Novel nanocomposite films, based on graphene oxide (GO) and TiO₂ nanotube arrays, were synthesized by assembling GO on the surface of self-organized TiO₂ nanotube arrays through a simple impregnation method. The composite films were characterized with field emission scanning electron microscopy, X-ray diffraction, Raman spectroscopy and UV–vis diffuse reflectance spectroscopy. The photoelectrochemical properties of the composite nanotube arrays were investigated under visible light illumination. Remarkably enhanced visible light photoelectrochemical response was observed for the GO decorated TiO₂ nanotube composite electrode compared with pristine TiO₂ nanotube arrays. The sensitizing effect of GO on the photoelectrochemical response of the TiO₂ nanotube arrays was demonstrated and about 15 times enhanced maximum photoconversion efficiency was obtained with the presence of GO. An enhanced photocatalytic activity of the TiO₂ nanotube arrays towards the degradation of methyl blue was also demonstrated after modification with GO. The results presented here demonstrate GO to be efficient for the improved utilization of visible light for TiO₂ nanotube arrays.

Preparing high-quality graphene through reduction of graphene oxide (GO) by oxalic acid is demonstrated in this paper. Transmission electron microscopy, scanning electron microscopy, Fourier transform infrared spectrometry, X-ray diffraction and Raman spectrometry were taken to confirm the reduction of GO and the formation of graphene under these mild conditions. Thermogravimetric analysis and conductivity measurements further testify the excellent thermal stability and conductivity of the obtained graphene. A possible mechanism for the reduction process was also proposed. Furthermore, a Pt–graphene composite was fabricated on a glassy carbon electrode and excellent electrocatalytic activity towards methanol oxidation was observed. With advantages of low toxicity, simple purification process and high quality of the product, oxalic acid provides a feasible route to prepare graphene from GO under mild conditions, thus facilitating the use of graphene-based materials for large-scale applications.

A novel nanocomposite of carbon quantum dots (CQDs) and TiO₂ nanotubes was fabricated and its enhanced photocatalytic and photoelectrochemical properties were demonstrated. Carbon quantum dots were obtained by electrochemical-etching graphite electrodes and TiO₂ nanotubes arrays were prepared by anodization methods. Subsequently, CQDs were assembled on the surface of vertically aligned TiO₂ nanotube arrays (CQDs/TiO₂ nanotubes) and the as-prepared samples were characterized by field-emission scanning electron microscopy, X-ray diffraction, UV-vis diffuse reflectance spectroscopy, photoelectrochemical and photocatalytic measurements. XPS measurement shows the presence of carbon species which come from CQDs. A red shift of light absorption edge and more absorption in the visible light region were observed for the resulting samples from the UV-vis diffuse reflectance spectra. An enhanced photocurrent and photopotential were demonstrated for the CQDs sensitized TiO₂ nanotubes under visible light irradiation and the photocurrent density was 2.7 times larger than that of pristine TiO₂ nanotubes. A solar cell was fabricated for further verifying the sensitization of CQDs over TiO₂ nanotubes. Moreover, the photocatalytic activity of CQDs/TiO₂ nanotubes towards the degradation of methylene blue was demonstrated and about 14% enhanced degradation efficiency was obtained with the presence of CQDs. This work developed a simple method to fabricate CQDs and demonstrated the introduction of CQDs to be a new approach for improving the utilization of visible light for TiO₂ nanotube arrays.