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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.

Atomically-thin two-dimensional (2D) nanostructure, an emerging class of nanomaterials, has achieved tremendous progress in the past decade and will remain a significant research topic in the near future Apart from the discoveries of various 2D nanomaterials, remarkable attentions have been paid to the rational design and alteration of architectures based on pristine 2D materials to meet demands of next-generation functional applications in the near term Herein, this review intends to provide an instant survey over the recent key advances on the structural engineering strategies of 2D nanomaterial-based architectures for new sensing capabilities and opportunities Through intrinsic or extrinsic alterations, the pristine 2D nanostructures of some unique intrinsic properties could be modified and functionalized in a predictable pattern to overcome existing drawbacks and obtain synergistic abilities, demonstrating a great potential in promoting sensing device performance In parallel with the review of latest development of structural engineering strategies and related highly functionalized sensor devices including sensing capabilities towards Covid-19 antigens, the underlying mechanisms of these designs are revealed to offer a unique insight in understandings of architecture design and property tailoring for boosting new sensing capabilities and performances

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.

Gas flaring in oil/gas drilling and gas leakage in natural gas power plant lead to significant energy loss and environmental burden. Here, solar-driven efficient methane oxidation was demonstrated under high velocity continuous flow over the ZnO/La0.8Sr0.2CoO3 (ZnO/LSCO) heterojunctions. The ZnO/LSCO heterojunctions enable a unique epitaxial hetero-interface, which effectively regulates the electron transfer between Zn 3d-O 2p hybrid orbital in ZnO and Co eg orbital in LSCO and promotes the rapid generation and refill of oxygen vacancy with unpaired electron (Vo ), thus enhancing the activity and mobility of surface lattice oxygen in ZnO/LSCO. Under solar illumination, the synergy of photothermal and photocatalytic effect boosts the reversible electron transfer in the interface, which further activates surface lattice oxygen, resulting in a ∼2 times higher methane oxidation activity. Such a solar-driven system not only enables a promising pathway for emitted methane utilization, but also provides an advanced catalyst design concept of epitaxial interface construction.

We report a self-limiting growth method of ultrasmall Pd-containing bimetallic nanocatalysts supported on carbon through under temperature reduction (UTR). The UTR phenomenon is observed for the (co)reduction of a second base metal precursor (M = Co, Ni, and Zn) below its thermodynamic reduction temperature in the presence of Pd. The origin of the UTR phenomenon lies in the hydrogen spillover of Pd to drive the (co)reduction of the base metal at the temperature below its thermodynamic reduction temperature; while, the further reduction is shut down simultaneously to limit the overgrowth of nanoparticles when forming nanoalloys. Using nitrided carbon as a support, we demonstrate the self-limiting growth of three Pd-containing nanoalloys, including Pd1.7Co, Pd1.6Ni and Pd2Zn, with average sizes around 1.5 nm using UTR. Sub-2 nm Pd-containing bimetallic nanoalloys exhibited excellent catalytic properties in both methanol electrooxidation and selective hydrogenation of cinnamaldehyde.

Supported metal single atom catalysts (SACs) present an emerging class of low-temperature catalysts with high reactivity and selectivity, which, however, face challenges on both durability and practicality. Herein, we report a single-atom Pt catalyst that is strongly anchored on a robust nanowire forest of mesoporous rutile titania grown on the channeled walls of full-size cordierite honeycombs. This Pt SAC exhibits remarkable activity for oxidation of CO and hydrocarbons with 90% conversion at temperatures as low as ~160

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.

Layered protonated titanates (LPTs) are promising support materials for catalytic applications because their high surface area and cation exchange capacity provide the possibility of achieving a high metal dispersion. However, the reported LPT nanomaterials are mainly limited to free-standing nanoparticles (NPs) and usually require high temperature and pressure conditions with extended reaction time. In this work, a high-throughput microwave-assisted hydrothermal method was developed for the direct synthesis of conformal LPT nanoarray coatings onto the three-dimensional honeycomb monoliths as well as other substrate surfaces at low temperature (75–95 °C) and pressure (1 atm). Using TiCl3 as the titanium source, H2O2 as the oxidant, and hydrochloric acid as the pH controller, a peroxotitanium complex (PTC) was formed and identified to play an essential role for the formation of LPT nanoarrays. The gaseous O2 released during the decomposition of PTC promotes the mass transfer of the precursors, making this ...

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 ...

A series of large scale Mx Co3-x O4 (M=Co, Ni, Zn) nanoarray catalysts have been cost-effectively integrated onto large commercial cordierite monolithic substrates to greatly enhance the catalyst utilization efficiency. The monolithically integrated spinel nanoarrays exhibit tunable catalytic performance (as revealed by spectroscopy characterization and parallel first-principles calculations) toward low-temperature CO and CH4 oxidation by selective cation occupancy and concentration, which lead to controlled adsorption-desorption behavior and surface defect population. This provides a feasible approach for scalable fabrication and rational manipulation of metal oxide nanoarray catalysts applicable at low temperatures for various catalytic reactions.

A variety of stannate nanostructures have been fabricated for UV photocatalysis, including zinc- and cadmium-based stannates. As the template nanostructures, high surface-area mesoporous metal hydroxystannate [ZnSn(OH) 6 and CdSn(OH) 6 ] nanoparticles (>100 m 2 /g) have been synthesized using a simple, low-temperature substitution chemical process with controlled porosity, morphology and crystallinity. Post-synthetic thermal treatments were employed to obtain amorphous ZnSnO 3 , CdSnO 3 , ilmenite CdSnO 3 , and crystalline Zn 2 SnO 4 –SnO 2 nanoparticles. As a result, the band gaps can be tuned from 5.4 eV to 3.3 eV and from 4.9 eV to 2.1 eV for Zn-based and Cd-based stannates, respectively. Amorphous ZnSnO 3 porous nanoparticles showed highest activity toward dye degradation under UV illumination followed by the Zn 2 SnO 4 -SnO 2 and ilmenite CdSnO 3 nanostructures due to their beneficial band structure alignment, high conductivities, and high specific surface areas. This study may provide an important strategy for high throughput synthesis and screening of functional complex metal oxide nanomaterials, while the enabled stannate nanomaterials could be utilized in various applications.

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.

Cobalt oxide (Co3O4), a low-cost transitional metal oxide, has been widely studied for catalytic total oxidation of CO and hydrocarbons. However, much less attention has been paid to the poisoning effect due to alkali metals in such oxidation catalysts despite the ease of access to such poisons in both mobile and stationary combustion sources. In this study, the alkali metals such as Na, K and Li were readily deposited onto a Co3O4 catalyst to study their poisoning effect on CO and propane oxidation. 1% doping onto Co3O4 was found to increase the light-off temperature by 50 °C in CO oxidation and over 160 °C in propane oxidation. It is also noted that alkali metals exhibited a ‘locking-effect’ on oxygen of Co3O4, leading to poor oxygen mobility of alkali metal doped catalysts. During the reaction, alkali metals can significantly promote the adsorption of CO2 to form robust surface carbonate species even at high temperature, another reason for the poisoning effect of alkali metals on Co3O4 nanocatalysts for catalytic oxidation.

Manganese oxide (OMS-2), a promising exhaust treatment catalyst, has been successfully integrated onto the channel wall surfaces of cordierite honeycombs in the form of a nanoarray forest. To further improve their catalytic carbon monoxide (CO) oxidation activity, a series of different metal oxide nanocoatings were uniformly grown on the surface of OMS-2 nanoarrays by utilizing a facile and fast microwave-assisted synthesis method. The manganese-cobalt oxide core-shell nanoarray based monolithic catalysts exhibit the highest catalytic performance with a 100% CO conversion at 150 °C, in a sharp contrast with 325 °C for the bare OMS-2 nanoarrays. Different growth mechanisms of the layered Co(OH)2 and spinel Co3O4 induced by different cobalt precursors are used to explain the morphology evolution of the manganese-cobalt oxide core-shell nanoarrays. The Co3+ distributed on the surface of the nanoarrays acts as the active sites for CO oxidation. The small grain size, abundant surface-adsorbed oxygen, and interfacial effects between MnO2 and Co3O4 were found to favor the observed high catalytic activity. A thermal annealing study showed that high temperature induces the sintering of the nanoarray catalyst, which further affects the CO oxidation activity. This fast, cost-effective, and scalable method will provide a new route for synthesizing efficient core-shell nanoarray monolithic catalysts for low temperature catalysis.

Chemical sensors usually adopted a single resistance-metric mode, which has a drawback of significant cross-sensitivities in mixed analytes scenario, thus showing a poor selectivity towards target analytes. By integration of multiple sensors in a so-called sensor array platform, multi-analyte detection and enhanced selectivity can be achieved. However such a strategy requires significant device integration work involving sophisticated electrical interconnects with increasing circuit complexity. In the past few years, we has been focused on establishing a multi-mode sensing platform based on a single nanostructure array (nanoarray) device. Such a single device platform can be operated at variable temperatures with different modes such as photocurrent, resistance, and impedance. These nanoarray sensors have been fabricated with good scalability using a variety of synthetic methods ranging from vapor to solution phase depositions. The array composition could be rationally designed from pristine homogeneous to dissimilar heterogeneous across metals, ceramics, and polymers. Depending on the comprised material composition, decoration of a nanolayer of perovskite, metal oxide, or noble metal may enable formation of electronic and catalytic sensitizers, as well as hetero-contacts that can drastically boost sensor performance under various gaseous analyte atmospheres. Toward mixed multi-analyte conditions, such a new type of multi-mode nanoarray sensors allows selective and sensitive detection of multiple chemical species in a single-device configuration. By relating the multiple sensing modes using a single nanoarray sensor, efficient and selective detection of multiple gas analytes has been accomplished. A database was established based on multiple sets of data, namely the sensitivities for different analytes obtained from the outputs of different sensing modes. The signals of multiple gas analytes could be differentiated through the principal component analysis process. The linear and non-linear decision boundaries could be readily derived for pattern recognition, and the multi-analyte quantification and detection.

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.

We employ first principles methods to simulate the steady-state catalytic interaction of oxygen with the LaMnO3(001) surface under varying levels of oxidative environment. First, density-functional theory together with transition-state theory is used to obtain the energetics and activation barriers of the relevant elementary surface chemical processes. The results were subsequently fed into a kinetic Monte Carlo (kMC) simulation scheme to explicitly account for the correlations, fluctuations, and spatial distributions of the oxygen ad-atoms and ad-molecules at the surface of the catalyst under steady-state conditions. Adsorbate–adsorbate lateral interactions and influence of the local environment on the reaction kinetics were adequately taken into account to compute (p, T) surface phase diagrams under equilibrium and metastable situations. The kMC-based approach allows us to describe the kinetics-related effects such as the influence of the thermochemical history of the sample on the metastable regions of...

Efficient bifunctional catalysts for electrochemical oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are highly desirable due to their wide applications in fuel cells and rechargeable metal air batteries. However, the development of nonprecious metal catalysts with comparable activities to noble metals is still challenging. Here we report a one-step wet-chemical synthesis of Ni-/Mn-promoted mesoporous cobalt oxides through an inverse micelle process. Various characterization techniques including powder X-ray diffraction (PXRD), N2 sorption, transmission electron microscopy (TEM), and scanning electron microscopy (SEM) confirm the successful incorporation of Ni and Mn leading to the formation of Co-Ni(Mn)-O solid solutions with retained mesoporosity. Among these catalysts, cobalt oxide with 5% Ni doping demonstrates promising activities for both ORR and OER, with an overpotential of 399 mV for ORR (at -3 mA/cm(2)) and 381 mV (at 10 mA/cm(2)) for OER. Furthermore, it shows better durability than precious metals featuring little activity decay throughout 24 h continuous operation. Analyses of cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), Raman, and O2-temperature-programmed desorption (O2-TPD) reveal that redox activity of Co(3+) to Co(4+) is crucial for OER performance, while the population of surface oxygen vacancies and surface area determine ORR activities. The comprehensive investigation of the intrinsic active sites for ORR and OER by correlating different physicochemical properties to the electrochemical activities is believed to provide important insight toward the rational design of high-performance electrocatalysts for ORR and OER reactions.

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.

A generic one-pot hydrothermal synthesis route has been successfully designed and utilized to in situ grow uniform manganese oxide nanorods and nanowires onto the cordierite honeycomb monolithic substrates, forming a series of nanoarray-based monolithic catalysts. During the synthesis process, three types of potassium salt oxidants have been used with different reduction potentials, i.e., K2Cr2O7, KClO3, and K2S2O8, denoted as HM-DCM, HM-PCR, and HM-PSF, respectively. The different reduction potentials of the manganese source (Mn(2+)) and oxidants induced the formation of manganese oxide nanoarrays with different morphology, surface area, and reactivity of carbon monoxide (CO) oxidation. K2Cr2O7 and KClO3 can induce sharp and long nanowires with slow growth rates due to their low reduction potentials. In comparison, the nanoarrays of HM-PSF presented shorter nanorods but displayed an efficient 90% CO oxidation conversion at 200 °C (T90) without noble-metal loading. Reducibility tests for the three monolithic catalysts by hydrogen temperature-programmed reduction revealed an activation energy order of HM-PSFHM-DCMHM-PCR for CO oxidation. The characterizations of oxygen temperature-programmed desorption and X-ray photoelectron spectroscopy indicated the abundant surface-adsorbed oxygen and lattice oxygen contributing to the superior reactivity of HM-PSF. The straightforward synthetic process showed a scalable, low-cost, and template-free method to fabricate manganese oxide nanoarray monolithic catalysts for exhaust treatment.

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...

By directly heating a surface-dezincified polycrystalline brass (Cu70Zn30 alloy) in ambient conditions, semiconductor nanostructure networks composed of CuO nanofilm and ZnO nanowires have been successfully fabricated. The dezincification induced by metallographic etching on Cu70Zn30 polycrystalline alloy surface created heterogeneous networks composed of individual Cu-rich micrograin surfaces and surrounding grain boundaries. The comparative thermal oxidation investigations show distinct growth of semiconductor nanowire/nanofilm network junctions on dezincified Cu70Zn30 substrates compared to the random ZnO nanowires growth on nonetched brass substrates. The grain boundary region is mostly dominated by ZnO nanowires, while each grain surface was overgrown with CuO nanofilm, forming nanowire/nanofilm network junctions. The different growth habits of ZnO nanowires and CuO nanofilms are due to composition and geometry distribution difference between dezincified Cu70Zn30 grain surface and grain boundary, res...

The scalable three-dimensional (3-D) integration of functional nanostructures into applicable platforms represents a promising technology to meet 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 talk, 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 and systems applicable with ultrahigh efficiency, good robustness and improved functionality. Selective examples of nano-array integration, scalable nanomanufacturing and representative monolithic devices will be highlighted, including automotive catalytic converters, thermo-catalytic and photo-activated reactors for fuel/chemical production, and sensors, with practical nanotechnology implications ranging from energy, environmental, to chemical and biotechnology areas.

CuxO (x = 1, 2) nanocrystals have been synthesized by the composite-hydroxide-mediated approach. The obtained nanocrystals were characterized by X-ray diffraction, energy dispersive X-ray spectroscopy, scanning electron microscopy, transmission electron microscopy, and UV–vis spectrum. The morphology of the nanocrystals changed from sphere-shaped nanostructures to flower-shaped nanostructures, and finally to nanowires associated with phase transformation from CuO to Cu2O by increasing the temperature. The possible phase transformation mechanism was discussed. The catalytic degradation activity of the CuxO (x = 1, 2) nanocrystals to methyl orange was also investigated. The photocatalytic ability of the sphere-shaped nanostructures is much higher than that of the nanowires, owing to its absorption of wider range of light energy. This work provides a new facile synthesis route of CuxO (x = 1, 2) nanocrystals and suggests their possible application in organic pollutants removal.

Large scale zigzag zinc blende single crystal ZnS nanowires have been successfully synthesized during a vapor phase growth process together with a small yield of straight wurtzite single crystal ZnS nanowires. AuPd alloy nanoparticles were utilized to catalyze a vapor-solid-solid growth process of both types of ZnS nanowires, instead of the more common vapor-liquid-solid growth process. Surprisingly, the vapor-phase grown zigzag zinc blende ZnS nanowires are metastable under high-energy electron irradiation in a transmission electron microscope, with straight wurtzite nanowires being much more stable. Upon exposure to electron irradiation, a wurtzite ZnO nanoparticle layer formed on the zigzag zinc blende ZnS nanowire surface with concomitant displacement damage. Both electron inelastic scattering and surface oxidation as a result of electron-beam heating occur during this structure evolution process. When prolonged higher-voltage electron irradiation was applied, local zinc blende ZnS nanowire bodies evolved into ZnS-ZnO nanocables, and dispersed ZnS-ZnO nanoparticle networks. Random AuPd nanoparticles were observed distributed on zigzag ZnS nanowire surfaces, which might be responsible for a catalytic oxidation effect and speed up the surface oxidation-induced structure evolution. Open image in new window

Single crystalline zinc hydroxystannate [ZnSn(OH)6] micro- and nanocubes have been successfully grown on pure tin substrates via a seedless hydrothermal synthesis method. Each ZnSn(OH)6 cube is enclosed by six equivalent {001} crystal planes. Cube size and aerial density were adjusted by controlling the reaction time and addition of diaminopropane (DAP) reagent. The hexamethylenetetramine- and DAP-assisted etching of the oxidized tin metal surface is found to play an important role in the nucleation and growth of the zinc hydroxystanate cubes. Synthesis at higher zinc nitrate concentrations as well as secondary growth resulted in the formation of ZnO nanorods in addition to the cubes. In situ scanning electron microscopy and transmission electron microscopy, thermal gravimetric analysis, and differential scanning calorimetry have been utilized to investigate the ZnSn(OH)6 thermal decomposition process. The appearance of endothermic peak near ∼540 K, attributed to the ZnSn(OH)6 decomposition, was correlate...

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.