Materials Letters 110 (2013) 57–60

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Preparation of titania/hydroxyapatite (TiO2/HAp) compositephotocatalyst with mosaic structure for degradationof pentachlorophenol

Juan Xie a,b, Xiaocai Meng a, Zhao Zhou b,c, Ping Li a, Lan Yao a, Li Bian a, Xiaorui Gao a,Yu Wei b,c,n

a College of Science, Hebei University of Engineering, Handan 056038, Chinab Key Laboratory of Inorganic Nanomaterials of Hebei Province, Hebei Normal University, Shijiazhuang 050024, Chinac College of Chemistry and Materials Science, Hebei Normal University, Shijiazhuang 050024, China

a r t i c l e i n f o

Article history:Received 25 May 2013Accepted 26 July 2013Available online 3 August 2013

Keywords:TitaniaHydroxyapatiteComposite materialsMicrostructurePhotocatalysisPentachlorophenol

7X/$ - see front matter & 2013 Elsevier B.V. Ax.doi.org/10.1016/j.matlet.2013.07.108

esponding author at: Key Laboratory of Inorge, Hebei Normal University, Shijiazhuang, 050311 8626 8342; fax: +86 311 8589 3425.

ail address: hbsdweiyu@163.com (Y. Wei).

a b s t r a c t

Titania/hydroxyapatite (TiO2/HAp) composite with mosaic structure was successfully synthesized via afacile route without any structure-directing agent. XRD, EDS and FESEM were used to characterize theproduct. It is found that lots of spherical TiO2 particles with average diameter of about 24 nm are formed,like pearls densely inlaid in the ravines of HAp carrier surface. Meanwhile, a possible surface adsorptionlayer hydrolysis mechanism for the formation of such novel structure has been proposed. Experimentalresults show that the obtained TiO2/HAp composite, which combines the excellent adsorption capacity ofHAp and the high photocatalytic activity of TiO2, can effectively degrade persistent organic pollutantpentachlorophenol under UV irradiation.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Pentachlorophenol (PCP) with stable aromatic ring system andchlorine substituents is a ubiquitous soil and water contaminantbecause of its widespread application in agriculture and industry[1]. This compound can not only alter the electrical conductivitiesof biomembranes and inhibit cellular enzymes but also producemutations in animal (including human) cells and exhibit terato-genic, carcinogenic, and reproductive effects [2]. Besides theconventional physicochemical and biological processes, photoca-talytic methods in which TiO2 is widely used have also beenemployed to treat chlorinated phenols [3,4]. However, TiO2 canonly make use of UV light and its adsorption ability to organics isrelatively weak. Hence, research on modification of TiO2 forimprovement of photocatalytic efficiency has received increasingattention in recent years [5,6].

Hydroxyapatite Ca10(PO4)6(OH)2, abbreviated as HAp, has greatimportance in materials chemistry. It is known as an excellentadsorbent for adsorption and separation of biomolecules, pollu-tants and heavy metal ions [7–9] for its special chemical structure

ll rights reserved.

anic Nanomaterials of Hebei024, China.

makes HAp have a strong ability to adsorb organic molecules andact to anchor various cations [10]. Some approaches have beendeveloped to synthesize TiO2–HAp composite, such as chemicalco-precipitation [11], in situ precipitation [12], photo-inducedformation [13], sol-gel dip coating [14], and gas tunnel typeplasma spraying [15], though research purposes are not to treatenvironmental pollutants. However, complicated procedures,sophisticated equipments or rigid experimental conditions usedin these methods severely restrict their application.

Herein, we present a simple, fast and reliable process forsynthesis of TiO2/HAp composite. This new kind of catalyst caneffectively degrade PCP under UV irradiation in a short time. Toour knowledge, there is no similar report about TiO2/HAp compo-site for photocatalytic degradation of PCP.

2. Experimental

Materials: PCP (99% purity) was purchased from Sigma Chemical,St. Louis, MO, USA. All other chemicals were of analytical grade andused as received without further purification. Deionized water wasused throughout.

Synthesis: Synthesis of HAp carrier: Adopting the molar ratio ofCa/P¼1.67, 25 mL of 0.5 mol L�1 Ca(NO3)2 was added dropwiseinto 15 mL of 0.5 mol L�1 NH4H2PO4 under stirring. Then, pH ofthe system was adjusted to �10.5 with concentrated ammonia

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solution (25�28%). After stirring at room temperature for 1 h,white product was collected, washed with distilled water anddried in natural environment.

Synthesis of TiO2/HAp composite: 1 g of HAp carrier and 0.5 mLof deionized water were added in 50 mL of absolute alcohol to geta suspension, which was treated in an ultrasonic cleaning bath(250 W) for 30 min and then was stirred at room temperature for5 h. After that, 3.4 mL Ti(OC4H9

n)4 diluted in 6.6 mL absolutealcohol was introduced into the aforementioned dispersive solu-tion, followed by continuous stirring for 5 h. Product was collectedby filtration, washed with absolute alcohol, air-dried at ambienttemperature, and finally calcined at 500 1C for 2 h in air.

Characterization: X-ray diffraction (XRD) data were collectedfrom powder samples by using a Bruker D8 Advance diffract-ometer with Cu Kα radiation (λ¼1.5418 Å). Elemental compositionof the product was determined by using OXFORD INCA 350 energydispersive spectroscopy (EDS). Morphology of samples was char-acterized by using field emission scanning electron microscopy(FESEM, HITACHI S-4800).

Photocatalytic experiments: A 125 W low pressure mercurylamp with main wavelength of 253.7 nm was used as light source.The distance between the lamp and the solution was about 10 cm.A total of 0.1 g catalyst powder was added into 100 mL of the PCPaqueous solution (10 mg L�1, pH 9�10) with ultrasonic treatmentfor 15 min. Before illumination, the suspension was stirred con-tinuously in dark for 30 min to ensure adsorption equilibrium. Atregular intervals, a certain amount of suspension containing PCPand powder catalyst was taken out and filtrated to completelyremove the catalyst. The kinetic photodecomposition process wasmonitored by measuring the residuary concentration of PCP with aUV–vis spectrophotometer (TU1900, Beijing Purkinje GeneralInstrument Co. Ltd., China) at 220 nm, and each concentrationwas repeatedly determined at least three times.

3. Results and discussion

Fig. 1 shows the XRD pattern of TiO2/HAp composite. Thediffraction peaks with 2θ values of 25.21, 37.91, 48.11, 54.21, 55.11,62.71 and 69.01 can be exactly indexed to anatase phase TiO2

(JCPDS 21-1272), while other discernible diffraction peaks in therange 30–501 are assigned to hexagonal phase HAp (JCPDS 09-0432). Average crystallite size of TiO2 particles calculated byapplying the Debye–Scherrer's equation is approximately 23.6 nm.

Fig. 2 demonstrates that only four elements, Ca, P, O and Ti existin TiO2/HAp composite, and Ca/P ratio determined from the

Fig. 1. XRD pattern of TiO2/HAp composite.

semiquantitative analysis of the atomic concentration (At%) is1.696, a little higher than the stoichiometric Ca/P ratio of 1.667.This phenomenon could be attributed to calcination treatment,because phosphorus atoms are able to disengage from the materialsurface and volatilize as oxides during heating process [16].Titanium element with content of 5.27% clearly appears in thefigure, confirming the incorporation of TiO2 on HAp and simulta-neously supporting the XRD analysis.

Fig. 3 shows the changes observed in the morphology of HApcarrier after calcination with and without TiO2 addition. PristineHAp carrier (Fig. 3a1 and a2) is in the irregular plate-like shape,and its surface is full of honeycomb pits. After high temperaturetreatment, the size of plate-like HAp carrier gets smaller, andhoneycombs are replaced by winding ravines (Fig. 3b1 and b2).Fig. 3c1 and c2 display the microstructure of TiO2/HAp composite.There are lots of spherical TiO2 nanoparticles with a diameter of20�28 nm, like pearls densely inlaying in the ravines of HApcarrier surface. Although long-time ultrasonic treatment wasemployed before FESEM observation, we can hardly see anyscattered TiO2 nanoparticles, indicating that TiO2 and HAp arecombined closely with each other, and the structure of TiO2/HApcomposite is very stable.

As known, HAp is a kind of polar material with stronghydrophilic property, for its surface possesses massive hydroxylgroups [17] and is easy to form hydration layer [18]. So after a longtime of stirring, adsorption equilibrium of trace water on pristineHAp carrier surface is reached, forming a stable adsorbed waterlayer. When added Ti(OC4H9

n)4 contacts with the adsorbed water,TiO2 nanoparticles are gradually prepared by hydrolysis reactionunder the condition of agitating. Finally, calcination treatmentchanges the morphology of HAp carrier and firmly immobilizesTiO2 nanoparticles on its surface.

Our previous study shows that mesoporous TiO2 microspheres ofsizes varying from 0.5 to 4.5 mm, which are composed of numerouspure anatase TiO2 nanoparticles of diameter �18 nm, can beobtained under the same experimental conditions without HApcarrier [19]. For comparison, the degradation efficiency of PCP byusing this kind of TiO2 microspheres was also investigated. Inaddition, blank study (absence of catalyst) was carried out as abackground check. From Fig. 4, we can see that without photocata-lyst, only �40% of PCP was degraded after 135 min, due to photo-lysis. But in the same amount of time, degradation efficiency of PCPwas raised to 97.52% in the presence of TiO2 microspheres. Althoughthe catalytic effect of TiO2 microspheres was better than that of TiO2/HAp composite in the first 52 min of the reaction, greatly relating tothe fact that the actual content of TiO2 in TiO2/HAp composite is

Fig. 2. EDS spectrum of TiO2/HAp composite.

Fig. 3. Low- and high-magnification FESEM images of different samples.

Fig. 4. Relationship between degradation efficiency of PCP and irradiation timewith error bars.

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lower than 0.1 g, PCP was almost completely eliminated by TiO2/HApcomposite within 90 min—a whole 45 min early. Because hydroxya-patite is not an effective catalyst for photooxidation [20], comparative

experimental results just indicate that the existence of HAp withexcellent adsorption capacity can really help to improve the photo-catalytic performance of TiO2.

4. Conclusions

In summary, spherical TiO2 nanoparticles were successfullyimmobilized on the surface of HAp carrier by a facile method.This TiO2/HAp composite has both excellent adsorption perfor-mance and superior photocatalytic redox ability, and can be usedas photocatalyst to effectively degrade PCP under UV irradiation.

Acknowledgments

This work was financially supported by grants from the NationalNatural Science Foundation of China (21206026), the Natural ScienceFoundation of Hebei Province (B2012402011; B2012402006), theScience and Technology Research Foundation of Hebei EducationDepartment for Outstanding Young Teachers in University(Y2012028), and the Science and Technology Research and Develop-ment Projects of Handan City (1128130092; 1223120094).

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