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Ultraviolet-Emitting Bi2O2.33 Nanosheets Prepared by Electrolytic Corrosion of Metal Bi Guoli Fang, G. Chen,* Jinqiang Liu, and Xiong Wang Engineering Research Center of Materials BehaVior and Design, Ministry of Education, Joint Laboratory of Nanostructured Materials and Technology, Nanjing UniVersity of Science and Technology, Nanjing, 210094, China ReceiVed: October 1, 2009; ReVised Manuscript ReceiVed: October 28, 2009
Nonstoichiometric Bi2O2.33 nanosheets have been synthesized via electrolytic corrosion of metal Bi. The novel route is neither template- nor catalyst-assisted, and is more simple than commonly used methods. Bismuth complex compound was used to increase bismuth concentration, and to enhance the diffusion rate of bismuth ions. The thicknesses of the as-obtained Bi2O2.33 nanosheets were 10-20 nm. One strong UV emission was detected in the high crystal quality Bi2O2.33 nanosheets at room temperature. The broad PL emission peak can be attributed to the near band-edge emission. 1. Introduction Currently, wide band gap semiconductor materials are very interesting because of their potential application in blue/ ultraviolet (UV) optoelectronic devices.1,2 GaN-based materials have been applied successfully for UV emission and detection.3,4 However, the greatest shortcoming of the lasers based on III-V nitrides is that the nitrides can be oxidized in air. The wide band gap semiconductor oxides may be a good candidate to replace nitrides, because of their higher chemical and thermal stability.5-7 In particular, the size effect is favorable to the intensity of UV emission properties of wide band gap semiconductor oxides.8-11 More efforts have been focused on the fabrication of their nanostructures and the optimization of their optical performances in the past few years. It has been proved that the low dimensional ZnO (Eg ) 3.37 eV) nanostructures, such as nanowires,10,12 nanorods,13 nanowhiskers,14 and nanosheets, are better short-wavelength semiconductor laser materials than III-V nitrides. The one-dimensional In2O3 (Eg ) 3.6 eV) nanowires have been prepared via sol-gel approach, which exhibit UV emission related with the singly ionized oxygen vacancies in In2O3 nanowires at room temperature.15 The binary oxides of bismuth are important semiconductor materials, which have been widely used in photocatalysis, optical coating, photovoltaic cells, gas sensors, and so on.16-19 The band gap energy of a Bi-O system (including BiO, Bi2O3, Bi2O2.33, Bi2O2.75) can change from 2.0 to 3.96 eV, while the band gap of BiO, R-Bi2O3, β-Bi2O3, and δ-Bi2O3 is 3.31, 2.85, 2.6, and 3.5 eV, respectively.20-23 However, most of the work on binary bismuth oxides concentrates on Bi2O3,24 especially in Bi2O3 nanostructures, such as nanoparticle, nanotube, and nanosheets.16,20,25 Nonstoichiometric Bi2O2.33 attracts our attention, and it has been previously observed only as thin films at room temperature.19,22,23,26-30 It also commonly appears as impurities in the films of bismuth oxides19,22,23,27 and bismuth oxide-based materials,29 although pure nonstoichiometric Bi2O2.33 thin film has been deposited on substrates by chemical vapor deposition.28 The optical properties of Bi2O2.33 and its fundamental dependence on the microstructure are still not well understood. In * Corresponding author. Fax: + 86 25 84315159. E-mail: gchen@ mail.njust.edu.cn.
this study, we attempt to synthesize a substrate-free Bi2O2.33 nanostructure, and understand its photoluminescence (PL) properties. 2. Experimental Section Inspired by electrolytic polishing of metals and alloys, we utilize electrochemical corrosion to synthesize Bi2O2.33 nanosheets. The anode material was metal bismuth foil with size of 2 cm in length and 1 cm in width, which could be employed as the bismuth source. A nickel foil was used as the cathode material. In a typical procedure, 5.85 g of NaCl, 2.12 g of disodium ethylenediamine tetraacetate (EDTA), and 20 mL of NaOH solution (5 mol/L) were added to a 200 mL beaker with 80 mL distilled water to form electrolyte. The electrochemical synthesis process was potentiostatically performed in a direct current electrolysis system at an applied voltage of 10 V at room temperature. After electrolyzing for 2 h under stirring, large quantities of yellow products were obtained. The products were rinsed with distilled water and ethanol several times, and then dried at 80 °C for 3 h in air. The chemical composition of the samples was determined by using inductively coupled plasma (ICP, IRIS intrepid II XSP, Thermo). X-ray diffraction (XRD) patterns of the samples were recorded on an X-ray diffractometer (Rigaku D/Max 2200PC) equipped with a graphite monochromator and Cu KR radiation (λ ) 1.5406 Å) in the 2θ range of 10-80°, while the voltage and electric current were held at 40 kV and 30 mA, respectively. X-ray photoelectron spectra (XPS) were recorded on a PHI5300 ESCA spectrometer (Perkin-Elmer) to characterize the surfaces of the nanosheets with its energy analyzer working in the pass energy mode at 71.55 eV, and the Al KR line was used as the excitation source. The binding energy reference was taken at 284.7 eV for the C1s peak arising from surface hydrocarbons. The morphology and microstructure of the products were determined by scanning electron microscopy (SEM, JEOL JSM-6380LA), transmission electron microscopy (TEM), and high-resolution TEM (HR-TEM, JEOL JEM-2100) with an accelerating voltage of 200 kV. PL was detected with a RF-5301 fluorescence spectrophotometer at room temperature. 3. Results and Discussion Figure 1a shows the XRD pattern of the collected products taken at room temperature. The most diffraction peaks could
10.1021/jp909431w 2010 American Chemical Society Published on Web 11/13/2009
Bi2O2.33 Nanosheets Prepared by Bi Electrolytic Corrosion
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Figure 1. (a) XRD pattern of the product: the diffraction peaks corresponding to the crystal face of the body-centered tetragonal Bi2O2.33 phase, and the impurity peak at 44.3° is observed. TEM (b) and SEM (c) images of the high crystal quality Bi2O2.33 nanosheets dried at 80 °C. (d) HR-TEM image of a typical portion of the Bi2O2.33 nanosheets shows a well-crystallized material without defects, and its FFT image (inset) indicates the single-crystal nature of the nanosheet with the surface orientation of (001).
be indexed to the body-centered tetragonal Bi2O2.33 phase (JCPDS file No.27-0051) with lattice parameters of a ) 3.857 Å and c ) 35.14 Å.26 The other diffraction peak at 44.3° detected is unknown. Because the bismuth content of 91.89% measured by ICP is in full agreement with the theoretic content 91.81% of Bi2O2.33, the dominant phase should be nonstoichiometric Bi2O2.33 phase. The resulting nonstoichiometric Bi2O2.33 might be attributed to the unique electrolysis system. According to the XPS spectra for the surface of the obtained nanosheets (Figure S1, Supporting Information), the main elements are Bi and O, so the trace impurity phase corresponding to diffraction angle 44.3° may be one of bismuth oxides. Further studies should be required to ascertain the impurity phase. It is also noted that the XRD peaks are obviously broadened, implying a small crystallite size characteristic of the product. As illustrated by Figure 1b, TEM demonstrate that well-defined Bi2O2.33 nanosheets are observed. The thicknesses of the as-synthesized nanosheets are measured to be in the range 10-20 nm, as evidenced by the SEM image (Figure 1c). Figure 1d is the HRTEM image of a nanosheet, taken with the incident electron beam perpendicular to the in-plane face of the nanosheet. The two different interplanar distances of 0.2726 and 0.1928 nm have been observed, which are in good agreement with the lattice interplanar distances of (110) and (020) planes of Bi2O2.33, respectively. The fast Fourier transform (FFT) image (the inset of Figure 1d) indicates the single crystal nature of the nanosheet with the surface orientation of (001). Figure S2 shows an HRTEM image of a Bi2O2.33 nanosheet (Figure S3) naturally dried at room temperature. Obviously, the Bi2O2.33 naturally dried is not well-crystallized. Its lattice is heavily distorted with a large number of defects. In comparison with the microstructure of Bi2O2.33 nanosheets dried at 80 °C, as shown in Figure 1d, the higher temperature drying can decrease impurities and structure defects.
To understand the formation procedure of the Bi2O2.33 nanosheets, analysis on the effects of electrolyte has been performed in detail. Figure 2 shows the XRD patterns of the products obtained at different pH values of the electrolyte. It can be seen that the final products are sensitive to the pH value of the electrolyte. A pure tetragonal BiClO phase revealed by the XRD pattern (Figure 2a, JCPDS file No.06-0249), was indeed synthesized with a pH value of 10. Raising pH value leads to an increase of the Bi2O2.33 content. The prerequisite to produce a pure Bi2O2.33 nanosheet is a larger pH value than 14. Therefore, the alkaline condition of pH g 14 must be satisfied for the electrochemical corrosion synthesis of pure Bi2O2.33. Furthermore, we found that the nanosheets could not be formed without the addition of EDTA. Thus, a series of electrochemical corrosion experiments have been performed by varying the amounts of EDTA. The reaction conditions (including synthesis temperature, applied voltage, electrode materials, drying temperature, and other reactants concentration) were kept unchanged during the experiments. Pure Bi2O2.33 could be synthesized with different amounts of EDTA, and the morphologies of the products exhibit a gradual change with increasing EDTA content (Figure 3). Consequently, the Bi2O2.33 in the form of nanosheets appears in the product synthesized with 3 mmol of EDTA (Figure 3b), and more than 5 mmol of the added EDTA is necessary to form uniform Bi2O2.33 nanosheets (Figure 3c). However, at a 0.015 mol/L concentration of EDTA, the reaction stops after a few minutes, and the products cover the surface of the Bi corrosion electrode. Additionally, the role of NaCl is found to only affect the corrosion rate, when the pH value of the electrolyte is above 14. Therefore, EDTA is a crucial factor in the synthesis of Bi2O2.33 nanosheets during the process of electrolytic corrosion for metal Bi. In the process of electrolytic corrosion, bismuth ions, which come from the Bi corrosion electrode, could undergo complex-
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Figure 2. XRD patterns of the products electrochemical corroded in different pH conditions: (a) the product is pure BiOCl, when pH ) 10; (b) the mixture of BiOCl and Bi2O2.33 is synthesized, when pH ) 12; (c) nonstoichiometric Bi2O2.33 is the predominant phase, when pH ) 14.
ation with EDTA. The complexation between bismuth ions and EDTA effectively increases the bismuth concentration and improves the diffusion rate of bismuth ions in electrolyte, which can restrain the Bi corrosion electrode covered by Bi2O2.33 and synthesize desired product continuously. Furthermore, the increase of bismuth concentration could raise the nucleation rate of Bi2O2.33 and decrease the size of products. The uneven corrosion of the Bi electrode and the lower diffusion rate of bismuth ions compared to the synthetic reaction rate of Bi2O2.33 result in local supersaturation of bismuth ion concentration, which are the main factors to form a large number of defects in Bi2O2.33 nanosheets. However, the higher temperature drying can decrease the defects and enhance the crystal quality of Bi2O2.33 nanosheets. Figure 4 shows the PL spectra of Bi2O2.33 nanosheets at room temperature excited at 305 nm. One intense UV emission was observed (Figure 4a) in the high crystal quality Bi2O2.33 nanosheets dried at 80 °C. The dominant peak is the P1 line at 360 nm corresponding to 3.44 eV in phonon energy. Additional weak features include a faint shoulder at 351 nm (P2, 3.53 eV) and an almost imperceptible emission at around 374 nm (P). In order to well understand the PL of the nanosheets, the excitation spectra are also examined. Two excitation peaks centered at 337 and 341 nm were detected with an emission wavelength of 374 nm (P), while only one peak (centered at 305 or 319 nm) was dectected with an emission wavelength of 360 (P1) or 351 nm (P2) (not shown), respectively. The inset of Figure 4 shows that the P line should consist of two features at 373 nm (P3, 3.32 eV) and 377 nm (P4, 3.29 eV), which is excited at 337 and 341 nm, respectively. Two weak steps of P5 (394 nm, 3.15
Figure 3. SEM images of the products synthesized with different amounts of EDTA: (a) 1.5 mmol, (b) 3 mmol, (c) 5 mmol.
eV) and P6 (401 nm, 3.09 eV) also are detected in the inset of Figure 4. The same emission peaks have not been observed in the Bi2O2.33 nanosheets dried at room temperature with a large number of defects (Figure 4b). The intense UV emission of high crystal quality Bi2O2.33 nanosheets at room temperature could be ascribed to the reduction of impurities and defects in the process of relatively higher temperature drying. In general, emission spectra can be divided into two broad categories: the near band-edge (NBE) emissions and deep-level (DL) emissions. The NBE-to-DL emission intensity ratio can increase with the reduction of impurity and structure defects, which results in detectable UV emission at room temperature.8,15 Moreover, the quantum confinement effect of two-dimensional (2D) nanosheets is favorable to excitonic emission. The excitonic emission peak position and strength are closely related to the excitons bound to neutral donors or acceptors.8,31,32 The UV emission of the high crystal quality Bi2O2.33 nanosheets can be
Bi2O2.33 Nanosheets Prepared by Bi Electrolytic Corrosion
J. Phys. Chem. C, Vol. 114, No. 2, 2010 867 Supporting Information Available: Figures show that the Bi2O2.33 nanosheets naturally dried at room temperature have a large number of defects. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 4. Room-temperature PL spectra of Bi2O2.33 nanosheets excited at 305 nm: (a) high crystal quality Bi2O2.33 nanosheets; (b) Bi2O2.33 nanosheets with a large number of defects. The inset shows detailed features of the line P, which is excited at 337 and 341 nm, respectively.
attributed to NBE emission at room temperature. Because the similar excitation spectra detected with an emission wavelength of P3 and P4 also have two main excitation peaks located at 337 and 341 nm, the P3 and P4 lines maybe assigned to heavy and light holes free excitonic radiative recombination emission.31,32 Additionally, the lines of P1, P2, P5, and P6 maybe mainly induced by nonstoichiometric structure and bound excitonic emission.32 4. Conclusions In summary, electrochemical corrosion has been successfully applied for the synthesis of body-centered tetragonal Bi2O2.33 nanosheets with thicknesses of 10-20 nm. This technique is neither template- nor catalyst-assisted, and is more simple than commonly used methods. It is very feasible to produce nanosized materials in industry. The high crystal quality Bi2O2.33 nanosheets dried at 80 °C exhibit extraordinary UV emission properties at room temperature. The PL emission peaks can be attributed to the NBE emission. The results demonstrate that Bi2O2.33 ought to be considered as a candidate material for UV light emitters. Acknowledgment. This work was supported by the National Natural Sciences Foundation of China (50431030 and 50871054) and the Outstanding Scholar Supporting Program of NUST. The authors thank Dr. Miguel Fuentes-Cabrera and Xingqiu Chen, from Oak Ridge National Laboratory, Oak Ridge, TN, for valuable suggestions and proofreading the manuscript.
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