Electrospinning of α - Fe2O3 and ZnFe2O4 nanofibers loaded with reduced graphene oxide (rgo) for h2s gas sensing application

The effect of the annealing temperature on the response of the

sensor based on 1.0 wt% RGO-loaded α-Fe2O3 NFs was similar to

that of pure α-Fe2O3 NFs. RGO enhanced the sensor response at low

annealing temperatures but decreased the response at high annealing

temperatures, compared to pure α-Fe2O3 NFs. This was similar to the

effect of the annealing temperature on DL of the sensors of pure α-

Fe2O3 NFs and 1.0 wt% RGO-loaded α-Fe2O3 NFs.

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nsors like long response time, irreversibility and low response. In particular, the sensor response to reducing gas was very low. On the other hand, the RGO-loaded SMO sensor had much higher response to reducing gas than the SMO-loaded RGO sensor thanks to their inherited gas sensing characteristics of SMO. The main conducting path of the sensor went through SMO. RGO concentration was usually below 5 wt% and RGO NSs were dispersed and disconnected in composites. The sensors behaved gas sensing characteristics of SMO. The RGO- loaded SMO sensors had higher response than the pure SMO sensors due to the formation of heterojunction between RGO and SMO. Sensors based on SMO NFs loaded with RGO combined advantages of RGO-loaded SMO sensors and NFs sensors. SMO NFs loaded with RGO were composed of SMO NFs and RGO NSs, in which RGO were distributed randomly and discontinuously among SMO nanograins or on NFs surface. The RGO-loaded SMO NFs structure had high porosity and large specific surface area; therefore, the sensor of this structure often had excellent sensitivity and fast response time. Many works reported that the RGO loaded- SMO NFs sensors had high response to both oxidizing and reducing 7 gases. The sensors also had good selectivity and fast response time. RGO enhanced the sensor response by forming heterojunctions between RGO and SMO. Besides, RGO had many functional groups, dangling bonds and defects that increased gas absorption, thereby increasing the sensor response. However, until now, H2S gas sensing properties of the RGO-loaded SMO NFs sensors in general and on Fe2O3 NFs loaded with RGO and ZFO NFs loaded with RGO in particular have not been investigated, which were studied on the flowing chapters. Finally, gas sensing mechanisms of NFs and RGO-loaded SMO NFs were also discussed in this chapter, which was related to the formation depletion surface on NFs surfaces and potential barriers at homojunctions among nanograins and heterojunctions between SMO and RGO. Moreover, the sensor gas sensing mechanisms to H2S was elaborately mentioned. CHAPTER 2. EXPERIMENTAL APPROACH This chapter presented the fabrication process of the sensing materials. Briefly, α-Fe2O3 and ZFO NFs were synthesized on chip by electrospinning. Precursor solution content, electrospun time and heat treatment conditions were changed to obtain the on-chip NFs sensors with different morphologies, structures and densities. RGO was reduced by L-ascorbic acid from graphene oxide (GO) synthesized from graphite power by Hummers method. A series of the sensors of 0, 0.5, 1.0, and 1.5 wt% RGO-loaded α-Fe2O3 and ZFO NFs was also fabricated on chip by electrospinning. The on- chip electrospun sensors were calcined at different temperatures to form RGO-loaded α-Fe2O3 and ZFO NFs. Then, some characterization methods like TGA, RAMAN, FESEM, TEM, HRTEM, SAED, EDX, and XRD were employed to analyze the synthesized NFs. Finally, gas sensing properties of the synthesized sensors were measured by flow-through technique which used a home-made system of a test chamber with controlled working temperature, a series of mass flow controllers to obtained a desired gas concentrations, and Keithley 2602 controlled by a software 8 program to record the electrical-resistance response of the test sensors under various concentrations and operating temperatures. CHAPTER 3. α-Fe2O3 NFs AND THEIR LOADING WITH RGO FOR H2S GAS SENSING APPLICATION 3.1. Introduction Hematite α-Fe2O3, an n-type semiconductor with the band gap Eg of 2.1 eV and rhombohedral crystal structure, has been widely used in gas sensors due to its high stability, low cost, non-toxicity, environmental friendliness and multiple functions. The H2S gas sensing properties of α-Fe2O3 with different nanostructures have been published in many works. However, H2S gas sensitivity at sub- ppm concentrations of α-Fe2O3 NFs sensors has not been investigated. Furthermore, despite some studies on effects of processing parameters on morphology, structure and gas sensitivity properties of the obtained NFs, similar studies on H2S gas sensing properties of α-Fe2O3 NFs have not been carried out. In addition, the RGO-loaded α-Fe2O3 NFs sensors have also attracted much attention. The studies proved that RGO enhanced gas sensitivity of the RGO-loaded α-Fe2O3 NFs sensor. However, H2S gas sensitivity, especially at low sub-ppm concentrations, of the RGO-loaded α-Fe2O3 NFs sensors has not been reported. In this chapter, α-Fe2O3 NFs were synthesized by electrospinning method. The precursor solution composition (i.e. polymer concentration and salt concentration) and technological parameters (i.e. electrospinning time and annealing temperature) were altered to obtain the different morphologies and structures of α-Fe2O3 NFs, leading to the effects on H2S gas sensing performance at sub-ppm concentration of α-Fe2O3 NFs sensors. Besides, RGO influence on morphologies, structures and H2S gas sensing properties of the RGO- loaded α-Fe2O3 NFs sensors was also discussed in detail. 9 3.2. H2S gas sensors based on α-Fe2O3 NFs 3.2.1. Morphologies and structures of α-Fe2O3 NFs The XRD results at different annealing temperatures confirmed rhombohedral structure of α-Fe2O3 NFs (JCPDS 33–0664). More diffraction peaks appeared and became sharper with the increased annealing temperature, indicating an increase in NFs crystallinity and nanograin size. The precursor solution content strongly influenced the morphology and structure of the synthesized α-Fe2O3 NFs. With low concentration of 7 wt % PVA, the NFs comprised a network of small beads interconnected by thin NFs. The higher PVA concentration was, the bigger fiber diameter became due to an increase in viscoelastic force which counteracted the electric field force. The fibers failed to form when the PVA concentration was too low or too high. The α-Fe2O3 NFs had the belt-like morphology at 2 wt% ferric salt and become quite round and uniform, and smooth surfaces with Figure 3.7. FESEM images of as-spun fibers (a) and α-Fe2O3 NFs prepared at different annealing temperatures: 400 (b), 500 (c), 600 (d), 700 (e), and 800°C (f). Inset figures are low- magnification images. 150 nm 3 µm (a) 150 nm 3 µm (d) 150 nm 3 µm (b) 150 nm 3 µm (e) 150 nm 3 µm (f) 150 nm 3 µm (c) 10 4 wt% ferric salt. With further increased ferric salt of 8 wt%, NFs diameters increased and the NFs surfaces became rough. The FE-SEM images of on-chip α-Fe2O3 NFs with electrospun time from 10 to 120 min were illustrated. When the electrospun time went up, the number of NFs connecting two electrodes also increased, especially the number of intersections among ZFO NFs significantly got bigger. The morphologies of NFs with different annealing temperatures were shown in Fig. 3.7. The NFs were 50–100 nm in diameter. The surface of the NFs became rough because the NFs were made up of many nanograins. The higher the annealing temperature was, the rougher the surface of the NFs was because of nanograin growth. At high annealing temperature of 800°C, NFs had the same shape as a bamboo due to coalescence and grain growth process. TEM, HRTEM, and EDX analyses further examined the morphologies, structures, and compositions of α-Fe2O3 NFs calcined at 600°C. TEM images showed that the NFs were composed of many nanograins; however, the NFs structure was quite tight. HRTEM image and FFT inset image confirmed that the NFs had a good crystal structure with parallel lattice fringes. The composition of α- Fe2O3 NFs with the presence of Fe and O elements was indicated in EDX spectrum results. Figure 3.9. Sensing transients of α-Fe2O3 NF sensors to 1 ppm H2S at various operating temperatures (a), sensor resistances (b), sensor response (c), response time and recovery time (d) as a function of operating temperatures. 100 1000 1 10 R (M  ) Time (s) H 2 S@ 1ppm & 250 o C H 2 S@ 1ppm & 300 o C H 2 S@ 1ppm & 350 o C H 2 S@ 1ppm & 400 o C H 2 S@ 1ppm & 450 o C (a) 0 5 (b) R ( M  ) Air 250 300 350 400 450 100 1000(d)  r e s p -r e c o v ( s ) Operating Temp. ( o C) Recovery time Response time 0 10 20(c) R e s p . (R a /R g ) 1ppm H 2 S 11 3.2.2. H2S gas sensing properties of sensors based on α- Fe2O3 NFs 3.2.2.1. Effects of operating temperature The effect of working temperature on the gas sensing performances of the sensor was shown in Fig. 3.9. The sensor response also decreased sharply with the increased working temperature because the gas desorption became stronger than gas adsorption and the height of the potential barrier at the grain boundaries decreased with increased working temperature. Conversely, the recovery time also became too long with the decreased working temperature because of the reduced reaction rate and diffusion rate along the grain boundaries. Therefore, to optimize the sensor response and recovery time, the working temperature of Figure 3.12. H2S sensing transients of α-Fe2O3 NF sensors with various annealing temperatures (400−800°C) (a–e) and different electrospinning time (10−120 min) (f–i). Sensor response to H2S gas as a function of annealing temperatures (k) and electrospinning time (l). 12 350°C was selected for further investigating gas sensing properties of the α-Fe2O3 NFs sensors. 3.2.2.2. Effects of solution contents The sensor response decreased with the increased PVA concentration from 7 to 15 wt% PVA because of increased NFs diameters. However, the NFs comprised a network of small beads interconnected by thin fibers with 7 wt% PVA concentration. Whereas, the NFs prepared from precursor solution with 11 wt% PVA showed the typical spider-net morphology with many round and uniform NFs fabricated by electrospinning as reported in many works. The sensor response to 1 ppm H2S gas was 2 at 2 wt%, and reached a maximum of 6.2 at 4 wt% g and then went down to 4.9 with 8 wt% ferric salt. Therefore, to optimize the NFs morphology and gas response, the sensor prepared from precursor solution with 11 wt% PVA and 4 wt% ferric salt was chosen for further study. 3.2.2.3. Effects of annealing temperature and electrospinning time As shown in Fig. 3.12, the response of α-Fe2O3 NFs sensors fluctuated with changed annealing temperature, which could possibly be explained by the change in crystallinity and grain size of NFs with different annealing temperatures. When the temperature went down from 600 to 500°C, the response decreased due to the strong influence of decreased crystallinity caused by decreased annealing temperature. When the temperature further decreased from 500 to 400°C, the sensor response increased because of the strong effect of decreased nanograin size. Meanwhile, when the temperature increased from 600 to 800°C, the sensor response decreased remarkably because of grain growth. The densities of the NFs on the microelectrode chip, which could be controlled by electrospinning time, strongly affected gas-sensing performance. The gas response showed a bell-shape relation with electrospun time at working temperature of 350°C and the response peak was obtained at the electrospun time of 30 min. The NFs sensor response increased with increased electrospun time due to an increase in the NFs-NFs junctions between Pt electrodes. Such junctions improved the sensor sensitivity when the sensors were exposed to H2S gas. The response decreased with the further 13 increased electrospun time because of the increased thickness of the sensing layer, resulting in the increased gas diffusion length. In short, the sensor based on α-Fe2O3 NFs was calcined at 600°C and electrospun for 30 min with the precursor solution of 11 wt% PVA and 4 wt% Fe (NO3)3 salt for optimizing among structures, morphologies and sensor response. 3.2.2.4. Selectivity and stability The sensor also had good selectivity to reducing gases like H2 and NH3 but its selectivity to oxidizing gas SO2 was still limited. The sensor also showed good repeatability, which highlighted practical applicability of the α-Fe2O3 NFs sensor. 3.3. H2S gas sensors based on α-Fe2O3 NFs loaded with RGO 3.3.1. Morphologies and structures of α-Fe2O3 NFs loaded with RGO Morphologies of RGO-loaded α-Fe2O3 NFs were not significantly affected by the changed RGO contents (0–1.5 wt%). RGO could not be found in FESEM images of RGO-loaded α-Fe2O3 NFs since RGO Figure 3.18. TEM images at different magnifications (a-b), SAED pattern (c), and HRTEM image (d) with corresponding fast Fourier transform (FFT) inset image of 1%wt RGO loaded α-Fe2O3 annealed at 600°C for 3 hours in air. 5 nm (104) 0.27 nm [441] (104) 2 nm -1 (c) 50 nm α- Fe 2 O 3 RGO (b) 200 nm (a) 10 nm -1 (d) (223) (2110) (1310) (404) 14 amount in NFs was relatively little. The effect of annealing temperatures on morphologies of 1.0 wt% RGO-loaded α-Fe2O3 NFs was similar to that of pure α-Fe2O3 NFs. The rhombohedral structure of α-Fe2O3 of the NFs was confirmed by the XRD results (JCPDS 33–0664). The EDX spectrum showed the presence of Fe, O, and C elements from the RGO-loaded α-Fe2O3 NFs. Fig. 3.18a showed a low-magnification TEM image of the 1.0 wt% RGO-loaded α-Fe2O3 NFs. The NFs with the diameter of 50– 100 nm consisted of many nanograins. The presence of RGO NSs on the NF surface was shown in Fig. 3.18b. Parallel lattice fringes were clearly visible in HRTEM images in Fig. 3.18c, which indicated a good crystalline structure. The SAED result confirmed the polycrystalline nature of the single-phase rhombohedral structure of hematite α-Fe2O3 (JCPDS 33–0664). All observed results proved that well-crystalline RGO-loaded α-Fe2O3 NFs were successfully fabricated on chip by electrospinning. 3.3.2. H2S gas sensing properties of RGO-loaded α-Fe2O3 NFs sensors 3.3.2.1. Effects of RGO contents As shown in Fig. 3.19a–d, all the sensors presented a typical n- type sensing behaviour, which confirmed that the conducting channel in RGO-loaded α-Fe2O3 NFs mainly went through α-Fe2O3 NFs nanograins. The sensor response increased with increased RGO contents up to 1.0 wt%, and then the response decreased with further increased RGO contents. The similar results were obtained with the effects of RGO content on the sensor DL in Fig. 3.19f. The enhanced response of the RGO-loaded α-Fe2O3 NFs sensors was possibly explained by the formation of a heterojunction between RGO and α- Fe2O3 and a homojunctions among α-Fe2O3 grain boundaries. Furthermore, the presence of RGO and RGO/Fe2O3 interfaces in RGO-loaded α-Fe2O3 NFs caused additional active gas-adsorption sites like vacancies, defects, and oxygen functional groups; this consequently enhanced the sensor response. However, when the RGO content went up to 1.5 wt%, the sensor response declined because RGO sheets connected together to form an individual conducting path, which decreased overall sensor resistance (Fig. 15 3.19g). As a result, exposure of the sensor to H2S gas also decreased the resistance modulation and led to a weaker sensor response. 3.3.2.2. Effects of working temperature The effects of working temperature on gas sensing properties of RGO-loaded sensor were similar to those of pure α-Fe2O3 NF sensors, which indicated that the loading of RGO in the NFs did not affect the working temperature. 3.3.2.3. Effects of annealing temperatures The effect of the annealing temperature on the response of the sensor based on 1.0 wt% RGO-loaded α-Fe2O3 NFs was similar to that of pure α-Fe2O3 NFs. RGO enhanced the sensor response at low annealing temperatures but decreased the response at high annealing temperatures, compared to pure α-Fe2O3 NFs. This was similar to the effect of the annealing temperature on DL of the sensors of pure α- Fe2O3 NFs and 1.0 wt% RGO-loaded α-Fe2O3 NFs. 3.3.2.4. Selectivity and stability The RGO-loaded sensors showed high selectivity and short-term stability. Regarding the selectivity to above test gases of the pure α- Figure 3.19. H2S sensing transients of α-Fe2O3 NFs sensors loaded with different RGO concentrations: 0 (a), 0.5 (b) 1.0 (c) and 1.5 wt% (d). Sensor resistance (e), gas response (f), and response time and recovery time (g) as a function of RGO concentrations at working temperature of 350°C. 0 1 (f) D L ( p p b ) 0.0 0.5 1.0 1.5 0 10 20 30(g) R ( M  ) RGO Conc. (wt.%) @Air&350 o C 0 500 1000 1500 2000 5 10 10 10 1 Time (sec) 1.5 wt.% RGO R e s is ta n c e ( M  ) 1 wt.% RGO 0.5 wt.% RGO (d) (c) (b) -Fe 2 O 3 @ H 2 S&350 o C 1 ppm 0.5 ppm 0.25 ppm 0.1 ppm (a) 2 4 6 8 10 3.1 6.1 7.3 (e) G a s R e s p o n s e ( R a /R g ) H2S@350C &1 ppm H 2 S@350C &0.5 ppm H 2 S@350C &0.25 ppm H 2 S@350C &0.1 ppm 9.2 16 Fe2O3 NFs sensor and the 1.0 wt% RGO-loaded α- Fe2O3 NFs sensor, the latter sensor had better selectivity to H2S gas (Fig. 3.24). Conclusion of chapter 3 This chapter studied the effects of annealing temperature, electrospun time and precursor solution contents (i.e. PVA concentration and salt concentration) on morphology and structure of α-Fe2O3 NFs fabricated on chip by electrospinning. The optimal results showed that the α-Fe2O3 NFs sensor calcined at 600 ° C and electrospun for 30 min with the precursor solution of 11 wt% PVA and 4 wt% Fe (NO3)3 gave a response of 6.1 to 1 ppm H2S gas at 350°C. In addition, RGO enhanced the sensing properties of RGO-loaded α-Fe2O3 NFs sensor compared to that of pure α-Fe2O3 NFs. The response of 1.0 wt% RGO-loaded α-Fe2O3 NFs sensors reached 9.2 to 1 ppm H2S at 350°C (1.5 times higher than that of pure α-Fe2O3 NFs at the same conditions). However, the response and selectivity of the sensors based on α- Fe2O3 NFs and their incorporation with RGO were not high. Therefore, improving the sensor selectivity and response is essential, which will be studied in the next chapter. CHAPTER 4. ZFO NFs AND THEIR LOADING WITH RGO FOR H2S GAS SENSING APPLICATION 4.1. Introduction Sensors based on binary α-Fe2O3 have low selectivity because they are sensitive to many different gases and their sensor response is also quite low. Many methods including doping binary α-Fe2O3 with noble metals and combining binary α-Fe2O3 with other metal oxides to form composites or ternary compounds have been used to improve Figure 3.24. Comparative selectivity of sensors based on α-Fe2O3 NFs and 1.0 wt% RGO loaded α-Fe2O3 NFs to various gases at 350°C. 0 2 4 6 8 10 SO 2 @350 o C& 10 ppm H 2 @350 o C&1000 ppm NH 3 @350 o C&1000 ppm H 2 S@350 o C& 1ppm 6.1 5.6 1.6 2.2 1.61.7 9.2 102 1.6S ( R a /R g o r R g /R a ) Fe 2 O 3 NFs Fe 2 O 3 NFs loaded 1 wt.% RGO SO 2 H 2 NH 3 H 2 S 3.8 17 the sensor selectivity and response. Particularly, ternary ZFO, a typical normal spinel with cubic crystal structure, is a promising material for detecting gases because of its good chemical and thermal stability, low toxicity, high specific surface area and excellent selectivity. The gas sensing properties of ZFO, especially to H2S, have been investigated in many works. However, researches on H2S gas sensitivity, especially at sub-ppm concentrations, of NFs ZFO sensors have not been published. In addition, the effects of heat treatment parameters such as annealing temperature, annealing time and annealing rate on the sensor morphology, structure and H2S gas sensing properties of NFs ZFO sensors have not been investigated. Furthermore, the H2S gas-sensing performance of ZFO NFs loaded with RGO has not been also studied. Therefore, in this chapter, ZFO-NFs sensors and their incorporation with RGO were fabricated by facile on-chip electrospinning. Then, the effects of heat treatment conditions on morphology, structure and H2S gas sensing performances of the ZFO NFs sensors were investigated. 100 nm (b) 200 nm (a) (d) 0.49 nm 0.42 nm 5 nm [101] (020) (000) (1ī ī) (220) (311) (400) (422) (511) (440) 5 nm -1 (c) Figure 4.7. TEM images at different magnifications (a-b), SAED pattern (c), and HRTEM image (d) with corresponding fast Fourier transform (FFT) inset image of ZFO-NFs calcined at 600°C for 3 h in air. 18 Simultaneously, the effects of RGO concentration and annealing temperature on the H2S gas sensing properties of the RGO-loaded ZFO NFs sensors were also discussed in detail. 4.2. H2S gas sensors based on ZFO NFs In this section, the morphology and structure of the ZFO NFs as well as the influence of heat treatment conditions (i.e. annealing temperature, annealing time, and heating rate) on morphology, structure and H2S gas sensing characteristics of ZFO NFs were systematically investigated. 4.2.1. Microstructure characterization The cubic spinel structure of ZFO NFs at different calcination conditions was confirmed in XRD pattern. The nanograins and crystallinity of the ZFO NFs increased with increased annealing temperature from 400 to 700°C and with the increased annealing time from 0.5 to 48 hours. Whereas, the grain size and crystallinity of ZFO NFs declined with the increased heating rate between 0.5 and 2°C/min because of a dramatic decrease in calcination duration. However, with a further increase in the heating rate, the grain size and crystallinity also rose. FESEM confirmed the effect of annealing temperature and annealing time on the NFs morphologies. Whereas, the heating rate changed from 0.5 to 5°C/min, the ZFO NFs were still spider-net-like and continuous, however, when the heating rate went up to 20°C/min, almost NFs with thinner diameters were fractured. Only NFs with larger diameters were still continuous. The EDX detected four elements (Fe, O, Si and Zn). The morphology and microstructure of ZFO NFs were further examined by TEM and HRTEM images (Fig. 4.7). Obviously, the synthesized ZFO sample was the multi-porous NFs composed of many nanograins with the average grain size of about 5–25 nm (Fig. 4.7a–b). The SAED pattern of the ZFO NFs in Fig. 4.7c revealed that the diffraction rings combined with the spots of polycrystalline nature of the cubic spinel ferrite phase. The HRTEM image and corresponding FFT inset image in Fig. 4.7d further confirmed the crystalline nature of the synthesized ZFO NFs. The HRTEM image exhibited parallel lattice fringes with spacing approximately 4.9 and 4.2 Å, corresponding to lattice planes (020) and (11 1 ), which was proved by the FFT inset. 19 4.2.2. Gas sensing properties 4.2.2.1. Effects of the operating temperature The sensor response and recovery time strongly increased when the operating temperature decreased from 450 to 250°C. The working temperature of 350°C was selected to further investigate gas sensing properties by compensation between the sensor response and recovery time. 4.2.2.2. Effects of the annealing temperature When the annealing temperature increased from 400 to 600°C, the sensor response also went up. The sensor response fell down with the further increased annealing temperature. This was explained by the as-mentioned effects of grain size and crystallinity on gas sensing properties of the sensor. The DL calculation was 0.048 ppb corresponding to the sensor calcined at 600°C. Therefore, 600°C was 20 40 60 80 100 0 12 24 36 48 1 10 100 R e s p . (R a /R g ) H 2 S@ 1 ppm & 350 o C Calcinated time (h) (b) Response time  r e s p ./ re c o v . ( s ) Recovery time (c) 20 40 60 80 100 0 5 10 15 20 10 100 R e s p . (R a /R g ) H 2 S@ 1 ppm & 350 o C Heating rate ( o C/min) (e) Response time  r e s p ./ re c o v . ( s ) Recovery time (f) 0.25 0.50 0.75 1.00 20 40 60 80 100 R e s p o n s e ( R a /R g ) H 2 S conc. (ppm) Calcinated @0.5 h Calcinated @3 h Calicnated @12 h Calcinated @48 h (a) 0.25 0.50 0.75 1.00 20 40 60 80 100 H 2 S conc. (ppm) Heating rate @0.5 o C/min Heating rate @2 o C/min Heating rate @5 o C/min Heating rate @20 o C/min R e s p o n s e ( R a /R g ) (d) Figure 4.10. Response at working temperature of 350°C as a

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