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The redox reaction between KMnO4 and Co(NO3)2 was designed and readily utilized for the scalable integration of spinel MnxCo3−xO4 nanosheet arrays in three-dimensional ceramic honeycombs by controlling the reaction temperature. The Co2+ can reduce MnO4− to form Mn–Co spinel oxide nanosheet arrays uniformly on the channel surface of cordierite honeycomb. The novel platinum group metal free oxide nanosheet array integrated ceramic honeycomb monolith shows good low-temperature catalytic activity for propane oxidation, with the 50 % conversion temperature achieved at 310 °C, which is a much lower temperature than that over the wash-coated commercial Pt/Al2O3. These integrated Mn–Co composite oxide nanoarrays may hold great promise for the construction of advanced monolithic catalyst for high-performance and low-cost emission control.

Scalable and cost-effective synthesis and assembly of technologically important nanostructures in three-dimensional (3D) substrates hold keys to bridge the demonstrated nanotechnologies in academia with industrially relevant scalable manufacturing. In this work, using ZnO nanorod arrays as an example, a hydrothermal-based continuous flow synthesis (CFS) method is successfully used to integrate the nano-arrays in multi-channeled monolithic cordierite. Compared to the batch process, CFS enhances the average growth rate of nano-arrays by 125%, with the average length increasing from 2 μm to 4.5 μm within the same growth time of 4 hours. The precursor utilization efficiency of CFS is enhanced by 9 times compared to that of batch process by preserving the majority of precursors in recyclable solution. Computational fluid dynamic simulation suggests a steady-state solution flow and mass transport inside the channels of honeycomb substrates, giving rise to steady and consecutive growth of ZnO nano-arrays with an average length of 10 μm in 12 h. The monolithic ZnO nano-array-integrated cordierite obtained through CFS shows enhanced low-temperature (200 °C) desulfurization capacity and recyclability in comparison to ZnO powder wash-coated cordierite. This can be attributed to exposed ZnO {100} planes, better dispersion and stronger interactions between sorbent and reactant in the ZnO nanorod arrays, as well as the sintering-resistance of nano-array configurations during sulfidation–regeneration cycles. With the demonstrated scalable synthesis and desulfurization performance of ZnO nano-arrays, a promising, industrially relevant integration strategy is provided to fabricate metal oxide nano-array-based monolithic devices for various environmental and energy applications.

Monolithic catalysts have been widely used in automotive, chemical, and energy relevant industries. Nanoarray-based monolithic catalysts have been developed, demonstrating high catalyst utilization efficiency and good thermal/mechanical robustness. Compared with the conventional washcoat-based monolithic catalysts, they have shown advances in precise and optimum microstructure control and feasibility in correlating materials structure with properties. Recently, the nanoarray-based monolithic catalysts have been studied for low temperature oxidation of automotive engine exhaust and exhibited interesting and promising catalytic activities. This review focuses on discussing the key structural parameters of nanoarray catalyst that affect the catalytic performance from the following aspects: (1) geometric shape and crystal planes, (2) guest atom doping and defects, (3) array size and size-assisted active species loading, and (4) the synergy effect of metal oxide in composite nanoarrays. Prior to the discussion, an overview of the current status of synthesis and development of the nanoarray-based monolithic catalysts is introduced. The performance of these materials in low temperature simulated engine exhaust oxidation is also demonstrated. We hope this review will elucidate the science and chemistry behind the good oxidation performance of the nanoarray-based monolithic catalysts and serve as a timely and useful research guide for rational design and further improvement of the nanoarray-based monolithic catalysts for automobile emission control.

Nanostructured cerium oxide (CeO2) with outstanding physical and chemical properties has attracted extensive interests over the past few decades in environment and energy-related applications. With controllable synthesis of nanostructured CeO2, much more features were technologically brought out from defect chemistry to structure-derived effects. This review highlights recent progress on the synthesis and characterization of nanostructured ceria-based materials as well as the traditional and new applications. Specifically, several typical applications based on the desired ceria nanostructures are focused to showcase the importance of nanostructure-derived effects. Moreover, some challenges and perspectives on the nanostructured ceria are presented, such as defects controlling and retainment, scale-up fabrication, and monolithic devices. Hopefully, this review can provide an improved understanding of nanostructured CeO2 and offer new opportunities to promote the further research and applications in the future.

In recent years, engineered nanostructure assemblies such as nanowire arrays have attracted much research attention due to their unique chemical and functional characteristics collectively. The engineered nano-assemblies usually carry the characteristics distinct from bulk as a result of a size effect in their comprised elemental building blocks. The nanoscale size induced high surface-to-volume ratio is a fundamental attribute responsible for various chemical and physical properties required in various technologically important applications such as catalysts and sensors. This review article surveys the latest progress in engineered metal oxide nanostructure arrays, i.e., nanoarrays, for advanced chemical sensors' design and application. It starts with an overview of gaseous chemical sensors followed by surveys of various fabrication methods and routes for metal oxide nanoarrays. Different sensing modes and corresponding applications have been highlighted in the mixed gaseous chemical sensing, which provides new approaches and perspectives to meet the challenges of selective gas sensing, such as the cross-sensitivity and inter-correlation of multiple sensing signals.

The scalable three-dimensional (3-D) integration of functional nanostructures into applicable platforms represents a promising technology for meeting the ever-increasing demands of fabricating high performance devices featuring cost-effectiveness, structural sophistication and multi-functional enabling. Such an integration process generally involves a diverse array of nanostructural entities (nano-entities) consisting of dissimilar nanoscale building blocks such as nanoparticles, nanowires, and nanofilms made of metals, ceramics, or polymers. Various synthetic strategies and integration methods have enabled the successful assembly of both structurally and functionally tailored nano-arrays into a unique class of monolithic devices. The performance of nano-array based monolithic devices is dictated by a few important factors such as materials substrate selection, nanostructure composition and nano-architecture geometry. Therefore, the rational material selection and nano-entity manipulation during the nano-array integration process, aiming to exploit the advantageous characteristics of nanostructures and their ensembles, are critical steps towards bridging the design of nanostructure integrated monolithic devices with various practical applications. In this article, we highlight the latest research progress of the two-dimensional (2-D) and 3-D metal and metal oxide-based nanostructural integrations into prototype devices applicable with ultrahigh efficiency, good robustness and improved functionality. Select examples of nano-array integration, scalable nanomanufacturing and representative monolithic devices such as catalytic converters, sensors and batteries will be utilized as the connecting dots to display a roadmap from hierarchical nanostructural assembly to practical nanotechnology implications ranging from energy, environmental, to chemical and biotechnology areas.

High-performance photodetectors are costly due to either the expensive nature of active materials or sophistication in the fabrication process required to meet performance targets. In this work, low-cost metal stannate based nanostructures are demonstrated as active materials for high-performance UV photodetectors. A robust, inexpensive, and scalable drop-casting process was successfully developed to fabricate thin film devices composed of amorphous ZnSnO3 nanocubes and/or polycrystalline Zn2SnO4–SnO2 nanoparticles with different optical and electronic structures. Moreover, the Zn2SnO4–SnO2 heterojunction nanoparticles were directly obtained by thermal treatment of amorphous ZnSnO3 nanocubes. Large-area, uniform, and continuous film photodetectors were achieved with pronounced and fast response upon exposure to UV illumination (λ 102 and ∼103 was achieved for ZnSnO3 and Zn2SnO4–SnO2 photodetectors, respectively) and excellent responsivity (as high as 0.5 A W−1 at 5.0 V bias from the Zn2SnO4–SnO2 photosensors) under low light intensity. This work provides a simple, cost-effective and tunable processing strategy to synthesize and apply earth-abundant metal stannate based nanomaterials for high performance UV photodetectors and other optoelectronic devices.

Monolithic catalysts, also known as structured catalysts, represent an important catalyst configuration widely used in automotive, chemical, and energy industries. However, several issues associated with washcoat based monolithic catalyst preparation are ever present, such as compromised materials utilization efficiency due to a less-than-ideal wash coating process, difficulty in precise and optimum microstructure control and lack of structure–property correlation. In this mini-review, we introduce the concept of nano-array catalyst, a new type of monolithic catalyst featuring high catalyst utilization efficiency, good thermal/mechanical robustness, and catalytic performance tunability. A comprehensive overview is presented with detailed discussion of the strategies for nano-array catalyst preparation and rational catalytic activity adjustment enabled by the well-defined nano-array geometry. Specifically their scalable fabrication processes are reviewed in conjunction with discussion of their various catalytic oxidation reaction performances at low temperature. We hope this review will serve as a timely and useful research guide for rational design and utilization of the new type of monolithic catalysts.

Heterojunctions are an important strategy for designing high performance electrical sensor materials and related devices. Herein, a new type of metal-semiconductor hybrid nanoparticle has been successfully used to remarkably sensitize the surface of ZnO nanowires for detecting NO2 with high responses over a broad temperature window ranging from room temperature to 600 °C. These hybrid nanoparticles are comprised of iron oxide nanowires with well dispersed single crystalline Au nanoparticles. The hybrid nanoparticle decorated ZnO nanowires have achieved a giant response, as high as 74 500 toward NO2 gas, about 42 times that of Au decorated ZnO nanowire sensors. This dramatic enhancement may be attributed to the efficient charge transfer across the Au-Fe2O3 Schottky and Fe2O3-ZnO semiconductor heterojunction interfaces. Due to the incorporation of thermally-stable Fe2O3 nanoparticles as the support of Au nanoparticles, the working temperature of nanowire sensors was successfully extended to higher temperatures, with an increase of 200 °C, from 400 °C to 600 °C. Such a combination of semiconductor heterojunction and semiconductor-metal Schottky contact presents a new strategy for designing high performance electrical sensors with high sensitivity, stability, selectivity, and wide operation temperature window, which are potentially suitable for advanced energy systems such as automotive engines and power plants.

Three-dimensional (3-D) assemblies of nanomaterials into applicable platforms or devices represent the need for meeting the practical requirements in cost-effectiveness, structure sophistication, multi-function enabling, and efficient operations. Such an integration process involves a diverse array of dissimilar building-blocks such as nanoparticles, nanowires, and nanofilms made of metals, ceramics, or polymers. In this talk, I will review our decade-long research effort in the assemblies and integrations of various ceramics and metals in the forms of nanostructures or nanostructure arrays (nanoarrays) onto different macroscale surfaces as function-carrying ensembles. Departing from catalytic converters, sensors, water electrolysis, to CO2-methanol conversion, a unique nanomaterials roadmap is being drawn with function-enabling, manufacturing capability, system integratability, and economic affordability as the underlying bases. Modular, multifunctional, and scalable nanoarray devices and systems are being designed and enabled for conversion, monitoring, production, and redistribution of fuels and chemicals such as hydrogen and methanol.

Multi-analyte chemical sensor aims to transform subtle variations in multiple analytes’ physical or chemical properties into distinct output signals. Chemically responsive nanostructure array (nanoarray) promises as a competitive sensor platform due to its robust physical properties, tunable chemical composition, and high surface area for analyte interaction. Specifically, the well-defined size, shape, and tunable surface structure and properties make it feasible to develop either new sensing modes on single device or integrated multi-modular sensors. In conjunction with the well-developed resistor-type sensors and sensor arrays, the complementary utilization of and intercorrelation with the electrochemical, optical, voltammetry modes in the multi-modular sensing strategies could provide multi-dimensional measurements to different analytes in a complex mixture form, where species information could be accurately and robustly separated from spatially collective responses. This review intends to provide a survey of the recent progress on multi-analyte sensing strategies and their unique structure design, as well as the related sensing mechanics in interaction of analytes and sensitizer and the behind mechanism for analytes’ differentiation.

Janus heteronanostructures (HNs), as an important class of anisotropic nanomaterials, could facilitate synergistic coupling of diverse functions inherited by their comprised nanocomponents. Nowadays, synthesizing deterministically targeted Janus HNs remains a challenge. Here, a general yet scalable technique is utilized to fabricate an array of programmable Janus HNs based on anodic aluminum oxide binary-pore templates. By designing and employing an overetching process to partially expose four-edges of one set of nanocomponents in a binary-pore template, selective deposition and interfacing of the other set of nanocomponents is successfully achieved along the exposed four-edges to form a densely packed array of Janus HNs on a large scale. In combination with an upgraded two-step anodization, the synthesis provides high degrees of freedom for both nanocomponents of the Janus HNs, including morphologies, compositions, dimensions, and interfacial junctions. Arrays of TiO2–Au and TiO2/Pt NPs–Au Janus HNs are ...

The design and discovery of green and highly efficient visible-light driven catalysts hold significant importance towards efficient harvesting, conversion and utilization of the full-spectrum solar energy. Here in this work, we report several wide band gap (>3.1 eV) semiconducting nanostructures counterintuitively showing excellent catalytic activity under solar light towards organic dye degradation. These nanostructured stannates include amorphous ZnSnO3 nanocubes and Zn2SnO4–SnO2 nanocrystal assemblies, which possess the merits of high activity, low cost, absence of toxicity, and ease of synthesis. Hydroxyl radicals (˙OH) are confirmed to be the major active species responsible for the dye degradation reactions. The catalysis tests under monochromatic light and optical characterization revealed remarkable activities in the visible light range due to populated defect states in these semiconductor nanostructures. Finally, the Zn2SnO4–SnO2 hetero-junction nanocrystal assemblies produced by slow-ramping thermal decomposition demonstrated very high photocatalytic efficiency under visible light, with dye molecules almost fully degraded in 20 min. Besides more visible-light-active defect states and larger crystallite size, the coherent (hetero-epitaxial) interfaces and strong type II heterojunction interaction between the spinel Zn2SnO4 and rutile SnO2 nano-grains might be the main reasons for the drastically improved photocatalytic performance of the slow-ramp Zn2SnO4–SnO2.

Constructed with parallel or honeycomb channels micrometer to millimeter in diameter, monolithic catalysts and reactors have been utilized in various business sectors ranging from mechanical, automotive, fine chemicals, pharmaceutical, to biotechnology industries. However, the performance of state-of-the-art washcoating monolithic catalysts in industry suffers from some long-standing problems such as mediocre-uniformity, need for high catalyst loading, low materials utilization, random catalytic sites, poor washcoat adhesion, short life-time, degradation tendency, all of which lead to much compromised and less-than desirable catalytic performance. In this work, we significantly mitigated these issues and reduced precious metal and metal oxide usages by 10–40 folds, by directly integrating bare monolith structures with ultra-efficient, thermally stable, and physically and chemically well-defined nanostructure arrays. The well-defined nanostructure array monolithic catalysts represent a new and effective model platform for bridging catalytic nanomaterials science and engineering with the practical industrial catalysis.

Noble metal nanoparticles are extensively used for sensitizing metal oxide chemical sensors through the catalytic spillover mechanism. However, due to earth-scarcity and high cost of noble metals, finding replacements presents a great economic benefit. Besides, high temperature and harsh environment sensor applications demand material stability under conditions approaching thermal and chemical stability limits of noble metals. In this study, we employed thermally stable perovskite-type La(0.8)Sr(0.2)FeO3 (LSFO) nanoparticle surface decoration on Ga2O3 nanorod array gas sensors and discovered an order of magnitude enhanced sensitivity to carbon monoxide at 500 °C. The LSFO nanoparticle catalysts was of comparable performance to that achieved by Pt nanoparticles, with a much lower weight loading than Pt. Detailed electron microscopy and X-ray photoelectron spectroscopy studies suggested the LSFO nanoparticle sensitization effect is attributed to a spillover-like effect associated with the gas-LSFO-Ga2O3 triple-interfaces that spread the negatively charged surface oxygen ions from LSFO nanoparticles surfaces over to β-Ga2O3 nanorod surfaces with faster surface CO oxidation reactions.

Tapered zinc oxide (ZnO) nanorods have been grown directly on Fe and Cu electrode surfaces using a facile, one-step seedless (catalyst-free) solution processing method. Diaminopropane (DAP) is used to facilitate the growth of tapered nanorods. Fe electrodes remain nearly intact for the duration of synthesis, yielding low areal density ZnO nanorod arrays, while Cu electrodes were continuously etched and grew relatively high areal density arrays with a strong interfacial connection. However, despite the presence of dissolved metal ions within the synthesis solution, Auger analysis reveals dopant-free ZnO nanorods. The fabricated nanorod arrays were demonstrated as a field emission source.

Composite nanoarchitectures represent a class of nanostructured entities that integrates various dissimilar nanoscale building blocks including nanoparticles, nanowires, and nanofilms toward realizing multifunctional characteristics. A broad array of composite nanoarchitectures can be designed and fabricated, involving generic materials such as metal, ceramics, and polymers in nanoscale form. In this review, we will highlight the latest progress on composite nanostructures in our research group, particularly on various metal oxides including binary semiconductors, ABO3-type perovskites, A2BO4 spinels and quaternary dielectric hydroxyl metal oxides (AB(OH)6) with diverse application potential. Through a generic template strategy in conjunction with various synthetic approaches—such as hydrothermal decomposition, colloidal deposition, physical sputtering, thermal decomposition and thermal oxidation, semiconductor oxide alloy nanowires, metal oxide/perovskite (spinel) composite nanowires, stannate based nanocompostes, as well as semiconductor heterojunction—arrays and networks have been self-assembled in large scale and are being developed as promising classes of composite nanoarchitectures, which may open a new array of advanced nanotechnologies in solid state lighting, solar absorption, photocatalysis and battery, auto-emission control, and chemical sensing.

A nanocomposite film of La0.67Sr0.33MnO3 (LSMO):ZnO is synthesized by depositing LSMO solution on a vertical array of ZnO nanorods grown on (0001) Al2O3 substrate. The magnetic behavior of the composite film differs from that of a pure LSMO film, possibly due to smaller grain size in the composite, small amount of Zn doping, or the presence of nonmagnetic ZnO phase near the LSMO grain boundaries. Magnetotransport measurements show that the low-field magnetoresistance (LFMR) of the nanocomposite film is significantly enhanced as compared to that observed for pure LSMO film. The highest value of the LFMR of the nanocomposite film at 10 K is –23.9% with a magnetic field of 0.5 T applied parallel to the current.

Ordered TiO 2 /(La,Sr)MnO 3 composite nanorod array catalysts have been successfully fabricated by using a combination of hydrothermal synthesis and radio-frequency (RF) magnetron sputtering method on Si and glass substrates. Electron microscopy, energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and Brunauer–Emmett–Teller (BET) method are used to characterize the structure, morphology, composition and surface area of the as-prepared nanorod arrays. The TiO 2 /(La,Sr)MnO 3 core–shell nanorod arrays exhibit an enhanced CO oxidation activity compared to TiO 2 nanorods, which might be due to the lattice oxygen in (La,Sr)MnO 3 . The TiO 2 /(La,Sr)MnO 3 composite nanorod array may provide a promising class of nanocatalysts for energy and environment applications.

An in situ carbothermal process has been utilized to successfully convert sputtered (0001) oriented ZnO nanofilm into large scale [0110] directed nanowires at 800−900 °C in a tubular reaction chamber. To understand the conversion process and unravel the driving force behind this growth phenomenon, a comparative study has been carried out on the thermal annealing processes of ZnO nanofilms under different gas flows (with or without Ar flow) and atmosphere (with or without graphite source input) controls. It is found that the graphite source and Ar flow are both necessary to induce the conversion of ZnO (0001) oriented nanofilm into ZnO nanowires grown along [0110]. By heating the graphite source, a C- or CO-rich reducing atmosphere was generated and carried downstream by Ar flow during the conversion process, which proved to be the key. Complementary first principles computations suggest that changes in the ordering of surface energies in the presence of CO occur. As compared with the clean facets with a...

Spherical CuO was synthesized by a simple hydrothermal reaction with the yield as high as ∼90%. The size of the CuO spheres can be facilely and selectively controlled to range from several hundred nanometers to a few micrometers simply by adjusting the concentration of the reactants. The CuO spheres consist of many small and uniform nanoneedles with tips about tens of nanometers in diameter. Interestingly, it was found that the initial concentration of reactants plays an important role in the formation of spherical CuO with different size. A growth mechanism of spherical CuO was also proposed based on the observed results. The cyclic voltammogrametry reveals that CuO nanospheres exhibit a remarkable electrocatalytic activity for the oxidation/reduction of H 2 O 2 . This study provides a promising route for the facile and cost-effective synthesis of inorganic nano- and micromaterials, and the synthesized CuO shows great promise in electrochemical applications.

Growth of vertically aligned ZnO nanowire arrays has been extensively studied on a variety of important semiconductor substrates, such as SiC and GaN. Systematic experiments were carried out to investigate the effect of growth parameters to the quality of the nanowires. In addition, the growth of nanowalls connecting individual aligned nanowires was studied and a growth mechanism was proposed. These conductive and interconnected nanowalls are indispensable for nanodevices to be fabricated on nonconductive substrates for serving as a common electrode. Finally, these nanowire arrays have been integrated as ultra violet detectors, which show good optical performance.

Well-tailored three-dimensional (3D) ZnO nanowire architectures have been successfully grown on Si microtrenches fabricated using nanoimprinting lithography by a low-temperature hydrothermal approach. Au nanoparticles or ZnO nanofilms were used as templates to tailor the orientation ordered nanowire growth normal to the microtrench surface. Au produced sparse nanowire growth, while ZnO seeds created densely packed growth. Optically, other than displaying a primary color when viewed from one incident angle, the 3D nanowire architecture periodically displayed multiple primary color domains covering all microtrenches and the local orientation ordered nanowire arrays. A pre-growth annealing of ZnO nanoseeds resulted in nonuniformity and non-periodic distribution of the grown nanoarchitectures and thus reduced the multicolor effect.

The ZnO nanobelts were synthesized by a hydrothermal method. The XRD spectrum indicates that the sample is wurtzite (hexagonal) structured ZnO with lattice constants of a = 3.249 A , c = 5.206 A . SEM and TEM images show the nanobelts to have lengths of 10–20 μm, widths of 50–500 nm, thicknesses of about 30–60 nm, and growth direction of [0001]. Gas sensitivity experiments on ZnO nanobelts were carried out under different temperatures. The results indicated high sensitivities with an operating temperature of only 220 ∘C for the oxidative gas O2, and 305 ∘C for the gas N2. The mechanism of gas sensitive effects is analyzed in detail.

Research applications in biomedical science and technology usually require various portable, wearable, easy-to-use, and/or implantable devices that can interface with biological systems. Organic or hybrid organic–inorganic microelectronics and nanoelectronics have long been a possibility. However, these devices require a power source, such as electrochemical cells or piezoelectric, thermoelectric, and pyroelectric transducers, to generate or store the electrical energy created through chemical, mechanical, or thermal processes. Finding a suitable power source has remained a major challenge for many devices in bioengineering and medical fields. ZnO is a typical piezoelectric and pyroelectric inorganic semiconducting material used for electromechanical and thermoelectrical energy conversion. Nanostructures of ZnO, such as nanowires (NWs), nanobelts (NBs), nanotubes, nanorings, nanosprings, and nanohelices, have attracted extensive research interest because of their potential applications as nanoscale sensors and actuators. While most of the current applications focus on its semiconducting properties, only a few efforts have utilized the nanometerscale piezoelectric properties of ZnO. Using ZnO NW arrays grown on a single-crystal sapphire substrate, we have successfully converted mechanical energy into electrical energy at the nanoscale. A conductive atomic force microscopy (AFM) tip was used in contact mode to deflect the aligned NWs. The coupling of piezoelectric and semiconducting properties in ZnO creates a strain field and charge separation across the NWs as a result of their bending. The rectifying characteristic of the Schottky barrier formed between the metal tip and the NW leads to electrical current generation. This is the principle behind piezoelectric nanogenerators. The ceramic and semiconducting substrates used for growing ZnO NWs are hard and brittle and cannot be used in applications that require a foldable or flexible power source, such as implantable biosensors. In this Communication, by using ZnO NW arrays grown on a flexible plastic substrate, we demonstrate the first successful flexible power source built on conducting-polymer films. This approach has two specific advantages: it uses a cost-effective, large-scale, wet-chemistry strategy to grow ZnO NW arrays at temperatures lower than 80 °C, and the growth of aligned ZnO NW arrays can occur on a large assortment of flexible plastic substrates. The latter advantage could play an important role in the flexible and portable electronics industry. Various dimensions, shapes, and orientations of ZnO NWs and microwires on flexible plastic substrates have been shown to be capable of producing piezoelectric voltage output, giving a real advantage for energy harvesting using large-scale ZnO NW arrays. The voltage generated from a single NW can be as high as 50 mV, which is large enough to power many nanoscale devices. The ZnO NWs were grown in solution using a synthetic chemistry approach. Figure 1 shows a series of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of typical ZnO NW arrays grown on a conductive plastic substrate. Figure 1a shows a low-resolution, topdown view of the densely aligned NW arrays. The aligned NWs have a uniform diameter of 200–300 nm and a hexagoC O M M U N IC A IO N

Heterogeneous catalysis is among the major solutions and processes for cost-effective and sustainable industrial application and processing. Search and discovery of highly efficient and stable heterogeneous catalysts represent an emergent frontier for overcoming the energy and environmental challenges we are facing. Fortunately, the growing nanotechnology offers one of the solutions through development of nanocatalysts with high activity, selectivity, and stability. Among various nanocatalysts, nano-array catalysts drew a great deal of attention not only owing to its model structure for exploring the origin of catalytic activity and mechanism but also because of its high collective catalytic activity and structural stability. In this chapter, we systematically reviewed recent progresses on the nano-array catalysts and their catalytic performance as oxidation catalysts, electrocatalysts, and photocatalysts.

The mechanical resonance behavior of a ZnO nanowire/nanorod at ambient condition has been studied under optical microscope by cutting its length using focused ion beam microscopy. Nanobalance using a ZnO nanowire as the cantilever has been demonstrated for measuring the mass in the order of pico-grams in working atmosphere (see optical microscopy images). The measurement limit of the balance is estimated to be ∼1 pg. The technique demonstrated here has potential for commercial applications in general laboratories, especially for measuring the mass of wet biological cells or species.

Rectifying diodes of single nanobelt/nanowire-based devices have been fabricated by aligning single ZnO nanobelts/nanowires across paired Au electrodes using dielectrophoresis. A current of 0.5 microA at 1.5 V forward bias has been received, and the diode can bear an applied voltage of up to 10 V. The ideality factor of the diode is approximately 3, and the on-to-off current ratio is as high as 2,000. The detailed IV characteristics of the Schottky diodes have been investigated at low temperatures. The formation of the Schottky diodes is suggested due to the asymmetric contacts formed in the dielectrophoresis aligning process.

The low melting point of Zn and the high melting point of ZnO, as well as their hexagonal crystal structures, present great advantages for designing and fabricating various metal/semiconductor core/shell nanostructures. By controlling the kinetics in the Zn and ZnO system, the lower-energy facets, and the oxidation rates of different surfaces, we can control the fabrication of Zn/ZnO core/shell single-crystal, polycrystalline, and mesoporous nanodisks, as well as a variety of ZnO nanotubes. The oxidation of a Zn nano-object leads to the formation of Zn/ZnO core/shell nanodisks. A lower oxidation temperature results in the formation of a single-crystal-like Zn/ZnO core/shell structure, while a higher oxidation temperature leads to the formation of textured and even polycrystalline nanostructures. A re-sublimation process of Zn in the core leaves a ZnO shell structure. This is an approach for synthesizing metal/semiconductor core/shell or composite nanostructures. This article offers a detailed description of the kinetics controlling the procedures, the nanostructures obtained, their morphological and crystal structures, and their formation mechanisms.

Polar surface induced asymmetric growth of single-side teethed ZnO nanocombs was attributed to the self-catalysis of the Zn-terminated (0 0 0 1) surface (Z.L. Wang, X.Y. Kong, J.M. Zuo, Phys. Rev. Lett. 91 (2003) 185502). In this Letter, nanocombs of ZnO with double-sided teeth have been observed. This symmetric growth of the fish-ribbon like teeth has been identified due to the existence of an inversion domain boundary along the ribbon, so that both side surfaces of the ribbon are terminated with the chemically active Zn-(0 0 0 1) plane. A model is also given about the formation of � 110� double-sided nanocombs based on the nucleus composed of multiply twinned pyramids. The data show that the Zn-terminated (0 0 0 1) surface is responsible for the formation of the teeth, while the oxygen-terminated (0 0 0 1) surface is chemically inactive and does not grow teeth. � 2005 Elsevier B.V. All rights reserved.

In this Letter, three-dimensional (3D) interconnected networks of ZnO nanowires and nanorods are synthesized by a high temperature solid–vapor deposition process. The nanorods and nanowires have diameters of 20–100 nm and they grow along the c-axis. Due to the diverse orientation of the nanowires grown from a polycrystalline substrate, the networks are formed by a sintering process of the crossed nanowires during growth. The thickness of the multilayer nano-network could be as thick as � 30 lm. The sharp nanowire tips, the high degree of networking, and high surface area of these unique nanonetworks make them a potential candidate for field emission, ultra-sensitive gas sensing, catalysts and filtering. � 2005 Elsevier B.V. All rights reserved.

Zinc oxide, an important semiconducting and piezoelectric material, has three key characteristics. First, it is a semiconductor, with a direct bandgap of 3.37 eV and a large excitation binding energy (60 meV), and exhibits near-UV emission and transparent conductivity. Secondly, due to its non-centrosymmetric symmetry, it is piezoelectric, which is a key phenomenon in building electro-mechanical coupled sensors and transducers. Finally, ZnO is bio-safe and bio-compatible, and can be used for biomedical applications without coating. With these unique advantages, ZnO is one of the most important nanomaterials for integration with microsystems and biotechnology. Structurally, due to the three types of fastest growth directions— , , and —as well as the ±(0001) polar surfaces, a diverse group of ZnO nanostructures have been grown in our laboratory. These include nanocombs, nanosaws, nanosprings, nanorings, nanobows, and nanopropellers. This article reviews our recent progress in the synthesis and characterization of polar-surface-induced ZnO nanostructures, their growth mechanisms, and possible applications as sensors, transducers, and resonators. It is suggested that ZnO could be the next most important nanomaterial after carbon nanotubes.

In the vapor-liquid-solid (VLS) growth of 1D nanostructures, the electronic structure of the substrate surface may critically affect the morphology of the grown nanowires/nanorods. In this paper, using a model system of the Sn-catalyzed growth of ZnO nanostructures on a single-crystal ZnO substrate, we demonstrate the effect of substrate surface termination on nanowire growth. Symmetric nanoribbons have been grown on the nonpolar surfaces of ((21 h10) (or ((011 h0)), but the polar surface ((0001) substrates have asymmetrically grown nanostructures. For the Zn-terminated (0001) substrate surface, uniform, long, and epitaxially aligned nanowires have been grown. For the oxygen-terminated (0001 h) substrate surface, short nanotrunks have been grown. These asymmetrical growth features are related to the atomic termination of the substrate, surface charges, and interface adhesion. These observations provide some insight into the physical chemical process in VLS growth.

Vapor-liquid-solid (VLS) is a well-established process in catalyst-guided growth of nanowires. The catalyst particle is generally believed to be in liquid state during growth, and it is the site for adsorbing incoming molecules; the crystalline structure of the catalyst may not have any influence on the structure of the grown one-dimensional nanostructures. In this paper, using tin particle guided growth of ZnO nanostructures as a model system, we show that the interfacial region of the tin particle with the ZnO nanowire/nanobelt could be ordered (or partially crystalline) during the VLS growth, although the local growth temperature is much higher than the melting point of tin, and the crystallographic lattice structure at the interface is important in defining the structural characteristics of the grown nanowires and nanobelts. The interface prefers to take the least lattice mismatch; thus, the crystalline orientation of the tin particle may determine the growth direction and the side surfaces of the nanowires and nanobelts. This result may have important impact on the understanding of the physical chemical process in the VLS growth.

The work function at the tips of individual ZnO nanobelts has been measured by an electromechanical resonant method using in-situ transmission electron microscopy. The work function of the ZnO nanobelts is ∼5.2 eV, which shows no significant dependence on the geometrical size. Using a ZnO nanobelt as a carrier beam, the work function of a single nanoparticle of a different material has also been measured.

We report the synthesis and characterization of nanowire−nanoribbon junction arrays of ZnO, which were grown by thermal evaporation of the mixture of ZnO and SnO2 powders at 1300 °C through a vapor−liquid−solid process. The Sn particles produced by the reduction of SnO2 act as the catalyst; the structure is formed due to a fast growth of ZnO nanowires along [0001] and the subsequent “epitaxial” radial growth of the ZnO nanoribbons along the six 〈0110〉 directions around the nanowire. The “liana” shape nanostructure could be a candidate for fabricating ultrahigh sensitive sensors.

The monolithic catalyst, namely the structured catalyst, is one of the important categories of catalysts used in various fields, especially in catalytic exhaust after-treatment. Despite its successful application in conventional wash-coated catalysts in both mobile and stationary catalytic converters, washcoat-based technologies are facing multi-fold challenges, including: (1) high Pt-group metals (PGM) material loading being required, driving the market prices; (2) less-than ideal distribution of washcoats in typically square-shaped channels associated with pressure drop sacrifice; and (3) far from clear correlations between macroscopic washcoat structures and their catalytic performance. To tackle these challenges, the well-defined nanostructure array (nano-array)-integrated structured catalysts which we invented and developed recently have been proven to be a promising class of cost-effective and efficient devices that may complement or substitute wash-coated catalysts. This new type of structured catalysts is composed of honeycomb-structured monoliths, whose channel surfaces are grown in situ with a nano-array forest made of traditional binary transition metal oxide support such as Al2O3, CeO2, Co3O4, MnO2, TiO2, and ZnO, or newer support materials including perovskite-type ABO3 structures, for example LaMnO3, LaCoO3, LaNiO, and LaFeO3. The integration strategy parts from the traditional washcoat technique. Instead, an in situ nanomaterial assembly method is utilized, such as a hydro (solva-) thermal synthesis approach, in order to create sound structure robustness, and increase ease and complex-shaped substrate adaptability. Specifically, the critical fabrication procedures for nano-array structured catalysts include deposition of seeding layer, in situ growth of nano-array, and loading of catalytic materials. The generic methodology utilization in both the magnetic stirring batch process and continuous flow reactor synthesis offers the nano-array catalysts with great potential to be scaled up readily and cost-effectively. The tunability of the structure and catalytic performance could be achieved through morphology and geometry adjustment and guest atoms and defect manipulation, as well as composite nano-array catalyst manufacture. Excellent stabilities under various conditions were also present compared to conventional wash-coated catalysts.

Monitoring and control of the gaseous combustion process are critically important in advanced energy systems such as power plants, gas turbines, and automotive engines. However, very limited gas sensing solutions are available in the market for such applications due to the inherent high temperature of the combustion gaseous atmosphere. In this study, we fabricated and demonstrated high-performance metal oxide based nanorod array sensors assisted with ultra-violet (UV) illumination for in situ and real-time high-temperature gas detection. Without UV-illumination, it was found that surface decoration of either 5 nm LSFO or 1 nm Pt nanoparticles can enhance the sensitivity over CO at 500 °C by an order of magnitude. Under the 254 nm UV illumination, the CO gas-sensing performance of Ga2O3-based nanorod array sensors was further enhanced with the sensitivity boosted by 125% and the response time reduced by 30% for the La0.8Sr0.2FeO3(LSFO)-decorated sample. The UV-enhanced detection of CO might be due to the i...

Helical nanomaterials represent an emerging group of nanostructures with unique spiral geometry as well as multiple functionalities owing to their enriched physical and chemical properties. With the novel properties enabled by their nanoscale dimension and unique geometry, the helical nanostructures may open opportunities to develop our understanding of new physics, chemistry and biology, and enable new nanodevice design and fabrication. This review article presents a comprehensive and in-depth overview of the latest progress in helical nanostructures synthesis, properties and potential applications. Specific attention is concentrated on the crystal growth theory for helical nanostructures, summary of the helical nanomaterials obtained so far, and their fabrication techniques as well as typical physical properties that can be potentially utilized for various applications.

This review presents a comprehensive overview of the hierarchical nanostructured materials with either geometry or composition complexity in environmental applications. The hierarchical nanostructures offer advantages of high surface area, synergistic interactions, and multiple functionalities toward water remediation, biosensing, environmental gas sensing and monitoring as well as catalytic gas treatment. Recent advances in synthetic strategies for various hierarchical morphologies such as hollow spheres and urchin-shaped architectures have been reviewed. In addition to the chemical synthesis, the physical mechanisms associated with the materials design and device fabrication have been discussed for each specific application. The development and application of hierarchical complex perovskite oxide nanostructures have also been introduced in photocatalytic water remediation, gas sensing, and catalytic converter. Hierarchical nanostructures will open up many possibilities for materials design and device fabrication in environmental chemistry and technology.

This final report to the Department of Energy (DOE) and National Energy Technology Laboratory (NETL) for DE-EE0000210 covers the period from October 1, 2009 to July 31, 2013. Under this project, DOE awarded UConn about $1,248,242 to conduct the research and development on a new class of 3D composite nanostructure based catalysts for lean NOx emission control. Much of the material presented here has already been submitted to DOE/NETL in quarterly technical reports. In this project, through a scalable solution process, we have successfully fabricated a new class of catalytic reactors, i.e., the composite nanostructure array (nano-array) based catalytic converters. These nanocatalysts, distinct from traditional powder washcoat based catalytic converters, directly integrate monolithic substrates together with nanostructures with well-defined size and shape during the scalable hydrothermal process. The new monolithic nanocatalysts are demonstrated to be able to save raw materials including Pt-group metals and support metal oxides by an order of magnitude, while perform well at various oxidation (e.g., CO oxidation and NO oxidation) and reduction reactions (H{sub 2} reduction of NOx) involved in the lean NOx emissions. The size, shape and arrangement of the composite nanostructures within the monolithic substrates are found to be the key in enabling the drastically reduced materials usage while maintaining the good catalytic reactivity in the enabled devices. The further understanding of the reaction kinetics associated with the unique mass transport and surface chemistry behind is needed for further optimizing the design and fabrication of good nanostructure array based catalytic converters. On the other hand, the high temperature stability, hydrothermal aging stability, as well as S-poisoning resistance have been investigated in this project on the nanocatalysts, which revealed promising results toward good chemical and mechanical robustness, as well as S-poisoning resistance. Further investigation is needed for unraveling the understanding, design and selection principles of this new class of nanostructure based monolithic catalysts.

This final report to the Department of Energy/National Energy Technology Laboratory for DE-FE0000870 covers the period from 2009 to June, 2013 and summarizes the main research accomplishments, which can be divided in sensing materials innovation, bimodular sensor demonstration, and new understanding and discoveries. As a matter of fact, we have successfully completed all the project tasks in June 1, 2013, and presented the final project review presentation on the 9th of July, 2013. Specifically, the major accomplishments achieved in this project include: 1) Successful development of a new class of high temperature stable gas sensor nanomaterials based on composite nano-array strategy in a 3D or 2D fashion using metal oxides and perovskite nanostructures. 2) Successful demonstration of bimodular nanosensors using 2D nanofibrous film and 3D composite nanowire arrays using electrical resistance mode and electrochemical electromotive force mode. 3) Series of new discoveries and understandings based on the new composite nanostructure platform toward enhancing nanosensor performance in terms of stability, selectivity, sensitivity and mass flux sensing. In this report, we highlight some results toward these accomplishments.