Facile Synthesis and Fine Morphological Tuning of Ag2O

Article pubs.acs.org/crystal

Facile Synthesis and Fine Morphological Tuning of Ag2O Myeong-Jin Kim,† Young-Sik Cho,† Seong-Hun Park,‡ and Young-Duk Huh*,† †

Department of Chemistry, Institute of Nanosensor and Biotechnology, Dankook University, Gyeonggi-Do 448-701, Republic of Korea ‡ Department of Chemistry, Faculty of Liberal Art and Teacher Education, University of Seoul, Seoul 130-743, Republic of Korea ABSTRACT: The morphological evolution of the Ag2O cubic crystal system was examined with the goal of controlling and fine-tuning the morphologies of the microcrystalline products. A variety of Ag2O microcrystalline shapes could be prepared through the reduction of a silver-pyridine complex in solution at room temperature. The concentrations NaOH and AgNO3 significantly affected the morphology of the Ag2O final product. The Ag2O morphology evolved from a simple truncated cube to an 8-pod with three arms along the ⟨111⟩ directions as the concentration of NaOH increased. The Ag2O morphology evolved from an edge-truncated cube to a rhombic dodecahedron as the total reactant concentrations increased, holding the AgNO3/pyridine/NaOH molar ratio constant. The morphological evolution from an 8-pod to a rhombic dodecahedron via an 8 × 3-pod, a 6 × 4-pod, a smoothed squared 6-pod, a concave rhombic dodecahedron, and a truncated rhombic dodecahedron was also observed. The crystal growth mechanism underlying the morphological evolution of Ag2O microcrystals is discussed.

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INTRODUCTION Morphology-controlled synthesis is an important issue in inorganic materials, which display unique morphology-dependent optical, magnetic, and catalytic properties.1−8 Facets with distinct atomic structures provide different photocatalytic activities and chemical reactivities. Several studies have recently reported the morphology-controlled synthesis of inorganic oxides. A variety of methods for preparing inorganic oxides using hard and soft templates have been developed. These methods can suffer from drawbacks associated with the template in that the removal of the hard template can be difficult, and residual soft template material at the crystal surfaces can affect the physical properties of the inorganic oxide. Wet simple chemical methods that are adaptable to the large-scale production of inorganic oxides without the need for hard or soft templates would be advantageous. Among the metal oxides of interest, cuprous oxides (Cu2O) have been extensively investigated in the context of morphologycontrolled synthetic approaches.9−13 The cubic unit cell of the Cu2O yields crystals with simple closed-shaped morphologies, such as cubes, truncated cubes, cuboctahedra, octahedra, and rhombic dodecahedra. A variety of Cu2O crystals with open shapes, such as 6-pods, 8-pods, and 12-pods, have also been widely observed. The morphology-dependent photocatalytic, antibacterial, and adsorption properties of Cu2O crystals have been studied.14−18 Crystal morphologies are affected by both the crystal habit formation and the mechanism for branching growth.19,20 Crystals with simple closed shapes, such as cubes and octahedra, minimize the total surface free energy under thermodynamic equilibrium during crystal habit formation and growth. Crystals with open shapes and branched morphologies, such as 6-pods and 8-pods, form by branching growth © 2012 American Chemical Society

processes. The development of chemical methods that permits the independent fine-tuning of crystal habit formation and growth provides an important strategy for the selective morphology-controlled preparation of inorganic oxides. Silver oxide (Ag2O) has been widely used as photocatalyst, antibacterial material, colorant, and electrode material. The morphology-dependent antibacterial activities of Ag2O were recently studied.21,22 The photocatalytic properties and antibacterial activities of Ag2O depend strongly on the surface atomic structures of the crystals. Because the {100}, {110}, and {111} crystalline surfaces of Ag2O display different surface atomic structures, monodisperse Ag2O crystals of a specific morphology must be selectively prepared. Although Cu2O and Ag2O assume similar cubic crystal structures, relatively little is known about the morphology-controlled synthesis and physical properties of Ag2O. To our knowledge, only one study has described the complete morphological evolution of Ag2O from a cube to a 6-pod structure.23 Most Ag2O synthetic efforts have focused on the preparation of stable morphologies rather than developing strategies for producing a variety of morphologies. A new method involving adjustments to the experimental conditions would provide a useful synthetic strategy for finetuning the morphologies. Thus, studies that focus on morphology-controlled synthetic approaches to the fine-tuning of various Ag2O morphologies have become important for understanding crystal growth mechanisms. This paper reports a systematic method for preparing a variety of Ag2O crystal morphologies, including cubes, 8-pods, 8-pods (3-arms), 8 × 3Received: May 18, 2012 Revised: June 18, 2012 Published: July 11, 2012 4180

dx.doi.org/10.1021/cg300681b | Cryst. Growth Des. 2012, 12, 4180−4185

Crystal Growth & Design

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pods, 6 × 4-pods, truncated cubes, concave rhombic dodecahedra, and rhombic dodecahedra using simple adjustments to the reactant concentrations without the need for hard or soft templates.

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Article

RESULTS AND DISCUSSION The crystal structures of the samples were characterized by XRD analysis. Figure 1 shows the XRD patterns of typical Ag2O

EXPERIMENTAL SECTION

Ag2O crystals were synthesized by a simple precipitation method at room temperature. All chemical reagents were commercial analyticgrade reagents and were used without further purification. The experimental conditions for the preparation of Ag2O crystals are summarized in Table 1. In a typical process for sample 1, a mixed

Table 1. Brief Summary of the Experimental Conditions Tested in This Work

no.

[AgNO3] (mmol)

[pyridine] (mmol)

[NaOH] (mmol)

[AgNO3]/ [pyridine]/ [NaOH] molar ratio

1

2.0

80.0

5.0

1:40:2.5

2 3 4

2.0 2.0 2.0

80.0 80.0 80.0

8.0 12.0 20.0

1:40:4 1:40:6 1:40:10

5 6

2.0 2.0

80.0 80.0

28.0 40.0

1:40:14 1:40:20

7

2.0

80.0

5.0

1:40:2.5

8 9 10 11 12 13

3.0 3.5 4.0 5.0 6.0 2.0

120.0 140.0 160.0 200.0 240.0 80.0

7.5 8.75 10.0 12.5 15.0 20.0

1:40:2.5 1:40:2.5 1:40:2.5 1:40:2.5 1:40:2.5 1:40:10

14 15 16 17 18 19

3.0 4.0 5.0 14.0 15.0 2.0

120.0 160.0 200.0 560.0 600.0 80.0

30.0 40.0 50.0 140.0 150.0 40.0

1:40:10 1:40:10 1:40:10 1:40:10 1:40:10 1:40:20

20 21 22 23 24

3.0 4.0 5.0 9.0 15.0

120.0 160.0 200.0 360.0 600.0

60.0 80.0 100.0 180.0 300.0

1:40:20 1:40:20 1:40:20 1:40:20 1:40:20

results Figures 2a, 3a Figure 2b Figure 2c Figures 2d, 4a Figure 2e Figures 2f, 5a Figures 2a, 3a Figure 3b Figure 3c Figure 3d Figure 3e Figure 3f Figures 2d, 4a Figure 4b Figure 4c Figure 4d Figure 4e Figure 4f Figures 2f, 5a Figure 5b Figure 5c Figure 5d Figure 5e Figure 5f

Figure 1. Typical XRD pattern of the as-prepared Ag2O products.

Figure 2. SEM images of Ag2O products prepared at different NaOH concentrations: (a) sample 1; (b) sample 2; (c) sample 3; (d) sample 4; (e) sample 5; and (f) sample 6. The insets show high-magnification SEM images with scale bars of 1 μm.

solution containing water (96.82 mL), a 0.1 M AgNO3 aqueous solution (2.0 mL), and pyridine (0.68 mL) was prepared. To the mixed solution was quickly added a 1.0 M NaOH aqueous solution (0.5 mL) at room temperature. The final mixed solution was incubated for 4 h. The products were collected by centrifugation at 4000 rpm for 5 min, were washed with water and ethanol several times, and were dried for 24 h at room temperature. The morphological evolution of the crystals was monitored as a function of the NaOH concentration, holding other reactant concentrations constant (samples 1−6). The morphological evolution of the crystals was also monitored at three AgNO3 concentrations by testing three sets of samples with AgNO3/ pyridine/NaOH molar ratios of 1/40/2.5 (sample 7−12), 1/40/10 (sample 13−18), and 1/40/20 (sample 19−24). The structures of the Ag2O products were analyzed by powder X-ray diffraction (XRD, X’pert-pro MPD) using Cu Kα radiation. The morphologies were characterized by scanning electron microscopy (SEM, Hitachi S-4300).

products. All peaks in Figure 1 corresponded to those reported for bulk Ag2O (JCPDS 12-0793, a = 0.4736 nm) with a cubic structure. No other impurity peaks were observed in the XRD pattern, indicating that Ag2O had been successfully synthesized. The Ag2O products were formed in a silver−pyridine complex solution. Since pyridine (Py) is a strong ligand, it is expected to chelate Ag+ ion to form [Ag(Py)2]+ complex. The formation constant of [Ag(Py)2]+ complex at 25 °C is about 2.2 × 104.24 This complex is linearly coordinated due to the sp hybridization of Ag+ ion.25,26 This complex then reacted with OH− to release the pyridine and form a water-insoluble Ag(OH)2 product upon the addition of NaOH. Finally, Ag2O formed by the dehydration of Ag(OH)2. 4181



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Figure 5. SEM images of the Ag2O products prepared at different AgNO3 concentrations for a fixed AgNO3/pyridine/NaOH molar ratio of 1/40/20: (a) sample 19; (b) sample 20; (c) sample 21; (d) sample 22; (e) sample 23; and (f) sample 24. The insets show highmagnification SEM images with scale bars of 1 μm.

Figure 3. SEM images of the Ag2O products prepared at different AgNO3 concentrations for a fixed AgNO3/pyridine/NaOH molar ratio of 1/40/2.5: (a) sample 7; (b) sample 8; (c) sample 9; (d) sample 10; (e) sample 11; and (f) sample 12. The insets show high-magnification SEM images with scale bars of 1 μm.

pyridine molar ratio of 1/40. At 5.0 × 10−3 M NaOH (sample 1), an edge-truncated cubic Ag2O product with a mean length of 1.3 μm was prepared, as shown in Figure 2a. At 8.0 × 10−3 M NaOH (sample 2), each plane of the cube featured a concave center, as shown in Figure 2b. As the NaOH concentration was increased to 1.2 × 10−2 M, open-shaped Ag2O crystals featuring eight identical horns were obtained from an increase in the concave areas within the edge-truncated cubic Ag2O product, as shown in Figure 2c. At a NaOH concentration of 4.0 × 10−2 M, Ag2O 8-pods were obtained. SEM images of the 8-pods revealed branched structures in which three arms were attached perpendicular to the main axis of each 8-pod. These structures were denoted 8-pods (3-arms), as shown in Figure 2d−f. The average thickness of the 8-pods decreased with increasing NaOH concentration. As the NaOH concentration increased, the closed truncated cubic structures first grew branches to form open structures with eight identical horns (8-pod structures); then they subsequently branched to form thin 8pod (3-arms) hyperstructures at higher NaOH concentrations. Generally, crystal growth processes are controlled by both crystal habit formation and branching growth. Crystal habits are associated with the surface energy of the crystal planes, whereas branching growth is related to diffusion effects. Branching growth is driven by kinetically controlled reactions, whereas crystal habit formation is driven by thermodynamically controlled reactions. Hyperstructures form preferentially by branching crystal growth, prior to establishing thermodynamic equilibrium. An 8-pod structure is usually observed in cases in which the crystal growth rate of the ⟨111⟩ direction is much larger than that of the ⟨100⟩ direction. Therefore, the kineticcontrolled crystal growth of the ⟨111⟩ direction increases significantly with the NaOH concentration for a fixed AgNO3/ pyridine molar ratio. The kinetic process drives the

Figure 4. SEM images of the Ag2O products prepared at different AgNO3 concentrations for a fixed AgNO3/pyridine/NaOH molar ratio of 1/40/10: (a) sample 13; (b) sample 14; (c) sample 15; (d) sample 16; (e) sample 17; and (f) sample 18. The insets show highmagnification SEM images with scale bars of 1 μm.

The morphologies of the Ag2O products were examined by SEM. A wide range of Ag2O product morphologies was obtained, depending on the concentrations of NaOH or AgNO3. The sets of synthetic reaction conditions are listed in Table 1. Figure 2 shows SEM images of the Ag2O products prepared with different concentrations of NaOH at AgNO3/ 4182



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Figure 6. Schematic diagram of the morphological evolution of Ag2O microcrystals as a function of the NaOH concentration and the total AgNO3/ pyridine/NaOH reactant concentration. The red, yellow, and green colors represent {100}, {110}, and unidentified planes, respectively.

truncated cubes formed without formation of corner-truncated forms. The body-centered cubic Ag2O crystal structure grew preferentially at both the {100} and {110} planes. As the ratio of the crystal growth rates along the {110} and {100} planes increased, the crystals evolved from cubes to rhombic dodecahedra. The crystal growth rates of the {110} planes relative to the {100} planes increased with the reactant concentrations for fixed AgNO3/pyridine/NaOH molar ratios. At very low concentrations of NaOH, as shown in Figure 3, only habit formation growth was involved. Figure 4 shows SEM images of the Ag2O products prepared at different AgNO3 concentrations for a fixed AgNO3/pyridine/ NaOH molar ratio of 1/40/10. The concentration of NaOH in this reaction system was 4 times the concentration in the reaction system used to prepare the crystals shown in Figure 3. At 2.0 × 10−3 M AgNO3, the products were 8-pods (3-arms), as shown in Figure 4a. At 3.0 × 10−3 M AgNO3, a second branching process occurred to form hyperbranched crystals, as shown in Figure 4b. Each 8-pod split into three pods to form 24-pods, denoted 8 × 3-pods. As the concentration of AgNO3 increased to 4.0 × 10−3 M, more complicated Ag2O product structures formed. The structures included six squared pods, denoted 6 × 4-pods. The surfaces of the squared pods were closed forms. The open 8-pods and 8 × 3-pods began forming

morphological evolution of Ag2O from a truncated cube to an 8-pod and further to a thin 8-pod (3-arms) hyperstructure. Figure 3 shows the SEM images of the Ag2O products prepared at different AgNO3 concentrations at a fixed AgNO3/ pyridine/NaOH molar ratio of 1/40/2.5. At 2.0 × 10−3 M and 3.0 × 10−3 M AgNO3, the products were edge-truncated cubes, as shown in Figure 3a and b. At AgNO3 concentrations of 3.5 × 10−3 M and 4.0 × 10−3 M, truncated rhombic dodecahedra were obtained, as shown in Figure 3c and d. Finally, rhombic dodecahedra were obtained at higher AgNO3 concentrations, 5.0 × 10−3 M and 6.0 × 10−3 M. As the total AgNO3/pyridine/ NaOH reactant concentration increased, the majority of Ag2O products were found to convert from edge-truncated cubes to rhombic dodecahedra via a truncated rhombic dodecahedron without passing through an open-shaped structure. The growth mechanism underlying the formation of rhombic dodecahedra from cubes, with retention of a closed structure, is driven mainly by the crystal habit formation. The crystal habit is determined by the thermodynamic surface free energies of the various crystal facets. A cube has six {100} facets, whereas a rhombic dodecahedron has twelve {110} facets. As shown in Figure 3a, truncated cubes have six {100} facets and twelve {110} facets. In this work, no crystals with eight {111} facets in truncated cubes were observed. This suggests that the edge4183



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dron as the total reactant concentration increased via a thermodynamically controlled habit formation mechanism. Both the crystal habit formation and the branching growth were involved in the morphological evolution at higher NaOH concentrations, and the Ag2O morphology evolved from an 8pod (3-arms) to 8 × 3-pods, 6 × 4-pods, smoothed squared 6pods, concave rhombic dodecahedra, and rhombic dodecahedra.

closed-shaped structures at higher AgNO3 concentrations for a AgNO3/pyridine/NaOH molar ratio of 1/40/10. As the AgNO3 concentration was increased further to 5.0 × 10−3 M, six smoothed squared pods formed, as shown in Figure 4d. The smoothed squared 6-pods resulted from crystal growth perpendicular to the surfaces of the 6 × 4-pods. As the concentration of AgNO3 increased to 1.4 × 10−2 M AgNO3, the crystals appeared to form concave rhombic dodecahedra by filling the open spaces of the six smoothed squared pods. Finally, rhombic dodecahedra were observed at 1.5 × 10−2 M AgNO3, as shown in Figure 4f. The branching growth and habit formation growth occurred simultaneously in the reaction system shown in Figure 4. For an AgNO3/NaOH molar ratio of 10, the branching growth mainly formed branched and hyperbranched structures, such as 8-pods (3-arms), 8 × 3pods, and 6 × 4-pods, at relatively low reactant concentrations. However, the branching growth processes occurred during the first stages of growth, and the habit formation crystal processes occurred in the second stages of growth, at higher reactant concentrations. During the second stage, the open spaces of the branched and hyperbranched structures formed during the first stage became filled in. Therefore, the NaOH and the total reactant concentrations played important roles in branching growth and habit formation, respectively. Figure 5 shows SEM images of the Ag2O products prepared with different AgNO3 concentrations at a fixed AgNO3/ pyridine/NaOH molar ratio of 1/40/20. The NaOH concentration in the reaction system shown in Figure 5 was 8 times higher than that in the reaction system shown in Figure 3. The morphological evolution in this reaction system was similar to that in the reaction system shown in Figure 4. As the total concentration of the reactants increased, the Ag2O morphology evolved from an 8-pod (3-arms), through the 8 × 3-pod and 6 × 4-pod, to a concave rhombic dodecahedron. In this case, the morphological evolution began with a thin 8pod (3-arms), as shown in Figure 5a, as distinct from the thick 8-pods (3-arms) shown in Figure 4a, because a higher concentration of NaOH was used in the samples shown in Figure 5a than those shown in Figure 4a. The thin 8-pods (3arms) included larger open spaces in the crystal. The open spaces filled to a lesser degree at higher total reactant concentrations, compared with the reaction system shown in Figure 4. Therefore, concave rhombic dodecahedra instead of rhombic dodecahedra were obtained as the final products, even at 1.5 × 10−2 M AgNO3, as shown in Figure 5f. In this reaction system, both the branching growth and habit formation growth were involved, as in the reaction system shown in Figure 4. Figure 6 shows a schematic diagram of the morphological evolution of Ag2O crystals as a function of the NaOH concentration and the total concentration of reactants.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +82 31 80053154. Fax: +82 31 80053148. E-mail address: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (KRF-2012-0002853).

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CONCLUSIONS We developed a simple wet chemical strategy for fine-tuning the morphology-controlled synthesis of Ag2O microcrystals without the need for hard or soft templates. Ag2O microcrystals were prepared by a precipitation reaction from a mixture of AgNO3, pyridine, and NaOH at room temperature. Various Ag2O microcrystal morphologies were achieved simply by adjusting the reactant concentrations. Kinetically controlled branching growth occurred at higher NaOH concentrations, and the Ag2O morphology evolved from an edge-truncated cube to a thin 8-pod (3-arms). The morphology of the Ag2O evolved from an edge-truncated cube to a rhombic dodecahe4184



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Facile Synthesis and Fine Morphological Tuning of Ag2O

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