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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)
• Instrumentation - Spectrometer- Excitation sources
• Raman in catalysis- In situ cells- In situ Raman (of working catalysts)
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
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)
Classical description
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
Classical description
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)
• Instrumentation - Spectrometer- Excitation sources
• Raman in catalysis- In situ cells- In situ Raman (of working catalysts)
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
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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)
• Instrumentation - Spectrometer- Excitation sources
• Raman in catalysis- In situ cells- In situ Raman (of working catalysts)
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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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)
• Instrumentation - Spectrometer- Excitation sources
• Raman in catalysis- In situ cells- In situ Raman (of working catalysts)
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
• Basic principles - Resonant Raman spectroscopy - Surface Enhanced Raman Scattering (SERS)
• Instrumentation - Spectrometer- Excitation sources
• Raman in catalysis- In situ cells- In situ Raman (of working catalysts)
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)
• Basic principles - Resonant Raman spectroscopy - Surface Enhanced Raman Scattering (SERS)
• Instrumentation - Spectrometer- Excitation sources
• Raman in catalysis- In situ cells- In situ Raman (of working catalysts)
C. Hess, 2006
In situ Raman cells
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Raman-scattered light
Gas inlet
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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
• Basic principles - Resonant Raman spectroscopy - Surface Enhanced Raman Scattering (SERS)
• Instrumentation - Spectrometer- Excitation sources
• Raman in catalysis- In situ cells- In situ Raman (of working catalysts)
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
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High-temperaturecement
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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?