Microwave-Assisted Synthesis of Titania Nanocubes, Nanospheres and Nanorods for Photocatalytic Dye Degradation
© to the authors 2008
Received: 4 September 2008
Accepted: 11 November 2008
Published: 26 November 2008
TiO2nanostructures with fascinating morphologies like cubes, spheres, and rods were synthesized by a simple microwave irradiation technique. Tuning of different morphologies was achieved by changing the pH and the nature of the medium or the precipitating agent. As-synthesized titania nanostructures were characterized by X-ray diffraction (XRD), UV–visible spectroscopy, infrared spectroscopy (IR), BET surface area, photoluminescence (PL), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and atomic force microscopy (AFM) techniques. Photocatalytic dye degradation studies were conducted using methylene blue under ultraviolet light irradiation. Dye degradation ability for nanocubes was found to be superior to the spheres and the rods and can be attributed to the observed high surface area of nanocubes. As-synthesized titania nanostructures have shown higher photocatalytic activity than the commercial photocatalyst Degussa P25 TiO2.
KeywordsNanocubes Nanorods Nanospheres Photocatalytic activity Microwave irradiation Dye degradation
Nanomaterials of transition metal oxides have attracted a great deal of attention from researchers in various fields due to their numerous technological applications [1–4]. Among them, nanocrystalline titania has been attracting increasing attention due to its fascinating properties and potential applications. Titanium dioxide is a versatile material which is being investigated extensively due to its unique optoelectronic and photochemical properties such as high refractive index, high dielectric constant, excellent optical transmittance in the visible and near IR regions as well as its high performance as a photocatalyst for water splitting and degradation of organics . With a band gap of 3.0–3.3 eV, titanium dioxide has been photocatalytically active only under ultraviolet light (wavelength λ < 400 nm) . Titanium dioxide mainly exists in three crystalline phases: anatase, rutile, and brookite . Among the three crystalline forms, anatase titanium dioxide is attracting more attention for its vital use as pigments , gas sensors , catalysts [10, 11], photocatalysts [12–14] in response to its application in environmentally related problems of pollution control and photovoltaics . The properties and catalytic activities of titania strongly depend upon the crystallinity, surface morphology, particle size, and preparation methods. The increased surface area of nanosized TiO2 particles may prove beneficial for the decomposition of dyes in aqueous media. Ohtani et al. proposed that high photocatalytic activity of titania can be achieved by imparting large surface area to adsorb substrates and by making high crystallinity to minimize the photoexcited electron-hole recombination rate. In general anatase titania is observed to be more active compared to its rutile phase. This difference in activity can be due to the high electron-hole recombination rate observed in rutile titania. Many synthetic methods have been reported for the preparation of nanotitania, including sol–gel reactions [17–19], hydrothermal reactions [20, 21], non-hydrolytic sol–gel reactions [22, 23], template methods [24–26], reactions in reverse micelles , and microwave irradiation. Nanotitania with various morphologies and shapes such as nanorods , nanotubes [29, 30], nanowires [31, 32], and nanospheres [33, 34] can be produced depending upon the synthetic method used. These different morphologies have different photocatalytic activities. In the present work, we report a simple microwave method to synthesize phase pure anatase and rutile nanotitania with different morphologies viz., cubes, spheres, and rods. Photocatalytic activity studies of the synthesized samples were carried out using the dye, methylene blue in aqueous solution under ultraviolet light irradiation. The photoluminescence (PL) features of the synthesized titania nanostructures were also compared in the present study.
All reagents were purchased from Merck, Germany. Titanium trichloride (15 wt% TiCl3, 10 wt% HC1) was used as the titanium precursor. NH4OH (1.5 M), NaCl (5.0 M), and NH4Cl (5.0 M) were employed for the synthesis. A typical microwave oven (Whirlpool, 1200W) operating at a frequency of 2,450 MHz was used for the synthesis.
Synthesis of TiO2Nanostructures
A general synthetic strategy adopted for the synthesis of titania nanostructures was using TiCl3as Ti precursor by varying the precipitating agents under different pH conditions. The precipitated sol was irradiated in a microwave oven in on and off mode for different durations depending upon the precipitation rate in each case. The completion of the reaction is checked by noting the color change (blue to colorless) of the reaction mixture. The white precipitate formed in each case was aged for 24 h and washed thoroughly with distilled water. The precipitated titania was then dried in an air oven at 100 °C and further calcined in a muffle furnace at 400 °C for 4 h.
In the case of sample 1 (S1) TiCl3 (20.0 mL) was added drop by drop to 200 mL of 1.5 M NH3 (pH = 11) solution  and the irradiation was done for 20 min for complete precipitation. In sample 2 (S2), TiCl3 (5.0 mL) was added dropwise with continuous stirring to 200 mL of 5.0 M NaCl solution (pH = 7)  and the reaction mixture was irradiated for 60 min for complete precipitation. In sample 3 (S3), TiCl3 (5.0 mL) was added dropwise to 200 mL of 5.0 M NH4Cl solution (pH = 5.9) and irradiated in a similar manner as in the previous case for 60 min.
Characterization of Titania Nanostructures
The X-ray diffraction (XRD) patterns of the titania were recorded on a Brucker D8 advance diffractometer with CuKαradiation. The crystallite size of TiO2was calculated using Debye Scherrer equation,L = k λ/(βcosθ), whereL is the average crystallite size, λ is the wavelength of the radiation, θ is the Bragg’s angle of diffraction, β is the full width at half maximum intensity of the peak andk is a constant usually applied as ~0.89. Scanning electron microscopic images were taken on a JEOL JSM-5600 SEM equipped with energy dispersive X-ray analysis (EDX). High resolution transmission electron micrographs and electron diffraction patterns were recorded using a JEOL JEM-3010 HRTEM microscope at an accelerating voltage of 300 kV. The TEM specimens were prepared by drop casting the sample on the surface of the carbon coated copper grid. The tapping mode AFM images of the samples deposited on a mica sheet were taken using Nanoscope-IV scanning probe microscope. The BET surface area, pore size distribution, and pore volume of the samples were measured on a Micromeritics ASAP 2010 analyzer based on N2adsorption at 77 K in the pressure range from 0.1 to 760 mmHg. The pore size distribution was calculated by the Barrett Joyner Halenda (BJH) method. IR spectra was recorded using Shimadzu 8400S FTIR spectrophotometer in the range of 400–4,000 cm−1. The ultraviolet–visible absorption (UV–vis) spectra were recorded using a UV-2450 Shimadzu UV–visible spectrophotometer. The photoluminescence (PL) spectral measurements were made using Perkin Elmer LS-55 luminescence spectrometer at an excitation wavelength of 325 nm.
Photocatalytic Activity Measurements
Photocatalytic activity of TiO2 was evaluated by the degradation of the dye, methylene blue (MB) in aqueous solution under ultraviolet light irradiation in the presence of as-synthesized TiO2 and the commercial Degussa P25 TiO2. The changes in the concentrations of methylene blue in the aqueous solution were examined by absorption spectra measured on a UV-2450 Shimadzu UV–visible spectrophotometer. Before examining the photocatalytic activity for degradation of aqueous methylene blue, TiO2 sol was prepared. About 100 mg of the synthesized TiO2 was dispersed ultrasonically in 50 mL of deionized water. For photodegradation experiments, 50 mL of 4 × 10−5 M methylene blue solution was added to the as-synthesized titania sol in a quartz reactor. To maximize the adsorption of the dye onto the TiO2 surface, the resulting mixture was kept in the dark for 30 min under stirring conditions . The solution was then irradiated for 180 min using a mercury lamp (100 W, Toshiba SHLS-1002 A). The degradation of the dye was monitored by measuring the absorption maximum of methylene blue at 661 nm at 30 min intervals of reaction.
Results and Discussion
X-ray Diffraction Studies
BET Surface Area Analysis
Textural analysis of mesoporous TiO2Nanostructures
Crystallite size from XRD (nm)
BET surface area (m2g−1)
Pore size (nm)
Pore volume (cm3 g−1)
Electron Microscopic Analysis
UV–Visible Absorption Studies
Summary of band gap and absorption onset of the synthesized nanotitania
Band gap (Eg) eV
Absorption onset (λmax)
Photocatalytic Activity Studies
Nanotitania with fascinating morphologies, particle size, and surface area can be effectively synthesized by a simple microwave irradiation technique. The morphology of the samples was effectively controlled by changing the pH of the media. The synthesized nano TiO2was structurally and physicochemically characterized. Structural and physicochemical characterization revealed the dependence of photocatalytic activity of nanotitania on different morphologies. The TEM images clearly reveal that the samples have cubical, spherical and rod shaped morphologies. The surface area and porosity of the three titania nanostructures were determined by BET and BJH methods. Anatase nanocubes (S1) exhibit a much higher BET specific surface area than rutile nanospheres (S2) and nanorods (S3). The band gap energy for anatase nanocubes is blue shifted (3.2 eV) compared to that of the rutile nanospheres (S2) and nanorods (S3). The UV–vis absorption and the photoluminescence emission spectral data demonstrated that the indirect transition is the exclusive route for the charge carrier recombination, indicating the strong coupling of wave functions of the trapped exciton pair with lattice phonons. The synthesized mesoporous anatase nanotitania with cubical morphology exhibit higher photocatalytic activity than spherical and rod shaped rutile titania nanostructures. Moreover, the synthesized mesoporous anatase TiO2with BET surface area 372 m2 g−1exhibit much higher photocatalytic activity than the commercial Degussa P25 TiO2photocatalyst in the degradation of the dye, methylene blue in aqueous solution under UV light irradiation. The higher photocatalytic activity of the anatase nanocubes may be due to the higher surface area and the lesser electron-hole recombination rate compared to the rutile nanostructures.
We are grateful to Dr. K. George Thomas of Regional Research Laboratory, Trivandrum and Prof. T. Pradeep of Indian Institute of Technology, Chennai for the AFM and HRTEM imaging.
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