Renewable H₂ is a carbon-free energy vector and sustainable alternative to the use of fossil fuels. Solar-driven photocatalytic H₂ generation from water is envisioned as an attractive technology for this purpose.[1] In recent years, metal-organic frameworks (MOFs) have emerged as potential heterogeneous photocatalysts for water splitting reactions.[2] One of the current challenges is the development of efficient MOFs for solar-driven overall water splitting (OWS) into H₂ and O₂.[2]

Herein, I present some of the strategies developed in our group to boost the photocatalytic performance of MOFs for solar-assisted water splitting reactions including hydrogen evolution (HER), oxygen evolution (OER) and OWS reactions.[2] The nature of metal node and organic ligand composition is identified as a methodology that determines both their energy band diagram and ability to boost water splitting reactions.[3] The development of mixed-metal porphyrin-based MOFs will also be discussed.[4]  The influence of co-catalysts on photocatalytic activity of MOFs will be briefly shown.[5] MOF defect engineering via in situ or post-synthetic modification also can boost photocatalytic activity.[6] Finally, recent methodologies based on engineering MOF-on-MOF heterojunctions with the record  activities for challenging OWS are presented.[7] Experimental evidence on the observed photocatalytic activities and insights about reaction mechanisms will be discussed through the presentation based on several techniques including spectroscopic and electrochemical characterizations combined with computational chemistry.

Keywords: Photocatalysis, Homogeneous Catalysis, Heterogeneous Catalysis, Single Atom Catalyst

Background and motivation. Among various solar energy conversion techniques, photocatalysis is deemed as a promising, environmentally benign, and cost-effective strategy to generate both fuels and high-value chemicals. While in this domain homogeneous photocatalysts prevail due to higher selectivity but the reutilisation of the catalyst is next to impossible. On the other hand, heterogeneous photocatalysts are recyclable but not highly selective. Therefore, to make a bridge between these two, a new strategy has been developed by synthesizing single metal atom photocatalysts that are selective as well as recyclable. In general, single-atom photocatalysts (SACs) have shown their compelling potential and arguably become the most active research direction in photocatalysis due to their fascinating strengths in enhancing light-harvesting, charge transfer dynamics, and surface reactions of a photocatalytic system.

Materials and methods. Based on the advantages of photocatalysis, we have synthesized several homogeneous catalysts as well as heterogeneous catalysts such as COFs by following routine techniques. However, for the SACs, we have designed our new synthetic technique.

Results and discussion. Based on this, recently we have developed several photocatalytic strategies for the synthesis of fuels as well as high valued chemicals via C-H bond functionalization, CO2 utilisation, water oxidation, plastic valorisation where the catalysts exhibited excellent selectivity as well as recyclability.

Increasing global energy demands have led to a rapid rise in environmental concerns, largely driven by the growing CO₂ levels in the atmosphere. Inspired by photosynthesis in nature, photocatalytic materials have gained significant attention as a method of producing value-added chemicals using CO₂ as feedstock material, mitigating the escalating greenhouse effect.

Owing to their high surface area, stability and tuneability, porous materials such as Metal Organic Frameworks (MOFs) and Covalent Organic Frameworks (COFs) are promising candidates for photocatalysis. Despite this, the absence of catalytically active metal sites and poor stabilities have hindered COFs and MOFs respectively. In addition, in terms of photocatalysis, MOFs and COFs have faced limitations due to factors including insufficient light absorption capabilities and quenching processes.[1,2] These problems could be addressed by introducing metal complexes into stable COF backbones, forming porous Metalloligand Covalent Organic Frameworks (MLCOFs), as demonstrated by Han et al., who used a Ru-MLCOF to photocatalytically produce syngas from CO₂.[3,4]

However, a better understanding of the atomic structure of MLCOFs is expected to improve their performance and design. Due to the low crystallinity of these frameworks, obtaining a complete structural understanding of MLCOFs is a challenge. Nevertheless, using a theoretical and experimental approach, MLCOFs can be fully characterised, as illustrated in a Ru(tpy)₂-based MLCOF.[5] Taking inspiration from this, we have synthesised a novel MLCOF, instead using Fe(tpy)₂ metalloligands and tetrasubstituted pyrene nodes. With the aim of producing a more environmentally friendly alternative, we use iron, an earth abundant transition metal. Morphological changes in response to the modulation of monomer ratios, shifting from globular to coral-like will be presented. In addition, the use of a freeze-thaw processing step, further shifts the morphology to stacked elongated sheet-like structures. A complete structural characterisation has been conducted, including crystallographic investigations using 3D electron diffraction and X-ray diffraction techniques. Prelimianry CO₂ reduction tests have been carried out and the results will be presented.

Understanding interfacial charge transfer dynamics in photoelectrochemical (PEC) systems is essential for optimizing the performance of semiconductor-based photoelectrodes. In this work, we systematically investigate electron transfer from the conduction band minimum (CBM) of a p-type semiconductor to redox states in the electrolyte under AM1.5G, as described by the Gerischer charge transfer model [1]. This framework considers the energetic overlap between the semiconductor CBM and a Gaussian distribution of redox states, defined by the reorganization energy (λ).

Here in this study, we focus on p-type Ag-alloyed Cu(In,Ga)Se₂ (ACIGS) photocathodes and demonstrate that their PEC response is strongly governed by the alignment between the CBM and the redox energy distribution of protons (H⁺) in the electrolyte. A compositional series with varying Ag-to-(Ag+Cu) atomic ratios (AAC) is examined, revealing that shifts in CBM position relative to λ significantly affect interfacial charge transfer efficiency.

By fitting chopped-light linear sweep voltammetry (LSV) responses of ACIGS photocathodes with various AAC using the Gerischer model, we quantitatively extract key physical parameters including λ, CBM position, charge transfer rate constant (k₀), and the effective photovoltage (E_Ph). The best-performing composition (AAC = 20%) shows the strongest overlap between CBM and λ, with a minimal energy offset (~30 meV), supporting highly efficient charge transfer and validating the model.

Overall, this study provides a quantitative application of the Gerischer charge transfer framework, offering valuable insight into the relationship between interfacial energetics and PEC performance, and guiding the rational design of high-efficiency photocathodes.

Carbon nitride, a polymeric material consisting of heptazine-based networks, has been extensively studied in photocatalytic processes owing to associated narrow band-gaps which allow visible light activity, as well as structural tunability which further modulate electronic and surface properties in favor of enhanced photoactivity for target chemical turn-overs. Particularly, poly(heptazine imide) (PHI) networks exhibit permanent ionic character, these charges being counter-balanced by common alkali such as Na⁺ or K⁺, which may be easily exchanged by other cationic species, targeting different structural or electronic modifications with implications in photocatalytic applications [1,2].

In this talk, I will highlight the importance of this ionic character in PHI networks, in conjuncture with observed photocatalytic performances. Mainly, the nature of such counter charge substituents plays an essential role in improving the lifetime of photogenerated electrons, phenomenology previously described for hybrid perovskites, while also facilitating and modulating catalytic processes dependent on electron/proton-transfer steps, the main applications of our group being either H₂O₂ evolution or the hydrogenation of CO₂ and alkenes. Moreover, I will describe how chemical modifications of this nature have further implications in storing/stabilizing charge carriers over extended periods of time for potential dark-photocatalytic reactions or modified kinetic behaviors [3].

Graphitic carbon nitrides absorb photons with wavelength shorter than approx. 450 nm. Absorption of such a photon produces short-lived electronically-excited state of the material. The excited state can oxidize and reduce certain chemical species that are available in the reaction mixture. When the reaction mixture contains only molecules able to donate electrons (easily oxidizable species) and it is free of electron acceptors, among which the most ubiquitous is O2, the excited state is quenched reductively. It leads to accumulation of electrons and charge-compensating cations in the material. This process – excitation of carbon nitride with light followed by its reductive quenching – is denoted as photocharging. Photocharging produces a state of carbon nitride carrying multiple uncoupled electrons. The talk offers a succinct overview of factors affecting the ability of carbon nitride to accumulate electrons via photocharging, such as type of electron donor and graphitic carbon nitride. Several examples of photocharged carbon nitride application as a reductant of organic and inorganic molecules in the dark are highlighted.

Photodeposition has emerged as a promising technique to grow metallic nanoparticles (NPs) on semiconductor supports, through reduction of metal ions by photoexcited electrons in the support. Recombination with holes is a competing process that can be suppressed, by including a hole scavenger to extract holes. Various studies have indicated the importance of including a hole scavenger, but the exact role and interactions of hole scavengers during photodeposition remains unclear. In particular, the effect of various hole scavengers on the resulting material properties are not well understood e.g., optical response, particle size, particle distribution, and growth kinetics of the deposited NP. Controlling synthesis is crucial for optimizing optical properties and thus applications in photocatalytic performance. In this work we have investigated the influence of the presence and type of four alcohol-based hole scavengers on the photodeposition of gold (Au) on zinc oxide (ZnO).

Ex-situ High-Resolution (Scanning) Transmission Electron Microscopy combined with Energy Dispersive X-ray spectroscopy (HR-(S)TEM-EDX) results show that without a hole scavenger, a broad particle size and shape distribution (20 ± 15 nm) and less nucleation sites are obtained. The optical response of Au NPs prepared without a hole scavenger show two Au Localized Surface Plasmon Resonance (LSPR) peaks, one located at ~ 515 nm and one at ~ 575 nm. From electromagnetic simulations we suspect that the LSPR peak at ~ 515 nm arises due to smaller, low-aspect-ratio NPs, whereas the LSPR peak at ~ 575 nm is due to a few large NPs with a high aspect ratio. In contrast, the use of different alcohol-based scavengers results in a uniform spatial distribution of monodisperse NP sizes of 9 ± 3 nm, leading to a well-defined Au LSPR peak centered at ~ 540 nm. Unique in-situ UV-Vis spectroscopy measurements reveal the formation of an adsorbed intermediate of Au on ZnO under dark equilibrium conditions, which reacts upon UV irradiation. These measurements also show that the photodeposition of Au on ZnO in the presence of ethanol as a hole scavenger leads to a faster nucleation and substantially accelerates particle growth compared to the non-scavenged case.

These results show that in the presence of a hole scavenger, efficient charge separation generates a high density of photogenerated electrons. Combined with numerous nucleation sites leads to the formation of many small Au NPs. In contrast, in the absence of a hole scavenger fewer charge carriers result in fewer nucleation events, which lead to a limited number of larger Au NPs. This work highlights how in-situ UV-Vis spectroscopy monitoring provides mechanistic and kinetic insight, enabling optimization of photodeposition and tailoring of Au/ZnO nanocomposite properties for photocatalysis.

In recent years, two-dimensional organic materials have attracted great interest for their ease of synthesis, for their catalytic activities and semiconducting properties. The appeal of these materials is that they are layered and easily exfoliated to obtain a mono (or few) layer material with interesting optoelectronic properties. Moreover, they have great potential for photocatalysis to obtain solar fuels. While the experimental procedure is well established, the computational counterpart is still not fully developed due to inherent methodological difficulties. In this talk, we present a novel method that we have recently developed to tackle these issues. In particular, in our novel computational protocol for photocatalysis  we explicitly consdier the exited state reactions pathways, going beyond the common band alignment strategy. By usign a density funcional theory approach called detla-SCF, we can excite an electron to the conduction band of the semiconductor and study the mechanism for both reduction and oxidation reactions. As examples, we show how graphitic carbon nitride with Co single atom can be used as catalytic centre for oxygen evolution photocatalysis.

Solar energy conversion via photoelectrochemical (PEC) cells offers a promising solution to current energy challenges, but significant advancements in materials and cell configurations are necessary. Until now, inorganic semiconductors such as metal oxides have been widely studied due to their affordability, non-toxicity and high stability. However, challenges such as charge recombination, low electron mobility and in some cases limited visible absorption limit their ability to obtain high conversion efficiencies. There are several strategies to improve the performance of photoelectrodes, ranging from the modification of materials to the incorporation of more systems that favor the reaction. In our group we are tackling several of them, such as the modification of electrode surfaces for optoelectronic modification, the incorporation of cocatalysts and, as a more innovative part, the use of organic semiconductors to prepare hybrid electrodes. Organic polymers, particularly conjugated polymers (CPs), show potential due to their light-harvesting and conductive properties. In particular, conjugated porous polymers (CPPs), with their 3D structure, provide improved stability and higher surface area. The combination of CPPs with widely studied materials like TiO₂ that suffer from limited visible light absorption due to their high bandgap energy can enhance the general performance.

In this talk, I will present to the audience different strategies to use CPP as multifunctional layers in photoelectrodes, which has been a challenge due to the normally used methodology to obtain these materials, generally composed of micron-sized particles. In this sense, our group has made an effort to design and develop several routes for the use of these materials in photoelectrochemical systems, in particular nanostructuring and electropolymerisation. In fact, these polymers have proven to be efficient as multifunctional layers since they have light absorption properties while they are good conductors of holes in the case of p and electrons in the case of n. We have been able to observe improved photocurrents and photopotentials in these hybrid electrodes. Furthermore, advanced characterizations have been carried out using electrochemical impedance spectroscopy and transient absorption spectroscopy and it has been shown that in most cases, when well designed, these heterojunctions can promote better charge transfer and longer lifetimes of the photogenerated charges. This finding opens the door to the use of these systems not only in photoelectrochemical devices but in any optoelectronic device where the preparation of quality thin films is of vital importance.

The photoreduction of CO₂ using perovskite materials has gained increasing attention due to the need for efficient solar-driven carbon conversion. Halide perovskites offer strong visible-light absorption, high carrier mobility, and tunable band structures, but their limited stability and lead toxicity raise significant challenges. Lead-free double perovskites provide a more sustainable alternative, combining environmental compatibility, structural robustness, and bandgap tunability, although they typically exhibit wider band gaps and lower carrier transport efficiencies. To mitigate these limitations, we have developed hybrid catalysts based on the lead-free Cs₂AgBiBr₆ double perovskite coated with a semiconducting organic polymer. The polymer enhances charge-transport dynamics and significantly influences photocatalytic selectivity, shifting CO production at lower polymer concentrations toward CH₄ generation at higher loadings while maintaining structural stability. We have performed spectroscopic and electrochemical analyses to understand the relationship between charge transfer and product selectivity. Overall, this hybrid strategy offers a promising route to optimize sustainable lead-free perovskite-based photocatalysts for CO₂ reduction.

Photoelectrochemical (PEC) water oxidation is gaining increasing attention as a sustainable pathway for solar-driven hydrogen production. Developing efficient photoanodes capable of catalyzing the inherently sluggish water oxidation reaction is essential for enabling practical solar-assisted water splitting. Beyond the Oxygen Evolution Reaction (OER), alternative oxidation pathways leading to value-added chemicals are also being explored, as they are energetically less demanding and can enhance overall process efficiency. 

Hematite (Fe₂O₃) stands out as a promising photoanode material due to its chemical stability, earth abundance and suitable bandgap (1.9-2.1 eV) corresponding to a theoretical Solar-to-Hydrogen efficiency of 16.8% [1]. Its application, however, remains hindered by intrinsically low electrical conductivity and pronounced bulk electron-hole recombination. Cation doping with Ti(IV) or Sn(IV), the latter frequently introduced via thermal diffusion from the FTO substrate during annealing above 750°C, can partially mitigate these limiting factors [2]. Despite extensive research efforts, the beneficial role of dopants in hematite photoanodes, as well as the kinetic processes governing bulk charge-transport and interfacial charge-transfer reactions, are still not fully understood [3].

In this work, we employ a comprehensive set of electrochemical techniques, including Linear Sweep Voltammetry (LSV), Cyclic Voltammetry (CV), Electrochemical Impedance Spectroscopy (EIS), and especially Intensity Modulated Photocurrent Spectroscopy (IMPS), to investigate Ti and Sn-doped hematite photoanodes for OER and Methanol Oxidation Reaction (MOR). IMPS, a powerful in-operando technique for PEC technology, enables us to extract characteristic time constants associated with charge-transfer and recombination pathways. A central focus of this work is the adoption of a physically consistent IMPS model, which is essential for the accurate interpretation of complex spectral features and for distinguishing between competing kinetic processes [4]. Structural and morphological characterization via XRD and SEM complete the electrochemical analysis. 

Our results show that under back illumination, photocurrent generation is mainly limited by inefficient electron collection, whereas under front illumination the dominant bottleneck arises from slow hole-transfer kinetics during the OER. The introduction of Ti or Sn dopants mitigates these effects. Additionally, EIS analysis reveals that, unlike the OER, the MOR proceeds without the involvement of surface states typically active at the hematite/electrolyte interface.

By integrating a rigorous IMPS modeling framework with complementary photoelectrochemical methods, this work provides a coherent mechanistic picture of the processes limiting hematite-based photoanodes. The insights gained offer valuable guidelines for the rational design of improved, earth-abundant materials for solar-driven chemical transformations.

Among visible-light semiconductors for Photoelectrochemical (PEC) water splitting, bismuth vanadate (BiVO₄) has emerged as a leading candidate due to its suitable bandgap (~2.4 eV), relative abundance, and good photochemical stability. Nevertheless, its performance is hindered by short carrier diffusion lengths, sluggish oxygen evolution kinetics, and severe bulk recombination [1]. To address these intrinsic limitations, recent strategies have focused on heterojunction engineering and, more recently, on entropy-driven material design as a means to modulate electronic structure and interfacial reactivity [2,3].

In this work, we establish a platform for the rational integration of BiVO₄ with multicationic high-entropy oxide (HEO) architectures through entropy- and interface-engineered design to construct tailored heterojunctions capable of enhancing charge separation, catalytic activity and long-term PEC durability. HEO offer unprecedented compositional flexibility and entropy-stabilized phase formation [4], while their locally disordered environments can tune electronic properties, increase defect tolerance, and provide catalytically active sites [5]. To deepen the understanding of entropic effects on PEC behavior, we explore two complementary approaches. First, we examine the one-pot synthesis of high-entropy bismuth vanadates incorporating five distinct cations at low concentrations (<2%) substituting either Bi or V sites. This controlled substitution aims to preserve the optical absorption properties of BiVO₄ while introducing catalytic functionality and mitigating recombination pathways. Second, we compare this bulk entropy-driven strategy with a heterostructure approach, in which a conventional BiVO₄ layer is coupled to an externally deposited HEO overlayer. To fabricate these BiVO₄/HEO heterojunctions, we employ a solution-based thin-film process that ensures compositional precision and scalability. By tuning precursor concentration, spin-coating cycles, and thermal profiles, we obtain dense, adherent HEO films with controlled microstructure, while maintaining compatibility with low-cost, large-area manufacturing [6]. Through complementary advanced characterization, we confirm the formation of single-phase high-entropy layers and intimate interfacial contact with BiVO₄, modulated surface states, multivalent active sites within the HEO component, and  band offsets. Overall, this study positions HEO heterojunctions as a versatile and scalable strategy for cost-effective solar-to-hydrogen technologies.

Wide-gap chalcogenides without In, such as CuGaSe₂ and CuGa₃Se₅, serve as excellent photocathodes for unassisted water splitting. These thin films cover 86% and 68% of the maximum theoretical photocurrents with band gaps of 1.68 and 1.85 eV, respectively [1]. However, Fermi level pinning is one of the major challenges that hinders the charge transfer at the semiconductor/electrolyte junction, leading to overpotentials between E_on and E_fb. Over the years, Ag incorporation has gained focus to address the challenges in PEC (Photoelectrochemical), as partial substitution of Cu by Ag deepens the VBM and reduces the formation of Cu-deficient phases. We prepared wide-band-gap Ag_xCu_1-xInGaSe₂ thin film photocathodes via a three-stage co-evaporation process. We investigated different compositions where [Ag]/[Ag]+[Cu] (AAC) = 20% emerges as highly efficient thin film in both PV (η = 11.56%) and PEC (J_ph= 24.08 mA/cm₂), and also examine the influence of (AAC) and [Ga]/[Ga]/[In] (GGI) depth profiles on both PEC and PV cells. To enhance the PEC properties, we further modify the surface to form a p-n junction on the surface of the electrode, and also study the impact of catalyst layers. Finally, we present the experimental results with improved onset potentials and photocurrent density, for both with and without surface modification.

Metal halide perovskites have emerged as promising alternatives among established light absorbers, enabling unassisted PEC water splitting^[1,2] and CO₂ reduction to syngas.^[3,4] While the bare perovskite light absorber is rapidly degraded by moisture, recent developments in the device structure have led to substantial advances in the device stability. Here, I will give an overview of the latest progress in perovskite PEC devices, introducing design principles to improve their performance and reliability. For this purpose, I will discuss the role of charge selective layers in increasing the device photocurrent and photovoltage, by fine-tuning the band alignment and enabling efficient charge separation. A further beneficial effect of hydrophobicity is revealed by comparing devices with different hole transport layers (HTLs).^[1,2] On the manufacturing side, I will reveal new insights into how appropriate encapsulation techniques can extend the device lifetime to a few days under operation in aqueous media.^[2,3] To this end, low melting alloys are replaced with graphite epoxy paste as a conductive, hydrophobic and low-cost encapsulant.^[2,5] These design principles are successfully applied to an underexplored BiOI light absorber, increasing the photocathode stability towards hydrogen evolution from minutes to months.^[6] Finally, we will explore the next steps required for scalable solar fuels production, showcasing the latest progress in terms of device manufacturing. A suitable choice of materials can decrease the device cost tenfold and expand the device functionality, resulting in flexible, floating artificial leaves.^[4] Those materials are compatible with large-scale, automated fabrication processes, which present the most potential towards future real-world applications.^[7,8] Similar PEC systems approaching a m² size can take advantage of the modularity of artificial leaves,^[9] whereas thermoelectric generators can further bolster water splitting by utilizing waste heat to provide an additional Seebeck voltage.^[10,11] Lastly, I will introduce PEC devices as versatile platforms to produce value-added chemicals including C₂ hydrocarbons (ethene, ethylene) and glycerol oxidation products, by interfacing the perovskite semiconductor with copper nanoflower catalysts and silicon nanowires (Fig. 1).^[12]

Photocatalysis is one of the most desired approaches for several energy production-related and sustainable applications, i.e., hydrogen production from water splitting or wastewater treatment. Nevertheless, it is crucial to use appropriate photocatalytic materials, endowed with specific features such as low charge carriers recombination, stability and activity in the visible range. The latter represents the most required property considering that 40% of sunlight is composed by visible light and the target of any photocatalytic application is the use of Sun as energetic source. Yet, the use of visible light-active materials is a countertrend compared to the current state of the art, which has seen TiO₂ as the most used photocatalyst. Even though it has high activity for many photocatalytic applications,^[1] TiO₂ suffers from a wide bandgap (3.0 eV)^[2] and consequently it is not active in visible light. Hence, our materials are in line with the tendency of research for new photocatalytic materials.

We chose transition metal hydroxides as target materials due to their interesting, layered morphology, which allows tunability of their bandgap up to the visible-light range.^[3] These materials suffer from radiation damage and high carriers recombination,^[4] so they need to be modified to be efficiently applied in photocatalysis. We synthesized different materials based on Co(OH)₂, i.e., Co₃O₄@Co(OH)₂ and Eu@Co(OH)₂ and tested them in photocatalytic hydrogen production from water splitting and degradation of microplastics and methylene blue. Comparing the results with the bare Co(OH)₂, we could appreciate the improvements obtained by modifying the material. Indeed, the stability under light and the catalytic activity were enhanced and the charge carriers recombination was diminished. Moreover, we studied the synthesis of a novel Ni carbonate hydroxide. In this case we could obtain for the first time a hierarchical structure of Ni₂(CO₃)(OH)₂·4H₂O with high potential for photocatalytic hydrogen evolution from water splitting. The materials presented were synthesized using a low-temperature approach and they were fully characterized to confirm our findings. Indeed, the UV-Visible spectra of the materials confirmed their low bandgap (2.0-2.4 eV) and the use of photocurrent and electron impedance spectroscopy showed the lower charge recombination compared to the bare hydroxides.

The direct conversion of solar energy into chemical energy in the form of solar fuels (e.g. green hydrogen), has the potential to contribute significantly to cover our energy needs [1,2]. Due to its comparably simple setup photoelectrochemical (PEC) water splitting shows promise as a cost-effective method for producing green hydrogen in the future [3,4]. A PEC system consists of two spatially separated electrodes connected via an ohmic contact, with the water oxidation taking place on the anode side and the hydrogen reduction on the cathode side of the cell [5].

While there have been significant advancements regarding the efficiency of PEC systems, their stability remains a major challenge [2,4]. So far, mainly two approaches have been pursued to improve the stability of photoelectrodes. Material-focused strategies (such as protective layers, surface engineering, cocatalysts, and doping) and device-level strategies (such as electrolyte engineering, reactor configuration, and operation conditions) [6, 7, 8, 9]. The best choice between those depends primarily on the design of the photoelectrode and its primary degradation mechanism. For example, in BiVO₄-based photoanodes, where dissolution is the main issue, electrolyte engineering, particularly saturating the electrolyte with V⁵⁺, has proven highly effective in reducing dissolution and thus enhancing photoelectrode stability [10]. Meanwhile for LaTiO₂N (LTON) based photoanodes, where degradation is dominated by cocatalyst related performance loss, stability improvements should focus on increasing cocatalyst stability.

In this study the stability of LTON based photoanodes was improved by tuning cocatalyst deposition. LTON was synthesized via solid state synthesis followed by thermal ammonolysis. The photoanodes were prepared by electrophoretic deposition followed by TiCl₄ necking. The cocatalysts were deposited via dip coating in ethanolic Co(NO₃)₂ and Ni(NO₃)₂ solutions, followed by annealing at elevated temperatures. To improve their stability the concentration of the solution, the dipping time as well as the annealing time and temperature have been varied. Further the annealing was performed in both oxidative (air) and reductive atmosphere (ammonia) followed by a further annealing step in air. The stability of the photoanodes was evaluated via long time chronoamperometries at 1.23 V vs RHE. Compositional and morphological changes of the photoanodes were investigated using SEM and HREM, while cocatalyst dissolution and electrolyte compositions were determined by ICPMS.

The electrochemical reduction of CO₂ (CO₂RR) is a sustainable technology that can be used to convert CO₂ into valuable products when electricity is generated from renewable sources. The focus of application-oriented research is currently mainly on the electrolysis of CO₂ to produce hydrogen-rich fuels or chemical feedstock material. In contrast, our project aims to produce stable, carbon-rich products (e.g. oxalate and carbon flakes) that can be safely and permanently disposed of in geological repositories. This negative emission technology has been developed to sustainably remove CO₂ from the global cycle.

The continuous electrochemical formation of solid carbon flakes from CO₂ has been reported on liquid GaInSn-M alloys (M: Ce, V) in water containing DMF^1,2. To further develop this approach, we studied GaInSn with and without additional Cerium alloying in DMF/H₂O/ TBAPF₆ electrolyte. Pure GaInSn shows a significant activity for CO₂RR to carbon monoxide (CO) and formate, depending on the content of water. We found evidence that the chemical composition of the liquid GaInSn interface changes with the applied electrode potential and water content, what can explain the observed product selectivity³. Our results suggest that the carbon monoxide (CO) generated on GaInSn serves as an intermediate product in the formation of carbon flakes on the alloyed cerium particles⁴. However, our study shows that numerous challenges still need to be overcome for this technically complex approach, which currently shows rather comparatively low carbon production rates.

As an alternative, the reduction of CO₂ to oxalate on solid, activated lead electrodes in anhydrous propylene carbonate (PC+TEA-Cl) was investigated. Faraday efficiencies for CO formation of over 80% at current densities of over 20 mA/cm² make it suitable for use as a cathode in a standalone solar-powered electrolyser. For this purpose, the electrolyzer was coupled with 5-junction or 3-junction solar cells and operated under 1sun-AM-1.5G illumination. To keep the required cell voltage sufficiently low, sacrificial electrodes were used and investigated as anodes. Using Zn anodes, we achieved solar-to-carbon conversion efficiencies of over 10%, around five to ten times higher than it is observed in natural photosynthesis. This demonstrates that our technical approach could require a smaller valuable land area than biomass-based methods.

Light-induced chemical transformations involve the synchronization of several steps starting with light absorption by a molecular chromophore or semiconductor followed by the transfer of the absorbed energy to the reaction substrate or to the catalyst responsible for the chemical transformation. In this talk, the use of organic chromophores or materials as photocatalysts for organic transformations or solar fuel production (such as hydrogen) will be discussed. In addition, the description of new electron donor-acceptor (EDA) complexes that open new chemical space in organic chemistry will be presented as a source of inspiration for catalytic transformations using organic materials. Emphasis will be given to the mechanistic pathways leading to the transformations proposed on basis of electrochemical analysis, steady state or time resolved spectroscopic techniques and the detection or trapping of reaction intermediates. Thus, important insights into energy transfer and electron transfer phenomena happening between photocatalyst, reaction's substrates and additional components (mainly sacrificial agents) will be discussed.

Figure 1. Representative photocatalysts, EDA complexes and resulting organic transformations. CTF: Covalent Triazine Framework. COF: Covalent Organic Framework.

References

[1] A. Gallego-Gamo, A. Granados, R. Pleixats, C. Gimbert-Suriñach, A. Vallribera, J. Org. Chem., 2023, 88, 12585. [2] A. Gallego-Gamo, D. Reyes-Mesa, A. Guinart, R. Pleixats, C. Gimbert-Suriñach, A. Vallribera, A. Granados, RSC Adv., 2023, 13, 23359.

[3] P. Sarró, A. Gallego-Gamo, R. Pleixats, A. Vallribera, C.  Gimbert-Suriñach, A. Granados, Adv. Synth. Catal., 2024, 366, 2587.

[4] A. Gallego-Gamo, Y. Ji, P. Sarró, R. Pleixats, E. Molins, C. Gimbert-Suriñach, A. Vallribera, A. Granados, J.Org. Chem., 2024, 89, 11682. [5] A. Gallego-Gamo, R. Pleixats, C. Gimbert-Suriñach, A. Vallribera, A. Granados, Chem. Eur. J., 2023, e202303854. [6] Y. Ji, A. Jaafar, C. Gimbert-Suriñach, M. Ribagorda, A. Vallribera, A. Granados, M. J. Cabrera-Alonso, Org. Chem. Front., 2024, 6660. [7] M. Gil, A. Gallego-Gamo, P. Sarró, R. Pleixats, C. Gimbert-Suriñach, A. Vallribera, A. Granados, J. Org. Chem., 2025, 90, 2500. [8] P. Sarró, A. Gallego-Gamo, C. Gimbert-Suriñach, A. Vallribera, A. Granados, Org. Chem. Front., 2025, 12, 3475. [9] P. Sarró, A. Gallego-Gamo, E. Molins, R. Pleixats, C. Gimbert-Suriñach, A. Vallribera, A. Granados, Org. Let., 2025, 27. 11372. [10] Y. Ji, R. Pleixats, I. Fernández, C. Gimbert-Suriñach, A. Vallribera, A. Granados, Org. Chem. Front., 2025, asap. [11] M. Salati, F. Dorchies, J.-W. Wang, M. Ventosa, S. González-Carrero, C. Bozal-Ginesta, J. Holub, O. Rüdiger, S. DeBeer, C. Gimbert-Suriñach, J. R. Durrant, M. Z. Ertem, M. Gil-Sepulcre, A. Llobet, Small, 2024, 2406375. [12] D. Reyes-Mesa, P. Sarró, M. F. Gusta, A. Jiménez-Solano, S. Das, B. P. Biswal, H. Vignolo-González, L. Velasco, A. Llobet, N. G. Bastús, V. F. Puntes, A. Vallribera, R. Pleixats, A. Granados, B. V. Lotsch, C. Gimbert-Suriñach, J. Am. Chem. Soc., 2025, asap.

Although significant advances have been made in the understanding, fine characterization, and synthesis control of photocatalytic materials, as well as in the design of photo-reactors, some challenges still remain for the development of systems that can be effectively integrated into devices for applications in the fields of Energy, and Environment.

To overcome the main limitations of photocatalytic materials, particularly semiconductor oxides (SCs) such as TiO₂, various strategies are currently being explored. Among the key challenges are (i) the enhancement of photocatalytic conversion efficiency, which involves utilizing photons more effectively, and (ii) the direct use of solar light, specifically wavelengths within the visible spectrum.
To achieve this, several strategies are under investigation (chemical doping, formation of heterojunctions between SCs, deposition of (bi-)metallic nanoparticles inducing co-catalytic and/or plasmonic effects, optimization of SC morphology (1D, 2D, 3D), modulation of surface area and porosity, association with adsorbents,…). In this context, coupling with carbon-based nanostructures is an area of significant interest.
This presentation will focus on the elaboration and optimization of TiO₂ carbon-based nanostructures, such as activated carbon, nanodiamonds, and 2D nanostructures such as Few-Layer Graphene (FLG) or derivatives, such as g-C₃N₄, for photocatalytic applications in pollution abatement and solar fuel production (H₂ production by solar-light driven photocatalytic water splitting or alcohols reforming, or gas-phase CO₂ photocatalytic reduction).