Direct Correlation of the Morphologies of Metal Carbonates, Oxycarbonates, and Oxides Synthesized by...

Direct Correlation of the Morphologies of Metal Carbonates,Oxycarbonates, and Oxides Synthesized by Dry Autoclaving to theIntrinsic Properties of the MetalsAnustup 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 undersimilar conditions resulted in various phases and morphologiesof metal carbonates, oxycarbonates, and oxides. The acetates ofcalcium (Group IIA) gave ∼100 nm thick CaCO3 nanosheets,and 3d transition metals (Mn and Fe) gave MnCO3microcubes and Fe3O4 tetragonal bipyramids. The lanthanides(La, Ce, and Pr) presented hierarchical flower-shapedstructures with self-assembled 85−250 nm thick nanosheetscomprising of La2O2CO3 and LaOHCO3 mixed phases, CeO2and Pr2O2CO3, respectively. The Gibb’s free energy offormation (ΔGf

o) of the oxide or decomposition of thecarbonates at elevated temperatures >700 °C controls the finalphase. Elemental line scans showed carbon coating on the nanosheets, whereas carbon existed as separate microspheres wheneverthe micron-sized morphologies were obtained. The solidification kinetics of the supercritical metal intermediates and carbonwere comparable when the freezing point (FP) of the metals is <1000 °C such that once the nanosheets formed from stacking ofthe solidified nuclei, carbon could wrap and stabilize the nanosheets. The autoclaved products were air heated to obtain the metaloxide phases. Defect-related emissions were observed from the rare-earth oxides.

1. INTRODUCTION

Since the advent of the research area of nanotechnology, one ofthe major focuses has been to synthetically design nanostruc-tures in various morphologies such as nanoparticles of differentshapes,1,2 nanowires,3 nanorods,4 nanodiscs,5 nanosheets, etc.6

The hierarchical morphologies of nanostructured materialspossess novel properties which attract scientists because of theirpotentiality in the fabrication of next-generation nanodevices.The prime focus is to tune the intrinsic properties which arelargely dependent on the size, shape, and morphology of thematerials of interest.7 To this effect, mechanistic explanationswere based on the type of precursors used, synthesis conditionssuch as temperature or time of reaction, and the role ofsurfactants or binding agents to the surface of thenanostructures in governing the morphology.8,9 High pressuresynthesis such as hydrothermal and solvothermal techniquesprovide facile routes to stabilize metastable shapes and sizesmaintaining good control over the crystalline composition.10,11

There have been lot of explanations on how a certainmorphology was obtained. Another high pressure approachinvolves the pyrolysis of metal precursors under dry conditionsin the absence of solvent, and the pyrolyzed intermediatesattain supercritical phases which undergo nucleation andgrowth of the product phases based on their thermodynamicand kinetic stabilities.12

A simple, straightforward approach of dry autoclaving hasbeen used to synthesize a wide variety of materials such ascarbon coated metal oxide,13−15 chalcogenide, carbide, boride,nitride, phosphide nanostructures,12,16−22 and various carbonmicro- and nanostructures such as carbon nanotubes andnanowhiskers.23−25 The metal alkoxides are sealed inside aclosed reactor, and the reactions occur under autogenicpressure due to decomposition of the alkoxides at elevatedtemperatures. It has the advantage of reaching highertemperatures and pressures close to ∼3 MPa.26 Inside thesealed reactor, the pyrolyzed chemical precursors undergometastable solid−liquid, solid−gas, liquid−gas, or gas−gassupercritical phases.12 However, the nucleation and growth ofthe products depend on the kinetic factors governing the finalmorphology of the materials. The pressure assisted techniquessuch as solvothermal and dry autoclaving have their pros andcons. The interesting aspect is that different morphologiescould be obtained without any surface-directing agents, and thedisadvantage lies in the unpredictability of these morphologies.This work deals with the formation of metal carbonates/oxycarbonates/oxides from dry autoclaving of representativemetal acetates across the periodic table and subsequent

Received: May 11, 2014Revised: June 9, 2014Published: July 7, 2014

Article

pubs.acs.org/crystal

© 2014 American Chemical Society 4060 dx.doi.org/10.1021/cg5006924 | Cryst. Growth Des. 2014, 14, 4060−4067

pubs.acs.org/crystal

conversion to metal oxides by air heating. The differentmorphologies obtained under similar reaction conditions wereexplained based on the different reaction kinetics whichdepends on the intrinsic properties of the metals and freeenergy of formation of their oxide or carbonates. To the best ofour knowledge, the trends in morphology evolution were neverexplained based on the basic physical properties of the metallicconstituents of the resultant phase.It is well-known that the physical properties of the elements

demonstrate certain trends in the Periodic Table of theelements. For example, on moving from left to right across the

groups and moving up the periods, the electronegativity andfirst ionization energy increase whereas the atomic radiidecrease. Also, the metallic character increases diagonallyfrom the top right to the bottom left of the Periodic Table. Inour effort to show the dependence of morphology on theelements in the periodic table, we chose the precursors of analkaline earth metal, i.e., calcium (Group IIA), 3d transitionmetals (manganese of Group VIIB and iron of Group VIIIB),and the lanthanides, namely, lanthanum, cerium, andpraseodymium. Since metals relate to the ability to loseelectrons, higher metallic character should lead to lower IE.

Figure 1. FESEM images of the autoclaved products synthesized from the acetate precursors of (a) Ca, (b) Mn, (c) Fe, (d) La, (e) Ce, and (f) Pr.The double arrows in d−f indicate the thickness of the nanosheets.

Figure 2. XRD patterns of the autoclaved products: (a) CaCO3, (b) MnCO3, (c) Fe3O4, (d) La2O2CO3, (e) CeO2, and (f) Pr2O2CO3. The symbols# and * indicate the secondary phases La(OH)CO3 and Pr6O11, respectively.

Crystal Growth & Design Article

dx.doi.org/10.1021/cg5006924 | Cryst. Growth Des. 2014, 14, 4060−40674061

Also, metallicity leads to higher melting/freezing points. In fact,Mn and Fe are less metallic and also possess higher FPs ascompared to the other four elements. We demonstrate herethat the morphology of the metal oxides has a direct correlationwith the FP of the elements for this particular synthesisstrategy. The present approach could pave the way forpredicting the morphologies of the products obtained by dryautoclaving.

2. EXPERIMENTAL SECTION2.1. Materials and Characterization. All reagents were of

analytical grade pure. Calcium(II) acetate hydrate (Ca(OOCCH3)2·xH2O; Alfa Aesar 99.9965%), manganese(II) acetate tetrahydrate(Mn(OOCCH3)2·4H2O; Merck ≥99.5%), iron(II) acetate (Fe-(OOCCH3)2 Sigma-Aldrich 95%), lanthanum(III) acetate hydrate(La(OOCCH3)3·xH2O; Sigma-Aldrich 99.9%), cerium(III) acetatehydrate (Ce(OOCCH3)3·xH2O; Sigma-Aldrich 99.99%), andpraseodymium(III) acetate hydrate (Pr(OOCCH3)3·xH2O; AlfaAesar 99.9%) were used without further purification. The reactionswere carried out in a Carbolite wire-wound tube furnace-single zone;model MTF 12/38/400. The X-ray diffraction (XRD) measurementswere carried out with a Rigaku (mini flex II, Japan) powder X-raydiffractometer having Cu Kα = 1.54059 Å radiation. Field emission

scanning electron microscopy (FESEM) images were recorded in CarlZeiss SUPRA 55VP FESEM. EDAX measurements were performedwith the Oxford Instruments X-Max with INCA software coupled tothe FESEM. The accelerating voltage was maintained at 5 kV forimaging and 20 kV for EDAX and elemental line scans. The samplepowders were dispersed in ethanol and drop casted on the aluminumstubs. After the solvent was dried, the loose powders were removedfrom the stub, and the latter was introduced inside the microscopechamber. Photoluminescence (PL) spectra were measured with aHoriba Scientific Fluoromax-4 spectrofluorometer using a Xe lamp asthe excitation source.

2.2. Synthesis Methodology. One gram each of the metalacetates was separately introduced in 2 mL Ham-Let Union (SS316)3/8″ stainless steel Swagelok reactors. The tightly capped filledreactors were heat treated at 700 °C for 6 h with a heating rate of 5°C/min followed by normal cooling. The resultant products afteropening the reactors were further treated as follows. (i) Theautoclaved products from Ce-, Pr-, Fe-, and Mn-acetates were airheated at 700 °C, and (ii) those from La- and Ca-acetates were airheated at 900 °C for 3 h, both with a heating rate of 5 °C/minfollowed by normal cooling. The different temperatures were requiredto obtain the right crystallographic phases.

Figure 3. FESEM images of the products obtained by air heating the autoclaved samples. (a) CaO, (b) Mn2O3, (c) α-Fe2O3, (d) La2O3, (e) CeO2,and (f) Pr6O11.



3. RESULTS AND DISCUSSION

Figure 1 shows the FESEM images of the products obtained bydry autoclaving the metal acetates, and their XRD patterns aredisplayed in Figure 2. Under exactly the same synthesisconditions, a Periodic Table trend in the morphologies couldbe observed. Ca-acetate resulted in ∼100 nm thick stackednanosheets of CaCO3, which crystallized in the rhombohedralcrystal structure with space group R3̅c. The XRD peaks matchwell with the JCPDS card No. 83-0578. Autoclaving of theacetates of 3d transition metals presented three-dimensional(3D) micron-sized structures. MnCO3 microcubes of ∼17 μmside length were obtained from Mn-acetate, whereas Fe-acetategave Fe3O4 micron-sized particles with tetragonal bipyramidalshapes where the base length is ∼3.6 μm, and the distance fromthe base to the apex is ∼2.6 μm. In Figure 1c, 1−2 μm diametercarbon spheres are also observed. MnCO3 crystallized in therhombohedral structure with space group R3̅c according toJCPDS card No. 86-0173 and the face centered cubic (fcc)Fe3O4 has the space group Fd3 ̅m (JCPDS card No. 75-0033).Also autoclaving of copper acetate (not dealt in this work) gave1−2 μm irregular shapes comprising spheres and bipyramids ofelemental copper. All the three autoclaved lanthanide acetatespresented flower-like structures where the self-assemblednanosheets are oriented from a common center. The thicknessof the nanosheets from dry autoclaving of La-, Ce-, and Pr-acetates was on average 90, 85, and 250 nm, respectively. Theflower morphologies of lanthanide oxides synthesized bymicrowave, hydrothermal, and other solution routes havealready been reported.27−29 The autoclaved product from La-acetate consisted of a mixture of phases. La-oxycarbonateLa2O2CO3 with a hexagonal structure according to the spacegroup P63/mmc (JCPDS 84-1963) was the major phase withhydroxycarbonate LaOHCO3 in orthorhombic (JCPDS 49-981) and hexagonal (JCPDS 26-815) crystal structures as theminor. The autoclaving of Pr-acetate resulted in Pr2O2CO3 inthe hexagonal P63/mmc structure as per JCPDS 37-0805. Aminor fraction of Pr6O11 was also observed. On the contraryCe-acetate was directly converted to the metal oxide CeO2,which has the fcc structure and space group Fm3 ̅m andmatched well with JCPDS 81-0782.When the autoclaved products were heated in air, the metal

carbonates/oxycarbonates decomposed to the respectiveoxides. Fe3O4 was converted to rhombohedral α-Fe2O3 withspace group R3 ̅c (JCPDS 33-0664), and the crystallinity ofCeO2 slightly increased after air heating (SupportingInformation, SI 1). Figure 3 shows the FESEM images of theair-heated products, and Figure 4 displays XRD patterns of thenewly formed metal oxides. The nearly smooth CaCO3nanosheets decomposed to 500−600 nm particles clusteredin the form of sheets. The XRD pattern of the product inFigure 4 displays the cubic crystal structure of CaO accordingto JCPDS 48-1467, where Ca2+ forms the fcc lattice and O2−

occupies the octahedral voids. The MnCO3 microcubesdecomposed to porous microcubes of Mn2O3 in the bodycentered cubic structure with space group Ia3̅ according toJCPDS 41-1442. The surface area of the Mn2O3 microcubeswas found to be ∼9 m2/g. The morphology of the tetragonalbipyramidal Fe3O4 particles did not alter appreciably after airheating. The flower-like structures of the autoclaved productsfrom La-, Ce-, and Pr-acetates disintegrated after being treatedat 900 °C in air. The growth center of the self-assemblednanosheets collapsed to give thicker sheets stacked on each

other (Figure 3d−f). La2O2CO3 and LaOHCO3 decomposedto give La2O3 in both hexagonal (space group P3̅m1, JCPDS05-0602) and monoclinic (JCPDS 22-0641) phases with thethird phase of La(OH)3 which exists in the hexagonal spacegroup of P63/m according to JCPDS 36-1481. Pr2O2CO3decomposed to the fcc structure of Pr6O11 with space groupFm3m according to JCPDS 06-329. The crystallite size of CeO2did not change after heating in air.From the morphological and structural characterizations, the

following observations can be made. (A) The oxides of Fe andCe can be obtained directly by autoclaving, whereas the rest ofthe oxide phases were obtained only after air heating theautoclaved products. (B) Nanosheet morphologies wereobtained from the acetates of alkaline earth metal (Ca) andrare-earth metals (La, Ce, Pr), whereas the transition metals(Mn and Fe) gave micron-sized 3D shapes. (C) The nanosheetstructures disintegrate after air heating and thicker sheets wereobserved. When heated to 700 °C inside the autoclave, themetal acetates decompose to their supercritical states and insidethe closed reactor the gaseous and/or liquid intermediatesformed from the dissociation reaction float inside the sealedreactor. The products start to solidify soon after the furnacecommences cooling.22 The following successive reactions can

Figure 4. XRD patterns of the metal oxides formed from thedecomposition of metal carbonates/oxycarbonates/hydroxycarbon-ates: (a) CaO, (b) Mn2O3, (c) hexagonal (h) and monoclinic (m)La2O3, La(OH)3 (o), and (d) Pr6O11.



take place inside the autoclave reactor. (i) M(CH3COO)2·xH2O → M(CH3COO)2 + xH2O (M = metal), (ii)M(CH3COO)2 → MCO3/M2(CO3)3 + CH3COCH3, (iii)M2(CO3)3 → M2O2CO3 + 2CO2, (iv) M2O2CO3 + H2O +CO2 → 2MOHCO3, (v) MCO3/M2(CO3)3 → MO + CO2 +CO (vi) CH3COCH3 → CO + C2H6, (vii) 2CO → C + CO2,(viii) MO + CO → M + CO2, (ix) M + CO + CO2 → MCO3/M2(CO3)3, and (x) MCO3/M2(CO3)3 → MO + CO2 + CO.Steps (i) and (ii) are obvious reactions from dry autoclaving

of the metal acetates. Step (iii) is common to both La- and Pr-products, whereas step (iv) is only valid for La. In steps (v) and(x), the decomposition of the metal carbonate to thecorresponding oxide is largely dependent on ΔGf

o of theoxide or decomposition of the carbonate in the oxygenchallenged environment of the autoclave. From the availabledata in the literature, it was observed that ΔGf

o of the formationof Fe3O4 by the reaction 3FeCO3 → Fe3O4 + 2CO2 + CO is+126.4 kJ/mol at 25 °C and −201.8 kJ/mol at 700 °C.30,31

Thus, decomposition of FeCO3 is favored at the elevatedreaction temperature. Similarly ΔGf

o of CeO2 is −1149.3 kJ/mol at 700 °C. On the contrary, the final reaction (x) forCaCO3 → CaO + CO2 was hindered since ΔGf

o of CaO is+233.7 kJ/mol at 700 °C. Likewise ΔGf

o of MnCO3 is −822.2kJ/mol in the range 500−800 °C and 100−1700 bar.32 Thus, insitu decomposition of MnCO3 to MnO or Mn2O3 is difficult.Also at 700 °C the free energy of conversion of La2O2CO3 toLa2O3 was calculated to be +26.4 kJ/mol and because of whichthe oxycarbonate was retained in the autoclave product.33 Step(v) becomes feasible for all the metals at the elevatedtemperature and pressure; however, step (x) becomesincreasingly difficult when the system starts cooling downfavoring the backward reaction. Fe3O4 and CeO2 were still the

final products due to the more favorable ΔGfo of the oxides.

Usually the forward reaction according to equation (viii) isquite difficult since the standard reduction potential of most ofthe metals is highly negative. However, inside the autoclave theelevated temperatures and extremely high autogenic pressure(∼3 MPa) can overcome the negative electrode potential andthus the respective metal intermediates can form at theirsupercritical states. When the temperature inside the autoclavestarts decreasing, elemental metal intermediates becomeunstable leading to equation (ix), and the reaction accordingto final equation (x) depends on ΔGf

o of the oxides. Wheneverthe standard reduction potential is positive, equation (viii)proceeds in the forward direction favoring the formation ofelemental metals such as copper.To explain the differences in morphology under similar

synthesis conditions, the nano-/microstructures were analyzedby following the alterations of elemental intensity underFESEM (Figure 5). From these experiments, it is evident thatelemental carbon plays a significant role in controlling theshape of the autoclaved products. One interesting observationwas that without any electrically conducting coating such as Auor Pt, the autoclaved nanosheets did not charge under theelectron beam of FESEM, whereas the air heated productsheavily charged. So it was likely that the nanosheets werewrapped by a conducting carbon envelope. The presence ofcarbon was however not evident from the XRD spectra (Figure2), where the C (002) reflection at 2θ ∼25° was absent due tothe higher intensity reflections from the dominant inorganicphases and the semigraphitic nature of carbon. Figure 5 showsthe elemental line scans on samples before and after air heating.The high intensity counts of carbon in the metal carbonatesand oxycarbonates is obvious. In the case of the autoclaved

Figure 5. Elemental line scans of (a) CaCO3 nanosheets, (b) CaO sheets, (c) MnCO3 microcubes and carbon spheres, (d) Mn2O3 cubes, (e) Fe3O4tetragonal bipyramid after autoclaving, (f) La2O2CO3 + LaOHCO3 flower, (g) La2O3 sheets, (h) autoclaved CeO2, (k) Pr2O2CO3 flowers, and (l)Pr6O11 sheets. EDAX patterns of (i) autoclaved and (j) air heated CeO2 nanosheets. All scale bars correspond to 200 nm.



nanosheets, carbon is highly concentrated at the edges (Figure5a,f,k). In Figure 5a, the high intensity Ca line is not shown tohighlight the behavior of the C line around the nanosheetedges. The original image is presented in Figure S2, SupportingInformation. In Figure 5f,h,k, the C line peaks over eachvertically aligned nanosheet. In these autoclaved nanosheets,the intensity of C line is close to the O line, indicating a highconcentration of carbon in these samples. After conversion tothe respective oxides, the carbon content on the sheets wereexpectedly reduced (Figure 5b,g,l). The residual carbon in theoxides can result from the leftover minor fractions ofundecomposed carbonate/oxycarbonate linkages in additionto probable contamination inside the sample chamber. For theautoclaved Fe3O4 and MnCO3 products, the carbon content onthe microstructures was pretty low (Figure 5c,e) since majorityof carbon coexisted as microspheres. In fact, these carbonspheres were largely absent whenever the nanosheets wereformed. Although the carbon content in MnCO3 is higher thanMn2O3, the carbon intensity on the microcubes in Figure 5c,dappears similar. This is because of the presence of carbonspheres in the same frame of Figure 5c, which suppresses the Cline on MnCO3 microcube. The authentic proof of carboncoating on the nanosheets can be obtained from the analysis ofCeO2 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 andafter air heating (Figure 5i, j). The 10 wt % carbon in the airheated CeO2 sheets might result from the trapped carbonintercalated between the sheets, which were not removed at700 °C. Because of the lubricating carbon coating on theautoclaved nanosheets, the surface of each nanosheet wasextremely smooth as is evident from the enlarged nanosheet inFigure 1d. Likewise ∼200 nm thick carbon film was previouslyobserved on other inorganic sheets prepared by dryautoclaving.15 Air decomposition of the carbon coated metalcarbonate and oxycarbonate nanosheets resulted in clustered

submicron particles or thicker sheets after removal of carbon.Metal oxide nanosheets such as CeO2 were also morpholog-ically stabilized by an encapsulating carbon layer, and thenanosheet stacking collapsed once the carbon layer wasremoved (Figure 3e). Thus, it can be inferred that carboncoating is solely responsible for the morphological stability ofthe nanosheets synthesized by dry autoclaving.The fact that Ca-, La-, Ce-, and Pr-autoclaved products had

carbon coating on the sheets whereas in Fe- and Mn-productscarbon remained as separate microspheres is quite intriguing.The nature of the autoclave products depends on the mutualstability of MCO3/M2O2CO3 ↔ MO ↔ M according to whichcarbonates/oxides/metals were obtained. In any case, thesupercritical intermediates at elevated temperatures >700 °Chave to solidify once the furnace starts cooling down and themorphology of the autoclaved products depends on thesolidification 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 theinorganic core is also well-known.13−15 For carbon to form astable coating on the nanosheets, the solidification kinetics ofthe elements and carbon should be comparable such that oncethe nanosheets form from the stacking of the solidified nuclei ofthe product, carbon will immediately encapsulate the nanosheetlayers (Figure 6). The inhomogeneous carbon coating can beattributed to the nonisotropic shapes which results in a gradientof carbon density. Carbon likely starts wrapping the nanosheetsfrom the regions of higher surface energy such as the edges(Figure 5a,f,h,k). In the case of microstructures where carbonforms separate spheres, the solidification of the inorganicelements and their compounds was much faster than that ofcarbon, such that once elemental carbon starts solidifying, theformation and growth of the inorganic phases were complete.In these kinetically controlled reactions, the solidification ratecan be directly correlated to FP of the elements. From thestandard data available and also shown in Figure 6, the FPs of

Figure 6. Schematics of formation of the autoclaved products (microcubes and nanosheets) and plot of FP of the metals. The gray clouds and theconstituents indicate the supercritical intermediates inside the autoclave. Larger fonts inside the clouds show higher concentration of theintermediates and vice versa.



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 froman elevated temperature >700 °C if FP of the components ismuch higher than the autoclave temperature. In that case,carbon cannot compete with the solidification rate of inorganiccomponents which result in separate MnCO3 microcubes/Fe3O4 tetragonal bipyramids. Carbon solidifies later as separatecarbon microspheres (Figure 6). Our logic applies to the earlierobtained irregular cubic/tetragonal nanoparticulate clusters ofTiO2 along with carbon microspheres from titanium(IV)isopropoxide,13 where FP of Ti is 1668 °C.34 With lowerfreezing temperatures, carbon encapsulated nanosheets orflower-like structures can be obtained. In the case of very lowFPs such as Zn (419.5 °C), carbon forms ∼60 nm thick shellover the ∼55 nm ZnO nanocrystals.14 If FP of the metal is toolow, carbon does not give enough time for the nanocrystals toarrange in the form of nanosheets and core−shell nanocrystalswere obtained. Thus, a direct correlation could be drawnbetween the morphologies and FP of the metallic constituentsof the respective phases. In fact, 1D or 2D architectures of 3dtransition metal oxides were obtained only if directed bycatalytic growth or van der Waals interactions between themetal chalcogenide layers or induced by external magneticfields.19,22,35

Lastly, the rare-earth oxides obtained by air heating of theautoclaved flowers were characterized for their optical proper-ties (Figure 7). PL spectra were recorded for the autoclavedsample (Ac), air heated autoclave product (Ac-air), and theoxides obtained by direct decomposition of the metal acetatesat 700 °C for 6 h in air without autoclaving (Air). La2O3, CeO2,

and Pr6O11 were excited at their characteristic wavelengths of350, 304, and 350 nm, respectively. The autoclaved samplecontaining La2O2CO3 and LaOHCO3 shows no emission in thevisible range of the electromagnetic spectrum. However, the Ac-air and Air La2O3 samples show two distinct emission peaks at466 and 575 nm. It is well-known that La3+ ion cannot beregarded as an emission center because of the absence ofelectrons in the inner 4f atomic orbital. Also, the sample wasexcited at 350 nm (≡ 3.5 eV), which is less than the bandgapenergy (Eg = 3.8 eV) of La2O3.

36 The emissions of 466 and 575nm are therefore assigned to singly oxygen vacancies in theLa2O3 samples.

36,37 Similar oxygen defect related emissions at467 and 560 nm were also observed in the Ac-air and Air CeO2

samples. The emissions occur from various defect states to theO 2p band.38 Ac CeO2 did not show any emission, probablydue to quenching of luminescence by the carbon coating, whichalso passivates the defect centers at the surface of thenanosheets. The Ac product containing Pr2O2CO3 did notshow any emission, and the possible luminescence from a smallfraction of Pr6O11 was similarly quenched by carbon. In the Ac-air Pr6O11, the emission peaks centered at 475, 572, 642, and665 nm are due to the 3H4 →

3P1,3P0 →

3H5,3P0 →

3F2, and3P1 → 3F3 transitions, respectively.39 Thus, dry autoclavingresulted in structural defects in the self-assembled nanosheetsof rare-earth oxides. Pressure induced syntheses also have thepossibility of introducing lattice strain which can be verifiedfrom the defect-related emissions.

4. CONCLUSIONS

In summary, the crystallographic phases and morphology of theproducts obtained by dry autoclaving of metal acetates wereexplained based on the ΔGf

o of the oxide or decomposition ofthe carbonates at >700 °C and solidification kinetics correlatedto FP of the metals, respectively. Negative ΔGo

f of theformation of Fe3O4 and CeO2 oxides at 700 °C resulted inthese oxide phases, whereas unfavorable ΔGf

o retained the Ca-,Mn-, La-, and Pr-carbonates and/or oxycarbonates. Thenanosheet structures of CaCO3, La2O2CO3 + LaOHCO3,CeO2, and Pr2O2CO3 were stabilized by an encapsulatingcarbon film. All these metals have FP below 1000 °C, andhence the solidification rate of the supercritical metalintermediates was comparable to that of carbon. When FP ofthe metals is above 1000 °C, the metal intermediates solidifiedmuch faster than carbon, and hence the microstructures wereobtained along with separate carbon microspheres. Thus, themorphologies obtained for different metal carbonates, oxy-carbonates, and oxides could be correlated to the intrinsicproperties of the metals. Air decomposition of the respectivecarbonates and oxycarbonates resulted in the crystalline metaloxides. PL spectra of the La2O3 and CeO2 samples showedemissions due to oxygen vacancies and surface defects. Directcorrelation between the intrinsic properties of the metals to themorphology of the metal compounds could help in resolvingthe unpredictability of morphology evolution of the latter.

■ ASSOCIATED CONTENT

*S Supporting InformationXRD and EDAX data. This material is available free of chargevia the Internet at http://pubs.acs.org.

Figure 7. Room temperature PL spectra of the rare-earth oxides. Ac:autoclaved sample, Ac-air: air heated autoclave product, and Air: directair heating of the metal acetates.



http://pubs.acs.org

■ AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work is supported by the Council of Scientific andIndustrial Research (CSIR), India, under Sanction No.01(2689)/12/EMR-II. A.S. thanks University Grants Commis-sion (UGC), New Delhi, for his fellowship. The infrastructuralsupport from IISER Kolkata is duly acknowledged.

■ REFERENCES(1) Tsao, Y.-C.; Rej, S.; Chiu, C.-Y.; Huang, M. H. J. Am. Chem. Soc.2014, 136, 396−404.(2) Sadhu, A.; Bhattacharyya, S. Chem. Mater. 2014, 26, 1702−1710.(3) Cheng, B.; Ouyang, Z.; Chen, C.; Xiao, Y.; Lei, S. Sci. Rep. 2013,3 (3249), 1−6.(4) Zhang, Y.; Geng, D.; Kang, X.; Shang, M.; Wu, Y.; Li, X.; Lian,H.; Cheng, Z.; Lin, J. Inorg. Chem. 2013, 52, 12986−12994.(5) Leidinger, P.; Popescu, R.; Gerthsen, D.; Feldmann, C. Chem.Mater. 2013, 25, 4173−4180.(6) Emdadi, L.; Wu, Y.; Zhu, G.; Chang, C.-C.; Fan, W.; Pham, T.;Lobo, R. F.; Liu, D. Chem. Mater. 2014, 26, 1345−1355.(7) Moulin, E.; Cid, J.-J.; Giuseppone, N. Adv. Mater. 2013, 25, 477−487.(8) Patra, B. K.; Sarkar, S.; Guria, A. K.; Pradhan, N. J. Phys. Chem.Lett. 2013, 4, 3929−3934.(9) Acharya, S.; Sarkar, S.; Chakraborty, S.; Pradhan, N. Chem.Eur.J. 2014, 20, 3922−3926.(10) Hu, B.; Wang, K.; Wu, L.; Yu, S.-H.; Antonietti, M.; Titirici, M.-M. Adv. Mater. 2010, 22, 813−828.(11) Rao, B. M.; Roy, S. C. J. Phys. Chem. C 2014, 118, 1198−1205.(12) Pol, V. G.; Pol, S. V.; Gedanken, A. Adv. Mater. 2011, 23, 1179−1190.(13) Bhattacharyya, S.; Pucci, A.; Zitoun, D.; Gedanken, A.Nanotechnology 2008, 19 (495711), 1−8.(14) Bhattacharyya, S.; Gedanken, A. J. Phys. Chem. C 2008, 112,4517−4523.(15) Sadhu, A.; Bhattacharyya, S. J. Phys. Chem. C 2013, 117, 26351−26360.(16) Bhattacharyya, S.; Zitoun, D.; Gedanken, A. Nanotechnology2011, 22 (075703), 1−7.(17) Bhattacharyya, S.; Estrin, Y.; Rich, D. H.; Zitoun, D.; Koltypin,Y.; Gedanken, A. J. Phys. Chem. C 2010, 114, 22002−22011.(18) Bhattacharyya, S.; Estrin, Y.; Moshe, O.; Rich, D. H.; Solovyov,L.; Gedanken, A. ACS Nano 2009, 3, 1864−1876.(19) Bhattacharyya, S.; Zitoun, D.; Estrin, Y.; Moshe, O.; Rich, D. H.;Gedanken, A. Chem. Mater. 2009, 21, 326−335.(20) Estrin, Y.; Rich, D. H.; Moshe, O.; Bhattacharyya, S.; Gedanken,A. Appl. Phys. Lett. 2009, 95, 181903.(21) Bhattacharyya, S.; Zitoun, D.; Gedanken, A. J. Phys. Chem. C2008, 112, 7624−7630.(22) Bhattacharyya, S.; Perelshtein, I.; Moshe, O.; Rich, D. H.;Gedanken, A. Adv. Funct. Mater. 2008, 18, 1641−1653.(23) Datta, A.; Sadhu, A.; Sen, B.; Kaur, M.; Sharma, R.; Das, S. C.;Bhattacharyya, S. Corros. Sci. 2013, 73, 356−364.(24) Shanmugam, S.; Gedanken, A. J. Phys. Chem. B 2006, 110,2037−2044.(25) Datta, A.; Dutta, P.; Sadhu, A.; Maiti, S.; Bhattacharyya, S. J.Nanopart. Res. 2013, 15 (1808), 1−15.(26) Pol, V. G.; Thiyagarajan, P. Ind. Eng. Chem. Res. 2009, 48,1484−1489.(27) Liu, J.; Zou, X.; Xu, B.; Luo, H.; Lv, H.; Han, L.; Yu, X.CrystEngComm 2012, 14, 3149−3155.

(28) Wei, J.; Yang, Z.; Yang, H.; Sun, T.; Yang, Y. CrystEngComm2011, 13, 4950−4955.(29) Wang, S.; Zhao, Y.; Chen, J.; Xu, R.; Luo, L.; Zhong, S.Superlattices Microstruct. 2010, 47, 597−605.(30) Holland, T.; Powell, R. J. Metamorph. Geol. 1998, 16, 309−343.(31) Powell, R.; Holland, T.; Worley, B. J. Metamorph. Geol. 1998, 16,577−588.(32) Fazeli, A. R.; Tareen, J. A. K.; Basavalingu, B.; Bhandage, G. T.Proc. Indian Acad. Sci. (Earth Planet Sci.) 1991, 100, 37−40.(33) Olafsen, A.; Fjellvag̊, H.; Stølen, S.; Atake, T.; Kawaji, H.;Matsuo, K. J. Chem. Thermodynamics 1999, 31, 433−449.(34) CRC Handbook of Chemistry and Physics, 92nd ed; Haynes, W.M.; CRC Press: Boca Raton, FL, 2011; pp 4.121−4.123.(35) Pol, S. V.; Pol, V. G.; Gedanken, A.; Felner, I.; Sung, M.-G.;Asai, S. Inorg. Chem. 2007, 46, 4951−4959.(36) Hu, C.; Liu, H.; Dong, W.; Zhang, Y.; Bao, G.; Lao, C.; Wang,Z. L. Adv. Mater. 2007, 19, 470−474.(37) Mu, Q.; Chen, T.; Wang, Y. Nanotechnology 2009, 20, 345602−1−7.(38) Kempaiah, D. M.; Yin, S.; Sato, T. CrystEngComm 2011, 13,741−746.(39) Lakshminarayana, G.; Yang, H.; Teng, Y.; Qiu, J. J. Lumin. 2009,129, 59−68.



mailto:[email protected]

Direct Correlation of the Morphologies of Metal Carbonates, Oxycarbonates, and Oxides Synthesized by...

Documents

Transcript of Direct Correlation of the Morphologies of Metal Carbonates, Oxycarbonates, and Oxides Synthesized by...