Summary of material science doctoral thesis: Synthesis and study of the hybrid materials between polypyrrole and nanostructured NiO for NH3 gas sensitivity

MINISTRY OF EDUCATION AND VIETNAM ACADEMY OF SCIENCE

TRAINING AND TECHNOLOGY

GRADUATE UNIVERSITY SCIENCE AND TECHNOLOGY

-----------------------------

HOANG THI HIEN

SYNTHESIS AND STUDY HYBRID MATERIALS BETWEEN PPY AND NANO STRUCTURED NIO FOR NH3 GAS SENSITIVITY

Major: Electronic materials

Code: 9440123

SUMMARY OF MATERIAL SCIENCE DOCTORAL THESIS

Ha Noi – 2021

The work was completed at: Graduate University Science and Technology – Vietnam Academy of Science and Technology

Advisors 1: Prof. Dr. Tran Trung

2: Dr. Ho Truong Giang

Reviewer 1: …

Reviewer 2: …

Reviewer 3: …

The thesis will be protected before the doctoral dissertation thesis, meeting at the Academy of

Science and Technology - Vietnam Academy of Science and Technology at … hour …'…, …

2021

The thesis can be found at:

- Library of the Academy of Science and Technology

- National Library of Vietnam

INTRODUCTION

1. The thesic’s urgency

Currently, the environmental problems such as soil, air and water, etc. along with

natural disasters are a big challenge for mankind. In which, the air environment is one of the

directly influenced and strongly diffuse wide environments.

Among the polluting gases, NH3 is a gas that is easily released into the environment, diffusing and polluting the air. Furthermore, NH3 can affect human health seriously, even though it causes death in humans and animals when exposed to high concentrations of 500 ppm. NH3 gas is very toxic with the permissible human exposure limit of 25 ppm for 8 hours, and it can cause explosion when in concentration range 16 - 25% by volume in the

atmosphere [1]. However, it is an important chemical used in many industries such as fertilizer production, oil and gas industry, rubber industry, food processing technology, household detergents, etc. Recently, the analysis of NH3 gas from human breath for the purpose of medical diagnosis (non-patient form) has attracted a lot of attention of scientists

and it promises to be application in the near future [2]. So, it is necessary and important that the detection and analysis NH3 gas protect environment, ensuring human health and promoting its application potential in industries.

The conductivity sensors (resistance sensors) based on typical nanostructured metal

oxide materials such as SnO2, ZnO, WO3, TiO2, In2O3, NiO, ... for high sensitivity are studied commonly. However, these sensors often have to operate at high temperatures (a

few hundred degrees Celsius), which leads to a change in the microstructure, particle size

effect of the sensitive membrane during operation. gas in the nanostructured. This effect

causes the sensor to encounter limitations with time-varying parameters such as reduced

reliability, reduced operation stability, “0” point drift, output signal routing error, etc. So,

the research direction of materials which can operate at room temperature is a trend that it is

received great interest in conductivity gas sensor research.

Conductive polymer materials such as Polyacetylene (PA); Polythiophen (PTh);

Polypyrole (PPy); Poly (p-phenylene vinylen) (PPV); Polyanilin (PANi); and Poly (3,4-

ethylene-dioxythiophen) (PEDOT) with the advantages of rich physical and chemical

properties, flexibility in the fabrication of component structures, low cost, durable and

friendly environment. Therefore, they are studied and used in many scientific and

engineering fields such as field-effect transistors [3], organic light-emitting diodes (OLED)

[4], processing and autoclaving. filler of environmental pollutants (toxic gas, fine dust, ...),

solar cell [5], super capacitor [6], metal corrosion protection [7], metal ion detection sensor

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heavy type [8,9], biomolecule detection sensor [10,11], gas sensor (NH3, CO2, H2S, SO2, NO2, H2, CH4) [12-15], etc. In the field of gas sensors, CPs have a huge advantage in being able to operate in the room temperature area with quite good sensitivity and fast response

time. Therefore, they attract the interest of many research groups around the world to

develop into gas sensor components. However, the gas sensitivity of CPs is currently

recorded to be low compared to metal oxides.

A recent research trend that is particularly interested by scientists around the world is

the use of hybrid composite material families of CPs with precious metals (Au, Ag, Pt…); nano carbon (carbon nanotubes - CNT, graphene); and metal oxides (SnO2, ZnO, WO3, TiO2, In2O3, NiO…). This approach not only combines the individual advantages of each material, but also creates a composite (hybrid) material with specific properties for good gas

sensitivity (high sensitivity, performance). in room temperature zone, response time is fast).

A typical example is the hybridization of conductive polymers (PANi, PPy) with metal oxides (SnO2, ZnO, WO3, TiO2, In2O3, NiO) which showed a lot of valuable and interesting gas sensitivity to some gases. NH3, NO2, H2 [16-18]. However, the disadvantages of composite materials are not the stability in the organic-inorganic hybrid structures, the

complexity and the difficult to control of many material components. Furthermore, the

mechanism of the hybrid materials formation, the gas-sensitive mechanism of hybrid

materials, or the evidence of inorganic-organic hybridization is still unclear and needs to be

clarified. Therefore, in addition to finding new inorganic-organic composites, studies aimed

at enhancing the properties of known hybrid materials are not only carried out in the field of

gas sensors but also for other areas.

On these bases, the thesis has selected a topic with the called name "Synthesis and study of the hybrid materials between polypyrrole and nanostructured NiO for NH3 gas sensitivity". In which the main idea is to combine PPy (good NH3 gas sensitivity) with NiO (an oxide containing a 3d transition metal that has the nature to flexibly convert the valence

state), thereby forming an inorganic - organic hybrid material with good gas sensitivity characteristics to NH3 in the low temperature region (room temperature). In addition, with the same material fabrication method, the dissertation studied on PPy nanostructures as well

as hybridization between PPy and NiO nanoparticles to demonstrate the clear existence of the inorganic - organic hybrid structure.

2. The goal of the thesis

- The thesis examined the effect of different nanostructured morphologies in

conductive polymer films (PPy, PANi) on NH3 gas sensitivity.

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- The thesis fabricated the nanostructured hybrid films between metal oxide (NiO) and conductive polymer (PPy) successfully and examined the NH3 gas sensitivity properties of the NiO / PPy hybrid film.

CHAPTER 1: OVERVIEW

Chapter 1 introduces an overview about the history of formation and development of

conductive polymers which are currently being researched and applied popularly in many

fields, especially in the field of gas sensors. General knowledge of conducting gas sensors

based on conductive polymer materials and hybrid conductive polymers including structure, basic properties, gas sensitivity mechanisms and factors influencing the basic properties of a

conductivity sensor will be discussed later in this chapter.

1.1. Conducting polymer

Polymer is polymeric compounds, present everywhere. Conductive mechanism of conductive polymers (CPs): The first reason is due CPs with alternating single (C-C) and

double bonds (C = C) and the second reason is related to the presence of dopant.

1.1.4. Conductive mechanism of CPs and inorganic hybrid CPs

1.1.4.2. Conductiving mechanism of the doped polymer

Undoped polymers are insulators, but when polymers doped their conductivity can

vary from insulating to metal. Fig. 1.5 shows that the conductivity scale of doped PEDOT,

PPy, and PANi conductive polymers, some metals, semiconductors and insulators.

Figure 1.1. The position of the CPs (PEDOT, PPy, PANi) conductivity scale is

compared with some metals, semiconductors and insulators.

1.1.6. PPy synthesis method

Among the mentioned conductive polymers, PPy is a p-type organic conductive

polymer produced by the oxidation of the Py monomer. In fact, the synthesis of nanostructured PPy conductive polymers mainly follows two methods: electrochemical polymerization and chemical polymerization.

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1.2. Gas sensor based on conductive polymer

1.2.1. Introduction to gas sensors

b) Some basic parameters of gas sensor

- Sensitivity, Response

- Selectivity

- Stabilization

- Response and recovery time

1.2.5. Gas-sensing mechanism

a) Gas sensitivity mechanism based on conductive polymer materials

Interactions between gas and conductive polymer molecules are the mainly chemical

or weak interactions. The physical properties of the CPs depend strongly on the doping

level, and this level can change by the chemical reaction with the analyzed gas at room

temperature range. This makes a simple method for the analyzed gases detection by the

sensor based on conductive polymer operating at room temperature.

CHAPTER 2: STUDYING AND MANUFACTURING PPY AND PANI CONDUCTIVE POLYME FILMS FOR NH3 GAS SENSITIVITY

The grown PANi and PPy films directly on the electrode surface by two methods, cyclic voltammetry (CV) electrochemical and vapor phase polymerization to make products

with different surface morphologies such as nanoparticles, nanowire, nanofiber clusters or porous structures and they are examined with NH3 gas in dry air (80% N2 + 20% O2) at room temperature. To show that, aspects of the surface morphology and structure of the doped PPy and PANi conductive polymer films influence to the NH3 gas sensitivity properties of the NH3 gas sensor such as the response, selectivity and response/recovery time.

2.1. Synthesize and NH3 gas sensitivity characteristics of PANi morphological structures from the CV electrochemical method

2.1.1. Synthesized polyaniline film by the CV electrochemical method

The PANi films on Si/SiO2 substrate were synthesized by the CV electrochemical method using electrochemical equipment (Potentiostat). The Electrochemical system diagram for PANi films fabrication in this study is shown in Fig. 2.2, which includes 3 electrodes: WE,

RE and CE.

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Figure 2.2. Diagram of polymer film manufacturing system using CV electrochemical

method.

2.1.2. Morphological structure and properties of PANi films

b) SEM image of PANi films

Fig. 2.4 shows that the SEM images of the PANi films fabricated by electrochemical

method when we change the number of CV scan cycle corresponding to the sample names

PA-4, PA-7, and PA-12. This result shown that the morphological structure of the polymer

film gradually turns into a complete nano-fiber form and the fiber density increases when the

number of CV scanning cycles is changed from 4 to 12.

Figure 2.4. SEM image of the PA-4, PA-7 and PA-12 film samples.

Fig. 2.5 indicates the surface SEM images of prepared PANi film samples by electrochemical method when we change the different aniline monomer concentrations in electrolyte solution including 0.1; 0.2; 0.5 and 1.0 M, corresponding to the captioned

samples in the figure are PA-0.1 (a, b); PA-0.2 (c, d); PA-0.5 (e, f) and PA-1.0 (g, h).

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Figure 2. 5. SEM image of PANi films for the varied aniline monomer concentration of

(a,b) PA-0.1; (c,d) PA-0.2; (e,f) PA-0.5 and (g,h) PA-1.0.

Thus, the nanostructured PANi films have been successfully synthesized by CV

electrochemical method with the number of scanning cycles (4 -12) and the concentration of

aniline monomers (0.1, 0.2, 0. 5 and 1.0 M) in electrolyte solution. For the concentration of

aniline monomers from 0.1 to 1.0 M, the film morphology changes from nanoparticles to

nanofibers, and the PANi film density on the substrate becomes more uniform. Among the

above samples, sample PA-1.0 with uniform film distribution and nanofiber morphology is

considered to have lots of promises for good gas response.

c) FTIR spectra of the PANi film

Fig. 2.6 is the typical FTIR spectrum for the PANi film (sample PA-1.0) evaluated in

the wavenumber range from 500 to 2000 cm-1.

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Figure 2.6. FTIR spectra of PANi membrane fabricated by electrochemical method.

Analyzing from the FTIR spectrum indicates the typical bonds in PANi film films have been successfully synthesized on Pt that PANi

structure, proving microelectrodes, Si/SiO2 substrates by electrochemical method.

2.1.3. NH3 gas sensitivity characteristics of PANi morphological structures

PANi films are fabricated by electrochemical method with samples PA-0.1; PA-0.5 and PA-1.0 selected for the NH3 gas response resistance test when its concentration change were 350, 180, 90 and 45 ppm at room temperature (25 oC), as shown in Fig. 2.7a-c. The response of the PANi sensor films to the NH3 gas concentrations was calculated and shown in Fig. 2.7d. It can be seen that the response of each sensor increases as the concentration of NH3 gas increases. Response of the long fiber PANi sensor (sample PA-1.0) was the greatest, reaching of 53% at 350 ppm NH3. The response and recovery time of the sensors are calculated for the gas concentration of 350 ppm NH3 and shown in Fig. 2.7e, we observed that the response and recovery time are reduced from 171 to 87 s and 387 to 117 s,

respectively, corresponding to the variable morphological structure from nanofibers to nanoparticles.

The results showed that the PANi nanowires are synthesized in the thesis, they exhibited short response and quite along response/recovery time when exposed to NH3 gas at room temperature.

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Figure 2. 7. NH3 gas sensitivity characteristics with concentrations of 45, 90, 180 and 350 ppm at 25 oC of nanoparticle structure PANi films (PA-0.1); nanofibers short (PA-0.5) and long nanofibers (PA-1.0).

2.2. Synthesis and NH3 gas sensitivity characteristics of the PPy morphological structures from vapor phase polymerization method

2.2.1. Polypyrole film is synthesized by vapor phase polymerization method

Fig. 2.10 illustrates the fabrication of conductive polymer (PPy) film on Al2O3

substrate by polymerization method using FeCl3.

Figure 2.10. Steps to synthesize PPy films by vapor phase polymerization method.

2.2.2. Morphological structure and properties of PPy films

a) Morphological structure of the PPy films

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Fig. 2.11 shows SEM images of the synthesized PPy films for the different used FeCl3

concentrations of 0.01, 0.02, 0.04 and 0.06 M.

Figure 2.11. SEM images of the PPy films for the used FeCl3 concentrations of (a, A) 0,01 M, (b, B) 0,02 M, (c, C) 0,04 M, and (d, D) 0,06.

When increasing the FeCl3 concentrations from 0.01 to 0.06 M, the surface morphologies with nanoparticles-cluster, nanofibers-matrix and porous-structure were obtained. We also claimed that the PPy film’s density increased with increasing the used FeCl3 oxidant concentration.

c) FTIR spectrum

Fig. 2.14 shows the FTIR spectra of the synthesized PPy films for the used FeCl3

concentrations of 0.01, 0.02, 0.04 and 0.06 M in the band of 500 – 2000 cm-1.

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Figure 2.14. FTIR spectrum of the synthesized PPy films for the used FeCl3 concentrations

of 0.01, 0.02, 0.04 and 0.06 M.

The slight shifts of the C=C stretching vibration of pyrrole ring (1527 and 1548 cm-1) and the C–H in plane deformation vibration (1041 cm-1) between the synthesized PPy films tends to shift toward the higher wave count.

e) PL spectrum

Fig. 2.16 shows the PL spectra at room temperature of the fabricated PPy film

samples.It was also found that the broad bands near at 600 and 750 nm of the all PPy films

dominated in the spectrum, as assigned to doping/oxidizing states in the PPy structure.

Figure 2.16. PL spectrum of the PPy films for the used FeCl3 concentrations of 0.01, 0.02, 0.04 and 0.06 M.

Thus, it was believed again that the FeCl3 oxidant was a main role in contribution to

the doping and growing of the PPy films.

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2.2.3. NH3 gas sensitivity characteristics of PPy morphological structures

a) Response to NH3 gas at room temperature

Fig. 2.17a-d shows the typical result of resistance responses for the PPy films in corresponding to the used FeCl3 concentrations of 0.01, 0.02, 0.04 and 0.06 M when exposed to NH3/air cycles of 45, 90, 180 and 350 ppm at 25 oC. It was obvious that the resistances of all the samples clearly changed according to the each NH3/air cycle, and their resistance increased when exposed to NH3 gas. Thus, the PPy films could be considered as p-type semiconductor.

Hình 2. 17. Resistance responses of the PPy films for the used FeCl3 concentrations of (a) 0.01 M, (b) 0.02 M, (c) 0.04 M and (d) 0.06 M when exposed to NH3/air cycles of 350, 180, 90 and 45 ppm; and (e) dependence of the gas-sensing responses on the NH3 concentration.

The gas-sensing responses depending on the NH3 concentrations were calculated and showed in Fig. 2.18. It was found that the NH3 sensitivity decreased with increasing the used FeCl3 concentration. Notably, in comparison, the nanoparticles-cluster PPy film (0.01 M FeCl3) presented the much higher gas-sensing response while the other PPy films were almost the same.

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Figure 2.18. Responses dependence of the PPy_0.01MFeCl3, PPy_0.02MFeCl3, PPy_0.04MFeCl3, and PPy_0.06MFeCl3 films on the NH3 concentrations

The effect of PPy morphological structures on response and recovery times is studied on the synthetic PPy film samples at different FeCl3 concentrations when exposed to 350 ppm NH3 at room temperature, the results are shown in Fig 2.20.

Figure 2.20. (a) One resistance response cycle of PPy films and (b) dependence of the response and recovery times on the used FeCl3 concentration when investigated at 350 ppm NH3.

The result indicated that the PPy films were the fast response to NH3 gas. The response time decreased with increasing the used FeCl3 concentration. In similar, the recovery time decreased with increasing the used FeCl3 concentration, with an exception for the case of 0.01 M FeCl3. For all the PPy films, the response times were in the range of 3 – 10 s, and the recovery times were in range of 44 – 90 s.

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b) Reversible response properties and selectivity to NH3 gas at room temperature

Fig. 2.21 shows the resistance responses in repeat to 5 cycles of 350 ppm NH3/air of the synthesized PPy films with the FeCl3 concentrations of 0.01, 0.02, 0.04 and 0.06 M. The results indicated that all the PPy films exhibited a well reversible and repeatable gas sensing behavior. It was noticed that the PPy films for the higher used FeCl3 concentration presented higher repetitive behavior, which may be due to their high electrical conductance.

Figure 2.21. Resistance responses in repeat to 5 cycles of 350 ppm NH3/air of the PPy films at room temperature (a) PPy_0.01MFeCl3, (b) PPy_0.02MFeCl3, (c) PPy_0.04MFeCl3, (d) PPy_0.06MFeCl3.

For investigating the sensing selectivity, the PPy_0.01MFeCl3, PPy_0.02MFeCl3, and PPy_0.06MFeCl3 films for the used FeCl3 concentrations of 0.01, 0.02 and 0.06 M were compared their responses to some common gases (NO2, H2, CO, and CH4) and relative humidity. Fig. 2.22 indicates comparison of the typical sensing responses of the PPy films to 45 ppm NH3, 50 ppm NO2, 1000 ppm H2, 1000 ppm CO, 1000 ppm CH4 and 94 %RH. It was found that all the PPy films presented the highest response to NH3 gas, even in case of highly active oxidizing/reducing gases.

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Figure 2.22. Response of the PPy_0.01MFeCl3; PPy_0.02MFeCl3 and PPy_0.06MFeCl3 films when exposed to 45 ppm NH3, 50 ppm NO2, 1000 ppm H2, 1000 ppm CO, 1000 ppm CH4 and 94% RH.

c) Influence of operating temperature and humidity

Response and response/recovery time of the PPy_0.01M FeCl3 sample depends on temperature and are shown in Fig. 2.24. When the temperature increased from 25 to 100 oC, the response of PPy_0.01M FeCl3 sample decreased significantly (Fig. 2.24a), both response time and recovery time decreased from 10 to 4 s and 44 to 25, respectively.

Figure 2.24. (a) responses with NH3 gas concentrations and (b) response / recovery times of PPy_0.01MFeCl3 film depend on operating temperatures of 25, 60 and 100 °C.

Fig. 2.25a shows a typical result of resistance of the PPy film for the 0.06 M FeCl3 responding to the relative humidity concentrations of 11 %RH, 33 %RH, 75 %RH, 85 %RH

and 94 %RH. Fig. 2.25b indicated that dependence of the responses of the PPy films for the

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used FeCl3 concentrations of 0.01, 0.02, and 0.06 M on the relative humidity concentration (with the resistances in dry air (Ro) extracted for 0 %RH).

Figure 2.25. Typical resistance response of the porous PPy film when exposed to relative

humidity of 11–94 %RH (a); dependence of the humidity responses of the PPy films with the FeCl3 concentrations of 0.01, 0.02 and 0.06 M on the relative humidity concentration (b).

The results indicated that the relative humidity response increased with increasing the used FeCl3 concentrations. It was noted that the nanoparticles cluster PPy film presented the smallest humidity influence in comparison with the others.

2.2.4. Discussing on the gas-sensitive mechanism of PPy structures

For gas sensing mechanism, PPy can be considered as p-type semiconductor by doping. When interacting with reducing/oxidizing gases (e.g. NH3), the PPy can take electrons from donating molecules (NH3) which decreases the doping (denoted as Cl-), as showed in Eq. (2.1). The other mechanism can be related by the proton (H+) transfer between the PPy and NH3, as showed in Eq. 2. These mechanisms can be described as [140-142]:

(2.1)

+Cl

(2.2) PPy+/Cl + NH3 PPy+/Cl + NH3

 PPy0/NH3 +, Cl  PPy+(-H)0 + NH4

CHAPTER 3: SYNTHESIS AND STUDY ON NH3 GAS SENSITIVITY CHARACTERISTICS OF NiO / PPy HYBRID MATERIALS

Recently, researches in NH3 gas sensor have been focused on inorganic materials (metal oxides: SnO2, ZnO, TiO2 and NiO) and organic materials (conducting polymer: polypyrrole – PPy, polyaniline – PANi and poly (3, 4-ethylene-dioxythipphene) – PEDOT). The metal-oxides based sensor presented the high sensing response to NH3 gas, but they usually operate at high temperature (about hundreds degree Celsius) and poor selectivity.

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Although the conducting polymer based sensor had usually the lower sensing response in

comparison, they could exhibit some advantages such as operating at room temperature, easy

fabrication, low cost and flexible design of device structure. Mostly, polypyrrole (PPy) nanostructures have presented high selectivity to NH3 at room temperature, as reported in Refs. [50, 148, 149]. In the other approach, combinations between inorganic and organic to form hybrid structures have been explored to obtain the advantages for NH3 gas sensing performance [16, 50].

3.1. Synthesis of NiO/PPy hybrid films

Synthesis processes and gas sensitivity analysis of NiO/PPy hybrid films are

generalized as illustrated in Fig. 3.3.

Figure 3. 1. Demonstration of steps in the fabrication and gas sensitivity consideration of NiO/PPy hybrid materials on Al2O3 substrates.

3.2. Properties of the NiO/PPy hybrid films

3.2.1. Morphological structures

b) NiO/PPy hybrid films on Al2O3 substrates

Fig. 3.5a-e presented SEM images of the synthesized NiO/PPy hybrid films on Al2O3 substrates for the used FeCl3 concentrations of 0.15 M (a), 0.2 M (b), 0.4 M (c), 0.8 M (d), and 1.5 M (e). It was clearly seen that the surface morphology of the NiO/PPy hybrid films strongly depended on the used FeCl3 concentration. These results provided that different surface-morphologies of the NiO/PPy hybrid films from the simple vapor polymerization were obtained by regulating the used FeCl3 oxidant concentration.

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Figure 3.2. SEM images of the NiO/PPy hybrid films for the various used FeCl3 concentrations of 0.15 M (a); 0.2 M (b); 0.4 M (c); 0.8 M (d); 1.5 M (e), and the PPy film for used FeCl3 concentration of 1.5 M (f).

Fig 3.6a presented HRTEM image of NiO/PPy1.5M-FeCl3 film sample. NiO particles have the spherical nanostructure and are fairly uniform in size, the representation for the

PPy identified by HRTEM image is not very clear. NiO nanoparticles have good

crystallinity and are in the form of a cubic crystal structure.

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Figure 3.6. HRTEM image (a) and SAED pattern analysis of NiO nanoparticles (b) of the NiO/PPy1.5M-FeCl3 film sample.

3.2.3. FTIR spectra

Fig. 3.8 showed FTIR spectra of the synthesized films with NiO, NiO/PPy0.2M-FeCl3, NiO/PPy0.4M-FeCl3, NiO/PPy0.8M-FeCl3, NiO/PPy1.5M-FeCl3 and PPy1.5M-FeCl3 in wavenumber range of 400 – 2000 cm-1. For the NiO/PPy hybrid samples, these specific peaks of vibrations in structures of PPy and NiO appeared with slight shifts in wavenumber.

Figure 3.8. FTIR spectrum of the NiO, NiO/PPy0.2M-FeCl3, NiO/PPy0.4M-FeCl3, NiO/PPy0.8M- FeCl3, NiO/PPy1.5M-FeCl3, và PPy1.5M-FeCl3 film samples.

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3.2.4. Raman spectra

Fig. 3.9 presented Raman spectra of the film samples for NiO, NiO/PPy0.2M-FeCl3, NiO/PPy0.4M-FeCl3, NiO/PPy0.8M-FeCl3 and PPy1.5M-FeCl3 in the range of 400 – 1200 cm-1. It was notable that the one-phonon TO and LO modes (522 cm-1) became broaden and shifted toward longer wavenumber for the NiO/PPy hybrid samples when the PPy content increased.

This characteristic could be explained by in relation to defect or surface effect of NiO, when

hybrid with PPy. Based on the results, it was possible to believe that the hybrid structure

between PPy and NiO formed and existed in the synthesized film samples

Figure 3.9. Raman spectrum of the NiO, NiO/PPy0.2M-FeCl3, NiO/PPy0.4M-FeCl3, NiO/PPy0.8M- FeCl3, and PPy1.5M-FeCl3 film samples.

3.3. Gas sensitivity properties of NiO /PPy hybrid films

Fig. 3.10 showed resistance responses of the film samples for NiO/PPy0.2M-FeCl3, NiO/PPy0.4M-FeCl3, NiO/PPy0.8M-FeCl3, NiO/PPy0.8M-FeCl3 and PPy1.5M-FeCl3 when exposed to NH3/air cycles of 45, 90, 180 and 350 ppm at operating temperature of 25 oC. This result indicated that the resistances of these samples clearly changed and increased according to the exposed NH3 gas. Gas-sensing responses (S) of the film samples for NiO, NiO/PPy0.15M-FeCl3, NiO/PPy0.2M-FeCl3, NiO/PPy0.4M-FeCl3, NiO/PPy0.8M-FeCl3, NiO/PPy1.5M-FeCl3 and PPy1.5M-FeCl3 depending on the NH3 concentrations at operating temperature of 25 oC were calculated and showed in Fig. 3.11a. In comparison, the sensing responses for 350 ppm NH3 of the all film samples were illustrated in Fig. 3.11b. The result indicated that the NH3 sensing response was

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maximum value S  246 % for 350 ppm NH3 for the NiO/PPy0.2M-FeCl3 film and gradually decreased when increasing the used FeCl3 concentration.

Figure 3.10. Resistance response of the NiO/PPy0.2M –FeCl3; NiO/PPy0.4M–FeCl3; NiO/PPy0.8M– FeCl3; NiO/PPy1.5M–FeCl3 và PPy1.5M–FeCl3 film samples to cycles air/ NH3 for NH3 concentrations of 350, 180, 90 và 45 ppm at operating temperature of 25 oC.

Figure 3.11. Response of the NiO, NiO/PPy0.15M-FeCl3, NiO/PPy0.2M-FeCl3, NiO/PPy0.4M-FeCl3, NiO/PPy0.8M-FeCl3, NiO/PPy1.5M-FeCl3 và PPy1.5M-FeCl3 film samples as the funciton of NH3 concentration (a); and comparing gas sensor responses for 350 ppm NH3 of all film samples (b).

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Figure 3.12. Resistance responses in repeat to 4 cycles of 350 ppm NH3/air of the films of NiO/PPy0.2M-FeCl3, NiO/PPy0.4M-FeCl3, NiO/PPy0.8M-FeCl3, NiO/PPy1.5M-FeCl and PPy1.5M-FeCl3 at operating temperature of 25 oC.

Fig. 3.12 showed the resistance responses in repeat to 4 cycles of 350 ppm NH3/air of the films of NiO/PPy0.2M-FeCl3, NiO/PPy0.4M-FeCl3, NiO/PPy0.8M-FeCl3, NiO/PPy1.5M-FeCl and PPy1.5M-FeCl3 at operating temperature of 25 oC. The results indicated that all the films exhibited a well reversible and repeatable gas sensing behavior.

Fig. 3.13a indicated dependence of gas sensing response of the NiO/PPy0.2M-FeCl3 film on NH3 concentration at operating temperatures of 25, 40, 50, 70, 90 and 110 oC. The response presented a light decrease with increasing the operating temperature. However, the

response was observed to be nearly independent on the operating temperature from 70 to 110 oC, and the film resistance begun to be unstable when the operating temperature increase above 130 oC due to the NiO/PPy structure destroyed. The response and recovery times (90) depended on the operating temperature of the NiO/PPy0.2M-FeCl3 film were calculated and presented in Fig. 3.13b. The result showed that the response time increased with increasing the temperature in range of 25 – 70 oC, and then slightly decreasing above 70 oC while the recovery time decreased with increasing the temperature in whole range. Magnitudes on the NH3 gas-sensing response (S) were approximately 160 – 250% for 350 ppm, while the response and recovery times (90) in range of 10 – 50 s and 30 – 210 s, respectively.

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Hình 3.13. The sensor response as a function of NH3 gas concentration (a) and the response / recovery times for 350 ppm NH3 (b) of NiO / PPy0.2M-FeCl3 film sample at operating temperatures of 25, 40. , 50, 70, 90 and 110 oC.

Fig. 3.14 indicated the typical comparison of gas-sensing responses of the NiO/PPy0.2M-FeCl3 film to 45 ppm NH3, 25 ppm NO2, 1000 ppm H2, 1000 ppm CO, and 94 %RH at different operating temperatures of 25, 50 and 90 oC.

Figure 3.14. Comparing sensing responses of NiO / PPy0.2M-FeCl3 film at temperatures 25; 50 and 90 oC for gases to 45 ppm NH3, 25 ppm NO2, 1000 ppm H2, 1000 ppm CO and 94% RH.

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The result indicated that the NiO/PPy0.2M-FeCl3 film was the very high selectivity at low temperature (25 oC), however, the selectivity strongly deceased with the higher operating temperatures (50 and 90 oC).

Gas-sensing mechanism:

In case of the synthesized NiO/PPy films, the gas sensing performance could be

considered by contributions of three agents including NiO, PPy and NiO-PPy hybrid.

The proposed model for NiO/PPy hybrid films with partial resistance components and

energy band structure is shown in Fig. 3.15. Basing on this model, the resistance of the

NiO/PPy films are contributed to gas sensitivity by three main components connected in parallel including: the resistance of the NiO nanoparticles is RNiO, the resistance of the PPy is the RPPy, and the resistance of the contact regions of NiO/PPy hybrid film is RNiO/PPy. Resistance response to NH3 gas of NiO / PPy hybrid film is resolved by RNiO/PPy.

Figure 3.15. Illustrated model for 3 resistance components (a) and energy band structure (b)

of the NiO / PPy hybrid films.

23

GENERAL CONCLUSION

1. The nanostructures of PANi and PPy:

 PANi films have been successfully synthesized by the electrochemical CV method on the surface of the Pt/(Si/SiO2) microelectrodes with the nanostructured morphologies (particles, bars and fibers). NH3 gas sensitivity characteristics at room temperature of the morphological structures showed that the responses are still quite

low, the response/recovery times are quite

 PPy films have been successfully fabricated by the simple vapor phase

polymerization method with the various morphology structures (nanoparticles, nanofiber networks and porous structures). For FeCl3 plays an important role for the electrical conductivity of PPy films. This expressed through resistance reductions of PPy films when increasing the used FeCl3 concentrations.

 The morphological structure and electrical conductivity of PPy films have shown strong influences on gas sensitivity characteristics. All samples have quite good responses, good reversible gas interactions, and short response/recovery times to NH3 at room temperature. In which, the PPy nanoparticle cluster was for the greatest response (reaching of S = 2.15 for 350 ppm NH3 at 25 oC), the nanofiber showed the best selectivity, and the porous structure is affected better by relative humidities.

2. Hybrid structure of PPy with NiO nanoparticles:

 Hybrid structure films between NiO nanoparticles and PPy have been successfully fabricated on Al2O3 substrates by vapor phase polymerization method with their different ratios when using various FeCl3 concentrations.

 Evidence of the existence of a hybrid structure between NiO and PPy is clearly shown through the studies of FTIR spectrum, Raman spectrum, and gas sensitivity.

 NiO/PPy hybrid film samples exhibited good gas sensitivity (with high gas-sensing responses, good reversible responses, short response and recovery times, and high selectivity for NH3 gas at the room temperature of 25 oC). The good response characteristic of the NiO/PPy film samples to NH3 gas was explained by the main contribution of the "NiO/PPy hybrid structure" when the NiO/PPy0.2M-FeCl3 sample had a dominant gas response (up to S = 3.46 for 350 ppm NH3 at 25 oC).

24

NEW CONTRIBUTIONS OF THE THESIS

 The thesis is shown a simple material synthesis method from vapor phase polymerization through adjusting FeCl3 oxidation concentrations and distributions on Al2O3 substrates to fabricate PPy films successfully with the different morphology structures including: nanoparticles, nanofiber networks, and porous structures. At the

same time, it is also clearly presented the influence of surface morphology structures on conductivities and NH3 gas sensitivity characteristics of PPy films at room temperature.

 The thesis has successfully hybridized between NiO nanoparticles and PPy by simple vapor phase polymerization method for using FeCl3 and found out the optimal hybridized ratio for the good NH3 gas sensitivity characteristics at room temperature with the highest response was reached of (S = 3.46 for 350 ppm NH3 at 25 oC) compared with pure PPy or NiO films.

 The thesis has contributed to clarify the properties and gas sensitivity mechanism of

inorganic - organic hybrid materials at room temperature.

25

PUBLISHED LISTS OF THE THESIS

1. Hoang Thi Hien, Chu Van Tuan, Do Thi Anh Thu, Pham Quang Ngan, Giang Hong Thai, Sai Cong Doanh, Ho Truong Giang, Nguyen Duc Van, Tran Trung, Influence

of surface morphology and doping of PPy film simultaneously polymerized by

vapour phase oxidation on gas sensing, Synthetic Metals, 250 (2019) 35-41. 2. Hoang Thi Hien, Do Thi Anh Thu, Pham Quang Ngan, Giang Hong Thai, Do Thanh Trung, Tran Trung, Man Minh Tan and Ho Truong Giang, NH3 gas high sensing performance of NiO/PPy hybrid nanostructures, Sensors and Actuators B, (2020),

under review.

3. Hoàng Thị Hiến, Ngô Thành Hiếu, Phạm Quang Ngân, Giang Hồng Thái, Đỗ Thị Anh Thư, Đỗ Thanh Trung, Lê Ngọc Thành Vinh, Mẫn Minh Tân, Trần Trung, Hồ

Trường Giang, Chế tạo cấu trúc nano lai vô cơ-hữu cơ NiO/PPy định hướng tăng

cường nhạy khí NH3 tại nhiệt độ phòng, Hội nghị Vật lý Chất rắn và Khoa học Vật

liệu Toàn quốc – SPMS 2019, p. 766-769.

4. Hoàng Thị Hiến, Phan Thế Dương, Trần Viết Thứ, Đỗ Thị Anh Thư, Giang Hồng Thái, Hồ Trường Giang, Trần Trung, Chu Văn Tuấn, Nghiên cứu tổng hợp

polypyrrole từ pha hơi cho nhạy khí NH3 tại nhiệt độ phòng, Hội nghị Vật lý Chất

rắn và Khoa học Vật liệu Toàn quốc – SPMS 2017, p. 496-498.

5. Hoàng Thị Hiến, Phan Thế Dương, Chu Văn Tuấn, Trần Viết Thứ, Đỗ Thị Anh Thư, Giang Hồng Thái, Hồ Trường Giang, Trần Trung, Tính chất nhạy khí NH3 tại nhiệt

độ phòng của polyaniline và polypyrrole được tổng hợp từ pha hơi, Hội nghị Vật lý

Chất rắn và Khoa học Vật liệu Toàn quốc – SPMS 2017, p. 505-507.