www.usra.edu

X-Ray Photoelectron Spectroscopy (XPS)

Applied to Soot &

What It Can Do for You

Randy L. Vander Wal, Vicki Bryg & Michael D. Hays USRA The U.S. EPA

Acknowledgements: 1. Former funding through NASA NRA 99-HEDs-01 (RVW) and the U.S. EPA 2. Soot samples supplied by the U.S. EPA, Sandia National Labs 3. Vicki Bryg, Patrick Rodgers, Y.L.Chen, David R. Hull and Dr. Chuck

Mueller, Sandia National Labs

Results Regarding Soot Nanostructure

Soot Nanostructure: (Definition) * Soot Nanostructure refers to carbon lamella (layer plane) length, orientation, separation and tortuosity. * Nanostructure is variable, dependent upon temperature, residence time and fuel identity.

Fringe Analysis Algorithm: (Quantification) * Lattice fringe analysis can be used to analyze HRTEM image data and quantify carbon nanostructure through statistical analysis.

Oxidation Rates: (Implications) * Oxidation rates are dependent upon nanostructure - suggests using nanostructure to control (accelerate) oxidation. * Source apportionment via analysis of nanostructure? * Health consequences related to nanostructure? * Environmental impact dependent upon nanostructure?

Pure HydrocarbonApplication I: Emissions Reduction

Oxygenated Additive

Fringe Length Analysis

Reference Fuel-n-hexadecane + heptamethylnonane 25

20

15

10

5

00.00 0.72 1.44 2.16 2.88 3.60 4.32 5.04 5.76

Fringe Length (nm)

Diethylene glycol ether (DGE)25

20

15

10

5

00.00 0.72 1.44 2.16 2.88 3.60 4.32 5.04 5.76

Fringe Length (nm)

Fringe Tortuosity Analysis

Reference Fuel- n-hexadecane + heptamethylnonane 20

15

10

5

01.00 1.12 1.24 1.36 1.48 1.60 1.72 1.84 1.96

Fringe Tortuosity

Diethylene glycol ether (DGE)20

15

10

5

01.00 1.12 1.24 1.36 1.48 1.60 1.72 1.84 1.96

Fringe Tortuosity

Smoke Meter (Engine Out)

Microscopic and Spectroscopic Analysis Techniques for Soot Characterization

HRTEMHRTEM(high resolution transmission(high resolution transmissionelectron microscopy)electron microscopy)

Microscopy TechniqueMicroscopy Technique

Physical StructurePhysical Structure(nanostructure)(nanostructure)

XPS (X-ray photoelectron spectroscopy)

Spectroscopy Technique

Chemical Composition (& bonding states)

Outline - XPS

1. Motivation & Background 2. Introduction to XPS 3. Analytical capabilities:

A. Elemental Composition (Identification of source; wear, etc.)

B. Carbon Oxidation State - Oxygen Functional Groups (Oxidation conditions)

* Consistency of samples within the same class * Distinctness between different classes of samples

C. Carbon (nano)structure

4. Conclusions

Introduction to X-Ray Photoelectron Spectroscopy (XPS)

* XPS provides information about elemental composition and oxidation state of the surface.

* A monochromatic X-ray beam of known energy displaces an electron from a K-shell orbital.

* The kinetic energy of the emitted electron is measured in an electron spectrometer.

* The binding energy Eb = hv – Ek is characteristic of the atom and orbital from which the electron is emitted.

Introduction to XPS (continued)

* A low-resolution wide-scan (survey) spectrum serves as the basis for the determination of the elemental composition of samples.

* At higher resolution, chemical shifts are observed depending upon oxidation state.

Ek = hv - Eb

X-ray e-

Eb

Outline

1. Motivation & Background 2. Introduction to XPS 3. Analytical capabilities:

A. Elemental Composition (Identification of source; wear, etc.)

B. Carbon Oxidation State - Oxygen Functional Groups (Oxidation conditions)

* Consistency of samples within the same class * Distinctness between different classes of samples

C. Carbon (nano)structure

4. Conclusions

Low-Resolution Survey Scan - Jet Aircraft Emissions

Low-Resolution Survey Scan - Diesel Engine Soot

6.0

5.0

4.0

% 3.0

2.0

1.0

0.0

XPS - Survey Elemental Analysis

S

Plant OFB

Wildfire

Jet Engines

Industrial OFB

Biodiesel A

Residential OFF

Biodiesel B

Biodiesel Soot

Na N Ca P

Elements

Outline

1. Motivation & Background 2. Introduction to XPS 3. Analytical capabilities:

A. Elemental Composition (Identification of source; wear, etc.)

B. Carbon Oxidation State - Oxygen Functional Groups (Oxidation conditions)

* Consistency of samples within the same class * Distinctness between different classes of samples

C. Carbon (nano)structure

4. Conclusions

Identify Oxygen Functional Groups

C-C sp2

Carbonyls

Phenols

Plasmon Carboxylic

-C-OH Phenol -C=O Carbonyl

-C-OOH Carboxylic

% A

rea

Oil Fired Boiler versus Residential Oil Fired Furnace

% A

rea

50

40

30

20

10

0

C1s Peak Histogram 80

C1s Peak Histogram

C-C sp2 60

C-C sp2

Fullerenic

40

-C-OH

C-C sp3

-C=O

0

20

-C-OOH -C-OOH

C-C sp3

-C=O

Industrial OFB1 Industrial OFB2

Res. OFF1 Res. OFF2

281.6 283.2 284.4 285.2 286.3 288.2 289.5 281.6 283.2 284.4 285.2 286.3 288.2 289.5

C1s Peak Position (eV) C1s Peak Position (eV)

0

10

20

30

40

50

60

% A

rea

Comparison Between Biodiesel Soots

C1s Peak Histogram

Biodiesel 15&16 Biodiesel 17&18

0

20

40

60

80

100 C1s Peak Histogram

% A

rea

C-C sp2

-C-OH -C-OOH

C-C sp2

-C-OH

-C=O

Fullerenic

Biodiesel 13Biodiesel 14

281.6 283.2 284.4 285.2 286.3 288.2 289.5 282.8 283.6 284.8 285.8 287 288.8

C1s Peak Position (eV) C1s Peak Position (eV)

Comparative Peak Intensities - Oxygen Functional GroupsAverage C1s Peaks

(normalized)

Diesel Soot

Plant OFB

Wildfire%

Jet Engine%

Industrial OFB

Residential OFF

Biodiesel A

Biodiesel B

281.4

283.4

284.2

284.7

285.0

285.9

286.3

C1s position (eV) 286.6

287.4

288.8

289.5

100%

90%

80%

70%

60%

50%

40%

30%

20%

10%

0%

Outline

1. Motivation & Background 2. Introduction to XPS 3. Analytical capabilities:

A. Elemental Composition (Identification of source; wear, etc.)

B. Carbon Oxidation State - Oxygen Functional Groups (Oxidation conditions)

* Consistency of samples within the same class * Distinctness between different classes of samples

C. Carbon (nano)structure

4. Conclusions

Motivation for Alternative Analysis Techniques

1. Lattice Fringe Analysis is time consuming

2. Analysis can be difficult to apply (in some cases)

Hmmm….

Identify Types of Carbon Nanostructure

C-C sp2

Carbonyls Phenols

Fullerenic

Carboxylic

C-C sp3

Identify Types of Carbon Nanostructure

Carbonyls Phenols

Carboxylic

C-C sp3 C-C sp2

Fullerenic

Identify Types of Carbon Nanostructure

C-C sp2

Carbonyls

Phenols

Fullerenic

Carboxylic

C-C sp3

Ave

rage

Inte

nsity

(raw

dat

a)

281.

4

283.

1

283.

6

284.

2

284.

4

284.

6

285.

0

285.

7

286.

0

286.

3

286.

5

286.

8

287.

4

287.

9

288.

8

289.

5

Comparative Peak Intensities - Carbon Nanostructure12000

Average XPS peaks

10000

Diesel Soot Plant OFB

8000 Wildfire Jet engines Industrial OFB6000 Residential OFF Biodiesel A

4000 Biodiesel B

2000

0

Peak positions

High Resolution Scan - C1s Region - HOPG Edge Planes

C-C sp2

C-C sp3

Carboxylic

Phenols

Carbonyl

Graphitic

Defects

High Resolution Scan - C1s Region - HOPG Layer Planes

C-C sp2

C-C sp3

Phenols

Graphitic

Defects

XPS Sensitivity to Carbon Nanostructure

C-C sp2

C-C sp3

Phenols

C-C sp2

C-C sp3

Carboxylic

Phenols

Carbonyl

* Developing Correlations with Carbon Reactivity*

Sample: Edge Sites Basal Plane Intensity Sum Intensity Sum

Planar Graphite ( ~ 284 eV, sp2)

1749 13%

11265 87%

Edge Graphite ( ~ 285 eV, sp3)

4021 37%

6723 63%

Ratio (G/D) 0.35 1.38

Edge site carbons can be nearly 10-fold more reactive than basal plane sites

Goal & Result

HRTEM & Lattice Fringe Analysis

XPS Carbon nanostructure Oxidative Reactivity

Conclusions

1. XPS analysis can identify & quantify trace elements. Utility: Can be used to identify source based on specific elements

present and their distribution. * Track fuel and/or oil elements * Analysis of engine wear

2. XPS can identify oxygen groups by bonding type; C-OH, C=O, and C-OOH. These reflect the soot oxidation history.

Utility: Identify the occurrence and degree of oxidation, such as the soot cake within a DPF

3. XPS can identify the types of carbon present, sp2, sp3 and fullerenic Therein it can provide a complimentary method to HRTEM and image analysis.

Utility: Correlate nanostructure with soot reactivity, and changes new fuels, e.g. biodiesel

Graphitic

Normal Particle

DPF Soot

Hollow Interiors

outer shell

Normal Particle Normal Particle

Partially Oxidized Partially Oxidized

Application III: DPFs

Application II: Source Specific NanostructureWildfire Emissions Oil Fired Boiler

Comparison between carbon lamella;short, disconnected versus longer range structure and order

Jet Aircraft Engine Exhaust

FullerenicStructure

Jet Engine Emission Diesel Emission Wildfire Emissiona1-1c-ROI_14 e2-3a-roi_1c4-hb-ROI_1 252525

202020

151515

101010

555

000 0.00 0.72 1.44 2.16 2.88 3.60 4.32 5.04 5.76 0 0.72 1.44 2.16 2.88 3.6 4.32 5.04 5.76 0.00 0.72 1.44 2.16 2.88 3.60 4.32 5.04 5.76

Fringe Length (nm) Fringe Length (nm) Fringe Length (nm)

C4-HB-SEPSKEL_1 A1-1C-SEPSKEL_1 E2-3A-ROI_1_4 25 25

25

202020

151515

101010

555

000 0.32 0.35 0.37 0.40 0.43 0.46 0.48 0.32 0.35 0.37 0.40 0.43 0.46 0.48 0.32 0.35 0.37 0.40 0.43 0.46 0.48

Fringe Separation (nm) Separation Distance (nm) Separation Distance (nm)Separation Distance (nm)

c4-hb-tort_1 a1-1c-tort_1 e2-3a-_1 202020

151515

101010

555

000 1.00 1.12 1.24 1.36 1.48 1.60 1.72 1.84 1.96 1.00 1.12 1.24 1.36 1.48 1.60 1.72 1.84 1.96 1.00 1.12 1.24 1.36 1.48 1.60 1.72 1.84 1.96

Tortuosity Tortuosity Tortuosity

Average C1s Peaks Average C1s PeaksIntensitiies and Locations Intensitiies and Locations

Ave

rage

Inte

nsity

2500

2000

1500

1000

500

0 0

500

1000

1500

2000

281.4 283.6 284.4 285.2 286.4 287 288.8

Bio 13-18

Ave

rage

Inte

nsity

Average Peak Position (eV)

281.4 283.6 284.4 285.2 286.4 287 288.8

Bio 13-14

Average Peak Position

0

2000

4000

6000

8000

1 104

Average C1s PeaksIntensitiies and Locations

BBB2&WVU

Ave

rage

Inte

nsity

Biodiesel 1 Biodiesel 2

Ordinary diesel

Averaged by Intensity

Averaged by Position

281.4 283.6 284.4 285.2 286.4 287 288.8

Average Peak Position (eV)