Abstract
Nanostructured anatase TiO2 with controllable morphology has been fabricated via the oxidation of TiC with H2O2. At room temperature, the reaction of TiC with H2O2 leads to dissolution of TiC into H2O2 aqueous solution, producing an acidic solution. By drying the acidic solution at 80°C in air, an amorphous powder of polytitanic acid with oxalate ligands is obtained, and its morphology is found to rely on the reaction time. By annealing the amorphous acidic powder at T > 350°C, the nanostructured anatase TiO2 with controllable morphology is generated. Depending on the oxidation time, the morphology can be fabricated as sponge-like shape, flower-like shape, spongy balls, and so forth. The nanostructured anatase TiO2 is stable under the heating treatments until 900°C, and its morphology can be tuned to the nanocrystalline grains. In addition to the annealing way, rice-shaped anatase nanocrystals can be directly formed by aging the acidic solution under ambient conditions.
1. Introduction
Titania (TiO2) is one of the mostly studied semiconductors. Since the discovery of the ability of TiO2 in splitting water [1], great interest has been aroused in nanostructured TiO2 owing to the various applications in solar energy conversion and photodegradation of organic pollutants [2, 3]. In those applications, one of the important parameters is the specific surface area, which is strongly related to the nanostructured morphology. To date, many synthetic routes have been developed to prepare the nanostructured TiO2 with different morphologies, for example, fabrication of TiO2 nanoparticles with sol-gel method [4, 5], successful syntheses of TiO2 nanowires [6, 7], nanorods [8, 9], and nanotubes [10, 11] with hydrothermal method, and preparation of TiO2 nanowires [12, 13], thin film [14], and nanorods [15, 16] with solvothermal method. In these synthetic routes, however, the subsequent thermal treatment is usually required for crystallization and removal of residual contaminants. Recently, direct oxidation of pure titanium has been reported for the preparation of TiO2 nanomaterials. By inserting a cleaned Ti plate into hydrogen peroxide solution, TiO2 nanorods can be formed through the oxidation of Ti with H2O2 at low temperature [17, 18]. By using acetone as the oxygen source, the oxidation of Ti with acetone at high temperature is also able to produce TiO2 nanorods [19]. It has been reported that TiO2 nanotubes can be fabricated by anodic oxidation of a Ti foil [20].
Similar to Ti, TiC can be also easily dissolved into H2O2 aqueous solution at room temperature via the oxidation with H2O2, producing an acidic solution. By drying the acidic solution, an amorphous powder of polytitanic acid with oxalate ligands is able to be obtained as precursor for fabrication of nanostructured anatase TiO2 [21, 22]. In the present work, detailed investigations including XRD, Raman [23], SEM, and TEM have been performed on fabrication of nanostructured anatase TiO2 via the oxidation of TiC with H2O2.
2. Experimental
2.1. Synthesis
Commercial TiC powders (99.5% purity, 3 μm in average size) and 30% H2O2 aqueous solution were used as the original materials in the syntheses of nanostructured anatase TiO2. TiC powders were dissolved into H2O2 aqueous solution to produce an acidic solution. By drying the acidic solution in air at 80°C, amorphous powders of polytitanic acid with oxalate ligands were then obtained. Nanostructured anatase TiO2 was fabricated by annealing the amorphous acidic powders in a homemade tubular furnace at high temperatures.
2.2. Characterization
X-ray diffraction (XRD) patterns were collected using XRD diffractometer (Smartlab Rigaku, Japan) with Cu Ka radiation of l = 1.54056 Å. Raman scattering was performed in a Raman spectrometer (Renishaw, New Mills, UK) with an Ar laser (514.5 nm). The infrared pattern was measured with the FTIR-8100 FT-IR spectrometer (Shimadzu, Kyoto, Japan). The morphologies of the samples were checked in a scanning electron microscopy (S4800, Hitachi, Japan) and transmission electron microscopy (JEM-2010, JEOL, Japan). In the TEM and HRTEM observations, the samples were prepared as follows: a small amount of the fabricated powders dispersed in ethanol. After the ultrasonic vibration for 15 min, a drop of the dispersion was placed onto a copper grid coated with a layer of amorphous carbon.
3. Results and Discussion
3.1. Oxidation of TiC with H2O2
TiC powders can be dissolved in colorless H2O2 aqueous solution at room temperature via the oxidation of TiC with H2O2, and a solution of polytitanic acid with oxalate ligands is produced [22]. In the dissolution process, a great deal of gas is released via serious bubbles and the solution color changes quickly to orange (see inset of Figure S1 in supporting information). The monitoring of IR spectroscopy indicates that the released gas is dominantly CO2 (see Figure S1 in supporting information, available online at http://dx.doi.org/10.1155/2016/6325615). It is observed that the color of acidic solution depends on the reaction time. By maintaining the reaction, the acidic solution changes gradually to yellow in color (see Figure S2 in supporting information), and a yellow gel can be finally obtained. By drying the acidic solution or gel at 80°C in air, amorphous powder can be obtained, and its color relies on the oxidation duration (see Figure S2 in supporting information). The infrared spectrum of the amorphous powder (see Figure S3 in supporting information) is found to be similar to the previously reported ones of polytitanic acid with oxalate ligands [21, 22].
In the present work, the nanostructured anatase TiO2 was fabricated by annealing the amorphous powders of polytitanic acid with oxalate ligands at high temperatures. To investigate the influence of oxidation time on the morphology of nanostructured anatase TiO2, the dissolution of TiC into H2O2 aqueous solution was performed in a series of glass beakers (marked with first reaction time of 3 h, 10 h, 14 h, 20 h, and 7 h) for different durations. When the desired time was over, the residual TiC particles and/or any insoluble impurities were removed out. The amorphous powders of polytitanic acid with oxalate ligands were then obtained by drying the acidic solution or gel in air at 80°C. The amorphous acidic powders were finally annealed in a homemade tubular furnace at high temperatures in air to fabricate nanostructured anatase TiO2.
3.2. Nanostructured Anatase TiO2 with Controllable Morphology
When the reaction was kept for 3 hours, a little amount of the supernatant or acidic solution was abstracted to check the oxidation product via the TEM measurements. As shown in the TEM image of Figure 1(a), the oxidation of TiC with H2O2 leads to the formation of spherical nanoparticles. After the reaction was maintained for 7 hours, the residual TiC particles were filtered out. The obtained supernatant was checked with TEM. As shown in the TEM image of Figure 1(b), the oxidation-produced nanoparticles become smaller in size and deviate from the spherical shape, but they are connected with each other to form an irregular network. The collected SAED pattern (the left inset in Figure 1(b)) indicates that the nanoparticles are in the amorphous state. The supernatant was firstly dried at 80°C. The XRD pattern of dried powders (right inset in Figure 1(b)) confirms the amorphous state. By annealing the dried powders at 500°C in air for one hour, the TEM image in Figure 1(c) shows that the connected nanoparticles (~20 nm in average size) become more regular in shape. The XRD pattern (inset of Figure 1(c)) displays that the annealing leads to the formation of anatase phase. Figure 1(d) gives the HRTEM image for one of the nanoparticles, revealing the good crystallinity. The corresponding fast Fourier transform (FFT) power pattern (inset in Figure 1(d)) supports that the annealed nanoparticles are in anatase phase.
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After 10-hour reaction of TiC with H2O2, the residual TiC particles were then removed, and the filtered-out supernatant was observed by the TEM measurements. As shown in Figure 2(a), the TEM images indicate that the oxidation product evolved into a sponge-like porous structure. It can be recognized that the sponge-like structure is formed by entwining 2D nanosheets. In Figure 2(b), the HRTEM image of a nanosheet displays distorted atomic fringes, and consistently the weak diffraction rings can be recognized in the SAED of inset. Therefore, the nanosheets are crystallized to a certain extent. After drying the supernatant at 80°C, the obtained powder was characterized by XRD. In inset on the top right corner of Figure 2(a), the XRD pattern demonstrates the appearance of several broad reflection peaks, confirming the observations of HRTEM and SAED measurements. The dried powder was annealed at 350°C in air for 2 hours. The TEM image in Figure 2(c) displays that the annealed product is highly porous, similar to a sponge in shape. In Figure 2(d), the XRD and Raman scattering characterizations reveal that the annealing induces the formation of anatase phase.
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When the oxidation of TiC with H2O2 was kept longer than 10 hours, precipitates were observed to appear gradually. The oxidation was maintained for 14 hours; the residual TiC particles were then removed. As shown in the TEM image of Figure 3(a), the oxidation product was developed into urchin-like particles in the microscale. Clearly, these urchin-like particles are formed by aggregations of 2D nanosheets. In the collected SAED in inset of Figure 3(a), clear diffraction rings can be observed. The HRTEM image of a nanosheet in Figure 3(b) also reveals the presence of well crystallized grains, being consistent with the SAED observation. After drying at 80°C in air, however, the XRD pattern of the dried powder is found to be similar to the one in inset of Figure 2(b). The dried powder was annealed in air at 700°C for 2 hours. As shown in Figure 3(c), the TEM image indicates that the annealed powder exhibits a flower-like shape. As demonstrated in Figure 3(d), the XRD and Raman scattering pattern reveal the annealing-induced formation of anatase phase.
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When the oxidation reaction was kept longer than 14 hours, more precipitates were observed to appear. After the reaction of 20 hours, the residual TiC particles were filtered out, and the oxidation product was checked by the TEM measurements. The TEM image in Figure 4(a) shows that the oxidation product exhibits the morphology of fluffy balls. The fluffy balls can be imagined to originate from aggregations of 2D nanosheets in a huge amount. In the collected SAED of inset in Figure 4(a), diffraction rings are clearly observed, and some diffraction spots are also recognized. In the amplified TEM image in Figure 4(b), many grains are observed inside the nanosheets, and the HRTEM image of one grain in inset shows the good crystallinity. After drying in air at 80°C, however, the obtained powder still produces the XRD pattern similar to the previous ones in inset of Figure 2(b). At 750°C in air, the annealing was performed on the dried powder for 2 hours. As shown in the SEM image of Figure 4(c) and the enlarged one in inset, the annealed product is found to become highly porous microspheres. The XRD and Raman pattern in Figure 4(d) are indicative of the annealing-induced formation of anatase phase.
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The oxidation of TiC with H2O2 was maintained for 7 hours, same as the reaction of 10 h. By removing the residual TiC particles, however, the orange supernatant or acidic solution was kept for aging under the ambient conditions. By aging for 3 days, the orange supernatant was transformed to opaque yellow gel. The TEM observations of the gel in Figure 5(a) reveal the morphology of cottony hairs. Inside the cottony hairs, the TEM image in inset of Figure 5(a) is indicative of the presence of many nanocrystals with the average size less than 2 nm. The further aging leads to the gradual appearance of precipitates. By aging for 7 days, the precipitates became white in color (see Figure S4 in supporting information). Figure 5(b) shows the TEM image of the precipitates after the aging for 7 days. On a network background made up of the cottony hairs, many rice-shaped nanoparticles are observed to be present. Inset of Figure 5(b) shows the TEM image of a rice-shaped nanoparticle. On the top end, aggregation of some cottony hairs is still observable, indicating that the nanoparticle is not completely well developed. Moreover, as marked out by red ovals in Figure 5(b), it can be recognized that the aggregations of cottony hairs tend to evolve into the rice-shaped nanoparticles. Thereby, the rice-shaped nanoparticles should evolve from aggregation of the cottony hairs. On the TEM image (inset of Figure 5(b)), it can be seen clearly that the rice-shaped nanoparticle is actually constructed with many nanocrystals with the average size less than 5 nm. Figure 5(c) is the HRTEM image of the rice-shaped nanoparticle, and inset is the corresponding FFT pattern. It can be identified that the nanocrystals are assembled with smaller mosaic blocks, which arranged almost along the same orientation with only a small deviation. The presence of a small deviation in orientation leads to the splitting of bright spots on the FFT pattern. As marked out with the red circles, the presence of nanopores can be recognized between the nanocrystals, which formed owing to the deviations between those smaller mosaic blocks. Based on the HRTEM image and the corresponding FFT pattern, it can be determined that the rice-shaped nanoparticle has the anatase structure.
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As shown in the TEM image of Figure 6(a), the aging for two months leads to the complete formation of rice-shaped nanoparticles. The good crystallinity of the nanoparticles is displayed by the sharp rings in inset of Figure 6(a). The Raman and XRD patterns in Figure 6(b) indicate the anatase phase of the nanoparticles. Figure 6(c) shows the HRTEM image of rice-shaped nanoparticles and the corresponding FFT pattern (inset). It can be determined that the nanoparticle is well crystallized along the zone axis. As shown in the enlarged HRTEM image of Figure 6(d), the presence of nanopores is still observable in the rice-shaped nanoparticle, indicating that it evolves from the one constructed with the smaller nanocrystals.
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3.3. Annealing Effect on Nanostructured Anatase TiO2
After 10-hour reaction of TiC with H2O2, the amorphous powder of polytitanic acid with oxalate ligands was obtained. As shown in Figures 2(c) and 2(d), nanostructured anatase TiO2 with a sponge-like shape was fabricated by annealing of the amorphous acidic powder in air at 350°C for two hours. Figure 7(a) shows the XRD pattern for the nanostructured anatase TiO2 obtained by annealing the amorphous acidic powder at different temperatures above 350°C for two hours. Even after the annealing at 900°C, the obtained TiO2 is dominantly in the anatase phase, indicating that the fabricated anatase TiO2 is stable until 900°C. Moreover, the reflection peaks are shifted owing to the annealing at different temperatures, but the shift directions of peaks are not consistent. As shown clearly in inset of Figure 7(a), some reflection peaks are shifted consistently towards the low angle with the increase of annealing temperature, while the others are shifted firstly towards the high angle and then towards the low angle. The lattice parameters are determined from the XRD patterns and are shown in Figure 7(b). With the rise of annealing temperature, the lattice parameter exhibits a monoclinic increase, while for the lattice parameter or , a quick decrease occurs firstly below ~700°C and then a slow increase appears above ~700°C. The volume of unit cell was calculated and shown as inset in Figure 7(b). Obviously, the unit cell is induced to shrink with the increase of annealing temperature below 700°C but expand above 700°C.
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As shown in Figure 7(a), the reflection peak width becomes narrower with the increase of annealing temperatures, being suggestive of the annealing-induced increase in the grain size. The average grain sizes were calculated from the most intense refection peak (110) using the Scherrer formula of , where is the X-ray wavelength, is the diffraction angle of the (110) peak, and is the half-width after subtracting the instrumental broadening. Inset of Figure 7(b) demonstrates the variation of with the annealing temperature. The rise of annealing temperature leads to the monoclinic increase in the average grain size. Even after the heating treatment at 900°C for 2 hours, the average grain size for the dominant anatase TiO2 is still in the nanoscale. The annealing-induced change in morphology was checked by the SEM measurements. Figure 8 shows the SEM images obtained after the heating treatments at four typical temperatures. As shown in Figure 8(a), after the annealing at temperatures below 700°C, the sponge-like morphology is maintained for the obtained nanostructured anatase TiO2, but the pores become smaller with the rise of annealing temperature. After the annealing at 700°C, Figure 8(b) reveals that the nanostructured morphology is still similar to the sponge-like ones, but nanograins can be recognized to appear. When the annealing was performed at 800°C and above, Figures 8(c) and 8(d) reveal clearly the formation of nanocrystalline grains. This annealing-induced change in nanostructured morphology could be considered as the origin for the abnormal variations of the lattice parameters and unit cell volume.
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3.4. Conclusions
TiC can be easily dissolved into H2O2 aqueous solution at room temperature via the oxidation with H2O2, producing an acidic solution. By drying the acidic solution at 80°C in air, an amorphous powder of polytitanic acid with oxalate ligands is obtained as precursor for fabrication of nanostructured anatase TiO2. It is observed that morphology of the prepared amorphous acidic powder depends on the oxidation time. Relying on the oxidation time, annealing of the obtained amorphous acidic powder is able to produce the nanostructured anatase TiO2 with controllable morphology. By heating the nanostructured anatase TiO2 with a sponge-like morphology, it is found that the anatase phase is stable until 900°C, and the morphology is changed to the nanocrystalline grains. In addition to the annealing way, it is found that rice-shaped anatase nanocrystals can be directly formed by aging the acidic solution under the ambient conditions.
Additional Points
(i) Hydrogen peroxide is used as the oxidant, producing no environmental contaminations. (ii) By controlling the reaction time of TiC with H2O2, the morphology of the produced nanostructured anatase TiO2 can be easily tuned. (iii) Aging at room temperature is able to induce the crystallization of nanostructured anatase TiO2, thus favorable for energy conservation.
Competing Interests
The authors have declared that no competing interests exist.
Acknowledgments
The authors would like to thank the financial supports from the National Basic Research Program of China (Grant no. 511CB808205) and Natural Science Foundations of China (Grant nos. 51272225, 51121061, and 51102206).
Supplementary Materials
The IR spectroscpoy spectra of the gas released during the dissolution process. Images of color variance for the aqueous solution and products. The infrared spectra of the amorphous powder obtained from the reaction of TiC and H2O2. The EESL of the anatase TiO2 obtained after annealing treatment.