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Rapid room-temperature synthesis and visible-light photocatalytic properties of BiOI nanoflowers
Journal of Alloys and Compounds 639 (2015) 445–451
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Rapid room-temperature synthesis and visible-light photocatalytic properties of BiOI nanoflowers Feng Cao, Jianmin Wang, Song Li, Jiajia Cai, Wanhong Tu, Xin Lv, Gaowu Qin ⇑ Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang 110819, China
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Article history: Received 10 December 2014 Received in revised form 23 March 2015 Accepted 24 March 2015 Available online 30 March 2015 Keywords: Hierarchical nanostructure Room-temperature solution synthesis Visible-light Photocatalysis
a b s t r a c t Uniform hierarchical BiOI nanoflowers composed of nanoplates with rough surfaces have been synthesized by a mild and rapid room-temperature solution synthesis approach. The effects of reaction parameters on the formation of 3D flower-like nanostructure were discussed. The morphology variations of the nanostructures were achieved by simply varying the reaction parameters in the system. The photocatalytic properties of the obtained samples for photodegradation of RhB dye under visible light irradiation were investigated, indicating that the flowerlike BiOI hierarchical nanostructure exhibits much higher photocatalytic activity than the sphere-like BiOI nanostructures owing to the larger surface areas, higher light absorption, and better photo-induced charge separation. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Recently, environmental problems such as air and water pollution have provided the impetus for sustained fundamental and applied research in the area of environmental remediation [1–3]. The photocatalytic technique for the degradation of organic contaminants has become a powerful way to remedy water/air pollution due to its simple and thorough decomposition process [4,5]. Nano-sized TiO2 photocatalyst, as a most prominent and suitable material for environmental purification and solar energy harvesting, has attracted wide research interest so far [6,7]. However, its solar energy conversion efficiency is too low to be economically sound from the viewpoint of technology. The main barriers are the rapid recombination of photo-generated electron/hole pairs and the poor activation of TiO2 by visible light [8,9]. In response to these deficiencies, many investigators have been conducting research with an emphasis on developing visible light responsed photocatalysts where there is effective separation of the photogenerated exciton pairs [10,11]. As a p-type narrow band-gap semiconductor (1.9 eV), BiOI has been paid a great deal of attention for its superior photocatalytic performance in contaminant decomposition under visible light irradiation, which accounts for about 42% usage of the solar energy [12,13]. In addition, the excellent photocatalytic performance is also due to its peculiar layer structure characterized by [Bi2O2] slabs interleaved by double slabs of iodine atoms and indirect⇑ Corresponding author. E-mail address: [email protected] (G. Qin). http://dx.doi.org/10.1016/j.jallcom.2015.03.185 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
transition band-gap semiconductor feature [14,15]. To date, multifarious micro/nanostructured BiOI materials have also been produced by various synthetic techniques, including sonochemical, solvothermal, hydrothermal, microwave-assisted hydrothermal methods, and thermal oxidation at high temperature [16–21]. Many of them demand harsh reaction conditions such as high temperature or high pressure. For industrial application, it is highly desirable to develop a simple and facile synthesis which requires only mild reaction conditions under normal atmospheric pressure and room temperature. Most importantly, they can be completely synthesized in a short period of time. For example, Li et al. reported a room-temperature ionic liquid-assisted microemulsion method to synthesize BiOI hollow microspheres with a reaction time of only 1 h [14,22]. However, many of ionic liquids are obviously more expensive than common organic solvents. In this work, we developed a simple and rapid room-temperature solution synthesis approach for the fabrication of uniform hierarchically BiOI nanoflowers in the presence of PVP and isopropanol. The preparation conditions are much milder and simpler than those of conventional methods. In addition, the effects of reaction parameters on the formation of 3D flower-like nanostructure were discussed. The morphology variations of the nanostructures were achieved by simply varying the reaction parameters in the system. Furthermore, the photocatalytic properties of the obtained samples with different morphologies were investigated by the photodegradation of Rhodamine B under visible light irradiation. The enhanced photocatalytic activity of the flowerlike BiOI hierarchical nanostructure is attributed to its novel and special nanostructure.
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2. Experimental 2.1. Materials All the reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), and were used as received without further purification. All reagents were of analytical grade. All solutions were prepared with high-purity bidistilled deionized water (Millipore Co., 18.3 MX cm) unless otherwise mentioned. 2.2. Preparation of samples In a typical synthesis, 0.5 mmol of Bi(NO3)34H2O and 0.5 g PVP were dissolved and stirred in a solvent of 10 ml of isopropanol, and then a 10 mL aqueous isopropanol solution containing 1.5 mmol KI was added into the above solution. The resultant suspension was stirred intensively and the color of the suspension turned from yellow to red in a few minutes. The precipitate was separated by centrifugation, washed with distilled water and absolute ethanol several times, and dried at 60 °C for 2 h. Fig. 1. XRD pattern of the as-synthesized BiOI nanoflowers. 2.3. Characterization The X-ray diffraction pattern of the products was performed on a Rigaku-D/max 2500 V X-ray diffractometer with Cu Ka radiation (k = 1.5418 Å), with an operation voltage and current maintained at 40 kV and 40 mA. Field-emission scanning electron microscopy (FESEM) images were obtained with a XL30 ESEM FEG microscope. Transmission electron microscopic (TEM) images and selected area electron diffraction (SAED) patterns were obtained on a JEOL JEM-2010 microscope. X-ray photoelectron spectrometry (XPS) analysis was measured by using a Thermo ESCALAB 250 electron spectrometer. N2 adsorption–desorption isotherms were measured at liquid-nitrogen temperature (77 K) using a Micromeritics Tristar II instrument. Samples were degassed at 120 °C overnight before measurements. Specific surface areas were calculated using the Brunauer–Emmett–Teller (BET) model, and pore size distributions were evaluated from the desorption branches of the nitrogen isotherms using the Barrett–Joyner–Halenda (BJH) model. The photoluminescence (PL) spectra were measured with the PerkinElmer LS 55 Fluorescence spectrometer at room temperature. The UV–vis solid absorption spectroscopy was measured on a UV/Vis spectrophotometer (PerkinElmer Lambda 750S, Japan) in the range of 200–800 nm with BaSO4 as reflectance standard sample. 2.4. Photoelectrochemical measurements For the preparation of photoanode, 10 mg of the final BiOI nanomaterials was dispersed in 2 ml Nafion solution (0.1%). 2.5 ll of the suspension with dispersed BiOI was cast on a fluorine doped tin oxide (FTO) conducting glass and allowed to dry under the 160 °C for 10 min. Electrochemical measurements were taken by using an electrochemical analyzer (CHI660D, CHI Shanghai, Inc.) in a standard three electrode system with a platinum foil as the counter electrode, the prepared photoanode (1 1 cm2) as the working electrode, and a KCl saturated Ag/AgCl as the reference electrode. The working electrode with illuminated area 1.0 cm2 was immersed in 0.5 M Na2SO4 (deaerating with a nitrogen flow for 0.5 h) and the light source was provided by a 300 W xenon lamp (Perfect Light, PLS-SXE300) through a 420 nm UV-cut filter. Time dependent photocurrent curves were measured with amperometric I–t curve method. Photocurrent over time was detected on visiblelight illumination repeated every 50 s at a bias potential of +0.5 V vs. RHE. 2.5. Photocatalytic activity studies Photocatalytic experiments in aqueous solution were performed in a watercooled quartz vessel. Photodegradation studies of RhB were carried out under 300 W xenon lamp (Perfect Light, PLS-SXE300) through a UV-cut filter. For a typical photocatalytic experiment, a suspension containing a powdered catalyst (80 mg) and fresh dye aqueous solution (100 mL, 5.0 10 5 M) was magnetically stirred in the dark for 15 min to establish an adsorption/desorption equilibrium of the dye species. At given irradiation time intervals, a series of aqueous solutions in a certain volume were collected and filtered through a Millipore filter for analysis. The absorption spectrum of the filtrate was measured on a UV/Vis spectrophotometer (PerkinElmer Lambda 750S, Japan). The concentration of dye was determined by monitoring the changes in the main absorbance centered at 545 nm for RhB dye.
3. Results and discussion 3.1. Morphology and structure The structures of the as-prepared samples were characterized by the XRD analysis, as shown in Fig. 1. All the peaks in this pattern can be identified as tetragonal BiOI with lattice constants
a = 3.994 Å, b = 3.994 Å, c = 9.149 Å (JCPDS Card No. 10-0445). No impurity phase is observed, which indicate that the product is highly pure. The sharp and narrow diffraction peaks indicate the high crystallization of the product. The morphology and size of the as-synthesized products were characterized by FE-SEM. The low-magnification FE-SEM image (Fig. 2A) demonstrates that the typical products consist of a large quantity of uniform 3D BiOI flowerlike nanostructures. The as-obtained flowerlike nanomaterial has a diameter of about 300– 500 nm constructed by curved nanoflakes. The yield of hierarchical nanostructures is high (>95%). The high-magnification FE-SEM image (Fig. 2B) reveals that the flowerlike nanomaterial consists of many intercrossed nanoplatelets with a thickness of about 30– 50 nm. Additionally, the rough surface of the nanoplates can be observed. The chemical composition of these nanoflowers was further characterized by using EDS (Fig. 2A, inset). Peaks of the elements Bi, O, and I are detected in the EDS pattern (the Au signal comes from the substrate). Further evidence for the quality and composition of the products can be provided by using XPS, and the XPS spectrum of BiOI was identified. The binding energies obtained in the XPS analysis were corrected for specimen charging, through referencing the C 1s to 286.40 eV. The survey-scan spectrum in Fig. 3A indicates that the sample consists of Bi, O and I as major elements, besides small amounts of O, N, and C. The N 1s, C 1s, and O 1s peaks are attributed to a trace amount of polyvinylpyrrolidone (PVP) molecules adsorbed on the surfaces of the BiOI nanoflowers. As shown in Fig. 3B, there are two sets of spin–orbit coupling peaks with the first set at 158.4 and 163.7 eV and the second set at 159.8 and 165.1 eV for Bi 4f7/2 and Bi 4f5/2, respectively. The spectrum of O 1s consists of peaks situated at binding energies of 529.9 eV, 531.5 eV, and 533.0 eV. The peak at 529.9 eV can be assigned to Bi–O bands and the peak at 531.5 eV was attributed to the I–O bands in BiOI. The peak at 533.0 eV was originated from the adsorbed water (Fig. 3C). In addition, two peaks at about 619.2 eV and 630.6 eV appear in the I 3d spectrum (Fig. 3D), corresponding to the binding energies of I 3d5/2 and I 3d3/2, respectively [22]. Further insight into the morphology and microstructure of flowerlike BiOI nanostructures were gained by using TEM, SAED and HRTEM. Fig. 4A shows the low-magnification TEM image of a single BiOI nanoflower. It can be clearly seen that the nanoflowers are self-assembled of nanosheets, which is consistent with the results of FE-SEM observation. In Fig. 4B, The SAED pattern of the nanopetal (Fig. 4A) taken from the nanoflower shows that the nanopetal is well-crystallized, and these diffraction spots can readily be indexed to tetragonal BiOI crystal. The HR-TEM image (Fig. 4C) and FFT pattern (inset in Fig. 4C) show a crystalline
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Fig. 2. (A) Low-magnification FESEM image of the BiOI nanoflowers. And the EDS pattern (inset). (B) Enlarged FESEM image of the BiOI nanoflowers.
Fig. 3. XPS spectra of the BiOI nanoflowers: (A) survey; (B) Bi 4f; (C) O 1s; (D) I 3d.
Fig. 4. (A) Representative TEM image; (B) SAED pattern; (C) HRTEM image and FFT pattern (inset) of the edge area of the BiOI nanoflowers.
character with a lattice spacing of 0.458 nm, which can be indexed to the (0 0 2) plane of BiOI nanoflowers. 3.2. Effects of experimental parameters on BiOI morphologies It is worthwhile to mention that the addition of PVP has strong effects on the formation of the well-defined flowerlike
morphologies. Fig. 5 shows the SEM images of the samples obtained with different amounts of PVP. Without the addition of PVP, BiOI crystals tend to grow into irregular flake morphology with a wide distribution range from 10 to 20 lm (Fig. 5A). The size of BiOI flakes decrease as the amount of PVP increases (Fig. 5B). When 1 mmol of PVP is added, 3D flowerlike BiOI form although the size distribution is irregular (Fig. 5C). When the addition of
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Fig. 5. FESEM images of the BiOI samples with different amounts of PVP: (A) 0, (B) 0.5, (C) 1 and (D) 2 mmol. In all cases, samples were prepared at 180 °C, and a reaction time of 12 h.
PVP is up to 2 mmol, uniform 3D flowerlike BiOI architectures are the only morphology (Fig. 5D). When the quantity of PVP is further increased (5 mmol), the morphology remains flowerlike without an obvious change (Fig. 2A and B). These results clearly indicate that the good dispersancy and high viscosity of PVP is the key to make the BiOI nanoflowers smaller and more regular as they grow gradually [23,24]. In addition, the morphology of BiOI is also strongly affected by the addition of isopropanol solvent. As shown in Fig. 6A, when only water is used as solvent instead of isopropanol, microrods in a big size are observed in the products. When DEG instead of isopropanol is used, hierarchical nanospheres appear (Fig. 6B). When EG and PEG are used, uniform nanospheres with rough
surfaces are detected (Fig. 6C and D). These observations clearly show that the presence of alcohol solvent is beneficial to the control of both morphology and grain size of the nanostructured BiOI. It is well-known that isopropanol is a strong coordinating agent with a relatively high boiling point and viscosity, and has been widely used in the polyol process to synthesize monodisperse and fine semiconductor nanoparticles [25]. 3.3. Photocatalytic degradation properties toward RhB solution The photocatalytic activity of the as-prepared BiOI nanoflowers and nanospheres is examined by visible-light degradation of RhB molecules in aqueous solution, as shown in Fig. 7. A blank test
Fig. 6. FESEM images of products prepared under similar conditions but with isopropanol replaced by (A) H2O, (B) DEG, (C) EG, (D) PEG.
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Fig. 7. Photocatalytic activities for degradation of RhB dye under visible-light irradiation at room temperature in the presence of different BiOI samples: without catalyst (red); without light (black); flower-like BiOI (green); spherical BiOI (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(RhB without any catalyst) exhibits no degradation under visible light irradiation, which indicates the photolysis of RhB could be ignored. At the same time, our experimental data on the RhB decolorization with BiOI nanoflowers show that the concentration of RhB dye almost does not change under dark conditions after the BiOI nanoflowers and dye solutions reach the adsorption–desorption equilibrium. However, the data of green and blue curves in Fig. 7B clearly indicate that the dye could be completely degraded in about 50 min in the presence of BiOI nanoflowers, whereas it takes 80 min to decompose 100% RhB dye when the BiOI nanospheres prepared by EG as capping agent were used as the photocatalyst. The photodecomposition performance by the as-obtained BiOI nanoflowers in the visible light band is clearly better than that by the BiOI nanospheres although they have similar particle sizes. The different photocatalytic activities of these two BiOI samples may be due to the difference in the effective light absorption, specific surface area, and even band-gap energy [26,27]. 3.4. Photocatalytic mechanism Nitrogen adsorption–desorption isotherms and the corresponding Barret–Joyner–Halenda (BJH) pore size distribution curves of the obtained BiOI products with different morphologies are shown in Fig. 8. We can find that the isotherms of obtained BiOI nanomaterials can be categorized as type IV with a distinct hysteresis loop observed in the range of 0.5–1.0 P/P0 according to IUPAC classification. The measured Brunauer–Emmett–Teller (BET) surface area of flower-like nanoarchitectures is 26 m2 g 1 from N2 adsorption– desorption isotherm, larger than the BiOI nanospheres (15 m2 g 1). The hierarchical BiOI nanoflowers will potentially exhibit superior performance in many fields such as electrochemistry, gas sensor, and photocatalysis properties [28,29]. The energy band gap of a semiconductor is closely related to its photo-absorption and photocatalytic ability. Fig. 9A shows the typical UV–vis solid absorption spectrum of the as-prepared BiOI samples with different morphologies. It can be seen that both BiOI nanomaterials have nearly the same light absorption region from 490 nm to 700 nm, which means the BiOI nanomaterials can be used as a visible-light active semiconductor photocatalyst. Additionally, flower-like sample displays strong absorption in the visible light region mainly due to its perfect light-trapping structure, in contrast, the spherical sample shows the comparatively weaker absorption. Strong light absorption may lead to an increase in the generation of electron–hole pairs, which enhances the photocatalytic activity of the nanomaterials. At the same time, the visible light absorption ability is advantageous for enhancing
Fig. 8. N2 adsorption–desorption isotherm for the BiOI nanomaterials with special morphologies: nanoflowers (red) and nanospheres (black). The inset shows BJH pore size distributions of the sample. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the usability of solar light [30–32]. Additionally, as shown in the inset of Fig. 9A, the derived bandgaps are estimated to be 1.80 and 1.87 eV for the nanoflowers and nanospheres, which confirms that the size, morphology, and structure of the semiconductor materials have an great influence on related optical properties and offer a way of tuning the band gap [33,34]. The charge-carriers separation/recombination rates of the photo-excited carriers are further investigated by photoluminescence spectra (PL) with excitation at 247 nm (an excitation slit width of 10 nm and an emission slit width of 10 nm), because the recombination of photo-induced charge carriers gives rise to the PL emission. As shown in Fig. 9B, all BiOI samples exhibit a strong emission peak centering at about 530 nm, which can be attributed to the band-band PL phenomenon with the energy of light which is approximately equals to the band-gap energy (1.94 eV) of BiOI. In comparison with the spectra of the spherical BiOI nanostructure, the intensity of the emission peak of the flowerlike BiOI nanostructure is relatively weaker, indicating a higher suppression in charge recombination. In other words, the BiOI nanoflowers have higher photogenerated charge separation efficiency. Furthermore, photoelectrochemical properties are performed to study the excitation and transfer of photogenerated charge carriers in photocatalysts under visible light irradiation. Fig. 10 shows the transient photocurrent responses of the samples under visible light
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Fig. 9. (A) UV–vis solid absorption spectra and the corresponding band gap values (inset) and (B) Photoluminescence (PL) spectra under 363 nm excitation of BiOI nanomaterials with special morphologies: nanoflowers (red) and nanospheres (black). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 10. The photocurrent densities vs. time (j–t) curves collected at a potential of 0.5 V vs. RHE for BiOI nanoflowers (red) and BiOI nanospheres (black). Wherein the photocurrent and dark current were measured at least 50 s each during repeated ON/OFF illumination cycles. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 11. Photocatalytic mechanism for BiOI nanoflowers: trapping experiment of active species during the photocatalytic reaction with 30 min visible light irradiation: EDTA-2Na as scavenger for holes; N2 as scavenger for superoxide radicals (O2 ); TBA (tert-Butyl alcohol) as scavenger for hydroxyl radicals (OH).
3.5. Regeneration and reusability irradiation at a bias potential of 0.5 V vs. RHE. The photocurrent increases sharply once there is irradiation of visible light, and decreases when the irradiation is removed. The irradiation process was repeated over 600 s. The photocurrent of the BiOI electrodes does not show obvious changes, that is, the photocurrent responses of these electrodes are very stable. Furthermore, it can be seen that the flowerlike BiOI hierarchical nanostructures exhibit much higher photocurrent intensity than the spherical BiOI electrode, indicating a more efficient photo-induced charge separation and transfer in flower-like BiOI hierarchical nanostructures, possibly due to the hierarchical structure and the visible light absorption ability [35–37]. The result is in good agreement with the PL, DRS results above, and confirms that the nanostructure and morphology of the material have a significant effect on photoelectrochemical and photocatalytic properties. In order to better understand the photocatalytic mechanism, it is necessary to detect main active oxidative species in the photocatalytic reaction. To do so, various scavengers, including EDTA-2Na (2 mmol/L); N2 (0.2 L/min); TBA (tert-Butyl alcohol, 10 mmol/L) are introduced into the solution of RhB. The experiment was similar to the photodegradation experiment. The results (Fig. 11) show that the photodegradation rate shows almost no change in the presence of TBA, and the photocatalytic activity decreases slightly after the addition of N2. However, it is inhibited greatly in the presence of EDTA-2Na, indicating that the holes and O2 are the main reactive species for photodegradation of RhB.
In addition to photocatalytic activity, the stability of photocatalysts is another important issue for their practical applications. Therefore, we carried out cycling degradation experiments by evaluating the concentration change of RhB under visible light irradiation, and all processes and parameters were kept unchanged during the cycling tests. As shown in Fig. 12, even after 5 successive cycles, BiOI nanoflowers still give 94% degradation rate of
Fig. 12. Cycling runs for the photocatalytic degradation of RhB over BiOI nanoflowers under visible light irradiation.
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(N130410002), Doctoral Program of Higher Education of China (20130042120011), Science Foundation of Liaoning Province (L2013109), Open Project of State Key Laboratory of Rare Earth Resource Utilizations (RERU2014002), and Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (47-1). References
Fig. 13. Typical FESEM image and XRD pattern (inset) of the BiOI nanoflower photocatalysis after a 5-cycle reaction.
RhB after 40 min visible-light irradiation, indicating its high stability and great promise in practical applications. Fig. 13 shows the SEM and XRD patterns of BiOI which have been used for 5 recycling running for the photodegradation of RhB. Notably, both crystal structure and morphology of the nanoflowers can be well remained in the photocatalysis reaction. These results indicate that the BiOI flowerlike structure photocatalyst is sufficiently stable during the photodegradation of the organic dyes. 4. Conclusions In summary, we reported a simple and rapid room-temperature synthesis route to fabricate the uniform 3D hierarchical BiOI nanoflowers. The morphology variations of the nanostructures were achieved by simply varying the reaction parameters in the system. Furthermore, the photocatalytic properties of the obtained samples were investigated by the photodegradation of Rhodamine B under visible light irradiation. The as-prepared 3D BiOI nanoflowers exhibit superior photoreactivity compared to the spherical ones under the visible light irradiation due to larger specific surface areas, higher light trapping, and lower carrier recombination rates, all of which ensure much higher photocatalytic activity. Moreover, this work provides some new insights into the design and fabrication of advanced photocatalytic materials with complex hierarchical architectures for high efficiency in the solar energy conversion. Acknowledgments The work was grateful to the financial support by National Natural Science Foundation of China (51402047), the Fundamental Research Funds for the Central Universities
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Rapid room-temperature synthesis and visible-light photocatalytic properties of BiOI nanoflowers Feng Cao, Jianmin Wang, Song Li, Jiajia Cai, Wanhong Tu, Xin Lv, Gaowu Qin ⇑ Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang 110819, China
a r t i c l e
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Article history: Received 10 December 2014 Received in revised form 23 March 2015 Accepted 24 March 2015 Available online 30 March 2015 Keywords: Hierarchical nanostructure Room-temperature solution synthesis Visible-light Photocatalysis
a b s t r a c t Uniform hierarchical BiOI nanoflowers composed of nanoplates with rough surfaces have been synthesized by a mild and rapid room-temperature solution synthesis approach. The effects of reaction parameters on the formation of 3D flower-like nanostructure were discussed. The morphology variations of the nanostructures were achieved by simply varying the reaction parameters in the system. The photocatalytic properties of the obtained samples for photodegradation of RhB dye under visible light irradiation were investigated, indicating that the flowerlike BiOI hierarchical nanostructure exhibits much higher photocatalytic activity than the sphere-like BiOI nanostructures owing to the larger surface areas, higher light absorption, and better photo-induced charge separation. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Recently, environmental problems such as air and water pollution have provided the impetus for sustained fundamental and applied research in the area of environmental remediation [1–3]. The photocatalytic technique for the degradation of organic contaminants has become a powerful way to remedy water/air pollution due to its simple and thorough decomposition process [4,5]. Nano-sized TiO2 photocatalyst, as a most prominent and suitable material for environmental purification and solar energy harvesting, has attracted wide research interest so far [6,7]. However, its solar energy conversion efficiency is too low to be economically sound from the viewpoint of technology. The main barriers are the rapid recombination of photo-generated electron/hole pairs and the poor activation of TiO2 by visible light [8,9]. In response to these deficiencies, many investigators have been conducting research with an emphasis on developing visible light responsed photocatalysts where there is effective separation of the photogenerated exciton pairs [10,11]. As a p-type narrow band-gap semiconductor (1.9 eV), BiOI has been paid a great deal of attention for its superior photocatalytic performance in contaminant decomposition under visible light irradiation, which accounts for about 42% usage of the solar energy [12,13]. In addition, the excellent photocatalytic performance is also due to its peculiar layer structure characterized by [Bi2O2] slabs interleaved by double slabs of iodine atoms and indirect⇑ Corresponding author. E-mail address: [email protected] (G. Qin). http://dx.doi.org/10.1016/j.jallcom.2015.03.185 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
transition band-gap semiconductor feature [14,15]. To date, multifarious micro/nanostructured BiOI materials have also been produced by various synthetic techniques, including sonochemical, solvothermal, hydrothermal, microwave-assisted hydrothermal methods, and thermal oxidation at high temperature [16–21]. Many of them demand harsh reaction conditions such as high temperature or high pressure. For industrial application, it is highly desirable to develop a simple and facile synthesis which requires only mild reaction conditions under normal atmospheric pressure and room temperature. Most importantly, they can be completely synthesized in a short period of time. For example, Li et al. reported a room-temperature ionic liquid-assisted microemulsion method to synthesize BiOI hollow microspheres with a reaction time of only 1 h [14,22]. However, many of ionic liquids are obviously more expensive than common organic solvents. In this work, we developed a simple and rapid room-temperature solution synthesis approach for the fabrication of uniform hierarchically BiOI nanoflowers in the presence of PVP and isopropanol. The preparation conditions are much milder and simpler than those of conventional methods. In addition, the effects of reaction parameters on the formation of 3D flower-like nanostructure were discussed. The morphology variations of the nanostructures were achieved by simply varying the reaction parameters in the system. Furthermore, the photocatalytic properties of the obtained samples with different morphologies were investigated by the photodegradation of Rhodamine B under visible light irradiation. The enhanced photocatalytic activity of the flowerlike BiOI hierarchical nanostructure is attributed to its novel and special nanostructure.
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2. Experimental 2.1. Materials All the reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), and were used as received without further purification. All reagents were of analytical grade. All solutions were prepared with high-purity bidistilled deionized water (Millipore Co., 18.3 MX cm) unless otherwise mentioned. 2.2. Preparation of samples In a typical synthesis, 0.5 mmol of Bi(NO3)34H2O and 0.5 g PVP were dissolved and stirred in a solvent of 10 ml of isopropanol, and then a 10 mL aqueous isopropanol solution containing 1.5 mmol KI was added into the above solution. The resultant suspension was stirred intensively and the color of the suspension turned from yellow to red in a few minutes. The precipitate was separated by centrifugation, washed with distilled water and absolute ethanol several times, and dried at 60 °C for 2 h. Fig. 1. XRD pattern of the as-synthesized BiOI nanoflowers. 2.3. Characterization The X-ray diffraction pattern of the products was performed on a Rigaku-D/max 2500 V X-ray diffractometer with Cu Ka radiation (k = 1.5418 Å), with an operation voltage and current maintained at 40 kV and 40 mA. Field-emission scanning electron microscopy (FESEM) images were obtained with a XL30 ESEM FEG microscope. Transmission electron microscopic (TEM) images and selected area electron diffraction (SAED) patterns were obtained on a JEOL JEM-2010 microscope. X-ray photoelectron spectrometry (XPS) analysis was measured by using a Thermo ESCALAB 250 electron spectrometer. N2 adsorption–desorption isotherms were measured at liquid-nitrogen temperature (77 K) using a Micromeritics Tristar II instrument. Samples were degassed at 120 °C overnight before measurements. Specific surface areas were calculated using the Brunauer–Emmett–Teller (BET) model, and pore size distributions were evaluated from the desorption branches of the nitrogen isotherms using the Barrett–Joyner–Halenda (BJH) model. The photoluminescence (PL) spectra were measured with the PerkinElmer LS 55 Fluorescence spectrometer at room temperature. The UV–vis solid absorption spectroscopy was measured on a UV/Vis spectrophotometer (PerkinElmer Lambda 750S, Japan) in the range of 200–800 nm with BaSO4 as reflectance standard sample. 2.4. Photoelectrochemical measurements For the preparation of photoanode, 10 mg of the final BiOI nanomaterials was dispersed in 2 ml Nafion solution (0.1%). 2.5 ll of the suspension with dispersed BiOI was cast on a fluorine doped tin oxide (FTO) conducting glass and allowed to dry under the 160 °C for 10 min. Electrochemical measurements were taken by using an electrochemical analyzer (CHI660D, CHI Shanghai, Inc.) in a standard three electrode system with a platinum foil as the counter electrode, the prepared photoanode (1 1 cm2) as the working electrode, and a KCl saturated Ag/AgCl as the reference electrode. The working electrode with illuminated area 1.0 cm2 was immersed in 0.5 M Na2SO4 (deaerating with a nitrogen flow for 0.5 h) and the light source was provided by a 300 W xenon lamp (Perfect Light, PLS-SXE300) through a 420 nm UV-cut filter. Time dependent photocurrent curves were measured with amperometric I–t curve method. Photocurrent over time was detected on visiblelight illumination repeated every 50 s at a bias potential of +0.5 V vs. RHE. 2.5. Photocatalytic activity studies Photocatalytic experiments in aqueous solution were performed in a watercooled quartz vessel. Photodegradation studies of RhB were carried out under 300 W xenon lamp (Perfect Light, PLS-SXE300) through a UV-cut filter. For a typical photocatalytic experiment, a suspension containing a powdered catalyst (80 mg) and fresh dye aqueous solution (100 mL, 5.0 10 5 M) was magnetically stirred in the dark for 15 min to establish an adsorption/desorption equilibrium of the dye species. At given irradiation time intervals, a series of aqueous solutions in a certain volume were collected and filtered through a Millipore filter for analysis. The absorption spectrum of the filtrate was measured on a UV/Vis spectrophotometer (PerkinElmer Lambda 750S, Japan). The concentration of dye was determined by monitoring the changes in the main absorbance centered at 545 nm for RhB dye.
3. Results and discussion 3.1. Morphology and structure The structures of the as-prepared samples were characterized by the XRD analysis, as shown in Fig. 1. All the peaks in this pattern can be identified as tetragonal BiOI with lattice constants
a = 3.994 Å, b = 3.994 Å, c = 9.149 Å (JCPDS Card No. 10-0445). No impurity phase is observed, which indicate that the product is highly pure. The sharp and narrow diffraction peaks indicate the high crystallization of the product. The morphology and size of the as-synthesized products were characterized by FE-SEM. The low-magnification FE-SEM image (Fig. 2A) demonstrates that the typical products consist of a large quantity of uniform 3D BiOI flowerlike nanostructures. The as-obtained flowerlike nanomaterial has a diameter of about 300– 500 nm constructed by curved nanoflakes. The yield of hierarchical nanostructures is high (>95%). The high-magnification FE-SEM image (Fig. 2B) reveals that the flowerlike nanomaterial consists of many intercrossed nanoplatelets with a thickness of about 30– 50 nm. Additionally, the rough surface of the nanoplates can be observed. The chemical composition of these nanoflowers was further characterized by using EDS (Fig. 2A, inset). Peaks of the elements Bi, O, and I are detected in the EDS pattern (the Au signal comes from the substrate). Further evidence for the quality and composition of the products can be provided by using XPS, and the XPS spectrum of BiOI was identified. The binding energies obtained in the XPS analysis were corrected for specimen charging, through referencing the C 1s to 286.40 eV. The survey-scan spectrum in Fig. 3A indicates that the sample consists of Bi, O and I as major elements, besides small amounts of O, N, and C. The N 1s, C 1s, and O 1s peaks are attributed to a trace amount of polyvinylpyrrolidone (PVP) molecules adsorbed on the surfaces of the BiOI nanoflowers. As shown in Fig. 3B, there are two sets of spin–orbit coupling peaks with the first set at 158.4 and 163.7 eV and the second set at 159.8 and 165.1 eV for Bi 4f7/2 and Bi 4f5/2, respectively. The spectrum of O 1s consists of peaks situated at binding energies of 529.9 eV, 531.5 eV, and 533.0 eV. The peak at 529.9 eV can be assigned to Bi–O bands and the peak at 531.5 eV was attributed to the I–O bands in BiOI. The peak at 533.0 eV was originated from the adsorbed water (Fig. 3C). In addition, two peaks at about 619.2 eV and 630.6 eV appear in the I 3d spectrum (Fig. 3D), corresponding to the binding energies of I 3d5/2 and I 3d3/2, respectively [22]. Further insight into the morphology and microstructure of flowerlike BiOI nanostructures were gained by using TEM, SAED and HRTEM. Fig. 4A shows the low-magnification TEM image of a single BiOI nanoflower. It can be clearly seen that the nanoflowers are self-assembled of nanosheets, which is consistent with the results of FE-SEM observation. In Fig. 4B, The SAED pattern of the nanopetal (Fig. 4A) taken from the nanoflower shows that the nanopetal is well-crystallized, and these diffraction spots can readily be indexed to tetragonal BiOI crystal. The HR-TEM image (Fig. 4C) and FFT pattern (inset in Fig. 4C) show a crystalline
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Fig. 2. (A) Low-magnification FESEM image of the BiOI nanoflowers. And the EDS pattern (inset). (B) Enlarged FESEM image of the BiOI nanoflowers.
Fig. 3. XPS spectra of the BiOI nanoflowers: (A) survey; (B) Bi 4f; (C) O 1s; (D) I 3d.
Fig. 4. (A) Representative TEM image; (B) SAED pattern; (C) HRTEM image and FFT pattern (inset) of the edge area of the BiOI nanoflowers.
character with a lattice spacing of 0.458 nm, which can be indexed to the (0 0 2) plane of BiOI nanoflowers. 3.2. Effects of experimental parameters on BiOI morphologies It is worthwhile to mention that the addition of PVP has strong effects on the formation of the well-defined flowerlike
morphologies. Fig. 5 shows the SEM images of the samples obtained with different amounts of PVP. Without the addition of PVP, BiOI crystals tend to grow into irregular flake morphology with a wide distribution range from 10 to 20 lm (Fig. 5A). The size of BiOI flakes decrease as the amount of PVP increases (Fig. 5B). When 1 mmol of PVP is added, 3D flowerlike BiOI form although the size distribution is irregular (Fig. 5C). When the addition of
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Fig. 5. FESEM images of the BiOI samples with different amounts of PVP: (A) 0, (B) 0.5, (C) 1 and (D) 2 mmol. In all cases, samples were prepared at 180 °C, and a reaction time of 12 h.
PVP is up to 2 mmol, uniform 3D flowerlike BiOI architectures are the only morphology (Fig. 5D). When the quantity of PVP is further increased (5 mmol), the morphology remains flowerlike without an obvious change (Fig. 2A and B). These results clearly indicate that the good dispersancy and high viscosity of PVP is the key to make the BiOI nanoflowers smaller and more regular as they grow gradually [23,24]. In addition, the morphology of BiOI is also strongly affected by the addition of isopropanol solvent. As shown in Fig. 6A, when only water is used as solvent instead of isopropanol, microrods in a big size are observed in the products. When DEG instead of isopropanol is used, hierarchical nanospheres appear (Fig. 6B). When EG and PEG are used, uniform nanospheres with rough
surfaces are detected (Fig. 6C and D). These observations clearly show that the presence of alcohol solvent is beneficial to the control of both morphology and grain size of the nanostructured BiOI. It is well-known that isopropanol is a strong coordinating agent with a relatively high boiling point and viscosity, and has been widely used in the polyol process to synthesize monodisperse and fine semiconductor nanoparticles [25]. 3.3. Photocatalytic degradation properties toward RhB solution The photocatalytic activity of the as-prepared BiOI nanoflowers and nanospheres is examined by visible-light degradation of RhB molecules in aqueous solution, as shown in Fig. 7. A blank test
Fig. 6. FESEM images of products prepared under similar conditions but with isopropanol replaced by (A) H2O, (B) DEG, (C) EG, (D) PEG.
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Fig. 7. Photocatalytic activities for degradation of RhB dye under visible-light irradiation at room temperature in the presence of different BiOI samples: without catalyst (red); without light (black); flower-like BiOI (green); spherical BiOI (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(RhB without any catalyst) exhibits no degradation under visible light irradiation, which indicates the photolysis of RhB could be ignored. At the same time, our experimental data on the RhB decolorization with BiOI nanoflowers show that the concentration of RhB dye almost does not change under dark conditions after the BiOI nanoflowers and dye solutions reach the adsorption–desorption equilibrium. However, the data of green and blue curves in Fig. 7B clearly indicate that the dye could be completely degraded in about 50 min in the presence of BiOI nanoflowers, whereas it takes 80 min to decompose 100% RhB dye when the BiOI nanospheres prepared by EG as capping agent were used as the photocatalyst. The photodecomposition performance by the as-obtained BiOI nanoflowers in the visible light band is clearly better than that by the BiOI nanospheres although they have similar particle sizes. The different photocatalytic activities of these two BiOI samples may be due to the difference in the effective light absorption, specific surface area, and even band-gap energy [26,27]. 3.4. Photocatalytic mechanism Nitrogen adsorption–desorption isotherms and the corresponding Barret–Joyner–Halenda (BJH) pore size distribution curves of the obtained BiOI products with different morphologies are shown in Fig. 8. We can find that the isotherms of obtained BiOI nanomaterials can be categorized as type IV with a distinct hysteresis loop observed in the range of 0.5–1.0 P/P0 according to IUPAC classification. The measured Brunauer–Emmett–Teller (BET) surface area of flower-like nanoarchitectures is 26 m2 g 1 from N2 adsorption– desorption isotherm, larger than the BiOI nanospheres (15 m2 g 1). The hierarchical BiOI nanoflowers will potentially exhibit superior performance in many fields such as electrochemistry, gas sensor, and photocatalysis properties [28,29]. The energy band gap of a semiconductor is closely related to its photo-absorption and photocatalytic ability. Fig. 9A shows the typical UV–vis solid absorption spectrum of the as-prepared BiOI samples with different morphologies. It can be seen that both BiOI nanomaterials have nearly the same light absorption region from 490 nm to 700 nm, which means the BiOI nanomaterials can be used as a visible-light active semiconductor photocatalyst. Additionally, flower-like sample displays strong absorption in the visible light region mainly due to its perfect light-trapping structure, in contrast, the spherical sample shows the comparatively weaker absorption. Strong light absorption may lead to an increase in the generation of electron–hole pairs, which enhances the photocatalytic activity of the nanomaterials. At the same time, the visible light absorption ability is advantageous for enhancing
Fig. 8. N2 adsorption–desorption isotherm for the BiOI nanomaterials with special morphologies: nanoflowers (red) and nanospheres (black). The inset shows BJH pore size distributions of the sample. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the usability of solar light [30–32]. Additionally, as shown in the inset of Fig. 9A, the derived bandgaps are estimated to be 1.80 and 1.87 eV for the nanoflowers and nanospheres, which confirms that the size, morphology, and structure of the semiconductor materials have an great influence on related optical properties and offer a way of tuning the band gap [33,34]. The charge-carriers separation/recombination rates of the photo-excited carriers are further investigated by photoluminescence spectra (PL) with excitation at 247 nm (an excitation slit width of 10 nm and an emission slit width of 10 nm), because the recombination of photo-induced charge carriers gives rise to the PL emission. As shown in Fig. 9B, all BiOI samples exhibit a strong emission peak centering at about 530 nm, which can be attributed to the band-band PL phenomenon with the energy of light which is approximately equals to the band-gap energy (1.94 eV) of BiOI. In comparison with the spectra of the spherical BiOI nanostructure, the intensity of the emission peak of the flowerlike BiOI nanostructure is relatively weaker, indicating a higher suppression in charge recombination. In other words, the BiOI nanoflowers have higher photogenerated charge separation efficiency. Furthermore, photoelectrochemical properties are performed to study the excitation and transfer of photogenerated charge carriers in photocatalysts under visible light irradiation. Fig. 10 shows the transient photocurrent responses of the samples under visible light
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Fig. 9. (A) UV–vis solid absorption spectra and the corresponding band gap values (inset) and (B) Photoluminescence (PL) spectra under 363 nm excitation of BiOI nanomaterials with special morphologies: nanoflowers (red) and nanospheres (black). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 10. The photocurrent densities vs. time (j–t) curves collected at a potential of 0.5 V vs. RHE for BiOI nanoflowers (red) and BiOI nanospheres (black). Wherein the photocurrent and dark current were measured at least 50 s each during repeated ON/OFF illumination cycles. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 11. Photocatalytic mechanism for BiOI nanoflowers: trapping experiment of active species during the photocatalytic reaction with 30 min visible light irradiation: EDTA-2Na as scavenger for holes; N2 as scavenger for superoxide radicals (O2 ); TBA (tert-Butyl alcohol) as scavenger for hydroxyl radicals (OH).
3.5. Regeneration and reusability irradiation at a bias potential of 0.5 V vs. RHE. The photocurrent increases sharply once there is irradiation of visible light, and decreases when the irradiation is removed. The irradiation process was repeated over 600 s. The photocurrent of the BiOI electrodes does not show obvious changes, that is, the photocurrent responses of these electrodes are very stable. Furthermore, it can be seen that the flowerlike BiOI hierarchical nanostructures exhibit much higher photocurrent intensity than the spherical BiOI electrode, indicating a more efficient photo-induced charge separation and transfer in flower-like BiOI hierarchical nanostructures, possibly due to the hierarchical structure and the visible light absorption ability [35–37]. The result is in good agreement with the PL, DRS results above, and confirms that the nanostructure and morphology of the material have a significant effect on photoelectrochemical and photocatalytic properties. In order to better understand the photocatalytic mechanism, it is necessary to detect main active oxidative species in the photocatalytic reaction. To do so, various scavengers, including EDTA-2Na (2 mmol/L); N2 (0.2 L/min); TBA (tert-Butyl alcohol, 10 mmol/L) are introduced into the solution of RhB. The experiment was similar to the photodegradation experiment. The results (Fig. 11) show that the photodegradation rate shows almost no change in the presence of TBA, and the photocatalytic activity decreases slightly after the addition of N2. However, it is inhibited greatly in the presence of EDTA-2Na, indicating that the holes and O2 are the main reactive species for photodegradation of RhB.
In addition to photocatalytic activity, the stability of photocatalysts is another important issue for their practical applications. Therefore, we carried out cycling degradation experiments by evaluating the concentration change of RhB under visible light irradiation, and all processes and parameters were kept unchanged during the cycling tests. As shown in Fig. 12, even after 5 successive cycles, BiOI nanoflowers still give 94% degradation rate of
Fig. 12. Cycling runs for the photocatalytic degradation of RhB over BiOI nanoflowers under visible light irradiation.
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(N130410002), Doctoral Program of Higher Education of China (20130042120011), Science Foundation of Liaoning Province (L2013109), Open Project of State Key Laboratory of Rare Earth Resource Utilizations (RERU2014002), and Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (47-1). References
Fig. 13. Typical FESEM image and XRD pattern (inset) of the BiOI nanoflower photocatalysis after a 5-cycle reaction.
RhB after 40 min visible-light irradiation, indicating its high stability and great promise in practical applications. Fig. 13 shows the SEM and XRD patterns of BiOI which have been used for 5 recycling running for the photodegradation of RhB. Notably, both crystal structure and morphology of the nanoflowers can be well remained in the photocatalysis reaction. These results indicate that the BiOI flowerlike structure photocatalyst is sufficiently stable during the photodegradation of the organic dyes. 4. Conclusions In summary, we reported a simple and rapid room-temperature synthesis route to fabricate the uniform 3D hierarchical BiOI nanoflowers. The morphology variations of the nanostructures were achieved by simply varying the reaction parameters in the system. Furthermore, the photocatalytic properties of the obtained samples were investigated by the photodegradation of Rhodamine B under visible light irradiation. The as-prepared 3D BiOI nanoflowers exhibit superior photoreactivity compared to the spherical ones under the visible light irradiation due to larger specific surface areas, higher light trapping, and lower carrier recombination rates, all of which ensure much higher photocatalytic activity. Moreover, this work provides some new insights into the design and fabrication of advanced photocatalytic materials with complex hierarchical architectures for high efficiency in the solar energy conversion. Acknowledgments The work was grateful to the financial support by National Natural Science Foundation of China (51402047), the Fundamental Research Funds for the Central Universities
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