WO2019050472A1

WO2019050472A1 - A method of producing a nanomaterial from a vanadium source and related nanomaterials

Info

Publication number: WO2019050472A1
Application number: PCT/SG2017/050454
Authority: WO
Inventors: Guowu ZHAN; Chi-Hwa Wang; Wei Cheng NG
Original assignee: National University Of Singapore; Sembcorp Industries Ltd
Priority date: 2017-09-08
Filing date: 2017-09-08
Publication date: 2019-03-14

Abstract

There is provided a method of producing a nanomaterial, the method comprising adding an alkaline earth metal salt to a leaching solution containing vanadate ions to form a mixture, the leaching solution being derived from a vanadium source, and precipitating an alkaline earth metal vanadate nanomaterial from the mixture. In particular, said vanadium source is an oil refinery waste or a carbon black waste. Also provided are nanomaterials derived from said vanadium source, the nanomaterial comprising alkaline earth metal vanadate nanostructures, and an electronic device comprising said nanomaterials.

Description

A METHOD OF PRODUCING A NANOMATERIAL FROM A VANADIUM SOURCE AND RELATED NANOMATERIALS

TECHNICAL FIELD

Various embodiments disclosed herein relate broadly to alkaline earth metal vanadate nanomaterials derived from a vanadium source, and methods of producing alkaline earth metal vanadate nanomaterials. Vanadium source includes but is not limited to vanadium-based waste such as fossil fuel waste.

BACKGROUND The amount of waste generated has dramatically increased with the rising consumption and resource use by a rapidly growing world population. An increasing amount of research interest has therefore been focused on building opportunities from waste, including food waste, biomass waste, oil fly ash, and the like. Such waste may sometimes contain useful components that still possess good market value, but the components are simply disposed of instead of recovered due to technological or economical limitations. An economically feasible recovery of the useful components may also contribute to a sustainable society. For instance, waste could be used as a valuable resource for the preparation of fuels and high-value chemicals/materials.

Vanadium is an excellent but toxic heavy metal that occurs in crude oil, coal, oil shale and tar sands. In particular, vanadium is the most abundant metal in crude oil, reaching concentration of up to 1 580 ppm of total crude, which is largely derived from chlorophyll of dead organisms. As crude oil is processed and refined into more useful fuels and intermediate products by oil refinery plants, vanadium will finally exist in carbon black waste (a major oil refinery waste). Carbon black waste is therefore a potential but untapped source of vanadium. Further, disposal of carbon black waste containing vanadium poses a pollution risk as vanadium is hazardous to human health and was found to impair the antioxidant enzymatic activities of human cell lines.

Typically, for metal recovery from carbon black waste, complete dissolution of valuable metals from carbon black waste is achieved by advanced leaching processes. Various technologies have been developed for the subsequent separation and recovery of metal in the leaching solutions, such as chemical precipitation, reactive crystallization, adsorption, ion exchange, electrochemical removal, biotechnological processes, and membrane separations. However, separation and recovery of metal, particularly vanadium, in the leaching solutions remains a challenge. Therefore, the studies on transformation of waste metal ions, particularly vanadium ions, into value-added products are still insufficient in both academia and industry. In view of the above, there is thus a need to address or at least ameliorate one of the above problems.

SUMMARY In one aspect, there is provided a method of producing a nanomaterial, the method comprising:

adding an alkaline earth metal salt to a leaching solution containing vanadate ions to form a mixture, the leaching solution being derived from a vanadium source;

precipitating an alkaline earth metal vanadate nanomaterial from the mixture.

In one embodiment, said adding at least one of an acid or a base to the vanadium source to form a leaching solution containing vanadate ions.

In one embodiment, the method further comprises, prior to the step of adding the alkaline earth metal salt to the leaching solution: (i) adding at least one of an acid or a base to the vanadium source to obtain a first leachate containing vanadate ions;

(ii) adding the leachate obtained from the preceding step to the vanadium source to obtain a second leachate containing a higher concentration of vanadate ions;

(iii) optionally repeating step (ii) for one or more times to obtain a third leachate or subsequent leachate;

wherein the leachate obtained after completion of the above steps forms the leaching solution containing vanadate ions for use in adding to the alkaline earth metal salt.

In one embodiment, the method further comprises recovering the precipitated alkaline earth metal vanadate nanomaterial from the mixture. In one embodiment, the precipitating step comprises stirring the mixture at room temperature.

In one embodiment, the precipitating step comprises heating the mixture to a temperature of more than 1 00 ^°C.

In one embodiment, the heating step is performed for a time period of from 2 hours to 40 hours.

In one embodiment, the heating step comprises a hydrothermal heating step at a temperature ranging from 120^°C to 250^°C.

In one embodiment, the alkaline earth metal salt comprises at least one of a nitrate salt, a hydroxide salt, a chloride salt, an acetate salt or a carbonate salt.

In one embodiment, the alkaline earth metal salt comprises at least one of a calcium salt, a strontium salt or a barium salt.

In one embodiment, the amount of alkaline earth metal salt added to the leaching solution is in the range of (0.1 - 20.0) w/v%. In one embodiment, the vanadium source is an oil refinery waste.

In one embodiment, the vanadium source is a carbon black waste. In one embodiment, the alkaline earth vanadate nanomaterial has an energy band gap in the range of from 3.7 eV to 4.5 eV.

In one aspect, there is provided a nanomaterial derived from a vanadium source, the nanomaterial comprising alkaline earth metal vanadate nanostructures.

In one embodiment, the alkaline earth metal vanadate nanostructures comprise precipitates from a mixture formed by adding an alkaline earth metal salt to a leaching solution containing vanadate ions, the leaching solution being derived from the vanadium source.

In one embodiment, the alkaline earth metal vanadate nanostructures comprise nanostructures selected from the group consisting of calcium vanadate nanostructures, strontium vanadate nanostructures, barium vanadate nanostructures and combinations thereof.

In one embodiment, the calcium vanadate nanostructures comprise at least one of the following properties:

a) has a chemical formula of CaioV6O2s;

b) is in a form of a single-crystal nanorod with an aspect ratio of no less than 15:2; or

c) has an X-ray diffraction pattern as shown in FIG. 9A,

the strontium vanadate nanostructures comprise at least one of the following properties:

a) is a hexagonal phase of SrioV602s;

b) has a structure of an ellipsoid formed via self-assembly of one or more one dimensional nanorods of no less than 105 nm; or

c) has an X-ray diffraction pattern as shown in FIG. 9B, and the barium vanadate nanostructures comprise at least one of the following properties:

a) is barium orthovanadate Ba3V20s;

b) is in a form of isolated polyhedral nanoparticles with a diameter of in the range of from 35 nm to 77 nm; or

c) has an X-ray diffraction pattern as shown in FIG. 9C.

In one embodiment, the alkaline earth metal vanadate nanostructures have an energy band gap in the range of from 3.7 eV to 4.5 eV.

In one aspect, there is provided an electronic device comprising the nanomaterial.

In one embodiment, the electronic device is a semiconductor device.

DEFINITIONS

The term "vanadium source" as used herein broadly refers to materials that contain detectable amounts of elemental vanadium having a ground oxidation state and/or species containing vanadium that exists in a variety of oxidation states. Examples of "vanadium source" include but are not limited to vanadium- based waste which in turn includes but is not limited to fossil fuel waste.

The term "waste" as used herein broadly refers to unwanted materials that are left over from a process and are intended to be disposed of. The term encompasses but is not limited to waste generated from industrial process plants such as oil refinery or petroleum refinery, power plant, chemical plant and water and wastewater treatment plant. Accordingly, the term "vanadium-based waste" as used herein broadly refers to materials that contain detectable amounts of elemental vanadium having a ground oxidation state and/or species containing vanadium that exists in a variety of oxidation states. Examples of "vanadium- based waste" include but are not limited to fossil fuel waste, oil refinery waste, petroleum coke and carbon black waste. "Carbon black waste" may be understood to be a carbon-rich solid residue generated from incomplete combustion of hydrocarbon or cracking of oil under high temperatures in an oil refinery.

The term "leaching" as used herein refers to a process of extracting metal species from a material containing the metal species with a leaching agent. The term "leaching solution" as used herein refers to a solution resulting from the addition of the leaching agent to the materials containing the metal species and the dissolution of the metal species. The leaching process may comprise a step of removing insoluble solids from the leaching solution subsequent to the dissolution of the metal species to form the leaching solution. Accordingly, the leaching solution may be substantially free of insoluble solids. The leaching solution may include both an intermediate and a final leaching solution/leachate. In some embodiments, as an intermediate leaching solution, it may be cycled repeatedly with the materials containing the metal species to eventually obtain a final leaching solution with an increased metal concentration. In some embodiments, the leaching process may comprise adjusting the pH of the leaching solution with an acid or a base.

The term "vanadium" as used herein broadly refers to elemental vanadium having a ground oxidation state and species containing vanadium that exists in a variety of oxidation states such as +2, +3, +4 and +5. Examples of such species encompass but are not limited to vanadium (II), vanadium(lll), vanadium(IV), vanadyl, vanadium(V), vanadate ions, salts, compounds or complexes. The term "vanadate ions" as used herein broadly refers to species containing an oxoanion of vanadium. Examples include but are not limited to VO3^" , VO4³-, V2O7⁴-, V3O9³-, V4O12⁴-, VsO ³- , protonated vanadate ions such as HVO4²- , H2VO4^" and polyvanadate ions. The term "oxidation state" as used herein refers to the degree of oxidation of an atom which is represented by zero, or a positive or negative number. An increase in oxidation state is referred to as an oxidation process, while a decrease in oxidation state is referred to as a reduction process. The term "structures" as used herein broadly refer to a discrete entity or discrete body. The structure described herein may be in the form of, or substantially in the form of, but not limited to, a rod or a particle. The structure described herein may be non-spherical, irregularly-shaped or ellipsoidally shaped structures. The structure described herein may also be formed by an aggregate of a plurality of sub-structures or a fragment of a small structure.

The term "material" as used herein broadly refers to materials having structures or constituents.

The term "nano" as used herein is to be interpreted broadly to include dimensions in a nanoscale, i.e., the range between 1 and 100 nm. Accordingly, a term carrying "nano" as prefix as used herein may include structures of sizes that are no more than about 1 0 nm, no more than about 20 nm, no more than about 30nm, no more than about 40 nm, no more than about 50 nm, no more than about 60 nm, no more than about 70 nm, no more than about 80 nm, no more than about 90 nm and no more than about 100 nm. The term "micro" as used herein is to be interpreted broadly to include dimensions no more than about 1000 pm. Accordingly, the term "microstructure" and the like as used herein may include a structure of sizes that are no more than about 1000 μη-Ί, no more than about 900 μιτη, no more than about 800 μιτη, no more than about 700 μη-Ί, no more than about 600 μιτη, no more than about 500 μιτη, no more than about 400 μη-Ί, no more than about 300 μιτη, no more than about 200 μιτη, or no more than about 100 μιτι.

The term "size" when used to refer to a "nanostructure", "nanorod" "nanoparticle", "nanomaterial", "microstructure" or the like may broadly refer to the largest dimension of the nanostructure, nanorod, nanoparticle, nanomaterial, microstructure or the like. For example, when the structure is substantially spherical, the term "size" can refer to the diameter of the structure; or when the structure is substantially non-spherical, the term "size" can refer to the largest length of the structure.

The term "alkaline earth metal" as used herein broadly refers to a chemical element found in Group 2 of the Periodic Table of elements (for example, beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba) and radium (Ra)) having a ground oxidation state and species containing the alkali earth metal that exists in a variety of oxidation states such as +1 and +2. Accordingly, the terms "alkaline earth metal salts" and "alkaline earth metal vanadates", as used herein broadly refer to salts and vanadates that contain the said alkaline earth metal having a ground oxidation state and/or species that contain the said alkali earth metal that exists in a variety of oxidation states.

The term "precipitate" as used herein broadly refers to a solid formed from a solution and the terms "precipitating" and "precipitated" shall be construed accordingly.

The term "and/or", e.g., "X and/or Y" is understood to mean either "X and Y" or "X or Y" and should be taken to provide explicit support for both meanings or for either meaning.

The terms "coupled" or "connected" when used in this description are intended to cover both directly connected or connected through one or more intermediate means, unless otherwise stated.

The term "associated with", used herein when referring to two elements refers to a broad relationship between the two elements. The relationship includes, but is not limited to a physical, a chemical or a biological relationship. For example, when element A is associated with element B, elements A and B may be directly or indirectly attached to each other or element A may contain element B or vice versa.

Further, in the description herein, the word "substantially" whenever used is understood to include, but not restricted to, "entirely" or "completely" and the like. In addition, terms such as "comprising", "comprise", and the like whenever used, are intended to be non-restricting descriptive language in that they broadly include elements/components recited after such terms, in addition to other components not explicitly recited. Further, terms such as "about", "approximately" and the like whenever used, typically means a reasonable variation, for example a variation of +/- 5% of the disclosed value, or a variance of 4% of the disclosed value, or a variance of 3% of the disclosed value, a variance of 2% of the disclosed value or a variance of 1 % of the disclosed value.

Furthermore, in the description herein, certain values may be disclosed in a range. The values showing the end points of a range are intended to illustrate a preferred range. Whenever a range has been described, it is intended that the range covers and teaches all possible sub-ranges as well as individual numerical values within that range. That is, the end points of a range should not be interpreted as inflexible limitations. For example, a description of a range of 1 % to 5% is intended to have specifically disclosed sub-ranges 1 % to 2%, 1 % to 3%, 1 % to 4%, 2% to 3% etc., as well as individually, values within that range such as 1 %, 2%, 3%, 4% and 5%. The intention of the above specific disclosure is applicable to any depth/breadth of a range.

Additionally, when describing some embodiments, the disclosure may have disclosed a method and/or process as a particular sequence of steps. However, unless otherwise required, it will be appreciated the method or process should not be limited to the particular sequence of steps disclosed. Other sequences of steps may be possible. The particular order of the steps disclosed herein should not be construed as undue limitations. Unless otherwise required, a method and/or process disclosed herein should not be limited to the steps being carried out in the order written. The sequence of steps may be varied and still remain within the scope of the disclosure.

DESCRIPTION OF EMBODIMENTS Exemplary, non-limiting embodiments of a method of producing a nanomaterial from a vanadate-based waste and related nanomaterials are disclosed hereinafter. In various embodiments, there is provided a method of producing a nanomaterial, the method comprising adding an alkaline earth metal salt to a leaching solution containing vanadate ions to form a mixture, the leaching solution being derived from a vanadium source; precipitating an alkaline earth metal vanadate nanomaterial from the mixture.

In various embodiments, the method may further comprise a reaction predominantly between the alkaline earth metal salt and vanadate ions of the leaching solution. The said vanadate ions may be VO4^3" ions in the leaching solution.

In various embodiments, the method may comprise a recovery reaction in a liquid phase. In some embodiments, the recovery reaction may comprise a reactive crystallisation method.

Advantageously, embodiments of the method disclosed herein provide a simplified method of recovering metal (e.g. vanadium) by complete dissolution of valuable metals from vanadium source through advanced leaching processes. Various embodiments of the method provide wet chemical methods involving precipitation and/or reactive crystallisation which are one of the simplest and effective processes available due to their low cost and ease to handle in the industry. Therefore, embodiments of said method may be desirable for industrial application. Additionally, a reactive crystallisation method may allow for an ultimate conversion of metal ions to value-added products with well-controlled size/morphology in a one-pot process.

Even more advantageously, the method disclosed herein may provide a technical method of preparing nanomaterials. In various embodiments of the method, the method may be used to prepare alkaline earth vanadate nanomaterials such as calcium vanadate nanomaterials, strontium vanadate nanomaterials and barium vanadate nanomaterials. Said alkaline earth vanadate nanomaterials may comprise single-crystal calcium vanadate nanorods oriented along the [001 ] direction (with chemical formula of CaioV602s), strontium vanadate nanorods with ellipsoid-like assembly (with chemical formula of SrioV602s) and/or barium vanadate polyhedral nanoparticles (with chemical formula of Ba3V20s). It would be appreciated that vanadate nanomaterials may be attractive due to its abundant potential applications in technical fields such as in chemical sensors, in transparent conductors, as photocatalysts, in lithium batteries, in biological imaging and in therapy. The vanadate nanomaterials may also have potential applications in ion-conducting glasses, electron-conducting glasses, and energy storage materials, such as electrochemical devices in sensor, cathode materials in lithium ion batteries with high energy capacity, high power density, high recyclability and low manufacture cost. Further, it would also be appreciated that vanadate nanomaterials may be attractive due to the electrical and/or magnetic properties arising from the presence of magnetic V⁴⁺ ions. As an example, rare earth vanadate nanoparticles may be promising candidates for applications as in vivo multifunctional probes, as said nanoparticles may perform a target-oriented delivery of active compounds.

In various embodiments, the method further comprises adding at least one of an acid or a base to the vanadium source to form a leaching solution containing vanadate ions. In some embodiments, the acid may be added before adding the base. In some other embodiments, only base may be added.

In various embodiments, the acid or the base added to the vanadium source acts as a leaching agent to form the leaching solution. In some embodiments, the acid added to the vanadium source may be hydrochloric acid, or nitric acid, or sulfuric acid, or phosphoric acid, or mixture thereof. Any suitable acid that effectively and/or preferentially leaches vanadium from the vanadium source may be used in the method disclosed herein. In some embodiments, the alkali added to the vanadium source may be sodium hydroxide, or potassium hydroxide, or ammonium hydroxide, or sodium carbonate, or potassium carbonate, or sodium bicarbonate, or potassium bicarbonate, or mixture thereof. Any suitable alkali that effectively and/or preferentially leaches vanadium from the vanadium source may be used in the method disclosed herein.

In certain embodiments, alkali may be further added to the leaching solution prepared. For example, if acid was first used to prepare the leaching solution, alkali may be added to adjust the pH of the leaching solution. In certain embodiments, alkali may be further added to the leaching solution such that the final leaching solution has an alkaline pH. For example, the final leaching solution may have a pH of more than about 7.0, more than about 7.5, more than about 8.0, more than about 8.5, more than about 9.0, more than about 9.5, more than about 10.0, more than about 1 0.5, more than about 1 1 .0, more than about 1 1 .5, more than about 1 2.0, more than about 12.5, more than about 13.0, more than about 13.5 or about 14.0. In certain embodiments, alkali may be further added to the leaching solution such that the pH of the final leaching solution may be in a range of more than about 1 2.0 to 14.0. In one embodiment, the pH of the final leaching solution is 13.5. Advantageously, adjusting the pH of the leaching solution to an alkaline pH may allow the leaching solution to be used to synthesise alkaline earth metal vanadate nanomaterials.

In various embodiments, a leaching process comprises a step of removing insoluble solids from the leaching solution subsequent to the dissolution of the metal species to form the leaching solution containing vanadate ions. In various embodiments, the leaching solution is substantially free of insoluble solids. Advantageously, the leaching process may be performed to purify the composition of the leaching solution/leachate obtained. In some embodiments, a substantially pure vanadium solution may be obtained, which may ensure a high purity of the final product.

In various embodiments, the leaching solution may be an intermediate leaching solution or a final leaching solution. As an intermediate leaching solution, it may be further used to leach more vanadium ions from the vanadium source. For example, it may be cycled repeatedly with the vanadium source to obtain a leaching solution with an increased vanadium concentration or a final leaching solution having a vanadium concentration that is suitable for use in subsequent processing steps to eventually produce the alkaline earth metal vanadate nanomaterials. It may be appreciated that if the concentration of vanadium present in the leaching solution/leachate is insufficient for subsequent steps of producing the alkaline earth metal vanadate nanomaterials, cycling leaching can be performed to increase the concentration of vanadium in the leaching solution/leachate. In various embodiments, the cycling leaching comprises subjecting a leaching solution/leachate having a low concentration of vanadium to one or more additional leaching process(es), without introducing any additional leaching agents. In other words, the leaching solution/leachate having a low concentration of vanadium may act as the leaching agent during the cycling leaching process. In various embodiments, the concentration of vanadium in the leaching solution/leachate increases with the number of times the leaching solution/leachate is cycled with the vanadium source. For example, if the concentration of vanadium in the first leaching solution/leachate is low, a vanadium source may be added to the first leaching solution/leachate to perform a second leaching process. The second leaching process may comprise the steps of dissolving vanadium and removing insoluble solid to form the second leaching solution/leachate, which has a higher vanadium concentration than the first leaching solution/leachate. In each leaching cycle, the leaching process may be repeated at least once, at least twice, or at least thrice to obtain a second leaching solution/leachate, third leaching solution/leachate or subsequent leaching solution/leachate, wherein the final leaching solution/leachate forms the final leaching solution containing vanadate ions.

In some embodiments, the concentration of vanadium in the first, second, third subsequent or final leaching solution/leachate is at least but not limited to about 100 ppm, about 200 ppm, about 300 ppm, about 400 ppm, about 500 ppm, about 600 ppm, about 700 ppm or about 800 ppm. In some embodiments, the concentration of vanadium in the second, third, subsequent or final leaching solution/leachate is at least but not limited to about 1000 ppm. In one embodiment, the concentration of vanadium in a first leaching solution/leachate is in a range of about 500 ppm to about 800 pm. The first leaching solution/leachate may act as a leaching agent in a second leaching process. After undergoing the second leaching process, the concentration of vanadium in the second leaching solution/leachate may increase to about 1 000 ppm. In some embodiments, if the concentration of vanadium in the second leaching solution/leachate is determined to be low, the second leaching solution/leachate obtained may be further cycled through the cycling leaching process in order to increase the concentration of vanadium in the final leaching solution/leachate. In some other embodiments, if the concentration of vanadium in the second leaching solution/leachate is determined to be sufficient, the second leaching solution/leachate may form the final leaching solution/leachate containing vanadate ions. In various embodiments, the minimum concentration of vanadium in a final leaching solution/leachate containing vanadate ions may be, but not limited to, 100 ppm.

In various embodiments, the method of producing nanomaterials further comprises, prior to the step of adding the alkaline earth metal salt to the leaching solution, step (i) adding at least one of an acid or a base to the vanadium source to obtain a first leachate containing vanadate ions; step (ii) adding the leachate obtained from the preceding step to the vanadium source to obtain a second leachate containing a higher concentration of vanadate ions; optionally repeating step (ii) for one or more times to obtain a third leachate or subsequent leachate, wherein the leachate obtained after completion of the above steps forms the leaching solution containing vanadate ions for use in adding to the alkaline earth metal salt. In various embodiments, the method further comprises recovering the precipitated alkaline earth metal vanadate nanomaterials from the mixture. In various embodiment, the method further comprises recovering an alkaline earth vanadate solid from the mixture.

In various embodiments, recovering the precipitated alkaline earth metal vanadate may comprise washing with deionised water. In some embodiments, recovering the precipitated alkaline earth metal vanadate may comprise washing with deionised water for up to 1 time, up to 2 times, up to 3 times, or up to 4 times.

In one embodiment, recovering the precipitated alkaline earth metal vanadate may comprise washing with deionised water for 3 times.

In various embodiments, the precipitating step of the method comprises stirring the mixture at room temperature. In various embodiments, the precipitating step may be performed at ambient/room temperature and atmospheric pressure.

Further, in various embodiments, the precipitating step may comprise stirring the mixture for up to about 5 minutes, up to about 6 minutes, up to about 7 minutes, up to about 8 minutes, up to about 9 minutes, up to about 10 minutes, up to about 1 1 minutes, up to about 1 2 minutes, up to about 13 minutes, up to about 14 minutes, or up to about 15 minutes.

In one embodiment, the precipitating step may comprise stirring the mixture for about 10 minutes.

In various embodiments, said stirring may comprise vigorous stirring.

It may be appreciated that in various embodiments, stirring the mixture at room temperature helps the homogeneously mixing of the raw materials which may promote the reaction between the vanadate ions (e.g. VO4^3" ions) and the alkaline earth metal salts. In various embodiments, the precipitating step of the method comprises heating the mixture to a temperature of more than 100 °C.

In various embodiments, the abovementioned temperature may be no less than about 100 °C, no less than about 1 10 °C, no less than about 120 °C, no less than about 130 °C, no less than about 140 °C, no less than about 150 °C, no less than about 160 °C, no less than about 170 °C, no less than about 180 °C, no less than about 190 °C, no less than about 200 °C, no less than about 210 °C, no less than about 220 °C, no less than about 230 °C, no less than about 240 °C, or no less than about 250 °C.

In various embodiments, the precipitating step of the method comprises a heating step performed for a time period of from 2 hours to 40 hours. In various embodiments, the said time period may be about 3 hours, about

4 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours, about 9 hours, about 10 hours, about 1 1 hours, about 12 hours, about 13 hours, about 14 hours, about 15 hours, about 16 hours, about 17 hours, about 18 hours, about 19 hours, about 20 hours, about 21 hours, about 22 hours, about 23 hours, about 24 hours, about 25 hours, about 26 hours, about 27 hours, about 28 hours, about 29 hours, about 30 hours, about 31 hours, about 32 hours, about 33 hours, about 34 hours, about 35 hours, about 36 hours, about 37 hours, about 38 hours, about 39 hours, or about 40 hours. In one embodiment, the precipitating step of the method comprises a heating step performed for a time period of about 12 hours.

In various embodiments, the precipitating step of the method comprises a heating step comprising a hydrothermal heating step at a temperature ranging from 120 ^°C to 250 ^°C.

In various embodiments, the heating step of the method comprising a hydrothermal heating step may be carried out at a temperature of no less than about 100 °C, no less than about 1 10 °C, no less than about 120 °C, no less than about 130 °C, no less than about 140 °C, no less than about 150 °C, no less than about 160 °C, no less than about 170 °C, no less than about 180 °C, no less than about 190 °C, no less than about 200 °C, no less than about 210 °C, no less than about 220 °C, no less than about 230 °C, no less than about 240 °C, or no less than about 250 °C.

In some embodiments, the hydrothermal heating step may be carried out at a temperature of no less than about 160 °C, no less than about 170 °C, no less than about 180 °C, no less than about 190 °C, or no less than about 200 °C. Advantageously, performing the hydrothermal heating step at these temperatures may be optimal for achieving higher vanadium recovery and better quality of the product size/morphology. In one embodiment, the precipitating step of the method comprises a heating step comprising a hydrothermal heating step at a temperature of about 200 °C.

In various embodiments, vanadium ions can be recovered and transferred to alkaline earth vanadate materials at room temperature conditions. Advantageously, performing the hydrothermal heating step in the method disclosed herein may result in achieving higher vanadium recovery and better quality of the product size/morphology in comparison to performing the precipitating step at room temperature conditions. In one embodiment, the precipitating step comprises performing the heating step and the stirring step simultaneously.

In various embodiments, performing precipitating step with a hydrothermal heating step allows for a vanadium recovery efficiency that is higher than performing the precipitating step under ambient or atmospheric conditions.

In various embodiments, performing the hydrothermal heating step in the method disclosed herein, may result in achieving a vanadium recovery efficiency of at least about 75%, at least about 80%, at least about 85%, at least about 90%, at least about 95% or about 100%. In comparison, in various embodiments, performing the precipitating step at room temperature conditions may result in achieving a vanadium recovery efficiency of no more than about 55%. In various embodiments, the vanadium recovery efficiency achieved when the precipitating step is performed at room temperature conditions is about 0%, no more than about 5%, no more than about 1 0%, no more than about 1 5%, no more than about 20%, no more than about 25%, no more than about 30%, no more than about 35%, no more than about 40%, no more than about 45%, no more than about 50%, or no more than about 55%.

In various embodiments, the alkaline earth metal salt of the method comprises at least one of a nitrate salt, a hydroxide salt, a chloride salt, an acetate salt or a carbonate salt. The alkaline earth metal salt may be in the form of a solid or a salt solution at the time of adding the leaching solution. The alkaline earth metal salt may be in the hydrous form.

In various embodiments, the said alkaline earth metal salt may be at least one of calcium nitrate (e.g. Ca(N03)2), calcium hydroxide (e.g. Ca(OH)2), calcium chloride (e.g. CaCl2), calcium acetate (e.g. Ca(OAc)2) , calcium carbonate (e.g. CaCOs), strontium nitrate (e.g. Sr(N03)2), strontium hydroxide (e.g. Sr(OH)2), strontium chloride (e.g. SrCl2), strontium acetate (e.g. Sr(OAc)2), strontium carbonate (e.g. SrCOs), barium nitrate (e.g. Ba(N03)2), barium hydroxide (e.g. Ba(OH)2), barium chloride (e.g. BaCl2), barium acetate (e.g. Ba(OAc)2) , or barium carbonate (e.g. BaCOs).

In various embodiments, the alkaline earth metal salt of the method comprises at least one of a calcium salt, a strontium salt or a barium salt. In various embodiments, the alkaline earth metal salt of the method disclosed herein may comprise any other alkaline earth metal salt. In these embodiments, the alkaline earth metal salt may be at least one of a beryllium salt, a magnesium salt, or a radium salt. In various embodiments, the amount of alkaline earth metal salt added to the leaching solution in the method is in the range of 0.1 - 20.0 w/v%. In various embodiments, the amount of alkaline earth metal salt added to the leaching solution in this method may be in the range of about 0.5 - 19.5 w/v%, about

I .0 - 19.0 w/v%, about 1 .5 - 18.5 w/v%, about 2.0 - 18.0 w/v%, about 2.5 - 17.5 w/v%, about 3.0 - 17.0 w/v%, about 3.5 - 16.5 w/v%, about 4.0 - 16.0 w/v%, about 4.5 - 15.5 w/v%, about 4.5 - 14.0 w/v%, about 5.0 - 13.5 w/v%, about 5.5 - 13.0 w/v%, about 6.0 - 12.5 w/v%, about 6.5 - 12.0 w/v%, about 7.0 - 1 1 .5 w/v%, about 7.5 -

I I .0 w/v%, about 8.0 - 10.5 w/v%, about 8.5 - 10.0 w/v%, about 9.0 - 9.5 w/v%.

In one exemplary embodiment, the amount of alkaline earth metal salt added to the leaching solution in the method is about 0.2 w/v%.

In another exemplary embodiment, the amount of alkaline earth metal salt added to the leaching solution in the method is about 2.0 w/v%.

In various embodiments, the step of recovering the precipitated alkaline earth vanadate nanomaterial of the method comprises centrifugation, filtration or a combination thereof.

In various embodiments, the method has a vanadium recovery efficiency of at least 80% from the leaching solution.

In various embodiments, the method may have a vanadium recovery efficiency of at least about 75%, at least about 80%, at least about 85%, at least about 90%, at least about 95%, or about 100%. Advantageously, the vanadium recovery efficiency of the method disclosed herein may be useful and cost-effective in recovering metals and other valuable materials from industrial waste. In various embodiments, the vanadium source of the method is an oil refinery waste.

In various embodiments, the vanadium source of the method is a carbon black waste. In some embodiments, the carbon black waste is substantially free of impurities such as phosphorous and silicon. It would be appreciated that industrial waste (e.g. oil refinery waste, carbon black waste) may contain metals (e.g. vanadium) and other valuable materials. In some embodiments, the method offers an environmentally friendly and sustainable approach of resource recovery transforming industrial waste into a source for the metals and/or other valuable materials. In this regard, in various embodiments, the vanadium source disclosed herein is distinct and different from a purified vanadium source which is commercially available and which is substantially free from other metals and/or chemical compounds.

In various embodiments, the alkaline earth vanadate of the method has an energy band gap in the range of from about 3.7 eV to about 4.5 eV.

In one embodiment where the alkaline earth vanadate of the method is calcium vanadate, the energy band gap is about 4.1 eV. In another embodiment where the alkaline earth vanadate of the method is strontium vanadate, the energy band gap is about 4.3 eV. In yet another embodiment where the alkaline earth vanadate is barium vanadate, the energy band gap is about 3.9 eV.

It would be appreciated that alkaline earth vanadates possess semiconducting properties associated with the hopping process. Advantageously, in various embodiments, the alkaline earth vanadates of the method disclosed herein have energy band gaps that are larger than that of micron-sized alkaline earth vanadates. Such alkaline earth vanadates may thus be used in electronic device operated at higher temperature and/or larger voltages, or wide band-gap semiconducting devices. This is unlike the case for transition metal vanadates (e.g. BiV04, MnV206) with narrow band gap which are used as photocatalysts.

In various embodiments, there is provided a preparation procedure of alkaline earth vanadate nanomaterials or wide band-gap semiconductors contain the following steps:

(a) Prepare acid or alkaline leachate of vanadium rich-industrial waste;

(b) Adding a certain amount of alkaline earth metal salts into the leachate;

(c) Mixing the alkaline earth metal salts and leachate;

(d) Post treatment of the resulting solution;

(e) Collection of alkaline earth vanadate solid by filtration or centrifugation methods.

In various embodiments, there is provided a nanomaterial derived from a vanadium source, the nanomaterial comprising alkaline earth metal vanadate nanostructures.

In various embodiments, the vanadium source from which the nanomaterial is derived from is an oil refinery waste. In various embodiments, the vanadium source from which the nanomaterial is derived from is a carbon black waste.

Advantageously, in various embodiments of the nanomaterial, the said nanomaterial is an environmentally friendly and sustainable product because the nanomaterial is derived from a vanadium source including, but not limited to, vanadium-based wastes and fossil fuel waste.

Even more advantageously, in various embodiments of the nanomaterial, there is better control over the morphology and size of the alkaline earth metal vanadate nanostructures. In one embodiment of the alkaline earth vanadate nanostructures of the nanomaterials disclosed herein, the morphology and size of the barium orthovanadate product produced are better controlled than using pure Na3VO4 chemical as a vanadium ion precursor under microwave solvothermal treatment followed by further heat-treatment (performed at a temperature of 600°C for a duration of 3 hours). The increased control over the morphology and the size of the alkaline earth metal vanadate nanostructures in the nanomaterial may be useful in its application to electronic device operated at higher temperature and/or larger voltages, or wide band-gap semiconducting devices.

It would be appreciated that in various embodiments of the nanomaterial, the nanomaterial may be attractive due to its abundant potential applications in technical fields such as in chemical sensors, in transparent conductors, as photocatalysts, in lithium batteries, in biological imaging and in therapy. The nanomaterials may also have potential applications in ion-conducting glasses, electron-conducting glasses, and energy storage materials, such as electrochemical devices in sensor, cathode materials in lithium ion batteries with high energy capacity, high power density, high recyclability and low manufacture cost. Further, it would also be appreciated that the nanomaterials may be attractive due to the electrical and/or magnetic properties arising from the presence of magnetic V⁴⁺ ions.

In various embodiments, the alkaline earth metal vanadate nanostructures of the nanomaterial comprise precipitates from a mixture formed by adding an alkaline earth metal salt to a leaching solution containing vanadate ions, the leaching solution being derived from the said vanadium source.

In various embodiments, the precipitate may be formed via a reaction predominantly between the alkaline earth metal salt and vanadate ions of the leaching solution. The said vanadate ions may be VO4^3" ions in the leaching solution.

In various embodiments, the alkaline earth metal salt added to form the precipitate may comprise at least one of, but not limited to, a nitrate salt, a hydroxide salt, a chloride salt, an acetate salt or a carbonate salt.

In various embodiments, the said alkaline earth metal salt added to form the precipitate may be at least one of calcium nitrate (e.g. Ca(N03)2) , calcium hydroxide (e.g. Ca(OH)2) , calcium chloride (e.g. CaCl2), calcium acetate (e.g. Ca(OAc)2), calcium carbonate (e.g. CaCOs), strontium nitrate (e.g. Sr(N03)2) , strontium hydroxide (e.g. Sr(OH)2), strontium chloride (e.g. SrCl2) , strontium acetate (e.g. Sr(OAc)2), strontium carbonate (e.g. SrCOs), barium nitrate (e.g. Ba(N03)2) , barium hydroxide (e.g. Ba(OH)2), barium chloride (e.g. BaCl2) , barium acetate (e.g. Ba(OAc)2), or barium carbonate (e.g. BaCOs).

In various embodiments, the alkaline earth metal salt added to form the precipitate may comprise one of a calcium salt, a strontium salt or a barium salt.

In various embodiments, the alkaline earth metal salt of the method disclosed herein may comprise any other alkaline earth metal salt. In these embodiments, the alkaline earth metal salt may be at least one of, but not limited to, a beryllium salt, a magnesium salt, or a radium salt.

Advantageously, in various embodiments of the nanomaterial, said nanomaterial is non-hazardous even though the nanomaterial is derived from industrial waste such as carbon black waste containing vanadium. In various embodiments, the alkaline earth metal vanadate nanostructures of the nanomaterials comprise nanostructures selected from the group consisting of calcium vanadate nanostructures, strontium vanadate nanostructures, barium vanadate nanostructures and combinations thereof. In various embodiments, the alkaline earth metal vanadate nanostructures of the nanomaterial may comprise any other alkaline earth metal nanostructures. In various embodiments, the alkaline earth metal vanadate nanostructures may comprise nanostructures selected from the group consisting of beryllium vanadate nanostructures, magnesium vanadate nanostructures, or radium vanadate nanostructures. In various embodiments, the alkaline earth metal vanadate nanostructures have well-defined shapes (e.g., single-crystal nanorod, or isolated polyhedral nanoparticles). In various embodiments, the nanomaterial comprises calcium vanadate nanostructures. In these embodiments, the said calcium vanadate nanostructures comprise at least one, at least two or all of the following properties: a) has a chemical formula of CaioV6O2s; b) is in a form of a single-crystal nanorod oriented along the [001 ] direction with an aspect ratio of no less than 15:2; or c) has an X-ray diffraction pattern as shown in FIG. 9A.

In various embodiments, the calcium vanadate nanostructures may comprise bush-like assembly of one-dimensional nanorods. In one embodiment, said nanomaterial may comprise calcium vanadate having the chemical formula of CaioV6O2s and is in the form of single-crystal nanorod oriented along the [001 ] direction with an aspect ratio of about 8:1 .

Advantageously, calcium vanadate nanostructures of the nanomaterial disclosed herein may be characterised with high surface area to volume ratio. This may especially be the case for one dimensional vanadate nanostructures (for example, nanorods, nanobelts and nanotubes). As a result, said nanomaterials may exhibit better electrochemical and photocatalytic properties and may be more promising than their bulk counterparts.

In various embodiments, the calcium vanadate nanorods have a length of no less than about 715 nm, no less than about 720 nm or no less than about 725 nm.

In various embodiments, the calcium vanadate nanorods have a diameter of no less than about 85 nm, no less than about 90 nm or no less than about 95 nm. In various embodiments, the aspect ratio of the calcium vanadate nanorods have an aspect ratio of no less than about 15:2, no less than about 8:1 or no less than about 17:2. In various embodiments, the nanomaterial comprises strontium vanadate nanostructures. In these embodiments, the said strontium vanadate nanostructures comprise at least one, at least two or all of the following properties: a) is a hexagonal phase of SnoV6025; b) has a structure of an ellipsoid formed via self-assembly of one or more one dimensional nanorods of no less than 105 nm; or c) has an X-ray diffraction pattern as shown in FIG. 9B.

In one embodiment, said nanomaterial may comprise hexagonal phase of SrioV6O25 and has a structure of an ellipsoid formed via self-assembly of many individual one dimensional short nanorods as building blocks.

Advantageously, the strontium vanadate nanostructures of the nanomaterial disclosed herein may be characterised with high surface area to volume ratio. This may especially be the case for one dimensional vanadate nanostructures (for example, nanorods, nanobelts and nanotubes). As a result, the said nanomaterials may exhibit better electrochemical and photocatalytic properties and may be more promising than their bulk counterparts.

In various embodiments, the morphology and size of the strontium vanadate nanostructures are different from that of calcium vanadate nanostructures.

In various embodiments, the strontium vanadate nanorods have a length of no less than about 105 nm, no less than about 1 10 nm, or no less than about 1 1 5 nm. In various embodiments, the nanomaterial comprises barium vanadate nanostructures. In these embodiments, the said barium vanadate nanostructures comprise at least one, at least two or all of the following properties: a) is barium orthovanadate Ba3V20s; b) is in a form of isolated polyhedral nanoparticles with a diameter of in the range of from 35 nm to 77 nm; or c) has an X-ray diffraction pattern as shown in FIG. 9C.

In one embodiment, said nanomaterial may comprise barium vanadate orthovanadate Ba3V20s in the form of isolated polyhedral nanoparticles.

In various embodiments, the isolated polyhedral nanoparticles of the nanomaterial comprising barium vanadate nanostructures may have a diameter of at least about 35 nm, at least about 36 nm, at least about 37 nm, at least about 38 nm, at least about 39 nm, at least about 40 nm, at least about 41 nm, at least about 42 nm, at least about 43 nm, at least about 44 nm, at least about 45 nm, at least about 46 nm, at least about 47 nm, at least about 48 nm, at least about 49 nm, at least about 50 nm, at least about 51 nm, at least about 52 nm, at least about 53 nm, at least about 54 nm, at least about 55 nm, at least about 56 nm, at least about 57 nm, at least about 58 nm, at least about 59 nm, at least about 60 nm, at least about 61 nm, at least about 62 nm, at least about 63 nm, at least about 64 nm, at least about 65 nm, at least about 66 nm, at least about 67 nm, at least about 68 nm, at least about 69 nm, at least about 70 nm, at least about 71 nm, at least about 72 nm, at least about 73 nm, at least about 74 nm, at least about 75 nm, at least about 76 nm or at least about 77 nm.

In one embodiment of the barium vanadate nanostructures of the nanomaterials disclosed herein, the morphology and size of the barium orthovanadate product produced are better controlled than using pure Na3VO4 chemical as a vanadium ion precursor under microwave solvothermal treatment followed by further heat-treatment (performed at a temperature of 600°C for a duration of 3 hours).

In various embodiments, the alkaline earth metal vanadate nanostructures of the nanomaterial have an energy band gap in the range of from 3.7 eV to 4.5 eV.

It would be appreciated that alkaline earth vanadates possess semiconducting properties associated with the hopping process. Advantageously, in various embodiments, the alkali earth metal vanadate nanostructures of the nanomaterial give rise to energy band gaps that are larger than that of micron- sized alkaline earth vanadates. Such nanomaterials may thus be used in electronic device operated at higher temperature and/or larger voltages, or wide band-gap semiconducting devices. This is unlike the case for transition metal vanadates (e.g. BiV04, MnV206) with narrow band gap which are used as photocatalysts.

In various embodiments, there is provided an electronic device comprising the nanomaterial.

In various embodiments, the electronic device comprising the nanomaterial is a semiconductor device. In one embodiment, the semiconductor device is a wide band-gap semiconductor device. In another embodiment, the semiconductor device is a semiconducting glass.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 is a graph showing the vanadium recovery efficiencies when calcium vanadate nanomaterials are prepared at room temperature via stirring and under hydrothermal condition in accordance with various embodiments disclosed herein.

FIGS. 2A and 2B are Transmission Electron Microscope (TEM) images of calcium vanadate nanorods prepared by using alkaline leaching solution with calcium nitrate under hydrothermal treatment at a temperature of 200 °C for a period of 12 hours in accordance with various embodiments disclosed herein.

FIG. 3 is a graph showing the effect of the amount of calcium nitrate tetrahydrate on the vanadium recovery efficiency in accordance with various embodiments disclosed herein.

FIG. 4 is a graph showing the vanadium recovery efficiencies when barium vanadate nanomaterials are prepared at room temperature via stirring and under hydrothermal condition in accordance with various embodiments disclosed herein. FIGS. 5A, 5B and 5C are TEM images of barium vanadate nanomaterials prepared by treating alkaline leaching solution with barium acetate under hydrothermal condition at a temperature of 200 °C for a period of 12 hours in accordance with various embodiments disclosed herein.

FIGS. 6A, 6B, 6C and 6D are TEM images of barium vanadate nanomaterials prepared by treating alkaline leaching solution with barium chloride under hydrothermal condition at a temperature of 200 °C for a period of 12 hours in accordance with various embodiments disclosed herein.

FIG. 7 is a graph showing the vanadium recovery efficiencies when strontium vanadate nanomaterials are prepared at room temperature via stirring and under hydrothermal condition in accordance with various embodiments disclosed herein.

FIGS. 8A, 8B and 8C are exemplary TEM images of strontium vanadate nanorod with ellipsoid-like structure prepared by treating alkaline leaching solution with strontium chloride hexahydrate in accordance with various embodiments disclosed herein.

FIGS. 9A, 9B and 9C are graphs showing the X-ray diffraction (XRD) patterns of three alkaline earth vanadate products, namely, calcium vanadate, strontium vanadate and barium vanadate respectively in accordance with various embodiments disclosed herein.

FIG. 10 is a graph showing the experimentally determined band gap energies for the prepared alkaline earth vanadates (namely, calcium vanadate, strontium vanadate and barium vanadate) by using Tauc plots of (ahv)² versus E {viz., hv), where a is the absorption coefficient, h is the Planck constant, and v is the photon's frequency.

FIG. 1 1 is a schematic flowchart for illustrating a method of producing a nanomaterial from a vanadium source in an exemplary embodiment. DETAILED DESCRIPTION OF FIGURES FIG. 1 is a graph showing the vanadium recovery efficiencies when calcium vanadate nanomaterials were prepared at room temperature via stirring and under hydrothermal condition in accordance with various exemplary embodiments disclosed herein. Vanadium recovery efficiencies were investigated using calcium nitrate, calcium hydroxide, calcium chloride, calcium acetate and calcium carbonate as alkali earth metal salts. Overall, the vanadium recovery efficiencies differed when calcium vanadate nanomaterials were prepared at room temperature via stirring and under hydrothermal condition when different calcium salts were used. Generally, as shown by the figure, higher vanadium recovery efficiencies could be achieved under hydrothermal condition.

FIGS. 2A and 2B are TEM images of calcium vanadate nanostructures prepared by using alkaline leaching solution with calcium nitrate under hydrothermal treatment at a temperature of 200 °C for a period of 12 hours in accordance with an exemplary embodiment disclosed herein. FIG. 2A is taken at 50,000χ magnification and FIG. 2B is taken at 90,000χ magnification. Based on the TEM images, the calcium vanadate nanorods obtained were one-dimensional nanorods with bush-like assembly. The length and diameter of the nanorods were about 720 nm and 90 nm respectively and the aspect ratio of the nanostructure is about 8:1 . FIG. 3 is a graph showing the effect of the amount of calcium nitrate tetrahydrate on the vanadium recovery efficiency in accordance with various embodiments disclosed herein. Incomplete vanadate recovery was observed when the amount of calcium nitrate tetrahydrate was lower than 30 mg. When the amount of calcium nitrate tetrahydrate was 30 mg or higher, almost 100% vanadium in the solution was converted to calcium vanadate.

FIG. 4 is a graph showing the vanadium recovery efficiencies when barium vanadate nanomaterials are prepared at room temperature via stirring and under hydrothermal condition in accordance with various embodiments disclosed herein. Four kinds of barium precursors were used, including barium hydroxide, barium chloride, barium acetate and barium carbonate. The vanadium recovery efficiency differed when barium vanadate nanomaterials were prepared at room temperature via stirring and under hydrothermal condition when barium carbonate was used as the barium precursor. The vanadium recovery efficiencies were about the same when barium vanadate nanomaterials were prepared at room temperature via stirring and under hydrothermal condition when barium hydroxide, barium chloride or barium acetate were used as barium precursors.

FIGS. 5A, 5B and 5C are TEM images of barium vanadate nanostructures prepared by treating alkaline leaching solution with barium acetate under hydrothermal condition at a temperature of 200 °C for a period of 12 hours in accordance with various embodiments disclosed herein. FIG. 5A is taken at 50,000χ magnification, FIG. 5B is taken at 100,000χ magnification and FIG. 5C is taken at 200,000χ magnification. Based on the TEM images, the average size of the barium vanadate (obtained in the form of isolated polyhedral nanoparticles) was in the range of from 40 nm to 72 nm. FIGS. 6A, 6B, 6C and 6D are TEM images of barium vanadate nanostructures prepared by treating alkaline leaching solution with barium chloride under hydrothermal condition at a temperature of 200 °C for a period of 12 hours in accordance with various embodiments disclosed herein. FIG. 6A is taken at 20,000χ magnification, FIG. 6B is taken at 50,000χ magnification, FIG. 6C is taken at 10Ο,ΟΟΟχ magnification and FIG. 6D is taken at 800,000χ magnification.

FIG. 7 is graph showing the vanadium recovery efficiencies when strontium vanadate nanomaterials are prepared at room temperature via stirring and under hydrothermal condition in accordance with various embodiments disclosed herein. The vanadium recovery efficiency was about the same when strontium vanadate nanomaterials were prepared at room temperature via stirring and under hydrothermal condition when strontium chloride was used as the alkali earth metal salt. FIGS. 8A, 8B and 8C are TEM images of strontium vanadate nanostructures with ellipsoid-like structure prepared by treating alkaline leaching solution with strontium chloride hexahydrate in accordance with various embodiments disclosed herein. FIG. 8A is taken at 3,000χ magnification, FIG. 8B is taken at 20,000χ magnification and FIG. 8C is taken at 80,000χ magnification. The strontium vanadate nanostructures comprised ellipsoid-like structures formed via self-assembly of numerous individual one dimensional nanorods with an average length of about 1 10 nm. FIGS. 9A, 9B and 9C are graphs showing the X-ray diffraction (XRD) patterns of three alkaline earth vanadate products, namely, calcium vanadate, strontium vanadate and barium vanadate respectively in accordance with various embodiments disclosed herein. The XRD pattern shows that the calcium vanadate product can be ascribed to the hexagonal CaioV6O25 (JCPDS card no. 52-0649), the strong and sharp XRD peaks of strontium vanadate product can be indexed to the hexagonal phase of SrioV6O25 and the barium vanadate product can be identified as barium orthovanadate

FIG. 10 is a graph showing the experimentally determined band gap energies for the prepared alkaline earth vanadates (namely, calcium vanadate, strontium vanadate and barium vanadate) by using Tauc plots of (ahv)² versus E {viz., hv), where a is the absorption coefficient, h is the Planck constant, and v is the photon's frequency. As shown, the measured band gap energies for calcium vanadate, strontium vanadate and barium vanadate were 4.1 eV, 4.3 eV and 3.9 eV respectively.

FIG. 1 1 is a schematic flowchart for illustrating a method of producing a nanomaterial from a vanadium source in an exemplary embodiment. In the exemplary embodiment, the vanadium source is a vanadium-based waste. It will be appreciated by a person skilled in the art that other suitable vanadium source for the method disclosed herein may also be used. To produce the nanomaterial, either base or acid is added to the vanadium source for leaching. If base is added to the vanadium source, an alkaline leaching solution is obtained. If acid is added to the vanadium source, after obtaining an acid leaching solution, base is added to adjust the pH. Next, after obtaining the final leaching solution, an alkaline earth metal salt is added to the final leaching solution. The resulting solution is then subjected to post-treatment before alkaline earth metal vanadate solids are collected. EXAMPLES

Example embodiments of the disclosure will be better understood and readily apparent to one of ordinary skill in the art from the following examples, and if applicable, in conjunction with the figures.

In the following examples, embodiments of the method disclosed herein are capable of effectively recovering vanadium from the leaching solution of oil refinery wastes / carbon black wastes in the form of alkaline earth vanadate nanomaterials. The examples demonstrate that embodiments of the method disclosed herein are safe, environmentally friendly and sustainable for recovering metal and other valuable materials from industrial waste. The examples further demonstrate that the vanadium recovery efficiencies may be improved when the alkaline earth metal nanomaterials are produced under hydrothermal condition in comparison to producing the same under room temperature conditions via stirring.

Further, as will be shown in the following examples, the alkaline earth metal nanomaterials prepared by embodiments of the method disclosed herein may display controlled morphology and size with relatively wide energy band gaps. Advantageously these properties of the alkaline earth metal nanomaterials may allow the materials to be used in applications such as in electronic device operated at higher temperature and/or larger voltages, or wide band-gap semiconducting devices.

Example 1 - Synthesis of alkaline earth vanadate nanomaterials The synthesis of alkaline earth vanadate nanomaterials is demonstrated in this

Example at a laboratory scale to primarily prove the principles involved. It will be understood that a further scale-up of the method may be carried out, for example by scaling it to an industrial process. In this Example, the carbon black waste collected from an oil refinery factory in Singapore. Further in this Example, chemicals used (without further purification) comprise sodium hydroxide (99%, VWR Chemicals), nitric acid (69%, VWR Chemicals), calcium nitrate tetrahydrate (99%, Sigma-Aldrich), calcium chloride (98%, Merck), calcium acetate monohydrate (>99.9%, Fisher Chemicals), calcium hydroxide (>96%, Fluka), calcium carbonate (Sigma-Aldrich), barium hydroxide monohydrate (>98%, Sigma-Aldrich), barium chloride dehydrate (99%, Merck), barium acetate (99%, Sigma-Aldrich), barium carbonate (Fisher Chemicals), strontium chloride hexahydrate (>99%, Merck), and strontium acetate (Strem Chemicals). Deionised water was used in this Example. The leaching solution of carbon black waste obtained by using acid or base is referred to as acid leaching solution and alkali leaching solution respectively. To synthesise alkaline earth vanadate nanomaterials, an acid or alkali leaching solution was prepared. In general, to prepare the acid leaching solution, 2.5 g of dry waste solid was added to 50 mL of nitric acid or hydrochloric acid. The concentrations of nitric acid and hydrochloric acid used were 0.25 M, 0.5 M or 1 M, with the most frequently used concentration being 0.5 M. To prepare the alkali leaching solution, 2.5 g of dry waste solid was added to 50 mL of sodium hydroxide. The concentrations of sodium hydroxide used were 0.25 M, 0.5 M or 1 M, with the most frequently used concentration being 1 .0 M. Although 2.5 g of dry waste solid was used in this example, it will be appreciated that 5.0 g of waste solid may also be used, which will lead to a higher concentration of vanadium ions in the leaching solution.

In a case where an alkali leaching solution was prepared, 5 mL of the leaching solution was then mixed with a pre-determined amount of alkaline earth metal salts. Next, the mixture was stirred at room temperature for 10 minutes before performing the hydrothermal treatment at a temperature of 200 °C for a duration of 12 hours. Subsequently, a white solid was collected by centrifugation and washing with deionised water for three times. In a case where an acid leaching solution was prepared, 5 mL of sodium hydroxide solution (1 M) was added to the acid leaching solution before the addition of alkaline earth metal salts. Sodium hydroxide solution was added to adjust the pH of the final leaching solution to be in a range of more than about 12.0 to 14.0, In general, the pH of the final leaching solution was 13.5. Next, the mixture was stirred at room temperature for 10 minutes before performing the hydrothermal treatment at a temperature of 200 °C for a duration of 12 hours. Subsequently, a white solid was collected by centrifugation and washing with deionised water for three times In comparison experiments, in a case where an alkali leaching solution was created, 5 mL of the leaching solution was mixed with a pre-determined of alkaline earth metal salts. Next, the mixture was stirred at room temperature for a duration of 12 hours. Subsequently, a white solid was collected by centrifugation and washing with deionised water for three times.

Further in comparison experiments, in a case where an acid leaching solution was created, 5 mL of sodium hydroxide solution (1 M) was added to the acid leaching solution before the addition of alkaline earth metal salts. Next, the mixture was stirred at room temperature for a duration of 12 hours. Subsequently, a white solid was collected by centrifugation and washing with deionised water for three times.

Example 2 - Vanadium recovery efficiencies

In this Example, the effects of using various alkali earth metal salts and preparing the alkali earth vanadate nanomaterials at different conditions (i.e. hydrothermal treatment vs. room temperature stirring) using embodiments of the method disclosed in the present disclosure on vanadium recovery efficiencies are demonstrated. (i) Vanadium recovery efficiencies using calcium precursors

To investigate the vanadium recovery efficiencies when calcium precursors are used, five different kinds of calcium precursors were used. The calcium precursors used were namely, calcium nitrate, calcium hydroxide, calcium chloride, calcium acetate and calcium carbonate.

Briefly, 5 imL of alkaline leaching solution of carbon black waste was mixed with different calcium salts, wherein, the amount of calcium ion was 0.42 mmol. Subsequently, the mixed solution was subjected to hydrothermal treatment at a temperature of 200 °C for a duration of 12 hours Subsequently, the product was collected by centrifugation and washing with deionised water for three times.

In comparison experiments, 5 imL of alkaline leaching solution of carbon black waste was mixed with different calcium salts, wherein, the amount of calcium ion was 0.42 mmol. Subsequently, the mixed solution was subjected to room temperature stirring for a duration of 12 hours. Subsequently, the product was collected by centrifugation and washing with deionised water for three times.

The vanadium recovery efficiencies when different calcium precursors were used and when calcium vanadate nanomaterials were prepared under different conditions are shown in FIG. 1 . As shown in FIG. 1 , vanadium recovery efficiencies differed when the calcium vanadate nanomaterials were prepared under hydrothermal condition and at room temperature.

In hydrothermal synthesis, there was no pronounced effect of the anions on vanadium recovery, as almost complete recovery of vanadium could be achieved by using any of the alkaline earth salts (with the exception of calcium carbonate). When calcium carbonate was used, a significantly lower recovery efficiency was observed, which suggested that solvable raw materials may have important roles on the formation of vanadate product.

When the mixed solution was subjected to room temperature stirring, different vanadium recovery efficiencies were observed when different calcium salts were used. In particular, no vanadium was recovered when calcium carbonate was used at room temperature condition. In general, it was shown that higher vanadium recovery could be achieved at hydrothermal condition.

Next, the effect of the amount of calcium precursor (calcium nitrate tetrahydrate) used on vanadium recovery efficiency is shown in FIG. 3. As shown in FIG. 3, the vanadium recovery was dependent on the amount of calcium salt used. Incomplete vanadate recovery was observed when the amount of calcium nitrate tetrahydrate used was lower than 30 mg. When the amount of calcium nitrate tetrahydrate used was higher than 30 mg, almost 100% of the vanadium in the solution was converted to calcium vanadate.

(ii) Vanadium recovery efficiencies using barium precursors

To investigate the vanadium recovery efficiencies when barium precursors are used, four different kinds of barium precursors were used. The barium precursors were namely, barium hydroxide, barium chloride, barium acetate, and barium carbonate.

Briefly, 5 imL of alkaline leaching solution of carbon black waste was mixed with different barium salts, wherein, the amount of barium ion was 0.42 mmol. Subsequently, the mixed solution was placed under hydrothermal treatment at a temperature of 200 °C for a duration of 12 hours. Subsequently, the product was collected by centrifugation and washing with deionised water for three times.

In comparison experiments, 5 imL of alkaline leaching solution of carbon black waste was mixed with different barium salts, wherein, the amount of barium ion was 0.42 mmol. Subsequently, the mixed solution was subjected to room temperature stirring for a duration of 12 hours (with vigorous stirring). Subsequently, the product was collected by centrifugation and washing with deionised water for three times. The vanadium recovery efficiencies when different barium precursors were used and when barium vanadate nanomaterials were prepared under different conditions are summarised in FIG. 4. As shown in FIG. 4, vanadium recovery efficiencies differed when the barium vanadate nanomaterials were prepared under hydrothermal condition and at room temperature when barium carbonate was used as the barium precursor. When barium hydroxide, barium chloride or barium acetate were used as barium precursors, vanadium recovery efficiencies between preparing the nanomaterials under hydrothermal condition and at room temperature did not differ significantly.

(iii) Vanadium recovery efficiency using strontium precursor

To investigate the vanadium recovery efficiency when a strontium precursor is used, strontium chloride was used as the alkaline earth metal source.

Briefly, 5 imL of alkaline leaching solution of carbon black waste was mixed with strontium chloride, wherein, the amount of strontium ion was 0.42 mmol. Subsequently, the mixed solution was placed under hydrothermal treatment at a temperature of 200 °C for a duration of 12 hours. Subsequently, the product was collected by centrifugation and washing with deionised water for three times.

In comparison experiments, 5 imL of alkaline leaching solution of carbon black waste was mixed with strontium chloride, wherein, the amount of strontium ion was 0.42 mmol. Subsequently, the mixed solution was subjected to room temperature stirring for a duration of 12 hours. Subsequently, the product was collected by centrifugation and washing with deionised water for three times.

The vanadium recovery efficiencies when strontium vanadate nanomaterials were prepared under different conditions are summarised in FIG. 7. As shown in FIG. 7, the vanadium recovery efficiencies were about 100% when the strontium vanadate nanomaterials were prepared under hydrothermal condition or at room temperature.

Example 3 - Characterisation of alkaline earth metal vanadate nanomaterials

In this Example, the morphology and sizes of the alkaline earth metal vanadate nanostructures in accordance with various embodiments disclosed herein were studied using Transmission Electron Microscopy (TEM). In addition, X-ray Diffraction (XRD) was used for phase composition identification of the alkaline earth metal vanadate nanostructures. Further, UV-vis-NIR diffuse reflectance spectra were used in this Example to show the optical properties of the as-prepared alkaline earth vanadates.

(i) Transmission Electron Microscopy

Generally, to prepare the samples of the alkaline earth metal vanadate nanomaterials for imaging using TEM, 5 imL of alkaline leaching solution of carbon black waste was mixed 100 mg of the respective alkaline earth metal salt under hydrothermal conditions (200 °C) for 12 hours. The alkaline earth metal salts used herein, were calcium nitrate, strontium chloride and barium acetate (or barium chloride), for the preparation of calcium vanadate nanomaterials, strontium vanadate nanomaterials, and barium vanadate nanomaterials, respectively. To image the samples, JEOL JEM-2010 with an electron kinetic energy of 200 kV was used.

FIGS. 2A and 2B show exemplary TEM images of calcium vanadate nanostructures obtained using calcium nitrate. The calcium vanadates were in the form of a bush-like assembly of one-dimensional nanorods. The measured length and diameter of the nanorods were about 720 nm and 90 nm respectively. The aspect ratio of the nanostructure was thus about 8:1 .

FIGS. 5A, 5B and 5C show exemplary TEM images of barium vanadate nanostructures obtained using barium acetate. The barium vanadates were in the form of isolated polyhedral nanoparticles with an average size of about 56 ± 16 nm in size.

FIGS. 6A, 6B, 6C and 6D show exemplary TEM images of barium vanadate nanostructures obtained using barium chloride. The barium vanadates were also in the form of isolated polyhedral nanoparticles with an average size of about 56 ± 16 nm in size.

FIGS. 8A, 8B and 8C show exemplary TEM images of strontium vanadate nanostructures obtained using strontium chloride. The strontium vanadates were in the form of ellipsoid-like structures formed via self- assembly of numerous individual one dimensional short nanorods. The nanorods had an average length of about 1 10 nm.

(ii) X-Rav Diffraction patterns

In order to study the XRD patterns of the alkaline earth metal vanadates obtained, the following synthesis methods were used.

To prepare calcium vanadates, 10 mg of calcium nitrate tetrahydrate was added to 5 mL of alkaline leaching solution and mixed. Subsequently, the mixed solution was placed under hydrothermal treatment at a temperature of 200 °C for a duration of 12 hours. Then, the product was collected by centrifugation and washing with deionised water for three times.

To prepare strontium vanadates, 10 mg of strontium chloride hexahydrate was added to 5 mL of alkaline leaching solution and mixed. Subsequently, the mixed solution was placed under hydrothermal treatment at a temperature of 200 °C for a duration of 12 hours. Then, the product was collected by centrifugation and washing with deionised water for three times.

To prepare barium vanadates, 10 mg of barium acetate was added to 5 mL of alkaline leaching solution and mixed. Subsequently, the mixed solution was placed under hydrothermal treatment at a temperature of 200 °C for a duration of 12 hours. Then, the product was collected by centrifugation and washing with deionised water for three times.

The crystallographic microstructure was obtained by an X-ray diffractometer (XRD, Bruker D8 Advance) equipped with Cu Ka radiation source.

FIGS. 9A, 9B and 9C show the XRD patterns of three alkaline earth vanadate products, namely, calcium vanadate, strontium vanadate and barium vanadate. The XRD pattern in FIG. 9A shows that the calcium vanadate product can be ascribed to the hexagonal CaioV₆O25 phase (JCPDS card no. 52-0649). The XRD pattern in FIG. 9B shows the strong and sharp XRD peaks of strontium vanadate product which can be indexed to the hexagonal phase of SrioV6O25. The XRD pattern in FIG. 9C shows that the barium vanadate product can be identified as barium orthovanadate Ba3V2Os.

(iii) UV-vis-NIR Spectroscopy

In order to study the optical properties of the as-prepared alkaline earth vanadates by UV-vis-NIR diffuse reflectance spectrum, the following methodology was used.

Band gap energies of solid samples were examined by diffused reflectance spectra in a UV-VIS-NIR spectrophotometer (Shimadzu 3600).

The measured band gap energies for calcium vanadate, strontium vanadate, and barium vanadate are shown in FIG. 10. As shown in FIG. 10, the measured band gap energies for calcium vanadate, strontium vanadate, and barium vanadate were 4.1 eV, 4.3 eV, and 3.9 eV, respectively.

APPLICATIONS

It would be appreciated that industrial waste (e.g. oil refinery waste, carbon black waste) may contain metals (e.g. vanadium) and other valuable materials. Advantageously, various embodiments of the method disclosed herein may offer an environmentally friendly and sustainable method of recovering said metals and other valuable materials from industrial waste, the industrial waste being a source of the metals and/or other valuable materials. Further, various embodiments of the method disclosed herein may provide a simplified method of recovering metal (e.g. vanadium) by complete dissolution of valuable metals from a vanadium source through advanced leaching processes. Even further, various embodiments of the method disclosed herein may provide wet chemical methods involving precipitation and/or reactive crystallisation which are one of the simplest and effective processes available due to their low cost and ease to handle in the industry. Therefore, the said method may be desirable for industrial application.

Various embodiments of the method disclosed herein may comprise a reactive crystallisation method that allows ultimate conversion of metal ions to value-added products in a one-pot process.

Various embodiments of the present disclosure provide a nanomaterial derived from a vanadium source, the nanomaterial comprising alkaline earth metal vanadate nanostructures. In various embodiments of the nanomaterial disclosed herein, the nanomaterials may have favourable size-dependent properties which may benefit their applications.

For example, advantageously, the alkaline earth metal vanadate nanostructures of embodiments of the nanomaterial disclosed herein may have an energy band gap that is larger than that of micron-sized alkaline earth vanadates. The said nanomaterials may thus be used in various industrial applications such as electronic device operated at higher temperature and/or larger voltages, or wide band- gap semiconducting devices. This is unlike the case for transition metal vanadates (e.g. B1VO4, MnV2O6) with narrow band gap which are used as photocatalysts. In various embodiments of the alkaline earth metal vanadate nanostructures of the nanomaterial disclosed herein, said nanostructures may be characterised with high surface area to volume ratio. This may especially be the case for one dimensional vanadate nanostructures (for example, nanorods, nanobelts and nanotubes). As a result, the said nanomaterials may exhibit better electrochemical and photocatalytic properties and may be more promising than their bulk counterparts.

In one embodiment of the alkaline earth vanadate nanostructures of the nanomaterials disclosed herein, the morphology and size of the barium orthovanadate product produced are better controlled than using pure Na3VO4 chemical as a vanadium ion precursor under microwave solvothermal treatment followed by further heat-treatment (performed at a temperature of 600°C for a duration of 3 hours). In various embodiments, the nanomaterial comprising alkaline earth metal vanadate nanostructures may be attractive due to its abundant potential applications in technical fields such as in chemical sensors, in transparent conductors, as photocatalysts, in lithium batteries, in biological imaging and in therapy. The nanomaterials may also have potential applications in ion-conducting glasses, electron-conducting glasses, and energy storage materials, such as electrochemical devices in sensor, cathode materials in lithium ion batteries with high energy capacity, high power density, high recyclability and low manufacture cost. Further, it would also be appreciated that the nanomaterials may be attractive due to the electrical and/or magnetic properties arising from the presence of magnetic V⁴⁺ ions. As an example, rare earth vanadate nanoparticles may be promising candidates for applications as in vivo multifunctional probes, as said nanoparticles may perform a target-oriented delivery of active compounds.

It will be appreciated by a person skilled in the art that other variations and/or modifications may be made to the embodiments disclosed herein without departing from the spirit or scope of the disclosure as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.

Claims

1 . A method of producing a nanomaterial, the method comprising:

precipitating an alkaline earth metal vanadate nanomaterial from the mixture.

2. The method of claim 1 , further comprising adding at least one of an acid or a base to the vanadium source to form a leaching solution containing vanadate ions.

3. The method of claim 1 , further comprising, prior to the step of adding the alkaline earth metal salt to the leaching solution:

(i) adding at least one of an acid or a base to the vanadium source to obtain a first leachate containing vanadate ions;

4. The method of any one of claim 1 , further comprising recovering the precipitated alkaline earth metal vanadate nanomaterial from the mixture.

5. The method of claim 1 , wherein the precipitating step comprises stirring the mixture at room temperature.

6. The method of claim 1 , wherein the precipitating step comprises heating the mixture to a temperature of more than 100 ^°C.

7. The method of claim 6, wherein the heating step is performed for a time period of from 2 hours to 40 hours.

8. The method of claim 6, wherein the heating step comprises a hydrothermal heating step at a temperature ranging from 120^°C to 250^°C.

9. The method of claim 1 , wherein the alkaline earth metal salt comprises at least one of a nitrate salt, a hydroxide salt, a chloride salt, an acetate salt or a carbonate salt.

10. The method of claim 1 , wherein the alkaline earth metal salt comprises at least one of a calcium salt, a strontium salt or a barium salt.

1 1 . The method of claim 1 , wherein the amount of alkaline earth metal salt added to the leaching solution is in the range of (0.1 - 20.0) w/v%.

12. The method of claim 1 , wherein the vanadium source is an oil refinery waste.

13. The method of claim 1 , wherein the vanadium source is a carbon black waste.

14. The method of claim 1 , wherein the alkaline earth vanadate nanomaterial has an energy band gap in the range of from 3.7 eV to 4.5 eV.

15. A nanomaterial derived from a vanadium source, the nanomaterial comprising alkaline earth metal vanadate nanostructures.

16. The nanomaterial of claim 1 5, wherein the alkaline earth metal vanadate nanostructures comprise precipitates from a mixture formed by adding an alkaline earth metal salt to a leaching solution containing vanadate ions, the leaching solution being derived from the vanadium source.

17. The nanomaterial of claim 15, wherein the alkaline earth metal vanadate nanostructures comprise nanostructures selected from the group consisting of calcium vanadate nanostructures, strontium vanadate nanostructures, barium vanadate nanostructures and combinations thereof.

18. The nanomaterial of claim 17, wherein:

the calcium vanadate nanostructures comprise at least one of the following properties:

a) has a chemical formula of CaioV6O2s;

c) has an X-ray diffraction pattern as shown in FIG. 9A,

a) is a hexagonal phase of SrioV602s;

c) has an X-ray diffraction pattern as shown in FIG. 9B, and

the barium vanadate nanostructures comprise at least one of the following properties:

a) is barium orthovanadate Ba3V20s;

c) has an X-ray diffraction pattern as shown in FIG. 9C.

19. The nanomaterial of claim 15, wherein the alkaline earth metal vanadate nanostructures have an energy band gap in the range of from 3.7 eV to 4.5 eV.

20. An electronic device comprising the nanomaterial of claim 15.

21 . The electronic device of claim 15, wherein the electronic device is a semiconductor device.