Stretchable and conformal humidity sensors that can be attached to the human body for continuously monitoring the humidity of the environment around the human body or the moisture level of the human skin can play an important role in electronic skin and personal healthcare applications. However, most stretchable humidity sensors are based on the geometric engineering of non-stretchable components and only a few detailed studies are available on stretchable humidity sensors under applied mechanical deformations. In this paper, we propose a transparent, stretchable humidity sensor with a simple fabrication process, having intrinsically stretchable components that provide high stretchability, sensitivity, and stability along with fast response and relaxation time. Composed of reduced graphene oxide-polyurethane composites and an elastomeric conductive electrode, this device exhibits impressive response and relaxation time as fast as 3.5 and 7 s, respectively. The responsivity and the response and relaxation time of the device in the presence of humidity remain almost unchanged under stretching up to a strain of 60% and after 10,000 stretching cycles at a 40% strain. Further, these stretchable humidity sensors can be easily and conformally attached to a finger for monitoring the humidity levels of the environment around the human body, wet objects, or human skin.

Transparent, stretchable, and rapid-response humidity sensor for body-attachable wearable electronics

https://www.rsc.org/suppdata/cc/c2/c2cc36747e/c2cc36747e.pdf

Selective surface functionalization at regions of high local curvature in graphene

Efficient and selective methods for covalent derivatization of graphene are needed because they enable tuning of graphene’s surface and electronic properties, thus expanding its application potential. However, existing approaches based mainly on chemistry of graphene and graphene oxide achieve only limited level of functionalization due to chemical inertness of the surface and nonselective simultaneous attachment of different functional groups, respectively. Here we present a conceptually different route based on synthesis of cyanographene via the controllable substitution and defluorination of fluorographene. The highly conductive and hydrophilic cyanographene allows exploiting the complex chemistry of −CN groups toward a broad scale of graphene derivatives with very high functionalization degree. The consequent hydrolysis of cyanographene results in graphene acid, a 2D carboxylic acid with pKa of 5.2, showing excellent biocompatibility, conductivity and dispersibility in water and 3D supramolecular asse...

http://pubs.acs.org/doi/pdf/10.1021/acsnano.6b08449

Cyanographene and Graphene Acid: Emerging Derivatives Enabling High-Yield and Selective Functionalization of Graphene

With growing demands of energy and enormous consumption of fossil fuels, the world is in dire need of a clean and renewable source of energy. Hydrogen (H2) is the best alternative, owing to its high calorific value (144 MJ/kg) and exceptional mass-energy density. Being an energy carrier rather than an energy source, it has an edge over other alternate sources of energy like solar energy, wind energy, and tidal energy, which require a constant energy source dependent upon weather conditions. However, its utilization as an energy carrier has not yet been commercialized due to its poor storage performance, which is attributed to low gravimetric and volumetric densities of adsorbed hydrogen at ambient temperature and technological limitations in meeting the stringent parameters set by Department of Energy, USA. With exceptionally large surface area (2630 m2/g), porous nature, lightweight, and high chemical and thermal stability (melting point ~ 4510 K) along with the possibility of economical and scalable production, graphene-based solid-state porous materials have shown promising applications in efficient hydrogen storage. In this context, the present review discusses the recent advances and progress on the utilization of functionalized graphene, graphene oxide, and its derivatives for effective storage of hydrogen, along with important theoretical advancements via DFT calculations, first-principle calculations, and Monte Carlo Simulations, etc. Pristine graphene has poor hydrogen storage characteristics, and addition of dopants like boron and nitrogen or decoration by transition metals significantly improves the performance. In addition, graphene allows the tuning of surface curvature which can help in achieving a reversible hydrogen storage system with fast kinetics. The impact of external stimuli like electric field and strain on electronic structure of graphene is discussed with the applicability in achieving a highly controllable adsorption–desorption system. Finally, the review concludes with life cycle assessment of graphene-engineered composites for effective hydrogen storage applications, along with their energy and environmental implications.

Functionalized graphene materials for hydrogen storage

Monolayer graphene was deposited on a Si wafer substrate decorated with SiO2 nanoparticles (NPs) and then exposed to aryl radicals that were generated in situ from their diazonium precursors. Using micro-Raman mapping, the aryl radicals were found to selectively react with the regions of graphene that covered the NPs. The enhanced chemical reactivity was attributed to the increased strain energy induced by the local mechanical deformation of the graphene.

Applied to graphene oxide, the molecular theory of graphene considers its oxide as a final product in the succession of polyderivatives related to a series of oxidation reactions involving different oxidants. The graphene oxide structure is created in the course of a stepwise computational synthesis of polyoxides of the (5,5) nanographene molecule governed by an algorithm that takes into account the molecule’s natural radicalization due to the correlation of its odd electrons, the extremely strong influence of the structure on properties, and a sharp response of the molecule behavior on small actions of external factors. Taking these together, the theory has allowed for a clear, transparent and understandable explanation of the hot points of graphene oxide chemistry and suggesting reliable models of both chemically produced and chemically reduced graphene oxides.

Molecular theory of graphene oxide

The standard D-G-2D pattern of Raman spectra of sp2 amorphous carbons is considered from the viewpoint of graphene domains presenting their basic structure units (BSUs) in terms of molecular spectroscopy. The molecular approximation allows connecting the characteristic D-G doublet spectra image of one-phonon spectra with a considerable dispersion of the C=C bond lengths within graphene domains, governed by size, heteroatom necklace of BSUs as well as BSUs packing. The interpretation of 2D two-phonon spectra reveals a particular role of electrical anharmonicity in the spectra formation and attributes this effect to a high degree of the electron density delocalization in graphene domains. A size-stimulated transition from molecular to quasi-particle phonon consideration of Raman spectra was experimentally traced, which allowed evaluation of a free path of optical phonons in graphene crystal.

Graphene Domain Signature of Raman Spectra of sp2 Amorphous Carbons.

sp2 Amorphous carbons are considered as objects of the modern nanotechnology. A particular set of the highest‑carbon-content sp2 species, two natural amorphous carbons (shungite carbon and antraxolite) as well as two engineered carbon blacks were subjected to analytical study by using a set of selected modern structural and compositional analytical techniques. The suggested approach occurred quite efficient and allowed disclosing a number of steady points that are common to the whole class of this carbon allotrope and are able to get a vision of atom-molecular representation of the solids. Among the commonalities observed, there are the following: i) sp2 Amorphous carbons are conglomerates of nanographites, basic structure units of which lay the foundation of the atom-molecular description of the solids. ii) The units represent framed graphene molecules of 1÷2 nm and of 1÷ (1÷3)*10 nm in size in the case of natural and engineered products, respectively. iii) The molecule framing, predominantly incomplete with respect to the number of vacant places, concerns only edge atoms and is implemented by the related chemical additives, such as hydrogen, oxygen, nitrogen, sulfur and halogens which are attached to the carbon core via chemical bonding. iv) Inelastic neutron scattering and X-ray photoelectron spectroscopy allow attributing the bonds to chemical compositions involving hydrogen and oxygen while quantum chemistry ensures reliable support. v) The basic structure units of sp2amorphous carbons are strongly radicalized due to which the latter acquire new properties, being the largest repository of stable radicals.

sp2 amorphous carbons in view of multianalytical consideration: Normal, expeсted and new

This paper highlights the molecular essence of graphene and presents its hydrogenation from the viewpoint of the odd-electron molecular theory. This chemical transformation was performed computationally, using a particular algorithm, through the stepwise addition of either hydrogen molecules or hydrogen atoms to a pristine graphene molecule. The graphene was considered to be a membrane, such that either both sides or just one side of the membrane was accessible to adsorbate, and the atoms on the perimeter of the membrane were either fixed (fixed membrane) or free to move (free-standing membrane). The algorithm explored the spatial distribution of the number of effectively unpaired electrons N
 DA over the carbon skeleton of the molecule. The highest ranked N
 DA values were considered to indicate the target atoms at each reaction step. The dependence of the hydrogenation itself and the final graphene hydrides on external factors such as whether the membrane was fixed, if both sides or only one side of the membrane were accessible to hydrogen, and whether the hydrogen was in the molecular or atomic state. Complete hydrogenation followed by the formation of a regular chairlike graphane structure (CH)
 n
 was only found to be possible for a fixed pristine graphene membrane for which the basal plane is accessible to hydrogen atoms from both sides.

Odd-electron molecular theory of graphene hydrogenation

The paper presents a joint consideration of Raman spectra of sp2 amorphous carbons alongside with the nature and type of their amorphicity. The latter was attributed to the enforced fragmentation. The fragments, presented with size-restricted graphene domains with heteroatom necklaces in the circumference, are the basic structural units (BSUs) of the solids, determining them as amorphics with molecular structure. The standard G-D-2D pattern of Raman spectra of polycyclic aromatic hydrocarbons, sp2 amorphous carbons, graphene and/or graphite crystal is attributed to BSUs graphene domains. The molecular approximation allows connecting the G-D spectra image of one-phonon spectra with a considerable dispersion of the C=C bond lengths within graphene domains, governed by size, heteroatom necklace of BSUs as well as BSUs packing. The interpretation of 2D two-phonon spectra reveals a particular role of electrical anharmonicity in the spectra formation and attributes this effect to a high degree of the electron density delocalization in graphene domains.

Nadezhda A. Popova

Papers

Molecular theory of graphene oxide

Graphene Domain Signature of Raman Spectra of sp2 Amorphous Carbons.

sp2 amorphous carbons in view of multianalytical consideration: Normal, expeсted and new

Odd-electron molecular theory of graphene hydrogenation

Graphene Domain Signature of Raman Spectra of sp2 Amorphous Carbons