Fundamentals of (Semiconducting)Metal Oxide Gas Sensors...2012/10/01 · Overview of sensing with...

Fundamentals of (Semiconducting)Metal Oxide Gas SensorsNicolae Barsan and Udo Weimar

Outline • Overview of sensing with SMOX based gas sensors

- What is a sensor?- Reception/transduction functions- Impact of sensor parts and test conditions

• Sensing of CO with (undoped)SnO2 based sensors- Dry air- Humid air

• Impact of „doping“ of SnO2 based sensing materials- WF investigations- XAS investigations

• Bringing an idea from the lab onto the highway

2 | Fundamentals of (Semiconducting)Metal Oxide Gas Sensors, Nicolae Barsan © 2012 University of Tübingen

Overview of sensing with SMOX based gas sensors; what is a sensor


Overview of sensing with SMOX based gas sensors; reception/transduction functions



SeO gas +⋅+ −αβ 22α

β−

SO

SeCOOCO gaskSgas react +⋅+⋅ →+⋅ −− αββ αβ 2

Surface chemistry (receptor function)



Charge transfer

depletion layer at the surface of the grains



Conduction in the layer

back to back Schottky barriers



Transduction

reducing of the potential barriers between the grains overall decrease in the resistance depends on the morphology of the layer

1~~ +αβ

COS pnG


Overview of sensing with SMOX based gas sensors; impact of sensor parts and test conditions

Material type

np SS =



Material: additives/dopants



chemical/catalytic effect of electrodes



Electrical effect of electrodes



Polarization


Overview of sensing with SMOX based gas sensors; what is a sensor


Studies of sensing with SMOX based gas sensors;


Sensing of CO with (undoped)SnO2 based sensorDry conditions: effect of exposure to oxygen


Sensing of CO with (undoped)SnO2 based sensorDry conditions: effect of exposure to oxygen

−≈

kTqVG Sn exp


Sensing of CO with (undoped)SnO2 based sensorDry conditions: effect of exposure to oxygen (WF and DC resistance)


Sensing of CO with (undoped)SnO2 based sensorDry conditions: effect of exposure to CO


Sensing of CO with (undoped)SnO2 based sensorDry conditions: effect of exposure to CO (WF and DC resistance)


Sensing of CO with (undoped)SnO2 based sensorEffect of humidity


Sensing of CO with (undoped)SnO2 based sensorEffect of humidity (DRIFTS results)


Sensing of CO with (undoped)SnO2 based sensorEffect of humidity (WF and DC resistance)


Sensing of CO with (undoped)SnO2 based sensorExposure to CO in humid air


Sensing of CO with (undoped)SnO2 based sensorExposure to CO in humid air (DC resistance)


Sensing of CO with (undoped)SnO2 based sensorExposure to CO in humid air (DRIFTS)



Material: additives/dopants


Doping: goals• Improving of sensing performance

- Noble metals additives to improve sensitivity and/or selectivity, decrease operation temperature, response and recovery times

- Ideally will be present at the surface- Bulk properties of the base material not expected to change

• Adjustment of baseline resistance- Donors or acceptors- Need to be in the bulk- Not expected to directly influence the sensing performance


Doping: WF investigationsExample of undoped SnO2


Doping: WF investigations


Doping: WF investigations

Material EC-EF[meV]

nb[cm-3]

LD[nm]

x0 in N2 [nm]

x0,dep [nm]

x0,acc [nm]

SnO2 80 2.14.1018 3.6 0 7.2 4.9

0.2 wt.% Au

80 2.14.1018 3.6 0 10 7.6

3 wt.% Al 270 4.57.1016 24.4 85.2 97.1 30.0

0.2 wt.% Pd

290 3.05.1016 30.0 124.9 130.6 39.4


Doping Pd: XAS investigations

XANES of Sensor: 0.2 wt.% Pd:SnO2 (A), dry air(B) exposure to 50 ppm CO in dry air (C) exposure to 30 ppm H2 in dry air at 300 °C(D) at 300 °C in 1000 ppm H2 in He


Doping Pt: XAS investigations• with Au electrodes in dry air at 300°C• after reducing conditions (2 vol. % H2/He at

600°C)• SnO2 with Pt-electrodes at 300°C in air• PtO2 powder


Doping Au: XAS investigations

HERFD-XANES spectra at Au L3-edge of: (a) 0.2 wt. % Au doped SnO2 sensor (red), (b) 2 wt. % Au doped SnO2 sensor (blue) and (c) 5 Au foil as reference (black) at RT in air.

HERFD-XANES spectra at Au L3-edge of a 0.2 wt. % Au:SnO2 sensor recorded at 300°C in: (a) dry air, (b) 50 ppm 30 CO in air and (c) 30 ppm H2 in air; lower part:


Doping: XAS investigations

Pd: No spillover or Fermi-level controll mechanism Pd in oxidized form well distributed Providing sites for oxygen adsorption

Au: Au in metallic form Small particles Spillover mechanism

Pt: Pt in highly oxidized form well distributed Strongly incorporated in the SnO2 matrix Surface effect like für Pd + bulk sensitization


Doping: WF and XAS investigations• Good match

- Pd2+ in XAS, acceptor levels in WF!- Separate Au metallic phase in XAS, no electronic effect in WF!

• Need to rethink sensitization models- Only Au fits to one „classical“ model, spill over- Pt and Pd are atomically distributed, „site“ effect!

• No pure volume or surface effects for Pt, Au and Al!• Pt is special, it makes the matrix (SnO2) easy to reduce with

consequences on the sensing mechanism!


Outlook Technology: FSP

Oxygen

Methane

Oxygen

OxygenPrecursor liquid

Syringe pump

MFCs

Exhaust ventFilter housing

Spray flame

Support flame

Shield gas

PIPI

Water inWater out

Depositionsubstrate

sensing area: 7 x 3.5 mm2


Outlook Technology: FSP

50 m

m

their direct deposition as functional layers

Preparation of functional materials

and


Outlook New materials


Outlook Surface engineering

1.27%NiO-SnO2

SnO2

“The Role of NiO Doping in Reducing the Impact of Humidity on the Performance of SnO2-Based Gas Sensors: Synthesis Strategies, and Phenomenological and Spectroscopic Studies”, Hae-Ryong Kim et al, Advanced Functional Materials 21, 23 (2011) 4456-4463


Outlook Modeling


Outlook Modeling

20

0

)(2

)( zznqzV b −⋅⋅⋅

⋅=

εε

( ) 200

0 2znqVzV bSz ⋅⋅⋅

⋅=== εε

21

21

00 2 S

b Vnqz ⋅

⋅⋅

⋅=⇒−

εε


Bringing an idea from the lab onto the highway

• Thick films/microhotplates combination- Why?- How?- Industrialization- Applications


• For n-type MOX one needs porous sensing layers for better performance

• The presence of noble metal dopants/sensitizers is needed to decrease response time and operation temperature

• One can obtain such layers by using powders, „dope“ them, make them into an ink and deposit them (screen-printing, drop coating, etc..)

• Micromachined substrates allow for decreasing the power consumption and simplify packaging

Thick films/microhotplates: why?


Institute of Microengineering (IMT), Samlab, Neuchâtel, Switzerland

Microhotplate

Thick films/microhotplates: how?


Institute of Microengineering (IMT), Samlab, Neuchâtel, Switzerland

Microhotplate



Microsensor: drop coated microhotplate



1 10 1001

10

100

f:\daten\gardner\Temphum.opj

300°C

250°C

200°C

350°C

400°C

30% r.h.

50% r.h.

70% r.h.

Sen

sor r

espo

nse

G(C

O)/G

0

CO concentration (ppm)

Performance evaluation



Simultaneous chemoresistive and thermal effects



120 240 360 480 600 720 840 960 1080 1200 1320 1440 15601k

10k200

400600

100020002000

1000500

200100

200100

6030

15107

C2H5OH [ppm]CH4 [ppm]CO [ppm]

time [min]

Pt doped / 50% r.h.

Rse

nsor

[Ω]

120 240 360 480 600 720 840 960 1080 1200 1320 1440 1560

402

404

406

Selectivity enhancement of SnO2 gas sensors: simultaneous monitoring of resistances and temperatures,Arnd Heilig

Figure 2

T sen

sor [

°C]

Simultaneous chemoresistive and thermal effects



0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.0 4.4 4.81

10

Selectivity enhancement of SnO2 gas sensors: simultaneous monitoring of resistances an d temperatures,Arnd HeiligFigure 4c

Theater = 400°C / Pt doped / 50% r.h.

CO CH4 C2H5OH

sens

or s

igna

l Rai

r / R

gas

- ∆ T [°C]

Gas discrimination



Application exploration

CO, NO2

sensor systema)



Test drive: prototype chemical sensor system



-500 0 500 1000 1500 2000 2500 3000 3500 40000

5

10

15

20

25

30

35

40

CO

con

cent

ratio

n (p

pm)

Time (seconds)

Electrochemical Cell signal

5

10

15

20

25

30

35

40

Log

(sen

sor c

ondu

ctan

ce)

Microsensor signal

Test drive: comparison with EC



Thick films/microhotplates: industrialization



sensing layer

Rsensing ~ 100Ω – 100MΩ

Rheater = 100Ω

2 mm

1 mm 0.45 mmSi

Si3N4 – membrane

heater

electrodes

sensing layer

Commercial microsensor

MOSMetal Oxide Semiconductor?R (resistance)

MOSMetal Oxide Semiconductor?R (resistance)


Sam

ple

Mea

n

191715131197531

280000

240000

200000

__X=213182UCL=226851

LCL=199514

Sam

ple

StD

ev

191715131197531

30000

15000

0

_S=10620

UCL=20917

LCL=322

Sample

Valu

es

2015105

280000

240000

200000

3200

00

2800

00

2400

00

200000

1600

00

1200

0080

000

LSL USL

LSL 72800USL 321699

Specifications

250000200000150000

Within

O v erall

Specs

StDev 11160,5C p 3,72C pk 3,24

WithinStDev 16781,9Pp 2,47Ppk 2,16C pm *

O v erall

11

11

1

T-605089-001 drop_sizeXbar Chart

Tests performed with unequal sample sizes

S Chart

Tests performed with unequal sample sizes

Last 20 Subgroups

Capability Histogram

Normal Prob PlotA D: 0,691, P : 0,069

Capability Plot



CO, NO2

sensor systema)

Thick films/microhotplates: applications


stop and go entering underground garage

-4

0

4

8

12

16

20

0 500 1000 1500 2000 2500 3000-0.16

-0.08

0.00

CO

[ppm

]

NO

2 [pp

m]

time [sec]

14

16

18

20

22

24

26

0 500 1000 1500 2000 2500 300020

22

24

26

28

30

32

hum

idity

[%r.h

.]

tem

pera

ture

[°C

]

time [sec]

stop and go

0

6

12

18

0 500 1000 1500 2000 2500 3000

-100000

-50000

0

50000

100000 TGS2620 off2600 offASP offASM

CO

[ppm

]

sign

al [a

. u.]

time [sec]

Humidity

Temperature

CO

NO2



From prototype to commercial chemical sensor system



Conclusions

• You need to (better/cheaper) solve an application; nobody pays for technology

• It takes time: - 1994 first tries with pastes and micro hotplates, 2001 first

commercial products (already more than 20 million are sold!)

• You need good people/doctoral students

• You need money/investors


Thank you.Contact: Dr. Nicolae BARSAN

Institute of Physical and Theoretical Chemistry / AG WeimarAuf der Morgenstelle 15, 72076 TübingenPhone: +49 7071 29-78761Fax: +49 7071 [email protected]


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Fundamentals of (Semiconducting)Metal Oxide Gas Sensors...2012/10/01 · Overview of sensing with...

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