Multi-metallic Hydrate Hollow Structures in Cobalt Hydrate Based

Mar 8, 2017 - The resulting structure and elemental distribution were analyzed to investigate the transformation from discs to rings. A representative...
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Multi-metallic Hydrate Hollow Structures in Cobalt Hydrate Based Systems Yanhui Chen,*,†,‡ Junfeng Zhou,‡ Felim Vajda,‡ Pierce Maguire,‡ Robert O’Connell,‡ Wolfgang Schmitt,§ Yonghe Li,† Zhengguang Yan,† Yuefei Zhang,† and Hongzhou Zhang‡ †

Institute of Microstructure and Property of Advanced Materials, Beijing University of Technology, Beijing 100124, China School of Physics and Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College Dublin, Dublin 2, Republic of Ireland § School of Chemistry and Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College Dublin, Dublin 2, Republic of Ireland ‡

S Supporting Information *

ABSTRACT: The preparation and analysis of Ni doped Co1−xNix(OH)2 (x = 0.6−0.8) nanorings are reported as produced using core−shell structured Co0.9Ni0.1(OH)2 nanodiscs as a starting template material. The resulting structure and elemental distribution were analyzed to investigate the transformation from discs to rings. A representative example involving the doping of Cu in the Co−Cu hydrate system is also presented.

1. INTRODUCTION Hollow structures of the same size as their solid counterparts have a much higher surface to volume ratio which is beneficial for applications including catalysis,1 energy storage and conversion,2 and biomedicine.3,4 Hollow metal oxide/hydrate nanomaterials have attracted intense research interest due to their low mass density, high porosity, and increased surface area.5−7 These characteristics make them good candidate materials for high surface area devices such as lithium-ion batteries and gas sensors.8,9 Nanorings are one example of these hollow structures and have been successfully fabricated in many material systems while exhibiting astonishing properties due to their ring-like structure.10−12 Divalent metal oxides or their hydrate nanorings are the most intensively studied hollow materials due to their high surface to volume ratio and special magnetic properties.13−15 For the preparation of hollow structures there are four typical categories of synthesis technique which include template-mediated approaches,16 the nanoscale-Kirkendall effect method, galvanic replacement, and those which use an entirely chemical etch method. The most popular and facile method to synthesize the hollow nanostructure is through a template-mediated approach. By coating the surface of the template particles with desired © XXXX American Chemical Society

materials and removing the template in a post-treatment, various hollow particles can be easily obtained.17 Another popular method is to synthesize hollow structures using the Kirkendall effect. The Kirkendall effect, which occurs as a consequence of the difference in diffusion rates of two adjacent metal atoms, can be used to produce multi-metallic alloys18 and corresponding oxides.19 Cobalt nanocrystal is a typical hollow forming material and can be used as a sacrificial template for ball-like CoO and CoSe nanocrystals.20 Cobalt containing materials have been reported in many systems such as Co3O415 and Co−Sn−O21 nanoring systems. However, controlled synthesis of Co−Ni−OH or other varieties of hollow ringlike binary materials remain a substantial challenge, acting as a barrier to potential supercapacitor applications. Furthermore, a universal method using cobalt hydrate nanodiscs as a sacrificial template to produce multi-metallic hydrate nanorings seems feasible. In our previous works, we have prepared hexagonal polycrystalline Co−Ni hydrate nanorings at room temperature and they exhibit a high electrical capacity up to 439.7 F g−1.22 It Received: October 23, 2016 Revised: February 27, 2017 Published: March 8, 2017 A

DOI: 10.1021/acs.cgd.6b01555 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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is proposed that the capacity can be further enhanced by improving the crystallinity of the hexagonal rings. In this work, we use improved temperature to prepare a kind of single-crystallized hexagonal rings with high concentration of doped nickel hydrate. We find that the addition of Ni/Cu into a Co(OH)2 system can cause chemical modification which tailors the morphology of the product. Detailed preparation of Ni doped Co1−xNix(OH)2 (x = 0.6−0.8) nanorings using core− shell structured Co0.9Ni0.1(OH)2 nanodiscs as a starting template will be reported and the results analyzed. Structural and elemental distributions will be analyzed to illuminate the transformation from discs to rings. A typical exploration in a Cu doped Co−Cu hydrate system will also be presented.

2. EXPERIMENTAL SECTION Chemical Synthesis. In a typical synthesis, two precursor solutions, CoCl2·6H2O at 0.018 M and NiCl2·6H2O at 0.002 M were dissolved together in 20 mL of deionized water. A small amount of 28 wt % N2H4·H2O from 0.05 to 0.2 mL was added to the solutions. During the reaction, the pH value of the solution was tuned by adding 20 mL of NaOH with a molar ratio from 0.05 to 2 M. This solution was stirred for 2 h followed by heating at a temperature of 120−175 °C for 6 h in a sealed 50 mL Teflon-lined autoclave. After heating, the autoclave was allowed to cool to room temperature in air. Then the black precipitate products were separated through the use of a strong magnet and the materials without magnetism were obtained. The product of the room temperature prepared samples has a pink color while the high temperature product has a gray-black color. All of the products were stored in a sealed ethanol tube while powders for XRD were dried at room temperature. Characterization. A BRUKER-D8 ADVANCE X-ray diffractometer (XRD) was used while employing a graphite monochromatized Cu Kα radiation flux at a scanning rate of 0.02° s−1 in the 2θ range of 10−90°. A Carl Zeiss Ultra scanning electron microscope (SEM) was used to image the morphologies of the Ni−Co oxide nanoparticles. An energy dispersive X-ray spectrum was acquired from the sample in the SEM. The precipitate itself was stored in ethanol for later use and ultrasonicated to ensure the nanoparticles were well-dispersed. Before imaging in the SEM and transmission electron microscope (TEM), the samples were dropped from solution by pipet onto an amorphous carbon film and allowed to dry in air. The TEM used for this imaging was a 300 kV FEI Titan capable of Energy Dispersive X-ray Spectroscopy (EDX). In all cases a wide selection of individual nanoparticles was examined to determine their typical structure and morphology.

Figure 1. XRD patterns of (a) products after reaction at 20 °C and (b) purified products after reaction at 175 °C.

diameter of about 250 nm with a range from 100 to 300 nm. A typical TEM image in Figure 2b with a 220 nm outer diameter nanodisc shows that it has uniform thickness. The electron diffraction pattern inset in Figure 2b indicates its hcp structure with a lattice parameter of a = b = 3.2 Å which corresponds to β-Co(OH)2. For the sample prepared at high temperature (175 °C in this case), hollow nanorings have the same outer diameter with a wall thickness of about 20 nm. The corresponding electron diffraction pattern inset indicates its β-Co(OH)2 or β-Ni(OH)2 structure, which agrees with the XRD data. High resolution transmission electron microscope (HRTEM) images (Figure 2c,f) from the border and inner space of the nanodisc and rings both indicate that they have a single-crystal structure. A lattice contraction from about 3.08 to 3.02 Å (∼2%) in the direction of the nanodiscs can be detected from average measurement results. The fast Fourier transformation (FFT) images in a nanodisc are shown in Supporting Information Figure S1 and also indicated a slight decrease of the lattice parameters from border to center. This contraction can be attributed to the nanomaterials’ border effect or structural variation from Co(OH)2 (lattice parameter, a = 3.18−3.19 Å) to Ni(OH)2 (lattice parameter, a = 3.11− 3.13 Å). The lattice contraction is nearly negligible in the nanorings, although 1−3 nm amorphous layers can be detected in the outer border of most rings. It has to be mentioned that if only CoCl2 was applied in the original solution, there was no hollow structure to be found. Though the initial cobalt and nickel ions are well-defined, the EDX results in Figure 3a show the change in the atomic ratio of nickel among the total metals by the equation of 100 × Ni/(Ni + Co) progressing from nanodiscs to nanorings. The nanodiscs usually contain a nickel ratio from 6 to 11% while it increases to 60−80% in nanorings. The total quantity of Co atoms in nanoparticles decreased from about 90 to 20−40 at. % after reaction. It is clear that the products change from Co(OH)2 rich to Ni(OH)2 rich by high temperature treatment. A further study on the elemental distributions of the room temperature nanodiscs, intermediate nanorings, and final nanorings is shown in Figure 3b−d. It can be seen that in the initial nanodiscs, the nickel is preferentially distributed within 20−30 nm of the border, while cobalt has a lower concentration in the border compared to the center. A

3. RESULTS AND DISCUSSION Figure 1a shows the XRD profiles of the products reacted at 20 °C (black line), and Figure 1b gives the purified products after reaction at 175 °C (red line). All of the diffraction peaks in the black line (from samples prepared in 20 °C) can be exclusively indexed to the hexagonal trigonal β-Co(OH)2 structure (JCPDS file No. 30-0443; space group, P3̅m1 (166); lattice constants, a = 3.2 Å, c = 4.6 Å, α = 90°, and γ = 120°). Due to the same structure occurring in β-Ni(OH)2 and βCo(OH)2, the main peaks from the red line (samples prepared in 175 °C) can be attributed to β-Ni(OH)2/Co(OH)2 and the CoOOH structure (JCPDS file No. 73-1213; space group, Fd3̅m (227); lattice constants, a = 4.7 Å and α = 35.5°). Some tiny small peaks as indexed by red balls can be assigned to spinel Co3O4 (JCPDS file No. 42-1467; space group, Fd3̅m (227); lattice constant, a = 8.1 Å). Figure 2 shows typical microstructures of purified products prepared in 20 and 175 °C. Figure 2a is a typical SEM image of room temperature formed nanodiscs. It has an average outer B

DOI: 10.1021/acs.cgd.6b01555 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 2. SEM images of (a) room temperature formed nanodiscs and (d) high temperature formed nanorings. TEM images and diffraction patterns of (b) room temperature formed nanodiscs and (e) high temperature formed nanorings. HRTEM images from the border to inner areas of (c) nanodiscs and (f) nanorings.

schematic illustration of the elemental distribution of the nanodiscs is shown in the last part of Figure 3b with a Co(OH)2 core (purplish red color) wrapped by a Ni(OH)2 shell (azure color). Figure 3c shows the elemental distribution in a typical intermediate nanoring with small holes. Such nanorings were created by adding less hydrazine hydrate during the reaction resulting in a core−shell structure which still exists. Hydrazine hydrates, usually 60−80 at. % of the usual liquid volume, can produce nanorings under the same temperature. The elemental distribution results in nanorings with large holes. Figure 3d indicates that cobalt nearly disappears leaving a large amount of Ni(OH)2 with a core−shell hollow ring-like skeleton. A series of experiments with different amounts of N2H4 had been done to test the effect of it. The rings can be found in a range from 0.05 to 0.2 mL in different temperatures. Usually we got rings by adding 0.15 mL of N2H4 into 20 mL of CoCl2 and NiCl2 solution totally, and then adding 20 mL of 2 M NaOH, and then sealed them into a 50 mL autoclave to heat at 175 °C for 6 h. The intermediate nanorings forms usually by using 1 M NaOH if other conditions were not changed. It is known that cobalt oxide or its hydrate have the tendency to form a hollow structure due to the Kirkendall effect.23 Nanorings prepared by our method have a hollow structure coming from the transformation from the original solid nanodiscs. However, the elemental distribution from EDX shows that the original nanodiscs have a core−shell structure with Co(OH)2 wrapped by Ni(OH)2. This core−shell structure can be explained by the precipitation sequence of the metal hydrate by an equation in an alkali solution:

pH = 14 −

1 (log[Mn +] − log K sp) n

where n is the valence of the metal, [M] represents effective concentration of the metal, and Ksp is the solubility product constant. The effective concentration can also be called the activity of ions and can be expressed as ai = γiNi. This applies in a dilute solution, where ai, γi, and Ni represent activity, the activity coefficient, and the concentration of the ion labeled i, respectively. Co2+ and Ni2+, have the same valence value and can be regarded as having the same activity coefficient. The activity is thus monotonically dependent on the nominal concentration. Since the effective concentration and nominal concentrations have the same tendency, we will use nominal concentrations for a qualitative analysis. Calculation results indicate that if the concentration of Co2+ is 0.018 M, the pH value needed is 7.53. Similarly, the pH value for a concentration of 0.002 M Ni is 7.93. This suggests that the Co2+ precipitate first forms disc-like structures due to its intrinsic hcp structure and provides borders and surfaces to which Ni2+ can then adhere. As the reaction occurs, two kinds of product form, one is a core−shell structured Co0.33Ni0.67(OH)2 metal hydrate and another is Co0.88Ni0.12 alloy. The formation of the core−shell ring-like metal hydrate can be explained as follows as shown in Figure 4: in the beginning, all the nanodiscs at room temperature were in a set of environmental conditions which was mildly reductive. Though not strong enough for the reduction to happen at room temperature only with metal complex forms, the reduction will happen when the system is kept at high temperature. Then, nickel will first be reduced on the surface and then the inner cobalt. At the same time, the first C

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Figure 3. (a) Experimental atomic ratio of Ni using the equation 100 × Ni/(Ni + Co) from nanodiscs to nanorings. HAADF and EDX mapping of (b) nanodisc, (c) intermediate nanoring, and (d) final nanoring.

Figure 4. Schematic illlustration of the formation process of the nanorings.

the outer border of the nanodiscs then grows quickly in high temperature into a polyhedral alloy. The outer part of the nanodiscs formed an intermediate metal which was easily

formed Ni will also react as reduction to prompt the formation of metal cobalt. A competitive diffusion between nickel and cobalt leads to the precipitation of cobalt from the center, and D

DOI: 10.1021/acs.cgd.6b01555 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 5. Microstructures of CoCu hydrate of (a−c) nanodiscs prepared at room temperature and (d, e) nanorings prepared at 145 °C/(6 h). (e) Ratio of copper among the total metals using 100 × Cu/(Co + Cu) calculation.

temperature exhibit superior crystallinity over the polycrystalline nanorings fabricated at room temperature. Second, for the room temperature synthesis, Ni can be replaced by a range of cations, e.g., Mn and Fe, and their hybrid cobalt hydrate nanorings have been synthesized with high yields, while it has not been successful to obtain hybrid single-crystalline cobalt hydrate nanorings with a cation other than Ni. Finally, the concentration of nickel in the single-crystalline nanorings can be varied from 10% to 67%, in contrast to the low nickel concentration (∼10 at. %) in the nanorings. In addition, the formation of the single-crystalline nanorings is due to the galvanic and Kirkendall effects, while galvanic corrosion and thermal oxidation process are responsible for the formation of the polycrystalline nanorings formed at low temperatures.

hydrolyzed in a hydrate solution. However, the center part then grew quickly to form a metal polyhedra which can be separated easily by the use of a strong magnet, and its detailed data are shown in the Supporting Information in Figure S2. As an easy hollow structure formed material, similar systems can be explored such as the CoCu system. Figure 5 shows typical images of copper doped cobalt hydrate synthesized using a similar method. In Figure 5a−c, typical nanodiscs have an outer diameter of about 200 nm with thickness down to 10 nm. The EDX results indicate that the average copper concentration in the border is 16%, while in the center it is 7%, which also suggests a core−shell structure. TEM images and diffraction patterns in Figure 5b show a hexagonal shape with a β-Co(OH)2 structure. After reaction at 145 °C for 6 h, ring-like structure forms with a Cu/(Cu + Co) ratio of 27− 33%. The TEM image and diffraction pattern in Figure 5e also shows a β-Co(OH)2 structure. Experimental results indicated that improved temperature in the Co−Cu system can increase the concentration of Cu atoms in the product. In all, core−shell structured nanodiscs formed in room temperature with an average Cu/(Cu + Co) ratio of 12% and nanorings formed at high temperature with an average Cu/(Cu + Co) ratio of 30%. Furthermore, improved temperature increase the yield of the nanorings: products grown at medium temperatures (e.g., 80 and 100 °C) do not contain nanorings (the reaction time was 6 h), while products obtained at 120 °C contain a smaller number of nanorings than that obtained at 145 °C if other parameters were the same. The single-crystal nanostructures are distinct from the polycrystalline rings reported in our previous work.22 First, the Co−Ni hydrate nanostructures synthesized at high

4. CONCLUSIONS In conclusion, we have reported the preparation of tunable multi-metallic cobalt based hydrate and Co−Ni−OH systems which were systematically analyzed. Combining the galvanic and Kirkendall effects, core−shell Co−Ni−OH nanodiscs were transformed into nanorings by removing the center of the cobalt hydrate nanodisc template. A similar method can be used to prepare Co−Cu−OH nanorings.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b01555. FFT images and images of polyhedral byproducts particles (PDF) E

DOI: 10.1021/acs.cgd.6b01555 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 010-67396521. Fax:01067396147. ORCID

Yanhui Chen: 0000-0002-3215-9302 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the staff at the Advanced Microscopy Laboratory (AML), CRANN, Trinity College Dublin. We also acknowledge support from the following funding bodies: Science Foundation Ireland (Grants 11/PI/1105, 07/SK/I1220a, 12/ TIDA/I2433, and 08/CE/I1432) and the Irish Research Council (Grants EPSPG/2011/239 and IRCSET-SolarPrint2010-02).

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REFERENCES

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DOI: 10.1021/acs.cgd.6b01555 Cryst. Growth Des. XXXX, XXX, XXX−XXX