Unraveling Charge Transfer in CoFe Prussian Blue Modified BiVO4 Photoanodes

Catalyst modification of metal oxide photoanodes can result in markedly improved water oxidation efficiency. However, the reasons for improvement are often subtle and controversial. Upon depositing a CoFe Prussian blue (CoFe-PB) water oxidation catalyst on BiVO4, a large photocurrent increase and onset potential shift (up to 0.8 V) are observed, resulting in a substantially more efficient system with high stability. To elucidate the origin of this enhancement, we used time-resolved spectroscopies to compare the dynamics of photogenerated holes in modified and unmodified BiVO4 films. Even in the absence of strong positive bias, a fast (pre-ms), largely irreversible hole transfer from BiVO4 to CoFe-PB is observed. This process retards recombination, enabling holes to accumulate in the catalyst. Holes in CoFe-PB remain reactive, oxidizing water at a similar rate to holes in pristine BiVO4. CoFe-PB therefore enhances performance by presenting a favorable interface for efficient hole transfer, combined with the catalytic function necessary to drive water oxidation

Impact of Oxygen Vacancy Occupancy on Charge Carrier Dynamics in BiVO4 Photoanodes

Oxygen vacancies are ubiquitous in metal oxides and critical to performance, yet the impact of these states upon charge carrier dynamics important for photoelectrochemical and photocatalytic applications remains contentious and poorly understood. A key challenge is the unambiguous identification of spectroscopic fingerprints which can be used to track their function. Herein, we employ five complementary techniques to modulate the electronic occupancy of states associated with oxygen vacancies in situ in BiVO4 photoanodes, allowing us to identify a spectral signature for the ionization of these states. We obtain an activation energy of ∼0.2 eV for this ionization process, with thermally activated electron detrapping from these states determining the kinetics of electron extraction, consistent with improved photoelectrochemical performance at higher temperatures. Bulk, un-ionized states, however, function as deep hole traps, with such trapped holes energetically unable to drive water oxidation. These observations help address recent controversies in the literature regarding oxygen vacancy function, providing new insights into their impact upon photoelectrochemical performance

Correction to "Tracking Charge Transfer to Residual Metal Clusters in Conjugated Polymers for Photocatalytic Hydrogen Evolution"

Tracking charge transfer to residual metal clusters in conjugated polymers for photocatalytic hydrogen evolution (Journal of the American Chemical Society (2020) 142:34 (14574-14587) DOI: 10.1021/jacs.0c06104) Page 14585. Appreciation for Dr. Yan-Gu Lin was inadvertently left out of the Acknowledgments. The scientific part of the paper remains unchanged. The complete correct Acknowledgments paragraph is as follows: ¦ ACKNOWLEDGMENTS M.S. is grateful to Imperial College for a President’s Ph.D. Scholarship and to the EPSRC for a Doctoral Prize Fellowship. J.R.D. and I.M. acknowledge support from KAUST (project numbers OSR-2015-CRG4-2572 and OSR-2018-CRG7- 3749.2). C.M.A., A.I.C., and R.S.S. acknowledge the Engineering and Physical Sciences Research Council (EPSRC, EP/ N004884/1). L.F. thanks the EU for a Marie Curie fellowship (658270). S.C. thanks Imperial College London for a Schro¨dinger Scholarship. R.G. is grateful to the FRQNT for a postdoctoral award and NSERC Discovery Grant funding. C.-L.C. appreciates his supervisor, Dr. Yan-Gu Lin, for his efforts on the beamtime support of XAS beamline and corresponding equipment/technical setup. All plotted data have been deposited on the open-access repository Zenodo and can be accessed via dx.doi.org/10.5281/zenodo.3932340

Correction to "Tracking Charge Transfer to Residual Metal Clusters in Conjugated Polymers for Photocatalytic Hydrogen Evolution"

Tracking charge transfer to residual metal clusters in conjugated polymers for photocatalytic hydrogen evolution (Journal of the American Chemical Society (2020) 142:34 (14574-14587) DOI: 10.1021/jacs.0c06104) Page 14585. Appreciation for Dr. Yan-Gu Lin was inadvertently left out of the Acknowledgments. The scientific part of the paper remains unchanged. The complete correct Acknowledgments paragraph is as follows: ¦ ACKNOWLEDGMENTS M.S. is grateful to Imperial College for a President’s Ph.D. Scholarship and to the EPSRC for a Doctoral Prize Fellowship. J.R.D. and I.M. acknowledge support from KAUST (project numbers OSR-2015-CRG4-2572 and OSR-2018-CRG7- 3749.2). C.M.A., A.I.C., and R.S.S. acknowledge the Engineering and Physical Sciences Research Council (EPSRC, EP/ N004884/1). L.F. thanks the EU for a Marie Curie fellowship (658270). S.C. thanks Imperial College London for a Schro¨dinger Scholarship. R.G. is grateful to the FRQNT for a postdoctoral award and NSERC Discovery Grant funding. C.-L.C. appreciates his supervisor, Dr. Yan-Gu Lin, for his efforts on the beamtime support of XAS beamline and corresponding equipment/technical setup. All plotted data have been deposited on the open-access repository Zenodo and can be accessed via dx.doi.org/10.5281/zenodo.3932340

Transient optical studies of metal oxides for water oxidation

As the next energy crisis looms threateningly before us and we teeter on the edge of the irreparable and undeniably catastrophic loss of global ecosystems as a result of man-made climate change, the quest for alternative fuels is of paramount importance. Solar water splitting is an active area of research which seeks to produce renewable hydrogen fuel from two abundant resources: water and sunlight. However, it is the formation molecular oxygen, the necessary by-product of water splitting, that presents the major chemical challenge, both in terms of kinetics and thermodynamics. As such, high-performing, stable and Earth-abundant materials for water oxidation are highly sought after. The focus of this thesis is the understanding of two such materials. While Chapter 1 gives a more detailed overview of the current environmental situation, the challenges in global energy and fuel supply, and the field of artificial photosynthesis, Chapter 2 introduces transition metal oxides, the workhorses of the water oxidation reaction. The frequently studied oxides are discussed and particular focus is given to current kinetic understanding and performance limitations. The methods applied in this thesis are then detailed in Chapter 3. This body of research concerns the spectroscopic analysis of two metal oxide systems for water oxidation: tungsten trioxide (WO3) and mixed nickel-iron oxides, which become oxyhydroxides during catalysis (Nix Fey OOH). WO3, a visible and ultraviolet light absorber, has been studied as a photoanode for photoelectrochemical water oxidation from the first conceptualisation of this reaction in the 1970s. Despite many years of intensive research, much regarding the mechanism and factors limiting the performance of this robust and relatively abundant oxide remain unclear and are frequently debated. Chapter 4 sets the stage for WO3 within the growing field of water oxidation, with direct comparison to other metal oxides. The kinetics of water oxidation and electron extraction are examined, revealing some unexpected trends. The timescale of water oxidation is found to be remarkably fast, t50% < 1 ms, while electron extraction is limited by trap-mediated transport. In Chapter 5, I delve deeper into this complex material to understand the role of the most common intrinsic defects to transition metal oxides: oxygen vacancies. This chapter begins by probing the initial charge separation of photogenerated carriers on ultrafast timescales, through which I uncover that electrons trap into defect states on pre-picosecond timescales. I then go on to examine the effects of altering band-bending before investigating samples with different oxygen content to deduce the importance of the resultant defect states generated. The space-charge layer was found to boost the attainable concentration of surface holes from ultrafast timescales, while an intermediate concentration of oxygen vacancies was deemed vital to adequately separate photogenerated charges. I conclude by highlighting the wider significance that defect control has across all timescales monitored, from picoseconds to seconds, and emphasise this as a means to the betterment of existing photoanodes for water oxidation. In the final results chapter, Chapter 6, I examine a different approach to water oxidation. This chapter explores Ni/Fe oxyhydroxides; dark catalytic materials that can be used as co-catalysts in conjunction with a photoanode (such as WO3) or employed independently for dark electrolysis using renewably-generated electricity. This chapter presents spectroelectrochemical analyses and examines the rate law for water oxidation on these materials. In particular, the relationship between nickel and iron (the latter an often unintended dopant of the former) is examined, with the aim of unearthing the origin of the synergistic benefit observed when both metals are present. I find that the reactive intermediates accumulated under catalytic conditions are nickel centred at low iron concentrations, but become iron-centred at greater Fe:Ni ratios. However, the rate order with respect to these species is four in each case, suggesting a similar catalytic mechanism between all samples examined. In Chapter 7, I conclude by summarising this body of work and discuss the impact it may have on the next steps in water oxidation research. Finally, I give my insights into the role that transitional metal oxides may have in the future of solar energy conversion.Open Acces

Determining the Role of Oxygen Vacancies in the Photocatalytic Performance of WO3 for Water Oxidation

Oxygen vacancies are common to most metal oxides, whether intentionally incorporated or otherwise, and the study of these defects is of increasing interest for solar water splitting. In this work, we examine nanostructured WO3 photoanodes of varying oxygen content to determine how the concentration of bulk oxygen-vacancy states affects the photocatalytic performance for water oxidation. Using transient optical spectroscopy, we follow the charge carrier recombination kinetics in these samples, from picoseconds to seconds, and examine how differing oxygen vacancy concentrations impact upon these kinetics. We find that samples with an intermediate concentration of vacancies (~2% of oxygen atoms) afford the greatest photoinduced charge carrier densities, and the slowest recombination kinetics across all timescales studied. This increased yield of photogenerated charges correlates with improved photocurrent densities under simulated sunlight, with both greater and lesser oxygen vacancy concentrations resulting in enhanced recombination losses and poorer J-V performances. Our conclusion, that an optimal – neither too high nor too low – concentration of oxygen vacancies is required for optimum photoelectrochemical performance, is discussed in terms of the impact of these defects on charge separation and transport, as well as the implications held for other highly doped materials for photoelectrochemical water oxidation

C–H activation and nucleophilic substitution in a photochemically generated high valent iron complex

The photochemical oxidation of a (TAML)FeIII complex 1 using visible light generated Ru(bpy)33+ produces valence tautomers (TAML)FeIV (1+) and (TAML˙+)FeIII (1-TAML˙+), depending on the exogenous anions. The presence of labile Cl− or Br− results in a ligand-based oxidation and stabilisation of a radical-cationic (TAML˙+)FeIII complex, which subsequently leads to unprecedented C–H activation followed by nucleophilic substitution on the TAML aryl ring. In contrast, exogenous cyanide culminates in metal-based oxidation, yielding the first example of a crystallographically characterised S = 1 [(TAML)FeIV(CN)2]2− species. This is a rare report of an anion-dependent valence tautomerisation in photochemically accessed high valent (TAML)Fe systems with potential applications in the oxidation of pollutants, hydrocarbons, and water. Furthermore, the nucleophilic aromatic halogenation reaction mediated by (TAML˙+)FeIII represents a novel domain for high-valent metal reactivity and highlights the possible intramolecular ligand or substrate modification pathways under highly oxidising conditions. Our findings therefore shine light on high-valent metal oxidants based on TAMLs and other potential non-innocent ligands and open new avenues for oxidation catalyst design.ASTAR (Agency for Sci., Tech. and Research, S’pore)MOE (Min. of Education, S’pore)Published versio

Water Oxidation Kinetics of Nanoporous BiVO4 Photoanodes Functionalised with Nickel/iron Oxyhydroxide Electrocatalysts

In this work, spectroelectrochemical techniques are employed to analyse the catalytic water oxidation performance of a series of three nickel/iron oxyhydroxide electrocatalysts deposited on FTO and BiVO4, at neutral pH. Similar electrochemical water oxidation performance is observed for each of the FeOOH, Ni(Fe)OOH and FeOOHNiOOH electrocatalysts studied, which is found to result from a balance between degree of charge accumulation and rate of water oxidation. Once added onto BiVO4 photoanodes, a large enhancement in the water oxidation photoelectrochemical performance is observed in comparison to the un-modified BiVO4. To understand the origin of this enhancement, the films were evaluated through time-resolved optical spectroscopic techniques, allowing comparisons between electrochemical and photoelectrochemical water oxidation. For all three catalysts, fast hole transfer from BiVO4 to the catalyst is observed in the transient absorption data. Using operando photoinduced absorption measurements, we find that water oxidation is driven by oxidised states within the catalyst layer, following hole transfer from BiVO4. This charge transfer is correlated with a suppression of recombination losses which result in remarkably enhanced water oxidation performance relative to un-modified BiVO4. Moreover, despite similar electrocatalytic performance of all three electrocatalysts, we show that variations in water oxidation performance observed among the BiVO4/MOOH photoanodes stem from differences in photoelectrochemical and electrochemical charge accumulation in the catalyst layers. Under illumination, the amount of accumulated charge in the catalyst is driven by the injection of photogenerated holes from BiVO4, which is further affected by the recombination loss at the BiVO4/MOOH interface, and thus leads to deviations from their behaviour as standalone electrocatalysts

Water oxidation kinetics of nanoporous BiVO4 photoanodes functionalised with nickel/iron oxyhydroxide electrocatalysts

In this work, spectroelectrochemical techniques are employed to analyse the catalytic water oxidation performance of a series of three nickel/iron oxyhydroxide electrocatalysts deposited on FTO and BiVO4, at neutral pH. Similar electrochemical water oxidation performance is observed for each of the FeOOH, Ni(Fe)OOH and FeOOHNiOOH electrocatalysts studied, which is found to result from a balance between degree of charge accumulation and rate of water oxidation. Once added onto BiVO4 photoanodes, a large enhancement in the water oxidation photoelectrochemical performance is observed in comparison to the un-modified BiVO4. To understand the origin of this enhancement, the films were evaluated through time-resolved optical spectroscopic techniques, allowing comparisons between electrochemical and photoelectrochemical water oxidation. For all three catalysts, fast hole transfer from BiVO4 to the catalyst is observed in the transient absorption data. Using operando photoinduced absorption measurements, we find that water oxidation is driven by oxidised states within the catalyst layer, following hole transfer from BiVO4. This charge transfer is correlated with a suppression of recombination losses which result in remarkably enhanced water oxidation performance relative to un-modified BiVO4. Moreover, despite similar electrocatalytic behaviour of all three electrocatalysts, we show that variations in water oxidation performance observed among the BiVO4/MOOH photoanodes stem from differences in photoelectrochemical and electrochemical charge accumulation in the catalyst layers. Under illumination, the amount of accumulated charge in the catalyst is driven by the injection of photogenerated holes from BiVO4, which is further affected by the recombination loss at the BiVO4/MOOH interface, and thus leads to deviations from their behaviour as standalone electrocatalysts