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Nithin Jacob, from the research group LCP, presented his work at the eMRS Fall 2024 conference on the advances in the field of materials science in Warsaw, Poland. Direct Z-scheme junctions have found applications in the field of air and water purification, hydrogen production and production of hydro-carbon fuels from CO₂ reduction. These junctions which often utilize dual n-type material have a theoretical maximum conversion efficiency of 11.4% taking into account charge carrier generation, separation and recombination. Chalcogenide materials have high absorption coefficients and are often made with earth abundant materials. These materials are potential candidates for Direct Z-scheme junctions that can be used on a large scale. In this study we simulate the current voltage characteristics of different heterojunctions (both dual n- and p-type as well as p-n junctions) to find promising photo-cathode chalcogenide materials that can be used as photocatalysts. The selection is made based on the photovoltaic performance of these photocatalytic materials, therefore this work reports on their ability to generate and separate charge carriers under illumination with the standard solar spectrum. Conventional solar cell parameters such as open circuit voltage, short circuit current and efficiency are used to evaluate the performance of these junctions. The study finally gives some case studies on direct Z-scheme junctions that could be used for CO₂ reduction to methanol and discusses their working principles.

Hybrid halide perovskites (HaPs) represent a class of material with excellent optoelectronic properties providing distinct avenues for disruptive photo(-electro) catalytic technologies. However, their photocatalytic activity, selectivity and stability remains a scientific and technological hurdle. In this perspective, we discuss fundamental aspects of perovskite based photocatalytic systems, specifically for CO₂ conversion and high value oxidation reactions, and highlight critical limiting factors and on-going challenges in the field. We critically assess the recent advances in designing halide perovskite hetero-interfaces and characterization methodologies which are often used to define the performance metrics. Furthermore, we outline important questions and identify emerging trends in relation to the remediation strategy towards improved photocatalytic performance and stability from halide perovskite semiconductors.

Michele Del Moro, from the research group ELCAT, presented his work at the 38th Topical Meeting conference in Manchester, UK. Photoelectrochemical devices harnesses solar power to convert carbon dioxide and water into valuable chemicals such as H₂, CO, HCOOH, ethylene and ethanol. The development of novel semiconductors and efficient photoelectrode architectures for that purpose remains critical. Despite years of research, the most studied materials are still Cu₂O-based ones, since it is one of the few native p-type semiconductors that is also known to be active for CO₂ reduction. Among the various activity enhancements strategies found in the literature, nano-structuring Cu₂O with optimal aspect ratio was recently reported to reduce the recombination of charges, therefore resulting in high photocurrents and efficiencies. In this work, we synthesized Cu2O nanowires (NWs) with a length of 2-5 μm and 100-500 nm diameter dfollowing Grätzel et al. The challenges we have encountered during the preparation of these nanowires (e.g. substrate choice, layer thickness, anodization parameter, thermal treatment) will be discussed in order to streamline possible future replication of Cu₂O NWs electrodes. Figure 1b shows that a net photocurrent density as high as 1.6 mA cm^-2 could be obtained during chopped light – linear sweep voltammetry (CL-LSV) measurements. However, since the formation of Cu was observed, the photocurrent was attributed to Cu₂O photoelectrodemical reduction to Cu, rather than to CO₂ reduction. Therefore, an AZO/TiO₂ layer was implemented by atomic layer deposition (ALD) as a protection layer and subsequently covered with Ag as a CO₂ reduction catalyst (Figure 1c). By implementing this protection strategy, the stability of the photoelectrode could be increased. Lastly, the effect of TiO₂ protection layer thickness and Ag loading on photocurrent, stability and CO faradaic efficiency will be discussed.

TiO₂ is the most widely used material in photoelectrocatalytic systems. A key parameter to understand its efficacy in such systems is the band bending in the semiconductor layer. In this regard, knowledge on the band energetics at the semiconductor/current collector interface, especially for a nanosemiconductor electrode, is extremely vital as it will directly impact any charge transfer processes at its interface with the electrolyte. Since direct investigation of interfacial electronic features without compromising its structure is difficult, only seldom are attempts made to study the semiconductor/current collector interface specifically. This work utilizes ultraviolet photoelectron spectroscopy (UPS) to determine the valence band maximum (E_VBM) and Fermi level (E_F) at different depths in a nano-TiO₂/TiN thin-film system reached using an Ar gas-clustered ion beam (GCIB). By combining UPS with GCIB depth profiling, we report an innovative approach for truly mapping the energy band structure across a nanosemiconductor/current collector interface. By coupling it with X-ray photoelectron spectroscopy (XPS), correlations among chemistry, chemical bonding, and electronic properties for the nano-TiO₂/TiN interface could also be studied. The effects of TiO₂ in situ electrochemical reduction in aqueous electrolytes are also investigated where UPS confirmed a decrease in the semiconductor work function (WF) and an associated increase in n-type Ti³⁺ centers of nano-TiO₂ electrodes post use in a 0.2 M potassium chloride solution. We report the use of UPS to precisely determine the energy band diagrams for a nano-TiO₂/TiN thin-film interface and confirm the increase in TiO₂ n-type dopant concentrations during electrocatalysis, promoting a much more comprehensive and intuitive understanding of the TiO₂ activation mechanism by proton intercalation and therefore further optimizing the design process of efficient photocatalytic materials for solar conversion.

Järi Van den Hoek, from the research group ELCAT, presented his work at the 75^th Annual Meeting conference on the advances in the field of materials science in Montréal, Canada. One of the most promising techniques for Carbon Capture and Utilization (CCU) is the electrochemical reduction of atmospheric or industrial waste CO₂ reduction to a variety of value-added products, including carbon monoxide (CO), formate/formic acid, ethylene (C₂H₄), etc. The production of formate using this sustainable and innovative technology holds significant potential for industrial applications given the high market value and the increasingly stringent regulations on CO₂ emissions. To advance this technology to a technological readiness level (TRL) 5, the combined development of high-performance electrolyzers and active, selective, and stable electrocatalysts is crucial. While electrocatalysts based on Bi, In, and Sn have been proposed for their high catalytic performance, the understanding of their interface properties and their impact on the electrochemical reactor performance remains limited. In this work, we employed two distinct atomic layer deposition (ALD) techniques (Plasma-enhanced (PE) and Thermal (T) ALD) to synthesize low-loading, uniform β-In₂S₃ crystal surface with exceptional performance for formate production and altered triple-phase boundary (TPB). Characterization of the obtained In₂S₃ ALD thin films was conducted using X-ray photoelectron spectroscopy (XPS), grazing-incidence wide angle X-ray scattering (GIWAXS), contact angle (CA) and scanning electron microscope (SEM). The reactive plasma-enhanced deposition allowed for modification of the gas diffusion electrode (GDE) substrate, thereby influencing its hydrophobicity and electrochemical performance through alteration of the TPB. Consequently, the significance of these TPB adaptations was further highlighted for the electroreduction to formate in two diverse electrochemical systems: (i) a small-scale flow-by electrolyzer with a current density exceeding 500 mA cm^-2, selectivity exceeding 90%, and stability of 30 h at 300 mA cm^-2, and (ii) a zero-gap electrolyzer at a current density of 150 mA cm^-2. The high catalytic performance of these uniform In₂S₃ ALD deposited thin films allowed for detailed electrochemical analysis of the reaction pathway and reaction order using a mass-transport coupled Tafel analysis and diluted CO₂ streams, providing clear insights into the electrocatalytic properties and performance of In₂S₃ for formate production. Our results demonstrate that altering the TPB by reducing its hydrophobicity impacts selectivity, activity, and stability but additionally modifies the reaction kinetics, including the limiting current density, exchange density, and the Tafel slope.

Check out the latest publication from SURF on UPS to investigate Ti₂O/Cu₂O Z-scheme junctions. Beatriz de la Fuente recently published in collaboration with Jan Bomnuter, Michele del Moro, Lien Smeesters, Vanina Cristaudo, Tom Breugelmans, Vera Meynen, Pegie Cool, Annick Hubin and Tom Hauffman a full length paper on the relationship between synthesis, electronic properties and visible light absorption of TiO₂/Cu₂O Z-schemes for photocatalytic applications with a combination of surface sensitive techniques and optical absorption studies. This work is the first application of Ultraviolet Photoelectron Spectroscopy (UPS) to study the relationship between Cu₂O electrodeposition time and electronic properties of the fabricated Z-schemes. The novelty of this work concerns the several insights that can be derived from UPS on previously fabricated TiO₂/Cu₂O heterojunctions and underlines which important electronic parameters can be extracted from the spectra to help with the band diagrams tracing task. Furthermore, a laboratory scale investigation is implemented to evaluate the visible light absorption performance of Cu₂O||TiO₂ under various electrodeposition conditions.

Nithin Jacob, from the research group LCP, presented his poster at the MATSUS conference on ‘Materials for sustainable development’ in Barcelona, Spain. Direct Z-scheme heterojunctions named after their charge transfer mechanism, use sunlight to conduct various photocatalytic reactions, similar to photosynthesis in plants. It is a promising candidate that can be used for CO₂ reduction reactions. Solar cell simulation techniques can be used to obtain material properties and insights into the electronic characteristics of these materials. By solving semiconductor differential equations that model the behavior of semiconductors under different light intensities and applied biases, the solar cell simulator program (SCAPS) can evaluate the energy band edges, carrier concentrations, and output characteristics of the device. In this study, various materials are simulated that could be used as direct Z-scheme junctions that can reduce CO₂ to methanol. These materials are modelled direct Z-scheme junctions in SCAPS by simulating the Shockley Read Hall (SRH) recombination using defect densities at the interface of the recombination junction (RJ). An initial screening methodology of Z-scheme junctions that can conduct the CO₂ reduction to methanol is presented.

During this conference Beatriz De La Fuente (SURF, VUB) talked about the combination of ultraviolet photoelectron spectroscopy (UPS) with optical absorption studies to investigate Cu₂O||TiO₂ direct Z-scheme junctions with different Cu₂O loading. This work is part of a recently accepted publication by the Applied Surface Science Journal and demonstrates the complementarity of surface sensitive techniques with optical absorption studies to design efficient Z-schemes photocatalysts for CO₂ reduction reactions via solar energy.

Check out the latest publication from LCP on the model for direct Z-scheme heterojunctions. This paper presents a comprehensive study on a compact model and the detailed balance limit for a dual n-type direct Z-scheme heterojunction. The compact model developed in this work describes the current–voltage (IV) characteristics of the staggered heterojunction under one-sided illumination. The model incorporates charge neutrality, surface recombination, thermionic emission over the barrier, and surface potentials. By considering these factors, the IV curve of the staggered heterojunction is captured, shedding light on the charge transfer and separation processes within the device. The heterojunction device consists of two photosystems: photosystem one (PSI) with a wide band gap and photosystem two (PSII) with a narrow band gap. Furthermore, the paper establishes the detailed balance limit for the efficiency of the dual n-type direct Z-scheme heterojunction. The maximum achievable efficiency, estimated to be 11.4%, is determined by the interplay between the band gap of PSII and the empirical relation for the maximum barrier for electrons leaving PSII. This detailed balance limit represents the highest efficiency that can be attained, accounting for carrier generation, recombination, and charge transfer mechanisms. The compact model and the derived detailed balance limit provide insights for designing and improving the performance of direct Z-scheme heterojunctions.

Check out the latest publication from CoCooN on the deposition of SnS2 via thermal and plasma-enhanched ALD for battery and CO2 electroreduction applications. This study discusses a plasma-enhanced atomic layer deposition (PE-ALD) method for depositing crystalline SnS₂ thin films using the tetrakis(dimethylamino)tin(IV) precursor and H₂S plasma at temperatures of 80 and 180 °C. X-ray diffraction confirms a layered hexagonal crystal structure and strong c-axis-oriented film growth, with the alignment of the basal planes mainly parallel to the substrate. At 80 °C, the film surface consists of continuous grain-like structures, whereas at 180 °C, the smooth film surface during the initial growth evolves to out-of-plane oriented structures when more SnS₂ is deposited. The influence of crystallinity and surface morphology on the electrochemical performance of crystalline SnS₂ thin films deposited by the PE-ALD process is evaluated for Li-ion battery and electrochemical CO₂ reduction applications. A comparison is made with those of amorphous SnS₂ thin films deposited by the corresponding thermal ALD process. As an anode material in Li-ion batteries, the SnS₂ thin film with out-of-plane oriented structures outperforms the other films with 77% capacity retention after 100 charge/discharge cycles despite its lower initial capacity. In contrast, the crystalline SnS₂ with grain-like morphology and amorphous SnS₂ retain only 65 and 34%, respectively, of the initial capacity after 100 charge/discharge cycles regardless of their higher initial capacity. In a similar fashion, the SnS₂ thin films with out-of-plane oriented structures exhibit lower Faradaic efficiencies for formate production by CO₂ electroreduction at 100 mA cm^–2 as compared to SnS₂ with grains (i.e., 64 vs 80%) albeit at lower overpotentials (i.e., 260 mV less negative) and maintaining a better structural and electrochemical stability. The amorphous SnS₂ thin film showed similar Faradaic efficiencies (i.e., 80%), stability, and overpotentials (i.e., −0.84 V vs RHE) compared to the crystalline SnS₂ thin film with a grain-like morphology.