Nanomaterials: improving gassensor performance

John Saffell

Alphasense Ltd.

Technical Director

jrs@alphasense.com

Paul Midgley

Professor of Materials Science

NANOMATERIALS 2010 University of Cambridge

We will consider:

Technologies and markets for gas sensing

Nanometrology

Nanomaterials as catalysts

Nanomaterials in optical gas sensing

Technologies and markets ingas detection

A roadmap, which includes the matrix oftechnologies and markets is availableon:

www.gas-sensor-roadmap.com

Gas detection has manymarkets

Market segments

Domestic safety

Automotive

Industrial safety

Process control

Military

Emerging markets

Niche

Air quality

Homeland security- Explosives/ terrorism

Asthma, allergies

Medical

Hydrogen: fuel cells

Extreme environments (space, volcanoes, oil)

Breath analysis & capnography

Existing markets

Fire and home safety

Leak detection

Car emissions

PM10, PM2.5

Industrial safety & LEL

Confined space entry

Stack emissions

Process control and analysis

Food processing, transport and storage

Breathalyser / alcohol & drugs

Ammonia

Benzene, BTEX

Outdoor air, Indoor air

Odours (WWT, landfill)

Many technologies areemployed

Components

Lasers and optics

UV, IR, microplasma sources

Wavelength separation MEMS

Low cost optics, detector arrays

Fibre optics

Micro GC

Micro MS

PID, IMS

QMB, SAW, BAW

Sensor arrays

Microprocessors/ FPGAs/ PICs/ ASIC

Wireless

Technologies

MEMS

Nanomaterials (QDs, CNT, catalysts, nano MO)

Polymers, liquid crystals

Electrochemistry

Separation science

Physical chemistry (enthalpy, speed of sound)

Products

NIR spectrometers

IR single line absorption

IMS

Micro GC/MS

Nanoparticle fluorescence

IR, Visible, THz gas cameras

Ultrasound, thermal conductivity imaging

Electrochem/ optical/polymer/ nano arrays

LIDAR, DOAS

Nanometrology

Electron microscopy and AFM are regular toolsfor both R&D and quality control

Scanning Electrochemical Microscopy,improved Raman and near field microscopy

are offering new opportunities

TEM: Daresbury analysis of our Pt/Ru catalyst,identifying oxides and allowing us to determine

the growth pattern

3nm

(010)

[001]

(001)

(011)

55o

(100)

+12.72%

+4.6

5%

RuO2 Ru

- 4.28%

(010)

(100)

(001) (110)

+12.72%

-4.2

8%

(001)

(100)

(i)

(ii)

(iii)

+4.6

5%

Zngrain

Pt islanddeposited

fromsolution

RuO2

deposited –columnar

growth

Runanocrystals at oxidesurface

Growthmechanism

time

Oxygen Map

We also use Energy Filtered TEM toidentify the surface activity of our catalysts

SEM is routinely used to quantify catalystprimary particle size distribution

2 4 6 8 10 120

5

10

15

20

25

30

35

Part

icle

Counts

Particle Diameter (nm)

Dart 181A

2 4 6 8 10 12 14 16 18 200

2

4

6

8

10

12

14

Part

icle

Counts

Particle diameter (nm)

PtBO2

Nanoparticles agglomerate, so primary particlesize does not tell the entire story

Ru black

5nm

100nm

Small 2-6 nm particles can agglomerate tolarge particles

Nanomaterials as catalysts

• We have been using nanomaterials ascatalysts for decades- they have justbeen rebranded as nanoparticles.

• With better analytical tools, we nowhave better control of our catalysts.

Nanomaterials for gas detection:many choices

•• CNTsCNTs (MW)(MW)

•• CNTsCNTs + polymer+ polymer

•• CNTS + metal oxidesCNTS + metal oxides

•• CNT + metal catalystsCNT + metal catalysts

•• ZnO nanowiresZnO nanowires

•• SnOSnO22 nanonano powderpowder

•• Tungsten oxidesTungsten oxides

•• IIIIII--V quantum dotsV quantum dots

Many growth/ deposition methods

• CVD

• PVD

• Nanopipette: QDs, MMOs, polymers

• electropolymerisation (polymers)

• in-situ CNT growth

• Flame ablation

Molecular structure of [Et2In(OS2CNMenBu)]2

NOAH: DTI funded project to make gas sensors fromquantum dots and nanorods using single component

CVD

(Universities of Manchester & Keele, Alphasense, Teer, Epichem)

SEM image of InS nanorods

-In2S3 films grown at 375 °C

TEM shows straight In2S3 nanorods

with average diameter of

ca. 20 nm and ca. 400–500 nm

in length.

High-resolution TEM confirms

crystallinity by indicating well-resolved

(103) lattice planes. The experimental

lattice spacing, 0.66 nm is consistent

with the 0.62 nm separation in bulkcrystals.

Good deposition, but poor gas response

TEM image of InS nanorods

Flame Spray Pyrolysis

SnO2 particles generated byflame spray pyrolysis

SnO2 by flame pyrolysis shows goodresponse and strong temperature

dependence

10 ppm C2H5OH (C). The sensors with Pd/Al2O3 filter (filled symbols) and without filter (open symbols)for both undoped SnO2 (black squares) and Pd-doped SnO2 (grey circles) are measured at 50% r.h. at 25°C.

Carbons

• Graphite

• CNT (single and multiwalled)

• boron doped diamond

• glassy carbon

• graphene?

5nm

TEM can also be used to follow a processsuch as ball milling of graphite

2nm

2nm

Increased ball milling increases theamorphous layer thickness

5nm

5nm

PECVD Chamber for direct growth ofCNT

• Graphite heater usedto heat substrates

• (Plasma) DC Voltage-630V

• Temperature ofGrowth: 550 – 900oC

Rotary PumpConnected to thebell jar

Gas Inlet for Ammonia,Acetylene and Nitrogen

GraphiteStage heaterconnected

Gas Exposureoutlet for thesamples

Top View Showingsamples on agraphite stage

Direct Growth of Carbon Nanotubes

• Novel Technique to growCNTs direct on chip

• Microheater heated to growCNTs locally on the desiredarea in 5mins in vacuum at0.2mbar.

• MWCNTs grown locally on thesmall heaters , radius 12um.

• SWCNTs can be grown athigher temperature and thinnercatalyst deposition.

Small heater withCNTs

CNTs Grown on SOI Membranes

ResistiveElectrodeswith CNT

on SOIMembrane

Depositionfor 15minsusing 2nmFe catalyst

How the CNTs will work assensors

• Gases like NO2 areelectrophillic so it canremove electrons fromCNTs (For SWCNTs)

• For MWCNTs – chargetransfer mechanism.

• CNT conductanceincreases and thereforethe resistance of the filmdecreases.

Reported CNT response to NO2

Room temperature Response Time (2ppm) = 30sec, Sensitivity = ~15%F.Udrea et al , IEDM 2007, December

ZnO nanowires

• Nanowire grew properly in case of resistive sensor withAl metallization(Au plated)

• Resistance 10 k – 300 k

Growth on microhotplate: combining MEMS and nanomaterials

Nanomaterials in optical gassensing

Quantum dots re-emit light at much longer wavelengths thanexcitaion wavelength- this allows us to shift LED emissions tomuch longer wavelengths (Trackdale)

Controlled nanoparticles on surfaces give repeatable SurfaceEnhanced Resonant Raman Spectroscopy (SERRS)

Nanoparticles can replace metal surfaces as the conductinglayer for surface plasmons (SPR)

Conclusion

• Improved, lower cost analytical tools (electronmicroscopy and AFM) bring quality control tonanomaterials

• Catalyst are being improved with III-V and carbonbased materials now added to our catalyst choices

• Optics are using the unusual emission andconduction properties of nanomaterials

Acknowledgements

• Paul O’Brien Manchester Chemistry

• Rod Jones Cambridge Chemistry

• Nicolae Barsan University of Tuebingen Physics

• Bill Milne, Sumita Santra and Florin UdreaCambridge Engineering

• James Covington and Julian GardnerWarwick Engineering

• Paul Midgeley and Cate DucattiCambridge Materials Science and Metallurgy

• Cambridge CMOS Sensors

• Daresbury Laboratory

• Technology Strategy Board (ULoGS project funding)

Thank you for your attention