Guanjia Zhao

Graphenes prepared by Staudenmaier, Hofmann and Hummers methods with consequent thermal exfoliation exhibit very different electrochemical properties

Large-scale fabrication of graphene is highly important for industrial and academic applications of this material. The most common large-scale preparation method is the oxidation of graphite to graphite oxide using concentrated acids in the presence of strong oxidants and consequent thermal exfoliation and reduction by thermal shock to produce reduced graphene. These oxidation methods typically use concentrated sulfuric acid (a) in combination with fuming nitric acid and KClO(3) (Staudenmaier method), (b) in combination with concentrated nitric acid and KClO(3) (Hofmann method) or (c) in the absence of nitric acid but in the presence of NaNO(3) and KMnO(4) (Hummers method). The evaluation of quality and applicability of the graphenes produced by these various methods is of high importance and is attempted side-by-side for the first time in this paper. Full-scale characterization of thermally reduced graphenes prepared by these standard methods was performed with techniques such as transmission and scanning electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. Their applicability for electrochemical devices was further evaluated by means of cyclic voltammetry techniques. We showed that while Staudenmaier and Hofmann methods (methods that do not use potassium permanganate as oxidant) generated thermally reduced graphenes with comparable electrochemical properties, the graphene prepared by the Hummers method which uses permanganate as oxidant showed higher heterogeneous electron transfer rates and lower overpotentials as compared to graphenes prepared by the Staudenmaier or Hofmann methods. This clearly shows that the methods of preparations have dramatic influences on the materials properties and, thus, such findings are of eminent importance for practical applications as well as for academic research.

Noble metal (Pd, Ru, Rh, Pt, Au, Ag) doped graphene hybrids for electrocatalysis

Metal decorated graphene materials are highly important for catalysis. In this work, noble metal doped-graphene hybrids were prepared by a simple and scalable method. The thermal reductions of metal doped-graphite oxide precursors were carried out in nitrogen and hydrogen atmospheres and the effects of these atmospheres as well as the metal components on the characteristics and catalytic capabilities of the hybrid materials were studied. The hybrids exfoliated in nitrogen atmosphere contained a higher amount of oxygen-containing groups and lower density of defects on their surfaces than hybrids exfoliated in hydrogen atmosphere. The metals significantly affected the electrochemical behavior and catalysis of compounds that are important in energy production and storage and in electrochemical sensing. Research in the field of energy storage and production, electrochemical sensing and biosensing as well as biomedical devices can take advantage of the properties and catalytic capabilities of the metal doped graphene hybrids.

Beyond platinum: bubble-propelled micromotors based on Ag and MnO2 catalysts.

Autonomous bubble-propelled catalytic micro- and nanomachines show great promise in the fields of biomedicine, environmental science, and natural resources. It is envisioned that thousands and millions of such micromachines will swarm and communicate with each other, performing desired actions. To date, mainly platinum catalyst surfaces have been used for the decomposition of a fuel, hydrogen peroxide, to oxygen bubbles. Here we propose Pt-free, low-cost inorganic catalysts for powering micromotors based on silver and manganese dioxide surfaces. Such Ag- and MnO2-based bubble-powered micromotors show fast motion even at very low concentrations of fuel, down to 0.1% of H2O2. These catalysts should enable unparalleled widespread use of such motors in real applications, as it will be possible to make them in large quantities at low cost.

External-Energy-Independent Polymer Capsule Motors and Their Cooperative Behaviors

The design and development of mobile nano-, micro-, and millimeter-scale autonomous systems have been perused over several decades. Here, we introduce a millimeter-sized polymer capsule motor with specific features and functionalities. It runs without any external energy sources or the consumption of external fuels such as H(2)O(2) or glucose. The occurrence of motion is due to the asymmetric release of organic solvent from the capsule and the asymmetric change in the surface tension of the surrounding liquid. The capsule moves from a place of lower surface tension to a place of higher surface tension (Marangoni effect) in an attempt by the system to attain the desirable lowest-free-energy state. The operation of the motor is versatile in terms of the environment, as it moves on a wide variety of liquid/air interfaces, including water, sea water, organic solvent/water mixtures, and acids. A high-motion velocity was observed, with a travelling distance of over 20 meters. The manipulation of its motion was achieved upon functionalization with nickel powder and application of an external magnetic field. Long-range interaction behaviors and surface-cleaning effects due to the chemotaxis effect were also demonstrated when the capsule was functionalized with sodium dodecyl sulfate (SDS). We believe that a plethora of applications can be envisioned with this motor, such as cargo delivery, manipulation of matter, sensing and detection, biorecognition, and environmental remediation.

Inherently Electroactive Graphene Oxide Nanoplatelets As Labels for Single Nucleotide Polymorphism Detection

Graphene materials are being widely used in electrochemistry due to their versatility and excellent properties as platforms for biosensing. However, no records show the use of inherent redox properties of graphene oxide as a label for detection. Here for the first time we used graphene oxide nanoplatelets (GONPs) as electroactive labels for DNA analysis. The working signal comes from the reduction of the oxygen-containing groups present on the surface of GONPs. The different ability of the graphene oxide nanoplatelets to conjugate to DNA hybrids obtained with complementary, noncomplementary, and one-mismatch sequences allows the discrimination of single-nucleotide polymorphism correlated with Alzheimers disease. We believe that our findings are very important to open a new route in the use of graphene oxide in electrochemistry.

Biomimetic Artificial Inorganic Enzyme‐Free Self‐Propelled Microfish Robot for Selective Detection of Pb2+ in Water

The availability of drinking water is of utmost importance for the world population. Anthropogenic pollutants of water, such as heavy-metal ions, are major problems in water contamination. The toxicity assays used range from cell assays to animal tests. Herein, we replace biological toxicity assays, which use higher organisms, with artificial inorganic self-propelled microtubular robots. The viability and activity of these robots are negatively influenced by heavy metals, such as Pb(2+) , in a similar manner to that of live fish models. This allows the establishment of a lethal dose (LD50 ) of heavy metal for artificial inorganic microfish robots. The self-propelled microfish robots show specific response to Pb(2+) compared to other heavy metals, such as Cd(2+) , and can be used for selective determination of Pb(2+) in water. It is a first step towards replacing the biological toxicity assays with biomimetic inorganic autonomous robotic systems.

Micromotors with built-in compasses.

We demonstrate here that iron containing rolled-up microtubular engines can be magnetized and act as compass needles - they sense the direction of an external magnetic field from afar and align the directionalities of their movements according to the external field, in a similar fashion to magnetotactic bacteria.

Concentric bimetallic microjets by electrodeposition

Self-propelled micro and nanosystems are at the forefront of nanotechnology research. Here we describe a method for fabrication of concentric bimetallic microjets via an electrochemical deposition route. These microjet engines consist of an inner platinum layer which is responsible for the catalytic decomposition of H2O2, which subsequently results in bubble-propelled movement. The outer copper layer allows for further functionalization of the microjet engines. These microjet engines are able to move at speeds of ∼7 bodylengths s-1 at fuel concentrations as low as 0.2% (wt.) of H2O2. The described method obviates the need to use physical vapor deposition (sputtering) and thus is applicable in any low-end equipped laboratory. Such an accessible method is expected to lead to a dramatic increase in the research activity on artificial self-propelled systems.

Magnetotactic artificial self-propelled nanojets.

Self-propelled catalytic bubble-ejecting nanotubes (nanojets) are expected to perform a variety of autonomous tasks. Herein, we will show that with the introduction of a Ni segment into the Au/Ni/Pt nanotube design followed by consequent magnetization a permanent change in the magnetic domain orientation of the Ni segment can be induced. Consequently, this results in the presence of a permanent magnet within the nanojet with its north/south domains oriented along the tube axis. Such a magnetized nanojet orients itself according to the external magnetic field and propels itself toward or away from the source of the magnetic field depending on its orientation. This behavior is similar to that of the magnetotactic bacteria. The ability to sense the magnetic field is expected to have a strong impact on future applications of autonomous self-propelled nanojets.

Macroscopic self-propelled objects.

Self-propelled systems are currently in the spotlight of the research community. We review the progress of the construction of both millimeter- and centimeter-sized self-propelled macroscopic objects. We will also discuss the various sources of energy used by these systems, such as the electromagnetic field, electric field, thermal gradient, and chemical energy, and present how these millimeter- and centimeter-sized devices can move at velocities of tens cm s(-1) and distances of several tens of meters.