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Water oxidation is the key kinetic bottleneck of photoelectrochemical devices for fuel synthesis. Despite advances in the identification of intermediates, elucidating the catalytic mechanism of this multi-redox reaction on metal–oxide photoanodes remains a significant experimental and theoretical challenge. Here, we report an experimental analysis of water oxidation kinetics on four widely studied metal oxides, focusing particularly on haematite. We observe that haematite is able to access a reaction mechanism that is third order in surface-hole density, which is assigned to equilibration between three surface holes and M(OH)–O–M(OH) sites. This reaction exhibits low activation energy (Ea ≈ 60 meV). Density functional theory is used to determine the energetics of charge accumulation and O–O bond formation on a model haematite (110) surface. The proposed mechanism shows parallels with the function of the oxygen evolving complex of photosystem II, and provides new insights into the mechanism of heterogeneous water oxidation on a metal oxide surface.

Hematite (α-Fe 2 O 3 ) is the most studied artificial oxygen-evolving photo-anode and yet its efficiency limitations and their origin remain unknown. A sub-picosecond reorganisation of the hematite structure has been proposed as the mechanism which dictates carrier lifetimes, energetics and the ultimate conversion yields. However, the importance of this reorganisation for actual device performance is unclear. Here we report an in situ observation of charge carrier self-localisation in a hematite device, and demonstrate that this process affects recombination losses in photoelectrochemical cells. We apply an ultrafast, device-based optical-control method to resolve the subpicosecond formation of small polarons and estimate their reorganisation energy to be ~0.5 eV. Coherent oscillations in the photocurrent signals indicate that polaron formation may be coupled to specific phonon modes (<100 cm −1 ). Our results bring together spectroscopic and device characterisation approaches to reveal new photophysics of broadly-studied hematite devices. The efficiency of Hematite (α-Fe 2 O 3 ) photo-anodes is thought to be limited by ultrafast lattice distortions or polarons. Here, we use an optical-control method with photocurrent detection to track small polarons in real time and demonstrate that they impact photoelectrochemical cell activity

Ni/Fe oxyhydroxides are the best performing Earth-abundant electrocatalysts for water oxidation. However, the origin of their remarkable performance is not well understood. Herein, we employ spectroelectrochemical techniques to analyse the kinetics of water oxidation on a series of Ni/Fe oxyhydroxide films: FeOOH, FeOOHNiOOH, and Ni(Fe)OOH (5% Fe). The concentrations and reaction rates of the oxidised states accumulated during catalysis are determined. Ni(Fe)OOH is found to exhibit the fastest reaction kinetics but accumulates fewer states, resulting in a similar performance to FeOOHNiOOH. The later catalytic onset in FeOOH is attributed to an anodic shift in the accumulation of oxidised states. Rate law analyses reveal that the rate limiting step for each catalyst involves the accumulation of four oxidised states, Ni-centred for Ni(Fe)OOH but Fe-centred for FeOOH and FeOOHNiOOH. We conclude by highlighting the importance of equilibria between these accumulated species and reactive intermediates in determining the activity of these materials. Multimetallic oxyhydroxides are highly active electrocatalysts for water oxidation but their mechanism and the role of each metal is poorly understood. Here, authors use spectroelectrochemical techniques to probe the species accumulated during catalysis in Ni/Fe oxyhydroxide films.

Multi-electron heterogeneous catalysis is a pivotal element in the (photo)electrochemical generation of solar fuels. However, mechanistic studies of these systems are difficult to elucidate by means of electrochemical methods alone. Here we report a spectroelectrochemical analysis of hydrogen evolution on ruthenium oxide employed as an electrocatalyst and as part of a cuprous oxide-based photocathode. We use optical absorbance spectroscopy to quantify the densities of reduced ruthenium oxide species, and correlate these with current densities resulting from proton reduction. This enables us to compare directly the catalytic function of dark and light electrodes. We find that hydrogen evolution is second order in the density of active, doubly reduced species independent of whether these are generated by applied potential or light irradiation. Our observation of a second order rate law allows us to distinguish between the most common reaction paths and propose a mechanism involving the homolytic reductive elimination of hydrogen. Understanding reaction mechanisms in heterogeneous (photo)electrochemical catalysts is key to improving solar-to-fuel conversion efficiencies. Here the authors compare the mechanism of hydrogen evolution on ruthenium oxide as an electrocatalyst and as part of a photocathode via an optical/electrochemical approach.

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-Fe 2 O 3 ) photoelectrodes. We demonstrate that regions of α-Fe 2 O 3 , 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. Physicochemical heterogeneity poses a significant constraint in photocatalyst advancement. Here the authors introduce a multimodal optical microscopy platform to assess activity and defects concurrently in photoelectrocatalysts, revealing that disorder can unexpectedly enhance local photoelectrocatalytic performance in certain instances.

We demonstrate a passive all-chalcogenide all-optical perceptron scheme. The network’s nonlinear activation function (NLAF) relies on the nonlinear response of Ge Sb Te to femtosecond laser pulses. We measured the sub-picosecond time-resolved optical constants of Ge Sb Te at a wavelength of 1500 nm and used them to design a high-speed Ge Sb Te -tuned microring resonator all-optical NLAF. The NLAF had a sigmoidal response when subjected to different laser fluence excitation and had a dynamic range of −9.7 dB. The perceptron’s waveguide material was AlN because it allowed efficient heat dissipation during laser switching. A two-temperature analysis revealed that the operating speed of the NLAF is ns. The percepton’s nonvolatile weights were set using low-loss Sb -tuned Mach Zehnder interferometers (MZIs). A three-layer deep neural network model was used to test the feasibility of the network scheme and a maximum training accuracy of 94.5% was obtained. We conclude that combining Sb -programmed MZI weights with the nonlinear response of Ge Sb Te to femtosecond pulses is sufficient to perform energy-efficient all-optical neural classifications at rates greater than 1 GHz.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.An amendment to this paper has been published and can be accessed via a link at the top of the paper.

A deep understanding of defects is essential for the optimization of materials for solar energy conversion. This is particularly true for metal oxide photo(electro)catalysts, which typically feature high concentrations of charged point defects that are electronically active. In photovoltaic materials, except for selected dopants, defects are considered detrimental and should be eliminated to minimize charge recombination. However, photocatalysis is a more complex process in which defects can have an active role, such as in stabilizing charge separation and in mediating rate-limiting catalytic steps. In this Review, we examine the behaviour of electronic defects in metal oxides, paying special attention to the principles that underpin the formation and function of trapped charges in the form of polarons. We focus on how defects alter the electronic structure of metal oxides, statically or transiently upon illumination, and discuss the implications of such changes in light-driven catalytic reactions. Finally, we compare oxide defect chemistry with that of new photocatalysts based on carbon nitrides, polymers and metal halide perovskites. Defects have a key role in determining the functionality of solids and can make them powerful catalysts. This Review examines defect chemistry in metal oxides and discusses the role that charged defects and polarons have in enabling photoelectrochemical reactions.

Inspired by the period-four oscillation in flash-induced oxygen evolution of photosystem II discovered by Joliot in 1969, Kok performed additional experiments and proposed a five-state kinetic model for photosynthetic oxygen evolution, known as Kok’s S-state clock or cycle 1 , 2 . The model comprises four (meta)stable intermediates (S 0 , S 1 , S 2 and S 3 ) and one transient S 4 state, which precedes dioxygen formation occurring in a concerted reaction from two water-derived oxygens bound at an oxo-bridged tetra manganese calcium (Mn 4 CaO 5 ) cluster in the oxygen-evolving complex 3 – 7 . This reaction is coupled to the two-step reduction and protonation of the mobile plastoquinone Q B at the acceptor side of PSII. Here, using serial femtosecond X-ray crystallography and simultaneous X-ray emission spectroscopy with multi-flash visible laser excitation at room temperature, we visualize all (meta)stable states of Kok’s cycle as high-resolution structures (2.04–2.08 Å). In addition, we report structures of two transient states at 150 and 400 µs, revealing notable structural changes including the binding of one additional ‘water’, Ox, during the S 2 →S 3 state transition. Our results suggest that one water ligand to calcium (W3) is directly involved in substrate delivery. The binding of the additional oxygen Ox in the S 3 state between Ca and Mn1 supports O–O bond formation mechanisms involving O5 as one substrate, where Ox is either the other substrate oxygen or is perfectly positioned to refill the O5 position during O 2 release. Thus, our results exclude peroxo-bond formation in the S 3 state, and the nucleophilic attack of W3 onto W2 is unlikely. Crystallography and spectroscopy are used to solve high-resolution structures of the intermediates of Kok’s S-state clock in photosystem II.

In the prevalent picture of ultrafast structural phase transitions, atomic motion occurs in a slowly varying potential energy surface adiabatically determined by fast electrons. However, this ignores non-conservative forces caused by electron–lattice collisions, which can substantially influence atomic motion. Most ultrafast techniques only probe the average structure and are less sensitive to random displacements and therefore do not detect the role played by non-conservative forces in phase transitions. Here we show that the lattice dynamics of the prototypical insulator–metal transition of vanadium dioxide cannot be described by potential energy alone. We use the sample temperature to control the preexisting lattice disorder before ultrafast photoexcitation across the phase transition and our ultrafast diffuse scattering experiments show that the fluctuations characteristic of rutile metal develop equally fast (120 fs) at initial temperatures of 100 and 300 K. This indicates that additional non-conservative forces are responsible for the increased lattice disorder. These results highlight the need for more sophisticated descriptions of ultrafast phenomena beyond the Born–Oppenheimer approximation as well as ultrafast probes of spatial fluctuations beyond the average unit cell measured by diffraction.The Born–Oppenheimer approximation is the prevailing assumption for interpreting ultrafast electron dynamics in solids. Evidence now suggests that collisions between electrons and lattice not captured by this approximation play an important role.