A method for synthesizing a lepidocrocite titania (TIO2) nanofilamentous product

Abstract

The disclosure relates to a method for synthesizing a lepidocrocite TiO2 nanofilamentous product, comprising contacting titanium oxysulphate or a titanium sulphate or a titanium phosphate or a titanium chloride or titanium perchlorate with a quaternary ammonium salt and / or base, heating to a temperature of from 25 to 95 °C and stirring for 5 from about 1 hour to about one week.

Description

[0001]A method for synthesizing a lepidocrocite titania (TiO2) nanofilamentous productTechnical field The present disclosure relates to a method for synthesizing a lepidocrocite TiO2 nanofilamentous product. More specifically, the disclosure relates to a method for synthesizing a lepidocrocite TiO2 nanofilamentous product as defined in the introductory parts of the independent claims. Background art Nanometric titanium dioxide (TiO2), also denoted titania, can be synthesized in variousforms, including one-dimensional (1D) nanofilament structures such as nanotubes or nanowires. One-dimensional TiO2 structures have unique properties and find applications in various fields, including photocatalysis, sensors, and energy storage. Making such nanostructures is typically slow and / or hazardous, which renders their bulk productionchallenging and expensive. They are also not truly one-dimensional, 1D, in the quantummechanical sense that the reduced dimensions lead to increased band gap energies, Eg. Allprevious 1D titanias reported were at best quasi-1D since their Egs were closer the Egs of bulktitania. One-dimensional (1D) and two-dimensional (2D) materials possess characteristics andproperties that their three-dimensional (3D) counterparts do not. Arguably the most important difference is in their much higher surface areas. In terms of properties, low- dimensional solids allow for quantum confinement alluded to above, and more active catalytic sites. Nanofilaments are thin fibres with thickness or diameter in the nanometre (10−9metres) range, typically 1 to 100 nanometres. Examples of nanofilaments are nanowires, nano threads, and nanotubes. Commonly used precursors for synthesizing TiO2 nanofilaments are e.g., Ti-carbides,-borides, -nitrides, -phosphides, and -silicides. In the synthesis of nanoparticles, top-down approach means reducing the size of the structure toward the nanoscale and involves mechanical methods to crush / breaking of bulk into several parts to form nanoparticles. The bottom-up approach is the formation of large nanostructure from smaller atoms and molecules by means of chemical reactions among the atoms / ions / molecules. WO 2022 / 1742664 A1 discloses a bottom-up, scalable synthesis of oxide-based sub-nano and nanofilaments and nanofilament-based two-dimensional flakes and mesoporouspowders. Binary and ternary titanium carbides, nitrides, borides, phosphides, and silicides are converted into 2D flakes. Titanium oxysulphate (TiOSO4), also denoted titanyl sulphate, is a precursor used in thesynthesis of nanosized TiO2, for example by aqueous sol-gel methods, microwave-assisted andhydrothermal methods. Aproblem with the solutions of the prior art is that high temperatures must be used,and these processes are neither cost-effective nor energy efficient. There is thus a need for improved methods for synthesizing lepidocrocite TiO2. Summary It is an object of the present disclosure to mitigate, alleviate or eliminate one or more of the above-identified deficiencies and disadvantages in the prior art and solve at least theabove-mentioned problem.It is thus an object of the present invention to provide an improved bottom-up synthesis process. It is a further object of the present invention to provide a simplified and cost-effective synthesis process. According to a first aspect there is provided a method for synthesizing a lepidocrocite TiO2 nanofilamentous product, comprising contacting titanium oxysulphate or a titaniumsulphate or a titanium phosphate or a titanium chloride or titanium perchlorate with aquaternary ammonium salt and / or base, heating to a temperature of from 25 to 95 °C and stirring for from about 1 hour to about one week. According to some embodiments, the conditions comprise heating to a temperature offrom 50 to 80 °C and stirring for from about 10 hours to three days.According to some embodiments, the quaternary ammonium salt and / or base comprises an ammonium hydroxide. According to some embodiments, the quaternary ammonium hydroxide comprises tetramethylammonium hydroxide (TMAOH), tetraethylammonium hydroxide (TEAOH), tetrapropylammonium hydroxide (TPAOH), tetrabutylammonium hydroxide (TBAOH), ammonium hydroxide (NH4OH), their amine derivatives, or any combination thereof. According to some embodiments, titanium oxysulphate or a titanium sulphate or atitanium phosphate or a titanium chloride or titanium perchlorate is mixed withtetramethylammonium hydroxide (TMAH) in a molar ratio of Ti:tetramethylammonium of 0.2 to 0.6. According to some embodiments, titanium oxysulphate or a titanium sulphate or atitanium phosphate or a titanium chloride or titanium perchlorate is mixed withtetramethylammonium hydroxide (TMAH) to a 5-25 wt % aqueous solution.According to some embodiments, the method comprises washing with a C1-C3alcohol until pH is about 6-8. According to some embodiments, the method comprises washing with ethanol untilthe pH is about 7.According to some embodiments, the method comprises drying to obtain three-dimensional (3D) TiO2 mesoparticles comprising one-dimensional (1D) lepidocrocite TiO2.According to some embodiments, the drying is air drying, freeze drying, or vacuum drying. According to some embodiments, the method comprises dispersing in deionized waterto obtain 1D lepidocrocite TiO2.According to some embodiments, the method comprises vacuum filtering to obtaintwo-dimensional (2D) TiO2 flakes.According to some embodiments, the titanium oxysulphate or the titanium sulphate isselected from titanium (IV) oxysulphate (TiOSO4), titanium (II) sulphate (TiSO4), titanium (III)sulphate (Ti2(SO4)3), and titanium (IV) sulphate (Ti(SO4)2). According to some embodiments, the method is performed in a bottom-up manner. The present disclosure will become apparent from the detailed description given below. The detailed description and specific examples disclose preferred embodiments of the disclosure by way of illustration only. Those skilled in the art understand from guidance in the detailed description that changes and modifications may be made within the scope of the disclosure. Hence, it is to be understood that the herein disclosed disclosure is not limited to the particular component parts of the device described or steps of the methods described since such device and method may vary. It is also to be understood that the terminology usedherein is for the purpose of describing particular embodiments only and is not intended to belimiting. It should be noted that, as used in the specification and the appended claim, the articles "a", "an", "the", and "said" are intended to mean that there are one or more of the elements unless the context explicitly dictates otherwise. Thus, for example, reference to "a unit" or "the unit" may include several devices, and the like. Furthermore, the words"comprising", "including", "containing" and similar wordings do not exclude other elements orsteps. The term TiO2 as used in the present application (in the form of 1D or 2D filaments, 2Dflakes, and 3D mesoporous particles) cover both pure TiO2, titanates, where the O:Ti ratio is >2, and TiO2 that contains defects in the form of both Ti- and / or O-vacancies.The terms pseudo two-dimensional or quasi two-dimensional sheets or flakes meanthat the structure looks two-dimensional, but the flakes / sheets are composed of 1D or 2Dlepidocrocite TiO2. Brief of the The above objects, as well as additional objects, features, and advantages of thepresent disclosure, will be more fully appreciated by reference to the following illustrative and non-limiting detailed description of example embodiments of the present disclosure, when taken in conjunction with the accompanying drawings. Figure 1 shows a block diagram of the synthetic process of 1D lepidocrocite titania andits self-assembly into 2D flakes and 3D mesoporous particles according to an embodiment ofthe present disclosure.Figures 2(a)-2(b) show (a) X-ray diffraction (XRD) pattern and (b) Raman spectrum of2D film consisting of 1D lepidocrocite TiO2.Figure 3 shows XRD pattern of mesoparticles consisting of 1D lepidocrocite TiO2.Figures 4(a)-4(b) show cross-sectional scanning electron microscope (SEM) image of 2Dfilm consisting of 1D lepidocrocite TiO2.Figures 5(a)-5(b) show SEM image of 3D mesoparticles consisting of 1D lepidocrociteTiO2. Detailed description The present invention relates to the field of 1D and 2D materials and to the field ofmetal oxide-based nanomaterials. The present invention more specifically relates to theconversion of titanium oxysulphate or a titanium sulphate or a titanium phosphate or atitanium chloride or titanium perchlorate to 1D Lepidocrocite titania (TiO2) via a bottom-upsolution processing method. The present disclosure will now be described with reference to the accompanying drawings, in which preferred example embodiments of the disclosure are shown. The disclosure may, however, be embodied in other forms and should not be construed as limited to the herein disclosed embodiments. The disclosed embodiments are provided to fully convey the scope of the disclosure to the skilled person. The first aspect of this disclosure shows a method for synthesizing a lepidocrocite TiO2 nanofilamentous product, comprising contacting titanium oxysulphate or a titanium sulphate or a titanium phosphate or a titanium chloride or titanium perchlorate with a quaternary ammonium salt and / or base, heating to a temperature of from 25 to 95 °C and stirring for from about 1 hour to about one week. The heating may be to a temperature from about 40 °C to about 90 °C, or from about40 °C to about 80 °C, or from about 50 °C to about 80 °C.The stirring may be from about 10 hours to about one week, or from about 10 hours toabout five days, or from about 10 hours to about three days, or from about one day to aboutsix days, or from about one day to about three days, or from about two days to about fivedays, or from about two days to about four days. The titanium oxysulphate or the titanium sulphate is selected from titanium (IV) oxysulphate (TiOSO4), titanium (II) sulphate (TiSO4), titanium (III) sulphate (Ti2(SO4)3), and titanium (IV) sulphate (Ti(SO4)2). The titanium phosphate is titanium (IV) phosphate (TiPO4). The titanium chloride is selected from titanium (III) chloride (TiCl3), titanium (IV) chloride (TiCl4). The titanium perchlorate is titanium (IV) perchlorate Ti(ClO4)4. The quaternary ammonium salt and / or base comprises an ammonium hydroxide. The quaternary ammonium hydroxide comprises tetramethylammonium hydroxide(TMAOH), tetraethylammonium hydroxide (TEAOH), tetrapropylammonium hydroxide (TPAOH), tetrabutylammonium hydroxide (TBAOH), ammonium hydroxide (NH4OH), their amine derivatives, or any combination thereof. In an embodiment of the method according to the invention, titanium oxysulphate or atitanium sulphate or a titanium phosphate or a titanium chloride or titanium perchlorate ismixed with tetramethylammonium hydroxide (TMAH) in a molar ratio of Ti:tetramethylammonium (TMA) of 0.2 to 0.6. In an embodiment of the method according to the invention, titanium oxysulphate or atitanium sulphate or a titanium phosphate or a titanium chloride or titanium perchlorate ismixed with tetramethylammonium hydroxide (TMAH) to a 5-25 wt % aqueous solution. In anembodiment, the TMAH concentration might be 10-25 wt %.In an embodiment of the method according to the invention, TiOSO4 is mixed withtetramethylammonium hydroxide (TMAH, chemical formula N(CH3)4OH) in a molar ratio ofTi:tetramethylammonium (TMA, chemical formula N(CH3)4+) of 0.2 to 0.6 and heated andstirred for 1 h to five days at a temperature of 25-90 °C. The reaction is an exothermic reactionand based on acid-base reaction. TiOSO4 is an inexpensive starting material. The reaction mechanism of TiOSO4 in TMAH is different from other Ti-precursors (e.g., Ti-carbides, borides, nitrides, phosphides, and silicides). Hence, selecting the precursor is an unconventional move and not the apparent nextcourse of action. From prior art, it would be expected that the Ti-precursor should be waterinsoluble, and that the oxidation state of the Ti should need to be less than IV. The inventorssurprisingly found that TiOSO4 works even though it is water soluble, and the Ti oxidation stateis IV. Further, TiOSO4 does not provide a solid precursor / reaction picture interface, which from prior art was thought to be important for the formation of 1D. The titanium sulphate, titanium phosphate, titanium chloride, and titaniumperchlorate react with an ammonium hydroxide in the same manner as described for TiOSO4and provide the same unexpected results as with TiOSO4.Higher temperature and higher concentration and lower molar ratios will provide faster conversion. The method further comprises washing with a C1-C3 alcohol until pH is about 6-8. The C1-C3 alcohol can be methanol, ethanol, 1-propanol, or 2-propanol. The washing can be performed with any of these alcohols, alone or in combination. In an embodiment, the pH is adjusted to about 7, by washing with any of the above-listed alcohols, or any of their combinations. Preferably, the method comprises washing with ethanol until pH is about 7. Theresultant product can be dried to obtain three-dimensional (3D) TiO2 mesoparticles comprising1D lepidocrocite TiO2. The 3D TiO2 particles are free flowing, i.e., the particles do not sticktogether. The drying may for example be air drying, freeze drying, or vacuum drying.Instead of drying and obtaining 3D TiO2 mesoparticles comprising 1D lepidocrociteTiO2, the resultant product after the washing with a C1-C3 alcohol may be dispersed indeionized (DI) water to obtain a 1D lepidocrocite TiO2 colloid. Said colloid can be vacuumfiltered to obtain quasi-two-dimensional 2D TiO2 flakes.The method according to the invention is performed in a bottom-up manner. At least some of the 1D lepidocrocite TiO2 nanofilaments produced by the methodaccording to the invention have a width in the range of from about 3 to about 50 Å. At leastsome of the nanofilaments have an average width in the range of from about 7 to about 20 Å.The 1D lepidocrocite TiO2nanofilaments produced by the method according to theinvention define a non-circular cross-section. The nanofilaments define a cross-sectionalaspect ratio of from greater than 1 to about 10, e.g., from about 2 to about 5. The 1D lepidocrocite TiO2 nanofilaments produced by the method according to theinvention have an average cross-sectional area in the range of from about 10 to about 100 Å2. At least some of the 1D lepidocrocite TiO2 nanofilaments produced by the methodaccording to the invention have a length in the range of from 1 nm to about 25 µm. At leastsome of the nanofilaments have a length in the range of from 1 nm to about 1 µm. Fig.1 shows a block diagram of the synthesis process of 1D lepidocrocite titania in anembodiment according to the invention. In the first step, TiOSO4 is mixed with TMAH 25 wt %in water at 80 °C for 48 hours. The solution is washed with ethanol until the pH is around 7,and the resultant product is either air dried, resulting in 3D TiO2 mesoparticles, or dispersed in deionized (DI) water, resulting in 1D lepidocrocite TiO2 colloid. The 1D lepidocrocite TiO2colloid is vacuum filtered and thereby converted to TiO2 pseudo 2D sheets.The simple solution processing method according to the invention yields a 1Dlepidocrocite structure of titania from a titanium oxysulphate or a titanium sulphate or atitanium phosphate or a titanium chloride or titanium perchlorate. The resultant product shows various morphologies through the self-assembly of the nanofilaments, as shown in theblock diagram in Fig. 1. At first, the resultant forms nearly spherical mesoporous particles (3D)comprised of 1D nanofilaments after several washing with ethanol. Upon vacuum filtration ofthe colloidal solution, 1D lepidocrocite TiO2 forms 2D flakes.The synthesis of 1D lepidocrocite TiO2 from a titanium oxysulphate or a titaniumsulphate or a titanium phosphate or a titanium chloride or titanium perchlorate is cheaper,more facile, and more scalable compared to the prior art syntheses. With the synthesis of 1Dlepidocrocite TiO2 from TiOSO4, it was found that the conversion was complete (100 %) forTi:TMA = 0.2 molar ratio in just 48 h at 80 °C in 25 wt % TMAH solution, and the end productwas of high quality (no precursor state was found). With the method according to the invention, it was produced short (≈ 100 nm) 1Dlepidocrocite TiO2nanofilaments (NFs), with minimal cross-sections of ≈ 5x7 Å2. The resulting1D lepidocrocite TiO2 NFs self-assemble in a plethora of nanostructures, such as two-dimensional flakes, nano bundles, or mesoporous particles, when dispersed in different solvents. The NFs self-assemble into nano bundles that, in turn, self-assemble intoribbons / fibres microns long. The ribbons / fibres self-assemble into 2D flakes, which are moreor less amorphous. The mesoparticles are free-flowing and more or less spherical with diameters in the ≈ 5to 10 μm range. The morphologies of the mesoparticles are quite unique and can be bestdescribed as spheres comprised of entangled 1D lepidocrocite titania NFs. The latter self-assemble into bundles at the sub- and micrometre scale that then entangle to form porousspheres. The 1D lepidocrocite TiO2 obtained by the method according to the present inventionwas analysed by XRD (Panalytical; model: X’pert, Netherlands, the scan was conducted at 2θvalues ranging from 5 to 80°, with a step size of 0.02° and a scan rate of 0.5 s / step. Theradiation used was CuKα (40 kV, 40 mA)), Raman spectroscopy (integrated with an invertedmicroscope (Nikon Ti-E) with a 60× air objective (Nikon, NA = 0.7). A 532 nm DPSS laser (λ-beam, RGB Photonics) was used for excitation with the output power set to 5 mW. Aspectrograph (Andor Kymera 328i) with a 600 l mm–1 diffraction grating (blazed at 500 nm)was used for analysis. The signal was collected using a EMCCD camera (Andor NewtonDU970P-BVF)), and SEM (Zeiss; Model: Sigma 300, Germany at varying magnifications).Figure 2(a)-2(b) shows (a) XRD pattern and (b) Raman spectrum of the 2D filmconsisting of 1D lepidocrocite TiO2. The XRD pattern in Fig. 2(a) reveals peaks at 6.7 and15.1 °θ. The Raman spectrum in Fig. 2(b) reveals distinct bands of lepidocrocite, confirmingthe structure is lepidocrocite not anatase. Figure 3 shows the XRD pattern of the mesoparticles consisting of 1D lepidocrocite TiO2(TiO2 Mpps). The peaks indicate that the (010) diffraction peak at the lower 2θ° angle (9.4°)and its higher 0k0 reflections (indicated by asterisks) represent the spacing between NFs along the b-direction. The fundamental peaks for lepidocrocite structure are at 2θ values of 48.6° and 62.5°, indexed as 200 and 002, respectively. Figure 4(a)-4(b) shows cross-sectional SEM image of 2D film consisting of 1Dlepidocrocite TiO2. The cross-sectional image shows the stacked structure of layers consistingof 1D lepidocrocite titania nanofilaments. Figure 5(a)-5(b) shows SEM image 3D mesoparticles consisting of 1D lepidocrocite TiO2.The micrographs show the structure is primarily spherical, with average diameters of 5-30 µm, mesoporous, and comprised of 1D NF bundles. The resulting nanostructures find applications in energy storage (electrodes for supercapacitors and batteries), photocatalysis, dye degradation, and water purification (antibiotic absorption). Example 1 - TiOSO4 to Lepidocrocite TiO2Titanium (IV) oxysulphate (TiOSO4) was mixed in molar ratios Ti:TMA = 0.2 to 0.6 andwas reacted with 10-25 wt % tetramethyl ammonium hydroxide (TMAH) aqueous solution at60-80 °C for 2 to 5 days under ambient pressures as shown in Table 1 and produced short (≈ 100 nm) 1D lepidocrocite TiO2 nanofilaments (NFs), with minimal cross-sections of ≈ 5x7 Å2.The resulting 1D lepidocrocite TiO2 NFs self-assembled in a plethora of nanostructures, such asquasi-two-dimensional flakes, nano bundles, or mesoporous particles, when dispersed indifferent solvents. The NFs self-assembled into nano bundles that, in turn, self-assembled intoribbons / fibres microns long. The latter self-assembled into 2D flakes, which were more or lessamorphous. From the results of the X-ray diffraction, Raman spectroscopy, and scanning electronmicroscope (SEM) micrographs as shown in Figures 2-5, it was concluded that regardless ofthe experimental conditions or final morphologies, the 1D lepidocrocite TiO2 NFs are theessential building blocks in all of the tested microstructures. Table 1: Different conditions for conversion of TiOSO4to 1D lepidocrocite TiO2. Molar ratioTemperature (°C) Time (days) TMAH(Ti:TMA) Concentration (wt%) 0.2 80 2 250.3 60 3 200.5 70 4 150.2 80 5 100.6 80 5 25Having described preferred embodiments of the invention it will be apparent to those skilled in the art that other embodiments incorporating the concepts may be used. These and other examples of the invention illustrated above are intended by way of example only and the actual scope of the invention is to be determined from the following claims.

Claims

CLAIMS 1. A method for synthesizing a lepidocrocite TiO2 nanofilamentous product, comprisingcontacting titanium oxysulphate or a titanium sulphate or a titanium phosphate or a titaniumchloride or titanium perchlorate with a quaternary ammonium salt and / or base, heating to atemperature of from 25 to 95 °C and stirring for from about 1 hour to about one week.

2. The method of claim 1, wherein the conditions comprise heating to a temperature offrom 50 to 80 °C and stirring for from about 10 hours to about three days.

3. The method of claim 1 or claim 2, wherein the quaternary ammonium salt and / or base comprises an ammonium hydroxide.

4. The method of claim 3, wherein the quaternary ammonium hydroxide comprises tetramethylammonium hydroxide (TMAOH), tetraethylammonium hydroxide (TEAOH), tetrapropylammonium hydroxide (TPAOH), tetrabutylammonium hydroxide (TBAOH), ammonium hydroxide (NH4OH), their amine derivatives, or any combination thereof.

5. The method of any one of the claims 1-4, wherein titanium oxysulphate or a titaniumsulphate or a titanium phosphate or a titanium chloride or titanium perchlorate is mixed withtetramethylammonium hydroxide (TMAH) in a molar ratio of Ti:tetramethylammonium (TMA)of 0.2 to 0.

6.

6. The method of any one of the claims 1-5, wherein titanium oxysulphate or a titaniumsulphate or a titanium phosphate or a titanium chloride or titanium perchlorate is mixed withtetramethylammonium hydroxide (TMAH) to a 5-25 wt % aqueous solution.

7. The method of any one of the claims 1-6, further comprising washing with a C1-C3alcohol until pH is about 6-8.

8. The method of claim 7, comprising washing with ethanol until the pH is about 7.

9. The method of any one of claims 7-8, further comprising drying to obtain three-dimensional (3D) TiO2 mesoparticles comprising one-dimensional (1D) lepidocrocite TiO2.

10. The method of claim 9, wherein the drying is air drying, freeze drying, or vacuum drying.

11. The method of any one of claims 7-8, comprising dispersing in deionized (DI) water to obtain 1D lepidocrocite TiO2.

12. The method of claim 11, comprising vacuum filtering to obtain two-dimensional (2D) TiO2 flakes.

13. The method of any one of claims 1-12, wherein the titanium oxysulphate or the titanium sulphate is selected from titanium (IV) oxysulphate (TiOSO4), titanium (II) sulphate (TiSO4), titanium (III) sulphate (Ti2(SO4)3), and titanium (IV) sulphate (Ti(SO4)2).

14. The method of any one of the claims 1-13, wherein the method is performed in a bottom-up manner.

Images