Controlled synthesis of three-dimensional hierarchical Bi2WO6 microspheres with optimum photocatalytic activity

Materials Research Bulletin 47 (2012) 315–320

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Controlled synthesis of three-dimensional hierarchical Bi2WO6 microspheres with optimum photocatalytic activity Hong Wang, Jimei Song *, Hui Zhang, Fei Gao, Shaojuan Zhao, Haiqin Hu College of Chemistry and Chemical Engineering, Anhui University, Key Laboratory of Anhui Province of Functional Inorganic Materials Chemistry, Hefei 230039, PR China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 29 May 2011 Received in revised form 19 September 2011 Accepted 9 November 2011 Available online 18 November 2011

Three-dimensional (3D) hierarchical Bi2WO6 microsphere and octahedral Bi2WO6 have been synthesized by a facile hydrothermal method using KNO3 solution and distilled water as solvent, respectively. The obtained products were characterized by X-ray diffraction, scanning electron microscopy, N2 adsorption/ desorption, and UV–vis diffuse reflectance spectroscopy in detail. The concentration of KNO3 played a key role in the formation of 3D hierarchical Bi2WO6 microspheres. A possible formation mechanism of Bi2WO6 microsphere was proposed. The photocatalytic activity of the as-synthesized products was evaluated by monitoring the degradation of MB solution under sunlight irradiation. It was found that the photocatalytic activity of the 3D hierarchical Bi2WO6 microsphere was superior to the octahedral Bi2WO6, which was attributed to the larger surface area and special hierarchical structure of Bi2WO6 microsphere. ß 2011 Elsevier Ltd. All rights reserved.

Keywords: A. Inorganic compounds A. Microporous materials B. Chemical synthesis C. X-ray diffraction D. Catalytic properties

1. Introduction There have been many studies on the preparation of hierarchical architectures with anisotropic structures, such as one-dimensional and two-dimensional shape [1,2]. The architectures are expected to exhibit unique physical and chemical properties depending on the size, shape, orientation, alignment, and dimensionality. Hierarchical architectures can be formed through several methods, such as biomimetic mineralization [3], polymer directed self-assembly [1], oriented aggregation of nanoscale building units [4], Kirkendall-type diffusion [5], hardand soft-templating synthesis [6,7], sequential nucleation and growth [8], etc. Generally, the chemical-solution-phase selfassembly method is considered as the most simple, effective, and inexpensive. Bismuth tungstate (Bi2WO6) is one of the simplest members of the Aurivillius oxide family of layered perovskites with the general formula Bi2An1BnO3n+3 (A = Ca, Sr, Ba, Pb, Na, K; B = Ti, Nb, Ta, Mo, W, Fe; and n = number of perovskite-like layers (An1BnO3n+1)2), which is structurally composed of alternating perovskite-like and fluorite-like blocks [9]. It is widely used in electrode materials [10], solar energy conversion [11] and catalysis [12]. In 1999 Kudo and Hijii first reported that Bi2WO6 exhibited the photocatalytic activity for O2 evolution, and then Zou et al. revealed that the organic compound could be degraded under visible light irradia-

* Corresponding author. Tel.: +86 551 5107342; fax: +86 551 5107342. E-mail address: [email protected] (J. Song). 0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2011.11.017

tion in the presence of Bi2WO6 [13,14]. However, all Bi2WO6 samples in above works were prepared by solid-state reaction and their photocatalytic activity was also poor. It is well-known that nanoscale photocatalysts usually exhibit excellent photocatalytic activity. Many efforts to synthesize nanosized Bi2WO6 have been made [15–18]. Nevertheless, it is difficult to separate and recycle the nanoscale photocatalysts. 3D microscale architectures fabricated from nanosized building blocks hold many advantages, such as high photocatalytic activity, abundant transport paths for small organic molecules, and easy operation of separation and recyclability. Recently, 3D hierarchical architecture Bi2WO6 has been reported by several research groups. Dekun Ma [19] reported that 3D hierarchical umbilicate Bi2WO6 microspheres were synthesized by a hydrothermal self-assembly process using citrate anions as a complexing agent. Huang et al. illustrated that the porous Bi2WO6 was prepared via the ultrasonic spray pyrolysis using bismuth citrate and tungstic acid as precursors [20]. Moreover, caddice clew-like, nest-like, flower-like and platelike Bi2WO6 hierarchical nano/microstructures were also prefabricated by using CTAB as template [21]. However, the morphologies and phases of Bi2WO6 exhibited a strong dependence on the pH of the precursor solution. Researchers unanimously believe that adjusting pH value or using organic compound (surfactant or polymer) would make the synthetic procedure complicated. Hence, it is no doubt that designing a simple, low-cost, morphologycontrolled method is very meaningful and significant. In this work, 3D hierarchical Bi2WO6 microsphere and octahedral Bi2WO6 were synthesized by the designed hydrothermal method using KNO3 solution and distilled water as solvent,

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respectively. The formation mechanism and morphology evolution of Bi2WO6 microsphere was proposed, and the photocatalytic activities of the as-synthesized products were evaluated by monitoring the degradation of MB solution under sunlight irradiation. It was found that the photocatalytic activity of the 3D hierarchical Bi2WO6 microsphere was superior to the octahedral Bi2WO6. 2. Experimental All reagents used in this work were analytical grade and without any further purification. 2.1. Synthesis of 3D hierarchical Bi2WO6 microsphere In a typical procedure, 2 mmol Bi(NO3)35H2O and 1 mmol Na2WO42H2O were dissolved in 20 mL solution of KNO3, respectively. Then, the Bi(NO3)3/KNO3 solution was added into the Na2WO4/KNO3 solution with vigorous magnetic stirring. After further agitation for half an hour, the mixed solution was poured into a stainless steel autoclave with a Teflon liner and heated at 140 8C for 6 h. When the autoclave had been cooled to room temperature, the products were separated by centrifugation and washed with distilled water and absolute ethanol several times. Subsequently, the products were dried under vacuum at 60 8C for 6 h. The product was labeled as sample 1. 2.2. Fabrication of octahedral Bi2WO6 During the above preparation process, KNO3 solution was substituted by distilled water, and the pH value of the solution was adjusted to 9 with NaOH. The product was labeled as sample 2.

Fig. 1. XRD patterns of the as-synthesized samples (a) sample 1 and (b) sample 2.

adsorption/desorption equilibrium. Then, the system was illuminated under natural solar light with constant stirring for 50 min. At different irradiation time intervals, about 5 mL solution of MB was collected, and centrifugalized to remove the photocatalysts. The remnant MB was measured by UV–vis spectroscopy. The photocatalytic degradation ratio was calculated by c/c0. c: the concentration of organic dyes solution irradiated for a certain time; c0: the original concentration of organic dyes solution. 3. Results and discussion

2.3. Characterization

3.1. Catalyst characterization

The crystalline structure of the products was characterized by powder X-ray diffractometer (XRD, Japan Rigaku D/max-RA X-ray diffractometer, with graphite monochromatized Cu Ka1 radiation, l = 0.15406 nm), the morphologies of the as-prepared products were observed by scanning electron microscopy (SEM, HITACHI S4800) at an acceleration voltage of 5.0 kV. The measurements of optical property were performed on an ultraviolet spectrometer (UV-3600, Japan). The Brunauer–Emmett–Teller (BET) surface area of the products was measured at nitrogen adsorption surface area pore size distribution analyzer (ASAP 2020M+C) using adsorption data.

Fig. 1 shows the XRD patterns of the obtained products. All diffraction peaks could be readily indexed to the pure orthorhombic Bi2WO6 (JCPDS no. 39-0256). No other diffraction peaks arising from possible impurities were detected. Compared with sample 1(Fig. 1a), the intensity of the diffraction peaks of sample 2 (Fig. 1b) was much stronger. It was indicated that sample 2 was of higher crystallinity [22]. The morphology of the as-synthesized Bi2WO6 was observed by SEM. Fig. 2 shows the typical panoramic FE-SEM image of sample 1. Spherical ball-like particles with an average diameter of 5–6 mm were observed. The surface of these microspheres was coarse. An amplified SEM image indicated that the microsphere was composed of many nanoplates with a lateral size of a few hundred nanometers and thickness of about 10 nm. Clearly, Bi2WO6 microspheres held three levels of structure. The primary structure was the layered crystal structure of Bi2WO6. The secondary

2.4. Photocatalytic property of the as-prepared products The as-prepared product (0.05 g) was dispersed in a 100 mL solution of MB (10 mg/L) in darkness for 30 min to establish an

Fig. 2. SEM images of sample 1.

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Fig. 3. SEM images of sample 2.

structure was the nanosheet assembled from nanoplates. The tertiary structure was the self-assembled spherical structure from the as-formed nanosheets. Fig. 3 described the morphology of the sample 2 in large scale. It consisted of a large quantity of octahedrons in which the edge length was about 300 nm. In order to clarify the formation of 3D hierarchical Bi2WO6 microspheres, a series of experiments were conducted. Table 1 shows the detailed reaction parameters and relevant results. It implied that the concentration of KNO3 was an important factor to induce the formation of 3D hierarchical Bi2WO6 microspheres. According to the experimental results and phenomena, relevant chemical reactions could be proposed as follows: Na2 WO4 Ð 2Naþ þ WO4 2

(1)

BiðNO3 Þ3 Ð Bi3þ þ 3NO3 

(2)

NO3  þ H2 O þ Bi3þ Ð BiONO3

(3)

2Bi3þ þ WO4 2 Ð Bi2 WO6

(4)

concentration of KNO3 was higher (1.5 mol/L or 2.0 mol/L), Bi3+ easily react with H2O and NO3 to form slightly soluble BiONO3, as shown in formula (3). So the concentration of free Bi3+ ion would reduce, and that of the free WO42 besieged Bi3+ would also decreased. This held back the direct collision between Bi3+ and WO42, thus decreased the rate of nucleation and growth of Bi2WO6. At this moment, Bi2WO6 nanoparticles reunited into a ball to minimize the surface energy [23]. In the following Ostwald ripening process, the ball was prone to form 2D flake-like structure with the reaction time lasting [24]. Finally, 3D hierarchical microspheres were formed (Fig. 4b). In a word, we deduced that 3D hierarchical Bi2WO6 microsphere was produced through crystal plane-selective growth starting from cores, not through aggregation of rectangular platelets which were produced independently. Therefore, the morphology of Bi2WO6 might be controlled by changing the concentration of KNO3 solution. 3.2. BET surface areas and pore structure

When the concentration of KNO3 was lower (0.5 mol/L or 1.0 mol/L), Bi3+ might rapidly react with WO42 to form Bi2WO6. The assembly of Bi2WO6 nanoparticles behaved inactive and preferred to form two-dimensional structures (Fig. 4a). When the Table 1 The detailed reaction parameters and corresponding results. Samples

Inorganic salt

c (mol/L)

Morphology

1 2 3 4

KNO3 KNO3 KNO3 KNO3

0.5 1.0 1.5 2.0

Flake-like Flake-like Microsphere Microsphere

Fig. 5 shows the nitrogen absorption/desorption isotherms of the samples. The Brunauer–Emmett–Teller (BET) specific surface area of the sample 1 was about 25.6 m2 g1, which was much larger than that of sample 2 (1.2 m2 g1). The physioadsorption isotherm of sample 1 with a distinct hysteresis loop in the range of 0.5–1.0 P/P0, which can be classified to type IV in the IUPAC classification is characteristic of porous materials [25]. The inset graph in Fig. 5 shows the Barrett–Joyner–Halenda (BJH) pore-size distribution plot for N2 sorption isothermal of sample 1. It was showed that the maximum mesopores diameter of Bi2WO6 microsphere was about 10 nm. 3.3. UV–vis diffuse reflectance spectroscopy (DRS) Fig. 6 shows UV–vis diffuse reflection spectra of the samples. The absorption edges of the samples 1 and 2 extended from the UV

Fig. 4. SEM images of the as-prepared samples (a) 0.5 mol/L KNO3 and (b) 2 mol/L KNO3.

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Fig. 6. UV–vis DRS for samples. Fig. 5. N2 absorption and desorption isotherms for (a) sample 1 and (b) sample 2.

light region to ca. 440 nm and ca. 460 nm, which implied the possibility of high photocatalytic activity of the obtained products under sunlight irradiation. The indirect band gap energy (Eg) was estimated from the intercept of a straight line fitting to a plot of [F(R1)hn]0.5 against hn, where F(R1) is the Kubelka–Munk function and hn is the incident photo energy [26]. Samples 1 and 2 exhibited Eg of 2.54 eV and 2.51 eV, which was consistent with the density functional theory (DFT) calculation result [16]. 3.4. Photocatalytic activity The photocatalytic activity of the as-synthesized products was evaluated by monitoring the degradation of MB solution under sunlight irradiation. The characteristic absorption of MB at l = 662 nm was selected for monitoring the photocatalytic degradation. Fig. 7 illustrated the time-dependent absorption spectra of MB aqueous solutions in the presence of samples. It was clear that the maximum absorption of the MB solution gradually decreased with reaction time lasting. In the presence of sample 1,

MB solution was completely degraded (the photocatalytic degradation ratio was about 99.12%) after irradiation for 50 min; while for sample 2, the photocatalytic degradation ratio was only 33.56%. The experiments of the direct photolysis of MB and the absorption for longer time of the as-synthesized products in the dark were also carried out. The results were given in Fig. 8a. It was shown that the degradation of MB was not significant at the absence of the catalyst, which implied the neglectable photolysis of MB under sunlight irradiation. It was also seen that about 53.42% and 9.98% of MB were absorbed for samples 1 and 2, respectively. In other word, the sample 1 has higher adsorption capacity than sample 2. This might be due to the special 3D hierarchical structure of sample 1. These data suggested that the adsorption of the catalysts played an important role in the photocatalytic degradation of MB. Considering the effect of photosensitization, the phenol solution was checked at the same conditions. Fig. 8b displayed the concentration changes curves of phenol solution at the

Fig. 7. Time-dependent absorption spectra of MB (a) sample 2 and (b) sample 1.

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Fig. 8. Photocatalytic degradation ratio of (a) MB and (b) phenol.

wavelength of 269 nm. It was found that photocatalytic degradation ratio of samples 1 and 2 was, respectively, 79.58% and 26.69%. The photocatalytic degradation ratio of MB for the as-synthesized samples (Fig. 7) was obviously superior to phenol. This fact demonstrated that there might be photosensitization existed when MB dyes are used. To understand the variation of photocatalytic activity of the assynthesized Bi2WO6, several factors such as electronic structure, surface property, morphology, and crystallinity must be addressed. The broadened band gap energy is regarded as the most likely reason [24], since it would enhance the redox potentials of the photogenerated electrons and holes. As stated above, the band gap energy of sample 2 slightly shifted toward the higher energy compared to sample 1. The slightly shift (only 0.03 eV) of band gap energy is not applicable to illustrate the cause. High crystallinity is beneficial to photocatalysts because it generally means fewer traps and superior photocatalytic activity [22]. However, the crystallinity of sample 2 was better than sample 1. Therefore, there must be other factors resulting in the different photocatalytic activities between samples 1 and 2. It is well-documented that the surface area of semiconductors has a great influence on the photocatalytic activity and in some cases it can be a dominant factor [27]. For large surface area, the ratio of the surface charge carrier transfer rate to the electron–hole recombination rate can be greatly improved, resulting in highly concentrated surface active sites and a higher photonic efficiency [28]. As shown in Fig. 5, the surface area of samples 1 and 2 were about 25.6 m2 g1 and 1.2 m2 g1, respectively. Hence, the surface area might be a major factor for the photocatalytic activity of the as-synthesized products in our experiment. As we know, the hierarchical architecture with large surface area and pore system plays an important role in catalyst design for its being able to improve the molecular transport of reactants and products [29,30]. The photocatalytic reaction could take place in the interior void, and the resulting products could move out of the inner space through diffusion. The hierarchical structure is also favorable for the multiple reflections of light within the sphere interior voids, allowing more efficient light harvesting and offering an enhanced photocatalytic activity [31]. Therefore, the enhanced photocatalytic activity of 3D hierarchical Bi2WO6 microsphere could be ascribed to the synergistic consequence of high surface area, large pore volume, and the special hierarchical structure. 4. Conclusions In conclusion, octahedral Bi2WO6 and Bi2WO6 microsphere have been successfully synthesized by the designed hydrothermal route. The concentration of KNO3 played a key role in the formation

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