Single-Particle Measurements Reveal the Origin of Low Solar-to-Hydrogen Efficiency of Rh-Doped SrTiO3 Photocatalysts

Solar-powered photochemical water splitting using suspensions of photocatalyst nanoparticles is an attractive route for economical production of green hydrogen. SrTiO₃-based photocatalysts have been intensely investigated due to their stability and recently demonstrated near-100% external quantum yield (EQY) for water splitting using wavelengths below 360 nm. To extend the optical absorption into the visible, SrTiO₃ nanoparticles have been doped with various transition metals. Here we demonstrate that doping SrTiO₃ nanoparticles with 1% Rh introduces midgap acceptor states which reduce the free electron concentration by 5 orders of magnitude, dramatically reducing built-in potentials which could otherwise separate electron-hole (e-h) pairs. Rhodium states also function as recombination centers, reducing the photocarrier lifetime by nearly 2 orders of magnitude and the maximum achievable EQY to 10%. Furthermore, the absence of built-in electric fields within Rh-doped SrTiO₃ nanoparticles suggests that modest e-h separation can be achieved by exploiting a difference in mobility between electrons and holes.

Mechanistic Insights on Permeation of Water over Iron Cations in Nanoporous Silicon Oxide Films for Selective H2 and O2 Evolution

Electrocatalysts encapsulated by an ultrathin and semipermeable oxide layer offer a promising avenue for efficient, selective, and cost-effective production of hydrogen through photoelectrochemical water splitting. This architecture is especially attractive for Z-scheme water splitting, for which a nanoporous oxide film can be leveraged to mitigate undesired, yet kinetically facile, reactions involving redox shuttles, such as aqueous iron cations, by limiting transport of these species to catalytically active sites. In this work, molecular dynamics simulations were combined with electrochemical measurements to provide a mechanistic understanding of permeation of water and Fe(III)/Fe(II) redox shuttles through nanoporous SiO₂ films. It is shown that even for SiO₂ pores with a width as small as 0.8 nm, water does not experience any energy barrier for permeating into the pores due to a favorable interaction with hydrophilic silanol groups on the oxide surface. In contrast, permeation of Fe(III) and Fe(II) into microporous SiO₂ pores is limited due to high energy barriers, which stem from a combination of distortion and dehydration of the second and third ion solvation shells. Our simulations and experimental results show that SiO₂ coatings can effectively mitigate undesired Fe(III)/Fe(II) redox reactions at underlying electrodes by attenuating permeation of iron cations, while allowing water to permeate and thus participate in water splitting reactions. In a broader context, our study demonstrates that selectivity of solvated cations can be manipulated by controlling the pore size and surface chemistry of oxide films.

Quantifying the Influence of Defects on Selectivity of Electrodes Encapsulated by Nanoscopic Silicon Oxide Overlayers

Encapsulation of electrocatalysts and photocatalysts with semipermeable nanoscopic oxide overlayers that exhibit selective transport properties is an attractive approach to achieve high redox selectivity. However, defects within the overlayers─such as pinholes, cracks, or particle inclusions─may facilitate local high rates of parasitic reactions by creating pathways for facile transport of undesired reactants to exposed active sites. Scanning electrochemical microscopy (SECM) is an attractive method to determine the influence of defects on macroscopic performance metrics thanks to its ability to measure the relative rates of competing electrochemical reactions with high spatial resolution over the electrode. Here, we report the use of SECM to determine the influence of overlayer defects on the selectivity of silicon oxide (SiO_x) encapsulated platinum thin-film electrocatalysts operated under conditions where two competing reactions─the hydrogen evolution and Fe(III) reduction reactions─can occur. After an SECM methodology is described to determine spatially resolved selectivity, representative selectivity maps are correlated with the location of defects that are characterized by optical, electron, and atomic force microscopies. This analysis reveals that certain types of defects in the oxide overlayer are responsible for ∼60-90% of the partial current density toward the undesired Fe(III) reduction reaction. By correcting for defect contributions to Fe(III) reduction rates, true Fe(III) permeability values for the SiO_x overlayers were determined to be over an order of magnitude lower than permeabilities determined from analyses that ignore the presence of defects. Finally, different types of defects were studied revealing that defect morphology can have varying influence on both redox selectivity and calculated permeability. This work highlights the need for spatially resolved measurements to evaluate the performance of oxide-encapsulated catalysts and understand their performance limits.

Quantification of Excited-State Brønsted-Lowry Acidity of Weak Photoacids Using Steady-State Photoluminescence Spectroscopy and a Driving-Force-Dependent Kinetic Theory

Photoacids and photobases constitute a class of molecules that upon absorption of light undergoes a reversible change in acidity, i.e. pK_a. Knowledge of the excited-state pK_a value, pK_a*, is critical for predicting excited-state proton-transfer behavior. A reasonable approximation of pK_a* is possible using the Förster cycle analysis, but only when the ground-state pK_a is known. This poses a challenge for the study of weak photoacids (photobases) with less acidic (basic) excited states (pK_a* (pK_b*) > 7), because ground-state pK_a (pK_b) values are >14, making it difficult to quantify them accurately in water. Another method to determine pK_a* relies on acid-base titrations with photoluminescence detection and Henderson-Hasselbalch analysis. This method requires that the acid dissociation reaction involving the thermally equilibrated electronic excited state reaches chemical quasi-equilibrium, which does not occur for weak photoacids (photobases) due to slow rates of excited-state proton transfer. Herein, we report a method to overcome these limitations. We demonstrate that liquid water and aqueous hydroxide are unique proton-accepting quenchers of excited-state photoacids. We determine that Stern-Volmer quenching analysis is appropriate to extract rate constants for excited-state proton transfer in aqueous solutions from a weak photoacid, 5-aminonaphthalene-1-sulfonate, to a series of proton-accepting quenchers. Analysis of these data by Marcus-Cohen bond-energy-bond-order theory yields an accurate value for pK_a* of 5-aminonaphthalene-1-sulfonate. Our method is broadly accessible because it only requires readily available steady-state photoluminescence spectroscopy. Moreover, our results for weak photoacids are consistent with those from previous studies of strong photoacids, each showing the applicability of kinetic theories to interpret driving-force-dependent rate constants for proton-transfer reactions.

Reconciliation of Differences in Apparent Diffusion Coefficients Measured for Self-Exchange Electron Transfer between Molecules Anchored to Mesoporous Titanium Dioxide Thin Films

Redox-active sites present at large concentrations as part of a solid support or dissolved as molecules in fluid solutions undergo reversible self-exchange electron-transfer reactions. These processes can be monitored using a variety of techniques. Chronoamperometry and cyclic voltammetry are common techniques used to interrogate this behavior for molecules bound to mesoporous thin films of wide-bandgap semiconductors and insulators. In order to use these techniques to obtain accurate values for apparent diffusion coefficients, which are proxies for rate constants for self-exchange electron transfer, it is imperative to take into consideration nonidealities in redox titrations, parasitic currents, and ohmic resistances. Using spectroelectrochemical measurements taken concurrently with measurements of chronoamperometry data, we show that the spectroscopic data is not confounded from effects of parasitic currents or electroinactive dyes. However, we show that the thickness of the thin film over the region that is optically probed by the measurements must be known. When each of these considerations is included in data analyses, calculated apparent diffusion coefficients are, within error, independent of the method used to obtain the data. These considerations help reconcile variations in apparent diffusion coefficients measured using different techniques that have been reported over the past several decades and allow correct analyses to be performed in the future, independent of the method used to obtain the data.

Reinvigorating electrochemistry education

Electrochemistry is an established discipline with modern frontiers spanning energy conversion and storage, neuroscience, and organic synthesis. In spite of the expanding opportunities for academic and industrial electrochemists, particularly in the growing energy-storage sector, rigorous training of electrochemists is generally lacking at academic institutions in the United States. In this perspective, we highlight the core concepts of electrochemistry and discuss ways in which it has been historically taught. We identify challenges faced when teaching inherently interdisciplinary electrochemical concepts and discuss how technology provides new tools for teaching, such as inexpensive electronics and open-source software, to help address these challenges. Finally, we outline example programs and discuss how new tools and approaches can be brought together to prepare scientists and engineers for careers in electrochemical technology where they can accelerate the research, development, and deployment of the clean energy technology essential to combat climate change in the coming decades.

Photoacid-Modified Nafion Membrane Morphology Determined by Resonant X-ray Scattering and Spectroscopy

Covalent attachment of photoacid dye molecules to perfluorinated sulfonic acid membranes is a promising route to enable active light-driven ion pumps, but the complex relationship between chemical modification and morphology is not well understood in this class of functional materials. In this study we demonstrate the effect of bound photoacid dyes on phase-segregated membrane morphology. Resonant X-ray scattering near the sulfur K-edge reveals that introduction of photoacid dyes to the end of the ionomer side chains enhances phase segregation among ionomer domains, and the ionomer domain spacing increases with increasing amount of bound dye. Furthermore, relative crystallinity is marginally enhanced within semicrystalline domains composed of the perfluorinated backbone. X-ray absorption spectroscopy coupled with first-principles density functional theory calculations suggest that above a critical concentration, the multiple hydrophilic groups on the attached photoacid dye may help increase residual water content and promote hydration of adjacent sulfonic acid side chains under dry or ambient conditions.

Interfacial Electron Transfer of Ferrocene Immobilized onto Indium Tin Oxide through Covalent and Noncovalent Interactions

The immobilization of molecular species onto electrodes presents a direct route to modifying surface properties with molecular fidelity. Conventional methods include direct covalent attachment and physisorption of pyrene-appended molecular compounds to electrodes with aromatic character through π-π interactions. A recently reported hybrid approach extends the synthetic flexibility of the latter to a broader range of electrode materials. We report an application of this approach to immobilization of pyrene-appended ferrocene onto pyrene-functionalized indium tin oxide (ITO). The modified ITO surfaces were characterized using X-ray photoelectron spectroscopy, fluorescence spectroscopy, and electrochemical techniques. An electron-transfer rate constant ( k_app) of 100 ± 8 s^-1 was measured between the electrode and immobilized ferrocene using electrochemical methods. For comparison, a ferrocene-modified electrode using conventional covalent attachment of vinylferrocene was also prepared, and k_app was measured to be 9 ± 2 s^-1.

Use of bipolar membranes for maintaining steady-state pH gradients in membrane-supported, solar-driven water splitting

A bipolar membrane can maintain a steady-state pH difference between the sites of oxidation and reduction in membrane-supported, solar-driven water-splitting systems without changing the overall thermodynamics required to split water. A commercially available bipolar membrane that can serve this purpose has been identified, its performance has been evaluated quantitatively, and is demonstrated to meet the requirements for this application. For effective utilization in integrated solar-driven water-splitting systems, such bipolar membranes must, however, be modified to simultaneously optimize their physical properties such as optical transparency, electronic conductivity and kinetics of water dissociation.

Photodriven spin change of Fe(II) benzimidazole compounds anchored to nanocrystalline TiO(2) thin films

Ferrous tris-chelate compounds based on 2-(2'-pyridyl)benzimidazole (pybzim) have been prepared and characterized for studies of spin equilibria in fluid solution and when anchored to the surface of mesoporous nanocrystalline (anatase) TiO(2) and colloidal ZrO(2) thin films. The solid state structure of Fe(pybzim)(3)(ClO(4))(2).CH(3)CN.H(2)O was determined by single-crystal X-ray diffraction at 110 K to be triclinic, P-1, a = 11.6873(18), b = 12.2318(12), c = 14.723(4) A, alpha = 89.864(13) degrees , beta = 71.430(17) degrees , gamma = 73.788(11) degrees , V = 1907.1(6) A(3), Z = 2, and R = 0.0491. The iron compound has a meridional FeN(6) distorted octahedral geometry with bond lengths expected for a low-spin iron center at 110 K. The visible absorption spectra of Fe(pybzim)(3)(2+) and Fe(pymbA)(3)(2+), where pymbA is 4-(2-pyridin-2-yl-benzimidazol-1-ylmethyl)-benzoic acid, in methanol solution were dominated by metal-to-ligand charge-transfer (MLCT) bands. Variable-temperature UV-visible absorption spectroscopy revealed dramatic changes in the extinction coefficient consistent with a high-spin ((1)A) left harpoon over right harpoon low-spin ((5)T) equilibrium. Thermodynamic parameters for the temperature-dependent spin equilibrium of Fe(pymbA)(3)(2+) in methanol were determined to be DeltaH(HL) = 3270 +/- 210 cm(-1) and DeltaS(HL) = 13.3 +/- 0.8 cm(-1) K(-1). The corresponding values for Fe(pybzimEE)(3)(2+), where pybzimEE is (2-pyridin-2-yl-benzimidazol-1-yl)-acetic acid ethyl ester, in acetonitrile solution were determined to be 3072 +/- 34 cm(-1)and 10.5 +/- 0.1 cm(-1) K(-1). The temperature-dependent effective magnetic moments of Fe(pybzimEE)(3)(2+) in acetonitrile solution were also quantified by the Evans method. Pulsed 532 nm light excitation of Fe(pybzim)(3)(2+) or Fe(pymbA)(3)(2+) in solution resulted in an immediate bleach of the MLCT absorption bands. Relaxation back to the equilibrium state followed a first-order reaction mechanism. Arrhenius analysis of the (5)T --> (1)A rate constant yielded an activation energy, E(a), of 1090 +/- 20 cm(-1) and 710 +/- 10 cm(-1) for Fe(pybzim)(3)(2+) and Fe(pymbA)(3)(2+) in methanol, respectively. The compound Fe(pymbA)(3)(2+) was found to bind to colloidal TiO(2) and ZrO(2) thin films. The absorption spectra of the surface-attached compounds were quantified from 295 to 193 K. Pulsed light excitation of Fe(pymbA)(3)/TiO(2) and Fe(pymbA)(3)/ZrO(2) resulted in the immediate bleach of the MLCT absorption bands. Relaxation was nonexponential but was well described by kinetic models based on a Gaussian distribution of activation energies or a Levy distribution of lifetimes. An Arrhenius analysis of the Gaussian data yielded average activation energies of 660 +/- 80 cm(-1) and 730 +/- 40 cm(-1) for Fe(pymbA)(3)(ClO(4))(2) on TiO(2) and ZrO(2) surfaces, respectively. The Levy distribution analysis did not adequately fit the Arrhenius model.

Photodriven heterogeneous charge transfer with transition-metal compounds anchored to TiO2 semiconductor surfaces

A critical review of light-driven interfacial charge-transfer reactions of transition-metal compounds anchored to mesoporous, nanocrystalline TiO2 (anatase) thin films is described. The review highlights molecular insights into metal-to-ligand charge transfer (MLCT) excited states, mechanisms of interfacial charge separation, inter- and intra-molecular electron transfer, and interfacial charge-recombination processes that have been garnered through various spectroscopic and electrochemical techniques. The relevance of these processes to optimization of solar-energy-conversion efficiencies is discussed (483 references).