Exploring inhibited interfaces using Vibrational Sum-Frequency Spectroscopy

UoM administered thesis: Phd

  • Authors:
  • Michael Dowhyj

Abstract

The goal of this project was to develop and utilise instrumentation for Vibrational Sum- Frequency Spectroscopy (VSFS) for the study of metallic interfaces treated with surfactants commonly used as corrosion inhibitors. This work is motivated primarily by the mechanistic knowledge gap surrounding the adsorption of such molecules, and the general lack of detail about the in situ interface. In particular, a corrosion inhibitor dodecyltrimethylammnonium chloride, abbreviated as DDTMAC, (an exemplar inhibitor of carbon steel in acidic environ- ments) has been focussed on, and its adsorption on gold and on carbon steel has been studied by VSFS and other complementary means, such as x-ray photoelectron spectroscopy (XPS), quartz crystal microgravimetry (QCM), and ellipsometry. VSFS is a second order non-linear optical technique, and possesses interface specificity (in- sensitive to bulk phases), making it a uniquely powerful for studying surfaces treated with such molecules, both ex and in situ. In VSFS, two photons strike a surface simultaneously (to within one picosecond), generating a third photon, and this third photon carries geometric and conformational information about the interface it was generated from. If incoming photon energies are swept, it also provides vibrational fingerprinting. The instrumentation described in this work uses an ultrashort (spectrally broad) probe pulse in the mid-IR region, and a longer, narrower pump pulse at 800 nm to produce broadband spectra rapidly (as opposed to tuning the probe pulse), meaning time resolved measurements of the in situ interface are possible. This instrumentation has been developed using ultrafast laser facilities in the Photon Science Institute at the University of Manchester, based on Ti:Sapphire sources, with a spectral The instrumentation has been tested using the benchmark system of self-assembled monolayers (SAMs) of octadecanethiol on gold, using the CH3 stretch mode region, and geometric informa- tion has been obtained about such a monolayer. This benchmark system has then been further studied using a more conventional surface probe, XPS, and geometric information has been verified. Following successful benchmarking, the VSFS instrument was applied to gold surfaces treated with DDTMAC. Initial measurements were ex situ, and once again verified using XPS, and coverage was determined to be comparable to the ODT benchmark at high concentrations for much shorter immersion times. Further measurements were applied to the in situ interface with the development of a liquid cell, exploiting the inherent surface specificity of VSFS, and it was noted that the in situ interface significantly differed from the ex situ surface. In situ measurements were then verified using common in situ probes, ellipsometry and quartz crystal microgravimetry (QCM), and coverage and thickness were calculated. This toolbox was then applied to the engineering interface of carbon steel treated with DDTMAC, both in water and in 1 molar hydrochloric acid, ex situ and in situ, and the symmetry and conformation of adlayers of DDTMAC was studied. We observed centrosymmetry at the in situ interface (with evolving weak resonances on a strong non-resonant background indicat- ing changes in ordering), and asymmetry at the ex situ interface resulting in much stronger resonances. This indicates disruption to the ordering of the overlayer upon removal from solution. The localised conformation and geometry of such adlayers was also studied using Sum-Frequency Microscopy in a collaboration with the University of Houston, and a domain structure of DDTMAC adsorption on the gold and steel surface was observed ex situ. Finally, VSFS in conjunction with near-ambient pressure XPS (NAP-XPS) was applied to study the initial adsorption of water on the native oxide film of a polycrystalline zinc surface for atmospheric corrosion applications. With VSFS, the OD resonance of deuterated water was probed (i.e. the non-hydrogen-bonded dangling OD bond at the air-water interface), and then the amount of water uptake was quantified using NAP-XPS, as a function of relative humidity. These measurements indicate that water adsorption occurs in an initially centrosymmetric layer, inaccessible to VSFS but not NAP-XPS, and subsequent water adsorption occurs in the form of droplets (islands) which grow as a function of relative humidity.

Details

Original languageEnglish
Awarding Institution
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Award date1 Aug 2020