Nitrogen atoms were found to exhibit a strong ability to promote the self-assembly of nitrogen-doped carbon nanotubes (NCNTs) from gaseous carbons, without an assistance of metal atoms. On the basis of this discovery, pure metal-free CNTs with a nitrogen-doping level as high as 20 atom % can be directly synthesized using melamine as a C/N precursor. This offers a novel pathway for carbon nanotube synthesis. Furthermore, the metal-free and intact characteristics of the NCNT samples facilitate a clear verification of the intrinsic catalytic ability of NCNTs. The results show that the NCNTs intrinsically display excellent catalytic activity for oxygen reduction in fuel cells, comparable to traditional platinum-based catalysts. More notably, they exhibit outstanding stability, selectivity, and resistance to CO poisoning, much superior to the platinum-based catalysts.

Incorporation of low-dimensional carbon nanostructures such as carbon nanotubes (CNTs) and graphene sheets into the semiconductor electrodes is a common approach to improve the charge collection and photovoltaic performance of dye-sensitized solar cells. In this work, we clarify the role of CNTs in the semiconductor electrodes by investigating and comparing the electronic process in the dye-sensitized TiO2-based photovoltaic devices. The results show that the formed CNT–TiO2 Schottky junction plays a crucial role in the photovoltaic characteristics. According to the thermionic emission theory, the variation of the photocurrent over the voltage of the cells strongly depends on the height of the Schottky barrier. When the output voltage is low, the intrinsic one-dimensional carbon nanostructures can facilitate electron transport. With the voltage of the cell increasing, the energy dissipation on the Schottky junction increases dramatically and CNTs gradually lose the role of electron transport channels. At the high voltage range, however, leakage of electrons via the CNTs becomes predominant. By virtue of the charge transport channels of CNTs, increments of 44% in photocurrent at short-circuit condition and 18.7% in the overall energy conversion are achieved. Our results provide a basic understanding of the role of CNTs in solar energy conversion.

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The modulation of TiO 2 structure can effectively alter the oxidation kinetics and pathway of sacrificial ethanol during the water-splitting reaction and dramatically adjust the selectivity of the valuable coupling product 2,3-butanediol from 0.0% to 96.6%, showing the possibility of photohydrogen production in green and economical indexes.Light-driven water splitting to produce hydrogen has drawn great attention due to increasing global concerns on fossil energy resources and climate problems. 1,2 Since the electrochemical photolysis of water on a titanium oxide (TiO 2 ) electrode was first discovered by Fujishima and Honda, 3 great progress has been achieved in understanding the microscopic processes involved in the photochemical systems 4-6 and catalysts, 7-10 and in the modulation of catalyst structures for visible light harvesting. 11-13 However, light-driven hydrogen production still faces many challenging issues for practical application.Light-driven water splitting intrinsically undergoes two reaction moieties: the reduction of protons (H + ) to molecular hydrogen by photo-excited electrons, and the oxidation of water to dioxygen by holes. 14 In an ideal photocatalytic system for permanent high-rate hydrogen generation, the oxidation half-reaction should match well with the reduction half-reaction; however, in most cases, it proceeds at a considerably low rate and greatly limits the total photocatalytic oxidation-reduction cycle. Sacrificial reductants, such as methanol and ethanol, are normally required to speed up hole consumption and slow down electron-hole recombination, 15,16 which can dramatically enhance hydrogen evolution by several orders of magnitude. 17 However, the use of sacrificial reductants undesirably brings along several problems, such as: (1) the water-splitting reaction is actually changed into a redox reaction between water and the sacrificial reductants, greatly increasing the system cost; and (2) sacrificial alcohols are normally converted to CO 2 and other oxidation products, 18,19 causing the photocatalytic H 2 O-to-H 2 energy-storage process to lose its intrinsic clean, non-carbon feature. These problems seriously influence the progress and realization of solar hydrogen production. Great efforts have been made to develop new catalysts for H 2 -O 2 co-generation, so that sacrificial reductants become unnecessary. 7,[20][21][22][23][24] This strategy seems ideal for the H 2 O-to-H 2 process, but the achievement of efficient, highly stable, and low-cost catalysts appears to require further research. Additionally, the H 2 -O 2 mixture produced may suffer from engineering difficulties in terms of

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