Micelle-Templated Oxides and Carbonates of Zinc, Cobalt, andAluminum and a Generalized Strategy for Their SynthesisBjorn Eckhardt,† Erik Ortel,† Denis Bernsmeier,† Jorg Polte,† Peter Strasser,† Ulla Vainio,‡

Franziska Emmerling,§ and Ralph Kraehnert*,†

†Technical University of Berlin, Department of Chemistry, Strasse des 17. Juni 124, D-10623 Berlin, Germany‡Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, D-22607 Hamburg, Germany§BAM Federal Institute of Materials Research and Testing, Richard-Willstatter-Strasse 11, D-12489 Berlin, Germany

*S Supporting Information

ABSTRACT: Catalysis, energy storage, and light harvesting require functional materials with tailored porosity andnanostructure. However, common synthesis methods that employ polymer micelles as structure-directing agents fail for zincoxide, for cobalt oxide, and for metal carbonates in general. We report the synthesis of the oxides and carbonates of zinc, cobalt,and aluminum with micelle-templated structure. The synthesis relies on poly(ethylene oxide)-block-poly(butadiene)-block-poly(ethylene oxide) triblock copolymers and a new type of precursor formed by chemical complexation of a metal nitrate withcitric acid. A general synthesis mechanism is deduced. Mechanistic insights allow for the prediction of optimal processingconditions for different oxides and carbonates based on simple thermogravimetric analysis. Employing this synthesis, films ofZnO and Co3O4 with micelle-controlled mesoporosity become accessible for the first time. It is the only soft-templating methodreported so far that also yields mesoporous metal carbonates. The developed synthesis is generic in nature and can be applied tomany other metal oxides and carbonates.

KEYWORDS: EISA, pore templating, metal oxide, metal carbonate, zinc oxide, cobalt oxide

■ INTRODUCTION

Many applications in catalysis, energy storage, and photo-voltaics rely on metal oxides that feature a specificnanostructure. The nanostructure of a metal oxide oftendetermines its optical, magnetic, and catalytic properties. Theoxides of cobalt and zinc provide some of the most prominentexamples. Nanostructured cobalt-based oxides are promisingmaterials for electrodes in supercapacitors1,2 and in lithium ionbatteries3,4 with superior charging rates.5 Moreover, theyrepresent very active catalysts for the oxygen evolution reactionin electrochemical water splitting6 and the oxygen reductionreaction in fuel cells.7 Also, ZnO nanostructures feature uniqueproperties. They are used in display technologies, photovoltaics,photocatalysis, and piezoelectric nanogenerators8 and allow theconstruction of self-powered nanodevices.9−11 Moreover, thephotonic band gap of ZnO can be tailored by the introductionof an inverse opal structure.12 Hence, control over the

morphology of metal oxides on a nanometer scale is of vitalimportance.So-called “templating” is one of the most versatile methods

for controlling the nanostructure of a metal oxide during itswet-chemical synthesis. It employs preformed nanostructures(templates) as structure-directing agents. The templatestypically possess the inverse shape of the desired poremorphology. Open porosity results from solidification of theoxide framework followed by template removal. Template-based syntheses have been reported for metal oxides withmesopores,13,14 macropores,12,15−17 and hierarchical poros-ity.16−18 Depending on the nature of the employed template,so-called hard and soft templating can be distinguished. Hardtemplating is commonly employed for the synthesis of

Received: February 14, 2013Revised: June 21, 2013

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macroporous oxides in powder form (ZnO12,15 andCo3O4

16,17). Although hard templating can also in generalproduce mesopores (ZnO13 and Co3O4

19,20), soft templating isby far the most common synthesis approach for mesoporousoxides.Common soft-templating routines such as evaporation-

induced self-assembling (EISA)21 employ micelles of amphi-philic block copolymers as the pore templates. In a typical EISAsynthesis, a solution containing an oxide precursor and theamphiphilic block copolymers is deposited onto a substrate.The solvent evaporates during deposition while the templatemolecules are arranged into micelles. Micelles and the partiallycondensed precursor assemble into an ordered mesophase. Asubsequent calcination converts this mesophase into amesoporous oxide film. EISA-based syntheses offer threemajor advantages: (I) The pore size, pore shape, and thicknessof pore walls22 can be controlled by the structure andconcentration of the template. (II) Synthesis protocols aresimple and reproducible. (III) A wide range of metal oxides isaccessible.14 However, neither the micelle-templated synthesisof Co3O4 films nor ZnO has been reported so far.The failure of EISA-based syntheses to produce templated

zinc oxide or cobalt oxide films originates from the propertiesof commonly employed metal precursors. Typically, oxideswere synthesized from (I) alkoxides or partially alkoxylatedmetal chlorides (e.g., SiO2,

23,24 TiO2,25,26 Al2O3,

27 andZrO2

28,29), (II) preformed colloidal nanocrystals [e.g., TiO2,30

Mn3O4,31 MnFe2O4, and InxSnyOz (ITO)31,32], and (III)

thermally decomposable metal compounds (e.g., IrO233). The

reasons for failure are intrinsically tied to the mechanisms ofmesophase formation and template removal. Route I based onhydrolysis and condensation of alkoxy groups fails forprecursors with high hydrolysis and condensation rates, becauserapid condensation results in undesired precipitation prior tomesophase assembly. Route II requires high-quality buildingblocks such as a redispersible nanocrystalline colloid with asmall particle diameter (d < ∼5 nm) and a narrow sizedistribution. However, for many metal oxides, such nanoparticle

syntheses remain challenging. Approach III is limited to metalprecursors that do not show excessive crystallization upondrying; otherwise, the ordered mesostructure cannot beformed. Moreover, oxide formation must occur at temperaturessignificantly below the typical temperatures of templatecombustion (∼300 °C); otherwise, pores collapse because ofpremature template removal. Other constraining factors are thelimited solubility of many precursor compounds, meltingduring calcination, and rapid crystallite growth of the metaloxide before template removal. Additionally, a generallimitation of all methods described here is that they do notprovide access to the soft-templated synthesis of mesoporousmetal carbonates.The synthesis of fine-grained metal oxides without templated

pore structure can be achieved, e.g., via the Pechini method orthe citrate method. The so-called Pechini method was originallypatented for the preparation of (untemplated) nanocrystallinemetal titanates and niobates.34 It relies on the initial formationof chelate complexes of metal ions (originally titan, niobium,and zirconium) with α-hydroxycarboxylic acids (for example,lactic, citric, or glycolic acid). Subsequent heating in thepresence of a polyhydroxy alcohol (e.g., ethylene glycol)induces polyesterification of the chelate complex, yielding anamorphous gel. Calcination at moderate temperatures typicallyconverts this gel into the corresponding crystalline metal oxide[e.g., LiMn2O4,

35 YVO4:Eu,36 LaPO4:Ce,Tb,

37 Eu2(WO4)3,38

and CaIn2O4:Eu39].

Alternatively, nanocrystalline metal oxides can be obtainedalso by heating of the corresponding metal carbonates toinduce thermal decomposition into the respective metal oxide.The synthesis has been used to prepare nanocrystalline MgO,40

ZnO,41 Co3O4,42 and Al2O3.

43

We recently reported the first synthesis of micelle-templatedmagnesium oxide.44 The preparation borrows from threedifferent approaches, i.e., the initial steps of the Pechini method(complexing the metal ion), the carbonate decompositionstrategy, and pore templating with polymer micelles. The MgOsynthesis44 relied on the initial preparation of a chemical

Figure 1. Synthesis scheme and deduced requirements for the synthesis of mesoporous metal oxides via metal carbonate intermediates. (1)Formation of a soluble complex from metal nitrate and a complexing agent such as citric acid. (2) Film deposition and concurrent self-assembly ofthe micelles of the polymer template with the precursor complex into an ordered mesophase. (3) Decomposition of the precursor complex into astructurally stable metal−carbonate intermediate at low temperatures while the ordered mesophase is retained. (4) Thermal treatment in air toremove the polymer template leading to open mesopores in the metal carbonate film. (5) Controlled decomposition of the amorphous carbonatethat forms the pore walls into the nanocrystalline metal oxide.

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precursor complex consisting of magnesium nitrate bonded tocitric acid and on thermally induced formation of a mesoporousMgCO3 intermediate. In this synthesis, water/ethanol solutionscontaining the complex and micelles of poly(ethylene oxide)-block-poly(butadiene)-block-poly(ethylene oxide) (PEO−PB−PEO) were dip-coated onto a substrate. The deposited filmswere converted into mesoporous MgO in two subsequentcalcination steps performed at 400 and 600 °C.This paper demonstrates that the self-assembly of triblock

copolymers with citric acid-based metal complexes providesaccess to micelle-templated oxides and carbonates of zinc,cobalt, and aluminum. Thus, mesoporous films of ZnO, Co3O4,ZnCO3, and Al2(CO3)3 with micelle-controlled pore structurebecome accessible for the first time. On the basis of amechanistic understanding, general criteria for the successfulsynthesis of metal oxides and metal carbonates with controlledporosity are deduced. Factors that are crucial for the synthesisas well as remaining limitations are critically discussed.

■ SYNTHESIS STRATEGY

From the recently reported synthesis of mesoporous MgO,44

five criteria that a synthesis strategy with general applicabilitywould have to fulfill can be deduced. The deduced criteria areillustrated in Figure 1 along with the proposed synthesisstrategy. (1) The metal salt and ligands with carboxylic acidfunctionality must form a chemical complex. Chelating ligandsare preferred because of the high stability of the complexes. (2)Polymer micelles and the metal complex must undergo self-assembly during deposition and drying to form an orderedmesostructure. (3) The chemical complex should decomposeinto a structurally stable metal carbonate at low temperatureswhile the templating micelles stabilize the formed mesostruc-ture. (4) Subsequent template removal should yield themesoporous metal carbonate; hence, decomposition of thetemplate polymer should occur at a temperature where thecarbonate remains thermally stable. (5) The final thermaltreatment should transform the carbonate into a nanocrystallinemetal oxide while retaining the templated pores.Guided by these requirements, we analyzed for metals Zn, Al,

Co, and Mg the physical and chemical processes that wouldconstitute the synthesis of the respective mesoporous carbonateand oxide. Formation of (1) a stable metal complex was studiedby electrospray ionization mass spectroscopy (ESI-MS) ofprecursor solutions. Employing highly amphiphilic surfactantsPEO−PB−PEO that form stable spherical micelles alreadyprior to solution deposition22 assured (2) robust reproduciblemesophase formation. Ordering of micelles and pore structureswas assessed by small-angle X-ray scattering (SAXS). Adequate

thermal treatment procedures for (3) carbonate formation, (4)template removal, and (5) oxide formation were establishedbased on thermogravimetric (TG) analysis of precursorcomplexes and templates. Additional characterization revealedthe structural evolution of the pore morphology [scanningelectron microscopy (SEM) and transmission electronmicroscopy (TEM)] and surface area (Kr sorption) as well asthe phase composition [Fourier transform infrared spectrosco-py (FTIR) and X-ray diffraction (XRD)] and crystallinity of thepore walls [selected area electron diffraction (SAED) andXRD]. The derived mechanistic picture explains the formationof mesoporous carbonates and oxides. It reveals also why thesynthesis of mesoporous CoCO3 necessarily fails.Mesoporous oxides and carbonates were synthesized as

summarized in Table 1. Briefly, for the synthesis of, e.g., ZnO, asolution containing template polymer (PEO213−PB184−PEO213), metal precursor [Zn(NO3)2·6H2O], complexingagent (citric acid), water, and ethanol was dip-coated onto Sisubstrates at a controlled temperature (25 °C) and a relativehumidity of 40%. Deposited films were calcined with procedure(i) to obtain mesoporous carbonate (ZnCO3, 1 h at 250 °C)and thereafter with procedure (ii) to obtain mesoporous oxide(ZnO, 25 min at 400 °C).

■ MESOPOROUS ZINC CARBONATE AND ZINCOXIDE

Figure 2 presents for the Zn-based material (i) calcined at 250°C the analysis by SEM (panel a), FTIR (panel b), and XRD(panel c). Moreover, Figure 2 shows properties of thecorresponding material (ii) calcined in addition at 400 °Cstudied by SEM (panel d), FTIR (panel e), XRD (panel f), andTEM (panels g−i). SEM analysis of sample (i) indicates thatcalcination at 250 °C yields a homogeneous film. Cross-sectionSEM images (Figure 2a) reveal that the formed film is ∼1100nm thick and completely penetrated by mesopores. The poresshow elliptical shapes ∼21 nm × ∼19 nm in size. Theappearance of the pore walls is smooth and unstructured; nocrystallite shapes can be distinguished (Figure 2a). FTIRspectra recorded on corresponding powder samples featureintense symmetric (1384 cm−1) and asymmetric (1583 cm−1)vibrations (Figure 2b). These bands can be assigned to zinccarbonate,45 whereas only negligible contributions at wave-numbers indicative of ZnO (e.g., 577 cm−1) are observed.Moreover, XRD analysis of the sample (Figure 2c) shows onlyreflections that can be attributed to the substrate (silicon wafer)and no indications of a crystalline Zn-containing phase. Krphysisorption indicates a surface area of 86.1 m2/g. This valueis slightly smaller than the surface area typically observed for

Table 1. Synthesis Conditions Employed for the Preparation of Micelle-Templated Mesoporous Carbonates and Oxides of Zn,Al, Co, and Mga

precursor system template solventcalcination (i),carbonate calcination (ii), oxide

Zn(NO3)2·6H2O (444 mg) and citric acid(144 mg)

PEO213−PB184−PEO213(70 mg)

H2O and ethanol (1.5 mLeach)

60 min at 250 °C 25 min at 400 °C

Al(NO3)3·9H2O (563 mg) and citric acid(144 mg)

PEO213−PB184−PEO213(70 mg)

H2O and ethanol (1.5 mLeach)

60 min at 300 °C 30 min at 900 °C

Co(NO3)2·6H2O (437 mg) and citric acid(144 mg)

PEO213−PB184−PEO213(70 mg)

H2O and ethanol (1.5 mLeach)

60 min at 200 °C 20 min at 300 °C

Mg(NO3)2·6H2O (385 mg) and citric acid(144 mg)

PEO213−PB184−PEO213(70 mg)

H2O and ethanol (1.5each)

60 min at 400 °C 120 min at 400 °C and 60 min at600 °C

aColumns 1−3 detail the compositions of the dip-coating solution. Column 4 lists the calcination procedure (i) that yields the carbonate and column5 the respective calcination (ii) that transforms the carbonate into the corresponding oxide.

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micelle-templated oxides (∼100−250 m2/g).14 Hence, com-bined analytical data indicate the successful synthesis of micelle-templated zinc carbonate comprising amorphous walls andinterconnected accessible mesopores.Further calcination (ii) at 400 °C transforms the carbonate

film (i) into ZnO while preserving the mesopore structure.FTIR spectra recorded for sample (ii) (Figure 2e) show astrong signal at 577 cm−1 indicative of ZnO, whereas only small

bands assigned to carbonate are retained.45 X-ray diffractiondata of the sample (Figure 2f) feature broad reflections atpositions of 2θ = 31.7° (100), 34.5° (002), 36.0° (101), 62.9°(103), and 67.8° (112). These reflections can be assigned toZnO in the hexagonal zincite structure (PDF-No. 00-036-1451)with crystallite sizes of ∼7 nm (Scherrer equation). Hence,calcination at 400 °C transforms the carbonate film almostcompletely into nanocrystalline ZnO. Corresponding top-view

Figure 2. Analysis of (i) mesoporous ZnCO3 calcined for 1 h at 250 °C and (ii) mesoporous ZnO calcined for 1 h at 250 °C followed by 25 min at400 °C by (a and d) SEM, (b and e) FTIR, (c and f) XRD, and (g−i) TEM. (a) Cross-section SEM of a mesoporous ZnCO3 film with the inset at ahigher magnification. (b) Infrared spectrum of the precursor complex calcined at 250 °C recorded in transmission mode. (c) Grazing incident XRDanalysis of amorphous ZnCO3 (i). (d) SEM top-view image of ZnO (ii). (e) FTIR spectrum of the dried precursor complex calcined by procedures(i) and (ii). (f) XRD analysis of ZnO (ii) with reflection positions corresponding to ZnO in the hexagonal zincite structure (PDF-No. 00-036-1451).(g−i) Electron microscopy analysis of (ii) ZnO by bright-field TEM, high-resolution TEM, and selected-area electron diffraction SAED, respectively(indexing: hexagonal zincite structure, PDF-No. 00-036-1451).

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SEM images (Figure 2d) indicate that films remainhomogeneous and macroscopically crack-free. The films arecompletely penetrated by mesopores ∼22 nm in diameter. Thepores are locally ordered and open toward the outer filmsurface. TEM analysis (Figure 2g) confirms the presence oftemplated mesopores throughout the sample volume. High-resolution TEM (Figure 2h) indicates crystallites and latticefringes, which confirms that the pore walls are crystalline.Furthermore, SAED analysis (Figure 2i) shows isotropicdiffraction rings with ring positions that match the reflectionsof hexagonal zincite structure (PDF-No. 00-036-1451). Thehomogeneous diffraction rings indicate that the pore wallsconsist of randomly oriented crystallites. Hence, calcination (ii)at 400 °C transforms the amorphous carbonate into nano-crystalline ZnO with a templated mesopore structure. However,the surface area of the mesoporous ZnO amounts to 250 m2/g(Kr physisorption), which is 3 times higher than that for thecorresponding ZnCO3. This increase in surface area can beattributed to additional microporosity observed in thecrystalline pore walls of ZnO, whereas the pore walls ofamorphous ZnCO3 appeared to be rather dense.Mesoscale ordering in Zn-containing films was analyzed after

different thermal treatments of deposited films. Figure 3 detailsthe evolution of order from deposited micelles (a and b) to (i)porous ZnCO3 (c and d) and (ii) ZnO (e and f) as indicated bytwo-dimensional 2D SAXS recorded in transmission at beamincident angles of 90° (a, c, and e) and of 6° (b, d, and f)relative to the substrate. The 2D SAXS pattern (a) recorded at90° for the as-deposited film features an isotropic ringcorresponding to a d spacing of 38 nm. Both the d spacingand the isotropic ring are preserved upon heat treatment at 250°C (Figure 3c) and during further calcination at 400 °C (Figure3e). In contrast, all 2D SAXS patterns recorded at a lowincident angle of 6° (Figure 3b,d,f) show diffraction rings withan elliptical shape. Such diffraction patterns have been reportedalso for conventional EISA-based syntheses, where the pore axisthat is oriented perpendicular to the substrate progressivelyshrinks during thermal treatments.46,47 This deformation iscaused by a loss of film volume resulting in homogeneousanisotropic film shrinkage upon drying as well as calcination.Hence, the d spacing perpendicular to the substrate decreasesin the studied Zn-containing films from 25 nm (as deposited)to 21 nm [carbonate (i)] and 19 nm [oxide (ii)] (Figure3b,d,f). However, the d spacing parallel to the substrate of 38nm remains unchanged (Figure 3a,c,e). This observation

confirms that film shrinkage is anisotropic and restricted tothe direction perpendicular to the substrate. Moreover, theobserved ellipsoidal 2D SAXS patterns (Figure 3b,d,f) indicatealso a certain degree of lattice distortion in the cubicmesostructure.22 SAXS analysis thus confirms that thedeposited micelles and precursor complex form a locallyordered mesophase, and that corresponding pore ordering ispreserved also during carbonate formation, template removal,and transition into a mesoporous zinc oxide.

■ FORMATION OF THE PRECURSOR COMPLEXThe proposed synthesis strategy requires the initial formationof a stable metal−precursor complex in solution (Figure 1,condition 1). The ability of citric acid to form complexes withthe nitrate compounds of Zn, Al, and Co was therefore assessedby electrospray ionization mass spectroscopy (ESI-MS) of thecomplex solutions. Mass spectra recorded in anion mode areprovided in the Supporting Information (Figure S2 for Zn,Figure S4 for Al, and Figure S9 for Co).The mass spectrum for zinc nitrate hexahydrate and citric

acid in ethanol (Figure S2 of the Supporting Information)shows characteristic mass fragments along with the correspond-ing isotope pattern. All observed masses can be assigned to zincions bonded to citric acid with nitrate as the counterion [i.e.,m/z 315.93 (C6H6NO10Zn)

−, m/z 378.92 (C6H7N2O13Zn)−,

m/z 507.95 (C6H5N2O13Zn2)−, m/z 507.97 (C12H14NO17Zn)

−,and m/z 571.87 (C12H12NO17Zn2)

−]. Hence, zinc (metal M)and citric acid (ligand L) form complexes with ML, M2L, ML2,and M2L2 stoichiometries. Moreover, the ESI-MS spectra ofcomplexed aluminum nitrate (Figure S4 of the SupportingInformation) and cobalt nitrate (Figure S9 of the SupportingInformation) also show a similar composition. Massescorresponding to ML and M2L2 complex stoichiometries areobserved for both metals. Moreover, Co-based solutions alsocontained M2L and ML2 stoichiometries. Hence, citric acidforms stable complexes with all studied metal ions, underliningthe generic nature of this initial synthesis step.

■ THERMAL STABILITY OF PORE TEMPLATES ANDPRECURSOR COMPLEXES

The proposed synthesis strategy requires the decomposition ofthe precursor complex into carbonate at temperatures wherethe template polymer remains sufficiently stable (Figure 1, step3). Moreover, access to the mesoporous carbonate implies thattemplate removal (Figure 1, step 4) occurs prior to

Figure 3. 2D SAXS pattern of films deposited from solutions with the complex of zinc nitrate and citric acid as well as micelles of the PEO213−PB184−PEO213 polymer template after different thermal treatments. From left to right: (a and b) as deposited, (c and d) zinc carbonate (i) calcinedfor 1 h at 250 °C, and (e, f) zinc oxide (ii) calcined for 1 h at 250 °C followed by 25 min at 400 °C. Samples were analyzed by SAXS with twodifferent incident angles of the X-ray beam of 90° (top) and 6° (bottom) relative to the substrate surface (linear intensity scale).

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decomposition of the carbonate into the oxide (Figure 1, step5). The thermal stability of the template polymer and ofdifferent precursor complexes was therefore investigated by TGanalysis in air. The recorded TG curves are presented in Figure4, contrasting the behavior of the template polymer (Figure 4a)with that of the citric acid complexes of Mg (Figure 4b), Al(Figure 4c), Zn (Figure 4d), and Co (Figure 4e).TGA indicates that the template starts to decompose at a

temperature of ∼250 °C (Figure 4a).33 However, a rapid massloss related to combustion of the polymer occurs between 375and 425 °C. The thermal stability of the polymer is therefore inline with literature reports, where decomposition temperaturesbetween ∼200 °C (PEO106−PPO70−PEO106, Pluronic F127)

48

and ∼400 °C (PEO79−PHB89, KLE)49 have been observed.

The TGA curves of all studied metal complexes show thesame typical shape (Figure 4b−e). An initial mass loss ofapproximately 30−50% is followed by a plateau with a constantmass and additional mass loss. Between 20 and 35% of theinitial mass is retained in the final stage. In the case of Zn, thefirst significant mass loss occurs between 160 and 225 °C(Figure 4d). The plateau of constant mass extends to 320 °C,whereas a constant mass is reached at ∼380 °C. In combinationwith XRD and IR analysis of phases (i) and (ii) (Figure 2), theobserved behavior is interpreted as decomposition of thecomplex into carbonate (first mass loss), the presence of astable carbonate (plateau), and decomposition of the carbonateinto the oxide (second mass loss). This interpretation is furthersupported by IR analysis of the gas phase during the secondmass loss, which detects CO2 as the main gas-phasedecomposition product (Figure S3 of the Supporting

Information). CO2 is a typical decomposition product ofmetal carbonates.Hence, calcination procedure (i) employed for Zn-based

films [1 h at 250 °C (see Table 1)] exploits the temperaturerange in which zinc carbonate remains stable and retains thepore walls made of carbonate (Figure 2a). However, atemperature of 250 °C is sufficient to slowly decompose thetemplate polymer (Figure 4a). Calcination (i) therefore yieldszinc carbonate with templated open mesoporosity (Figure 2a).Moreover, thermal treatments at temperatures exceeding theplateau range, i.e., >320 °C (Figure 4d), decompose thecarbonate into the oxide. Thus, the synthesis of mesoporousZnO requires a secondary calcination at 400 °C (Figure 2d).Similar TGA curves are obtained also for the metal

complexes of Mg, Al, and Co (Figure 4b,c,e). Thedecomposition of the complex and its conversion intocarbonate start in all three cases below 100 °C. Moreover,FTIR analysis of the solid samples provides in the plateauregion evidence of the presence of the corresponding metalcarbonate (Figure S6 of the Supporting Information for Al,Figure S11 of the Supporting Information for Co, and ref 44 forMg). Decomposition of the carbonate at temperatures beyondthe plateau region is supported by detection of CO2 as the maindecomposition product also for Al (Figure S8 of the SupportingInformation), Co (Figure S13 of the Supporting Information),and Mg.44 Thus, for all studied metals, the decomposition ofthe precursor complex into the corresponding carbonate (firstmass loss) and its transformation into the respective oxide(second mass loss) can be assumed.Depending on the metal, the position of the plateau shifts

(Figure 4). Hence, also the optimal calcination condition for

Figure 4. Thermogravimetric analysis recorded for (a) the PEO213−PB184−PEO213 polymer and (b−e) dried complexes of citric acid with differentmetal nitrates: (b) Mg(NO3)2, (c) Al(NO3)3, (d) Zn(NO3)2, and (e) Co(NO3)2. The colors mark different temperature ranges of thermalmodification of the template polymer and of the metal complexes. The green frame highlights the temperature range of the decomposition of thetemplate polymer. Colored areas indicate the evolution of the metal complexes: (blue) decomposition of the precursor complex into thecorresponding carbonate, (yellow) existence of the metal carbonate, and (red) decomposition of the carbonate into the metal oxide. Heating wasconducted in air at a rate of 5 K/min.

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obtaining the micelle-templated mesoporous carbonates andoxides should differ. To confirm the proposed mechanism,calcination procedures (i) and (ii) were adjusted for Al and Co(Table 1) based on TGA data. Additionally, Mg-based filmswere prepared.44 SEM images of the synthesized micelle-templated oxides and carbonates are presented in Figure 5.

■ ALUMINUM CARBONATE AND OXIDE

TGA of the complexed aluminum nitrate shows a plateaubetween 225 and 310 °C (Figure 4c). The template polymerdecomposes at 250 °C. Hence, calcination at (i) 300 °C waschosen to produce mesoporous aluminum carbonate andadditional calcination (ii) at 900 °C to produce mesoporousaluminum oxide and induce its crystallization.The cross-section SEM image in Figure 5a shows the film

after calcination at 300 °C. The film is fully penetrated bytemplated mesopores with the typical elliptical shape and a sizeof ∼20 nm (width) × ∼9 nm (height). FTIR analysis (FigureS6 of the Supporting Information, i) reveals two characteristicbands indicative of aluminum carbonate, i.e., asymmetric (1608cm−1) and symmetric (1469 cm−1) stretching vibrations.50 Thefilm is amorphous according to XRD (Figure S7 of theSupporting Information, i) and features a surface area of 44.8m2/g (Kr physisorption). Hence, thermal treatment (i) of thedeposited film forms amorphous mesoporous aluminumcarbonate.SEM images of the film formed by additional calcination at

900 °C show that the film contains templated mesopores ∼22nm in diameter (Figure 5d). TEM analysis confirms that thefilm is fully mesoporous (Figure S5a,c of the SupportingInformation) and composed of small crystallites ∼6 nm indiameter (Figure S5d of the Supporting Information). Twovibrations observed at 538 and 732 cm−1 indicative of Al−Obonds and the absence of carbonate-related vibrations in IR

evidence the formation of aluminum oxide (Figure S6 of theSupporting Information).51,52 XRD analysis detects two broadreflections at 46.0° and 66.6° (Figure S7 of the SupportingInformation), which correspond well with the positions of(400) and (440) reflections reported for γ-Al2O3 (PDF-No. 00-050-0741). The crystallite size estimated via the Scherrerequation amounts to 5 nm. Diffraction rings observed in SAEDcorrespond to (311), (400), (511), (440), (444), and (800)reflections of γ-Al2O3 (Figure S5b of the SupportingInformation) and confirm the phase assignment. SAXS analysis(Figure S14a of the Supporting Information) evidences poreordering in the as-deposited film, the aluminum carbonate, andthe aluminum oxide at a d spacing of 39 nm parallel to thesubstrate (Figure S14a of the Supporting Information, 90°).The corresponding periodic distance perpendicular to thesubstrate decreases from 27 nm (as deposited) to 14 nm(carbonate) and 5 nm (oxide) (Figure S14a of the SupportingInformation, 6°). The oxide Kr BET surface area amounts to286 m2/g. Hence, calcination (ii) at 900 °C transformsmesoporous aluminum carbonate into crystalline γ-aluminawith templated and ordered mesopore structure.

■ COBALT CARBONATE AND OXIDE

TGA of the cobalt complex indicates less favorable behavior(Figure 4e). Also here, a plateau can be observed and attributedto the presence of cobalt carbonate. However, the plateau isshifted to temperatures as low as 190−245 °C, i.e., below thevalue of ∼250 °C required for thermal decomposition of thetemplate polymer. Hence, cobalt carbonate decomposes beforethe template can be removed. Deposited films were thereforecalcined at (i) 200 °C to stabilize the carbonate and (ii) 300 °Cto produce a mesoporous oxide.IR analysis of the film calcined at 200 °C (Figure S11 of the

Supporting Information, i) shows asymmetric (1585 cm−1) and

Figure 5. Electron microscopy images demonstrating the ability of the synthesis strategy to address different metals (Al, Mg, and Co). Images of thecarbonates of (a) Al, (b) Mg, and (c) (nonporous) Co. Oxides of (d) Al, (e) Mg, and (f) Co. Imaging methods: (a and b) cross-sectional SEM,(inset in panel b) top-view SEM, and (d−f) top-view SEM. See the Supporting Information for further TEM analysis of Al (Figure S5) and Comaterials (Figure S10).

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symmetric (1392 cm−1) stretching vibrations that can beassigned to carbonate.53 The formed film is amorphousaccording to XRD (Figure S12 of the Supporting Information,i). Unfortunately, electron microscopy imaging of the film failedbecause of its instability in the electron beam. Moreover, thefilm’s surface area was too small to be detected by Krphysisorption analysis. Both observations suggest that thepolymer template is still present in the film. SAXS analysisreveals a diffraction ring for the sample (Figure S14b of theSupporting Information) with a d spacing of 37 nm parallel and18 nm perpendicular to the substrate, both indicating thepresence of an ordered mesostructure. Hence, micelle structurecobalt carbonate is formed during calcination at 200 °C, yet itspore structure is blocked by the remaining template micelles.This effect is in complete agreement with the TG analysis ofthe precursor complex (Figure 4e) and template (Figure 4a)and demonstrates a limitation of the proposed synthesisstrategy in its present form. However, cobalt carbonate withopen mesoporosity should become accessible when the PEO−PB−PEO template is removed by alternative (nonthermal)methods or when alternative templates with lower decom-position temperatures are employed.Nevertheless, analysis of the film calcined in addition at (ii)

300 °C indicates that the mesoporous oxide can still be formed.Top-view SEM images of the film show spherical pores ∼22 nmin diameter (Figure 5f). TEM analysis confirms that the film isfully porous (Figure S10c of the Supporting Information) withpore walls composed of crystallites ∼6 nm in size (Figure S10dof the Supporting Information). FTIR spectra contain strongbands indicative of cobalt oxide formation, i.e., Co−Ovibrations at 663 and 570 cm−1,54 whereas only small signalsfor carbonates remained (Figure S11 of the SupportingInformation, ii). The corresponding XRD pattern (Figure S12of the Supporting Information, ii) reveals numerous reflectionsthat match with the reported phase PDF-No. 00-042-1467 ofcrystalline Co3O4 in a spinel structure [31.1° (220), 36.9°(311), 44.8° (400), 55.7° (422), 59.3° (511), and 65.3° (440)].The crystallite size estimated via the Scherrer equation (311)amounts to 7 nm. SAED analysis (Figure S10b of theSupporting Information) evidences numerous diffraction ringsthat confirm the crystallinity of the samples as well as theassignment of the crystalline spinel phase. Furthermore, 2DSAXS confirms the presence of an ordered pore structure(Figure S14b of the Supporting Information). The sample’ssurface area amounts to ∼257 m2/g (Kr physisorption). Hence,calcination (ii) of the micelle-structured (nonporous) cobaltcarbonate at 300 °C forms a nanocrystalline Co3O4 film with aspinel structure and the desired open templated porosity.

■ COMPARISON TO MAGNESIUM CARBONATE ANDOXIDE

TGA of the complex of magnesium nitrate with citric acidfeatures a broad plateau of constant mass between 275 and 425°C (Figure 4b). According to the proposed mechanistichypothesis, calcination (i) at temperatures within this temper-ature window should yield mesoporous carbonate, whereassubsequent calcination (ii) at temperatures exceeding 425 °Cshould result in mesoporous magnesium oxide. We recentlyreported the corresponding synthesis of magnesium carbonatewith templated mesopore structure employing 400 °C forcalcination (i).44 The transformation into mesoporous MgOsucceeded by calcination (ii) at 600 °C. Panels b and e ofFigure 5 show images of the corresponding materials (see ref

44 for details). Both employed calcination temperatures are inagreement with the current TGA analysis. Hence, the validity ofthe developed mechanistic hypothesis and synthesis approach(Figure 1) is also confirmed for the case of MgCO3 and MgO.

■ CONCLUSIONS

We present a new approach for the synthesis of micelle-templated oxides and carbonates of zinc, cobalt, and aluminum.The method employs a unique precursor to overcomelimitations of classical EISA-based syntheses, i.e., complexesformed from citric acid and metal nitrates. The precursorsreliably self-assemble with amphiphilic block copolymers. Usingthis approach, films of ZnO, Co3O4, ZnCO3, and Al2(CO3)3were synthesized for the first time with micelle-controlled openmesoporosity. The respective pores are locally ordered andresult in surface areas in the ranges of 44−86 m2/g(carbonates) and 250−286 m2/g (oxides). The pore systemsshow the uniaxial shrinkage that is typical for EISA-basedoxides. The pore sizes of the materials were readily adjusted bychanging the size of the template polymer (ZnO; Figure S15 ofthe Supporting Information).A general mechanism is proposed for the developed synthesis

(Figure 1). As in the Pechini method, the first step requires theformation of a metal complex in solution. The complexassembles with template micelles into an ordered mesophaseduring film deposition and solvent evaporation. Sequentialthermal treatments convert the precursor complex first into amicelle-structured amorphous carbonate, remove the templateyielding the porous carbonate, and finally convert the carbonateinto the mesoporous oxide. Carbonates and oxides feature acubic ordered mesopore structure. The calcination temper-atures required for carbonate and oxide synthesis can bederived from simple TG analysis of the correspondingprecursor complex. Moreover, a comparison between TGAdata of metal complex and template polymer predicts if thermaltemplate removal from the carbonate is feasible. Theamorphous character of the intermediate carbonate appearsto be of particular importance, because it facilitates templateremoval without sintering of wall-forming crystallites andtherefore avoids degradation of the templated pore structure.The synthesis is generic in nature and therefore applicable to awide range of metal oxides and carbonates.The presented synthesis strategy for creating functional metal

oxides and carbonates can be easily tuned and optimized forenergy storage, electro catalysis, sunlight harvesting, orbiomedical applications. In particular, amorphous CaCO3

could be of interest for biocompatible implant coating anddefined model systems in bone cell culturing55 andbiomineralization research.56 However, the synthesis shouldbe further refined on the basis of an improved fundamentalunderstanding. Physicochemical investigations could reveal thetype of interaction between the precursor complex andmicelles, guide the tailoring of the complex structure, andexplain how crystallites are formed. Most importantly,extending the synthesis strategy to yield also bimetalliccarbonates and oxides with optimized pore structure such asthe Cu/ZnOx-based catalysts employed in industrial methanolsynthesis57 or indium-free transparent conductive oxides couldresult for many applications in a tremendous improvement inperformance.

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■ EXPERIMENTAL SECTIONChemicals. Zinc nitrate hexahydrate (98%, extra pure) was

obtained from Acros. Aluminum nitrate nonahydrate (>99%, proanalysis), magnesium nitrate hexahydrate (>99%, pro analysis), andcobalt nitrate hexahydrate (>98%, for analysis) were purchased fromMerck. Water-free citric acid (>99.5%, pro analysis), ethanol (>99.9%,absolute), and the HCl solution (37 wt %, pro analysis) were obtainedfrom Roth. Concentrated sulfuric acid (95 wt %, puriss) was purchasedfrom Th. Geyer. PEO−PB−PEO polymers were synthesized byPolymer Service GmbH Merseburg.22 All chemicals were used withoutfurther purification.Film Synthesis. Prior to film deposition, substrates (Si wafers)

were cleaned with ethanol and heated in air (2 h at 600 °C). The dip-coating solution for zinc films was prepared by joining powders ofZn(NO3)2·6H2O, citric acid, and a template in the amounts listed inTable 1. The powders were dissolved in a mixture of Milli-Q water(1.5 mL) and ethanol (1.5 mL) by being stirred overnight, resulting ina colorless solution. Films were prepared by dip-coating substrates at awithdrawal rate of 150 mm/min under a controlled atmosphere (25°C, 40% relative humidity). Afterward, films were allowed to dry for atleast 10 min before being transferred into the preheated mufflefurnace. Mesoporous ZnCO3 films were obtained after calcination for1 h at 250 °C. Mesoporous ZnO films required calcination for 1 h at250 °C, natural cooling to room temperature, and a second calcinationfor 25 min at 400 °C (preheated furnace). ZnO films with 8 and 15nm pore diameters as well as bimodal films were obtained in a similarfashion (see Figure S15 of the Supporting Information).Films of magnesium carbonate and magnesium oxide were prepared

according to the method described previously44 employing thecomposition and conditions listed in Table 1.The dip-coating solution for the alumina films was prepared by

mixing the powders of Al(NO3)3·9H2O, citric acid, and PEO213−PB184−PEO213 (Table 1). The powders were dissolved in a mixture ofMilli-Q water and ethanol by being stirred overnight, resulting in aslightly yellow solution. Films were prepared by dip-coating substratesat a withdrawal rate of 150 mm/min at 25 °C and 40% relativehumidity. Afterward, films were allowed to dry for at least 10 minbefore being transferred into the preheated muffle furnace. Themesoporous aluminum carbonate films were obtained by calcinationfor 1 h at 300 °C. The mesoporous aluminum oxide films wereobtained after being calcined for 1 h at 300 °C, naturally cooled toroom temperature, and heated for 30 min to 900 °C (preheatedfurnace).The dip-coating solution for the cobalt-based films was prepared by

mixing the powders of Co(NO3)2·6H2O, citric acid, and PEO213−PB184−PEO213 (Table 1). The powders were dissolved in a mixture ofMilli-Q water and ethanol by being stirred overnight, resulting in ared/pink solution. Films were prepared by dip-coating substrates at awithdrawal rate of 150 mm/min at 25 °C and 40% relative humidity.Afterward, films were allowed to dry for at least 10 min before beingtransferred into the preheated muffle furnace. The cobalt carbonatefilms employed calcination for 1 h at 200 °C. The mesoporous cobaltoxide films were obtained after being calcined for 1 h at 200 °C,naturally cooled to room temperature, and heated for 20 min at 300°C (preheated furnace).All dip-coating solutions remained clear and without precipitants

even after 1 month but were used only in the first 5 days afterpreparation to avoid depletion effects of the polymer template.Characterization. TEM was conducted on a FEI Tecnai G 2 20 S-

TWIN instrument that operated at 200 kV on films scraped off fromthe substrates and transferred onto a copper grid coated with laceycarbon. SEM imaging was performed using a JEOL 7401F instrumentat an acceleration voltage of 10 kV and a working distance of 4 mm.Image J version 1.44o (http://rsbweb.nih.gov/ij) was employed todetermine the pore diameter and film thickness. Kr adsorptionisotherms were measured at 77 K with a Quantachrome Autosorb-1-Cinstrument. The film samples were degassed in vacuum at 150 °C for 2h prior to physisorption. The surface area was calculated using theBrunauer−Emmett−Teller (BET) method. To determine the coating

mass, the oxide films were dissolved and the concentration wasmeasured in a Varian 715-ES ICP-OES instrument. The Al2O3 filmswere dissolved in a mixture of H2SO4 (3 mL, 95 wt %) and H3PO4 (3mL, 85 wt %) in 30 min at 200 °C and 20 bar in a microwave (200W). The ZnO films were dissolved in an aqueous HCl solution (8 mL,37 wt %) while being stirred for 30 h at 25 °C. The Co3O4 films weredissolved in an aqueous HCl solution (8 mL, 37 wt %) while beingstirred for 40 h at room temperature.

2D SAXS patterns were recorded at DORIS III storage ring,beamline B1 at DESY Hamburg with a PILATUS 1 M detector(Dectris) at a sample−detector distance of 3589 or 1785 mm and anX-ray energy of 16029 eV. 2D SAXS patterns were also recorded atPETRA III storage ring, beamline P03 at DESY Hamburg with aPILATUS 1 M detector at a sample−detector distance of 3161 mmand an X-ray energy of 12956 eV. FTIR spectra were recorded on aPerkin-Elmer Spectrum 100 instrument on samples pressed in KBr.XRD was measured on a Bruker D8 Advance instrument (Cu Kαradiation) with a grazing incident beam (1°). Reflections were assignedusing PDFMaintEx library version 9.0.133. TG FTIR was measured ona Netzsch STA 409 connected to a Bruker Optik Equinox 55instrument in air at a heating rate of 5 K/min. Gas-phase IR spectrawere assigned using the EPA vapor phase FTIR library. Electrosprayionization mass spectra (ESI-MS) were measured with a ThermoScientific Orbitrap LTQ XL instrument operating at a source voltageof 10 kV. The spray solutions were prepared by codissolving metalnitrates and citric acid in a 2:1 molar ratio in ethanol and sprayeddirectly into the ESI-MS instrument at a flow rate of 5 μL/min.

■ ASSOCIATED CONTENT*S Supporting InformationAdditional data. This material is available free of charge via theInternet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: ralph.kraehnert@tu-berlin.de.

Author ContributionsR.K. and B.E. designed the study. B.E. and D.B. conducted thesynthesis and material characterization. B.E., J.P., U.V., and F.E.analyzed the materials with X-ray-based methods. B.E., E.O.,R.K., and J.P. prepared the manuscript. P.S. contributed editingof the manuscript and helpful discussion throughout the study.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSR.K., D.B., and E.O. acknowledge generous funding fromBMBF (FKZ 03EK3009). B.E. is thankful for financial supportfrom the German Cluster of Excellence in Catalysis (UNICAT)funded by the German National Science Foundation (DFG)and managed by the Technical University of Berlin (TUBerlin). R.K. is grateful for support from Einstein-StiftungBerlin. Analytical support by Zentraleinrichtung Elektronen-mikroskopie (ZELMI) at TU Berlin (TEM), Oliver Goerke(TG FTIR), Gregor Koch (TG), and Maria Schlangen (ESI-MS) is acknowledged. Portions of this research were conductedon beamline B1 at light sources DORIS III and PETRA III atDESY, a member of the Helmholtz Association (HGF).

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