AN AMORPHOUS TITANIUM DIOXIDE PRECURSOR MATERIAL, METHOD OF PRODUCING THEREOF AND METHOD OF CONTROLLING CRYSTALLINE PHASES THEREOF

Abstract

Titanium dioxide (TiO2) forms the basis of devices for applications including sensing devices, solar cells, photo-electrochromics, and photocatalysis. Such devices exploit different phases of TiO2 within such devices and accordingly it would be beneficial to have an amorphous TiO2 precursor which allows crystalline phase spatial patterning, for the crystallization of the amorphous TiO2 precursor to be triggered at low energies, and with the crystalline phase controllable at room-temperature without necessitating complex handling whilst providing TiO2 phases that ate stable over a prolonged period of time. Accordingly, there ate provided processes for providing a TiO2 precursor and controlling the conversion of the TiO2 precursor from amorphous-to-anatase, amorphous-to-rutile, amorphous-to-mixture of anatase/rutile or from amorphous-to-anatase-to-rutile in a simple and efficient manner.

Claims

1. A method of converting amorphous titanium dioxide nanoparticles comprising: establishing a precursor of amorphous titanium dioxide (TiO2) nanoparticles; depositing the precursor onto a substrate to form a film of amorphous TiO2 nanoparticles; and selectively exposing one or more regions of the film of amorphous TiO2 nanoparticles to controlled optical illumination at a predetermined optical wavelength and optical power density, wherein for each region the amorphous TiO2 panicles are convened to either an anatase TiO2 phase, a mixed anatase-rutile TiO2 phase and a rutile TiO2 phase in dependence upon the optical power density employed within that region.

2. The method according to claim 1, wherein the precursor of amorphous titanium dioxide (TiO2) nanoparticles is formed using an incomplete nonhydrolytic sol-gel ester elimination reaction.

3. The method according to any one of claims 1 to 2, wherein the precursor of amorphous titanium dioxide (TiO2) nanoparticles is formed using an incomplete nonhydrolytic sol-gel ester elimination reaction exploiting at least of titanium (IV) isopropoxide and oleic acid.

4. The method according to any one of claims 1 to 3, wherein the amorphous titanium dioxide (TiO2) nanoparticles are undoped.

5. The method according to any one of claims 1 to 4, wherein the predetermined optical wavelength is 532 nm.

6. The method according to any one of claims 1 to 4, wherein the predetermined optical wavelength is within the range of selected energy that are sufficient to trigger the amorphous TiO2 to anatase TiO2 transformation and carrier excitation within the anatase TiO2 phase from the valence band to the carrier band in order to support the subsequent oxygen scavenging process for forming the intermediate TiO2 phase and subsequently the rutile TiO2 phase.

7. The method according to any one of claims 1 to 6, wherein establishing the precursor of amorphous TiO2 nanoparticles does not require at least one of high temperature processing and high pressure processing.

8. The method according to any one of claims 1 to 2, wherein illumination between a first minimum optical power density and a first maximum optical power density converts the film of amorphous TiO2 particles to crystalline anatase TiO2; illumination above a second minimum optical power density converts the film of amorphous TiO2 particles to crystalline rutile TiO2; and illumination above the first maximum optical power density but below the second minimum optical power density converts the film of amorphous TiO2 particles to a mixture of crystalline anaphase TiO2 and crystalline rutile TiO2.

9. A method comprising: establishing a precursor of amorphous titanium dioxide (TiO2) nanoparticles; depositing the precursor onto a substrate to form a film of amorphous TiO2 nanoparticles; and selectively exposing one or more regions of the film of amorphous TiO2 nanoparticles to a controlled electron beam illumination at a predetermined energy and power density, wherein for each exposed region the electron beam illumination triggers nucleation and crystallization of the amorphous TiO2 particles.

10. The method according to claim 9, wherein the irradiation voltage of the electron beam is established in dependence upon the thickness of the film of amorphous TiO2 nanoparticles.

11. A method of producing an amorphous titanium dioxide nanoparticle precursor material.

12. The method according to claim 11, wherein the amorphous titanium dioxide nanoparticle precursor material is produced using an incomplete nonhydrolytic sol-gel ester elimination reaction.

13. The method according to claim 12, wherein the sol-gel ester elimination reaction employs titanium (IV) isopropoxide and oleic acid.

14. The method according to claim 13, wherein the sol-gel ester elimination reaction comprises: mixing the titanium (IV) isopropoxide and oleic acid under a predetermined gas at a predetermined first temperature; raising the temperature to a predetermined second temperature for a predetermined duration; and terminating the reaction by reducing the temperature to a predetermined third temperature; adding a predetermined chemical; and recuperating the amorphous titanium dioxide nanoparticles through a predetermined process.

15. The method according to claim 14, further comprising at least one of: storing the amorphous titanium dioxide nanoparticles as a powder for subsequent use; and re-dispersing the amorphous titanium dioxide nanoparticles within a solvent.

16. The method according to any one of claims 11 to 15, wherein the amorphous titanium dioxide (TiO2) nanoparticle precursor material supports conversion to is formed by an incomplete nonhydrolytic sol-gel ester elimination reaction.

17. The method according to any one of claims 11 to 16, wherein the amorphous TiO2 nanoparticle precursor material can be converted to a TiO2 film which is either an anatase TiO2 phase, a mixed anatase-rutile TiO2 phase or a rutile TiO2 phase in dependence upon an optical power density of an optical illumination within a predetermined wavelength range applied to the amorphous TiO2 nanoparticle precursor material.

18. The method according to any of claims 11 to 17, wherein the amorphous TiO2 nanoparticle precursor material can be converted to a TiO2 film comprising two or more regions in a single process step; wherein each region is either an anatase TiO2 phase, a mixed anatase-rutile TiO2 phase or a rutile TiO2 phase; and the phase is established in dependence upon an optical power density of an optical illumination within a predetermined wavelength range applied to the amorphous TiO2 nanoparticle precursor material.

19. An amorphous titanium dioxide nanoparticle precursor material.

20. The method according to claim 19, wherein the amorphous titanium dioxide (TiO2) nanoparticle precursor material can be convened to a TiO2 film which is either an anatase TiO2 phase, a mixed anatase-rutile TiO2 phase or a rutile TiO2 phase in dependence upon an optical power density of an optical illumination within a predetermined wavelength range applied to the amorphous TiO2 nanoparticle precursor material.

21. The method according to claim 20, wherein the amorphous TiO2 nanoparticle precursor material can be converted to a TiO2 film comprising two or more regions in a single process step; wherein each region is either an anatase TiO2 phase, a mixed anatase-rutile TiO2 phase or a rutile TiO2 phase; and the phase is established in dependence upon an optical power density of an optical illumination within a predetermined wavelength range applied to the amorphous TiO2 nanoparticle precursor material.

22. The method according to claim 20, wherein the regions are adjacent to one another.

23. A product having been manufactured with a converted amorphous titanium dioxide nanoparticle according to the method of claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein:

[0032] FIG. 1A depicts a TEM image of amorphous TiO2 nanoparticles as synthetized using a sol-gel process according to an embodiment;

[0033] FIG. 1B depicts an AFM image of a thin film of amorphous TiO2 nanoparticles produced using a sol-gel process according to an embodiment with an average nanoparticle diameter around 10 nm;

[0034] FIG. 1C depicts an SEM image of the amorphous TiO2 films after focusing under SEM;

[0035] FIG. 1D depicts the Raman spectroscopy of the amorphous film of TiO2 prior to conversion wherein the absence of any defined crystalline structure yields no peaks in this analysis as expected for an amorphous phase of TiO2.

[0036] FIG. 2A depicts a schematic of the experimental setup used for laser-induced photo-activation of the TiO2 structure according to an embodiment,

[0037] FIG. 2B depicts a laser scanning microscope image of the amorphous TiO2 films after crystalline phase alteration via a process according to an embodiment wherein the patterned letters ÉTS consist of anatase-phase crystalline TiO2, while the underline consists of rutile-phase crystalline TiO2, and the darker background consists of amorphous TiO2

[0038] FIG. 2C depicts Raman micro-spectroscopy measurements of the anatase crystalline phase of the TiO2 formed using an embodiment;

[0039] FIG. 2D depicts Raman micro-spectroscopy measurements of the rutile crystalline phase of the TiO2 formed using an embodiment,

[0040] FIG. 3 depicts Raman spectra of the various TiO2 crystalline structures obtained using different excitation laser power densities with a process according to an embodiment;

[0041] FIG. 4 depicts the evolution of crystalline structure as function of time for the transition from amorphous to pure anatase, mixed anatase/rutile and pure rutile at different power densities respectively taken over 1 minute together with Raman peak intensity evolution for the dominant Raman peak recorded over 1 hour for the conversion to anatase, mixed anatase/rutile and rutile respectively generated with a process according to an embodiment;

[0042] FIG. 5A depicts a schematic of laser printing a checkered pattern onto an amorphous TiO2 film to pattern anatase TiO2 and rutile TiO2 regions adjacent to one another;

[0043] FIG. 5B depicts a laser scanning microscope image of a checkered pattern over amorphous TiO2 films wherein anatase TiO2 is generated in squares 1 and 2 and rutile TiO2 is generated in the squares 3 and 4 with a process according to an embodiment;

[0044] FIGS. 5C and 5D depict different false coloured maps for a micro-Raman image of the pattern depicted in FIGS. 5A and 5B; and

[0045] FIG. 5E depicts a topographic 3D surface reconstruction for the checkered pattern depicted in FIGS. 5A and 5B;

[0046] FIG. 6 depicts a proposed Jablonski diagram for the laser-induced phase transition from anatase to rutile TiO2 according to an embodiment;

[0047] FIG. 7 depicts via a schematic the sequential conversion process from amorphous TiO2 to anatase phase, mixed anatase/rutile phase, and final rutile phase using a sol-gel nanoparticle film and photonic excitation method according to an embodiment; and

[0048] FIG. 8 depicts unit cells for the anatase TiO2 and rutile TiO2 respectively.

DETAILED DESCRIPTION

[0049] The present specification is directed to titanium dioxide (TiO.sub.2, TiO2) and more particularly to the provisioning of a chemically stable amorphous TiO2 precursor, processes for producing the TiO2 precursor, and processes for forming from the TiO2 precursor a thin film of titanium oxide having regions with one or more crystallized phases teach phase being a monocrystalline phase). Forming the thin film of TiO2 with one or more crystallized phases being achievable at room temperature under ambient atmospheric conditions by controllably defining at least one stable converted crystallization phase at a desired region of the film. The crystallization phases being one or more crystalline structures of titanium dioxide which cart be formed next to one another within a single manufacturing step.

[0050] The ensuing description provides representative embodiment(s) only, and is not intended to limit the scope, applicability or configuration of the disclosure. Rather, the ensuing description of the embodiment(s) will provide those skilled in the art with an enabling description for implementing an embodiment or embodiments of the invention. It being understood that various changes can be made in the function and arrangement of elements without departing from the spirit and scope as set forth in the appended claims. Accordingly, an embodiment is an example or implementation of the inventions and not the sole implementation. Various appearances of “one embodiment,” “an embodiment” or “some embodiments” do not necessarily all refer to the same embodiments. Although various features of the invention may be described in the context of a single embodiment, the features may also be provided separately or in any suitable combination. Conversely, although the invention may be described herein in the contest of separate embodiments for clarity, the invention can also be implemented in a single embodiment or any combination of embodiments.

[0051] Reference in the specification to “one embodiment”, “an embodiment”, “some embodiments” or “other embodiments” means that a particular feature, structure, or characteristic described in connection with the embodiments is included in at least one embodiment, but not necessarily all embodiments, of the inventions. The phraseology and terminology employed herein is not to be construed as limiting but is for descriptive purpose only. It is to be understood that where the claims or specification refer to “a” or “an” element, such reference is not to be construed as there being only one of that element. It is to be understood that where the specification states that a component feature, structure, or characteristic “may”, “might”, “can” or “could” be included, that particular component, feature, structure, or characteristic is not required to be included.

[0052] Reference to terms such as “left”, “right”, “top”, “bottom”, “front” and “back” are intended for use in respect to the orientation of the particular feature, structure, or element within the figures depicting embodiments of the invention. It would be evident that such directional terminology with respect to the actual use of a device has no specific meaning as the device can be employed in a multiplicity of orientations by the user or users.

[0053] Reference to terms “including”, “comprising”, “consisting” and grammatical variants thereof do not preclude the addition of one or more components, features, steps, integers or groups thereof and that the terms are not to be construed as specifying components, features, steps or integers. Likewise, the phrase “consisting essentially of”, and grammatical variants thereof, when used herein is not to be construed as excluding additional components, steps, features integers or groups thereof but rather that the additional features, integers, steps, components or groups thereof do not materially alter the basic and novel characteristics of the claimed composition, device or method. If the specification or claims refer to “an additional” element, that does not preclude there being more than one of the additional element.

[0054] A “sol-gel” as used herein may refer to, but is not limited to, a process for producing a solid material from one or more starting materials wherein a solution (“sol”) evolves towards the formation of a gel-like diphasic system comprising both a liquid phase and a solid phase which is then, typically, processed to yield only the solid phase. The precursor “sol” may be deposited onto a substrate to form a film (e.g., by dip coating or spin coating), cast into a suitable container with the desired shape (e.g., to obtain monolithic ceramics, glasses, fibers, membranes, aerogels), or used to synthesize powders (e.g., microspheres, nanospheres). The sol-gel approach allows for fine control of the product's chemical composition as even small quantities of dopants, such as organic dyes and rare-earth elements, can be introduced in the sol and end up uniformly dispersed in the final product. “Sol-gel” processing may be employed for, but not limited to, ceramic materials, glassy materials, metal oxides and hybrid materials.

[0055] “Titanium dioxide” as used herein may refer to, but is not limited to, the oxide of titanium having the chemical formal TiO.sub.2 and is also known as titanium(IV) oxide and titania. Titanium dioxide having a variety of forms including the naturally occurring minerals rutile, anatase and brookite, and additionally two high pressure forms, a monoclinic baddeleyite-like form and an orthorhombic α-PbO2-like form. Titanium dioxide has eight modifications in addition to rutile, anatase, and brookite, comprising three metastable phases which can be produced synthetically, namely monoclinic [TiO.sub.2(B)], tetragonal [TiO.sub.2(H)] and orthorhombic [TiO.sub.2(R)], and five high-pressure forms, namely an α-PbO2-like [TiO.sub.2(II)], a baddeleyite-like [akaogiite], a cotunnite-like (TiO.sub.2(OII)], an orthorhombic phase [TiO.sub.2(OI)], and a cubic phase.

[0056] “Rutile” as used herein may refer to, but is not limited to, titanium dioxide (TiO.sub.2) in one of its mineral foims, the others being anatase and brookite. Rutile being categorized by a tetragonal unit cell in the crystal class ditetragonal dipyramidial with unit cell parameters a=b=4.584 Å, and c=2.953 Å.

[0057] “Anatase” as used herein may refer to, but is not limited to, titanium dioxide (TiO.sub.2) in one of its mineral forms, the others being rutile and brookite. Anatase being categorized by a tetragonal unit cell in the crystal class &tetragonal dipyramidial with unit cell parameters a=b=3.785 Å, and c=9.5143 Å.

[0058] “Brookite” as used herein may refer to, but is not limited to, titanium dioxide (TiO.sub.2) in one of its mineral forms, the others being anatase and rutile. Brookite being categorized by an orthorhombic unit cell in the crystal class dipyramidial with unit cell parameters a=5.4558 Å, b=9.1819 Å and c=5.1429 Å.

A: Sol-Gel Precursor Formation

[0059] According to one embodiment, there is a process of forming an amorphous TiO2 sol-gel precursor. The amorphous TiO2 sol-gel precursor is adapted to produce various TiO2 crystallization phases when controllably triggered low-energies. According to one embodiment, the crystallization of the amorphous TiO2 sol-gel precursor is controllably triggered at room-temperature and in ambient air with low-power laser-induced photo-activation.

[0060] The formation process of the TiO2 sol-gel precursor, according to an embodiment, exploits an incomplete nonhydrolytic sol-gel ester elimination reaction of titanium (IV) isopropoxide (TTIP) and oleic add (OA). For instance, 3.36 g of TTIP and 10 g of OA are mixed at room temperature under a nitrogen atmosphere. The resulting mixture has a light-yellow coloration is heated to 280° C. over a period of 20 minutes (at a rate of 14° C. min.sup.−1) and kept at this temperature for 2 hours during which time the light-yellow solution gradually turns to a dark brown coloration. At this point, the solution is cooled down to room temperature and ethanol is added in excess to yield a beige precipitate. The solution is then centrifuged for 30 minutes to recuperate the TiO2 nanoparticles. The recuperated TiO2 nanoparticles can then be re-dispersed in hexane for re-use or be kept as a powder for further processing. The recuperated TiO2 nanoparticies advantageously requires low energy to be activated (e.g. low activation energy) and can easily achieve crystalline phase transitioning with low laser energy irradiation, under atmospheric conditions, without any additive or dopant.

[0061] A transmission electron micrograph (TEM) of recuperated TiO2 nanoparticles is depicted in FIG. 1A. FIG. 1A presents a TEM image of the pristine amorphous TiO2 nanoparticles as synthesized via the sol-gel formation process (i.e. sol-gel route) wherein a TiO2 nanoparticle suspension in hexane was held at room temperature within a glass substrate at the bottom of a container and the hexane allowed to slowly evaporate to yield a glass substrate with a 23 μm film of amorphous TiO2 nanoparticles upon it. According to an embodiment, the recuperated TiO2 nanoparticles have a size that is smaller or equal to 14 nm.

[0062] Within other embodiments of the invention the recuperated TiO2 nanoparticles may include one or more organic and/or inorganic components provided that these are not adversely affected from the physical and chemical processes occurring during the optically initiated conversion process(es). Optionally, different organic and/or inorganic compounds may be selectively incorporated during a single deposition process of the precursor or they may be added through multiple deposition processes of the precursor. Within other embodiments of the invention the organic and/or inorganic compounds may be incorporated with the precursor during its formation which are subsequently deposited.

B: Fabrication of the Patternable Amorphous TiO2 Nanoparticle Films Using Sol-Gel Chemistry

[0063] According to one embodiment there is a method of patterning a substrate using sol-gel chemistry for producing patterned TiO2 films. Initially, a glass substrate is placed at the bottom of a beaker or container tilled a TiO2 nanoparticles dispersion such as a dispersion produced by sol-gel chemistry or produced by an other suitable method. The beaker is then covered with a perforated parafilm membrane to allow slow evaporation of the hexane solvent at room-temperature. When the solvent is completely evaporated, which could take under certain conditions approximately 24 hours, the glass substrate is removed yielding a 23 μm thickness film of amorphous TiO2 nanoparticles created over its surface. FIG. 1B depicts an atomic force microscopy (AFM) image of the thin film of amorphous TiO2 nanoparticles synthesized via the sol-gel chemistry. FIG. 1C depicts a scanning electron microscope (SEM) image of the amorphous TiO2 film after focusing under the SEM. The arrows point out the charging effect areas where the electron beam was focused whilst the light areas in the film correspond to crystallized areas where the focused electron beam triggered nucleation and transformation from amorphous to crystalline TiO2. The amorphous nature of the TiO2 nanoparticle film formed is noticeable in FIGS. 1B to 1D. FIG. 1D depicts the result of Raman spectroscopy upon an as deposited film of TiO2 nanoparticles. In addition, an X-ray diffraction (XRD) analysis is performed to further confirm the amorphous nature of the as depicted TiO2 nanoparticle film.

[0064] By observing the SEM image of this amorphous nanoparticle film in FIG. 1C, it can be noticed that the focused electron beam from the SEM is sufficient to initiate a nucleation and crystallization of the amorphous TiO2 film. The lighter “fern leaf-shaped” areas indicate where the nucleation process occurred. From there, it spreads along the film following fractal patterns. Accordingly, this nucleation and crystallization depicted in FIG. 1C confirms that the recuperated TiO2 requires relatively low photo-activation energy. It shall be recognized that when the TiO2 film is not perfectly uniform, the thicker areas of the TiO2 film require an increased irradiation voltage ranging between 5 kV to 15 kV to trigger nucleation under the SEM. Unfortunately, it was not possible to obtain Raman signatures of these crystallized features since they were too small for the spatial resolution of the confocal Raman microscope available to the inventors.

C: Patterning On-Demand Crystalline Structure Over Amorphous TiO2 Films

[0065] It can be noticed from the SEM that the amorphous TiO2 nanoparticles have a very low activation energy to initiate the phase transformation. Whilst direct write electron beam (e-beam) lithography for patterning the amorphous TiO2 nanoparticles may be employed in common with other direct write e-beam process such as e-beam lithography etc., due to the processing speed of such e-beam processes these tend to be limited to the highest resolution devices such as those requiring resolutions below 100 nm. Accordingly, low-power laser-induced photoactivation is used to generate mixed structures of anatase and/or rutile. Whilst the embodiments described below exploit direct write techniques using a laser it shall be recognized that the process may be adapted to exploit reticle or mask-based processes wherein illumination over a larger area is performed with masking and the time and/or intensity of the illumination adjusts the optical power density coupled to the amorphous TiO2 nanoparticle film to form either an anatase, rutile or mixed anatase-rutile crystallization phase within the film at a desired region.

[0066] Accordingly, the experimental configuration employed for the results presented below exploits direct writing and process adjustment by controlling the power density of the laser. According to one embodiment, a WI Tec Alpha300 confocal Raman microscope equipped with a 60 mW fiber-coupled continuous-wave laser at 532 nm and a mechanical attenuator are controlled to produce a source beam for phase patterning the amorphous TiO2 film, i.e. defining the pattern of crystalline phases of the resulting TiO2 material such that different regions may be converted to different phases by adjustment of the irradiating beam energy. The source beam is coupled through a 10× objective for excitation of the amorphous TiO2 and is mounted on a motorized high-precision XYZ stage.

[0067] Referring to FIG. 2A there is depicted a schematic of the spatially-resolved laser-induced photoactivation process for the amorphous TiO2 nanoparticle film. FIG. 2B depicts a laser scanning microscope image of the amorphous TiO2 nanoparticle film after crystalline phase alteration using the spatially-resolved laser-induced photoactivation process. As can be noticed the letters “ÉTS” have been patterned to consist of anatase-phase crystalline TiO2, while the underline mark consists of rutile-phase crystalline TiO2. The darker background consists of the unaltered amorphous TiO2 film. The different crystalline phases are confirmed by Raman micron-spectroscopy measurements as depicted in FIGS. 2C and 2D respectively. The process can controllably photo-activate the phase transition from the amorphous TiO2 to the anatase and/or rutile phase at room-temperature and in ambient environment by adjusting the excitation power density. In this embodiment, a power density of above 75 W mm.sup.−2 is required to complete the transition to anatase TiO2 (the “ÉTS” letters in FIG. 2B) The transition to rutile TiO2 is completed using an incident power density of above 445 W mm.sup.−2 (the horizontal line under “ÉTS” in FIG. 2B). Referring to FIG. 3 the anatase and rutile crystalline phases are confirmed by Raman micro-spectroscopy results in the uppermost and lowermost curves respectively for high-resolution Raman spectra of each crystalline structure obtained at these different laser power densities. However, at an intermediate power density, e.g. 275 W mm.sup.−2, as depicted in the middle curve of FIG. 3 the process yields a mixture of both anatase and rutile crystalline TiO2 phases.

[0068] In order to analyse the influence of the incident laser power density on the crystallization process, the transient evolution of the Raman signature is recorded over a period of 1 minute after opening the laser shutter at the 5 second mark. Additionally, the Raman peak intensity evolution for the dominant Raman peak is recorded over 1 hour for the conversion process. These results are depicted in FIG. 4A to FIG. 4F where:

[0069] FIG. 4A depicts the transient evolution of the Raman signature for the laser-assisted crystallization kinetics to reach anatase at 75 W mm.sup.−2;

[0070] FIG. 4B depicts the transient evolution of the Raman signature for the laser-assisted crystallization kinetics to reach mixed anatase and rutile at 275 W mm.sup.−2;

[0071] FIG. 4C depicts the transient evolution of the Raman signature for the laser-assisted crystallization kinetics to reach rutile at 445 W mm.sup.−2;

[0072] FIG. 4D depicts the Raman peak intensity evolution for the dominant Raman peak for the laser-assisted crystallization kinetics to reach anatase at 75 W mm.sup.−2;

[0073] FIG. 4E depicts the Raman peak intensity evolution for the dominant Raman peak for the laser-assisted crystallization kinetics to reach mixed anatase and rutile at 275 W mm.sup.−2; and

[0074] FIG. 4F depicts the Raman peak intensity evolution for the dominant Raman peak for the laser-assisted crystallization kinetics to reach rutile at 445 W mm.sup.−2.

[0075] The crystallization process for all three TiO2 polymorphs starts as soon as the amorphous film is exposed to the laser irradiation. In each of FIGS. 4A to 4C the laser shutter opens at the 5-second mark. In FIGS. 4D to 4F the dashed vertical line marks the time required to reach 90% of maximum Raman intensity. However, it is evident in each that the crystallization process for all three TiO2 polymorphs begins as soon as the amorphous film is exposed to the laser irradiation.

[0076] For the pure anatase crystalline phase obtained using a 75 W mm.sup.−2 power density, the identifying peak at 152 cm.sup.−1 starts to appear during the first 6 seconds of laser exposure. The other characteristic peaks of the anatase phase at 395 cm.sup.−1, 513 cm.sup.−1 and 634 cm.sup.−1 are well defined after only 30 seconds of exposure. The other TiO2 crystalline phase transitions observed exhibit similar behavior shown in FIG. 4D.

[0077] In the mixed crystalline structure generated under 275 W mm.sup.−2 exposure, well-defined rutile characteristic peaks such as 152 cm.sup.−1, 261 cm.sup.−1, 420 cm.sup.−1 and 610 cm.sup.−1 can be observed instantly after laser exposure. As depicted in FIG. 4B, the intensity of all these peaks increases over time but, contrary to the characteristic peak ratios present in rutile, the peak at 152 cm.sup.−1 still dominates, suggesting incomplete transition from anatase to rutile. Indeed, the transient evolution of the 152 cm.sup.−1 peak behaves similarly to what can be observed for anatase, indicating the presence of a mixed, anatase and rutile, crystalline phase achieved at this power density. FIG. 4E depicts the evolution of the 152 cm.sup.−1 rutile peak, which rapidly increases upon laser exposure before reaching saturation. On the other hand, the mixed anatase/rutile phase behaves more as a transitional state, where contributions of both phases are perceived simultaneously. As such, the mixture phase displays both strong 152 cm.sup.−1 (from anatase) and 420 cm.sup.−1 (from rutile) peaks. With higher portion of anatase phase dominating the Raman spectra, it is possible that the peak at 420 cm.sup.−1 displays different kinetics due to the contribution of the adjacent anatase peaks and displays its distorted and irregular behavior.

[0078] For the rutile crystalline structure generated at higher power densities, it is possible to observe in FIG. 4C the well-defined mile peaks at 152 cm.sup.−1, 261 cm.sup.−1, 420 cm.sup.−1 and 610 cm.sup.−1 immediately after laser exposure. FIG. 4F depicts the evolution of the dominant 420 cm.sup.−1 rutile peak.

[0079] In all cases, only small changes in the intensity of the Raman signal after the first 500 seconds of exposure can be noticed, suggesting that conversion is complete after a short exposure time and that the process is not cumulative over time.

[0080] in order to study in detail, the patterning process and its effect on the amorphous TiO2 film structure, anatase and rutile crystalline structures were patterned in a checkered pattern upon the same substrate from the same deposited amorphous TiO2 nanoparticle film as depicted schematically in FIG. 5A. The dominant crystalline phase is confirmed by performing a Raman mapping of the photo-activated areas. Each transition, from amorphous to anatase or rutile is selectively achieved by controlling the laser power density. It is important to note that the patterning velocity need not be uniform during the processing. In fact, all the patterns that are shown in the Figures are produced by manually moving the XY positioner using a joystick rather than computer based control of the motorized stages. It shall be recognized that despite this limitation the processing yields good results however automation of this process would yield improved results. Since laser exposure induces crystalline conversion in the amorphous TiO2 films, the excitation power density within the confocal Raman system is kept below 75 W mm-2 for the acquisition of the Raman data.

[0081] FIGS. 5B to 5D present the anatase areas having a well-defined shape. Referring to FIG. 5B the Trace A corresponds to the path left by the laser during the anatase patterning process. This path is narrow with an approximate width of 9.3 μm. The rutile areas are also well defined, but the crystalized area was bigger than intended. This arises as the result of the wider trace left by the laser during the rutile pattering process. The laser spot of the laser does not change while increasing the power density but the affected area in the film is increased. The width of the trace R in FIG. 5B is approximately 17.2 μm. Referring to FIG. 5B the anatase TiO2 is formed in squares 1 and 2 whilst rutile TiO2 is generated in squares 3 and 4. FIGS. 5C and 5D depict two false colour maps of a micro-Raman image of the pattern. In each figure the darker “squares” are rutile TiO2 whilst the lighter “squares” are the anatase TiO2 respectively. In FIG. 5E a topographic 3D surface reconstruction for the checkered pattern is depicted using image processing of data acquired during laser scanning microscope analysis.

[0082] As noted supra the visible difference between traces A and R in FIG. 5B arise from the different values of power density required to crystalize each phase; i.e. an increment in power density during the patterning process translated in an increment in the width of the trace left by the laser. Within FIG. 5B it also possible to view the clear contrast difference between the amorphous area and the crystallized regions in the patterned sample. In the crystallized regions, the color changes from dark to light gray and cracks can be clearly observed in all crystallized regions, especially in the rutile TiO2 regions. These cracks appear as a direct consequence of the natural densification of the TiO2 during the crystallization process. Indeed, the atoms reorganize during the laser-induced photo-activation process. This reorganization at the atomic level results in an important reduction in volume which led to the generation of the cracks in the photo-converted regions. The density values reported in the literature are 3830 kg m-3 and 4240 kg m-3 for anatase and rutile, respectively. Such reduction generates cracks in the photoconverted regions. Cracks observed in the rutile TiO2 regions are more pronounced than cracks formed in anatase regions.

[0083] This contrast is explained by the smaller unit-cell volume for rutile TiO2. As shown below in Table 1, the volume of the anatase unit-cell is more than twice the volume of the rutile unit-cell. In other words, during rutile patterning, atoms get closer and the reorganization of its lattice generates more shrinkage, resulting in larger and deeper cracks when the material crystallizes. This loss of volume on the material becomes more noticeable when analyzing the structure using a 3D topographic surface reconstruction of the checkered pattern. In FIG. 5D, it is noticeable that the anatase tiles are shallower, smoother and more regular than the rutile tiles. The rutile regions also exhibit deeper cracks as the densification of the TiO2 occurs to a greater extent in rutile compared with anatase. FIG. 8 depicts in first and second images 800A and 800B respectively the unit cells of the anaphase and rutile phases of TiO2 respectively.

TABLE-US-00001 TABLE 1 Unit Cell Dimensions and Volume for Anatase TiO2 and Rutile TiO2 Phase Anatase Rutile a 3.73 Å 4.59 Å b 3.73 Å 4.59 Å c 9.37 Å 2.95 Å Unit Cell 130.36 Å.sup.3 62.42 Å.sup.3 Volume

[0084] It shall be recognized that phase stability within nanocrystalline TiO2 is known for its great dependence on the particle size.

[0085] With the sol-gel precursor process described herein, the synthetized amorphous TiO2 nanoparticles have a size of around 10 nm. When processed with the laser illumination at different intensities anatase, rutile and mixed anatase-rutile phases can be formed within a single process on the deposited TiO2 film. Indeed, complex patterns are achieved on the deposited film through laser patterning. It is therefore shown that localized thermal effects caused by the focused laser are responsible for the crystalline phase transition.

[0086] Within the laser irradiation processes and results described and depicted with respect to FIGS. 1A to 5E respectively the irradiation source is a 532 nm laser. Although this laser energy is smaller than the 3.2 eV bandgap of TiO2, the absorption of this visible light is attributed to the presence of oxygen vacancies generated during the synthesis of the TiO2 nanoparticles. Accordingly, the oxygen vacancies create band states within the TiO2 bandgap that effectively allow the absorption of the 532 nm laser irradiation while raising the energy of the system. Further, this also promotes lattice relaxation, helping with the rearrangement process of the ions that participate in the phase transition.

[0087] The phase transition phenomenon relies on the ability of the material to absorb or desorb molecular oxygen. In other words, the high concentration of oxygen facilitates the phase transition due to the ionic mobility created in oxygen vacancies wherein these vacancies relax the lattice and facilitate the rearrangement of ions for the transition from anatase to rutile.

[0088] In contrast with processes that require an oxygen rich atmosphere, the present process is performed at room temperature, in ambient air and at normal atmospheric conditions, the oxygen molecules present in ambient air act as very efficient photoexcited electron scavengers, trapping the excited electrons from the conduction band into the surface states of the TiO2. The oxygen molecules are then adsorbed at the surface of the TiO2 nanoparticles to partially compensate these oxygen vacancies wherein the presence of these oxygen vacancies then promotes the formation of Ti3 + sites in the crystal structure as the electrons left behind by the vacancy are distributed on neighboring, Ti sites, reducing them from Ti4+ to Ti3+. At this point, assisted by the continuous laser irradiation, the adsorbed oxygen molecule passivates the TiO2 by bridging the metallic ions. As a result of the phase transition and the relaxation of the electrons, a vibrational phonon is emitted as heat. This sequence is depicted by FIG. 6 which presents a proposed Jablonski diagram for the laser-induced phase transition from anatase to rutile TiO2, according to an embodiment.

[0089] In fact, the process of forming the anatase phase produces additional oxygen vacancies that advantageously promote the formation of the rutile phase if the anatase phase region is further excited at the required laser intensity.

[0090] Accordingly, a low-power laser-assisted photoactivation process for TiO2 nanostructured amorphous films can be performed at room-temperature in an ambient environment. This approach allows to selectively and controllably convert to the anatase and/or rutile crystalline phases by controlling the laser power density incident on a film of amorphous TiO2 nanoparticles synthesized through an incomplete nonhydrolytic sol-gel ester elimination reaction. According to one embodiment, the incomplete nonhydrolytic sol-gel ester elimination reaction employs titanium isopropoxide and oleic acid.

[0091] FIG. 7 depicts schematically the sequential conversion process from amorphous TiO2 to anatase phase TiO2, mixed anatase/rutile phase TiO2, and final rutile phase TiO2 using a sol-gel nanoparticle film and photonic excitation method. The initial amorphous TiO2 is a film of TiO2 nanoparticles most of which have a dimension of approximately 14 nm or less.

[0092] Raman micro-spectroscopy allows for observation of the controllable transition to pure anatase, mixed anatase/rutile and pure rutile crystalline phases generated using different laser power densities. Transient evolutions are also measured using time-resolved Raman micro-spectroscopy that show that the crystallization occurs in the very first seconds of irradiation, that the effect is permanent and non-cumulative. Laser scanning microscope images and 3D topographic surface reconstruction of the photo-activated areas also verified the loss in volume after crystallization due to the denser atomistic reorganization after phase transition. The present method can be used to spatially-organize different crystalline phases with high level of precision to study new synergistic effects.

[0093] The present amorphous TiO2 nanoparticles have a size that is under 14 nm and allows laser-assisted phase transition to anatase. Moreover, the present process allows to provide additional oxygen vacancies in the anatase phase. The additional oxygen vacancies facilitate the transition from the anatase to rutile phase. Indeed, the subsequent transition from anatase to rutile, it relies upon the high capacity of the synthetized TiO2 to absorb molecular oxygen.

[0094] Importantly, all the phase transitions presented within the present sol-gel chemistry and laser based processing are achieved without any dopant in the TiO2 film prior to photoactivation of the crystallization process. Accordingly, this facilitates integration of TiO2 into many photo-electro-catalytic applications. Additionally, the ability to process with an accurate, reproducible and scalable method crystalline TiO2 with varying composition in one region or differing compositions at adjacent regions within a single fabrication process sequence offer the opportunity to establish novel pathways of TiO2-based platforms, especially for energy conversion and environmental applications.

[0095] According to one embodiment, there is a provide a fabrication methodology for a pure amorphous TiO2 ink which can be easily included within printing platforms, e.g. 3D printing platforms, as well as conventional semiconductor style device manufacturing techniques. The photonic excitation employed and presented supra was at 532 nm. Optical sources at this wavelength may exploit an infra-red pumped, frequency doubled diode-pumped solid-state laser. Alternatively, the laser may be an argon ion laser emitting at 528.7 nm, a krypton laser emitting at 531 nm, a frequency doubled Nd: YAG laser at 532 nm, a frequency doubled Nd: YLF laser, a frequency doubled, or diode pumped Nd: YVO4 laser at 532 nm, a diode pumped KTP laser at 532 nm, or a sapphire laser at 532 nm for example.

[0096] According to another embodiment, there is a fabrication of a pure amorphous TiO2 powder which can be provided as a base to produce an ink, a slurry or a tablet (e.g. compressed powder) depending on the area of application or available storage facilities and storage duration.

[0097] Whilst in the present embodiment a 532 nm optical source is used and discussed supra in respect of FIGS. 1 to 6, it shall be recognized that other optical wavelengths may be employed provided that the selected wavelength or wavelengths and energy are sufficient to trigger the amorphous TiO2 to anatase TiO2 transformation and carrier excitation within the anatase TiO2 phase from the valence band to the carrier band in order to support the subsequent oxygen scavenging process for forming the intermediate TiO2 phase and subsequently the rutile TiO2 phase. Replacement of the 532 nm wavelength laser employed in the conversion processes described above with an optical source emitting at 355 nm, e.g. a frequency tripled Nd: YAG, has also demonstrated conversion of the amorphous TiO2 film to one or more crystalline phases under ambient environmental condition.

[0098] Beneficially an incomplete nonhydrolytic sol-gel ester elimination reaction is used, and the process does not require any dopant thereby allowing the purity of the TiO2 nanoparticles and subsequent amorphous film to be defined.

[0099] Beneficially the process defined and described herein does not require any high temperature and/or high pressure processing of the TiO2 precursor.

[0100] Beneficially the process defined and described herein exploits amorphous TiO2 as the precursor rather than anatase TiO2.

[0101] Beneficially the process defined and described herein exploits undoped TiO2 as the precursor rather than doped TiO2.

[0102] Beneficially the process defined and described herein allows direct patterning within a single amorphous TiO2 nanoparticle film of regions of anatase, rutile and mixed rutile-anatase.

[0103] It is important to note that the process defined and described herein provides for spatially controlled, via focused optical beam excitation or patterned optical excitation, and high quality conversion of the amorphous TiO2 into either the anatase phase, the rutile phase or a mixed anatase-rutile phase from an amorphous TiO2 precursor.

[0104] Whilst the present process uses solvent evaporation to deposit the TiO2 nanoparticles from suspension it shall be recognized that the solvent suspended solution, or ink, may be employed with other well established fabrication techniques including, but not limited to, spin coating, spray coating, dip coating, evaporation etc.

[0105] The foregoing disclosure of the embodiments of the present solution have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit to the precise forms disclosed. Many variations and modifications of the embodiments described herein will be apparent to one of ordinary skill in the art in light of the above disclosure. The scope of the present is to be defined only by the claims appended hereto, and by their equivalents.

[0106] Further, in describing representative embodiments of the present solutions, the specification may have presented the method and/or process of the present solution as a particular sequence of steps. However, to the extent that the method or process does not rely on the particular order of steps set forth herein, the method or process should not be limited to the particular sequence of steps described. As one of ordinary skill in the art would appreciate, other sequences of steps may be possible. Therefore, the particular order of the steps set forth in the specification should not be construed as limitations on the claims. In addition, the claims directed to the method and/or process of the present solution should not be limited to the performance of their steps in the order written, and one skilled in the art can readily appreciate that the sequences may be varied and still remain within the spirit and scope of the present solution.