Communication pubs.acs.org/crystal
Highly Efficient Bi2O2CO3 Single-Crystal Lamellas with Dominantly Exposed {001} Facets Hongwei Huang,*,† Jinjian Wang,† Fan Dong,§ Yuxi Guo,† Na Tian,† Yihe Zhang,*,† and Tierui Zhang‡ †
Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China § Chongqing Key Laboratory of Catalysis and Functional Organic Molecules, College of Environmental and Biological Engineering, Chongqing Technology and Business University, Chongqing 400067, China ‡ Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *
ABSTRACT: Herein we report the Bi2O2CO3 single-crystal nanoplates with dominant {001} exposing facets fabricated via a controllable hydrothermal means. Exposed {001} reactive facets enable BOC-001 nanoplates efficient separation and migration of photoinduced electron−hole pairs, thereby resulting in highly enhanced photoreactivity pertaining to rhodamine B degradation, NO removal, and photocurrent generation. The present work provides a new reference for manipulation of facet-dependent photocatalytic activity of semiconductors.
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arrangement of the [CO3] anionic group.19 Nevertheless, the property of most concern for Bi 2O 2CO3 is the high photocatalytic performance.20−29 Zheng etc. first reported the synthesis and photocatalytic activity of Bi2O2CO3 samples with different morphologies,20 including a flower-like sample, sponge-like porous sphere sample, and plate-like sample. Among them, the flower-like Bi2O2CO3 exhibits the highest rhodamine B degradation activity. On the basis of the HRTEM results, the flower-like Bi2O2CO3 products are considered to be composed of {001}-plane exposed flakes. Thus, the high photocatalytic activity of flower-like Bi2O2CO3 was thought to be mainly from the exposed reactive {001} plane of the flakes. Besides, the Bi2O2CO3-based composite photocatalysts also show high photoactivity; e.g., the ternary composite photocatalyst Bi2O3/Bi2O2CO3/Sr6Bi2O9 can degrade sulfamethoxazole effectively.30 The degradation efficiency relying on different condition parameters, including initial sulfamethoxazole concentrations, pH values, and photocatalyst concentrations, was also investigated. Finally, the mineralization mechanism involving formation of reactive superoxide radicals was proposed. In addition, the plasmonic composite photocatalyst Ag-AgBr/Bi2O2CO3 also exhibited an enhanced photocatalytic activity for degradation of methylene blue and
owadays, semiconductor crystal-facet engineering is very active, which paves a new way for physicochemical property tailoring and the photoactivity enhancement of photocatalysts.1−5 The successful synthesis of active {001} facets of anatase TiO2 crystals via HF serving as a capping agent opened the door of research on facet-dependent photocatalytic performance.6,7 Two-dimensional (2D) bismuth nanomaterials have also drawn considerable attention because their layered crystal configuration can afford an internal driving force promoting the effective separation of photoinduced charge carriers.8−12 As a typical 2D layered photocatalyst, bismuth oxychloride (BiOCl) gains special interest for facet-controlled manufacture of its single crystal. BiOCl also exhibits facetdependent photocatalytic activity that strongly lies on the dominant exposing facets of the crystal.13−17 It was disclosed that BiOCl single-crystalline nanosheets with exposed {001} facets enable more efficient separation of photoexcited electron−hole pairs, thus displaying higher photoreactivity than the counterpart with exposed {010} facets.13 That is to say, the {001} facets are the highly reactive facets in BiOCl. Bismuth carbonate (Bi2O2CO3), as a newcomer of the “sillén” family that is a kind of bismuth compound composed of [Bi2O2]2+ layers and interlaid halogen ions or anionic groups, also features a 2D layered crystal structure built by [Bi2O2]2+ layers and [CO3]2− triangles.18 Lately, we found that Bi2O2CO3 exhibits a large nonlinear optical (NLO) effect with strong second-harmonic generation (SHG), which results from its non-centrosymmetrical crystal structure and the aligned © 2015 American Chemical Society
Received: October 14, 2014 Revised: January 13, 2015 Published: January 16, 2015 534
DOI: 10.1021/cg501527k Cryst. Growth Des. 2015, 15, 534−537
Communication
Crystal Growth & Design
{200} and {110} facets. This SAED pattern is assigned to the [001] zone-axis diffraction spots of orthorhombic Bi2O2CO3. The angle is also in good agreement with the value from the crystal structure of Bi2O2CO3 (Figure 1e). We can also observe that the (110) plane contains more Bi atoms, while the (200) plane features a high density of O atoms. The high-resolution TEM (HRTEM) image (Figure 1c) indicates that the interplanar spacing of lattice fringes is 0.273 nm, which can be indexed into the (110) lattice planes. On the basis of the above analyses, we can conclude that the top and bottom exposing surfaces are {001} facets (Figure 1d). This is further confirmed by the following XRD patterns. Figure 2 shows the XRD patterns of as-obtained BOC and BOC-001 products. All diffraction peaks observed could be
rhodamine B under visible light, which is ascribed to its heterostructure and Ag nanoparticles surface-plasmon-resonance (SPR) effect.31 In the study, the AgBr nanoparticles were deposited on the flat surfaces of Bi2O2CO3 nanosheets with exposed {001} facets according to the HRTEM images. Though the {001} facet was determined to be the exposed facet of Bi2O2CO3 crystals by HRTEM in the previous papers, nevertheless, the diffraction peaks of their X-ray diffraction (XRD) patterns corresponding to the (002), (004), and (006) planes belonging to {001} facets are much lower than most of the other peaks, such as (013), (110), (011), (020), (123), (114), etc. It indicates that the ratios of exposed {001} facets of Bi2O2CO3 crystals in previously reported papers are relatively low. Besides, the facet-dependent photocatalytic activity was studied only by pollutant degradation. Especially, investigations on separation, migration, and recombination properties of photogenerated electron−hole pairs strongly depending on reactive facets are not included. Herein, we first disclose the controllable synthesis and the highly enhanced photoactivity for rhodamine B (RhB) degradation, NO removal, and photocurrent generation of Bi2O2CO3 single-crystal nanoplates with dominant {001} exposing facets. The Bi2O2CO3 samples with dominantly exposed {013} and {001} facets are hydrothermally obtained and denoted as BOC and BOC-001, respectively. The BOC exhibits a microspheric structure assembled by plenty of nanosheets by scanning electron microscopy (SEM), while the BOC-001 products are composed of large-scale round-pill-like nanoplates with diameters of 100−800 nm (Figure S1 in the Supporting Information). Transmission electron microscopy (TEM) images (Figure 1a) further verified such a plate-like structure. The single-crystalline nature of BOC-001 sample was confirmed by a selected-area electron diffraction (SAED) pattern (Figure 1b). The angle indicated is 45°, corresponding to the theoretical calculation obtained for the angle between the
Figure 2. XRD patterns of the BOC and BOC-001 nanoplates.
indexed into the pristine orthorhombic phase Bi2O2CO3 (ICSD-94740) with space group Imm2. Fascinatingly, the (002), (004), and (006) peaks belonging to the {001} facets of BOC-001 exhibit overwhelmingly higher relative intensities than other peaks and that of BOC. It discloses that the major exposed surfaces of BOC-001 are {001} facets. That is to say, the Bi2O2CO3 nanoplates with predominantly exposed {001} facets were synthesized. It is also consistent with the results from the TEM and HRTEM analysis. Figure 3a presents the UV−visible diffuse reflectance spectra (DRS) of BOC and BOC-001. They have similar absorption
Figure 3. (a) DRS and (b) band gaps of BOC and BOC-001.
edges around 400 nm. As an indirect-band gap semiconductor, the band energy of Bi2O2CO3 could be acquired from the plot of (Ahν)1/2 versus energy (hν).31 On the basis of the xintercept, we can determine that the band gaps of BOC and BOC-001 are 3.22 and 3.17 eV, respectively (Figure 3b). Thus, both BOC and BOC-001 have strong light absorption in the UV region.
Figure 1. (a) TEM image, (b) SAED pattern, (c) HRTEM image, (d) schematic orientation illustration, and (e) crystal structure of the BOC-001 crystals. 535
DOI: 10.1021/cg501527k Cryst. Growth Des. 2015, 15, 534−537
Communication
Crystal Growth & Design
4c, the prompt photocurrent responses with good reproducibility were observed for both BOC and BOC-001 electrodes. In contrast to the case of BOC, BOC-001 exhibits a remarkably enhanced photocurrent density, confirming its more efficient charge separation. It is in accordance with their photodegradation efficiencies. We also utilized electrochemical impedance spectra (EIS) to survey the separation and transfer efficiency of photoexcited holes and electrons occurring on BOC and BOC-001 electrodes. As revealed in Figure 4d, BOC001 presents a smaller arc radius than BOC, validating the higher efficiency of charge transfer of BOC-001. Besides, we can determine the lifetime of the injected electrons (τ) via the equation (τ = 1/2πf), where f represents the minimum inverse frequency.35,36 Accordingly, the lifetimes of the electrons of BOC and BOC-001 were calculated to be 0.14 and 0.25 μs, respectively (Figure 4e). This greatly prolonged lifetime of BOC-001 also confirmed the more efficient separation and migration of electron−hole pairs occurring in {001} facets. Photoluminescence (PL) spectra can provide evidence for monitoring the recombination efficiency of photogenerated electrons (e−) and holes (h+) as the recombination of photoexcited h+ and e− often results in the energy release in the form of PL emission.21,34 As shown in Figure 4f, the PL intensity of BOC-001 is obviously lower than that of BOC, demonstrating that the lower recombination rate of e− and h+ occurred in BOC-001. That is to say, the {001} exposing facet holds a stronger ability in separating the electron−hole pairs, which contributes to the more excellent photocatalytic performance of BOC-001. It is also consistent with the results from above EIS and Bode-phase spectra. Moreover, the stability and durability of BOC-001 were evaluated. As shown in Figure S3, Supporting Information, there is no obvious efficiency decay after five recycling runs, indicating the BOC-001 photocatalyst is stable and resistant to photocorrosion in the degradation process. The XRD results also confirmed this observation. The XRD pattern of BOC-001 sample after irradiation remained the same as that before irradiation (Figure S4, Supporting Information). It further demonstrated the stability of crystalline phase and exposed facets. In summary, the Bi2O2CO3 single-crystal nanoplates with dominantly exposed {001} facets were synthesized by a controllable hydrothermal means. The as-prepared BOC-001 nanoplates exhibit much more excellent photocatalytic performance and photoelectrochemical properties, which can be attributed to the high separation and fast transfer of charge carriers derived from the {001} exposing facets. These findings may pave an alternative way to engineer the facet-dependent photoreactivity of 2D photocatalysts.
The photocatalytic properties of BOC and BOC-001 were studied via degradation of dye RhB, NO removal in the gas phase, and photocurrent generation under irradiation of UV light. The self-photolysis of RhB was found to be negligible in the absence of photocatalyst (Figure S2, Supporting Information). After 30 min irradiation, only about 58% RhB was degraded over BOC. Nevertheless, the photodegradation efficiency of RhB over BOC-001 can reach as high as 95% in the same period. Though the BET specific surface area of BOC001 (10.6 g/cm2) is slightly smaller than that of BOC (14.5 g/ cm2), BOC-001 displays an evidently superior apparentreaction-rate constant (k), which is approximately 3 times that of BOC (Figure 4a). In order to confirm the higher
Figure 4. (a) Apparent reaction rate constants for photodecomposition of RhB and (b) NO removal ratios in gas phase over BOC and BOC-001 samples under UV light irradiation. (c) Transient photocurrent responses, (d) EIS Nynquist plots, and (e) the Bodephase of BOC and BOC-001 electrodes. (f) PL emission spectra of BOC and BOC-001.
photocatalytic activity of BOC-001 than BOC and exclude the photosensitization effect of RhB, we have carried out the photocatalytic NO removal experiment in the gas phase over BOC and BOC-001. As shown in Figure 4b, no degradation of NO was observed without the addition of photocatalyst. In contrast, the NO gas can be effectively removed in the presence of BOC and BOC-001 photocatalysts, and the removal ratios are 33% and 45% for BOC and BOC-001, respectively. This further validated the more efficient photocatalytic reactivity of Bi2O2CO3 with dominating {001} exposing facets. As a typical 2D nanostructure, along the [001] direction, the effective separation of photoexcited charge carriers can be induced by the self-induced internal electric field. Thus, the more efficient separation of electron−hole pairs occurred in BOC-001 compared to the case of BOC. It was also verified by the photocurrent generation, which can be employed to illuminate the interfacial charge transfer dynamics.33,34 As shown in Figure
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ASSOCIATED CONTENT
S Supporting Information *
SEM images of BOC and BOC-001, photodegradation curves of RhB over BOC and BOC-001, repeated photodegradation curves, and XRD patterns before and after irradiation of BOC001 sample. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*(H.H.) E-mail:
[email protected]. *(Y.Z.) E-mail:
[email protected]. 536
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Crystal Growth & Design Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundations of China (Grant No. 51302251, 51322213, 51172245), the Fundamental Research Funds for the Central Universities (2652013052), the National High Technology Research and Development Program (863 Program 2012AA06A109) of China.
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