The preparation process can be summarized in Figure 1a. The fundamental process is dependent on liquid-phase growth of ultrathin lamellar nickel hydroxide precursor under microwave irradiation. This stage undergoes a homogeneous alkalinization of nickel(II) nitrate solutions by urea hydrolysis through inductive effect of microwave irradiation under low-temperature atmospheric conditions. This process is conducted in a 1000 mL three-necked flask in microwave reactor (Figure S1a). The comfortable thermodynamic and kinetic factors are favorable for growth of ultrathin intermediate24,25. The formation of ultrathin nanosheets is dominated by a self-assembly and oriented attachment mechanism. The rapid microwave heating can facilitate the super saturation of reactant species, leading to the instantaneous formation of ultrafine nanocrystals and then spontaneous self-assembling or oriented attachment by intrinsic driving force of lamellar nickel hydroxide for 2D anisotropic growth. After heat treatment at 300°C (Figure S2a), α-Ni(OH) 2 nanosheets are completely decomposed into NiO. Figure S2b shows that single crystalline phase of α-Ni(OH) 2 with a hexagonal layered structure (JCPDS 22-0444) was obtained. All the diffraction peaks of NiO are consistent with a face-centered cubic phase (JCPDS 04-0835). The as-synthesized α-Ni(OH) 2 precursor exhibits a geometrically sheet-like 2D structure (Figure 1b). They are uniform and freestanding with a micron-sized planar area. The nanosheets are comprised of ultrafine nanocrystals arranging in planar direction (Figure S3). The sheet-like 2D morphology can be perfectly retained after heat treatment (Figure 1c) and there are no apparent broken or collapsed structures in the final sample, suggesting a good structural stability of this 2D structure. Figure 1d further reveals a highly flexible and gauze-like morphology of non-layered NiO.

Figure 1 (a) Schematic illustrating synthesis of nanosheets; FESEM images of (b) α-Ni(OH) 2 and (c), (d) NiO nanosheets. Full size image

Figure 2a further shows the freestanding and large-area sheet-like morphology of α-Ni(OH) 2 , none of them assembles into a 3D hierarchical architecture. Figure 2b–c reveal their clear and well-defined outline with highly flexible and transparent features, suggesting fundamental characteristics in common of this kind of ultrathin sheet-like nanostructure. HRTEM investigation in the edge areas of ultrathin nanosheets is a common and direct method to determine the layer thickness microscopically9. Figure S4a collected from the folded edge or protuberant ridge of α-Ni(OH) 2 nanosheets demonstrates an average thickness of ~1.52 nm, suggesting that the nanosheets is comprised of 2–3 layers of octahedral Ni(OH) 6 arranging in hexagonal symmetry on planar direction. The XPS analysis is further carried out to determine the composition and the surface electronic state of the as-synthesized α-Ni(OH) 2 nanosheets (Figure S5). Only oxygen and nickel species are detected.

Figure 2 (a) low and (b) high magnification FESEM images of α-Ni(OH) 2 nanosheets; (c) TEM image (the inset showing SAED pattern), (d) a planar HRTEM image, (e), (f) the corresponding FFT pattern and enlarged HRTEM image recorded from (d) and (g) a vertical HRTEM image of NiO nanosheets. Full size image

SAED pattern (inset in Figure 2c) indicates a polycrystalline nature of the overall NiO nanosheets. The three marked diffraction rings correspond to the {111}, {200} and {220} planes, respectively. Figure 2d shows that the NiO nanosheet in the selected region shares the same lattice fringe and crystallographic orientation, indicating the same behavior in short range during the crystal transformation. Some observed small pores are irregularly distributed on surface causing a high roughness, which could be created by the evacuation of gaseous contents. FFT pattern (Figure 2e) collected from the area of regular lattice fringes reveals single-crystalline feature. Figure 2f shows some visible lattice fringes with an equal interplanar distance of 2.4 Å, corresponding to (111) planes of cubic NiO. Figure 2g demonstrates a ~1.16 nm layer thickness of NiO. As expected, the specific surface area of α-Ni(OH) 2 and NiO nanosheets is as high as 190.15 and 196.01 m2 g−1 (Figure S6). Such high values are associated with the unique structure of ultrathin nanosheets with extended and rough surfaces. More interestingly, it is found that the NiO nanosheets can form a stable and uniform dispersion in ethanol for weeks (Figure S1d).

Compared with traditional wet-chemical syntheses, the microwave-assisted liquid-phase growth can decrease reaction time to less than 20 minutes26. The formation of precursor nanosheets is finished in a very short time. After 3 minutes under microwave irradiation with subsequent heat treatment, the perfect non-layered NiO nanosheets can be obtained (Figure S7a). Obviously different from the previously reported wet-chemical syntheses and the analogous microwave-activated procedure22,26, the current reaction systems can avoid the morphology transformation to 3D hierarchical structures with prolonged reaction time. Figure S7a–d show that the non-layered NiO nanosheets synthesized with different microwave irradiating time (3, 5, 15 and 30 minutes) nearly exhibit the same morphology. It could be attributed to the fast local heating from microwave activation and the good structural stability of those nanosheets. This is the first report about a synthesis of freestanding low-dimensional nanostructures, especially ultrathin 2D nanosheets, that independent of reaction time.

In the experiments, it is found that the crucial reaction parameter is water. The formation of nanosheets strongly depends on the effects of the water molecules. Under the optimal amounts of water, the NiO samples exhibit a freestanding and large-area sheet-like morphology. Whereas with the contained water decreasing, the final products become folded and assemble into a flower-like quasi-spherical 3D hierarchical architecture (Figure S8a), although retaining their sheet-like building blocks (Figure S8b). In the absence of water, the final morphology further evolves into spherical aggregates (Figure S8c), where nanosheets completely disappearing. Instead, ultrafine nanocrystals tend to spontaneously together forming 3D spherical structures (Figure S8d) rather than arranging in 2D planar direction. So the current experiments demonstrate that directional hydrophobic attraction plays a crucial role in determining morphologies of final products. The formation of ultrathin nanosheets in our microwave-assisted liquid-phase growth procedure is attributed to two factors: layered-structural nature and hydrophobicity. It is well known that the 2D anisotropic growth of nanomaterials needs larger driving force. In case of the layered crystals, they have the tendency to growth into layers. So the intrinsic driving force of lamellar nickel hydroxide is adequate for their 2D anisotropic growth under microwave activation. The layered-structural nature is believed to be a prerequisite for the formation of 2D network in the present facile method. The required hydrophobicity can bring about directional hydrophobic attraction between nanocrystals and water molecules, forming two-phase interfaces where the excessive surface energy can be accommodated. A balance of anisotropic hydrophobic attraction and electrostatic interaction can be realized, which is favorable for the spontaneous organization of nanocrystals into nanosheets27,28. The resulted interaction could allow for the epitaxial orientation of ultrafine nanocrystals and hinder their potential of shrinking and aggregating. Furthermore, the presence of the hydrophobicity could also terminate their stacking and packing, leading to ultrathin 2D structure rather than 3D graphite-like layered framework.

To investigate local atomic arrangements and electronic structures of ultrathin α-Ni(OH) 2 nanosheets, X-ray absorption fine structure spectroscopy (XAFS) measurements at Ni K-edge are conducted. Figure 4a shows the typical layered structure model. Figure 3b shows that the observed spectral peaks for α-Ni(OH) 2 nanosheets slightly shift to the higher energy direction. Particularly, the most remarkable difference appears in the main peak locating at around 8351 eV in terms of position and intensity. This result evidently manifests the affection of interlayer scattering toward the observed spectrum. The ultrathin thickness (1.52 nm) means the few layers (2–3) of the α-Ni(OH) 2 nanosheets, which deliver a little interlayer scattering. The removing of interlayer scattering causes the shift of the three major peaks to the higher energy side29. Ni K-EXAFS k2x(k) oscillation curve for nanosheets (Figure 3c) exhibits a small reduction in amplitude and a little difference in spectral shape compared with that of bulk counterpart, implying the different local atomic arrangement. The nanosheets exhibit three obvious differences in Fourier transform curve (Figure 3d). First, the Ni-O2/O3 peak intensity of the nanosheets decreases significantly to noise level, which can be attributed to the missing contribution from the O3 atoms. This result once again demonstrates the few layer structure of the ultrathin nanosheets. Secondly, the nanosheets show a shift toward a shorter distance and an intensity decrease in the Ni-Ni3 peak. The shift can be attributed to the presence of surface structure distortion on nanosheets. While the particularly strong Ni-Ni3 peak for the bulk mainly caused by the focusing effect from the Ni1 atom situated halfway on the Ni-Ni3 path29. Thirdly, the Ni-Ni1 peak for the nanosheets slightly shifts toward a shorter distance. But the Ni-O1 peak shows no shift with intensity similar to the corresponding bulk peak. These facts suggest that the structural contraction occurs parallel to the Ni layer (i.e. along the ab plane) in nanosheets, which also means a noticeable distortion on surface of the nanosheets that in turn helps to balance their excessive surface energy and then allow them with excellent stability11.

Figure 3 (a) Schematic structure model of α-Ni(OH) 2 single layer along c axis. The shadow represents the (001) plane. The third-nearest neighbor oxygen atom (O3) is situated in the adjacent layer (not shown); (b) Ni K-edge XANES spectra of the α-Ni(OH) 2 nanosheets and their bulk counterpart; (c) Ni K-EXAFS oscillation functions k2x(k) and (d) the corresponding Fourier transforms. Full size image

Figure 4 Typical CV curves of α-Ni(OH) 2 nanosheets at different scan rates in 6 M KOH electrolyte. The CV curve for the pristine Ni foam current collector at 5 mV s−1 is also listed for comparison demonstrating a negligible CV background. Full size image

Ultrathin 2D nanomaterials represent a great promising application in next generation batteries and supercapacitors. With respect to the ultrathin α-Ni(OH) 2 and NiO nanosheets, which are almost completely made up by surfaces with the most active material exposed outside for the highly surface-dependent Faradaic redox reactions, their potential in supercapacitor are systematically investigated. Figure 4 shows typical Cyclic voltammogram (CV) curves of the ultrathin α-Ni(OH) 2 nanosheets. A pair of current peaks can be clearly observed during the cathodic and anodic sweeps, which correspond to the reversible conversion between Ni(II) and Ni(III)30,31,32. The reaction involves the reversible process of insertion and extraction of OH−1 ions. This result reveals that the charge storage mechanism of the α-Ni(OH) 2 nanosheet electrodes is mainly ascribed to the pseudocapacitance from the Faradaic processes. Furthermore, the redox peaks show a symmetric characteristic, suggesting a high reversibility of α-Ni(OH) 2 nanosheets. Apparently, the current density increases with the increasing scan rate and all CV curve maintain a similar shape, indicating that the ultrathin α-Ni(OH) 2 nanosheets are beneficial to fast redox reactions. The CV curves show more prominent symmetry at higher scan rates indicating better high-rate response of the α-Ni(OH) 2 nanosheets. In addition, as the scan rate increased, the potential of the anodic and cathodic peaks shifted in more positive and negative directions, respectively, most likely attributed to a high electron hopping resistance or the limitation of the ion diffusion rate to satisfy electronic neutralization during the redox reaction.

As shown in Figure 5a, all the nonlinear discharge curves confirm the pseudocapacitive characteristic. Encouragingly, the ultrathin α-Ni(OH) 2 nanosheets deliver a ultrahigh specific capacitance of 4172.5 F g−1 at 1 A g−1 (Figure 5b). This specific capacitance is the highest value reported up to now. The ever reported high specific capacitance is 3500 F g−1 for Co(OH) 2 on ultra-stable Y zeolite33, but still lower than our current result. As the discharge current density increased, the ultrahigh specific capacitance is still maintained. The specific capacitances are 3650, 3270, 2820 and 2680 F g−1 at 2, 4, 8 and 16 A g−1, respectively. The specific capacitance gradually decreases at higher current density due to the incremental (iR) voltage drop. It is found that all the obtained specific capacitances are higher than the corresponding results in previous reports respectively checked at the same current density. It is well known that fabricating direct nanostructured electrodes is the most attractive strategy, such as highly ordered nanostructured array electrode, to achieve ultrahigh electrochemical performances. For example, Yang reported electrodeposited Ni(OH) 2 on nickel foam as direct nanostructured electrodes and obtained a high specific capacitance of 3152 F g−1 at 4 A g−1 current density34, yet slightly lower than our 3270 F g−1; Yuan fabricated the Ni foam supported ultrathin mesoporous NiCo 2 O 4 nanosheets electrodes achieving excellent pseudocapacitance of 1694 F g−1 at 8 A g−1 current density35; Shang reported the coaxial Ni x Co 2x (OH) 6x /TiN nanotube arrays as supercapacitor electrodes, exhibiting a high specific capacitance of 2543 F g−1 calculated from the CV curves (5 mV s−1)36. In practice, using of direct nanostructured electrodes is favorable for proof-concept studies or for microdevices, but unlikely for commercial applications, especially in electric vehicles or power grid storage. Remarkably, the specific capacitances of our α-Ni(OH) 2 nanosheets are distinctly superior to that of the conceptual designing direct nanostructured electrodes.

Figure 5 Electrochemical characterization of α-Ni(OH) 2 nanosheets: (a) discharge curves; (b) specific capacitance as a function of current densities; (c) average specific capacitance versus cycle number, the inset shows the galvanostatic charge-discharge curves at current density of 4 A g−1; (d) Nyquist plots before and after cycling at current density of 4 A g−1, (inset) equivalent fitting circuit and impedance at high frequency region. Full size image

when current density is increased to 16 A g−1, the specific capacitance is still 2680 F g−1 and retain 64.2% of its initial value with a current density increase of 16 times, indicating excellent rate capability due to short diffusion path distances. The specific capacitances are larger than the theoretical value of 2602 F g−1 (within 0.4 V). Besides pseudocapacitance derived from the Faradaic redox reactions, contributions from double-layer capacitance are obvious. Considering the high specific surface area from α-Ni(OH) 2 nanosheets, contribution from double-layer capacitance certainly supply an additional boost to the observed value. As shown in Figure 5c, the α-Ni(OH) 2 nanosheets exhibit excellent cycling stability. The average specific capacitance at 4 A g−1 increases gradually up to 3320 F g−1 in the course of 2000 cycles with a high capacitance retention of 101.5%, due to full activation of electrode material. Even at high rates, 8 and 16 A g−1, the capacitance retention after 2000 cycles is still maintained at 100% and 98.5%, respectively, indicating stable cycling performance at each current density. The insert in Figure 5c further indicates that the fast charge-discharge process of the electrode is highly reversible.