Nghiên cứu chế tạo sợi nano fe2o3 và znfe2o4 lai graphene khử từ ôxit graphene (RGO) bằng phương pháp phun tĩnh điện và ứng dụng cho cảm biến khí h2s tt tiếng anh

MINISTRY OF EDUCATION AND TRAINING HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY Nguyen Van Hoang ELECTROSPINNING OF α-Fe2O3 AND ZnFe2O4 NANOFIBERS LOADED WITH REDUCED GRAPHENE OXIDE (RGO) FOR H2S GAS SENSING APPLICATION Major: Materials Science Code: 9440122 ABSTRACT OF DOCTORAL DISSERTATION OF MATERIALS SCIENCE Hanoi - 2020 The Dissertation was completed at: Hanoi University of Science and Technology Supervisor: Prof PhD Nguyen Van Hieu Reviewer 1: Prof PhD Luu Tuan Tai Reviewer 2: Prof PhD Pham Thanh Huy Reviewer 3: Prof PhD Vu Dinh Lam The dissertation will be defended at the University Council of Doctoral dissertation held at Hanoi University of Science and Technology At ……… , date……… month… year……… The dissertation can be found at the libraries: Ta Quang Buu, Hanoi University of Science and Technology Vietnam National Library INTRODUCTION Background of the thesis Recently, 1D nanostructures including nanowires (NWs), nanorods (NRs), nanotubes (NTs), and nanofibers (NFs) have attracted much attention for a wide application including optical catalysis, electronic devices, optoelectronic devices, storage devices, and gas sensors due to their high surface-to-volume ratio Especially, NFs are widely used in many fields such as catalysis, sensor, and energy storage because of their outstanding properties like their large surface area-to-volume ratio and flexible surface functionalities There are several approaches for NFs fabrication, for example, drawing, template, phase separation, self-assembly, and electrospinning, among which electrospinning is a simple, costeffective and versatile method for NFs production Regarding gas sensing applications, semiconductor metal oxide (SMO) NFs sensors have a lot of promise due to their advantages of SMO materials like low cost, simple fabrication, and high compatibility with microelectronic processing Furthermore, NFs consist of many nanograins, therefore, grain boundaries are large, surface-to-volume ratio is very high, and gases easily diffuse along grain boundaries As a result, an exceptionally high response was observed in in SMO NFs gas sensors by electrospinning Among various SMO NFs prepared by electrospinning, α-Fe2O3 has become a potential gas sensing material because of its low cost and thermal stability and ability to detect many gases such as NO2, NH3, H2S, H2, and CO Besides, zinc ferrite ZnFe2O4 (ZFO), a Fe2O3-based ternary spinel compounds, has been a promising material for detecting gases thanks to its good chemical and thermal stability, low toxicity, high specific surface area and excellent selectivity Otherwise, H2S is a colorless, corrosive, inflammable and extremely toxic gas which can be rapidly absorbed by human lungs and easily causes diseases in respiratory and nervous system, even deaths However, until now, very few studies on H2S gas sensing properties of α-Fe2O3 and ZFO NFs, especially effects of parameters of fabrication process (i.e solution composition, heat treatment, and electrospun time) on morphology, structure and H2S gas sensing properties of the sensors have been carried out although there have been some reports about H2S gas sensitivity of the sensor of other nanostructures of α-Fe2O3 or ZFO sensors (e.g nanochains, porous nanospheres, and porous nanosheets (NSs)) Furthermore, RGO, a GP reduced from GO produced from graphite by Hummer method, has recently received world-wide attention owing to its exceptional physicochemical properties The combination between SMO NFs and RGO to enhance gas sensing performance through the formation of heterojunction was mentioned in many works However, up to present, there have been no reports on incorporation of RGO in α-Fe2O3 and ZFO NFs for enhanced H2S gas sensing performance Therefore, the thesis titled "Electrospinning of α-Fe2O3 and ZnFe2O4 nanofibers loaded with reduced graphene oxide (RGO) for H2S gas sensing application” was carried out to answer the concerns mentioned above The study objective The study objective of the thesis are listed as follows: - To successfully fabricate on-chip sensors based on α-Fe2O3, ZFO NFs and their loading with RGO by on-chip electrospinning - To explore the effect of parameters (i.e solution composition, heat treatment, electrospun time, and RGO concentration) of fabrication process on the NF morphology, structure and H2S gas sensing properties - To clarify H2S gas sensing mechanism of the sensors of α-Fe2O3, ZFO NFs and their incorporation with RGO Research scope and content The thesis uses α-Fe2O3, ZFO NFs and their loading with RGO, as well as harmful gas H2S as object studies The study focuses on the following contents: - To optimize some process parameters (i.e solution composition, heat treatment, electrospun time, and RGO concentration) for onchip sensor fabrication of α-Fe2O3, ZFO NFs and their loading with RGO via electrospinning method - To characterize the NFs and to analyze the relationship between their morphology and microstructure of NFs with fabrication process parameters - To examine H2S gas-sensing properties of the NFs sensors for clarifying the relationship among morphology, microstructure with gas-sensing properties of the NFs sensors - To understand the H2S gas-sensing mechanisms of α-Fe2O3, ZFO NFs and their loading with RGO Research Methodology To achieve the objectives, the thesis research was conducted by experimental methods, namely: - The on-chip electrospinning method was employed for the fabrication of α-Fe2O3, ZFO NFs and their loading with RGO - Morphology and structure of the NFs were characterized by TGA, RAMAN, FE-SEM, TEM, HR-TEM, SAED, EDX, and XRD - The gas-sensing properties of the NFs were measured by a home-made system using flow-through technique Practical and scientific significance of the thesis The scientific relevance: The thesis results elaborated the relationship among processing parameters, microstructure, and gassensing properties of α-Fe2O3, ZFO NFs and their loading with RGO In addition, the thesis also clarified H2S gas-sensing mechanisms of α-Fe2O3, ZFO NFs and their loading with RGO Furthermore, the research results have been reviewed by domestic and foreign scientists, and published in prestigious journals such as Journal of Hazardous Materials and Sensors and Actuator B, which shows scientific significance of the dissertation The practical relevance: This dissertation focused on the development of the effective sub-pp H2S gas sensor of α-Fe2O3, ZFO NFs and their loading with RGO by on-chip electrospinning method The optimized results provide a premise to develop the sensors for environmental monitoring, occupational health, petrochemical plant, which showed significantly practical relevance of the dissertation The original contributions of the dissertation Currently, almost NFs sensors are prepared by two-step process: synthesis of sensing materials and then fabrication of the sensors, which is not cost-effective for large-scale production and difficult for reproducible sensor fabrication In this thesis, the on-chip NFs sensors were successfully synthesized by electrospinning The effect of morphology, structure and composition which were changed in fabrication process by varying solution concentration, electrospun time, heat treatment conditions, and RGO concentration on H2S gas sensing properties of the sensor of α-Fe2O3, ZFO NFs and their incorporation with RGO was systematically investigated In addition, H2S gas sensing mechanisms of α-Fe2O3, ZFO NFs and their incorporation with RGO, especially when annealing temperature was changed, were also discussed in detail The main research results of the thesis were published in 02 ISI articles, 01 location article, and 02 proceedings of conference The structure of the thesis This thesis is interpolated from the articles by the author Apart from the introductions, conclusions and recommendations, there are four main chapters and a list of references and publications in this thesis Chapter 1: Overview on SMO NFs and their loading with RGO for gas-sensing application Chapter 2: Experimental approach Chapter 3: α-Fe2O3 NFs and their loading with RGO for H2S gas sensing application Chapter 4: ZFO NFs and their loading with RGO for H2S gas sensing application CHAPTER OVERVIEW ON SMO NFs AND THEIR LOADING WITH RGO FOR GAS-SENSING APPLICATION In this chapter, an overview on electrospinning, one of the most simple, cost-effective and flexible methods for NFs fabrication with such various kinds of materials as polymers, SMO, and composites, was introduced NFs formation made use of electrostatic forces to stretch a viscoelastic solution A high voltage was applied to a solution droplet suspended at a tip of a syringe needle When the electric field reached a critical value, a charged jet of the solution was ejected and stretched to form a continuous and thin fiber from the tip of needle to a collector Subsequently, the as-spun fibers were calcined to decompose polymer and crystallite to form SMO NFs Some parameters in fabrication process, which affected morphologies and microstructures of NFs, were also mentioned NFs morphologies and microstructures depended on such factors as electrospinning parameters, solution, and environmental conditions Furthermore, collectors and needles also had a strong influence on morphologies and microstructures of obtained NFs The most commonly used collector was the rotary drum collector which was suitable for mass production of aligned NFs In addition, the conditions of heat treatment process greatly affected NFs morphologies and microstructures Any changes in the annealing temperature, annealing time, or heating rate could lead to changes in NFs morphologies and microstructures, resulting in the varied NFs properties SMO NFs have been widely used in gas sensing application Many works showed that the NFs sensor have high response and fast response-recovery time due to their high porosity and large specific surface area structures Especially, there have been many studies on H2S gas sensing properties of SMO NFs and composite NFs The results showed that NFs structure had higher response and faster response time than other nanostructures In addition, the response and selectivity of the composites sensors were enhanced compared to those of binary SMO sensors However, there are not many researches on the sensors based on NFs of α-Fe2O3 or ZFO to different gases in general and to H2S gas in particular, especially at sub-ppm concentrations This chapter also reviewed on RGO and its application in gas sensing field In general, RGO was widely used in gas sensors thanks to its incompletely reduced functional groups, and many dangling bonds or defects that created favorable positions for gases to absorb However, RGO sensors also had some limitations such as low response, drift resistance, irreversibility, long response and recovery time Therefore, combining RGO with other materials like noble metals or SMO helped to solve these problems There were two trends to combine RGO and SMO to enhance the gas sensing properties On the one hand, SMO particles were attached to the surface of RGO NSs, resulting in SMO-loaded RGO sensors The sensors were conducted through continuously connected RGO NSs The sensors showed gas sensing characteristics of a p-type semiconductor of RGO while SMO enhanced the response and response-recovery time However, the SMO-loaded RGO sensors failed to solve some inherent limitations of RGO sensors 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 wt% and RGO NSs were dispersed and disconnected in composites The sensors behaved gas sensing characteristics of SMO The RGOloaded 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 loaded6 SMO NFs sensors had high response to both oxidizing and reducing 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 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 onchip 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 program to record the electrical-resistance response of the test sensors under various concentrations and operating temperatures CHAPTER 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 subppm 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 RGOloaded α-Fe2O3 NFs sensors was also discussed in detail 3.2.2 H2S gas sensing properties of sensors based on αFe2O3 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) 11 350°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 to 15 wt% PVA because of increased NFs diameters However, the NFs comprised a network of small beads interconnected by thin fibers with 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 ppm H2S gas was at wt%, and reached a maximum of 6.2 at wt% g and then went down to 4.9 with wt% ferric salt Therefore, to optimize the NFs morphology and gas response, the sensor prepared from precursor solution with 11 wt% PVA and 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 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 12 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 with the precursor solution of 11 wt% PVA and 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 (c) (a) [441] (104) -1 nm (104) 0.27 nm 200 nm (b) nm (d) RGO (1310) (223) (404) α- Fe2O3 (2110) 50 nm -1 10 nm 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 hours in air 13 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 ntype 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 αFe2O3 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 14 0.25 ppm H2S@350C &0.5 ppm 9.2 ppm 10 H2S@350C &0.1 ppm 6.1 7.3 (b) 0.5 wt.% RGO (e) H2S@350C &0.25 ppm 0.1 ppm 0.5 ppm Resistance (M) H2S@350C &1 ppm (a) DL (ppb) Gas Response (Ra/Rg) -Fe2O3@ H2S&350oC 3.1 10 (f) (c) wt.% RGO 10 o 1.5 wt.% RGO @Air&350 C (d) (g) 30 R (M) 10 20 10 0 500 1000 1500 2000 Time (sec) 0.0 0.5 1.0 RGO Conc (wt.%) 1.5 Figure 3.19 H2S sensing transients of α-Fe2O3 NFs sensors loaded with different RGO concentrations: (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 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 αFe2O3 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 α15 Fe2O3 NFs sensor and the 10 Fe O NFs 9.2 Fe O NFs loaded wt.% RGO 1.0 wt% RGO-loaded α8 SO @350 C& 10 ppm Fe2O3 NFs sensor, the latter 6.1 H @350 C&1000 ppm 5.6 102 NH @350 C&1000 ppm sensor had better selectivity H S@350 C& 1ppm to H2S gas (Fig 3.24) 2.2 1.6 1.6 1.7 1.6 3.8 Conclusion of chapter This chapter studied the NH HS SO H effects of annealing Figure 3.24 Comparative selectivity temperature, electrospun of sensors based on α-Fe2O3 NFs time and precursor solution and 1.0 wt% RGO loaded α-Fe2O3 contents (i.e PVA NFs to various gases at 350°C 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 with the precursor solution of 11 wt% PVA and wt% Fe (NO3)3 gave a response of 6.1 to 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 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 αFe2O3 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 S (Ra/Rg or Rg/Ra) 3 o o o o 2 CHAPTER 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 16 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 (a) (c) (440) (511) (422) (400) (311) (220) -1 200 nm (b) nm (d) [101] (020) (000) (1ī ī) 0.49 nm 0.42 nm 100 nm nm 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 h in air 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 17 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 (1 1 ), which was proved by the FFT inset 18 80 H2S@ ppm & 350oC Calcinated @0.5 h Calcinated @3 h Calicnated @12 h Calcinated @48 h (b) (a) (s) 60 100 80 60 40 20 Recovery time 100 40 (c) 20 resp./recov Response (Ra/Rg) 100 Resp (Ra/Rg) 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 10  Response time 0.50 0.75 1.00 12 24 36 48 Calcinated time (h) H2S conc (ppm) 60 (e) 100 80 60 40 20 (d) Recovery time 40 (f) Response time 100 10  20 (s) 80 H2S@ ppm & 350oC o Heating rate @0.5 C/min o Heating rate @2 C/min o Heating rate @5 C/min o Heating rate @20 C/min resp./recov Response (Ra/Rg) 100 Resp (Ra/Rg) 0.25 0.25 0.50 0.75 H2S conc (ppm) 1.00 10 15 20 Heating rate (oC/min) Figure 4.10 Response at working temperature of 350°C as a function of H2S concentration for different annealing time (a) and heating rate (d) Response and response-recovery time as a function of annealing time (b, c) and heating rate (e, f) 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 19 selected as the optimal annealing temperature for ZFO NFs Response time and recovery time also decreased with the increased annealing temperature from 400 to 700°C 4.2.2.3 Effects of annealing time and heating rate According to Fig 4.10, the sensor response increased with the increased annealing time from 0.5 to h; however, the sensor response decreased with the further increased annealing time up to 48 h The recovery time also relatively decreased with increased annealing time Regarding the heating rate, two peak responses were observed at 0.5 and 5°C/min The sensor calcined at the heating rate of 0.5°C/min showed the highest response, however, the heating rate could not further decrease because of the limitation of furnace temperature controller As observed in Fig 4.10h, response-recovery time as a function of the heating rate decreased 100 when heating rate increased from 0.5 to 20 5°C/min, but slightly increased when heating 10 rate was further increased to 20°C/min The effects NH HS SO H of crystallinity, grain size, Figure 4.12 Comparative selectivity and high porosity of ZFO NFs-based sensors and αstructure were employed Fe2O3-based NFs sensors to various to explain these gas gases at 350°C sensing results 4.2.2.4 Selectivity and stability The results showed that the ZFO NFs sensors displayed an excellent selectivity to H2S gas among other gases Moreover, as shown in Fig 4.12, the ZFO NFs sensor exhibited the higher selectivity and response than the sensors based on α-Fe2O3 NFs and their loading with RGO, which may be due to unique spinel crystal structure of ZFO as reported in many works The sensors also had a good stability throughout the cycle test 4.3 H2S gas sensors based on ZFO NFs loaded with RGO 4.3.1 Microstructure characterization Fe2O3 NFs S (Ra/Rg or Rg/Ra) Fe2O3 NFs loaded wt.% RGO ZFO NFs SO2@350 oC& 10 ppm H2@350 oC&1000 ppm NH3@350 oC&1000 ppm H2S@350 oC& 1ppm 20 The FE-SEM images of RGO-loaded ZFO NFs with different amounts of RGO confirmed that the NFs morphologies were not significantly affected by the change of the RGO content (0–1.5 wt%) and RGO could not be found from FESEM of RGO-loaded ZFO NFs FESEM images also showed that NF surfaces became rough due to the increased nanograin size with the increased annealing temperature The XRD patterns proved the cubic spinel structure of ZFO (JCPDS 89–7412) The EDX spectrum indicated the presence of C, Fe, Zn and O elements from the NFs The TEM, HRTEM, SAED, and FFT results further determined that well-crystalline RGO-loaded ZFO NFs had been successfully fabricated by electrospinning 4.3.2 Gas-sensing properties 4.3.2.1 Effects of RGO contents The sensor response reached a maximum at 1.0 wt% RGO when the RGO content changed from to 1.5 wt% The explanation for this result was similar to that of the RGO-loaded α-Fe2O3 NFs sensor as mentioned in Section 3.3.2.1 The recovery time increased when the RGO weight percentage rose between and 1.0% due to the increase in the amount of absorbed H2S gas However, when the RGO concentration further increased to 1.5 wt%, the individual conducting path was formed, leading to the increased electron mobility and decreased sensor recovery time Meanwhile, the response time of the sensor did not change much when RGO content was varied The response time of all RGO-load ZFO NF sensors was quite short, below 10 s 4.3.2.2 Effects of operating temperature The response decreased with the increased working temperature from 250 to 450°C, which was as similar as that of pure ZFO NF sensors in Section 4.2.2.1 This confirmed that the loading of RGO in the NFs did not affect the working temperature as mentioned in Section 3.3.2.2 4.3.2.3 Effects of annealing temperatures The response reached a maximum at 600°C for all H2S concentrations with the increased annealing temperature from 400 to 21 (a) 147 100 61 ZFO NFs (d) 21.8 8.2 15.8 10 Response time 600 700 1000 100 Recovery time 500 0.2 0.0 ZFO NFs lai wt.% RGO 43.4 400 0.4 (c) 77.6 50 0.6 wt% RGO+ZFO ZFO 102 (s) 150 (b) DL (ppb) SZFO+RGO/SZFO 1wt.% RGO-loaded ZnFe2O4@ 1ppm H2S &350 oC 400 500 600 700 resp./recov o @ ppm H2S & 350 C ZnFe2O4@ 1ppm H2S&350 oC  Response (Ra/Rg) 200 Annealing temp (oC) Figure 4.20 Comparison of response (a), change level of response (b), detection limit (DL) (c) and response-recovery time (d) of bare-ZFO and 1%wt RGO loaded ZFO NFs sensors to ppm H2S gas at 350°C as a function of annealing temperatures 700°C because of the inverse effects of nanograin size and crystallinity In addition, the weight loss of RGO also declined with the decreased annealing temperature, making the rest of the RGO amount be enlarged, which caused the sensor response to change The response time remained almost unchanged when the annealing temperatures varied while the recovery time fluctuated with increased annealing temperatures because of the effects of nanograin size and RGO weight loss Fig 4.20ab presented the comparative effects of annealing temperature on the sensor response of the pure ZFO and RGO-loaded ZFO NF sensor The magnitude of the effect on the response of the RGO-loaded sensor significantly decreased with the increased annealing temperature due to the increased weight loss of RGO It was the same as the effects of annealing temperature on the DL of the sensors of pure ZFO NFs and RGO-loaded ZFO NFs in Fig 4.20c The response and recovery time of the pure ZFO-sensor were longer than those of the RGO-loaded sensor because of the larger electron mobility in RGO-loaded sensors 4.3.2.4 Selectivity, stability and RH effects The sensor had good reproducibility and short-term stability The RGO-loaded ZFO NFs sensor also had excellent selectivity to H2S gas The RGO-loaded ZFO NFs sensor also presented better selectivity to H2S than pure ZFO sensor Besides, the effects of RH on the response of the sensor were also investigated 22 The H2S sensors in this thesis had much higher response than sensors based on other materials or nanostructures due to RGO influence, morphologies and structures of the as-synthesized NFs Obviously, the H2S response of ZFO sensor was higher than that of the α-Fe2O3 sensor, which was because ZFO NFs had a multi-porous structure while α-Fe2O3 NFs had a quite dense structure as seen in TEM images In addition, the unique crystal structure and high surface activity with many defects of ZFO made it become a superior gas sensing material, especially to reducing gases Moreover, the sensor loaded with RGO had a higher H2S response than the pure sensor in same working conditions because of the heterojunction between RGO and α-Fe2O3 or ZFO Conclusion of chapter The chapter optimized heat treatment conditions (temperature annealing, time annealing and heating rate) which strongly affected morphologies, microstructures and gas sensing properties of ZFO NFs sensors The optimal condition showed that the ZFO NFs calcined at 600°C for h with heating rate of 0.5°C/min gave the best response of 102 to ppm H2S gas at the working temperature of 350°C The results also confirmed that RGO enhanced sensitivity and selectivity The responses of wt% RGO-ZFO NFs were 1.5 times higher than that of pure ZFO NFs at the same conditions, which was significantly affected by formation of heterojunctions between RGO and ZFO CONCLUSIONS AND RECOMMENDATIONS Conclusion Based on the above-mentioned research results, some conclusions were drawn and listed hereafter  The α-Fe2O3 NFs, ZFO NFs and their loading with RGO were successfully on-chip fabricated by electrospinning method  The precursor solution contents, electrospun time, and annealing temperature, which strongly affected morphologies, microstructures and gas sensing performances of α-Fe2O3 NFs, were optimized The optimized α-Fe2O3 NFs get response of 6.1 to ppm H2S at 350ºC, which corresponding to PVA 23    concentration of 11 wt%, ferric salt concentration of wt%, and electrospun time of 30 The heat treatment conditions (i.e annealing temperature, annealing time and heating rate) which significant impacts on nanograin size, crystallinity and gas sensing performances of ZFO NFs, also were optimized The optimized ZFO NFs had response of 102 to ppm H2S at 350ºC, which corresponding to annealing temperature of 600ºC, annealing time of h, and heating rate of 0.5ºC/min RGO enhanced the sensing properties The response increased about 1.5 times (9.2 with RGO-loaded α-Fe2O3 NFs and 147 with RGO-loaded ZFO NFs to ppm H2S at 350ºC) and selectivity also improved The sensors of ZFO NFs and their loading with RGO also expressed excellent sensitivity and selectivity which were better than those of the sensors of α-Fe2O3 NFs and their loading with RGO Recommendations for future works The electrospinning method is a simple technique to fabricate NFs composite with various materials from their precursor salts Whereas, it is quite easy to mix spinel structures with other SMO, which has significant impacts on NFs structure, composition and gas sensing properties Thus, some directions for future research were suggested as follows:  Doping spinel structures with noble metals (Au, Pt, and Pd)  Mixing spinel structures with one or more SMO to form multicomponent compounds like MxZn1-xFe2O4  Studying other spinel types MFe2O4 (M: Cu, Mg, Mn…) 24 LIST OF PUBLICATIONS Nguyễn Văn Hoàng, Nguyễn Văn Dũng, Nguyễn Thị Hồng Phước, Đặng Thị Thanh Lê, Chử Mạnh Hưng, Nguyễn Văn Hiếu (2017), “Nghiên cứu chế tạo sợi nano ZnO phương pháp phun tĩnh điện”, Hội nghị Vật lý Chất rắn Khoa học Vật liệu Toàn quốc, pp 420 – 423 Nguyen Van Hoang, Phan Hong Phuoc, Chu Manh Hung, Nguyen Van Hieu (2017), “Investigating NO2 sensing capabilities of the electrospun α-Fe2O3 nanofibers-based sensors”, Proceeding of The 12th Asian Conference on Chemical Sensors (ACCS2017), pp 340-343 Nguyen Van Hoang, Nguyen Van Dung, Do Quang Dat, Quan Thi Minh Nguyet, Chu Manh Hung, and Nguyen Van Hieu (2017), “On-chip ZnO nanofibers prepared by electrospinning method for NO2 gas detection”, Communications in Physics, Vol 27, pp 317-326 Nguyen Van Hoang, Chu Manh Hung, Nguyen Duc Hoa, Nguyen Van Duy, Nguyen Van Hieu (2018), “Facile on-chip electrospinning of ZnFe2O4 nanofiber sensors with excellent sensing performance to H2S down ppb level”, Journal of Hazardous Materials, Vol 360, pp 6-16 Nguyen Van Hoang, Chu Manh Hung, Nguyen Duc Hoa, Nguyen Van Duy, Inkyu Park, Nguyen Van Hieu (2019), “Excellent detection of H2S gas at ppb concentrations using ZnFe2O4 nanofibers loaded with reduced graphene oxide”, Sensors & Actuators: B Chemical, Vol 282, pp 876-884 ... in α -Fe2O3 and ZFO NFs for enhanced H2S gas sensing performance Therefore, the thesis titled "Electrospinning of α -Fe2O3 and ZnFe2O4 nanofibers loaded with reduced graphene oxide (RGO) for H2S. .. Background of the thesis Recently, 1D nanostructures including nanowires (NWs), nanorods (NRs), nanotubes (NTs), and nanofibers (NFs) have attracted much attention for a wide application including... H2S@ 350C &0.5 ppm 9.2 ppm 10 H2S@ 350C &0.1 ppm 6.1 7.3 (b) 0.5 wt.% RGO (e) H2S@ 350C &0.25 ppm 0.1 ppm 0.5 ppm Resistance (M) H2S@ 350C &1 ppm (a) DL (ppb) Gas Response (Ra/Rg)  -Fe2O3@ H2S& 350oC