Ag nanoparticles anchored NiO/GO composites for enhanced capacitive performance

Hierarchical nickel oxide/graphene oxide (NiO/GO) and nickel oxide/graphene oxide/silver (NiO/GO/Ag) heterostructures were sucessfully fabricated as high-performance supercapacitors electrode materials by using a hydrothermal process and a photoreduction process. The experimental results showed that the NiO/GO/Ag heterostructure electrodes showed better electrochemical performance than those of NiO/GO and bare NiO nanosheets. The NiO/GO/Ag electrode exhibited a higher specific capacitance of 229 F g⁻¹ at a current density of 1 A g⁻¹, higher than that of 161 F g⁻¹ for NiO/GO composites. Furthermore, NiO/GO/Ag electrode also showed good rate capability (still 200 F g⁻¹ at 6 A g⁻¹) and cycling stability (24% loss after 2000 repetitive cycles at a scan rate of 20 mV s⁻¹). The enhanced capacitive performance of the NiO/GO/Ag composites was mainly attributed to the introduction of Ag nanoparticles, which increased the electrical conductivities of the composites, and promoted the electron transfer between the active components. This study suggested that NiO/GO/Ag composites were a promising class of electrode materials for high performance energy storage applications.     

Effect of heterojunction on photocatalytic properties of multilayered ZnO-based thin films

In the present study, the effects of the heterojunctions on the optical and structural characteristics and the resulting photocatalytic properties of multilayered ZnO-based thin films were investigated. The junctions were composed of semiconducting ZnO nano-porous films coated on the In₂O₃ and SnO₂ counterpart layers. The multilayered ZnO films based on the triple-layered Ag-doped indium oxide (AIO)/tin oxide (TO)/zinc oxide (ZnO), indium oxide (IO)/Ag-doped tin oxide (ATO)/zinc oxide (ZnO), indium oxide (IO)/tin oxide (TO)/zinc oxide (ZnO) and tin oxide (TO)/indium oxide (IO)/zinc oxide (ZnO) have been fabricated by subsequent sol–gel dip coating. Their structural and optical properties combined with photocatalytic characteristics were examined toward degradation of Solantine Brown BRL (C.I. Direct Brown), an azo dye using in Iran textile industries as organic model under UV light irradiation. Effects of operational parameters such as initial concentration of azo dye, irradiation time, solution pH, absence and presence of Ag doping and consequent of sublayers on the photodegradation efficiencies of ZnO nultilayered thin films were also investigated and optimum conditions were established. It was found that the photocatalytic degradation of azo dye on the composite films followed pseudo-first order kinetics. Photocatalytic activity of AIO/TO/ZnO interface composite film was higher compared with other films and the following order was observed for films activities: AIO/TO/ZnO > IO/TO/ZnO > ATO/IO/ZnO > TO/IO/ZnO. Differences in the film efficiencies can be attributed to differences in crystallinity, interfacial lattice mismatch, and surface morphology. Besides, the presence of Ag doping between layers that may act as trap for electrons generated in the ZnO over layer thus preventing electron–hole recombination.     

Microwave assisted synthesis of rGO/ZnO composites for non-enzymatic glucose sensing and supercapacitor applications

Zinc oxide (ZnO) and Graphene Oxide (GO) are known to show good electrochemical properties. In this paper, rGO/ZnO nanocomposites have been synthesised using a simple microwave assisted method. The nanocomposites are characterized using XRD, Raman, SEM and TEM. XRD reveals the wurtzite structure of ZnO and TEM shows the heterogeneous nucleation of ZnO nanocrystals anchored onto graphene sheets. The electrochemical properties of the rGO/ZnO nanocomposite enhanced significantly for applications in glucose sensors and supercapacitors. The non-enzymatic glucose sensor of this nanocomposite tested using cyclic voltammetry (CV) and chronoamperometry, exhibits high sensitivity (39.78 mA cm⁻² mM⁻¹) and a lower detection limit of 0.2 nM. The supercapacitor electrode of rGO/ZnO nanocomposite exhibits a significant increase in specific capacitance.     

A green and facile one-pot synthesis of Ag–ZnO/RGO nanocomposite with effective photocatalytic activity for removal of organic pollutants

In this study, Ag–ZnO/reduced graphene oxide (Ag–ZnO/RGO) composite was synthesized by a green and facile one-step hydrothermal process. Aqueous suspension containing Ag and ZnO precursors with graphene oxide (GO) sheets was heated at 140 °C for 2 h. The morphology and structure of as-synthesized particles were characterized by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, and Photoluminescence (PL) spectroscopy which revealed the formation of composite of metal, metal oxide and RGO. It was observed that the presence of Ag precursor and GO sheets in the hydrothermal solution could sufficiently decrease the size of ZnO flowers. The hybrid nanostructure, with unique morphology, obtained from this convenient method (low temperature, less time, and less number of reagents) was found to have good photocatalytic and antibacterial activity. The perfect recovery of catalyst after reaction and its unchanged efficiency for cyclic use showed that it will be an economically and environmentally friendly photocatalyst.     

The size and dispersion effect of modified graphene oxide sheets on the photocatalytic H2 generation activity of TiO2 nanorods

Nano graphene oxide (NGO) was produced by further refluxing graphene oxide (GO) sheets in HNO₃, and carboxylic acid functionalized graphene oxide (GO–COOH) was obtained by a simple etherification reaction between GO and chloroacetic acid. The GO, GO–COOH and NGO sheets are combined with TiO₂ nanorods by a two-phase assembling method, and confirmed by transmission electronic microscopy. The GO–TiO₂, GO–COOH–TiO₂ and NGO–TiO₂ composites are used in a comparative study of photocatalytic H₂ generation activity under UV light irradiation. The H₂ generation rate of TiO₂ nanorods was slightly increased from 15 to 30 mL h⁻¹ g⁻¹ by replacing oleic acid ligands with hydrophilic dopamine, and significantly increased to 105 mL h⁻¹ g⁻¹ after combining with GO sheets. The further comparative study shows that GO–COOH–TiO₂ composite has higher H₂ generation rate of 180 mL h⁻¹ g⁻¹ than that of GO–TiO₂ and NGO–TiO₂ composites.     

Bio-inspired cross-linking with borate for enhancing gas-barrier properties of poly(vinyl alcohol)/graphene oxide composite films

The demand for flexible and transparent barrier films in industries has been increasing. Learning from nature, borate ions were used to cross-link poly(vinyl alcohol) (PVA) and graphene oxide (GO) to produce flexible, transparent high-barrier composite films with a bio-inspired structure. PVA/GO films with only 0.1 wt% GO and 1 wt% cross-linker exhibited an O₂ transmission rate <0.005 cc m⁻² day⁻¹, an O₂ permeability <5.0 × 10⁻²⁰ cm³ cm cm⁻² Pa⁻¹ s⁻¹, and a transmittance at 550 nm >85%; thus, they can be used for flexible electronics. Fourier transform infrared spectrometry and X-ray photoelectron spectroscopy indicated that the outstanding barrier properties are attributed to the formation of chemical cross-linking involving borate ions, GO sheets, and PVA, similar to the borate cross-links in high-order plants. Comparing our experimental data with the Cussler model, we found that the effective aspect ratio was significantly increased after cross-linking, suggesting that cross-linking networks connected GO with each other to form ultra-large impermeable regions. A feasible green technique, with potential for commercial production of barrier films for flexible electronics was presented.     

One-step synthesis of ZnO and Ag/ZnO heterostructures and their photocatalytic activity

By following a one-step, novel methodology, ZnO and Ag/ZnO heterostructures were successfully synthesized at room-temperature. This route is simple, effective, high yield (91%), environmentally friendly (green synthesis) and consists of a mechanically assisted metathesis reaction. The metathesis reaction used in this investigation showed two results: the in-situ generation of alkaline nitrates, LiNO₃/NaNO₃, and the direct crystallization of the desired Zn-based compounds in milling media; revealing a true mechanochemical synthesis of ZnO and Ag/ZnO (1.25, 2.50 and 4.50 mol% of Ag) heterostructures. Particles showed spherical-like morphologies and sizes smaller than 20 nm. The Ag/ZnO heterostructures exhibited higher photocatalytic activity than ZnO for degrading methylene blue (MB) dye. It was also shown that the presence of Ag (up to 1.25 mol%) nanoparticles (NPs) in ZnO accelerates the photodegradation reaction and then slows down with further increases in Ag contents. The 1.25-Ag/ZnO sample (10 mg) showed the highest photocatalytic activity (96%) for degrading MB (100 ml, 10 mg L^?1) within 100 min under UV–Vis light irradiation (λ = 310 nm).     

Improved charging/discharging behavior of electropolymerized nanostructured composite films of polyaniline and electrochemically reduced graphene oxide

In this work, a simple electrochemical reduction procedure has been applied to nanostructured composite films of polyaniline (PANI) and graphene oxide (GO) having a globular surface morphology with the grain size of 50 nm. The reduction converts GO to reduced GO (rGO) which improves the electroactivity of the PANI composite films with 30%. Cyclic voltammetry confirmed the reduction of GO to rGO whereas electrochemical impedance spectroscopy showed that the rGO network increases the redox capacitance of the composite films with 15% to 77 mF cm⁻². In a three-electrode cell, the anodic charge of the PANI film containing GO increased with 18.7% during the potential cycling stability test for 10,000 cycles between −0.2 and 0.5 V, indicating that the film had a good stability against degradation. This composite film type still maintained a high capacitance of 15 mF cm⁻² in a symmetric two-electrode cell after 10,000 potential cycles between 0 and 0.4 V. The electrochemically prepared PANI composite films reported here are aimed to be used in capacitor applications where it is crucial to deposit thin PANI layers on well-defined small surfaces where other polymerization or deposition techniques cannot be used and in solid-state chemical sensors as ion-to-electron transducer interfaces.     

Structural,optical and photocatalytic properties of ZnO nanorods: Effect of aging time and number of layers

ZnO thin films were prepared by sol–gel dip coating method onto glass substrates. The effects of aging time of the starting solution (2, 10 and 30 days) and the number of coats (2, 5 and 10 coatings) on structural, morphological and optical properties were investigated. Photocatalytic efficiency was also assessed. X-ray diffraction analysis indicates that all the films exhibit a Zincite-type structure with a preferred grains orientation along the [002] direction. The preferred orientation factor (POF) increases with aging time while the crystallite size decreases. The field emission scanning electron microscopy observations reveals nanorods morphology. The length of ZnO nanorods increase with increasing number of layers whereas their length decreases as a function of aging time while adopting a random orientation. A high optical transparency is observed for all ZnO thin films, ranging from 90 up to 96%. Methylene Blue (MB) dye photocatalytic degradation was found increases with aging time, reaching almost 94% after 10 h under UV irradiation. The apparent reaction rate (K_app) obtained by Langmuir-Hinshelwood model increases with increasing aging time from, from 0.218 h⁻¹ for 2 days to reach a steady state around 0.270 h⁻¹. Nevertheless, a small variation of K_app was recorded when varying the number of coats; 0.223–0.226 h⁻¹.     

Vitis labruska skin extract assisted green synthesis of ZnO super structures for multifunctional applications

The formation of unique morphologies of zinc oxide (ZnO) super structured frameworks were reported via a simple and eco-friendly route employing Vitis labruska fruit Black Grape Skin (BGS) extract as a fuel. XRD, FTIR, UV–vis, PL, SEM and TEM studies are performed to analyse the formation and characterization of ZnO. XRD confirmed the crystalline nature of the material with hexagonal Wurtzite structure having average crystallite size of ~50 nm. FTIR spectrum shows a band at 532 cm⁻¹ due to the vibrational mode of Zn-O bending. The band gap of the ZnO was found to be 3.26 eV. SEM images confirm the formation of different morphologies like Mysore pak (a popular Indian dessert), canine teeth, hollow pyramids and gems were obtained by varying the weight of BGS. These superstructures were obtained due to the interaction of Zn²⁺ with BGS extract. TEM images clearly shows lattice spacing of 0.29 nm corresponding to the (002) plane of ZnO. Photoluminescence (PL) spectrum shows strong yellow light emission upon excitation at 320 nm due to the Zn-O defects. Synthesized ZnO nanoparticles (Nps) exhibited good photocatalytic activity for the degradation of Methylene blue (MB) dye. The photocatalytic activity was due to the production of OH˙ radicals during photo irradiation on ZnO Nps. ZnO Nps also exhibited superior antibacterial activity against Staphylococcus aureus and Escherichia coli bacteria. Further, ZnO Nps were also used in the development of novel electrochemical sensing platform towards the electrochemical detection of hydrazine as a model system at very low concentrations having a detection limit of 0.3 µM.     

Incorporated ZnO onto nano clinoptilolite particles as the active centers in the photodegradation of phenylhydrazine

ZnO was incorporated into nano particles of clinoptiloite by ion exchanging the zeolite in a zinc nitrate aqueous solution followed by calcinations process. All raw and modified samples were characterized by XRD, FT-IR, DRS, TG-DTG, SEM, BET and TEM. The prepared catalyst was used in the photocatalytic degradation of phenylhydrazine (PHZ) and the best experimental parameters were obtained as: 0.25 g L⁻¹ of the catalyst, 20 ppm pollutant concentration and pH = 8. The degradation extent was monitored by UV–vis spectroscopy and the results were confirmed by HPLC and the chemical oxygen demand (COD).     

Photodegradation of tetrahalobisphenol-A (X = Cl,Br) flame retardants and delineation of factors affecting the process

The photoassisted degradation (HPLC-UV absorption), dehalogenation (HPLC-IC) and mineralization (TOC decay) of the flame retardants tetrabromobisphenol-A (TBBPA) and tetrachlorobisphenol-A (TCBPA) were examined in UV-irradiated alkaline aqueous TiO₂ dispersions (pH 12), and for comparison the parent bisphenol-A (BPA, an endocrine disruptor) in pH 4–12 aqueous media to assess which factor impact most on the photodegradative process. Complete degradation (2.7–2.8 × 10⁻² min⁻¹) and dehalogenation (1.8 × 10⁻² min⁻¹) of TBBPA and TCBPA occurred within 2 h of UV irradiation, whereas only 45–60% mineralization (2.3–2.7 × 10⁻³ min⁻¹) was complete within 5 h for the flame retardants at pH 12 and ca. 80% for the parent BPA. Factors examined in the pH range 4–12 that impact the degradation of BPA were the point of zero charge of TiO₂ particles (pH_pzc; electrophoretic method), particle or aggregate sizes of TiO₂ (light scattering), and the relative number of OH radicals (as DMPO–OH adducts; ESR spectroscopy) produced in the UV-irradiated dispersion. Dynamics of BPA degradation (2.0–2.4 × 10⁻² min⁻¹) were pH-independent and independent of particle/aggregate size, but did correlate with the number of OH radicals, at least at pHs 4 to 8–9, after which the rates decreased somewhat at pH > 9 with decreasing adsorption owing to Coulombic repulsive forces between the very negative TiO₂ surface and the anionic forms of BPA (pK_as ca. 9.6–11.3), even though the number of OH radicals continued to increase at the higher pHs.     

Promoting role of transition metal oxide on ZnTiO3–TiO2 nanocomposites for the photocatalytic activity under solar light irradiation

Transition metal oxide (Fe₂O₃, Co₃O₄ and CuO) loaded ZnTiO₃–TiO₂ nanocomposites were successfully prepared by solid state dispersion method. The structural, morphological and optical properties of samples were characterized by TGA/DTA, XRD, BET, FT-IR, DRS, PL, XPS and SEM techniques. The photocatalytic activity of samples was investigated by degradation of 4-chlorophenol in water under sunlight. The Fe₂O₃ loaded sample was found to exhibit much higher photocatalytic activity than the other composite powders. 7Fe₂O₃/ZnTi sample has the highest percentage of 4-chlorophenol degradation (100%) and highest reaction rate (1.27 mg L⁻¹ min⁻¹) was obtained in 45 min. The enhancement of photocatalytic activity for ZnTiO₃–TiO₂ sample with Fe₂O₃ addition may be attributed to its small particle size, the presence of more surface OH groups, lower band gap energy than other samples in this paper and the presence of more hexagonal ZnTiO₃ phase in the morphology.     

Synthesis of AgCl/Bi3O4Cl composite and its photocatalytic activity in RhB degradation under visible light

This work presents a novel composite photocatalyst, AgCl/Bi₃O₄Cl, which was prepared using an ion-exchange method. The synthesized composite was characterized by various techniques and its photocatalytic activity was investigated in RhB degradation under visible light irradiation. Results indicated that the introduction of AgCl into Bi₃O₄Cl promoted the specific surface area, light absorption performance and the separation efficiency of electron–hole pairs, which resulted in a high photocatalytic activity of the composite. The optimal AgCl/Bi₃O₄Cl sample showed a RhB degradation rate of 0.048 min^− 1, which was 2.2 and 2.4 times higher than those of AgCl and Bi₃O₄Cl, respectively.     

Multifunctional ZnO/Fe-O and graphene oxide nanocomposites: Enhancement of optical and magnetic properties

ZnO is a semiconductor with a great interest, but it has several deficiencies which limit its use in technologic applications. One important limitation is having the band gap in the UV which reduces its use in optical devices. To solve this problem, in this work, composites based in ZnO with goethite and graphene oxide (GO) by sol-gel are prepared. The obtained samples (powders and thin films) were characterized microstructurally (DTA, XRD, micro-Raman, FE-SEM), optically (transmittance and photoluminescence) and magnetically (SQUID). The ZnO band gap of multifunctional composites shows a red-shift towards visible range (E_g ∼3.01 eV) with high transmittance ∼85% (thickness of 362 nm) over the visible wavelength range. A long-range magnetic order at room temperature appears in these nanocomposites (Ms = 1.60·10⁻² emu/g). The combination of both dopants allows modifying the functional properties of ZnO, opening a great field of applications in ZnO composites, such as spintronic and optoelectronic devices.     

Synthesis of cube-like Ag/AgCl plasmonic photocatalyst with enhanced visible light photocatalytic activity

Cube-like Ag/AgCl plasmonic photocatalyst was successfully synthesized through a one-pot precipitation method by simply adding an aqueous solution of AgNO₃ into the natural hot spring, wherein the hot spring acted as the chlorine source. The cube-like Ag/AgCl with a size of 0.5–0.9 μm exhibited enhanced visible light photocatalytic performance for the degradation of organic MO dye due to the localized surface plasmon resonance (LSPR) of the photoexcited Ag species. The trapping experiments confirmed that O₂⁻ and h⁺ were the main active species during the photocatalytic process.     

Enhancing polymer/graphene oxide gas barrier film properties by introducing new crystals

A transparent, gas barrier film comprised of poly(vinyl alcohol) (PVA) and graphene oxide (GO) is synthesized through combined methods of solution blending and isothermal recrystallization. The recrystallized PVA/GO film with only 0.07 vol% GO gives an O₂ transmission rate <0.005 cc m⁻² day⁻¹ and an O₂ permeability <5.0 × 10⁻²⁰ cm³ cm cm⁻² Pa⁻¹ s⁻¹; hence, it is far superior to other blend polymer/inorganic composites. The excellent O₂ barrier properties are attributed to a unique hybrid of PVA crystals and GO sheets. PVA crystals form around the GO during isothermal recrystallization, indicating that a GO sheet can act as a nucleating agent. The newly formed PVA crystals fill in the spaces between the GO sheets, and together they become ultra-large impermeable regions, which can prevent the passage of O₂. The hybrid film has potential applications in flexible electronics, pharmaceuticals, and food packaging.     

Photocatalytic degradation of 17-β-oestradiol on immobilised TiO2

Micro-molar concentrations of aqueous 17-β-oestradiol were 98% destroyed in 3.5 h by photocatalysis over the titanium dioxide powder immobilised on Ti-6Al-4V alloy. The concentration of oestradiol was determined by HPLC with fluorescence detection. The degradation kinetics were fitted to a Langmuir–Hinshelwood model with k(S) = 4.4 × 10⁻² μmol dm⁻³ min⁻¹ and K(S) = 0.347 dm³ μmol⁻¹. The pseudo-first-order rate constant (1.57 × 10⁻² min⁻¹) was in line with the 50% degradation time of 40 min. The apparent quantum yield per electron was φ_e′  = 0.41%. The effect of pH on the initial rate of degradation was similar to that reported for phenol.     

In situ synthesis of ultrathin 2-D TiO2 with high energy facets on graphene oxide for enhancing photocatalytic activity

Two kinds of TiO₂ with novel structures, interpenetrating anatase TiO₂ tablets (IP-TiO₂), and overlapping anatase TiO₂ nanosheets (OL-TiO₂) with exposed {0 0 1} facets, are synthesized. The graphene oxide (GO) supported ultrathin TiO₂ nanosheets (OL-TiO₂/GO) is also prepared by one-pot hydrothermal method. The microscopic feature, morphology, phase, and nitrogen adsorption–desorption isotherms are characterized. The performance of photocatalytic degradation of methyl blue is also measured. Compared with IP-TiO₂, the OL-TiO₂ with GO possess higher photocatalytic efficiency. The GO can improve the photocatalytic property by increasing specific surface area, accelerating the separation of electron–hole pairs, as well as extending the electron life. The growth process of TiO₂ nanosheets on graphene oxide layers probably follows a step-growth mechanism with F⁻ as morphology controlling agent. The steps on the surface can improve the photocatalytic activity further due to the increase of dangling bonds of 5-coordinated Ti (Ti_5c) which are considered to be the active sites in the photocatalytic reaction.     

An easy one-step electrosynthesis of graphene/polyaniline composites and electrochemical capacitor

An easy electrochemical technique is proposed to prepare electrochemically reduced graphene oxide (ERGO)/polyaniline (PANI) composites in a single step. The technique uses a two-electrode cell in which a separator soaked with an acid solution is sandwiched between graphene oxide (GO)/aniline films deposited on conductive substrates and an alternating voltage was applied to the electrodes. Successful preparations of ERGO/PANI composites were evidenced by characterizations due to UV–vis-NIR, FT-IR, XPS, XRD, and SEM measurements with free-standing films of ERGO/PANI obtained easily by disassembling the two-electrode cells. The ERGO/PANI films exhibited a high mechanical stability, flexibility, and conductivity (68 S cm⁻¹ for the composite film containing 80% ERGO) with nanostructured PANI particles (smaller than 20 nm) embedded homogeneously between the ERGO layers. The two-electrode cells acted as electrochemical capacitors (ECs) after a sufficient voltage cycling and exhibited relatively large specific capacitances (195–243 F g⁻¹ at a scan rate of 100 mV s⁻¹) with an excellent cycle life (retention of 83% capacitance after 20,000 charge–discharge cycles). Influences of the GO/aniline ratio, the sort of electrolytes, and the weight of the composite on the energy storage characteristics of ECs comprising the ERGO/PANI composites were also studied.