WO2016098127A1

WO2016098127A1 - NOVEL TITANIUM DIOXIDE - GRAPHENE QUANTUM DOTS (TiO2-GQDS) HYBRID MULTIFUNCTIONAL MATERIAL AND PREPARATION THEREOF

Info

Publication number: WO2016098127A1
Application number: PCT/IN2015/050203
Authority: WO
Inventors: Pankaj Poddar; Anupam BISWAS
Original assignee: Council Of Scientific & Industrial Research
Priority date: 2014-12-16
Filing date: 2015-12-16
Publication date: 2016-06-23

Abstract

The present invention disclosed a novel surface disordered rutile Titanium dioxide- graphene quantum dots (TiO₂-GQDs) hybrid multifunctional material and preparation thereof.

Description

NOVEL TITANIUM DIOXIDE - GRAPHENE QUANTUM DOTS (Ti0₂-GQDS) HYBRID MULTIFUNCTIONAL MATERIAL AND PREPARATION THEREOF FIELD OF THE INVENTION:

The present invention relates to novel surface disordered rutile Ti0₂-graphene quantum dots (Ti0₂-GQDs) hybrid multifunctional material and preparation thereof.

BACKGROUND AND PRIOR ART:

Semiconductors have remained at the forefront of materials research since development of first transistor back in 1048. The usefulness of the semiconductors lies in a tight control over the impurities, surface/interface environment, morphology etc. The controlled defects, in the form of doping, vacancies, porosity, amorphicity and other crystalline defects have actually contributed into development of new technologies due to tailored electronic structure of these multifunctional materials which led to fascinating optical, electrical, magnetic, catalytical, antimicrobial properties over the years. However, development of new technologies based on multifunctional semiconductors still continues due to far better control of these band-gap engineered materials fuelled by novel chemical synthesis breakthroughs in recent years. After the initial thrust led of electronic devices, the recent revival of interest in three keywords- (1) energy technologies (photovoltaic s, water splitting etc.), (2) environment (degradation of organic pollutants, clean and cheap potable water etc.), and (3) healthcare (antimicrobial consumer products, medical devices, body implants, nano-medicines, bio-imaging etc), has thrown newer and tougher challenges and opportunities in front of materials chemists and physicists. It is clear that in order to improve some of these properties, better control over crystalline defects is necessary through the development of simple chemistry and devising new composite or hybrid materials with mutlifunctinal properties. Among the different types of semiconducting materials, bismuth oxy-chloride (BiOCl) and titanium di-oxide (Ti0₂) were regarded superior over few others. The focus of past research was to modify the electronic properties of the host material or by forming the hybrid and functionalized nanostructures. The facet dependent photocatalytic degradation of pollutants revealed varying level of photocatalytic activity of different facets. Among various photocatalytic materials, titania is most widely researched. Apart from solar power harvesting ability, Ti0₂ has created considerable attention in other potential fields as we discussed in the earlier section. Under the solar light illumination; electrons are excited from valence band to conduction band to form excitons, which are generally responsible for photocatalytic activity. Typically, the Ti0₂ exists in rutile and anatase phases. Among these two phases, it was observed that the anatase- Ti0₂ generally exhibits better photocatalytic performance over rutile-Ti0₂. The photocatalytic performance is greatly affected by its morphology, crystallinity, lattice doping, porosity and most importantly the particle-size of the material. Increased photocatalytic performance in nanoparticles was attributed to the quantum- size effect and increased surface area. In general, Ti0₂ has low valence band maxima (VBM), typically 1.6 eV which hinders its visible and near infrared (NIR) absorption.

Therefore, there is a great need to increase the VBM, which, in turn will increase its solar harvesting efficiency. In literature, different methods have been adopted to achieve higher catalytic performance, such as - introduction of defects or metallic and non-metallic impurities.

In the search of a multifunctional material, the combination of nanostructured titania and GQDs, has huge benefits with potential applications targeted towards energy, health and environment related technological challenges. No doubt that recently, the Ti0₂-GQDs hybrid materials are reported to give superior photocatalytic degradation of pollutants.. In Ti0₂, the charge recombination increases which, in turn, reduces its applicability in photocatalysis. The charge recombination probability in Ti0₂ is greatly reduces by the introduction of graphene, particularly GQDs. For an improved catalytic performance, one requires making a hybrid out of surface disordered Ti0₂ and graphene quantum dots as in surface-disordered anatase Ti0₂ the VBM increases, which increases its solar absorption. The Ti0₂-GQDs hybrid will comprise of large electron-hole pair separation, wide wavelength solar light harvesting capabilities and enhanced surface area which is required for good photocatalyst.

Earlier, it was reported that, high-pressure synthesised surface disordered anatase Ti0₂ completed photodegradation of methylene bule (MB) within 8 min. Whereas, the graphene- Ti0₂ (P25, 80% anatase, 20% rutile) nanowire composite reported by Pan et al. could do the same job in 10 min. Graphene-Ti0₂ hybrid always improves photocatalytic performance. However, there is no report of surface disordered rutile Ti0₂-GQDs hybrid. Therefore, there is a great need to develop new materials, which, in turn will increase the solar harvesting efficiency.

Chinese Pat. No. CN102963934 discloses preparation method of bismuth tungstate quantum dot and preparation method of bismuth tungstate quantum dot-graphene composite material. The preparation method comprises the steps: a) dissolving a soluble bismuth salt and sodium oleate in water, stirring for more than 1 hour to form a first emulsion-like precursor solution containing bismuth ions, wherein the molar concentration of the sodium oleate in the first precursor solution is smaller than 0.3mol/L; b) dissolving the soluble bismuth salt in water, and stirring and ultrasonically dispersing to form a uniform second precursor solution containing tungstate ions; and c) mixing the first precursor solution and the second precursor solution, and carrying out hydro-thermal synthesis for more than 12 hours at 120-180 DEG C. Chinese Pat. No. CN102176382 discloses method for preparing grapheme-quantum dot composite film and solar battery structured by using same. A graphene - quantum dot composite film, wherein the preparation process comprises the following steps: (1) natural graphite powder as a raw material, adding a certain amount of sodium nitrate, cold concentrated sulfuric acid and potassium permanganate, below were uniformly mixed to obtain a mixed solution at 20 °C, then the reaction was warmed to 30-40 °C 25-35min, to the mixed solution of deionized water was slowly added, the reaction temperature was raised to 95-100 °C 10-20min, then adding a certain the amount of hydrogen peroxide solution; centrifugal added a solution of hydrogen peroxide was then filtered, and washed with dilute hydrochloric acid solution to remove metal ions, and then washed with deionized water to remove excess acid, and repeatedly washed with water until neutral, to give a final oxide aqueous graphite sheets, which were then sonicated to give a brown solution graphene oxide uniformly dispersed; per Ig graphite powder were added an amount of sodium nitrate 0.1 -5g, the amount of concentrated sulfuric acid was added to 10- 100 ml, the amount of potassium permanganate is added 0.5- lOg, the amount of hydrogen peroxide solution was added 5- 50ml; graphene oxide solution (2) to take the above step (1) is uniformly dispersed, 1: 1-1:50 quality ratio of added particles or quantum dots quantum dots reaction precursor solution to form an oxide graphene - a mixture of quantum dots; (3) to the step (2) of the graphene oxide - a mixed quantum dot was added hydrazine hydrate reducing agent, at 80 °C was stirred for 24h, after the reduction reaction, the product was subjected to sonication, then centrifuged to remove a small amount of precipitate obtained by centrifugation of the suspension is stable graphene - quantum dot solution; (4) the above step (3) of graphite ene-quantum dot solution was diluted and stirred until a homogeneous and stable suspension; (5) in the above step (4) of graphene-quantum dot suspension by vacuum filtration, the untreated membrane forming a thin film; said step (5) or to (5) using the treated membrane, which is treatment process: The suspension is vacuum filtration oxide particles in the untreated membrane layer deposited oxide film; and then suction filtered Graphene - suspension quantum dot deposited on the treated membrane; (6) a large number of water washing after the tight junctions (5) is fixed on the filter film on a substrate material; (7) is then placed in an oven 55-65 °C and dried under vacuum 6h-24h, the filters were then dissolved away with acetone, to obtain graphene - quantum dot thin film on a conductive substrate. Oxide film is Ti0₂, ZnO, ZnS, Sn0₂, Nb₂Os, A1₂0₃, ln₂0₃, CuO, and one or more of Si0₂ oxide film.

Article titled "A visible-light-driven composite photocatalyst of Ti0₂ nanotube arrays and graphene quantum dots" by Donald K. L. et al. published in Beilstein Journal of Nanotechnology, 2014, 5, pp 689-695 reports a visible-light-driven photocatalyst was fabricated by covalently bonding GQDs onto amine-modified Ti0₂ nanotube arrays. The GQDs/TNAs composite retains the highly ordered nanotube morphology and well crystallized anatase phase. The high visible-light photocatalytic activity could be attributed to photosensitization of Ti0₂ nanotube arrays by GQDs. This research shows the potential of GQD-based photocatalysts for visible-light-driven photocatalytic and photoelectrochemical applications.

Article titled "Photocatalytic hydrogen evolution on graphene quantum dots anchored Ti0₂ nanotubes-array" by Yifu Yu et al. published in International Journal of Hydrogen Energy, 2013, 38 (28), pp 12266-12272 reports the inversely structured Ti0₂ nanotubes-array (Ti0₂- NA) and CdS-modified Ti0₂ nanotubes-array (CdS/Ti0₂-NA) with graphene quantum dots (GQDs) anchored inside were prepared through a facile impregnation method. The activity evaluation results show that the hydrogen evolution rate during photocatalytic water splitting was greatly improved after loading GQDs into Ti0₂-NA and CdS/Ti0₂-NA. By breaking graphene into GQDs, the light-filtering effect of graphene was remarkably inhibited as compared with that of conventional large graphene sheets. Moreover, the overall morphology of Ti0₂ nanotube array could be well maintained after anchoring GQDs inside, which is favorable to mass transfer.

Article titled "Hot electron injection from Graphene Quantum Dots to Ti0₂" by Kenrick J. Williams et al. published in ACS Nano, 2013, 7 (2), pp 1388-1394 reports hot electron injection and charge recombination dynamics for graphene quantum dots (QDs, each containing 48 fused benzene rings) anchored to the Ti0₂ surface via carboxyl linkers. It find ultrafast electron injection from photoexcited graphene QDs to the Ti0₂ conduction band with time constant Ti < 15 fs and charge recombination dynamics characterized by a fast channel (Tri = 80-130 fs) and a slow one (T_r2 = 0.5-2 ps). The fast decay channel is attributed to the prompt recombination of the bound electron-hole pair across the interface. The slow channel depends strongly on excitation photon energy or sample temperature and can be explained by a "boomerang" mechanism, in which hot electrons are injected into bulk Ti0₂, cooled down due to electron-phonon scattering, drifted back to the interface under the transient electric field, and recombine with the hole on graphene QDs.

Article titled "Facile synthesis of anatase Ti0₂ quantum-dot/graphene-nanosheet composites with enhanced electrochemical performance for lithium-ion batteries" by Mo R et al. published in Advanced Materials, 2014; 26(13) pp 2084-8 reports a facile method to synthesize well-dispersed Ti0₂ quantum dots on graphene nanosheets (Ti0₂ -QDs/GNs) in a water-in-oil (W/O) emulsion system. The Ti0₂ /graphene composites display high performance as an anode material for lithium-ion batteries (LIBs), such as having high reversible lithium storage capacity, high Coulombic efficiency, excellent cycling stability, and high rate capability.

Article titled "Hydrothermal synthesis of CdTe quantum dots-Ti0₂-graphene hybrid" by Jinghua Liu et al. published in Physics Letters A, 2014, 378 (4), pp 405-407 reports CdTe- Ti0₂-graphene nanocomposites were successfully synthesized via a simple and relatively general hydrothermal method. During the hydrothermal environment, GO was reduced to reduced graphene oxide (RGO), accompanying with the anchoring of Ti0₂ nanoparticles on the surface of RGO. In the following process, CdTe quantum dots (QDs) were then in situ grown on the carbon basal planes.

Article titled "Preparation of graphene-Ti0₂ composites with enhanced photocatalytic activity" by Kangfu Zhou et al. published in New Journal of Chemistry, 2011, 35, 353-359 reports preparation of graphene-Ti0₂ (G-Ti0₂) composites through a one-pot solvothermal reaction by using graphite oxide (GO) and tetrabutyl titanate as starting materials. Ti0₂ particles with anatase phase and a narrow size distribution were dispersed on the surface of graphene sheets uniformly. The fluorescence quenching confirmed that graphene acted as an electron-acceptor material to effectively hinder the electron-hole pair recombination of Ti0₂. The product prepared with 30 mg of GO and 8 h of reaction time exhibited excellent photocatalysis to methylene blue (MB) degradation under irradiation of simulated sunlight. Such intriguing photocatalyst may find significant applications in various fields.

Article titled "Highly photoactive, low bandgap Ti0₂ nanoparticles wrapped by Graphene" by Joon Seok Lee et al. published in Advanced Materials, 2012, 24(8), pp 1084-1088 reports highly photoactive, graphene-wrapped anatase Ti0₂ nanoparticles are synthesized through one-step hydrothermal reduction of graphene oxide (GO) and Ti0₂ crystallization from GO- wrapped amorphous Ti0₂ NPs. Graphene-Ti0₂ nanoparticles exhibit a red-shift of the band- edge and a significant reduction of the bandgap (2.80 eV). Graphene-Ti0₂ nanoparticles possess excellent photocatalytic properties under visible light for the degradation of methylene blue.

Article titled "Green synthesis of biphasic Ti0₂-reduced Graphene Oxide nanocomposites with highly enhanced photocatalytic activity" by Md. Selim Arif Sher Shah et al. published in ACS Applied Material Interfaces, 2012, 4 (8), pp 3893-3901 reports a series of Ti0₂- reduced graphene oxide (RGO) nanocomposites were prepared by simple one-step hydrothermal reactions using the titania precursor, TiCl₄ and graphene oxide (GO) without reducing agents. Hydrolysis of TiCl₄ and mild reduction of GO were simultaneously carried out under hydrothermal conditions.

Article titled "Fabrication of Graphene Quantum Dots via size- selective precipitation and their application in Upconversion-based DSSCs" by Eunwoo Lee et al. published in Chemical Communication, 2013, 49, pp 9995-9997 reports a novel approach to synthesize highly luminescent graphene quantum dots (GQDs) with well-defined sizes was explored based on simple oxidation of herringbone-type carbon nanofibers (HCNFs) and size- selective precipitation.

Article titled "Highly luminescent S, N co-doped graphene quantum dots with broad visible absorption bands for visible light photocatalysts" by Qu D et al. published in Nanoscale, 2013, 5(24), pp 12272-7 reports a facile hydrothermal synthesis route to N and S, N co-doped graphene quantum dots (GQDs) was developed by using citric acid as the C source and urea or thiourea as N and S sources. Both N and S, N doped GQDs showed high quantum yield (78% and 71%), excitation independent under excitation of 340-400 nm and single exponential decay under UV excitation. A broad absorption band in the visible region appeared in S, N co-doped GQDs due to doping with sulfur, which alters the surface state of GQDs.

Article titled "Titanium Dioxide-based nanomaterials for photocatalytic fuel generations" by Yi Ma et al. published in Chemical Reviews., 2014, 114 (10), pp 9987-10043 reports recent research progress in solar fuel generation on Ti0₂-based photocatalysts, especially for production of hydrogen by biomass conversion and water splitting as well as generation of carbon-based chemical fuels by photoreduction of C0₂.

OBJECTIVE OF INVENTION:

The main objective of the present invention is to provide a novel surface disordered rutile Ti0₂-graphene quantum dots (Ti0₂-GQDs) hybrid multifunctional material. Another objective of the present invention is to provide a process for the preparation of novel surface disordered rutile Ti0₂-graphene quantum dots (Ti0₂-GQDs) hybrid multifunctional material.

SUMMARY OF THE INVENTION:

Accordingly, the present invention provides a novel surface disordered rutile Ti0₂-graphene quantum dots (Ti0₂-GQDs) hybrid multifunctional material.

In an embodiment, the present invention provides a process for the preparation of novel surface disordered rutile Ti0₂-graphene quantum dots (Ti0₂-GQDs) hybrid multifunctional material comprising the steps of:

a) mixing multi-walled carbon nanotubes (MWCNTs), sodium bismuthate, and acid in distilled water and stirring for 30-60 mins to obtain a black mixture;

b) heating the mixure of step (a) in the range of 100-250 °C for 10-15 hours to obtain the reacted colourless solution;

c) treating the reacted colourless solution of step (b) with base to precipitate-out bismuth ions from the solution;

d) adding titanium iso-butoxide, alcohol, and acid to the as-synthesized GQDs of step (c) to obtain an aqueous mixture;

e) stirring the mixture of step (d) in the range of 50-100 °C for 30 to 60 mins and then transferring into a Teflon lined autoclave followed by heating in the range of 100-250 °C for 10-15 hours to obtain a precipitate which on work- up furnish the desired product.

In still another embodiment, the present invention provides the said acid in step (a) may be nitric acid, sulfuric acid or mixture of both.

In yet another embodiment, the present invention provides the said heating in step (b) is in Teflon-lined autoclave.

In a preferred embodiment, the present invention provides a said base used in step (c) may be ammonium hydroxide or sodium hydroxide.

In another embodiment, the present invention provides the said acid used in step (d) is glycolic acid or oxalic acid. BRIEF DESCRIPTION OF THE DRAWINGS:

Figure 1: A comparison of room temperature XRD patterns of rutile-Ti0₂, rutile-Ti0₂-GQDs hybrid, and PCPDF#894920 data of rutile Ti0₂. The # marked peaks represent graphitic carbon.

Figure 2: A) The SEM images of the rutile Ti0₂-GQDs hybrid, B, C, D) EDX mapping of the sample showing the presence of Ti, O, C and E) overlapped C, Ti and O. F) overlap of SEM and EDX mapping. Here, Ti, O, and C atoms are denoted as yellow, green, and red respectively. Scale bars 50 μιη.

Figure 3: TEM images of A) Ti0₂-GQDs hybrid, and B) rutile Ti0₂. Scale bars 50 nm.

Figure 4: A, B) Comparison of bright-field and dark-field {(101) diffracted beam of Ti02} images of the same area of rutile-Ti0₂-GQDs hybrid. Images clearly displayed the uniform distribution of Ti0₂ and GQDs. C, D) HRTEM images of Ti0₂-GQDs hybrid. The areas marked with black and white closed curves are for GQDs and rutile-Ti0₂ grains, respectively. The lattice- spacing calculated from black and white areas are 0.33 and 0.24 nm respectively. Scale bars are 20, 20, 2, and 5 nm for A, B, C, and D, respectively.

Figure 5: (A) HRTEM images of surface-disordered rutile-Ti0₂ with (B, C) the panels showing line-scan analyses of disordered lattice of rutile Ti0₂. Black and red line-scans in HRTEM image corresponds to the top and bottom panels, respectively.

Figure 6: 13 C magic angle spinning (MAS) NMR spectra of rutile Ti0₂-GQDs hybrid.

Figure 7: A comparison between Raman spectra of rutile-Ti0₂ and surface disordered rutile- Ti0₂ hybrid.

Figure 8: A comparison between FTIR spectra of rutile-Ti0₂ and Ti0₂-GQDs hybrid.

Figure 9: A) UV-visible absorption and B) solid PL spectra of rutile-Ti0₂ and rutile Ti0₂- GQDs hybrid. The PL spectra were taken with an excitation wavelength of 370 nm.

Figure 10: XPS spectra of rutile Ti0₂-GQDs hybrid showing binding energies of A) C Is, B) Ols, and C) Ti 2p core levels.

Figure 11: XPS of rutile Ti02 showing binding energy spectra of A) Ols, and B) Ti 2p core- level electrons.

Figure 12: TGA thermogram of rutile-Ti0₂-GQDs hybrid.

Figure 13: The curves show methylene blue (MB) photodegradation in natural sunlight. A) absorption spectra of MB taken at various exposure time and the inset shows the relative concentration (C/C0) of MB plotted as a function of exposure time B) absorption spectra of rhodamine B (RhB) taken at various exposure time, the inset shows relative concentration (C/CO) of RhB plotted as a function of exposure time . Where C was the concentration at time t and CO was the initial concentration.

Figure 14: Absorption spectra dye after various cycles of photodegradation of A) MB after 6 min of natural sun light exposure and B) RhB after 4 min of natural sun light exposure.

Figure 15: A comparison of room temperature XRD patterns of rutile-Ti0₂-GQDs hybrid, rutile-TiOi-GQDs hybrid after 4th cycle and PCPDF#894920 data of rutile Ti0₂. The # marked peaks represent graphitic carbon.

Figure 16: A) The SEM image of the rutile Ti0₂-GQDs hybrid. EDX mapping of the sample showing the presence of B) Ti, C) O, D) C atoms and E) overlapped C, Ti and O. F) EDX data of the sample ~ 6% of C and T and O 32% and 61%, respectively. Here, Ti, O, and C atoms are denoted as yellow, green, and red respectively. Scale bar was 50 μιη.

Figure 17: A, B) Comparison of bright-field and dark-field {(101) diffracted beam of Ti0₂} images same area of rutile-Ti0₂-GQDs hybrid. Inset of B) shows the corresponding SAED pattern. Images clearly display the uniform distribution of Ti0₂ and GQDs. C, D) TEM images of Ti0₂-GQDs hybrid. Scale bars are 20, 20, 100, and 50 nm for A, B, C, and D, respectively.

Figure 18: A, B) HRTEM images show the distribution of rutile-Ti0₂ and GQDs in Ti0₂- GQDs hybrid. 0.33 nm and 0.21 nm lattice spacing are for GQDs (111) and (100) plane. 0.32 nm and 0.24 nm lattice spacing are for rutile-Ti0₂ (110) and (101) plane.

Figure 19: A, B, C) TEM images D) HRTEM image of rutile-Ti0₂ nanorods. Scale bars are 100, 50, 20, and 5 nm for A, B, C, and D, respectively.

DETAILED DESCRIPTION OF THE INVENTION:

The invention will now be described in detail in connection with certain preferred and optional embodiments, so that various aspects thereof may be more fully understood and appreciated.

In view of above, the present invention provides a novel surface disordered rutile Ti0₂- graphene quantum dots (Ti0₂-GQDs) hybrid multifunctional material and preparation thereof.

In an embodiment, the present invention provides a matrix comprising inorganic semiconductor and graphitic carbon, said graphitic carbon embedded in said matrix, such that the matrix is photo active in solar spectrum in the range of 200 to 800 nm. In another embodiment, the present invention provides a matrix comprising Titania and graphitic carbon, said graphitic carbon embedded in said matrix, such that the matrix is photo active in solar spectrum range of 200 to 800 nm.

In still another embodiment, the present invention provides a hydrothermal process of preparation of matrix comprising the steps of:

e) Stirring the mixture of step (d) in the range of 50-100 °C for 30 to 60 mins and then transferring into a Teflon lined autoclave followed by heating in the range of 100-250

°C for 10-15 hours to obtain a precipitate which on work- up furnish the desired product.

In another embodiment, said acid is used in step (a) is nitric acid.

In still another embodiment, said heating of step (b) is in Teflon-lined autoclave.

In yet another embodiment, said base of step (c) is ammonium hydroxide.

In still yet another embodiment, said acid is used in step (d) is glycolic acid.

In still yet another embodiment, said alcohol is used in step (d) is isopropyl alcohol.

In still yet another embodiment, said process is simple and efficient process.

In still yet another embodiment, the present invention provides a surface disordered Ti0₂- GQDs hybrid showing 98% degradation of MB within 6 min in the presence of natural sunlight.

In still yet another embodiment, the present invention provides a surface disordered Ti0₂- GQDs hybrid having potential for huge applications in various technologies related to energy, health, and environment.

EXAMPLES:

The following examples, which include preferred embodiments, will serve to illustrate the practice of this invention, it being understood that the particulars shown are by way of example and for purpose of illustrative discussion of preferred embodiments of the invention. Example 1:

15 mg of multi-walled carbon nanotubes (MWCNTs), 1 mmol of sodium bismuthate, and 3 mL of nitric acid in 27 mL of distilled water were mixed for half an hour. The black- coloured mixture was transferred inside a 70 mL Teflon-lined autoclave and was heated at 200 °C for 12 h. After that, the reacted colourless solution was treated with ammonium hydroxide to precipitate-out bismuth ions from the solution. From the as- synthesized GQDs, 20 mL volume was taken in a beaker and into that 5 mL of titanium iso-butoxide, 5 mL of isopropyl alcohol, and 3 g of glycolic acid were added. This aqueous mixture was stirred in 70 °C for 30 min. Then, it was transferred into a 70 mL Teflon lined autoclave and was heated at 200 °C for 12 h. After the reaction, a black coloured precipitate was observed at the bottom of the Teflon vessel. The obtained precipitate was washed by centrifuging thrice at 5000 rpm for 5 min periods, with intermediate re-dispersion of the precipitate in ethanol to remove the unreacted reactants and impurities.

For the synthesis of white-Ti0₂, analogous reaction procedure was performed in aqueous medium without adding the GQDs and was used after following the similar purification steps as mentioned above for the hybrid material. These ethanol-washed samples were used for further characterizations and photocatalytic reactions. Refer figures below for characterization data.

The powder x-ray diffraction (PXRD) patterns of both white and black-coloured titania samples matched with PCPDF#894920 for rutile-Ti0₂ (Fig. 1). The XRD data of black- coloured sample was collected at a slow scan-rate but it appeared to be still noisy, indicating the formation of disordered titania in the Ti0₂-GQDs hybrid. Additionally, in the black- coloured sample, at 2Θ -25° and -47°, humps were observed, which are characteristic of graphitic carbon (Fig. 1). The hump at -25° was generally regarded as (002) peak of graphite. There were no indications of impurity peaks in both the XRD spectra. Energy dispersive X- ray (EDX) analysis of as-synthesized hybrid material showed ~6 weight percent of carbon in Ti0₂-GQDs hybrid. The EDX mapping of hybrid sample showed the carbon, titanium and oxygen uniformity over the entire grain (Fig. 2 and 16). The EDX analysis gives oxygen in double weight percent of titanium which confirms the stoichiometry of the rutile-Ti0₂ (Fig. 16). The high resolution transmission electron microscopy (HRTEM) images showed that the as-synthesized hybrid material was not regular rod-shaped where as rutile-Ti0₂ showed nanorod-type of appearance (Fig. 3 and 10). The rutile-Ti0₂ was prepared in an aqueous medium containing GQDs, so the growth of the Ti0₂ was hindered during the reaction. Therefore, instead of regular Ti0₂ structure, found some small grains assembled together containing rutile-Ti0₂ and GQDs (Fig. 4 and 18). In HRTEM images at higher magnification, find this type of texturing in the hybrid material. The lattice-disordering was also confirmed by the HRTEM line-scan analysis on a single nanocrystallite grain of rutile Ti0₂ (Fig. 5). Figure 5 shows the surface disorderness in the surface-disordered rutile-Ti0₂ with (B, C) the panels showing line-scan analyses of disordered lattice of rutile Ti0₂. Bright and dark field HRTEM imaging of the same region of the sample was performed to confirm the Ti0₂ and GQDs distribution. It was observed that Ti0₂ was not a continuous matrix rather GQDs were present all over the matrix containing rutile Ti0₂ (Fig. 4 and 17). This further confirms the SEM EDX mapping results the formation of uniform Ti0₂-GQDs hybrid. The reaction was carried out in high pressure hydrothermal condition and GQDS were inserted in the Ti0₂ precursor mixture. After the gentle heating at 70 °C for 30 min, the titanium hydroxide, which appears to be in the lumps form, gets dissolved in the warm condition and consequently get a homogeneous precursor of Ti0₂ and GQDs. In the hydrothermal condition, the rutile phase Ti0₂ starts forming. But GQDs were there in the solution, so the rutile phase Ti0₂ don't gets enough opportunity to grow. But in absence of GQDs, the Ti0₂ growth was not hindered and appears to be rod type texture. So this condition gave rise to Ti0₂ and GQDs domains site next to each other. In hydrothermal condition the pressure goes inside very high and the domains of rutile phase Ti0₂ were very small. This drastic condition may cause to the disordering in the surface of rutile phase Ti0₂. Another possible reason for disordering was that the Ti0₂ was being forming by bottom-up approach and GQDs were attached to the surface during its growth resulting was surface disordered rutile Ti0₂.

The 13 C magic angle spinning (MAS) studies revealed the presence of graphitic carbon with carboxylic functionalities in the as-synthesized rutile Ti0₂-GQDs hybrid. A chemical shift observed at 179.1 (0=C-0), 131 (graphitic-sp2), 75.7 (C-OH) and 59.1 (C-O-C) p.p.m. This signature chemical shift confirms the presence of graphitic carbon and carboxylic functionalities in the hybrid (Fig. 6). Further, Fourier transformed infrared spectroscopy (FTIR) was recorded for the rutile-Ti0₂ and rutile Ti0₂-GQDs hybrid. The band at 680 cm-1 was attributed to the Ti-O-Ti vibration which was present in both pure rutile Ti0₂ and rutile Ti0₂-GQDs hybrid. However, in the case of pure rutile Ti0₂, the above mentioned vibration band appeared to be very intense in contrast to the rutile Ti0₂-GQDs hybrid. In the hybrid material find the signature of C=C bend (1653 cm-1) and C-0 stretch (1343 cm-1) (Fig. 8). Raman spectroscopy was recorded for the as-prepared samples. The rutile Ti0₂ exhibits strong Raman peaks situated at ~ 443 and ~ 604 cm^"1 which were attributed to Eg and Alg transitions, respectively. In addition, a strong order effect was observed as a peak at 231 cm^" (Fig. 7). Five additional weak peaks were also observed that were characteristics of rutile- Ti0₂. In case of the hybrid, the 150 cm-1 peak appears to be stronger in comparison to the other peaks. The shift of the 443, 604 and 231 cm^"1 peak might be due to disorder behaviour of rutile-Ti0₂ or attachment of GQDs on Ti0₂.

UV-vis spectroscopy was measured for the rutile Ti0₂ and surface disordered rutile Ti0₂- GQDs hybrid. In both the cases got the absorption peak around 340 nm which was expected for rutile phase Ti0₂. In surface disordered rutile Ti0₂-GQDs hybrid, due to the presence of GQDs get a absorption feature in lower wavelength. Apart from that in surface disordered rutile Ti0₂-GQDs hybrid the absorption feature was broad in nature (Fig. 9). For both rutile Ti0₂ and surface disordered rutile Ti02-GQDs hybrid, photoluminescence spectra (PL) was taken and it was found that the spectral intensity for the hybrid was reduced by almost 10 order magnitude in comparison to rutile Ti0₂ (Fig. 9). The lower PL intensity in Ti0₂-GQDs hybrid may be due to higher charge recombination of electron and hole. The GQDs were having high electron accepting and transfer capability so the electron can move very fast from Ti to C. This mare fact helps to increase the photocatalytic performance.

The x-ray photoelectron spectroscopy data (XPS) were taken for rutile-Ti0₂ and surface disordered rutile Ti0₂-GQDs hybrid. Titanium (Ti) 2p binding energy peaks for hybrid sample was shifted towards higher energy by ~1 eV in comparison to rutile-Ti0₂ (Fig. 10 and 11). This shift may be due to the uniform distribution of GQDs in Ti0₂ matrix which leading to the increase in binding energy. A comparison between the O ls spectra of rutile-Ti0₂ and disordered rutile Ti0₂-GQDs hybrid showed a marked difference. In case of disordered Ti0₂, observed two binding energy peaks situated at ~ 531.7 eV and -533.2 eV respectively. The peak at 533.2 eV mainly appears due to oxygen attached to the graphitic carbon (Fig. 10 and 11). Whereas, in rutile Ti0₂, observed only one binding energy peak situated at -530.4 eV which appears due to Ti0₂ binding energy. Similar to the Ti 2p, in case of Ti0₂-GQDs hybrid, the O ls peak shifts to higher biding energy by - 1 eV due to the effect of GQDs association. The possibilities of GQDs oxygen functionalities cannot be ruled-out. But this also gives an indication of disordered behaviour of rutile Ti0₂ in Ti0₂-GQDs hybrid. Carbon Is spectrum of Ti0₂-GQDs hybrid showed binding energy peaks situated at - 285 eV, 286.5 eV and 288.7 eV which can be assigned to C=C, C-OH and C=0 respectively (Fig.10).

Quantitative information of the synthesized hybrid material was obtained by thermo gravimetric analysis (TGA). In the spectrum, found an initial weight-loss (-60 °C) which can be due to some amount of adsorbed ethanol as the material was washed several time with ethanol in the purification process (Fig. 12). The second weight-loss region was noticed after ~ 130 °C. This was due to the fact that the GQDs were terminated with surface carboxylic group and oxygen functionalities which start decomposing around -130 °C. In the case of hybrid, there was a strong interaction between rutile Ti0₂ and GQDs. The third weight- loss region extending from ~ 350 °C to -460 °C was due to graphitic carbon (Fig. 12). After the TGA experiment, it was observe that the black coloured hybrid material turned into yellowish coloured material. Qualitatively, it can be concluded from the TGA analysis that the hybrid material contains 6-10% of carbonous material. The EDX analysis also showed that the synthesized hybrid contains 6% of carbon which was in rough agreement with the TGA analysis (Fig. 16 and 12).

The photocatalytic activity of as-synthesized hybrid was investigated with Methylene Blue (MB) degradation in presence natural solar light. Ten mL of 5* 10^"5 M MB solution was taken for the photocatalytic degradation with its optical density (O.D) around be 1.6. In the dye solution, added 10 mg of as- synthesized disordered rutile Ti0₂-GQDs hybrid material and the resultant dye-hybrid solution was irradiated with natural solar-light. The optical density measurements using UV-vis spectroscopy at various exposure time of sun-light showed that after 15 sec -91% of the MB degraded with natural solar light illumination. Moreover, after 30 sec, 45 sec, 2 min, 4 min and 6 min exposure of sun-light, noticed the degradation of MB to be -92%, -93%, -95%, -96% and -98% respectively (Fig. 13). To check the sustainability of the as-synthesized hybrid material, centrifuged-out the material and used it for three more cycles. It was noticed that even after 3rd cycle, the disordered rutile Ti0₂-GQDs hybrid material was able to degrade -93% of MB dye within -6 min (Fig. 14). There was no shift of peak position of degraded MB dye which indicates a single step degradation process. After 3rd cycle, the remaining hybrid material was collected by ethanol washing and the PXRD pattern was recorded in order to verify the stability of the material after 3rd cycle of dye- degradation. It was observed that there was no change in crystalline-phase of the material. Both the peaks for rutile Ti0₂ and graphitic carbon hump at -25° were present in XRD pattern (Fig. 15). This shows that after 3rd cycle of dye-degradation the materials remain same and can be used further. The catalytic behaviour was also tested on rodamine B (RhB) dye. For RhB degradation, took 10 mL of 10^"5 M concentrated solution and 10 mg of as- synthesized disordered rutile Ti0₂-GQDs hybrid material and rest of the procedure was kept the same as above. In case of RhB, observed -90% of degradation in 1st cycle within 4 min of natural solar light exposure (Fig. 13). However, after 3rd cycle of dye-degradation, we observed -52% degradation of RhB after 4 min of exposure (Fig. 14). A drastic increase in solar light driven photocatalysis in disordered rutile Ti0₂-GQDs hybrid was mainly due to its increased solar light harvesting efficiency. In the case of this surface disordered rutile Ti0₂- GQDs hybrid, it is not only the disordered nano titina surface which increases the solar harvesting efficiency but also the quantum size effect of graphene plays an important role in solar spectrum absorption. Due to high pressure reaction in hydrothermal vessel, the small domains of Ti0₂ gets disordered surface. It was observed that GQDs are terminated with surface carboxylic group. This also acts as a surface active moiety, which, in turn helps to increase photocatalytic activity. In addition, the rough surface in the as-prepared enhances the chances of multiple scattering to increases absorption possibilities.

Table 1: Raman spectral peaks of rutile- Ti0₂ and Ti0₂-GQDs hybrid.

ADVANTAGES OF INVENTION:

1. Novel multifunctional material.

2. Simple and efficient process.

3. The material can be used in solar light harvesting

Claims

A matrix comprising surface disordered inorganic semiconductor or inorganic semiconductor and graphitic carbon.

The matrix as claimed in claim 1, wherein said graphitic carbon is embedded in said matrix such that the matrix is photo active in solar spectrum range of 200 to 800 nm. The matrix as claimed in claim 1, wherein said inorganic semiconductor is Titania. A hydrothermal process of preparation of matrix as claimed in claim 1, wherein said process comprising the steps of:

a) mixing multi-walled carbon nanotubes (MWCNTs), sodium bismuthate, and acid in distilled water and stirring for a period of time ranging between 30-60 mins to obtain a black mixture;

b) heating the mixure of step (a) at a temperature in the range of 100-250 °C for a period of time in the range of 10-15 hours to obtain the reacted colourless solution;

d) adding titanium iso-butoxide, alcohol, and acid to the as- synthesized GQDs of step (c) to obtain an aqueous mixture;

e) stirring the mixture of step (d) at a temperature in the range of 50-100 °C for a period of time in the range of 30 to 60 mins and then transferring into a Teflon lined autoclave followed by heating at a temperature in the range of 100-250 °C for a period of time in the range of 10-15 hours to obtain a product.

The process as claimed in claim 4, wherein said acid in step (a) are nitric acid, sulfuric acid or mixture of both.

The process as claimed in claim 4, wherein said heating in step (b) is in Teflon-lined autoclave.

The process as claimed in claim 4, wherein said base used in step (c) is ammonium hydroxide or sodium hydroxide.

The process as claimed in claim 4, wherein acid used in step (d) is glycolic acid or oxalic acid.