Chapter 3 Zero-Dimensional Nanostructures: Nanoparticles

Why are nanoparticles important? The properties of nanoparticles can be controlled by engineering the size, shape, and composition of the particles. Engineers can incorporate them into other materials to create new nanocomposite materials with enhanced or entirely different properties from their parent materials. With the inclusion of appropriately selected nanoparticles: • metals can be made stronger and harder • ceramics can have enhanced toughness and formability • insulating materials can be made to conduct heat or electricity • protective coatings can become transparent

Why are nanoparticles important? Ceramics for use in a variety of demanding environments based on nanocrystalline aluminum oxide and ZTA (zirconia-toughened alumina). Transparent coatings for attenuating IR and UV radiation, abrasion resistance, static dissipation, and conductive films fabricated from antimony and indium/tin oxides. Catalysts for chemical and environmental applications, such as cerium oxide and iron oxide. Materials to improve topical healthcare products, including zinc oxide.

Applications

Immediate future nanomaterials applications: next few years polymer-based paints that defy scratching and corrosion iron-polymer batteries that generate twice as much power resilient metal-composite car-body panels that pop back into place woven nanotubes that are 100 times stronger yet lighter than steel

Why are nanoparticles important? “Certain metal oxides such as calcium or magnesium oxide are relatively inert in their naturally occurring forms. However, in reactive nanoparticle form, their chemical reactivity is dramatically enhanced and they are capable of destroying many hazardous substances such as chlorinated hydrocarbons, polychlorinated biphenyls (PCBs), insecticides, acid gases, organophosphorous compounds, and even military chemical agents.”

LITTLE DOTS ON THE PRAIRIE: KANSAS COMPANY CULTIVATES NANOCRYSTALS By Michael Becker, Small Times Correspondent Sept. 11, 2001 – Few people would include Manhattan, Kan., on their list of hotbeds of technological innovation in the United States. The [Chemistry] department has been leveraging its experience to break into the emerging field of nanocrystals. This has resulted in the creation of Nanoscale Materials Inc. The company is now preparing two nanocrystal-based products – a skin cream designed to protect against chemical and environmental hazards and a spray applied from a fire extinguisher type canister that provides similar protection. http://www.nanoscalematerials.com/content/nanoactive_materials/applications.asp

Synthesis Methods Top down: by lithography or other mechanical methods Bottom up: chemical or electrochemical

Mechanical Process Chemical Engineering Science 58 (2003) 815 . 821

Disadvantages – Mechanical Process Impurities can be introduced into the particle surface. Such impurity may serve as sintering aid, or may form eutectic liquid so as to introduce liquid phase sintering. Particle surface damage or defects can readily occur so that the surface energy of particles are very high, and is favorable to densification.

Synthesis Methods—bottom up 1. Thermodynamic Equilibrium approach: a. generation of supersaturation, b. nucleation, c. subsequent growth. 2. Kinetic approach: a. microemulsion, aerosol pyrolysis, template based deposition

Requirements of Nanoparticles 1. identical size of all particles (monosized or with uniform size distribution), 2. identical shape or morphology, 3. identical chemical composition and crystal structure among different particles and within individual particles, such as core and surface composition much be the same, 4. individually dispersed, or redispersible.

Nanoparticles through homogeneous nucleation

Change of Surface Energy The surface energy γ can be changed by 1) when it is close to the roughening temperature of the nuclei, 2) use of different solvent, 2) additives in solution, and 3) incorporation of impurities into solid phase.

Supersaturation σ or ΔGνIncreases as Temperature Decreases

Synthesis of Monosized Nanoparticles

Growth controlled by diffusion

More Detailed Considerations d(v)/dt = hA(C – Cs) where v =4πr3/3, A= 4πr2. dr/dt = h(C – Cs); where Sh = h2r/D = 2 when there is only mass diffusion. With initial condtion of to = ro, r = ht(C – Cs) + ro if h is constant. However, h is actually = D/r when there is only mass diffusion. If h is a constant, this leads to δr = δro. The radius difference becomes smaller when the size of nanoparticles grow bigger.

By analogy from Heat Transfer For forced convection around sphere, NuD = 2 + 0.6ReD1/2Pr1/3, and NuD = 2 when there is only heat conduction. By analogy for mass convection around sphere, ShD = h2r/D = 2 + 0.6ReD1/2Sc1/3, where Sc = ν/D. ShD = 2 when there is only mass diffusion.

Derivation for Eqn(3.12) r12 = kDt + r012 (1) r22 = kDt + r022 (2) (1) – (2) leads to r12 - r22 = r012 - r022 When r1 r2, then r01 r02 2rδr = 2r0δr0 δr= r0δr0/r

Growth Controlled by Surface Process

Growth Controlled by Surface Process – Mononuclear Growth

Growth Controlled by Surface Process – Poly-nuclear Growth

One can infer that For monolayer growth, dA/dt/V = constant, this will lead to Eqn (3.15) For poly-nucleation growth, dV/dt/A = constant, this will lead to Eqn (3.19)

Methods to Achieve Diffusion-Controlled Growth It appears that the use of diffusion barrier, such as a polymeric stabilizer, on the surface of a growing particle is the best approach since it can make large amount of nanoparticles.

Synthesis of Metallic Nanoparticles Diffusion controlled process can be achieved by low concentration of solute or polymeric monolayer adhered onto the growth surface. Precursor: elemental metals, inorganic salts and metal complexes. Reduction reagent: sodium citrate, hydrogen peroxide, hydroxylamine hydrochloride, citric acid, CO,… Polymeric stabilizer: Polyvinyl alcohol (PVA), sodium polyacrylate

This method is known as Turkevich method.

Silver Nanoparticles Monodisperse samples of silver nanocubes were synthesized in large quantities by reducing silver nitrate (AgNO3) with ethylene glycol in the presence of poly(vinylpyrrolidone) (PVP) at 160oC. These cubes were single crystals and were characterized by a slightly truncated shape bounded by {100}, {110}, and {111} facets. The presence of PVP and its molar ratio (in terms of repeating unit) relative to silver nitrate both played important roles in determining the geometric shape and size of the product (13 DECEMBER 2002 VOL 2176 298 SCIENCE) .

The shape of Silver Nanoparticles changed by Growth Conditions The morphology and dimensions of the product were found to strongly depend on reaction conditions such as temperature, the concentration of AgNO3, and the molar ratio between the repeating unit of PVP and AgNO3. For example, when the temperature was reduced to 120°C or increased to 190°C, the product was dominated by nanoparticles with irregular shapes. The initial concentration of AgNO3 had to be higher than 0.1 M, otherwise silver nanowires were the major product. If the molar ratio between the repeating unit of PVP and AgNO3 was increased from 1.5 to 3, MTPs (multiply twinned particles (MTPs) with their surfaces bounded by the lowest-energy {111} facets (16).) became the major product. Silver nanocubes of various dimensions could be obtained by controlling the growth time (25).

Gold Nanoparticles Chlorauric acid dissolves into water to make 20 ml very dilute solution of 2.5x10-4M. Then 1 ml 0.5% sodium citrate is added into the boiling solution. The mixture is kept at 100oC till color changes, while maintaining the overall volume of the solution by adding water.

Synthesis of Pt nanoparticles Precursor: K2PtCl4, H2PtCl6 Reduction reagent: radiolysis, hydrogen, sodium citrate Stabilizer: PVA 1. radiolysis: The γ-ray of Co is used to generate hydrated electrons, hydrogen atoms and 1-hydorxylmethyl radicals. These radicals is used to reduce Pt2+ in K2PtCl4 to the zero valence state and form Pt particles of 1.8 nm. 2. sodium citrate: H2PtCl6 was mixed with sodium citrate and boiled for 1 hr, yields Pt particles of 2.5 nm (Turkevich method).

Synthesis of Pt nanoparticles 3. Hydrogen: a) K2PtCl4 in dilute aqueous solution were hydrolyzed to form hydroxides where sodium hydroxide is used as catalyst. The hydrogen reduction is used and PVA is used as stabilizer to obtain the Pt particles.

Synthesis of Pt nanoparticles by Hydrogen reduction

Synthesis of Tetrahexahedral Platinum Nanocrystals with High-Index Facets and High Electro-Oxidation Activity The shapes of noble metal nanocrystals (NCs) are usually defined by polyhedra that are enclosed by {111} and {100} facets, such as cubes, tetrahedra, and octahedra. Platinum NCs of unusual tetrahexahedral (THH) shape were prepared at high yield by an electrochemical treatment of Pt nanospheres supported on glassy carbon by a square-wave potential. The single-crystal THH NC is enclosed by 24 high-index facets such as {730}, {210}, and/or {520} surfaces that have a large density of atomic steps and dangling bonds. These high-energy surfaces are stable thermally (to 800°C) and chemically and exhibit much enhanced (up to 400%) catalytic activity for equivalent Pt surface areas for electro-oxidation of small organic fuels such as formic acid and ethanol. Starting from Pt nanospheres electrodeposited on glassy carbon (GC) substrate instead of bulk Pt, we obtained THH Pt NCs at high yield. Electrochemical preparation was carried out in a standard three-electrode cell at room temperature (25). All electrode potentials are reported on the scale of a saturated calomel electrode (SCE). In a typical experiment, polycrystalline Pt nanospheres of size about 750 nm (fig. S1) were electrodeposited on a GC electrode in a solution of 2 mM K2PtCl6 + 0.5MH2SO4. The Pt nanospheres were then subjected to a square-wave treatment, with upper potential 1.20 V and lower potential between –0.10 and –0.20 V, at 10 Hz in a solution of 0.1 M H2SO4 + 30 mM ascorbic acid for 10 to 60 min. As illustrated schematically by Fig. 1A, THH Pt NCs were grown exclusively on GC surface at the expense of Pt nanospheres. (4 MAY 2007 VOL 316 SCIENCE)

Fig. 1. (A) Scheme of electrochemical preparation of THH Pt NCs from nanospheres. The Pt nanosphere is an agglomeration of tiny Pt nanoparticles of irregular shapes (fig. S1). Under the influence of the squarewave potential, new Pt NCs of THH shape grow at the expense of the large nanospheres (the large nanosphere is “dissolved” into smaller ones, which eventually transform into THH shape) (B) Low-magnification SEM image of THH Pt NCs with growth time of 60 min. (C and D) High-magnification SEM images of Pt THH viewed down along different orientations, showing the shape of the THH. (E) Geometrical model of an ideal THH. (F) High-magnification SEM image of a THH Pt NC, showing the imperfect vertices as a result of unequal size of the neighboring facets. Scale bars in (C), (D), and (F), 100 nm.

Fig. 3. Size control of THH Pt NCs and their thermal stability Fig. 3. Size control of THH Pt NCs and their thermal stability. SEM images of THH Pt NCs grown at (A) 10, (B) 30, (C) 40, and (D) 50 min. The insets in (A) and (B) are the high-magnification SEM images that confirm the shape of THH. Scale bars, 200 nm. (E) Size distributions of THH Pt NCs in (A), B), (C), and (D), respectively, after counting more than 500 particles for each sample. (F) In situ TEM observation on the thermal stability of THH Pt NCs. The images were recorded at various temperatures in TEM at a heating rate of 7°C/min. The NC preserves its shape to ~815°C and even higher with a slight truncation at the corners and apexes, as seen in the TEM image.

Influences of Reduction Reagents

Effect of Reduction Reagents Reduction Reagent Shape Sodium citrate spherical Hydrogen peroxide spherical Hydroxylamine hydrochloride cubical {100} facets Citric acid trigons or very thin platelets of o trigonal symmetry {100} facets Concentration and pH value of the reduction reagent have noticealbe effect on the shape of nanoparticles. For example, lowering the pH value caused the {111} facets to develop at the expense of the {100} facets.

Stronger reduction reagent

Explanations For transition metals, in general, the stronger the reduction reagent, the smaller the nanoparticles will be produced. The stronger reduction reagent would generate an abrupt surge of the concentration of growth species, resulting in a very high supersaturation. Consequently, a large number of initial nuclei would form. For a given concentration of metal precursor, the formation of a large number of initial nuclei would result in a smaller size of the grown nanoparticles.

Increasing amount of Cl affects shape and size of nanoparticles

Effect of Na2CO3

Effect of Na2CO3 Silver particles of 7-20 nm with spherical shape were obtained for the mixture ratio from 1 to 1.5. More Na2CO3 results in higher pH value or higher concentration of hydroxyl ions, which would promote the reduction rate and lead to production of a large quantity of growth species and shift the growth away from diffusion limited process.

Effects of Polymer A strong absorption of polymer would occupy the growth sites and reduce the growth rate of nanoparticles. A full coverage of polymer can hinder the diffusion of growth species from the surrounding solution to the surface of the particles. In addition, polymer may interact with solute, catalyst, or solvent and affect reaction.

Effect of Polymer

Effect of Pt ions/polymer

Without Using Polymer

Synthesis of Semiconductor Nanocrystal Precursor: Dimethylcadmium (Me2Cd), trioctylphosphine selenide (TOPSe), [trioctylphosphine telluride (TOPTe)] Solvents and capping materials (coordinating solvents): mixed tri-n-octylphosphine (TOP) and tri-n-octylphosphine oxide (TOPO) solutions Purification Process

Synthesis Process (I) 50 gm of TOPO is dried and degassed in the reaction vessel by heating to 200oC at 1 torr for 20 min, flushing periodically with argon. The temperature of the reaction flask is stabilized at 300oC under 1 atm of argon.

Synthesis Process (II) 1.0mL of Me2Cd is added to 25.0 mL of TOP in the dry box. 10.0 mL of 1.0 M TOPSe stock solution is added to 15.0 mLof TOP. Both solutions are combined and loaded into a syringe in the dry box. Heat is removed from the reaction vessel. The cold solution is injected into the vigorously stirring, hot reaction flask. This leads to homogeneous nucleation. Heat is added, due to sudden decrease in temperature to 180oC, to raise temperature and maintain at 230-260oC. This can promote slow growth of initial nuclei, nuclei with smaller size may dissolve back into solution and cause further growth of large particles—Ostwald ripening—leading to monodispersed colloidal solution.

Purification Process (I) The colloidal dispersion is cooled down to 60oC and added with 20 mL of anhydrous methanol, resulting in the reversible flocculation of the nano crystallites. The flocculate is separated from the supernatant by centrifugation. After further centrifugation, the flocculate is dispersed in 25 mL of anhydrous 1-butanol. Adding 25 mL of anhydrous methanol to the supernatant produces flocculation of the crystallites, and removes excess TOP and TOPO.

Purification Process (II) and narrowing of the size distribution A final rinse of the flocculate with 50 mL of methanol and subsequent vacuum drying produces 300 mg of free flowing TOP/TOPO capped nanocrystallites. The purified nanocrystallites are dispersed in anhydrous 1-butanol. Then, anhydrous methanol is added drop wise until opalescence persists upon stirring or sonication. Separation of supernatant and flocculate by centrifugation produces a precipitate enriched with largest crystallites. Dispersion of the precipitate in 1-butanol and size-selective precipitation with methanol is repeated until no narrowing of the size distribution.

Attenuation and broadening in (102) and (103) peaks are due to stacking faults along (002) axis.

Thermal Decomposition of Complex Precursor in a High-Boiling Solvent Precursor 1: mix GaCl3 and P(SiMe3)3 in a molar ratio of Ga:P of 1:1 in toluene to produce [Cl2GaP(SiMe3)2]2, a precursor of GaP. Precursor 2: mix Chloroindium oxalate and P(SiMe3)3 in CH3CN to form InP precursor. Precursor 3: mix Chlorogallium oxalate, Chloroindium oxalate and P(SiMe3)3 in toluene at room temperature to form GaInP2 precursor. Stabilizer: a mixture of TOP and TOPO.

Thermal Decomposition of Complex Precursor in a High-Boiling Solvent The precursor is dissolved in the colloidal stabilizer at elevated temperature for several days to produce nanocrystals capped with TOPO. Different sizes ranging from 2.0 to 6.5 nm are obtained by changing the precursor concentration or by changing the temperature. The narrow size distribution is achieved by 1) the slow process rate of the decomposition reaction of the precursors, 2) the steric diffusion barrier of TOP and TOPO monolayer on the surface of nanoparticles.

Thermal Decomposition to Produce GaAs Nanoparticles Precursor:Li(THF)2As(SiMe3)2 (THF = tetrahydrofuran) is added to a pentane solution of [(C5Me5)2GaCl]2, followed by filtration, evaporation of the solvent, and recrystallization, to form pure (C5Me5)GaAs(SiMe3)2, a precursor of GaAs. The precursor is dissolved in organic solvent such as alcohol and undergoes thermal decomposition at T>60oC exposed to air to form GaAs nanoparticles.

CdS Nanopaticles Mixing Cd(ClO4)2 and (NaPO3)6 solutions with pH adjusted with NaOH and bubbled with argon gas. H2S is injected into the gas phase and the solution is vigorously shaken.

Synthesis of Oxide Nanoparticles Reaction and growth in the formation of oxide nanoparticles are more difficult to manipulate, since oxides are generally more stable thermally and chemically than most semiconductors and metals. For example, Ostwald ripening process applied in the synthesis of oxide nanoparticles to reduce size; the results may be less effective than in other materials.

Sol-Gel Process What is sol-gel ? Reaction of chemicals in solution to produce nanometer-sized primary particles called “sols” “sols” can then be linked to form a three-dimensional solid network, called “gel”. Why sol-gel ? A typical gel structure is characteristically very uniform and is porous. Thus, has high surface area. Low processing temperature, molecular level homogeneity.

Sol-Gel Process Particularly useful in making complex metal oxide, temperature sensitive organic-inorganic hybrid materials, and thermodynamically unfavorable or metastable materials.

Sol Gel Processing Precursors: metal alkoxides, inorganic and organic salts Process: hydrolysis and condensation of precusors Organic or aqueous solvents may be used to dissolve precursors, and catalysts are often added to promote hydrolysis and condensation.

Hydrolysis and Condensation Metal alkoxides

Key Issues The key issue is to promote temporal nucleation followed by diffusion-controlled subsequent growth. The particle size can be varied by changing the concentration and aging time and is in the region from 1 to 100 nm. However, the stabilization of colloids is achieved by electrostatic double layer mechanism. Therefore, the diffusion controlled growth is achieved through other mechanisms (not by polymeric barrier) , such as controlled release and a low concentration of growth species in the sol.

Preparation of Silica Spheres Precursors: silicon alkoxides with different alkyl ligand sizes Catalyst: ammonia Solvent: alcohol Process: mix alcohol, solvent and water, then add silicon alkoxide precursor under vigorous stirring. Depending on the different kinds of precursors and solvents, and the amount of water and ammonia used, spherical silica particles with mean sizes ranging from 50 nm to 2 μm can be obtained. The reaction rate also depends on the above parameters.

Preparation of Silica Spheres For different kind alcoholic solvents, reaction rates are fastest with methanol, slowest with n-butanol. Final particle sizes are the smallest with methanol, the biggest with n-butanol. In addition, smaller ligand [e.g. Si(OC2H5)4] results in faster reaction rate, larger particle size, and vise versa. Both hydrolysis and condensation reaction rate strongly depend on temperature.

Preparation of α-Fe2O3 Nanoparticles Precursor: FeCl3 Solvent: HCl Process: FeCl3 solution is mixed with HCl and diluted. The mixture is added into preheated water at 95 – 99oC with constant stirring. The solution is kept in a sealed bottle at 100oC for 24 hr before quenched in cold water. The high temperature favors a fast hydrolysis reaction and results in high supersaturation, which leads to formation of a large number of small nuclei. Dilution is to ensure a controlled nucleation and diffusion controlled growth. The long period of heating allows occurrence of Ostwald ripening process to further narrow the size distribution.

Controlled Release of Ions Controlled release of anions and/or cations has a significant effect on the kinetics of nucleation and subsequent growth of oxide nanoparticles, and is achieved by the spontaneous release of anions from organic molecules. For example, heating solution of urea, CO(NH2)2, causes liberating hydroxide ions, which causes precipitation of metal oxide or hydroxide. Certain types of anions are used as catalyst. Anions can have other effects on the processing and morphology of the nanoparticles.

ZnO Nanoparticles Precursor: zinc acetate is dissolved into methanol to form zinc alkoxide precursor.

Vapor Phase Reaction to form GaAs Nanoparticles Synthesis is at elevated temperature and low pressure. The low pressure is to ensure a low concentration of growth species so as to promote diffusion-controlled growth. Precursor: Trimethyl gallium and AsH Reduction agent: H2 (also used as carrier gas) Homogeneous nucleation and reaction occurs at 700oC. Nanoparticles are collected on a holey carbon film downstream at 350oC.

Solid State Phase Segregation Nanoparticles of metals and semiconductors in glass matrix are formed by homogeneous nucleation in the solid state. First, the metal or semiconductor precursors are introduced to and homogeneously distributed in the liquid glass melt at high temperature during glass making, before quenching to room temperature. Then the glass is annealed by heating to a temperature about the glass transition point and held for a desired period of time. During annealing, the precursors are converted into metals or semiconductors. The supersaturated metals can form nanoparticles through nucleation and subsequent growth via solid-state diffusion.

concentration

Synthesis of Nanoparticles through Heterogeneous Nucleation

Through Heterogeneous Nucleation

Kinetically Confined Synthesis of Nanoparticles – Inside Micelles 1. Synthesis Inside Micelles: Fabrication of Micelles: Mesoporous materials, see figure in next page. 2. Reaction proceeds only inside the micelles and particle stops growing when the reactants are consumed.

Use of Microemulsion for Synthesis of Nanoparticles Microemulsion: a dispersion of fine liquid droplets of an organic solution (oil droplets) in an aqueous solution (water). The chemical reaction can take place at the interfaces between the organic droplets and aqueous solution, when the reactants are introduced into two non-mixable solution, or inside the organic droplets when all the reactants are dissolved into the organic droplets.

Synthesis of Nanoparticles using microemulsions

Aerosol Synthesis 1. top down approach 2. particles are polycrystalline 3. particles prepared need be collected and redispersed for many applications.

Synthesis Process Precursor: liquid precursor, a mixture solution or a colloidal dispersion. Making into liquid aerosol: dispersion of liquid droplets in air. This can be done by sonication or spining. Liquid droplets solidify through evaporation of solvent or further react with the chemicals that are present in the gas.

Growth Termination for CdS Nanoparticles The growth surface attached by organic components or alien ions. Growth process stops when the growth sites are all occupied. 1. Cadmium acetate is dissolved into methanol, [Cd] = 0.1 M. 2. Sodium sulfide in a mixture of water and methanol in 1:1 volume ratio, [S2-] = 0.1 M. 3. Thiophenol in methanol, [PhSH] = 0.2 M.

Synthesis Process Solutions B and C are mixed first, then solution A is added in an overall volume ration of A:B:C = 2:1:1. CdS particle sizes varies with the relative ratio of sulfide to thiophenol and ranges from less than 1.5 nm to 3.5 nm.

Spray Pyrolysis This technique is used for preparation of metal and metal oxides. The process is to convert microsized liquid droplets of precursor or precursor mixture into solid particles through heating. Example for Ag nanoparticles: Precursor solutions: Ag2CO3, Ag2O and AgNO3 with NH4HCO3 at temperature of 400oC.

Template-Based Synthesis Iron oxide, Fe3O4 nanoparticles dispersed in a solid polymer matrix can be made by infiltration of ion chloride solution. The polymer matrix are cation exchange resins, which are formed by beads of 100 - 300μm in diameter and contain micropores. The microporous medium can also be zeolite – crystalline microporous aluminosilicates.