WO2011071213A1

WO2011071213A1 - Phosphor-titanium dioxide nanocomposite useful as photocatalyst and method for preparation thereof

Info

Publication number: WO2011071213A1
Application number: PCT/KR2010/002940
Authority: WO
Inventors: Ho Jin Ryu; Tae Sun Chang; Dilip Raosaheb Thube
Original assignee: Korea Research Institute of Chemical Technology KRICT
Current assignee: Korea Research Institute of Chemical Technology KRICT
Priority date: 2009-12-07
Filing date: 2010-05-10
Publication date: 2011-06-16
Anticipated expiration: 2012-06-07
Also published as: KR20110064130A

Abstract

Provided are a phosphor-titanium dioxide nanocomposite useful as a photocatalyst and a method for preparing the same, more particularly a novel phosphor-titanium dioxide nanocomposite including nanoparticles prepared from a CaAl₂O₄:Eu²⁺,Nd³⁺phosphor and a titanium(IV) precursor by a sol-gel method, having superior thermal stability to be present as anatase crystal phase even after heat treatment at 400 ºC to 600 ºC and exhibiting continued catalytic activity in a broad wavelength region of UV to visible and under a light-free condition using the energy stored in the phosphor, and a method for preparing the same.

Description

PHOSPHOR-TITANIUM DIOXIDE NANOCOMPOSITE USEFUL AS PHOTOCATALYST AND METHOD FOR PREPARATION THEREOF

The present invention relates to phosphor-titanium dioxide nanocomposite useful as a photocatalyst and a method for preparing the same, more particularly to a novel phosphor-titanium dioxide nanocomposite comprising nanoparticles prepared from a CaAl₂O₄:Eu²⁺,Nd³⁺phosphor and a titanium(IV) precursor by a sol-gel method, having superior thermal stability to be present in anatase crystal phase even after heat treatment at 400 ºC to 600 ºC and exhibiting continued catalytic activity in a broad wavelength region of UV to visible and under a light-free condition using the energy stored in the phosphor, and a method for preparing the same.

Titanium dioxide has been widely used as a photocatalyst to remove organic contaminants from water, purify air, remove offensive odors, remove dust and prevent frost. When titanium dioxide particles are exposed to UV irradiation, there occurs oxidation-reduction reaction on the particle surface, which is useful in removing organic contaminants from water. In addition, desorption of oxygen atom occurs on the surface of the titanium dioxide particles. Due to this superhydrophilicity, titanium dioxide is useful in air purification, dust removal and frost prevention. Titanium dioxide used as photocatalyst exists mostly in anatase crystal phase, with a crystal size of 35 nm or smaller. Since titanium dioxide in the anatase crystal phase has a band gap energy of 3.2 eV corresponding to the UV region (wavelength < 388 nm), it acts as a photocatalyst only under UV light.

As is well known, UV accounts for only 3-5% of the sunlight reaching the surface of the earth. Accordingly, it is required that the absorption property of the titanium dioxide, i.e., exhibiting photocatalytic activity only under UV irradiation, needs to be improved. The light absorption property of titanium dioxide may be improved by introducing a transition metal such as Fe or Cr or adding nitrogen to enable photocatalytic reaction in the visible region. However, these methods have problems that the activity of the photocatalytic reaction decreases rapidly because of inclusion of heteromolecules in titanium dioxide and of increase of electron-hole recombination.

Therefore, there is a need for the development of a novel photocatalyst that exhibits photocatalytic activity in a wide wavelength region.

An object of the present invention is to provide a novel photocatalyst exhibiting photocatalytic activity in a wide wavelength region, thus maximizing utilization of solar energy, and having superior light absorption and storage ability, thereby exhibiting continued catalytic activity even under a light-free condition, and a method for preparing the same.

The present invention relates to a nanocomposite comprising a CaAl₂O₄:Eu²⁺,Nd³⁺phosphor and titanium dioxide.

The present invention also relates to a use of the nanocomposite which exhibits photocatalytic activity in a wide wavelength region of UV to visible when heat-treated at 400 ºC to 600 ºC as a photocatalyst.

Further, the present invention also relates to a method for preparing a phosphor-titanium dioxide nanocomposite, comprising: (a) dispersing a phosphor in ethanol and ultrasonically degrading it; (b) adding an acid and water to the ethanol solution obtained in (a) in which the phosphor is dispersed to prepare an acidic solution of pH 3-5; (c) adding a titanium(IV) ethanol solution to the acidic solution to obtain precipitate; and (d) filtering, washing, drying and sintering the precipitate to obtain a nanocomposite of the phosphor and titanium dioxide.

The present invention enables successful preparation of a nanocomposite comprising a phosphor and titanium dioxide using a sol-gel method.

The phosphor-titanium dioxide nanocomposite of the present invention exhibits photocatalytic activity in a wide wavelength region, thereby maximizing utilization of solar energy, and exhibits continued catalytic activity even under a light-free condition, thereby providing an improved efficiency as a photocatalyst.

With a small nanoscale particle size, the phosphor-titanium dioxide nanocomposite of the present invention minimizes electron-hole recombination during degradation, thereby increasing band gap energy. Further, the increase in surface area due to the decrease in particle size leads to the increase in the adsorption of contaminants on the nanoparticle surface, thereby improving efficiency of photocatalytic degradation.

The phosphor-titanium dioxide nanocomposite of the present invention exhibits superior efficiency of photocatalytic degradation of methylene blue (MB), an organic contaminant.

The above and other objects, features and advantages of the present invention will become apparent from the following description of preferred embodiments given in conjunction with the accompanying drawings, in which:

Fig. 1 schematically shows a process of preparing nanocomposite of a phosphor and titanium dioxide according to a sol-gel method;

Fig. 2 shows X-ray diffraction patterns of a phosphor-titanium dioxide nanocomposite treated at 400 ºC and 600 ºC, respectively;

Fig. 3 shows UV-visible diffuse reflectance spectrums of a phosphor-titanium dioxide nanocomposite and a titanium dioxide particle;

Fig. 4 shows Fourier transform infrared spectroscopy (FT-IR) spectrums of a phosphor-titanium dioxide nanocomposite and a titanium dioxide particle;

Fig. 5 shows a scanning electron micrograph (SEM) of a phosphor-titanium dioxide nanocomposite;

Fig. 6 shows a transmission electron micrograph (TEM) of a phosphor-titanium dioxide nanocomposite;

Fig. 7 shows an energy dispersive X-ray spectroscopy (EDS) analysis result of a phosphor-titanium dioxide nanocomposite;

Fig. 8 shows specific surface area and pore size distribution of a phosphor-titanium dioxide nanocomposite heat-treated at 400 ºC;

Fig. 9 shows degradation of methylene blue (MB) with time by a phosphor-titanium dioxide nanocomposite; and

Fig. 10 compares degradation of MB by a phosphor-titanium dioxide nanocomposite and a titanium dioxide particle under UV radiation.

Hereinafter, the embodiments of the present invention will be described in detail with reference to accompanying drawings.

The present invention provides a phosphor-titanium dioxide nanocomposite prepared from a titanium precursor and a CaAl₂O₄:Eu²⁺,Nd³⁺phosphor by a sol-gel method.

The nanocomposite of the present invention exhibits photocatalytic activity in the UV to visible region. Whereas the existing titanium dioxide photocatalyst exhibits catalytic activity only in the UV region, the phosphor-titanium dioxide nanocomposite of the present invention exhibits catalytic activity in a wide wavelength region. In addition, the employment of the CaAl₂O₄:Eu²⁺,Nd³⁺phosphor enables the nanocomposite of the present invention to store energy from the environment and internal energy for a day.

The CaAl₂O₄:Eu²⁺,Nd³⁺phosphor used to prepare the photocatalyst in the present invention is an inorganic material which absorbs or stores light. By providing the stored light to the photocatalyst under a light-free condition, the phosphor enables the photocatalyst to exhibit photocatalytic activity continuously. The CaAl₂O₄:Eu²⁺,Nd³⁺phosphor is a blue emitting phosphor mainly comprising calcium carbonate (CaCO₃) and alumina (Al₂O₃) and doped with europium (Eu) and neodymium (Nd) [Journal of the Ceramic Society of Japan 41 (2006), No. 8]. The blue emitting CaAl₂O₄:Eu²⁺,Nd³ phosphor is known to be stable in visible to UV region, not to include environmental toxic substance and to exhibit better brightness and afterglow properties than other oxide phosphors.

In the method for preparing a phosphor-titanium dioxide nanocomposite according to the present invention, a sol-gel method is selected. The sol-gel method enables preparation of the phosphor-titanium dioxide nanocomposite in a very short time using a simple equipment. The method for preparing a phosphor-titanium dioxide nanocomposite of the present invention by the sol-gel method is schematically illustrated in Fig. 1.

The method for preparing a phosphor-titanium dioxide nanocomposite illustrated in Fig. 1 will be described in detail.

First, the phosphor is dispersed in ethanol and ultrasonically degraded for 1 minute to 1 hour. The CaAl₂O₄:Eu²⁺,Nd³⁺phosphor used in the present invention is an inorganic material capable of absorbing and storing light in UV to visible region. During the calcination, it prevents the transition of the titanium dioxide crystal phase from anatase to rutile. Thus, titanium dioxide in the resultant nanocomposite maintains a stable anatase crystal phase not only during the calcination at 400 ºC to 600 ºC but also at high temperature above 600 ºC.

Then, an acid and water are added to the ethanol solution in which the phosphor is dispersed to prepare an acidic solution with pH 3 to 5. The acid and water are added to adjust the pH of the solution. Based on 1 part by weight of the phosphor, water may be added in an amount of 2 to 10 parts by weight and the acid may be added in an amount of 0.01 to 1 part by weight. The acid may be selected from a common inorganic or organic acid including hydrochloric acid, sulfuric acid, phosphoric acid, acetic acid, etc. The acid and the amount thereof may be selected adequately to adjust the pH within the aforesaid range.

Then, a titanium(IV) ethanol solution is added to the acidic solution to cause precipitation. The titanium(IV) ethanol solution is prepared by dissolving a titanium(IV) precursor in ethanol. The titanium(IV) precursor may be titanium acetate, titanium nitrate, titanium sulfate, titanium C₁-C₆alkoxide, or the like. In the examples that follow, titanium isopropoxide was used as the titanium(IV) precursor. After the addition of the titanium(IV) ethanol solution, the mixture is vigorously stirred at 30 ºC to 80 ºC for 10 minutes to 1 hour.

Then, the resulting precipitate is filtered, washed, dried and calcined to obtain a phosphor-titanium dioxide nanocomposite. More specifically, the precipitate is filtered, washed with ethanol and distilled water, and then dried in an oven at 60 ºC to 90 ºC for 15 to 30 hours. Subsequently, after keeping at room temperature for at least 12 hours, cacination is performed at 400 ºC to 600 ºC for 1 to 5 hours to obtain the phosphor-titanium dioxide nanocomposite. Through X-ray diffraction analysis, it was confirmed that the phosphor-titanium dioxide nanocomposite maintains anatase crystal phase of titanium dioxide even after the heat treatment at 400 ºC to 600 ºC. This shows that the nanoparticles prepared by the sol-gel method according to the present invention have sufficient thermal stability.

Physical and chemical properties of thus prepared phosphor-titanium dioxide nanocomposite were analyzed as follows. Average particle size of the phosphor-titanium dioxide nanocomposite was measured using an X-ray diffraction analyzer and UV/DRS was measured at room temperature at 250-880 nm in order to investigate the band gap energy of the phosphor-titanium dioxide nanocomposite. The phosphor-titanium dioxide nanocomposite was prepared into KBr pellets and FT-IR was measured at 4000-200 cm^-1.

Microstructure and texture of the phosphor-titanium dioxide nanocomposite were observed using a scanning electron microscope and a transmission electron microscope. It was observed that the phosphor-titanium dioxide nanocomposite had small-sized titanium dioxide nanoparticles dispersed well on the surface of the phosphor. Surface characteristics of catalyst (BET method), pore volume and average pore diameter (BJH method) are determined by adsorption and desorption of nitrogen. The phosphor-titanium dioxide nanocomposite exhibited large surface area and high band gap energy. Photocatalytic activity of the nanocomposite was measured through degradation of methylene blue, which was measured using a spectrometer.

The examples and experiments will now be described. The following examples and experiments are for illustrative purposes only and not intended to limit the scope of this disclosure.

Example 1: Synthesis of titanium dioxide nanoparticles using sol-gel method

Ethanol (225 g, Merck, 99.7%), distilled water (DI, 6.8 g) and glacial acetic acid (0.07 g) were mixed and sufficiently stirred for 10 minutes. Then, titanium(IV) isopropoxide (TIP, Ti(OC₃H₇)₄, Aldrich, 97%, 35 g) was added dropwise for about 20 minutes. Upon the addition of the TIP precursor, titanium dioxide (TiO₂) nanoparticles were produced as white precipitate. The precipitate was subjected to ultrasonic degradation for 1 hour and kept at room temperature for about 40 minutes. The precipitate was filtered to obtain the TiO₂nanoparticles, which were washed with distilled water and dried in a desiccator at 80 ºC for about 24 hour. The dried TiO₂nanoparticles were heat-treated at 400 ºC for 3 hours, at a rate of 2 ºC/min.

Example 2: Synthesis of CaAl ₂ O ₄ :Eu ²⁺ ,Nd ³⁺ -TiO ₂ nanocomposite using sol-gel method

A phosphor (CaAl₂O₄:Eu²⁺,Nd³⁺, 1.0 g) was dispersed in ethanol (75 mL, Merck, 99.7%). After ultrasonic degradation for 50 minutes, distilled water (3.5 g) and glacial acetic acid (0.035 g) were added and stirred for 10 minutes. Then, TIP (Ti(OC₃H₇)₄, Aldrich, 97%) and 0.24 M ethanol (75 mL) were added dropwise for about 20 minutes. The weight proportion of the phosphor to TiO₂was 1 : 3.4. After stirring at 50 ºC for 5 minutes and drying in an oven at 70 ºC for 12 hours, white precipitate was formed quickly. The precipitate was filtered and washed sequentially with ethanol and distilled water. The precipitate was dried in an oven at 80 ºC for 24 hours and heat-treated at 400 ºC for 3 hours to obtain a CaAl₂O₄:Eu²⁺,Nd³⁺-TiO₂ nanocomposite.

Example 3: Photocatalytic activity

In order to investigate photocatalytic activity of the CaAl₂O₄:Eu²⁺,Nd³⁺-TiO₂ nanocomposite prepared in Example 2, photocatalytic degradation of methylene blue (MB, C₁₆H₁₈CIN₃S) was observed in an aqueous solution of the nanocomposite.

A 5 ppm MB aqueous solution (1.8×10^-5M, 1000 mL) exhibiting maximum absorbance at about 665 nm was prepared and added to a photocatalysis reactor. Then, the CaAl₂O₄:Eu²⁺,Nd³⁺-TiO₂ nanocomposite (400 mg, 0.04 wt%) prepared in Example 2 was dispersed in the MB aqueous solution (1000 mL). 1 minute later, UV (wavelength < 388 nm) was irradiated to the aqueous solution using a 9 W mercury lamp. 5 mL of the aqueous solution was collected and then centrifuged (3,000 rpm). UV-visible spectroscopy measurement (2401 PC, Shimadzu) was made at 200-800 nm.

Result and discussion

Characteristics of the CaAl₂O₄:Eu²⁺,Nd³⁺-TiO₂ nanocomposite were identified through X-ray diffraction analysis, UV-visible spectroscopy, IR spectroscopy, scanning electron microscopy, transmission electron microscopy and specific surface area analysis (BET).

Fig. 2 shows X-ray diffraction patterns of the CaAl₂O₄:Eu²⁺,Nd³⁺-TiO₂ nanocomposite calcined at 400 ºC and 600 ºC, respectively. It can be seen that TiO₂ exists in anatase crystal phase in the nanocomposite. Also, it can be seen from the X-ray diffraction pattern that the TiO₂ nanocrystal has a particle size of about 13 nm, smaller than that of the titanium dioxide nanoparticles (~21 nm) prepared in Example 1 by the sol-gel method without using the phosphor. As such, when the titania nanoparticles are formed on the phosphor surface of the CaAl₂O₄:Eu²⁺,Nd³⁺-TiO₂ nanocomposite, they are dispersed with small particle size. As a result, the photocatalyst has increased specific surface area and, thus, improved photocatalytic activity.

Fig. 3 shows UV-visible spectrum of the CaAl₂O₄:Eu²⁺,Nd³⁺-TiO₂ nanocomposite. Absorption in the blue wavelength region of 393-417 nm is clearly seen. And, the titanium dioxide sample exhibits increased band gap energy (3.16 eV) because of minimized electron-pair recombination during the photocatalytic degradation of MB.

Fig. 4 shows Fourier transform infrared spectroscopy (FT-IR) spectrums of the CaAl₂O₄:Eu²⁺,Nd³⁺-TiO₂ nanocomposite (Example 2) and the titanium dioxide powder (Example 1), respectively. Vibration of TiO₂ is observed over 200-800 cm^-1, and O-H stretching vibration and vibration band of adsorbed H₂O are observed at about 3405-3415 cm^-1 and 1630-1635 cm^-1, respectively.

Fig. 5 and Fig. 6 show a scanning electron micrograph (SEM) and a transmission electron micrograph (TEM) of the CaAl₂O₄:Eu²⁺,Nd³⁺-TiO₂ nanocomposite. As seen in the micrographs, the phosphor-titanium dioxide nanoparticles are spherical particles with an average crystal size of 10-15 nm. The selected area electron diffraction (SAED) pattern reveals a good crystal phase with distinct rings and bright spots. Well dispersed TiO₂ nanocrystals are present on the phosphor surface.

Fig. 7 shows an energy dispersive X-ray spectroscopy (EDS) analysis result of the CaAl₂O₄:Eu²⁺,Nd³⁺-TiO₂ nanocomposite.

Fig. 8 shows specific surface area and pore size distribution of the CaAl₂O₄:Eu²⁺,Nd³⁺-TiO₂ nanocomposite. Pore size distribution was calculated from the adsorption branch according to the BRJ method. Nitrogen adsorption-desorption isotherm of the phosphor-titanium dioxide nanocomposite shows that adsorption decreases because of increased hysteresis phenomenon. It may be due to the difference in the shape of the pores of the TiO₂ particles. The pores will allow a quick diffusion of MB molecules while the photocatalyst functions. The phosphor-titanium dioxide nanocomposite had a specific surface area of 178.31 m²/g.

Fig. 9 shows photocatalytic degradation of MB aqueous solution with time by the CaAl₂O₄:Eu²⁺,Nd³⁺-TiO₂ nanocomposite.

Fig. 10 compares photocatalytic degradation of MB by the CaAl₂O₄:Eu²⁺,Nd³⁺-TiO₂ nanocomposite (Example 2), commercially available TiO₂nanoparticle (Degussa, P-25) and the TiO₂nanoparticle (Example 1) under UV (365 nm) radiation. The CaAl₂O₄:Eu²⁺,Nd³⁺-TiO₂ nanocomposite exhibits better degradation result than the commercially available TiO₂nanoparticle or the TiO₂nanoparticle of Example 1. The phosphor-titanium dioxide nanocomposite resulted in degradation of about 80% of MB in 60 minutes. This shows that the phosphor-titanium dioxide nanocomposite has very superior photocatalytic efficiency. The superior photocatalytic efficiency of the phosphor-titanium dioxide nanocomposite is owing to reduced particle size, minimized electron-hole recombination, increased surface area and increased adsorption of MB on the photocatalyst surface. The improved photocatalytic action on the surface means that the phosphor increased the photocatalyst action.

The present application contains subject matter related to Korean Patent Application No. 2009-0120592, filed in the Korean Intellectual Property Office on December 7, 2009, the entire contents of which is incorporated herein by reference.

While the present invention has been described with respect to the specific embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.

The phosphor-titanium dioxide nanocomposite according to the present invention is useful as a photocatalyst and exhibits improved photocatalytic activity under UV radiation.

Accordingly, the phosphor-titanium dioxide nanocomposite of the present invention is useful as a photocatalyst for removing organic contaminants or the like.

Claims

A nanocomposite comprising a CaAl₂O₄:Eu²⁺,Nd³⁺phosphor and titanium dioxide.
The nanocomposite according to claim 1, wherein said nanocomposite is present in anatase crystal phase by calcining at 400 ºC to 600 ºC.
The nanocomposite according to claim 1, which has photocatalytic activity in UV to visible region.
A photocatalyst which is the nanocomposite according to any one of claims 1 to 3.
The photocatalyst according to claim 4, which is used for removing organic contaminants.
A method for preparing a phosphor-titanium dioxide nanocomposite, comprising:(a) dispersing a phosphor in ethanol and ultrasonically degrading it;(b) adding an acid and water to the ethanol solution obtained in (a) in which the phosphor is dispersed to prepare an acidic solution of pH 3-5;(c) adding a titanium(IV) ethanol solution to the acidic solution to obtain precipitate; and(d) filtering, washing, drying and calcining the precipitate to obtain a nanocomposite of the phosphor and titanium dioxide.
The method for preparing a phosphor-titanium dioxide nanocomposite according to claim 6, wherein the phosphor is CaAl₂O₄:Eu²⁺,Nd³⁺.
The method for preparing a phosphor-titanium dioxide nanocomposite according to claim 6, wherein the titanium(IV) ethanol solution is a solution prepared by dissolving a titanium C₁-C₆ alkoxide precursor in ethanol.
The method for preparing a phosphor-titanium dioxide nanocomposite according to claim 6, wherein the titanium C₁-C₆ alkoxide precursor is titanium isopropoxide.
The method for preparing a phosphor-titanium dioxide nanocomposite according to claim 6, wherein said calcination is performed at 400 ºC to 600 ºC.