Mesoporous yolk-shell structure Bi2MoO6 microspheres with enhanced visible light photocatalytic activity

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Mesoporous yolk-shell structure Bi2MoO6 microspheres with enhanced visible light photocatalytic activity Jinliang Li, Xinjuan Liu, Zhuo Sun, Likun Pan

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S0272-8842(15)00473-3 http://dx.doi.org/10.1016/j.ceramint.2015.03.068 CERI10147

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Cite this article as: Jinliang Li, Xinjuan Liu, Zhuo Sun, Likun Pan, Mesoporous yolkshell structure Bi2MoO6 microspheres with enhanced visible light photocatalytic activity, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2015.03.068 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Mesoporous yolk-shell structure Bi2MoO6 microspheres with enhanced visible light photocatalytic activity

Jinliang Li,a Xinjuan Liu,*b Zhuo Suna and Likun Pan*a

a

Engineering Research Center for Nanophotonics & Advanced Instrument, Ministry of Education, Shanghai Key Laboratory of Magnetic Resonance, Department of Physics,Department of Physics, East China Normal University, Shanghai 200062 China, Fax: +86 21 62234321; Tel: +86 21 62234132; E-mail: [email protected] (L. K. Pan) b

Center for Coordination Bond and Electronic Engineering, College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, China; E-mail: [email protected] (X. J. Liu)

Abstract Mesoporous yolk-shell structure Bi2MoO6 (BMO-YS) microspheres were successfully synthesized via a facile solvothermal route in Bi2MoO6 precursor solution. The morphology, structure and photocatalytic performance of the BMO-YS in the degradation of Rhodamine B (RhB) were characterized by scanning electron microscopy,

transmission

adsorption-desorption,

electron

UV-vis

microscopy,

absorption

X-ray

spectroscopy

diffraction, and

nitrogen

electrochemical

impedance spectra, respectively. The as-prepared BMO-YS mainly consists of 1

microspheres with diameters of about 1.5 µm. The photocatalytic studies reveal that the BMO-YS not only exhibits optimum photocatalytic performance, which may be attributed to the excellent charge separation characteristics and the enhanced light absorption offered by its unique yolk-shell structure, but also possesses excellent recyclability for photocatalysis.

Keywords: yolk-shell structure; Bi2MoO6; Rhodamine B; photocatalysis 1. Introduction Semiconductor photocatalysis is a potentially promising approach to solve current environmental and energy issues [1-3]. As a green chemical technology, photocatalysis utilizes solar energy to decompose harmful organic and inorganic pollutants presented in air and aqueous systems or to split water to supply clean and recyclable hydrogen energy [4]. Bi2MoO6 as one of Aurivillius oxide semiconductors with narrow band gap of 2.66 eV, has been used as a selective oxidation catalyst and pigment for many years [5, 6]. Furthermore, Bi2MoO6 has been proved as an effective photocatalyst for decomposing organic pollutants in waste water into inorganic substances [7-10]. However, Bi2MoO6 prefers to crystallize in the tetragonal matlockite structure, a layered structure characterized by [Bi2O2]2+ and slabs interleaved by double slabs of [MoO4]2- [11, 12]. Therefore, it is very difficult to control the morphologies of Bi2MoO6. Under the efforts of the researchers, many structures such as nanosheets, nanotubes, nanofibers, solid and hollow microspheres of Bi2MoO6 have been synthesized [7-9, 13]. Despite the progress to date, it is still 2

highly desirable to investigate Bi2MoO6 with other morphologies. Core-shell structure is powerful a structure for its mechanical, optical, electrical, and chemical properties [14-20]. These advantages turn into a driving force for the exploration in a growing list of applications of this functional structure [21-23]. For instance, researchers synthesized concentric multilayer semiconductor nanoparticles in order to improve their optical property in early 1990s [24]. Inspired by nature, a special class of core-shell structure with a distinctive core-in-hollow-shell configuration has attracted tremendous interest in recent years [25-27]. Some researchers vividly called it ‘yolk-shell’ structure or ‘rattle-type’ structure [28]. With the unique properties of movable core, interstitial hollow space, and the functionality of shell, yolk-shell structure has great potential for application in various fields, such as nanoreactors, biomedicine, lithium-ion batteries and gas sensors [29-34]. Besides, such structure is conducive to photocatalysis because of multiple reflections of light within the sphere interior voids [28, 35]. To date, several studies synthesized yolk-shell structure by template-assisted selective etching approach [36]. For example, Wang et al. [25] described a surface-protected etching approach to prepare yolk-shell SnO2 structures for gas sensing. Wang et al. [37] described a soft template assembly method to synthesize ZnO double-yolk structure for photocatalysis. Although this approach can be easily implemented in conception, it is often associated with some disadvantages, such as tedious processing steps and low production rate. As an alternative, template-free approach is also developed to synthesize yolk-shell structures. Currently, several groups have reported the preparation of yolk-shell 3

structures by template-free approach. For example, Li et al. [28, 38] synthesized yolk-shell TiO2 microsphere and yolk-shell Au@TiO2 by solvothermal method based on Ostwald ripening for photocatalysis. Despite the progress to date, such an approach has been used for the synthesis of relatively few materials and it is still a significant challenge to explore more semiconductor yolk-shell structures by using template-free approach. In this work, mesoporous yolk-shell structure Bi2MoO6 (BMO-YS) microspheres were synthesized via a solvothermal method in Bi2MoO6 precursor solution. The possible mechanism for the formation of mesoporous BMO-YS microspheres was discussed. The BMO-YS microspheres exhibit excellent photocatalytic performance under visible light irradiation.

2. Experimental 2.1. Synthesis The BMO-YS microspheres were obtained by a solvothermal reaction. In a typical case, 0.3638 g Bi(NO3)3•5H2O and 0.0908 g Na2MoO4•2H2O were dissolved in 7.5 mL ethylene glycol (EG) under magnetic stirring, respectively. The two solutions were mixed together, and then 45 mL ethanol was slowly added into the above solution, followed by stirring for 20 min. The resulting clear solution was transferred into a 100 mL teflon-lined stainless steel autoclave, which was heated to 160 °C for 12 h. The obtained sample was isolated by washing five times with distilled water, dried in a vacuum oven at 80 °C for 24 h and then annealed at 350 °C 4

in air atmosphere for 1 h. In this work, Bi2MoO6 nanoparticles (BMO-NP), Bi2MoO6 nanosheets (BMO-NS) and Bi2MoO6 microspheres (BMO-MS) were also synthesized for comparison. The BMO-MS and BMO-NP were prepared according to similar process except that the temperature is changed to 120 °C and the deionized water is used instead of ethanol, respectively. BMO-NS was synthesized by a hydrothermal reaction. 0.3638 g Bi(NO3)3•5H2O was dissolved in 7.5 mL nitric acid (3 mol L-1) and 0.0908 g Na2MoO4•2H2O was dissolved in 7.5 mL aqueous solution. Then the two solutions were mixed together, and 45 mL NaOH solution (0.5 mol L-1) was slowly added into the above solution, followed by stirring for 20 min. The following procedure is same to that of BMO-YS. For the electrochemical impedance spectra (EIS) testing, 90 mg sample with 0.2 mL 2.5 wt% polyvinyl alcohol binder was homogenously mixed in water to form a slurry. Then, the resultant slurries were coated on the graphite flake (2 cm×2 cm). Finally, these prepared electrodes were dried in a vacuum oven at 60 °C for 24 h. 2.2. Characterization The surface morphology, structure and composition of the samples were characterized by field-emission scanning electron microscopy (FESEM, Hitachi S-4800), X-ray diffraction (XRD, Holland Panalytical PRO PW3040/60) with Cu-Kα radiation (V =30 kV, I = 25 mA), energy dispersive X-ray spectroscopy (EDS, JEM-2100) and high-resolution transmission electron microscopy (HRTEM, JEOL-2010). The Brunauer-Emmett-Teller specific surface areas of the samples were evaluated on the basis of nitrogen adsorption isotherms measured at 300 °C using a 5

BELSORP-max nitrogen adsorption apparatus (Micrometitics, Norcross, GA). The UV-Vis diffusion absorption spectra were recorded on a UV-vis spectrophotometer (Hitachi U-3900) equipped with an integrated sphere attachment by using BaSO4 as a reference. EIS measurement was carried out on an electrochemical workstation (AUTOLAB PGSTAT302N) under dark condition using a three electrode configuration with a Pt foil as counter electrode and a standard calomel electrode as reference electrode. The electrolyte was 10 mg L-1 RhB aqueous solution. EIS were recorded in the frequency range of 0.1 Hz-1 MHz, and the applied bias voltage and ac amplitude were set at open-circuit voltage and 10 mV. Total organic carbon (TOC) of the solution was analysed using the TOC analyser (Shimadzu, TOC-L CPN). 2.3. Photocatalytic experiments The photocatalytic performance of the samples was evaluated through the photocatalytic degradation of RhB (10 mg L-1) under visible light irradiation. The samples (80 mg) were dispersed in 80 mL RhB aqueous solutions. The mixed suspensions were first magnetically stirred in the dark for 30 min to reach the adsorption-desorption equilibrium. Under ambient conditions and stirring, the mixed suspension was exposed to the visible irradiation (λ>400 nm) produced by a 400 W metal halogen lamp. At certain time intervals, 2 ml of the mixed suspensions were extracted and centrifuged to remove the photocatalysts. The filtrates were analysed by recording the UV-vis spectra of RhB using Hitachi U-3900 UV-vis spectrophotometer. The apparent quantum efficiency (AQE) was measured under the similar photocatalytic reaction conditions. The LED lamp (2.5 W, 400 nm) was used as light 6

source to trigger the photocatalytic reaction. The lamp was positioned on the top of the reactor and the distance between the lamp and the reactor was 6 cm. The focused intensity and area on the flask for lamp were ca. 2.5 mW cm-2 (measured by reference solar cell, Oriel) and 20 cm2, respectively. The AQE was calculated according to the following equation:

AQE =

∓ (d[x] / dt) × 100% d[hv]inc / dt

(1)

Where d[x]/dt is the concentration change rate of the reactant and d[hv]inc/dt is the total optical power impinging on the sample [28, 35].

Fig. 1 FESEM images (a and b), EDS (c) and XRD pattern (d) of BMO-YS.

3. Results and discussion Fig. 1a shows the unique yolk-shell structures of the BMO-YS by FESEM measurement. It can be observed that a number of perfect BMO-YS microspheres 7

with diameters of about 1.5 µm are evenly distributed. From the cross-section of the cracked microspheres, the hierarchically structured outer spherical shell can be visualized to consist of constituent ultrathin nanosheets, as shown in the high magnification micrographs (Fig. 1b). Compared to the shell, the inner sphere has smoother surface due to the close accumulation of tiny Bi2MoO6 nanocrystals. It should be noticed that the two kinds of hierarchical structures of shell and yolk may enormously enlarge the surface area of Bi2MoO6 and thereby enhance the dye adsorption [39]. In addition, it is expected that the multilayered hollow-sphere structure should produce multiple reflections of incident light in-between the hierarchical hollow spheres, so as to improve the efficiency of light harvesting [39]. The BMO-YS is further identified by EDS linked to FESEM (Fig. 1c). The peaks of Bi, Mo and O in EDS spectra prove the existence of Bi2MoO6. The atom ratios of Bi and Mo are 2:1, indicating that the pure Bi2MoO6 was synthesized. Fig. 1d shows the XRD pattern of BMO-YS, which exhibits several diffraction peaks at 28.3º, 32.7º, 47.2º, 55.5º and 58.5º, indexed to (131), (002), (062), (133) and (262) crystal planes of Bi2MoO6 (JCPDS 21-0102) [40]. The FESEM images, XRD patterns and EDS of BMO-MS, BMO-NS and BMO-NP are shown in Fig. S1 and S2 (ESI†) for comparison. The results show that BMO-MS, BMO-NS and BMO-NP display solid microsphere, sheet and particle morphologies, respectively, and all of them are pure Bi2MoO6.

8

Fig. 2 (a) Low-magnification and (b) high-magnification HRTEM images of BMO-YS.

Fig. 2(a) and (b) show the low-magnification and high-magnification HRTEM images of BMO-YS. It is clearly seen that BMO-YS shows the yolk-shell structure. The lattice fringes with an interplanar distance of 0.315 nm can be assigned to the (131) plane of Bi2MoO6 (JCPDS 21-0102) [40].

Fig. 3 FESEM images of BMO-YS obtained at different solvothermal treatment time: (a) 1 h, (b) 4 h, (c) 8 h and (d) 12 h. 9

To

understand

the

growth

mechanism

of

the

as-prepared

products,

time-dependent experiments were carried out to reveal the formation process of BMO-YS. Fig. 3 shows the FESEM images of BMO-YS prepared via solvothermal treatment for different reaction time. Uniform, smooth, solid microspheres are obtained at 1 h (Fig. 3a). Some surface platelets are formed at 4 h and an urchin-like prickly surface can be observed (Fig. 3b). Subsequently, the platelets form shells on the solid cores, creating a core-shell structure at 8 h (Fig. 3c). These inner sphere cores shrink with time, forming a unique yolk-shell structure at 12 h (Fig. 3d). Interestingly, the diameter of outer sphere, however, is almost not changed during the structural evolution. Finally, the samples are annealed at 350 °C for 1 h to remove organic contaminants adsorbed on the surface, as shown in Fig. 1b.

Fig. 4 Scheme of the formation of BMO-YS microspheres.

Based on the SEM analysis, the growth mechanism of the BMO-YS is proposed, which is similar to previous report [28]. It is well known that the formation of smaller crystallites is kinetically favored and the formation of larger crystallites is thermodynamically favored. EG is a well-known complexing agent that coordinates to 10

Bi3+ ions and decreases the concentration of free Bi3+ ions [7, 41]. As a result, the rate of precipitation reaction was slowed down, resulting in the separation of the nucleation and growth process. Therefore, Bi2MoO6 microspheres were formed in a relatively slow rate (Fig. 4, step 1) at the earlier stage of reaction. With the increasing hydrothermal time (4 h), the Ostwald ripening was proceeded. Meanwhile, the growth of a uniform microsphere structure consisting of plates was observed under hydrothermal conditions due to the high intrinsic anisotropic nature of Bi2MoO6 (Fig. 4, step 2). Moreover, there might be the presence of free EG molecules absorbed on the amorphous Bi2MoO6 in the inner microsphere, which offered many nucleation sites for further growth. In this time, the yolk-shell structure appeared (Fig. 4, step 3). With the further Ostwald ripening, the inner yolk in yolk-shell structure shrank, and the homogeneous yolk-shell structure Bi2MoO6 was finally formed at 12 h (Fig. 4, step 4). The growth of nanocrystals might take place from the outer side of the aggregates toward the inside, resulting in the formation of a dense structure in the center.

Fig. 5 Nitrogen adsorption-desorption isotherm and corresponding pore size distribution curve (inset) of BMO-YS. 11

The nitrogen adsorption-desorption isotherm and pore size distribution curve (inset) of BMO-YS are shown in Fig. 5, and those of other samples are shown in Fig. S3 (EIS†). All of them show type IV isotherms with H3 hysteresis loops [42-44]. This behaviour may be caused by the existence of non-rigid aggregates [45]. The pore size distribution curve of BMO-YS indicates that the sample mainly consists of mesopores. The specific surface areas and mean pore diameters of BMO-YS, BMO-MS, BMO-NS and BMO-NP are displayed in Table 1. It is clearly observed that the yolk-shell structure shows larger specific surface area than others, which can supply more surface active sites and is beneficial to the photocatalytic performance [46].

Table 1 Specific surface areas and mean pore diameters of BMO-YS, BMO-MS, BMO-NS and BMO-NP. Sample

Specific surface areas (m2 g−1)

Mean pore diameter (nm)

BMO-YS

27.53

23.25

BMO-MS

10.26

20.72

BMO-NS

14.03

32.40

BMO-NP

13.18

26.77

The UV-vis diffuse absorption spectra of BMO-YS, BMO-MS, BMO-NS and BMO-NP were measured using a UV-vis spectrophotometer with an integrating sphere, as shown in Fig. 6. It can be clearly observed that all of the samples exhibit spectral response in visible range owing to the narrow energy gap of Bi2MoO6. 12

Compared with BMO-MS, BMO-NS and BMO-NP, BMO-YS exhibits higher light absorption, which is due to the enhanced multiple light reflections offered by its yolk-shell structure [28, 35], and the corresponding schematic is shown in the inset of Fig. 6 .

Fig. 6 UV-vis diffuse absorption spectra of BMO-YS, BMO-MS, BMO-NS and BMO-NP. Inset shows a schematic illustration of multiple light reflections within the yolk-shell structure.

The charge transfer and recombination behaviours of the samples were studied by analysing the EIS under dark condition. Fig. 7 shows the typical Nyquist plots of BMO-YS, BMO-MS, BMO-NS and BMO-NP. The semicircle in the EIS spectra is ascribed to the contribution from the charge transfer resistance (Rct) and constant phase element (CPE) at the photocatalyst/electrolyte interface. The inclined line, resulting from the Warburg impedance (ZW), corresponds to the ion diffusion process in the electrolyte [46-48]. The corresponding equivalent circuit and fitted parameters are shown in the inset of Fig. 7 and Table S1 (EIS†), where the Rs denotes the electrolyte resistance. The Rct values of BMO-YS, BMO-MS, BMO-NS and 13

BMO-NP are 2.38, 4.03, 2.58 and 3.30 kΩ, respectively, and the BMO-YS carries the smallest Rct value, indicating that the recombination of photo-induced electrons and holes in BMO-YS is more effectively inhibited [49-52].

Fig. 7 Nyquist plots of BMO-YS, BMO-MS, BMO-NS and BMO-NP. Inset is the corresponding equivalent circuit model.

Photocatalytic degradation of RhB by BMO-YS, BMO-MS, BMO-NS and BMO-NP was performed under visible light irradiation, as shown in Fig. 8. The normalized temporal concentration changes (C/C0) of RhB during the photocatalytic process are proportional to the normalized maximum absorbance (A/A0), which can be derived from the change in the RhB absorption profile at a given time interval. The results of corresponding adsorption experiments before photocatalysis were shown in Fig. S4 (EIS†), and the BMO-YS shows the higher adsorption capacity, due to its larger specific surface area [53]. It is observed that the degradation rates of RhB for BMO-MS, BMO-NS and BMO-NP are 32%, 54% and 39%, respectively, while the value for BMO-YS is 97%, much higher than those of others. Fig. 9 shows the TOC removal efficiency of RhB during the process of photocatalytic degradation. It can be 14

seen that the mineralization ratios of RhB for BMO-YS, BMO-MS, BMO-NS and BMO-NP are 60%, 21%, 32% and 23%, respectively. The AQEs of BMO-YS, BMO-MS, BMO-NS and BMO-NP were also measured and the corresponding values were 7.3%, 3.1%, 1.6% and 1.5%. These above results demonstrate that the BMO-YS exhibits higher photocatalytic performance than others. The enhanced photocatalytic performance of BMO-YS should be mainly ascribed to the reduced electron-hole pair recombination and the increased light absorption due to the enhanced multiple light reflections within the interior cavity, which can be confirmed by the EIS and UV-vis absorption measurements. In addition, the increased specific surface area of BMO-YS should contribute to the improvement of its photocatalytic performance.

Fig. 8 Photocatalytic degradation of RhB by BMO-YS, BMO-MS, BMO-NS and BMO-NP under visible light irradiation.

15

Fig. 9 TOC removal efficiency of RhB by BMO-YS, BMO-MS, BMO-NS and BMO-NP under visible light irradiation.

A possible pathway for RhB photocatalytic degradation was proposed here. Under visible light irradiation, in Bi2MoO6, the electrons could transfer from valence band to conduction band, and then react with O2 adsorbed on the surface of Bi2MoO6 to produce radical •O2− and radical •OH, which could decompose RhB [54]. Due to the yolk-shell structure, the multiple light reflections could be offered, and more electron hole pairs could be generated [20]. The photocatalytic process is shown as follows:

→

Catalyst + hv e- + h+

→•O +2H → •OH + OH (h , •OH) →products

e - + O2 2e- + •O2− RhB + •O2-

(2)

2

−

+

-

+

(3) (4) (5)

The photo-stability of BMO-YS by investigating its photocatalytic performance under visible light irradiation with four times of cycling uses was studied, as shown in Fig. 10. It can be seen that the recycled use of BMO-YS does not conspicuously affect 16

its photocatalytic activity.

Fig. 10 Photo-stability of BMO-YS by investigating its photocatalytic activity under visible light irradiation with 4 times of cycling uses.

4. Conclusions In conclusion, the BMO-YS microspheres with diameters of about 1.5 µm were synthesized by the self-assembly of nanosheets via a simple solvothermal method. They achieve high RhB degradation rate of 97% with good photo-stability under the exposure of visible light. The excellent photocatalytic performance stems from the unique yolk-shell structure of Bi2MoO6, which can enhance the light absorption, increase the specific surface area and promote the charge transfer.

Acknowledgements Financial support from the National Natural Science Foundation of China (No. 21401180) is gratefully acknowledged.

17

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