Rapid Topochemical Modification of Layered Perovskites via Microwave Reactions

The Grafting of Hydroxyaromatic Organics within Layered Perovskites via a Microwave-Assisted Method

A new series of inorganic-organic hybrid perovskite materials were prepared by microwave-assisted grafting reactions. Simple carboxylic acids, acetic acid, and propionic acid, as well as hydroxyaromatic carboxylic acids, 3,5-dihydroxy benzoic acid (DBA), 5-hydroxyisophthalic acid (HPA), 4-hydroxybenzoic acid (HBA), and 4-hydroxy-4-biphenyl carboxylic acid (HBCA), were reacted with the Dion-Jacobson double-layered perovskite, HLaNb2O7, and its alcoxy derivatives. Grafting was found to not occur with simple carboxylic acids, while those molecules with hydroxyls were all attached to the perovskite interlayers. Reactivity of the hydroxyaromatic carboxylic acids varied with the different layered perovskite hosts where reactions with HLaNb2O7 did not occur, and those with n-propoxy-LaNb2O7 were limited; the greatest extent of reactivity was seen with n-decoxy-LaNb2O7. This is attributed to the larger interlayer spacing available for the insertion of the various hydroxyaromatic carboxylic acid compounds. The loading exhibited by the grafting species was less than that seen with well-known long-chain alkoxy grafting groups. It is expected that the width of the molecules contributes to this where, due to the benzyl groups, the interlayer volume of the grafted moieties occupies a larger horizontal fraction, therefore minimizing the loading to the below half. X-ray powder diffraction and transmission electron microscopy studies found that grafting of the n-decoxy-LaNb2O7 intermediates with the series of hydroxyaromatics resulted in a reduction in crystallinity along with a disruption of the layer structure. Raman data on the series show little variation in local structure except for HBCA, where there appears to be a lengthening of the Nb-O apical linkage and a possible reduction in the distortion of inner-layer NbO6 octahedra. The optical properties of the hydroxyaromatic carboxylic acid grafted perovskites were also investigated using diffuse-reflectance UV-Vis spectroscopy. The band gaps of DBA, HPA, and HBA were found to be similar to the parent (Eg ≈ 3.4 eV), while the HBCA was significantly less by ca. 0.6 eV. This difference is attributed to electron withdrawal from the perovskite block to the HBCA ligand, leading to a lower band gap for the HBCA compound. The methods described herein allow for the formation of a new series of inorganic-organic hybrid materials where the products are of interest as precursors to more complex architectures as well as models for band gap modification of metal oxide photocatalysts.

Rapid Synthesis of Kaolinite Nanoscrolls through Microwave Processing

Kaolinite nanoscrolls (NScs) are halloysite-like nanotubular structures of great interest due to their ability to superimpose halloysite's properties and applicability. Especially attractive is the ability of these NScs to serve as reaction vessels for the uptake and conversion of different chemical species. The synthesis of kaolinite NScs, however, is demanding due to the various processing steps that lead to extended reaction times. Generally, three intercalation stages are involved in the synthesis, where the second step of methylation dominates others in terms of duration. The present research shows that introducing microwave processing throughout the various steps can simplify the procedure overall and reduce the synthesis period to less than a day (14 h). The kaolinite nanoscrolls were obtained using two final intercalating agents, aminopropyl trimethoxy silane (APTMS) and cetyltrimethylammonium chloride (CTAC). Both produce abundant NScs, as corroborated by microscopy measurements as well as the surface area of the final products; APTMS intercalated NScs were 63.34 m²/g, and CTAC intercalated NScs were 73.14 m²/g. The nanoscrolls averaged about 1 μm in length with outer diameters of APTMS and CTAC intercalated samples of 37.3 ± 8.8 nm and 24.9 ± 6.1 nm, respectively. The availability of methods for the rapid production of kaolinite nanoscrolls will lead to greater utility of these materials in technologically significant applications.

Photocatalytic Activity of n-Alkylamine and n-Alkoxy Derivatives of Layered Perovskite-like Titanates H2Ln2Ti3O10 (Ln = La, Nd) in the Reaction of Hydrogen Production from an Aqueous Solution of Methanol

Two series of hybrid inorganic-organic derivatives, obtained via the modification of protonated Ruddlesden–Popper phases H2Ln2Ti3O10 (Ln = La, Nd) with intercalated n-alkylamines and grafted n-alkoxy groups, have been systematically investigated in relation to photocatalytic hydrogen production from a model of 1 mol % aqueous solution of methanol for the first time. Photocatalytic measurements were performed both for bare samples and for their composites with Pt nanoparticles as a cocatalyst using an advanced scheme, including dark stages, monitoring of the volume concentration of the sample in the reaction suspension during the experiment, shifts of its pH and possible exfoliation of layered compounds into nanolayers. It was found that the incorporation of organic components into the interlayer space of the titanates increases their photocatalytic activity up to 117 times compared with that of the initial compounds. Additional platinization of the hybrid samples’ surface allowed for achieving apparent quantum efficiency of hydrogen evolution of more than 40%. It was established that the photocatalytic activity of the hybrid samples correlates with the hydration degree of their interlayer space, which is considered a separate reaction zone in photocatalysis, and that hydrogen indeed generates from the aqueous methanol solution rather than from organic components of the derivatives.

Fast and efficient shear-force assisted production of covalently functionalized oxide nanosheets

HYPOTHESIS

While controlled and efficient exfoliation of layered oxides often remains a time consuming challenge, the surface modification of inorganic nanosheets is of outmost importance for future applications. The functionalization of the bulk material prior to exfoliation should allow the application of tools developped for Van der Waals materials to directly produce functionalized oxide nanosheets.

EXPERIMENTS

The Aurivillius phase Bi₂SrTa₂O₉ is functionalized by a linear aliphatic phosphonic acid via microwave-assisted reactions. The structure of the hybrid material and the coordination of the phosphonate group is scrutinized, notably by Pair Distribution Function. This functionalized layered oxide is then exfoliated in one hour in organic solvent, using high shear force dispersion. The obtained nanosheets are characterized in suspension and as deposits to check their chemical integrity.

FINDINGS

The covalent functionalization decreases the electrostatic cohesion between the inorganic layers leading to an efficient exfoliation in short time under shearing. The functionalization of the bulk material is preserved on the nanosheets upon exfoliation and plays a major role to enable liquid-phase exfoliation and in the stability of the resulting suspensions. This strategy is very promising for the straighforward preparation of functionalized nanosheets, paving the way for versatile design of new (multi)functional hybrid nanosheets for various potential applications.

Synthesis of Organic-Inorganic Hybrids Based on Perovskite-like Bismuth Titanate H2K0.5Bi2.5Ti4O13·H2O and n-Alkylamines

New organic-inorganic hybrids have been synthesized by the intercalation of n-alkylamines (methylamine, ethylamine, n-propylamine, n-butylamine, n-hexylamine, and n-octylamine) into the structure of the protonated and hydrated form of the perovskite-like layered titanate H₂K_0.5Bi_2.5Ti₄O₁₃·H₂O (HKBT₄·H₂O). The possibility of the synthesis of the hybrid materials was studied in a wide range of conditions. It was found that interlayer water plays a crucial role in the formation of intercalated hybrids. The obtained compounds were characterized with powder X-ray diffraction analysis; Raman, IR, and NMR spectroscopies; thermogravimetry (TG), TG coupled with mass spectrometry, and CHN analyses; and scanning electron microscopy. It was suggested that the intercalated n-alkylamines exist in the form of alkylammonium ions forming a paraffin-like bilayer with an average tilting angle of ∼77.5°. The obtained HKBT₄×RNH₂ compounds contain 0.4-0.7 n-alkylamine molecules per formula unit as well as the varied amount of intercalated water. By gentle heating, they can be obtained as dehydrated forms, which are thermally stable up to 250 °C.

Surface Modification of Layered Perovskite Nanosheets with a Phosphorus Coupling Reagent in a Biphasic System

Oleyl phosphate-modified HLaNb₂O₇· xH₂O nanosheets (OP_HLaNb nanosheets) were prepared via phase transfer from an aqueous phase, comprising a dispersion of HLaNb₂O₇· xH₂O (HLaNb) nanosheets, formed through the intercalation of tetrabutylammonium ion (TBA⁺) in the interlayer space of HLaNb and subsequent delamination, to a cyclohexane phase containing oleyl phosphate (OP, a mixture of monoester and diester). The modification of HLaNb nanosheets with OP was essentially completed within 3 days at a pH value of 2 or 4. Both infrared and solid-state ¹³C cross-polarization and magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra of the OP_HLaNb nanosheets showed the presence of OP and/or related species and TBA⁺ on the HLaNb nanosheet surface. The solid-state ³¹P MAS NMR spectra of OP_HLaNb nanosheets exhibited new signals at -2 and 0 ppm, the former of which indicates the formation of Nb-O-P bonds. These whole data set obtained by complementary techniques clearly point out the modification of the HLaNb nanosheet surface by OP moieties causing a phase transfer. OP_HLaNb nanosheets showed higher dispersibility in cyclohexane than the OP_HLaNb_interlayer nanosheets, which were prepared via stepwise substitution reactions in the interlayers of HLaNb to achieve surface modification with OP and subsequent exfoliation in cyclohexane. The presence of TBA⁺ on the HLaNb nanosheets and the use of a liquid-liquid biphasic system were likely to improve the dispersibility. These results show that the preparation of OP-modified HLaNb nanosheets which could be well-dispersed in the cyclohexane phase was successful because of the use of a liquid-liquid biphasic system.

Microwave-assisted functionalization of the Aurivillius phase Bi2SrTa2O9: diol grafting and amine insertion vs. alcohol grafting

The microwave-assisted functionalization of an Aurivillius phase is investigated towards various molecules – alcohols, diols and amino-alcohols – and the preferential reactivity of the various moieties is studied as a function of the reaction conditions.

Microwave-assisted functionalization of the layered Aurivillius phase Bi₂SrTa₂O₉ by alcohols is thoroughly investigated. The grafting of linear aliphatic and bulky alcohols is studied as a function of the starting material, underlining the importance of the prefunctionalization of the layered perovskite, for instance by butylamine. In addition, the functionalization by α,ω-alkanediols is explored. α,ω-alkanediols bearing long alkyl chains (n_C > 3) adopt an unprecedented pillaring arrangement, whereas 1,3-propanediol and ethyleneglycol adopt a bilayer arrangement, only one out of the two hydroxyl groups being coordinated. Finally, the reactivities of alcohols and amines towards insertion are compared: the preferential reactivity of the two functional groups appears to be strongly dependent of the reaction conditions, and especially of the water content. This study is further extended to the case of amino-alcohol insertion. In this case, the amine group is preferentially bound, but it is possible to control the grafting of the alcohol moiety, thus going from a bilayer arrangement to a pillaring one. This work is of particular importance to be able to functionalize easily and rapidly layered oxides with elaborated molecules, bearing several different potentially reactive groups.

Microwave-assisted routes for rapid and efficient modification of layered perovskites

Recent advances in exploiting microwave radiation in the topochemical modification of layered oxide perovskites are presented. Such methods work well for rapid bulk synthetic steps used in the production of novel inorganic-organic hybrids (protonation, grafting, intercalation, and in situ click reactions), exfoliation to produce dispersed nanosheets, and post-exfoliation processing to rapidly vary nanosheet surface groups. Compared to traditional methods that often take days, microwave methods can produce quality products in as little as 1-2 h.

Soft chemistry of ion-exchangeable layered metal oxides

Disassembly and re-assembly of layered metal oxides by soft chemical approaches can be used to tailor functionalities in artificial photosynthesis, energy storage, optics, and piezoelectrics.