Raman spectroscopy: Basic principles and applications€¦ · Raman spectroscopy: Basic principles...

Christian Hess

Raman spectroscopy:Basic principles and applications

• Basic principles - Resonance Raman scattering - Surface Enhanced Raman Scattering (SERS)

• Instrumentation - Spectrometer- Excitation sources

• Raman in catalysis- In situ cells- In situ Raman (of working catalysts)

C.V. Raman (1928)

• Basic principles- Resonance Raman scattering - Surface Enhanced Raman Scattering (SERS)



C. Hess, 2006

Why Raman spectroscopy?

• Information on rotational and vibrational levels

• Raman effect small but accessible by use of lasers

• Complementary information to IR spectroscopy

homonuclear diatomic molecules, low frequency range

• In situ analysis of organic and inorganic compounds

• Analysis of aqueous solutions and solids (powders)

• Using resonance and surface enhancement effects ~1010

Trace gas/single molecule analysis - molecular structure

Introduction

C. Hess, 2006

• Spatial charge separation under influence of electric field E

induced dipole moment μ:

μ = α E (1)

α: polarizability

Example: polarizability changes during CO2 vibrations

Classical description

C. Hess, 2006

Example: Polarizability changes CO2

Vibrational modes of CO2

C. Hess, 2006

• Spatial charge separation under influence of electric field E

induced dipole moment μ:

μ = α E (1)

α: polarizability

• Electric field E due to electromagn. wave with frequency ν0

E = E0 cos 2π ν0t (2)

μ = α E0 cos 2π ν0t (3)

emission of light at same frequency ν0

I = (2/3c3) μ* μ* == (16π4α2E0

2/3c3) ν04 (4)


2

2

2

⎟⎟⎠

⎞⎜⎜⎝

⎛dtd μ

C. Hess, 2006

• Internal vibrational motion with Eigenfrequency vM

q = q0 cos 2π νMt (5)

• Polarizability α develop in series

α = αq=0 + (∂α/∂q)q=0 q + higher order terms (6)

μ = α E (7)

= (αq=0 + (∂α/∂q)q=0 q0 cos 2π νMt) E0 cos 2π ν0t

= αq=0 E0 cos 2π ν0t + ½ (∂α/∂q)q=0 q0 E0 [cos 2π (ν0 - νM)t

+ cos 2π (ν0 + νM)t

Rayleigh Stokes/Anti-Stokes


C. Hess, 2006

Q.M. description

Stokes Anti-StokesRayleigh

inelastic impact inelastic impactelastic impact

ΔEM = 0 ΔEM < 0 ΔEM > 0

ν = ν0 – νvib < ν0 ν‘ = ν0 – νvib > ν0ν = ν0

ν = 0

ν = 1

ν = 0

ν = 1

ν = 0

ν = 1

ν = 0

ν = 1

hν0 h(ν0 - νvib)

hν0 h(ν0 + νvib)

hν0 hν0

hν0 hν0

~~~~ ~~~~

~~~~ ~~~~ ~~~~ ~~~~

~~~~~~~~

harmonic oscillator: Δv = ±1

C. Hess, 2006

Raman intensity?

Is = Ni σR(i→f) IL (8)

Ni: initial state population

σR(i→f): Raman cross section for transition Ei→Ef

IL: Laser intensity

thermal equilibrium: Boltzmann distribution for state Ni at T

Ni = N0 exp(-ihνvib/kT) (9)

lower energy state: higher initial state population

I(Stokes) > I(Anti-Stokes)

Q.M. description

Example: Stokes/Anti-Stokes intensities of CHCl3C. Hess, 2006

Q.M. description

StokesAnti-Stokes

-800 -400 0 400 800

0

20000

40000

60000

80000

In

tens

ity

Raman shift (cm-1)

514nm excitation neat CHCl3

C. Hess, 2006

• Basic principles- Resonance Raman scattering- Surface Enhanced Raman Scattering (SERS)



C. Hess, 2006

Introduction to Resonance Raman scattering

Polarizability tensor α (single e state):

:ˆ,ˆ βα μμ Electric dipole moment operator i.a. with incident/scattered light

∑ ⎟⎟⎠

⎞⎜⎜⎝

⎛

Γ+−=

v iwwgevevg

ev0g0ev,

0ˆˆ11 βααβ

μμα

h

•

•

Gas inlet

Gas outlet

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Quart z chips

O-ring

High-t empera tureceme ntQuart z woolHeatin g wireQuart z windo wSamp le

••

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•

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O-ring


••

non-resonant Raman

g1g0

Stokes

resonance virtual level

Vis

resonant Raman

g1g0

Stokes

electronic resonance virtual level

Vis

ev

C. Hess, 2006

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non-resonant Raman

v=1v=0

Stokes

resonance virtual level

Vis

resonant Raman

v=1v=0

Stokes

electronic resonance virtual level

Vis

Example: β-Carotin

514nm 632nm

Wavelength (nm)

Abs

orba

nce

C. Hess, 2006

Example: β-Carotin

C=C stretch

0 400 800 1200 1600 2000

0

10000

20000

30000

In

tens

ity

Raman shift (cm-1)

β-carotin, 9mW, 20s exp 514nm excitation 632nm excitation

enhancementfactor = 10

C. Hess, 2006

• Basic principles- Resonant Raman spectroscopy - Surface Enhanced Raman Scattering (SERS)



C. Hess, 2006

Introduction to SERS

‘Normal’ Raman scattering

I(νL)

N molecules with σR,free

I(νS)

INRS(νS) = N I(νL) σR,free

C. Hess, 2006

SERS mechanism - enhancement factors

Surface-enhanced Raman scattering

I(νL) A(νL)2

N’ with σR,ads, N’ N

I(νS) A(νS)2

ISERS(νS) = N’ I(νL) A(νL)2 A(νS)2σR,ads

“Rough” surfacee.g. metal nanostructure

(1) E.m. field enhancement (2) Chemical enhancementmax. 10-100max. ~ 106 (isolated Ag,Au)

r d

Molecule

metalε

ε0

EM(ν)A(ν) = E0(ν)

max. ~ 108 (coupled)

≤

C. Hess, 2006

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SERS substrate

SEM images of the silver nanowire ML on Si wafer

Deposited ML shows domains of aligned silver nanowires

A. Tao, F. Kim, C. Hess, J. Goldberger, R. He, Y. Sun, Y. Xia, P. Yang, Nano Lett. 3 (2003) 1229

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SERS substrate

UV-VIS absorption spectra of silver nanowire ML

Broadened extinction 500-600 nm due to wire-wire coupling

transverse longitudinal

Raman

•

•

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SERS example: 1-hexadecanthiol/Ag-LB film

ν(C

-S) t

ν(C

-C)

CH

3ro

ck

CH

2w

ag CH

2tw

ist/

scis

sor

• Enhancement factor EF:

EF = [ISERS]/[IRaman] × [Mb]/[Mads]

• thiol head group (2.3 Å)2, closed-packed layer

molecule conc. on surface:2.5 × 1014/cm2

532nm, band at 1295 cm-1

EF = 2 × 105

C. Hess, 2006

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•

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••

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O-ring


••

SERRS example: Rhodamine 6G/Ag-LB film

SERRS = Surface Enhanced Resonance Raman Scattering

• Large EF = 109 as result of SERS and Resonant Raman

• Linear relationship between intensity and concentration

surface not saturated

• Langmuir description using

Mads= Mads,max Ka0/(1+Ka0)

yields ΔGads = 46 KJ/mol

C. Hess, 2006

• Basic principles - Resonant Raman spectroscopy - Surface Enhanced Raman Scattering (SERS)



C. Hess, 2006

Raman spectrometer

Triple spectrograph• first 2 gratings subtractive

removal Rayleigh (40% E) • grating 3/4 creates dispersion• detection: diode array

Challenges to record (good) spectra • Separate inelastically scattered from Rayleigh (99.99999%) light• Collect the maximum number of inelastically scattered photons

Fourier Transform (FT) - Raman• Michelson interferometer

(Jacquinot, multiplex)• NIR (1.064 µm) excitation,

less fluorescence self absorption

Modern Raman spectrometer

Light source(Laser)

Dispersion grating

Detector(CCD)

samplesampling

optics(objective,

immersion probe)

Notchfilter

• holographic notch filters80% T of Raman light,324 - 1339 nm available

• single transmission grating0 - 4400 cm-1 (multiplex)“clean”, no moving parts, high light throughput

• cooled CCD (~ 40% QE)400 - 1000 nm detection




C. Hess, 2006

Laser excitation wavelengths

Fluorescence(organic samples/water)

choice of laser wavelength:• scattering intensity ~ν4

• detectors• resonance enhancement• self-absorption of sample• fluorescence ( NIR, UV)

(typical range)




C. Hess, 2006

In situ Raman cells

•

•

Gas inlet

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O-ring


••

•

•

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••

Raman-scattered light

Gas inlet

Gas outlet

Thermocouple well

Quartz chips

O-ring

High-temperaturecement

Quartz woolHeating wire

Quartz windowSample

••

• Laser excitation atlow laser power tominimize heating

• Raman cell design mimics plug-flow reactor: gases flow through catalyst bed

• 50 – 100 mg sample

Plug-flow reactor

C. Hess, 2006

In situ Raman cells

Fluidized bed reactor • Particle motionreduces laser

beam exposure

• Raman cell mimics fluidized bed reactor: gases flow through catalyst bed

• UV laser excitation

• ~200 mg sample

In situ Raman cells

Rotating Raman cell • Sample rotationreduces laser

beam exposure

• Gas flow over catalyst bed

• ~200 mg sample

• Requires pellets




C. Hess, 2006

The NO storage-reduction conceptX

Support

BaOBaO PtPt

NOO2

NO2

NO

BaNO3

NO O2

NO2

* Jang et al., Catal. Lett. 77 (2001) 1

• Lower fuel consumption by use of engines operating with excess oxygen

• Typical storage catalysts consist of storage material (BaO) and metal (Pt)

Problem: Reduction of NOx

Solution: Storage of NOx followed by short rich periods for reduction

• BaO/Al2O3 deactivates at higher temperatures (due to Ba-Al alloying)*Focus on BaO/MgO which is stable up to at least 900°C

C. Hess, 2006

Experimental setup: Raman spectroscopyIn situ

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Raman-scattered light

Gas inlet

Gas outlet

Thermocouple well

Quartz chips

O-ring

High-temperaturecement

Quartz woolHeating wire

Quartz windowSample

••

• Laser excitation at 532 nm,low laser power of ~6 mWminimizes heating of sample

• Raman cell design mimicstypical plug-flow reactor:Gases flow through catalystbed (50 mg) at ~40 ml/min

C. Hess, 2006

Preparation of the 14 mol % BaO/MgO catalyst

Jeol 2010, 200 kV, x40k200 nm

• Impregnation of Ba(NO3)2 on MgO (Fisher)• Drying at 100°C, heating at 120°C for 2h• Heating to 900°C in He for 2h to decompose Ba(NO3)2 and BaCO3

• BaO formation confirmed by XRD

• TEM shows formation ofhomogeneous BaO phase

• X-ray analysis confirmsthe presence of BaO/MgO

C. Hess, 2006

10501020Raman shift (cm-1)

1080

Minutes:20.0

9.0

7.5

10.0

0

5

10

5030

Min

utes

134731

1048

1% NO 400 C

2

14001200Raman shift (cm-1)

1327

1337

Minutes:6.0

9.08.5

7.58.0

Ba(NO )2+

Ba -NO2+

2

Ba(NO )3 2

Ba -NO2+

2

822 1337

1327

259

Formation of nitro species precedes that of nitrates

In situ Raman spectra during NO2 exposure

C. Hess, 2006

10491% NO 300 C

2

0

5

10

6030

Min

utes

1335

135733

Ba(NO )2+

Ba -NO2+

2

825

1327

259

Ba(NO )3 2

1% NO 475 C

2

Min

utes

0

5

10

20

151

1048

1337724 81430

Ba -NO2+

2

1327

253

Temperature dependence

300°C: Formation of nitro species precedes nitrates (as at 400°C)475°C: Higher rate of NO3 formation but smaller storage capacity-

In situ Raman spectra during NO2 exposure

In situ Raman spectra after catalyst pre-oxidation

Dependence on pre-oxidation time

150010005000 Raman shift (cm-1)

Min

utes

0

5

10

1% NO 400 C

2

25% O pre-flow: 1 min 2

140 10521336

Ram

anin

tens

ity(a

.u.)

Ba -NO2+

2

1052R

aman

inte

nsity

(a.u

.)

150010005000 Raman shift (cm-1)

Min

utes

0

5

10

2015

1% NO 400 C

2

25% O pre-flow: 4 min 2

134730

1048

Ba(NO )3 2

Ba -NO2+

2

1052

Pre-oxidation of BaO enables direct nitrate formation at 400°C

C. Hess, 2006

2

Temperature dependence

In situ Raman spectra during NO2 /O2 exposure

Presence of oxygen surpresses formation of nitro speciesincreases the rate of nitrate formation

Min

utes

0

5

10

730

10481% NO / 20% O400 C

2 2

134

Ba(NO )3 2

136

732

1049

0

5

1030

20

Min

utes

1% NO / 20% O300 C

2 2

Ba(NO )2+

1337

Ba(NO )3 2

824

BaO Ba O

NO

BaO 2 Ba(NO 23)

+2+ NO /He2

He+ O /He2 NO

2

+ NO /He2

+ NO /He2

+ NO /He2 [Ba(NO ] ) + [Ba(NO ])32++

Summary - Mechanism for NO storage2

C. Hess and J.H. Lunsford, JPCB 106 (2002) 6358C. Hess and J.H. Lunsford, JPCB 107 (2003) 1982

Summary - NO storage capacity2

400 C500 C400 C/O300 C/O

22

,

C. Hess, 2006

Summary: Study reveals Raman key features

• Raman spectra without interference of gas phase

• simple glass/quartz cell

• in situ spectra can be recorded at high temperatures (500°C)

• time-dependent in situ experiments

• Raman spectra down to ~100 cm-1

• detection of IR inactive peroxide vibrations

• detection of adsorbed species and catalyst bulk phase

• quantification using internal reference

• self scattering of MgO support low

Raman spectra of supported phase

other supports?C. Hess, 2006

IR vs Raman of typical oxide supports

IR Raman

I.E. Wachs, Catal. Today 27 (1996) 437

n-pentane isomerization over 17 wt% WO3/ZrO2

S. Kuba and H. Knözinger J. Raman Spectrosc. 33 (2002) 325

S(2-methylbutane)~30%

W=OC=C (coke)

250ºC

1% n-pentane in He, 10 ml/min

Relation of Raman intensity to sample reflectance

Raman intensity based on Kubelka-Munk formalism:

ρ: Raman coefficient (~ν4) s: scattering coefficient

( )( ) )(1

1 0∞

∞

∞∞∞ =

−+

= RGsI

RRR

sIo ρρψ

[ ][ ]

{ }[ ]( ) [ ] 40

4

)(

)(1)(1)()(

ν

ψ

vRGs

cIvR

vRvRsIcv

v o

∞

∞

∞∞∞

=

−+

=

)(vR∞ : can be measured bydiffuse reflectance UV-Vis

Raman intensity correction

corrected dataoriginal data

plasma line

Structure of vanadia in VxOy/Al2O3

Z. Wu, H.-S. Kim, P.C. Stair, S. Rugmini, S.D. Jackson, JPCB (2005)

UV laser Visible laser

1.2 V/nm2

4.4 V/nm2

0.03 V/nm2

LMCT:O V

alumina

O

OV

OOH

O

OV

OO

OV

OO

dehydrated vanadia: isolated vs polymeric

Preferential excitationof structurally different VxOy species possible?

UV Raman – selective resonance enhancement

UVVisV2O5 VxOy V2O5 VxOyV=O in: V=O in:

1030 1018

alumina

O

OV

OOH

O

OV

OO

OV

OO

MeOH partial oxidation on polycryst. Ag (SERS)

C.-B. Wang, G. Deo, I.E. Wachs, JPCB 103 (1999) 5645

thermal cycles2 diff. atomic O species(isotope shifts ~20 cm-1)

MeOH/O2 = 3

Ag-O-AgAg=O

O2 flow after pretreatment

decreasingAg-O-Ag intin MeOH/O2

active site?assignment?subsurface?

Summary: Raman for catalytic applications

• Raman spectra accessible without interference of gas phase

• simple glass/quartz cell

• in situ spectra can be recorded at high temperatures (500°C)

• time-dependent in situ experiments

• Raman spectra of adsorbates and catalyst down to ~100 cm-1

• quantification using internal reference

correction for changes in sample reflectance ( DR UV-Vis)

• self scattering of common support materials low

Raman spectra of supported phase

• Signal enhancement as result of resonance Raman and SERS

C. Hess, 2006

Questions?

Raman spectroscopy: Basic principles and applications€¦ · Raman spectroscopy: Basic principles...

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