Mianheng Jiang

Visible-light photocatalytic, solar thermal and photoelectrochemical properties of aluminium-reduced black titania

Utilizing solar energy for hydrogen generation and water cleaning is a great challenge due to insufficient visible-light power conversion. Here we report a mass production approach to synthesize black titania by aluminium reduction. The obtained sample possesses a unique crystalline core–amorphous shell structure (TiO2@TiO2−x). The black titania absorbs ∼65% of the total solar energy by improving visible and infrared absorption, superior to the recently reported ones (∼30%) and pristine TiO2 (∼5%). The unique core–shell structure (TiO2@TiO2−x) and high absorption boost the photocatalytic water cleaning and water splitting. The black titania is also an excellent photoelectrochemical electrode exhibiting a high solar-to-hydrogen efficiency (1.7%). A large photothermic effect may enable black titania “capture” solar energy for solar thermal collectors. The Al-reduced amorphous shell is proved to be an excellent candidate to absorb more solar light and receive more efficient photocatalysis.

Fast growth of inch-sized single-crystalline graphene from a controlled single nucleus on Cu–Ni alloys

Wafer-scale single-crystalline graphene monolayers are highly sought after as an ideal platform for electronic and other applications. At present, state-of-the-art growth methods based on chemical vapour deposition allow the synthesis of one-centimetre-sized single-crystalline graphene domains in ∼12 h, by suppressing nucleation events on the growth substrate. Here we demonstrate an efficient strategy for achieving large-area single-crystalline graphene by letting a single nucleus evolve into a monolayer at a fast rate. By locally feeding carbon precursors to a desired position of a substrate composed of an optimized Cu-Ni alloy, we synthesized an ∼1.5-inch-large graphene monolayer in 2.5 h. Localized feeding induces the formation of a single nucleus on the entire substrate, and the optimized alloy activates an isothermal segregation mechanism that greatly expedites the growth rate. This approach may also prove effective for the synthesis of wafer-scale single-crystalline monolayers of other two-dimensional materials.

Effective nonmetal incorporation in black titania with enhanced solar energy utilization

Nonmetal-doped black titania is achieved in a core–shell structure by a two-step synthesis. The nonmetal dopants in amorphous TiO2−x shells decrease e–h recombination centers, and more than 6.6 at.% N further improves solar energy absorption from 65% up to 85%. The photocatalytic H2 generation of the N-doped black titania is 15.0 mmol h−1 g−1 under 100 mW cm−2 of full-sunlight and 200 μmol h−1 g−1 under 90 mW cm−2 of visible-light irradiation, superior to TiO2−x and reported titania photocatalysis.

Scotch-tape-like exfoliation of graphite assisted with elemental sulfur and graphene–sulfur composites for high-performance lithium-sulfur batteries

A new composite structure of graphene–sulfur with a high electrochemical performance is proposed. Scotch-tape-like sulfur-assisted exfoliation of graphite is developed to produce the graphene–sulfur composites and freestanding low-defect graphene sheets. The intimate interaction between sulfur and graphene, attributed to the similar electronegativities of the two elements, is stronger than the van der Waals forces between adjacent π–π stacked graphene layers. This causes cleavage of the graphene layers when the sulfur molecules stick to the surface and edges of the graphite, similar to Scotch tape in micromechanical exfoliation processes. This approach enables us to obtain graphene with an electrical conductivity as high as 1820 S cm−1 and a Hall mobility as high as 200 cm2 V−1 s−1, superior to most reported graphene. Furthermore, the graphene sheets which uniformly anchor sulfur molecules provide a superior confinement ability for polysulfides, sufficient space to accommodate sulfur volumetric expansion, a large contact area with the sulfur and a short transport pathway for both electrons and Li+. The unique structure containing 73 wt.% sulfur exhibits excellent overall electrochemical properties of 615 mA h g−1 at the 1 C (1 C = 1675 mA g−1) rate after 100 cycles (corresponding average Coulombic efficiency of over 96%) and 570 mA h g−1 at 2 C. These encouraging results represent that sulfur molecules bound onto graphene sheets could be a promising cathode material for lithium batteries with a high energy density.

Large-scale fabrication of heavy doped carbon quantum dots with tunable-photoluminescence and sensitive fluorescence detection

Carbon quantum dots (CQDs) have been intensively investigated due to their interesting electrochemical and photoluminescent properties. Here, we report a novel method for large-scale preparation of heavy doped CQDs with tunable photoluminescence. In the present synthetic process, the carbon nanoparticles from Chinese ink were oxidized and cut simultaneously using a mature process to obtain oxidized-CQDs as precursors, and then the heteroatom (N, S or Se) doped CQDs were obtained by a one-step hydrothermal reduction and in situ doping treatment. The heavy doped CQDs are just 1–6 nm size, and have improved photoluminescence with different emission wavelengths, higher quantum yield, longer lifetime and good photostability. Further experiments suggested that these N and S doped CQDs were very sensitive for the detection of Cu2+ and Hg2+, respectively.

Precisely aligned graphene grown on hexagonal boron nitride by catalyst free chemical vapor deposition.

To grow precisely aligned graphene on h-BN without metal catalyst is extremely important, which allows for intriguing physical properties and devices of graphene/h-BN hetero-structure to be studied in a controllable manner. In this report, such hetero-structures were fabricated and investigated by atomic resolution scanning probe microscopy. Moiré patterns are observed and the sensitivity of moiré interferometry proves that the graphene grains can align precisely with the underlying h-BN lattice within an error of less than 0.05°. The occurrence of moiré pattern clearly indicates that the graphene locks into h-BN via van der Waals epitaxy with its interfacial stress greatly released. It is worthy to note that the edges of the graphene grains are primarily oriented along the armchair direction. The field effect mobility in such graphene flakes exceeds 20,000 cm2·V−1·s−1 at ambient condition. This work opens the door of atomic engineering of graphene on h-BN, and sheds light on fundamental research as well as electronic applications based on graphene/h-BN hetero-structure.

Transparent conductive graphene films synthesized by ambient pressure chemical vapor deposition used as the front electrode of CdTe solar cells

Transparent conducting fi lms (TCFs) are used as front electrodes in a-Si, CdTe, and CuInGaSe 2 (CIGS) thin fi lm solar cells, dye-sensitized solar cells (DSCs), etc. [ 1 ] The most common TCFs are In 2 O 3 :Sn (ITO) and SnO 2 :F (FTO). However, the need for substitutes is ever increasing because of the limited availability of In, high production cost, and poor performance (cell deterioration and brittleness) as a result of ion diffusion. [ 2 ]

Direct growth of few-layer graphene films on SiO2 substrates and their photovoltaic applications

We first demonstrate the use of few layer graphene films directly grown on SiO2 substrates obtained by ambient pressure chemical vapor deposition (APCVD) as counter electrodes in dye-sensitized solar cells (DSSCs). The layer number and crystal size of graphene films can be tuned by changing growth temperature, growth time and gas flow ratio (CH4 : H2). The continuous graphene films exhibit extremely excellent electrical transport properties with a sheet resistance of down to 63.0 Ω sq−1 and extremely high mobility of up to 201.4 cm2 v−1 s−1. The highly conductive graphene films as counter electrodes of DSSCs achieve a photovoltaic efficiency of 4.25%, which is comparable to the DSSC efficiency (4.32%) based on FTO counter electrodes. Our work indicates the great potential of CVD graphene films directly grown on dielectric substrates for photovoltaic and electronic applications.

Large-scale preparation of highly conductive three dimensional graphene and its applications in CdTe solar cells

High-yield three dimensional (3D) graphene networks were prepared on Ni foams by ambient pressure chemical vapour deposition (APCVD). The layer number of graphene can be tuned by changing the gas flow ratio and growth time. The assembled films from the vacuum filtration of the 3D graphene possessed excellent electrical transport properties (Rs ∼ 0.45 Ω/sq, σ ∼ 600 S cm−1), superior to the reported graphene and carbon nanotube films. Highly conductive films as back electrodes of CdTe solar cells significantly improved the photovoltaic efficiency (9.1%).

Direct PECVD growth of vertically erected graphene walls on dielectric substrates as excellent multifunctional electrodes

A novel uniform multi-level matrix of vertically erected graphene walls is directly grown on a dielectric substrate by plasma enhanced chemical vapor deposition (PECVD) at 900 °C without the presence of any catalyst and post-transfer treatment. Such a two-level structure is composed of continuous vertically erected graphene sheets (the second level) on a nanocrystalline graphene film (the first level). A nanocrystalline film is formed in the first stage (<15 min), and the graphene walls initialize on the boundary C sp3 atoms as nucleation centers to grow the erected graphene walls as the second-level component. The microstructure of the graphene walls can be modified by plasma power, growth time and seed layer coating. The unique three-dimensional graphene structure possessed high hydrophobicity (contact angle: 141°), outstanding electron conductivity (sheet resistance: 198 Ω sq−1), and tunable transparency (91.9–38.0% at 550 nm). The three-dimensional structure enables the graphene to act as an excellent electron transport network with high surface area in many aspects. The highly conductive graphene walls were used as the counter electrode of dye-sensitized solar cells (DSSC) with a photovoltaic efficiency of 6.01%, comparable to FTO-based DSSCs (6.10%). This in situ one-step growth indicates the great potential to fabricate excellent electrodes for photovoltaic and electronic applications.