Semiconductor photocatalysis has attracted much interest for its potential in air pollution, solid waste and water pollution treatments [1, 2, 3, 4]. Various semiconductors have been explored, with tantalum pentoxide (Ta2O5) being attractive because of its high photocatalytic activity, dielectric constant, refractive index, and chemical stability [5, 6, 7, 8, 9, 10]. Nanomaterials with well- defined crystal structures and morphologies have been applied in various areas because of their size and structure-dependent properties [11, 12, 13]. Much effort has focused on one- dimensional (1D) nanostructures, such as nanorods and nanowires [14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24]. These exhibit different physical and chemical properties to their bulk counterparts.
Nanoparticulate Ta2O5 has been prepared by sol-gel [25, 26], hydrothermal [27, 28, 29], solvothermal [30, 31], hot filament metal vapor deposition [32], and microemulsion [33, 34] methods. Most of these methods yield poorly crystalline Ta2O5 nanoparticles. Subsequent high-temperature calcination is also generally required to yield the final product, which can lower the surface area and decrease the catalytic activity. Hydrothermal synthesis can yield nanostructures with high crystallinity, high purity and narrow size distribution [35]. The product morphology can be controlled by adjusting the hydrothermal synthesis conditions. A few tantalates have been hydrothermally prepared at low temperature [36, 37, 38, 39, 40, 41]. The hydrothermal synthesis of nanoparticulate Ta2O5 is difficult because Ta2O5 is much more inert than TiO2 [34, 42]. Ta2O5 nanowires [27] and nanoflowers [28] were recently hydrothermally prepared using hydrofluoric acid, which is impractical for green syntheses and industrial application. Gömpel et al. [29] hydrothermally prepared Ta2O5 nanorods without using hydrofluoric acid, instead using an expensive tantalum(V) n-butoxide source. A hydrothermal synthesis of Ta2O5 nanoparticles using an economical Ta source in the absence of hydrofluoric acid is required.
In the present study, 1D Ta2O5 nanorods with tunable morphology were hydrothermally synthesized with the assistance of polyethylene glycol (PEG) and Sr(OH)2. Ta2O5 powder was used as the Ta source. The effect of the synthesis parameters on the product morphology was investigated, and a growth mechanism was proposed. The photocatalytic performance of the Ta2O5 nanorods in the photodegradation of rhodamine B (RhB) under UV irradiation was investigated. The nanorods exhibited high activity, a high apparent reaction rate constant, and good recyclability.
All chemicals were purchased from Alfa Aesar and used as received. In a typical synthesis, 2 mmol of Ta2O5 powder (99.99%) was dispersed in anhydrous ethanol, and a solution of Sr(OH)2 was added dropwise under stirring. PEG (Mw = 300, 50 mg) with certain deionized water was added. The mixture was transferred to a polytetrafluoroethylene-lined autoclave (30 mL) for static crystallization at 200 °C for 5, 24, or 48 h. The resulting precipitate was collected by centrifugation and thoroughly washed with deionized water and ethanol. The product was dried at 80 °C overnight to yield Ta2O5 nanorods, which were denoted by Ta2O5-x-y, where x is the Ta2O5/Sr(OH)2 molar ratio used during synthesis, and y is the reaction duration.
X-ray diffraction (XRD) patterns were recorded on a Bruker D8 ADVANCE powder diffractometer using Cu-Ka radiation (l = 0.1542 nm) at a scanning rate of 12°/min at 2θ = 10°-80°. Specific surface areas were determined through low- temperature N2 adsorption-desorption isotherms, collected on a Quantachrome iQ-MP gas adsorption analyzer. Samples were dehydrated at 300 °C for 2 h prior to measurement. The total surface area was calculated via the BET equation. The morphologies and compositions were examined by field emission scanning electron microscopy (FE-SEM, Hitachi S-4800). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were collected on a Philips Tecnai G2 20 S-TWIN microscope at 200 kV. A few drops of a sample suspension in ethanol were allowed to evaporate on a carbon-coated copper grid at ambient temperature. Ultraviolet-visible (UV-vis) diffuse reflectance spectra were recorded in the air against BaSO4 at 200-700 nm using a Varian Cary 300 spectrophotometer. Photoluminescence (PL) spectra were recorded on a Spex FL201 fluorescence spectrophotometer. Samples were dry-pressed into self-supporting wafers and then excited by a 325-nm He-Cd laser at ambient temperature.
The photocatalytic degradation of RhB was performed in a top-irradiation-type double-walled quartz cell cooled by water. A 250 W mercury lamp (lmax = 365 nm) was used as the light source. About 0.1 g of catalyst was added to 200 mL of RhB solution (10 mg/L) in the quartz cell. The suspension was stirred in the dark until the RhB concentration was constant (~30 min), which indicated adsorption equilibrium. After commencing the photocatalytic reaction, aliquots were removed at regular time intervals and analyzed by UV-vis absorption spectrometry (Varian Cary 300).
The presence of PEG and Sr(OH)2 during the hydrothermal synthesis of Ta2O5 nanostructures affects their resulting morphology. Figure 1 shows that irregular nanoparticles of large (~2 μm) and small (~200 nm) size were formed in the absence of PEG (Ta2O5-1/1-ref) (Fig. 1(b)). The starting material Ta2O5 consisted of particles of ~200 nm in diameter (Fig. 1(a)). The larger particles indicated the further growth of Ta2O5 during hydrothermal treatment in Sr(OH)2 solution. No growth of Ta2O5 was observed in the absence of Sr(OH)2. Homogeneous nanorods with an average length of ~200 nm and diameter of ~25 nm were obtained (Fig. 1(c) and (d)) when Sr(OH)2 and PEG were present (Ta2O5-1/1-48h). The TEM image in Fig. 1(e) shows the crystalline lattice along two blue lines. The interplanar spacings of 0.388 and 0.315 nm corresponded to the (0 0 1) and (1 11 0) lattice planes of orthorhombic Ta2O5 (JCPDF 25-0922), respectively [18]. The product morphology was the same as that of raw Ta2O5 when only PEG was present (not shown). The role of Sr(OH)2 was thought to be inducing the growth of Ta2O5, and that of PEG to induce anisotropic Ta2O5 growth by acting as a capping agent.
XRD patterns of the products formed under different hydrothermal conditions are shown in Fig. 2. Phase-pure Ta2O5 (JCPDF 25-0922) was obtained after the hydrothermal treatment of Ta2O5 in Sr(OH)2 solution in the presence or absence of PEG at a Ta2O5/Sr(OH)2 ratio of 1/1. Diffraction peaks at 22.9°, 28.3°, 28.8°, 36.7°, 37.1°, 46.7°, 49.2°, 51.1°, and 55.5° corresponded to the (0 0 1), (1 11 0), (2 0 0), (1 11 1), (2 0 1), (0 0 2), (1 18 1), (0 9 2), and (1 11 2) lattice planes of Ta2O5, respectively. Additional diffraction peaks at 29.6°, 34.9°, 48.9°, and 58.2° were observed at a Ta2O5/Sr(OH)2 ratio of 2/1, probably due to the formation of SrTa2O6 (JCPDF 51-1683).
UV-vis diffuse reflectance spectra of Ta2O5 prepared from different conditions are shown in Fig. 3. All samples exhibited the same UV absorption edge at ~315 nm, with negligible red or blue shifts. The corresponding band gaps of the Ta2O5 samples and reference Ta2O5 sample were ~3.9 eV, as determined from absorption onsets by plots of the square root of the Kubelka-Munk function versus photon energy [43]. This value was in agreement with that of Maeda et al. [44]. The Ta2O5 products were n-type semiconductors, which were expected to be photocatalytically active under UV irradiation.
The effects of crystallization duration, Ta2O5/Sr(OH)2 ratio, and PEG dosage on the product morphology were investigated. Figure 4(a-c) shows that the crystallization duration significantly affected the product morphology. Ta2O5 nanoparticles with average length-diameter ratios of 1.6, 3.6, and 8 were obtained after reaction for 5, 24, and 48 h, respectively. No further changes were observed after longer reaction durations. These observations indicated the orientated re-growth of Ta2O5 under hydrothermal conditions. The HRTEM image of Ta2O5-1/1-48h (Fig. 1(e)) indicated that re-growth occurred along the (0 0 1) plane.
The Ta2O5/Sr(OH)2 ratio affected the morphology and crystal phase of the product. Nanoparticles of mixed phase (Ta2O5 and SrTa2O6) were obtained at a Ta2O5/Sr(OH)2 ratio of 2/1. Phase-pure orthorhombic Ta2O5 nanorods were obtained at Ta2O5/Sr(OH)2 ratios of 1/1 and 1/2. The average length- diameter ratio of Ta2O5-1/2-48h nanorods was 18, higher than that of the Ta2O5-1/1-48h nanorods (25 × 200 nm), as shown in Fig. 4(e-f). Mixed metal oxides would be observed at Ta2O5/ Sr(OH)2 ratios of < 1/2. The properties of Ta2O5 samples obtained from different synthesis conditions are summarized in Table 1.
Crystalline Ta2O5 is insoluble under normal conditions, even in highly alkaline media. However, Ta2O5 can be partly dissolved and hydroxylated to Ta2O5·xH2O or Ta(OH)5 at high temperature and pressure [1]. PEG has been shown to be effective for the synthesis of nanorods in metal oxide systems, such as zinc oxide [45] and cerium oxide [46].
In the present study, Ta2O5 in the presence of Sr(OH)2 dissolved and hydroxylated to its corresponding hydroxide under hydrothermal conditions. PEG adsorbed to the resulting Ta2O5·xH2O or Ta(OH)5 via hydrogen bonding and van der Waals forces. The 1D PEG chains controlled the nucleation and growth of the Ta2O5 nanorods by favoring the formation of elongated nuclei. PEG continuously adsorbed on nuclei surfaces, with different affinities for different facets. This caused preferential growth in specific directions and the formation of 1D structure. In the presence of PEG, the growth units, i.e. the single Ta2O5 crystal lattice, preferentially landed on the (0 0 1) faces of Ta2O5, so nanorods gradually formed along this direction (Fig. 1). Increasing the reaction duration resulted in longer nanorods (Fig. 4). This is similar to the growth of ZnO nanorods, in which (0 0 1) is the fastest growth direction [45]. The XRD results indicated the good crystallinity of Ta2O5 nanorods after 5 h of hydrothermal treatment. Some irregular nanoparticles of different sizes were also formed in the absence of PEG (Fig. 1(b)). The proposed formation mechanism for the Ta2O5 nanorods is shown in Fig. 5.
The photocatalytic activities of the Ta2O5 samples were evaluated in the degradation of RhB under UV irradiation. Dye solutions were first photolyzed in the absence of a photocatalyst to examine their stability. RhB was not degraded in the dark, and only slightly degraded under UV irradiation in the absence of a catalyst (< 2%). Thus, the photolysis and effect of catalyst adsorption were negligible. Degradation curves and corresponding plots of ln(C0/C) versus time-on-stream (TOS) catalyzed by different samples are shown in Fig. 6. The maximum absorption of the degraded solutions exhibited hypsochromic shifts with increasing TOS. This indicated cleavage of the RhB conjugated structure, and decomposition via a series of N-deethylated intermediates. The degradation of RhB followed pseudo-first-order kinetics. The different Ta2O5 samples exhibited differing photocatalytic activities. The surface area, crystal phase, morphology, and crystallinity are the main factors affecting the photocatalytic activity of semiconductor oxides [47]. All samples (except Ta2O5-2/1-48h) possessed the same orthorhombic phase with similar crystallinities. Thus, the Ta2O5 surface area and morphology were thought to be important factors controlling the photocatalytic activity. For Ta2O5 nanorods of the same orthorhombic phase, the photocatalytic activity correlated well with surface area. Higher surface areas corresponded to higher apparent rate constants (kapp) and higher photocatalytic activity. Larger surface areas reportedly favor larger numbers of active sites, where photogenerated electron-hole pairs can induce more hydroxyl and superoxide radicals to react with organic absorbates [48]. Therefore, Ta2O5 nanorods with larger surface areas exhibited higher photocatalytic activity.
The Ta2O5 morphology was determined by the crystallization duration and Ta2O5/Sr(OH)2 ratio, so both of these could be used to influence the photocatalytic activity. The lower Ta2O5/Sr(OH)2 ratio in Ta2O5-x-48h resulted in a higher kapp and higher photocatalytic activity (Table 2). The highest photocatalytic activity with a kapp of 0.156 min-1 was obtained for Ta2O5-1/2-48h. BaTa2O6 nanorods have a reported kapp of 0.05495 min-1 [37]. Longer hydrothermal reaction durations for Ta2O5-1/1-y resulted in higher kapp values. Figure 7 shows a direct correlation between the length-diameter ratio and kapp. These observations were consistent with the reported view that 1D (nanowires and nanorods) facilitate the separation of photogenerated electron-hole pairs [49, 50, 51].
PL spectroscopy can reveal the efficiency of charge carrier trapping, immigration, and transfer in semiconductors [52, 53]. PL emission generally originates from the radiative recombination of photogenerated electrons and holes. Room temperature PL emission spectra of the Ta2O5 samples are shown in Fig. 8. PL peaks at 400-500 nm were observed for all samples due to surface and bulk irradiative recombination. The changes in PL intensity correlated with the observed photocatalytic activity (i.e., lower PL intensity correlated with higher photocatalytic activity).
The stability of the highest activity sample (Ta2O5-1/2-48h) was investigated by the successive photodegradation of RhB, as shown in Fig. 9. The photocatalyst sample was separated by centrifugation, thoroughly washed with distilled water, and then used for the subsequent photodegradation without thermal treatment. Ta2O5-1/2-48h exhibited good recyclability over five cycles, as evidenced by the constant photodegradation rate. Thus, Ta2O5-1/2-48h was stable during the photodegradation of RhB and has potential in the treatment of dye-containing waste water.
Ta2O5 nanorods were hydrothermally prepared from commercial Ta2O5. The presence of PEG and Sr(OH)2 was necessary to form the nanorods. Sr(OH)2 induced the surface dissolution and re-growth of Ta2O5. PEG induced the anisotropic growth of Ta2O5 by acting as a capping agent. The crystallization duration and Ta2O5/Sr(OH)2 ratio had a strong influence on the morphology of the hydrothermal products. The Ta2O5 nanorods exhibited similar UV absorption properties and were used in the photodegradation of RhB under ultraviolet irradiation. The Ta2O5 morphology strongly affected the photocatalytic activity, and a direct correlation between the length-diameter ratio and apparent rate constant was observed. An apparent reaction rate constant of 0.156 min-1 was obtained for Ta2O5 nanorods with a length-diameter ratio of ~18. The good recyclability of the Ta2O5 nanorods indicated their potential for treating dye-containing waste water.
2015, Vol. 36 Issue (3): 432-438
PDF (1125 KB)
2015, Vol. 36 Issue (3): 432-438
PDF (1125 KB)