Synthesis of surfactant crystals containing gallium ions and their hydrolysis in the interlayer spaceThe surfactant crystals were prepared by mixing surfactant and metal ion solutions. As a typical example, we focused on gallium because of its relatively simple and controllable soluble nature in aqueous solutions to understand the formation process26. First, Ga(NO3)3 was dissolved in water and mixed with sodium octadecylsulfate aqueous solution. The obtained precipitate was dried to form surfactant crystals with Ga3+ species (Fig. 2a). The surfactant crystals containing Ga3+ species had ordered lamellar structures with the Lc phase and an interlayer distance of 5.8 nm, as shown by the X-ray diffraction (XRD) pattern (Supplementary Fig. 1a). The alkyl chain of the surfactant was arranged in an order different from that of the liquid crystalline lamella (Supplementary Fig. 1b). The composition of the obtained surfactant crystals was estimated as C18H37OH:C18H37SO4:Ga(OH)1.8:H2O = 0.12:1:0.83:2.8, as evaluated by the thermogravimetry (TG) curve and carbon–hydrogen–nitrogen–sulfur (CHNS) analysis (Supplementary Fig. 1c and Supplementary Table 1). The octadecanol was derived from the impurity in the reagent surfactant (Supplementary Table 1). The Ga3+ species dissolved in water were partially hydrolyzed, forming [Ga(OH)(H2O)5]2+ or [Ga(OH)2(H2O)4]+ 26; and mixing with anionic surfactant solutions led to precipitation through ligand exchange from water into surfactant anions. The detailed characterization of the lamellar hybrids is presented in the supplementary information.Fig. 2: Synthesis of precursor surfactant crystals and intermediate crystals containing gallium ions.Optical microscope images, scale bar 40 μm, (inset) appearances, and estimated compositions and predicted structures of a precursor surfactant crystals containing gallium species and b intermediate crystals prepared by humid ammonia vapor treatment. c Radial structure function of the surfactant crystals containing gallium species obtained by Fourier transform of EXAFS oscillations (Supplementary Fig. 2c), d TEM image, scale bar 25 nm, and (inset) appearance and e EDS spectrum of the solution containing Ga species with TMS groups.Then, the obtained surfactant crystals were treated with humid ammonia vapor to obtain intermediates of the molecularly thin amorphous 2D nanosheets. The surfactant retained its solid state, and the size and morphology almost remained the same, even after humid ammonia vapor treatment (Fig. 2b). The composition of the intermediate was investigated by TG, CHNS, and Fourier transform infrared (FT-IR) spectroscopy analyses and was estimated as C18H37SO4:C18H37OH:Ga2O3-xHy:NH4:H2O = 1:1:0.72:1:0.82 (Supplementary Fig. 3c, d and Supplementary Table 1). The (NH4)2SO4 formed by hydrolysis of C18H37SO4 was selectively removed via a washing process with water. The gallium species in the interlayer space of precursor surfactant crystals were hydrolyzed and formed a gallium oxide/oxyhydroxide group. The X-ray absorption near edge structure (XANES) spectrum clearly showed that the Ga species changed their coordination state, similar to gallium oxide (Supplementary Fig. 2b). The radial structure function obtained by extended X-ray absorption fine structure (EXAFS) oscillation indicated the formation of gallium oxide/oxyhydroxide because of the strong peak derived from Ga–O–Ga or Ga–OH–Ga, despite the lack of apparent peaks in the precursor crystals (Fig. 2c). In addition, ammonium ions were introduced as the counter ions instead of gallium species, as supported by the molar ratio of nitrogen and sulfur (N/S = 1) because the condensation of the gallium species decreased the electrical charge neutralizing the surfactant head groups (Supplementary Table 1). Then, the structure and morphology of the obtained intermediate were evaluated from XRD patterns, transmission electron microscopy (TEM) images, and scanning electron microscopy (SEM) images. The XRD pattern of the intermediate showed that the lamellar structure was broadened but remained, and the Lβ structure of the surfactant was formed (Supplementary Fig. 3a, b). The lamellar structure and plate-like morphology of the obtained intermediate were evaluated by SEM and TEM images (Supplementary Fig. 3e, f). The molar ratio distribution of S and Ga was homogeneous, and no outside deposition of gallium species was found. From these results, we assumed that the gallium oxide/oxyhydroxide cluster was formed homogeneously in the surfactant crystals, as illustrated in Fig. 2b.We investigated the detailed structure of the gallium species by dissolving the intermediate in tetrahydrofuran (THF) after protecting the surface of the cluster with trimethylsilyl (TMS) groups. The obtained THF solution did not show any clear Tyndale effect, indicating that there was insufficient large scatterer to show the bright line of the scattered light (Fig. 2d (inset)). Although TEM observation of the samples did not show definite structures, except for the collodion membrane (Fig. 2d), energy-dispersive X-ray spectroscopy (EDS) spectrum showed strong signals derived from the Si and Ga species (Fig. 2e). Therefore, the intermediate was to be composed of the gallium cluster and surfactants. The gallium cluster did not form a continuous structure, such as a 2D nanosheet. The gallium species formed a noncontinuous cluster in the interlayer space of the surfactant crystals, despite gallium species normally forming continuous structures under strongly basic conditions, such as humid ammonia vapor. The limited nanospace in the surfactant crystals should be effective to inhibit the diffusion of the gallium species, leading to homogeneously distributed gallium clusters without formation of the continuous structure, even with activated hydroxy groups.Formation of amorphous 2D nanosheets via formamide immersion of intermediatesThe obtained intermediates were immersed in formamide at 50 °C for five days, and a colloidal solution with obvious Tyndale effects was obtained. The solution was dropped on the Si wafer, and the thicknesses of the nanosheets were evaluated by atomic force microscopy (AFM) imaging. The nanosheets on the Si substrate were 3.0 nm thick, and calcination of the nanosheets decreased the thickness to 1.5 nm because of the removal of slightly remained organic species and the increase in density by the condensation of hydroxy groups (Fig. 3a). The TEM observations of the obtained samples showed sheet-like structures with definite morphologies (Fig. 3b); the selected area electron diffraction (SAED) analysis showed a halo pattern derived from the amorphous structure (Fig. 3b (inset)), which strongly indicates that nanosheets composed of non-layered structures were formed. The EDS spectra of the nanosheets clearly showed the existence of the gallium species, and their mapping images corresponded to the scanning transmission electron microscopy (STEM) image (Fig. 3d, e). The EDS mapping of the S element obviously showed that the surfactant was removed, indicating that the nanosheets were stable without surfactant. Furthermore, the nanosheets were composed of gallium oxide/oxyhydroxide, as shown by the peaks at 1119 eV derived from Ga3+ 2p3/2 in the X-ray photoelectron spectroscopy (XPS) spectrum (Fig. 3c). The spectrum derived from O1s showed multiple peaks, which should indicate the formation of Ga–O–Ga, in addition to Ga–OH–Ga or Ga–OH groups in the amorphous nanosheets (Supplementary Fig. 4). The calcination process of the amorphous nanosheets also lead to the decrease of the peaks at 532 eV, which may suggest the formation of oxide. Therefore, we could conclude that molecularly thin amorphous gallium oxide/oxyhydroxide was successfully formed. The nanosheets also showed a robust stability against post treatments such as sonication and washing processes. Even after the sonication process, gallium derived nanosheets retained their sheet-like morphology without decomposition into small pieces. For removing the surfactant adsorbed on the nanosheets, we used centrifugation and redisperse process. Even after such a harsh washing, there was no significant change in the morphology and structure of the nanosheets. These results indicate that our amorphous nanosheets have a robust stability against the post treatments; gallium species were connected contentiously and formed stabilized nanosheet structures (detailed experiments were shown in Supplementary Information). Moreover, the thickness of the amorphous 2D nanosheets could be controlled by changing the concentration of the intermediates in the formamide. Nanosheets with thicknesses of 3.0, 6.0, and 20 nm were obtained by changing the dispersion conditions, and calcination of those nanosheets decreased the thicknesses to 1.5, 3.0 and 10 nm, respectively (Supplementary Fig. 5). Therefore, this process has potential for designing the thicknesses of nanosheets.Fig. 3: Molecularly thin amorphous 2D gallium oxide/oxyhydroxide nanosheets.a AFM image with height (solid line) and the corresponding evaluated place (dashed line), scale bar 15 μm, b TEM image, scale bar 5 μm, and (inset) SAED pattern, c XPS spectrum, d STEM image and EDS mappings of Ga and S elements and e EDS spectrum of amorphous 2D gallium oxide/oxyhydroxide nanosheets, scale bar 2 μm.Key factors for the formation of amorphous nanosheetsAmorphous 2D gallium oxide/oxyhydroxide nanosheets were obtained after formamide immersion despite the non-continuous gallium species in the intermediate. Therefore, the formation of the nanosheets could not be caused only by the simple exfoliation of layered compounds. We investigated the composition of the solution and the residual composites to elucidate the formation mechanism. Immersing the intermediates in formamide resulted in the partial dissolution of the intermediates and dispersing gallium clusters within the formamide after a day. Furthermore, the composition of the intermediate was altered from C18H37SO4:C18H37OH:Ga2O3-xHy = 1:1:0.72 to 1:27.4:33.8 with slightly changing the morphology of the remaining composites, suggesting selective dissolution of C18H37SO4 and movement of gallium clusters within the remaining powders (Supplementary Fig. 8). Following 5 days immersion, the powders immersed in formamide further dissolved in the solution, resulting in lamellar composites of gallium oxide/oxyhydroxide nanosheets and octadecanol (C18H37SO4:C18H37OH:Ga2O3-xHy = 1:42:102) while retaining the morphology after one day of immersion (Supplementary Fig. 9). The small nanosheets might arise from exfoliating lamellar hybrids of octadecanol and nanosheets. On the other hand, octadecanol crystals with distinct morphology were also formed in the formamide solution, reflecting the morphology of the obtained amorphous nanosheets (Supplementary Fig. 10). From these results, we proposed the following formation mechanism: gallium clusters diffusing outside should adsorb on the surface of octadecanol crystals or introduced into the interlayer space of the lamellar hybrids, and formed amorphous nanosheets. Detailed discussions regarding key factors are provided in the Supplementary information (Supplementary Fig. 11).Synthesis of amorphous 2D metal oxide/oxyhydroxide nanosheets with various compositionsThis surfactant template method using solid-state crystals allowed the synthesis of amorphous 2D nanosheets for gallium oxide, among other oxides and oxyhydroxides. Anionic surfactants acted as ligands of various metal ions that stably formed aqua complexes or polycationic clusters in aqueous solutions, leading to the formation of surfactant crystals. Therefore, the surfactant crystals were prepared using metal salts with Mg2+, Al3+, Ca2+, Sc3+, V3+, Cr3+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Ga3+, Sr2+, Y3+, Zr4+, Ru3+, Rh3+, Cd2+, In3+, Sn2+, Ba2+, La3+, Ce3+, Sm3+, Gd3+, Pb2+ and Bi3+ ions as precursors. Lamellar crystals were successfully formed by mixing aqueous solutions of sodium octadecylsulfate and metal salt aqueous solutions, as shown by the XRD patterns (Fig. 4b). The lamellar crystals containing divalent cations and lanthanoids formed single phase components because they formed a stable aqua complex without the coordination of the OH- group under weakly acidic conditions. In contrast, the surfactant crystals containing elements Al3+, Sc3+, Cr3, Fe3+, Ga3+, Rh3+ and In3+ tended to have broad XRD patterns, probably because these ions dissolved in the solution as a complex containing both H2O and OH- ligands (M(OH)v(OH2)n-v(z-v)+) under weakly acidic conditions, forming various polynuclear complexes27. Therefore, metal complexes were distributed in their oligomeric form, forming various surfactant crystals and broadening the XRD patterns. Metal ions with relatively high valence states (z ≥ 4), such as Zr4+, strongly reacted with water, even under highly acidic conditions, making it difficult to prepare the surfactant crystals.Fig. 4: Synthesis of surfactant crystals with various kinds of metal ions.a Appearances of the surfactant crystals and the results from each process. b XRD patterns of the surfactant crystals prepared using various metal ions.The obtained surfactant crystals were treated with humid ammonia vapor, and almost all lamellae showed structural changes due to the hydrolysis and condensation of metal species, with remaining peaks derived from the packing of alkyl chains. The crystals with Ba2+, Sr2+ and Ca2+ retained their structures because of their stabilities against hydrolysis, even under basic conditions, due to their low electronegativities (χ = 0.89, 0.99 and 1.04) (Supplementary Fig. 14). Although the surfactant crystals containing V3+ and Sn2+ reacted with ammonia vapor treatment, products including ammonium vanadate and tin oxide were obtained, possibly due to their high reactivity to changes in valence to V5+ and Sn4+. Although Bi3+ is a trivalent cation, its high electronegativity (χ = 2.03) allowed a drastic reaction, leading to an apparent polycrystalline metal oxide. In contrast, the surfactant crystals containing other elements did not show obvious peaks derived from any metal hydroxide, oxyhydroxide, oxide, or other related compounds, except cerium species. While the XRD patterns may suggest the presence of crystalline samples, the observed peaks are likely attributable to the packing of alkyl-chains and ammonium sulfate (Supplementary Fig. 15). The metal species generally reacted with OH- groups and formed continuous structures or metalate under strongly basic conditions; however, we assumed that the obtained intermediate probably formed molecular hydroxide for divalent cations and lanthanoid by considering the results of La3+ as a typical example (Supplementary Fig. 16), and we assumed the formation of neutral clusters for Al3+, Sc3+, Cr3, Fe3+, Ga3+, Rh3+ and In3+ due to the results of the gallium species. The interlayer space of the surfactant crystals could act as a unique space considering the chemical statement of the obtained metal species, as some of the intermicellar space exhibited similar phenomena28,29.Then, the obtained lamellar hybrids were immersed in formamide and heated for seven days. Distinct inorganic nanosheets were formed when we used intermediates containing Al3+, Sc3+, Cr3+, Fe3+, Ga3+, Rh3+, and In3+, in addition to cerium, manganese, and cobalt species. The cerium, manganese, and cobalt species were probably oxidized in the obtained nanosheets, which could promote the formation of nanosheets (Supplementary Fig. 17). The other intermediate containing Mg2+, Ni2+, Cu2+, Zn2+, Y3+, Cd2+, La3+, Sm3+, Gd3+ and Pb2+ dissolved in the formamide completely, and obvious nanosheets containing metal species were not observed. The thicknesses of the nanosheets prepared by using intermediates with Al3+, Sc3+, Cr3+, Mn3+, Fe3+, Co3+, Ce4+, Rh3+ and In3+ were 1.5, 2.5, 1.0, 0.7, 2.5, 0.8, 1.5, 1.2 and 5.5 nm, respectively, which were the thinnest to our knowledge (Fig. 5a and Supplementary Figs. 18 and 19). Furthermore, the combination of these elements allowed the formation of nanosheets, such as Fe3+ & Ga3+ and Ga3+ & In3+, showing the possibility of the combination of desired metal ions into the nanosheets. These elements were homogeneously distributed over the amorphous nanosheets, as shown by EDS mapping images (Supplementary Fig. 20a, c). The composition of the amorphous nanosheets can be intentionally modified by controlled composition of the precursor surfactant crystals, which is very helpful in the rational design of the amorphous nanosheets. (Supplementary Fig. 20b). The TEM images showed a sheet-like structure with a folded part, indicating that free-standing nanosheets were obtained and folded during the transfer into TEM grids. The SAED patterns of nanosheets showed halo patterns derived from the amorphous structures due to nanosheets or support membranes; ceria nanosheets exhibited different trends (Fig. 5b and Supplementary Fig. 9). The ceria nanosheets were polycrystalline, probably because of their high reactivity and valence. Therefore, we demonstrated that the solid surfactant crystal method was apparently effective for synthesizing various kinds of metal oxide/oxyhydroxide nanosheets. In addition, the obtained nanosheets have a large lateral size, which is suitable for fabricating the electrode on the nanosheets, which is useful for future investigation of their properties (Supplementary Figs. 21 and 22).Fig. 5: Molecularly thin amorphous 2D nanosheets with various compositions.a AFM images with height (solid line) and the corresponding evaluated place (dashed line), and b TEM images and (inset) SAED patterns of nanosheets synthesized with various metal ions.Furthermore, it is essential to clarify the stability of the obtained amorphous nanosheets for their various applications. The obtained nanosheets with Al3+, Fe3+, Co3+, In3+, and Rh3+ species had high long-term stability because they maintained their morphology and amorphous nature even after one year of standing and subsequent sonication process (Supplementary Fig. 23). While the nanosheets containing Sc3+, Cr3+, and Mn3+ species were no longer observable after one year of standing, those not subjected to long-term standing retained their morphology and amorphous nature even after the sonication process (Supplementary Fig. 24). Additionally, these nanosheets exhibited wrinkled or folded structures, suggesting soft and resilient mechanical properties. In addition, we transferred obtained amorphous nanosheets containing Fe3+ to the porous membrane. The obtained nanosheets retained their morphology even after transfer and following drying process (Supplementary Fig. 25), which means obtained nanosheets had enough mechanical stability. Recent theoretical calculations on 2D amorphous carbon nanosheets indicate robust mechanical stability even compared to crystalline monolayers, suggesting their potential applicability across various classes of 2D materials30. These findings suggest that our nanosheets exhibit sufficient stability for a wide range of applications and possess potential for demonstrating unique mechanical properties.For further understanding of the essence of this method, the differences in the metal species regarding the formation of nanosheets were discussed. Considering the possible formation mechanism of the amorphous nanosheets, it is essential to generate the interaction among octadecanol and metal species for templating the morphology of the crystals. One of the key factors for the synthesis of the amorphous nanosheets is the formation of the clusters in the intermediates because immersing the precursor crystals containing metal ions did not lead to the formation of the nanosheets. Recent research clarified that the nano-sized clusters have super chaotropic characteristics31, which lead to strong interaction with the octadecanol. Therefore, it should be essential to keep the clusters in the solution stable without degradation and promote condensation after the adsorption on the template crystals for the synthesis of amorphous nanosheets.For considering the stability of metal species, it is widely known that the pH, electronegativity, and valence of cations are important because they can define the condensation characteristics of M-OH bonds in the solutions27,32. The apparent pH levels of the dispersed solutions were almost neutral, and then we discussed the condensation of the metal species and clusters in the neutral condition. In the case of the trivalent cations with relatively high electronegativities, they can form electrically neutral species under neutral pH conditions, which form continuous structure via concentration (Supplementary Fig. 26). Therefore, the clusters slowly dispersed from surfactant crystals should adsorb on the octadecanol crystals, and probably leading to the condensation with templating the crystalline octadecanol morphology. On the other hand, the metal species that did not form the nanosheets were divalent or trivalent cations with low electronegativity; these species tend to form soluble metal complexes, such as [M(H2O)6]2+ at neutral pH condition (Supplementary Fig. 26), and continuous structures cannot be stably formed. Therefore, we concluded that the formation of clusters in the intermediate and the stability and condensation of clusters in the formamide dominated the formation of the amorphous 2D nanosheets. Utilizing stronger basic vapor conditions for the preparation of intermediates with clusters and further precise control of pH in an aging solution may allow for the preparation of amorphous 2D nanosheets with other compositions. This sold-state surfactant templating method will further develop the complex design techniques of amorphous 2D nanosheets, including high-entropy compounds.In summary, molecularly thin amorphous 2D nanosheets with various kinds of metal oxides/oxyhydroxides were successfully synthesized via a stepwise reaction using solid surfactant crystals as templates. Surfactant crystals with various kinds of metal ions were successfully prepared by simply mixing the anionic surfactant and metal ion solutions. The humid ammonia vapor treatment for the surfactant crystals formed clusters inside the interlayer space of solid surfactant crystals. Immersing the obtained crystals into formamide formed molecularly thin amorphous 2D oxide/oxyhydroxide nanosheets. The limited space in the solid surfactant crystals promoted the stepwise reaction of the hydrolysis, the condensation of metal ions and the 2D assembly of building units, thus generating diverse amorphous 2D nanosheets. This solid surfactant crystal templating system created amorphous 2D oxide/oxyhydroxide nanosheets composed of Al3+, Sc3+, Mn3+, Fe3+, Co3+, Ga3+, In3+ and Rh3+ species. Our solid surfactant crystalline templating approach could significantly impact the science of amorphous 2D nanosheets synthesized via the bottom-up approach.
