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

2 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 nanostr

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:

1 Ta Quang Buu, Hanoi University of Science and Technology

2 Vietnam National Library

INTRODUCTION

1 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,

cost-effective 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

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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-Fe 2 O 3 and ZnFe 2 O 4 nanofibers loaded with reduced graphene oxide (RGO) for

H 2 S gas sensing application” was carried out to answer the concerns

mentioned above

2 The study objective

The study objective of the thesis are listed as follows:

- To successfully fabricate on-chip sensors based on α-Fe2O3, ZFONFs 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, ZFONFs and their incorporation with RGO

3 Research scope and content

The thesis uses α-Fe2O3, ZFONFs 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 chip sensor fabrication of α-Fe2O3, ZFONFs and their loading with RGO via electrospinning method

on 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, ZFONFs and their loading with RGO

4 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, ZFONFs 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

5 Practical and scientific significance of the thesis

The scientific relevance: The thesis results elaborated the

relationship among processing parameters, microstructure, and sensing properties of α-Fe2O3, ZFONFs and their loading with RGO

gas-In addition, the thesis also clarified H2S gas-sensing mechanisms of α-Fe2O3, ZFONFs 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, ZFONFs 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

6 The original contributions of the dissertation

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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

7 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 1 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

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 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

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

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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 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

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-Fe 2 O 3

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-Fe 2 O 3 NF sensors to 1 ppm

H 2 S at various operating temperatures (a), sensor resistances (b), sensor response (c), response time and recovery time (d) as a

function of operating temperatures

0 10 20 (c)

3.2.2 H2S gas sensing properties of sensors based on Fe2O3 NFs

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 H 2 S sensing transients of α-Fe 2 O 3 NF sensors with various annealing temperatures (400−800°C) (a–e) and different electrospinning time (10−120 min) (f–i) Sensor response to H 2 S gas as a function of annealing temperatures (k)

and electrospinning time (l)