Transcript Nanostructured Ag/SnO2 Sandwich Sensors for Acetone Sensing

SnO2 with Ag Nanoelectrodes for
Sensing Ultra Low Acetone
Concentrations
Final Presentation
Semester Project FS09
E. Buitrago
Advisors: Dr. H. Keskinen and A. Tricoli
Particle Technology Laboratory
Swiss Federal Institute of Technology (ETHZ)
1
Outline
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Motivation
Tin Oxide
Silver
Experimental
Results
Conclusion
Outlook
Questions?
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Motivation: Gas sensors for VOCs
• Certain VOCs in human breath = disease biomarkers:
Examples for disease markers in human breath:1
VOCs
Disease
Ethane and pentane
Oxidative stress
Methylated hydrocarbons
Lung or breast cancer
Hydrocarbons (especially ethane and pentane)
Oxidative stress
Isoprene
Cholesterol metabolism
Acetone
Diabetes mellitus, ketonemia
– Acetone2
– diabetic patients:
– healthy individuals:
1.Boguslaw et al., Biomed. Chromatogr., 21, 2007, 544.
2. Wang et al., Chem. Mater.,20, 2008, 4894.
1.8 ppm
0.8 ppm.
3
Tin Oxides
(35%)
Eranna et al., Crit. Rev. Solid State Mater. Sci., 29, 2004, 171.
4
Sensing of different metal oxides to various gaseous species.
Eranna et al., Crit. Rev. Solid State Mater. Sci., 29, 2004, 171.
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SnO2 Sensitivity to Low Concentrations of
Acetone
Sensitivity
SnO2 Dip coating
21 °C
Dry Air
dXRD = 5 nm
Acetone(ppm)
Zhao et al., Sens. Actuators, B., 115, 2006, 460.
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Acetone in Breath Detection Challenges
• > 200 VOCs in human
breath. 1
• VOCs present at trace
levels:
– i.e. ammonia: 0.8 ppm ,
ethanol: 0.1 ppm.2
1. Dang et al., J. Chromatogr., B810, 2004, 274.
2. Boguslaw et al., Biomed. Chromatogr., 21, 2007, 554.
3. Gaman et al., Russian Physics Journal, 51, 2008, 833.
• Breath saturated in H2O,
– H2O decreases SnO2
resistivity.3
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Nanostructured SnO2
Gas Sensitivity and Resistivity
1011
60
300 °C
Dry Air
300 320
°C °C, 10 ppm EtOH
Dry FSP
Air
1010
800
109
ppm H2
Dry Air
108
800 ppm CO
50
800 ppm H2
20
107
0
106
0
Xu
et al.,
Sens.
B., 3, 1991, 149.
Tricoli
et al.,
ToAct.
be submitted.
200
400
SnO2 Bulk Thickness (nm)
600
800
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Film Resistance and Sensitivity
• Electrode geometry and
minimal distance.1
• Film characteristics
(porosity, thickness,
material, etc.).
Interdigitated Electrodes
SnO2/CuO
Multi - Layer
• Divide Sensitive and
Conductive Functions!2
Au Electrode
40 mm
1. Shukla et al., International Journal of Hydrogen Energy. 33, 2008, 470.
2. Tricoli et al., To be submitted.
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Ag Nanoparticles as Nanoelectrodes
• Advantages
–Ag lowest resistivity of all metals
Ag: 15.87 nΩ·m,1 CuO: 0.1 Ω·m2 (20°C).
–Can produce metallic Ag by flames.3
–Relatively cheap.4
–Ag can enhance sensitivity.4
1.http://en.wikipedia.org/wiki/Resistivity
2.Tsai et al., Acta Materialia, 57, 2008, 1570.
3.Keskinen et al., Journal of Nanoparticle Research. 9, 2007, 569.
4.http://www.kitco.com/market/us_charts.html
5.Kim et al., Thin Solid Films. 516, 2008,198.
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FSP Direct Deposition and In-situ
Flame Annealing
• SnO2:
– 0.5M Tin (II) ethylhexanoate in
Xylene
• Ag:
– 0.01 M AgNO3 in ethanol,
ethylhexanoate acid (1:1 ratio)
5/5 Flame
Dep time: 15 s
• Anneal:
– Xylene
– 12/5 Flame
– Anneal time: 25 s
Mädleretetal.al.Adv.
Tricoli
Sens.
Mater.,
Actuators,
20, 2006,
B. 2006.
3005.
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Ag Nanoelectrodes-Anneal
Before in-situ anneal
15 s
After in-situ anneal
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Deposition Time
15 seconds
60 seconds
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Deposition of Functional SnO2
Ag on Alumina Substrate
SnO2 on Ag and Alumina
Substrate
Ag-Bottom
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Qualitative Effect of Anneal on Glass
Substrate
~3.3 μm
~0.4 μm
Ag-Bottom- No Anneal
Glass Substrate
Ag-Bottom- Annealed
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Sensor Testing
T = 320 °C
Synthetic
dry air
Water Vapor
S = Rair/Ranalyte
Acetone
(1) Tubular furnace, (2) Quartz tube
(3) Sensor, (4) Gold wiring
Teleki et al., Sens. Actuators, B.,119, 2006, 684.
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Characterization of Ag Nanoelectrodes
6
10
Ag-Not Annealed
Ag-Annealed
SnO2
Substrate
5
Baseline Resistance RAir [M]
10
Ag-Bottom
4
320 °C
Ag-Bottom
Dry Air
SnO2
10
3
10
2
10
1
10
0
15
30
45
Deposition Time (seconds)
60
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Response
R
1/R =1/RAg +1/RSnO2
RSnO2
CH3COCH3 CO2, H2O
O-
O-
O-
O-
O-
e-  e-  e- 
R SnO2
+
R Ag
O-
Substrate
S = RDry Air/RAcetone
CH3COCH3 (gas) + 8O- (adsorbed)  3CO2 (gas) +3H2O (gas) +8e- (conduction band)
Qin et al., Nanotechnology. 19, 2008, 7.
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Reproducibility
Sensor Response S = RAir /RAcetone
2.2
2.0
SnO2-1
1.8
SnO2-Average
1.6
SnO2-2
320 °C
0% RH
1.4
1.2
1.0
0.0
0.2
0.4
Acetone (ppm)
0.6
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Ag-Bottom vs. SnO2 under Dry
Conditions
Sensor Response S = RAir /RAcetone
4
Ag-Bottom
SnO
2
SnO2
Wang et al. 2008
3
~40%
350 °C
10% Cr doped WO3
320 °C
Dry Air
2
1
-0.1
0.0
Wang et al., Chem. Mater.,20, 2008, 4894.
0.1
0.2
0.3
0.4
Acetone (ppm)
0.5
0.6
0.7
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Sensor Response S = RRH=80% /RAcetone RH=80%
Effect of RH, Closer to Real Conditions
1.15
1.10
Ag-Bottom
SnO2
SnO2
~9%
320 °C
80% RH
1.05
1.00
S =RRH=80%/RAcetone RH=80%
0.0
0.2
0.4
Acetone (ppm)
0.6
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Ag-Bottom Selectivity under Dry
Conditions
Sensor Response S = RAir /RAnalyte
4
Acetone
Ethanol
Ethanol
~40%
320 °C
Dry Air
2
0.0
0.2
0.4
Analyte(ppm)
0.6
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Sensor Response S = RRH=80% /RAnalyte RH=80%
Ag-Bottom Acetone Selectivity 80% RH
Acetone
Ethanol
1.2
1.1
320 °C
80% RH
1.0
0.0
0.2
0.4
Analyte(ppm)
0.6
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Conclusions
• Conductive path already with Ag 15 s,
annealed.
• Detection of < 0.6 ppm acetone possible with
ultra thin SnO2 and nanostructured Ag/SnO2.
• Ag-Bottom 40% more sensitive than SnO2 0%
RH, 9% in 80% RH, acetone.
• Ag-Bottom selective to acetone 0% RH.
• Acetone and ethanol sensitivity comparable
80% RH.
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Outlook
• TiO2 doped Ag-Bottom sensor testingdecrease cross sensitivity to humidity.
• Repetition ethanol humidity Testing.
• “Home-made” FSP-made sensor testing and
characterization.
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Acknowledgments
• Dr. Helmi Keskinen
• Antonio Tricoli
• PTL Lab
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Thank you for
your attention,
Questions?
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Appendix
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XRD
Thermal Stability Ag
Effect Ag addition, Resistance
Dry and Humid Air Trace
Portable Gas sensors
High Concentration mini-p results
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: SnO2
: Ag
: Al2O3
: Au
XRD Results
Ag 8 min
dXRD = 20 nm
SnO2 Filter
dXRD = 12 nm
Ag-Bottom
15 s. Deposition
time
SnO2-Only
Au + Al2O3 Substrate
20
25
30
35
2degree
40
45
50
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Ag Nanoparticles as Nanoelectrodes
• Disadvantages
– Low thermal stability in air, < 500°C.
Akhavan et al. Applied Surface Science., 2007, 254, 548.
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Low Thermal Stability High
Resistances
a) As deposited Ag
b) 500 °C
c) 700 °C
1 hour anneal in dry air
Sheet resistance variation with Ag Thickness,
different temperatures. SEM.
Akhavan et al. Applied Surface Science., 254, 2007, 548.
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Ag Nanoparticles as Nanoelectrodes
• Disadvantages
– Low thermal stability at low temperatures
in air, < 500°C.1
– Melting point depression for decreasing
grain sizes. 10 nm  < 760 K, bulk: 1233 K.
Shyjumon et al. The Eur. Phys. J. D., 37, 2006, 309.
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Ag Nanoparticles as Nanoelectrodes
Gibbs-Thomson Equation
σ = 1.02 J/m 2 (surface energy)
M = 107.9 g/mol (Molar mass)
ρ = 10.5 g/cm3 (density)
∆Hm = 11.3 kJ/mol melting enthalpy
Tbulk = 1233K(bulk melting pt.)
r = radius of cluster size.
Shyjumon et al. The Eur. Phys. J. D., 37, 2006, 309.
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Ag Nanoelectrodes
1 hour, O2 Atmosphere
Kim et al., Thin Solid Films. 2008, 516, 198
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Sensitive to Ultra Low Concentrations
of Acetone
9
1.6x10
Ag-Bottom
320 °C
0% RH
RH=0, 0 ppm
9
1.2x10
Ohms
0.1 ppm
0.2
8
8.0x10
0.5
8
4.0x10
3500
S= RDryAir/RAnalyte
4000
4500
0.6
5000
time (secs)
5500
6000
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Ag-Anneal 80%, Acetone Response
6
3.5x10
Ag-Bottom
80% RH, 0 ppm Acetone
0.1 ppm
6
Ohms
3.0x10
0.2 ppm
6
2.5x10
0.5 ppm
6
2.0x10
0.6 ppm
S= RRH=80%/RRH=80%, Analyte
6
1.5x10
23000
23500
24000
24500
Time (secs)
25000
25500
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Portable Micro Gas Sensors
Microhotplate back-heating
SnO2
300 mm
Baseline  109 ohm, Optimal Baseline  107 ohm
Kühne et al., J. Micromech. Microeng., 18, 2008, 035040
Tricoli et al., Adv. Mater., 20, 2008, 3005
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Sensor Response, S
Acetone Sensor Response, Low Concentrations
100
S = Rair/Ranalyte
10
Acetone Ag-Bottom
Acetone Ag-Top
Acetone SnO2-Only
1
T = 320 °C
Synthetic dry air
0
5
10
15
20
25
30
35
40
45
50
55
Acetone Concentration, ppm
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CO Response Compared
CO CO2 O2
O- O - O- O -
Sensor Response, S=Rair/Ranalyte
CO SnO2-Top
CO SnO2-Only
e-  e-  e- 
e-  e-  e- 
CO Ag-Top
O- O- O- O1
T = 320 °C
Synthetic dry air
Catalytic CO consumption
without electron transfer
1
10
CO Concentration, ppm
Mädler et al. J. Mater. Res., 22, 2007, 854.
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