Catalyst design for the reduction of CO₂ to valuable fuels needs property-function relationships to identify more generalized material design guidelines. A large body of work has been developed studying defect chemistry and especially oxygen vacancy chemistry in oxide systems for the water oxidation reaction, since typically these surfaces are unprotected, offering the investigation of the semiconductor-liquid junction in a photoanode directly.[1, 2] Recent works by Profs. Wang and Domen developed a new p-type visible light absorber (La,Sr)(Rh,Ti)O₃ employed in the Z-scheme photocatalyst sheet device with a record 1% solar-to-hydrogen efficiency,[3] turning the focus to investigating defect chemistry in absorbers driving the reduction reaction.[4] Our latest work explores defect chemistry further, studying the CO₂ photohydrogenation reaction with doped SrTiO₃.[5] A key parameter we identify is surface area-normalized activity, which enables the identification of such material property-function relationships, in analogy to the insights gained from our recent studies in electrochemical CO₂ reduction.[6]

Recent progress in photocatalysis highlights the critical role of self-assembled monolayers (SAMs) in enhancing the performance of single-atom catalysts (SACs) on semiconductor surfaces. This study investigates the effect of a phosphonic acid-based SAM on the photocatalytic hydrogen evolution of iridium-decorated titanium dioxide nanotube (TiO₂ NT) surfaces. TiO₂ NTs were synthesized by anodization followed by thermal annealing. Iridium atoms were anchored using an ultrasonication-assisted deposition method under inert conditions. Subsequently, the phosphonic SAM was embedded in the solution under dark conditions via immersion. Co-presence of Ir and the SAM was confirmed by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). Under UV irradiation, SAM-functionalized samples exhibited significantly higher hydrogen evolution rates compared to untreated controls. This enhancement is attributed to improved charge separation and the prevention of Ir atom aggregation. The formation of the anatase phase was verified by X-ray diffraction (XRD), while surface chemistry and functionalization were validated using X-ray photoelectron spectroscopy (XPS) and ToF-SIMS. Hydrogen evolution rates were quantified via gas chromatography (GC).These results demonstrate a synergistic effect between SAM modification and single-atom catalysis, offering a promising strategy to boost photocatalytic efficiency in solar-to-fuel conversion applications.

Herein, we report a novel approach to enhance the photocatalytic degradation of organic pollutants under visible light mediated by nanocrystalline TiO₂ (nc-TiO₂) films¹. This enhancement results from embedding a nc-TiO₂ film in a Fabry–Perot optical resonator. This approach significantly improve the nc-TiO₂ absorption in the visible range, enabling efficient activation under narrowband LED illumination and simulated solar light. The nanoporous system allows infiltration of methylene blue aqueous solutions, providing a versatile platform for environmental remediation. Photodegradation kinetic was systematically studied at different wavelengths (385 nm, 405 nm), revealing an improvement up to 200% of the kinetic constant compared to conventional TiO₂ films. It was found that the incidence angle critically influences the coupling between resonator optical modes and LED irradiance, offering a new degree of control over photocatalytic performance. Photodegradation kinetics of methylenen blue under a solar simulator were also studied, revealing a higher rate in the case of the optical cavity. This work illustrates how optical engineering can synergize with material chemistry to advance light-driven pollutant degradation. Our findings demonstrate that optical resonator-assisted TiO₂ films effectively overcome the inherent limitations of conventional photocatalysts under visible light. This strategy opens up new opportunities for efficient solar-driven water treatment and a range of other environmental remediation applications by tailoring absorption through resonant structures and optimising illumination parameters.

¹ B. de Sola, L. Caliò, M. Romero, M. Herrera-Collado, M. Calvo, H. Míguez. Nanocrystallline TiO₂ based Fabry Pérot Resonators for Enhanced Visible Light Photocatalysis. In preparation.

Nanoparticle-based aerogels are highly porous materials with a large surface area consisting of crystalline nanoparticles that are assembled into three-dimensional networks. They can be synthesized from a large variety of preformed nanoparticles by gelling the corresponding colloidal solutions. Due to their unique structural and physicochemical properties, aerogels have great potential as catalysts for light-driven gas-phase reactions, including hydrogen production, degradation of organic molecules, and CO₂ reduction. However, in order to fully exploit this potential, their composition, microstructure, optical transparency, and macroscopic geometry must be precisely controlled so that they are optimized for the reaction to be catalyzed on the one hand, and meet the specific requirements of the photoreactor on the other hand, in particular ensuring efficient gas transport and uniform light penetration.

This presentation will discuss various methods for synthesizing metal oxide-based aerogels from preformed nanocrystals under different reaction conditions and in the presence of various co-catalysts. Strategies for tailoring their macroscopic morphology from monolithic architectures to spherical or worm-like granules to improve catalytic performance in light-driven gas-phase reactions will be highlighted. Recent advances in introducing chirality into these materials will also be presented.

Photocatalysis offers an environmentally friendly strategy for transforming pollutants into valuable chemicals while enabling efficient water purification. Realizing this potential requires materials that operate under visible light, maintain stability in continuous operation, and display high selectivity toward targeted products. In this talk, I will present two photocatalytic systems developed in our group that address these challenges from complementary perspectives.

The first focuses on gas-phase CO₂ conversion using EDOT-based donor–acceptor–donor trimers as visible-light-responsive organic photocatalysts.^[1],[2] By tuning the molecular acceptor units, we modulate packing, electronic structure, and excited-state behavior, enabling selective formation of C₂ hydrocarbons under continuous flow. This modular molecular design provides a pathway to guide charge separation and reaction selectivity using fully organic-based photocatalysts.

The second system targets microplastic removal and degradation using 3D-printed silicon carbide architectures with interconnected porous networks. Their tailored geometry enhances light utilization, solid–solid contact, and mechanical robustness, enabling efficient microplastic capture and oxidation in both static and flow conditions.^[3]

Together, these results illustrate how molecular design and material engineering can advance photocatalysis toward practical, sustainable environmental remediation.

Heterogeneous photocatalysts provide an ideal platform for sustainable chemical synthesis as they are often inexpensive to synthesize at scale and allow facile catalyst recovery and reuse.[1-3] Particulate photocatalysts are most commonly used as dispersions in the bulk of a liquid phase, but this poses intrinsic limitations to efficient catalyst use and scale-up as well as photon harvesting and utilization (Beer-Lambert law), as particles away from the outermost layers of the reaction vessel may not be exposed to light.[4]

Immobilization strategies of photocatalysts on solid supports are therefore being explored, with the aim of replacing dispersed powders with monolithic materials or thin films. Floatable photocatalysts[5,6] are assembled by immobilizing semiconductors on floatable (low-density) support matrices. These materials have recently been demonstrated in emerging technologies for water purification,[7,8] energy harvesting[9,10] solar fuel synthesis,[11] and plastic photoreforming.[6] 

Unlike monolithic packed-bed reactors and photosheets, floatable photocatalysts offer a potential platform to compartmentalize products and charges in photoredox catalysis. While they have been shown at the gas-liquid interface, there are no reports on the segregation of liquid species. Efforts to achieve the compartmentalization of liquid phase redox reactions have been mostly inspired by biological systems,[12] using artificial synthetic and colloidal nanoreactors based on liposomal structures.[13-17]

We introduce liquid|solid|liquid (L|S|L) photocatalysis.[18] Here, macroscopic separation of stacked liquid aqueous phase, photocatalytic solid layer, and liquid organic phase allows paired redox synthesis of hydrophilic and hydrophobic chemical species with continuous operation of both liquid streams, which makes it an ideal framework for liquid-liquid flow photocatalysis. 

In our first demonstration of this innovative concept, we use a carbon nitride/polypropylene (CNx/PP) composite immobilized between water and organic solvents to pair the synthesis of clean aqueous H2O2 and added-value aldehydes from biomass-derived chemicals, including 1-butanol from fermentation processes, or lignocellulose as a by-product of the pulp industry. 

We demonstrate a facile and robust method to prepare metal-free, resource-benign floatable photocatalysts based on solvent-free thermal processing with readily scalable carbon nitride and low-density plastics. The protocol provides a general approach for the preparation of low-density photocatalysts from shredded plastic and any powdered material of interest for catalytic or other purposes, with limited constraints on their thermal or chemical stability. Our fabrication procedure also provides a route for plastic upcycling, in the interest of circular economy.

In applications for paired aqueous|organic photocatalysis, we report the synthesis of 2.7±0.5 mmol L^–1 h^–1 aqueous H₂O₂ and 1.5±0.4 mmol L^–1 h^–1 butanal from 1-butanol oxidation at room temperature under blue LED irradiation. We also demonstrate kraft lignin upcycling in ethyl acetate using concentrated solar irradiation for combined solar light and heat management in a continuous flow process, and we observe kraft lignin depolymerization with a drop in weight-average molecular weight (Mw) from 2746 Da in pristine kraft lignin to 1400 Da with of 5.55±1.90 µmoles of H₂O₂ extracted after 16 h photocatalysis. We emphasize that the possibility of seamless flow operation highlights the versatility and potential for sustainable chemical synthesis using liquid|solid|liquid photocatalysis.

Our integrated, compartmentalized L|S|L photocatalytic reactors target sustainable synthesis on multiple fronts, including plastic upcycling, biomass and industrial byproduct valorization, abatement of separation costs, and it opens promising unexplored avenues for continuous flow photocatalysis in immiscible liquid-liquid media.

The escalating global energy crisis, coupled with the urgent need to transition away from fossil fuels, has intensified the search for sustainable energy solutions. Photocatalytic water splitting—using sunlight, water, and a catalyst to generate hydrogen—represents a particularly promising approach to clean energy production. Yet this process faces a critical limitation: the formation of unwanted hydrogen peroxide (H₂O₂) byproducts due to uncontrolled radical spin states, severely compromising both efficiency and commercial viability.^[1]

A breakthrough may lie in exploiting molecular chirality. Beyond its recognition since the 19th century, chirality has revealed a remarkable quantum mechanical property: chiral molecules can selectively filter electron spins through the chiral-induced spin selectivity (CISS) effect. This phenomenon opens an unprecedented pathway to controlling spin states in water splitting reactions, potentially eliminating problematic byproduct formation.^[2]

Meanwhile, organic semiconductors (OSCs) have emerged as transformative materials across electronic applications, from transistors and OLEDs to flexible photovoltaics. Their appeal stems from tuneable electronic properties, mechanical flexibility, solution-based processing, and cost-effectiveness. Combining these advantages with chiral spin selectivity could revolutionize hydrogen production, creating efficient and scalable clean energy systems.^[3]

This research presents the development of a novel chiral OSC that not only exhibits the desired CISS effect but also enables comprehensive analysis of OSC performance in water splitting applications. Through comparison with both achiral reference materials and racemic analogues, we demonstrate the unique advantages of chirality. Our results reveal a striking four-fold enhancement in current density—directly correlating to hydrogen evolution—when comparing our chiral OSC to non-chiral counterparts. This dramatic improvement demonstrates how incorporating chirality alone can achieve remarkable advances in water splitting efficiency. The enhancement stems from CISS-mediated spin control, enabling optimized catalytic pathways and substantially improved hydrogen generation for renewable energy applications.

In light of the global energy crisis and rapidly increasing carbon footprint in the atmosphere, there’s a high call for solar powered energy solutions, particularly through carbon capture and utilization (CCU) technologies. Photosynthetic biohybrid systems combining light harvesting materials with microbial biocatalysts is a promising approach for sustainable synthesis of chemicals from CO₂.¹ We demonstrate this through integrating CO₂ reducing microbes with earth earth-abundant, non-toxic elements derived semiconductors performing CO₂ reduction into chemicals powered by sunlight.  In this talk, I’ll be presenting two projects exploring semiconductor-microbe biohybrids for solar chemical technologies.

The first project focuses on developing a photosynthetic biohybrid chassis coupled with fermentation for fatty acid production from carbon dioxide. We developed a biohybrid system Integrating a metal chalcogenide semiconductor Cu₂ZnSnS₄ (CZTS) with the CO₂-fixing electrotrophic bacterium Sporomusa ovata. This photocatalytic biohybrid system efficiently produced acetate and ethanol over five days of stable light-driven operation. The acetate and ethanol produced were subjected for a fermentative chain elongation by a Clostridum species, forming C₄ (butyrate) and C₆ (caproate) fatty acids. The second project is about bioinspired solar methanogenesis coupled with alcohol upgrading. Here we develop a dual-function semi artificial biohybrid system made of organic semiconductor Integrated with methanogenic archaea (Methanosarcina species). This photocatalytic biohybrid efficiently converts CO₂ into CH₄, paired with a hole induced alcohol oxidation reaction on solar illumination. Additionally, to further elucidate the extracellular electron transfer mechanisms in the two Methanosarcina species employed, we performed complementary cellular characterization using Raman spectroscopy, ICP-OES, and UV-vis analysis on the cells – which results in corroborating findings in line with genomic/transcriptomic evidence of the organisms.

Halide perovskites (HPs) are currently the most attractive photoactive materials for triggering solar–driven photo(electro)chemical (PEC) reactions, involving fuel and energy generation. However, the fast deterioration of HPs into polar solvents limits their applicability making the protective layers or the use of non–polar systems, pivotal strategies to keep stable their intrinsic features and structural integrity. Here, the ligand engineering concept is introduced using diverse bulky alkylammonium bromide (AlkylBr) molecules with the purpose of promoting the surface passivation of HPs, avoiding the permeation of polar molecules such as alcohols and inducing their partial ionization for producing alkoxide species, suitable for perovskite protection. In this context, high–quality AlkylBr–capped 3D CsPbX3 nanocrystals are stable in this polar media up to 7300 h (10 months), retaining a PL quantum yield of 100%.[1,2]  Accordingly, these photomaterials demonstrate a highly oxidizing power to carry out the PEC methanol-to-formaldehyde conversion with a Faradaic efficiency ~60%. Furthermore, by applying composition engineering, lead-free 2D A2SnX4 microcrystals with band-to-band emission are stabilized in aqueous environments, promoting hydrogen production ~20 μmol.g-1, a photocurrent ~ 1 mAcm-2 and a operational durability up to 2000 s. These features make HPs prominent candidates for the fabrication of advanced Solar-PEC technologies.

Halide perovskite semiconductors can merge the highly efficient operational principles of conventional inorganic semiconductors with the low‑temperature solution processability of emerging organic and hybrid materials, offering a promising route towards cheaply generating electricity as well as light. Following a surge of interest in this class of materials, research on colloidal halide perovskite nanocrystals (NCs) has gathered momentum in the last decade. This talk will highlight several findings of our group on their synthesis, for example our recent studies on the influence of various factors on the growth of perovskite NCs, which can lead to the formation of NCs with peculiar shapes (for example hollow structures) and to NC heterostructures (for example CsPbBr3/PbS, CsPbBr3/Pb4S3B2 and Bi/Bi13S18Br2   heterostructures) by promoting/suppressing the heterogenous nucleation of selected materials. Detailed Structural and optical investigations of these heterostructures will be covered, and their potential in a set of photocatalytic reactions will be discussed.

Halide perovskites have become in the last years enormously appealing optoelectronic materials because of their high sunlight-harvesting efficiency and notable charge carrier generation/transfer capabilities. These features have been recognized as the key factors for enhancing photoconversion efficiencies in solar photovoltaic devices. Furthermore, their fascinating photoluminescence quantum yield (PLQY) especially in the case of colloidal nanocrystals and can explain the expansion of their applicability in the wide optoelectronic field. The outstanding PLQY of perovskite nanocrystals is the clear evidence of the significant reduction of non-radiative recombination pathways. Consequently, after photoexcitation, these systems present a pool of photoexcited carriers whose extra energy can be used in an efficient radiative emission in LEDs or taken advantage of in different ways, such as providing work in solar cells or driving diverse chemical reactions. In this talk, our interest will focus on this last application. Here, we shown the application of halide perovskite nanocrystals in the photocatalytical and photoelectrochemical degradation of Organic compounds, highlighting the potentiality to drive both oxidation and reduction reactions. However, their use in most of the practical applications is limited due to the instability of the perovskite nanocrystals in polar environments. We will also discuss about the preparation of non-encapsulated CsPbX3 nanocrystals dispersed in fully alcohol environments, with outstanding stability through surface defect passivation strategy. Eventually, the use of Pb-free Sn-based halide perovskites will be discussed for application in HI splitting and beyond.

To overcome the inefficient surface charge transfer and sluggish OER kinetics that limit hematite (α-Fe₂O₃) photoanodes paired with oxygen-evolution co-catalysts (OECs), we introduce an oxygen-deficient double perovskite, PrBa₀.₅Sr₀.₅Co₁.₅Fe₀.₅O₆–δ (PBSCF), as a highly active OEC for surface modification. This approach enables the formation of an in-situ S-scheme heterojunction with hematite, confirmed by theoretical analyses, which accelerates electron transfer and significantly improves reaction kinetics. As a result, the Si:Ti–Fe₂O₃/PBSCF photoanode achieves a photocurrent density of 3.70 mA cm⁻² at 1.23 V_RHE, displays a lowered onset potential, and retains exceptional operational stability for more than 120 hours. These results underscore the capability of perovskite oxide layers to alleviate the intrinsic limitations of hematite photoanodes and offer a promising pathway toward efficient, durable PEC water-splitting systems for clean energy production.

The CO2 transformation via electrochemical reduction has been a longstanding target, considering the application of intermittent renewable energy sources. In such a system, the ability to produce liquid fuels is highly desirable due to their high energy density and security in storage and transportation, to which the design of electrocatalytic materials is the main focus. In this direction, Copper materials showed great promise to promote the selective electroreduction of CO2 to C2+ products with a high conversion efficiency. Research efforts have been made to improve the activity and selectivity of Cu-based electrocatalysts through doping or alloying with other transition metals. Nevertheless, the selectivity analysis requires a lot of time and several tests in order to find the proper configuration. This trial-and-error procedure is one of the main bottlenecks in the realization of performative electrodes.

Moreover, these electrocatalysts can be coupled to semiconductors to obtain photocathodes. Nowadays, metal–oxide-based ones are very popular. Nevertheless, Cu-based oxides are a known exception, and their rapid photo-corrosion under cathodic bias in aqueous media poses as their most prohibitive limitation.

In this study, we investigate Cu–Ti [1] and Cu–Sn [3] heterostructured catalysts to elucidate how compositional variations influence electrocatalytic performance and product selectivity during the CO2 reduction reaction (CO2RR). Using magnetron co-sputtering in a high-throughput configuration, we fabricated compositional gradients of Ti and Sn on super-polished Si substrates and FTO supports. Local electrochemical characterization was performed with a scanning flow (photo)electrochemical cell, enabling rapid screening of catalyst libraries and identification of optimal compositions. This approach revealed that 5 at.% Ti in Cu–Ti and 10 at.% Sn in Cu–Sn yields the most promising electrochemical responses. Following compositional optimization, selectivity analyses were conducted using HPLC and micro-GC to quantify liquid and gaseous CO2RR products, respectively. Overall, the combined high-throughput synthesis and localized characterization strategy significantly accelerated catalyst evaluation and facilitated the identification of highly selective Cu-based heterostructures for CO2 reduction.

In addition to that, those catalysts were coupled with a semiconductor, in particular, electrodeposited Cu2O electrodes with TiO2 as a passivation layer, to study their stability and performance under both light and dark configurations.

The development of stand-alone solar-to-fuel devices is essential for scalable and sustainable hydrogen production. We present an integrated photoelectrochemical–photovoltaic (PEC–PV) system that employs a dichroic optical filter to fully exploit the solar spectrum. By spectrally splitting sunlight, high-energy photons are directed to a PEC reactor for water oxidation, while lower-energy photons are transmitted to a photovoltaic (PV) cell for electricity generation, enabling simultaneous hydrogen and power production without external bias.

As a proof of concept, a molybdenum-doped bismuth vanadate (Mo:BiVO₄) photoanode was coupled with a commercial silicon (Si) solar cell, and both solar-to-hydrogen (STH) and power conversion efficiencies (PCE) were quantified. The dichroic-based PEC–PV system demonstrated enhanced performance upon introducing a hole scavenger, which facilitates bias-free operation and opens pathways for coupling hydrogen generation with parallel oxidative reactions of industrial relevance.

The modular architecture enables independent optimization of the PEC and PV components, as well as of the dichroic filter, allowing fine-tuning of the spectral distribution to balance STH and PCE. The optimized configuration achieved a combined Solar-to-X efficiency of 18.2%, approaching the standalone Si-PV performance (18.4%) and distributed between chemical fuel generation (7.1% STH) and electrical power output (11.1% PCE).

Importantly, this performance was achieved using low-cost, commercially available Si PV cells, making the architecture competitive with state-of-the-art PEC-PV systems based on expensive multi-junction modules, as well as commercial PV-electrolysis setups. By leveraging optical selectivity and modular integration, this approach advances the development of practical, high-efficiency Solar-to-X technologies for sustainable energy applications.

Among the most promising photocatalysts for solar energy conversion into hydrogen as a clean fuel via photoelectrochemical (PEC) water splitting, two ternary metal oxides able to absorb a relatively large portion of the solar spectrum, i.e., BiVO₄ and CuWO₄, represent good candidates to be employed as photoanodes in the demanding water oxidation reaction [1].

In particular, BiVO₄, having a 2.4 eV band gap energy, has emerged as the leading photocatalyst for this application despite its poor electron transfer ability and slow water oxidation kinetics. Two main strategies have thus been pursued to improve the PEC performance of BiVO₄, consisting in either i) coupling it with another suitable semiconductor oxide in a heterojunction, or ii) doping it with hexavalent metal ions such as Mo⁶⁺.

Indeed, significantly higher values of Incident Photon to Current Efficiency (IPCE) have been attained at wavelengths shorter than 500 nm when BiVO₄ was coupled with WO₃ in a WO₃/BiVO₄ heterojunction, by exploiting the excellent visible light harvesting properties of BiVO₄ combined with the superior conductivity of photoexcited electrons, typical of WO₃. Due to the favourable band alignment between the two oxides, photopromoted electrons in BiVO₄ are expected to migrate into WO₃ and then rapidly to the external circuit, while photoproduced holes may accumulate in BiVO₄. Selected case studies dealing with the performances exhibited under different irradiation configurations by home-made thin coupled electrodes, prepared through variable deposition techniques, will be presented [2-4]. At the same time, the multifaceted role of Mo⁶⁺ doping onto both the bulk and surface properties of BiVO₄ will be clarified by means of a unique combination of morphological and PEC analyses [5, 6].

On the other hand, an efficient use of CuWO₄ as photoanode material requires to overcome its severe internal charge recombination due to intra-gap states, acting as electron traps, as revealed through a PEC investigation coupled with ultrafast transient absorption analyses [7]. This issue has been mitigated by the 50 at.% molybdenum for tungsten substitution [8], with the development of CuW_0.5Mo_0.5O₄ photoanodes, exhibiting a 4-fold increase of the bulk charge carrier separation efficiency compared to pristine CuWO4, as clearly evidenced by intensity modulated photocurrent spectroscopy (IMPS) analysis [9], in full agreement with the results of PEC measurements performed in the presence of sacrificial agents or cocatalysts [10]. Optimized CuW_0.5Mo_0.5O₄ combined with BiVO₄ in a heterojunction finally exhibited a definitely superior PEC performance compared to the individual components, with a synergistic charge separation improvement in the 350-480 nm range under frontside irradiation through a BiVO₄ layer thick enough to absorb most of the incident light [9].

The extensive combustion of fossil fuels has led to severe environmental pollution and global warming. Achieving carbon neutrality requires accelerating the development and deployment of clean, renewable energy sources. Converting small molecules such as water and carbon dioxide into green fuels and value-added chemicals via photo- and electrocatalysis offers a promising route to replace fossil fuels while reducing CO₂ emissions. This approach also enables the storage of solar and renewable electricity, addressing their intermittency and geographical constraints, and holds significant scientific and practical potential. In this talk, I will outline the background and importance of photo- and electrocatalytic fuel and chemical synthesis, and present our recent progress on photoelectrodes, electrocatalysts, and integrated devices for water splitting and CO₂ reduction.

The advancement of photocatalytic systems for solar fuel production requires careful control over the structures and properties of metal oxide semiconductor materials. In this talk, I will present integrated synthesis strategies that combine Physical Vapor Deposition with Rapid Photonic Annealing,^[1] demonstrating how these methods address different material challenges across the semiconductor synthesis spectrum to unlock the potential of solar fuels.

Using Bi₂O₃ as a model binary system (chosen for its relatively accessible processing temperatures and well-characterized polymorphism), we demonstrate kinetic control of crystal polymorphism through Flash Photonic Heating (FPH, heating rates of 10⁶-10⁷ °C/s), achieving reversible α↔β phase transformations. Controlling polymorphism is critical because metastable phases often exhibit superior optoelectronic properties - the β-phase shows substantially enhanced photocurrent density and reduced bandgap compared to the thermodynamically stable α-phase. While pulsed laser deposition enhances film quality by delivering energies orders of magnitude higher than those of chemical methods for precise stoichiometric control, FPH enables polymorph control at accessible temperatures, providing insights into kinetic versus thermodynamic crystal-formation pathways.

At the opposite end of the synthesis-complexity spectrum, high-entropy rare earth oxides (HEREOs) such as (Ce₀.₂Zr₀.₂La₀.₂Pr₀.₂Y₀.₂)O₂ require integrated strategies to fulfill their potential for enhanced catalytic performance and structural stability. These chemically complex, refractory materials need the combined high-energy deposition techniques of physical vapor deposition with the unique thermal processing capabilities of rapid photonic annealing to ensure proper crystallization and controlled nanostructuring. FPH allows the formation of nanoscale grains and increased surface areas that are practically impossible to achieve with conventional thermal processing, unlocking the catalytic potential crucial for next-generation energy conversion systems.

This materials-spanning approach shows how synthesis requirements shift from controlling polymorphs for better optoelectronic properties in binary systems to integrated nanostructuring strategies for improved catalytic performance in high-entropy materials. The presentation will illustrate how understanding these different challenges offers key pathways for developing advanced metal oxide semiconductors for solar fuel applications.

Photoelectrochemical (PEC) water splitting provides a compelling route to sustainable hydrogen production, and organic semiconductors offer unique advantages for this technology through their low cost, tuneable optoelectronic properties and compatibility with scalable, large-area solution processing. However, their application in PEC systems remains limited by modest photocurrent densities, instability in aqueous environments, and poor intrinsic photostability of many bulk-heterojunction (BHJ) blends, which require to be investigated and addressed in order to achieve long-term device operation.

In this talk, I will first present our work using a high-performing PM6:D18:L8-BO BHJ photoanode protected by a multifunctional graphite sheet that is electrically conductive, functionalised by an earth-abundant NiFeOOH catalyst, and readily fabricated using scalable processing. This protective layer prevents water-induced degradation while enabling efficient charge transfer to the catalyst, allowing photocurrent densities exceeding 25 mA cm^–2 at 1.23 V_RHE for water oxidation. Furthermore by integrating two photoactive layers with complementary absorption into a monolithic tandem photoanode, we demonstrate bias-free hydrogen generation with a solar-to-hydrogen (STH) efficiency of 5% [1].

I will then discuss how monolithic organic photoelectrodes can more broadly exploit the excellent optoelectronic properties of polymer:non-fullerene BHJs, while also addressing the remaining limitations of high voltage losses, poor photostability, and the high synthetic complexity typical of many donor–acceptor combinations. To overcome these constraints, we introduce a new BHJ comprising the low-synthetic-complexity polymer PTQ10 and the near-infrared-absorbing acceptor L8-BO. When paired with the NiFeOOH-functionalised graphite sheet, the resulting monolithic photoanodes achieve an onset potential of +0.64 V_RHE, a photocurrent density of 21 mA cm^–2 at +1.23 V_RHE, and a t₈₀ operational stability of 22 h under full AM 1.5G illumination. Compared to our earlier PM6:D18:L8-BO system, this represents a 40 mV increase in photovoltage and a sevenfold improvement in operational stability (t₈₀ extended from 3 h to 22 h). I will show how the superior photochemical and morphological stability of the PTQ10:L8-BO blend underpins these improvements, addressing the key degradation pathways that limit long-term operation. Building on this improved system, I will demonstrate monolithic tandem photoanodes based on PTQ10:IDIC and PTQ10:L8-BO absorbers, achieving bias-free water splitting with a record STH efficiency of 6.2%.

Finally, I will outline key directions for further enhancing both the stability and efficiency of integrated tandem organic photoelectrodes for bias-free photoelectrochemical hydrogen production.