We report that graphene films with thickness ranging from 1 to 7 layers can be controllably synthesized on the surface of polycrystalline copper by a chemical vapour deposition method. The number of layers of graphene is controlled precisely by regulating the flow ratio of CH 4 and H 2 , the reaction pressure, the temperature and the reaction time. The synthesized graphene films were characterized by scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, X-ray diffraction and Raman spectroscopy. In addition, the graphene films transferred from copper to other substrates are found to have a good optical transmittance that makes them suitable for transparent conductive electrodes.

a b s t r a c t N-doped TiO 2 nanowire/N-doped graphene (NeTiO 2 /NG) heterojunctions are fabricated by a simple hydrothermal method in a solution containing urea. In this hybrid structure, a three-dimensional hybrid photocatalyst was fabricated by using one-dimensional N-doped TiO 2 nanowires penetrating through twodimensional graphene nanosheets. Compared with TiO 2 nanowire/graphene and N-doped TiO 2 nanowire/ graphene composites, the NeTiO 2 /NG heterostructures demonstrate a better photocatalytic performance for the degradation of methylene blue under visible light irradiations, as well as displaying a better recyclability. It is found that the nitrogen atoms originated from the decomposition of urea were not only entered into the lattice of TiO 2 nanowires but also doped into the skeleton of graphene nanosheets. Results show that N doping expands the visible light absorption region of TiO 2 , and N-doped graphene additionally improves the separation and transportation of photogenerated electronehole pairs plus generating a higher photocurrent, which plays a critical role for enhancing the photocatalytic activity.

We have investigated the performance of Schottky junction solar cells based on silicon and graphene with 1–6 layers. The open-circuit voltage of solar cells shows an increase when increasing the number of graphene layers. However, the power conversion efficiency and short-circuit current density increase monotonically when the number of graphene layers is less than 4 and reduces as the number of graphene layers further increases. Our results demonstrate that the number of layers related to the work function and transmission of graphene plays a critical role in determining the performance of solar cells.

A novel green lubricating oil additive (carbon quantum dot (CQD) particle-doped nickel (Ni-CQD)) was synthesized from citric acid and nickel acetate. The effects of CQD and Ni-CQD nanoparticles on the tribological behaviors of polyethylene glycol (PEG200) were investigated under different loads and reciprocation speeds. The results indicate that CQD and Ni-CQD particles can both enhance the lubrication properties of PEG200. However, the Ni-CQD nanoparticles enhanced the lubrication properties more than the plain CQD particles did. The average friction coefficient and wear rate of PEG200 containing 2 wt% Ni-CQDs were reduced by 35.5% and 36.4%, respectively, compared to PEG200 containing pure CQDs under a load of 8 N and reciprocation speed of 25 mm/s over 60 min. The friction and wear mechanisms are attributed to the fact that friction induces the Ni-CQDs to participate in the formation of a tribofilm, resulting in a low friction coefficient and wear rate.

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