Macroscale and Nanoscale Photoelectrochemical Behavior of p-Type Si(111) Covered by a Single Layer of Graphene or Hexagonal Boron Nitride.

Two-dimensional (2D) materials may enable a general approach to the introduction of a dipole at a semiconductor surface as well as control over other properties of the double layer at a semiconductor/liquid interface. Vastly different properties can be found in the 2D materials currently studied due in part to the range of the distribution of density-of-states. In this work, the open-circuit voltage (Voc) of p-Si-H, p-Si/Gr (graphene), and p-Si/h-BN (hexagonal boron nitride) in contact with a series of one-electron outer-sphere redox couples was investigated by macroscale measurements as well as by scanning electrochemical cell microscopy (SECCM). The band gaps of Gr and h-BN (0-5.97 eV) encompass the wide range of band gaps for 2D materials, so these interfaces (p-Si/Gr and p-Si/h-BN) serve as useful references to understand the behavior of 2D materials more generally. The value of Voc shifted with respect to the effective potential of the contacting solution, with slopes (ΔVoc/ΔEEff) of -0.27 and -0.38 for p-Si/Gr and p-Si/h-BN, respectively, indicating that band bending at the p-Si/h-BN and p-Si/Gr interfaces responds at least partially to changes in the electrochemical potential of the contacting liquid electrolyte. Additionally, SECCM is shown to be an effective method to interrogate the nanoscale photoelectrochemical behavior of an interface, showing little spatial variance over scales exceeding the grain size of the CVD-grown 2D materials in this work. The measurements demonstrated that the polycrystalline nature of the 2D materials had little effect on the results and confirmed that the macroscale measurements reflected the junction behavior at the nanoscale.

In Situ Quantification of Surface Intermediates and Correlation to Discharge Products on Hematite Photoanodes Using a Combined Scanning Electrochemical Microscopy Approach.

Hematite is a promising photoanode for solar driven water splitting. Elucidating its surface chemical pathways is key to improving its performance. Here, we use redox titrations in the Surface Interrogation mode of Scanning Electrochemical Microscopy (SI-SECM) to quantitatively probe in situ the reactivity and time evolution of surface species formed on hematite during photo assisted water oxidation. Using SI-SECM, two distinct populations of oxidizing surface species were resolved with measured ksi of 316 m3/(mol·s) and 2 m3/(mol·s) for the more and less reactive species, respectively. While the surface coverage of both species was found to increase as a function of applied bias, the rate constants did not change appreciably, suggesting that the mechanism of water oxidation is independent of bias potential. In the absence of applied potential, both populations exhibit decay that is well described by second order kinetics, with kd values of 1.2 × 105 ± 0.2 × 105 and 6.3 × 103 ± 0.9 × 103 m2/(mol·s) for the fast and slow reacting adsorbates, respectively. Using transient substrate generation/tip collection mode, we detected the evolution of as much as 1.0 µmol/m2 of H2O2 during this decay process, which correlates with the coverage observed by one of the titrated species. By deconvoluting the reactivity of multiple adsorbed reactants, these experiments demonstrate how SI-SECM enables direct observation of multiple adsorbates and reaction pathways on operating photoelectrodes.

High-Throughput Preparation of Metal Oxide Nanocrystals by Cathodic Corrosion and Their Use as Active Photocatalysts.

Nanoparticle metal oxide photocatalysts are attractive because of their increased reactivity and ease of processing into versatile electrode formats; however, their preparation is cumbersome. We report on the rapid bulk synthesis of photocatalytic nanoparticles with homogeneous shape and size via the cathodic corrosion method, a simple electrochemical approach applied for the first time to the versatile preparation of complex metal oxides. Nanoparticles consisting of tungsten oxide (H2WO4) nanoplates, titanium oxide (TiO2) nanowires, and symmetric star-shaped bismuth vanadate (BiVO4) were prepared conveniently using tungsten, titanium, and vanadium wires as a starting material. Each of the particles were extremely rapid to produce, taking only 2-3 min to etch 2.5 mm of metal wire into a colloidal dispersion of photoactive materials. All crystalline H2WO4 and BiVO4 particles and amorphous TiO2 were photoelectrochemically active toward the water oxidation reaction. Additionally, the BiVO4 particles showed enhanced photocurrent in the visible region toward the oxidation of a sacrificial sulfite reagent. This synthetic method provides an inexpensive alternative to conventional fabrication techniques and is potentially applicable to a wide variety of metal oxides, making the rapid fabrication of active photocatalysts with controlled crystallinity more efficient.

Interrogating Charge Storage on Redox Active Colloids via Combined Raman Spectroscopy and Scanning Electrochemical Microscopy.

Redox active colloids (RACs) are dispersible, cross-linked polymeric materials that incorporate a high concentration of redox-active motifs, making them attractive for next-generation size-exclusion redox flow batteries. In order to tap into their full potential for energy storage, it is essential to understand their internal charge mobility, capacity, and cyclability. Here we focus on using a combined suite of Raman spectroscopy and scanning electrochemical microscopy (SECM) tools for evaluating three important parameters that govern charge storage in viologen-RACs: their intraparticle redox active concentration, their reduction/oxidation mechanism, and their charge transfer rate. We addressed RACs using SECM imaging and single-particle experiments, from which the intraparticle diffusion and concentration parameters were elucidated. By using Raman spectroscopy coupled to surface interrogation SECM, we further evaluated their reversible redox properties within monolayer films of 80- and 135-nm-sized RACs. Most notably we have confirmed that the concentration and redox mechanisms are essentially unchanged when varying the RAC size. As expected, we see that larger particles inherently require longer times for electrolysis independent of the methodology used for their study. Our simulations further verify the internal concentration of RACs and suggest that their porosity enables solution redox active mediators to penetrate and titrate charge in their interior. The combined methodology presented here sets an important analytical precedent in decoupling the charge storage properties of new bulk materials for polymer batteries starting from probing low-dimensional assemblies and single particles using nano- and spectroelectrochemical approaches.

Soft Surfaces for Fast Characterization and Positioning of Scanning Electrochemical Microscopy Nanoelectrode Tips.

The testing of nanoelectrode tips for scanning electrochemical microscopy (SECM) is a slow and cumbersome task that often results in untimely electrode breakage due to crashing against a substrate. Here, we evaluated approach curves of nano- and microelectrodes to soft surfaces using SECM for a rapid and more convenient characterization and positioning protocol. Soft surfaces consisted of either a submerged argon bubble or a thin polydimethylsiloxane (PDMS) layer. While approach curves to Ar bubbles in the presence of a surfactant were promising for the characterization of microelectrode tips, their performance with nanoelectrodes was deficient. In contrast, approach curves to PDMS films allowed the rapid positioning of nanoelectrodes as small as 30 nm radius at speeds up to 5 µm/s without the risk of breakage. The nanoelectrodes were able to approach the polymer films multiple times without affecting their electrochemical performance. Furthermore, using a half-coated substrate with PDMS, nanoelectrodes could be retracted and positioned very close to the bare, hard substrate for characterization with traditional approach curves. We estimate time savings on tip characterization/positioning on the order of 10- to 100-fold. This simple procedure is easily implemented without the requirement of additional devices supplementing existing commercial SECM instruments.

Electrochemical Imaging of Photoanodic Water Oxidation Enhancements on TiO2 Thin Films Modified by Subsurface Aluminum Nanodimers.

Detecting metal plasmonic enhancements on the activity of semiconducting photoanodes for water oxidation is often obscured by the inherent electroactivity and instability of the metal in electrolyte. Here, we show that thin TiO2 photoanodes modified by subsurface Al nanodimers (AlNDs) display enhancements that are consistent with plasmon modes. We directly observed enhancements by mapping the oxygen evolution rates on TiO2/AlND patterns using scanning electrochemical microscopy (SECM) while exciting the surface plasmons of the nanodimers. This study highlights the importance of sample configuration for the in situ characterization of metal/photoanode interactions and suggests a route for Al-based plasmonics applied to photoelectrochemistry.

Structure of the Photo-catalytically Active Surface of SrTiO3.

A major goal of energy research is to use visible light to cleave water directly, without an applied voltage, into hydrogen and oxygen. Although SrTiO3 requires ultraviolet light, after four decades, it is still the "gold standard" for the photo-catalytic splitting of water. It is chemically robust and can carry out both hydrogen and oxygen evolution reactions without an applied bias. While ultrahigh vacuum surface science techniques have provided useful insights, we still know relatively little about the structure of these electrodes in contact with electrolytes under operating conditions. Here, we report the surface structure evolution of a n-SrTiO3 electrode during water splitting, before and after "training" with an applied positive bias. Operando high-energy X-ray reflectivity measurements demonstrate that training the electrode irreversibly reorders the surface. Scanning electrochemical microscopy at open circuit correlates this training with a 3-fold increase of the activity toward the photo-induced water splitting. A novel first-principles joint density functional theory simulation, constrained to the X-ray data via a generalized penalty function, identifies an anatase-like structure as the more active, trained surface.

Electrochemical Imaging and Redox Interrogation of Surface Defects on Operating SrTiO3 Photoelectrodes.

We introduce electrochemical imaging and nano-resolved measurements of catalytic intermediates on operating SrTiO3 photoelectrodes. Spatially resolved redox titrations of photogenerated reactive oxygen species (ROS) were used to profile changes in ROS coverage and reactivity at pristine and ion-milled defective areas on n-doped (100) SrTiO3. Adsorbed ROS reached a potential-dependent limiting coverage of ∼0.1 monolayer and did not differ significantly between milled and pristine areas. However, the reaction kinetics between a solution-phase mediator and adsorbed ROS were found to be significantly decreased over ion-milled areas. Using a nanoelectrode, we resolved ksi values of 5 and 300 m(3)/s·mol for these bimolecular reactions at defective and pristine sites, respectively. Ion-milled areas also showed significantly decreased activity toward photo-oxidations, providing evidence that photogenerated ROS mediate fast charge-transfer reactions with solution-phase species at the semiconductor-electrolyte interface. Our results provide spatially resolved direct evidence of the impact of surface defects on the performance of photoelectrochemical systems. Scanning electrochemical microscopy offers a powerful method for evaluating the reactivity of an operating electrochemical interface by using redox titrations that detected as few as 30 attomoles of adsorbed ROS.