TiO2 and Al2O3 nanoporous oxide layers decorated with silver nanoparticles—active substrates for SERS measurements

Marcin Pisarek,Andrzej Kudelski,Maria Janik-Czachor,Agata Roguska,Marcin Holdynski

doi:10.1007/s10008-013-2375-x

Abstract

Self-organized nanoporous oxide layers (TiO2, Al2O3) exhibiting specific properties, obtained by anodic oxidation at a constant voltage in neutral electrolyte, may serve as attractive SERS substrates for investigating the interactions between an adsorbate and adsorbent, or as a stable platform for detecting various organic compounds. This paper presents the influence of the size of the nanotubes/nanopores and the structure of the porous oxide layers on the SERS enhancement factor, E F. We used pyridine and mercaptobenzoic acid as probe molecules, since they have a large cross-section for Raman scattering. To characterize the morphology and structure of the oxide layer substrates, before and after vacuum vapor deposition of silver nanoparticles, we applied scanning electron microscopy, X-ray diffraction and surface analytical techniques: AES, XPS and SERS. The results obtained show that for the same amount of Ag (0.02 mg/cm2) the size of the nanopores significantly affects the E F, which reaches, at a properly chosen nanopore size, distinctly higher values than that characteristic of a standard silver surface roughened by electrochemical cycling, i.e. E F > 106. The new Ag/MeO x –NT composites layer, ensure a good reproducibility of the SERS measurements and exhibit stability over a limited period of time.

Highlights

Characterization of the self-organized porous oxide substrates The optimal anodization conditions applied resulted in the Fig. 1 Effect of anodic voltage Vmax on average diameter of nanopores formation of TiO2 and Al2O3 nanotubes (“hollow cylinders”)
No evidence of extra peaks in the patterns suggests that the as-anodized oxides may be amorphous, which is in close agreement with the data from the literature [13, 18, 32, 33]
This is in agreement with existing literature data which suggest that the phase transition from anatase to rutile takes place at an annealing temperature of about 550–600 °C [13, 18, 33, 34]

Summary

Introduction

The growing interest in titania and alumina nanoporous materials is due to their wide application in such fields as photocatalysis (TiO2) [1,2,3], electrocatalysis (TiO2) [4,5,6], lithography (Al2O3) [7, 8] and optoelectronics (TiO2, Al2O3) [3, 6, 9], and as self-cleaning coatings (TiO2) [3], bio-sensor coatings (TiO2) [10] and biomedical materials (TiO2) [11,12,13]. SERS investigations with a probe molecule (pyridine, mercaptobenzoic acid - MBA) adsorbed on the Ag/ MeOx–NT composite layers provide spectroscopic data for determining the influence of the size of the nanotubes on the SERS enhancement factor (EF); 2.

Objectives

Results

Conclusion