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.

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.

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.

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.

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.

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.

Novel eco-friendly low cost and energy efficient synthesis of (Nd-Pr-Dy)2Fe14B magnetic powder from monazite concentrate.

Syntheses of Nd2Fe14B magnetic powder by conventional method is a complicated multi-step process, which produces harmful pollutants and consumes a huge amount of energy and resources. Herein we report a simple chemical route for the preparation of (Nd-Pr)2Fe14B magnetic powder using monazite concentrate as a precursor. Th, U, Sm, and La impurities were removed from monazite leachate by roasting, solvent extraction and leaching the concentrate. Purified leachate consisting of Nd and Pr Chlorides was added to the FeCl3 solution, and the solution produced was co-precipitated with NaOH. RE and Fe hydroxide precipitates were converted to the oxides by annealing at 700 °C. Boric acid and CaH2 were added in the RE and Fe oxides produced, and this mixture was reduced and diffused to (Nd-Pr)2Fe14B. Magnetic properties of the (Nd-Pr)2Fe14B produced were enhanced by introducing antiferromagnetic coupling, induced by Dy addition and efficient removal of CaO byproduct through ball milling in ethanol which increased the BHmax from 3.9 to 11.45 MGOe. Process reported is energy efficient, environment-friendly, time saving and low-cost.

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.

Determination of Dy substitution site in Nd2-xDyxFe14B by HAADF-STEM and illustration of magnetic anisotropy of "g" and "f" sites, before and after substitution.

Nd2Fe14B and Nd2-xDyxFe14B (x = 0.25, 0.50) particles were prepared by the modified co-precipitation followed by reduction-diffusion process. Bright field scanning transmission electron microscope (BF-STEM) image revealed the formation of Nd-Fe-B trigonal prisms in [- 101] viewing zone axis, confirming the formation of Nd2Fe14B/Nd2-xDyxFe14B. Accurate site for the Dy substitution in Nd2Fe14B crystal structure was determined as "f" site by using high-angle annular dark field scanning transmission electron microscope (HAADF-STEM). It was found that all the "g" sites are occupied by the Nd, meanwhile Dy occupied only the "f" site. Anti-ferromagnetic coupling at "f" site decreased the magnetic moment values for Nd1.75Dy0.25Fe14B (23.48 µB) and Nd1.5Dy0.5Fe14B (21.03 µB) as compared to Nd2Fe14B (25.50 µB). Reduction of magnetic moment increased the squareness ratio, coercivity and energy product. Analysis of magnetic anisotropy at constant magnetic field confirmed that "f" site substitution did not change the patterns of the anisotropy. Furthermore, magnetic moment of Nd2Fe14B, Nd2-xDyxFe14B, Nd ("f" site), Nd ("g" site) and Dy ("f" site) was recorded for all angles between 0° and 180°.

Interfacial Engineering at Quantum Dot-Sensitized TiO2 Photoelectrodes for Ultrahigh Photocurrent Generation.

Metal oxide semiconductor/chalcogenide quantum dot (QD) heterostructured photoanodes show photocurrent densities >30 mA/cm2 with ZnO, approaching the theoretical limits in photovoltaic (PV) cells. However, comparative performance has not been achieved with TiO2. Here, we applied a TiO2(B) surface passivation layer (SPL) on TiO2/QD (PbS and CdS) and achieved a photocurrent density of 34.59 mA/cm2 under AM 1.5G illumination for PV cells, the highest recorded to date. The SPL improves electron conductivity by increasing the density of surface states, facilitating multiple trapping/detrapping transport, and increasing the coordination number of TiO2 nanoparticles. This, along with impeded electron recombination, led to enhanced collection efficiency, which is a major factor for performance. Furthermore, SPL-treated TiO2/QD photoanodes were successfully exploited in photoelectrochemical water splitting cells, showing an excellent photocurrent density of 14.43 mA/cm2 at 0.82 V versus the Reversible Hydrogen Electrode (RHE). These results suggest a new promising strategy for the development of high-performance photoelectrochemical devices.

Magnetic spin exchange interaction in SmCo5/Co nanocomposite magnet for large energy product.

Magnetic spin exchange-coupled magnets have been investigated for obtaining an enhanced energy product, however, approaches at the nanoscale have been greatly restricted because of the lack of consideration of the relationships among the individual components. Here, we suggest a facile strategy for fabricating exchange-coupled nanomagnets with a large energy product. As a bottom-up approach, this work introduces a combined thermal decomposition and reduction/diffusion process to obtain a magnetic spin exchange coupled SmCo5/Co nanocomposite magnet. The SmCo5/Co nanocomposite magnet was fabricated through a three-step approach: (1) chemical synthesis of Co@SmOx nanoparticles and Co nanoparticles as hard and soft magnetic phases, respectively, (2) 3-dimensional alternating arrangement of both magnetic phases and (3) a reduction/diffusion process for the magnetic spin exchange interaction. Our results demonstrate that an effective magnetic spin exchange interaction strongly depends on the dimension and arrangement of the hard and soft phases, which were synthetically tuned to be within the magnetic domain wall size.

Chemical synthesis of Nd2Fe14B/Fe-Co nanocomposite with high magnetic energy product.

Nd2Fe14B is one of the most popular permanent magnets (PMs) possessing the best energy product (BH)max among the common PM materials. However, exchange-coupled nanocomposite magnets fabricated by embedding nanostructures of soft-phase magnetic materials into a hard-phase magnetic matrix manifest higher remanence and a higher energy product. Here we present the fabrication of exchange coupled Nd2Fe14B/Fe-Co magnetic nanocomposites using gel-combustion and diffusion-reduction processes. Pre-fabricated CoFe2O4 nanoparticles (NPs) of ∼5 nm diameter were incorporated into a Nd-Fe-B oxide matrix during its synthesis by gel-combustion. The obtained mixed oxide was further processed with oxidative annealing at 800 °C for 2 h and reductive annealing at 900 °C for 2 h to form a Nd2Fe14B/Fe-Co nanocomposite. Nanocomposites with different mol% of soft-phase were prepared and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and physical property measurement system (PPMS) to study their crystalline phase, morphology and magnetic behavior. Addition of 7.7 mol% of soft-phase was found to be optimum, producing a coercivity (H c) of 5.6 kOe and remanence (M r) of 54 emu g-1 in the nanocomposite.

Enhanced Efficiency of Functional Smart Window with Solar Wavelength Conversion Phosphor-Photochromic Hybrid Film.

Here, the high sensitivity and enhanced photochromic efficiency of the hybrid film of solar wavelength conversion phosphor and photochromic film for functional smart window have been achieved by fabricating a double-layered hybrid structure of wavelength conversion phosphor and photochromic film. Y2SiO5:Pr3+ phosphor nanoparticles, which upconvert visible light to UV light, were synthesized by simple hydrothermal method. The synthesized Y2SiO5:Pr3+ nanoparticle was coated as layered structured film on photochromic H3PW12O40 film. The Y2SiO5:Pr3+/H3PW12O40 hybrid film showed an enhanced sensitivity and efficiency of photochromic process with solar light irradiation due to the increased UV portion of solar light through upconversion process of visible light by wavelength conversion phosphor layer. The increased UV portion by upconversion process of Y2SiO5:Pr3+ layer through excited-state absorption and energy transfer upconversion process, contributed to an enhancement of coloration rate of photochromic H3PW12O40 film by 7 times with 50 min of 1 sun irradiation due to fast conversion of W6+ state to W5+ state in H3PW12O40 film with 5 times enhanced photochromic sensitivity compared with pristine photochromic film.

Dual-Function Au@Y2O3:Eu3+ Smart Film for Enhanced Power Conversion Efficiency and Long-Term Stability of Perovskite Solar Cells.

In the present study, a dual-functional smart film combining the effects of wavelength conversion and amplification of the converted wave by the localized surface plasmon resonance has been investigated for a perovskite solar cell. This dual-functional film, composed of Au nanoparticles coated on the surface of Y2O3:Eu3+ phosphor (Au@Y2O3:Eu3+) nanoparticle monolayer, enhances the solar energy conversion efficiency to electrical energy and long-term stability of photovoltaic cells. Coupling between the Y2O3:Eu3+ phosphor monolayer and ultraviolet solar light induces the latter to be converted into visible light with a quantum yield above 80%. Concurrently, the Au nanoparticle monolayer on the phosphor nanoparticle monolayer amplifies the converted visible light by up to 170%. This synergy leads to an increased solar light energy conversion efficiency of perovskite solar cells. Simultaneously, the dual-function film suppresses the photodegradation of perovskite by UV light, resulting in long-term stability. Introducing the hybrid smart Au@Y2O3:Eu3+ film in perovskite solar cells increases their overall solar-to-electrical energy conversion efficiency to 16.1% and enhances long-term stability, as compared to the value of 15.2% for standard perovskite solar cells. The synergism between the wavelength conversion effect of the phosphor nanoparticle monolayer and the wave amplification by the localized surface plasmon resonance of the Au nanoparticle monolayer in a perovskite solar cell is comparatively investigated, providing a viable strategy of broadening the solar spectrum utilization.

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.