DOI: 10.1021/cg900486r
Seedless Synthesis and Thermal Decomposition of Single Crystalline Zinc Hydroxystannate Cubes
2009, Vol. 9 4456–4460
Gregory Wrobel,† Martin Piech,‡ Sameh Dardona,‡ Yong Ding,§ and Pu-Xian Gao*,† †
Department of Chemical, Materials and Biomolecular Engineering & Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269-3136, ‡Department of Physical Sciences, United Technologies Research Center, East Hartford, Connecticut 06108, and §School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245 Received May 3, 2009; Revised Manuscript Received June 18, 2009
ABSTRACT: Single crystalline zinc hydroxystannate [ZnSn(OH)6] micro- and nanocubes have been successfully grown on pure tin substrates via a seedless hydrothermal synthesis method. Each ZnSn(OH)6 cube is enclosed by six equivalent {001} crystal planes. Cube size and aerial density were adjusted by controlling the reaction time and addition of diaminopropane (DAP) reagent. The hexamethylenetetramine- and DAP-assisted etching of the oxidized tin metal surface is found to play an important role in the nucleation and growth of the zinc hydroxystanate cubes. Synthesis at higher zinc nitrate concentrations as well as secondary growth resulted in the formation of ZnO nanorods in addition to the cubes. In situ scanning electron microscopy and transmission electron microscopy, thermal gravimetric analysis, and differential scanning calorimetry have been utilized to investigate the ZnSn(OH)6 thermal decomposition process. The appearance of endothermic peak near ∼540 K, attributed to the ZnSn(OH)6 decomposition, was correlated with morphology changes induced via resistive thermal annealing and localized electron beam heating.
1. Introduction The inorganic tin-containing compounds, mainly zinc stannate (ZnSnO3) and zinc hydroxystannate [ZnSn(OH)6, ZHS], are well-documented as nontoxic flame-retardant and smokesuppressant additives for a wide range of plastics, rubbers, paints, and other polymeric materials.1-4 They have been the most promising alternative synergists to the toxic antimony trioxide (Sb2O3) in halogen-containing polymer formulations.3 Their nontoxic nature combined with smoke-suppressant properties superior to Sb2O3 contributed to an increasing market share following development in the mid-1980s.5 Furthermore, ZHS thermal decomposition products including amorphous ZnSnO3 and crystalline SnO2 and Zn2SnO4 are used in lithium ion battery anodes and gas/vapor sensors.6-8 The preparation of metal hydroxystannate materials has been accomplished using mainly coprecipitation of hydroxides.9 Other approaches included ion exchange and sonochemical and hydrothermal methods.10,11 The resultant materials were micrometer-sized powders comprised of aggregated nanosized crystallites. In contrast, free-standing ZHS nanocubes were prepared hydrothermally using a mixture of ZnCl2, Na2SnO3 3 3H2O, and polyvinylpyrrolidone.6 Similarly, the hydrothermal method was used to synthesize ZnSnO3 and ZHS microcube films on zinc oxide-seeded indium tin oxide substrates.12 However, the ZHS generated using these methods either lack the robustness for making conformal and continuous coatings or require further additives and processing to control particle dispersion stability. Therefore, the discovery of robust, low-cost fabrication methods would greatly contribute to the wider commercial use of these nontoxic, multifunctional materials including ZHS, ZnSnO3, SnO2, and Zn2SnO4. *To whom correspondence should be addressed. E-mail: puxian.
[email protected]. pubs.acs.org/crystal
Published on Web 08/07/2009
While the synthesis of ZHS powders is well-understood, the thermal decomposition process has not been thoughtfully researched to date. A rough decomposition temperature region has been estimated at ∼200-380 °C.13 However, studies relating the decomposition process to structural and morphological transformations within ZHS crystallite are not available. This information would help widen the applicability of these materials in the area of flame retardation and smoke suppression. Herein, we report a robust, large-scale, and low-cost method of preparing size-tunable, density-controlled, and conformal ZHS cube films on Sn metal substrates via a seedless, low-temperature (60 °C) hydrothermal synthesis. Using an array of microscopy and spectroscopy techniques, we present a systematic thermal decomposition study of the fabricated ZHS micro- and nanocubes. 2. Experimental Section All chemicals were used as received from the manufacturer without further purification. Three types of tin substrates were used as follows: polycrystalline tin rods (99.95%, Alfa Aeasar), single crystal tin [orientation (100), (110), and (111); Princeton Scientific], and tin foil (99.9985%, Alfa Aesar). The tin rod was machined into pucks with their top faces ground and polished using a standard procedure. The single crystal and foil were cut to desired sizes with a sharp razor blade. Prior to synthesis, all substrates were ultrasonically cleaned in hexanes (Chromasolv grade, Aldrich) for 3 min, dried with compressed argon gas (UHP, Matheson), and plasma cleaned (Harrick Plasma) in Ar gas for 2 min. The substrates were mounted upside down (pucks and single crystal samples) or suspended vertically (foils) in sealed, 125 mL PTFE bottles containing 50 mL of 20 mM Zn(NO3)2 3 6H2O (ZNH, >99.0%, Fluka), 20 mM hexamethylenetetramine (HMT; 99%þ, Aldrich), and 0 or 140 mM 1,3-diaminopropane (DAP; 98%, Alfa Aesar) aqueous solution. Following incubation in an oven at 60 °C for 2-18 h, the substrates were removed from the growth solution, rinsed with deionized water, and dried with Ar gas. r 2009 American Chemical Society
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Figure 1. ZHS cube films on a polished, polycrystalline Sn substrate: (a) SEM image of micrometer-sized cubes grown for 17 h from the ZNH, HMT, and DAP solution. (b) SEM image of nanosized cubes grown without DAP. (c) SEM image of interpenetrated cubes grown for ∼24 h from the ZNH, HMT, and DAP solution. (d) XRD spectrum of a dense cube film; the miller-indexed peaks correspond to the cubic-structured ZnSn(OH)6, while the stars (*) and crosses (þ) identify R-Sn and SnO peaks, respectively. (e) EDXS spectrum of a 10 μm cube. The dissolved oxygen content and pH of growth solutions were probed with a dissolved oxygen meter (Hanna HI98186) and pH meter (Cole Parmer Chemcadet), respectively. The amount of dissolved tin used to calculate the metal etch rate was estimated using inductively coupled plasma atomic emission spectrometer (IPAAES; Spectro Gensesis) equipped with a concentric gas nebulizer and Ar torch. The characterization of chemical composition, morphology, and structure was carried out using JEOL 6335F field emission scanning electron microscope (FESEM) equipped with an energy dispersive X-ray spectrometer (EDXS), a Philip E420 transmission electron microscope (TEM), a JEOL 4000X TEM, and an X-ray diffractometer (XRD; BRUKER AXS D5005, Cu KR radiation, λ = 1.540598 A˚). The oxide layer thickness on Sn substrates was estimated using sputter-assisted X-ray photoelectron spectroscopy (XPS; PHI 5000 Versa Probe). Thermogravimetric analysis (TGA) was carried out using the TA Instruments Q500 with Ar purge gas (UHP, Matheson) and platinum sample pans. Differential scanning calorimetry (DSC) was performed using a TA Instruments Q20 with aluminum sample pans.
3. Results and Discussion Figure 1a shows a typical SEM image of a ZHS cube film on a polished, polycrystalline Sn puck substrate following a 17 h growth in zinc nitrate hexahydrate (ZNH), HMT, and DAP solution. The microcubes are ∼15 μm on the side and exhibit an aerial density of ∼700 mm-2. Exclusion of DAP from the growth solution under the same synthesis conditions resulted in significantly denser (∼55,800 mm-2) ZHS films comprised of much smaller, ∼350 nm nanocubes (Figure 1b). Similar results have been obtained with unpolished tin foils. Allowing the reaction to proceed for ∼24 h yielded a continuous ZHS film comprised of overlapping and interpenetrating cubes (Figure 1c). Complementary characterization methods were utilized to determine the structure and composition of the grown
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Figure 2. Typical TEM images of a representative ZHS cube before (a) and after (b) tilting of the cube by ∼27° with respect to the left edge ([010] axis); (c and d) electron diffraction patterns corresponding to panels a and b, respectively.
ZHS films. A typical XRD spectrum of a dense cube film is illustrated in Figure 1d with a 2θ angle ranging from 20 to 80°. The peaks of R-Sn (I41/amd, JCPDS 4-673) and SnO (P4/nmm, JCPDS 6-395) have been identified and labeled with stars (*) and crosses (þ), respectively. The remaining peaks in the figure are identified and indexed as the cubic ZnSn(OH)6 phase (Pn3m, a = 7.80 A˚, JCPDS 01-073-2384). The EDXS analysis of representative 10 μm cube is depicted in Figure 1e. The cube was found to have the following composition: O ∼ 76.8 ( 2.3 atomic (at.) %, Zn ∼ 12.6 ( 0.5 at. %, and Sn ∼ 10.6 ( 2.7 at. %. Within the experimental error, the atomic ratio of O:Zn:Sn is close to the 6:1:1, matching the stochiometric ZnSn(OH)6. Figures 2a shows a bright field TEM image of a micrometer-sized ZHS cube. Its select area electron diffraction pattern in Figure 2c can be indexed with an incident electron beam along the [001] direction according to the cubic-phased ZnSn(OH)6 structure. Furthermore, combining the diffraction pattern and image, we can conclude that the ZHS cube is enclosed by six equivalent {001} crystal planes. After the sample was tilted around the [010] axis by ∼27°, the projected image of the cube is displayed in Figure 2b. The corresponding diffraction pattern in Figure 2d can be indexed with incident electron beam along the [102] direction. Such a tilting angle is the right angle between the [001] and the [102] orientations of the cubic-phased ZnSn(OH)6 structure. Figure 1a,b clearly shows that varying the ligand type (i.e., HMT vs DAP) can be utilized to control the cube size and aerial density. Similarly, changing the reaction time was found to alter the ZHS cube dimensionality. Specifically, a shorter synthesis time (∼2 h) in the presence of DAP yielded smaller, ∼1.5 μm-sized cubes at a higher density of ∼28000 mm-2. Prolonged reaction for ∼24 h, on the other hand, resulted in the formation of a dense ZHS cubes layer, with cubes overlapping and interpenetrating each other (Figure 1c). At that point, the cubes did not increase in size due to the Sn surface passivation by the continuous ZHS cube film.
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Figure 3. SEM images of ZHS cubes grown on single crystal tin with (a) (100), (b) (110), and (c) (111) orientations, respectively. Typical SEM images of mixed ZHS cube and ZnO rod films on polycrystalline tin substrates grown using (d) a one-step hydrothermal synthesis with 150 mM ZNH and (e) a two-step process.
As evidenced by Figure 1a-c, ZHS cubes grew in random orientations on the polycrystalline Sn substrate. Similarly, when grown on single crystal (100), (110), and (111) surfaces under identical reaction conditions, the cubes did not exhibit any preferred orientation (Figure 3). The three single crystal samples nonetheless differed with respect to the cube size and aerial density of the grown ZHS films. In the case of (100) Sn sample (Figure 3a), the ZHS cubes have an average size of ∼5-10 μm with a density of ∼1340 mm-2. Growth on the (110) plane yielded relatively small, ∼2-3 μm cubes with a density of ∼3000 mm-2 (Figure 3b), while the (111) planes contained ∼3-5 μm cubes at a density of ∼3000 mm-2 (Figure 3c). Therefore, the ZHS cube growth was the most favorable on polycrystalline substrates followed by (100), (111), and (110) single crystal surfaces. Because the only source of Sn4þ ions available for the ZHS growth is from the tin metal oxidation/dissolution, the same trend is expected for metal surface etching rate. Hence, HMT- and DAPassisted Sn etch rate increases in the order polycrystalline > (100) > (111) > (110) substrates. More detailed investigation is necessary, however, to better quantify this effect. Further examination of Figure 3c reveals a concurrent growth of micrometer-sized zinc oxide (ZnO) nanorods on the (111) single crystal Sn surface, which has been identified using EDXS analysis and TEM characterization. This is not surprising, as ZnO rod growth was reported for a variety of different substrates under similar hydrothermal conditions.14-18 In fact, raising the ZNH concentration in the growth solution from 20 to 150 mM promoted the formation of ZnO nanorods on polycrystalline Sn samples (Figure 3d). In this case, however, the nanorods appeared to grow simultaneously with neighboring ZHS cubes forming an interpenetrating network. To explore whether the ZnO nanorods nucleate on the Sn substrate or ZHS cubes, a two-step growth experiment was performed. In the first step, sparse ZHS films similar to those illustrated in Figure 1a were synthesized on the polycrystalline Sn substrate. Following deionized water rinse, these samples were then incubated in a fresh growth solution. The resultant ZHS/ZnO films shown in Figure 3e indicate the preferential growth of ZnO nanorods from
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the Sn substrate. This can be attributed to the positively charged Sn surface, which favors surface binding of negative hydroxyl ions and dissolved zinc hydroxide complexes. The availability of free and complexed hydroxyl groups near the surface enables facile ZnO nanorod nucleation and growth through zinc hydroxide formation and precipitation upon thermal activation.19 In contrast, ZHS remains negatively charged, offering a lower concentration of surface hydroxyl ions, which decreases the probability of precipitating zinc hydroxide nuclei and the subsequent ZnO nanorod growth. As can be seen in Figure 3e, upon successful nucleation from the ZHS cube facets, the ZnO nanorods grow in sparse, flowerlike bundles. The formation of similar ZnO structures has been reported earlier in a zinc suflate, HMT, and ethylenediamine solutions.20 While a similar nucleation process is likely to occur on both the Sn and the ZHS surfaces, competitive growth from densely packed nuclei results in termination of the ZnO crystal side branches, yielding more aligned uniform nanorods on the Sn surface as compared to welldeveloped flowerlike structures on the ZHS.21 The competitive growth of ZHS cubes and ZnO nanorods observed here may enable an intricate morphology control and warrants a more detailed investigation. The ZHS cubes on tin substrates are most likely formed by heterogeneous nucleation followed by a “dissolution-precipitation” growth process.12,22,23 This process starts from the HMT decomposition to ammonia and formaldehyde.24 Subsequent protonation of ammonia provides a low concentration of hydroxide anions (pH ∼ 6.7). When DAP is added to these solutions, the hydroxide concentration increases significantly (pH ∼ 11.0) due to the greater basicity of the DAP molecules present at a higher concentration. The next step is hydroxide-facilitated dissolution of the thin oxide layer generally present on metallic tin.25 Using sputter-assisted XPS, the oxide layer thickness was determined to be ∼32 and ∼6 nm for the untreated and polished sample surfaces, respectively. The oxide layer dissolution yields stannite and stannate ions, with the former further oxidized to the latter in the presence of dissolved oxygen.26 Devoid of protective oxide coating, the exposed Sn metal oxidizes and dissolves, while the resulting Sn4þ ions most likely complex with ammonia and DAP. SnðsÞ ¼ Sn4þ ðaqÞþ4e -
ð1Þ
Sn4þ ðaqÞþxOH - ðaqÞþyNH3 ðaqÞþzDAPðaqÞ ¼ SnðOHÞx ðNH3 Þy ðDAPÞz ðx -4Þ - ðaqÞ
ð2Þ
Metal oxidation described by reaction 1 is facilitated by dissolved oxygen and possible reduction of nitrate to nitrite.27 Meanwhile, the extent of metal dissolution is strongly dependent on the concentration of hydroxide, ammonia, and DAP ligands available for complexation as expressed by reaction 2. The most severe surface etching with a dissolution rate of ∼ 60 nm/h occurred when DAP was present in the growth solution. On the contrary, tin samples incubated in solutions containing HMT alone dissolved at ∼0.12 nm/h. This difference can be attributed to the dual effect of increased solution pH and higher stability of the Sn(OH)x(DAP)z(x-4)- complexes as compared with Sn(OH)x(x-4)-. Subsequent formation of ZHS from solvated Sn4þ and Zn2þ species in alkaline solutions has been documented elsewhere.28 In the absence of other ligands, OH- complexes with
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Figure 4. Thermally induced dehydration and weight loss of ZHS cube films during heating: The solid curve corresponds to the DSC trace, while the dashed curve corresponds to TGA data.
Sn4þ and Zn2þ to form Sn(OH)62- and Zn(OH)42-, respectively. Under hydrothermal conditions, ZHS nuclei form quickly, followed by growth to yield cube-shaped crystals. In the presence of ammonia and DAP, the process is likely similar, albeit with altered nucleation and growth kinetics as reported previously for the hydrothermal synthesis of zinc oxide crystals.24 On the basis of the above data and discussion, the proposed growth mechanism of ZHS cubes on tin surfaces is as follows: (1) Hydroxide and ligand initiated oxide dissolution followed by metal oxidation and dissolution, (2) heterogeneous nucleation at most favorable defect sites brought about by local supersaturation of Sn(OH)62- and Zn(OH)42- precursors as well as their ammonia/DAP complexes near the substrate surface, (3) growth of ZHS cubes from the formed nuclei, and (4) passivation of the metal surface by a continuous ZHS film. More detailed studies aimed specifically at clarifying the mechanism responsible for the ZHS cube film formation on Sn substrates are currently ongoing and will be reported in the future. In addition to exploring facile one-step synthesis of single crystal ZHS cubes, their thermal decomposition properties have also been examined. Both TGA and DSC have been utilized to this end. The DSC was performed over a 323673 K range at a heating rate of 15 K/min, while the TGA was carried out at a rate of 20 K/min from 323 to 1073 K. Figure 4 illustrates the results of this investigation. The sharp peak centered at ∼506 K in the DSC plot corresponds to the melting point of Sn at 504 K. It is due to the sample preparation method whereby ZHS crystals were collected together with a thin film of the metal substrate (i.e., ZHS cubes were physically scraped off the metal surface). The endothermal dehydration of ZHS into amorphous ZnSnO3 is observed as a broad peak at ∼540 K:13 ZnSnðOHÞ6 ðsÞ f ZnSnO3 ðsÞþ3H2 OðgÞ
ð3Þ
This transition temperature is ∼37 K higher than that reported previously12,29 and can be attributed to sample contamination by SnO and/or ZnO impurities. Correspondingly, the TGA data exhibit a sharp decrease in mass starting at ∼475 K and ending at ∼535 K due to the loss of water upon
Figure 5. Effect of thermal annealing on the ZHS crystallite morphology: (a) high-resolution SEM image of as-grown cubes and (b) following thermal annealing at 400 °C for 2.8 h. Effect of electron beam irradiation on ZHS cubes: (c) SEM image of a twinned intersection of four single crystal cubes following 15 keV electron beam irradiation for ∼10 min, (d) TEM image of a microcube with its top portion subjected to a focused 400 keV electron beam for ∼2 min, (e) a diffusive electron diffraction ring pattern corresponding to the top portion of the cube in panel d, and (f) a single crystal [001] electron diffraction pattern corresponding to the lower part of the cube in panel d.
ZHS decomposition. The total weight loss at 723 K was calculated to be ∼6.1%, as compared with the expected 18.1% based on the stoichiometry of eq 3.12 This discrepancy is due to the presence of impurities including Sn, SnO, and ZnO, which did not contribute to the weight loss. Figure 5 illustrates the induced morphology changes in ZHS cube films due to thermal annealing. Figure 5a,b are typical high-resolution SEM images acquired before and after treatment at 400 °C for 2.8 h, respectively. Annealing results in significant roughening of the crystal faces along with the appearance of nanosized asperities. Similar morphology changes have been observed upon exposure of the ZHS cubes to a focused 15 keV electron beam, as illustrated in Figure 5c. The changes in morphology were more pronounced in the case of the higher energy electron irradiation (400 keV), as shown in Figure 5d. The exposed cube region became irregularly shaped with concurrent phase transformation to amorphous structure as confirmed by a diffusive ring electron diffraction pattern (Figure 5e). On the other hand, the unexposed region of the cube shows a single crystalline diffraction pattern (Figure 5f). Therefore, it is possible that electron beam irradiation resulted in ZHS dehydration into amorphous ZnSnO3 due to localized heating.13 4. Conclusion In summary, single crystalline ZHS cube films on pure tin substrates were synthesized in a one-step hydrothermal process. The cube size and aerial density were affected by the choice of reagent concentration, growth temperature,
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and time. Reaction at higher zinc nitrate concentration resulted in the formation of zinc oxide nanorods in addition to the ZHS cubes, both nucleated from the underlying tin substrate. Sequential synthesis, on the other hand, demonstrated ZnO nanorod nucleation from the ZHS crystallites, although less preferentially as compared with the tin surface. DAP- and hexamethylene-tetraamine-assisted etching of oxidized Sn surfaces was found to play an important role in the nucleation and growth of zinc hydroxystanate cubes on these substrates. DSC and TGA confirmed endothermal dehydration of ZHS into amorphous ZnSnO3 at ∼540 K. Thermally induced dehydration process resulting from either direct heating or electron beam irradiation led to similar ZHS crystal morphology changes. Specifically, the crystal faces were roughened along with the appearance of nanosized asperities. Nonetheless, more detailed studies of ZHS decomposition characteristics are needed to further the understanding of this material’s fire-retardant and smoke-suppressant properties. Acknowledgment. We acknowledge the financial support from the UConn New Faculty start-up funds and the United Technologies Research Center. We also thank Dr. Joseph Mantese and Dr. Donald Potter for their kind help and stimulating discussions.
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