Artificial leaf technology for co2 transformation into cn- oxygenates with metal oxides photocatalyst

Abstract

The present disclosure relates to development of a bifunctional photocatalytic thin film with artificial leaf technology for reduction of carbon dioxide. The present disclosure also relates to process of preparation of a bifunctional photocatalytic thin film for reduction of carbon dioxide. Further, the present disclosure also discloses a process of preparation of products like oxygenates up to C1-C4 alcohols, aldehydes / ketones, and H2.

Description

[0001] ARTIFICIAL LEAF TECHNOLOGY FOR CO2 TRANSFORMATION INTO CN- OXYGENATES WITH METAL OXIDES PHOTOCATALYST FIELD OF THE INVENTION The present invention provides artificial leaf technology for CO2 transformation into Cn- oxygenates with metal oxides photocatalyst. Particularly, the present invention provides artificial photosynthesis and / or photocatalytic thin films and a process for production thereof. More particularly, the present disclosure provides an artificial photosynthesis device and a process for the fabrication thereof. Further the present invention relates to a process for the production of Cn-oxygenates (n = 1-4) from CO2 and H2O with the help of the earth's abundant metal-based oxides bifunctional photocatalyst with a 10-65% conversion of CO2 in sunlight or any suitable light source. BACKGROUND OF THE INVENTION There's considerable interest in the development of photocatalytic CO2conversion by inexhaustible sources, such as, sunlight, since this process as parallels with natural photosynthesis on which life on earth is grounded. At the moment, the critical effort in this field has been aimed at adding productivity, rather than at the control of product distribution / selectivity. Particularly, organic molecules with two or more carbon atoms (C2+) have advanced added value such as ethanol, propanol, propane, butanols, and acetone than methane, carbon monoxide, or formate, which are generally the major products of CO2reduction. Many unsuccessful attempts have been made in the past with photocatalytic CO2 reduction to products, like C-C coupled products such as, C2 and higher oxygenates (eg. C2H5OH), C3-C4 hydrocarbons (propanes and / or butanes). The article entitled “Ti3+Defective TiO2 / CdS Z-Scheme Photocatalyst for Enhancing Photocatalytic CO2 Reduction to C1−C3 Products” by Hua Wang et al., [Industrial & Engineering Chemistry Research, 2022, 61, 8724-8737] reports direct z-scheme photocatalyst developed by Ti3+defective TiO2 / CdS nanoflower (NF) helps to enhance C1-C3hydrocarbons production from CO2reduction. Herein, NF-TiO2 / CdS heterojunctions are constructed via the sulfurization of TiO2 / Cd-MOF. Photocatalytic CO2 reduction activity was measured with 30 mg of catalyst powder in 20 ml of 0.1 M NaOH and 10 mL of triethanolamine (TEOA) under 300W Xe lamp (UV) irradiation. The cumulative yields of CO and methane obtained with optimal heterojunction photocatalyst NF-TiO2 / CdS0.6 are 1007.0 μmol / g and 83.5 μmol / g (in 2.5 h), respectively; in addition to a combined yield of 55.3 μmol / g (2.5 h) of C2H4(ethylene) and C3H6(propylene) is observed. This enhanced yield of C1−C3 products is attributed to the synergistic effects of abundant Ti3+defects that increase adsorption and activation of CO2, the large surface area that provides sufficient active sites, and a direct Z-scheme heterojunction with a good intimate contact interface that effectively separates charge carriers and accumulates electrons with strong reducibility. Though the photocatalyst has a tendency to form C2-C3 hydrocarbons as CO2 reduction product but applicability of large-scale production is still limited as a powder form of catalyst is incorporated in the reaction. Also, in the absence of NaOH, a sacrificial TEOA reagent, and in the visible light (or sunlight) no C1-C3 hydrocarbons could form. Hence it limits the use of the reported catalyst in direct sunlight. Further, use of NaOH alkali would make the CO2to transform to carbonate and bicarbonates, which is not a preferred form (of CO2) as it requires more energy for conversion. Above all, CdS is known to undergo photo-corrosion under irradiation conditions and Cd should be avoided due to its poisonous nature, especially in view of the products will be in contact with catalyst surface as well as leached Cd in solution. The article entitled, “Artificial Photosynthesis of C1–C3 Hydrocarbons from Water and CO2 on Titanate Nanotubes Decorated with Nanoparticle Elemental Copper and CdS Quantum Dots” by Michael R. Hoffmann et al., [J. Phys. Chem. A2015, 119, 4658–4666] reports ternary catalyst contains Cds / Cu-TNT where sodium trititanate (NaxH2−xTi3O7) nanotubes decorated with surface deposits of elemental copper (Cu0) and sequential deposits of CdS quantum dots. This ternary photocatalyst is capable of conversion of CO2and water into C1−C3 hydrocarbons (e.g., CH4, C2H6, C3H8, C2H4, and C3H6) upon irradiation with 450 W Xe lamp in visible light above 420 nm. This report contains utilization of simple inorganic photocatalytic system; however CdS is known to undergo photo-corrosion under irradiation conditions and Cd should be avoided due to its poisonous nature. Hence the stability of this ternary photocatalyst for a longer period is still an unresolved matter. The article entitled, “Photocatalytic reduction of CO2over Ag / TiO2nanocomposites prepared with a simple and rapid silver mirror method” by Zhigang Zou et al., [Nanoscale, 2016, 8, 11870]. Mainly methane and methanol, as well as a small amount of C2 and C3 species, such as, acetaldehyde and acetone, were observed. However, Ag / TiO2 works in UV light, and the reaction was carried out in CO2 saturated NaHCO3 solution, which is not the preferred method. The article entitled “Facet-dependent active sites of as single Cu2O particle photocatalyst for CO2 reduction to methanol” by Y. A Wu et al., [Nature Energy, 4, 2019, 957] also reports a very high CO2conversion to methanol, at the rate of 1.2 mol / h.g of the catalyst with solar to fuel (STF) efficiency of 10 %. However, experiments were carried out with 10 mg of Cu2O nanoparticles in powder form and suspended in water, and continuously purged with CO2 / H2O gas mixture till the solution is saturated with CO2. CO2saturated Cu2O containing solution is illuminated with a 300 W Xe lamp between 0 and 60 min. under continuous CO2 / H2O flow and the product is analysed periodically.10 mg of particulate catalyst produces 0.133 mmol / s.g of methanol and yet to be demonstrated at higher scale; although it is claimed to produce 1.2 mol / h.g methanol through simple weight normalization, 0.133 mmol / s.g and 1.2 mol / h.g do not match by extrapolation from second to hour by a factor of 3600. With this photocatalyst, only methanol is observed as a product and no C-C coupled products, such as ethanol or ethylene, were observed. It is to be noted that, unlike conventional catalysis, there are many difficulties / issues associated with scaling up the photocatalysis experiments with larger amounts of powder photocatalysts and indeed lower activity is reported at higher scale (e.g.100 mg and above) of catalysts. Indian patent Application no 202211074765 by Chinnakonda Subramanian Gopinath, Kranti Nishikant Salgaonkar, provides an artificial photosynthesis device and a process for fabrication thereof. The application further discloses a process for production of C1 / C2 compounds from CO2 and H2O with the help of the artificial photosynthesis device with a solar to fuel efficiency of 14.7 %. The work reported by Naresh and Gopinath in US2022 / 0048018 A1 deals with Cu2O deposited on the surface of TiO2 and coated as a thin film by drop-casting method. It is to be underscored that the above mentioned work leads to the conversion of methanol to formaldehyde along with hydrogen generation, and no CO2 reduction was reported. Hence, there is still a need to develop an effective photocatalyst system or device which can provide effective and better CO2 reduction into value added products, especially drop-in-fuels, making it the green technology considering conversion of pollutant CO2 into industrially useful value added products such as C1-C4 oxygenated products, etc. Present invention also could lead to longer chain carbon-containing molecules (C5-C10molecules), that could be similar to gasoline fraction typically used in automotive vehicles and many other applications. OBJECTIVES OF THE INVENTION The primary objective of the present invention is to develop a bifunctional photocatalytic thin film for reduction of carbon dioxide. Secondly, the present invention is to provide a process of preparation of the bifunctional photocatalytic thin film for reduction of carbon dioxide. Another object of the present invention is to provide a process of preparation of C1-4-oxygenates from carbon dioxide and water. Yet another object of the present invention is to provide an improved process for carbon dioxide reduction without any co-catalyst and photoanode. SUMMARY OF THE INVENTION Accordingly, the present invention provides a bifunctional photocatalytic thin film for reduction of CO2, wherein the film comprises of: i) metal oxide quantum dots MxOy, ii) a porous semiconductor metal oxide, and iii) a support; wherein the porous semiconductor metal oxide is pre-coated onto said support and said metal oxide quantum dots MxOy is assembled from ionic precursors and integrated into the pores of said porous semiconductor metal oxide as quantum dots of MxOy(where x = 1 to 3; y = 1-4). In an embodiment of present invention, the MxOy is 3d or 4d transitions metal oxide, wherein metal M is selected from the group consisting of Cu, Ni, Co, Mo, Ag and a combination thereof into a porous semiconductor metal oxide is selected from TiO2, ZnO and polymorphs thereof. In another embodiment of present invention, the said MxOy is CuxO and said porous semiconductor is TiO2. In still another embodiment of present invention, the support is selected from Fluorine doped tin oxide (FTO) and Indium Tin Oxide (ITO) plate. In still another embodiment of present invention, the bifunctional photocatalytic thin film is characterized in that; a. the metal Cu is integrated, structurally and electronically, into pores of the wide band gap semiconductor TiO2 in the form of metal oxide CuxOy quantum dots, b. metal oxide CuxOy quantum dots are integrated, structurally and electronically, 90-100% into pores of the semiconductor and 1-10% on the exterior surface of the semiconductor, c. pore formed inside said TiO2 with size of up to 10 nm, d. quantum dots have a particle size ranging from 1 nm to 10 nm, e. the thin film having a stability and electrons population due to structural and electronic integration of MxOy and TiO2, leading to heterojunctions along the periphery of MxOy, as they are contained within the pores of TiO2, and f. the thin film is having a dimension of at least 1x1 cm2to 25 cm2and / or up to 1x1 square feet. In still another, the present invention provides, the process of preparation of bifunctional photocatalytic thin film by SILAR method, wherein the process comprises the steps of: i. pre-coating the metal oxide (TiO2) onto said support (FTO plate) to obtain pre-coated thin film, ii. immersing said pre-coated thin film obtained at step i) into metal-ion precursor solution (copper nitrate solution) at a temperature in the range of 15-45°C for a time period of 20-70 seconds to obtain copper-ion coated in the nanopores of TiO2thin film, iii. washing said metal coated film using water, and iv. then treating with solution of base (NaOH) and hydrazine hydrate followed by final washing with water, drying, and calcination to obtain said artificial photosynthesis device or bifunctional photocatalytic thin film. In yet another, the present invention provides, a process for the preparation of value-added products, wherein the process comprises of solubilizing CO2 in water and reacting with bifunctional photocatalytic thin film under static and flowing conditions in the presence of light. In yet another of present invention provides, the illumination conditions are created by exposing the photocatalytic thin film to a light selected from a UV-visible light, a visible light, visible + NIR light and direct sunlight. In yet another of present invention provides, the said value-added products are selected from the group comprising of C1-C4 oxygenates selected from alcohols and aldehydes / ketones, and H2. In yet another of present invention provides, the conversion efficiency of carbon dioxide to Cn oxygenates is ~65% under direct sunlight in 5h, where n is between 1-4. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 provides schematic picture of SILAR process carried out for preparation of bifunctional photocatalyst-Cu2O quantum dots incorporated and integrated into the pores of TiO2. CTU and CTC indicating the device made without calcination (or uncalcined) and after calcination in inert atmosphere, respectively. Fig. 2 provides (a) UV-visible absorption spectra of TiO2, Cu2O and CTC and CTU. (b) Band gap of Cu2O@TiO2 bifunctional photocatalyst (CTC), realized in accordance with an embodiment of the present disclosure. Note the CTU shows only marginal visible light absorption, in contrast to visible light and NIR absorption in a wide wavelength region for CTC. Cu2O also shows light absorption only up to 600 nm. Indeed Cu2O@TiO2 electronic integration brings light absorption beyond 600 nm and up to 800 nm (as shown in Fig.9 too). Fig. 3 provides representative HRTEM images of Cu2O-TiO2 bifunctional photocatalyst, realized in accordance with an embodiment of the present disclosure, showing Cu2O QDs formed inside pores of TiO2with the formation of heterojunction between them. Uniform size (~2 nm) quantum dots are observed over the entire range of Cu2O-TiO2 underscores the uniqueness of the present synthetic protocol and its preference to occupy micropores (A) 20nm and (B) 5 nm. Fig. 4 provides CO2 reduction products observed with Cu2O-TiO2 bifunctional photocatalyst, realized in accordance with an embodiment of the present disclosure, and analyzed by NMR methods. Fig. 5 provides Products yield obtained from Cu2O-TiO2 bifunctional photocatalyst as a function of irradiation time under one sun condition realized in accordance with an embodiment of the present disclosure. Fig. 6 provides Products yield obtained from Cu2O-TiO2 bifunctional photocatalyst as a function of irradiation time under direct sunlight realized in accordance with an embodiment of the present disclosure.

[0002] Fig. 7: The scalability of the photoanodes of different device sizes. (a) Products yield obtained over a 5-hour exposure to sunlight reaction time of 1, 2.25 and 6.25 cm2device sizes and (b) Individual solar to fuel efficiency (STFE) for various products obtained in 5 h along with CO2 conversion and combined STFE for CTC Bi-APS. A linear increase in products yield observed with increasing area of the device size demonstrates the scalability to even bigger sizes. Fig. 8: Powder X-ray diffraction (PXRD) pattern of bulk Cu2O, CTC, TiO2with JCPDS pattern for Cu2O cubic is also given for reference. Only selected Cu2O diffraction features are observed in CTC, while majority of Cu2O features not observed underscores the selected crystallographic interaction between pore embedded Cu2O with TiO2. Due to structural integration, many of the TiO2 features too shifts to lower diffraction angle, ascertain the integration aspects. Fig. 9. Apparent quantum yield (AQY) calculated at different wavelengths for CTC device and compared with UV-Vis absorption spectrum; a good correlation is observed among them underscores the light harvesting occurs in visible and NIR regions, due to Cu2O QDs. Fig.10. Wavelength-dependent incident photon-to-current efficiency (IPCE) performance of TiO2, Cu2O and Cu2O-TiO2 photoanode; inset shows the integrated current density calculated as a function of light wavelength. DETAILED DESCRIPTION OF THE INVENTION The term “Earth abundant metals” used throughout specification is defined as those metals available at least 1000 times larger than that of Ir precious metal in terms of atom fraction of the elements in Earth’s upper continental crust (Ref. https: / / en.wikipedia.org / wiki / Abundance_of_the_chemical_elements). “Integration” in the present description demonstrates the structural and electronic integration of constituent components into one (photocatalyst or photoanode) material. After integration of individual components, as defined in the synthesis method, they lose, at least partially, their individual identity and acquire a new identity. This is supported by the changes observed in visible light absorption of the integrated material, embedding small QDs (quantum dots) of visible light absorbing oxide in the pores of other oxide (wide band-gap semiconductor oxide), new reactivity observed through unique products etc., and are not limited to the above. Integration also includes the constituent components are in electrical contact with each other, which allow electron-hole separation at the heterojunctions and conduction / dispersion of charge carriers to the reaction sites. Interaction of pre-formed Cu2O QDs interacting with TiO2 on latter’s surface through heterojunction formation, which is generally mentioned as Cu2O / TiO2 in the prior arts, is different from the present synthesis method described, which involves assembling of ionic components of copper and hydroxyl ions in the nanopores of TiO2 and QDs compulsorily forms within the restricted nanopores. While the generally reported procedure in prior arts involves the external interaction of QDs and TiO2 particles. “Heterojunction” in the present disclosure is different from the conventional heterojunctions; contact between two different semiconductor particles occurs within a limited area / edges. However, in the present document, QDs of Cu2O or other semiconductors are in contact along its entire periphery with another wide band-gap semiconductor (i.e. TiO2), hence the charge separation is highly facilitated across the entire periphery of the QD. In other words, pore embedded QDs can be considered as core and TiO2 may be considered as shell, and hence the entire surface of core is in electrical contact with shell material. Conventional core@shell forms by making core first followed by deposition of shell material; however, the present synthesis method allows the diffusion of ionic precursors in the pores of shell material and employs the “bottom-up” approach to make core, as described in synthesis procedures. Bi-functional photocatalyst in the present invention is defined as follows: (a) regular functions of any photocatalyst that absorbs sunlight, generates charge carriers and they are separated into electron and holes at the heterojunctions; (b) charge carriers are also utilized for oxidation (water oxidation to oxygen) as well as reduction (CO2 reduction to Cn oxygenate products) reactions, without employing any additional co-catalyst. It is to be noted that any prior art involves at least one (or two) co-catalyst(s) for oxidation and / or reduction reaction(s). Although it is mentioned as bi-functional for simplicity in the present document, the devices reported actually performs multifunctions (from the first few initial steps of sunlight absorption, water and CO2 activation to product molecules formation as the last step), and they are broadly classified into two functions. The term semiconductor, semiconductor substrate, and the like is generally used to refer to compositions, structures, devices, etc. that include materials that have semiconductive properties, unless stated. The term visible light absorbing semiconductor, light absorbing oxide, light absorbing material, sunlight absorbing material and the like include materials that absorbs visible light and near-IR photons anywhere in the range between 400 and 1000 nm. This is in addition to the possible absorption of UV light below 400 nm photons. The term Cu2O-TiO2, Cu2O@TiO2, Cu2O embedded in TiO2 pores and the like is used to refer to the Cu2O quantum dots integrated within the nanopores of TiO2. The term artificial photosynthesis, artificial leaf, photoanode with / without co-catalyst and the like is generally used for the devices that converts light energy into chemical energy and store them in the product molecules, much like natural photosynthesis. This disclosure pertains to the domain of artificial photosynthesis, with a specific emphasis on crafting an artificial photosynthesis device by using earth-abundant metal oxides. Additionally, it encompasses the innovation of a photocatalytic material and the associated production methodology. Furthermore, this disclosure encompasses a technique for generating Cnoxygenates (n=1-4) from carbon dioxide and water using the artificial photosynthesis device. One critical aspect of the current disclosure offers the bifunctional photocatalytic characteristic of the material. Water oxidation as well as CO2 reduction occurs on the same catalyst, but on different sites, and without the aid of any co-catalyst / cathode material for redox reactions. This is in contrast to the earlier findings as well as our earlier patent disclosures. This material encompasses metal oxide semiconductor quantum dots combined with a porous semiconductor, where the semiconductor quantum dots consist of 3d or 4d transition metal oxide. The invention relates to a bifunctional photocatalytic thin film for the reduction of CO2, the film comprising: i) metal oxide MxOy QDs (e.g. Cu2O, etc.) assembled and integrated with, (ii) another metal oxide (e.g. TiO2, etc.) and iii) support (e.g. glass slide, FTO plate, ITO plate etc.); wherein the metal oxide is pre-coated onto said support and said metal oxide quantum dots MxOy (where x = 1 to 3; y = 1-4) is assembled from ionic precursors and integrated into the pores of said metal oxide as Quantum dots exhibiting special electronic and catalysis properties due to their small size than its counterpart with bigger size particles. When quantum dots are electronically and structurally integrated with another material, the photocatalytic performance increases enormously due to the separation of charge carriers at the interface of Cu2O-TiO2. The said bifunctional material acts as artificial photosynthesis material, which is free from co-catalyst, and operates independently to oxidize water and reduce CO2 in the presence of light to give Cn-Oxygenates (n=1-4) as products. This also minimizes the diffusion of charger carriers to the long distances, as the reduction and oxidation sites are available in close proximity, which is not the case in any photocatalyst system. The above mentioned bifunctional photocatalyst operates as a standalone system that helps harness the light to reduce CO2 to useful products. The present bifunctional photocatalyst simplifies the system and avoids the use of additional materials hence becoming advantageous for practical applications. The present invention further relates to a process of preparation of said bifunctional photocatalytic thin film by SILAR method, comprising steps of: i) pre-coating the metal oxide (TiO2) onto said support (FTO plate / ITO plate) to obtain the pre-coated thin film, ii) dipping or immersing said precoated thin film into metal-ion precursor solution at specific conditions and for a specific time period to obtain the metal-ion coated thin film, iii) washing said metal coated film using water, iv) than treating with a solution of base (NaOH) and hydrazine hydrate followed by final washing with water, drying, and calcination to obtain said photocatalytic thin film. Last step is required to produce hydroxyl form of metal-ion, i.e. Cu(OH)2, followed by its conversion to Cu2O. The SILAR method employed in present embodiments is for M = Cu-ion. Indeed, it can be further applied to any 3d and 4d transition metal-ions. The particular emphasis of copper in the present photocatalyst system is to make the CO2reduction process for the formation of C2+ products. Cu-based catalyst is well known for C-C coupling. To make a perfect photocatalytic system to give Cn-oxygenates (n = 1-4) while maintaining the copper oxidation state is crucial for the stabilization reaction and to facilitate the C-C coupling. Thus, photocatalysts with precisely tailored and stabilized copper oxidation state is highly desired due to their intriguing properties. The metal oxide used in the present invention is to fabricate a thin film-based bi-functional photocatalyst consists of Cu as source metal. Integration of copper oxide-based material with porous TiO2 is made by employing SILAR method. NaOH and hydrazine hydrate play a role in the integration of metal oxides into TiO2 pores. Metal-ion adsorbed on TiO2 pores reacts with alkaline NaOH leading to the formation of Mx(OH)z, (where x = 1 to 3; z = 1-12), depending on the oxidation state of M. Thus, the formed metal hydroxide is then reduced by hydrazine hydrate leading to the formation of metal oxide in pores of wide band gap semiconductors, such as TiO2. The said reagent addition to the solution followed by sequential dipping for an optimized time leads to QDs formation of metal oxide in porous semiconductors. Use of appropriate solvents and controlling synthesis parameters provides the required oxidation state of copper or any metal ion, which facilitates CO2 reduction reaction for the formation of Cn-oxygenates. The SILAR method can be further applied to any 3d and 4d transition metal such as M = Ni, Mn, Co, Mo, Ag etc. which can be employed in the form of metal oxide QDs to wide band gap semiconductors, such as TiO2, and can be further explored for photocatalytic CO2 reduction. The bifunctional photocatalyst discussed in the present disclosure relates to a process of harnessing light for the co-conversion of CO2 and H2O to give economically viable and higher value Cn oxygenates (n=1-4); this process comprising an artificial photosynthesis device consisting of bi-functional photocatalyst of metal oxide (MxOy) (where x = 1 to 3; y = 1-4) quantum dots. In another embodiment of the present disclosure relates to a process for the production of Cn-oxygenate compounds from CO2 and H2O; the process comprising exposing an artificial photosynthesis device, comprising bifunctional photocatalyst of earth-abundant metal oxide (MxOy) quantum dots integrated with a porous semiconductor, wherein earth-abundant metal oxide quantum dots comprise a 3d transition metal oxide, a 4d transition metal oxide mainly M = Cu, Co, Ni, Mo, Ag etc. In the presence of light and CO2 and H2O, this process leads to Cn-oxygenates (n = 1-4) compounds. Yet another objective is the following: The artificial photosynthesis device is placed in a quartz reactor before being exposed to the light. In some embodiments, the artificial photosynthesis device is exposed to a light selected from a UV-visible light, a visible-NIR light, and all forms of sunlight. In some embodiments, the Metal (M= Cu, Ni, Co, Mo, Ag etc.) is integrated, structurally and electronically, exclusively into pores of the wide band gap of semiconductors such as TiO2 in the form of metal oxides (MxOy) (where x = 1 to 3; y = 1-4) quantum dots. In the present invention when an artificial photosynthetic device made up of M = Cu, Cu2O in pores of TiO2 semiconductor forms thin film and activity performed under one sun condition as well as in direct sunlight shows photocatalytic CO2and H2O conversion to C1-C4 oxygenates with ~10-65% conversion from within 1 to 5 h of reaction time duration. The present invention further relates to process of preparation of said bifunctional photocatalytic thin film by SILAR method, comprising steps of: i) pre-coating the metal oxide (TiO2) onto said support (FTO plate / ITO plate) to obtain precoated thin film, ii) dipping or immersing said precoated thin film into metal precursor solution at specific conditions and for specific time period to obtain metal coated thin film, iii) washing said metal coated film using water, iv) then treating with solution of base (NaOH) and hydrazine hydrate followed by final washing with water, drying, and calcination to obtain said photocatalytic thin film. Step (iv) is specifically applied to avoid the formation of divalent cupric oxide (CuO), which exhibits a low (1.2 eV) bandgap and does not meet the water oxidation potential. The present invention also relates to process of preparation of products like alcohols C1-C4, H2, and aldehyde / ketone, etc. comprising reacting CO2with said bifunctional photocatalytic thin film under specific reaction conditions and for specific time period. The product / structural features of said bifunctional photocatalytic thin film are: i) quantum dots are formed in the form of M(x)O(y) (x = 1 to 3; y=1-4), for metal used therein; ii) mesoporous TiO2 thin film is formed, iii) Normally, the TiO2 is active under UV radiation and not in visible sunlight, hence, by making combination with said MxOy , decreases band gap and making it active in visible sunlight, iv) pores filling inside said TiO2 with size in the range of 1-10 nm, preferably in the smaller pores around 2 nm, and this is possible because of calcination step, v) higher stability and electrons population in case of thin film due to structural and electronic integration of MxOyand TiO2, vi) step v leads to high quality heterojunctions along the periphery of MxOy, as they are contained within the pores of TiO2, vii) the thin film can be prepared and used with dimensions of 1x1 cm2which can easily goes upto 25 cm2with the present doctor blade method, and up to 1x1 square feet by spray-coating. Larger size devices made up to 2.25 and 6.25 cm2has been demonstrated to show a liner increase in product yield (Fig. 7) underscoring the large potential associated with the present findings. The following discussion provides many example embodiments of the inventive subject matter. Although each embodiment represents a single combination of inventive elements, the inventive subject matter is considered to include all possible combinations of the disclosed elements. Thus, if one embodiment comprises elements A, B, and C, and a second embodiment comprises elements B and D, then the inventive subject matter is also considered to include other remaining combinations of A, B, C, or D, even if not explicitly disclosed. In an embodiment, the present invention provides a bifunctional (Twin effect) photocatalytic thin film for reduction of CO2, the film comprising: i) metal oxide quantum dots MxOy(e.g. Cu2O, Ag2O, CoO, Co2O3, MoO3etc.) assembled and integrated with, (ii) another metal oxide (e.g. TiO2, etc.) as a porous semiconductor, and iii) a support (e.g. glass slide, FTO plate, ITO plate etc.); wherein the metal oxide is precoated onto said support and said metal oxide quantum dots MxOyis assembled from ionic precursors and integrated into the pores of said metal oxide as quantum dots of M(x)O(y) (where x = 1 to 3; y = 1-4). In another embodiment, the present invention provides a process of preparation of said bifunctional photocatalytic thin film by SILAR method, comprising steps of: i) pre-coating the metal oxide (TiO2) onto said support (FTO plate / ITO plate) to obtain precoated thin film, ii) dipping or immersing said precoated thin film into metal-ion precursor solution at specific conditions and for specific time period to obtain metal-ion coated thin film, iii) washing said metal coated film using distilled water, iv) then treating with solution of base (NaOH) and hydrazine hydrate followed by final washing with water, drying, and calcination to obtain said artificial photosynthesis device or photocatalytic thin film. In some embodiments specific condition at step ii) is at a temperature in the range of 15-45°C.In some embodiment, specific time period at step ii) is in the range of 20-70 seconds. In yet another embodiment, the present invention provides a process of preparation of products like alcohols C1-C4, H2, aldehyde / ketone, etc. comprising reacting CO2 and water with said bifunctional photocatalytic thin film under static / flowing / continuous flow and illumination conditions. In some embodiments, the product / structural features of said bifunctional photocatalytic thin film are: i) quantum dots are formed in the form of M(x)O(y) (x = 1 to 3; y = 1-4), for metal used therein; ii) mesoporous TiO2 thin film is formed, iii) Normally, the TiO2 is active under UV radiation and not in visible sunlight, hence, by making combination with said M(x)O(y) (metal), decreases band gap and making it active in visible sunlight, iv) pore formed inside said TiO2 with size in the range of 1-10 nm, preferably in the smaller pores around 2 nm, and this is possible because of calcination step, v) higher stability and electrons population in case of thin film, due to structural and electronic integration of MxOy and TiO2, vi) step v leads to high quality heterojunctions along the periphery of MxOy, as they are contained within the pores of, vii) the thin film can be prepared in a typical wet-chemistry laboratory and used with dimensions of 1 to 6.25 cm2. Bigger size (25 cm2and above) is possible with TiO2 spray coating unit. The present disclosure employed the integration of Cu2O with titania in thin film form and demonstrated for the conversion of photocatalytic co-reduction of CO2and water to value-added products. The present disclosure and the prior art disclosure are based on different synthetic procedures as the prior art includes a wet impregnation method for metal deposition onto TiO2thin film which is again made up of the drop-casting method. In other words, Cu2O was deposited on the external surfaces of TiO2. The present disclosure involves the integration of Cu2O in the micro and mesopores of titania film by successive ionic layer and adsorption and reaction (SILAR) method with Cu(II) and hydroxyl precursors; with structural and electronic integration of assembled ionic precursors with TiO2, Cu2O QDs are generated in the pores of titania leading to abundant heterojunctions throughout the film. The present invention provides solution to the problem of the lack of efficient integration in prior arts. Thus, the present invention is focused on the development of visible light absorbing earth abundant metal oxide quantum dots in micro and mesopores of TiO2, which also act as efficient co-catalyst to drive multielectron artificial photosynthetic CO2reduction i.e., Cn-oxygenates products. An aspect of the present disclosure provides the development of bifunctional photocatalytic material. The fabricated bifunctional material is a thin film photocatalyst, free from co-catalyst, operated independently to reduce CO2 and H2O in the presence of light to give Cn-Oxygenates (n = 1-4) as products. There is no sacrificial agents (electron or hole consuming), whatsoever, such as alcohols, metal-ions, were used in the present invention. Currently developed bifunctional photocatalyst thin film consists of metal oxide semiconductor quantum dots combined with a porous semiconductor, where the semiconductor quantum dots consist of 3d or 4d transition metal oxide and TiO2 as porous semiconductor applied. The said co-catalyst-free bifunctional photocatalyst has the ability to perform the CO2 reduction reaction without the need for additional material to enhance the reaction. This helps to simplify the overall process and hence can make the system applicable for practical large-scale applications. The bifunctional photocatalyst device helps to reduce overall production costs as it simplifies the fabrication method. The developed device does not rely on additional material maintenance or any degradation, hence it results in improving the durability and stability of the bifunctional photocatalyst device. The fabricated bifunctional photocatalyst eliminates issues such as suitable co-catalyst selection and their integration in the proper way to make the overall process efficient. Further, 1 cm2device was re-used several days (at least 12 days) in direct sunlight with freshly prepared CO2-saturated water every day. Results reported in Table 1 is observed within an error margin of 10% with no significant decrease in activity and no change in selectivity (Table 3) too. These results make the present findings really an inventive as well as innovative. In the present invention, a bifunctional photocatalyst consists of metal oxide quantum dots in TiO2 porous semiconductor where mentioned metal oxides are mainly from 3d or 4d transitional metals. The metal oxides are low-cost, earth-abundant, environmentally friendly, and have an exceptional ability to facilitate charge carriers upon light absorption with easily tunable optical and electrical properties. The use of earth-abundant metal oxide with integration in porous TiO2enables the photocatalyst to harness a broader spectrum of light making the whole system more efficient in generating the electron-hole pair and can help to increase the photocatalytic activity in comparison to TiO2 alone. Developing efficient and stable metal oxide is still a challenge that can accelerate the photocatalytic CO2reduction reaction with good yield and lower production cost of process. In some embodiments, the metal (M = Cu, Ni, Co etc.) is integrated, structurally and electronically, exclusively into pores of the wide band gap of semiconductor such as TiO2in form of metal oxides (MxOy) quantum dots. In some embodiments, metal oxide (MxOy) quantum dots are integrated, structurally and electronically, into pores of the semiconductor and on the exterior surface of the semiconductor. In some embodiments, the porous semiconductor comprises TiO2, ZnO and polymorphs thereof. In some embodiments of the present invention, the metal oxide quantum dots comprise M = Cu, Ni, Co, Mo, Ag etc. 3d or 4d transition metals integrated with TiO2semiconductor, wherein the quantum dots are integrated, structurally and electronically, at least into the pores of TiO2 semiconductor. In some embodiments, the quantum dots have a particle size ranging from 1 nm to 10 nm within the micro and mesopores of porous wide bandgap semiconductors, such as TiO2. Yet another aspect of the present invention provides a process of preparation of said bifunctional photocatalytic thin film by SILAR method, comprising steps of: i) pre-coating the wide bandgap porous metal oxide (TiO2) semiconductor onto said support (FTO plate / ITO plate) to obtain the precoated thin film, ii) dipping or immersing said precoated thin film into metal-ion precursor solution at specific conditions and for a specific time period to obtain the metal-ion coated predominantly in the pores of thin film, iii) washing said metal-ion coated film using water to remove any / only physically held metal-ions, iv) then treating with a solution of base (NaOH) and hydrazine hydrate followed by final washing with water, drying, and calcination in inert atmosphere to obtain said photocatalytic thin film (Fig.1). UV-visible absorption spectra and corresponding band gap of Cu2O@TiO2bifunctional photocatalyst, realized in accordance with an embodiment of the present disclosure. The CTU shows only marginal visible light absorption, in contrast to visible light absorption in a wide wavelength region for CTC as shown in Fig. 2. CTU and CTC indicating the device made without calcination (or uncalcined) and after calcination in inert atmosphere, respectively. Only Cu2O prepared on FTO plate shows light absorption only up to 600 nm, while CTC absorption extends at least up to 800 nm, demonstrating the electronic integration of Cu2O with TiO2in CTC brings in more light absorption with very small amount of Cu2O integrated in it. AQY result shown in Fig.8 also attest this fact. The mentioned SILAR method employed in present embodiments is for M = Cu and can be further applied to 3d or 4d transition metal. The particular use of copper in the present bifunctional photocatalyst system to make the CO2reduction process for the formation of Cn- Oxygenate products. Cu-based catalyst is well known for C-C coupling. To make a perfect photocatalytic system to give Cn-oxygenates (n = 1-4) maintaining the copper oxidation state is crucial for the stabilization reaction and to facilitate the C-C coupling. Thus, the photocatalysts with precisely tailored copper oxidation states are highly desired due to their intriguing properties. The precise oxidation state of copper is brought to photocatalyst by specific synthesis SILAR methods and structural modification. Strategies within the design in bifunctional photocatalysts by generating a large number of heterojunctions in a co-catalyst-free system can be implemented to reduce the recombination of photogenerated charge carriers, enhancing the overall photocatalytic CO2reduction and H2O oxidation to Cn-oxygenates (n = 1-4). Efficient separation and dispersion of charge carriers at heterojunctions are fully supported by the double-digit (14-18 %) solar-to-fuel efficiency in Fig. 7b as well as by IPCE (incident photon to current efficiency) as a function of wavelength in Fig. 10. Unless separation of electron and hole can happen, both STFE and IPCE would not show higher values.15-33 % and 15 % IPCE performance observed below 550 nm and above 550 nm (Fig. 10) reiterates the efficient light absorption from the entire visible light wavelength range. Similarly, apparent quantum yield (AQY, which is a measure of the efficiency of photocatalytic reactions by comparing the amount of light energy supplied to the catalyst surface to the amount of products formed), displays non-zero value in NIR region too with maximum AQY at 420 nm. Indeed, AQY and light absorption plots are in excellent agreement with each other, making the present work really inventive through integration of Cu2O QDs in the pores of TiO2. Use of suitable concentrations of NaOH and hydrazine hydrate (N2H4) leads towards the formation of metal oxide quantum dots. In the case of M = Cu, Cu2O quantum dots are integrated, structurally and electronically into the pores of the TiO2 semiconductor and are formed by the reduction of copper hydroxide by N2H4as the concentration of hydroxide is maintained more than metal precursor. The following reaction path could take place for the formation of metal oxide (MxOy) where M = Cu, into pores of the TiO2 semiconductor. 4Cu(OH)2 + N2H4 2Cu2O + N2 + 6H2O (upon calcination in N2 or inert atm.) Cu(OH)2 + 2OH- Cu(OH)42-4Cu(OH)42-+ N2H42Cu2O + N2+ 6H2O + 8OH- (upon calcination in N2or inert atm.) In some embodiments, the metal precursor and reducing compounds, such as NaOH etc, used are aqueous solutions containing water. In some embodiments, the step of contacting the porous semiconductor for metal oxide quantum dots formation with the precursors is affected for a time period ranging from 20-70 seconds. In some embodiments, the process further includes the step of after the successive ionic layer adsorption and reaction (SILAR) is affected, the step of annealing in an inert atmosphere comprised: of exposing the porous semiconductor adsorbed with the transition metal hydroxide / oxide to a temperature ranging from 200 °C to 600 °C for a time period ranging from 1 to 6 hours. In some embodiments, the metal oxide quantum dots comprise Cu2O and for the formation of Cu2O quantum dots in pores of TiO2 semiconductor dipping time is maintained at 60 seconds in each precursor solution. For the formation of Cu2O (Cu +1 Oxidation state), quantum dots are annealed in an inert atmosphere in the presence of argon or helium or nitrogen at 300 °C for 3h. In an exemplary embodiment, a thin film of porous semiconductor (TiO2) is prepared by the doctor blade method, for example, of uniform thickness of 9^1 µm. To introduce transition metal oxide quantum dots (QDs), for example, of Cu2O, into the pores of titania thin film SILAR method is used. The employment of the SILAR (Successive Ionic Layer Adsorption and Reaction) method facilitates the uniform assembly of Cu2O quantum dots (QDs) with particle sizes ranging from 1 to 5 nm within the micro and mesopores of TiO2, over the entire thickness of the film. High-resolution TEM images reveal spherical Cu2O QDs in the 1-5 nm size range. Uniform size (~2 nm) quantum dots are observed over the entire range of Cu2O@TiO2underscores the uniqueness of the present synthetic protocol and its preference to occupy micropores as shown in Fig.3. TEM results also demonstrate the complete integration of Cu2O with TiO2, and particularly without any agglomeration, in spite of high temperature calcination in N2atmosphere. Several 2 nm Cu2O QDs observed in the nanopores of any TiO2 particle (of about 20 nm) is evident for the embedded nature of QDs in the pores; the same cannot be made by depositing preformed metal oxide on TiO2 or by any impregnation method. None of the prior arts demonstrated these aspects and any heating / calcination makes the copper oxide to agglomerate as well as to oxidize to cupric oxide, while the present invention demonstrate the non-agglomerative and non-oxidizing nature of Cu2O on heating / annealing, as they are integrated thoroughly with TiO2 matrix. The SILAR method leads to the formation of a significant number of hetero structures between Cu2O and TiO2, attributed to the precise QD assembly within the mesoporous TiO2 particles. Following the procedure adopted by Salgaonkar et al. [Journal of Material Chemistry A 2023, 11, 15168], simple calculation suggests the number of heterojunctions to be few hundred trillions, especially due to 2 nm size Cu2O QDs, the heterostructure of Cu2O and TiO2 across the entire device thickness enhances both electron-hole separation and thus becomes enough to facilitate the photocatalytic CO2 and H2O reduction reaction in the presence of light to form Cn-oxygenates without any help of cocatalyst. Hence the entire device is named as bifunctional photocatalyst. In some embodiments, the artificial photosynthesis device is placed in a quartz reactor before being exposed to the light. In some embodiments, the artificial photosynthesis device is exposed to a light selected from a UV-visible light, visible light, visible + NIR light and direct sunlight. In some embodiments, the Metal (M = Cu, Ni, Co, etc.) is integrated, structurally and electronically, exclusively into pores of the wide band gap of semiconductors such as TiO2in the form of metal oxides (MxOy) quantum dots. In the present invention when a bifunctional photocatalyst-artificial leaf device made up of metal oxide quantum dots herein, M = Cu, Cu2O quantum dots in pores of TiO2 semiconductor forms a thin film (10±1 micron) and activity performed under one sun condition as well as direct sunlight shows photocatalytic CO2 and H2O conversion to C1-C4 oxygenates. Cu2O-TiO2 bifunctional photocatalyst shows conversion of CO2 and H2O reduction to hydrogen as a gaseous product with the formation of liquid Cn-oxygenates products, where n = 1-4 such as methanol, ethanol, acetone, tertiary butanol. These results highlights the active centres for CO2 reduction and C-C coupling should be closer to each other for facile production of higher homologues of Cn-oxygenates. CO2 reduction products observed with Cu2O@TiO2bifunctional photocatalyst were analyzed by NMR method as shown in Fig.4 and the results in Fig. 5-7. Reduction product yields were observed within the ranges of 1-12 (methanol), 5-32 (ethanol), 7-50 (acetone), and 3-20 (tertiary butanol) µmol / mg.cm2(1 mg catalyst, Cu2O + TiO2, coated over 1 cm2area), respectively, under one sun conditions. Under one sun condition, the maximum conversion efficiency of carbon dioxide to all products fell within the 10-60% range, while the selectivity for C3-C4 oxygenates was approximately 60% after a 5-hour reaction. However, when experiments were conducted in direct sunlight, more favorable product yields were observed. Specifically, with direct sunlight exposure for a 5-hour reaction period, were recorded in the ranges of 1-13 (methanol), 5-38 (ethanol), 8-62 (acetone), and 3-22 (tertiary butanol) µmol / mg·cm2, respectively. Moreover, the maximum conversion efficiency of carbon dioxide to Cn oxygenates increases to ~65% under direct sunlight in 5h duration. It is to be reiterated that products concentration is given in micromoles for all product molecules; that is, in terms of molarity, product concentrations are given. Nonetheless, it is also to be underscored that while a methanol molecule formation requires one CO2 molecule, acetone and tert-butyl alcohol would require three and four CO2 molecules, respectively. Hence the amount of CO2converted for longer chain product molecules are also larger, accordingly. EXAMPLES The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention. Example 1: Photoanode TiO2Thin film (Porous Semiconductor) Preparation FTO plates were selected as the substrate in this technique. Initially, the FTO plates underwent a thorough cleaning process with isopropyl alcohol, followed by a treatment with TiCl3on the conductive side of the FTO. A TiO2paste was applied to a 1 cm2area of the plate using the doctor blade method. It was then dried at 60°C for 2 hours and subsequently calcined at 450°C for 30 minutes. The detailed procedure for preparing the TiO2paste is as follows: 1 gram of TiO2powder (Degussa P25) was stirred for 10 minutes with 33 ml of ethanol and 0.33 ml of glacial acetic acid. This mixture was then sonicated for 10 minutes. Next, 0.5 gm of ethyl cellulose was introduced to the mixture and stirred for 10 minutes, followed by an additional 10 minutes of sonication. Finally, 3 ml of terpineol was added to the mixture and left for 30 minutes of stirring and sonication. Following this step, the solvent was evaporated using a rota vapour to obtain a uniformly thick paste. This titania paste was evenly coated onto the FTO plate, resulting in films with a thickness ranging from 8-14 µm. Example 2: Preparation of Bifunctional photocatalytic Film (MxOy-TiO2) Precoated TiO2 thin film by doctor blade method, as stated in example 1, is used for assembling and integration of another metal oxide quantum dots into the pores of titania structurally and electronically. Herein earth abundant metal oxide (Cu2O) is used. Cu ion-containing solution, prepared by 5mM of copper nitrate solution in water, in one beaker, and a mixture of 75mM NaOH and 60mM Hydrazine hydrate in aqueous solution another beaker was taken. Both solutions were used in SILAR method to deposit Cu2O QDs in the porous structure of titania film. Pre-coated TiO2film on FTO was immersed in copper nitrate solution for 60 seconds at 15-45°C followed by immersion in water. After this, film was immersed in NaOH and NH2-NH2 solution for 60 seconds at ambient temperatures (~25 deg C) followed by washing with water. This is considered as one SILAR cycle. Precursor concentration and SILAR cycle time can be varied. Amount / content of QD in the titania film was gradually increased as the number of SILAR cycle subjected was increased. Film was rinsed with deionized water and dried in air after each cycle. After the application of SILAR cycles, the film was annealed in an inert atmosphere at 300oC for 3h to form uniform Cu2O QDs in the pores of TiO2. This film can be considerd as a bifunctional AL device and designated as CTC. Example 3: Photocatalytic CO2and H2O reduction reaction with 1 cm2thin film of Cu2O-TiO2 bifunctional photocatalyst. To prepare a 1 cm2bifunctional photocatalyst in thin film form of 1 mg of Cu2O / TiO2 was coated, consisting of 92-99wt % titania and 1-5wt % Cu2O Quantum Dots (QDs) incorporated into the titania pores. The CTC AL device was immersed in 10 ml of deionized water at pH = 7, where water served as the hydrogen source. The quartz reactor containing the device was sealed with a septum and saturated with 99.9% CO2gas for approximately 40 minutes to maximize CO2 dissolution in the water. For enhanced dissolution, the reaction flask was placed in an ice bath (1-3 °C) during saturation. The reaction flask was then exposed to one sun illumination under static conditions for a minimum of 5 hours. Periodically, samples were withdrawn using a leak-tight syringe for analysis of products in both the gas and liquid phases, using GC and NMR methods. Using this bifunctional photocatalytic device, reduction products yields were observed within the ranges of 1-12 (methanol), 5-32 (ethanol), 7-50 (acetone) and 3-20 (tertiary butanol) µmol / mg.cm2(1 mg catalyst, Cu2O + TiO2, coated over 1 cm2area), respectively, under one sun conditions. Under one sun condition, the maximum conversion efficiency of carbon dioxide to all products fell within the 10-60% range, while the selectivity for C3-C4 oxygenates was approximately 60% after a 5-hour reaction as shown in Fig. 5. However, when experiments were conducted in direct sunlight, more favourable product yields were observed. Specifically, with direct sunlight exposure for a 5-hour reaction period, were recorded in the ranges of 1-13 (methanol), 5-38 (ethanol), 8-62 (acetone) and 3-22 (tertiary butanol) µmol / mg·cm2, respectively. Moreover, the maximum conversion efficiency of carbon dioxide to Cnoxygenates increases to ~65% under direct sunlight in 5h as shown in Fig.6. Overall, direct sunlight exposure resulted in a notable enhancement of CO2 conversion into Cn-oxygenates. Generally, the peak direct sunlight conditions in summer months, characterized by a high solar flux, exhibited the highest catalytic activity. The table below provides product yields in µmol / mg.cm2, selectivity & CO2 conversion rate as a function of irradiation time for a 1 cm2CTC under both one sun and direct sunlight conditions (see Table 1-3). Table 1: Products yield in µmol / mg.cm2obtained from 1 cm2thin film of Cu2O@TiO2 bifunctional photocatalyst as a function of irradiation time under one sun condition (100 mW / cm2). First five rows give the cumulative yield of products, while the last three rows gives the average yield observed per hour from the continuous reaction for a period of 5 h. Time / h Yield / µmol / mg.cm2Hydrogen Methanol Ethanol Acetone Ter. Butyl Alcohol 1 0.63 1.8 5.7 7.4 3.3 2 1.1 4.9 11.6 19.7 6.9 3 2.8 7.2 14.6 31.8 11.9 4 4.1 9.8 21.2 39.2 15.1 5 5.2 11.3 32 50.8 18.7 Rate[#]1.04 2.26 6.4 10.2 3.74 Rate[a]0.53 1.25 3.60 5.64 2.11 Rate[b]0.46 0.97 3.02 4.80 1.67[#]Average product yield observed per hour after continuous 5 h reaction[a]Measured with 455 nm filter and the average yield observed after continuous 5 h reaction. 455 nm filter allows photons of wavelength higher than 455 nm, and all lower wavelength photons are filtered[b]Measured with 550 nm filter and the average yield observed after continuous 5 h reaction. 550 nm filter allows photons of wavelength higher than 550 nm, and all lower wavelength photons are filtered Table 2: Products yield in µmol / mg.cm2obtained from 1cm2thin film of Cu2O-TiO2bifunctional photocatalyst as a function of irradiation time under direct sunlight. Time / h Direct sunlight Hydrogen Methanol Ethanol Acetone Ter. Butyl Alcohol 1 0.8 2.1 6.3 8.2 3.9 2 1.2 5.4 13.5 19.6 7.6 3 3.1 7.8 19.8 31.7 12.8 4 4.7 10.6 26.4 44.5 16.2 5 6 13.5 38 62 21.5 Table 3: Selectivity & CO2 conversion rate in percentage obtained from 1cm2thin film of Cu2O-TiO2 bifunctional photocatalyst as a function of irradiation time under exposure of direct sunlight. Time / h Selectivity (%) CO2 Conversion H2 Methanol Ethanol Acetone Ter. ButylRate / %Alcohol 1 3.3 8.6 25.9 38.5 16 9.9 2 2.3 10.5 26.3 41.4 14.8 21.9 3 3.8 9.6 24.4 42.2 15.7 35.1 4 4.4 9.9 24.8 43.4 15.2 47.5 5 4.3 9.6 26.9 43.9 15.2 65.6 Example 4: Photocatalytic CO2 and H2O reduction reaction with 2.25 and 6.26 cm2thin film of Cu2O-TiO2 bifunctional photocatalyst. 2.25 and 6.25 cm2bifunctional photocatalyst in thin film form was prepared, as described in example 2, but with bigger area TiO2 films. Both films consisting 92-99 wt % titania and 1-5wt % Cu2O Quantum Dots (QDs) incorporated into the titania pores. The CTC AL device was immersed in 25 and 50 ml deionized water at pH = 7, in a custom-built quartz reactor. All the other conditions remain the same, as in example 3 and the reactions were carried out in direct sunlight. The reactor with APS device was then exposed to sunlight illumination under static conditions for a minimum of 5 hours. Periodically, samples were withdrawn using a leak-tight syringe for analysis of products in both the gas and liquid phases, using GC and NMR methods. Product analysis results are shown in Fig. 7. Same reaction was carried out under dynamic flow conditions in a custom-built reactor in direct sunlight. Same products and product patterns are observed, as shown in Fig. 7, with enhanced CO2 conversion by 10-30%. Example 5: STFE Calculation from the Product Yield results The solar-to-fuel efficiency (STFE) was computed to assess the performance of the CTC APS devices, with detailed calculation procedures given below. Utilizing the average production rate measured experimentally under direct sunlight and an average solar power density of 65 mW / cm2, a combined STFE of 17.8% was observed for the 1 cm2CTC device. Even with a higher assumed solar power density of 70 mW / cm2, the combined STFE remained around 17 %. The integration of Cu2O QDS with TiO2 APS devices met the critical requirement of STFE > 10% while generating C1 -C4 value-added products, rendering the current system highly promising for commercial applications. Although a decrease in STFE was noted with larger-sized devices, 13.6% for 2.25 and 12.5% for 6.25 cm2compared to the 1 cm2device, both larger devices still exhibited STFE values ≥10%, as depicted in Fig. 7. Device configuration optimization is likely to increase the STFE values for larger size devices. Equations employed for calculating STFE is given below: STFCH3OH=STFC2H5OH=STFAcetone=STFTert-butanol=Power density (Ptotal) was assumed to be 65 mW / cm2for all calculations, and irradiation area was 1 cm2. Respective products formation yield were taken in µmol / h. Table 3 below provides solar to fuel efficiency of fabricated devices in direct sunlight. The present invention converts water and CO2 together to many C-C coupled products, such as ethanol, acetone, and tert butyl alcohol. The present invention relates reduction of CO2 to above C-C coupled products, and the nature of reactions are entirely opposite to the prior art US2022 / 0048018 A1. Visible light absorption from the wide wavelength range of sunlight is a characteristic of the present invention along with electronically and structurally integrated Cu2O-TiO2composite. In fact, the present preparation method involves few steps, without which it would not be possible to stabilize integrated Cu2O-TiO2 and the same has been characterized very well by HRTEM (Fig.3). ADVANTAGES OF THE INVENTION 1. The said bifunctional material is derived from low-cost earth-abundant material, which also simplifies the synthetic process and results in an economic production. The absence of a co-catalyst makes the thin film fabrication process easier and thus becomes more applicable for large-scale production. 2. The Cu2O-TiO2 bifunctional photocatalyst of the present invention produces higher selectivity and maximum CO2Conversion Rate.

Claims

We Claim, 1. A bifunctional photocatalytic thin film for reduction of CO2, wherein the film comprises of: i) a metal oxide quantum dots MxOy, ii) a porous semiconductor metal oxide, and iii) a support; wherein the porous semiconductor metal oxide is pre-coated onto said support and said metal oxide quantum dots MxOy is assembled from an ionic precursors and integrated into pores of said porous semiconductor metal oxide as quantum dots of MxOy(where x = 1 to 3; y = 1-4).

2. The bifunctional photocatalytic thin film as claimed in claim 1, wherein MxOy is 3d or 4d transitions metal oxide, wherein metal M is selected from the group consisting of Cu, Ni, Co, Mo, Ag and a combination thereof; and wherein the porous semiconductor metal oxide is selected from the group consisting of TiO2, ZnO and polymorphs thereof.

3. The bifunctional photocatalytic thin film as claimed in claim 2, wherein said MxOyis CuxO and said porous semiconductor is TiO2.

4. The bifunctional photocatalytic thin film as claimed in claim 1, wherein said support is selected from Fluorine doped tin oxide (FTO) and Indium Tin Oxide (ITO) plate.

5. The bifunctional photocatalytic thin film as claimed in claims 1 or 2, characterized in that; a. the metal Cu is integrated, structurally and electronically, into pores of the wide band gap semiconductor TiO2in the form of metal oxide CuxOyquantum dots, b. metal oxide CuxOy quantum dots are integrated, structurally and electronically, 90-100% into pores of the semiconductor and 1-10% on the exterior surface of the semiconductor, c. pore formed inside said TiO2with size of up to 10 nm, d. quantum dots have a particle size ranging from 1 nm to 10 nm, e. the thin film having a stability and electrons population due to structural andelectronic integration of MxOy and TiO2, leading to heterojunctions along the periphery of MxOy, as they are contained within the pores of TiO2, and f. the thin film is having a dimension of at least 1x1 cm2to 25 cm2and / or up to 1x1 square feet.

6. A process for preparation of the bifunctional photocatalytic thin film as claimed in claim 1, wherein the process comprises the steps of: i. pre-coating a metal oxide (TiO2) onto a support (FTO plate) to obtain a pre-coated thin film, ii. immersing said pre-coated thin film obtained at step i) into a metal-ion precursor solution (copper nitrate solution) at a temperature in the range of 15-45°C for a time period of 20-70 seconds to obtain copper-ion coated in the nanopores of TiO2 thin film, iii. washing said metal coated film of step ii) using water, and iv. treating said metal coated film of step iii) with a solution of base (NaOH) and hydrazine hydrate followed by washing with water, drying, and calcination to obtain said bifunctional photocatalytic thin film.

7. A process for the preparation of value-added products (Cnoxygenates), wherein the process comprises of solubilizing CO2 in water and reacting with the bifunctional photocatalytic thin film as claimed in claim 1 under static and flowing illumination conditions.

8. The process as claimed in claim 7, wherein said illumination conditions are created by exposing the photocatalytic thin film to a light selected from a UV-visible light, a visible light, visible + NIR light and direct sunlight.

9. The process as claimed in claim 7, wherein said value-added products are selected from the group comprising of C1-C4 oxygenates selected from alcohols and aldehydes / ketones, and H2.

10. The process as claimed in claim 7, wherein conversion efficiency of carbon dioxide to Cn oxygenates is ~65% under direct sunlight in 5h, where n is between 1-4.

Images