Facile precursor-mediated synthesis of porous core–shell-type Co₃O₄ octahedra with large surface area for photochemical water oxidation

Abstract

A porous Co₃O₄ material with unique octahedron-in-octahedron core–shell-type morphology is prepared via a facile precursor-mediated synthetic route. This material possesses large surface area and good catalytic activity for water oxidation reaction.

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Natural photosynthesis in plants is based on a notable reaction—the biocatalytic conversion of water and carbon dioxide into oxygen and carbohydrates. The first step of the natural photosynthesis is the sunlight-driven water oxidation reaction, which takes place in photosystem II (PS II, a membrane protein complex).¹ This photosystem is built around a Mn₄CaO₅ cubane-type cluster that is the catalytically active center for water oxidation.^1,2 Similar to natural photosynthesis, its artificial version (e.g., water splitting) also requires efficient synthetic water-oxidation catalysts based on earth-abundant elements.^3,4

Over the past decades, several oxides based on transition metals such as Fe, Ni, Mn and Co have been investigated for water oxidation reaction (WOR).⁵ Cobalt(II,III) oxide (Co₃O₄) among them has been shown to be a promising WOR catalytic material due to its high activity, good stability and earth-abundance.^6–12 In view of the fact that the WOR with Co₃O₄ is a heterogeneous catalytic process, considerable amount of research effort was recently made to optimize Co₃O₄'s catalytic performance by increasing its surface area. Large surface area is anticipated to give high surface reactive sites, large contact area with reactants, and thereby improved catalytic activity. To increase Co₃O₄'s surface area, convenient and efficient strategies mainly include (1) making it in nanostructured form,^6–9 and (2) creating porous structure in it.^10–12 In 2009, Esswein et al. revealed the size-dependent water oxidation activities of Co₃O₄ nanoparticles.⁶ Since then, there are several studies on new methods for the preparation of nanostructured Co₃O₄ WOR catalysts with unsupported, ultrasmall-sized, and/or ligand- and surfactant-free features.^7–9 For example, Müller and co-workers synthesized surfactant-free, unsupported Co₃O₄ nanoparticles (<5 nm) with high WOR activity using a pulsed laser ablation method.⁸ Besides nanostructured Co₃O₄ materials, porous Co₃O₄ materials with large surface areas were also synthesized and applied for water-oxidation applications. Jiao et al. prepared a series of mesoporous silica/carbon supported Co₃O₄ nanoclusters as efficient WOR catalysts.¹⁰ And hard template method was also used by some groups to fabricate mesoporous Co₃O₄ WOR catalysts.¹² Despite many achievements made in this field, as outlined above, template-free synthesis of porous Co₃O₄ materials with clean surface, large surface area and catalytic activity were not reported.

Herein, we report a facile, precursor-mediated, template-free synthetic route to prepare a porous Co₃O₄ material (hereafter denoted as Co₃O₄-1) that possesses an octahedron-in-octahedron core–shell-type morphology, a large surface area, and catalytic activities towards water oxidation. This method involves the synthesis of an octahedron-shaped cobalt alkoxide (denoted as Co-PDO) by the solvothermal self-assembly of Co²⁺ and 1,3-propanediol (PDO), followed by a simple thermal treatment (200 °C) of the resulting Co-PDO in air to remove organic component in it as well as to form porous Co₃O₄-1 directly (Fig. 1, see experimental details in ESI†). Previously, some Co₃O₄ materials with an octahedral morphology were reported, but they had a non-porous structure (or low surface area), and were not used for catalytic water oxidation reaction.¹³

Fig. 1 Schematic representation of synthesis of porous octahedron-in-octahedron core–shell-type Co₃O₄ material (i.e., Co₃O₄-1) from Co-PDO. F_c and F_a mean contraction force and adhesion force, respectively.

The Co-PDO precursor used herein was prepared by a solvothermal reaction between cobalt acetate tetrahydrate and 1,3-propanediol (PDO), which served as both a solvent and a reactant. It is worth noting here that the Co-PDO precursor could be formed over wide ranges of both Co²⁺ concentration (3.1–31 mmol L⁻¹) and reaction temperature (140–200 °C), while the crystal structure and morphology of Co-PDO remained unchanged (see Fig. S1 in ESI†); and this also ensures that the synthesis of Co-PDO is easily controllable and reproducible. Hereafter, Co-PDO denotes the product with Co²⁺ concentration of 15.5 mmol L⁻¹ and reaction temperature of 160 °C, if not particularly indicated. The structure of the Co-PDO precursor was further characterized by FT-IR spectroscopy, X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA) (see Fig. S2–S4†). All the results suggested that the Co-PDO precursor was a crystalline cobalt(II) alkoxide with a cobalt content of ∼22.6 wt% (see ESI† section for detailed discussion of the characterization results); however, its accurate crystal structure has not been resolved yet.

The morphology of the Co-PDO precursor was observed by scanning electron microscopy (SEM). The low-magnification SEM images (Fig. 2A) show that the product consists of octahedron-like particles with a side length of ∼7 μm. The TEM image of the Co-PDO precursor (Fig. 2B) appears as a regularly hexagonal shape, which is in agreement with the Co-PDO's octahedron-like morphology (Fig. S5,† please see a simulation of how an octahedral particle can show a hexagonal shape during TEM measurement).

Fig. 2 (A) SEM and (B) TEM images of the Co-PDO precursor, and (C) SEM, (D) TEM, (E) HTEM images and (F) XRD pattern of Co₃O₄-1. SEM image of a broken Co₃O₄-1 particle was shown in the inset of (C).

Fig. 2C and S6† show typical SEM images of Co₃O₄-1, which was synthesized by thermal treatment of Co-PDO at 200 °C. The SEM images reveal that Co₃O₄-1 maintains the morphology and size of the Co-PDO precursor. However, different from the Co-PDO precursor with a solid structure, the Co₃O₄-1 material possesses a unique core–shell-type structure with a shell thickness of ∼300 nm, as shown by a broken particle (Fig. 2C, inset and S7†). The TEM image of Co₃O₄-1 (Fig. 2D) shows a darker hexagonal region at the center and a continuous lighter hexagonal region around the edge of the Co₃O₄-1 microparticle, indicating again the formation of an octahedron-in-octahedron core–shell-type structure. The HRTEM images (Fig. 2E and S8†) clearly show that nanoparticles with a size of <5 nm are randomly arranged and interconnected in the whole visible area. The observed lattice spacing is about 0.24 nm, which corresponds to the distance between the (311) crystal planes of the cubic Co₃O₄ phase.

The morphology evolution from Co-PDO's solid structure to Co₃O₄-1's core–shell-type structure could be explained based on a thermal-driven chemical transformation process, in which the decomposition of Co-PDO and the formation of Co₃O₄ occur simultaneously. Upon the heat treatment, there would be a large contraction force (F_c) induced by the decomposition of organic components in the Co-PDO precursor (Fig. 1). At the same time, there is also an opposite adhesion force (F_a) that could be attributed to the formation of Co₃O₄ crystallites, especially at/near the precursor particle–air interface. The synergy of F_c and F_a finally results in the separation of the core and the shell (or the formation of the core–shell-type structure). Similar explanation was also reported by Zhang et al. in a study that showed the formation of ZnMn₂O₄ ball-in-ball microspheres.¹⁴

The complete conversion of Co-PDO into Co₃O₄ was further confirmed by XRD (Fig. 2F) and FT-IR (Fig. S9†) measurements. It is seen in Fig. 2F that the XRD peaks of Co₃O₄-1 are perfectly indexed to cubic Co₃O₄ (JCPDS no. 42-1467). In the IR spectrum of Co₃O₄-1 (Fig. S9†), those IR absorption bands related to the organic component in Co-PDO completely disappeared (Fig. S2†); and the characteristic IR absorption related to Co₃O₄ appeared.

The porous structure and surface area of Co₃O₄-1 were studied by N₂-adsorption measurement. The N₂ adsorption–desorption isotherms (Fig. 3A) of Co₃O₄-1 are characteristic of type-IV with H3-type hysteresis loops, demonstrating the presence of a highly porous texture in the material. The corresponding BJH pore-size distribution (Fig. 3B) derived from the desorption isotherm shows a narrow pore-size distribution ranging from 1 nm to 4 nm. It is presumable that the formation of porous structure in Co₃O₄-1 might be originated from the thermal-driven removal of organic components in Co-PDO and some related structural rearrangement. Furthermore, the BET surface area of Co₃O₄-1 is ∼190 m² g⁻¹, demonstrating that Co₃O₄-1 is a high-surface-area material.

Fig. 3 (A) N₂ adsorption–desorption isotherms of Co₃O₄-1 and (B) the corresponding pore size distribution derived from the desorption isotherm.

Next, we investigated the visible-light-driven water oxidation activity of the as-obtained Co₃O₄ material using the established [Ru(bpy)₃]²⁺/persulfate system (bpy = 2,2′-bipyridine) in a Na₂SiF₆–NaHCO₃ buffer solution (pH = 5.8).¹⁵ In this system, [Ru(bpy)₃]²⁺ and persulfate were used as sensitizer and sacrificial electron acceptor, respectively. In addition, [Ru(bpy)₃]²⁺ was reported previously to have the lowest decomposition rate in the Na₂SiF₆–NaHCO₃ buffer solution (pH = 5.8).^9,10

For comparative purpose, two more Co₃O₄ samples were prepared by thermal treatment of Co-PDO at 300 and 400 °C, respectively, and correspondingly they are labeled as Co₃O₄-2 and Co₃O₄-3 (see material characterization in Fig. S10†). Furthermore, the catalytic activities of Co₃O₄-1, Co₃O₄-2, Co₃O₄-3 and a commercially available Co₃O₄ sample (Com-Co₃O₄) were measured under the same condition by keeping the same weight of samples in each case. Before photocatalytic experiments, several control experiments such as without a catalyst, light, [Ru(bpy)₃]²⁺, or persulfate were performed, and in these cases no O₂ evolution was detected. The results demonstrate that in such a visible-light-driven water oxidation system, each of these components is required.

As shown in Fig. 4A, all the four Co₃O₄-based materials exhibit obvious catalytic activities towards WOR (or oxygen evolution) under visible-light irradiation. The catalytic activities of the materials towards WOR increase in the order of Com-Co₃O₄ ≪ Co₃O₄-3 < Co₃O₄-2 < Co₃O₄-1. To be precise, Co₃O₄-1, Co₃O₄-2, Co₃O₄-3 and Com-Co₃O₄ afford O₂ evolution rates of 211.5, 134.5, 68.1 and 12.2 μmol g⁻¹ min⁻¹, respectively. Thus, the activity of Co₃O₄-1 (the most active catalyst in our work) can be roughly said to be ca. 17.3 times as high as that of Com-Co₃O₄. This trend in the catalytic activity of the materials would correlate with the materials' surface areas, as the BET surface area of Co₃O₄-1 is 190 m² g⁻¹, which is much larger than those of Co₃O₄-2 (61 m² g⁻¹), Co₃O₄-3 (20 m² g⁻¹) and Com-Co₃O₄ (<10 m² g⁻¹). To further compare the catalytic activities of the three Co₃O₄ materials we synthesized, their BET surface areas were used to normalize the O₂ evolution rates. The normalized O₂ evolution rates are 1.1, 2.2 and 3.4 μmol m⁻² min⁻¹ for Co₃O₄-1, Co₃O₄-2, Co₃O₄-3, respectively. This result clearly demonstrates that the surface area of Co₃O₄ material is not the only factor affecting its catalytic activity. Considering the structural difference of the three materials, the crystallinity of Co₃O₄ might be the other important factor affecting the catalytic activity. A higher crystallinity, which is generally achieved at a higher reaction temperature, could decrease the recombination of photo-generated electron–hole pairs, and thus improve the catalytic activity. The above results indicate that simultaneous achievement of high surface area and high crystallinity in a Co₃O₄ material might be a feasible strategy to enhance its catalytic activity towards water oxidation.

Fig. 4 (A) Typical O₂ evolution–time curves with Co₃O₄-1, Co₃O₄-2, Co₃O₄-3, Com-Co₃O₄ and no catalyst under visible-light irradiation; and (B) cycles of the catalytic water oxidation in the presence of Co₃O₄-1. The photochemical O₂ evolution was continued for 60 min with the removal of O₂ in the reaction cell every 5 min. Reaction conditions: catalyst, 5 mg; reaction solution: Na₂SiF₆–NaHCO₃ buffer solution (20 mL, pH = 5.8); Na₂S₂O₈, 65 mg; [Ru(bpy)₃]Cl₂·6H₂O, 15 mg; light source, 500 W Xe lamp (λ > 420 nm; light intensity, 190 mW cm⁻²).

To further assess the catalytic activity of the Co₃O₄-1 material, its turnover frequency (TOF) was calculated based on the assumption that all the Co ions present in the material were catalytically active (this method was also used previously³). The as-obtained TOF might be a gross underestimation, because not every Co in the material is catalytically accessible or active. The TOF of Co₃O₄-1 was found to be 2.86 × 10⁻⁴ S⁻¹ (Table S1†), which is comparable with those of the existing Co₃O₄-based catalysts for photochemical water oxidation (e.g., 2.4 × 10⁻⁴ S⁻¹ for a mesoporous Co₃O₄ material;^12a 2.12–4.05 × 10⁻⁴ S⁻¹ for porous silica/carbon-supported Co₃O₄ nanoclusters¹⁰). However, the TOF value of Co₃O₄-1 is lower than those of Co₃O₄ materials that were used for electrochemical water oxidation (e.g., 7.8 × 10⁻³ for Co₃O₄ film;^16a 2.76 × 10⁻² for Co₃O₄ electrode;^16b 9.3 × 10⁻³ for Co₃O₄ nanoparticle⁶).

The stability of the Co₃O₄-1 catalyst was demonstrated by a recycling experiment (Fig. 4B). Although Co₃O₄ is known to be unstable at neutral and lower pH,¹⁷ our results indicate that our material Co₃O₄-1 is stable for up to 30 minutes (six cycles) under our experimental conditions. But further catalytic experiment shows that the catalytic activity of Co₃O₄-1 starts to decrease. To find out the reason behind the decrease of catalytic activity, the sample after catalytic reaction was carefully characterized (Fig. S11†). After the catalytic water oxidation reaction, the material, e.g., Co₃O₄-1, did not show any change in its crystal structure (see XRD pattern in Fig. S11A†). The octahedron-like morphology of Co₃O₄-1 was also largely maintained after catalytic reaction (see SEM in Fig. S11B†). But there seemed be some impurities that were deposited on the Co₃O₄-1 particles' surface (Fig. S11B†). Further XPS characterization (Fig. S11C and D†) demonstrated that a considerable amount of silica were present in the sample after catalytic reaction. The formation of silica may be due to the hydrolysis of Na₂SiF₆ in the buffer during photolysis.^12a Silica on the surface of Co₃O₄-1 particles will reduce the surface reactive sites of the material, leading to the decrease of the catalytic activity.

Conclusions

In summary, we report a facile synthetic approach to a porous, high-surface-area, core–shell-type Co₃O₄ material that has efficient catalytic activity towards WOR. We also believe that the work could encourage the exploration for the template-free synthesis of high-surface-area WOR catalysts.

Acknowledgements

This work was supported by the NSFC (21071060, 21371070); the National Basic Research Program of China (2013CB632403); Jilin province science and technology development projects (20140101041JC, 20130204001GX).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and supporting results. See DOI: 10.1039/c4ra02134g

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