Abstract
Heterogeneous photocatalysis using TiO2 is a non-selective technique used for the degradation of organic molecules. Controlling the morphology of TiO2 has recently been considered one of the important approaches for controlling the selectivity of TiO2 In this work, TiO2 nanotubes and nanosheets were synthesized from spherical TiO2 nanoparticles using the hydrothermal method. The starting and prepared samples were characterized by XRD, TEM and FESEM. The selectivity of the three morphologies towards the photocatalytic degradation of three food dyes (colours yellow sunset, red allura and red carmoisine) was tested. Importantly, changes in morphology led to each dye being adsorbed preferentially by one of the three morphologies and decomposing more rapidly, where the optimum rate of degradation for sunset yellow, red allura and red carmoisine was achieved by TiO2 nanosheets, spherical TiO2 and TiO2 nanotubes, respectively.
1. Introduction
Photocatalytic activity of TiO2 is greatly influenced by a range of factors including surface area, crystal structure, crystallite size, morphology, shape, particle aggregation, phase composition, surface defects and surface hydroxyl group content, among others [19, 16; 13]. These factors are strongly related to synthesis and processing routes [13]. Tsai and Cheng (1997) [14] compared several commercial and lab-made TiO2 samples in the photodegradation of phenol and found that the lab-made rutile without thermal annealing showed the greatest activity in the complete oxidation of phenol to CO2. On systematically studying the photocatalytic activity of a variety of hydrothermally prepared TiO2 particles, Testino et al. (Testino, Bellobono et al. 2007) [13] found that high crystallinity and high aspect ratio are key parameters for substantially improving the photocatalytic activity of rutile. These findings prompted us to prepare well-crystallized rutile TiO2 nanorods with high aspect ratio, where carriers can freely move along the length of the rods, which is expected to reduce electron-hole recombination. Furthermore, nanorods with a high aspect ratio and large size are highly desirable, as these nanoparticles are easier to be separated in water treatment [4; 6; 2]. TiO2 nanocrystals with various morphologies and shapes have been prepared using many methods, but primarily electrospining, templating and hydrothermal preparation [9; 3; 7; 11; 5; 16; 17]. A distinct advantage of hydrothermal preparation over electrospinning or templating fabrication is that the as-made product is crystalline, thus avoiding a thermal annealing step that consequently decreases the surface hydroxyl group content. However, it is relatively difficult to achieve nanorods with high aspect ratios like those obtained with electrospinning or templating fabrication by means of hydrothermal preparation. Controlling of the reaction rate is crucial in obtaining TiO2 nanocrystals with the desired crystalline structure and/or shapes [5].
In the process of using photocatalytic reactions to chemically degrade a contaminant, the problem arises that such reactions have low selectivity. Selectivity controls a reaction in such a way that one target molecule can be adsorbed preferentially by the system in order to decompose more rapidly [8; 18]. Research in the field of selective photocatalysis is relatively novel; accordingly, this work presents a different approach to selective heterogeneous photocatalysis, based on the selectivity of TiO2 with different morphologies regarding the degradation of a group of food azo dyes.
2. Experimental Section
2.1 Synthesis of TiO2 nanotubes and TiO2 nanosheets
All of the applied reactants and solvents were of analytical grade and were used without further purification. In a typical procedure, 5g of pure anatase phase TiO2 bulk-powder (spherical morphology) was mixed with 250ml of 10N NaOH aqueous solution under constant stirring for roughly 1 h. A milky-white solution appeared, which was then transferred to a Teflon-lined stainless steel autoclave with 500 ml capacity and heat-treated at 160°C for 4h and 16h, respectively, in order to prepare nanosheets and nanotubes. The autoclave chambers were air-cooled to room temperature following the reaction. The formed white precipitates were recovered and washed several times with distilled water. A treatment of the products with 0.1N HCl solution was carried out and the precipitates were finally calcinated at 500°C for 4 h in air [1].
The crystalline phases of the products were detected by X-ray diffraction. The morphologies of the samples were studied using a transmission electron microscope (TEM).
2.2 Photocatalytic experiments
Solutions of yellow sunset (YS), red allura (RA) and red carmoisine (RC) were prepared by dissolving the coloured powder in distilled water to obtain a solution of 1×10−6M concentration. The photocatalysis experiments were carried out in a 100mL beaker containing about 25mL of dye aqueous solution and about 0.05g of the catalyst. The irradiation was carried out using a 12W UV lamp as a source of UV radiation, which was placed vertically on the reaction vessel at a distance of 12cm. At specific time intervals, a certain amount of the sample solution was withdrawn and the changes in concentration of the dye were observed from its characteristic absorption at 480, 508 and 502nm for SY, RA and RC, respectively, using a UV-vis spectrophotometer model (Thermo Scientific, Evolution 600). The structural formulae of the three dyes are shown in Figure 1.
Structural formula of the colour dyes yellow sunset, red carmoisine and red allura
3. Results and Discussion
3.1 Physical characterization of the photocatalysts
The commercial TiO2 (spherical particles) and prepared TiO2 nanosheets and nanotubes powder were characterized by X-ray diffraction (XRD) analysis, field emission scanning electron microscopy (FESEM) and transmission electron microscope (TEM). The XRD patterns of the investigated TiO2 samples with different morphologies are shown in Figure 2(a-c). In Figure 2a, the bulk TiO2 reveals that anatase formed with excellent crystallinity, obvious from the peak intensities. It crystallized in the well-known tetragonal symmetrical manner, with four molecules per unit cell. The data were compared and indexed with ICDD card no 21–1272.
XRD patterns of (a) spherical TiO2; (b) TiO2 nanotubes; (c) TiO2 nanosheets. A = anatase; B = TiO2(B).
For the synthesized nanotubes: Figure 2(b) shows that planes (004) and (112) disappeared, while (105) and (211) decreased sharply in intensity. An observed improvement for the intensity of the (200) plane is very clear, which is a preferred orientation in the case of the tubular (1-D) shape, as clarified and seen in the TEM micrographs.
The nanosheets exhibit different features; the (101) plane became sharper, while all other planes were broader. Planes (004), (105) and (211) reappear again, owing to the 2D sheet morphology, but always remained broad and with low intensities, compared with the bulk sample (see Figure 2(a)). The appearance of a broad peak at roughly 30° can be assigned to the possible transformation of some anatase crystals into rutile, as was previously reported in nanoregime a possible transition can take place at about 550° [10].
The main differences between the three diffractograms are the intensities of the main peaks, as well as the existence of a preferred orientation (200) for nanotubes and for nanosheets (101). Moreover, the broad peaks point to the small crystallite size, as reported in Table 1.
Structural and kinetic parameters of TiO2 nanostructures
The TEM images of commercial TiO2 (spherical particles) are shown in Figure 3a. It can be seen that the TiO2 powder consists of nanosized grains with the presence of agglomerated particles. Figure 3b shows the TEM image of the prepared multilayered TiO2 nanosheets, while Figure 3c shows the TEM images of the prepared TiO2 nanotubes. The tubes have a diameter range of 16–70 nm and are arranged parallel to one another with nearly homogenous dimensions, with some intercalated tubes pointing in a different direction. The FESEM images of the three morphologies – spherical, nanosheets and nanotubes – are shown in Figure 4(a-c), respectively. In Figure 4a, TiO2 grains appear to have homogenous distribution with a small degree of coalescence. This was primarily due to electrostatic attraction between grains. Figure 4b clarifies that nanosheets overlap with one another, with no preferred direction of orientation. The sheets possess nearly similar dimensions, i.e., narrow distribution. The agglomerated appearance of grains originates from the absence of surfactant during preparation. In Figure 4c it is clear that the nanotubes are randomly oriented and appear to have uniform dimensions in terms of cross-section and length, and form a cluster-like network.
(a) TEM image of spherical TiO2; (b) TEM image of multilayered TiO2 nanosheets; (c) TEM image of TiO2 nanotubes
(a) FESEM image of spherical TiO2; (b) FESEM image of multilayered TiO2 nanosheets; (c) FESEM image of TiO2 nanotubes
3.2 Photocatalytic activity of the three morphologies
The three colours – yellow sunset, red allura and red carmoisine – were completely decolorized as shown in Figure 5a and changes in concentration were recorded as a function of UV-irradiation exposure time for different TiO2 nanostructures; this is illustrated in Figure 5b and Figure 6(a-c). It is clear that photocatalytic degradation was strongly dependent on TiO2 morphology. The photocatalytic activity of the three morphologies was tested as a model first on YS and it was found that as it moved from a spherical to a tubular structure through the sheets' structure, the time of degradation reduced from 400 min for spherical, 75 min for nanotubes and 55 min for nanosheets. When we tested the catalytic activity in the degradation of the other two colours – RA and RC – it was found that the previous trend did not occur again. As shown in Figure 7(a-c) and as summarized in Table 1, it is clear that the best rate of degradation for sunset yellow was achieved by TiO2 nanosheets, by spherical TiO2 for allura and by TiO2 nanotubes for carmoisine. These results are extremely promising and allow us to tune the morphology of TiO2 based on the targeted dye. From these observations, it is clear that the preferred orientation of each morphology rendered it more specific in terms of action; each dye is adsorbed preferentially by one of the three morphologies and decomposes more rapidly. In compliance with these results, Sofianou et al. [12] found that calcined TiO2 anatase nanoplates exhibited the best photocatalytic activity for oxidizing the NO gas to NO2 and NO3−, whereas the washed TiO2 anatase nanoplates, which preserved the initial morphology, exhibited the best photocatalytic activity in terms of decomposing acetaldehyde. They concluded the dominant exposed {1 0 1} or {0 0 1} crystal facets of the TiO2 anatase nanoplates to be the key factor in tuning the adsorption selectivity of air pollutants. Xiang et al. [15] also found that TiO2 films composed of flower-like TiO2 microspheres with exposed {001} facets exhibited tuneable photocatalytic selectivity towards the decomposition of azo dyes in water by modifying the surface of TiO2 microspheres, as well as by varying the degree of the etching of {001} facets.
(a) Optical image of colour yellow sunset, colour red carmoisine and colour red allura before and after degradation; (b) Changes in UV-vis spectra of colour yellow sunset, red carmoisine and red allura following irradiation with UV light in the presence of TiO2 nanoparticles with different morphologies
(a) Photocatalytic activity of Spherical TiO2; (b) TiO2 nanosheets; (c) TiO2 nanotubes.
Linear transform Ln (Co/C) = f(t) of kinetic curves
To understand this phenomenon, we have to know that the adsorption of reactants on the catalyst surface is one of the prerequisites in any heterogeneous catalytic chemical reaction. Selectivity in the adsorption stage can be achieved by changing the amount, size, morphology and surface of the catalyst used, as well as the type or size of the target compounds. As the adsorption phase becomes faster, the degradation time also decreases. In this work, experiments were designed to alter the exposed surfaces of TiO2 nanoparticles by controlling the morphologies of these particles. When the reaction time increased from four to 16 hrs, the nanoparticles showed different morphologies with preferred orientations. A preferred orientation (200) for nanotubes and for nanosheets (101), as well as the broad peaks, point to the small crystallite size as reported in Table 1, caused the variation in degradation rates and controlled selectivity.
A good correlation between crystal structure, morphology and photocatalytic activity will be established in order to recommend the use of such TiO2 in the selective photodegradation of organic dyes.
4. Conclusion
TiO2 nanotubes and nanosheets were successfully synthesized from commercial TiO2 (spherical particles) using a hydrothermal method. The photocatalytic activity of the three morphologies was tested for the degradation of yellow sunset, red allura and red carmoisine. It was concluded that nanoparticle morphology serves as the primary influence in the selectivity of catalysts. The optimum degradation of sunset yellow was achieved by TiO2 nanosheets, by spherical TiO2 for allura and by TiO2 nanotubes for carmoisine. These results are extremely promising and present the potential for tuning the morphology of TiO2 based on the targeted dye, thereby issuing in a new era in the field of selective heterogeneous photocatalysis.