Electrochemistry for Engineers LECTURE 12 Lecturer: Dr. Brian Rosen Office: 128 Wolfson Office Hours: Sun 16:00

Final Exam No books No notes Calculator required (no graphing calculators) Equation sheet provided Standard reduction potentials provided Question types – Open answer – Drawing graphs / identifying graphs – Calculations 3 hours (test designed for 2 hours)

Coupled Characterization

Coupled Characterization Examples Spectraelectrochemistry Sum Frequency Generation Scanning Electrochemical Microscopy Synchrotron Radiation – In Situ XRD, XPS Electrochemical Quartz Crystal Microbalance Electrochemical AFM

Transmission Spectraelectrochemistry OTE = optical transparent electrode: SnO 2, In 2 O 3

Specialized Cells

Spectral Data as a function of Potential

Sum Frequency Generation

Polarization using incident E-M field No net polarization (Centrosymmetric) Molecules have a polarizability, P, which describes how E-M radiation polarizes molecules P net =0 P net ≠0

Linear vs. Nonlinear Optics Linear Optics: Polarizability depends on the first order susceptibility term χ (1) :E Nonlinear Optics: Polarizability depends on higher order terms e.g. χ (2) :EE SFG is a second order nonlinear process and is dependent on χ (2) :EE P=ε o (χ (1) :E + χ (2) :EE + χ (3) :EEE +……) Second order (and ALL even orders) susceptibility term is zero in Centrosymmetric media (i.e. bulk of most materials) because it is a third rank tensor

SFG is Surface Selective χ (2) is nonzero at the surface because the molecules are no longer centrosymmetric Consequence 1: SFG does NOT probe the bulk Consequence 2: SFG can be a good in-situ electrochemical technique Bulk vs. Surface

Sum Frequency Generation (SFG) must be Infrared and Raman active Virtual electronic state (i.e. no resonance) ν=0 ν=1 InfraredRaman-StokesSFG ω IR ω VIS ω SFG ω IR ω SHG ν=2 SHG

Sum Frequency Generation (SFG) must be Infrared and Raman active Virtual electronic state (i.e. no resonance) ν=0 ν=1 InfraredRaman-StokesSFG ω IR ω VIS ω SFG SAME SELECTION RULES AS IR AND RAMAN

Conventional SFG In conventional SFG, an IR beam coherently and resonantly excites the first transition. The excited population (υ=1) is up-regulated by a visible laser to a virtual (non-resonant) state. Frequency resolution is given by scanning the IR laser across different infrared frequencies. Disadvantage: No time resolution! Not so useful for in-situ electrochemical system when you want to scan electrode bias.

Second order Susceptibility has 2 contributions χ (2) = χ(nr) (2) + χ(r) (2) Susceptibility has a non-resonant and resonant contribution. (we only care about the resonant) – Resonant: vibrational modes – Non-Resonant: electronic transitions of metal surface. Both contributions are subject to so-called Free Induction Decay (covered later)

Coherent Waves Coherence: Waves always have same relative phase, θ. (Leads to stationary interference i.e. constructive and destructive)

Free Induction Decay (FID) Illustrated = Molecule with polarization, P, and a vibrational mode oscillating at a frequency, ω ω ω ωω+dω Weak H-bonding Moderate H-bonding Strong H-bonding ω-dω Due to inhomogeneous environment, the SFG signal from different molecules (from the same resonant transition) will have slightly different frequencies and will eventually become incoherent, causing decay in the SFG signal.

Consequences of Time-Bandwidth Product Because Fourier Transform sets a constant limit for the value of the TB-P for a given waveform, longer pulses will have narrower bandwidths and shorter pulses will have wider bandwidths.

Broadband (BB-SFG) Rather than scanning the IR laser, a spectrally broad femtosecond (fs) IR beam pulses with a large linewidth, Γ, to probe the surface. All vibrations with resonances in the band will have a population excited (υ=0  1). BB-SFG resolution comes from a temporally long (spectrally short) VIS pulse such that the resonant population is excited to a virtual state.

Frequency Resolved BB-SFG 1. IR pulse coherently excites υ=0  1 transition and VIS pulse (overlapping temporally and spatially) up-regulates this population to virtual electronic state. 2. Spectrally narrow and temporally long VIS pulse gives good frequency resolution and no time resolution.

Time Resolved: Offset IR – VIS 1. IR pulse coherently and resonantly excites υ=0  1 transition 2. Coherence decays at T due to FID 3. Duration of VIS pulse MUST be less than T or no coherence will remain (VIS pulse spectrally wide).

Disadvantage: multiple resonant modes In many experiments, having two resonant transitions is unwanted because it convolutes the analysis of the SFG spectra. If the both the IR and VIS pulses are spectrally wide, the probability of overlapping with a resonance in another electronic state is high, leading to poor ω-resolution.

Spectraelectrochemistry Cell

CO Stripping with SFG

CO Accumulation During CV

Scanning Electrochemical Microscopy (SECM)

SECM Collection Modes Substrate generation tip collection (SG/TC) Tip generation substrate collection (TG/SC) Competitive Constant current or constant height Competitive

SECM for ORR Activity Fernandez et al. (Bard), JACS, 127,(2004) ORR Activity of Pd x Co (1-x) Alloys

Synchrotron Radiation Acceleration of electrons using powerful magnets creates radiation in the tangential direction. At 99.9% the speed of light, this is most X-ray radiation 10,000+’s more powerful than laboratory X-ray sources

XANES and EXAFS

Extended X-ray Fluorescence Spectroscopy (EXAFS)

Identify pre and post-edge Background

Subtract Background, Convert Energy Space to K-Space k is the “wavenumber” and had units of inverse distance

FFT into “R-Space”

Producing X-Rays at APS kdI kdI

Electrochemical Quartz Crystal Microbalance Quartz is a piezoelectric Resonant frequency dependent on the mass of the quartz electrode

Electrochemical AFM