Visible‐light‐driven self‐cleaning SERS substrate of silver nanoparticles and graphene oxide decorated nitrogen‐doped titania nanotube array

Graphene oxide (GO) and silver nanoparticles (Ag NPs) sequentially decorated nitrogen‐doped titania nanotube array (N‐TiO₂ NTA) had been designed as visible‐light‐driven self‐cleaning surface‐enhanced Raman scattering (SERS) substrate for a recyclable SERS detection application. N‐TiO₂ NTA was fabricated by anodic oxidation and then doping nitrogen treatment in ammonia atmosphere, acting as a visible‐light‐driven photocatalyst and supporting substrate. Ag/GO/N‐TiO₂ NTA was prepared by decorating GO monolayer through an impregnation process and then depositing Ag NPs through a polyol process on the surface of N‐TiO₂ NTA, acting as the collection of organic molecule and Raman enhancement. The SERS activity of Ag/GO/N‐TiO₂ NTA was evaluated using methyl blue as an organic probe molecule, revealing the analytical enhancement factor of 4.54 × 10⁴. Ag/GO/N‐TiO₂ NTA was applied as active SERS substrate to determine a low‐affinity organic pollutant of bisphenol A, revealing the detection limit of as low as 5 × 10^?7 m . Ag/GO/N‐TiO₂ NTA could also achieve self‐cleaning function for a recycling utilization through visible‐light‐driven photocatalytic degradation of the adsorbed organic molecules. Ag/GO/N‐TiO₂ NTA has been successfully reused for five times without an obvious decay in accuracy and sensitivity for organic molecule detection. The unique properties of this SERS substrate enable it to have a promising application for the sensitive and recyclable SERS detection of low‐affinity organic molecules. Copyright © 2016 John Wiley & Sons, Ltd.     

Co‐doped Pt Nanowire Networks with Clean Surfaces for Enhanced Oxygen Reduction Reactions

Surfactants or capping agents are usually employed to control the shapes and sizes of metal nanowires (NWs). Polyvinylpyrrolidone (PVP) and oleylamine (OAm) are the most common capping agents used in the synthesis of metal nanowires. However, these capping agents bind strongly onto the surface of the nanowires and severely prevent the reactant molecules from entering the active sites. In the present research, a facile acetic acid/NaBH₄ treatment technology is reported to effectively remove PVP and OAm from the surface of the co‐doped Pt NWs. Interestingly, the morphology of poor crystalline platinum nanowires treated with NaBH₄ solution is transformed into nanowire networks (NWNs) with higher crystallinity. Furthermore, in comparison with the commercial Pt/C catalyst, the catalytic activity of co‐doped Pt NWNs with clean surfaces shows improvements of up to 4.1 times for mass activity and 5.1 times for specific activity, respectively.     

Facile Fabrication of Multi‐targeted and Stable Biochemical SERS Sensors

The direct transfer of single‐crystalline Au nanowires (NWs) onto Au substrates was achieved by a simple attachment and detachment process. In the presence of a lubricant, Au NWs grown vertically on a sapphire substrate were efficiently moved to an Au substrate through van der Waals interactions. We demonstrate that the transferred Au NWs on the Au substrate can act as sensitive, reproducible, and long‐term‐stable surface‐enhanced Raman scattering (SERS) sensors by detecting human α‐thrombin as well as Pb²⁺ and Hg²⁺ ions. These three biochemically and/or environmentally important analytes were successfully detected with high sensitivity and selectivity by Au NW‐SERS sensors bound by a thrombin‐binding aptamer. Furthermore, the as‐prepared sensors remained in working order after being stored under ambient conditions at room temperature for 80 days. Because Au NWs can be routinely transferred onto Au substrates and because the resultant Au NW‐SERS sensors are highly stable and provide with high sensitivity and reproducibility of detection, these sensors hold potential for practical use in biochemical sensing.     

Preparation of Ag Nanowire @ Au Nanoparticle Hybrid Nanowires and their Influence on the Fluorescence Properties of P3HT

In this study, the galvanic displacement reaction between silver and AuCl₄⁻ was carried out to synthesize a series of silver nanowire (Ag NW) @ gold nanoparticle (Au NP) hybrid nanowires. The influence of Ag NW @ Au NP hybrid nanowires on the fluorescence properties of the poly (3‐hexylthiophene) (P3HT) was investigated. The particle sizes of Au NPs on the hybrid nanowires could be adjusted by varying the reaction time and the concentration of the HAuCl₄ solution. Furthermore, steady‐state fluorescence measurements showed that the fluorescence intensity of the P3HT films was higher on various Ag NW @ Au NP hybrid nanowires compared to that on a bare silicon substrate. This was due to the increase in the intensity of electromagnetic field by the localized surface plasmon resonances of Au NPs and surface plasmon polaritons of Ag NWs from the hybrid nanowires. The results were further confirmed by the Raman spectra of the P3HT films on different substrates.     

Ag Nanowire‐Ag Nanoparticle Hybrids for the Highly Enhanced Fluorescene Detection of Protoporphyrin IX Based on Surface Plasmon‐Enhanced Fluorescence

In this study, we developed an approach to fabricate novel 1D Ag NWs‐Ag NPs hybrid substrate for enhanced fluorescene detection of protoporphyrin IX (PpIX) based on surface plasmon‐enhanced fluorescence. The Ag NWs‐Ag NPs hybrid was synthesized by combining the hydrothermal method and self‐assembly method with the asisstance of polyvinylpyrrolidone (PVP). When the Ag NWs‐Ag NPs hybrid was deposited on the glass substrate and employed as active substrate to detect PpIX, the fluorescence intensity of PpIX was enhanced greatly due to the coupling effect of localized surface plasmon‐localized surface plasmon (LSP‐LSP) and localized surface plasmon‐surface plasmon propagation (LSP‐SPP) which induced great enhancement of the electromagnetic field. Furthermore, the enhancement effect was approximately linear when the concentration of PpIX was ranged from 1×10^?7 mol/L to 2×10^?5 mol/L, and the photobleaching phenomenon of PpIX was reduced greatly, indicating that the fabricated Ag NWs‐Ag NPs hybrid substrate had well performance for PpIX imaging. This work provides an effective approach to prepare highly sensitive and stable fluorescence enhancement substrate, and has great potential application in fluorescence imaging.     

A New p‐Metal‐n Structure AgBr‐Ag‐BiOBr with Superior Visible‐Light‐Responsive Catalytic Performance

A novel visible‐light‐driven AgBr‐Ag‐BiOBr photocatalyst was synthesized by a facile hydrothermal method. Taking advantage of both p‐n heterojunctions and localized surface plasmon resonance, the p‐metal‐n structure exhibited a superior performance concerning degradation of methyl orange under visible‐light irradiation (λ>420 nm). A possible photodegradation mechanism in the presence of AgBr‐Ag‐BiOBr composites was proposed, and the radical species involved in the degradation reaction were investigated. HO₂^?/^?O₂^? played the same important role as ^?OH in the AgBr‐Ag‐BiOBr photocatalytic system, and both the electron and hole were fully used for degradation of organic pollutants. A dual role of metallic Ag in the photocatalysis was proposed, one being surface plasmon resonance and the other being an electron‐hole bridge. Due to the distinctive p‐metal‐n structure, the visible‐light absorption, the separation of photogenerated carriers and the photocatalysis efficiency were greatly enhanced.     

A Two‐Dimensional Ruddlesden–Popper Perovskite Nanowire Laser Array based on Ultrafast Light‐Harvesting Quantum Wells

Miniaturized nanowire nanolasers of 3D perovskites feature a high gain coefficient; however, room-temperature optical gain and nanowire lasers from 2D layered perovskites have not been reported to date. A biomimetic approach is presented to construct an artificial ligh-harvesting system in mixed multiple quantum wells (QWs) of 2D-RPPs of (BA)₂(FA)_n−1Pb_nBr_3n+1, achieving room-temperature ASE and nanowire (NW) lasing. Owing to the improvement of flexible and deformable characteristics provided by organic BA cation layers, high-density large-area NW laser arrays were fabricated with high photostability. Well-controlled dimensions and uniform geometries enabled 2D-RPPs NWs functioning as high-quality Fabry–Perot (FP) lasers with almost identical optical modes, high quality (Q) factor (ca. 1800), and similarly low lasing thresholds.     

Stereoaligned Epitaxial Growth of Single‐Crystalline Platinum Nanowires by Chemical Vapor Transport

Epitaxial Pt nanowire (NW) arrays are synthesized for the first time by a chemical vapor transport method by using a metal halide as a precursor. Here we report that the epitaxial growth direction of NWs can be steered by seed crystal morphology. Octahedral seeds grow into inclined NWs possessing six growth directions, whereas half‐octahedral seeds grow into vertical and horizontal NWs. Interfacial energies between the seed material and the substrate are critical in determining the morphology of seed crystals. We also demonstrate that non‐SERS‐active Pt NWs can show strong surface‐enhanced Raman scattering (SERS) spectra by placing them on Ag films. The active SERS observation would help to elucidate platinum‐catalyzed chemical reactions.     

Urchin‐like Ag Nanowires as Non‐enzymatic Hydrogen Peroxide Sensor

Urchin‐like Ag nanowires were prepared by reacting AgNO₃(aq) with Cu metal in the presence of cetyltrimethylammonium chloride and HNO₃(aq) on a screen printed carbon electrode at room temperature. The diameters of the nanowires were about 100 nm, while the lengths were up to 10 μm. Cyclic voltammetric experiments using the Ag nanowires as the working electrode showed electrocatalytic H₂O₂ reduction. The electrode exhibited a high sensitivity of 4705 μA mM^‐1 mg^‐1 cm^‐2 from 50 μM to 10.35 mM and a measurable detection limit of 10 μM in amperometric detection. This is the first report on Ag NWs for non‐enzymatic H₂O₂ sensing.     

Synthesis and direct interactions of silver colloidal nanoparticles with pollutant gases

Silver nanoparticles (NPs) were synthesized in organic solvents. Spontaneous reduction of silver salts takes place in N,N′-dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO) at room temperature. The formed colloids are not stable without a stabilizing agent, hence rarely used, and inexpensive organic molecules (β-cyclodextrin and cholic acid) were used as surface modifiers in DMF. The stabilization was successful; the Ag NPs remained stable for more than 3 months. Additionally, Ag NPs were prepared using Ag-2-ethylhexanoate and Na-citrate as capping agent in DMSO. The resulting NPs are stable, of 4.4 nm average size, and at the same time reactive for catalytic purposes. The interaction of Ag NPs with pollutant atmospheric gases (NO and SO₂) was studied. UV–visible spectra show the oxidation of silver and the very efficient reduction of NO at room temperature. SO₂ molecules are adsorbed on the NPs surface, causing their aggregation and precipitation.     

Two‐Dimensional Metal‐Organic Layers for Electrochemical Acceptorless Dehydrogenation of N‐Heterocycles

The catalytic acceptorless dehydrogenation (CAD) is an attractive synthetic route to unsaturated compounds because of its high atomic efficiency. Here we report electrochemical acceptorless dehydrogenation of N‐heterocycles to obtain quinoline or indole derivatives using metal‐organic layer (MOL) catalyst. MOL is the two‐dimensional version of metal‐organic frameworks (MOF), and it can be constructed on conductive multi‐walled carbon nanotubes via facile solvothermal synthesis to overcome the conductivity constraint for MOFs in electrocatalysis. TEMPO‐OPO₃^2? was incorporated into the system through a ligand exchange with capping formate on the MOL surface to serve as the active catalytic centers. The hybrid catalyst is efficient in the organic conversion and can be readily recycled and reused.     

Enhancing Electron Collection Efficiency and Effective Diffusion Length in Dye‐Sensitized Solar Cells

Intensity‐modulated photocurrent spectroscopy and intensity‐modulated photovoltage spectroscopy are employed to measure the dynamics of electron transport and recombination in the ZnO nanowire (NW) array‐ZnO/layered basic zinc acetate (LBZA) nanoparticle (NP) composite dye‐sensitized solar cells (DSSCs). The roles of the vertical ZnO NWs and insulating LBZA in the electron collection and transport in DSSCs are investigated by comparing the results to those in the TiO₂–NP, horizontal TiO₂–NW and vertical ZnO–NW‐array DSSCs. The electron transport rate and electron lifetime in the ZnO NW/NP composite DSSC are superior to those in the conventional TiO₂–NP cell due to the existence of the vertical ZnO NWs and insulating LBZA. It indicates that the ZnO NW/NP composite anode is able to sustain efficient electron collection over much greater thickness than the TiO₂–NP cell does. Consequently, a larger effective electron diffusion length is available in the ZnO composite DSSC.     

Fabrication of Hierarchically Structured MOF‐Co3O4 on Well‐aligned CuO Nanowire with an Enhanced Electrocatalytic Property

Engineering appropriate shape and size of three‐dimensional inorganic nanostructures materials is of one the main critical problems in pursuing high‐performance electrode materials. Herein, we fabricate a metal‐organic framework derived cobalt oxide (Co₃O₄) are grown on copper oxide nanowire (CuO NWs) supported on the surface of 3D copper foam substrate. The highly aligned CuO NWs were prepared by using electrochemical anodization of copper foam in ambient temperature and followed by MOF Co₃O₄ was grown via a simple in situ solution deposition then consequent calcination process. The obtained binder‐free 3D CuO NWs@Co₃O₄ nanostructures were further characterized by using X‐ray diffraction, X‐ray photoelectron spectroscopy, field‐emission scanning electron microscopy, and transmission electron microscopy. Furthermore, electrochemical sensing of glucose was studied by using Cyclic Voltammetry, and chronoamperometry techniques. Interestingly, 3D CuO NWs@Co₃O₄ electrode exhibits excellent performance for the oxidation of glucose compared with individual entities. The proposed sensor shows wide linear ranges from 0.5 μM to 0.1 mM with the sensitivity of 6082 μA/μM and the lowest detection limit (LOD) of 0.23 μM was observed with the signal to noise ratio, (S/N) of 3. The superior catalytic oxidation of glucose mainly is endorsed by the excellent electrical conductivity and synergistic effect of the Co₃O₄ and CuO NWs.     

A Nonenzymatic Hydrogen Peroxide Sensor Based on Silver Nanowires and Chitosan Film

A nonenzymatic amperometric electrochemical sensor for the detection of hydrogen peroxide (H₂O₂) was fabricated based on highly dense silver nanowires (Ag NWs) and chitosan (CS) film. Ag NWs were synthesized by a poly(vinyl pyrrolidone) (PVP)‐mediated polyol process in the presence of manganese chloride (MnCl₂), and were characterized by scanning electron microscopy (SEM), energy‐dispersive X‐ray spectroscopy (EDS), and X‐ray diffraction (XRD). Under the optimal conditions, the proposed nonenzymatic sensor exhibited good electrocatalytic activity towards the reduction of H₂O₂, and could detect H₂O₂ in the linear range of 0.008–1.35 mM, with a detection limit of 2 µM (S/N=3).     

Bimetallic Ag‐Pt Sub‐nanometer Supported Clusters as Highly Efficient and Robust Oxidation Catalysts

A combined experimental and theoretical investigation of Ag‐Pt sub‐nanometer clusters as heterogeneous catalysts in the CO→CO₂ reaction (COox) is presented. Ag₉Pt₂ and Ag₉Pt₃ clusters are size‐selected in the gas phase, deposited on an ultrathin amorphous alumina support, and tested as catalysts experimentally under realistic conditions and by first‐principles simulations at realistic coverage. In situ GISAXS/TPRx demonstrates that the clusters do not sinter or deactivate even after prolonged exposure to reactants at high temperature, and present comparable, extremely high COox catalytic efficiency. Such high activity and stability are ascribed to a synergic role of Ag and Pt in ultranano‐aggregates, in which Pt anchors the clusters to the support and binds and activates two CO molecules, while Ag binds and activates O₂, and Ag/Pt surface proximity disfavors poisoning by CO or oxidized species.     

Microwave‐Assisted Modular Fabrication of Nanoscale Luminescent Metal‐Organic Framework for Molecular Sensing

Miniaturizing the size of metal‐organic framework (MOF) crystals to the nanometer scale is challenging, but it provides more advanced applications without changing the characteristic features itself. It is especially useful to investigate the correlation between the porous properties and the interfacial structures of nanocrystals. Using amino acids as capping agents, nanoscale Tb‐MOF‐76 is fabricated rapidly by means of microwave‐assisted methods. Both the modular effects of the amimo acids and the acid–base environment of the reaction medium have an important impact on the morphologies and dimensions of Tb‐MOF‐76. The structures of the samples are confirmed by powder X‐ray diffraction, and the morphologies are characterized by SEM. Photoluminescence studies reveal that these Tb‐MOF‐76 materials exhibit a green emission corresponding to the transition ⁵D₄→⁷F_J of Tb³⁺ ions under UV‐light excitation, which is sensitive to small organic molecules in solution.     

Novel Silver/Poly(vinyl alcohol) Nanocomposites for Surface‐Enhanced Raman Scattering‐Active Substrates

Summary: Surface‐enhanced Raman scattering (SERS)‐active substrates with high enhancement were prepared by an in situ reduction method. Novel silver/poly(vinyl alcohol) (PVA) nanocomposite films were obtained, in which the silver nitrate, poly(γ‐glutamic acid) (PGA), and PVA acted as precursor, stabilizer, and polyol reducant, respectively. The UV‐visible spectra of the as‐fabricated films showed that the surface plasmon resonance (SPR) absorption band was narrow and of a stronger intensity, which indicates that the Ag nanoparticle size distribution on the substrate was highly uniform. This finding was further confirmed by X‐ray diffraction (XRD), transmission electron microscopy (TEM), and field‐emission scanning electron microscope (FE‐SEM) measurements. It was found that a PGA‐stabilized PVA nanocomposite film revealed the presence of well‐dispersed spherical silver nanoparticles with an average diameter of 90 nm. The new substrate presents high SERS enhancement and the enhanced factor is estimated to be 10⁶ for the detection of benzoic acid.

The Raman scattering enhancement factor for the Raman spectra of benzoic acid on the various nanocomposite films.     

Fluorine‐Doped Porous Single‐Crystal Rutile TiO2 Nanorods for Enhancing Photoelectrochemical Water Splitting

Fluorine‐doped hierarchical porous single‐crystal rutile TiO₂ nanorods have been synthesized through a silica template method, in which F^? ions acts as both n‐type dopants and capping agents to make the isotropic growth of the nanorods. The combination of high crystallinity, abundant surface reactive sites, large porosity, and improved electronic conductivity leads to an excellent photoelectrochemical activity. The photoanode made of F‐doped porous single crystals displays a remarkably enhanced solar‐to‐hydrogen conversion efficiency (≈0.35 % at ?0.33 V vs. Ag/AgCl) under 100 mW cm^?2 of AM=1.5 solar simulator illumination that is ten times of the pristine solid TiO₂ single crystals.     

WS2 Nanowires as a High‐Performance Anode for Sodium‐Ion Batteries

We report the synthesis and anode application for sodium‐ion batteries (SIBs) of WS₂ nanowires (WS₂ NWs). WS₂ NWs with very thin diameter of ≈25 nm and expanded interlayer spacing of 0.83 nm were prepared by using a facile solvothermal method followed by a heat treatment. The as‐prepared WS₂ NWs were evaluated as anode materials of SIBs in two potential windows of 0.01–2.5 V and 0.5–3 V. WS₂ NWs displayed a remarkable capacity (605.3 mA h g^?1 at 100 mA g^?1) but with irreversible conversion reaction in the potential window of 0.01–2.5 V. In comparison, WS₂ NWs showed a reversible intercalation mechanism in the potential window of 0.5–3 V, in which the nanowire‐framework is well maintained. In the latter case, the interlayers of WS₂ are gradually expanded and exfoliated during repeated charge–discharge cycling. This not only provides more active sites and open channels for the intercalation of Na⁺ but also facilitates the electronic and ionic diffusion. Therefore, WS₂ NWs exhibited an ultra‐long cycle life with high capacity and rate capability in the potential window of 0.5–3 V. This study shows that WS₂ NWs are promising as the anode materials of room‐temperature SIBs.     

Electrocatalytic Production of C3‐C4 Compounds by Conversion of CO2 on a Chloride‐Induced Bi‐Phasic Cu2O‐Cu Catalyst

Electrocatalytic conversion of carbon dioxide (CO₂) has recently received considerable attention as one of the most feasible CO₂ utilization techniques. In particular, copper and copper‐derived catalysts have exhibited the ability to produce a number of organic molecules from CO₂. Herein, we report a chloride (Cl)‐induced bi‐phasic cuprous oxide (Cu₂O) and metallic copper (Cu) electrode (Cu₂O_Cl) as an efficient catalyst for the formation of high‐carbon organic molecules by CO₂ conversion, and identify the origin of electroselectivity toward the formation of high‐carbon organic compounds. The Cu₂O_Cl electrocatalyst results in the preferential formation of multi‐carbon fuels, including n‐propanol and n‐butane C3–C4 compounds. We propose that the remarkable electrocatalytic conversion behavior is due to the favorable affinity between the reaction intermediates and the catalytic surface.