Fabrication of Copper Hydroxyphosphate with Complex

all peaks can be clearly indexed as the pure orthorhombic phase and are consistent ..... In addition, pumpkin-like, dumbbell-shaped, and double-fl...
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J. Phys. Chem. B 2006, 110, 7750-7756

Fabrication of Copper Hydroxyphosphate with Complex Architectures Jiasheng Xu and Dongfeng Xue* State Key Laboratory of Fine Chemicals, Department of Materials Science and Chemical Engineering, School of Chemical Engineering, Dalian UniVersity of Technology, 158 Zhongshan Road, Dalian 116012, P. R. China ReceiVed: December 22, 2005

Copper hydroxyphosphate [Cu2(OH)PO4] with complex architectures has been synthesized through a simple and mild hydrothermal route in the absence of any external inorganic additives or organic structure-directing templates. Powder X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectrometry are used to characterize various properties of the obtained samples. Single-crystals, twinned-crystals, and various novel architectures of copper hydroxyphosphate can be constructed through a careful control of synthetic parameters, such as the molar ratio of initial reagents, reagent concentration, reaction time, and temperature. On the basis of structure and chemical bond analysis, copper hydroxyphosphate crystals tend to grow along the c-axis and have a rotation twinned-crystal growth habit, which is essential for the formation of various complex architectures. The current approach provides a facile strategy to synthesize copper hydroxyphosphate crystals with unique morphologies and complex architectures, which may be applicable to the synthesis of other inorganic materials.

Introduction The chemical synthesis of inorganic materials with unusual and complex architectures has attracted considerable attention, since the shape and texture of materials strongly determine their properties.1-3 Synthesizing inorganic materials with complex architectures could therefore be relevant to the design of new types inorganic materials. It is well-known that both intrinsic and extrinsic factors (i.e., the crystal structure and growth environment) have significant effects on the final crystal morphology.4 The geometric shape (habit) of a crystal is determined by the external expression of a selected set of symmetry-related faces. In general, faces perpendicular to the fast growth direction have smaller surface areas, and slow growing faces therefore dominate the morphology. The relative growth rate reflects the difference in the interplay among internal lattice structures and the external environment (of the crystallization system).5 Recently, the strategy of using organic templates and/or additives to control the nucleation and growth of inorganic particles has been widely applied to the morphogenesis of inorganic materials with complex forms.6,7 Due to various expected advantages, the template-free method (generally, through controlling the crystallization) seems to be one of the most promising chemical strategies for the “one-pot” synthesis of inorganic materials with complex architectures. In this regard, many successful attempts have been reported to manipulate the shape of inorganic materials.8-11 The ability to control crystallization is a critical requirement in the synthesis of many technologically important materials. Metal phosphates have various potential applications in the area of catalysis, ion exchange, proton conductivity, intercalation chemistry, photochemistry, and chemistry materials.11-14 For example, hydroxyapatite [Ca10(OH)2(PO)6, HAp] is an important functional biomaterial being often used as a bone substitute material in orthopedic, dental, and maxillofacial applications, due to its chemical and structural similarity to the mineral part * Address correspondence to this author. E-mail: [email protected].

of human hard tissues.15,16 The numerous investigations of the Cu-P-O and Cu-P-O-H systems have shown their very rich crystal chemistry, and 13 different structures of copper phosphates or hydroxyphosphates have been discovered to date.17 Copper hydroxyphosphate [Cu2(OH)PO4] with some impurities was first observed in mineral products, which has been widely investigated due to its novel catalytic effect for an important environmentally benign oxidation reaction. This excellent catalytic effect may have a relationship with its special crystal structure,18,19 which consists of both 50% 5-coordinated Cu and 50% 6-coordinated Cu. It is well accepted that there is a close relationship between the morphology and the properties of inorganic materials, that is, morphologies determine the properties since the crystal shape dictates the interfacial atomic arrangement of the material. However, the reports on the growth and morphology evolution of copper hydroxyphosphate crystals are very few. Herein, we report a rational synthesis of copper hydroxyphosphate architectures by a mild hydrothermal method, and demonstrate a feasible, large-scale, and controllable synthesis of copper hydroxyphosphate architectures. Single-crystals, twinned-crystals, and various novel architectures of copper hydroxyphosphate have been manipulatively synthesized by simply tuning the initial reagent’s molar ratio of Cu2+ and PO43-. Especially, copper hydroxyphosphate crystals tend to grow along the c-axis and have the rotation twinned-crystal growth habit, which is essential for the formation of various architectures. This approach (through well controlling the basic crystallization process to design new types of inorganic materials) provides a facile strategy to synthesize copper hydroxyphosphate crystals with unique morphologies and complex architectures, which may be applicable to the synthesis of other inorganic materials. Experimental Section The copper hydroxyphosphate products were synthesized by a solution phase approach. In a typical experimental procedure, the architectures were obtained by the hydrothermal treatment

10.1021/jp0574448 CCC: $30.25 © 2006 American Chemical Society Published on Web 03/30/2006

Copper Hydroxyphosphate with Complex Architectures

J. Phys. Chem. B, Vol. 110, No. 15, 2006 7751

TABLE 1: A Brief Summary of Representative Experiments in This Work no.

preparation procedure and reaction conditionsa

1

5 mL of CuCl2 (1.0 M) + 20 mL of (NH4)2HPO4 (1.0 M) at 150 °C for 72 h 5 mL of CuCl2 (1.0 M) + 5 mL of (NH4)2HPO4 (1.0 M) at 150 °C for 72 h 10 mL of CuAc2 (0.25 M) + 10 mL of (NH4)2HPO4 (1.0 M) at 180 °C for 48 h 20 mL of CuAc2 (0.25 M) + 10 mL of (NH4)2HPO4 (1.0 M) at 180 °C for 48 h 20 mL of CuAc2 (0.25 M) + 2.5 mL of (NH4)2HPO4 (1.0 M) at 180 °C for 48 h 10 mL of CuAc2 (0.25 M) + 1.25 mL of (NH4)2HPO4 (1.0 M) at 180 °C for 48 h 10 mL of CuAc2 (0.25 M) + 1.25 mL of (NH4)2HPO4 (1.0 M) at 180 °C for 48 h

2 3 4 5 6 7b

Cu2+/PO34 molar ratio

result

1:4

stout prismatic (Figures 2 and S6)

1:1

long prismatic (Figure S7)

1:4

rotation twinned-crystals (Figures 3 and S8)

1:2

2:1

a quadruple petal pumpkin-like morphology (Figures 5 and S9) a double-mushroom-like morphology (Figures 6 and S10) a double-flower-like morphology (Figures 7 and S12)

2:1

a well-defined aggregate (Figures 8 and S13)

2:1

a Prior to the reaction, the solution mixture was sonicated in an ultrasonic water bath for 10 min. The bluish slurry mixture was then transferred to a Teflon-lined stainless steel autoclave, which was filled with deionized water up to 80% of its capacity (80 mL). b In this experiment, the Teflon-lined stainless steel autoclave was filled with ethanol up to 80% of its capacity.

of a Cu(CH3COO)2 (hereafter abbreviated as CuAc2) solution in the presence of (NH4)2HPO4 at 180 °C for 48 h. The starting solutions of analytical grade CuAc2 and (NH4)2HPO4 were freshly prepared. Synthetic copper hydroxyphosphate crystals were prepared by the titration of a 0.25 M CuAc2 solution with a 1.0 M (NH4)2HPO4 solution under vigorous stirring at room temperature for 30 min. The molar ratio of Cu2+ and PO43was varied at 2:1, 1:2, and 1:4, respectively. In some cases, CuAc2 was replaced by CuCl2 or (NH3)2HPO4 was replaced by K2HPO4 for the parallel experiments (as shown in Table 1). Prior to the reaction, the solution mixture was sonicated in an ultrasonic water bath for 10 min. The bluish slurry mixture was then transferred to a Teflon-lined stainless steel autoclave, which was filled with deionized water up to 80% of its capacity (80 mL). The autoclave was heated to 180 °C in 60 min and maintained at this temperature for 48 h, and then cooled to room temperature naturally. The deep green crystals were collected and washed several times with distilled water and absolute ethanol to remove impurities. The final products were dried at 50 °C (more than 5 h) for further characterizations. The phase and crystallographic structure of the samples were determined by powder X-ray diffraction (XRD, D/Max 2400, Rigaku, by a diffractometer equipped with the graphite monochromatized Cu KR radiation) in the 2θ angles ranging from 10° to 70°. The morphology and size of the crystals were characterized by scanning electron microscope (SEM, JSM5600LV, JEOL) equipped with an energy-dispersive X-ray spectrometry (EDX). FT-IR spectra were recorded on a Fourier transform infrared spectrometer (FT-IR, KBr disk method; NEXUS) at wavenumbers 400-4000 cm-1. Results and Discussion The deep green copper hydroxyphosphate crystallizes in the orthorhombic space group of Pnnm with a ) 8.062(5) Å, b ) 8.384(4) Å, and c ) 5.881(2) Å.19 The structure (as shown in Figures S3-S5, Supporting Information) consists of a PO4 tetrahedron, a CuO4(OH) trigonal bipyramid, a CuO4(OH)2 octahedron, and an OH group between two Cu species but no P-O-P chains. Chains of edge-sharing CuO4(OH)2 octahedra extend parallel to the c-axis and are linked by the corner-sharing PO4 tetrahedron to form channels in that direction. The octahedra, which are elongate due to the Jahn-Teller effect, share corners with pairs of edge-sharing CuO4(OH) trigonal bipyramids that occur in these channels. XRD patterns of the as-prepared copper hydroxyphosphate samples are shown in

Figure 1. Representative XRD patterns of Cu2(OH)PO4 crystals: (A) Experimental conditions correspond to entry no. 2 in Table 1. (B) Experimental conditions correspond to entry no. 1 in Table 1. (C) XRD patterns of Cu2(OH)PO4 polycrystalline powders ground from crystals (B). The standard diffraction pattern of Cu2(OH)PO4 (JCPDS No. 360404) is shown as a reference.

Figures 1 and S2, all peaks can be clearly indexed as the pure orthorhombic phase and are consistent with the reported data (JCPDS card File No. 36-0404). No diffraction peaks for other phases or materials (such as ludjibaite [Cu5(OH)4(PO4)2] or copper phosphate hydrate [Cu3(PO4)2‚3H2O]) are observed in XRD patterns, indicating a high purity and crystallinity of the final products. The relative intensities of peaks in parts A and B of Figure 1 are obviously different. The relative intensities of (110), (220), (330), (440), and (011) planes provide information regarding the crystal orientation against the substrate (which was used to prepare XRD samples) because the powder diffractometer collects reflections only from the crystallographic planes parallel to the substrate. For example, XRD patterns (Figure 1A) of orthorhombic crystals show a weak intensity of the (011) plane compared to Figure 1B, since the elongated octahedra are seldom oriented along {011} planes parallel to the substrate as shown in typical SEM images (Figure S6 is different from Figure S7). However, Figure 1B shows the strong intensity of the (011) plane and the weak intensity of the (110), (220), (330), and (440) planes, which means that the {011} planes of these elongated octahedra particles have more chances parallel to the substrate (as shown in Figure S6). From the relative intensity of different reflection peaks, we can conclude that the elongated octahedron crystals form faces with the {110} and {011} planes (four {110} tetragonal prism faces and four {011} trigonal end caps), and the elongated direction is along the c-axis. The explicit descriptions of the schematic illustration are shown in Figures 2D and S7C).

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Figure 2. SEM images of typical copper hydroxyphosphate crystals (experimental conditions: 5 mL of 1.0 M CuCl2 + 20 mL of 1.0 M (NH4)2HPO4 at 150 °C for 72 h). (A, B, and C) Detailed views at different visual angles, scale bar ) 50 µm (panoramic morphologies are shown in the Supporting Information, Figure S6). (D) Schematic illustration for a primary stout prismatic crystal, expressing the {110} tetragonal prism faces and {011} trigonal end caps.

SEM images of the as-prepared copper hydroxyphosphate single crystals are shown in Figure 2, which exhibit a novel prism morphology. The prism is bounded with four trapeziform prism faces and four trigonal end caps. The newly formed crystal faces can be identified by stereological analysis of scanning electron micrographs taken from crystal samples at different viewing directions. The assignment of {110} and {011} faces is derived from the XRD analysis and the results are compared to those of computer models of copper hydroxyphosphate elongated octahedron (Figure 2D).20 Surprisingly, a delicate morphology with two kinds of elaborate patterns at different faces appeared when we tuned the experimental conditions (10 mL of CuAc2 (0.25 M) + 10 mL of (NH4)2HPO4 (1.0 M) at 180 °C for 48 h, the initial reagents molar ratio of Cu2+ and PO43- was fixed as 1:4, as shown in Figure 3) in our present synthetic system. Four enlaced faces of the delicate cube show a butterfly-like pattern (com-

Xu and Xue prised of two symmetrical trapeziform patterns), while the other two opposite faces express four uniform pairs of triangle pattern, i.e., eight-triangle pattern (Figures 3C and 3D). We have proposed a rotation twinned-crystal growth for this delicate cube, as shown in Figure 4. An ideal of the elongated octahedron subunit is rotated 90° about the c-axis and incorporated with itself, the rotation twinned-crystal formed (Figure 4B,D). The rotation axis is the c-axis and the rotation angle is 90°. There are four almost equivalent reentrant angles from the top view (looking from the c-axis). The cause of the twinned-crystal is one of the most complicated issues for crystal growth engineering.21-23 The formation of most twinned-crystals is due to the atomic stacking fault and the intrinsic equilibrium structures with lower energys the geometric and atomic characteristic of the crystal structure is thus the mainly intrinsic factor. The slight discrepancy of lattice constants (a ) 8.062 Å and b ) 8.384 Å) and its growth habit along the c-axis are two intrinsic factors for the formation of rotation twinned-crystals. The crystal growth environments are also an important extrinsic factor in the crystallization of twinned-crystals. The existence of acetate (as the only organic compound in our present synthetic system) may have an effect on copper hydroxyphosphate crystal growth. When CuAc2 was replaced by CuCl2, the morphology is changed drastically (comparing Figure 2 to Figure 3). The main reason is likely to be that the diffuse rate of Cu2+ ions in solution is different in both growth environmentssthe Cu2+ ions can easily form complexes with CH3COO- anions during the formation of copper hydroxyphosphate architectures.27 The existence of lattice OH- ions has an essential influence on the morphology of copper hydroxyphosphate during the hydrothermal crystallization process. It is well-known that27 the Cu2+ ion forms complexes with anions in the solution, the bond strength of which follows the order OH- > NH3 > acetate. At high pH values the Cu2+ ion can form complexes with OHanions with a high probability during the crystallization, and vice versa. Hence, the initial pH value of the solution plays an important role during the crystallization of copper hydroxyphosphate. A similar analysis for the crystal morphology has been reported in our previous work,11 based on the calculation

Figure 3. SEM images of copper hydroxyphosphate rotation twinned-crystals, expressing a delicate cube morphology with elaborate patterns at different faces (experimental conditions: 10 mL of CuAc2 (0.25 M) + 10 mL of (NH4)2HPO4 (1.0 M) at 180 °C for 48 h). (A and B) Panoramic morphologies, scale bar ) 10 µm. (C and D) Detailed views on two kinds of faces, scale bar ) 5 µm. (E) Schematic illustration of a theoretical rotation twinned-crystal (stereoview).

Copper Hydroxyphosphate with Complex Architectures

Figure 4. Schematic illustration of a theoretical rotation twinnedcrystal. (A) An ideal of the elongated octahedron subunit. (B) A theoretical rotation twinned-crystal (after self-rotated 90°). (C and D) Top views (looking from the c-axis) of the twinned-crystal represented in parts A and B.

of the bond strength (considering different chemical bonds in the crystal lattice and different growth units during the crystallization process). At a high pH value the Cu2+ ion forms complexes with OHanions and we consider that Cu-OH bonds are formed before the crystallization (type I), while at low pH values, we consider that Cu-OH bonds are formed during the crystallization (type II). In the lattice of copper hydroxyphosphate crystals (Figures S3-S5), Cu-OH bonds lie between the {001} planes. Therefore, different types of Cu-OH bonds (so-called type I and type II) have a great effect on the growth rate of {001} planes, i.e., along the c-axis direction (the growth rate is determined by the bond number and bond strength, which formed during the crystallization). At a high pH value (more Cu-OH bonds are formed before the crystallization), the growth rate along the

J. Phys. Chem. B, Vol. 110, No. 15, 2006 7753 c-axis direction is low, and the morphology is the typical elongated octahedron (Figure S6), while at a low pH value (more Cu-OH bonds are formed during the crystallization), the growth rate along the c-axis direction is high, and the crystal appearance is the elongated octahedron with a high length-diameter ratio (Figure S7). Obviously, increasing the molar ratio of Cu2+ and PO43- can inevitably lead to a decreased pH value of the solution. At the same time, the concentration of PO43- also has an effect on the crystallization process. The whole synthetic system is a buffer solution environment, which can keep the pH value of the solution in a certain range during the whole crystallization. When the concentration of PO43- is high, the pH value changes slowly, which also provides a kind of relatively stable growth environment for the crystal growth. In a word, the whole reaction system provides an appropriate crystal growth environment in our present process. Therefore, we can conclude that the initial molar ratio of Cu2+ and PO43- plays a pivotal role in controlling the crystallization of copper hydroxyphosphate crystals. By gradually increasing the initial reagent’s molar ratio of Cu2+ and PO43- (from 1:4 to 1:2, and 2:1 respectively), two kinds of novel architectures were observed based on the rotation twinned-crystal growth, and self-similar growth (as shown in Figures 5 and 6). SEM images of copper hydroxyphosphate with a nontrivial superstructure are shown in Figure 5, which expresses a quadruple petal pumpkin-like morphology. From the top visual angle, this morphology has some similarities with the rotation twinned-crystals (Figure 3) based on the regular staking of prismatic crystals. Figure 6 shows a dumbbell-shaped copper hydroxyphosphate aggregate (also called a double-mushroom-like morphology). The fractal growth and architecture start with long prismatic seeds followed by self-similar branching and end up with anisotropic dumbbell-shaped aggregates. Fractal objects are best known as self-similar patterns that cannot be explained by classic geometry. Fractals are scale invariant, meaning that an object will look statistically similar at different length scales. For example, if a piece of a fractal object is cut out and magnified, the resulting object will, on average, look the same as the original one. This so-called self-similarity is a characteristic of fractal objects.24 The fractal progressive stages of the morphogenesis are clearly shown in Figure 6C-E. Figure S10 shows the coexist-

Figure 5. SEM images of copper hydroxyphosphate nontrivial superstructure, expressing a quadruple petal pumpkin-like morphology (experimental conditions: 20 mL of CuAc2 (0.25 M) + 10 mL of (NH4)2HPO4 (1.0 M) at 180 °C for 48 h). (A and B) Panoramic morphologies, scale bar ) 10 µm. (C, D, and E) Detailed views at different visual angles, scale bar ) 5 µm.

7754 J. Phys. Chem. B, Vol. 110, No. 15, 2006

Xu and Xue

Figure 6. SEM images of a dumbbell-shaped copper hydroxyphosphate architecture (experimental conditions: 20 mL of CuAc2 (0.25 M) + 2.5 mL of (NH4)2HPO4 (1.0 M) at 180 °C for 48 h). (A and B) Panoramic morphologies, scale bar ) 10 µm. (C, D, and E) Selected sequence of SEM images of progressive stages of self-assembled (hierarchical) growth of copper hydroxyphosphate aggregates (morphogenesis): from an elongated crystal to dumbbell shape, scale bar ) 5 µm.

Figure 7. SEM images of copper hydroxyphosphate nontrivial superstructure, expressing a double-flower-like morphology (experimental conditions: 10 mL of CuAc2 (0.25 M) + 1.25 mL of (NH4)2HPO4 (1.0 M) at 180 °C for 48 h). (A and B) Panoramic morphologies, scale bar ) 100 and 50 µm, respectively. (C, D, and E) Detailed views at different visual angles, scale bar ) 20 µm.

ence of different stages of aggregates; it is clearly shown that our dumbbell-shaped copper hydroxyphosphate aggregate is a self-similarity. The mechanism (i.e., the branching growth at two ends of the primary crystal and the formation of a dumbbell morphology) seems a general growth phenomenon and has been observed in several systemssthe exact growth mechanism is still under investigation, although some explanations were given in the literature based on the role of intrinsic electric fields (that direct the growth of dipole crystals).3,25,26 Our present findings (the investigation of copper hydroxyphosphate crystallization reveals the rotation twinned-crystal growth habit and general elongation along the c-axis) may also shed light on the general aspects of this morphology-formation mechanism. Our work also shows that other synthetic conditions have great effects on the formation of copper hydroxyphosphate architectures, such as the concentration of reagents, reaction temperature, and time (some parallel experimental results are shown in Figures S14 and S15, see the Supporting Information). When the initial concentration of reactants is half (i.e., the

dosage of CuAc2 and (NH4)2HPO4 is half), copper hydroxyphosphate with the double-flower-like morphology can be obtained (as shown in Figures 7 and S12). The reactant concentration exerts an important impact on the crystal nucleation and growth, which is responsible for the morphology of products. At the low concentration of initial reagents, crystals grow gradually with enough room and time (i.e., at the low supersaturation level, the crystal growth rate is a dominant step). The particle size is enlarged (comparing Figure 6 to Figure 7, the average particle size is about 20 and 50 µm, respectively) and the degree of branching growth is increased. When an amount of ethanol is added in the synthetic process to increase the supersaturation of solution (experimental conditions: 10 mL of CuAc2 (0.25 M) + 1.25 mL of (NH4)2HPO4 (1.0 M) at 180 °C for 48 h, the Teflon-lined stainless steel autoclave was filled with ethanol up to 80% of its capacity), the morphology of the as-prepared copper hydroxyphosphate is changed drastically (comparing Figure 7 with Figure 8). Figure 8 shows a well-defined aggregate based on the regular

Copper Hydroxyphosphate with Complex Architectures

J. Phys. Chem. B, Vol. 110, No. 15, 2006 7755

Figure 8. SEM images of copper hydroxyphosphate nontrivial superstructure, which are well-defined aggregates (experimental conditions: 10 mL of CuAc2 (0.25 M) + 1.25 mL of (NH4)2HPO4 (1.0 M) at 180 °C for 48 h; in this experiment, the Teflon-lined stainless steel autoclave was filled with ethanol up to 80% of its capacity). (A and B) Panoramic morphologies, scale bar ) 50 and 20 µm, respectively. (C, D, and E) Detailed views at different visual angles, scale bar ) 10 µm.

Figure 9. SEM images of various copper hydroxyphosphate architectures: (A) a single crystal, scale bar ) 50 µm; (B) a rotation twinned-crystal, scale bar ) 5 µm; (C) a quadruple petal pumpkin-like architecture, scale bar ) 5 µm; (D) a dumbbell-shaped copper hydroxyphosphate architecture, scale bar ) 5 µm; (E) a double-flower-like architecture, scale bar ) 20 µm; and (F) a well-defined aggregate, scale bar ) 10 µm.

staking of typical prismatic crystals. As a whole, the general morphology of aggregates has some similarities with the rotation twinned-crystals (comparing Figure 8 with Figure 3). Simultaneously, the particle size becomes smaller (comparing Figure 7 with Figure 8, the average particle size is about 50 and 30 µm, respectively) and the degree of branching growth is decreased. In addition, the esterification reaction and its reverse reaction between ethanol and acetic acid in this synthetic system also have an effect on the formation of this well-defined aggregate, and ethanol molecules may affect copper hydroxyphosphate morphology by interacting with hydroxyl (Cu-OH) during the crystallization process. In principle, the observed fractal growth occurs at synthetic conditions far from thermodynamic equilibrium where the chemical perturbation plays an important role. On the basis of our growth experiments, it is clear now that the branching is extremely sensitive to the provided chemical environment (especially, the initial reagent’s molar ratio of Cu2+ and PO43-, and the supersaturation level), which consequently determines the final organization and crystal morphology.

Finally, various typical copper hydroxyphosphate architectures are shown in Figure 9 for a clear comparison. A basic crystal shape is determined by the habit formation and branching growth. A good control of these processes leads to an enormous degree of synthetic freedom in generating various complex architectures. Copper hydroxyphosphate crystals tend to grow along the c-axis and have the rotation twinned-crystal growth habit, which is essential for the formation of various complex architectures. Furthermore, the growth environments exert an exquisite control over the crystallization process for various complex architectures. In this hydrothermal crystallization process, the variation of the pH value and Cu2+ and PO43concentrations in the microenvironment of the interface (between the crystal and solution) directly affect the form of growth units, which drastically affects the final morphology. Meanwhile, there is the most stable square-planar amino complex [Cu(NH3)42+] in our synthetic system. Cu2+ can be gradually released from the amino complex [Cu(NH3)42+] during the crystallization process. In the parallel experiment, when (NH3)2HPO4 was replaced by K2HPO4, the morphology of the obtained particles

7756 J. Phys. Chem. B, Vol. 110, No. 15, 2006 has been changed drastically, which can be clearly observed from Figures 5 and S15. In addition, pumpkin-like, dumbbellshaped, and double-flower-like morphologies are various forms of the spherulitic polycrystalline aggregate at different stages. We have obtained these architectures by tuning the Cu2+/PO43molar ratio and reagent concentration. These architectures are self-similar objects, also called fractal growth, which are used qualitatively without measuring fractal dimensions. Conclusions In summary, we have demonstrated a simple and mild hydrothermal approach by elucidating and manipulating key conditions that affect shape-guiding processes to precisely control the morphology of copper hydroxyphosphate. A variety of novel copper hydroxyphosphate architectures can be constructed through a careful control of synthetic parameters, such as the molar ratio of initial reagents, reagent concentrations, and reaction time and temperature. On the basis of structure and chemical bond analysis, copper hydroxyphosphate crystals tend to grow along the c-axis and have the rotation twinnedcrystal growth habit, which is essential for the formation of various architectures. The ability to spontaneously form such remarkable complex architectures as described in this work can offer a novel approach to fabricate new materials with a wide range of technological applications. The simple hydrothermal process does not need any inorganic and organic templates, which can easily avoid introducing impurities into the final product. The present hydrothermal process may also be applicable to the preparation of other basic copper(II) compounds such as gerhardtite (Cu2(OH)3NO3), atacamite (Cu2(OH)3Cl), and posnjakite (Cu4(OH)6SO4‚H2O). Acknowledgment. The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (NSFC #20471012), a Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China (FANEDD #200322), the Research Fund for the Doctoral Program of Higher Education (RFDP #20040141004), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. Supporting Information Available: XRD, FT-IR, SEM, and EDX results of the reported samples and the crystal structure of copper hydroxyphosphate. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Mann, S. Angew. Chem., Int. Ed. 2000, 39, 3392. (b) Mann S.; Ozin, G. A. Nature 1996, 382, 313. (c) Colfen H.; Antonietti, M. Angew. Chem., Int. Ed. 2005, 44, 5576. (2) (a) Bowden, B.; Weck, M.; Choi, I. S.; Whitesides, G. M. Acc. Chem. Res. 2001, 34, 231. (b) Yang, P.; Rizvi, A. H.; Messer, B.; Chmelka, B. F.; Whitesides, G. M.; Stucky, G. D. AdV. Mater. 2001, 13, 427. (c) Alivisatos, A. P. Nat. Biotechnol. 2004, 22, 47. (3) (a) Kniep, R.; Busch, S. Angew. Chem., Int. Ed. Engl. 1996, 35, 2623. (b) Busch, S.; Dolhaine, H.; DuChesne, A.; Hernz, S.; Hochrein, O.; Laeri, F.; Podebrad, O.; Vietze, U.; Weiland, T.; Kniep, R. Eur. J. Inorg. Chem. 1999, 1643. (c) Busch, S.; Schwarz, U.; Kniep, R. AdV. Funct. Mater. 2003, 13, 189.

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