Chế tạo và nghiên cứu tính chất quang học của vật liệu tio2 có cấu trúc nano pha tạp ion đất hiếm tt tiếng anh

The research object is nano structure TiO2 material doped with the rare earth ions.. 2 systematically presents research results on the physical properties of nano TiO2 materials doped wi

ACKNOWLEDGMENT OF THE AUTHOR

I guarantee this is my own research work, conducted under the guidance of Associate Professor Nguyen Manh Son, at the Department of Physics, University of Sciences, Hue University The data and results of the thesis are guaranteed to be accurate, truthful and have never been published in any other works

I also certify that I do not submit this PhD thesis to any other training institution for a degree

In: Hue, Vietnam

On:

Signature:

SUMMARY

Due to many anomalous properties and its applicability in various fields, nano-sized TiO2 has been

of interest to scientists Nano TiO2 is an important agent in photocatalytic [7], [28], converting solar energy into electrical energy [26], [27], photosynthesis of water into hydrogen fuel [21], [ 32], [66], [88]

With its high thermal stability, durability, non-toxicity and outstanding optical properties, structure TiO2 is considered a potential new substrate for doping rare earth ions (RE) The transfer of energy from nano TiO2 to rare earth ions is made easier because they have many energy levels For example, 5D1 →

nano-7

F1, 5D0 → 7FJ transitions (J = 0, 1, 2, 3, 4) of Eu3+ ions will emit radiation in the visible region at 543, 579,

595, 615, 655 and 701 nm [73], [81] Because TiO2 has many polymorphs and RE ions have a special electronic structure, therefore, studying their luminescent properties will bring new information

Thus, the study of the above issues is not only scientifically significant but also practical So far, the question of the energy transfer mechanism between TiO2 network with different crystal structure and RE ions, as well as the position of RE ions in TiO2 network is still questionable The inverted fluorescence effect (for Stocks) of RE ions in the nano TiO2 network is an attractive research object [44], [87] Nanomaterials are characterized by physical and chemical properties that depend on size and structure Meanwhile, size, structure and its applicability depend on manufacturing technology Therefore, to be proactive in researching and applying properties of materials in real life, we focus on developing technology to product nano TiO2 by ultrasonic – hydrothermal, and sulfuric acid methods These are simple methods of synthesizing materials, low cost, suitable for laboratory conditions of training institutions

For the above reasons, we chose the thesis topic: “Synthesis and investigation optical properties

of TiO2 nanostructure doped with earth rare ions”

The research object is nano structure TiO2 material doped with the rare earth ions Research content includes:

Basic research:

 Research and fabricate rare earth TiO2 nanoparticles with sulfuric acid method and ultrasonic - hydrothermal method

 Study the effect of manufacturing conditions on the structure, microstructure and spectral properties

of RE3+ TiO2 doped materials when heated at different temperatures

 Study the energy transfer effect between TiO2 network and trigger centers

 Study the fluorescent effect of nanoTiO2 doped RE

 Calculate and simulate the energy band structure of TiO2, nano TiO2 doped RE by the density function theory (DFT)

Application development research, We focus on photocatalytic ability of TiO2 nano and doped TiO2 materials The theoretical and practical meanings are reflected in the achieved results The thesis

2

systematically presents research results on the physical properties of nano TiO2 materials doped with rare earth ions The results of the thesis are new contributions in terms of basic research and application of this material

The main contents of the thesis are presented in 4 chapters

Chapter 1 Overview of theory;

Chapter 2 Manufacturing technology, structure, microstructure of TiO2 nano materials doped rare earth ions (Eu3+, Sm3+);

Chapter 3 Spectroscopic characteristic of TiO2 nanoparticles doped RE3+ ions;

Chapter 4 Application of nano TiO2 in photocatalytic;

CHAPTER 1 LITERATURE REVIEW 1.1 OVERVIEW OF NANO – STRUCTURED TiO2

1.1.1 Introduction of nano-structured TiO2

1.1.1.1 The structural forms and physical properties of TiO2

TiO2 are a typical semiconductor, formed at high temperatures when Ti reacts with O The most typical and stable oxidation state of Ti is + 4 (TiO2) due to Ti4 + ions having a durable configuration of noble gases (18 electrons) In addition, Ti can exist in the lower oxidation states of +2 (TiO) and +3 (Ti2O3), but it is easier to switch to a more stable +4 state

Depending on the fabrication conditions, TiO2 can have anatase, rutile, brookite or all three types of polymorphic structure, in which the anatase and rutile structures are the most common (Figure 1.1)

Figure 1 1 Anatase and rutile structure of TiO2

These two structures differ due to the deformation of each octahedron and the way in which the octahedra are connected Each Ti4+ ion is in an octahedron surrounded by 6 O2- ions The octahedral mass corresponding to the rutile phase is uneven due to the weak diamond face deformation, while, the octahedral

of the anatase phase is strongly deformed Therefore, the symmetry of the anatase system is lower than the symmetry of the rutile system Differences in the network structure of TiO2 create differences in density, energy region structure and a range of other physical properties between anatase and rutile phases

1.1.1.2 Energy band structure of TiO2

TiO2 is a semiconductor with a relatively large band gap, the valence band filled with electrons, and the blank conductivity TiO2 in the anatase phase has a band width of 3.2 eV, which corresponds to the

energy of a light quantum with a wavelength of about 388 nm, while TiO2 rutile phase has a band width of 3.0 eV corresponding to the energy amount of a light quantum with a wavelength of about 413 nm

Figure 1.3 Diagram of the energy band structure of TiO2

1.1.1.3 Applications of nano TiO2

+ Application in the field of photocatalyst

Due to its extremely strong photocatalytic effect, nano-sized TiO2 is effectively used to treat the environment [18], [57], [60]

+ Application of color-sensitive solar cells (DSSC)

TiO2 can absorb light in the visible region and convert solar energy into electrical energy for applications in solar cells [11], [26], [62]

1.1.2.2 Sol – gel method

The sol - gel method is the process of converting sol into gel with two stages: sol and gel generation Synthesis of nano TiO2 by this method we can obtain materials with the desired state such as mass, embryo film, fiber and powders of uniform size [10], [31], [53], [58 ], [71], [77]

1.1.2.3 Microwave method

e

-e

-λ ≤ 413 nm Vùng cấm Vùng dẫn

Vùng hóa trị

e

-e

-λ ≤ 388 nm Vùng cấm Vùng dẫn

Vùng hóa trị

4

When using the microwave method, the heat addition by creating molecular vibrations at very high speed Rapid and uniform heating, which is similar to hydrothermal processes at high temperatures Heat is generated by friction between molecules and the conversion of microwave energy into heat The advantage

of this method is that the synthesis is fast, simple and easy to repeat [84]

1.1.2.4 Ultrasonic method

The methods using ultrasound waves (called ultrasound method for short) are new approaches developed in recent years [74] This method uses high-power ultrasound source to create chemical reactions through the cavitation effect

1.1.2.5 Electrochemical method

The electrochemical method is an important method in the synthesis of TiO2 nanotubes in tubes, fibers or films [80], [54], [52] In general, the electrochemical method has good control over the shape and size of nano TiO2 materials by creating anodic molding

1.2 OPTICAL CHARACTERISTICS OF RARE EARTH IONS

1.2.1 Overview of rare earth elements

Rare-earth elements (RE) are elements of the Lanthan group, characterized by the 4f-filled electronic layer shielded by the outer-filled electronic layer of 5s2 and 2p6 Therefore, the effect of the master lattice field on the optical shifts in the 4f n configuration is small (but necessary)

Rare-earth elements: Ce, Pr, Nd, Pm, Eu, Gb, Tb, Dy, Ho, Er, Tm, Yb have atomic numbers from 58

to 70 which play a very important role in the luminescence of crystals Diagram of the energy level structure

of valence rare earth ions, also called Dieke diagram (Figure 1.4)

Figure 1.1 Energy level diagram of RE3+ ions - Dieke diagram

1.2.2 Optical characteristics of Europium and Samarium

1.2.2.1 Optical characteristics of Europium

Europium (Eu) is a rare earth element of the Lantanite family in the 63rd cell (Z = 63) in the Mendeleev periodic table Europium usually exists in the form of valence 2 and valence oxides, but in the trivalent form (Eu2O3) is more common Electronic configuration of atoms and ions:

Eu: 1s 2 2s 2 2p 6 …(4f 7

)5s 2 5p 6 6s 2

Eu2+: 1s 2 2s 2 2p 6 … (4f 7 )5s 2 5p 6

Eu3+: 1s 2 2s 2 2p 6 … (4f 6 )5s 2 5p 6

The emission spectra of Eu2+ ions and of Eu3+ ions are shown in Figure 1.5

Figure 1 2 The emission spectra of Eu2+ ions and Eu3+ ion

1.2.2.2 Optical characteristics of Samarium

Samarium (Sm) is a rare earth element of the Lantanite family located in the 62nd cell (Z = 62) in the Mendeleev periodic table Samarium usually exists in the form of Sm2O3¬ oxide, a solid crystalline structure, pale yellow, with a cubic structure Electronic configuration of atoms and ions:

Sm (Z=62): 1s 2 2s 2 2p 6 …(4f 6

)5s 2 5p 6 6s 2

Sm3+: 1s 2 2s 2 2p 6 …(4f 5

) 5s 2 5p 6 The emission spectra of Sm3+ ion is located in the orange-red region, corresponding to transitions: 4 G 5/26

H J (J =

5/2; 7/2; 9/2; 11/2; 13/2; 15/2) (figure 1.6)

550 575 600 625 650 675 700 725 750 0.0

0.5 1.0 1.5 2.0 2.5 3.0 3.5

E Coli [3] The authors of Thai Thuy Tien, Le Van Quyen, Au Van Tuyen, Ha Hai Nhi, Nguyen Huu Khanh Hung, Huynh Thi Kieu Xuan studied synthesizing TiO2 nanotubes by electrochemical method applied in photocatalyst [4] Only Le Viet Phuong, Nguyen Duc Chien and Do Phuc Hai (ITIMS) have studied the optical properties of Ca1-xEuxTiO3 red light-emitting material

The study optical properties of rare earth ions on nano TiO2 has not been much researched in Vietnam

1.3.2 Research situation of scientific issues abroad

Nano TiO2 materials are very much interested in research by many scientists around the world Since

1994, D Philip Colombo et al Synthesized nano TiO2 by sol - gel method [55] With many outstanding physical properties, especially when doped into this network, some metal or nonmetal ions to change the structure as well as geometry, nano TiO2 has brought many practical applications In 1997, Md Mosaddeq-ur-Rahman et al Synthesized lead-doped nano TiO2 (Pb) for solar cell fabrication applications [51] Shi-Jane Tsai, Soofin Cheng studied photocatalyst properties of nano TiO2 to decompose phenolic [69] In the following years, nano TiO2 was soon applied in other fields such as making electrodes for electronics and biomedical applications [13], [41] In addition, scientists have sought to manipulate the size and geometry of nanomaterials to meet specific research objectives in basic and applied research Although researched and applied very early in many fields, but today, nano TiO2 is still an attractive and topical research object

In 2007 Jie Zhang, Xin Wang, Wei-Tao Zheng, Xiang-Gui Kong, Ya-Juan Sun and Xin Wang studied the fabrication of Er3+ doped nano TiO2 by chemical method combining heat treatment in different modes The authors obtained TiO2 material: Er3+ hollow sphere As the heat treatment time increases, the thickness and smoothness of the shell increases, the connection between the orbs increases When heated to 8000C, transfer phase anatase - rutile formed in TiO2 material However, they do not occur in Er3+ doped TiO2 material This result shows that Er3+ ions play an important role in preventing this phase transition [83]

In 2008, Quingkun Shang and colleagues studied the reverse conversion effect of Eu3+ - Yb3+ in nano-TiO2 based fabrication by sol-gel method The authors found two emission bands in the region of 520 -

570 nm (2H11/2, 4S3/2 - 4I15 / 2) and 640 - 690 nm (4F9 / 2 - 4I15 / 2) when stimulated by wavelength laser

980 nm [64] Chenguu Fu studied the fluorescence spectrum of Er3+ doped TiO2 by wet chemical method The author has observed relatively strong narrow line luminescence in the infrared region of about 1.53 μm The author states that it is the luminescence of the Er3+ ion occupying the lattice position in the nano TiO2

crystal and is the result of the energy transfer from the TiO2 background lattice to this spurious [15]

In 2017, Vesna ĐorđevićBojana, Bojana Milicevic and Miroslav D Dramicanin published an depth overview of TiO2 nano manufacturing methods and optical properties of nano TiO2 doped with rare earth ions [72] This report has shown that introducing trivalent rare earth ions into the nano TiO2 network has changed the structure and some physical properties of the system In addition, because TiO2 (anatase)

in-with a band gap of about 3.2 eV, while the energy gap (from the ground state to the lowest stimulus level) of rare earth ions is relatively large, there is only one rare earth (Nd3+, Sm3+, Eu3+, Ho3+, Er3+, Tm3+, Yb3+) when doped into this substrate causes luminescence phenomenon

CHAPTER 2 MANUFACTURE TECHNOLOGY, STRUCTURE AND MICROSTRUCTURE OF RARE EARTH

(Eu 3+ , Sm 3+ ) DOPED NANO TIO2 MATERIAL 2.1 SYNTHESIS NANO TiO2 MATERIAL

2.1.1 Synthesis of nano TiO2 by ultrasonic - hydrothermal method

Using hydrothermal ultrasound method to synthesize nano TiO2 has been interested in research by domestic and foreign scientists because this method has many outstanding advantages, simple manufacturing process, and easy to repeat The structure of the material after fabrication is in the form of nanotubes or

nanorods with sizes of several nanometers

Add TiO2 powder (anatase, Merck 98%) to 16 M NaOH solution (Merck) by ratio TiO2: NaOH = 1:

2 ratio The mixture is dispersed by ultrasonic power of 100W during 30 minute The mixture was hydrothermal at 150°C for 16 hours The mixture after hydrothermal process is neutralized with 0.1 M HCl solution, then washed several times to remove unwanted components and dried at 70 ° C for 24 hours The final product obtained is TiO2.nH2O which is thermal treatmented at different temperatures between 250°C and 950°C for 2 hours

2.1.2 Synthesis of nano TiO2 by using sulfuric acid method

The mixture of TiO2 and H2SO4 solution (98%) in the ratio TiO2 (g): H2SO4 (mL) = 1: 2 is dispersed

by ultrasonic power of 100W for 15 minutes, then Heat at 100oC for 1h After being heated, the mixture is hydrolyzed and neutralized with NH4OH solution to a pH of 8, creating a white precipitate, and then washed several times to remove unwanted components then dry at 70oC for 24 hours The final product obtained is TiO2.nH2O powder This powder is processed at temperatures between 250oC and 1000oC for 2 hours

2.1.3 Fabrication of RE doped nano TiO2 materials

RE3+ ion-doped TiO2 nanomaterials are fabricated in 2 steps

+ Fabrication of nano TiO2 solution Add 0.5 gram of TiO2.nH2O powder to a mixture of 20 ml of

H2O2 solution and 10 ml of NH4OH When TiO2 is completely dissolved, add 20 ml of H2O

+ Fabrication of nano TiO2 doped RE3+ ions Dissolve RE2O3 in HNO3 solution with a sufficient amount of distilled water to obtain a 0.01 M salt solution Finally, add Eu(NO3)3 or Sm(NO3) 3 solution to TiO2 solution with different concentration ratios (RE3+ / (Ti + RE)) (from 0.1% mol to 15% mol) The mixture is then stirred with magnetic stirrer combined with heating to regain the mixture in powder form This powder is heat-treated at different temperatures (from 350oC to 950oC) for 2 hours

2.2 STRUCTURE AND MICROSTRUCTURE OF RE DOPED NANO TiO2

2.2.1 Structure and microstructure of nano TiO2

2.2.1.1 Microstructure of nano TiO2

Nano TiO2 materials after fabrication by ultrasonic - hydrothermal method and the method of using sulfuric acid with sizes from several nm to several tens of nm are shown by TEM anhier in Figures 2.5 and 2.6 The shape and dimensions of the samples depend on the technological conditions and the fabricating material method

8

Figure 2 1 TEM image of nano TiO2 prepared by ultrasonic - hydrothermal method at 550oC for 2 hours

Figure 2 2 TEM image of nano TiO2 prepared by using sulfuric acid calcined at 550oC for 2 hours From TEM images in Figures 2.5 and 2.6, it has been shown that TiO2 is synthesized by both methods with high uniformity, ranging in size from a few nm to several tens of nm Samples fabricated by hydrothermal method have the form of nano bar While, samples manufactured using sulfuric acid are

spherical in shape

2.2.1.2 The crystal structure of nano TiO2

Usually, the crystal structure of a material depends on technological factors such as sample heating temperature and manufacturing method The following is an X-ray diffraction diagram of samples heated at different temperatures from 250°C to 950°C for 2 hours

A R A R

20 30 40 50 60 70 80 90

A A A A A

R R

R - rutile

Figure 2 9 X-ray diffraction diagram of nano TiO2 fabricated by by using sulfuric acid method X-ray diffraction diagrams in Figures 2.8 and 2.9 show that, when the sample heating temperature is lower than 350°C, the nano TiO2 samples have an amorphous structure, when the temperature ranges from 350°C to less than 650°C the nano TiO2 particles have crystal anatase phase structure characterized by diffraction peaks at angles 2θ equal 25,28o; 37.78o; 48.05o; 54.1o; 55.01o; 62,61o; 68.9o; 70.7o and 75.3o have Miller numbers, respectively (101), (004), (200), (105), (211), (204), (116), (220) and (215) [7], [17], [70], [50], [79], [82], [76], [9] When the calcination temperature of the sample is about 650oC, the rutile phase is formed which is characterized by diffraction peaks at 2θ equal 27.41; 36.05; 41.34; 54.32; and 68.99, respectively, with Miller indexes (110), (101), (111), (211), and (301) [70], [50], [82], [76The ratio of anatase phase, XA, in the material is calculated by equation (2.1) [23], [50]:

where is a constant with a value of 0.89 (in case of fabrication by hydrothermal method) and 0.9 (fabricated by acid method)); is the wavelength of X-ray radiation ( 1,5406 Å), is the half-width of diffraction peak (101) for the anatase phase and (110) for the rutile phase, is the diffraction angle corresponding to the vertices (101) and (110)

The phase ratio and particle size of materials manufactured by two different methods are shown in Tables 2.1 and 2.2

Table 2.1 The ratio of anatase phase (XA), rutile (XR) and crystal size (D) of TiO2 are synthesized by

ultrasonic - hydrothermal method

10

Table 2.2 The ratio of anatase phase (XA), rutile (XR) and crystal size (D) of TiO2 are synthesized by using

sulfuric acid method Temperature (oC) 350 450 550 650 750 850 950

516 609 637

(a)

TiO 2 350 o C TiO 2 550 o C TiO 2 750 o C TiO 2 850 o C TiO 2 1000 o C

DÞch chuyÓn Raman (cm -1 )

235

294 447

516

609 637

(b)

Figure 2 4 The Raman spectrum of TiO2 is made by ultrasonic - hydrothermal method (a), sulfuric acid

method (b) From the Raman spectra, we found that for the TiO2 samples heated at 350°C and 550°C the Raman peaks appeared at 144.1; 198; 394.4; 516 and 637.7 cm-1 correspond to the Eg, Eg, B1g, A1g, and Eg oscillation modes of the anatase phase For samples calcined at 950°C, Raman peaks appear at 142; 447 and

609 cm-1 correspond to the vibration modes B1g, Eg, and A1g of the rutile phase, the mode at 235 cm-1corresponds to the lattice vibrations of many phonons (Figure 2.12) [82], [76], [33], [24], [34], [43], [12], [61] The Raman analysis results are completely consistent with the X-ray diffraction analysis as presented

Figure 2 5 Absorption spectrum of TiO2 samples according temperature Figure 2.13 is the UV-Vis absorption spectra of TiO2 prepared by method of using sulfuric acid From here, it is allowed to determine the band gap of the material according to Kubelka Munk theory [19], [46], [59], [63] The results of calculating the band gap of TiO2 samples are listed in Table 2.3

Table 2 3 Energy band gap of TiO2

2.2.2 Structure, microstructure of RE 3+ doped nano TiO2

2.2.2.1 Microstructure of of RE 3+ doped nano TiO2

TEM image of 1% molar TiO2 doped Ti: Eu and TiO2 doped with 1% mol Sm3+ calcined at 500oC is shown in Figures 2.14 and 2.15 TEM images show that the samples are about 10 to 20 nm in size This is consistent with the result of calculating particle size from diffraction spectrum by the Debye - Scherrer equation

Figure 2 6 TEM images of TiO2:Eu3+ (1% mol) calcined at 500oC taken at different positions

Figure 2 7 TEM images of TiO2:Sm3+ (1% mol) calcined at 500oC taken at different positions Morphologically, RE-doped samples are generally spherical in shape similar to un-doped samples The Eu3+ doped samples show clumping and the image of particles is not clear Whereas, the Sm3+ doped samples have a very clear image, the separated particles are more like the less doped samples

2.2.2.2 The crystal structure of the RE doped nano TiO2

The crystal structure of the RE doped nano TiO2 material was investigated through X-ray diffraction (XRD), Raman spectrometry and UV-Vis absorption spectrometry at room temperature X-ray diffraction measurement of Eu3+ and Sm3+ doped samples heated at 550oC with a concentration of 0.1% mol to 6% mol

is depicted in Figure 2.16

12

Figure 2 8 X-ray diffraction diagram of TiO2:Eu3+ (a), TiO2:Sm3+ (b) according to the doped concentration

is calcined at 550oC for 2 hours

Figure 2 9 X-ray diffraction diagram of TiO2: Eu3+ (1% mol) (a), TiO2: Sm3+ (1% mol) (b) heated from

450oC to 950oC

From the X-ray diffraction scheme, it is shown that samples heated at 450°C are mostly amorphous Meanwhile, un-doped TiO2 samples have anatase structure When increasing the temperature of sample heating, TiO2 doped materials with 1% mol Eu3+ and TiO2 doped with 1% mol Sm3+ have anatase phase crystal structure with increasing crystallinity with calcination temperature Eu3+ doped samples have higher anatase phase crystallinity When the calcination temperature reaches 750oC, there is the appearance of rutile phase From the temperature range of 750oC to 950oC, the Eu3+ and Sm3+ doped samples have a crystal phase structure, which is a mixture of anatase and rutile phases Eu3+ doped samples have higher rutile crystallinity than Sm3+ doped samples This is shown by the observation that at the same firing temperature, the peak at 27.41° corresponds to the lattice (110) of the rutile phase of the Eu3+ doped samples than the Sm3+ doped samples In addition, we also use the Debye - Scherrer equation to calculate the particle size for the above samples Anatase - rutile phase ratio and crystal size are listed in Table 2.4: