Photolysis-Induced Mineralization of Self-Assembled Witherite

May 12, 2009 - ABSTRACT: The natural phenomenon that calcium carbonate secretion in coral tissues falls at night or in darkness and rises during the d...
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Photolysis-Induced Mineralization of Self-Assembled Witherite Hierarchical Architectures Yu Zhao,† Yi Xie,*,† Si Yan,† and Yawei Dong‡ DiVision of Nano-materials and Nano-chemistry, Hefei National Laboratory for Physical Sciences at Microscale, and Department of Chemistry, UniVersity of Science and Technology of China, Hefei, Anhui 230026, P. R. China

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 7 3072–3078

ReceiVed July 10, 2008; ReVised Manuscript ReceiVed April 15, 2009

ABSTRACT: The natural phenomenon that calcium carbonate secretion in coral tissues falls at night or in darkness and rises during the day allows us to glean some clues that the light irradiation, either directly or indirectly, should contribute to the mineralization of natural minerals. This paper describes the synthesis of self-assembled witherite hierarchical architectures originating from the photolysis of living matters, for example, pyruvic and barbituric acid. Most of the witherite architectures have seldom been reported to date. Furthermore, the witherite crystals that are obtained in the absence of commonly used polymers or surfactants show impressive hierarchical order and mesoscale assembly. Investigation into the formation process of the crystals suggests that the adsorbate effect caused by low molecular weight ions under UV irradiation have a remarkable kinetic effect on crystallization, particularly with regard to polymorph selectivity and habit modification. Inasmuch the avoidance of extra additives and different kinds of matrices can achieved through such a process, this method provides the possibility to utilize such carbonates to serve as phase-changematerial (PCM) supporter in future “smart” wall. Introduction Materials with potentially interesting properties can be achieved through the self-assembly of nanoparticle building blocks into ordered superstructures by bottom-up approaches, which is regarded as one of the key topics of modern colloid and materials.1 Materials researchers have drawn their inspiration from biological systems such as the formation of peals and bones with excellent optical or mechanical properties2 to mimic and fabricate materials with complex morphologies and hierarchical order that may potentially contribute to superior properties over the corresponding artificial crystals.3 Generally, organized macromolecules, which contain well-defined arrays of functional groups, control polymorph selection and the oriented nucleation of crystals by lowering the nucleation energy of specific crystal faces,4 and thus considerable attention has been focused on selfassembled media, such as blockcopolymer aggregates, microemulsions, and surfactant micelles for controllable growth of highly organized crystallites.5 Particularly, morphosynthesis of barium/calcium carbonate with exquisite morphologies, hierarchical order and functional values is of great importance and long-term interest in industrial applications.6 In addition to the generation of mesostructured barium/ calcium carbonate by using different organic additives or templates, we try to illustrate the mineralization of witherite from the effect of light irradiation. Photolysis based on the ultraviolet (UV) light of solar spectrum is common in nature, from photodegradation of air pollutant7a to metabolism of living matter7b,c or even to sterilization in modern life. In the realm of inorganic/hybrid materials synthesis, photolysis has been successfully applied to synthesize well-defined noble metal particles by taking advantage of photolyzing silver halide under UV-light irradiation.8 Our recent research has shown that various selenium9a and * Corresponding author. E-mail: [email protected]. Fax: (86)-551-3603987. † Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China. ‡ Department of Chemistry, University of Science and Technology of China.

tellurium9b structures can be achieved under visible-light irradiation by using poly(vinylpyrrolidone) as crystal growth modifier. These strategies yield uniform products and thus can be regarded as promising routes for the preparation of materials with well-defined morphologies. However, photolytic synthesis has seldom been mentioned in the possible relationship between solar irradiation and the formation of natural minerals, for instance, the generation of carbonates, inasmuch the generation of CO2 by photolysis of organic matter would serve as promising CO32- source. An edificatory phenomenon can be found in the daily or circadian variation of calcium carbonate secretion in recent corals based on the physiological studies first made by Kawaguti and Sakumotos10 and subsequently and more elegantly by Goreau.11 Their studies show that the rate of calcium carbonate uptake in coral tissues falls at night or in darkness and rises during the day. In this paper, such a phenomenon clues us to investigate and distinguish the crystallization behavior of carbonates under light irradiation, rather than to tell the direct or indirect contribution of light irradiation to the mineralization of carbonate minerals. Previous methods usually rely on the decomposition of ammonium carbonate to generate CO2 in the gas phase and transportation of CO2 into the solution phase via gas-liquid diffusion,12 whereas we concentrate on the photolysis of some living matter such as pyruvic acid or barbituric acid, which has potentials to serve as the CO32- source,13,14 to mimic the mineralization of witherite in a homogeneous reaction system. Herein, we try to use the light irradiation to mineralize witherite with complex morphologies and hierarchical order. A mercury lamp (wavelength distribution: 350-450 nm) is used to provide the UV irradiation. We must mention that the relative irradiance of a mercury lamp is much higher than the natural system, though our aim is to investigate and distinguish the crystallization behavior from the commonly used methods. The result turns out to be a confirmative and exciting one, witherite crystals with distinctive styles can be readily obtained by photolysis of a precursor solution under ambient temperature and pressure. The feature of our approach exists in the way that takes account of light irradiation on the mineralization of natural minerals, which

10.1021/cg8007409 CCC: $40.75  2009 American Chemical Society Published on Web 05/12/2009

Photolysis-Induced Mineralization of Witherite

has been proven to be a promising strategy that benefit the generation of witherite architectures with hierarchical order.

Crystal Growth & Design, Vol. 9, No. 7, 2009 3073 Scheme 1. Schematic Illustration of the Formation Process of Witherite from the Photolysis of (a) Pyruvic Acid and (b) Barbituric Acid

Experimental Section Sample synthesis. All reagents were of analytical grade and used without further purification. All the glassware (glass bottle and small pieces of glass substrate) was cleaned and sonicated in ethanol for 5 min before being rinsed with distilled water and further soaked in a H2O/HNO3 (65%)/H2O2 (1:1:1, v/v/v) solution. Finally, the glassware is rinsed with doubly distilled water and dried in air with acetone. In a typical procedure to synthesize witherite crystals by photolysis of pyruvic acid, barium hydroxide (1.0 mmol, 0.17 g) and pyruvic acid (1.0 mmol, 0.09 g) was dissolved into doubly distilled water (100 mL) and bubbled with nitrogen for 2 h before use. The precursor solution was transferred into a pretreated reagent bottle with the capacity of 200 mL. The bottle was then sealed and exposed to UV irradiation (provided by a high-voltage mercury lamp, 60 W, wavelength distribution: 350-450 nm) for 4 days. The white deposit was collected by centrifugation, rinsed with doubly distilled water and ethanol, and allowed to dry at room temperature. To synthesize witherite crystals in buffer solutions, we dissolved barium chloride (1.0 mmol, 0.25 g) and barium acetate (1.0 mmol, 0.27 g) into NH4Cl/NH3 · H2O (pH 8.5, 100 mL, cNH4Cl ) 2 M) and HAc/NaAc (pH 8.5, 200 mL, cNaAc ) 2 M) buffer solutions and then took the rest of the steps described above. To synthesize witherite crystals by photolysis of barbituric acid, we used equimolar barbituric acid to instead of pyruvic acid. The rest steps were the same as described above to synthesize witherite crystals by photolysis of pyruvic acid in the buffer-free condition. Calcite crystals could also be synthesized by using calcium chloride or calcium acetate in the corresponding reaction systems. Characterization. The X-ray diffraction patterns (XRD) were recorded with Philips X’Pert Pro Super diffract meter with Cu KR radiation (λ ) 1.54178 Å); the field emission scanning electron microscopy (FESEM) was performed on FEI Sirion-200 SEM; transmission electron microscopy (TEM) images associated with select area electron diffraction (SAED) were performed on Hitachi H-800 TEM with an acceleration voltage of 200 kV; and high-resolution TEM (HRTEM) images were performed on JEOL-2010 TEM with an acceleration voltage of 200 kV. The Fourier transform infrared (FTIR) experiments were carried out on a Magna-IR750 FTIR spectrometer in a KBr pellet, scanning from 4000 to 400 cm-1 at room temperature. The microscope observation was performed on an Olympus DP20 digital camera.

Results and Discussion Pyruvic acid, the end zymolysis product of saccharide and initiator substrate of tricarboxylic acid cycle in organism,15 can release CO2 gas by oxidative decarboxylation under UV irradiation. By taking advantage of this, the photolytic process of pyruvic acid can be applied to serve as the CO2 source and further applied to mimic the generation of carbonate minerals. The mineralization reaction of witherite is carried out under UV irradiation using a high-voltage mercury lamp as irradiation source. The photolysis of pyruvic acid is presented in Scheme 1a. When a starting solution containing equimolar barium hydroxide and pyruvic acid are exposed to UV irradiation for four days, white witherite deposit is obtained. X-ray diffraction (XRD) analysis indicates that the as-obtained product is phasepure orthorhombic barium carbonate (curve A in Figure 1). Figure 2a shows the panoramic field emission scanning electron microscopy (FESEM) image of the as-obtained nearly monodispersed witherite spheres. These spheres have uniform size distribution of 3-4 µm and are porous in appearance. The higher-resolution FESEM image shown in Figure 2b reveals that the spheres exhibit a highly hierarchical order, which is constructed from self-assembled, homogeneous, rodlike nanocrystals with the diameter of about 30 nm. The nanorods spreading out from the center of the sphere in a radial way that

can be clearly observed from the lateral view as shown in Figure 2b inset. It is worthy mentioning that the pH value decreases because of the proton release caused by the conversion of CO2 to CO32-. In addition, the formation of such crystals with hierarchical order is surprising because of the absence of macromolecules or surfactants that are proved to have significant influences on the generation of hierarchical order.5a,16,19 Inasmuch the avoidance of extra additives, this method exhibits the potential for industrial application. Because biomineralization that takes place in the organism possesses a relatively stable pH condition, we have also introduced some buffer systems to further mimic the mineralization of witherite as well (the highermagnification FESEM image and optical micrograph are presented in the Supporting Information, S1). Figure 2c shows a macroscopic flowerlike witherite crystals obtained in the NH4Cl/NH3 · H2O buffering system (corresponding XRD pattern, curve B in Figure 1). The bottom part of the flower is loosely attached to the glassware thus turns out to be flat (see the Supporting Information, Figure S1a). The petals show a radial arrangement and hierarchical structure as shown in Figure 2d, which illustrates that the petals are not simply dense rods but have sharp edges overlapping in a regular arrangement as the scales of a fish (Figure 2d inset). The sharp edges show a preferential growth rate along the direction (see the

Figure 1. XRD analysis of the witherite crystals obtained through the photolysis of pyruvic and barbituric acid in buffer-free and buffer solutions. All six of the XRD patterns are consistent with the standard value (JCPDS 41-0373) of orthorhombic barium carbonate (space group Pnma).

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Figure 2. Electron microscopy images of witherite crystals synthesized by photolysis of pyruvic acid. (a, b) Low- and high-magnification FESEM image of nearly monodispersed witherite spheres. Inset in b: Side elevation of the aligned rods, scale bar: 0.2 µm. (c, d) Low- and high-magnification FESEM image of macroscopic flowerlike witherite crystals obtained in NH4Cl/NH3 · H2O buffer solution. Inset in d: Petal details, scale bar: 1 µm. (e, f) Low- and high-magnification FESEM image of needlelike witherite crystals obtained in HAc/NaAc buffer solution. Inset in f: Transverse section of the needle suggesting the 6-fold symmetry along its central axis, scale bar: 2 µm.

Figure 3. Electron microscopy images of witherite crystals synthesized by photolysis of barbituric acid. (a, b) Low- and high-magnification FESEM image of urchinlike witherite crystals obtained in buffer-free solution. Inset in b: Transverse section of a unit nanorod, scale bar: 100 nm. (c, d) Low- and high-magnification FESEM image of polyhedral witherite crystals obtained in NH4Cl/NH3 · H2O buffer solution. Inset in d: Transverse section revealing the witherite crystals are composed of parallel-aligned rods. Scale bar: 500 nm. (e, f) Lowand high-magnification FESEM image of coral-like witherite crystals obtained in HAc/NaAc buffer solution.

Supporting Information, Figure S2). When the photolysis process is carried out in the HAc/NaAc buffering system, witherite needles with uniform size distribution have been obtained as presented in Figure 2e (corresponding XRD pattern, curve C in Figure 1). Differing from the reported one-dimensional witherite structures fabricated by using a double hydrophilic block copolymer16a or microemulsion16b as the crystal modifier, the witherite needles have rough faces (Figure 2f) and show a 6-fold symmetry (Figure 2f inset), which seems in contrary to the classical crystallization theory. Because the external morphologies of mesocrystals are not, in the majority of cases, related to the primary crystal symmetry, sometimes they have the typical rough faces, but a higher symmetry than the primary units.1b Besides, it is evident from previous studies that low molar-mass additives can influence crystal habit by selective adsorption processes that lead to preferential growth inhibition for distinct crystal facets, in addition to the adsorption of soluble macromolecules and inorganic ions.17 If such a process takes place, orthorhombic witherite needles with forbidden 6-fold symmetry can be produced. Such a reaction system can also be readily extended to synthesize other carbonates such as calcite with hierarchical order (see the Supporting Information, S3). The UV irradiation of nucleic acids and their components has been intensively studied in vitro and in vivo.18 Particularly, barbituric acid, an important intermediate in photolysis of nucleic acids,14 tends to form activated carboxylic compounds which would easily decarboxylate and thus provides the possibility of producing carbonate. Witherite crystals can be synthesized by taking advantages of this as shown in Scheme 1b. The corresponding XRD analysis of the as-obtained witherite crystals is severally presented in curves D-F in Figure 1. Figure 3 shows a set of FESEM images of witherite crystals obtained by photolysis of barbituric acid in both buffer and buffer-free conditions. Figure 3a shows the urchinlike witherite

crystals obtained in buffer-free solution. This morphology is obviously different from the ones obtained through gas-liquid diffusion method without the existence of crystal modifier,19 suggesting the fact that the UV irradiation as well as adsorbate effect does influence the crystal growth behavior as observed elsewhere.9 The higher-resolution FESEM image of the witherite urchins reveals the alignment of the nanocrystals within the superstructure (Figure 3b) and the transverse section of a unit nanorod (Figure 3b inset). Tetragonal-prismlike nanorods aligned in a radial way are several micrometers long and 100-150 nm in diameter and show a preferential growth direction along (see the Supporting Information S4). Figure 3c shows a polyhedral witherite crystal obtained in NH4Cl/NH3 · H2O buffer solution. Although the crystals are apparently well-faceted, they show an inner texture and cracks and are clearly composed of dense nanofibers. Sites parallel to the lateral axis display smooth surfaces, whereas sites on the longitudinal axis have a rough shape and show superstructure. More information of the inner texture can be observed in Figure 3d and inset. The rough surface is actually constructed with closely parallel-aligned nanorods with average diameter of 200 nm. The wavy surfaces and irregular edges observed are atypical for single crystals and are likely to be the effect of an imperfect structuring of subunits.20 Figure 3e presents the FESEM image of coral-like witherite crystals obtained in HAc/NaAc buffer solution. The branches of the coral-like crystals are well-faceted, as shown in Figure 3f. The distinguishable morphologies of witherite crystals obtained in the same reaction conditions but with different carbonate precursors should be contributed from the different adsorbate effects, which will be discussed in the following relevant sections. Because the reaction system does not need the assistance of macromolecules or surfactants that are widely used and proven to have significant influences on the self-assembly of basic building blocks into integrated crystals, the formation of

Photolysis-Induced Mineralization of Witherite

Figure 4. Digital camera image showing the effects of heat and UV irradiation on the formation of witherite crystals. The starting bufferfree solution (a) kept in darkness, (b) maintained at 40 °C, (c) exposed to UV irradiation, and (d) exposed to UV irradiation in HAc/NaAc buffering system for 4 days.

witherite crystals with different hierarchical orders should be contributed to the UV irradiation as well as adsorbate effects. Moreover, it has to be emphasized that the structural dimensions and appearances of the witherite crystals obtained are distinguishable even in the same reaction conditions but with different precursors. For instance, the witherite needles and corals obtained by photolysis of pyruvic and barbituric acid show different growth behaviors, which implies that various morphologies can be readily achieved if the proper precursor of CO2 source is introduced. Therefore, the crucial roles of adsorbate effects and UV irradiation on the generation of different kinds of witherite crystals can be deduced. A series of control experiments have been performed to determine the role of UV irradiation in the formation of these witherite crystals. A digital camera image showing the effects of heat and UV irradiation on the mineralization of witherite is presented in Figure 4. In this approach, we found that the solution-phase temperature was about 40 °C. The starting solution that is kept in darkness for 4 days remains transparent, suggesting that the oxidative decarboxylation of pyruvic acid hardly takes place. Another starting solution which is maintained at 40 °C for 4 days also remains transparent, revealing that such a temperature can not lead to the oxidative decarboxylation of pyruvic acid. In comparison, when exposed to the UV irradiation for 4 days, the starting solutions turns out to be suspended, indicating the oxidative decarboxylation of pyruvic acid and the generation of witherite product. The formation process of the witherite crystals is investigated by analyzing the early stages during their growth process, witherite needles for example. Figure 5a shows the transmission electron microscopy (TEM) image of the particles obtained after being exposed to UV irradiation for 5 h. The corresponding selected area electron diffraction (SAED) pattern confirms the particles are amorphous. When the irradiation time is prolonged to 15 h, crystalline nanorods and amorphous particles shown in Figure 5b are obtained. Some amorphous particles shown in the bottom of Figure 5b with the same size distribution as the ones presented in Figure 5a can still be observed. The witherite needles with smaller size (Figure 5) and single crystallinity (Figure 5d and its inset) appeared after 2 days of UV irradiation. The shape revolution from small amorphous particles to polycrystalline rods and finally to single-crystalline needles reveals a mesoscale transformation process accompanied with crystallographic fusion as suggested by Co¨lfen and Mann.25,30 This phenomenon has also been observed by Antonietti and coworkers in the preparation of BaCO3 helices.5a Differing from the reported methods that use macromolecules or surfactants

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Figure 5. TEM images of (a) amorphous nanoclusters and (b) crystalline rods collected after being exposed to UV irradiation for 5 hours, respectively. Insets: the corresponding SAED patterns. (c) TEM image showing the needle tip. (d) HRTEM image of the witherite needle. The lattice spacings of 3.65 and 5.29 Å can be assigned to the spacing of adjacent {102} and {040} crystal planes. The needle shows a preferential growth rate along the β direction. Inset: Corresponding SAED pattern.

to control the morph selectivity of witherite crystals,5a,16,19 the distinctive nucleation behavior and adsorption of low-molecularweight ions under UV irradiation conditions apparently contribute to the formation of the various witherite hierarchical architectures. It is evident that the nucleation and growth process in our reaction system differs from the previous reports on the mineralization of witherite crystals. The nucleation process in our system mainly depends on supersaturation (σ) of the solution,21 which can be defined by eq 1

σ≡

()

a ∆µ ) ln kBT ac

(1)

or for BaCO3 in aqueous solution, the supersaturation can be defined by eq 2

(

σ ) ln

a(Ba2+)a(CO23 ) Ksp

)

(2)

where ∆µ is the change in chemical potential per molecule, kB is the Boltzmann constant, T is absolute temperature, a and ac are actual and equilibrium activity products, a(Ba2+) is the activity of Ba2+, a(CO32-) is the activity of CO32-, and Ksp is the equilibrium solubility product at zero ionic strength of the experimental solutions at experimental temperature. In the commonly used gas-liquid diffusion method, the nucleation process is determined by both the supersaturation and the gas/ liquid interface diffusion rate. The latter one can be formulated by eq 3 according to Fick’s law22

JCO2 ) -DCO2 / H2O

dcCO2 dz

(3)

where JCO2 is the mass flux of CO2, DCO2/H2O is the diffusivity of CO2 gas into H2O, and dcCO2/dz is the change in CO2 concentration divided by the change in distance. In the photolytic mineralization process, the nucleation mainly depends on the supersaturation and is determined by activity of Ba2+ and CO32-, which shows variance in different buffering conditions. Distinguishable from the gas-liquid diffusion nucleation process that takes place under nonequilibrium condition, the nucleation process under UV irradiation possesses a more stable nucleation rate. Because the pho-

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Figure 6. FTIR spectra of the amorphous intermediate and witherite crystals synthesized by photolysis of pyruvic acid in different buffering conditions.

tolysis velocity is relatively slow and depends on the intensity of illumination and temperature that remains constant during the reaction process, the nucleation rate is stable with a relatively slow decrease during the reaction process. Under the circumstance of UV irradiation, the different nucleation behavior in addition to the adsorption of low-molecularweight ions during the growth process would lead to the generation of different kinds of witherite crystals. The adsorption of low-molecular-weight ions to form functionalized building blocks and subsequent self-assembly of such building blocks into integrated crystals has been studied in previous works.23,24 Similar to macromolecules, low-molecular-weight ions usually have a remarkable kinetic effect on crystallization, particularly with regard to polymorph selectivity and habit modification.25 To confirm such an effect, as with CH3COO- for example, we have examined the amorphous intermediate (Figure 5a) and the products obtained by photolysis of pyruvic acid in buffer and bufferfree reaction conditions (Figure 2a) by Fourier transform infrared (FTIR) analysis, which is presented in Figure 6. Acetic acid is expected to produce three strong peaks near 1590, 1432-1405, and 1335 cm-1,26 whereas witherite is expected to produce three strong and narrow peaks near 1437, 857, and 696 cm-1.27 The absorption peak centered at 2827 cm-1 associates with the stretching vibration of C-H bond in acetic acid, and the absorption peaks centered at 1602 and 1353 cm-1 correspond with symmetrical and asymmetrical stretching vibration of carboxyl, respectively. The absorption peak located at 1740 cm-1 corresponds with stretching vibration of -COOH in acetic acid, while the absorption peak near 1400 cm-1 corresponds with deformation vibration of -CH3 in acetic acid. It is noticeable that the transformation from hybrid amorphous particles to partially included CH3COO- crystals through crystallographic fusion during the mesoscale transformation process should be responsible for weakness of the characteristic peaks of acetic acid.25 The result shows that under certain circumstances, the interactions between CH3COO- and witherite building blocks severely inhibit lattice construction such that colloidal aggregates containing hybrid primary particles with metastable amorphous particles are deposited. With time, however, the amorphous nanoparticles slowly crystallize within the ag-

Zhao et al.

Figure 7. Demonstration of thermal insulation capability of octadecane@BaCO3 porous spheres, where 1 cm3 of water at 50 °C was allowed to cool in a water-ice mixture in glass vials covered with different insulation jackets and then heated in a 50 °C furnace. Curves A-C show the change in temperature of the glass vial covered with conventional tinfoil jacket, tinfoil/BaCO3 jacket, and tinfoil/ PCM@BaCO3 jacket, respectively.

gregates, and this process can be strongly coupled with mesophase transitions involving surface-adsorbed low-molecular-weight ions. Therefore, the concentration of such lowmolecular-weight ions would seriously affect the selfassembly of these particles into integrated crystals with hierarchical order during the mesoscale transformation process. By halving the concentration of pyruvic acid to decrease the concentration of CH3COO- (as a photolytic product of pyruvic acid), witherite crystals distinguishable from the hierarchically structured witherite flowers (Figure 2c) can be obtained (see the Supporting Information, S5). In analogy to the CH3COO-, other low-molecular-weight ions presented in Scheme 1b by photolysis of barbituric acid also have a remakeable kinetic effect on crystallization, because the corresponding witherite crystals obtained by using pyruvic and barbituric acid as CO32- source show completely different morphologies. Solar energy is available only during the day, and hence its application requires efficient thermal energy storage so that the excess heat collected during sunshine hours may be stored for later use during the night. Similar problems arise in heat recovery systems where the waste heat availability and utilization periods are different, requiring some thermal energy storage. The encapsulation of phase change materials (PCM) provides the possibility to overcome such problems because it can provide large heat transfer area, reduce the PCMs reactivity toward the outside environment, and control the changes in volume of the storage materials as phase change occurs.28 On the one hand, the BaCO3 porous spheres (Figure 2a) possess nanoscaled matrices that can serve as ideal PCM supporters to fabricate thermal insulators with the desired ability to stabilize the temperature; on the other hand, the BaCO3 porous spheres are composed of nanoscaled particles whose heat capacities should be greater than those of the polycrystalline bulk suggested by previous experimental evidence and computer simulations.29 Figure 7 demonstrates the thermal capability of octadecane@BaCO3 porous spheres (Figure 2a) to stabilize temperature.30 In this experiment, the encapsulation of phase change materials (PCM) was carried out in a filtering flask containing 1.0 g of BaCO3 sample and excessive octadecane. The temperature

Photolysis-Induced Mineralization of Witherite

of the filtering flask was kept above 35 °C to make sure that octadecane is in its liquid phase. The filtering flask was then vacuumed for 5 min to make the liquid octadecane mount onto the matrices of the BaCO3 porous spheres. The octadecane@BaCO3 porous spheres was washed with distilled water and absolute ethanol several times and dried in air at room temperature. The as-prepared octadecane@BaCO3 sample as well as BaCO3 porous spheres was mixed with poly(vinylidene fluoride) (PVDF) with an 85:15 mass ratio in N-methylpyrrolidinone before being painted on a tin foil. A glass vial was covered with a different insulation jacket, filled with 1 cm3 of 50 °C water, and then allowed to cool in a water-ice mixture before being heated in a 50 °C furnace. The water temperature in the vial was measured every 20 s and recorded until it reached 15 °C when cooling and 45 °C when heating. Curves A1-C1 show the cooling process where there was conventional tinfoil, tinfoil/BaCO3 porous spheres, and tinfoil/octadecane@BaCO3 porous spheres insulation jacket on the glass vial. It is evident that during the cooling process, the insulating capacity was increased by the addition of PCM insulation. The duration of the cooling down process is ca. 6, 8, and 12 min covered with tinfoil, tinfoil/BaCO3 porous spheres, and tinfoil/PCM@BaCO3 porous spheres, respectively. Accordingly, curves A2-C2 severally shows the heating process using different insulation jackets. During the heating process, the insulating capacity was also increased by the addition of PCM insulation, with the sample reaching 45 °C after ca. 10, 13, and 19 min for the sample covered by tinfoil, tinfoil/BaCO 3 porous spheres, and tinfoil/ PCM@BaCO3 porous spheres, respectively. The tinfoil and BaCO3 porous spheres were less effective compared with the PCM-filled BaCO3 porous spheres during either the cooling or heating process. More importantly, the jacket based on PCM-filled BaCO3 porous spheres allowed one to stabilize the temperature (close to the melting point of octadecane) in the vial for about 2 min both in the cooling and heating processes. In contrast, the temperature continuously dropped with time for all other types of insulating jackets. Such carbonate minerals obtained through photolysis-induced mineralization possesses different kinds of matrices and can be applied as ideal PCM supporters thus exhibit their potential in a future “smart” wall. Conclusions In conclusion, witherite crystals of multiple spatiotemporal scales have been fabricated for the first time through a photolysis-induced mineralization process. These crystals exhibit remarkable hierarchical order that differs from the reported witherite mesocrystals fabricated by using the prevalent gas-liquid diffusion method. In consideration of the absence of macromolecule or surfactant that serves as a crystal growth modifier to generate mesostructures, the adsorbate effect caused by lowmolecular-weight ions under UV irradiation, which is used as the driving force to generate CO2 as carboxylate source by photolysis of pyruvic or barbituric acid, is considered to be responsible for the formation of witherite mesocrystals. Insomuch as the avoidance of extra additives, this method exhibits the potential for industrial application. Such a method is expected to be further developed for the preparation of other carbonate minerals, and the intrinsic relationship between the solar irradiation and the formation of natural minerals is also expected to be further investigated. Insomuch as different kinds of matrices can achieved through the photolysis-induced mineralization of carbonate minerals, the as obtained carbonates

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exhibit their potential to serve as PCM supporter in a future “smart” wall. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (20621061) and National Basic Research Program of China (2009CB939901). The authors acknowledge Mr. Fei Zheng and Mr. Linfeng Fei for their technical assistance with FESEM and HRTEM observations, respectively. Supporting Information Available: XRD analysis, FESEM, TEM, and HRTEM images of the as-synthesized samples (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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