Hongzhou Dong

Significant effects of reaction temperature on morphology, crystallinity, and photoelectrical properties of rutile TiO2 nanorod array films

Oriented single-crystalline TiO2 nanorod arrays have been extensively studied as the electrode of photoelectrochemical cells due to their unique properties. In this study, oriented rutile TiO2 nanorod arrays were directly synthesized on fluorine-doped tin oxide glass substrates by a facile hydrothermal method, and the effects of growth conditions (i.e. reaction temperature, growth time and titanium precursor) on their morphologies, crystal structures and photoelectrical properties were investigated. Reaction temperature played a more critical role in tailoring the surface morphology, crystal structures (i.e. length, diameter and crystallinity of nanorods) and photoelectrical properties of the nanorods than growth time did. With the increase in reaction temperature from 140 °C to 200 °C, both photocurrent density and external quantum efficiency (EQE) increased initially and then decreased, with a maximum value of 5.6 × 10−2 mA cm−2 at 170 °C and 2.7% at 160 °C, respectively. In addition, photoelectric measurements demonstrated that TiO2 nanorod arrays synthesized from TiCl4 at a relatively low reaction temperature exhibited a much higher EQE value than those obtained from titanium isopropoxide.

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.

Solvothermal synthesis of boron-doped graphene and nitrogen-doped graphene and their electrical properties

In this work, pristine graphene, nitrogen-doped graphene and boron-doped graphene were synthesized by a facile solvothermal process using potassium or lithium nitride as catalyst. The formation mechanism of graphene and doped graphene was discussed, and the chlorine gas generated during the reaction performed a significant role. High yield of graphene and doped graphene can be produced via the solvothermal route with relatively mild conditions, and X-ray photoelectron energy spectroscopy analysis confirmed the doping status and concentration of nitrogen or boron within graphene sheets. Especially, electrical properties of graphene-based field effect transistors revealed that the introduction of nitrogen or boron atoms into graphene sheets can effectively tailor electrical property of graphene from conducting characteristics to semiconducting behaviors.

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%).

Copper quantum dots on TiO2: A high-performance, low-cost, and nontoxic photovoltaic material

The surface decoration of TiO2 with Cu quantum dots (QDs) was carried out through a simple chemical redox deposition method. The QDs in the form of Cu(I)/(II)-O-Ti(IV) network were attached tightly and highly dispersed onto the pre-sintered TiO2 surface, and no obvious change could be detected from the lattice and surface morphology of TiO2 after the modification. Quantum size effect was evidenced by diffuse reflectance spectra (DRS), from which the absorption spectrum extended from 380 nm to 440 nm. The concentration of Cu measured by energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) was higher than its theoretical value, a result which suggested that the QDs resided on the TiO2 surface and that the QD sensitization was primarily a surface process. In comparison to pure TiO2, the introduction of 1.0 wt. % Cu QDs increased the photocurrent density from 3.1 to 17.0 μA/cm2. Even a trace amount of Cu (only about 0.25 wt. %) can strongly enhance the photoelectric activity of TiO2. When excessive Cu was coated onto the TiO2 surface, the recombination of the photo-induced charges would be aggravated by the aggregation of QDs, and the growth of Cu grains made the bandgap of the QDs and TiO2 unmatched so that the photovoltaic conversion was restrained. Besides Cu QDs, the photoelectrical properties of TiO2 nanoparticles can be enhanced by the incorporation of other copper-based QDs (e.g., CuO, Cu 2O, CuS, and Cu 2S); the elementary Cu QDs demonstrated the best characteristics among them.

High Performance of N-Doped Graphene with Bubble-like Textures for Supercapacitors

Nitrogen-doped graphene (NG) with wrinkled and bubble-like texture is fabricated by a thermal treatment. Especially, a novel sonication-assisted pretreatment with nitric acid is used to further oxidize graphene oxide and its binding with melamine molecules. There are many bubble-like nanoflakes with a dimension of about 10 nm appeared on the undulated graphene nanosheets. The bubble-like texture provides more active sites for effective ion transport and reversible capacitive behavior. The specific surface area of NG (5.03 at% N) can reach up to 438.7 m2 g-1 , and the NG electrode demonstrates high specific capacitance (481 F g-1 at 1 A g-1 , four times higher than reduced graphene oxide electrode (127.5 F g-1 )), superior cycle stability (the capacitance retention of 98.9% in 2 m KOH and 99.2% in 1 m H2 SO4 after 8000 cycles), and excellent energy density (42.8 Wh kg-1 at power density of 500 W kg-1 in 2 m KOH aqueous electrolyte). The results indicate the potential use of NG as graphene-based electrode material for energy storage devices.

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.

Novel plate-stratiform nanostructured Bi 12 O 17 Cl 2 with visible-light photocatalytic performance

Novel plate-stratiform nanostructured Bi 12 O 17 Cl 2 was studied with its visible-light photocatalytic performance. The Bi 12 O 17 Cl 2 photocatalyst synthesized by a solid-state reaction was constructed of dozens of primary nanosheets, which were stacked by a parallel array of ultrathin secondary nanosheets. The microstructure and crystal structure of Bi 12 O 17 Cl 2 primary and secondary nanosheets were discovered by high-resolution transmission electron microscopy and selected-area electron diffraction analyses. Its absorption edge was determined as about 590 nm and the band gap energy was 2.1 eV. The Bi 12 O 17 Cl 2 nanomaterial exhibited superior visible-light-responsive photocatalytic activity and confirmed successful photodegradation of methyl orange (MO) under visible-light irradiation. The degradation efficiency was up to 97% in 90 min. Furthermore, the Bi 12 O 17 Cl 2 photocatalyst exhibited excellent photostability and high mineralization capacity for MO photodegradation reaction. The MO photodegradation process was dominated by the direct photocatalytic mechanism. The contribution from its morphology and microstructure to superior photocatalytic activity was discussed.

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.