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Thermoelectricity School
Annecy, June 2014
Synthesis of nanomaterials by soft chemistry
methods for thermoelectric applications
Lorette Sicard
Figure of merit:
Introduction
𝑍𝑇 =𝑆2𝜎
κT
• S: Seebeck coefficient
• σ: electronic conductivity
• κ: thermal conductivity
κ = κe+ κl
S, σ, κ are closely linked properties
Semi-conducting materials
Isolating materials Metals
Carrier concentration (cm-3)
2
S ZT κ
S2
Key issue: to decouple S, σ, κ
• σ • S • ↘ κ
« Phonon glass – Electron crystal »
concept
Strategies to increase ZT:
Engineering the crystal structure at atomic scale
Cage-like materials: skutterudites, clathrates
« rattling » atoms
Complex crystals with defaults: half-Heusler compounds, …
Misfits oxides: cobaltites, …
….
Engineering the material structure at the nanoscale
Low-dimension materials; nanostructuring
3
Introduction
Dramatic change of the DOS by quantum confinement effect
Phonon scattering of phonons at the interfaces Decrease of κ
Increase of S
Low-energy carrier filtering at the boundaries Increase of power factor
Semi-metal to semi-conducting transition
Contribution of nano-engineering:
4
Introduction
Nanostructured materials:
5
From theory…
M. Dresselhaus, Microscale Thermophysical
Engineering, 3 (1999) 100.
…to experiment.
T.C. Harman, Science 297 (2002) 2229
Bi2Te3/Sb2Te3
Super-lattices
ZT = 2.4
Nano_materials
Nano-composites
Nano-composites
PbTe matrix
PbS
PbTe
M.G. Kanatzidis Chem. Mater. 22 (2010) 648.
R. Venkatasubramanian Nature. 413 (2001) 597.
Sb2SexTe1-x
based quantum
dots supperlatices
ZT = 1.6
Introduction
Nanostructured materials in devices:
6
Nano-bulk materials
Nano-composites
Nano-inclusions: LAST (AgPbmSbTem+2), … Phase segregation Chemical engineering approach
Super-lattices Costly physical processes
Sintered nano-particles Physical/chemical processes
Interest restricted to applications at moderate temperature: mostly Bi2Te3, PbTe, CoSb3
Introduction
Outline
I. Fundamentals of soft-chemistry synthesis
II. Reduction in aqueous medium
III. Reduction in organic solution
IV. Solvothermal synthesis
V. Synthesis in confined medium
VI. Decomposition of organometalic complexes
VII. Post-synthesis treatments
Conclusions/Perspectives
Outline
I. Fundamentals of soft-chemistry synthesis 1. Generalities
2. Mechanisms of formation of NPs
3. Chemical reactions involved in the obtention of NPs
II. Reduction in aqueous medium
III. Reduction in organic solution
IV. Solvothermal synthesis
V. Synthesis in confined medium
VI. Decomposition of organometalic complexes
VII. Post-synthesis treatments
Conclusions/Perspectives
I. Fundamentals / 1. Generalities
2 approaches:
volume
powder
nanoparticles
atoms
clusters
Physical methods :
Ball milling
Lithography
Laser ablation
Electroexplosion of a wire
….
Chemical methods :
CVD
Solution chemistry
« Top-down »
« Bottom-up »
Buk-nanomaterial
Sintering at moderateT
7
Advantages and drawbacks of soft-chemistry synthesis
8
Drawbacks:
Advantages:
Easy to perform
Often scalable, high yields
Low temperature, low cost
Versatility over crystalline phase, morphology, size
Good control over particle size, distribution, morphology
Good chemical homogeneity, low strain
Difficulties to control accurately the stoichiometry
Possible oxide/organic coating of the particles
detrimental to densification and electronic properties
I. Fundamentals / 1. Generalities
General procedure for the obtention of nanomaterials
9
Centrifugation
Structure
XRD • Cristallographic phase • Impurities • Cristallinity • Cristallite size • Defaults, strains TEM • HRTEM • Electronic diffraction
ICP-AES X-ray fluorescence EDX XPS FTIR
Stoichiometry Morphology Physical
SEM TEM
Electronic properties Thermal conductivities
I. Fundamentals / 1. Generalities
Characterization
10
Characterization mechanisms of formation
Intermediate species in solution • NMR • UV-visible spectroscopy
Intermediate precipitates • XRD • TEM • …
Nucleation/growth • SAXS • UV-visible spectroscopy • Solution chemical analysis
General procedure for the obtention of nanomaterials
I. Fundamentals / 1. Generalities
Centrifugation
Adapted from V.K. La Mer et R.H. Dinegar, J. Am. Ceram. Soc. 17 (1950) 4847.
Fundamentals of the formation of nanoparticles
time
2. Nucleation
3. Growth
4. Aging
Mo
lecu
lar
sp
ec
ies c
on
cen
trati
on
Nu
mb
er
of
pa
rtic
les
11
1. Induction
1. 2. 3. 4.
1. Induction period
Dissolution of the precursors
I. Fundamentals / 2. Mechanisms of formation of NPs
12
-200
-150
-100
-50
0
50
100
150
200
0 0.5 1 1.5 2
r/rc
DG
Esurface
Evolume Etotal
rc
Nucleation Growth
Nucleus small solid spherical particle
∆𝐺(𝑟) =4π𝑟2𝛾 −4
3𝜋𝑟3 𝑅𝑇𝑙𝑛𝑆
𝑉𝑚
∆𝐺 𝑟 = ∆𝐺𝑆 + ∆𝐺𝑖𝑛𝑡
∆𝐺𝑖𝑛𝑡: free enthalpie of formation of the condensed phase
∆𝐺𝑆: free enthalpie of the surface
𝛾 : surface tension
𝑆 =𝐶𝐿
𝐶𝑆; Vm: molar volume
r
𝑑(∆𝐺 𝑟 )
𝑑𝑟= 0 Critical radius: rc 𝑟𝑐 =
2𝛾
𝑅𝑇𝑙𝑛𝑆
2. Nucleation
I. Fundamentals / 2. Mechanisms of formation of NPs
3. Growth
13
I. Fundamentals / 2. Mechanisms of formation of NPs
2 growth models:
Growth limited by the diffusion of species in solution
Growth limited by the precipitation of monomers
14
Growth limited by the diffusion of species in solution
𝐽 =4𝜋𝑟2
𝑉𝑚
𝑑𝑟
𝑑𝑡 The growth rate depends solely of the flux of the
monomers supplied to the particles
𝐽 = 4𝜋𝑟𝐷(𝐶𝑏𝑢𝑙𝑘 − 𝐶𝑠) Fick law ; CS: concentration at the particle surface D: diffusion coefficient
𝑑𝑟
𝑑𝑡=
𝑉𝑚𝐷
𝑟(𝐶𝑏𝑢𝑙𝑘 − 𝐶𝑠)
Growth rate inversely proportional to the radius
𝑑𝜎2
𝑑𝑡= 2𝑉𝑚𝐷 (𝐶𝑏𝑢𝑙𝑘 − 𝐶𝑠) 1 − 𝑟
1
𝑟
<0
The variance of the size distribution ↘ as the particles are growing
If no additional nucleation growth occurs
The smaller the particles are, the faster they grow Focusing effect (dispersion in size ↘)
I. Fundamentals / 2. Mechanisms of formation of NPs
15
Growth limited by the precipitation of monomers
4. Aging
2 mechanisms:
Coalescence
Ostwald ripening
I. Fundamentals / 2. Mechanisms of formation of NPs
K; DamKöhler number proportional to diffusion rate/precipitation rate
Competitive effect: dissolution and precipitation
Growth rate: with:
𝐶0𝑆𝑒𝑞
: monomer concentration in solution
𝑘0𝑝: precipitation rate constant of bulk
K low diffusion-controlled growth (focusing effect) K high reaction-controlled growth (focusing effect weakened)
: transfer coefficient
𝑑𝑟∗
𝑑𝜏=
𝑆 − exp (1 𝑟∗ )
𝑟∗ + 𝐾𝑒𝑥𝑝(𝛼 𝑟∗ ) 𝑟∗=
𝑅𝑇
2𝛾𝑉𝑚𝑟
𝜏 =𝑅2𝑇2𝐷𝐶0
𝑆𝑒𝑞
4𝛾2𝑉𝑚𝑡
K=𝑅𝑇
4𝛾𝑉𝑚
𝐷
4𝑘0𝑝
Seed-mediated growth
16
Co65Ni35
Pt/CoNi = 2,5 10-4
dm = 50 nm
Co50Ni50
Pt/CoNi = 0,5 10-2
dm = 7 nm
40 nm40 nm150 nm150 nm
Co80Ni20
homogeneous nucleation
dm = 1400 nm
10-6
10-5
10-4
10-3
10-2
10
100
dm (
nm
)
[Pt]/[Co+Ni] (mol/mol)
3
1
CoNigerme
germegermeCoNim
]NiCo[
]germe[
M
VM6d
Strategies to control the particle size
Control of the size
or Core/shell
I. Fundamentals / 2. Mechanisms of formation of NPs
17
Conc. and nature of the reactants, nature of the solvent, reaction time
Homogeneous nucleation
I. Fundamentals / 2. Mechanisms of formation of NPs
hot-injection heating-up
T, P, heating rate, heating mode
Co-precipitation
Heating rate:
Heating mode:
Microwave conventional
Used for polar solvent exclusively
• High heating rate • Uniform heating • High penetration depth of the wave • Rapid reaction (a few minutes) • High yield • Softer conditions can be used
18
2NH3 +CO2
Reactivity of the precursors
Homogeneous nucleation
Urea
Trioctylphosphine telluride
thiourea
+ H2O
Growth oriented by ligands
I. Fundamentals / 2. Mechanisms of formation of NPs
TiO
iPr
iPrO
iPrO
iPrO
Ti OiPr
iPrO
O
OiPr
O
CH3
CH3
1 acac
4
Ti4+
J. Livage, L’actualité Chimique, 1997
titanium isopropoxide titanium acetate
(Cl-)4Te4+
Tellurium chloride
Use of a hard/soft template
Templating strategies
19
Coalescence:
Electrostatic stabilization:
Steric stabilization:
- - -
- -
- -
- -
- -
- -
- -
- -
- -
- -
Modification of the pH, counter-ion…
Addition of polymer, surfactant Use of chelating agents, solvent
Electro-steric stabilization: - - - -
- -
- -
- - -
- - - - -
-
Templating strategies:
Use of a hard/soft template
Strategies to avoid agregation
I. Fundamentals / 2. Mechanisms of formation of NPs
20
In summary:
Control of nucleation/growth/aging through the synthesis parameters (T, P,
time, heating rate and mode, precursor nature, additives, …)
linked to the chemical reactions which depends of the nature of the NPs to synthesize
I. Fundamentals / 3. Chemical reactions
a- Oxides
b- Metals
c- Chalcogenides
I. Fundamentals / 3. Chemical reactions
21
Sol-gel synthesis
a. Synthesis of oxide nanoparticles
Precursors
Sol
Gel Aerogel/Xerogel
Precipitate
Powder
Film
Oxide/glass
Drying Calcination
Condensation
Sol-gel synthesis
Co-precipitation
Specific for oxide synthesis
22
• Dissolution/complexation Mz+ + N H2O [M(OH2)N]z+
• Hydrolysis
[M(OH2)N]z+ + h OH- [M(OH)h (OH2)N-h]z+ + hH2O In basic medium:
• Condensation
M-OH + HO-M M-O-M + H2O oxolation
Aquo complex
M-OH + H2O-M M-OH-M + H2O olation
In the presence of water
From metal alkoxyde precursors
• Hydrolysis
• Condensation
M-OR + H2O M-OH + ROH
M-OH +HO-M M-O-M + H2O
M-OR +HO-M M-O-M + ROH
TEOS
From metal salts
Sol Gel
Hydrolysis rate >> Condensation rate
I. Fundamentals / 3. Chemical reactions
R: alkyl group
Ca(NO3)2 Co(NO3)2.4H2O
water citric acid
H2O
Dried gel
Poudre
70°C – 3h
130°C, 12h
calcination
400°C
600°C
650°C
800°C
700°C
750°C
Calcination 600°C, Furnace 900°C, 8h
Calcination 600°C, SPS 700°C, 5min
Y. Liu, J. Am. Ceram. Soc., 88 [5] 1337–1340 (2005
Example: preparation of cobaltite Ca3Co4O9
23
I. Fundamentals / 3. Chemical reactions
24
In the absence of water
Metal halide in alcohol
The oxygen comes from the solvent
MX4 + R-OH RX + M-OH
M-OH +HO-M M-O-M + H2O
Metal halide in ether
R-OR + MX4 RX + M-OR
M-OR +RO-M M-O-M + ROR
I. Fundamentals / 3. Chemical reactions
25
Co-precipitation method in aqueous solution
Oxide preparation
aq. NiCl2 aq. NH4OH aq. Fe(NO3)3
Ni(NO3)2
Zn(NO3)2
aq. NaOH
Calcination necessary
In aqueous medium
In organic medium
Controlled hydrolysis
Diethyleneglycol:
- high boiling T solvent
- stabilizing agent
I. Fundamentals / 3. Chemical reactions
26
Mn+ + ne- M0 : E0
M
Xm - ne- Xm-n : E0R < E0
M
N2H4.H2O → N2H5+ + OH-
N2 + 5H+ + 4e− → N2H5+ (ion hydrazinium) E0 = -0,23V
Usual reductants:
• Hydrazine
• Borohydride
Co2(CO)8 → 2Co + 8CO
Reduction
b. Synthesis of metal NPs
Thermolysis
B(OH)4- + 4H2O +8e-
BH4- + 8OH- E = -1,24V at pH = 14
Strong reducing agent
I. Fundamentals / 3. Chemical reactions
27
E0Te/Te2−=−1.14V
E0Pb2+/Pb =−0.13V
E0Bi3+/Bi=0.200V
E0Te4+/Te=0.53 V
c. Synthesis of metal chalcogenide NPs
Pb2+ + Te2- PbTe
or Pb + Te PbTe
2Bi3+ + 3Te2- Bi2Te3
or 2Bi + 3Te Bi2Te3
Reduction of Te and/or Pb/Bi in some cases
Use of a reductant: often NaBH4 or N2H4
For PbTe: Pb(+II) and Te(+IV) or Te(0)
Precursors:
For Bi2Te3: Bi(+III) and Te(+IV) or Te(0)
2 possible reaction pathways:
Reduction
Example of PbTe and Bi2Te3:
I. Fundamentals / 3. Chemical reactions
28
I. Fundamentals / 3. Chemical reactions
In water/organic solvent
At ambiant pressure/solvothermal conditions
In confined medium
Soft template: micelles/microemulsions
Hard template
29
Thermolysis
I. Fundamentals / 3. Chemical reactions
Te(-II)-TOP complex
Bi(III)/Pb(II) complex with ligands:
=M-S CnH2n+1
=MO2C-CnH2n+1
+
in organic solvent
hot-injection heating-up Microwave
Outline
I. Fundamentals of soft-chemistry synthesis
II. Reduction in aqueous medium 1. Use of organic agents
2. Templating method
III. Reduction in organic solution
IV. Solvothermal synthesis
V. Synthesis in confined medium
VI. Decomposition of organometalic complexes
VII. Post-synthesis treatments
Conclusions/Perspectives
Synthesis of Bi2Te3, PbTe, CoSb3 based-NPs
30
II. Synthesis in aqueous medium
Advantages/Drawbacks of reduction in aqueous medium
Advantages:
Very easy to perform
Water = environmental friendly solvent
Low cost reactants
Absence of organic shells in some cases
Drawbacks:
Distribution in size generally high
Agregation difficult to prevent
Surface oxidation possible
31
Strategies to control the morphology and size of the nanoparticules
1. Control of the precursor reactivity and/or particle growth and agregation: Addition of an organic structuring agent
EDTA
Ethylenediaminetetraacetic
disodium salt
Bi3+
Bi(III)/EDTA
complex
Control of the reactivity of ions Control of growth/agregation
Addition of a polymeric agent
Control of growth/agregation by steric effect
2. Control of the NPs form and size by sacrificial templating
PVP
Polyvinyl
pyrrolidone
II. Synthesis in aqueous medium
32
II. Synthesis in aqueous medium / 1. Use of organic agents
W.X. Zhang, Mater. Research. Bulletin 35 (2009) 2009.
Synthesis of PbTe in the absence of an organic agent
Te
aq. KOH
Pb(CH3CO2)2●3H2O
H2O
Agregated NPs 35 nm-sized Polydispersity
Role of the organic directing agent
1. Control of the precursor reactivity and/or particle growth and agregation
1) BiCl3 + Na2TeO3
2) NaBH4
24 hours
Reflux
Preheated H2O
+ NaOH
+ organic agent
Pure Bi2Te3
Influence of the nature of the directing agent
EDTA PVP ethylene diamine
Nanorods d 20-70nm
Agregated nanoparticles
Importance of the directing agent on the growth and aging of the NPs Control over shape and size
33 J.Z. Hu, Phys. Scr. T129 (2007) 120.
Nanotubes d 20-70nm
II. Synthesis in aqueous medium / 1. Use of organic agents
1) BiCl3 + Na2TeO3
2) NaBH4
Preheated H2O
+ NaOH
+ organic agent
With EDTA
Using Na2TeO3 Using Te
20 nm-sized particles congregated into flakes
Nanotubes d 20-70nm
34 J.Z. Hu, Phys. Scr. T129 (2007) 120.
Importance also of the precursors on the shape and size
Influence of the nature of the directing agent
II. Synthesis in aqueous medium / 1. Use of organic agents
35 C. Kim, Powder Technol.. 214 (2011) 463.
Bi(NO3)3 in H2O
Ascorbic acid
EDTA
+ aq. NaOH
3h
Te
aq. NaBH4
100°C
Bi2Te3
Bi2Te3
[EDTA] size ↘
Bi2Te3+
Bi2TeO5+Te
Bi2Te3 Bi2Te3
100nm 100nm 100nm 100nm
Influence of the concentration of the directing agent
Tuning of the reaction product Tuning of the size
with the directing agent concentration
II. Synthesis in aqueous medium / 1. Use of organic agents
BiCl3 in aq. HNO3
+ thioglycolic acid H6TeO6 + H20
=Bi-SCH2CO2H
complex
H20, 70°C
N2H4.H2O
100°C
A. Purkayastha, Adv. Mater. 18 (2006) 496.
36
Multiple role of the directing agent
Bi2Te3 nanorods 54 nm
N. Gaponik, J. Phys. Chem. B 106 (2002) 7177.
Refluxing time in an
aq. thioglycolic acid
solution Thioglycolic acid serves as - directing agent - doping agent
Incorporation of S n-type semi-conductor
II. Synthesis in aqueous medium / 1. Use of organic agents
37
II. Synthesis in aqueous medium / 2. Templating method
0) NaOH/H20
100°C
1) Na2TeO3+NaBH4
1/2h
2) Pb(CH3CO2).3H2O
t
t=0 Pure Te
1) TeO32- + BH4
- + 2H2O Te + H2BO3- + 2OH- + 2H2
X. Chen, J. Crystal Gowth 311 (2009) 3179.
2. Control of the NPs form and size by sacrificial templating
37
t=0 Pure Te
2) 3Pb2+ + 2BH4- + 6OH- 3Pb + 2H2BO3
- + 5H2
Pb + Te Pb Te
t=0 Te + PbTe
X. Chen, J. Crystal Gowth 311 (2009) 3179.
0) NaOH/H20
100°C
1) Na2TeO3+NaBH4
1/2h
2) Pb(CH3CO2).3H2O
t
II. Synthesis in aqueous medium / 2. Templating method
2. Control of the NPs form and size by sacrificial templating
37 X. Chen, J. Crystal Gowth 311 (2009) 3179.
t=0 Pure Te
t=0 Te + PbTe
t=48h Pure PbTe
Nanorods
L=0.3-2μm d=50-200nm
Nanorods constituted of NPS (30-50nm)
Absence of organic agents
1) NaOH/H20
100°C
2) Na2TeO3+NaBH4
1/2h
3) Pb(CH3CO2).3H2O
t
II. Synthesis in aqueous medium / 2. Templating method
2. Control of the NPs form and size by sacrificial templating
Outline
I. Fundamentals of soft-chemistry synthesis
II. Reduction in aqueous medium
III. Reduction in organic solution
IV. Solvothermal synthesis
V. Synthesis in confined medium
VI. Decomposition of organometalic complexes
VII. Post-synthesis treatments
Conclusions/Perspectives
III. Synthesis in organic medium
Advantages of organic solvents over water:
Higher boiling temperature
Favors cristallinity
Some are chelating agents
Control the reactivity of precursors
Orientate the form and shape
Control the dispersity in size
Favors colloidal dispersion
Some are reducing agents
Avoid surface oxidation
38
Drawbacks:
Adsorption of organic species at the particle surface
III. Synthesis in organic medium
• Polar solvent, high εr and Tboiling
O H
O
O H O H O H
C H 3
O H O H
T b
( ° C) 198 245 192
1,2 - ethanediol 1 , 2 - butanediol Diethyleneglycol
C H 3
O H
e r
ethanol water
H 2 O
100 78,5
78 , 5 24 38 32 22
39
Polyol synthesis
• Reducing agent
• Polar solvent, high εr and Tboiling
39
• Complexing agent
M
OAc
OH 2
OAc
OH 2
DEG
H 2 O
H 2 O O
O
O
OAc
O
OAc
M
échange
acidobasique M
OH
O
OAc
O
O
O
D. Chiche, Phys. Chem. Chem. Phys., 11 (2009) 11310.
Adsorption on the particle surface
Complex metal ions
Control of nucleation and growth
Avoid surface oxidation
Polyol synthesis
III. Synthesis in organic medium
40
Optimisation of the reaction parameters: the case of CoSb3
CoCl2
SbCl3
NaBH4
TetraEG
PVP
Argon
CoCl2 + 2NaBH4 → Co + 2BH3 + 2NaCl + H2
2SbCl3 + 6NaBH4 → 2Sb + 6BH3 + 6NaCl + 3H2
Co+Sb→CoSb (165°C)
CoSb + 2Sb → CoSb3 (T 180°C)
Possibility of obtaining CoSb3 pure and well crystallized with the adequate parameters
15 min
240°C size < 30nm
L. Yang, Materials Letters 62 (2008) 2483–2485
Very well crystallized
tetraEG:
III. Synthesis in organic medium
Reaction parameters: T, time, reactant nature, concentration, organic agents…
Influence of temperature
Temperature=
key parameter
41
Possibility of obtaining CoSb3 pure with the adequate parameters
L. Yang, Materials Letters 62 (2008) 2483–2485
CoSb3 is metastable (idem ball milling)
CoSb2 and traces of Co appear with prolonged time
III. Synthesis in organic medium Optimisation of the reaction parameters: the case of CoSb3
Influence of time
CoCl2
SbCl3
NaBH4
TetraEG
PVP
Argon
60 min
tetraEG:
CoSb3 → CoSb2 +Co
Control over stoechiometry
Synthesis of solid solutions: the case of Bi2(Te,Se)3
III. Synthesis in organic medium
42 A. Soni, ..M. Dresselhauss Nano Lett. 12 (2012) 1203.
Bi(NO3)3.H2O
K2TeO3.H2O
Na2SeO3
240°C, 4/5h
Bi2Te3 Bi2Te2.7Se0.3 Bi2Se3
Nanoplates
SPS 250°C, 40MPa
1μm 1μm 1μm
1μm
Formation of solid solution with controlled stoichiometry
Different precursors in solution
Orientation of the NPs after pelletization
Te/Se ratio
Outline
I. Fundamentals of soft-chemistry synthesis
II. Reduction in aqueous medium
III. Reduction in organic solution
IV. Solvothermal synthesis 1. Control of the stoichiometry
2. Control of the morphology
3. Nanoplating
V. Synthesis in confined medium
VI. Decomposition of organometalic complexes
VII. Post-synthesis treatments
Conclusions/Perspectives
Supercritical
fluid
Solu
tion c
hem
istr
y
Hydrothermal
conditions
water
43
Solvothermal conditions
IV. Solvothermal synthesis
T > TB in an autoclave P > Patm.
Teflon
liner Steel body
Reactive
solution
Idea: to reproduce the geological conditions
of e and
higher mobility of dissolved species
lower polarity
Equilibrium constants modified (Ke ↗, …)
Modification of the solvent properties modification of chemical reactions
P depends of the volume introduced in the autoclave
44
IV. Solvothermal synthesis
Versatile method (over cristalline phase, morphology, size)
High cristallinity
Good chemical homogeneity
Excellent control over morphology and particle size distribution
Advantages of solvothermal conditions over ambiant pressure
Same principle as ambiant pressure synthesis:
1. Control of the stoichiometry optimization of the synthesis conditions
2. Control of the morphology use of organic agents sacrificial templating 3. Obtention of nanocomposites by nanoplating
45
1. Control of the stoichiometry
IV. Solvothermal synthesis / 1. Control of the stoichiometry
Phase segregation in the case of CoSb3 ?
J.Q. Li, Mater. Chem. Phys. 112 (2008) 57-62
CoCl2.6H2O
SbCl3
NaBH4
TriEG
290°C
RT
290°C
12h: CoSb3
3 h: CoSb3, Sb, CoSb2
1 h: CoSb3, Sb, CoSb2
Sb CoSb2
CoSb3
Sb + CoSb2 CoSb3
time ↗
Possibility of obtaining CoSb3 pure with the adequate parameters
Optimisation of the synthesis conditions: solvothermal synthesis of CoSb3
46
Possibility of controling the stoichiometry of solid solution
J.Q. Li, Mater. Chem. Phys. 112 (2008) 57-62
NPs size: 10nm
NPs size: 20nm
Evolution of the cell parameter with [Fe]
Vegard law
FeCl3.6H2O
CoCl2.6H2O
SbCl3
NaBH4
TriEG
290°C
Obtention of solid solution: the case of CoSb3-xFex
IV. Solvothermal synthesis / 1. Control of the stoichiometry
47
NaTeO3
Bi(NO3).5H2O
EG
N2H4.H2O
CH3CO2H
PVP
140°C, 1h
1
Growth direction: 112 0 top-bottom facets: 0001 ; side facets 112 0
5 quintuple layers
1.5 unit cell
Q. Yuan, Chem. Commun. 47 (2011à) 12131.
Very well crystallized Bi2Te3 flower-like nanocrystals
Use of organic agents: the case of Bi2Te3 flower-like nanocrystals
Control of the direction growth
2. Control of the morphology
IV. Solvothermal synthesis / 2. Control of the morphology
48
Uniformity in form and size
Role of acetic acid:
Acetic acid: structure directing agent
Role of PVP:
PVP: stabilizing agent
with without
Absence of acetic acid
Self-assembly organisation:
2D-film Honey-comb like spheres
Q. Yuan, Chem. Commun. 47 (2011à) 12131.
IV. Solvothermal synthesis / 2. Control of the morphology
49
Templating strategy
Pb(NO3)2
NaOH
Te nanorods
160°C, 6h
Topotactic transformation
J. He, Materials Science in Semiconductor Processing 12 (2009) 217.
Sacrificial templating strategy to control the form and size
Example of the synthesis of PbTe from Te nanorods
IV. Solvothermal synthesis / 2. Control of the morphology
50
3. Obtention of nanocomposites by nanoplating
Bulk CoSb3
CoCl2
SbCl3
Absolute ethanol
Ethylenediamine
NaBH4 240°C, 80h
Nanostructuring lowers the thermal conductivity X. Ji, Phys. stat. sol. (RRL) 1 (2007) 229.
Hot pressing
N2, 500°C, 30 min.
NPs precipitates on the bulk
CoSb3 surface
Nanostructing retained
after densification
Formation of nanocomposites
5%nano 20%nano
bulk
IV. Solvothermal synthesis / 3. Nanoplating
Outline
I. Fundamentals of soft-chemistry synthesis
II. Reduction in aqueous medium
III. Reduction in organic solution
IV. Solvothermal synthesis
V. Synthesis in confined medium 1. Use of reverse micelles
2. Use of non stabilized biphasic medium
VI. Decomposition of organometalic complexes
VII. Post-synthesis treatments
Conclusions/Perspectives
Control of morphology/ size/agregation by : - organic agents - sacrificial templating
Reactions carried out in nano-reactors
V. Synthesis in confined medium / 1. Reverse micelles
reverse micelle
direct micelle
Surfactant
51
Reaction inside the nano-reactor by content exchange
Merged micelles during a few nanoseconds
Content exchange Reaction
M.P. Pileni Nature Materials 2 (2003) 145.
Stable emulsions
1. Use of reverse micelles as nano-reactors
Principle
V. Synthesis in confined medium / 1. Reverse micelles
Modulation of the size of reverse micelles
Modulation of the form of the reverse micelle
52
M.P. Pileni Nature Materials 2 (2003) 145.
w=[H2O] /[AOT]
w=1 w=3 w=5 w=20
Example of Co NPS
Particle size in the nanometer range
Modulation of the size of the NPs to some extend
Modulation of the form of the NPS
Possibility of obtaining solid solutions
use of a ligand as dopant
use of multiple micellar solutions
Difficult to scale up
Advantages:
Drawbacks:
V. Synthesis in confined medium / 1. Reverse micelles Use of a ligand as a dopant
L=thioglycolic acid
After annealing: Bi2Te3 + Bi2Te2S
Micelles of dioctyl sodium sulfosuccinate (AOT) in isooctane
53 A. Purkayastha, Adv. Mater. 18 (2006) 2958.
isooctane
ω=2.4
ω=[H2O] /[AOT]
ω=2.4 ω=4.1
Well crystallized NP Non agglomerated NPs Size with ω (2.5 to 10.4nm)
Incorporation of S from thioglycolic acid Protection against oxidation
AOT
ω=4.1
2.5nm 10.4nm
V. Synthesis in confined medium / 1. Reverse micelles Use of multiple micellar solutions
54 A.J. Karkamkar, M.G. Kanatzidis, J. Am.
Chem. Soc. 128 (2006) 6002.
Reverse micelles of SDS in octane + 1-butanol
SDS: Sodium dodecylsulfate
1) Micellar solution containing Ag(NO3)
+ micellar solution containing K2Sb(C4H2O6)2
+ micellar solution containing Pb(NO3)2
2) Addition of a micellar solution containing Na2TeO3/K2Te
3) Addition of a micellar solution containing NaBH4
AgPb2SbTe4
AgPbSbTe3
AgSbTe2
Formation of pure LAST alloys
d=5.2nm
High atomic order
Nanometric size Well dispersed NPs Size dispersity
d=4.3nm
d=4.6nm
100nm
50nm
50nm
2nm
V. Synthesis in confined medium / 2. Emulsions
Bi3+ Te4+
EDX EDX
J. Fouineau, L. Sicard et al. Chem. Mater. submitted. 55
BiCl3
TeCl4
EG
C12H25SH
2. Use of non-stabilized biphasic medium
V. Synthesis in confined medium / 2. Emulsions
Bi3+ Te4+
J. Fouineau, L. Sicard et al. Chem. Mater. submitted. 55
BiCl3
TeCl4
EG
C12H25SH
Hollow spheres
1 mm 2 mm
2. Use of non-stabilized biphasic medium
V. Synthesis in confined medium / 2. Emulsions
Bi3+ Te4+
J. Fouineau, L. Sicard et al. Chem. Mater. submitted. 55
BiCl3
TeCl4
EG
C12H25SH
Hollow spheres constituted of monocristaline platelets of Bi2Te3
1 mm 2 mm 1 mm 2 mm
2. Use of non-stabilized biphasic medium
+ Bi3+
+ Te4+
Aligned platelets 200 nm
Outline
I. Fundamentals of soft-chemistry synthesis
II. Reduction in aqueous medium
III. Reduction in organic solution
IV. Solvothermal synthesis
V. Synthesis in confined medium
VI. Decomposition of organometalic complexes 1. Synthesis of simple chalcogenides
2. Synthesis of solid solutions
3. Synthesis of hetero-nanostructures
VII. Post-synthesis treatments
Conclusions/Perspectives
Reductions
56
VI. Decomposition of complexes
Bi(III) complex
Te(-II) complex
Ligand:
CnH2n+1SH
CnH2n+1CO2H
Te-TOP
Very small nanoparticles (d < 10nm) in the case of hot injection
Monodispersity in size and form
Possibility of tuning the size and form
Colloidal solutions (very well dispersed capped particles) or films
Decomposition of organometalic complexes
Example of Bi2Te3
Advantages:
Presence of an organic shell
Drawbacks:
Heating up or Hot injection Burst of nucleation
Inert atmosphere T150 – 200°C
57
1. Synthesis of simple chalcogenides
M.R. Dyrmier, Small, 5 (2009) 933.
CnH2n+1SH (n=8, 12, 18)
Bi3+--SCnH2n+1 complex
1h
50/150°C
With C12H25SH at 150°C
100nm
Importance of the formation of the Bi-thiol complex:
Well crystallized Bi2Te3 particles
Without thiol
Particle morphology, size, dispersity control
Te(-II)-TOP complex
T = 50/150°C
Synthesis of Bi2Te3 from Bi-thiol complexes
VI. Decomposition of complexes / 1. Simple chalcogenides
58 Possibility of tuning the particle size
M.R. Dyrmier, Small. 5 (2090) 933.
Alkanethiol chain length
C8H17SH C12H25SH C18H37SH
100nm 100nm 100nm
Influence of the capping agent chain length:
T = 50 °C
Influence of the temperature:
Steric protection of the newly formed particles inhibition of particle growth
Particle size (nm) 50°C 150°C
C8H17SH 295 9815
C12H25SH 233 456
C18H37SH 172 527
High temperatures favors growth
The alkanethiol remains adsorbed at the NPs surface
VI. Decomposition of complexes / 1. Simple chalcogenides
59
High temperature favors growth Change of shape with size (Wulff construction)
J. Urban, J. Am. Chem. Soc. 128 (2006) 3248.
Influence of the temperature:
Bi3+-carboxylate
complex
(CH3CO2-)3 Bi3+
+ oleic acid
Te(-II)-TOP complex
T
T < 170°C Monodisperse cubo-
octahedral NPS
T > 200°C Less monodisperse
cubic NPS Larger size
Synthesis of PbTe from Bi-alcoolate complexes
VI. Decomposition of complexes / 1. Simple chalcogenides
60
Possibility of tuning the particle size with the ligand
Influence of the oleic acid/Pb acetate ratio
Oleic acid/Pb acetate molar ratio
The size with the addition of oleic acid
J. Urban, J. Am. Chem. Soc. 128 (2006) 3248.
Stability of lead oleate > lead acetate Lead oleate reacts more slowly Less nuclei
VI. Decomposition of complexes / 1. Simple chalcogenides
High degree of orders ; interesting structures for thermoelectricity
Tape casting of a PbTe colloidal solution
J. Urban, J. Am. Chem. Soc. 128 (2006) 3248. 61
Deposition of QDs lattices
VI. Decomposition of complexes / 1. Simple chalcogenides
62
Pure I doped-PbTe nanoparticles (4-5% depending on the initial ratio) with tunable size
n-type semi-conductors (non-doped PbTe is p-type) S2σ ↗ with I doping for a same size
Doping of PbTe by iodine
H. Fang, Nano Letters 14 (2014) 1153.
Pb/oleate
complex
in 1-octadecene
Te/I-TOP complex
N2
2. Synthesis of solid solutions
VI. Decomposition of complexes / 2. Solid solutions
63
Formation of a pure solid solution of AgPbmSbTem+2 (m=1–18)
Synthesis of quaternary nanocrystals
I. U. Arachchige, Adv. Mater. 20 (2008) 3638.
1) Pb(II)oleate
Sb(II) oleate
octadecene
2) Ag-dodecanethiol complex
Te(-II)-TOP complex
N2, 150 °C, 2 min Pure LAST solid
phase
Vegard law
VI. Decomposition of complexes / 2. Solid solutions
AgPbmSbTem+2
64
Well crystallized Highly monodisperse NPs Diameter = 6.8-8.4nm
I. U. Arachchige, Adv. Mater. 20 (2008) 3638.
AgPbmSbTem+2
mth=2 mexp=2.44
mth=4 mexp=3.96
mth=6 mexp=7.69
mth=8 mexp=9.87
mth=18 mexp=25.7
Morphology of the NPs
VI. Decomposition of complexes / 2. Solid solutions
65
At 150°C, crystallite size from 6.3 to 14.5nm
Phase separation upon annealing at 150°C within the nanocrystals but no
morphological changes observed
Effect of annealing
I. U. Arachchige, Adv. Mater. 20 (2008) 3638.
AgPb4SbTe6
*: AgSbTe2
#: Ag2Te
@: Sb2Te3
25°C
150°C
400°C
VI. Decomposition of complexes / 2. Solid solutions
66
PbTe-PbSe core-shell nanostars
M. Scheele, ACS Nano 5 (201) 8541.
Pb/oleate
complex
In diphenylether
Te(-II)-TOP complex
N2 N2
+ Pb/oleate complex
in diphenylether
Se(-II)-TOP complex
PbTe
PbTe-PbSe
Epitaxial growth of PbSe in the [100] direction onto each of 6 the octahedron tips
3. Preparation of hetero-nanostructures
PbTe NPs
VI. Decomposition of complexes / 3. Hetero-nanostructures
67
4. Preparation of composite films
Binary QDs lattice
PbTe (6.5nm) + Ag2Te (3.2nm)
PbTe (10.1nm) + Ag2Te (6.5nm)
PbTe + AgTe colloidal solutions followed by self-organization evaporation
High degree of orders Structure depending upon the NPs size
J. Urban, Nature Mater. 6 (2007) 115.
Modulation of
Measures after N2H4 treatment (see after)
VI. Decomposition of complexes / 3. Hetero-nanostructures
Outline
I. Fundamentals of soft-chemistry synthesis
II. Reduction in aqueous medium
III. Reduction in organic solution
IV. Solvothermal synthesis
V. Synthesis in confined medium
VI. Decomposition of organometalic complexes 1. Synthesis of simple chalcogenides
2. Synthesis of solid solutions
3. Synthesis of hetero-nanostructures
VII. Post-synthesis treatments 1. Thermal treatments
2. Chemical treatments
Conclusions/Perspectives
Reductions
68
VII. Post-synthesis treatments
detrimental to densification and electronic properties
Possible organic/oxide coating: drawback of the solution synthesis Is it a fatality ?
1. Thermal solutions (annealing)
2. Chemical solutions
69
VII. Post-synthesis treatments / 1. Thermal treatments
Insulating
alkanethiol layer
(C18H17SH)
M.R. Dyrmier, Small. 5 (2009) 933.
Hydraulic
press
Use of C18H17SH Particle size (nm)
σ (S cm-1) S (μV K-1) S2σ (μV2cm-1K-1)
Bulk Bi2Te3 12.2 -125 1.9 105
NPs synthesized at T = 50°C
527 5.89 10-1 -85 2.5 103
Poor electronic properties
100 nm
Annealing under N2flow
1. Thermal treatments
NPs obtained by decomposition of organometallic complexes
(Bi-SCnH2n+1 and Te-TOP)
thermal treatment:
350°C, 24h, under N2 flow
Nanostructuration retained
70 M.R. Dyrmier, Small. 5 (2009) 933.
Use of C18H17SH Particle size (nm)
σ (S cm-1)
S (μV K-1)
S2σ (μWcm-1K-1)
NPs before annealing 172 1.07 10-3 -49 2.6 10-6
NPs after annealing 397 30.4 -150 0.69
densification, d 87% after cold compaction
Improvement of the power factor due to • removal of the organic shell • sulfur doping during annealing
Absence of T dependence different from bulk Bi2Te3
at 300 K: κ(NPs) = 0,51 W m-1K-1 << κ(bulk)
VII. Post-synthesis treatments / 1. Thermal treatments
71 M.R. Dyrmier, Small. 5 (2009) 933.
Removal of the organic surface layer electronic properties
Keeping nanostructuration layer low κ
Advantages of thermal treatment under N2 flow:
Drawbacks:
VII. Post-synthesis treatments / 1. Thermal treatments
Small increase of oxide content
Thermal treatment under N2 flow do not permit to remove the oxide layer
72
Annealing under Ar flow
Y. Zhao, J.S. Dyck, B.M. Hernandez, C. Burda, J. Phys.Chem. C 114 (2010) 11607.
No oxide formation
NPs obtained by decomposition of Bi–thiol (C12H25SH) and Te-TOP complexes
Annealing for 5 hours under Ar flow
Cold-pressing
Vacuum-dried T = 300°C T = 350°C T = 380°C
Bi2Te3
53 nm 62 nm 71 nm 100 nm
Nanostructured materials up to 350°C
VII. Post-synthesis treatments / 1. Thermal treatments
73 Y. Zhao, J.S. Dyck, B.M. Hernandez, C. Burda, J. Phys. Chem. C 114 (2010) 11607.
Bi2Te3
Ar flow annealing permits to remove the capping ligand without oxidation
Low σ before annealing σ ↗ with annealing T
n-type semi-conductor S max forT=300°C
Sσ2↗ with annealing T
Simultaneous ↗ of σ and S at annealing T of 300°C
Data non-corrected from radiation loss
Weak ↗ of κL with T due to phonon scattering through nanostructuring Low κL values which increases with annealing
VII. Post-synthesis treatments / 1. Thermal treatments
B. Zhang, Appl. Mater. Interf. 2 (2010) 3170.
Bi2Te3 prepared by ball milling treated in a 5%wt HCl solution
Composites of Bi2Te3 in PEDOT:PSS
Removal of the oxide layer
XPS
Acidic treatment to remove the oxide shell:
VII. Post-synthesis treatments / 2. Chemical treatments 2. Chemical treatments
Both and S increase
74
75
Chemical exchange to remove organic ligands:
M. Scheele, Adv. Funct. Mater. 19 (2009) 3476.
1) Exchange of dodecanethiol ligand shell by oleic acid on the surface of nanoparticles obtained by complexes decomposition
2) Hydrazine (or ammoniac) etching treatment Hexane + NPs
N2H4
Hexane
N2H4 + NPs
N2H5+ -O2C17H33
N2H4: - base which can remove the capping agent - reducing agent
sample Resistivity (m cm)
S (μV K-1) (300K)
κ (W m-1 K-1) (200K)
zT (300K)
n-type bulk Bi2Te3 1.4 -180 2.2 0.32
this work 1.3 -80 0.8 0.2
3) SPS treatment (50°C)
Same resistivity as bulk n-Bi2Te3
Lower thermal conductivity.
VII. Post-synthesis treatments / 2. Chemical treatments
76 J.F. Urban J. Am. Chem. Soc. 128 (2006) 3248.
Same resistivity as bulk n-Bi2Te3
Lower thermal conductivity.
N2H4 treatment of a PbTe film followed by thermal treatment
Quantum dots nature preserved with: - hydrazine treatment - thermal treatment up to 200°C (vacuum)
Interparticle distance from 1.8 down to 0.3 nm
N2H4
Insulator σ = 10-13Scm-1
n-type conductor σ x1010-1012
σ ↗
150 °C
VII. Post-synthesis treatments / 2. Chemical treatments
SAXS
77 D. Yang, Nanoscale 5 (2013) 7290.
Same crystalline structure Inter-particle distance from 1.4 to <0.3nm
Molecular Metal Chalcogenide complexes (MMC)
Formation of MMC: dissolution in hydrazine of bulk metal chalcogenides in the presence of an excess of chalcogenide
NPs/Hexane
MMC/N2H4
Hexane
N2H4 +
MMC-capped NPs
PbSe obtained by decomposition of complexes (insets) and after MMC treatment (main TEM images)
M.V. Kovalenko, Science 324 (2009) 1417.
20 nm
VII. Post-synthesis treatments / 2. Chemical treatments
Use as capping agents
78 D. Yang, Nanoscale 5 (2013) 7290.
VII. Post-synthesis treatments-2. Chemical treatments
n-type (due to MMC) S ↗ when NPs size
(quantum confinement) S ↗ at high T (release
of low energy carriers and ↗ of minority hole carriers)
Electronic properties:
σx108 after MMC treatment
σ=110-320 S cm-1 at 300K
σ ↗ with NPs size σ ↗ with T (low energy
carrier filtering)
low
↗ with NPs size ( interface density)
High ZT
Thermal properties:
Thermoelectric properties:
79
Spin casting
Post-synthesis colloidal atomic layer deposition Removal of the capping agent and engineering stepwise the surface stoichiometry
After 1 cycle
10 nm
After 2-3 cycles
Oleic acid capped PbSe NCs
Connection along the (100) facets first (due to weaker bound oleic ligand)
oxidated surface
non oxidated surface
High electron mobility: 4.5 cm2/(V.s)
VII. Post-synthesis treatments / 2. Chemical treatments
S.J. Oh, Nano Letters14 (2014) 1559.
Post-synthesis charge carrier control
80
Conclusions/Perspectives
Advantages of soft-chemistry synthesis:
Variety of chemical routes and strategies to control:
• the cristalline phase
• the stoichiometry
• the morphology
• the particle size, dispersity
Scalable, high yields, low temperature, low cost
Good chemical homogeneity, high cristallinity, low strain
Possibility of designing a high variety of simple or complex materials:
• simple chalcogenides (PbTe, Bi2Te3, CoSb3)
• solid solutions ((Bi,Sb)2Te3, Bi(Te,Se)3, Co (Sb,Fe)3, AgPbmSbTe2+m)
• core-shell NPs
• nanocomposites
Obtention of dense and non oxidized materials through post-treatments
81
Conclusions/Perspectives
Improvement ways:
From « essay-error » to a reasonned approach
Pursuit of efforts to design in a same material both:
• nanostructuring
• chemical stoichiometry (solid solution)
optimization of , S and κ in the same material
Towards advanced materials:
D.-K. Ko, Nano Letters11 (2011) 2841.
Pt nanocubes encapsulated in Sb2Te3
S improved due to filtering of low energy holes but decrease of σ due to mobility reduction Slightly improved power factor
Y. Zhang, Nano Letters. 12 (2012) 1075.
Thank you for your attention Some questions ?
R.J. Mehta, Nature Mater., 11 (2012) 233.
Te/Se-P(C8H17)3 complex
Quick, efficient and scaling up synthesis
Te/Se
(C8H17)3P
Microwave 90-120s
Bi/Sb-S(CH2CO2H) complex
BiCl3/SbCl3
A B + 30-60 s
2-10 g min-1
100 % yield
top-bottom facets: 0001
Single crystals
High crystallinity, shape control
Thioglycolic acid:
directing agent
Sulfur doping via the thioglycolic acid Protection against oxidation thanks to thioglycolic acid Bi2Te3: slight excess of Bi
Energy (eV)
WDX
0.1-O.3 at.% Sulfur controlable through synthesis conditions
R.J. Mehta, Nature Mater., 11 (2012) 233.
After sintering at 300/400°C under vacuum :
Cold-compaction (hydraulic press) and sintering density : 923%
Good compaction
TGA replaced by neodecanoate: n-type
Importance of Sulfur doping
S-doped Bi2Te3 (Bi-rich): n-type
κ: 40-75%
κL: 60-75% lower than bulk materials
attributed to nanostructuring
50nm
Microwave dose (kJ g-1) κL (Wm-1K-1) Bi2Te3
0.59
0.38
50-80
5-10
10-100nm grains 5-50nm inter and intragranular pores
Electronic properties controlled by Sulfur doping
Thermal conductivity controlled by Nanostructuring
R.J. Mehta, Nature Mater., 11 (2012) 233.
Bi2Te3: 1-2 at. % Bi-rich
Bi2Se3: 1-5 at. % Bi-deficient
Sb2Te3: stoeichiometric
0.1-O.3 at.% Sulfur
25% higher than conventional alloys
2-4 times higher than their bulk
counterparts (same composition and
stoichiometry)
High performances nanostructured materials through
- nanostructuring
- doping