1.

Introduction

With the serious energy crisis and increasing environmental concerns, development of better energy storage materials is urgently needed.^1,2 Photocatalytic water splitting is an environmentally friendly and promising way to store unexploited solar energy in the form of hydrogen because of its high energy density.^3–5 The photocatalyst is one of the key factors in the photocatalytic water splitting.^6–9 The major challenge in this area is to create a sustainable and efficient photocatalyst that can work well with visible light, which is the major component (i.e., $\sim 50\%$) of the solar spectrum.¹⁰ Nanostructured metal oxides (such as ${\mathrm{TiO}}_2$, ${\mathrm{SnO}}_2$, ${\mathrm{Co}}_3{\mathrm{O}}_4$, ${\mathrm{Fe}}_2{\mathrm{O}}_3$, CuO, and ${\mathrm{WO}}_3$) are novel materials for photocatalytic water splitting under visible light irradiation.^11–15 Among them, ${\mathrm{Co}}_3{\mathrm{O}}_4$ as a p-type semiconductor is particularly attractive in this area because of its suitable bandgap of 3.95 to 2.13 eV.¹⁶

Synthesis of ${\mathrm{Co}}_3{\mathrm{O}}_4$ nanoparticles has been reported using various methods, such as thermal decomposition,¹⁷ template method,¹⁸ hydrothermal method,¹⁹ microwave-assisted,²⁰ and chemical spray pyrolysis.²¹ These methods usually consume more energy, are capital intensive, and use toxic chemicals in the preparation process.^16,22 As an alternative to these traditional methods, green synthesis is considered to be a safe, economic, and ecological method for the preparation of nanoparticles.²³ Green synthesis by naturally derived materials, including plant extract and components and microorganisms, has been studied for mass production to detoxify and degrade hazardous pollutants.^24,25 For example, Abukhadra et al.²⁶ reported MCM-41/${\mathrm{Co}}_3{\mathrm{O}}_4$ nanocomposite was synthesized from rice husk silica gel and peach leaves, for enhanced photocatalytic degradation of acephate pesticide. Salam et al.²⁷ prepared organo-bentonite/${\mathrm{Co}}_3{\mathrm{O}}_4$ green nanocomposite and investigated it as a potential eco-friendly, low-cost adsorbent and photocatalyst for promising removal of both malachite green dye and Cr(VI) ions.

Therefore, developing an efficient and stable photocatalyst by an environmentally friendly, low-cost, and efficient green-synthesis method to accomplish the overall water splitting without using sacrificial agents, noble metals, and external bias is still a challenge. In this paper, we report on the green synthesis, characterization, and catalytic effect of the green-synthesized ${\mathrm{Co}}_3{\mathrm{O}}_4$ nanoparticles on overall water splitting. The effects of the calcination temperature on the structure and properties, and the ambient temperature on the photocatalytic process, have also been reported.

2.

Experimental

3.

Results and Discussion

To investigate the morphology of ${\mathrm{Co}}_3{\mathrm{O}}_4$ nanoparticles, SEM was employed, and the results are shown in Fig. 1. As shown in Fig. 1(a), the nanoparticles calcined at 700°C show the hexagonal structure, some of which are very large in size. As can be seen from Figs. 1(b)–1(d), the nanoparticles calcined at 600°C, 500°C, and 400°C have spherical structure, and the distribution is very uniform. On the other hand, the nanoparticles calcined at 400°C are very large and agglomerated as shown in Fig. 1(e), which are not suitable for use as photocatalysts.

Fig. 1

SEM images of ${\mathrm{Co}}_3{\mathrm{O}}_4$ nanoparticles calcined at the different temperature: (a) 700°C, (b) 600°C, (c) 500°C, (d) 450°C, and (e) 400°C.

Figure 2 presents the XRD pattern of ${\mathrm{Co}}_3{\mathrm{O}}_4$ nanoparticles calcined at different temperatures. The peaks at $2\theta$ values [19.1 deg, 31.2 deg, 36.9 deg, 38.5 deg, 44.8 deg, 55.6 deg, 59.4 deg, 65.3 deg, 74.1 deg, and 77.3 deg corresponding to (111), (200), (311), (222), (400), (422), (511), (440), (620), and (533) planes of ${\mathrm{Co}}_3{\mathrm{O}}_4$, respectively] are indexed to the cubic ${\mathrm{Co}}_3{\mathrm{O}}_4$ (JCPDS42-1467).^28,29 It can be seen from Fig. 2, there is no other impurity peak, indicating high purity of ${\mathrm{Co}}_3{\mathrm{O}}_4$. At the same time, the diffraction peak is sharp, suggesting that the crystallinity of ${\mathrm{Co}}_3{\mathrm{O}}_4$ is relatively high. But with the increase in temperature, the small peaks according to (222), (422), (620), or (533) planes of ${\mathrm{Co}}_3{\mathrm{O}}_4$ appeared, which may be due to the morphology of ${\mathrm{Co}}_3{\mathrm{O}}_4$ as shown in Fig. 1. By using Sheerer formula $D=0.94\lambda /\beta \cos \theta$, the size of ${\mathrm{Co}}_3{\mathrm{O}}_4$ nanoparticles according to the diffraction peak of (311) crystal plane is 15, 14, 19, and 18 nm for the calcination temperature at 450°C, 500°C, 600°C, and 700°C, respectively.

Fig. 2

XRD spectrum of ${\mathrm{Co}}_3{\mathrm{O}}_4$ calcined at different temperatures.

To verify the micro-structure of ${\mathrm{Co}}_3{\mathrm{O}}_4$, the TEM image of ${\mathrm{Co}}_3{\mathrm{O}}_4$ nanoparticles calcined at 500°C is shown in Fig. 3. From Fig. 3(a), it can be seen that ${\mathrm{Co}}_3{\mathrm{O}}_4$ nanoparticles are uniformly dispersed and their size is less than 20 nm. As shown in Fig. 3(b), the lattice fringes of ${\mathrm{Co}}_3{\mathrm{O}}_4$ can be observed clearly. The fringes with spacing of 0.46 nm correspond to (111) lattice plane (JCPDS42-1467) of the cubic ${\mathrm{Co}}_3{\mathrm{O}}_4$, suggesting ${\mathrm{Co}}_3{\mathrm{O}}_4$ nanoparticles exhibit good crystallinity.

Fig. 3

TEM images of ${\mathrm{Co}}_3{\mathrm{O}}_4$ nanoparticles calcined at 500°C: (a) low magnification and (b) high magnification.

The UV–Vis spectra of ${\mathrm{Co}}_3{\mathrm{O}}_4$ nanoparticles are shown in Fig. 4. As can be seen from Fig. 4, there are two broad peaks in the wavelength range of 300 to 450 nm and 600 to 750 nm. The first peak can be assigned to ${\mathrm{O}}^{2-}$ to ${\mathrm{Co}}^{2+}$ charge transfer, and the second one can be assigned to ${\mathrm{O}}^{2-}$ to ${\mathrm{Co}}^{3+}$ charge transfer.³⁰ The peak intensity of ${\mathrm{Co}}_3{\mathrm{O}}_4$ nanoparticles calcined at 500°C is the highest, which indicates that ${\mathrm{Co}}_3{\mathrm{O}}_4$ nanoparticles calcined at 500°C can absorb more light. When the temperature is higher than 500°C, the peak intensity decreases gradually. As discussed by XRD, the size of ${\mathrm{Co}}_3{\mathrm{O}}_4$ nanoparticles calcined at 500°C is the smallest, and the nanoparticles size increases with the increase of temperature. This suggests that the calcination temperature not only affects the grain size but also the light absorption.

Fig. 4

UV–Vis spectrum of ${\mathrm{Co}}_3{\mathrm{O}}_4$ calcined at different temperatures.

Figure 5 shows the amount of the hydrogen and oxygen evolution under visible irradiation over ${\mathrm{Co}}_3{\mathrm{O}}_4$ calcined at 450°C and 500°C as the function of the irradiation time. From Fig. 5(a), the evolution of the hydrogen is $247.1\mu {\mathrm{molg}}^{-1}$ and that of the oxygen is $109.5\mu {\mathrm{molg}}^{-1}$ for ${\mathrm{Co}}_3{\mathrm{O}}_4$ photocatalyst calcined at 450°C in 5 h without use of any cocatalyst and sacrificial agent. Moreover, the ratio of the hydrogen to the oxygen is almost 2:1. ${\mathrm{Co}}_3{\mathrm{O}}_4$ calcined at 500°C also showed the same trend. For overall water splitting, the amount of the hydrogen evolution is $259.4\mu {\mathrm{molg}}^{-1}$ and that of the oxygen is $135.7\mu {\mathrm{molg}}^{-1}$ in 5 h using ${\mathrm{Co}}_3{\mathrm{O}}_4$ calcined at 500°C as shown in Fig. 5(b). Obviously, ${\mathrm{Co}}_3{\mathrm{O}}_4$ photocatalyst calcined at 500°C shows the better photocatalytic activity than that of ${\mathrm{Co}}_3{\mathrm{O}}_4$ calcined at 450°C and is chosen for further photocatalytic experiments by controlling the ambient temperature.

Fig. 5

Reaction time courses of the hydrogen and the oxygen evolution under visible irradiation over ${\mathrm{Co}}_3{\mathrm{O}}_4$ calcined at different temperatures: (a) 450°C and (b) 500°C.

To study the effect of the ambient temperature on the photocatalytic process, the reaction temperature was controlled at 40°C, 50°C, and 60°C, and the results about the amount of the hydrogen and the oxygen evolution are shown in Fig. 6. With the increase of temperature, the evolution of hydrogen and oxygen also increases. The increase of the evolution may be due to the increase in kinetics of the reaction. The increase of temperature effect may increase the mobility of carriers. These charge carriers then move on to the surface with less recombination rate and react fast to produce hydrogen and oxygen. As shown in Fig. 6, the amount of hydrogen at 40°C, 50°C, and 60°C is 268.9, 284.6, and $310.7\mu {\mathrm{molg}}^{-1}$, respectively, while the amount of oxygen is 133.1, 146.4, and $175.3\mu {\mathrm{molg}}^{-1}$, respectively. Obviously, the ratio of the hydrogen to oxygen is near 2:1 in all experiments, which means that ${\mathrm{Co}}_3{\mathrm{O}}_4$ photocatalyst can completely split pure water without using any cocatalyst and sacrificial agent.

Fig. 6

Amount of the hydrogen and the oxygen evolution under visible irradiation in 5 h over ${\mathrm{Co}}_3{\mathrm{O}}_4$ calcined at 500°C as the function of the ambient temperature.

The photocatalytic stability of ${\mathrm{Co}}_3{\mathrm{O}}_4$ nanoparticles was evaluated under visible light irradiation at room temperature for 12 h. The experiment was done in three cycles. After 4 h of each cycle, the photocatalyst was recovered by centrifugation and drying and used in the next cycle. Figure 7 shows the stability performance of ${\mathrm{Co}}_3{\mathrm{O}}_4$ over three cycles. It can be seen from Fig. 7 that the hydrogen and the oxygen evolution of the latter cycle is only slightly lower than that of the previous cycle, and the reduction rate is less than 9%, which suggests that the ${\mathrm{Co}}_3{\mathrm{O}}_4$ for overall water splitting is stable.

Fig. 7

Reaction time courses of the hydrogen and the oxygen evolution under the visible irradiation over ${\mathrm{Co}}_3{\mathrm{O}}_4$ calcined at 500°C at room temperature.

4.

Conclusion

Through this environmentally friendly and inexpensive method, ${\mathrm{Co}}_3{\mathrm{O}}_4$ nanoparticles were successfully synthesized using the bread fungus as green materials. These nanoparticles calcined at 500°C had the smallest grain size, the uniform distribution, and the best light absorption. Furthermore, ${\mathrm{Co}}_3{\mathrm{O}}_4$ nanoparticles were used as a photocatalyst for overall water splitting under visible light irradiation without any cocatalyst and sacrificial agent. The evolution rates of the hydrogen and the oxygen increased by increasing the photocatalytic temperature, and the ratio of the hydrogen to oxygen is close to 2:1. In addition, ${\mathrm{Co}}_3{\mathrm{O}}_4$ photocatalyst had a good stability. Especially, ${\mathrm{Co}}_3{\mathrm{O}}_4$ calcined at 500°C showed a good photocatalytic performance. Within 5 h, the amount of hydrogen and oxygen is 259.4 and $135.7\mu {\mathrm{molg}}^{-1}$, respectively. Therefore, the green synthesis of cobalt oxide is a promising method for overall water splitting.