Local Environment and Interactions of Liquid and Solid Interfaces

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Local Environment and Interactions of Liquid and Solid Interfaces Revealed by Spectral Lineshape of Surface Selective Nonlinear Vibrational Probe Shunli Chen, Li Fu, Zizwe A. Chase, Wei Gan, and Hongfei Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10215 • Publication Date (Web): 17 Oct 2016 Downloaded from http://pubs.acs.org on October 18, 2016

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The Journal of Physical Chemistry

Local Environment and Interactions of Liquid and Solid Interfaces Revealed by Spectral Lineshape of Surface Selective Nonlinear Vibrational Probe Shun-Li Chen,† Li Fu,‡ Zizwe A. Chase, #,║ Wei Gan,*,§ and Hong-Fei Wang,*,¶ †

Laboratory of Environmental Science and Technology, Key Laboratory of Functional Materials and Devices for Special Environments of Chinese Academy of Sciences, Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, 40-1 South Beijing Road, Urumqi, Xinjiang 830011 (China) ‡

William R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352 (USA) #

Institute for Integrated Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352 (USA)

║

School of Chemical and Biological Engineering, Washington State University, Pullman, WA 99364 (USA)

§

Department of Natural Science and Humanities, Shenzhen Graduate School, Harbin Institute of Technology, University Town, Shenzhen 518055 (China)

¶

Physical Sciences Division, Physical & Computational Sciences Directorate, Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, Washington 99352 (USA)

Corresponding Author * Email: [email protected], [email protected]

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ABSTRACT: Vibrational spectral lineshape contains important detailed information of molecular vibration and reports its specific interactions and couplings to its local environment. In this work, recently developed sub-1 cm-1 high-resolution broadband sum frequency generation vibrational spectroscopy (HR-BB-SFG-VS) was used to measure the -C≡N stretch vibration in the 4-n-octyl-4’-cyanobiphenyl (8CB) Langmuir or Langmuir-Blodgett (LB) monolayer as a unique vibrational probe, and the spectral lineshape analysis revealed the local environment and interactions at the air/water, air/glass, air/calcium fluoride and air/α-quartz interfaces for the first time. The 8CB Langmuir or LB film is uniform and the vibrational spectral lineshape of its -C≡N group has been well characterized, making it a good choice as the surface vibrational probe. Lineshape analysis of the 8CB -C≡N stretch SFG vibrational spectra suggests the coherent vibrational dynamics and the structural and dynamic inhomogeneity of the -C≡N group at each interface are uniquely different. In addition, it is also found that there are significantly different roles for water molecules in the LB films on different substrate surfaces. These results demonstrated the novel capabilities of the surface nonlinear spectroscopy in characterization and in understanding the specific structures and chemical interactions at the liquid and solid interfaces in general.


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1. Introduction Surface/Interface is ubiquitous in nature and man-made world. Investigating the specific local environment and molecular interactions at the surface/interface is crucial in understanding and controlling heterogeneous physical, chemical and biological processes related to atmospheric, environmental, energy, industrial and biological science and technology activity.1-3 For molecules adsorbed on surface/interface, their vibrational spectroscopy gives the frequency domain information on the ensemble-averaged molecular vibrational transitions that are coupled to and interact with their local environment, and the vibrational spectral peak positions and lineshapes reflect the properties and interactions of different surfaces or even the same surface under different treatment or conditions.4 Vibrational spectral lineshape analysis on the spectral frequency shifts, linewidth and lineshape changes, degeneracy and frequency splitting has long been recognized as powerful tool for understanding molecular dynamics and interactions in the condensed phases and surfaces. This is because the vibrational spectral lineshape contains the details of the microscopic dynamics, intermolecular couplings and time dependent behavior of the solvation that influences the energy of the vibrational transitions.5-10 Even though these studies have been well established for bulk condensed phase studies,7-8 vibrational spectral lineshape analysis on molecular surface and interfaces has been rather limited.11-18 The main reason is that direct probe of the vibrational spectra of the molecular interactions at the molecular surface and interface requires in-situ and surface-specific technique that can accurately measure the spectral lineshapes. The surface selective and sub-monolayer sensitive sum-frequency generation vibrational spectroscopy (SFG-VS) is one unique technique that can directly measure the surface vibration motion of molecules.19-25 Particularly, the recently developed of the sub-1 cm-1 high-resolution broadband sum frequency generation vibrational spectroscopy (HR-BB-


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SFG-VS) has greatly expanded the power of SFG-VS in obtain molecular details of surfaces and interfaces through the measurement of the intrinsic and accurate spectral lineshape of surface vibrations.18, 25-28 With HR-BB-SFG-VS measurement for the intrinsic spectral lineshape, the quantitative contributions to the Lorentzian (linear dephasing) and the Gaussian (quadratic dephasing) components under the Bloch dynamics description of the surface vibrational spectra of the -C≡N groups of the 4-n-octyl-4’-cyanobiphenyl (8CB) Langmuir or Langmuir-Blodgett (LB) monolayer were explicitly determined through lineshape analysis of high-resolution spectra and free-induction decay (FID) coherent vibrational dynamics.17, 27, 29-30 Since the -C≡N groups in the 8CB monolayer are well-ordered and can directly interact with the surface in contact, these make it an excellent choice as the molecular probe for local environment and chemical interactions of surfaces and interfaces. In this report, we use the -C≡N stretch vibration of the 8CB Langmuir and LB monolayer to probe its HR-BB-SFG-VS spectral lineshapes at different liquid and solid surfaces. The quantitative contributions to the Lorentzian (linear dephasing) and Gaussian (quadratic dephasing) components in the -C≡N stretch vibrational spectral lineshape from the 8CB monolayer at the air/water, air/glass, air/calcium fluoride surfaces and air/α-quartz were analyzed to reveal the local environment and molecular interactions of these interfaces.30 Here, the -C≡N stretch vibration in the uniform 8CB Langmuir and LB monolayers is shown to be an excellent probe for interfacial molecular structure, organization and the molecule-substrate interaction. The same measurement and analysis can be applied to other probes, as the nitrile (C≡N), carbonyl (-C=O), alkyne (-C≡CH) and azido (-N3) vibrations of different probe molecules have been extensively explored as vibrational probes for local chemical environment and chemical interactions in surface and biological studies.31-42


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2. Experimental details Sample preparation and HR-BB-SFG-VS experimental details were described in detail in the literature and are briefly presented here.17, 26-27, 30 2.1 Sample preparation The 4-n-octyl-4-cyanobiphenyl (8CB) (98% purity) from Sigma–Aldrich was used as received and dissolved in HPLC-grade chloroform (Sigma–Aldrich, >99.9% purity) to the concentration of ~ 0.31mM. For the preparation of 8CB Langmuir monolayer, the stock solution was spread onto the surface of water (18.2 MΩ, Millipore, Inc.) contained in a clean Teflon Petri dish using a glass microsyringe. The Teflon dish was cleaned multiple times with a mixture of methanol and Millipore water, rinsed with copious amounts of Millipore water, and then treated with a digital UV/Ozone cleaner (Novascan Technologies, Inc.) for 20 min and with a plasma cleaner (PDC-001-HP, Harrick Plasma) for 20-30min. The measurement of the surface pressure-area isotherm of 8CB Langmuir monolayer at the air/water interface and the preparation of LB monolayers on the glass (FisherScientific, 12-544-5CY) and CaF2 (size Φ30×3(mm), Conex System Technology, Inc.) substrate were carried out by the Wilhelmy plate-method using a computer-controlled KSV 1000-2 minithrough (KSV, Inc.). The π-A phase diagrams of the 8CB Langmuir monolayers at the air/water interface at a room temperature of ~22 ᵒC is shown in the supporting information.17 Three surface densities tested selected for SFG measurements are three typical densities in the uniform liquid expanded phase (uniform monolayer: 49.5, 45.5, 43 Å2 per molecule).17, 29, 43 The clean procedure of various substrates is the same as that of Teflon dish. After spreading the 8CB/chloroform solution on water, it was


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kept for ~10 min before compression to allow the evaporation of chloroform and the equilibration of a uniform 8CB film. The film was compressed by the computerized LB trough with the compression rate of 10 mm/min. After the target pressure was reached, the film was allowed to equilibrate for about 10 min before transfer. The films were vertically transferred onto the substrates with the surface pressure kept constant. An elevation rate of 1 mm/min was used for all experiments. The prepared 8CB LB films on glass and CaF2 surface were kept for 30-60 min with the room humidity ~ 40% before spectra measurement. 2.2. Sub-1 cm -1 resolution broadband SFG-VS measurement Sum-frequency generation vibrational spectroscopy (SFG-VS) is the surface selective and sub-monolayer sensitive spectroscopic technique that measures the vibrational spectra of molecular groups at surfaces and interfaces.19, 24 Here the sub-wavenumber resolution version of the broadband SFG-VS (BB-SFG-VS) is used.44 In the BB-SFG-VS, usually a non-resonant visible laser beam with a fixed frequency and a broadband resonant infrared (IR) laser beam are simultaneously focused on the molecular surface or interface, and the broad band SFG-VS signal at the sum of the visible and IR frequencies are dispersed by spectrograph and detected by low noise high sensitivity CCD camera. The spectral resolution is determined by the narrow width of the visible frequency, the dispersion power of the spectrograph and the size of the pixels of the CCD camera.26, 28 The setup of sub-1 cm-1 resolution SFG-VS spectrometers were described in detail in previous work.

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In brief, the system consists of two sets of synchronized

(Synchrolock-AP, Coherent Inc.) Ti: Sapphire oscillators/amplifiers, running at 1kHz at the fundamental of 800nm, one with

Local Environment and Interactions of Liquid and Solid Interfaces

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