Micelle-Templated Oxides and Carbonates of Zinc, Cobalt, and

Article pubs.acs.org/cm

Micelle-Templated Oxides and Carbonates of Zinc, Cobalt, and Aluminum and a Generalized Strategy for Their Synthesis Björn Eckhardt,† Erik Ortel,† Denis Bernsmeier,† Jörg 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-Willstätter-Strasse 11, D-12489 Berlin, Germany ‡

S Supporting Information *

ABSTRACT: Catalysis, energy storage, and light harvesting require functional materials with tailored porosity and nanostructure. However, common synthesis methods that employ polymer micelles as structure-directing agents fail for zinc oxide, 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)-blockpoly(ethylene oxide) triblock copolymers and a new type of precursor formed by chemical complexation of a metal nitrate with citric acid. A general synthesis mechanism is deduced. Mechanistic insights allow for the prediction of optimal processing conditions for different oxides and carbonates based on simple thermogravimetric analysis. Employing this synthesis, films of ZnO and Co3O4 with micelle-controlled mesoporosity become accessible for the first time. It is the only soft-templating method reported so far that also yields mesoporous metal carbonates. The developed synthesis is generic in nature and can be applied to many other metal oxides and carbonates. KEYWORDS: EISA, pore templating, metal oxide, metal carbonate, zinc oxide, cobalt oxide

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INTRODUCTION

morphology of metal oxides on a nanometer scale is of vital importance. So-called “templating” is one of the most versatile methods for controlling the nanostructure of a metal oxide during its wet-chemical synthesis. It employs preformed nanostructures (templates) as structure-directing agents. The templates typically possess the inverse shape of the desired pore morphology. Open porosity results from solidification of the oxide framework followed by template removal. Templatebased syntheses have been reported for metal oxides with mesopores,13,14 macropores,12,15−17 and hierarchical porosity.16−18 Depending on the nature of the employed template, so-called hard and soft templating can be distinguished. Hard templating is commonly employed for the synthesis of

Many applications in catalysis, energy storage, and photovoltaics rely on metal oxides that feature a specific nanostructure. The nanostructure of a metal oxide often determines its optical, magnetic, and catalytic properties. The oxides of cobalt and zinc provide some of the most prominent examples. Nanostructured cobalt-based oxides are promising materials for electrodes in supercapacitors1,2 and in lithium ion batteries3,4 with superior charging rates.5 Moreover, they represent very active catalysts for the oxygen evolution reaction in electrochemical water splitting6 and the oxygen reduction reaction in fuel cells.7 Also, ZnO nanostructures feature unique properties. They are used in display technologies, photovoltaics, photocatalysis, and piezoelectric nanogenerators8 and allow the construction of self-powered nanodevices.9−11 Moreover, the photonic band gap of ZnO can be tailored by the introduction of an inverse opal structure.12 Hence, control over the © XXXX American Chemical Society

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

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dx.doi.org/10.1021/cm400535d | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

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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 of the micelles of the polymer template with the precursor complex into an ordered mesophase. (3) Decomposition of the precursor complex into a structurally stable metal−carbonate intermediate at low temperatures while the ordered mesophase is retained. (4) Thermal treatment in air to remove the polymer template leading to open mesopores in the metal carbonate film. (5) Controlled decomposition of the amorphous carbonate that forms the pore walls into the nanocrystalline metal oxide.

macroporous oxides in powder form (ZnO 12,15 and Co3O416,17). Although hard templating can also in general produce mesopores (ZnO13 and Co3O419,20), soft templating is by far the most common synthesis approach for mesoporous oxides. Common soft-templating routines such as evaporationinduced self-assembling (EISA)21 employ micelles of amphiphilic block copolymers as the pore templates. In a typical EISA synthesis, a solution containing an oxide precursor and the amphiphilic block copolymers is deposited onto a substrate. The solvent evaporates during deposition while the template molecules are arranged into micelles. Micelles and the partially condensed precursor assemble into an ordered mesophase. A subsequent calcination converts this mesophase into a mesoporous oxide film. EISA-based syntheses offer three major advantages: (I) The pore size, pore shape, and thickness of pore walls22 can be controlled by the structure and concentration of the template. (II) Synthesis protocols are simple and reproducible. (III) A wide range of metal oxides is accessible.14 However, neither the micelle-templated synthesis of 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 properties of commonly employed metal precursors. Typically, oxides were synthesized from (I) alkoxides or partially alkoxylated metal chlorides (e.g., SiO2,23,24 TiO2,25,26 Al2O3,27 and ZrO228,29), (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 of mesophase formation and template removal. Route I based on hydrolysis and condensation of alkoxy groups fails for precursors with high hydrolysis and condensation rates, because rapid condensation results in undesired precipitation prior to mesophase assembly. Route II requires high-quality building blocks such as a redispersible nanocrystalline colloid with a small particle diameter (d < ∼5 nm) and a narrow size distribution. However, for many metal oxides, such nanoparticle

syntheses remain challenging. Approach III is limited to metal precursors that do not show excessive crystallization upon drying; otherwise, the ordered mesostructure cannot be formed. Moreover, oxide formation must occur at temperatures significantly below the typical temperatures of template combustion (∼300 °C); otherwise, pores collapse because of premature template removal. Other constraining factors are the limited solubility of many precursor compounds, melting during calcination, and rapid crystallite growth of the metal oxide before template removal. Additionally, a general limitation of all methods described here is that they do not provide access to the soft-templated synthesis of mesoporous metal carbonates. The synthesis of fine-grained metal oxides without templated pore structure can be achieved, e.g., via the Pechini method or the citrate method. The so-called Pechini method was originally patented for the preparation of (untemplated) nanocrystalline metal titanates and niobates.34 It relies on the initial formation of chelate complexes of metal ions (originally titan, niobium, and zirconium) with α-hydroxycarboxylic acids (for example, lactic, citric, or glycolic acid). Subsequent heating in the presence of a polyhydroxy alcohol (e.g., ethylene glycol) induces polyesterification of the chelate complex, yielding an amorphous gel. Calcination at moderate temperatures typically converts 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 obtained also by heating of the corresponding metal carbonates to induce 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-templated magnesium oxide.44 The preparation borrows from three different approaches, i.e., the initial steps of the Pechini method (complexing the metal ion), the carbonate decomposition strategy, and pore templating with polymer micelles. The MgO synthesis44 relied on the initial preparation of a chemical B



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Table 1. Synthesis Conditions Employed for the Preparation of Micelle-Templated Mesoporous Carbonates and Oxides of Zn, Al, Co, and Mga precursor system Zn(NO3)2·6H2O (444 mg) and citric acid (144 mg) Al(NO3)3·9H2O (563 mg) and citric acid (144 mg) Co(NO3)2·6H2O (437 mg) and citric acid (144 mg) Mg(NO3)2·6H2O (385 mg) and citric acid (144 mg)

template PEO213−PB184−PEO213 (70 mg) PEO213−PB184−PEO213 (70 mg) PEO213−PB184−PEO213 (70 mg) PEO213−PB184−PEO213 (70 mg)

solvent H2O and each) H2O and each) H2O and each) H2O and each)

calcination (i), carbonate

calcination (ii), oxide

ethanol (1.5 mL

60 min at 250 °C

25 min at 400 °C

ethanol (1.5 mL

60 min at 300 °C

30 min at 900 °C

ethanol (1.5 mL

60 min at 200 °C

20 min at 300 °C

ethanol (1.5

60 min at 400 °C

120 min at 400 °C and 60 min at 600 °C

a

Columns 1−3 detail the compositions of the dip-coating solution. Column 4 lists the calcination procedure (i) that yields the carbonate and column 5 the respective calcination (ii) that transforms the carbonate into the corresponding oxide.

thermal treatment procedures for (3) carbonate formation, (4) template removal, and (5) oxide formation were established based on thermogravimetric (TG) analysis of precursor complexes and templates. Additional characterization revealed the structural evolution of the pore morphology [scanning electron microscopy (SEM) and transmission electron microscopy (TEM)] and surface area (Kr sorption) as well as the phase composition [Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD)] and crystallinity of the pore walls [selected area electron diffraction (SAED) and XRD]. The derived mechanistic picture explains the formation of mesoporous carbonates and oxides. It reveals also why the synthesis of mesoporous CoCO3 necessarily fails. Mesoporous oxides and carbonates were synthesized as summarized in Table 1. Briefly, for the synthesis of, e.g., ZnO, a solution containing template polymer (PEO213−PB184− PEO213), metal precursor [Zn(NO3)2·6H2O], complexing agent (citric acid), water, and ethanol was dip-coated onto Si substrates at a controlled temperature (25 °C) and a relative humidity 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).

precursor complex consisting of magnesium nitrate bonded to citric acid and on thermally induced formation of a mesoporous MgCO3 intermediate. In this synthesis, water/ethanol solutions containing 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 films were converted into mesoporous MgO in two subsequent calcination steps performed at 400 and 600 °C. This paper demonstrates that the self-assembly of triblock copolymers with citric acid-based metal complexes provides access 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 structure become accessible for the first time. On the basis of a mechanistic understanding, general criteria for the successful synthesis of metal oxides and metal carbonates with controlled porosity are deduced. Factors that are crucial for the synthesis as well as remaining limitations are critically discussed.

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SYNTHESIS STRATEGY From the recently reported synthesis of mesoporous MgO,44 five criteria that a synthesis strategy with general applicability would have to fulfill can be deduced. The deduced criteria are illustrated in Figure 1 along with the proposed synthesis strategy. (1) The metal salt and ligands with carboxylic acid functionality must form a chemical complex. Chelating ligands are preferred because of the high stability of the complexes. (2) Polymer micelles and the metal complex must undergo selfassembly during deposition and drying to form an ordered mesostructure. (3) The chemical complex should decompose into a structurally stable metal carbonate at low temperatures while the templating micelles stabilize the formed mesostructure. (4) Subsequent template removal should yield the mesoporous metal carbonate; hence, decomposition of the template polymer should occur at a temperature where the carbonate remains thermally stable. (5) The final thermal treatment should transform the carbonate into a nanocrystalline metal 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 would constitute the synthesis of the respective mesoporous carbonate and oxide. Formation of (1) a stable metal complex was studied by electrospray ionization mass spectroscopy (ESI-MS) of precursor solutions. Employing highly amphiphilic surfactants PEO−PB−PEO that form stable spherical micelles already prior to solution deposition22 assured (2) robust reproducible mesophase formation. Ordering of micelles and pore structures was assessed by small-angle X-ray scattering (SAXS). Adequate

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MESOPOROUS ZINC CARBONATE AND ZINC OXIDE 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 the corresponding material (ii) calcined in addition at 400 °C studied by SEM (panel d), FTIR (panel e), XRD (panel f), and TEM (panels g−i). SEM analysis of sample (i) indicates that calcination at 250 °C yields a homogeneous film. Cross-section SEM images (Figure 2a) reveal that the formed film is ∼1100 nm thick and completely penetrated by mesopores. The pores show elliptical shapes ∼21 nm × ∼19 nm in size. The appearance of the pore walls is smooth and unstructured; no crystallite shapes can be distinguished (Figure 2a). FTIR spectra recorded on corresponding powder samples feature intense symmetric (1384 cm−1) and asymmetric (1583 cm−1) vibrations (Figure 2b). These bands can be assigned to zinc carbonate,45 whereas only negligible contributions at wavenumbers indicative of ZnO (e.g., 577 cm−1) are observed. Moreover, XRD analysis of the sample (Figure 2c) shows only reflections that can be attributed to the substrate (silicon wafer) and no indications of a crystalline Zn-containing phase. Kr physisorption indicates a surface area of 86.1 m2/g. This value is slightly smaller than the surface area typically observed for C



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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 at 400 °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 a higher magnification. (b) Infrared spectrum of the precursor complex calcined at 250 °C recorded in transmission mode. (c) Grazing incident XRD analysis 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).

micelle-templated oxides (∼100−250 m2/g).14 Hence, combined analytical data indicate the successful synthesis of micelletemplated zinc carbonate comprising amorphous walls and interconnected 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 a strong signal at 577 cm−1 indicative of ZnO, whereas only small

bands assigned to carbonate are retained.45 X-ray diffraction data of the sample (Figure 2f) feature broad reflections at positions of 2θ = 31.7° (100), 34.5° (002), 36.0° (101), 62.9° (103), and 67.8° (112). These reflections can be assigned to ZnO 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 almost completely into nanocrystalline ZnO. Corresponding top-view D



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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) calcined for 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 two different incident angles of the X-ray beam of 90° (top) and 6° (bottom) relative to the substrate surface (linear intensity scale).

confirms that film shrinkage is anisotropic and restricted to the direction perpendicular to the substrate. Moreover, the observed ellipsoidal 2D SAXS patterns (Figure 3b,d,f) indicate also a certain degree of lattice distortion in the cubic mesostructure.22 SAXS analysis thus confirms that the deposited micelles and precursor complex form a locally ordered mesophase, and that corresponding pore ordering is preserved also during carbonate formation, template removal, and transition into a mesoporous zinc oxide.

SEM images (Figure 2d) indicate that films remain homogeneous and macroscopically crack-free. The films are completely penetrated by mesopores ∼22 nm in diameter. The pores are locally ordered and open toward the outer film surface. TEM analysis (Figure 2g) confirms the presence of templated mesopores throughout the sample volume. Highresolution TEM (Figure 2h) indicates crystallites and lattice fringes, which confirms that the pore walls are crystalline. Furthermore, SAED analysis (Figure 2i) shows isotropic diffraction rings with ring positions that match the reflections of hexagonal zincite structure (PDF-No. 00-036-1451). The homogeneous diffraction rings indicate that the pore walls consist of randomly oriented crystallites. Hence, calcination (ii) at 400 °C transforms the amorphous carbonate into nanocrystalline 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 the corresponding ZnCO3. This increase in surface area can be attributed to additional microporosity observed in the crystalline pore walls of ZnO, whereas the pore walls of amorphous ZnCO3 appeared to be rather dense. Mesoscale ordering in Zn-containing films was analyzed after different thermal treatments of deposited films. Figure 3 details the evolution of order from deposited micelles (a and b) to (i) porous ZnCO3 (c and d) and (ii) ZnO (e and f) as indicated by two-dimensional 2D SAXS recorded in transmission at beam incident angles of 90° (a, c, and e) and of 6° (b, d, and f) relative to the substrate. The 2D SAXS pattern (a) recorded at 90° for the as-deposited film features an isotropic ring corresponding to a d spacing of 38 nm. Both the d spacing and the isotropic ring are preserved upon heat treatment at 250 °C (Figure 3c) and during further calcination at 400 °C (Figure 3e). In contrast, all 2D SAXS patterns recorded at a low incident angle of 6° (Figure 3b,d,f) show diffraction rings with an elliptical shape. Such diffraction patterns have been reported also for conventional EISA-based syntheses, where the pore axis that is oriented perpendicular to the substrate progressively shrinks during thermal treatments.46,47 This deformation is caused by a loss of film volume resulting in homogeneous anisotropic film shrinkage upon drying as well as calcination. Hence, the d spacing perpendicular to the substrate decreases in the studied Zn-containing films from 25 nm (as deposited) to 21 nm [carbonate (i)] and 19 nm [oxide (ii)] (Figure 3b,d,f). However, the d spacing parallel to the substrate of 38 nm remains unchanged (Figure 3a,c,e). This observation

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FORMATION OF THE PRECURSOR COMPLEX The proposed synthesis strategy requires the initial formation of a stable metal−precursor complex in solution (Figure 1, condition 1). The ability of citric acid to form complexes with the nitrate compounds of Zn, Al, and Co was therefore assessed by electrospray ionization mass spectroscopy (ESI-MS) of the complex solutions. Mass spectra recorded in anion mode are provided 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 corresponding isotope pattern. All observed masses can be assigned to zinc ions 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 of complexed aluminum nitrate (Figure S4 of the Supporting Information) and cobalt nitrate (Figure S9 of the Supporting Information) also show a similar composition. Masses corresponding to ML and M2L2 complex stoichiometries are observed for both metals. Moreover, Co-based solutions also contained M2L and ML2 stoichiometries. Hence, citric acid forms stable complexes with all studied metal ions, underlining the generic nature of this initial synthesis step.

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THERMAL STABILITY OF PORE TEMPLATES AND PRECURSOR COMPLEXES The proposed synthesis strategy requires the decomposition of the precursor complex into carbonate at temperatures where the template polymer remains sufficiently stable (Figure 1, step 3). Moreover, access to the mesoporous carbonate implies that template removal (Figure 1, step 4) occurs prior to E



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Figure 4. Thermogravimetric analysis recorded for (a) the PEO213−PB184−PEO213 polymer and (b−e) dried complexes of citric acid with different metal 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 thermal modification of the template polymer and of the metal complexes. The green frame highlights the temperature range of the decomposition of the template polymer. Colored areas indicate the evolution of the metal complexes: (blue) decomposition of the precursor complex into the corresponding carbonate, (yellow) existence of the metal carbonate, and (red) decomposition of the carbonate into the metal oxide. Heating was conducted in air at a rate of 5 K/min.

Information). CO2 is a typical decomposition product of metal carbonates. Hence, calcination procedure (i) employed for Zn-based films [1 h at 250 °C (see Table 1)] exploits the temperature range in which zinc carbonate remains stable and retains the pore walls made of carbonate (Figure 2a). However, a temperature of 250 °C is sufficient to slowly decompose the template polymer (Figure 4a). Calcination (i) therefore yields zinc carbonate with templated open mesoporosity (Figure 2a). Moreover, thermal treatments at temperatures exceeding the plateau range, i.e., >320 °C (Figure 4d), decompose the carbonate into the oxide. Thus, the synthesis of mesoporous ZnO 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). The decomposition of the complex and its conversion into carbonate start in all three cases below 100 °C. Moreover, FTIR analysis of the solid samples provides in the plateau region evidence of the presence of the corresponding metal carbonate (Figure S6 of the Supporting Information for Al, Figure S11 of the Supporting Information for Co, and ref 44 for Mg). Decomposition of the carbonate at temperatures beyond the plateau region is supported by detection of CO2 as the main decomposition product also for Al (Figure S8 of the Supporting Information), Co (Figure S13 of the Supporting Information), and Mg.44 Thus, for all studied metals, the decomposition of the precursor complex into the corresponding carbonate (first mass 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

decomposition of the carbonate into the oxide (Figure 1, step 5). The thermal stability of the template polymer and of different precursor complexes was therefore investigated by TG analysis in air. The recorded TG curves are presented in Figure 4, 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 mass loss related to combustion of the polymer occurs between 375 and 425 °C. The thermal stability of the polymer is therefore in line with literature reports, where decomposition temperatures between ∼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 the same typical shape (Figure 4b−e). An initial mass loss of approximately 30−50% is followed by a plateau with a constant mass and additional mass loss. Between 20 and 35% of the initial mass is retained in the final stage. In the case of Zn, the first 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 combination with XRD and IR analysis of phases (i) and (ii) (Figure 2), the observed behavior is interpreted as decomposition of the complex into carbonate (first mass loss), the presence of a stable carbonate (plateau), and decomposition of the carbonate into the oxide (second mass loss). This interpretation is further supported by IR analysis of the gas phase during the second mass loss, which detects CO2 as the main gas-phase decomposition product (Figure S3 of the Supporting F



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Figure 5. Electron microscopy images demonstrating the ability of the synthesis strategy to address different metals (Al, Mg, and Co). Images of the carbonates 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 Co materials (Figure S10).

evidence the formation of aluminum oxide (Figure S6 of the Supporting Information).51,52 XRD analysis detects two broad reflections at 46.0° and 66.6° (Figure S7 of the Supporting Information), which correspond well with the positions of (400) and (440) reflections reported for γ-Al2O3 (PDF-No. 00050-0741). The crystallite size estimated via the Scherrer equation amounts to 5 nm. Diffraction rings observed in SAED correspond to (311), (400), (511), (440), (444), and (800) reflections of γ-Al2 O3 (Figure S5b of the Supporting Information) and confirm the phase assignment. SAXS analysis (Figure S14a of the Supporting Information) evidences pore ordering in the as-deposited film, the aluminum carbonate, and the aluminum oxide at a d spacing of 39 nm parallel to the substrate (Figure S14a of the Supporting Information, 90°). The corresponding periodic distance perpendicular to the substrate decreases from 27 nm (as deposited) to 14 nm (carbonate) and 5 nm (oxide) (Figure S14a of the Supporting Information, 6°). The oxide Kr BET surface area amounts to 286 m2/g. Hence, calcination (ii) at 900 °C transforms mesoporous aluminum carbonate into crystalline γ-alumina with templated and ordered mesopore structure.

obtaining the micelle-templated mesoporous carbonates and oxides 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 films were prepared.44 SEM images of the synthesized micelletemplated oxides and carbonates are presented in Figure 5.

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ALUMINUM CARBONATE AND OXIDE TGA of the complexed aluminum nitrate shows a plateau between 225 and 310 °C (Figure 4c). The template polymer decomposes at 250 °C. Hence, calcination at (i) 300 °C was chosen to produce mesoporous aluminum carbonate and additional calcination (ii) at 900 °C to produce mesoporous aluminum 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 by templated mesopores with the typical elliptical shape and a size of ∼20 nm (width) × ∼9 nm (height). FTIR analysis (Figure S6 of the Supporting Information, i) reveals two characteristic bands indicative of aluminum carbonate, i.e., asymmetric (1608 cm−1) and symmetric (1469 cm−1) stretching vibrations.50 The film is amorphous according to XRD (Figure S7 of the Supporting Information, i) and features a surface area of 44.8 m2/g (Kr physisorption). Hence, thermal treatment (i) of the deposited film forms amorphous mesoporous aluminum carbonate. SEM images of the film formed by additional calcination at 900 °C show that the film contains templated mesopores ∼22 nm in diameter (Figure 5d). TEM analysis confirms that the film is fully mesoporous (Figure S5a,c of the Supporting Information) and composed of small crystallites ∼6 nm in diameter (Figure S5d of the Supporting Information). Two vibrations observed at 538 and 732 cm−1 indicative of Al−O bonds and the absence of carbonate-related vibrations in IR

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COBALT CARBONATE AND OXIDE TGA of the cobalt complex indicates less favorable behavior (Figure 4e). Also here, a plateau can be observed and attributed to the presence of cobalt carbonate. However, the plateau is shifted to temperatures as low as 190−245 °C, i.e., below the value of ∼250 °C required for thermal decomposition of the template polymer. Hence, cobalt carbonate decomposes before the template can be removed. Deposited films were therefore calcined at (i) 200 °C to stabilize the carbonate and (ii) 300 °C to 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 G



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symmetric (1392 cm−1) stretching vibrations that can be assigned to carbonate.53 The formed film is amorphous according to XRD (Figure S12 of the Supporting Information, i). Unfortunately, electron microscopy imaging of the film failed because of its instability in the electron beam. Moreover, the film’s surface area was too small to be detected by Kr physisorption analysis. Both observations suggest that the polymer template is still present in the film. SAXS analysis reveals a diffraction ring for the sample (Figure S14b of the Supporting Information) with a d spacing of 37 nm parallel and 18 nm perpendicular to the substrate, both indicating the presence of an ordered mesostructure. Hence, micelle structure cobalt carbonate is formed during calcination at 200 °C, yet its pore structure is blocked by the remaining template micelles. This effect is in complete agreement with the TG analysis of the precursor complex (Figure 4e) and template (Figure 4a) and demonstrates a limitation of the proposed synthesis strategy in its present form. However, cobalt carbonate with open mesoporosity should become accessible when the PEO− PB−PEO template is removed by alternative (nonthermal) methods or when alternative templates with lower decomposition 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 nm in diameter (Figure 5f). TEM analysis confirms that the film is fully porous (Figure S10c of the Supporting Information) with pore walls composed of crystallites ∼6 nm in size (Figure S10d of the Supporting Information). FTIR spectra contain strong bands indicative of cobalt oxide formation, i.e., Co−O vibrations at 663 and 570 cm−1,54 whereas only small signals for carbonates remained (Figure S11 of the Supporting Information, ii). The corresponding XRD pattern (Figure S12 of the Supporting Information, ii) reveals numerous reflections that match with the reported phase PDF-No. 00-042-1467 of crystalline 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 the Supporting Information) evidences numerous diffraction rings that confirm the crystallinity of the samples as well as the assignment of the crystalline spinel phase. Furthermore, 2D SAXS confirms the presence of an ordered pore structure (Figure S14b of the Supporting Information). The sample’s surface area amounts to ∼257 m2/g (Kr physisorption). Hence, calcination (ii) of the micelle-structured (nonporous) cobalt carbonate at 300 °C forms a nanocrystalline Co3O4 film with a spinel structure and the desired open templated porosity.

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

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CONCLUSIONS

We present a new approach for the synthesis of micelletemplated oxides and carbonates of zinc, cobalt, and aluminum. The method employs a unique precursor to overcome limitations of classical EISA-based syntheses, i.e., complexes formed from citric acid and metal nitrates. The precursors reliably self-assemble with amphiphilic block copolymers. Using this approach, films of ZnO, Co3O4, ZnCO3, and Al2(CO3)3 were synthesized for the first time with micelle-controlled open mesoporosity. The respective pores are locally ordered and result in surface areas in the ranges of 44−86 m2/g (carbonates) and 250−286 m2/g (oxides). The pore systems show the uniaxial shrinkage that is typical for EISA-based oxides. The pore sizes of the materials were readily adjusted by changing the size of the template polymer (ZnO; Figure S15 of the Supporting Information). A general mechanism is proposed for the developed synthesis (Figure 1). As in the Pechini method, the first step requires the formation of a metal complex in solution. The complex assembles with template micelles into an ordered mesophase during film deposition and solvent evaporation. Sequential thermal treatments convert the precursor complex first into a micelle-structured amorphous carbonate, remove the template yielding the porous carbonate, and finally convert the carbonate into the mesoporous oxide. Carbonates and oxides feature a cubic ordered mesopore structure. The calcination temperatures required for carbonate and oxide synthesis can be derived from simple TG analysis of the corresponding precursor complex. Moreover, a comparison between TGA data of metal complex and template polymer predicts if thermal template removal from the carbonate is feasible. The amorphous character of the intermediate carbonate appears to be of particular importance, because it facilitates template removal without sintering of wall-forming crystallites and therefore avoids degradation of the templated pore structure. The synthesis is generic in nature and therefore applicable to a wide range of metal oxides and carbonates. The presented synthesis strategy for creating functional metal oxides and carbonates can be easily tuned and optimized for energy storage, electro catalysis, sunlight harvesting, or biomedical applications. In particular, amorphous CaCO3 could be of interest for biocompatible implant coating and defined model systems in bone cell culturing 55 and biomineralization research.56 However, the synthesis should be further refined on the basis of an improved fundamental understanding. Physicochemical investigations could reveal the type of interaction between the precursor complex and micelles, guide the tailoring of the complex structure, and explain how crystallites are formed. Most importantly, extending the synthesis strategy to yield also bimetallic carbonates and oxides with optimized pore structure such as the Cu/ZnOx-based catalysts employed in industrial methanol synthesis57 or indium-free transparent conductive oxides could result for many applications in a tremendous improvement in performance.

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COMPARISON TO MAGNESIUM CARBONATE AND OXIDE TGA of the complex of magnesium nitrate with citric acid features a broad plateau of constant mass between 275 and 425 °C (Figure 4b). According to the proposed mechanistic hypothesis, calcination (i) at temperatures within this temperature window should yield mesoporous carbonate, whereas subsequent calcination (ii) at temperatures exceeding 425 °C should result in mesoporous magnesium oxide. We recently reported the corresponding synthesis of magnesium carbonate with templated mesopore structure employing 400 °C for calcination (i).44 The transformation into mesoporous MgO succeeded by calcination (ii) at 600 °C. Panels b and e of Figure 5 show images of the corresponding materials (see ref H



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Article

mass, the oxide films were dissolved and the concentration was measured in a Varian 715-ES ICP-OES instrument. The Al2O3 films were dissolved in a mixture of H2SO4 (3 mL, 95 wt %) and H3PO4 (3 mL, 85 wt %) in 30 min at 200 °C and 20 bar in a microwave (200 W). 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 were dissolved in an aqueous HCl solution (8 mL, 37 wt %) while being stirred 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 an X-ray energy of 16029 eV. 2D SAXS patterns were also recorded at PETRA III storage ring, beamline P03 at DESY Hamburg with a PILATUS 1 M detector at a sample−detector distance of 3161 mm and an X-ray energy of 12956 eV. FTIR spectra were recorded on a Perkin-Elmer Spectrum 100 instrument on samples pressed in KBr. XRD was measured on a Bruker D8 Advance instrument (Cu Kα radiation) with a grazing incident beam (1°). Reflections were assigned using PDFMaintEx library version 9.0.133. TG FTIR was measured on a Netzsch STA 409 connected to a Bruker Optik Equinox 55 instrument in air at a heating rate of 5 K/min. Gas-phase IR spectra were assigned using the EPA vapor phase FTIR library. Electrospray ionization mass spectra (ESI-MS) were measured with a Thermo Scientific Orbitrap LTQ XL instrument operating at a source voltage of 10 kV. The spray solutions were prepared by codissolving metal nitrates and citric acid in a 2:1 molar ratio in ethanol and sprayed directly into the ESI-MS instrument at a flow rate of 5 μL/min.

EXPERIMENTAL SECTION

Chemicals. Zinc nitrate hexahydrate (98%, extra pure) was obtained from Acros. Aluminum nitrate nonahydrate (>99%, pro analysis), magnesium nitrate hexahydrate (>99%, pro analysis), and cobalt nitrate hexahydrate (>98%, for analysis) were purchased from Merck. Water-free citric acid (>99.5%, pro analysis), ethanol (>99.9%, absolute), and the HCl solution (37 wt %, pro analysis) were obtained from Roth. Concentrated sulfuric acid (95 wt %, puriss) was purchased from Th. Geyer. PEO−PB−PEO polymers were synthesized by Polymer Service GmbH Merseburg.22 All chemicals were used without further purification. Film Synthesis. Prior to film deposition, substrates (Si wafers) were cleaned with ethanol and heated in air (2 h at 600 °C). The dipcoating solution for zinc films was prepared by joining powders of Zn(NO3)2·6H2O, citric acid, and a template in the amounts listed in Table 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 in a colorless solution. Films were prepared by dip-coating substrates at a withdrawal rate of 150 mm/min under a controlled atmosphere (25 °C, 40% relative humidity). Afterward, films were allowed to dry for at least 10 min before being transferred into the preheated muffle furnace. Mesoporous ZnCO3 films were obtained after calcination for 1 h at 250 °C. Mesoporous ZnO films required calcination for 1 h at 250 °C, natural cooling to room temperature, and a second calcination for 25 min at 400 °C (preheated furnace). ZnO films with 8 and 15 nm pore diameters as well as bimodal films were obtained in a similar fashion (see Figure S15 of the Supporting Information). Films of magnesium carbonate and magnesium oxide were prepared according to the method described previously44 employing the composition 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 of Milli-Q water and ethanol by being stirred overnight, resulting in a slightly yellow solution. Films were prepared by dip-coating substrates at a withdrawal rate of 150 mm/min at 25 °C and 40% relative humidity. Afterward, films were allowed to dry for at least 10 min before being transferred into the preheated muffle furnace. The mesoporous aluminum carbonate films were obtained by calcination for 1 h at 300 °C. The mesoporous aluminum oxide films were obtained after being calcined for 1 h at 300 °C, naturally cooled to room temperature, and heated for 30 min to 900 °C (preheated furnace). 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 of Milli-Q water and ethanol by being stirred overnight, resulting in a red/pink solution. Films were prepared by dip-coating substrates at a withdrawal rate of 150 mm/min at 25 °C and 40% relative humidity. Afterward, films were allowed to dry for at least 10 min before being transferred into the preheated muffle furnace. The cobalt carbonate films employed calcination for 1 h at 200 °C. The mesoporous cobalt oxide 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 after preparation to avoid depletion effects of the polymer template. Characterization. TEM was conducted on a FEI Tecnai G 2 20 STWIN instrument that operated at 200 kV on films scraped off from the substrates and transferred onto a copper grid coated with lacey carbon. SEM imaging was performed using a JEOL 7401F instrument at 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 to determine the pore diameter and film thickness. Kr adsorption isotherms were measured at 77 K with a Quantachrome Autosorb-1-C instrument. The film samples were degassed in vacuum at 150 °C for 2 h prior to physisorption. The surface area was calculated using the Brunauer−Emmett−Teller (BET) method. To determine the coating

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ASSOCIATED CONTENT

S Supporting Information *

Additional data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

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

R.K. and B.E. designed the study. B.E. and D.B. conducted the synthesis 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 editing of the manuscript and helpful discussion throughout the study. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.K., D.B., and E.O. acknowledge generous funding from BMBF (FKZ 03EK3009). B.E. is thankful for financial support from the German Cluster of Excellence in Catalysis (UNICAT) funded by the German National Science Foundation (DFG) and managed by the Technical University of Berlin (TU Berlin). R.K. is grateful for support from Einstein-Stiftung Berlin. Analytical support by Zentraleinrichtung Elektronenmikroskopie (ZELMI) at TU Berlin (TEM), Oliver Goerke (TG FTIR), Gregor Koch (TG), and Maria Schlangen (ESIMS) is acknowledged. Portions of this research were conducted on beamline B1 at light sources DORIS III and PETRA III at DESY, a member of the Helmholtz Association (HGF).

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Micelle-Templated Oxides and Carbonates of Zinc, Cobalt, and

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