Enhanced photocatalytic activity of graphene oxide/titania nanosheets composites for methylene blue degradation

In the present work, we report enhanced photocatalytic degradation of methylene blue dye in aqueous solution by using ultra-thin anatase TiO₂ nanosheets (NSs) combined with graphene oxide (GO) as a photocatalyst. The two-dimensional ultra-thin anatase TiO₂ NSs are fabricated via chemical exfoliation. By completely delaminating a lepidocrocite-type layered protonic titanate H_xTi_2−x/4□_x/4O₄·H₂O (x=0.7, □: vacancy) into individual layers through ion exchange with tetrabutylammonium (TBA⁺) cations, well-dispersed ultra-thin colloidal Ti_0.91O₂ NSs with a lateral size up to a few micrometers are obtained. Subsequent acid treatment induces colloidal Ti_0.91O₂ to reassemble and precipitate into a gelation form, followed by thermal annealing to convert the Ti_0.91O₂ gelation into anatase TiO₂ nanosheets as photocatalyst for methylene blue degradation. TiO₂ NSs show a high photocatalytic degradation efficiency of 53.2% due to the ultra-thin thickness for facile electron transfering and large surface area for methylene blue absorption. Moreover, photocatalytic effect can be further improved by simply adding GO suspension to achieve colloidal self-assembly of GO and TiO₂ NSs. An optimal GO content of 3 wt% further increases the photocatalytic degradation efficiency to 91.2% due to faster electron–hole seperation and improved surface area provided by GO. This work provides a simple but effective approach by combing graphene oxide with TiO₂ nanosheets synthesized via the exfoliation method for methylene blue degradation.     

超级电容器用石墨烯纳米片的制备及性能

通过在低温、常压条件下热剥离氧化石墨(GO)前驱体制备了石墨烯纳米片,然后用其制成了超级电容器。利用XRD、FT-IR、SEM和TEM对所制石墨烯纳米片的物相组成和形貌进行了分析,另外,采用循环伏安、恒流充放电和交流阻抗谱技术对所制超级电容器的超级电容性能进行了研究。结果表明:GO在200℃、常压下即可被有效热剥离;所制超级电容器在6 mol/L KOH体系中的最大比电容约为276 F/g。     

Visible photoluminescence from Ge nanoclusters implanted in nanoporous aluminum oxide films

Nanoporous aluminum oxide (Al₂O₃) films with uniform porous size of 45 nm prepared by the electrochemical process in inorganic acid medium were implanted at room temperature (RT) with 120 keV Ge⁺ ions with a fluence of 1.2×10¹⁶ cm⁻². The nucleation and growths of Ge nanoparticles, were obtained by thermal annealing of the implanted samples at the temperature range of 200-600 °C. The size and distribution of the nanoparticles were characterized by photoluminescence (PL) measurements. The photoluminescence measurements as a function of the annealing temperature shows that at low annealing temperature (200 °C), the sample presents a low intensity and broad emission band centered at 5456 Å consistent with emission band characteristics of nanocluster of Ge with diameter in the range of 4-8 nm, as the annealing temperature increases to 400 °C the PL intensity increases by a factor of almost 20 and the emission band suffers a small red shift. The intensity increases can be related to the increase of the number of Ge nanocluster. At the annealing temperature of 600 °C, the emission band is considerably red shifted by almost 172 Å and the emission intensity decreases significantly, strongly suggesting that nanocrystalline Ge having a character of direct optical transitions exhibits the visible photoluminescence.     

石墨烯-Au纳米复合体系的构筑及其光限幅效应

首先对Au纳米颗粒进行巯基修饰,再对其采用表面活性剂十六烷基三甲基溴化铵(CTAB)进行二级修饰,并将其自组装负载于石墨烯纳米毯(GNSs)上。通过紫外-可见吸收光谱证明Au纳米颗粒在石墨烯纳米毯(GNSs)上的成功负载。通过透射电子显微镜探明其微观结构,表明Au纳米颗粒在石墨烯纳米毯上呈现局部规整排列,其原因与石墨烯纳米毯自身的平整结构有关。采用开孔Z-扫描技术研究了负载Au纳米颗粒的石墨烯纳米毯的非线性光限幅性能,结果表明:其光限幅起始阈值明显下降,在低入射能量时即产生光限幅特性。并发现入射光强增大时非线性散射增强,说明非线性散射是产生复合体系的非线性光限幅效应的重要机理。     

Improvement of visible light photocatalytic activity over graphene oxide/CuInS2/ZnO nanocomposite synthesized by hydrothermal method

In this study, graphene oxide/CuInS₂/ZnO as a new photocatalyst with light absorption properties in the visible region were successfully synthesized via hydrothermal route. The UV–vis absorption spectra of the catalyst suggested that the graphene oxide/CuInS₂/ZnO is active under visible light. It was evaluated the photocatalytic activities of graphene oxide/CuInS₂/ZnO on the degradation of Rhodamine B under visible light irradiation and was found that the graphene oxide/CuInS₂/ZnO obtained exhibit photocatalytic activity higher than single ZnO and CuInS₂/ZnO. Presence of graphene oxide with high specific surface area and great conductivity make it as a good support for CuInS₂/ZnO and improves removal efficiency for degradation of Rhodamine B.     

Effect of characteristic properties of graphene oxide on reduced graphene oxide/Si schottky diodes performance

We investigated the effect of the size of graphene oxide (GO) sheets made with two different types of GO solution on the performance of Si-based solar cells. Large-sized reduced GO (rGO) with an in-plane crystalline diameter of 3.42 nm has smaller defect sites and thus the Si/rGO Schottky junction solar cell shows a lower leakage current than the solar cell with small-sized rGO (i.e. an in-plane crystalline diameter of 3.03 nm). Enhanced open-circuit voltage (V_oc) and improved short-circuit current (J_sc) are observed for the solar cell with large-sized rGO due to the increased work function and Schottky barrier height at the Si and rGO junction. In other words, an increased built-in potential and a wider depletion region of the solar cell with large-sized rGO contribute to the increased carrier absorption and generation. These findings indicate that (i) rGO acts as a good transparent conducting layer and hole-transporting layer, and (ii) the control of rGO size in Si/rGO Schottky junction solar cell is important to improve the performance.     

Investigation of graphene nanosheets as counter electrodes for efficient dye-sensitized solar cells

In this study, graphene nanosheets (GNs) were used to fabricate novel counter electrodes (CEs) for dye-sensitized solar cells (DSSCs). The electrode properties of various CEs were comprehensively analyzed using scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectrometry (EDS), and cyclic voltammetry (CV). DSSCs with various GN CEs were characterized using current density–voltage (J–V), incident photo-to-current conversion efficiency (IPCE), and electrochemical impedance spectroscopy (EIS) measurements. The results show that GN CEs sintered at 400 °C in a nitrogen atmosphere for 30 min yielded the optimal electrode properties and DSSC efficiency. This study also fabricated GN–Pt composite and GN–Pt stacked CEs for the DSSCs, and the influences of the CEs on the efficiency of the DSSCs were investigated. The results show that the GN–Pt stacked CEs yielded the optimal electrochemical catalytic properties and DSSC efficiency. The power conversion efficiency of the DSSCs based on GN–Pt stacked CEs yielded a 16.7% improvement compared with conventional Pt CEs.     

Squared-like BiOCl nanosheets synthesized by ethylene glycol-assisted solvothermal method and their photocatalytic performance

Bismuth oxychloride(Bi OCl) with morphology of squared-like nanosheet is synthesized by solvothermal method using ethylene glycol aqueous reaction solution. The product is characterized by X-ray powder diffraction(XRD), scanning electron microscopy(SEM), energy dispersive spectroscopy(EDS), transmission electron microscopy(TEM) and ultraviolet-visible(UV-Vis) diffuse reflection spectroscopy, respectively. The layered structure, the hydrogen bonding between hydroxyl groups and their selective adsorption cause the formation of the squared-like nanosheets. The photocatalytic degradation activity of the as-prepared Bi OCl is tested by the degradation of methyl orange under UV light irradiation. Repeating the degradation process four times under the same condition, the results show that the squared-like Bi OCl nanosheets present high photocatalytic activity and stability.     

Growth and visible photoluminescence of highly oriented (1 0 0) zinc oxide film synthesized on silicon by plasma immersion ion implantation

High-quality (1 0 0) ZnO films with smooth surface topography have been synthesized on Si substrate by plasma immersion ion implantation. The materials exhibit compressive stress because of room temperature growth. After annealing at different temperatures, various visible photoluminescence bands are observed. The optical phenomenon as well as the transition mechanism which may involve defects such as [Zn_I], [V_Zn], and [O_i⁻] induced by the high substrate bias are discussed in this paper.     

Specific features of ir photoluminescence of bismuth-doped silicon dioxide synthesized by plasmachemical method

Photoluminescence spectra and kinetics of the bismuth-doped silicon dioxide are studied in the wavelength interval 0.7?C1.6 ??m in the absence and in the presence of aluminum co-doping. It is demonstrated that a broad spectral band that is centered at 1420 nm and exhibits a decay time of 600 ??s dominates in the long-wavelength part of the spectral interval regardless of the presence of aluminum in the directly deposited unfused bismuth-doped silicon dioxide. The melting of the plasma-deposited aluminosilicate material leads to the shift of the center of the band to a wavelength of 1100 nm, whereas the melting of the aluminum-free material weakly affects the spectrum. It is demonstrated that an increase in the temperature to 700 K causes an increase in the intensity of the band centered at a wavelength of 1420 nm, which is most clearly seen under the excitation at a wavelength of 808 nm. An increase in the intensity is accompanied by an increase in the lifetime and a sharp decrease in the intensity of the band peaked at a wavelength of 825 nm. The spectral features and kinetics of photoluminescence are interpreted in the framework of the three-level scheme of electronic transitions for two types of bismuth defects that simultaneously exist in the aluminosilicate materials and exhibit close energies of the stationary electronic states.     

Synthesis and photoluminescence characterizations of the Er3+-doped ZnO nanosheets with irregular porous microstructure

Er-doped ZnO nanosheets with high quality were synthesized by the hydrothermal and post-annealing techniques, and the effect of erbium dopant on the structures, morphologies and photoluminescence properties of the as-synthesized samples were determined using XRD, SEM, TEM, EDS, PL and Raman spectroscopy. The results showed that Er³⁺ ions were successfully incorporated into the crystal lattice of ZnO host, and some irregular porous microstructure with diameter of 3–10 nm could be seen on ZnO nanosheets as various doping concentrations. It was found that the crystallization and photoluminescence properties of ZnO nanosheets were strongly influenced by erbium doping concentration. The ultraviolet emission and deep level emission were both appeared in PL spectra, and the intensity of the whole deep level emission was enhanced with erbium doping, indicating the deep-level-defect luminescent centers were increased in the doped samples. Moreover, the crystallization of the samples became worse due to more defects by erbium doping.     

Aligned carbon tubes synthesized using porous aluminum oxide

The fabrication of a matrix of highly ordered vertically aligned carbon tubes synthesized using a porous anodic aluminum oxide template is considered. The effect of synthesis regimes on the order and topological characteristics of carbon tubes is investigated. The effect of structural and morphological changes in the porous aluminum oxide which take place during the high-temperature synthesis of carbon tubes on the origin and growth of the latter is discussed.     

Synthesis of metal oxide composite nanosheets and their pressure sensing properties

This study presents the synthesis of metal oxides composite nanosheets(oxides of cobalt, zinc and iron) and their pressure sensing properties. A transducer has been fabricated to directly measure the resistance–pressure and impedance–pressure relationships of pristine nanopowder. At the initial stage, a nanopowder sample of 10 mm diameter and 1 mm height was placed in the transducer and by applying pressure of up to 8.15 kN/m2;the DC resistance and the impedance are reduced by 44% on average. It can be explained by the densification of the samples and a decrease in porosity due to the effect of pressure. It was also observed that the DC resistance increases with time and saturated within 8 min. It is considered that this phenomenon is based on the effect of displacement currents of bound charges. The dependences of the impedance phase(θ) on frequency and pressure have also been investigated.     

Ethylene glycol assisted hydrothermal synthesis of graphene sheets supporting CdS nanospheres for quenched photoluminescence

In this paper, we report the obtention of graphene–cadmium sulfide (G/CdS) nanocomposites were successfully synthesized by the ethylene glycol assisted hydrothermal method. The structure and composition of the obtained nanocomposites were confirmed by means of X-ray diffraction (XRD), scanning electronic microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) measurements. XRD patterns showed that the CdS nanospheres belong to hexagonal structure. SEM and TEM images suggested a homogeneous distribution of CdS nanospheres coated on the graphene sheets successfully. FT-IR and XPS analyses indicated that GO has been simultaneously reduced to graphene nanosheets during the deposition of CdS nanocomposite. Moreover, PL investigations demonstrated that the G/CdS nanocomposites displayed significant decrease in PL emission compared with the corresponding sphere-like CdS nanoparticles. The investigation gave a promise to the development of original yet highly efficient graphene oxide-based novel electrode material in optical detectors.     

Photocatalytic properties of Bi2O2CO3 nanosheets synthesized via a surfactant-assisted hydrothermal method

Bi₂O₂CO₃ nanosheets were synthesized via a facile sodium dodecyl sulfate (SDS)-assisted hydrothermal process. X-Ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) surface area analysis and UV-Vis diffuse reflectance spectroscopy techniques were used to investigate the phase structure, morphology and optical properties of the Bi₂O₂CO₃ products. The SDS surfactant played an important role in the formation of Bi₂O₂CO₃ nanosheets. As-synthesized Bi₂O₂CO₃ nanosheets exhibited excellent photocatalytic activity for RhB degradation under irradiation with simulated sunlight. O₂^? radicals are considered to be the dominant active oxygen species in the photodegradation process.     

The effects of Al doping and post-annealing via intrinsic defects on photoluminescence properties of ZnO:Eu nanosheets

Europium (Eu) and Aluminum (Al) co-doped ZnO nanosheets were synthesized by a hydrothermal method. The effects of Al concentration as a dopant and post-annealing of ZnO:Eu nanosheets on its structural, electrical and optical properties were investigated in detail. Prepared samples were characterized structurally using X-ray diffraction (XRD), morphologically using scanning electron microscopy (SEM) and optically using photoluminescence (PL) spectroscopy analyses. No diffraction peak related to dopants in XRD spectrum along with shift in peaks angles relevant to ZnO proved that Al and Eu ions were doped successfully into ZnO nanosheets. This study recommends that extrinsic doping and intrinsic defects have impressive roles on transferring energy to Eu ions at indirect excitations. Based on photoluminescence observations, intra-4f transitions of Eu³⁺ ions at an excitation wavelength of 390 nm allow a sharp red luminescence. Also the results showed that optical properties of ZnO can be tuned by varying the amount of Al concentration. In comparison with annealed Al doped ZnO:Eu nanostructures, as-grown samples showed the stronger PL peaks which indicated the effective role of intrinsic defects beside of extrinsic doping on energy transfer from ZnO host to Eu³⁺ ions which consequently led to producing the strong red emission from these sites.     

Ultrathin free-standing graphene oxide film based flexible touchless sensor

Ultrathin free-standing graphene oxide (GO) films were fabricated by vacuum filtration method assisted with Ni(OH)2 nanosheets as the sacrifice layer.The surface of the obtained GO film is very clean as the Ni(OH)2nanosheets can be thoroughly etched by HC1.The thickness of the GO films can be well-controlled by changing the volume of GO dispersion,and the thinnest GO film reached ～12 nm.As a novel and transparent dielectric material,the GO film has been applied as the dielectric layer for the flexible touchless capacitive sensor which can effectively distinguish the approaching of an insulator or a conductor.     

Stable photovoltaic cells based on graphene oxide/indium zinc oxide bilayer anode buffer

We report a solution-processed graphene oxide (GO) functioned as an anode buffer layer in organic photovoltaic cells (OPVs). The OPVs using indium zinc oxide (IZO), IZO/GO, GO/IZO, and poly(3,4-thylenedioxythiophene) doped poly(styrene sulfonate) (PEDOT:PSS) as a control device, exhibited the conversion efficiency of 3.4%, 3.5%, 3.9% and 3.4%, respectively. No obvious degradation was discovered for the OPVs with incorporating GO as one of the anode buffer layers after 1 h continuous illumination under AM1.5. On the other hand, after only 1 h continuous illumination, a momentous degradation was observed for the OPVs without the presence of GO. All these results demonstrate that the GO layer plays an important role in the improvement of the stability with conventional device architecture.     

Spectra of photoluminescence from silicon nanocrystals

The evolution of time-resolved photoluminescence (PL) spectra in Au-doped nanocrystalline silicon films produced by laser ablation has been studied. The PL spectra with a relaxation time of nanoseconds are broad; they lie in the energy range 1.4–3.2 eV with a peak at 2.4–2.8 eV. At the longest times of tens of microseconds, the spectra become narrower, with a peak at 1.6 eV. At intermediate times, two bands are observed: low-energy (1.6 eV) and high-energy, with the peak shifting from 2.7 to 2.1 eV with time increasing. The data are discussed in terms of quantum confinement, dielectric amplification, and manifestation of kinetically coupled electron-hole and exciton subsystems. Ions and atoms of gold passivate dangling bonds on the Si surface and serve as catalysts for the oxidation of nanocrystals. The similarity between recombination processes responsible for the visible PL in oxidized por-Si layers and in the studied Au-doped films is discussed.