ARTICLE pubs.acs.org/JPCC
Investigation of Relative Stability of Different Facets of Ag2O Nanocrystals through Face-Selective Etching Lian-Ming Lyu and Michael H. Huang* Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan
bS Supporting Information ABSTRACT: Ag2O nanocubes, rhombicuboctahedra, octahedra, and extended hexapods were employed for the examination of the relative stability of different crystal planes to chemical etching through careful face-selective etching. Precise control of the amount of NH3 solution injected into a mixture of Ag2O nanocrystals and NaOH enables this face-selective etching. Ag(NH3)2+ formed from dissolved silver ions should drive the etching process while NaOH tunes the reaction equilibrium to control morphology of the etched nanocrystals. The order of facet stability in this reaction was found to be {111} > {110} > {100}. The {100} faces are most easily etched. By carefully adjusting the volume of NH3 solution introduced, novel Ag2O cubic nanoframes and rhombicuboctahedra with square depressions on all the {100} faces can be fabricated. The {111} facets contain significant terminal silver atoms, so hydroxide ions should interact strongly to maintain these surfaces. Hydroxide ions are less effective at adsorbing on the {100} faces with terminal oxygen atoms, so these faces are more susceptible to etching. ζ potential measurements support the argument of hydroxide ion adsorption. Interestingly, the {100} facets of Ag2O were found to be most stable in a weakly acidic HNO3 solution; octahedral nanocrystals can transform into particles consisted of {100} square terraces.
’ INTRODUCTION Silver(I) oxide (Ag2O) is a p-type semiconductor with a reported band gap of 1.46 eV.1 It has the same cuprite crystal structure as that of Cu2O (a body-centered cubic packing of oxygen atoms with silver atoms occupying one-half of the tetrahedral sites). Ag2O has been widely used as a cocatalyst in palladium-catalyzed reactions for organic synthesis.2 Hence, Ag2O nanocrystals may be potentially useful for organic synthesis. Despite its apparent simplicity in preparation, very few studies have reported the synthesis of Ag2O nanocrystals with morphology control.37 We have previously described a procedure for the formation of Ag2O nanocrystals with systematic shape evolution from cubic to edge- and corner-truncated cubic, rhombicuboctahedral, edge- and corner-truncated octahedral, octahedral, and hexapod structures by preparing an aqueous mixture of AgNO3, NH4NO3, and NaOH with fixed molar ratios.5 The work was inspired by the ability to synthesize Cu2O nanocrystals with similar shape evolution.812 It was noted that some of the Ag2O particles exhibit slightly etched faces. It is therefore interesting to evaluate the relative stability of the different exposed faces (i.e., {100}, {110}, and {111} faces) of these nanostructures by carefully etching a particular surface while maintaining the structural integrity of the other faces. Face-selective etching is also a good approach to enrich the morphological variety of Ag2O nanostructures. As an example, face-selective etching of truncated rhombic dodecahedral Cu2O nanocages by HCl can lead to the formation of Cu2O nanoframes.13 An investigation of the relative stability of different crystal planes of Cu2O nanocrystals in a weak acetic acid solution has also been reported by r 2011 American Chemical Society
examining the extent of face etching and the resulting exposed surfaces.14 In this study, we have used Ag2O nanocubes, rhombicuboctahedra, octahedra, and extended hexapods to examine the relative stability of different crystal faces of Ag2O by selectively etching the least stable faces. NH3 was used as the etchant. By carefully controlling the volume of NH3 solution injected, only a specific face is etched, resulting in the formation of new Ag2O nanostructures. Cubic nanoframes can also be synthesized. Structural and optical characterization of these etched nanocrystals has been performed. The relative instability of different crystal facets to chemical etching is explained in terms of their surface atomic structures and supported by ζ potential measurements. The relative facet stability of these Ag2O nanocrystals in a weak HNO3 solution was also examined.
’ EXPERIMENTAL SECTION Chemicals. Silver nitrate (AgNO3, 99.8%, Showa), ammonium nitrate (NH4NO3, 99%, Showa), ammonia (NH3, 2830%, Showa), sodium hydroxide (NaOH, 99%, Mallinckrodt), and nitric acid (HNO3, 65%, Fluka) were used without further purification. Ultrapure distilled and deionized water was used for all solution preparations. Received: June 24, 2011 Revised: August 2, 2011 Published: August 23, 2011 17768
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Synthesis of Ag2O Nanocrystals with Systematic Shape Evolution. Procedure for the synthesis of Ag2O crystals with
systematic shape evolution from cubic to rhombicuboctahedral and octahedral structures has been described previously.5 Briefly, 0.1 M AgNO3, 0.2 M NH4NO3, and 2.0 M NaOH were mixed with final AgNO3:NH4NO3:NaOH molar ratios of 1:2:11.8. The total solution volume is 6 mL. Since the amount of Ag2O nanocubes produced is relatively small, the volumes of the reagents used have been proportionally increased six-fold in this study for the synthesis of nanocubes. Thus, for the growth of Ag2O nanocubes, 26.865 mL of water, 1.5 mL of 0.2 M NH4NO3, 1.5 mL of 0.1 M AgNO3, 0.135 mL of 2.0 M NaOH, and 6.0 mL of 0.25 M NaOH were introduced in the order listed. The preadded 2.0 M NaOH can more efficiently produce NH3 from ammonium ions for the formation of Ag(NH3)2+ ions. The molar ratio of NH4NO3: preadded NaOH is 1:0.9. Extended hexapods were synthesized with AgNO3:NH4NO3:NaOH molar ratios of 1:2:41.8. After centrifugation and removal of the top solution, the Ag2O particles obtained were dispersed in deionized water. The amount of deionized water used is the same as that of 0.1 M AgNO3 introduced for the synthesis of Ag2O nanocrystals. For instance, 1.5 mL of water was used for the storage of Ag2O nanocubes. Etching Process. In a typical etching process, 9.7 mL of deionized water, 1.0 M NaOH, and 0.2 mL of Ag2O nanocrystal solution were mixed with sonication for 10 s. The mixture was then kept in a water bath set at 35 °C for 30 min. Different volumes of 0.15 M NH3 solution was injected into the mixture under stirring at a rate of 0.1 mL per min by using a syringe pump (see Supporting Information, Figure S4). The reaction was stopped after NH3 injection was completed. The sample was centrifuged at 4500 rpm for 80 s. The top solution was removed, and the precipitate was centrifuged again in 3 mL of water. Finally the precipitate was dispersed in 0.2 mL of deionized water. Instrumentation. Scanning electron microscopy (SEM) images of the synthesized Ag2O crystals were obtained using a JEOL JSM-7000F scanning electron microscope. TEM characterization was performed on a JEOL JEM-2100 electron microscope operating at 200 kV. XRD patterns were collected using a Shimadzu XRD-6000 diffractometer with Cu KR radiation. UVvis absorption spectra were taken on a Jasco V-570 spectrophotometer. ζ potential measurements were carried out on a Brookhaven 90Plus particle size analyzer. Here 60 μL of concentrated Ag2O particle solution dispersed in 3 mL of deionized water was used for the ζ potential measurements.
’ RESULTS AND DISCUSSION The Ag2O nanocrystals used for the face-selective etching study were synthesized following our reported procedure. An aqueous mixture of AgNO3, NH4NO3, and NaOH was prepared with fixed molar ratios of the reagents. Formation of Ag(NH3)2+ complex ions can control the reaction rate and allow the growth of nanocrystals with excellent shape control. The following reactions take place to form Ag2O: NH4 NO3 þ NaOH f NH3 þ H2 O þ Naþ þ NO3 ð1Þ AgNO3 þ 2NH3 T AgðNH3 Þ2 þ þ NO3
ð2Þ
AgðNH3 Þ2 þ þ NaOH T AgOH þ 2NH3 þ Naþ
ð3Þ
2AgOH T Ag2 O þ H2 O
ð4Þ
Equation 3 as written has an equilibrium constant K of 2.94 (inverse of the reaction AgOH + 2NH3 + Na+ T Ag(NH3)2+ + NaOH with K = Ksp of AgOH Kf of Ag(NH3)2+ = (2.0 108) (1.7 107) = 0.34). Compared to the value of K for the direct mixing of AgNO3 and NaOH at 5 107 (Ag+ + OH T AgOH with K = 1/Ksp), formation of the Ag(NH3)2+ species can drastically reduce the rate of rapid growth of AgOH and thus Ag2O to achieve excellent morphology control of the Ag2O nanocrystals. In working with Ag2O nanocrystals, we found that their extended storage in water can lead to partial dissolution. Some particles with etched faces were produced. The reversible reaction of eq 4 may be responsible for this observation. Therefore, it should be interesting to explore this phenomenon and examine the relative stability of the different facets of Ag2O crystals. For face-selective etching of Ag2O nanocrystals, reversing the reaction in eq 3 may be feasible. NH3 solution was chosen as the etchant. To better control the rate and extent of face-selective etching, NaOH was premixed with Ag2O crystals before the subsequent continuous injection of NH3 solution. Figure 1 gives SEM images of Ag2O nanocubes, rhombicuboctahedra, octahedra, and extended hexapods before and after chemical etching. The crystals have sizes of several hundred nanometers, while the hexapods are over 1 μm in size. Progressively increasing volumes of NH3 solution were added to follow the extent of a controlled face-selective etching process. The nanocubes possess slightly truncated edges and corners. It is obvious that upon increasing the volume of 0.15 M NH3 solution injected from 140 to 160 and 180 μL, the {100} faces of the nanocubes are increasingly etched. Square depressions are formed first. As the extent of the {100} face etching process grows, cubic nanoframes can be observed. These nanoframes represent a new nanostructure for Ag2O and are attainable only with careful face-selective etching. The results indicate that the {100} faces of Ag2O crystals are most susceptible to chemical etching under the current reaction conditions. The susceptibility of the {100} faces of Ag2O nanocrystals to etching can be easily recognized even without the addition of NH3 solution. Figure 1b1 shows that some Ag2O rhombicuboctahedra already exhibit slightly etched square faces before the addition of NH3 solution due to their reaction with water. It is suspected that the much smaller fraction of the {100} faces on rhombicuboctahedra as compared to that for the nanocubes makes the phenomenon of face-etching more pronounced in rhombicuboctahedra when these particles are stored in water. To assist in the discussion of the face-selective etching process, drawings of the four different Ag2O nanocrystal morphologies with labeled facets are provided in Figure 2. Representative SEM images of novel face-etched Ag2O nanocrystals are also shown in Figure 2. By increasing the amount of NH3 solution added from 140 to 180 μL, nearly all {100} faces of rhombicuboctahedra are removed, and the depressions deepen. Remarkably, other facets of rhombicuboctahedra are preserved. The results further confirm the instability of the {100} faces of Ag2O to chemical etching. To establish the relative order of stability of the {110} and {111} faces of Ag2O nanocrystals, octahedra and extended hexapods were employed for the same etching experiments. The octahedral nanocrystals contain slightly truncated edges and corners. With the use of just 60 μL of NH3 solution, only the {100} corners of the octahedral nanocrystals were etched. Upon increasing the volume of NH3 solution added to 100140 μL, the {110} edges of the octahedral nanocrystals also became etched. The large 17769
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Figure 1. SEM images of (a1a4) Ag2O nanocubes, (b1b4) rhombicuboctahedra, (c1c4) octahedra, and (d1d4) extended hexapods before and after chemical etching. The volumes of 0.15 M NH3 solution used are (a2, b2) 140, (a3, b3) 160, and (a4, b4) 180 μL for nanocubes and rhombicuboctahedra, (c2) 60, (c3) 100, and (c4) 140 μL for octahedra, (d2) 60, (d3) 70, and (d4) 80 μL for hexapods.
Figure 2. Representative SEM images of the etched Ag2O nanocrystals and drawings of these polyhedral nanostructures before etching with their facets labeled. 17770
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Figure 3. (a) TEM and (b) SEM images of etched Ag2O rhombicuboctahedra synthesized by introducing 140 μL of 0.15 M NH3 solution. (c,d) TEM image of a single Ag2O rhombicuboctahedron viewed along the [100] direction and its corresponding SAED pattern. Diffraction spots corresponding to those of Ag are produced as a result of electron beam irradiation.
{111} facets are still mostly preserved. Thus, the {110} faces of Ag2O are less resistant to etching than the {111} faces. The morphology of a Ag2O hexapod is related to that of an octahedron by stretching all its six corners to form six branches. Upon close examination, each square branch of an extended hexapod also shows slightly truncated edges and terminates with a tiny flat {100} face. The different crystal facets are indicated in Figure 2. Previous detailed SEM and TEM characterization establishes the assignments of these facets for the extended hexapods.5 Again it is evident that the {100} corners and thin {100} and {110} edges of each branch in the extended hexapods are most vulnerable to chemical etching (also see Figure 2). At higher NH3 concentrations, the relatively small {111} facets near the tip of each branch also display etching phenomenon. This series of investigation reveals for the first time the relative stability of the different faces of Ag2O nanocrystals to etching or dissolution in the order of {100} < {110} < {111}. Transmission electron microscopy (TEM) characterization of etched Ag2O rhombicuboctahedra synthesized by introducing 140 μL of 0.15 M NH3 solution was performed (see Figure 3). Consistent with the SEM characterization, Ag2O nanocrystals with partially to extensively hollow interiors can be clearly observed. Thus, the etching process can uniformly proceed far into the interior of the particles from all the {100} faces. Selectedarea electron diffraction (SAED) pattern of a partially etched rhombicuboctahedron viewed along the [100] direction is shown in Figure 3d. Diffraction spots from Ag and Ag2O were recorded, suggesting the instability of Ag2O to prolonged electron beam irradiation. The etched Ag2O nanocubes were further examined by taking their X-ray diffraction (XRD) patterns and UVvis absorption spectra (see the Supporting Information, Figures S1 and S2). XRD patterns of the nanocubes before and after adding 140 180 μL of 0.15 M NH3 solution look nearly identical. The (200) reflection peak is exceptionally strong in intensity as expected for
uniformly deposited nanocubes. UVvis absorption spectra of the Ag2O nanocubes before and after adding the same amounts of NH3 solution also look very similar. As a result of these nanocubes with sizes of several hundreds of nanometers, their extinction spectra are dominated by light scattering bands centered at around 350, 470, and 9301050 nm. The broad light scattering band in the near-infrared region blue-shifts from 1050 to 970 nm after introducing 140 μL of the NH3 solution. The band further shifts to 930 nm after adding 160180 μL of the NH3 solution. It is possible that the {100} facets of Ag2O nanocrystals are naturally more susceptible to dissolution in water, so that some as-grown Ag2O nanoparticles exhibit depressed {100} faces. One can also consider the crystal structures of different facets of Ag2O to explain the observed order of facet stability to etching by NH3 in a basic solution. Figure 4 displays the crystal models of Ag2O with different exposed surfaces. Vertical views over the {100}, {110}, and {111} planes and their rotated views are provided to show the arrangements of surface atoms. The {100} plane is terminated with oxygen atoms. The {111}face is terminated with significant terminal silver atoms and subsurface silver atoms. The {110} plane consists of rows of surface oxygen and silver atoms. From this analysis, it can be expected that the {111} surfaces should be more positively charged or polar and interact more strongly with negative charged ions. The {100} faces are neutral or less charged. The {110} planes have intermediate surface polarity. In this study, NaOH and NH3 solution were both added to the Ag2O nanocrystals for face-selective etching. Hydroxide ions should interact more strongly with the {111} facets, making the surfaces less accessible to direct reaction with NH3. Such a protecting ionic layer is not present or effective for the {100} faces, so NH3 can more readily react with the nanocrystals through these faces. This leads to the pronounced etching of the {100} faces. The {110} faces also lack this protecting layer, so they can also be etched easily. However, for Ag2O nanocrystals 17771
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Figure 4. Crystal models of Ag2O showing different exposed surfaces: (a1, a2) vertical view over the {100} plane and with a 15° tilt or rotation; (b1, b2) vertical view over the {110} plane and with a 75° rotation; (c1c4) vertical view over the {111} plane and after 45°, 63°, and 90° rotations. The areas within the triangles for the 63° and 90° rotation views are the {111} planes. Crystal models are rotated to clearly reveal the exposed surface atoms. The larger spheres represent the oxygen atoms.
with a large fraction of {100} faces, etching or dissolution takes place on the {100} faces first, making the {110} facets more resistant to attack by NH3. This explanation accounts for the observed order of facet stability. To support the argument of hydroxide ions adsorbing on the crystal surfaces, ζ potential measurements of these nanocrystals dispersed in deionized water were made. The measured ζ potentials for Ag2O nanocubes, rhmbicuboctahedra, octahedra, and extended hexapods are 38.0, 32.5, 24.1, and 22.7 mV, respectively. All four samples give negative ζ potential values, validating their surface negative charges with hydroxide ions. Interstingly, ζ potentials increase systematically from octahedra and hexapods to nanocubes as the fraction of {100} facets increase. Larger ζ potentials are associated with greater stability of colloidal particle suspension in the solution; particles with lower values are more likely to aggregate or coagulate. The results also explain why Ag2O nanocubes can disperse in a positively charged methylene blue solution, but octahedra and hexapods float to the top of a methylene blue solution after stirring the solution for 60 min.5 This phenonemon should arise from electrostatic repulsions between methylene blue molecules and Ag2O particles with significant {111} surfaces. Similar behavior has been observed for Cu2O nanocrystals.9 It is interesting to note that a study examining the relative stability of different crystal planes of Cu2O nanocrystals in a weak acetic acid solution at pH 3.5 found the order of facet stability as {100} . {111} > {110}.14 New {100} facets form at the expense of the less stable {111} and {110} crystal planes. This order is completely different from that observed in this work. Considering the same crystal structures for Cu2O and Ag2O, this drastically different facet stability is intriguing. We have also dispersed Ag2O octahedra in 60 and 130 μL of 0.03 M HNO3 solution for 5 min to see how the particle morphology evolves in an acidic solution. The following reaction may take place: AgOH þ HNO3 T H2 O þ Agþ þ NO3
ð5Þ
Remarkably, some particles with square {100} terraces were produced (Supporting Information, Figure S3), suggesting that the {100} faces can grow at the expense of {111} faces in an acidic condition. Rhombicuboctahedra show increased areas
of {100} depressions. Results from the acid-treated extended hexapods suggest instability of both {110} and {111} facets, so they can be easily destroyed. Thus, relative facet stability for Ag2O nanocrystals can be quite different depending on the solution pH. The low stabilty of {111} and {110} facets in an acidic solution may be related to more favorable formation of AgOH on these facets, or shifting of the equilibrium expression in eq 4 to the left.
’ CONCLUSIONS In this study, Ag2O nanocubes, rhombicuboctahedra, octahedra, and extended hexapods were used to examine the relative stability of different facets. NH3 solution injected into a mixture of Ag2O nanocrystals and NaOH enables face-selective etching and the determination of the relative facet stability. The order of stability to chemical etching or dissolution in this system is {111} > {110} > {100}. The {100} facets are most easily etched. Novel cubic Ag2O nanoframes and rhombicuboctahedra with square depressions can be fabricated. The observed order of facet stability can be explained by considering the surface atomic arrangements of these crystal planes. The {111} faces with significant terminal silver atoms should interact more strongly with hydroxide ions and are most stable. The {100} faces bounded by oxygen atoms lack this protecting layer and are most easily etched. ζ potential measurements support this hydroxide ion adsorption argument. In a weak nitric acid solution, Ag2O octahedra can transform into an unusual morphology with stepped {100} faces, revealing greater stability of the Ag2O {100} faces in a weakly acidic solution. ’ ASSOCIATED CONTENT
bS
Supporting Information. XRD patterns, UVvis absorption spectra, SEM images, and a photograph. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. 17772
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’ ACKNOWLEDGMENT We thank the National Science Council of Taiwan for the support of this work (Grant NSC 98-2113-M-007-005-MY3). ’ REFERENCES (1) Ida, Y.; Watase, S.; Shinagawa, T.; Watanabe, M.; Chigane, M.; Inaba, M.; Tasaka, A.; Izaki, M. Chem. Mater. 2008, 20, 1254–1256. (2) Zhou, M.-B.; Wei, W.-T.; Xie, Y.-X.; Lei, Y.; Li, J.-H. J. Org. Chem. 2010, 75, 5635–5642. (3) Murray, B. J.; Li, Q.; Newberg, J. T.; Menke, E. J.; Hemminger, J. C.; Penner, R. M. Nano Lett. 2005, 5, 2319–2324. (4) Wang, X.; Wu, H.-F.; Kuang, Q.; Huang, R.-B.; Xie, Z.-X.; Zheng, L.-S. Langmuir 2010, 26, 2774–2778. (5) Lyu, L.-M.; Wang, W.-C.; Huang, M. H. Chem.Eur. J. 2010, 16, 14167–14174. (6) Fang, J.; Leufke, P. M.; Kruk, R.; Wang, D.; Scherer, T.; Hahn, H. Nano Today 2010, 5, 175–182. (7) Yan, Z.; Bao, R.; Chrisey, D. B. Langmuir 2011, 27, 851–855. (8) Kuo, C.-H.; Huang, M. H. J. Phys. Chem. C 2008, 112, 18355–18360. (9) Ho, J.-Y.; Huang, M. H. J. Phys. Chem. C 2009, 113, 14159–14164. (10) Kuo, C.-H.; Huang, M. H. Nano Today 2010, 5, 106–116. (11) Kuo, C.-H.; Hua, T.-E.; Huang, M. H. J. Am. Chem. Soc. 2009, 131, 17871–17878. (12) Wang, W.-C.; Lyu, L.-M.; Huang, M. H. Chem. Mater. 2011, 23, 2677–2684. (13) Kuo, C.-H.; Huang, M. H. J. Am. Chem. Soc. 2008, 130, 12815–12820. (14) Hua, Q.; Shang, D.; Zhang, W.; Chen, K.; Chang, S.; Ma, Y.; Jiang, Z.; Yang, J.; Huang, W. Langmuir 2011, 27, 665–671.
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