METHOD FOR PREPARING METAL OXIDE NANOSHEETS

Abstract

The present invention generally relates to a method for preparing metal oxide nanosheets. In a preferred embodiment, graphene oxide (GO) or graphite oxide is employed as a template or structure directing agent for the formation of the metal oxide nanosheets, wherein the template is mixed with metal oxide precursor to form a metal oxide precursor-bonded template. Subsequently, the metal oxide precursor-bonded template is calcined to form the metal oxide nanosheets. The present invention also relates to a lithium-ion battery anode comprising the metal oxide nanosheets. In a further preferred embodiment, the battery anode may comprise a reduced template, which is reduced graphene oxide (rGO) or reduced graphite oxide.

Claims

1.-33. (canceled)

34. Metal oxide nanosheets having a thickness in the range of 0.5 nm to 10 nm.

35. The metal oxide nanosheets according to claim 34, wherein the metal oxide is a transition metal oxide or wherein the metal oxide nanosheets are doped binary oxide nanosheets or ternary oxide nanosheets.

36. The metal oxide nanosheets according to claim 34, wherein said metal oxide nanosheets are mesoporous.

37. The metal oxide nanosheets according to claim 34, wherein said metal oxide nanosheets are amorphous or crystalline.

38. The metal oxide according to claim 34, wherein the metal oxide nanosheets have a crystallite size in the range of 3 nm to 100 nm or wherein the metal oxide nanosheets have a Brunauer-Emmett-Teller (BET) surface area in the range of 15 m.sup.2/g to 350 m.sup.2/g.

39. A battery anode comprising metal oxide nanosheets comprising at least one transition metal, wherein the metal oxide nanosheets are binary oxide nanosheets, doped binary oxide nanosheets or ternary oxide nanosheets.

40. The battery anode according to claim 39, wherein the metal oxide nanosheets have a thickness in the range of 0.5 nm to 10 nm.

41. The battery anode according to claim 39, wherein the layered metal oxide further comprising a reduced template.

42. The battery anode according to claim 41, wherein the reduced template is reduced graphene oxide (rGO) or reduced graphite oxide.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0150] The accompanying drawings illustrate a disclosed embodiment and serves to explain the principles of the disclosed embodiment. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention.

[0151] FIG. 1 is a number of images and curves related to characterization of Nb.sub.2O.sub.5 nanosheets prepared according to Example 1a. FIG. 1A is the transmission electron microscopy (TEM) image and selected area electron diffraction (SAED) pattern (inset) of Nb.sub.2O.sub.5 nanosheets obtained after calcination in air at 500° C./one hour; FIG. 1B is the TEM image and SAED pattern (inset) of Nb.sub.2O.sub.5 nanosheets obtained after calcination in air at 550° C./3 hours; FIG. 1C shows the TEM image and SAED pattern (inset) of Nb.sub.2O.sub.5 nanosheets obtained after calcination in air at 600° C./one hour. FIG. 1D, FIG. 1E and FIG. 1F show the high-resolution TEM (HRTEM) images of Nb.sub.2O.sub.5 nanosheets obtained after calcination in air at 500° C./one hour, 550° C./3 hours and 600° C./one hour, respectively. FIG. 1G depicts the X-ray diffraction (XRD) patterns. FIG. 1H describes the thermal gravimetric analysis (TGA) profiles of Nb.sub.2O.sub.5 nanosheets in air (ramp=2° C./min).

[0152] FIG. 2A shows nitrogen adsorption-desorption isotherms and Barrett-Joyner-Halenda (BJH) pore size distribution curves (inset) of Nb.sub.2O.sub.5 nanosheets synthesized in air at 500° C./one hour and 550° C./3 hours as described in Example 1a. FIG. 2B describes the effect of increasing calcination temperature and duration on specific surface area and pore volume of the same samples as FIG. 2A.

[0153] FIG. 3 is a number of images and graphs related to the characterization of Nb.sub.2O.sub.5 nanosheets synthesized using Nb-ethoxide according to Example 1b. FIG. 3A, FIG. 3B, FIG. 3C and FIG. 3D are the TEM images of the nanosheets obtained at different calcining temperatures, at 350° C./one hour (FIG. 3A), at 450° C./one hour (FIG. 3B), at 500° C./one hour (FIG. 3C), and at 600° C./one hour (FIG. 3D). FIG. 3E describes the atomic force microscopy (AFM) analysis of the same sample of nanosheets as FIG. 3C. FIG. 3F depicts the XRD patterns of nanosheets obtained according to Example 1b.

[0154] FIG. 4 is a number of images and graphs related to characterization of TiO.sub.2 nanosheets calcined in air for one hour according to Example 1c. FIG. 4A, FIG. 4B and FIG. 4C are the TEM images of the nanosheets calcined at different temperature, at 350° C. (FIG. 4A), at 500° C. (FIG. 4B), at 550° C. (FIG. 4C). FIG. 4E describes the XRD patterns of nanosheets calcined at different temperatures (at 350° C., 500° C. and 550° C.). FIG. 4D describes the HRTEM image of the nanosheets calcined at 500° C. Inset: SAED pattern. FIG. 4F depicts the nitrogen adsorption-desorption isotherms of the nanosheets calcined at 350° C. and 500° C. FIG. 4G shows the AFM analysis of nanosheets calcined at 500° C.

[0155] FIG. 5 is a number of graphs and images related to characterization of doped binary oxide nanosheets as described in Example 2. FIG. 5A and FIG. 5B show the XRD patterns of Ti-doped Nb.sub.2O.sub.5 and Nb-doped TiO.sub.2 nanosheets at the specified doping levels. FIG. 5E shows the magnified (001) XRD peak in FIG. 5A. FIG. 5F shows the magnified (101) peak in FIG. 5B. FIG. 5C and FIG. 5D are the TEM images of doped binary oxide nanosheets Ti—Nb.sub.2O.sub.5-0.02 and Nb—TiO.sub.2-0.1, respectively; FIG. 5G and FIG. 5H are the HRTEM images; FIG. 5I and FIG. 5N are the X-ray photoelectron spectroscopy (XPS) Nb 3d spectra; FIG. 5J and FIG. 5M are the XPS Ti 2p spectra; FIG. 5K, FIG. 5L, FIG. 5O, and FIG. 5P are the energy dispersive X-ray spectroscopy (EDX) elemental maps of FIG. 5C, FIG. 5G, FIG. 5I, FIG. 5M, FIG. 5K, FIG. 5O, 0.02 at % Ti-doped Nb.sub.2O.sub.5 and FIG. 5D, FIG. 5H, FIG. 5J, FIG. 5N, FIG. 5L, FIG. 5P, 0.1 at % Nb-doped TiO.sub.2 nanosheets.

[0156] FIG. 6 is a number of images and graphs related to characterization of ternary nanosheets prepared according to Example 3. FIG. 6A is the TEM image of TiNb.sub.2O.sub.7 nanosheets, FIG. 6C and FIG. 6D are the EDX elemental maps of TiNb.sub.2O.sub.7 nanosheets, FIG. 6G is the XRD pattern of TiNb.sub.2O.sub.7 nanosheets, FIG. 6I is the XPS Nb 3d spectrum of TiNb.sub.2O.sub.7 nanosheets and FIG. 6K is the XPS Ti 2p spectrum of TiNb.sub.2O.sub.7 nanosheets. FIG. 6B is the TEM image of Ti.sub.0.61Nb.sub.1.29O.sub.4/rGO nanosheets, FIG. 6E and FIG. 6F are the EDX elemental maps of Ti.sub.0.61Nb.sub.1.29O.sub.4 nanosheets. Inset in FIG. 6B is the TEM image of Ti.sub.0.61Nb.sub.1.29O.sub.4/rGO. FIG. 6H is the XRD patterns of Ti.sub.0.61Nb.sub.1.29O.sub.4 and Ti.sub.0.61Nb.sub.1.29O.sub.4/rGO nanosheets. FIG. 6J is the XPS Nb 3d spectrum of Ti.sub.0.61Nb.sub.1.29O.sub.4/rGO nanosheets and FIG. 6L is the XPS Ti 2p spectrum of Ti.sub.0.61Nb.sub.1.29O.sub.4/rGO nanosheets.

[0157] FIG. 7 is a number of images and graphs related to characterization of a range of nanosheet materials synthesized according to Example 4. FIG. 7A, FIG. 7B, FIG. 7C, FIG. 7E, FIG. 7F, FIG. 7G, FIG. 7I, FIG. 7J, FIG. 7K, FIG. 7L, FIG. 7M, FIG. 7N, FIG. 7O and FIG. 7P are the TEM images of Fe.sub.2O.sub.3(FIG. 7A), SnO.sub.2 (FIG. 7B), NiO (FIG. 7C), Co.sub.3O.sub.4(FIG. 7E), Mn.sub.3O.sub.4(FIG. 7F), ZrO.sub.2 (FIG. 7G), Ni.sub.xCo.sub.3-xO.sub.4 (FIG. 7I, FIG. 7J, FIG. 7K and FIG. 7L), and Mn.sub.xCo.sub.3-xO.sub.4 (FIG. 7M, FIG. 7N, FIG. 7O and FIG. 7P), nanosheets, respectively. FIG. 7D, FIG. 7H, FIG. 7Q, FIG. 7R are the XRD patterns of Fe.sub.2O.sub.3, SnO.sub.2 and NiO (FIG. 7D), Co.sub.3O.sub.4, Mn.sub.3O.sub.4 and ZrO.sub.2 (FIG. 7H), Ni.sub.xCo.sub.3-xO.sub.4 (FIG. 7Q) and Mn.sub.xCo.sub.3-xO.sub.4 (FIG. 7R) nanosheets.

[0158] FIG. 8 is a histogram for comparing the Zeta potential of GO, GO-PDDA, Nb.sub.2O.sub.5, TiO.sub.2 and TiNb.sub.2O.sub.7 nanosheets. GO-PDDA was prepared according to Example 5.

[0159] FIG. 9 is a number of images and graphs related to ex-situ analysis of Nb.sub.2O.sub.5 and TiO.sub.2 nanosheets after cycling according to Example 6. FIG. 9A is the TEM image of Nb.sub.2O.sub.5 after the first discharge, FIG. 9B is the TEM image of Nb.sub.2O.sub.5 after the first charging, FIG. 9C, is the TEM image of Nb.sub.2O.sub.5 after 200 cycles. FIG. 9D and FIG. 9E are the HRTEM images; FIG. 9F is the XRD patterns (Al refers to Al current collector, *indicates carbon additives peak) of Nb.sub.2O.sub.5 nanosheets after (FIG. 9A, FIG. 9D, FIG. 9F) 1.sup.st discharge, (FIG. 9B, FIG. 9E, FIG. 9F) 1.sup.st charge and (FIGS. 9C, 9F) 200 cycles. FIG. 9G, is the TEM image of TiO.sub.2 after 200 cycles and FIG. 9H. shows the HRTEM image of sample FIG. 9G; FIG. 9I is the XRD patterns (Cu refers to Cu current collector, *indicates carbon additives peak, arrows indicate TiO.sub.2 peaks) of TiO.sub.2 nanosheets after 200 cycles.

[0160] FIG. 10 is a number of graphs related to the cycling stability and rate capability of high-voltage-anode nanosheets according to Example 6. FIG. 10A, FIG. 10B, FIG. 10E, FIG. 10F describe the cycling stability and FIG. 10C, FIG. 10D, FIG. 10G, FIG. 10H show the rate capability of FIG. 10A, FIG. 10C, pure and doped Nb.sub.2O.sub.5 (assuming 1 C=150 mA/g), FIG. 10B, FIG. 10D pure and doped TiO.sub.2 (assuming 1 C=168 mA/g), FIG. 10E, FIG. 10G, TiNb.sub.2O.sub.7 (assuming 1 C=387.6 mA/g) and FIG. 10F, FIG. 10H Ti.sub.0.61Nb.sub.1.29O.sub.4 nanosheets.

[0161] FIG. 11 is a number of graphs related to the cycling stability and rate capability of high-capacity anodes according to Example 7. FIG. 11A, FIG. 11B, FIG. 11C, FIG. 11G, FIG. 11H and FIG. 11I show the cycling stability and FIG. 11D, FIG. 11E, FIG. 11F, FIG. 11J, FIG. 11K and FIG. 11L describe the rate capability of FIG. 11A, FIG. 11D, iron oxide, FIG. 11B, FIG. 11E, SnO.sub.2, FIG. 11C, FIG. 11F, NiO, FIG. 11G, FIG. 11J, Co.sub.3O.sub.4, FIG. 11H, FIG. 11K, Ni.sub.1.29Co.sub.1.71O.sub.4 and FIG. 11I, FIG. 11L, Mn.sub.1.08Co.sub.1.92O.sub.4 nanosheets.

EXAMPLES

[0162] Non-limiting examples of the invention and a comparative example will be further described in greater detail by reference to specific Examples, which should not be construed as in any way limiting the scope of the invention.

Example 1: Synthesis of Binary Oxide

[0163] 1a. Preparation of Nb.sub.2O.sub.5 Nanosheets Using NbCl.sub.5 as Precursor

[0164] In the synthesis of Nb.sub.2O.sub.5 as the binary oxide, metal oxide precursor NbCl.sub.5 (purchased from Sigma-Aldrich of Saint Louis, Mo. of the United States of America) was thoroughly mixed with graphene oxide (GO) dispersion and washed, leaving only those strongly bonded to GO surface. The NbCl.sub.5-bonded GO was pre-calcined in Ar for 2 hours at 550° C., and then calcined in a temperature range of 500-600° C. in air to remove the template and form the nanosheets simultaneously. Nb.sub.2O.sub.5 synthesized herein was used to demonstrate the synthetic process and the tunability of the nanosheets' attributes.

[0165] FIG. 1A, FIG. 1B and FIG. 1C illustrate the transmission electron microscopy (TEM) images of the Nb.sub.2O.sub.5 nanosheets synthesized at different temperature i.e. at 500° C. for one hour, at 550° C. for 3 hours and at 600° C. for one hour, respectively. The nanosheets were mesoporous structures that were composed of interconnected primary nanoparticles, as revealed by high-resolution TEM (HRTEM) as shown in FIG. 1D, FIG. 1E and FIG. 1F. The crystal structures were determined to be orthorhombic Nb.sub.2O.sub.5(JCPDS #00-030-0873) by X-ray diffraction (XRD) (refer to FIG. 1G).

[0166] The nanosheet crystallinity, crystallite size, surface area, porosity and reduced graphene oxide (rGO) content were modified by varying the calcination temperature and duration. At a lower calcination temperature and a shorter calcination duration (500° C. and one hour), weakly crystalline nanosheets were obtained, as revealed by HRTEM FIG. 1D, selected area electron diffraction (SAED) FIG. 1A (inset) and XRD FIG. 1G (graph labelled as 500° C./one hour).

[0167] As calcination temperature and duration increased, crystallinity increased as shown by XRD FIG. 1G (graph labelled as 550° C./3 hours), and the crystallites grew in size, as observed in HRTEM images (refer to FIG. 1E and FIG. 1F). This was accompanied by a decreased Brunauer-Emmett-Teller (BET) specific surface area and increased porosity as a result of particle coarsening (refer to FIG. 2A and FIG. 2B, respectively). The growth of the nanocrystallites along [001] direction increased with calcination temperature and duration, as demonstrated by HRTEM as shown in FIG. 1D, FIG. 1E and FIG. 1F. This was confirmed by the increase in (001)/(180) XRD peak intensity ratio from 0.74 to 0.77 and 0.93, as calcination temperature/duration increased from 500° C./one hour to 550° C./3 hours and 600° C./one hour, respectively (refer to FIG. 1G). This enhanced Nb.sub.2O.sub.5's performance as Li-ion battery anode since Li.sup.+ ions were expected to move freely along the natural tunnels between (001) planes.

[0168] The GO template was not completely removed by calcination. Residual rGO of 44.7 wt %, 29.0 wt % and 17.0 wt % was detected for samples calcined at 500° C./1 h, 550° C./3 h and 600° C./1 h, respectively (FIG. 1H). The residual rGO was also detected as a characteristic XRD peak at 20=26.6°, the intensity of which decreased as calcination temperature/duration increased.

1b. Preparation of Nb.sub.2O.sub.5 Nanosheets Using Niobium (V) Ethoxide as Precursor

[0169] Nanosheets of Nb.sub.2O.sub.5 metal oxides can also be produced with different types of precursors following similar procedure as described in example 1a above, except that different metal oxide precursor and/or the calcination temperature was used and no pre-calcination was employed. For example, using niobium (V) ethoxide (Nb-eth) (purchased from Sigma-Aldrich of Saint Louis, Mo. of the United States of America) as precursor, Nb.sub.2O.sub.5 nanosheets were also obtained, as confirmed by TEM (refer to FIG. 3A, FIG. 3B, FIG. 3C and FIG. 3D).

[0170] The experimental results suggest that the nanosheets' crystallinity can be controlled by calcination temperature. Nb.sub.2O.sub.5 nanosheets were amorphous when calcination temperatures of <500° C. were used, and they were crystalline when calcined at >500° C. as shown by XRD patters in FIG. 3F. Compared to NbCl.sub.5 precursor as shown in example 1a above, synthesis with Nb-eth precursor yielded Nb.sub.2O.sub.5 nanosheets with almost 4-fold increase in weight (refer to FIG. 3F). The higher yield was probably due to a better interaction between the alkoxide precursor and GO. The thickness of Nb.sub.2O.sub.5 nanosheets was determined to be about 2 nm by atomic force microscopy (AFM) (refer to FIG. 3E), demonstrating the effectiveness of our strategy to derive ultrathin metal oxide nanosheets.

1c. Preparation of TiO.sub.2Nanosheets Using Ti(IV) Butoxide as Precursor

[0171] TiO.sub.2 nanosheets were also obtained using the GO planar-confined growth method similar to examples 1a and 1b.

[0172] TiO.sub.2 nanosheets were synthesized using GO template and Ti(IV) butoxide (purchased from Sigma-Aldrich of Saint Louis, Mo. of the United States of America) as precursor, followed by calcination in air. The nanosheet structure is shown by TEM images (refer to FIG. 4A, FIG. 4B and FIG. 4C). TiO.sub.2 nanosheets adopted anatase crystal structure (JCPDS #00-021-1272), as shown by XRD (FIG. 4E). SAED pattern (FIG. 4D: inset) confirmed the anatase phase, whose (101) and (200) planes were identified by HRTEM (refer to FIG. 4D). TiO.sub.2 nanosheets are mesoporous as shown by the BJH pore size distribution as shown in FIG. 4F: inset. Specific surface area (FIG. 4F) and crystallite size (Table 1) of TiO.sub.2 nanosheets decreased with the increase in calcination temperature and duration. The thickness of TiO.sub.2 (500° C./1 h) was found to be about 1.3 nm using AFM analysis (FIG. 4G).

[0173] The effects of synthesis and calcination parameters on the characteristics of Nb.sub.2O.sub.5 and TiO.sub.2 nanosheets are summarized in Table 1 below.

TABLE-US-00001 TABLE 1 Synthesis conditions and properties of binary oxide nanosheets BET specific rGO surface Metal Calcination Crystallite content area oxide conditions Precursor Phase size (nm).sup.a (wt %) (m.sup.2/g) Nb.sub.2O.sub.5 500° C./1 h/air .sup.b NbCl.sub.5 Orthorhombic 14.3 45 74.7 Nb.sub.2O.sub.5 550° C./3 h/air .sup.b NbCl.sub.5 Orthorhombic 22.3 29 64.5 Nb.sub.2O.sub.5 350° C./1 h/air Nb(V) Amorphous — 3.0 113 ethoxide Nb.sub.2O.sub.5 500° C./1 h/air Nb(V) Orthorhombic 28.9 0 69.4 ethoxide Nb.sub.2O.sub.5 600° C./1 h/air Nb(V) Orthorhombic 35.4 0 26.2 ethoxide TiO.sub.2 350° C./1 h/air Ti(IV) Tetragonal  8.6 3.0 98.7 butoxide TiO.sub.2 500° C./1 h/air Ti(IV) Tetragonal 12.6 0 42.8 butoxide .sup.aBased on (001) peak of Nb.sub.2O.sub.5 or (101) peak of TiO.sub.2. .sup.b Pre-calcined at 550° C./2 h/Ar.

Example 2: Synthesis of Doped Binary Oxide Nanosheets

[0174] The synthesis strategy presented in example 1 can also be adapted to prepare doped binary oxide nanosheets, such as Ti-doped Nb.sub.2O.sub.5 and Nb-doped TiO.sub.2 (refer to Table 2). The doped metal oxide nanosheets adopted the crystal structure of the host metal oxide, with no new phase observed (FIG. 5A and FIG. 5B). The successful doping was confirmed by XRD peak shifts. For Ti-doped Nb.sub.2O.sub.5 nanosheets, diffraction peaks shifted to higher 20 angles as Ti doping increased (refer to FIG. 5E, and Table 2), due to the incorporation of Ti.sup.4+, which has a smaller ionic radius, in the Nb.sub.2O.sub.5 crystal lattice.

[0175] In contrast, the XRD peaks shifted to lower 20 angles as Nb doping increased in Nb-doped TiO.sub.2 nanosheets (Refer to FIG. 5F and Table 2), due to the incorporation of Nb.sup.5+, which has a larger ionic radius, in the TiO.sub.2 crystal lattice. TEM confirmed the nanosheet morphology of Ti-doped Nb.sub.2O.sub.5 and Nb-doped TiO.sub.2 (FIG. 5C and FIG. 5D). These doped nanosheets showed similar lattice spacing as the host oxide in the HRTEM images of FIG. 5G and FIG. 5H. EDX elemental mapping of Ti-doped Nb.sub.2O.sub.5 as shown in FIG. 5K and FIG. 5O, and Nb-doped TiO.sub.2 (refer to FIG. 5L and FIG. 5P) showed homogeneous distributions of Ti and Nb within the nanosheets. XPS confirmed the doping, and illustrated the main oxidation states as Nb.sup.5+ and Ti.sup.4+, respectively (refer to FIG. 5I, FIG. 5M, FIG. 5J, and FIG. 5N).

TABLE-US-00002 TABLE 2 XRD peak shift in doped Ti—Nb.sub.2O.sub.5 and Nb—TiO.sub.2 nanosheets (001) peak (101) peak Doping position position Metal oxide Dopant atomic % 2θ (°) 2θ (°) Nb.sub.2O.sub.5 Ti 0 22.72 — Nb.sub.2O.sub.5 Ti 2 22.74 — Nb.sub.2O.sub.5 Ti 7.5 22.76 — Nb.sub.2O.sub.5 Ti 25 22.85 — TiO.sub.2 Nb 0 — 25.38 TiO.sub.2 Nb 4 — 25.38 TiO.sub.2 Nb 8 — 25.34 TiO.sub.2 Nb 10 — 25.33 TiO.sub.2 Nb 20 — 25.27

Example 3: Synthesis of Ternary Oxide Nanosheets

[0176] Ternary oxide nanosheets were synthesized using metal oxide precursors at the appropriate ratios. The experimental results suggest that the phase of ternary oxide nanosheets can be modified by the calcination conditions. For example, with a Ti:Nb precursor atomic ratio of 1:2, TiNb.sub.2O.sub.7 nanosheets were obtained when calcined in air, while oxygen-deficient Ti.sub.0.61Nb.sub.1.29O.sub.4/rGO nanosheets were obtained when calcined in argon.

[0177] Ti.sub.0.61Nb.sub.1.29O.sub.4 nanosheets were obtained by removing rGO from Ti.sub.0.61Nb.sub.1.29O.sub.4/rGO via calcination in air, with no phase change observed. The nanosheet morphology was confirmed by TEM (FIG. 6A and FIG. 6B). EDX elemental mapping showed homogeneous Ti and Nb distributions as can be seen from FIG. 6C, FIG. 6D, FIG. 6E and FIG. 6F. The crystalline phases were determined to be monoclinic TiNb.sub.2O.sub.7(JCPDS #01-077-1374) and tetragonal Ti.sub.0.95Nb.sub.0.95O.sub.4(JCPDS #00-047-0024) (FIG. 6G and FIG. 6H). XPS Nb 3d and Ti 2p peaks showed that Nb.sup.5+ and Ti.sup.4+ were the main species in TiNb.sub.2O.sub.7 nanosheets (refer to FIG. 6I and FIG. 6K).

[0178] The XPS peaks of Ti.sub.0.61Nb.sub.1.29O.sub.4/rGO nanosheets were shifted to higher binding energies, which could be attributed to residual rGO that could not be removed in argon (FIG. 6J and FIG. 6L). The appearance of a second peak for Nb 3d5/2 and Ti 2p3/2 at lower binding energies indicated the presence of Nb.sup.4+ and Ti.sup.3+, which corresponded to the oxygen-deficient phase. Synthesis conditions and properties of TiNb.sub.2O.sub.7 and Ti.sub.0.61Nb.sub.1.29O.sub.4 nanosheets are shown in Table 3.

TABLE-US-00003 TABLE 3 Synthesis conditions and properties of ternary oxide nanosheets Crystallite rGO BET specific Calcination size content surface area Metal oxide conditions Phase (nm).sup.a (wt %) (m.sup.2/g) TiNb.sub.2O.sub.7 700° C./1 h/air Monoclinic 14.9 0.0 23.8 Ti.sub.0.61Nb.sub.1.29O.sub.4 700° C./2 h/Ar Tetragonal 7.5 36.1 217.4 Ti.sub.0.61Nb.sub.1.29O.sub.4 500° C./1 h/air .sup.b Tetragonal 8.0 0.0 98.7 .sup.aBased on (020) peak of TiNb.sub.2O.sub.7 or (110) peak of Ti.sub.0.61Nb.sub.1.29O.sub.4. .sup.b Pre-calcined at 700° C./2 h/Ar.

Example 4: Synthesis of Other Metal Oxide Nanosheets

[0179] The GO planar-confined growth strategy as shown in examples 1, 2 and 3 above can also be extended to other types of metal oxides. Binary oxides, e.g., Fe.sub.2O.sub.3, SnO.sub.2, NiO, Co.sub.3O.sub.4, Mn.sub.3O.sub.4 and ZrO.sub.2 were synthesized as nanosheets (FIG. 7A, FIG. 7B, FIG. 7C, FIG. 7D, FIG. 7E, FIG. 7F, FIG. 7G and FIG. 7H). Doped and ternary oxide nanosheets with tunable compositions and crystalline phases were also prepared. Ni.sub.xCo.sub.3-xO.sub.4 nanosheets were derived with different Ni/Co atomic ratios of 0.08, 0.22, 0.38 and 0.75 (FIG. 7I and FIG. 7L).

[0180] The experimental results suggest that the ratio of Ni/Co affected the nanosheet crystal structure (FIG. 7Q). At a low Ni/Co ratio of 0.08 (i.e. Ni.sub.0.22Co.sub.2.78O.sub.4), Co.sub.3O.sub.4 phase was obtained (JCPDS #00-042-1467) with Ni ions only replaced a portion of the cobalt ions in the Co.sub.3O.sub.4 phase. As the Ni/Co ratio increased, a phase change to Co.sub.2NiO.sub.4 (JCPDS #00-002-1074) was necessary to accommodate more Ni ions. Mn.sub.xCo.sub.3-xO.sub.4 and Co.sub.xMn.sub.3-xO.sub.4 nanosheets (refer to FIG. 7M, FIG. 7N, FIG. 7O and FIG. 7P) were synthesized with Mn/Co atomic ratios of 0.56, 1.11, 1.68 and 2.33. The first two ratios resulted in nanosheets with Co.sub.2MnO.sub.4 or MnCo.sub.2O.sub.4 phase (JCPDS #00-001-1130 and #00-023-1237, respectively) (see FIG. 7R). The latter two ratios led to CoMn.sub.2O.sub.4 phase (JCPDS #00-055-0685).

[0181] The synthesis strategy described herein has been shown to be capable of deriving a wide variety of metal oxide nanosheets with tunable composition and phase. The experimental results revealed that the calcination conditions can be further manipulated to control the crystallite size, rGO content and specific surface area of the nanosheets (refer to Table 4).

TABLE-US-00004 TABLE 4 Synthesis conditions and properties of various metal oxide nanosheets Calcination Crystallite rGO BET specific conditions size content surface area Metal oxide (in air) Phase (nm).sup.a (wt %) (m.sup.2/g) Iron Oxide 300° C./1 h Amorphous — 38.3 44.20 Iron Oxide 325° C./1 h Amorphous — 7.40 118.4 SnO.sub.2 375° C./1 h Orthorhombic .sup.b 40.3 78.0 SnO.sub.2 400° C./1 h Orthorhombic 4.9 3.30 93.6 NiO 325° C./1 h Rhombohedral .sup.b 42.9 87.4 NiO 350° C./1 h Rhombohedral 8.4 2.70 70.1 Co.sub.3O.sub.4 325° C./0.7 h Cubic 10.5  21.9 31.9 Co.sub.3O.sub.4 350° C./1 h Cubic 11.8  4.90 28.1 Mn.sub.3O.sub.4 325° C./1 h Amorphous — 25.0 74.4 Mn.sub.3O.sub.4 350° C./1 h Tetragonal 17.5  3.60 34.7 Ni.sub.1.29Co.sub.1.71O.sub.4 325° C./0.8 h Cubic 4.2 31.3 85.9 Ni.sub.1.29Co.sub.1.71O.sub.4 350° C./1 h Cubic 4.5 <3 67.8 Mn.sub.1.08Co.sub.1.92O.sub.4 325° C./0.3 h Cubic 5.3 38.4 49.4 Mn.sub.1.08Co.sub.1.92O.sub.4 500° C./1 h Cubic 6.8 2.9 41.5 .sup.aBased on strongest XRD peak. .sup.b Crystallite size could not be calculated due to low XRD peak intensity.

[0182] Further, as indicated above that the reaction conditions for preparing the metal oxides above can be further optimized by for example varying the precursor concentration, processing temperature, the temperature of calcining step as well as the duration of calcining step. Table 5 below shows the optimized conditions for the synthesis of metal oxide nanosheets including binary oxide nanosheets, doped binary oxide nanosheets and ternary oxide nanosheets.

TABLE-US-00005 TABLE 5 Optimized conditions for synthesis of metal oxide nanosheets Stirring Calcination Calcination Precursor Temperature Temperature Duration Metal Oxide Metal Oxide Precursor Conc. .sup.a (° C.) (° C.) .sup.b (h) Nb.sub.2O.sub.5 NbCl.sub.5 2.96  .sup. RT .sup.c 500-600 .sup.d 1 Nb(OC.sub.2H.sub.5).sub.5 TiO.sub.2 Ti(OC.sub.4H.sub.9).sub.4 2.96 RT 500 1 TiNb.sub.2O.sub.7 Ti(OC.sub.4H.sub.9).sub.4 0.89 RT 700 1 Nb(OC.sub.2H.sub.5).sub.5 2.07 Ti.sub.0.61Nb.sub.1.29O.sub.4 Ti(OC.sub.4H.sub.9).sub.4 0.9 RT  .sup. 500 .sup.e 1 Nb(OC.sub.2H.sub.5).sub.5 2.1 ZrO.sub.2 Zr(OC.sub.3H.sub.7).sub.4 2.96 RT 400 1 (Alfa Aesar) SnO.sub.2 Dibutyltin dilaurate 2.96 RT 400 1 (Sigma-Aldrich) Fe.sub.2O.sub.3 Fe(III) acetylacetonate 2.96 45 400 1 (Merck) NiO Ni(II) acetylacetonate 2.96 45 350 1 (Merck) Co.sub.3O.sub.4 Co(II) acetylacetonate 2.96 45 350 1 (Sigma-Aldrich) Mn.sub.3O.sub.4 Mn(II) acetylacetonate 2.96 45 350 1 (Merck) Ni.sub.1.29Co.sub.1.71O.sub.4 Ni(II) acetylacetonate 2.37 45 350 1 Co(II) acetylacetonate 0.59 Mn.sub.1.08Co.sub.1.92O.sub.4 Mn(II) acetylacetonate 0.14 45 500 1 Co(II) acetylacetonate 2.82 Co.sub.0.9Mn.sub.2.1O.sub.4 Co(II) acetylacetonate 2.46 45 500 1 Mn(II) acetylacetonate 0.50 .sup.a In mmol per 240 mL of absolute ethanol. .sup.b Calcined in air. .sup.c RT: room temperature. .sup.d GO/NbCl.sub.5 was pre-calcined at 550° C./2 h/Ar. .sup.e Pre-calcined at 700° C./2 h/Ar.

Example 5: Synthesis of Metal Oxide/rGO Nanosheets Using Opposite-Charge Method

[0183] To further enhance the electrochemical properties of the metal oxides, a reduced graphene oxide was incorporated to the metal oxide nanosheets to afford a composite material. This incorporation is outlined below.

[0184] Positively charged GO was prepared by non-covalent functionalization using Poly(diallyldimethylammonium chloride) (PDDA) (purchased from Sigma-Aldrich of Saint Louis, Mo. of the United States of America) Briefly, 60 mg of GO was dispersed in 150 mL distilled water by ultrasonication. This was followed by the dissolution of 3.9 g NaCl in the GO dispersion. Subsequently, 7.8 mL of PDDA (20 wt % in H.sub.2O, MW: 100,000-200,000) was added to the dispersion, followed by ultrasonication for 1.5 hours and stirring overnight.

[0185] The resulting dispersion was centrifuged, washed three times with distilled water, freeze dried, and finally dried at 60° C. overnight. Positively charged GO-PDDA (zeta potential shown in FIG. 8) was dispersed in distilled water at a concentration of 0.5 mg/mL (dispersion A). Negatively charged metal oxide nanosheets (zeta potential shown in FIG. 8) were dispersed in a separate tube at a concentration of 0.4 mg/mL (dispersion B). Dispersions A and B were mixed rapidly, ultrasonicated briefly and vortexed. The aggregated nanocomposite was collected by centrifugation, dried at 60° C. overnight, and calcined at 400° C. in Ar for one hour.

Example 6: Transition Metal Oxides as Li-Ion Battery Anodes: High-Voltage Anodes

[0186] The transition metal oxides prepared as above used as Li-ion battery anodes was investigated. The first category is high-voltage anodes, which include Nb.sub.205, TiO.sub.2 and titanium niobium oxide. This category has the advantage of higher safety profile and low volume change.

[0187] A stable performance was demonstrated by Nb.sub.2O.sub.5, TiO.sub.2 and TiNb.sub.2O.sub.7 nanosheets, achieving 99.3, 172.0 and 143.6 mAh/g, respectively, after 100 cycles at 1 C (FIG. 9A, FIG. 9B, and FIG. 9E). Ti.sub.0.61Nb.sub.1.29O.sub.4 nanosheets also showed stable performance, attaining 148.9 mAh/g after 100 cycles at 0.5 C (FIG. 9F). This is the first report of using Ti.sub.0.95Nb.sub.0.95O.sub.4 phase as Li-ion battery anode. The good stability profiles of the high-voltage-anode nanosheets can be attributed to the low volume change during cycling, and the stability of the nanosheet structure. Ex situ electrode analysis of Nb.sub.2O.sub.5 and TiO.sub.2 after cycling confirmed the stability of the nanosheet morphology and phase over prolonged cycling (FIG. 9A, FIG. 9B, FIG. 9C, FIG. 9D, FIG. 9E, FIG. 9F, FIG. 9G, FIG. 9H and FIG. 9I).

[0188] The nanosheets were modified to enhance their electrochemical performance by incorporating rGO within the nanosheets and by doping. rGO was incorporated by adjusting the synthesis conditions to retain residual rGO within the nanosheets (metal oxide/rGO), or by introducing rGO after nanosheet synthesis using the opposite-charge method (metal oxide/rGO-OCM). Synthesis conditions and rGO contents of bare and modified nanosheets are summarized in Table 6.

TABLE-US-00006 TABLE 6 Synthesis conditions and rGO content of high-voltage-anode nanosheets rGO Calcination content Nanosheet Precursor conditions (wt %) Nb.sub.2O.sub.5 Nb(V) ethoxide 500° C./1 h/air 0 Nb.sub.2O.sub.5/rGO NbCl.sub.5 550° C./2 h/Ar 29.0 550° C./3 h/air Nb.sub.2O.sub.5/rGO-OCM Nb(V) ethoxide 500° C./1 h/air 22.9 400° C./1 h/Ar Ti—Nb.sub.2O.sub.5 (2 at % Ti) Ti(IV) butoxide 600° C./1 h/air 0 Nb(V) ethoxide TiO.sub.2 Ti(IV) butoxide 500° C./1 h/air 0 TiO.sub.2/rGO-OCM Ti(IV) butoxide 500° C./1 h/air 23.6 400° C./1 h/Ar Nb—TiO.sub.2 (10 at % Nb) Nb(V) ethoxide 500° C./1 h/air 0 Ti(IV) butoxide TiNb.sub.2O.sub.7 Ti(IV) butoxide 700° C./1 h/air 0 Nb(V) ethoxide TiNb.sub.2O.sub.7/rGO-OCM Ti(IV) butoxide 700° C./1 h/air 23.8 Nb(V) ethoxide 400° C./1 h/Ar Ti.sub.0.61Nb.sub.1.29O.sub.4 Ti(IV) butoxide 700° C./2 h/Ar 0 Nb(V) ethoxide 500° C./1 h/air Ti.sub.0.61Nb.sub.1.29O.sub.4/rGO-OCM Ti(IV) butoxide 700° C./2 h/Ar 27.0 Nb(V) ethoxide 500° C./1 h/air 400° C./1 h/Ar

[0189] After modification, the performance of Nb.sub.2O.sub.5 nanosheets was significantly enhanced at 1 C, reaching 137.5, 143.3 and 130.9 mAh/g for Nb.sub.2O.sub.5/rGO, Nb.sub.2O.sub.5/rGO-OCM and doped Ti—Nb.sub.2O.sub.5-0.02, respectively, after 100 cycles (refer to FIG. 10A). This could be attributed to enhanced conductivity. As for TiO.sub.2, TiNb.sub.2O.sub.7 and Ti.sub.0.61Nb.sub.1.29O.sub.4, the modified nanosheets had comparable performance to the bare ones (refer to FIG. 10B, FIG. 10E and FIG. 10F). This could be due to the ability of the bare nanosheets to tolerate the relatively lower rates of 1 C and 0.5 C.

[0190] Rate capability was varied among the high-voltage anode nanosheets. Bare Nb.sub.2O.sub.5 nanosheets achieved only 8.4 mAh/g at 20 C (refer to FIG. 10C). However, its rate performance was enhanced by the modified nanosheets, especially Nb.sub.2O.sub.5/rGO, which achieved a remarkable specific capacity of 120.3 mAh/g at 20 C. The excellent rate capability of Nb.sub.2O.sub.5/rGO can be attributed to the very good contact between residual rGO and Nb.sub.2O.sub.5 nanosheets.

[0191] TiO.sub.2, TiNb.sub.2O.sub.7 and Ti.sub.0.61Nb.sub.1.29O.sub.4 nanosheets showed very good rate capability, achieving 74.9, 72.8 and 46.7 mAh/g at 20 C, 15 C and 15 C, respectively (refer to FIGS. 10D, FIG. 10G and FIG. 10H). Their rate performance was further enhanced by modification, reaching 102.3, 85.7 and 40.3 mAh/g for doped Nb—TiO.sub.2 (10 at % Nb), TiNb.sub.2O.sub.7/rGO-OCM and Ti.sub.0.61Nb.sub.1.29O.sub.4/rGO-OCM nanosheets, at 20 C, 15 C and 25 C, respectively.

[0192] The performance of the optimized Nb.sub.2O.sub.5, TiO.sub.2 and TiNb.sub.2O.sub.7 nanosheets was compared to the literature (refer to Table 7 below), demonstrating their high energy storage capabilities. The high performance achieved by the nanosheets could be attributed to the 2D nanostructure, whose thinness facilitated Li.sup.+ and electron transport, and improved contact with the electrolyte. The micron-sized lateral dimensions provided continuous charge-transfer path, thus improving rate capability, and helped to sustain structural integrity during cycling. Doping and rGO introduction have been demonstrated to be successful strategies to improve Li-ion battery performance, especially at high current densities.

TABLE-US-00007 TABLE 7 Comparison of Nb.sub.2O.sub.5, TiO.sub.2 and TiNb.sub.2O.sub.7 nanosheet battery performance to literature Current Specific Density Capacity Material Morphology (mA/g) (mAh/g) Reference Nb.sub.2O.sub.5/rGO Nanosheets 150 216 This work 3000 120 Nb.sub.2O.sub.5/C .sup.a Nanocrystals 200 160 Previous 3000 100 work Nb.sub.2O.sub.5 Nanosheets 200 184 Previous 1000  90 work Nb.sub.2O.sub.5 3D nanowire 200 165 Previous superstructure 500 135 work Nb.sub.2O.sub.5 Nanowires 100 209 Previous 2000 175 work Nb.sub.2O.sub.5/C .sup.a Mesoporous 100 180 Previous nanocomposite 5000 115 work Nb—TiO.sub.2 Nanosheets 85 338 This work (10 at % Nb) 168 268 3360 105 TiO.sub.2 Microboxes 168 205 Previous 3360  63 work TiO.sub.2 Cubes 85 201 Previous 336  96 work TiO.sub.2 Hollow 85 215 Previous microspheres 3360  88 work TiO.sub.2 Mesoporous 85 160 Previous microspheres 1680 100 work TiO.sub.2/G .sup.b 3D network 200 185 Previous 2000 105 work TiO.sub.2 Porous 168 166 Previous microspheres 3360 130 work TiNb.sub.2O.sub.7/ Nanosheets 194  .sup. 263 .sup.c This work rGO-OCM 1940 150 5810  89 TiNb.sub.2O.sub.7/G .sup.b Nanoparticles 1550 156 Previous 3100 136 work TiNb.sub.2O.sub.7 Nanofibers 200 240 Previous 1000 150 work TiNb.sub.2O.sub.7@C .sup.a Microwires 2000 140 Previous 6000  75 work TiNb.sub.2O.sub.7 Microspheres 194  .sup. 270 .sup.c Previous 7750 100 work TiNb.sub.2O.sub.7 Nanoporous 1940 236 Previous framework 7740 195 work .sup.a C: carbon. .sup.b G: graphene. .sup.c Second cycle.

Example 7: Transition Metal Oxides as Li-Ion Battery Anodes: High-Capacity Anodes

[0193] The second category includes iron oxide, SnO.sub.2, NiO, Co.sub.3O.sub.4, Ni.sub.1.29Co.sub.1.71O.sub.4 and Mn.sub.1.08Co.sub.1.92O.sub.4, which operate by conversion and/or alloying-dealloying mechanisms; they are discharged until very low potentials and have high capacities.

[0194] Iron oxide, SnO.sub.2, NiO, Co.sub.3O.sub.4, Ni.sub.1.29Co.sub.1.71O.sub.4 and Mn.sub.1.08Co.sub.1.92O.sub.4 nanosheets were tested as high-capacity Li-ion battery anodes. For each metal oxide, two samples were selected, one with <8 wt % rGO (designated as metal oxide) and the other with around 22-43 wt % rGO (designated as metal oxide/rGO). Calcination conditions and rGO contents of all tested samples are shown in Table 8 below.

TABLE-US-00008 TABLE 8 Synthesis conditions, rGO content and battery performance of high-capacity-anode nanosheets rGO Current Calcination conditions content density Number Specific capacity Nanosheet (in air) (wt %) (A/g) of cycles (mAh/g) Iron oxide - Iron oxide/rGO 325° C./1 h-300° C./1 h 7.4-38.3 1 300 129.2-1394.4 7  .sup. 45 .sup.a  .sup. .sup.b-346.1 SnO.sub.2—SnO.sub.2/rGO 400° C./1 h-375° C./1 h 3.30-40.3  0.8 300 129.0-1271.0 4  .sup. 35 .sup.a 56.9-694.7 NiO—NiO/rGO 350° C./1 h-325° C./1 h 2.7-42.9 1 300 138.6-1624.3 5  .sup. 33 .sup.a 23.6-311.1 Co.sub.3O.sub.4—Co.sub.3O.sub.4/rGO 350° C./1 h-325° C./0.7 h 4.9-21.9 1 300 982.2-1509.9 5  .sup. 33 .sup.a 20.8-112.5 Ni.sub.1.29Co.sub.1.71O.sub.4—Ni.sub.1.29Co.sub.1.71O.sub.4/rGO 350° C./1 h-325° C./0.8 h  <3-31.3 1 300 534.5-1594.9 5  .sup. 35 .sup.a 34.8-109.6 Mn.sub.1.08Co.sub.1.92O.sub.4—Mn.sub.1.08Co.sub.1.92O.sub.4/rGO 500° C./1 h-325° C./0.3 h 2.9-38.4 1 300 264.9-1497.3 5  .sup. 35 .sup.a 44.5-159.8 .sup.a In the rate study shown in FIG. 11. .sup.biron oxide nanosheets were not cycled at 7 A/g because they lost almost all capacity at lower rates, reaching 22.2 mAh/g at 5 A/g (40 cycles).

[0195] It was found that for these transition metal oxides, which store Li.sup.+ by conversion and/or alloying-dealloying mechanisms, the presence of relatively large rGO content was essential for good performance. rGO served as a conductive support that prevented disconnection from the current collector, and as a buffer against the severe volume change during cycling. High-capacity anodes have also shown an activation step, whereby an initial capacity decline was followed by an increase in capacity over cycling.

[0196] Specific capacities as high as 1394, 1271, 1624, 1510, 1595 and 1497 mAh/g were achieved by iron oxide/rGO, SnO.sub.2/rGO, NiO/rGO, Co.sub.3O.sub.4/rGO, Ni.sub.1.29Co.sub.1.71O.sub.4/rGO and Mn.sub.1.08Co.sub.1.92O.sub.4/rGO nanosheets, respectively, after 300 cycles at 1 A/g (FIG. 11A, FIG. 11B, FIG. 11C, FIG. 11G, FIG. 11H and FIG. 11I). Excellent rate capabilities were also obtained; iron oxide/rGO, SnO.sub.2/rGO and NiO/rGO achieved 346.1, 694.7 and 311.1 mAh/g at 7, 4 and 5 A/g, respectively (refer to FIG. 11D, FIG. 11E and FIG. 11F). Co.sub.3O.sub.4/rGO and Ni.sub.1.29Co.sub.1.71O.sub.4/rGO had similar capacity of ˜110 mAh/g (FIG. 11J and FIG. 11K), while Mn.sub.1.08Co.sub.1.92O.sub.4/rGO retained 159.8 mAh/g (see Mn.sub.1.08Co.sub.1.92O.sub.4/rGO in FIG. 11L), at 5 A/g. The results are summarized in Table 8 above.

[0197] Capacity increase over cycling was observed in the case of high-capacity anodes. This phenomenon has been studied previously, and it was concluded that capacity increase would occur due to catalytically enhanced electrolyte decomposition at low potential, leading to the formation of a gel-like polymer that would dissolve during charging. Another hypothesis was that irreversible Li.sub.2O formation would take place during the initial cycles, which would be followed by gradual Li.sub.2O activation over cycling, leading to capacity increase. The activation of Li.sub.2O has been attributed to the metallic nanoparticles generated during the initial cycles.

[0198] In comparison to the literature, rGO-incorporated nanosheets have demonstrated excellent performance as high-capacity anodes (Tables 9, 10 and 11 below), demonstrating the enhanced electrochemical properties of the nanocomposites.

TABLE-US-00009 TABLE 9 Comparison of optimized iron oxide and SnO.sub.2 nanosheet battery performance with literature. Current Specific Density Capacity Material Morphology (mA/g) (mAh/g) Reference Iron Oxide/rGO Nanosheets 500 1070 This work 4000 480 5000 450 Fe.sub.2O.sub.3/MWCNTs .sup.a Nanoparticles 500 400 Previous 2000 320 work Iron oxide@C Yolk-shell 200 820 Previous 4000 370 work Iron oxide/G Nanoparticles 1000 730 Previous 5000 500 work Fe.sub.2O.sub.3/C Bubble 500 913 Previous nanorods 5000 491 work Fe.sub.3O.sub.4@C Yolk-shelled 500 890 Previous boxes 5000 575 work SnO.sub.2/rGO Nanosheets 400 1131 This work 1600 918 4000 713 SnO.sub.2/G Nanoparticles 500 672 Previous in 3D foam 3000 480 work SnO.sub.2/G Nanoparticles 500 1090 Previous 1000 790 work SnO.sub.2/G/polyaniline Nanoparticles 500 530 Previous 2000 240 work SnO.sub.2@C Submicroboxes 1000 583 Previous 5000 372 work SnO.sub.2/G Quantum dots 780 1100 Previous 3900 932 work .sup.a Multi-walled carbon nanotubes.

TABLE-US-00010 TABLE 10 Comparison of optimized NiO and Co.sub.3O.sub.4 nanosheet battery performance with literature. Current Specific Density Capacity Material Morphology (mA/g) (mAh/g) Reference NiO/rGO Nanosheets 500 952 This work 2000 503 5000 311 NiO/C Nanosheets 200 1043 Previous 800 824 work NiO/G Nanosheets 700 872 Previous 3600 492 work NiO/G Nanoparticles 400 509 Previous 800 369 work NiO/G Nanoparticles 450 400 Previous 3000 200 work Co.sub.3O.sub.4/rGO Nanosheets 500 986 This work 2000 304 4000 150 Co.sub.3O.sub.4/G Nanoparticles 500 484 Previous work Co.sub.3O.sub.4/G Nanoparticles 500 800 Previous 1000 600 work Co.sub.3O.sub.4/G Fibers 300 754 Previous 1000 295 work Co.sub.3O.sub.4/G Hollow spheres 1000 692 Previous 5000 259 work

TABLE-US-00011 TABLE 11 Comparison of optimized Ni.sub.1.29Co.sub.1.71O.sub.4 and Mn.sub.1.08Co.sub.1.92O.sub.4 nanosheet battery performance with literature. Current Specific Density Capacity Material Morphology (mA/g) (mAh/g) Reference Ni.sub.1.29Co.sub.1.71O.sub.4/rGO Nanosheets 500 913 This work 2000 291 NiCo.sub.2O.sub.4/MWCNTs Nanosheets 100 836 Previous 1000 392 work NiCo.sub.2O.sub.4/rGO Nanoplates 500 521 Previous 800 396 work NiCo.sub.2O.sub.4/rGO Nanosheets 100 1200 Previous 1000 437 work NiCo.sub.2O.sub.4 Nanowire arrays 1000 800 Previous on carbon 3000 600 work textiles Mn.sub.1.08Co.sub.1.92O.sub.4/rGO Nanosheets 500 1020 This work 2000 372 4000 196 MnCo.sub.2O.sub.4 Spheres 800 571 Previous work MnCoO.sub.x Microspheres 500 698 Previous 1500 417 work MnCo.sub.2O.sub.4 Microspheres 400 814 Previous 1800 513 work MnCo.sub.2O.sub.4/G Nanoparticles 1000 853 Previous 4000 462 work

Materials Characterization

[0199] The nanosheets were characterized using TEM (FEI Tecnai F20) fitted with EDX analyzer (OXFORD X-Max.sup.N), XRD (Bruker D8 ADVANCE), TGA (PerkinElmer Pyris 1 TGA), N.sub.2 adsorption (Micromeritics ASAP 2020), FT-IR (PerkinElmer Spectrum 100) and XPS (VG ESCALAB 220i-XL). Zeta potential was determined using Zetasizer Nano-SZ (Malvern Instruments). Nanosheet thickness was determined by AFM (Bruker Dimension ICON AFM, non-contact/tapping mode). Raman spectroscopy (Witec Alpha 300S) was performed using 532-nm laser source.

Electrochemical Measurements

[0200] The active materials were mixed with vapor-grown carbon fibers (VGCFs) and polyvinylidene fluoride (PVDF) at a weight ratio of 7:2:1, and dispersed in N-methyl-2-pyrrolidone (NMP) to form a slurry. The slurry was coated on copper or aluminum foil, dried at 90° C. overnight, and then pressed. Coin cells were assembled in an argon glove box using Li metal as the counter electrode and 1 M LiPF.sub.6 in ethylene carbonate and diethyl carbonate (1:1) as the electrolyte. Galvanostatic charge-discharge measurements were conducted at various current densities at a voltage range of 1.1-3 V for Nb.sub.2O.sub.5, 1-3 V for TiO.sub.2, TiNb.sub.2O.sub.7 and Ti.sub.0.61Nb.sub.1.29O.sub.4, and 0.005-3 V for all other metal oxide nanosheets.

INDUSTRIAL APPLICABILITY

[0201] The synthesis method for preparing the metal oxide nanosheets described herein can be used for the industrial production of metal oxide nanosheets with different variations, such as binary, doped binary, ternary or more complex oxide nanosheets. The resulting metal oxide nanosheets have many potential industrial applications, such as in the fields of energy storage, catalysis and sensors.

[0202] The metal oxides nanosheets described in the present disclosure can be used as active material for the anode in the Lithium-ion battery. Since, the anodes are shown to exhibit high capacity, excellent rate capacity, and/or long-term cycling stability, they therefore allow a broader application of lithium-ion battery using the anode comprising the layered metal oxide as described herein. The application of the present technology will allow the use of lithium-ion battery in many applications such as electronics (including communication, healthcare and transportation).

[0203] The lithium-ion batteries that use the metal oxide nanosheets as the active material as described in the present disclosure may be used as high density power sources for a wide variety of applications for example in automobile (electric vehicles including electric cars, hybrid vehicles, electric bicycles, personal transporters and advanced electric wheelchairs, radio-controlled models, model aircraft, aircraft), portable devices (mobile phone/smartphone, laptops, tablets, digital cameras and camcorders), in power tools (including cordless drills, sanders, and saws), or in healthcare (portable medical equipment such as monitoring devices, ultrasound equipment, and infusion pumps).

[0204] Further, the metal oxide nanosheets produced by the method described in the present disclosure may also be used as sensor such as gas sensor for domestic, commercial and industrial applications with many advantages such as low cost, easy production and compact size. Another potential application of the metal oxide nanosheets is their use as catalyst for various chemical reactions such as oxidation reaction.

[0205] It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.