Abstract: Using tetrabutyl titanate as a precursor, amorphous titania (TiO2) nanoparticles have beensynthesized by a sol-gel method. Thermal gravimetry, differential scanning calorimetry, powder X-raydiffraction, fourier transformed infrared spectroscopy, transmission electron microscopy, and X-rayphotoelectron spectroscopy were employed to characterize the amorphous titania nanoparticles. Theresults indicated that after calcination at 320?C for 2 hours, the product obtained is pure, amorphousTiO2 nanoparticle aggregates of about 30 nm in diameter with the primary particle size of 3-5 nm.
Key words: Nanoparticles; Titania; Amorphous particels
Titania (TiO2) is an important metal-oxide semiconductor and has been extensively studied ascatalyst supports, semiconductors, materials for solar-energy conversion,[3, 4] photocatalysis,[5,6] gas sensors, and electrochemical capacitors. For the synthesis of crystalline titania particles,a number of chemical methods including the sol-gel method, hydrothermal synthesis,reversed micelle method, cathodic electrodeposition, and chemical vapor deposition wereemployed. Usually, conventional titania precursors such as titanium (IV) chloride (TiCl4) andtitanium alkyloxide [Ti(OR)4] were used for preparation of TiO2.
Because of the lack of long range order and the metastability in thermodynamics, amorphousTiO2 materials will exhibit novel properties. For example, amorphous TiO2 films showed a highdielectric constant and might be candidates as the dielectric layer in ultra-thin film capacitors.Amorphous TiO2 films exhibited super-hydrophilicity which is crucial in self-cleaningtechniques.[16, 17] Amorphous TiO2 has shown potential applications as electrodes in solarbatteries and in semiconductors. Computational modeling of amorphous TiO2 showed thatthe structural characteristics of amorphous TiO2 nanoparticles were different from that of theamorphous TiO2 bulk.[20, 21] Zhang et al. synthesized amorphous titania by hydrolysis of titaniumethoxide at the ice point; the synthesized titania consisted of 0.2 μm TiO2
aggregates. However,amorphous TiO2 nanoparticles with aggregate sizes in nanoscale have not been reported so far.
In our present letter, we synthesized amorphous TiO2 nanoparticles using tetrabutyl titanate asthe precursor by a sol-gel method and subsequent calcination. The products obtained werecharacterized by thermal gravimetry (TG), differential scanning calorimetry (DSC), powder X-raydiffraction (XRD), Fourier transformed infrared spectroscopy (FTIR), transmission electronmicroscopy (TEM), and X-ray photoelectron spectroscopy (XPS).
Amorphous titania nanoparticles were prepared from tetrabutyl
titanate [Ti(OC4H9)4, TBOT],ethanol, and water by a sol-gel method. The overall reaction for the conversion of tetrabutyltitanate to titania isReaction (1) can be divided into two separate reactions,[23, 24] namely,
In the present study, R is C4H9. Since an exchange reaction can occur between the ethanolsolvent and the tetrabutyl-based reactant, the final product can be an oxo-alkoxide as shown inreaction (3) if the hydrolysis reaction is limited, or can be titania if the hydrolysis reaction iscomplete (x=4) and all the organic (R) groups are removed from the starting alkoxide. Thesereactions, as well as other theoretical and experimental results of the hydrolysis of titaniumalkoxides, have been reviewed by Bradley et al.
Firstly, the solution S1 [a mixture of 5 ml Ti(OC4H9)4 and 45 ml C2H5OH (Eth)] and thesolution S2 (a mixture of 50 ml C2H5OH and 50 ml H2O) were prepared. Then, with the solutionS2 being vigorous stirred by an electric magnetic stirrer, the solution S1 was slowly dripped intothe solution S2 at 2 drops per second. After the dripping process was complete, the solution wasmixed by the electric magnetic stirrer for 12 h; and the titania particles precipitated in the solution.Then the resulting precipitates were vacuum filtered. The resulting precipitates were washed withethanol by resuspending the precipitate in ethanol and filtering. The washed precipitates weredried at 100?C for 6 h and calcined at 320?C for 2 h.
Thermal analysis (TG and DSC) was performed on the as-synthesized titania precursorsample using a Netzsch STA 449C TA instrument in flowing nitrogen atmosphere at a heating rateof 10?C/min. XRD analysis was performed on Rigaku D/Max-2400 X-ray diffractometer with CuKα radiation to determine
the structure of the samples. To provide direct evidence of TiO2, FTIRspectra were recorded in the range of 400~4000 cm-1 at room temperature (RT) by using a Nexus670 spectrometer. The samples for FTIR were prepared using the KBr technique which werecalibrated by polystyrene. XPS measurements were carried out on a PHI-5702 (PhysicalElectronics, Inc.) spectrometer. During the XPS analysis, Al Kα X-ray beam with a power
of 250W was adopted. The vacuum of the instrument chamber was 1×10-7 Pa.
The binding energy wascalibrated with reference to C 1s peak (285.0 eV). TEM observations were conducted on a JEM2010 electron microscope operated at 200 kV. Energy dispersive X-ray spectroscopy (EDS)analysis was conducted on the TEM during the TEM observations The TEM samples wereprepared by depositing the powder suspended into ethanol onto a holey carbon-coated 200 meshcopper grid.
2 Results and discussion
Fig. 1 shows the TG and DSC traces of the as-dried titania precursor sample in the range of30~900?C. In the DSC trace (Fig. 1), there are two endothermic peaks at 78.5 and 317.5?C andtwo exothermic peaks at 430.1
and 853.1?C, respectively. The endothermic peak at 78.5?Ccorresponds
to the evaporation of ethanol. The endothermic peak at 317.5?C should be due to theevaporation of water and organic compounds. The exothermic peak at 430.1?C corresponds to thecrystallization of the anatase
phase. The exothermic peak at 853.1?C can be attributed to thecrystallization of the rutile phase.
In the TG trace, the weight loss below 153?C mainly corresponds to the evaporation of waterand solvent (ethanol) with increasing temperature. The weight loss is about 23% of the origin totalweight. This process was found to be associated with the endothermic peak in the DSC curve atabour 78.5?C. At temperatures around 200?C, the removal of the organic substances andphysically absorbed water starts mainly due to the desorption. This process is not complete untilabout 317.5?C and is correlated to the endothermic peak centered at 317.5 ?C in the DSC curve.The total weight loss during this process is about 9% of the origin total weight. With temperatureincreasing over 320?C, the loss in weight is much less.
The thermal analysis results of the as-synthesized titania precursor sample imply that theas-dried titania precursor should be calcined at 320?C for 2 h in this work to remove the organicsubstances and physically absorbed water.
The XRD patterns of the as-dried titania precursor sample and the sample calcinated at 320?Cfor 2 h were collected from 2θ =10~80? and
are shown in Fig. 2. The XRD patterns show diffusehalos of glassy materials and no sharp diffraction peaks of any crystalline phase. This indicatesthat the sample calcinated at 320?C for 2 h is amorphous in structure.
The full width at half maximum (FWHM) of the main maximum of each XRD patterns werecalculated by using Gaussian fitting. The FWHM value (2θ)
increases from 22.86? for the as-driedsample (curve A) to 27.48? for
calcinated sample (curve B). The increased FWHM value of thediffuse peak may be correlated to the enhanced atomic disorder.
The FTIR spectra for the as-dried titania precursor sample and the sample calcined at 320?Cfor 2 h collected in the range of 400~4000 cm-1
are shown in Fig. 3. In the curve A of Fig. 3, theband centered at 1625 cm-1 and the broad absorption band in the 3200~3600 cm-1 range should beassigned to the deformation and O-H stretching vibrations of the weak-bound water in theas-dried titania precursor sample, respectively. Two sharp absorption bands at 2856 and 2925 cm-1are from the C-H stretching vibration of the hydrocarbone. The sharp absorption bands in theregion of 1400~1610 cm-1 can be attributed to the deformation of -CH2- and -CH3 and stretches ofTi-OH.[26, 29] The signals between 800 and 1400 cm-1 are attributed to the lattice vibrations oftitanium oxide.
For the titania sample calcined at 320?C for 2 h, the absorption bands
in the curve of Fig. 3due to the hydrocarbone disappear, indicating the complete removal of the hydrocarbone from thesample. The absorption peaks of Ti-OH also disappear. The band centered at 1625 cm-1 and thebroad absorption band in the 3200~3600 cm-1 range are assigned to the deformation and O-Hstretching vibrations of the weak-bound water which are much weaker than for the as-dried titaniaprecursor sample. From Fig. 3, it can be concluded that, although the sample was calcined at320?C for 2h, the OH groups in the sample is still present. This should be due to the water in thesample physically adsorbed in air after calcination. The signals between 800 and 1400 cm-1 arefrom the lattice vibrations of titanium oxide.
The FTIR results indicate that except of the physically adsorbed water, the synthesizedsample is TiO2.
To analyze the morphology and size distribution of the amorphous titania particles, TEMobservations were carried out for the titania sample calcined at 320?C for 2 h. Fig. 4a shows themicrograph of the amorphous titania particles calcined at 320?C for 2 h. The sample consists ofequiaxial or spherical aggregates. The aggregates are composed of fine primary nanoparticles ofsize between 3-5 nm. It was reported that the TiO2 particles have a strong tendency to agglomerateto large particles. In our present case, the average size of the aggregates is about 30 nm indiameter.
The inset in Fig. 4a shows the selected area electron diffraction pattern of the calcined titaniasample. There are no visible diffraction rings or diffraction spots observed in the selected areaelectron diffraction pattern of the titania particle sample, indicating that the prepared titaniaparticles are amorphous, which is consistent with the XRD analysis results discussed above.
In order to clarify the element composition of the titania sample calcined at 320?C for 2 h, theEDS analysis was performed on the sample during the TEM observations. The measured EDSspectrum is shown in Fig. 4b. Oxygen, carbon, copper, and titanium are present in the sample.Carbon and copper are from the supporting carbon flim and copper grid. The signals of titaniumand oxygen detected are from the titania sample. The Ti/O atomic ratio is about 1/2. This confirmsthat the calcined samples are pure TiO2.
In order to ascertain the surface structure of the amorphous TiO2 nanoparticles. The XPSspectrum of the amorphous TiO2 nanoparticles calcined at 320?C for 2 h was measured within thebinding energy range
of 0~1000 eV and is shown in Fig. 5a; and the individual lines of Ti 2p aredisplayed in Fig. 5b. The binding energy scale was calibrated by the C 1s peak at 285.0 eV fromthe surface contamination. The XPS signals supply the binding energy information of the surfaceand subsurface layers (5 nm deep) of the calcined titania sample. The XPS analysis reveals that
thesample contains only titanium, oxygen, and carbon. And the C 1s peak at 285 eV could beattributed to the surface contamination. The individual lines of Ti 2p in Fig. 5b show two peaks of2p3/2 and 2p1/2 with a better symmetry at 458.6 and 464.3 eV, respectively. The calcined samplehas a peak separation of 5.7 eV between the 2p3/2 and 2p1/2 peaks, which corresponds to a 2p3binding energy of Ti (IV) ion. The binding energy values of the individual components andtheir possible interpretations are obtained by comparing these values with the reported data forvarious chemical states. The XPS results reveal that the binding energies (e. g., 458.6 eV for Ti2p3/2) and the Ti 2p spectrum structure are identical to those of anatase. It could be assumed thatthe -O-Ti-O-Ti- network is already organized into the anatase-like structure although the titaniaparticles are amorphous in structure.
Amorphous TiO2 nanoparticles were prepared by a sol-gel process, using tetrabutyl titanateas neutral inorganic precursor, and following calcination. The TiO2 particles are amorphous instructure. TEM observation indicated that the primary particle size of the synthesized TiO2 isabout 3-5 nm and the aggregate size is about 30 nm. TG and DSC analysis shows that the samplecould be calcinated at 320?C to remove the orhanic substances. In TEM observations of thesamples, neither obvious diffraction rings nor diffraction spots could be observed in the selectedarea electron diffraction pattern, indicating the sample was amorphous in structure. This wasconsistent with XRD measurements. FTIR, EDS and XPS measurements indicated that thecomposition of the particles were pure TiO2.
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