Facile Synthesis of Co3O4 with Different Morphologies via Oxidation

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Facile Synthesis of CoO with Different Morphologies via Oxidation Kinetic Control and its Application in Hydrogen Peroxide Decomposition Huihui Chen, Mei Yang, Sha Tao, Mingyue Ren, and Guangwen Chen Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00841 • Publication Date (Web): 07 Oct 2016 Downloaded from http://pubs.acs.org on October 8, 2016

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Crystal Growth & Design

Facile Synthesis of Co3O4 with Different Morphologies via Oxidation Kinetic Control and its Application in Hydrogen Peroxide Decomposition Huihui Chena, b, Mei Yanga,*, Sha Taoa, b, Mingyue Rena, b, Guangwen Chena,* a

Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese

Academy of Sciences, Dalian 116023, China b

University of Chinese Academy of Sciences, Beijing 100049, China

ABSTRACT Co3O4 nanoparticles (NPs) with tubular and hollow structures were successfully synthesized by the formation of CoOOH and subsequent high-temperature calcination in air. The as-synthesized Co3O4 retained the morphologies of CoOOH. Therefore, the key step for the synthesis of Co3O4 was the controllable preparation of CoOOH NPs with tubular and hollow structures, which were prepared through a facile strategy involving the oxidation of β-Co(OH)2 under strong basic condition (template-free, aqueous solution and mild temperature). The morphology of CoOOH was tuned by employing air and H2O2 as the oxidizing agents which possessed different oxidation abilities and thus resulted in different oxidation kinetics. The plausible formation mechanism of CoOOH NPs with tubular and hollow structures was both related to the Kirkendall effect. In contrast to the commercial Co3O4, the as-prepared Co3O4 NPs with tubular and hollow structures showed superior

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catalytic activities for the decomposition of H2O2. The reaction rate constants of the as-prepared Co3O4 NPs with tubular and hollow structures were over 100 times the value of the commercial one.

1. Introduction

Up to now, cobalt oxide (Co3O4) has attracted a great deal of interest due to its great potential in the field of electricity, sensing and catalysis, etc. Co3O4 is well regarded as an outstanding alternative to the commercial graphite anode for Li-ion batteries because of its high theoretical capacity, low cost and well-defined electrochemical redox activity

1-4

. For

example, Lou et al. reported that Co3O4 with a needlelike nanotube morphology possessed ultrahigh charge capacity of 950 mA·h·g-1 with nearly 100% capacity retention for over 30 cycles 4. Apart from the anode material for Li-ion batteries, Co3O4 can be used as a promising electrode material for supercapacitors. For instance, reduced Co3O4 nanowires showed a superior specific capacitance of 977 F·g-1 at a current density of 2 A·g-1 5. Porous aggregated nanorods of Co3O4 gave a high specific capacitance of 780 F·g-1 6. In addition, Co3O4 is also found to be highly active for CH4 combustion, low-temperature oxidation of CO and decomposition of H2O2 7-9.

Experimentally, the performances of Co3O4 are remarkably influenced by crystallinity, size and shape, etc. In order to get a better performance, many attempts have been made to synthesize Co3O4 with different morphologies. Varieties of Co3O4 nanostructures such as nanocubes, anisotropic nanostructures, nanoporous architectures, quasi-1D nanocomposites and nanocages have been reported

10-14

. Generally, Co3O4 can be prepared by the thermal


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decomposition of cobalt hydroxide (Co(OH)2), oxyhydroxide (CoOOH) or other precursors under oxidative condition. The as-prepared Co3O4 usually maintains the morphology of the precursor. As an example, mesoporous hexagonal Co3O4 was successfully synthesized by annealing hexagonal Co(OH)2 at 400 oC 3. Co3O4 hexagonal nanorings were formed through the hydrolysis of Co-based metal organic frameworks followed by high-temperature calcination

15

. Therefore, Co(OH)2, CoOOH or other precursors with diverse morphologies

are fabricated to tune the morphology of Co3O4. The most common preparation methods for Co(OH)2 include wet chemical precipitation, hydrothermal method and electrochemical deposition, etc. For example, hexagonal microplates, nanocones, microflowers and hollow structures have been successfully obtained

16-19

. Compared with Co(OH)2, the literatures

dedicated to the synthesis of CoOOH are relatively scarce. CoOOH is usually synthesized by the oxidation of Co(OH)2 with oxidizing reagents (O2, H2O2 or NaClO3)

20, 21

. In addition,

Co3O4 with hollow nanostructures can be formed in the thermal decomposition of the precursors under oxidative condition via the Kirkendall effect

22-24

. For example, Hu et al.

reported that Co3O4 porous nanocages were successfully synthesized by the Kirkendall effect in the thermal decomposition of Prussian blue analogue truncated nanocubes

24

. It is well

known that the Kirkendall effect is based on nonequilibrium interdiffusion during the reaction process. However, very few studies have paid attention to the effect of the reaction kinetics on the Kirkendall effect-induced morphology of nanoparticles.

In this manuscript, CoOOH with different morphologies were successfully synthesized by manipulating the oxidation kinetics. CoOOH NPs with tubular and hollow structures were synthesized through the oxidation of β-Co(OH)2 by air and H2O2 under strong basic



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condition, respectively. The formation of CoOOH NPs with tubular and hollow structures were both related to the Kirkendall effect. Due to the different oxidation abilities of air and H2O2, the oxidation kinetics differed and thus affected the Kirkendall effect-induced morphology of CoOOH. Co3O4 was prepared by the subsequent high-temperature calcination of CoOOH in air. The as-prepared Co3O4 retained the morphology of CoOOH and exhibited excellent activity in the catalytic decomposition of H2O2.

2. Experimental Section

2.1. Materials

For the synthesis of CoOOH with different morphologies, cobalt chloride (CoCl2·2H2O), aqueous ammonia (20 wt.%), sodium hydroxide (NaOH) and hydrogen peroxide (H2O2, 30 wt.%) were purchased with analytical grade. All chemicals were used as received without further treatment. Deionized water was used in all experiments. In addition, the commercial Co3O4 were purchased from Aladdin Industrial Corporation for comparison.

2.2. The synthesis of CoOOH NPs with tubular and hollow structures

For the synthesis of CoOOH NPs with tubular structure, CoCl2·2H2O (0.01 mol) and aqueous ammonia (0.02 mol) were dissolved into 100 ml deionized water, respectively. The process was carried out by dropwise addition of CoCl2 solution and aqueous ammonia into a stirred three-necked flask containing 50 ml deionized water at 50 oC under N2 atmosphere. A green suspension was obtained. After aging for 30 min, 175 ml NaOH aqueous solution with a concentration of 0.4 mol/L was added into the green suspension. After the addition of


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NaOH solution, the color of the suspension changed from green to pink. Subsequently, the pink suspension was kept at 50 oC for 2 h and then N2 flow was stopped. The pink precipitate was collected by centrifugation and washed several times with deionized water. Afterwards, the pink precipitate was dispersed into 175 ml NaOH aqueous solution (0.4 mol/L) and aged in air at 50 oC for 3 h. The color of the precipitate changed from pink to dark yellow and finally to black brown with the increasing reaction time. After cooling to room temperature, the black brown precipitate was centrifuged, washed several times with deionized water and dried. The as-prepared black brown precipitate was denoted as CoOOH-air. For comparison, some amount of the green and pink suspensions after aging for 30 min and 2 h was withdrawn

and

the

as-prepared

samples

were

denoted

as

Co(OH)2-NH3

and

Co(OH)2-NH3-NaOH, respectively.

CoOOH NPs with hollow structure were prepared following a similar protocol with CoOOH NPs with tubular structure. The difference was that the pink suspension obtained by adding NaOH aqueous solution into the green suspension was aged at 50 oC for 30 min under N2 atmosphere. Then, N2 flow was stopped and 10 ml H2O2 was added into the pink suspension. The color of the suspension changed from pink to black brown at the moment when H2O2 was added. Subsequently, the black brown suspension was kept at 50 oC for 2 h. The black brown precipitate was collected following the same procedure of CoOOH NPs with tubular structure. The as-prepared black brown precipitate was denoted as CoOOH-H2O2.

2.3. The synthesis of Co3O4 NPs with tubular and hollow structures



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The as-prepared CoOOH NPs with tubular and hollow structures were calcined at 300 o

C for 1 hour in air. The as-prepared Co3O4 derived from CoOOH NPs with tubular and

hollow structures were denoted as Co3O4-air and Co3O4-H2O2, respectively.

2.4. Characterization

The powder X-ray diffraction (XRD) patterns of the as-prepared samples were recorded by a PANalytical X'pert-Pro powder X-ray diffractometer, using Cu Ka monochromatized radiation (λ=0.1541 nm) at a scanning rate of 5°/min. The morphologies of the as-prepared samples were observed by scanning electron microscopy (SEM, JEOL JSM-7800F with the accelerating voltage of 3 kV) and transmission electron microscopy (TEM, JEOL JEM-2100 with the accelerating voltage of 120 kV). The specific surface areas of the samples were measured by the BET method on a Quadrasorb SI instrument using nitrogen adsorption isotherms at 77K (the cross section of the nitrogen molecule was taken to the 0.162 nm2). X-ray photoelectron spectroscopy (XPS) analysis was performed on an ESCALAB 250Xi system, using Al Ka radiation as the X-ray source, which was used to analyze elemental and chemical states of the samples.

2.5. Catalytic decomposition of H2O2

The reaction was initiated by introducing 12.5 ml H2O2 solution (0.4 mol/L) into a magnetic stirred three-necked flask containing 18 mg catalyst in 12.5 ml deionized water at 25 oC. The volume of gaseous product was measured using a gas burette connected to one neck of the three-necked flask. The cycled test was carried out by collecting the catalyst with centrifugation.


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3. Results and discussion

3.1. The Characterization of the precursor and Co3O4

Co3O4 NPs with tubular and hollow structures were synthesized by the preparation of CoOOH NPs with tubular and hollow structures followed by high-temperature treatment. In order to prepare CoOOH with different morphologies, air and H2O2 were employed to manipulate the oxidation kinetics. Figure 1a shows the XRD patterns of Co(OH)2-NH3, Co(OH)2-NH3-NaOH, CoOOH-air and CoOOH-H2O2. Every diffraction peak in the XRD pattern of Co(OH)2-NH3 can be assigned to α-Co(OH)2 or β-Co(OH)2 (JPCDS-30-0443). Cobalt hydroxide can crystallize in two polymorphic forms, namely α- and β-phase α-Co(OH)2 can transform spontaneously into more stable β-Co(OH)2

25

.

26, 27

. Therefore, it can

be predicted that Co(OH)2-NH3 is a mixture of α-Co(OH)2 and β-Co(OH)2. In addition, the diffraction peaks of α-Co(OH)2 is broad, indicating a poor crystalline nature of α-Co(OH)2. For Co(OH)2-NH3-NaOH, the diffraction peaks at 19.162o, 32.635o, 38.084o, 51.540o, 58.145o and 61.776o can be indexed to the (0 0 1), (1 0 0), (0 1 1), (0 1 2), (1 1 0) and (1 1 1) facets of β-Co(OH)2. The sharp and narrow peaks implies that Co(OH)2-NH3-NaOH has a high degree of crystallinity. Evidently, the addition of NaOH can result in a complete transformation from predominantly amorphous α-Co(OH)2 to highly crystalline β-Co(OH)2. After aging Co(OH)2-NH3-NaOH in air atmosphere or H2O2, the characteristic diffraction peaks of β-Co(OH)2 disappear, while the characteristic diffraction peaks corresponding to CoOOH are observed in the XRD patterns of CoOOH-air and CoOOH-H2O2. These results demonstrate that β-Co(OH)2 can be totally transformed to CoOOH by the oxidation of air or



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H2O2 under a strong basic condition. In addition to CoOOH, no other diffraction peaks are found in the XRD patterns of CoOOH-air and CoOOH-H2O2, indicating a high purity of the as-prepared samples.

Figure 1. The XRD patterns of (1) Co(OH)2-NH3 (2) Co(OH)2-NH3-NaOH (3) CoOOH-air (4) CoOOH-H2O2.

Figure

2

and

3

depict

the

SEM

and

TEM

images

of

Co(OH)2-NH3,

Co(OH)2-NH3-NaOH, CoOOH-air and CoOOH-H2O2. As shown in Figure 2A and 3A, Co(OH)2-NH3 is mainly composed of irregular and thin nanoplates with an average thickness of ~10 nm. Co(OH)2-NH3-NaOH consists of well-defined hexagonal nanoplates with smooth surfaces, which is in accordance with the XRD results (Figure 2B and 3B). β-Co(OH)2 with hexagonal morphology has been widely synthesized by chemical precipitation, hydrothermal and electrochemical methods, etc. When these hexagonal nanoplates are aged in air for 3 h, nanoparticles with tubular structure and extremely rough edge surfaces are obtained (Figure 2C, 3C and 3D). The enlarged SEM image (Figure 2D) reveals that the rough edge surfaces are constructed of a large number of nanoflakes. It can be clearly seen that the interiors of nanoparticles are not entirely empty. The average void diameter and wall thickness are ~200 nm and ~100 nm, respectively. In addition, a lot of scattered irregular nanoplates also exist, surrounding the nanoparticles with tubular structure. When Co(OH)2-NH3-NaOH is aged in H2O2 for 2 h, most nanoparticles show a similar hexagonal morphology with Co(OH)2-NH3-NaOH (Figure 2E and 3E). Notably, there is a degree of contrast (dark/bright) between the edge surfaces and interiors of the flat-lying nanoplates (Figure 3E). Additionally,


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the degree of contrast between the edge surfaces and interiors of the edge-standing nanoplates is also distinct (Figure 3F). These observations imply that CoOOH-H2O2 possesses a hollow inner structure. This hollow inner structure can be clearly seen from a broken piece of CoOOH-H2O2 (Figure 2E). In comparison with CoOOH-air, the morphology of CoOOH-H2O2 can be seen as nanoparticles with hollow structure. The surface of CoOOH-H2O2 is also exceedingly rough (Figure 2F). Additionally, there are many pores with the diameter ranging from 10-30 nm on the top or bottom surfaces, indicating a mesoporous structure of CoOOH-H2O2. The rough surface and mesoporous structure may be both formed due to the dehydration of β-Co(OH)2 to CoOOH. According to the aforementioned results, it was evident that the morphology of CoOOH-air was different from that of CoOOH-H2O2, which was supposed to be related to the different oxidation ability of air and H2O2.

Figure 2. The SEM images of (A) Co(OH)2-NH3 (B) Co(OH)2-NH3-NaOH (C, D) CoOOH-air (E, F) CoOOH-H2O2.

Figure 3. The TEM images of (A) Co(OH)2-NH3 (B) Co(OH)2-NH3-NaOH (C, D) CoOOH-air (E, F) CoOOH-H2O2.

Figure 4 displays the TEM images of Co3O4-air and Co3O4-H2O2. As shown in Figure 4, Co3O4-air and Co3O4-H2O2 exhibit a tubular and hollow structures, respectively, demonstrating that the morphologies of CoOOH-air and CoOOH-H2O2 are successfully retained. As depicted in Figure 5A, all diffraction peaks at 31.374o, 36.937o, 44.835o, 59.449o and 65.802o in the XRD patterns of Co3O4-air and Co3O4-H2O2 can be indexed as spinel Co3O4 (JPCDS-09-0418). According to the Sherrer’s equation, the crystalline size of Co3O4



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in Co3O4-air and Co3O4-H2O2 is both 14.3 nm. Additionally, the BET surface area and porosity of Co3O4-air and Co3O4-H2O2 were studied by nitrogen adsorption-desorption isotherms at 77 K. Co3O4-air and Co3O4-H2O2 both exhibit a Type IV adsorption isotherm with an H1 hysteresis loop, implying the disordered mesoporous nature of Co3O4-air and Co3O4-H2O2 (Figure 5B). The BET surface area of Co3O4-air and Co3O4-H2O2 are 81.3 and 79.8 m2/g, respectively. The pore volume of Co3O4-air and Co3O4-H2O2 are 0.18 and 0.15 cm3/g, respectively. The pore size distributions derived from adsorption branch of Co3O4-air and Co3O4-H2O2 exhibit a broad distribution from 3.5 nm to 40 nm, which can be attributed to the intra-aggregated pore within the agglomerated particles (Figure 5C). As shown in the high-resolution XPS spectra of Co element (Figure 1S), the peaks located at 779.7 and 794.8. eV can be ascribed to Co3+, and another two peaks located at 781.2 and 796.3 eV can be attributed to Co2+. Apparently, Co element is present in the chemical state of Co3+ and Co2+, implying the presence of Co3O4 on the surface.

Figure 4. The TEM images of (A) Co3O4-air and (B) Co3O4-H2O2.

Figure 5. The XRD patterns, N2 adsorption-desorption isotherm curve and BJH adsorption pore size distribution of (1) Co3O4-air and (2) Co3O4-H2O2.

3.2. The plausible formation mechanism of the precursor

To shed light on the formation mechanism for the unique morphologies of CoOOH induced by the different oxidation abilities of air and H2O2, the samples aged for different reaction times were collected and characterized by TEM and SEM. The TEM and SEM images of CoOOH-air synthesized by aging Co(OH)2-NH3-NaOH in air for different reaction


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times are shown in Figure 6 and 7. At the early stage of the reaction (0.5 h), the as-prepared samples retain the morphology of hexagonal nanoplates. β-Co(OH)2 hexagonal nanoplates are enclosed by the (0 0 1) facets (top and bottom surfaces) and (1 1 0) facets (edge surfaces) 28

. The (1 1 0) facets begin to become rougher, accompanied by the appearance of scattered

irregular nanoplates, while the (0 0 1) facets remain smooth (Figure 6A, 6B and 7A). When the reaction time increases to 1 h, the thickness of the rough edge surfaces increases to ~70 nm. Voids between the dark edge surfaces and interiors are detected for the flat-standing nanoplates. These voids still exist as the reaction time is prolonged to 1.5 h. The dark interiors become smaller while the rough edge surfaces turn thicker with the increasing reaction time. When the reaction time further increases to 2 h, the dark interiors disappear and irregular tubular structure with rough edge surfaces of ~100 nm in thickness is obtained. Based on the above observation, a formation mechanism of CoOOH NPs with tubular structure involving two stages was proposed and summarized in Figure 8A. Rough edge surfaces were formed in the first stage. Since the experiment was carried out under a strong basic solution, the (1 1 0) facet of β-Co(OH)2 hexagonal nanoplates was selectively dissolved into the solution to form Co(OH)42- 29. Subsequently, Co(OH)42- reacted with the deliquescent O2 in the water, reprecipitated and thus generated lots of scattered irregular CoOOH nanoplates. Accompanying the dissolution-oxidation-reprecipitation process, the residual β-Co(OH)2 on the (1 1 0) facet was oxidated in a solid phase and thereby rough edge surfaces composed of quantities of nanoflakes were obtained. In the second stage, only solid-phase oxidation of β-Co(OH)2 to CoOOH occurred. The formation of tubular structure was to some extent associated to the Kirkendall effect, which had been widely applied in the fabrication of



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the hollow structures 30, 31. It was hypothesized that the diffusion of O2 dissolved in the water to the interior of β-Co(OH)2 hexagonal nanoplate was slower than that of β-Co(OH)2 to the external surface. As a consequence, a gap between the edge surface and interior was formed and tubular structures were finally obtained undergoing shell-core like intermediates. The related reactions were expressed as Eq. 1-3.

Co(OH) 2 +2OH - → Co(OH) 421 1 Co(OH)2-4 + O 2 → CoOOH+ H 2 O+2OH 4 2 1 1 Co(OH)2 + O 2 → CoOOH+ H 2 O 4 2

(1)

(2)

(3)

Figure 6. The TEM images of CoOOH-air aged in air for different reaction times (A, B) 0.5 h (C) 1 h (D, E) 1.5 h (F) 2 h.

Figure 7. The SEM images of CoOOH-air aged in air for different reaction times (A) 0.5 h (B) 1 h (C) 1.5 h (D) 2 h.

Figure 8. A possible formation mechanism of (A) CoOOH-air (B) CoOOH-H2O2.

Figure 9 depicts the TEM images of CoOOH-H2O2 synthesized by aging Co(OH)2-NH3-NaOH in H2O2 for 1 h and 2 h. At the early stage of the reaction (1 h), hollow hexagonal nanoplates with two voids near the top and bottom surfaces (core-shell like structure) are obtained. As the reaction time increases to 2 h, the two voids merge and the top and bottom surfaces become thicker. The formation of this hollow structure could also be attributed to the Kirkendall effect. Since H2O2 had a much stronger oxidation ability than air,


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the (0 0 1) and (1 1 0) facets of β-Co(OH)2 nanoplates were oxidated quickly in a solid phase to form CoOOH the moment H2O2 contacted with β-Co(OH)2 nanoplates, which prevented β-Co(OH)2 to be dissolved in the solution containing excess NaOH. As a result, the dissolution-oxidation-reprecipitation process did not happen when H2O2 was used as the oxidizing agent. The Kirkendall effect was conducted by that the slower diffusion rate of H2O2 to the interior of β-Co(OH)2 than that of β-Co(OH)2 to the external surface. The formation mechanism of CoOOH-H2O2 was schematically illustrated in Figure 8B.

Figure 9. The TEM images of CoOOH-air aged in H2O2 for different reaction times (1) 1 h (2) 2 h.

3.3. The effect of other synthesis parameters

A series of controlled experiments were carried out to study the effect of experimental variables on the morphology of CoOOH. The aqueous ammonia was found to play an indispensable role in the synthesis of CoOOH NPs with tubular and hollow structures. To identify the role of aqueous ammonia, it was replaced by NaOH, and the other synthesis conditions were remained unchanged. The as-prepared samples were denoted as Co(OH)2-NaOH, Co(OH)2-NaOH-NaOH, CoOOH-air-NaOH and CoOOH-H2O2-NaOH, respectively.

The

TEM

images

of

Co(OH)2-NaOH,

Co(OH)2-NaOH-NaOH,

CoOOH-air-NaOH and CoOOH-H2O2-NaOH are shown in Figure S2. Obviously, Co(OH)2-NaOH is comprised of hexagonal nanoplates instead of irregular nanoplates. The edge length and thickness are ~50 nm and ~10 nm, respectively. All the diffraction peaks in the XRD pattern of Co(OH)2-NaOH can be assigned to β-Co(OH)2, indicating that



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α-Co(OH)2 can be transformed to β-Co(OH)2 more quickly when NaOH is employed as the precipitating agent (Figure S3). Co(OH)2-NaOH-NaOH consists of hexagonal nanoplates with the edge length of ~100 nm and thickness of ~20 nm. After aging Co(OH)2-NaOH-NaOH in air for 2 h, the XRD result demonstrates that β-Co(OH)2 is totally transformed to CoOOH. However, no tubular structure is obtained. Co(OH)2-NaOH-NaOH collapses into irregular nanoplates, indicating aqueous ammonia is essential in the synthesis of CoOOH NPs with tubular structure. Interestingly, CoOOH-H2O2-NaOH exhibits a unique nanoring structure with the edge length and thickness of ~50 nm and ~10 nm, respectively. The related studies are under way and the details will be reported in another work.

The effect of the reaction temperature for the oxidation of β-Co(OH)2 hexagonal nanoplates was also studied when H2O2 was used as the oxidizing agent. As shown in Figure S4, the as-prepared sample exhibits a similar morphology with β-Co(OH)2 and no hollow structure is obtained at 30 oC. As the reaction temperature increases to 50 and 70 oC, hollow structure is obtained. The XRD results (Figure S5) demonstrate that the sample prepared at 30 oC is mainly comprised of β-Co(OH)2 besides CoOOH, indicating that β-Co(OH)2 can not be totally oxidated to CoOOH at this low temperature. The diffraction peaks in the XRD patterns of the samples prepared at 50 and 70 oC can be all assigned to CoOOH. These results implied that a proper temperature was needed for the oxidation of β-Co(OH)2 to CoOOH.

3.4. The catalytic decomposition of H2O2

As is well known, H2O2 is widely used as an oxidant in many industrial process such as double-bond epoxidation for the production of propene oxide from propene 32. In general, an


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excess of H2O2 is needed to achieve a high conversion, which should be eliminated before the effluents discharge into the environment. One of the most effective methods to eliminate H2O2 is heterogeneous catalytic decomposition of H2O2. Additionally, H2O2 decomposition can be employed as a monopropellant for hydraulic and pneumatic actuators, oxygen source and a heat supplier due to its strongly exothermic nature

33

. According to the literature

studies, Co3O4 showed excellent catalytic activity for the decomposition of H2O2 9. Herein, the decomposition of H2O2 was selected as a model reaction to evaluate the catalytic performances of Co3O4-air and Co3O4-H2O2. As Figure 11 depicted, the commercial Co3O4 shows a poor catalytic activity for the decomposition of H2O2. Only 9.1 mL O2 is obtained within 100 min (theoretical value equals to 56.0 mL). Notably, Co3O4-air and Co3O4-H2O2 both exhibit a much better catalytic activity in comparison with the commercial Co3O4. Moreover, the catalytic activity of Co3O4-H2O2 is much higher than that of Co3O4-air. The O2 evolution volume equals to the theoretical value within 12 min over Co3O4-H2O2, while Co3O4-air shows the theoretical value of O2 evolution with 20 min. The specific surface area of commercial Co3O4 was measured to be only 2.3 m2/g, which led to a poor activity in H2O2 decomposition. The superior activities of Co3O4-air and Co3O4-H2O2 could be ascribed to the larger specific surface area induced by the unique morphologies. Furthermore, because Co3O4-air and Co3O4-H2O2 owned the similar specific surface area, pore volume, surface composition and chemical state, the difference in the catalytic activity can be attributed to the dissimilar morphologies of Co3O4-air and Co3O4-H2O2. Generally, the kinetic equation of the decomposition of H2O2 can be described as

ln(CH2 O2 ,0 CH2 O2 ) = kt


(4)


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Page 16 of 35

where k and t represented the pseudo-first-rate kinetic constant and reaction time, respectively. Therefore, the slope of the linear curve (reaction time versus ln(CH O ,0 CH O ) 2

2

2

2

represented the rate constant. The rate constant for catalytic composition of H2O2 over commercial Co3O4, Co3O4-air and Co3O4-H2O2 are summarized in Table 1. The rate constant of the commercial Co3O4 was 0.0009 min-1. The rate constants of Co3O4-air and Co3O4-H2O2 were calculated to be 0.15 and 0.28 min-1 during the first run, respectively. The rate constants of other catalysts used for H2O2 decomposition were summarized in Table S1. As shown in Table S1, the rate constants of Co3O4-air and Co3O4-H2O2 were comparable with that of Co3O4 hollow nanospheres but smaller than that of perovskite oxides34. Unfortunately, both of Co3O4-air and Co3O4-H2O2 exhibited a little deactivation in the cycled test. The rate constants of Co3O4-air and Co3O4-H2O2 decreased to 0.11 and 0.17 min-1 during the fourth run, respectively. Nevertheless, the catalytic activities of Co3O4-air and Co3O4-H2O2 were still much higher than that of the commercial Co3O4, indicating Co3O4-air and Co3O4-H2O2 were potential catalysts for the decomposition of H2O2 and thereby the degradation of organic pollutants.

Figure 10. The cycled test for the decomposition of H2O2 over Co3O4-air and Co3O4-H2O2.

Table 1 Reaction rate constant for the decomposition of H2O2 over Co3O4-air and Co3O4-H2O2.

4. Conclusions

In summary, CoOOH with different morphologies derived from β-Co(OH)2 hexagonal nanoplates were successfully synthesized by manipulating the oxidation kinetics. The


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oxidation process was carried out under strong basic aqueous solution without any template. CoOOH NPs with tubular structure were obtained when air was used as the oxidizing agent, while CoOOH NPs with hollow structure were formed by oxidizing β-Co(OH)2 with H2O2. The formation mechanism of CoOOH NPs with tubular and hollow structures were both related to the Kirkendall effect. It was found that the aqueous ammonia and proper reaction temperature were very important for the preparation of CoOOH NPs with tubular and hollow structures. Furthermore, Co3O4 prepared by calcining CoOOH NPs with tubular and hollow structures at high temperature retained the morphologies of the precursors. The as-prepared Co3O4 showed superior catalytic activity for the decomposition of H2O2 than the commercial one. The novel morphologies of CoOOH and Co3O4 can be expected to bring new opportunities for further fundamental research as well as for practical applications in heterogeneous catalysis, supercapacitors, Li-ion batteries and so on.



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FIGURE

Figure 1. The XRD patterns of (1) Co(OH)2-NH3 (2) Co(OH)2-NH3-NaOH (3) CoOOH-air (4) CoOOH-H2O2.








Figure 9. The TEM images of CoOOH-air aged in H2O2 for different reaction times (1) 1 h (2) 2 h.



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■ CoOOH ▲ β-Co(OH)2 ▼ α-Co(OH) 2

■

■ ■ ■

Intensity /a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


■ ■

■ ■ ■

(D) (C)

▲

▲ ▲

▲

▲ ▲

▲

▼

10

▲▲

(A)

▼

20

(B)

30

40

50

60

70

80

o

2 Theta /

Figure 1. The XRD patterns of (A) Co(OH)2-NH3 (B) Co(OH)2-NH3-NaOH (C) CoOOH-air (D) CoOOH-H2O2.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



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◆ Co 3 O4

(A)

Intensity /a.u.

◆ ◆

◆

◆

◆

(2)

(1)

25

35

45

55

o

65

75

2 Theta /

3

Adsorbed volume (cm/g)

(B)

Adsorption Adsorption

(1) (2)

0.0

0.2

0.4

Desorption Desorption

0.6

0.8

1.0

Relative Pressure (P/P0)

0.22

(C)

-1

0.20 0.18

(1)

0.16

3

dV(logd) /cm•g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.14 0.12 0.10 (2)

0.08 0.06 0.04 0.02 0

6

12

18

24

30

36

Pore diameter /nm


42


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 9. The TEM images of CoOOH-air aged in H2O2 for different reaction times (A) 1 h (B) 2 h.


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80 ■ Co 3 O4 -H2 O2 ▲ Co 3 O4 -air

O2 Evolution /mL

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


◆ commercial Co 3 O4

60

1st

3rd

2nd

4th

40

20

0 0

20

40

60

80

100

Time /min




1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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TABLES. Table 1 Reaction rate constant for the decomposition of H2O2 over Co3O4-air and Co3O4-H2O2. Sample

k (1st run)

k (4th run)

Co3O4-air

min-1 0.15

min-1 0.11

Co3O4-H2O2

0.28

0.17

Commercial

0.0009


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ASSOCIATED CONTENT

Supporting Information. The XPS spectrum of as-prepared Co3O4, TEM images and XRD patterns of the samples synthesized at different synthesis parameters were summarized in the supporting information.

AUTHOR INFORMATION

Corresponding Author *

Mei

Yang:

E-mail:

[email protected],

Tel.:

+86-411-8437-9816,

Fax.:

+86-411-8437-9327.

* Guangwen Chen: E-mail: [email protected], Tel.: +86-411-8437-9031, Fax.: +86-411-8437-9327.

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources National Natural Science Foundation of China (Nos. 21406226, 21225627).

ACKNOWLEDGMENT The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (Nos. 21406226, 21225627).



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For Table of Contents Use Only

Facile Synthesis of Co3O4 with Different Morphologies via Oxidation Kinetic Control and its Application in Hydrogen Peroxide Decomposition Huihui Chena, b, Mei Yanga,*, Sha Taoa, b, Mingyue Rena, b, Guangwen Chena,* a

Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese

Academy of Sciences, Dalian 116023, China b

University of Chinese Academy of Sciences, Beijing 100049, China

Co3O4 NPs with tubular and hollow structures were successfully synthesized by the formation of CoOOH and subsequent high-temperature calcination in air. The as-synthesized Co3O4 retained the morphologies of CoOOH, and thus the key step was the controllable synthesis of CoOOH NPs with tubular and hollow structures, which were selectively synthesized by manipulating the oxidation kinetics.


Facile Synthesis of Co3O4 with Different Morphologies via Oxidation

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