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Journal of Nanomaterials
Volume 2013 (2013), Article ID 627385, 11 pages
http://dx.doi.org/10.1155/2013/627385
Research Article

Solvothermal Synthesis of TiO2 Photocatalysts in Ketone Solvents with Low Boiling Points

1Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, 415 Chien-Kung Road, Kaohsiung 807, Taiwan
2Department of Chemical and Biotechnology Engineering, University of Colorado at Boulder, Boulder, CO 80309, USA
3Department of Chemical Engineering, Da Nang University of Technology, Da Nang, Vietnam

Received 4 February 2013; Revised 13 April 2013; Accepted 18 April 2013

Academic Editor: Yanbao Zhao

Copyright © 2013 Chau Thanh Nam et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The titanium dioxide (TiO2) photocatalysts were synthesized by a solvothermal process in highly alkaline 70 : 30 water : ketone solutions with a TiO2-P25 precursor and calcined at different temperatures. The ketone solvents, such as acetone and methyl ethyl ketone (MEK), had low boiling points (<100°C). The as-prepared samples were characterized by XRD, TEM, FTIR, UV-vis and Raman spectroscopy. The effects of the different solvents on the nanostructure, the morphology, and the photocatalytic performance of the TiO2 products were investigated. Nanotubes formed in water and water-MEK, while nanoparticle/nanowires formed in water-acetone and water-acetone-MEK. The ketone solvents played an important role in the improving nanostructure properties of these products, which affected their photocatalytic reactions. The results indicated that samples synthesized in solvents such as water and MEK had high adsorption and photocatalytic behaviors. The photocatalytic reactivity was the greatest for the TiO2 prepared in MEK and calcined at 100°C, which was even more reactive than the sample prepared in water and TiO2-P25 powder.

1. Introduction

Titanium dioxide ( ) is a common metaloxide semiconductor that is used in many key industrial applications, such as in the construction of photocatalysts, solar cells, microelectronics, and electrochemical devices. Many researchers have focused on applications of in photocatalytic materials due to of its low cost, nontoxicity, chemical stability, oxidizing strength, and powerful photocatalytic activity [1, 2]. Many methods of synthesizing nanotubes, including wet chemical techniques, such as sol-gel, precipitation, and hydrothermal/solvothermal, have been applied successfully to synthesize different nanostructures [38]. Currently, liquid-phase processing at low temperatures is an effective synthetic route for producing nanostructures [916] and hydrothermal/solvothermal techniques are used widely to synthesize nanostructures [1724]. Hydrothermal/solvothermal syntheses are advantageous given that they facilitate the synthesis of nanometer-sized crystalline powders at relatively low temperatures [20, 22, 23]. Solvothermal reactions, which use a solvent under conditions of high pressure and mild temperature, show promise for developing nanotechnologies [25]. The use of organic solvents in solvothermal synthesis helps to control the structure and morphology of the products due to their shape- and size-dependent properties [22, 23, 26]. The addition of an organic solvent to water provides an excellent reaction medium for the hydrothermal processing of nanoparticles given that these mixtures modulate the reaction rate and equilibrium by adjusting the dielectric constant and the density of the solvent with respect to pressure and temperature thus producing faster reaction rates and smaller particles [27]. Alcohol, which is utilized often as an organic solvent, is believed to play an important role in the formation of titania phases [23]. A number of different solvents were used in solvothermal syntheses to investigate the growth mechanisms and characteristics of titania [22, 26]. In previous studies, ketones were rarely used as solvents in the syntheses but were used (mainly acetone) in postsynthesis washing processes [17, 19, 2830]. Therefore, the effects of ketone solvents, especially solvents with low boiling points such as acetone (Ac) and methyl ethyl ketone (MEK), on the structures, morphologie, and optical properties of are unknown.

The photocatalytic behavior of titania depends significantly on its microstructure and preparation method [31]. However, the size, the morphology, and the structural properties of  nanostructures depend on the precursors and reaction parameters, such as the reaction temperature, the reaction time, and the solution pH during the reaction [32, 33]. In this study, we synthesized nanostructures by solvothermal routes (at 130°C, for 24 h) in highly alkaline solutions using ketone solvents with low boiling points (acetone and methyl ethyl ketone) and -P25 nanoparticles as precursors and calcined the products at different temperatures. We investigated the effects of different solvents on the nanostructure, the phase composition, the morphology, the pore volume and size, and the specific surface area of the products, as well as the efficiency of the photocatalytic reactions associated with the degradation of a dye (methylene blue). This solvothermal method shows promise as an efficient, energy-saving strategy because synthesis and calcination are performed at low temperatures. Moreover, the products are highly efficient photocatalysts as demonstrated by their ability to decolor organic dyes under visible-light irradiation.

2. Materials and Methods

2.1. Reagents

Analytical grade chemicals were used as received without any further purification. NaOH, (P25) powder, and methylene blue (MB) were purchased from Across Organics (NJ, USA). A 200 mL solution of MB (300 ppm) was prepared in double-distilled water and diluted as required.

2.2. Instrumentation

Glassware was washed thoroughly in tap water, followed by distilled water; it was then dried overnight in a hot-air oven at 50°C. The crystal structures of the products were characterized by X-ray diffraction (PANalytical, X Pert PRO X-ray diffractometer, CuKα radiation) and micro-Raman spectroscopy (Dimension-P2 Raman). A Micromeritics ASAP 2020 analyzer was used for BET analysis. TEM measurements were performed using a Philips, M-200 transmission electron microscope operated at 200 kV. Samples for TEM analysis were prepared by sonicating products in ethanol for 15 min, depositing 2-3 drops on thin carbon films supported on a copper grids, and drying overnight at 60°C. A Fourier transform infrared spectroscopy (FTIR) study was conducted with a PerkinElmer spectrophotometer. A Jasco V-600 UV-vis spectrophotometer was used to record diffused UV-vis reflectance spectra in the 300–800 nm range and to measure red shifts. Absorbance measurements were carried out using a UV-Vis spectrophotometer (Hitachi, U-2800) equipped with a 1 cm quartz cell.

2.3. Preparation Nanostructures

Solutions of 10 M NaOH were prepared by adding 28 g NaOH to 70 mL of a 70 : 30 mixed solvent of distilled water and ketones. The ketone solvents had low boiling points, less than the boiling point of water (100°C), such as acetone (Ac) and methyl ethyl ketone (MEK). The samples were named as shown in Table 1. In a typical procedure, 2.1 g -P25 powder was added to 70 mL of 10 M NaOH in a 70 : 30 mixture of water and ketone solvent and stirred continuously for 30 min with a magnetic stirrer. Then, the mixture was sonicated for 30 min. Next, this homogeneous suspension was hydrothermally treated in a stainless steel, Teflon-lined autoclave at 130°C for 24 h. Afterwards, the reactor cooled naturally in air to room temperature. The reaction products were washed thoroughly with large volumes of double-distilled water to achieve a neutral pH (7). The solids were collected by filtration and dried at 80°C for 6 h. The products were calcined at different temperatures for 2 h to obtain nanostructures. Lastly, the products were used to remove MB by photocatalytic degradation under visible-light irradiation. To study the effects of the ketone cosolvents on the morphologies and properties of structures, we used a 1 : 1 acetone : MEK solvent in a 70 : 30 water : solvent mixture. After mixing with NaOH, the pH values of the 70 : 30 water : ketone solvents were 9.96 (acetone), 10.28 (MEK), and 10.36 (A : MEK = 1 : 1).

tab1
Table 1: The effects of various solvents on specific surface areas ( ) and pore parameters.
2.4. Photocatalytic Degradation of Aqueous Methylene Blue (MB)

Photocatalytic degradation experiments were carried out by dispersing 0.01 g nanostructures in 100 mL MB solution with an initial dye concentration ( ) of 21 mg/L (dose of 0.1 g/L) and stirring for 1 h in darkness. Suspensions of dyes and catalysts were added to a photocatalytic reaction vessel under visible-light irradiation. A 300 W halogen lamp with a 410 nm cut-off glass filter was used as the light source. These photocatalytic degradation measurements were conducted at room temperature (25 ± 2°C) and atmospheric pressure. The reaction temperature was controlled by continuously circulating cool water outside of the vessel during the irradiation process. The total duration of visible-light irradiation was 2 h. Every 15 min, approximately 8 mL of solution was removed and immediately centrifuged at 4000 rpm for 10 min. The MB dye concentration remaining in the solution was measured with a UV-Vis spectrophotometer at the maximum absorption wavelength of 664 nm. The blue color of the solution gradually faded over time because MB was adsorbed on particles and was degraded by light.

3. Results and Discussion

3.1. Characterization of Nanostructures

The crystal structure, morphology, and size of the synthesized products were determined using XRD and TEM (Table 2). The MEK sample of (prepared using methyl ethyl ketone) and samples (prepared in different solvents) with the best photocatalytic performance were compared to samples synthesized in water and the -P25 precursor.

tab2
Table 2: The effect of different solvents on morphologies, sizes and structures of TiO2 nanostructures.

The XRD patterns of various samples are shown in Figure 1. The samples prepared using MEK and calcined at temperatures up to 400°C exhibited mainly diffraction lines of titanate phases, Figure 1(a); these results are similar to those of previous work [34]. However, when the sample was heated at 500°C, the diffraction pattern corresponded to a mixture of anatase (48°), rutile (44.6°), and sodium trititanate phases (28.9, 29.8, 33.1, and 67.1°). This XRD analysis was consistent with the phase composition that was determined quantitatively with the normalized RIR method, summarized in Table 2. Similar results reported by Zárate et al. [35] indicated that a sample synthesized in 10 M NaOH during 18 h was stable when heated up to 400°C and was transformed into sodium hexatitanate when calcined at 500°C. This transformation of into sodium trititanate showed that it was not possible to completely eliminate sodium from the samples, and after a long period of hydrothermal treatment, Na+ formed in the structure of the calcined samples [35]. The sample prepared using MEK was thermally stable up to 400°C, and the relative intensity of the peaks of the titanate phases underwent very little change although peak intensities increased significantly after calcination at 500°C. The average crystal size ( ), determined by applying the Scherrer equation [36] and using the full width at half maximum (FWHM) of the titanate phases, are given in Table 2: , where is the X-ray wavelength (0.15418 nm), is the Bragg angle, and B is the FWHM. These results show that the FWHM of the peaks decreased gradually when the calcination temperature increased, especially at 500°C, at which the FWHM decreased significantly, an effect that may be attributed to increased crystallinity. However, calcination at 500°C resulted in the growth of nanocrystals up to 28.6 nm, and this size increase is significant compared to the MEK100 (9.3 nm) sample that was calcined at a low temperature. Figure 1(b) shows that all of the samples contained peaks corresponding to titanate phases. The FWHM of the peaks decreased in the following order: MEK100 < A100 < W100 < AMEK200. This order suggested that, when products were calcined at the same temperature, smaller crystals formed when ketone solvents (acetone, MEK) were used than when water alone was used as the solvent (see results in Table 2).

fig1
Figure 1: XRD patterns. (a) Samples prepared using MEK and calcined at different temperatures, and (b) samples using various solvents and calcined at different temperatures. The notations T, A, and R correspond to titanate, anatase, and rutile, respectively. The reference patterns correspond to the JCPDS: number 21-1272 (anatase), number 21-1276 (rutile), number 31-1329 (sodium trititanate), and number 47–561 (hydrogen trititanate).

The phase components of the samples were characterized by Raman spectroscopy, Figure 2. The spectrum for the sample of -P25 precursor exhibited the characteristics of anatase phase scattering at approximately 144, 399, 515, and 638 cm−1 [3740]. Raman spectra of the rutile phase were rarely reported to contain a medium intensity peak at approximately 417 cm−1 [41]. However, to be compatible with the results of XRD patterns in Figure 1, this peak should be assigned to the rutile phase because the -P25 precursor included a mixture of anatase and rutile phases. The titanate nanotubes produced a unique spectrum with moderate to strong intensity bands at 445, 275, and 278 cm−1 [34, 35, 40]. Raman investigations complemented the XRD analysis, showing that titanate phases existed for the samples prepared using MEK and calcined at temperatures from 100 to 400°C. At 500°C, the Raman spectrum contained peaks for the anatase (638.4 cm−1) and rutile (417 cm−1) phases as well as some new peaks at 129.9 and 206.7 cm−1 (Figure 2(a)). These new peaks may belong to titanate phases (forms of sodium trititanate), and they are fully consistent with the XRD analysis of the sample prepared using MEK and calcined at 500°C. The thermal stability of these samples can be considered from Raman spectra and XRD patterns. Up to 400°C, the samples still exhibited the titanate phase characteristic of the as-synthesized samples. At 500°C, the sample began to partially transform to the anatase (peaks at 48° in the XRD and at 638.4 cm−1 in the Raman results) and the rutile (peaks at 44.6° in the XRD and at 417 cm−1 in the Raman results) phases. New peaks corresponding to sodium titanate also appeared (peaks at 28.9, 29.8, 33.1, and 67.1° in the XRD patterns and at 129.9 and 206.7 cm−1 in the Raman spectra). These results partially resembled results from previous studies [34, 35]. Similar to the XRD analysis, Figure 2(b) showed that all peaks corresponding to the titanate phase (at 275.3 and 445.5 cm−1) were more intense and clearer for the W100 and MEK100 samples than the A100 and AMEK200 samples.

fig2
Figure 2: Raman spectra. (a) Samples prepared using MEK and calcined at different temperatures, and (b) samples prepared using various solvents and calcined at different temperatures. The notations T, A, and R correspond to titanate, anatase, and rutile, respectively.

The nitrogen adsorption-desorption isotherms of samples prepared using MEK and various solvents and calcined at different temperatures are presented in Figure 3. Specific surface areas ( ), average pore volumes, and pore sizes of these samples are summarized in Table 1.

fig3
Figure 3: Nitrogen adsorption-desorption isotherms. (a) Samples prepared using MEK and calcined at the different temperatures, and (b) samples prepared using various solvents and calcined at different temperatures.

According to the BDDT classification system, the isotherms of samples W100, MEK100, MEK200, MEK300, and MEK400 were type IV with type H2 and type H3 hysteresis loops, and the isotherms of samples MEK500, A100, and AMEK200 were type II with type H3 hysteresis loops [42]. Generally, the W100 and MEK samples that were calcined from 100 to 400°C exhibited high specific surface areas ( ). As shown in Figure 3, the capillary cohesion of the MEK500 (Figure 3(a)) and A100 (Figure 3(b)) samples occurred at the highest pressure, suggesting that these samples had the largest pore sizes and the smallest specific surface areas (Table 1). For the sample prepared using MEK, decreased gradually as the calcination temperature increased. The hysteresis loops shifted to higher relative pressures, indicating that the pore size grew gradually because crystal growth leads to the formation of larger pores. The samples had high values and pore volumes, which greatly improved adsorption efficiency. It is remarkable that the specific surface area was higher for the products than for the -P25 precursor.

Figure 4 shows pore size distributions, which were relatively narrow for samples prepared in MEK and calcined at temperatures from 100 to 300°C (Figure 4(a)) and sample W100 (Figure 4(b)). The sizes were somewhat wider for sample MEK400 and the widest for sample MEK500 (Figure 4(a)). Similarly, samples A100 and AMEK200 (Figure 4(b)) exhibited a wider pore size distribution (5–90 nm) than samples W100 and MEK100 (3–15 nm).

fig4
Figure 4: Pore distribution. (a) Samples prepared using MEK and calcined at different temperatures, and (b) samples prepared using various solvents and calcined at different temperatures.

Figure 5 shows TEM images of samples with the highest photocatalytic efficiency for each of the different solvents. The insets show the corresponding selected area electron diffraction (SAED) patterns and the sample dimensions. These SAED patterns show that the as-prepared samples contain crystallites in polycrystalline phases. In addition, samples W100 (Figure 5(a)) and MEK100 (Figure 5(c)) exhibited predominantly tube-like structures, sample A100 (Figure 5(b)) showed both nanoparticle and nanowire structures, and sample AMEK200 (Figure 5(d)) contained a combination of nanoparticles, nanowires, and a small amount of nanotubes. The nanotubes in these samples exhibited an outside diameter of approximately 6-7 nm and lengths of several hundred nanometers. The ends of these nanotubes were open, which was extremely beneficial for adsorption and photocatalysis. The left inset in Figure 5 is an enlarged picture of these samples. Sample A100 (Figure 5(b)) showed nanoparticles with an average diameter of ca. 50 nm. These TEM images are very beneficial for interpreting the effectiveness of photocatalytic reactions that are presented in detail in a later section. In particular, the use of water and MEK in aqueous NaOH solutions promoted the formation of nanotubes, revealing that the ketone solvent MEK positively affected the solvothermal synthesis of nanotubes.

fig5
Figure 5: TEM images of samples prepared using various solvents and calcined at different temperatures. (a) W100, (b) A100, (c) MEK100, and (d) AMEK200.

Figure 6 shows FTIR spectra of samples that were prepared using different solvents and that showed the highest photocatalytic efficiency. As shown in Figure 6, all of these spectra have a common feature, namely, bands at 3000–3650 cm−1 due to adsorbed water and hydroxyl groups. This property can be attributed to stretching vibrations of –OH in water. In particular, the adsorbed water content was significantly higher in the synthesized samples than in the -P25 precursor due to the greater specific surface areas and pore volumes of the products. The band observed at 1630 cm−1 for all of the samples can be assigned to a water molecule bending mode [43]. Distinct broad bands in the 400–800 cm−1 region were assigned to Ti–O and Ti–O–Ti skeletal frequency regions [44]. Nearly every solvent used in this study has a low boiling point (under 100°C), facilitating its removal from crystals during calcination. These solvent characteristics may be the reason that FTIR spectra showed that nearly no organic components were present in these final products.

fig6
Figure 6: FTIR spectra. (a) Samples prepared using MEK and calcined at different temperatures, and (b) samples prepared using different solvents and calcined at different temperatures.

Figure 7 shows UV-vis absorption spectra of nanostructures. The band gaps of these samples were calculated from plots of versus photon energy [45], Table 2. The results indicated that samples prepared in MEK and calcined at temperatures from 100 to 400°C and the W100 sample exhibited slight red shifts relative to pure -P25. The estimated band gap energies were 3.20 eV for W100 and 3.21, 3.22, 3.20, and 3.22 eV for samples prepared in MEK calcined at 100, 200, 300, and 400°C, respectively, compared to pure -P25 (3.25 eV). These band gap energies indicated that these samples were not phase mixtures but rather contained only the titanate phase, shown by the XRD patterns and Raman spectra; these results are consistent with those for the phase compositions as shown in Table 2. In the mixed /titanate phase, the titanate nanostructures normally have smaller band gaps than pure anatase , and their band edges overlap, resulting in significant red shifts (reductions of band gap energies) [24, 46, 47]. The A100, AMEK200, and MEK500 samples exhibited blue shifts with estimated band gap energies of 3.36, 3.45, and 3.29 eV, respectively. These blue shifts were fully consistent with poor photocatalysis performance under visible-light irradiation by these samples (presented in the next section).

fig7
Figure 7: UV-vis absorbance spectra. (a) Samples prepared in MEK and calcined at different temperatures, and (b) samples prepared using various solvents and calcined at different temperatures.
3.2. Photocatalytic Degradation of MB

In this experiment, the solution was stirred in darkness to reach adsorption equilibrium. The initial concentration of the dye solution (MB) after stirring was . We selected samples prepared using MEK and different ketone solvents and calcined at various temperatures to carry out the photocatalytic reaction under visible light irradiation, Figure 8. The adsorption yield of MB solution in darkness and photodegradation efficiencies under visible light irradiation for all of the samples are listed in Table 3.

tab3
Table 3: Removal efficiency of MB in aqueous solution for different solvents.
fig8
Figure 8: Photodegradation ratio of MB under visible light irradiation. (a) Samples prepared using MEK and calcined at temperatures from 100 to 500°C, and (b) samples using various solvents and calcined at different temperatures.

The results show that the sample prepared using MEK and calcined at 100°C was the most efficient photocatalyst, with MB removal equal to 77.3% after irradiation for 120 min. The MB photodegradation efficiency decreased when the calcination temperature increased (Figure 8(a)). The MEK100 sample also exhibited higher photodegradation efficiency than the W100 sample and -P25 powder (Figure 8(b)). However, the W100 and MEK100 samples generally retained high adsorption efficiencies and were very efficient photocatalysts under visible light irradiation (Figure 8(b)). Given that samples W100 and MEK100 had better phase intensities (in their XRD patterns and Raman spectra), their morphology consisted of nanotubes (TEM) with high specific surface areas (BET), and they exhibited better red shifts than other samples (UV-vis). Although the crystalline phase composition was mainly titanate (Table 2), sample MEK100 had a smaller crystallite size and lower average pore size (see Tables 1 and 2), the number of active surface sites and the surface charge carrier transfer rate increased. Therefore, the surface charge carrier transfer rate increased, and the photodegradation reaction was more efficient [48]. Increasing the calcination temperature produced an increase in the crystallite size (Table 2), a reduction in the specific area (Table 1), and a decrease in the photocatalytic efficiency (Figure 8(a)). Figure 8(b) also shows that samples A100 and AMEK200 had lower photocatalytic efficiencies after 120 min of visible light irradiation. This finding was entirely consistent with the above results for samples A100 and AMEK200, that is, less intense phases and blue shifts relative to -P25 powder, with morphologies that were mainly nanoparticles or nanowires and with smaller specific surface areas.

The solubility, the diffusion process, and the chemical reactivity of the reactants are greatly enhanced [22] in solvothermal syntheses due to solvent properties, such as the density, the viscosity, and the diffusion coefficient, which can dramatically change product morphologies and sizes compared to conventional synthesis conditions [22, 49]. Thus, solvent selection is critical for controlling of chemical mechanisms leading to target materials. Reaction mechanisms induced during solvothermal reactions depend on physico-chemical solvent properties and can be indispensable for orienting the structural form of the product [25]. Every solvent used in this study had a low boiling point (below 100°C), such as acetone (56.5°C) and methyl ethyl ketone (79.6°C). The boiling point is an important solvent property because it determines the evaporation rate. When the synthesis temperature is 130°C, these solvents evaporate. Acetone, in particular, has a low boiling point of 56.5°C, allowing for easier evaporation at its boiling point. Acetone can partially evaporate while mixing NaOH and sonicating powder because this reaction is highly exothermic. Evaporation may also affect the efficiency of creating nanotubes by solvothermal methods (although acetone is miscible in water) and indirectly reduce the adsorption and the photocatalysis efficiencies. The volatile nature of acetone may create adverse conditions for the solvothermal production of nanotubes, which can be seen in the TEM images of nanoparticle products. Although MEK has properties similar to those of acetone and is not completely miscible with water, it boils at a higher temperature and evaporates more slowly than acetone. These properties may enhance the efficiency of MEK for synthesizing nanotubes, and the products may have larger surface areas, Table 1. The photocatalytic reaction performance of products showed that samples prepared using MEK had higher efficiencies than samples prepared using water or acetone and that they were calcined at the same temperature (Figure 8(b)). The sample prepared using the mixed cosolvent of acetone and MEK had lower efficiency due to the effect of acetone, which diminished the performance of , and the AMEK200 sample exhibited the best photocatalytic activity at 200°C (see Table 3). For Ac, AMEK, and MEK, adsorption in darkness, as well as the photocatalytic activity of the products prepared by the solvothermal method using ketone solvents, was in the following sequence: MEK > AMEK > Ac.

4. Conclusions

Various ketone solvents affected the morphology and the structure of products, as well as its photocatalytic degradation of MB dye. The samples prepared using MEK solvent and calcined at temperatures up to 400°C exhibited mainly titanate phases, but mixed phases of titanate (63.9%), anatase (19.1%), and rutile (17%) appeared at 500°C. Samples that were synthesized using solvents such as water and MEK were highly effective in adsorbing MB dye in darkness and photocatalytic reactions under visible light irradiation. The samples prepared in water and MEK were more efficient than their counterparts prepared in acetone or an acetone-MEK mixture and the -P25 precursor because of their better phase intensities, nanotube morphologies, higher specific surface areas, and larger red shifts. The MEK100 sample exhibited the most efficient photodegradation of MB due to its smallest crystallite size. Generally, for most of the products prepared using different solvents, MB adsorption and photocatalytic efficiencies were the highest for samples calcined at 100°C and lowest at the calcination temperature of 500°C. However, for prepared using a cosolvent of acetone-MEK, the photocatalytic performance was the best for samples calcined at 200°C.

Acknowledgment

The authors wish to thank the National Science Council, NSC 99–2221-E-151-062, for their financial support of this study.

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