Synthesis of Mesoporous Metal Oxide by the Thermal Decomposition

Mar 12, 2013 - ... of controlled oxidation states with identical textural properties. Jae Hwa Lee , Young Jin Sa , Tae Kyung Kim , Hoi Ri Moon , Sang ...
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Synthesis of Mesoporous Metal Oxide by the Thermal Decomposition of Oxalate Precursor Limin Guo,*,†,‡ Hiroyuki Arafune,† and Norio Teramae*,† †

Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan International Institute for Carbon-Neutral Energy Research, Kyushu University, 744 Motooka, Nishi-Ku Fukuoka 819-0395, Japan

‡

S Supporting Information *

ABSTRACT: A synthesis method was newly developed to prepare mesoporous transition metal oxides by thermal decomposition of transition metal oxalates, and the method was advantageous in its versatility, low cost, and environmental friendliness. Various mesoporous transition metal oxides were successfully synthesized by the newly developed method, such as magnetic γ-Fe2O3, CoFe2O4, and NiFe2O4, MnxOy, Co3O4, and NiO. Morphology, structure, and magnetic property of the synthesized mesoporous transition metal oxides were characterized by XRD, TG-DTA, SEM, TEM, quantum design SQUID, and N2 sorption techniques. From the dependency of the heating rate, calcination time, and calcination temperature on the metal oxide structures, it was revealed that the calcination temperature was the major factor to determine the final mesoporous structure of the metal oxides. The mesoporous structures were well constructed by their corresponding metal oxide nanoparticles resulting from oxalate thermal decomposition.

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INTRODUCTION Owing to high surface area, tunable pore size, and defined pore structure, mesoporous materials have attracted extensive attention and stimulated vast application research areas over the past two decades.1−10 Particularly, mesoporous transitionmetal oxide materials have great potential for applications in catalysis, photocatalysis, sensors, and electrode materials because of their characteristic catalytic, optical, and electronic properties.11−16 Toward various applications, mesoporous metal oxide with high crystallinity is a prerequisite to get excellent performance.16 For example, when mesoporous titanium dioxide is applied to photocatalysis, the amorphous regions are known to work as trap sites for the recombination of photoexcited electrons and holes, and such regions limit the photocatalytic performance.15,16 In addition, crystalline metal oxides also have high thermal and mechanical stability.13,16 Sometimes, the preparation of magnetic mesoporous metal oxides are extremely favorable for the separation and recycling processes after utilization.17,18 Up to now, the mesoporous metal oxide synthesis still focuses on the soft or hard templating method. For the soft templating, the as-synthesized mesoporous metal oxides usually have low stability and crystallinity.15,16,19−22 Some mesoporous metal oxides lose their pore structure once the removal of the soft template.15 The others cannot withstand high temperature treatment for crystal structure transformation, and subsequently their mesostructures also collapse.21,22 Some researchers12,16 have adopted a synthesis method similar to hard-templating in order to strengthen the mesostructure framework, which offers the © XXXX American Chemical Society

amorphous mesoporous metal oxides a tolerance to high temperature treatment for crystal structure transformation. For example, Wiesner’s group16 has utilized block copolymers with an sp2-hybridized carbon-containing hydrophobic block as a mesostructure-directing agent which can be converted into carbon under an inert atomsphere. This in-situ formed carbon can serve as a rigid mesostructure support, and it makes the mesopore of metal oxides intact while crystallizing at high temperature. For the hard templating,13,23−25 inorganic precursors are filled into preprepared ordered mesoporous silica or carbon templates that are needed to be removed during the final step. The hard template offers rigid mesostructured frameworks to withstand high temperature treatment, and highly crystalline materials without mesostructure collapse can finally be obtained. Many kinds of metal oxides with high crystallinity have been obtained by hard templating.13 However, this synthetic method is tedious and often involves the use of harmful hydrofluoric acid or sodium hydroxide during the removal of the hard template. Therefore, development of an easy, low cost, and environmental benign method for the synthesis of mesoporous transition-metal oxides with high crystallinity is a great benefit to application of such materials for practical research areas. Recently, Shi’s group has proposed a template-free strategy to prepare mesoporous metal oxides from metal sulfates or Received: January 24, 2013 Revised: March 10, 2013

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metal oxalates using thermal decomposition.26−29 Many mesoporous metal oxides with high surface area and well contolled pore structure, such as Al2O3,26 ZrO2,26 TiO2,26 Mn2O3,27 NiO,27 α-Fe2O3,28 and NiFe2O4,29 have already been successfully synthesized. However, the research on the template-free strategy for the synthesis of mesoporous metal oxides is still very insufficient compared to the well-known soft templating and hard templating synthesis method. Some existing deficiencies of the template-free strategy need to be further conquered. For example, the preparation of a metal oxalate precursor should be elaborately controlled, and surfactant addition is essentially necessary for morphology control,27−29 which indicates the process is somewhat timeconsuming and not favorable for large-scale production. In addition, the surface area of as-prepared mesoporous metal oxides would dramatically decrease when the crystallinity is continuously improved by increasing the crystallizing temperature.26−29 For magnetic mesoporous metal oxides, the saturation magnetization value is still low, indicating existence of some amorphous mesoporous metal oxides.29 In this paper, we propose an extremely simplified method for metal oxalate preparation. The proposed method does not need to use any surfactant, and simple pouring of one reaction solution into the other one is enough for metal oxalate precipitation. Thermal decompositions were further carried out to obtain various kinds of mesoporous metal oxides with good crystallinity, such as magnetic γ-Fe2O3, CoFe2O4, and NiFe2O4, Co3O4, and NiO. The effect of calcination temperature, time, and heating rate on the structures of final mesoporous metal oxides was also examined.

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varied as 1, 3, and 5 h, and the respectively obtained three samples were denoted as Mn-1, Mn-2, and Mn-3 for simplicity. Fixing the calcination temperature at 350 °C and time length of 5 h, the heating rate was changed to 5 °C/min and the obtained sample was denoted as Mn-4. Fixing the calcination time length as 1 h and heating rate of 1 °C/min, the calcination temperatures were varied as 400 and 500 °C. The respectively obtained samples were denoted as Mn-5 and Mn-6. Characterization. X-ray diffraction (XRD) patterns of metal oxalates were recorded with a MAC Science/Bruker M21X diffractometer using Cu Kα radiation (45 kV and 250 mA). XRD patterns of metal oxides were recorded with a Bruker D2 phase diffractometer equipped with a position-sensitive detector using Cu Kα radiation (40 kV and 20 mA). The scanning rate was 3°/min. Scanning electron microscopic (SEM) images were obtained with a Hitachi-4300 field emission electron microscope (FE-SEM). SEM observations were done after depositing of a thin Pt/Pd layer on the samples using a magnetron ion-sputter apparatus. Transmission electron microscopic (TEM) images were taken with a Topcon EM002B with an accelerating voltage of 200 kV. Samples for TEM observations were prepared by the following procedures: the obtained mesoporous metal oxide powder was dispersed in ethanol by ultrasonication and then mounted on a Cu grid. During the TEM observation, only porous structure of the thin edge area of sample particles could be clearly observed. Because the diameter of sample particles was too large for the electron beam to penetrate through. The TG-DTA results were obtained with a Rigaku Thermo Plus TG8120 in air, and the heating rate was 10 °C/min. Nitrogen adsorption− desorption measurements were carried out at −196 °C with a BELSORP-Minill (Bel Japan) instrument. Before measurements, samples were degassed at 100 °C for 10 h. The specific surface area and the pore size distribution were calculated according to the Brunauer−Emmett−Teller (BET) and Barrett−Joyner−Halenda (BJH) model, respectively.

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RESULTS AND DISCUSSION Thermal decomposition of inorganic salts (such as nitrates, sulfates, hydroxides, carboxylates, acetates, formates, and oxalates) have been extensively utilized to obtain metal oxide powders.30−35 During the thermal decomposition process, generation of gases (such as CO2, SO2, NO2, H2O, and so on) occurs accompanied by remarkable shrinkage in the framework of the materials, and finally metal oxides are obtained if the process is carried out in an air/O2 atmosphere.27 Therefore, it is possible to obtain porous materials by avoiding extensive materials shrinkage during the decomposition of metal salts precursors.25−29 Oxalate hydrates will be a very good choice as precursors for porous oxides preparation due to their large quantity mass loss during thermal decomposition (usually more than 50 wt %36 ), easy synthesis, low cost, and low decomposition temperature in air.27 According to the crystallographic structure, the dihydrated oxalates MC2O4·2H2O, where M stands for a divalent metal ion such as Mg2+, Mn2+, Fe2+, Co2+, and Ni2+, can be classified to the magnesium series oxalates.29 In this paper, all as-prepared metal oxalate hydrates belong to this family. When coexiting two metal cations such as Co 2+ and Fe 2+ (or Ni 2+ and Fe 2+ ) react with C 2 O 4 2− simultaneously, a single-phase crystalline hydrate is formed instead of mixture formation of each metal oxalate as confirmed by XRD data. Mesoporous Magnetic Metal Oxides. The as-prepared FeC2O4·2H2O, CoFe2(C2O4)3·6H2O, and NiFe2(C2O4)3·6H2O were well crystallized, which was confirmed from the XRD results. The XRD patterns of CoFe2(C2O4)3·6H2O and NiFe2(C2O4)3·6H2O are very similar to that of FeC2O4·2H2O (Figure S2), which is monoclinic and described with the space group C2/c.29 The SEM images of FeC2O4·2H2O (Figure 1a),

EXPERIMENTAL SECTION

Synthesis of metal oxalate precursors: Chemically pure grade FeSO4·7H2O, Co(NO3)2·6H2O, NiCl2·4H2O, MnCl2·4H2O, and sodium oxalate were used as starting materials without further purification. The experimental details for metal oxalate preparation are as follows. 100 mL of saturated sodium oxalate aqueous solution was added into a flask and stirred at room temperature. The proper amount of metal salt (7.2 g of FeSO4·7H2O for FeC2O4·2H2O, 6.2 g of FeSO4·7H2O and 2.5 g of Co(NO3)2·6H2O (molar ratio is 2:1) for CoFe2(C2O4)3·6H2O, 6.2 g of FeSO4·7H2O and 2.1 g of NiCl2·6H2O (molar ratio is 2:1) for NiFe2(C2O4)3·6H2O) was dissovled into 20 mL of water and then poured into the above-described sodium oxalate aqueous solution under stirring. Fifteen minutes later, the precipitates were collected by filtration and subsequently washed by distilled water and ethanol. Thereafter, the powder was dried at 60 °C overnight. For the preparation of MnC 2 O 4 ·2H 2 O, CoC 2 O 4 ·2H 2 O, and NiC2O4·2H2O, the procedures were similar to the above-described procedure. However, the precipitation process was carried out at 80 °C. The adding amounts of MnCl2·4H2O, Co(NO3)2·6H2O, and NiCl2·6H2O were 5.2, 7.6, and 6.2 g, respectively. Mesoporous transition metal oxides were prepared from corresponding metal oxalates using thermal decomposition. The mesoporous metal oxides were obtained by calcining the metal oxalates. The calcination temperatures for different metal oxalates were determined based on the TG-DTA results (Supporting Information Figure S1) of metal oxalates. The detailed calcination procedures are given below. For the prepartion of mesoporous γ-Fe2O3, CoFe2O4, NiFe2O4, and Co3O4, the corresponding metal oxalates were calcined at 300 °C for 3 h. The heating rate was 1 °C/min. For NiO, the corresponding metal oxalate was calcined at 350 °C for 3 h. The heating rate was 1 °C/min. For MnxOy, the calcination temperature, time, and heating rate were varied in order to examine thermal decomposition conditions. Fixing the calcination temperature at 350 °C and heating rate of 1 °C/min, the calcination time length was B

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Fe2(C2O4)3·6H2O, and NiFe2(C2O4)3·6H2O, respectively, at 300 °C for 3 h. Here, the temperature for the thermal decomposition was decided according to the TG-DTA results (Figure S1a−c). The three oxalate precursors have completely decomposed, and no obvious changes in sample mass and DTA curves were found above 300 °C. The SEM images of assynthesized γ-Fe2O3 (Figure 1b), CoFe2O4 (Figure 1d), and NiFe2O4 (Figure 1f) demonstrate that they overall inherit the morphology of their corresponding oxalate precursors. However, some obvious shrinkage can be observed, especially for the sample of γ-Fe2O3 (Figure 1b) compared with its oxalate precursor (Figure 1a). In addition, some cracks can also be observed on the surface of γ-Fe2O3 (Figure 1b), CoFe2O4 (Figure 1d), and NiFe2O4 (Figure 1f). The TEM image of γFe2O3 (Figure 2a) shows wormlike mesopores among these γFe2O3 nanocrystallites. The corresponding XRD pattern of γFe2O3 (Figure 2b) shows a series of diffraction peaks and can be ascribed to cubic γ-Fe2O3 (JCPDS No. 39-1346). The crystal size of γ-Fe2O3 was also estimated by using the Scherrer equation on the (311) peak of the XRD pattern, and the value was 32.4 nm. Here, the final iron oxide phase should be γFe2O3 not Fe3O4 due to the dark red not black color of obtained powder.37−39 In addition, the lifted background signal of the XRD pattern (Figure 2b) could be acribed to the glass sample holder used in the measurements and has no relationship with characteristics of the sample itself. N2 adsorption−desorption isotherms and the BJH pore size distribution of as-synthesized γ-Fe2O3 are shown in Figure 2c. The porous nature is proved by the irreversible type IV adsorption−desorption isotherms with a H1 hysteresis loop. The specific surface area of γ-Fe2O3 is 28.6 m2/g and the pore size centers at 4.8 nm. γ-Fe2O3 is well-known for its magnetic properties.37,38 So the magnetic property of γ-Fe2O3 was also investigated. Figure 2d shows the room-temperature magnetic hysteresis loop. The saturation magnetization of γ-Fe2O3 was 70.0 emu/g at a magnetic field of 7 T, implying a very strong magnetic response to the magnetic field. The coercivity value of as-synthesized γ-Fe2O3 was around 110 Oe. Differently, the αFe2O3 phase was obtained in our previous report under the similar thermal decomposition procedure.28 In the report, FeC2O4·2H2O was elaboratedly prepared and sodium dioctylsulfosuccinate was also used as a surfactant to control the precipitation of FeC2O4·2H2O.28 Although the exact reason is not yet understood, the starting FeC2O4·2H2O precursors prepared by different processes may account for the formation of the final different metal oxide phases formation. Figure 3a shows the TEM image of CoFe2O4, and a wormlike mesopore can be clearly observed among these CoFe2O4 nanocrystallites. The corresponding XRD pattern of CoFe2O4 (Figure 3b) shows a series of diffraction peaks which can be ascribed to a cubic CoFe2O4 spinel phase (JCPDS No. 03-0864), indicating a well-crystallized state of CoFe2O4. The crystal size of CoFe2O4 estimated by using the Scherrer equation on the (311) peak was 23.7 nm. N2 adsorption−desorption isotherms and the BJH pore size distribution of as-synthesized CoFe2O4 are shown in Figure 3c. The porous nature is proved by the irreversible type IV adsorption−desorption isotherms with a H1 hysteresis loop, which is similar to that of γ-Fe2O3. The specific surface area of CoFe2O4 is 49.9 m2/g and the pore size centers around 4.0 nm. CoFe2O4 is also well-known for its magnetic properties.40,41 Figure 3d shows a room-temperature magnetic hysteresis loop of CoFe2O4. The saturation magnetization of CoFe2O4 was 59.8 emu/g at a magnetic field of 7 T,

Figure 1. SEM images of (a) FeC 2 O 4 ·2H 2 O, (c) CoFe2(C2O4)3·6H2O, (e) NiFe2(C2O4)3·6H2O, and (b) γ-Fe2O3, (d) CoFe2O4, and (f) NiFe2O4 after calcined at 300 °C for 3 h. Scale bar corresponds to 2 μm.

CoFe2(C2O4)3·6H2O (Figure 1c), and NiFe2(C2O4)3·6H2O (Figure 1e) show similar rectangular block morphology with a size of several micrometers, and they also show smooth surfaces and straight edges, which reflects their silimar crystallographic structures and growing nature. The thermal analysis results of these three oxalates (Figure S1a−c) demonstrated that mass loss occurred by two steps when increasing the heating temperature in a static air atomosphere. One was the dehydration process, which took place at low temperature, around 160−200 °C, and was an endothermic process. The other was the decomposition of oxalic groups and the oxidation of the precursors, which took place at high temperature, around 200−280 °C, and was an exothermic process. The appearance of the only two mass loss stages also confirmed the formation of a single-phase crystalline hydrate of CoFe2(C2O4)3·6H2O and NiFe 2 (C 2 O 4 ) 3 ·6H 2 O. The total mass losses for FeC2O4·2H2O, CoFe2(C2O4)3·6H2O, and NiFe2(C2O4)3·6H2O are around 57, 58, and 57 wt %, respectively. If the as-prepared precursors were the mixture of two separate oxalates, there would be more peaks in the DTA curves.29 Therefore, CoFe2(C2O4)3·6H2O and NiFe2(C2O4)3·6H2O have a single crystallized phase state, similar to FeC2O4·2H2O. The γ-Fe2O3, CoFe2O4, and NiFe2O4 were obtained by thermal decomposition of FeC2O4·2H2O, CoC

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Figure 2. (a) TEM image, (b) XRD pattern, (c) N2 adsorption and desorption isotherm and pore size distribution (inset), and (d) magnetization curve of as-synthesized γ-Fe2O3.

implying a very strong magnetic response to the magnetic field. The coercivity value of as-synthesized CoFe2O4 was around 600 Oe. Figure 4a shows the TEM image of NiFe2O4, and a wormlike mesopore can also be clearly observed among these NiFe2O4 nanocrystallites. The corresponding XRD pattern of NiFe2O4 (Figure 4b) shows a series of diffraction peaks which can be ascribed to a cubic NiFe2O4 spinel phase (JCPDS No. 10-0325) similar to the case of γ-Fe2O3 and CoFe2O4, which demonstrates the well-crystallized state of NiFe2O4. The crystal size of NiFe2O4 estimated by using the Scherrer equation on the (311) peak was 18.8 nm. N2 adsorption−desorption isotherms and the BJH pore size distribution of as-synthesized NiFe2O4 are shown in Figure 4c. The adsorption−desorption isotherms are very similar to that of CoFe2O4, which indicates they have similar porous characteristics. The specific surface area of NiFe2O4 is 59.8 m2/g, and the pore size centers around 3.6 nm. Figure 4d shows the room-temperature magnetic hysteresis loop of NiFe2O4. The saturation magnetization of NiFe2O4 was 50.2 emu/g at a magnetic field of 7 T, implying a very strong magnetic response to the magnetic field. The coercivity value of as-synthesized NiFe2O4 was around 170 Oe. The as-synthesized CoFe2O4 and NiFe2O4 have much higher surface area than γ-Fe2O3. According to TEM images (Figures 2a, 3a, and 4a), the nanocrystallite size of γ-Fe2O3 is much larger than those of CoFe2O4 and NiFe2O4, and the mesoporous structures of these metal oxides were constructed

by many nanocystallites. Accordingly, the larger size of nanocrystallites could be the cause of the smaller surface area of the porous structure formed by the assembly of these nanocrystallites. The further addition of Ni2+ and Co2+ to the FeC2O4·2H2O structure may restrict the nanocrystallite growth during thermal decomposition process. This kind of grain refinement method is very popular during the alloy preparation.42,43 Here, the surface area of as-synthesized metal oxides was much smaller compared with previous reports that used similar thermal decomposition.26−29 However, accounting the crystallinity and the saturated magnetization value, the mesoporous metal oxides prepared in the present study show a much higher surface area. Taking the magnetic metal oxide NiFe2O4 as an example, both the saturated magnetization value (32.1 emu/g) and surface area (27.7 m2/g) of reported NiFe2O429 are much lower than those values (50.2 emu/g and 59.8 m2/g) of NiFe2O4 synthesized here. Mesoporous Manganese Oxides. The as-prepared manganese oxalate is crystallized and contains two crystallization waters, which can be concluded by its XRD pattern (Figure S3). Figure 5 shows the SEM morphologies of asprepared manganese oxalate and decomposed manganese oxide (Mn-1). The well-crystallized oxalate precursors are shieldlike or polyaggregates and with dozens of micrometers size. This morphology and size are not obviously changed after thermal decomposition, and only some crackings can be observed on D

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Figure 3. (a) TEM image, (b) XRD pattern, (c) N2 adsorption and desorption isotherm and pore size distribution (inset), and (d) magnetization curve of as-synthesized CoFe2O4.

from that of Mn-1 (Figure 6c). Further TEM observations (SFigure S4) and N2 sorption measurements of samples Mn-2, Mn-3, and Mn-4 were carried out, and the results were similar to those obtained for Mn-1, which means that the heating time (up to 5 h) and rate (up to 5 °C/min) have no obvious influences on the final phase state and pore structure of manganese oxides. However, the XRD patterns of Mn-5 and Mn-6 have very obvious difference compared to those of Mn-1 to Mn-4 (Figure 6c), and several new diffraction peaks appear, which can be ascribed to bixbyite Mn2O3 (JCPDS No. 411442), which means some Mn3O4 was transformed into Mn2O3 as increasing the calcination temperature. During the transformation process from Mn3O4 to Mn2O3, a slight mass loss should appear, and the slight mass loss around 430 °C was indeed clearly recorded by TG-DTA analysis (Figure S1d). The TEM image of Mn-5 (Figure 6b) shows much larger nanocrystallite compared to Mn-1 (Figure 6a), although some mesopores still can be clearly observed. The surface area of Mn5 was 30.5 m2/g, and the value greatly decreased comparing with 75.7 m2/g of Mn-1. The crystallite of Mn-6 further obviously grew (Figure S5), and most of the mesopores among nanocrystallites also disappeared, and the surface area of Mn-6 also dramatically decreased to 12.5 m2/g. Based on the analysis of Mn-5 and Mn-6, the thermal decomposition temperature has obvious influences not only on the pore structure but also on the phase state. Herein, very different results from the one

the surface. Because of the high total mass loss up to 57%, obvious pore structure should form within the obtained manganese oxide particles. The TEM image of the manganese oxide Mn-1 (Figure 6) confirms the existence of homogeneous pore structure and shows wormlike mesopores. The XRD pattern of Mn-1 (in Figure 6c) shows some weak and broad diffraction peaks which can be ascribed to hausmannite Mn3O4 (JCPDS No. 24-0734). N2 adsorption−desorption isotherms and the BJH pore size distribution of sample Mn-1 are shown in Figure 6d. The porous nature is proved by the irreversible type IV adsorption−desorption isotherms with a H1 hysteresis loop. The specific surface area of sample Mn-1 is 75.7 m2/g and the pore size centers at 2.0 nm. The thermal decomposition conditions were further optimized to examine the crystallinity and pore structure changes of manganese oxides. Fixing the calcination temperature at 350 °C and heating rate at 1 °C/ min, the calcination time length was prolonged from 1 to 3 and 5 h. Then, the two samples were obtained, and they were denoted as Mn-2 and Mn-3, respectively. Fixing the calcination temperature at 350 °C and the time length for 5 h, the heating rate was increased to 5 °C/min and the sample Mn-4 was obtained. Fixing the calcination time length for 1 h and heating rate at 1 °C/min, the calcination temperature was increased from 350 to 400 or 500 °C. Then the two obtained samples were denoted as Mn-5 and Mn-6, respectively. The XRD patterns of Mn-2, Mn-3, and Mn-4 have no obvious differences E

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Figure 4. (a) TEM image, (b) XRD pattern, (c) N2 adsorption and desorption isotherm and pore size distribution (inset), and (d) magnetization curve of as-synthesized NiFe2O4.

Figure 5. SEM images for (a) MnC2O4·2H2O and (b) Mn-1 after calcined at 350 °C for 1 h. Scale bar corresponds to 4 μm.

the final porous manganese oxide synthesis was specially emphasized, although no other evidence on the samples prepared using higher heating rates was offered. The details why the different results emerge are also not yet understood just like the above-described γ-Fe2O3 formation. However, we

reported previously27 also appear. In the previous report,27 Mn2O3 was the only phase state after thermal decomposition of manganese oxalate hydrate, and the Mn2O3 had an amorphous phase state below 400 °C of thermal decomposition. In addition, the importance of slow heating rate of 1 °C/min for F

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Figure 6. TEM images of (a) Mn-1 and (b) Mn-5, (c) XRD patterns of manganese oxides (Mn-1, Mn-2, Mn-3, Mn-4, Mn-5, and Mn-6) after different thermal treatment and (d) N2 adsorption and desorption isotherm and pore size distribution (inset) of Mn-1.

synthesis method was advantageous in its versatility, low cost, and environmental friendliness. The magnetic metal oxides have very high saturated magnitization values and will be a good candidate for magnetic separable applications. From the dependency of the heating rate, calcination time, and calcination temperature on the metal oxide mesostructures, it was revealed that the calcination temperature was the major factor to determine the final mesoporous structure of the metal oxides. The mesoporous structures were well constructed by their corresponding metal oxide nanoparticles resulting from oxalate thermal decomposition.

think that the different preparation procedures for manganese oxalate hydrate should have a close relationship with final products formation during the thermal decomposition. In addition, mesoporous Co3O4 (Figure S6), NiO (Figure S7), and other transition metal oxides with high crystallinity can also be synthesized by the similar method described above. Here, we will not describe them one by one due to the length of the paper. Indeed, the method presented in this paper is quite versatile, low cost, environmental benign, and suitable for the large-scale preparation of mesoporous transition metal oxides with high crystallinity. Because of the low decomposition temperature and very high mass loss ratio of metal oxalate hydrate, it is possible to further increase the surface area of the obtained metal oxides with high crystallinity by efficiently controlling the shrinkage and nanocrystallite growth of metal oxides during the thermal decomposition process. In addition, the influences of different metal oxalate precursor preparations should be further carefully studied.

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ASSOCIATED CONTENT

S Supporting Information *

TG-DTA results, XRD patterns, and TEM images. This material is available free of charge via the Internet at http:// pubs.acs.org.

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CONCLUSIONS In summary, various mesoporous transition metal oxides with high crystallinity were successfully synthesized by thermal decomposition of transition metal oxalates, such as magnetic γFe2O3, CoFe2O4, and NiFe2O4, Co3O4, and NiO, and the

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (L.G.); [email protected]. ac.jp (N.T.). Notes

The authors declare no competing financial interest. G

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ACKNOWLEDGMENTS L. M. Guo thanks JSPS (Japan Society for the Promotion of Science) for a fellowship. This work was partly supported by Grant-in-Aid for Scientific Research (S), No. 22225003, from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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dx.doi.org/10.1021/la400323f | Langmuir XXXX, XXX, XXX−XXX