Effect of ctab surfactant on properties of fepd nanoparticles

VIETNAM NATIONAL UNIVERSITY, HANOI VIETNAM JAPAN UNIVERSITY PHAM QUANG ANH EFFECT OF CTAB SURFACTANT ON PROPERTIES OF FePd NANOPARTICLES MASTER’S THESIS VIETNAM NATIONAL UNIVERSITY, HANOI VIETNAM JAPAN UNIVERSITY PHAM QUANG ANH EFFECT OF CTAB SURFACTANT ON PROPERTIES OF FePd NANOPARTICLES MAJOR: NANOTECHNOLOGY CODE: 8440140.11QTD RESEARCH SUPERVISORS: Prof Dr Sc NGUYEN HOANG LUONG Dr LUU MANH QUYNH Hanoi, 2022 COMMITMENT I have read and understood the plagiarism violations I pledge with personal honor that this research result is my own and does not violate the Regulation on prevention of plagiarism in academic and scientific research activities at VNU Vietnam Japan University (Issued together with Decision No 700/QD-ĐHVN dated 30/9/2021 by the Rector of Vietnam Japan University) Hanoi, 2022 Author Pham Quang Anh ACKNOWLEDGEMENTS At the end of the two years I have spent at Vietnam Japan University, there are a lot of people that I am deeply thankful towards Every moment passed, I realized that I was lucky to work with these wonderful people First of all, I have to mention Prof Dr Sc Nguyen Hoang Luong, who was generous enough to offer me an opportunity to work under his guidance and let me delve deeper into a topic that I have been interested in for a long time Despite his busy schedule, his door was always open to me so I could be honest with him about any troubles Secondly, I would like to express my gratitude and admiration towards Dr Luu Manh Quynh, who has been my boss, my teacher, my mentor, and my inspirer He was always the first person I shared my ideas with and, each and every time, he gave me the motivation I needed to dig deeper and think broader His optimism and willingness to appreciate new viewpoints made every discussion highly productive and full of creativity Most of my research duration was spent at Faculty of Physics, Hanoi University of Science I would like to thank Assoc Prof Dr Nguyen Hoang Nam, Dr Truong Thanh Trung, Mr Hoang Van Huy, who helped me with various experimental setups and gave me valuable advice I am also truly grateful to my classmates at Vietnam Japan University, Ms Huong Giang, Ms Hai Ly, Ms My Nga and Mr Hoang Giang I am happy that along this rocky trip, I have gained four true friends, who I can talk to when I was sad, happy, or just bored Finally, I am forever indebted to my family My parents, who always believe in me and stand by me unwaveringly, provided me with more love and care than I could wish for My sister had to carry my share of familial duty when I was too occupied with enormous amount of work Author Pham Quang Anh TABLE OF CONTENTS List of Tables i List of Figures ii List of Abbreviations iv INTRODUCTION CHAPTER 1: OVERVIEW 1.1 Motivation of research 1.2 FePd nanoparticles (NPs) as hard magnetic material 1.2.1 Phase diagram of FePd alloy 1.2.2 A1 to L10 phase transition 1.3 Ferromagnetism in nanoparticles 1.3.1 Size-dependent ferromagnetism .6 1.3.2 Ferromagnetism in FePd nanomaterials 1.4 Cetyltrimethylammonium bromide (CTAB) surfactant and micellar structure 1.5 Sonoelectrochemical synthesis of nanomaterials CHAPTER 2: SYNTHESIS AND CHARACTERIZATION METHODS 11 2.1 FePd NPs preparation 11 2.1.1 Sonoelectrochemical synthesis .11 2.1.2 Annealing with NaCl 13 2.2 Characterization of FePd NPs .13 2.2.1 Scanning Electron Microscopy – Energy Dispersive X-ray Spectroscopy (SEM-EDS) and Transmission Electron Microscopy (TEM) 13 2.2.2 Differential Scanning Calorimetry (DSC) 16 2.2.3 X-ray Diffraction (XRD) 16 2.2.4 Vibrating Sample Magnetometer (VSM) .17 CHAPTER 3: RESULTS AND DISCUSSION 19 3.1 Appearance of FePd NPs and particle size growth by annealing .20 3.2 Stoichiometry of synthesized FePd NPs .22 3.3 Thermal profile of FePd NPs 24 3.4 Crystallographic structures of FePd NPs 25 3.5 Magnetic properties of FePd NPs .33 3.5.1 Magnetization curves and coercivity values 33 3.5.2 Saturation magnetization and remanent magnetization 36 CONCLUSION .38 REFERENCES 39 LIST OF TABLES Table 3.1 Naming system for 16 samples, including as-prepared and 14 annealed at different temperatures for hours Sample names are in bold 19 Table 3.2 Lattice parameters and c/a ratio of fct-FePd at different annealing temperatures for FP0 samples At 700˚C, tetragonal phase was not detected 29 Table 3.3 Lattice parameters and c/a ratio of fct-FePd at different annealing temperatures for FP50 samples 30 Table 3.4 Comparison of crystallite size from XRD spectra and particle size from TEM images for FP0 and FP50 as-prepared samples 33 i LIST OF FIGURES Figure 1.1 Phase diagram of Fe-Pd alloy Reprinted from Ref [15] Figure 1.2 A1 to L10 phase transition scheme commonly assumed in FePd alloy Figure 1.3 Alternative A1 to L10 phase transition scheme proposed by Vlasova et al Reprinted from Ref [18] .5 Figure 1.4 Size effect on magnetic properties of nanoparticles Reprinted from Ref [20] .6 Figure 1.5 Possible configurations of L10 modification Reprinted from Ref [26] Figure 1.6 Ball-and-stick model of CTAB Long straight C16H33 tail is hydrophobic (CH3)N+ head is hydrophilic Br- counterion is not shown in this model as it does not contribute to surfactant capability of CTAB Figure 1.7 Illustration of a simple sonoelectrochemical synthesis setup Reprinted from Ref [33] .9 Figure 2.1 Overview of FePd NPs preparation process 11 Figure 2.2 Cycles of current on/off pulses and ultrasound pulse 12 Figure 2.3 Schematic of core components in a TEM instrument Reprinted from Ref [41] .15 Figure 2.4 Diffraction of X-ray caused by arbitrary (hkl) plane .16 Figure 2.5 Schematic of VSM operation 18 Figure 3.1 (a) TEM image and (b) particle size measured and distribution plotted from images of FP0 sample .20 Figure 3.2 (a) TEM image and (b) particle size measured and distribution plotted from images of FP50 sample .21 Figure 3.3 TEM images of (a) FP0-400 and (b) FP50-400 Average diameters were 23 and 24nm for FP0-400 and FP50-400 samples, respectively 21 Figure 3.4 EDS spectrum of FP0 sample 22 Figure 3.5 EDS spectrum of FP50 sample 23 Figure 3.6 Element mapping of FP50 sample 23 Figure 3.7 Differential Scanning Calorimetry (DSC) profile of synthesized FePd NPs Negative heat flow direction was exothermic and positive direction was endothermic 24 Figure 3.8 XRD spectra of as-prepared FP0 sample and samples annealed at temperatures ranging from 400˚C to 700˚C for hours Measuring step was set at 0.03˚ 26 Figure 3.9 XRD spectra of as-prepared FP50 sample and samples annealed at temperatures ranging from 400˚C to 700˚C for hours Measuring step was set at 0.03˚ 27 Figure 3.10 XRD spectra of FP0 as-prepared and annealed samples magnified at 38˚42˚ region Deconvolution of peaks were attempted using Gaussian fit model .27 ii Figure 3.11 XRD spectra of FP50 as-prepared and annealed samples magnified at 38˚42˚ region Deconvolution of peaks were attempted using Gaussian fit model .28 Figure 3.12 Lattice parameters – annealing temperature relationship of fcc-FePd, unknown fcc and fct-FePd phases in FP0 annealed samples FP0-700 lattice parameter was not included as there was only fcc-(Fe,Pd) phase .31 Figure 3.13 Lattice parameters – annealing temperature relationship of fcc-FePd, unknown fcc and fct-FePd phases in FP50 annealed samples 32 Figure 3.14 Room temperature magnetization curves for FP0 samples annealed at temperatures from 400 to 700˚C 34 Figure 3.15 Room temperature magnetization curves for FP50 samples annealed at temperatures from 400 to 700˚C 35 Figure 3.16 Coercivity (HC) – annealing temperature relationship of all annealed sample 35 Figure 3.17 Saturation magnetization (MS) dependence on annealing temperature .36 Figure 3.18 Remanent magnetization (MR) dependence on annealing temperature .36 iii LIST OF ABBREVIATIONS 3D-NAND: CTAB: DC: DRAM: DSC: EDS: fcc: fct: HC: HDD: MR: MRAM: MS: NP: SEM: SSD: TEM: VSM: XRD: three-dimensional NAND logic gate cetyltrimethylammonium bromide single-domain critical diameter dynamic random-access memory differential scanning calorimetry energy dispersive X-ray spectroscopy face-centred cubic face-centred tetragonal coercivity hard-disk drive remanent magnetization magnetoresistive random-access memory saturation magnetization nanoparticle scanning electron microscopy solid-state drive transmission electron microscopy vibrating sample magnetometer X-ray diffraction iv INTRODUCTION Ferromagnetism can be seen in various aspects of the modern life, from refrigerators, biosensors, transportation, to computers The case of superconducting magnet train, such as Japan’s Maglev train, is a familiar example of scaling up magnets to harvest advantages of magnetism However, in computers, especially data storage, the downscaling of individual magnets to nano size is not so straightforward Dependence of magnetic properties on particle size has long been aware of However, synthesizing uniform and homogeneous particles is both difficult and highly expensive Although nanomaterials such as FePt, FePd, CoPt, etc all showed decent results in magnetic tests and could be promising candidates to remarkably increase areal density of magnetic recording devices, there are various factors standing in the way leading to application in commercial products FePd has an important advantage over other ferromagnetic nanomaterials It has been well documented that the required processing temperature of FePd to reach ferromagnetic state is well below others This means less energy consumed, and safer manufacturing conditions In this thesis, FePd nanoparticles have been synthesized using sonoelectrochemical method, and cetyltrimethylammonium bromide (CTAB) was added to the synthesis process to evaluate its effects on various properties of FePd nanoparticles The main objectives of this thesis consist of:  Synthesizing Fe50Pd50 nanoparticles with and without presence of CTAB  Comparing appearance, structural properties and magnetic properties of FePd nanoparticles synthesized with and without CTAB  Attempting to produce single phase L10 FePd nanoparticles (111) plane and 46.6˚ peak of (200) plane correspond to disordered fcc-FePd phase, while ordered fct-FePd phase generated 40.7˚ peak of (111) plane and 47.1˚ peak of (200) plane High c/a ratio of fct-FePd at 400˚C showed a rather low degree of ordering at this temperature As the temperature was raised to 450˚C for FP50 and 500˚C for FP0, a third and unknown phase appeared at 40.45˚, co-existing with fcc-FePd and fct-FePd The unknown phase was determined to be fcc-structured or fct-like with c/a ratio approaching unity, from lattice parameter calculation, with a = 3.86 Å In figures used for this thesis, the third phase would be denoted as unknown-fcc, for there was no conclusive evidence of transformation to tetragonal lattice shape When this unknown phase totally disappeared, in FP0-650 and FP50-700 samples, the FePd NPs became truly single-phased with only fcc structure present This unresolved phase has also been observed by Trung in a similar temperature range [22] The unknown fcc phase could be an intermediate phase that preceded formation of highly ordered fct phase Table 3.2 Lattice parameters and c/a ratio of fct-FePd at different annealing temperatures for FP0 samples At 700˚C, tetragonal phase was not detected Annealing temperature (˚C) a (Å) c (Å) c/a 400 3.857 3.801 0.9854 450 3.857 3.798 0.9848 500 3.858 3.795 0.9835 550 3.879 3.713 0.9572 600 3.883 3.713 0.9564 650 3.903 3.680 0.9429 700 not detected not detected not detected 29 Table 3.3 Lattice parameters and c/a ratio of fct-FePd at different annealing temperatures for FP50 samples Annealing temperature (˚C) a (Å) c (Å) c/a 400 3.85894 3.80362 0.98567 450 3.85353 3.76699 0.97754 500 3.85507 3.75899 0.97508 550 3.85507 3.74122 0.97047 600 3.85507 3.73618 0.96916 650 3.87291 3.72906 0.96286 700 3.86824 3.71502 0.96039 From Table 3.2 and Table 3.3, it could be concluded that ordering of fct phase occurred gradually over a long temperature range as c/a ratio decreased with elevated temperature, reaching a minimum c/a ratio right before transformation to the γ-(Fe,Pd) phase at high temperature, as with the case of FP0 sample annealed at 700˚C The trend of changes in lattice parameters is visually presented in Figure 3.12 and Figure 3.13 For FP50 annealed samples, γ-(Fe,Pd) phase was not yet formed, indicating that smaller starting particles of FP0 could have reduced the transition temperature from L1 0-FePd to γ(Fe,Pd) Monophasic nature and high degree of ordering are greatly beneficial for magnetic properties of FePd NPs, as would be confirmed in the Section 3.5.1 Quantification of degree of ordering, S, can be roughly calculated using the formula [43] 𝑆 = − (𝑐 ⁄𝑎) /[1 − (𝑐⁄𝑎) ] (3.1) where (c/a)exp is the experimentally measured value and (c/a)std = 0.9665 (JCPDS 021440) is the standard value However, experimentally, it has been shown that annealing in NaCl environment can cause experimental value to be significantly lower than the standard [23] The lowest c/a ratio obtained from 16 samples in this research was 0.9429 30 of FP0 sample annealed at 650˚C NaCl annealing environment could be the key to achieve low axial ratio Size of fct-structured crystallites in FP0 sample annealed at 650˚C was 20.83±0.15 nm, while the number for FP50 sample annealed at 700˚C was 19.00±0.24 nm At other annealing temperatures, due to heavy overlapping of multiple peaks, as shown in Figure 3.11, the uncertainty become too large to assess growth of fct-FePd phase Although the difference between crystallite size of these two samples were rather small, its effect on magnetic properties was quite noticeable, as would be shown in Section 3.5 Figure 3.12 Lattice parameters – annealing temperature relationship of fcc-FePd, unknown fcc and fct-FePd phases in FP0 annealed samples FP0-700 lattice parameter was not included as there was only fcc-(Fe,Pd) phase 31 Figure 3.13 Lattice parameters – annealing temperature relationship of fcc-FePd, unknown fcc and fct-FePd phases in FP50 annealed samples Polycrystallinity in as-prepared sample Applying Scherrer’s equation to (111) plane peak of FP0 and FP50 samples yielded the crystallite size of 5.73 nm and 13.02 nm, respectively By comparison with rough estimation of mean particle size figure obtained from TEM (Table 3.4), a disparity between particle size and crystallite size could be seen for FP50 Although the crystallite size was still quite high, the contrast was stark when comparing with the numbers of FP0 sample As a result, FP50 could possibly be considered as polycrystalline NPs and FP0 as single crystal NPs CTAB micellar structure and reduced impact from ultrasound, as mentioned in Section 3.1, could be the reason for this phenomenon Large, polycrystalline NPs of FP50 sample were possibly produced by growth and aggregation 32 of smaller single-crystal particles in a micelle core Polycrystallinity is then a reasonable explanation for the formation of larger particles in presence of CTAB Table 3.4 Comparison of crystallite size from XRD spectra and particle size from TEM images for FP0 and FP50 as-prepared samples FP0 FP50 Crystallite size (nm) 5.73 13.02 Average particle size (nm) 6.07 18.30 Crystallite/particle volume ~85% ~35% Single crystal Possible polycrystalline fraction (%) Conclusion 3.5 Magnetic properties of FePd NPs 3.5.1 Magnetization curves and coercivity values Room temperature magnetization curves are provided in Figure 3.14 and 3.15 for annealed samples of FP0 and FP50, respectively In the bottom right corner of the figures, magnified curves around the origin of coordinates were also included The numerical values of coercivity (HC) were summarized in Figure 3.16 All annealed sample showed hard magnetic properties, which is explained by presence of fct-FePd, albeit with different degree of ordering From Table 3.2 and Table 3.3, it could be seen that degree of ordering of fct-FePd increased accordingly to annealing temperature; however, the trend of coercivity values did not follow the same rule Appearance and degree of ordering of hard magnetic fct-FePd phase is therefore not the only decisive factor of HC in samples Highest coercivity of annealed FP0 (1.46 kOe) and FP50 (0.91 kOe) samples were obtained at 650˚C and 700˚C, respectively This finding correlates with high degree of ordering in fct phase, as pointed out in Section 3.4 The temperature range for highest coercivity and complete L10 formation was in good agreement with figures reported by Shamis et al [19] As Fe and Pd atoms were rearranged into ideal positions, the magnetic anisotropy of the crystallites increased, resulted in a larger coercive force required to 33 flip magnetic moments Larger crystallite size in FP0 (20.83 nm) compared to FP50 (19.00 nm) might be the reason for larger coercivity value, if they were both below the single magnetic domain size of the NPs However, there exists no fixed value of for this parameter, as stated by Kim and colleagues [44] Aforementioned statement remained a hypothesis Another variable that could affect the HC value is the easy axis direction of crystallites in one particle Random coordination of crystallites tends to lower effective magnetocrystalline anisotropy of the particle, to a randomized extent, which is highly possible for both FP0-650 and FP50-700, as the crystallite size was relatively small Figure 3.14 Room temperature magnetization curves for FP0 samples annealed at temperatures from 400 to 700˚C Apart from the crystallite hypothesis, interaction of different phases in each particle must also be taken into account in order to gain a complete understanding of coercivity – temperature relationship, particularly the unusual drop of HC at 600˚C for FP0 and 550˚C for FP50 The key, but also the difficulty, would be the determination and examination of unknown phase Depending on the magnetic properties of this phase, magnetization curves obtained could look massively different As pointed out in Section 3.2, the distributions of Fe and Pd atoms were not completely uniform, which could also 34 lead to different particles having slightly different properties, a consequence of nonuniformity in phase interactions Figure 3.15 Room temperature magnetization curves for FP50 samples annealed at temperatures from 400 to 700˚C Figure 3.16 Coercivity (HC) – annealing temperature relationship of all annealed sample 35 3.5.2 Saturation magnetization and remanent magnetization Figure 3.17 Saturation magnetization (MS) dependence on annealing temperature Figure 3.18 Remanent magnetization (MR) dependence on annealing temperature Saturation magnetization generally followed an upward trend, shown in Figure 3.17 The linear relationship between MS and particle size beyond a certain value of diameter 36 has been well researched and proven [45, 46] As temperature increased, product NPs tend to grow larger FP50-550 was the only significant outlier of this trend Remanent magnetization is an important parameter of ferromagnetic materials, since it is the leftover magnetization after external magnetic field has been removed, directly related to the maximum energy product, also known as strength, of the magnet Here, MR obeyed a highly similar trend to HC (Figure 3.18), because the fct-FePd phase is the main contributor of both MR and HC Remanent magnetization reached highest value of 23.3 emu/g for FP0 at 650˚C and 22.7 emu/g for FP50 at 700˚C 37 CONCLUSION From the analyses of results as presented in Chapter 3, a few conclusions could be drawn:  Fe50Pd50 NPs were successfully synthesized using the same precursor ratio, with and without presence of CTAB surfactant  Addition of CTAB in sonoelectrochemical synthesis resulted in larger particles but size distribution was more uniform  FePd NPs synthesized in CTAB could be polycrystalline NPs  FePd NPs synthesized in CTAB had higher transition temperature from fct-FePd to fcc-(Fe,Pd), 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