Graphene Coupled with Nanocrystals: Opportunities and Challenges for Energy and Sensing Applications

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 energy storage/conversion systems and challenges for the widespread use of graphene-based sensors

A General Approach to One-Pot Fabrication of Crumpled Graphene-Based Nanohybrids for Energy Applications

Crumpled graphene oxide (GO)/graphene is a new type of carbon nanostructure that has drawn growing attention due to its three-dimensional open structure and excellent stability in an aqueous solution. Here we report a general and one-step approach to produce crumpled graphene (CG)–nanocrystal hybrids, which are produced by direct aerosolization of a GO suspension mixed with precursor ions. Nanocrystals spontaneously grow from precursor ions and assemble on both external and internal surfaces of CG balls during the solvent evaporation and GO crumpling process. More importantly, CG–nanocrystal hybrids can be directly deposited onto various current-collecting substrates, enabling their tremendous potential for energy applications. As a proof of concept, we demonstrate the use of hybrid electrodes of CG–Mn₃O₄ and CG–SnO₂ in an electrochemical supercapacitor and a lithium-ion battery, respectively. The performance of the resulting capacitor/battery is attractive and outperforms conventional flat graphene-based hybrid devices. This study provides a new and facile route to fabricating high-performance hybrid CG–nanocrystal electrodes for various energy systems

Carbon Nanotube with Chemically Bonded Graphene Leaves for Electronic and Optoelectronic Applications

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 intrinsic chemical bonding between the two constituent components. We further show that the resulting CNT–FLG structure exhibits remarkable optoelectronic and gas sensing properties superior to its CNT or CNT+G counterparts. The new structure reported here is thus attractive for various electronic and optoelectronic applications

Modulating Gas Sensing Properties of CuO Nanowires through Creation of Discrete Nanosized p–n Junctions on Their Surfaces

We report significant enhancement of CuO nanowire (NW) sensing performance at room temperature through the surface functionalization with SnO₂ nanocrystals (NCs). The sensitivity enhancement can be as high as ∼300% for detecting 1% NH₃ diluted in air. The improved sensitivity could be attributed to the electronic interaction between p-type CuO NWs and n-type SnO₂ 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 SnO₂ NCs on CuO NWs and doping of SnO₂ NCs)

Hg(II) Ion Detection Using Thermally Reduced Graphene Oxide Decorated with Functionalized Gold Nanoparticles

Fast and accurate detection of aqueous contaminants is of significant importance as these contaminants raise serious risks for human health and the environment. Mercury and its compounds are highly toxic and can cause various illnesses; however, current mercury detectors suffer from several disadvantages, such as slow response, high cost, and lack of portability. Here, we report field-effect transistor (FET) sensors based on thermally reduced graphene oxide (rGO) with thioglycolic acid (TGA) functionalized gold nanoparticles (Au NPs) (or rGO/TGA-AuNP hybrid structures) for detecting mercury­(II) ions in aqueous solutions. The lowest mercury­(II) ion concentration detected by the sensor is 2.5 × 10^–8 M. The drain current shows rapid response within less than 10 s after the solution containing Hg²⁺ ions was added to the active area of the rGO/TGA-AuNP hybrid sensors. Our work suggests that rGO/TGA-AuNP hybrid structures are promising for low-cost, portable, real-time, heavy metal ion detectors

Exploring Adsorption and Reactivity of NH₃ on Reduced Graphene Oxide

Sensors based on graphene and functionalized graphene are emerging as the state of the art for detecting extremely small quantities of target molecules under realistic working conditions with high selectivity. Although some theoretical work has emerged to understand such adsorption processes (Tang and Cao J. Phys. Chem. C 2012, 116, 8778; Leenaerts et al. Phys. Rev. B 2008, 77, 125416; Tang and CaoJ. Chem. Phys. 2011, 134, 044710), little experimental evidence detailing the dynamics of the adsorption and resulting surface species has been reported. Here, we study the adsorption of NH₃ on reduced graphene oxide (RGO) using in situ infrared (IR) microspectroscopy performed under realistic working conditions (i.e., ambient pressure), along with density functional theory (DFT) calculations to support experimental observations. Conclusions drawn from experiment and theory reveal the presence of various surface species that impact the conductivity of the substrate at varying rates. The species arising from adsorption and interactions between NH₃ and RGO include molecularly physisorbed NH₃, as well as chemisorbed fragments such as NH₂, OH, and CH due to dissociation of NH₃ at defects and epoxide groups

Fast and Selective Room-Temperature Ammonia Sensors Using Silver Nanocrystal-Functionalized Carbon Nanotubes

We report a selective, room-temperature NH₃ 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 NH₃. Upon exposure to 1% NH₃ 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 NH₃ to the Ag NC–MWCNTs hybrid leads to the conductance change in the hybrid

Facile Synthesis of Highly Dispersed Co₃O₄ Nanoparticles on Expanded, Thin Black Phosphorus for a ppb-Level NO_<i>x</i> Gas Sensor

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 NO_<i>x</i> 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

Evidence of Nanocrystalline Semiconducting Graphene Monoxide during Thermal Reduction of Graphene Oxide in Vacuum

As silicon-based electronics are reaching the nanosize limits of the semiconductor roadmap, carbon-based nanoelectronics has become a rapidly growing field, with great interest in tuning the properties of carbon-based materials. Chemical functionalization is a proposed route, but syntheses of graphene oxide (G-O) produce disordered, nonstoichiometric materials with poor electronic properties. We report synthesis of an ordered, stoichiometric, solid-state carbon oxide that has never been observed in nature and coexists with graphene. Formation of this material, graphene monoxide (GMO), is achieved by annealing multilayered G-O. Our results indicate that the resulting thermally reduced G-O (TRG-O) consists of a two-dimensional nanocrystalline phase segregation: unoxidized graphitic regions are separated from highly oxidized regions of GMO. GMO has a quasi-hexagonal unit cell, an unusually high 1:1 O:C ratio, and a calculated direct band gap of ∼0.9 eV

Evidence of Nanocrystalline Semiconducting Graphene Monoxide during Thermal Reduction of Graphene Oxide in Vacuum

As silicon-based electronics are reaching the nanosize limits of the semiconductor roadmap, carbon-based nanoelectronics has become a rapidly growing field, with great interest in tuning the properties of carbon-based materials. Chemical functionalization is a proposed route, but syntheses of graphene oxide (G-O) produce disordered, nonstoichiometric materials with poor electronic properties. We report synthesis of an ordered, stoichiometric, solid-state carbon oxide that has never been observed in nature and coexists with graphene. Formation of this material, graphene monoxide (GMO), is achieved by annealing multilayered G-O. Our results indicate that the resulting thermally reduced G-O (TRG-O) consists of a two-dimensional nanocrystalline phase segregation: unoxidized graphitic regions are separated from highly oxidized regions of GMO. GMO has a quasi-hexagonal unit cell, an unusually high 1:1 O:C ratio, and a calculated direct band gap of ∼0.9 eV