Influence of Fe-clustering on the water oxidation performance of two-dimensional layered double hydroxides

Among the two-dimensional (2D) materials family, layered double hydroxides (LDHs) represent a key member due to their unparalleled chemical versatility. In particular, Fe-based LDHs are distinguished candidates due to their high efficiency as oxygen evolution reaction (OER) electrocatalysts. Herein, we have selected MgFe-based LDH phases as model systems in order to decipher whether Fe-clustering exerts an effect on the OER performance. For that, we have optimized hydrothermal synthesis by using triethanolamine (TEA) as the chelating agent. The magnetic characterisation allows us to identify the Fe-clustering degree by following both magnetic susceptibility as well as magnetization values at 2 K. Thanks to this, we demonstrated that TEA induces an increment in Fe-clustering. Electrochemical OER measurements show that both samples behave identically by using glassy carbon electrodes. Interestingly, when the samples are tested in the most commonly employed electrode, nickel foam, striking differences arise. The sample exhibiting a lower Fe-clustering behaves as a better electrocatalyst with a reduction of the overpotential values of more than 50 mV to reach 100 mA cm−2, as a consequence of a favoured surface transformation of MgFe-LDHs phases into more reactive oxyhydroxide NiFe-based phases during the electrochemical tests. Hence, this work alerts about the importance of the electrocatalyst-electrode collector interactions which can induce misinterpretations in the OER performance

Influence of the Interlayer Space on the Water Oxidation Performance in a Family of Surfactant-Intercalated NiFe-Layered Double Hydroxides

Layered double hydroxides (LDHs) are low dimensional materials that act as benchmark catalysts for the oxygen evolution reaction (OER). Many LDH properties affecting the OER have been studied to reach the optimal efficiency but no systematic studies concerning the influence of the interlayer space have been developed. In this context, these materials allow a large tunability in their chemical composition enabling the substitution of the interlayer anion and therefore modifying exclusively the basal space. Here, we synthesize by anion exchange reactions a surfactantintercalated family of NiFe-LDHs with increasing basal spacing ranging from 8.0 to 31.6 Å (one of the largest reported so far for a NiFe-LDH) while the electrochemical OER performance of this family of compounds was explored to analyse the interlayer distance effect keeping similar morphology, dimensions and metallic composition. Results show the increase of the LDH basal space undergo to lower Tafel slopes, higher electrochemical surface area and a reduction of the resistance related to the chemisorption of oxygen leading to better kinetic behaviour, showing an optimum enhancement of the electrocatalytic performance for the NiFe-dodecyl sulphate (basal space of 25 Å). Interestingly, the NiFe-dodecyl sulphate exhibits optimum proton diffusion values, indeed a further increment in the basal space compromises the onset potential, a fact that could be related to an increase in the hydrophobicity between the layers. Moreover, by judicious tuning of the interlayer space, it is possible to reach a Tafel slope value for the most spaced LDH (NiFe-octadecyl sulphate, basal space of 31.6 Å), similar to the one obtained for exfoliated NiFe nanosheets, showing a much better long‐time stability due to the three‐dimensional robustness of the catalysts. This work illustrates the importance of molecular engineering in the design of novel highly active catalysts and provides important insights into the understanding of basic principles of oxygen evolution reaction in NiFe-LDHs

Boosting the supercapacitive behavior of CoAl-layered double hydroxides via tuning the metal composition and interlayer space

Layered double hydroxides (LDHs) are promising supercapacitor materials due to their wide chemical versatility, earth abundant metals and high specific capacitances. Many parameters influencing the supercapacitive performance have been studied such as the chemical composition, the synthetic approaches, and the interlayer anion. However, no systematic studies about the effect of the basal space have been carried out. Here, two-dimensional (2D) CoAl-LDHs were synthesized through anion exchange reactions using surfactant molecules in order to increase the interlayer space (ranging from 7.5 to 32.0 Å). These compounds exhibit similar size and dimensions but different basal space to explore exclusively the interlayer distance influence in the supercapacitive performance. In this line, Co:Al ratios of 2:1, 3:1 and 4:1 were explored. In all cases, an enhancement of the specific capacitance was observed by increasing the basal space, reaching ca. 50 % more than the value obtained from the less-spaced 2:1 CoAl-LDH (up to ca. 750 - 1100 F.g-1 at 1 A.g-1). This increment mainly occurs because of the increase in the electrochemical surface area (up to ca. 260 %) and the higher electrolyte diffusion. Interestingly, best performance is achieved for the lowest Co:Al ratio (i. e. the highest Al content) revealing the important role of the electrochemically inert Al in the structure

Fundamental Insights into the Covalent Silane Functionalization of NiFe Layered Double Hydroxides

Layered double hydroxides (LDHs) are a class of 2D anionic materials exhibiting wide chemical versatility and promising applications in different fields ranging from catalysis to energy storage and conversion. However, the covalent chemistry of this kind of 2D materials is still barely explored. Herein, we report the covalent functionalization with silanes of a magnetic NiFe-LDH. The synthetic route consists of a topochemical approach followed by the anion exchange reaction of a surfactant molecules prior to the covalent functionalization with the (3- aminopropyl)triethoxysilane (APTES) molecules. The functionalized NiFe-APTES was fully characterized using X-ray diffraction, infrared spectroscopy, electron microscopy, thermogravimetric analysis coupled with mass spectrometry and 29Si solid state nuclear magnetic resonance, among others. The effect on the electronic properties of the functionalized LDH was studied by a magnetic study in combination with Mössbauer spectroscopy. Moreover, the reversibility of the silane-functionalization at basic pH was demonstrated, and the quality of the resulting LDH proved by studying the electrochemical performance in the oxygen evolution reaction in basic media. Furthermore, the anion exchange capability for the NiFe-APTES was tested employing Cr(VI), resulting in an increase of 200 % of the anion retention. This report allows a new level in the tunability of LDHs opening the door to the synthesis of new hybrid materials and architectures

Hexagonal Hybrid Bismuthene by Molecular Interface Engineering

[EN] High-quality devices based on layered heterostructuresare typicallybuilt from materials obtained by complex solid-state physical approachesor laborious mechanical exfoliation and transfer. Meanwhile, wet-chemicallysynthesized materials commonly suffer from surface residuals and intrinsicdefects. Here, we synthesize using an unprecedented colloidal photocatalyzed,one-pot redox reaction a few-layers bismuth hybrid of "electronicgrade" structural quality. Intriguingly, the material presentsa sulfur-alkyl-functionalized reconstructed surface that preventsit from oxidation and leads to a tuned electronic structure that resultsfrom the altered arrangement of the surface. The metallic behaviorof the hybrid is supported by ab initio predictionsand room temperature transport measurements of individual nanoflakes.Our findings indicate how surface reconstructions in two-dimensional(2D) systems can promote unexpected properties that can pave the wayto new functionalities and devices. Moreover, this scalable syntheticprocess opens new avenues for applications in plasmonics or electronic(and spintronic) device fabrication. Beyond electronics, this 2D hybridmaterial may be of interest in organic catalysis, biomedicine, orenergy storage and conversion.This work has been supported by the European Union (ERC-2018-StG 804110-2D-PnictoChem & and ERC Proof of Concept Grant 101101079-2D4H2 to G.A.; ERC-2021-StG 101042680 2D-SMARTiES awarded to J.J.B.), the Spanish MICINN (PID2019-111742GA-I00, PID2020-115100GB-I00, MRR/PDC2022-133997-I00, TED2021-131347B-I00, and Excellence Unit Maria de Maeztu CEX2019-000919-M), and the Generalitat Valenciana (CIDEGENT/2018/001, CIDEGENT/2018/005, and CDEIGENT/2019/022). Financial support by Severo Ochoa centre of excellence program (CEX2021-001230-S) is gratefully acknowledged. M.K. and H.B.W. acknowledge support by the Deutsche Forschungsgemeinschaft (DFG), under Projektnummer 182849149 (SFB 953, projects B08 and B13). Electron microscopy work carried out at UCM (M.V., G.S.S.) sponsored by MICINN PID2021-122980OB-C51 and Comunidad de Madrid MAD2D-CM-UCM3. G.S.S. acknowledges financial support from Spanish MCI Grant Nos. RTI2018-099054-J-I00 (MCI/AEI/FEDER, UE) and IJC2018-038164-I. C.D. and Y.M.E. thank the cluster of excellence 3DMM2O funded by DFG under Germany's Excellence Strategy - 2082/1 - 390761711 for financial support. The authors thank Lukas Grunwald and Erich Muller for helpful discussions. A.M.R. thanks the Spanish MIU (Grant No FPU21/04195). A.S.-D. thanks the Universidad de Valencia, for an Atraccion del talento' predoctoral grant. F.G.-P. thanks ITQ, UPV-CSIC for concession of a contract (PAID 01-18).Dolle, C.; Oestreicher, V.; Ruiz, AM.; Kohring, M.; Garnes-Portoles, F.; Wu, M.; Sánchez-Santolino, G.... (2023). Hexagonal Hybrid Bismuthene by Molecular Interface Engineering. Journal of the American Chemical Society. 145(23):12487-12498. https://doi.org/10.1021/jacs.2c1303612487124981452

Hexagonal Hybrid Bismuthene by Molecular Interface Engineering

High-quality devices based on layered heterostructures are typically built from materials obtained by complex solid-state physical approaches or laborious mechanical exfoliation and transfer. Meanwhile, wet-chemically synthesized materials commonly suffer from surface residuals and intrinsic defects. Here, we synthesize using an unprecedented colloidal photocatalyzed, one-pot redox reaction a few-layers bismuth hybrid of “electronic grade” structural quality. Intriguingly, the material presents a sulfur-alkyl-functionalized reconstructed surface that prevents it from oxidation and leads to a tuned electronic structure that results from the altered arrangement of the surface. The metallic behavior of the hybrid is supported by ab initio predictions and room temperature transport measurements of individual nanoflakes. Our findings indicate how surface reconstructions in two-dimensional (2D) systems can promote unexpected properties that can pave the way to new functionalities and devices. Moreover, this scalable synthetic process opens new avenues for applications in plasmonics or electronic (and spintronic) device fabrication. Beyond electronics, this 2D hybrid material may be of interest in organic catalysis, biomedicine, or energy storage and conversion.This work has been supported by the European Union (ERC-2018-StG 804110-2D-PnictoChem & and ERC Proof of Concept Grant 101101079-2D4H2 to G.A.; ERC-2021-StG 101042680 2D-SMARTiES awarded to J.J.B.), the Spanish MICINN (PID2019-111742GA-I00, PID2020–115100GB–I00, MRR/PDC2022-133997-I00, TED2021-131347B-I00, and Excellence Unit María de Maeztu CEX2019-000919-M), and the Generalitat Valenciana (CIDEGENT/2018/001, CIDEGENT/2018/005, and CDEIGENT/2019/022). Financial support by Severo Ochoa centre of excellence program (CEX2021–001230–S) is gratefully acknowledged. M.K. and H.B.W. acknowledge support by the Deutsche Forschungsgemeinschaft (DFG), under Projektnummer 182849149 (SFB 953, projects B08 and B13). Electron microscopy work carried out at UCM (M.V., G.S.S.) sponsored by MICINN PID2021-122980OB-C51 and Comunidad de Madrid MAD2D-CM-UCM3. G.S.S. acknowledges financial support from Spanish MCI Grant Nos. RTI2018-099054-J-I00 (MCI/AEI/FEDER, UE) and IJC2018-038164-I. C.D. and Y.M.E. thank the cluster of excellence 3DMM2O funded by DFG under Germany’s Excellence Strategy – 2082/1 – 390761711 for financial support. The authors thank Lukas Grünwald and Erich Müller for helpful discussions. A.M.R. thanks the Spanish MIU (Grant No FPU21/04195). A.S.-D. thanks the Universidad de Valencia, for an ‘Atracción del talento’ predoctoral grant. F.G.-P. thanks ITQ, UPV–CSIC for concession of a contract (PAID 01-18)

Hexagonal Hybrid Bismuthene by Molecular Interface Engineering

High-quality devices based on layered heterostructures are typically built from materials obtained by complex solid-state physical approaches or laborious mechanical exfoliation and transfer. Meanwhile, wet-chemically synthesized materials commonly suffer from surface residuals and intrinsic defects. Here, we synthesize using an unprecedented colloidal photocatalyzed, one-pot redox reaction a few-layers bismuth hybrid of “electronic grade” structural quality. Intriguingly, the material presents a sulfur-alkyl-functionalized reconstructed surface that prevents it from oxidation and leads to a tuned electronic structure that results from the altered arrangement of the surface. The metallic behavior of the hybrid is supported by ab initio predictions and room temperature transport measurements of individual nanoflakes. Our findings indicate how surface reconstructions in two-dimensional (2D) systems can promote unexpected properties that can pave the way to new functionalities and devices. Moreover, this scalable synthetic process opens new avenues for applications in plasmonics or electronic (and spintronic) device fabrication. Beyond electronics, this 2D hybrid material may be of interest in organic catalysis, biomedicine, or energy storage and conversion

Magnetism in two-dimensional layered double hydroxides

Layered double hydroxides (LDHs) are a class of anionic clays known for a long time and properly classified in the beginning of 20th century. They are composed of positively charged metal hydroxide-based sheets endowed with interlayer anions and solvent molecules to keep the electro-neutrality. LDHs have attracted increasing attention during the last years because of their rich chemical versatility and the fact that they can be exfoliated -or directly synthesized- into two-dimensional (2D) nanosheets, impacting a wide range of potential applications. Among others, magnetism stands out as one of the most appealing properties of LDHs, this is mainly due to the possibility of modulating their magnetic interactions by judicious tuning of their composition, morphology or interlayer spacing. The combination of their modulable physical properties with good processability positions these nanosheets as excellent candidates for the development of hybrid materials and heterostructures. This review addresses from the first reports to the most recent advances in the magnetic properties of LDHs and their hybrids, showing the great potential they hold as 2D quantum materials. In addition, it is also shown how magnetic properties can be useful in energy-related applications, either evaluating the purity of materials of utmost importance such as NiFe-LDHs, or elucidating their cationic order at the atomic scale, which influences the catalytic performance

Crystallographic and geometrical dependence of water oxidation activity in Co-based layered hydroxides

Cobalt-based layered hydroxides (LHs) stand out as one of the best families of electroactive materials for the alkaline oxygen evolution reaction (OER). However, fundamental aspects such as the influence of the crystalline structure and its connection with the geometry of the catalytic sites remains poorly understood. Thus, to address this we have conducted a thorough experimental and in silico study on the most essential Co-LHs (i.e.: ɑ-LH, β-LH and LDH) which allows us to understand the role of the layered structure and coordination environment of Co atoms on the OER performance. The ɑ-LH, containing both octahedral and tetrahedral sites, behaves as the best OER catalyst in comparison to the other phases, pointing out the role of the chemical nature of the crystalline structure. Indeed, density functional theory (DFT) calculations confirm the experimental results which can be explained in terms of both, the significant reduction of the Egap, due to the presence of tetrahedral sites, as well as the more favourable reconstruction of the ɑ-LH structure into active Co(III)-based oxyhydroxide-like phase. Furthermore, ex-situ X-ray diffraction and absorption spectroscopy reveal the permanent transformation of ɑ-LH phase into an unprecedented highly reactive oxyhydroxide-like structure under ambient conditions. Hence, our findings highlight the key role of tetrahedral sites on the electronic properties of the LH structure as well as their inherent reactivity towards OER catalysis, paving the way for the rational design of more efficient and low-maintenance electrocatalysts

Influence of crystallographic structure and metal vacancies on the oxygen evolution reaction performance of Ni-based layered hydroxides

Nickel-based layered hydroxides (LHs) are a family of efficient electrocatalysts for the alkaline oxygen evolution reaction (OER). Nevertheless, fundamental aspects such as the influence of the crystalline structure and the role of lattice distortion of the catalytic sites remain poorly understood and typically muddled. Herein, we carried out a comprehensive investigation on ɑ-LH, β-LH and LDH phases, analysing the role exerted by Ni-vacancies by means of structural, spectroscopical, in-silico and electrochemical studies. Indeed, density functional theory (DFT) calculations, in agreement with X-ray absorption spectroscopy (XAS), confirm that the presence of Ni-vacancies produces acute distortions of the electroactive Ni sites (shortening in the Ni-O distances and changes in the O-Ni-O angles), triggering the appearance of Ni localised electronic states on the Fermi level, reducing of Egap, and therefore increasing the reactivity of the electroactive sites. Furthermore, post-mortem Raman and XAS measurements unveil the transformation of ɑ-LH phase into a highly reactive oxyhydroxide-like structure stable under ambient conditions. Hence, this work pinpoints the critical role of cationic vacancies on the structural and electronic properties of the LH structures, which controls their inherent reactivity towards OER catalysis. We envision Ni-based ɑ-LH as a perfect platform for trivalent cations hosting, closing the gap toward the next generation of benchmark efficient earth-abundant electrocatalysts