Synthesis of Nanorod-Shaped Cobalt Hydroxycarbonate and Oxide

Jan 13, 2010 - Co3O4 nanomaterials are widely used in chemical sensors,(1) lithium-ion ..... 47, 48) but unidentate acetate anions in the Co-160 precu...
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Synthesis of Nanorod-Shaped Cobalt Hydroxycarbonate and Oxide with the Mediation of Ethylene Glycol Xiaowei Xie,† Panju Shang,‡ Zhiquan Liu,*,‡ Yongge Lv,† Yong Li,† and Wenjie Shen*,† State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China, and Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China ReceiVed: NoVember 19, 2009; ReVised Manuscript ReceiVed: December 21, 2009

Cobalt hydroxycarbonate nanorods are prepared by precipitation of cobalt acetate with sodium carbonate in ethylene glycol. Structural and chemical analyses of the intermediate phases during the precipitation and aging process revealed that amorphous cobalt hydroxide acetate is formed at the initial stage where ethylene glycol acts as a simple solvent and a coordinating agent. With the slow addition of sodium carbonate, carbonate anions are gradually intercalated into the interlayers by replacing the acetate and hydroxyl anions. This anionexchange process induces a dissolution-recrystallization process in which ethylene glycol serves as a ratecontrolling agent, producing rod-like cobalt hydroxide carbonate. During the aging process, ethylene glycol gradually incorporates into the structure to replace the carbonate and acetate anions; the interlayer structure is collapsed, and the nanorod-shape turns into thin crimped sheets. Co3O4 nanorods with a diameter of about 10 nm and a length of 200-300 nm are then obtained by calcination of the nanorod-shaped cobalt hydroxycarbonate precursor. This spontaneous shape transformation from the precursor to the oxide is attributed to the unique thermal stability of the cobalt hydroxycarbonate nanorods with the presence of ethylene glycol and acetate anions in the interlayers. The Co3O4 nanorods show a much superior catalytic activity for CO oxidation to the conventional spherical Co3O4 nanoparticles, clearly demonstrating the morphology-dependent nanocatalysis. Introduction 1

Co3O4 nanomaterials are widely used in chemical sensors, lithium-ion batteries,1,2 optical and magnetic materials,3,4 and heterogeneous catalysts,5 where their properties are strongly dependent on their size and morphology. Accordingly, novel Co3O4 structures such as nanocubes,5 nanorods,6 nanowires,7 nanotubes,8 nanowalls,9 nanofibers,3 and nanoplatelets10 have been synthesized successfully. Even hierarchically flower-like nanomaterials,11-13 mircospheres,14,15 and hollow spheres16 have also been fabricated. These nanostructures are usually synthesized through a two-step process. The precursors, cobalt hydroxide salts expressed in a formula of Co(OH)2-x(An-)x/n · mH2O (A represents the anions), are initially synthesized in liquid phase. These lamellar precursors consist of positively charged Co(OH)2-x layers with anions and water molecules residing in the interlayer to restore charge neutrality. Not only inorganic anions but also organic anions could be intercalated into the layers, and the interlayer spacing of the resultant compounds is expanded, depending on the size of the specific anion.17 Co3O4 nanostructures are then obtained by thermal decomposition of the precursors at elevated temperatures. With such a procedure, the shape of the Co3O4 materials is largely dependent on the structure of the precursors. Therefore, control of uniform morphology of cobalt hydroxide salts through proper synthetic strategy plays a central role in obtaining Co3O4 materials with novel morphologies. * To whom correspondence should be addressed. (Z.L.) Tel/Fax: +8624-8370826. E-mail: [email protected]. (W.S.) Tel: +86-411-84379085. Fax: +86-411-8469447. E-mail: [email protected]. † Dalian Institute of Chemical Physics, Chinese Academy of Sciences. ‡ Institute of Metal Research, Chinese Academy of Sciences.

Among Co3O4 nanomaterials, one-dimensional Co3O4 nanorods have attracted special attention because of the unique optical and magnetic properties4 and the potential applications in lithium-ion battery electrodes.2 Solvothermal synthesis,18 molten salt approach,19 and inverse microemulsion route6 have been employed to prepare Co3O4 nanorods with the presence of organic surfactants/templates. The diameters of the obtained nanorods are usually in the range of tens or hundreds of nanometers and the lengths are up to several or tens of micrometers. Recently, cobalt hydroxycarbonate nanorods with the diameter in the range of 2-200 nm have been synthesized only using carbonate precipitation in water.20 The carbonate anion acts as a structure-directing agent, which selectively decreases the rates of crystal growth along both the [001] and the [100] directions, resulting in the [010]-elongated nanorods. This structure-directing effect of carbonate anion causing the formation of rod-like cobalt hydroxycarbonates has been further demonstrated under hydrothermal conditions.4,13,21,22 Unfortunately, these rod-like cobalt hydroxycarbonates have been transformed into one-dimensional arrays of Co3O4 nanoparticles or porous Co3O4 nanorods by removing the hydroxyl and carbonate species during the calcination process at 300-500 °C.4,13,20-22 Nevertheless, this synthesis of rod-like cobalt hydroxycarbonate precursors using a simple precipitation in liquid phase should be particularly stressed due to the advantages of simple apparatus, easy control, and large-scale fabrication. On the other hand, precipitation or hydrolysis of metal salts or alkoxides in polyols were found to affect strongly the manner of crystallites aggregation in the formation of metal hydroxide salts and to permit the generation of interconnected pores, resulting in enhanced surface area and well-defined porosity.23-27

10.1021/jp911011g  2010 American Chemical Society Published on Web 01/13/2010

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We have recently communicated that Co3O4 nanorods that preferentially expose reactive {110} planes are extremely active and stable for low-temperature CO oxidation compared to the conventional Co3O4 nanoparticles which have only a few exposed active sites, clearly demonstrating a morphologydependent effect.28,29 The nanorod-shaped Co3O4 was obtained by calcination of cobalt hydroxycarbonate nanorods prepared by precipitation of cobalt acetate with sodium carbonate in ethylene glycol. In this work, we have extended the study of the formation mechanism of the cobalt hydroxycarbonate precursors and the Co3O4 nanorods. The fabrication process of the cobalt hydroxycarbonate nanorods was elaborated by varying the synthetic parameters, and the nanorod-shape transformation from the precursors to the oxides was investigated as well. Experimental Section Materials Preparation. In a typical synthesis, 4.98 g of Co(OAc)2 · 4H2O was dissolved in 60 mL of ethylene glycol and heated to 80 or 160 °C to form a homogeneous solution. A total of 200 mL of 0.2 M Na2CO3 aqueous solution was then added to the mixture through a syringe pump with a flow rate of 1.11 mL/min. The precipitate was further aged in the mother liquid at that temperature for 1 h. The preparation course was performed under vigorous stirring and nitrogen flow. The precipitate was filtered and thoroughly washed with distilled water and ethanol, following by drying at 50 °C overnight under vacuum. These precursors were denominated as Co-T, where T refers to the temperature of precipitation. Further calcination of the Co-T precursors at 450 °C for 4 h in air produced the Co3O4 nanomaterials, which were assigned as Co3O4-T. In order to verify the formation mechanism of the Co-160 nanorods, the precipitates at different stages (10 min, 30 min, 45 min, 90 min, and 3 h) were extracted from the synthetic solution and then filtered and washed only with ethanol to exclude the influence of water. The obtained solids were dried at 50 °C overnight under vacuum. For examining the effect of the aging process, the precipitates obtained at 160 °C under the typical precipitation conditions were aged for a certain period ranging from 1 to 24 h. The samples were then filtrated and thoroughly washed with distilled water and ethanol. The resultant solids were dried at 50 °C overnight under vacuum. Characterization. The X-ray powder diffraction (XRD) patterns were recorded on a Rigaku D/MAX 2500 X-ray diffractometer using a Cu KR radiation source operated at 40 kV and 250 mA. The mean crystallite sizes of Co3O4 were calculated from the Scherrer equation using the most intensive (311) peak, where the particle shape factor was taken as 0.9.30 The transmission electron microscopy (TEM) images were recorded on a Philips Tecanai GP2P20 microscope. The specimens were prepared by ultrasonically suspending the sample in ethanol, and a drop of the solution was placed on the carbon-enhanced copper grids and dried in air. The Fourier transformed infrared (FTIR) spectra were collected on a Bruker Vector 22 spectrometer using KBr pellets. The pellet typically contained 5 wt % samples in KBr. The thermal gravity and differential thermal analysis (TGDTA) were recorded on a Pyris Diamond of Perkin-Elmer thermogravimetric analyzer. The samples were heated from room temperature to 800 °C at a rate of 10 °C/min with flowing air (40 mL/min). The N2 adsorption-desorption isotherm was recorded at -196 °C using a Micrometics ASAP 2000 instrument. Before the measurement, the sample was degassed at 150 °C for 5 h.

Figure 1. TEM images of the Co-80 (a and b) and Co-160 (c and d) precursors.

The specific surface area was calculated by multipoint Braunauer-Emmett-Teller (BET) analysis of the nitrogen adsorption isotherm. Catalytic Evolution. CO oxidation reaction was conducted with a fixed-bed quartz tubular reactor at atmospheric pressure. 200 mg samples were loaded and pretreated with a 20% O2/He mixture (50 mL/min) at 450 °C for 1 h. The catalyst sample was then cooled to -77 °C by putting the reactor into a dry ice trap. The feed gas of 1% CO/2.5% O2/He (50 mL/min) was introduced through a mass-flow controller. Selective oxidation of CO in H2-rich gas (the so-called PROX reaction) was also carried out with the same reactor system at atmospheric pressure. After the pretreatment as mentioned above, the catalyst sample (200 mg) was cooled to the desired reaction temperature and the feed gases of 0.5%CO/1%O2/60%H2/He were introduced with a flow rate of 50 mL/min. Effluents from the reactor were analyzed by the online gas chromatograph (HP 6890) equipped with a TCD and a flame ionization detector (FID). The conversion of CO was calculated based on the amounts of CO consumption and CO2 formation. To determine the concentration of carbon dioxide produced quantitatively, a nickel catalyst converter was placed before the FID and used for converting CO2 into methane. Results and Discussion Co3O4 Nanorods. Figure 1 shows the TEM images of the Co-T precursors. When the precipitation was conducted at 80 °C, short nanorods with a diameter of about 10 nm and a length of 50-100 nm were produced. With elevating the temperature to 160 °C, the diameter of the nanorods maintained at about 10 nm, but the length enlarged to 200-300 nm. Figure 2 shows the XRD patterns of the Co-T samples. The diffractions representing cobalt hydroxycarbonate are in good accordance with previous reports.13,20 The Co-160 sample has stronger (020), (221), and (412) diffractions but weaker (040), (231), (340), and (450) diffractions compared to those of the Co-80 sample. Figure 3 shows the FTIR spectra of the Co-T precursors. The broadband at about 3500 cm-1 is attributed to the stretching vibration of the O-H bond, υ(OH).20,31 The bands at about 1074, 835, 736, and 687 cm-1 are assigned to the stretching vibrations of υ(CdO), δ(CO3), δ(OCO), and F(OCO)

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Figure 2. XRD patterns of the Co-80 and Co-160 precursors.

Figure 4. TG-DTG profiles of the Co-80 and Co-160 precursors.

Figure 5. XRD patterns of the Co3O4-80 and Co3O4-160 samples. Figure 3. FTIR spectra of the Co-80 and Co-160 precursors.

in the carbonate anion, respectively.20,32,33 The band at about 942 cm-1 can be ascribed to the δ(Co-OH) bending vibration, whereas that at about 518 cm-1 represents the Fw(CoOH) vibration.20,32 This indicates that hydroxyl and carbonate anions are the intercalating species in the Co-T precursors. The major difference in the FTIR spectra becomes obvious in the range of 1000-2000 cm-1. The bands at 1524 and 1419 cm-1 of the Co-80 precursor correspond to the stretching vibrations of υ(OCO2) and υ(CO3) in the carbonate anion.20,32 Whereas the asymmetric stretching υas(COO-) (1596 cm-1) and the symmetric stretching υs(COO-) (1392 cm-1) bands in the Co-160 precursor with a wavenumber separation of 204 cm-1 confirm the presence of acetate anion in the structure, probably acting as a unidentate ligand toward cobalt cations.34-36 Therefore, the Co-80 precursor is cobalt hydroxycarbonate, but the Co-160 precursor is acetate-containing cobalt hydroxycarbonate. Figure 4 illustrates the thermal decomposition of the Co-T samples. The mass loss at about 100 °C is attributed to the elimination of molecular water. The Co-80 sample has two mass losses at about 221 and 246 °C, indicating the sequential dehydroxylation and decomposition of carbonate anions. For the Co-160 sample, the single mass loss at 232 °C represents

the simultaneous dehydroxylation and decomposition/combustion of carbonate and acetate anions, releasing water and carbon dioxide. No further mass loss has been detected above 400 °C, and the precursors are converted to cobalt oxides. Figure 5 shows the XRD patterns of the Co3O4-T samples obtained by calcination of the Co-T precursors at 450 °C. Apparently, typical diffractions of spinel crystalline structure of Co3O4 (JCPDS: 42-1467) were observed in both cases. The crystalline size of Co3O4 is 16 nm in the Co3O4-80 sample with a surface area of 62 m2/g, whereas it decreases to 12 nm in the Co3O4-160 sample having a surface area of 163 m2/g. Figure 6 shows the TEM images of the Co3O4-T samples. The Co3O480 sample contains irregular spherical particles with size of 10-30 nm which connected disorderly. The Co3O4-160 sample shows nanorod-shape with a diameter of about 10 nm and a length of 200-300 nm. Notably, the Co3O4-160 sample maintained the nanorod-shape structure of the precursor, showing the high rigidity and thermal stability of the acetatecontaining cobalt hydroxycarbonate. This result is quite different from the general observation that calcination of cobalt hydroxycarbonate nanorods often yields Co3O4 spherical nanoparticles or porous Co3O4 nanorods, instead of Co3O4 nanorods, due to the removal of hydroxyl and carbonate species.4,13,20-22 Morphology Evolution. The chemical composition and shape evolution of the Co-160 nanorods were then investigated

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Figure 6. TEM images of the Co3O4-80 (a-c) and Co3O4-160 (d-f) samples.

Figure 8. XRD patterns of the precipitates obtained in water (a) and with the addition of aqueous Na2CO3 solution for 10 min (b), 30 min (c), 45 min (d), 90 min (e), and 3 h (f).

Figure 7. TEM images of the precipitates obtained in water (a) and with the addition of aqueous Na2CO3 solution for 10 min (b), 30 min (c), 45 min (d), 90 min (e), and 3 h (f).

intensively. When the addition of sodium carbonate was conducted for 10 min, the precipitate had irregular sheet-shape (Figure 7b). The low-angle reflection at 13.146 Å is assigned to the (003) diffraction of the hydrotalcite-like structure, and the diffractions at 6.564 and 4.360 Å represent the (006) and (009) lines (Figure 8b). The d values of these diffractions, d003 ≈ 2d006 ≈ 3d009, also confirm the hydrotalcite-like structure.33,37 FTIR profile of this sample further validated the XRD observation. As shown in Figure 9b, the broadband at 3500 cm-1 represents the stretching vibration of the O-H group and the

C-H stretching vibrations appear at 2944 (asymmetric) and 2869 cm-1 (symmetric).38,39 The bands at 1580 and 1400 cm-1 with a wavenumber separation of 180 cm-1 corresponds to the asymmetric stretching υas(COO-) and symmetric stretching υs(COO-) vibrations of unidentate acetate species.35,36 The characteristic bands of 1091 and 1052 cm-1 are attributed to a neutrally adsorbed ethylene glycol on the surface without obvious shift in wavenumbers, compared to pure ethylene glycol.27,35 The bands at 1338 and 1024 cm-1 are assigned to the stretching vibrations of δ(CH3) and Fr(CH3), respectively.27 The band at 663 cm-1 is attributed to Co-O stretching vibration.33 Therefore, this initial precursor should be cobalt hydroxide acetate with a layered structure. For comparison, hydrolysis of cobalt acetate in ethylene glycol was conducted for 10 min under the same conditions with the addition of water, instead of sodium carbonate. Layered cobalt hydroxide acetate was obtained with almost the same chemical composition and structure (Figures 7-9a). This clearly demonstrates that hydrolysis of cobalt acetate in ethylene glycol is the major reaction during the initial stage of precipitation, instead of carbonate precipitation. With the gradual addition of sodium carbonate from 0.5 to 3 h, the irregular sheet was converted to rod-like shape (Figure 7c-f); the (003), (006), and (009) diffractions

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Figure 10. TEM images of the samples obtained by aging the typical precipitate for 6 h (a and b), 12 h (c and d), and 24 h (e and f).

Figure 9. FTIR spectra of the precipitates obtained in water (a) and with the addition of aqueous Na2CO3 solution for 10 min (b), 30 min (c), 45 min (d), 90 min (e), and 3 h (f).

representing the hydrotalcite-like structure gradually disappeared whereas the typical diffractions of cobalt hydroxycarbonate appeared. The representative bands of acetate anions at about 1600 and 1400 cm-1 distorted gradually and the wavenumber separation increased to more than 200 cm-1, indicating the releasing of acetate anion from the structure. Meanwhile, the bands representing carbonate anion at 1074 and 835 cm-1 enhanced significantly, confirming that the carbonate anions intercalated into the interlayer structure, replacing the acetate anions. When sodium carbonate was fully added at 3 h, the precipitate showed a nanorod-shape with a diameter of about 10 nm and a length of 200-300 nm where acetate and carbonate anions were presented in the structure. That is, acetate-containing cobalt hydroxycarbonate nanorods. Aging Effect. The typical acetate-containing cobalt hydroxycarbonate nanorods were obtained by precipitation for 3 h and further aging for 1 h at 160 °C. The morphology evolution and chemical composition of this precursor during the course of aging were examined. As shown in Figure 10, when the aging time was extended to 6 h, the sample had a rod-like structure with a diameter of about 10 nm and a length of 200-300 nm, similar to the Co-160 precursor. But the (100) reflection at about 10 Å (Figure 2) disappeared (Figure 11a) due to the collapse of the layers.31,40 The bands at 1063 and 993 cm-1 corresponding to υas(CO) and υs(CO) stretching vibrations enhanced considerably (Figure 12a), indicative of an increased amount of ethylene glycol in the structure.14,27,38 When the aging lasted for 12 h, the sample is composed of sheets and crimped nanorods

Figure 11. XRD patterns of the samples obtained by aging the typical precipitate for 6 h (a), 12 h (b), and 24 h (c).

(Figure 10c,d). Two sawtooth-shaped diffractions at 2θ ) 33° and 59° intensified largely (Figure 11b), evidencing the turbostratic disordered structure that is commonly observed in R-hydroxides.31 Meanwhile, the diffractions of cobalt hydroxide carbonate further weakened and almost disappeared. In the FTIR spectrum (Figure 12b), bending vibration of interlayer water molecule appeared with the typical band at 1638 cm-1.41 The vibrations of carbonate and acetate anions (1600-1400 cm-1) decreased significantly but the vibrations of ethylene glycol (1070 and 997 cm-1) increased considerably, indicating the incorporation of ethylene glycol into the structure. Further extending the aging time to 24 h, the sample contained aggregates of thin crimped sheets without definite shape (Figure

Cobalt Hydroxycarbonate Nanorods

Figure 12. FTIR spectra of the samples obtained by aging the typical precipitate for 6 h (a), 12 h (b), and 24 h (c).

10e,f). In the XRD pattern (Figure 11c), only two sawtoothshaped diffractions were detected, the low-angle reflection of the layered structure completely vanished. The interlayer water molecule with a bending vibration at 1641 cm-1 was enhanced (Figure 12c).41 The vibration bands of carbonate and acetate anions completely disappeared and there were only stretching vibrations of ethylene glycol. Apparently, acetate and carbonate anions are gradually replaced by ethylene glycol during the aging process and this anion-exchange phenomenon in double layered hydroxides has been well decumented.38,42,43 For example, ethylene glycol gradually incorporated into the structure of Zn-Al-CO3 double layered hydroxide and finally replaced the carbonate anions.38 Growth Mechanism. Based on the results described above, it seems that the formation of the acetate-containing cobalt hydroxycarbonate nanorods follows a multistage process depending on the role of ethylene glycol. When cobalt acetate is dissolved into ethylene glycol at 160 °C, cobalt alkoxyacetate is formed.27,35 The formation of this intermediate was proved and structurally characterized by dissolving zinc acetate in ethylene glycol and diethylene glycol.44,45 Here, ethylene glycol acts as a simple solvent and a coordinating agent to cationic cobalt species. With the addition of carbonate sodium, cobalt hydroxide acetate is formed initially by hydrogenolysis of the cobalt alkoxyacetate intermediate. As proved by XRD (Figure 8) and FTIR (Figure 9) measurements, hydrolysis of cobalt alkoxyacetate to cobalt hydroxide acetate occurs at the early 10 min, instead of carbonate precipitation. The factor governing the formation of cobalt hydroxide acetate is the hydrolysis ratio, defined as h ) nH2O/nCo2+, and the value should be more than 26 in the case of cobalt acetate dissolving in diethylene glycol at 160 °C.35 Under the current conditions, after adding sodium carbonate for 10 min, the hydrogenolysis ratio is as high as 60, apparently favoring the formation cobalt hydroxide acetate. The amorphous structure (Figure 7b) is formed during this fast nucleation. When the synthetic solution is turned to basic conditions with the gradual addition of Na2CO3 aqueous solution, however, carbonate anions are then gradually intercalated into the interlayers by replacing

J. Phys. Chem. C, Vol. 114, No. 5, 2010 2121 acetate and hydroxyl anions due to the strong affinity to the interlayers.46,47 This anion-exchange process is kinetically driven and induces a dissolution-recrystallization of the initially formed phase, producing novel crystalline phases. Because the addition rate of sodium carbonate is very slow, the growth of crystals proceeds very slowly, favoring an anisotropic growth owing to the structure-directing effect of the carbonate anions. Meanwhile, ethylene glycol serves as a rate-controlling agent by separating the nucleation of cobalt hydroxide acetate and the crystal growth of rod-like cobalt hydroxide carbonate. Hence, the acetatecontaining cobalt hydroxycarbonate nanorods are formed through the fast nucleation and the slow crystalline growth. When the precipitate is further aged in the mother liquid, ethylene glycol incorporates into the layered structure to replace carbonate and acetate anions. As confirmed by the FTIR measurements (Figure 12), ethylene glycol gradually replaces the carbonate and acetate anions in the structure and finally it presents as the sole anions. Together with this, the interlayer structure is collapsed and the nanorod-shape turns into large aggregates of thin crimped sheets. The inherent shape transformation from the Co-160 precursor to the Co3O4-160 nanorod is quite different from that of the Co-80 precursor, where the nanorod-shape precursor was transferred to nanoparticle (Figure 6). This verifies the thermal stability of the Co-160 nanorods because of the presence of ethylene glycol and acetate anions at a proper ratio in the interlayers. Ethylene glycol strongly affects the manner that the crystallites aggregate and promotes the generation of a network of small interconnected pores.23-27 On the other hand, the peculiar synthesis route results in large amounts of acetate anions in the Co-160 precursor, whereas the Co-80 precursor only contains carbonate and hydroxyl anions. Carbonate anions intercalate into the layers only as free species,43,47,48 but unidentate acetate anions in the Co-160 precursor bind to cobalt atom through a single oxygen atom, forming intermolecular H-bonding with nearby hydroxyl groups. This positive role of H-bonding in the formation of self-assembled Co3O4 nanotubes has been recently demonstrated.49 Upon calcination, cobalt complexes with a strong intermolecular hydrogen bonding formed one-dimensional Co3O4 nanotubes, but short Co3O4 nanotubes with defects and broken spheres were produced when cobalt complexes with a weak or no intermolecular H-bonding were used. Therefore, acetate anions acting as bridges enhance the connection of adjacent layers through intermolecular interaction during calcination, maintaining the rod-like structure of the Co-160 precursor. Catalytic Performance. Figure 13 compares the reaction performance of the Co3O4 nanoparticles and nanorods for lowtemperature oxidation of CO. The nanoparticles gave initial CO conversion of 30% and then rapidly stabilized at about 10% with time on-stream. Interestingly, the nanorods showed 100% CO conversion at the initial 6 h and then only slightly decreased with time on-stream. Even after reaction for 12 h, the conversion of CO was still maintained at 80%, clearly evidencing that the nanorods are more active and stable than the nanoparticles. Figure 14 shows the temperature dependence of CO oxidation in H2-rich gas over the Co3O4 nanoparticles and nanorods. Apparently, the nanorods exhibited remarkably higher CO and O2 conversions than the nanoparticles in the temperature range tested. At 120 °C, the conversion of CO was 99% over the nanorods, whereas it was only 85% over the nanoparticles; the corresponding O2 conversions were 50% and 32%, respectively. The conversion of CO reached 100% at 140 °C over the nanorods, but it was 96% over the nanoparticles. Meanwhile,

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Figure 13. CO oxidation over the Co3O4 nanoparticles and nanorods. Reaction conditions: 1.0 vol % CO/2.5 vol % O2/He; 15 000 mL/(g h); -77 °C.

Figure 14. CO oxidation in H2-rich gas over the Co3O4 nanoparticles and nanorods. Reaction conditions: 0.5 vol % CO/1.0 vol % O2/60 vol % H2/He; 15 000 mL/(g h).

the conversion of O2 over the nanorods approached 83% owing to the oxidation of hydrogen in the feed stream, whereas it was only 50% on the nanoparticles. These results clearly demonstrate that the nanorods are more active than the nanoparticles. As we have recently reported,28 the Co3O4 nanorods tend to preferentially expose the {110} planes that are rich in Co3+ species acting as active sites for CO oxidation. However, the Co3O4 nanoparticles usually expose the less reactive {001} and {111} planes, which contain only inactive Co2+ sites, resulting in a lower activity. Conclusions Calcination of acetate-containing cobalt hydroxycarbonate nanorods, obtained by precipitation of cobalt acetate with sodium carbonate in ethylene glycol, yielded nanorod-shaped Co3O4 materials. During the precipitation process, amorphous cobalt

hydroxide acetate was initially formed, followed by the gradual intercalation of carbonate anions into the interlayers, replacing the acetate and hydroxyl anions. This anion-exchange process induced a dissolution–recrystallization in which carbonate anions acted as a structure-directing agent and ethylene glycol serves as a rate-controlling agent, producing rod-like acetate-containing cobalt hydroxide carbonate. During the aging process, ethylene glycol gradually incorporated into the structure to replace carbonate and acetate anions and finally it presented as the sole anion. Because of this, the nanorod-shape turned into large aggregates of thin crimped sheets. Co3O4 nanorods with a diameter of about 10 nm and a length of 200-300 nm were obtained by calcination of the acetatecontaining cobalt hydroxycarbonate nanorods. This spontaneous shape transformation from the precursor to the oxide is attributed to the presence of ethylene glycol and acetate anions in the

Cobalt Hydroxycarbonate Nanorods nanorod-shaped precursor. The Co3O4 nanorods showed a much better catalytic activity for CO oxidation than the spherical Co3O4 nanoparticles, confirming the concept of morphologydependent nanocatalysis. References and Notes (1) Li, W. Y.; Xu, L. N.; Chen, J. AdV. Funct. Mater. 2005, 15, 851. (2) Zhang, H.; Wu, J. B.; Zhai, C. X.; Ma, X. Y.; Du, N.; Tu, J. P.; Yang, D. R. Nanotechnology 2008, 19, 035711. (3) Barakat, N. A. M.; Khil, M. S.; Sheikh, F. A.; Kim, H. Y. J. Phys. Chem. C 2008, 112, 12225. (4) Wang, G. X.; Shen, X. P.; Horvat, J.; Wang, B.; Liu, H.; Wexler, D.; Yao, J. J. Phys. Chem. C 2009, 113, 4357. (5) Hu, L. H.; Peng, Q.; Li, Y. D. J. Am. Chem. Soc. 2008, 130, 16136. (6) Liu, Y. K.; Wang, G. H.; Xu, C. K.; Wang, W. Z. Chem. Commun. 2002, 1486. (7) Salabas, E. L.; Rumplecker, A.; Kleitz, F.; Radu, F.; Schu¨th, F. Nano Lett. 2006, 6, 2977. (8) Lou, X. W.; Deng, D.; Lee, J. Y.; Feng, J.; Archer, L. A. AdV. Mater. 2008, 20, 258. (9) Yu, T.; Zhu, Y. W.; Xu, X. J.; Shen, Z. X.; Chen, P.; Lim, C. T.; Thong, J. T. L.; Sow, C. H. AdV. Mater. 2005, 17, 1595. (10) Hou, Y. L.; Kondoh, H.; Shimojo, M.; Kogure, T.; Ohta, T. J. Phys. Chem. B 2005, 109, 19094. (11) Yang, L. X.; Zhu, Y. J.; Li, L.; Zhang, L.; Tong, H.; Wang, W. W.; Cheng, G. F.; Zhu, J. F. Eur. J. Inorg. Chem. 2006, 4787. (12) Ding, Y. S.; Xu, L. P.; Chen, C. H.; Shen, X. F.; Suib, S. L. J. Phys. Chem. C 2008, 112, 8177. (13) Li, B. X.; Xie, Y.; Wu, C. Z.; Li, Z. Q.; Zhang, J. Mater. Chem. Phys. 2006, 99, 479. (14) Cao, A. M.; Hu, J. S.; Liang, H. P.; Song, W. G.; Wan, L. J.; He, X. L.; Gao, X. G.; Xia, S. H. J. Phys. Chem. B 2006, 110, 15858. (15) Cong, H. P.; Yu, S. H. Cryst. Growth Des. 2009, 9, 210. (16) Chen, Y. C.; Hu, L.; Wang, M.; Min, Y. L.; Zhang, Y. G. Colloid Surface. A 2009, 336, 64. (17) Meyn, M.; Beneke, K.; Lagaly, G. Inorg. Chem. 1993, 32, 1209. (18) Lian, S. Y.; Wang, E. B.; Gao, L.; Xu, L. Mater. Lett. 2006, 61, 3893. (19) Ke, X. F.; Cao, J. M.; Zheng, M. B.; Chen, Y. P.; Liu, J. S.; Ji, G. B. Mater. Lett. 2007, 61, 3901. (20) Xu, R.; Zeng, H. C. J. Phys. Chem. B 2003, 107, 12643. (21) Hosono, E.; Fujihara, S.; Honma, I.; Zhou, H. S. J. Mater. Chem. 2005, 15, 1938. (22) Wang, Z. H.; Chen, X. Y.; Zhang, M.; Qian, Y. T. Solid State Sci. 2005, 7, 13.

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