N-doped graphene quantum dots-functionalized titanium dioxide nanofibers and their highly efficient photocurrent respons
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nium dioxide (TiO2), a widely used inorganic semiconductor owing to its superb photoelectric properties, has frequently been fabricated into composites to reduce its relatively large band gap and overcome its limited visible light absorption. In this article, a “layer-by-layer” method has been developed to prepare the composite structure of nitrogen (N)-doped graphene quantum dots (GQDs)-sensitized TiO2 nanofibers. The as-prepared structure shows considerable luminescence and exhibits excellent photoelectric properties. Various factors including the crystalline phase of TiO2, amount of N in GQDs, and irradiation wavelength were investigated to find the optimal conditions for enhanced photoelectric activity. It is demonstrated that the combination of highest N amount GQDs with TiO2 nanofibers of mixed phases (750 °C-sintered TiO2 nanofibers) possess the best photoelectric properties. The enhancement of properties using TiO2 nanofibers with mixed phases mainly contributes to the transfer of electrons between conduction bands of different phases in TiO2 and the distinctive photoluminescence (PL) property of N-GQDs. Furthermore, this enhancement can be achieved in most areas of the visible light range. The general mechanism of the electron generation and transfer of the structure is based on the normal PL and upconversion PL property of N-GQDs which serve as the sensitizer. We consider it a feasible method to improve the photoelectric conversion efficiency in photovoltaic devices. I. INTRODUCTION
Graphene quantum dots (GQDs), a novel type of graphene materials, have aroused enormous interest due to their excellent chemical and physical properties.1–4 On top of unique electron transportation properties5 inherited from graphene, GQDs share with carbon dots advantages such as chemical inertness, low cytotoxicity, and excellent biocompatibility. Beyond that, GQDs possess a novel phenomenon which is related to quantum confinement and edge effects.6,7 These distinctive properties lead to their various applications in photovoltaics.2,8 Substitutional N-doping of graphitic carbon materials has proved to be an effective method to modify their electronic properties and chemical reactivity for their extensive application in numerous fields, such as lithium batteries,9 ultracapacitors,10 electrocatalysis,11 and hydrogen storage.12 Therefore, doping GQDs with nitrogen opens up new prospects for the utilization of GQDs as a consequence of the drastic modification of their electronic properties and creation new active sites.4 Li et al.13 synthesized N-doped GQDs by hydrothermal approach and studied their distinctive
Address all correspondence to these authors. a) email: [email protected] b) email: [email protected] DOI: 10.1557/jmr.2014.152 1408
J. Mater. Res., Vol. 29, No. 13, Jul 14, 2014
http://journals.cambridge.org
Downloaded: 11 Nov 2014
photoluminescence (PL) properties, which are different from those of N-free GQDs. Several reports applied N-GQDs in oxygen reduction reaction and they exhibit superior electrocatalytic ability.11,14 Tita
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