Synthesis and Characterization of Nanoparticles

Diplomarbeit von Stephan Dankesreiter

Introduction:

The development of small and smallest particle is one of today’s key features in modern science. The goal is to form materials with improved properties than their ‘classical’ ancestors with just a fractional amount of raw material.

Another key feature of nanoparticles is their different, and sometimes unexpected, behavior concerning reactivity, compared with their bulk materials. Because of this, nanoparticles have a wide range of applications, especially in the field of catalysis.

Here, characteristics of nanoparticles - more edges, corners, defects or oxygen vacancies – are used to obtain a high performance of the catalysts. Nanoscaled particles also exhibit larger surface area and higher metal dispersion, which further contributes to the catalytic possibilities.

To gain such particles, two different pathways are given: first, there is the so-called ‘top down’ pathway, considered as further developments of micro technology, where physical preparation methods like lithography are used. The second way is the ‘bottom up” method where self-assembling systems, formed by surfactants, are used.

Concerning gold nanoparticles, it is reported that the use of C16TAB at specified conditions, gives gold nanorods with a sharp size-distribution because the direction of growth is predetermined. Being a cationic surfactant, C16TAB affects the electrochemical potentials and introduces bromide-ions as an additional species to the reaction. To achieve gold nanoparticles from aqueous HAuCl4-solutions, the above-mentioned method needs a separate reducing agent such as ascorbic acid (Asc0), NaBH4 or N2H4.

A way of synthesizing spherical gold nanoparticles is the use of Nd:YAG laser with a salt induced agglomeration. By modifying the formulation of the salt solution, different sizes are obtained. This way of synthesis, a combination of physical top-down and self-assembling bottom-up processes, can be modified by adding surfactants, like PEG, to optimize size distribution and physical characteristics, like UV-Vis absorption.

This method is an elegant way of synthesis; however, problems may occur by functionalizing the particles, because of a high salt content. Here, a high influence of purity, concentration and composition to the size and shape of gold nanoparticles might be given. Therefore, a route of synthesis is needed, which shows high efficiency in producing gold nanoparticles and in stabilizing them with a manageable amount of parameters. As mentioned before, good results are obtained by adding surfactants, which limit growth. In the case of gold nanoparticles, this control can be performed by gel-forming agents. To avoid impurities and to limit the parameters of the system, an optimal gel-forming specie is also the reducing agent and can influence size and shape by varying only physical parameters such as temperature.

It is reported that Asc0 can be used as a reactant to reduce Au3+ to gold nanoparticles due to a low redox-potential of Asc0 (E0 versus NHE = +0.08 V) and a high redox-potential of Au3+ (E0 versus NHE = +1.498 V).

The problem of this reaction is agglomeration and growth control, because of the absence of a surfactant. However, Asc0 is not only a reducing agent, it is also a slightly weak acid (pka1 = 4.25) with two alcohol-groups, and therefore, it can react under acid conditions with fatty acids to form an alkyl-ester. This ester still has the reducing-ability of Asc0 and can form aggregates, because of their hydrophobic tail. Therefore, a surfactant with reducing properties and the ability of forming self-assembling structures is achieved. Using different carboxylic acids, different surfactants with different aggregation patterns can be produced, thus different sizes of nanoparticles can be gained. With the use of biodegradable agents, ascorbic acid and fatty acids, a step in the direction of ‘green chemistry” is taken. Therefore, the reaction-matrix, the gel formed by the different surfactants, is fully biodegradable.

In addition, gold nanoparticles are reported to exhibit a small degree of toxicity at low concentrations and are widely used in medical applications like biomedical imaging and diagnostic tests. Recent development shows a possible application of gold nanoparticles as active agent in cancer-therapy, where the nanoparticles are absorbed by the tumor cells eight times more than by normal cells. The excitement of these particles by X-rays destroys the tumor-cells due to gold’s significant high-Z X-ray absorption.

Technical applications of gold nanoparticles can be found at very small particles with diameters below 10 nm. The deposition on metal oxides or activated carbon is connected to catalytic properties, especially at low temperatures, for many reactions such as CO oxidation and propylene epoxidation.

A major problem of gold nanoparticles is the high price of gold. Therefore, other materials with good catalytic properties had to be found. With excellent thermal stability, catalytic properties and a comparable low-cost synthesis, zirconium dioxide is one of them. For example, the production of synthesis gas, the carbon dioxide reforming of methane, and the hydrogenation of carbon dioxide for the production of methanol, widely used as a feedstock for chemical industries and the use as an alternative fuel, which is cleaner and more efficient in fuel cells, are nowadays used applications for zircon nanoparticles.

There exist plenty of possible applications for zirconia nanoparticles, like advanced structural transformation-toughened ceramics for wear parts, engine and machine components, cutting and abrasive tools, sensors for oxygen transport and detection, solid electrolytes for solid oxide fuel cells and high-temperature water-vapor electrolysis cells, catalysts for automotive exhaust cleaning and the partial oxidation of hydrocarbons, pigments and much more.

However, there exist three different crystalline structures of zirconia, monoclinic, tetragonal and cubic, where the first one is stable at room temperature. The high temperature phases, which have potential applications as oxygen sensors, solid fuel cells, and several ceramic components, cannot be retained at room temperature because the transformation is reversible. The addition of small amounts of different oxides (such as yttria) can stabilize partially or fully these modifications.

However, unexpected metastable tetragonal zirconia my be present at room temperature in un-doped crystals.

It has been supposed by several works that either the occurrence of specific precursors or the control of crystal size can lead to tetragonal crystals, which are stable at room temperature. Several efforts have been made to explain this extraordinary behavior. In particular, it has been suggested that small particles are more stable by forming tetragonal particles instead of monoclinic ones because of surface energy-effects.

This means, the rational use of the preparation procedure gives particles of different sizes and structures.

Table of Contents:

I.	Introduction	1
II.	Fundamentals	7
1.	Plasma oscillation and Mie's theory	9
1.1	Principles of plasma oscillation	10
1.2	Scattering and absorption of small particles	12
2.	L(+)-ascorbic acid and its derivates	15
2.1	Chemical properties of ascorbic acid	15
2.2	AscX surfactants and their aggregates	17
2.3	The formation of gel and coagel	22
3.	Zirconia and its different structures and species	24
3.1	Zirconyl chloride in aqueous solutions	24
3.2	Structures of zirconium oxide	27
3.2.1	Crystal structures and martensic phase transformation	27
3.2.2	Tetragonal zirconia and critical crystal size	34
4.	Structural investigation techniques	37
4.1	X-ray based methods	37
4.1.1	The nature of X-rays	37
4.1.2	Small angle X-ray scattering (SAXS)	41
4.1.2.1	Scattering by one electron	42
4.1.2.2	The scattering vector	42
4.1.2.3	The electron density	44
4.1.2.4	The scattering intensity	45
4.1.2.5	The auto correlation and invariant	46
4.1.2.6	Scattering of spherical particles	47
4.1.2.7	The Guinier approximation	48
4.1.2.8	Correlation length and Porod's law of scattering	49
4.1.2.9	Scattering of particles with non-uniform electron density	50
4.1.3	X-ray diffraction (XRD)	52
4.1.3.1	Phase shift and intensity	53
4.1.3.2	Bragg's Law of diffraction	55
4.1.3.3	The reciprocal lattice and the system of Miller indices	57
4.1.3.4	The Scherrer equation	60
4.2	Electron based method: scanning electron microscopy (SEM)	62
4.2.1	Principal setup	62
4.2.2	The scanning process	64
III.	Experimental	65
5.	Chemicals	67
5.1	Preparation of Gold Nanoparticles	67
5.2	Preparation of zirconium-based nanoparticles	67
6.	Analytical Methods	68
6.1	Thermogravimetric analysis (TGA)	68
6.2	Differential scanning calorimetry (DSC)	68
6.3	Bright field and phase contrast microscopy	69
6.4	UV/Vis absorption	70
6.5	Raman measurements	71
6.6	Small angle X-ray scattering (SAXS)	72
6.7	X-ray diffraction (XRD)	73
6.8	Scanning electron microscopy (SEM)	74
7.	Synthesis of L(+)ascorbyl stearate (Asc18)	75
8.	Synthesis of Gold Nanoparticles	77
8.1	Preparation with Asc18 surfactant	77
8.2	Preparation with Asc12 surfactant	78
8.3	Preparation with Asc14 Asc10 and Asc8 surfactants	79
9.	Preparation of ZrO2- nanoparticles	80
9.1	Synthesis of zirconium hydroxide by coprecipitation in homogeneous phase (sol)	80
9.2	Preparation of hydrous zirconia gel	80
IV.	Results and Discussion	83
10.	Gold nanoparticles	85
10.1	Determination of the cmt of Asc18	85
10.2	Synthesized nanoparticles and their colors	86
10.2.1	Influence of reaction temperature	86
10.2.2	Comparison of different concentrations	88
10.3	UV-Vis characterization	90
10.3.1	Comparison of different concentrations	90
10.3.2	Comparison of different reaction temperatures	93
10.3.2.1	Asc10	94
10.3.2.2	Asc12	95
10.3.2.3	Asc14	97
10.3.2.4	Asc18	98
10.4	SAXS characterization	101
10.4.1	The Schulz Spheres fitting model	101
10.4.2	Comparison of reactions above and below the cmc	102
10.4.3	Comparison of different reaction temperatures	103
10.4.4	Comparison of AscX surfactants with different chain lengths	106
10.4.4.1	Asc12	106
10.4.4.2	Asc14	108
10.4.4.3	Asac18	110
10.5	Conclusion	112
11.	3. Zirconium hydroxide and oxide nanoparticles	114
11.1	Raman characterization	114
11.2	Dialysis of the sol and an aqueous ZrOCl2 solution	116
11.2.1	Conductivity of the sol and ZrOCl2 solution	116
11.2.2	Progress of pH and conductivity during gel-formation	116
11.3	Characterization with microscopic methods	118
11.3.1	LM-micrographs of untreated gel	118
11.3.2	LM-micrographs of squeezed gel	118
11.3.3	LM-micrographs of air-dried gel	120
11.3.4	LM-micrographs of collapsed gel	121
11.3.5	LM-micrographs of a freeze-dried gel	122
11.4	DSC measurements of the gel	123
11.5	Characterization by SEM	126
11.6	SAXS characterization of the gel	129
11.6.1	The unified fit model	129
11.6.2	Structural parameters of the gel	131
11.7	TGA and DTG measurements	134
11.8	XRD characterization of calcined samples	138
11.8.1	Diffractogram of the gel-synthesized particles	138
11.8.2	Diffractogram of the sol-synthesized particles	139
11.8.2.1	Samples containing NaCl	140
11.8.2.2	Samples without NaCl	142
11.9	Conclusion	143
V.	Annex	145
	List of Figures	147
	List of Tables	150
	Bibliography	150

Text Sample:

Chapter 4.2, Electron based method: scanning electron microscopy (SEM):

The use of X-rays for structural analysis is well established nowadays. However, problems occur when structures containing larger and smaller particles should be analyzed during one experiment. Compared with light microscopy, X-ray dependent methods are not able to give an image of the focused sample. Since light microscopy is limited in resolution because of the limited use of light due to optical components, the influence of the optical aperture and the Rayleigh criterion, which describes the limit in size of observable structures by using visible light, other ways of imaging had to be found.

By using electrons instead of electromagnetic waves, optical problems are bypassed. This led to the development of transmission electron microscopy (TEM) and scanning electron microscopy (SEM), where the latter has major advantages in sample preparation, diversity and resolution.

4.2.1, Principal setup:

Images are produced by scanning a sample with an electron-beam while displaying the signal from an electron detector on a computer monitor. By choosing the appropriate detection mode, either topographic or compositional contrast can be obtained. Spatial resolution better than 10 nm in topographic mode and 100 nm in compositional mode can be achieved.

The big advantage of the topographic mode is the large depth of field in SEM images. An important factor in the success of the SEM is that images of three-dimensional objects are usually accessible to immediate intuitive interpretation by the observer, just like in optical microscopy. The range of applications of SEM can be extended by adding other types of detection, e.g. for light emission caused by electron bombardment or cathodoluminescence (CL), or the use of an X-ray-detector for energy dispersive X-ray spectroscopy (EDX) to perform elemental analysis.

Fig. 4.11 gives a simplified overview of a typical SEM with two electron detectors. Classical SEM instruments are operating at vacuum, so an interaction of electrons with gas-molecules is minimized.

Electrons are emitted by an electron gun, which is typically a thermionic cathode. With the highest melting point of all metals (3422 °C), tungsten has optimal characteristics for high efficient electron-emitting filaments. A direct current heats the filament to about 2400 °C. At this temperature, tungsten emits electrons into the surrounding vacuum (thermionic emission).

The emitted electrons are accelerated in an electric field of about 30 kV. The result is an electron beam, which is focused on a small part of a sample by magnetic lenses.

The impact of electrons on the sample induces several interactions with the electrons. The detection of these interactions gives information on the specific part of the sample, where the emission of secondary electrons is the most common information source.

Electrons of the primary beam effect an emission of electrons of the outer shells of the atoms located at the sample’s surface. Therefore, topographic information is gained.

To obtain information on the chemical composition, back-scattered electrons can be detected. These electrons are reflected by atoms on the specimen’s surface, where big atoms have a greater ability to reflect electrons as small ones. The mechanism of reflection is supposed to be an elastically impact of electrons on surface electrons.