Optically Transparent Titanium Dioxide Particles ... - ACS Publications

J. Phys. Chem. C 2008, 112, 19979–19985

19979

Optically Transparent Titanium Dioxide Particles Incorporated in Poly(hydroxyethyl methacrylate) Thin Layers Va´clav Sˇtengl,† Vendula Housˇkova´,*,† Snejana Bakardjieva,† Nataliya Murafa,† and Vladimı´r Havlı´n‡ Institute of Inorganic Chemistry AS CR, V.V.i., 250 68, Czech Republic, and Asting, Roztoky 78, 252 63, Czech Republic ReceiVed: April 13, 2008; ReVised Manuscript ReceiVed: October 23, 2008

The optically transparent particles of titanium dioxide (rutile modification) were prepared by hydrolysis of aqueous solution of titanium(III) chloride in the presence of hydroxyethyl methacrylate. The transparent TiO2 doped with Ag or Pd nanoparticles was incorporated in a poly(hydroxyethyl methacrylate) thin layer and deposited on a quartz surface. The transparent particles of titania were characterized by measurement of particle size distribution, high-resolution transmission electron microscopy and selected area electron diffraction. Photocatalytic activity of prepared thin layers was determined by mineralization of Orange II dye, salicylic acid, butane, and acetone under UV radiation. The effects of the Ag and Pd doping on the morphology and microstructure of transparent TiO2 nanoparticles and their impacts on the photocatalytic activity were also studied. The photoactivity of the prepared transparent titania nanoparticles incorporated in thin poly(hydroxyethyl methacrylate) layers was assessed by the photocatalytic decomposition during irradiation at 365 or 254 nm. 1. Introduction Recently, a large number of studies devoted to the homopolymerization and copolymerization of 2-hydroxyethyl methacrylate (HEMA) have been reported. This growing interest is largely due to different application fields of the poly(HEMA) and its copolymers, e.g., optical lenses, implants, drug delivery devices, support for enzyme immobilization, holography.1-4 HEMA is a monomer with numerous applications. It is commercially available and prepared in a single step from methyl methacrylate or methacrylic acid. HEMA can also be easily polymerized like the majority of methacrylic derivatives. A primary alcohol function allows substitution reactions with the monomer or the corresponding polymer. The monomer, which is soluble in water, gives a hydrogel after polymerization whose applications in biomedical fields are important. Two types of inorganic-organic hybrid materials were prepared in a onestep process by mixing zirconium butoxide or propoxide and 2-hydroxyethyl methacrylate with or without benzoyl peroxide.5 Transparent monoliths made of a new interpenetrating network based on titanium-oxo-poly(hydroxyethyl methacrylate) nanocomposites were obtained through one-pot synthesis with low shrinking.6 Very small particles of titanium dioxide exhibited a quantum size effect.7 Their physicochemical properties depended on the particle dimension. Compared to macroscopic TiO2 powder materials, these particles showed a blue shift8 in their absorption spectra due to broadening of the band gap. Their aqueous dispersion does not practically scatter light. The synthesis of transparent colloidal solutions of extremely small titanium dioxide particles in water or ethyl alcohol by hydrolysis of TiCl49 or nanometer size Q-TiO2 particles10 prepared by a sol-gel * Corresponding author. Tel:420 2 6617 3534. Fax: 420 2 2094 0257. E-mail: [email protected]. † Institute of Inorganic Chemistry AS CR. ‡ Asting.

method are presented. In ref 11, a detailed kinetic study of the photocatalytic degradation of diuron in aqueous colloidal solutions of Q-TiO2 particles prepared by hydrolysis of TiCl4 is reported. In ref 12, the transparent particles of titanium dioxide (anatase or brookite modification) were prepared by hydrolysis of an aqueous solution of titanium(III) chloride in the presence of poly(ethylene glycol). The noble metals such as Pd,13 Pt,14 and Au15 deposited or doped on TiO2 have the highest Schottky barriers16 among the metals and thus act as electron traps, facilitating electron-hole separation and promoting interfacial electron transfer process.17 Ag is particularly suitable for industrial applications, due to its low cost and easy preparation. The effects of Ag dopants on the lattice or surface of TiO2 have been examined.18,19 TiO2 loaded with Ag enables the catalyst to perform more effectively and shortens the illumination period.20 In ref 21, the effect of silver doping on the microstructure and photocatalytic activity of TiO2 films prepared by the sol-gel method was investigated. It was found that suitable Ag dopant can increase the activity and the mechanism is mainly attributed to the change of anatase grain size. In this paper, the preparation, morphology, and photocatalytic activity of transparent particles (aqueous colloidal solutions of TiO2 particles) in thin layers prepared by hydrolysis of TiCl3 solution in HEMA is described. The effects of the Ag and Pd doping on the morphology and microstructure of transparent TiO2 nanoparticles and their impacts on the photocatalytic decomposition of a model compound (Orange II dye, salicylic acid, butane, and acetone) were studied. 2. Experimental Section 2.1. Chemicals and Raw Materials. 2-Hydroxyethyl methacrylate, AgNO3, PdCl2, and titanium evaporation slug (99.99%) were supplied by Fluka (Munich, Germany). Ten grams of metal titanium was diluted in 70 mL of hydrochloric acid in a roundbottom flask with reflux cooler and heated in an electrical

10.1021/jp803194p CCC: $40.75  2008 American Chemical Society Published on Web 11/18/2008

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19980 J. Phys. Chem. C, Vol. 112, No. 50, 2008 heating nest. A violet solution of titanium(III) chloride was obtained. The prepared solution of titanium(III) was filled to a total volume of 100 mL with hydrochloric acid. 2.2. Synthesis of Transparent Titanium Dioxide Particles Incorporated in 2-Hydroxyethyl Methacrylate. A solution of 50 g of 2-hydroxyethyl methacrylate in 100 mL of ethyl alcohol was prepared. Ten milliliters of the violet solution of titanium(III) chloride was added to the 2-hydroxyethyl methacrylate solution. The reaction mixture was stirred at temperature of 25 °C. The stirring was terminated after discoloration of the reaction mixture. The formation of transparent particles was determined by the Tyndall effect22 by means of a red laser. The sample prepared this way was named H1_Ti. The solution of hydroxyethyl metacrylate with transparent TiO2 particles (solution A) was deposited on a quartz tube and glass slide, respectively, as a thin transparent layer. 2.3. Synthesis of Pd and Ag Nanoparticles in 2-Hydroxyethyl Methacrylate. A 0.1 g amount of AgNO3 and PdCl2, respectively, was diluted in 50 mL of distilled water and 5 mL of 2-hydroxyethyl metacrylate. This reaction solution was magnetically stirred. After 10 min, 0.1 g of dimethylaminoborane as a reduction agent was added and the solution’s color change to a dark brown. Then the reaction mixture was stirred for another hour. Five milliliters of this solution with Ag nanoparticles and Pd nanoparticles, respectively, was mixed with 100 mL of solution A (hydroxyethyl metacrylate with transparent TiO2 particles) and deposited on a quartz tube and glass slide. The samples prepared by this way were named H1_TiAg and H1_TiPd, respectively. 2.4. Characterization Methods. The particle size distribution was determined by laser scaterring using ZEN 1600 equipment (Malvern Co.). The sample was tested in a square glass cuvette with round aperture (PCS8501). Transmission electron microscopy was carried out by using two instruments: Philips EM 201 at 80 kV and a JEOL JEM 3010 equipment at 300 kV (LaB6 cathode). A copper grid coated with a holey carbon support film was used to prepare samples for TEM observation. A drop of solution with transparent particles was applied on a carbon-coated grid. Scanning electron microscopy (SEM) studies were performed using a Philips XL30 CP microscope equipped with EDX (energy dispersive X-ray), Robinson, SE (secondary electron), and BSE (back-scattered electron) detectors. The sample was placed on an adhesive C slice and coated with a 10 nm thick Au-Pd alloy layer. Kinetics of the photocatalytic degradation of aqueous Orange II dye [sodium salt 4-[(2-hydroxy-1-naphtenyl)azo]benzenesulfonic acid]23 solutions, salicylic acid,24 and butane,25 respectively, was measured by using a photoreactor (see Figure 1). The photoreactor consists of a stainless steel cover (1) and the Teflon plate (2) with holes where Orange II dye solution (or salicylic acid solution or butane) circulated by means of peristaltic pump and membrane pump, respectively. The quartz tube (3) with photocatalytic layer was irradiated with a 4-W UV lamp (5). Solution inflow (7) and outflow (8) of the Orange II dye, salicylic acid solution or butane is in the cap (6) of photoreactor. The concentration of Orange II dye was determined by measuring absorbance at 480 nm with a vis spectrophotometer (ColorQuestXE). The fluorescence intensities of salicylic acid were measured by using a fluorescence detector (FL 2000, Spectraphysics). Salicylic acid analysis was performed by measuring the fluorescence intensity of emissive wavelength at

Figure 1. Schematic diagram of the photoreactor: 1, stainless steel cover; 2, Teflon plate; 3, quartz tube with tested photocatalytic layer; 4, quartz ampule; 5, UV lamp; 6, screw cap; 7, inflow of Orange II solution; and 8, outflow of Orange II solution.

430 nm (excitation wavelength at 315 nm). Standard solutions of salicylic acid in the concentration range 1-100 ppm were used to perform the standard calibration graph of the fluorescence intensities at each solution concentration. There is a good linearity between the fluorescence intensity and the concentration of salicylic acid. The concentration of butane was measured as a current signal from a gas detector (Figaro TGS 813). Kinetics of the photocatalytic degradation of acetone was measured by using a stainless steel photoreactor (constructed in our laboratory) with a fluorescent lamp (Narva) and black lamp (254 nm wavelength), with input power 8 W. The percentage concentrations of acetone, oxygen, carbon dioxide, and carbon monoxide were measured with a gas analyzer based on infrared detectors from Aseko Ltd. The photocatalytic layers were coated onto glass substrate of 10 × 15 cm size. The initial concentration of acetone in reactor was 1 mL. 3. Results and Discussion HEMA is prone to polymerize due to the vinyl bond in its molecule, while the hydrophilic hydroxy group makes it possible to form a water-compatible polymer. Although high molecular weight HEMA homopolymer is hydrophilic and has a relatively high degree of hydration (up to 42% water can be absorbed per unit mass of lightly cross-linked HEMA based gel26), it is generally regarded as being only water-swellable, rather than water-soluble. The most quoted example of side-group reorientation is that in polyHEMA surfaces, which exposes CH3 and CH2 groups to a hydrophobic environment and the OH groups to a hydrophilic environment. Spontaneous hydrolyzation of titanium(III) chloride in water occurs and by aging process crystalline TiO2 is formed:

TiCl3 + H2O f Ti(OH)3 + HCl f TiO2 + H2O

(1)

The process realizes similarly in the presence of HEMA, but more slowly (about 3 days):

CH2dC(CH3)COO(CH2)2OH + TiCl3 f CH2dC(CH3)COO(CH2)2OTiCl2 + HCl (2) The hydrolysis of TiCl3 in diluted HEMA with ethyl alcohol is substantially faster and the reaction mixture is decolorized after 1 day:

TiO2 Particles Incorporated in Poly(HEMA) Layers

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Figure 2. Particle size distribution of optically transparent particles: (a) tiania particles (H1Ti) and Pd particles (H1_Pd), (b) Pd-doped titania particles (H1_TiPd), (c) titania particles (H1_Ti) and Ag particles (H1_Ag), and (d) Ag-doped titania particles (H1_TiAg).

CH2dC(CH3)COO(CH2)2OH + TiCl3 + 2EtOH f CH2dC(CH3)COO(CH2)2O-Ti-(OEt)2 + 3HCl (3) At low concentrations of TiCl3 (to the limit 10 mL of TiCl3 in 50 g HEMA), the transparent particles of titania are formed. At higher concentration of TiCl3, white agglomerates of titania precipitate were observed. The particle size analysis of transparent TiO2 particles prepared by hydrolysis of TiCl3 in the presence of 2-hydroxyethyl metacrylate is presented in Figure 2. The primary particles of titania approach 60 nm particle size. Titania agglomerates have average agglomerate size within the range of about 280-340 nm. The primary Pd and Ag particles with particle size of about 12 and 35 nm, respectively, were obtained. Results obtained by high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) are shown in Figures 3-6. Figure 3 represents titania nanoparticles dispersed in poly(hydroxyethyl methacrylate). The crystalline areas form the islands in the amorphous parts of the poly(hydroxyethyl methacrylate). The fine fringe spacing is 0.32 nm (Figure 3d) and corresponds to the [110] plane of rutile (JC PDF 21-1276). Diffraction methods are the most important sources of structure information to identify individual microscopic-sized crystallites, i.e. to identify the crystallographic phase the crystallite corresponds to. Structure determination is generally based on SAED patterns in the TEM. A computer program called ProcessDiffraction27 helps to index a set of single crystal SAED patterns by determining which of the presumed structures can fit all the measured patterns simultaneously. Distances and angles are measured in the digitalized patterns with a graphical tool. Figure 4 depicts the selected area of electron diffraction patterns obtained by the analysis of the HRTEM micrograph.

Figure 3. HRTEM micrograph of transparent titania particles prepared by hydrolysis of TiCl3 in the presence of hydroxyethyl metacrylate (sample H1_Ti).

The diffraction patterns were analyzed by the program ProcessDiffraction and analyzed the prepared sample as rutile. Figure 5 shows HRTEM micrographs of titania nanoparticles in poly(hydroxyethyl methacrylate) doped with Pd nanoparticles. It followed from Figure 5 that the light-colored and dark-colored crystalline areas between amorphous parts of poly(hydroxyethyl metacrylate) areas are formed. The fringe (0.32 nm) of lightcolored crystalline areas ([110] diffraction plane) corresponds to rutile. The fringe spacing of 0.19 and 0.22 nm (Figure 5c,d) of dark-colored crystalline areas corresponds to Pd nanoparticles

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Figure 4. The SAED pattern of H1_Ti sample.

Figure 5. HRTEM micrograph of transparent titania particles prepared by hydrolysis of TiCl3 in the presence of hydroxyethyl metacrylate doped with Pd (sample H1_TiPd).

(diffraction planes [200] and [111], respectively, JC PDF 461043). The selected area of SAED patterns demonstrates the present of rutile phase in this sample. Titania nanoparticles dispersed in poly(hydroxyethyl methacrylate) doped with Ag metallic nanoparticles are presented in Figure 6a-c. It is obvious that the titania and Ag nanoparticles form crystalline islands in amorphous poly(hydroxyethyl metacrylate) phase. From Figure 6c it follows that the fringe spacing of 0.32 nm corresponds to rutile and the fringe spacing of 0.23 nm corresponds to Ag nanoparticles (diffraction plane [111], JC PDF 89-3722). The selected area of diffraction patterns confirmed the rutile phase. The SEM images of the thin layer of optically transparent titania particles incorporated in poly(hydroxyethyl methacrylate) deposited on glass slide are presented in Figure 7. The figure presents that very good crystalline particles of titania are precipitated from the layer of poly(hydroxyethyl methacrylate). The photocatalytic activity of the thin titania layers and tiania layers doped with Ag and Pd, respectively, was determined by degradation of Orange II aqueous solutions under UV radiation. The concentration is proportional to absorbance in the region of validity of the Lambert-Beer law (4):

A ) εcl

(4)

where A is the absorbance, c is the concentration of absorbing component, l is the length of absorbing layer, and ε is the molar

Figure 6. HRTEM micrograph of transparent titania particles prepared by hydrolysis of TiCl3 in the presence of hydroxyethyl metacrylate doped with Ag (sample H1_TiAg).

absorbing coefficient. The time dependence of Orange II dye decomposition can be described by using eq 5 for the firstorder kinetic reaction:11

d[OII] ) k(a0 - [OII]) dt

(5)

where [OII] is the concentration of Orange II dye, a0 is the initial concentration of Orange II dye, and k is the rate constant. It is obvious from Figure 8 that the first-order kinetic curves (plotted as lines) are fitted to all experimental points. In fact, illumination of thin layers by photons of energy greater than the band gap energy creates an electron (e-) and hole (h+) pair following reaction 6

TiO2 + hν f TiO2(e-,h+)

(6)

The presence of Ag nanoparticles efficiently promotes the electron-hole separation by attracting the conduction-band photoelectron, and recombination of e- and h+ is aborted. The photogenerated holes are free to react with hydroxyl ions OHadsorbed on the TiO2 surface and create hydroxyl radicals OH•. The photocatalytic oxidation of salicylic acid to carbon dioxide and water was performed at the pH range 3.0-3.5 in the presence of TiO2 particles and silver-modified TiO2 particles.28 The enhanced reduction of oxygen through better electron-hole separation in Ag/TiO2 particles compared to pure TiO2 particles increases the rate of mineralization. Photocatalytic efficiencies

TiO2 Particles Incorporated in Poly(HEMA) Layers

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Figure 7. SEM micrographs of titania particles deposited on the glass slide.

the recombination is avoided. The band electrons react with electron acceptors such as oxygen, creating oxygen radicals O2•-. These radicals show extremely strong oxidizing properties and they are able to degrade the pollutant:

OH• + O2•- + OII f intermediates f CO2 + H2O (7)

Figure 8. Photocatalytic degradation of Orange II dye on titania particles, Pd-doped titania particles, and Ag-doped titania particles deposited on the quartz tube (a); (b) butane on titania particles and Pd-doped titania particles deposited on the quartz tube; (c) salicylic acid on titania particles and Pd-doped titania particles deposited on the quartz tube.

of Ag-implanted titania were evaluated by means of decolorization of methylene blue solution under fluorescent light.29 Agimplanted titania showed photocatalytic efficiency higher than unimplanted rutile titania. The application of the palladium cocatalyst on thin titania films increased the rate constant of salicylic acid photooxidation.30 The addition of Pd nanoparticles separates the electron-hole pairs by trapping the holes. Thus,

The highest value of the photodegradation rate constant (k ) 0.0151 min-1) was observed for the thin layer made from titania transparent particles in poly(hydroxyethyl metacrylate) doped with Pd (see Figure 8a). The photodegradation rate constant of titania particles doped with Ag was 0.0146 min-1, which is 1.5 times greater than the constant of thin films with titania without the metal doping (k ) 0.0101 min-1). The photodegradation of salicylic acid30 is presented in Figure 8b. Amphlett et al.31 suggested that 2,3- and 2,5-dihydroxybenzoicacid and catechol are the major intermediates in the first step of the pathway of salicylic acid degraded by hydroxyl radicals. The rate constant of Pd-doped titania (k ) 0.0024 min-1) was 2 times greater than the constant determined for the thin films with titania particles without the Pd doping (k ) 0.0012 min-1). Low molecular weight alkanes, including propane, isobutane, and n-butane, can be completely oxidized to carbon dioxide and water vapor by using a tubular photoreactor containing supported ZrO2-TiO2 thin film photocatalyst.32 The reactivity of these alkanes is similar. Isobutane conversion is slightly better than that of n-butane, and propane conversion is about 5-10% lower than the butane conversion. Figure 8c showed the photocatalytic decomposition of butane. The concentration of butane was measured as a current signal by a Figaro TGS 813 detector. The rate constant of a thin layer made from titania transparent particles in poly(hydroxyethyl metacrylate) was 0.0011 min-1, and the rate constant of Pd-doped titania was 0.0014 min-1. These results characterize the good photocatalytic properties of the optically transparent titanium dioxide particles incorporated into the poly(hydroxyethyl methacrylate). The photodegradation of acetone is presented in Figure 9. Djeghri et al. 33 reported photoinduced oxidation of C2-C8 alkanes on TiO2 at ambient temperature. In general, they observed that alkanes (CnH2n+2) formed ketones (CnH2nO) and other aldehydes CmH2mO with 2 < m < n. If the alkane was branched, the ketone was CmH2mO with 3 < m < n. The reactivity of different types of carbon atoms followed the sequence: Ctert > Cquat > Csec > Cprim. Coronado et al.34 reported that acetone is adsorbed exclusively in a molecular form on TiO2. The photocatalytic oxidation yields acetate and formate complexes, along with adsorbed acetaldehyde and formic acid. These adsorbed molecules can act as intermediate species in

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and inorganic titania (TiO2) were synthesized by using 2-hydroxyethyl methacrylate as a coupling agent. The optically transparent titania nanoparticles doped with Pd and Ag nanoparticles were prepared and deposited as a thin layer on the quartz tube. As-prepared samples were used as photocatalysts for mineralization of organic pollutants. The photocatalytic activity of thin poly(hydroxyethyl metacrylate) layer was determined by degradation of Orange II dye, salicylic acid, and butane under UV radiation and demonstrated the good photocatalytic properties of these materials. It was found that Ag and Pd dopant increase the phototocatalytic activity by enhancing the electron-hole pairs separation and inhibiting their recombination and by decreasing the grain sizes of titania particles. Ag doping is the most suitable for photocatalytic decomposition of Orange II solution. Pd doping is suitable for photocatalytic decomposition of all systems, i.e., Orange II, salicylic acid, butane, and acetone. The resulting hybrid material exhibits minimal shrinking, high transparency, and high photocatalytic activity inherent in the titanium oxide component and offer the possibility to use these materials as a precursor for a thin photoactive layer. Acknowledgment. This work was supported by the Academy of Sciences of the Czech Republic (Project No. AV OZ 40320502) and the Ministry of Education of Czech Republic (Project No. LC 523). References and Notes

Figure 9. Photocatalytic degradation of acetone in the gaseous phase on sample (a) H1_Ti, (b) H1_TiAg, and (c) H1_TiPd.

the photooxidation of acetone. The reaction of acetone conversion to CO2 and H2O can be written as

CH3COCH3 + 4O2 f 3CO2 + 3H2O

(8) 35,36

Partial oxidation of acetone results in formation of CO. The stoichiometry of the oxidation reaction can be written (at least for the carbon balance) roughly as

CH3COCH3 + O2 f xCO + (3 - x)CO2 + yH2O

(9)

In our case, partial oxidation of acetone with various CO/ CO2 ratio has occurred. The rate constant of decomposition of acetone is 6.2 × 10-4 for sample H1_Ti, 8.6 × 10-4 for sample H1_TiAg, and 9.2 × 10-4 for sample H1_TiPd. The best photocatalytic decomposition of acetone and also higher production of CO2 than CO was achieved with the sample H1_TiPd (see Figure 9c). It demonstrated good acetone conversion (75%) after irradiation for about 1100 min. 4. Conclusions. Sol-gel-derived organic-inorganic hybrid materials consisting of organic poly(hydroxyethyl methacrylate)

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