Patent Application
20180050922
The present invention relates to a process for producing titanium dioxide (TiO2) nanoparticles with desired ratio of anatase and rutile phases. This method is a simple, cost effective and eco-friendly method, since it involves use of minimal chemicals and process steps. This process is useful to produce non doped and metal doped TiO2 nanoparticles with high surface area which is suitable for high efficiency dye-sensitized solar cells (DSSC) applications and high photocatalytic activities.
Titanium dioxide (TiO2) is of growing interest as it finds application in areas like paints and varnishes as well as paper and plastics. There are also other pigment applications like printing inks, fibers, rubber, cosmetic products and foodstuffs. TiO2 photocatalysis is widely used in a variety of applications and products in the environmental and energy fields, including self-cleaning surfaces, air and water purification systems, sterilization, hydrogen evolution, and photoelectrochemical conversion. Additionally, it can be used as antibacterial agent because of strong oxidation activity and superhydrophilicity.
Titanium dioxide occurs in nature as two important polymorphs; the stable rutile and metastable anatase. Both phases are tetragonal in nature with different lattice parameters as shown in
In particular, nanocrystalline TiO2 is well known as the most commonly used photoanode material for dye-sensitized solar cells (DSSC). The anatase phase (a-TiO2) gained much attention due to its more active surface chemistry and smaller particles for more dye adsorption. Anatase is metastable and can be transformed irreversibly to thermodynamically more stable and condense rutile phase at higher temperature. The rutile phase TiO2 (r-TiO2), due to the high refractive index, has excellent light-scattering characteristics, which is a profitable property from the perspective of effective light harvesting. Combination of anatase and rutile TiO2 can be more effective than the pure phase owing to the electron-holes separation at the interface between phases and the formation of interband gap trap which may influence interparticle carrier transportation.
US 2005/0164880 A1 describes a process for the preparation of a TiO2 containing catalyst or catalyst support by sol-gel method which leads to catalysts or catalyst supports with TiO2. The method for preparing the nano-crystalline TiO2 has been reported in U.S. Pat. No. 7,638,555.
Good conversion efficiency was found to be achieved from the DSSCs based on TiO2 nanocomposites with 24 wt % rutile nanorods, which was attributed to improved light harvesting caused by the enhancement of specific surface area and scattering effect from rutile nanorods (Wenquin Peng, Masatoshi Yanagida, Liyuan Han and Shahat Ahmed, Nanotechnology Vol. 22 (2011) 275709). As per Snejana Bakardjieva, Jan Subrt, Vaclav Stengl, Maria Jesus Dianez, Maria Jesus Sayagues, Applied Catalysis B, Environmental Vol. 58 (2005) 193-202), the photocatalytic activity of the sample containing 77.4% anatase and 22.6% rutile was higher than that of the nanocrystalline anatase powder.
In DSSC solar cell structures, mixing of 10 to 40 wt % of rutile with anatase is expected to increase the conversion efficiency due to high refractive index of rutile TiO2. This is also expected to considerably reduce the cost involved in associated with the production the solar cells. However, fabrication of the composite structure consisting of TiO2 nanoparticle matrix and scattering centers takes multiple processes, which include separate synthesis or acquisition of anatase TiO2 nanoparticles and rutile TiO2 nanoparticles and then mechanical mixing of the two forms. Such complex methods are not suitable for large-scale production. Moreover, the mechanical mixing can cause non-homogeneous distribution of scattering centers in TiO2 matrix and also aggregation formation, which will lead to the presence of cracks in the film during sintering. With respect to the existing methods, there also exists another limitation of the rate of photocatalytic degradation which is attributed to the recombination of photogenerated electron-hole (e−-h+) pairs, which is also accounted to the non uniform size and clumping of the TiO2 nano particles. There are no known processes for the production of TiO2 with desired ratio of anantase and rutile.
Therefore there is a need for a simple and efficient process to produce TiO2 nanoparticles which contains desired ratio of anatase and rutile with uniform particles, high surface area and minimum aggregation.
It is an objective of the present invention to provide a simple and efficient process to produce TiO2 which contains desired ratio of anatase and rutile nanoparticles with uniform particles, high surface area and minimum aggregation.
Accordingly In a primary aspect, the present invention relates to a process for the production of titanium dioxide (TiO2) with desired ratio of anatase and rutile phase, the method comprising
In a preferred embodiment, the process comprises doping TiO2 nanoparticles with water soluble metal precursor. The water soluble metal precursor may be a water soluble precursor of metals selected from the group consisting of Ag, Ni, Zn, Cr, Ge, Mo, Ru, Rh, Sn, W, Sr, Al, Si, Mn, Fe, Au, Pt, Co, V, Cu and Pd.
Accordingly this invention also provides a process for the production of titanium dioxide (TiO2) nanoparticles with desired ratio of anatase phase and rutile phase, the method comprising
Titanium trichloride (TiCl3) used in the instant process may be an aqueous solution of 0.1% to 30% of TiCl3 in double distilled de-ionized water.
In an important embodiment the present invention provides a solar cell comprising the mixed TiO2 nanoparticles produced by the instant process. Accordingly the present invention provides dye sensitized solar cells (DSSCs) with anatase and rutile mixed TiO2 resulted in high efficiency compared to 100% anatase.
Further scope and applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating embodiments of the invention, are given by way of illustration only, because various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
The foregoing summary, as well as the following detailed description of the invention will be better understood when read in conjunction with the appended drawings. For the purpose of assisting in the explanation of the invention, there are shown in the drawings embodiments which are presently preferred and considered illustrative. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown therein.
For the purposes of the following detailed description, it is to be understood that the invention may assume various alternative variations and step sequences, except where expressly specified to the contrary. Moreover, other than in any operating examples, or where otherwise indicated, all numbers expressing, for example, quantities of ingredients used in the specification are to be understood as being modified in all instances by the term “about”. It is noted that, unless otherwise stated, all percentages given in this specification and appended claims refer to percentages by weight of the total composition. Thus, before describing the present invention in detail, it is to be understood that this invention is not limited to particularly exemplified systems or process parameters that may of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments of the invention only, and is not intended to limit the scope of the invention in any manner.
The use of examples anywhere in this specification including examples of any terms discussed herein is illustrative only, and in no way limits the scope and meaning of the invention or of any exemplified term. Likewise, the invention is not limited to various embodiments given in this specification.
Unless otherwise defined, 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 invention pertains. In the case of conflict, the present document, including definitions will control.
It must be noted that, as used in this specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the content clearly dictates otherwise. Thus, for example, reference to “a flower extract” may include two or more such flower extracts.
The terms “preferred” and “preferably” refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the invention.
As used herein, the terms “comprising” “including,” “having,” “containing,” “involving,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to.
The term “nanopartcle” as herein described refers to ultrafine particles of TiO2, which between 1 and 100 nanometers in size.
“Anantase” and “rutile” as used in the invention refers to the two mineral forms of titanium dioxide.
“Solar cell” (also called a photovoltaic cell) as herein described refers to is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect. It is a form of photoelectric cell (in that its electrical characteristics e.g. current, voltage, or resistancevary when light is incident upon it) which, when exposed to light, can generate and support an electric current without being attached to any external voltage source, but do require an external load for power consumption.
The “fill factor”, as described herein is more commonly known by its abbreviation “FF”, of a solar cell refers to a parameter which, in conjunction with Voc and Isc, determines the maximum power from a solar cell. The FF is defined as the ratio of the maximum power from the solar cell to the product of Voc and Isc. Graphically, the FF is a measure of the “squareness” of the solar cell and is also the area of the largest rectangle which will fit in the IV curve.
“Calcination” (calcining) as herein described refers to a thermal treatment process in presence of air or oxygen applied to solid materials to bring about a thermal decomposition, phase transition, or removal of a volatile fraction. The calcination process normally takes place at temperatures below the melting point of the product materials.
“Annealing” as herein described refers to, a heat treatment in metallurgy that alters the microstructure of a material causing changes in properties such as strength, hardness, and ductility.
The present invention has been made in an effort to obtain TiO2 which comprises desired ratio of anatase to rutile wherein a conversion of 100% anatase to 100% rutile or 100% rutile to 100% anatase is made possible. This is achieved by a process of reacting a flower extract with TiCl3. The percentage conversion of anatase to rutile depends upon factors like the quantity of flower extract and the reaction temperature. Also, doping of different metals in TiO2 is of additional advantage in that it provides TiO2 nanoparticles with uniform particle size minimizing clumping and aggregation.
In the process of the present invention, the flower extract acts both as reducing and capping reagent in the preparation of TiO2 from TiCl3. In an important embodiment of the invention, the flower extract used in the process of the invention could be extract of flowers of plants selected from Peltophorum pterocarpum. However the present invention also could encompass the use of other flowers also for the process of the invention. The flower extract used in the instant process could be prepared by heating the flowers with double distilled deionized water at temperature ranging from 40° C. to 95° C. and filtering it.
TiO2 shows relatively high reactivity and chemical stability under ultraviolet light (λ<387 nm), whose energy exceeds the band gap of 3.3 eV in the anatase crystalline phase. The absorption and photocatalytic activity of visible light will allow utilization of the main part of the solar spectrum, even under poor illumination of interior lighting. So, it is very essential to prepare visible light activated TiO2 to improve the efficiency of the solar cells and photocatalytic activity. The metallic doping is expected to narrow the band gap of TiO2.
Accordingly in an important embodiment of the present invention, the process of this invention includes doping metal ions into the TiO2 lattice. TiO2 may be doped with metallic dopants including the noble metal by adding the dopant precursor in step (a). Various metal dopants like Ag, Ni, Zn, Cr, Ge, Mo, Ru, Rh, Sn, W, Sr, Al, Si, Mn, Fe, Au, Pt, Co, V, Cu, Pd etc. can be doped by this technique. In a preferred embodiment the metal could be selected from Ag, Ni, Mn, Fe, Au, Pt, Co, V, Cu and Pd. Doping precursors are mixed at the stage of step (a); i.e., water soluble metal precursor and TiCl3 are reacted with flower extract.
In an embodiment of the present invention, in step (a) diluted TiCl3 and flower extract are mixed together. In the process of the invention, by the reaction of TiCl3 and the flower extract, TiCl3 is converted into Ti(OH)4 and then TiO2.
The reaction in step (a) of claim 1 is operated inside constant temperature bath at a temperature of 40° C. to 200° C., and in a more preferred aspect the temperature could be 50° C. to 90° C. The reaction is carried out with stirring at a speed of 5 to 100 rpm. In a preferred embodiment the stirring speed could be 20-40 rpm and in a more preferred aspect the stirring speed could be 30 rpm. The reaction in step a) could be carried out for a period (residence time) of 60-360 minutes. In a preferred embodiment, the residence time may be 120 to 180 min and in a more preferred aspect the residence time may be 120 min.
In an embodiment of the invention, the temperature of drying the solution in step (b) could be 40° C.-110° C.
In an embodiment of the invention, the temperature of calcinations in step (c) may be 300° C.-800° C. In a preferred embodiment, the temperature of calcination may be 400° C. to 650° C. and in a more preferred aspect the temperature of calcination may be 600° C. The duration of calcination in step (c) could be 60-300 min. In a preferred embodiment, the duration of calcination may be 150-210 min and in a more preferred aspect it may be 180 min.
The ratio of anatase to rutile in the TiO2 obtained by this process could be 0:100 to 100:0 percentage by weight.
An important aspect of this invention provides a solar cell comprising the mixed (TiO2) nanoparticles produced by the process of the instant invention. The solar cell as per the present invention may be a dye sensitized solar cell (DSSC). The fill factor of the DSSC may be prepared with mixed TiO2. The TiO2 nanoparticles are coated on conducting FTO (flourine doped tin oxide) substrate and dye sensitized solar cells (DSSCs) were fabricated with anatase and rutile mixed TiO2. The dye sensitized solar cell (DSSC) as per the present invention exhibits high efficiency as compared to known solar cells which uses 100% anatase.
The green synthesized nanoparticles prepared by the present process are more stable even at high temperature up to 900° C. The prepared TiO2 nanoparticles can be used in the fields of water purification, air purification, self cleaning surface, antibacterial agent, catalyitic activity, superhydrophilicity activity, conversion of solar energy into electrical energy, etc. Doping metal ions into the TiO2 lattice reduces e−-h+ recombination in photocatalytic processes thereby helps to minimize aggregation and clumping of the TiO2 nano particles and helps to obtain uniform nanoparticles with higher surface area, which is another important requirement for solar cell devices. The doping of metals in TiO2 can also narrow the band gap and able to achieve visible light activated TiO2. Visible light activated TiO2 are expected to improve the efficiency of the solar cells and photocatalytic activity.
Peltophorum pterocarpum flowers obtained from Salem, Tamilnadu, India, were used to prepare the flower extract. Peltophorum pterocarpum is a species of Peltophorum which belong to Family Fabaceae (Leguminosae), which is native to tropical southeastern Asia and a popularly ornamental tree grown around the world.
The fresh Peltophorum pterocarpum flowers were weighed and thoroughly washed several times by using double distilled deionized water to remove the adhering soil and dust. The washed flowers were boiled with double distilled deionized water at a temperature of 40° C. to 95° C. for about 3 minutes and then filtered. The extract was stored at 4° C. for further experiments.
The titanium trichloride (TiCl3) was purchased from the market and prepared by mixing with the double distilled deionized water to make 0.1% to 30% aqueous solution of TiCl3.
The Peltophorum pterocarpum flower extract was filled in a clean burette, and slowly dropped into 1.35% aqueous solution of TiCl3 with constant stirring at 30 rpm at different temperatures ranging from 40° C. to 95° C. pH of the reaction mixture was about 1.5. The reaction mixture containing synthesized TiO2 nanoparticles was dried by heating at around 60° C. Calcination of the powder was done at 600° C. for 3 hrs. Other components resulted from the process of this invention along with TiO2 comprised of organic components such as carbon, titanium hydroxide and HCl. Carbon is expected to burn during the calcination of the powder around 600° C. Other by-products like titanium hydroxide and HCl will get evaporate during the drying and annealing process.
The Peltophorum pterocarpum flower extract was filled in a clean burette, and slowly dropped into 1.35% aqueous solution of TiCl3 and a 1 to 20 wt. % of AgNO3/NiCl2.6H2O with constant stirring at 30 rpm at different temperatures ranging from 40° C. to 95° C. pH of the reaction mixture was about 1.5. The reaction mixture containing synthesized TiO2 nanoparticles was dried by heating at around 60° C. Calcination of the powder was done at 600° C. for 3 hrs. Other components resulted from the process of this invention along with TiO2 comprised of organic components such as carbon, titanium hydroxide and HCl. Carbon is expected to burn during the calcination of the powder around 600° C. Other by-products like titanium hydroxide and HCl were expected to evaporate during the drying and annealing process.
The influence of different parameters such as concentration of the base material, dosage of the flower extract, temperature and reaction time on the synthesis of TiO2 nanoparticles were studied. The synthesized nanoparticles were characterized using X-Ray Diffraction (XRD), Raman spectroscopy, High Resolution Transmission Electron Microscopy (HR-TEM), High Resolution Scanning Electron Microscopy (HR-SEM), Electron Dispersive X-ray analysis (EDX), Electron Probe Micro-Analyzer (EPMA) and X-ray Photoelectron Spectroscopy (XPS) which reveal the formation TiO2, nano nature of the particles and weight percentage of doping materials.
The diffraction pattern was recorded by XRD with Cu-Kα radiation (λ=1.540598 A°) as the excitation source.
where, x is the weight fraction of rutile in the powders, and IA and IR are the X-ray intensities of the anatase and rutile peaks, respectively.
Table 1 shows the weight % (wt. %) of anatase and rutile in the undoped TiO2 produced by using different preparation conditions. Crystalline sizes for anatase and rutile were estimated from the Debye-Scherrer formula using the (101) peak of anatase and the (110) peak of rutile, respectively. The size of anatase particle vaired from 14-18 nm where as the rutile particles size varied from 71-33 nm.
XRD of the sample prepared with 5 ml of extract indicated that the sample was 100% rutile TiO2 as shown in
For Ag doped TiO2 as Ag doping increases two new peaks at about 37.8 and 44.2 were observed, which corresponds to (111) and (200) peaks for Ag. In the Ni doped samples the Ni (111) peak was observed at 44.3° and intensity increased as the doping concentration increases.
In order to investigate the influences of the annealing process of TiO2 the 100% anatase TiO2 was annealed at 600° C., 700° C., 800° C., 900° C. and 1000° C. for 2 hrs. The XRD of the 100% anatase annealed at different temperature is shown in
The samples were analyzed by HR-TEM to determine and compare the size and morphology of the particles prepared from various conditions.
Table 3 shows the anatase and rutile wt. % and crystalline size for different doping of NiCl2.6H2O precursors.
In order to confirm the chemical composition of the synthesized powders, the undoped, Ag doped and Ni doped samples were examined by EDX analysis. The EDX of the (a) undoped (b) Ag doped and (c) Ni doped TiO2 nanoparticles are shown in
From EPMA the Ti and O atomic ratio was 32.69% and 64.74% respectively. Also 2.07% of C was observed. These results show the presence of TiO2 with very less impurities.
The XPS survey spectrum reveals the peak of Ti, O, C and dopand materials.
The TiO2 Nanoparticles are coated on conducting FTO (fluorine doped tin oxide) substrate by Doctor blade method (removes the excess ink from the smooth non-engraved portion of the image carrier first used by Mann George in 1952) and dye sensitized solar cells (DSSCs) were fabricated with 100% anatase TiO2, anatase and rutile mixed TiO2.
The adsorption enhancement should be related to the increase in light scattering owing to the presence of the rutile nanopartilces. The lightscattering property of the dye-free TiO2 films and with dye loading were evaluated by diffuse reflectance spectroscopy.
The current-density-voltage (J-V) characteristics for the masked DSSCs with TiO2 composite films as photoanodes are presented in
The present invention is advantageous in that it provides a simple and cost effective process for the production of TiO2 with desired ratio of anatase and rutile nanoparticles without and with metallic dopants. The process has an additional advantage that it is a green synthesis method which is eco friendly, since it minimizes the use of chemical substances.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1615/CHE/2014 | Mar 2014 | IN | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2015/052192 | 3/25/2015 | WO | 00 |
