Biphase TiO2 Microspheres with Enhanced Photocatalytic Activity

Apr 14, 2014 - ABSTRACT: TiO2 microspheres (TMS) with perfect spherical morphology were synthesized by spray drying of a hydrothermally cured aqueous ...
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Biphase TiO2 Microspheres with Enhanced Photocatalytic Activity Sudipto Pal,*,† Anna Maria Laera,† Antonio Licciulli,† Massimo Catalano,‡ and Antonietta Taurino‡ †

Department of Engineering for Innovation, University of Salento, 73100 Lecce, Italy CNR-IMM University of Salento, 73100 Lecce, Italy

‡

S Supporting Information *

ABSTRACT: TiO2 microspheres (TMS) with perfect spherical morphology were synthesized by spray drying of a hydrothermally cured aqueous suspension of TiO2 nanoparticles. TiO2 powders (TP) obtained by drying the nanoparticle suspension were studied simultaneously to determine which was the most efficient photocatalyst. SEM images and laser granulometry on TMS show spherical morphology with the diameter ranging from 2 to 10 μm. TMS had high specific surface area after annealing as seen from BET analyses. XRD analyses show that TMS consist of anatase and rutile crystalline phases where the rutile fraction increases with annealing temperature and above 500 °C rutile dominates anatase. Raman spectroscopy shows several Raman bands from anatase and rutile phases and supports the XRD results of phase transformation with increasing annealing temperature. Photodegradation of organic pollutants in aqueous solution under UV light irradiation establishes the higher photocatalytic activity of TMS with respect to TP. The highest efficiency was found on the 400 °C annealed TMS.

1. INTRODUCTION Titanium dioxide (TiO2) is the most extensively studied semiconductor oxide with diverse applications in photocatalysis, pigments in paints, building materials, cosmetics, pharmaceutical industries, dye-sensitized solar cells (DSSCs), energy storage, and sensing applications.1−6 In the field of photocatalysis, nanosized TiO2 has gained a position as the most promising candidate toward organic pollutant degradation, atmospheric purification, and other environmental issues.7 Over its photocatalyst competitors, TiO2 has many advantages such as inertness to chemical environment, long-term photostability, nontoxicity, and relatively lower cost. The photocatalytic activity of TiO2 depends on several factors such as degree of crystallinity, specific surface area, anatase−rutile phase composition, size, and geometry.8 In the photocatalysis experiments, TiO2 is used either as suspended catalyst, mainly in powder form, or immobilized on various supports. Nevertheless, nanopowder handling and processing is a serious issue due to the harmful nature of air-suspended nanoparticles, which causes several health problems, and the tendency to form aggregates which limits their flowability.9,10 Moreover, the use of TiO2 nanopowders for the removal of water contaminants (natural organic matter, pollutants, and bacteria) is still a challenge because of the difficulty of catalyst recovery from treated samples. TiO2 microspheres could be a convenient alternative that can be easily separated under gravity from a wet reactor while preserving the advantageous properties of nanoscale materials such as the retention of exposed active areas, dispersibility, and higher degree of contact with the light source. In recent years, direct synthesis of microspheres of oxide materials by using aerosol-assisted or, more specifically, a spray drying process has drawn the attention of researchers and is a fast-growing research field due to the rapid and scalable production of solid and hollow particles with ease of control.9−18 The most economical and convenient way to obtain dried particles is by spraying the fluid in a hot drying medium. In the spray drying process, a feeding liquid, which © 2014 American Chemical Society

may be a solution, suspension, dispersion, emulsion, slurry, or gel, is pumped and sprayed through an atomizer nozzle into a hot gas chamber, where the liquid is evaporated and the droplets are converted to dry spherical microgranules. The dried spheres are collected through a high speed cyclone collector or filter bag. The morphological properties of the micrometer size particles are determined by several experimental factors, such as the nature of feeding solution (concentration, organic additives, solvent, etc.), the feed flow rate, the geometry of the atomizer, the carrier gas flow rate, the chamber temperature, and the collection rate.19,20 The spray drying method is widely used in food processing and the pharmaceutical industries18,21 and nowadays is well investigated for the production of photocatalytic TiO2 micropowders.9,10,13−17 In this work, we propose a synthetic strategy in a reproducible way, leading to the formation of TiO 2 microspheres (TMS) by spray drying an aqueous suspension of TiO2 nanoparticles prepared by hydrothermal treatment. Perfect spherical-shaped solid TMS were obtained without using any shape-forming surfactant or additives. TiO2 nanoparticle suspension, as feeding solution, led to stable and welldefined microspheres which retained their structure without any deformation after heat treatment at 300−500 °C. TMS were evaluated by comparing with TiO2 powder (TP) obtained by oven drying the hydrothermally treated nanoparticle suspension. TMS showed enhanced photocatalytic activity compared to TP over photodegradation of rhodamine B (RhB) dye. TP and TMS were characterized by Brunauer−Emmett− Teller (BET), X-ray diffraction (XRD), Raman spectroscopy, FTIR, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and UV−visible spectroscopy. Received: Revised: Accepted: Published: 7931

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2. EXPERIMENTAL DETAILS 2.1. Synthesis of TiO2 Microspheres. Prior to the spray drying process, an aqueous suspension containing crystalline TiO2 nanoparticles (1 wt %) was prepared by hydrothermal method. Titanium tetraisopropoxide (TTIP, 97% SigmaAldrich) was added dropwise to acidified (oxalic acid, 98%, Sigma-Aldrich) water at pH 1 after which a white precipitate of titanium hydroxide was formed. It turned into a clear sol after hydrolysis−condensation reaction at 75−80 °C for 2 h. At this stage, amorphous TiO2 is expected to form. The as-formed sol was placed in an autoclave at 135 °C for 30 min to let the amorphous colloidal TiO2 turn into crystalline nanoparticles. As some of the water was evaporated during hydrothermal treatment, constant solid content was maintained by adding the required amount of water to the suspension. This suspension was atomized in the spray drier (ICF Welko, commercial grade) and passed through a rotor with speed regulation to the high pressure atomizer nozzle to form the droplets inside the chamber. Compressed air pressure of 2 bar was maintained during the atomization process, and the inner chamber temperature was fixed to 200 °C. TMS were produced in the hot air system chamber during solvent evaporation and separated by the air hot stream through a cyclone. The spray drying setup is represented in Figure 1.

where D is the average crystallite size, k is the shape factor (0.9), λ is the wavelength of X-ray radiation, β is the full line width at half-maxima (fwhm) of the main diffraction peak, and θ is the Bragg angle of the corresponding diffraction peak. The weight fraction of anatase and rutile contents in the samples were calculated according to the following equations:22,23

2.2. Characterization. To evaluate the shape and size of the TMS, SEM measurements were performed with a Zeiss EVO-40 scanning electron microscope. Crystalline phases of the TiO2 samples annealed at different temperatures were investigated by X-ray diffraction (XRD) on a Rigaku Ultima Xray diffractometer using Cu Kα radiation (λ = 1.5406 Å) operating at 40 kV/30 mA with the step size of 0.02°. The average crystalline sizes of TP and TMS were estimated according to Scherrer’s equation, accounting for the most intense diffraction peaks of the corresponding anatase and rutile phases.

kλ β cos θ

1 1 + 1.26(IR110/IA101)

(2)

WR =

1 1 + 0.8(IA101/IR110)

(3)

where WA and WR are the weighted fraction of anatase and rutile in the mixed phase, and IA101 and IR110 are the integrated intensity of corresponding anatase (101) and rutile (110) diffraction peaks, respectively. FTIR spectra of the assynthesized and annealed TMS were carried out with a JASCO FTIR-6300 instrument over the range 4000−400 cm−1 with a resolution of 4 cm−1 by accumulating 256 scans for each measurement, adopting the KBr disc method. Raman measurements (FT-Raman) of the powders were directly performed on a JASCO RFT-6000 Raman attachment by using a 1064 nm CW 500 mW laser source and spectral resolution of 4 cm−1. Specific surface area of the samples were measured by BET method using a Quantachrome NOVA 2200e surface analyzer. UV−vis diffuse reflectance spectra of the powder samples were measured with a Agilent Cary 5000 spectrophotometer equipped with a 110 mm diameter integrating sphere. Measurements were performed in the 200−800 nm wavelength range by using poly(tetrafluoroethylene) (PTFE) as reflectance standard. Morphological properties of TiO2 nanoparticles were investigated with a Leo 922 transmission electron microscope operating at 200 kV. 2.3. Photocatalytic Experimental Setup. Photocatalytic activity of TMS and TP samples was accomplished by analyzing the degradation of rhodamine B (RhB), methylene blue (MB), and methyl orange (MO) dyes under UV light irradiation. The experiment was performed in a photocatlytic chamber provided by Salentec, composed of a UV lamp (Radium Sanolux 300 W emitting maximum intensity at 365 nm) on the top, magnetic multistirrer, air bubbler, and air cooling system. To perform the test in each cases, TiO2 powder samples (1 g/L) were dispersed in 200 mL of RhB aqueous solution (1 × 10−6 M) and irradiated with the UV light. Photocatalytic decomposition of RhB was monitored by measuring the absorption band at 554 nm. Photodegradation of MB and MO were also performed under similar conditions. Irradiated solution was taken out at regular time intervals, and the absorption spectra were acquired with an Agilent Cary 5000 series UV−visible spectrophotometer. In all cases, prior to the measurement step, the catalysts were separated from the solution by centrifugation. The absorption spectra of irradiated dye solutions in the absence of any catalysts were collected for comparison.

Figure 1. (a) Schematic presentation of the spray dryer: (1) feeding suspension, (2) regulated rotor, (3) atomizer, (4) droplet forming hot chamber, (5) cyclone, and (6) collection unit. (b) Scheme of the spray drying mechanism. Inset shows photograph of the spray-dried powders.

D=

WA =

3. RESULTS AND DISCUSSION TiO2 microspheres (TMS) with narrow size distribution were prepared by spray drying using a commercial grade spray dryer, as schematically depicted in Figure 1. To evaluate the features of spray-dried TiO2, some powders were prepared by grinding the solid product after oven drying the hydrothermally treated sol and designated as TiO2 powder (TP). TMS and TP powders were annealed at 300 °C, 400 °C, and 500 °C for 1 h

(1) 7932

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Figure 2. SEM micrographs of the (a, b) as-synthesized TMS and (c) TMS annealed at 400 °C. Particle size distribution of TMS obtained from laser granulometry is shown in part d.

and presented throughout this work as TP/TMS-200, TP/ TMS-300, TP/TMS-400, and TP/TMS-500, respectively. Morphological properties of TMS are depicted with the SEM micrographs in Figure 2. Micrographs of the as-synthesized TMS are shown in Figure 2a,b, whereas part c shows the image of the sample after heat treatment at 400 °C. It is evident from all the images that TMS, either as-synthesized or after thermal annealing, possess perfect spherical morphology without any evidently deformed shape. Solid spherical nature was also confirmed by observing the SEM images of hand-grinded TMS powder. The permanence of the morphology after thermal annealing at 400 °C confirms the higher thermal stability of the TMS. The average size of the TMS as seen from the micrographs could be estimated in the range of 2−12 μm with the most probable population at 4−6 μm. This was further confirmed by analyzing the TMS with a particle size analyzer. The size distribution is shown in Figure 2d from which the mean diameter was estimated to be ∼6 μm. Both TMS and TP showed high specific surface areas which are listed in Table 1. The specific surface area (SBET) of assynthesized TP and TMS were found to be 218 and 233 m2 g−1, respectively. After thermal treatment at 300 °C, 400 °C, and 500 °C, the specific surface area of all the samples decreased gradually. It is interesting to observe that either in the case of as-synthesized or annealed samples the surface area of TMS is greater than that of the TP samples, thus indicating the superiority of TMS over TP. Infrared spectroscopy (IR) was undertaken to evaluate the thermal decomposition and burnout of organic species. Figure 3 shows FTIR spectra of the TMS powder samples before and after calcination at different temperatures. The as-prepared TMS (Figure 3, curve a) shows several vibrational peaks, where the broad peak centered at about 3420 cm−1 is attributed to the O−H stretching vibration24,25 and other peaks at 1716, 1690, 1399, and 1242 cm−1 are indexed as the asymmetric and

Table 1. Physicochemical Properties of TP and TMS Powders phase contenta

crystalline size (nm)b

sample

WA

WR

⟨DA⟩

⟨DR⟩

SBET (m2 g−1)c

K (min−1)d

TP TP-300 TP-400 TP-500 TMS TMS-300 TMS-400 TMS-500

0.593 0.562 0.48 0.184 0.545 0.516 0.475 0.262

0.404 0.435 0.509 0.814 0.452 0.481 0.522 0.735

4.13 4.76 5.43 7.32 4.25 4.71 5.10 7.20

8.38 10.91 12.77 19.56 10.23 11.31 13.04 18.41

218 141 105 49 233 153 128 85

0.011 0.021 0.026 0.017 0.008 0.019 0.028 0.023

a

Weighted fraction of anatase (WA) and rutile (WR) from XRD results. Calculated from XRD using Scherrer’s formula. cBET surface area. d Apparent rate constant calculated from the relation −ln(C/C0) = Kt. b

symmetric stretches of the Ti-coordinated oxalate groups.24−27 Appearance of a broad absorption peak in the frequency range of 400−800 cm−1 for all the TMS samples can be assigned to the characteristic stretching vibration of Ti−O and Ti−O−Ti of pure TiO2.24,26,28 With increasing calcination temperature, all the peaks attributed to oxalates disappear, indicating their decomposition. Here we also observed weak peaks at 1618 cm−1 in samples annealed at 300 °C and 400 °C that are attributed to adsorbed water molecules. X-ray diffraction analyses were carried out to investigate the structural evolution with annealing temperature. The set of XRD patterns of TP and TMS calcinated at different temperatures are presented in Figure 4. Pattern of dried powder obtained from the sol before hydrothermal treatment is also presented (Figure 4a, curve indicated with T-80), which reflects the amorphous nature at this stage. After hydrothermal treatment, the several sharp diffraction peaks due to anatase and 7933

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in grain size could be explained by the Ostwald ripening phenomena of particle growth induced by thermal diffusion. Raman spectral measurement is a very sensitive tool for identification of different crystalline phases of TiO2 and is very effective to correlate with XRD. Raman spectra of TMS powders before and after thermal treatment are shown in Figure 5. All the spectra show several well-defined peaks due to

Figure 3. FTIR spectra of (a) as-prepared TMS and after thermal treatment at (b) 300 °C, (c) 400 °C, and (d) 500 °C.

rutile crystalline phases were observed. Peaks at 2θ values of 25.38°, 48.12°, and 54.36° could be assigned to (101), (200), and (211) crystalline planes of the anatase (A) phase29 (JCPDS no. 84-1286), whereas other diffraction peaks at 27.45°, 36.11°, 39.22°, 41.28°, 44.09°, and 56.65° arise from the (110), (101), (200), (111), (210), and (220) crystalline planes of the rutile (R) phase29 (JCPDS no. 88-1175). Average crystalline sizes of TP and TMS and their anatase−rutile phase contents are presented in Table 1. In the case of as-synthesized TP and TMS, the anatase phase (∼60%) is found to dominate, but as the annealing temperature increases, rutile content also gradually increases. After 500 °C, rutile (∼80%) is the major phase because it is the most thermodynamically stable among the three polymorphs anatase, brukite, and rutile.3 The patterns showed that with increasing thermal treatment the intense anatase and rutile peaks become sharp which is due to the increase in crystalline size, as seen from Table 1. This increase

Figure 5. Raman spectra of TMS powders (a) before and after calcination at (b) 300 °C, (c) 400 °C, and (d) 500 °C with excitation line at 1064 nm. A, R, and B in the figure stands for anatase, rutile, and brukite crystalline phases, respectively.

the crystalline phases of TiO2. Raman bands at 160, 202, 407, and 643 cm−1 could be attributed to the Eg, Eg, B1g, and Eg modes for the anatase phase whereas the bands at 509−520 cm−1 correspond to the A1g mode superimposed with the B1g mode due to the anatase phase.24,29−31 Other major Raman bands at 251, 453, and 616 cm−1 can be identified as twophonon scattering (combination), Eg and A1g modes of the rutile phase.29−31 Here we could also observe some weak bands

Figure 4. Evolution of the XRD patterns of (a) TP and (b) TMS powder samples annealed at different temperatures. In part a, T-80 curve represents the diffraction pattern of amorphous TiO2 powder obtained before hydrothermal treatment. 7934

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at 218 and 370 cm−1 due to trace amount of brookite phase (B1g and B2g modes, respectively)32 and a relatively prominent band at 328 cm−1 due to two-phonon scattering of the anatase phase.31 As shown in Figure 5, curves a−d, with increasing annealing temperature, the intensity of the band at 453 cm−1 due to the rutile phase increases compared to the 407 cm−1 band of the anatase phase, which is consistent with the results of XRD analyses. The increase of the other rutile bands at 251 and 616 cm−1 also supports this phenomena. To get an estimate of the band gap energy of the TMS powders, UV−visible diffuse reflectance spectra were collected and the Y-axis was transformed into the Kubelka−Munk function which is presented in Figure 6. Considering TiO2 an

anatase and rutile phases (3.2 eV for anatase and 3.0 eV for rutile).3 The trend of blue shifting of the binding energy with increasing annealing temperature indicates an increase of the rutile fraction, which is consistent with the XRD and Raman analyses. Transmission electron microscopic (TEM) measurements were performed on hydrothermally treated TP to investigate morphological behavior of the nanoparticles and are presented in Figure 7. In the TEM micrographs two types of structures are observed, i.e., uniform spherical nanoparticles, with an average diameter of 4 nm and elongated aggregates with an average width of 10 nm. The inset of Figure 7b shows the SAED pattern obtained from the area imaged in Figure 7b. The presence of two families of diffracted rings, one consisting of broad and continuous rings and the other one consisting of spotlike rings can be observed (Figure S4, Supporting Information). By deriving the “dhkl” values, it was found that the continuous rings are related to the anatase phase, whereas the spot rings are related to the rutile phase. Furthermore, on the basis of the different appearance of the diffracted rings, it can be derived that the spherical nanoparticles belong to the anatase phase and the elongated structures are in the rutile phase. The photocatalytic activity of as-synthesized and heat-treated TP and TMS was evaluated by observing the degradation of RhB, MB (Figure S1, Supporting Information), and MO (Figure S2, Supporting Information) in aqueous solution under UV light irradiation. The strong absorption peak at 554 nm of RhB aqueous solution was monitored at certain time intervals to realize the degradation kinetics. Photolysis of RhB solution without any catalyst was also performed under the same conditions for comparison. The RhB degradation kinetics of TP and TMS are shown in Figure 8 with the plot of C/C0 versus UV light irradiance. A fixed concentration of RhB with irradiation time ensures the stability of this dye under UV light and confirms the photoactivity of TP and TMS samples. Figure 8 shows that all the heat-treated samples follow an exponential decay path toward RhB degradation. To compare the reaction kinetics of different samples, we assume that RhB follows the first-order rate law, −ln(C/C0) = Kt, where K is the apparent rate constant for degradation. Figure 9 shows the linear relationship of −ln(C/C0) with irradiation time (except for the as-prepared samples TP and TMS), from which the

Figure 6. Kubelka−Munk treatment of the diffuse reflectance spectra of TMS powders annealed at different temperatures. The inset shows the diffuse reflectance spectra.

indirect band gap semiconductor, the plot of (F(R) hν)1/2 vs photon energy gives the direct evaluation of band gap energy from the intersection of the linear fit with the X-axis (photon energy). Extrapolation of the linear part of the curves gives an indirect band gap of 3.028, 3.016, 3.009, and 2.994 eV for TMS, TMS-300, TMS-400, and TMS-500 samples, respectively. These values are in between the band gap energy of pure

Figure 7. TEM images of hydrothermally treated TiO2 nanoparticles (TP). The upper inset of part b shows the selected area electron diffraction (SAED) performed on the image. 7935

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is noteworthy that in both cases, photocatalytic activity is highest for the 400 °C treated samples and the overall performance of TMS is higher than that of TP. The photodegradation efficiency of the heat-treated samples (400 °C and 500 °C) was also verified with MB and MO dyes (Figure S1, Figure S2, Table S1, Supporting Information), from where it is observed that TMS-400 sample is superior to the others. The recycling photocatalytic test of the best performing sample (TMS-400) shows good photostability of the microspheres (Figure S3, Supporting Information). Comparison of the above results shows that although the as-synthesized powders (TP and TMS) have higher specific surface area and a greater anatase/rutile ratio (Table 1), their photocatalytic efficiency is less than that of the annealed samples. From the IR spectral analysis (Figure 3, curve a), we can say that this is due to the presence of organic contaminants (carbonious species), originating from the oxalic acid during the synthesis procedure, which hinder the photodegradation efficiency. With increasing annealing temperature, the organic species decompose gradually and after 400 °C their decomposition is complete. At this level (at 400 °C), both samples show greater photocatalytic efficiency. At 400 °C, the fraction of anatase and rutile is almost equal for both the samples (Table 1) whereas above 500 °C rutile fraction is more present (∼0.82 for TP and 0.74 for TMS) than anatase. Although it is well-known from several studies that anatase is the most favored crystalline phase of TiO2 in photocatalytic experiments,7,33,34 numerous studies showed that the presence of an optimum amount of rutile in the mixed phase TiO2 system also increases the photocatalytic efficiency.7,23,35−38 In our case increasing photocatalytic efficiency up to 400 °C could be ascribed to the decomposition of organics while maintaining the optimum anatase to rutile ratio in the biphasic system within the synergism.37 As this ratio goes beyond this limit, the efficiency also begins to drop. On the other hand, the higher efficiency of TMS over TP could be due to its higher specific surface area, as seen from Table 1. Considering all the above findings, there are several variables which are responsible for the different results in photocatalysis experiments and could be summarized as follows: in the case of as-synthesized TP and TMS, the lower photoactivity is attributed to the presence of organic species and lower crystalline order, whereas 400 °C samples show higher activity due to the removal of organics and possession of a mixed crystalline phase with optimum anatase to rutile ratio. The decrease in efficiency above 500 °C annealing temperature is attributed to the excess amount of rutile content, and the superiority of TMS compared to TP could arise from the higher specific surface area of TMS.

Figure 8. Photocatalytic degradation of RhB dye under UV light irradiation with (a) TP and (b) TMS. C0 and C are the initial concentration of RhB after adsorption equilibrium and the concentration with exposure time, respectively.

4. CONCLUSIONS We have successfully synthesized biphasic TiO2 microspheres by spray drying. TiO2 water-suspended nanocrystals obtained by a hydrothermal process were finely atomized and agglomerated in the form of perfect spheres with dimensions in the range 2−10 μm. TiO2 microspheres with good crystallinity and high surface area displayed photocatalytic efficiency higher than that of ordinary powders obtained from the same colloidal suspension. The microspheres consist of tunable anatase and rutile phases where the rutile phase increases with annealing temperature. The photooxidation of RhB, MB, and MO dyes show that microspheres annealed at 400 °C have the best photocatalytic efficiency compared to that of ordinary powders obtained by drying and calcinating the

Figure 9. Photodegradation kinetics of RhB in the presence of (a) TP and (b) TMS. Apparent reaction rate constants (K) of the respective samples are mentioned in the figures.

reaction rate constants (K) were calculated and are listed in Table 1. The K value for TP, TP-300, TP-400, and TP-500 are found to be 0.011, 0.021, 0.026, and 0.017 min−1, respectively, and those for TMS are 0.008, 0.019, 0.028, and 0.023 min−1. It 7936

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colloidal suspension. Organic byproducts, specific surface area, and anatase to rutile ratio are the key factors to determine the photodegradation efficiency.

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ASSOCIATED CONTENT

* Supporting Information S

Photocatalytic degradation and kinetics of methylene blue (Figure S1) and methyl orange (Figure S2) dyes under UV light irradiation with TP and TMS samples annealed at 400 °C and 500 °C and their rate constants (Table S1). Five cycles of photodegradation tests of RhB dye with TMS-400 sample (Figure S3), selected area electron diffraction (SAED) pattern performed on the image of Figure 7b showing different dhkl planes of the corresponding anatase (A) and rutile (R) phases (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +39 0832 297321. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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