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Applied Catalysis B: Environmental 106 (2011) 398– 404

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Applied Catalysis B: Environmental

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Hydroxyapatite/titanium dioxide nanocomposites for controlled photocatalyticNO oxidation

Anastasios Mitsionisa, Tiverios Vaimakisa,∗, Christos Trapalisb, Nadia Todorovab,Detlef Bahnemannc, Ralf Dillert c

a Department of Chemistry, University of Ioannina, P.O. Box 1186, 451 10 Ioannina, Greeceb Institute of Materials Science, NCSR Demokritos, 153 10, Attiki, Greecec Institut fur Technische Chemie, Leibniz Universitaet Hannover, Callinstrasse 3, D-30167 Hannover, Germany

a r t i c l e i n f o

Article history:Received 21 March 2011Received in revised form 23 May 2011Accepted 28 May 2011Available online 6 June 2011

Keywords:HydroxyapatiteTitanium dioxideNanocompositesPhotocatalysisNO oxidation

a b s t r a c t

Biphasic photocatalytic powders consisted of hydroxyapatite (HA) and titanium dioxide (TiO2) wereprepared by precipitation of HA in presence of TiO2 (Evonik-Degussa P25). Depending on the volumeratio between HA and TiO2 in the initial solution the materials showed different textural properties.Aggregates consisted of spherical particles were formed at low HA/TiO2 ratio, while the decrease of theTiO2 amount resulted in formation of needle and lath like particles. The specific surface area (ssa) of thecomposites was higher than the ssa of the pure components and increased with the decrease of the TiO2

amount. The novel materials exhibited excellent activity in photocatalytic NO oxidation, while the activityin NO2 removal, either by oxidation or by adsorption to the HA, was extremely low for the compositeswith volume ratio VHA/VTiO2 more than 1. The selective behavior of these composites towards the twosteps of the NOx oxidation process was related to the preparation procedure followed. The composite with50% (v/v) TiO2 revealed higher •OH formation ability and photocatalytic activity in overall NOx removalthan the pure TiO2 component that was ascribed to the specific VHA/VTiO2 ratio and growth of HA on theTiO2 surface.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Photocatalysis is one of the most promising technologies appliedin gas and liquid pollutants degradation for environmental pro-tection purposes. Among the materials used as photocatalysts,titanium dioxide (TiO2) is currently regarded as most effective [1,2].The high stability, non-toxicity and low production cost of TiO2 inthe form of film and powder facilitate its exploitation for waterand air purification [3]. However, its large scale application is stillrestricted by the costly activation energy needed and low efficiencyof the catalyst. Among the important features of TiO2, influencingthe photocatalytic activity are, specific surface area, crystal phase,crystallinity and particle size [4–6]. The efficiency of the photo-catalyst is also influenced by the presence of active species likehydroxyl radical (•OH), superoxide radical (•O2) and hydrogen per-oxide (H2O2) on the surface of TiO2 [7–10].

Calcium phosphate materials are known for their biomedicalapplications either as artificial bone and teeth or drug deliverycarriers [11]. Recently, increasing interest in these materials as

∗ Corresponding author. Tel.: +30 2651008352; fax: +30 2651008795.E-mail address: tvaimak@cc.uoi.gr (T. Vaimakis).

adsorbents can be noticed due to their ionic exchange property,adsorption affinity, bonding with organic molecules, low water sol-ubility, high stability under both reducing and oxidizing conditions,availability and low cost [12]. Hydroxyapatite (HA, Ca5(PO4)3OH)in particular, apart from being important material in biology andchemistry [13], is known for its adsorption properties for differentbacteria and NO2 [14]. HA is also reported as a novel stable materialin heterogeneous photocatalytic degradation of pollutants underUV irradiation. It has been found that the surface of synthetic HAparticles possesses ca. 2.6 groups/nm2 of P–OH acting as adsorptionsites for CO2, CH3OH, H2O, pyridine, n-butylamine and acetic acid[15,16].

The photocatalytic activity of HA was studied for degradation ofmethyl mercaptane [17], methylene blue [18], dimethyl sulphide[19], calmagite [20], propane and propene. Recently, the photocat-alytic activity of Ti substitution HA was reported. The experimentalband gap of HA, Ti–HA and anatase calculated by Tsukada et al. [21]was found to be >6 eV, 3.65 eV, and 3.27 eV, respectively, while thecalculated band gap of HA was reported to be around 4.5–5.4 eV([21] and related references). Nishikawa et al. indicated that oxy-gen vacancies are formed on HA by UV irradiation and suggestedthat the O2

•− radicals formation takes place through electron trap-ping on these vacancies [17,22,23]. Tanaka et al. [19] have studied

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the oxidation of the methyl mercaptane and DMS in vacuum, that is,free from O2, and suggested that the trapped electron generated byUV irradiation forms the surface P–OH• radicals on HA. The forma-tion of such radicals has been reported for surface Ti–OH groups ofanatase TiO2 [7]. Matsumura et al. [24] have reported in the oxida-tion of CO over HA at 600 ◦C, basic O2− ions, which are generated bythe dehydration of hydroxyl groups, can activate molecular oxygenas O2

− radical species by UV irradiation, which react with CO.Recently, HA/TiO2 composites for biomedical applications have

been prepared. HA/TiO2 compounds were synthesized by sol–gelmethod [25], miscellany of sol–gel and spin coating methods[26,27], hydrothermal treatment of HA powders with colloidalTi(OH)4 solution [18], and combined high gravity and hydrothermalmethods [28,29]. Rod-like TiO2 was deposited on HA by hydrother-mal treatment using a water-soluble titanium amine complex [30].HA–TiO2 nanocomposites were successfully prepared by in situmethod and microwave techniques for acceleration [31]. NanosizedTiO2 island structure on the platy HA nanocrystals has been pre-pared by two-step emulsion process [32]. Different titanium-basedsystems with increasing surface area were synthesized on naturalphosphate through sol–gel process [33]. Bio-mimetic synthesis ofHA crystals was employed by immersed TiO2 into simulated bodyfluid (SBF) [34–37].

Nathanael et al. found that the TiO2 nanoparticles, in low con-centration, facilitate the HA nanorods growth by heterogeneousnucleation [29]. The resulting hydroxyapatite nanocrystals loadedon TiO2 nanoparticles showed higher photocatalytic activity com-paring to commercial TiO2 catalyst [32]. Porous ceramics bodycoated by TiO2 and HA showed a rapid and complete oxidativedecomposition of acetaldehyde, which could not be attributed tothe absorption capacity of HA alone, but to the synergic effect ofradical formation on HA under irradiation [38]. Ji et al. [39] pre-pared TiO2/HA nanocomposites by a process based on the liquidimmiscibility of TiO2/�-tricalcium phosphate mixture in aqueousgelatin solution. They observed that the hybridization of TiO2 andHA results in the blue-shift of the absorption edge and to theenhancement of the photoinduced charges separation.

The object of this work is the preparation and characterizationof HA/TiO2 biphasic nanocomposite materials, as well as the inves-tigation of the influence of the HA component on their activityin photocatalytic NO oxidation. In order to avoid agglomerationof HA nanoparticles in the absence of any external agents, highspeed dispersing element at high temperature was employed. Thus,agglomeration free pristine HA and HA/TiO2 nanocomposites withhomogeneous structure could be obtained.

2. Experimental

2.1. Materials

All experiments were performed with analytical reagent gradechemicals and solvents. CaCl2·2H2O (Fluka, Assay (KT) 99%),Ca(H2PO4)2·H2O (Riedel-de Haën, Assay 88%) were used for thesynthesis of the adsorbent. Ammonia solution 25% (Riedel-de Haën)was used as pH controller. Commercial TiO2 (Evonic Degussa P25)was used as photocatalytic component.

2.2. Preparation of photocatalysts

The apparatus used for the preparation of the nanocompositematerials is described in our previous work [40]. A supersatu-rated solution of 0.0538 mol Ca(H2PO4)2·H2O and 0.1254 mol CaCl2with a Ca/P molar ratio of 1.67 (HA stoichiometry) was transferredinto the reactor vessel containing 800 mL of distilled water andthe proper amount of TiO2. The TiO2 amount was calculated in

Table 1Volume ratio between HA and TiO2 components, width of the band gap and specificsurface area of the samples.

Sample VHA/VTiO2ratio Band gap (eV) Specific surface area (m2/g)

TiO2 – 3.12 55.4HT01 1 3.08 78.8HT02 2 3.07 84.4HT05 5 3.07 98.2HT10 10 3.06 101.1HA – – 60.4

order to produce nanocomposite materials with HA/TiO2 volumeratios from 1 to 10. The solution was heated at 97 ± 1 ◦C for 30 minunder airflow with rate 15 L/h. Consequently, high speed disper-sion equipment was used to obtain rotation speed of 5000 rpm.Then 18 mL of concentrated NH4OH solution (25% (w/w)) wasslowly (∼6 mL min−1) added to the solution. The pH of the mix-ture increased from 3.8 to 8.8 and white slurry was produced. Theslurry was aged overnight at room temperature, filtrated, washedwith distilled water and dried at 90 ◦C for 6 h. The prepared biphasicmaterials were nominated as HT01, HT02, HT05 and HT10 accord-ing to the HA/TiO2 volume ratio used (Table 1).

2.3. Characterization of photocatalysts

The crystallinity of the materials was investigated by X-raydiffraction (XRD) technique. A Brüker P8 Advance apparatus wasemployed and the measurements were taken for a 2� rangeof 10–60◦ in steps of 0.02◦. International Centre for Diffrac-tion Data (ICDD) cards were used for the identification of thecrystalline phases. FT-IR measurements were performed using aspectrophotometer Model Spectrum RX I FT-IR, Perkin–Elmer. TheKBr disk technique was used with ∼2 mg of powder in ∼200 mgof spectroscopic-grade KBr (Merck) which had been dried at100 ◦C. The infrared spectra of the samples were recorded in the1250–450 cm−1 region.

The textural properties of the materials were examined by Scan-ning Electron Microscopy (SEM) using a JEOL JSM-6300 instrument,as well as N2 adsorption–desorption porosimetry. QuantachromeAutosorb Automated Gas Sorption System was used for determi-nation of the pore size distribution. The samples were degassed at180 ◦C and pressure of 10–30 Torr for 6 h before measurement.

UV–Vis Diffuse Reflectance Spectra (DRS) were recorded at roomtemperature using Shimadzu UV-2401PC equipment with a BaSO4coated integration sphere.

2.4. Evaluation of active •OH radicals production

The production of active •OH radicals by the prepared materi-als was evaluated using Terephthalic Acid-Fluorescence (TPA-FL)probe method. The fluorescence intensity of the hydroxyltereph-thalic acid (TPAOH) produced, was estimated according to thereaction (1) [41–44].

(1)

Aqueous stock solution containing 0.01 M NaOH and 3 mM TPA(Sigma–Aldrich) was initially prepared. 15 mg of each catalyst weremixed with 3.5 cm3 of the stock solution in a Pyrex glass cell. The

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mixtures were stirred for 10 min in the dark prior irradiation. A150 W Xe lamp (Hamamatsu Photonics, C2499) was used as lightsource for the excitation of the materials. The wavelength wasadjusted to 387 ± 11 nm and the samples were irradiated for 5 minwith intensity of 40 W/m2. The photoluminescence spectra of thesamples were recorded at room temperature in air using a SHI-MADZU RF-5301 spectrofluorophotometer equipped with a 150 WXe lamp, a red sensitive photomultiplier and reflection gratingmonochromators with fixed slits of 0.5 nm. The wavelength accu-racy was ±1.5 nm. A long-wavelength passing filter (UV-35) wasused to cut off scattered and second-order light.

2.5. Photocatalytic experiments

The photocatalytic activity of the samples was studied by NOoxidation procedure according to standard ISO/DIS 22197-1 forevaluation of air purification performance of photocatalytic mate-rials. The experimental set-up for the photocatalytic tests consistedof a gas supply part, the photoreactor, and a chemiluminescentNO–NOx gas analyzer (Horiba ambient monitor APNA-360). TheHA/TiO2 composite materials were pressed in holders with 25 cm2

surface area. Each holder was placed in the gas-flow photocatalyticreactor. The gaseous reaction mixture was prepared by mixingstreams of dry air (1500 mL/min), wet air (1500 mL/min) and50 ppm NO/N2 (approximately 60 mL/min), in order to obtain a finalconcentration of NO of 1 ppm. The photoreactor was illuminatedwith four 8 W black lights with a mean irradiation wavelength of350 nm, the UV light intensity achieving 1 mW/cm2. Prior to thephotocatalytic tests, the photoreactor was purged with the NO/airmixture without illumination until a steady outlet NO concentra-tion was achieved. According to the standard, the photocatalystadsorbs and oxidizes the NO to nitrogen dioxide (NO2). A portionof the produced NO2 is released to the gas phase while anotherportion is oxidized to stable nitrate ion (NO3

−) on the catalytic sur-face. The concentrations of the nitrogen oxide species, remained inthe gas phase above the catalysts, were monitored in the dark andunder UV(A) illumination for 120 min.

The photonic efficiency (�%) of the materials was calculated asa ratio of the number of degraded molecules to the number ofincident photons during the illumination according to Eq. (2):

� % = n degraded moleculesn available photons

=∫ t1

t0AX(ppm) dt

I�T(2)

where t0, t1 start and end of the illumination time. �T = t1 − t0.A is the coefficient taken as a product of the gas flow rate valueexpressed in mol/s (A = 2.08 × 10−9 mol/s). X is either the differ-ence between the initial NO/NOx concentration and the monitoredNO/NOx concentration or directly the NO2 concentration in ppm.The incident photon flux I (mol/s) on the surface (S) of the sampleis calculated using the Eq. (3):

I = I′′�S

Nhc(3)

where I′′ is the intensity of the incident light and �, N, h, c are thewavelength, the Avogadro number, the Planck constant and thespeed of light, correspondingly.

3. Results and discussion

3.1. Characterization of photocatalysts

The XRD patterns of the composite materials and the pure HAand TiO2 components are presented in Fig. 1. The position and therelative intensity of the peaks of the TiO2 diagram are characteris-tic for both rutile and anatase crystalline phase of TiO2 (JCPDS cardNo. 21-1276 and No. 21-1272 correspondingly). The HA diffraction

Fig. 1. XRD patterns of the composite photocatalysts and the pure components TiO2

and hydroxyapatite, where: R – rutile, A – anatase and H – hydroxyapatite.

peaks can be identified according to JCPDS card No. 09-0432. Thecharacteristic peak for HA {0 0 2} at 2� = 25.9◦ can be observed in thepatterns of the pure HA sample as well as the HA containing com-posites. The intensity of the HA peaks for the composites increaseswith the increase of the HA/TiO2 ratio, while the intensity of theTiO2 peaks decreases.

Fig. 2 shows the FT-IR spectra of the composite materials andthe pure HA only in the 1250–450 cm−1 region, which will be dis-cussed in details. The FT-IR spectrum recorded for the TiO2 sample(not presented in Fig. 2) exhibited a broad band at 800–450 cm−1

corresponding to Ti–O stretching mode. Bands related to the watermolecules of hydration at about 1600 cm−1 (�OH of hydrationwater molecules) and at approximately 3400 cm−1 (mOH of hydra-tion water molecules), were also present. For the pure HA sample,the recorded symmetric P–O stretching vibration of the PO4

3− bandat 962 cm−1, the triply degenerated bending vibrations at 602 cm−1,as well as the band at 475 cm−1, are attributed to the HA [45]. Theweak shoulder at 1177 cm−1 and the weak peak at 869 cm−1 canbe assigned to P–O–H deformation modes, while the weak peakat 962 cm−1 can be assigned to out of plane deformation. Theseabsorption bands indicate the presence of the HPO4

2− ionic groupin calcium deficient HA. The characteristic absorption bands at

1200 1100 1000 900 800 700 600 500

HT01HT02HT05

HT10

569

523

563

60263

2

904

1093

1097

962

1033

106811

12

1177 47

5

1049

Abs

orba

nce,

A%

Wavenumber, cm-1

869

HA

Fig. 2. FT-IR spectra of the composite materials and the pure components TiO2 andhydroxyapatite.

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A. Mitsionis et al. / Applied Catalysis B: Environmental 106 (2011) 398– 404 401

0

100

200

300

0

100

200

0.00 0.25 0.50 0.75 1.00

100

200

0.25 0.50 0.75 1.00

Volu

me,

(cc/

g)

TiO2 HT01

HT02 HT05

HT10

P/P0

HA

Fig. 3. N2 adsorption/desorption isotherms of the composite photocatalysts and thepure components TiO2 and hydroxyapatite.

3570 and 631 cm−1 that correspond, respectively, to the stretch-ing vibration and bending deformation modes of the lattice OH−

ions, indicate the formation of the crystalline HA phase. A mediumpeak at 904 cm−1 is attributed to P(OH)3 symmetrical stretching.For the HA containing samples, the characteristic bands for PO4

3−

appear at 1068, 1033, 602, 563 and 475 cm−1. The composites showhybrid spectra with the shape and intensity of adsorption bandsproportional to HA/TiO2 ratio. In the spectrum of sample HT01 the1068 and 1033 cm−1 absorption bands, assigned to PO3 degener-ate and P–O antisymmetric stretching modes, correspondingly, areincorporated in one broad band at 1049 cm−1. The 563 cm−1 peakassigned to O–P–O bending mode is shifted to 569 cm−1 imply-ing stronger P–O bond in comparison to pure HA. This behavior isattributed to the bonding of PO4

3− groups with the Ti atoms on thesurface of TiO2. As the Ti is less electropositive than Ca, the P–Obond is stronger in the Ti–O–PO3

2− group than in the Ca–O–PO32−

one.The adsorption–desorption isotherms of the samples are shown

in Fig. 3. The isotherms have similar type III shape, which is char-acteristic for nonporous solids. The specific surface area (Table 1)of the composites seems to be dependent on the initial amountTiO2 in the solution. With the increase of the amount TiO2 the ssaof the composites decreases from 101.1 to 78.8 m2 g−1. The valuesare much higher than the ssa values of the pure components HA(60.4 m2 g−1) and TiO2 (55.4 m2 g−1). Considering the ssa of TiO2constant, the increase might be attributed to the formation of highssa HA component when precipitated in presence of TiO2. However,larger quantities of TiO2 in the initial solution act deteriorating inthe development of high-ssa HA/TiO2 composites.

SEM micrographs of the samples are presented in Fig. 4. It isrevealed that sample TiO2 (Fig. 4a) is consisted of spherical par-ticles with diameter of ∼50 nm. The pure HA sample (Fig. 5f) isconsisted of lath-like particles with main size around 200–300 nmin length and 50 nm in width. The shape and size observed are char-acteristic for HA prepared through modified precipitation method[40,46,47]. The use of high speed dispersion equipment creates theproper hydrodynamic conditions for growth of HA in the form oflath-like particle in {0 0 1} direction [40] in absence of TiO2. Thecomposite sample HT01 (Fig. 4b), whose theoretical HA volume isequal to TiO2 volume (VHA/VTiO2

= 1), is consisted of aggregates aswell as individual particles. The shape of the individual particlesis more similar to TiO2 than HA particle shape. Lath-like parti-cles, which can be expected owing to the HA, are not present. The

estimated sizes of the aggregates and the individual particles arebetween 0.5–1 �m and 50–200 nm, correspondingly. The increaseof the VHA/VTiO2

ratio (sample HT02, Fig. 4c) leads to the formationof needle-like HA particles with diameter ∼40 nm. Further increaseof the VHA/VTiO2

ratio (sample HT10, Fig. 4e) results in formation oflath-like HA particles with width up to ∼150 nm. For the samplesHT05 and HT10 (Fig. 4d and e) round particles attributed to the TiO2can be observed on the edge of the lath-like HA particles. It can besuggested that the dispersion of the initially precipitated HA and itsgrowth is facilitated by the presence of TiO2 matrix in the startingsolution.

3.2. Mechanism of photocatalyst formation

The point of zero charge of TiO2 is reported to be at pH ∼6.25[48,49]. In our experiment, the pH of the initial solution is 3.88[40]. The main ionic species available are H2PO4

−, Ca2+ and Cl− thatcreate positive �-potential of the TiO2 particles in the initial solu-tion. When the equilibrium is reached, negative H2PO4

− ions can beattached to the TiO2 surface. By increasing the pH of the solution,the H2PO4

− ions can be transformed to HPO42− and PO4

3− ions.At this point, calcium phosphate precipitation occurs. The HA pre-cipitation process in the presence of TiO2 particles can take placethrough the following pathways (Fig. 5): dimerization of two TiO2particles charged with H2PO4

− (Fig. 5a) and growth of HA on singleTiO2 particle (Fig. 5b). It can be suggested that the dimerization isthe predominant precipitation process, which explains the absenceof needle or lath-like particles in the HT01 sample. Moreover, theincrease of the specific surface area of the composites in compar-ison to the pure components, can also be related to the formationof HA bridged TiO2 particles through the dimerization pathway.The decrease of the TiO2 amount in the initial solution leads to theformation of needle instead of lath-like HA particles.

The diffuse reflectance spectra of the materials (not presentedhere) revealed slight transition of the main absorbance towards theUV region (blue shift) for the nanocomposite samples in compar-ison to the pure TiO2. However, the absorbance of the compositematerials decreases with the decrease of the TiO2 amount in them.The band gap widths calculated according to the Kubelka–Munktheory [50] are given in Table 1. The values for the composite sam-ples are between 3.06 and 3.08 eV while the band gap of pure TiO2is broader (3.12 eV). This fact could be related to the synergic effectof HA on TiO2.

3.3. Active •OH radicals production

The fluorescence emission spectra of TiO2 and the compositepowders (Fig. 6) exhibit a peak at ∼425 nm due to the formation ofTPAOH [41]. It can be observed that the intensity of the fluorescencepeak for the pure HA sample is extremely low revealing no •OHproduction on the surface of the HA. With the increase of the TiO2amount in the initial solution, the intensity of the peak increases.Moreover, the composite with highest amount TiO2 (sample HT01)demonstrates higher •OH production than pure TiO2. This effect canbe attributed to the specific texture of the composite which pro-vides higher specific surface area, as well as the optimum HA/TiO2ratio for both •OH formation and TPA molecules adsorption.

3.4. Photocatalytic oxidation of NO

The concentration profiles of the NO, NO2 and NOx obtainedduring the photocatalytic procedure are depicted in Fig. 7. Initially,NO adsorption/desorption equilibrium over the photocatalysts wasreached in dark that is visualized in the curves as a small peak beforeillumination point. After the light is turned on, a sharp decreaseof NO concentration and a simultaneous increase of NO2 concen-

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Fig. 4. SEM microphotographs of the composite photocatalysts and the pure components TiO2 and hydroxyapatite. The arrows point at round particles of TiO2 on the edgeof the lath-like HA particles.

tration followed by stabilization of the gas concentrations can beobserved for all samples. The differences in their photocatalyticbehavior are related mainly to the intensity of the two processes:NO to NO2 oxidation (decrease of NO) and NO2 to NO3

− oxida-tion (decrease of NOx). The prepared materials exhibited significanteffectiveness on NO to NO2 oxidation, depending on the TiO2 con-tent. The pure HA showed its own photocatalytic activity whichcould not be attributed to the •OH formation, as the TPA-FL resultsrevealed no such ability of HA. According to Nishikawa [23] the UVirradiation causes the formation of oxygen vacancies on the HA sur-face by change of PO4 groups. Formation of the labile O2

•− radicalstakes place through the electron transfer from the formed surfaceHA vacancies to the overflowing oxygen in the gas state (reaction(4)). The radicals must be very active and able to oxidize the NO.

We suggest oxidation mechanism similar to the CO oxidation [24],according the reactions (5) and (6). Consequently, NO2 could beoxidized from O− radicals to NO3

−, according to the reaction (7).

e− + O2 → O2•− (4)

NO + O2•− → NO2 + O− (5)

NO + 2O− → NO2 + O2− (6)

NO2 + O− → NO3− (7)

The sample HT01 with the largest content TiO2 among the com-posite catalysts exhibited maximum activity that is even higherthan pure TiO2. This finding can be related to the higher oxida-tive and reductive potentials of the photo generated h+ and e−,

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A. Mitsionis et al. / Applied Catalysis B: Environmental 106 (2011) 398– 404 403

Fig. 5. The proposed mechanisms of HA precipitation: (a) dimerization of two TiO2

particles charged with H2PO4− , (b) growth of HA on single TiO2 particle.

400 500 6000

20

40

60

HAHT10HT05HT02

HT01

Fluo

resc

ence

Inte

nsity

(a.u

.)

Wavelength (nm)

TiO2

Fig. 6. Fluorescence spectra of the composite photocatalysts and the pure compo-nents TiO2 and hydroxyapatite.

Fig. 7. Concentration profiles of NOx , NO and NO2 gases over the composite photo-catalysts and the pure components TiO2 and hydroxyapatite.

respectively, in case of blue-shift of the absorption edge [39]. Inaddition, the side surface of the HA crystal is negatively charged,while the a-face of the c-axis elongated HA is positively chargedwhich could attract the produced NO3

− anions and enhance thetotal NOx removal.

With the decrease of the TiO2 amount in the initial solution, theactivity in NO to NO2 oxidation decreases. Nevertheless, the activityof the sample with smaller amount of TiO2 (VHA/VTiO2

= 10) is stillcomparable to this of pure TiO2. On the other hand, the HA/TiO2composites, except for sample HT01, showed lower activity in NOx

removal in comparison to pure TiO2. The major part of the producedNO2 is released to the overflowing gas without further oxidation

Fig. 8. Photonic efficiencies of the materials in NO oxidation, NO2 emission and NOx removal.

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404 A. Mitsionis et al. / Applied Catalysis B: Environmental 106 (2011) 398– 404

to NO3− or absorption into the HA/TiO2 composites as expected.

This can be attributed to the lower effective active surface areaof anatase in composite granule with higher VHA/VTiO2

ratio aftercomplete HA precipitation. Nevertheless, the specific behavior ofthe prepared composite materials allows separation of the twosteps of the NO oxidation process.

The calculated photonic efficiencies of the materials aredepicted in Fig. 8. All composite samples showed improved pho-tonic efficiencies in NO oxidation when compared to pure HA andTiO2. Sample HT01 has not only photonic efficiency in NO oxi-dation twice this of TiO2, but a higher efficiency in NOx removalas well. This result can be related to the optimum 1:1 volumeratio between the components which ensures their excellent dis-persion and utilization of the photocatalytic properties of bothcomponents.

4. Conclusions

The prepared HA/TiO2 composites are consisted of TiO2 par-ticles connected by HA bridges, forming needle and/or lath-likeparticles. The precipitation of HA in presence of TiO2, can producespecific textures with well dispersed TiO2 particles and higher spe-cific surface area composites in comparison to the pure TiO2. Thecomposite with the highest amount of TiO2 in the initial solution(VHA/VTiO2

= 1) demonstrated higher •OH production capabilitythan pure TiO2. The decrease of the TiO2 amount increases the ssa ofthe composites and reduces the crystallinity and •OH concentrationon their surface.

The HA/TiO2 composite materials exhibited different activi-ties for each part of the NO → NO2 → NO3

− photocatalytic process.While enhanced photocatalytic activity in NO → NO2 oxidation wasrecorded for all composites in comparison to the pure compo-nents, the activity in the NO2 → NO3

− oxidation was extremelylow for all composites except for the sample with volume ratioVHA/VTiO2

equal to one. The unique behavior of the HA/TiO2 com-posites was attributed to the dispersion between the componentsand the influence of residual acidic groups due to the HA precipi-tation procedure.

Acknowledgements

This work has been supported by the program for the promo-tion of the exchange and scientific cooperation between Greece andGermany (IKYDA 2009).

The authors are thankful to the Evonik Degussa Corporation forproviding P25 powder used throughout the experiments.

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