Article pubs.acs.org/Langmuir
Design of Porous Silica Supported Tantalum Oxide Hollow Spheres Showing Enhanced Photocatalytic Activity Manu Sharma,† Debashree Das,† Arabinda Baruah,† Archana Jain,† and Ashok K. Ganguli*,†,‡ †
Department of Chemistry, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi-110016, India Institute of Nano Science & Technology, Habitat Centre, Phase-X, Sector-64, Mohali, Punjab-160062, India
‡
S Supporting Information *
ABSTRACT: Silica-supported tantalum oxide (ST) hollow spheres were designed for photocatalytic applications in the UV range of 4.1 to 4.8 eV. These nanostructures with a variable diameter of 100−250 nm and shell thickness of 24−58 nm were obtained by the hydrothermal treatment of tantalum isopropoxide and tetraethylorthosilicate at 120 °C for 48 h in the presence of cetyl trimethyl ammonium bromide, which was used as a capping agent. The maximum observed surface area was found to be 610 m2/g and pore size distribution of ST hollow spheres varied from 13.4 to 19.0 nm. Lewis acidity of silica and the contact area between SiO2 and Ta2O5 plays a crucial role in controlling the photocatalytic properties of the ST hollow spheres. We observe a remarkable 6× enhancement in the photoactivity of silica-supported tantalum oxide hollow spheres compared to pure Ta2O5.
1. INTRODUCTION Semiconductor photocatalysts have been considered for various applications like destruction of microorganisms such as bacteria and virus, inactivation of cancer cells, odor control, nitrogen fixation, hydrogen production, organic synthesis, water, and air purification.1−6 Semiconductor nanoparticles (e.g., TiO2, Ta2O5, ZnO, Fe2O3, CdS, and ZnS, etc.) have also been used as sensitizers for light-induced redox reactions.7−9 These redox processes have been possible due to the change in the electronic structure of the semiconductor nanoparticles within a filled valence band and an empty conduction band. Semiconductors exhibit good photoactivity in the UV and visible region due to the generation of a reductant (a conduction band electron) and an oxidant (a valence band hole). These charges separate and move about to the surface of the photocatalyst, which is affected by crystal structure, crystallinity, and particle size of the photocatalyst. Efficient charge migration can be achieved for crystalline catalysts with less number of defects.10−12 Among the most challenging problem related to photocatalysis is to mimic the water splitting reactions possible in a green leaf. The first reported photocatalyst for water splitting is TiO2, which works under UV irradiation.13 Ta2O5 is also an important semiconductor photocatalyst (band gap of 4.1 eV), which is able to decompose pure water into H2 and O2 with RuO2 or NiOx as cocatalysts, (pure Ta2O5 alone produces only trace amounts of H2 and no O2).14 The advantage of Ta2O5 as a photocatalyst is its large band gap which imparts the material with a range of tunability compared to that found in other transition metal oxide photocatalysts. Studies on the improvement of the photocatalytic activity of mesoporous Ta2 O5 have been possible by loading with NiO and pretreatment.15,16 © 2014 American Chemical Society
Silica particles have been of interest due to their permeable shells, which is helpful in drug delivery, as catalyst support, or as heat and sound insulation materials.17 They have been used in doxorubicin delivery, tumor therapy, and ibuprofen storage.18,19 Mesoporous silica particles are also attractive because of their low toxicity, large surface area, pore volume, and excellent chemical stability, which provide facile adsorption as well as loading capacity of therapeutic chemicals.20−23 Due to their high surface area, these mesoporous silica nanoparticles show good performance as a catalyst and mechanical support. These particles can be dispersed in solvents and the active sites of the material are highly accessible. Various inorganic nanoparticles such as Au, CdSe, and Fe3O4 decorated on silica surface have been in demand for the controlled release of encapsulated drugs.24−27 Since rough surfaces of silica particles provide a larger surface area than do smooth surfaces,28 silica-decorated nanostructures show better optical, magnetic, and biological properties compared with other functional materials (e.g. quantum dots, carbon nanotubes, plasmonic nanoparticles, and magnetic nanoparticles). The silica structures combined with other materials have applications in the field of optics, magnetic, electronics, and biology.29−36 In the present study, hollow spheres of silica-supported tantalum oxide have been successfully synthesized using an optimized hydrothermal method (in the presence of the surfactant) at low temperature. The main aim of the synthesis of such supported oxide is to design a highly efficient Received: December 20, 2012 Revised: February 27, 2014 Published: March 3, 2014 3199
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Nitrogen adsorption−desorption isotherms were recorded at liquid nitrogen temperature (77 K) using a Nova 2000e (Quantachrome Corp.) equipment, and the specific area was determined by the Brunauer−Emmett−Teller (BET) method. The as-prepared ST samples were degassed at 150 °C for 12 h prior to the surface area measurements. Conductivity measurement of CTAB and the water− ethanol system was carried out using a ELICO CM 183 EC-TDS Analyzer. Steady-state fluorescence measurements have been carried out using a Horiba FluoroMax-4.
photocatalyst. We have designed the nano structures to combine the efficient adsorption characteristics of mesoporous silica with the photocatalytic properties of Ta2O5. The design of the nanostructures enables a good interface between the SiO2 and Ta2O5, which also leads to close proximity of the adsorbed dye and the active site of Ta2O5 on the silica surface.
2. EXPERIMENTAL SECTION 2.1. Materials. All chemicals used were of analytical grade and were used without any further purification. Tantalum isopropoxide and tetraethylorthosilicate (TEOS) (Alfa Aesar), cetyl trimethyl ammonium bromide (CTAB) (Spectrochem Laboratories), ethanol (Merck KGaA), and ammonia solution (Fisher Scientific) were used. The solutions were made in deionized water. 2.2. Synthesis of Silica-Supported Tantalum Oxide SiO2− Ta2O5 (ST) Hollow Spheres. The silica supported tantalum oxide SiO2−Ta2O5 (ST) hollow spheres having different molar ratios (1:0.1, 1:0.85, and 1:1.7) of tetraethylorthosilicate and tantalum isopropoxide have been synthesized by the hydrolysis of tantalum isopropoxide and tetraethylorthosilicate, using the hydrothermal method. Cetyl trimethyl ammonium bromide (0.50 g) and tetraethylorthosilicate [0.25 mL (1 M)] were dissolved in 7 mL of water and 6.3 mL of ethanol solution. The reaction mixture was stirred for 30 min at room temperature and a different volume of tantalum isopropoxide 0.25, 4.5, and 9 mL (molar ratios 0.1, 0.85, and 1.7) were added and stirred for 10 min. Next, an ammonia solution (0.5 mL of 25%) was added dropwise and stirred for 15 min. The mixture was then loaded in a Teflon vessel kept in a hydrothermal bomb and heated at 120 °C for 48 h. The contents were centrifuged, washed with water followed by ethanol, and dried at 70 °C. All the precursors were heated at 550 °C for 5 h to remove the CTAB surfactant. Unsupported silica was synthesized by a known method.37 Nomenclature, concentration, and reaction conditions of different silica supported tantalum oxide composites are summarized in Table 1.
3. RESULTS AND DISCUSSION The powder X-ray pattern of silica-supported tantalum oxide (ST) composites show an amorphous nature after hydrothermal treatment. The calcined samples (at 550 °C) also show a poor crystalline nature (Figure S1 of the Supporting Information). The samples become crystalline when calcined at 850 °C and show peaks corresponding to the orthorhombic phase of Ta2O5 (Figure S2 of the Supporting Information). The assignment of the infrared bands has been shown in Table S1 of the Supporting Information. The presence of SiO2 was indicated by the weak band at ∼1630−1640 (−OH bending) and a Si−O−Si asymmetric stretching band from ∼1077 to 1090 cm−1. The symmetric stretching band due to Si−O−Si is observed at 805 and 675 cm−1. The presence of tantalum and binding of Ta with silica surface (Si−O−Ta) can be attributed due to the band observed from ∼950 to 968 cm−1. The band observed from ∼3420 to 3439 cm−1 is due to −OH stretching. TEM images of ST hollow spheres with 1:0.1 molar ratios of tetraethylorthosilicate and tantalum isopropoxide in Figure 1 show the porous nature of the synthesized material. TEM-EDX results confirm the presence of Ta and Si (Figure S3 of the Supporting Information). It is observed that at the molar ratio of 1:0.85, we obtain hollow spheres (Figures 2 and 4) of ST. The average shell thickness was found to be ∼28 nm (Figure 2, panels a and b). On calcination, the shell thickness increased to ∼50 nm (Figure 2, panels c and d). The diameter of ST particles observed from the TEM micrographs was ∼100 to 250 nm. Figure 3 (panels a−d) shows the TEM images of ST hollow spheres (using 1:1.7 molar ratio of SiO2:Ta2O5), where we observe the increase in shell thickness with the increase in the molar ratio. The thickness of the particles was found to be ∼40 nm (Figure 3, panels a and b). On calcination, the shell thickness further increased up to ∼60 nm (Figure 3, panels c and d). This may be due to grain-size enhancement on calcinations.38,39 From TEM studies, we could see an increase in the shell roughness along with the enhancement of shell thickness after calcinations; the most plausible cause of that could be the evaporation of leftover aqueous or organic part, causing grain rearrangement and voids in the shell surface. From point EDX studies (Figure S4 of the Supporting Information), both Si and Ta could be observed at the center and the periphery. The FE-SEM images of SiO2−Ta2O5 hollow spheres (ST) have been shown in Figure 4, which shows the spherical type morphology. The images of the broken spheres also confirm that they are hollow in nature. The calcined (550 °C) silica-supported tantalum oxide hollow spheres show the average particle size of ∼200 nm. The optical properties and the band gap measurements of the ST hollow spheres have been studied using diffuse reflectance spectroscopy (DRS) in the range from 200 to 800 nm. The resulting reflectance information was converted into band gap by using the Kubelka−Munk equation.40 DRS spectra
Table 1. Nomenclature, Concentration, And Reaction Conditions of Different Silica-Supported Tantalum Oxide Composites sample concentration
nomenclature
reaction condition
Si:Ta; 0:0.1 Si:Ta; 1:0.1
T ST1
Si:Ta; 1:0.1 Si:Ta; 1:0.85
ST2 ST3
Si:Ta; 1:0.85 Si:Ta; 1:1.7
ST4 ST5
Si:Ta; 1:1.7
ST6
calcined, 550 °C, 5 h precalcined, hydrothermal heated at 120 °C, 48 h calcined, 550 °C, 5 h precalcined hydrothermal heated at 120 °C, 48 h calcined, 550 °C, 5 h precalcined, hydrothermal heated at 120 °C, 48 h calcined, 550 °C, 5 h
2.3. Characterization. Powder X-ray diffraction (XRD) studies were carried out on a Bruker D8-Advance powder X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). FT-IR spectroscopy studies (using KBr) were carried out using a Nicolet Protege 460 Fourier transform infrared (FT-IR) spectrometer in the range of 400−4000 cm−1. The transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDAX) studied were carried out using FEI Technai G2 20 electron microscope operating at an accelerating voltage of 200 kV. The field emission scanning electron microscopy (FE-SEM) studies were carried out on gold-coated disks using FEI Quanta 3D FEG/FESEM at an accelerating voltage of 20 kV. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) was carried out on an Ultima-2 Jobin Yvon (HORIBA) spectrometer. For recording the diffuse reflectance spectra of the samples, a UV− visible spectrophotometer Shimadzu UV-2450, was used in the wavelength range of 200−800 nm with barium sulfate as the reference. 3200
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Figure 1. TEM images of ST using 1:0.1 molar ratios of tetraethylortho-silicate and tantalum isopropoxide. (a and b) Hydrothermal-treated (precalcined) material and (c and d) calcined at 550 °C for 5 h.
Figure 2. TEM images of (c and d) ST using 1:0.85 molar ratios of tetraethylortho silicate and tantalum isopropoxide. (a and b) Hydrothermal treated (precalcined) and calcined at 550 °C for 5 h. 3201
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Figure 3. TEM images of ST using (c and d) 1:1.7 molar ratios of tetraethylortho-silicate and tantalum isopropoxide and (a and b) hydrothermaltreated (precalcined) and calcined at 550 °C for 5 h.
Figure 4. FESEM images of ST using 1:0.85 molar ratios of tetraethylortho-silicate and tantalum isopropoxide hydrothermal-treated (a) ST3 (precalcined), (b) ST5 (precalcined), (c) ST4 (calcined), and (d) ST6 (calcined) at 550 °C for 5 h.
of hydrothermally treated (precalcined) ST hollow spheres (Figure S5 of the Supporting Information) shows that the
absorption ranges from 262 to 258 nm and corresponds to a band gap of 4.73−4.87 eV, with an increase in concentration of 3202
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Figure 5. Schematic diagram and mechanism for the formation of SiO2−Ta2O5 (ST) hollow spheres.
from 225 to 610 m2/g. The enhancement in surface area after calcination is due to the removal of surfactant from the pores of the sample. The high surface area of the ST with molar ratio (1:0.1) is due to the high content of silica. With calcination, the surface of hollow spheres becomes rougher providing more surface contact (more interfacial area) with supported Ta2O5, which plays a crucial role in deciding the photoactivity of the materials. The pore size varied from 13.4 to 19.0 nm, indicating a mesoporous nature of the material. The relative atomic concentrations of Si and Ta in the ST samples were analyzed using ICP-AES. The ratios of atomic concentrations of Si and Ta in the samples ST3 and ST5 were calculated from the ICP-AES data. These results have been compared to the theoretical values which are within 87% of the theoretical values (Table S3 of the Supporting Information). We now try to understand the mechanism for the formation of hollow spheres of the silica-supported Ta2O5. In a previous study, it has been reported that the solvent polarity plays an important role to create the hollow structure of particles.43 When CTAB was added to the ethanol−water system, CTA+ species form self-assembled micellar structures in solution.44,45 This micellar aggregation of the CTAB formed at 100 mM in the water−ethanol mixture, which was confirmed by a conductivity study and steady-state fluorescence measurement (detailed analysis of fluorescence and conductivity measurement is in Figures S9 and S10 and Table S4 of the Supporting Information). As soon as TEOS is added to the water− ethanol−CTAB mixture, it undergoes hydrolysis and condensation reactions, which proceed on the surface of CTAB self-assembled structures. TEOS in water leads to anionic species and readily gets adsorbed on the surface of the positively charged CTAB self-assembly. When tantalum isopropoxide was added to the reaction mixture, it immediately hydrolyzes and condenses on the silica surface. Finally, upon the addition of ammonia to the reaction mixture, silica gets etched out and CTAB gets involved in dissolution and redeposition of the etched out silica particles on the surface
tantalum. There is an increase in band gap of Ta2O5 in the SiO2-supported Ta2O5 hollow spheres as compared to pure tantalum oxide nanoparticles. Band gap of bulk tantalum pentoxide is known to be 4.1 eV.41 DRS spectra of calcined ST hollow spheres show (Figure S6 of the Supporting Information) lower band gaps (4.53 to 4.60 eV) with increase in tantalum concentration. The increase in particle size on calcination leads to decrease in the band gap. We observe a blue shift of the absorption edge of the as-obtained samples toward shorter wavelengths with an increase in the concentration of tantalum, indicating an increase in the band gap of silicasupported Ta2O5 in comparison with the unsupported oxide. The enhancement in the band gap of tantalum oxide nanoparticles has been explained due to silica as a support and size confinement effect of tantalum oxide nanoparticles. The Lewis acidity of the silica provides more acidic sites on the silica surface, which interact with the metal oxide electrons and are available to bind with the tantalum oxide nanoparticles. Silica-supported Ta2O5 leads to change in the binding energies for tantalum. This change in binding energies results in an increase in Ta orbital energy, thereby increasing the band gap and, hence, we observe a blue shift.42 The larger band gap of Ta2O5 in comparison to other transition metal oxide photocatalysts allows a range of band gap modification and yields more reactive species in aqueous media. In literature, it was reported that the valence band holes are powerful oxidants [+1.0 to +3.5 V with respect to the natural hydrogen electrode (NHE)] while the conduction band electrons are good reductants (+ 0.5 to −1.5 V). The conduction band of Ta2O5 is more negative than other transition metal oxides and hence is capable of reducing several of compounds. The surface area, pore size, and pore volume of the silicasupported tantalum oxide hollow spheres have been shown in Table S2 of the Supporting Information and Figure 8. The specific surface area of the synthesized ST was found to be between 84 and 89 m2/g, with the increase in the concentration of Ta for the hydrothermally treated materials. For the calcined ST samples, the surface area has increased and was in the range 3203
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Figure 6. TEM images of ST using different concentrations of ammonia (a) without ammonia, (b) 0.25 mL ammonia, and (c) 0.50 mL ammonia.
Figure 7. TEM images of ST using different concentrations of CTAB: (a) no CTAB, (b) 0.10 g CTAB, (c) 0.25 g CTAB, and (d) 0.50 g CTAB.
amount of CTAB was gradually increased (0.10, 0.25, and 0.50 g) more and more well-shaped hollow-sphere-type morphology evolved. This is due to the micellar aggregation of CTAB in the water−ethanol mixture at around 100 mM (0.5g CTAB), as discussed in the Supporting Information. From the TEM images, the particle size was found to be quite smaller than the silica particles formed in presence of CTAB. People have already reported and investigated the role of CTAB in the mechanism of hollow structure formation.47 From that study, it appeared that CTAB plays a crucial role in designing the hollow-sphere-type morphology by dissolving the silica (etched out from the core by ammonia) and redepositing it on the periphery of the spheres. Therefore the optimum concentration of CTAB in the reaction system is also an important factor in the formation of the precise shell structure, which is also evident from the TEM images. Role of Tantalum Isopropoxide. Concentration of tantalum isopropoxide is also an important factor for the
of tantalum pentaoxide, thereby forming a uniform shell consisting of both silica and tantalum pentaoxide (Figure 5). Role of NH3. In order to comprehend the role of ammonia in the formation of hollow spheres, a series of experiments has been carried out. In the absence of ammonia, no hollow structures were formed, but when 0.5 mL of ammonia was used, only a few of the particles were found to be hollow spheres (Figure 6). When the amount of ammonia is increased from 0.5 to 1 mL, the hollow spheres were formed. These results suggest that ammonia is necessary for the etching of silica from the core. This role of ammonia has also been discussed in literature.46 Role of CTAB. So as to understand the role of CTAB in the formation of the hollow spheres, a series of experiments were carried out by changing the CTAB concentration, while keeping all the other reaction parameters the same. From the TEM images, (Figure 7) it was observed that in the absence of CTAB, no spherical structure was formed. However, when the 3204
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Figure 8. TEM images of ST using different concentrations of tantalum isopropoxide: (a) tantalum isopropoxide (0.53 mL) and (b) tantalum isopropoxide (9 mL).
rate constants are listed (Table 2). With lowest concentration of Ta, the sample is more porous due to higher content of silica.
formation of hollow spherical structures. The etched out silica particles from the center get redeposited on the tantalum oxide surface which act as a support and leads to the formation of the shell, which is also evident from the TEM images obtained by varying the tantalum isopropoxide concentration. At very low concentration of tantalum isopropoxide, no hollow sphere was observed (Figure 8). Therefore, it is concluded that adjusting the rate of hydrolysis and optimizing the appropriate molar ratios of TEOS and tantalum isoporopoxide helps to construct the hollow spherical architecture of the synthesized nanoparticles. When the concentration of tantalum isoproxide is varied, the shell thickness could be modified. There have been other reports where shell thickness of silica nanoparticles is modified with varying concentration of TEOS.43,48−50 An important aspect of silica nanoparticles is their surface chemistry, which plays a specific role in the binding of metal oxide particles on the surface of silica. FT-IR studies have suggested that the surface properties and Lewis acidity of silica leads to its ability to bind with metal oxide nanoparticles. The Lewis acidity of silica at pH ∼ 9 to 10 has been more favorable to the formation of silica-supported metal oxide nanoparticles.51 3.1. Photocatalytic Activity of Silica-Supported Tantalum Oxide. The photocatalytic activity of the silicasupported Ta2O5 samples were evaluated by the degradation of Rhodamine B dye under UV irradiation. The photocatalytic reactions were carried out in a quartz reactor with a watercirculating jacket. Experiments were also performed in the absence of photocatalyst and illumination (blank tests). For the photocatalytic degradation of Rhodamine B, 50 mg of the asprepared catalyst was added to 200 mL of 12 μM Rhodamine B (pH = ∼8.9). Before illumination, the dye solution with the suspended catalyst was stirred for half an hour to ensure the adsorption/desorption equilibrium during the photocatalytic process. After 30 min of the solution stirring in the dark to attain the adsorption equilibrium, the UV lamp (125 W high pressure Hg lamp) was positioned above the reactor and switched on. The reaction was carried out at room temperature for 2 h. The concentration of Rhodamine B dye aqueous solution was analyzed using a UV−vis spectrophotometer at its maximum absorption wavelength of 546 nm. The photocatalytic dye degradation reaction is pseudo unimolecular. So, the rate constant is calculated from the slope of the graph obtained by plotting ln(Co/C) versus time. The errors on the
Table 2. Rate Constant and t1/2 of ST with Varying Amounts of Tantalum Isopropoxide sample
rate constant (h−1)
% error
t1/2
ST1 ST2 ST3 ST4 ST5 ST6 T
0.049 0.108 0.145 0.023 0.022 0.029 0.026
4% 4% 8% 7% 5% 6% 4%
14.15 6.42 4.78 30.13 31.5 23.90 26.66
Therefore, its adsorption capacity is very high, which leads to greater percentage of degradation. With increasing Ta concentration, adsorption efficiency decreases, so the net degradation also decreases. After calcination, the grain size of the particles increases leading to a decrease in the band gap, which affects the photoactivity of the particles. Probable reason for decrease in photoactivity with calcination is the decrease in the contact area between silica and tantalum oxide as a result of grain size enlargement (which causes a decrease in the surface area of the particles). Reasons for the degradation of catalyst with time: (a) photobleaching of the material (TEM image of ST3) and (b) adsorption of dye molecules on the catalyst surface, leading to a decrease in active sites and surface area. Figure 9 displays the photodegradation of rhodamine B over the silica-supported Ta2O5 under UV irradiation. A blank experiment in the absence of photocatalyst under UV irradiation showed that photolysis of rhodamine B was negligible. The rate constant and t1/2 of the photocatalytic reaction using ST with different concentrations of silica and tantalum have been shown in Table S3 of the Supporting Information. From the rate constant values, it is observed that all the synthesized ST shows higher photoefficiency compared to pure tantalum oxide. The best result is obtained at the composition with molar ratio of 1:0.85 (ST3) with a rate constant of 0.145 h−1 compared to pure Ta2O5 (rate constant 0.026 h−1) (i.e., nearly 6 times). It is observed that at composition with a 0.1 molar ratio of Ta, 64% degradation is achieved in 100 min, whereas the same concentration for the calcined sample shows a degradation of ∼78% in a time period of 20 min. Note that this composition has maximum silica 3205
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Figure 10. Cycling studies of photodecomposition of ST using the 1:0.85 molar ratio of tetraethyl-orthosilicate and tantalum isopropoxide under UV light in 100 min.
Figure 9. Photocatalytic degradation of rhodamine B at 546 nm as a function of irradiation time performed on hydrothermally treated (precalcined) and calcined ST, with varying a molar ratio of tetraethylorthosilicate and tantalum isopropoxide. (Inset shows bar diagram of ST samples).
We have compared the photocatalytic activity of our material with state-of-the-art photocatalysts and found that in terms of efficiency of photocatalytic degradation, although there are some reports on materials having better efficiency than ours,53,54 while our material is better than several other recently studied materials.55,56 Another key point is that our method of synthesis and choice of material (Ta2O5) is different compared to the normal studies on ZnO, TiO2, CeO2, or CdS.
content. Here, two processes, adsorption (36% by SiO2) and photodegradation (by Ta2O5) of Rhodamine B dye, proceed simultaneously. The precalcined sample shows photodegradation was mainly contributed by the photoactivity of Ta2O5. After calcination due to enlargement of the pore size of the particles, the dye adsorption by the catalyst increases. The percentage degradation corresponding to the catalyst with the molar concentration of 0.85 was observed to be 68%, and after calcination, the percentage decreased to 55%. When the Ta ratio was increased to 1.7 M, the degradation decreased to 38%, which was further reduced to 27% after calcination. A comparative diagram of the percentage degradation of the various synthesized materials is presented in the inset of Figure 9. It is observed that with an increase in the concentration of tantalum, we do not observe a continuous increase in the photoactivity of the synthesized sample. Therefore, an optimal concentration of tantalum isopropoxide is to be maintained to achieve maximum photodegradation. The enhanced degradation of the silica-supported Ta2O5 compared to bare Ta2O5 nanoparticles is due to the Lewis acid nature of the supported silica, which displaces the conduction band to more negative values and the valence band to more positive values on the electrochemical scale of potentials. This leads to an increase in the concentration of photoreactive species in the media by preventing the recombination of the reactive species leading to the photodegradation of the organic dye. The increased adsorption capacity of porous hollow spheres is attributed to the highly porous structure, hollow geometry, large surface area, and electrostatic attraction between the silica surface and dye. The hollow spheres provide more surface contact and space to absorb the molecule to make the arrival of the excited electron easier at the hollow surface compared to the smooth and solid surface.52 The reusability of the photocatalyst (Figure 10) material has been ascertained for five consecutive cycles. The TEM image (Figure S7 of the Supporting Information) and infrared bands (Table S1 of the Supporting Information) assignment of the ST samples also show the similar peak after 5 cycles which support the theory that the material is stable and reusable after photocatalysis.
4. CONCLUSIONS Highly efficient photocatalysts based on silica-supported tantalum oxide (ST) hollow spheres were synthesized by a simple hydrothermal method. The optical band gap of these ST nanoparticles was tuned from 4.1 to 4.8 eV by varying the SiO2:Ta2O5 content. TEM and FESEM study shows the formation of hollow spherical morphology for the different shell thicknesses of synthesized silica-supported oxides. This is the first report on hollow spherical type morphology of silicasupported Ta2O5. These silica-tantalum oxide nanospheres exhibit strong photocatalytic activity toward the degradation of rhodamine B under UV light irradiation with an enhancement of 6× the activity compared to that of pure Ta2O5. These results suggested that silica-supported tantalum oxide is of significance for the photo oxidation of organics and can be useful for organic waste remediation.
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ASSOCIATED CONTENT
S Supporting Information *
XRD and PXRD patterns of ST hollow sphere, EDX data of ST, point EDX image profile of ST, diffuse reflectance studies, TEM image of ST3, nitrogen adsorption−desorption isotherm of ST4, fluorescence spectra of pyrene, conductivity measurement of CTAB, FT-IR band assignment of ST hollow sphere, ICP-AES results of ST, surface area measurements of ST, and emission intensity ratio of pyrene. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: +91 11 2659 1511. Fax: +91 11 2658 1102. 3206
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Notes
(18) Barbe, C.; Bartlett, J.; Kong, L.; Finnie, K.; Lin, H. Q.; Larkin, M.; Calleja, S. Silica Particles: A Novel Drug-Delivery System. Adv. Mater. 2004, 16, 1959−1966. (19) Zhu, Y. F.; Shi, J. L.; Li, Y. S.; Chen, H. R.; Shen, W. H.; Dong, X. P. Storage and Release of Ibuprofen Drug Molecules in Hollow Mesoporous Silica Spheres with Modified Pore Surface. Microporous Mesoporous Mater. 2005, 85, 75−81. (20) Piao, Y.; Burns, A.; Kim, J.; Wiesner, U.; Hyeon, T. Designed Fabrication of Silica-Based Nanostructured Particle Systems for Nanomedicine Applications. Adv. Funct. Mater. 2008, 18, 3745−3758. (21) Burns, A.; Ow, H.; Wiesner, U. Fluorescent Core-Shell Silica Nanoparticles: Towards ‘‘Lab on a Particle’’ Architectures for Nanobiotechnology. Chem. Soc. Rev. 2006, 35, 1028−1042. (22) Trewyn, B. G.; Slowing, I. I.; Giri, S.; Chen, H. T.; Lin, V. S. Y. Functionalized Mesoporous Silica Nanoaprticles (MSNs) for Applications in Drug Delivery and Catalysis. Acc. Chem. Res. 2007, 40, 846−853. (23) Vallet-Regí, M.; Balas, F.; Arcos, D. Mesoporous Materials for Drug Delivery. Angew. Chem., Int. Ed. 2007, 46, 7548−7558. (24) Liu, R.; Zhang, Y.; Zhao, X.; Agarwal, A.; Mueller, L. J.; Feng, P. pH-responsive Nanogated Ensemble Based on Gold-Capped Mesoporous Silica Through an Acid-Labile Acetal Linker. J. Am. Chem. Soc. 2010, 132, 1500−1501. (25) Zhu, C. L.; Lu, C. H.; Song, X. Y.; Yang, H. H.; Wang, X. R. Bioresponsive Controlled Release using Mesoporous Silica Nanoparticles Capped with Aptamer-Based Molecular Gate. J. Am. Chem. Soc. 2011, 133, 1278−1281. (26) Lai, C. Y.; Trewyn, B. G.; Jeftinija, D. M.; Jeftinija, K.; Xu, S.; Jeftinija, S.; Lin, V. S. A Mesoporous Silica Nanosphere-Based Carrier System with Chemically Removable CdS Nanoparticle Caps for Stimuli-Responsive Controlled Release of Neurotransmitters and Drug Molecules. J. Am. Chem. Soc. 2003, 125, 4451−4459. (27) Giri, S.; Trewyn, B. G.; Stellmaker, M. P.; Lin, V. S. Y. StimuliResponsive Controlled-Release Delivery System Based on Mesoporous Silica Nanorods Capped with Magnetic Nanoparticles. Angew. Chem., Int. Ed. 2005, 44, 5038−5044. (28) Xu, S.; Hartvickson, S.; Zhao, J. X. Increasing Surface Area of Silica Nanoparticles with a Rough Surface. ACS Appl. Mater. Interfaces 2011, 3, 1865−1872. (29) Jin, Y.; Lohstreter, S.; Pierce, D. T.; Parisien, J.; Wu, M.; Hall, C., III; Zhao, J. X. Silica Nanoparticles with Continuously Tunable Sizes: Synthesis and Size Effects on Cellular Contrast Imaging. Chem. Mater. 2008, 20, 4411−4419. (30) Santra, S.; Zhang, P.; Wang, K. M.; Tapec, R.; Tan, W. H. Conjugation of Biomolecules with Luminophore-Doped Silica Nanoparticles for Photostable Biomarkers. Anal. Chem. 2001, 73, 4988− 4993. (31) Zhao, X.; Dytocio, R. T.; Tan, W. Ultrasensitive DNA Detection Using Highly Fluorescent Bioconjugated Nanoparticles. J. Am. Chem. Soc. 2003, 125, 11474−11475. (32) He, X. X.; Wang, K.; Tan, W.; Liu, B.; Lin, X.; He, C.; Li, D.; Huang, S.; Li, J. Bioconjugated Nanoparticles for DNA Protection from Cleavage. J. Am. Chem. Soc. 2003, 125, 7168−7169. (33) Bromley, S. T.; Moreira, I. D. R.; Neyman, K. M.; Illas, F. Approaching Nanoscale Oxides: Models and Theoretical Methods. Chem. Soc. Rev. 2009, 38, 2657−2670. (34) Stromme, M.; Brohede, U.; Atluri, R.; Garcia-Bennett, A. E. Mesoporous Silica-Based Nanomaterials for Drug Delivery: Evaluation of Structural Properties Associated with Release Rate. Wiley Interdiscip. Rev.: Nanomed. Nanobiotechnol. 2009, 1, 140−148. (35) Piao, Y.; Burns, A.; Kim, J.; Wiesner, U.; Hyeon, T. Designed Fabrication of Silica-Based Nanostructured Particle Systems for Nanomedicine Applications. Adv. Funct. Mater. 2008, 18, 3745−3758. (36) Tallury, P.; Payton, K.; Santra, S. Silica-Based Multimodal/ Multifunctional Nanoparticles for Bioimaging and Biosensing Applications. Nanomedicine 2008, 3, 579−592. (37) Ganguly, A.; Ahmad, T.; Ganguli, A. K. Silica Mesostructures: Control of Pore Size and Surface Area Using a Surfactant-Templated Hydrothermal Process. Langmuir 2010, 26, 14901−14908.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS A.K.G. thanks the Department of Science & Technology (Nanomission), Department of Electronics and Information Technology (DeitY), and CSIR, Govt. of India, for financial support. D.D. and A.B. thank CSIR and UGC for fellowships. The authors are also thankful to Dr. J. K. Tripathi and Mr. Sandeep Kumar Gautam, School of Environmental Sciences, Jawaharlal Nehru University, Delhi, for providing the ICP-AES facility.
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REFERENCES
(1) Ritleng, V.; Sirlin, C.; Pfeffer, M. Ru, Rh, and Pd-Catalyzed C-C Bond Formation Involving C-H Activation and Addition on Unsaturated Substrates: Reactions and Mechanistic Aspects. Chem. Rev. 2002, 102, 1731−1769. (2) Jang, H. Y.; Krische, M. J. Catalytic C-C Bond Formation via Capture of Hydrogenation Intermediates. Acc. Chem. Res. 2004, 37, 653−661. (3) Trost, B. M; Toste, F. D.; Pinkerton, A. B. Non-Metathesis Ruthenium-Catalyzed C-C Bond Formation. Chem. Rev. 2001, 101, 2067−2096. (4) Trost, B. M. A Biological Catalysis for Synthetic Efficiency. Pure Appl. Chem. 1992, 64, 315−322. (5) Trost, B. M. Atom Economy-A Challenge for Organic Synthesis: Homogeneous Catalysis Leads the Way. Angew. Chem., Int. Ed. Engl. 1995, 34, 259−281. (6) Sheldon, R. A. Atom Efficiency and Catalysis in Organic Synthesis. Pure Appl. Chem. 2000, 72, 1233−1246. (7) Kisch, H. Semiconductor Photocatalysis for Organic Synthesis. Adv. Photochem. 2001, 26, 93−143. (8) Pelizzetti, E.; Minero, C. Metal Oxides as Photocatalysts for Environmental Detoxification. Comments Inorg. Chem. 1994, 15, 297− 337. (9) Boer, K. W. Survey of Semiconductor Physics; Van Nostrand Reinhold: New York, 1990; Vol. 2, pp 1−249. (10) Hoffman, A. J.; Carraway, E. R.; Hoffmann, M. R. Photocatalytic Production of H2O2 and Organic Peroxides on Quantum-Sized Semiconductor Colloids. Environ. Sci. Technol. 1994, 28, 776−785. (11) Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R. Photocatalytic Production of H2O2 and Organic Peroxides in Aqueous Suspensions of TiO2, ZnO, and Desert Sand. Environ. Sci. Technol. 1988, 22, 798−806. (12) Xing, J.; Fang, W. Q.; Zhao, H. Z.; Yang, H. G. Inorganic Photocatalysts for Overall Water Splitting. Chem.−Asian J. 2012, 7, 642−657. (13) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37−38. (14) Abe, R.; Sayama, K.; Arakawa, H. Significant Effect of Iodide Addition on Water Splitting into H2 and O2 Over Pt-loaded TiO2 Photocatalyst: Suppression of Backward Reaction. Chem. Phys. Lett. 2003, 371, 360−364. (15) Noda, Y.; Lee, B.; Domen, K.; Kondo, J. N. Synthesis of Crystallized Mesoporous Tantalum Oxide and Its Photocatalytic Activity for Overall Water Splitting under Ultraviolet Light Irradiation. Chem. Mater. 2008, 20, 5361−5367. (16) Takahara, Y.; Kondo, J. N.; Takata, T.; Lu, D.; Domen, K. Mesoporous Tantalum Oxide Characterization and Photocatalytic Activity for the Overall Water Decomposition. Chem. Mater. 2001, 13, 1194−1199. (17) Liu, C.; Wang, A.; Yin, H.; Shen, Y.; Jiang, T. Preparation of Nanosized Hollow Silica Spheres from Na 2 SiO 3 using Fe 3 O4 Nanoparticles as Templates. Particuology 2012, 10, 352−358. 3207
dx.doi.org/10.1021/la500167a | Langmuir 2014, 30, 3199−3208
Langmuir
Article
(38) Ingham, B.; Linklater, R.; Kemmitt, T. Grain Growth Kinetics of ZnO:Al Nanocrystalline Powders During Calcination from Sol Gels. J. Phys. Chem. C 2011, 115, 21034−21040. (39) Khorramia, S. A.; Mahmoudzadeh, G.; Madani, S. S.; Gharib, F. Effect of Calcination Temperature on the Particle Sizes of Zinc Ferrite Prepared by a Combination of Sol-Gel Auto Combustion and Ultrasonic Irradiation Techniques. Journal of Ceramic Processing Research 2011, 12, 504−508. (40) Loyalka, S. K; Riggs, C. A. Inverse Problem in Diffuse Reflectance Spectroscopy: Accuracy of the Kubleka-Munk Equations. Appl. Spectrosc. 1995, 49, 1107−1110. (41) Ramprasad, R. First Principles Study of Oxygen Vacancy Defects in Tantalum Pentoxide. J. Appl. Phys. 2003, 94, 5609−5612. (42) Ndiege, N.; Chandrasekharan, R.; Radadia, A. D.; Harris, W.; Mintz, E.; Masel, R. I.; Shannon, M. A. Synthesis, Characterization, and Photoactivity of Ta2O5-Grafted SiO2 Nanoparticles. Chem.Eur. J. 2011, 17, 7685−7693. (43) Wang, Q.; Liu, Y.; Yan, H. Mechanism of a Self-Templating Synthesis of Monodispersed Hollow Silica Nanospheres with Tunable Size and Shell Thickness. Chem. Commun. 2007, 23, 2339−2341. (44) Li, W.; Zhang, M.; Zhang, J.; Han, Y. Self-assembly of Cetyl Trimethylammonium Bromide in Ethanol-Water Mixtures. Front. Chem. Eng. China 2006, 4, 438−442. (45) Wang, M.; Zeng, Q.; Zhao, B.; He, D. Application of Tailored Silica Microspheres in Coatings: Synthesis, Characterization, Thermal and Hydrophobic Properties. J. Mater. Chem. A 2013, 1, 11465− 11472. (46) Chen, Y.; Chen, H.; Guo, L.; He, Q.; Chen, F.; Zhou, J.; Feng, J.; Shi, J. Hollow/Rattle-Type Mesoporous Nanostructures by a Structural Difference-Based Selective Etching Strategy. ACS Nano 2010, 4, 529−539. (47) Fang, X.; Chen, C.; Liu, Z.; Liu, P.; Zheng, N. Cationic Surfactant Assisted Selective Etching Strategy to Hollow Mesoporous Silica Spheres. Nanoscale 2011, 3, 1632−1639. (48) Chen, M.; Wu, L. M.; Zhou, S. X.; You, B. A Method for the Fabrication of Monodisperse Hollow Silica Spheres. Adv. Mater. 2006, 18, 801−806. (49) Darbandi, M.; Thomann, R.; Nann, T. Hollow Silica Nanospheres: In situ, Semi-In situ, and Two-Step Synthesis. Chem. Mater. 2007, 19, 1700−1703. (50) Ge, C.; Zhang, D.; Wang, A.; Yin, H.; Ren, M.; Liu, Y.; Jiang, T.; Yu, L. Synthesis of Porous Hollow Silica Spheres using PolystyreneMethyl Acrylic Acid Latex Template at Different Temperatures. J. Phys. Chem. Solids 2009, 70, 1432−1437. (51) Deng, Z. W.; Chen, M.; Zhou, S. X.; You, B.; Wu, L. A Novel Method for the Fabrication of Monodisperse Hollow Silica Spheres. Langmuir 2006, 22, 6403−6407. (52) Yu, Y.; Zhang, L.; Wang, J.; Yang, Z.; Long, M.; Hu, N.; Zhang, Y. Preparation of Hollow Porous Cu2O Microspheres and Photocatalytic Activity under Visible Light Irradiation. Nanoscale Res. Lett. 2012, 7, 347−352. (53) Lu, J.; Zhang, P.; Li, A.; Su, F.; Wang, T.; Liu, Y.; Gong, J. MesoporousAnatase TiO2 Nanocups with Plasmonic Metal Decoration for Highly Active Visible-LightPhotocatalysis. Chem. Commun. 2013, 49, 5817−5819. (54) Yao, W.; Zhang, B.; Huang, C.; Ma, C.; Song, X.; Xu, Q. Synthesis and Characterization of High Efficiency and Stable Ag3PO4/ TiO2Visible Light Photocatalyst For the Degradation of Methylene Blue and Rhodamine B Solutions. J. Mater. Chem. 2012, 22, 4050− 4055. (55) Chowdhury, P.; Malekshoar, G.; Ray, M. B.; Zhu, J.; Ray, A. K. Sacrificial Hydrogen Generation from Formaldehyde with Pt/TiO2 Photocatalyst in Solar Radiation. Ind. Eng. Chem. Res. 2013, 52, 5023− 5029. (56) Yang, M. Q.; Xu, Y. J. Basic Principles for Observing the Photosensitizer Role of Graphene in the Graphene−Semiconductor Composite Photocatalyst from a Case Study on Graphene−ZnO. J. Phys. Chem. C 2013, 117, 21724−21734.
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