B18-Core EXAFS
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Caiwu
Liang
,
Reshma R.
Rao
,
Katrine L.
Svane
,
Joseph H. L.
Hadden
,
Benjamin
Moss
,
Soren B.
Scott
,
Michael
Sachs
,
James
Murawski
,
Adrian Malthe
Frandsen
,
D. Jason
Riley
,
Mary P.
Ryan
,
Jan
Rossmeisl
,
James R.
Durrant
,
Ifan E. L.
Stephens
Diamond Proposal Number(s):
[30396]
Open Access
Abstract: Understanding what controls the reaction rate on iridium-based catalysts is central to designing better electrocatalysts for the water oxidation reaction in proton exchange membrane electrolysers. Here we quantify the densities of redox-active centres and probe their binding strengths on amorphous IrOx and rutile IrO2 using operando time-resolved optical spectroscopy. We establish a quantitative experimental correlation between the intrinsic reaction rate and the active-state energetics. We find that adsorbed oxygen species, *O, formed at water oxidation potentials, exhibit repulsive adsorbate–adsorbate interactions. Increasing their coverage weakens their binding, thereby promoting O–O bond formation, which is the rate-determining step. These analyses suggest that although amorphous IrOx exhibits a higher geometric current density, the intrinsic reaction rates per active state on IrOx and IrO2 are comparable at given potentials. Finally, we present a modified volcano plot that elucidates how the intrinsic water oxidation kinetics can be increased by optimizing both the binding energy and the interaction strength between the catalytically active states.
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Jun 2024
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I18-Microfocus Spectroscopy
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Camilo A.
Mesa
,
Michael
Sachs
,
Ernest
Pastor
,
Nicolas
Gauriot
,
Alice J.
Merryweather
,
Miguel A.
Gomez-Gonzalez
,
Konstantin
Ignatyev
,
Sixto
Gimenez
,
Akshay
Rao
,
James R.
Durrant
,
Raj
Pandya
Diamond Proposal Number(s):
[30381]
Open Access
Abstract: Photo(electro)catalysts use sunlight to drive chemical reactions such as water splitting. A major factor limiting photocatalyst development is physicochemical heterogeneity which leads to spatially dependent reactivity. To link structure and function in such systems, simultaneous probing of the electrochemical environment at microscopic length scales and a broad range of timescales (ns to s) is required. Here, we address this challenge by developing and applying in-situ (optical) microscopies to map and correlate local electrochemical activity, with hole lifetimes, oxygen vacancy concentrations and photoelectrode crystal structure. Using this multi-modal approach, we study prototypical hematite (α-Fe2O3) photoelectrodes. We demonstrate that regions of α-Fe2O3, adjacent to microstructural cracks have a better photoelectrochemical response and reduced back electron recombination due to an optimal oxygen vacancy concentration, with the film thickness and extended light exposure also influencing local activity. Our work highlights the importance of microscopic mapping to understand activity, in even seemingly homogeneous photoelectrodes.
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May 2024
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B18-Core EXAFS
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Benjamin
Moss
,
Katrine L.
Svane
,
David
Nieto-Castro
,
Reshma R.
Rao
,
Soren B.
Scott
,
Cindy
Tseng
,
Michael
Sachs
,
Anuj
Pennathur
,
Caiwu
Liang
,
Louise I.
Oldham
,
Eva
Mazzolini
,
Lole
Jurado
,
Gopinathan
Sankar
,
Stephen
Parry
,
Veronica
Celorrio
,
Jahan M.
Dawlaty
,
Jan
Rossmeisl
,
Jose Ramon
Galán-Mascarós
,
Ifan E. L.
Stephens
,
James R.
Durrant
Diamond Proposal Number(s):
[30663]
Open Access
Abstract: A barrier to understanding the factors driving catalysis in the oxygen evolution reaction (OER) is understanding multiple overlapping redox transitions in the OER catalysts. The complexity of these transitions obscure the relationship between the coverage of adsorbates and OER kinetics, leading to an experimental challenge in measuring activity descriptors, such as binding energies, as well as adsorbate interactions, which may destabilize intermediates and modulate their binding energies. Herein, we utilize a newly designed optical spectroelectrochemistry system to measure these phenomena in order to contrast the behavior of two electrocatalysts, cobalt oxyhydroxide (CoOOH) and cobalt–iron hexacyanoferrate (cobalt–iron Prussian blue, CoFe-PB). Three distinct optical spectra are observed in each catalyst, corresponding to three separate redox transitions, the last of which we show to be active for the OER using time-resolved spectroscopy and electrochemical mass spectroscopy. By combining predictions from density functional theory with parameters obtained from electroadsorption isotherms, we demonstrate that a destabilization of catalytic intermediates occurs with increasing coverage. In CoOOH, a strong (∼0.34 eV/monolayer) destabilization of a strongly bound catalytic intermediate is observed, leading to a potential offset between the accumulation of the intermediate and measurable O2 evolution. We contrast these data to CoFe-PB, where catalytic intermediate generation and O2 evolution onset coincide due to weaker binding and destabilization (∼0.19 eV/monolayer). By considering a correlation between activation energy and binding strength, we suggest that such adsorbate driven destabilization may account for a significant fraction of the observed OER catalytic activity in both materials. Finally, we disentangle the effects of adsorbate interactions on state coverages and kinetics to show how adsorbate interactions determine the observed Tafel slopes. Crucially, the case of CoFe-PB shows that, even where interactions are weaker, adsorption remains non-Nernstian, which strongly influences the observed Tafel slope.
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Mar 2024
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B18-Core EXAFS
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Diamond Proposal Number(s):
[30396]
Open Access
Abstract: Understanding the effect of noncovalent interactions of intermediates at the polarized catalyst–electrolyte interface on water oxidation kinetics is key for designing more active and stable electrocatalysts. Here, we combine operando optical spectroscopy, X-ray absorption spectroscopy (XAS), and surface-enhanced infrared absorption spectroscopy (SEIRAS) to probe the effect of noncovalent interactions on the oxygen evolution reaction (OER) activity of IrOx in acidic and alkaline electrolytes. Our results suggest that the active species for the OER (Ir4.x+–*O) binds much stronger in alkaline compared with acid at low coverage, while the repulsive interactions between these species are higher in alkaline electrolytes. These differences are attributed to the larger fraction of water within the cation hydration shell at the interface in alkaline electrolytes compared to acidic electrolytes, which can stabilize oxygenated intermediates and facilitate long-range interactions between them. Quantitative analysis of the state energetics shows that although the *O intermediates bind more strongly than optimal in alkaline electrolytes, the larger repulsive interaction between them results in a significant weakening of *O binding with increasing coverage, leading to similar energetics of active states in acid and alkaline at OER-relevant potentials. By directly probing the electrochemical interface with complementary spectroscopic techniques, our work goes beyond conventional computational descriptors of the OER activity to explain the experimentally observed OER kinetics of IrOx in acidic and alkaline electrolytes.
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Mar 2024
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E01-JEM ARM 200CF
I20-EDE-Energy Dispersive EXAFS (EDE)
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Hui
Luo
,
Victor Y.
Yukuhiro
,
Pablo S.
Fernández
,
Jingyu
Feng
,
Paul
Thompson
,
Reshma R.
Rao
,
Rongsheng
Cai
,
Silvia
Favero
,
Sarah J.
Haigh
,
James R.
Durrant
,
Ifan E. L.
Stephens
,
Maria-Magdalena
Titirici
Diamond Proposal Number(s):
[28663, 25476]
Open Access
Abstract: Pt-based bimetallic electrocatalysts are promising candidates to convert surplus glycerol from the biodiesel industry to value-added chemicals and coproduce hydrogen. It is expected that the nature and content of the elements in the bimetallic catalyst can not only affect the reaction kinetics but also influence the product selectivity, providing a way to increase the yield of the desired products. Hence, in this work, we investigate the electrochemical oxidation of glycerol on a series of PtNi nanoparticles with increasing Ni content using a combination of physicochemical structural analysis, electrochemical measurements, operando spectroscopic techniques, and advanced product characterizations. With a moderate Ni content and a homogenously alloyed bimetallic Pt–Ni structure, the PtNi2 catalyst displayed the highest reaction activity among all materials studied in this work. In situ FTIR data show that PtNi2 can activate the glycerol molecule at a more negative potential (0.4 VRHE) than the other PtNi catalysts. In addition, its surface can effectively catalyze the complete C–C bond cleavage, resulting in lower CO poisoning and higher stability. Operando X-ray absorption spectroscopy and UV–vis spectroscopy suggest that glycerol adsorbs strongly onto surface Ni(OH)x sites, preventing their oxidation and activation of oxygen or hydroxyl from water. As such, we propose that the role of Ni in PtNi toward glycerol oxidation is to tailor the electronic structure of the pure Pt sites rather than a bifunctional mechanism. Our experiments provide guidance for the development of bimetallic catalysts toward highly efficient, selective, and stable glycerol oxidation reactions.
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Nov 2022
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I19-Small Molecule Single Crystal Diffraction
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Catherine M.
Aitchison
,
Michael
Sachs
,
Marc A.
Little
,
Liam
Wilbraham
,
Nick J.
Brownbill
,
Christopher M.
Kane
,
Frédéric
Blanc
,
Martijn
Zwijnenburg
,
James R.
Durrant
,
Reiner S.
Sprick
,
Andrew I.
Cooper
Diamond Proposal Number(s):
[21726]
Open Access
Abstract: So far, most organic semiconductor photocatalysts for solar fuels production have been linear polymers or polymeric networks with a broad distribution of molecular weights. Here, we study a series of molecular dibenzo[b,d]thiophene sulfone and fluorene oligomers as well-defined model systems to probe the relationship between photocatalytic activity and structural features such as chain length and planarity. The hydrogen evolution rate was found to vary significantly with bridge head atom, chain length, and backbone twisting. A trimer (S3) of only three repeat units has excellent activity for proton reduction with an EQE of 8.8% at 420 nm, approaching the activity of its polymer analogue and demonstrating that high molar masses are not a prerequisite for good activity. The dynamics of long-lived electrons generated under illumination in the S3 oligomer are also very similar to the corresponding polymer, both under transient and constant irradiation conditions.
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Jul 2020
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E01-JEM ARM 200CF
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Hui
Luo
,
Nikolaos
Papaioannou
,
Enrico
Salvadori
,
Maxie
Roessler
,
Gereon
Ploenes
,
Ernst R. H.
Van Eck
,
Liviu
Tanase
,
Jingyu
Feng
,
Yiwei
Sun
,
Yan
Yang
,
Mohsen
Danaie
,
Ana
Jorge Sobrido
,
Andrei
Sapelkin
,
James
Durrant
,
Stoichko D.
Dimitrov
,
Maria-Magdalena
Titirici
Diamond Proposal Number(s):
[17587]
Abstract: As a new class of sustainable carbon material, the term “carbon dots” represents an “umbrella term” as there are many types of materials included. We employ a broad range of techniques to develop understanding on hydrothermally synthesized carbon dots and show how fine tuning the structural features using simple reduction/oxidation reactions can drastically affect their excited state properties. Structural and spectroscopic studies found that photoluminescence originates from direct excitation of localized fluorophores involving oxygen functional groups, while the excitation at graphene‐like features leads to ultrafast phonon‐assisted relaxation and largely quenches the fluorescent quantum yields. This is arguably the first to identify the dynamics of photoluminescence including Stokes’ shift formation, allowing us to fully resolve the relaxation pathways in these carbon dots. The comprehensive investigation sheds light on how understanding the excited state relaxation processes in different carbon structure is crucial for tuning the optical properties for any potential commercial applications.
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Aug 2019
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I09-Surface and Interface Structural Analysis
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Carlos
Sotelo-Vazquez
,
Raul
Quesada-Cabrera
,
Min
Ling
,
David O.
Scanlon
,
Andreas
Kafizas
,
Pardeep
Kumar Thakur
,
Tien-Lin
Lee
,
Alaric
Taylor
,
Graeme W.
Watson
,
Robert G.
Palgrave
,
James R.
Durrant
,
Christopher S.
Blackman
,
Ivan P.
Parkin
Abstract: Semiconductor heterojunctions are used in a wide range of applications including catalysis, sensors, and solar-to-chemical energy conversion devices. These materials can spatially separate photogenerated charge across the heterojunction boundary, inhibiting recombination processes and synergistically enhancing their performance beyond the individual components. In this work, the WO3/TiO2 heterojunction grown by chemical vapor deposition is investigated. This consists of a highly nanostructured WO3 layer of vertically aligned nanorods that is then coated with a conformal layer of TiO2. This heterojunction shows an unusual electron transfer process, where photogenerated electrons move from the WO3 layer into TiO2. State-of-the-art hybrid density functional theory and hard X-ray photoelectron spectroscopy are used to elucidate the electronic interaction at the WO3/TiO2 interface. Transient absorption spectroscopy shows that recombination is substantially reduced, extending both the lifetime and population of photogenerated charges into timescales relevant to most photocatalytic processes. This increases the photocatalytic efficiency of the material, which is among the highest ever reported for a thin film. In allying computational and experimental methods, this is believed to be an ideal strategy for determining the band alignment in metal oxide heterojunction systems.
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Mar 2017
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Abstract: Multifunctional P-doped TiO2 thin films were synthesized by atmospheric pressure chemical vapor deposition (APCVD). This is the first example of P-doped TiO2 films with both P5+ and P3– states, with the relative proportion being determined by synthesis conditions. This technique to control the oxidation state of the impurities presents a new approach to achieve films with both self-cleaning and TCO properties. The origin of electrical conductivity in these materials was correlated to the incorporation of P5+ species, as suggested by Hall Effect probe measurements. The photocatalytic performance of the films was investigated using the model organic pollutant, stearic acid, with films containing predominately P3– states found to be vastly inferior photocatalysts compared to undoped TiO2 films. Transient absorption spectroscopy studies also showed that charge carrier concentrations increased by several orders of magnitude in films containing P5+ species only, whereas photogenerated carrier lifetimes—and thus photocatalytic activity—were severely reduced upon incorporation of P3– species. The results presented here provide important insights on the influence of dopant nature and location within a semiconductor structure. These new P-doped TiO2 films are a breakthrough in the development of multifunctional advanced materials with tuned properties for a wide range of applications.
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May 2015
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