Enhanced Photocatalytic Activity of TiO2 Nanoparticles Supported on

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Enhanced Photocatalytic Activity of TiO Nanoparticles Supported on Electrically Polarized Hydroxyapatite Xuefei Zhang, and Matthew Z. Yates ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03838 • Publication Date (Web): 27 Apr 2018 Downloaded from http://pubs.acs.org on April 28, 2018

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ACS Applied Materials & Interfaces

Enhanced Photocatalytic Activity of TiO2 Nanoparticles Supported on Electrically Polarized Hydroxyapatite Xuefei Zhang† and Matthew Z. Yates∗,†,‡ †Department of Chemical Engineering, University of Rochester, Rochester, NY 14627 ‡Laboratory for Laser Energetics, University of Rochester, Rochester, NY 14627 *E-mail: [email protected]

Abstract Fast recombination of photo-generated charge carriers in titanium dioxide (TiO2) remains a challenging issue limiting the photocatalytic activity. This study demonstrates increased photocatalytic performance of TiO2 nanoparticles supported on electrically polarized hydroxyapatite (HA) films. Dense and thermally stable yttrium and fluorine co-doped HA films with giant internal polarization were synthesized as photocatalyst supports. TiO2 nanoparticles deposited on the support were then used to catalyze the photochemical reduction of aqueous silver ions to produce silver nanoparticles. It was found that significantly more silver nanoparticles were produced on polarized HA supports than on depolarized HA supports. In addition, the photodegradation of methyl orange with TiO2 nanoparticles on polarized HA supports was found to be much faster than with TiO2 nanoparticles on depolarized HA supports. It is proposed that separation of photo-generated electrons and holes in TiO2 nanoparticles is promoted by the internal polarization of the HA support, and consequently, the recombination of charge carriers is mitigated. The results imply that materials with large internal polarization can be used in strategies for enhancing quantum efficiency of photocatalysts.

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Keywords hydroxyapatite; titania; photocatalysis; polarization; silver; methyl orange

1 Introduction Semiconductors, such as TiO2, CdS, and ZnO, have been extensively studied for photocatalytic applications, such as water splitting and dye degradation. 1–3 However, in most cases the photocatalytic activity remains unacceptably low for practical applications. The reported quantum efficiencies for water splitting are all below 2.5% while theoretical calculations indicate that the maximum efficiency could reach 14% with a 2.5 eV band gap semiconductor. 4,5 One major factor limiting the efficiency of photocatalysis is the fast recombination of photo-generated electron-hole pairs. 6 Internal electric fields have been used to separate photoinduced charge carriers and reduce the recombination of electron-hole pairs. 5 Various studies have utilized p-n junctions and metal catalyst-photocatalyst junctions to produce internal electric fields in photocatalytic systems that facilitate the separation of charge carriers and, consequently, improve photocatalytic activity. 7–9 The incorporation of ferroelectric materials in photocatalytic systems was recently reported to offer similar improvement of photocatalytic activity. 10,11 The spontaneous polarization in ferroelectrics produces charged surfaces that cause photoinduced charge carriers to move in opposite directions and thus separate electron-hole pairs. The effect of the electric field originating from the support impacts the photocatlytic activity of nano-scale photocatalysts. It was shown that the photocatalytic activity of nanoscale ( 15 nm) TiO2 coatings was significantly affected by the polarity of a ferroelectric support, while thicker ( 100 nm) TiO2 films on the same ferroelectric support were unaffected by the polarization of the support. 12 Yeredla et al also reported that a polar mineral tourmaline support increased the quantum efficiency of TiO2 in the photosplitting of water due to the internal electric fields of tourmaline. 13 Hydroxyapatite (HA) is a crystalline form of calcium phosphate with the stoichiometric for2


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mula Ca10(PO4)6(OH)2. Synthetic HA has been widely explored for a variety of applications, such as medical implants, 14,15 chemical sensors, 16 and catalysis, 17 due to its biocompatibility, unique electrical properties, and capacity for ion exchange. It was recently discovered that dense HA films synthesized by a two-step method display surprisingly large internal electrical polarization. 18 The stored charge density in the as-synthesized HA film was approximately 70,000 µC/cm2 , while the highest reported value in ferroelectrics is only around 150 µC/cm2 . 19 Although a number of studies have used various ferroelectrics as photocatalyst supports, there is no known study of strongly polarized HA as a support to enhance photocatalytic activity. Since it possesses giant internal polarization, the HA film has the potential to greatly affect photocatalytic activity when used as a support for semiconductor photocatalysts. In the present study, yttrium/fluorine co-doped hydroxyapatite films (YF-HA) were synthesized by a two-step method. TiO2 was chosen as a model photocatalyst because it is one of the most common photocatalysts due to its low cost, low toxicity, good availability and relatively high stability. 20 TiO2 nanoparticles were hydrothermally deposited on the YF-HA films and also on depolarized YF-HA films (dpYF-HA). Photoreduction of Ag+ ions to Ag0 nanoparticles was then carried out to evaluate the effect of the internal electrical polarization of the HA support on the photocatalytic activity of TiO2 nanoparticles. The results show that significantly more Ag nanoparticles were produced on YF-HA supports than on dpYF-HA supports. Photodegradation of methyl orange was also performed to evaluate the photocatalytic activity of TiO2 nanoparticles on the two types of supports. A possible mechanism of the observed enhanced photocatalytic activity is proposed.

2 Experimental 2.1

Materials

Tris(hydroxymethyl)aminomethane (>99.8%), NH4F (98%), K2HPO4 (99.99%), NaCl (≥99.0%), disodium ethylene diaminetetraacetate dihydrate (Na2EDTA · 2 H2O) (99.0-101.0%), CaCl2 · 2 H2O 3



(> 99%), Ca(NO3)2 · 4 H2O (99.0%), and TiCl4 (99.9%) were all obtained from Sigma-Aldrich. (NH4)2HPO4 (> 99.0%) was purchased from EMD. Hydrochloric acid (37%) and ammonium hydroxide (28.0-30.0%) were purchased from Mallinckrodt Chemicals. Y(NO3)3 · 6 H2O (99.9%), titanium (Ti) (0.89 mm thick) and platinum (Pt) foils (0.127 mm thick) were obtained from Alfa Aesar. Silver nitrate (AgNO3, ACS grade) was obtained from Amresco. All chemicals were used as received. Deionized water was used throughout the experiments.

2.2

Sample Preparation

For the electrochemical synthesis of HA nanocrystals, cleaned Ti foils (12.5×12.5×0.89 mm) were used as the cathode, and a Pt foil was used as the anode. The electrolyte solution consisted of 138 mM NaCl, 50 mM tris (hydroxymethyl)-aminomethane, 1.3 mM CaCl2 · 2 H2O, and 0.84 mM K2HPO4. The solution was adjusted to pH 7.2 using hydrochloric acid. The electrodes were immersed in the electrolyte solution with a 1 cm distance maintained between the electrodes. The reaction was carried out at 95°C for 5 min with a constant current density of 12.5 mA/cm2 . After the reaction, the sample was taken out, rinsed with deionized water, and dried in air. For hydrothermal growth of yttrium/fluorine co-doped HA crystals, the sample obtained from the first step was transferred to a Teflon-lined pressure vessel (Parr model 4744). The hydrothermal reaction solution consisted of 0.115 M Na2EDTA · 2 H2O, 0.1 M Ca(NO3)2 · 4 H2O, 0.01 M Y(NO3)3 · 6 H2O, 0.02 M NH4F, and 0.06 M (NH4)2HPO4. The pH of the solution was adjusted to 10.0 using ammonium hydroxide. The pressure vessel was filled with 30 mL of the reaction solution, sealed, and then placed in a convection oven at 200°C for 10 h. After the reaction, the sample was taken out, rinsed with deionized water and dried in air. The as-synthesized yttrium/fluorine co-doped HA films are denoted as YF-HA films. To investigate the effect of internal polarization of as-synthesized YF-HA films on photocatalytic activity, some of the films were subjected to depolarization. The sample was placed in a tube furnace (Lindberg Blue M, Thermo Scientific), heated at a heating rate of 5°C/min to 600°C, kept for 1 h, and naturally cooled down to room temperature. The YF-HA films after depolarization are 4


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denoted as dpYF-HA films.

2.3

Thermally stimulated depolarization current measurement

The stored charge of the YF-HA films was determined by thermally stimulated depolarization current (TSDC) method. The surface of the film was coated with approximately 10 nm of platinum (Pt) through sputter coating. Then the sample was placed between two Pt foils (1.25 cm X 1.25 cm) held together by a clamp. Two Pt wires were attached to the Pt foils and connected a picoammeter (Model 6487, Keithley Instruments). The sample was placed in a tube furnace (Lindberg Blue M, Thermo Scientific) and heated to 600°C at a heating rate of 5°C/min while continuously measuring the depolarization current.

2.4

Deposition of TiO2 nanoparticles

For hydrothermal deposition of TiO2 nanoparticles, a Ti(OH)4 dispersion was first prepared by adding 1.1 mL TiCl4 dropwise to 30 mL of deionized water. Then the pH of the solution was adjusted to 7.24 using 2 M ammonium hydroxide. The resulting Ti(OH)4 dispersion was filtered and washed with deionized water. 8 g of the paste-like product was re-dispersed in 100 mL deionized water by sonication. Before reaction, 1 mL of the Ti(OH)4 dispersion was mixed with 29 mL ethanol and then used as the hydrothermal reaction solution. An HA film was placed in a Teflonlined pressure vessel, and the Ti(OH)4 in ethanol was added to the vessel. The vessel was sealed and placed in a convection oven at 150°C for 12 h. Finally, the sample was rinsed with deionized water and dried in air. The YF-HA films are denoted as TiO2/YF-HA and TiO2/dpYF-HA after depositing TiO2 onto the polarized and depolarized YF-HA, respectively.

2.5

Photoreduction of Ag+ to Ag0

Photoreduction of Ag+ ions to Ag0 nanoparticles was carried out in order to investigate photocatalytic activity of TiO2 nanoparticles supported on YF-HA and dpYF-HA films. The prepared

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sample was immersed in 5 mL of 1 mM AgNO3 solution in a glass petri dish. A 254 nm wavelength UV lamp (6 W, UVP, LLC) placed 10 cm from the sample surface was used as the light source. The reaction was carried out for 15 min at room temperature while stirring the solution. After the reaction, the sample was rinsed with deionized water and dried in air.

2.6

Photodegradation of methyl orange

Photodegradation of methyl orange was also conducted to evaluate photocatalytic activity of TiO2 nanoparticles supported on YF-HA and dpYF-HA films. Three TiO2/YF-HA and three TiO2/dpYFHA films were placed in 6 mL of 10 mg/L methyl orange solutions, respectively. Bare TiO2/YF-HA and TiO2/dpYF-HA films were also tested as control groups. A 254 nm wavelength UV lamp (6 W, UVP, LLC) placed 10 cm from the sample surface was used as the light source. A small amount of the methyl orange solution was taken out for UV-Vis measurement and put back at 4 h, 8 h, 12 h, and 16 h. After the reaction, the sample was rinsed with deionized water, dried in air, and used for the second and third cycle measurements.

2.7

Sample characterization

Morphology of the HA film was examined using a field emission source scanning electron microscope (FESEM, DSM982, Zeiss-Leo). The crystal structure was determined by X-ray diffraction (XRD, PW3020, Philips). The surface composition was analyzed by an XPS spectrometer (AXIS Ultra DLD, Kratos).

3 Results and discussion 3.1

Synthesis and characterization of Yttrium/Fluoride co-doped HA films

To investigate the effect of the internal polarization on the photocatalytic activity of TiO2 nanoparticles, dense and thermally stable HA films were prepared using a two-step process. First, HA

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nanocrystals were electrochemically deposited onto Ti substrates as seed layers. As shown in Fig. 1(a), needle-like HA crystals with the width of approximately 20 nm and the length of roughly 500 nm were grown on the substrates. Then, the HA seed layers were subjected to a hydrothermal growth step. The HA films were co-doped with yttrium (Y) and fluoride (F) during the hydrothermal crystal growth by adding Y(NO3)3 · 6 H2O and NH4F to the reaction solutions. Due to their high adsorption affinity to csurfaces, yttrium ions have been found to promote lateral growth of HA crystals and thus produce dense thin films. 21 The yttrium-doped HA also absorbs more calcium ions compared with undoped HA crystals. 22 Therefore, the yttrium-doping was used in order to obtain dense HA films. When using this hydrothermal synthesis method, the resulting yttrium-doped HA is known to be calcium deficient relative to the stoichiometric HA composition. 21,23 The calcium deficiency causes a loss in thermal stability, as calcium-deficient HA dehydroxylates and further decompose into other forms of calcium phosphate like β-tricalcium phosphate (β-Ca3(PO4)2, β-TCP) at elevated temperature. 24,25 Some studies have shown that doping fluoride ions into HA crystals suppresses dehydroxylation, leading to higher thermal stability and better resistance to decomposition at high temperature. 25–27 In this study, HA films need to be heated up to 600°C to be fully depolarized. 18 Therefore, fluoride ions were co-doped with yttrium ions for preparing dense and thermally stable HA films. The as-synthesized yttrium/fluorine co-doped HA films are referred to as YF-HA, and after depolarization the films are referred to as dpYF-HA. Fig. 1(b)-(d) shows the morphology of YF-HA films prepared by hydrothermal growth before and after depolarization at 600°C. The YF-HA films are composed of closely packed YF-HA crystals that grow preferentially with the c-axis oriented normal to the substrate surface. The preferred crystal orientation is evident by the hexagonal facets visible in top-view SEM images shown in Fig. 1(c) and 1(d). The YF-HA crystals have smooth surfaces and are roughly 500 nm wide. The side view of a YF-HA film (Fig. 1 (b)) shows that the thickness of the film is approximately 4 µm. The SEM images suggest that the YF-HA films are thermally stable since no morphological changes were observed after depolarization (Fig. 1(d)). A previous study of

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Figure 1: SEM images of different HA coatings: (a) an electrochemically deposited HA seed layer, (b) side view of a hydrothermally synthesized YF-HA film, (c) top view of a hydrothermally synthesized YF-HA film, (d) top view of a dpYF-HA film.

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thermal stability showed that the formation of β-TCP would be clearly evident from the SEM images as the new crystal phase nucleates and grows on the surface during thermal decomposition of HA. 25 The elemental composition of a YF-HA film before and after depolarization was obtained by X-ray photoelectron spectroscopy (XPS), shown in Table 1. The results confirm that Y3+ and F– ions were incorporated into the film, which is consistent with our previous studies. 23,28 The detection of carbon was due to adventitious carbon contamination. The results also suggest that the composition of the film surface did not change after depolarization. Table 1: Elemental composition of a YF-HA film before and after depolarization (atomic percentage). O(%) C(%) F(%) Ca(%) Y(%) P(%) As-synthesized 47.82 21.97 1.63 15.52 1.59 11.47 Depolarized 47.30 22.86 1.70 15.03 1.65 11.46 X-ray diffraction (XRD) confirms that no change in crystal structure occurred during the depolarization process. Some YF-HA and dpYF-HA coatings were scraped from the Ti substrates to form powder samples for examining the crystal structures by XRD. The XRD patterns are shown in Figure 2. For YF-HA powders and dpYF-HA powders (Fig. 2(a) and (b)), all the peaks can be attributed to the apatite crystal structure according to the reference pattern of hydroxyapatite (JCPSD card 09-0432). The peak positions, however, are slightly shifted from the reference pattern, which is as expected because doping of the crystals with Y3+ and F– will affect the hydroxyapatite crystal lattice dimensions slightly. The XRD patterns from YF-HA powders and dpYF-HA powders appear to show identical apatite crystal phases. No other crystal phase was detected. For comparison, XRD patterns were also collected for the YF-HA and dpYF-HA films supported on the titanium substrate. As shown in Fig. 2(c) and (d), the XRD patterns from YF-HA and dpYF-HA films appear identical. The only peak not attributed to the apatite crystal structure is located near 2θ = 38° and is due to the underlying titanium support. Unlike the powder samples, the films supported on titanium have highly pronounced peaks located at around 2θ = 25° and 53° from the (0 0 2) and (0 0 4) apatite crystal planes. The enhanced intensity of these two peaks is due to the preferential orientation of the crystals with the c-axis normal to the surface. 21 9



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Figure 2: XRD patterns for different samples: (a) YF-HA powders, (b) dpYF-HA powders, (c) a YF-HA film, and (d) a dpYF-HA film. Thermally stimulated depolarization current (TSDC) measurement was carried out to characterize the electrical charge stored in YF-FA films. As shown in Figure 3, the measured current density reached a maximum at approximately 480°C and fell sharply to zero as the sample was heated above 500°C, which indicated that the sample was completely depolarized. The total stored charge density can be calculated by integrating the current density using the following equation: 29 1 Q= β

Z

J(T )dT

(1)

where Q is the total stored charge density (C/cm2 ), β is the heating rate (°C/s), and J(T) is the current density (A/cm2 ) at temperature T (°C). The calculated total stored charge density is 54.9 mC/cm2 , which is in agreement with our previous results. 18,28

3.2

Deposition of TiO2 nanoparticles on YF-HA and dpYF-HA films

After the YF-HA films and dpYF-HA films were obtained, TiO2 nanoparticles were deposited on the top surfaces of YF-HA crystals using a hydrothermal method. 30,31 Figure 4 shows the morphologies of TiO2 deposited onto the YF-HA and dpYF-HA films. The TiO2 appears as nanoparticles with a size of around 10 nm and some clusters of nanoparticles. No measurable morphological difference was found between the TiO2 nanoparticles formed on the two types of supports. To examine the crystal phase of the synthesized TiO2 nanoparticles, residual particles left in 10


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Figure 3: Thermally stimulated depolarization current (TSDC) measurement of a YF-HA film.

Figure 4: SEM images of (a) TiO2 nanoparticles deposited on YF-HA, and (b) TiO2 nanoparticles deposited on dpYF-HA

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the hydrothermal solution after the reaction were collected and scanned by XRD (Fig. 5). The XRD pattern is well matched to the pattern of anatase TiO2 according to the reference JCPDS card 21-1272. It is worth noting that apatite films containing deposited TiO2 nanoparticles were also scanned by XRD, but no diffraction peaks from TiO2 were observed. This is likely due to the fact that the quantity of TiO2 deposited is relatively low, so that the intensity of the XRD peaks from TiO2 nanoparticles cannot be differentiated from the background noise in the XRD spectra.

Figure 5: XRD pattern of TiO2 particles collected from the hydrothermal solution after the reaction. It is well matched to the reference pattern of anatase TiO2. A more sensitive analytical technique, high-resolution XPS, confirms the presence of TiO2 on the surface of YF-HA and dpYF-HA films after the reaction. As shown in Fig. 6, Ti was detected on both YF-HA and dpYF-HA films. The XPS spectra were calibrated using adventitious carbon (C 1s peak at 284.5 eV) as a reference. Interestingly, the Ti peaks of TiO2 nanoparticles on dpYFHA films show a shift of roughly 0.3 eV to higher binding energy compared to those on YF-HA films. The peak shift indicates that the internal polarization of YF-HA films is affecting the binding energy of Ti in the deposited TiO2 nanoparticles. Although a number of groups have studied the effect of internal electric fields on the photocatalytic activity of TiO2, very few studies have done similar XPS analysis. Arveux found that the binding energy of Ti in BaTiO3 was different when BaTiO3 thin films were deposited onto SrTiO3 single crystals with different orientations. 32 The shift of binding energy was attributed to the differences in the surface polar state of the different crystal facets. A possible explanation of the binding energy shift of Ti in this work will be discussed 12


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in section 3.3. In the following sections, the YF-HA and dpYF-HA films with surface deposited TiO2 are denoted as TiO2/YF-HA and TiO2/dpYF-HA, respectively.

Figure 6: High-resolution XPS spectra of TiO2 nanoparticles deposited on YF-HA and dpYF-HA films.

3.3

Photocatalytic activity of TiO2 nanoparticles

When TiO2 nanoparticles are irradiated by UV light, the photo-excited electrons can catalyze the photoreduction of Ag+ ions to produce Ag0 nanoparticles. 33 Therefore, photoreduction of Ag+ ions was conducted as a model reaction to evaluate the photocatalytic activity of TiO2 nanoparticles supported on YF-HA and dpYF-HA films. The prepared samples were immersed in AgNO3 solutions and irradiated by a 254 nm UV lamp for 15 min. Figure 7 shows the SEM images of TiO2/YF-HA and TiO2/dpYF-HA films after the photoreduction. Comparing Fig. 7(a) and (b) with the SEM images of TiO2/YF-HA and TiO2/dpYF-HA (Fig. 4), reveals that a large quantity of Ag nanoparticles formed on the TiO2 nanoparticles. The newly deposited Ag nanoparticles are whiter than the previously deposited TiO2 nanoparticles which are in gray color in the SEM images due to the higher contrast from the metallic Ag particles. The TiO2/dpYF-HA film appears 13



to have fewer Ag nanoparticles deposited compared to the TiO2/YF-HA film. To display the Ag nanoparticles more clearly, backscattered electron (BSE) mode in SEM was used. Since elements with high atomic number backscatter electrons more strongly than elements with low atomic number, heavy elements appear brighter in BSE images. 34 Therefore, Ag nanoparticles will be brighter than the TiO2 nanoparticles and underlying apatite support in BSE images. Fig. 7(c) and (d) are the BSE images of TiO2/YF-HA and TiO2/dpYF-HA films after photoreduction of Ag+ ions. It clearly shows that there are significantly more Ag nanoparticles (bright dots) on TiO2/YF-HA films than on TiO2/dpYF-HA films.

Figure 7: SEM images of (a) TiO2/YF-HA and (b) TiO2/dpYF-HA films after photoreduction of Ag+ ions by irradiation of 254 nm UV light; and backscattered electron (BSE) images of (c) TiO2/YF-HA and (d) TiO2/dpYF-HA films after photoreduction of Ag+ ions by irradiation of 254 nm UV light. High-resolution XPS was used to further confirm the presence of Ag nanoparticles. As shown 14


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in Fig. 8(b) and (d), a prominent Ag 3d signal was detected on both TiO2/YF-HA and TiO2/dpYFHA films after the photoreduction. The binding energy of Ag 3d in the samples (367.7 eV) is a little lower than the reference binding energy of metallic Ag (368.2 eV). 35 However, the narrow asymmetric peak shape and energy loss feature on the higher binding energy side are indicators that the deposited Ag nanoparticles are metallic. 35,36 The shift of Ag 3d peaks to lower binding energy is likely due to interaction between Ag nanoparticles and TiO2 nanoparticles.

Figure 8: High-resolution XPS spectra of TiO2/YF-HA and TiO2/dpYF-HA films after the photoreduction of Ag+ ions to Ag nanoparticles by irradiation of 254 nm UV light. (a) peaks of Ti 2p from the TiO2/YF-HA film, (b) peaks of Ag 3d from the TiO2/YF-HA film, (c) peaks of Ti 2p from the TiO2/dpYF-HA film, and (d) peaks of Ag 3d from the TiO2/dpYF-HA film. 15



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In the XPS spectrum, the peak area (integrated intensity) depends on the atomic concentration of the respective element. 35 Therefore, despite the fact that the absolute quantities of the deposited TiO2 nanoparticles and Ag nanoparticles are not obtainable, the ratio of the deposited Ag to TiO2 can be calculated using the following equation: 35 nAg IAg /STi = nTiO2 ITi /SAg

(2)

where n is the number of atoms of the element per cm3 of the sample, I is the peak area (integrated intensity), and S is atomic sensitivity factor. The higher the ratio of Ag to TiO2, the more Ag nanoparticles were produced per unit of TiO2 nanoparticles, which suggests higher photocatalytic activity. Peak areas were calculated from the XPS spectra of Ti 2p and Ag 3d, similar to those shown in Figure 8, and Equation (2) was used to determine the atomic ratio of Ag to TiO2. Figure 9 summarizes the calculated ratios of Ag to TiO2 obtained from different types of samples. As a control, TiO2 nanoparticles were also deposited onto commercial plasma sprayed HA films (PSHA) which do not possess large internal polarization. The same heat treatment process used to depolarize YF-HA was also applied to a number of the plasma sprayed HA films before deposition of TiO2 nanoparticles (dpPS-HA). The tests were performed in triplicate for each type of sample. It quantitatively shows that significantly more Ag nanoparticles were produced on TiO2/YF-HA films than on TiO2/dpYF-HA films. On the other hand, the control group shows negligible difference between TiO2/PS-HA and TiO2/dpPS-HA films. Hydroxyapatite is usually considered to be a wide band gap semiconductor. Its band gap ranges from 4 eV to 6 eV based on different measured data and modeling calculations. 37,38 YF-HA and dpYF-HA films without TiO2 nanoparticles displayed very low photocatalytic activity when the samples were immersed in AgNO3 solution and irradiated by 254 nm UV light. XPS analysis shows that the quantity of Ag produced by bare YF-HA and dpYF-HA films is negligible compared to that produced by YF-HA and dpYF-HA films with deposited TiO2 nanoparticles. Considering that there was no measurable change in the crystal structure and chemical composition during the

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Figure 9: Ag:TiO2 atomic ratios obtained from TiO2/YF-HA, TiO2/dpYF-HA, plasma sprayed HA (PS-HA), and heat-treated plasma sprayed HA (dpPS-HA) films. The numbers above error bars indicate the mean values measured, and error bars indicate standard deviation. depolarization process, the higher amount of Ag nanoparticles deposited on TiO2/YF-HA films can be attributed to the large internal electrical polarization of YF-HA films. It is postulated that the internal electrical polarization of the support enhances the photocatalytic activity by promoting the separation of photogenerated electron and hole pairs in the TiO2 nanoparticles. The lifetime of the photogenerated electrons must be long enough to allow reaction with Ag+ ions before recombination with the photogenerated holes. When TiO2 nanoparticles are supported on a YF-HA film, the electron and hole pairs are driven in opposite directions due to the electric potential originating from the polarized YF-HA support. The polarization direction is from the top surface of YF-HA crystals to the bottom based on our previous work. 18 As a result, the Ti 2p peaks in XPS spectra shift to lower binding energy. As illustrated in Fig. 10, photogenerated electrons in TiO2 nanoparticles are driven towards the electrolyte solution by the electric field originating from the polarized support. The holes in TiO2 nanoparticles are driven towards the YF-HA support. The electrons thus move toward the upper surface where they reduce Ag+ ions to Ag nanoparticles, while the holes move toward the interface between the TiO2 nanoparticles and the YF-HA support. It is likely that the holes react with OH– to produce OH · radicals. In this 17



way, the recombination of charge carriers is mitigated, and thus the photocatalytic activity of TiO2 nanoparticles is enhanced.

Figure 10: A schematic of the proposed mechanism of the photocatalytic activity enhancement by the internal polarization of a YF-HA support. The photodegradation of methyl orange by TiO2/YF-HA and TiO2/dpYF-HA films was also performed to evaluate the photocatalytic activity of the samples. Figure 11 shows the light absorbance of methyl orange solutions with different samples under UV light irradiation versus time. The TiO2/YF-HA films showed the highest rate of degrading methyl orange, while the TiO2/dpYFHA films displayed a much slower degradation rate. As control groups, bare YF-HA and bare 18


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dpYF-HA films showed almost no photocatalytic activity. The mechanism of dye degradation has been reported previously. 39,40 Under UV irradiation the photogenerated electrons in TiO2 nanoparticles can combine with oxygen molecules to produce superoxide radical anions, O2– . The photoinduced holes can react with OH– groups to form OH radicals or oxidize dye molecules directly. The internal polarization of YF-HA supports reduces the recombination of photogenerated charge carriers and thus enhances the photocatalytic activity of TiO2 nanoparticles in degradation of methyl orange.

Figure 11: Light absorbance at 465 nm for methyl orange solutions versus time of exposure to 254 nm UV light irradiation in the presence of different catalyst samples. The recyclability of the TiO2/YF-HA was examined, as shown in Figure 12. The TiO2/YFHA still showed relatively higher photocatalytic activity in photodegradation of methyl orange compared to TiO2/dpYF-HA after three repeated cycles of reactions. The slight decrease of photocatalytic activity after three cycles was probably due to a small loss of TiO2 nanoparticles from the films during the photodegradation and recycling process.

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Figure 12: Repeated photodegradation of methyl orange to test the recyclability of TiO2/YF-HA catalyst films.

4 Conclusions In conclusion, novel thin films of electrically polarized HA were synthesized through co-doping with Y3+ and F–. The doping with yttrium and fluoride produced dense thin films that are thermally stable. The electrical polarization was measured and thermal depolarization was confirmed to occur above 500°C. No changes in crystal structure or chemical composition were detected after depolarizing the films for 1 hour at 600°C. When TiO2 nanoparticle photocatalysts are supported on the apatite films, a significantly higher amount of Ag metal nanoparticles are reduced on the polarized support as compared to the depolarized support and plasma-sprayed hydroxyapatite support used as a control. It was also found that the TiO2 nanoparticles on the polarized support displayed a much faster degradation rate of methyl orange than on the depolarized support. It is proposed that photo-generated electrons and holes in TiO2 nanoparticles are driven to different directions by the internal polarization of the YF-HA support, reducing the recombination of charge carriers and thus improving the photocatalytic activity. The results imply that materials with large internal polarization can be designed and used in new strategies for enhancing quantum efficiency of photocatalysts. 20


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Acknowledgement The authors acknowledge support from the University of Rochester and the DOE through the Laboratory for Laser Energetics (DE-FC03-92SF19460). We thank Christine Pratt for assistance with XRD measurements.

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Enhanced Photocatalytic Activity of TiO2 Nanoparticles Supported on

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