Showing papers by "Nathan S. Lewis published in 2015"
TL;DR: In this paper, a monolithically integrated device consisting of a tandem-junction GaAs/InGaP photoanode coated by an amorphous TiO2 stabilization layer was used to effect unassisted, solar-driven water splitting in 1.0 M KOH(aq).
Abstract: A monolithically integrated device consisting of a tandem-junction GaAs/InGaP photoanode coated by an amorphous TiO2 stabilization layer, in conjunction with Ni-based, earth-abundant active electrocatalysts for the hydrogen-evolution and oxygen-evolution reactions, was used to effect unassisted, solar-driven water splitting in 1.0 M KOH(aq). When connected to a Ni–Mo-coated counterelectrode in a two-electrode cell configuration, the TiO2-protected III–V tandem device exhibited a solar-to-hydrogen conversion efficiency, ηSTH, of 10.5% under 1 sun illumination, with stable performance for >40 h of continuous operation at an efficiency of ηSTH > 10%. The protected tandem device also formed the basis for a monolithically integrated, intrinsically safe solar-hydrogen prototype system (1 cm2) driven by a NiMo/GaAs/InGaP/TiO2/Ni structure. The intrinsically safe system exhibited a hydrogen production rate of 0.81 μL s−1 and a solar-to-hydrogen conversion efficiency of 8.6% under 1 sun illumination in 1.0 M KOH(aq), with minimal product gas crossover while allowing for beneficial collection of separate streams of H2(g) and O2(g).
256 citations
TL;DR: In this paper, the use of thin-layer protection strategies to enable semiconductor-based solar-driven fuel production is discussed and an outlook for the future development of thin layer protection strategies is provided.
Abstract: The electrochemical instability of semiconductors in aqueous electrolytes has impeded the development of robust sunlight-driven water-splitting systems. We review the use of protective thin films to improve the electrochemical stability of otherwise unstable semiconductor photoelectrodes (e.g., Si and GaAs). We first discuss the origins of instability and various strategies for achieving stable and functional photoelectrosynthetic interfaces. We then focus specifically on the use of thin protective films on photoanodes and photocathodes for photosynthetic reactions that include oxygen evolution, halide oxidation, and hydrogen evolution. Finally, we provide an outlook for the future development of thin-layer protection strategies to enable semiconductor-based solar-driven fuel production.
243 citations
TL;DR: In this article, a suite of disparate methodologies has been proposed and used historically to evaluate the efficiency of systems that produce fuels, either directly or indirectly, with sunlight and/or electrical power as the system inputs.
Abstract: The energy-conversion efficiency is a key metric that facilitates comparison of the performance of various approaches to solar energy conversion. However, a suite of disparate methodologies has been proposed and used historically to evaluate the efficiency of systems that produce fuels, either directly or indirectly, with sunlight and/or electrical power as the system inputs. A general expression for the system efficiency is given as the ratio of the total output power (electrical plus chemical) divided by the total input power (electrical plus solar). The solar-to-hydrogen (STH) efficiency follows from this globally applicable system efficiency but only is applicable in the special case for systems in which the only input power is sunlight and the only output power is in the form of hydrogen fuel derived from solar-driven water splitting. Herein, system-level efficiencies, beyond the STH efficiency, as well as component-level figures of merit are defined and discussed to describe the relative energy-conversion performance of key photoactive components of complete systems. These figures of merit facilitate the comparison of electrode materials and interfaces without conflating their fundamental properties with the engineering of the cell setup. The resulting information about the components can then be used in conjunction with a graphical circuit analysis formalism to obtain “optimal” system efficiencies that can be compared between various approaches. The approach provides a consistent method for comparison of the performance at the system and component levels of various technologies that produce fuels and/or electricity from sunlight.
195 citations
TL;DR: It is demonstrated that a reactively sputtered NiOx layer provides a transparent, antireflective, conductive, chemically stable, inherently catalytic coating that stabilizes many efficient and technologically important semiconducting photoanodes under viable system operating conditions, thereby allowing the use of these materials in an integrated system for the sustainable, direct production of fuels from sunlight.
Abstract: Reactively sputtered nickel oxide (NiOx) films provide transparent, antireflective, electrically conductive, chemically stable coatings that also are highly active electrocatalysts for the oxidation of water to O2(g). These NiOx coatings provide protective layers on a variety of technologically important semiconducting photoanodes, including textured crystalline Si passivated by amorphous silicon, crystalline n-type cadmium telluride, and hydrogenated amorphous silicon. Under anodic operation in 1.0 M aqueous potassium hydroxide (pH 14) in the presence of simulated sunlight, the NiOx films stabilized all of these self-passivating, high-efficiency semiconducting photoelectrodes for >100 h of sustained, quantitative solar-driven oxidation of water to O2(g).
169 citations
TL;DR: A taxonomy and nomenclature for solar fuels generators based on the source of the asymmetry that separates photogenerated electrons and holes was developed in this article, and three basic device types have been identified: photovoltaic cells, photoelectrochemical cells, and particulate/molecular photocatalysts.
Abstract: A number of approaches to solar fuels generation are being developed, each of which has associated advantages and challenges. Many of these solar fuels generators are identified as “photoelectrochemical cells” even though these systems collectively operate based on a suite of fundamentally different physical principles. To facilitate appropriate comparisons between solar fuels generators, as well as to enable concise and consistent identification of the state-of-the-art for designs based on comparable operating principles, we have developed a taxonomy and nomenclature for solar fuels generators based on the source of the asymmetry that separates photogenerated electrons and holes. Three basic device types have been identified: photovoltaic cells, photoelectrochemical cells, and particulate/molecular photocatalysts. We outline the advantages and technological challenges associated with each type, and provide illustrative examples for each approach as well as for hybrid approaches.
167 citations
TL;DR: The stabilization of np(+)-Si(100) and n-Si(111) photoanodes for over 1200 h of continuous light-driven evolution of O2(g) in 1.0 M KOH(aq) by an earth-abundant, optically transparent, electrocatalytic, stable, conducting nickel oxide layer is reported.
Abstract: Semiconductors with small band gaps (<2 eV) must be stabilized against corrosion or passivation in aqueous electrolytes before such materials can be used as photoelectrodes to directly produce fuels from sunlight. In addition, incorporation of electrocatalysts on the surface of photoelectrodes is required for efficient oxidation of H2O to O2(g) and reduction of H2O or H2O and CO2 to fuels. We report herein the stabilization of np+-Si(100) and n-Si(111) photoanodes for over 1200 h of continuous light-driven evolution of O2(g) in 1.0 M KOH(aq) by an earth-abundant, optically transparent, electrocatalytic, stable, conducting nickel oxide layer. Under simulated solar illumination and with optimized index-matching for proper antireflection, NiOx-coated np+-Si(100) photoanodes produced photocurrent-onset potentials of −180 ± 20 mV referenced to the equilibrium potential for evolution of O2(g), photocurrent densities of 29 ± 1.8 mA cm–2 at the equilibrium potential for evolution of O2(g), and a solar-to-O2(g) co...
141 citations
TL;DR: In this paper, X-ray synchrotron radiation in conjunction with AP-XPS has enabled simultaneous monitoring of the solid surface, the solid/electrolyte interface, and the bulk electrolyte of a PEC cell as a function of the applied potential.
Abstract: Photoelectrochemical (PEC) cells based on semiconductor/liquid interfaces provide a method of converting solar energy to electricity or fuels. Currently, the understanding of semiconductor/liquid interfaces is inferred from experiments and models. Operando ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) has been used herein to directly characterize the semiconductor/liquid junction at room temperature under real-time electrochemical control. X-ray synchrotron radiation in conjunction with AP-XPS has enabled simultaneous monitoring of the solid surface, the solid/electrolyte interface, and the bulk electrolyte of a PEC cell as a function of the applied potential, U. The observed shifts in binding energy with respect to the applied potential have directly revealed ohmic and rectifying junction behavior on metallized and semiconducting samples, respectively. Additionally, the non-linear response of the core level binding energies to changes in the applied electrode potential has revealed the influence of defect-derived electronic states on the Galvani potential across the complete cell.
139 citations
TL;DR: In this paper, an ultrathin (2 nm) film of cobalt oxide (CoO_x) was introduced onto n-Si photoanodes prior to sputter-deposition of a thick multifunctional NiO-x coating, yielding stable photoelectrodes with photocurrent-onset potentials of ~−240 mV relative to the equilibrium potential for O2(g) evolution and current densities of ~28 mA cm^(−2) at the optimum for water oxidation.
Abstract: Introduction of an ultrathin (2 nm) film of cobalt oxide (CoO_x) onto n-Si photoanodes prior to sputter-deposition of a thick multifunctional NiO_x coating yields stable photoelectrodes with photocurrent-onset potentials of ~−240 mV relative to the equilibrium potential for O2(g) evolution and current densities of ~28 mA cm^(−2) at the equilibrium potential for water oxidation when in contact with 1.0 M KOH(aq) under 1 sun of simulated solar illumination. The photoelectrochemical performance of these electrodes was very close to the Shockley diode limit for moderately doped n-Si(100) photoelectrodes, and was comparable to that of typical protected Si photoanodes that contained np+ buried homojunctions.
123 citations
TL;DR: In this paper, conductive, amorphous TiO2 coatings deposited by atomic layer deposition, in combination with a sputter deposited NiCrOx oxygen-evolution catalyst, have been used to protect Si microwire arrays from passivation or corrosion in contact with aqueous electrolytes.
Abstract: Conductive, amorphous TiO2 coatings deposited by atomic-layer deposition, in combination with a sputter deposited NiCrOx oxygen-evolution catalyst, have been used to protect Si microwire arrays from passivation or corrosion in contact with aqueous electrolytes. Coated np+-Si/TiO2/NiCrOx as well as heterojunction n-Si/TiO2/NiCrOx Si microwire-array photoanodes exhibited stable photoelectrochemical operation in aqueous ferri-/ferro-cyanide solutions. The coatings also allowed for photoanodic water oxidation in 1.0 M KOH(aq) solutions for >2200 h of continuous operation under simulated 1 Sun conditions with ∼100% Faradaic efficiency for the evolution of O2(g).
120 citations
TL;DR: In this article, the branched CoP nanostructures were synthesized by reacting cobalt(II) acetylacetonate with trio-ctylphosphine in the presence of trio-cyclophosphine oxide.
Abstract: CoP nanostructures that exposed predominantly (111) crystal facets were synthesized and evaluated for performance as electrocatalysts for the hydrogen-evolution reaction (HER). The branched CoP nanostructures were synthesized by reacting cobalt(II) acetylacetonate with trioctylphosphine in the presence of trioctylphosphine oxide. Electrodes comprised of the branched CoP nanostructures deposited at a loading density of ∼1 mg cm−2 on Ti electrodes required an overpotential of −117 mV to produce a current density of −20 mA cm−2 in 0.50 M H2SO4. Hence the branched CoP nanostructures belong to the growing family of highly active non-noble-metal HER electrocatalysts. Comparisons with related CoP systems have provided insights into the impact that shape-controlled nanoparticles and nanoparticle–electrode interactions have on the activity and stability of nanostructured HER electrocatalysts.
111 citations
TL;DR: It is shown that TiO2 films with a variety of crystal structures and midgap defect state distributions form rectifying junctions with n-Si and are highly conductive toward photogenerated carriers in n- Si/TiO2/Ni photoanodes, and can be controlled to yield optimized photoelectrode performance.
Abstract: Light absorbers with moderate band gaps (1–2 eV) are required for high-efficiency solar fuels devices, but most semiconducting photoanodes undergo photocorrosion or passivation in aqueous solution. Amorphous TiO2 deposited by atomic-layer deposition (ALD) onto various n-type semiconductors (Si, GaAs, GaP, and CdTe) and coated with thin films or islands of Ni produces efficient, stable photoanodes for water oxidation, with the TiO2 films protecting the underlying semiconductor from photocorrosion in pH = 14 KOH(aq). The links between the electronic properties of the TiO2 in these electrodes and the structure and energetic defect states of the material are not yet well-elucidated. We show herein that TiO2 films with a variety of crystal structures and midgap defect state distributions, deposited using both ALD and sputtering, form rectifying junctions with n-Si and are highly conductive toward photogenerated carriers in n-Si/TiO2/Ni photoanodes. Moreover, the photovoltage of these electrodes can be modified...
TL;DR: This thematic issue contains reviews of various aspects of Solar Energy Conversion, where solar panels provide a known, scalable technology to capture and convert sunlight into electricity.
Abstract: This thematic issue contains reviews of various aspects of Solar Energy Conversion. The sun provides the largest energy source known to man, with more energy from sunlight striking the earth in 1 h than all of the energy consumed on the planet in an entire year. Solar panels provide a known, scalable technology to capture and convert sunlight into electricity. Moreover, the costs of Si-based photovoltaic panels have declined continuously in the past decade, to the point where solar electricity is now cost-competitive in certain regions and niche markets. Nevertheless, solar energy conversion continues to attract fervent efforts devoted to the discovery and development of new materials, concepts, devices, and systems that can provide new and/or dramatically improved functionality and scalability.
TL;DR: The louvered design provides a robust platform for implementation of various types of photoelectrochemical assemblies, and can provide an approach to significantly higher solar conversion efficiencies as new and improved materials become available.
Abstract: A fully integrated solar-driven water-splitting system comprised of WO3/FTO/p^(+)n Si as the photoanode, Pt/TiO_2/Ti/n^(+)p Si as the photocathode, and Nafion as the membrane separator, was simulated, assembled, operated in 1.0 M HClO_4, and evaluated for performance and safety characteristics under dual side illumination. A multi-physics model that accounted for the performance of the photoabsorbers and electrocatalysts, ion transport in the solution electrolyte, and gaseous product crossover was first used to define the optimal geometric design space for the system. The photoelectrodes and the membrane separators were then interconnected in a louvered design system configuration, for which the light-absorbing area and the solution-transport pathways were simultaneously optimized. The performance of the photocathode and the photoanode were separately evaluated in a traditional three-electrode photoelectrochemical cell configuration. The photocathode and photoanode were then assembled back-to-back in a tandem configuration to provide sufficient photovoltage to sustain solar-driven unassisted water-splitting. The current–voltage characteristics of the photoelectrodes showed that the low photocurrent density of the photoanode limited the overall solar-to-hydrogen (STH) conversion efficiency due to the large band gap of WO_3. A hydrogen-production rate of 0.17 mL hr^−1 and a STH conversion efficiency of 0.24 % was observed in a full cell configuration for >20 h with minimal product crossover in the fully operational, intrinsically safe, solar-driven water-splitting system. The solar-to-hydrogen conversion efficiency, ηS_TH, calculated using the multiphysics numerical simulation was in excellent agreement with the experimental behavior of the system. The value of ηSTH was entirely limited by the performance of the photoelectrochemical assemblies employed in this study. The louvered design provides a robust platform for implementation of various types of photoelectrochemical assemblies, and can provide an approach to significantly higher solar conversion efficiencies as new and improved materials become available.
TL;DR: The general conditions under which intrinsically safe, efficient solar-driven water-splitting cells can be operated are described.
Abstract: The solution transport losses in a one-dimensional solar-driven water-splitting cell that operates in either concentrated acid, dilute acid, or buffered near-neutral pH electrolytes have been evaluated using a mathematical model that accounts for diffusion, migration and convective transport, as well as for bulk electrochemical reactions in the electrolyte. The Ohmic resistance loss, the Nernstian potential loss associated with pH gradients at the surface of the electrode, and electrodialysis in different electrolytes were assessed quantitatively in a stagnant cell as well as in a bubble-convected cell, in which convective mixing occurred due to product-gas evolution. In a stagnant cell that did not have convective mixing, small limiting current densities (<3 mA cm^(−2)) and significant polarization losses derived from pH gradients were present in dilute acid as well as in near-neutral pH buffered electrolytes. In contrast, bubble-convected cells exhibited a significant increase in the limiting current density, and a significant reduction of the concentration overpotentials. In a bubble-convected cell, minimal solution transport losses were present in membrane-free cells, in either buffered electrolytes or in unbuffered solutions with pH ≤ 1. However, membrane-free cells lack a mechanism for product-gas separation, presenting significant practical and engineering impediments to the deployment of such systems. To produce an intrinsically safe cell, an ion-exchange membrane was incorporated into the cell. The accompanying solution losses, especially the pH gradients at the electrode surfaces, were modeled and simulated for such a system. Hence this work describes the general conditions under which intrinsically safe, efficient solar-driven water-splitting cells can be operated.
TL;DR: In this paper, a photoanode protection strategy using a multifunctional NiO_x coating is presented, and the ransparency/antireflectivity, low electrochromism, conduction of holes, corrosion protection, and active electrocatalysis for water-oxidation half-reaction are described.
Abstract: A photoanode protection strategy using a multifunctional NiO_x coating is presented. The ransparency/antireflectivity, low electrochromism, conduction of holes, corrosion protection, and active electrocatalysis for water-oxidation half-reaction are described.
TL;DR: The MW geometry allows the fabrication of photocathodes entirely comprised of earth-abundant materials that exhibit performance comparable to that of devices that contain Pt.
Abstract: The electrocatalytic performance for hydrogen evolution has been evaluated for radial-junction n+p-Si microwire (MW) arrays with Pt or cobalt phosphide, CoP, nanoparticulate catalysts in contact with 0.50 M H2SO4(aq). The CoP-coated (2.0 mg cm–2) n+p-Si MW photocathodes were stable for over 12 h of continuous operation and produced an open-circuit photovoltage (Voc) of 0.48 V, a light-limited photocurrent density (Jph) of 17 mA cm–2, a fill factor (ff) of 0.24, and an ideal regenerative cell efficiency (ηIRC) of 1.9% under simulated 1 Sun illumination. Pt-coated (0.5 mg cm–2) n+p-Si MW-array photocathodes produced Voc = 0.44 V, Jph = 14 mA cm–2, ff = 0.46, and η = 2.9% under identical conditions. Thus, the MW geometry allows the fabrication of photocathodes entirely comprised of earth-abundant materials that exhibit performance comparable to that of devices that contain Pt.
TL;DR: In this article, the tradeoff between the optical obscuration and kinetic overpotentials of electrocatalyst films patterned onto the surface of tandem light-absorber structures in model photoelectrosynthetic water-splitting systems was investigated using a 0-dimensional load-line analysis and experimental measurements.
Abstract: The trade-off between the optical obscuration and kinetic overpotentials of electrocatalyst films patterned onto the surface of tandem light-absorber structures in model photoelectrosynthetic water-splitting systems was investigated using a 0-dimensional load-line analysis and experimental measurements. The electrocatalytic performance of the catalyst at high current densities, normalized to the electrocatalyst surface area, is an important factor in the dependence of the optimal solar-to-hydrogen (STH) conversion efficiency, ηSTH,opt, on the filling fraction (fc) of the patterned catalysts, because even under conditions that produce minority-carrier current densities of ∼10 mA cm−2 at the solid/liquid interface, the current density at catalyst-bearing sites can be >1–2 A cm−2 in low filling-fraction films. A universal current-density versus potential relationship, up to current densities of 10 A cm−2, was obtained experimentally for the hydrogen-evolution reaction (HER) using patterned Pt ultramicroelectrode (UME) arrays with a range of filling fractions and disc diameters. The ηSTH,opt of system designs that utilize patterned electrocatalysts located on the illuminated side of tandem photoabsorbers was then evaluated systematically. The maximum STH conversion efficiency, ηSTH,max, using a hypothetical electrocatalyst that was optically transparent but which nevertheless exhibited a current-density versus potential behavior that is characteristic of the most active Pt films measured experimentally regardless of their optical obscuration, was 26.7%. By comparison, the maximum ηSTH,opt of 24.9% for real patterned Pt electrocatalyst films closely approached this ideal-case limit. The performance and materials utilization of the patterned electrocatalysts and of the uniformly coated electrocatalysts on tandem photoabsorbers were also compared in this study. Hence, patterned electrocatalysts with very low filling fractions can provide a potentially promising path to the realization of efficient large-scale photoelectrolysis systems while minimizing the use of scarce noble metals.
TL;DR: An n+p-Si microwire array coupled with a two-layer catalyst film consisting of Ni-Mo nanopowder and TiO2 light-scattering nanoparticles has been used to simultaneously achieve high fill factors and light-limited photocurrent densities from photocathodes that produce H2(g) directly from sunlight and water as discussed by the authors.
Abstract: An n+p-Si microwire array coupled with a two-layer catalyst film consisting of Ni–Mo nanopowder and TiO2 light-scattering nanoparticles has been used to simultaneously achieve high fill factors and light-limited photocurrent densities from photocathodes that produce H2(g) directly from sunlight and water. The TiO2 layer scattered light back into the Si microwire array, while optically obscuring the underlying Ni–Mo catalyst film. In turn, the Ni–Mo film had a mass loading sufficient to produce high catalytic activity, on a geometric area basis, for the hydrogen-evolution reaction. The best-performing microwire array devices prepared in this work exhibited short-circuit photocurrent densities of −14.3 mA cm−2, photovoltages of 420 mV, and a fill factor of 0.48 under 1 Sun of simulated solar illumination, whereas the equivalent planar Ni–Mo-coated Si device, without TiO2 scatterers, exhibited negligible photocurrent due to complete light blocking by the Ni–Mo catalyst layer.
TL;DR: In this paper, the authors evaluated the performance of a 6-electron/6-proton CO2 reduction system at the concentration of CO2 in the current atmosphere (pCO2 = 400 ppm) on a variety of scale lengths that span from laboratory scale to global scale.
Abstract: The operational constraints for a 6-electron/6-proton CO2 reduction system that operates at the concentration of CO2 in the current atmosphere (pCO2 = 400 ppm) have been evaluated on a variety of scale lengths that span from laboratory scale to global scale. Due to the low concentration of CO2 in the atmosphere, limitations due to mass transport of CO2 from the tropopause have been evaluated through five different regions, each with different characteristic length scales: the troposphere; the atmospheric boundary layer (ABL); the canopy layer; a membrane layer; and an aqueous electrolyte layer. The resulting CO2 conductances, and associated physical transport limitations, will set the ultimate limit on the efficiency and areal requirements of a sustainable solar-driven CO2 reduction system regardless of the activity or selectivity of catalysts for reduction of CO2 at the molecular level. At the electrolyte/electrode interface, the steady-state limiting current density and the concomitant voltage loss associated with the CO2 concentration overpotential in a one-dimensional solar-driven CO2 reduction cell have been assessed quantitatively using a mathematical model that accounts for diffusion, migration and convective transport, as well as for bulk electrochemical reactions in the electrolyte. At pCO2 = 400 ppm, the low diffusion coefficient combined with the low solubility of CO2 in aqueous solutions constrains the steady-state limiting current density to 10% efficient solar-driven CO2 reduction system (based on the solar collection area). This flux limitation is consistent with estimates of oceanic CO2 uptake fluxes that have been developed in conjunction with carbon-cycle analyses for use in coupled atmosphere/ocean general circulation models. Two strategies to improve the feasibility of obtaining efficient and sustainable CO2 transport to a cathode surface at pCO2 = 400 ppm are described and modeled quantitatively. The first strategy employs yet unknown catalysts, analogous to carbonic anhydrases, that dramatically accelerate the chemically enhanced CO2 transport in the aqueous electrolyte layer by enhancing the acid–base reactions in a bicarbonate buffer system. The rapid interconversion from bicarbonate to CO2 in the presence of such catalysts near the cathode surface would in principle yield significant increases in the steady-state limiting current density and allow for >10% solar-fuel operation at the cell level. The second strategy employs a thin-layer cell architecture to improve the diffusive transport of CO2 by use of an ultrathin polymeric membrane electrolyte. Rapid equilibration of CO2 at the gas/electrolyte interface, and significantly enhanced diffusive fluxes of CO2 in electrolytes, are required to increase the steady-state limiting current density of such a system. This latter approach however only is feasible for gaseous products, because liquid products would coat the electrode and therefore thicken the hydrodynamic boundary layer and accordingly reduce the diffusive CO2 flux to the electrode surface. Regardless of whether the limitations due to mass transport to the electrode surface are overcome on the laboratory scale, at global scales the ultimate CO2 flux limitations will be dictated by mass transport considerations related to transport of atmospheric CO2 to the boundary plane of the solar-driven reactor system. The transport of CO2 across the troposphere/ABL interface, the ABL/canopy layer interface, and the canopy layer/electrolyte interface have therefore been assessed in this work, to provide upper bounds on the ultimate limits for the solar-to-fuel (STF) conversion efficiency for systems that are intended to effect the reduction of atmospheric CO2 in a sustainable fashion at global scale.
TL;DR: A broad suite of experimental and computational tools that can be used to define the structure-property relationships of photoelectrode materials at small dimensions and on fast time scales is presented in this paper.
Abstract: Materials and photoelectrode architectures that are highly efficient, extremely stable, and made from low cost materials are required for commercially viable photoelectrochemical (PEC) water-splitting technology. A key challenge is the heterogeneous nature of real-world materials, which often possess spatial variation in their crystal structure, morphology, and/or composition at the nano-, micro-, or macro-scale. Different structures and compositions can have vastly different properties and can therefore strongly influence the overall performance of the photoelectrode through complex structure–property relationships. A complete understanding of photoelectrode materials would also involve elucidation of processes such as carrier collection and electrochemical charge transfer that occur at very fast time scales. We present herein an overview of a broad suite of experimental and computational tools that can be used to define the structure–property relationships of photoelectrode materials at small dimensions and on fast time scales. A major focus is on in situ scanning-probe measurement (SPM) techniques that possess the ability to measure differences in optical, electronic, catalytic, and physical properties with nano- or micro-scale spatial resolution. In situ ultrafast spectroscopic techniques, used to probe carrier dynamics involved with processes such as carrier generation, recombination, and interfacial charge transport, are also discussed. Complementing all of these experimental techniques are computational atomistic modeling tools, which can be invaluable for interpreting experimental results, aiding in materials discovery, and interrogating PEC processes at length and time scales not currently accessible by experiment. In addition to reviewing the basic capabilities of these experimental and computational techniques, we highlight key opportunities and limitations of applying these tools for the development of PEC materials.
TL;DR: Scanning microwave microscopy imaging of single layers of MoS2 and n- and p-doped WSe2 is performed to underscore the capability of SMM for the study of 2D materials to image, identify, and study electronic defects.
Abstract: Optimizing new generations of two-dimensional devices based on van der Waals materials will require techniques capable of measuring variations in electronic properties in situ and with nanometer spatial resolution. We perform scanning microwave microscopy (SMM) imaging of single layers of MoS2 and n- and p-doped WSe2. By controlling the sample charge carrier concentration through the applied tip bias, we are able to reversibly control and optimize the SMM contrast to image variations in electronic structure and the localized effects of surface contaminants. By further performing tip bias-dependent point spectroscopy together with finite element simulations, we distinguish the effects of the quantum capacitance and determine the local dominant charge carrier species and dopant concentration. These results underscore the capability of SMM for the study of 2D materials to image, identify, and study electronic defects.
TL;DR: In this paper, free-standing, membrane-embedded, Si microwire arrays have been used to affect the solar-driven, unassisted splitting of HI into H_2 and I_3−.
Abstract: Free-standing, membrane-embedded, Si microwire arrays have been used to affect the solar-driven, unassisted splitting of HI into H_2 and I_3−. The Si microwire arrays were grown by a chemical-vapor-deposition vapor–liquid–solid growth process using Cu growth catalysts, with a radial n+p junction then formed on each microwire. A Nafion proton-exchange membrane was introduced between the microwires and Pt electrocatalysts were then photoelectrochemically deposited on the microwires. The composite Si/Pt–Nafion membrane was mechanically removed from the growth substrate, and Pt electrocatalysts were then also deposited on the back side of the structure. The resulting membrane-bound Si microwire arrays spontaneously split concentrated HI into H_2(g) and I_3− under 1 Sun of simulated solar illumination. The reaction products (i.e. H_2 and I_3−) were confirmed by mass spectrometry and ultraviolet–visible electronic absorption spectroscopy.
TL;DR: In this article, a sensitivity analysis has been performed for a variety of generic designs for solar-Fuels generators, revealing the relative importance of reductions in the overpotentials of electrocatalysts, of improvements in the materials properties of light absorbers, and of optimization in the system geometry for various different types of solar-fuels generators.
Abstract: A sensitivity analysis has been performed for a variety of generic designs for solar-fuels generators. The analysis has revealed the relative importance of reductions in the overpotentials of electrocatalysts, of improvements in the materials properties of light absorbers, and of optimization in the system geometry for various different types of solar-fuels generators, while considering operation at a range of temperatures as well as under a variety of illumination intensities including up to 10-fold optical concentration.
TL;DR: In this paper, the authors present results selected from on-going studies of earth-abundant electrocatalysts for the reactions that underpin artificial photosynthesis: nickel-molybdenum alloys for the hydrogen evolution reaction, calcium birnessite as a heterogeneous analogue for the oxygen-evolving complex in natural photosynthesis, and single-crystalline copper in relation to the carbon dioxide reduction reaction.
TL;DR: In this article, transmission infrared spectroscopy (TIRS) and high-resolution electron energy-loss spectrograms (HREELS) were used to characterize Si(111) surfaces.
Abstract: Ethynyl- and propynyl-terminated Si(111) surfaces synthesized using a two-step halogenation/alkylation method have been characterized by transmission infrared spectroscopy (TIRS), high-resolution electron energy-loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), atomic-force microscopy (AFM), electrochemical scanning–tunneling microscopy (EC-STM) and measurements of surface recombination velocities (S). For the ethynyl-terminated Si(111) surface, TIRS revealed signals corresponding to ethynyl ≡C–H and C≡C stretching oriented perpendicular to the surface, HREELS revealed a Si–C stretching signal, and XPS data showed the presence of C bound to Si with a fractional monolayer (ML) coverage (Φ) of ΦSi–CCH = 0.63 ± 0.08 ML. The ethynyl-terminated surfaces were also partially terminated by Si–OH groups (ΦSi–OH = 0.35 ± 0.03 ML) with limited formation of Si3+ and Si4+ oxides. For the propynyl-terminated Si(111) surface, TIRS revealed the presence of a (C–H)...
TL;DR: As detected by X-ray photoelectron spectroscopy after the ALD process, theCH3- and mixed CH3-/HC(O)CH2CH2- functionalized Si(111) surfaces exhibited less interfacial SiOx than was observed for ALD of metal oxides on H-Si( 111) substrates.
Abstract: Silicon surfaces terminated with a mixed monolayer containing both a propyl aldehyde functionality and methyl groups were prepared and used to control the interfacial chemical and electronic properties of Si(111) surfaces during atomic-layer deposition (ALD) of Al2O3 or MnO. Si(111) surfaces functionalized only with the aldehyde moiety exhibited surface recombination velocities, S, of 2500 ± 600 cm s–1 whereas the mixed CH3–/HC(O)CH2CH2–Si(111) surfaces displayed S = 25 ± 7 cm s–1. During the ALD growth of either Al2O3 or MnO, both the HC(O)CH2CH2–Si(111) and CH3–/HC(O)CH2CH2–Si(111) surfaces produced increased metal oxide deposition at low cycle number, relative to H–Si(111) or CH3–Si(111) surfaces. As detected by X-ray photoelectron spectroscopy after the ALD process, the CH3– and mixed CH3–/HC(O)CH2CH2– functionalized Si(111) surfaces exhibited less interfacial SiOx than was observed for ALD of metal oxides on H–Si(111) substrates.
TL;DR: Simulations of light absorption and concentration in idealized lamellar arrays, in conjunction with all of the available data, indicated that a self-optimization of the periodicity of the nanoscale pattern, resulting in the maximizing of the anisotropy of interfacial light absorption in the three-dimensional structure, is consistent with the observed growth process of such films.
Abstract: Photoelectrochemical growth of Se–Te films spontaneously produces highly ordered, nanoscale lamellar morphologies with periodicities that can be tuned by varying the illumination wavelength during deposition. This phenomenon has been characterized further herein by determining the morphologies of photoelectrodeposited Se–Te films in response to tailored spectral illumination profiles. Se–Te films grown under illumination from four different sources, having similar average wavelengths but having spectral bandwidths that spanned several orders of magnitude, all nevertheless produced similar structures which had a single, common periodicity as quantitatively identified via Fourier analysis. Film deposition using simultaneous illumination from two narrowband sources, which differed in average wavelength by several hundred nanometers, resulted in a structure with only a single periodicity intermediate between the periods observed when either source alone was used. This single periodicity could be varied by man...
TL;DR: The interfacial shear strength between Si microwires and a Nafion membrane has been tailored through surface functionalization of the Si to influence the mechanical adhesion forces at a Si-Nafion interface.
Abstract: The interfacial shear strength between Si microwires and a Nafion membrane has been tailored through surface functionalization of the Si. Acidic (−COOH-terminated) or basic (−NH_2-terminated) surface-bound functionality was introduced by hydrosilylation reactions to probe the interactions between the functionalized Si microwires and hydrophilic ionically charged sites in the Nafion polymeric side chains. Surfaces functionalized with SiO_x, Si–H, or Si–CH_3 were also synthesized and investigated. The interfacial shear strength between the functionalized Si microwire surfaces and the Nafion matrix was quantified by uniaxial wire pull-out experiments in an in situ nanomechanical instrument that allowed simultaneous collection of mechanical data and visualization of the deformation process. In this process, an axial load was applied to the custom-shaped top portions of individual wires until debonding occurred from the Nafion matrix. The shear strength obtained from the nanomechanical measurements correlated with the chemical bond strength and the functionalization density of the molecular layer, with values ranging from 7 MPa for Si–CH3 surfaces to ∼16–20 MPa for oxygen-containing surface functionalities. Hence surface chemical control can be used to influence the mechanical adhesion forces at a Si–Nafion interface.
TL;DR: Atomically flat, terraced H-Ge(111) was prepared by annealing in H2(g) at 850 °C, and the formation of monohydride Ge-H bonds oriented normal to the surface was indicated by angle-dependent Fourier-transform infrared (FTIR) spectroscopy.
Abstract: Atomically flat, terraced H–Ge(111) was prepared by annealing in H2(g) at 850 °C. The formation of monohydride Ge–H bonds oriented normal to the surface was indicated by angle-dependent Fourier-transform infrared (FTIR) spectroscopy. Subsequent reaction in CCl3Br(l) formed Br-terminated Ge(111), as shown by the disappearance of the Ge–H absorption in the FTIR spectra concomitant with the appearance of Br photoelectron peaks in X-ray photoelectron (XP) spectra. The Br–Ge(111) surface was methylated by reaction with (CH3)2Mg. These surfaces exhibited a peak at 568 cm–1 in the high-resolution electron energy loss spectrum, consistent with the formation of a Ge–C bond. The absorption peaks in the FTIR spectra assigned to methyl “umbrella” and rocking modes were dependent on the angle of the incident light, indicating that the methyl groups were bonded directly atop surface Ge atoms. Atomic-force micrographs of CH3–Ge(111) surfaces indicated that the surface remained atomically flat after methylation. Electroc...
TL;DR: In this article, three-target reactive radiofrequency sputtering was used to synthesize non-degenerately doped semiconducting alloys having <10% atomic composition (x = 0.025).
Abstract: We report on the fabrication and structural and optoelectronic characterization of II-IV-nitride ZnSn_x Ge(1−x)N_2 thin-films. Three-target reactive radio-frequency sputtering was used to synthesize non-degenerately doped semiconducting alloys having <10% atomic composition (x = 0.025) of tin. These low-Sn alloys followed the structural and optoelectronic trends of the alloy series. Samples exhibited semiconducting properties, including optical band gaps and increasing in resistivities with temperature. Resistivity vs. temperature measurements indicated that low-Sn alloys were non-degenerately doped, whereas alloys with higher Sn content were degenerately doped. These films show potential for ZnSn_x Ge_(1−x)N_2 as tunable semiconductor absorbers for possible use in photovoltaics, light-emitting diodes, or optical sensors.