Phase-Sensitive Second-Harmonic Generation of Electrochemical Interfaces

Water flipping and the oxygen evolution reaction on Fe2O3 nanolayers

Hematite photoanodes are promising for the oxygen evolution reaction, however, their high overpotential (0.5-0.6 V) for water oxidation and limited photocurrent make them economically unviable at present. The work needed to orient dipoles at an electrode surface may be an overlooked contribution to the overpotential, especially regarding dipoles of water, the electron source in the oxygen evolution reaction (OER). Here, we employ second harmonic amplitude and phase measurements to quantify the number of net-aligned Stern layer water molecules and the work associated with water flipping, on hematite, an earth abundant OER semiconductor associated with a high overpotential. At zero applied bias, the pH-dependent potentials for Stern layer water molecule flipping exhibit Nernstian behavior. At positive applied potentials and pH 13, approximately one to two monolayers of water molecules points the oxygen atoms towards the electrode, favorable for the OER. The work associated with water flipping matches the cohesive energy of liquid water (44 kJ mol-1) and the OER current density is highest. This current is negligible at pH 5, where the work approaches 100 kJ mol-1. Our findings suggest a causal relationship between the need for Stern layer water flipping and the OER overpotential, which may lead to developing strategies for decreasing the latter.

Second harmonic generation null angle polarization analysis for determining interfacial potential at charged interfaces

Methods of quantifying the electrostatics of charged interfaces are important in a range of research areas. The surface-selective nonlinear optical technique second harmonic generation (SHG) is a sensitive probe of interfacial electrostatics. Recent work has shown that detection of the SHG phase in addition to its amplitude enables direct quantification of the interfacial potential. However, the experimental challenge of directly detecting the phase interferometrically with sufficient precision and stability has led to the proposal and development of alternative techniques to recover the same information, notably through wavelength scanning or angle scanning, each of which has their own associated experimental challenges. Here, we propose a new polarization-based approach to recover the required phase information, building upon the previously established nonlinear optical null ellipsometry (NONE) technique. Although NONE directly returns only relative phase information between different tensor elements of the second-order susceptibility, it is shown that a symmetry relation that connects the tensor elements of the potential-dependent third-order susceptibility can be used to generate the absolute phase reference required to calculate the interfacial potential. The sensitivity of the technique to potential at varying surface charge densities and ionic strengths is explored by means of simulated data of the silica:water interface. The error associated with the use of the linearized Poisson-Boltzmann approximation is discussed and compared to the error associated with the precision of the measured NONE null angles. Overall, the results suggest that NONE is a promising approach for performing phase-resolved SHG based quantification of interfacial potentials that experimentally requires only the addition of standard polarization optics to the basic single-wavelength, fixed-angle SHG apparatus.

A contactless in situ EFISH method for measuring electrostatic potential profile of semiconductor/electrolyte junctions

In photoelectrochemical cells, promising devices for directly converting solar energy into storable chemical fuels, the spatial variation of the electrostatic potential across the semiconductor-electrolyte junction is the key parameter that determines the cell performance. In principle, electric field induced second harmonic generation (EFISH) provides a contactless in situ spectroscopic tool to measure the spatial variation of electrostatic potential. However, the total second harmonic generation (SHG) signal contains the contributions of the EFISH signals of semiconductor space charge layer and the electric double layer, in addition to the SHG signal of the electrode surface. The interference of these complex quantities hinders their analysis. In this work, to understand and deconvolute their contributions to the total SHG signals, bias-dependent SHG measurements are performed on the rutile TiO2(100)-electrolyte junction as a function of light polarization and crystal azimuthal angle (angle of the incident plane relative to the crystal [001] axis). A quadratic response between SHG intensity and the applied potential is observed in both the accumulation and depletion regions of TiO2. The relative phase difference and amplitude ratio are extracted at selected azimuthal angles and light polarizations. At 0° azimuthal angle and s-in-p-out polarization, the SHG intensity minimum has the best match with the TiO2 flatband potential due to the orthogonal relative phase difference between bias-dependent and bias-independent SHG terms. We further measure the pH-dependent flatband potential and probe the photovoltage under open circuit conditions using the EFISH technique, demonstrating the capability of this contactless method for measuring electrostatic potential at semiconductor-electrolyte junctions.

Cation Modifies Interfacial Water Structures on Platinum during Alkaline Hydrogen Electrocatalysis

The molecular details of an electrocatalytic interface play an essential role in the production of sustainable fuels and value-added chemicals. Many electrochemical reactions exhibit strong cation-dependent activities, but how cations affect reaction kinetics is still elusive. We report the effect of cations (K+, Li+, and Ba2+) on the interfacial water structure using second-harmonic generation (SHG) and classical molecular dynamics (MD) simulation. The second- (χH2O(2)) and third-order (χH2O(3)) optical susceptibilities of water on Pt are smaller in the presence of Ba2+ compared to those of K+, suggesting that cations can affect the interfacial water orientation. MD simulation reproduces experimental SHG observations and further shows that the competition between cation hydration and interfacial water alignment governs the net water orientation. The impact of cations on interfacial water supports a cation hydration-mediated mechanism for hydrogen electrocatalysis; i.e., the reaction occurs via water dissociation followed by cation-assisted hydroxide/water exchange on Pt. Our study highlights the role of interfacial water in electrocatalysis and how innocent additives (such as cations) can affect the local electrochemical environment.

Optical method for quantifying the potential of zero charge at the platinum-water electrochemical interface

When an electrode contacts an electrolyte, an interfacial electric field forms. This interfacial field can polarize the electrode's surface and nearby molecules, but its effect can be countered by an applied potential. Quantifying the value of this countering potential ('potential of zero charge' (pzc)) is, however, not straightforward. Here we present an optical method for determining the pzc at an electrochemical interface. Our approach uses phase-sensitive second-harmonic generation to determine the electrochemical potential where the interfacial electric field vanishes at an electrode-electrolyte interface with Pt-water as a model experiment. Our method reveals that the pzc of the Pt-water interface is 0.23 ± 0.08 V versus standard hydrogen electrode (SHE) and is pH independent from pH 1 to pH 13. First-principles calculations with a hybrid explicit-implicit solvent model predict the pzc of the Pt(111)-water interface to be 0.23 V versus SHE and reveal how the interfacial water structure rearranges as the electrode potential is moved above and below the pzc. We further show that pzc is sensitive to surface modification; deposition of Ni on Pt shifts the interfacial pzc in the cathodic direction by ~360 mV. Our work demonstrates a materials-agnostic approach for quantifying the interfacial electrical field and water orientation at an electrochemical interface without requiring probe molecules and confirms the long-held view that the interfacial electric field is more intense during hydrogen electrocatalysis in alkaline than in acid.

The interfacial structure of super-concentration LiNO3 aqueous electrolyte studied by second harmonic generation

The interfacial structure of a super-concentration LiNO₃ aqueous electrolyte was studied using non-resonant second harmonic generation (SHG) and heterodyne-detected SHG spectra. First, we investigated the electric double layer structure at the air/LiNO₃ interface. As the concentration of LiNO₃ increased, the SHG intensity first increased and then remained unchanged, while the SHG phase changed by about 5°. These results reveal that there was only a small amount of NO₃ ^- at the interface. The increase of the SHG intensity resulted from the thickening of the interfacial water molecular layer. In addition, we studied the broadening mechanism of the electrochemical stability window (ESW) for the super-concentrated LiNO₃ aqueous electrolyte. During cyclic voltammetry scanning, the potential-dependent SHG curves of the Pt/LiNO₃ interface verify that at the cathodic end of the ESW, as the concentration of LiNO₃ increased, the orientation angle θ of Pt-H changed less and the number density N_s of Pt-H gradually decreased, which indicates the decrease of the number of adsorbed H atoms on the Pt electrode surface. Therefore, the decrease of the number of free water molecules on the Pt electrode surface resulted in an expanded ESW.

A New Imaginary Term in the Second-Order Nonlinear Susceptibility from Charged Interfaces

Nonresonant second harmonic generation (SHG) phase and amplitude measurements obtained from the silica-water interface at varying pH values and an ionic strength of 0.5 M point to the existence of a nonlinear susceptibility term, which we call χ_X⁽³⁾, that is associated with a 90° phase shift. Including this contribution in a model for the total effective second-order nonlinear susceptibility produces reasonable point estimates for interfacial potentials and second-order nonlinear susceptibilities when χ_X⁽³⁾ ≈ 1.5χ_water⁽³⁾. A model without this term and containing only traditional χ⁽²⁾ and χ⁽³⁾ terms cannot recapitulate the experimental data. The new model also provides a demonstrated utility for distinguishing apparent differences in the second-order nonlinear susceptibility when the electrolyte is NaCl versus MgSO₄, pointing to the possibility of using heterodyne-detected SHG to investigate ion specificity in interfacial processes.