Abstract
We reported a facile hydrothermal approach to synthesize BaTiO3 nanocubes with controlled sizes for degradation of methylene blue (MB). The nanocubes with reaction time of 48 hours exhibited the highest photocatalytic efficiency, owing to their narrower size distribution and better crystallinity compared to those of 24 hours and, at the meantime, smaller particle size than those of 72 hours. This work also demonstrated the degradation of methylene orange (MO) using BaTiO3 nanocubes synthesized for 48 hours. Compared with the removal of MB, BaTiO3 had lower photocatalytic activity on MO, mainly due to the poorer absorption behavior of MO on the surface of BaTiO3 nanocubes. The degradation efficiency for each photocatalytic reaction was calculated. The possible mechanism of the photocatalytic decomposition on MB has been addressed as well.
1. Introduction
In recent years, the elimination of toxic chemicals from wastewater has become increasingly important. In particular, it has been reported that 17–20% of all industrial water comes from the dyeing and treatment of textiles [1–4]. The problem of toxicity from dye wastewater has received considerable attention, and until now numerous physical or chemical remedies have been developed, such as membrane filtration, ion exchange, and absorption on activated carbon to treat dye wastewaters, whereas they are accompanied with disadvantages of producing concentrated sludge production, low efficiency, and high cost, respectively [1, 5]. Consequently, photocatalysis has been intensively studied owing to its simplicity, low toxicity, good chemical stability, and high degradation efficiency of the dye macromolecule [2, 4, 6].
In particular, oxide photocatalyst has drawn much attention for degrading organic chemicals from wastewater through the advanced oxidation process (AOP), generally with the assistance of UV irradiation. The dye macromolecule can be mineralized into smaller and less harmful substances through a sequence of advanced oxidation processes triggered by strong oxidizing species, such as ∙OH radicals produced in situ [2]. Among these oxides, perovskite-type materials have unique potential which displays photostability and preeminent photocatalytic activity, by virtue of possessing larger lattice distortion and defects, and therefore providing additional routes of trapping holes and inhibiting the recombination rate of electron-hole pairs [4, 6, 7]. Also the vacancy of metal cations and O2− anions in the perovskite-type structure promotes the adsorption of oxygen onto the cation sites of the surface, which boosts the photocatalytic reaction [7, 8].
BaTiO3 is a typical perovskite photocatalyst with unique physical and chemical properties, which highly depend on its morphology and particle size; thus high purity and nanoscale structure are highly desired [9]. Hydrothermal method is being used for the fabrication of perovskite oxide nanocrystals due to several advantages of high purity, homogeneous, crystalline, controllable particle size, and well-defined morphology [10, 11]. Preparation of various BaTiO3 nanostructures such as nanowires, nanocubes, and nanorods has been reported in the literatures [9, 12–15]. Among all these structures, cubic structure has a superior property to decrease crystal defects and enhance the surface-to-volume ratio. Conventionally, BaTiO3 nanocubes have been prepared by several growth methods: sol-gel methods, the conventional hydrothermal method, and recently the surfactant-assisted hydrothermal method [4, 9, 14–18]. The surfactant assisted hydrothermal method enables growing BaTiO3 nanocubes at lower temperature and the ease of controlling the size and morphology of the final product [13]. By controlling the shape and size of the BaTiO3 nanocrystals, the electronic structure and surface structure are considered to promote the photocatalyst activity. From the electronic structure aspect, smaller nanoparticles have size-dependent electronic states different from the bulk, which guarantee their unique properties [6, 11]. From the surface structure aspect, the cubic shaped nanoparticles have surfaces with well-defined atomic arrangements. The atomic arrangement on the crystal surface can influence the photocatalytic properties in terms of activity and selectivity [1].
Here, in this study, ultrasmall BaTiO3 nanocubes have been successfully fabricated using hydrothermal method. The effect of the reaction time on the size and morphology of the nanocubes was studied. Also, the photocatalytic activity of the as-prepared BaTiO3 nanoparticles on decomposition of methylene blue (MB) and methylene orange (MO) has been investigated.
2. Experimental
All chemicals were used as received without further purification. In a typical synthesis, Ba(OH)2 and TALH (0.05 mol L−1, Ba : Ti = 1 : 1) were dissolved into small amount of distilled water. 6 mL of 1 M NaOH was added to adjust the pH value of the solution to 13. tert-butylamine and oleic acid were then added to the solution in sequence (Ba : oleic acid : tert-butylamine = 1 : 8: 12 in molar ratio). Finally, the volume of the solution was adjusted to 30 mL. The resultant mixture was then transferred and sealed into autoclave at 200°C for 24 hrs, 48 hrs, and 72 hrs, respectively. After the hydrothermal process, the final product was washed using ethanol for four times.
As-prepared BaTiO3 products were characterized by X-ray diffraction using CuKa radiation in a 2θ range from 10 to 90 degree with a step size of 0.026. Morphology features of BaTiO3 products were investigated using TEM (Phillips CM200).
The photocatalytic activity of as-prepared BaTiO3 for decomposing methylene blue (MB) and methylene orange (MO) in aqueous solution was investigated by the bleaching of dye solvated. In a typical measurement, the obtained BaTiO3 powders were put into a quartz reactor with 60 mL of MB aqueous solution, and the initial concentration was 20 mg/L. The reactor was then kept in the dark with agitation for 30 min to obtain adsorption equilibrium, prior to light irradiation by a 110 W UV lamp. The efficiency of the degradation processes was evaluated by monitoring the dye decolorization at the maximum absorption around λ = 663 nm as a function of irradiation time in the separated MB solution with a UV-vis spectrometer (Perkin Elmer Lambda 950).
3. Results and Discussion
3.1. Characterization
Figure 1 shows the XRD patterns of samples prepared under different reaction times. It can be found that the samples showed a series of well-indexed peaks referred to as cubic structure. BaTiO3 crystalline phase contents can be observed with all the reaction times. However, a small amount of BaCO3 was also identified in all of the samples contributing to the reaction between Ba(OH)2 and airborne CO2, which dissolved as during the hydrothermal process [9, 13]. The phase of BaCO3 has been illustrated on the XRD results indexed as the weak peaks at 2θ = 35° and 42°.
The influence of reaction time on the evaluation of BaTiO3 nanocube was studied by performing the experiments with various reaction times ranging from 24 hrs to 72 hrs, maintaining other reaction parameters identical. According to the TEM images shown in Figure 2, the morphology of all BaTiO3 samples resembles a cube-like nanostructure. The particle size increased from 10 to 20 nm with an increase in synthesis duration, confirming that the particle size of as-prepared BaTiO3 can be tailored by varying the synthesis time. Also, with the increasing reaction time, the edges of the cube became sharper, indicating an increase in the crystallinity of the cubic phase. As shown in Figure 2, little nanocube formation was observed at shorter period of time (24 hrs); instead, particles agglomerated in a spherical shape. Subsequently, by extending the reaction time to 48 hrs, it can be clearly observed that these nanostructures became larger and a few aggregated small particles were found on the surface. By prolonging the reaction time to 72 hrs, well-defined BaTiO3 nanocubes with sharp corners were obtained.
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3.2. Degradation of Dyes
The photocatalytic degradation of MB was selected to evaluate the photocatalytic activities of the prepared BaTiO3 nanocubes using UV-vis spectrometer. The band gap of BaTiO3 is = 3.2 eV [19], which allows it to adsorb the UV light. Figure 3 shows typical absorption spectra of methylene blue during different degradation periods. As can be seen from Figure 3, the decreasing absorption value (A) with the irradiation time represented the reduction of dye concentration in aqueous solution.
The intensity of maximum absorption (λ = 663 nm) was recorded at different time intervals and converted to the concentration of MB solution. The photocatalytic activities of BaTiO3 synthesized at different reaction durations are exhibited in Figure 4.
As shown in Figure 4, the photocatalytic activities varied from the samples prepared for increasing reaction duration. The sample synthesized for 48 hrs exhibits a relatively higher photocatalytic performance than that synthesized for either shorter or longer period. After 40 min of UV illumination, the MB removal over BaTiO3 for 48 hrs is as low as 98%, whereas it takes as long as 55 min until the MB was completely decomposed by BaTiO3 synthesized over a period of 72 hrs.
The photocatalytic activity of BaTiO3 nanocubes for the degradation of methylene orange was conducted under the UV light, shown in Figure 5. Typically, BaTiO3 synthesized for 48 hrs was selected since it provided the optimal photocatalytic performance over the degradation of MB. Compared with the photocatalytic results obtained for degradation of methylene blue, MO was decomposed more slowly by BaTiO3 nanocubes. It took about 65 min of UV illumination to reach a 96% removal.
The degradation efficiency (%) has been calculated and presented in Table 1, defined as where is the initial concentration of the dye and is the concentration of dye at degradation time, .
For the removal of MB, BaTiO3 synthesized for 48 hrs has the highest degradation efficiency which is 99.2% after 45 min UV irradiation. BaTiO3 for 72 hrs has the lowest efficiency of 72.7% under the same period of irradiation time. The intrinsic activity discrepancy among the BaTiO3 nanoparticles prepared over different reaction times can be explained through several factors: particle morphology, particle size, and crystallinity. It is well understood that morphology and size of nanoparticles are the two most important impacts on the photocatalytic activity due to the difference in surface area, the number of the active sites, and consequently the catalytic selectivity [6, 8, 20–22]. In comparison of as-prepared BaTiO3 samples, the nanoparticles prepared for 48 hrs have a better and uniform morphology distribution compared to those synthesized for 24 hrs. Also by synthesizing the BaTiO3 nanocubes for extended reaction duration, higher crystallization can be achieved and therefore, in turn, suppress the recombination of photoinduced holes and electrons [6]. On the other hand, the BaTiO3 nanocubes synthesized for 48 hrs have a smaller size of 15 nm compared to those prepared for 72 hrs. Smaller nanoparticles result in a larger surface area with more active sites and therefore enhance the photocatalytic activity. Also the smaller the particle size, the wider the band gap. Consequently the oxidizing ability of photoexcited holes and the reducing ability of photoexcited electrons are expected to be stronger. Also the migrating time of photoexcited electrons and holes from the inner to surfaces is shorter for smaller particle size [20, 21]. Hence the BaTiO3 nanoparticles synthesized for 48 hrs have the highest photocatalyst activity in degradation of MB with smaller crystal size of 15 nm compared to the BaTiO3 synthesized for 72 hrs and with better cubic morphology and crystallinity compared to the BaTiO3 nanoparticles prepared for 24 hrs.
For the removal of MO, the degradation efficiency was as low as 76.2% using BaTiO3 nanocubes synthesized for 48 hrs as a photocatalyst. As the photocatalytic procedure carried out for the removal of MO was identical to that used for MB (the initial dye concentration, the dosage of BaTiO3 powder, and the initial solution pH were identical), the main reason for this degeneracy of the removal efficiency of MO compared to that of MB can be attributed to the chemical structures of MO and MB. It is well known that MB has a positive charged surface whereas MO is negatively charged [23]. The opposite charged surface of these two dyes may lead to a difference in the degree of adsorption on the surface of BaTiO3 nanocubes [23, 24].
3.3. Degradation Mechanism
The mechanism of photocatalysis is shown in Figure 6 and can be described as follows. Under the illumination of UV light irradiation , the BaTiO3 nanoparticles are photoexcited, promoting charge separation. The electron generated from charge separation will be promoted from the valence band to the conduction band generating. The conduction band electron can migrate to the surface of BaTiO3. Subsequently, oxygen adsorbed on the surface of BaTiO3 is able to react with the photoelectron to initiate a series of strong oxidative free radicals. At the meantime, a positive charged hole (h+) in the valence band is formed which can react with H2O to generate ∙OH radical. These generated radicals further react with MB producing a whole range of intermediates to achieve complete mineralization with the formation of less harmful carbon dioxide, water, and nitrogen [2, 25].
4. Conclusion
Nanocubic BaTiO3 particles were synthesized by hydrothermal methods using tert-butylamine and oleic acid as two surfactants. The results of TEM images show that the morphology and size of BaTiO3 nanoparticles can be tailored by changing the reaction time. By increasing the reaction duration, the BaTiO3 particles formed more cubic shape with sharp corners and the particle size increased from 10 nm to 20 nm. These results indicate that BaTiO3 synthesized for 48 hrs has the highest photocatalytic activity, which can be attributed to the relatively better morphology compared to the BaTiO3 synthesized for 24 hrs and smaller particle size compared with the BaTiO3 synthesized for 72 hrs. In addition, the photocatalytic degradation of MO was examined using BaTiO3 photocatalyst. In contrast to cationic MB, anionic MO has lower degradation efficiency on BaTiO3 nanocubes, indicating that the intrinsic charge of dye may lead to a difference in the adsorption behaviour on the surface of photocatalyst.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.