Exploration of octahedrally shaped MnCo₂O₄ catalyst particles for visible light driven photocatalytic water splitting reaction

Abstract

MnCo₂O₄ catalyst with a smooth octahedral geometry and a band gap of 2.11 eV was developed for a visible light driven photocatalytic water splitting process considering the crucial role of Co(III)OH centres in Co₃O₄ catalysts, as well as cobalt(II) oxide and Mn³⁺ centres in the most active Mn₂O₃ catalyst towards photocatalytic water splitting process. The formation of the MnCo₂O₄ catalyst was confirmed by XRD, EDAX and XPS. The smooth octahedral geometry of most of the crystals synthesized was visualized via FE-SEM analysis. The photocatalytic efficiency of the catalyst was analyzed with respect to time, presence of sacrificial agent (methanol) and in presence of direct sunlight. 0.08 g of the catalyst produced about 33 mL of gaseous products from 20 mL of water after 95 minutes of visible light irradiation (λ > 420 nm). The overall water splitting efficiency was increased in the presence of a small amount of methanol; hydrogen GC spectra revealed that evolved hydrogen gas produced in the presence of methanol was extra pure. The reusability analysis revealed that the synthesized catalyst could be reused after the activation process possibly due to the surface restructuring of the active centres of the catalyst during the continuous water splitting process. There was a slight decrease in the activity of the reused catalyst due to the decrease in concentration of the octahedrally shaped crystals, as evidenced by the FE-SEM analysis. These results revealed that the developed octahedrally shaped MnCo₂O₄ catalyst will be an essential part for the future generation of visible light driven water splitting catalysts.

---

Introduction

Cobalt and manganese based photocatalytic materials are emerging as efficient catalytic materials for water splitting reactions.^1,2 The manganese oxides are able to provide the thermodynamic driving force required to split water over a wide pH range.³ In the photosynthetic systems, the manganese also plays key role in water oxidation; Mn₄CaO₅ being the crucial component present in such system.⁴ Moreover, Mn₂O₃ is reported as the most active catalyst for the water splitting process.^5,6 The catalytic activity of manganese oxide catalyst significantly increased in basic media maybe due to the higher stability of Mn³⁺ ions in the basic medium.⁷ The presence of Mn³⁺ in the oxide surface is essential for the oxidation of water; however, Mn³⁺ ion disproportionates in to Mn²⁺ and Mn⁴⁺ at acidic and neutral pH and catalytic water oxidation efficiency is decreased.⁷ Manganese oxides are incorporated with [Ru(bpy)₃]³⁺ along with H₂O₂ or ozone to obtain higher catalytic activity under neutral and weakly acidic conditions.⁸ Silica supported MnO_x activated with [Ru(bpy)₃]³⁺ are also emerging as efficient catalysts for water oxidation catalysis.⁹ The spinel Co₃O₄ and micro-structured cobalt oxide were also used as efficient water oxidation catalysts in the presence of visible light; however, the cobalt oxide catalysts were severely deactivated due to the formation of cobalt hydroxide precipitates.¹⁰ This deactivation process could be significantly reduced in Co and Mn based spinel catalysts due to the Co–Mn synergistic interaction;^11,12 thus, these systems emerged as effective visible light driven water splitting catalysts. Moreover, the time-resolved observations of water oxidation intermediates on a cobalt oxide catalyst revealed the crucial role of Co(III)OH centres in Co₃O₄ for determining the photocatalytic efficiency.¹³ Moreover, F. Song et al. recently explored a Co²⁺ and Co³⁺ containing mixed-valence catalyst for efficient visible light driven water oxidation.¹⁴ In addition, the cobalt(II) oxide nanoparticles have recently emerged as visible photocatalyst with higher efficiency.¹⁵ These literature reports revealed that Mn and Co based spinel catalyst with Mn³⁺ (the most active oxidation state of Mn for water splitting), Co²⁺ (cobalt(II) oxide) and Co³⁺ (active centre in spinel Co₃O₄) with a band gap of less than three will be an efficient catalyst for water splitting process under visible light. Salek et al.¹⁶ reported that MnCo₂O₄ with Mn³⁺, Co³⁺ and Co²⁺ ions at the octahedral and tetrahedral voids possess a visible light active band gap. Moreover, J. Song et al. established the significance of shape selectivity in aerobic decontamination reactions.¹⁷ In addition, NiGa₂O₄ crystals with octahedral shape exhibited significantly higher photocatalytic water splitting efficiency than other geometries.¹⁸ In this context, the development of MnCo₂O₄ catalyst with octahedral shape and favorable orbital and electronic characteristics for visible light driven photo catalytic water splitting is the objective of the present study.

Experimental materials and methods

Synthesis procedure for MnCo₂O₄ catalyst

MnCl₂·4H₂O (AR) and CoCl₂·6H₂O (AR) with a mole ratio of 1 : 2 were thoroughly mixed to form a paste. Excessive amount of the paste was added to a water–isopropanol (1 : 1) mixture, heated at 100 ± 3 °C with stirring, and evaporated to dryness. The mixture obtained was powdered and a small quantity of water–isopropanol mixture was added, and the same process was repeated. The mixture obtained was again efficiently powdered and placed in a muffle furnace at 470 ± 5 °C for 12 hours. Every half an hour, the mixture was carefully removed, placed in air and stirred well. All the conditions were fixed based on the reproducibility of the desired characteristics of the products, obtained after physico-chemical analysis.

Physicochemical characterization of MnCo₂O₄ catalyst

XRD patterns of the samples were recorded using a Bruker AXS D8 Advance Diffractometer with Cu Kα radiation. XRF analysis was carried out using a Bruker Model S4 Pioneer Sequential Wavelength Dispersive X-ray Spectrometer. XPS investigations were carried out on an OMICRON EA-125 Photoelectron spectrometer at a base pressure of more than ∼1 × 10⁻¹⁰ Torr. A Hitachi SU6600 Variable Pressure Field Emission Scanning Electron Microscope (FESEM) was used for obtaining FE-SEM images. Energy dispersive spectroscopic analysis was conducted using a Horiba, EMAX (137 eV). To examine the particle size distribution of the catalysts, transmission electron microscopy (TEM) (model TecnaiG2-20) was employed. A reference sample was analyzed and the accuracy was confirmed each time prior to the analysis of the actual sample.

Photocatalytic water splitting analysis of MnCo₂O₄ catalyst using a photoreactor

Photocatalytic water splitting was investigated using a photoreactor manufactured by Popular Science Apparatus Workshops Pvt. Ltd. (PSAW India) with visible light (λ > 420 nm) having 8 tubes of 8 W (SANYO, Electric Co. Ltd. Japan) capacity, which were concentrically arranged to obtain a uniform illumination. 20 mL of distilled water was placed in a quartz tube containing an appropriate amount of powdered MnCo₂O₄ catalyst and was irradiated with visible light for an appropriate time with continuous stirring for photocatalytic water splitting. The gases evolved were collected using a measuring gar. 1 mL of methanol was used as the sacrificial agent. In this case, the evolved gas was passed through a saturated NaOH solution for the absorption of carbon dioxide gas. Direct sunlight was used to analyze the performance of the catalyst under sunlight and irradiation was applied on sunny days from 12.30 pm to 14.30 pm (Kerala, India) when solar intensity fluctuations were minimum. The same experiments were repeated for 5 days and average readings are included in the Fig. 5C.

Results and discussions

Standardization of the synthetic parameters for MnCo₂O₄

The entire synthetic parameters of the catalyst, such as precursors, solvent, temperature of evaporation and calcination temperatures, were optimized based on the geometry, band gap, oxidation states of Mn and Co and the overall stability of the catalyst MnCo₂O₄. The catalyst was synthesized from a chloride precursor for the preferred formation of smooth octahedral crystals with Co³⁺/Mn³⁺ at octahedral site.¹⁹ Kudo and Miseki²⁰ reported that a catalyst with smooth crystal structure will reduce the electron hole pair and result in increased photocatalytic activity.²⁰ The standardized synthetic parameters for unsupported MnCo₂O₄ catalyst are given in the Experimental materials and methods. The physicochemical characterization of MnCo₂O₄ and the extended assessment of its catalytic activity were carried out to fix the parameters based on the reproducibility obtained with different batches of the synthesized products.

Evidence for the formation of MnCo₂O₄

The nature of the formation of the inverse spinel MnCo₂O₄ catalyst was analyzed by comparing the XRD pattern of the synthesized MnCo₂O₄ catalyst with the available reports. The XRD pattern in Fig. 1 of ESI† revealed that no other phases, except MnCo₂O₄ spinel peaks, were formed. The peaks were found to be sharp, similar to those observed by Esko et al.²¹ and Nissinen et al.²² for MnCo₂O₄ inverse spinel phases at 900 °C. The confirmation of the formation of inverse spinel in MnCo₂O₄ can be analyzed from the binding energy spectra obtained from the XPS analysis; the results are discussed in the following section.

Confirmation of the formation of MnCo₂O₄ catalyst

The binding energy spectrum, shown in Fig. 1, confirmed the presence of Mn³⁺, Co³⁺ and Co²⁺ in the synthesized catalyst. The Mn2p-electron binding energy and Co2p electron binding were measured using XPS. The spectrum consists of spin–orbit-split 2p_3/2 and 2p_1/2 peaks at about 641 and 653 eV, respectively, in the case of Mn(2p), which is evidenced in the Fig. 1B. These observations revealed the preferred formation of Mn³⁺ of MnCo₂O₄ rather than Mn²⁺/Mn⁴⁺ in MnFe₂O₄.^19,23 The peak at 780 eV (Fig. 1A) reveals the presence of both Co³⁺ and Co²⁺, respectively, for Co2p_3/2.²³ The satellite peak at 786.4 eV corresponds to Co²⁺ ion.²⁴ The peak at 795.2 eV reveals Co2p_1/2, and the satellite peak at 803.6 eV reveals the presence of Co³⁺/Co²⁺ ions.²⁴ Zhang et al. have also reported a similar XPS spectral pattern for MnCo₂O₄ nanofibers.²⁵ Together, these results confirm the formation and existence of MnCo₂O₄.

Fig. 1 XPS pattern of MnCo₂O₄ confirming the formation of MnCo₂O₄ catalyst.

Evidence for decreased recombination probability in terms of particle size

The distance that the photogenerated electrons and holes have to travel to reach the reaction sites on the surface of the catalyst becomes shorter when the particle size is very small, and thus it results in a decreased recombination probability.²⁰ Therefore, the particle size of the catalyst was analyzed with TEM. The results of TEM analysis revealed that the synthesized particles possess an average particle size of 100 nm, as evidenced from Fig. 2. These results showed that the synthesized MnCo₂O₄ catalyst particles have a smaller size; therefore, it is predicted that the recombination possibility for electron hole pairs is reduced, and the catalyst will be accordingly efficient for the photocatalytic water splitting reactions.

Fig. 2 TEM images of MnCo₂O₄ evidenced the formation of nanoparticles of MnCo₂O₄.

Visualization of MnCo₂O₄ catalyst particles with octahedral morphological characteristics

The crystal quality of the catalysts are vital for obtaining competent photocatalytic activity. When the crystalline quality is higher, the number of defective sites will be lower. The defective sites will act as trapping and recombination centres for photogenerated electrons and holes, resulting in a decreased catalytic activity.²⁰ A FE-SEM analysis was carried out to investigate the morphology of the synthesized catalyst particles of MnCo₂O₄. The FE-SEM images, shown in Fig. 3(A1–A4), revealed that the synthesized catalyst particles have smooth octahedral geometry with clearly seen crystal faces. Zhou et al. reported that in the case of NiGa₂O₄ octahedron nanocrystals, the electron hole recombination rate was lower than in the other shapes. Therefore, there is an enhancement in the photocatalytic activity.¹⁸ The effectiveness of the synthetic procedure from chloride precursors resulted in the smooth octahedral geometry of MnCo₂O₄.¹⁹ The compositions of Mn and Co in the octahedrally shaped crystals were investigated using EDAX analysis. The results of EDAX analysis (ESI Fig. 4†) revealed that the composition of all the crystals, shown in the FE-SEM image (Fig. 3), was the same, and the Mn : Co ratio was 1 : 2.

Fig. 3 Visualization of octahedrally shaped particles in a fresh MnCo₂O₄ catalyst (A1 to A4), a surface restructured used MnCo₂O₄ catalyst particles after drying (B1 to B4), and a decrease in the concentration of octahedrally shaped particles in used catalysts after the activation process (C1 to C4).

Evidence for visible light driven photocatalytic activity of MnCo₂O₄ catalyst in terms of band gap

The band gap of a catalyst has a significant role in determining the photocatalytic water splitting efficiency; the band gap of the visible light driven photocatalysts should be narrower than 3 eV.²⁰ Appropriate band tailoring is critical for the design of visible light driven photocatalysts. The band gap of the synthesized MnCo₂O₄ catalyst was evaluated by following the approach of Pankove and Lampert.²⁶ The Tauc plot and the UV-absorption spectrum (inset) are included in Fig. 4. These results reveal that the MnCo₂O₄ possess a band gap of 2.11 eV, similar to that observed by Salek et al.¹⁶ MnCo₂O₄ is predicted to be an effective catalyst for visible light driven photocatalytic water splitting processes based on the results of TEM, FE-SEM and band gap analysis. The efficiency and practical feasibility of the synthesized MnCo₂O₄ towards visible light driven photo catalytic water splitting processes were analyzed to verify the prediction, and the results are discussed in the subsequent section.

Fig. 4 UV-Visible absorption spectrum of MnCo₂O₄ (inset) and corresponding Tauc plot used for calculating the band gap of MnCo₂O₄.

Efficiency and practical feasibility of unsupported MnCo₂O₄ catalyst for photo catalytic water splitting in presence of visible light

Optimization of the amount of catalyst required for photocatalytic water splitting. The photocatalytic water splitting efficiency of different amounts of the catalyst was analyzed for optimizing the amount of catalyst required for the water splitting process. The amount of the catalyst was increased from 0.01 to 1.5 g for 20 mL of water for 45 minutes of visible light irradiation (λ > 420 nm). As the amount of catalyst was increased from 0.01 to 0.08 g, the amount of gas collected as a result of the photocatalytic water splitting also increased from 7.9 mL to 22.6 mL. When the amount of the catalyst was further increased from 0.08 to 1.5 g, there was no further significant increase in the photocatalytic water splitting performance, as evidenced in the Fig. 2 of ESI.† Therefore, 0.08 g was fixed as the optimum amount for the water splitting reactions.

Evidence of excellent visible water splitting performance of unsupported MnCo₂O₄ catalyst (0.08 g) as a function of time. The performance of the catalyst was monitored under visible light using a photoreactor manufactured by PSAW, Pvt. Ltd. India with visible light (λ > 420 nm) having 8 tubes each of 8 W capacity, which are concentrically arranged to obtain a uniform illumination. The variation in photocatalytic water splitting performance in terms of the volume of gas evolved (mixture of H₂ and O₂ in mL) with time is shown in Fig. 5A. The effective water splitting process starts after 10 minutes of irradiation and the catalytic activity gradually increased to reach a maximum at 45 minutes, as evidenced from the Fig. 5A. The catalytic activity gradually decreased after 45 minutes even when the catalyst was active after 95 minutes, but the gaseous products liberated were significantly lower than the initial amounts. The catalyst exhibited almost a similar trend in the presence of direct sunlight, but the catalyst retained its activity even after 120 minutes. The total gaseous products (H₂ and O₂) collected were about 36 mL after 120 minutes from 0.08 g of the catalyst, as evidenced in Fig. 5C. As can be seen in Fig. 5, the gradual decrease in catalytic activity might be due to the deactivation of the catalyst because of the surface restructuring of the Mn centres by the continuous water splitting reactions.⁹ The most active type II heterostructured nanomaterials still suffer from this type of chemical instability, which significantly decreases the efficiency of the visible photo catalysts.²⁷ The FE-SEM images of the used catalyst, shown in Fig. 3(B1–B4), also revealed that there was a significant surface restructuring due to prolonged water splitting reactions.

Fig. 5 Photocatalytic water splitting efficiency of 0.08 g of MnCo₂O₄ catalyst in the presence of visible light (λ > 420 nm) at room temperature with time (A), in the presence of methanol (B) and in the presence of sunlight (C).

Evidence for enhanced water splitting performance of the unsupported MnCo₂O₄ in the presence of methanol. The sacrificial agents such as methanol act as hole scavengers and enrich electrons in a photocatalyst, as well as the photocatalytic hydrogen evolution reactions.²⁸ The effect of methanol on the water splitting behaviour of the MnCo₂O₄ catalyst was analyzed by adding 1 mL methanol to 19 mL of distilled water before photoirradiation. The gas evolved during the photocatalytic water splitting process was passed through a saturated NaOH solution for the absorption of CO₂ released during the oxidation of methanol, producing pure hydrogen at the measuring chamber. The enhanced water splitting process in the presence of methanol in terms of volume of gas collected (H₂ gas in mL) is shown in Fig. 5B. The water splitting process started rapidly, increased to a maximum value at 50 minutes and gradually decreased. However, the catalyst was effective even after 90 minutes, but with a comparatively lower activity. The hydrogen GC analysis revealed that the evolved hydrogen gas was pure; the GC spectra of evolved hydrogen is depicted in the Fig. 3 of ESI.†

Evidence for the reusability of the catalyst. The used catalyst was again applied in the photoreactor for analyzing the reusability of the catalyst under visible light (λ > 420 nm) at room temperature for 45 minutes. The used catalyst was filtered, dried, and activated at 470 ± 5 °C (muffle furnace) for 60 minutes, and then powdered for the reusability analysis. These parameters were fixed based on the activity, stability and life of the catalyst. The catalyst was active only after the activation process, which confirmed the surface restructuring⁹ of the catalyst that led to the deactivation of the catalyst after the continuous water splitting process. The FE-SEM images, shown in Fig. 3(C1–C4), of the catalyst after the activation process revealed that the extent of surface restructuring was lesser in the activated catalyst after the water splitting process. The activity of the reused catalyst with and without activation is shown in Fig. 6. There was a slight decrease in the activity of the used catalyst after the activation process possibly due to the decrease in the concentration of octahedrally shaped crystals and slight surface restructuring, as evidenced from the FE-SEM analysis shown in Fig. 3(A2–A4) and (C2–C4).

Fig. 6 Reusability of MnCo₂O₄ (A) with activation and (B) without activation.

Predicted mechanism for the overall performance of the catalyst

The octahedrally shaped MnCo₂O₄ possesses a band gap of 2.11 eV, and thus it would generate the necessary electrons and holes for the water splitting process in the presence of visible light.²⁰ The enhanced activity of the catalyst without the presence of a noble metal based co-catalyst/sensitiser might be due to the nano-sized smooth octahedral crystal structure with the presence of active Mn³⁺ and Co²⁺/Co³⁺. Moreover, there is a possibility of a lesser electron hole recombination for the octahedrally shaped crystals.¹⁸ The decrease in the activity of the catalyst after 45 minutes is attributed to the surface restructuring,²⁷ as evidenced from the FE-SEM analysis shown in Fig. 3(B1–B4). The higher activity of the reused catalyst after the activation process is due to the retention of the octahedral shape. However, the slight decrease in the overall activity of the catalyst is due to the decrease in the concentration of the octahedral crystals, as evidenced from the FE-SEM analysis shown in Fig. 3(C1–C4).

Conclusions

MnCo₂O₄ catalyst with a smooth octahedral geometry was tailored and synthesized by considering the crucial role of Co(III)OH centres in Co₃O₄ catalyst and cobalt(II) oxide and Mn³⁺ centres in the most active Mn₂O₃ catalyst towards the photocatalytic water splitting process. The formation of the catalyst was confirmed from the results of EDAX, XRD and XPS analyses. The smooth octahedral geometry of the MnCo₂O₄ catalyst was achieved from a chloride synthetic route and the resulting particles were visualized by FE-SEM analysis. Moreover, the synthesized particles possess lesser sizes, as evidenced from TEM. Therefore, the probability of the recombination of electron–hole pairs was decreased. The band gap analysis revealed that the synthesized MnCo₂O₄ catalysts possess a band gap of 2.11 eV, and therefore the catalyst was applied for visible light driven water splitting process. 0.08 g of the catalyst produced about 33 mL of gaseous products from 20 mL of water after 95 minutes of visible light irradiation (λ > 420 nm). The overall water splitting efficiency was increased in the presence of a small amount of methanol; hydrogen GC spectra revealed that the evolved hydrogen gas in the presence of methanol was extra pure. The reusability analysis revealed that the synthesized catalyst could be reused after the activation process possibly due to the surface restructuring of the active centres of the catalyst during the continuous water splitting process.

Acknowledgements

We thank M/s Sandhya Lekshmi, R. and Mr Aravind Mohan, AgriInformatics, Indian Institute of Information Technology and Management-Kerala for designing the graphical abstract.

Notes and references

1. M. W. Kanan and D. G. Nocera, In situ Formation of an Oxygen-Evolving Catalyst in Neutral Water Containing Phosphate and Co²⁺, Science, 2008, 321, 1072–1075.
2. A. Listorti, J. Durrant and J. Barber, Artificial photosynthesis: Solar to fuel, Nat. Mater., 2009, 8, 929–930.
3. M. Wiechen, I. Zaharieva, H. Dau and P. Kurz, Layered manganese oxide for water-oxidation: alkaline earth cations influence catalytic activity in a photosystem II-like fashion, Chem. Sci., 2012, 3, 2330–2339.
4. K. Sauer and V. K. Yachandra, A possible evolutionary origin for the Mn₄ cluster of the photosynthetic water oxidation complex from natural MnO₂ precipitates in the early ocean, Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 8631–8636.
5. M. Morita, C. Iwakura and H. Tamura, The anodic characteristics of manganese dioxide electrodes prepared by thermal decomposition of manganese nitrate, Electrochim. Acta, 1977, 22, 325–328.
6. A. Singh and L. Spiccia, Water oxidation catalysts based on abundant 1st row transition metals, Coord. Chem. Rev., 2013, 257, 2607–2622.
7. T. Takashima, K. Hashimoto and R. Nakamura, Mechanisms of pH-dependent activity for water oxidation to molecular oxygen by MnO₂ electrocatalysts, J. Am. Chem. Soc., 2012, 134, 1519–1527.
8. D. M. Robinson, Y. B. Go, M. Greenblatt and G. C. Dismukes, Water Oxidation by λ-MnO₂: Catalysis by the Cubical Mn₄O₄ Subcluster Obtained by Delithiation of Spinel LiMn₂O₄, J. Am. Chem. Soc., 2010, 132, 11467–11469.
9. F. Jiao and H. Frei, Nanostructured manganese oxide clusters supported on mesoporous silica as efficient oxygen-evolving catalysts, Chem. Commun., 2010, 46, 2920–2922.
10. G. L. Elizarova, L. G. Matvienko, N. V. Lozhkina, V. N. Parmon and K. I. Zamaraev, Homogeneous catalysts for dioxygen evolution from water. Water oxidation by trisbipyridylruthenium(III) in the presence of cobalt, iron and copper complexes, React. Kinet. Catal. Lett., 1981, 16, 191–194.
11. Y. Okuno, O. Yonemitsu and Y. Chiba, Manganese dioxide as specific redox catalyst in the photosensitized oxygen generation from water, Chem. Lett., 1983, 12, 815–818.
12. F. Conrad, M. Bauer, D. Sheptyakov, S. Weyeneth, D. Jaeger, K. Hametner, P.-E. Car and G. R. Patzke, New spinel oxide catalysts for visible-light-driven water oxidation, RSC Adv., 2012, 2, 3076–3082.
13. M. Zhang, M. D. Respinis and H. Frei, Time-resolved observations of water oxidation intermediates on a cobalt oxide nanoparticle catalyst, Nat. Chem., 2014, 6, 362–367.
14. F. Song, Y. Ding, B. Ma, C. Wang, Q. Wang, X. Du, S. Fu and J. Song, Energy Environ. Sci., 2013, 6, 1170–1184.
15. L. Liao, Q. Zhang, Z. Su, Z. Zhao, Y. Wang, Y. Li, X. Lu, D. Wei, G. Feng, Q. Yu, X. Cai, J. Zhao, Z. Ren, H. Fang, F. R. Hernandez, S. Baldelli and J. Bao, Efficient solar water-splitting using a nanocrystalline CoO photocatalyst, Nat. Nanotechnol., 2014, 9, 69–73.
16. G. Salek, S. G. Fritsch, P. Dufour and C. Tenailleau, A simple preparation process of pure Mn_3−xCo_xO₄ (x = 1, 1.5, 2) desert rose-like nanoparticles and their optical properties, Int. J. Chem., 2012, 4, 44–53.
17. J. Song, D. K. Britt, H. Furukawa, O. M. Yaghi, K. I. Hardcastle and C. L. Hill, J. Am. Chem. Soc., 2011, 133, 16839–16846.
18. S. X. Zhou, X. J. Lv, C. Zhang, X. Huang, L. Kang, Z. S. Lin, Y. Chen and W. F. Fu, ChemPlusChem, 2014, 79, 1–9.
19. Y. W. Wang, W. Tian, T. Zhai, C. Zhi, Y. Bando and D. Golberg, Cobalt(II,III) oxide hollow structures: fabrication, properties and applications, J. Mater. Chem., 2012, 22, 23310–23326.
20. A. Kudo and Y. Miseki, Heterogeneous photocatalyst materials for water splitting, Chem. Soc. Rev., 2009, 38, 253–278.
21. K. U. Esko, E. L. Rautama, M. Laitinen, T. Sajavaara and M. Karppinen, Control of oxygen nonstoichiometry and magnetic property of MnCo₂O₄ thin films grown by atomic layer deposition, Chem. Mater., 2010, 22, 6297–6300.
22. T. Nissinen, T. Valo, M. Gasik, J. Rantanen and M. Lampinen, J. Power Sources, 2002, 106, 109–115.
23. K. J. Kim and J. W. Heo, Electronic structures and optical properties of inverse spinel MnCo₂O₄ thin films, J. Korean Phys. Soc., 2012, 60, 1376–1380.
24. Y. Sharma, N. Sharma, G. V. Subba Rao and B. V. R. Chowdari, Studies on spinel cobaltites, FeCo₂O₄ and MgCo₂O₄ as anodes for Li-ion batteries, Solid State Ionics, 2008, 179, 587–597.
25. Y. Zhang, L. Luo, Z. Zhang, Y. Ding, S. Liu, D. Deng, H. Zhaoa and Y. Chen, J. Mater. Chem. B, 2014, 2, 529–535.
26. J. I. Pankove and M. A. Lampert, Model for electroluminescence in GaN, Phys. Rev. Lett., 1974, 33, 361–365.
27. Y. Wang, Q. Wang, X. Zhan, F. Wang, M. Safdar and J. He, Visible light driven type II heterostructures and their enhanced photocatalysis properties: a review, Nanoscale, 2013, 5, 8326–8339.
28. T. Kawai and T. Sakata, Conversion of carbohydrate into hydrogen fuel by a photocatalytic process, Nature, 1980, 286, 474–476.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra12646g

---