Hierarchical polymorphic MnCO₃ series induced by cobalt doping via a one-pot hydrothermal route for CO catalytic oxidation

Abstract

In this study, a series of cobalt-doped MnCO₃ hierarchical microstructures with different morphologies were synthesized by tuning a single variable (the dopant content) via a one-step, mild solvothermal synthesis in a N,N-dimethylformamide (DMF) solution system. Structure evolution of the polymorphic MnCO₃ took place with the morphology obviously transforming from an initial flower-microstructure to fan-like, then to hedgehog-like hemispheres and finally to flake-spheres as the Co²⁺ theoretical content (Co/(Co + Mn)) increased from 0 to 20% in the solvothermal process. Cobalt ion modulated reaction-limited aggregation (RLA) is proposed in the growth mechanism. The mechanism of Co²⁺-induced acceleration and full growth is further investigated. The Co²⁺ doped manganese carbonate displays wonderful catalytic performance towards CO oxidation.

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1 Introduction

Nowadays, one of the major focuses of the research area of nanotechnology has been to synthetically design micro-/nanostructures with various morphologies such as nanowires,^1,2 hollow microspheres,³ ball-flowers,^4,5 nanodiscs,⁶ nanosheets,^1,7 etc.The hierarchical morphologies mean that nanostructured materials possess novel properties which have attracted much attention from researchers to tune the intrinsic properties which are largely dependent on the shape, dimension, polymorph and morphology of the materials. Naturally, it is of great importance to explore an efficient way to exert highly accurate control over the morphology and polymorph and to arrange them into highly ordered and hierarchically organized structures from nanoscale to macroscale.^8–10 In recent research, the doping-induced control of size and shape has stood out among conventional morphological approaches^11–15 as a suitable modification to improve the function of the prepared nanomaterials. Liu et al. prepared copper doped CeO₂ nanospheres with different hollowness and size, and found the differences were due to the concentration of CuCl₂·2H₂O in the precursor solution; moreover, Cu²⁺ doping was found to enhance the reduction behavior and catalytic performance towards CO oxidation.¹⁶ Shen et al. discovered that the concentration of ZrO(NO₃)₂ influenced the film thickness of their prepared Zr-doped hematite nanorods.¹⁷ Lu’s group discussed that the morphology of SnO₂ could be controlled by varying the concentration of the added Zn²⁺.¹⁸ In fact, impurity doping was recently found to have crucial effects on the nucleation and growth of many functional materials, as well as the enhancement of the performance in practical applications.

Manganese carbonate (MnCO₃) materials have attracted the fascination of researchers because they are cheap, abundant, environmentally friendly and possess intriguing properties that are widely used in promising systems for biomedical applications, such as biosensors, bioreactors, and drug delivery devices,¹⁹ and in other applications such as potential electrode materials for super capacitors,²⁰ or as catalysts for the Fischer–Tropsch reaction.^21,22 Furthermore, MnCO₃ could also be used as a typical precursor for manganese oxides, which are of considerable importance due to their potential applications in catalysis,^23,24 rechargeable batteries,^25–27 or as a new template to prepare various hierarchical materials.^28,29 To date, many endeavors have been devoted to investigate the influence of the doping elements on the morphologies¹⁷ and the corresponding improvement in the functional behavior of doped metal oxides.^17,18,25 However, literature concerning the impurity doping-induced control on the morphology growth and phase transformations of MnCO₃ materials is rare. Hydro-/solvothermal and co-precipitation methods were used for the synthesis of the metal carbonate.^20,28 Different solvents and surfactants are most commonly used to direct and mediate metal carbonate crystallization.^30–32 Hollow MnCO₃ microspheres and MnCO₃ nanocubes were synthesized via an ionic liquid-assisted hydrothermal synthetic method by regulating the concentration of the ionic liquid.³³ Hollow CaCO₃ spheres were synthesized in water/[HMim]-[BF₄] emulsions using CaCl₂ as the precursor.³⁴ However, to the best of our knowledge, it still remains an open challenge to establish a suitable and facile synthetic methodology for the preparation of metal carbonate materials with easy control over phase, size, shape and dimensionality.

In this study, we demonstrate the control of the morphological patterns of a series of cobalt-doped MnCO₃ micro-structures by tuning a single variable (the dopant content) in a one-step, mild solvothermal process. As far as we know, this is the first time that the impact of cobalt ion doping on the morphology of manganese carbonate has been reported. When the Co²⁺ theoretical content (Co/(Co + Mn)) was increased from 0 to 20%, the morphology of the final products changed and cobalt ion modulated reaction-limited aggregation (RLA) is proposed in the growth mechanism. Moreover, the wonderful catalytic performance of the Co²⁺ doped manganese carbonate hierarchical architectures towards CO oxidation was demonstrated.

2 Experimental

2.1 Materials

All the chemical reagents were of analytical grade, purchased from Sinopharm Chemical Reagent Co. Ltd (China) and used as received without further purification. Commercial manganese carbonate was used: 90 wt% MnCO₃ and 10 wt% PC (Portland cement), BET surface area of 5.1 m² g⁻¹.

2.2 The synthesis of MnCO₃ microflowers (P₁)

0.19 g of polyvinylpyrrolidone (PVP, K30) was dissolved into 15 mL of N,N-dimethylformamide (DMF), and after stirring for 20 min, 180 μL of a Mn(NO₃)₂ solution (50 wt%) was added to the homogeneous solution. After stirring for another 10 min, the mixture was transferred to a 25 mL Teflon-lined stainless steel autoclave, which was maintained for 18 h at 210 °C with a ramp rate of 2 °C min⁻¹. When the autoclave was cooled to room temperature, the white products were collected and washed with absolute alcohol four times sequentially. Finally, the products were dried at 60 °C overnight.

2.3 The synthesis of Co-doped MnCO₃ microstructures (P₂–P₆)

Different amounts of cobalt acetate tetrahydrate, weighed in molar ratios (Co/(Co + Mn)) of 1.5%, 5%, 10%, 15% and 20%, were dissolved into 15 mL of N,N-dimethylformamide (DMF) that contained 0.19 g of PVP (K30). After stirring the above homogeneous solution for 20 min, 180 μL of a Mn(NO₃)₂ solution (50 wt%) was added to the reaction solution. After continuous stirring for another 10 min, the mixture was transferred to a 25 mL Teflon-lined stainless steel autoclave, which was maintained for 18 h at 210 °C with a ramp rate of 2 °C min⁻¹. When the autoclave was cooled to room temperature, the final precipitates were collected and washed with absolute alcohol four times sequentially. Finally, the products were dried at 60 °C overnight. The above products with different Co²⁺ doping concentration were labeled as the P₂–P₆ samples.

2.4 Physical characterization

The crystal structure information of the synthesized samples was established by powder X-ray diffraction (XRD Bruker D8 diffractometer with Cu-Kα radiation (λ = 0.15418 nm)). The micro-structure morphology of the powders was observed by using a transmission electron microscope (TEM, JEM 1011-CXII, 100 kV). A field-emission scanning electron microscope (FE-SEM, Hitachi, S4800) equipped with energy-dispersive X-ray spectroscopy (EDS) was used to characterize the specific morphologies. X-ray photoelectron spectroscopy (XPS) data were acquired on an ESCALAB 250 X-ray photoelectron spectrometer with Al Kα radiation and the binding energies were determined utilizing the C 1s spectrum as a reference at 284.7 eV. The surface areas were calculated by the Brunauer–Emmett–Teller (BET) method. Raman data were obtained using a Lab RAM HR4800 spectrometer using a 473 nm laser line as an excitation source. FTIR spectra were obtained on a Bruker Vector 22 model spectrometer.

2.5 Catalytic experiments

The catalytic activity of the as-obtained samples was evaluated on a continuous flow fixed-bed micro-reactor operating under atmospheric pressure. In a typical experiment, 25 mg catalyst particles with 250 mg quartz sand were placed into a stainless steel tube reactor. The composition of the raw material gas was CO/O₂/N₂ (1 : 10 : 89). The flow rate was 60 mL min⁻¹. The temperature of the reactor was monitored by the thermocouple placed on the catalysts, and the heating rate was 1.7 K min⁻¹. The products from the outlet of the reactor were analyzed using an online gas chromatograph (Gasboard-3121, China Wuhan Cubic Co.).

3 Results and discussion

3.1 Structural analysis

In our work, MnCO₃ products, including pure MnCO₃ (P₁) and the series of Co-doped MnCO₃ samples (P₂–P₆), were synthesized via a one-step hydrothermal method at 210 °C; it is worth noting that it’s crucial to maintain a slow warming with a ramp rate of 2 °C min⁻¹. The phases of the as-obtained products were characterized by powder XRD measurements, as shown in Fig. 1. The diffraction peaks of all the products illustrate that they can be perfectly indexed to a pure phase of a rhombohedral structure of MnCO₃ (JCPDS no. 44-1472). It was noticed that no other peaks stemming from manganese oxides, cobalt oxides or cobalt carbonate could be found, which indicated the formation of homogeneous Mn–Co carbonate solid solutions. Furthermore, the chemical composition, the bonding situation of the Co-doped MnCO₃ crystals, and the substitution will be ascertained in the following text.

Fig. 1 XRD patterns of the as-obtained P₁–P₆ samples.

The typical SEM images of the as-synthesized samples are displayed in Fig. 2. The low magnification SEM images of the P₁ sample show that the products consist of well-dispersed hierarchical microflowers with sizes ranging from 10 to 12 μm (Fig. 2A1). The magnified SEM image (Fig. 2A2) displays the panoramic view of our microflowers and makes it clear that the petals are branched out by inside-out evacuation.

Fig. 2 SEM images of the as-prepared P₁–P₆ samples (A–F) with Co²⁺ dopant content ranging from 0 to 20%.

In our experiment, an eye-catching phenomenon emerged when Co²⁺ was introduced to the manganese carbonate host. Varying the amount of cobalt acetate tetrahydrate could yield a series of Co–MnCO₃ microstructures with controllable sizes and morphologies. When the amount of cobalt acetate tetrahydrate reached 1.5%, the primary blooming flowers morphology (Fig. 2A1 and A2) disappeared and the P₂ sample showed ordered propeller-like morphology with an average radius of 6.5 μm as shown in Fig. 2B1 and B2; then a fan-shaped microstructure (Fig. 2C1 and C2) and a hedgehog-like structure with a radius of 3 μm (Fig. 2D1 and D2) were obtained when 5% and 10% cobalt acetate tetrahydrate were added to the system, respectively. Obviously, the incorporation of Co²⁺ lead to fully oriented growth for the Mn–Co carbonate solid solutions. Furthermore, the continuous increase in the amount of cobalt acetate tetrahydrate resulted in the formation of flake-microspheres with increased average radii: 6 μm for 15% doping (Fig. 2E1 and E2) and 8 μm for 20% doping (Fig. 2F1 and F2). The influence of Co²⁺ doping on the morphology of MnCO₃ was found to be significant due to the fact that the structures of the Co-doped products evolved from propeller-like to fan-like to structures almost like a half a sphere and finally into spherical structures during the whole process of increasing the amount of Co(AC)₂·4H₂O.

The compositional data of the as prepared P₁–P₆ samples are shown in Table 1 (ESI†). The Co/(Co + Mn) atom ratio increases as the theoretical doping increases. In addition, the EDS spectra corresponding to the doped samples are also displayed in Fig. S1.† In our work, the doping ions have the same valence and similar radii to the ones in the hosts, and are stable with the host lattice anion, which allowed some cobalt cations to substitute Mn²⁺ ions into the MnCO₃ lattice and thus drove the formation of a stable deficient rhombohedral structure. As shown in Fig. 3, the element mapping of Mn, Co, C and O in an individual microsphere (P₅ sample) and a fan-like structure (P₃ sample) demonstrates that all four elements are distributed throughout the MnCO₃ microstructures. As a result, it confirmed the occurrence of homogeneous distributions of Mn, Co, C and O, which is consistent with the XRD results.

Fig. 3 SEM images and corresponding EDS mapping images of an individual (A) P₅ microsphere and (B) fan-like P₃ microstructure: Mn (red), Co (green), C (grey) and O (blue).

All of the samples (P₁–P₆) were also characterized by Raman spectroscopy using an exciting laser wavelength of 473 nm, illustrated in Fig. 4. For the pure manganese carbonate (P₁ sample), the main characteristic Raman band is observed at 1088 cm⁻¹, which is due to the carbonate group.^35,36 Furthermore, the peaks for the P₂–P₆ samples display a regular variation in that the peak values decrease with an increase in the Co²⁺ doping content. This may be corresponding to lattice defects. In fact, the crystal lattice of our doped MnCO₃ contracts since Co²⁺ (small-size impurities) are doped to substitute the host ions (Mn²⁺),^16,37 which will inevitably result in smaller atomic spacing. Moreover, no characteristic peaks originating from metal oxides in the spectrum of the P₂–P₆ samples appear. This agrees with the previous XRD results, which further certify the incorporation of cobalt ions in the MnCO₃ rhombohedral lattice.

Fig. 4 Raman spectra of the synthesized pure (P₁) and Co-doped manganese carbonate (P₂–P₆) samples.

FTIR analysis was also carried out to investigate the influence of the impurity doping on the formation of MnCO₃ crystals. Fig. S2† shows the FTIR spectra of pure MnCO₃ (P₁) and Co-doped MnCO₃ (P₂–P₆). As we can see, these six spectra are almost the same. A weak broad band around 3430 cm⁻¹ is due to the stretching and bending vibrations of water molecules adsorbed on the sample surface. The presence of CO₃²⁻ in the as-prepared samples is evidenced by the characteristic peaks at 1402, 1075, 861.84, and 725 cm⁻¹, which are assigned to the vibrational modes ν3(E′), ν1(A1′), ν2(A′′), and ν4(E′′), respectively.^33,38–40 In fact, the peak located around 2490 cm⁻¹ is also associated to a vibrational mode of the carbonate anion. The weak peak at 1798 cm⁻¹ is attributed to an overtone or a combination band of the composition of the carbonate groups and divalent metal ions.⁴¹

To evaluate the oxidation state of the Mn and the Co dopant, XPS analysis was performed. Fig. 5a shows a wide scan spectrum of the doped P₅ sample as the typical example. The peaks located at 284.7, 532, 642, and 799 eV are assigned to the characteristic peaks of C 1s, O 1s, Mn 2p and Co 2p, respectively.^38,41 As expected, all of the above sharp peaks verify the abundant existence of carbon, oxygen and cobalt elements on the surface of the Co-doped MnCO₃. As displayed in Fig. 5b, several typical BE peaks (such as those at 642.2 and 654.2 eV) of the Mn 2p spectrum are observed in the P₅ sample, which suggest the existence of Mn(II) in the as-obtained products. The Co 2p spectrum (Fig. 5e) shows a doublet containing a low energy band (Co 2p_3/2) and a high energy band (Co 2p_1/2) at 781.8 and 798.4 eV, respectively. Thus, the energy difference between the peaks for Co 2p_3/2 and 2p_1/2 is approximately 16.6 eV, which indicates the presence of both Co²⁺/Co³⁺ species in the doped manganese carbonate samples.^42–44 The peak at 289 eV of the C 1s spectrum (Fig. 5c) is assigned to the carbon element in association with oxygen in the carbonate ions. Furthermore, the predominant O 1s peak (Fig. 5d) located at a binding energy of 531.7 ± 0.2 eV belongs to the lattice oxygen of MnCO₃. The XPS spectra are consistent with the theory that the cobalt ions substitute Mn²⁺ in the MnCO₃ lattice,⁴⁵ which verifies our view in EDS-mapping.

Fig. 5 Representative XPS spectra of the P₅ sample: (a) survey, (b) Mn 2p, (c) C 1s, (d) O 1s and (e) Co 2p.

3.2 Morphology mechanism

For the pure sample, highly monodisperse nanoblocks with a size of 360 nm were obtained at 4 h after starting the heat preservation at 210 °C, which may be supposed as the initial nucleating stage. Progressive processes of self-assembled upgrowths of the flakes and hierarchical arrangements lead to the final petal morphology, as illustrated in Fig. 2A1 and A2.

In order to understand the formation mechanism of the Co-doped samples and the growth process better, we studied their temporal morphological evolution by taking TEM images on samples obtained from the reaction at different time intervals. Herein, the P₃ sample served as an illustrative case study with its theoretical doping content of Co(AC)₂·4H₂O as 5%; the 15% sample is illustrated in Fig. S3.† Fig. 6 shows the evolution of the fan-like structures. When the solvothermal reaction time is 70 min at 210 °C, the nanoblocks are already observed. XRD (Fig. 7a) of the sample presents diffraction peaks of Mn₃O₄ (JCPDS no. 24-0734). As the reaction proceeds to 2 h, these nanoblocks start to gather in a predetermined direction (Fig. 6b). This oriented attachment following the aggregation is confirmed by Fig. 6c. Growth cycles of aggregation and oriented attachment increase the thickness of the fan-like structures and the TEM image becomes darker, as shown in Fig. 6d and e. As the solvothermal process is prolonged to 12 h, the morphology grows almost into its target morphology (Fig. 6f) and the size of the fan increases with the reaction time. XRD shows the same diffraction peaks of Mn₃O₄ for the sample collected at 2 h. Meanwhile, some peaks weaken and even disappear at 4 h, but no other peaks from the cobalt series appear (Fig. 7b and c). It’s worth noting the crystalline transition that occurred after 8 h when the crystals grew from Mn₃O₄ into MnCO₃ (JCPDS no. 44-1472), as displayed in Fig. 7d and e. The typical probable chemical reaction for the formation of Co–MnCO₃ can be described by the following steps:⁴⁰

HCON(CH₃)₂ + H₂O → HCOOH + NH(CH₃)₂ (1)

(1 − x)Mn²⁺·xCo²⁺ + 2OH⁻ + 1/6O₂ → Mn_(1−x)Co_xO_4/3 + H₂O (2)

Mn_(1−x)Co_xO_4/3 + HCOOH → Mn_(1−x)Co_xCO₃ + H₂O (3)

Fig. 6 Typical TEM images of the Co-doped P₃ sample obtained at 210 °C for different solvothermal times: (a) 70 min, (b) 2 h, (c) 3 h, (d) 4 h, (e) 8 h and (f) 12 h.

Fig. 7 XRD patterns of the Co-doped P₃ sample obtained at 210 °C for different solvothermal times: (a) 70 min, (b) 2 h, (c) 4 h, (d) 8 h and (e) 12 h.

Based on the above time-dependent transformation process, we thus propose the reaction-limited aggregation (RLA) process⁴⁶ to explain the formation mechanism of Co-doped manganese carbonate, and their shape evolution is illustrated in detail in Scheme 1. From the nanocrystal synthesis point of view, crystal growth occurs through nucleation followed by growth. Firstly, the initial Mn_1−xCo_xO_4/3 nuclei tend to be nanoblocks because of the crystalline nature of the hausmannite structure. Then, these initial nuclei aggregate and self-assemble to quickly form the rudimentary fan-like, hedgehog-like and spherical structures for the 5%, 10% and 15% samples, respectively. At the same time, the spherical structures were further maintained for the 20% sample. Such notable changes in the morphological variation originate from the inclusion of different amounts of Co(AC)₂·4H₂O as utilized for the Co doping in the reaction solution for 18 h. It has been demonstrated that the introduction of Co²⁺ increases the polarity of the reaction system.¹⁸ Thus, the addition of Co(AC)₂·4H₂O increases the driving force for aggregation which accelerates the aggregation speed. For the 1.5% to 10% Co-doped samples, oriented aggregation occurs among Co-doped Mn₃O₄ nanoblocks, followed by linear growth and further self-assembly into flower-like or hemispherical structures. However, when the content of Co²⁺ ions reaches 15–20%, the excessive aggregation speed tends to be deprived of directionality. Because of the reduced surface-to-volume ratios, and thus surface energy, the initial Co-doped Mn₃O₄ nanoblocks undergo anisotropic aggregation to form spherical structures, further integrating into the layered sheet structure and ultimately growing into compact hierarchical microspheres. What’s more, the initial aggregation will have more of a tendency to be a spherical structure with a higher content of Co²⁺ ions.

Scheme 1 Schematic illustration of the growth processes of the polymorphic Co-doped MnCO₃ microstructures.

In our synthesis, fan-like and hedgehog-like structures can also grow into the final flaked-spheres when the reaction time is prolonged to 36 h. Fig. S4a and b† show the flaked microstructures for our 5% and 10% samples. Thus, it is rational to infer that Co²⁺ ions play a great role in accelerating the evolutionary progresses. In a word, Co²⁺ ion modulated reaction-limited aggregation (RLA) is vital in the growth of the Co-doped MnCO₃ hierarchical microstructures.

3.3 Catalytic properties

To address the potential application of the prepared Co-doped MnCO₃ microstructures in catalysis, we chose CO oxidation as a model catalytic reaction. It is reported⁴⁷ that the Langmuir–Hinshelwood mechanism is generally proposed to be responsible for CO catalytic oxidation. The significant effect of manganese oxides on CO oxidation was confirmed in many other studies.^48–50 The CO oxidation process can be divided into three stages: the O₂ dissociative adsorption [eqn (4) and (5)] and its surface reaction with CO [eqn (6)]⁵¹

O_2(g) + ⊕ → O₂–⊕ (4)

O₂–⊕ + ⊕ → 2O–⊕ (5)

CO–* + O–⊕ → CO_2(g) + * + ⊕ (6)

* and ⊕ denote metal and support sites, respectively.

In general, the catalytic activity of the materials is related to their surface areas and superficial support sites. Fig. 8 shows the catalytic profiles of the pure and Co-doped manganese carbonate series. For both the pure and the Co-doped manganese carbonates, it can be obviously seen that the catalytic performance has been sharply improved in comparison with commercial manganese carbonate. As shown in Table 1 (ESI†), the surface area data can be the first evidence to explain the above phenomenon because the higher the surface area, the more active sites there are for surface reaction (6).^52,53 The catalytic oxidation of CO is achieved during the range of 100 to 295 °C for all of our samples. However, the P₂–P₆ samples with different Co²⁺ doping content show better catalytic activity compared with the as-prepared undoped manganese carbonate (P₁). For example, the initial reaction temperature for P₅ is 100 °C whereas P₃ begins to react with CO at 105 °C and the P₁ sample starts to react with CO at 175 °C. Furthermore, the T₈₀ temperatures of the prepared manganese carbonates with and without Co²⁺ doping are as follows: 205 °C (P₅) < 220 °C (P₃) < 250 °C (P₁). As the adsorbed phase steps (4) and (5) are reported to be the determining steps in the Langmuir–Hinshelwood mechanism, the surface area is of little importance when a number of oxygen vacancies exist.^16,47 On the one hand, the oxygen vacancy theory can be used to make the above question clear. The extra oxygen vacancies are generated by the incorporation of Co²⁺ and Co³⁺ into rhombohedral MnCO₃. At the same time, the additional lattice perturbation and structural stress improves O²⁻ mobility.^54,55 On the other hand, tricobalt tetraoxide is most active for CO oxidation and Co³⁺ is the most active site for CO oxidation, especially in the face of Co²⁺.⁵⁶ As a result, the cobalt ions replacing the divalent Mn²⁺ ion in the rhombohedral MnCO₃ lattice enhance the O₂ dissociative adsorption (eqn (4) and (5)), further the surface reaction with CO and finally improve the catalytic performance for CO oxidation among our Co-doped samples. The enhancement would be more apparent with the increase of Co²⁺ content. However, the excessive increase of the Co²⁺ content (20%) in the DMF system leads to the decrease of the catalytic activity (P₆), which indicates that an appropriate doping content is needed to produce the optimal Mn–Co carbonate solid solutions to achieve the best catalytic activity.

Fig. 8 Conversion of CO over the as-prepared pure (P₁) and Co-doped manganese carbonate (P₂–P₆ samples) and commercial manganese carbonate.

To explore the thermal stability of Co-doped MnCO₃, the catalytic tests were performed for six cycles. The recycled catalytic profiles were represented and compared in Fig. 9, which takes the Co-doped P₅ as the typical sample. Obviously, the sample retains high catalytic activity during the second to sixth run. The initial conversion temperatures are 95 degrees lower than the first run. This phenomenon can be assigned to the decomposition of organic matter in the microspheres by the high-temperature treatment in the first run, which leads to the increase of surface-active oxygen sites.⁴⁸ After the catalysis, the features of the catalysts remain almost unchanged, which can be observed from the FTIR and SEM images (see Fig. S4 and S5†). It is obvious that the Co-doped MnCO₃ catalysts possess a desirable stability.

Fig. 9 Catalytic performance of the obtained P₅ in different runs.

4 Conclusions

In summary, a series of designed Co²⁺-doped manganese carbonates were obtained via a one-pot solvothermal approach. The plumpness of their three-dimensional shapes increased with the Co²⁺ doping content while the sizes remained consistent. In our report, the whole growth mechanism is explained as follows: (1) Mn₃O₄ nanoblocks were formed in the nucleation process for both the pure and the Co²⁺ doped system; and (2) Co²⁺ ion modulated reaction-limited aggregation (RLA) is vital to the growth of the Co-doped MnCO₃ hierarchical microstructures. Furthermore, Co²⁺ ions can accelerate the rapid growth in preferential directions. With prolonged reaction times or increased Co²⁺ doping, perfect flaked spheres would finally be obtained. The catalytic properties of the Co-doped and undoped manganese carbonates were investigated, which demonstrates the as-prepared products are promising catalysts in CO oxidation, especially the 15% Co²⁺ doped manganese carbonate, which shows a high catalytic ability for CO oxidation. These results may be a primary step in understanding and designing manganese carbonates with desirable morphology and size, as well as other functional materials.

Acknowledgements

This work was supported by the Natural Science Foundation of China (grant no. 21276142 and 21476129).

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Footnote

† Electronic supplementary information (ESI) available: The surface areas, compositional data and EDS spectra of Co-doped samples; FTIR spectra of the as prepared doped and undoped MnCO₃ samples; SEM images of the Co-doped P₅ sample obtained at 210 °C for different solvothermal times; SEM images of the 5%, 10% Co-doped samples with a prolonged reaction time of 36 h and of the P₅ samples after repeating the catalysis 6 times; FTIR spectra of the P₅ sample before and after repeating the catalysis 6 times. See DOI: 10.1039/c5ra04708k

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