Surface morphology engineering of metal-oxide
films by chemical spray pyrolysis
Juan Rodríguez, José Solís , Mónica Gómez and
Walter Estrada
Universidad Nacional de Ingeniería, Facultad de Ciencias,
Casilla 31-139, Lima, Perú
I. Introduction
Thin film technology is nowadays a
widespread
technique
for
materials
fabrication, and numerous materials have
been prepared in the form of thin films.
The fabrication techniques are based on
different physical-chemical principles, but
generally speaking most procedures can be
distinguished as either vacuum-based or
chemical-based techniques.
• Owing to their simplicity and inexpensiveness,
chemical techniques have been studied
extensively for the preparation of thin films.
• Those
techniques facilitate the desing of
materials at the molecular level.
•
The chemical spray pyrolysis technique (SPT)
is one of the major techniques to deposit a
wide variety of thin films. It can use a variety of
atomization techniques such as ultrasonic
nebulisation, spray hydrolysis, corona spray
pyrolysis, electrostatic spray pyrolysis
I.1 Conventional Spray Pyrolysis Technique
I.2 Novel Spray Pyrolysis Technique (SPT)
Gas exhausting
Air compressor
temperature
controller
heater
substrate
Fluxmeter
hose
Pressure gauge
Chamber
spraying
peristaltic
pump
gel
This spray system has the following advantages:
-
Good size selectivity of the droplets
-
Vorticity suppression of the spray gas in the
spraying chamber
-
Reduced convection flow of the spraying gas in
the deposition chamber.
I.2 Sol-Gel Technique
• The sol-gel process constitute an important
part of so-called “soft-chemistry”. It has a low
cost and is rather simple: a network is
progressively
built
through
inorganic
polymerization reactions at room or moderate
temperature.
• Depending
on
the
regularity
of
the
macromolecular structure, crystalline or
amorphous materials may be prepared.
Sol-Gel Process
11
4
1
2
2
Uniform particles
5
3
solvent
extraction
Sol
Aerogel
Gel
5
6
6
7
fibres
Xerogel film
11
heat
10
solvent
evaporation
Xerogel
8
heat
9
Thin film
Ceramic
Sol-Gel Process
• A ceramic is formed by reactions between
molecular precursors, then homogeneity of the
material is perfect at the atomic scale.
• This makes the sol-gel technique, for instance,
very suitable for producing optical coatings.
• A common process for coatings is by dipping
the substrate in a colloidal solution; however
one of the major drawbacks of this method is to
produce films of limited thickness.
I.3 Morphology engineering
•
According to the kind of application, a specular
or porous thin film is pursued
•
SPT has proved to be a versatile technique in
order to control the morphology of the coating
•
Specular coatings are required mainly for
optical
applications:
selective
surfaces,
mirrors, electrochromic devices, etc.
Surface roughness can be monitored from
rough to specular by controlling the precursor
solution: type of solvent and pH, and keeping
fixed optimal spraying conditions: substrate
temperature, gas carrier flux and pressure,.
In this work we shall discuss the morphology of
NiOx and ZnO films.
Porous materials are of interest in a variety
of devices such as electrochromic ‘smart’
windows, nanocrystalline solar cells, batteries,
photocatalytic reactors, and gas sensors.
•
Porosity in thin films can be achieved in
many ways. Standard procedure relies on
atomistic deposition of species under conditions
giving a low ad-atom mobility so that a fine-grained
crystalline or amorphous structure is grown.
•
In this work we take an alternative route by
means of soft chemistry.
•
When SPT and sol-gel are properly combined
the resulting spray-gel technique (SGT) can be very
useful for large area applications, letting the film´s
morphology be monitored by controlling the
precursors and deposition conditions.
•
The technique basically consists in
producing an aerosol from a gel which is sprayed
over a hot substrate where the film will grow.
•
In this work we shall discuss the morphology
of tungsten-oxide and tin-oxide-based coatings
using the SGT.
II. Morphology and related optical
properties of coatings obtained by
SPT
•
Schematic representation of an improved
spray system associated to an optical system for
in-situ measurement of the film thickness.
•
The optical system also allows
determination of the roughness of the films.
the
Spray system with an optical set-up for
in-situ film thickness measurement.
Gas out
Moving plate
Heater
Air compress
Substrate
pipe
Beam splitter
Spray nozzle
He-Ne Laser
Pressure gauge
Photodetectors and signal amplifiers
II.1 Pyrolytic
coatings
zinc-oxide-based
•
Zinc-oxide-based coatings have interesting
potential applications: thermoelectric and gas
sensor devices, transparent electrodes, selective
surfaces and piezo-electric devices, etc.
•
Using the novel system we have grown
undoped zinc oxide thin films. In a previous work
we analyzed their growing characteristics, atomic
composition, optical properties, photocatalitic
response and morphology.
•
In this work the influence of spraying
solution on morphology is analyzed.
•
Droplets of the spraying solution was
obtained with a medical nebulizer and
compressed air was used as gas carrier.
•
In all experiments the gas carrier pressure
was kept at 1.7x105 Pa during deposition.
•
The fog of the spraying solution was
brought to the hot substrate (350 °C) where the
solvent evaporates taking place the pyrolytic
reaction and the film starts to grow.
•
The pH of the spraying solution was kept
constant at 5. Deposition was stopped when the
number of interference fringes measured in-situ
during deposition were three.
Ethanol
Water
3/1
100
80
0.6/1
0.4/1
0.2/1
0.1/1
0/1
0
200
400
600
800
1000
1200
1400
1600
1800
Time (s)
In-situ normal reflectance as
a function of time for
pyrolytic ZnO-based films.
Each curve corresponds to
the shown ethanol / water
proportion in the precursor
solution.
TD at 550 nm
Reflexion (arb. units)
1/1
60
40
20
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Ethanol to water Ratio
Diffuse transmittance, TD, of
ZnO - films obtained with
different ethanol / water
proportion in the spraying
solution.
2 mm
a)
b)
c)
d)
e)
f)
g)
Morphology
of
ZnO-based
films
obtained by SEM from pyrolytic
solutions at different etanol / water
proportions: (a) 0/1, (b) 0.1/1, (c) 0.2/1,
(d) 0.4/1, (e) 0.6/1, (f) 1/1, and (g) 3/1.
•
ZnO film prepared at an ethanol / water ratio
of 3/1, well defined interference fringes are
observed to decrease in intensity as the film
becomes thicker.
•
As the amount of ethanol decreases in the
precursor solution, the films become rougher as
deduced from the fast dimming of their
interference fringes
•
This trend is observed up to a ratio of
ethanol to water of 0.2/1. For films prepared at a
lower ethanol / water ratio the surface becomes
smoother. Defined interference fringes are
observed in a film prepared at a ethanol / water
ratio of 0/1.
•
Similar behavior is founded by measuring
the diffuse transmittance, TD, at 550 nm
wavelength for ZnO-based films obtained at
different proportions of ethanol / water. Films
obtained at ratios of 0.1/1, 0.2/1 and 0.4/1 show
optical diffusing surfaces, while films obtained
with either pure water or pure ethanol show
specular surfaces.
II.2 Pyrolytic Nickeloxide-based coatings
•
Nickel-oxide-based coatings have
been largely studied for many kind of
applications, like gas sensors, batteries,
fuel cells, electrochromism, etc.
•
They are particularly promising as
electrochromic materials possessing anodic
coloration
Glass or plastic substrates
Transparent
conducting
electrodes
V
Ion storage
Electrochromic
Ion conductor/electrolyte
Electrochromic materials
are characterized by a
reversible and persistent
change of the optical
properties under the
action of an applied
electric field.
•
Many techniques has been used to get a
nickel-oxide thin film and most of them are
based on vacuum processes. However,
spray pyrolysis is also a good alternative for
growing electrochromic-nickel-oxide-based
coatings.
•
Smooth nickel-oxide-based coatings with
specular optical properties were produced in
two ways:
(a) using an alcoholic solution (ethanol) of
Ni(NO3)26H2O at 0.25 M
(b) using aqueous solutions of the mixture
Ni(NO3)2.6H2O / Co(NO3)2.6H2O at 90 / 10M
ratio.
•
Those solutions were sprayed on to substrates
at 300 C during 30 minutes giving rise to
coatings with 0.5 - 1 mm thickness.
•
Electrochromism was studied by cyclic
voltammetry using a potentiostat connected to a
three electrodes cell, where a platinum foil and a
saturated calomel electrode were used as
counter and reference electrode, respectively.
•
Aqueous solution of 0.1 M KOH was used as
electrolyte for cation insertion/extraction. In-situ
optical transmittance measurements together
with electrochemical measurements were carried
out using a He-Ne laser beam ( = 632.8 nm)
Films obtained from alcoholic solutions
a)
b)
(b) Ex-situ
optical
transmittance
(a) Cyclic voltammetric curve for a
NiOx film obtained from an alcoholic
solution (bottom) and the in-situ
transmittance at  = 632.8 nm (top).
The scan rate was 10 mV/s and the
electrolyte was aqueous 0.1 M KOH.
Films obtained from aqueous solutions
50
Transmittance (%)
45
40
35
30
25
2
Current density (mA/cm )
20
1.0
0.5
0.0
-0.5
-1.0
-1.5
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
Voltage (V)
Cyclic voltametric curve and
associated transmittance at
632.8 nm.
Optical transmittance for the
bleached and colored state
of the nickel - cobalt oxide
film prepared by spray
pyrolysis.
5 mm
SEM micrographs of specular
surface of sprayed nickel-oxide
based coating obtained from
alcoholic solution
SEM micrographs of rough
surface of nickel-oxide based
coating obtained from aqueous
solution
II.3
Morphology
and
related
optical-electrical properties of
coatings obtained by SGT
•
Porous materials are of interest in a
variety of devices requiring a large interface
between a solid and either a liquid or gas
medium. Interesting applications are related
to gas sensors.
•
Semiconductor gas sensors use
changes in the electrical conductance of a
polycrystalline sensing ceramic to detect
gas components in air.
Gas sensors
Gold
electrodes
Heating
resistor
Al2O3
substrate
Nanocrystalline
WO3 film
•
In this work, we report highly porous WO3,
CuWO4 and SnO2 coatings as gas sensing
materials produced by SGT.
•
Mixed WO3 and CuWO4 films were
prepared from a gel via acidification of 0.1 M
sodium tungstate aqueous solution through a
proton exchange resin.
Flow Chart of Gel Preparation for
CuWO4-x Film
Na2WO4.2H2O
0,1M(ac)
pH = 7,8
Acidificatión by ion exchange
using a strongly acid resin
[WO2(OH)2]
o
Acetone 2% Vol.
Polymerizatión
(t = 45min.)
(WO3.H2O)n
pH = 1,1
Cu(SO4).5H 2O(s) , in
~1mL H 2O.( molar ratio
Cu:W between 0,01 – 1,0)
(WO3.H2O)n
2+
2+ (Cu , SO 4 )
pH = 1,5 – 2,0
spraying
Film (CuWO4-x.H2O)n
Experimental conditions
Substrate temperature(°C)
220
Air flux (L/ min)
9
Spraying time (min.)
45
Air pressure (kPa)
30
Nozzle- Substrate
- distance (cm)
0.5
The obtained thin films were subjected to heat treatment at
600 °C for 3 h. Alumina slides were used as substrate.
Characterization - SEM
5 mm
0.5 mm
SEM micrograph of WO3 film
0.5mm
Cu/W : 3%
0.5mm
Cu/W : 7%
0.5mm
Cu/W : 20%
0.5mm
Cu/W : 30%
0.5mm
Cu/W : 10%
0.5mm
Cu/W : 100%
Gas-Sensing Properties
60
Sensitivity
50
40
30
20
10
0
0
20
40
60
80
Cu/W molar ratio in the solution (%)
100
Flow Chart of Gel Preparation for SnO2
Film
A
I or T
SnCl4.5H2O
(C5H11O)4Sn
+ H2O
+ H2O
SnO2 (Sol)
(SnO2)n
SnO2 (Sol)
+ H2O + NH3
(T=600C)
(ac)
(SnO2)n
Centrifugal Separation
Liquid
(SnO2)n (s)
Wash
(SnO2)n (ac)
+ NH3 (15 M) or TEA (15M)
PH=8,8
(SnO2)n (ac)
Spraying
(SnO2)n film
+ H2O + NH3
(T=600C)
(ac)
Centrifugal Separation
Liquid
(SnO2)n (s)
Wash
(SnO2)n (ac)
+ NH3 (15 M)
PH=8,8
(SnO2)n (ac)
Spraying
(SnO2)n film
Experimental conditions
Substrate temperature(°C)
130
Air flux (L/ min)
40
Spraying time (min.)
60
Air pressure (kPa)
150
Nozzle- Substrate
- distance (cm)
0.1
The obtained thin films were subjected to heat treatment at
500 °C for 2 h. Alumina slides were used as substrate.
Characterization - SEM
I
A
0.5 mm
T
0.5 mm
SEM micrograph of SnO2 film
0.5 mm
Gas-Sensing Properties
300
50 ppm
To= 400 °C
250
T
40 ppm
30 ppm
A
Sensibility (S)
200
Ethanol out
150
I
20 ppm
10 ppm
100
Ethanol in
50
0
0
10
20
30
Time (min)
40
50
III. Conclusions
SPT and SGT have proved to be a versatile
technique in order to control the morphology of
the coating
Combining the spray pyrolysis and the sol-gel
techniques (spray-gel) gives the possibility to
produce very rough and highly diffuse films,
under appropriate conditions.
This technique brings new possibilities for
fabricating highly porous films like WO3, CuWO4-x
and SnO2.
Gracias