Growth of PbSO4 Single-Crystal Nanorods and Optical Properties by a Microemulsion Approach Jun-Hua Xiang, Shu-Hong Yu,* Xin Geng, Bian-Hua Liu, and Yang Xu Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Department of Materials Science and Engineering, Department of Chemistry, University of Science and Technology of China, Hefei 230026, People’s Republic of China Received October 31, 2004;
CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 3 1157-1161
Revised Manuscript Received December 11, 2004
ABSTRACT: Single-crystal PbSO4 (anglesite) nanorods with different diameter and aspect ratios have been synthesized by a microemulsion technique in a SDS/hexane/hexanol/water microemulsion. The effect of concentration of reactants, concentration of surfactants, and temperature on the crystallization of PbSO4 nanorods was investigated. Room-temperature photoluminescence spectra of well-defined uniform PbSO4 nanorods give two strong emission bands at 410 and 435 nm, and the weak emission at 340 nm almost cannot be detected at an excitation energy of 4.6-5.17 eV. As the excitation energy decreases, the 380 nm emission becomes intensified. When the excitation energy is larger than 5 eV in the range of 5.39-5.90 eV, only the blue band at 380 nm was observed. 1. Introduction Lead sulfate, which is known as a natural mineral called anglesite, is an important scintillator material,1,2 electrode material for storage batteries,3,4 and also a pigment for decoration.5 Many procedures are reported for the preparation of PbSO4, e.g., from Pb,6,7 PbO,8,9 PbO2,10 or solutions containing Pb2+.11 Hydrothermal growth of PbSO4 single crystals has been reported by Kikuta et al.12 Platelike PbSO4 nanocrystals have been prepared by a microemulsion approach using a quaternary water-in-oil (w/o) microemulsion system made of water, Triton X-100, cyclohexane, and n-pentanol.13 Recently, lead sulfate crystals were precipitated in a solution of polyethyleneimine (PEI) using a double-jet crystallizer.14 To our best knowledge, a soft synthesis of single-crystal PbSO4 nanorods has not been achieved previously. Recent advances in utilizing soft templates such as micelles and emulsions to control the shape of inorganic nanocrystals have been reviewed by Pileni.15 It has been demonstrated that reverse micelles and microemulsions are ideal nanoreactors for the synthesis of diverse anisotropic nanoparticles and are widely used for the preparation of various inorganic crystals such as noble metal nanorods,16-19 Ag nanodisks,20 and semiconductor nano-objects such as CdS, CdSe nanotubes/nanowires,21 and CdS nanotriangles.22 In addition, this approach can also be used for the synthesis of V2O5 nanowires/ nanorods,23 cubic-shaped KMnF3 nanocrystallites,24 CaSO4,25 BaCO3,26 and CaCO3 nanowires,27 BaWO4 nanorods/nanowires,28 high-order structures such as BaCrO4 chains and filaments,29 and BaSO4 filaments and cones.30,31 Recently, the combination of microemulsion techniques with a hydrothermal/solvothermal process has also been explored for the preparation of nanocrystals such as ZnS nanocrystallites,32 CdS nanoclusters and nanorods,33 Cu2S flakes and nanodisks,34 PbS nanow* To whom correspondence should be
[email protected]. Fax: 0086 551 3603040.
addressed.
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ires,35 NiS nanosheets,36 TiO2 nanocrystallites,37 SnO2 nanorods,38 BaF2 whiskers,39 and molecular sieve fibers.40 In this paper, PbSO4 single-crystal nanorods have been synthesized in a sodium dodecyl sulfate (SDS)/ hexane/hexanol/water microemulsion system by either a room temperature or an elevated temperature route. The influence of the concentrations of reactants and sulfate sources and the temperature on the shape and crystallization of PbSO4 nanocrystallites was investigated. The optical properties of these single-crystal nanorods were studied. 2. Experimental Section 2.1. Preparation of PbSO4 Nanorods Using Na2SO4 and Pb(NO3)2 as Reactants. SDS (0.01 mol) used as a surfactant was dissolved in a mixed solution of 30 mL of hexane and 9 mL of hexanol, which was used as a cosurfactant. Pb(NO3)2 aqueous solution (7.2 mL) (0.02 M) was added to the mixture under stirring. The molar ratio of water and surfactant (w) was 40, and the concentration of SDS was 0.22 M based on overall volume of the microemulsion. After stirring for 5 min, 0.04 g of Na2SO4 was added. The resulting microemulsion was aged at room temperature for 7 days. For hydrothermal treatment experiments, the solution was transferred into a 55 mL capacity stainless Teflon-lined autoclave after stirring for 30 min at room temperature. The autoclave was sealed, maintained at 120 °C for 12 h, and then allowed to cool to room temperature naturally. The obtained precipitates were centrifuged, washed several times using distilled water and absolute ethanol, and dried in a vacuum at 60 °C for 6 h. 2.2. Preparation of PbSO4 Nanorods Using CS2 and Pb(NO3)2 as Reactants. In a typical experiment, 0.02 mol SDS was dissolved in a mixed solution of 40 mL of hexane and 12 mL of hexanol. Then 7.2 mL of 0.1 M Pb(NO3)2 aqueous solution was added to the mixture under stirring, so that the molar ratio of water and surfactant (w) was 20 and the concentration of SDS was about 0.33 M based on overall volume of the microemulsion. After stirring for 5 min, 22 µL of CS2 was added. The pH value of this system was 5. The microemulsion was transferred into a 70 mL capacity stainless Teflon-lined autoclave after being stirred for 30 min. The autoclave was sealed, maintained at 150 °C for 15 h, and then allowed to cool to room temperature naturally. The obtained precipitate was centrifuged and washed using distilled water
10.1021/cg049630t CCC: $30.25 © 2005 American Chemical Society Published on Web 01/28/2005
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Figure 1. XRD patterns of PbSO4 nanocrystals. (a) 0.01 mol SDS/30 mL hexane/9 mL hexanol/7.2 mL 0.1 M Pb(NO3)2/0.2 g Na2SO4, room temperature, 7 days. (b) 0.01 mol SDS/30 mL hexane/9 mL hexanol/7.2 mL 0.02 M Pb(NO3)2/0.04 g Na2SO4, 120 °C, 12 h. (c) 0.01 mol SDS/30 mL hexane/9 mL hexanol/ 7.2 mL 0.02 M Pb(NO3)2/0.04 g Na2SO4, room temperature, 7 days. (d) 0.02 mol SDS/40 mL hexane/12 mL hexanol/7.2 mL 0.1 Pb(NO3)2/22 µL CS2, 150 °C, 15 h. (9) PbS. and absolute ethanol several times and finally dried in a vacuum at 60 °C for 6 h. 2.3. Characterization. The products were characterized by X-ray diffraction (XRD) pattern, recorded on a MAC Science Co. Ltd. MXP 18 AHF X-ray diffractometer with monochromatized Cu KR radiation (λ ) 1.54056 Å). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) analysis was performed on a Hitachi (Tokyo, Japan) H-800 transmission electron microscope at an accelerating voltage of 200 kV and a JEOL-2010 high-resolution transmission electron microscope, also at 200 kV, respectively. Photoluminescence (PL) spectra were acquired on a Fluorolog3-TAU-P at room temperature.
3. Results and Discussion PbSO4 can be crystallized either at room temperature or at elevated temperature using Na2SO4 as the sulfate source (Figure 1a-c). Pure PbSO4 product was obtained
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by using 0.1 M Pb(NO3)2 aqueous solution and Na2SO4 as reactants after aging for 1 week at room temperature as shown in Figure 1a. All diffraction peaks shown in Figure 1a can be indexed as a primitive orthorhombic lattice of PbSO4 (anglesite) with cell parameters a ) 6.97, b ) 8.47, and c ) 5.39 Å, which are in good agreement with those reported in the literature (JCPDS card number 36-1461). As an alternative way, another sulfate source such as CS2 was used instead of the sulfate anions for the formation of PbSO4 when higher temperature was applied using a combined hydrothermal-microemulsion approach as shown in Figure 1d. PbSO4 phase can be produced together with PbS phase if CS2 is used as a sulfate source by a hydrothermal process at elevated temperature. The TEM image in Figure 2a showed the product was made of nanorods with a diameter of about 40 nm and length up to 1 µm. The nanorods tend to align in a parallel fashion. A typical nanorod is shown in Figure 2b. The electron diffraction pattern in Figure 2c taken along the 〈100〉 zone axis indicated that the nanorod was perfect single crystal and preferentially grows along the c axis. Hydrothermal treatment of a resulting microemulsion at 120 °C for 12 h resulted in the formation of nanorods with a diameter of 250-350 nm and a length of 2-5 µm as shown in Figure 3a. The products were also pure PbSO4 single crystals (Figure 1b). Applying temperature in this case is not favorable for the formation of uniform nanorods which could be due to the disturbance of the rod-shaped reverse micelles under boiling conditions. When the concentrations of Pb(NO3)2 and Na2SO4 increase, the product is composed of conglomerated nanoparticles with diverse morphologies such as rods and irregular shapes, as shown in Figure 3b. The result suggested the concentration of reactants has a significant effect on the shape of the products. For effective control over the shape of the nanorods, the concentration of the reactants in the microemulsion system is a key parameter. The effect of the sulfate source on the formation of PbSO4 crystals was investigated. The dominant PbSO4 phase was formed and only a small amount of PbS phase was detected as shown in Figure 1d if CS2 was
Figure 2. (a,b) TEM images of PbSO4 nanorods synthesized at room temperature. 0.01 mol SDS/30 mL hexane/9 mL hexanol/ 7.2 mL 0.02 M Pb(NO3)2/0.04 g Na2SO4, room temperature, 7 days. (c) Electron diffraction pattern taken on a single PbSO4 nanorod shown in image b. The scale bar for images a and b is, respectively, 500 and 125 nm.
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Figure 5. The XRD pattern of Pb(DS)2 obtained in SDS/ hexane/hexanol/Pb(NO3)2 microemulsion at 120 °C for 12 h. Figure 3. TEM images of PbSO4 nanocrystals obtained by a hydrothermal microemulsion route. (a) 0.01 mol SDS/30 mL hexane/9 mL hexanol/7.2 mL 0.02 M Pb(NO3)2/0.04 g Na2SO4, 120 °C, 12 h. (b) 0.01 mol SDS/30 mL hexane/9 mL hexanol/ 7.2 mL 0.1 M Pb(NO3)2/0.2 g Na2SO4, room temperature, 7 days. The scale bar is, respectively, 1.25 µm and 500 nm.
cos φ ) (h1h2/a2 + k1k2/b2 + l1l2/c2)/(h12/a2 + k12/b2 + l12/c2)1/2(h22/a2 + k22/b2 + l22/c2)1/2 where cell parameters for the orthorhombic PbSO4 are a ) 6.957 Å, b ) 8.476 Å, and c ) 5.398 Å (JCPDS card number 36-1461). The calculated angle between (002) and (101) is 37.8°, which is consistent with that measured value. In addition, the angle between (100) and (101) is 52.3°, which fits the measured value very well too. These results confirmed that the nanorods grow preferentially along the c axis, which is consistent with the result of the electron diffraction pattern shown in Figure 4c. The reaction mechanism of prismatic PbSO4 nanorods using CS2 as the sulfate source was proposed. When CS2 was used as the sulfate source, the product was a mixture of prismatic-like PbSO4 nanorods and a small amount of PbS after hydrothermal treatment. If no CS2 was added to the system and other conditions were not changed, the obtained products were a lamellar Pb(DS)2 as identified by the XRD pattern shown in Figure 5, which is consistent with that reported previously.41 The possible chemical reactions involved in the hydrothermal treatment process can be formulated as the following:
Pb2+ + 2NaDS f Pb(DS)2 + 2Na+
(1)
CS2 + 4O2 + 2H2O f 2SO42- + CO2 + 4H+ (2)
Figure 4. (a,b) TEM images of PbSO4 nanorods. 0.02 mol SDS/40 mL hexane/12 mL hexanol/7.2 mL 0.1 M Pb(NO3)2/22 µL CS2, 150 °C, 15 h. (c) Electron diffraction pattern of PbSO4 nanorods shown in image b. (d) A HRTEM image of a typical PbSO4 nanorod. The scale bar for images a and b is, respectively, 250 and 125 nm.
used as the sulfate source after hydrothermal treatment at 150 °C for 15 h. TEM images (Figure 4) showed the obtained PbSO4 nanocrystals were prismatic-like nanorods with a diameter of 200-250 nm and a length of 600-1250 nm, which are stacked in a parallel fashion. The electron diffraction pattern in Figure 4c taken along the 〈010〉 zone axis indicated that the nanorods are single crystalline and the nanorods grow preferentially along the c axis. The lattice resolved HRTEM images are shown in Figure 4d. The lattice spacing of 4.36 Å corresponds to that for (101) planes. The angle between (002) and (101) can be calculated from the following formula for orthorhombic structure:
Pb(DS)2 + SO42- f PbSO4V + 2DS-
(3)
CS2 + H2O f H2S + CO2
(4)
Pb(DS)2 + H2S f PbSV + 2DS- + 2H+
(5)
The two competitive reactions eqs 3 and 5 happened in such system. Because the amount of CS2 was not superfluous and the microemulsion was weakly acidic, this condition was not favorable for the formation of PbS crystals. Therefore, PbSO4 phase was the dominant product. The adsorption of surfactant onto the specific faces of PbSO4 crystals will slow the growth rates of the (100) and (010) faces, which resulted in preferential growth along 〈001〉. The room-temperature photoluminescence properties of PbSO4 nanorods obtained under different conditions were investigated. The precipitate obtained at room temperature in the surfactant-free solution system gives very weak emission bands centered at 340 and 430 nm
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Figure 6. The room-temperature luminescence spectra of PbSO4 nanorods obtained under different conditions recorded at an excitation energy of 4.6 eV (270 nm). (a) 0.02 M Pb(NO3)2 and 0.02 M Na2SO4, room temperature, 7 days. (b) 0.01 mol SDS/30 mL hexane/9 mL hexanol/7.2 mL 0.1 M Pb(NO3)2/0.2 g Na2SO4, room temperature, 7 days. (c) 0.01 mol SDS/30 mL hexane/9 mL hexanol/7.2 mL 0.02 M Pb(NO3)2/0.04 g Na2SO4, room temperature, 7 days. (d) 0.02 mol SDS/40 mL hexane/12 mL hexanol/7.2 mL 0.1 M Pb(NO3)2/22 µL CS2, 150 °C, 15 h.
Figure 7. Room-temperature excitation energy dependent emission spectra of PbSO4 nanorods obtained on the PbSO4 nanorods as shown in Figure 2a. Curves a-d were obtained for the same sample using the excitation wavelengths 210, 220, 230, and 240 nm, respectively.
(Figure 6a). The presence of a 340 nm peak (UV band) is consistent with that reported previously which was attributed to radiative dissociation of an anion exciton.1a However, the peak at 360 nm1b was not observed in the present samples but a broad emission peak located at 435 nm was observed, which is similar to that measured at low temperature for large single crystals.2b,c The photoluminescence spectrum of the PbSO4 nanocrystals mineralized in microemulsion at room temperature (Figure 3b) is shown in Figure 6b, in which the emission band at 340 nm is already very weak but the emission band at 435 nm is much intensified. The welldefined uniform PbSO4 nanorods shown in Figure 2a give two strong emission bands at 410 and 435 nm (Figure 6c), and the weak emission at 340 nm cannot be detected. In comparison, the prismatic PbSO4 nanorods obtained by the hydrothermal approach give a more
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intensified emission band centered at 435 nm and the 340 nm emission band is totally missing (Figure 6d). The presence of broad emission bands in the range of 400-500 nm was not reported previously, and their origins need to be further investigated. The relationship of the excitation energy with the luminescence emission peak position was studied for the PbSO4 nanorods (the sample shown in Figure 2a). As the excitation energies are in the range of 5.39-5.90 eV, only a broad blue band at 380 nm was observed (curves a-c in Figure 7). Interestingly, the obvious enhancement of the 380 nm emission intensity was observed as the excitation energy decreased. Such dependence between the excitation energy and 340 nm emission intensity was not found previously. When the excitation energy was 5.17 eV, the strong emission with two overlapping components at 380 and 434 nm was observed (curve d in Figure 7). The low-temperature luminescence spectra need to be further studied. 4. Conclusions In summary, PbSO4 single-crystal nanorods with round or prismatic-like surfaces were prepared using either Na2SO4 or CS2 as sulfate sources which reacted with Pb(NO3)2 in a SDS/hexane/hexanol/water microemulsion. The concentration of reactants, the kind of reactants, the concentrations of surfactants, and the temperature have a significant influence on the growth of PbSO4 nanorods. Room-temperature photoluminescence spectra of well-defined uniform PbSO4 nanorods give two strong emission bands at 410 and 435 nm, and the weak emission at 340 nm almost cannot be detected at an excitation energy of 4.6-5.17 eV. When the excitation energy is larger than 5 eV in the range of 5.39-5.90 eV, only a blue band at 380 nm was observed and the obvious enhancement of the 380 nm emission intensity was found as the excitation energy decreases in this range. Such relationship of the excitation energy with the emission intensity and the emission peak shift is still not clear. The luminescence properties of the single-crystal PbSO4 nanorods could be of interest for engineering of the scintillation properties and fundamental study. Acknowledgment. This work was supported by special funding support from the Centurial Program of the Chinese Academy of Sciences, the Distinguished Youth Fund, the Distinguished Team (Grants No. 20325104 and No. 20321101), Contract No. 50372065 from the Natural Science Foundation of China, and the Scientific Research Foundation for Returned Overseas Chinese Scholars, State Education Ministry. References (1) (a) Blasse, G. Chem. Phys. Lett. 1975, 35, 299. (b) Tarashchan, A. N. Kiev: Naukova Dumka 1978. (c) Firk, F. W. K. Nucl. Instrum. Methods Phys. Res. Sect. A 1990, 297, 532. (d) Mose, W. W.; Derenzo, S. E.; Shlichta, P. J. IEEE Trans. Nucl. Sci. 1992, 39, 1190. (e) Zhang, J. G.; Lund, J. C.; Cirignano, L.; Shah, K. S.; Squillante, M. R. IEEE Trans. Nucl. Sci. 1994, 41, 669. (2) (a) Kamenskikh, I. A.; Kirm, M.; Kolobanov, V. N.; Mikhailin, V. V.; Orekhanov, P. A.; Shpinkov, I. N.; Spassky, D. A.; Vasil’ev, A. N.; Zimmerer, G. Radiat. Eff. Defects Solids 2001, 154, 307. (b) Kamenskikh, I. A.; Kirm, M.; Kolobanov, V. N.; Mikhailin, V. V.; Orekhanov, P. A.; Shpinkov, I. N.;
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