Facile Hydrothermal Synthesis of Sodium Tantalate (NaTaO3

Oct 19, 2009 - Nanosize NaTaO3 photocatalysts have been synthesized by a single-step hydrothermal method. The product was characterized using various ...
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J. Phys. Chem. C 2009, 113, 19411–19418

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Facile Hydrothermal Synthesis of Sodium Tantalate (NaTaO3) Nanocubes and High Photocatalytic Properties Xia Li* and Jinling Zang College of Materials Science and Technology, Qingdao UniVersity of Science and Technology, Qingdao 266042 P.R. China ReceiVed: April 14, 2009; ReVised Manuscript ReceiVed: September 13, 2009

Nanosize NaTaO3 photocatalysts have been synthesized by a single-step hydrothermal method. The product was characterized using various techniques, such as X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and a volumetric adsorption method (Brunauer-Emmett-Teller (BET)). The influence of reaction temperature and time, as well as alkaline concentration, on the morphology of the photocatalysts was investigated. The results showed that pure-phase NaTaO3 could be synthesized in a NaOH solution of 0.5 mol/L concentration at 140 °C for 12 h. This process was dominated by a dissolution/ precipitation mechanism, in which the growth mechanism of the NaTaO3 nanoparticles was also discussed based on its crystal structure. The NaTaO3 nanostructures show a higher photocatalytic activity in the degradation of Safranine T dye and gaseous formaldehyde than those of solid-state-reacted counterparts and P25, due to their perfective crystallinity and larger surface areas. 1. Introduction In the past two decades, oxide semiconductor photocatalysis has attracted extensive attention due to its wide potential application in environmental procedures such as air purification, water disinfection, hazardous waste remediation, and water purification.1-3 Among these semiconductors, TiO2 is the most widely studied because of its chemical stability, nontoxicity, and high photocatalytic activity. Unfortunately, shortcomings such as poor solar efficiency hinders its extensive application. Although much work has been devoted to the improvement of the photocatalytic performance of TiO2, the improvement is limited.4-9 Therefore, some researchers have diverted their attention to exploit novel photocatalysts. Among these active materials, tantalates exhibit high photocatalytic performance because tantalates possess conduction bands consisting of a Ta5d orbital located at a more negative position than that of titanates (Ti3d), which have conduction bands at a high potential, so a variety of mixed metal oxides containing closed-shell Ta5+ transition-metal ions have been studied recently. For example, Kudo and other groups have reported that many tantalates, K3Ta3Si2O13,10 alkaki tantalates ATaO3 (A ) Li, Na, and K),11 alkaline earth tantalates BTa2O6 (B ) Ca,Sr, and Ba),12 K3Ta3B2O1213, lanthanum-doped NaTaO3,14 SnMn2O6, and SnM2O7 (M ) Nb and Ta),15 lanthanide tantalates LnTaO4 (Ln ) La, Ce, Pr, Nd, and Sm),16 NiOx/ In0.9Ni0.1TaO4,17 R3MO7 (R ) Y, Ga, La; M ) Nb, Ta),18,19 BaTa4O15,20 BaTa2O6,21 Y1-xTaxO1.5+x,22 and so on. These tantalates show high activities for photocatalytic water splitting under UV light irradiation. Furthermore, most of these active photocatalysts are perovskite-related oxides compounds, such as Sr2M2O7 (M ) Nb, Ta),23,24 KTaO3,25,26 RbNdTa2O7,27 and so forth. In particular, NaTaO3 with a perovskite structure is the most active among tantalates. Wiegel27 investigated that tatalates with perovskite structure consist of corner-shared TaO6 octahedra in their crystal * Corresponding author.

structure, and the bond angle of Ta-O-Ta is close to 180°. Thus the photogenerated electron-hole pairs can migrate easily in the corner-shared framework of TaO6 units, which is benefical to the photocatalysis reaction. Kudo reported that NiO/NaTaO3 photocatalysts showed the hightest activity when the NaTaO3 photocatalysts were prepared in the presence of excess sodium.11 Therefore, tantalate photocatalysts with perovskite-like structure are currently a hot research topic. In general, these tantalate photocatalysts are mainly synthesized via a conventional solid-state reaction (SSR)11-17 or polymerized complex (PC)18,19 method at high temperature (typically 1000-1300 °C). So, it is difficult for the SSR route to control crystallinity and particle size, localized segregation of components, and stoichiometry because of high temperatures. For several applications, particularly as catalysts and for screen printing resistors, synthesis at low temperature is desirable to produce powders with a small particle size and high surface area. Many low-temperature methods such as sol-gel or precipitation routes require a calcination tip in the process for the formation of the final product, which results in some loss of surface area. Hydrothermal synthesis as a well-known traditional wet chemical process is promising for the direct preparation of advanced nanostructures because of its advantages.28 It is believed that this method generates highly crystalline products with high purity, narrow size distribution, and low aggregation. Moreover the morphology and crystal form of the products can also be controlled by adjusting the hydrothermal reaction conditions. Zhu and co-workers have reported ultrafine CaNb2-xTaxO7(0 e × e 2) nanoparticle photocatalysts with pyrochlore structure and monomolecular-layer Ba5Ta4O15 nanosheet with hexagonal structure synthesized by hydrothermal method.29,20 Both nanosized particles and nanosheets showed high photocatalytic activity in the degradation of Rhodamine B and gaseous formaldehyde. In this work, we report a simple hydrothermal route for synthesis NaTaO3 nanocube photocatalysts from tantalum oxide. The optimum reaction parameters, microstructure, and morphol-

10.1021/jp907334z CCC: $40.75  2009 American Chemical Society Published on Web 10/19/2009

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ogy of resultant samples were investigated to improve photocatalytic activities, and a possible growth mechanism is presented here. The photocatalytic performance of these NaTaO3 nanocubes for the photo degradation of various dyes and gaseous formaldehyde has been investigated and found to be distinctly more active than that of Degussa P25 and NaTaO3 microparticles under UV light irradiation. 2. Experimental Section Hydrothermal treatment was applied to synthesize NaTO3 nanocubes. All the reagents were analytically pure, commercially available, and used without further purification. In a typical synthesis procedure, 0.60 g of NaOH (the NaOH solution with different concentrations) and 0.442 g of Ta2O5 were added into a Teflon-lined stainless steel autoclave (50 mL capacity) that was filled with the mixture to 80% of the total volume. The autoclave was kept at a temperature between 60 and 140 °C for a designed period of time under autogenerous pressure, so that we could examine the influence of the reaction temperature and time on the structure and morphology of the resultant products. Then the autoclave was cooled to room temperature naturally, and the resulting precipitates were collected by centrifugation at 3000 rpm several times, washed with deionized water and ethanol thoroughly, and dried at 80 °C in an oven for 5 h before further characterization. Other samples were prepared by following a similar procedure under slightly different conditions. X-ray diffraction (XRD) measurements were carried out with a Bruker D 8 Advance Powder X-ray diffractometer with a CuKR (λ ) 0.1541784 nm) radiation source. Diffraction patterns were collected from 10° to 80° at a speed of 4°/min. The morphologies and composition of the samples were examined with field emission scanning electron microcopy (FE-SEM, Hitachi Model) transmission electron microscopy (TEM), which was performed on a Hitachi Model H-800 transmission electron microscope using an accelerating voltage of 200 KV with a tungsten filament. The microstructure of the samples was analyzed by high-resolution transmission electron microscopy (HRTEM) observation, which was performed with a Philips Tecnai 20v-Twin transmission electron microscope using an accelerating voltage of 200 KV. Samples for the TEM and HRTEM were prepared by ultrasonically dispersing the assynthesized products into absolute ethanol, then placing a drop of this suspension onto a copper grid coated with an amorphous carbon film and then drying in air. Fourier transform infrared (FTIR) spectra were measured on Shimadzu FTIR-8200PC spectrophotometer at room temperature. Ultraviolet-visible (UV-vis) diffuse reflectance spectra were recorded on a UV-vis-NIR spectrophotometer (Cary500, America Varian) and were converted from reflection to absorption by the standard Kubelka-Munk method. The specific areas of the powders were determined by using a Micromeritics ASAP 2020 specific area andporosityanalyzerusingthemethodofBrunauer-Emmett-Teller (BET). The photodegradation property of the products was tested in our homemade instruments. In this case, 0.1 g of product was dispersed into a beaker, which was filled with 100 mL of 10 mg/L Safranine T (referred to later as ST). Prior to irradiation, the suspensions were ultrasonically sonicated for 10 min and then magnetically stirred in dark conditions for 30 min to establish adsorption/desorption equilibrium. The suspensions were then irradiated under UV-light. A 10 W lamp with a maximum emission at about 254 nm was used as the light source. The concentration of the dyes during the degradation

Figure 1. XRD patterns of the as-prepared samples at 140 °C for 12 h, with different NaOH concentrations (a) 0.25 M, (b) 0.50 M, and (c) 0.75 M.

was determined by measuring the absorbance in step time using the UV-vis-NIR spectrophotometer (Cary500, America Varian). Simultaneously, the collected solution was filtered to remove the residual before characterization. Gaseous photodecomposition experiments were carried in an outer-irradiationtype quartz cell. The as-prepared sample (0.1 g) was dispersed on the surface of a glass plate with 7.5 cm × 2.5 cm area. The glass plate was placed on the bottom of the reaction vessel, and the distance between the sample and the light source was 5 cm. The vaporized gaseous formaldehyde was forced to flow through the photoreactor. After adsorption equilibrium was reached, the photoreactor was sealed, and the photocatalytic reaction was started. The concentration was measured with a SP-502 gas chromatograph. 3. Result and Discussion 3.1. Effect of Alkaline Concentration. In the hydrothermal process, the concentration of alkali solution is the key factor that affects the crystal structure and component. Therefore, the investigations have been conducted with different NaOH concentrations in the NaTaO3 preparation. Figure 1 shows the representative XRD patterns of the hydrothermal samples derived from different alkaline concentrations. As can be seen, when the NaOH solution concentration was 0.25 M or lower, only the Ta2O5 phase was detected, which meant that the reaction between NaOH and Ta2O5 did not take place at all. When the concentration was increased to 0.5 M, most of the diffraction peaks of the phase appeared, but the diffraction peaks of the raw material (Ta2O5) were still observed in samples, suggesting that the hydrothermal reaction was not completed. On the other hand, all diffraction peaks can be readily assigned to a pure phase and no diffraction peaks from impurity phase were observed when the NaOH concentration was 0.75 M. From the XRD pattern, we can see that all the reflection peaks of the

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Figure 2. XRD patterns of samples produced by hydrothermal method at different temperatures (a, b, c, d and e are the samples prepared by hydrothermal method under 60, 80, 100, 120, and 140 °C for 12 h, respectively).

products can be readily indexed to as pure NaTaO3 with an orthorhombic structure [space group: (Pcmn)], identical with the reported data in the JCPDS cards (25-0863).30,31 Perovskitelike NaTaO3 has three polymorphs such as cubic perovskite, orthorhombic phase, and monoclinic phase. Only NaTaO3 with orthorhombic phase can be obtained under the present hydrothermal conditions. A similar phenomenon was also observed by other research groups. For example, Teng et al. have synthesized NaTaO3 powder with different structures from the solid-state, sol-gel, and hydrothermal methods. The refinement results showed that the sol-gel specimen has a monoclinic phase, while the hydrothermal and solid-state specimens have an orthorhombic phase.31 According to the results described above, we can conclude that the high NaOH concentration favored the NaTaO3 formation. The difference in the final products was ascribed to the different alkaline hydroxylated Ta-O ionic groups formed under different alkaline concentrations during the hydrothermal process.20 These hydroxylated Ta-O ionic groups were the building blocks for NaTaO3. When no NaOH was used, no hydroxylated Ta-O ionic groups can be formed. By increasing the NaOH concentration, the amount of the hydroxylated Ta-O ionic groups can be improved gradually, resulting in the increasing contents of NaTaO3 was obtained. The same conclusion was also obtained in the BaTaO system during the hydrothermal process.20,21,32 3.2. Effect of the Reaction Temperature and Time. To determine the hydrothermal temperatures for NaTaO3 to be wellcrystallized, the precursor suspension was heat treated for 12 h at different temperatures from 60 to 140 °C with the 0.75 M NaOH solution. The resulting XRD patterns are shown in Figure 2. It is clearly seen that the XRD pattern of resulting sample remains the same as that of Ta2O5, implying that NaTaO3 did not form when the reaction temperature was as low as 60 °C. With the increase of hydrothermal temperature, the relative intensity of the peaks attributed to the Ta2O5 phase decrease gradually, and a coexistent phase appears and strengthens. The XRD pattern of the sample obtained at 120 °C are profoundly different from that of Ta2O5, suggesting that the reaction takes place substantially. When the temperature was further increased to140 °C, pure NaTaO3 is obtained completely. Therefore, the reaction temperature plays an important role to govern the

Figure 3. XRD patterns of the as-prepared samples at 140 °C for different times: (a) 3 h, (b) 6 h, (c) 9 h, and (d)12 h.

formation of NaTaO3 crystals, and in our study the optimum temperature for the preparation of pure phase NaTaO3 is ca. 140 °C or higher. The higher reaction temperature needed in the present system can be interpreted in terms of thermodynamic aspects. From a thermodynamic point, the free energy of Ta-O covalent bond in Ta2O5 is too high to be broken under normal conditions, but under high temperature and high pressure provided by the hydrothermal conditions, the Ta2O5 powders can be partly dissolved and hydroxylated into the corresponding hydroxide,20,33 forming a variety of hydroxylated Ta-O ionic groups. In this case, the activation energy for synthesis of NaTaO3 could be reduced and the nanocubes growth could happen when enough foreign input overcomes the reaction barrier under hydrothermal conditions. However, at low temperature (80-120 °C), because of the insolubility of Ta2O5 powders, tantalum hydroxide can hardly be produced. On the other hand, the low reaction temperature could not provide enough energy in overcoming the reaction barrier, resulting in the failure to synthesize the final products. Thus only an appropriate reaction temperature (140 °C) was employed, the formation of NaTaO3 nanocubes can be obtained successfully. The hydrothermal method has a great advantage over the conventional SSR method in that a pure phase NaTaO3 can be prepared at reduced temperature. For instance, hydrothermal temperature needed to obtain pure phase NaTaO3 was 140 °C in the work. However, other groups have reported that the alkali tantalates, ATaO3 (A ) Li, Na, and K) powders were prepared by SSR or sol-gel method about at 1000 K for 10 h in the air.14,31 Compared to the conventional high temperature reaction, the hydrothermal method is obviously a low-cost, energy saving and easy way to obtain crystalline NaTaO3 with nanosize. In addition, it is worthwhile to mention that the reaction time has strong effects on the formation of pure crystalline NaTaO3. Figure 3 shows the XRD patterns of the samples synthesized at 140 °C for various reaction times with 0.75 M NaOH solution. It was clear that the phase transformation was developed

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Figure 4. FE-SEM images of the reaction intermediates taken from the hydrothermal reaction after (a)3 h,(b) 6 h,(c) 9 h,(d)12 h.

Figure 5. FE-SEM image of obtained NaTaO3 nanocubes: (a) low-magnification and (b) high-magnification SEM images.

gradually in the time series condition, and shorter reaction time was not in favor of forming the NaTaO3 phase. As can be seen, the diffraction patterns of the resultant products were mainly attributed to Ta2O5 when the reaction time was less than 3 h. Until 6 h of the hydrothermal reaction, the feeble NaTaO3 crystalline phase appeared. The intensity of the diffraction peak becomes stronger as the reaction time prolongs. A well crystallized NaTaO3 phase was formed from the poorly crystallized one in the subsequent period when the reaction time reached 12 h. So in the present work, NaTaO3 photocatalysts were prepared by a hydrothermal method at 140 °C for 12 h with 0.75 M NaOH solution. 3.3. Morphology and Structure. A series of control experimens were carried out to further investigate the influence of the reaction time on the morphology evolution and microstructures, the well-defined results are shown in detail in Figures 4 and 5, respectively. Figure 4 shows the representative SEM images of the samples prepared from different reaction time. After 3 h of hydrothermal reaction, the sample comprised nearly spherical colloids particle formed by agglomeration of nanoparticles, which are ascribed to the reactant of Ta2O5 powder. With the time prolonged to 6 h, NaTaO3 nanocubes start to emerge, but there were many small irregular particles coexisted on the surface of the regular NaTaO3 particles. With the time increasing, more nanocubes formed and grew further at the cost of the smaller particles, suggesting that the nanocubes grow at the consummation of the small particles due to the difference in solubility between the large particles and the small particles

according to Gibbs - Thomson law.34 Finally, the smaller particles disappeared completely when the reaction time was extended to 12 h, almost all the products exhibited the square morphology with the size of 150-300 nm as shown Figure 4d. The morphology and lattice structure of NaTaO3 nanocubes were examined by FE-SEM. Figure 5a gives typical lowmagnification images of NaTaO3 nanocubes and indicates that the large quantity of products was achieved by using the hydrothermal approach, and almost all of the samples exhibited the cubic morphology. The size of the NaTaO3 sample was in the range of 200-400 nm, and the average size was about 300 nm. Figure 5b shows that a magnified SEM image of the nanocubes, and it can be seen that NaTaO3 nanocubes possess perfect cubic morphology with sharp corners and well-defined edges. The cube lengths range from 100 to 300 nm. In contrast, the particle size of NaTaO3 synthesized from SSR was ca. 2-3 µm as reported in ref 14. As the particle size was decreased, the probability of the surface reaction of electrons and holes with water molecules is increased in comparison with recombination in the bulk. So it can be anticipated that our as-prepared sample with a nanoneter size exhibits improved photocatalyst activity; this point would be further justified in a later section. Further structural characterization was performed with TEM as shown in Figure 6. Figure 6a gave the low-magnification TEM image of these NaTaO3 nanocubes with a small size of about 150 nm, which facilitated structure characterization using TEM. The crystallinity of the NaTO3 nanocubes was confirmed with the selected area electron diffraction (SAED) experiment.

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Figure 6. (a) TEM image of the NaTaO3 nanocube sample. (b) SAED pattern of a NaTaO3 nanocube. (c) HRTEM image of a NaTaO3 nanocube showing clear lattice fringes. (d) EDS spectrum of NaTaO3 nanocubes.

A spot pattern from the single-cube is shown in Figure 6b. In this case, the electron beam was incident along the [001] direction, and the spot array had a 4-fold axis that can be indexed with hk0 (i.e.,[001] zone spots, in accordance with the extinction rule of electron diffraction of space group Fd3m), indicating a cuboid symmetry for the single-crystal nanocube. The HRTEM image of the NaTaO3 product is shown in Figure 6c. The measured d spacing of about 0.389 nm corresponds to the (002) planes of orthorhombic phase NaTO3. The corresponding energy dispersive X-ray (EDS) spectrum of the NaTaO3 nanocubes sample shows that it is composed of Na, Ta, and O, no other impurity was detected. The result reveals that the ratio of Na/ Ta/O ) 1:1:2.98. 3.4. Growth Mechanism of NaTaO3 Nanocubes. Considering that there are no additional templates and surfactants in the present case, it is reasonable that the growth and formation of nanostructure is neithor a catalyst-assisted nor a templateassisted since the only source materials used in this case are pure Ta2O5 and NaOH. Therefore, a disslution-recrystallization mechanism may be proposed to explain the formation of the corresponding nanostructure based on the above SEM images and XRD pattern results of the products obtained in the different time and temperature series. At the first stage, the crystalline Ta2O5 is insoluble under normal conditions even though high alkaline concentrations were employed. Then under hydrothermal conditions, Ta2O5 powder can be partly dissolved and hydroxylated into the corresponding hydroxide with increasing temperature and pressure.35 Namely, they formed a highly disordered colloidal precursor such as Ta2O5 · xH2O, Ta(OH)5, and so forth. With the time prolonged and temperature elevated, Ta(OH)5 could rapidly grow to form nanosized particles with near spherical shape. Then, as a result of the relatively higher solubility of Ta(OH)5 compared with that of NaTaO3 under certain hydrothermal conditions, they would dissolve gently, and when the concentration of Na+ reaches or exceeds supersaturation, NaTaO3 crystallites begin to nucleate. This can be confirmed by the FTIR spectra of product, as the NaTaO3 nanocube sample shows main bands at 500-800 cm-1, which is attributed to Ta-O stretching and Ta-O-Ta bridging stretching modes. This band is mainly due to a totally symmetric combination of coupled Ta-O stretching modes in the chains of tantalum octahedral (see the Supporting Information, Figure S1). In the subsequent stage, Ostwald ripening process occurred. As the reaction continued, the NaTaO3 nuclei grew gradually,

the growth of large particles at the cost of the smaller ones, due to tendency of the solid phase in the system to self-adjust to reduce the total surface free energy, according to the wellknown Gibbs-Thomson law. Finally, NaTaO3 nanocubes with a size of about 200 nm were formed after 12 h of hydrothermal processing. For the growth mechanism of nanocubes, Murphy36 pointed out that the preferential adsorption of molecules and ions in solution to different crystal faces directed the growth of nuclei into various shapes by controlling the growth rates along different crystal axes. For the crystal shape with a face-centered cubic (fcc) nanocrystal, Wang37 suggested that the shape was mainly determined by the ratio of the growth rate in [100] to that in [111], and cubes bound by the six {100} planes will be formed when the ratio is relatively lower, as the plane with the fastest growth rate will disappear quickly. According to our experimental results, the growth process of NaTaO3 nanocubes was consistent with the above reports. It is meaningful to note that, in the conventional synthesis strategies, the growth of nuclei was controlled by the short-range diffusion of ions in a limited space. Therefore, it was difficult to control the morphology of the final products. However, under the hydrothermal environment, nuclei could grow freely in aqueous solution (an open space) to form the nanocrystals with their natural habit, i.e., nanorods, nanosheet, nanocubes, and so forth. From this point of view, the hydrothermal method exhibited a particular advantage in the synthesis of nanostructured materials. 3.5. Photoabsorbance Property. Previous studied have reported the relationship between crystal structure and energy delocalization for perovskite-type alkali tantalates, although tantalates of different chemical formulas were used in these studies. The excited energy is efficiently delocalized at a bond angle of Ta-O-Ta close to 180°, while the excited energy would be localized at bond angles deviating from 180°. The bond angles of the NaTaO3 specimens synthesized in the present study are 163° (hydrothermal method) and 157° (SSR), respectively. The optical properties of the specimens must be different as a result of the difference in their bond angle. The photoabsorbance of the NaTaO3 specimens was examined by using a UV-vis spectrometer. Figure 7 shows the typical diffuse reflection spectra. It is of interest to observe that the onsets of the absorption for the specimens are different and have values of 304 and 310 nm for the hydrothermal and solid states, respectively. As is known, for a crystalline semiconductor, the

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Figure 7. Diffuse reflectance spectra of NaTaO3 (a) nanocubes prepared at 140 °C for 12 h and (b) SSR sample.

optical absorption near the band edge follows the equation38 R ) A(hV - Eg)n/2/hV, where R, V, A, Eg, and n are the absorption coefficient, incident light frequency, band gap, constant, and an integer, respectively. Among them, n decides the characteristics of the transition in a semiconductor. By using this equation, the value of n for NaTaO3 was 1; this means that the optical transition for NaTaO3 is directly allowed. According to this equation, the band gap of the NaTaO3 nanocubes and NaTaO3 microparticles could be estimated from the onset of the absorption edge to be 3.78 and 4.0 eV, respectively. The band gap of perovskite-like alkaline tantalum oxides was reported to increase with the deviation from 180° of Ta-O-Ta bond angle due to the decreased conduction bandwidth.39,40However, we can see that the trend of the band gap variation with the bond angle for the NaTaO3 specimens was opposite to the reported argument. This contradiction may result from the fact that phonon absorption is involved in the interband transition process. It was suggested that the intensity of phonon absorption increases as the Ta-O-Ta bond angle in NaTaO3 approaches 180°.41,42 On the other hand, the absorption edge energy has been shown to exhibit crystallite size for small semiconductor particles. This may also be the reason for the difference in band gap observed. 3.6. Phtocatalytic Properties of NaTO3 Nanocubes. The photocatalytic activity of the NaTaO3 nanocubes has been studied in photodegeradation reactions of Safranine T (ST) and Methylene Blue (MB), as well as gaseous formaldehyde under UV irradiation. Before the photocatalytic reaction, the dye solutions were first photolyzed in the absence of the photocatalysts to examine their stability. The results show that the dyes and gas are not decomposed, even after long illumination with UV light. In addition, the concentrations of dyes almost do not change under dark conditions after the NaTaO3 and dye solutions reach the adsorption-desorption equilibrium. Therefore, the presence of both catalysts and illumination is necessary for efficient degradation. Figure 8a shows the degradation curves for ST in the presence of NaTaO3 nanocubes prepared at different temperatures. As shown in the figure, the apparent reaction constant for the sample prepared by a 12 h reaction at 100° is 0.01201 min-1. The photocatalytic activity of the samples increases with the reaction

Figure 8. The photocatalytic performance of samples synthesized at various temperatures for 12 h: (a) the degradation of ST, and (b) degradation curve for ST dye in the presence of NaTaO3 nanostructure under UV irradiation.

temperature used for the preparing the nanocubes, owing to the better crystalline order of structure formed at higher temperatures. When synthesis temperature is 120 °C or 140 °C, the photocatalytic activity of the fabricated nanocubes is significantly increased and the apparent reaction constants are 0.02623 and 0.03978 min-1, respectively. However, nanocubes prepared at 160 °C show reduced photocatalytic activity and lower apparent reaction constants. In addition, compared with TiO2 (P25, Degussa), the sample has a higher photocatalytic activity. The model substrates ST is completely degraded upon being subjected to UV irradiation for 60 min in the presence of NaTaO3 nanocubes prepared at 140 °C (Figure 8b). This photocatalytic performance is 10 times better than that of SSR NaTaO3 particles. The degradation of formaldehyde over the NaTaO3 nanocubes is shown in Figure 9. Samples synthesized at different temperatures shows similar photocatalytic performance, and gaseous formaldehyde is completely degraded over the tantalate nanocubes within 60 min under UV irradiation. The photocatalytic activity of the nanocubes is about twice that of SSR tantalates for the decomposition of formaldehyde. For samples synthesized at higher reaction temperature, the crystal structure becomes more perfect, resulting in increased photocatalytic activity. A corresponding XRD result confirmed that the sample with lower crystallinity was unstable after UV light irradiation because some of impurity appeared in the patterns that can be indexed as Figure 2.When the reaction temperature

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J. Phys. Chem. C, Vol. 113, No. 45, 2009 19417 catalytic activity of the nanocrystalline samples mainly results from the large surface area of the nanosized sample and the good crystallinity. This facile preparation method is expected to be utilized in fabrications of nanostructure of various semiconductor functional materials with advanced properties. Acknowledgment. The authors are grateful for the financial aid from the National Natural Science Foundation of China (Grant No. 50772051). Supporting Information Available: FTIR spectra. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes

Figure 9. The photocatalytic performance of samples synthesized at various temperatures for degradation of gaseous formaldehyde (C/C0: concentration/initial concentration).

is increased to 140 °C, the photocatalytic performance of the nanocubes reaches a peak. Further increasing the reaction temperature for the preparation of the nanocubes does not yield any further improvements in the photocatalytic activity. For the photocatalytic activity of semiconductor oxides, even though the principle and activity-controlling factors of the photomineralization process in semiconductor-based photocatalysis have been discussed previously,43 many aspects of the function of inorganic photocatalysts are still unclear, such as the detailed mechanism reduction and oxidation on the semiconductor surface, electrons and holes transferring in and out of the catalyst, and the effect of variable material preparations and surface defects on the catalytic activity of semiconductor.44 Considering our prepared samples, the enhancement may be attributed to large surface area and smaller particle size, and less surface defect. In our experimental results, the sample from the solid-state routes was aggregated particles, and the surface area was 0.45m2/g. However, the samples from the hydrothermal process usually possess smaller crystallite size and lower agglomeration (BET 32.4m2/g), which led to high photocatalytic activity under light irradiation. It has been reported that the large value of the surface-to-volume ratio can increase the number of active surface sites where the photogenerated electron-hole pairs are able to induce more hydroxyl and superoxide radicals to react with absorbed molecules.45 Compared to bulk tantalates, the nanosized structure has a significant promotion of the holes generated inside the crystal, which can be transferred to the surface and can interact with the organic molecules. Therefore, the NaTaO3 nanocubes show enhanced photocatalytic activity. 4. Conclusion Nanosize NaTaO3 photocatalysts with cubic morphology have been successfully synthesized by a simple hydrothermal approach. The synthesis parameters such as alkaline concentration, reaction temperature, and reaction time played important roles in the formation of NaTaO3 nanocubes. The growth mechanism of the nanocubes obeys a dissolution-crystallization process. The experiments of photocatalytic degradation of solution dye, and gaseous formaldehyde indicated that nanosized NaTaO3 had high photocatalytic activity under UV radiation. High photo-

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