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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

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Influence of the focusing

Wavenumbers /cm-1

Inte

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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

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arb

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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