Well-dispersed mesoporous Ta2O5 submicrospheres: Enhanced ...
Chemical Engineering Journal 229 (2013) 371–377
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Well-dispersed mesoporous Ta2O5 submicrospheres: Enhanced photocatalytic activity by tuning heating rate at calcination Can Tao, Leilei Xu, Jianguo Guan ⇑ State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Well-dispersed mesoporous Ta2O5
submicrospheres were obtained by a facile strategy. The nucleation and growth behaviors are closely related to heating rates (R). R impacts the dispersibility, mesoporosity and crystallinity of the products. The products exhibit an exceptional photocatalytic activity for hydrogen evolution.
a r t i c l e
i n f o
Article history: Received 29 January 2013 Received in revised form 4 May 2013 Accepted 9 June 2013 Available online 17 June 2013 Keywords: Ta2O5 submicrosphere Heating rate Mesoporosity Dispersibility Photocatalytic hydrogen production
a b s t r a c t Well-dispersed mesoporous Ta2O5 submicrospheres were obtained by tailoring the heating rates (R) of calcining Ta2O5 precursor colloidal spheres which were synthesized by the hydrolysis of tantalum glycolate in a mixture of acetone and water. With increasing R, the pore size and the dispersibility of the asprepared submicrospheres decrease gradually whereas the specific surface area and the pore volume reach maximum values at R = 5 °C/min. These phenomena are reasonably explained by the dependence of the temperature gradient within the Ta2O5 precursor colloidal spheres on R, which modulates the nucleation and growth behaviors of the Ta2O5 crystallites during the calcination process. Because of the good dispersibility, unique mesoporosity and high crystallinity, the Ta2O5 submicrospheres obtained at R = 5 °C/min show a significantly enhanced photocatalytic activity not only for hydrogen evolution but also for photodegradation of organic pollutants such as methylene blue and rhodamine B. The results here suggest that the meticulous control over the structural characteristics is an effective alternative to obtain highly efficient photocatalytic materials. The as-prepared well-dispersed mesoporous Ta2O5 submicrospheres are promising for the applications in clean energy access and environment remediation. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Tantalum pentoxide (Ta2O5) has a wide range of applications such as dynamic random access memory, antireflective coating layer, gas sensor, capacitor and photocatalyst due to the high dielectric constant and refractive index, as well as the good chemical resistance [1–7]. Especially, the wide band gap of 3.9 eV and
⇑ Corresponding author. Tel.: +86 27 87218832; fax: +86 27 87879468. E-mail address: [email protected] (J. Guan). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.06.012
the relatively high conduction band (CB) endow it a unique superiority for photocatalytic hydrogen evolution [8–10]. Besides the intrinsic characters of photocatalyst particles, the dispersibility, size, specific surface area and active reaction sites, which are closely related to their morphologies and/or structures, also strongly affect the photocatalytic performances. Therefore, since commercial Ta2O5 (C–Ta2O5) was discovered to use as a photocatalyst for hydrogen production [9], diverse nanostructured Ta2O5-based materials have been fabricated for photocatalytic hydrogen production [11–24]. Among them, mesoporous Ta2O5 [25] and Ta2O5-based composites [26,27] demonstrate unique
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advantages for photocatalytic hydrogen production. For example, their large surface areas and nanoscale pore walls can greatly increase the reaction active sites and decrease the transfer distance of photogenerated charge carriers, thus enhance the photocatalytic activity. The crystallized Ta2O5 mesoporous structures, which are obtained by the aid of SiO2 reinforcement [28,29], can not only increase the transfer velocity but also decrease the recombination opportunity of photoexcited electrons and holes. Thus, they can show a further increased activity compared to amorphous Ta2O5-based mesoporous materials. However, all the reported Ta2O5-based mesoporous materials are almost bulk and/or micron-sized. Undoubtedly, they show poor dispersibility and will rapidly become sediment in the reaction system, resulting in the depression of the light harvesting and the reactant accessibility. Therefore, well-dispersed, crystalline Ta2O5 mesoporous submicrospheres are expected as an excellent photocatalyst. It seems that monodispersed Ta2O5 microspheres have been prepared by a two-step process [30], but they tend to form dumbbell-like or chain-like structures. Especially after calcination, they agglomerate badly, and are hardly dispersible in solution. This suggests that the fabrication of well-dispersed, crystalline Ta2O5 mesoporous spheres is still a challenge because of the fast hydrolysis rates and sensitive nature of the tantalum precursors as well as the high crystalline temperature of Ta2O5. Herein, we attempt to optimize the heating rates (R) in the calcination process to prepare well-dispersed mesoporous Ta2O5 submicrospheres for photocatalytic hydrogen evolution and dye decolorization via hydrolysis of tantalum glycolate in a mixture of acetone and water followed by calcinations. The influences of R on the structure, textural characteristics and optical absorption properties of the Ta2O5 submicrospheres, as well as some factors influencing on the photocatalytic performances are investigated in detail. The submicrospheres with a unique mesoporosity, a high crystallinity and a good dispersibility, which show significantly enhanced photocatalytic performances, are obtained by tuning R. The elaborate modulation of kinetic parameters used in the calcination process of the colloid precursors is an effective approach to tailor the structural characteristic of photocatalysts, and thus achieves an excellent photocatalytic activity.
2. Experimental section 2.1. Catalyst preparation The synthesis route of the well-dispersed Ta2O5 colloids is similar to the previous report of monodispersed TiO2 colloids [31]. In a typical synthesis, 0.4 g tantalum ethylate (Ta(OC2H5)5, 99.9%, Shanghai Bangcheng Chemical Co. Ltd.) was dissolved into 10 mL ethylene glycol (99.9%, Sinopharm Chemical Reagent Co. Ltd.) by vigorously stirring for 8 h at room temperature in a glove box purged with nitrogen gas. Afterwards, the mixture was taken out from the glove box and immediately poured into a 100 mL acetone containing 0.5 vol.% of water, and then vigorous stirred for 10 min. After aging for 30 min, the white precipitate was separated by centrifugation, and then washed with distilled water and ethanol three times to remove most of the ethylene glycol. The final product was dried in a vacuum oven at 60 °C for 24 h to obtain the well-dispersed Ta2O5 precursor spherical colloids. The well-dispersed mesoporous Ta2O5 submicrospheres were obtained by calcining the Ta2O5 spherical colloid precursor at 800 °C with a heating rate (R) of 5 °C/min and then maintained for 2 h in air. The Ta2O5 submicrospheres with different textural characteristics and dispersibilities were obtain by changing R to investigate the R influences.
2.2. Catalyst characterization Scanning electron microscopy (SEM) images of the samples were obtained in a Hitachi S-4800 field-emission scanning electron microscope. The crystalline structures of the samples were characterized by a D/Max-RB X-ray diffractometer at an accelerating voltage of 40 kV and a current of 20 mA with Cu Ka radiation (k = 0.15406 nm), in the 2h range from 10° to 70° at a scanning rate of 0.05 °/s. Thermogravimetric and differential thermal analysis (TG–DTA) were performed on a STA449C thermal analyzer (Netzsch, Germany) in the air with a heating rate of 10 °C/min. The N2 adsorption–desorption data were obtained on a Micromertics ASAP 2020 instrument (USA). UV–Vis diffusion reflectance spectra (DRS) were performed on a UV-2550 PC UV–Vis spectrophotometer (Shimadzu, Japan). Transmission electron microscopy (TEM) analyses were conducted on an F20 S-TWIN electron microscope (Tecnai G2, FEI Company) with an accelerating voltage of 200 kV. 2.3. Photocatalytic test The photocatalytic activities of the as-prepared mesoporous Ta2O5 submicrospheres for hydrogen evolution were measured in a Labsolar H2 closed-gas circulation system with external-irradiation Pyrex cell, which was placed about 10 cm under a 300 W xenon lamp (PLS-SXE300, Beijing Trusttech Co. Ltd., China). The diameter of the reactor is ca. 63 cm, almost equal to the facula size of the light source. 20 mg of the as-prepared samples without any cocatalysts were dispersed in 100 mL 20 vol.% methanol aqueous solution, and then ultrasonicated for 10 min to get a good dispersion. The overall water-splitting was also conducted under the same conditions without the addition of methanol. The platinization of the catalyst was tested by an in situ photodeposition method. H2PtCl6.6H2O with different contents (0.5, 0.7 and 1 wt%) was added into the 20 vol.% methanol aqueous solution. Before irradiation, the closed system was repeatedly evacuated to 0.1 MPa to remove dissolved oxygen and other gases. The generated hydrogen was in situ analyzed with a GC 7890-II TCD gas chromatograph (TECHCOMP) using an MS-5 A column, which was connected to the gas circulating line with nitrogen carrier. The column temperature is 45 °C, the injection and detection temperatures are both 120 °C, and the current is 100 mA. The photocatalytic decolorizations of methylene blue (MB) and rhodamine B (RhB) aqueous solutions were preformed under simulant sunlight irradiation using a 300 W Xe lamp, respectively. 20 mg of the as-prepared submicrospheres are dispersed in 20 mL of RhB or MB aqueous solution with a concentration of 20 mg L 1. The adsorption–desorption equilibrium between dye molecules and the photocatalysts was achieved by continuously stirring for 30 min in dark. The concentrations of the MB and RhB in the aqueous solutions after removing the catalyst were monitored using a UV–Vis spectrophotometer (Japan Shimadzu UV–Vis 2550) at every 20 min intervals, respectively. The photocatalytic activities of the dyes decolorization were evaluated by the values of C/C0. C0 and C denoted the initial concentration and the concentration at each irradiated time interval, respectively. 3. Results and discussion The as-prepared Ta2O5 precursor spherical colloid particles before calcination were characterized by SEM, TEM, XRD and TG–DTA, respectively. Fig. 1 manifests that the prepared samples are almost monodispersed spheroidal particles of 330 nm in size, which possess smooth surfaces and solid inner structures. Fig. 1c confirms that the precursor particles are obviously amorphous. In order to set a calcination temperature for crystallization, we have
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Fig. 1. SEM (a), TEM (b) images, XRD pattern (c) and TG–DTA curves (d) of the as-prepared Ta2O5 colloid precursor.
Table 1 The textural characteristics of the as-prepared mesoporous Ta2O5 submicrospheres.
Fig. 2. XRD patterns of the mesoporous Ta2O5 submicrospheres calcined at different R.
carried out the TG–DTA analysis of the as-prepared colloid precursor particles. As illustrated in Fig. 1d, the precursor particles have three differentiated steps of weight loss in the temperature range of 50–700 °C. This is originated from the removal of physically adsorbed water and the decomposition of the residual organic groups in the precursor particles [28,31]. At the temperature higher than 800 °C, no weight loss is observed in the TG curve. The small exothermic peak centered at 861 °C is mainly attributed to the phase transformation of the precursor particles from an amorphous state into a crystallized state. Based on the result of TG–DTA analysis, the as-prepared colloid precursor was calcined at 800 °C for 2 h to crystallize. In order to regulate the textural properties and dispersibility, the mesoporous Ta2O5 submicrospheres were obtained by calcined with different R. Fig. 2 shows the XRD patterns of the as-prepared Ta2O5 samples. Obviously, all the characteristic diffraction peaks at 2h = 22.8°,
R (oC/min)
SBET (m2/g)
Dp (nm)
Vp (cm3/g)
Dg (nm)
2 5 10
8.11 11.29 8.73
19.41 15.84 15.58
0.039 0.045 0.034
26.54 24.94 22.48
28.2°, 28.7°, 36.6°, 46.7° and 55.6° correspond to the d spacings of 3.89, 3.16, 3.10, 2.45, 1.95 and 1.65 Å. They can be assigned to (0 1 0), (4 1 1), (0 0 2), (1 1 0 2), (0 2 0), (0 2 2) of the orthorhombic Ta2O5, respectively (JCPDS 79-1375). The intensities of the diffraction peaks for the samples obtained at R = 5 °C/min become relatively strong, indicating a better crystallinity of the samples. The average crystallite sizes (Dg) of the as-prepared Ta2O5 samples have been measured by the Scherrer equation and listed in Table 1. It is found that with increasing R from 2 °C/min to 10 °C/min, Dg decreased gradually. This is because the high heating rate may bring about a relatively large temperature gradient within the submicrospheres, resulting in the formation of solid/liquid biphase core–shell structures during the non-equilibrium heating process [32] As the heterogeneous nucleation only requires a low activation energy, the co-existence of heterogeneous phases is helpful for the crystalline nucleation of Ta2O5. It is imaginable that the more the generated nuclei are, the smaller the as-obtained crystallite size is. Additionally, the rapid heating rate also causes a sharp decomposition of the residual organic groups in the obtained colloidal precursors, suddenly releasing vast gases. As a result, the crystallite growth is inhibited around the pores. Both of the two reasons may contribute to the deceased Dg with increasing R. The morphology and mesoporosity of the Ta2O5 products were characterized by SEM, TEM and N2 adsorption–desorption analysis. Fig. 3a–c manifest that after the spherical colloid precursors were thermally treated at 800 °C for 2 h with different R, all of the resultant particles almost maintained the initial spherical morphology.
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Fig. 3. SEM images of the mesoporous Ta2O5 submicrospheres calcined at various R of (a) 2 °C/min, (b) 5 °C/min, (c) 10 °C/min and (d) TEM image of the mesoporous Ta2O5 submicrospheres calcined at R = 5 °C/min.
Fig. 4. Nitrogen adsorption–desorption isotherms of the Ta2O5 submicrospheres calcined at different R. The inset shows the corresponding pore size distribution profiles.
But the surfaces became rough and the diameter decreased to 250 nm. It is obvious that the resultant spheres obtained at R 6 5 °C/min are well-dispersed, while those obtained at R of 10 °C/min tend to break up and adhere to decrease the dispersibility of the samples. This may be explained by the assumption that the latter have a lower melting point because of the decreased size of the Ta2O5 nanocrystallites on the surfaces. They easily fuse together via the adhesion to contiguous submicrospheres when treated at the elevated temperature. Thus, the dispersibility of the samples decreases. Fig. 3d obviously shows that the resultant submicrospheres obtained at R of 5 °C/min exhibit a unique porous structure, which favors the reactants adsorption and the light scattering. Fig. 4 shows the N2 adsorption–desorption isotherms and the corresponding pore size distribution curves of the as-prepared samples. It can be seen that all the as-prepared samples represent type IV isotherms with a pronounced H3 type hysteresis loop,
Fig. 5. UV–Vis DR spectra for the mesoporous Ta2O5 submicrospheres calcined at different R; the inset shows the plots of (ahm)1/2 versus hm.
indicating the characteristic of mesoporous materials with assemblages of slit-shaped pores [33]. This is in good agreement with the result of the TEM image. But the samples have varied average pore diameters and distributions with different R. Compared with the samples obtained at R of 2 °C/min and 10 °C/min, the sample calcined at an R of 5 °C/min shows the narrowest size distribution as well as the largest BET specific surface area (SBET) (Table 1). The sample calcined at R = 10 °C/min has a decreased SBET, but the smallest Dg. This further confirms the occurrence of the surface adhesion of the nanocrystallites caused by the decreased melting point as well as a mass of crystallization heat. This reasonably suggests that R not only influences the morphology of the as-prepared Ta2O5 submicrospheres, but also influences the textural characteristics. The optical properties of the as-prepared mesoporous Ta2O5 submicrospheres were characterized by UV–Vis DRS. Fig. 5 reveals
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that a significant absorption edge at 320–330 nm is assigned to the intrinsic absorption of Ta2O5. Compared with C–Ta2O5, the band edges of the as-prepared mesoporous Ta2O5 submicrospheres exhibited obvious blueshifts. As shown in the inset of Fig. 5, the band gaps of all the as-prepared samples can be estimated according to the Kubelka–Munk method [34]. They are 3.86 eV, 3.87 eV and 3.85 eV, respectively. All are bigger than that of C–Ta2O5 (3.81 eV), suggesting that the as-prepared mesoporous Ta2O5 submicrospheres have an increased band gap, which endows them with an enhanced oxidizing and reducing ability of the photogenerated charges. This is mostly ascribed to their decreased particle size. In addition, the sample calcined at an R of 5 °C/min showed a slightly increased light absorption compared with the other calcined samples. This may be related to the textural characteristics of the samples. Photocatalytic activities for hydrogen production of the as-prepared mesoporous Ta2O5 submicrosphere samples were evaluated in 20 vol.% methanol aqueous solution under the irradiation of simulant sunlight in the absence of any cocatalysts. For comparison, the activity of C–Ta2O5 was also tested at the same conditions. From Fig. 6a, it can be observed that all the mesoporous submicrospheres calcined at different R show much higher photocatalytic activities than C–Ta2O5. The mesoporous Ta2O5 submicrospheres calcined at R = 5 °C/min show the highest photocatalytic activity of 11.2 lmol/h0.02 g, 4 times larger than that of C–Ta2O5. In contrast, the amorphous mesoporous Ta2O5 loaded by 4% NiO only exhibits the hydrogen evolution efficiency of 150 lmol/h0.5 g under a high pressure Hg lamp irradiation [25]. The photocatalytic
activity of the Ta2O5 nanowires and the hierarchical Ta2O5 singlecrystalline nanorods in the absence of cocatalysts only achieved 214 lmol/h g [14] and 200 lmol/h g [16], respectively. Additionally, the photocatalytic overall water-splitting experiments were also conducted over the as-prepared mesoporous Ta2O5 submicrospheres and C–Ta2O5. Fig. 6b shows that the photocatalytic activity for overall water-splitting was 0.24 lmol/h0.02 g,. This value is 2 times higher than that of the C–Ta2O5 and 1.5 times higher than that of hierarchical Ta2O5 single-crystalline nanorods [14]. In order to enhance the photocatalytic activities of photocatalysts by promoting the separation of photoinduced electrons and holes, platinum (Pt) nanoparticles were usually photodeposited on the photocatalyst surface and acted as active sites for H2 production. As shown in Fig. 7, the photocatalytic hydrogen production activity achieves an optimal value when 0.5% Pt were loaded onto the surfaces of the as-prepared mesoporous Ta2O5 submicrospheres. It indicated that the content of Pt loadings was directly related to the surface characters of the photocatalytic materials [35]. At higher Pt loading (>0.5 wt%), the excessive Pt particles may aggregate and cover active sites on the mesoporous Ta2O5 submicrospheres surface, which decreases the hydrogen production efficiency. Except for photocatalytic hydrogen production, the as-prepared mesoporous Ta2O5 submicrospheres can also be used for the decolorization of dyes such as MB and RhB. Fig. 8 showed the photocatalytic decolorization of RhB and MB by the as-prepared mesoporous Ta2O5 submicrospheres and C–Ta2O5, respectively. It can be observed that the decolorization of the MB and RhB solutions in the absence of any photocatalyst occurs very slowly under the simulant light irradiations. Whereas, the photodecolorization efficiency of the MB and RhB solutions over the as-prepared mesoporous Ta2O5 submicrospheres achieves 96.9% and 93.5% after 80 min and 140 min, respectively. both of them were 3 times faster than those of C–Ta2O5. The above results indicate that the as-prepared well-dispersed mesoporous Ta2O5 submicrospheres are remarkably advantageous for hydrogen evolution and environment remediation, and will be a promising candidate of photocatalysts. The enhanced photocatalytic activity of the as-prepared mesoporous Ta2O5 submicrospheres can be reasonably attributed to the following factors. The even particle size and good dispersibility make the as-prepared submicrospheres suspend stably in the reaction solution. This can harvest more lights as well as provide adequate access to the reactant. Additionally, the increased crystalline compared to the amorphous mesoporous Ta2O5 should greatly decrease the inner defects of the submicrospheres, resulting in the rapid transfer
Fig. 6. Photocatalytic hydrogen production activities of the mesoporous Ta2O5 submicrospheres calcined at different R and C–Ta2O5 from 20 vol.% methanol aqueous solution (a) and pure water (b) under the simulant solar light irradiation.
Fig. 7. Influences of the loaded Pt contents on the photocatalytic hydrogen evolution activities of the mesoporous Ta2O5 submicrospheres calcined at R = 5 °C/min under the simulant solar light irradiation.
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Acknowledgments This work financially was supported by the National Natural Science Foundation of China (51002111), the Fundamental Research Funds for the Central Universities (2012-IV-085), the Natural Science Foundation of Hubei Province (2010CDA030), and the Subject Leadership Project of Wuhan City (201150530145).
References
Fig. 8. Photocatalytic decolorization of the MB (a) and RhB (b) aqueous solutions in the presence/absence of photocatalysts. The inset pictures indicate that the color of the aqueous solution containing the mesoporous Ta2O5 submicrosphere photocatalysts fades away swiftly with the prolonged irradiation time of the simulant solar light. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
and utilization of photogenerated charge carriers. The unique mesoporosity and thin pore walls can also decrease the transfer distance of the photogenerated charge carriers and effectively restrain their recombination. Consequently, the photocatalytic efficiencies for both hydrogen production and dye decolorization are enhanced. 4. Conclusions Well-dispersed mesoporous Ta2O5 submicrospheres were successfully prepared by a two-step approach involving the hydrolysis of tantalum glycolate in the mixture of acetone and water and the subsequent calcination at a certain heating rate (R). The dispersibility, crystallinity, textural characteristics and the optical property as well as the photocatalytic activity for hydrogen evolution were investigated as functions of R. The sample calcined at R of 5 °C/min exhibits the highest photocatalytic activity, which is 4 times larger than that of C–Ta2O5 and also superior to that of the previously reported Ta2O5 nanostructures. Simultaneously, it also shows efficient photocatalytic activities for RhB and MB decolorization due to the outstanding dispersibility and mesoporosity. The results suggest that the structural characteristic of photocatalysts can be meticulously controlled via tuning kinetic parameters such as R at calcination, which provides a facile alternative to obtain highly efficient photocatalysts. The well-dispersed mesoporous Ta2O5 submicrospheres prepared here have potential applications not only in hydrogen production, but also in environment remediation.
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Well-dispersed mesoporous Ta2O5 submicrospheres: Enhanced photocatalytic activity by tuning heating rate at calcination Can Tao, Leilei Xu, Jianguo Guan ⇑ State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Well-dispersed mesoporous Ta2O5
submicrospheres were obtained by a facile strategy. The nucleation and growth behaviors are closely related to heating rates (R). R impacts the dispersibility, mesoporosity and crystallinity of the products. The products exhibit an exceptional photocatalytic activity for hydrogen evolution.
a r t i c l e
i n f o
Article history: Received 29 January 2013 Received in revised form 4 May 2013 Accepted 9 June 2013 Available online 17 June 2013 Keywords: Ta2O5 submicrosphere Heating rate Mesoporosity Dispersibility Photocatalytic hydrogen production
a b s t r a c t Well-dispersed mesoporous Ta2O5 submicrospheres were obtained by tailoring the heating rates (R) of calcining Ta2O5 precursor colloidal spheres which were synthesized by the hydrolysis of tantalum glycolate in a mixture of acetone and water. With increasing R, the pore size and the dispersibility of the asprepared submicrospheres decrease gradually whereas the specific surface area and the pore volume reach maximum values at R = 5 °C/min. These phenomena are reasonably explained by the dependence of the temperature gradient within the Ta2O5 precursor colloidal spheres on R, which modulates the nucleation and growth behaviors of the Ta2O5 crystallites during the calcination process. Because of the good dispersibility, unique mesoporosity and high crystallinity, the Ta2O5 submicrospheres obtained at R = 5 °C/min show a significantly enhanced photocatalytic activity not only for hydrogen evolution but also for photodegradation of organic pollutants such as methylene blue and rhodamine B. The results here suggest that the meticulous control over the structural characteristics is an effective alternative to obtain highly efficient photocatalytic materials. The as-prepared well-dispersed mesoporous Ta2O5 submicrospheres are promising for the applications in clean energy access and environment remediation. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Tantalum pentoxide (Ta2O5) has a wide range of applications such as dynamic random access memory, antireflective coating layer, gas sensor, capacitor and photocatalyst due to the high dielectric constant and refractive index, as well as the good chemical resistance [1–7]. Especially, the wide band gap of 3.9 eV and
⇑ Corresponding author. Tel.: +86 27 87218832; fax: +86 27 87879468. E-mail address: [email protected] (J. Guan). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.06.012
the relatively high conduction band (CB) endow it a unique superiority for photocatalytic hydrogen evolution [8–10]. Besides the intrinsic characters of photocatalyst particles, the dispersibility, size, specific surface area and active reaction sites, which are closely related to their morphologies and/or structures, also strongly affect the photocatalytic performances. Therefore, since commercial Ta2O5 (C–Ta2O5) was discovered to use as a photocatalyst for hydrogen production [9], diverse nanostructured Ta2O5-based materials have been fabricated for photocatalytic hydrogen production [11–24]. Among them, mesoporous Ta2O5 [25] and Ta2O5-based composites [26,27] demonstrate unique
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advantages for photocatalytic hydrogen production. For example, their large surface areas and nanoscale pore walls can greatly increase the reaction active sites and decrease the transfer distance of photogenerated charge carriers, thus enhance the photocatalytic activity. The crystallized Ta2O5 mesoporous structures, which are obtained by the aid of SiO2 reinforcement [28,29], can not only increase the transfer velocity but also decrease the recombination opportunity of photoexcited electrons and holes. Thus, they can show a further increased activity compared to amorphous Ta2O5-based mesoporous materials. However, all the reported Ta2O5-based mesoporous materials are almost bulk and/or micron-sized. Undoubtedly, they show poor dispersibility and will rapidly become sediment in the reaction system, resulting in the depression of the light harvesting and the reactant accessibility. Therefore, well-dispersed, crystalline Ta2O5 mesoporous submicrospheres are expected as an excellent photocatalyst. It seems that monodispersed Ta2O5 microspheres have been prepared by a two-step process [30], but they tend to form dumbbell-like or chain-like structures. Especially after calcination, they agglomerate badly, and are hardly dispersible in solution. This suggests that the fabrication of well-dispersed, crystalline Ta2O5 mesoporous spheres is still a challenge because of the fast hydrolysis rates and sensitive nature of the tantalum precursors as well as the high crystalline temperature of Ta2O5. Herein, we attempt to optimize the heating rates (R) in the calcination process to prepare well-dispersed mesoporous Ta2O5 submicrospheres for photocatalytic hydrogen evolution and dye decolorization via hydrolysis of tantalum glycolate in a mixture of acetone and water followed by calcinations. The influences of R on the structure, textural characteristics and optical absorption properties of the Ta2O5 submicrospheres, as well as some factors influencing on the photocatalytic performances are investigated in detail. The submicrospheres with a unique mesoporosity, a high crystallinity and a good dispersibility, which show significantly enhanced photocatalytic performances, are obtained by tuning R. The elaborate modulation of kinetic parameters used in the calcination process of the colloid precursors is an effective approach to tailor the structural characteristic of photocatalysts, and thus achieves an excellent photocatalytic activity.
2. Experimental section 2.1. Catalyst preparation The synthesis route of the well-dispersed Ta2O5 colloids is similar to the previous report of monodispersed TiO2 colloids [31]. In a typical synthesis, 0.4 g tantalum ethylate (Ta(OC2H5)5, 99.9%, Shanghai Bangcheng Chemical Co. Ltd.) was dissolved into 10 mL ethylene glycol (99.9%, Sinopharm Chemical Reagent Co. Ltd.) by vigorously stirring for 8 h at room temperature in a glove box purged with nitrogen gas. Afterwards, the mixture was taken out from the glove box and immediately poured into a 100 mL acetone containing 0.5 vol.% of water, and then vigorous stirred for 10 min. After aging for 30 min, the white precipitate was separated by centrifugation, and then washed with distilled water and ethanol three times to remove most of the ethylene glycol. The final product was dried in a vacuum oven at 60 °C for 24 h to obtain the well-dispersed Ta2O5 precursor spherical colloids. The well-dispersed mesoporous Ta2O5 submicrospheres were obtained by calcining the Ta2O5 spherical colloid precursor at 800 °C with a heating rate (R) of 5 °C/min and then maintained for 2 h in air. The Ta2O5 submicrospheres with different textural characteristics and dispersibilities were obtain by changing R to investigate the R influences.
2.2. Catalyst characterization Scanning electron microscopy (SEM) images of the samples were obtained in a Hitachi S-4800 field-emission scanning electron microscope. The crystalline structures of the samples were characterized by a D/Max-RB X-ray diffractometer at an accelerating voltage of 40 kV and a current of 20 mA with Cu Ka radiation (k = 0.15406 nm), in the 2h range from 10° to 70° at a scanning rate of 0.05 °/s. Thermogravimetric and differential thermal analysis (TG–DTA) were performed on a STA449C thermal analyzer (Netzsch, Germany) in the air with a heating rate of 10 °C/min. The N2 adsorption–desorption data were obtained on a Micromertics ASAP 2020 instrument (USA). UV–Vis diffusion reflectance spectra (DRS) were performed on a UV-2550 PC UV–Vis spectrophotometer (Shimadzu, Japan). Transmission electron microscopy (TEM) analyses were conducted on an F20 S-TWIN electron microscope (Tecnai G2, FEI Company) with an accelerating voltage of 200 kV. 2.3. Photocatalytic test The photocatalytic activities of the as-prepared mesoporous Ta2O5 submicrospheres for hydrogen evolution were measured in a Labsolar H2 closed-gas circulation system with external-irradiation Pyrex cell, which was placed about 10 cm under a 300 W xenon lamp (PLS-SXE300, Beijing Trusttech Co. Ltd., China). The diameter of the reactor is ca. 63 cm, almost equal to the facula size of the light source. 20 mg of the as-prepared samples without any cocatalysts were dispersed in 100 mL 20 vol.% methanol aqueous solution, and then ultrasonicated for 10 min to get a good dispersion. The overall water-splitting was also conducted under the same conditions without the addition of methanol. The platinization of the catalyst was tested by an in situ photodeposition method. H2PtCl6.6H2O with different contents (0.5, 0.7 and 1 wt%) was added into the 20 vol.% methanol aqueous solution. Before irradiation, the closed system was repeatedly evacuated to 0.1 MPa to remove dissolved oxygen and other gases. The generated hydrogen was in situ analyzed with a GC 7890-II TCD gas chromatograph (TECHCOMP) using an MS-5 A column, which was connected to the gas circulating line with nitrogen carrier. The column temperature is 45 °C, the injection and detection temperatures are both 120 °C, and the current is 100 mA. The photocatalytic decolorizations of methylene blue (MB) and rhodamine B (RhB) aqueous solutions were preformed under simulant sunlight irradiation using a 300 W Xe lamp, respectively. 20 mg of the as-prepared submicrospheres are dispersed in 20 mL of RhB or MB aqueous solution with a concentration of 20 mg L 1. The adsorption–desorption equilibrium between dye molecules and the photocatalysts was achieved by continuously stirring for 30 min in dark. The concentrations of the MB and RhB in the aqueous solutions after removing the catalyst were monitored using a UV–Vis spectrophotometer (Japan Shimadzu UV–Vis 2550) at every 20 min intervals, respectively. The photocatalytic activities of the dyes decolorization were evaluated by the values of C/C0. C0 and C denoted the initial concentration and the concentration at each irradiated time interval, respectively. 3. Results and discussion The as-prepared Ta2O5 precursor spherical colloid particles before calcination were characterized by SEM, TEM, XRD and TG–DTA, respectively. Fig. 1 manifests that the prepared samples are almost monodispersed spheroidal particles of 330 nm in size, which possess smooth surfaces and solid inner structures. Fig. 1c confirms that the precursor particles are obviously amorphous. In order to set a calcination temperature for crystallization, we have
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Fig. 1. SEM (a), TEM (b) images, XRD pattern (c) and TG–DTA curves (d) of the as-prepared Ta2O5 colloid precursor.
Table 1 The textural characteristics of the as-prepared mesoporous Ta2O5 submicrospheres.
Fig. 2. XRD patterns of the mesoporous Ta2O5 submicrospheres calcined at different R.
carried out the TG–DTA analysis of the as-prepared colloid precursor particles. As illustrated in Fig. 1d, the precursor particles have three differentiated steps of weight loss in the temperature range of 50–700 °C. This is originated from the removal of physically adsorbed water and the decomposition of the residual organic groups in the precursor particles [28,31]. At the temperature higher than 800 °C, no weight loss is observed in the TG curve. The small exothermic peak centered at 861 °C is mainly attributed to the phase transformation of the precursor particles from an amorphous state into a crystallized state. Based on the result of TG–DTA analysis, the as-prepared colloid precursor was calcined at 800 °C for 2 h to crystallize. In order to regulate the textural properties and dispersibility, the mesoporous Ta2O5 submicrospheres were obtained by calcined with different R. Fig. 2 shows the XRD patterns of the as-prepared Ta2O5 samples. Obviously, all the characteristic diffraction peaks at 2h = 22.8°,
R (oC/min)
SBET (m2/g)
Dp (nm)
Vp (cm3/g)
Dg (nm)
2 5 10
8.11 11.29 8.73
19.41 15.84 15.58
0.039 0.045 0.034
26.54 24.94 22.48
28.2°, 28.7°, 36.6°, 46.7° and 55.6° correspond to the d spacings of 3.89, 3.16, 3.10, 2.45, 1.95 and 1.65 Å. They can be assigned to (0 1 0), (4 1 1), (0 0 2), (1 1 0 2), (0 2 0), (0 2 2) of the orthorhombic Ta2O5, respectively (JCPDS 79-1375). The intensities of the diffraction peaks for the samples obtained at R = 5 °C/min become relatively strong, indicating a better crystallinity of the samples. The average crystallite sizes (Dg) of the as-prepared Ta2O5 samples have been measured by the Scherrer equation and listed in Table 1. It is found that with increasing R from 2 °C/min to 10 °C/min, Dg decreased gradually. This is because the high heating rate may bring about a relatively large temperature gradient within the submicrospheres, resulting in the formation of solid/liquid biphase core–shell structures during the non-equilibrium heating process [32] As the heterogeneous nucleation only requires a low activation energy, the co-existence of heterogeneous phases is helpful for the crystalline nucleation of Ta2O5. It is imaginable that the more the generated nuclei are, the smaller the as-obtained crystallite size is. Additionally, the rapid heating rate also causes a sharp decomposition of the residual organic groups in the obtained colloidal precursors, suddenly releasing vast gases. As a result, the crystallite growth is inhibited around the pores. Both of the two reasons may contribute to the deceased Dg with increasing R. The morphology and mesoporosity of the Ta2O5 products were characterized by SEM, TEM and N2 adsorption–desorption analysis. Fig. 3a–c manifest that after the spherical colloid precursors were thermally treated at 800 °C for 2 h with different R, all of the resultant particles almost maintained the initial spherical morphology.
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Fig. 3. SEM images of the mesoporous Ta2O5 submicrospheres calcined at various R of (a) 2 °C/min, (b) 5 °C/min, (c) 10 °C/min and (d) TEM image of the mesoporous Ta2O5 submicrospheres calcined at R = 5 °C/min.
Fig. 4. Nitrogen adsorption–desorption isotherms of the Ta2O5 submicrospheres calcined at different R. The inset shows the corresponding pore size distribution profiles.
But the surfaces became rough and the diameter decreased to 250 nm. It is obvious that the resultant spheres obtained at R 6 5 °C/min are well-dispersed, while those obtained at R of 10 °C/min tend to break up and adhere to decrease the dispersibility of the samples. This may be explained by the assumption that the latter have a lower melting point because of the decreased size of the Ta2O5 nanocrystallites on the surfaces. They easily fuse together via the adhesion to contiguous submicrospheres when treated at the elevated temperature. Thus, the dispersibility of the samples decreases. Fig. 3d obviously shows that the resultant submicrospheres obtained at R of 5 °C/min exhibit a unique porous structure, which favors the reactants adsorption and the light scattering. Fig. 4 shows the N2 adsorption–desorption isotherms and the corresponding pore size distribution curves of the as-prepared samples. It can be seen that all the as-prepared samples represent type IV isotherms with a pronounced H3 type hysteresis loop,
Fig. 5. UV–Vis DR spectra for the mesoporous Ta2O5 submicrospheres calcined at different R; the inset shows the plots of (ahm)1/2 versus hm.
indicating the characteristic of mesoporous materials with assemblages of slit-shaped pores [33]. This is in good agreement with the result of the TEM image. But the samples have varied average pore diameters and distributions with different R. Compared with the samples obtained at R of 2 °C/min and 10 °C/min, the sample calcined at an R of 5 °C/min shows the narrowest size distribution as well as the largest BET specific surface area (SBET) (Table 1). The sample calcined at R = 10 °C/min has a decreased SBET, but the smallest Dg. This further confirms the occurrence of the surface adhesion of the nanocrystallites caused by the decreased melting point as well as a mass of crystallization heat. This reasonably suggests that R not only influences the morphology of the as-prepared Ta2O5 submicrospheres, but also influences the textural characteristics. The optical properties of the as-prepared mesoporous Ta2O5 submicrospheres were characterized by UV–Vis DRS. Fig. 5 reveals
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that a significant absorption edge at 320–330 nm is assigned to the intrinsic absorption of Ta2O5. Compared with C–Ta2O5, the band edges of the as-prepared mesoporous Ta2O5 submicrospheres exhibited obvious blueshifts. As shown in the inset of Fig. 5, the band gaps of all the as-prepared samples can be estimated according to the Kubelka–Munk method [34]. They are 3.86 eV, 3.87 eV and 3.85 eV, respectively. All are bigger than that of C–Ta2O5 (3.81 eV), suggesting that the as-prepared mesoporous Ta2O5 submicrospheres have an increased band gap, which endows them with an enhanced oxidizing and reducing ability of the photogenerated charges. This is mostly ascribed to their decreased particle size. In addition, the sample calcined at an R of 5 °C/min showed a slightly increased light absorption compared with the other calcined samples. This may be related to the textural characteristics of the samples. Photocatalytic activities for hydrogen production of the as-prepared mesoporous Ta2O5 submicrosphere samples were evaluated in 20 vol.% methanol aqueous solution under the irradiation of simulant sunlight in the absence of any cocatalysts. For comparison, the activity of C–Ta2O5 was also tested at the same conditions. From Fig. 6a, it can be observed that all the mesoporous submicrospheres calcined at different R show much higher photocatalytic activities than C–Ta2O5. The mesoporous Ta2O5 submicrospheres calcined at R = 5 °C/min show the highest photocatalytic activity of 11.2 lmol/h0.02 g, 4 times larger than that of C–Ta2O5. In contrast, the amorphous mesoporous Ta2O5 loaded by 4% NiO only exhibits the hydrogen evolution efficiency of 150 lmol/h0.5 g under a high pressure Hg lamp irradiation [25]. The photocatalytic
activity of the Ta2O5 nanowires and the hierarchical Ta2O5 singlecrystalline nanorods in the absence of cocatalysts only achieved 214 lmol/h g [14] and 200 lmol/h g [16], respectively. Additionally, the photocatalytic overall water-splitting experiments were also conducted over the as-prepared mesoporous Ta2O5 submicrospheres and C–Ta2O5. Fig. 6b shows that the photocatalytic activity for overall water-splitting was 0.24 lmol/h0.02 g,. This value is 2 times higher than that of the C–Ta2O5 and 1.5 times higher than that of hierarchical Ta2O5 single-crystalline nanorods [14]. In order to enhance the photocatalytic activities of photocatalysts by promoting the separation of photoinduced electrons and holes, platinum (Pt) nanoparticles were usually photodeposited on the photocatalyst surface and acted as active sites for H2 production. As shown in Fig. 7, the photocatalytic hydrogen production activity achieves an optimal value when 0.5% Pt were loaded onto the surfaces of the as-prepared mesoporous Ta2O5 submicrospheres. It indicated that the content of Pt loadings was directly related to the surface characters of the photocatalytic materials [35]. At higher Pt loading (>0.5 wt%), the excessive Pt particles may aggregate and cover active sites on the mesoporous Ta2O5 submicrospheres surface, which decreases the hydrogen production efficiency. Except for photocatalytic hydrogen production, the as-prepared mesoporous Ta2O5 submicrospheres can also be used for the decolorization of dyes such as MB and RhB. Fig. 8 showed the photocatalytic decolorization of RhB and MB by the as-prepared mesoporous Ta2O5 submicrospheres and C–Ta2O5, respectively. It can be observed that the decolorization of the MB and RhB solutions in the absence of any photocatalyst occurs very slowly under the simulant light irradiations. Whereas, the photodecolorization efficiency of the MB and RhB solutions over the as-prepared mesoporous Ta2O5 submicrospheres achieves 96.9% and 93.5% after 80 min and 140 min, respectively. both of them were 3 times faster than those of C–Ta2O5. The above results indicate that the as-prepared well-dispersed mesoporous Ta2O5 submicrospheres are remarkably advantageous for hydrogen evolution and environment remediation, and will be a promising candidate of photocatalysts. The enhanced photocatalytic activity of the as-prepared mesoporous Ta2O5 submicrospheres can be reasonably attributed to the following factors. The even particle size and good dispersibility make the as-prepared submicrospheres suspend stably in the reaction solution. This can harvest more lights as well as provide adequate access to the reactant. Additionally, the increased crystalline compared to the amorphous mesoporous Ta2O5 should greatly decrease the inner defects of the submicrospheres, resulting in the rapid transfer
Fig. 6. Photocatalytic hydrogen production activities of the mesoporous Ta2O5 submicrospheres calcined at different R and C–Ta2O5 from 20 vol.% methanol aqueous solution (a) and pure water (b) under the simulant solar light irradiation.
Fig. 7. Influences of the loaded Pt contents on the photocatalytic hydrogen evolution activities of the mesoporous Ta2O5 submicrospheres calcined at R = 5 °C/min under the simulant solar light irradiation.
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Acknowledgments This work financially was supported by the National Natural Science Foundation of China (51002111), the Fundamental Research Funds for the Central Universities (2012-IV-085), the Natural Science Foundation of Hubei Province (2010CDA030), and the Subject Leadership Project of Wuhan City (201150530145).
References
Fig. 8. Photocatalytic decolorization of the MB (a) and RhB (b) aqueous solutions in the presence/absence of photocatalysts. The inset pictures indicate that the color of the aqueous solution containing the mesoporous Ta2O5 submicrosphere photocatalysts fades away swiftly with the prolonged irradiation time of the simulant solar light. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
and utilization of photogenerated charge carriers. The unique mesoporosity and thin pore walls can also decrease the transfer distance of the photogenerated charge carriers and effectively restrain their recombination. Consequently, the photocatalytic efficiencies for both hydrogen production and dye decolorization are enhanced. 4. Conclusions Well-dispersed mesoporous Ta2O5 submicrospheres were successfully prepared by a two-step approach involving the hydrolysis of tantalum glycolate in the mixture of acetone and water and the subsequent calcination at a certain heating rate (R). The dispersibility, crystallinity, textural characteristics and the optical property as well as the photocatalytic activity for hydrogen evolution were investigated as functions of R. The sample calcined at R of 5 °C/min exhibits the highest photocatalytic activity, which is 4 times larger than that of C–Ta2O5 and also superior to that of the previously reported Ta2O5 nanostructures. Simultaneously, it also shows efficient photocatalytic activities for RhB and MB decolorization due to the outstanding dispersibility and mesoporosity. The results suggest that the structural characteristic of photocatalysts can be meticulously controlled via tuning kinetic parameters such as R at calcination, which provides a facile alternative to obtain highly efficient photocatalysts. The well-dispersed mesoporous Ta2O5 submicrospheres prepared here have potential applications not only in hydrogen production, but also in environment remediation.
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