Materials Research Bulletin, Vol. 34, No. 7, pp. 1135–1142, 1999 Copyright © 1999 Elsevier Science Ltd Printed in the USA. All rights reserved 0025-5408/99/$–see front matter
HYDROTHERMAL SYNTHESIS OF Pb2SnO4 Mingmei Wu1*, Xiuling Li1, Guoping Shen1, Dian He1, Aihong Huang1, Yuji Luo1, Shouhua Feng2, and Ruren Xu2 1 Department of Chemistry, Zhongshan University, Guangzhou 510275, P.R. China 2 Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130023, P.R. China (Refereed) (Received July 6, 1998; Accepted September 1, 1998)
ABSTRACT Fine, yellowish powder of Pb2SnO4 with elongated crystalline morphology was synthesized hydrothermally in an alkaline system. To prepare a pure product, the mineralizer NaOH should be at a suitable concentration. In comparison with the solid-state method, the hydrothermal synthesis of Pb2SnO4 requires much milder reaction conditions. For some metastable phases such as PbO, 3PbO–H2O and PbSnO3–2H2O transformed into Pb2SnO4 with increase in hydrothermal reaction temperature and reaction time. Pb2SnO4 is considered to be the thermodynamically stable phase in the alkaline hydrothermal system of PbO and SnO2. Reaction temperature and reaction time influenced the size and aspect ratio of the elongated particles. In this work, no PbSnO3 phase was found under any hydrothermal conditions, even though the molar ratio of the starting material was 1:1. © 1999 Elsevier Science Ltd
KEYWORDS: A. inorganic compounds, A. oxides, B. chemical synthesis INTRODUCTION Hydrothermal synthesis has been used to prepare phase-pure, high-quality ceramic (oxides) materials. It requires milder reaction conditions, compared with the high-temperature solidstate method . Many ABO3-typed complex oxides, such as MTiO3 [2–5] and MZrO3 [6,7]
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(M ⫽ Ca, Sr, Ba, and Pb) with perovskite structures, have been prepared by using the hydrothermal method. The thermodynamic modeling based on systems of MO and TiO2 (or ZrO2) has shown that ABO3-type compounds with perovskite structure are stable phases under alkaline conditions, in agreement with reported experimental results [2– 8]. No thermodynamic modeling on a system of MO and SnO2 under hydrothermal condition, however, has been proposed so far, although the hydrothermal synthesis of BaSnO3 has been investigated . Although PbSnO3 crystallizes in a tetragonal system (JCPDS, PDF 4-550), while BaSnO3 crystallizes in a cubic system, both of these ABO3-type compounds belong to perovskite structures. Hydrothermal preparation of PbSnO3 has not been described so far. As far as we are aware, no studies have been reported on direct hydrothermal synthesis of complex oxides with K2NiF4 structure, the so-called layered perovskite structure. Pb2SnO4, a complex oxide of K2NiF4 structure with Pbam space group (No. 55)  has been prepared conventionally by high-temperature solid-state reaction and used as sensor material, pigment, and catalyst [10,11]. In this article, the hydrothermal synthesis of Pb2SnO4 from lead acetate and sodium stannate is presented for the first time. EXPERIMENTAL Lead acetate (Pb(CH3COO)2–3H2O, analytical grade, Guangzhou Chemical Reagent Factory) and sodium stannate (Na2SnO3–3H2O, analytical grade, Beijing Shuanghuan Shiji) were each dissolved into water to prepare a lead and a tin solution, respectively. The two types of solution were mixed under magnetic stirring, and a white, milk-like mixture formed. Sodium hydroxide (NaOH, analytical grade, Guanghua Chemical Reagent Factory of Shantou City) solution, used as the mineralizer, was added dropwise into the lead–tin mixture. The final mixture of 40 ml with a total concentration of [Pb] and [Sn] fixed at 0.5 mol L⫺1 was charged into a 50 ml PTFE-lined stainless steel autoclave for hydrothermal reaction. After hydrothermal reaction at a desired temperature for a period of time was completed, the precipitate was filtered, washed with distilled water, and kept in a desiccator for drying at ambient temperature. The product was identified using X-ray diffraction (XRD) performed on a Shimadzu XD-3A diffractometer using graphite monochromator filtered Cu K␣ radiation at 30 kV and 20 mA, by scanning at 2°2 min⫺1. The morphology of the product was observed using a Hitachi S-520 scanning electron microscope. RESULTS AND DISCUSSION NaOH Concentrations on Products. The mineralizer concentration generally plays a fundamental role in hydrothermal reaction. It has been shown  that TiO2 phase emerges under acidic conditions, while BaTiO3 is the product under basic conditions in the TiO2/BaO system. The situation is similar for other TiO2/MO systems and ZrO2/MO systems [1–9,12, 13]. Here the hydrothermal reactions were conducted on a lead acetate (PbAc2) concentration of 0.34 mol L⫺1 and a sodium stannate (Na2SnO3) concentration of 0.16 mol L⫺1 at 210°C for 24 h. XRD patterns of the hydrothermal reaction products deduced from the feedstocks with different NaOH concentrations are shown in Figure 1. According to Figure 1a, the hydrothermal product from PbAc2 and Na2SnO3 was mainly PbSnO3–2H2O with a small amount of SnO2. Pb2SnO4 with a trace of PbSnO3–2H2O was synthesized with a NaOH
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FIG. 1 XRD patterns of products formed from lead acetate and sodium stannate at different concentrations of NaOH: (a) 0.0 mol L⫺1, (b) 1.0 mol L⫺1, and (c) 2.0 mol L⫺1. concentration of 1.0 mol L⫺1 (Fig. 1b). Pure Pb2SnO4 product was obtained with a NaOH concentration of 2.0 mol L⫺1 (Fig. 1c). The Miller indices are shown in Figure 1c in light of JCPDS data (PDF 24-589).
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FIG. 2 Scanning electron micrographs of the Pb2SnO4 particles hydrothermally synthesized (a) at 100°C for 24 h, (b) at 210° for 24 h, and (c) at 210°C for 3 days.
The SEM image of the Pb2SnO4 powder is depicted in Figure 2b. The Pb2SnO4 particles showed rodlike morphology, according to Figure 2b. The particle sizes were about 13 by 3 m. When the NaOH concentration was 3.0 mol L⫺1, a trace of PbSnO3–2H2O coexisted with Pb2SnO4 in the product, the same as when the NaOH concentration was 1.0 mol L⫺1. These results indicate that a NaOH concentration of 2.0 mol L⫺1 could be considered as the preferable concentration to synthesize the pure phase of Pb2SnO4. Reaction Temperatures and Time on the Products. Reaction temperature is another influential thermodynamic factor on products of hydrothermal synthesis. It has a remarkable effect not only on the morphologies, but also on phases. Here the hydrothermal reaction between 0.34 mol L⫺1 PbAc2 and 0.16 mol L⫺1 Na2SnO3 in 2.0 mol L⫺1 NaOH at different temperatures was considered. When the reaction was carried out at 50°C for 24 h, the product identified by XRD method was chiefly attributed to 3PbO–H2O with some PbSnO3–2H2O (Fig. 3a). The target product, Pb2SnO4, could be obtained when the reaction temperature was increased to 100°C (Fig. 3b). The corresponding particle sizes were about 6 by 3 m (Fig. 2a). In this work, the pure phase of Pb2SnO4 could be synthesized at temperatures higher than 100°C for 24 h. When the reaction was carried out at 210°C for only 2 h, the product mainly consisted of red PbO with a small amount of PbSnO3–2H2O and Pb2SnO4 (Fig. 4a). The Pb2SnO4 diffraction peaks in the XRD pattern became more and more apparent as the hydrothermal reaction time became longer. Pb2SnO4 phase emerged after 4 h (Fig. 4b). According to the XRD pattern in Figure 4c, pure phase of Pb2SnO4 was synthesized in 6 h. These results indicate that other phases would gradually transform into Pb2SnO4 with increase in the reaction temperature and/or reaction time. These results also imply that Pb2SnO4 was the stable phase under such an alkaline condition in the system of SnO2 and PbO. The pure phase
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FIG. 3 XRD patterns of products synthesized at (a) 50°C and (b) 100°C for 1 day.
of Pb2SnO4 could be isolated from the hydrothermal precipitation at higher temperatures and/or for longer reaction duration, such as at 210°C for 3 days (Fig. 2c). The hydrothermal product of Pb2SnO4 powder was yellowish. It is observed from Figure 2a and b that the Pb2SnO4 particles became longer and longer with increase in the reaction temperature. The rodlike particles of Pb2SnO4 hydrothermally treated at 210°C for 3 days were about 20 by 3 m (Fig. 2c). The aspect ratio of the particles shown in Figure 2c (3 days) was greater than that of the particles shown in Figure 2b (24 h). These phenomena implied that particles of Pb2SnO4 tended to become longer with increasing reaction temperature and reaction time. It is interesting that the relative intensities of (220) and (310) diffraction peaks to the (211) diffraction peak gradually increased with increase in reaction time (Fig. 5) and/or reaction temperatures. The intensity of the (211) peak in each XRD pattern was primarily defined as 100. The relative intensities of (220) and (310) for the product formed at 100°C in 24 h were about 16 and 42, respectively (Fig. 3b). The intensities shown in Figure 1b for the product
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FIG. 4 XRD patterns of products synthesized at 210°C for (a) 2 h, (b) 4 h, and (c) 6 h. formed at 210°C in 24 h became 96 and 109, respectively. These results revealed that the fine crystals of Pb2SnO4 demonstrated their oriented crystallization habit along the  direction, and some of their longer faces in Figure 2 should be attributed to the (220) and (310) peaks.
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FIG. 5 Reaction time and relative diffraction intensities of (220) and (310) peaks compared to that of (211) peaks of those products formed at 210°C.
The Pb:Sn Ratios in the Feedstocks. For hydrothermal synthesis, various products were obtained from feedstocks with different component ratios. The products generated from mixtures with a Pb:Sn ratio of 9:1–1:9 in 2.0 mol L⫺1 NaOH heated at 210°C for 72 h are listed in Table 1. According to Table 1, the products shifted from PbO to Pb2SnO4 and finally to SnO2, with decreasing Pb:Sn ratio. At a NaOH concentration of 2.0 mol L⫺1 and Pb:Sn ratios less than 7:3, no PbSnO3–2H2O phase was found (Table 1). At a NaOH concentration of 3.0 mol L⫺1, however, the PbSnO3–2H2O phase formed at Pb:Sn ratios of 5:5 and 3:7. It appears that PbSnO3–2H2O formed more easily in a more alkaline aqueous solution of NaOH. On the other hand, when the NaOH concentration decreased to 1.0 mol L⫺1, the phase PbSnO3–2H2O was also yielded, as shown in Figure 1b. The concentration of NaOH should be at an appropriate value, to obtain a phase-pure product of Pb2SnO4. It should be noted that PbAc2 could be dissolved in a higher concentrated NaOH solution, such as 5.0 mol L⫺1. It is well known that the Pb component will finally exist as [HPbO2]⫺  or [Pb(OH)4]2⫺  and the Sn as [Sn(OH)6]2⫺ in a more alkaline aqueous solution. CONCLUSION Pb2SnO4 powder was prepared under much milder conditions, compared with the solid-state method. That the ABO3-type compound PbSnO3 was not found in this work means that the thermodynamic factor of the SnO2/PbO system is much different than that of the TiO2/PbO or ZrO2/PbO system. In fact, Kutty and Vivekanadan  reported that BaSnO3 was not
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TABLE 1 Hydrothermal Products Formed from Raw Materials with Different Pb:Sn Ratios Products Pb:Sn ratio 9:1 7:3 5:5 3:7 1:9
red and yellow PbO Pb2SnO4 Pb2SnO4 SnO2 SnO2
Pb2SnO4 and PbSnO3–2H2O SnO2 Pb2SnO4 Pb2SnO4
directly formed in a tightly closed vessel. The features of other SnO2/MO systems are not similar to those of other TiO2/MO or ZrO2/MO systems either. The investigation results on other SnO2/MO systems will be submitted in the near future. ACKNOWLEDGMENTS This work was supported by National Natural Foundation of China (No. 59702008 and No. 29871035) and Provincial Natural Foundation of Guangdong (No. 950015 and No. 97015) and the Key Lab of ISPC of Jilin University. REFERENCES 1.
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