Guanghui Liu

Magnesiothermic synthesis of sulfur-doped graphene as an efficient metal-free electrocatalyst for oxygen reduction

Efficient metal-free electrocatalysts for oxygen reduction reaction (ORR) are highly expected in future low-cost energy systems. We have successfully prepared crumpled, sheet-like, sulfur-doped graphene by magnesiothermic reduction of easily available, low-cost, nontoxic CO2 (in the form of Na2CO3) and Na2SO4 as the carbon and sulfur sources, respectively. At high temperature, Mg can reduce not only carbon in the oxidation state of +4 in CO32− to form graphene, but also sulfur in SO42− from its highest (+6) to lowest valence which was hybridized into the carbon sp2 framework. Various characterization results show that sulfur-doped graphene with only few layers has an appropriate sulfur content, hierarchically robust porous structure, large surface area/pore volume, and highly graphitized textures. The S-doped graphene samples exhibit not only a high activity for ORR with a four-electron pathway, but also superior durability and tolerance to MeOH crossover to 40% Pt/C. This is mainly ascribed to the combination of sulfur-related active sites and hierarchical porous textures, facilitating fast diffusion of oxygen molecules and electrolyte to catalytic sites and release of products from the sites.

In situ growth of spinel CoFe2O4 nanoparticles on rod-like ordered mesoporous carbon for bifunctional electrocatalysis of both oxygen reduction and oxygen evolution

The lack of efficient electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) has been a fatal issue for the development of metal–air batteries in large-scale commercialization. In this paper, spinel CoFe2O4 (CFO) nanoparticles were successfully in situ grown onto rod-like ordered mesoporous carbon (RC) by a facile, scalable hydrothermal method, followed by annealing at different temperatures. The as-acquired CFO/RC nanohybrid pyrolyzed at 400 °C (CFO/RC-400) has a high specific surface area (150.3 m2 g−1) and two sets of uniform mesopore systems (3.38 and 19.1 nm), all of which are favorable for the improvement of the electrocatalytic activity. The hybridization of CFO nanoparticles and the RC matrix results in increased ORR and OER electrocatalytic activity of the CFO/RC nanohybrids, which is significantly superior to that of unsupported CFO nanoparticles and pure RC. CFO/RC-400 shows better catalytic activity for the ORR with a direct four-electron reaction pathway than those prepared at other temperatures in terms of the onset potential and limiting current density. Furthermore, the CFO/RC-400 nanohybrid exhibits outstanding durability for both the ORR and OER, and can outperform commercial Pt/C. The excellent bifunctional electrocatalytic activities of the CFO/RC nanohybrids are mainly owing to the hierarchical mesoporous structures of the nanohybrids and strong coupling between the CFO nanoparticles and the RC matrix.

In situ formation of nitrogen-doped carbon nanoparticles on hollow carbon spheres as efficient oxygen reduction electrocatalysts

In situ formation of nitrogen-doped carbon nanoparticles on hollow carbon spheres (NHCSs) were successfully realised via a simple, scalable emulsion polymerization route using melamine as the nitrogen precursor, followed by thermal treatment at 1000 °C in N2. All NHCSs show large BET specific surface areas (648.2-837.7 cm2 g-1) and pore volumes (0.91-1.16 cm3 g-1), evidently superior to N-free hollow carbon spheres (HCSs) (524.3 cm2 g-1 and 0.48 cm3 g-1, respectively). This unique nanocomposite has hierarchical micro-/mesoporosity (1.9 nm and 16.2-19.0 nm). The X-ray photoelectron spectroscopy (XPS) measurements indicate the successful introduction of N atoms into the carbon framework and that the N-doping level can be controlled by changing the amount of melamine. The N-doping by adding melamine during the hydrothermal process not only affects the morphologies and porosities of the final samples, but also improves the electrocatalytic activity compared to N-free HCSs. NHCS-2, prepared with the molar melamine/hexamethylentetramine ratio of 1, showed the best electrocatalytic activity for the oxygen reduction reaction (ORR) in terms of onset potential, half-wave potential and limit current density. The NHCS-2 exhibited not only excellent activity with a mainly four-electron reaction pathway, but also superior long-term durability and methanol tolerance to that of commercial Pt/C in alkaline solution. The excellent electrocatalytic activity of the NHCS-2 is mainly due to its high relative content of pyridinic- and graphitic-N groups as well as unique hierarchical micro-/mesoporosity and a large specific surface area, advantageous for mass transfer and thus improving the electrocatalytic activity.

Solvothermally synthesized graphene nanosheets supporting spinel NiFe2O4 nanoparticles as an efficient electrocatalyst for the oxygen reduction reaction

The production of efficient and low-cost electrocatalysts for the oxygen reduction reaction (ORR) is one of the key issues for the extensive commercialization of fuel cells. In this paper, we describe a facile one-pot hydrothermal synthesis route to in situ grow spinel NiFe2O4 nanoparticles onto the graphene nanosheets which were produced in advance by a scalable solvothermal reduction of chloromethane and metallic potassium. The resultant NiFe2O4/graphene nanohybrid exhibits superior electrocatalytic activity for the ORR to pure graphene nanosheets and unsupported NiFe2O4 nanoparticles, which mainly favours a desirable direct 4e− reaction pathway during the ORR process. Meanwhile, the NiFe2O4/graphene nanohybrid exhibits the outstanding long-term stability for the ORR, outperforming the commercial 20 wt% Pt/C based on the current–time chronoamperometric test. The excellent catalytic activity and stability of NiFe2O4/graphene nanohybrid are ascribed to the strong coupling and synergistic effect between NiFe2O4 nanoparticles and graphene nanosheets.

A novel Eu2+ activated G-La2Si2O7 phosphor for white LEDs: SiC-reduction synthesis, tunable luminescence and good thermal stability

A new Eu2+ activated, G-type La2Si2O7 phosphor was synthesized successfully via a novel SiC-reduction route. The valence state of the Eu2+ ions was identified with XRD and XPS analysis and the luminescence spectrum presented Eu2+ broad bands. The G-La2Si2O7:Eu2+ (LPS:Eu2+) phosphor exhibited tunable emission colors depending on the excitation wavelength or the Eu concentration, enabling the production of white light. The color tunable property is ascribed to the component ratio of the two specific luminescent centers, Eu(1) and Eu(2). Eu2+ ions prefer to occupy the La3+ crystallographic sites selectively, which was identified by electron paramagnetic resonance (EPR) spectroscopy. Furthermore, the relative emission intensity of the phosphor at 100 °C and 160 °C can maintain 89% and 76% of the value measured at room temperature, which is much better than that of most of Eu2+ doped silicon oxides phosphors. The Eu(1) emission possesses a better fluorescence thermal stability than the Eu(2) emission, and an energy transition from Eu(1) to Eu(2) occurs. This better thermal stability and energy transition have been explained by the schematic configuration coordination. A w-LED device was fabricated by combining the prepared La2Si2O7:Eu2+ and commercial BaMgAl10O17:Eu2+ phosphors with a 365 nm n-UV chip. The w-LED device generates white light (color rendering index Ra = 93.9), and its CIE chromaticity coordinates and correlated color temperature (CCT) are (x, y) = (0.3429, 0.3523) and 5090 K, respectively. These results suggest that LPS:Eu2+ has a great potential for use in UV-LED-driven white emitting diodes.

Combinatorial optimization of La, Ce-co-doped pyrosilicate phosphors as potential scintillator materials.

A combinatorial method was employed to rapidly screen the effects of La, Ce-co-doping on the luminescent properties of Gd2Si2O7 pyrosilicate using an 8 × 8 library. The candidate formulations (Gd1-x-yLax)2Si2O7:Ce2y were evaluated by luminescence pictures under ultraviolet excitation. The optimal composition was found to be (Gd0.89La0.1)2Si2O7:Ce0.02 after scaled-up preparation and detailed characterization of powder samples, which shows an excellent light output under both ultraviolet and X-ray excitation (about 5.43 times of commercial YAG:Ce powders). The XRD results indicate that the phase structure sequence is tetragonal-orthorhombic-triclinic for different calcination temperatures and doping ions. The (Gd0.89La0.1)2Si2O7:Ce0.02 powder sample also demonstrated excellent temperature stability of luminescence up to 200 °C and a short decay time of several tens of nanoseconds, suggesting that this may represent a new kind of scintillation material, such as single crystals, ceramics, glass, or phosphors.

Y2Si4N6C:Ce3+ carbidonitride green-yellow phosphors: novel synthesis, photoluminescence properties, and applications

The Y2Si4N6C:Ce3+ carbidonitride phosphor has been successfully synthesized via a novel acid-driven carbonization and carbothermal reduction nitridation method (ADC–CRN). This novel approach for Y2Si4N6C:Ce3+ promises lower heating temperature and shorter heating time than classical methods, indicative of a cost-effective and facile way to search for new silicon-based carbidonitrides. In contrast to Ce3+ activated (oxy)nitrides showing blue-green emissions, Y2Si4N6C:Ce3+ exhibits an individual green-yellowish emission band centered at 550 nm which is ascribed to the incorporation of highly covalent C4− into the host lattice. The sp3 hybrid C4− was identified through high resolution electron energy loss spectroscopy analysis (EELS). Direct evidence for sole substitution of Ce3+ for Y3+ in Y2Si4N6C is represented for the first time using electron paramagnetic resonance (EPR) spectra. The red shift induced by the increasing Ce3+ content in Y2Si4N6C is reasonably deduced by the energy transfer model of intra-Ce3+ and inter-Ce3+ ions. A pc-w-LED packaging was fabricated via a combination of the yellow Y2Si4N6C:Ce3+ and blue La2Si4N6C:Ce3+ phosphors prepared using a 365 nm n-UV chip. The w-LED device shows a good color rendering index (Ra), CIE chromaticity coordinates and correlated color temperature (CCT) of 83.8, (0.3258, 0.3314) and 5819 K, respectively. These results suggest that Y2Si4N6C:Ce3+ has great potential for use in UV-LED-driven white emitting diodes.

Co-doping effect of Mn2+ on fluorescence thermostability of Ca-α-sialon:Eu2+ phosphors

To reveal transition metal manganese ion (Mn2+) co-doping influence on Ca-α-sialon:Eu2+ phosphors, Mn2+ and europium ion (Eu2+) co-doped Ca-α-sialon phosphors were synthesized using a solid state reaction at 1600 °C under an ambient nitrogen atmosphere, and the effects of the co-dopant Mn2+ on the fluorescence thermostability of Ca-α-sialon:Eu2+ phosphors were systematically investigated. With increasing Mn2+ concentration, X-ray diffraction analysis shows a phase-pure host Ca-α-sialon structure and unit cell shrinkage (or a tighter structure). The photoluminescence spectra of all samples, with or without Mn2+ co-doping, exhibit a strong yellow emission. For the energy transfer between Mn2+ and Eu2+, the emission intensity is strongest when the co-doping concentration of Mn2+ is 0.02 molar and the CIE chromaticity index of the strongest emission composition is (0.474, 0.513) with a high internal quantum efficiency of 72.4%. Importantly, the fluorescence thermal quenching behavior of the as-prepared phosphors is remarkable and is over 80% of the initial emission intensity tested at room temperature, at a higher temperature of 275 °C. The major energy transition mechanism between co-dopants Mn2+ and Eu2+ during the heating process was deduced and considered to be a non-radiative energy transfer and phonon-assisted tunneling. Using further calculations, the thermal activation energy, ΔE, is 0.28 eV. In consequence, Mn2+ and Eu2+ co-doped Ca-α-sialon is an attractive and competitive phosphor candidate for applications in white light emitting diodes.

Combinatorial discovery of self-color-mixing phosphors Bi4(1−x)Si3O12:RE4x3+ (RE3+ = Dy3+, Eu3+) for direct white light emission

Phosphors with direct white light emission have been successfully found in the rare earth doped bismuth silicate systems, Bi4(1−x)Si3O12(BSO) : RE4x3+ (RE3+ = Eu3+, Dy3+), using a fast combinatorial design and screening method. The material libraries were synthesized by ink-jetting precursor solutions into microreactor wells and sintering at high temperatures. The candidate compositions of the white light phosphors with varied dopant concentrations were evaluated by screening the recorded luminescence pictures under UV light excitation. The optimal compositions showing white light emission were further confirmed to be Bi3.96Si3O12 : Eu0.043+ and Bi3.84Si3O12 : Dy0.163+ via scale-up experiments. Additionally, the CIE chromaticity coordinate of the white light emission is located at (0.333, 0.326) for the optimal Bi3.96Si3O12 : Eu0.043+ and the CIE coordinate of the Bi3.84Si3O12 : Dy0.163+ phosphor is located at (0.302, 0.333), which are both very close to the CIE coordinate of soft sunlight (0.330, 0.330) and good for human eyesight. For the BSO : RE3+ (RE3+ = Eu3+, Dy3+) samples, the generation of direct white light emission can be realized by properly controlling the amount of doped RE3+ ions. It has been concluded that the white light emission directly results from a self-color-mixing mechanism of intrinsic host luminescence and doping ion luminescence. Besides that, the luminescence properties, energy transfer mechanism, and decay time of the direct white light phosphors have been discussed to set up a close relationship between the doping compositions and luminescence characteristics of the BSO : RE3+ (RE3+ = Eu3+, Dy3+) phosphors.

Preparation and luminescence properties of SiO2/Lu2Si2O7:Ce composite starting from mesopore template

A chemical processing method has been developed for preparing the nanosized phosphors of Lu2Si2O7:Ce3+ (LPS:Ce) by choosing mesoporous template SBA-15 as Si source. Accordingly, the luminescent ceramic composite of SiO2/LPS:Ce has also been densified by hot pressing at elevated temperatures using composited powders with the prepared LPS:Ce phosphor particles embedded in SiO2 matrix. XRD analysis and TEM observation have been carried out for both the prepared LPS:Ce phosphors and the final SiO2/LPS:Ce ceramics, indicating complete crystallization and a well distribution of LPS:Ce nanoparticles in SiO2 component. The ultra-violet fluorescence spectra, X-ray excited luminescence spectra, and decay time of the resultant SiO2/LPS:Ce ceramic composite have also been measured, showing a blue emission from Ce3+ and a fast decay time of about 37 ns. Experimental results show that an attractive strategy has been successfully developed for producing translucent or even transparent ceramics with phosphor nanoparticles embedded in matrix. The specific route can provide an effective control of reaction channels and a uniform distribution of phosphor particles in matrix by the template guiding role from ordered mesoporous SiO2. The fabrication method thus developed could be employed in other kinds of composited ceramics.