WO2020222260A1

WO2020222260A1 - A process of preparing transition metal doped hollow carbon nano-bubble and applications thereof

Info

Publication number: WO2020222260A1
Application number: PCT/IN2020/050397
Authority: WO
Inventors: Kiran Prakash SHEJALE; Devika LAISHRAM; Rakesh Kumar SHARMA; Krishnapriya R
Original assignee: Indian Institute Of Technology Jodhpur
Priority date: 2019-05-02
Filing date: 2020-05-01
Publication date: 2020-11-05

Abstract

The present disclosure discloses a process for preparing transition metal doped hollow carbon nano-bubble, the process comprising: (a) contacting at least one first base, at least one solvent, resorcinol, and formaldehyde solution to obtain a first mixture; (b) contacting the first mixture, and at least one tetraalkoxysilane, to obtain a second mixture; (c) contacting the second mixture, and at least one second base to obtain a third mixture; (d) stirring the third mixture to obtain a hollow carbon nano-bubble; (e) treating the hollow carbon nano-bubble with at least one transition metal salt to obtain a fourth mixture; (f) processing the fourth mixture to obtain a solid product; and annealing the solid product under inert atmosphere followed by processing to obtain transition metal doped hollow carbon nano-bubble.

Description

A PROCESS OF PREPARING TRANSITION METAL DOPED HOLLOW

CARBON NANO-BUBBLE AND APPLICATIONS THEREOF

FIELD OF INVENTION

[001] The present disclosure relates to the field of nanomaterials and particularly to a process of preparing metal nano-particles doped hollow carbon nano-bubbles and its application.

BACKGROUND OF THE INVENTION

[002] Alternative renewable energy is an imperative today with the need to find a sustainable energy source with a smaller carbon footprint. Materials and devices are required to remedy various environmental issues such as water pollution, CO2 emission, etc. High performing nanomaterials that are economically viable and ecologically benign are intriguing as these will bring about great changes to solve the energy crisis and fix other environmental issues. Photoanodes for solar cell applications, such as dye sensitized solar cells (DSSC) reveal that the use of carbon-based nanomaterials, such as SWCNTs, MWCNTs, and graphene increase the electrocatalytic activity and conductivity. The diverse morphology of nitrogen -doped carbon cubes, fibers, and solid and hollow spheres has grabbed great research attention because of the versatility that allows it to be used in a wide range of applications in the field of energy storage, CO2 capture and storage, and to some extent energy conversion.

[003] Hollow carbon nanosphere is one such versatile material with tremendous potential as active materials for several applications. Properties like electrical conductivity, porous carbon supporting material, high chemical and mechanical stability, hollow interior structure, high surface area, and biocompatibility make them a prominent candidate for water and air purification, drug delivery, adsorption, separation, catalysis, energy storage, and conversion.

[004] C. Zhang et al. (ACS Omega, 2018, 3, 96 - 105), and N. Zhang et.al. (Chem. Commun., 2018, 54, 1205 - 1208) reported that catalytic performance and electrochemical stability can be improved dramatically by confining metal nanoparticles in the carbon assemblies, such as carbon.

[005] Metal nanoparticles doped hollow carbon nano-bubbles is one of the important futuristic material due to its broad range of unique optical, absorption, separation, electrical, and photocatalytic properties. These nano-bubbles exhibit excellent electronic conductivity for electrochemical sensing and energy harvesting and the unique hollow carbon structure protects and provides stable morphology. Moreover, the highly active sites show superior catalytic activity with enhanced selectivity which will accelerate the mass diffusion of electrons that accounts for the high charge storage capacity.

[006] There are a lot of preparative methods for hollow carbon nano-bubbles preparation. Moreover, most of them are tedious synthetic procedures. It is a significant challenge to synthesize uniform sized metal nanoparticle doped hollow carbon nano-bubbles. A templating approach for preparing hollow carbon nano-bubbles is known, wherein spherical core-shell configuration is firstly formed. The core acts as a template on which carbon precursor is mounted. Then the template is generally removed by carbonization or post-etching process to form a hollow interior.

[007] Most often, hollow carbon bubbles are prepared by the template and ionic liquid method. However, these methods are not productive for doping metal nanoparticle due to undesirable morphological change, non-uniformity, high cost, poor stability with complicated non-Si template and removal of the same. Also, bulk synthesis limits larger scale application.

[008] Thus, a simple, stable and cost-effective process of preparation of transition metal doped hollow carbon nano-bubbles nanostructures is desired.

SUMMARY OF THE INVENTION

[009] In an aspect of the present disclosure, there is provided a process for preparing transition metal doped hollow carbon nano-bubble, the process comprising: (a) contacting at least one first base, at least one solvent, resorcinol, and formaldehyde solution to obtain a first mixture; (b) contacting the first mixture, and at least one tetraalkoxysilane, to obtain a second mixture; (c) contacting the second mixture, and at least one second base to obtain a third mixture; (d) stirring the third mixture to obtain a hollow carbon nano-bubble; (e) treating the hollow carbon nano bubble with at least one transition metal salt to obtain a fourth mixture; (f) processing the fourth mixture to obtain a solid product; and (g) annealing the solid product under inert atmosphere followed by processing to obtain transition metal doped hollow carbon nano-bubble.

[0010] In an aspect of the present disclosure, there is provided a transition metal doped carbon nano-bubble obtained by the process comprising: (a) contacting at least one first base, at least one solvent, resorcinol, and formaldehyde solution to obtain a first mixture; (b) contacting the first mixture, and at least one tetraalkoxysilane, to obtain a second mixture; (c) contacting the second mixture, and at least one second base to obtain a third mixture; (d) stirring the third mixture to obtain a hollow carbon nano-bubble; (e) treating the hollow carbon nano-bubble with at least one transition metal salt to obtain a fourth mixture; (f) processing the fourth mixture to obtain a solid product; and (g) annealing the solid product under inert atmosphere followed by processing to obtain transition metal doped hollow carbon nano-bubble.

[0011] These and other features, aspects, and advantages of the present subject matter will be better understood with reference to the following description and appended claims. This summary is provided to introduce a selection of concepts in a simplified form. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

BRIEF DESCRIPTION OF DRAWING

[0012] The detailed description is described with reference to the accompanying figure. In the figure, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the drawings to reference like features and components.

[0013] Figure 1 illustrates the X-ray diffraction (XRD) pattern of (a) carbon nano-bubble; and (b) transition metal doped hollow carbon nano-bubbles of the present disclosure, in accordance with an implementation of the present disclosure.

[0014] Figure 2 illustrates the field emission scanning electron microscopic (FESEM) images of carbon nano-bubble (CNB) at different magnifications, in accordance with an implementation of the present disclosure.

[0015] Figure 3 illustrates the transmission electron microscopic (TEM) images of carbon nano bubble (CNB) at different magnifications, in accordance with an implementation of the present disclosure.

[0016] Figure 4 illustrates the BET N2 adsorption-desorption isotherm (a) and pore distribution (b) radius of carbon nano-bubble (CNB), in accordance with an implementation of the present disclosure.

[0017] Figure 5 illustrates the cyclic voltammetry plots (0.1 mM Na2SC>4, applied voltage 0-0.8 V and scan rate 100 mV/s), of the transition metal doped hollow carbon nano-bubbles, in accordance with an implementation of the present disclosure.

[0018] Figure 6 illustrates plot summing up the photo-conversion efficiency of loading percentage of various transition metal doped hollow carbon nano-bubbles-based photoanodes from the dyesensitized solar cell (DSSC) device, in accordance with an implementation of the present disclosure. [0019] Figure 7 illustrates the TEM images of Ni-doped hollow carbon nano-bubble with different concentrations at (a) 0.01 M; (b) 0.009 M; and (c) 0.6 M, in accordance with an implementation of the present disclosure.

[0020] Figure 8 illustrates the FESEM images of Ni doped hollow carbon nano-bubble with different concentrations at (a) 0.1 M; (b) 0.009 M; and (c) 0.6 M, in accordance with an implementation of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

[0021] Those skilled in the art will be aware that the present disclosure is subject to variations and modifications other than those specifically described. It is to be understood that the present disclosure includes all such variations and modifications. The disclosure also includes all such steps, features, compositions, and compounds referred to or indicated in this specification, individually or collectively, and any and all combinations of any or more of such steps or features.

Definitions

[0022] For convenience, before further description of the present disclosure, certain terms employed in the specification, and examples are collected here. These definitions should be read in the light of the remainder of the disclosure and understood as by a person of skill in the art. The terms used herein have the meanings recognized and known to those of skill in the art, however, for convenience and completeness, particular terms and their meanings are set forth below.

[0023] The articles“a”,“an” and“the” are used to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article.

[0024] The terms“comprise” and“comprising” are used in the inclusive, open sense, meaning that additional elements may be included. It is not intended to be construed as“consists of only”.

[0025] Throughout this specification, unless the context requires otherwise the word“comprise”, and variations such as“comprises” and“comprising”, will be understood to imply the inclusion of a stated element or step or group of element or steps but not the exclusion of any other element or step or group of element or steps.

[0026] The term“including” is used to mean“including but not limited to”.“Including” and “including but not limited to” are used interchangeably.

[0027] The phase“acid solution” refers to HF solution, HC1 solution, HNO3 solution. The concentration of acid varies in the range of 5-35 % with respect to the acid solution. [0028] Ratios, concentrations, amounts, and other numerical data may be presented herein in a range format. It is to be understood that such range format is used merely for convenience and brevity and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a temperature range of about 80 °C - 120 °C should be interpreted to include not only the explicitly recited limits of about 80 °C to about 120 °C, but also to include sub-ranges, such as 85 °C, 100 °C, 110 °C, and so forth, as well as individual amounts, including fractional amounts, within the specified ranges, such as 85.5 °C, 100.5 °C, 110.5 °C, for example.

[0029] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the disclosure, the preferred methods, and materials are now described. All publications mentioned herein are incorporated herein by reference.

[0030] The present disclosure is not to be limited in scope by the specific implementations described herein, which are intended for the purposes of exemplification only. Functionally- equivalent products, compositions, and methods are clearly within the scope of the disclosure, as described herein.

[0031] To overcome the inadequacies of the template and ionic liquid method as described in the background section, the present disclosure provides a process of preparation of transition metal doped hollow carbon nano-bubble and its efficient use in several applications. Doping the carbon nano-bubble with transition metal salt alleviates the absolute strain generated during heterojunction between carbon species and metal nanoparticles which have a large potential to enhance all the properties of these versatile hollow carbon nanosphere. Conducting carbon can prevent agglomeration, instability and nonuniformity of the metal nanoparticles all over the porous carbon shell. Moreover, extremely less amount of these metals adds more number of active sites and morphological manipulation leading to new or enhanced functions.

[0032] The carbon shell and inner free space accounts for the total volume of hollow carbon nano bubbles with the inner volume of the nano-bubble maintaining around 90% of the uniform nature. The nominal diameter of the hollow carbon nano-bubbles ranges between 70 - 160 nm whereas the carbon shell may have a thickness of 5 - 15 nm. The carbon shell of the present disclosure with excellent porosity with large number of sites among carbon grains for diffusion and anchoring other metal nanoparticles. The metal embedded hollow carbon nano-bubble can be used as electro active photoanode and counter electrodes in solar energy harvesting. With metal impregnation, these materials are promising electrochemical sensor with high sensitivity and selectivity. Thus, the object of present invention is to provide a simple, stable and cost-effective process of preparation of transition metal doped hollow carbon nano-bubbles nanostructures.

[0033] In an embodiment of the present disclosure, there is provided a process for preparing transition metal doped hollow carbon nano-bubble, the process comprising: (a) contacting at least one first base, at least one solvent, resorcinol, and formaldehyde solution to obtain a first mixture; (b) contacting the first mixture, and at least one tetraalkoxysilane, to obtain a second mixture; (c) contacting the second mixture, and at least one second base to obtain a third mixture; (d) stirring the third mixture to obtain a hollow carbon nano-bubble; (e) treating the hollow carbon nano bubble with at least one transition metal salt to obtain a fourth mixture; (f) processing the fourth mixture to obtain a solid product; and (g) annealing the solid product under inert atmosphere followed by processing to obtain transition metal doped hollow carbon nano-bubble.

[0034] In an embodiment of the present disclosure, there is provided a process for preparing transition metal doped hollow carbon nano-bubble, the process comprising: (a) contacting at least one first base, at least one solvent, resorcinol, and formaldehyde solution to obtain a first mixture; (b) contacting the first mixture, and at least one tetraalkoxysilane, to obtain a second mixture; (c) contacting the second mixture, and at least one second base to obtain a third mixture; (d) stirring the third mixture to obtain a hollow carbon nano-bubble; (e) treating the hollow carbon nano bubble with at least one transition metal salt to obtain a fourth mixture; (f) processing the fourth mixture to obtain a solid product; and (g) annealing the solid product under inert atmosphere followed by processing to obtain transition metal doped hollow carbon nano-bubble, wherein the hollow carbon nano-bubble to the at least one transition metal salt weight ratio is in the range of 100: 1 - 100: 10.

[0035] In an embodiment of the present disclosure, there is provided a process for preparing transition metal doped hollow carbon nano-bubble, the process comprising: (a) contacting at least one first base, at least one solvent, resorcinol, and formaldehyde solution to obtain a first mixture; (b) contacting the first mixture, and at least one tetraalkoxysilane, to obtain a second mixture; (c) contacting the second mixture, and at least one second base to obtain a third mixture; (d) stirring the third mixture to obtain a hollow carbon nano-bubble; (e) treating the hollow carbon nano bubble with at least one transition metal salt to obtain a fourth mixture; (f) processing the fourth mixture to obtain a solid product; and (g) annealing the solid product under inert atmosphere followed by processing to obtain transition metal doped hollow carbon nano-bubble, wherein the at least one transition metal salt molar concentration is in the range of 0.01 M - 0.5 M.

[0036] In an embodiment of the present disclosure, there is provided a process for preparing transition metal doped hollow carbon nano-bubble, the process comprising: (a) contacting at least one first base, at least one solvent, resorcinol, and formaldehyde solution to obtain a first mixture; (b) contacting the first mixture, and at least one tetraalkoxysilane, to obtain a second mixture; (c) contacting the second mixture, and at least one second base to obtain a third mixture; (d) stirring the third mixture to obtain a hollow carbon nano-bubble; (e) treating the hollow carbon nano bubble with at least one transition metal salt to obtain a fourth mixture; (f) processing the fourth mixture to obtain a solid product; and (g) annealing the solid product under inert atmosphere followed by processing to obtain transition metal doped hollow carbon nano-bubble, wherein the at least one transition metal salt molar concentration is 0.1 M.

[0037] In an embodiment of the present disclosure, there is provided a process for preparing transition metal doped hollow carbon nano-bubble, the process comprising: (a) contacting at least one first base, at least one solvent, resorcinol, and formaldehyde solution to obtain a first mixture; (b) contacting the first mixture, and at least one tetraalkoxysilane, to obtain a second mixture; (c) contacting the second mixture, and at least one second base to obtain a third mixture; (d) stirring the third mixture to obtain a hollow carbon nano-bubble; (e) treating the hollow carbon nano bubble with at least one transition metal salt to obtain a fourth mixture; (f) processing the fourth mixture to obtain a solid product; and (g) annealing the solid product under inert atmosphere followed by processing to obtain transition metal doped hollow carbon nano-bubble, wherein the at least one first base to the at least one solvent weight ratio is in the range of 4:3 - 2: 1.

[0038] In an embodiment of the present disclosure, there is provided a process for preparing transition metal doped hollow carbon nano-bubble, the process comprising: (a) contacting at least one first base, at least one solvent, resorcinol, and formaldehyde solution to obtain a first mixture; (b) contacting the first mixture, and at least one tetraalkoxysilane, to obtain a second mixture; (c) contacting the second mixture, and at least one second base to obtain a third mixture; (d) stirring the third mixture to obtain a hollow carbon nano-bubble; (e) treating the hollow carbon nano- bubble with at least one transition metal salt to obtain a fourth mixture; (f) processing the fourth mixture to obtain a solid product; and (g) annealing the solid product under inert atmosphere followed by processing to obtain transition metal doped hollow carbon nano-bubble, wherein the hollow carbon nano-bubble has a diameter in the range of 100 - 200 nm.

[0039] In an embodiment of the present disclosure, there is provided a process for preparing transition metal doped hollow carbon nano-bubble, the process comprising: (a) contacting at least one first base, at least one solvent, resorcinol, and formaldehyde solution to obtain a first mixture; (b) contacting the first mixture, and at least one tetraalkoxysilane, to obtain a second mixture; (c) contacting the second mixture, and at least one second base to obtain a third mixture; (d) stirring the third mixture to obtain a hollow carbon nano-bubble; (e) treating the hollow carbon nano bubble with at least one transition metal salt to obtain a fourth mixture; (f) processing the fourth mixture to obtain a solid product; and (g) annealing the solid product under inert atmosphere followed by processing to obtain transition metal doped hollow carbon nano-bubble, wherein the at least one first base is selected from the group consisting of ammonia solution, urea, and combination thereof.

[0040] In an embodiment of the present disclosure, there is provided a process for preparing transition metal doped hollow carbon nano-bubble, the process comprising: (a) contacting at least one first base, at least one solvent, resorcinol, and formaldehyde solution to obtain a first mixture; (b) contacting the first mixture, and at least one tetraalkoxysilane, to obtain a second mixture; (c) contacting the second mixture, and at least one second base to obtain a third mixture; (d) stirring the third mixture to obtain a hollow carbon nano-bubble; (e) treating the hollow carbon nano bubble with at least one transition metal salt to obtain a fourth mixture; (f) processing the fourth mixture to obtain a solid product; and (g) annealing the solid product under inert atmosphere followed by processing to obtain transition metal doped hollow carbon nano-bubble, wherein the at least one solvent is selected from the group consisting of distilled water, ethanol, isopropyl alcohol, and combinations thereof.

[0041] In an embodiment of the present disclosure, there is provided a process for preparing transition metal doped hollow carbon nano-bubble, the process comprising: (a) contacting at least one first base, at least one solvent, resorcinol, and formaldehyde solution to obtain a first mixture; (b) contacting the first mixture, and at least one tetraalkoxysilane, to obtain a second mixture; (c) contacting the second mixture, and at least one second base to obtain a third mixture; (d) stirring the third mixture to obtain a hollow carbon nano-bubble; (e) treating the hollow carbon nano bubble with at least one transition metal salt to obtain a fourth mixture; (f) processing the fourth mixture to obtain a solid product; and (g) annealing the solid product under inert atmosphere followed by processing to obtain transition metal doped hollow carbon nano-bubble, wherein the at least one tetraalkoxysilane is selected from the group consisting of tetraethoxysilane, tetramethyl orthosilicate, and combination thereof.

[0042] In an embodiment of the present disclosure, there is provided a process for preparing transition metal doped hollow carbon nano-bubble, the process comprising: (a) contacting at least one first base, at least one solvent, resorcinol, and formaldehyde solution to obtain a first mixture; (b) contacting the first mixture, and at least one tetraalkoxysilane, to obtain a second mixture; (c) contacting the second mixture, and at least one second base to obtain a third mixture; (d) stirring the third mixture to obtain a hollow carbon nano-bubble; (e) treating the hollow carbon nano bubble with at least one transition metal salt to obtain a fourth mixture; (f) processing the fourth mixture to obtain a solid product; and (g) annealing the solid product under inert atmosphere followed by processing to obtain transition metal doped hollow carbon nano-bubble, wherein the at least one second base is selected from the group consisting of ethylenediamine, N,N,N',N'- tetramethyldiaminomethane, and combinations thereof.

[0043] In an embodiment of the present disclosure, there is provided a process for preparing transition metal doped hollow carbon nano-bubble, the process comprising: (a) contacting at least one first base, at least one solvent, resorcinol, and formaldehyde solution to obtain a first mixture; (b) contacting the first mixture, and at least one tetraalkoxysilane, to obtain a second mixture; (c) contacting the second mixture, and at least one second base to obtain a third mixture; (d) stirring the third mixture to obtain a hollow carbon nano-bubble; (e) treating the hollow carbon nano bubble with at least one transition metal salt to obtain a fourth mixture; (f) processing the fourth mixture to obtain a solid product; and (g) annealing the solid product under inert atmosphere followed by processing to obtain transition metal doped hollow carbon nano-bubble, wherein the at least one transition metal salt is selected from nitrate or chloride salt of Co, Ni, Fe, Cu, Zn, Pd, Pt, Ag, or Au.

[0044] In an embodiment of the present disclosure, there is provided a process for preparing transition metal doped hollow carbon nano-bubble, the process comprising: (a) contacting at least one first base, at least one solvent, resorcinol, and formaldehyde solution to obtain a first mixture; (b) contacting the first mixture, and at least one tetraalkoxysilane, to obtain a second mixture; (c) contacting the second mixture, and at least one second base to obtain a third mixture; (d) stirring the third mixture to obtain a hollow carbon nano-bubble; (e) treating the hollow carbon nano bubble with at least one transition metal salt to obtain a fourth mixture; (f) processing the fourth mixture to obtain a solid product; and (g) annealing the solid product under inert atmosphere followed by processing to obtain transition metal doped hollow carbon nano-bubble, wherein contacting at least one first base, at least one solvent, resorcinol, and formaldehyde solution is carried out at a temperature in a range of 30 °C to 45 °C for a period in a range of 0.5 hour to 1 hour to obtain a first mixture.

[0045] In an embodiment of the present disclosure, there is provided a process for preparing transition metal doped hollow carbon nano-bubble, the process comprising: (a) contacting at least one first base, at least one solvent, resorcinol, and formaldehyde solution to obtain a first mixture; (b) contacting the first mixture, and at least one tetraalkoxysilane, to obtain a second mixture; (c) contacting the second mixture, and at least one second base to obtain a third mixture; (d) stirring the third mixture to obtain a hollow carbon nano-bubble; (e) treating the hollow carbon nano bubble with at least one transition metal salt to obtain a fourth mixture; (f) processing the fourth mixture to obtain a solid product; and (g) annealing the solid product under inert atmosphere followed by processing to obtain transition metal doped hollow carbon nano-bubble, wherein contacting the first mixture, and at least one tetraalkoxysilane is carried out for a period in a range of 25 mins to 50 mins to obtain a second mixture.

[0046] In an embodiment of the present disclosure, there is provided a process for preparing transition metal doped hollow carbon nano-bubble, the process comprising: (a) contacting at least one first base, at least one solvent, resorcinol, and formaldehyde solution to obtain a first mixture; (b) contacting the first mixture, and at least one tetraalkoxysilane, to obtain a second mixture; (c) contacting the second mixture, and at least one second base to obtain a third mixture; (d) stirring the third mixture to obtain a hollow carbon nano-bubble; (e) treating the hollow carbon nano bubble with at least one transition metal salt to obtain a fourth mixture; (f) processing the fourth mixture to obtain a solid product; and (g) annealing the solid product under inert atmosphere followed by processing to obtain transition metal doped hollow carbon nano-bubble, wherein contacting the second mixture, and at least one base at a temperature in the range of 30 °C to 45 °C for a period in the range of 5 mins to 15 mins at a stirring speed in the range of 350 rpm to 650 rpm to obtain a third mixture.

[0047] In an embodiment of the present disclosure, there is provided a process for preparing transition metal doped hollow carbon nano-bubble, the process comprising: (a) contacting at least one first base, at least one solvent, resorcinol, and formaldehyde solution to obtain a first mixture; (b) contacting the first mixture, and at least one tetraalkoxysilane, to obtain a second mixture; (c) contacting the second mixture, and at least one second base to obtain a third mixture; (d) stirring the third mixture to obtain a hollow carbon nano-bubble; (e) treating the hollow carbon nano bubble with at least one transition metal salt to obtain a fourth mixture; (f) processing the fourth mixture to obtain a solid product; and (g) annealing the solid product under inert atmosphere followed by processing to obtain transition metal doped hollow carbon nano-bubble, wherein stirring the third mixture is carried out at a stirring speed in the range of 350 rpm to 650 rpm for a period in the range of 18 hours to 30 hours to obtain a hollow carbon nano-bubble.

[0048] In an embodiment of the present disclosure, there is provided a process for preparing transition metal doped hollow carbon nano-bubble, the process comprising: (a) contacting at least one first base, at least one solvent, resorcinol, and formaldehyde solution to obtain a first mixture; (b) contacting the first mixture, and at least one tetraalkoxysilane, to obtain a second mixture; (c) contacting the second mixture, and at least one second base to obtain a third mixture; (d) stirring the third mixture to obtain a hollow carbon nano-bubble; (e) treating the hollow carbon nano bubble with at least one transition metal salt to obtain a fourth mixture; (f) processing the fourth mixture to obtain a solid product; and (g) annealing the solid product under inert atmosphere followed by processing to obtain transition metal doped hollow carbon nano-bubble, wherein treating the hollow carbon nano-bubble with at least one transition metal salt is carried out in a teflon autoclave at a temperature in the range of 80 °C to 120 °C, for a period in a range of 20 hours to 26 hours to obtain a fourth mixture.

[0049] In an embodiment of the present disclosure there is provided a process for preparing transition metal doped hollow carbon nano-bubble, the process comprising: (a) contacting at least one first base, at least one solvent, resorcinol, and formaldehyde solution to obtain a first mixture; (b) contacting the first mixture, and at least one tetraalkoxysilane, to obtain a second mixture; (c) contacting the second mixture, and at least one second base to obtain a third mixture; (d) stirring the third mixture to obtain a hollow carbon nano-bubble; (e) treating the hollow carbon nano- bubble with at least one transition metal salt to obtain a fourth mixture; (f) processing the fourth mixture to obtain a solid product; and (g) annealing the solid product under inert atmosphere followed by processing to obtain transition metal doped hollow carbon nano-bubble, wherein processing the fourth mixture comprises processes selected from washing, filtering, drying, and combination thereof to obtain a solid product.

[0050] In an embodiment of the present disclosure there is provided a process for preparing transition metal doped hollow carbon nano-bubble, the process comprising: (a) contacting at least one first base, at least one solvent, resorcinol, and formaldehyde solution to obtain a first mixture; (b) contacting the first mixture, and at least one tetraalkoxysilane, to obtain a second mixture; (c) contacting the second mixture, and at least one second base to obtain a third mixture; (d) stirring the third mixture to obtain a hollow carbon nano-bubble; (e) treating the hollow carbon nano bubble with at least one transition metal salt to obtain a fourth mixture; (f) processing the fourth mixture to obtain a solid product; and (g) annealing the solid product under inert atmosphere followed by processing to obtain transition metal doped hollow carbon nano-bubble, wherein annealing the solid product is carried out a temperature in the range of 720°C to 750 °C for a period in the range of 1 hour to 3 hours under N2 atmosphere followed by processing to obtain transition metal doped hollow carbon nano-bubble.

[0051] In an embodiment of the present disclosure there is provided a process for preparing transition metal doped hollow carbon nano-bubble, the process comprising: (a) contacting at least one first base, at least one solvent, resorcinol, and formaldehyde solution to obtain a first mixture; (b) contacting the first mixture, and at least one tetraalkoxysilane, to obtain a second mixture; (c) contacting the second mixture, and at least one second base to obtain a third mixture; (d) stirring the third mixture to obtain a hollow carbon nano-bubble; (e) treating the hollow carbon nano bubble with at least one transition metal salt to obtain a fourth mixture; (f) processing the fourth mixture to obtain a solid product; and (g) annealing the solid product under inert atmosphere followed by processing to obtain transition metal doped hollow carbon nano-bubble, wherein processing the fourth mixture comprises processes selected from washing, filtering, drying, and combination thereof to obtain a solid product, wherein the washing involves treating with acid solution followed by washing and drying.

[0052] In an embodiment of the present disclosure there is provided a transition metal doped carbon nano-bubble obtained by the process, as described herein, wherein the process comprises: (a) contacting at least one first base, at least one solvent, resorcinol, and formaldehyde solution to obtain a first mixture; (b) contacting the first mixture, and at least one tetraalkoxysilane, to obtain a second mixture; (c) contacting the second mixture, and at least one second base to obtain a third mixture; (d) stirring the third mixture to obtain a hollow carbon nano-bubble; (e) treating the hollow carbon nano-bubble with at least one transition metal salt to obtain a fourth mixture; (f) processing the fourth mixture to obtain a solid product; and (g) annealing the solid product under inert atmosphere followed by processing to obtain transition metal doped hollow carbon nano bubble.

[0053] In an embodiment of the present disclosure there is provided a transition metal doped carbon nano-bubble as described herein, wherein the transition metal doped carbon nano-bubble has a charge capacitance in the range of 57 F/g - 408 F/g.

[0054] In an embodiment of the present disclosure there is provided a transition metal doped carbon nano-bubble as described herein, wherein the transition metal doped carbon nano-bubble has a specific surface area in the range of 340 - 500 m²/g.

[0055] In an embodiment of the present disclosure there is provided a transition metal doped carbon nano-bubble as described herein, wherein the transition metal doped carbon nano-bubble has a porosity in the range of 0.25 - 0.40 cm³/g.

[0056] In an embodiment of the present disclosure there is provided a transition metal doped carbon nano-bubble as described herein, for use in photo-conversion efficient solar cells and energy storage.

EXAMPLES

[0057] The disclosure will now be illustrated with working examples, which is intended to illustrate the working of disclosure and not intended to take restrictively to imply any limitations on the scope of the present disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice of the disclosed methods and compositions, the exemplary methods, devices and materials are described herein. It is to be understood that this disclosure is not limited to particular methods, and experimental conditions described, as such methods and conditions may apply. Materials and Methods

[0058] Iron (III) nitrate nonahydrate, cobalt (II) nitrate hexahydrate, nickel (II) nitrate hexahydrate, copper (II) nitrate trihydrate, zinc nitrate hexahydrate, palladium (II) nitrate, chloroplatinic acid, gold (III) chloride trihydrate, silver nitrate in their hexahydrate form, NH3 solution, tetraethoxysilane, resorcinol, ethylenediamine, formaldehyde solution, urea, HF, absolute ethanol and propanol were procured from Alfa aesar and scientific fisher. All the chemicals were analytical reagent (AR) grade used without further purification.

[0059] The nano-bubble of present disclosure, as synthesized in Example 1 , were characterized by scanning electron microscopy (EV018 special edition Carl Ziess) with an accelerating voltage of 20 kV, X-ray diffractometer (Bruker D8 advance) with Cu Ka as X-ray source (l = 1.54056A0) at 40 Kv and 40 mA, UV-visible diffuse reflectance spectroscopy (Cary 4000 scan Varian UV-vis system), Fourier-transform infrared spectra (Vertex 70v spectrometer - Bruker) in 400-4000 cm-1 range with KBr as a reference sample and TGA-6000 thermal analyzer (Perkin Elmer) under a nitrogen atmosphere at heating rate of 100 C min^-1. All measurements were done at room temperature.

EXAMPLE 1

General process of preparation of metal nanoparticle embedded hollow carbon nanobubbles

of the present disclosure

[0060] A process of preparing the transition metal doped hollow carbon nano-bubble were prepared by modified template approach. All used reagents were highly purity AR grade and used without further purification. The process of preparation of the Metal nanoparticle embedded hollow carbon nano-bubbles is discussed as below:

[0061] Metal nanoparticle embedded hollow carbon nano-bubbles were synthesized by a modified template method. The NH3 solution was mixed to a solution of distilled water and ethanol in the ratio of wt % 3.2: 1.1:7.5, and stirred for 45 min (first mixture). Then 3 g of tetraethoxysilane was added dropwise and followed by resorcinol and formaldehyde solution (37 wt%) in the ratio of (2.1 :3.1) to obtain the second mixture and finally 0.10 mL of ethylenediamine to obtain third mixture. The third mixture was vigorously stirred at intervals of 10 min until 24 hours at 40°C to obtain hollow carbon nano-bubble. 0.1 M concentration of the respective metal nitrates (Co, Ni, Fe, Cu, Zn, Pd, Pt, Ag, Au) were taken and used as doping agent after mixing with urea and to obtain fourth mixture. Then this solution was transferred to the Teflon-lined autoclave and kept at 100°C for 24 hours. The resultant product was obtained by vaccum filtration with several washings by ethanol and dried at 100°C under vacuum oven for 18 hours to obtain a solid product. Finally, solid product was annealed/carbonized by annealing at 750°C for 2 hours under N2 atmosphere. And hollow assemblies were achieved by washing S1O2 template in dilute HF solution for 10 hours and subsequently washed with ethanol and water and final transition metal doped hollow carbon nano-bubbles were obtained by drying at 100 °C for 12 hours.

[0062] Process of preparing Cobalt doped carbon nano-bubble (Co-CNB): The NH3 solution was mixed to a solution of distilled water and ethanol in the ratio of wt % 3.2: 1.1 :7.5, and stirred for 45 min (first mixture). Then 3 g of tetraethoxysilane was added dropwise and followed by resorcinol and formaldehyde solution (37 wt%) in the ratio of (2.1 :3.1) to obtain the second mixture and finally 0.10 mL of ethylenediamine to obtain third mixture. The third mixture was vigorously stirred at intervals of 10 min until 24 hours at 40°C to obtain hollow carbon nano bubble. 0.1 M concentration of the cobalt nitrate was taken and used as doping agent after mixing with urea and to obtain fourth mixture. Then this solution was transferred to the Teflon-lined autoclave and kept at 100°C for 24 hours. The resultant product was obtained by vaccum filtration with several washings by ethanol and dried at 100°C under vacuum oven for 18 hours to obtain a solid product. Finally, solid product was annealed/carbonized by annealing at 750°C for 2 hours under N2 atmosphere. And hollow assemblies were achieved by washing S1O2 template in dilute HF solution for 10 hours and subsequently washed with ethanol and water and cobalt doped hollow carbon nano-bubbles was obtained by drying at 100 °C for 12 hours.

[0063] Process of preparing Nickel doped carbon nano-bubble: The NH3 solution was mixed to a solution of distilled water and ethanol in the ratio of wt % 3.2: 1.1 :7.5, and stirred for 45 min (first mixture). Then 3 g of tetraethoxysilane was added dropwise and followed by resorcinol and formaldehyde solution (37 wt%) in the ratio of (2.1:3.1) to obtain the second mixture and finally 0.10 mL of ethylenediamine to obtain third mixture. The third mixture was vigorously stirred at intervals of 10 min until 24 hours at 40°C to obtain hollow carbon nano-bubble. 0.1 M concentration of the nickel nitrate was taken and used as doping agent after mixing with urea and to obtain fourth mixture. Then this solution was transferred to the Teflon-lined autoclave and kept at 100°C for 24 hours. The resultant product was obtained by vaccum filtration with several washings by ethanol and dried at 100°C under vacuum oven for 18 hours to obtain a solid product. Finally, solid product was annealed/carbonized by annealing at 750°C for 2 hours under N2 atmosphere. And hollow assemblies were achieved by washing S1O2 template in dilute HF solution for 10 hours and subsequently washed with ethanol and water and nickel doped hollow carbon nano-bubbles were obtained by drying at 100 °C for 12 hours.

[0064] Similarly, using the process as described above various transition metal doped hollow carbon nano-bubbles can be prepared.

[0065] The reaction between the resorcinol and formaldehyde resulted in intermediates diffusing on the surface, an effect of the presence of NH₄ ⁺, resulted in the formation of a polymeric resin sphere. A stable colloidal suspension of the negatively charged sphere surrounded by positively charged NFL_t ⁺ was formed, the interaction of which stabilized and prevented the spheres from agglomerating. Further, after the hydrothermal reaction, core S1O2 was etched by HF with the above carbonization step as discussed above to obtain hollow carbon nano-bubbles.

[0066] It is the process of the present disclosure with specific steps and parameters that resulted in the formation of transition metal doped hollow carbon nano-bubble. Additionally, high surface area and selectivity with highly active nature was achieved following the process as described above. The weight ratio of the hollow carbon nano-bubble to the at least one transition metal salt played a crucial role in the formation transition metal doped hollow carbon nano-bubbles. Experiments were conducted, wherein the molar concentration of transition metal salt was varied with respect to the carbon nano-bubble. The desired transition metal doped hollow carbon nano bubble were not obtained and resulted into loss of monodisperse and uniform spherical morphology nature.

EXAMPLE 2

Characterization of the nano-bubble

[0067] The XRD data for all carbon nano-bubble showed two broad peaks were absorbed at 43.77°, 23.72° which corresponds to (100) and (002) planes which can be because of more carbonization at nano-bubbles (Figure la). Figure lb depicts the XRD pattern for the cobalt doped carbon nano-bubble. The sharp peaks clearly indicate the formation of cobalt doped hollow carbon nano-bubble. [0068] FE-SEM and TEM were used for morphology and microstructure analysis. FESEM images at different magnification can be observed in Figure 2 indicating the uniform distribution of the synthesized nanomaterials. The hollow nature of the carbon nano-bubble can be established from the higher magnification images in Figure 2 whereby the underlying spherical layers can be seen through the transparent spheres of the first layers. The carbon nano-bubbles showed monodisperse uniform spherical morphology with diameter of 180 nm to 380 nm.

[0069] The above results were furthermore segmented by the TEM results. Figure 3 shows hollow CNB Also, both techniques revealed that the nano-bubbles have a hollow structure especially HRTEM exhibits big and uniform void space with thin shell made up of carbon (15nm to 20 nm).

[0070] To realize the change in the pore structure of the carbon nano-bubble of the present disclosure N2 adsorption-desorption BET was performed and the pore size was estimated using the Brauner-Joyner-Halenda (BJH) method (Figure 4). The carbon nano-bubble showed an H4 type hysteresis loop with an isotherm similar to type I. This indicated that the synthesized nanomaterials were microporous. The filling of micropore type observes high uptake at relatively low pressure because of high adsorption potential due to the narrow pore structure. The hysteresis did not show any limiting absorption at high P/Po with narrow pores with slits even within the micropores. The hysteresis loop at low pressure was possibly due to swelling of nonrigid pores and uptake of adsorbent molecules with pore sizes similar to that of the adsorptive molecule. Additionally, the high uptake observed at relatively low pressure was an indication of the presence of nanopores within the shell. Analysis of the surface area revealed that the surface area of carbon nano-bubble was found to be 359.87 m²/g with pore volume 0.3163 cm³/g and the average pore radius was calculated to be 175.8 nm (Figure 4).

EXAMPLE 3

Preparation of electrodes for energy storage

[0071] Embedded hollow carbon nano-bubbles (CNB) decorated electrode for the electrochemical study was prepared by drop casting of solution containing cobalt doped carbon nano-bubbles, nickel doped carbon nano-bubble, and the carbon nano-bubble (without doping transition metal), Nafion and isopropyl alcohol. This highly active electrode was dried and used as a device to analyze energy storage capacity

[0072] The synthesized electrode was analyzed as potential materials for its energy storage properties by electrochemical characterizations such as cyclic voltammetry (CV). All the electrochemical studies were performed using three electrode configurations in 0.1 mM Na2SC>4 aqueous electrolyte, Cyclic voltammetry plots (0.1 mM Na2SC>4, applied voltage 0-0.8 V and scan rate 100 mV/s).

[0073] Figure 5 shows the CV curve obtained from various CNB, i.e., CNB, Cobalt doped carbon nano-bubble (Co-CNB), nickel doped carbon nano-bubble (Ni-CNB) at 100 mV/s scan rate. It was observed that CNB exhibited near rectangular- shaped curves without any prominent redox potential peaks, indicating a double-layer capacitive behavior, good electrochemical reversibility, and high-power characteristics for the transition metal doped hollow carbon nano-bubbles. The current density and area under the CV curve for the various CNB at the 100 mV/s scan rate was high which lead to specific capacitance of around 408 F/g, 348 F/g, 57 F/g for CNB, Cobalt doped carbon nano-bubble (Co-CNB), nickel doped carbon nano-bubble (Ni-CNB) respectively.

EXAMPLE 4

Preparation of electrodes and solar cell device fabrication

[0074] Fluorine tin oxide glass was ultrasonically cleaned for 15 min with soap solution, distilled water and HCl-ethanol solution (1 : 10) and absolute ethanol. The cleaned FTO were pre-treated with aqueous TiCU solution (40 mM) at 70°C for 30 min then washed with D/W and ethanol and dried at room temperature. The prepared metal doped carbon nano-bubbles samples were mixed with P25 at a ratio 4:6 (weight ratio), screen printing paste was prepared with the addition of acetic acid, ethyl cellulose, terpineol and ethanol (wt% ratio 0.1:2.7: 1:3.38). Photoanodes were prepared by screen printing method keeping the area of the photoanode film precisely controlled by screen printing mesh size dimension and thickness of the film was controlled by repeating printing process for several times. The prepared photoanode was sintered at 500°C for 15 min at 3°C/min ramp rate and after cooling was immersed into 0.5 mM N719 dye in ethanol for 20 hours. The cleaned fluorine tin oxide glass was coated by platinum sol using brush painting and then films were annealed at 450°C for 30 min. Then the Pt counter electrode and dye loaded photoanode were sandwiched together with iodide triiodide electrolyte solution to form a dye-sensitized solar cell device (DSSC).

[0075] The performance of fabricated DSSCs revealed that the overall photo-conversion efficiency of the device showed great improvement in its performance as given Figure 6. Figure 6 shows a graph showing photo-conversion efficiency calculated from the DSSC device for photoanodes prepared from various carbon nano-bubbles prepared by the process of the present disclosure. It can be observed that from Figure 6 that 40% loading of Ni-doped hollow carbon nanonbubble showed the highest efficiency among all with high current density nearly 20 mA/cm². The increase in the Voc and Jsc strongly suggests that the reduced charge recombination and improvement in the charge collection and transport of charge carrier efficiency in the suggested photoanode composition. Additionally, the increased efficiency can be a cumulative factor of the increased surface area leading to better absorption of dye and efficient trapping of photons. This can be attributed to the spherical morphology increasing the pathway and scattering of light. Thus, the measured current density is highly influenced by the amount of dye absorbed over the active area of the fabricated photoanode.

EXAMPLE 5

Comparative example

[0076] When the molar concentration of the at least one transition metal salt was varied from the disclosed range, i.e., 0.01 M to 0.5M in the process of the present disclosure, the desired transition metal doped hollow carbon nano-bubble were not obtained and resulted into loss of monodisperse and uniform spherical morphology nature.

[0077] Figure 7 shows the TEM images of Ni doped CNB with different concentrations at (a) 0.01 M; (b) 0.009 M; and (c) 0.6 M, where the formation of metal doped CNB is negatively affected by the increase/decrease in concentration of the metal dopant. Figure 7 (a) shows the nickel doped carbon nano-bubble formed by the process of the present disclosure, wherein the concentration of the nickel nitrate (transition metal salt) was 0.1 M. However, when the metal salt concentration was 0.009 M or 0.6 M agglomeration and cluster like formation was observed. Similarly, Figure 8 depicts the FESEM images of Ni doped CNB with different concentrations at (a) 0.1 M; (b) 0.009 M; and (c) 0.6 M where (a) represents the transition metal doped hollow carbon nano-bubbles formed by the process of the present disclosure and (b) and (c) shows that the decrease and increase of the concentration of transition metal salt lead to agglomeration and cluster like formation.

ADVANTAGES OF THE PRESENT DISCLOSURE

[0078] The present disclosure provides a process for preparing transition metal doped hollow carbon nano-bubble, the process comprising: (a) contacting at least one first base, at least one solvent, resorcinol, and formaldehyde solution to obtain a first mixture; (b) contacting the first mixture, and at least one tetraalkoxysilane, to obtain a second mixture; (c) contacting the second mixture, and at least one second base to obtain a third mixture; (d) stirring the third mixture to obtain a hollow carbon nano-bubble; (e) treating the hollow carbon nano-bubble with at least one transition metal salt to obtain a fourth mixture; (f) processing the fourth mixture to obtain a solid product; and (g) annealing the solid product under inert atmosphere followed by processing to obtain transition metal doped hollow carbon nano-bubble. The metal nanoparticles doped hollow carbon nano-bubbles obtained from cost modified template method of the present disclosure has versatile applications, such as, for efficient energy harvesting and storage. With photon scattering morphology and electro- active doping these hollow carbon nano-bubbles can harvest extra light and exhibits high photo-conversion efficiency. Also, high charge capacity and stability proves the high potential for use in energy storage devices. Additionally, high surface area and selectivity with highly active nature indicates these doped hollow carbon sphere as an efficient material for energy harvesting and storage. The site-specific and accessibility of the transition metal nanoparticles doped hollow carbon nano-bubbles prepared by the process of present disclosure can be considered as a highly efficient material for nonenzymatic direct electrochemical biosensor for glucose and H₂O₂.

Claims

We Claim:

1) A process for preparing transition metal doped hollow carbon nano-bubble, the process comprising:

(a) contacting at least one first base, at least one solvent, resorcinol, and formaldehyde solution to obtain a first mixture;

(b) contacting the first mixture, and at least one tetraalkoxysilane, to obtain a second mixture;

(c) contacting the second mixture, and at least one second base to obtain a third mixture;

(d) stirring the third mixture to obtain a hollow carbon nano-bubble;

(e) treating the hollow carbon nano-bubble with at least one transition metal salt to obtain a fourth mixture;

(f) processing the fourth mixture to obtain a solid product; and

(g) annealing the solid product under inert atmosphere followed by processing to obtain transition metal doped hollow carbon nano-bubble.

2) The process as claimed in claim 1 , wherein the hollow carbon nano-bubble to the at least one transition metal salt weight ratio is in the range of 100: 1 - 100: 10.

3) The process as claimed in claim 1, wherein the at least one transition metal salt molar concentration is in the range of 0.01 M - 0.5 M.

4) The process as claimed in claim 1 , wherein the at least one first base to the at least one solvent weight ratio is in the range of 4:3 - 2: 1.

5) The process as claimed in claim 1, wherein the hollow carbon nano-bubble has a diameter in the range of 100 - 200 nm.

6) The process as claimed in claim 1 , wherein the at least one first base is selected from the group consisting of ammonia solution, urea, and combination thereof.

7) The process as claimed in claim 1 , wherein the at least one solvent is selected from the group consisting of distilled water, ethanol, isopropyl alcohol, and combinations thereof.

8) The process as claimed in claim 1, wherein the at least one tetraalkoxysilane is selected from, the group consisting of tetraethoxysilane, tetramethyl orthosilicate, and combination thereof. 9) The process as claimed in claim 1 , wherein the at least one second base is selected from the group consisting of ethylenediamine, N,N,N',N'-tetramethyldiaminomethane, and combinations thereof.

10) The process as claimed in claim 1, wherein the at least one transition metal salt is selected from nitrate or chloride salt of Co, Ni, Fe, Cu, Zn, Pd, Pt, Ag, or Au.

11) The process as claimed in claim 1, wherein contacting at least one first base, at least one solvent, resorcinol, and formaldehyde solution is carried out at a temperature in a range of 30 °C to 45 °C for a period in a range of 0.5 hour to 1 hour to obtain the first mixture.

12) The process as claimed in claim 1, wherein contacting the first mixture and at least one tetraalkoxysilane is carried out for a period in a range of 25 mins to 50 mins to obtain a second mixture.

13) The process as claimed in claim 1, wherein contacting the second mixture, and at least one base at a temperature in the range of 30 °C to 45 °C for a period in the range of 5 mins to 15 mins at a stirring speed in the range of 350 rpm to 650 rpm to obtain the third mixture.

14) The process as claimed in claim 1, wherein stirring the third mixture is carried out at a stirring speed in the range of 350 rpm to 650 rpm for a period in the range of 18 hours to 30 hours to obtain the hollow carbon nano-bubble.

15) The process as claimed in claim 1, wherein treating the hollow carbon nano-bubble with at least one transition metal salt is carried out in a teflon autoclave at a temperature in the range of 80 °C to 120 °C, for a period in a range of 20 hours to 26 hours to obtain the fourth mixture.

16) The process as claimed in claim 1, wherein processing the fourth mixture comprises processes selected from washing, filtering, drying, and a combination thereof to obtain a solid product.

17) The process as claimed in claim 1, wherein annealing the solid product is carried out a temperature in the range of 720°C to 750°C for a period in the range of 1 hour to 3 hours under N2 atmosphere followed by processing to obtain transition metal doped hollow carbon nano-bubble.

18) The process as claimed in claim 16, wherein the processing is selected from steps selected from treating with an acid solution, washing, drying, and combinations thereof. 19) A transition metal doped carbon nano-bubble obtained by the process as claimed in any one of the claims 1- 18.

20) The transition metal doped carbon nano-bubble as claimed in claim 19, has a charge capacitance in the range of 57 F/g - 408 F/g.

21) The transition metal doped carbon nano-bubble as claimed in claim 19, for use in photo conversion efficient solar cells and energy storage.