This Review focuses on noncovalent functionalization of graphene and graphene oxide with various species involving biomolecules, polymers, drugs, metals and metal oxide-based nanoparticles, quantum dots, magnetic nanostructures, other carbon allotropes (fullerenes, nanodiamonds, and carbon nanotubes), and graphene analogues (MoS2, WS2). A brief description of π–π interactions, van der Waals forces, ionic interactions, and hydrogen bonding allowing noncovalent modification of graphene and graphene oxide is first given. The main part of this Review is devoted to tailored functionalization for applications in drug delivery, energy materials, solar cells, water splitting, biosensing, bioimaging, environmental, catalytic, photocatalytic, and biomedical technologies. A significant part of this Review explores the possibilities of graphene/graphene oxide-based 3D superstructures and their use in lithium-ion batteries. This Review ends with a look at challenges and future prospects of noncovalently modified graph...

/pdf/noncovalent-functionalization-of-graphene-and-graphene-oxide-49z1h0rufs.pdf

Noncovalent Functionalization of Graphene and Graphene Oxide for Energy Materials, Biosensing, Catalytic, and Biomedical Applications

In recent years, nanocrystals of metal sulfide materials have attracted scientific research interest for renewable energy applications due to the abundant choice of materials with easily tunable electronic, optical, physical and chemical properties. Metal sulfides are semiconducting compounds where sulfur is an anion associated with a metal cation; and the metal ions may be in mono-, bi- or multi-form. The diverse range of available metal sulfide materials offers a unique platform to construct a large number of potential materials that demonstrate exotic chemical, physical and electronic phenomena and novel functional properties and applications. To fully exploit the potential of these fascinating materials, scalable methods for the preparation of low-cost metal sulfides, heterostructures, and hybrids of high quality must be developed. This comprehensive review indicates approaches for the controlled fabrication of metal sulfides and subsequently delivers an overview of recent progress in tuning the chemical, physical, optical and nano- and micro-structural properties of metal sulfide nanocrystals using a range of material fabrication methods. For hydrogen energy production, three major approaches are discussed in detail: electrocatalytic hydrogen generation, powder photocatalytic hydrogen generation and photoelectrochemical water splitting. A variety of strategies such as structural tuning, composition control, doping, hybrid structures, heterostructures, defect control, temperature effects and porosity effects on metal sulfide nanocrystals are discussed and how they are exploited to enhance performance and develop future energy materials. From this literature survey, energy conversion currently relies on a limited range of metal sulfides and their composites, and several metal sulfides are immature in terms of their dissolution, photocorrosion and long-term durability in electrolytes during water splitting. Future research directions for innovative metal sulfides should be closely allied to energy and environmental issues, along with their advanced characterization, and developing new classes of metal sulfide materials with well-defined fabrication methods.

Recent advances in metal sulfides: from controlled fabrication to electrocatalytic, photocatalytic and photoelectrochemical water splitting and beyond

Nickel sulfide microsphere film on Ni foam as an efficient bifunctional electrocatalyst for overall water splitting

Transition-metal carbides (MXenes) are an emerging class of two-dimensional materials with promising electrochemical energy storage performance. Herein, for the first time, by direct chemical synthesis, nanocrystalline e-MnO2 whiskers were formed on MXene nanosheet surfaces (e-MnO2/Ti2CTx and e-MnO2/Ti3C2Tx) to make nanocomposite electrodes for aqueous pseudocapacitors. The e-MnO2 nanowhiskers increase the surface area of the composite electrode and enhance the specific capacitance by nearly 3 orders of magnitude compared to that of pure MXene-based symmetric supercapacitors. Combined with enhanced pseudocapacitance, the fabricated e-MnO2/MXene supercapacitors exhibited excellent cycling stability with ∼88% of the initial specific capacitance retained after 10000 cycles which is much higher than pure e-MnO2-based supercapacitors (∼74%). The proposed electrode structure capitalizes on the high specific capacitance of MnO2 and the ability of MXenes to improve conductivity and cycling stability.

/pdf/direct-chemical-synthesis-of-mno2-nanowhiskers-on-transition-3dsyplggfl.pdf

Direct Chemical Synthesis of MnO2 Nanowhiskers on Transition-Metal Carbide Surfaces for Supercapacitor Applications

With the profuse amount of two-dimensional (2D) materials discovered and the improvements in their synthesis and handling, the field of 2D heterostructures has gained increased interest in recent years. Such heterostructures not only overcome the inherent limitations of each of the materials, but also allow the realization of novel properties by their proper combination. The physical and mechanical properties of graphene mean it has a prominent place in the area of 2D heterostructures. In this review, we will discuss the evolution and current state in the synthesis and applications of graphene-based 2D heterostructures. In addition to stacked and in-plane heterostructures with other 2D materials and their potential applications, we will also cover heterostructures realized with lower dimensionality materials, along with intercalation in few-layer graphene as a special case of a heterostructure. Finally, graphene heterostructures produced using liquid phase exfoliation techniques and their applications to energy storage will be reviewed.

/pdf/synthesis-structure-and-applications-of-graphene-based-2d-nv3tlbcbvr.pdf

Synthesis, structure and applications of graphene-based 2D heterostructures

The two-dimensional (2D) transition metal dichalcogenide nanosheet-carbon composite is an attractive material for energy storage because of its high Faradaic activity, unique nanoconstruction and electronic properties. In this work, a facile one step preparation of a molybdenum disulfide (MoS2) nanosheet-graphene (MoS2/G) composite with the in situ reduction of graphene oxide is reported. The structure, morphology and composition of the pure MoS2 and composites were comparatively analyzed by various characterization techniques. The electrochemical performance of the pure MoS2, graphene oxide and the MoS2/G composite electrode materials was evaluated by cyclic voltammogram, galvanostatic charge-discharge and electrochemical impedance spectroscopy. The MoS2/G composite showed a higher specific capacitance (270 F g(-1) at a current density of 0.1 A g(-1)) compared to the pure MoS2 (162 F g(-1)) in a neutral aqueous electrolyte. Moreover, the energy density of the composite electrode is also higher (12.5 Wh kg(-1)) with a high power density (2500 W kg(-1)) compared to the pure MoS2. In addition, the MoS2/G composite electrode showed excellent cyclic stability even after 1000 cycles. The enhancement in specific capacitance, excellent cyclic stability and high energy density of the composite electrode are mainly due to the interconnected conductive network of the composite as well as the synergetic effect of the pure MoS2 and graphene. The experimental results demonstrated that the MoS2/G composite is a promising electrode material for high-performance supercapacitors.

Graphene decorated with MoS2 nanosheets: a synergetic energy storage composite electrode for supercapacitor applications

Nanoparticle single-phase nickel sulfides such as NiS, NiS2, Ni3S4, and Ni7S6 were prepared from elemental sulfur and nickel nitrate hexahydrate, using a temperature-controlled precursor injection method. The initial ratio of the concentrations of the sources was used to control the size and phase of the final product. Phase control was confirmed using X-ray diffraction and transmission electron microscopy. The synthesized nickel sulfide phases, which had metallic characteristics, were used to study the catalytic reduction of 4-nitrophenol. The results showed that the catalytic activity of the NiS nanoparticles in the reduction of 4-nitrophenol to 4-aminophenol was higher than those of the other nickel sulfide phases. In addition, the nanocrystals showed good separation ability and reusability for reduction of 4-nitrophenol.

Single-step synthesis and catalytic activity of structure-controlled nickel sulfide nanoparticles

Hierarchical structures of nickel sulfide have been grown by the hydrothermal method. Nickel nitrate hexahydrate and thiourea were used as precursor materials to synthesize nickel sulfide. Ethylenediaminetetraacetic acid was used as a capping agent to achieve monodispersity. The different phases of nickel sulfide and its dependency on the precursor concentration were analyzed by X-ray diffractometry. Transmission electron microscopy analysis was used to confirm the phase changes and morphological behavior of the synthesized material. The morphological evolution of the hierarchical structure formation was studied systematically by scanning electron microscopy. In this study, we explore a novel method to control the synthesis of nickel sulfide hierarchical structures by varying the precursor concentration. The two mixed phases enhanced the catalytic activity in the 4-nitro phenol reduction reaction.

Shape controlled synthesis of hierarchical nickel sulfide by the hydrothermal method.

Cu2ZnSnS4 (CZTS) nanoparticles were synthesized by the precursor injection method using oleylamine as a solvent. Preliminary characterization indicated that the synthesized nanoparticles belonged to the kesterite structure with a bulged sphere-like morphology. Reduced graphene oxide (rGO) was synthesized by an improved Hummers method and was used for nanoparticle functionalization. CZTS nanocrystals were decorated on rGO by two different methods. One was oleylamine-based nanoparticle functionalization, and the other was in situ nanoparticle growth. Transmission electron microscopy analysis of CZTS-functionalized rGO showed that the synthesized nanoparticles were uniformly spread on the surface of rGO sheets. Single phase CZTS nanoparticles were grown on rGO without any impurity phase in the in situ growth. Tuned absorption of the pure CZTS was observed by the decoration of CZTS nanoparticles on the surface of rGO in the visible and UV regions.

Growth and optical properties of Cu2ZnSnS4 decorated reduced graphene oxide nanocomposites

A near infra-red (NIR) active NaGdF4:Yb:Er/Ag/TiO2 nanocomposite photocatalyst was successfully synthesized by a one-pot thermal decomposition method. The composite structure, morphology and elemental mapping of the synthesized NaGdF4:Yb:Er/Ag/TiO2 nanocomposite were characterized by X-ray diffraction and transmission electron microscopy analysis. The energy transfer among NaGdF4:Yb:Er, Ag, and TiO2 was revealed by upconversion photoluminescence measurements at 980 nm. The Ag and NaGdF4 nanoparticles enhanced the visible and NIR light absorption property of the NaGdF4:Yb:Er/Ag/TiO2 nanocomposite. The NIR and UV-visible light induced photocatalytic study of the NaGdF4:Yb:Er/Ag/TiO2 composite was examined by rhodamine B degradation. The energy transfer among NaGdF4:Yb:Er, Ag, and TiO2 significantly influenced the photocatalytic activity under NIR irradiation. The catalysts produced oxidative species during NIR irradiation, which are responsible for the photocatalytic degradation of rhodamine B. NaGdF4:Yb:Er/Ag/TiO2 showed photocatalytic activity under NIR and UV-visible radiation (full solar irradiation), which is superior to a UV or visible light active photocatalyst. The study provided a UV-visible and NIR-responsive photocatalyst and its energy transfer mechanism.

Rajan Karthikeyan

Papers

Graphene decorated with MoS2 nanosheets: a synergetic energy storage composite electrode for supercapacitor applications

Single-step synthesis and catalytic activity of structure-controlled nickel sulfide nanoparticles

Shape controlled synthesis of hierarchical nickel sulfide by the hydrothermal method.

Growth and optical properties of Cu2ZnSnS4 decorated reduced graphene oxide nanocomposites

UV-visible and near-infrared active NaGdF4:Yb:Er/Ag/TiO2 nanocomposite for enhanced photocatalytic applications