Rapid One-Pot Solvothermal Batch Synthesis of Porous Nanocrystal

Sep 15, 2017 - Synopsis. Facile one-pot solvothermal synthesis of porous transition metal mixed-oxides were demonstrated. The multiple transition meta...
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Rapid One-Pot Solvothermal Batch Synthesis of Porous Nanocrystal Assemblies Composed of Multiple Transition-Metal Elements Masataka Ohtani,*,†,‡,§ Tomoyuki Muraoka,† Yuki Okimoto,† and Kazuya Kobiro*,†,‡,§ †

School of Environmental Science and Engineering, ‡Laboratory for Structural Nanochemistry, and §Research Center for Material Science and Engineering, Kochi University of Technology, 185 Miyanokuchi, Tosayamada, Kami, Kochi 782-8502, Japan S Supporting Information *

ABSTRACT: The ability of a rapid-heating solvothermal process to synthesize porous nanocrystal assemblies composed of the multiple transition metals was demonstrated. The rapid heating facilitated the quick formation of nascent nanocrystals to generate homogeneous mixed transition-metal oxides. Systematic studies of the synthesis of mixed-metal oxides under various experimental conditions indicated that the present simple method is suitable to develop a wide variety of binary and ternary transition-metal systems such as Co/Mn, Ni/Mn, and Co/Mn/Fe mixed-metal oxides. The products obtained from the rapid heating process were hierarchically assembled porous nanospheres composed of sub-10 nm nanocrystals, which had an extraordinarily high surface area and nano/mesopores. Electrochemical tests revealed the high catalytic ability of the porous nanocrystal assemblies in water oxidation.

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INTRODUCTION Porous nanomaterials have received much attention over the past decade because of their favorable chemical, physical, and optical properties compared with those of the bulk state.1−6 In particular, porous metal oxides of abundant 3d transition metals (Mn, Fe, Co, and Ni) are prime candidates as catalyst materials in terms of durability, activity, and cost.7,8 However, synthesis of mixed-metal oxides with well-controlled morphologies and porous nanostructures is difficult. Porous transition-metal oxides have been fabricated by a wide variety of synthetic approaches such as solid-state reactions, coprecipitation, and sacrificial template methods.7−10 Recently, solvothermal (or hydrothermal) methods have been used extensively to obtain fine nanocrystals of metal oxides.11−14 However, the situation is considerably more complex regarding the fabrication of mixed-metal oxides containing multiple transition metals. Because of the unique intrinsic redox reactivity of each transition metal, the nucleus formation and particle growth of a metal oxide often proceed independently for each metal component, resulting in segregated mixtures of metal oxides containing each single metal component. To regulate the size and morphology of the metal nanoparticles in the solvothermal or hydrothermal synthesis, several factors in reaction conditions have been considered in recent years. One of the most important approaches is control of the heating rate in the beginning of hydrothermal or solvothermal reactions. In this context, there are important pioneering studies in developing the nanosized metal oxides. © XXXX American Chemical Society

Kawasaki et al. demonstrated the synthesis of nanosized TiO2 and NiO by using hydrothermal reactions with a special continuous flow-type reactor, which realize rapid heating up to ca. 100 °C/s.15,16 Aymonier and co-workers also substantiated the usefulness of the alcohol-based solvothermal synthesis of CeO2 nanocrystals with the controlled heating rate in a continuous flow reactor.17−20 They also successfully synthesized nanocrystalline mixed-metal oxide (super-paramagnetic ferrite nanoparticles: MnFe2O4, Fe3O4) in continuous supercritical ethanol.21 However, despite extensive studies, substantial progress in synthetic methodology using the heatingrate control in the solvothermal conditions is still needed to realize mixed-metal oxide systems containing multiple transition-metal components, such as binary or ternary mixed-metal oxides. Furthermore, the one-pot synthesis of porous mixed oxides based on transition metals also remains a challenging issue. To overcome the limitations of traditional solution-based syntheses of transition-metal composite oxides with multiple metal components, herein, we demonstrate a rapid one-pot synthetic approach based on a rapid-heating technique in a batch-type reactor to prepare porous metal oxides containing multiple transition-metal species (Figure 1a). Recently, we demonstrated a new synthetic approach to fabricate porous metal oxides using an alcohol-based solvothermal reaction.22−30 In our method, the methanol-based solvothermal reaction was Received: May 11, 2017

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DOI: 10.1021/acs.inorgchem.7b01192 Inorg. Chem. XXXX, XXX, XXX−XXX

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was examined using SEM. Figure 2 reveals that the materials prepared under different heating conditions possessed different

Figure 1. (a) Schematic illustration of one-pot rapid heating solvothermal synthesis. (b) The tubular batch-type reactor used for the high-temperature/high-pressure reaction. (c) Temperature rises inside the reactor during heating processes with different heating media. (i) Rapid heating at 500 °C/min with a molten-salt bath; (ii) slow heating at 5 °C/min with an electric furnace.

conducted in a closed reactor consisting of an SUS-316 stainless steel tube with pressure-resistant caps (Figure 1b). The inner temperature was traced by a thermocouple. Additionally, our experimental setup used a preheated molten-salt bath, enabling rapid temperature control to perform the solvothermal reaction at high temperature and pressure using water and various organic media as solvents (Figure 1c).22,30 Here, we extend the batch-type rapid-heating synthetic methodology to fabricate porous nanocrystal assemblies composed of multiple transition metals. Using this method, porous transition-metal oxides having high surface area and porosity can be easily synthesized by a one-pot reaction without any sacrificial templates and/or post annealing/calcination.

Figure 2. SEM images of the products prepared using (a) rapid and (b) slow heating. (i) Low and (ii) high magnification.

morphologies. The particles obtained from the rapid heating reaction were spherical with uniform size and shape (Figure 2a). In contrast, a mixture with heterogeneous size and morphology was obtained from the slow heating reaction (Figure 2b). The particles formed random micrometer-scale aggregates. These observations clearly show that the heating rate of the solvothermal reaction strongly affects the morphology of the resultant particles. The nanostructure of the obtained particles was further characterized by TEM. Figure 3a displays a typical TEM image of a spherical particle obtained by the rapid heating method. The spherical particle is composed of a large number of nanoparticles. Judging from size analysis using the several highresolution (HR) TEM images, the average size of these primary particles was in the range of 5−10 nm (Figure 3b and S1). Additionally, HR-TEM and fast Fourier transform (FFT) images show that each primary nanoparticle has a clear lattice fringe with interplanar spacings of 0.24 and 0.21 nm (Figure 3b). These spacings agree with the lattice parameters estimated from diffraction peaks at 36.5° and 42.5° in the corresponding powder XRD pattern (Figure S2). The size of the nanocrystals calculated using Scherrer’s equation were 6.0 nm, which is in good agreement with that observed by HR-TEM. In addition, judging from the systematic analysis in the HR-TEM image by using the FFT processing for each primary particle (Figure S3), these primary nanoparticles were randomly assembled into a secondary particle with nano convex−concave surface morphology. These observations indicate that the spherical particles are produced by self-assembly of nanosized crystals formed during the rapid heating solvothermal reaction. Detailed analysis by STEM/EDX was also performed to evaluate the elemental composition of the materials. As shown in Figure 3c, cobalt and manganese were equally distributed throughout the spherical particles. The estimated atomic compositions of cobalt, manganese, and oxygen were 19.4, 18.7, and 61.9%, respectively, which correspond to the initial

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RESULTS AND DISCUSSION Synthesis of Mixed-Metal Oxides by the Rapid Heating Solvothermal Reaction. A typical synthetic procedure was as follows. First, cobalt nitrate, manganese nitrate, and solvent [methanol (MeOH) and diethylene glycol (DEG)] were mixed in an SUS-316 stainless steel tubular reactor, which was sealed by a screw cap. Then, the reactor was heated to 300 °C at a heating rate of 500 °C/min using a preheated molten-salt (mixture of NaNO3 and KNO3) bath. The temperature inside the reactor was detected by a thermocouple. After it was heated for 10 min, the reaction was rapidly quenched by immersing the reactor in an ice−water bath. The resultant products were centrifuged, washed with MeOH several times, and then dried under vacuum. The products were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDX), powder X-ray diffraction (XRD), and nitrogen adsorption/desorption analysis. As a control experiment, a slow heating reaction using a heating rate of 5 °C/min was also conducted by placing the reactor containing the same precursors in an electric furnace. Nanostructural Analysis of Porous Mixed-Metal Oxides Prepared by Rapid Heating. To elucidate the effect of the heating rate on the synthesis of mixed-metal oxides, the structure of the products obtained using different heating rates B

DOI: 10.1021/acs.inorgchem.7b01192 Inorg. Chem. XXXX, XXX, XXX−XXX

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two main processes: nucleation and subsequent growth.31−33 From this viewpoint, plausible pathways for the present reactions are depicted in Figure 4. As pointed out in the

Figure 4. Schematic illustration of the formation of composite metal oxides through (i) slow and (ii) rapid heating processes.

Introduction, the different types of transition-metal ions have inherently different reactivity. For example, the similar solvothermal reaction of the cobalt ion solution tends to form the sheetlike structures that contain the heterogeneous mixture of cobalt hydroxide and cobalt oxide (Figure S5a). In contrast to this, the case of the manganese ion afforded much smaller size nanocrystals of manganese oxide (Figure S5b). According to them, in the slow heating reaction, the initial nucleation step takes a long time, resulting in nascent nanoparticles with a broad size distribution (Figure 4(i)). As a result, heterogeneous aggregates form (Figures 2b and S4b). In contrast, the present approach heated the whole solution rapidly to reach the supercritical state of MeOH.17−20,34−37 This environment would allow quick nucleation of nascent particles of mixed-metal oxides to give a narrow size distribution, as illustrated in Figure 4(ii). Judging from the product yield (>90%) based on the initial molar concentration of the metal ions, the metal ions in the precursor solution were immediately consumed within the temperature rising. This prevented further crystal growth of more than 10 nm. It may also determine the size of assemblies. In fact, the similar spherical porous nanocrystal assemblies were also obtained by the shorter reaction time (Figure S6). During these solvothermal reactions, the low solvent polarity of supercritical MeOH may also contribute to the efficient formation of the homogeneous spherical porous assemblies, as discussed in our previous reports.22,27,29 The surfactant-like interaction of supercritical MeOH and diethylene glycol also affected the formation of nanosized crystals and the spherical structure of assemblies, as pointed out in the literature.17,18 To support this, similar rapid-heating reaction without diethylene glycol was also conducted. The TEM images of the product exhibited the bunching structure of the nanoparticle assemblies (Figure S7). Additionally, other organic solvents, such as acetone and acetonitrile, were also tested. Figure S8 showed that the choice of the solvent affected the spherical morphologies of the resultant nanocrystal assemblies. Furthermore, the addition of another organic additive instead of the DEG, such as triethylene glycol, also gave the different size and morphologies of the spherical assemblies (Figure S9). However, according to the EDX analysis, these solvent and organic additives were not observed in the resultant materials. To support this, the Fourier transform infrared (FT-IR) spectra of the product clearly showed the absence of the peaks

Figure 3. (a) TEM image, (b) HR-TEM image (inset) FFT image, (c) STEM and EDX mapping images (green: cobalt, blue: manganese, and red: oxygen), (d) nitrogen absorption/desorption isotherm, and (e) pore-size distribution of porous Co/Mn nanocrystal assemblies.

ratio of cobalt to manganese (1:1) in the precursor solution. These observations demonstrate that the assembled nanocrystals in the spherical particles were composed of a binary metal oxide of cobalt and manganese. The nanocrystal assemblies were further characterized by nitrogen adsorption/desorption measurements. Brunauer− Emmett−Teller analysis of the obtained nitrogen adsorption/ desorption isotherm (Figure 3d) revealed that the specific surface area of the product was 155 m2/g, which was markedly higher surface area as compared with that of previous examples of the mixed-metal oxides. In addition, the pore-size distribution determined from the isotherm by micropore analysis revealed the product contained micropores with a size of ca. 1.3 nm (Figure 3e). These results show that the present rapid heating method is capable of fabricating a microporous material in a single step in one pot without a template. Mechanistic Insights into the Rapid Heating Solvothermal Process. The SEM images in Figure 2 revealed that the heating rate during the initial stage of the solvothermal reaction strongly affected the resultant particle morphology and size. The TEM images also clarified the effect of heating rate on the resultant nanostructure. A TEM image of the product obtained from the slow-heating reaction showed that it was a complex mixture of agglomerates with heterogeneous size and morphology (Figure S4a). Meanwhile, STEM/EDX analysis suggested this product contained a heterogeneous distribution of metal elements (Figure S4b). These results clearly indicate that the initial formation step of the primary particles was strongly affected by the heating rate. According to extensive studies on the solution-based synthesis of nanocrystals, their growth mechanism involves C

DOI: 10.1021/acs.inorgchem.7b01192 Inorg. Chem. XXXX, XXX, XXX−XXX

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Catalytic Activity of a Porous Mixed-Metal Oxide. As a preliminary test to investigate the catalytic application of a porous mixed-metal oxide sample, we used the porous Co/Mn binary metal oxide as an oxidation catalyst in the electrochemical water oxidation reaction.38−45 As shown in Figure 6a,

corresponding to the organic additive (Figure S10), confirming that the removal of the organics by washing and vacuum drying was complete. These results suggest that MeOH and DEG were weakly interacted on the surface of the spherical particles during the solvothermal reaction. Synthesis of Porous Mixed-Metal Oxides by the Rapid Heating Solvothermal Process. To emphasize the scope of the present rapid heating solvothermal method, we also synthesized different types of binary and ternary composite metal oxide systems. Judging from the systematic synthesis and structural analysis of a series of mixed-metal oxides, including Ni/Mn binary metal oxide and Co/Mn/Fe ternary metal oxide (Figure 5a,b, respectively), the present rapid heating approach

Figure 6. Electrocatalytic performance of the porous Co/Mn mixedmetal oxide. (a) Schematic illustration of the experimental setup and (b) hydrodynamic voltammogram of the water oxidation reaction at electrodes functionalized with Co/Mn mixed-metal oxide prepared with rapid heating (red), Co/Mn mixed-metal oxide prepared with slow heating (gray), RuO2 (green), and IrO2 (blue). Reference electrode: Ag/AgCl, counter electrode: Pt coil, working electrode: glassy carbon rotating ring electrode, rotating speed: 1600 rpm, electrode area: 0.071 cm2, catalyst loading: 0.2 mg/cm2, sweep rate: 100 mV/s, and electrolyte: 0.1 mol/L KOH.

the working electrode was formed by drop-casting Co/Mn oxide powder on a rotating ring electrode with conductive carbon powder and binder (Nafion). As control experiments, iridium oxide (IrO2) and ruthenium oxide (RuO2) were also tested under the same experimental conditions. Comparison of the hydrodynamic linear voltammograms of water oxidation over the three electrodes (Figure 6b) clearly revealed the higher catalytic activity of the porous Co/Mn oxide compared with those of RuO2 and IrO2, although these particles have similar size (100−500 nm) as compared with that of our porous assemblies (Figure S14). In addition to this, the clear activity difference was also appeared in comparison of voltammograms of the Co/Mn oxides prepared in rapid and slow heating reactions (Figure 6b, red and gray line). Same tendency was also observed in Ni/Mn oxides prepare by rapid and slow heating reaction (Figure S15), although the porous Ni/Mn oxides exhibited less catalytic activity in water oxidation reaction as compared with the porous Co/Mn oxides. Furthermore, these porous Co/Mn oxides showed adequate catalyst stability during the several cycles of the electrochemical loading (Figure S16). These observations proved that the high surface area of the porous Co/Mn nanocrystal assemblies provided the highly active surface for electrocatalytic oxidation reaction. These results also demonstrate the ability of the present rapid one-pot synthetic approach to accelerate screening of metal combinations appropriate for use as catalyst materials.

Figure 5. TEM (i), STEM (ii), and EDX mapping (iii−v) images of (a) Ni/Mn binary mixed-metal oxide and (b) Co/Mn/Fe ternary mixed-metal oxide porous nanoassemblies. The oxygen atom mapping for the ternary system was omitted for simplicity.

is a simple but effective process that tolerates various metal combinations to afford porous mixed-metal oxides. Notably, according to the results of STEM/EDX (Figure 5(ii−v)) and XRD analyses (Figure S11), all the transition-metal species used in the reaction were homogeneously distributed in the resultant fine nanocrystals of mixed-metal oxides. The HRTEM observations and their FFT images also supported the high crystallinity of the primary particles (Figure S12). Additionally, the Ni/Mn binary oxide and Co/Mn/Fe ternary oxide possessed extraordinarily high specific surface areas of 224 and 123 m2/g, respectively (Figure S13). The slight differences in the specific surface area in these binary and trinary metal oxides (Co/Mn, Ni/Mn, and Co/Mn/Fe) may be caused by the differences of the pore size and porosity of the resultant nanocrystal assemblies (BJH pore size: 1.1 nm for Ni/ Mn oxide; 3.0 nm for Co/Mn/Fe oxide), although the surface area per unit weight hinges on the crystal density of the composite nanocrystals. These results demonstrate the general versatility of the rapid heating solvothermal method to fabricate various types of composite metal-oxide nanomaterials.

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CONCLUSION We demonstrated that the rapid-heating solvothermal reaction in methanol is a facile approach to prepare composite metal oxides. It is particularly worth noting that two or three different transition-metal species completely blended together in the resultant mixed-metal oxides. This indicates that rapid heating D

DOI: 10.1021/acs.inorgchem.7b01192 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry *E-mail: [email protected]. (K.K.)

in high-temperature/high-pressure methanol strongly affects the reaction environment and formation/growth of the mixedmetal oxide nanoparticles. The developed approach could be used to synthesize porous catalyst materials for water oxidation. This investigation demonstrated the ability of the rapid-heating solvothermal approach to access highly integrated metal oxide catalysts.

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ORCID

Masataka Ohtani: 0000-0003-1016-1812 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Creation of New Business and Industry program through the Kochi Prefectural Industry-Academia-Government Collaboration Research Promotion Operation and funding from the Japan Society for the Promotion of Science KAKENHI Grant No. 15K06560 (K.K.) and 17K14858 (M.O.). The authors also thank the Research Center for Nanotechnology of Kochi Univ. of Technology for performing TEM and SEM analyses.

EXPERIMENTAL SECTION

Materials. MeOH, DEG, cobalt(II) nitrate hexahydrate (Co(NO3)2·6H2O), manganese(II) nitrate hexahydrate (Mn(NO3)2· 6H2O), nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O), iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O), RuO2, and IrO2 were purchased from Wako Pure Chemical Industries Ltd., Japan. All the reagents and chemicals used were obtained from commercial sources and used as received unless otherwise noted. Synthetic Procedure. Mixed-metal oxides (Co/Mn, Ni/Mn, and Co/Mn/Fe) were synthesized using various mixtures of Co(NO3)2/ Mn(NO3)2, Ni(NO3)2/Mn(NO3)2, and Co(NO3)3/Mn(NO3)2/Fe(NO3)3, respectively, in MeOH as precursor solutions. For example, equimolar amounts of Co(NO3)2 (175 μmol) and Mn(NO3)2 (175 μmol) were added to a mixture of MeOH (3.5 mL) and DEG (1.05 mL, 11.1 mmol). The resultant precursor solution was transferred to an SUS-316 stainless steel tubular reactor with an inner volume of 10 mL. The reactor was then sealed with an SUS-316 screw cap. The sealed reactor was placed in a preheated molten-salt bath that was maintained at 300 °C for 10 min. Caution! Under these experimental conditions, the maximum heating rate is ca. 550 °C/min. Thus, the inner pressure of the reactor increases drastically within 30 s. The reactor should be treated caref ully, and users should wear protective equipment in case there is an unexpected accident. After a certain time period, the reaction was immediately quenched by placing the reactor in an ice−water bath. The obtained mixture was centrifuged, washed with MeOH, and then dried under vacuum to give a powder. The other mixed-metal oxides were synthesized in the same manner from appropriate precursor solutions. The obtained samples were characterized by SEM, TEM, STEM/EDX, XRD, and nitrogen adsorption/desorption measurements. Characterization Methods. SEM was performed on a Hitachi SU8020 FE-SEM microscope. TEM images were collected on a JEOL JEM-2100F microscope. EDX mapping and line-scan plots for STEM images were obtained on an Oxford INCA X-max 80 EDX spectrometer. XRD patterns were obtained using a Rigaku SmartLab diffractometer with graphite-monochromatized Cu Kα radiation (Xray wavelength: 1.5418 Å) in steps of 0.02° over the 2θ range of 10− 90°. The powder samples were set on a nonrefractive silicon holder. Nitrogen adsorption−desorption isotherms were measured using a BEL Japan Inc. Belsorp Mini (II). FT-IR spectra were recorded using a Shimadzu IR Affinity-1 spectrometer with ATR unit. Electrochemical performance tests were performed on an ECstat-300 potentiostat/ galvanostat (EC Frontier Co., Ltd., Japan) with an RRDE-3A rotating disk electrode (BAS Inc., Japan).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01192. HR-TEM, STEM, TEM, SEM, EDX, FT-IR, XRD patterns, FFT analysis images, adsorption/desorption isotherms, pore-size distributions showing the characterization of the obtained mixed-metal oxides (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (M.O.) E

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DOI: 10.1021/acs.inorgchem.7b01192 Inorg. Chem. XXXX, XXX, XXX−XXX