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Feeding America Eastern Wisconsin (FAEW) distributed 81% more food to community members in need during the COVID-19 pandemic than in the prior year. Though systems were in place to manage food receipt and distribution data, social distancing requirements and technological barriers revealed inefficiencies in how these data were being utilized. In pursuit of data-driven decision making and in the context of a global pandemic, FAEW partnered with Marquette University data scientists through an industry supported grant. Applying newly learned skills in Business Intelligence, students have produced detailed reports of data cleanliness in FAEW’s source systems to improve underlying data quality and better support analytic efforts. Additionally, students have synchronized Human Centered Design Thinking and Visual Analytics to produce an interactive application that is being used to optimize inventory management, storage availability, and product distribution. Finally, students are utilizing Business Analytics techniques such as supervised and unsupervised data mining to provide new insights about food receipt and distribution patterns that will have a sustainable impact on FAEW operations. This unique partnership is providing experiential learning opportunities for students, tangible data science solutions that FAEW will use to ensure best practices, and real-world solutions to collaboratively end hunger in our communities.

Hydrogen detection devices based on gold-tin oxide/reduced graphene oxide (Au–SnO2/rGO) nanohybrids were fabricated by combining a hydrothermal method with sputter coating. The gas sensing performance of the Au–SnO2/rGO sensor was investigated under different concentrations of hydrogen from 0.04% to 1% at room temperature, which indicated a notable sensitive response even for 0.04% hydrogen. The activation energies of hydrogen adsorption/desorption were extracted via Arrhenius analysis which revealed the acceleration effect of gold dopants. This acceleration led to a faster response and recovery during hydrogen sensing. The activation energy analysis provided a more comprehensive understanding on the gas sensing mechanism. A hydrogen detection handheld device is demonstrated by integrating the sensor chip with a portable digital meter for direct readout of test results.

Expanded few-layer black phosphorus nanosheets (FL-BP NSs) were functionalized by branched polyethylenimine (PEI) using a simple noncovalent assembly to form air-stable overlayers (BP-PEI), and a Co3O4@BP-PEI composite was designed and synthesized using a hydrothermal method. The size of the highly dispersed Co3O4 nanoparticles (NPs) on the FL-BP NSs can be controlled. The BP-C5 (190 °C for 5 h) sensor, with 4–6 nm Co3O4 NPs on the FL-BP NSs, exhibited an ultrahigh sensitivity of 8.38 and a fast response of 0.67 s to 100 ppm of NOx at room temperature in air, which is 4 times faster than the response of the FL-BP NS sensor, and the lower detection limit reached 10 ppb. This study points to a promising method for tuning properties of BP-based composites by forming air-stable overlayers and highly dispersed metal oxide NPs for use in high-performance gas sensors.

Atomically thin black phosphorus (BP) field-effect transistors have excellent potential for sensing applications. However, commercial scaling of PFET sensors is still in the early stage due to various technical challenges, such as tedious fabrication, low response% caused by rapid oxidation, non-ideal response output (spike/bidirectional), and large device variation due to poor control over layer thickness among devices. Attempts have been made to address these issues. First, a theoretical model for response% dependence on the number of layers is developed to show the role of atomically thin BP for better responses. A position-tracked, selected-area-exfoliation method has been developed to rapidly produce thin BP layers with a narrow distribution (∼1–7 layers), which can harness excellent gate control over the PFET channel. The typical current on/off ratio is in the range of ∼300–500. The cysteine-modified Al2O3-gated PFET sensors show high responses (∼30–900%) toward a wide detection range (∼1–400 ppb) of lead ions in water with a typical response time of ∼10–30 s. A strategy to minimize device variation is proposed by correlating PFETs’ on/off ratio with sensitivity parameters. The thickness variation of the gate oxide is investigated to explain non-ideal and ideal response transient kinetics.

Expanded few-layer black phosphorus nanosheets (FL-BP NSs) were functionalized by branched polyethylenimine (PEI) using a simple noncovalent assembly to form air-stable overlayers (BP-PEI), and a Co

In this study, a buckypaper (BP), a foil of carbon nanotubes, with covalently bonded graphene leaves is prepared as a scaffold of pseudocapacitors. The capacitive electrodes are fabricated by an in situ growth of graphene on BP followed by electrochemical deposition of MnO2. The specific capacitance, energy/power-density, and cycle stability of the hybrid electrodes are explored by means of electrochemical impedance analysis, cyclic voltammetry measurements, and galvanostatic charge–discharge analysis. Due to the graphene interconnection between carbon nanotubes, the internal resistances of the electrodes decrease by a factor of ∼2. The MnO2/graphene/BP (MnO2/g-BP) hybrid electrode delivers a specific capacitance as high as 1022 F g−1 at a scan rate of 5 mV s−1, based on the mass of MnO2. It exhibits excellent rate capability and cycle stability. The critical role of the covalently bonded graphene leaves in enhancing charge transport is further clarified by comparison with the electrodes without them. Therefore, this study provides a new understanding for improving the performance of metal oxide-based electrochemical supercapacitors and can be generalized for designing next-generation high-performance energy-storage devices.

Meeting the increasing demand for sensors with high sensitivity, high selectivity, and rapid detection presents many challenges. In the last decade, electronic sensors based on field-effect transistors (FETs) have been widely studied due to their high sensitivity, rapid detection, and simple test procedure. Among these sensors, two-dimensional (2D) nanomaterial-based FET sensors have been demonstrated with tremendous potential for the detection of a wide range of analytes which is attributed to the unique structural and electronic properties of 2D nanomaterials. This comprehensive review discusses the recent progress in graphene-, 2D transition metal dichalcogenide-, and 2D black phosphorus-based FET sensors, with an emphasis on rapid and low-concentration detection of gases, biomolecules, and water contaminants.

Real-time lead ion monitoring for drinking water is in an urgent demand, due to the high biotoxicity of lead. We fabricated a reduced graphene oxide (rGO) percolation network based field-effect transistor (FET) by using an easy and scalable micromolding-in-capillary method for lead ion detection in water. The percolation theory analysis elucidates that the required GO mass concentration for a 2D continuum connection converges at a predictable value. Guided by the theoretical analysis, the prepared rGO network was constructed with 1–4 layers of rGO flakes and exhibits comparable electrical properties with single-layer rGO devices. A thin Al2O3 layer was deposited on the device to isolate the analyte from the FET device. With the specific L-Glutathione reduced (GSH) probe, the sensor can reach a limit of detection (LOD) in ppb-level to lead ions. In addition, good selectivity and the high sensing response to Pb2+ concentrations around 15 ppb (maximum contaminant level of lead for drinking water, US Environmental Protection Agency) suggest our sensor holds great potential for lead ion monitoring in drinking water.

Graphene coupled with nanocrystals (NCs) represents a new type of hybrid nanostructure that has attracted wide attention in energy and sensing applications. The interaction between graphene and NCs provides the hybrids with additional properties, offering rich opportunities to tune the material structure and properties. This Perspective highlights some recent progress in the research on graphene–NC hybrid structures with a focus on their energy storage/conversion and sensing applications. The structural characteristics of graphene–NC hybrids and the advantages of coupling NCs with graphene are demonstrated and discussed. Recent studies have shown the great potential of graphene–NC hybrids to improve the performance of energy storage/conversion devices (e.g., Li ion batteries, supercapacitors, fuel cells, and solar cells) and sensing devices (e.g., chemical sensors, biosensors, and water sensors). Further understanding and development of graphene–NC hybrids could therefore help address the demand for new e...

A facile and reliable method has been developed to synthesize nanohybrids of anatase TiO2 nanoparticles-decorating carbon nanotubes (CNTs@TiO2). The nanohybrid displays a unique feature of having CNTs encapsulated inside and anatase TiO2 nanoparticles coating on the CNT surface. Comprehensive characterizations suggest that CNTs@TiO2 nanohybrids exhibit unique properties of CNTs and TiO2 nanoparticles with one-dimensional structure, excellent electrical conductivity, high surface area, and good biocompatibility. The electrochemical properties of CNTs@TiO2 nanohybrids were systematically investigated, it was revealed that the CNTs@TiO2 nanohybrids showed great promise as anode materials of microbial fuel cells (MFCs). Compared with pure TiO2 nanoparticles and CNTs alone, CNTs@TiO2 nanohybrids exhibited a much higher output current, power density, and Coulombic efficiency when used as anode materials of MFCs. The as-developed synthetic route opens up a new avenue for designing CNTs-based nanohybrid materials for various applications benefiting from the synergetic effect among constituents.

This work describes a promising strategy for large-scale fabrication of silicon (Si) nanotubes. The process began with preparation of silica nanotubes using rod-like NiN2H4 as a template and the resulting silica nanotubes were then converted to Si nanotubes by a thermal reduction process assisted with magnesium powder. The electrochemical properties of Si nanotubes were investigated as anode of lithium-ion batteries. It was demonstrated that the as-developed Si nanotubes showed significantly improved rate capability and long-term cycling performance compared with commercial silicon meshes. Keywords: Silicon nanotubes, Lithium-ion batteries, Anode

A semi-empirical equation is derived to provide a correlation between the ozone generation rate of a negative wire-to-plate corona discharge in both dry and humid air and a series of design/operating parameters. A basic correlation is first derived by applying dimensional analysis on negative wire-to-plate corona discharge in dry air. Further development on the basic correlation is carried out by integrating the influence of humidity. The derived equation is validated by previously reported experimental data and numerical model. The new semi-empirical equation is comprehensive and useful in guiding the design/operation of indoor corona devices under actual ambient operating conditions.

We report on a one-step binder-free fabrication method for electrochemical double layer (EDL) capacitor electrodes consisting of vertically-oriented graphene uniformly grown on a metallic current collector. The double-layer capacitive behavior of the resulting electrode is studied in both aqueous and organic electrolytes. Compared with conventional graphene-based EDL capacitor electrode fabrication methods, this method offers the following advantages: (a) no need to use a binder, (b) open channels for better ion access, and (c) exposed edge planes for improved material wettability. These unique features lead to excellent capacitive behavior in organic electrolytes, including a specific capacitance slightly higher than that in aqueous electrolytes at the same potential scan rate and a high knee frequency (∼3174 Hz in the current work).

We report on the ordered assembly of sorted single-walled carbon nanotubes (SWCNTs) (95% semiconducting tubes) by drying aqueous droplets on meshed grids with an enhanced evaporation rate. Besides the commonly observed "coffee ring" deposit of aligned SWCNTs on the droplet contact line after water evaporation, tubes self-organized in the central area of a grid and covered up to -1/3 of the whole surface area of a grid. In addition, parallel-aligned and straightened SWCNTs were seen to span across cracks in the SWCNT-surfactant film. The evaporation-driven assembly of sorted SWCNTs has the potential to produce ordered SWCNT structures that are attractive for the fabrication of electronic devices comprising mostly semiconducting or metallic tubes.

We report a selective, room-temperature NH(3) gas-sensing platform with enhanced sensitivity, superfast response and recovery, and good stability, using Ag nanocrystal-functionalized multiwalled carbon nanotubes (Ag NC-MWCNTs). Ag NCs were synthesized by a simple mini-arc plasma method and directly assembled on MWCNTs using an electrostatic force-directed assembly process. The nanotubes were assembled onto gold electrodes with both ends in Ohmic contact. The addition of Ag NCs on MWCNTs resulted in dramatically improved sensitivity toward NH(3). Upon exposure to 1% NH(3) at room temperature, Ag NC-MWCNTs showed enhanced sensitivity (~9%), very fast response (~7 s), and full recovery within several minutes in air. Through density functional theory calculations, we found that the fully oxidized Ag surface plays a critical role in the sensor response. Ammonia molecules are adsorbed at Ag hollow sites on the AgO surface with H pointing toward Ag. A net charge transfer from NH(3) to the Ag NC-MWCNTs hybrid leads to the conductance change in the hybrid.

We report significant enhancement of CuO nanowire (NW) sensing performance at room temperature through the surface functionalization with SnO(2) nanocrystals (NCs). The sensitivity enhancement can be as high as ∼300% for detecting 1% NH(3) diluted in air. The improved sensitivity could be attributed to the electronic interaction between p-type CuO NWs and n-type SnO(2) NCs due to the formation of nanosized p-n junctions, which are highly sensitive to the surrounding gaseous environment and could effectively manipulate local charge carrier concentration. Our results suggest that the NC-NW structure is an attractive candidate for practical sensing applications, in view of its outstanding room-temperature sensitivity, excellent dynamic properties (rapid response and quick recovery), and flexibility in modulating the sensing performance (e.g., by adjusting the coverage of SnO(2) NCs on CuO NWs and doping of SnO(2) NCs).

Hybrid nanomaterials composed of carbon nanotubes (CNTs) and graphene could potentially display outstanding properties that are superior to either CNTs or graphene alone. However, the inherent CNT–graphene loose junctions present in the CNT–graphene composites synthesized by existing methods significantly hinder the realization of the full potential held by CNT–graphene hybrids. In this letter, we report on a brand-new, three-dimensional (3D) carbon nanostructure comprising few-layer graphene (FLG) sheets inherently connected with CNTs through sp2 carbons, resembling plant leaves (FLGs) growing on stems (CNTs). The resulting hybrid nanostructures were characterized using scanning electron microscopy, transmission electron microscopy (TEM), Raman spectroscopy, and X-ray photoelectron spectroscopy. The evolution of FLG sheets on CNTs was tracked by high-resolution TEM. Distinct from a random mixture of CNTs and graphene sheets (CNT+G) suffering from poor CNT–graphene contacts, our CNT–FLG structure has intr...

We report the fabrication of a highly sensitive field-effect transistor (FET) biosensor using thermally-reduced graphene oxide (TRGO) sheets functionalized with gold nanoparticle (NP)-antibody conjugates. Probe antibody was labeled on the surface of TRGO sheets through Au NPs and electrical detection of protein binding (Immunoglobulin G/IgG and anti-Immunoglobulin G/anti-IgG) was accomplished by FET and direct current (dc) measurements. The protein binding events induced significant changes in the resistance of the TRGO sheet, which is referred to as the sensor response. The dependence of the sensor response on the TRGO base resistance in the sensor and the antibody areal density on the TRGO sheet was systematically studied, from which a correlation of the sensor response with sensor parameters was found: the sensor response was more significant with larger TRGO base resistance and higher antibody areal density. The detection limit of the novel biosensor was around the 0.2 ng/mL level, which is among the best of reported carbon nanomaterial-based protein sensors and can be further optimized by tuning the sensor structure. Open image in new window

Synthesis of carbon nanowalls (CNWs) at atmospheric pressure is realized by using a negative normal glow discharge, which differs from prior low-pressure plasma-enhanced chemical vapor deposition methods for CNW growth and holds great potential for mass-production of CNWs. The glow discharge is characterized for electrical and optical properties. The resulting CNW structures are analyzed using transmission electron microscopy, scanning electron microscopy, and Raman spectroscopy. The CNW growth process is examined for effects of discharge regime, discharge duration (growth time), hydroxyl radicals (water vapor), and local current density. Possible CNW growth mechanism and critical process parameters to the CNW growth and evolution are identified. OH radicals play an essential role in the initial nucleation process, but excess OH radicals accelerate the oxidation of CNWs. For a fixed growth time, there exists an optimum feed gas relative humidity and an optimum current density (∼40% and ∼9.17 A/m2, respectively, in the current work) for the growth of large and less defective CNWs with a high degree of crystallinity. Results obtained in this study can be used to tailor final properties of the as-grown CNWs for various applications.

Graphene has attracted growing interest in the past few years. Growing vertically oriented graphene sheets with a designed pattern is practically attractive for device applications based on graphene. Here we report a patterned synthesis of vertical graphene nanosheets using plasma-enhanced chemical vapor deposition. Both experimental and modeling results suggest that the electric field distribution above the substrate material plays a key role in the graphene coverage. Vertical graphene patterns can thus be designed through artificially designing the surface electric field distribution. A field-effect transistor (FET) sensor device has been demonstrated for detection of low-concentration gases using vertically patterned graphene sheets bridging a metal electrode gap.

The growth of CuO nanowires (NWs) through direct oxidation of copper is widely utilized. We give further evidence of a short-circuit, grain boundary diffusion mechanism. First, we show enhanced CuO NW growth through oxidizing nanocrystalline Cu. Second, we show the presence of a bi-crystal structure with a Cu rich (1 1 −2)/(0 0 −1) boundary along the entire length of the NW. Our analysis suggests that the growth of CuO NWs occurs via the short-circuit diffusion of Cu ions across the Cu 2 O layer, followed by short-circuit diffusion along the CuO NW bi-crystal grain boundary and to the NW tip, where subsequent oxidation occurs.

Stimuli-responsive field-effect transistors (FETs) based on 2D nanomaterials have been considered as attractive candidates for sensing applications due to their rapid response, high sensitivity, and real-time monitoring capabilities. Here we report on an impedance spectroscopy technique for FET sensor applications with ultra-high sensitivity and good reproducibility. An alumina-gated FET, using an ultra-thin black phosphorus flake as the channel material, shows significantly improved stability and ultra-high sensitivity to lead ions in water. In addition, the phase angle in the low frequency region was found to change significantly in the presence of lead ion solutions, whereas it was almost unchanged in the high frequency region. The dominant sensing performance was found at low frequency phase spectrum around 50 Hz and a systematic change in the phase angle in different lead ion concentrations was found. Applying the impedance spectroscopy technique to insulator-gated FET sensors could open a new avenue for real-world sensor applications.

Novel nanomaterials, such as nanowires and carbon nanotubes (CNTs), have attracted considerable attention in electrical detection of chemical and biological species for clinical diagnosis and practical pharmaceutical applications during the past decade. [ 1–3 ] Electrical detection of biomolecules using nanomaterials can often achieve high sensitivity because nanomaterials are extremely sensitive to electronic perturbations in the surrounding environment. By using CNTs and CNT-based fi eldeffect transistors (FETs), biosensors have been demonstrated for the detection of protein binding [ 4–7 ] and DNA hybridization events. [ 8 , 9 ] The detection limit of reported CNT protein sensors is normally at 0.1–10 nM level, [ 2 , 5 , 10 ] and an improved detection limit could reach 1 ng/ml through cleaving the protein using an enzyme. [ 7 ] Although CNT devices are promising candidates for biosensors with high sensitivity, the variation in the device characteristics is an obstacle to the device reliability and the device sensitivity is still limited by surface area and electrical properties of CNTs. Graphene, a single layer of carbon atoms in a two-dimensional honeycomb lattice, has potential applications in the electrical detection of biological species due to their unique physical properties. [ 11–13 ] Intrinsic graphene is a zero-gap semiconductor that has remarkably high electron mobility ( ∼ 15 000 cm 2 ⋅ V − 1 ⋅ s − 1 ) at room temperature, [ 12 ] which is even higher than that of CNTs. [ 14 ] Although graphene has been explored for various applications, [ 15–26 ] there are only limited reports on graphenebased biosensors until recently. [ 27–33 ] For instance, large-sized graphene fi lm FETs were fabricated for the electrical detection of DNA hybridization; [ 27 ] graphene oxide (GO) was used in single-bacterium and label-free DNA sensors. [ 29 ] In addition, electrolyte-gated graphene FETs for electrical detection of pH and protein adsorption were reported. [ 30 ] Despite the sparse demonstration of graphene for biosensing applications, graphene-based FETs have not been reported for detection of protein binding (antibody to antigen) events. Because the carrier

This study demonstrates the electrical detection of protein binding by the introduction of Au nanoparticle–antibody conjugates in a carbon nanotube field effect transistor (CNTFET), in which the nanoparticle-functionalized carbon nanotube serves as the electrical conducting channel. Antibody (anti-horseradish peroxidase) and antigen (horseradish peroxidase) binding events lead to the amplitude change in the drain current, which can be sensitively detected by FET measurements. The sensor shows negligible response to mismatched proteins such as Immunoglobulin G (IgG), confirming the specificity of the biosensor. The reported CNTFET-based biosensor could be adapted to detect a variety of proteins for in vitro diagnostics.

In this article, we introduce and provide details on a large-scale, cost-effective pathway to fabricating ultrahigh dense CuO nanowire arrays by thermal oxidation of Cu substrates in oxygen ambient. The CuO nanowires that are produced at ∼500 °C for ∼150 min feature an average length and diameter of ∼15 μm and ∼200 nm, respectively. The room temperature device-related characteristics such as transport, analyte detection and opto-electronic response of individual CuO nanowires have been probed by fabricating single CuO nanowire devices with the use of lift-off photolithographical techniques. The experiments confirm that as-grown nanowires are of p-type, have a band gap of ∼1.4 eV, and show strong sensitivity to both NO2 and NH3 gases. The devices also showed strong response to white light with device responsivity approaching ∼8 A/W for optical power densities of only ∼1 mW/cm2. Additionally, a complex interaction of photoproduced electron−hole pairs with the surface-originating chemisorbed agents including...

Ag nanoparticles with different loadings were systematically coated onto uniformly aligned TiO2 nanorod arrays. The enhanced photocatalytic decay rates were found to increase linearly with the Ag nanoparticle load until a maximum photodecay rate was reached at 0.25 wt.% Ag loading, and then decrease almost linearly with further increasing Ag loading. A simple reaction model based on the Ag nanoparticle coverage and charge trapping is proposed, and the predicted results agree very well with the experimental results quantitatively. The effective lifetime of photogenerated holes due to the trapping of the Ag nanoparticles on the surface of TiO2 nanorods was estimated to be approximately three times that of pure TiO2 nanorods.

One-dimensional (1D) tungsten oxide crystals (wires, wire bundles, and tubes) were synthesized by a catalyst-free, physical evaporation–condensation process using a mini-arc plasma source. The growth process of the 1D tungsten oxide structure was deciphered through sequential scanning electron microscopy observations. Crystalline tungsten oxide grains form first and then rotate and align their positions to build up 1D crystals as wires, wire bundles, and tubes to minimize the surface energy because of the anisotropic nature of the tungsten oxide crystal. The influence of temperature and growth time on the crystal synthesis is analyzed.

A new gas-sensing platform for low-concentration gases (NO{sub 2}, H{sub 2}, and CO) comprises discrete SnO{sub 2} nanocrystals uniformly distributed on the surface of multiwalled carbon nanotubes (CNTs). The resulting hybrid nanostructures are highly sensitive, even at room temperature, because their gas sensing abilities rely on electron transfer between the nanocrystals and the CNTs.

Facile dry decoration of graphene oxide sheets with aerosol Ag nanocrystals synthesized from an arc plasma source has been demonstrated using an electrostatic force directed assembly technique at room temperature. The Ag nanocrystal-graphene oxide hybrid structure was characterized by transmission electron microscopy (TEM) and selected area diffraction. The ripening of Ag nanocrystals on a graphene oxide sheet was studied by consecutive TEM imaging of the same region on a sample after heating in Ar at elevated temperatures of 100 °C, 200 °C, and 300 °C. The average size of Ag nanocrystals increased and the number density decreased after the annealing process. In particular, migration and coalescence of Ag nanocrystals were observed at a temperature as low as 100 °C, suggesting a van der Waals interaction between the Ag nanocrystal and the graphene oxide sheet. The availability of affordable graphene-nanocrystal structures and their fundamental properties will open up new opportunities for nanoscience and nanotechnology and accelerate their applications.

Aerosol nanoparticles draw great attention due to their unique physico-chemical properties that can be harnessed for innovative applications as well as for their potential impact on public health and the environment. Microscopic characterization of these particles is critically important to understand the unique particle properties and their interaction with biological systems. Here we report on a simple and efficient method for size, morphology, and crystallinity analyses of aerosol nanoparticles through transmission electron microscopy (TEM). The method uses carbon nanotubes (CNTs) as the particle support on conventional TEM grids. The CNT support significantly improves the quality of high resolution TEM (HRTEM) imaging due to the unique properties of CNTs, the high probability of particles sitting at the edge of CNTs, and the minimization of the interference interaction between the electron beam and the supporting substrate. The improved HRTEM images are useful for detailed characterization of nanoparticles, and particularly suitable for analyzing core-shell nanoparticles.

Tin oxide (SnO2) nanocrystals of a few nanometers are of great interest for electronic applications. Here we present a mini-arc plasma method to produce aerosol tin oxide nanocrystals at atmospheric pressure. The product SnO2 nanocrystals are then assembled onto the external surface of carbon nanotubes (CNTs) to form hybrid SnO2–CNT structures. The absorption properties of both the SnO2 nanocrystals and the SnO2–CNT hybrid structures have been characterized. Quantum size effects have been observed for as-produced SnO2 nanocrystals. The intrinsic nanoparticle size selection during the assembly process results in a blue shift of the absorption spectrum for hybrid nanostructures.

A facile dry route electrostatic-force-directed assembly (ESFDA) was used to deposit silver nanocrystals onto the external surface of vertically aligned multiwalled carbon nanotubes (MWCNTs). Charged and nonagglomerated aerosol silver nanocrystals were first produced using a mini-arc plasma source. The nanocrystals were then assembled onto the electrically biased MWCNT array through the enhanced electric field near the CNT surface. The electrostatic field plays a crucial role in the assembly process. Gradients of the nanocrystal size and the nanocrystal areal density along the length of the CNT have been observed, and these are likely due to the variation of the electric field near the CNT surface. The ESFDA technique enables the in situ coating of CNT arrays with nanocrystals to create novel hybrid nanomaterials with tailored properties.

An electrostatic-force-directed-assembly technique was used to coat multiwalled carbon nano-tubes (MWCNTs) with aerosol Ag nanoparticles produced from a mini-arc plasma source. The deposition of Ag nanoparticles onto CNTs was confirmed by transmission electron microscopy (TEM), high-resolution TEM, scanning electron microscopy, and X-ray photoelectron spectroscopy. Ripening of Ag nanoparticles on CNTs was observed via successive TEM imaging after heating the nanoparticle–nanotube hybrid structures in air to three different temperatures ranging from 100°C to 300°C. With temperatures at and above 200°C, the areal density of Ag nanoparticles decreased and the average particle size increased. In particular, migration and coalescence of Ag nanoparticles have been observed at this relatively low temperature, which suggests a van der Waals nanoparticle–nanotube interaction.

This article reviews the research on the use of graphene and related materials in the photoanode of dye-sensitized solar cells (DSSCs). Graphene-based materials, such as pristine graphene, graphene oxide, and reduced graphene oxide, have properties attractive for various components of the DSSC photoanode. We first provide a brief introduction to graphene properties and analyze requirements for making a high-performance photoanode. Then, we introduce applications of graphene-based materials in each part of the DSSC photoanode, i.e., the transparent conducting electrode, the sensitizing material, and the semiconducting layer. Particularly, we discuss how the incorporation of graphene-based materials in those components can enhance the photoanode performance. It is clear that the outstanding properties of graphene, such as the fast electron transfer ability, high Young’s modulus, and good transparency, benefit DSSC photoanode research, and doping or surface modifications of graphene nanosheets with other materials can also improve the photoanode and, thus, the resulting cell performance. Finally, we present an outlook for current issues and further trends for using graphene materials in DSSC photoanodes.

Nanocrystals in the lower-nanometer-size range are attracting growing interest due to their unique properties. A simple and versatile atmospheric direct current mini-arc plasma source has been developed to produce nanoparticles as small as a few nanometers. The nanoparticles are formed by direct vaporization of solid precursors followed by a rapid quenching. Both semiconductor tin oxide and metallic silver nanoparticles have been produced at rates of 1–10 mg/h using the mini-arc source. Transmission electron microscopy and X-ray diffraction analyses indicate that most nanoparticles as produced are nonagglomerated and crystalline. Size distributions of nanoparticles measured with an online scanning electrical mobility spectrometer are broader than the self-preserving distribution, suggesting that the nanoparticle growth is coagulation-dominated, and that the particles experience a range of residence times. The electrical charges carried by as-produced aerosol nanoparticles facilitate the manipulation of nanoparticles. The new mini-arc plasma source hence shows promise to accelerate the exploration of nanostructured materials.

We present an efficient gas-phase route to decorate both single-walled and multiwalled carbon nanotubes with nanoparticles based on electrostatic force directed assembly. We show that, in addition to an intrinsic nanoparticle size selection, the packing density and the final size distribution of nanoparticles docking on nanotubes can be controlled during the assembly process. Our method enables in situ coating of nanotubes with nanoparticles. Due to the inherent material-independent nature of the electrostatic force, various compositions of such nanoparticle–nanotube structures can be produced using this new technique. These novel structures will enable new opportunities for exploring nanoscience, nanotechnology, and biotechnology.

Energy storage devices not only are widely used in nearly all aspects of our daily lives, but also become an essential necessity for the development of renewable energy, which features intermittency. Many researchers around the world are working to improve the energy density, power density, safety, and charging speed of energy storage devices to meet the ever-increasing demands of storing electricity. In this context, extensive research has been and is being carried out on improving the performance of the two major electrochemical energy storage devices, namely batteries and supercapacitors. Traditional rechargeable batteries have shortcomings such as high cost, short life, long charging time, low power density, and potential environmental issues. On the other hand, electrochemical capacitors (i.e., the so-called supercapacitors or ultracapacitors) show great potential in energy storage because of excellent charge/discharge rates, long cycle life, and environmental friendliness. Vertically-oriented graphene (VG) has many characteristics that are ideal for building supercapacitors with improved performance and has been investigated as an electrode material for supercapacitors. In this chapter, supercapacitors using VG-based materials as electrodes are introduced and discussed.

Vertically-oriented graphenes (VGs) have outstanding electrical conductivity, robust binding with the substrate, and high chemical tolerance in electrolytes. These features of VGs offer great promise in electrochemical energy storage and conversion devices. Following our discussion in the previous chapter on the application of VGs as the electrode materials in supercapacitors, in this chapter we introduce the potential of VGs in other energy storage and conversion devices, specifically, rechargeable batteries, fuel cells, and dye-sensitized solar cells (DSSCs). Without the use of any binder and conductive additives, VGs grown on current collectors (e.g., metal foils) can directly serve as the anode in lithium-ion batteries (LIBs) and the electrode in vanadium redox flow batteries (VRFBs), which can significantly simplify the battery manufacturing process. More important, batteries using VG electrodes have shown improved performance. For energy conversion applications, VGs can be used in fuel cells and DSSCs. The unique features of VGs make them an ideal catalyst support for anode reactions in a fuel cell. In addition to working as the catalyst support, VGs with a controlled number of oxygen functional groups can work as catalysts themselves for DSSCs.

The unique properties of vertically-oriented graphene (VG) are discussed in this chapter. VG is intrinsically graphene, but it also possesses unique structural features, i.e., being arranged perpendicularly to the substrate surface. Therefore, VG possesses not only the properties of graphene but also some unique characteristics induced by its oriented arrangement. We start this chapter with a brief introduction of some general properties of graphene, which is deemed reasonable and necessary before we elaborate on the uniqueness of VG. To illustrate the attractive characteristics of VG, we compare VG with planar (or horizontal) graphene structures and emphasize the benefits that can be brought about due to VG’s vertical orientation. The unique properties of VG are summarized at the end of this chapter. Understanding of the VG’s uniqueness is critical to appreciating why VG has attracted so much interest and is also an essential step toward tailoring VG properties for various applications.

The plasma-enhanced chemical vapor deposition (PECVD) is an effective method for producing vertically-oriented graphene (VG) sheets, but it is also a very complex one because of the complexity associated with the plasma chemistry. In addition to the type of plasma sources discussed in Chap. 3, the morphology and structure of the PECVD-produced VG sheets are also strongly affected by a set of operating parameters, including precursors (e.g., feedstock gas type and composition, plasma gas type), the substrate temperature, and the operating pressure. In this chapter, we discuss two important operating parameters for the synthesis of VG in the PECVD process, specifically precursor and temperature.

Vertically-oriented graphene (VG) sheets have the potential for many exciting energy and environmental applications due to their unique morphology and properties. However, it is necessary to realize the large-scale synthesis of this novel material at a low cost before we can make it economically viable for its widespread applications. It is theoretically suggested and experimentally proven that the productivity of VG is dependent on pressure, and atmospheric pressure growth could improve the efficiency and lower the cost of VG production. In the first part of this chapter, we examine how the operating pressure affects the VG production in the plasma-enhanced chemical vapor deposition (PECVD). It is also important to have the capability to grow VG on different substrates so that VG can be integrated into devices/systems for specific applications. Fortunately, the successful synthesis of VG on different substrates in PECVD processes has been realized, which is introduced in the second part of this chapter.

The plasma-enhanced chemical vapor deposition (PECVD) method is a key method for synthesizing vertically-oriented graphene (VG). Because the plasma region provides active species (e.g., energetic electrons, excited molecules and atoms, free radicals, and photons), PECVD offers several advantages in nanostructure synthesis, e.g., a relatively low substrate temperature, a high growth selectivity, and good control in nanostructure ordering/patterning. These features make PECVD the most suitable method for VG growth. On the other hand, the growth of VG using PECVD is a quite complex process due to the complexity of plasma chemistry. The morphology and structure of the VG sheets produced by PECVD are strongly dependent on the types of plasma sources and a series of operating parameters, such as feedstock gas type and composition, the substrate temperature, and the operating pressure. In this chapter, we first discuss the growth mechanism of VG in a PECVD process and then discuss how plasma sources affect the VG growth. Characterization of PECVD-produced VG from various plasma sources using Raman spectroscopy, a powerful tool to study carbon nanostructures, is also discussed in this chapter.