Effects of Morphology and Crystallinity on the Photocatalytic Activity of

Article pubs.acs.org/IECR

Effects of Morphology and Crystallinity on the Photocatalytic Activity of (BiO)2CO3 Nano/microstructures Wanglai Cen,† Ting Xiong,‡ Chiyao Tang,‡ Shandong Yuan,§ and Fan Dong*,‡ †

College of Architecture and Environment and §Institute of New Energy and Low Carbon, Sichuan University, Chengdu 610065, P. R. China ‡ Chongqing Key Laboratory of Catalysis and Functional Organic Molecules, College of Environmental and Biological Engineering, Chongqing Technology and Business University, Chongqing 400067, China ABSTRACT: (BiO)2CO3 microspheres and nanoparticles with different crystallinities have been successfully synthesized via a simple hydrothermal route. The as-prepared samples were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, UV−vis diffuse-reflectance spectroscopy, and nitrogen adsorption−desorption isotherms. The carbonate ions play a key role in controlling the morphology and crystallinity of (BiO)2CO3. The photocatalytic performance of the as-prepared (BiO)2CO3 samples was evaluated toward the removal of NO upon UV-, visible-, and simulated-solar-light irradiation. The results indicated that the photocatalytic activity of (BiO)2CO3 microspheres was higher than that of (BiO)2CO3 nanoparticles, which was mainly attributed to the synergistic effects of hierarchical structure, low crystallinity, and large surface areas.

1. INTRODUCTION Interest in the use of semiconductors in photocatalysis for energy crisis and environmental pollution has dramatically increased since TiO2 was discovered to harvest solar energy to split water into oxygen and hydrogen.1−4 Especially, enormous effort has been devoted to exploring the factors associated with the photocatalytic activity of semiconductors in order to make them available for practical application.5−7 Studies have revealed that the photocatalytic activity of semiconductors can be affected by many factors. Take TiO2 as an example, whose photocatalytic activity can be enhanced by tuning the crystal face, surface area, surface properties, crystallinity. and morphology.8−14 From a crystallinity point of view, high crystallinity can promote charge transfer from the center to the surface and then increase the photocatalytic activity,15 while low crystallinity is also reported to enhance the photocatalytic activity because it provides active centers for oxidation/reduction reactions via the production of defects (impurities, microvoids, and oxygen vacancies).16 Therefore, crystallinity is a complex factor to be considered for affecting the photocatalytic activity. Recently, bismuth-containing layered semiconductors have been an important subject for photocatalysis.17 Nanostructures of Bi2WO6, BiVO4, BiOX (X = Cl, Br, I), Bi2S3, and (BiO)2CO3 have been fabricated by various methods and applied as photocatalysts for removing pollutants.18−22 Among which, (BiO)2CO3 draws increasing attention because of its promising antibacterial performance,23 photocatalytic activity, sensing, and supercapacitance.24−29 (BiO)2CO3 compounds with one-dimensional (1D) nanostructure (nanotubes and nanoparticles),23,30 two-dimensional (2D) structure (nanosheets and nanoplates),31−33 and three-dimensional (3D) microstructure (rose, flower, and persimmon-like)34−37 have been successfully prepared and applied in photocatalysis. Although advances have been made on the synthesis of © 2014 American Chemical Society

(BiO)2CO3 nanostructures, little is known about the relationship between the photocatalytic activity and crystallinity of (BiO)2CO3 with different morphologies. Herein, we try to synthesize (BiO)2CO3 microspheres and (BiO)2CO3 nanoparticles with distinct crystallinity via a simple hydrothermal method. It was found that the crystallinity and morphology can be tuned by simply changing the amount of CO32−. The crystallinity and morphologydependent photocatalytic activities for removing NO under UV-, visible-, and simulated-solar-light irradiation were investigated. The results indicated that (BiO)2CO3 microspheres displayed high photocatalytic activity compared with (BiO)2CO3 nanoparticles, which mainly resulted from the hierarchical structure, low crystallinity, and large surface area. The present work provides new insight into the relationship between the crystallinity, morphology, and the corresponding photocatalytic activity.

2. EXPERIMENTAL SECTION 2.1. Synthesis. All chemicals used in this study were analytical grade (Sigma-Aldrich) and were used without further purification. Distilled water was used in all experiments. In a typical synthesis, an appropriate amount of bismuth citrate (1.60 g) and a certain amount of sodium carbonate were mixed with 75 mL of water in a 100 mL autoclave Teflon vessel and stirred for 30 min. The resulting aqueous precursor suspension was then hydrothermally treated at 180 °C for 24 h. The sample obtained was filtered, washed with ionized water and ethanol three times, and dried at 60 °C for 12 h to obtain final products with no further Received: Revised: Accepted: Published: 15002

July 4, 2014 September 8, 2014 September 9, 2014 September 9, 2014 dx.doi.org/10.1021/ie502670n | Ind. Eng. Chem. Res. 2014, 53, 15002−15011

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3. RESULTS AND DISCUSSION 3.1. Phase Structure. XRD patterns of the two samples are shown in Figure 1. Only dominant peaks of (BiO)2CO3

treatment. Depending on the amount of sodium carbonate (0.23 and 1.53 g), the resulting (BiO)2CO3 materials were denoted as BOC-M and BOC-P, respectively. 2.2. Characterization. The crystal phases of the sample were analyzed by X-ray diffraction (XRD) with Cu Kα radiation (model D/max RA, Rigaku Co., Japan). X-ray photoelectron spectroscopy (XPS) with Al Kα X-rays (hm = 1486.6 eV) radiation (Thermo ESCALAB 250, USA) was used to investigate the surface properties. Scanning electron microscopy (SEM; model JSM-6490, JEOL, Japan) was used to characterize the morphology of the obtained products. The morphology and structure of the samples were examined by transmission electron microscopy (TEM; JEM-2010, Japan). Fourier transform infrared (FTIR) spectra were recorded on a Nicolet Nexus spectrometer on samples embedded in KBr pellets. Raman spectra were recorded at room temperature using a micro-Raman spectrometer (Renishaw InVia) in the backscattering geometry with a 514.5 nm Ar+ laser as a 75 nm excitation source. The UV−vis diffuse-reflectance spectrometry (DRS) spectra were obtained for the dry-pressed disk samples using a Scan UV−vis spectrophotometer (TU-1901, China) equipped with an integrating sphere assembly, using BaSO4 as the reflectance sample. Nitrogen adsorption− desorption isotherms were obtained on a nitrogen adsorption apparatus (ASAP 2020, USA). All of the samples were degassed at 150 °C prior to measurements. 2.3. Evaluation of the Photocatalytic Activity. The photocatalytic activity was investigated by the removal of NO at ppb levels in a continuous-flow reactor at room temperature. The volume of the rectangular reactor, made of stainless steel and covered with Saint-Glass, was 4.5 L (30 cm × 15 cm × 10 cm). For photocatalytic activity testing under simulated solar light, a 300 W commercial tungsten halogen lamp (General Electric) was vertically placed outside the reactor. For the visible-light photocatalytic activity test, a UV cutoff filter (420 nm) was adopted to remove UV light in the light beam. For the UV-light photocatalytic activity test, two UV lamps (6 W) were used. The photocatalyst (0.15 g) was coated on a dish with a diameter of 12.0 cm. The coated dish was then pretreated at 60 °C to remove the water in the suspension. The NO gas was acquired from a compressed gas cylinder at a concentration of 100 ppm of NO (N2 balance) with a traceable National Institute of Standards and Technology standard. The initial concentration of NO was diluted to about 450 ppb by the air stream supplied by a zero air generator (model 111, Thermo Environmental Inc.). The desired relative humidity level of the NO flow was controlled at 70% by passing the zero air streams through a humidification chamber. The gas streams were premixed completely by a gas blender, and the flow rate was controlled at 3.3 L/min by a mass flow controller. After the adsorption− desorption equilibrium was achieved in the dark, the lamp was turned on. The concentration of NO was continuously measured by a chemiluminescence NO analyzer (model 42c, Thermo Environmental Instruments Inc.), which monitors NO, NO2, and NOx (NOx represents NO + NO2) with a sampling rate of 0.7 L/min. The removal ratio (η) of NO was calculated as η (%) = (1 − C/C0) × 100%, where C and C0 are concentrations of NO in the outlet and feeding streams, respectively. The kinetics of the photocatalytic NO removal reaction is a pseudo-first-order reaction at low NO concentration as ln(C0/C) = kt, where k is the initial apparent rate constant.

Figure 1. XRD patterns of the BOC-M and BOC-P samples.

(JCPDS file no. 25-1464) can be observed in the BOC-M sample, and the weak and wide diffraction peaks suggest the low crystallinity of BOC-M. All of the diffraction peaks of orthorhombic (BiO)2CO3 crystallites are detected in the BOC-P sample, and no other impurity peak can be observed, implying that the sample is highly pure. The sharp diffraction peaks for BOC-P also indicate high crystallinity. In addition, the diffraction peak of the (002) plane in BOC-P becomes much pronounced compared with the standard card, indicating that the sample has a particular anisotropic growth along the {001} plane. On the basis of these results, we can find that the amount of CO32− exerts a significant influence on the crystallinity of the samples. For BOC-P, excessive CO32− leads to the production of a great number of nuclei and accelerates crystal growth, which promotes the crystallization of (BiO)2CO3, thereby generating a sample with high crystallinity. 3.2. Chemical Compositions. Further evidence for the chemical composition and chemical states of the BOC-M and BOC-P samples was obtained by XPS (Figure 2). XPS survey spectra in Figure 2a demonstrate that the two samples contain Bi, O, and C elements, and no other elements can be detected. In Figure 2b, two peaks at 159.03 and 164.35 eV are attributed to the Bi 4f7/2and Bi 4f5/2 states of Bi3+ in the BOC-M sample, respectively.38 Compared to BOC-M, these peaks in BOC-P are shifted to 159.12 and 164.45 eV, separately, suggesting the strong binding energy of Bi 4f in BOC-P resulting from high crystallinity. Likewise, the four peaks corresponding to C 1s of BOC-P are shifted to higher binding energy compared to BOC-M (Figure 2c). The peaks at about 284, 286, and 287 eV can be ascribed to adventitious carbon species from XPS measurement, while the peak located at about 288.0 eV is related to the carbonate ion in (BiO)2CO3.34 In addition, the O 1s spectra for the samples are presented in Figure 2d; contrary to the C 1s spectra, the chemical shift of the three peaks associated with O 1s of BOC-P is shifted to lower values in comparison with BOC-M. The peak at around 530 eV is typically attributed to the Bi−O 15003

dx.doi.org/10.1021/ie502670n | Ind. Eng. Chem. Res. 2014, 53, 15002−15011


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Figure 2. XPS spectra of the BOC-M and BOC-P samples: (a) survey; (b) Bi 4f; (c) C 1s; (d) O 1s.

further confirm its nanoparticle-shaped structure. The calculated lattice spacing d value is 0.273 nm (Figure 4e), corresponding to the spacing of the (110) crystal plane. The SAED pattern in Figure 4f shows an array of clear and regular diffraction spots of a single nanoparticle, indicating that the nanoparticle is single-crystalline. According to the previous research (the amount of sodium carbonate is set at 0.46 g with other conditions unchanged),39 near-amorphous particles were first generated through reaction, nucleation, crystallization, and aggregation processes. Followed by the formation of stacked embryos with attached particles due to dissolution and recrystallization, the stacked uniform solid microspheres with small particles attached on the edges were generated by the consumption of particles through Ostwald ripening. The stacked microspheres further grew to produce monodisperse hierarchical microspheres. Later, uniform monodisperse (BiO)2CO3 hierarchical hollow microspheres constructed by nanosheets were obtained. Here, the BOC-M sample is prepared at a relatively low concentration of CO32−. The low concentration of CO32− may be unfavorable for the growth of (BiO)2CO3 nanosheets. Namely, the growth of nanosheets through Ostwald ripening is quite slow; hence, some particles are not consumed but are present around the nanosheets. However, the BOC-P product generated at excessive CO32− ions shows nanoparticle morphology. Excessive CO32− makes nucleation and growth turn into a kinetic control, generating irregular morphology. Moreover, excessive CO32− induces the preferred growth orientation, yielding thick nanoparticles, which cannot be selfassembled into microspheres. Thus, (BiO)2CO3 with nanosheets morphology is generated.

bond in (BiO)2CO3, and the other two peaks at about 531 and 532 eV can be assigned to carbonate species and water adsorbed on the surface.38 XPS combined with XRD analysis confirms that the two samples are pure (BiO)2CO3 and the crystallinity of BOC-P is higher than that of BOC-M. 3.3. Shape and Morphology. Figures 3 and 4 present the SEM and TEM images of the samples obtained from precursors with different amounts. From a low-magnification SEM image of the BOC-M sample (Figure 3a), we can see many interconnected microspheres with sizes ranging from 3 to 6.72 μm. A high-magnification SEM image of the BOC-M sample (Figure 3b) reveals that many pores are formed on the surface of the microspheres because of the self-arrangement of nanosheets. The open pores could promote multiple scattering of the incident light, leading to an enhanced light-harvesting capacity. In the meantime, some nanoparticles are still adhered to the surface of the sample. The TEM images (Figure 3c,d) show that the entire microsphere is solid. A high-resolution TEM image (Figure 3e) on the edge of a microsphere shows the lattice with two different lattice spacings of 0.295 and 0.273 nm, matching the spacing of the (013) and (110) crystal planes of (BiO)2CO3, respectively. The selected-area electron diffraction (SAED) pattern of the edge of a microsphere recorded in Figure 3f shows the polycrystalline nature. (BiO)2CO3 nanoparticles (BOC-P) with thicknesses of 35.2−210.9 nm are achieved when the sodium carbonate dosage is increased, as shown in Figure 4a, which is markedly different from the microsphere morphology of BOC-M. The high-magnification SEM image (Figure 4b) reveals that these nanoparticles are not uniform. TEM images (Figure 4c,d) 15004



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Figure 3. SEM (a and b) and TEM (c−e) images of the as-prepared BOC-M sample. (f) SAED pattern of the edge of a microsphere.

3.4. Vibration Spectra (FTIR and Raman). FTIR spectra (Figure 5a) were adopted to investigate the chemical composition and chemical bonding. The “free” CO32− of (BiO)2CO3 (point group symmetry D3h) has four internal vibrations (ν1, ν2, ν3, and ν4).40 The strong absorption peaks at 1468 and 1391 cm−1 are assigned to antisymmetric vibration ν3, and the peaks at 698 and 670 cm−1 are indexed to the in-plane deformation ν4, while the peaks at 1065 and 844 cm−1 are ascribed to the symmetric stretching mode ν1 and the out-of-plane bending mode ν 2 , respectively. Obviously, the vibrations of these peaks in BOC-M are much weaker than those in BOC-P because of the low crystallinity. Figure 5b shows the Raman spectra of the two samples. The strong bands below 600 cm−1 are mainly due to “lattice vibrations”. Two bands at 667 and 668 cm−1 in the Raman spectrum are indexed to the internal vibration ν4 of CO32−. The band at 1067 cm−1 is ascribed to the internal vibration ν1 of CO32−. Two bands at 1362 and 1391 cm−1 in the Raman spectrum are indexed to the internal vibration ν3 of CO32−.40 Similar to the FTIR spectral analysis, the peak

intensity of BOC-M in Raman spectra is quite weak compared with BOC-P. 3.5. UV−Vis DRS Analysis. The optical property of the BOC-M and BOC-P samples was examined by the UV−vis DRS technique, as shown in Figure 6. The BOC-M sample exhibits higher UV-light absorption than BOC-P (Figure 6a). Moreover, the absorption edge of BOC-M is red-shifted with respect to BOC-P, which may result from the special hierarchical structure and low crystallinity. On the one hand, the multiple reflections and scattering of light in the hierarchical structure will contribute to the absorption and utilization of incident light.41 On the other hand, the low crystallinity is considered to produce defects. Defects such as oxygen vacancies can give rise to local states below the conduction band edge, which can participate in a new photoexcitation process.42 Namely, electrons are excited to the oxygen vacancy states from the valence band with a low energy of light, which leads to typical excitations in the longwavelength region of the spectra. In addition, the band gaps of as-synthesized BOC-M and BOC-P (Eg) samples estimated from the intercept of the tangents to the plots of (αhν)1/2 15005



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Figure 4. SEM (a and b) and TEM (c−e) images of the as-prepared BOC-P sample. (f) SAED pattern of a single nanoparticle.

Figure 5. FTIR (a) and Raman (b) spectra of the BOC-M and BOC-P samples.

νersus photon energy (Figure 6b) are 3.14 and 3.41 eV,

3.6. Brunauer−Emmett−Teller Surface Area and Pore Structure. The specific surface area and porosity property of BOC-M and BOC-P were analyzed by nitrogen adsorption− desorption measurement. A typical IV isotherm with H3-type

respectively. Obviously, BOC-M exhibits a narrower band gap than BOC-P. 15006



Figure 6. UV−vis DRS (a) and plots of (αhν)

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versus photon energy (b) of BOC-M and BOC-P.

Figure 7. Nitrogen adsorption−desorption isotherms (a) and the corresponding pore diameter distributions (b) of the BOC-M and BOC-P samples.

Figure 8. Photocatalytic activity (a) and reaction rate constants k (b) of the BOC-M and BOC-P samples under UV-light irradiation.

size distribution curve (Figure 7b) of BOC-P displays a horizontal line, implying that the BOC-P sample has no pores, which coincides with the observation in SEM (Figure 4). Additionally, the specific surface areas of BOC-M and BOC-P are 34.5 and 2.5 m2/g, respectively. The large surface area of BOC-M benefiting from the special hierarchical structure is expected to accelerate the photocatalytic reaction by providing more active sites and promoting the separation efficiency of photocarriers. 3.7. Photocatalytic Activities under UV-, Visible-, and Simulated-Solar-Light Irradiation. The photocatalytic

hysteresis loops is observed for BOC-M, suggesting the existence of mesoporous structure (Figure 7a).43 The corresponding pore-size distribution (Figure 7b) of BOC-M shows micropores with pore diameters of about 1 nm and mesopores at a wide range of 4−40 nm, demonstrating the formation of micropores and mesopores. Micropores are formed between the (BiO)2CO3 particles, and mesopores are created by aggregation of the (BiO)2CO3 nanosheets. However, BOC-P displays a horizontal isotherm (Figure 7a), which indicates that nearly no interactions take place between nitrogen molecules and the sample.43 Also, the pore15007



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Figure 9. Photocatalytic activity (a) and reaction rate constants k (b) of the BOC-M and BOC-P samples under visible-light irradiation.

Figure 10. Photocatalytic activity (a) and reaction rate constants k (b) of the BOC-M and BOC-P samples under simulated-solar-light irradiation.

that are favorable for the diffusion of reaction intermediates and products to accelerate the reaction rate. The special hierarchical structures can also improve the separation efficiency of photocarriers. Besides, the enhanced light absorption of BOC-M can be achieved by multiple reflections and scattering of the incident light in the hierarchical structure, which enhance the light-harvesting efficiency and then increase the quantity of the electrons and holes to take part in the photocatalytic reaction. Second, the low crystallinity of BOC-M is expected to generate defects. These defects can increase light absorption and can behave as important adsorption and active sites for the photocatalytic reactions. Third, the large surface area of BOC-M can provide more catalytic active sites and may also facilitate separation of the photoinduced electrons and holes. 3.8. Long-Term Photocatalytic Activity. Aiming to test the durability of BOC-M, the experiment on long-term photocatalytic activity was performed under simulated-solarlight irradiation (Figure 11a). Obviously, there is no obvious deactivation in the photocatalytic removal of NO even after 20 h of solar exposure, which indicates the stable and efficient photocatalytic activity of BOC-M. In addition, SEM, XRD, FTIR, and UV−vis DRS experiments of BOC-M after longterm irradiation are conducted to investigate its stability determined from the work reported by Song et al.44 As shown in Figure 11a (inset), we can find that the SEM image of BOC-M after long-term irradiation is almost the same as the SEM image of a fresh sample in Figure 3. Furthermore, the XRD pattern in Figure 11b of the BOC-M used displays that

performance of the two samples has been investigated by the removal of NO under UV-, visible-, and solar-light irradiation. According to the previous research, NO could not be photolyzed without photocatalysts under UV- (360 nm), visible-, and solar-light irradiation.33 Figure 8a depicts variation of the NO concentration (C/C0 %) with the irradiation time over the BOC-M and BOC-P samples under UV-light irradiation. We can find that BOC-M shows enhanced photocatalytic activity (42.6%) compared with BOC-P (24.8%). Moreover, the reaction rate constant k of BOC-M is 15.1 × 10−2 min−1, which is higher than that of BOC-P (9.2 × 10−2 min−1; Figure 8b). Under visible-light irradiation (Figure 9a), a removal ratio of only 3.3% toward NO is observed when using BOC-P as a photocatalyst due to its low photoresponse to visible light, which is much lower than that when BOC-M (36.1%) is used. Furthermore, the reaction rate constant k for BOC-M (9.7 × 10−2 min−1) is high in contrast to BOC-P (0.3 × 10−2 min−1; Figure 9b). Figure 10a shows the photocatalytic activity of the two samples under simulated-solar-light irradiation. The removal ratios of NO by BOC-M and BOC-P products are 42.7% and 21.2%, respectively. Likewise, BOC-M displays a higher reaction rate constant k of 15.5 × 10−2 min−1 than BOC-P (6.9 × 10−2 min−1; Figure 10b). Namely, BOC-M exhibits higher activity than BOC-P for NO removal under UV-, visible-, and simulated-solar-light irradiation. This could mainly be attributed to the synergistic effect of the special hierarchical structure, low crystallinity, and large surface area. First, the hierarchical structures of BOC-M offer many pores 15008



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Figure 11. Long-term photocatalytic activity of BOC-M under simulated-solar-light irradiation (a), SEM image (inset of a), XRD (b), FTIR spectra (c), and UV−vis DRS (d) of BOC-M after long-term irradiation.

removal. This work provides new insight into understanding the photocatalytic activity over semiconductors with different crystallinities and morphologies.

the crystal structure was not changed after the photocatalytic reaction, demonstrating its phase stability. No reaction intermediates and products (such as HNO2 and HNO3) are present on the surface of the BOC-M used, as revealed by FTIR spectra (Figure 11c), which suggests that the reaction intermediates and products could diffuse rapidly because of the hierarchical structure. The UV−vis DRS of BOC-M after long-term irradiation (Figure 11d) consistent with the fresh sample further confirms the stable photocatalytic performance. These results indicate that the as-prepared BOC-M sample is highly active and stable under simulated-solar-light irradiation. Namely, BOC-M is a promising photocatalytic material to harness solar energy for environmental application.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 23 62769785 605. Fax: +86 23 62769785 605. Author Contributions

All authors have seen and given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is financially supported by the National Natural Science Foundation of China (Grants 51478070 and 51108487) and the Natural Science Foundation Project of CQ CSTC (Project cstc2013jcyjA20018).

4. CONCLUSION In summary, (BiO)2CO3 microspheres with low crystallinity and (BiO)2CO3 nanoparticles with high crystallinity have been successfully fabricated through a simple hydrothermal method. The concentration of CO32− ions in the precursor plays a decisive role in the morphology and crystallinity. Compared with (BiO)2CO3 nanoparticles synthesized at excessive CO32−, (BiO)2CO3 microspheres displayed enhanced photocatalytic activity toward the removal of NO upon UV-, visible-, and simulated-solar-light irradiation. The enhanced photocatalytic activity was mainly attributed to the synergistic effect of hierarchical structure, low crystallinity, and large surface area. In addition, the (BiO)2CO3 microspheres showed highly active and stable photocatalytic activity under simulated-solarlight irradiation, possessing potential application in pollutant

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