Article pubs.acs.org/crystal
Direct Correlation of the Morphologies of Metal Carbonates, Oxycarbonates, and Oxides Synthesized by Dry Autoclaving to the Intrinsic Properties of the Metals Anustup Sadhu, Shiv Prakash Singh, and Sayan Bhattacharyya* Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur 741252, W.B., India S Supporting Information *
ABSTRACT: Dry autoclaving of the metal acetates under similar conditions resulted in various phases and morphologies of metal carbonates, oxycarbonates, and oxides. The acetates of calcium (Group IIA) gave ∼100 nm thick CaCO3 nanosheets, and 3d transition metals (Mn and Fe) gave MnCO 3 microcubes and Fe3O4 tetragonal bipyramids. The lanthanides (La, Ce, and Pr) presented hierarchical flower-shaped structures with self-assembled 85−250 nm thick nanosheets comprising of La2O2CO3 and LaOHCO3 mixed phases, CeO2 and Pr2O2CO3, respectively. The Gibb’s free energy of formation (ΔGof ) of the oxide or decomposition of the carbonates at elevated temperatures >700 °C controls the final phase. Elemental line scans showed carbon coating on the nanosheets, whereas carbon existed as separate microspheres whenever the micron-sized morphologies were obtained. The solidification kinetics of the supercritical metal intermediates and carbon were comparable when the freezing point (FP) of the metals is 700 °C have to solidify once the furnace starts cooling down and the morphology of the autoclaved products depends on the solidification kinetics of the elements or their compounds.19 The solidification rate of metals is faster than elemental carbon, and thus formation of a nanostructured carbon shell over the inorganic core is also well-known.13−15 For carbon to form a stable coating on the nanosheets, the solidification kinetics of the elements and carbon should be comparable such that once the nanosheets form from the stacking of the solidified nuclei of the product, carbon will immediately encapsulate the nanosheet layers (Figure 6). The inhomogeneous carbon coating can be attributed to the nonisotropic shapes which results in a gradient of carbon density. Carbon likely starts wrapping the nanosheets from the regions of higher surface energy such as the edges (Figure 5a,f,h,k). In the case of microstructures where carbon forms separate spheres, the solidification of the inorganic elements and their compounds was much faster than that of carbon, such that once elemental carbon starts solidifying, the formation and growth of the inorganic phases were complete. In these kinetically controlled reactions, the solidification rate can be directly correlated to FP of the elements. From the standard data available and also shown in Figure 6, the FPs of
nanosheets, carbon is highly concentrated at the edges (Figure 5a,f,k). In Figure 5a, the high intensity Ca line is not shown to highlight the behavior of the C line around the nanosheet edges. The original image is presented in Figure S2, Supporting Information. In Figure 5f,h,k, the C line peaks over each vertically aligned nanosheet. In these autoclaved nanosheets, the intensity of C line is close to the O line, indicating a high concentration of carbon in these samples. After conversion to the respective oxides, the carbon content on the sheets were expectedly reduced (Figure 5b,g,l). The residual carbon in the oxides can result from the leftover minor fractions of undecomposed carbonate/oxycarbonate linkages in addition to probable contamination inside the sample chamber. For the autoclaved Fe3O4 and MnCO3 products, the carbon content on the microstructures was pretty low (Figure 5c,e) since majority of carbon coexisted as microspheres. In fact, these carbon spheres were largely absent whenever the nanosheets were formed. Although the carbon content in MnCO3 is higher than Mn2O3, the carbon intensity on the microcubes in Figure 5c,d appears similar. This is because of the presence of carbon spheres in the same frame of Figure 5c, which suppresses the C line on MnCO3 microcube. The authentic proof of carbon coating on the nanosheets can be obtained from the analysis of CeO2 nanosheets. The line scan on the autoclaved CeO2 nanosheets shows a distinct carbon coating (Figure 5h), which is also confirmed from the EDAX analyses before and after air heating (Figure 5i, j). The 10 wt % carbon in the air heated CeO2 sheets might result from the trapped carbon intercalated between the sheets, which were not removed at 700 °C. Because of the lubricating carbon coating on the autoclaved nanosheets, the surface of each nanosheet was extremely smooth as is evident from the enlarged nanosheet in Figure 1d. Likewise ∼200 nm thick carbon film was previously observed on other inorganic sheets prepared by dry autoclaving.15 Air decomposition of the carbon coated metal carbonate and oxycarbonate nanosheets resulted in clustered 4065
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Article
Ca, La, Ce, and Pr are 842, 920, 795, and 931 °C, respectively. On the contrary, Mn and Fe have MPs of 1244 and 1535 °C, respectively.34 The intermediates can immediately solidify from an elevated temperature >700 °C if FP of the components is much higher than the autoclave temperature. In that case, carbon cannot compete with the solidification rate of inorganic components which result in separate MnCO3 microcubes/ Fe3O4 tetragonal bipyramids. Carbon solidifies later as separate carbon microspheres (Figure 6). Our logic applies to the earlier obtained irregular cubic/tetragonal nanoparticulate clusters of TiO2 along with carbon microspheres from titanium(IV) isopropoxide,13 where FP of Ti is 1668 °C.34 With lower freezing temperatures, carbon encapsulated nanosheets or flower-like structures can be obtained. In the case of very low FPs such as Zn (419.5 °C), carbon forms ∼60 nm thick shell over the ∼55 nm ZnO nanocrystals.14 If FP of the metal is too low, carbon does not give enough time for the nanocrystals to arrange in the form of nanosheets and core−shell nanocrystals were obtained. Thus, a direct correlation could be drawn between the morphologies and FP of the metallic constituents of the respective phases. In fact, 1D or 2D architectures of 3d transition metal oxides were obtained only if directed by catalytic growth or van der Waals interactions between the metal chalcogenide layers or induced by external magnetic fields.19,22,35 Lastly, the rare-earth oxides obtained by air heating of the autoclaved flowers were characterized for their optical properties (Figure 7). PL spectra were recorded for the autoclaved sample (Ac), air heated autoclave product (Ac-air), and the oxides obtained by direct decomposition of the metal acetates at 700 °C for 6 h in air without autoclaving (Air). La2O3, CeO2,
and Pr6O11 were excited at their characteristic wavelengths of 350, 304, and 350 nm, respectively. The autoclaved sample containing La2O2CO3 and LaOHCO3 shows no emission in the visible range of the electromagnetic spectrum. However, the Acair and Air La2O3 samples show two distinct emission peaks at 466 and 575 nm. It is well-known that La3+ ion cannot be regarded as an emission center because of the absence of electrons in the inner 4f atomic orbital. Also, the sample was excited at 350 nm (≡ 3.5 eV), which is less than the bandgap energy (Eg = 3.8 eV) of La2O3.36 The emissions of 466 and 575 nm are therefore assigned to singly oxygen vacancies in the La2O3 samples.36,37 Similar oxygen defect related emissions at 467 and 560 nm were also observed in the Ac-air and Air CeO2 samples. The emissions occur from various defect states to the O 2p band.38 Ac CeO2 did not show any emission, probably due to quenching of luminescence by the carbon coating, which also passivates the defect centers at the surface of the nanosheets. The Ac product containing Pr2O2CO3 did not show any emission, and the possible luminescence from a small fraction of Pr6O11 was similarly quenched by carbon. In the Acair Pr6O11, the emission peaks centered at 475, 572, 642, and 665 nm are due to the 3H4 → 3P1, 3P0 → 3H5, 3P0 → 3F2, and 3 P1 → 3F3 transitions, respectively.39 Thus, dry autoclaving resulted in structural defects in the self-assembled nanosheets of rare-earth oxides. Pressure induced syntheses also have the possibility of introducing lattice strain which can be verified from the defect-related emissions.
4. CONCLUSIONS In summary, the crystallographic phases and morphology of the products obtained by dry autoclaving of metal acetates were explained based on the ΔGof of the oxide or decomposition of the carbonates at >700 °C and solidification kinetics correlated to FP of the metals, respectively. Negative ΔGof of the formation of Fe3O4 and CeO2 oxides at 700 °C resulted in these oxide phases, whereas unfavorable ΔGof retained the Ca-, Mn-, La-, and Pr-carbonates and/or oxycarbonates. The nanosheet structures of CaCO3, La2O2CO3 + LaOHCO3, CeO2, and Pr2O2CO3 were stabilized by an encapsulating carbon film. All these metals have FP below 1000 °C, and hence the solidification rate of the supercritical metal intermediates was comparable to that of carbon. When FP of the metals is above 1000 °C, the metal intermediates solidified much faster than carbon, and hence the microstructures were obtained along with separate carbon microspheres. Thus, the morphologies obtained for different metal carbonates, oxycarbonates, and oxides could be correlated to the intrinsic properties of the metals. Air decomposition of the respective carbonates and oxycarbonates resulted in the crystalline metal oxides. PL spectra of the La2O3 and CeO2 samples showed emissions due to oxygen vacancies and surface defects. Direct correlation between the intrinsic properties of the metals to the morphology of the metal compounds could help in resolving the unpredictability of morphology evolution of the latter.
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ASSOCIATED CONTENT
* Supporting Information S
Figure 7. Room temperature PL spectra of the rare-earth oxides. Ac: autoclaved sample, Ac-air: air heated autoclave product, and Air: direct air heating of the metal acetates.
XRD and EDAX data. This material is available free of charge via the Internet at http://pubs.acs.org. 4066
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the Council of Scientific and Industrial Research (CSIR), India, under Sanction No. 01(2689)/12/EMR-II. A.S. thanks University Grants Commission (UGC), New Delhi, for his fellowship. The infrastructural support from IISER Kolkata is duly acknowledged.
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