Showing papers by "Nathan S. Lewis published in 2020"

Effects of bubbles on the electrochemical behavior of hydrogen-evolving Si microwire arrays oriented against gravity

Abstract: The size-distribution, coverage, electrochemical impedance, and mass-transport properties of H2 gas-bubble films were measured for both planar and microwire-array platinized n+-Si cathodes performing the hydrogen-evolution reaction in 0.50 M H2SO4(aq). Inverted, planar n+-Si/Ti/Pt cathodes produced large, stationary bubbles which contributed to substantial increases in ohmic potential drops. In contrast, regardless of orientation, microwire array n+-Si/Ti/Pt cathodes exhibited a smaller layer of bubbles on the surface, and the formation of bubbles did not substantially increase the steady-state overpotential for H2(g) production. Experiments using an electroactive tracer species indicated that even when oriented against gravity, bubbles enhanced mass transport at the electrode surface. Microconvection due to growing and coalescing bubbles dominated effects due to macroconvection of gliding bubbles on Si microwire array cathodes. Electrodes that maintained a large number of small bubbles on the surface simultaneously exhibited low concentrations of dissolved hydrogen and small ohmic potential drops, thus exhibiting the lowest steady-state overpotentials. The results indicate that microstructured electrodes can operate acceptably for unassisted solar-driven water splitting in the absence of external convection and can function regardless of the orientation of the electrode with respect to the gravitational force vector.

Isotopically Selective Quantification by UPLC-MS of Aqueous Ammonia at Submicromolar Concentrations Using Dansyl Chloride Derivatization

Abstract: Ammonia (NH₃) is essential for food production and is commercially synthesized from nitrogen (N₂) and hydrogen (H₂) using the Haber–Bosch process. Enormous amounts of ammonia are made every year in a reaction that requires high temperatures and pressures, with the Haber–Bosch accounting for ∼1.6% of total annual global energy consumption. Electrocatalysts are also being explored to convert in an environmentally friendly manner N₂ to NH₃ at ambient temperature and pressure.

Modeling the Performance of A Flow-Through Gas Diffusion Electrode for Electrochemical Reduction of CO or CO2

Abstract: A flow-through gas diffusion electrode (GDE) consisting of agglomerate catalysts for CO or CO2 reduction, gas channels for reactants, aqueous electrolytes for ionic transport, and metallic current collectors was simulated and evaluated using a numerical model. The geometric partial current densities and Faradaic Efficiencies (FE) for CH4, C2H4 and H2 generation in GDEs were calculated and compared to the behavior of analogous aqueous-based planar electrodes. The pH-dependent kinetics for CH4 and C2H4 generation were used to represent the intrinsic catalytic characteristics for the agglomerate catalyst. The modeling indicated that relative to planar electrodes for either CO reduction (COR) or CO2 reduction (CO2R), substantial increases in electrochemical reduction rates and Faradaic efficiencies are expected when flow-through GDEs are used. The spatially resolved pH and reaction rates within the flow-through GDEs were also simulated for two different operating pHs, and the resulting transport losses were analyzed quantitatively. For CO2 reduction (CO2R), substantial loss of CO2 via chemical reaction with the locally alkaline electrolyte was observed due to the increased pH in operating GDEs.

Si Microwire-Array Photocathodes Decorated with CuAllow CO 2 Reduction with Minimal Parasitic Absorption ofSunlight

Abstract: High loadings of Cu were integrated on the light-facing side of Si microwire arrays used under simulated sunlight for the photoelectrochemical reduction of CO2(aq) to hydrocarbons in 0.10 M KHCO3(a...

Atomic force microscopy: Emerging illuminated and operando techniques for solar fuel research.

Abstract: Integrated photoelectrochemical devices rely on the synergy between components to efficiently generate sustainable fuels from sunlight. The micro- and/or nanoscale characteristics of the components and their interfaces often control critical processes of the device, such as charge-carrier generation, electron and ion transport, surface potentials, and electrocatalysis. Understanding the spatial properties and structure–property relationships of these components can provide insight into designing scalable and efficient solar fuel components and systems. These processes can be probed ex situ or in situ with nanometer-scale spatial resolution using emerging scanning-probe techniques based on atomic force microscopy (AFM). In this Perspective, we summarize recent developments of AFM-based techniques relevant to solar fuel research. We review recent progress in AFM for (1) steady-state and dynamic light-induced surface photovoltage measurements; (2) nanoelectrical conductive measurements to resolve charge-carrier heterogeneity and junction energetics; (3) operando investigations of morphological changes, as well as surface electrochemical potentials, currents, and photovoltages in liquids. Opportunities for research include: (1) control of ambient conditions for performing AFM measurements; (2) in situ visualization of corrosion and morphological evolution of electrodes; (3) operando AFM techniques to allow nanoscale mapping of local catalytic activities and photo-induced currents and potentials.

Reductant-Activated, High-Coverage, Covalent Functionalization of 1T′-MoS2

Abstract: Recently developed covalent functionalization chemistry for MoS2 in the 1T′ phase enables the formation of covalent chalcogenide–carbon bonds from alkyl halides and aryl diazonium salts. However, t...

Failure modes of protection layers produced by atomic layer deposition of amorphous TiO2 on GaAs anodes

Abstract: Amorphous titanium dioxide (a-TiO2) films formed by atomic layer deposition can serve as protective coatings for semiconducting photoanodes in water-splitting cells using strongly alkaline aqueous electrolytes. Herein, we experimentally examine the mechanisms of failure for p+-GaAs anodes coated with a-TiO2 films (GaAs/a-TiO2). Galvanic displacement of exposed GaAs by Au allowed imaging of pinholes in the a-TiO2 coatings, and enabled collection of quantitative and statistical data associated with pinhole defects. A combination of imaging, electrochemical measurements, and quantitative analyses of corrosion products indicated that extrinsic pinholes were present in the a-TiO2 films before electrochemical operation. During electrochemical operation these pinholes led to pitting corrosion of the underlying GaAs substrate. The dominant source of pinholes was the presence of atmospheric particulate matter on the GaAs surface during deposition of the a-TiO2 layer. The pinhole density decreased substantially when the thickness of the a-TiO2 coating increased beyond 45 nm, and approached zero when the thickness of the film exceeded 112 nm. The density of pinholes in films thinner than 45 nm decreased when the a-TiO2 coating was deposited in an environmentally controlled cleanroom. Pinhole-free GaAs/a-TiO2 devices were also tested via chronoamperometry to quantify the rate of pinhole formation during electrochemistry. The time-to-failure increased with thickness, suggesting that the failure mechanism may involve diffusion or migration through the film. However, other mechanisms may also contribute to the degradation of thicker films (>112 nm). Nevertheless, as previously hypothesized, extrinsic pinhole defects formed during deposition and testing control the short-term protective performance of the a-TiO2 film for GaAs anodes evolving O2 from water.

High Broadband Light Transmission for Solar Fuels Production Using Dielectric Optical Waveguides in TiO2 Nanocone Arrays.

Abstract: We describe the fabrication and use of arrays of TiO2 nanocones to yield high optical transmission into semiconductor photoelectrodes covered with high surface loadings of light-absorbing electrocatalysts. Covering over 50% of the surface of a light absorber with an array of high-refractive-index TiO2 nanocones imparted antireflective behavior ( 85% transmission of broadband light to the underlying Si, even when thick metal contacts or opaque catalyst coatings were deposited on areas of the light-facing surface that were not directly beneath a nanocone. Three-dimensional full-field electromagnetic simulations for the 400-1100 nm spectral range showed that incident broadband illumination couples to multiple waveguide modes in the TiO2 nanocones, reducing interactions of the light with the metal layer. A proof-of-concept experimental demonstration of light-driven water oxidation was performed using a p+n-Si photoanode decorated with an array of TiO2 nanocones additionally having a Ni catalyst layer electrodeposited onto the areas of the p+n-Si surface left uncovered by the TiO2 nanocones. This photoanode produced a light-limited photocurrent density of ∼28 mA cm-2 under 100 mW cm-2 of simulated air mass 1.5 illumination, equivalent to the photocurrent density expected for a bare planar Si surface even though 54% of the front surface of the Si was covered by an ∼70 nm thick Ni metal layer.

Conformal SnOx heterojunction coatings for stabilized photoelectrochemical water oxidation using arrays of silicon microcones

Abstract: The efficiency of photoelectrodes towards fuel-forming reactions is strongly affected by surface-based charge recombination, charge-transfer losses, and parasitic light absorption by electrocatalysts. We report a protective tin oxide (SnOx) layer formed by atomic-layer deposition that limits surface recombination at n-Si/SnOx heterojunctions and produces ∼620 mV of photovoltage on planar n-Si photoanodes. The SnOx layer can be deposited conformally on high aspect-ratio three-dimensional structures such as Si microcone arrays. Atomic-level control of the SnOx thickness enabled highly conductive contacts to electrolytes, allowing the direct electrodeposition of NiFeOOH, CoOx, and IrOx electrocatalysts for photoelectrochemical water oxidation with minimal parasitic absorption losses. SnOx-coated n-Si microcone arrays coupled to electrodeposited catalysts exhibited photocurrent densities of ∼42 mA cm−2 and a photovoltage of ∼490 mV under 100 mW cm−2 of simulated solar illumination. The SnOx layer can be integrated with amorphous TiO2 to form a protective SnOx/TiO2 bilayer that exhibits the beneficial properties of both materials. Photoanodes coated with SnOx/TiO2 exhibited a similar photovoltage to that of SnOx-coated photoanodes, and showed >480 h of stable photocurrent for planar photoelectrodes and >140 h of stable photocurrent for n-Si microcone arrays under continuous simulated solar illumination in alkaline electrolytes.

Enhanced stability of silicon for photoelectrochemical water oxidation through self-healing enabled by an alkaline protective electrolyte

Abstract: Alkaline electrolytes impede the corrosion of Si photoanodes under positive potentials and/or illumination, due to the formation of a SiOx layer that etches 2–3 orders of magnitude more slowly than Si. Hence during water oxidation under illumination, pinholes in protection layers on Si photoanodes result in the local formation of a protective, stabilizing passive oxide on the Si surface. However, operation under natural diurnal insolation cycles additionally requires protection strategies that minimize the dark corrosive etching rate of Si at pinholes. We show herein that addition of [Fe(CN)6]3− to 1.0 M KOH(aq) results in a self-healing process that extends the lifetime to >280 h of an np+-Si(100) photoanode patterned with an array of Ni catalyst islands operated under simulated day/night cycles. The self-healing [Fe(CN)6]3− additive caused the exposed Si(100) surface to etch >180 times slower than the Si etch rate in 1.0 M KOH(aq) alone. No appreciable difference in etch rate or facet preference was observed between Si(100) and Si(111) surfaces in 1.0 M KOH(aq) with [Fe(CN)6]3−, indicating that the surface conformally oxidized before Si dissolved. The presence of [Fe(CN)6]3− minimally impacted the faradaic efficiency or overpotential of p+-Si/Ni electrodes for the oxygen-evolution reaction.

Cathodic NH4+ leaching of nitrogen impurities in CoMo thin-film electrodes in aqueous acidic solutions

Abstract: Electrocatalytic reduction of dinitrogen (N2) to ammonium (NH4+) in acidic aqueous solutions was investigated at ambient temperature and pressure using a cobalt–molybdenum (CoMo) thin-film electrode prepared by magnetron reactive sputtering. Increased concentrations of ammonium ions (NH4+) were consistently detected in the electrolyte using ion chromatography (IC) after constant-potential electrolysis at various potentials (≤−0.29 V vs. RHE). Using a newly developed analytical method based on ammonia derivatization, performing the experiments with 15N2-labelled gas led however to the detection of increased 14NH4+ concentrations instead of 15NH4+. X-ray photoelectron spectroscopic (XPS) analysis of the electrode surface revealed the presence of MoN and Mo–NHx species. Several contamination sources were identified that led to substantial increases in the concentration of ammonium ions, including 15NH3 impurities in 15N2 gas. The observed ammonium concentrations can be consistently ascribed to leaching of nitrogen (14N) impurities incorporated in the CoMo film during the sputtering process. Researchers in the field are therefore urged to adopt extended protocols to identify and eliminate sources of ammonia contamination and to very carefully monitor the ammonium concentrations in each experimental step.

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

Abstract: 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.

Evaluation of sputtered nickel oxide, cobalt oxide and nickel–cobalt oxide on n-type silicon photoanodes for solar-driven O2(g) evolution from water

Abstract: Thin films of nickel oxide (NiOx), cobalt oxide (CoOx) and nickel–cobalt oxide (NiCoOx) were sputtered onto n-Si(111) surfaces to produce a series of integrated, protected Si photoanodes that did not require deposition of a separate heterogeneous electrocatalyst for water oxidation The p-type transparent conductive oxides (p-TCOs) acted as multi-functional transparent, antireflective, electrically conductive, chemically stable coatings that also were active electrocatalysts for the oxidation of water to O2(g) Relative to the formal potential for water oxidation to O2, Eo′(O2/H2O), under simulated Air Mass (AM)15 illumination the p-TCO-coated n-Si(111) photoanodes produced mutually similar open-circuit potentials of −270 ± 20 mV, but different photocurrent densities at Eo′(O2/H2O), of 28 ± 03 mA cm−2 for NiOx, 18 ± 03 mA cm−2 for CoOx and 24 ± 05 mA cm−2 for NiCoOx The p-TCOs all provided protection from oxide growth for extended time periods, and produced stable photocurrent densities from n-Si in 10 M KOH(aq) (ACS grade) under potential control at Eo′(O2/H2O) for >400 h of continuous operation under 100 mW cm−2 of simulated AM15 illumination

Optically tunable mesoscale CdSe morphologies via inorganic phototropic growth

Abstract: Inorganic phototropic growth using only spatially conformal illumination generated Se–Cd films that exhibited precise light-defined mesoscale morphologies including highly ordered, anisotropic, and periodic ridge and trench nanotextures over entire macroscopic substrates Growth was accomplished via a light-induced electrochemical method using an optically and chemically isotropic solution, an unpatterned substrate, and unstructured, incoherent, low-intensity illumination in the absence of chemical directing agents or physical templates and masks The morphologies were defined by the illumination inputs: the nanotexture long axes aligned parallel to the optical E-field vector, and the feature sizes and periods scaled with the wavelength Optically based modeling of the growth closely reproduced the experimental results, confirming the film morphologies were fully determined by the light–matter interactions during growth Solution processing of the Se–Cd films resulted in stoichiometric, crystalline CdSe films that also exhibited ordered nanotextures, demonstrating that inorganic phototropic growth can effect tunable, template-free generation of ordered CdSe nanostructures over macroscopic length scales

Defect-Seeded Atomic Layer Deposition of Metal Oxides on the Basal Plane of 2D Layered Materials

Abstract: Atomic layer deposition (ALD) on mechanically exfoliated 2D layered materials spontaneously produces network patterns of metal oxide nanoparticles in triangular and linear deposits on the basal surface. The network patterns formed under a range of ALD conditions and were independent of the orientation of the substrate in the ALD reactor. The patterns were produced on MoS2 or HOPG when either tetrakis(dimethylamido)titanium or bis(ethylcyclopentadienyl)manganese were used as precursors, suggesting that the phenomenon is general for 2D materials. Transmission electron microscopy revealed the presence, prior to deposition, of dislocation networks along the basal plane of mechanically exfoliated 2D flakes, indicating that periodical basal plane defects related to disruptions in the van der Waals stacking of layers, such as perfect line dislocations and triangular extended stacking faults networks, introduce a surface reactivity landscape that leads to the emergence of patterned deposition.

Spontaneous Formation of >90% Optically Transmissive, Electrochemically Active CoP Films for Photoelectrochemical Hydrogen Evolution.

Abstract: Earth-abundant catalysts for the hydrogen-evolution reaction require increased mass loadings, relative to Pt films, to achieve comparable activity and stability in acidic electrolytes. We report he...

Genesis and Propagation of Fractal Structures During Photoelectrochemical Etching of n-Silicon

Abstract: The genesis, propagation, and dimensions of fractal-etch patterns that form anodically on front- or back-illuminated n-Si(100) photoelectrodes in contact with 11.9 M NH4F (aqueous) have been invest...

Surface Passivation and Positive Band-Edge Shift of p-Si(111) Surfaces Functionalized with Mixed Methyl/Trifluoromethylphenylacetylene Overlayers

Abstract: Chemical functionalization of semiconductor surfaces can provide high-efficiency photoelectrochemical devices through molecular-level control of the energetics, surface dipole, surface electronic d

Path-Dependent Morphological Evolution of Se-Te Mesostructures Prepared by Inorganic Phototropic Growth.

Abstract: We describe herein a path-dependent "history" effect wherein the film morphology generated in the second step of a two-step inorganic phototropic growth process depends on a preexisting structure that has been first grown under different optical stimulation conditions. Se-Te generated with static illumination exhibited a highly anisotropic lamellar morphology with a characteristic feature pitch proportional to the input wavelength. Growth using first a short wavelength of light, followed by growth using a longer wavelength, resulted in the second-stage morphology exhibiting termination of lamellae formed during the first growth step. The lamellar pitch at the end of the second growth step was larger than that effected in the first step. In contrast, use of the same input wavelengths but in the opposite order produced no change in the feature pitch but rather only linear feature extension. Analysis of light absorption in simulated structures, in tandem with the empirical data, indicated that the history effect and asymmetric path dependence are a result of emergent nanophotonic processes at the growth interface that dynamically shape the optical field and direct morphological evolution of the photodeposit in a continuous feedback loop.

Increased spatial randomness and disorder of nucleates in dark-phase electrodeposition lead to increased spatial order and pattern fidelity in phototropically grown Se-Te electrodeposits.

Abstract: The role of nucleation was investigated during phototropic growth of Se–Te. Under low levels of mass deposition (mass equivalent of −3.75 mC cm−2 of charge passed) that produced small nucleate spacings, patterns in photoelectrochemically deposited Se–Te films converged at relatively earlier levels of mass deposition and ultimately exhibited higher pattern fidelity throughout pattern development as compared to pattern formation from larger initial nucleate spacings. Consistently, use of an applied striking potential during very early levels of mass deposition produced more spatially random dark-phase electrodeposited nucleates and led to phototropic Se–Te photoelectrodeposited films that exhibited improved pattern fidelity relative to depositions performed with no striking step. Collectively, the data indicate that increases in randomness and spatial disorder of the dispersion of the initial nucleates produces increases in the fidelity and spatial order in the resulting phototropically grown electrodeposits.

Solar fuels generator with ph separation

Abstract: A solar fuels generator includes an anolyte and a catholyte in contact with a separator. The separator is configured such that the pH of the anolyte and the pH of the catholyte are each held at a steady state pH level during operation of the solar fuels generator. The steady state pH level of the anolyte is different from the steady state pH level of the catholyte.

Antimonate electrocatalyst for an electrochemical reaction

Abstract: Disclosed are stable, active non-precious metal oxide catalysts, such as transition metal antimonates (TMAs), for electrochemical reactions in harsh media conditions, such as the chlorine evolution reaction (CER). A disclosed electrocatalyst includes a metal oxide film containing a crystalline transition metal antimonite (TMA). The crystalline TMA may include NiSb2Ox, CoSb2Ox, or MnSb2Ox. The metal oxide film may be formed on a conductive substrate, for example, a substrate including an antimony-doped tin oxide (ATO) film, using an annealing process.