Synthesis of MicroâNano-assembled Manganese Carbonate via

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Synthesis of micro-nano assembled manganese carbonate via aqueous precipitation assisted by ethanol Yanqing Tang, Yangcheng Lu, and Guangsheng Luo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02039 • Publication Date (Web): 17 Aug 2017 Downloaded from http://pubs.acs.org on August 21, 2017

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Industrial & Engineering Chemistry Research

Synthesis of micro-nano assembled manganese carbonate via aqueous precipitation assisted by ethanol Yanqing Tang, Yangcheng Lu*, Guangsheng Luo State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing, 100084, China

KEYWORDS: MnCO3, Ethanol, Precipitation method, micro-nano assembly

ABSTRACT: The micro-nano assembled manganese carbonate (MnCO3) was synthesized via aqueous precipitation assisted by ethanol. The effects of the time and the flow rate to introduce ethanol on the morphology and size of products were carefully investigated. It reveals that ethanol may participate in the precipitation of MnCO3 nanoparticles, the retardant of aggregate fusion, and the reassembly of aggregates at different stage. By a delayed but prompt introduction of ethanol, we can regulate these effects to endow the micro-nano assembled MnCO3 with high surface area, uniform morphology, and calcination-resistant skeleton simultaneously. The synthesis of monodispersed micro-nano assembled MnCO3 at elevated concentrations of reactants is also available by introducing ethanol earlier.

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ACS Paragon Plus Environment


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1.

INTRODUCTION

Guided by the growing appetite for high energy-storage materials, manganese carbonate has attracted more and more attention as electrode material since it showed high theoretical Li-storage capacity (467mA h g-1) for lithium storage than transition oxides in lithium storage ability1-4. However, the large volume changes during the delithiation/lithiation process and the inherent low conductivity of carbonate compounds lead to the unwanted capacity fading of manganese carbonate and limit its further applications5-7. In order to overcome these shortcomings, plenty of efforts have been made either on designing unique morphology structures or on introducing composite materials5,8,9. Micro-nano assembled material possessing the advantages of both microsized material and nanosized materials now supposed to be a potential candidate. The nanosized units can provide short distances for Li+ diffusion and large surface area to provide enriched contact sites for the reaction. For the micro-structured materials, the volume changing during the delithiation/lithiation process can be buffered effectively due to the presence of pores between nano-particles. From the point of this view, it’s of great significance to synthesize micro-nano assembled particles with a strong skeleton and high surface area. Due to the widely applications of manganese carbonate in catalysts10, high-density magnetic storage media2,5,11-13, electronics, drug delivery14,15 and additives in ceramics16 and so on6,17,18, many groups have been studying it on the relationship between morphology and function. So far, there are a variety of methods to synthesize micro-nano assembly particles, such as precipitation method8,11,18-21, hydrothermal and solvothermal method4,6,22-27, surfactant assisted method17,24,28, micelle method1,2,29, ultrasonic30,31 and electrodeposition32. However, the commercial synthesis of micro-nano structured MnCO3 is still challenging due to tedious operation and low productivity. A facile, environmental friendly and effective method for preparing micro-nano assembled MnCO3 is highly demanded. Herein, ethanol was introduced to control the assembly behavior of MnCO3 nanoparticles 2


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generated by aqueous precipitation, and monodispersed micro-nano assembled MnCO3 with high surface area and calcination-resistant skeleton was obtained by optimizing the addition time and amount of ethanol. Through investigating the evolution of the morphology and size of products, we explored the multiple effects of ethanol on the formation of micro-nano assembled MnCO3, which could be regulated to achieve mass production of various micro-nanostructured materials potentially. 2.

EXPERIMENTAL SECTION

2.1 Chemicals All of the reagents used during the experiments including manganese sulphate monohydrate (MnSO4·H2O, 99 %), sodium carbonate and ethanol were of analytical reagent grade. MnSO4·H2O were purchased from J&K Scientific (China). Sodium carbonate were purchased from West Long Chemical Co. Ltd. Ethanol were obtained from Beijing Tong Guang Fine Chemicals Company. All the chemicals were used without any further purification. Furthermore, the deionized water was used to make up solutions. 2.2 Procedures of MnCO3preparation An aqueous precipitation method assisted by ethanol was exploited for the preparation of MnCO3. The schematic of experimental set-up is given in Figure 1. First we started up the precipitation reaction in the microreactor which was designed and fabricated by using laser etching techniques (Lajamin Laser). The geometric size of the microchannel was 15 mm ×0.5 mm × 0.5 mm (length × width × height). We used a stainless steel membrane with an average pore diameter of 5 µm as the dispersion medium. In a typical synthesis, 0.1 mol/L MnSO4 aqueous solution and 0. /L Na2CO3 aqueous solution were used as dispersed fluid and continuous fluid, respectively; both of them were delivered at the flow rate of 60 mL/min and mixed in the microreactor; after 10s residence time in the outlet pipe of the microreactor, ethanol was introduced at specific flow rate through a tee to obtain MnCO3 slurry with ethanol solution; the 3



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slurry was aged for 5 min and then separated by centrifugation to obtain white MnCO3 precipitate; the MnCO3 precipitate was washed with deionized water, ethanol, acetone and hexane for several times, and dried in a vacuum oven at 70 °C overnight for further characterization. The sample obtained by introducing x mL/min ethanol was denoted as MnCO3-x, and the MnCO3-0 was the sample obtained by aqueous precipitation directly. Without specific statement, the experimental conditions in typical synthesis would be adopted.

Figure 1. Schematic of the experimental set-up 2.3 Analysis and Characterization The morphology of the products was observed by scanning electron microscope (SEM; JSM-7401, JEOL) and transmission electron microscopy (TEM; JEM-2010, 120 kV). The phase of samples was characterized by X-ray diffraction (XRD; D8-Aduance, BRUKER) using Cu Kα radiation (40 kV and 40 mA) at a scanning rate of 10o min-1. The surface area and porous structure of samples were characterized by nitrogen adsorption-desorption (QUADRASORB SI) at 77 K. Thermogravimetric analysis (STA409PC) was also carried out at a heating rate of 10°Cmin-1 from 30 °Cto 800 °C and kept stable at 100 °C for 30 min to remove the residual solvents under nitrogen atmosphere. 3.

RESULTS AND DISCUSSION

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3.1 Morphology and composition of particles The samples of MnCO3-0 and MnCO3-60 were characterized by XRD, SEM, TEM, and TGA to determine their morphology and composition. Figure 3 shows SEM and TEM images. As seen in Figure 2(a), the MnCO3-0 has many conjoint particles. The inset image is just a particle composed of two spherical-like daughter particles. Differently, most of the MnCO3-60 particles shown in Figure 2(b) were of monodispersed spherical-like particles. We measured the particle size of around 300 secondary particles from SEM images. The average particle size is 510 nm and the standard deviation is 0.1038. From the TEM images we can also confirm that there is no hollow structure in these particles. While, close observation reveals that these particles were composed of massive nanoparticles and have rough surface, as shown in Figures 2(c) and 2(d).

Figure 2. SEM and TEM images of MnCO3-0 (a, c) and MnCO3-60 (b, d) and size distribution of MnCO3-60. 5



Figure 3 shows the XRD patterns of MnCO3-0 and MnCO3-60. As seen, all the peaks could be well fitted to the rhombohedral phase of MnCO3 (JCPDS card no.44-1472) in the R3 space group, indicating that the MnCO3-0 and MnCO3-60 were both of the proper crystallinity without impurity. Thermal analysis of these MnCO3 samples were also conducted to ensure the composition. As shown in Figure 4, the first step (about 5 % weight loss) occurs in the temperature range between 30 and 300 oC, corresponding to the evaporation of physically adsorbed solvent. The second step mainly occurs at 300 oC associating with the thermal decomposition of MnCO3 particles. The samples have the weight loss between 300 oC to 500 oC about 36.6 % and 35.4 %, which was very closely to the theoretical weight loss from MnCO3 to MnO. From the TG curves, we can also find that the ending temperature of weight loss for MnCO3-60 is a little bit higher than that for MnCO3-0, which indicating a better thermal stability. It may be resulted from their differences in primary particles and aggregation state. However, all these evidences verify that we have obtained monodispersed spherical-like micron-nano assembled particles with the assistance of ethanol. MnCO3 - 60 MnCO3 - 0

30

60

(300)

50

(214)

(113)

40

(122)

20

(024)

10

(202)

(110)

(012)

(018) (116)

(104)

Relative Intensities(a.u.)

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70

2 theta (degree)

Figure 3. XRD patterns of as-prepared MnCO3-0 and MnCO3-60

6


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Figure 4. TG and DTA curves of MnCO3-0 and MnCO3-60. 3.2 The influences of ethanol during the formation of MnCO3 particles In order to understand the details of the formation of MnCO3 particles and clarify the effects of ethanol, we changed the factors in preparation procedures one by one and made careful comparisons on the various samples. Firstly, we changed the flow rate of ethanol and the SEM images of various samples are shown in Figure 5. Compared with Figure 3(a), the particles are all monodispersed as ethanol being introduced. It can be sure that the ethanol can effectively prevent the secondary particles from generating conjoint particles by fusion. Besides, the average size of secondary particles is decreased with the increasing of the flow rate of ethanol. The average size is around 700 nm at the flow rate of 30 mL/min, around 510 nm at the flow rate of 60 mL/min, and around 370 nm at the flow rate of 100 mL/min, respectively.

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Figure 5. SEM, TEM images and size distributions of MnCO3-30 (a, c) and MnCO3-100 (b, d). We could also calculate the size of crystal grain from XRD pattern through Scherrer equation. It is 6.0 nm for MnCO3-0, 8.5 nm for MnCO3-30, 8.2 nm for MnCO3-60, and 6.7 nm for MnCO3-100, respectively. Besides, from the close observation on particles, as shown in Figures 2(c&d) and Figures 5(c&d), we can find that the primary particles on the surface of the particles become smaller with the increasing of the flow rate of ethanol. Based on these two evidences, we think that there may be a second nucleation stage caused by ethanol. When we introduced ethanol as the aqueous precipitation is proceeding, the supersaturation ratio of MnCO3 in the solution may increase abruptly to result in the second nucleation. It will generate new primary particles that could adhere to the original secondary particles. When the flow rate of ethanol increased, the supersaturation ratio becomes higher so that the new primary particles become 8


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smaller. While, for MnCO3-0 without secondary nucleation induced by ethanol, the primacy particles on the surface will experience growth under relatively low supersaturation ratio to achieve larger size.

Figure 6. SEM images of MnCO3-60obtained by introducing ethanol after (a) 20s, (b)30s, (c)60s, and (d)5min aqueous precipitation process.

We further investigated the morphology and structure of particles obtained by introducing ethanol after aqueous precipitation process with different duration comparatively, where SEM andN2 sorption analysis were exploited. The SEM images were shown in Figure 6 and Figure 3(b). We can see that the secondary particles in Figure 3(b), 6(a) and 6(b) are all monodispersed and spherical-like, and seem to be similar in size (around 600 nm). They correspond to 10 s, 20 s, and 30 s aqueous precipitation process, respectively. When the aqueous precipitation process extended to 60 s, there existed some irregular particles, as seen in Figure 6(c). After 5 min 9



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aqueous precipitation process, the morphology of the particles is almost independent on whether ethanol was introduced or not. These results indicate that the aqueous precipitation process as well as the self-assembly of primary particles has almost finished after 5 min, and ethanol should be introduced earlier to affect the formation of micro-nano assembly. The N2 sorption analysis could characterize the aggregation state of primary particles quantitatively. All the nitrogen sorption isotherms can be classified as type IV with hysteresis loops (Figure S1, Supporting Information). The results of specific surface area, total cumulative pore volume as well as average pore diameter are tabulated in Table 1. From entries 5 and 6, we can find that when ethanol was introduced after 5 min aqueous precipitation process, the changes of surface area and average pore size were little. It indicates that the aggregation state of primary particles has become stable after long-term aging. On the contrary, for the entries 1-4 with short aging time before the introduction of ethanol, both the surface area and the cumulative pore volume were distinctly smaller than those of MnCO3-0. A plausible explanation is that the original aggregation of primary particles in aqueous solution is relatively loose and can become dense with the introduction of ethanol. Another interesting finding from entries 1-4 is that the surface area becomes smaller as the time to introduce ethanol postpones from 10 s to 60 s. It may be determined by the secondary nucleation induced by ethanol. The secondary nucleation is weakened with the elongation of aqueous precipitation process that decreases the soluble MnCO3 consistently. In detail, the introduction of ethanol at 10 s can cause intensive secondary nucleation, generate large amount of primary particles to embed into the original secondary particles, and resist the densification effect of ethanol. Comparatively, the introduction of ethanol at 60s can only generate small amount of primary particles and has less impact on the densification effect of ethanol.

Table 1. Surface area and porosity data of MnCO3 Entry

Samples

Quenching time

Specific surface Area (m2g-1)

Average pore Diameter (nm)

10


Cumulative pore volume (cm3g-1)

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1

MnCO3-60

10 s

115.3

4.528

0.07761

2

MnCO3-60

20 s

93.83

4.494

0.1193

3

MnCO3-60

30 s

83.89

4.619

0.07506

4

MnCO3-60

60 s

75.54

6.690

0.1115

5

MnCO3-60

5 min

166.5

4.448

0.2203

6

MnCO3-0

-

151.0

3.913

0.2527

Figure 7. SEM images of Mn2O3 samples: (a) Mn2O3-0; (b) Mn2O3-30; (c) Mn2O3-60; (d) Mn2O3-100 An important usage of MnCO3 is to prepare Mn2O3 by calcination as precursor. As we can control the appearance and structure of MnCO3 as expected, it will be welcome to keep them after calcination. Herein, we calcined the MnCO3-x at 560 oCfor 6h under flowing oxygen and 11



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obtained samples denoted as Mn2O3-x. The XRD patterns of these samples are shown in Figure S2. All the diffraction peaks in the pattern match well with the standard pattern of Mn2O3 (JCPDS No 41-1442)33.Then SEM was used to characterize the morphology of these samples, as shown in Figure 7.We can see that the particles of Mn2O3-0 look like walnut, relatively different from other three samples. The particles had a more stable shell, reflecting that the stability during the calcination was highly dependent on the size of primary particles. The primary particles on the surface were larger and had better skeleton stability in calcination. Table 2. Surface area and porosity data of MnCO3 and Mn2O3 Samples

Specific surface Average pore

Cumulative pore

Area (m2g-1)

Diameter (nm) volume (cm3g-1)

MnCO3-0

151.0

3.913

0.2527

MnCO3-30

119.2

3.663

0.05040

MnCO3-60

115.3

4.528

0.07761

MnCO3-100 169.8

4.340

0.1050

Mn2O3-0

15.19

15.74

0.06416

Mn2O3-30

18.21

33.08

0.1591

Mn2O3-60

14.83

11.67

0.04692

Mn2O3-100

10.81

11.01

0.03264

The Nitrogen sorption measurements were also conducted to characterize the skeleton structure of Mn2O3. The specific surface area and porosity data of MnCO3 and corresponding Mn2O3 were shown in Table 2. We can see that for MnCO3-0, MnCO3-60, and MnCO3-100, calcination could reduce the specific surface area and cumulative pore volume much, and increase the average pore diameter distinctly. It indicates serious agglomeration of primary particles and skeleton collapse of secondary particles. While, from MnCO3-30 to Mn2O3-30, the cumulative pore volume increased a lot due to the good stability of skeleton in calcination. The difference of various samples in Table 2 is the flow rate of ethanol to be introduced, which mainly affects the size of primary particles generated by ethanol induced secondary nucleation. Compared with MnCO3-60 12


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and MnCO3-100, this part of primary particles of MnCO3-30 is larger in size. Large size of primary particles embedded in secondary particles may enhance the stability of the micro-nano assembly in calcination. Thus, a proper introduction of ethanol could endow the micro-nano assembly of MnCO3-100 with monodispersed morphology and calcination-resistant skeleton. 3.3 The proposed mechanism of MnCO3 particles formation According to above observations, analysis and discussion, we attempted to propose a formation mechanism of micro-nano assembled MnCO3 based on the contributions of nucleation, growth and aggregation, as shown in Figure 8.

Figure 8. Schematic illustration of the formation processes of MnCO3

In general, the MnCO3 particles are formed probably via a “aggregation-subunits mechanism”34,35. At the initial nucleation stage, Mn2+ and CO are mixed to yield a supersaturated solution leading to form MnCO3 nuclei, and then the nuclei grow into primary particles. In the second step, the particles grow by aggregating and form secondary particles. The secondary particles can further fuse into conjoint particles in aqueous solution. The fast nucleation, growth and aggregation at the high supersaturation ratio lead to a relatively irregular and loose core. The original structure can be self-assembled into a little tighter one as time went 13



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by. Besides, the consuming of reactants leads to the decrease of supersaturation ratio. Then large subunits are formed and then adhere to the surface of the original particles to form the shell of the particles, which is more stable than the core during the calcination process. However, when we introduced the ethanol, the solution was suddenly changed and it may cause three different but related effects. Firstly, the sudden change of the solution can increase the supersaturation ratio and result in the secondary nucleation of MnCO3. The new primary particles from secondary nucleation can either adhere on the surface of the original particles or embed into the loose particles. When the secondary nucleation particles were small, they can easily embed into the original secondary particles which may hinder the densification effect of ethanol. Otherwise, they will be prone to adhering to the surface of the secondary particles. Secondly, the sudden change of the solution can cause the densification of the original loose particles formed in the water, while the embedded primary particles generated by secondary nucleation can help resist this effect. Thirdly, the sudden change of the solution can stop the fusion of secondary particles, partly due to the coating of secondary particles by the primary particles generated by secondary nucleation. The mechanism of micro-nano assembled MnCO3 formation implies the formation and fusion of secondary particles are tandem processes dependent on the time to introduce ethanol much. Accordingly, high concentration reactants, favorable for massive production, can accelerate the whole process, and ethanol should be introduced earlier to obtain monodispersed and well assembled secondary particles. Figure 9 shows the SEM images of samples prepared by using high concentration reactants. For the samples in Figure 3(b) and Figures 9(a&b), only the reactants concentrations are different. By comparison, we can find that the fusion of secondary particles becomes serious with the increasing of reactants concentrations. As the time to introduce ethanol was shifted from 10 s to 1 s, we got the samples shown in Figures 9(c&d). Comparing Figures 9(c&d) with Figures 9(a&b), it is obvious that the monodispersity was much improved by introducing ethanol earlier. Meanwhile, the size of secondary particles decreases remarkably with the increasing of reactants concentrations. In evidence, the aqueous 14


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precipitation assisted by ethanol could be facile and productive preparation method for monodispersed micro-nano assembled MnCO3.

Figure 9. SEM images of MnCO3 samples with different time to introduce ethanol. (a) t = 10 s, CMnSO4=CNaCO3=0.5 mol/L; (b)t = 10 s; CMnSO4=CNaCO3=1.0 mol/L;(c) t = 1 s, CMnSO4=CNaCO3=0.5 mol/L; (d) t = 1 s; CMnSO4=CNaCO3=1.0 mol/L.

4.

CONCLUSIONS

The aqueous precipitation assisted by ethanol was found to be facile and productive preparation method for the monodispersed micro-nano assembled MnCO3 with high specific area. Based on characterization on evolution of morphology and structure with the time to introduce 15



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ethanol and the flow rate of ethanol, we proposed the formation mechanism of micro-nano assembledMnCO3andrevealed the multiple effects of ethanol during the process: inducing secondary nucleation and new primary particles, densifying the aggregation of primary particles in secondary particles, and preventing the fusion of secondary particles. Thus, by introducing ethanol after short aqueous precipitation process, less than 60 s in this work, we could obtain secondary particles composed with primary particles generated in water and primary particles generated in ethanol-water solution. A proper assembly type of these primary particles is helpful for keeping stable skeleton during calcination from MnCO3 to Mn2O3.The aqueous precipitation assisted by ethanol has the potential to be applied in preparing various micro-nano assembly particles with high surface area and flexible structure due to its convenience and universal mechanism. Based on the characterization results of as prepared MnCO3, a good performance in lithium ion battery may be expected, and related electrochemical testing is ongoing in our group.

ASSOCIATED CONTENT Supporting Information. The Nitrogen adsorption/desorption isotherms and pore distribution plots of MnCO3 samples The XRD patterns and the Nitrogen adsorption/desorption isotherms and pore distribution plots of as-prepared Mn2O3 calcinedfrom different MnCO3

AUTHOR INFORMATION Corresponding Author Tel: +86-10-6277-3017. Fax: +86 10 62770304. Email: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS 16


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The authors are gratefully thankful for the support of the National Natural Science Foundation of China (21422603, U1662120) on this work.

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