High-Performance Battery-Type Supercapacitors Based on Self-Oriented Growth of Nanorods/Nanospheres Composite Assembled on Self-Standing Conductive GO/CNF Frameworks.

MnOx-based materials have limited capacity and poor conductivity over various voltages, hampering their potential for energy storage applications. This work proposes a novel approach to address these challenges. A self-oriented multiple-electronic structure of a 1D-MnO2-nanorod/2D-Mn2O3-nanosphere composite was assembled on 2D-graphene oxide nanosheet/1D-carbon nanofiber (GO/CNF) hybrids. Aided by K+ ions, the MnO2 nanorods were partially converted to Mn2O3 nanospheres, while the GO nanosheets were combined with CNF through hydrogen bonds resulting in a unique double binary 1D-2D mixed morphology of MnO2/Mn2O3-GO/CNF hybrid, having a novel mechanism of multiple Mn ion redox reactions facilitated by the interconnected 3D network. The morphology of the MnO2 nanorods was controlled by regulating the potassium ion content through a rinsing strategy. Interestingly, pure MnO2 nanorods undergo air-annealing to form a mixture of nanorods and nanospheres (MnO2/Mn2O3) with a distinct morphology indicating pseudocapacitive surface redox reactions involving Mn2+, Mn3+, and Mn4+. In the presence of the GO/CNF framework, the charge storage properties of the MnO2/Mn2O3-GO/CNF composite electrode show dominant battery-type behavior because of the unique mesoporous structure with a crumpled morphology that provides relatively large voids and cavities with smaller diffusion paths to facilitate the accumulation/intercalation of charges at the inner electroactive sites for the diffusion-controlled process. The corresponding specific capacity of 800 C g-1 or 222.2 mAh g-1 at 1 A g-1 and remarkable cycling stability (95%) over 5000 cycles at 3 A g-1 were considerably higher than those of the reported electrodes of similar materials. Moreover, a hybrid supercapacitor device is assembled using MnO2/Mn2O3-GO/CNF as the positive electrode and activated carbon as the negative electrode, which exhibits a superior maximum energy density (∼25 Wh kg-1) and maximum power density (∼4.0 kW kg-1). Therefore, the as-synthesized composite highlights the development of highly active low-cost materials for next-generation energy storage applications.

Green syntheses of graphene and its applications in internet of things (IoT)-a status review.

Internet of Things (IoT) is a trending technological field that converts any physical object into a communicable smarter one by converging the physical world with the digital world. This innovative technology connects the device to the internet and provides a platform to collect real-time data, cloud storage, and analyze the collected data to trigger smart actions from a remote location via remote notifications, etc. Because of its wide-ranging applications, this technology can be integrated into almost all the industries. Another trending field with tremendous opportunities is Nanotechnology, which provides many benefits in several areas of life, and helps to improve many technological and industrial sectors. So, integration of IoT and Nanotechnology can bring about the very important field of Internet of Nanothings (IoNT), which can re-shape the communication industry. For that, data (collected from trillions of nanosensors, connected to billions of devices) would be the 'ultimate truth', which could be generated from highly efficient nanosensors, fabricated from various novel nanomaterials, one of which is graphene, the so-called 'wonder material' of the 21st century. Therefore, graphene-assisted IoT/IoNT platforms may revolutionize the communication technologies around the globe. In this article, a status review of the smart applications of graphene in the IoT sector is presented. Firstly, various green synthesis of graphene for sustainable development is elucidated, followed by its applications in various nanosensors, detectors, actuators, memory, and nano-communication devices. Also, the future market prospects are discussed to converge various emerging concepts like machine learning, fog/edge computing, artificial intelligence, big data, and blockchain, with the graphene-assisted IoT field to bring about the concept of 'all-round connectivity in every sphere possible'.

Graphene and its derivatives as biomedical materials: future prospects and challenges.

Graphene and its derivatives possess some intriguing properties, which generates tremendous interests in various fields, including biomedicine. The biomedical applications of graphene-based nanomaterials have attracted great interests over the last decade, and several groups have started working on this field around the globe. Because of the excellent biocompatibility, solubility and selectivity, graphene and its derivatives have shown great potential as biosensing and bio-imaging materials. Also, due to some unique physico-chemical properties of graphene and its derivatives, such as large surface area, high purity, good bio-functionalizability, easy solubility, high drug loading capacity, capability of easy cell membrane penetration, etc., graphene-based nanomaterials become promising candidates for bio-delivery carriers. Besides, graphene and its derivatives have also shown interesting applications in the fields of cell-culture, cell-growth and tissue engineering. In this article, a comprehensive review on the applications of graphene and its derivatives as biomedical materials has been presented. The unique properties of graphene and its derivatives (such as graphene oxide, reduced graphene oxide, graphane, graphone, graphyne, graphdiyne, fluorographene and their doped versions) have been discussed, followed by discussions on the recent efforts on the applications of graphene and its derivatives in biosensing, bio-imaging, drug delivery and therapy, cell culture, tissue engineering and cell growth. Also, the challenges involved in the use of graphene and its derivatives as biomedical materials are discussed briefly, followed by the future perspectives of the use of graphene-based nanomaterials in bio-applications. The review will provide an outlook to the applications of graphene and its derivatives, and may open up new horizons to inspire broader interests across various disciplines.

"Electro-Typing" on a Carbon-Nanoparticles-Filled Polymeric Film using Conducting Atomic Force Microscopy.

Next-generation electrical nanoimprinting of a polymeric data sheet based on charge trapping phenomena is reported here. Carbon nanoparticles (CNPs) (waste carbon product) are deployed into a polymeric matrix (polyaniline) (PANI) as a charge trapping layer. The data are recorded on the CNPs-filled polyaniline device layer by "electro-typing" under a voltage pulse (VET , from ±1 to ±7 V), which is applied to the device layer through a localized charge-injection method. The core idea of this device is to make an electrical image through the charge trapping mechanism, which can be "read" further by the subsequent electrical mapping. The density of stored charges at the carbon-polyaniline layer, near the metal/polymer interface, is found to depend on the voltage amplitude, i.e., the number of injected charge carriers. The relaxation of the stored charges is studied by different probe voltages and for different devices, depending on the percolation of the CNPs into the PANI. The polymeric data sheet retains the recorded data for more than 6 h, which can be refreshed or erased at will. Also, a write-read-erase-read cycle is performed for the smallest "bit" of stored information through a single contact between the probe and the device layer.

Improved electrochemical properties of morphology-controlled titania/titanate nanostructures prepared by in-situ hydrothermal surface modification of self-source Ti substrate for high-performance supercapacitors.

ABSTARCT: Ti substrate surface is modified into two-dimensional (2D) TiO2 nanoplatelet or one-dimensional (1D) nanorod/nanofiber (or a mixture of both) structure in a controlled manner via a simple KOH-based hydrothermal technique. Depending on the KOH concentration, different types of TiO2 nanostructures (2D platelets, 1D nanorods/nanofibers and a 2D+1D mixed sample) are fabricated directly onto the Ti substrate surface. The novelty of this technique is the in-situ modification of the self-source Ti surface into titania nanostructures, and its direct use as the electrochemical microelectrode without any modifications. This leads to considerable improvement in the interfacial properties between metallic Ti and semiconducting TiO2. Since interfacial states/defects have profound effect on charge transport properties of electronic/electrochemical devices, therefore this near-defect-free interfacial property of Ti-TiO2 microelectrode has shown high supercapacitive performances for superior charge-storage devices. Additionally, by hydrothermally tuning the morphology of titania nanostructures, the electrochemical properties of the electrodes are also tuned. A Ti-TiO2 electrode comprising of a mixture of 2D-platelet+1D-nanorod structure reveals very high specific capacitance values (~7.4 mF.cm-2) due to the unique mixed morphology which manifests higher active sites (hence, higher utilization of the active materials) in terms of greater roughness at the 2D-platelet structures and higher surface-to-volume-ratio in the 1D-nanorod structures.

Oxygen Vacancy-Induced Structural, Optical, and Enhanced Supercapacitive Performance of Zinc Oxide Anchored Graphitic Carbon Nanofiber Hybrid Electrodes.

Zinc oxide (ZnO) nanoparticles (NPs) anchored to carbon nanofiber (CNF) hybrids were synthesized using a facile coprecipitation method. This report demonstrates an effective strategy to intrinsically improve the conductivity and supercapacitive performance of the hybrids by inducing oxygen vacancies. Oxygen deficiency-related defect analyses were performed qualitatively as well as quantitatively using Fourier transform infrared spectroscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. All of the analyses clearly indicate an increase in oxygen deficiencies in the hybrids with an increase in the vacuum-annealing temperature. The nonstoichiometric oxygen vacancy is mainly induced via the migration of the lattice oxygen into interstitial sites at elevated temperature (300 °C), followed by diffusion into the gaseous phase with further increase in the annealing temperature (600 °C) in an oxygen-deficient atmosphere. This induction of oxygen vacancy is corroborated by diffuse reflectance spectroscopy, which depicts the oxygen-vacancy-induced bandgap narrowing of the ZnO NPs within the hybrids. At a current density of 3 A g(-1), the hybrid electrode exhibited higher energy density (119.85 Wh kg(-1)) and power density (19.225 kW kg(-1)) compared to a control ZnO electrode (48.01 Wh kg(-1) and 17.687 kW kg(-1)). The enhanced supercapacitive performance is mainly ascribed to the good interfacial contact between CNF and ZnO, high oxygen deficiency, and fewer defects in the hybrid. Our results are expected to provide new insights into improving the electrochemical properties of various composites/hybrids.

Anchoring Mechanism of ZnO Nanoparticles on Graphitic Carbon Nanofiber Surfaces through a Modified Co-Precipitation Method to Improve Interfacial Contact and Photocatalytic Performance.

A facile three-step co-precipitation method is developed to synthesize graphitic carbon nanofibers (CNFs) decorated with ZnO nanoparticles (NPs). By interchanging intermediate steps of the reaction processes, two kinds of nanohybrids are fabricated with stark morphological and physicochemical differences. The morphologies differ because of the different chemical environments of the NP/nanocluster formation. The hybrid with larger and non-uniform ZnO nanocluster size is formed in liquid phase and resulted in considerable interfacial defects that deteriorate the charge-transfer properties. The hybrid with smaller and uniform ZnO NPs was formed in a dry solid phase and produced near-defect-free interfaces, leading to efficient charge transfer for superior photocatalytic performance. The results broaden the understanding of the anchoring/bonding mechanism in ZnO/CNF hybrid formation and may facilitate further development of more effective exfoliation strategies for the preparation of high-performance composites/hybrids.

Morphology-dependent low macroscopic field emission properties of titania/titanate nanorods synthesized by alkali-controlled hydrothermal treatment of a metallic Ti surface.

One-dimensional (1D) and two-dimensional (2D) titania/titanate nanostructures are fabricated directly on a self-source metallic titanium (Ti) surface via in situ surface re-construction of a Ti substrate using potassium hydroxide (KOH) under a hydrothermal (HT) condition. The effect of temperature and the concentration of KOH on the variations in morphology and titania-to-titanate phase changes are studied and explained in detail. A growth model is proposed for the formation process of the platelet-to-nanorod conversion mechanism. The field emission (FE) properties of titania/titanate nanostructures are studied, and the effects of the morphologies (such as 1D nanorods, 2D nanoplatelets, and a mixture of 1D nanorods and 2D platelets) on the FE properties of the samples are investigated. The samples depict a reasonable low turn-on field and emission stability. The FE mechanism is observed to follow standard Fowler-Nordheim (FN) electron tunneling. The geometrical field enhancement factor (ß) is measured to be very high, and is compared with theoretical values calculated from various existing models to explore the feasibility of these models. The surface modification of metallic Ti by a simple non-lithographic bottom-up method and the low-macroscopic FE properties can provide a potential alternative to field emission displays for low-power panel technology.

Biofilm formation on a TiO2 nanotube with controlled pore diameter and surface wettability.

Titania (TiO2) nanotube arrays (TNAs) with different pore diameters (140 - 20 nm) are fabricated via anodization using hydrofluoric acid (HF) containing ethylene glycol (EG) by changing the HF-to-EG volume ratio and the anodization voltage. To evaluate the effects of different pore diameters of TiO2 nanotubes on bacterial biofilm formation, Shewanella oneidensis (S. oneidensis) MR-1 cells and a crystal-violet biofilm assay are used. The surface roughness and wettability of the TNA surfaces as a function of pore diameter, measured via the contact angle and AFM techniques, are correlated with the controlled biofilm formation. Biofilm formation increases with the decreasing nanotube pore diameter, and a 20 nm TiO2 nanotube shows the maximum biofilm formation. The measurements revealed that 20 nm surfaces have the least hydrophilicity with the highest surface roughness of ∼17 nm and that they show almost a 90% increase in the effective surface area relative to the 140 nm TNAs, which stimulate the cells more effectively to produce the pili to attach to the surface for more biofilm formation. The results demonstrate that bacterial cell adhesion (and hence, biofilm formation) can effectively be controlled by tuning the roughness and wettability of TNAs via controlling the pore diameters of TNA surfaces. This biofilm formation as a function of the surface properties of TNAs can be a potential candidate for both medical applications and as electrodes in microbial fuel cells.

Nanocrystalline ZnO thin film deposition on flexible substrate by low-temperature sputtering process for plastic displays.

A low temperature sputter deposition process is adopted to fabricate nanocrystalline ZnO thin films on plastic (polyethylene terepthalate) substrate. Very good crystalline films are synthesized at a substrate temperature around 120 degrees C. Structural and microstructural analyses confirm the proper phase formation of the nanomaterial with an average nanoparticle size around 5-10 nm. Optical transmission analysis of the film deposited on plastic substrate depicts nearly 90% visible transmittance with a direct bandgap around 3.56 eV. This cost-effective, low-temperature fabrication of nanocrystalline thin film with very good structural and optical properties will find important applications in plastic display technology. Also the process is a vacuum-based clean process, which is compatible to CMOS-IC fabrication techniques and therefore, can easily be integrated with modern solid state device fabrication processes for diverse device applications.

Poole-Frenkel effect in sputter-deposited CuAlO(2+x) nanocrystals.

Field-assisted thermionic emission within a sputter-deposited, nanocrystalline thin film of CuAlO2.06 is observed for the first time, and explained in terms of the Poole-Frenkel model. The presence of adsorbed oxygen ions as trap-states at the grain boundary regions of the nanostructured thin film is considered to manifest this phenomenon. Under an applied field, the barrier of the trap potential is lowered and thermal emission of charge carriers takes place at different sample temperatures to induce nonlinearity in the current (I)-voltage (V) characteristics of the nanomaterial. Fitting of the Poole-Frenkel model with the I-V data shows that the nonlinearity is effective above 50 V under the operating conditions. Calculations of the energy of the trap level, acceptor level and Fermi level reveal the existence of deep level trap-states and a shallow acceptor level with acceptor concentration considerably higher than the trap-states. Hall measurements confirm the p-type semiconductivity of the film, with a hole concentration around 10(18) cm(-3). Thermopower measurements give a room-temperature Seebeck coefficient around 130 µV K(-1). This temperature-dependent conductivity enhancement within CuAlO2 nanomaterial may find interesting applications in transparent electronics and high-voltage applications for power supply networks.

Low-macroscopic field emission properties of wide bandgap copper aluminium oxide nanoparticles for low-power panel applications.

Field emission properties of CuAlO(2) nanoparticles are reported for the first time, with a low turn-on field of approximately 2 V µm(-1) and field enhancement factor around 230. The field emission process follows the standard Fowler-Nordheim tunnelling of cold electron emission. The emission mechanism is found to be a combination of low electron affinity, internal nanostructure and large field enhancement at the low-dimensional emitter tips of the nanoparticles. The field emission properties are comparable to the conventional carbon-based field emitters, and thus can become alternative candidate for field emission devices for low-power panel applications.

High-speed droplet actuation on single-plate electrode arrays.

This paper reports a droplet-based microfluidic device composed of patterned co-planar electrodes in an all-in-a-single-plate arrangement and coated with dielectric layers for electrowetting-on-dielectric (EWOD) actuation of discrete droplets. The co-planar arrangement is preferred over conventional two-plate electrowetting devices because it provides simpler manufacturing process, reduced viscous drag, and easier liquid-handling procedures. These advantages lead to more versatile and efficient microfluidic devices capable of generating higher droplet speed and can incorporate various other droplet manipulation functions into the system for biological, sensing, and other microfluidic applications. We have designed, fabricated, and tested the devices using an insulating layer with materials having relatively high dielectric constant (SiO(2)) and compared the results with polymer coatings (Cytop) with low dielectric constant. Results show that the device with high dielectric layer generates more reproducible droplet transfer over a longer distance with a 25% reduction in the actuation voltage with respect to the polymer coatings, leading to more energy efficient microfluidic applications. We can generate droplet speeds as high as 26 cm/s using materials with high dielectric constant such as SiO(2).

Large field enhancement at electrochemically grown quasi-1D Ni nanostructures with low-threshold cold-field electron emission.

Ni nanorod arrays have been vertically grown on a Ta-coated Si substrate via an electrodeposition process through the nanopores of a porous alumina membrane. Field emission studies of the samples are performed which show a considerable low-threshold field around 5 V µm(-1). The field emission mechanism followed Fowler-Nordheim tunneling due to large field enhancement at the emitter tips. Low-dimensional structures of the nanorod tips provided the large geometrical field enhancement and thus produce a high enough local or barrier field for low-threshold cold-field electron emission. The cost-effective synthesis of vertically aligned Ni nanorods on an Si substrate and low-threshold field emission properties can provide a potential alternative to conventional carbon-based field emitters for low power panel applications.

The design, fabrication, and photocatalytic utility of nanostructured semiconductors: focus on TiO2-based nanostructures.

Recent advances in basic fabrication techniques of TiO2-based nanomaterials such as nanoparticles, nanowires, nanoplatelets, and both physical- and solution-based techniques have been adopted by various research groups around the world. Our research focus has been mainly on various deposition parameters used for fabricating nanostructured materials, including TiO2-organic/inorganic nanocomposite materials. Technically, TiO2 shows relatively high reactivity under ultraviolet light, the energy of which exceeds the band gap of TiO2. The development of photocatalysts exhibiting high reactivity under visible light allows the main part of the solar spectrum to be used. Visible light-activated TiO2 could be prepared by doping or sensitizing. As far as doping of TiO2 is concerned, in obtaining tailored material with improved properties, metal and nonmetal doping has been performed in the context of improved photoactivity. Nonmetal doping seems to be more promising than metal doping. TiO2 represents an effective photocatalyst for water and air purification and for self-cleaning surfaces. Additionally, it can be used as an antibacterial agent because of its strong oxidation activity and superhydrophilicity. Therefore, applications of TiO2 in terms of photocatalytic activities are discussed here. The basic mechanisms of the photoactivities of TiO2 and nanostructures are considered alongside band structure engineering and surface modification in nanostructured TiO2 in the context of doping. The article reviews the basic structural, optical, and electrical properties of TiO2, followed by detailed fabrication techniques of 0-, 1-, and quasi-2-dimensional TiO2 nanomaterials. Applications and future directions of nanostructured TiO2 are considered in the context of various photoinduced phenomena such as hydrogen production, electricity generation via dye-sensitized solar cells, photokilling and self-cleaning effect, photo-oxidation of organic pollutant, wastewater management, and organic synthesis.