Janus Nanosheets Derived from K₄Nb₆O₁₇·3H₂O via Regioselective Interlayer Surface Modification

Abstract

Inorganic Janus nanosheets were successfully prepared using the difference in reactivity between interlayers I and II of layered hexaniobate K 4 Nb6O 17 ·3H 2 O. Janus nanosheets exhibit the highest anisotropy among Janus compounds due to their morphology. It is therefore important to prepare Janus nanosheets with stable shapes in various solvents, robust chemical bonds between nanosheets and fuctional groups and high versatility due to surface functional groups. K 4 Nb 6 O 17 ·3H 2 O, which possesses two types of interlayers and two types of organophosphonic acids that react with metal oxides to form robust covalent bonds, was employed to prepare Janus nanosheets for this study. Interlayer I was modified by octadecylphosphonic acid, followed by modification by carboxypropylphosphonic acid mainly at interlayer II. Preparation of Janus nanosheets with two organophosphonate moieties was confirmed by 31 P MAS NMR. After these regioselective and sequential modifications, the products were exfoliated into single-layered nanosheets in THF. Two types of derivatives with different repeating distances were recovered from a dispersion containing nanosheets exfoliated by different processes, centrifugation, and solvent evaporation. AFM analysis of the exfoliated nanosheets revealed that the products were Janus compounds. There are high expectations for application of these types of Janus nanosheets in various fields and for design of various Janus nanosheets using this preparation method.There are various methods of preparing Janus nanoparticles. Regioselective surface modification of nanoparticles can produce Janus nanoparticles [11]. By forming nanoparticles with two raw materials, Janus nanoparticles with different compositions can be prepared [2]. Self-assembly and subsequent cross-linking of polymer chains can also produce Janus nanoparticles [12]. Janus rods can be prepared by forming silica using TEOS by the sol-gel method on Fe 3 O 4 regioselectively [13]. Janus cylinders can be prepared by masking one side of a cylinder and conducting subsequent modification of the other side [14].Janus nanosheets exhibit the highest anisotropy among Janus compounds due to their unique morphology. They are also useful as emulsifiers, because Janus nanosheets cannot rotate at the interfaces of micelles [15]. Most Janus nanosheets reported so far have consisted of polymers. Stupp et al. reported the preparation of Janus nanosheets by polymerization of oligomers with polymerizable groups [16]. Polymerization of oligomers led to the formation of sheet morphology, because the polymerizable groups were located at the center of an oligomer. Walther et al. used triblock copolymers, polystyrene-block-polybutadiene-block-poly(tert-butyl methacrylate), for preparing Janus nanosheets [17]. Janus nanosheets were prepared by cross-linking polybutadiene domains of the triblock copolymers, and the resulting Janus nanosheets had two types of surfaces, polystyrene, and poly(tertbutyl methacrylate) moieties. These methods are based on selective polym...

Preparation of water-dispersible Janus nanosheets from K4Nb6O17·3H2O and their behaviour as a two-dimensional surfactant on air–water and water-toluene interfaces

“…28 It is therefore concluded that Nb–O–P bonds were additionally formed between the PA moiety and niobate layer. Based on our previous study using the same set of alkylammonium ions for preparing intermediates, 25 it can be concluded that PPA and PA moieties were mobilized in interlayers I and II by two-step surface modification. Based on our previous study on interlayer surface modification of K 4 Nb 6 O 17 ·3H 2 O by PPA, both PPA and moieties should be present in PhP(O)(OH)O–Nb and/or (HO) 2 P(O) O–Nb monodentate environments.…”

“…Furthermore, weak signals assignable to n -dodecyl chains in C 12 N + were observed at 16–41 ppm, a result indicating the presence of C 12 N + in PPA_PA_NbO. 25…”

“…In PPA_NbO, which is estimated to be an A-type derivative, the Nb : P molar ratio was calculated to be 6.0 : 1.9. Since the molar ratio for the maximum modification amount of guest species in interlayer I is equal to the molar ratio of potassium in interlayer I, Nb : P = 6.0 : 2.0, 25 95% of the reactive sites in interlayer I of PPA_NbO were modified with PPA. In PPA_NbO, no N atoms were present, indicating complete removal of 2C 18 2MeN + from interlayer I of PPA_NbO.…”

“…Here we report the preparation of K 4 Nb 6 O 17 ·3H 2 O-based single-layered Janus nanosheets with one surface modified by phenylphosphonic acid and the other with phosphoric acid. These Janus nanosheets could be dispersed in water by increasing their hydrophilicity compared to the Janus nanosheets reported in our previous study, 25 which have octadecylphosphonate moiety and carboxypropylphosphonate moiety on the opposite surfaces of each nanosheet. In order to clarify their surfactant behaviour, the nanosheets’ surface behaviour at the air–water interface was investigated by dynamic surface tension measurement.…”

“…It should be noted that stable Nb–O–P bonds are present between the niobite nanosheet surface and surface functional groups, which is very advantageous for their use as a two-dimensional surfactant. These K 4 Nb 6 O 17 ·3H 2 O-based Janus nanosheets furthermore exhibit versatility in the selection of surface functional groups achieved by changing the surface modifiers, 25 and one type of K 4 Nb 6 O 17 ·3H 2 O-based Janus nanosheets bearing hydrophilic and hydrophobic groups was reported. It is therefore expected that another type of K 4 Nb 6 O 17 ·3H 2 O-based Janus nanosheets with appropriate hydrophilicity and hydrophobicity on opposite sides of the individual nanosheets for their stable dispersion in water can be prepared by proper selection of surface modifiers and behaves as a two-dimensional surfactant.…”