Article pubs.acs.org/Langmuir
Fully Understanding the Photochemical Properties of Bi2O2(CO3)1−xSx Nanosheets Chao Chang, Fei Teng,* and Zailun Liu Jiangsu Engineering and Technology Research Center of Environmental Cleaning Materials (ECM), Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control (AEMPC), Jiangsu Joint Laboratory of Atmospheric Pollution Control (APC), Collaborative Innovation Center of Atmospheric Environment and Equipment Technology (AEET), School of Environmental Science and Engineering, University of Information Science & Technology, 219 Ningliu Road, Nanjing 210044, China S Supporting Information *
ABSTRACT: The photochemical properties of crystal facets with obviously distinct atomic and geometric structures have been studied widely to date. However, little work has been performed for two or more facets with very similar atomic and geometric structures. Herein, we mainly report the photochemical properties of {001} and {100} facets of Bi2O2(CO3)1−xSx with very similar atomic and geometric structures. The simulation and experimental results show that over {100} facets, sulfur prefers to substitute for the carbonate anion, leading to the formation of an interesting serpentine internal electric field that greatly inhibits the charge recombining of electrons and holes, which has rarely been demonstrated; over {001} facets, however, sulfur preferentially adsorbs in oxygen vacancies, which greatly reduces the surface energy of {001} facets, leading to 80% of the high-energy {001} facets exposed. As a result, the photochemical properties of nanosheets have been greatly improved. This study could help us to fully understand the photochemical properties of semiconductors. widely to date.8,9 However, little work has been performed for two or more facets with very similar atomic and geometric structure. Herein, we select layered Bi2O2CO310 as a model material with the aim of revealing the effects of bulk and surface structures on the photochemical properties. Recently, Cao et al.11 have synthesized Bi2O2CO3 hierarchical nanoflowers by a solvothermal method, and the nanoflowers exhibited high photocatalytic activity under UV light. Wei et al.12 have prepared Ag3PO4−Bi2O2CO3 with high visible light photocatalytic activity. Hu et al.13 have synthesized flowerlike βBi2O3/Bi2O2CO3 microspheres using Bi2O2CO3 as the selfsacrificing precursor, which exhibited high photocatalytic activity under visible light irradiation for the degradation of ophenylphenol. For Bi2O2CO3, its {001} and {100} facets have very similar atomic and geometric structure, except for a slightly different spatial orientation of O in CO32− ((Figure S2 in SI, Figure 1). However, the photoelectric property difference in {001} and {100} facets is still unknown. Typically, sulfur is introduced in the synthesis of crystal on the basis of the following facts: First, because both sulfur and oxygen are chalcogen elements with the same number of outermost electrons (NOE), we expect that sulfur could be easy to be introduced into Bi2O2CO3. Second, the Bi−O−S system has
1. INTRODUCTION In recent years, increasing attention has been paid to the surface properties of crystalline material by materials scientists, especially by catalysis researchers. Normally, the different facets of materials possess different atomic, electronic, and geometric structures, which endow them with distinct properties.1−4 Thus, numerous studies have focusing on the surface control of materials. For example, anatase TiO2 with a high percentage of {001} facets exposed is reported to have better photocatalytic activity than that of low-energy {101};5 bismuth-containing compounds with a unique layered structure,6 which consists of the staggered [Bi2O2] double slabs and anionic slabs (Figure S1, Supporting Information (SI)), have also been reported recently. The {001} facets of BiOCl exhibit higher degradation activity than does methyl orange (MO) under ultraviolet light irradiation, and the {010} facets exhibit high activity under visible light irradiation as a result of their quite different geometric structures.7 For photoelectric materials, however, the bulk structure of materials is also another key factor that cannot been ignored. It is desirable to fully understand the bulk and surface properties of materials. Typically, the low charge separation efficiency is one of the most fatal limiting factors, which is still a challenge for us. Thus, it is not enough to efficiently improve the charge separation efficiency merely by surface control.7 The photochemical properties of crystal facets with obviously distinct atomic and geometric structures have been studied © XXXX American Chemical Society
Received: January 14, 2016 Revised: March 15, 2016
A
DOI: 10.1021/acs.langmuir.6b00149 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
Figure 1. Atomic structures of (001) and (100) facets after introducing sulfur: (a) A001-Bi, adsorption of sulfur ion on the (001) facet terminated with Bi; (b) A001-O, adsorption of sulfur ion on the (001) facet terminated with O; (c) A100-Bi, adsorption of sulfur ion on the (100) facet terminated with Bi; (d) A100-O, adsorption of sulfur ion on the (100) facet terminated with O; (e) S001-Bi, substitution of sulfur ion for CO32− on the (001) facet terminated with Bi; (f) S001−O, substitution of sulfur ion for CO32− on the (001) facet terminated with O; (g) S100-Bi, substitution of sulfur ion for CO32− on the (100) facet terminated with Bi; (h) S100-O, substitution of sulfur ion for CO32− on the (100) facet terminated with Bi; (i) *A001-Bi, adsorption of sulfur ion on the oxygen vacancy of the (001) facet terminated with Bi.
2. EXPERIMENTAL SECTION
been reported to present obvious superconducting properties.14−19 Thus, we expect that the introduction of sulfur could lead to a high electric conductivity that favors charge transformation, similar to that in the Bi−O−S system. It is interesting that over {001} facets ending with oxygen (001-O), sulfur prefer to adsorb on an oxygen vacancy site, greatly reducing its surface energy. Over {100} facets ending with bismuth (100-Bi), sulfur prefers to substitute for CO32−, resulting in the formation of a serpentine internal electric field that favors charge separation. As a result, an improved photocatalytic property is obtained by both bulk and surface structures. This could aid the overall understanding of the photochemical properties of semiconductors.
2.1. Sample Preparation. All reagents were of analytical grade, purchased from Beijing Chemical Reagents Industrial Company of China, and used without further purification. To prepare the U1 sample, 0.2 mmol of Bi(NO3)3·5H2O was placed into an 80 mL round-bottomed flask that contained 50 mL of distilled water. The mixture was continuously stirred for 30 min. At the end, the pH of the system was determined to be 1 due to the strong hydrolysis of Bi3+. Next, 10 mL of a 0.02 mol L−1 urea solution was added to the solution above, and then the pH of the system was adjusted to 9 by adding concentrated ammonia dropwise. The white suspension was transferred to a Teflon-lined stainless steel autoclave and then treated hydrothermally at 120 °C for 24 h. After being cooled naturally to room temperature, the solids were collected by B
DOI: 10.1021/acs.langmuir.6b00149 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
number of atoms in the supercell, μ is the chemical potential, and A is the surface area. The factor of 1/2 corresponds to two equivalent surfaces in the slab models. The chemical potential μslab Bi2O2CO3 of a condensed and stoichiometric phase of Bi2O2CO3 is written as a sum of the chemical potential of each species within the crystal. At 0 K and constant pressure, the chemical potential of the surface is in bulk equilibrium with that of the bulk (μslab Bi2O2CO3 = EBi2O2CO3). Thus, the surface energy can be expressed as follows:
centrifugation and washed with deionized water five times. Finally, the sample was dried in a desiccator for 5 h at 60 °C. For U2−U5 samples, the same preparation conditions were used, except that different amounts of sulfourea were added. The asprepared samples at 0.066, 0.1, 0.2, and 0.3 mmol of sulfourea added are designated as U2, U3, U4, and U5, respectively. 2.2. Characterization. The crystal structures of the samples were determined by an X-ray powder polycrystalline diffractometer (Rigaku D/max-2550VB) using graphite-monochromatized Cu Kα radiation (λ = 0.154 nm), operating at 40 kV and 50 mA. The XRD patterns were obtained in the range of 10−70° (2θ) at a scanning rate of 5° min−1. The morphologies of the samples were characterized on a scanning electron microscope (SEM, Hitachi SU-1510) at an acceleration voltage of 15 keV. The powders were coated with a 5-nm-thick gold layer before observation. The fine structures of the samples were determined by high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100F) equipped with an electron diffraction (ED) attachment with an acceleration voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were made on a VG ESCALAB MKII XPS system with a Mg Kα source and a charge neutralizer. All of the binding energies were referenced to the C 1s peak at 284.8 eV of surface adventitious carbon. UV−vis diffuse reflectance spectra of the samples were obtained using a UV−vis spectrophotometer (UV-2550, Shimadzu, Japan). BaSO4 was used as a reflectance standard in a UV−vis diffuse reflectance experiment. Nitrogen sorption isotherms were performed at 77 K and
