Jackson S.D., Hargreaves J.S.J. Metal Oxide Catalysis

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16.3 Synthesis 629

material in the presence of cetyltrimethylammonium chloride and ammonia

provided higher surface areas. Moreover, surface area and thermal stability were

found to depend on digestion time. In their study, digestion at 110 ° C for 100 hours

provided high thermal stability and the highest surface area of 370 m

− 1

; the same

sample when calcined at 700 ° C possessed a surface area of 160 m

− 1

, which was

the best value obtained to date. Zirconia samples that were digested more than

100 hours had high thermal stability but no increases in surface area were observed.

The same samples that were dried by applying supercritical techniques did not

yield better results.

As previously stated, a common method for producing crystalline nanoparticle

oxides is the co - precipitation of metal cations as carbonates, dicarbonates or oxa-

lates, followed by their subsequent calcination and decomposition. Unfortunately,

the calcination invariably leads to agglomeration or, at high temperatures, aggrega-

tion and sintering. As an example, CeO

nanopowders have been prepared by cal-

cining the product of the precipitation between Ce(NO

)

and (NH

)

, resulting

in crystalline 6 nm particles of CeO

at calcination temperatures as low as 300 ° C

[106] . NiO with 10 – 15 nm particles has been similarly prepared by precipitating

aqueous Ni

solutions with (NH

)

and calcining the products at 400 ° C [107] .

In some rare instances, crystalline oxides can be precipitated from aqueous solu-

tion, eliminating the need for a calcination step and greatly reducing the risk of

agglomeration. For example, TiO

can be prepared by precipitation using aqueous

TiCl

with NH

(aq) under ambient conditions. The products were 50 – 60 nm aggre-

gates and stabilized with poly(methyl methacrylate) [108] .

The direct co - precipitation of complex ternary oxides is somewhat uncommon,

but is nonetheless possible, particularly when the product assumes a very thermo-

dynamically favorable structure such as spinel. In such cases, the precipitation

reactions are normally carried out at elevated temperatures (50 – 100 ° C). Condensa-

tion of the two hydroxide intermediates into oxides and the induction of co -

Table 16.2 Survey from the literature of ZrO

prepared using precipitation methods.

Starting

material

Ppt agent Surfactant

Conditions Surface area

− 1

)

Ref.

ZrOC1

N H

(aq) none calcined 450 ° C 247 [97]

ZrOC1

N H

(aq) C

TACl calcined 450 ° C 274 [97]

ZrOC1

N H

(aq) C

TACl calcined 450 ° C 300 [97]

ZrOC1

N H

(aq) C

TACl calcined 450 ° C 312 [97]

ZrOC1

N H

(aq) C

TACl calcined 450 ° C 313 [97]

ZrOC1

NaOH P

123

calcined 450 ° C 103 [98]

ZrOC1

NaOH P

123

calcined 600 ° C 51 [98]

Zr(NO

)

TEA CTAB calcined 500 ° C – [99]

ZrOC1

N H

(aq) CTAB calcined 600 ° C 168 [100]

ZrOC1

N H

(aq) CTAB calcined 800 ° C 105 [100]

a C

TACl, dodecyltrimethylammonium chloride; P

123

, poly(ethylene oxide)-b-poly(propylene

oxide)-b-poly(ethylene oxide) triblock copolymer; CTAB, cetyltrimethylammonium bromide.

630 16 Properties, Synthesis and Applications of Highly Dispersed Metal Oxide Catalysts

precipitation were carried out in the same reaction vessel. For instance, Pr

- doped

ceria has been precipitated to yield monodispersed 13 nm particles by aging

aqueous solutions of Ce(NO

)

and PrCl

at 100 ° C in the presence of a hexamethy-

lenetetramine stabilizer [109] . In such a case, the stabilizer indirectly serves as the

precipitating agent by raising the pH sufﬁ ciently to induce precipitation of the

metal hydroxides. Likewise, MnFe

was prepared from aqueous Mn

and Fe

at temperatures up to 100 ° C to yield 5 – 25 nm particles [110] .

Chinnasamy and coworkers have reported an extensive series of experiments

for the preparation of spinel - structured CoFe

[111] . The sizes of the products

were determined by the inﬂ uence of reaction temperature, reactant concentration

and reactant addition rate. In each case, aqueous solutions of Fe

and Co

were

precipitated with dilute NaOH. The results were predominantly in line with expec-

tations based on the considerations outlined in reaction temperature, reactant

concentration and reactant addition rate. Increasing the temperature from 70 ° C

to 98 ° C increased the average particle size from 14 to 18 nm. Increasing the NaOH

concentration from 0.73 to 1.13 M increased particle size from 16 to 19 nm. NaOH

concentrations of 1.5 M or greater resulted in the formation of a secondary FeOOH

phase and slowing the NaOH addition rate appeared to broaden the particle size

distribution. Li and coworkers also prepared 12 nm CoFe

by a similar route but

stabilized the product by acidiﬁ cation with dilute nitric acid [112] .

A summary of oxides precipitated from aqueous solutions, including the rele-

vant reaction conditions, is given in Table 16.3 [113 – 119] .

16.3.2.2 Sonochemical Co - precipitation

The principle of sonochemistry is breaking the chemical bond with the application

of high - power ultrasound waves, usually between 10 and 20 MHz. The physical

Table 16.3 Summary of reactions for the precipitation of oxides from aqueous solution.

Oxide Starting

material

Ppt agent Stabilizer Conditions Product size

(nm)

Ref.

FeCl

N H

(aq) H

calcined 550 ° C 53.5 [113]

TiO

Ti(SO

)

N H

(aq) H

calcined 550 ° C 9.2 [113]

Al(NO

)

N H

(aq) H

calcined 550 ° C 13.2 [113]

CeO

Ce(NO

)

N H

(aq) none calcined 650 ° C 12 – 15 [114]

TiO

TiCl

N a

NaCl calcined 700 ° C 30 [115]

N H

· H

O none calcined 300 ° C 35 [116]

· H

O none calcined 500 ° C 30 [116]

γ - Mn

KMnO

· H

O none – 8 [116]

FeCl

N H

(aq) H

atm 8 – 50 [117]

NiO NiCl

N H

(aq) CTAB annealed 500 ° C 22 – 28 [118]

ZnO ZnCl

N H

(aq) CTAB annealed 500 ° C 40 – 60 [118]

SnO

SnCl

N H

(aq) CTAB annealed 500 ° C 11 – 18 [118]

SbCl

NaOH PVA annealed 350 ° C 10 – 80 [119]

16.3 Synthesis 631

phenomenon responsible for the sonochemical process is acoustic cavitation.

According to published theories for the formation of nanoparticles by sonochem-

istry, the main events that occur during the preparation are creation, growth and

collapse of the solvent bubbles that are formed in the liquid. These bubbles are in

the nanometer size range. Solute vapors diffuse into the solvent bubble and when

the bubble reaches a certain size, its collapse takes place. During the collapse very

high temperatures of 5000 – 25 000 K [127] are obtained, which is enough to break

chemical bonds in the solute. The collapse of the bubble takes place in less than

a nanosecond [128, 129] hence a high cooling rate (1011 K s

− 1

) is also obtained. This

high cooling rate hinders the organization and crystallization of the products.

Since the breaking of bonds in the precursor occurs in the gas phase, amorphous

nanoparticles are obtained. Though the reason for the formation of amorphous

products is well understood, the formation of nanostructures is not. One possible

explanation is that in each collapsing bubble a few nucleation centers are formed

and while the fast kinetics does not stop the growth of nuclei that growth is limited

by the collapse. Another possibility is that the precursor is a non - volatile com-

pound and the reaction occurs in a 200 nm ring surrounding the collapsing bubble

[130] . In the latter case, the sonochemical reaction occurs in a liquid phase and

the products could be either amorphous or crystalline depending on the tempera-

ture in the ring region of the bubble. Suslick has estimated the temperature of the

ring region as 1900 ° C. The sonochemical method has been found useful in many

areas of material science, from the preparation of amorphous products [131, 132]

through the insertion of nanomaterials into mesoporous materials [102, 103] to

the deposition of nanoparticles on ceramic and polymeric surfaces [104, 105] .

Sonochemical methods for the preparation of nanoparticles were pioneered by

Suslick in 1991 [127] . If a reaction is carried out in the presence of oxygen by

similar methods, the product formed will be an oxide. NiFe

has been synthe-

sized by sonicating the mixture of Fe(CO)

and Ni(CO)

in decalin under 1 – 1.5 atm

pressure of oxygen [133] . ZrO

was synthesized using Zr(NO)

and NH

(aq) in

aqueous phase under ultrasonic irradiation, followed by calcination at 300 – 1200 ° C

to improve the crystallinity or, for temperatures of 800 ° C and above, to induce

the tetragonal to monoclinic phase transformation [100, 134] . Nanocrystalline

1 - x

MnO

was prepared in a similar manner [135] . The method has also been

successfully applied to synthesize Ni(OH)

and Co(OH)

nanoparticles [136] and

iron(III) oxide [137] . Polycrystalline CeO

nanorods 5 – 10 nm in diameter and 50 –

150 nm in length were synthesized via ultrasonication using polyethylene glycol

(PEG) as a structure - directing agent at room temperature. The content of PEG,

the molecular weight of PEG and the sonication time were conﬁ rmed to be the

crucial factors determining the formation of one - dimensional CeO

nanorods. A

possible ultrasonic formation mechanism is suggested in Figure 16.9 [138] .

16.3.2.3 Microwave - Assisted Co - precipitation

The microwave processing of nanoparticles results in rapid heating of the reaction

mixtures, particularly those containing water. As a consequence, the precipitation

632 16 Properties, Synthesis and Applications of Highly Dispersed Metal Oxide Catalysts

of particles from such solutions tends to be rapid and nearly instantaneous,

leading to very small particle sizes and narrow size distributions within the prod-

ucts. The method offers the additional beneﬁ t of requiring very short reaction

times and has similarly been used for the synthesis of Ag and Au nanoparticles

and transition metal oxides. Tu and coworkers [139] adapted a continuous ﬂ ow

reactor which is depicted in Figure 16.10 . For example, two nano - structured

cerium(IV) oxide powders were synthesized by different synthetic routes: two

samples were obtained by precipitation from a basic solution of cerium nitrate and

treated at 523 and 923 K, respectively. A broad particle size distribution is observed

for CeO

obtained by precipitation (8.0 – 15.0 nm). Smaller particles (sizes around

3.3 – 4.0 nm) with a narrow particle size distribution characterize the ceria obtained

by microwave irradiation. Uniform α - Fe

nanoparticles were prepared by forced

hydrolysis of ferric salts under microwave irradiation. Gedanken ’ s group has also

published extensively on various oxides and chalcogenides prepared by microwave -

assisted irradiation [120 – 122, 140] .

Figure 16.9 Possible formation mechanism of CeO

nanorods (from [139] ).

Figure 16.10 Schematic of a continuous - ﬂ ow microwave

reactor consisting of (a) a liquid column as a pressure

regulator, (b) a metal salt solution container, (c) a microwave

oven cavity, (d) a metal cluster dispersion receiver and (e) a

spiral tube reactor (from [139] ).

16.3 Synthesis 633

16.3.3

Solvothermal Technique

Hydrothermal processes [123 – 126] involve using water at elevated temperatures

and pressures in a closed system, often in the vicinity of its critical point. A more

general term, “ solvothermal, ” refers to a similar reaction in which a non - aqueous

solvent (organic or inorganic) is used. Under solvo(hydro)thermal conditions,

certain properties of the solvent, such as density, viscosity and diffusion coefﬁ -

cient, change dramatically and the solvent behaves much differently from what is

expected under ambient conditions [126] . Consequently, the solubility, the diffu-

sion process and the chemical reactivity of the reactants (usually solids) are greatly

increased or enhanced, enabling the reaction to take place at a much lower tem-

perature than normal. The method has been widely applied and adopted for crystal

growth of many inorganic materials, such as zeolites, quartz, metal carbonates,

phosphates and other oxides and halides [141 – 148] .

Solvothermal techniques have been extensively developed for the synthesis of

metal oxides [149 – 152] . Unlike many other synthetic techniques, solvothermal

synthesis concerns a much milder and softer chemistry conducted at low tempera-

tures. The mild and soft conditions make it possible to leave polychalcogen build-

ing - blocks intact while they reorganize themselves to form various new structures,

many of which might be promising for applications in catalysis, electronic, mag-

netic, optical and thermoelectronic devices [153 – 155] . They also allow the forma-

tion and isolation of phases that may not be accessible at higher temperatures

because of their metastable nature [156, 157] .

Although some solvothermal processes involve supercritical solvents, most

simply take advantage of the increased solubility and reactivity of metal salts and

complexes at elevated temperatures and pressures without bringing the solvent to

its critical point. The metal complexes are decomposed thermally either by boiling

the contents in an inert atmosphere or by using an autoclave. A suitable capping

agent or stabilizer such as a long - chain amine, thiol, trioctylphosphine oxide

( TOPO ), etc. is added to the reaction contents at a suitable point to hinder the

growth of the particles and hence stabilize them against agglomeration. The sta-

bilizers also help in dissolution of the particles in different solvents. Unlike the

cases of co - precipitation and sol – gel methods, solvothermal processes also allow

substantially reduced reaction temperatures, and the products of solvothermal

reactions are usually crystalline and do not require post - annealing treatments.

The synthesis of nanocrystalline TiO

, which is an important photocatalyst for

the decomposition of toxic chemicals, is one of the more thoroughly investigated

solvothermal/hydrothermal reactions. Approaches to this preparation have

involved the decomposition of metal alkoxides, [158] a TOPO - capped autoclave

synthesis of TiO

by metathetic reaction [159] and decomposition of a metal N -

nitroso - N - phenyl hydroxylamine complex [160, 161] . In 1988, Oguri and coworkers

reported the preparation of anatase by hydrothermally processing hydrous titania

prepared by the controlled hydrolysis of Ti(OEt)

in ethanol [162] . The reaction

conditions leading to monodispersed anatase nanoparticles by this approach

634 16 Properties, Synthesis and Applications of Highly Dispersed Metal Oxide Catalysts

were elucidated by others [163] . Extension of the method to the preparation of

lanthanide - doped titania particles has also been reported [164] . Cheng and cowork-

ers developed a method for preparing nano - scale TiO

by hydrothermal synthesis

using an aqueous TiCl

solution [165] . They found that acidic conditions favored

rutile while basic conditions favored anatase. It was also found that higher tem-

perature favored the highly dispersed product and that grain size could be con-

trolled by the addition of minerals such as SnCl

or NaCl, although the presence

of NH

Cl led to agglomeration of particles. The approach was extended, and

revealed that phase purity of the products depends primarily on concentration,

with higher concentrations of TiCl

favoring the rutile phase, while particle size

depends primarily on reaction time [166] . Yin and coworkers produced 2 – 10 nm

crystallites of monodispersed, phase - pure anatase by using citric acid to stabilize

the TiO

nanoparticles and treating the precursors hydrothermally in the presence

of KCl or NaCl mineralizers [167] .

Niederberger and coworkers [150] reported a widely applicable solvothermal

route to nanocrystalline iron, indium, gallium and zinc oxides based on the reac-

tion between the corresponding metal acetylacetonate as metal oxide precursor

and benzylamine as solvent. They proved that with the exception of the iron oxide

system, in which a mixture of the two phases magnetite and maghemite is formed,

only phase - pure materials are obtained, γ - Ga

, zincite ZnO and cubic In

. The

particle sizes lie in the ranges 15 – 20 nm for the iron, 10 – 15 nm for the indium,

2.5 – 3.5 nm for the gallium and around 20 nm for the zinc oxide (Figure 16.11 ).

Moreover, the same group developed mixed nanocrystalline BaTiO

, SrTiO

and

(Ba,Sr) - TiO

. BaTiO

nanoparticles are nearly spherical in shape, with diameters

ranging from 4 to 5 nm, while SrTiO

particles display less - uniform particle shapes,

the size vaying between 5 and 10 nm [168] .

Masui and coworkers [169] reported the hydrothermal synthesis of nanocrystal-

line, monodispersed CeO

with a very narrow size distribution. They combined

CeCl

· 6H

O and aqueous ammonia with a citric acid stabilizer and heated the

solution in a sealed Teﬂ on container at 80 ° C. The CeO

nanoparticles exhibited a

3.1 nm average diameter. The ceria particles were subsequently coated with tur-

bostratic boron nitride by combining them with a mixture of boric acid and 2,2 ′ -

iminodiethanol, evaporating the solvent and heating at 800 ° C under ﬂ owing

ammonia. Inoue and coworkers were able to reduce the particle size of hydrother-

mally prepared colloidal CeO

to 2 nm using a similar approach by autoclaving a

mixture of cerium metal and 2 - methoxyethanol at 250 ° C [170] . In this case, the

autoclave was purged with nitrogen prior to heating and the colloidal particles

were coagulated after heating with methanol and ammonia. The higher reaction

temperatures resulted in increased particle size.

In many cases, anhydrous metal oxides have been prepared by solvothermal

treatments of sol – gel or micro - emulsion - based precursors. Wu and coworkers

prepared anatase and rutile TiO

by a micro - emulsion - mediated method, in which

the micro - emulsion medium was further treated by hydrothermal reaction [171] .

This micro - emulsion - mediated hydrothermal ( MMH ) method could lead to the

formation of crystalline titania powders under much milder reaction conditions.

16.3 Synthesis 635

The micro - emulsion medium was heated to 120 – 200 ° C in a stainless steel auto-

clave. Micro - emulsions acidiﬁ ed with HNO

produced monodispersed anatase

nanoparticles while those acidiﬁ ed with HCl produced rutile nanorods. Titanium

dioxide (TiO

) nanoparticles prepared this way have been shown to be active

toward the photocatalytic oxidation of phenol [172] .

Metal oxides can also be synthesized by the decomposition of metal – cupferron

complexes, M

Cup

(Cup = C

N(NO)O

−

) [160] . Seshadri and coworkers were

able to replace amine - based solvents with toluene, and prepared ≈ 10 nm diameter

γ - Fe

and ≈ 7 nm CoFe

by hydrothermal processes [161] . They synthesized

maghemite γ - Fe

nanoparticles from a Fe

III

– cupferron complex and spinel

CoFe

nanoparticles starting from Co

– cupferron complex and Fe

III

– cupferron

Figure 16.11 TEM overview images of (a) iron oxide,

nanoparticles (scale bar 100 nm) and their respective selected -

area electron diffraction patterns (b, d, f and i); (i) corresponds

to (g) (from [150] ).

636 16 Properties, Synthesis and Applications of Highly Dispersed Metal Oxide Catalysts

complex. However, the authors found that the presence of at least trace quantities

of a strongly coordinating amine was necessary to act as a capping agent and

prevent aggregation. They were also able to synthesize nanoparticulate ZnFe

by a similar approach [173] .

16.3.4

Micro - Emulsion Technique

Micro - emulsions or micelles (including reverse micelles) represent an approach

based on the formation of micro/nano reaction vessels for the preparation of

nanoparticles, and has received considerable interest in recent years [174 – 180] . A

literature survey indicates that ultra - ﬁ ne nanoparticles in the size range 2 – 50 nm

can be easily prepared by this method. This technique uses an inorganic phase in

water - in - oil micro - emulsions, which are isotropic liquid media with nano - sized

water droplets dispersed in a continuous oil phase. In general, micro - emulsions

consist of, at least, a ternary mixture of water, a surfactant (or a mixture of surface -

active agents) and oil. The dispersion of the aqueous phase is shown in Figure

16.12 [68] . The classical examples of emulsiﬁ ers are sodium dodecyl sulfate ( SDC )

and aerosol bis(2 - ethylhexyl) sulfosuccinate (AOT). The surfactant (emulsiﬁ er)

molecule stabilizes the water droplet because they have polar head groups and

non - polar organic tails. The organic (hydrophobic) portion faces towards the oil

phase and the polar (hydrophilic) group towards water. In diluted water (or oil)

solutions, the emulsiﬁ er dissolves and exists as a monomer, but when its concen-

tration exceeds a certain limit called the critical micelle concentration ( CMC ), the

molecules of emulsiﬁ er associate spontaneously to form aggregates called micelles.

These micro - water droplets then form nano - reactors for the formation of nanopar-

ticles. The nanoparticles formed usually have monodisperse properties. One

method of formation consists of mixing two micro - emulsions or macro - emulsions

and aqueous solutions carrying the appropriate reactants in order to obtain the

Figure 16.12 Schematic representation of (a) a micelle and

(b) an inverse micelle (from [68] ).

16.3 Synthesis 637

desired particles. The interchange of the reactants takes place during the collision

of the water droplets in the micro - emulsions. The interchange of the reactant is

very fast so that, for the most commonly used micro - emulsions, it occurs simply

during the mixing process. The reduction, nucleation and growth occur inside the

droplets, which controls the ﬁ nal particle size. The chemical reaction within the

droplet is very fast, so the rate determining step will be the initial communication

step of the micro - droplets with different droplets. The rate of communication

has been deﬁ ned by a second - order communication - controlled rate constant and

represents the fastest possible rate constant for the system. The reactant concentra-

tion has a major inﬂ uence on the reaction rate. The rate of both nucleation and

growth are determined by the probabilities of the collisions between several atoms,

between one atom and a nucleus and between two or more nuclei. Once a nucleus

forms with the minimum number of atoms, the growth process starts. For the

formation of monodisperse particles, all of the nuclei must form at the same time

and grow simultaneously and with the same rate.

The preparation of metal oxide nanoparticles within micelles involves forming

two micro - emulsions, one with the metal salt of interest and the other with the

reducing or oxide - containing agent and mixing them together. When the two dif-

ferent reactants mix, the interchange of the reactants takes place through the

collision of water micro - droplets. The reaction (reduction, nucleation and growth)

takes place inside the droplet, which controls the ﬁ nal size of the particles. The

interchange of nuclei between two micro - droplets does not take place owing to the

special restrictions from the emulsiﬁ er. Once the particle inside the droplets

attains its full size, the surfactant molecules attach to the metal surface, thus sta-

bilizing and preventing further growth.

Reverse micelles are used to prepare nanoparticles by using an aqueous solution

of reactive precursors that can be converted to insoluble nanoparticles. Nanoparti-

cle synthesis inside the micelles can be achieved by a variety of methods, including

hydrolysis of reactive precursors, such as alkoxides, and precipitation reactions of

metal salts [181, 182] . Solvent removal and subsequent calcination leads to the ﬁ nal

product. A variety of surfactants can be used in these processes such as pentadeca-

oxyethylene nonylphenylether (TNP - 35) [182] , decaoxyethylene nonylphenyl ether

(TNT - 10) [182] , poly(oxyethylene)

nonylphenylether (NP5) [183] and many others

that are commercially available. Several parameters, such as the concentration of

the reactive precursor in the micelle and the weight percentage of the aqueous

phase in the micro - emulsion, affect the properties, including particle size, particle

size distribution, agglomerate size and phases of the ﬁ nal oxide powders. There

are several advantages to using this method, including the preparation of very small

particles and the ability to control the particle size. Disadvantages include low pro-

duction yields and the need to use large amounts of solvents and surfactants.

This method has been successfully applied for the synthesis of metals, metal

oxides, alloys and core – shell nanoparticles. The synthesis of metal oxides from

reverse micelles is similar in most aspects to their synthesis in aqueous phase by

a precipitation process. For example, precipitation of hydroxides is obtained

by addition of a base such as NH

(aq) or NaOH to a reverse micelle solution

638 16 Properties, Synthesis and Applications of Highly Dispersed Metal Oxide Catalysts

containing aqueous metal ions at the micellar cores. This method has been also

used to synthesize mixed metal iron oxides [162] ,

MFeOHexcess MFeOHO

24 2

++−

()

⎯→⎯⎯ + ↑

∆,

where M = Fe, Mn or Co. The resultant particles obtained are of the order of 10 –

20 nm in diameter. The cation distribution in the case of spinels depends on the

temperature used in the reaction [184] . If the transition metal cation is unstable

or insoluble in aqueous media, nanoparticles of those metals can be prepared by

hydrolysis of suitable precursors; for example, TiO

has been prepared from tet-

raisopropyl titanate [185] in the absence of water. The hydrolysis occurs when a

second water - containing solution of reverse micelles is added to the solution of

the ﬁ rst micelle containing the metal precursor. Adjusting the concentration of

reactants can change the ﬁ nal crystal form, that is between amorphous and crystal-

line. The reaction can be described as follows,

Ti O H O TiO Pr-OH

()

+→+

()

Micro - emulsions have been also employed to prepare precursors that decompose

during calcination, resulting in desired mixed metal oxides. A series of mixed -

metal ferrites have been prepared by precipitating the metal precursor in a H

O -

AOT - isooctane system and calcining the products at 300 – 600 ° C [186] . A

superconductor material YBa

7 - δ

with 10 nm particles have been prepared by

combining a micellar solution of Y

, Ba

and Cu

prepared in an Igepal CO -

430 – cyclohexene system with a second micellar solution containing oxalic acid in

the aqueous cores (Note: Igepal CO - 430 is a surfactant with chemical name “ Nonyl

phenol 4 mole ethoxylate ” ). The precipitate was subsequently calcined to 800 ° C to

remove oxalic precursors [187] . Other metals and metal oxides that are prepared

by a similar method include tungsten and tungsten oxide nanoparticles [188] and

high - surface - area Al

[189] .

16.3.5

Combustion Methods

The combustion synthesis technique consists of bringing a saturated aqueous

solution of the desired metal salts and a suitable organic fuel to the boil, until the

mixture ignites a self - sustaining and rather fast combustion reaction, resulting in

a dry, usually crystalline, ﬁ ne oxide powder. By simple calcination, the metal

nitrates can, of course, be decomposed into melt oxides upon heating to or above

the phase transformation temperature.

Flame processes have been widely used to synthesize nanosize powders of oxide

materials. Chemical precursors are vaporized and then oxidized in a combustion

process using a fuel/oxidant mixture such as propane/oxygen or methane/air

[190] . They combine the rapid thermal decomposition of a precursor/carrier gas

stream in a reduced pressure environment with thermophoretically driven deposi-