Hua Chun Zeng

Synthetic architecture of interior space for inorganic nanostructures

One of the major technological challenges in nanoscience and nanotechnology is the self-assembly of tiny nano-building units (e.g., nanokits and nanoparts) into larger (i.e., mesoscale or microscale) organized conformations and geometrical architectures for device applications. To meet the requirements of new applications, an interior space for the nanostructures may be further required. When coupled with chemical functionality of boundary materials, the interior “nanospace” of the nanostructures possesses both aesthetic beauty and scientific attraction. For example, in addition to well studied core–shell nanostructures, there has been increasing research interest in the fabrication of hollow inorganic nanostructures owing to their potential applications in optical, electronic, magnetic, catalytic and sensing devices ranging from photonic crystals to drug-delivery carriers and nanoreactors. In this feature article, we report our recent research progress in this emerging field; our research is concentrated on the exploration of various novel organizing schemes through which interior spaces with architectural design can be created for inorganic nanostructures. These template-free “one-pot” synthetic methods include “oriented attachment”, Ostwald ripening and Kirkendall effect etc. for direct solid evacuation under mild reaction conditions. Future research directions will also be addressed in this article.

Ostwald Ripening: A Synthetic Approach for Hollow Nanomaterials

Fabrication of nanomaterials with hollow interiors is an important research area in nanoresearch, owing to their potential applications in photonic devices, drug delivery, material encapsulation, ionic intercalation, surface functionalization, nanocatalysts, membrane nanoreactors, and many other technologies. The common preparative methods for this new class of materials can be broadly divided into hard and soft template-assisted syntheses. In recent years, furthermore, the interest in template-free techniques for these materials has also increased, as the new processes involved in these techniques are relatively simple and less demanding, compared to the template-assisted processes. In this short review, we will introduce the application of a well-known physical phenomenon of crystal growth – Ostwald ripening – in the fabrication of hollow nanomaterials. It has been demonstrated that formation of the interior spaces of nanostructures depends on the aggregative states of the primary crystallites during the synthesis. With this new development, many inorganic nanomaterials with interior spaces can now be fabricated in solution media together with the materials synthesis. Different types of Ostwald ripening observed in this synthetic approach have been reviewed. In particular, various geometric structures and configurations prepared with these methods have been discussed. The prepared hollow materials also allow further compositional and structural modifications under the similar process conditions. Future directions in this research area are also discussed.

Synthesis, morphological control, and antibacterial properties of hollow/solid Ag2S/Ag heterodimers.

Ag(2)S and Ag are important functional materials that have received considerable research interest in recent years. In this work, we develop a solution-based synthetic method to combine these two materials into hollow/solid Ag(2)S/Ag heterodimers at room temperature. Starting from monodisperse Cu(2)O solid spheres, CuS hollow spheres can be converted from Cu(2)O through a modified Kirkendall process, and the obtained CuS can then be used as a solid precursor for preparation of the Ag(2)S/Ag heterodimers through ion exchange and photo-assisted reduction. We have found that formation of the Ag(2)S/Ag heterodimers is instantaneous, and the size of Ag nanocrystals on the hollow spheres of Ag(2)S can be controlled by changing the concentration and power of reducing agents in the synthesis. The growth of Ag nanoparticles on hollow spheres of Ag(2)S in the dimers is along the [111] direction of the silver crystal; the light absorption properties have also been investigated. Furthermore, coupling or tripling of Ag(2)S/Ag heterodimers into dumbbell-like trimers ((Ag(2)S)(2)/Ag, linear) and triangular tetramers ((Ag(2)S)(3)/Ag, coplanar) can also be attained at 60 degrees C by adding the bidentate ligand ethylenediamine as a cross-linking agent. To test the applicability of this highly asymmetric dipolar composite, photocatalytic inactivation of Escherichia coli K-12 in the presence of the as-prepared Ag(2)S/Ag heterodimers has been carried out under UV irradiation. The added Ag(2)S/Ag heterodimers show good chemical stability under prolonged UV irradiation, and no appreciable solid dissolution is found. Possible mechanisms regarding the enhanced antibacterial activity have also been addressed.

Synthesis of complex nanomaterials via Ostwald ripening

In recent years, nanostructured materials with interior cavities and surface porosity have received extensive interest in the materials research community, owing to their many important applications in emerging technologies. Among many synthetic strategies, Ostwald ripening has become a direct matter-relocation approach to create interior spaces for this class of new functional materials. In this short article, we review the general processes of Ostwald ripening in various types of crystalline and noncrystalline materials, as well as stoichiometric and nonstoichiometric solid precursors. Because Ostwald ripening can be operated in a solution environment, this approach provides a simple hollowing means to prepare complex nanostructured materials, such as core–shell, yolk-shell, and multi-shelled solids, when coupled with additional chemical reactions in synthesis. Furthermore, it is possible to perform multiple rounds of Ostwald ripening when fresh solid precursors are deposited into the internal void space or onto the external surface of pre-existing intermediate hollow structures. In addition to the structural and geometrical architectures, compositional tailoring can also be achieved by preparing the solid precursors into multicomponent composites and/or by post-ripening modification and functionalisation. Future research directions and possible improvements of this approach are also addressed.

Synthesis and self-assembly of complex hollow materials

Hollow materials with interiors or voids and pores are a class of lightweight nanostructured matters that promise many future technological applications, and they have received significant research attention in recent years. On the basis of well-known physicochemical phenomena and principles, for example, several solution-based protocols have been developed for the general preparation of these complex materials under mild reaction conditions. This article is thus a short introductory review on the synthetic aspects of this field of development. The synthetic methodologies can be broadly divided into three major categories: (i) template-assisted synthesis, (ii) self-assembly with primary building blocks, and (iii) induced matter relocations. In most cases, both synthesis and self-assembly are involved in the above processes. Further combinations of these methodologies appear to be very important, as they will allow one to prepare functional materials at a higher level of complexity and precision. The synthetic strategies are introduced through some simple case studies with schematic illustrations. Salient features of the methods developed have been summarized, and some urgent issues of this field have also been indicated.

Highly Monodisperse MIII-Based soc-MOFs (M = In and Ga) with Cubic and Truncated Cubic Morphologies

In this work, we carry out an investigation on shape-controlled growth of In(III)- and Ga(III)-based square-octahedral metal-organic frameworks (soc-MOFs). In particular, controllable crystal morphological evolution from simple cubes to complex octadecahedra has been achieved, and resultant highly uniform crystal building blocks promise new research opportunities for preparation of self-assembled MOF materials and related applications.

TiO2 Thin Films Prepared via Adsorptive Self-Assembly for Self-Cleaning Applications

Low-cost controllable solution-based processes for preparation of titanium oxide (TiO(2)) thin films are highly desirable, because of many important applications of this oxide in catalytic decomposition of volatile organic compounds, advanced oxidation processes for wastewater and bactericidal treatments, self-cleaning window glass for green intelligent buildings, dye-sensitized solar cells, solid-state semiconductor metal-oxide solar cells, self-cleaning glass for photovoltaic devices, and general heterogeneous photocatalysis for fine chemicals etc. In this work, we develop a solution-based adsorptive self-assembly approach to fabricate anatase TiO(2) thin films on different glass substrates such as simple plane glass and patterned glass at variable compositions (normal soda lime glass or solar-grade borofloat glass). By tuning the number of process cycles (i.e., adsorption-then-heating) of TiO(2) colloidal suspension, we could facilely prepare large-area TiO(2) films at a desired thickness and with uniform crystallite morphology. Moreover, our as-prepared nanostructured TiO(2) thin films on glass substrates do not cause deterioration in optical transmission of glass; instead, they improve optical performance of commercial solar cells over a wide range of incident angles of light. Our as-prepared anatase TiO(2) thin films also display superhydrophilicity and excellent photocatalytic activity for self-cleaning application. For example, our investigation of photocatalytic degradation of methyl orange indicates that these thin films are indeed highly effective, in comparison to other commercial TiO(2) thin films under identical testing conditions.

Catalytic decomposition of nitrous oxide on alumina-supported ruthenium catalysts Ru/Al2O3

Abstract The catalytic decomposition of N 2 O (28 mol%) has been studied for the Ru/Al 2 O 3 system with various ruthenium contents (Ru wt% = 0.00 – 0.26). The optimized catalyst of the studied system has a high catalytic activity of 4.8 × 10 4 μmol (N 2 O) g −1 h −1 400°C. The study shows that the surface area of Ru/Al 2 O 3 decreases with increase of loading of the metal. Two different types of surface structure are formed, and the optimized catalyst is found in the intermediate region between them. The catalyst has been investigated further with various background gases (O 2 , CO 2 , and H 2 O). It is found that O 2 has little inhibitive effect on the high conversion rate reaction while it shows a certain effect at low conversion (low temperature). Acidity-basicity of the catalyst has been examined with the probe gas CO 2 . In contrast to CO 2 , which does not adsorb on the catalyst, water shows a strong tendency to hamper the decomposition. The reaction on the optimized catalyst can be described as first order with respect to N 2 O partial pressure. Using the findings of this work, a two-step process has been proposed for the adipic acid off-gas treatment.

Preparation of a Ru‐Nanoparticles/Defective‐Graphene Composite as a Highly Efficient Arene‐Hydrogenation Catalyst

The catalytic hydrogenation of aromatic compounds is of particular importance in the refining industry. Common heterogeneous hydrogenation catalysts are supported noble-metal (Ru, Pt, Pd, Rh, etc.) nanoparticles (NPs). Although many interesting results have been reported with different catalyst systems, such as the chemoselective hydrogenation of nitrobenzene over TiO2or Fe2O3-supported Au catalysts, [2] the partial hydrogenation of dinitrobenzene into nitroanilines with Ru/C, and the selective hydrogenation of p-phenylphenol into p-cyclohexylphenol with Pd/C, only a limited number of catalysts have achieved the hydrogenation of arenes with complete conversion and exclusive selectivity. In most of these supported-metal-NP catalysts, the support materials are catalytically inert ; they only provide a large surface area for the active NPs and keep them well-separated for reuse. In contrast, some support materials have shown synergetic effects with the metal NPs and effectively enhance/modulate their activities or selectivities. Amorphous active carbon materials, which are usually considered to be inert, are widely used as support materials for various catalysts, owing to their large surface area, excellent stabilities, ready availability, and low cost. On the other hand, as the newest allotrope in the carbon materials family, graphene has been extensively investigated since it was first reported in 2004. Although the majority of these studies have focused on its applications in physics, it is increasingly appreciated that it has great potential as a support material for catalysis because its tunable electronic structure may significantly affect the activities of the supported catalysts. We recently demonstrated by using first-principle calculations that the use of defective graphene as a support would enhance the stability of Ru13 NPs, as well as prompt the adsorption of benzene and hydrogen molecules; moreover, we predicted that a Ru/defective-graphene composite would be an excellent catalyst for the hydrogenation of arenes. Herein, we report a facile method that enables the in situ production of fine (d 3 nm) Ru NPs on graphene oxide. With the formation of Ru NPs, graphene oxide was simultaneously reduced and Ru NPs were firmly anchored onto the reduced graphene oxide (rGO) through interactions with its dangling surface bonds. When used for arene (toluene and benzene)-hydrogenation reactions, the obtained Ru/rGO composite exhibited high catalytic activities and also good stability with little leaching or aggregation of the Ru NPs during multiple reaction runs. In comparison with amorphous carbon materials, rGO showed a remarkable promoting ability for Ru NPs, thus giving much-higher turnover frequencies (TOFs). These experimental results verified our theoretical expectations. With the help of high-resolution TEM, Gomez-Navarro et al. identified the presence of various types of defects in rGO, including clustered pentagons and heptagons, vacancies, edges, and contaminations, among which “single vacancies” and “di-vacancies” had the greatest potential for trapping metal atoms. Therefore, for simplicity, our calculations used “single vacancies” with sp dangling bonds to represent the defects in rGO. Our recent work indicated that Ru NPs could be stabilized by a defective graphene substrate, owing to the hybridization between the dsp states of Ru NPs and the sp dangling bonds at the graphene defective sites. The calculated binding energy (Eb) of a Ru13 cluster on single-vacancy graphene (SVG) is as high as 7.41 eV. By comparison, the Eb of a Ru13 cluster on primitive graphene (PG) is only 2.55 eV. It has also been pointed out that locating Ru atoms on graphene defects is highly thermodynamically favorable and that the local curvature that is formed at the metal/graphene interface raises the diffusion barriers for Ru atoms, which prohibit the particles from sintering. These results imply the importance of defective sites for anchoring Ru NPs onto graphene, the high stability of such a composite catalyst, as well as the difficulty of depositing Ru NPs onto PG. Microporous active carbon materials and mesoporous carbon materials are essentially amorphous materials that consist of both spand sp-hybridized carbon atoms with ill-defined atomic structures. Owing to their large surface areas and abundant structural defects, amorphous carbon (AC) materials are commonly used to support metal NPs for catalysis. To provide greater insight into the impact of the substrate on the activity of the catalysts, first-principles-based calculations were performed to investigate the electronic structures of the Ru/ SVG and Ru/AC composite materials. For Ru/SVG, as revealed by density-of-states (DOS) analysis (Figure 1 a), Ru-sp states and Ru-d states were overlapped over a large energy range, from 6.1 eV to the Fermi level (EF), thereby showing the hybridization of these states. The peaks of Ru at about 6.0 eV [a] Dr. K. X. Yao, Dr. X. Liu, Prof. Dr. Y. Han Advanced Membranes and Porous Materials Center Chemical and Life Sciences and Engineering Division King Abdullah University of Science and Technology Thuwal 23955-6900 (Saudi Arabia) E-mail : yu.han@kaust.edu.sa [b] Dr. X. Liu School of Chemistry Dalian University of Technology Dalian 116024 (China) [c] Z. Li, C. C. Li, Prof. Dr. H. C. Zeng Department of Chemical and Biomolecular Engineering Faculty of Engineering National University of Singapore 10 Kent Ridge Crescent, 119260 (Singapore) [] These authors contributed equally to this work. Supporting information for this article, including the methods for firstprinciples calculations, is available on the WWW under http://dx.doi.org/ 10.1002/cctc.201200354.

Calcium Carbonate Nanotablets: Bridging Artificial to Natural Nacre

Single-crystalline CaCO(3) nanotablets are synthesized in large quantities through oriented attachment of pristine nanoparticles. The prepared nanotablets can serve as genuine building blocks for the construction of nacreous inorganic-organic hybrids, through which freestanding films and monoliths with tunable composition and mechanical properties are fabricated. These newly available CaCO(3) crystal tablets may also serve as a starting platform for future CaCO(3) research.