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We report a facile method to enhance the photoelectrochemical (PEC) performance of Sb 2 Se 3 photocathodes by controlling the growth of bilayer Sb 2 Se 3 consisting of vertically oriented nanorods on a compact Sb 2 Se 3 layer. Sb 2 Se 3 thin films with controllable nanorod diameters were achieved by manipulating the substrate temperatures during metallic Sb thin film deposition. The lower temperature-derived Sb 2 Se 3 photocathode, with a larger nanorod diameter (202 ± 48 nm), demonstrated a photocurrent density of -15.2 mA cm -2 at 0 V RHE and an onset potential of 0.21 V RHE . In contrast, the higher temperature-derived Sb 2 Se 3 photocathode, with a smaller nanorod diameter (124 ± 28 nm), exhibited an improved photocurrent density of -22.1 mA cm -2 at 0 V RHE and an onset potential of 0.31 V RHE . The enhanced PEC performance is attributed to reduced charge recombination, facilitated by a shorter charge transport path in the [ hk 0] direction. This study highlights the significance of morphology control in optimizing Sb 2 Se 3 photocathodes, providing insights for future material and device design., Competing Interests: The authors declare no competing financial interests., (This journal is © The Royal Society of Chemistry.)

Molecularly-modified electrode materials that maintain stability over a broad pH range are rare. Typically, each electrochemical transformation necessitates a specifically tuned system to achieve strong binding and high activity of the catalyst. Here, we report the functionalisation of mesoporous indium tin oxide (mITO) electrodes with the macrocyclic host molecule pillar[6]arene (PA[6]). These electrodes are stable within the pH range of 2.4-10.8 and can be equipped with electrochemically active ruthenium complexes through host-guest interactions to perform various oxidation reactions. Benzyl alcohol oxidation serves as a model reaction in acidic media, while ammonia oxidation is conducted to assess the systems performance under basic conditions. PA[6]-modified electrodes demonstrate catalytic activity for both reactions when complexed to different guest molecules and can be reused by reabsorption of the catalyst after its degradation. Furthermore, the system can be employed to perform subsequent reactions in electrolyte with varying pH, enabling the same electrode to be utilised in multiple different electrocatalytic reactions., (© 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

The use of amorphous TiO 2 (a-TiO 2 ), deposited by atomic layer deposition, is a common strategy to protect semiconductors from degradation when used in water-splitting photoelectrochemical (PEC) cells. Electrochemical impedance spectroscopy (EIS) is a suitable technique to study these PEC cells because it is capable of deconvoluting multiple processes occurring during operation, therefore providing information about mechanisms leading to the overall device performance. When biased under hydrogen evolution conditions, EIS shows that two simultaneous processes occur in a-TiO 2 -protected photocathodes, which introduces an ambiguity in choosing the correct equivalent circuit to describe the operating device. In this report, a model p-Si|a-TiO 2 |Pt photocathode system is used to show that the Maxwell circuit best describes the operating mechanism, as opposed to the more commonly used Voight and nested circuits. This indicates that, under hydrogen evolution conditions, both faradaic and nonfaradaic processes are occurring. Whereas the faradaic process corresponds to the hydrogen evolution reaction itself, the nonfaradaic process is traced to the p-Si|a-TiO 2 interface.

The use of conductive and corrosion-resistant protective layers represents a key strategy for improving the durability of light absorber materials in photoelectrochemical water splitting. For high performance photoanodes such as Si, GaAs, and GaP, amorphous TiO 2 protective overlayers, deposited by atomic layer deposition, are conductive for holes via a defect band in the TiO 2 . However, when coated on simply prepared, low-cost photoanodes such as metal oxides, no charge transfer is observed through amorphous TiO 2 . Here, we report a hybrid polyethyleneimine/TiO 2 layer that facilitates hole transfer from model oxides BiVO 4 and Fe 2 O 3 , enabling access to a broader scope of available materials for practical water oxidation. A thin polyethyleneimine layer between the light absorber and the hybrid polyethyleneimine/TiO 2 acts as a hole-selective interface, improving the optoelectronic properties of the photoanode devices. These polyethyleneimine/TiO 2 modified photoanodes exhibit high photostability for solar water oxidation over 400 h., (© 2024. The Author(s).)

Molybdenum sulfide serves as an effective nonprecious metal catalyst for hydrogen evolution, primarily active at edge sites with unsaturated molybdenum sites or terminal disulfides. To improve the activity at a low loading density, two molybdenum sulfide clusters, [Mo 3 S 4 ] 4+ and [Mo 3 S 13 ] 2- , were investigated. The Mo 3 S x molecular catalysts were heterogenized on Sb 2 Se 3 with a simple soaking treatment, resulting in a thin catalyst layer of only a few nanometers that gave up to 20 mA cm -2 under one sun illumination. Both [Mo 3 S 4 ] 4+ and [Mo 3 S 13 ] 2- exhibit catalytic activities on Sb 2 Se 3 , with [Mo 3 S 13 ] 2- emerging as the superior catalyst, demonstrating enhanced photovoltage and an average faradaic efficiency of 100% for hydrogen evolution. This superiority is attributed to the effective loading and higher catalytic activity of [Mo 3 S 13 ] 2- on the Sb 2 Se 3 surface, validated by X-ray photoelectron and Raman spectroscopy., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Antimony selenide (Sb 2 Se 3 ) has recently been intensively investigated and has achieved significant advancement in photoelectrochemical (PEC) water splitting. In this work, a facile one-step hydrothermal method for the preparation of Sn-doped Sb 2 Se 3 photocathodes with improved PEC performance was investigated. We present an in-depth study of the performance enhancement in Sn-doped Sb 2 Se 3 photocathodes using capacitance-voltage (CV), drive-level capacitance profiling (DLCP), and electrochemical impedance spectroscopy (EIS) techniques. The incorporation of Sn 2+ into the Sb 2 Se 3 results in increased carrier density, reduced surface defects, and improved charge separation, thereby leading to improved PEC performance. With a thin Sb 2 Se 3 absorber layer (270 nm thickness), the Sn-doped Sb 2 Se 3 photocathode exhibits an improved photocurrent density of 17.1 mA cm -2 at 0 V versus RHE ( V RHE ) compared to that of the undoped Sb 2 Se 3 photocathode (14.4 mA cm -2 ). This work not only highlights the positive influence of Sn doping on Sb 2 Se 3 photocathodes but also showcases a one-step method to synthesize doped Sb 2 Se 3 with improved optoelectronic properties., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Abundantly available biomass-based platform chemicals, including 5-hydroxymethylfurfural (HMF), are essential stepping stones in steering the chemical industry away from fossil fuels. The efficient catalytic oxidation of HMF to its diacid derivative, 2,5-furandicarboxylic acid (FDCA), is a promising research area with potential applications in the polymer industry. Currently, the most encouraging approaches are based on solid-state catalysts and are often conducted in basic aqueous media, conditions where HMF oxidation competes with its decomposition. Efficient molecular catalysts are practically unknown for this reaction. In this study, we report on the synthesis and electrocatalysis of surface-bound molecular ruthenium complexes for the transformation of HMF to FDCA under acidic conditions. Catalyst immobilisation on mesoporous indium tin oxide electrodes is achieved through the incorporation of phosphonic acid anchoring groups. Screening experiments with HMF and further reaction intermediates revealed the catalytic route and bottlenecks in the catalytic synthesis of FDCA. Utilising these immobilised electrocatalysts, FDCA yields of up to 85 % and faradaic efficiencies of 91 % were achieved, without any indication of substrate decomposition. Surface analysis by X-ray photoelectron spectroscopy (XPS) post-electrocatalysis unveiled the desorption of the catalyst from the electrode surface as a limiting factor in terms of catalytic performance., (© 2024 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Macrocyclic host molecules bound to electrode surfaces enable the complexation of catalytically active guests for molecular heterogeneous catalysis. We present a surface-anchored host-guest complex with the ability to electrochemically oxidize ammonia in both organic and aqueous solutions. With an adamantyl motif as the binding group on the backbone of the molecular catalyst [Ru(bpy-NMe 2 )(tpada)(Cl)](PF 6 ) ( 1 ) (where bpy-NMe 2 is 4,4'-bis(dimethylamino)-2,2'-bipyridyl and tpada is 4'-(adamantan-1-yl)-2,2':6',2″-terpyridine), high binding constants with β-cyclodextrin were observed in solution (in DMSO- d 6 :D 2 O (7:3), K 11 = 492 ± 21 M -1 ). The strong binding affinities were also transferred to a mesoporous ITO (mITO) surface functionalized with a phosphonated derivative of β-cyclodextrin. The newly designed catalyst ( 1 ) was compared to the previously reported naphthyl-substituted catalyst [Ru(bpy-NMe 2 )(tpnp)(Cl)](PF 6 ) ( 2 ) (where tpnp is 4'-(naphthalene-2-yl)-2,2':6',2″-terpyridine) for its stability during catalysis. Despite the insulating nature of the adamantyl substituent serving as the binding group, the stronger binding of this unit to the host-functionalized electrode and the resulting shorter distance between the catalytic active center and the surface led to better performance and higher stability. Both guests are able to oxidize ammonia in both organic and aqueous solutions, and the host-anchored electrode can be refunctionalized multiple times (>3) following the loss of the catalytic activity, without a reduction in performance. Guest 1 exhibits significantly higher stability in comparison to guest 2 toward basic conditions, which often constitutes a challenge for anchored molecular systems. Ammonia oxidation in water led to the selective formation of NO 3 - with Faradaic efficiencies of up to 100%.

Sb 2 Se 3 , consisting of one-dimensional (Sb 4 Se 6 ) n nanoribbons has drawn attention as an intriguing light absorber from the photovoltaics (PVs) research community. However, further research is required on the performance-limiting factors in Sb 2 Se 3 PVs. In this study, we investigated the charge carrier behavior in Sb 2 Se 3 PVs by impedance spectroscopy (IS) under light illumination. (Sb 4 Se 6 ) n nanoribbons with two different orientations were used to investigate the effect of crystal orientation on the device performance. Regardless of the (Sb 4 Se 6 ) n orientation, negative capacitance was observed at forward bias, representing a recombination pathway at the TiO 2 /Sb 2 Se 3 interface. A comparison of the recombination resistances and lifetimes of two different Sb 2 Se 3 PVs showed that a better interface could be formed by placing the (Sb 4 Se 6 ) n ribbons parallel to the TiO 2 layer. Based on these observations, an ideal structure of the Sb 2 Se 3 /TiO 2 interface is proposed, which will enhance the performance of Sb 2 Se 3 PVs toward its theoretical limit.

Antimony selenide (Sb 2 Se 3 ) is an auspicious material for solar energy conversion that has seen rapid improvement over the past ten years, but the photovoltage deficit remains a challenge. Here, simple and low-temperature treatments of the p-n heterojunction interface of Sb 2 Se 3 /TiO 2 -based photocathodes for photoelectrochemical water splitting were explored to address this challenge. The FTO/Ti/Au/Sb 2 Se 3 (substrate configuration) stack was treated with (NH 4 ) 2 S as an etching solution, followed by CuCl 2 treatment prior to deposition of the TiO 2 by atomic layer deposition. The different treatments show different mechanisms of action compared to similar reported treatments of the back Au/Sb 2 Se 3 interface in superstrate configuration solar cells. These treatments collectively increased the onset potential from 0.14 V to 0.28 V vs. reversible hydrogen electrode (RHE) and the photocurrent from 13 mA cm -2 to 18 mA cm -2 at 0 V vs. RHE as compared to the untreated Sb 2 Se 3 films. From SEM and XPS studies, it is clear that the etching treatment induces a morphological change and removes the surface Sb 2 O 3 layer, which eliminates the Fermi-level pinning that the oxide layer generates. CuCl 2 further enhances the performance due to the passivation of the surface defects, as supported by density functional theory molecular dynamics (DFT-MD) calculations, improving charge separation at the interface. The simple and low-cost semiconductor synthesis method combined with these facile, low-temperature treatments further increases the practical potential of Sb 2 Se 3 for large-scale water splitting., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Cu 2 S is a promising solar energy conversion material due to its suitable optical properties, high elemental earth abundance, and nontoxicity. In addition to the challenge of multiple stable secondary phases, the short minority carrier diffusion length poses an obstacle to its practical application. This work addresses the issue by synthesizing nanostructured Cu 2 S thin films, which enables increased charge carrier collection. A simple solution-processing method involving the preparation of CuCl and CuCl 2 molecular inks in a thiol-amine solvent mixture followed by spin coating and low-temperature annealing was used to obtain phase-pure nanostructured (nanoplate and nanoparticle) Cu 2 S thin films. The photocathode based on the nanoplate Cu 2 S (FTO/Au/Cu 2 S/CdS/TiO 2 /RuO x ) reveals enhanced charge carrier collection and improved photoelectrochemical water-splitting performance compared to the photocathode based on the non-nanostructured Cu 2 S thin film reported previously. A photocurrent density of 3.0 mA cm -2 at -0.2 versus a reversible hydrogen electrode ( V RHE ) with only 100 nm thickness of a nanoplate Cu 2 S layer and an onset potential of 0.43 V RHE were obtained. This work provides a simple, cost-effective, and high-throughput method to prepare phase-pure nanostructured Cu 2 S thin films for scalable solar hydrogen production., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

Ammonia solution etching was carried out on thermally-oxidised cuprous oxide (TO-Cu 2 O) in photocathode devices for water splitting. The etched devices showed increased photoelectrochemical (PEC) performance compared to the unetched ones as well as improved reproducibility. -8.6 mA cm -2 and -7 mA cm -2 photocurrent density were achieved at 0 V and 0.5 V versus the reversible hydrogen electrode (V RHE ), respectively, in the champion sample with an onset potential of 0.92 V RHE and a fill factor of 44%. An applied bias photon-to-current efficiency of 3.6% at 0.56 V RHE was obtained, which represents a new record for Cu 2 O-based photocathode systems. Capacitance-based profiling studies showed a strong pinning effect from interfacial traps in the as-grown device, and these traps were removed by ammonia solution etching. Moreover, the etching procedure gave rise to a diverse morphology of Cu 2 O crystals based on the different crystallographic orientations. The distribution of crystallographic orientations and the relationship between the crystal orientation and the morphology after etching were examined by electron backscatter diffraction (EBSD) and scanning electron microscopy (SEM). The high-index crystal group showed a statistically higher PEC performance than the low-index group. X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) revealed metallic copper at the Cu 2 O/Ga 2 O 3 interface, which we attribute as the dominant trap that limits the PEC performance. It is concluded that the metallic copper originates from the reduction of the CuO impurity layer on the as-grown Cu 2 O sample during the ALD process, while the reduction from Cu 2 O to Cu is not favourable., Competing Interests: W. N. received sponsored research funding from PERA Complexity for the work in this article. The methodology described above is part of a system with the potential for commercialization as technology to be led by PERA Complexity within its patent portfolio., (This journal is © The Royal Society of Chemistry.)

We report a regenerable and redox-inactive ZnO x H y layer that was in situ deposited onto metal oxides MO z (M = Co, Fe, and Ni) in alkaline media containing [Zn(OH) 4 ] 2- species during water oxidation. An interface dipole was developed at the MO z /Zn interface, resulting in a decrease of the OER overpotential. Exemplified by the CoO z /ZnO x H y bilayer structure, it presented a 155 mV lower overpotential to deliver 10 mA cm -2 and long-term stability relative to the unmodified CoO z film.

Cu 2 S is a promising solar energy conversion material owing to its good optical properties, elemental earth abundance, and low cost. However, simple and cheap methods to prepare phase-pure and photo-active Cu 2 S thin films are lacking. This study concerns the development of a cost-effective and high-throughput method that consists of dissolving high-purity commercial Cu 2 S powder in a thiol-amine solvent mixture followed by spin coating and low-temperature annealing to obtain phase-pure crystalline low chalcocite Cu 2 S thin films. After coupling with a CdS buffer layer, a TiO 2 protective layer and a RuO x hydrogen evolution catalyst, the champion Cu 2 S photocathode gives a photocurrent density of 2.5 mA cm -2 at -0.3 V vs. reversible hydrogen electrode (V RHE ), an onset potential of 0.42 V RHE , and high stability over 12 h in pH 7 buffer solution under AM1.5 G simulated sunlight illumination (100 mW cm -2 ). This is the first thiol-amine-based ink deposition strategy to prepare phase-pure Cu 2 S thin films achieving decent photoelectrochemical performance, which will facilitate its future scalable application for solar-driven hydrogen fuel production., (© 2021 The Authors. ChemSusChem published by Wiley-VCH GmbH.)

Anchoring molecular catalysts on electrode surfaces combines the high selectivity and activity of molecular systems with the practicality of heterogeneous systems. Molecular catalysts, however, are far less stable than traditional heterogeneous electrocatalysts, and therefore a method to easily replace anchored molecular catalysts that have degraded could make such electrosynthetic systems more attractive. Here we applied a non-covalent 'click' chemistry approach to reversibly bind molecular electrocatalysts to electrode surfaces through host-guest complexation with surface-anchored cyclodextrins. The host-guest interaction is remarkably strong and enables the flow of electrons between the electrode and the guest catalyst. Electrosynthesis in both organic and aqueous media was demonstrated on metal oxide electrodes, with stability on the order of hours. The catalytic surfaces can be recycled by controlled release of the guest from the host cavities and the readsorption of fresh guest.

This work explores the solid-liquid interface of a rhenium-tricarbonyl complex embedded in a layer of zirconium oxide deposited by atomic layer deposition (ALD). Time-resolved and steady state infrared spectroscopy were applied to reveal the correlations between the thickness of the ALD layer and the spectroscopic response of the system. We observed a transition of the molecular environment from flexible to rigid, as well as limitations to ligand exchange and excited state quenching on the embedded complexes, when the ALD layer is roughly of the same height as the molecules.

To accelerate the deployment of hydrogen produced by renewable solar energy, several technologies have been competitively developed, including photoelectrochemical (PEC), photocatalytic, and photovoltaic-electrolysis routes. In this review, we place PEC in context with these competing technologies and highlight key advantages of PEC systems. After defining the unique performance metrics of the PEC water splitting system, recently developed strategies for enhancing each performance metric, such as the photocurrent density, photovoltage, fill factor, and stability are surveyed in conjunction with the relevant theoretical aspects. In addition, various advanced characterization methods are discussed, including recently developed in situ techniques, allowing us to understand not only the basic properties of materials but also diverse photophysical phenomena underlying the PEC system. Based on the insights gained from these advanced characterization techniques, we not only provide a resource for researchers in the field as well as those who want to join the field, but also offer an outlook of how thin film-based PEC studies could lead to commercially viable water splitting systems.

TiO 2 and WO 3 are two of the most important, industrially relevant earth-abundant oxides. Although both materials show complementary functionality and are promising candidates for similar types of applications such as catalysis, sensor technology, and energy conversion, their chemical stability in reactive environments differs remarkably. In this study, anodic barrier oxides are grown on solid-solution W x Ti 1- x alloy precursors covering a wide compositional range (0 ≤ x ≤ 1) with the goal of creating functional oxides with tailored stability. A strong Ti-cation enrichment in the surface region of the grown W x Ti 1- x O n layer is observed, which can be controlled by both the anodizing conditions and precursor composition. For Ti concentrations above 50 at. %, a continuous nanometer-thick TiO 2 protective coating is achieved on top of a homogeneous W x Ti 1- x O n film as evidenced by X-ray photoelectron spectroscopy and transmission electron microscopy analyses. A comprehensive electrochemical assessment demonstrates a very stable passivation of the surface in both acidic and alkaline environments. This increase in chemical stability correlates directly with the presence of this protective TiO 2 film. The results of this work provide insights into the oxidation behavior of W 1- x Ti x alloys, but more importantly demonstrate how controlled oxidation of self-passivating alloys can lead to oxide alloys with thin, protective surface layers that otherwise would require more sophisticated deposition methods.

In this work, we demonstrate that buried junction photocathodes featuring an ALD TiO 2 protective overlayer can be readily characterized using a variation of the dual working electrode (DWE) technique, where the second working electrode (WE2) is spatially isolated from the hydrogen-evolving active area. The measurement of the surface potential during operation enables the operando deconvolution of the photovoltaic and electrocatalytic performance of these photocathodes, by reconstructing J -Δ V curves (reminiscent of photovoltaic J - V curves) from the 3-electrode water splitting data. Our method provides a clearer understanding of the photocathode degradation mechanism during stability tests, including loss of the catalyst from the surface, which is only possible in our isolated WE2 configuration. A pn + Si/TiO 2 photocathode was first investigated as a well behaved model system, and then the technique was applied to an emerging material system based on Cu 2 O/Ga 2 O 3 , where we uncovered an intrinsic instability of the Cu 2 O/Ga 2 O 3 junction (loss of photovoltage) during long term stability measurements.

The solar resource is immense, but the power density of light striking the Earth's surface is relatively dilute, necessitating large area solar conversion devices in order to harvest substantial amounts of power for renewable energy applications. In addition, energy storage is a key challenge for intermittent renewable resources such as solar and wind, which adds significant cost to these energies. As the majority of humanity's present-day energy consumption is based on fuels, an ideal solution is to generate renewable fuels from abundant resources such as sunlight and water. In this account, we detail our recent work towards generating highly efficient and stable Earth-abundant semiconducting materials for solar water splitting to generate renewable hydrogen fuel.

Intermolecular vibrational energy transfer in monolayers of isotopically mixed rhenium carbonyl complexes at solid-liquid interfaces is investigated with the help of ultrafast 2D Attenuated Total Reflectance Infrared (2D ATR IR) spectroscopy in dependence of plasmonic surface enhancement effects. Dielectric and plasmonic materials are used to demonstrate that plasmonic effects have no impact on the vibrational energy transfer rate in a regime of moderate IR surface enhancement (enhancement factors up to ca. 30). This result can be explained with the common image-dipole picture. The vibrational energy transfer rate thus can be used as a direct observable to determine intermolecular distances on surfaces, regardless of their plasmonic properties.

Protective overlayers for light absorbers in photoelectrochemical water-splitting devices have gained considerable attention in recent years. They stabilize light absorbers which would normally be prone to chemical side reactions leading to degradation of the absorber. Atomic layer deposition (ALD) enables conformal and reproducible ultrathin protective layer growth even on highly structured substrates. One of the most widely investigated protective layers is amorphous TiO 2 , deposited by ALD at a relatively low temperature (120-150 °C). We have deposited protective layers from tetrakis(dimethylamido)titanium(IV) at two different temperatures and investigated their chemical composition as well as optical and electrochemical properties. Our main findings reveal a change in the flat band potential with thickness, reaching a stable value of about -50 to -100 mV versus reversible hydrogen electrode for films >30 nm, with doping densities of ∼10 20 cm 3 . Practical thicknesses to achieve pinhole-free films are evaluated and discussed.

A new strategy of using forward gradient self-doping to improve the charge separation efficiency in metal oxide photoelectrodes is proposed. Gradient self-doped CuBi 2 O 4 photocathodes are prepared with forward and reverse gradients in copper vacancies using a two-step, diffusion-assisted spray pyrolysis process. Decreasing the Cu/Bi ratio of the CuBi 2 O 4 photocathodes introduces Cu vacancies that increase the carrier (hole) concentration and lowers the Fermi level, as evidenced by a shift in the flat band toward more positive potentials. Thus, a gradient in Cu vacancies leads to an internal electric field within CuBi 2 O 4 , which can facilitate charge separation. Compared to homogeneous CuBi 2 O 4 photocathodes, CuBi 2 O 4 photocathodes with a forward gradient show highly improved charge separation efficiency and enhanced photoelectrochemical performance for reduction reactions, while CuBi 2 O 4 photocathodes with a reverse gradient show significantly reduced charge separation efficiency and photoelectrochemical performance. The CuBi 2 O 4 photocathodes with a forward gradient produce record AM 1.5 photocurrent densities for CuBi 2 O 4 up to -2.5 mA/cm 2 at 0.6 V vs RHE with H 2 O 2 as an electron scavenger, and they show a charge separation efficiency of 34% for 550 nm light. The gradient self-doping accomplishes this without the introduction of external dopants, and therefore the tetragonal crystal structure and carrier mobility of CuBi 2 O 4 are maintained. Lastly, forward gradient self-doped CuBi 2 O 4 photocathodes are protected with a CdS/TiO 2 heterojunction and coated with Pt as an electrocatalyst. These photocathodes demonstrate photocurrent densities on the order of -1.0 mA/cm 2 at 0.0 V vs RHE and evolve hydrogen with a faradaic efficiency of ∼91%.

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.

Energy band alignments at heterointerfaces play a crucial role in defining the functionality of semiconductor devices, yet the search for material combinations with suitable band alignments remains a challenge for numerous applications. In this work, we demonstrate how changes in deposition conditions can dramatically influence the functional properties of an interface, even within the same material system. The energy band alignment at the heterointerface between Cu2O and ZnO was studied using photoelectron spectroscopy with stepwise deposition of ZnO onto Cu2O and vice versa. A large variation of energy band alignment depending on the deposition conditions of the substrate and the film is observed, with valence band offsets in the range ΔEVB = 1.45-2.7 eV. The variation of band alignment is accompanied by the occurrence or absence of band bending in either material. It can therefore be ascribed to a pinning of the Fermi level in ZnO and Cu2O, which can be traced back to oxygen vacancies in ZnO and to metallic precipitates in Cu2O. The intrinsic valence band offset for the interface, which is not modified by Fermi level pinning, is derived as ΔEVB ≈ 1.5 eV, being favorable for solar cell applications.

Sunlight-driven water splitting to produce hydrogen fuel is an attractive method for renewable energy conversion. Tandem photoelectrochemical water splitting devices utilize two photoabsorbers to harvest the sunlight and drive the water splitting reaction. The absorption of sunlight by electrocatalysts is a severe problem for tandem water splitting devices where light needs to be transmitted through the larger bandgap component to illuminate the smaller bandgap component. Herein, we describe a novel method for the deposition of an optically transparent amorphous iron nickel oxide oxygen evolution electrocatalyst. The catalyst was deposited on both thin film and high-aspect ratio nanostructured hematite photoanodes. The low catalyst loading combined with its high activity at low overpotential results in significant improvement on the onset potential for photoelectrochemical water oxidation. This transparent catalyst further enables the preparation of a stable hematite/perovskite solar cell tandem device, which performs unassisted water splitting.

Water oxidation is a key chemical reaction, central to both biological photosynthesis and artificial solar fuel synthesis strategies. Despite recent progress on the structure of the natural catalytic site, and on inorganic catalyst function, determining the mechanistic details of this multiredox reaction remains a significant challenge. We report herein a rate law analysis of the order of water oxidation as a function of surface hole density on a hematite photoanode employing photoinduced absorption spectroscopy. Our study reveals a transition from a slow, first order reaction at low accumulated hole density to a faster, third order mechanism once the surface hole density is sufficient to enable the oxidation of nearest neighbor metal atoms. This study thus provides direct evidence for the multihole catalysis of water oxidation by hematite, and demonstrates the hole accumulation level required to achieve this, leading to key insights both for reaction mechanism and strategies to enhance function.

Though Cu2O has demonstrated high performance as a photocathode for solar water splitting, its band gap is too large for efficient use as the bottom cell in tandem configurations. Accordingly, copper chalcopyrites have recently attracted much attention for solar water splitting due to their smaller and tunable band gaps. However, their fabrication is mainly based on vacuum evaporation, which is an expensive and energy consuming process. Here, we have developed a novel and low-cost solution fabrication method, and CuInS2 was chosen as a model material due to its smaller band gap compared to Cu2O and relatively simple composition. The nanostructured CuInS2 electrodes were synthesized at low temperature in crystalline form by solvothermal treatment of electrochemically deposited Cu2O films. Following the coating of overlayers and decoration with Pt catalyst, the as-fabricated CuInS2 electrode demonstrated water splitting photocurrents of 3.5 mA cm(-2) under simulated solar illumination. To the best of our knowledge, this is the highest performance yet reported for a solution-processed copper chalcopyrite electrode for solar water splitting. Furthermore, the electrode showed good stability and had a broad incident photon-to-current efficiency (IPCE) response to wavelengths beyond 800 nm, consistent with the smaller bandgap of this material.

The splitting of water into hydrogen and oxygen molecules using sunlight is an attractive method for solar energy storage. Until now, photoelectrochemical hydrogen evolution is mostly studied in acidic solutions, in which the hydrogen evolution is more facile than in alkaline solutions. Herein, we report photoelectrochemical hydrogen production in alkaline solutions, which are more favorable than acidic solutions for the complementary oxygen evolution half-reaction. We show for the first time that amorphous molybdenum sulfide is a highly active hydrogen evolution catalyst in basic medium. The amorphous molybdenum sulfide catalyst and a Ni-Mo catalyst are then deposited on surface-protected cuprous oxide photocathodes to catalyze sunlight-driven hydrogen production in 1 M KOH. The photocathodes give photocurrents of -6.3 mA cm(-2) at the reversible hydrogen evolution potential, the highest yet reported for a metal oxide photocathode using an earth-abundant hydrogen evolution reaction catalyst., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Although sunlight-driven water splitting is a promising route to sustainable hydrogen fuel production, widespread implementation is hampered by the expense of the necessary photovoltaic and photoelectrochemical apparatus. Here, we describe a highly efficient and low-cost water-splitting cell combining a state-of-the-art solution-processed perovskite tandem solar cell and a bifunctional Earth-abundant catalyst. The catalyst electrode, a NiFe layered double hydroxide, exhibits high activity toward both the oxygen and hydrogen evolution reactions in alkaline electrolyte. The combination of the two yields a water-splitting photocurrent density of around 10 milliamperes per square centimeter, corresponding to a solar-to-hydrogen efficiency of 12.3%. Currently, the perovskite instability limits the cell lifetime., (Copyright © 2014, American Association for the Advancement of Science.)

Transient absorption spectroscopy on subpicosecond to second time scales is used to investigate photogenerated charge carrier recombination in Si-doped nanostructured hematite (α-Fe2O3) photoanodes as a function of applied bias. For unbiased hematite, this recombination exhibits a 50% decay time of ~6 ps, ~10(3) times faster than that of TiO2 under comparable conditions. Anodic bias significantly retards hematite recombination dynamics, and causes the appearance of electron trapping on ps-μs time scales. These ultrafast recombination dynamics, their retardation by applied bias, and the associated electron trapping are discussed in terms of their implications for efficient water oxidation.

The kinetic competition between electron-hole recombination and water oxidation is a key consideration for the development of efficient photoanodes for solar driven water splitting. In this study, we employed three complementary techniques, transient absorption spectroscopy (TAS), transient photocurrent spectroscopy (TPC), and electrochemical impedance spectroscopy (EIS), to address this issue for one of the most widely studied photoanode systems: nanostructured hematite thin films. For the first time, we show a quantitative agreement between all three techniques. In particular, all three methods show the presence of a recombination process on the 10 ms to 1 s time scale, with the time scale and yield of this loss process being dependent upon applied bias. From comparison of data between these techniques, we are able to assign this recombination phase to recombination of bulk hematite electrons with long-lived holes accumulated at the semiconductor/electrolyte interface. The data from all three techniques are shown to be consistent with a simple kinetic model based on competition between this, bias dependent, recombination pathway and water oxidation by these long-lived holes. Contrary to most existing models, this simple model does not require the consideration of surface states located energetically inside the band gap. These data suggest two distinct roles for the space charge layer developed at the semiconductor/electrolyte interface under anodic bias. Under modest anodic bias (just anodic of flatband), this space charge layer enables the spatial separation of initially generated electrons and holes following photon absorption, generating relatively long-lived holes (milliseconds) at the semiconductor surface. However, under such modest bias conditions, the energetic barrier generated by the space charge layer field is insufficient to prevent the subsequent recombination of these holes with electrons in the semiconductor bulk on a time scale faster than water oxidation. Preventing this back electron-hole recombination requires the application of stronger anodic bias, and is a key reason why the onset potential for photocurrent generation in hematite photoanodes is typically ~500 mV anodic of flat band and therefore needs to be accounted for in electrode design for PEC water splitting.

Concerns over climate change resulting from accumulation of anthropogenic carbon dioxide in the atmosphere and the uncertainty in the amount of recoverable fossil fuel reserves are driving forces for the development of renewable, carbon-neutral energy technologies. A promising clean solution is photoelectrochemical water splitting to produce hydrogen using abundant solar energy. Here we present a simple and scalable technique for the deposition of amorphous molybdenum sulphide films as hydrogen evolution catalyst onto protected copper(I) oxide films. The efficient extraction of excited electrons by the conformal catalyst film leads to photocurrents of up to -5.7 mA cm(-2) at 0 V versus the reversible hydrogen electrode (pH 1.0) under simulated AM 1.5 solar illumination. Furthermore, the photocathode exhibits enhanced stability under acidic environments, whereas photocathodes with platinum nanoparticles as catalyst deactivate more rapidly under identical conditions. The work demonstrates the potential of earth-abundant light-harvesting material and catalysts for solar hydrogen production.

This account describes a strategy for directly forming three of the six rings found in the polyketide natural product hirsutellone B via a novel cyclization cascade. The key step in our approach comprises two transformations: a large-ring-forming, nucleophilic capture of a transient acylketene and an intramolecular Diels-Alder reaction, both of which occur in tandem through thermolyses of appropriately functionalized, polyunsaturated dioxinones. These thermally induced "double cyclization" cascades generate three new bonds, four contiguous stereocenters, and a significant fraction of the polycyclic architecture of hirsutellone B. The advanced macrolactam and macrolactone intermediates that were synthesized by this process possess key features of the hirsutellone framework, including the stereochemically dense decahydrofluorene core and the strained para-cyclophane ring. However, attempts to complete the carbon skeleton of hirsutellone B via transannular carbon-carbon bond formation were undermined by competitive O-alkylation reactions. This account also documents how we adapted to this undesired outcome through an evaluation of several distinct strategies for synthesis, as well as our eventual achievement of a formal total synthesis of hirsutellone B.

Cu2ZnSnS4 (CZTS) is a promising p-type semiconductor that has not yet been extensively investigated for solar fuel production via water splitting. Here, we optimize and compare two different electrodeposition routes (simultaneous and sequential) for preparing CZTS electrodes. More consistent results are observed with the simultaneous route. In addition, the effect of etching and the presence of a CdS buffer layer on the photocurrent are investigated. Finally, we demonstrate for the first time the stabilization of these electrodes using protecting overlayers deposited by atomic layer deposition (ALD). Our best performing protected electrodes (Mo/CZTS/CdS/AZO/TiO2/Pt) exhibited a photocurrent of over 1 mA cm(-2) under standard one sun illumination conditions and a significant improvement in stability over unprotected electrodes.

The reactive intermediates known as acylketenes exhibit a rich chemistry and have been extensively utilized for many types of inter- and intramolecular bond-forming reactions within the field of organic synthesis. Characteristic reactions of acylketenes include cycloadditions, carbon-carbon bond-forming reactions, and nucleophilic capture with alcohols or amines to give beta-keto acid derivatives. In particular, the intramolecular capture of acylketene intermediates with pendant nucleophiles represents a powerful method for forming both medium-sized rings and macrocycles, often in high yield. This tutorial review examines the history, generation, and reactivity of acylketenes with a special focus on their applications in the synthesis of natural products.

A tandem ketene-trapping/Diels-Alder cyclization sequence was the pivotal transformation in an efficient, asymmetric synthesis of a decahydrofluorene tricyclic structure possessing eight stereogenic centers and key features of the hirsutellone class of antitubercular natural products. The hirsutellone-like beta-keto ester that was fashioned by this sequence (13 steps; 6% overall yield) demonstrated significant inhibitory activity against Mycobacterium tuberculosis. The mechanism of action of this antitubercular compound is not yet known.

A new protein modification reaction has been developed based on a palladium-catalyzed allylic alkylation of tyrosine residues. This technique employs electrophilic pi-allyl intermediates derived from allylic acetate and carbamate precursors and can be used to modify proteins in aqueous solution at room temperature. To facilitate the detection of modified proteins using SDS-PAGE analysis, a fluorescent allyl acetate was synthesized and coupled to chymotrypsinogen A and bacteriophage MS2. The tyrosine selectivity of the reaction was confirmed through trypsin digest analysis. The utility of the reaction was demonstrated by using taurine-derived carbamates as water solubilizing groups that are cleaved upon protein functionalization. This solubility switching technique was used to install hydrophobic farnesyl and C(17) chains on chymotrypsinogen A in water using little or no cosolvent. Following this, the C(17) alkylated proteins were found to associate with lipid vesicles. In addition to providing a new protein modification strategy targeting an under-utilized amino acid side chain, this method provides convenient access to synthetic lipoproteins.