Regulation of d-Band Centers in Localized CdS Homojunctions through Facet Control for Improved Photocatalytic Water Splitting.

The accelerated kinetic behaviour of charge carrier transfer and its unhindered surface reaction dynamic process involving oxygenated-intermediate activation and conversion are urgently required in photocatalytic water (H2 O) overall splitting, which has not been nevertheless resolved yet. Herein, localized CdS homojunctions with optimal collocation of high and low index facets to regulate d-band center for chemically adsorbing and activating key intermediates (*-OH and *-O) have been achieved in H2 O overall splitting into hydrogen. Density functional theory, hall effect, and in situ diffuse reflectance infrared Fourier transform spectroscopy confirm that, electrons and holes are kinetically transferred to reductive high index facet (002) and oxidative low index facet (110) of the localized CdS homojunction induced by facet Fermi level difference to dehydrogenate *-OH and couple *-O for hydrogen and oxygen evolution, respectively, along with a solar conversion into hydrogen (STH) of 2.20 % by Air Mass 1.5 Global filter irradiation. These findings contribute to solving the kinetic bottleneck issues of photocatalytic H2 O splitting, which will further enhance STH.

Photocatalytic H2O Overall Splitting into H2 Bubbles by Single Atomic Sulfur Vacancy CdS with Spin Polarization Electric Field.

Low efficient transfer of photogenerated charge carriers to redox sites along with high surface reaction barrier is a bottleneck problem of photocatalytic H2O overall splitting. Here, in the absence of cocatalysts, H2O overall splitting has been achieved by single-atomic S vacancy hexagonal CdS with a spin polarization electric field (PEF). Theoretical and experimental results confirm that single-atomic S vacancy-induced spin PEF with opposite direction to the Coulomb field accelerates charge carrier transport dynamics from the bulk phase to surface-redox sites. By systematically tuning the spin PEF intensity with single-atomic S vacancy content, common pristine CdS is converted to a photocatalyst that can efficiently complete H2O overall splitting by releasing a great number of H2 bubbles under natural solar light. This work solves the bottleneck of solar energy conversion in essence by single atom vacancy engineering, which will promote significant photocatalytic performance enhancement for commercialization.

Graphitic C2N3: An Allotrope of g-C3N4 Containing Active Azide Pentagons as Metal-Free Photocatalyst for Abundant H2 Bubble Evolution.

A g-C3N4 allotrope, a curved leaf-like graphitic C2N3 (g-C2N3) with an intrinsic spontaneous polarization electric field (ISPEF), has been constructed for efficient solar energy conversion into H2 energy via photocatalytic H2O splitting. The curved leaf-like π-delocalization g-C2N3 was composed of aromatic azide pentagons and normal triazine hexagons obtained by cycloaddition between -C≡N groups from dicyandiamide polymerization and azide from the heat-treated polypyrrole fibers. Under light irradiation (λ > 420 nm), photo-generated charges are driven to separate efficiently and transfer from bulk to active sites of the surface under ISPEF that is opposite to the Coulomb field. Consequently, without any cocatalyst, g-C3N4 allotrope demonstrates a very high H2-production activity of 14.9 mmol g-1 h-1 accompanied by a lot of H2 bubbles, which is 2.6 times of g-C3N4 loading with Pt. In comparison with the reported metal-free photocatalysts or those supported with noble metals, g-C3N4 allotrope (i.e., leaf-like g-C2N3) is confirmed to be the best metal-free photocatalyst for H2O splitting into H2 fuel so far. The contructed leaf-like g-C2N3 with SPEF supplies a suitable platform for solar energy conversion into H2 fuel, which actively contributes to clean energy production.

Low-Energy Facets on CdS Allomorph Junctions with Optimal Phase Ratio to Boost Charge Directional Transfer for Photocatalytic H2 Fuel Evolution.

Low-energy facets on CdS allomorph junctions with optimal phase ratio are designed to boost charge directional transfer for photocatalytic H2 fuel evolution. Fermi energy level difference between low-energy facets as driving force promotes electrons directional transfer to hexagonal CdS(102) facet and holes to cubic CdS(111) facet. The optimal allomorphs CdS presents superior photocatalytic H2 evolution rate of 32.95 mmol g-1 h-1 with release in a large amount of visible H2 bubbles, which is much higher than single-phase CdS with high-energy facets and even supports noble metal photocatalysts. This scientific perspective on low-energy facets of allomorph junctions with optimal phase ratio breaks the long-held view of pursuing high-energy crystal surfaces, which will break the understanding on surface structure crystal facet engineering of photocatalytic materials.

Fabrication and Characteristics of Heavily Fe-Doped LiNbO3/Si Heterojunction.

A series of heavily Fe-doped LiNbO3 (LN:Fe) crystals were grown via the Czochralski method. The dark- and photo-conductivity of the 5.0 wt.% Fe-doped LiNbO3 crystal reached 3.30 × 10-8 Ω-1 cm-1 and 1.46 × 10-7 Ω-1 cm-1 at 473 nm, which are about 7 and 5 orders of magnitude higher than that of congruent LiNbO3, respectively. Then, a p-n heterojunction was fabricated by depositing the heavily Fe-doped LiNbO3 on a p-type Si substrate using the pulsed laser deposition. The current-voltage curve of the LN:Fe/Si heterojunction presents a well-defined behavior with a turn-on voltage of 2.9 V. This LN:Fe/Si heterojunction gives an excellent prototype device for integrated optics and electro-photonics.

Coordination-driven synthesis of perfected π-conjugated graphitic carbon nitride with efficient charge transfer for oxygen activation and gas purification.

Efficient yield of reactive-oxygen species (ROS) is greatly important for environmental purification and engineering. In this study, the perfected π-conjugated g-C3N4 (PNa-g-C3N4) photocatalysts were constructed by coordination between 3p orbits of Na and N 2p lone electron at vacancy structure of tri-s-triazine polymer for ROS evolution and elimination of HCHO and NO. The perfected π-conjugated structure enhances the visible-light capturing capability, enriches active sites for O2 activation, and promotes the directional charge transfer from N 2p of C3-N to Na and C. Therefore, the superior activities including the evolution of O2- (35 µmol.L-1h-1), and H2O2 (517 µmol.L-1h-1) have been achieved over PNa-g-C3N4 photocatalyst. As a result, PNa-g-C3N4 photocatalysts demonstrate high performances removal efficiency of NO (53% for 6 min), and HCHO (almost 100% for 55 min) in the elimination process. The results may provide the promising strategy to construct efficient photocatalytic system to yield ROS for environmental purification.

Highly Stretchable Conductive Fibers from Few-Walled Carbon Nanotubes Coated on Poly(m-phenylene isophthalamide) Polymer Core/Shell Structures.

A core/shell stretchable conductive composite of a few-walled carbon nanotube network coated on a poly(m-phenylene isophthalamide) fiber (FWNT/PMIA) was fabricated by a dip-coating method and an annealing process that greatly enhanced interactions between the FWNT network and PMIA core as well as within the FWNT network. The first strain-conductivity test of the as-prepared FWNT/PMIA fiber showed a stretching-induced alignment of nanotubes in the shell during the deformation process and a good conductivity stability with a slight conductivity drop from 109.63 S/cm to 98.74 S/cm (Δσ/σ0 = 10%) at a strain of ∼150% (2.5 times the original length). More importantly, after the first stretching process, the fiber can be recovered with a slight increase in length but a greatly improved conductivity of 167.41 S/cm through an additional annealing treatment. The recovered fiber displays a similarly superb conductivity stability against stretching, with a decrease of only ∼13 S/cm to 154.49 S/cm (Δσ/σ0 = 8%) at a strain of ∼150%. We believe that this conductivity stability came from the formation and maintaining of aligned nanotube structures during the stretching process, which ensures the good tube-tube contacts and the elongation of the FWNT network without losing its conductivity. Such stable conductivity in stretchable fibers will be important for applications in stretchable electronics.

Tuning nondoped carbon nanotubes to an efficient metal-free electrocatalyst for oxygen reduction reaction by localizing the orbital of the nanotubes with topological defects.

Breaking the electron delocalization of sp(2) carbon materials by heteroatom doping is a practical strategy to produce metal-free electrocatalysts of oxygen reduction reaction (ORR) for fuel cells. Whether carbon nanotubes (CNTs) can be efficiently tuned into ORR electrocatalysts only by intrinsic defects rather than heteroatom doping has not been well studied yet in experiment and theory. Here we introduce topological defects of nonhexagon carbon rings into CNTs to break the delocalization of their orbitals and make such type of CNTs to be a high-performance ORR catalyst. The electrochemical tests and theoretical studies indicate that the O2 chemisorption and the following electrocatalytic activity are promoted by the introduced topological defects and show a strong dependence on the defect amount. Such topological-defect CNTs (TCNTs) have an excellent ORR performance owing to a 3.8-electron-transferring process, ∼4 times higher current density and ∼120 mV more positive peak potential than normally straight CNTs. Moreover, TCNTs show a higher steady-state diffusion current density and much better stability and immunity to crossover effect as compared with commercial Pt/C catalyst. Hence, our results strongly suggest that tuning the surface structure of CNTs with nonhexagon carbon rings is a novel strategy for designing advanced ORR electrocatalysts for fuel cells.

Facile synthesis of Fe(3)O(4)/MWCNTs by spontaneous redox and their catalytic performance.

Fe(3)O(4) nanoparticles with a size range of 4-8 nm were formed by the spontaneous redox reaction between Fe(3 + ) and multi-walled carbon nanotubes (MWCNTs). Cyclic voltammetry, Raman spectroscopy and x-ray photoelectron spectroscopy were employed to study the thermodynamic and dynamic conditions for the Fe(3)O(4)/MWCNTs formation. It is found that the high defect density of MWCNTs was thermodynamically favorable for the spontaneous reduction of Fe(3 + ) ions and a reaction time of above 2.5 h should be guaranteed. As the catalysts for benzene hydroxylation to phenol, the as-obtained Fe(3)O(4)/MWCNTs exhibit superior catalytic performance to those prepared by the hydrothermal method. Therefore, the spontaneous redox between the Fe(3 + ) and MWCNTs supplies an attractive facile route for the preparation of Fe(3)O(4) nanoscale catalysts.