Electrochemical CO2 reduction with low overpotential by a poly(4-vinylpyridine) electrode for application to artificial photosynthesis.

Pyridine molecules have been used as a catalyst to reduce the activation energy of the CO2 reduction reaction. It has been reported that CO2 is reduced by pyridine catalysts at low overpotential around -0.58 V vs. SCE. Poly(4-vinylpyridine), which has pyridine functional groups shows similar catalytic properties to reduce CO2 at low overpotential like pyridinium catalysts. Different thickness of P(4-VP) coated Pt electrodes were analyzed to determine the catalytic properties for CO2 reduction. Cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy methods showed the catalytic CO2 reduction properties of a P(4-VP)/Pt electrode. Thin P(4-VP)/Pt film showed a low current density of -0.16 mA cm-2 under CO2 atmosphere and the current density reached -0.45 mA cm-2 with increase of the P(4-VP) thickness. The increase of current density was explained by an increased surface concentration of adsorbed pyridinium groups of the thick P(4-VP) layer. Nyquist plots also showed decrease of impedance with increase of the P(4-VP) layer indicating fast charge transfer between Pt and the P(4-VP) layer due to the increase of hybrid ionic complex formation on the Pt surface. However, charge transfer is restricted when the P(4-VP) layer becomes more thick because of slowed protonation of pyridine groups adjacent to the Pt surface due to the suppressed permeability of electrolyte solution into the PVP membrane. This electrochemical observation provides a new aspect of P(4-VP) polymer for CO2 reduction.

Surfactant free fabrication and improved charge carrier separation induced enhanced photocatalytic activity of {001} facet exposed unique octagonal BiOCl nanosheets.

Unique octagonal shaped BiOCl nanosheets (NS) dominantly exposed with high energy {001} crystal facets have been fabricated via a simple hydrothermal route without using organic surfactants. The dynamics of photogenerated charge carriers have been studied by time-resolved photoluminescence spectroscopy. The fitting parameters of the decay kinetics were used to calculate both the intensity weighted average lifetime (〈τ〉int.), as well as the amplitude weighted average lifetime (〈τ〉amp.) of the photogenerated charge carriers. The 〈τ〉int. and 〈τ〉amp. values for {001} BiOCl NS, i.e., 17.23 ns and 1.94 ns, respectively, were observed to be significantly higher than the corresponding values obtained for pristine BiOCl such as 2.52 ns and 1.07 ns, respectively. Significant quenching of the PL emission intensity of {001} BiOCl NS reflected the enhanced separation of the photogenerated charge carriers. Reduced thickness and in situ iodine doping was favorable to minimize the recombination tendency. The photocatalytic activity was monitored via the photodegradation of RhB under visible light illumination (λ > 400 nm). {001} BiOCl NS exhibited superior performance when compared to pristine BiOCl in terms of the rapid degradation kinetics and higher photonic efficiency. The photocatalytic efficiency of {001} BiOCl NS was 2.8 times higher than pristine BiOCl. Iodine doping induced extended the optical absorption in the visible region and improved the separation of the photogenerated charge carriers, which played an important role to enhance the photocatalytic activity. The photodegradation mechanism was systematically studied using various radical quenchers and it was revealed that photogenerated holes (h(+)) and superoxide radicals (˙O(2-)) actively participated whereas hydroxyl (OH˙) radicals had a negligible contribution in the photodegradation of RhB. {001} BiOCl NS has shown a higher photocurrent density and lower charge transfer resistance analyzed through photoelectrochemical and electrochemical impedance measurements. This study highlights the fabrication of unique octagonal BiOCl NS with improved separation of charge carriers across high energy crystal facts to design a highly efficient photocatalyst.

Enhanced photocurrent density of hematite thin films on FTO substrates: effect of post-annealing temperature.

Fluorine doped tin oxide (FTO) is widely used as a substrate in the synthesis of a photo-reactive semiconductor electrode for solar water splitting. The hematite film on the surface of the FTO substrate annealed at 700 °C showed an enhanced photocurrent value with a maximum photocurrent of 0.39 mA cm(-2) at 1.23 V vs. RHE under 1 sun illumination. This is a much enhanced photocurrent value of the hematite films than that of those annealed at temperatures lower than 700 °C. This is a promising approach for the enhancement of the photoelectrochemical properties of metal oxide thin films. This work reports on the mechanism of the annealing process of the synthesized hematite film to enhance the photocurrent value. Furthermore, this can be used for the enhanced efficiency of the solar water splitting reaction.

Axis-oriented, continuous anatase titania films with exposed reactive {100} facets.

Homogeneous TiO2 single crystals with high exposure of {100} reactive facets were constructed as a seed monolayer on transparent conductive substrates with the desired orientation of reactive facets. A secondary growth process was subsequently carried out on the monolayer seed film to form an axis-oriented continuous reactive film. Performing secondary growth with different precursors led to optimized conditions for high-performance photoelectrochemical activity of anatase TiO2 films. Experimental techniques such as UV/Vis absorption spectroscopy, X-ray diffraction, high-resolution SEM, and photoelectrochemistry were used to characterize the structural, optical, and photoelectrochemical properties of the as-synthesized films. As a photoanode in a photoelectrochemical cell, the axis-oriented reactive film shows a maximum photocurrent density of 0.3 mA cm(-2), as opposed to 0.075 mA cm(-2) for non-axis-oriented (randomly oriented) TiO2 film.

Manganese-doped highly ordered mesoporous silicate with high efficiency for oxidation suppression.

Herein, we demonstrate a facile approach to manganese-doped highly ordered mesoporous silicate with oxidation-suppression function. As biocompatible supports of guest ions, the ordered mesoporous silicate was synthesized by evaporation-induced self-assembly. The phase-transition from disordered to lamellar structures in the highly ordered mesoporous structure of these porosity-tuned materials was controlled by adjusting the concentration of a lab-made polystyrene-b-polyethylene oxide copolymer. Manganese was successfully incorporated as a guest in the hexagonally packed mesoporous silicate by using an ultrasound-assisted technique. The incorporation of manganese ions into the pores of a mesoporous silicate support could be induced for host-guest functional applications. Manganese-doped mesoporous silicate structures have been examined for their use as antioxidizing agents by electron spin resonance (ESR) measurements and radical-scavenging tests. The manganese atoms in the mesoporous structures could act in a free-radical-scavenging capacity, much like manganese nanoparticles. The high efficiency of their oxidation-suppression function is extended for application to catalytic products.

One pot synthesis of exchange coupled Nd2Fe14B/alpha-Fe by pechini type sol-gel method.

In this work, a combination of nanoparticles of Nd2Fe14B hard magnetic phase and alpha-Fe soft magnetic phase were synthesized by one pot chemical synthesis technique using sol-gel method. A gel of Nd-Fe-B was prepared using NdCl3 x 6H2O, FeCl3 x 6H2O, H3BO3, citric acid, and ethylene glycol by pechini type sol-gel method. The gel was subsequently calcined and annealed to obtain the mixed oxide powders. The produced metal oxide particles were identified with XRD, SEM, TEM to obtain the crystal structure, shape and domain structure of them. The nanoparticles of mixed phase of Nd2Fe14B/alpha-Fe were obtained from these oxides by a process of reduction-diffusion in vacuum by employing CaH2 as reducing agent. During this process it was optimized by controlling temperature, reaction time and concentration of the reducing agent (CaH2). The phase formation of Nd2Fe14B was resulted by the direct diffusion of NdH2, Fe and B. The magnetic property of produced hard and soft phases was successfully identified with vibrating sample magnetometer (VSM). The mixed domains of the hard and soft phases were identified with selected area electron diffraction method (SAED) patterns.

DFT-Based Study for the Enhancement of CO2 Adsorption on Metal-Doped Nitrogen-Enriched Polytriazines.

Nitrogen-enriched polytriazine (NPT), a carbon nitride-based material, has received much attention for CO2 storage applications. However, to enhance the CO2 uptake capacity more efficiently, it is necessary to understand the interaction mechanism between CO2 molecules and NPT through appropriate modification of the structures. Here, we introduce a method to enhance the CO2 adsorption capacity of NPT by incorporating metal atoms such as Sn, Co, and Ni into the polytriazine network. DFT calculations were used to investigate the CO2 adsorption mechanism of the polytriazine frameworks by tracking the interactions between CO2 and the various interaction sites of NPT. By optimizing the geometry of the pure and metal-containing NPT frameworks, we calculated the binding energy of metal atoms in the NPT framework, the adsorption energy of CO2 molecules, and the charge transfer between CO2 molecules and the corresponding adsorption systems. In this work, we demonstrate that the CO2 adsorption capacity of NPT can be greatly enhanced by doping transition-metal atoms into the cavities of NPT.

Synergistic material modification-induced optimization of interfacial charge transfer and surface hydrogen adsorption.

Being chemically stable, low cost and made from abundant resources, titanium dioxide (TiO2) possesses the most desired advantages for photocatalytic applications. However, the intrinsic limits of high surface hydrogen adsorption energy, wide band gap, low separation rate and rapid recombination of the photogenerated charge carriers greatly hamper its utilization. To address these issues, the present work combines density functional theory (DFT) calculations with rational modifications of TiO2 with nickel doping and an ultra-thin shield of fluorinated carbon (FNT) for application in the photocatalytic hydrogen evolution reaction (HER). Comprehensive studies imply that the synergistic modifications not only optimize the surface H adsorption, but also facilitate the interfacial charge transfer and simultaneously prevent the photochemical and chemical corrosion of the catalysts. In good agreement with the theoretical predictions, the resulting FNT photocatalysts demonstrate an optimal HER efficiency of 13.0 mmol g-1 h-1, nearly 33-times and over three-times beyond that of the pristine TiO2 (0.4 mmol g-1 h-1) and the Ni-doped TiO2 (4.2 mmol g-1 h-1), respectively. Moreover, the composite also exhibits excellent stability with a well-reproducible HER performance over a 66-hour cyclic HER test of 15 cycles.

Synthesis of multifunctional metal- and metal oxide core@mesoporous silica shell structures by using a wet chemical approach.

We demonstrate a facile wet chemical approach for fabricating spherical metal/metal-oxide core@mesoporous silica shell hybrid nanoparticles with different core and shell thicknesses. Vertically aligned mesoporous silica (mSiO(2)) shells were fabricated over the pre-synthesized spherical SiO(2) nanoparticles through a three-step strategy: 1) synthesis of core materials, 2) covering the core with an organic-inorganic composite layer, and 3) removing the organic template through calcinations in air. The mechanisms of hybrid structure formation are proposed. The multifunctional nature of the hybrid structures could be induced by incorporating guest ions/molecules, such as Ag, Mn, and TiO(2), into the pores of an mSiO(2) shell. Mn and TiO(2) cluster-incorporated composite structures have been tested to be antioxidizing agents and effective photocatalysts through electron spin resonance, radical scavenging tests, and the photocatalytic degradation of rhodamine B. The possibility of incorporating several hetero-element guest clusters in these mesoporous composite particles makes them highly attractive for multifunctional applications.

Concentration and temperature effect on controlling pore size and surface area of mesoporous titania by using template of F-68 and F-127 co-polymer in the sol-gel process.

Mesoporous titania with crystalline pore walls and controlled pore sizes was fabricated through triblock copolymer (pluronic series) templated sol-gel process by changing the copolymer concentration and by adjusting their calcination temperature. Compared with mesoprous silicate, the synthetic condition of mesoporous titania would be sensitive to calcination temperature. Their pore arrangement and pore size depend strongly on the concentration of copolymer used as a template. Their arrangement of pores and specific surface area increases with the increase of calcination temperature up to critical limit, 320 degrees C. Beyond the critical temperature, the orderness of pores and specific surface area decreases due to the collapse of the pore walls. The specific surface area, pore size and pore orderness can be controlled by optimizing calcination temperature as well as polymer concentration. We demonstrate the mechanism of pore formation and their collapse in the sol-gel synthesis of mesoporous titania.

Silver based photocatalysts in emerging applications.

The infinite availability of solar energy grants the potential of fulfilling the energy demands and environmental sustainability requirements with more feasible and reliant renewable energy forms through photocatalysis. In the past decade, the intensive plasmonic effect, suitable work function, superior electrical conductivity and physiochemical properties have made Ag-based photocatalysts attractive components for emerging applications. The local surface plasmon resonance effect (LSPR) provides extra hot-carriers to participate in the photocatalytic process, and Schottky/Ohmic contacts would facilitate charge transfer. Here, recent studies focused on Ag-based photocatalysts for emerging applications are reviewed. Notably, the mechanisms of LSPR, the Schottky barrier and ohmic contacts are introduced together with urgent issues in CO2 reduction, antibacterial application, H2 generation, and environmental hazard removal. Additionally, some perspectives and directions on more comprehensive designs on material system, band alignment and functionalization are given to further the exploration in this research area.

Durable CuxO/mesoporous TiO2 photocatalyst for stable and efficient hydrogen evolution.

Heterogeneous structures containing highly dispersed semiconductor nanoparticles on a photoactive support are effective for the photocatalytic hydrogen evolution reaction (HER). In this work, the interlayer ion-exchange and space confining nature of layered titanate nanosheets was used to embed copper ions in titanates, which were then transitioned to mesoporous CuxO/TiO2 with highly dispersed CuxO nanostructures. Both experimental and density functional theory (DFT) studies demonstrated that the fine-decoration of CuxO nanostructures and the reducible valence of the copper species enabled stable superior photocatalytic activity. The HER efficiency was enhanced to 12.45 mmol g-1 h-1 for the mesoporous CuxO/TiO2 composites in comparison to an efficiency of 0.38 mmol g-1 h-1 for the non-modified TiO2. Steady HER performances over 10 h, cyclic HER measurement over 60 h, and testing of the composite kept under ambient conditions for over one year, demonstrated excellent stability of the composite against photochemical and wet-chemical erosion.

Significance of Ionic Character Induced by Ga-Doped γ-Al2O3 on Polyethylene Degradation to the Precursors of Gasoline and Diesel Oil with a Trace Amount of Wax.

Polyethylene degradation has a significant ecological impact but is also economically beneficial because it generates fuels and useful chemical products. Our study mainly describes the cleavage of C-C and C-H bonds when polyethylene (dispersed in 1-octadecene) was low-temperature heat-treated in two steps, at 180 and 250 °C, for 24 h for each step. Finally, it was converted to a mixture of the precursors of gasoline and diesel oil with a trace amount of wax. A series of reactions resulted in cracking, dehydrogenation and oxidation, hence producing polycarboxylic acids and saturated and unsaturated hydrocarbons. ESI-MS analysis revealed that mixed oil consisted of low carbon number hydrocarbons and their derivatives of carboxylic acids, with the carbon number ranging from C-6 to C-18. In the trace amount of wax, complicated carboxylic acids and hydrocarbons with carbon number C-22 to C-58 were also identified. FT-IR analysis further confirmed the presence of carboxylic acid derivatives and double bonds in the degradation products. γ-Al2O3 nanorods effectively catalyzed the degradation process by enhancing the C-C chain length in the products. Lewis acid (Al) and Lewis base (oxygen) in the γ-Al2O3 induced ionic character of the C-C bond chain, which led to the efficient cracking of the C-C bond. Poor shielding effect, smaller atomic size and greater ionization energy made Ga a stronger Lewis acid compared to Al; hence, Ga-doped γ-Al2O3 catalyzed the degradation process even more effectively.

Synthesis of Uniform Size Rutile TiO2 Microrods by Simple Molten-Salt Method and Its Photoluminescence Activity.

Uniform-size rutile TiO2 microrods were synthesized by simple molten-salt method with sodium chloride as reacting medium and different kinds of sodium phosphate salts as growth control additives to control the one-dimensional (1-D) crystal growth of particles. The effect of rutile and anatase ratios as a precursor was monitored for rod growth formation. Apart from uniform rod growth study, optical properties of rutile microrods were observed by UV-visible and photoluminescence (PL) spectroscopy. TiO2 materials with anatase and rutile phase show PL emission due to self-trapped exciton. It has been observed that synthesized rutile TiO2 rods show various PL emission peaks in the range of 400 to 900 nm for 355 nm excitation wavelengths. All PL emission appeared due to the oxygen vacancy present inside rutile TiO2 rods. The observed PL near the IR range (785 and 825 nm) was due to the formation of a self-trapped hole near to the surface of (110) which is the preferred orientation plane of synthesized rutile TiO2 microrods.

Defectronics based photoelectrochemical properties of Cu2+ ion doped hematite thin film.

The concentration of guest elements (dopants) into host materials play an important role in changing their intrinsic electrical and optical properties. The existence of hetero-element induce defect in crystal structure, affecting conductivity. In the current work, we report Cu2+ ion into hematite in the defectronics point of view and their photoelectrochemical properties. Crystal distortion in the structure of hematite is observed as the amount of dopant increases. Among 1, 3 and 5 mol% of Cu2+ doped hematite, the existence of 1 mol% of Cu2+ ion into hematite crystal structure produce photocurrent value of 0.15 mA/cm2, IPCE value of ~ 4.7% and EIS value of ~ 2000 Ω/cm2 as best performances. However, further increasing dopants increases the number of interstitial defects, which cause the deformation of intrinsic lattice structure.

Hydrothermal synthesis of anatase TiO2 nanorods with high crystallinity using ammonia solution as a solvent.

Anatase TiO2 nanorods with high crystallinity were synthesized using ammonia solution (28%) as a solvent by through the hydrothermal method. The X-ray diffraction pattern confirmed the product's anatase phase and high crystallinity, and the transmission electron microscope (TEM) image demonstrated the unique morphologies of the two ends of the TiO2 nanorods (two tringle-horn shapes and one round-horn shape), whose lengths and widths were within the ranges of 200-300 and 60-110 nm, respectively. The high-resolution TEM image clearly displayed the crystal lattices of the (101) planes lying along the direction of the lengthes of the TiO2 nanorods. The energy dispersive X-ray spectrum of a TiO2 nanorod revealed the presence of about 4 atm% nitrogen element as a trace in the anatase TiO2 nanorod. The Raman spectrum of the TiO2 nanorods also showed the typical bands of anatase TiO2 and very weak peaks resulting from the TiN first-order defect-induced Raman scattering. The UV-vis diffuse-reflectance spectra showed a slight red shift (about 3 nm) of the anatase TiO2 nanorods compared with P25, which probably resulted from the trace of TiN on the surfaces of the anatase TiO2 nanorods. A three-stage-process mechanism model is proposed for the formation of the nanorods: Rhombus crystallites bounded by four {101} faces are first formed through anisotropic growth, then longer rhombus crystallites are grown via oriented attachment, finally, nanorods with a unique morphology are self-assembled by Van Der Waals forces.

Four-step eco-friendly energy efficient recycling of contaminated Nd2Fe14B sludge and coercivity enhancement by reducing oxygen content.

Complete recycling of Nd2Fe14B sludge by chemical methods has gained significance in recent years, however, it is not easy to recycle highly contaminant sludge and obtain product with good magnetic properties. Herein we report a simple four-step process to recycle the Nd2Fe14B sludge containing ~ 10% of contaminants. Sludge was leached in H2SO4 and selectively co-precipitated in two steps. In the first co-precipitation, Al3+ and Cu2+ were removed at pH 6. Thereafter, in the second co-precipitation Fe2+ and RE3+ sulfates were converted to the Fe and RE hydroxides. By annealing at 800 °C RE and Fe hydroxides precipitates were converted to the oxides and residual carbon was oxidized to CO2. After the addition of boric acid, Fe and RE oxides were reduced and diffused to the (Nd-RE)2Fe14B by calciothermic reduction diffusion. Removal of CaO by washing with D.I. water in glove box reduced the oxygen content (~ 0.7%), improved crystallinity and enhanced the magnetic properties significantly. Coercivity increased more than three times (from 242.71 to 800.55 kA/m) and Mr value was also enhanced up to more than 20% (from 0.481 to 0.605 T). In this green process Na2SO4 and Ca(OH)2 were produced as by-product those are non-hazardous and were removed conveniently.

Enhancement of anisotropy energy of SmCo5 by ceasing the coupling at 2c sites in the crystal lattice with Cu substitution.

SmCo5 and SmCo5-xCux magnetic particles were produced by co-precipitation followed by reduction diffusion. HRTEM confirmed the Cu substitution in the SmCo5 lattice. Non-magnetic Cu was substituted at "2c" site in the SmCo5 crystal lattice and effectively stopped the coupling in its surroundings. This decoupling effect decreased magnetic moment from SmCo5 (12.86 µB) to SmCo4Cu (10.58 µB) and SmCo3Cu2 (7.79 µB) and enhanced anisotropy energy from SmCo5 (10.87 Mega erg/cm3) to SmCo4Cu (14.05 Mega erg/cm3) and SmCo3Cu2 (14.78 Mega erg/cm3). Enhancement of the anisotropy energy increased the coercivity as its values for SmCo5, SmCo4Cu and SmCo3Cu2 were recorded as 4.5, 5.97 and 6.99 kOe respectively. Being six times cheaper as compared to Co, substituted Cu reduced the price of SmCo3Cu2 up to 2%. Extra 15% Co was added which not only enhanced the Mr value but also reduced the 5% of the total cost because of additional weight added to the SmCo3Cu2. Method reported in this work is most energy efficient method on the synthesis of Sm-Co-Cu ternary alloys until now.