Multi-electron transfer by visible light irradiation has been attracting attention of chemists (Zou et al. 2001; Fujishima and Honda 1972; Sayama et al. 2001; Kato et al. 2004; Konta et al. 2004; Zong et al. 2010; Zhang et al. 2010; Wang 2009; Furukawa et al. 2007; Kawahara et al. 2007a, b), because such a transfer will be a key for obtaining functional materials such as electronic and/or magnetic devices, oxidation–reduction catalysts, artificial photosynthesis catalysts, solar cells, and so on. In our previous works, we found that the calcination of hybrid materials composed of organic polymers and heavy metals such as Ti, Zr, Hf, W, Ta and V under a reducing atmosphere produced new types of carbon cluster composite materials uniformly dispersed with the corresponding nano-sized metal oxides, i.e., SnO2 (Matsui et al. 2007a), TiO2 (Miyazaki et al. 2008), ZrO2 (Miyazaki et al. 2009), HfO2 (Matsui et al. 2009a), WO3 (Matsui et al. 2007b) and V2O3 (Matsui et al. 2009b). The composite materials can stimulate the electron transfer between the metal oxides and the carbon clusters under visible light irradiation with a light-responsive oxidation–reduction function. Here, it should be indicated that these oxides essentially have too high band gaps and impossible to exhibit a visible light-responsive electron excitation. However, we assume that in our systems some bonding between the interface of oxides and carbon clusters along with the doping of carbon atom inside the oxides could be formed to affect the features of the band gaps and/or electron movements of oxides (Miyazaki et al. 2009; Matsui et al. 2007b, 2009a, b). Further, carbon clusters are considered to enhance the light-absorption ability (Zhao et al. 2010). If this assumption is true, then the combination of carbon clusters and insulating materials with higher band gaps is also expected to cause visible light-responsive electron excitation. In the present study, we selected Al2O3 with the band gap of 9.0 eV for a suitable insulating material to synthesize composite materials and subsequently studied their water splitting activity under visible irradiation. Here, we report a simple synthetic route for the synthesis of novel nano-sized Al2O3/carbon cluster composite materials and their effective water splitting activity under visible light irradiation.

A mixture of 2.95 g (9.30 mmol) of Al2(C2O4)3·H2O, 5.34 g (56.8 mmol) of phenol, 4.61 g (154 mmol) of a 37 % formalin solution and 40 mL of a 1 mol/L sodium hydroxide solution was heated for 4 h to obtain Al2(C2O4)3-dispersed phenol resin I (Al 6.85 %, H 3.83 %, C 48.5 %. [C]/[Al] = 16). Inductively coupled plasma-atomic emission spectrometer (ICP-AES) and energy dispersive X-ray spectroscopy (EDX) measurements were used to analyze composition of the I and Ic. Elemental analysis was performed for C and H using Yanaco MT-6. Al composition was measured using ICP-AES using Shimadzu ICP-7500. The scanning electron microscope-energy dispersive X-ray (SEM-EDX) analysis of I showed that Al atom was uniformly dispersed in the resin. One gram of I in a porcelain crucible was heated under an argon atmosphere with a heating rate of 5 °C/min and kept at 500 °C for 1 h using a Kouyou KTF045N furnace to obtain black-colored material Ic (Al 18.9 %, H 1.48 %, C 45.1 %. [C]/[Al] = 5).

The X-ray photoelectron spectroscopy (XPS) measurement of Ic revealed the presence of a peak at 74.9 eV due to the Al2p orbital of Al2O3. The transmission electron microscopic (TEM) image of Ic showed the presence of particles with the diameters of ca. 5 nm, possibly Al2O3, in carbon matrix (Fig. 1). These results suggest that the calcined material was composed of nano-sized Al2O3 and carbon clusters. To evaluate the electron transfer process in Ic, the electron spin resonance (ESR) spectra of Ic in the presence of either a reductant (1,4-hydroquinone) or an oxidant (1,4-benzoquinone) under the irradiation of light (λ > 460 nm) were measured. The addition of the reductant to Ic decreased the ESR signal intensity and the addition of the oxidant increased the intensity (Fig. 2), suggesting that the material has a photo-responsibility and the electron transfer process in Ic is carbon clusters → Al2O3 particles to form an oxidation site at the carbon clusters and a reduction site at the Al2O3 parts.

Fig. 1
figure 1

TEM image of Ic

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Fig. 2
figure 2

ESR spectra of Ic in the presence of either 1,4-hydroquinone or 1,4-benzoquinone under the irradiation of light (λ > 460 nm)

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The visible light-responsive oxidation–reduction function of the present newly synthesized composite material was examined. First, the reduction reaction of methylene blue with Ic under the irradiation of visible light (λ > 460 nm) was spectrometrically performed. The absorption peak intensity of methylene blue was found to decrease with the irradiation time (Fig. 3), but essentially no degradation took place in the dark. Next, the visible light-irradiated oxidation–reduction reaction of an aqueous 0.05 mmol/L silver nitrite solution with Ic was performed to give a [O2]:[Ag] ratio of 1:4. Here, if a four-electron oxidation–reduction reaction takes place, then a [O2]:[Ag] ratio is given to be 1:4. These findings indicate an occurrence of smooth visible light-responsive oxidation–reduction ability.

Fig. 3
figure 3

UV-Vis spectra of methylene blue in the presence of Ic under the irradiation of visible light (λ > 460 nm)

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It is known that the loading of Pt particles on the semiconductors increases the reduction ability. The surface of Ic was thus modified with Pt particles by reacting with a hydrogen hexachloroplatinate aqueous methanol solution under visible light irradiation to obtain Pt-loaded material denoted as Ic-Pt. The TEM image of Ic-Pt revealed the presence of highly dispersed Pt particles of ca. 5 nm diameters on the surface of Ic (figure is not shown). The ICP-AES analysis of Ic-Pt showed a Pt content of 0.5 wt%. Water splitting experiments using Ic and Ic-Pt were performed in the following way. A stirred mixture of water (0.2 mL) and the calcined materials (10 mg) in a sealed tube was irradiated by visible light (λ > 460 nm) at room temperature for 12 h, and the evolved gases were analyzed by gas chromatography. The results are shown in Table 1. It is quite interesting to note that H2 and O2 were obtained with a H2/O2 molar ratio of 2 for Ic-Pt, while no gas was evolved with Ic.

Table 1 Water decomposition with the use of Ic and Ic-Pt under the irradiation of visible light (λ > 460 nm)
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We have fundamentally succeeded in constructing a visible light-responsive electron excitation of Al2O3 by simple carbonization technique. The novel nano-sized composite materials effectively split the water and produce H2 and O2 with the [H2]/[O2] ratios of ca.2.under visible light irradiation. We believe that similar photo-reactive excitation of either other insulating materials or metal oxides with high band gaps must be achieved by this approach and our observations will contribute to the development of new photoscience.