Simple hydrothermal synthesis and photocatalytic performance of coral-like BaTiO₃ nanostructures

Abstract

Coral-like BaTiO₃ nanostructures were successfully synthesized via a simple hydrothermal route at 150 °C for 15 h employing BaCl₂, tetrabutyl titanate [(C₄H₉O)₄Ti] and NaOH as reactants without the assistance of any surfactant or template. The phase of the as-obtained BaTiO₃ was characterized by X-ray powder diffraction (XRD). Energy dispersive spectrometry (EDS), scanning electron microscopy (SEM) and (high-resolution) transmission electron microscopy (TEM/HRTEM) were employed for the composition and morphology analyses of the final product. Some factors influencing the formation of the coral-like BaTiO₃ nanostructures were investigated, including the amount of NaOH, the barium ion source, and the reaction temperature and time. Experiments showed that the as-prepared coral-like BaTiO₃ nanostructures presented good photocatalytic activity for the degradation of methyl orange (MO) dye under the irradiation of artificial sunlight. It was found that the photocatalytic activity of the coral-like BaTiO₃ nanostructures could be affected by the pH value of the system.

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1. Introduction

With the blooming development of industrialization process, environmental issues have become increasingly severe due to the discharge of a large amount of wastewater globally every year. To maintain the sustainable development of the human society, people must fight environmental pollution. Nowadays, many methods are employed for the treatment of pollution, including adsorption, ion-exchange, membrane separation, and photocatalysis.¹⁻³ Among them, photocatalysis is believed to be one of the most efficient technologies for the degradation of organic pollutants. Therefore, many photocatalysts including TiO₂ have been designed and produced.

As one of the prominent ferroelectric and piezoelectric materials, perovskite-type BaTiO₃ is mainly applied in multilayer capacitors, nonvolatile memories, thermistors, transducers, and electro-optical devices due to its large dielectric constant and nonlinear optical coefficient.^4–6 Recently, however, some titanate-based materials including BaTiO₃ were found to bear good photocatalytic activities.^7–11 Obviously, these are useful exploration for environmental treatments.

Many studies have discovered that the properties of a material are usually dependent on its shape and/or size. Thus, it has become a goal to prepare BaTiO₃ nanostructures with a certain morphology. However, the conventional method to synthesize BaTiO₃ is the solid-phase reaction between BaCO₃ and TiO₂ at a high temperature, ca. 1100 °C. The morphology and size of BaTiO₃ cannot be easily controlled by the solid-phase route; thus, only irregular particles with micrometer scales are often obtained. Unlike the solid-phase process, the morphology- and size-controlled synthesis of a material can be easily realized in wet chemical routes through simply adjusting experimental parameters. To date, employing various solution-phase approaches, many BaTiO₃ micro/nanostructures with different morphologies and sizes have been successfully obtained including core–shell particles, plate-like particles, nanoporous structures, nanotubes, tetragonal and dendritic structures.^12–17 In 2008, Maxim et al. hydrothermally synthesized BaTiO₃ micro/nanostructures by the reaction of titanate layered nanotubes and barium hydroxide.¹⁷ Simultaneously, they investigated the influence of the reaction temperature and time on the shape of the final product, and found that dendritic BaTiO₃ microstructures could be generated at 200 °C for 24 h or 110 °C for 114 h. However, the performance of the product was ignored. Recently, Lee and coworkers prepared cubic BaTiO₃ nanoparticles through a hydrothermal route at 100–180 °C for 24–72 h, employing P25-TiO₂ and Ba(OH)₂ as the reactants. Simultaneously, the UV light-assisted photocatalytic activity of the as-obtained BaTiO₃ nanoparticles was investigated.¹⁸

Distinctly, the hydrothermal synthesis of BaTiO₃ micro/nano-structures has been reported in literature.^17,18 However, high reaction temperatures or/and long reaction times were often needed. Simultaneously, some specific reactants were used. In the present work, we designed a mild hydrothermal route to successfully prepare coral-like BaTiO₃ nanostructures. BaCl₂, tetrabutyl titanate and NaOH were used as reactants. The reactions were carried out at 150 °C for 15 h, without the assistance of any surfactant or template. Compared with previous reports,^17,18 a lower temperature, shorter time, and/or common reactants were employed. At the same time, the photocatalytic performance of the BaTiO₃ nanostructures under the illumination of an artificial sunlight was studied. More importantly, considering the practical applications, the influence of pH on the photocatalytic performance of BaTiO₃ was also investigated.

2. Experimental section

All the reagents and chemicals were analytically pure, purchased from Shanghai Chemical Company and used without further purification.

2.1 Synthesis of coral-like BaTiO₃

In a typical experiment, 0.92 g of BaCl₂·6H₂O (∼2.9 mmol) was first dissolved in distilled water to form a 20 mL solution. Then, 1 mL of tetrabutyl titanate (∼2.9 mmol) was added into the above solution with vigorous magnetic-stirring and continuously stirred for another 20 min to get a uniform solution. Here, slightly excess NaOH (6 mmol) was introduced with stirring to assure the complete reaction of Ba and Ti source. After successively stirring for another 10 min, the system was poured into a Teflon-lined stainless steel autoclave of 50 mL capacity, and heated at 150 °C for 15 h. Finally, the autoclave was allowed to cool to room temperature naturally. The white precipitate was collected, washed with distilled water and absolute ethanol for several times to remove the impurities, and dried in vacuum at 60 °C for 6 h.

To investigate the influence of the amount of NaOH, the above preparation process was repeated under the presence of 0, 12 and 24 mmol NaOH.

2.2 Characterization

X-ray powder diffraction (XRD) patterns were obtained on a Shimadzu XRD-6000 X-ray diffractometer equipped with Cu Kα radiation (λ = 0.154060 nm), employing a scanning rate of 0.02° s⁻¹. TEM images were obtained on a JEOL JEM-2010 transmission electron microscope, employing an accelerating voltage of 200 kV. SEM images and energy dispersive spectrometry (EDS) of the product were obtained on a Hitachi S-4800 field emission scanning electron microscope, employing the accelerating voltage of 5 kV and 15 kV, respectively. The FTIR spectrum of the product was obtained on an IR Prestige-21 Infrared spectrometer (Shimadzu Corporation). The TGA of the porous superstructures was obtained on a differential thermal analyzer (ThermoElectron Company of America, SDT Q600). UV-vis diffuse reflectance spectrum (DRS) was recorded by a Shimadzu UV2450 spectrometer using MgO as a standard at room temperature. N₂ adsorption measurements were performed on a Micromeritics ASAP 2020 M + C volumetric adsorption equipment at 77 K using Brunauer–Emmett–Teller (BET) calculations for surface area.

2.3 Photocatalytic property

To investigate the photocatalytic properties of the as-prepared coral-like BaTiO₃ nanocrystals, methyl orange (MO) dye was selected as the pollutant model. In a typical experiment, 20, 30 and 40 mg of coral-like BaTiO₃ nanocrystals were dispersed in 50 mL 10 mg L⁻¹ MO aqueous solution. The pH value of the system was adjusted by the addition of either an NaOH or HCl solution. Prior to irradiation, the system was dispersed by ultrasonic waves for 15 min and then stirred for 30 min in the dark to ensure an adsorption/desorption equilibrium. After the system had been irradiated by a 500 W Xe lamp without a filter with continuous stirring for a given time, the photocatalyst was removed by centrifugation at 10 000 rpm for 5 min. The concentration change of MO dye was monitored by a Metash 6100 UV-vis absorption spectrophotometer (Shanghai).

3. Results and discussion

3.1 Structure and morphology characterization

Fig. 1a shows a typical XRD pattern of the product prepared under the current hydrothermal conditions. By comparison with the data from JCPDS card files no. 86-1569 (see the bottom in Fig. 1a), all of the diffraction peaks can be indexed to the cubic BaTiO₃ phase. The strong diffraction peaks indicate that the product has good crystallinity. The bottom curve shown in Fig. 1b is the energy dispersive spectrometry (EDS) analysis of the as-prepared product. The peaks belonging to C, Ba, Ti and O elements are visible. Based on the calculation of peak areas, the molar ratio of Ti : Ba : C : O is 1 : 1.11 : 3.44 : 4.14. Markedly, Ba is slightly more than Ti according to the stoichiometry of BaTiO₃. According to Maxim and Lee's reports,^17,18 small amounts of BaCO₃ always exist in the final product. In the present work, it is also possible that a small amount of BaCO₃ exists in the product, although it cannot be detected in the XRD pattern due to the limitation of the instrument. Furthermore, the high carbon content might be appearing from the adsorption of the final product on organic molecules, which were produced during the formation of BaTiO₃. This will be proven in the subsequent section.

Fig. 1 (a) The XRD pattern, (c) FTIR spectrum and (d) TG analysis of the product prepared by the current hydrothermal route; (b) EDS analyses of the product before and after heat-treatment.

In the present simple hydrothermal system, the formation of BaTiO₃ should be obtained by following two reactions:

(C₄H₉O)₄Ti + 4H₂O = Ti(OH)₄ + 4C₄H₉OH (1)

BaCl₂ + Ti(OH)₄ + 2NaOH = BaTiO₃ + 2NaCl + 3H₂O (2)

The produced BaTiO₃ nanostructures could adsorb the n-butyl alcohol generated in eqn (1) owing to the high surface energy of the nanoparticles, which resulted in the high C content. To prove the abovementioned speculation, the FTIR spectrum of the final product was obtained. As shown in Fig. 1c, the broad and strong band centered at 3435 cm⁻¹ is assigned to the O–H stretching vibration of the interlayer water molecules and the H-bound OH group. Another peak observed at 1634 cm⁻¹ is assigned to the bending vibration of water molecules.¹⁹ A weak peak at 2925 cm⁻¹ belongs to the stretching vibration of –CH₂– group. The two broad characteristic bands below 700 cm⁻¹ are assigned to the Ti–O vibration modes. Another characteristic band of the crystalline barium titanate phase is observed at 1444 cm⁻¹.²⁰ Fig. 1d shows the TGA curve of the obtained product. Although the total weight loss is only 8% in the range from room temperature to 600 °C, two weight-loss regions can be observed. The first weight loss located at 100–200 °C should be ascribed to the desorption of physically adsorbed water on the surfaces of the coral-like BaTiO₃ nanostructures.¹⁰ The second one is found in the range of 200–470 °C, which should be appearing due to the loss of surface organic molecules and chemically bonded hydroxyl groups.¹⁰ Here, EDS technology was again employed for the component analysis of the calcined product. As shown in the upper curve of Fig. 1b, the C content in the product dramatically decreases. This fact proves that the high C content in the uncalcined product was due to the adsorption of organic compounds.

The morphology of the as-obtained BaTiO₃ was observed by SEM and TEM techniques. Fig. 2a shows a representative low-magnification SEM image of the product. Coral-like nanostructures can be observed easily. Further enlargement shows that the ends of the coral-like nanostructures present dendritic structures (see Fig. 2b), which were confirmed by the TEM observations. As shown in Fig. 2c, the products are solid dendrites with diameters of 100–200 nm. An HRTEM image of the product is depicted in Fig. 2d. The clear lattice fringes are visible. The distance between neighbouring planes is measured to be 0.397 nm, which is very close to 0.4003 nm of the (010) plane of cubic BaTiO₃ (JCPDS no. 86-1569). Furthermore, the SAED pattern shown in Fig. 2d confirms the good crystallinity of the coral-like BaTiO₃ nanostructures.

Fig. 2 Electron micrographs of the product prepared by the current hydrothermal route: (a) a representative low-magnification SEM image, (b) a high-magnification SEM image, (c) a representative TEM image, and (d) an HRTEM image and (inset) SAED pattern.

3.2 Influencing factors

In the present work, NaOH was another important component in addition to BaCl₂ and Ti(OC₄H₉)₄. Experiments showed that the amount of NaOH could affect the phase and morphology of the final product. When no NaOH was introduced, the product comprised of aggregated grains and films (see Fig. 3a). XRD analysis showed that the final product was TiO₂ (see the below curve in Fig. 3d). After 12 mmol of NaOH was employed, many irregular particles and few coral-like nanostructures coexisted in the product (Fig. 3b). Here, XRD analysis confirmed the production of small amounts of BaCO₃ in addition to BaTiO₃ (see the middle curve in Fig. 3d). Upon further increasing the amount of NaOH to 24 mmol, the product was composed of abundant particles with a mean size of ∼300 nm (Fig. 3c). Moreover, the content of BaCO₃ further increased in the final product based on the result of XRD (see the above curve in Fig. 3d). The abovementioned facts clearly indicate that appropriate amounts of NaOH are indispensable in the formation of coral-like BaTiO₃ nanostructures.

Fig. 3 SEM images of the products prepared under the same experimental conditions from the systems with various amounts of NaOH: (a) 0 mmol NaOH, (b) 12 mmol NaOH and (c) 24 mmol NaOH. (d) Corresponding XRD patterns.

Furthermore, the Ba²⁺ ion source could also affect the formation of the coral-like BaTiO₃ nanostructures. Fig. 4a and b shows the typical SEM images of the products prepared from the systems containing Ba(CH₃COO)₂ and Ba(NO₃)₂, respectively, under the same experimental conditions. Obviously, aggregated nanoparticles and rods were generated when Ba(CH₃COO)₂ was used as the Ba²⁺ ion source instead of BaCl₂ (see Fig. 4a). However, when Ba(NO₃)₂ was selected as the Ba²⁺ ion source, coral-like nanostructures were still the main product in addition to some big grains (see Fig. 4b). The abovementioned facts demonstrate that the counter anion also plays an important role in the formation of BaTiO₃ with various shapes. Generally, CH₃COO⁻ anions have a stronger coordinative ability to some metal ions than the Cl⁻ and NO₃⁻ anions. Thus, it is possible that the BaTiO₃ nuclei were surrounded by CH₃COO⁻ anions, which was unfavorable to the oriented growth of the BaTiO₃ crystals. As a result, no coral-like nanostructures were formed. Due to the weak interaction to BaTiO₃, however, Cl⁻ or NO₃⁻ ions could only occupy small amounts of active sites, and the BaTiO₃ nuclei could grow oriented along the directions unoccupied by Cl⁻ or NO₃⁻ ions. Thus, coral-like nanostructures were finally obtained.

Fig. 4 SEM images of the products prepared under the same experimental conditions with different Ba²⁺ ion sources: (a) Ba(CH₃COO)₂ and (b) Ba(NO₃)₂.

Further investigations showed that coral-like BaTiO₃ nanostructures were always obtained in the proper temperature and time ranges. Namely, the reaction temperature and time hardly affected the morphology of the final product in the present experimental conditions. Based on the abovementioned experimental facts, in the present simple hydrothermal system, the possible formation mechanism of coral-like BaTiO₃ nanostructures can be described as follows: with the production of BaTiO₃, the saturation of the system increased. BaTiO₃ gradually nucleated and grew. Because some active sites of the BaTiO₃ nuclei were occupied by Cl⁻ ions, BaTiO₃ nanocrystals grew oriented along the directions unoccupied by Cl⁻ ions. Finally, coral-like BaTiO₃ nanostructures were formed.

3.3 Photocatalytic performance of the coral-like BaTiO₃

Generally, BaTiO₃ is a direct band gap semiconductor with an energy band gap of about 3.2 eV,^21,22 and mostly absorbs in the ultraviolet portion of the solar spectrum. Fig. 5 depicts the UV-vis diffuse reflectance spectra of the BaTiO₃ nanostructures prepared from the systems containing 6, 12 and 24 mmol NaOH. Indeed, various BaTiO₃ nanostructures mainly absorb the UV light below 400 nm. The band gaps (E_g) of three BaTiO₃ nanostructures are estimated to be ∼3.26, 3.29 and 3.30 eV. The coral-like BaTiO₃ nanostructures present the lowest band gap.

Fig. 5 UV-vis diffuse reflectance spectra of BaTiO₃ nanostructures prepared from the systems containing various amount of NaOH. The inset is the plot of (ahν)² ∼ hν.

Based on the consideration of practical applications, sunlight should be the most ideal irradiation source. In our experiments, a 500 W of Xe lamp without a filter was used as the illumination source. Thus, an artificial sunlight containing UV light was obtained. To investigate the photocatalytic performance of the as-obtained BaTiO₃ nanostructures, certain amounts of BaTiO₃ nanostructures were dispersed in a methyl orange (MO) solution of 10 mg L⁻¹. Fig. 6a shows the absorption spectra of the MO aqueous solutions irradiated by the artificial sunlight source for various durations in the presence of 30 mg of coral-like BaTiO₃ nanostructures. The characteristic peak of MO gradually decreases with the extension of the irradiation time. After 90 min, the MO solution is almost colorless (see the insert in Fig. 6a). Under the irradiation of the light, generally, photogenerated electrons and holes can be formed in a photocatalyst.²³

Photocatalyst + hv → photocatalyst + e⁻ + h⁺ (3)

Fig. 6 (a) Absorption spectra and photographs (see the inset) of MO aqueous solutions of 10 mg L⁻¹ irradiated by a 500 W Xe lamp for various durations in the presence of 30 mg coral-like BaTiO₃ nanostructures. (b) The correlation curves between the photodegradation efficiency of MO and the amount of catalyst.

Then, the holes react with water molecules or OH⁻ to produce ˙OH radicals with strong oxidative activity.²⁴

h⁺ + H₂O → ˙OH + H⁺ (4)

Thus, organic molecules such as MO dyes adsorbed on the surface of the photocatalyst are finally oxidized by ˙OH radicals. The abovementioned degradation process should also be applicable in our work.

Furthermore, it was found that the catalytic efficiency of the photocatalyst could be affected by its amount. As seen from Fig. 6b, when no catalyst was used, ∼40% MO was degraded within 90 min under the same light source. After 20 mg BaTiO₃ was employed, ∼75% MO was degraded under the same experimental conditions. When the amount of BaTiO₃ was increased to 30 mg, ∼92% MO was removed from the aqueous solution. Upon further increasing the amount of BaTiO₃ to 40 mg, however, only ∼71% MO was degraded. Namely, the degradation efficiency of the catalyst decreased. It was believed that the solid catalyst could prevent the utilization of the light, especially at high concentrations.²⁵ Thus, excess BaTiO₃ could not promote the catalytic efficiency.

In practical applications, wastewater is often acidic or alkaline. Therefore, it is very significant to investigate the influence of the pH of the system on the catalytic efficiency of the catalyst. Fig. 7 depicts the correlation curves between the degradation efficiency of the catalyst to the MO dye and the pH of the system in the presence of 30 mg BaTiO₃ under the same experimental conditions. One can find that the degradation efficiency of the catalyst first decreased with the increase of the pH value from 1 to 7, and then increased with the pH from 7 to 13. Under the neutral system, the catalyst presents the lowest catalytic efficiency. Moreover, the degradation efficiencies are higher under acidic conditions than those under basic conditions (see the inset in Fig. 7). Lee et al. considered that the surfaces of photocatalysts with a negative charge easily adsorbed cationic species, while anionic ones were readily adsorbed on the surfaces of positively charged photocatalysts.¹⁸ In the present solution with 10 mg L⁻¹ MO, the pH of the system was ∼5. When the pH was adjusted by HCl to a lower value, the surfaces of the coral-like BaTiO₃ nanostructures became positive, which was made available for adsorbing more negatively charged MO molecules on their surfaces. Markedly, this facilitated the degradation of the MO dye. In addition, Ba²⁺ ions could be leached from BaTiO₃ nanocrystals in neutral aqueous solution and the leaching rate became faster with the pH decrease of the system.²⁶ Here, TiO₂ was produced on the surface of the BaTiO₃ nanocrystals according to the following reaction:^20,26

BaTiO₃(s) + 2H⁺(aq) = Ba²⁺(aq) + TiO₂(s) + H₂O(aq) (5)

Fig. 7 The correlations between the degradation efficiency of the catalyst and the system pH in the presence of 30 mg BaTiO₃ nanostructures. The inset is the correlation curve of the degradation efficiency of the catalyst and the system pH after MO solution was irradiated for 90 min.

Thus, the catalytic efficiency of the product was improved in the acidic system. Different from the acidic system, however, more hydroxyl radicals with high oxidation capability were generated at the higher pH values, which also promoted the degradation of MO dye.^27–29

Moreover, we also investigated the influence of the morphology of the catalyst on the photocatalytic performance. As shown in Fig. 8, when the BaTiO₃ particles prepared under the presence of 12 mmol or 24 mmol NaOH were used as the catalyst, respectively, the degradation efficiencies were in turn 50% and 40%, respectively, within 90 min, which are far lower than 92% of the coral-like nanostructures. Obviously, the abovementioned degradation efficiency change should be related to the morphology of the final product. Generally, the morphology change of a catalyst can cause its surface area to change. Accordingly, its adsorptive capacity will be also different. Fig. 9 shows the BET surface areas and BJH adsorption average pore diameters of the final products prepared from the systems containing 6 and 12 mmol NaOH. The BET surface areas and BJH adsorption average pore diameters are 74.1 m² g⁻¹ and 7.7 nm, and 17.1 m² g⁻¹ and 9.4 nm, respectively. Markedly, the surface area of the product decreased with the increase of NaOH amount in the initial system. As a result, the adsorptive capacity of the catalyst decreased, and accordingly the degradation efficiency decreased. Moreover, as mentioned previously, the content of BaCO₃ increased in the final product with the increase of NaOH amount. This is another possible reason to cause the photocatalytic capacity decrease of the catalyst in addition to the influence of the morphology.

Fig. 8 The comparison of the photocatalytic efficiency of the catalyst in the presence of 30 mg BaTiO₃ nanostructures with various shapes prepared from systems containing 6, 12 and 24 mmol of NaOH.

Fig. 9 BET surface areas and pore size distribution of the BaTiO₃ nanostructures prepared under the same conditions with different NaOH concentrations: (a) 6 mmol and (b) 12 mmol.

4. Conclusions

In summary, coral-like BaTiO₃ nanostructures have been successfully prepared by a simple hydrothermal route at 150 °C for 15 h without the assistance of any surfactant or template. It was found that the amount of NaOH played a crucial role in the formation of the coral-like BaTiO₃ nanostructures. With the increase of NaOH amount from 6 mmol to 24 mmol, the content of BaCO₃ gradually increased in the final product and the coral-like nanostructures retrogressed into nanoparticles. Experiments showed that the coral-like BaTiO₃ nanostructures could be used as a photocatalyst for the degradation of methyl orange. Under the irradiation of artificial sunlight, 92% of MO was degraded under the presence of 30 mg of catalyst within 90 min. Furthermore, the pH value of the system could markedly affect the photocatalytic efficiency of the catalyst. The catalyst presented the lowest catalytic efficiency in the system of pH 7.0, and the catalyst exhibited a better catalytic efficiency in acidic or alkaline solution. This is significant in practical applications, indicating that the as-obtained coral-like BaTiO₃ nanostructures have potential applications in environmental treatment and protection.

Acknowledgements

The authors thank the National Natural Science Foundation of China (21171005) and Key Foundation of Chinese Ministry of Education (210098) for the fund support.

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