scispace - formally typeset
Search or ask a question
Author

Yan Lu

Bio: Yan Lu is an academic researcher from Helmholtz-Zentrum Berlin. The author has contributed to research in topics: Nanoparticle & Polyelectrolyte. The author has an hindex of 48, co-authored 166 publications receiving 10653 citations. Previous affiliations of Yan Lu include Dresden University of Technology & Humboldt University of Berlin.


Papers
More filters
Journal ArticleDOI
Stefanie Wunder1, Frank Polzer1, Yan Lu1, Yu Mei1, Matthias Ballauff1 
TL;DR: In this article, a study on the catalytic reduction of 4-nitrophenol by sodium borohydride in the presence of metal nanoparticles is presented, where the nanoparticles are embedded in spherical polyelectrolyte brushes.
Abstract: We present a study on the catalytic reduction of 4-nitrophenol by sodium borohydride in the presence of metal nanoparticles. The nanoparticles are embedded in spherical polyelectrolyte brushes, which consist of a polystyrene core onto which a dense layer of cationic polyelectrolyte brushes are grafted. The average size of the nanoparticles is approximately 2 nm. The kinetic data obtained by monitoring the reduction of 4-nitrophenol by UV/vis-spectroscopy could be explained in terms of the Langmuir−Hinshelwood model: The borohydride ions transfer a surface-hydrogen species in a reversible manner to the surface. Concomitantly 4-nitrophenol is adsorbed and the rate-determining step consists of the reduction of nitrophenol by the surface-hydrogen species. The apparent reaction rate can therefore be related to the total surface S of the nanoparticles, to the kinetic constant k related to the rate-determining step, and to the adsorption constants KNip and KBH4 of nitrophenol and of borohydride, respectively. In...

1,047 citations

Journal ArticleDOI
TL;DR: This tutorial review a subset of well-studied reactions that take place in aqueous phase and for which a comprehensive kinetic analysis is available, namely the reduction of p-nitrophenol and hexacyanoferrate, both by borohydride ions.
Abstract: Catalysis by metallic nanoparticles is certainly among the most intensely studied problems in modern nanoscience. However, reliable tests for catalytic performance of such nanoparticles are often poorly defined, which makes comparison and benchmarking rather difficult. We tackle in this tutorial review a subset of well-studied reactions that take place in aqueous phase and for which a comprehensive kinetic analysis is available. Two of these catalytic model reactions are under consideration here, namely the reduction of (i) p-nitrophenol and (ii) hexacyanoferrate (III), both by borohydride ions. Both reactions take place at the surface of noble metal nanoparticles at room temperature and can be accurately monitored by UV-vis spectroscopy. Moreover, the total surface area of the nanoparticles in solution can be known with high precision and thus can be directly used for the kinetic analysis. Hence, these model reactions represent cases of heterogeneous catalysis that can be modelled with the accuracy typically available for homogeneous catalysis. Both model reactions allow us to discuss a number of important concepts and questions, namely the dependence of catalytic activity on the size of the nanoparticles, electrochemistry of nanoparticles, surface restructuring, the use of carrier systems and the role of diffusion control.

935 citations

Journal ArticleDOI
TL;DR: It is demonstrated that thermosensitive core–shell networks may indeed be used as such a nanoreactor by modulating the activity of nanoparticles through a thermodynamic transition that takes place within the carrier system.
Abstract: Metal nanoparticles have properties that are significantly different from the bulk properties of the metals. Moreover, their high surface-to-volume ratio renders them ideal candidates for application as catalysts. However, the pronounced tendency of nanoparticles to aggregate must be overcome by using suitable carrier systems. Recently, a number of systems have been discussed that are suitable for applications in aqueous environments. These include polymers, dendrimers, microgels, 18] and other colloidal systems. 20] In all the cases studied so far, these carrier systems only provide a suitable support for the nanoparticles and prevent them from aggregating. In this way the carrier system of, for example, dendrimers or microgels acts much in the same way as a “nanoreactor” that immobilizes the particles and leads to their more convenient handling. Here we report on the first system that allows us to modulate the activity of nanoparticles through a thermodynamic transition that takes place within the carrier system. Figure 1 displays the principle. Metallic nanoparticles are embedded in a polymeric network attached to a colloidal core particle. In all the cases discussed here the core consists of poly(styrene) (PS) while the network consists of poly(Nisopropylacrylamide) (PNIPA) cross-linked with N,N’-methylenebisacrylamide (BIS). The particles are suspended in water, which swells the PNIPA at room temperature. The PNIPA network, however, undergoes a phase transition around 30 8C, during which most of the water is expelled. Previous experiments have demonstrated that this transition is perfectly reversible and the process of shrinking and reswelling can be repeated without degradation or coagulation of the particles. Metallic nanoparticles embedded in such a network are fully accessible to reactants at low temperature. Above the phase transition, however, the marked shrinkage of the network should be followed by a concomitant slowing down of the diffusion of the reactants within the network. The rate of reactions catalyzed by the nanoparticles should thus be slowed down considerably. In this way, the network could act as a “nanoreactor” that can be opened or closed to a certain extent. Herein we demonstrate that thermosensitive core–shell networks may indeed be used as such a nanoreactor. The activity of the catalyst can be modulated by temperature over a wide range. As the model reaction we chose the reduction of 4-nitrophenol to 4-aminophenol by sodium borohydride. The reaction was repeatedly performed to check the catalytic activity of the metal nanoparticles, and the results obtained in the present study can be directly compared to literature data. The carrier particles having a PS core and a PNIPA shell were prepared as described recently. 24] Figure 2 shows a schematic representation of the silver nanoparticles being

699 citations

Journal ArticleDOI
Yu Mei1, Yan Lu1, Frank Polzer1, Matthias Ballauff1, Markus Drechsler1 
TL;DR: In this paper, a quantitative comparison of the catalytic activity of palladium nanoparticles immobilized in different colloidal carrier systems, namely, in spherical polyelectrolyte brushes (SPB) and core−shell microgels, is presented.
Abstract: We present a quantitative comparison of the catalytic activity of palladium nanoparticles immobilized in different colloidal carrier systems, namely, in (i) spherical polyelectrolyte brushes (SPB) and (ii) core−shell microgels. The first system given by the SPB carrier particles consist of a solid core of polystyrene onto which long chains of poly((2-methylpropenoyloxyethyl) trimethylammonium chloride) (PMPTAC) are grafted. These positively charged polyelectrolyte chains form a dense layer on the surface of the core particles which binds the divalent PdCl42- ions. Reduction leads to metallic Pd particles. System 2 is given by core−shell microgels which consists of a solid core of polystyrene and a shell of cross-linked poly(N-isopropylacrylamide) (PNIPA). The metal ions were strongly localized within the network because of complexation of the PdCl42- ions and the nitrogen atoms of PNIPA. Reduction of these ions leads to nearly monodisperse nanoparticles of metallic palladium that are only formed within th...

661 citations

Journal ArticleDOI
TL;DR: In this paper, the catalytic activity of gold nanoparticles in aqueous solution as a function of temperature was analyzed using the Langmuir-Hinshelwood model.
Abstract: We present the analysis of the catalytic activity of gold nanoparticles in aqueous solution as a function of temperature. As a model reaction, the reduction of p-nitrophenol (Nip) by sodium borohydride (BH4–) is used. The gold nanoparticles are immobilized on cationic spherical polyelectrolyte brushes that ensure their stability against aggregation. High-resolution transmission electron microscopy shows that the Au nanoparticles are faceted nanocrystals. The average size of the nanoparticles is 2.2 nm, and the total surface area of all nanoparticles could be determined precisely and was used in the subsequent kinetic analysis. Kinetic data have been obtained between 10 and 30 °C by monitoring the concentrations of Nip and BH4– by UV–vis spectroscopy. The reaction starts after an induction time t0, and the subsequent stationary phase yields the apparent reaction rate, kapp. All kinetic data could be modeled in terms of the Langmuir–Hinshelwood model; that is, both reactants must be adsorbed onto the surfac...

492 citations


Cited by
More filters
Journal ArticleDOI
TL;DR: This work reviews recent advances and challenges in the developments towards applications of stimuli-responsive polymeric materials that are self-assembled from nanostructured building blocks and provides a critical outline of emerging developments.
Abstract: Responsive polymer materials can adapt to surrounding environments, regulate transport of ions and molecules, change wettability and adhesion of different species on external stimuli, or convert chemical and biochemical signals into optical, electrical, thermal and mechanical signals, and vice versa. These materials are playing an increasingly important part in a diverse range of applications, such as drug delivery, diagnostics, tissue engineering and 'smart' optical systems, as well as biosensors, microelectromechanical systems, coatings and textiles. We review recent advances and challenges in the developments towards applications of stimuli-responsive polymeric materials that are self-assembled from nanostructured building blocks. We also provide a critical outline of emerging developments.

4,908 citations

01 Dec 1991
TL;DR: In this article, self-assembly is defined as the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent bonds.
Abstract: Molecular self-assembly is the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent bonds. Molecular self-assembly is ubiquitous in biological systems and underlies the formation of a wide variety of complex biological structures. Understanding self-assembly and the associated noncovalent interactions that connect complementary interacting molecular surfaces in biological aggregates is a central concern in structural biochemistry. Self-assembly is also emerging as a new strategy in chemical synthesis, with the potential of generating nonbiological structures with dimensions of 1 to 10(2) nanometers (with molecular weights of 10(4) to 10(10) daltons). Structures in the upper part of this range of sizes are presently inaccessible through chemical synthesis, and the ability to prepare them would open a route to structures comparable in size (and perhaps complementary in function) to those that can be prepared by microlithography and other techniques of microfabrication.

2,591 citations