2.1.5 Surface coatings of NaYF4:20%Yb,2%Er UC nanoshells Undoped NaYF4 nanoshells were first synthesized using the synthesis method of NaYF4:20%Yb,2%Er nanoshells Sec.. 2.3 Assembly of A
Trang 1Chapter 2 Experimental Methods
2.1 Material synthesis: UC nanoshells
UC nanoparticles with hcp phase and diameter size ~10 nm were
synthesized using thermal decomposition.71,72 A high UC emission was achieved for the UC nanoparticles produced using this method.73 In this method, precursors of trifluoroacetates of metal ions were decomposed at high temperature above 300 oC in high boiling point solvents (e.g 1-octadecene) and surfactants (e.g oleylamine and oleic acid), leading to the formation of
UC nanoparticles stabilized by the surfactants
Recent reports showed the hollow nanoparticles were synthesized via Kirkendall effect.74,75 The first Kirkendall effect was reported in 1942 and the result was confirmed in 1947.76,77 The Kirkendall effect was first studied for the synthesis of hollow structures of cobalt sulfide nanoparticles from room temperature to 182oC.74,78 The formation mechanism of voids inside the particles was dominated by outward diffusion of cobalt cations and balanced
by inward diffusion of vacancies The small voids in each particle were observed between the cobalt core and sulfide shell due to condensation of vacancies at the boundary These small voids coalesced into bigger ones, followed by disappearance of the cobalt cores Finally, a single void in the center of the cobalt sulfide nanoparticles was formed
More recently, the Kirkendall effect has been applied to synthesize hollow UC nanoshells For example, hollow NaYF4:Yb,Er UC nanoshells were synthesized using a controlled ion exchange process from cubic-phase Y2O3 nanospheres79 or thermal decomposition of a mixture of trifluoroacetate
Trang 2precursors.80 These hollow nanoshells were formed due to the Kirkendall effect In this thesis, NaYF4:Yb,Er nanoshells with hcp crystal structure were synthesized using thermal decomposition of trifluoroacetate precursors in oleylamine at 340 °C.81
2.1.1 Preparation of trifluoroacetate precursors
Trifluoroacetate precursors of Y, Yb, and Er [(CF3COO)3M, M = Y,
Yb, and Er ions] were prepared by dissolving their respective oxides or hydroxides in trifluoroacetic acid (CF3COOH), followed by drying in oven at
80 oC.73 Sodium trifluoroacetate (CF3COONa) was prepared by dissolving sodium carbonate (Na2CO3) in trifluoroacetic acid, followed by drying in oven
at 80 oC
2.1.2 Synthesis of NaYF4:20%Yb,2%Er UC nanoshells
In the synthesis of NaYF4:20%Yb,2%Er nanoshells, 20 mL of oleylamine was first reacted with 3 mL of trifluoroacetic acid in a 50-mL three-neck flask under a continuous flow of Ar gas A mixture of (CF3COO)3Y (0.488 mmol), (CF3COO)3Yb (0.125 mmol), (CF3COO)3Er (0.013 mmol), and CF3COONa (1.252 mmol) was then added and followed by 0.6 mL of deionized water under vigorous stirring at 60 °C for 5 min This mixture was heated to 340 °C using a heating mantle under refluxing condition and in the presence of Ar gas for protection from oxidation After 30 min, the mixture was allowed to cool to 80 °C The oleylamine-capped UC nanoshells were isolated by centrifugation at 10000 rpm for 3 min, followed by washing and redispersing in hexane for characterizations To investigate the formation
Trang 3mechanism of the UC nanoshells, the samples were collected at 300 °C, 5 min,
10 min, and 30 min at 340 °C for structure and microstructure analyses
2.1.3 Synthesis of solid NaYF4 core/NaYF4:20%Yb,2%Er shell
nanoparticles
In the synthesis of solid NaYF4 core (~10 nm)/NaYF4:20%Yb,2%Er
UC shell (~3 nm), the solid NaYF4 core was first prepared and then coated by
UC shell, following the reported methods.32,35 In the preparation of solid NaYF4 core, CF3COONa (1.252 mmol) and (CF3COO)3Y (0.626 mmol) was dissolved in oleylamine (20 ml) in a 50-mL three-neck flask at 90 ºC under Ar until a clear solution was formed The mixture was then heated to 340 °C using a heating mantle and kept at such temperature for 30 min The solution was allowed to cool to 100 ºC A mixture of (CF3COO)3Y (0.976 mmol), (CF3COO)3Yb (0.250 mmol), (CF3COO)3Er (0.026 mmol), and CF3COONa (2.504 mmol) were added to the solution This mixture was heated to 340 °C under refluxing condition and Ar After 30 min, the mixture was allowed to cool to 80 °C The particles were isolated by centrifugation at 10000 rpm for
3 min, followed by washing and redispersing in hexane
2.1.4 Synthesis of solid NaYF4:20%Yb,2%Er nanoparticles
In the typical synthesis of solid NaYF4:20%Yb,2%Er nanoparticles (~15 nm), (CF3COO)3Y (0.976 mmol), (CF3COO)3Yb (0.250 mmol), (CF3COO)3Er (0.026 mmol), and CF3COONa (2.504 mmol) were dissolved
in oleylamine (20 ml) in a 50-mL three-neck flask at 90 ºC under the presence
of Ar gas until a clear solution was formed The mixture was then heated to
Trang 4340 °C using a heating mantle and held at this temperature for 40 min The solution was allowed to cool to 80 ºC The particles were isolated by centrifugation at 10000 rpm for 3 min, followed by washing and redispersing
in hexane
2.1.5 Surface coatings of NaYF4:20%Yb,2%Er UC nanoshells
Undoped NaYF4 nanoshells were first synthesized using the synthesis method of NaYF4:20%Yb,2%Er nanoshells (Sec 2.1.2) For the preparation
of undoped NaYF4 nanoshells, only (CF3COO)3Y (0.626 mmol) and CF3COONa (1.252 mmol) were used in the first step without (CF3COO)3Yb and (CF3COO)3Er The surface coatings were done by coating the undoped NaYF4 nanoshells with Yb,Er doped NaYF4 shell, followed by another undoped NaYF4 shell on top The detailed procedure is given as follows A mixture of (CF3COO)3Y (0.976 mmol), (CF3COO)3Yb (0.250 mmol), (CF3COO)3Er (0.026 mmol), and CF3COONa (2.504 mmol) were added to the solution of as-synthesized undoped NaYF4 nanoshells in a 50-mL three-neck flask and then heated to 340 °C using a heating mantle under refluxing condition and Ar After 30 min, the mixture was allowed to cool to 100 °C Then (CF3COO)3Y (1.952 mmol) and CF3COONa (5.008 mmol) were added and followed by heating to 340 oC After 30 min, the mixture was allowed to cool to 80 °C The particles were isolated by centrifugation at 10000 rpm for
3 min, followed by washing and redispersing in hexane
Trang 52.2 Material synthesis: Au-Ag metallic nanoshells
Galvanic replacement reaction is a powerful method to produce hollow metallic nanostructures.52,82 In galvanic replacement reaction, Ag nanoparticles are commonly used as the sacrificial metal template in synthesis
of hollow Au-Ag nanoshells Au would be formed via the reduction of HAuCl4 and deposited on Ag templates being oxidized.83 The deposition of
Au and oxidation of Ag solids lead to the formation of Au-Ag nanoshells with interior cavity
2.2.1 Synthesis of Ag nanoparticles
In a typical synthesis of Ag nanoparticles consisted of decahedral (~43
nm in size) and triangular prism (~53 nm in edge length) shapes, 5 mL of 1-octadecene (ODE) and 3 mL of oleylamine were mixed using a magnetic stirrer at a spin rate of 700 rpm in a 25-mL three-neck flask ODE was selected because of its high boiling point (~315 oC) and good compatibility with oleylamine, allowing the reaction at 160 oC The ODE-oleylamine mixture was then heated to 160 oC using a heating mantle under a continuous flow of N2 gas A solution of 10 mg of AgNO3 (58.9 mol) was immediately injected to the mixture The temperature decreased to ~155 oC after injection
of the AgNO3 solution, then increased to 160 oC again within a few minutes After 30 min at 160 oC, the solution was cooled to 60 oC, followed by dilution with 8 mL of toluene The solution of Ag nanoparticles was kept in a vial wrapped with aluminum foil and stored in the dark until further use As-synthesized Ag nanoparticles were used as a sacrificial template in synthesis
of Au-Ag nanoshell via the galvanic replacement reaction with HAuCl4
Trang 62.2.2 Synthesis of Au–Ag nanoshells
As-synthesized Ag nanoparticles (16 mL in the solution) were added to
a 50-mL three-neck flask (with magnetic stirrer at a spin rate of ~700 rpm) and then heated to 60 oC in a water bath A 5 mM HAuCl4 solution was prepared
by dissolving 29.5 mg of HAuCl4•3H2O (75 mol) in 13.5 mL of toluene and 1.5 mL of oleylamine Fresh HAuCl4 solution was prepared and kept in the dark before use The 5 mM HAuCl4 solution was added to the 50-mL reaction flask at an injection rate of 0.25 mL/min The samples were collected after injection of various amounts of 5 mM HAuCl4 solution as shown in Table 2.1 The samples were isolated by centrifugation at 10,000 rpm for 3 min The obtained particles were washed three times with 8 mL of toluene, followed by centrifugation, and re-dispersed in 8 mL of toluene for characterizations
Table 2.1 Samples were collected after the injection of a variable amount of 5
mM HAuCl4 solution for the measurement of microstructure, morphology, chemical composition, and extinction spectra
Sample Volume of 5 mM HAuCl4 solution (mL) HAuCl4 (mol) Amount of
Trang 72.3 Assembly of Au-Ag nanoshells and UC nanoshells
To study the plasmonic effects of Au-Ag nanoshells on fluorescence properties of UC nanoshells, single layer of Au-Ag nanoshells on glass substrates were prepared using spin-coating, followed by the coating of silica film using sputtering UC nanoshells were further deposited on the silica film-coated Au-Ag nanoshell layer to form an assembly of Au-Ag nanoshell layer/silica film/UC nanoshell layer In this assembly, the distance between Au-Ag nanoshell layer and UC nanoshell layer was controlled by the thickness
of silica film The assemblies with different surface coverage % of Au-Ag nanoshell layer were also prepared The surface coverage- and distance-dependent fluorescence of the UC nanoshells was investigated
2.3.1 Preparation of Au-Ag nanoshell layer
Glass substrate was prior soaked in aqua regia (3 parts concentrated HCl (37%)/1 part concentrated HNO3 (65%)) for removal of contamination at the glass surface, followed by washing three times with ethanol The washed glass was then dried in oven at 80 oC for 24 h The glass was kept in vacuum chamber before use
Au-Ag nanoshell layer on glass substrate was prepared using spin-coating The Au-Ag nanoshell solution (25 L) of was dropped on a glass substrate (1.5 cm x 1.5 cm), followed by spin-coating at a speed of 1500 rpm for 30 seconds for each cycle Samples were prepared by 5 – 40 cycles of spin coating to obtain Au-Ag nanoshell layer with different surface coverage % on the substrates The samples were kept in the vacuum chamber before use Scanning electron microscopy (SEM) was performed for each sample The
Trang 8surface coverage % of the Au-Ag nanoshells on the substrate was calculated from the SEM images using Java image processing and analysis program (ImageJ) The surface coverage % is defined by the ratio of area occupied by the nanoshells to the total analyzed area
2.3.2 Assembly of Au-Ag nanoshell layer/silica film/UC nanoshell layer
Silica deposition was performed using a magnetron sputtering system
at a deposition rate of 15 nm per h at room temperature The operating conditions were 150 W (RF power), 0.5 Pa (chamber pressure), and 50 standard cubic centimeter per minute (sccm) (Ar flow rate) The silica deposition rate was calibrated using surface profiler (KLA Tencor Alpha-Step Q) The glass and silica have similar appearance and properties For the thickness measurement using the surface profiler, the glass substrate was first coated with Ti film and followed by silica film The silica thickness was obtained from the total (Ti + silica) thickness normalized by the thickness of
Ti film The average silica deposition rate was measured to be ~15 nm per h
A similar silica deposition rate (~15 nm per h) was obtained from the silica film sputtered on silicon substrates using the same operating conditions of the magnetron sputtering system (Fig 2.1) In this study, silica film was selected due to its good chemical and thermal stability, and low thermal conductivity (1.4 Wm−1K−1).84 Its inertness to solvents (e.g hexane and toluene) would allow the deposition of hexane solution of UC nanoshells on the silica film-coated Au-Ag nanoshells by solvent evaporation method, whereas the low thermal conductivity would reduce the photothermal effects of Au-Ag nanoshells to UC nanoshells
Trang 9Fig 2.1 (a) SEM image of cross-section of silica film sputtered on silicon
substrate for 4 hours (b) Average thickness of silica films obtained at different sputtering time The average silica deposition rate was calculated to
be ~15 nm per h
The Au-Ag nanoshell layer with different surface coverage % on the glass was coated by 30-nm silica films using a magnetron sputtering system The silica film-coated Au-Ag nanoshell layer was then coated with UC nanoshells The procedure is given as follows The sample of silica film-coated Au-Ag nanoshell layer was placed in a 10-mL beaker glass Then 2
mL of 0.07 wt% hexane solutions of UC nanoshells were added to the 10-mL beaker glass The UC nanoshells were then deposited on the Au-Ag nanoshell layer/silica film by solvent evaporation in a vacuum chamber (Fig 2.2) To obtain the similar concentration of UC nanoshells deposited on the Au-Ag nanoshell layer/silica film, same concentration and volume of hexane solutions
of UC nanoshells was used to fabricate all the assemblies in this study The effects of the Au-Ag surface coverage on the fluorescence of the UC nanoshells were studied
To investigate the distance-dependent fluorescence of UC nanoshells, the Au-Ag nanoshell layer (prepared by 20 cycles of spin-coating) was coated
Trang 10by silica films with different thickness (5 - 180 nm) using the magnetron sputtering system The UC nanoshells were then deposited on the Au-Ag nanoshell layer/silica film to form an assembly of Au-Ag nanoshell layer/silica film/UC nanoshell layer The distance between the Au-Ag nanoshell layer and
UC nanoshell layer was controlled by the thickness of the silica film
Fig 2.2 Schematic of deposition of UC nanoshells (0.07 wt% in hexane) by
solvent evaporation in a vacuum chamber
2.4 Material characterizations
2.4.1 Crystal structure
X-ray diffraction (XRD) is a non-destructive characterization technique which can provide crystal structure information of materials In this technique, monochromated X-ray striking a sample is scattered by the atoms
in the sample The scattered intensity of the X-ray is collected as a function of
Trang 11samples were investigated using powder XRD diffractometer system (Cu K radiation) (Bruker AXS, Karlsruhe, Germany) XRD spectra of the samples were compared with their corresponding standard XRD spectra [Joint Committee on Powder Diffraction Standards (JCPDS), for example, file
number PDF 16-334 for hcp sodium yttrium fluoride and PDF 4-783 for fcc
silver]
2.4.2 Microstructure and surface morphology
Transmission electron microscopy (TEM) is a microscopy technique in which an electron beam is transmitted through an ultra-thin sample, interacting with the specimen as it passes through An image is formed from the interaction of the electrons transmitted through the sample In this thesis, the microstructure of the nanoparticle samples was studied using a JEOL JEM 2010F transmission electron microscope (JEOL, Tokyo, Japan) operated at
200 kV Carbon-coated copper grids (400 meshes) were used to support the nanoparticles The average particle size was estimated by random measurements of 100 – 200 particles from the TEM images Elemental composition of the samples were performed using energy dispersive X-ray (EDX) in the transmission electron microscope The average concentration of the elements in the samples was determined from the EDX data randomly collected at least 5 different selected area
Different from the TEM which produces an image by detecting the electrons transmitted through the sample, scanning electron microscopy (SEM) produces an image by detecting the electrons such as secondary electrons which are emitted from the sample surface due to excitation by the