Design of Ordered Mesoporous Composite Materials and Their

Article pubs.acs.org/cm

Design of Ordered Mesoporous Composite Materials and Their Electrocatalytic Activities for Water Oxidation Tobias Grewe, Xiaohui Deng, Claudia Weidenthaler, Ferdi Schüth, and Harun Tüysüz* Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germany S Supporting Information *

ABSTRACT: The controlled synthesis of a series of ordered mesoporous composite materials via solid−solid reaction of ordered mesoporous Co3O4 with various transition metal precursors is reported. This versatile methodology allows preparation of a range of composites with precisely controllable material compositions. The textural parameters of the heterostructured compounds are highly dependent on the oxidation state of the dopant. Electrocatalytic activities of the prepared materials were investigated as oxygen evolution catalysts for the electrolysis of water. Among the ordered mesoporous composite materials, Co3O4−CuCo2O4 shows a significant enhancement for electro-catalytic water splitting with a lower onset potential and higher current density. Following these results, a series of ordered mesoporous composite materials based on cobalt and copper oxides with different atomic ratios were prepared through a nanocasting route. Enhanced electrocatalytic performance was obtained for all composite samples in comparison with Co3O4. KEYWORDS: ordered mesoporous materials, nanocasting, mixed oxides, water oxidation

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INTRODUCTION In the last decades, the synthesis of ordered mesoporous materials (OMMs) has become a major research field. Various applications of OMMs have been suggested, due to their distinctive properties, such as high surface area, uniform and narrow pore size distributions, large pore volume, controllable compositions, crystallinity, thermal and chemical stability, and surface functionalities.1−6 Synthetic routes to produce OMMs are based on soft and hard templating techniques.7−14 Soft templating relies on solidification of an organic monomer or inorganic precursor around a surfactant or a block polymer. In the nanocasting route a solid material is used as a template to create an inverse replica by the following three steps. First, a hard template (mostly an ordered mesostructured silica) is prepared. In the second step, the template is impregnated with a suitable precursor (an organic compound or a metal salt), followed by thermal treatment under an inert atmosphere or in air. In the final step, the silica template can be removed by using dilute HF or NaOH, resulting in formation of the inverse replica of the hard template. Nanocasting is a very effective technique to manufacture crystalline porous materials that are hard to produce by conventional pathways.10,15,16 Another methodology to produce ordered mesoporous materials is the pseudomorphic transformation where an ordered mesoporous material is converted to another one by a posttreatment, such as an oxidation or reduction in which composition and/or chemical and physical behavior of the material is changed but its structural order is retained.17−22 Ordered mesoporous oxides can also be modified via solid−solid reaction, where the OMM is first loaded with a metal salt and then calcined to fabricate heterostructured nanocomposites by © 2013 American Chemical Society

retaining the order of the original OMM skeleton. This type of surface modification can result in novel properties that could be interesting in a range of applications.5,23 The photo- and electrochemical water splitting has recently attracted considerable attention due to the possibility of utilizing solar radiation to produce clean energy in the form of hydrogen.24−29 In addition to semiconductors themselves, cocatalysts are always playing an essential role to accommodate excited electrons and holes on specific active sites and thus improve the redox reaction kinetics. They can also reduce the recombination of electrons and holes.30 Water oxidation is a half reaction in overall water splitting as it offers electrons for the proton reduction. Due to the fact that this reaction is always limited by a relatively slow kinetic and high overpotential, numerous efforts have been made for the discovery of suitable water oxidation cocatalysts (WOC). With the advantages of reasonable activity, relative low overpotential, and abundance, transition metal oxides are emerging as WOC candidates, which could be superior to noble metal oxides, such as RuO2 and IrO2.31,32 Kanan and Nocera reported the water oxidation under neutral conditions by a Co2+ based material.33 Besides, electrodeposited Mn oxide films and nanostructured Co3O4 supported by mesoporous silica were found to be efficient catalysts for water oxidation in base solution.34,35 The size effect of Co3O4 nanoparticles on catalytic activity was investigated as well.36 A comprehensive review on the application of nanostructured Co and Mn oxide clusters in water oxidation has Received: September 23, 2013 Revised: November 19, 2013 Published: November 29, 2013 4926

dx.doi.org/10.1021/cm403153u | Chem. Mater. 2013, 25, 4926−4935

Chemistry of Materials

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been published.37 Very recently, Co3O4/Co2MnO4 nanocomposites were tested as catalysts for both oxygen evolution and oxygen reduction reaction, which makes the synthesis of composite material more interesting for electrocatalytic application.38 Moreover, it was reported that integration of CoOx nanoparticles with semiconductor photoanodes leads to higher quantum efficiency and better stability in the photochemical process.39,40 Recently, it was shown that nanocast Co3O4 is an efficient material as cocatalyst for the oxygen evolution in water splitting: Co3O4 with the highest surface area showed the highest current density.9,41 It was demonstrated that Co3O4 in ordered mesoporous and nanoparticle morphology showed comparable activities. However, one big advantage of using ordered mesoporous structures over nanoparticles was the simple dispersion and formation of a steady film on the working electrode. Even though nanocasting is more time-consuming in contrast to nanoparticle synthesis, the flexible synthetic process allows the preparation of surfactant free composite nanostructures with precisely controlled composition, crystallite size, and interconnectivity for electrochemical applications.9 Further, Jiao et al. developed a selective leaching process of Mg substitution from as-made MgCo2O4 spinel structure which results in a significant amount of defects and remarkable higher oxygen evolution activities.42 Evolved gases were analyzed by a Clark electrode and gas chromatography, and the evolved oxygen could be determined with both analysis techniques, which indicates that the gas trapping is not an issue for these kinds of mesostructured materials. Herein, following a versatile solid−solid reaction route, we describe the synthesis of a group of cobalt oxide-based ordered mesoporous composites. The structural and electrocatalytic properties toward water oxidation were investigated. Among the composite materials, particularly Cu(II) doped Co3O4 exhibited enhanced water oxidation activities in alkali solution. In addition, a series of Co3O4−CuCo2O4 nanocomposites with different copper amounts was prepared through nanocasting for a systematic study to evaluate the effect of the loading amount of copper on the catalytic activities of the materials.

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Preparation of CuxCoyO4 Samples. In a typical procedure, 0.2 g of KIT-6 was dispersed in 2 mL of ethanol solution with Co2+ and Cu2+ in a certain molar ratio (x/y = 1/2, 1/4, 1/8, 1/16) and stirred for 1 h. The same precursors (Co(NO3)2·6H2O and Cu(NO3)2· 3H2O) as aforementioned were used as metal salts. Afterward, the sample was dried at 60 °C and calcined at 250 °C for 4 h. The same impregnation procedure was repeated followed by an intermediate calcination step at 200 °C for 4 h and a final calcination at 550 °C for 6 h. Then the silica was removed by the same leaching process as mentioned before and the samples were collected by centrifugation. Electrochemical Measurement. Electrocatalytic water oxidation measurements were carried out in a three-electrode configuration (Model: AFMSRCE, PINE Research Instrumentation) with Ag/AgCl as reference electrode and Pt wire as counter electrode. KOH aqueous solution with various concentrations was used as electrolyte and argon was bubbled through to remove the oxygen during the measurement. Working electrodes were fabricated by depositing target materials on glassy carbon electrodes (PINE, 5 mm diameter, 0.196 cm2 area). The surface of glass carbon (GC) electrodes was polished with Al2O3 suspension (5 and 0.25 μm, Allied High Tech Products, Inc.) before use. After that, 5 μL of material/ethanol suspension was sonicated for 20 min and dropped on the GC surface. The electrode was then dried at room temperature. Subsequently, 5 μL of 0.25 wt % Nafion solution was dropped on GC as the binding agent, and finally the electrode was dried under light irradiation. All current versus potential curves were measured in a rotating disc electrode (RDE) configuration with a sweep rate of 20 mV/s at 2000 rpm. The electrochemical measurements were carried out at least on two GC working electrodes to check the reproducibility, and their average was taken into account. Stability tests were carried out in 0.1 M KOH solution by applying a bias of 0.8 V (vs Ag/AgCl) for 10 min and 0 V (vs Ag/AgCl) for an additional 10 min sequentially in a time period of 2 h. Potentials are reported vs Ag/AgCl reference electrode, and overpotentials are calculated based on the formula Eoverpotential = EAg/AgCl + 0.197 V + 0.0591 pH − 1.23 V. General Information and Characterization. All the chemicals and reagents were purchased from Sigma Aldrich and used without further purification. Wide and low angle XRD patterns collected at room temperature were recorded on a Stoe theta/theta diffractometer in Bragg−Brentano geometry (Cu Kα1/2 radiation). The measured patterns were evaluated qualitatively by comparison with entries from the ICDD-PDF-2 powder pattern database or with calculated patterns using literature structure data. TEM images of samples were obtained with an H-7100 electron microscope (100 kV) from Hitachi. EDS spectroscopy was conducted on a Hitachi S-3500N. The microscope is equipped with a Si(Li) Pentafet Plus-Detector from Texas Instruments. N2-sorption isotherms of the Co3O4−transition metal composites were measured with a NOVA 3200e at liquid nitrogen temperature. The Co3O4−Cu doped samples have been investigated with an ASAP 2010 adsorption analyzer (Micrometrics) at 77 K. Prior to the measurements, the samples were degassed at 150 °C for 10 h. Total pore volumes were determined using the adsorbed volume at a relative pressure of 0.97. BET surface areas were determined from the relative pressure range between 0.06 and 0.2. Pore size distribution curves were calculated by the BJH method from the desorption branch. HR-SEM images of the samples were taken using a Hitachi S-5500 ultrahigh resolution cold field emission scanning electron microscope operated at 2, 10, and 30 kV. All samples were prepared on lacey carbon films supported by a copper grid. The obtained images were analyzed using the Scandium 5.0 software package from Soft Imaging System GmbH. XPS measurements were performed with a Kratos HSi spectrometer with a hemispherical analyzer. The monochromatized Al Kα X-ray source (E = 1486.6 eV) was operated at 15 kV and 15 mA. For the narrow scans, an analyzer pass energy of 40 eV was applied. The hybrid mode was used as the lens mode. The base pressure during the experiment in the analysis chamber was 4 × 10−7 Pa. To account for charging effects, all spectra have been referenced to C 1s at 284.5 eV.

EXPERIMENTAL SECTION

Preparation of Ordered Mesoporous Co3O4. Synthesis of KIT-6 was conducted as described before.43 KIT-6 (hydrothermal treatment at 35 °C) was used as a hard template to prepare mesoporous Co3O4. In an exemplary reaction, 7 g of KIT-6 was dispersed in 20 mL of 0.8 M Co(NO3)2·6H2O ethanol solution and stirred for 1 h. Afterward, the sample was dried at 60 °C, and then it was calcined at 200 °C for 4 h. The impregnation step was repeated two more times with 20 and 10 mL of solution with an intermediate calcination step at 200 °C for 4 h and a final calcination at 550 °C for 6 h. The silica template was removed with 2 M aqueous NaOH solution. For this the Co3O4−silica composite was dispersed in hot 2 M NaOH (∼70 °C, 25 mL for 1 g of silica), shaken for 2 h, and kept in the oven for another 2 h. After sedimentation, the solution was decanted and fresh hot NaOH solution was added. It was shaken again for 1 h and left in the oven at 60 °C for 12 h. The Co3O4 was filtered off, followed by washing with water until neutralization, and dried at 50 °C. Preparation of Co3O4−Composite Oxides. Nanocast Co3O4 (0.2 g, 0.8 mmol) was impregnated with other metal salts such as Fe(NO 3 ) 3 ·9H 2 O, Mn (NO 3 ) 2 ·4H 2 O, [MoO 3 ] 1 2 [H 3 P O 4 ], [WO3]12[H3PO4], Cu(NO3)2·3H2O, and Ni(NO3)2·6H2O (0.6 M metal ion concentration, 0.5 mL of ethanol) under stirring for 1 h at room temperature with a Co/M ratio of 8. Subsequently, the composite was dried overnight at 60 °C and then calcined at 550 °C in air for 4 h. 4927



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Figure 1. Wide and low angle XRD patterns (a), N2-physisorption isotherm and pore size distribution (b), and TEM (c) and HR-SEM (d) images of cubic ordered Co3O4. The scale bars are 100 nm.

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RESULTS AND DISCUSSION Earlier studies indicated that ordered mesoporous Co3O4 nanocasted from KIT-6 with low aging temperature shows superior catalytic activities in various reactions.44,45 This was attributed to smaller crystallite size of the replica, its high surface area that provides more catalytic centers, open subframework structure, and larger pores that facilitates diffusion. Therefore, in this study KIT-6 aged at lower temperature (35 °C) is used as a hard template in order to prepare ordered mesoporous Co3O4 with high surface area, open subframework structure, and larger pores. Detailed analyses results like low angle XRD pattern, N2 adsorption−desorption isotherms and pore size distribution, TEM surveys, and related explanation for KIT-6 can be found in the Supporting Information in Figure S1. The low temperature during the hydrothermal synthesis step of KIT-6 leads to fabrication of a replica with a lower symmetry and bimodal pore size distribution due to the lower interconnectivity between two channels of the KIT-6 silica template.46 The structure of the Co3O4 was investigated with different analytical techniques to gain information concerning the crystal structure, ordering, and textural parameters of the material. The wideangle X-ray diffraction (XRD) pattern of the produced Co3O4 replica shows the pure cobalt spinel phase with broad reflections (Figure 1a), indicating small crystallites with an average size of 8.7 nm, which was determined by using the Scherrer approach. The low angle XRD pattern shows the (110) and (211) reflections (inset in Figure 1a) indicative for the cubic ordered mesoporous structure with a lower symmetry that can

be assigned as I4132, as expected from the low connectivity of the pore systems in the KIT-6 template.47 As seen in Figure 1b, the nitrogen sorption isotherm is of type IV, characteristic for mesoporous materials. It shows a pronounced capillary condensation step from 0.8 to 0.98 P/P0. The material has a BET surface area of 141 m2/g and a total pore volume of 0.471 cm3/g. It has a bimodal pore size distribution with maxima around 4 and 11 nm (insert in Figure 1b). This indicates that the Co3O4 grows in some regions in both channels of KIT-6 that results in small pores around 4 nmand in some part only in one channel of the template that gives the larger pores around 11 nm, which is equal to the total value of the one of pore size and wall thickness of the silica hard template. Furthermore, the structure of Co3O4 was investigated by transmission electron microcopy (TEM) and high resolution scanning electron microscopy (HR-SEM) to check the homogeneity of the sample. The TEM micrograph in Figure 1c shows the high ordering present in the particles over a long distance. The open and accessible pores of the cobalt oxide are visible in HR-SEM as well in TEM images (Figure 1c,d). The open pores have a diameter of roughly 11 nm, estimated from Figure 1c,d, which is in good agreement with the pore size measured by physisorption. The average crystallite size was estimated to be around 6 nm, and the particle (domain) size is in the range of several hundred nanometers. The SEM and TEM analysis confirmed the replication of the ordered mesoporous structure without the presence of bulk Co3O4. After the synthesis of a large batch of ordered mesoporous Co3O4 (around 4 g), it was used as a skeleton for the 4928



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Figure 2. Schematic illustration of post-treatment of ordered mesoporous Co3O4.

Table 1. Theoretical Phase Ratios of Co/Dopant and Their Textural Parameters Co3O4 Co3O4/composite phase ratio SBET [m2/g] pore volume [cm3/g] pore size [nm]

Fe(III)

Mn(III)

Mo(VI)

W(VI)

Cu(II)

Ni(II)

−

5/1

5/1

2.3/1

2.3/1

2/1

2/1

141 0.471 4 and 11 bimodal

99 0.360 4 and 11 bimodal

113 0.271 4 and 11 bimodal



66 0.299 11 monomodal

67 0.215 11 monomodal

presented in Table 1. This high amount of the second phases would in all cases be detectable by XRD. In order to check this, the crystal structures of the prepared materials were examined by XRD. As seen in Figure 3a, materials doped with Ni, Cu,

fabrication of ordered mesoporous composites. The synthesis route is based on a solid−solid reaction of ordered mesoporous Co3O4 with other transition metals/metal oxides. This versatile strategy allows the preparation of composite materials with similar symmetry, morphology, and textural parameters, where the effects of the doping with other metals can be precisely evaluated in various applications, in particularly in heterogeneous catalysis. To modify the inner surface of Co3O4 in a controlled manner, the porous structure was filled with metal salt by a wet-impregnation route with a Co/dopant molar ratio of 8, as depicted in Figure 2 (step I). After drying, the pore volume of cobalt oxide is filled with dopant salt that can be decomposed at low temperatures (Figure 2, step II), supplying a reactive dopant species that will react with cobalt ions from the framework and form a new mixed oxide layer in the pores (Figure 2, step III) during high temperature treatment.11 Transition metal precursors with different oxidation states were chosen as dopants such as Ni(II), Cu(II), Fe(III), Mn(III) nitrates, and Mo(VI) and W(VI) heteropolyphosphates. These metals are known to form mixed spinels or mixed oxides with cobalt at high temperature.48−55 The obtained mixed spinels will later be referenced as MCo2O4, CoM2O4, or CoMO4, depending on the oxidation state of the dopant M2+, M3+, or M6+, respectively. It should be kept in mind, after the impregnation and drying procedure, that all samples were calcined at 550 °C. At this kind of high temperature, the starting precursors decompose completely by resulting in crystalline compounds. The lattice of Co3O4 could host many other cations by replacing the cobalt cations. In spite of expectation, if the solid−solid reaction between the dopant and the Co3O4 does not occur at high temperature, an oxide phase based on the doped metal will be obtained beside the Co3O4 crystal structure. This is illustrated by taking into account Cu(II) as an example. As aforementioned, Cu(II) can react with the ordered mesoporous Co3O4 skeleton by forming CuCo2O4 spinel inside the Co3O4 pores. Considering the molar ratio between cobalt and copper that is, 8/1the ratio of the crystal phases of Co3O4/CuCo2O4 will be 2/1. When copper does not react with Co3O4 and forms CuO, the ratio of Co3O4 to CuO will be 2.7/1. The molar phase ratios of all expected composite spinels to Co3O4 are

Figure 3. Wide (a) and low angle (b) XRD patterns of Co3O4 and mixed composite oxides.

Mn, and Fe have the same patterns as neat Co3O4. A second phase, be it oxide or spinel, is not detected. Since the unit cell parameters of CuCo2O4, NiCo2O4, CoFe2O4, and CoMn2O4 are very close to the ones of Co3O4, one cannot distinguish these phases with XRD analysis, particularly in the case of small particles with very broadened reflections. In a control experiment, when copper and cobalt nitrate precursors are mixed in an atomic ratio of 1/2 and calcined at 550 °C, the obtained 4929



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Figure 4. HR-SEM images of surface doped Co3O4 based materials: Co3O4−CoFe2O4 (a), Co3O4−CoMn2O4 (b), Co3O4−CoMoO4 (c), Co3O4− CoWO4 (d), Co3O4−CuCo2O4 (e), and Co3O4−NiCo2O4 (f). The scale bars are 100 nm.

(Supporting Information). The obtained XPS data confirm the expected oxidation states of cobalt (+II and +III) and dopants (Ni+II, Cu+II, Fe+III, W+VI, Mo+VI).48,56−61 The results of the XPS measurements support the formation of the expected composite phases. In the case of Mn as dopant, the characterization of the oxidation state is difficult via XPS because the binding energies of manganese +II and +III are very similar.62 However, in a control experiment, pure Mn(NO3)2·4H2O was decomposed at 550 °C that resulted in formation of manganese(III) oxide, which suggests that probably CoMn2O4 rather than MnCo2O4 spinel is formed. With knowledge about the formation of the expected composites, the structural ordering of the samples was examined by low angle XRD (Figure 3b). Mesoporous Co3O4 shows the characteristic (110) and (211) reflections, and the composites exhibit similar reflexes at the same two theta values, which indicates thermal stability of this type of ordered mesoporous structure. Moreover, the materials were examined by HRSEM and TEM after the post-treatment. TEM images of the

product was the pure crystalline CuCo2O4, and the formation of the CuO was not observed by XRD. In the event of the hexavalent dopants (Mo and W), the unit cell parameters of the mixed metal oxides are noticeably different from those of pure Co3O4 since the cobalt spinel has a cubic crystal structure while CoMO4 is monoclinic. The additional reflections in the XRD pattern of Mo and W doped Co3O4 suggest formation of CoMoO4 and CoWO4 phases. The observation of these phases indicates that the crystallite size of material is in the detectable range. Therefore, we can corroborate a solid−solid reaction of di- and trivalent dopants with the Co3O4 and formation of a mixed oxide structure. After the solid− solid reaction, the weight of all samples increased and our elemental analysis results indicated the presence of the expected amount of the dopants (Supporting Information Figure S2). In order to gain information concerning the oxidation state of the incorporated cations, we investigated the surface of the composite materials by X-ray photoelectron spectroscopy (XPS). The spectra of the samples are presented in Figure S3 4930



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the doped metal ion. As discussed before, the formation of the mixed metal oxide layer consumes Co3O4 to form the composite metal oxide layer. In case of the trivalent ions, the Co3O4 and CoM2O4 phases are in a ratio of 5/1 (5 Co3O4 + 1 CoM2O4 → 16 Co atoms + 2 M atoms → 8/1). At this ratio, only a small amount of the parent material is transformed, and the ordered structure is preserved. The M(VI) ions create a phase ratio of Co3O4/CoMO4 = 2.3/1, which means a significant amount of Co3O4 is consumed during the formation of CoMO4. This explains the partial filling of the small and bigger pores. Finally, the Co3O4−MCo2O4 composites show the highest phase ratio of 2/1, which matches the analysis results. At this ratio the small pores are completely blocked, and the bigger pores are filled to a certain extent. The data discussed above indicated the successful fabrication of composite ordered mesoporous metal oxides. In the following part, we will focus on the electrochemical activities of the prepared materials as oxygen evolution catalysts for the electrolysis of water. Recently it was shown that the nanocast Co3O4 indicated high activity for oxygen evolution in an alkaline medium (0.1 M KOH, similar conditions that are used in this work) and the obtained activity was reported to be comparable to the most active Ir/C catalyst and better than Co3O4 nanoparticles and the Pt/C catalyst.41 Herein, syntheses of Co3O4 nanoparticles, mesoporous Co3O4 from disordered mesoporous silica template and Co3O4 in bulk form, were conducted for comparison in water oxidation reaction. The TEM images (Supporting Information, Figure S5) show the nanoparticles possess a size of ∼8 nm, while the bulk Co3O4 has a much bigger domain size, which is in good agreement with the measured BET surface area that is shown in Table S2 (Supporting Information). The Co3O4 prepared by the nanocasting method from silica gel possesses the desired disordered mesoporous structure and a surface area of 65 m2/g. From the linear sweeps presented in Figure S6 (Supporting Information), it can be seen that the ordered mesoporous Co3O4 clearly outperformed disordered Co3O4 from the silica gel template as well as the nanosized counterparts. The bulk Co3O4 is showing the lowest activity due to the lowest surface area. A detailed comparison can be found in Table S2 (Supporting Information). A distinguished advantage of mesoporous Co3O4 over the nanoparticle is that the skeleton can be quite stable during the storage and harsh reaction conditions while the nanoparticles tend to aggregate and result in larger size and consequently lower catalytic active sites. The linear sweeps of Co3O4, Co3O4−CoM2O4 (M = Fe, Mn), Co3O4−CoMO4 (M = Mo, W), and Co3O4−MCo2O4 (M = Cu, Ni) in 0.1 M KOH solution (pH 13) are shown in Figure 6. The nanocast Co3O4 samples doped with Fe, W, Mo, and Ni on the surface show similar catalytic behavior as pure Co3O4 for oxygen evolution though the current densities are slightly different at higher applied voltage. Co3O4−CoMoO4 indicates a little higher activity while Co3O4−NiCo2O4 shows very similar activity as pure nanocast Co3O4. It should be kept in mind that the surface areas of all composite materials are lower than Co3O4 and thus they possess less catalytically active surface sites. Despite that, composite samples indicate superior or comparable electrocatalytic activities as nanocast Co3O4 for water oxidation. Among the composite samples, Co3O4− CoMn2O4 exhibits the lowest current densities. On the other hand, Co3O4−CuCo2O4 shows a significantly lower onset potential for water oxidation and a higher current density in spite of the lower BET surface area in comparison with ordered

composite sample show ordered mesoporous structure that is consisted of coupled and uncoupled subframework structure (Supporting Information, Figure S4), while SEM images of the samples reveal the intact three-dimensional open structure with pores throughout the particles (Figure 4). However, some regions of the samples seem to be denser due to partial pore filling of the materials with the additional compound. In addition, the textural parameters of the composite materials were determined by N2-sorption. All materials still show type IV isotherms (Figure 5a). After the post-treatment, the surface area

Figure 5. Nitrogen adsorption−desorption isotherms of ordered mesoporous Co3O4 and composite materials (a) and their pore size distributions (b).

and pore volume of the composite materials clearly changed. There is a clear trend, dependent on the oxidation state of the dopants. The textural parameters of the composite materials are summarized in Table 1. The impregnation with the trivalent metal cations reduced the surface area of the parent Co3O4 from 141 m2/g to around 100 m2/g, and pore volume drops by about 0.1 cm3/g. As seen in Figure 5b, the post-treatment affects the pore size distribution as well. After the impregnation and solid− solid reaction, the ratio of the smaller pore size to the bigger one decreases clearly. After impregnation with Mo(VI) or W(VI) the surface area is in the range of 70 − 80 m2/g. From the pore size distribution it becomes more obvious that the small pores at ∼4 nm are less available. Composites doped with Cu(II) and Ni(II) show an even stronger effect of pore filling. The BET surface area is in the range of 65 m2/g, and the small pores are completely filled. The trend of decreased surface area and the change in the pore size distributions can be related to the oxidation state of 4931



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materials. A deeper study on electronic structure, conductivity, and charge transport and separation ability of the composite materials needs to be carried out to find out the main reason for the superior catalytic activity of the copper doped material, which will be the task of another study. The surface doping method described above serves as an easy way to modify the surface and make it more convenient to compare the activity between materials with a similar structure but different surface species. For comparison of the surface doped and nanocast composite materials, a series of the sample were also prepared by direct nanocasting. Our electrochemical investigation indicated that the materials show a trend for electrochemical water oxidation similar to that of surface doped ones, with Cu being the highest and Mn the lowest. Herein, copper is chosen as an example for a deeper analysis. Inspired by the activity enhancement from the Co3O4−CuCo2O4 composite, a series of CuxCoyO4 (y/x = 2, 4, 8, 16) composites was synthesized by a direct nanocasting method. In a typical experiment for nanocast composite synthesis, instead of posttreatment by surface doping, certain ratios of Co and Cu precursors were impregnated simultaneously to obtain a uniform distribution throughout the materials. After the removal of the silica template, the obtained CuxCoyO4 samples show the same spinel phase as pure Co3O4, according to wide angle XRD measurements (Supporting Information, Figure S7a). This suggests that no CuO phases were formed. In all cases type IV adsorption and desorption isotherms (Supporting Information, Figure S8) are observed in N2-sorption measurement, and BET surface areas in the range of 120−140 m2/g were calculated (Supporting Information, Table S1). Low angle XRD patterns (Supporting Information, Figure S7b) as well as TEM images (Supporting Information, Figure S9) further confirm the presence of ordered mesoporous structures. The electrocatalytic activity of the samples was first investigated in 0.1 M KOH solution (pH 13). As shown in Figure 7, all the CuxCoyO4 samples show a higher activity than pure ordered mesoporous Co3O4, which is in agreement with the previous results obtained for the surface doped Co3O4−CuCo2O4 composite. The composite samples indicate also slightly better activity than pure CuCo2O4. Since morphology, particle size, and surface area of all samples are similar (in the range of 125− 145 m2/g), the diverse activities might be related to electronic structure and conductivity. Furthermore, junctions between CuCo2O4 and Co3O4 crystal phases might also affect the material conductivity and electron transfer ability, and thus its electrocatalytic activity. A more detailed investigation should be conducted to comprehend reasons of the catalytic activity enhancement. In addition, directly nanocast CuxCoyO4 samples show better activities than the surface doped sample, which can be attributed to the higher surface areathe surface areas of doped Co3O4−CuCo2O4 and direct nanocast Cux Coy O4 samples are 66 and 126 m2/g, respectivelythat provides more active sites for the catalytic reaction. Another reason of diverse catalytic activities might be dissimilarity of surface species. It should be noted that the solid−solid reaction of

Figure 6. Oxygen evolution currents of as-made Co3O4, Co3O4− CoM2O4 (M = Fe, Mn), Co3O4−CoMO4 (M = Mo, W), and Co3O4− MCo2O4 (M = Cu, Ni) composites dispersed on glassy carbon electrode in 0.1 M KOH electrolyte (catalyst loading ∼0.12 mg/cm2 for all the samples). The inset figure shows the current increment in a narrow voltage range (0.54−0.70 V vs Ag/AgCl).

mesoporous Co3O4. The improvement is even more remarkable in a narrow overpotential range, as shown in the inset of Figure 6. A quantitative comparison of the activity between nanocast Co3O4 and the Co3O4−CuCo2O4 composite is made, and the results are presented in Table 2. After the surface modification with Cu2+, the obtained composite material shows a higher current density (31.2 mA/cm2) at 1 V (vs Ag/AgCl, or overpotential of 735 mV) than pure Co3O4 (28.6 mA/cm2), and the overpotential required to achieve a certain current density decreases. There are several physical and chemical properties that affect electrocatalytic activity of a material, such as morphology, topology, particle and crystallite size, textural parameters including surface area, porosity, pore volume and size, crystallinity, structural defects, conductivity, charge transport, and separation ability. In this study, fabricated materials have very similar morphology, particle, and crystallite size: therefore, their diverse catalytic activities might be related to their textural parameters and crystal and electronic structures. The textural parameters of a material are key points in heterogeneous catalysis since higher surface area provides more catalytic center for catalytic reactions and porosity conveniences diffusion. As seen in Table 1, the pure Co3O4 sample has the highest surface area and largest pore volume of 141 m2/g and 0.47 cm3/g, respectively. Among the composite samples, copper doped composite has the lowest surface area (66 m2/g) while manganese doped sample possesses the highest surface area (113 m2/g); however, their electrocatalytic behaviors are not following this trend. Copper based composite material shows the best activity while manganese exhibits the worst activity. It seems that electronic and crystal structures of the materials are more dominant than the textural parameters of the composite

Table 2. Overpotential (at 5, 10, and 20 mA/cm2) and Current Density (at 1 V vs Ag/AgCl) of Nanocast Co3O4 and Co3O4− CuCo2O4 Composite

Co3O4 Co3O4−CuCo2O4 composite

overpotential (mV) at 5 mA/cm2



current density (mA/cm2) at overpotential 735 mV

462 428

528 498

643 614

28.6 31.2

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Figure 9. Chronoamperometric measurement of the nanocast Co3O4 and CuxCoyO4 (y/x = 8) under a bias of 0.8 V vs Ag/AgCl at pH 13 for 6000 s. Figure 7. Oxygen evolution currents of as-made mesoporous Co3O4, CuxCoyO4 (y/x = 2, 4, 8, 16) samples dispersed on glassy carbon electrode in 0.1 M KOH electrolyte (catalyst loading ∼ 0.12 mg/cm2 for all the samples). The inset figure shows the current increment in a narrow voltage range (0.54−0.70 V vs Ag/AgCl).

The sample with a Co/Cu ratio of 8 is later selectedwith the best performancefor further investigations to study material stability and the effect of the pH. Since electrolyte concentration is an important parameter for electrochemical applications, a series of KOH solutions with different pH values were prepared and tested to investigate the effect of OH− concentration on the catalytic activity of mesoporous Co3O4 and CuxCoyO4 (y/x = 8) samples. Plots of current density vs potential for different pH values are presented in Figure 8. The activity difference is negligible at lower pH conditions (11 and 12) since relatively low current densities are observed in both cases. However, the effect of the pH value becomes more significant as the current density reaches 28.2 and 36.6 mA/cm2 for Co3O4 and the composite sample, respectively, at 1 V (vs Ag/AgCl) at pH 13. Further increasing the pH of the electrolyte gives a higher current density and clearer activity distinction between the two investigated samples. Results of the quantitative comparison are given in Table 3. These results indicate that higher concentration of OH− favors the electrocatalytic water oxidation for both materials where a higher activity is obtained with copper based ordered mesoporous oxide. A more detailed comparison of our materials with the literature data is presented in the Supporting Information, in Table S2. Stability has always been an essential aspect in catalyst evaluation since material durability has to be considered for long-term utilization. A stability study on mesostructured Co3O4 and the most active copper−cobalt based composite sample was carried out. Chronoamperometric measurements with a bias of 0.8 V vs Ag/AgCl were conducted at pH 13 for both materials, and Figure 9 shows the current density vs time plots. The initial current spike observed when the voltage was applied is probably related to the capacitance component at the solid−liquid interface. As expected, the CuxCoyO4 (y/x = 8) electrode shows

Figure 8. Linear voltammograms of nanocast Co3O4 (solid line) and CuxCoyO4 (y/x = 8, dashed line) in solutions with various KOH concentrations.

copper precursor with ordered mesoporous Co3O4 might result in a composite that has CuCo2O4 surface shell and Co3O4 core. A similar behavior has been observed when iron nitrate was impregnated into ordered mesoporous Co3O4 and calcined at a high temperature; the obtained composite was based on a core− shell structure.63 Direct nanocast composite is expected to have a homogeneous distribution of cobalt and copper atoms in the structure, while in case of the surface doped sample, the copper will be more concentrated at the surface. Therefore, the surface compositions of both the sample series are different, and this difference affects the catalytic activities of the materials as well.

Table 3. Overpotential (at 10 and 20 mA/cm2) and Current Density (at 1 V vs Ag/AgCl) of Nanocast Co3O4 and CuxCoyO4 (y/x = 8) at Various pH overpotential (mV) at 10 mA/cm2 pH pH pH pH pH

11 12 13 13.5 14


current density (mA/cm2) at 1 V vs Ag/AgCl

Co3O4

CuxCoyO4 (y/x = 8)

Co3O4

CuxCoyO4 (y/x = 8)

Co3O4

CuxCoyO4 (y/x = 8)

528 451 427

471 413 391

643 492 454

572 463 416

0.3 3.0 28.2 97.1 273.8

0.3 3.3 36.6 108.8 307.8

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better catalytic activity than pure Co3O4, with average current densities of 13.5 mA/cm2 and 11.5 mA/cm2, respectively. Despite the mesoporous structure and high surface area, both samples show slight deactivation over 6000 s. The current drop within each independent step is largely due to the bubble accumulation on the electrode surface. After the stability test, the structure of the CuxCoyO4 (y/x = 8) has been further investigated with TEM and EDS analyses. It was found that the material shows excellent structural stability by preserving the ordered mesoporous structure (Supporting Information, Figure S10), and EDS results indicate that the molar ratio of cobalt to copper did not change dramatically after the electrochemical stability test.

CONCLUSIONS It was demonstrated that the surface of nanocast Co3O4 can be successfully modified by a solid−solid reaction with various transition metal salts. The structural parameters of the resulting composite materials highly depend on the oxidation state of the dopant. This method provides a blueprint for the production of composite materials based on ordered porous oxides that could have interesting physical and chemical properties. The catalytic activity of the composite materials was investigated in electrochemical water splitting, and the Co3O4−CuCo2O4 was found to be the most active one, with a lower onset potential and higher current density than pure Co3O4. Furthermore, CuxCoyO4 (y/x = 2, 4, 8, 16) with a uniform elemental distribution was synthesized via nanocasting and tested as electrocatalyst for water oxidation. The performance of the electrocatalyst was further enhanced where superior catalytic activities were observed for all copper based materials in comparison with pure Co3O4. The current density of the CuxCuyO4 (y/x = 8) electrode increases with alkaline concentrationwith superior performance to Co3O4and the material shows acceptable stability during long-term water electrolysis; it thus holds promise as a cost-effective anode material. This work opens up new possibilities in developing functional materials with ordered structure and high surface area based on nonprecious metal catalyst. An additional study concerning the combination of this developed anode material with a high surface area semiconductor material for photo−electrochemical water splitting is in progress. ASSOCIATED CONTENT

S Supporting Information *

TEM images, EDS results, and XPS spectra of the nanocomposite series, XRD patterns, N2 sorption, and TEM images of KIT-6 silica and nanocast CuxCoyO4 samples, and TEM image of CuxCoyO4 sample after the stability test. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank our TEM department for SEM images and EDS analysis in addition to Max-Planck-Society and Fonds der Chemischen Industrie (FCI) for funding. T.G. thanks FCI for a Chemiefonds Fellowship. 4934



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Design of Ordered Mesoporous Composite Materials and Their

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