Deep eutectic solvents (DESs) have received increasing attention since the pioneering work of Abbott et al. reported in 2001 [1, 2]. DESs is broadly informed as green substitute solvents to the conventional ionic liquids (ILs). These solvents are usually a combination of two or more cheap and safe hydrogen-bonding acceptors (HBAs) and hydrogen-bonding donors (HBDs) compounds which have a melting point lower than that of their individual components [3-6]. They are liquids at temperatures of 100 ℃ or below and show duplicate solvent properties to ILs such as inertness with water, non-flammable and non-volatile. Besides, DESs have low synthesis expense due to the low price of raw materials and extensively applied as dynamic solvents for organic synthesis, extraction, separation, materials, and electrochemistry [7-23].

To date, most work on the synthesis of DESs systems that are liquid at ambient temperature exploits an organic cation-based moieties, including ammonium, phosphonium, metal salts, and sulphonium where Cl^-, Br^-, I^-, NO₃^- anions usually acted as HBAs [24-29]. Developing basic types of eutectic solvents is indispensable for extending the application of DESs in various fields including nanotechnology. Recently, we found a new kind of DESs composed of crown ether, polyethylene glycol (PEG) and hydroxides, which can be used to efficiently extract non-basic nitrogen compounds from fuel oil [30]. However, more recently, we found crown ether is unnecessary. Thus, in this communication, we proved that PEG and hydroxides can be form a new serial of green and low-cost basic DESs or DESs analogues.

On the other hand, two-dimensional (2D) ultrathin transition metal oxide nanomaterials have bestowed great intentness due to their unique, diverse properties and extensive applications in catalysis, magnetism, electronics, optical and electrochemical technologies [31-35]. Based on these remarkable properties, the synthesis of nanostructured metal oxides, such as NiCo₂O₄, have previously been reported using various conventional solvents, surfactants, and time-consuming methods combined with a post-thermal treatment in the extremely high temperatures [36-43]. Therefore, it remains a challenging task to prepare transition metal oxide nanomaterials under very mild conditions due to the lack of desired solvents.

In this work, new solvent system acts as a structuring framework, whereas the basic DESs are acting simultaneously as a solvent, shape-control agent, and reactant. The results showed in the immensely simple, fast and energy-saving preparation of cubic NiCo₂O₄ ultrathin nanosheets at 80 ℃.

We designed a new series of highly basic DESs are made up by solely strong hydroxides including LiOH, NaOH, and KOH as HBAs, while EG and low molecular PEG including PEG-200, PFG-400 and PEG-600 serve as HBDs as shown in Fig. 1A. The photograph view of basic DESs in different molar ratios of EG and PEG with the set amount of hydroxides is shown in Fig. 1B. The illustration of the molecular dynamic (MD) simulation structure of the basic DES was performed with NaOH and PEG by using PEG-458 (n = 10) as a pattern of PEG polymer (Fig. 1C). It was found that PEG was bent to form the hydrogen bondings with the OH^- acceptor of sodium hydroxide. The bond lengths of two H—O—H bonds in DESs were 1.69 and 1.75 Å, respectively. The results showed that the two compounds were intimately connected by the electrostatic and hydrogen-bonding interactions to form very stable basic DESs. Fig. S1 (Supporting information) shows the conductivity of the basic DESs was measured with different molar ratios of NaOH:PEG-200 to find the optimum composition. The two linear fragments were drawn based on the change curve of the conductivity, and the molar ratio which corresponding to the intersection of the two linear fragments is the critical aggregation of molar ratio. The aggregation value was nearly 1:44, which was found as the best eutectic molar ratio of these basic DESs.

The structure of basic DESs was further confirmed by ¹H NMR and FT-IR (Figs. S2 and S3 in Supporting information). Fig. S2A shows the ¹H NMR of pure EG, NaOH:EG (1:10), and NaOH:EG (1:44). The singlet signal was observed at 4.44 ppm for —OH proton of pure EG. However, in case of DESs, the corresponding singlet was shifted to low field region indicating the existence of H-bonding interaction between the two compounds during the formation of DESs, while the peak position was different in different molar ratios due to the variation of interactions. On the other hand, Figs. S2B and C show the ¹H NMR of pure PEG-200 and PEG-based basic DESs in different molar ratios. As shown in Fig. S2B, the multiple was assigned between 3.60-3.74 ppm for —CH₂ proton of pure PEG-200, which shifted to the higher field in case of DESs. Furthermore, the singlet of —OH proton in PEG-200 was observed at 4.61 ppm (Fig. S2C), while the corresponding —OH proton singlet position in DESs was shifted based on the molar ratios of both compounds, indicating the variation of interactions and existence of H-bonding.

Compared with commonly used DESs, OH^- acceptors-based DESs are a new kind of strong basic solvents. The essential physical properties including density, viscosity, conductivity, melting point (T_m), freezing point (T_f) and glass trasition temperature (T_g) of these DESs were determined at 298.15 K. As listed in Table 1, net EG has a T_m, and T_f is -11.5 and -42.8 ℃, respectively. However, in NaOH/EG, (molar ratio, 1:44), the T_m slightly decreases to -16.0 ℃ and the T_f significantly decreases to -65.0 ℃.

Furthermore, pure PEG-200 had a T_g to -80.2 ℃, and the T_g slightly decreases to -83.1 and -85.8 ℃ for NaOH/PEG-200 system at 1:10 and 1:44 molar ratios, respectively. These changes of the T_m, T_f, and T_g must come from the interaction between HBDs molecules and the OH^- acceptors ion, and the lowest T_g was observed than organic cation-based ordinary reported DESs [44]. Moreover, PEG-400 and PEG-600 with NaOH show a lower T_m and T_f compared to their real constituents, which are consistent with the common reported DESs properties [45]. In addition, the T_g of LiOH/PEG-200, KOH/PEG-200 system was approximately similar to the NaOH/PEG-200 system at 1:44 M ratio. It is noteworthy that merely those compounds that can form hydrogen bonds with acceptor ions can show the construction of a homogeneous liquid with significant changes in the T_m, T_f and T_g.

On the other hand, all the DESs have lower density (ρ), viscosity (cP), and higher ionic conductivity (ʌ). The density of EG and PEG-based DESs were different from that of either of the constituents. The distinction in density among the eutectic mixtures indicates that there is a difference in packing among the eutectic liquids. Furthermore, PEG-600 based DESs has shown little higher density, which suggests a reduction in the average hole radius in this system and hence a decrease in mass transport properties and an increase in viscosity. Moreover, all DESs was shown the lower viscosity at 298.15 K which means these liquids have better mass transport and should lead to a decrease in electrical inhibition. Besides, these basic DESs are remarkably conducting, and the conductivity was approximately compatible with all the reported DESs system previously studied, and in some extent, the conductivity is higher than organic cation-based conventional DESs system [46], which proves that the ionic species are dissociated in the liquid and can move liberally. This result implies that these eutectic liquids properties would be tailored for other applications such as electrolytes and supercapacitors.

As one of the possible application of basic DESs, NiCo₂O₄ nanosheets were synthesised using NaOH/PEG-200 (1:44, molar ratio) DESs. To the best of our knowledge, NiCo₂O₄ nanosheets have few been obtained below 100 ℃. This solvent showed very high solubility of metal salts during the synthesis process. Therefore, it is expected that highly functional nanomaterials could be synthesized with improved chemical stability, increased catalytic properties, and more distinctive optical properties. In this case, the formation of the Ni-Co-based nanosheets comprises the noted hydrolysis/precipitation process. First, NaOH in basic DESs generated OH^- ions in the reaction medium, which reacted with Ni²⁺ and Co²⁺ nitrate salts to quick decompose and the formation of Ni-CoOOH precursors, while the precursors growth was affected by the controlled rate of OH^- ions. The nanosheets precursor was washed by pure ethanol several times with sonication to remove the loosely attached product from the surface. The NiCo₂O₄ were obtained by drying of Ni-CoOOH at 70 ℃ in the electric oven (see the experimental section for details procedure). Therefore, the reactions involved in the formation of NiCo₂O₄ in basic DESs can be described by the below equations [47].

With the assistance of PEG, the generated precursors formed an ultrathin nanosheets structure, because PEG is an efficient shape-control agent for nanomaterial formation for its two —OH groups which can enhance the growth rate of the crystal faces and the steric hindrance effect to obtain well-structured nanomaterials. To compare the results whether net PEG-200 and NaOH aqueous solution are capable of preparing NiCo₂O₄ were investigated. It is observed that using net PEG-200 to prepare NiCo₂O₄ was not found any precipitate under the same conditions of basic DESs is shown in Fig. S4 (Supporting information). On the other hand, NaOH aqueous solution could produce the precipitate without well-structured morphology which was confirmed by SEM and TEM is shown in Fig. S5 (Supporting information). These results indicate that the composition of both NaOH and PEG molecules in basic DESs is crucial to obtain the well-structured nanomaterials.

The X-ray diffraction (XRD) analysis is shown in Fig. 2. A series of characteristic peak appears at 2θ values, which can be indexed to the (111), (220), (311), (222), (400), (422), (511), (440) and (533) planes, and are in good agreement with the cubic NiCo₂O₄ (JCPDS No. 20-0781), respectively [48, 49]. Albeit the peak intensity is comparatively weak indicating a tiny crystallite size of NiCo₂O₄ phase.

Fig. 3 shows the typical scanning electron microscopy (SEM) (Figs. 3A and B), transmission electron microscopy (TEM) (Figs. 3C and D), HRTEM image (Fig. 3E), EDX spectrum (Fig. 3F) and EDX elemental mappings (Fig. 3G) of NiCo₂O₄ (1:2) nanosheets. As can be shown, the nanosheets are uniformly grown in the basic DESs during the preparation. From the enlarged views (Fig. 3B), the nanosheets are interlinked with each other to form a sheet-like structure, which may be able to retain excellent electrochemical and mechanical strengths. TEM images further confirmed the successful preparation of nanosheets (Figs. 3C and D). The lattice fringe HRTEM image shown in Fig. 3E can be readily indexed to the (311) crystal plane of the cubic NiCo₂O₄ phase with an interplane spacing of 0.24 nm. The molar ratio between Ni and Co is 1:2, as confirmed by the EDX spectrum, in agreement with the stoichiometric ratio of NiCo₂O₄ (Fig. 3F). Furthermore, the EDX elemental mappings indicate that the nanosheet was composed of Ni, Co, and O elements, and the three elements are homogeneously distributed in the nanosheets (Fig. 3G). Its is worth to remark that these basic DES-based synthetic deftness can be elongated to the preparation of other technologically crucial metal oxides nano-materials such as MnCo₂O₄, NiMn₂O₄, CoCu₂O₄, and Co₃O₄ at 80 C (Figs. S6–S9 in Supporting information).

The Brunauer-Emmett-Teller (BET) surface area of the NiCo₂O₄ nanosheets is determined by the N₂ adsorption/desorption isotherms measurement at 76 K was found as high as 122 m²/g (Fig. S10 in Supporting information). In addition, the NiCo₂O₄ nanosheets feature further confirmed by the corresponding Barrett–Joyner–Halenda (BJH) pore-size distribution analysis, implying BJH mean pore diameter were found to be 3.0 nm (Fig. S10 inset).

In summary, a novel hydroxide-based basic DESs was designed towards NiCo₂O₄ ultrathin nanosheets synthesis. The structure of DESs was confirmed using ¹H NMR, FT-IR, and MD simulation, and correlated this with their physical properties. Initially, NaOH/PEG-200 (1:44) DESs were selected and found that this DESs is exceptionally efficient to produce rapidly cubic ultrathin NiCo₂O₄ nanosheets under very mild conditions with nano size, high pore volumes and large surface area without using any post-thermal treatment in the high temperature. Therefore, these solvents are expected to provide new avenues for obtaining other advanced nanomaterials in the future. Moreover, this new basic DESs may also be hopeful for other applications such as catalysis and organic synthesis.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Nos. 21822407, 21675164) and the CAS-President International Fellowship Initiative (No. 2017PC0014).

Appendix A. Supplementary data

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cclet.2019.09.055.