Leilei Kang

Mechanistic understanding of surface plasmon assisted catalysis on a single particle: cyclic redox of 4-aminothiophenol

Surface plasmon assisted catalysis (SPAC) reactions of 4-aminothiophenol (4ATP) to and back from 4,4′-dimercaptoazobenzene (DMAB) have been investigated by single particle surface enhanced Raman spectroscopy, using a self-designed gas flow cell to control the reductive/oxidative environment over the reactions. Conversion of 4ATP into DMAB is induced by energy transfer (plasmonic heating) from surface plasmon resonance to 4ATP, where O2 (as an electron acceptor) is essential and H2O (as a base) can accelerate the reaction. In contrast, hot electron (from surface plasmon decay) induction drives the reverse reaction of DMAB to 4ATP, where H2O (or H2) acts as the hydrogen source. More interestingly, the cyclic redox between 4ATP and DMAB by SPAC approach has been demonstrated. This SPAC methodology presents a unique platform for studying chemical reactions that are not possible under standard synthetic conditions.

Highly Sensitive Surface-Enhanced Raman Spectroscopy (SERS) Platforms Based on Silver Nanostructures Fabricated on Polyaniline Membrane Surfaces

Here, we demonstrate a facile synthesis of homogeneous Ag nanostructures fully covering the polyaniline (PANI) membrane surface simply by introducing organic acid in the AgNO(3) reaction solution, as an improved technique to fabricate well-defined Ag nanostructures on PANI substrates through a direct chemical deposition method [Langmuir2010, 26, 8882]. It is found that the chemical nature of the acid is crucial to create a homogeneous nucleation environment for Ag growth, where, in this case, homogeneous Ag nanostructures that are assembled by Ag nanosheets are produced with the assistance of succinic acid and lactic acid, but only scattered Ag particles with camphorsulfonic acid. Improved surface wettability of PANI membranes after acid doping may also account for the higher surface coverage of Ag nanostructures. The Ag nanostructures fully covering the PANI surface are extremely sensitive in the detection of a target analyte, 4-mercaptobenzoic acid (4-MBA), using surface-enhanced Raman spectroscopy (SERS), with a detection limit of 10(-12) M. We believe the facilely fabricated SERS-active substrates based on conducting polymer-mediated growth of Ag nanostructures can be promising in the trace detection of chemical and biological molecules.

Recent progress in the applications of graphene in surface-enhanced Raman scattering and plasmon-induced catalytic reactions

Graphene continues to attract tremendous interest, owing to its excellent optical and electronic properties. Based on its unique features, graphene has been employed in the ever-expanding research fields. Surface-enhanced Raman scattering (SERS) may be one of the significant applied fields where graphene can make a difference. Since its discovery, the SERS technique has been capable of ultra sensitively detecting chemical and biological molecules at very low concentration, even at the single molecule level, but some problems, such as irreproducible SERS signals, should be overcome before being practically applied for spectral analysis. Graphene can be a promising candidate to make up the deficiency of a conventional metal SERS substrate. Furthermore, graphene, serving as an enhancement material, is usually deemed as a chemically inert substance to isolate the interactions between metal and probe molecules. While, irradiated by laser, structure changes of graphene under specific conditions and the contributions of its high electron mobility in plasmon-induced catalytic reactions are often ignored. In this review, we mainly focus on the state-of-the-art applications of graphene in the fields of SERS and laser-induced catalytic reactions. The advances in informative Raman spectra of graphene are firstly reviewed. Then, the graphene related SERS substrates, including graphene-enhanced Raman scattering (GERS) and graphene-mediated SERS (G-SERS), are summarized. We finally highlight the catalytic reactions occurring on graphene itself and molecules adsorbed onto graphene upon laser irradiation.

Synthesis and characterization of polyaniline nanoparticles with enhanced microwave absorption

A series of polyaniline (PANI) materials have been prepared by a reverse dropping method with the assistance of polyvinylpyrrolidone (PVP). It can be found that PVP and the dropping rate play critical roles in determining the morphology evolution of PANI, and the restriction of PVP is highly dependent on the dropping rate. Well dispersed PANI nanoparticles can only be obtained under moderate conditions. Thanks to the unique preparative process, these samples show significant changes in the length and oxidation state of conjugated chains, as proved by the results of UV/vis absorption spectra and FT-IR spectra, which result in their distinguishable conductivity and microwave absorption. Very interestingly, PANI nanoparticles exhibit substantially enhanced microwave absorption properties. In particular the optimum one, PANI-NP2, presents very strong reflection loss (−40.5 dB at 5.8 GHz) and wide response bandwidth (3.2–18 GHz over −10 dB), which are indeed comparable to those composites of PANI with various magnetic particles, implying promising applications as a kind of light-weight and highly effective microwave absorber. By systematically investigating the electromagnetic parameters, it can be concluded that suitable complex permittivity, improved characteristic impedance and multiple relaxation processes in PANI nanoparticles should be responsible for their good microwave absorption.

In Situ Surface-Enhanced Raman Spectroscopy Study of Plasmon-Driven Catalytic Reactions of 4-Nitrothiophenol under a Controlled Atmosphere

We demonstrate the plasmon‐driven catalytic reactions of 4‐nitrothiophenol (4NTP) on a single Ag microsphere by an in situ surface‐enhanced Raman spectroscopy (SERS) technique. The highly SERS‐active hierarchical Ag structures served as an ideal platform to study plasmon‐driven catalytic reactions. This single‐particle surface‐enhanced Raman spectroscopy (SP‐SERS) technique coupled with inbuilt apparatus allow us to study the impact of reaction atmospheres and laser power on the rate of dimerization and reduction of 4NTP. Contrary to that found in previous studies, 4NTP could be transformed into 4‐aminothiophenol under H2O or H2 atmosphere. The broadening and splitting of the ν(CC) band during the reaction results from the frequency shift of the ν(CC) band that arises from different products. Our results suggest that the SP‐SERS technique is ideally suited to study plasmon‐driven catalytic reactions because of the possibility to monitor the reaction under controlled atmospheres in real time.

Precursor-directed synthesis of quasi-spherical barium ferrite particles with good dispersion and magnetic properties

The precursor-directed method has been firstly used for the preparation of M-type barium ferrite (BaFe12O19) particles in this article, where Fe3O4 microspheres were selected as the precursor. During the preparation process, it can be found that the volume ratio of water to ethanol plays an important role in determining the crystalline phase, and a high-purity magnetoplumbite structure can be achieved under the optimized conditions. Thanks to the morphology and dispersion of Fe3O4 microspheres, the obtained product presents a quasi-spherical shape, good dispersion, and relatively uniform particle size even after high-temperature heat treatment. Especially, the as-prepared barium ferrite exhibits excellent magnetic properties with both high intrinsic coercivity (5342 Oe) and large saturation magnetization (68.3 emu g−1), which are better than most results from some unconventional techniques. A series of Co–Ti substituted barium ferrite particles [Ba(CoTi)xFe12−2xO19, 0.25 ≤ x ≤ 1.0] are also prepared through the same technique by using Co–Ti substituted Fe3O4 microspheres as precursors, and they also display high-purity crystalline phase, quasi-spherical shape, and good dispersion, indicating that this route can be versatile for barium ferrite with different chemical compositions. However, with an increasing heteroatom content, the intrinsic coercivity and saturation magnetization drastically decrease because of the selective substitution of Fe3+ sites by Co2+ and Ti4+ in the magnetoplumbite structure.

Fast fabrication of homogeneous silver nanostructures on hydrazine treated polyaniline films for SERS applications

We here demonstrate a fast fabrication of homogeneous silver nanostructures on polyaniline films within one minute, which show highly sensitive response in SERS applications.

Improvement of stability of ZnO/CH3NH3PbI3 bilayer by aging step for preparing high-performance perovskite solar cells under ambient conditions

To date, most of fabricating processes of efficient perovskite solar cells (PSCs) have been carried out in high vacuum conditions or nitrogen protection atmosphere. Here we report a simple and novel strategy to fabricate efficient and stable ZnO-based PSCs. Firstly, ZnO nanoparticles were prepared by zinc acetate hydrolysis in methanol, and then the growth of methylamine lead iodide (CH3NH3PbI3) crystal layer was realized by a two-step method. With aging treatment of ZnO film, high-quality ZnO/CH3NH3PbI3 bilayer was obtained. Most importantly, the fabrication, measurement and store of solar cells in our processes were totally carried out under ambient conditions without a high vacuum or oxygen-free and water-free environment. A maximum efficiency of 9.4% was achieved. Moreover, the prepared devices exhibited promising stability at 30% humidity and 25 °C without any protection. We believe that the proposed strategy would push the applications of PSCs.

Chemical deposition of Ag nanostructures on polypyrrole films as active SERS substrates

A facile synthesis of homogeneous Ag nanostructures on modified polypyrrole (PPy) films through direct chemical deposition has been demonstrated. The as-prepared Ag nanostructures are highly sensitive and reproducible in the SERS detection of the target analyte, methylene blue (MB).

Gamma-irradiation induced direct fabrication of SERS-active Ag nanoparticles on glass substrates

We have demonstrated here a facile gamma-irradiation induced direct fabrication of Ag nanoparticles on glass substrates for SERS applications. It has been found that the agents complexing with the Ag+ ions play a dominant role in enabling Ag particle growth directly on the glass substrates, whereas using bare AgNO3 solution only produced Ag particles in the solution but not on the glass substrate. Moreover, the complexing agent also decides the size and morphology of the Ag nanoparticles, where using ammonia leads to much larger Ag particles than when using ethylenediamine. The γ-ray dose can also influence the size of the Ag nanoparticles, and a higher dose usually results in larger Ag nanoparticles. The SERS performances of the as-fabricated Ag nanoparticles supported on glass substrates have been compared. The uniform Ag nanoparticles with smaller sizes prepared by using ethylenediamine as the complexing agent typically present superior SERS sensitivities. We believe that this facile and cost-effective gamma-irradiation induced fabrication of Ag nanoparticles will be of interest in SERS studies.