Che5700 陶瓷粉末處理 Synthesis of ionic compounds Majority liquid phase method involves ionic compounds, calculation of supersaturation requires ionic products Ksp; Another characteristics: charge neutrality Ions in solution: often hydrated!, dehydration process is always involved; ions with slow dehydration – often form hydrated compounds. In general: hydration number, for cations: 4-6; for anions: 0-2; Interaction between ions and water molecule: electrostatic forces, thus electronic structure or charge density – main factor to determine this force pH always an important factor: affects ion-pairing; complex ion formation etc.;

Characteristics of ionic solutions Che5700 陶瓷粉末處理 Solvent molecule: may adsorb on particle surface, thus shows effect, different solvents (or small additions, co-solvent) will affect nucleation and growth behavior; Other factors: e.g. ultrasonic, electrical field, magnetic field, etc. may also show effects Ion pairs formation  change distribution of species  C A = [A] + [AB] + 2 [A 2 B] + [AB 2 ] + …. driving force in ionic solution: (I.P) 1/2 – Ksp 1/2 (IP = ionic product; Ksp solubility product)

Right figure: 5,6,7 region: suspended solution, 5,6 with charged, stable suspension

Different pH: often obtain different ionic compounds, in theory, it is the one with the lowest solubility (thermodynamic reasons) Yet different ionic compounds involves complex chemistry, difficult to predict (kinetic reasons)

Nielsen found: solubility of ionic compound related to its surface energy:  /kT = 2.82 – ln(Cs) Cs: mol/m 3

Metal Ions in Solution Che5700 陶瓷粉末處理 Ionic radius Na+ < K+< Ag+ ; number of salts containing water of crystallization  19 for Na+; 14 for K+; 4 for Ag+ Metal ions: in addition to ion pair formation, may also form polynuclear complex (sort of precursor to nucleus) All metal ions in solution are hydrated; when z/d increase, easier to release H+, or first hydrolysis constant Bronsted acid: provide proton (proton donor) Lewis acid: to provide empty electron orbital to accept electrons

Classification of metal ions Che5700 陶瓷粉末處理 ======================

Another classification Che5700 陶瓷粉末處理 Metal ion can associate with OH in water, to release H +, act like an acid

Stability Sequence Che5700 陶瓷粉末處理 To the left: A type metal cations B type

Stepwise Hydrolysis Reaction for Al == 6 Al  8 Al

Distribution of Fe Species Calculated based on stability constant

From Fe(OH) 3 to Fe 2 O 3 source: 李崧助 MS thesis (NTHU, 2008) Two transformation routes from hydroxide to oxide under boiling condition: (a) dissolution – re- crystallization; (b) in-situ solid state transformation Case 1: adding Fe +2 as catalyst to accelerate dissolution and re-crystallization process; Case 2: adding urea or NaHCO 3 to accelerate solid state transformation process (possible via action of CO 2 bubbles – need to be proven)

Fe +2 concentration dropped at first, then gradually increase back to original concentration; Fe +3 concentration increase with time, while pH of solution decrease with time FeOOH formed immediately when Fe +2 was added;

Some Possible Reactions Fe(OH) 3 + Fe +2 + H 2 O  FeOOH + Fe(OH) 2 + H + Fe(OH) 3 + H + + e -  Fe +2 + H 2 O (reductive dissolution) e- may come from: Fe +2  Fe +3 + e- or Fe(OH) 2 + OH -  Fe(OH) 3 + e- hydrolysis of Fe+3 and subsequent transformation into oxide Fe H 2 O  Fe(OH) H + 2 Fe(OH) 2 +  Fe 2 O 3 + H 2 O + H +

Initial pH = 7; t= 20, 50 & 90 minutes; (final pH = 5.4 (20 min; 3.1 (90 min)) Initial pH = 9, t = 4 hours, still pure Fe 3 O 4 phase; (final pH = 7.3)

Possible Mechanism for Solid State Transformation Dirk Walter ( Thermochimica Acta, 445 (2006) 195 ) 含水氧化鐵 O2-O2- OH - continue to move toward interface Urea and sodium bicarbonate decompose to produce CO 2, possibly exhibiting similar effects H2OH2O H + and O 2 - combine together

First Hydrolysis Constant * In figure d=M-O bond distance; four types of metal ions

Solubility Product & Hydrolysis Constant Overall reaction K = (*K 1 ) z K so As shown in previous figure, according to its charge, four types can be found; Often when the first water molecule left, the precipitation occurs;

Solubility and supersaturation Due to changes in species, at different pH, solubility and corresponding supersaturation also changes Che5700 陶瓷粉末處理

Hydroxo, Oxo complexes from metal ions

Start with simple hydration  lost one H +, to become hydroxo  lost another H + to become oxo complex

Effects of anions Che5700 陶瓷粉末處理 Mostly: chloride, nitrate, carbonate, sulfate, perchlorate, phosphate, etc; [counter-ions] They will affect particle size, shape, composition, etc. Anions entering particle: may not be easy to eliminate by calcination, may require higher temperature to lower to ppm level; Different anions – different morphology, e.g. Fe(OH) FeSO H 2 O  hexagonal Fe 3 (OH) 5 (SO 4 ) 2 2H 2 O + 4 H + 3 Fe(OH) +2 + Fe(SO 4 )+ + 4 H 2 O  monoclinic Fe 4 (SO 4 )(OH) H +

Mixing Che5700 陶瓷粉末處理 Cation and anions from two different sources, always require mixing, though very fast, still take several seconds, its effect may not be negligible. Method of addition: direct strike, reverse strike (add alkaline to metal ion solution or reverse), it will affect reaction rate and thus rate of supersaturation. Sequence of events: mixing  reaction  supersaturation  nucleation/growth/agglomeration  aging  final particle size distribution, shape, crystallinity;

Co-precipitation Che5700 陶瓷粉末處理 Classification:  Isomorphous mixed crystal formation: structure similar or formation of compounds Anomalous mixed crystal formation: one of which should not precipitate, yet was removed from solution Adsorption Occlusion (inclusion): trapped during growth Post-precipitation: after precipitation of the first compound, either independent or in coating form distribution coefficient (B/A) in whole crystal = D (B/A) sol (B/A) on surface = (B/A) sol D: homogeneous distribution coefficient; : logarithmic distribution coefficient (more often)

Chemical Reactions Examples  NiSO FeSO (NH 4 ) 2 C 2 O 4. H 2 O  (at 60 o C)  5 Ni 0.2 Fe 0.8 C 2 O 4. 2H 2 O + 10 NH SO 4 -2  Ba(OC 3 H 7 ) 2 + Ti(OC 5 H 11 ) 4 in alcohol solution + H 2 O  BaTiO C 3 H 7 OH + 4 C 5 H 11 OH

Monodispersed Colloids Che5700 陶瓷粉末處理 example: micelles, latex, micro-emulsion, photographic emulsion, ceramics, inorganic chemicals (pigment, pharmacy, catalyst, etc); stress micron or sub-micron size; now: nanosize Applications: model system for fundamental studies; may have better packing and sintering results; Formation: (a) separate nucleation and growth steps: best strong dependency of nucleation on supersaturation and low growth rate; (b) resist agglomeration (based on electrostatic force, surfactant, polymers, gel network etc.); (c) choice of growth modes: diffusion or surface reaction controlled

Typical Monodispersed Systems Homogeneous systems :  Redox reaction  Hydrolysis: such as alkoxide in alcohol; high temperature forced hydrolysis in acid; etc.  Decomposition of compounds: Cd ion + TAA  CdS  Reaction of chelates: M-EDTA complex Heterogeneous systems :  Emulsion polymerization  Reaction in microemulsions  Reaction in aerosols  Recrystallization: involves dissolution

Homogeneous system: depend on control of reaction rate, in a clean system, rapid nucleation to consume supersaturation, so particle grow to the same size in a short period

Core may have time to become crystallized; material in diffusion – may be nanosized colloids

ZnS colloids: use TAA, TAA = thioacetamide (S=C- (CH 3 )(NH 2 ); it will slowly hydrolyze to release S -2, to do the precipitation reaction, slower than use of Na 2 S, with more uniform particle size 但粒徑均勻.

Reaction or Diffusion Control Che5700 陶瓷粉末處理 Diffusion control or surface reaction control of growth rate; Results show: diffusion control – size distribution become narrower with size; similar for reaction control (different scale)

Results from semi-batch Stober process SiO2 (from 陳 貞志 )

Hydrothermal Method Che5700 陶瓷粉末處理 Higher reaction temperature (higher pressure), need to use autoclave, thus higher reaction rate. To some strong acid solid, we can obtain oxide directly, or at least oxo-hydroxides (e.g. Zr, Zn…)

Use hetereoaggregation to prepare core- shell particles example: latex particle + polyallylamine hydrochloride to make it positive charge, easy to adsorb negative charged CdTe colloids (from adv. Mater. 13(22), 1685, 2001