SnO2 with Ag Nanoelectrodes for Sensing Ultra Low Acetone Concentrations

Final PresentationSemester Project FS09

E. Buitrago Advisors: Dr. H. Keskinen and A. Tricoli

Particle Technology Laboratory

Swiss Federal Institute of Technology (ETHZ)

1

Outline

• Motivation• Tin Oxide• Silver• Experimental• Results• Conclusion• Outlook• Questions?

2

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

3

–Acetone2

–diabetic patients: 1.8 ppm– healthy individuals: 0.8 ppm.

1.Boguslaw et al., Biomed. Chromatogr., 21, 2007, 544.2. Wang et al., Chem. Mater.,20, 2008, 4894.

4Eranna et al., Crit. Rev. Solid State Mater. Sci., 29, 2004, 171.

Tin Oxides (35%)

5

Sensing of different metal oxides to various gaseous species.

Eranna et al., Crit. Rev. Solid State Mater. Sci., 29, 2004, 171.

6Zhao et al., Sens. Actuators, B., 115, 2006, 460.

SnO2 Dip coating 21 °CDry Air dXRD = 5 nm

Acetone(ppm)

Sens

itivi

ty

SnO2 Sensitivity to Low Concentrations of Acetone

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

• Breath saturated in H2O,

– H2O decreases SnO2

resistivity.3 7

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.

Nanostructured SnO2 Gas Sensitivity and Resistivity

SnO2 Bulk Thickness (nm)

1011

1010

109

108

107

106

0 200 400 600 800

50

60

20

0

320 °C, 10 ppm EtOHFSP

8Xu et al., Sens. Act. B., 3, 1991, 149.

300 °CDry Air

Dry Air

800 ppm H2

300 °CDry Air

Tricoli et al., To be submitted.

800 ppm H2

800 ppm CO

Base

line

Resi

stan

ce R

Air,

Ohm

s

Sens

itivi

ty

Film Resistance and Sensitivity

• Electrode geometry and minimal distance.1

• Film characteristics (porosity, thickness, material, etc.).

• Divide Sensitive and Conductive Functions!2

9

Interdigitated Electrodes

1. Shukla et al., International Journal of Hydrogen Energy. 33, 2008, 470.2. Tricoli et al., To be submitted.

SnO2/CuO Multi - Layer

Au Electrode40 mm

• 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

10

Ag Nanoparticles as Nanoelectrodes

1.http://en.wikipedia.org/wiki/Resistivity2.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.html5.Kim et al., Thin Solid Films. 516, 2008,198.

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 FlameDep time: 15 s

• Anneal:– Xylene– 12/5 Flame– Anneal time: 25 s

Mädler et al. Sens. Actuators, B. 2006.11

Tricoli et al. Adv. Mater., 20, 2006, 3005.

Ag Nanoelectrodes-Anneal

12

Before in-situ anneal15 s

After in-situ anneal

Deposition Time

13

15 seconds 60 seconds

Deposition of Functional SnO2

14

Ag on Alumina Substrate SnO2 on Ag and AluminaSubstrate

Ag-Bottom

Qualitative Effect of Anneal on Glass Substrate

15

Ag-Bottom- No AnnealGlass Substrate

Ag-Bottom- Annealed

~3.3 μm~0.4 μm

Sensor Testing

Teleki et al., Sens. Actuators, B.,119, 2006, 684.16

Synthetic dry air

Acetone

T = 320 °C

Water Vapor

(1) Tubular furnace, (2) Quartz tube(3) Sensor, (4) Gold wiring

S = Rair/Ranalyte

Characterization of Ag Nanoelectrodes

17

320 °C Dry Air

Substrate

0 15 30 45 60101

102

103

104

105

106

B

asel

ine

Res

ista

nce

RA

ir [M

]

Deposition Time (seconds)

Ag-Not Annealed

0 15 30 45 60101

102

103

104

105

106

B

asel

ine

Res

ista

nce

RA

ir [M

]

Deposition Time (seconds)

Ag-Not Annealed Ag-Annealed

0 15 30 45 60101

102

103

104

105

106

B

asel

ine

Res

ista

nce

RA

ir [M

]

Deposition Time (seconds)

Ag-Not Annealed Ag-Annealed SnO

2

SnO2

0 15 30 45 60101

102

103

104

105

106

B

asel

ine

Res

ista

nce

RA

ir [M

]

Deposition Time (seconds)

Ag-Not Annealed Ag-Annealed SnO

2

Ag-Bottom

Ag-Bottom

18

Substrate

+ -

e- e- e-

O -

O -

O -

O - O

- O -

CH3COCH3 CO2, H2O

R SnO2

R Ag

1/R =1/RAg +1/RSnO2

RSnO2

CH3COCH3 (gas) + 8O- (adsorbed) 3CO2 (gas) +3H2O (gas) +8e- (conduction band)

Qin et al., Nanotechnology. 19, 2008, 7.

R Response

S = RDry Air/RAcetone

Reproducibility

190.0 0.2 0.4 0.6

1.0

1.2

1.4

1.6

1.8

2.0

2.2

SnO2-1

Sen

sor R

espo

nse

S =

RA

ir /R

Ace

tone

Acetone (ppm)

0.0 0.2 0.4 0.6

1.0

1.2

1.4

1.6

1.8

2.0

2.2

SnO2-1

SnO2-2

Sen

sor R

espo

nse

S =

RA

ir /R

Ace

tone

Acetone (ppm)

0.0 0.2 0.4 0.6

1.0

1.2

1.4

1.6

1.8

2.0

2.2

SnO2-1

SnO2-2

SnO2-Average

Sen

sor R

espo

nse

S =

RA

ir /R

Ace

tone

Acetone (ppm)

320 °C0% RH

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

1

2

3

4

SnO2

Sen

sor R

espo

nse

S =

RA

ir /R

Ace

tone

Acetone (ppm)

Ag-Bottom vs. SnO2 under Dry Conditions

20

320 °CDry Air

Wang et al., Chem. Mater.,20, 2008, 4894.

350 °C10% Cr doped WO3

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

1

2

3

4

Ag-Bottom SnO

2

Sen

sor R

espo

nse

S =

RA

ir /R

Ace

tone

Acetone (ppm)

~40%

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

1

2

3

4

Ag-Bottom SnO

2

Wang et al. 2008

Sen

sor R

espo

nse

S =

RA

ir /R

Ace

tone

Acetone (ppm)

0.0 0.2 0.4 0.6

1.00

1.05

1.10

1.15 SnO2

Sen

sor R

espo

nse

S =

RR

H=80%

/RA

ceto

ne R

H=80%

Acetone (ppm)

0.0 0.2 0.4 0.6

1.00

1.05

1.10

1.15 Ag-BottomSnO

2

Sen

sor R

espo

nse

S =

RR

H=80%

/RA

ceto

ne R

H=80%

Acetone (ppm)

Effect of RH, Closer to Real Conditions

21

320 °C80% RH

S =RRH=80%/RAcetone RH=80%

~9%

Ag-Bottom Selectivity under Dry Conditions

22

320 °CDry Air

~40%

0.0 0.2 0.4 0.6

2

4

Ethanol

Sen

sor R

espo

nse

S =

RA

ir /R

Analy

te

Analyte(ppm)

0.0 0.2 0.4 0.6

2

4

Acetone Ethanol

Sen

sor R

espo

nse

S =

RA

ir /R

Analy

te

Analyte(ppm)

0.0 0.2 0.4 0.6

1.0

1.1

1.2

Acetone Ethanol

Sen

sor R

espo

nse

S =

RR

H=80%

/RA

naly

te R

H=80%

Analyte(ppm)

Ag-Bottom Acetone Selectivity 80% RH

23

320 °C80% RH

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.

24

Outlook

• TiO2 doped Ag-Bottom sensor testing- decrease cross sensitivity to humidity.

• Repetition ethanol humidity Testing.• “Home-made” FSP-made sensor testing and

characterization.

25

Acknowledgments

• Dr. Helmi Keskinen• Antonio Tricoli• PTL Lab

26

Thank you for your attention,

Questions?27

Appendix

• XRD• Thermal Stability Ag• Effect Ag addition, Resistance• Dry and Humid Air Trace• Portable Gas sensors• High Concentration mini-p results

28

20 25 30 35 40 45 50

2degree20 25 30 35 40 45 50

2degree20 25 30 35 40 45 50

2degree

XRD Results

29

: SnO2

: Ag: Al2O3

: Au

Au + Al2O3 Substrate

SnO2 FilterdXRD = 12 nm

Ag-Bottom

Ag 8 mindXRD = 20 nm

SnO2-Only

15 s. Depositiontime

Ag Nanoparticles as Nanoelectrodes

• Disadvantages– Low thermal stability in air, < 500°C.

30Akhavan et al. Applied Surface Science., 2007, 254, 548.

Low Thermal Stability High Resistances

31

Sheet resistance variation with Ag Thickness, different temperatures. SEM. Akhavan et al. Applied Surface Science., 254, 2007, 548.

a) As deposited Agb) 500 °Cc) 700 °C1 hour anneal in dry air

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.

32Shyjumon et al. The Eur. Phys. J. D., 37, 2006, 309.

Ag Nanoparticles as Nanoelectrodes

33

Gibbs-Thomson Equation

Shyjumon et al. The Eur. Phys. J. D., 37, 2006, 309.

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 enthalpyTbulk = 1233K(bulk melting pt.) r = radius of cluster size.

Ag Nanoelectrodes

34

1 hour, O2 Atmosphere

Kim et al., Thin Solid Films. 2008, 516, 198

Sensitive to Ultra Low Concentrations of Acetone

353500 4000 4500 5000 5500 6000

4.0x108

8.0x108

1.2x109

1.6x109

Ohm

s

time (secs)

RH=0, 0 ppm

0.1 ppm

0.2

0.5

0.6

Ag-Bottom320 °C0% RH

S= RDryAir/RAnalyte

Ag-Anneal 80%, Acetone Response

36

80% RH, 0 ppm Acetone

0.1 ppm

0.2 ppm

0.5 ppm

0.6 ppm

23000 23500 24000 24500 25000 255001.5x106

2.0x106

2.5x106

3.0x106

3.5x106

Ohm

s

Time (secs)

Ag-Bottom

S= RRH=80%/RRH=80%, Analyte

Portable Micro Gas Sensors

37Kühne et al., J. Micromech. Microeng., 18, 2008, 035040 Tricoli et al., Adv. Mater., 20, 2008, 3005

Baseline 109 ohm, Optimal Baseline 107 ohm

Microhotplate back-heating

SnO2

300 mm

0 5 10 15 20 25 30 35 40 45 50 55

1

10

100

Acetone Ag-Bottom Acetone Ag-Top Acetone SnO

2-Only

Sensor

Response, S

Acetone Concentration, ppm

Acetone Sensor Response, Low Concentrations

38

T = 320 °CSynthetic dry air

S = Rair/Ranalyte

1 10

1

CO SnO2-Top

CO SnO2-Only

CO Ag-Top

Sensor

Response, S

=R

air/

Ranaly

te

CO Concentration, ppm

CO Response Compared

39

T = 320 °CSynthetic dry air

O- O- O-

O-O-O- O-e- e- e-

O- O-

O-O-O- O-

e- e- e-

O2

Mädler et al. J. Mater. Res., 22, 2007, 854.

CO CO2

Catalytic CO consumptionwithout electron transfer