Division of Chemistry and Chemical Engineering, Caltech

NRC Ford Foundation Postdoctoral Fellow

Jesus worked at Division of Chemistry and Chemical Engineering, Caltech as a NRC Ford Foundation Postdoctoral Fellow

NASA Ames Research Center

Intern

Synthesis and Characterization of ZnO nanostructures for piezoelectric applications

Beckman Coulter

Validation Specialist

Designing and reproducing software for medical devices and laboratory equipment

NRC-Ford Foundation Fellowship

Fellow

Jesus worked at NRC-Ford Foundation Fellowship as a Fellow

University at Buffalo (UB)

NASA-Harriett G. Jenkins Fellow

(Full stipend and full remission of tuition and fees)

University at Buffalo

Research Assistant

Jesus worked at University at Buffalo as a Research Assistant

Methods of Photoelectrode Characterization with High Spatial and Temporal Resolution

Energy & Enviromental Science

Methods of Photoelectrode Characterization with High Spatial and Temporal Resolution

Energy & Enviromental Science

Electrochemical surface science twenty years later: Expeditions into the electrocatalysis of reactions at the core of artificial photosynthesis

Surface Science

Surface science research fixated on phenomena and processes that transpire at the electrode-electrolyte interface has been pursued in the past. A considerable proportion of the earlier work was on materials and reactions pertinent to the operation of small-molecule fuel cells. The experimental approach integrated a handful of surface-sensitive physical–analytical methods with traditional electrochemical techniques, all harbored in a single environment-controlled electrochemistry-surface science apparatus (EC-SSA); the catalyst samples were typically precious noble metals constituted of well-defined single-crystal surfaces. More recently, attention has been diverted from fuel-to-energy generation to its converse, (solar) energy-to-fuel transformation; e.g., instead of water synthesis (from hydrogen and oxygen) in fuel cells, water decomposition (to hydrogen and oxygen) in artificial photosynthesis. The rigorous surface-science protocols remain unchanged but the experimental capabilities have been expanded by the addition of several characterization techniques, either as EC-SSA components or as stand-alone instruments. The present manuscript describes 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.

Methods of Photoelectrode Characterization with High Spatial and Temporal Resolution

Energy & Enviromental Science

Electrochemical surface science twenty years later: Expeditions into the electrocatalysis of reactions at the core of artificial photosynthesis

Surface Science

Surface science research fixated on phenomena and processes that transpire at the electrode-electrolyte interface has been pursued in the past. A considerable proportion of the earlier work was on materials and reactions pertinent to the operation of small-molecule fuel cells. The experimental approach integrated a handful of surface-sensitive physical–analytical methods with traditional electrochemical techniques, all harbored in a single environment-controlled electrochemistry-surface science apparatus (EC-SSA); the catalyst samples were typically precious noble metals constituted of well-defined single-crystal surfaces. More recently, attention has been diverted from fuel-to-energy generation to its converse, (solar) energy-to-fuel transformation; e.g., instead of water synthesis (from hydrogen and oxygen) in fuel cells, water decomposition (to hydrogen and oxygen) in artificial photosynthesis. The rigorous surface-science protocols remain unchanged but the experimental capabilities have been expanded by the addition of several characterization techniques, either as EC-SSA components or as stand-alone instruments. The present manuscript describes 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.

Improving O2 production of WO3 photoanodes with IrO2 in acidic aqueous electrolyte

Physical Chemistry Chemical Physics

WO3 is a promising candidate for a photoanode material in an acidic electrolyte, in which it is more stable than most metal oxides, but kinetic limitations combined with the large driving force available in the WO3 valence band for water oxidation make competing reactions such as the oxidation of the acid counterion a more favorable reaction. The incorporation of an oxygen evolving catalyst (OEC) on the WO3 surface can improve the kinetics for water oxidation and increase the branching ratio for O2 production. Ir-based OECs were attached to WO3 photoanodes by a variety of methods including sintering from metal salts, sputtering, drop-casting of particles, and electrodeposition to analyze how attachment strategies can affect photoelectrochemical oxygen production at WO3 photoanodes in 1 M H2SO4. High surface coverage of catalyst on the semiconductor was necessary to ensure that most minority-carrier holes contributed to water oxidation through an active catalyst site rather than a side-reaction through the WO3/electrolyte interface. Sputtering of IrO2 layers on WO3 did not detrimentally affect the energy-conversion behavior of the photoanode and improved the O2 yield at 1.2 V vs. RHE from [similar]0% for bare WO3 to 50–70% for a thin, optically transparent catalyst layer to nearly 100% for thick, opaque catalyst layers. Measurements with a fast one-electron redox couple indicated ohmic behavior at the IrO2/WO3 junction, which provided a shunt pathway for electrocatalytic IrO2 behavior with the WO3 photoanode under reverse bias. Although other OECs were tested, only IrO2 displayed extended stability under the anodic operating conditions in acid as determined by XPS.

Methods of Photoelectrode Characterization with High Spatial and Temporal Resolution

Energy & Enviromental Science

Electrochemical surface science twenty years later: Expeditions into the electrocatalysis of reactions at the core of artificial photosynthesis

Surface Science

Surface science research fixated on phenomena and processes that transpire at the electrode-electrolyte interface has been pursued in the past. A considerable proportion of the earlier work was on materials and reactions pertinent to the operation of small-molecule fuel cells. The experimental approach integrated a handful of surface-sensitive physical–analytical methods with traditional electrochemical techniques, all harbored in a single environment-controlled electrochemistry-surface science apparatus (EC-SSA); the catalyst samples were typically precious noble metals constituted of well-defined single-crystal surfaces. More recently, attention has been diverted from fuel-to-energy generation to its converse, (solar) energy-to-fuel transformation; e.g., instead of water synthesis (from hydrogen and oxygen) in fuel cells, water decomposition (to hydrogen and oxygen) in artificial photosynthesis. The rigorous surface-science protocols remain unchanged but the experimental capabilities have been expanded by the addition of several characterization techniques, either as EC-SSA components or as stand-alone instruments. The present manuscript describes 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.

Improving O2 production of WO3 photoanodes with IrO2 in acidic aqueous electrolyte

Physical Chemistry Chemical Physics

WO3 is a promising candidate for a photoanode material in an acidic electrolyte, in which it is more stable than most metal oxides, but kinetic limitations combined with the large driving force available in the WO3 valence band for water oxidation make competing reactions such as the oxidation of the acid counterion a more favorable reaction. The incorporation of an oxygen evolving catalyst (OEC) on the WO3 surface can improve the kinetics for water oxidation and increase the branching ratio for O2 production. Ir-based OECs were attached to WO3 photoanodes by a variety of methods including sintering from metal salts, sputtering, drop-casting of particles, and electrodeposition to analyze how attachment strategies can affect photoelectrochemical oxygen production at WO3 photoanodes in 1 M H2SO4. High surface coverage of catalyst on the semiconductor was necessary to ensure that most minority-carrier holes contributed to water oxidation through an active catalyst site rather than a side-reaction through the WO3/electrolyte interface. Sputtering of IrO2 layers on WO3 did not detrimentally affect the energy-conversion behavior of the photoanode and improved the O2 yield at 1.2 V vs. RHE from [similar]0% for bare WO3 to 50–70% for a thin, optically transparent catalyst layer to nearly 100% for thick, opaque catalyst layers. Measurements with a fast one-electron redox couple indicated ohmic behavior at the IrO2/WO3 junction, which provided a shunt pathway for electrocatalytic IrO2 behavior with the WO3 photoanode under reverse bias. Although other OECs were tested, only IrO2 displayed extended stability under the anodic operating conditions in acid as determined by XPS.

Finite size effects on the structural progression induced by lithiation of V2O5: A combined diffraction and Raman spectroscopy study

Journal of Materials Chemistry A

Developing an understanding of the structural changes induced during the insertion of Li-ions into the layered framework of nanostructured V2O5 is necessary to unravel the origin of the dramatically increased power densities characteristic of nanostructured electrodes. In this work, we have contrasted the sequence of structural progressions induced within V2O5 micron-sized powders, hydrothermally grown nanowires, and CVD-grown nanoplatelet arrays as a function of chemical lithiation using powder diffraction and Raman spectroscopy. Raman spectroscopy serves as a powerful and highly sensitive probe for investigating the local structure of the lithiated V2O5 phases. We note a profound size dependence of the structural progression with the kinetics of Li-ion uptake following: CVD-grown nanoplatelet arrays ≫ hydrothermally grown nanowires > micron-sized powders. For bulk powders, Raman spectroscopy indicates conversion to the α-phase at 30 s and to the ε-phase at 30 min. The ε-phase continues to grow in spatial extent for the remaining 2 h duration. In contrast, the hydrothermally grown nanowires convert to the α-phase after 30 s and have the ε-phase as the predominant surface species after just 1 min. The CVD grown nanoplatelets show a much accelerated response with the ε-phase nucleated within just 30 s and the Li-rich ε′-phase stabilized after 5 min. After 30 min of lithiation, these nanowires convert to the δ/γ phase and are subsequently irreversibly amorphized after 2 h. Chemical delithiation is seen to result in reversion to the α-phase for bulk and hydrothermally grown nanowire powders for chemical lithiations up to 2 h. In contrast, the unlithiated orthorhombic phase is recovered upon delithiation of the δ/γ-phase nanoplatelet arrays.

Methods of Photoelectrode Characterization with High Spatial and Temporal Resolution

Energy & Enviromental Science

Electrochemical surface science twenty years later: Expeditions into the electrocatalysis of reactions at the core of artificial photosynthesis

Surface Science

Surface science research fixated on phenomena and processes that transpire at the electrode-electrolyte interface has been pursued in the past. A considerable proportion of the earlier work was on materials and reactions pertinent to the operation of small-molecule fuel cells. The experimental approach integrated a handful of surface-sensitive physical–analytical methods with traditional electrochemical techniques, all harbored in a single environment-controlled electrochemistry-surface science apparatus (EC-SSA); the catalyst samples were typically precious noble metals constituted of well-defined single-crystal surfaces. More recently, attention has been diverted from fuel-to-energy generation to its converse, (solar) energy-to-fuel transformation; e.g., instead of water synthesis (from hydrogen and oxygen) in fuel cells, water decomposition (to hydrogen and oxygen) in artificial photosynthesis. The rigorous surface-science protocols remain unchanged but the experimental capabilities have been expanded by the addition of several characterization techniques, either as EC-SSA components or as stand-alone instruments. The present manuscript describes 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.

Improving O2 production of WO3 photoanodes with IrO2 in acidic aqueous electrolyte

Physical Chemistry Chemical Physics

WO3 is a promising candidate for a photoanode material in an acidic electrolyte, in which it is more stable than most metal oxides, but kinetic limitations combined with the large driving force available in the WO3 valence band for water oxidation make competing reactions such as the oxidation of the acid counterion a more favorable reaction. The incorporation of an oxygen evolving catalyst (OEC) on the WO3 surface can improve the kinetics for water oxidation and increase the branching ratio for O2 production. Ir-based OECs were attached to WO3 photoanodes by a variety of methods including sintering from metal salts, sputtering, drop-casting of particles, and electrodeposition to analyze how attachment strategies can affect photoelectrochemical oxygen production at WO3 photoanodes in 1 M H2SO4. High surface coverage of catalyst on the semiconductor was necessary to ensure that most minority-carrier holes contributed to water oxidation through an active catalyst site rather than a side-reaction through the WO3/electrolyte interface. Sputtering of IrO2 layers on WO3 did not detrimentally affect the energy-conversion behavior of the photoanode and improved the O2 yield at 1.2 V vs. RHE from [similar]0% for bare WO3 to 50–70% for a thin, optically transparent catalyst layer to nearly 100% for thick, opaque catalyst layers. Measurements with a fast one-electron redox couple indicated ohmic behavior at the IrO2/WO3 junction, which provided a shunt pathway for electrocatalytic IrO2 behavior with the WO3 photoanode under reverse bias. Although other OECs were tested, only IrO2 displayed extended stability under the anodic operating conditions in acid as determined by XPS.

Finite size effects on the structural progression induced by lithiation of V2O5: A combined diffraction and Raman spectroscopy study

Journal of Materials Chemistry A

Developing an understanding of the structural changes induced during the insertion of Li-ions into the layered framework of nanostructured V2O5 is necessary to unravel the origin of the dramatically increased power densities characteristic of nanostructured electrodes. In this work, we have contrasted the sequence of structural progressions induced within V2O5 micron-sized powders, hydrothermally grown nanowires, and CVD-grown nanoplatelet arrays as a function of chemical lithiation using powder diffraction and Raman spectroscopy. Raman spectroscopy serves as a powerful and highly sensitive probe for investigating the local structure of the lithiated V2O5 phases. We note a profound size dependence of the structural progression with the kinetics of Li-ion uptake following: CVD-grown nanoplatelet arrays ≫ hydrothermally grown nanowires > micron-sized powders. For bulk powders, Raman spectroscopy indicates conversion to the α-phase at 30 s and to the ε-phase at 30 min. The ε-phase continues to grow in spatial extent for the remaining 2 h duration. In contrast, the hydrothermally grown nanowires convert to the α-phase after 30 s and have the ε-phase as the predominant surface species after just 1 min. The CVD grown nanoplatelets show a much accelerated response with the ε-phase nucleated within just 30 s and the Li-rich ε′-phase stabilized after 5 min. After 30 min of lithiation, these nanowires convert to the δ/γ phase and are subsequently irreversibly amorphized after 2 h. Chemical delithiation is seen to result in reversion to the α-phase for bulk and hydrothermally grown nanowire powders for chemical lithiations up to 2 h. In contrast, the unlithiated orthorhombic phase is recovered upon delithiation of the δ/γ-phase nanoplatelet arrays.

A VO-Seeded Approach for the Growth of Star-Shaped VO2 and V2O5 Nanocrystals: Facile Synthesis, Structural Characterization, and Elucidation of Electronic Structure

CrystEngComm, Royal Society of Chemistry

Obtaining shape and size control of strongly correlated materials is imperative to obtain a fundamental understanding of the influence of finite size and surface restructuring on electronic instabilities in the proximity of the Fermi level. We present here a novel synthetic approach that takes advantage of the intrinsic octahedral symmetry of rock-salt-structured VO to facilitate the growth of six-armed nanocrystallites of related, technologically important binary vanadium oxides VO2 and V2O5. The prepared nanostructures exhibit clear six-fold symmetry and most notably show remarkable retention of electronic structure. The latter has been evidenced through extensive X-ray absorption spectroscopy measurements.

Methods of Photoelectrode Characterization with High Spatial and Temporal Resolution

Energy & Enviromental Science

Electrochemical surface science twenty years later: Expeditions into the electrocatalysis of reactions at the core of artificial photosynthesis

Surface Science

Surface science research fixated on phenomena and processes that transpire at the electrode-electrolyte interface has been pursued in the past. A considerable proportion of the earlier work was on materials and reactions pertinent to the operation of small-molecule fuel cells. The experimental approach integrated a handful of surface-sensitive physical–analytical methods with traditional electrochemical techniques, all harbored in a single environment-controlled electrochemistry-surface science apparatus (EC-SSA); the catalyst samples were typically precious noble metals constituted of well-defined single-crystal surfaces. More recently, attention has been diverted from fuel-to-energy generation to its converse, (solar) energy-to-fuel transformation; e.g., instead of water synthesis (from hydrogen and oxygen) in fuel cells, water decomposition (to hydrogen and oxygen) in artificial photosynthesis. The rigorous surface-science protocols remain unchanged but the experimental capabilities have been expanded by the addition of several characterization techniques, either as EC-SSA components or as stand-alone instruments. The present manuscript describes 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.

Improving O2 production of WO3 photoanodes with IrO2 in acidic aqueous electrolyte

Physical Chemistry Chemical Physics

WO3 is a promising candidate for a photoanode material in an acidic electrolyte, in which it is more stable than most metal oxides, but kinetic limitations combined with the large driving force available in the WO3 valence band for water oxidation make competing reactions such as the oxidation of the acid counterion a more favorable reaction. The incorporation of an oxygen evolving catalyst (OEC) on the WO3 surface can improve the kinetics for water oxidation and increase the branching ratio for O2 production. Ir-based OECs were attached to WO3 photoanodes by a variety of methods including sintering from metal salts, sputtering, drop-casting of particles, and electrodeposition to analyze how attachment strategies can affect photoelectrochemical oxygen production at WO3 photoanodes in 1 M H2SO4. High surface coverage of catalyst on the semiconductor was necessary to ensure that most minority-carrier holes contributed to water oxidation through an active catalyst site rather than a side-reaction through the WO3/electrolyte interface. Sputtering of IrO2 layers on WO3 did not detrimentally affect the energy-conversion behavior of the photoanode and improved the O2 yield at 1.2 V vs. RHE from [similar]0% for bare WO3 to 50–70% for a thin, optically transparent catalyst layer to nearly 100% for thick, opaque catalyst layers. Measurements with a fast one-electron redox couple indicated ohmic behavior at the IrO2/WO3 junction, which provided a shunt pathway for electrocatalytic IrO2 behavior with the WO3 photoanode under reverse bias. Although other OECs were tested, only IrO2 displayed extended stability under the anodic operating conditions in acid as determined by XPS.

Finite size effects on the structural progression induced by lithiation of V2O5: A combined diffraction and Raman spectroscopy study

Journal of Materials Chemistry A

Developing an understanding of the structural changes induced during the insertion of Li-ions into the layered framework of nanostructured V2O5 is necessary to unravel the origin of the dramatically increased power densities characteristic of nanostructured electrodes. In this work, we have contrasted the sequence of structural progressions induced within V2O5 micron-sized powders, hydrothermally grown nanowires, and CVD-grown nanoplatelet arrays as a function of chemical lithiation using powder diffraction and Raman spectroscopy. Raman spectroscopy serves as a powerful and highly sensitive probe for investigating the local structure of the lithiated V2O5 phases. We note a profound size dependence of the structural progression with the kinetics of Li-ion uptake following: CVD-grown nanoplatelet arrays ≫ hydrothermally grown nanowires > micron-sized powders. For bulk powders, Raman spectroscopy indicates conversion to the α-phase at 30 s and to the ε-phase at 30 min. The ε-phase continues to grow in spatial extent for the remaining 2 h duration. In contrast, the hydrothermally grown nanowires convert to the α-phase after 30 s and have the ε-phase as the predominant surface species after just 1 min. The CVD grown nanoplatelets show a much accelerated response with the ε-phase nucleated within just 30 s and the Li-rich ε′-phase stabilized after 5 min. After 30 min of lithiation, these nanowires convert to the δ/γ phase and are subsequently irreversibly amorphized after 2 h. Chemical delithiation is seen to result in reversion to the α-phase for bulk and hydrothermally grown nanowire powders for chemical lithiations up to 2 h. In contrast, the unlithiated orthorhombic phase is recovered upon delithiation of the δ/γ-phase nanoplatelet arrays.

A VO-Seeded Approach for the Growth of Star-Shaped VO2 and V2O5 Nanocrystals: Facile Synthesis, Structural Characterization, and Elucidation of Electronic Structure

CrystEngComm, Royal Society of Chemistry

Obtaining shape and size control of strongly correlated materials is imperative to obtain a fundamental understanding of the influence of finite size and surface restructuring on electronic instabilities in the proximity of the Fermi level. We present here a novel synthetic approach that takes advantage of the intrinsic octahedral symmetry of rock-salt-structured VO to facilitate the growth of six-armed nanocrystallites of related, technologically important binary vanadium oxides VO2 and V2O5. The prepared nanostructures exhibit clear six-fold symmetry and most notably show remarkable retention of electronic structure. The latter has been evidenced through extensive X-ray absorption spectroscopy measurements.

Operando Synthesis of Macroporous Molybdenum Diselenide Films for Electrocatalysis of the Hydrogen-Evolution Reaction

ACS Catalysis

The catalytically inactive components of a film have been converted, through an operando method of synthesis, to produce a catalyst for the reaction that the film is catalyzing. Specifically, thin films of molybdenum diselenide have been synthesized using a two-step wet-chemical method, in which excess sodium selenide was first added to a solution of ammonium heptamolydbate in aqueous sulfuric acid, resulting in the spontaneous formation of a black precipitate that contained molybdenum triselenide (MoSe3), molybdenum trioxide (MoO3), and elemental selenium. After purification and after the film had been drop cast onto a glassy carbon electrode, a reductive potential was applied to the precipitate-coated electrode. Hydrogen evolution occurred within the range of potentials applied to the electrode, but during the initial voltammetric cycle, an overpotential of ∼400 mV was required to drive the hydrogen-evolution reaction at a benchmark current density of −10 mA cm–2. The overpotential required to evolve hydrogen at the benchmark rate progressively decreased with subsequent voltammetry cycles, until a steady state was reached at which only ∼250 mV of overpotential was required to pass −10 mA cm–2 of current density. During the electrocatalysis, the catalytically inactive components in the as-prepared film were (reductively) converted to MoSe2 through an operando method of synthesis of the hydrogen-evolution catalyst. The initial film prepared from the precipitate was smooth, but the converted film was completely covered with pores ∼200 nm in diameter. The porous MoSe2 film was stable while being assessed by cyclic voltammetry for 48 h, and the overpotential required to sustain 10 mA cm–2 of hydrogen evolution increased by <50 mV over this period of operation.

Methods of Photoelectrode Characterization with High Spatial and Temporal Resolution

Energy & Enviromental Science

Electrochemical surface science twenty years later: Expeditions into the electrocatalysis of reactions at the core of artificial photosynthesis

Surface Science

Surface science research fixated on phenomena and processes that transpire at the electrode-electrolyte interface has been pursued in the past. A considerable proportion of the earlier work was on materials and reactions pertinent to the operation of small-molecule fuel cells. The experimental approach integrated a handful of surface-sensitive physical–analytical methods with traditional electrochemical techniques, all harbored in a single environment-controlled electrochemistry-surface science apparatus (EC-SSA); the catalyst samples were typically precious noble metals constituted of well-defined single-crystal surfaces. More recently, attention has been diverted from fuel-to-energy generation to its converse, (solar) energy-to-fuel transformation; e.g., instead of water synthesis (from hydrogen and oxygen) in fuel cells, water decomposition (to hydrogen and oxygen) in artificial photosynthesis. The rigorous surface-science protocols remain unchanged but the experimental capabilities have been expanded by the addition of several characterization techniques, either as EC-SSA components or as stand-alone instruments. The present manuscript describes 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.

Improving O2 production of WO3 photoanodes with IrO2 in acidic aqueous electrolyte

Physical Chemistry Chemical Physics

WO3 is a promising candidate for a photoanode material in an acidic electrolyte, in which it is more stable than most metal oxides, but kinetic limitations combined with the large driving force available in the WO3 valence band for water oxidation make competing reactions such as the oxidation of the acid counterion a more favorable reaction. The incorporation of an oxygen evolving catalyst (OEC) on the WO3 surface can improve the kinetics for water oxidation and increase the branching ratio for O2 production. Ir-based OECs were attached to WO3 photoanodes by a variety of methods including sintering from metal salts, sputtering, drop-casting of particles, and electrodeposition to analyze how attachment strategies can affect photoelectrochemical oxygen production at WO3 photoanodes in 1 M H2SO4. High surface coverage of catalyst on the semiconductor was necessary to ensure that most minority-carrier holes contributed to water oxidation through an active catalyst site rather than a side-reaction through the WO3/electrolyte interface. Sputtering of IrO2 layers on WO3 did not detrimentally affect the energy-conversion behavior of the photoanode and improved the O2 yield at 1.2 V vs. RHE from [similar]0% for bare WO3 to 50–70% for a thin, optically transparent catalyst layer to nearly 100% for thick, opaque catalyst layers. Measurements with a fast one-electron redox couple indicated ohmic behavior at the IrO2/WO3 junction, which provided a shunt pathway for electrocatalytic IrO2 behavior with the WO3 photoanode under reverse bias. Although other OECs were tested, only IrO2 displayed extended stability under the anodic operating conditions in acid as determined by XPS.

Finite size effects on the structural progression induced by lithiation of V2O5: A combined diffraction and Raman spectroscopy study

Journal of Materials Chemistry A

Developing an understanding of the structural changes induced during the insertion of Li-ions into the layered framework of nanostructured V2O5 is necessary to unravel the origin of the dramatically increased power densities characteristic of nanostructured electrodes. In this work, we have contrasted the sequence of structural progressions induced within V2O5 micron-sized powders, hydrothermally grown nanowires, and CVD-grown nanoplatelet arrays as a function of chemical lithiation using powder diffraction and Raman spectroscopy. Raman spectroscopy serves as a powerful and highly sensitive probe for investigating the local structure of the lithiated V2O5 phases. We note a profound size dependence of the structural progression with the kinetics of Li-ion uptake following: CVD-grown nanoplatelet arrays ≫ hydrothermally grown nanowires > micron-sized powders. For bulk powders, Raman spectroscopy indicates conversion to the α-phase at 30 s and to the ε-phase at 30 min. The ε-phase continues to grow in spatial extent for the remaining 2 h duration. In contrast, the hydrothermally grown nanowires convert to the α-phase after 30 s and have the ε-phase as the predominant surface species after just 1 min. The CVD grown nanoplatelets show a much accelerated response with the ε-phase nucleated within just 30 s and the Li-rich ε′-phase stabilized after 5 min. After 30 min of lithiation, these nanowires convert to the δ/γ phase and are subsequently irreversibly amorphized after 2 h. Chemical delithiation is seen to result in reversion to the α-phase for bulk and hydrothermally grown nanowire powders for chemical lithiations up to 2 h. In contrast, the unlithiated orthorhombic phase is recovered upon delithiation of the δ/γ-phase nanoplatelet arrays.

A VO-Seeded Approach for the Growth of Star-Shaped VO2 and V2O5 Nanocrystals: Facile Synthesis, Structural Characterization, and Elucidation of Electronic Structure

CrystEngComm, Royal Society of Chemistry

Obtaining shape and size control of strongly correlated materials is imperative to obtain a fundamental understanding of the influence of finite size and surface restructuring on electronic instabilities in the proximity of the Fermi level. We present here a novel synthetic approach that takes advantage of the intrinsic octahedral symmetry of rock-salt-structured VO to facilitate the growth of six-armed nanocrystallites of related, technologically important binary vanadium oxides VO2 and V2O5. The prepared nanostructures exhibit clear six-fold symmetry and most notably show remarkable retention of electronic structure. The latter has been evidenced through extensive X-ray absorption spectroscopy measurements.

Operando Synthesis of Macroporous Molybdenum Diselenide Films for Electrocatalysis of the Hydrogen-Evolution Reaction

ACS Catalysis

The catalytically inactive components of a film have been converted, through an operando method of synthesis, to produce a catalyst for the reaction that the film is catalyzing. Specifically, thin films of molybdenum diselenide have been synthesized using a two-step wet-chemical method, in which excess sodium selenide was first added to a solution of ammonium heptamolydbate in aqueous sulfuric acid, resulting in the spontaneous formation of a black precipitate that contained molybdenum triselenide (MoSe3), molybdenum trioxide (MoO3), and elemental selenium. After purification and after the film had been drop cast onto a glassy carbon electrode, a reductive potential was applied to the precipitate-coated electrode. Hydrogen evolution occurred within the range of potentials applied to the electrode, but during the initial voltammetric cycle, an overpotential of ∼400 mV was required to drive the hydrogen-evolution reaction at a benchmark current density of −10 mA cm–2. The overpotential required to evolve hydrogen at the benchmark rate progressively decreased with subsequent voltammetry cycles, until a steady state was reached at which only ∼250 mV of overpotential was required to pass −10 mA cm–2 of current density. During the electrocatalysis, the catalytically inactive components in the as-prepared film were (reductively) converted to MoSe2 through an operando method of synthesis of the hydrogen-evolution catalyst. The initial film prepared from the precipitate was smooth, but the converted film was completely covered with pores ∼200 nm in diameter. The porous MoSe2 film was stable while being assessed by cyclic voltammetry for 48 h, and the overpotential required to sustain 10 mA cm–2 of hydrogen evolution increased by <50 mV over this period of operation.

Synthesis and hydrogen-evolution activity of tungsten selenide thin films deposited on tungsten foils

Journal of Electroanalytical Chemistry,

Thin films of WSe2 have been deposited onto a conductive substrate (tungsten foil) using a relatively simple chemical-vapor-transport technique. X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, X-ray powder diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy indicated that the films consisted of micron-sized single crystals of WSe2 that were oriented perpendicular to the surface of the tungsten foil substrate. Linear sweep voltammetry was used to assess the ability of the WSe2 films to catalyze the hydrogen-evolution reaction and chronopotentiometry was used to gauge the temporal stability of the catalytic performance of the films under cathodic conditions. A 350 mV overpotential (η) was required to drive the hydrogen-evolution reaction at a current density of −10 mA cm−2 in aqueous 0.5 M H2SO4, representing a significant improvement in catalytic performance relative to the behavior of macroscopic WSe2 single crystals. The WSe2 thin films were relatively stable under catalytic conditions, with the overpotential changing by only ∼10 mV after one hour and exhibiting an additional change of ∼5 mV after another hour of operation.

Methods of Photoelectrode Characterization with High Spatial and Temporal Resolution

Energy & Enviromental Science

Electrochemical surface science twenty years later: Expeditions into the electrocatalysis of reactions at the core of artificial photosynthesis

Surface Science

Surface science research fixated on phenomena and processes that transpire at the electrode-electrolyte interface has been pursued in the past. A considerable proportion of the earlier work was on materials and reactions pertinent to the operation of small-molecule fuel cells. The experimental approach integrated a handful of surface-sensitive physical–analytical methods with traditional electrochemical techniques, all harbored in a single environment-controlled electrochemistry-surface science apparatus (EC-SSA); the catalyst samples were typically precious noble metals constituted of well-defined single-crystal surfaces. More recently, attention has been diverted from fuel-to-energy generation to its converse, (solar) energy-to-fuel transformation; e.g., instead of water synthesis (from hydrogen and oxygen) in fuel cells, water decomposition (to hydrogen and oxygen) in artificial photosynthesis. The rigorous surface-science protocols remain unchanged but the experimental capabilities have been expanded by the addition of several characterization techniques, either as EC-SSA components or as stand-alone instruments. The present manuscript describes 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.

Improving O2 production of WO3 photoanodes with IrO2 in acidic aqueous electrolyte

Physical Chemistry Chemical Physics

WO3 is a promising candidate for a photoanode material in an acidic electrolyte, in which it is more stable than most metal oxides, but kinetic limitations combined with the large driving force available in the WO3 valence band for water oxidation make competing reactions such as the oxidation of the acid counterion a more favorable reaction. The incorporation of an oxygen evolving catalyst (OEC) on the WO3 surface can improve the kinetics for water oxidation and increase the branching ratio for O2 production. Ir-based OECs were attached to WO3 photoanodes by a variety of methods including sintering from metal salts, sputtering, drop-casting of particles, and electrodeposition to analyze how attachment strategies can affect photoelectrochemical oxygen production at WO3 photoanodes in 1 M H2SO4. High surface coverage of catalyst on the semiconductor was necessary to ensure that most minority-carrier holes contributed to water oxidation through an active catalyst site rather than a side-reaction through the WO3/electrolyte interface. Sputtering of IrO2 layers on WO3 did not detrimentally affect the energy-conversion behavior of the photoanode and improved the O2 yield at 1.2 V vs. RHE from [similar]0% for bare WO3 to 50–70% for a thin, optically transparent catalyst layer to nearly 100% for thick, opaque catalyst layers. Measurements with a fast one-electron redox couple indicated ohmic behavior at the IrO2/WO3 junction, which provided a shunt pathway for electrocatalytic IrO2 behavior with the WO3 photoanode under reverse bias. Although other OECs were tested, only IrO2 displayed extended stability under the anodic operating conditions in acid as determined by XPS.

Finite size effects on the structural progression induced by lithiation of V2O5: A combined diffraction and Raman spectroscopy study

Journal of Materials Chemistry A

Developing an understanding of the structural changes induced during the insertion of Li-ions into the layered framework of nanostructured V2O5 is necessary to unravel the origin of the dramatically increased power densities characteristic of nanostructured electrodes. In this work, we have contrasted the sequence of structural progressions induced within V2O5 micron-sized powders, hydrothermally grown nanowires, and CVD-grown nanoplatelet arrays as a function of chemical lithiation using powder diffraction and Raman spectroscopy. Raman spectroscopy serves as a powerful and highly sensitive probe for investigating the local structure of the lithiated V2O5 phases. We note a profound size dependence of the structural progression with the kinetics of Li-ion uptake following: CVD-grown nanoplatelet arrays ≫ hydrothermally grown nanowires > micron-sized powders. For bulk powders, Raman spectroscopy indicates conversion to the α-phase at 30 s and to the ε-phase at 30 min. The ε-phase continues to grow in spatial extent for the remaining 2 h duration. In contrast, the hydrothermally grown nanowires convert to the α-phase after 30 s and have the ε-phase as the predominant surface species after just 1 min. The CVD grown nanoplatelets show a much accelerated response with the ε-phase nucleated within just 30 s and the Li-rich ε′-phase stabilized after 5 min. After 30 min of lithiation, these nanowires convert to the δ/γ phase and are subsequently irreversibly amorphized after 2 h. Chemical delithiation is seen to result in reversion to the α-phase for bulk and hydrothermally grown nanowire powders for chemical lithiations up to 2 h. In contrast, the unlithiated orthorhombic phase is recovered upon delithiation of the δ/γ-phase nanoplatelet arrays.

A VO-Seeded Approach for the Growth of Star-Shaped VO2 and V2O5 Nanocrystals: Facile Synthesis, Structural Characterization, and Elucidation of Electronic Structure

CrystEngComm, Royal Society of Chemistry

Obtaining shape and size control of strongly correlated materials is imperative to obtain a fundamental understanding of the influence of finite size and surface restructuring on electronic instabilities in the proximity of the Fermi level. We present here a novel synthetic approach that takes advantage of the intrinsic octahedral symmetry of rock-salt-structured VO to facilitate the growth of six-armed nanocrystallites of related, technologically important binary vanadium oxides VO2 and V2O5. The prepared nanostructures exhibit clear six-fold symmetry and most notably show remarkable retention of electronic structure. The latter has been evidenced through extensive X-ray absorption spectroscopy measurements.

Operando Synthesis of Macroporous Molybdenum Diselenide Films for Electrocatalysis of the Hydrogen-Evolution Reaction

ACS Catalysis

The catalytically inactive components of a film have been converted, through an operando method of synthesis, to produce a catalyst for the reaction that the film is catalyzing. Specifically, thin films of molybdenum diselenide have been synthesized using a two-step wet-chemical method, in which excess sodium selenide was first added to a solution of ammonium heptamolydbate in aqueous sulfuric acid, resulting in the spontaneous formation of a black precipitate that contained molybdenum triselenide (MoSe3), molybdenum trioxide (MoO3), and elemental selenium. After purification and after the film had been drop cast onto a glassy carbon electrode, a reductive potential was applied to the precipitate-coated electrode. Hydrogen evolution occurred within the range of potentials applied to the electrode, but during the initial voltammetric cycle, an overpotential of ∼400 mV was required to drive the hydrogen-evolution reaction at a benchmark current density of −10 mA cm–2. The overpotential required to evolve hydrogen at the benchmark rate progressively decreased with subsequent voltammetry cycles, until a steady state was reached at which only ∼250 mV of overpotential was required to pass −10 mA cm–2 of current density. During the electrocatalysis, the catalytically inactive components in the as-prepared film were (reductively) converted to MoSe2 through an operando method of synthesis of the hydrogen-evolution catalyst. The initial film prepared from the precipitate was smooth, but the converted film was completely covered with pores ∼200 nm in diameter. The porous MoSe2 film was stable while being assessed by cyclic voltammetry for 48 h, and the overpotential required to sustain 10 mA cm–2 of hydrogen evolution increased by <50 mV over this period of operation.

Synthesis and hydrogen-evolution activity of tungsten selenide thin films deposited on tungsten foils

Journal of Electroanalytical Chemistry,

Thin films of WSe2 have been deposited onto a conductive substrate (tungsten foil) using a relatively simple chemical-vapor-transport technique. X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, X-ray powder diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy indicated that the films consisted of micron-sized single crystals of WSe2 that were oriented perpendicular to the surface of the tungsten foil substrate. Linear sweep voltammetry was used to assess the ability of the WSe2 films to catalyze the hydrogen-evolution reaction and chronopotentiometry was used to gauge the temporal stability of the catalytic performance of the films under cathodic conditions. A 350 mV overpotential (η) was required to drive the hydrogen-evolution reaction at a current density of −10 mA cm−2 in aqueous 0.5 M H2SO4, representing a significant improvement in catalytic performance relative to the behavior of macroscopic WSe2 single crystals. The WSe2 thin films were relatively stable under catalytic conditions, with the overpotential changing by only ∼10 mV after one hour and exhibiting an additional change of ∼5 mV after another hour of operation.

Effective Piezoelectric Response of Substrate-Integrated ZnO Nanowire Array Devices on Galvanized Steel

ACS Applied Materials & Interfaces

Harvesting waste energy through electromechanical coupling in practical devices requires combining device design with the development of synthetic strategies for large-area controlled fabrication of active piezoelectric materials. Here, we show a facile route to the large-area fabrication of ZnO nanostructured arrays using commodity galvanized steel as the Zn precursor as well as the substrate. The ZnO nanowires are further integrated within a device construct and the effective piezoelectric response is deduced based on a novel experimental approach involving induction of stress in the nanowires through pressure wave propagation along with phase-selective lock-in detection of the induced current. The robust methodology for measurement of the effective piezoelectric coefficient developed here allows for interrogation of piezoelectric functionality for the entire substrate under bending-type deformation of the ZnO nanowires.

Methods of Photoelectrode Characterization with High Spatial and Temporal Resolution

Energy & Enviromental Science

Electrochemical surface science twenty years later: Expeditions into the electrocatalysis of reactions at the core of artificial photosynthesis

Surface Science

Surface science research fixated on phenomena and processes that transpire at the electrode-electrolyte interface has been pursued in the past. A considerable proportion of the earlier work was on materials and reactions pertinent to the operation of small-molecule fuel cells. The experimental approach integrated a handful of surface-sensitive physical–analytical methods with traditional electrochemical techniques, all harbored in a single environment-controlled electrochemistry-surface science apparatus (EC-SSA); the catalyst samples were typically precious noble metals constituted of well-defined single-crystal surfaces. More recently, attention has been diverted from fuel-to-energy generation to its converse, (solar) energy-to-fuel transformation; e.g., instead of water synthesis (from hydrogen and oxygen) in fuel cells, water decomposition (to hydrogen and oxygen) in artificial photosynthesis. The rigorous surface-science protocols remain unchanged but the experimental capabilities have been expanded by the addition of several characterization techniques, either as EC-SSA components or as stand-alone instruments. The present manuscript describes 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.

Improving O2 production of WO3 photoanodes with IrO2 in acidic aqueous electrolyte

Physical Chemistry Chemical Physics

WO3 is a promising candidate for a photoanode material in an acidic electrolyte, in which it is more stable than most metal oxides, but kinetic limitations combined with the large driving force available in the WO3 valence band for water oxidation make competing reactions such as the oxidation of the acid counterion a more favorable reaction. The incorporation of an oxygen evolving catalyst (OEC) on the WO3 surface can improve the kinetics for water oxidation and increase the branching ratio for O2 production. Ir-based OECs were attached to WO3 photoanodes by a variety of methods including sintering from metal salts, sputtering, drop-casting of particles, and electrodeposition to analyze how attachment strategies can affect photoelectrochemical oxygen production at WO3 photoanodes in 1 M H2SO4. High surface coverage of catalyst on the semiconductor was necessary to ensure that most minority-carrier holes contributed to water oxidation through an active catalyst site rather than a side-reaction through the WO3/electrolyte interface. Sputtering of IrO2 layers on WO3 did not detrimentally affect the energy-conversion behavior of the photoanode and improved the O2 yield at 1.2 V vs. RHE from [similar]0% for bare WO3 to 50–70% for a thin, optically transparent catalyst layer to nearly 100% for thick, opaque catalyst layers. Measurements with a fast one-electron redox couple indicated ohmic behavior at the IrO2/WO3 junction, which provided a shunt pathway for electrocatalytic IrO2 behavior with the WO3 photoanode under reverse bias. Although other OECs were tested, only IrO2 displayed extended stability under the anodic operating conditions in acid as determined by XPS.

Finite size effects on the structural progression induced by lithiation of V2O5: A combined diffraction and Raman spectroscopy study

Journal of Materials Chemistry A

Developing an understanding of the structural changes induced during the insertion of Li-ions into the layered framework of nanostructured V2O5 is necessary to unravel the origin of the dramatically increased power densities characteristic of nanostructured electrodes. In this work, we have contrasted the sequence of structural progressions induced within V2O5 micron-sized powders, hydrothermally grown nanowires, and CVD-grown nanoplatelet arrays as a function of chemical lithiation using powder diffraction and Raman spectroscopy. Raman spectroscopy serves as a powerful and highly sensitive probe for investigating the local structure of the lithiated V2O5 phases. We note a profound size dependence of the structural progression with the kinetics of Li-ion uptake following: CVD-grown nanoplatelet arrays ≫ hydrothermally grown nanowires > micron-sized powders. For bulk powders, Raman spectroscopy indicates conversion to the α-phase at 30 s and to the ε-phase at 30 min. The ε-phase continues to grow in spatial extent for the remaining 2 h duration. In contrast, the hydrothermally grown nanowires convert to the α-phase after 30 s and have the ε-phase as the predominant surface species after just 1 min. The CVD grown nanoplatelets show a much accelerated response with the ε-phase nucleated within just 30 s and the Li-rich ε′-phase stabilized after 5 min. After 30 min of lithiation, these nanowires convert to the δ/γ phase and are subsequently irreversibly amorphized after 2 h. Chemical delithiation is seen to result in reversion to the α-phase for bulk and hydrothermally grown nanowire powders for chemical lithiations up to 2 h. In contrast, the unlithiated orthorhombic phase is recovered upon delithiation of the δ/γ-phase nanoplatelet arrays.

A VO-Seeded Approach for the Growth of Star-Shaped VO2 and V2O5 Nanocrystals: Facile Synthesis, Structural Characterization, and Elucidation of Electronic Structure

CrystEngComm, Royal Society of Chemistry

Obtaining shape and size control of strongly correlated materials is imperative to obtain a fundamental understanding of the influence of finite size and surface restructuring on electronic instabilities in the proximity of the Fermi level. We present here a novel synthetic approach that takes advantage of the intrinsic octahedral symmetry of rock-salt-structured VO to facilitate the growth of six-armed nanocrystallites of related, technologically important binary vanadium oxides VO2 and V2O5. The prepared nanostructures exhibit clear six-fold symmetry and most notably show remarkable retention of electronic structure. The latter has been evidenced through extensive X-ray absorption spectroscopy measurements.

Operando Synthesis of Macroporous Molybdenum Diselenide Films for Electrocatalysis of the Hydrogen-Evolution Reaction

ACS Catalysis

The catalytically inactive components of a film have been converted, through an operando method of synthesis, to produce a catalyst for the reaction that the film is catalyzing. Specifically, thin films of molybdenum diselenide have been synthesized using a two-step wet-chemical method, in which excess sodium selenide was first added to a solution of ammonium heptamolydbate in aqueous sulfuric acid, resulting in the spontaneous formation of a black precipitate that contained molybdenum triselenide (MoSe3), molybdenum trioxide (MoO3), and elemental selenium. After purification and after the film had been drop cast onto a glassy carbon electrode, a reductive potential was applied to the precipitate-coated electrode. Hydrogen evolution occurred within the range of potentials applied to the electrode, but during the initial voltammetric cycle, an overpotential of ∼400 mV was required to drive the hydrogen-evolution reaction at a benchmark current density of −10 mA cm–2. The overpotential required to evolve hydrogen at the benchmark rate progressively decreased with subsequent voltammetry cycles, until a steady state was reached at which only ∼250 mV of overpotential was required to pass −10 mA cm–2 of current density. During the electrocatalysis, the catalytically inactive components in the as-prepared film were (reductively) converted to MoSe2 through an operando method of synthesis of the hydrogen-evolution catalyst. The initial film prepared from the precipitate was smooth, but the converted film was completely covered with pores ∼200 nm in diameter. The porous MoSe2 film was stable while being assessed by cyclic voltammetry for 48 h, and the overpotential required to sustain 10 mA cm–2 of hydrogen evolution increased by <50 mV over this period of operation.

Synthesis and hydrogen-evolution activity of tungsten selenide thin films deposited on tungsten foils

Journal of Electroanalytical Chemistry,

Thin films of WSe2 have been deposited onto a conductive substrate (tungsten foil) using a relatively simple chemical-vapor-transport technique. X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, X-ray powder diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy indicated that the films consisted of micron-sized single crystals of WSe2 that were oriented perpendicular to the surface of the tungsten foil substrate. Linear sweep voltammetry was used to assess the ability of the WSe2 films to catalyze the hydrogen-evolution reaction and chronopotentiometry was used to gauge the temporal stability of the catalytic performance of the films under cathodic conditions. A 350 mV overpotential (η) was required to drive the hydrogen-evolution reaction at a current density of −10 mA cm−2 in aqueous 0.5 M H2SO4, representing a significant improvement in catalytic performance relative to the behavior of macroscopic WSe2 single crystals. The WSe2 thin films were relatively stable under catalytic conditions, with the overpotential changing by only ∼10 mV after one hour and exhibiting an additional change of ∼5 mV after another hour of operation.

Effective Piezoelectric Response of Substrate-Integrated ZnO Nanowire Array Devices on Galvanized Steel

ACS Applied Materials & Interfaces

Harvesting waste energy through electromechanical coupling in practical devices requires combining device design with the development of synthetic strategies for large-area controlled fabrication of active piezoelectric materials. Here, we show a facile route to the large-area fabrication of ZnO nanostructured arrays using commodity galvanized steel as the Zn precursor as well as the substrate. The ZnO nanowires are further integrated within a device construct and the effective piezoelectric response is deduced based on a novel experimental approach involving induction of stress in the nanowires through pressure wave propagation along with phase-selective lock-in detection of the induced current. The robust methodology for measurement of the effective piezoelectric coefficient developed here allows for interrogation of piezoelectric functionality for the entire substrate under bending-type deformation of the ZnO nanowires.

A Scanning Probe Investigation of the Role of Surface Motifs in the Behavior of p-WSe2 Photocathodes

Energy & Environmental Science

The spatial variation in the photoelectrochemical performance of bare and Pt-decorated p-type WSe2 photocathodes has been investigated by scanning photocurrent microscopy (SPCM). The measurements revealed significant differences in the charge-collection performance (quantified by the values of external quantum yields, Φext) on various macrosopic terraces. This dissimilarity in the quality of the terraces rather than the effects of the macro-scale step edges dominated the charge-collection efficiency of the samples. Local topographic and spectral response measurements indicated that the local electronic structures of terraces was not uniform between terraces and that low-quality terraces (Φext < 0.15) had many more sub-band-gap states than in high-quality terraces. Hence, the photoelectrochemical performance of bulk single crystals of two-dimensional layered p-WSe2 photocathodes is dictated predominantly by the photoactivity of terraces and not by the charge collection properties of the macro-scale edge sites, as it has been widely understood formerly.

Methods of Photoelectrode Characterization with High Spatial and Temporal Resolution

Energy & Enviromental Science

Electrochemical surface science twenty years later: Expeditions into the electrocatalysis of reactions at the core of artificial photosynthesis

Surface Science

Surface science research fixated on phenomena and processes that transpire at the electrode-electrolyte interface has been pursued in the past. A considerable proportion of the earlier work was on materials and reactions pertinent to the operation of small-molecule fuel cells. The experimental approach integrated a handful of surface-sensitive physical–analytical methods with traditional electrochemical techniques, all harbored in a single environment-controlled electrochemistry-surface science apparatus (EC-SSA); the catalyst samples were typically precious noble metals constituted of well-defined single-crystal surfaces. More recently, attention has been diverted from fuel-to-energy generation to its converse, (solar) energy-to-fuel transformation; e.g., instead of water synthesis (from hydrogen and oxygen) in fuel cells, water decomposition (to hydrogen and oxygen) in artificial photosynthesis. The rigorous surface-science protocols remain unchanged but the experimental capabilities have been expanded by the addition of several characterization techniques, either as EC-SSA components or as stand-alone instruments. The present manuscript describes 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.

Improving O2 production of WO3 photoanodes with IrO2 in acidic aqueous electrolyte

Physical Chemistry Chemical Physics

WO3 is a promising candidate for a photoanode material in an acidic electrolyte, in which it is more stable than most metal oxides, but kinetic limitations combined with the large driving force available in the WO3 valence band for water oxidation make competing reactions such as the oxidation of the acid counterion a more favorable reaction. The incorporation of an oxygen evolving catalyst (OEC) on the WO3 surface can improve the kinetics for water oxidation and increase the branching ratio for O2 production. Ir-based OECs were attached to WO3 photoanodes by a variety of methods including sintering from metal salts, sputtering, drop-casting of particles, and electrodeposition to analyze how attachment strategies can affect photoelectrochemical oxygen production at WO3 photoanodes in 1 M H2SO4. High surface coverage of catalyst on the semiconductor was necessary to ensure that most minority-carrier holes contributed to water oxidation through an active catalyst site rather than a side-reaction through the WO3/electrolyte interface. Sputtering of IrO2 layers on WO3 did not detrimentally affect the energy-conversion behavior of the photoanode and improved the O2 yield at 1.2 V vs. RHE from [similar]0% for bare WO3 to 50–70% for a thin, optically transparent catalyst layer to nearly 100% for thick, opaque catalyst layers. Measurements with a fast one-electron redox couple indicated ohmic behavior at the IrO2/WO3 junction, which provided a shunt pathway for electrocatalytic IrO2 behavior with the WO3 photoanode under reverse bias. Although other OECs were tested, only IrO2 displayed extended stability under the anodic operating conditions in acid as determined by XPS.

Finite size effects on the structural progression induced by lithiation of V2O5: A combined diffraction and Raman spectroscopy study

Journal of Materials Chemistry A

Developing an understanding of the structural changes induced during the insertion of Li-ions into the layered framework of nanostructured V2O5 is necessary to unravel the origin of the dramatically increased power densities characteristic of nanostructured electrodes. In this work, we have contrasted the sequence of structural progressions induced within V2O5 micron-sized powders, hydrothermally grown nanowires, and CVD-grown nanoplatelet arrays as a function of chemical lithiation using powder diffraction and Raman spectroscopy. Raman spectroscopy serves as a powerful and highly sensitive probe for investigating the local structure of the lithiated V2O5 phases. We note a profound size dependence of the structural progression with the kinetics of Li-ion uptake following: CVD-grown nanoplatelet arrays ≫ hydrothermally grown nanowires > micron-sized powders. For bulk powders, Raman spectroscopy indicates conversion to the α-phase at 30 s and to the ε-phase at 30 min. The ε-phase continues to grow in spatial extent for the remaining 2 h duration. In contrast, the hydrothermally grown nanowires convert to the α-phase after 30 s and have the ε-phase as the predominant surface species after just 1 min. The CVD grown nanoplatelets show a much accelerated response with the ε-phase nucleated within just 30 s and the Li-rich ε′-phase stabilized after 5 min. After 30 min of lithiation, these nanowires convert to the δ/γ phase and are subsequently irreversibly amorphized after 2 h. Chemical delithiation is seen to result in reversion to the α-phase for bulk and hydrothermally grown nanowire powders for chemical lithiations up to 2 h. In contrast, the unlithiated orthorhombic phase is recovered upon delithiation of the δ/γ-phase nanoplatelet arrays.

A VO-Seeded Approach for the Growth of Star-Shaped VO2 and V2O5 Nanocrystals: Facile Synthesis, Structural Characterization, and Elucidation of Electronic Structure

CrystEngComm, Royal Society of Chemistry

Obtaining shape and size control of strongly correlated materials is imperative to obtain a fundamental understanding of the influence of finite size and surface restructuring on electronic instabilities in the proximity of the Fermi level. We present here a novel synthetic approach that takes advantage of the intrinsic octahedral symmetry of rock-salt-structured VO to facilitate the growth of six-armed nanocrystallites of related, technologically important binary vanadium oxides VO2 and V2O5. The prepared nanostructures exhibit clear six-fold symmetry and most notably show remarkable retention of electronic structure. The latter has been evidenced through extensive X-ray absorption spectroscopy measurements.

Operando Synthesis of Macroporous Molybdenum Diselenide Films for Electrocatalysis of the Hydrogen-Evolution Reaction

ACS Catalysis

The catalytically inactive components of a film have been converted, through an operando method of synthesis, to produce a catalyst for the reaction that the film is catalyzing. Specifically, thin films of molybdenum diselenide have been synthesized using a two-step wet-chemical method, in which excess sodium selenide was first added to a solution of ammonium heptamolydbate in aqueous sulfuric acid, resulting in the spontaneous formation of a black precipitate that contained molybdenum triselenide (MoSe3), molybdenum trioxide (MoO3), and elemental selenium. After purification and after the film had been drop cast onto a glassy carbon electrode, a reductive potential was applied to the precipitate-coated electrode. Hydrogen evolution occurred within the range of potentials applied to the electrode, but during the initial voltammetric cycle, an overpotential of ∼400 mV was required to drive the hydrogen-evolution reaction at a benchmark current density of −10 mA cm–2. The overpotential required to evolve hydrogen at the benchmark rate progressively decreased with subsequent voltammetry cycles, until a steady state was reached at which only ∼250 mV of overpotential was required to pass −10 mA cm–2 of current density. During the electrocatalysis, the catalytically inactive components in the as-prepared film were (reductively) converted to MoSe2 through an operando method of synthesis of the hydrogen-evolution catalyst. The initial film prepared from the precipitate was smooth, but the converted film was completely covered with pores ∼200 nm in diameter. The porous MoSe2 film was stable while being assessed by cyclic voltammetry for 48 h, and the overpotential required to sustain 10 mA cm–2 of hydrogen evolution increased by <50 mV over this period of operation.

Synthesis and hydrogen-evolution activity of tungsten selenide thin films deposited on tungsten foils

Journal of Electroanalytical Chemistry,

Thin films of WSe2 have been deposited onto a conductive substrate (tungsten foil) using a relatively simple chemical-vapor-transport technique. X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, X-ray powder diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy indicated that the films consisted of micron-sized single crystals of WSe2 that were oriented perpendicular to the surface of the tungsten foil substrate. Linear sweep voltammetry was used to assess the ability of the WSe2 films to catalyze the hydrogen-evolution reaction and chronopotentiometry was used to gauge the temporal stability of the catalytic performance of the films under cathodic conditions. A 350 mV overpotential (η) was required to drive the hydrogen-evolution reaction at a current density of −10 mA cm−2 in aqueous 0.5 M H2SO4, representing a significant improvement in catalytic performance relative to the behavior of macroscopic WSe2 single crystals. The WSe2 thin films were relatively stable under catalytic conditions, with the overpotential changing by only ∼10 mV after one hour and exhibiting an additional change of ∼5 mV after another hour of operation.

Effective Piezoelectric Response of Substrate-Integrated ZnO Nanowire Array Devices on Galvanized Steel

ACS Applied Materials & Interfaces

Harvesting waste energy through electromechanical coupling in practical devices requires combining device design with the development of synthetic strategies for large-area controlled fabrication of active piezoelectric materials. Here, we show a facile route to the large-area fabrication of ZnO nanostructured arrays using commodity galvanized steel as the Zn precursor as well as the substrate. The ZnO nanowires are further integrated within a device construct and the effective piezoelectric response is deduced based on a novel experimental approach involving induction of stress in the nanowires through pressure wave propagation along with phase-selective lock-in detection of the induced current. The robust methodology for measurement of the effective piezoelectric coefficient developed here allows for interrogation of piezoelectric functionality for the entire substrate under bending-type deformation of the ZnO nanowires.

A Scanning Probe Investigation of the Role of Surface Motifs in the Behavior of p-WSe2 Photocathodes

Energy & Environmental Science

The spatial variation in the photoelectrochemical performance of bare and Pt-decorated p-type WSe2 photocathodes has been investigated by scanning photocurrent microscopy (SPCM). The measurements revealed significant differences in the charge-collection performance (quantified by the values of external quantum yields, Φext) on various macrosopic terraces. This dissimilarity in the quality of the terraces rather than the effects of the macro-scale step edges dominated the charge-collection efficiency of the samples. Local topographic and spectral response measurements indicated that the local electronic structures of terraces was not uniform between terraces and that low-quality terraces (Φext < 0.15) had many more sub-band-gap states than in high-quality terraces. Hence, the photoelectrochemical performance of bulk single crystals of two-dimensional layered p-WSe2 photocathodes is dictated predominantly by the photoactivity of terraces and not by the charge collection properties of the macro-scale edge sites, as it has been widely understood formerly.

A Substrate Integrated and Scalable Templated Approach Based on Rusted Steel for the Fabrication of Polypyrrole Nanotube Arrays

Applied Materials and Interfaces, American Chemical Society

We report here a facile, generalizable, and entirely scalable approach for the fabrication of vertically aligned arrays of Fe2O3/polypyrrole core—shell nanostructures and polypyrrole nanotubes. Our ―all electrochemical‖ approach is based on the fabrication of R-Fe2O3 nanowire arrays by the simple heat treatment of commodity low carbon steel substrates, followed by electropolymerization of conformal polypyrrole sheaths around the nanowires. Subsequently, electrochemical etching of the nanowires yields large-area vertically aligned polypyrrole nanotube arrays on the steel substrate. The developed methodology is generalizable to functionalized pyrrole monomers and represents a significant practical advance of relevance to the technological implementation of conjugated polymer nanostructures in electrochromics, electrochemical energy storage, and sensing.