanofibers is respectively.The decrease of E g for the rGO-wrapped TiO 2 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.
3.2.Gas-Sensing Properties.The gas-sensing properties of pristine TiO 2 nanofibers, pure rGO sheets, and rGOwrapped TiO 2 nanofibers were evaluated by using the gasmeasurement 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 TiO 2 nanofiber gas sensor exposed to H 2 S, C 2 H 2 , H 2 , CH 4 , and CO 2 with a concentration of 10, 20, 50, 70, and 100 ppm at room temperature.It can be seen that the baseline resistance of pristine TiO 2 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 TiO 2 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].
Since the response of pristine TiO 2 nanofiber gas sensor for H 2 S sensing is very low at room temperature, only the results of rGO and rGO-wrapped TiO 2 nanofiber gas sensors will be more reported.The changes in resistance of rGOwrapped TiO 2 nanofiber and rGO gas sensors upon exposure to H 2 S 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 rGOwrapped TiO 2 nanofiber gas sensor shows a high response to H 2 S, while the response of rGO gas sensor is very low.In addition, both gas sensors exhibit increments of resistance upon exposure to H 2 S 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 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 H 2 S 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 con-centrations, the values of gas response for the rGO-wrapped TiO 2 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 TiO 2 nanofiber gas sensor in the better H 2 S response than pure semiconducting state of rGO gas sensor.The sensitivity was defined by the slope of linear graph in the relation of gas response versus gas concentration [51][52][53].Figure 11(a) shows linear graphs of gas response as a function of H 2 S concentration for the rGO-wrapped TiO 2 nanofiber and rGO gas sensors.It can be found that the calculated values of slopes for rGO-wrapped TiO 2 nanofiber and rGO gas sensors are 0.341 and 0.029 ppm -1 , respectively.The coefficients of determination (R 2 ) for rGOwrapped TiO 2 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 TiO 2 nanofibers leads to a better sensitivity of the H 2 S gas sensor at room temperature.The response time  7 Adsorption Science & Technology was defined as the time to reach 90% of the change for the sensor resistance after exposure to a target gas [51][52][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][52][53].The trend lines of response and recovery times for rGO-wrapped TiO 2 nanofiber gas sensor exposed to H 2 S 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 H 2 S 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 H 2 S concentrations.
The changes in resistance of rGO-wrapped TiO 2 nanofiber sensors upon exposure to H 2 S with concentrations of 70 and 10 ppm are displayed in Figures 12(a) and 12(b), respec-tively.It can be seen that the resistance of sensor exposed to 70 ppm H 2 S could not return back to its baseline resistance.This is due to the fact that the H 2 S 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 TiO 2 nanofiber sensor under 10 ppm H 2 S, 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 H 2 S molecules after the purging process.
As can be found from the preliminary results, the thickness of rGO wrapped around the TiO 2 nanofibers is dependent on the GO content.The SEM image of rGO-wrapped TiO 2 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 TiO 2 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 H 2 S response of rGO-wrapped TiO 2 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 TiO 2 nanofibers before the rGO-wrapped TiO 2 nanofibers were formed by hydrothermal process.As shown in Figure 13(b), the values of H 2 S 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 TiO 2 nanofiber to obtain an effective room-temperature H 2 S gas sensor is 50 mg.In the case of rGO-wrapped TiO 2 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 rGOwrapped TiO 2 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   8 Adsorption Science & Technology response of three-dimensional graphene-carbon nanotube nanostructures to toluene under the condition of high carbon content [54].The generated p-n heterojunctions of nanocomposite materials have been affirmed by baseline resistance of gas sensor [54,55].For the n-type semiconducting TiO 2 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 TiO 2 nanofiber gas sensor falls within ~2.5 MΩ while the baseline resistance of rGOwrapped TiO 2 nanofiber is found to be ~3.7 kΩ.This could be affirmed to the formation of p-n heterojunctions between TiO 2 nanofibers and rGO sheets.
Figure 14 shows the selectivity histogram of rGOwrapped TiO 2 nanofiber and rGO gas sensors toward different gases, including 100 ppm H 2 S, 500 ppm C 2 H 2 , 500 ppm H 2 , 1000 ppm CH 4 , and 1000 ppm CO 2 at room temperature.It is seen that the rGO-wrapped TiO 2 nanofiber gas sensor exhibits a high response to H 2 S and low responses to C 2 H 2 , H 2 , CH 4 , and CO 2 , 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 C 2 H 2 , H 2 , CH 4 , and CO 2 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 TiO 2 nanofiber gas sensor to H 2 S at room temperature.
The H 2 S-sensing measurement of rGO-wrapped TiO 2 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 roomtemperature storage indicates that the gas responses are almost stable under ambient conditions.Moreover, the H 2 S-sensing performance of the room-temperature rGOwrapped TiO 2 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 H 2 S response, the high operation temperature of them is still an obstacle [41,56].
The H 2 S-sensing mechanisms of rGO-wrapped TiO 2 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 TiO 2 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 TiO 2 nanofiber, causing an increase of baseline resistance from ~0.93 kΩ to ~3.54 kΩ after wrapping the rGO on the TiO 2 nanostructures (see Figure 10).In air, the chemisorbed oxygen groups will trap free electrons from the conduction band of TiO 2 and the Fermi level of rGO, according to the  − ads [37].It should be noted that H 2 S represents a reducing gas property.When rGO-wrapped TiO 2 nanofiber gas sensor is exposed to H 2 S, the gas molecules will react with adsorbed oxygen groups.The free electrons trapped by O 2 -groups will return back to conduction band of rGO-wrapped TiO 2 nanofibers, resulting in the increase of resistance of p-type semiconducting gas sensor.The reaction can be labeled as follows: 2H 2 S g + 3O 2 − ads ⟶ 2H 2 O g + 2SO 2 g + 3e − [28,57].The schematic illustration of the proposed H 2 S-sensing mechanism is illustrated in Figure 16(a).The increase of resistance for the gas sensor upon exposure to H 2 S can be explained by the increase of the height for the potential barrier at p-n heterojunction, according to the following: where R is the sensor resistance, R o is a constant, q is the charge of electron, Δϕ 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 TiO 2 nanofiber gas sensor is illustrated in Figure 16(b).The nonrecovery of resistance for the sensor under high H 2 S concentration was presented after the stream of H 2 S gas was cut off.This is due to the highly strong bonding between H 2 S molecules and oxygen containing groups on rGO-wrapped TiO 2 nanofiber surface.This may lead to a creation of baseline drift for the resistance of the rGO-wrapped TiO 2 nanofibers at the higher concentration of H 2 S (70 ppm).
For the second possible mechanism, the rGO-wrapped TiO 2 nanofiber gas sensor is exposed to lower concentration of H 2 S (10 ppm).It has been discussed based on the direct charge-transfer process between H 2 S molecules and rGOwrapped TiO 2 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 TiO 2 nanofibers.When the H 2 S molecules are adsorbed onto active areas of rGO-wrapped TiO 2 nanofiber, the holes of rGO-wrapped TiO 2 nanofiber will react to free electrons from H 2 S molecules.Then, the resistance of the p-type semiconducting rGO-wrapped TiO 2 nanofiber gas sensor increases.Moreover, the H 2 S 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 H 2 S (10 ppm).

Conclusions
The rGO-wrapped TiO 2 nanofibers were successfully synthesized by the combination of electrospinning and hydrothermal methods.The confirmation of existence for rGO sheets on the surface of TiO 2 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 TiO 2 nanofibers.The nanofibers were evaluated for H 2 S adsorption at room temperature.As the results, the rGO-wrapped TiO 2 nanofiber gas sensor presented a p-type semiconducting behaviour and good response upon exposure to H 2 S at room temperature.The hydrothermal wrapping of rGO sheets around the TiO 2 nanofibers with an optimal GO content of 50 mg led to the optimal enhancement of H 2 S response at room temperature.Moreover, the rGO-wrapped TiO 2 nanofiber gas sensor exhibited a good selectivity and long-term stability toward H 2 S as well.The H 2 S-sensing mechanism of rGOwrapped TiO 2 nanofibers gas sensor was proposed based on the formation of p-n heterojunctions of p-type rGO sheets and n-type TiO 2 nanofibers.Finally, the direct charge-transfer process between H 2 S molecules and rGOwrapped TiO 2 nanofiber surfaces by physisorption was mentioned as a second possible H 2 S-sensing mechanism.All the results proved that the rGO-wrapped TiO 2 nanofibers can be considered as a good selection for applications in H 2 S adsorption at room temperature.

Figure 2 :
Figure 2: Photograph of dimensional Ag-IDE (a) before and (b) after drop-casting with a sensing suspension on PET substrate.

Figure 3 :
Figure 3: Schematic illustration of gas-measurement system (inset shows a photograph of stainless-steel chamber with electrical signal wires).

Figure 9 :Figure 10 :
Figure 9: Resistance changes of pristine TiO 2 nanofiber gas sensor exposed to various gases with different concentrations at room temperature.

Figure 11 :
Figure 11: (a) Gas responses of rGO-wrapped TiO 2 nanofiber and rGO gas sensors and (b) response/recovery times of rGO-wrapped TiO 2 nanofiber gas sensor as a function of H 2 S concentration at room temperature.

Figure 12 :
Figure 12: Resistance changes of rGO-wrapped TiO 2 nanofiber gas sensor exposed to H 2 S with concentrations of (a) 70 ppm and (b) 10 ppm at room temperature.

Figure 13 :
Figure 13: (a) SEM image of rGO-wrapped TiO 2 nanofiber prepared by 100 mg of GO powder.(b) Gas responses of rGO-wrapped TiO 2 nanofiber gas sensor as a function of prepared GO content.

Figure 14 :Figure 15 :
Figure 14: Selectivity histogram of rGO-wrapped TiO 2 nanofiber and rGO gas sensors exposed to various gases with different concentrations at room temperature.

Table 1 :
Comparison of H 2 S gas sensors from previous works and this work.