WO2015034437A1

WO2015034437A1 - Metal oxide nanostructured material and electrochromic devices containing the material

Info

Publication number: WO2015034437A1
Application number: PCT/SG2014/000418
Authority: WO
Inventors: Wenbin KANG; Pooi See Lee
Original assignee: Nanyang Technological University
Priority date: 2013-09-03
Filing date: 2014-09-03
Publication date: 2015-03-12
Also published as: SG11201601553UA

Abstract

A method for preparing a metal oxide nanostructured material is provided. The method includes providing an electrolyte solution comprising a metal oxide precursor, a polyphenol, and an oxidizing agent; and heating the electrolyte solution under hydrothermal conditions to obtain the metal oxide nanostructured material. A method for preparing a metal oxide layer, and an electrochromic device comprising a metal oxide layer are also provided.

Description

METAL OXIDE NANOSTRUCTURED MATERIAL AND ELECTROCHROMIC DEVICES CONTAINING THE MATERIAL

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority of US provisional application No. 61/873,078 filed on 3 September 2013, the content of which is incorporated herein by reference in its entirety for all purposes.

TECHNICAL FIELD

[0002] The invention relates to metal oxide nanostructured material, and electrochromic devices comprising the metal oxide nanostructured material.

BACKGROUND

[0003] Electrochromism may be described as a persistent, reversible color and/or opacity change of a material upon application of a voltage across the material. Generally, electrochromic materials demonstrate a memory effect, in that reversal of color and/or opacity does not take place until an oppositely polarized potential is applied. In other words, a burst of electricity is required to change color and/or opacity of the electrochromic materials, but once the change has been effected, electricity is not required to maintain the change.

[0004] Typically, energy required to drive electrochromic materials is very low, involving an operating potential of only a few volts. This makes electrochromic devices very attractive due to their low energy consumption.

[0005] Although this phenomenon has been extensively studied for more than 40 years, there are only a very limited number of commercial electrochromic products due to problems faced by existing electrochromic materials. Firstly, low chromic contrasts of less than 50 % in the visible range are exhibited by the materials. Further, long durations of up to hundreds of seconds may be required to effect a change in color and/or opacity. In addition, the electrochromic materials exhibit poor stability, which may manifest in the form of severe peeling after several color and/or opacity changing cycles, thereby limiting their practicality in commercial applications. [0006] In view of the above, there exists a need for improved materials suitable for use in electrochromic devices that overcome or at least alleviate one or more of the above- mentioned problems. SUMMARY

[0007] In a first aspect, the invention refers to a method for preparing a metal oxide nanostructured material. The method comprises

a) providing an electrolyte solution comprising a metal oxide precursor, a polyphenol, and an oxidizing agent; and

b) heating the electrolyte solution under hydrothermal conditions to obtain the metal oxide nanostructured material.

[0008] In a second aspect, the invention refers to a metal oxide nanostructured material prepared by a method according to the first aspect.

[0009] In a third aspect, the invention refers to a method for preparing a metal oxide layer. The method comprises

a) providing a suspension comprising a metal oxide nanostructured material prepared by a method according to the first aspect, and

b) depositing the suspension on a substrate to obtain the metal oxide layer.

[0010] In a fourth aspect, the invention refers to an electrochromic device comprising a metal oxide layer prepared by a method according to the third aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings, in which:

[0012] FIG. lA is a field emission scanning electron microscopy (FESEM) image of hybrid structured vanadium oxide at low magnification. FIG. IB and 1C are transmission electron microscopy (TEM) images at high magnification revealing the membrane-nanobelt hybrid structure. FIG. ID is a high-resolution transmission electron microscopy (HRTEM) image showing lattice structure of the hybrid structured sample. Line in FIG. ID shows d₍₀₂₂₎ = 0.204 nm (inset shows the selected area diffraction pattern which tells the polycrystalline nature). Scale bar in FIG. 1A denotes 1 μιη; FIG. IB denotes 500 nm; FIG. 1C denotes 100 nm; and FIG. ID denotes 2.5 nm. Scale bar in inset of FIG. ID denotes 1 nm.

[0013] FIG. 2 A shows X-ray diffraction (XRD) spectrum of the hybrid structured vanadium oxide. FIG. 2B shows Fourier transform infrared spectroscopy (FTIR) spectrum of the vanadium oxide under test.

[0014] FIG. 3 A to 3D depict energy-dispersive X-ray spectroscopy (EDX) mapping and lattice structure of the membrane, where FIG. 3A shows selected area for EDX mapping with pure membrane without any nanobelt; FIG. 3B is a HRTEM image showing the polycrystalline property of the membrane; FIG. 3C shows oxygen atom (O) distribution; and FIG. 3D shows vanadium atom (V) distribution. Scale bar in FIG. 3A denotes 100 nm; FIG. 3B denotes 10 nm; FIG. 3C denotes 100 nm; and FIG. 3D denotes 100 nm.

[0015] FIG. 4 shows EDX spectrum of a hybrid structured vanadium oxide according to an embodiment.

[0016] FIG. 5A and 5B show electrochromic performance of the hybrid structured vanadium oxide sample, where FIG. 5A is a graph of transmission versus wavelength for electrochromic contrast test, and FIG. 5B is a graph of transmission versus time for switching time calculation at wavelength of .700 nm.

[0017] FIG. 6 is an atomic force microscopy (AFM) image of a 5 μτη by 5 μηι region for root-mean-square (RMS) roughness testing, which suggests a very smooth surface with RMS of 7.72 nm.

[0018] FIG. 7 is a graph depicting poor cycling stability of the as-prepared vanadium oxide on ITO/glass without LPEI surface treatment.

[0019] FIG. 8 is a graph depicting peeling condition manifested by transmission increase at colored state for samples with and without surface treatment subjected to a continuous electrochemical cycling. (For a reasonable comparison, cycle number is only taken for the first 70 as after that the surface untreated sample faces a severe peeling).

[0020] FIG. 9 is a graph depicting cycling stability test of the LPEI surface treated sample.

[0021] FIG. 10 is a graph showing cycling stability of samples with/without surface treatment.

[0022] FIG. 11 shows photographs of nanobelt - membrane hybrid structured V₂0₅ at A) bleached state; B) colored state. [0023] FIG. 12 depicts graphs showing 2-electrode electrochromic performance test of the hybrid structured vanadium oxide. FIG. 12A shows the contrast and FIG. 12B shows the switching time. DETAILED DESCRIPTION

[0024] As disclosed herein, polyphenols are used to control structure of metal oxide nanostructured material formed. The metal oxide nanostructured material may comprise nanobelts or possess a nanobelt-membrane hybrid structure. Advantageously, electrochromic films comprising the metal oxide nanostructured materials may exhibit high contrast levels of more than 60 %, measured using a three electrode test at a wavelength of 700 nm. Further, by surface treating substrates such as ITO-glass with a polyimine such as linear polyethylenimine prior to deposition, stability of the deposited electrochromic film may be greatly enhanced with less than 20 % reduction in transmission after 100 cycles. In terms of the preparation method, as natural polyphenols such- as tannic acid and flavonoids may be used, this renders the process environmentally friendly. The electrochromic metal oxide layers may be prepared using a drop casting method, which does not require use of expensive equipment.

[0025] With the above in mind, the present invention refers in a first aspect to a method for preparing a metal oxide nanostructured material. The method comprises providing an electrolyte solution comprising a metal oxide precursor, a polyphenol, and an oxidizing agent, and heating the electrolyte solution under hydrothermal conditions to obtain the metal oxide nanostructured material.

[0026] As used herein, the term "nanostructured material" refers to a material having at least one dimension that is in the nanometer range. At least one dimension of the nanostructured material may be less than 100 nm. In various embodiments, a nanostructured material has a dimension typically ranging from 1 nm to 100 nm (where 10 angstrom = 1 nm = 1/1000 micrometer). Examples of nanostructured material may include nanotubes, nanoflowers, nanowires, nanofibers, nanoflakes, nanoparticles, nanodiscs, nanosheets, and combinations thereof.

[0027] In various embodiments, the metal oxide nanostructured material comprises nanobelts. The term "nanobelt" as used herein refers to a one-dimensional nanostructure having a long and flat structure, and which bears resemblance to a belt, strip or ribbon. Typically, nanobelts are chemically pure, structurally uniform single crystals, possessing rectangular cross-sections, clean edges and smooth surfaces.

[0028] Advantageously, presence of nanobelts in the metal oxide nanostructured material may significantly increase surface area of the metal oxide nanostructured material to result in increase in intercalation sites to electrolytes in an electrochromic device when the metal oxide nanostructured material is used as an electrochromic material. Easy access of the electrolyte to the intercalation sites may in turn translate into improved chromic contrasts, as the contrast may relate to extent and amount at which active sites of the electrochromic material intercalate with electrolyte ions.

[0029] Size of the nanobelts may be characterized by their width and/or their length. Average width of the nanobelts may be calculated by dividing the sum of the width of each nanobelt by the total number of nanobelts.

[0030] In various embodiments, the nanobelts have an average width in the range of about 20 nm to about 40 nm, such as about 25 nm to about 40 nm, about 30 nm to about 40 nm, about 35 nm to about 40 nm, about 20 nm to about 35 nm, about 20 nm to about 30 nm, about 20 nm to about 25 nm, about 25 nm to about 35 nm, about 20 nm, about 25 nm, about 30 nm, about 35 nm or about 40 nm.

[0031] The nanobelts may have an average length in the range of about 0.5 μη to about 8 μηι. For example, the nanobelts may have an average length in the range of about 0.5 μιη to about 6 μηι, about 0.5 μηι to about 4 μιτι, about 0.5 μπι to about 2 μπι, about 2 μηι to about 8 μπι, about 4 μπι to about 8 μιη, about 6 μιη to about 8 μπι, about 1 μπι to about 5 μιη, about 2 μιη to about 6 μηι, or about 3 μπι to about 5 μιη.

[0032] A plurality of the nanobelts may be grouped together, or otherwise joined or attached together, to form a nanobelt bundle. In various embodiments, the metal oxide nanostructured material comprises nanobelt bundles.

[0033] In some embodiments, the metal oxide nanostructured material is a nanobelt- membrane hybrid nanostructured material. As used herein, the term "membrane" refers to a thin sheet or layer. By the term "hybrid", this means that a nanobelt structure and a membrane structure co-exist in the metal oxide nanostructured material. For example, a portion of the metal oxide nanostructured material may comprise one or more nanobelts, and another portion of the metal oxide nanostructured material may comprise a membrane. The nanobelt-membrane hybrid nanostructured material may be polycrystalline. [0034] Without wishing to be bound by theory, it is postulated by the inventors that formation of the nanobelt-membrane hybrid nanostructured material may involve the following mechanism. Initially, metal ions of the metal oxide precursor may be chelated by phenol groups of the polyphenol. Under hydrothermal conditions, clustering of the metal ions may take place to induce crystallization and nucleation of metal oxide phase. The metal oxide phase may adopt a nanobelt morphology. In embodiments wherein the metal oxide is vanadium oxide, for example, the vanadium oxide may form nanobelts that expose _,(00L) faces outward, which is one of the preferred orientations adopted by vanadium oxide. At the same time, part of the metal oxide species may spread laterally and grow in a two- dimensional manner to result in a membrane. This lateral growth of metal oxide species may be attributed to steric hindrance effect exerted by aromatic rings that present in the polyphenol that chelate the metal ions.

[0035] In various embodiments, the metal oxide nanostructured material comprises one or more nanobelt bundles which are held in place by a membrane. Advantageously, the hybrid membrane-nanobelt structure may possess a smoother surface, due to filling or replacement of the original interstitials or spacings between nanobelts by the membrane. The smoother surface translates into a lower extent of light scattering, which may result in enhanced optical contrasts in electrochromism exhibited by the metal oxide nanostructured material.

[0036] Size of the membrane may, likewise, be characterized using a similar method as that for nanobelts described above. In various embodiments, the membranes have an average width and/or an average length in the range of about 100 nm to about 1000 nm. Thickness of the membrane may be at least substantially uniform, and may be in the range of about 10 nm to about 30 nm.

[0037] The method comprises providing an electrolyte solution comprising a metal oxide precursor, a polyphenol, and an oxidizing agent.

[0038] Providing an electrolyte solution comprising a metal oxide precursor, a polyphenol, and an oxidizing agent may include providing an aqueous reagent comprising the metal oxide precursor and the polyphenol, adding an electrolyte to the aqueous reagent to form an electrolytic aqueous reagent, and adding an oxidizing agent to the electrolytic aqueous reagent.

[0039] The term "electrolyte" as used herein refers to ionic or molecular substances which, when in solution, break down or disassociate to form differently charged ions or differently charged particles. Addition of an electrolyte to the aqueous reagent forms ions that may be beneficial for formation of the metal oxide nanostructured material, as efficient separation of the metal oxide nanostructured material may be achieved, particularly where thin nanobelts are formed. In embodiments where the metal oxide precursor, polyphenol and/or oxidizing agent are able to form ions in solution to obtain an electrolyte solution, addition of an electrolyte may not be required.

[0040] Examples of an electrolyte include, but are not limited to, a salt, a base, and acids such as an organic acid and an inorganic acid.

[0041] In various embodiments, the electrolyte is a salt. For example, the salt may be an inorganic salt. An inorganic salt may be formed from the neutralization reaction of an acid and a base. The inorganic salt may dissociate in the aqueous solution to form an ionic aqueous reagent. For example, KG may dissociate in water to form a cation of K⁺ and an anion of CI^". Other examples of inorganic salts include, but are not limited to NaCl, CaCl₂, BaCl₂, MgCl₂, NaBr, KBr, Nal, KBr, NaN0₃, KN0₃, Mg(N0₃)₂, Ca(N0₃)₂, Na₂S0₄, K₂S0₄, NaC10₄, NH CI, NH₄N0₃, (NH₄)₂S0₄, CH₃COONa, CH₃COONH₄, to name only a few. At least one type of salt can be added. When two or more salts are added, they may be different types. For example, Na₂S0₄ may be added with NaCl.

[0042] The electrolyte may be a base. Non-limiting examples of a base include potassium carbonate, calcium carbonate, sodium carbonate, barium carbonate, zinc carbonate hydroxide hydrate, magnesium carbonate hydroxide hydrate, calcium hydroxide, sodium hydroxide, magnesium hydroxide and aluminum hydroxide. ,

[0043] In various embodiments, the electrolyte is an acid. The acid may be an organic acid or an inorganic acid. Non-limiting examples of an organic acid include carboxylic acids, sulphonic acids such as butanesulphonic acid, butanedisulphonic acid, benzenesulphonic acid, methylbenzenesulphonic acid, ethylbenzenesulphonic acid, dodecylbenzenesulphonic acid, 2,4,6-trimethylbenzenesulphonic acid, 2,4-dimethylbenzenesulphonic acid, 5- sulphosalicylic acid, 1-sulphonaphthalene, 2-sulphonaphthalene, hexanesulphonic acid, octanesulphonic acid and dodecanesulphonic acid, and amino acids such as glycine, alanine, valine, a-aminobutyric acid, Y-aminobutyric acid, alanine, taurine, serine, e-amino-nhexanoic acid, leucine, norleucine and phenylalanine.

[0044] Non-limiting examples of an inorganic acid include hydrochloric acid, sulfuric acid, sulfurous acid, nitric acid, nitrous acid, phosphoric acid, boric acid, and carbonic acid. [0045] In various embodiments, the electrolyte is a strong acid. In specific embodiments, the acid comprises or consists of sulfuric acid.

[0046] In embodiments where the electrolyte contains or is an acid, the acid may be added to the aqueous reagent in any suitable amount that is able to render pH of the aqueous reagent less than 3. In specific embodiments, adding an acid to the aqueous reagent comprises adjusting pH of the aqueous reagent to be in the range of about 1 to about 2. For example, pH of the aqueous reagent may be adjusted to be in the range of about 1.2 to about 2, about 1.4 to about 2, about 1 to about 1.8, about 1 to about 1.6, about 1.2 to about 1.8, about 1.4 to about 1.8, or about 1.5 to about 1.7. In so doing, an acidic aqueous reagent is formed.

[0047] The metal oxide precursor may be selected from the group consisting of metal sulfates, metal acetates, metal alkoxides, metal halides, metal phosphates, metal nitrates, and combinations thereof In some embodiments, the metal oxide precursor comprises or consists of a metal sulfate.

[0048] Metal of the metal oxide may be selected from Group 3 to Group 12 of the Periodic System of Elements. In various embodiments, the metal oxide is an oxide of a transition metal. Examples of transition metal include, but are not limited to, scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), and alloys thereof.

[0049] In some embodiments, metal of the metal oxide is selected from the group consisting of vanadium, niobium, tantalum, and combinations thereof. The metal oxide precursor may accordingly be one or more of vanadium sulfate, vanadium acetate, vanadium alkoxides, vanadium halide, vanadium phosphate, vanadium nitrate, niobium sulfate, niobium acetate, niobium alkoxide, niobium halide, niobium phosphate, niobium nitrate, tantalum sulfate, tantalum acetate, tantalum alkoxide, tantalum halide, tantalum phosphate, and tantalum nitrate.

[0050] In specific embodiments, the metal oxide is an oxide of vanadium. The metal oxide precursor may, for example, comprise or consist of vanadium sulfate.

[0051 ] The metal oxide precursor is mixed with a polyphenol in the electrolyte solution. As used herein, the term "polyphenol" refers to compounds having two or more hydroxyl groups attached to one or more aromatic groups, which also includes glycosidic polyphenols and/or their derivatives in the form of acids, esters, ethers, and the like. [0052] The term "aromatic group" as used herein refers to aromatic hydrocarbon groups as well as heterocyclic aromatic groups. Heterocyclic aromatic groups include those containing oxygen, nitrogen, or sulphur, such as those derived from furan, pyrazole or thiazole. Aromatic groups may be monocyclic, such as in the case of benzene; bicyclic, such as in the case of naphthalene; or polycyclic, such as in the case of anthracene.

[0053] Examples of monocyclic aromatic groups include five-membered rings, such as those derived from pyrrole; and six-membered rings, such as those derived from pyridine. The aromatic groups may comprise fused aromatic groups comprising rings that share their connecting bonds.

[0054] The polyphenol may be a natural polyphenol. For example, the polyphenol may be a plant-derived polyphenol. With rise in pollution levels following rapid urbanization and technological advancement, there is greater emphasis in green chemistry in recent years. As mentioned above, natural polyphenols such as tannic acid and flavonoids, may be used, which render the process environmentally friendly.

[0055] Examples of natural polyphenol include, but are not limited to, tannins such as tannic acid, phenylpropanoids, flavonoids, or combinations thereof.

[0056] Tannic acid is an example of a green material, which is extracted from natural tree bark and leaves. Accordingly, tannic acid is a type of plant-derived polyphenol. It may have formula C₇₆H₅₂0₄₆, and may comprise a mixture of polygalloyl glucoses or polygalloyl quinic acid esters with the number of galloyl moieties per molecule in the range from 2 to 12.

[0057] Examples of flavonoids include flavones, flavonols, flavan-3-ols, flavanones, 3- hydroxyflavanones, isoflavones, neoflavonoids, and anthocyanidins.

[0058] Examples of flavones include, but are not limited to, luteolin, apigenin, baicalin, tangeritin, or combinations thereof. Examples of flavonols, include, but are not limited to, quercetin, galantin, kaempferol, myricetin, fisetin, isorhamnetin, pachypodol, rhamnazin, rutin, hydroxyethylrutosides, or combinations thereof. Examples of flavan-3-ols include, but are not limited to catechins, theaflavins, or combinations thereof. Examples of flavanones include, but are not limited to, hesperetin, naringenin, eriodictyol, or combinations thereof. Examples of 3-hydroxyflavanones include, but are not limited to, dihydroquercetin, dihydrokaempferol, or combinations thereof. Examples of isoflavones include, but are not limited to, genistein, daidzein, glycitein, and combinations thereof. Examples of anthocyanidins include, but are not limited to, cyanidin, delphinidin, malvidin, pelargonidin, peonidin, petunidin, or combinations thereof.

[0059] In various embodiments, the polyphenol is selected from the group consisting of tannins such as tannic acid; catechins and derivatives thereof, such as epigallocatechin, epigallocatechin gallate, and catechin gallate; afzelin, miquelianin, eriocitrin, cinchonain-lb, and combinations thereof.

[0060] In specific embodiments, the polyphenol is selected from the group consisting of tannic acid; epigallocatechin, epigallocatechin gallate, catechin gallate, afzelin, miquelianin, eriocitrin, cinchonain-lb, derivatives thereof, and combinations thereof.

[0061 ] For example, the polyphenol may be selected from the group consisting of

[0062]

derivatives thereof, and combinations thereof.

[0063] In some embodiments, the polyphenol comprises or consists of tannic acid. As mentioned above, tannic acid is a type of plant-derived polyphenol, and may comprise a mixture of polygalloyl glucoses or polygalloyl quinic acid esters with the number of galloyl moieties per molecule in the range from 2 to 12.

[0064] In specific embodiments, the polyphenol comprises or consists essentially of

[0065] As mentioned above, a polyphenol, such as tannic acid, may be used to control structure of metal oxide nanostructured material formed, where it may shape and define morphology of the metal oxide nanostructured material during synthesis. As may be seen from the structure of tannic acid, for example, presence of multiple phenol groups in tannic acid renders its suitability for use as a chelating agent with metal ions. Steric hindrance effect from the aromatic rings prevents metal particles that are reduced by tannic acid from aggregation.

[0066] An oxidizing agent may be added to the electrolytic aqueous reagent. In various embodiments, the oxidizing agent is selected from the group consisting of peroxides, ozone, peracetic acid, and combinations thereof. In specific embodiments, the oxidizing agent comprises or consists of hydrogen peroxide.

[0067] The method of the first aspect includes heating the electrolyte solution under hydrothermal conditions to obtain the metal oxide nanostructured material.

[0068] The term "hydrothermal" as used herein refers to treatment conditions of a reagent in a sealed system such as a closed vessel or an autoclave, whereby temperatures in the system are raised to a temperature above normal boiling point of the reagent at a pressure that is equal to or greater than the pressure required to prevent boiling of the reagent. [0069] As mentioned above, clustering of the metal ions may take place under hydrothermal conditions to induce crystallization and nucleation of metal oxide phase. As mentioned above, ions that are present in the electrolyte solution may aid in efficient separation of the metal oxide nanostructured material. The metal oxide phase may adopt a nanobelt morphology. Concurrently, part of the metal oxide species may spread laterally and grow in a two-dimensional manner to result in a membrane, due to steric hindrance effect exerted by aromatic rings that present in the polyphenol that chelate the metal ions.

[0070] The temperature at which the electrolyte solution comprising the metal oxide precursor, the polyphenol, and the oxidizing agent is heated may depend on the type of metal oxide precursor, polyphenol, and oxidizing agent present.

[0071] In various embodiments, heating the electrolyte solution under hydrothermal conditions comprises heating the electrolyte solution, preferably in an autoclave, at a temperature in the range of about 120 °C to about 200 °C. For example, heating the electrolyte solution may be carried out at a temperature in the range of about 140 °C to about 200 °C, about 160 °C to about 200 °C, about 170 °C to about 200 °C, about 120 °C to about 170 °C, about 120 °C to about 150 °C, about 120 °C to about 130 °C, about 130 °C to about 160 °C, about 140 °C to about 170 °C, about 150 °C, about 180 °C, or about 200 °C.

[0072] In various embodiments, heating the electrolyte solution under hydrothermal conditions includes heating the electrolyte solution, preferably in an autoclave, for a time period in the range of about 1 hour to about 48 hours.

[0073] For example, heating the electrolyte solution may be carried out for a time period in the range of about 6 hour to about 48 hours, about 12 hours to about 48 hours, about 18 hours to about 48 hours, about 24 hours to about 48 hours, about 1 hour to about 24 hours, about 1 hour to about 12 hours, about 12 hour to about 48 hours, about 24 hour to about 48 hours, or about 15 hours to about 30 hours.

[0074] In a second aspect, the invention refers to a metal oxide nanostructured material prepared by a method according to the first aspect.

[0075] As mentioned above, the metal oxide nanostructured material may possess a hybrid membrane-nanobelt structure, which comprises one or more nanobelt bundles which are held in place by a membrane. Advantageously, the hybrid membrane-nanobelt structure may possess a smoother surface, due to filling or replacement of the original interstitials or spacings between nanobelts by the membrane. The smoother surface translates into a lower extent of light scattering, which may result in enhanced optical contrasts in electrochromism exhibited by the metal oxide nanostructured material.

[0076] The invention refers in a further aspect to a method for preparing a metal oxide layer. The method includes providing a suspension comprising a metal oxide nanostructured material prepared by a method according to the first aspect, and depositing the suspension on a substrate to obtain the metal oxide layer.

[0077] In various embodiments, the substrate is selected from the group consisting of glass, ITO-coated glass, FTO-coated glass, ITO-coated poly(ethylene terephthalate), graphene-coated glass, carbon nanotube-coated glass, graphene-coated poly(ethylene terephthalate), carbon nanotube-coated poly( ethylene terephthalate), metal nano wires, metal nanoparticles, metal grids, and combinations thereof.

[0078] Depositing the suspension on a substrate to obtain the metal oxide layer may include treating surface of the substrate with a polyimine prior to depositing the suspension on the substrate. The polyimine may comprise at least one of a polyalkyl imine and a polyalkylene imine. Examples of polyalkyl imine include, but are not limited to, polyethyl imine, polypropyl imine, and polybutyl imine. Examples of polyalkylene imine include, but are not limited to, polyethylene imine, polypropylene imine, and polybutylene imine. In specific embodiments, the polyimine comprises or consists of linear polyethyl enimine.

[0079] Depositing the suspension on a substrate may include drop-casting the suspension comprising a metal oxide nanostructured material on the substrate. During drop-casting, the suspension is added drop-wise on a substrate, where it is allowed to spread to form a layer on the substrate. Spreading of the suspension may take place via spin-coating. At the same time, solvent in the suspension is allowed to evaporate. In so doing, a metal oxide layer may be obtained. In various embodiments, the metal oxide layer is an electrochromic metal oxide layer.

[0080] The invention refers in a further aspect to an electrochromic device comprising a metal oxide layer prepared by a method according to the third aspect.

[0081] In various embodiments, the metal oxide is an oxide of vanadium. Use of vanadium oxide, such as V₂0₅, is advantageous as it possess a layered structure which facilitates ion intercalation.

[0082] Electrochromic materials may be used to control the amount of light and heat allowed to pass through windows, such as for use in smart glass. When activated, the glass changes from transparent to translucent, blocking some (or all) wavelengths of light. For example, the smart glass may be configured to block ultraviolet light. Advantageously, use of smart glass allows cost savings for heating, air-conditioning and lighting, and may avoid costs of installing and maintaining motorized light screens, blinds, or curtains.

[0083] Electrochromic materials may also be used, for example, in automobile industry to automatically tint rear-view mirrors under different lighting conditions

[0084] Hereinafter, the present invention will be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, lengths and sizes of layers and regions may be exaggerated for clarity.

[0085] As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

[0086] The invention illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms "comprising", "including", "containing", etc. shall be read expansively and without limitation. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the inventions embodied therein herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention.

[0087] The invention has been described broadly and generically herein. Each of the narrower species and subgeneric groupings falling within the generic disclosure also form part of the invention. This includes the generic description of the invention with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein.

[0088] Other embodiments are within the following claims and non- limiting examples. In addition, where features or aspects of the invention are described in terms of Markush groups, those skilled in the art will recognize that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group.

EXPERIMENTAL SECTION

[0089] In embodiments disclosed herein, green synthesis of an unique hybrid structured vanadium oxide with aid of a green extract from plants, namely, tannic acid, has been demonstrated. Tannic acid was used as a shape and structure controller for the metal oxide synthesis. In embodiments, a hybrid structure consists of nanobelt membrane of V₂0₅ was formed using a green synthetic structural directly agent. Method according to embodiments involves a room temperature tannin solution-phase chemical reduction method through changing pH of solution.

[0090] The resultant unique hybrid structure was a construct of nanobelt and membrane and delivered a high transmittance contrast of 62 % in the visible range. The electrochromism of this nanobelt-membrane hybrid structure showed high optical modulation of 62 % with good cycling stability, making it a suitable choice for applications such as smart windows.

[0091] Existence of nanobelts significantly increased the surface area, providing abundant intercalation sites of easy access. The membrane structure played an important role in achieving such a high contrast. The original interstitials or spacing between nanobelts were partially replaced by the membrane, which smoothened the rough structure (root mean square roughness of 7.72 nm based on Atomic Force Microscope). This smooth hybrid nanobelt membrane structure reduced light scattering, which helped to boost the transmittance at bleach state. This facilitated the coloring state of the film that gave a dark color approaching blue, thus managing to keep the transmittance at a low level, resulting in a high contrast at 62 %.

[0092] The hybrid structured vanadium oxide demonstrated a fast switching behavior with coloration and bleaching time of 7.0 s, and 9.9 s respectively at 90 % modulation. The membrane confined the nanobelts into bundles between which electrolyte was allowed to infiltrate freely into the hybrid structure and enhanced the switching kinetics.

[0093] Example 1: Synthesis of ViOg hybrid membrane sample

[0094] Hydrothermal synthesis was carried out with vanadium oxide. 0.15 g VOS0₄ was dissolved in 25 ml DI water, with subsequent addition of 0.025 g tannic acid to the mixture to form a dark blue solution. 1 M sulfuric acid (H₂S0₄) was then added dropwise to tune pH of the solution to about 1.6, before addition of 8 ml 30 % H₂0₂ to the solution.

[0095] An orange solution was obtained, which was then put into an autoclave and kept at 180 °C for 24 hours to form a gel-like orange aggregate. The weak gel was shaken to redisperse it in water, and washed several times with water and ethanol in an alternative fashion via centrifuge. This final suspension of vanadium oxide was drop-casted on indium- tin oxide (ITO) glass to prepare the thin film.

[0096] Example 2: ITO surface treatment

[0097] The surface treatment was carried out using the following procedure. Briefly, ITO/glass was washed thoroughly in ethanol and water, and was dried via purging with nitrogen gas before immersing in 1 mgL^"1 aqueous Linear polyethylenimine (LPEI, Mw 25000) solution with pH of 2.5 adjusted by 1 M hydrochloric acid (HC1) for 20 min.

[0098] Example 3: Characterization of V_jOg hybrid membrane

[0099] The synthesized sample was investigated by Field Emission Scanning Electron Microscopy (FESEM) JEOL 7600F and Transmission Electron Microscopy (TEM) JEOL 21 OOF to obtain the morphology and lattice images.

[00100] X-Ray Diffraction (XRD) measurement was carried out on Shimazu XRD-6000 (Cu target, 2°/min). Fourier transform infrared spectroscopy (FT-IR) spectrum was collected on a Frontier FT-IR spectrometer. Sample roughness was measured using Atomic Force Microscope (AFM).

Γ00101] Example 4: Electrochromic performance testing

[00102] Electrochromic testing was carried out in a conventional three electrode environment with the active material on ITO/glass as the working electrode, silver (Ag) wire as the reference, and platinum (Pt) wire as the counter electrode. 1 M lithium perchlorate in propylene carbonate was the electrolyte. The voltage supply was from Solartron 1470E and the active material was polarized between -0.7V and 1 V versus Ag wire reference for electrochromic contrast testing and stability testing.

[00103] Example 5: Results and discussion

[00104] After washing the hydrothermal sample, a belt-like structure may be seen lying flat on the silicon wafer as shown in the FESEM image of FIG. 1 A. TEM analysis revealed that the belts are actually composed of finer nanobelts with a width of about 20 nm to 40 nm. These nanobelts were confined in a bundle by a thin solid membrane as shown in FIG. IB and FIG. 1C, which results in a hybrid structure.

[00105] High resolution TEM (HR-TEM) image and the ring-like electron diffraction pattern (FIG. ID, inset) indicated that the hybrid structured oxide is in polycrystalline form.

[00106] Without wishing to be bound by theory, it is postulated that formation mechanism of the hybrid structured material is as follows: First, the vanadium ion groups were chelated by the abundant phenol groups from tannic acid. High pressure and temperature environment in the autoclave caused clustering of the metal ions, and induced crystallization and nucleation of vanadium oxide phase. The vanadium oxide phase adopted a nanobelt morphology, exposing (00L) faces outward, which is one of the preferred orientations vanadium oxide takes.

[00107] Despite having the vanadium species homogenously aggregate into nanobelts, part of the vanadium species spread laterally and grew in a two-dimensional manner, resulting in a membrane-like growth.

[00108] This may be attributed to the steric hindrance effect exerted by the aromatic rings present in the tannic acid that chelate the vanadium ions. The hybrid membrane- nanobelt structure with a smoother surface relieves the light scattering and gives rise to enhanced optical contrast in electrochromism that will be discussed later.

[00109] XRD (X-ray diffraction) spectrum of the membrane-nanobelt structure is shown in FIG. 2A. The well-defined peaks in the XRD pattern correspond to orthorhombic vanadium oxide hydrate with a composition of V₂0₅* 1.6H₂0. These peaks belong to (00L) series and confirm that the hybrid structure is lying flat and exposing these (00L) faces.

[00110] FTIR (Fourier-transform Infra-Red) spectrum also confirms presence of the hydrated vanadium oxide. The spectrum data is listed in FIG. 2B in the range of 400 cm^"1 to 4500 cm^"1, the strong absorption peaks at 3418 cm^"1, 2361 cm^"1 and 1625 cm^"1 are ascribed to vibrational modes of water molecules, which indicate that water molecules are retained inside the nanostructures after hydrothermal synthesis.

[001 1 1] The peak at 1000 cm^"1 corresponds to V=0 terminal oxygen stretching mode, and the peak at around 760 cm^"1 attributes to V-O-V asymmetric stretching mode. Finally, the peak at around 508 cm^"1 is related to the stretching mode of the oxygen atom shared between three vanadium atoms.

[001 12] To confirm chemical structure of the membrane in this hybrid, EDX (Energy Dispersive X-ray Spectroscopy) mapping of the distribution of vanadium and oxygen atoms was conducted and is shown in FIG. 3C and FIG. 3D. The area (FIG. 3A) was carefully chosen, in which no nanobelts were observed. The dots in FIG. 3C and FIG. 3D refer to oxygen and vanadium distribution respectively. Uniform signal coverage of the two elements across the selected area demonstrated that the membrane also contains vanadium oxide. Atom percent of vanadium and oxygen, which suggest hydrated form of vanadium oxide, are shown in FIG. 4. The HRTEM analysis showed easily distinguished lattice structure of the membrane which confirms its polycrystalline property as shown in FIG. 3B.

[001 13] The color change mechanism involved a reversible electrochemical reaction whereby cathodic current through vanadium oxide caused V⁺⁵ (orange color) reduction to V⁺⁴ (green-bluish color). When an anodic current passes, the reverse reaction occurs. The reactions may be generalized using the following equation:

[001 14] V₂0₅ + xLi⁺ + xe -> Li_xV₂0₅

[00115] Contrast was taken at wavelength of 700 nm for this hybrid structured material. According to FIG. 5A, the contrast yielded was up to 62 %, which is higher than other reported vanadium oxide within the visible spectrum. It has been suggested that, contrast of electrochromic materials is related to the extent and amount of active material that may be intercalated with electrolyte ioris.

[001 16] Existence of nanobelts significantly increased the surface area, providing abundant intercalation sites of easy access. Besides, the membrane structure plays an important role in achieving such a high contrast as demonstrated herein. The original interstitials or spacing between nanobelts were partially replaced by the membrane, which smoothen the rough structure (root mean square roughness of 7.72 nm based on Atomic Force Microscope, FIG. 6). Generally speaking, an increase of the porosity with roughened surface reduces contrast. This smooth hybrid nanobelt membrane structure reduces light scattering, which helps to increase transmittance at bleached state. This is in complement to the strong absorption behavior of the electrochomic film in the colored state that gives a dark color approaching blue, which keeps the transmittance at a low level. Finally, a large difference in transmission levels is reflected by the high contrast.

[001 17] Switching time is described as 90 % of the time consumed for completion of color conversion. The hybrid structured vanadium oxide demonstrated a fast switching behavior with coloration and bleaching time of 7.0 s, and 9.9 s respectively (calculated from FIG. 5B), which is comparable to other systems with nanowire, nanorod, and amorphous structures. This may be explained from the FESEM image shown in FIG. 1A. The membrane did not form a continuous integrate like a thin film, but confined the nanobelts into bundles between which electrolyte was allowed to infiltrate freely into the hybrid structure and enhanced the switching kinetics.

[001 18J As for coloration efficiency, it may be defined by the following equations: [001 19] η = , where AOD = log ^Tb,eac"^ed

^Q ^ " colored

[00120] Herein, T_bi_eached and T_coi₀red refer to sample transmission at the bleached and colored state respectively. AOD refers to change of optical density, and q refers to charge inserted to vanadium oxides per area. Extracted coloration efficiency was 20.7 cm²/C at a wavelength of 700 ran. This value was similar to other reported crystalline vanadium oxide structures.

[00121] Apart from being generally low in electrochromic contrast, another problem faced by vanadium oxide eletrochromism is its poor stability. The contrast usually drops drastically within the first few cycles (refer to FIG. 7) and the film was found to slowly peel off from the ITO/glass substrate under no surface treatment condition. This eventually led to a gradual increase of the sample transmission at the colored state. To characterize the peeling intensity quantitatively, difference in sample transmission at the colored state before and after a certain number of cycles is adopted as AT_coi₀ied- This parameter describes the extent of film peeling condition when cycle number goes up. The film peeling condition of the sample with and without LPEI treatment is compared in FIG. 8. The active material is much harder to peel off after LPEI surface treatment, which gives rise to only 0.8 % increase in AT_coi_oied after around 70 cycles compared to the unsatisfactory 15.33 % rise in AT_coiored for the untreated sample. [00122] With the LPEI surface treatment, which increases interaction between the electrode and active material, a much better cycling stability may be achieved as shown in FIG. 9. A contrast degradation of only 18.6 % was obtained after 100 cycles for samples on LPEI treated ITO/glass substrate, as compared to a severe decrease in contrast up to 92.5 % for the sample without surface treatment (shown in FIG. 7). In fact, the fast contrast degradation occurs only during the first few cycles and the contrast is gradually stabilized in the later cycles as shown in FIG. 10.

[00123] The stability enhancement is due to coexistence of electrostatic and hydrogen bonding interaction with the substrate. In short, LPEI is a cationic polymer with secondary amine groups; vanadium oxide usually absorbs negative charges on the surface. The electrostatic interaction as well as the N H— 0=V hydrogen bonding interaction improved cycling stability of the film. As a comparison for the untreated sample, repeated intercalation and deintercalation strains the crystalline structure and diminishes the interaction between ITO surface and vanadium oxide, causing the film to peel off easily with increased cycling number.

[00124] As may be seen from the results above, a nanobelt-membrane hybrid structured polycrystalline vanadium oxide using tannic acid as a green and shape directing agent was synthesized. A film was made by a simple drop casting method on ITO glass and its electrochromic property was investigated which revealed a high coloration contrast of 62 % under a three electrode test at a wavelength of 700 nm. This is an improvement over state o the art electrochromic materials which exhibit a chromic contrast of less than 50 %. Furthermore, with linear polyethylenimine (LPEI) surface treatment on ITO glass, the stability of the drop-casted film was greatly enhanced with only 18.6 % reduction in transmission after 100 cycles. The hybrid structured vanadium oxide with improved coloration contrast and enhanced cycling stability is promising for commercial applications.

[00125] Example 6: Commercial applications

[00126] Regular glass may only allow a constant amount of light, the 'smart' window may be tuned, permitting any amount of light to pass, resulting in an estimated $ 1 1 billion to $20 billion dollars a year savings in heating, lighting and air-conditioning costs. According to the Environment Protection Agency in US, even a $7 billion savings would equate to a reduction in carbon emissions at power generating plants equal to taking 336,000 cars off the road, and the energy savings would be enough to light every home in New York City. [00127] The technology disclosed herein may be well-positioned for this endeavor, aiming at the global architectural glass market that produces an estimated 20 billion square feet of flat glass each year. In US, sales of residential window units have grown over 50 million units. Commercial window sales have increased to nearly 500 million square feet a year. Companies marketing electrochromic products are now mainly using vacuum evaporation coating techniques for the smart glass applications, automotive mirrors, helmet visors.

[00128] As disclosed herein, a green synthesis of a unique hybrid structured vanadium oxide with the aid of a green extract from plants, namely, the tannic acid was demonstrated. Tannic acid was employed as a shape and structure controller for the metal oxide synthesis. The resultant unique hybrid structure is a construct of nanobelt and membrane. The nanobelt membrane film is prepared by a facile drop casting onto a LPEI surface treated ITO glass for electrochemical testing. The electrochromism of this nanobelt-membrane hybrid structure shows high optical modulation of 62 % in the visible range with a good cycling stability, making it a suitable choice for applications such as smart windows.

[00129] Example 7: Two electrode electrochromic performance test

[00130] Two electrode electrochromic performance test of the hybrid structured vanadium oxide sample was also carried out for further validation. The film was polarized between -2.0 V and +2.8 V in LiC10₄ electrolyte. The contrast obtained at 700 nra is 64 %. T_bi_eached and Tcoiored measurement (time consumed for 90 % change of the contrast) shows bieached ⁼ 32.5 s and T_CO|_ored = 46.6 s.

[00131] According to the growth mechanism for the development of the unique nanobelt- membrane hybrid structured vanadium oxide suggested herein, it is postulated that the following chemicals that possess both chelating ability as well as steric hindrance ability will result in similar nanostructures as tannic acid does. Their individual molecular structures are shown below:

[00132]

Catechin

Eriocitrin Cinchonain-lb

[00133] While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.

Claims

Method for preparing a metal oxide nanostructured material, the method comprising a) providing an electrolyte solution comprising a metal oxide precursor, a polyphenol, and an oxidizing agent; and

b) heating the electrolyte solution under hydrothermal conditions to obtain the metal oxide nanostructured material:

Method according to claim 1 , wherein providing an electrolyte solution comprising a metal oxide precursor, a polyphenol, and an oxidizing agent comprises

a) providing an aqueous reagent comprising the metal oxide precursor and the polyphenol,

b) adding an electrolyte to the aqueous reagent to form an electrolytic aqueous reagent; and

c) adding an oxidizing agent to the electrolytic aqueous reagent. Method according to claim 2, wherein the electrolyte is an acid.

Method according to claim 3, wherein the acid comprises or consists of sulfuric acid.

Method according to claim 3 or 4, wherein adding an acid to the aqueous reagent comprises adjusting pH of the aqueous reagent to be in the range of about 1 to about 2.

Method according to any one of claims 1 to 5, wherein the metal oxide precursor is selected from the group consisting of metal sulfates, metal acetates, metal alkoxides, metal halides, metal phosphates, metal nitrates, and combinations thereof.

Method according to any one of claims 1 to 6, wherein the metal oxide precursor comprises or consists of a metal sulfate.

Method according to any one of claims 1 to 7, wherein the metal oxide precursor comprises or consists of vanadium sulfate. Method according to any one of claims 1 to 8, wherein metal of the metal oxide is selected from Group 3 to Group 12 of the Periodic System of Elements.

Method according to any one of claims 1 to 9, wherein metal of the metal oxide is selected from the group consisting of vanadium, niobium, tantalum, and combinations thereof.

Method according to any one of claims 1 to 10, wherein metal of the metal oxide is vanadium.

12. Method according to any one of claims 1 to 11, wherein the polyphenol is a natural polyphenol. 13. Method according to any one of claims 1 to 12, wherein the polyphenol is a plant- derived polyphenol.

14. Method according to any one of claims 1 to 13, wherein the polyphenol is selected from the group consisting of

derivatives thereof, and combinations thereof.

Method according to any one of claims 1 to 14, wherein the polyphenol comprises or consists of tannic acid.

Method according to any one of claims 1 to 15, wherein the polyphenol comprises or consists essentially of

17. Method according to any one of claims 1 to 16, wherein the oxidizing agent is selected from the group consisting of peroxides, ozone, peracetic acid, and combinations thereof.

18. Method according to any one of claims 1 to 17, wherein the oxidizing agent comprises or consists of hydrogen peroxide.

19. Method according to any one of claims 1 to 18, wherein heating the electrolyte solution under hydrothermal conditions comprises heating the electrolyte solution, preferably in an autoclave, at a temperature in the range of about 120 °C to about 200

20. Method according to any one of claims 1 to 19, wherein heating the electrolyte solution under hydrothermal conditions comprises heating the electrolyte solution, preferably in an autoclave, for a time period in the range of about 1 hour to about 48 hours.

21. Method according to any one of claims 1 to 20, wherein the metal oxide nanostructured material comprises nanobelts.

22. Method according to any one of claims 1 to 21 , wherein the metal oxide nanostructured material is a nanobelt-membrane-hybrid nanostructured material.

23. Metal oxide nanostructured material prepared by a method according to any one of claims 1 to 22.

24. Method for preparing a metal oxide layer, the method comprising

a) providing a suspension comprising a metal oxide nanostructured material prepared by a method according to any one of claims 1 to 22, and

b) depositing the suspension on a substrate to obtain the metal oxide layer.

Method according to claim 24, wherein the substrate is selected from the group consisting of glass, ITO-coated glass, FTO-coated glass, ITO-coated poly(ethylene terephthalate), graphene-coated glass, carbon nanotube-coated glass, graphene-coated poly(ethylene terephthalate), carbon nanotube-coated poly(ethylene terephthalate), metal nanowires, metal nanoparticles, metal grids, and combinations thereof.

Method according to claim 24 or 25, wherein depositing the suspension on a substrate to obtain the metal oxide layer comprises treating surface of the substrate with a polyimine prior to depositing the suspension on the substrate.

Method according to claim 26, wherein the polyimine comprises at least one of a polyalkyl imine and a polyalkylene imine.

Method according to claim 26 or 27, wherein the polyimine comprises or consists of linear polyethylenimine.

29. Method according to any one of claims 24 to 28, wherein depositing the suspension on a substrate comprises drop-casting the suspension comprising a metal oxide nanostructured material on the substrate. 30. Method according to any one of claims 24 to 29, wherein the metal oxide layer is an electrochromic metal oxide layer.

31. Electrochromic device comprising a metal oxide layer prepared by a method according to any one of claims 24 to 30.