Hydrogen production using zinc-doped carbon nitride catalyst irradiated with visible light

Recently, graphitic carbon nitride (g-C₃N₄) has been investigated as a photocatalyst for water splitting and organic dye degradation. In this study, we have developed a simple soft-chemical method of doping Zn into g-C₃N₄ to prepare a metal-containing carbon nitride. The doping was confirmed by x-ray photoelectron spectroscopy, and diffusion reflectance spectra revealed a significant red shift in the absorption edge of Zn/g-C₃N₄. This hybrid material shows high photocatalytic activity and good stability for hydrogen evolution from an aqueous methanol solution under visible light irradiation (λ≥420 nm). The hydrogen evolution rate was more than 10 times higher for a 10%-Zn/g-C₃N₄ sample (59.5 μmol h⁻¹) than for pure g-C₃N₄. The maximum quantum yield was 3.2% at 420 nm.     

Hydrogen production using zinc-doped carbon nitride catalyst irradiated with visible light

Abstract

Recently, graphitic carbon nitride (g-C₃N₄) has been investigated as a photocatalyst for water splitting and organic dye degradation. In this study, we have developed a simple soft-chemical method of doping Zn into g-C₃N₄ to prepare a metal-containing carbon nitride. The doping was confirmed by x-ray photoelectron spectroscopy, and diffusion reflectance spectra revealed a significant red shift in the absorption edge of Zn/g-C₃N₄. This hybrid material shows high photocatalytic activity and good stability for hydrogen evolution from an aqueous methanol solution under visible light irradiation (λ≥420 nm). The hydrogen evolution rate was more than 10 times higher for a 10%-Zn/g-C₃N₄ sample (59.5 μmol h^?1) than for pure g-C₃N₄. The maximum quantum yield was 3.2% at 420 nm.     

Deformation mechanisms for carbon and boron nitride nanotubes

Molecular dynamics simulation in the density functional tight-binding approximation is used to assess the energetics and atomic mechanisms of the bending and twisting of carbon and boron nitride nanotubes.     

Preparation of platinum-loaded cubic tungsten oxide: A highly efficient visible light-driven photocatalyst

Multistructural tungsten oxide samples were prepared using the hydrothermal method in the presence of different sulfates. In this paper, we present WO₃ nanorods, WO₃ toothpicks and cubic WO₃ samples prepared in the presence of Na₂SO₄, Li₂SO₄ and FeSO₄, respectively. These catalysts were characterized by XRD, SEM, TEM, EDS and UV-vis DR. It is found that Fe₂O₃ was impregnated in the cubic WO₃ which is different from other two samples. After Pt loading, Pt-loaded WO₃ with different morphology acting as novel visible light-driven photocatalysts showed remarkably high photocatalytic activity under visible light radiation. Significantly, the maximum efficiency of photodegradation was observed at 1 wt.% Pt loading amount in the cubic WO₃ sample. The highest photocatalytic activity of the cubic Pt/Fe₂O₃/WO₃ photocatalyst is attributed to the synergistic action of Pt nanoparticles and Fe₂O₃.     

Hydrogenase-coated carbon nanotubes for efficient H2 oxidation

Multiwalled carbon nanotubes grown on gold electrodes manufactured by microtechnology techniques have been used as a platform for oriented and stable immobilization of a Ni-Fe hydrogenase. Microscopic and electrochemical characterization of the system are presented. High-density currents due to H2 oxidation electrocatalysis, stable for over a month under continuous operational conditions, were measured. The functional properties of this nanostructured hydrogenase electrode are suitable for hydrogen biosensing and biofuel applications.     

Oxygen reduction activity of carbon nitride supported on carbon nanotubes

Fuel cells offer an alternative to burning fossil fuels, but use platinum as a catalyst which is expensive and scarce. Cheap, alternative catalysts could enable fuel cells to become serious contenders in the green energy sector. One promising class of catalyst for electrochemical oxygen reduction is iron-containing, nanostructured, nitrogen-doped carbon. The catalytic activity of such N-doped carbons has improved vastly over the years bringing industrial applications ever closer. Stoichiometric carbon nitride powder has only been observed in recent years. It has nitrogen content up to 57% and as such is an extremely interesting material to work with. The electrochemical activity of carbon nitride has already been explored, confirming that iron is not a necessary ingredient for 4-electron oxygen reduction. Here, we synthesize carbon nitride on a carbon nanotube support and subject it to high temperature treatment in an effort to increase the surface area and conductivity. The results lend insight into the mechanism of oxygen reduction and show the potential for carbon nanotube-supported carbon nitride to be used as a catalyst to replace platinum in fuel cells.     

Self-assembly of carbon nanotubes and boron nitride nanotubes into coaxial structures

Coaxial carbon nanotube/boron nitride nanotube (CNT/BNNT) multi-walled structures are ideal components in nanoelectronic systems. Our molecular dynamics simulations show that separate CNTs and BNNTs can self-assemble into stable coaxial structures in water under appropriate conditions. In case study three types of representative coaxial structures: (5, 5) CNT/(10, 10) BNNT, (5, 5) BNNT/(10, 10) CNT and (5, 5) BNNT/(10, 10) BNNT are obtained. Simulation results also reveal that the self-assembly time between two separate BNNTs is increased remarkably due to the polarization of BNNTs in water. The mechanism of self-assembly among these tubes is demonstrated in detail. Further, coaxial (10, 10) BNNT/(10, 10) CNT/(15, 15) BNNT nanoheterojunctions are achieved for potential application in nanoelectronic systems. The present work shows the feasibility to fabricate the coaxial nanodevices such as insulating high-strength cables, high frequency oscillators and nanojunctions using self-assembly approach.     

Encapsulation of cellulose chain into carbon nanotubes and boron nitride nanotubes

Encapsulation of cellulose chain into carbon nanotubes and boron nitride nanotubes was investigated to find out the possibility of band gap engineering in these nanotubes. The structural stability and the electronic properties of the zigzag carbon nanotubes and boron nitride nanotubes filled with cellulose chain were studied using density functional theory. It was found that encapsulation of cellulose chain into nanotubes was an exothermic process. The metallic properties of the carbon nanotubes did not change by cellulose encapsulation. The semiconductor and insulator nanotubes filled with cellulose were shown semiconducting properties. The energy band gap of these tubes was decreased by cellulose encapsulation. The results demonstrated the ability of band gap engineering through the encapsulation of cellulose chain into carbon nanotubes and boron nitride nanotubes.     

Electrodes based on nano-tree-like vanadium nitride and carbon nanotubes for micro-supercapacitors

Vanadium nitride(VN) was deposited by DC-sputtering on a vertically aligned carbon nanotube(CNTs)template for the purpose of nano-structuration. This led to the fabrication of hierarchically composite electrodes consisting of porous and nanostructured VN grown on vertically aligned CNTs in a nano-treelike configuration for micro-supercapacitor application. The electrodes show excellent performance with an areal capacitance as high as 37.5 m F cm~(-2) at a scan rate of 2 mV s~(-1) in a 0.5 MK_2SO_4 mild electrolyte solution. Furthermore, the capacitance decay was only 15% after 20,000 consecutive cycles. Moreover,the capacitance was found to increase with VN deposit thickness. The X-ray photoelectron spectroscopy analyses of the electrodes before and after cycling suggest that the oxide layers that form at the VN surface is the responsible for the redox energy storage in this material. Such electrodes can compete with other transition metal nitride based electrodes for micro-supercapacitors.     

Facile synthesis of boron nitride coating on carbon nanotubes

Boron nitride (BN) coating on the surface of carbon nanotubes (CNTs) was synthesized by the direct reaction of NaBH₄ and NH₄Cl in the temperature range of 500–600 °C. X-ray diffraction, field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) confirm the formation of BN coating. It is revealed that the BN coating follows the shape of CNTS without damaging the surface of CNTs, and the elements B and N distribute homogenously along the whole CNTs without chemical bonds between carbon and BN layers. Besides, the oxidation resistance of the CNTs improved a lot after being coated with BN.     

Field emission properties of carbon nanotubes coated with boron nitride

Field emission properties of carbon nanotubes coated with a single layer of boron nitride are calculated using the first-principles pseudopotential method. At lower bias voltage, the emission current of the coated nanotube is comparable to that of the bare carbon nanotube and is dominated by the contribution from localized states at the tip of the tube. At higher voltage, newly generated hybridized states between the carbon nanotube tip and the even-membered boron nitride rings contribute significantly to the emission current because they experience a low tunneling barrier compared with the bare carbon nanotube case. Our results suggest that the insulator coating can, besides protecting the nanotube tip from the attack of gas molecules, substantially enhance the field emission current.     

Facile preparation of porous carbon nitride for visible light photocatalytic reduction and oxidation applications

In this work, we have employed melamine, cyanuric acid and thymine to fabricate triazine-based carbon nitrides (CNs) by supramolecular approach. The resultant CNs possess large specific surface area, hierarchical porous structure, better light absorption capacity and high separation rate of photoinduced carriers. Then, the photocatalytic reduction and oxidation performance has been evaluated. The obtained CNs exhibit enhanced photocatalytic reduction performance on water splitting to H₂, the largest hydrogen evolution rate can reach 8466.3 μmol g^?1 h^?1, which is 81.9 times as high as that of bulk CN. Simultaneously, the porous CNs show excellent photocatalytic reduction ability on the conversion of CO₂ to H₂, CO and CH₄. Of particular interest is that they have high selectivity for CO. It’s worth noting that the porous CNs also possess outstanding photocatalytic oxidation ability on high concentration nitric oxide (NO), and the highest NO conversion rate can reach 79.3% under visible light. The enhanced photocatalytic performance for the multifunctional porous CN can be ascribed to the synergic effect of large specific surface area, strong light absorption capacity and fast separation of photoinduced electron–hole pairs. Finally, the photocatalytic reduction and oxidation mechanism of the porous CN is also proposed and discussed.     

Electroless-hydrothermal construction of nickel bridged nickel sulfide@mesoporous carbon nitride hybrids for highly efficient noble metal-free photocatalytic H_2 production

Metallic Ni bridged Ni S@mesoporous carbon nitride hybrids were for the first time fabricated through a one-pot electroless-assisted hydrothermal method. The intimate Ni bridge between the interface of mesoporous carbon nitride and Ni S was confirmed by HRTEM and in-depth XPS analysis using an Ar+-cluster sputtering gun and a possible mechanism was put forward to elucidate the formation process of the unique structure. Without adding any noble metals as cocatalysts, the optimized catalyst 10 % NiS/mCN-160-12 showed a H_2 evolution rate of 1419 μmol·g~(-1)·h~(-1), which is about 34 and 14 fold higher than that of bare mesoporous carbon nitride and Ni S, respectively. The dramatically enhanced photocatalytic performance was mainly ascribed to the synergistic effect of Ni S cocatalyst loading and the formation of metallic Ni between the interface of mesoporous carbon nitride and Ni S, which served as a charge-transfer bridge to facilitate the transfer and separation of photo-induced electron-hole pairs.     

An efficient numerical model for vibration analysis of single-walled carbon nanotubes

This paper presents a linear spring-based element formulation for computation of vibrational characteristics of single-walled carbon nanotubes (CNTs). Three-dimensional nanoscale elements and corresponding elemental equations are developed for the numerical treatment of the dynamic behaviour of single-walled CNTs, including appropriate stiffness and mass characteristics. The atomistic microstructure of nanotubes is used to assemble the elemental equations and construct the dynamic equilibrium equation. The developed elements simulate the relative translations and rotations between atoms as well as the mass of the atoms. In this way, molecular mechanics theory can be applied directly because the atomic bonds are modelled by using exclusively physical variables such as bond stretching. The modelling is regenerative and can provide simulations for different geometric characteristics of the nanotubes. Numerical results are presented that illustrates new natural frequencies and mode shapes, going beyond the usual ones for various nanotubes under different support conditions and defects. Comparisons with corresponding numerical predictions from the literature, where they are possible, show very good agreement.     

Phosphorus-doping activates carbon nanotubes for efficient electroreduction of nitrogen to ammonia

The electrochemical nitrogen reduction reaction (NRR) as an energy-efficient approach for ammonia synthesis is hampered by the low ammonia yield and ambiguous reaction mechanism. Herein, phosphorus-doped carbon nanotube (P-CNTs) is developed as an efficient metal-free electrocatalyst for NRR with a remarkable NH₃ yield of 24.4 μg·h⁻¹·mg⁻¹_cat. and partial current density of 0.61 mA·cm⁻². Such superior activity is found to be from P doping and highly conjugated CNTs substrate. Experimental and theoretical investigations discover that the electron-deficient phosphorus sites with Lewis acidity should be genuine active sites and NRR on P-CNTs follows the distal pathway. These findings provide insightful understanding on NRR processes on P-CNTs, opening up opportunities for the rational design of highly-active cost-effective metal-free catalysts for electrochemical ammonia synthesis.

     

Thin-film polarizers for visible spectral range with nanostructured surface modified by carbon nanotubes

The effect of carbon nanotubes (CNTs) on the optical and mechanical properties of iodinated poly(vinyl alcohol) (PVA) polarization films for the visible spectral range is briefly considered. The application of CNTs onto iodinated PVA films increases their optical transmission in the 200–750 nm wavelength range and produces their surface hardening.     

Theoretical investigation of methane adsorption onto boron nitride and carbon nanotubes

Methane adsorption onto single-wall boron nitride nanotubes (BNNTs) and carbon nanotubes (CNTs) was studied using the density functional theory within the generalized gradient approximation. The structural optimization of several bonding configurations for a CH₄ molecule approaching the outer surface of the (8,0) BNNT and (8,0) CNT shows that the CH₄ molecule is preferentially adsorbed onto the CNT with a binding energy of −2.84 kcal mol⁻¹. A comparative study of nanotubes with different diameters (curvatures) reveals that the methane adsorptive capability for the exterior surface increases for wider CNTs and decreases for wider BNNTs. The introduction of defects in the BNNT significantly enhances methane adsorption. We also examined the possibility of binding a bilayer or a single layer of methane molecules and found that methane molecules preferentially adsorb as a single layer onto either BNNTs or CNTs. However, bilayer adsorption is feasible for CNTs and defective BNNTs and requires binding energies of −3.00 and −1.44 kcal mol⁻¹ per adsorbed CH₄ molecule, respectively. Our first-principles findings indicate that BNNTs might be an unsuitable material for natural gas storage.     

Stability and electronic properties of armchair boron nitride/carbon nanotubes

In this work are studied the electronic and structural properties of armchair boron nitride/carbon nanotubes using first principles calculations. The density functional within the generalized gradient approximation (HSEh1PBE-GGA) is used. For each composition, different bonding schemes for the construction of the hybrid systems were employed. Among them, structural stability with neutral charge was determined for the following compositions: T1: B₄₀N₃₅C₇₅H₂₀, T2: B₃₅N₄₀C₇₅H₂₀, T3: B₃₇N₃₈C₇₅H₂₀, T4 : B₃₇N₃₇C₇₆H₂₀, and T7: B₃₅N₃₅C₈₀H₂₀. All these hybrid nanotubes have high polarity; the T3, T4 and T7 are semiconductors: whereas T1 and T2 are conductor in character. The formers also have magnetic behavior. These properties together with a low-chemical potential suggest applications as nano-vehicle for drug delivery. These mixed nanotubes also have potential applications in the electronic devices based on the small work function.