1
Gas Phase Reactions
Heating: furnace, laser, plasma, flame, arc
Gas-Metal Rxn
Ti + N2  TiN 1800 K
Ti + CH4  TiC + H2 mp 2940 C
3 Si + 2 N2  Si3N4
W + CH4  WC + H2 mp 2720 C
WC dissolved in Co = cemented carbides
(Widia materials)
Cementite
steel + H2/CO + CH4 + NH3  Fe3C + nitrides
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Gas Phase Reactions
Gas-Gas Rxn
Flame hydrolysis
volatile compounds are passed through an oxygen-hydrogen
stationary flame, homogeneous nucleation from supersaturated
vapor (nano):
SiCl4 + 2 H2 + O2  SiO2 + 4 HCl
fumed silica
Reagent bp/C Product
SiCl4 57 SiO2
AlCl3 180 (subl.) Al2O3
TiCl4 137 TiO2
CrO2Cl2 117 Cr2O3
Fe(CO)5 103 Fe2O3
GeCl4 84 GeO2
Ni(CO)4 42 NiO
SnCl4 114 SnO2
ZrCl4 331 (subl.) ZrO2
VOCl3 127 V2O5
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SiCl4 + H2O  OSiCl2 + 2 HCl
OSiCl2 + H2O  SiClOOH + HCl
SiClOOH     SiO2 + HCl
Gas Phase Reactions
Si60O90Cl60
4
Y2O3 Particles by Flame Aerosol Process
oxygen
hydrogen
Y(NO3)3
Particle size control by precursor concentration
Higher concentration = larger size
5
Calcium phosphate nanoparticles Ca/P molar ratios 1.43 to 1.67
Synthesized by simultaneous combustion of
Ca(OAc)2 + OP(OnBu)3 in a flame spray reactor
Fluoro-apatite and zinc or magnesium doped calcium phosphates
adding trifluoroacetic acid or metal carboxylates into the fuel
Nanoparticle morphology
At a molar ratio of Ca/P < 1.5 promoted the formation of dicalcium pyrophosphate
(Ca2P2O7)
Phase pure tricalcium phosphate TCP - Ca3(PO4)2
obtained with a precursor Ca/P ratio of 1.52 after subsequent calcination at 900 °C
Micropores and the facile substitution of both anions and cations
Possible application as a biomaterial
Flame Aerosol Process
Spray Pyrolysis
6
(1) mass flow controller – O2 1 L/min
(2) ultrasonic nebulizer – aqueous
solution 2 Co(OAc)2 : 1 Ni(OAc)2
(3) 3-zone heater - 400 C
(4) temperature controller
(5) electrostatic precipitator
SEM micrographs of NiCo2O4 particles obtained
from different concentrations of Co(OAc)2 and
Ni(OAc)2 precursor solutions –
Lower concentration reduces particle size
400 C
Tubular furnace reactor
Morphology Control
7
(a) HAADF-STEM of a
rutile@anatase core@shell
microsphere; (b) titanium L2,3
core-loss EELS spectra acquired
from the indicated areas compared
to reference TiO2 polymorphs
[rutile (green) and anatase (red)]
(d−f) EELS maps: (d) rutile (green), (e) anatase (red), and (f) rutile and anatase
overlaid color map. (c) 3D tomographic reconstruction of another typical
rutile@anatase core−shell microsphere, together with the corresponding HAADFSTEM
image (inset)
Rutile@Anatase core@shell microspheres
8
Gas Phase Reactions
High-power CO2 lasers
3 SiH4 + 4 NH3  Si3N4 + 12 H2
HN(SiMe3)2 + NH3  Si3N4 + SiC
DC-Ar Plasma
TiCl4 + NH3  TiN + HCl
Electric arc synthesis (Krätschmer)
Graphite  C + C2 + …. soot + C60 + C70
He atmosphere (100 torr),
U = 10–20 V, I = 0–250 A
Fullerene C60 extracted from the soot with toluene
Yields 1 – 10 %
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traces of a transporting agent B (e.g., I2)
T1T2
A AB (g)
A
Vapor Phase Transport Syntheses
Sealed glass tube reactors
Solid reactant(s) A + gaseous transporting agent B (O2, Cl2, I2, CO…..)
Temperature gradient furnace T ~ 50 - 1000 C
A + B react at T2 to form gaseous AB (g)
Equilibrium established A (s) + B (g) ⇄ AB (g)
Equilibrium constant K
Gaseous transport of AB (g) to the other end
Concentration gradient of AB (g) = driving force for gaseous diffusion
AB (g) decomposes back to A (s) at T1, crystals of pure A
Temperature dependent K
Equilibrium concentration of AB (g) changes with T, different at T2 and T1
A (s) + B (g)  AB (g) A (s) + B (g)  AB (g)
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Whether T1 < T2 or T1 > T2 depends on the thermochemical balance of
the reaction !
Transport can proceed from higher to lower or from lower to higher
temperature
Vapor Phase Transport Syntheses
Example: Pt (s) + O2 (g) ⇄ PtO2 (g)
Endothermic reaction, PtO2 forms at hot end, diffuses to cool end,
deposits well formed Pt crystals, observed in furnaces containing Pt
heating elements or thermocouples (thermometers)
Chemical vapor transport, T2 > T1, provides concentration gradient and
thermodynamic driving force for gaseous diffusion of vapor phase
transport agent AB (g)
Uses of VPT
• Synthesis of new solid state materials
• Growth of single crystals
• Purification of solids
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Thermodynamics of VPT









21
0
1
2
12
11
lnlnln
TTR
H
K
K
KK
van’t Hoff equation
Reversible equilibrium needed: G = RT lnKeq = H  T S
* Exothermic H < 0
Smaller T implies larger Keq
AB (g) forms at cooler end, decomposes at hotter end of reactor
W + 3 Cl2 ⇄ WCl6 400/1400 C (exo)
Ni + 4 CO ⇄ Ni(CO)4 50/190 C (exo)
* Endothermic H > 0
Larger T implies larger Keq
AB (g) forms at hotter end, decomposes at cooler end of reactor
2 Al + AlCl3 ⇄ 3 AlCl 1000/600 C (endo)
4 Al + Al2S3 ⇄ 3 Al2S 1000/900 C (endo)
A (s) + B (g) ⇄ AB (g)
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Estimation of the thermochemical balance (H) of a transport
reaction:
ZnS(s) + I2(gas) ⇄ ZnI2(gas) + S(g) H = ??
Zn(s) + I2(g) ⇄ ZnI2(gas) Hf = 88 kJ mol-1
ZnS(s) ⇄ Zn(s) + S(g) (Hf) = +201 kJ mol-1
----------------------------------------------------------------
 ZnS(s) + I2(gas) ⇄ ZnI2(gas) + S(g) H = +113 kJ mol-1
Endothermic reaction, transport from hot to cold end!
Vapor Phase Transport Syntheses
Applications of VPT Methods
13
Ti + 2 I2 ⇄ TiI4
H = 376 kJ mol-1
Exothermic:
transport from cold to hot
Purification/crystallization of metals: Van Arkel Method
Cr (s) + I2 (g) ⇄ CrI2 (g) Exothermic
Exothermic, CrI2 (g) forms at cold end, pure Cr (s) deposited at hot end
Useful for Ti, Hf, V, Nb, Cu, Ta, Fe, Th
Removes metals from carbide, nitride, oxide impurities
W-filament (ca. 1500 K)
Ti-powder (ca. 800 K)
I2
Ti-crystals
Applications of VPT Methods
14
Double Transport involving opposing Exothermic-Endothermic reactions
Endothermic:
WO2 (s) + I2 (g) (800 C) ⇄ WO2I2 (g) (1000 C)
Exothermic:
W (s) + 2 H2O (g) + 3 I2 (g) (1000 C) ⇄ WO2I2 (g) + 4 HI (g) (800 C)
The antithetical nature of these two reactions allows W/WO2 mixtures to be
separated at different ends of the gradient reactor using H2O/I2 as the
transporting VP reagents
Applications of VPT Methods
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Vapor Phase Transport for Synthesis
A (s) + B (g) (T1) ⇄ (T2) AB (g)
AB (g) + C (s) (T2) ⇄ (T1) AC (s) + B (g)
Concept: couple VPT with subsequent reaction to give overall reaction:
A (s) + C (s) (T2) ⇄ (T1) AC (s)
Direct reaction sluggish even at high T
SnO2 (s) + 2 CaO (s)  Ca2SnO4 (s) Phosphor material for light-emitting diodes
The reaction speeded up with CO as VPT agent:
SnO2 (s) + CO (g) ⇄ SnO (g) + CO2 (g)
SnO (g) + CO2 (g) + 2 CaO (s) ⇄ Ca2SnO4 (s) + CO (g)
Applications of VPT Methods
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Direct reaction is sluggish:
Cr2O3 (s) + NiO (s)  NiCr2O4 (s) Magnetic materials
Cr2O3 (s) + 3/2 O2 ⇄ 2 CrO3 (g) Greatly enhanced rate with O2
2 CrO3 (g) + NiO (s) ⇄ NiCr2O4 (s) + 3/2 O2 (g)
Overcoming Passivation Through VPT
2 Al (s) + 3 S (s)  Al2S3 (s) Passivating skin stops reaction
In presence of surface cleansing VPT agent I2:
Endothermic:
Al2S3 (s) + 3 I2 (g) (700 C) ⇄ 2 AlI3 (g) + 3/2 S2 (g) (800 C)
Applications of VPT Methods
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Vapor Phase Transport for Synthesis
Zn (s) + S (s)  ZnS (s) passivation prevents reaction to completion
Endothermic:
ZnS (s) + I2 (g) (800 C) ⇄ ZnI2 (g) + ½ S2 (g) (900 C)
VPT Synthesis of ZnWO4 from WO3 and ZnO
a phosphor host crystal for Ag+, Cu+, Mn2+
WO3 (s) + 2 Cl2 (g) (980 C) ⇄ WO2Cl2 (g) + Cl2O (g) (1060 C)
ZnO (s) + WO2Cl2 (g) + Cl2O (g) (1060 C) ⇄ ZnWO4 (s) + Cl2 (g)
Growth of epitaxial GaAs films or single crystals by VPT
GaAs (s) + HCl (g) ⇄ GaCl (g) + ½ H2 (g) + ¼ As4 (g) Endothermic
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Laser-induced Homogeneous Pyrolysis
Sensitizer
SF6
948 cm-1
Isopropanol
958 cm-1
Laser wavelength
10.60 ± 0.05 m
• Overlap between the vertical
reactant gas stream and the
horizontal laser beam
• Reaction zone away from
the chamber walls
• Nucleation of nanoparticles
• Less contamination
• Narrow size distribution
Fe(CO)5  Fe + 5 CO
C2H4 + h  C2H4
*
Excitation energy
transferred to vibrationaltranslational
modes
 T increases
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Iron-oxide Nanoparticles
by Laser-induced Homogeneous Pyrolysis
2 Fe(CO)5 + 3 N2O  Fe2O3 + 10 CO + 3 N2
PXRD
TEM
Maghemite Fe2O3