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Constructing robust catalysts by growing nanostructure arrays directly onto honeycomb monoliths is a promising technology for fabricating highly active reactors with low materials usage and cost-effectiveness. In this study, catalytically active NiO nanosheet arrays were uniformly grown onto the channel surfaces of cordierite honeycombs to form monolithic catalysts with ultra-low loading. Ce doping was found to significantly enhance the catalytic propane oxidation performance of pristine NiO nanosheet arrays. With a Ce doping of ∼4 wt.% in NiO, a conversion of 90 % propane oxidation into CO2 was achieved at 440 °C, a ∼40 °C decrease compared to the pristine NiO under a space velocity of 24,000 h−1. With the promoting effect of Ce, the Ce-doped NiO nanosheet arrays exhibited a much better thermal stability, with only 1.1 % loss of conversion after a 48 h operation at 425 °C, in comparison with a 21.5 % loss over the pristine NiO catalyst.

One-dimensional titanium dioxide (TiO2) nanostructure arrays (nanoarrays) are important metal oxide nanomaterials that can be synthesized via facile solvothermal methods. The properties of the organic solvents can impose significant effects on the microstructures and properties of the final products. However, the discussions were limited to the homogeneously nucleated TiO2 nanomaterials in free-standing powder form, while the solvent effects are less understood during the heterogeneous growth of TiO2 nanoarrays on a substrate surface. In this work, six organic compounds, namely 2-butanone, n-decane, n-hexane, toluene, ethylene glycol and ethanol, were selected as the solvents for the solvothermal synthesis of TiO2 nanoarrays on the cordierite monolithic substrates. Special attentions are paid to the morphology, crystallinity, specific surface area, and porosity of the samples. The heterogeneous growth of TiO2 nanoarrays on substrate surfaces is found to favor the solvents with moderate dielectric constants, which can be partially dissolved in the aqueous solution and modulate the reaction rate during the solvothermal synthesis. Organic solvents with low dielectric constants may result in a complete separation between the precursors and aqueous solution, and therefore slow down the overall reaction, causing the insufficient growth of the nanoarrays. The TiO2 nanoarrays are obtained with optimum morphology from the combination of 2-butanone and titanium (IV) butoxide as solvent and precusor, respectively, with a high specific surface area up to 56 m2/g including cordierite substrate given a micron thickness. When loaded with Pt catalyst, the TiO2 nanoarray-based monolithic catalysts show excellent low-temperature catalytic activity and hydrothermal stability for the CO and hydrocarbon oxidation under the simulated exhausted conditions. This work shall shed light on a better understanding of the growth mechanism and rational design of TiO2 nanoarrays for high-performance catalytic converters.

Nanoarray-based monolithic catalysts have been developed for various applications, including CO oxidation, hydrocarbon combustion, lean NOx trapping, and low-pressure CO2 hydrogenation. In this work, SO2 adsorption properties have been explored and evaluated on the cordierite honeycomb monoliths grown with zinc oxide nanoarray (ZnO), zinc oxide nanoarray washcoated by BaCO3 nanoparticles (ZnO/BaCO3), and manganese oxide nanowire array with cryptomelane structure (MnOx) at a temperature range from 50 to 425 °C. All samples show temperature-dependent SO2 adsorption behaviors. The adsorption results reveal the performance order: MnOx > ZnO/BaCO3 > ZnO, with ~ 90% SO2 adsorbed in MnOx at 425 °C. Washcoated BaCO3 contributes to the improvement of SO2 adsorption in ZnO nanoarray, and the best performance displayed in MnOx may be attributed to their high specific surface area. After regeneration, nanoarrays all exhibit good thermal stability during test-regeneration cycles. No additional phase is formed in regenerated ZnO nanoarrays (ZnO-R), while BaCO3 is converted to BaSO4 in the regenerated ZnO/BaCO3 nanoarrays (ZnO/BaCO3-R), and the sulfur species (possibly MnSO4) and Mn2O3 are found in regenerated MnOx nanoarrays (MnOx-R). It is noted that a small amount of sulfur species (possibly MnSO4) may promote the SO2 adsorption of MnOx-R at a lower temperature, while the formed Mn2O3 contributes to the deactivation of MnOx-R.

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 oC under simulated diesel exhaust conditions while using 5 times less Pt-group metals than a commercial oxidation catalyst. Such an excellent low-temperature performance is sustained over hydrothermal aging and sulfation as a result of highly dispersed and isolated active single Pt ions bonded at the Ti vacancy sites with 5 or 6 oxygen ions on titania nanowire surfaces., Supported metal single-atom catalysts face challenges on both durability and practicality. Here, the authors demonstrate that a sustained 90% diesel oxidation conversion at ~160 oC is achieved by single-atom Pt on TiO2 nanowire-array integrated catalytic converter.

Stabilized Cu+ species have been widely considered as catalytic active sites in composite copper catalysts for catalytic reactions with industrial importance. However, few examples comprehensively ...

Amongst various gaseous pollutants, NO2 is one of the major exhausts originating from fossil fuel and gas combustions in vehicle engines and power plants at high temperature. Under such a scenario, in situ and real-time gas detection and monitoring solutions are largely limited, which hinders the energy-efficient and resource-saving operations of these advanced energy systems. In this report, a new type of perovskite-sensitized β-Ga2O3 nanorod-array has been successfully fabricated to detect NOx at high temperature selectively and sensitively. The demonstrated gas sensors are highly sensitive to NO2 at 800 °C, with excellent reversible and reproducible response characteristics. Through the surface decoration of perovskite-type La0.8Sr0.2CoO3 (LSCO) nanoparticles, the sensitivity of β-Ga2O3 nanorod array gas sensors is enhanced by nearly an order of magnitude, along with much faster response dynamics. A remarkable selectivity toward oxidative gases was also demonstrated with the robust differentiation of NO2.

Photoabsorption, charge separation efficiency and surface reactive catalytic sites are three critical factors in semiconductor photocatalytic process, which determine the photocatalytic activity. For bulk g-C3N4 derived from direct pyrolysis of C/N rich precursors, reactive sites distributed on the lateral edges are very scarce. In this work, we report the template-free preparation of three novel structured g-C3N4, namely, porous tubular (PT) g-C3N4, porous tubular yolk-shell (PTYS) g-C3N4, and porous split yolk-shell (PSYS) g-C3N4, by an unprecedented precursor microstructure regulation of melamine crystals in a gas-pressure mediated re-crystallization process. Enhanced photoabsorption, increased surface area, largely improved separation and migration efficiencies of photoinduced charge carriers are simultaneously realized in these g-C3N4 structures. Noticeably, selective photo-deposition test uncovers that the porous outer-walls and inner-rods of PTYS g-C3N4 are enriched by abundant reductive reactive sites, which consumedly boost the photo-reduction activity. Collectively promoted by these advantages, PTYS g-C3N4 shows not only an efficient H2 production activity with a high apparent quantum efficiency (AQE) of 11.8% at λ = 420 ± 15 nm, but also a superior CO2 reduction for CO production than bulk g-C3N4 by a factor 5.6, which is verified by the 13C isotopic labeling. This work develops precursor microstructure engineering as a promising strategy for rational design of unordinary g-C3N4 structure for renewable energy production.

Diesel soot combustion suffers from ignition temperatures (T10) as high as > 450 °C in the absence of catalysts, which are unavailable in diesel exhaust during normal driving cycles (normally 200–400 °C). A catalytic diesel particulate filter (CDPF) could decrease T10 greatly, but it is often inadequate due to the poor contact associated with the solid (catalyst)-solid (soot) interactions. Herein, a highly significant T10, as low as ˜200 °C, was achieved on noble metal-free ultrathin MnO2-x nanosheet array fabricated by in situ etching of a La layer from LaMnO3 under loose contact conditions in a NO-containing atmosphere. A number of advantages were found with such a system including the improved reducibility. Then, the nanosheet array ensures high dispersion of soot on the catalyst. Finally, high NO-to-NO2 oxidation activity further facilitates contact between catalyst and soot via NO2, a stronger oxidant than O2.

Supported metal catalysts are one of the major classes of heterogeneous catalysts, which demand good stability in both the supports and catalysts. Herein, layered protonated titanate-derived TiO2 (LPT-TiO2) nanowire arrays were synthesized to support platinum catalysts using different loading processes. The Pt ion-exchange loading on pristine LPTs followed by thermal annealing resulted in superior Pt catalysts supported on the LPT-TiO2 nanoarrays with excellent hydrothermal stability and catalytic performance toward CO and NO oxidations as compared to the Pt catalysts through wet-impregnation on the anatase TiO2 (ANT-TiO2) nanoarrays resulted from thermal annealing of LPT nanoarrays. Both loading processes resulted in highly dispersed Pt nanoparticles (NPs) with average sizes smaller than 1 nm at their pristine states. However, after hydrothermal aging at 800 °C for 50 h, highly dispersed Pt NPs were only retained on the ion-exchanged LPT-TiO2 nanoarrays with the support structure consisting of a mixture of 74% anatase and 26% rutile TiO2. For the wet-impregnation loading directly on anatase TiO2 nanoarrays derived from LPT, the Pt catalysts experienced severe agglomeration after hydrothermal aging, with the nanoarray supports consisting of 86% anatase and 14% rutile TiO2. Spectroscopy analysis suggested that Pt2+ cations intercalated into the interlayers of the titanate frameworks through ion-exchange impregnation procedure, which altered the chemical and electronic structures of the catalysts, resulting in the shifts of the electronic binding energy, Raman bands, and optical energy bandgap. The ion-exchangeable nature of LPT nanoarrays clearly provides a structural modification in Pt-doped LPT that has resulted in a strong interaction between the Pt catalysts and LPT-TiO2 nanoarray supports, leading to the enhanced hydrothermal stability of the catalysts. Considering the wide applications of the LPT and TiO2 nanomaterials as supports for catalysts, this finding provides a new pathway to design highly stable supported metal catalysts for different reactions.

A new, surfactant-free hydrothermal method has been developed for the growth of CeO2-based nanoflake arrays onto three-dimensional-channeled cordierite honeycomb substrates. Herein, a leaching-crystallization mechanism was proposed where the hydrothermal reaction, only involving cerium nitrate and water, leached the cordierite surface slightly and induced the formation of CeO2 nanoparticles subsequently. Further continued reaction reincorporated Al and Si atoms leached from cordierite into CeO2, finally recrystallizing Ce-Al-Si composite nanoflake structures. By using atomic layer deposition process, well-dispersed, size-controlled Pt nanoparticles were uniformly decorated on the CeO2-based nanoflakes to form the Pt/ CeO2 nano-array-based monolithic catalyst. Despite 5-50 times reduction in the active material usage compared with the traditional wash-coated catalyst, the Pt/CeO2 nano-array monolithic catalyst exhibited good catalytic oxidation activities over various individual gases, such as propylene, propane, CO, and NO oxidation, with 90% conversion efficiencies at temperatures below 200 °C. Under the simulated exhaust condition of low-temperature diesel combustion (LTC-D) developed by US DRIVE, the monolithic catalyst with low Pt loading (˜1 g/l) exhibits 90% conversion of catalytic oxidation over CO and hydrocarbonsat temperatures as low as ˜180 °C, much superior to the performance of traditional washcoated catalysts.

High performance of an ultra-low Pt loading diesel oxidation catalyst can be achieved by using a combination of novel nano-array structured support, precise control of ultrafine active Pt particles, and an addition of H2 as a promoter into the exhausts. Highly stable mesoporous rutile TiO2 nano-array was uniformly grown on three-dimensional (3-D) cordierite honeycomb monoliths using a solvothermal synthesis. Atomic layer deposition was employed for precise dispersion of ultrafine Pt particles (0.95 ± 0.24 nm) on TiO2 nano-array with a Pt loading of 1.1 g/ft3. Despite low Pt loading, the Pt/TiO2 nano-array catalyst shows impressive low-temperature oxidation reactivity, with the conversion of CO and total hydrocarbon (THC) reaching 50% at 224 and 285 °C, respectively, in the clean diesel combustion (CDC) simulated exhaust conditions. The excellent activity is attributed to the unique nano-array structure that promotes gas-solid interaction and ultra-small Pt particle dispersion that increase surface Pt atoms. We also demonstrate that addition of more H2 into the exhaust can lower light-off temperature for CO and THC by up to ∼60 °C and ∼30 °C, respectively.

Channel surfaces of 3-D cordierite honeycomb were readily etched with basic solution and became rougher with formation of a uniform layer of silicate nanosheet array structures. The reaction between KMnO4 and Co(NO3)2 was applied for integrating Mn-Co-O nanosheet array on both blank and modified cordierite honeycombs. The etched cordierite channels can induce the Mn-Co-O arrays with larger unit size, higher density and thickness. Compared to the blank cordierite, the Mn-Co-O array on etched cordierite has much better catalytic performance in oxidation of hydrocarbons, which can be due to its varied features such as more abundant porosity, higher surface area, more Mn4+, Co3+ and adsorbed oxygen species. This study provides an efficient way for tuning the nanostructures of Mn-Co composite oxide nano-array based monolith, further boosting their catalytic activities in combustion of hydrocarbons.

The catalytic total oxidation of short-chain alkanes released from automobile exhausts is still a big challenge in volatile organic compound (VOC) elimination. The significant degradation of catalytic activity after hydrothermal aging is a widely existing issue. Herein, we report a facile one-pot hydrothermal method to successfully grow TiO2–Al2O3 binary nanoarrays on the 3D channel surfaces of ceramic honeycombs. Such a binary nanoarray was heterogeneously integrated on the cordierite honeycomb channel surface with closely separated nanowire arrays of anatase-TiO2 and mesoporous γ-Al2O3, which exhibited excellent robustness under mechanical vibration and thermal and hydrothermal aging. Moreover, propane conversion with the Pt/TiO2–Al2O3 binary nanoarray catalyst rapidly reached 80% at a temperature as low as 224 °C, suggesting that the binary nanoarray catalyst is a promising candidate for practical applications.

In this study, a new type of lean NOx traps (LNT) based on Pt:La0.8Sr0.2MnO3 (LSMO) nanoparticles decorated on ZnO nanorod array integrated monoliths have been successfully designed and demonstrated. Compared to the commercial catalyst, the nanorod array based catalyst demonstrates competitive NO oxidation activity, and exceptional hydrothermal stability and recoverability after sulfur poisoning. With uniform distribution of perovskite and Pt nanoparticles, the ZnO nanorod array retains its structure after long-time exposure to the reducing gaseous atmosphere. The H2 treatment induces the surface Mn4+ reduction to Mn3+ and subsequently to Mn2+, affecting oxygen mobility, and decreasing the amount of surface lattice oxygen on the LSMO perovskite, thus leading to the catalyst performance decay according to a Mars-Van Krevelen reaction mechanism. Such surface state change was found to be reversible, with the catalytic performance fully recovered after O2 re-treatment, making it suitable for NOx storage and reduction (NSR). The complete lean NOx traps are formulated with rationally-designed sequential BaCO3 and Pt loading, improving NOx storage capacity. The ZnO nanorod array supported LNT shows good water compatibility under simulated exhaust. The nanorod array catalysts may be potentially suitable for vehicular NOx emission control.

Copper manganese oxide (CuMn2O4) was introduced into the nanoarray-based monolithic catalysts system for advanced exhaust after-treatment. Through scalable and cost-effective hydrothermal reactions, nanosheet layers of copper manganese oxide were uniformly coated onto the manganese oxide nanoarrays (HM-PCR), which were grown on the cordierite honeycomb monoliths. The core nanoarray support, HM-PCR, a well-defined array architecture for active material deposition, contributed to an increase of open surface area and thus enhanced catalytic oxidation performance. The CuMn2O4 coated nanoarray-based catalyst, NA-CuMn2O4, shows efficient 90% propane (C3H8) conversion at around 400 °C, which is 50 °C and 75 °C lower than CuMn2O4 wash-coated catalyst (WC-CuMn2O4) and Pd loaded catalyst (WC-Pd), respectively. Compared to monolithic catalysts with a traditional alumina support, the benefit of nanoarray morphology was demonstrated by correlating the variation of surface area to the reactivity. The incorporation of cobalt ions was found to increase the specific surface area and thus enhance C3H8 conversion of CuMn2O4. The CuMn2O4/MnO2 nanoarray-based monoliths are promising types of emission control devices.

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.

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.

Significant reduction of noble metal catalyst loading and simplification of electrode fabrication are urgently needed in order to lower the cost of proton exchange membrane electrolyzer cells (PEMECs) for large-scale hydrogen production. Herein, we report an integrated electrode design comprising in-situ grown platinum nanowires (PtNW) on ultrathin titanium liquid/gas diffusion layers (LGDLs) via a cost-effective and green chemical synthesis approach. The ultrathin integrated PtNW electrodes showed a low cell voltage of 1.643 V and high efficiency of 90.08% at 1000 mA cm−2 using about 15 times lower catalyst loadings than a conventional catalyst-coated membrane in PEMEC tests. Ex-situ electrochemical characterizations and microscale visualizations further reveal that PtNW electrodes display highly efficient hydrogen evolution reactions and excellent electrode durability due to high active surface area, favorable bubble detachment, and structural stability. This work provides new insights into catalyst layer design and facile ultrathin electrode fabrication for more compact and low-cost PEM electrolyzers, fuel cells and other systems.

Reducing the mass transfer resistance globally of a catalyst is a key to enhancing the catalytic reaction kinetics and fully utilizing the catalyst activity. Despite the success in tailoring the external mass transfer in the widely studied washcoat monoliths, the internal mass transfer resistance is difficult to be reduced due to the requirement of increasing macroporosity while maintaining high specific surface area and mechanical stability. Therefore, nanostructured array-based monolithic catalysts (nanoarray catalysts) have been developed in the past decade as a promising class of structured catalysts that may complement or substitute washcoat catalysts. This work fundamentally elucidates the enhanced mass transport properties of the nanoarray monolithic catalysts by a combination of experimental measurements and theoretical modeling. Using a low-dimensional model, the relative contributions of resistances were quantified in terms of chemical kinetics, internal and external mass transfers based on a probe model of C2H4 oxidation over the TiO2 supported Pt-based monolithic catalysts. The nanoarray catalysts displayed a lower internal mass transfer resistance than the washcoat counterparts as a result of the high macroporosity and small thickness of nanoarray layers. The nanoarray configuration provides a new pathway towards designing high-performance monolithic reactors and catalysts with low internal diffusion limitations for various gas phase reactions.

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.

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.

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.

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.

Single phase materials with both spontaneous electric polarization and magnetization are rare, despite remarkable efforts in developing magnetoelectric multiferroics. In this work, a single-phase polycrystalline GdFe0.5Cr0.5O3 (GFCO) thin film was spin-coated onto a platinized silicon substrate. X-ray diffraction data suggest that the film exhibits an orthorhombic perovskite structure with a Pbnm space group. No other impurity phases were detected. Magnetization measurements reveal the Néel temperature of the GFCO film to be ∼220 K and illustrate a weak ferromagnetic component at 5 K, which could be due to spin canting. Frequency dependent ferroelectric–paraelectric transition was observed around 480 K, indicating the diffuse relaxor-like behavior. The electric field dependent polarization measurements show a lossy behavior below 200 K. The electric field dependent dielectric constant (tunability) measured at 1 MHz in a wide temperature range reveals that the tunability maximizes near the observed dielectric maxima, which further confirms the ferroelectric to paraelectric transition in the present film.

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

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 hydrothermal strategy combined with colloidal deposition synthesis was successfully used to grow ZnO/perovskite (LaBO 3 , B = Mn, Co, Ni) core–shell nanorod arrays within three dimensional (3-D) honeycomb cordierite substrates. A facile sonication assisted colloidal wash coating process is able to coat a uniformly dispersed perovskite nanoparticles onto the large scale ZnO nanorod arrays rooted on the channel surfaces of the 3D cordierite substrate achieved by hydrothermal synthesis. Compared to traditional wash-coated perovskite catalysts, an enhanced catalytic performance was observed for propane oxidation with 25 °C lower light-off temperature than powder-form coated perovskite catalyst of similar LaMnO 3 loading (4.3 mg). Temperature programmed reduction and desorption under H 2 and O 2 atmosphere, respectively, were used to study the reducibility and oxygen activity of these core–shell nanorod array based monolithic catalysts, revealing a catalytic activity sequence of LaCoO 3 > LaMnO 3 > La 2 NiO 4 at the initial stage of catalytic reaction. The good dispersion and size control in La-based perovskite nanoparticles and their interfaces to ZnO nanorod array support may contribute to the enhancement of catalytic performance. This work may provide a new type of Pt-group metals (PGM) free catalysts with improved catalytic performance for hydrocarbon oxidations at low temperature.

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.

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.

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.

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.

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.