Introduction

Construction of a stable charge-separated electron excitation under visible light irradiation is required for achieving an artificial photosynthesis function, and semiconductors have been expected to provide such a function (Fujihara et al. 1998; Domen et al. 2000; Konta et al. 2004; Kudo et al. 2004; Kobayashi et al. 2005; Aki et al. 2006). Effective oxidation–reduction function will be achieved by increasing the stability of charge separation state without the recombination of holes and excited electrons. We have assumed that the calcination of metal–organic hybrid materials will provide new types of nano-sized metal compound-carbon clusters composite materials, in which some bonding on the interfaces of carbon clusters and metal compounds will be formed. The bonding on the interfaces of carbon clusters and metal compounds may affect the natures of band gaps and/or electron transfer, and carbon clusters will enhance the light absorption ability (Yamamoto et al. 2006; Matsui et al. 2007a; Furukawa et al. 2007; Kawahara et al. 2007a; Matsui et al. 2007b; Kawahara et al. 2007b; Miyazaki et al. 2008; Matsui et al. 2009a). We have thus been examining to clarify the electron transfer feature of various metal/oxides carbon clusters composite materials (Miyazaki et al. 2009a, b; Matsui et al. 2009b; Ge et al. 2010; Zhang et al. 2010).

In the previous paper, we showed the electronic behaviors of MoO3/carbon clusters/ZrO2 composite materials obtained by the microwave treatment of MoCl5/ZrCl4/starch/graphite complexes, and the photo-responsive catalysis with an electron transfer process of MoO3 → carbon clusters → ZrO2 was shown to take place (Kawahara et al. 2007b). Here, we considered that MoO3-loading onto carbon clusters/ZrO2 composite material could cause an effective and/or selective electron transfer feature. In the present work, nano-sized MoO3 particles were modified on ZrO2/carbon clusters composite materials Ic’s which were obtained by the calcination of ZrCl4/starch complexes I’s (Scheme 1). The electronic feature and photo-catalytic activities of Pt- and/or MoO3-loaded ZrO2/carbon clusters composite materials were also investigated under visible light irradiation.

Scheme 1
scheme 1

Synthesis of the materials

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Experimental

Reagents

Commercially available zirconium chloride ZrCl4, hexaammonium heptamolybdate tetrahydrate (NH4)6Mo7O24·4H2O, starch, methylene blue, citric acid, hydrogen hexachloroplatinate hexahydrate H2PtCl6·6H2O, and potassium permanganate were used as received.

Synthesis of complexes I’s

A mixture of ZrCl4 and starch in 300 mL of distilled water was stirred at room temperature for 1 h. Then the water was evaporated under a reduced pressure and subsequently, the residues were dried under vacuum to obtain complexes I’s. The amounts of reagents used for the synthesis are shown in Table 1.

Table 1 Charged amounts of reagents for preparing complexes I1, I2 and I3
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Calcination of complexes I’s

3 g of I1, I2 and I3 in porcelain crucible was heated under an argon atmosphere with a heating rate of 5°C/min and kept at 400, 500, and 600°C for 1 h to obtain calcined materials I1c400–I1c600, I2c400–I2c600 and I3c400–I3c600, respectively.

Pt-loading on I3c500

A mixture of 0.1 g of I3c500 and 0.5 mL of an aqueous 4.99 mmol/L hydrogen hexachloroplainate solution was irradiated by light (λ > 380 nm) with stirring for 30 min. 0.5 mL of methanol was added to the mixture and subsequently the resulting mixture was again irradiated by light (λ > 380 nm) with stirring for 30 min. The precipitates were collected, washed with distilled water, and dried under a vacuum to obtain Pt-loaded material denoted as I3c500Pt.

MoO3-loading on I3c500Pt

A mixture of 50 mg of I3c500Pt and 0.62 mg (0.5 μmol), 3.13 mg (2.5 μmol) and/or 6.26 mg (5.01 μmol) of (NH4)6Mo7O24·4H2O in 100 mL of distilled water was stirred at room temperature for 24 h. Water was evaporated under vacuum to yield the precipitates, which were dried and calcined in a porcelain crucible in the air at 300°C for 10 min using a Barnstead Thermolyne FB1300 electric furnace to obtain MoO3-loaded materials denoted as I3c500PtMo’s, i.e., I3c500PtMo1, I3c500PtMo2 and/or I3c500PtMo3, respectively.

MoO3-modification on I3c500

A mixture of 50 mg of I3c500 and 6.26 mg (5.01 μmol) of (NH4)6Mo7O24·4H2O in 100 mL of distilled water was stirred at room temperature for 24 h. Water was evaporated to obtain the precipitates, which were then dried and calcined in a porcelain crucible in the air at 300°C for 10 min to yield MoO3-loaded material denoted as I3c500Mo.

Pt-loading on I3c500Mo

A mixture of 0.1 g of I3c500Mo and 0.5 mL of an aqueous 4.99 mmol/L hydrogen hexachloroplatinate solution was then treated by the similar procedure described in “Pt-loading on I3c500” to obtain Pt-loaded material denoted as I3c500MoPt.

MnO2-loading on I3c500Pt

A mixture of 50 mg of I3c500Pt, 0.08 mg (0.5 μmol) of KMnO4 and 3 mL of ethanol in 100 mL of distilled water was stirred at room temperature for 24 h. Water was evaporated, and subsequently the residues were dried and calcined in a porcelain crucible in the air at 300°C for 30 min to obtain MnO2-loaded material denoted as I3c500PtMn.

Characterization

Elemental analyses were performed for C and H using Yanaco MT-6, and for Zr, Mo and Pt using Shimadzu ICPS-7500. XPS analyses were done using Shimadzu ESCA-850. TEM observations were carried out using Jeol JEM-3010. ESR spectra were measured using Jeol JEM-3010. TCD gas chromatography was taken with Shimadzu GC-8A. Visible light was generated using a Hoya-Schott Megalight 100 halogen lamp. UV–Vis spectra were measured using Hitachi U-4000. The reduction reaction of meyhylene blue with the calcined materials was carried out in the following way. 3 mg of the calcined materials was added into 10 mL of a 0.03 mmol/L methylene blue-0.12 mmol/l citric acid aqueous solution and the mixture was stirred in the dark for 48 h. The mixture was irradiated with visible light (λ > 460 nm) and the concentration of methylene blue was estimated by UV–Vis spectral analysis.

The oxidation–reduction reaction of an aqueous silver nitrate solution with the calcined materials was also performed in the following way. A mixture of 10 mg of the materials and 1 mL of an aqueous 0.05 mmol/L AgNO3 solution was irradiated by visible light (>460 nm) under an argon atmosphere for 3 h, and the evolved O2 gas was analyzed with gas chromatography and the formed Ag was estimated by ICP analysis.

Water photo-decomposition reaction with the calcined materials was carried out in the following way. 10 mg of the materials in 0.2 mL of degassed water was irradiated by visible light (λ > 460 nm) at room temperature for 12 h under an argon atmosphere, and the evolved H2 and O2 gases were analyzed by gas chromatography.

Results and discussion

The results of the elemental analyses of complexes I1–I3 are shown in Table 2. Zr atom was detected in the complexes. The SEM–EDX measurements of the complexes showed that Zr atom was uniformly dispersed in the materials. The calcination of complexes I’s produced black-colored materials Ic’s. Table 2 also summarizes the results of the elemental analyses of the calcined materials. The observed H contents decreased with the increase of the calcination temperature, suggesting that the carbonization of the materials proceeded. [C]/[Zr] ratios in the calcined materials increased with the increase of [Starch]/[ZrCl4] ratios in the complexes. The XPS measurements of the calcined materials showed peaks at 182.0–182.3 eV due to the Zr3d orbital of ZrO2. The TEM observations revealed the presence of particles, possibly ZrO2, with the diameters of 3–5 nm for I1c500 and of 2–3 nm for I2c500 and I3c500 (Fig. 1). The results suggest that the calcined materials were composed of nano-sized ZrO2 and carbon clusters.

Table 2 Elemental analysis of complexes I1–I3 and calcined materials I1cs–I3cs
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Fig. 1
figure 1

TEM images of I1c500, I2c500 and I3c500

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In order to examine the electron transfer process of the calcined materials, the ESR spectra of I3c400 in the presence of either an oxidant (1,4-benzoquinone) or a reductant (1,4-hydroquinone) under the irradiation of visible light (λ > 460 nm) were measured (Fig. 2). A peak at 337 mT (g = 2.003) was found to increase with the addition of the oxidant and decrease with the addition of the reductant, indicating that the signal is due to a cation radical. Our opinion is that a visible light-sensitive electron transfer in the process of carbon clusters → ZrO2 took place to form an oxidation site at the carbon clusters and a reduction site at the ZrO2 parts.

Fig. 2
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ESR spectra of I3c400 in the presence of either 1,4-benzoquinone or 1,4-hydroquinone under the irradiation of visible light (λ > 460 nm)

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The photo-catalytic abilities of the calcined materials were examined. Figure 3 is the UV–Vis spectra of methylene blue in the presence of I3c500 under visible light (>460 nm) irradiation. The absorption peak intensity of methylene blue was found to decrease with the irradiation time, indicating that I3c500 had photo-responsive reduction ability. The reduction activities (ra) of the calcined materials were determined by the equation ra = (the amount of methylene blue) × (g of the calcined material)−1 × (hour)−1 and the results are shown in Table 3. The highest ra value was observed for I3c500, indicating that I3c500 had the highest reduction ability.

Fig. 3
figure 3

UV–Visible spectra of methylene blue in the presence of I3c500 under the irradiation of visible light (λ > 460 nm)

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Table 3 Reduction activities (ra) of calcined materials Ics
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The surface of I3c500 was modified with Pt particles according to the procedure described in “Pt-loading on I3c500” to yield Pt-loaded material I3c500Pt. Subsequently I3c500Pt was also modified with MoO3 according to the method described in “MoO3-loading on I3c500Pt” to obtain MoO3-loaded materials I3c500PtMo’s. The results of the elemental analyses of I3c500Pt and I3c500PtMo’s are summarized in Table 4, indicating the presence of Pt and/or Mo atoms in the materials. The XPS measurements of the materials showed peaks at 70.9–71.0 eV due to the 4f orbital of Pt and/or at 232.4–232.6 eV due to the Mo3d orbital of MoO3. The SEM–EDX analysis of the materials revealed that Pt and/or MoO3 particles were uniformly dispersed on the surfaces of the materials. The TEM images of the materials showed the presence of particles with the diameters of a few nanometers, possibly Pt particles, and of 50–100 nm, possibly MoO3 particles, in the matrix (Fig. 4). The oxidation–reduction reaction of an aqueous silver nitrate solution with the materials under the irradiation of visible light (λ > 460 nm) were performed and the results are shown in Table 5. The calcined materials were found to form both Ag and O2, indicating that the materials had visible light-responsive oxidation–reduction abilities. The amounts of Ag and O2 formed for I3c500Pt (No. 2) were slightly higher than those for Pt-unloaded material I3c500 (No. 1), indicating that Pt-loading enhanced the photocatalysis ability. It is noted that the amounts of Ag and O2 formed in I3c500PtMo’s (Nos. 3–5) were higher than those in I3c500Pt (No. 2), indicating that MoO3-loading on the surface of I3c500Pt enhanced the photo-catalysis ability. The amount of O2 formed for I3c500PtMos increased with the increase of the amount of MoO3 in the materials, suggesting that an oxidative MoO3 may enhance the oxidation ability of the calcined material. However, the amount of Ag formed for I3c500PtMo’s was found to decrease with the increase of MoO3 in the materials. Our assumption is that MoO3 may indiscriminately be modified on the surface of I3c500Pt and thus, MoO3 loaded at the reduction site may decrease the reduction ability at the reduction site to decrease the Ag formation. In order to confirm this assumption, the surface of I3c500 was initially modified with MoO3, followed by subsequent Pt-loading according to the procedure described in “MoO3-modification on I3c500” to obtain I3c500MoPt. The visible-light irradiated decomposition reaction of methylene blue with I3c500MoPt was examined and the results are also shown in Table 5. As expected, the amounts of both Ag and O2 formed in I3c500MoPt (No. 6) were considerably lower than those in I3c500PtMo1 (No. 3). The normal electrode potential (E0) of MoO3 is +0.32 V. Here, the loading of metal oxide with a higher E0 value is expected to enhance the oxidation ability of the material. Thus, MnO2 with the E0 value of +1.23 V was modified on the surface of I3c500Pt according to the procedure described in “Pt-loading on I3c500Mo” and “MnO2-loading on I3c500Pt” to obtain I3c500PtMn. The photo-catalysis ability of I3c500PtMn was examined and the results are also shown in Table 5. The amount of O2 formed for I3c500PtMn (No. 7) was found to be higher than that for I3c500PtMo’s (Nos. 3–5), indicating that MnO2 enhanced the oxidation ability. However, the amounts of Ag formed for I3c500PtMo1 (No. 3) and I3c500PtMn (No. 7) were nearly equal, suggesting that a partial reduction of MnO2 may take place for I3c500PtMn. Water photo-decomposition examinations were performed and the results are shown in Table 6. No H2 and O2 evolution was observed for I3c500 (No. 1) and I3c500Pt (No. 2). On the other hand, I3c500PtMo1 (No. 3) was found to evolve both H2 and O2, however, the [H2]/[O2] ratio was smaller than an ideal ratio of 2. Our consideration is that a partial reduction of MoO3 deposited on the reduction site of the material may take place to decrease the reduction ability. Another interesting finding is that the amount of O2 evolved for I3c500PtMn (No. 4) was higher than that for I3c500PtMo1 (No. 3) but no H2 evolution was observed for I3c500PtMn. Higher O2 evolution may be due to higher oxidation ability of MnO2, as described above. Non-formation of H2 may be due to an occurrence of reduction reaction of MnO2 formed at the reduction site.

Table 4 Elemental analyses of Pt-loaded materials I3c500Pt and I3c500PtMo’s
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Fig. 4
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TEM image of MoO3-loaded PtI3c500

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Table 5 Amounts of Ag and O2 formed in the visible light-irradiated decomposition reaction of an aqueous AgNO3 solution with the calcined materials
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Table 6 Water decomposition reaction with the calcined materials under the irradiation of visible light (λ > 460 nm)
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Conclusions

Nano-sized ZrO2/carbon clusters composite materials denoted as Ic’s were successfully synthesized by the calcination of ZrCl4/starch complexes denoted as I’s. Pt-loaded ZrO2/carbon clusters composite materials were also prepared by doping the Pt particles on the surfaces of Ic’s. The surfaces of the Pt-loaded materials were further modified with MoO3 particles and the MoO3-loaded materials thus obtained were found to show a visible light-sensitive oxidation–reduction function.