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Fabrication, modification and application of (BiO)2CO3-based photocatalysts: A review
Accepted Manuscript Title: Fabrication, modification and application of (BiO)2 CO3 -based photocatalysts: A review Author: Zilin Ni Yanjuan Sun Yuxin Zhang Fan Dong PII: DOI: Reference:
S0169-4332(15)03236-5 http://dx.doi.org/doi:10.1016/j.apsusc.2015.12.231 APSUSC 32214
To appear in:
APSUSC
Received date: Revised date: Accepted date:
25-10-2015 25-12-2015 31-12-2015
Please cite this article as: Z. Ni, Y. Sun, Y. Zhang, F. Dong, Fabrication, modification and application of (BiO)2 CO3 -based photocatalysts: A review, Applied Surface Science (2016), http://dx.doi.org/10.1016/j.apsusc.2015.12.231 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 proof before it is published in its final 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.
Fabrication, modification and application of (BiO)2CO3-based photocatalysts: A review
Zilin Ni a, Yanjuan Sun a, Yuxin Zhang b, Fan Dong a,∗ a
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Chongqing Key Laboratory of Catalysis and Functional Organic Molecules, College of Environment and Resources, Chongqing Technology and Business University, Chongqing, 400067, China. b College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China.
∗To whom correspondence should be addressed. E-mail: [email protected] (Fan Dong). Tel/Fax: + 23-62769785-605. 1 Page 1 of 42
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Abstract: (BiO)2CO3 (BOC), a fascinating material, belongs to the Aurivillius-related oxide family with an intergrowth texture in which Bi2O22+ layers and CO32- layers are orthogonal to each other. BOC is a suitable candidate for various fields, such as healthcare, photocatalysis, humidity sensor, nonlinear optical application and supercapacitors. Recently, the photocatalysis properties of (BiO)2CO3 have been gained increased attention. BOC has a wide band gap (3.1-3.5 eV), which constrains its visible light absorption and utilization. In order to enhance the visible light driven photocatalytic performance of BOC, many modification strategies have been developed. According to the discrepancies of different coupling mechanisms, six primary systems of BOC-based nanocomposites can be classified and summarized: namely, metal/BOC heterojunction, single metal oxides (metal sulfides)/BOC heterostructure, bismuth-based metallic acid salts (BixMOy)/BOC, bismuth oxyhalides (BiOX)/BOC, metal-free semiconductor/BOC and the BOC-based complex heterojunction. Doping BOC with nonmetals (C, N and oxygen vacancy) is unique strategy and warrants a separate categorization. In this review, we first give a detailed description of the strategies to fabricate various BOC micro/nano structures. Next, the mechanisms of photocatalytic activity enhancement are elaborated in three parts, including BOC-based nanocomposites, nonmetal doping and formation of oxygen vacancy. The enhanced photocatalytic activity of BOC-based systems can be attributed to the unique interaction of the p–n junction (semiconductor/semiconductor heterostructures), the Schottky junction (metal/semiconductor heterostructures), the surface plasmon resonance (SPR) e ect, the surface scattering and reflecting (SSR) e ect, the well-matched band structures, tunable electronic band structure, photosensitization and excellent electronic conductivity. Besides, multi-functional applications of BOC based materials are presented. Finally, prospective about the rational design, mechanistic understanding and application of BOC based materials is demonstrated, aiming to broaden the perspective and provide guidelines for future work. Keywords: (BiO)2CO3; photocatalysis; morphology control; heterojunction; doping.
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1. Introduction During the past few decades, environmental remediation and solar energy utilization have been attached tremendous importance for the severity of environmental pollution and global energy crisis. Semiconductor photocatalysis has received wide research interests worldwide as it could remove pollutants in a green manner. Apart from pollutant removal, photocatalysis is also considered as one of the most promising avenues for solar energy conversion, such as photocatalytic solar fules production [16]. Traditional TiO2 is a popular photocatalyst since Fujishima reported the generation of H2 and O2 by photoelectrochemical water splitting [7]. However, TiO2 shows poor visible-light photocatalytic activity due to its large band gap. Doping metals/nonmetals, coupling with other narrow band gap semiconductors and hydrogen treatment, have been successfully applied to enhance the visible light activity of TiO2. Various TiO2-based photocatalysts have been developed for improved visible light photocatalysis [7-17]. Alternatively, seeking for non-TiO2 based novel visible-light photocatalysts is another choice. A wave of Bi-based photocatalysts has hit the world. Most of Bi-based materials own controlled morphology and high photocatalysis efficiency. Bismuth, a typical semimetal material, has attracted great interest due to a set of unique properties, such as very small e ective masses, a large mean free path, a long Fermi wavelength, high carrier motilities and small band overlap energy. Moreover, nano-confinement e ects, observed in the Bi element, allow a semimetal-to-semiconductor transition at diameters of a few tens of nanometers [18-19]. Semimetal Bi was found to exhibit the strong SPR-mediated e ect and exploited in many applications, including sensors, fluorescence and surface-enhanced spectroscopy [18-19]. For instance, with strong SPR-mediated e ect, semimetal Bi nanoparticles exhibited high photocatalytic activity [19]. On the other hand, bismuth-containing semiconductor photocatalysts such as Bi2O3 [20-22], Bi2S3 [23-24], Bi2WO6 [25-34], BiVO4 [35-41], and BiOX (X = Cl, Br, I) [42-58], BiPO4 [59-60], aroused much interests because of their high visible light photocatalytic activity. As a member of Aurivillius-based oxide family, (BiO)2CO3 (BOC) is composed of alternate Bi2O22+ and CO32− layers, and the plane of the CO32− group is orthogonal to the plane of the Bi2O22+ layer [18,61]. Based on lattice match, (BiO)2CO3 orthogonal networks (2DONWs) can be transformed into β-Bi2O3, BiOCl 2DONWs, and Bi2S3 nested self-similar 2DONWs [62]. (BiO)2CO3 nanostructures are fascinating materials with remarkable properties and have been widely used in a diverse fields. (BiO)2CO3 has long been used for medical and healthcare purposes [63-65]. Recently, (BiO)2CO3 is demonstrated to be a highly promising photocatalyst [66-70]. Also, (BiO)2CO3 can be applied in other fields, such as supercapacitors [71,72], the humidity sensor [73], and nonlinear optical element [74]. (BiO)2CO3 compounds with one-dimensional (1D) nanostructure (nanotubes and nanoparticles) [65,69,73,75-79], two-dimensional (2D) structure (nanosheets and nanoplates) [70,80-81], and three-dimensional (3D) microstructure (rose, flower, 3
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persimmon-like, honeycomb-like, dandelion-like,hydrangea-like) [72,82-86] have been successfully prepared and applied in photocatalysis. Designed synthesis of nanostructured materials, especially those ordered and superstructured semiconductors with facet dominated growth and properties, is significant for high performance applications. (BiO)2CO3 single-crystal nanoplates with dominant {001} facets were fabricated for enhanced photocatalysis [69,87]. The band gap of (BiO)2CO3 is 3.1-3.5 eV [88]. With such a wide band gap, the (BiO)2CO3 has a low utilization efficiency of solar light [89]. A series of solutions have been reported to broaden the absorption spectra of (BiO)2CO3. Noble metal Ag (semimetal Bi) loaded BOC becomes highly active under visible light irradiation due to the formation of Schottky junction and SPR e ect [71,90,91]. BOC could also act as an eminent candidate for coupling with various functional materials to enhance the photocatalytic performance. For example, single metal oxide/BOC, BixMOy/BOC, BiOX/BOC as well as metal sulfides/BOC are constructed to form p–n junction/n–n junction. By coupling with semiconductors of narrow band gaps, these BOC-based heterostructures present excellent visible light absorption and enhanced charge separation and migration. Recently, π-conjugated materials are applied to modify BOC as well, such as graphene/BOC [78,92], Polyaniline (PANI)-decorated BOC nanosheets [93], polypyrrole (PPy)/BOC [94]. The excellent electronic conductivity of π-conjugated materials could promote the charge transfer and thus enhance photocatalytic performance. Nonmetal doping of BOC is an effective methods to modify the band structure and enhance the visible light activity of BOC [85-86,9599]. In this perspective, we make a comprehensive review on recent progress in the fabrication, mechanistic understanding, and potential applications of BOC-based photocatalysts. First, we review different synthesis methods for (BiO)2CO3 hierarchical micro/nano structures. Then, according to the discrepancies of different coupling agents, we classify BOC-based nanocomposites into six primary systems: namely, metal/BOC heterojunction, single metal oxides (metal sulfides)/BOC heterostructure, bismuth-based metallic acid salts (BixMOy)/BOC, bismuth oxyhalides (BiOX)/BOC, metal-free semiconductor/BOC, and the BOC-based complex heterojunction. The main photocatalytic mechanisms have been emphasized. We highlight the enhancement of the photocatalytic performance of BOC via nonmetals doping. Various applications of BOC and its derivatives in diverse areas were demonstrated. Finally, we present the conclusion and future prospective on the photocatalysis mechanism, theoretical calculation, controlled synthesis and applications of BOC-based materials.
2. Synthesis of (BiO)2CO3 micro/nano structures The photocatalytic performance of semiconductors is predominantly controlled by the intrinsic properties, including band gap structure, charge separation kinetics, surface area, and morphology [6,100-103]. The physicochemical properties are connected with preparation method, hydrothermal temperature, hydrothermal time, assistance and precursors. Employing an appropriate synthesis method is of great 4 Page 4 of 42
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significance to optimizing the structural features. 2.1 The effects of preparation methods During the past few years, different preparation methods for (BiO)2CO3 hierarchical micro/nano structures have been reported. Hydrothermal treatment is most widely used. For example, Xie’s group synthesized (BiO)2CO3 nanoflowers, nanosponges and nanoplates by a simple hydrothermal process [69]. As shown in Fig. 1(a), persimmon-like (BiO)2CO3 hierarchical structures with uniform size were prepared hydrothermally and demonstrated high photocatalytic efficiency under simulated solar irradiation [104]. Dai and co-workers synthesized flower-like (BiO)2CO3 by hydrothermal treatment of bismuth nitrate, citric acid and urea in water and then investigated the effects of citrate ion on the morphology and photocatalytic activity of (BiO)2CO3 [105]. Other methods were also employed for preparation of (BiO)2CO3, such as solutionphase method, reflux method and solvothermal method. Lee et al prepared (BiO)2CO3 nanosheetsby a solution-phase route shown in Fig. 1 (b) [106]. Chen et al. synthesized nanotubes, nanobars (Fig. 1 (c)), nanoplates, nanoparticles and cube-like (BiO)2CO3 by simple reflux and solvothermal processes [65,80]. Dai et. al prepared (BiO)2CO3 nanosheet using urea as carbon source via solvothermal method [70]. Through a facile solvothermal method, Chen et al. synthesized (BiO)2CO3 nanotubes (NTs) with ethylene glycol (EG), which was depicted in Fig. 1 (d) [107].
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Fig. 1 (a) Persimmon-like BOC (Reprinted with permission from ref. 104. Copyright (2011) The Royal Society of Chemistry.); (b) Sheet-like BOC (Reprinted with permission from ref. 106. copyright 2007, Elsevier.); (c) BOC nanobars by using (NH4)2CO3 as reagent (Reprinted with permission from ref. 80. copyright 2010, Elsevier.); (d) BOC nanotubes (Reprinted with permission from ref. 107. copyright 5 Page 5 of 42
2012, Wiley.)
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2.2 The effects of hydrothermal conditions Hydrothermal temperature and time are important factors in controlling the morphology of BOC. As shown in Fig. 2, Dong et al. prepared the flower-like, pinonlike and faceted nanoplates-like BOC nanostructures by a one-pot template-free hydrothermal treatment of bismuth citrate and sodium carbonate at 150, 180 and 210 o C [108]. With the reaction temperatures increasing, the thickness of nanoplates was increased due to the preferred growth along the (110) plane. Flower-like (150 oC), pinon-like (180 oC), faceted nanoplates-like (210 oC) BOC nanostructures were selectively formed. Among these morphology, the flower-like BOC showed the highest photocatalytic activity toward removal of aqueous RhB, ascribing to the multiple light reflections between the nanoplates. Dong et al also investigated the effects of temperature on hydrothermal treatment of bismuth citrate and dicyandiamide. BOC nanostructures of irregular particles (150 oC), honeycomb-like hierarchical microspheres (180 oC) and loose nanosheet aggregates (210 oC) can be produced [86].
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Fig. 2 flower-like (a), pinon-like (b) and faceted nanoplates (c) (BiO)2CO3 samples with hydrothermal temperature 150 oC, 180 oC, 210 oC. (Reprinted with permission from ref. 108. copyright 2013, Elsevier.) The morphology of BOC varied with time in hydrothermal reaction. Dong et al obtained time-dependent BOC samples by a hydrothermal treatment of bismuth citrate and sodium carbonate for elucidating the growth mechanism of BOC hierarchical hollow microspheres. During the initial 6h, bismuth citrate was hydrolyzed by the abundant OH- ions produced by sodium carbonate to produce BiO+. (Reaction (1) and (2)) With the reaction time increasing to 7.5h, bismuth subcarbonate hydroxide was generated at this reaction stage. (Reaction (3)) From 9 to 24h, the CO32- ions may substitute OH- ions in the (BiO)4CO3(OH)2 layers (Reaction (4)) [68]. Na2CO3 + 2H2O → CO32- + 2OH- + 2Na+ + 2H+ (1) 3+ + + Bi (citrate) + 3OH → BiO + citrate + 3H (2) + 24BiO + CO3 + 2OH → (BiO)4CO3(OH)2(s) (3) (BiO)4CO3(OH)2(s) + CO32- → (BiO)2CO3(s) + 2OH- (4) Fig. 3 showed the SEM images of BOC samples obtained with different reaction stages. The formation mechanism of BOC hierarchical hollow microspheres was revealed as following. First, near amorphous particles are produced through reaction, nucleation, crystallization, and aggregation processes. Then, intermediate 6 Page 6 of 42
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(BiO)4CO3(OH)2 microspheres emerge due to dissolution and recrystallization. Through Ostwald ripening, stacked uniform solid microspheres grow with consumption of particles, subsequently forming microspheres with a hole in the center, like flower buds. Finally, owing to layers splitting, uniform monodisperse (BiO)2CO3 hierarchical hollow microspheres were produced and exhibit excellent photocatalytic performance in removal of NO.
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Fig. 3 SEM images of the as-prepared BOC-7.5h (a), BOC-9h (b), BOC-12h (c), BOC-24h (d) samples. (Reprinted with permission from ref. 68. copyright 2012, The Royal Society of Chemistry.) 2.3 The effects of templates The wet chemical route has been proved to be one of the most effective and convenient approaches in preparing BOC with diversely controllable morphologies and architectures especially for large production. Generally, the synthesis of BOC can be classified as two types: template method and template-free method. A specific structure can be easily constructed by the template approach. For instance, Chen et al. utilized the assistance of cetyltrimethylammonium bromide (CTAB) to controll the synthesis of flower-like BOC, which exhibited high photocatalytic degradation of wastewater dyes under UV-vis light irradiation [82]. Cheng et al. reported the synthesis of hierarchical rose-like BOC microstructures assembled by single-crystalline nanosheets with the help of PVP [76]. Using sodium dodecyl sulfate (SDS) as a soft template, the as-synthesized BOC nanosheets showed excellent photocatalytic activity for RhB degradation under simulated sunlight [109]. However, template synthesis usually requires tedious procedures such as template modification, precursor attachment and core removal, which consequently limit their large-scale production and further applications. In addition, the inadequate removal of the templates may result in undesired impurities. To obtain pure hollow and 7 Page 7 of 42
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hierarchical structures, template-free approaches are highly desirable. Huang et al prepared BOC nanosheets and flower-like microstructures employing a template-free, and low-temperature solution method [110]. The growth mechanism of uniform BOC hierarchical hollow microspheres via template-free fabrication was reported by Dong and co-workers [67]. Chen et al. synthesized BOC nanotubes (NTs) through a facile solvothermal method without the need for any surfactants or templates [107].
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2.4 The effects of precursors Morphology of materials varies with different precursors, concentration and pH. Using tri-sodium citrate as both the coordinating agent and carbon source, Qian et al. hydrothermally synthesized three kinds of BOC microstructures and found spongelike BOC owned the highest photocatalytic performance. By tuning the concentration of tri-sodium citrate, BOC of sponge-like, rose-like and plate-like shapes could be selectively obtained [88]. Dai et al. investigated the effects of citrate ion on the morphology and photocatalytic activity and discovered that BOC microspheres synthesized in the solution with the C6H8O7/Bi(NO3)3 molar ratio of 2:1 showed the highest photocatalytic activity [105]. The microstructure of the BOC structures could also be modulated by varying the concentration of Na2CO3 [110]. Huang et al. found that only nanoplates with a thickness of about 30 nm can be prepared when the molar ratio of Bi(NO3)3/Na2CO3 was increased to 1:3. If the ratio was decreased to 1:9, irregular curving multisheets were formed [110]. By changing the amount of precursor (sodium carbonate), Dong et al. synthesized BOC microspheres and BOC particles with different morphology via a simple hydrothermal method. It was found that the crystallinity and morphology can be tuned by simply changing the amount of CO32−, providing a new insight into the relationship between the crystallinity, morphology, and the corresponding photocatalytic activity. Based on different amount of sodium carbonate (0.23 g and 1.53 g), the as-prepared (BiO)2CO3 materials were denoted as BOC-M (Fig. 4 (a)) and BOC-P (Fig. 4 (b)), respectively [111].
Fig. 4 SEM images of BOC-M (a) and BOC-P (b) with the amount of sodium carbonate 0.23g and 1.53g respectively. (Reprinted with permission from ref. 111. copyright 2014, American Chemical Society.) For BOC-P nanoparticles, excessive CO32− leads to the production of a great number of nuclei and accelerates crystal growth, which promotes the crystallization of BOC, 8 Page 8 of 42
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thereby generating a sample with high crystallinity. However, compared with BOC-P nanoparticles, BOC-M microspheres show higher photocatalytic activity toward the removal of NO upon UV-light, visible-light and simulated-solar-light irradiation, attributing to the synergistic e ects of its hierarchical structure, low crystallinity and large surface area. In addition, The morphology of BOC can be controlled by varying the pH value. Xie et al adjusted the pH of the solution to 7-9 and 4-5 to acquire the flower-like BOC hierarchitectures and sponge-like BOC porous spheres, utilizing Bi(NO3)3·5H2O and C6H8O7·H2O as precursors [69]. Recently, as a new raw material, CO2 is introduced into the preparation of BOC nanosheets. Instead of organic precursors, atmospheric CO2 is easily available and economic. The using of CO2 will reduce the amount of undesirable organic byproducts during the synthesis process and promote the large scale production of (BiO)2CO3. Zhou et al. synthesized (BiO)2CO3 nanosheets with exposed {001} facets, benefiting from the capture of atmospheric CO2 as a low-cost carbon source [73]. (BiO)2CO3 single-crystal nanosheets were successfully fabricated by an eco-friendly process under mild conditions with atmospheric CO2 within 30 min. In the process, bismuth nitrate and CO2 in air were used without extra separation [81]. This rapidly prepared (BiO)2CO3 nanosheets exhibited an efficient and durable photocatalytic performance for NO removal [81].
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3. (BiO)2CO3 based nanocomposites 3.1 Metal/(BiO)2CO3 nanocomposites Three types of metal/(BiO)2CO3 nanocomposites have been reported, including Ag/BOC, Au/BOC and Bi/BOC whose photocatalytic mechanisms are similar (Table 1). In general, their enhanced photocatalytic activity and photocurrent generation benefit from the cooperative contribution of the surface plasmon resonance (SPR e ect), e cient separation of electron–hole pairs and prolonged lifetime of charge carriers induced by metal nanoparticles. Dong et al fabricated plasmonic Ag nanocrystal decorated 3D (BiO)2CO3 hierarchical microspheres (Ag/BOC), which exhibited greatly enhanced visible light photocatalytic activity, photocurrent generation and promoted NO2 oxidation compared to the pure (BiO)2CO3 microspheres [90]. The enhanced photocatalytic activity of Ag/BOC is ascribed to SPR e ect of Ag nanocrystals, e cient charge separation and prolonged lifetime of charge carriers. On the other hand, the surface scattering and reflecting (SSR e ect) resulting from the special 3D hierarchical architecture of BOC also contributes to the increased photocatalytic activity. Fig. 5 illustrates the scheme of photocatalysis mechanism of Ag-decorated (BiO)2CO3 under visible light irradiation. The deposited Ag nanoparticles propel the Ag/BOC samples to absorb more visible light owing to the SPR e ect. Simultaneously, Ag nanoparticles can be photoexcited, thus enhancing the surface electron excitation and interfacial electron transfer. Because the Fermi level of metallic Ag (0.4 eV) is lower than the conduction band of (BiO)2CO3 (0.20 eV), the photogenerated electrons will probably transfer from (BiO)2CO3 to the deposited Ag nanoparticles (as shown in Fig. 5), creating a Schottky barrier at the interface that reduces the recombination of 9 Page 9 of 42
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electron–hole pairs and increases the lifetime of the charge carriers. Subsequently, photogenerated charge carriers will be transformed into active species (•OH) that are responsible for the degradation of pollutants. Due to different redox potentials, the photoexcited electrons of Ag together with the electrons transferred from (BiO)2CO3 can reduce O2 to OH−, and the holes from (BiO)2CO3 can oxidize OH− to •OH. Finally, OH− radicals, as the major reactive oxidation species, could oxidize NO to the NO3−.
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Fig. 5 (a, b) SEM images of Ag/BOC-5%; (c) Scheme of photocatalytic mechanism of Ag-decorated (BiO)2CO3 under visible light irradiation. (Reprinted with permission from ref. 90. Copyright (2014) The Royal Society of Chemistry.) Also, Peng et al. deposited Ag particles on the surface of (BiO)2CO3 via a facile photoreduction process [71]. Loading of 0.6 wt% Ag on the (BiO)2CO3 microspheres could significantly enhance the photocatalytic activity for degradation of methyl orange dye and improve the supercapacitive behavior with lower electrical resistance. PVP, KCl and hexamethylenetetramine (HMT) play key roles in the formation of such hierarchical microspheres. In the synthesis process, PVP is used as a reactant that not only provides C and O sources but also serves as a template to induce the nanoplateassembly to form microspheres. In addition, the size of the (BiO)2CO3 microspheres is reduced from 6.0 mm to 1.0 mm by the addition of KCl in the synthesis. A possible formation mechanism of the (BiO)2CO3 microspheres is proposed in Fig. 6. And the proposed reactions can be summarized in Eqn. (1)–(6): [Bi3+]PVP → Bi3+ + PVP (1) HMT + H2O → OH- + NH4+ + HCHO (2) 3+ + + Bi + OH → BiO + H (3) 2+ PVP + H2O → CO3 + 2H (4) BiO+ + CO32- → (BiO)2CO3 (5) overall reaction → (BiO)2CO3 + NH4NO3+HCHO (6) Bi(NO3)3 + PVP +HMT + H2O
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Fig. 6 A schematic illustration of the formation process of the hierarchical (BiO)2CO3 microspheres. (Reprinted with permission from ref. 71. Copyright (2013) The Royal Society of Chemistry.)
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In the hydrothermal process, due to the intrinsic tetragonal layer-structure and selective adsorption of surfactant PVP molecules on the crystal faces, (BiO)2CO3 crystal nuclei anisotropically grow into nanoplates. When the amount of PVP is low in the reaction, the obtained (BiO)2CO3 presents nanoplate structures. With the increased amount of PVP, these nanoplates can assemble into a hierarchical conglomeration through molecular interaction between surfactant molecules, and then transform into 3-dimensional nanostructures driven by the minimization of surface energy [71]. Novel 3D Au/(BiO)2CO3 (Au/BOC) heterostructures with size-controlled Au nanoparticles (NPs) (2–10 nm) were synthesized by Dong’s group (Fig. 7(a) and 7(b)) [112]. As elucidated in Fig. 7 (c), the enhanced visible light photocatalytic activity of Au/BOC can be attributed to the synergetic e ects of SPR, Schottky Barrier, and the strong light scattering and reflecting e ects (SSR).
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Fig. 7 (a, b) TEM and HRTEM images of Au/BOC composites with 2 ml Au NPs of 2–4 nm. (c) Photocatalytic mechanism of Au/BOC under visible light irradiation. (Reprinted with permission from ref. 112. Copyright (2015) The Royal Society of Chemistry.)
Recently, Dong et al. firstly reported that plasmonic Bi nanoparticles deposited on (BiO)2CO3 microspheres (Bi/BOC) via an one-pot hydrothermal treatment of bismuth citrate, sodium carbonate, and thiourea. Thiourea functions as a reductant and could reduce Bi3+ to metallic Bi. It is economic and efficient to utilize low-cost Bi nanoparticles as a substitute for noble metals to enhance visible light photocatalysis and photochemical stability [91]. The mechanisms for enhanced photocatalytic 11 Page 11 of 42
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performance of Bi/BOC are illustrated in Fig. 8. Firstly, the SPR e ect of the spatially confined electrons in the deposited Bi nanoparticles favors the visible light absorption. Secondly, owing to the SPR e ect, the Bi nanoparticles can be photoexcited, hence strengthening the surface electron excitation and interfacial electron transfer. Thirdly, given that the Fermi level of metallic Bi (-0.17 eV) is higher than the conduction band of (BiO)2CO3 (0.20 eV), the photo-generated electrons can transfer from Bi particles to (BiO)2CO3. Once releasing electrons, Bi can shift to more positive potentials and then return to its primary state by accepting electrons from the valence band of (BiO)2CO3. This process restrains the recombination of electron-hole pairs in (BiO)2CO3 because of the interface transfer of electrons (I) from (BiO)2CO3 to the Bi nanoparticles as shown in Fig. 8. Fourthly, the SPR-mediated local electromagnetic field of Bi (II) in Fig. 8 leads to the separation of the electron-hole pairs in (BiO)2CO3. These mechanisms synergistically boost the photocatalytic activity [91]. Recently, Sun et al. realized one-pot solvent-controlled synthesis of Bi/BOC using bismuth citrate, sodium carbonate and ethylene glycol as precursors [113].
Fig. 8 Photocatalytic mechanism scheme of Bi/BOC under visible light irradiation: interface transfer of electrons from (BiO)2CO3 to Bi(I) and local electromagnetic field of Bi(II). (Reprinted with permission from ref. 91. Copyright (2014) American Chemical Society.) Table 1 Photocatalytic properties of Metal/(BiO)2CO3 nanocomposite photocatalysts Composite photocatalyst
Photocatalyst of the highest activity
Typical parameters
Photocatalytic activity
Reference photocatalyst
Enhanced factor over the reference photocatalys
Ag/(BiO)2CO3
Ag(0.6%)/BOC microspheres (with KCl)
photodegradation of methyl orange (MO) under UV-vis light
91% in 50 min
BOC microspheres (with KCl) BOC microspheres (without KCl) BOC microplates
1.2 1.4 2.0
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Au/(BiO)2CO3
Au(2-4nm)/BOC with 4ml Au solution
BOC microspheres BOC nanosheet
2.5 13.5
203 ppb/min NP
BOC microspheres Au(4ml,4-6nm)/BOC Au(4ml,8-10nm)/BOC Au(2ml,2-4nm)/BOC Au(8ml,2-4nm)/BOC Bi/BOC-1% Bi/BOC-10% Bi BOC light source 100W
58% in 30 min
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Bi/BOC-5% λ >420 nm light source 100W (The mass ratio of thiourea to (BiO)2CO3 was 5%)
Bi/(BiO)2CO3
37.2% in 30 min
Bi/BOC-5%-32W Bi/BOC-5%-5W different light sources (BiO)2CO3
1.8 1.1 1.3 1.1 1.1 1.6 1.7 19.3 2.6
1.3 1.9 1.9
M
Bi/(BiO)2CO3
54% in 30 min
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Bi/(BiO)2CO3
NO removal under visible light and the same below
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Ag/BOC-5%
2.2
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Ag/(BiO)2CO3
P25
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3.2 Metal oxides (sulfides)/(BiO)2CO3 heterostructure So far, various single metal oxides (metal sulfides) have been coupled with BOC for enhanced visible light photocatalysis. The p-n heterostructures are full of interest with an internal electric field at interface, thus promoting the separation of photogenerated electron–hole pairs and increasing their photocatalytic activity. Photocatalytic enhancement mechanisms of the p-n heterojunctions have been proposed based on the relative band gap position of these two semiconductors and the integrated heterostructures [72,114-118], as shown in Table 2. Wei et al prepared Ag3PO4/(BiO)2CO3 composites by a combination of hydrothermal technique and precipitation. Ag3PO4 nanoparticles with ca. 200 nm in size were attached to the surface of (BiO)2CO3 microspheres with diameter of 1–2 µm, which resulted in higher photocatalytic activity toward degradation of Rhodamine B (RhB) under visible light irradiation [114]. Ag2O/(BiO)2CO3 p-n heterojunctions were prepared with commercial (BiO)2CO3 as precursor via a simple photosynthesis process [115]. The (BiO)2CO3@Fe2O3 nanosheets with exposed active {001} facet were fabricated by Hu et al. [116]. Sphere-like and flower-like Fe3O4/(BiO)2CO3 photocatalysts were obtained by Zhu et al. with the adjusting of the pH from 4.0 and 6.0, respectively. The composites photocatalysts displayed not only wonderful activity towards degradation of methyl orange and methylene blue, but also superparamagnetic behavior at room temperature, making Fe3O4/(BiO)2CO3 easily be recovered and recycled [117]. Very recently, as supercapacitors, flower-like (BiO)2CO3@MnO2 and Bi2O3@MnO2 nanocomposites prepared by Zhang et al. demonstrated high specific capacitance performance and good cycling stability [72]. This work opens a new avenue for development of BOC-based supercapacitors. 13 Page 13 of 42
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Flower-like p-n heterostructured β-Bi2O3/(BiO)2CO3 microspheres were synthesized through calcinations of a (BiO)2CO3 self-sacrifice precursor for the visible-light photocatalytic degradation of o-phenylphenol [118]. Xu et al also reported the synthesis of β-Bi2O3/(BiO)2CO3 nanosheet composites through rational heat treatment of the precursor (BiO)2CO3 [63]. Currently, loading noble metals is becoming increasingly prevalent ascribing to its effective enhancement of the photocatalytic activity. However, these metals are rare and valuable. As a transition metal sulfide, molybdenum disulfide (MoS2) is an emerging photocatalytic co-catalyst to substitute noble metals. Wang et al. synthesized novel MoS2/(BiO)2CO3 photocatalysts by a simple hydrothermal method. The special structure of MoS2 provides more active sites to adsorb dyes, making the MoS2/(BiO)2CO3 exhibit excellent degradation of rhodamine B under UV light irradiation [119]. The size quantization of Bi2S3 enables its nanoparticles (1.3 eV for bulk) to show tunable band gap energy and photosensitization in the visible region [120]. The combination of Bi2S3 with BOC could not only increase the absorption of visible light, but also benefit the separation of the photogenerated carriers, thus making (BiO)2CO3/Bi2S3 composites a good choice to optimize the visible light-driven photocatalytic properties of (BiO)2CO3. To date, the integration of (BiO)2CO3 and Bi2S3 to construct novel heterojunctions has been reported by three articles [120-122]. Huang et al. reported (BiO)2CO3/Bi2S3 heterojunctions through a facile partial ion exchange method between the (BiO)2CO3 microspheres and TAA, from which S2ions reacted with (BiO)2CO3 to form Bi2S3 on the (BiO)2CO3 nanosheets [120]. Novel heterojunctions Bi2S3/(BiO)2CO3 were prepared by a simple chemical reaction with commercial (BiO)2CO3 from Zai et al [121]. The Bi2S3/(BiO)2CO3 exhibits the enhanced photocatalytic activity for degradation of RhB, which is related to the unique band gap structure of n–n-type Bi2S3/(BiO)2CO3 heterojunctions and special morphology with the partly exposed core. It’s worth noting that 5 mol% Bi2S3/(BiO)2CO3 can benefit the migration of electrons or holes to the reaction sites (Fig. 9 left), while the Bi2S3/(BiO)2CO3 of 25 mol% (Fig. 9 right) is not favorable for the charge transfer because the photogenerated holes will largely accumulate in the (BiO)2CO3 side, counteracting the built-in potential produced by n-n-type heterojunctions [121].
Fig. 9 The e ect of heterojunction structures of photocatalysts. (Reproduced with 14 Page 14 of 42
permission from ref. 121. Copyright 2014 The Royal Society of Chemistry.）
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of
metal
oxides
Photocatalyst of the highest activity
Typical parameters
(sulfides)/(BiO)2CO3
Photocatalytic activity
Reference photocatalyst
Enhanced factor over the reference photocatal
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Ag3PO4/(BiO)2CO3
Ag3PO4/BOC-100% (the molar ratio of Ag3PO4:BOC=1:1)
degradation of 97% in 30 min rhodamine B (RhB) under visible light
BOC microspheres Ag3PO4-BOC-200% Ag3PO4-BOC-50%
2.2 1.0 1.9
Ag2O/(BiO)2CO3
Ag2O/BOC-50% (the molar ratio of Ag2O: BOC =1:2)
degradation of RhB, methyl blue (MB), methyl orange (MO) under visible light
100% in 12 min 95% in 12 min 100% in 12 min
1.5 1.5 1.4 1.3 1.6 4.8
Fe2O3/(BiO)2CO3
Fe2O3/BOC (5 mol% Fe2O3)
degradation of RhB under UV-vis light
99% in 25 min
Ag2O/BOC-8.3% Ag2O/BOC-12.5% Ag2O/BOC-25% Ag2O/BOC-100% Ag2O BOC degradation of RhB in 12 min Fe2O3/BOC (0 mol% Fe2O3) (1 mol% Fe2O3) (3 mol% Fe2O3) (10 mol% Fe2O3) (20 mol% Fe2O3)
1.3 1.3 1.1 1.2 1.4
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β-Bi2O3/(BiO)2CO3
β-Bi2O3/BOC (calcined at 300 ◦C) β-Bi2O3/BOC (calcined at 300 ◦C)
degradation of MB under visible light
99% in 40min
degradation of MO under visible light
99% in 40min
degradation of MB 95% in 150 min under visible light degradation of o- 99.8% in 45 min phenylphenol under visible light
sphere-like Fe3O4/BOC (PH=4)
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β-Bi2O3 (330 ◦C) α-Bi2O3 (400/500 ◦C) BOC (250 ◦C) N-doped TiO2 mixed β-Bi2O3 and BOC commercial β-Bi2O3
2 13 827 80 2.6
0.2wt.% MoS2/BOC 1wt.% MoS2/BOC 2wt.% MoS2/BOC MoS2 BOC Bi2S3/BOC-20 min Bi2S3/BOC-2 h 15 mol% Bi2S3/BOC 25 mol% Bi2S3/BOC BOC
1.1 1.0 1.0 1.7 1.1 1.1 1.3 1.5 2.9 2.9
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MoS2/BOC with 0.5wt% MoS2
degradation of RhB under UV light
Bi2S3/(BiO)2CO3
Bi2S3/BOC-1 h
93% in 3 h
Bi2S3/(BiO)2CO3
5 mol% Bi2S3/BOC
degrading RhB under visible light degrading RhB under visible light
degrading RhB under sunlight
100% in 75 min
15 mol% Bi2S3/BOC 25 mol% Bi2S3/BOC BOC
1.3 1.1 1.3
Bi2S3/BOC-0.25 (the molar ratio of Bi2S3:BOC = 0.25)
NO removal under visible light
57.1% in 30 min
Supercapacitors with higher specific capacitance
Typical parameters
specific capacitance 196 Fg-1
Bi2S3/BOC-0.05 Bi2S3/BOC-0.1 Bi2S3/BOC-0.5 Bi2S3/BOC-1.0 BOC Reference supercapacitors Bi2O3/BOC
1.2 1.2 1.3 No activity 2.7 Enhancemen factor over reference 1.4
100% in 30 min
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3.3 BixMOy/(BiO)2CO3 heterostructures Table 3 shows the photocatalytic properties of BixMOy/(BiO)2CO3 heterostructures. Bi2WO6 is the simplest member among the Aurivillius-based compounds and a visible light responsive photocatalyst owing to its relatively narrow band gap. Huang et al. obtained a novel (BiO)2CO3/Bi2WO6 composites photocatalyst via a one-pot hydrothermal route, using Bi(NO3)3·5H2O, citric acid, PEG and Na2WO4 as the raw materials. The as-prepared (BiO)2CO3/Bi2WO6 were assembled by many interconnecting nanosheets and exhibited a RhB degradation efficiency of 98% after 60 min [123]. Zhang et al. successfully fabricated the (BiO)2CO3/Bi3NbO7 composite 16 Page 16 of 42
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via a simple hydrothermal method and found it an effective visible-light-driven photocatalyst for inactivation of Escherichia coli [124]. BiVO4 has a narrow band gap for visible light absorption and other merits are plentiful abundance, low cost, and good stability. Yu et al. developed a novel hydrothermal approach to synthesize hierarchical BiVO4/(BiO)2CO3 nanocomposites with reactive crystalline facets using urea as a morphology mediator as shown in Fig. 11. Their results indicate that both physical parameters and associated photocatalytic activity of BiVO4/(BiO)2CO3 nanocomposites can be tuned by the urea concentration and reaction time in the synthesis process. With the urea concentration added, the specific surface areas, pore volume and average pore size increased. Compared to BiVO4 and (BiO)2CO3 bulk counterpart, BiVO4/(BiO)2CO3 nanocomposites show enhanced photocatalytic activity under visible-light irradiation [125].
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Fig. 11 (a) SEM image of the BiVO4/(BiO)2CO3 composite (U20) where BiVO4 particles are marked with arrow heads; (b) HRTEM image of BiVO4/(BiO)2CO3 nanocomposite showing the lattice fringes (inset in b shows TEM image of BiVO4/(BiO)2CO3 nanocomposite). (Reproduced with permission from ref. 125. Copyright 2011, Elsevier.） Table 3 Photocatalytic properties of BixMOy/(BiO)2CO3 heterostructures Composite photocatalyst
Photocatalyst of highest activity
the
Bi2WO6/(BiO)2CO3
Bi2WO6/BOC
Bi3NbO7/(BiO)2CO3
Bi3NbO7/BOC-7.2% (the mass ratio of urea: Bi(NO3)3·5H2O=7.2%)
BiVO4/(BiO)2CO3
BiVO4/BOC-8% (U20) (the mass ratio of BiVO4:urea=8%)
Typical parameters
Photocatalytic activity
Reference photocatalyst
degradation of RhB under Xe light inactivation of Escherichia coli under visible light degradation of RhB under visible light
98% in 60 min
BOC Bi2WO6
Enhanced factor over the reference photocataly 1.1 1.1
4.7 log of E. coli completely killed after 5 h
Bi3NbO7/BOC-2.4% Bi3NbO7/BOC-12% Degussa P25 Bi3NbO7 bulk BiVO4 BOC nanosheet BiVO4/BOC-27% BiVO4/BOC-13%
1.5 1.7 1.3 1.9 1.8 2.1 1.1 1.0
97% in 60 min
3.4 BiOX/(BiO)2CO3 (X=Cl, I) heterostructures Photocatalytic properties of BiOX/(BiO)2CO3 heterostructures are shown in Table 4. 17 Page 17 of 42
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BiOI has a narrow band gap of 1.70−1.83 eV and can absorb most of the visible light (λ < 700 nm). However, BiOI itself is always poor in photocatalytic activity. In view of the fact that (BiO)2CO3 and BiOI are similar in structure, it is envisaged that a p-n BiOI/(BiO)2CO3 composite could integrate the merits of (BiO)2CO3 and BiOI, consequently showing high photocatalytic activity under visible light. Yin’s group fabricated BiOI/(BiO)2CO3 composites at room temperature by a facile method with the assistance of CTAB. Owing to the formation of heterojunctions, BiOI/(BiO)2CO3 shows excellent photocatalytic activity under the irradiation of visible light [89]. In order to develop efficient visible light photocatalysts for air purification, three dimensional (3D) BiOI/(BiO)2CO3 solid solutions were synthesized by a template free method at room temperature from Dong and his co-workers. As shown in Fig. 12, the solid solutions can adjust the band gap of BiOI/(BiO)2CO3 and improve photocatalytic activity [126].
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Fig. 12 Schematic illustration of the band gap structures of all samples (A) BOI, (B) BOC-BOI-50%, (C)BOC-BOI-25%, (D)BOC-BOI-5%, (E)BOC. (Reproduced with permission from ref. 126. Copyright 2014, Elsevier.） Similarly, BiOCl is used to couple with (BiO)2CO3. Xu et al. synthesized (BiO)2CO3/BiOCl nanocomposites with different molar ratios by a simple homogeneous precipitation at room temperature [127]. Fan et al. found that the enhanced photocatalytic performance of BiOCl/(BiO)2CO3 was closely related to the suitable conduction band interaction and structure characteristic, extending the optical response range and improving the efficient separation of photo-induced electron-hole pairs by the synergistic effect of BiOCl and (BiO)2CO3 [128]. Through acid etching method, a series of (BiO)2CO3/BiOX (X = Cl, Br, I) heterostructured photocatalysts were synthesized, displaying much higher photocatalytic activity than pure (BiO)2CO3 and the corresponding BiOX for the degradation of methyl orange (MO) [129]. Table 4 Photocatalytic properties of BiOX/Bi2O2CO3 heterostructures Composite photocatalyst
Photocatalyst of the highest activity
Typical parameters
Photocatalytic activity
Reference photocatalyst
Enhanced factor over the reference
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Photocatal BiOI/BOC (the molar ratio of Bi3+/CO32−/I−= 1/7/1)
degradation of RhB, MB, crystal violet respectively or their mixture under visible-light
100% RhB in 15 min 95% MB in 80 min 97% crystal violet in 60 min 50.8% in 30 min
BiOI/(BiO)2CO3
BOC-BiOI-25% (the molar ratio of BOC:BiOI = 3:1)
NO removal under visible light
BiOCl/(BiO)2CO3
BiOCl/BOC-1/6 (the molar ratio of CO32−/Cl = 1/6)
degradation of RhB under visible light
BiOCl/(BiO)2CO3
BiOCl/BOC (NaOH concentration is 0.90 mol/L)
degradation of MO under simulated sunlight
95% in 8 h k= 0.027 min-1
BiOX/(BiO)2CO3 (X = Cl, Br, I)
BiOI/BOC
degradation of MO under visible light
92.8% in 3 h
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BiOI BOC BiOI BOC BiOI BOC BOC-BiOI-5% BOC-BiOI-50% BOC BiOI BiOCl/BOC-1/2 BiOCl/BOC-1/4 BiOCl/BOC-1/8 BOC BiOCl BiOCl/CO3-1 BiOCl/CO3-3 BiOCl BOC (different NaOH concentrations) BiOCl/BOC BiOBr/BOC
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6 7 2.6 1.5 1.4 1.6 3.5 1.3 8.5 25 1.1 1.1 1.2 1.4 1.3 2 1.6 6.8 2.5
2.7 2.1
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3.5 Metal-free semiconductor/(BiO)2CO3 Graphitic carbon nitride (g-C3N4) is a low-cost metal-free substance that shows high thermal and chemical stability as well as amenability to chemical modification. Coupling BOC with g-C3N4 can be capable of harvesting visible light as well as showing e ective separation of charge carriers. The enhanced photocatalytic activity of g-C3N4/(BiO)2CO3 can be mainly ascribed to the well-matched band structures, dye photosensitization and e cient crystal facets coupling interaction. (Table 5) The g-C3N4/(BiO)2CO3 nanojunctions showed much higher visible-light photocatalytic activity than pure g-C3N4 and (BiO)2CO3 for the degradation of RhB and phenol [130-132]. Huang’s group synthesized g-C3N4/(BiO)2CO3 by a simple mixed-calcinations [130]. By varying the reaction conditions, Yin’s group found that photocatalytic degradation of dyes over g-C3N4/(BiO)2CO3 under visible light was a dye-sensitized process [131]. Through a one-pot e cient capture of atmospheric CO2 method at room temperature, Dong et al. prepared novel g-C3N4/(BiO)2CO3 organicinorganic nanojunctioned photocatalysts with (BiO)2CO3 nanoflakes deposited onto the surface of g-C3N4 nanosheets [132]. The photocatalytic mechanism of gC3N4/(BiO)2CO3 is shown in Fig. 13.
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Fig. 13 Mechanism of the photocatalytic degradation of RhB over g-C3N4/(BiO)2CO3 semiconductor under visible light irradiation (λ > 420 nm). (Reproduced with permission from ref. 132. Copyright 2014, The Royal Society of Chemistry.） As shown in Fig. 13, the mechanism of indirect dye photosensitization induced degradation with CN-BOC might proceed in the following procedure. First, the RhB molecules, which are absorbed on the surface of BOC, transfer into its excited state (RhB*) under visible-light irradiation. Then, the electrons from the RhB* are trapped by the molecules oxygen on the conduction band of the BOC. Next, the electrons in the CB of BOC react with O2 to produce•O2− radicals for further degradation of RhB+•. During the reactive process, the •O2− radicals is identified as the main active species and the BOC is served only as an electron-mediator, which may be favorable for the e ective separation and transfer of the injected electrons and cationic RhB radicals [132]. In addition to the metal-semiconductor or semiconductor-semiconductor junctions, a series of the π-conjugated material-semiconductor photocatalyst junctions was reported such as graphite/TiO2 [133], C60/ZnO [134-135], polyaniline (PANI)/ZnO [136-137], PANI/g-C3N4 [138], PANI/BiOCl [139], PANI/Bi12TiO20 [140], and PANI/Bi2WO6 [141]. The transfer of photon-generated carrier to the π-conjugated material is promoted by the formation of heterostucture The interactional area between the large π-conjugated system and the electron clouds on the surface could improve the charge separation efficiency [135]. Lately, π-conjugated materials are applied to modify BOC as well, such as BOC/graphene [78,92,142], Polyaniline (PANI)-decorated {001} facets of (BiO)2CO3 nanosheets [93], polypyrrole (PPy)/(BiO)2CO3 [94]. Graphene (GR), as a two-dimensional carbon atom monolayer arranged in a honeycomb network, exhibits many unique properties, such as superior charge carrier mobility, high transparency, large surface area, high thermal/chemical stability and excellent flexibility [78]. Recently, graphene (GR)-based semiconductor photocatalysts have attracted extensive attention. Because the excellent electronic conductivity of GR promotes the transfer of photogenerated electrons through p-p bond interactions and inhibits recombination of the photoexcited electron-hole pairs, magnificently enhancing photocatalytic performance. Yu et al. fabricated hierarchical graphene-(BiO)2CO3 composites via a template-free hydrothermal method to improve the quantum e ciency of (BiO)2CO3 [92]. Zhang et al. utilized an one-pot efficient 20 Page 20 of 42
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capture of atmospheric CO2 at room temperature to obtain self-assembly of (BiO)2CO3 nanoflakes on graphene and graphene oxide nanosheets, which was an effective strategy to improve the photocatalytic performance of two-dimensional nanostructured materials [142]. It is noteworthy that the graphene has strong light absorption ability and could act as a shield against light absorption in semiconductors. Therefore, only when (BiO)2CO3 is loaded with an appropriate amount such as 1.0 wt% graphene can it owns the best photocatalytic performance [92]. However, only a small fractional area of (BiO)2CO3 makes direct contact with the graphene, leading to a relatively low quantum e ciency, and the (BiO)2CO3 particles tend to agglomerate, decreasing the e ective surface area during the photocatalytic degradation of organic pollutants [78]. To overcome the disadvantages above, Zhang and his colleagues prepared graphenewrapped (BiO)2CO3 (WBGR) core–shell structures for highly enhanced quantum e ciency as well as the photocatalytic activity of (BiO)2CO3 by maximizing its contact area using graphene encapsulation [78]. This enhanced quantum e ciency is attributed to the maximized contact area between the (BiO)2CO3 cores and the GR shells, not merely inhibiting aggregation of the(BiO)2CO3 microspheres but also protecting them from structural destruction. (Fig. 14 (a) and (b)) The electrons from WBGR can react with O2 to generate •O2- radicals that can then participate in the photocatalytic process. In contrast to the physical absorption in graphene (BiO)2CO3, the interfacial connection between (BiO)2CO3 and GR is achieved by chemical bonds (C-Bi bonds) in WBGR, which facilitates ultrafast charge transfer and separation and achieves a high charge separation yield with WBGR.
Fig. 14 (a) Illustration of the preparation procedure for WBGR core-shell structures; (b) Schematic illustration of the enhanced quantum e ciency and the radical reaction mechanism of WBGR. (Reproduced with permission from ref. 78. Copyright 2014, The Royal Society of Chemistry.) Polyaniline (PANI)-decorated {001} facets of (BiO)2CO3 nanosheets were synthesized by Zhou et al. It is demonstrated that the strong interfacial interactions between (BiO)2CO3 {001} facets and PANI could promote in situ formation of oxygen vacancy at the interface due to the high oxygen density characteristic of (BiO)2CO3 {001} facets [93]. Novel PPy/(BiO)2CO3 composites were synthesized by photocatalytic degradation of Rhodamine-B (RhB) under ultra violet (UV) irradiation and 0.75 wt.% PPy/(BiO)2CO3 composite showed the highest photocatalytic activity 21 Page 21 of 42
[94]. Table 5 Photocatalytic properties of metal-free semiconductor/(BiO)2CO3 Photocatalyst of the highest activity
Typical parameters
Photocatalytic activity
Reference photocatalyst
Enhanced factor over the reference photocatalyst
g-C3N4/(BiO)2CO3
g-C3N4/BOC (with molar ratio 1:2)
45% in 4 h
g-C3N4 BOC
3.1 14.2
g-C3N4/(BiO)2CO3
g-C3N4/BOC-10wt% CN-BOC-10wt%
k = min-1
g-C3N4 BOC
4 2
g-C3N4/(BiO)2CO3
g-C3N4/BOC
Graphene-wrapped (BiO)2CO3
Graphene-wrapped rose-like BOC
100% RhB in 5 h 19% phenol in 5 h k = 2.81×10-4 s-1
g-C3N4 BOC g-C3N4 BOC BOC Graphene-BOC
1.3 1.4 1.4 4.2 8.7 4.2
Graphene-(BiO)2CO3
Graphene-BOC (1.0 wt% graphene)
degrading RhB under visible light degrading RhB under visible light degrading RhB and phenol under visible light carbamazepine degradation under UV-vis light degrading RhB under visible light
97% RhB in 75 min
BOC
1.3
Graphene/(BiO)2CO3
Graphene/BOC
NO removal under solar light degrading RhB under visible light
63.5% in 30 min
BOC Graphene Oxide/BOC
1.16 1.03
k = 0.286 min-
degrading RhB under UV light
95% in 30 min
BOC BOC/PANI2.8wt% BOC/PANI-10.1wt% BOC/PANI-12.4wt% PPy/BOC-0.25wt% PPy/BOC-0.50wt% PPy/BOC-1.00wt% BOC
4.5 3.2 2.0 2.4 2.2 3.3 5.6 1.9
polypyrrole/(BiO)2CO3 (PPy/BOC)
PPy/BOC-0.75wt%
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3.6 Complex heterostructure Other complex heterojunctions, such as Ag-AgBr/(BiO)2CO3 [143], Bi2O3/(BiO)2CO3/Sr6Bi2O9 [144] and Graphene/TiO2/(BiO)2CO3 [145], have also been developed. (Table 6) Zhu et al applied a two-step synthesis method to obtain a plasmonic Ag-AgBr/BOC composite photocatalyst with enhanced photocatalytic activity for the degradation of RhB and MB under visible light, which was attributed to the heterostructure and surface plasmon resonance (SPR) exhibited by Ag nanoparticles [143]. Photocatalytic degradation of sulfamethoxazole (SMX) was investigated by Niu’s group using Bi2O3/(BiO)2CO3/Sr6Bi2O9 (BSO) photocatalyst under visible light (>420 nm) irradiation [144]. Ao et al prepared graphene and titania co-modified flower-like 22 Page 22 of 42
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The condition or photocatalyst of the highest activity
Typical parameters
Ag-AgBr/(BiO)2CO3
Ag-AgBr/BOC
degradation of RhB and MB under visible light
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Table 6 Photocatalytic properties of (BiO)2CO3-based complex heterostructures
degrading 10 mg/L sulfamethoxazole (SMX) under visible light degrading MO under UV light
100% MB in 12 min
100% RhB in 12 min
90% of SMX degradation and 36% of TOC reduction at pH 7.0 after 120 min 94% in 4 min
Reference conditions or photocatalysts
Enhanced factor over the reference photocatal
BOC Ag/BOC Ag/AgBr AgBr/BOC BOC Ag/BOC AgBr/BOC pH=3 pH=5 pH=9 pH=11 BOC GR/BOC
3.3 1.9 1.5 1.3 1.4 1.2 1.1 0 1.7 1.1 2.1 3.8 2.4
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including rose-like, hydrangea flower-like and peony flower-like microspheres could be controllably fabricated [97]. Interestingly, the ammonia works as not only a nitrogen resource doping (BiO)2CO3, but also a solvent hydrolyzing the bismuth citrate. In addition, we obtained N-doped(BiO)2CO3 honeycomb-like hierarchical microspheres, using bismuth citrate and dicyandiamide as precursors. The ammonium ions from the decomposition of dicyandiamide react with bismuth ions and carbonate ions from the decomposition of citrate ions, producing in situ N-doped (BiO)2CO3 microspheres with exhibited excellent visible light activity [86]. Moreover, persimmon-like, flower-like and nanoflakes nano/microstructures were also prepared by one-step hydrothermal treatment of bismuth citrate and urea with diverse temperature [99]. To gain new insight into the N-doped (BiO)2CO3, our group revealed a new growth mechanism of self-organized N-doped (BiO)2CO3 hierarchical microspheres which were fabricated by hydrothermal treatment of bismuth citrate and urea without additives [98]. The facile and convenient method, free of any additive, can be applied to synthesize other hierarchical materials with a special shape.
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Fig. 15 SEM images of as-synthesized samples obtained at di erent hydrothermal times: (a) 3 h, (b) 9 h, (c) 24 h, (d) 48 h. (Reprinted with permission from ref. 98. Copyright (2014) The Royal Society of Chemistry.)
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NH2CONH2 + H2O → 2NH3 + CO2 (1) NH3 + H2O → NH4+ + OH(2) CO2 + 2OH- → CO32- + H2O (3) + Bi(C6H5O7) + 3OH → BiO + C6H5O73- + 3H+ (4) 4BiO+ + CO32- + 2OH- → (BiO)4CO3(OH)2 (s) (5) (BiO)4CO3(OH)2 (s) + CO32- → 2(BiO)2CO3(s) + 2OH- (6) C6H5O73- → CO2+H2O (7) To obtain a better understanding of the formation mechanism of the self-organized Ndoped (BiO)2CO3 microspheres, products generated from di erent growth stages are collected for SEM measurements as shown in Fig. 15. And the reaction equations (reaction (1-7)) are presented above. When the reaction is carried out for 3 h, the morphology of the obtained samples (Fig. 15 (a)) is similar to that of the samples collected at 0 h. When the reaction lasts for 6 h, large amounts of regular particles with uniform size are produced, because bismuth citrate is hydrolyzed by OH− arising from reaction (2), generating di erent types of ions (reaction(4)). Then some ions react with each other (reaction (5)) to form the (BiO)4CO3(OH)2 growth nuclei, which subsequently grow to crystallize. (BiO)4CO3(OH)2 can react with CO32− to produce 24 Page 24 of 42
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(BiO)2CO3. (reaction (6)) Thus, it is supposed that the (BiO)2CO3 growth nuclei are initially generated on the surface of CO2 bubbles and then tend to aggregate together as growth centers with increasing concentration of OH− (reaction (6)). Upon further increasing the time to 9 h, it is found in Fig. 15 (b) that the size of the particles becomes smaller, which can be considered as the consequence of the dissolution and recrystallization of (BiO)4CO3(OH)2 particles. The microspheres become bigger by the consumption of particles around. After 24 h of hydrothermal treatment, the number of particles decreased dramatically and microspheres self-assembled by nanosheets can be clearly observed, while few particles are still adhered to the surface of the microspheres (Fig. 15 (c)). As all particles disappear, all (BiO)4CO3(OH)2 transformed to the (BiO)2CO3 phase accompanied by the doping of N element into the lattice of (BiO)2CO3 when the reaction time is further extended to 48 h (Fig. 15 (d)). The plausible formation mechanism of self-organized N-doped (BiO)2CO3 microspheres is proposed as displayed in Fig. 16.
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Fig. 16 The formation mechanism of self-organized N-doped (BiO)2CO3 microspheres. (Reprinted with permission from ref. 98. Copyright (2014) The Royal Society of Chemistry.) 4.2 Carbon doping and carbonates self-doping Carbon doping and carbonates self-doping, facile and e cient methods, can narrow the band gap, extend the light absorption range and thus increase visible light photocatalytic activity of semiconductor photocatalysts. Dong et al applied glucose as the carbon source to construct C-doped (BiO)2CO3 microspheres via a facile hydrothermal method [146]. A schematic band structure is proposed for the undoped and C-doped (BiO)2CO3 samples as shown in Fig. 17 (e). Carbon doping could reduce the band gap of (BiO)2CO3 and generate localized states above the valence band, resulting in the extension of the light response to the visible region. On the other hand, carbon doping promotes the e ective separation of photogenerated charge carriers. As a result, C-doped (BiO)2CO3 samples exhibit enhanced visible photocatalytic activity in comparison with the undoped (BiO)2CO3. The sample C-doped (BiO)2CO3 with 0.1160 g glucose (CBOC-M) shows the highest visible light photocatalytic activity and excellent stability (Fig. 17 (c, d)). Its SEM images are depicted in Fig. 17 (a, b). 25 Page 25 of 42
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Fig. 17 (a, b) SEM images of the as-prepared CBOC-M sample; (c, d) Photocatalytic NO removal performance of the as-prepared samples under visible light irradiation and the cycle photocatalytic activity of the CBOC-M sample. (e) Proposed schematic energy band structure of the as-prepared samples. The samples C-doped (BiO)2CO3 with different glucose contents (0.0580 g, 0.1160 g, and 0.20 g) were labeled as CBOC-L, CBOC-M and CBOC-H, respectively. (Reprinted with permission from ref. 146. Copyright (2015) The Royal Society of Chemistry.)
The work from Huang’s group demonstrated self-doping of the CO32− into wide bandgap (BiO)2CO3 can narrow the band and extend the photoresponsive range from UV to visible light, which is verified by both the experimental and theoretical results [147]. The self-doped (BiO)2CO3 (C-BOC) exhibits drastically enhanced visible-light photoreactivity for NO removal. Density functional theory (DFT) calculation is used 26 Page 26 of 42
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to investigate the crystal and electronic structure of C-BOC. In Figure 18 (a), the result from crystal structure optimizing indicates that the foreign CO32− ions are doped in the caves constructed by the four adjacent CO32− ions. As shown in Figure 18 (b) and 18 (c), the denser band density of self-doped BOC is resulted from its 2 × 1 × 2 super cell. The photoresponse of C-BOC is extended and an obvious absorption band at 1.2 eV appears, confirming that self-doped CO32− ions could e ectively narrow the band gap.
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Fig. 18 (a) Crystal structure of C-BOC. (b) Electronic band structures of BOC and CBOC. (c) Calculated imaginary dielectric functions versus energy of BOC and CBOC. (Reprinted with permission from ref. 147. Copyright (2015) American Chemical Society.) Table 7 Photocatalytic removal of NO by (BiO)2CO3 doped with nonmetals. or
Photocatalytic activity
Reference conditions or photocatalysts
Enhanced factor over the reference photocatalyst
42.6% in 30 min visible light 46.1% in 30 min UV light 41% in 30 min visible light 49.4% in 30 min visible light 56.8% in 30 min UV-vis light 41.6% in 30 min visible light
undoped BOC C-doped TiO2 undoped BOC P25 C-doped TiO2 N-doped TiO2 C-doped TiO2 N-doped TiO2 C-doped TiO2 P25 C-doped TiO2 N-doped TiO2 undoped BOC undoped BOC P25 BOC-150 BOC-210 BOC-pure P25
8.5 2.1 1.8 1.4 1.6 1.1 2.4 1.4 1.9 1.2 2.0 1.2 5 1.2 1.1 1.4 1.0 9.9 4.7
P25
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The condition photocatalyst of the highest activity
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Composite photocatalyst
N-doped (BiO)2CO3
N-doped (BiO)2CO3
N-doped BOC (5.33 mL ammonia solution ) N-doped BOC (1.6 mL ammonia solution ) N-doped BOC-180 °C
N-doped (BiO)2CO3
N-doped BOC
N-doped (BiO)2CO3
N-doped BOC
under under under under under under
50.0% after 60 min under solar light 49.2% in 30 min under visible light
N-doped (BiO)2CO3
N-doped BOC-180°C
N-doped (BiO)2CO3
N-doped BOC-180°C 48 h
49.6% in 30 min under visible light 57% after 60 min under
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N-doped BOC-180°C 48 h
C-doped BOC-0.1160 g (glucose content is 0.1160 g)
CO32--doped (BiO)2CO3
CO32--doped BOC (the molar ratio of sodium citrate:Bi(NO3)3·5H2O = 1.5)
undoped BOC
4.5
undoped BOC
1.2
undoped BOC C-doped TiO2 C-doped BOC-0.0580 g C-doped BOC-0.20 g P25 C 3 N4 BiOBr
7.5 2.9 1.2 1.5 4 3.7 3
48% removal in 30 min under visible light
cr
C-doped (BiO)2CO3
UV light 49.3% removal in 30 min under visible light 46% after 30 min under simulated solar light 59.7% removal in 30 min under visible light
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N-doped (BiO)2CO3
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4.3 Formation of oxygen vacancy Interesting, Dong et al. recently found a novel environmental catalyst, black defective (BiO)2CO3 microspheres, which was prepared by a vacuum heat treatment method and could be regenerated by secondary vacuum heat treatment [148]. The black sample BOC-275 (Fig. 19 (b,c)) is obtained by vacuum heat treatment of the pure (BiO)2CO3 (Fig. 19 (a)) with 275 oC for 3 h and exhibits excellent NO removal performance both at room temperature in the dark and under visible light irradiation. Oxygen vacancy is discovered in black defective (BiO)2CO3, which can not only narrow the band gap of (BiO)2CO3 but also activate O2 to react with NO. The mechanism of the catalysis in the dark and under visible light is illustrated in Fig. 19 (d). The active sites (Bi0, Bi5+, and oxygen defects) contained in black (BiO)2CO3 tend to absorb the NO molecules and then weaken their bonds, while the oxygen defects can directly activate the adsorbed O2 to generate the active species. Then the adsorbed NO molecules react with the active species to create nitrogen dioxide, nitrite and nitrate in the dark. Under visible light irradiation, thermal energy from the transformation of partial light energy can accelerate the surface thermocatalytic reaction. What’s more, the visible light activity of black (BiO)2CO3 can be enhanced by the plasmonic e ect of the Bi element and the narrowed band gap of (BiO)2CO3 caused by the oxygen defects. According to the mechanisms above, the e cient dark catalytic activity is ascribed to thermocatalysis and the elevated visible light photocatalytic performance can be attributed to the synergy of thermocatalysis and photocatalysis.
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Fig. 19 (a, b) Color of the as-prepared samples before the catalytic test; (c) SEM image of BOC-275; (d) The illustration of the mechanisms of the catalysis in the dark and under visible light for NO removal. (Reprinted with permission from ref. 148. Copyright (2015) The Royal Society of Chemistry.)
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5. Multi-functional applications 5.1 Photocatalytic degradation of pollutants One of the most promising applications of (BiO)2CO3 is photocatalytic degradation of pollutants, including wastewater dyes, gaseous NO and heavy metals. Dyes in wastewater are stable and resistant to biodegradation due to their complex aromatic molecular structure, which come from various industries, such as textiles, printing, pulp mills, leather, food, dyestu s, and plastics. So far, various technologies, including biological treatment, coagulation/flocculation, ozone treatment, chemical oxidation, membrane filtration, ion exchange, photocatalysis, and adsorption, have been developed for the treatment of dye-containing e uents [149]. Among these methods, photocatalysis could completely degrade dyestu without secondary pollution. (BiO)2CO3 and (BiO)2CO3-based derivatives could effectively remove mixed wastewater dyes (rhodamine-B, methylene blue, methylene orange, crystal violet or their mixture) [63,70,76,82,88]. For example, RhB can be well removed by (BiO)2CO3 [69], Sb-doped (BiO)2CO3 nanoplates [150], Polyaniline (PANI)(BiO)2CO3 [93], and so on. In addition, (BiO)2CO3 nanotubes exhibited excellent Cr(VI) removal capacity [107]. NOx in the atmosphere (NO and NO2) is a kind of harmful acidic gas with low concentration. NOx transfers, transforms, and participates in the formation of secondary pollutants such as PM2.5 and photochemical smog [151]. NOx is also one of the typical pollutants in indoor air. Industrial emissions of high concentration of NOx are usually removed by selective catalytic reduction (SCR), chemical absorption, and biological filtration method. But these methods are not suitable for indoor purification of low concentration NOx. The traditional purification technology for indoor pollution uses the activated carbon adsorption, which just transfers the pollutants from the gas phase to solid phase, bringing a series of problems such as post-processing 29 Page 29 of 42
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and regeneration of activated carbon. Photocatalysis, as a green chemistry technology, has become the most promising technology to control indoor pollution. Dong et al. demonstrated a series of research on (BiO)2CO3, nonmetal-doped (BiO)2CO3 and (BiO)2CO3-based composites, which exhibited excellent photocatalytic performance for NO removal under both UV and visible light irradiation [66-68,85-86,132,146]. It is worth to mention that the defective (BiO)2CO3 superstructures showed high room temperature catalytic activity toward NO removal [148].
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5.2 Medical and healthcare (BiO)2CO3 has long been used for medicinal purposes as an anti-inflammatory, antibacterial, and antacid, especially in the treatment of Helicobacter pylori (H. pylori) infection, peptic ulcers and other gastrointestinal disorders [75]. Chen et al. successfully synthesized well-crystallized bismuth subcarbonate nanoparticles by water-in-oil (w/o) microemulsion-assisted hydrothermal method. The (BiO)2CO3 nanoparticles exerted comparable anti-Helicobacter pylori activities to the clinically used drug, colloidal bismuth subcitrate [65,80]. Also, (BiO)2CO3-based composites could be applied in the processing of drugs, making a contribution to healthcare. For example, Bi2O3/(BiO)2CO3/Sr6Bi2O9 could degrade sulfamethoxazole (SMX) [144]. In addition, β-Bi2O3/(BiO)2CO3 was utilized to the degradation of o-phenylphenol, which was a widely used fungicide and preservative agent [118]. Besides, carbamazepine (CBZ) could be disposed of by graphene-wrapped (BiO)2CO3 [78].
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5.3 Supercapacitors As a new class of energy storage device, supercapacitors have been widely used to power up electrical vehicles, consumer electronics, and military devices due to their unique properties, such as high power density, long cycle life, and faster charging and discharging within short time. The performance of supercapacitors to a great extent depends on their electrode materials [71,72]. Recently, (BiO)2CO3 materials have been researched as the electrodes for supercapacitors. Uniform (BiO)2CO3 microspheres composed of multilayered nanoplates had been successfully synthesized through a hydrothermal route in the presence of PVP and KCl. Peng et al. demonstrated that the Ag/(BiO)2CO3 composite as an active material delivered specific capacities of 620 and 361 F g-1 at current densities of 1 and 5 A g-1, respectively [71]. Also, Zhang et al. fabricated MnO2-decorated flower-like bismuth subcarbonate ((BiO)2CO3) via a hydrothermal approach, and further investigated its performance as the electrodes for supercapacitor. The core-shell nanostructures display moderate capacities (196 F g-1 or 54.5 mAh g-1 for ((BiO)2CO3@MnO2) with superb cycling stability (125% for (BiO)2CO3@MnO2 after 1000 cycles). The electrochemical properties of the electrode are strongly related to their high surface area, porous structure and good conductivity, which can not only provide rich active sites but also shorten the ion transport pathways [72]. 5.4 Other applications Humidity sensors play a very important role in environmental monitoring, industrial 30 Page 30 of 42
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process control and in our daily life. Therefore, it is highly demanded that stable humidity sensors, with high sensitivity, fast response and recovery times, stability, little humidity hysteresis and low price. Recently, (BiO)2CO3 nanosheets with exposed {001} facets were obtained from the straightforward and economic room temperature conversion of commercial Bi2O3 with CO2 as atmospheric carbon source. And Zhou’s group discovered that this (BiO)2CO3 sensor showed high sensitivity, quick response/recovery time, and good reproducibility with a narrow humidity hysteresis [73]. Nonlinear optical (NLO) materials have received much attention due to their significant applications in the laser field, including laser micromachining, semiconductor photolithography, photochemical synthesis and high-resolution photoemission spectrometer. Huang et al. utilized a hydrothermal method to obtain a novel nonlinear optical (NLO) material (BiO)2CO3. The powder second-harmonic generation (SHG) measurement indicates that the NLO efficiency is approximately 5 times as large as that of KDP (KH2PO4) standard, making the (BiO)2CO3 a potential NLO material [74].
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6. Conclusion and perspectives In summary, the recent progress on synthesis of (BiO)2CO3 nano/micro structures, BOC-based nanocomposites, nonmetal doped (BiO)2CO3 and multifunctional applications have been comprehensively reviewed. The morphological structure of (BiO)2CO3 was dependent on the preparation method, hydrothermal temperature, hydrothermal time, and selection of precursors. The understanding of the fabrication, mechanism and potential applications of (BiO)2CO3 will be beneficial for developing other novel photocatalysts with magnificent performance. Although considerable advances have been achieved, some crucial mechanisms and modification strategies in engineering the (BiO)2CO3-based nanocomposites still need to be addressed. Below are some existing challenges and possible future directions. First, many efforts have been contributed to the morphology control and modification of (BiO)2CO3. However, the investigation on the surface or interface of the materials is scarce. The interface of heterojunctions can be well-controlled if we understand the charge generation, separation and transportation process across the nanoscale interfaces. Detailed mechanism of the charge transfer process is more requisite for complex multi-component heterojunction photocatalysts than simple two component heterostructures. Advanced in situ techniques can be employed to gain more insights to address this challenge. Second, more insightful evidences should be obtained to support the proposed transfer pathway of photoinduced electrons and holes and the functions of internal electric field during photocatalysis. Also, the effects of doping on the band structure and electronic structure of (BiO)2CO3 should be investigated with theoretical calculations. Based on theoretical calculations, the designed synthesis and modification of (BiO)2CO3 can be well-controlled to achieve more efficient performance. Third, the controlled synthesis of (BiO)2CO3-based systems with dominant facets 31 Page 31 of 42
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remains to be excavated. The semiconductors with dominant facet are significant for design and synthesis of high performance devices and systems. Apart from the {001} crystal face of (BiO)2CO3, other high-energy crystal facets are also desirable. The atomic steps, dangling bonds of (BiO)2CO3 with other exposed facets may also act as active sites to capture electrons to facilitate charge separation and transfer, and favor the adsorption of reactive molecules and desorption of products. Based on new discovered materials, there are still great opportunities to develop (BiO)2CO3 based heterojunction with active facets, such as 2D-2D heterojunction with coupled exposed facets. Fourth, the current photocatalytic applications of (BiO)2CO3-based nanocomposites mainly focus on the degradation of pollutants. Although it has been applied in humidity sensor, nonlinear optical element, antibacterial, supercapacitor, reports in these area are still scare. Up to now, numerous versatile photocatalysts are in demand to broaden the application in other areas such as H2 generation, CO2 photoreduction, selective chemical synthesis and clean fuel production. (BiO)2CO3-based photocatalysts may also find wide applications in these important areas.
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This research is financially supported by the National Natural Science Foundation of China (51478070, 21501016, 51108487). References
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air with hierarchical bismuth oxybromide nanoplate microspheres under visible light, Environ. Sci. Technol. 43 (2009) 4143-4150.
Highlights:
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The (BiO)2CO3 with Aurivillius structure y is an emergent material. Synthesis of (BiO)2CO3 micro/nano structures was reviewed. The mechanisms of (BiO)2CO3 based nanocomposites were discussed. Doping (BiO)2CO3 with nonmetals for enhanced activity was highlighted. Multi-functional applications of (BiO)2CO3 based derivatives was demonstrated.
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S0169-4332(15)03236-5 http://dx.doi.org/doi:10.1016/j.apsusc.2015.12.231 APSUSC 32214
To appear in:
APSUSC
Received date: Revised date: Accepted date:
25-10-2015 25-12-2015 31-12-2015
Please cite this article as: Z. Ni, Y. Sun, Y. Zhang, F. Dong, Fabrication, modification and application of (BiO)2 CO3 -based photocatalysts: A review, Applied Surface Science (2016), http://dx.doi.org/10.1016/j.apsusc.2015.12.231 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 proof before it is published in its final 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.
Fabrication, modification and application of (BiO)2CO3-based photocatalysts: A review
Zilin Ni a, Yanjuan Sun a, Yuxin Zhang b, Fan Dong a,∗ a
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Chongqing Key Laboratory of Catalysis and Functional Organic Molecules, College of Environment and Resources, Chongqing Technology and Business University, Chongqing, 400067, China. b College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China.
∗To whom correspondence should be addressed. E-mail: [email protected] (Fan Dong). Tel/Fax: + 23-62769785-605. 1 Page 1 of 42
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Abstract: (BiO)2CO3 (BOC), a fascinating material, belongs to the Aurivillius-related oxide family with an intergrowth texture in which Bi2O22+ layers and CO32- layers are orthogonal to each other. BOC is a suitable candidate for various fields, such as healthcare, photocatalysis, humidity sensor, nonlinear optical application and supercapacitors. Recently, the photocatalysis properties of (BiO)2CO3 have been gained increased attention. BOC has a wide band gap (3.1-3.5 eV), which constrains its visible light absorption and utilization. In order to enhance the visible light driven photocatalytic performance of BOC, many modification strategies have been developed. According to the discrepancies of different coupling mechanisms, six primary systems of BOC-based nanocomposites can be classified and summarized: namely, metal/BOC heterojunction, single metal oxides (metal sulfides)/BOC heterostructure, bismuth-based metallic acid salts (BixMOy)/BOC, bismuth oxyhalides (BiOX)/BOC, metal-free semiconductor/BOC and the BOC-based complex heterojunction. Doping BOC with nonmetals (C, N and oxygen vacancy) is unique strategy and warrants a separate categorization. In this review, we first give a detailed description of the strategies to fabricate various BOC micro/nano structures. Next, the mechanisms of photocatalytic activity enhancement are elaborated in three parts, including BOC-based nanocomposites, nonmetal doping and formation of oxygen vacancy. The enhanced photocatalytic activity of BOC-based systems can be attributed to the unique interaction of the p–n junction (semiconductor/semiconductor heterostructures), the Schottky junction (metal/semiconductor heterostructures), the surface plasmon resonance (SPR) e ect, the surface scattering and reflecting (SSR) e ect, the well-matched band structures, tunable electronic band structure, photosensitization and excellent electronic conductivity. Besides, multi-functional applications of BOC based materials are presented. Finally, prospective about the rational design, mechanistic understanding and application of BOC based materials is demonstrated, aiming to broaden the perspective and provide guidelines for future work. Keywords: (BiO)2CO3; photocatalysis; morphology control; heterojunction; doping.
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1. Introduction During the past few decades, environmental remediation and solar energy utilization have been attached tremendous importance for the severity of environmental pollution and global energy crisis. Semiconductor photocatalysis has received wide research interests worldwide as it could remove pollutants in a green manner. Apart from pollutant removal, photocatalysis is also considered as one of the most promising avenues for solar energy conversion, such as photocatalytic solar fules production [16]. Traditional TiO2 is a popular photocatalyst since Fujishima reported the generation of H2 and O2 by photoelectrochemical water splitting [7]. However, TiO2 shows poor visible-light photocatalytic activity due to its large band gap. Doping metals/nonmetals, coupling with other narrow band gap semiconductors and hydrogen treatment, have been successfully applied to enhance the visible light activity of TiO2. Various TiO2-based photocatalysts have been developed for improved visible light photocatalysis [7-17]. Alternatively, seeking for non-TiO2 based novel visible-light photocatalysts is another choice. A wave of Bi-based photocatalysts has hit the world. Most of Bi-based materials own controlled morphology and high photocatalysis efficiency. Bismuth, a typical semimetal material, has attracted great interest due to a set of unique properties, such as very small e ective masses, a large mean free path, a long Fermi wavelength, high carrier motilities and small band overlap energy. Moreover, nano-confinement e ects, observed in the Bi element, allow a semimetal-to-semiconductor transition at diameters of a few tens of nanometers [18-19]. Semimetal Bi was found to exhibit the strong SPR-mediated e ect and exploited in many applications, including sensors, fluorescence and surface-enhanced spectroscopy [18-19]. For instance, with strong SPR-mediated e ect, semimetal Bi nanoparticles exhibited high photocatalytic activity [19]. On the other hand, bismuth-containing semiconductor photocatalysts such as Bi2O3 [20-22], Bi2S3 [23-24], Bi2WO6 [25-34], BiVO4 [35-41], and BiOX (X = Cl, Br, I) [42-58], BiPO4 [59-60], aroused much interests because of their high visible light photocatalytic activity. As a member of Aurivillius-based oxide family, (BiO)2CO3 (BOC) is composed of alternate Bi2O22+ and CO32− layers, and the plane of the CO32− group is orthogonal to the plane of the Bi2O22+ layer [18,61]. Based on lattice match, (BiO)2CO3 orthogonal networks (2DONWs) can be transformed into β-Bi2O3, BiOCl 2DONWs, and Bi2S3 nested self-similar 2DONWs [62]. (BiO)2CO3 nanostructures are fascinating materials with remarkable properties and have been widely used in a diverse fields. (BiO)2CO3 has long been used for medical and healthcare purposes [63-65]. Recently, (BiO)2CO3 is demonstrated to be a highly promising photocatalyst [66-70]. Also, (BiO)2CO3 can be applied in other fields, such as supercapacitors [71,72], the humidity sensor [73], and nonlinear optical element [74]. (BiO)2CO3 compounds with one-dimensional (1D) nanostructure (nanotubes and nanoparticles) [65,69,73,75-79], two-dimensional (2D) structure (nanosheets and nanoplates) [70,80-81], and three-dimensional (3D) microstructure (rose, flower, 3
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persimmon-like, honeycomb-like, dandelion-like,hydrangea-like) [72,82-86] have been successfully prepared and applied in photocatalysis. Designed synthesis of nanostructured materials, especially those ordered and superstructured semiconductors with facet dominated growth and properties, is significant for high performance applications. (BiO)2CO3 single-crystal nanoplates with dominant {001} facets were fabricated for enhanced photocatalysis [69,87]. The band gap of (BiO)2CO3 is 3.1-3.5 eV [88]. With such a wide band gap, the (BiO)2CO3 has a low utilization efficiency of solar light [89]. A series of solutions have been reported to broaden the absorption spectra of (BiO)2CO3. Noble metal Ag (semimetal Bi) loaded BOC becomes highly active under visible light irradiation due to the formation of Schottky junction and SPR e ect [71,90,91]. BOC could also act as an eminent candidate for coupling with various functional materials to enhance the photocatalytic performance. For example, single metal oxide/BOC, BixMOy/BOC, BiOX/BOC as well as metal sulfides/BOC are constructed to form p–n junction/n–n junction. By coupling with semiconductors of narrow band gaps, these BOC-based heterostructures present excellent visible light absorption and enhanced charge separation and migration. Recently, π-conjugated materials are applied to modify BOC as well, such as graphene/BOC [78,92], Polyaniline (PANI)-decorated BOC nanosheets [93], polypyrrole (PPy)/BOC [94]. The excellent electronic conductivity of π-conjugated materials could promote the charge transfer and thus enhance photocatalytic performance. Nonmetal doping of BOC is an effective methods to modify the band structure and enhance the visible light activity of BOC [85-86,9599]. In this perspective, we make a comprehensive review on recent progress in the fabrication, mechanistic understanding, and potential applications of BOC-based photocatalysts. First, we review different synthesis methods for (BiO)2CO3 hierarchical micro/nano structures. Then, according to the discrepancies of different coupling agents, we classify BOC-based nanocomposites into six primary systems: namely, metal/BOC heterojunction, single metal oxides (metal sulfides)/BOC heterostructure, bismuth-based metallic acid salts (BixMOy)/BOC, bismuth oxyhalides (BiOX)/BOC, metal-free semiconductor/BOC, and the BOC-based complex heterojunction. The main photocatalytic mechanisms have been emphasized. We highlight the enhancement of the photocatalytic performance of BOC via nonmetals doping. Various applications of BOC and its derivatives in diverse areas were demonstrated. Finally, we present the conclusion and future prospective on the photocatalysis mechanism, theoretical calculation, controlled synthesis and applications of BOC-based materials.
2. Synthesis of (BiO)2CO3 micro/nano structures The photocatalytic performance of semiconductors is predominantly controlled by the intrinsic properties, including band gap structure, charge separation kinetics, surface area, and morphology [6,100-103]. The physicochemical properties are connected with preparation method, hydrothermal temperature, hydrothermal time, assistance and precursors. Employing an appropriate synthesis method is of great 4 Page 4 of 42
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significance to optimizing the structural features. 2.1 The effects of preparation methods During the past few years, different preparation methods for (BiO)2CO3 hierarchical micro/nano structures have been reported. Hydrothermal treatment is most widely used. For example, Xie’s group synthesized (BiO)2CO3 nanoflowers, nanosponges and nanoplates by a simple hydrothermal process [69]. As shown in Fig. 1(a), persimmon-like (BiO)2CO3 hierarchical structures with uniform size were prepared hydrothermally and demonstrated high photocatalytic efficiency under simulated solar irradiation [104]. Dai and co-workers synthesized flower-like (BiO)2CO3 by hydrothermal treatment of bismuth nitrate, citric acid and urea in water and then investigated the effects of citrate ion on the morphology and photocatalytic activity of (BiO)2CO3 [105]. Other methods were also employed for preparation of (BiO)2CO3, such as solutionphase method, reflux method and solvothermal method. Lee et al prepared (BiO)2CO3 nanosheetsby a solution-phase route shown in Fig. 1 (b) [106]. Chen et al. synthesized nanotubes, nanobars (Fig. 1 (c)), nanoplates, nanoparticles and cube-like (BiO)2CO3 by simple reflux and solvothermal processes [65,80]. Dai et. al prepared (BiO)2CO3 nanosheet using urea as carbon source via solvothermal method [70]. Through a facile solvothermal method, Chen et al. synthesized (BiO)2CO3 nanotubes (NTs) with ethylene glycol (EG), which was depicted in Fig. 1 (d) [107].
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Fig. 1 (a) Persimmon-like BOC (Reprinted with permission from ref. 104. Copyright (2011) The Royal Society of Chemistry.); (b) Sheet-like BOC (Reprinted with permission from ref. 106. copyright 2007, Elsevier.); (c) BOC nanobars by using (NH4)2CO3 as reagent (Reprinted with permission from ref. 80. copyright 2010, Elsevier.); (d) BOC nanotubes (Reprinted with permission from ref. 107. copyright 5 Page 5 of 42
2012, Wiley.)
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2.2 The effects of hydrothermal conditions Hydrothermal temperature and time are important factors in controlling the morphology of BOC. As shown in Fig. 2, Dong et al. prepared the flower-like, pinonlike and faceted nanoplates-like BOC nanostructures by a one-pot template-free hydrothermal treatment of bismuth citrate and sodium carbonate at 150, 180 and 210 o C [108]. With the reaction temperatures increasing, the thickness of nanoplates was increased due to the preferred growth along the (110) plane. Flower-like (150 oC), pinon-like (180 oC), faceted nanoplates-like (210 oC) BOC nanostructures were selectively formed. Among these morphology, the flower-like BOC showed the highest photocatalytic activity toward removal of aqueous RhB, ascribing to the multiple light reflections between the nanoplates. Dong et al also investigated the effects of temperature on hydrothermal treatment of bismuth citrate and dicyandiamide. BOC nanostructures of irregular particles (150 oC), honeycomb-like hierarchical microspheres (180 oC) and loose nanosheet aggregates (210 oC) can be produced [86].
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Fig. 2 flower-like (a), pinon-like (b) and faceted nanoplates (c) (BiO)2CO3 samples with hydrothermal temperature 150 oC, 180 oC, 210 oC. (Reprinted with permission from ref. 108. copyright 2013, Elsevier.) The morphology of BOC varied with time in hydrothermal reaction. Dong et al obtained time-dependent BOC samples by a hydrothermal treatment of bismuth citrate and sodium carbonate for elucidating the growth mechanism of BOC hierarchical hollow microspheres. During the initial 6h, bismuth citrate was hydrolyzed by the abundant OH- ions produced by sodium carbonate to produce BiO+. (Reaction (1) and (2)) With the reaction time increasing to 7.5h, bismuth subcarbonate hydroxide was generated at this reaction stage. (Reaction (3)) From 9 to 24h, the CO32- ions may substitute OH- ions in the (BiO)4CO3(OH)2 layers (Reaction (4)) [68]. Na2CO3 + 2H2O → CO32- + 2OH- + 2Na+ + 2H+ (1) 3+ + + Bi (citrate) + 3OH → BiO + citrate + 3H (2) + 24BiO + CO3 + 2OH → (BiO)4CO3(OH)2(s) (3) (BiO)4CO3(OH)2(s) + CO32- → (BiO)2CO3(s) + 2OH- (4) Fig. 3 showed the SEM images of BOC samples obtained with different reaction stages. The formation mechanism of BOC hierarchical hollow microspheres was revealed as following. First, near amorphous particles are produced through reaction, nucleation, crystallization, and aggregation processes. Then, intermediate 6 Page 6 of 42
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(BiO)4CO3(OH)2 microspheres emerge due to dissolution and recrystallization. Through Ostwald ripening, stacked uniform solid microspheres grow with consumption of particles, subsequently forming microspheres with a hole in the center, like flower buds. Finally, owing to layers splitting, uniform monodisperse (BiO)2CO3 hierarchical hollow microspheres were produced and exhibit excellent photocatalytic performance in removal of NO.
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Fig. 3 SEM images of the as-prepared BOC-7.5h (a), BOC-9h (b), BOC-12h (c), BOC-24h (d) samples. (Reprinted with permission from ref. 68. copyright 2012, The Royal Society of Chemistry.) 2.3 The effects of templates The wet chemical route has been proved to be one of the most effective and convenient approaches in preparing BOC with diversely controllable morphologies and architectures especially for large production. Generally, the synthesis of BOC can be classified as two types: template method and template-free method. A specific structure can be easily constructed by the template approach. For instance, Chen et al. utilized the assistance of cetyltrimethylammonium bromide (CTAB) to controll the synthesis of flower-like BOC, which exhibited high photocatalytic degradation of wastewater dyes under UV-vis light irradiation [82]. Cheng et al. reported the synthesis of hierarchical rose-like BOC microstructures assembled by single-crystalline nanosheets with the help of PVP [76]. Using sodium dodecyl sulfate (SDS) as a soft template, the as-synthesized BOC nanosheets showed excellent photocatalytic activity for RhB degradation under simulated sunlight [109]. However, template synthesis usually requires tedious procedures such as template modification, precursor attachment and core removal, which consequently limit their large-scale production and further applications. In addition, the inadequate removal of the templates may result in undesired impurities. To obtain pure hollow and 7 Page 7 of 42
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hierarchical structures, template-free approaches are highly desirable. Huang et al prepared BOC nanosheets and flower-like microstructures employing a template-free, and low-temperature solution method [110]. The growth mechanism of uniform BOC hierarchical hollow microspheres via template-free fabrication was reported by Dong and co-workers [67]. Chen et al. synthesized BOC nanotubes (NTs) through a facile solvothermal method without the need for any surfactants or templates [107].
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2.4 The effects of precursors Morphology of materials varies with different precursors, concentration and pH. Using tri-sodium citrate as both the coordinating agent and carbon source, Qian et al. hydrothermally synthesized three kinds of BOC microstructures and found spongelike BOC owned the highest photocatalytic performance. By tuning the concentration of tri-sodium citrate, BOC of sponge-like, rose-like and plate-like shapes could be selectively obtained [88]. Dai et al. investigated the effects of citrate ion on the morphology and photocatalytic activity and discovered that BOC microspheres synthesized in the solution with the C6H8O7/Bi(NO3)3 molar ratio of 2:1 showed the highest photocatalytic activity [105]. The microstructure of the BOC structures could also be modulated by varying the concentration of Na2CO3 [110]. Huang et al. found that only nanoplates with a thickness of about 30 nm can be prepared when the molar ratio of Bi(NO3)3/Na2CO3 was increased to 1:3. If the ratio was decreased to 1:9, irregular curving multisheets were formed [110]. By changing the amount of precursor (sodium carbonate), Dong et al. synthesized BOC microspheres and BOC particles with different morphology via a simple hydrothermal method. It was found that the crystallinity and morphology can be tuned by simply changing the amount of CO32−, providing a new insight into the relationship between the crystallinity, morphology, and the corresponding photocatalytic activity. Based on different amount of sodium carbonate (0.23 g and 1.53 g), the as-prepared (BiO)2CO3 materials were denoted as BOC-M (Fig. 4 (a)) and BOC-P (Fig. 4 (b)), respectively [111].
Fig. 4 SEM images of BOC-M (a) and BOC-P (b) with the amount of sodium carbonate 0.23g and 1.53g respectively. (Reprinted with permission from ref. 111. copyright 2014, American Chemical Society.) For BOC-P nanoparticles, excessive CO32− leads to the production of a great number of nuclei and accelerates crystal growth, which promotes the crystallization of BOC, 8 Page 8 of 42
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thereby generating a sample with high crystallinity. However, compared with BOC-P nanoparticles, BOC-M microspheres show higher photocatalytic activity toward the removal of NO upon UV-light, visible-light and simulated-solar-light irradiation, attributing to the synergistic e ects of its hierarchical structure, low crystallinity and large surface area. In addition, The morphology of BOC can be controlled by varying the pH value. Xie et al adjusted the pH of the solution to 7-9 and 4-5 to acquire the flower-like BOC hierarchitectures and sponge-like BOC porous spheres, utilizing Bi(NO3)3·5H2O and C6H8O7·H2O as precursors [69]. Recently, as a new raw material, CO2 is introduced into the preparation of BOC nanosheets. Instead of organic precursors, atmospheric CO2 is easily available and economic. The using of CO2 will reduce the amount of undesirable organic byproducts during the synthesis process and promote the large scale production of (BiO)2CO3. Zhou et al. synthesized (BiO)2CO3 nanosheets with exposed {001} facets, benefiting from the capture of atmospheric CO2 as a low-cost carbon source [73]. (BiO)2CO3 single-crystal nanosheets were successfully fabricated by an eco-friendly process under mild conditions with atmospheric CO2 within 30 min. In the process, bismuth nitrate and CO2 in air were used without extra separation [81]. This rapidly prepared (BiO)2CO3 nanosheets exhibited an efficient and durable photocatalytic performance for NO removal [81].
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3. (BiO)2CO3 based nanocomposites 3.1 Metal/(BiO)2CO3 nanocomposites Three types of metal/(BiO)2CO3 nanocomposites have been reported, including Ag/BOC, Au/BOC and Bi/BOC whose photocatalytic mechanisms are similar (Table 1). In general, their enhanced photocatalytic activity and photocurrent generation benefit from the cooperative contribution of the surface plasmon resonance (SPR e ect), e cient separation of electron–hole pairs and prolonged lifetime of charge carriers induced by metal nanoparticles. Dong et al fabricated plasmonic Ag nanocrystal decorated 3D (BiO)2CO3 hierarchical microspheres (Ag/BOC), which exhibited greatly enhanced visible light photocatalytic activity, photocurrent generation and promoted NO2 oxidation compared to the pure (BiO)2CO3 microspheres [90]. The enhanced photocatalytic activity of Ag/BOC is ascribed to SPR e ect of Ag nanocrystals, e cient charge separation and prolonged lifetime of charge carriers. On the other hand, the surface scattering and reflecting (SSR e ect) resulting from the special 3D hierarchical architecture of BOC also contributes to the increased photocatalytic activity. Fig. 5 illustrates the scheme of photocatalysis mechanism of Ag-decorated (BiO)2CO3 under visible light irradiation. The deposited Ag nanoparticles propel the Ag/BOC samples to absorb more visible light owing to the SPR e ect. Simultaneously, Ag nanoparticles can be photoexcited, thus enhancing the surface electron excitation and interfacial electron transfer. Because the Fermi level of metallic Ag (0.4 eV) is lower than the conduction band of (BiO)2CO3 (0.20 eV), the photogenerated electrons will probably transfer from (BiO)2CO3 to the deposited Ag nanoparticles (as shown in Fig. 5), creating a Schottky barrier at the interface that reduces the recombination of 9 Page 9 of 42
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electron–hole pairs and increases the lifetime of the charge carriers. Subsequently, photogenerated charge carriers will be transformed into active species (•OH) that are responsible for the degradation of pollutants. Due to different redox potentials, the photoexcited electrons of Ag together with the electrons transferred from (BiO)2CO3 can reduce O2 to OH−, and the holes from (BiO)2CO3 can oxidize OH− to •OH. Finally, OH− radicals, as the major reactive oxidation species, could oxidize NO to the NO3−.
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Fig. 5 (a, b) SEM images of Ag/BOC-5%; (c) Scheme of photocatalytic mechanism of Ag-decorated (BiO)2CO3 under visible light irradiation. (Reprinted with permission from ref. 90. Copyright (2014) The Royal Society of Chemistry.) Also, Peng et al. deposited Ag particles on the surface of (BiO)2CO3 via a facile photoreduction process [71]. Loading of 0.6 wt% Ag on the (BiO)2CO3 microspheres could significantly enhance the photocatalytic activity for degradation of methyl orange dye and improve the supercapacitive behavior with lower electrical resistance. PVP, KCl and hexamethylenetetramine (HMT) play key roles in the formation of such hierarchical microspheres. In the synthesis process, PVP is used as a reactant that not only provides C and O sources but also serves as a template to induce the nanoplateassembly to form microspheres. In addition, the size of the (BiO)2CO3 microspheres is reduced from 6.0 mm to 1.0 mm by the addition of KCl in the synthesis. A possible formation mechanism of the (BiO)2CO3 microspheres is proposed in Fig. 6. And the proposed reactions can be summarized in Eqn. (1)–(6): [Bi3+]PVP → Bi3+ + PVP (1) HMT + H2O → OH- + NH4+ + HCHO (2) 3+ + + Bi + OH → BiO + H (3) 2+ PVP + H2O → CO3 + 2H (4) BiO+ + CO32- → (BiO)2CO3 (5) overall reaction → (BiO)2CO3 + NH4NO3+HCHO (6) Bi(NO3)3 + PVP +HMT + H2O
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Fig. 6 A schematic illustration of the formation process of the hierarchical (BiO)2CO3 microspheres. (Reprinted with permission from ref. 71. Copyright (2013) The Royal Society of Chemistry.)
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In the hydrothermal process, due to the intrinsic tetragonal layer-structure and selective adsorption of surfactant PVP molecules on the crystal faces, (BiO)2CO3 crystal nuclei anisotropically grow into nanoplates. When the amount of PVP is low in the reaction, the obtained (BiO)2CO3 presents nanoplate structures. With the increased amount of PVP, these nanoplates can assemble into a hierarchical conglomeration through molecular interaction between surfactant molecules, and then transform into 3-dimensional nanostructures driven by the minimization of surface energy [71]. Novel 3D Au/(BiO)2CO3 (Au/BOC) heterostructures with size-controlled Au nanoparticles (NPs) (2–10 nm) were synthesized by Dong’s group (Fig. 7(a) and 7(b)) [112]. As elucidated in Fig. 7 (c), the enhanced visible light photocatalytic activity of Au/BOC can be attributed to the synergetic e ects of SPR, Schottky Barrier, and the strong light scattering and reflecting e ects (SSR).
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Fig. 7 (a, b) TEM and HRTEM images of Au/BOC composites with 2 ml Au NPs of 2–4 nm. (c) Photocatalytic mechanism of Au/BOC under visible light irradiation. (Reprinted with permission from ref. 112. Copyright (2015) The Royal Society of Chemistry.)
Recently, Dong et al. firstly reported that plasmonic Bi nanoparticles deposited on (BiO)2CO3 microspheres (Bi/BOC) via an one-pot hydrothermal treatment of bismuth citrate, sodium carbonate, and thiourea. Thiourea functions as a reductant and could reduce Bi3+ to metallic Bi. It is economic and efficient to utilize low-cost Bi nanoparticles as a substitute for noble metals to enhance visible light photocatalysis and photochemical stability [91]. The mechanisms for enhanced photocatalytic 11 Page 11 of 42
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performance of Bi/BOC are illustrated in Fig. 8. Firstly, the SPR e ect of the spatially confined electrons in the deposited Bi nanoparticles favors the visible light absorption. Secondly, owing to the SPR e ect, the Bi nanoparticles can be photoexcited, hence strengthening the surface electron excitation and interfacial electron transfer. Thirdly, given that the Fermi level of metallic Bi (-0.17 eV) is higher than the conduction band of (BiO)2CO3 (0.20 eV), the photo-generated electrons can transfer from Bi particles to (BiO)2CO3. Once releasing electrons, Bi can shift to more positive potentials and then return to its primary state by accepting electrons from the valence band of (BiO)2CO3. This process restrains the recombination of electron-hole pairs in (BiO)2CO3 because of the interface transfer of electrons (I) from (BiO)2CO3 to the Bi nanoparticles as shown in Fig. 8. Fourthly, the SPR-mediated local electromagnetic field of Bi (II) in Fig. 8 leads to the separation of the electron-hole pairs in (BiO)2CO3. These mechanisms synergistically boost the photocatalytic activity [91]. Recently, Sun et al. realized one-pot solvent-controlled synthesis of Bi/BOC using bismuth citrate, sodium carbonate and ethylene glycol as precursors [113].
Fig. 8 Photocatalytic mechanism scheme of Bi/BOC under visible light irradiation: interface transfer of electrons from (BiO)2CO3 to Bi(I) and local electromagnetic field of Bi(II). (Reprinted with permission from ref. 91. Copyright (2014) American Chemical Society.) Table 1 Photocatalytic properties of Metal/(BiO)2CO3 nanocomposite photocatalysts Composite photocatalyst
Photocatalyst of the highest activity
Typical parameters
Photocatalytic activity
Reference photocatalyst
Enhanced factor over the reference photocatalys
Ag/(BiO)2CO3
Ag(0.6%)/BOC microspheres (with KCl)
photodegradation of methyl orange (MO) under UV-vis light
91% in 50 min
BOC microspheres (with KCl) BOC microspheres (without KCl) BOC microplates
1.2 1.4 2.0
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Au/(BiO)2CO3
Au(2-4nm)/BOC with 4ml Au solution
BOC microspheres BOC nanosheet
2.5 13.5
203 ppb/min NP
BOC microspheres Au(4ml,4-6nm)/BOC Au(4ml,8-10nm)/BOC Au(2ml,2-4nm)/BOC Au(8ml,2-4nm)/BOC Bi/BOC-1% Bi/BOC-10% Bi BOC light source 100W
58% in 30 min
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Bi/(BiO)2CO3
37.2% in 30 min
Bi/BOC-5%-32W Bi/BOC-5%-5W different light sources (BiO)2CO3
1.8 1.1 1.3 1.1 1.1 1.6 1.7 19.3 2.6
1.3 1.9 1.9
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3.2 Metal oxides (sulfides)/(BiO)2CO3 heterostructure So far, various single metal oxides (metal sulfides) have been coupled with BOC for enhanced visible light photocatalysis. The p-n heterostructures are full of interest with an internal electric field at interface, thus promoting the separation of photogenerated electron–hole pairs and increasing their photocatalytic activity. Photocatalytic enhancement mechanisms of the p-n heterojunctions have been proposed based on the relative band gap position of these two semiconductors and the integrated heterostructures [72,114-118], as shown in Table 2. Wei et al prepared Ag3PO4/(BiO)2CO3 composites by a combination of hydrothermal technique and precipitation. Ag3PO4 nanoparticles with ca. 200 nm in size were attached to the surface of (BiO)2CO3 microspheres with diameter of 1–2 µm, which resulted in higher photocatalytic activity toward degradation of Rhodamine B (RhB) under visible light irradiation [114]. Ag2O/(BiO)2CO3 p-n heterojunctions were prepared with commercial (BiO)2CO3 as precursor via a simple photosynthesis process [115]. The (BiO)2CO3@Fe2O3 nanosheets with exposed active {001} facet were fabricated by Hu et al. [116]. Sphere-like and flower-like Fe3O4/(BiO)2CO3 photocatalysts were obtained by Zhu et al. with the adjusting of the pH from 4.0 and 6.0, respectively. The composites photocatalysts displayed not only wonderful activity towards degradation of methyl orange and methylene blue, but also superparamagnetic behavior at room temperature, making Fe3O4/(BiO)2CO3 easily be recovered and recycled [117]. Very recently, as supercapacitors, flower-like (BiO)2CO3@MnO2 and Bi2O3@MnO2 nanocomposites prepared by Zhang et al. demonstrated high specific capacitance performance and good cycling stability [72]. This work opens a new avenue for development of BOC-based supercapacitors. 13 Page 13 of 42
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Flower-like p-n heterostructured β-Bi2O3/(BiO)2CO3 microspheres were synthesized through calcinations of a (BiO)2CO3 self-sacrifice precursor for the visible-light photocatalytic degradation of o-phenylphenol [118]. Xu et al also reported the synthesis of β-Bi2O3/(BiO)2CO3 nanosheet composites through rational heat treatment of the precursor (BiO)2CO3 [63]. Currently, loading noble metals is becoming increasingly prevalent ascribing to its effective enhancement of the photocatalytic activity. However, these metals are rare and valuable. As a transition metal sulfide, molybdenum disulfide (MoS2) is an emerging photocatalytic co-catalyst to substitute noble metals. Wang et al. synthesized novel MoS2/(BiO)2CO3 photocatalysts by a simple hydrothermal method. The special structure of MoS2 provides more active sites to adsorb dyes, making the MoS2/(BiO)2CO3 exhibit excellent degradation of rhodamine B under UV light irradiation [119]. The size quantization of Bi2S3 enables its nanoparticles (1.3 eV for bulk) to show tunable band gap energy and photosensitization in the visible region [120]. The combination of Bi2S3 with BOC could not only increase the absorption of visible light, but also benefit the separation of the photogenerated carriers, thus making (BiO)2CO3/Bi2S3 composites a good choice to optimize the visible light-driven photocatalytic properties of (BiO)2CO3. To date, the integration of (BiO)2CO3 and Bi2S3 to construct novel heterojunctions has been reported by three articles [120-122]. Huang et al. reported (BiO)2CO3/Bi2S3 heterojunctions through a facile partial ion exchange method between the (BiO)2CO3 microspheres and TAA, from which S2ions reacted with (BiO)2CO3 to form Bi2S3 on the (BiO)2CO3 nanosheets [120]. Novel heterojunctions Bi2S3/(BiO)2CO3 were prepared by a simple chemical reaction with commercial (BiO)2CO3 from Zai et al [121]. The Bi2S3/(BiO)2CO3 exhibits the enhanced photocatalytic activity for degradation of RhB, which is related to the unique band gap structure of n–n-type Bi2S3/(BiO)2CO3 heterojunctions and special morphology with the partly exposed core. It’s worth noting that 5 mol% Bi2S3/(BiO)2CO3 can benefit the migration of electrons or holes to the reaction sites (Fig. 9 left), while the Bi2S3/(BiO)2CO3 of 25 mol% (Fig. 9 right) is not favorable for the charge transfer because the photogenerated holes will largely accumulate in the (BiO)2CO3 side, counteracting the built-in potential produced by n-n-type heterojunctions [121].
Fig. 9 The e ect of heterojunction structures of photocatalysts. (Reproduced with 14 Page 14 of 42
permission from ref. 121. Copyright 2014 The Royal Society of Chemistry.）
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Recently, Dong and co-workers applied a facile anion exchange reaction between (BiO)2CO3 and Na2S to develop 3D hierarchical (BiO)2CO3/amorphous Bi2S3 heterostructures with increased solar absorption and enhanced visible light photocatalytic activity towards NO removal, as shown in Fig. 10 [122].
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Photocatalyst of the highest activity
Typical parameters
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Reference photocatalyst
Enhanced factor over the reference photocatal
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Fig. 10 Formation of 3D (BiO)2CO3/amorphous Bi2S3 heterostructured hierarchical microspheres by a controllable anion exchange process. (Reproduced with permission from ref. 122. Copyright 2015 The Royal Society of Chemistry.）
Ag3PO4/(BiO)2CO3
Ag3PO4/BOC-100% (the molar ratio of Ag3PO4:BOC=1:1)
degradation of 97% in 30 min rhodamine B (RhB) under visible light
BOC microspheres Ag3PO4-BOC-200% Ag3PO4-BOC-50%
2.2 1.0 1.9
Ag2O/(BiO)2CO3
Ag2O/BOC-50% (the molar ratio of Ag2O: BOC =1:2)
degradation of RhB, methyl blue (MB), methyl orange (MO) under visible light
100% in 12 min 95% in 12 min 100% in 12 min
1.5 1.5 1.4 1.3 1.6 4.8
Fe2O3/(BiO)2CO3
Fe2O3/BOC (5 mol% Fe2O3)
degradation of RhB under UV-vis light
99% in 25 min
Ag2O/BOC-8.3% Ag2O/BOC-12.5% Ag2O/BOC-25% Ag2O/BOC-100% Ag2O BOC degradation of RhB in 12 min Fe2O3/BOC (0 mol% Fe2O3) (1 mol% Fe2O3) (3 mol% Fe2O3) (10 mol% Fe2O3) (20 mol% Fe2O3)
1.3 1.3 1.1 1.2 1.4
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β-Bi2O3/BOC (calcined at 300 ◦C) β-Bi2O3/BOC (calcined at 300 ◦C)
degradation of MB under visible light
99% in 40min
degradation of MO under visible light
99% in 40min
degradation of MB 95% in 150 min under visible light degradation of o- 99.8% in 45 min phenylphenol under visible light
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β-Bi2O3 (330 ◦C) α-Bi2O3 (400/500 ◦C) BOC (250 ◦C) N-doped TiO2 mixed β-Bi2O3 and BOC commercial β-Bi2O3
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0.2wt.% MoS2/BOC 1wt.% MoS2/BOC 2wt.% MoS2/BOC MoS2 BOC Bi2S3/BOC-20 min Bi2S3/BOC-2 h 15 mol% Bi2S3/BOC 25 mol% Bi2S3/BOC BOC
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MoS2/BOC with 0.5wt% MoS2
degradation of RhB under UV light
Bi2S3/(BiO)2CO3
Bi2S3/BOC-1 h
93% in 3 h
Bi2S3/(BiO)2CO3
5 mol% Bi2S3/BOC
degrading RhB under visible light degrading RhB under visible light
degrading RhB under sunlight
100% in 75 min
15 mol% Bi2S3/BOC 25 mol% Bi2S3/BOC BOC
1.3 1.1 1.3
Bi2S3/BOC-0.25 (the molar ratio of Bi2S3:BOC = 0.25)
NO removal under visible light
57.1% in 30 min
Supercapacitors with higher specific capacitance
Typical parameters
specific capacitance 196 Fg-1
Bi2S3/BOC-0.05 Bi2S3/BOC-0.1 Bi2S3/BOC-0.5 Bi2S3/BOC-1.0 BOC Reference supercapacitors Bi2O3/BOC
1.2 1.2 1.3 No activity 2.7 Enhancemen factor over reference 1.4
100% in 30 min
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3.3 BixMOy/(BiO)2CO3 heterostructures Table 3 shows the photocatalytic properties of BixMOy/(BiO)2CO3 heterostructures. Bi2WO6 is the simplest member among the Aurivillius-based compounds and a visible light responsive photocatalyst owing to its relatively narrow band gap. Huang et al. obtained a novel (BiO)2CO3/Bi2WO6 composites photocatalyst via a one-pot hydrothermal route, using Bi(NO3)3·5H2O, citric acid, PEG and Na2WO4 as the raw materials. The as-prepared (BiO)2CO3/Bi2WO6 were assembled by many interconnecting nanosheets and exhibited a RhB degradation efficiency of 98% after 60 min [123]. Zhang et al. successfully fabricated the (BiO)2CO3/Bi3NbO7 composite 16 Page 16 of 42
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via a simple hydrothermal method and found it an effective visible-light-driven photocatalyst for inactivation of Escherichia coli [124]. BiVO4 has a narrow band gap for visible light absorption and other merits are plentiful abundance, low cost, and good stability. Yu et al. developed a novel hydrothermal approach to synthesize hierarchical BiVO4/(BiO)2CO3 nanocomposites with reactive crystalline facets using urea as a morphology mediator as shown in Fig. 11. Their results indicate that both physical parameters and associated photocatalytic activity of BiVO4/(BiO)2CO3 nanocomposites can be tuned by the urea concentration and reaction time in the synthesis process. With the urea concentration added, the specific surface areas, pore volume and average pore size increased. Compared to BiVO4 and (BiO)2CO3 bulk counterpart, BiVO4/(BiO)2CO3 nanocomposites show enhanced photocatalytic activity under visible-light irradiation [125].
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Fig. 11 (a) SEM image of the BiVO4/(BiO)2CO3 composite (U20) where BiVO4 particles are marked with arrow heads; (b) HRTEM image of BiVO4/(BiO)2CO3 nanocomposite showing the lattice fringes (inset in b shows TEM image of BiVO4/(BiO)2CO3 nanocomposite). (Reproduced with permission from ref. 125. Copyright 2011, Elsevier.） Table 3 Photocatalytic properties of BixMOy/(BiO)2CO3 heterostructures Composite photocatalyst
Photocatalyst of highest activity
the
Bi2WO6/(BiO)2CO3
Bi2WO6/BOC
Bi3NbO7/(BiO)2CO3
Bi3NbO7/BOC-7.2% (the mass ratio of urea: Bi(NO3)3·5H2O=7.2%)
BiVO4/(BiO)2CO3
BiVO4/BOC-8% (U20) (the mass ratio of BiVO4:urea=8%)
Typical parameters
Photocatalytic activity
Reference photocatalyst
degradation of RhB under Xe light inactivation of Escherichia coli under visible light degradation of RhB under visible light
98% in 60 min
BOC Bi2WO6
Enhanced factor over the reference photocataly 1.1 1.1
4.7 log of E. coli completely killed after 5 h
Bi3NbO7/BOC-2.4% Bi3NbO7/BOC-12% Degussa P25 Bi3NbO7 bulk BiVO4 BOC nanosheet BiVO4/BOC-27% BiVO4/BOC-13%
1.5 1.7 1.3 1.9 1.8 2.1 1.1 1.0
97% in 60 min
3.4 BiOX/(BiO)2CO3 (X=Cl, I) heterostructures Photocatalytic properties of BiOX/(BiO)2CO3 heterostructures are shown in Table 4. 17 Page 17 of 42
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BiOI has a narrow band gap of 1.70−1.83 eV and can absorb most of the visible light (λ < 700 nm). However, BiOI itself is always poor in photocatalytic activity. In view of the fact that (BiO)2CO3 and BiOI are similar in structure, it is envisaged that a p-n BiOI/(BiO)2CO3 composite could integrate the merits of (BiO)2CO3 and BiOI, consequently showing high photocatalytic activity under visible light. Yin’s group fabricated BiOI/(BiO)2CO3 composites at room temperature by a facile method with the assistance of CTAB. Owing to the formation of heterojunctions, BiOI/(BiO)2CO3 shows excellent photocatalytic activity under the irradiation of visible light [89]. In order to develop efficient visible light photocatalysts for air purification, three dimensional (3D) BiOI/(BiO)2CO3 solid solutions were synthesized by a template free method at room temperature from Dong and his co-workers. As shown in Fig. 12, the solid solutions can adjust the band gap of BiOI/(BiO)2CO3 and improve photocatalytic activity [126].
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Fig. 12 Schematic illustration of the band gap structures of all samples (A) BOI, (B) BOC-BOI-50%, (C)BOC-BOI-25%, (D)BOC-BOI-5%, (E)BOC. (Reproduced with permission from ref. 126. Copyright 2014, Elsevier.） Similarly, BiOCl is used to couple with (BiO)2CO3. Xu et al. synthesized (BiO)2CO3/BiOCl nanocomposites with different molar ratios by a simple homogeneous precipitation at room temperature [127]. Fan et al. found that the enhanced photocatalytic performance of BiOCl/(BiO)2CO3 was closely related to the suitable conduction band interaction and structure characteristic, extending the optical response range and improving the efficient separation of photo-induced electron-hole pairs by the synergistic effect of BiOCl and (BiO)2CO3 [128]. Through acid etching method, a series of (BiO)2CO3/BiOX (X = Cl, Br, I) heterostructured photocatalysts were synthesized, displaying much higher photocatalytic activity than pure (BiO)2CO3 and the corresponding BiOX for the degradation of methyl orange (MO) [129]. Table 4 Photocatalytic properties of BiOX/Bi2O2CO3 heterostructures Composite photocatalyst
Photocatalyst of the highest activity
Typical parameters
Photocatalytic activity
Reference photocatalyst
Enhanced factor over the reference
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Photocatal BiOI/BOC (the molar ratio of Bi3+/CO32−/I−= 1/7/1)
degradation of RhB, MB, crystal violet respectively or their mixture under visible-light
100% RhB in 15 min 95% MB in 80 min 97% crystal violet in 60 min 50.8% in 30 min
BiOI/(BiO)2CO3
BOC-BiOI-25% (the molar ratio of BOC:BiOI = 3:1)
NO removal under visible light
BiOCl/(BiO)2CO3
BiOCl/BOC-1/6 (the molar ratio of CO32−/Cl = 1/6)
degradation of RhB under visible light
BiOCl/(BiO)2CO3
BiOCl/BOC (NaOH concentration is 0.90 mol/L)
degradation of MO under simulated sunlight
95% in 8 h k= 0.027 min-1
BiOX/(BiO)2CO3 (X = Cl, Br, I)
BiOI/BOC
degradation of MO under visible light
92.8% in 3 h
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2.7 2.1
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3.5 Metal-free semiconductor/(BiO)2CO3 Graphitic carbon nitride (g-C3N4) is a low-cost metal-free substance that shows high thermal and chemical stability as well as amenability to chemical modification. Coupling BOC with g-C3N4 can be capable of harvesting visible light as well as showing e ective separation of charge carriers. The enhanced photocatalytic activity of g-C3N4/(BiO)2CO3 can be mainly ascribed to the well-matched band structures, dye photosensitization and e cient crystal facets coupling interaction. (Table 5) The g-C3N4/(BiO)2CO3 nanojunctions showed much higher visible-light photocatalytic activity than pure g-C3N4 and (BiO)2CO3 for the degradation of RhB and phenol [130-132]. Huang’s group synthesized g-C3N4/(BiO)2CO3 by a simple mixed-calcinations [130]. By varying the reaction conditions, Yin’s group found that photocatalytic degradation of dyes over g-C3N4/(BiO)2CO3 under visible light was a dye-sensitized process [131]. Through a one-pot e cient capture of atmospheric CO2 method at room temperature, Dong et al. prepared novel g-C3N4/(BiO)2CO3 organicinorganic nanojunctioned photocatalysts with (BiO)2CO3 nanoflakes deposited onto the surface of g-C3N4 nanosheets [132]. The photocatalytic mechanism of gC3N4/(BiO)2CO3 is shown in Fig. 13.
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Fig. 13 Mechanism of the photocatalytic degradation of RhB over g-C3N4/(BiO)2CO3 semiconductor under visible light irradiation (λ > 420 nm). (Reproduced with permission from ref. 132. Copyright 2014, The Royal Society of Chemistry.） As shown in Fig. 13, the mechanism of indirect dye photosensitization induced degradation with CN-BOC might proceed in the following procedure. First, the RhB molecules, which are absorbed on the surface of BOC, transfer into its excited state (RhB*) under visible-light irradiation. Then, the electrons from the RhB* are trapped by the molecules oxygen on the conduction band of the BOC. Next, the electrons in the CB of BOC react with O2 to produce•O2− radicals for further degradation of RhB+•. During the reactive process, the •O2− radicals is identified as the main active species and the BOC is served only as an electron-mediator, which may be favorable for the e ective separation and transfer of the injected electrons and cationic RhB radicals [132]. In addition to the metal-semiconductor or semiconductor-semiconductor junctions, a series of the π-conjugated material-semiconductor photocatalyst junctions was reported such as graphite/TiO2 [133], C60/ZnO [134-135], polyaniline (PANI)/ZnO [136-137], PANI/g-C3N4 [138], PANI/BiOCl [139], PANI/Bi12TiO20 [140], and PANI/Bi2WO6 [141]. The transfer of photon-generated carrier to the π-conjugated material is promoted by the formation of heterostucture The interactional area between the large π-conjugated system and the electron clouds on the surface could improve the charge separation efficiency [135]. Lately, π-conjugated materials are applied to modify BOC as well, such as BOC/graphene [78,92,142], Polyaniline (PANI)-decorated {001} facets of (BiO)2CO3 nanosheets [93], polypyrrole (PPy)/(BiO)2CO3 [94]. Graphene (GR), as a two-dimensional carbon atom monolayer arranged in a honeycomb network, exhibits many unique properties, such as superior charge carrier mobility, high transparency, large surface area, high thermal/chemical stability and excellent flexibility [78]. Recently, graphene (GR)-based semiconductor photocatalysts have attracted extensive attention. Because the excellent electronic conductivity of GR promotes the transfer of photogenerated electrons through p-p bond interactions and inhibits recombination of the photoexcited electron-hole pairs, magnificently enhancing photocatalytic performance. Yu et al. fabricated hierarchical graphene-(BiO)2CO3 composites via a template-free hydrothermal method to improve the quantum e ciency of (BiO)2CO3 [92]. Zhang et al. utilized an one-pot efficient 20 Page 20 of 42
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capture of atmospheric CO2 at room temperature to obtain self-assembly of (BiO)2CO3 nanoflakes on graphene and graphene oxide nanosheets, which was an effective strategy to improve the photocatalytic performance of two-dimensional nanostructured materials [142]. It is noteworthy that the graphene has strong light absorption ability and could act as a shield against light absorption in semiconductors. Therefore, only when (BiO)2CO3 is loaded with an appropriate amount such as 1.0 wt% graphene can it owns the best photocatalytic performance [92]. However, only a small fractional area of (BiO)2CO3 makes direct contact with the graphene, leading to a relatively low quantum e ciency, and the (BiO)2CO3 particles tend to agglomerate, decreasing the e ective surface area during the photocatalytic degradation of organic pollutants [78]. To overcome the disadvantages above, Zhang and his colleagues prepared graphenewrapped (BiO)2CO3 (WBGR) core–shell structures for highly enhanced quantum e ciency as well as the photocatalytic activity of (BiO)2CO3 by maximizing its contact area using graphene encapsulation [78]. This enhanced quantum e ciency is attributed to the maximized contact area between the (BiO)2CO3 cores and the GR shells, not merely inhibiting aggregation of the(BiO)2CO3 microspheres but also protecting them from structural destruction. (Fig. 14 (a) and (b)) The electrons from WBGR can react with O2 to generate •O2- radicals that can then participate in the photocatalytic process. In contrast to the physical absorption in graphene (BiO)2CO3, the interfacial connection between (BiO)2CO3 and GR is achieved by chemical bonds (C-Bi bonds) in WBGR, which facilitates ultrafast charge transfer and separation and achieves a high charge separation yield with WBGR.
Fig. 14 (a) Illustration of the preparation procedure for WBGR core-shell structures; (b) Schematic illustration of the enhanced quantum e ciency and the radical reaction mechanism of WBGR. (Reproduced with permission from ref. 78. Copyright 2014, The Royal Society of Chemistry.) Polyaniline (PANI)-decorated {001} facets of (BiO)2CO3 nanosheets were synthesized by Zhou et al. It is demonstrated that the strong interfacial interactions between (BiO)2CO3 {001} facets and PANI could promote in situ formation of oxygen vacancy at the interface due to the high oxygen density characteristic of (BiO)2CO3 {001} facets [93]. Novel PPy/(BiO)2CO3 composites were synthesized by photocatalytic degradation of Rhodamine-B (RhB) under ultra violet (UV) irradiation and 0.75 wt.% PPy/(BiO)2CO3 composite showed the highest photocatalytic activity 21 Page 21 of 42
[94]. Table 5 Photocatalytic properties of metal-free semiconductor/(BiO)2CO3 Photocatalyst of the highest activity
Typical parameters
Photocatalytic activity
Reference photocatalyst
Enhanced factor over the reference photocatalyst
g-C3N4/(BiO)2CO3
g-C3N4/BOC (with molar ratio 1:2)
45% in 4 h
g-C3N4 BOC
3.1 14.2
g-C3N4/(BiO)2CO3
g-C3N4/BOC-10wt% CN-BOC-10wt%
k = min-1
g-C3N4 BOC
4 2
g-C3N4/(BiO)2CO3
g-C3N4/BOC
Graphene-wrapped (BiO)2CO3
Graphene-wrapped rose-like BOC
100% RhB in 5 h 19% phenol in 5 h k = 2.81×10-4 s-1
g-C3N4 BOC g-C3N4 BOC BOC Graphene-BOC
1.3 1.4 1.4 4.2 8.7 4.2
Graphene-(BiO)2CO3
Graphene-BOC (1.0 wt% graphene)
degrading RhB under visible light degrading RhB under visible light degrading RhB and phenol under visible light carbamazepine degradation under UV-vis light degrading RhB under visible light
97% RhB in 75 min
BOC
1.3
Graphene/(BiO)2CO3
Graphene/BOC
NO removal under solar light degrading RhB under visible light
63.5% in 30 min
BOC Graphene Oxide/BOC
1.16 1.03
k = 0.286 min-
degrading RhB under UV light
95% in 30 min
BOC BOC/PANI2.8wt% BOC/PANI-10.1wt% BOC/PANI-12.4wt% PPy/BOC-0.25wt% PPy/BOC-0.50wt% PPy/BOC-1.00wt% BOC
4.5 3.2 2.0 2.4 2.2 3.3 5.6 1.9
polypyrrole/(BiO)2CO3 (PPy/BOC)
PPy/BOC-0.75wt%
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3.6 Complex heterostructure Other complex heterojunctions, such as Ag-AgBr/(BiO)2CO3 [143], Bi2O3/(BiO)2CO3/Sr6Bi2O9 [144] and Graphene/TiO2/(BiO)2CO3 [145], have also been developed. (Table 6) Zhu et al applied a two-step synthesis method to obtain a plasmonic Ag-AgBr/BOC composite photocatalyst with enhanced photocatalytic activity for the degradation of RhB and MB under visible light, which was attributed to the heterostructure and surface plasmon resonance (SPR) exhibited by Ag nanoparticles [143]. Photocatalytic degradation of sulfamethoxazole (SMX) was investigated by Niu’s group using Bi2O3/(BiO)2CO3/Sr6Bi2O9 (BSO) photocatalyst under visible light (>420 nm) irradiation [144]. Ao et al prepared graphene and titania co-modified flower-like 22 Page 22 of 42
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(BiO)2CO3 multi-heterojunctions with excellent activity for dye degradation, which could be ascribed to the formation of multi-heterojunctions and the high surface area of GR and TiO2 [145].
The condition or photocatalyst of the highest activity
Typical parameters
Ag-AgBr/(BiO)2CO3
Ag-AgBr/BOC
degradation of RhB and MB under visible light
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Graphene/(BiO)2CO3/TiO2 (GR/BOC/TiO2)
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Table 6 Photocatalytic properties of (BiO)2CO3-based complex heterostructures
degrading 10 mg/L sulfamethoxazole (SMX) under visible light degrading MO under UV light
100% MB in 12 min
100% RhB in 12 min
90% of SMX degradation and 36% of TOC reduction at pH 7.0 after 120 min 94% in 4 min
Reference conditions or photocatalysts
Enhanced factor over the reference photocatal
BOC Ag/BOC Ag/AgBr AgBr/BOC BOC Ag/BOC AgBr/BOC pH=3 pH=5 pH=9 pH=11 BOC GR/BOC
3.3 1.9 1.5 1.3 1.4 1.2 1.1 0 1.7 1.1 2.1 3.8 2.4
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4. Doping with nonmetals 4.1 Nitrogen doping Nitrogen doping has been utilized as one of the most effective methods to improve the visible absorption and activity of (BiO)2CO3 through narrowing the band gap or forming a mid-gap [85-86,95-99]. (Table 7) Our group has systematically carried out N-doped (BiO)2CO3 including the effect of different precursors, hydrothermal temperature and reaction time to achieve controlled morphology and composition. The as-obtained N-doped (BiO)2CO3 hierarchical structures exhibit magnificent photocatalytic performance under both visible and ultraviolet light irradiation for NO removal. We conducted tunable synthesis of 3D monodisperse in situ N-doped (BiO)2CO3 hierarchical architectures composed of 2D single-crystal nanosheets by a one-pot template-free hydrothermal method, utilizing bismuth citrate and ammonia solution as precursors. Depending on the different concentration of ammonia solution, the morphologies of various types of N-doped (BiO)2CO3, including dandelion-like, hydrangea-like and peony flower-like microspheres, could be selectively constructed due to different self-assembly patterns of nanosheets [85]. By adjusting the hydrothermal temperature, the fascinating morphologies of N-doped (BiO)2CO3, 23 Page 23 of 42
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including rose-like, hydrangea flower-like and peony flower-like microspheres could be controllably fabricated [97]. Interestingly, the ammonia works as not only a nitrogen resource doping (BiO)2CO3, but also a solvent hydrolyzing the bismuth citrate. In addition, we obtained N-doped(BiO)2CO3 honeycomb-like hierarchical microspheres, using bismuth citrate and dicyandiamide as precursors. The ammonium ions from the decomposition of dicyandiamide react with bismuth ions and carbonate ions from the decomposition of citrate ions, producing in situ N-doped (BiO)2CO3 microspheres with exhibited excellent visible light activity [86]. Moreover, persimmon-like, flower-like and nanoflakes nano/microstructures were also prepared by one-step hydrothermal treatment of bismuth citrate and urea with diverse temperature [99]. To gain new insight into the N-doped (BiO)2CO3, our group revealed a new growth mechanism of self-organized N-doped (BiO)2CO3 hierarchical microspheres which were fabricated by hydrothermal treatment of bismuth citrate and urea without additives [98]. The facile and convenient method, free of any additive, can be applied to synthesize other hierarchical materials with a special shape.
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Fig. 15 SEM images of as-synthesized samples obtained at di erent hydrothermal times: (a) 3 h, (b) 9 h, (c) 24 h, (d) 48 h. (Reprinted with permission from ref. 98. Copyright (2014) The Royal Society of Chemistry.)
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NH2CONH2 + H2O → 2NH3 + CO2 (1) NH3 + H2O → NH4+ + OH(2) CO2 + 2OH- → CO32- + H2O (3) + Bi(C6H5O7) + 3OH → BiO + C6H5O73- + 3H+ (4) 4BiO+ + CO32- + 2OH- → (BiO)4CO3(OH)2 (s) (5) (BiO)4CO3(OH)2 (s) + CO32- → 2(BiO)2CO3(s) + 2OH- (6) C6H5O73- → CO2+H2O (7) To obtain a better understanding of the formation mechanism of the self-organized Ndoped (BiO)2CO3 microspheres, products generated from di erent growth stages are collected for SEM measurements as shown in Fig. 15. And the reaction equations (reaction (1-7)) are presented above. When the reaction is carried out for 3 h, the morphology of the obtained samples (Fig. 15 (a)) is similar to that of the samples collected at 0 h. When the reaction lasts for 6 h, large amounts of regular particles with uniform size are produced, because bismuth citrate is hydrolyzed by OH− arising from reaction (2), generating di erent types of ions (reaction(4)). Then some ions react with each other (reaction (5)) to form the (BiO)4CO3(OH)2 growth nuclei, which subsequently grow to crystallize. (BiO)4CO3(OH)2 can react with CO32− to produce 24 Page 24 of 42
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(BiO)2CO3. (reaction (6)) Thus, it is supposed that the (BiO)2CO3 growth nuclei are initially generated on the surface of CO2 bubbles and then tend to aggregate together as growth centers with increasing concentration of OH− (reaction (6)). Upon further increasing the time to 9 h, it is found in Fig. 15 (b) that the size of the particles becomes smaller, which can be considered as the consequence of the dissolution and recrystallization of (BiO)4CO3(OH)2 particles. The microspheres become bigger by the consumption of particles around. After 24 h of hydrothermal treatment, the number of particles decreased dramatically and microspheres self-assembled by nanosheets can be clearly observed, while few particles are still adhered to the surface of the microspheres (Fig. 15 (c)). As all particles disappear, all (BiO)4CO3(OH)2 transformed to the (BiO)2CO3 phase accompanied by the doping of N element into the lattice of (BiO)2CO3 when the reaction time is further extended to 48 h (Fig. 15 (d)). The plausible formation mechanism of self-organized N-doped (BiO)2CO3 microspheres is proposed as displayed in Fig. 16.
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Fig. 16 The formation mechanism of self-organized N-doped (BiO)2CO3 microspheres. (Reprinted with permission from ref. 98. Copyright (2014) The Royal Society of Chemistry.) 4.2 Carbon doping and carbonates self-doping Carbon doping and carbonates self-doping, facile and e cient methods, can narrow the band gap, extend the light absorption range and thus increase visible light photocatalytic activity of semiconductor photocatalysts. Dong et al applied glucose as the carbon source to construct C-doped (BiO)2CO3 microspheres via a facile hydrothermal method [146]. A schematic band structure is proposed for the undoped and C-doped (BiO)2CO3 samples as shown in Fig. 17 (e). Carbon doping could reduce the band gap of (BiO)2CO3 and generate localized states above the valence band, resulting in the extension of the light response to the visible region. On the other hand, carbon doping promotes the e ective separation of photogenerated charge carriers. As a result, C-doped (BiO)2CO3 samples exhibit enhanced visible photocatalytic activity in comparison with the undoped (BiO)2CO3. The sample C-doped (BiO)2CO3 with 0.1160 g glucose (CBOC-M) shows the highest visible light photocatalytic activity and excellent stability (Fig. 17 (c, d)). Its SEM images are depicted in Fig. 17 (a, b). 25 Page 25 of 42
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Fig. 17 (a, b) SEM images of the as-prepared CBOC-M sample; (c, d) Photocatalytic NO removal performance of the as-prepared samples under visible light irradiation and the cycle photocatalytic activity of the CBOC-M sample. (e) Proposed schematic energy band structure of the as-prepared samples. The samples C-doped (BiO)2CO3 with different glucose contents (0.0580 g, 0.1160 g, and 0.20 g) were labeled as CBOC-L, CBOC-M and CBOC-H, respectively. (Reprinted with permission from ref. 146. Copyright (2015) The Royal Society of Chemistry.)
The work from Huang’s group demonstrated self-doping of the CO32− into wide bandgap (BiO)2CO3 can narrow the band and extend the photoresponsive range from UV to visible light, which is verified by both the experimental and theoretical results [147]. The self-doped (BiO)2CO3 (C-BOC) exhibits drastically enhanced visible-light photoreactivity for NO removal. Density functional theory (DFT) calculation is used 26 Page 26 of 42
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to investigate the crystal and electronic structure of C-BOC. In Figure 18 (a), the result from crystal structure optimizing indicates that the foreign CO32− ions are doped in the caves constructed by the four adjacent CO32− ions. As shown in Figure 18 (b) and 18 (c), the denser band density of self-doped BOC is resulted from its 2 × 1 × 2 super cell. The photoresponse of C-BOC is extended and an obvious absorption band at 1.2 eV appears, confirming that self-doped CO32− ions could e ectively narrow the band gap.
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Fig. 18 (a) Crystal structure of C-BOC. (b) Electronic band structures of BOC and CBOC. (c) Calculated imaginary dielectric functions versus energy of BOC and CBOC. (Reprinted with permission from ref. 147. Copyright (2015) American Chemical Society.) Table 7 Photocatalytic removal of NO by (BiO)2CO3 doped with nonmetals. or
Photocatalytic activity
Reference conditions or photocatalysts
Enhanced factor over the reference photocatalyst
42.6% in 30 min visible light 46.1% in 30 min UV light 41% in 30 min visible light 49.4% in 30 min visible light 56.8% in 30 min UV-vis light 41.6% in 30 min visible light
undoped BOC C-doped TiO2 undoped BOC P25 C-doped TiO2 N-doped TiO2 C-doped TiO2 N-doped TiO2 C-doped TiO2 P25 C-doped TiO2 N-doped TiO2 undoped BOC undoped BOC P25 BOC-150 BOC-210 BOC-pure P25
8.5 2.1 1.8 1.4 1.6 1.1 2.4 1.4 1.9 1.2 2.0 1.2 5 1.2 1.1 1.4 1.0 9.9 4.7
P25
1.6
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N-doped (BiO)2CO3
N-doped (BiO)2CO3
N-doped BOC (5.33 mL ammonia solution ) N-doped BOC (1.6 mL ammonia solution ) N-doped BOC-180 °C
N-doped (BiO)2CO3
N-doped BOC
N-doped (BiO)2CO3
N-doped BOC
under under under under under under
50.0% after 60 min under solar light 49.2% in 30 min under visible light
N-doped (BiO)2CO3
N-doped BOC-180°C
N-doped (BiO)2CO3
N-doped BOC-180°C 48 h
49.6% in 30 min under visible light 57% after 60 min under
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N-doped BOC-180°C 48 h
C-doped BOC-0.1160 g (glucose content is 0.1160 g)
CO32--doped (BiO)2CO3
CO32--doped BOC (the molar ratio of sodium citrate:Bi(NO3)3·5H2O = 1.5)
undoped BOC
4.5
undoped BOC
1.2
undoped BOC C-doped TiO2 C-doped BOC-0.0580 g C-doped BOC-0.20 g P25 C 3 N4 BiOBr
7.5 2.9 1.2 1.5 4 3.7 3
48% removal in 30 min under visible light
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C-doped (BiO)2CO3
UV light 49.3% removal in 30 min under visible light 46% after 30 min under simulated solar light 59.7% removal in 30 min under visible light
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4.3 Formation of oxygen vacancy Interesting, Dong et al. recently found a novel environmental catalyst, black defective (BiO)2CO3 microspheres, which was prepared by a vacuum heat treatment method and could be regenerated by secondary vacuum heat treatment [148]. The black sample BOC-275 (Fig. 19 (b,c)) is obtained by vacuum heat treatment of the pure (BiO)2CO3 (Fig. 19 (a)) with 275 oC for 3 h and exhibits excellent NO removal performance both at room temperature in the dark and under visible light irradiation. Oxygen vacancy is discovered in black defective (BiO)2CO3, which can not only narrow the band gap of (BiO)2CO3 but also activate O2 to react with NO. The mechanism of the catalysis in the dark and under visible light is illustrated in Fig. 19 (d). The active sites (Bi0, Bi5+, and oxygen defects) contained in black (BiO)2CO3 tend to absorb the NO molecules and then weaken their bonds, while the oxygen defects can directly activate the adsorbed O2 to generate the active species. Then the adsorbed NO molecules react with the active species to create nitrogen dioxide, nitrite and nitrate in the dark. Under visible light irradiation, thermal energy from the transformation of partial light energy can accelerate the surface thermocatalytic reaction. What’s more, the visible light activity of black (BiO)2CO3 can be enhanced by the plasmonic e ect of the Bi element and the narrowed band gap of (BiO)2CO3 caused by the oxygen defects. According to the mechanisms above, the e cient dark catalytic activity is ascribed to thermocatalysis and the elevated visible light photocatalytic performance can be attributed to the synergy of thermocatalysis and photocatalysis.
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Fig. 19 (a, b) Color of the as-prepared samples before the catalytic test; (c) SEM image of BOC-275; (d) The illustration of the mechanisms of the catalysis in the dark and under visible light for NO removal. (Reprinted with permission from ref. 148. Copyright (2015) The Royal Society of Chemistry.)
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5. Multi-functional applications 5.1 Photocatalytic degradation of pollutants One of the most promising applications of (BiO)2CO3 is photocatalytic degradation of pollutants, including wastewater dyes, gaseous NO and heavy metals. Dyes in wastewater are stable and resistant to biodegradation due to their complex aromatic molecular structure, which come from various industries, such as textiles, printing, pulp mills, leather, food, dyestu s, and plastics. So far, various technologies, including biological treatment, coagulation/flocculation, ozone treatment, chemical oxidation, membrane filtration, ion exchange, photocatalysis, and adsorption, have been developed for the treatment of dye-containing e uents [149]. Among these methods, photocatalysis could completely degrade dyestu without secondary pollution. (BiO)2CO3 and (BiO)2CO3-based derivatives could effectively remove mixed wastewater dyes (rhodamine-B, methylene blue, methylene orange, crystal violet or their mixture) [63,70,76,82,88]. For example, RhB can be well removed by (BiO)2CO3 [69], Sb-doped (BiO)2CO3 nanoplates [150], Polyaniline (PANI)(BiO)2CO3 [93], and so on. In addition, (BiO)2CO3 nanotubes exhibited excellent Cr(VI) removal capacity [107]. NOx in the atmosphere (NO and NO2) is a kind of harmful acidic gas with low concentration. NOx transfers, transforms, and participates in the formation of secondary pollutants such as PM2.5 and photochemical smog [151]. NOx is also one of the typical pollutants in indoor air. Industrial emissions of high concentration of NOx are usually removed by selective catalytic reduction (SCR), chemical absorption, and biological filtration method. But these methods are not suitable for indoor purification of low concentration NOx. The traditional purification technology for indoor pollution uses the activated carbon adsorption, which just transfers the pollutants from the gas phase to solid phase, bringing a series of problems such as post-processing 29 Page 29 of 42
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and regeneration of activated carbon. Photocatalysis, as a green chemistry technology, has become the most promising technology to control indoor pollution. Dong et al. demonstrated a series of research on (BiO)2CO3, nonmetal-doped (BiO)2CO3 and (BiO)2CO3-based composites, which exhibited excellent photocatalytic performance for NO removal under both UV and visible light irradiation [66-68,85-86,132,146]. It is worth to mention that the defective (BiO)2CO3 superstructures showed high room temperature catalytic activity toward NO removal [148].
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5.2 Medical and healthcare (BiO)2CO3 has long been used for medicinal purposes as an anti-inflammatory, antibacterial, and antacid, especially in the treatment of Helicobacter pylori (H. pylori) infection, peptic ulcers and other gastrointestinal disorders [75]. Chen et al. successfully synthesized well-crystallized bismuth subcarbonate nanoparticles by water-in-oil (w/o) microemulsion-assisted hydrothermal method. The (BiO)2CO3 nanoparticles exerted comparable anti-Helicobacter pylori activities to the clinically used drug, colloidal bismuth subcitrate [65,80]. Also, (BiO)2CO3-based composites could be applied in the processing of drugs, making a contribution to healthcare. For example, Bi2O3/(BiO)2CO3/Sr6Bi2O9 could degrade sulfamethoxazole (SMX) [144]. In addition, β-Bi2O3/(BiO)2CO3 was utilized to the degradation of o-phenylphenol, which was a widely used fungicide and preservative agent [118]. Besides, carbamazepine (CBZ) could be disposed of by graphene-wrapped (BiO)2CO3 [78].
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5.3 Supercapacitors As a new class of energy storage device, supercapacitors have been widely used to power up electrical vehicles, consumer electronics, and military devices due to their unique properties, such as high power density, long cycle life, and faster charging and discharging within short time. The performance of supercapacitors to a great extent depends on their electrode materials [71,72]. Recently, (BiO)2CO3 materials have been researched as the electrodes for supercapacitors. Uniform (BiO)2CO3 microspheres composed of multilayered nanoplates had been successfully synthesized through a hydrothermal route in the presence of PVP and KCl. Peng et al. demonstrated that the Ag/(BiO)2CO3 composite as an active material delivered specific capacities of 620 and 361 F g-1 at current densities of 1 and 5 A g-1, respectively [71]. Also, Zhang et al. fabricated MnO2-decorated flower-like bismuth subcarbonate ((BiO)2CO3) via a hydrothermal approach, and further investigated its performance as the electrodes for supercapacitor. The core-shell nanostructures display moderate capacities (196 F g-1 or 54.5 mAh g-1 for ((BiO)2CO3@MnO2) with superb cycling stability (125% for (BiO)2CO3@MnO2 after 1000 cycles). The electrochemical properties of the electrode are strongly related to their high surface area, porous structure and good conductivity, which can not only provide rich active sites but also shorten the ion transport pathways [72]. 5.4 Other applications Humidity sensors play a very important role in environmental monitoring, industrial 30 Page 30 of 42
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process control and in our daily life. Therefore, it is highly demanded that stable humidity sensors, with high sensitivity, fast response and recovery times, stability, little humidity hysteresis and low price. Recently, (BiO)2CO3 nanosheets with exposed {001} facets were obtained from the straightforward and economic room temperature conversion of commercial Bi2O3 with CO2 as atmospheric carbon source. And Zhou’s group discovered that this (BiO)2CO3 sensor showed high sensitivity, quick response/recovery time, and good reproducibility with a narrow humidity hysteresis [73]. Nonlinear optical (NLO) materials have received much attention due to their significant applications in the laser field, including laser micromachining, semiconductor photolithography, photochemical synthesis and high-resolution photoemission spectrometer. Huang et al. utilized a hydrothermal method to obtain a novel nonlinear optical (NLO) material (BiO)2CO3. The powder second-harmonic generation (SHG) measurement indicates that the NLO efficiency is approximately 5 times as large as that of KDP (KH2PO4) standard, making the (BiO)2CO3 a potential NLO material [74].
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6. Conclusion and perspectives In summary, the recent progress on synthesis of (BiO)2CO3 nano/micro structures, BOC-based nanocomposites, nonmetal doped (BiO)2CO3 and multifunctional applications have been comprehensively reviewed. The morphological structure of (BiO)2CO3 was dependent on the preparation method, hydrothermal temperature, hydrothermal time, and selection of precursors. The understanding of the fabrication, mechanism and potential applications of (BiO)2CO3 will be beneficial for developing other novel photocatalysts with magnificent performance. Although considerable advances have been achieved, some crucial mechanisms and modification strategies in engineering the (BiO)2CO3-based nanocomposites still need to be addressed. Below are some existing challenges and possible future directions. First, many efforts have been contributed to the morphology control and modification of (BiO)2CO3. However, the investigation on the surface or interface of the materials is scarce. The interface of heterojunctions can be well-controlled if we understand the charge generation, separation and transportation process across the nanoscale interfaces. Detailed mechanism of the charge transfer process is more requisite for complex multi-component heterojunction photocatalysts than simple two component heterostructures. Advanced in situ techniques can be employed to gain more insights to address this challenge. Second, more insightful evidences should be obtained to support the proposed transfer pathway of photoinduced electrons and holes and the functions of internal electric field during photocatalysis. Also, the effects of doping on the band structure and electronic structure of (BiO)2CO3 should be investigated with theoretical calculations. Based on theoretical calculations, the designed synthesis and modification of (BiO)2CO3 can be well-controlled to achieve more efficient performance. Third, the controlled synthesis of (BiO)2CO3-based systems with dominant facets 31 Page 31 of 42
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remains to be excavated. The semiconductors with dominant facet are significant for design and synthesis of high performance devices and systems. Apart from the {001} crystal face of (BiO)2CO3, other high-energy crystal facets are also desirable. The atomic steps, dangling bonds of (BiO)2CO3 with other exposed facets may also act as active sites to capture electrons to facilitate charge separation and transfer, and favor the adsorption of reactive molecules and desorption of products. Based on new discovered materials, there are still great opportunities to develop (BiO)2CO3 based heterojunction with active facets, such as 2D-2D heterojunction with coupled exposed facets. Fourth, the current photocatalytic applications of (BiO)2CO3-based nanocomposites mainly focus on the degradation of pollutants. Although it has been applied in humidity sensor, nonlinear optical element, antibacterial, supercapacitor, reports in these area are still scare. Up to now, numerous versatile photocatalysts are in demand to broaden the application in other areas such as H2 generation, CO2 photoreduction, selective chemical synthesis and clean fuel production. (BiO)2CO3-based photocatalysts may also find wide applications in these important areas.
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This research is financially supported by the National Natural Science Foundation of China (51478070, 21501016, 51108487). References
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air with hierarchical bismuth oxybromide nanoplate microspheres under visible light, Environ. Sci. Technol. 43 (2009) 4143-4150.
Highlights:
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The (BiO)2CO3 with Aurivillius structure y is an emergent material. Synthesis of (BiO)2CO3 micro/nano structures was reviewed. The mechanisms of (BiO)2CO3 based nanocomposites were discussed. Doping (BiO)2CO3 with nonmetals for enhanced activity was highlighted. Multi-functional applications of (BiO)2CO3 based derivatives was demonstrated.
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Graphical Abstract
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