Liyan Yu

The optical and electronic properties of graphene quantum dots with oxygen-containing groups: a density functional theory study

The effects of five types of oxygen-containing functional groups (–COOH, –COC–, –OH, –CHO, and –OCH3) on graphene quantum dots (GQDs) are investigated using time-dependent density functional theory (TD-DFT). Their absorption spectra and HOMO–LUMO gaps are quantitatively analyzed to reveal the influence of different oxygen-containing groups including their locations and quantities on the optical properties of GQDs. Compared with those on the edge of the GQD plane, oxygen-containing groups located on the surface have more evident effects on the optical properties. The calculated HOMO–LUMO gaps of pristine GQDs and edge-functionalized GQDs with –OH, –COOH, –OCH3, –CHO, and –COC– are 2.34, 2.32, 2.31, 2.30, 2.27, and 2.15 eV, respectively, whereas the HOMO–LUMO gaps of surface-functionalized GQDs with the groups above are 0.36, 0.32, 0.37, 0.39, and 1.86 eV, respectively. Interestingly, the influence of surface and edge functionalization on the HOMO–LUMO gap of GQDs is almost opposite. The absorption process is investigated along with excited state analysis, which includes the oscillator strengths, natural transition orbitals, and charge difference density. It is found that functionalization on the basal plane greatly changes the distribution of electron density in surface-functionalized GQDs.

N-doped graphene-supported Pt and Pt-Ru nanoparticles with high electrocatalytic activity for methanol oxidation

In this work, Pt and Pt-Ru nanoparticles were synthesized on both graphene and nitrogen (N)-doped graphene sheets, and their effects on electrocatalytic activity for methanol oxidation were investigated using cyclic voltammetry and electrochemical impedance spectroscopy. Experimental results show that, in comparison to pure graphene as catalyst support, N-doped graphene-supported Pt and Pt-Ru nanoparticles demonstrate enhanced characteristics for methanol electro-oxidations with regard to oxidation potential, forward peak oxidation current density, and charge transfer resistance. For instance, the forward peak current densities of graphene-supported Pt and Pt-Ru nanoparticles were 9.5 mA/cm2 and 7.3 mA/cm2, respectively; however, the current densities of N-doped graphene-supported Pt and Pt-Ru nanoparticles were 19.9 mA/cm2 and 16.2 mA/cm2, respectively. The doping of nitrogen into graphene can effectively improve the currently density by twice. Our findings suggest the use of N-doped graphene sheets as promising catalyst supports for direct methanol fuel cells.

Performance of FTO-free conductive graphene-based counter electrodes for dye-sensitized solar cells

The attractiveness of graphene arises from its low cost, transparency, high electrical conductivity, chemical robustness, and flexibility, as opposed to the rising cost and brittleness of FTO. In particular, graphene is emerging as a possible substitute for FTO in flexible displays, touch screens, and solar cells. The main goal of our work is to develop new conductive oxide free graphene-based counter electrodes for dye sensitized solar cells (DSSCs). Graphene nanoplates are modified by silane coupling agent to introduce vinyl groups, and then mixed with polyurethane adhesive and cast on glass substrate. The film is irradiated by UV source and heat treated under Ar/H2. A network graphene film is formed and tightly bonded on glass substrate with enhanced electrical conductivity. The structure of network graphene is investigated by XPS, TGA and SEM. The DSSCs with network graphene counter electrode exhibit power conversion efficiencies of 9.33%, much better than those with FTO electrodes (4.05%).

Device optimization of CsSnI2.95F0.05-based all-solid-state dye-sensitized solar cells with non-linear charge-carrier-density dependent photovoltaic behaviors

Determining how the intrinsic kinetics of photo-generated charge carriers affect extrinsic photovoltaic performance is difficult yet essential work for the optimization of novel types of solar cells. However, contributions of several coexistent internal reactions can rarely be differentiated from one to another solely by means of experimental approaches. In this contribution, we propose the optimization of all-solid-state dye-sensitized solar cells by applying the inorganic hole-transport material (HTM) CsSnI2.95F0.05, experimentally focusing on enhancement of the interconnection between electrolyte precursor and the TiO2 nanorod array. More importantly, by taking advantage of a physics-based device-level model that describes the diverse kinetics occurring among active TiO2/dye/HTM junctions, we quantified the correlation between electrolyte precursor adsorbed onto the TiO2 electrode and hole injection from dye to HTM. We attribute the significant impact of hole injection rate (khi) on non-linear charge carrier density-dependent photovoltaic response to one physical interpretation of experimental observations concerning abnormal photovoltaic responses following variable intensity illumination. Eventually, we achieved an average power conversion efficiency of approximately 7.7% over a large number of fabricated cells, the best one of which attained 9.8%.

High-Performance Supercapacitor of Graphene Quantum Dots with Uniform Sizes

Graphene quantum dots (GQDs) with uniform sizes of less than 5 nm are synthesized by a novel top-down strategy. Nitric acid as a strong oxidant can be used to cut graphene oxide via sonication and hydrothermal processes. Moreover, purified GQDs are obtained from removing oxygen-containing functional groups in a heat treatment process. Both nanoscale size and edge effect of GQDs improve their abundant active sites and restrain the restack of graphene nanosheets. Meanwhile, their electrochemical performance demonstrates the properties of the GQDs for practical application in energy storage. The GQD electrode material shows an ideal electric double-layer capacitance behavior such as a high specific capacitance of 296.7 F g-1, a satisfactory energy density of 41.2 W h kg-1 at 1 A g-1, a low internal resistance, a small relaxation time, and an excellent cycling stability. The results illustrate excellent electrochemical activity, high conductivity, and enhanced ion transport rate on the surface of electrolyte and electrode. The advantages of GQDs confirm their unique characteristics for potential applications in the field of electrode materials for supercapacitors.

Array fabrication and photoelectrochemical properties of Sn-doped vertically oriented hematite nanorods for solar cells

We report on the synthesis and characterization of Sn-doped hematite nanorods as well as their implementation as the photoanode for solar cells. Hematite nanorods are prepared on fluorine-doped tin oxide (FTO) substrates by a hydrothermal method, followed by a two-step sintering in air, and Sn-doping is achieved by adding SnCl4 into the mixture solution during the hydrothermal process. In comparison to un-doped hematite, Sn-doped hematite nanorods exhibit a higher array growth density along the direction [110], which indicates that the Sn-doping can facilitate the vertically oriented growth of the hematite nanorod arrays; moreover, the Sn-doping can result in enhanced photocurrent density and photoelectrical efficiency due to the improved carrier density. These new findings will provide new information to enhance the photoelectrochemical characteristics of hematite, one of the best potential photoanode materials.

Effect of Cs–Ce–Zr Catalysts/Soot Contact Conditions on Diesel Soot Oxidation

Cs–Ce–Zr catalysts with various weight ratios are prepared by the sol–gel method in this paper. The main crystalline phases were identified by X-ray diffraction. The activities of catalysts during soot combustion were tested by thermogravimetric and differential scanning calorimetry. The contact conditions of soot/catalysts (sintered at 450 and 380 °C, respectively, under loose and tight contact conditions) were observed by scanning electron microscopy to study the effect of contact conditions on catalytic activity, and it was determined that the catalytic activities under tight contact conditions are superior to those under loose contact conditions. However, the soot oxidation rate speeds up after the peak temperature of about 450 °C under loose contact conditions, which is due to the fact that the contact condition is enhanced by melting CsNO3. The soot onset ignition temperature is lower for the catalysts with more Cs content under loose contact conditions. The minimum gaps of the soot onset ignition temperature and soot oxidation rates under the two contact conditions are 32 and 7 °C, which shows that the gap of catalytic activities under the respective contact conditions can be decreased by the formation of different crystalline phases.

Effects of proton-conducting electrolyte microstructure on the performance of electrolyte-supported solid oxide fuel cells

Three kinds of proton-conducting electrolyte powder BaCe0.8Sm0.2O2.9 (BCS) with different microstructures are synthesized by three different methods: EDTA-citrate method, EDTA-citrate and ball-milling method, and hydrothermal method. X-ray diffraction and scanning electron microscopy are used to investigate the microstructure and morphology of the BCS powders, and electrochemical measurements and impedance spectroscopy are employed to analyze electrical characteristics of the electrolyte-supported solid oxide fuel cells (SOFCs). It is found that the performance of electrolyte-supported SOFCs strongly depends upon the electrolyte microstructure, which is dominated by the synthesis methods. At the operating temperature of 650 °C, the highest SOFC performance (80 mW/cm2) is obtained from the cell with nanostructured proton conducting electrolyte powder synthesized by the hydrothermal method, while the lowest performance (17 mW/cm2) is the cell with the largest grain powder synthesized by the EDTA-citrate method without ball-milling treatment.

Research on K-V-rare Earth Metal Catalysts for Diesel Soot Oxidation

Five types of KNO3-NH4VO3-rare earth metal nitrate (K-V-rare earth metal) catalysts supported on a-porous alumina ceramic substrates were prepared by a coating method. All the catalysts were characterized by X-ray diffraction and thermogravimetry/differential scanning calorimetry. Catalytic activities were evaluated by a soot oxidation reaction using a temperature-programmed reaction system. The experimental results show that the addition of rare earth metal compound could obviously improve the catalytic activities of the K-V-based catalysts. The proper ratio of K-V-rare earth metal catalysts can not only lower the soot onset ignition temperature, but also quicken the soot oxidation rate. The crystalline phases formed by K, V, and rare earth metal are stable.

Tuning Electronic and Optical Properties of Graphene Quantum Dots by Selective Boronization

The optical properties of graphene quantum dots (GQDs) can be modified through introducing heteroatoms, including doping heteroatoms and covalent bonding with specific groups. Hence, we use density functional theory (DFT) and time-dependent (TD) DFT to understand the effects of boron doping configurations (i.e., BC3, BC2O and BCO2) on the electronic and optical properties of GQDs. Absorption spectra and HOMO–LUMO gaps are quantitatively calculated to study the correlations between the optical properties and electronic structure with different boronization and oxidation patterns. It demonstrates that BC2O can induce a red shift of absorption spectra, while the absorption spectra of the surface doped GQD with BCO2 exhibits a blue shift. According to the excited state analysis, BC3 plays an important role in determining the electronic transition, while the effects of BC2O and BCO2 on tuning the electronic and optical properties of GQDs are dictated by their hybridization form of carbon. Meanwhile, it indicates that the coexistence of B atoms and oxidized B bonding configurations can help charge transfer in the absorption process.