Room Temperature Synthesis of Hierarchical SrCO3 Architectures by

transmission electron microscopy. The SrCO3 architectures are in dandelion-like and flower-like morphologies with diameters of. 5–10 μm and compose...
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Room Temperature Synthesis of Hierarchical SrCO3 Architectures by a Surfactant-Free Aqueous Solution Route Wen-Shou Wang, Liang Zhen,* Cheng-Yan Xu, Li Yang, and Wen-Zhu Shao School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150001, People’s Republic of China

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 5 1734–1740

ReceiVed June 21, 2007; ReVised Manuscript ReceiVed December 26, 2007

ABSTRACT: Novel SrCO3 architectures were prepared by a facile aqueous solution route at room temperature using SrCl2, Na2CrO4, and NaOH as the starting reaction reagents and distilled water as the solvent. The synthesized products were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, selected-area electron diffraction and high-resolution transmission electron microscopy. The SrCO3 architectures are in dandelion-like and flower-like morphologies with diameters of 5–10 µm and composed of numerous well-aligned single-crystalline nanorods of 20–40 nm in diameter and 1–2 µm in length. It is found that the morphology of the final products is strongly dependent on the experimental parameters, such as the concentration of aqueous NaOH solution and the reaction temperature. Various controlled synthetic experiments indicate that the growth process of SrCO3 architectures involves the growth of single-crystalline SrCrO4 nanowires, the formation of CO32- ions through the reaction between CO2 gas from air and aqueous NaOH solution, and finally the formation of SrCO3 architectures through the reaction between SrCrO4 and CO32- ions controlled by the difference of the solubility product. In addition, a “rod to dumbbell to sphere” mechanism is proposed for the formation of the SrCO3 architectures. Introduction One-dimensional (1D) nanostructures have attracted great attention because of their remarkable properties and potential applications in nanoscale circuits and optoelectronic, electrochemical, and electromechanical devices during the past few decades.1,2 Many recent efforts have been focused on the hierarchical assembly of 1D nanoscale building blocks, such as nanowires, nanorods, nanotubes, nanobelts, and so forth, into ordered complex hierarchical architectures or superstructures, which is not only a crucial step for the realization of “bottomup” techniques toward future nanodevices but also offers opportunities to explore their novel collective optical, mechanical, magnetic, and electronic properties.3–5 Novel applications might emerge if complex hierarchical architectures could be successfully synthesized in a controllable manner. The hierarchical structures are expected to play an important role in fabricating the next generation of microelectronic and optoelectronic devices because they can be used as both building units and interconnections.6 There have been extensive studies exploring various approaches to the synthesis of hierarchical architectures, and most of the methods developed so far can be categorized mainly as either chemical vapor deposition methods or solution-phase chemical routes. The chemical vapor deposition methods have been employed to fabricate a variety of novel hierarchical architectures; examples include GaP/Ga(As)-P nanotrees,7a boron nanowires Y-junctions,7b MgO nanoflowers,7c ZnS dandelion-like microspheres7d and dart-shaped nanoribbons,7e ZnO architectures with various shapes, such as nanohelices,8a nanobridges,8b nanonails8c and nanocombs,8d nanoplate-nanorod junctions,8e and so forth. On the other hand, a recent advance in this area shows that solution-phase chemical synthesis is realized in achieving a variety of inorganic hierarchical architectures, which has been considered as the most promising route in terms of cost, throughput, and the potential for large-scale production. Complex nanorods/wires superstructures, such as dendrite-like,9 snow-flake-like,10 dandelion-like,11 * To whom correspondence should be addressed. Fax: +86-451-8641-3922. Tel.: +86-451-8641-2133. E-mail: [email protected].

penniform,12 multiarmed13 nanostructures and so on, have been fabricated in recent years. Although great progress has been achieved on the synthesis approaches for hierarchical architectures by the above methods, they usually require catalysts, expensive and even toxic templates or surfactants, hightemperature, and a series of complicated procedures. The introduction of catalysts, templates, surfactants, or other additives into the synthetic process undoubtedly brings impurities into the final products, increases the production cost, and leads to difficulty for scale-up production. The synthetic strategy operating at high-temperature is also an energy consumption route and a disadvantage for procedure control and equipment design.14 Thus, a low-temperature, surfactant-free, solutionphase chemical fabricating procedure is a desirable synthesis condition that chemists pursue to modify the high-temperature, surfactant-assisted methods. Moreover, low-temperature synthesis in aqueous solutions represents an environmentally benign and user-friendly approach, which may be considered to be a relatively green chemical alternative of practical significance.15 However, there has only been limited success in synthesizing inorganic hierarchical architectures composed of 1D nanostructures at relative low temperatures, especially at room temperature. Therefore, it still remains a significant challenge to develop facile and effective surfactant-free methods for the synthesis and architecture control of hierarchical nanostructures at room temperature. Strontium carbonate (SrCO3) is a very important reagent and has many applications in industry: as a constituent of ferrite magnets for small direct current motors, as an additive in the production of glass for color television tubes,16 and in the production of iridescent and specialty glasses, pigments, driers, paints, pyrotechnics, strontium metal and other strontium compounds.17 Recently, SrCO3 has attracted intensive attention because of its interesting additional application in many other fields, such as SrCO3-based chemiluminescence sensors exhibiting high selectivity to ethanol18a and as a Co/SrCO3 catalyst displaying high activity for dry reforming of methane with small amount of carbon deposition.18b However, compared with the great progress in the investigation of its properties, studies on

10.1021/cg070564f CCC: $40.75  2008 American Chemical Society Published on Web 04/01/2008

Room Temperature Synthesis of SrCO3 Architectures

the synthesis of SrCO3, especially its nanostructures, have been lagging far behind, and there are only a few reports on either the direct conversion process or the black ash process.19 Very recently, Zhu et al.20a reported the synthesis of SrCO3 nanowires and their catalytic activity to ethanol. SrCO3 nanostructures with various morphologies, such as nanowires, nanorods, and spherelike and ellipsoid-like particles, were successfully synthesized by a microemulsion-mediated solvothermal method.20b Fabrication of SrCO3 complex architectures with three-dimensional (3D) or highly ordered nanostructures is highly desired in current materials synthesis because this material holds the promise of additional advanced applications. However, to the best of our knowledge, controlled synthesis of SrCO3 hierarchical nanostructures has not been achieved to date. Recently, we have demonstrated a facile room temperature aqueous solution method for the large-scale synthesis of SrCrO4 nanowires, PbCrO4 nanorods,21 and CdMoO4 hollow microspheres22 without using templates or surfactants. In this work, we further present a similar simple surfactant-free aqueous solution route to the large-scale synthesis of SrCO3 hierarchical architectures assembled by nanorods or nanoribbons. This bottom-up technique is based on an aqueous solution method at room temperature and does not require any surfactants nor hightemperature processing, which makes it suitable for industrial production applications. The SrCO3 hierarchical architectures are synthesized by a one-pot method from the mixture of aqueous solutions of SrCl2, Na2CrO4, and NaOH at room temperature without any additives. The shape and size of the SrCO3 hierarchical architectures can be perfectly controlled by the OH- concentration and reaction temperature in the reaction system. Various controlled synthetic experiments indicate that the formation process of SrCO3 architectures in the present reaction system involves the growth of single-crystalline SrCrO4 nanowires, the formation of CO32- ions through the reaction between CO2 gas from air and aqueous NaOH solution, and finally the formation of SrCO3 architectures through the reaction between SrCrO4 and CO32- ions controlled by the difference of the solubility product (Ksp) of SrCrO4 and SrCO3. Experimental Section Synthesis. All chemicals were of analytical grade and used as received without further purification. In a typical procedure, 0.2 mol · L-1 of aqueous SrCl2 solution (25 mL) was added into 25 mL of aqueous Na2CrO4 solution (0.2 mol · L-1) drop by drop under strong magnetic stirring at room temperature, resulting in the precipitation of a light-yellow product indicating the formation of SrCrO4 nanowires. The mixture was kept under stirring for about 10 min, and then a certain concentration of NaOH aqueous solution (25 mL) was added into 12.5 mL of the above mixture suspensions and stirred for another 10 min. The color of the light-yellow precipitation changed slowly to white, indicating the transformation of SrCrO4 to SrCO3. The resulting suspension was placed at room temperature for 5 days without further stirring or shaking. Then, the products were collected by centrifugation, washed several times with distilled water and absolute ethanol, and finally dried in air at 60 °C for 1 h. Characterization. The X-ray diffraction (XRD) pattern of the asobtained samples was recorded on a Rigaku D/max-rA diffractometer with Cu KR radiation (λ ) 1.5405 Å). Scanning electron microscopy (SEM) images were taken on a Hitachi S-4700 field-emission scanning electron microscope. Transmission electron microscopy (TEM) characterization was carried out on a Phillips Tecnai 20 microscope at an accelerating voltage of 200 kV. The high-resolution TEM (HRTEM) characterization and selected area electron diffraction (SAED) pattern was performed on a JEOL3010 microscope at an accelerating voltage of 300 kV, equipped with X-ray energy dispersive spectrometer (EDS). For TEM and HRTEM experiments, the as-synthesized powders were

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Figure 1. SEM images of the SrCO3 hierarchical architectures with an OH- concentration of 5 mol · L-1 (25 mL). (a, b) An overall SEM image of the spherical architectures. (c, d) Low- and high-magnification SEM image of an individual flower-like SrCO3 superarchitecture. (e, f) Low- and high-magnification SEM of an individual dandelion-like SrCO3 hierarchical architecture. first dispersed in ethanol by ultrasonic treatment. Then a small drop of the dispersions was transferred to a holey carbon film supported on a copper grid.

Results and Discussion The typical SrCO3 hierarchical architectures synthesized in aqueous solution at room temperature with an OH- concentration of 5 mol · L-1 were observed by field-emission SEM. Figure 1a displays a representative overview of the SrCO3 hierarchical architectures, which shows that the as-obtained products are composed of large-scale spherical architectures. The high magnification SEM image in Figure 1b shows that the spherical architectures have diameters in the range of 5 to 10 µm and are composed of numerous well-aligned nanorods. Detailed SEM observations reveal that there exist two types of spherical architectures: flower-like superarchitectures and dandelion-like 3D architectures. Figure 1c shows the morphology of an individual flower-like SrCO3 superarchitecture with a diameter of about 10 µm. The high magnification SEM image (Figure 1d) of the superarchitecture indicates that the superarchitecture is composed of uniform nanorods with a diameter of about 30 nm and lengths of 1 to 2 µm. All the nanorods are selfassembled to construct the superarchitecture. Figure 1e shows an individual dandelion-like SrCO3 3D architecture with a diameter of about 5 µm. The high-magnification SEM image in Figure 1f exhibits detailed information about the dandelionlike architecture, which is also composed of nanorods with a diameter of about 30 nm and lengths of 1 to 2 µm. All the nanorods are radially oriented to its center and self-organized into a spherical-like assembly. In addition, besides the flowerlike and the dandelion-like SrCO3 architectures, a few hierarchical wheel-like superarchitectures with a diameter of about 20

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Figure 2. XRD patterns of (a) SrCrO4, (b) SrCO3 hierarchical architectures with an OH- concentration of 5 mol · L-1 (25 mL) and (c) Sr(OH)2 · 8H2O.

µm can be occasionally observed (Supporting Information, Figure S1). The high-magnification SEM images display that high-quality, uniform nanorods with diameter of 30 nm and 0.5 to 1 µm in length are aligned vertically in the outer part and randomly in the center of the wheel-like architectures. The phase structure of the SrCO3 hierarchical architectures obtained with the OH- concentration of 5 mol · L-1 was examined by its XRD pattern, as shown in Figure 2. The strong and sharp diffraction peaks in Figure 2b indicate that the asobtained SrCO3 hierarchical architectures are principally crystalline. All of the diffraction peaks can be perfectly indexed to the orthorhombic phase of SrCO3 (JCPDS 05–0418), with lattice parameters of a ) 5.107 Å, b ) 8.414 Å, c ) 6.029 Å. Compared with the XRD results of SrCrO4 (Figure 2a, JCPDS Card No. 15-0368) and Sr(OH)2 (Figure 2c, JCPDS Card No. 01-1263), no characteristic peaks from other impurities, such as SrCrO4 or Sr(OH)2, can be detected from the XRD pattern, indicating that the as-synthesized product has high phase purity. The XRD result also indicates the complete conversion of SrCrO4 to SrCO3 when aqueous NaOH solution is added into the reaction system of aqueous solutions of SrCl2 and Na2CrO4. The morphology and structure of the dandelion-like SrCO3 hierarchical architectures are further characterized by TEM, SAED, and HRTEM as shown in Figure 3. Figure 3a displays the representative TEM image of an individual dandelion-like SrCO3 hierarchical architecture with a diameter of 5 µm, which consists of nanorods with perfect spherical assembly, confirming the above SEM observation in Figure 1e. This dandelion-like SrCO3 architecture cannot be destroyed into discrete nanorods even under unltrasonication, indicating that the architectures are actually integrated and are not just aggregations of these nanorods. The high-magnification TEM image (Figure 3b) suggests that the building nanorods have uniform diameters of 20–40 nm and lengths of 1–2 µm. Figure 3c shows the TEM image of an individual SrCO3 nanorod broken down from the hierarchical architectures. The selected nanorod has a diameter of about 50 nm and a length of 800 nm. Figure 3d shows the SAED pattern taken from the selected nanorod (shown in Figure 3c). As indexed (inset in Figure 3d), the zone axis is [1,0,0]. Figure 3e shows a typical HRTEM image of the nanorod. The fringe spacing is determined to be 0.34 nm, which is close to the (021) lattice spacing of SrCO3. Both the SAED pattern and

Figure 3. (a) TEM image of an individual dandelion-like SrCO3 hierarchical architecture, (b) high-magnification TEM image on the fringe of an individual SrCO3 hierarchical architecture, (c) TEM image of an individual nanorod broken down from the hierarchical architecture, (d) SAED pattern taken from the selected nanorod (Figure 3c) and schematic illustration of SAED spots in inset, and (e) HRTEM image of the selected nanorod. Scale bar in Figure 3c: 200 nm. The sample was obtained with an OH- concentration of 5 mol · L-1 (25 mL).

Figure 4. EDS spectrum of the as-synthesized SrCO3 hierarchical architectures with an OH- concentration of 5 mol · L-1 (25 mL).

the HRTEM image clearly demonstrate the single-crystalline nature of the nanorod. The chemical composition of the as-synthesized SrCO3 hierarchical architectures obtained with the OH- concentration of 5 mol · L-1 is checked using EDS, as shown in Figure 4. The EDS spectrum shows the presence of Sr, C and O peaks together with a Cu signal coming from the TEM grid. No Cr signal is detected in the EDS spectrum, indicating the absence of SrCrO4 in the sample. The EDS result also implies that all the SrCrO4 has been converted to SrCO3, which is in agreement with the XRD result. Our synthetic parameters allow further shape and size manipulation of SrCO3 hierarchical architectures. For example, the

Room Temperature Synthesis of SrCO3 Architectures

Figure 5. SEM images of dandelion-like SrCO3 hierarchical architectures synthesized with an OH- concentration of 10 mol · L-1 (25 mL). (a, b) Low-magnification SEM images of dandelions-like hierarchical architectures with high yield. (c, d) High-magnification SEM images of an individual dandelions-like hierarchical architectures composed of nanoribbons.

concentration of OH- can significantly affect the size and shape of the products. Figure 5 presents typical SEM images of the SrCO3 hierarchical architectures prepared with an OH- concentration of 10 mol · L-1 (25 mL) while keeping other reaction conditions the same. Figure 5a shows that the sample mainly consists of dandelion-like architectures with a diameter of about 5 µm. From the enlarged SEM image shown in Figure 5b, the SrCrO3 “dandelions” comprise numerous nanoribbons pointing toward the center of dandelion sphere. High magnification SEM images of the dandelion (Figure 5c,d) indicate that the diameter and length of these nanoribbons are in the range of 20–80 nm and 0.5–2 µm, respectively. The phase structure and chemical composition of the dandelion-like SrCO3 hierarchical architectures is determined by the XRD pattern and EDS (Supporting Information, Figure S2). The XRD and EDS results indicate that phase-purity SrCO3 products are synthesized under this condition. When the OH- concentration is decreased to 2 mol · L-1 (25 mL) and the other reaction conditions are kept the same, the product was characterized by SEM observation (shown in Figure 6), XRD pattern, and EDS (Supporting Information, Figure S3). The low-magnification SEM images show that the product consists of a large-scale “double-trumpet-like” hierarchical architecture (Figure 6a,b). The SrCO3 double-trumpet-like hierarchical architecture also displays a complex feature from high magnification SEM observations. As shown in Figure 6c, the SrCO3 has a trunk with a diameter of about 800 nm and a length of 2 µm. Both ends of the trunk have a “trumpet” with an average diameter of 2.5 µm. High-magnification SEM image (Figure 6d) reveals that each “trumpet” in the hierarchical structure is formed through the assembly of nanorods with an uniform diameter of about 30 nm and lengths from 0.5 to 2 µm. The XRD and EDS results also indicate that phase-purity SrCO3 products can be obtained with OH- concentration of 2 mol · L-1 (25 mL). It is found that the reaction temperature also has significant influence on the morphology of the as-synthesized SrCO3 products. At the reaction temperature of 40 °C, large-scale irregular nanorod bundles together with a few dandelion-like SrCO3 architectures with large size distribution are obtained, as shown in Figure 7a,b. When the reaction temperature is

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Figure 6. SEM images of double-trumpet-like hierarchical SrCO3 architectures synthesized with an OH- concentration of 2 mol · L-1 (25 mL). (a, b) Low-magnification SEM images of double-trumpetlike architectures with high yield. (c) SEM image of an individual double-trumpet-like architecture. (d) High-magnification SEM image of one end of the double-trumpet-like architectures.

Figure 7. SEM images of the SrCO3 products obtained at different reaction temperature while keeping other reaction conditions the same: (a, b) 40 °C and (c, d) 80 °C.

further increased to 80 °C, the sample contains mainly dumbbelllike SrCO3 architectures (Figure 7c). High magnification SEM image (Figure 7d) shows that the dumbbell-like architectures are composed of numerous nanorods with lengths of 5–10 µm and diameters of 2–4 µm for the tip part and 2 µm for the middle part. The EDS results (Supporting Information, Figure S4) shows that the products synthesized at 40 and 80 °C only contain Sr, C, and O peaks, together with a Au peak generated by coating Au on the sample for SEM observation. This result indicates that appropriate reaction temperature is important for the preparation of SrCO3 architectures, which might be related to the reaction rate increased by high temperature in the present reaction system. To understand the growth mechanism of the SrCO3 hierarchical architectures, we performed a series of experiments to explore the formation process. The experimental conditions are listed in Table 1. The first experiment was carried out in the absence of NaOH aqueous solution. Without the addition of aqueous NaOH solution into the reaction system, large-scale

1738 Crystal Growth & Design, Vol. 8, No. 5, 2008 Table 1. Typical Samples Prepared under the Controlled Synthetic Conditions experimental condition

product

typical morphology

1. SrCl2 + Na2CrO4 2. SrCl2 + Na2CO3

SrCrO4 SrCO3

3. SrCl2 + Na2CrO4 + Na2CO3 4. SrCl2 + NaOH 5. Sr(OH)2 + Na2CO3

SrCO3

nanowires, Figure S5a plate-like aggregates, Figure S6b porous irregular rod-like structures, Figure S7c nanorod bundles, Figure S8d plate-like aggregates, Figure S9e

SrCO3 SrCO3

a Supporting Information, Figure S5. b Supporting Information, Figure S6. c Supporting Information, Figure S7. d Supporting Information, Figure S8. e Supporting Information, Figure S9.

SrCrO4 nanowires with relative uniform diameter of about 100 nm and lengths of tens of micro meters (Supporting Information, Figure S5) were formed via the direct reaction between aqueous SrCl2 and Na2CrO4 solutions at room temperature; the formation mechanism for this process was documented elsewhere.21 The second experiment was carried out by the direct reaction between aqueous SrCl2 and Na2CO3 solutions under the same synthetic conditions. SEM and EDS results show that the product consists of plate-like SrCO3 aggregates composed of numerous nanoparticles with diameters of 20 to 50 nm (Supporting Information, Figure S6). In the third experiment, the reaction took place under the same synthetic condition but using Na2CO3 aqueous solution instead of the NaOH aqueous solution. Only porous irregular rod-like SrCO3 aggregates composed of nanoparticles were obtained (Supporting Information, Figure S7). In the fourth experiment, no Na2CO3 aqueous solution was added into the reaction system but SrCl2 aqueous solution was used to react directly with a NaOH aqueous solution, while keeping the other reaction conditions the same. The product consists of large-scale rod-like bundles, with diameter of 400 nm and length of 0.8 to 1.2 µm (Supporting Information, Figure S8). For the last experiment, only using aqueous Sr(OH)2 and Na2CO3 solutions as the precursors, also only plate-like SrCO3 aggregates were obtained (Supporting Information, Figure S9). On the basis of the above experiments, we found that the initial formation of SrCrO4 nanowires, the product of CO32ions through the reaction between CO2 gas from air and aqueous NaOH solution, and the subsequent anion-exchange reaction between CO32- and CrO42- ions to create SrCO3 play key important roles in the final formation of the SrCO3 hierarchical architectures. In the recent reaction system, four reactions described in the following are believed to be responsible for the formation of the SrCO3 hierarchical architectures: (1) Sr2+ + CrO42- ) SrCrO4, (2) CO2 + 2OH- ) CO32-, (3) SrCrO4 ) Sr2+ + CrO42-, and (4) Sr2+ + CO32- ) SrCO3. SrCrO4 nanowires are first obtained through the reaction between SrCl2 and Na2CrO4 aqueous solution, as shown in reaction 1. When NaOH aqueous solution is introduced, it is well-known that the gas of CO2 from the air can react with OH- in the solution to form CO32- ions (reaction 2).19 The conversion from SrCrO4 to SrCO3 is attributed to the difference of the Ksp for SrCrO4 (5.2 × 10-4) and SrCO3 (1.1 × 10-10) at room temperature. Compared with SrCrO4, SrCO3 is more thermodynamically stable due to its lower Ksp. Thus, the SrCrO4 nanowires can be in situ dissociated slowly into Sr2+ and CrO42- ions in the reaction system (reaction 3). When the CO32- ions are formed in the reaction system, the anion-exchange reaction between CO32- and CrO42- ions has spontaneously taken place to form SrCO3 (reaction 4). It is obvious from reaction 2 that the variation of the OH- concentration results in the changes of the CO32- ion concentration, which would have profound effect on the growth process of the SrCO3 hierarchical architectures.

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In addition, a higher reaction temperature will also accelerate the reaction rate for the formation of CO32- ions. Therefore, the significant influence of the OH- concentration and reaction temperature on the shape and size of the SrCO3 hierarchical architectures would be related to the concentration of CO32-. To further demonstrate the conversion of SrCrO4 to SrCO3, 0.4 mol · L-1 of NaOH (25 mL) was introduced into the reaction system of aqueous solutions of SrCl2 and Na2CrO4 while keeping other reaction conditions the same as in a typical synthesis. The XRD result (Supporting Information, Figure S10) shows that the obtained products are composed of SrCrO4 and SrCO3 compounds, which clearly demonstrate the transformation from SrCrO4 to SrCO3. Although the synthetic procedure for SrCO3 hierarchical architectures is very facile, the formation mechanism seems to be still unclear. The formation process of inorganic hierarchical architectures is a complex process, which is affected by both crystal growth environments and crystal structures, including the degree of supersaturation, diffusion of the reaction, surface energy, crystal structures, and so forth.23 It is well-known that the slow reaction rate in the solution phase is favorable for the nucleation and crystallization for nanocrystals. Well-controlled complex morphologies of inorganic materials are usually difficult to obtain by directly mixing two incompatible aqueous solutions of metal salts because of a rapid decrease in supersaturation and further depletion of reaction nutrients in a short period of time.23b According to reactions 1-4, the reaction speed for formation of SrCO3 hierarchical architectures is quite slow compared with the directly mixing of the Sr2+ and CO32- ions. The initially formed SrCrO4 nanowires act as a strontium source, which release Sr2+ ions slowly to subsequently react with CO32ions. The formation rate of CO32- ions from the reaction between OH- and CO2 gas from air is also slow compared with using Na2CO3 salt as a reagent. When the CO32- ions are formed in the reaction solution, the SrCrO4 nanowires dissociate into Sr2+ and CrO42- ions and the SrCO3 is crystallized by the reaction between the Sr2+ and CO32- ions in the solution, where the solubility difference is the driving force. Therefore, the slow reaction rate controlled by the formation of Sr2+ and CO32ions would offer a favorable chemical environment for the formation of the SrCO3 hierarchical architectures. To further understand the evolution of the SrCO3 hierarchical architectures, we carefully examined the sample obtained from the aqueous solution with an OH- concentration of 5 mol · L-1 (25 mL) at room temperature after 5 days using SEM analysis. Figure 8a shows the low-magnification SEM image of the SrCO3 crystals with various intermediates such as rod-like nanorod bundles, dumbbell-like nanorod bundles, and dandelion-like architectures. High-magnification SEM images shown in Figure 8b-g indicate that the growth of the final SrCO3 hierarchical architectures is from nuclei to rod-like nanorod bundles (Figure 8b), dumbbell-like nanorods bundles (Figure 8c-e), and final dandelion-like architectures (Figure 8g). Colfen and co-workers24 have reported on the synthesis of CaCO3 microspheres with similar growth process evolution from rods to dumbbells and spheres with the assistance of double-hydrophilic black copolymers. They suggest that the formation mechanism is controlled by the “rod-to-dumbbell-to sphere” transformation, which is further demonstrated in the formation of MnCO3 and CdCO3.25 This “rod-to-dumbbell-to sphere” transformation seems to be a general phenomenon in crystal growth and is observed for several material system, such as carbonates24–26 (CaCO3, BaCO3, MnCO3, CdCO3, and so forth), nickel phosphate (Ni11(HPO3)8(OH)6),27 fluoroapatites (Ca5(PO4)3OH),28

Room Temperature Synthesis of SrCO3 Architectures

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Figure 8. (a) Low-magnification SEM image of the product SrCO3 with various morphologies, obtained from the aqueous solution with an OHconcentration of 5 mol · L-1 (25 mL) at room temperature after 5 days. (b-g) A growth process for SrCO3 hierarchical architectures could be demonstrated in which the architectures develop starting from nuclei and grow further to form rods (b), dumbbells (c-f), and finally evolve into complete spherical architectures (g).

and so forth. However, the reason for the branching from rods to dumbbells remains unknown although the shape evolutions have been observed for the above different material system. Very recently, Tang and Alivisatos29 proposed a splitting crystal growth mechanism for the formation of sheaflike Bi2S3 nanostructures, which might provide some insight for the initial branching from rods to dumbbells. Crystal splitting is possible if the oversaturation exceeds a certain “critical” level, which is specific for each mineral and for the given conditions. On the basis of our experiments, the formation mechanism for the SrCO3 hierarchical architectures might be due to the crystal splitting-induced “rod-to-dumbbell-to sphere” transformation process. In this mechanism, rodlike bundles are formed first. Then these rodlike bundles can grow at their ends resulting in dumbbell-like bundles, where crystal splitting is the main reason for the initial branching from rods to dumbbells. These dumbbell-like bundles grow further into closed spheres when the reaction is performed in a suitable reaction time. Conclusions In summary, a facile, room-temperature, surfactant-free aqueous solution route has been developed for the synthesis of SrCO3 3D architectures. The shape and size of the SrCO3 hierarchical architectures can be perfectly manipulated by controlling the OH- concentration and the reaction temperature. The key steps of this process involve the growth of singlecrystalline SrCrO4 nanowires, the formation of CO32- ions through the reaction between CO2 gas from air and NaOH aqueous solution, and, finally, the creation of SrCO3 hierarchical architectures by a replacement reaction between CrO42- and CO32- ions controlled by the difference of the solubility product. Although a detailed investigation concerning the growth mechanism is still in progress, we suggest that the evolution mechanism of the SrCO3 hierarchical architectures is mainly through a “rod to dumbbell to sphere” growth mechanism. Our study might open a novel, facile, and environmentally friendly solution-phase chemical route to the large-scale synthesis of other hierarchical architectures at room temperature. Supporting Information Available: Typical SEM images of wheellike SrCO3 hierarchical architectures, XRD pattern, SEM images, and EDS results of the as-prepared SrCrO4 nanowires, SEM images and EDS results of SrCO3 products obtained under different controlled experiments corresponding to Table 1, and XRD result of the SrCrO4

and SrCO3 compounds (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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