Nanoscale TiO2 Protection Layer Enhances the Built-In Field and Charge Separation Performance of GaP Photoelectrodes

Operando Contactless EFISH Study of the Rate-Determining Step of Light-Driven Water Oxidation on TiO2 Photoanodes

For many slow solar-fuel-forming reactions, the accumulation of photogenerated minority carriers on the photoelectrode surface leads to light-induced band edge unpinning, affecting the junction properties by decreasing band bending in the semiconductor space charge layer and increasing the driving force of surface reactions in the electric double layer. In this study, we demonstrate a contactless operando electric field-induced second harmonic generation (EFISH) method for measuring the band bending change (δΔΦSCRL) on photoelectrodes upon photoexcitation. For n-doped rutile TiO2 water oxidation photoanodes at pH 7, δΔΦSCRL increases at more positive potentials or higher illumination power density until it reaches saturation values. We show that under fast mass transport conditions, δΔΦSCRL is exclusively attributed to the accumulated charged rate-determining species that can be regarded as temporary surface states, and the relationship between the photocurrent and δΔΦSCRL can be well modeled by assuming that hole trap states function as the reaction center. Kinetic isotope experiments identify proton-coupled electron transfer as the rate-determining step and suggest a possible chemical nature of the key intermediate. We demonstrate that light-induced band edge unpinning is a beneficial feature under high illumination conditions for oxygen evolution reaction on TiO2 because it maintains the photon-to-current conversion efficiency by enhancing the surface reaction driving force, shedding light on the actual device application.

Electrochemical Model for Field Effect Conductivities of Electrolytes in Equilibrium Systems

Field effect electronic and ionic conductivities come from the electronic and ionic resonances and polarizations in an electrolyte. An external atmosphere field causes field enhanced electronic conductivity and field depressed ionic conductivity. The higher molar formation Gibbs energy of the pi side than that of the ni side in a pin electrolyte heterostructure results in field enhanced electronic conductivity and field depressed ionic conductivity on the pi side as well as field depressed electronic conductivity and field enhanced ionic conductivity on the ni side. An impedance spectrum arises from polarizations at high temperature but from both polarizations and resonances at low temperature. The electrochemical model demonstrates the origins of field effect conductivities in an electrolyte under equilibrium.

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.

Establishing Multiple-Order Built-In Electric Fields Within Heterojunctions to Achieve Photocarrier Spatial Separation

Hybridizing two heterocomponents to construct a built-in electric field (BIEF) at the interface represents a significant strategy for facilitating charge separation in carbon dioxide (CO2)-photoreduction. However, the unidirectional nature of BIEFs formed by various low-dimensional materials poses challenges in adequately segregating the photogenerated carriers produced in bulk. In this study, leveraging zinc oxide (ZnO) nanodisks, a sulfurization reaction is employed to fabricate Z-scheme ZnO/zinc sulfide (ZnS) heterojunctions featuring a multiple-order BIEF. These heterojunctions reveal distinctive interfacial structures characterized by two semicoherent phase boundaries. The cathodoluminescence 2D maps and density functional theory calculation results demonstrate that the direction of the multiple-order BIEF spans from ZnS to ZnO. This directional alignment significantly fosters the spatial separation of photogenerated electrons and holes within ZnS nanoparticles and enhances CO2-to-carbon monoxide photoreduction performance (3811.7 µmol h-1 g-1). The findings present a novel pathway for structurally designing BIEFs within heterojunctions, while providing fresh insights into the migratory behavior of photogenerated carriers across interfaces.

Direct In Situ Measurement of Quantum Efficiencies of Charge Separation and Proton Reduction at TiO2-Protected GaP Photocathodes

Photoelectrochemical solar fuel generation at the semiconductor/liquid interface consists of multiple elementary steps, including charge separation, recombination, and catalytic reactions. While the overall incident light-to-current conversion efficiency (IPCE) can be readily measured, identifying the microscopic efficiency loss processes remains difficult. Here, we report simultaneous in situ transient photocurrent and transient reflectance spectroscopy (TRS) measurements of titanium dioxide-protected gallium phosphide photocathodes for water reduction in photoelectrochemical cells. Transient reflectance spectroscopy enables the direct probe of the separated charge carriers responsible for water reduction to follow their kinetics. Comparison with transient photocurrent measurement allows the direct probe of the initial charge separation quantum efficiency (ϕ_CS) and provides support for a transient photocurrent model that divides IPCE into the product of quantum efficiencies of light absorption (ϕ_abs), charge separation (ϕ_CS), and photoreduction (ϕ_red), i.e., IPCE = ϕ_absϕ_CSϕ_red. Our study shows that there are two general key loss pathways: recombination within the bulk GaP that reduces ϕ_CS and interfacial recombination at the junction that decreases ϕ_red. Although both loss pathways can be reduced at a more negative applied bias, for GaP/TiO₂, the initial charge separation loss is the key efficiency limiting factor. Our combined transient reflectance and photocurrent study provides a time-resolved view of microscopic steps involved in the overall light-to-current conversion process and provides detailed insights into the main loss pathways of the photoelectrochemical system.