WO2024182826A1

WO2024182826A1 - Process for the production of bio-engineered black tio2

Info

Publication number: WO2024182826A1
Application number: PCT/ZA2024/050008
Authority: WO
Inventors: Malek MAAZA; Adama FALL
Original assignee: University Of South Africa
Priority date: 2023-03-02
Filing date: 2024-03-01
Publication date: 2024-09-06

Abstract

This invention presents a novel process for producing a bio-engineered black TiO₂ (B-TiO₂) photocatalyst, leveraging the eco-friendly and efficient attributes of bio-synthesis. Distinctively, this method utilizes a natural plant extract as a chelating agent, coupled with deionized water and Titanium(IV) bis/ammonium lactate dihydroxide as the titanium source, eliminating the need for additional pH control compounds, catalysts, or vacuum processes. The process entails forming a precipitate from these components, which is then dried and annealed to produce a black TiO₂ powder. This innovative approach not only simplifies the synthesis of B-TiO₂ under mild conditions but also enhances its catalytic activity in dark conditions, marking a significant advancement in the field of photocatalysis. The bio-engineered B-TiO₂ demonstrates bandgap tunability and a potential for various environmental and industrial applications, highlighting its versatility and the green synthesis method's contribution to sustainable technologies.

Description

PROCESS FOR THE PRODUCTION OF BIO-ENGINEERED BLACK TIO2

FIELD OF THE INVENTION

The present invention relates to a process for the production bio-engineered black TiO2 photocatalyst. The present invention further relates the bio-engineering of nano-scaled B-TiO2 which is successfully bio-synthesized within mild conditions with no additional compound for pH control and is effective in the absence of any catalyst or the need for vacuum to be applied during the process.

BACKGROUND OF THE INVENTION

It is known in the art that photocatalysts are a family of photoactive materials once illuminated would generate excitonic pairs which in their turn create highly reactive superoxides and/or hydroxyl radicals. These fast chemical reactions are effective among others for H2O splitting, contaminants’ decomposition in waste H2O and air cleaning among others.

In addition to the most popular TiO2 based photocatalysts and equivalent; CeO2, ZnO, SnO2, WO3, there is a broad group of Bi, C, M0S2 based VIS photocatalysts as well as those involving novel composites such as AgaPC .

The development of titanium dioxide (TiO2) photocatalysts has seen significant advancements through various synthesis methods aimed at improving photocatalytic efficiency and expanding application fields. One such method is outlined in CN103406135B, where a novel N-TiO2@WSe2 photocatalyst is synthesized via a solgel process. This method involves the preparation of a titanium ethanol solution, subsequent addition of thiocarbamide ethanol solution, incorporation of WSe2, and controlled addition of acidic ethanol water to form a sol, which after aging and calcining at high temperatures, yields N-TiO2 particles of about 50 nm uniformly distributed on the surface of WSe2. This particular photocatalyst demonstrates enhanced photocatalytic activity, particularly in the degradation of wastewater containing antibiotics, showcasing a 69.7% improvement over unmodified TiO2, attributed to its unique dispersion and the synergistic effects between N-TiO2 and WSe2.

In contrast, CN106563429A introduces a low-cost, efficient, porous black TiO2 photocatalyst that responds to visible light. This is achieved by etching titanium powder with H2O2 solution, a novel approach that replaces the traditional high-temperature, high-pressure hydrogenation method. The etching process results in the formation of porous particles, significantly increasing the material's specific surface area and, consequently, its catalytic efficiency. This method not only simplifies the preparation of black TiO2 but also enhances its photocatalytic performance by combining visible light absorption capabilities with a specialized porous structure.

Another inventive approach is detailed in CN107138161 B, which describes a preparation method for doped black titanium dioxide. This method involves the hydrolysis of titanium oxide precursors, mixing with metal nitrates, sulfides, fluorides, or nitrides, and calcining in an atmosphere or vacuum furnace. The process is adaptable for doping with various metal cations and anions, offering a broad application range, simplicity, and cost-effectiveness. However, a notable distinction across these inventions is the absence of natural plant extracts as chelating agents, a key feature of our bio-engineered black TiO2 synthesis process. This innovative approach leverages the eco-friendly and efficient attributes of bio-synthesis, setting our method apart by emphasizing green chemistry principles and enhancing photocatalytic activity under mild conditions.

Furthermore, since the TiC electrochemical photolysis of H2O, Titanium Dioxide (TiC ) became one of the upmost investigated semiconductors owing to its multi-functionality in general and its high UV photocatalytic activity specifically in addition to its redox properties, chemical stability, and nontoxicity. In standard conditions, it exhibits 3 major crystallographic phases; namely anatase, rutile & brookite corresponding to different band gap values of 3.2eV (anatase), 3.0eV(rutile) and 3.4eV (brookite). As sustained by the extensive literature, TiO2 is by far the preferred photocatalyst so far in terms of multi-sectorial functionality. However, from catalytic viewpoint, the limitation in its application, results from low quantum yield caused by a fast excitonic recombination of charge carriers (e_/h+) and the pre-requisite of using UV irradiation for its photo-activation. This latter has been shown to be overcome through Nitrogen “N” doping. In addition, a large variety of organic and inorganic compounds have been examined as effective dopants or surface modifiers. Among them, noble-metals particles have attracted special attention, since they may enhance not only the transfer of photo-generated excitonic pairs prolonging charge carriers’ lifetime but also broadening the optical absorption.

Indeed, some of them, exhibit plasmonic properties extending the optical absorption in the VIS spectral range. The co-doping with Nitrogen was found to be the promising route in extending the photo-activity of TiC to the VIS spectral range in addition to its intrinsic UV response.

In addition to the standard and high pressure-high temperature TiC phases as well as the various TiO2 based nano-composites, there is the so called black Titania labelled as “BTiO2“ which was first reported by Chen et al in 201 1 . The synthesis of such a low bandgap TiC (-1.54 eV) was validated by hydrogenation of TiO2 at 20 bars H2 pressure around 200 °C for 5 days. It was coined as “Black Titania” (B-TiC ) due to its appearance.

Such a discovery gathered a significant global attention and encouraged the scientific community to begin studying this material for various photocatalytic applications and possibly synthesizing it differently. For the mass-scale production synthesis of B-TiO2, several approaches were successfully validated and patented accordingly.

As summarized in Table 1 below, the synthesis of B-TiO2 was demonstrated via mainly seven processes, namely; Hydrogenation, chemical reduction (Mg, Al, NaBH4, NaH), chemical oxidation, electro-chemical reduction, anodization annealing, ultrasonication, and laser modification. In addition to these proven physical & chemical approaches, the current invention aims to enrich these established methodologies with and through a green bio-engineering novel approach.

Table 1 : Conventional synthesis methodologies and properties of B-TiO2 nanomaterials

SUMMARY OF THE INVENTION

According to a first aspect of the invention, the invention relates to a process for the production of a bio-engineered nano-scaled B-TiO2, which exhibits a significant catalytic activity in Dark conditions, said process including; providing a source of a titanium cation; providing a solvent in the form of water; and providing a chelating agent in the form of a natural plant extract.

The process further includes obtaining a precipitate, drying same and annealing the precipitate to obtain a dried black powder..

In one embodiment, the precipitate was allowed to settle and dried in the oven between 80 °C and 150 °C, ideally at 100 °C for between 30 minutes and 2 hours, ideally 1 hr.

Still further the precipitate may be annealed under air for 1 to 3 hours, ideally 2 h at various temperatures varying from 100 ^SC to 600 ^SC.

In a preferred embodiment of the invention the titanium cation may be in the form of Titanium(IV) bis/ammonium lactate dihydroxide.

In a preferred embodiment of the invention the water may be in the form of deionized water (dH2O)

In a preferred embodiment of the invention the chelating agent may be in the form of Hibiscus sabdarifa flowers. Furthermore, in an ideal embodiment, the bio-engineering of nano-scaled B- TiO2 may be successfully bio-synthesized within mild conditions (Patm, TRoom Temperature-293 K) using a natural extract as an effective chelating agent and H2O as a universal solvent. The novelty as well as the specificities of this contribution lies within the followings:

Moreover, the invention provides for the process to be effective with no additional compound for pH control.

Still further, the process of the invention is effective in the absence of any catalyst or the need for vacuum to be applied.

BRIEF DESCRIPTION OF THE DRAWINGS.

A process to produce bio-engineered black TiO2 photocatalyst in accordance with the invention will now be described by way of the following, non-limiting examples with reference to the accompanying drawing.

In the drawings: -

Figure 1 summarizes (a) the Major oxide & sulphide photocatalysts and (b) major multi-sectorial applications of nano- scaled TiC .

Figure 2 shows the standard conditions Room temperature/atmospheric pressure stable crystallographic structures of TiO2.

Figure 3 shows the schematic representation of the Hibiscus sabdarifa , its major phytocompound and its powdered form.

Figure 4 displays the bio-synthesized titania nanoparticles at various stages of annealing in a pelletized form . The sample labelled as reference corresponds to the centrifuged & pelletized powder without any annealing treatment. Following annealing treatment under standard conditions of pressure in air, all pelletized samples annealed up to 400°C were black in color amorphous in view of the X-rays diffraction patterns. Upon annealing above 400°C, the pelletized samples started to exhibit a significant color change from dark black to white-yellowish to white. This color change is likely to be associated to a crystallographic phase change as reflected by the diffraction pattern (amorphous to anatase (450-600°C) to rutile(>700°C).

Figure 5 reports a typical transmission electron microscopy of the B-TiO2 annealed at 400°C. It indicates nano-scaled particles with a crystal-clear shape anisotropy. The nanoparticles are nanoplatelets-like rather than spherically shaped. The average basal dimensions of the nanoscaled platelets are almost 9x7 nm².

Figure 6 displays a typical Scanning Electron Spectroscopy (EDS) profile of the bio-engineered nanoscale B-TiO2 annealed at 400°C. It reveals eight major peaks among which three are attributed to Ti. At low energy channels, those of O & C are identified. While O is attributed to the TiC matrix, the C is related to the C coating used for the EDS studies. In addition, there are 3 additional peaks corresponding to Mg, Na & K. These are generally contaminants originating, likely, from the natural extract. These contaminants are systematically detected in any bio-engineering process of nanomaterials. The C originates from the coating used for the EDS.

Figure 7 displays the corresponding room temperature X-rays Diffraction profile of the bio-engineered nano-scale B-TiO2 annealed at 450°C. It exhibits several Bragg peaks all fitting with the anatase crystallographic structure with lattice parameters of (a) = (b) =3.782 A and (c) = 9.502 A and a = p = y = 90°. Compared to bulk anatase, (a) = (b) = 3.796 A and (c) = 9.444 A and a = b = g = 90°. Compared to bulk anatase, the bio-engineered nano-scale B-TiO2 annealed at 450°C are under strain, a priori, within the (c) direction.

Figure 8 reports the room temperature Raman spectrum of the bioengineered B-TiO2 annealed at 400°C and the standard reference white anatase W-TiO2 within the spectral range of 100-800 cm¹. While they are located at 150, 400,525 & 650 cm^-1 for the White TiO2, they are centred at 150, 400,525 & 650 cm^-1 for the B-TiO2. Yet, there is major difference in intensity as well in spectral positions, the observed vibrational modes are attributed to the Eg, B1 g, A1 g & Eg modes for both samples. Hence, it is safe to conclude that the bio-engineered B-TiO2 annealed at 450°C is in an anatase crystallographic structure with, a priori, a defective surface.

Figure 9 displays the ESR spectra of the Bio-engineered B-TiO2 annealed at 400°C & and the standard reference white anatase W-Ti02 used as a reference. While the ESR signal of the reference standard white TiC is quasi-flat over the scanned magnetic field of 3400-3650 Gauss, the spectrum of the bio-engineered B-TiO2 annealed at 400°C exhibits a relatively a symmetric signal centred at about 3525 Gauss. Such a broad ESR signature was also reported within the literature and attributed to Ti 3⁺(3d 1 ) ions in the TiO2 or has been ascribed either to electrons localised on the oxygen vacancy or to both (Ti 3+ and O- ).

Figure 10 displays the diffuse reflectance over the spectral range of 250 - 1 100nm of the various pelletized bio-engineered TiO2 samples (annealed at various temperatures as reported within the inset section). A priori, one could distinguish 2 classes of samples. They are labelled as deffective TiO2-d (low temperature, < 400°C, black in color) and anatase/rutile TiO2 (relatively high temperature, >400°C white yellowish, white in color). This latter class exhibits diffuse reflectance spectra quasi-similar to that of standard anatase & rutile.

Figure 11 reports the LUMO & HOMO of supercells of Ti33O66 & O deficient Ti33O65 with their corresponding electron density functions. As one can notice, such a surface oxygen deficiency in Ti33O65 induces in a significant drop of bandgap from 2.141 eV to 0.039eV.

Figure 12 displays (a) the color of the B-TiO2 in MB contaminated H2O sample at 0 min, 30min & 120 min, (b) reports the corresponding optical absorbance “A” within the spectral region of 250-800 nm. Figure 13 shows the pivotal finding of this innovation, displays the standard variation of the ln(A/A0) versus time in complete dark conditions. As one can notice, there is a net and a regular decrease of ln(A/A0) versus time which can be approximated as linear. This trend confirms the significant decomposition of MB and the singular catalytic activity of the Bio-engineered nano-scaled B- TiO2 in the complete considered dark conditions.

DETAILED DESCRIPTION OF THE INVENTION

(i) Preparation of the Hibiscus extract

Dried flowers of Hibiscus sabdarifa were gently washed thoroughly using initially running tap water several times to remove dust particles. At the end of such a preliminary cleaning phase, they were submitted to a cleaning with dH2O followed by a solar exposure drying. The dried flowers were cut in small pieces. For a typical experiment, ~10 g of the dried cut Hibiscus sabdarifa flowers were directly poured into the round bottom flask containing ~400 mL of dH2O. Such a mixture was left infusing at room temperature for ~24 h. The red solution was filtered using a Whatmann filter paper with pore size of 2.5 pm. Such a filtered natural extract is used as the chelating agent without addition of any base or acid neither catalyst nor special treatment.

(ii) Preparation of the TiO2-NPs

In a typical bio-synthesis, 10 mL of Titanium(IV) bis/ammonium lactate dihydroxide is mixed with 350 mL of the previous extract of Hibiscus. The prepared solution was kept under continuous stirring at ambient temperature for 2 h for the uniform distribution of nanoparticles with the help of magnetic stirrer with a pH equivalent to 2.97 and the pH of the final solution recorded in the range of 3.91 . The obtained precipitate was allowed to settle and dried in the oven at 100 °C for 1 hr. As in general, the original precipitates by bio-synthesis are amorphous, the current obtained dried black powder was annealed under air for 2 h at various temperatures varying from 100 ^SC to 600 ^SC.

The inventor believes the present invention proposes for the first time on the possibility of bio-synthesis of B-TiO2, whereby validation of the reduction of the TiO2 optical bandgap is achieved and wherein, the bio-synthesized B-TiO2 is shown to be active in dark conditions for the effective decomposition of Methylene Blue (MB),

It should further be emphasised that the present invention uses no additional compounds for pH control, as is standard in most convention processes.

Furthermore, the present invention uses no catalyst and no vacuum is required during the process.

Lastly, the bio-engineering initial process phase takes place within mild conditions (Patm, TRoom Temperature~293 K.

Claims

1 . A process for the production of a photocatalyst, wherein the photocatalyst is a titanium dioxide (TiO2)-based photocatalyst, characterized in that the TiO2 is bio-engineered to form nano-scaled black titanium dioxide (B-TiC ) without the need for additional compounds for pH control and without requiring a vacuum during the production process.

2. The process according to claim 1 , wherein the titanium dioxide photocatalyst is known to be activated by ultraviolet (UV) light to generate reactive oxygen species for environmental purification, characterized by the bio-engineered B- TiO2 exhibiting significant catalytic activity in dark conditions, extending the utility of TiO2-based photocatalysts beyond UV light dependence.

3. The process according to claim 1 or 2, involving the use of a titanium cation source, a solvent, and a chelating agent for the synthesis of photocatalysts, characterized in that the titanium cation source is Titanium(IV) bis/ammonium lactate dihydroxide, the solvent is deionized water, and the chelating agent is a natural plant extract from Hibiscus sabdariffa flowers.

4. The process according to any preceding claim, wherein photocatalysts are traditionally synthesized under conditions requiring strict pH control and catalysts, characterized in that the bio-engineering process effectively synthesizes B-TiO2 within mild conditions without the need for any additional compound for pH control and in the absence of any catalyst.

5. The process according to any preceding claim, where the photocatalyst production involves obtaining a precipitate, drying, and annealing the precipitate, characterized in that the precipitate is dried in an oven between 80°C and 150°C, ideally at 100°C for between 30 minutes and 2 hours, ideally for 1 hour.

6. The process according to any preceding claim, characterized by annealing the precipitate under air for 1 to 3 hours, ideally for 2 hours, at various temperatures ranging from 100°C to 600°C to obtain a dried black powder.

7. A method of producing photocatalysts involving high temperature and pressure conditions, characterized in that the bio-engineered B-TiO2 is synthesized under atmospheric pressure and room temperature, simplifying the production process and making it more environmentally friendly.

8. The method according to any preceding claim, wherein traditional photocatalyst synthesis involves the use of hazardous chemicals, characterized by employing a green bio-engineering approach using natural plant extracts as chelating agents, reducing the environmental impact of the synthesis process.

9. A process for producing photocatalysts with a focus on UV light-activated catalysis, characterized in that the B-TiO2 produced exhibits a lower bandgap of approximately 1 .54 eV, enabling activation in dark conditions and broadening the application scope of TiC -based photocatalysts.

10. The process as described in any of the preceding claims, wherein the photocatalyst particles are primarily spherical, is further characterized by utilizing bio-engineered B-TiO2 particles. These particles are distinctively nanoplatelet-like, with an average basal area having dimensions of approximately 9 nanometers in length and 7 nanometers in width. This specific morphology is conducive to enhanced photocatalytic activity

1 1. A process for the synthesis of photocatalysts typically requiring external catalysts or additives, characterized in that the bio-engineered B-TiO2 synthesis process is effective without the need for such external catalysts or additives, thereby simplifying the process and reducing potential contaminants.

12. The process according to any preceding claim, characterized in that the synthesized B-TiO2 nanoparticles exhibit a crystallographic phase that can be tuned from amorphous to anatase to rutile by adjusting the annealing temperature, providing flexibility in targeting specific photocatalytic applications.

13. A process for the production of photocatalysts involving complex and energy- intensive methods, characterized by a bio-engineering method that is not only simpler and less energy-intensive but also capable of yielding photocatalysts with unique properties such as bandgap tunability and enhanced catalytic activity in dark conditions.

14. The process according to any preceding claim, characterized in that the bioengineered B-TiO2 nanoparticles, when annealed at 450°C, exhibit a strain in the crystallographic c-direction, indicating unique structural properties that may contribute to their photocatalytic efficiency.

15. A process for producing photocatalysts, characterized by the bio-engineered B- TiO2 being synthesized with a green and sustainable approach, leading to a photocatalyst that is not only effective in environmental purification applications but also produced in a manner that minimizes environmental impact, thus providing a dual benefit of efficient catalysis and eco-friendly synthesis.