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Effect of the substrate charge and morphology
on the SERS spectra of some benzohydroxamic acids
Orlin Blajiev, Kitty Baert, Annick Hubin
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Adsorption related SERS
Two diverging directions
Analytical detection of organic molecules in solutions
Adsorption of organic molecules on metal substrates
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Molecular adsorption geometry by means of SERS
Parameters needed
• Local direction of the electric field vector
• Vector alignment of the scattered electric field and transition dipole moment
• Influence of the chemical adsorption on the directions of the transitional dipole moments
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Aim of the work
Experimental part• To decrease the influence of the SERS
substrate morphology by crystallizing of round and single Ag particles.
Computational part • To reduce the effect of the substrate size by
enlarging it as much as possible. • To investigate the effect of the substrate
charge on the geometry optimisation and on the vibration spectra.
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Experimental details
Preparation of the SERS substrate
•Solution A 0.1 M AgNO3+KOH+NH
4OH
•Solution B 0.05 M dextrose•Copper plates were dipped for 2 seconds and immediately rinsed in water
Organic compound
•Benzohydroxamic acid (BHA)
Solutions
•90 % H2O+10% MeOH+10-4 M BHA•x% H2O+(100-x)% MeOH+10-4 M BHA
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Morphology of the substrate
SEM pictures of the SERS substrate
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Influence of the substrate preparation
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Influence of the position
Wavenumbers /cm-1
Inte
nsi
ty /
arb
. unit
s
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Influence of the focusing
Wavenumbers /cm-1
Inte
nsi
ty /
arb
. unit
s
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Influence of the solvent
90% water+10% MeOH+10-4 BHA
100% MeOH+10-4 BHA
90% water+10% MeOH+10-4 BHA
Wavenumbers /cm-1
Inte
nsi
ty /
arb
. unit
s
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Previous computational models
Single-atom “substrate”1
DFT, periodic boundarySIESTA, DZP basis set
Slab substrate1,2
1. O. Blajiev et al. J. Raman Spectr. 34 p.295 2003, 2. O. Blajiev et al. J. Raman Spectr. 37 p.777 2006
DFT, PC-Gamess6-311G* and 3-21G (Ag)
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Charge topology on a slab substrate
In the plane of the hydroxamic group
In the topmost plane of the Ag slab
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Quantum-chemical approach used in this work
• Embedded cluster method-ONIOM• Large substrate-39 Ag atoms in total, 12 of
them treated explicitly• Basis sets: MIDIX (BHA), LANL2DZ (Ag), UFF
for the low level system• Restriction of the charge flow
• Charges of –2, -1, 0, 1, and 2 electrons
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Effect of the charge on the optimization geometry
Charge of -2 Charge of +2
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Charge distribution between the molecule and substrate
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Charge distribution within the hydroxamic group
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Effect of the charge on the quadrant stretch at 1588 cm-1
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Effect of the charge on the quadrant stretch at 1486 cm-1
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Conclusions
• Yet another SERS substrate is proposed. • Stable, well resolved spectra of BHA on it
were obtained.• Successful quantum-chemical calculations of
BHA on charged substrates were done.• Important parameters were found to vary
with the excess charge.
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Future work
• Experimental investigation of the substrate charge influence on the adsorption parameters of some BHA derivatives
• Exploration of the SERS feasibility to unravel molecular orientation in view of the variable direction of the near field