The electronic behaviors of visible light sensitive Nb2O5/Cr2O3/carbon clusters composite materials

doi:10.1016/j.cap.2008.05.006

Current Applied Physics

Volume 9, Issue 3, May 2009, Pages 592-597

https://doi.org/10.1016/j.cap.2008.05.006 Get rights and content

Abstract

Nano-sized Nb₂O₅/Cr₂O₃/carbon clusters composite material has been successfully obtained by the calcination of a Nb(HC₂O₄)₅/CrCl₃/starch complex under an argon atmosphere. The compositions of the resulting composite materials were determined using ICP, elemental analysis and surface characterization by XRD and TEM. The UV–VIS and XPS spectra of the composites were also obtained. ESR spectral examinations suggest the possibility of an electron transfer in the process of Nb₂O₅→ carbon clusters → Cr₂O₃. The reduction reaction of methylene blue with the resulting composite material has also been examined.

Introduction

Realization of charge-separated electron transfer with photo-responsive function will lead to an achievement of a novel photocatalytic function. Such a transfer has been achieved by semiconductors [1], [2], [3], [4], [5], [6], however an electron transfer under whole visible light irradiation has not been established yet. We considered that such a transfer could be achieved by the combination of carbon clusters and nano-sized semiconductors, in which carbon clusters are expected to act as a visible light absorption site and an electron transfer bed, and semiconductors to act as an electron excitation site, resulting in the realization of visible light-responsive electron transfer.

We have recently reported the synthesis of metal–organic moiety hybrid copolymers [7], [8], [9], [10], and also the formation of nano-sized inorganic metal compounds/carbon clusters composite materials by the calcination of such metal–organic moiety hybrid copolymers under a reducing atmosphere. The resulting novel composite material shows a visible light induced electron transfer between metal compounds particles and carbon clusters [11], [12], [13], [14]. Especially, CeO₂/carbon clusters/Ho₂O₃ particles loaded with Pt particles showed an electron transfer process of CeO₂→ carbon clusters → Ho₂O₃→ Pt which decomposed water to H₂ and O₂ with the H₂/O₂ molar ratio of 2 under whole visible light irradiation [15]. These findings strongly indicate that the combination of two metal oxides in the carbon phase is important to achieve an effective photocatalyst. However, we are unable to give a principle for achieving an effective two step electron transfer at present. Therefore, we have been investigating to clarify the electron transfer features of the combination of various two metal oxides/carbon clusters composite materials.

On the other hand, we have considered that such composite materials could also be obtained by the calcination of aggregated complexes composed of inorganic metal compounds and organic polymers. The main advantage of this approach is easy to prepare the nano-sized metal oxide particles uniformly distributed on carbon clusters. In this work, we studied the electron transfer features of Nb₂O₅/Cr₂O₃/carbon clusters composite material. Nb₂O₅ has the band positions of B.G = 3.4 eV and VB = 3.31 eV [16], and Cr₂O₃ has those of B.G = 3.5 eV and C.B = −0.57 eV [16], respectively. Thus, it is expected that the combination of these two metal oxides in carbon clusters may show a two step electron transfer [15]. Therefore, Nb(HC₂O₄)₅/CrCl₃/starch complex III was prepared and heated under an argon atmosphere to obtain calcined material IIIc, Nb₂O₅/Cr₂O₃/carbon clusters (Scheme 1), and finally the electronic behavior of IIIc was also examined.

Section snippets

Reagents

Commercially available niobium oxalate hexahydrate Nb(HC₂O₄)₅· 6H₂O, chromium chloride hexahydrate CrCl₃· 6H₂O, starch, 1,4-benzoquinone, 1,4-hydroquinone, methylene blue, and citric acid were used.

Synthesis of complexes I–III

A mixture of Nb(HC₂O₄)₅ · 6H₂O or CrCl₃ · 6H₂O and starch in 150 mL of distilled water was stirred at 80 °C for 30 min. Water was evaporated under a reduced pressure to get the precipitates, which were then dried under a reduced pressure to obtain complex I, II or III. The amounts of reagents used for the

Results and discussion

The results of the elemental analysis of complexes I, II and III are shown in Table 2. Nb and/or Cr have been found to be involved in the complexes. The calcination of I and II at 400, 500, 600 and 700 °C under an argon atmosphere produced black-colored materials of Ic-400–Ic-700 and IIc-400–IIc-700. The calcined materials of IIIc-500–IIIc-700 were obtained by the calcination of III at 500, 600 and 700 °C. Table 2 also shows the results of the elemental analysis of the calcined materials. The

Conclusions

We have successfully synthesized the nano-sized Nb₂O₅/Cr₂O₃/carbon clusters composite material by the calcination of a Nb(HC₂O₄)₅/CrCl₃/starch complex under an argon atmosphere. The surface characterizations of the resulting composites indicate that they are composed of nano-sized particles of Nb₂O_5, Cr₂O₃ and carbon clusters. ESR spectral examinations of the composite materials suggest that the possibility of an electron transfer in the process of Nb₂O₅ → carbon clusters → Cr₂O₃. The

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The electronic behaviors of visible light sensitive Nb2O5/Cr2O3/carbon clusters composite materials

Abstract

Introduction

Section snippets

Reagents

Synthesis of complexes I–III

Results and discussion

Conclusions

Mater. Chem. Phys.

J. Colloid Interface Sci.

Nature

J. Chem. Soc. Faraday Trans.

J. Phys. Chem.

Nature

J. Phys. Chem. B

Chem. Lett.

The electronic behaviors of visible light sensitive Nb₂O₅/Cr₂O₃/carbon clusters composite materials