There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Sustainable hydrogen production is an essential prerequisite of a future hydrogen economy. Water electrolysis driven by renewable resource-derived electricity and direct solar-to-hydrogen conversion based on photochemical and photoelectrochemical water splitting are promising pathways for sustainable hydrogen production. All these techniques require, among many things, highly active noble metal-free hydrogen evolution catalysts to make the water splitting process more energy-efficient and economical. In this review, we highlight the recent research efforts toward the synthesis of noble metal-free electrocatalysts, especially at the nanoscale, and their catalytic properties for the hydrogen evolution reaction (HER). We review several important kinds of heterogeneous non-precious metal electrocatalysts, including metal sulfides, metal selenides, metal carbides, metal nitrides, metal phosphides, and heteroatom-doped nanocarbons. In the discussion, emphasis is given to the synthetic methods of these HER electrocatalysts, the strategies of performance improvement, and the structure/composition-catalytic activity relationship. We also summarize some important examples showing that non-Pt HER electrocatalysts could serve as efficient cocatalysts for promoting direct solar-to-hydrogen conversion in both photochemical and photoelectrochemical water splitting systems, when combined with suitable semiconductor photocatalysts.

Noble metal-free hydrogen evolution catalysts for water splitting

Since the discovery of mechanically exfoliated graphene in 2004, research on ultrathin two-dimensional (2D) nanomaterials has grown exponentially in the fields of condensed matter physics, material science, chemistry, and nanotechnology. Highlighting their compelling physical, chemical, electronic, and optical properties, as well as their various potential applications, in this Review, we summarize the state-of-art progress on the ultrathin 2D nanomaterials with a particular emphasis on their recent advances. First, we introduce the unique advances on ultrathin 2D nanomaterials, followed by the description of their composition and crystal structures. The assortments of their synthetic methods are then summarized, including insights on their advantages and limitations, alongside some recommendations on suitable characterization techniques. We also discuss in detail the utilization of these ultrathin 2D nanomaterials for wide ranges of potential applications among the electronics/optoelectronics, electrocat...

Recent Advances in Ultrathin Two-Dimensional Nanomaterials

Graphene oxide: preparation, functionalization, and electrochemical applications.

Transition metal carbides and nitrides (MXenes), a family of two-dimensional (2D) inorganic compounds, are materials composed of a few atomic layers of transition metal carbides, nitrides, or carbonitrides. Ti3C2, the first 2D layered MXene, was isolated in 2011. This material, which is a layered bulk material analogous to graphite, was derived from its 3D phase, Ti3AlC2 MAX. Since then, material scientists have either determined or predicted the stable phases of >200 different MXenes based on combinations of various transition metals such as Ti, Mo, V, Cr, and their alloys with C and N. Extensive experimental and theoretical studies have shown their exciting potential for energy conversion and electrochemical storage. To this end, we comprehensively summarize the current advances in MXene research. We begin by reviewing the structure types and morphologies and their fabrication routes. The review then discusses the mechanical, electrical, optical, and electrochemical properties of MXenes. The focus then turns to their exciting potential in energy storage and conversion. Energy storage applications include electrodes in rechargeable lithium- and sodium-ion batteries, lithium-sulfur batteries, and supercapacitors. In terms of energy conversion, photocatalytic fuel production, such as hydrogen evolution from water splitting, and carbon dioxide reduction are presented. The potential of MXenes for the photocatalytic degradation of organic pollutants in water, such as dye waste, is also addressed, along with their promise as catalysts for ammonium synthesis from nitrogen. Finally, their application potential is summarized.

Applications of 2D MXenes in energy conversion and storage systems

Direct electrodeposition of reduced graphene oxide on glassy carbon electrode and its electrochemical application

The catalytic activity of molybdenum sulfide (MoS 2 ) for hydrogen evolution reaction (HER) strongly depends on the number of exposed active edges of MoS 2 nanosheets. Making single or few-layer MoS 2 nanosheets vertically stand on a substrate is a very effective way to maximally expose the edge sites of MoS 2 nanosheets. Vertically standing single or few-layer MoS 2 nanosheets on porous TiO 2 nanofibers (TiO 2 @MoS 2 ) are successfully prepared via a simple hydrothermal reaction. Due to plenty of pores in the electrospun TiO 2 nanofibers, the MoS 2 nanosheets vertically grow from the inside to the outside, and the growth mode of the MoS 2 nanosheets rooting into the TiO 2 nanofibers endows not only intimate contact between TiO 2 and MoS 2 for fast electrons transfer but also high structural stability of TiO 2 @MoS 2 heterostructure. The vertical orientation of MoS 2 nanosheets enables the active edge sites of MoS 2 to be maximally exposed. Without using Pt cocatalyst, the TiO 2 @MoS 2 heterostructure achieves high photocatalytic hydrogen production rates of 1.68 or 0.49 mmol h −1  g −1 under UV–vis or visible light illumination, respectively. This high photocatalytic activity arises from the positive synergetic effect between the MoS 2 and TiO 2 components in this novel heterostructure. In addition, the TiO 2 @MoS 2 heterostructure exhibits a high durability as evidenced by the invariable hydrogen production rate after continuous illumination over 30 h. The work advances the development of highly efficient molybdenum sulfide-based HER catalysts.

Vertical single or few-layer MoS2 nanosheets rooting into TiO2 nanofibers for highly efficient photocatalytic hydrogen evolution

Graphene, a 2D sp 2 -hybridized carbon sheet, has recently attracted great attention due to its unique electrical, optical, and mechanical properties as well as its potential use in various fi elds, such as electronics, supercapacitors, sensors, and composite materials. [ 1 ] Many methods have been proposed for graphene production, among which the chemical reduction of graphene oxide (GO) obtained from ultrasonic exfoliation of oxidized graphite is the most convenient way to yield large quantities of graphene sheets. [ 2 ] However, the practical applications of graphene are challenged by its irreversible agglomeration both in the drying state and in common solvents, which signifi cantly reduces its effectiveness. Introducing metal nanoparticles (NPs) was initially proposed in order to separate graphene sheets. [ 3 ] Nowadays, it is well realized that the dispersion of metal NPs on graphene sheets also potentially provides a new way to develop novel catalytic, magnetic, and optoelectronic materials. [ 4 ] The design and synthesis of graphene–metal nanohybrid assemblies are therefore of great interest for the exploration of their applications. Graphene–metal nanocomposites have typically been prepared by chemical or thermal reduction of mixtures of graphene (or GO) and metallic precursors. [ 3 , 5 ] Obviously, these methods involve highly toxic chemicals, such as hydrazine hydrate, or high temperature and, moreover, multiple steps are required that are time or labor consuming. Recently, electrochemical reduction of GO has been developed. [ 6 ] It is attractive for graphene-fi lm synthesis due to its simple, fast, and green nature. Typically, the electrochemical synthesis of graphene is carried out via two steps, namely, GO being fi rst assembled on the electrodes by dip-coating, drop-casting, or spray-coating methods and then being subjected to electrochemical reduction. Shortly thereafter, an approach, using GO-coated electrodes being immersed in a metallic precursor solution to perform one-step coelectrochemical reduction, was proposed to fabricate graphene– metal nanocomposite fi lms. [ 7 ] However, the above-mentioned

Yanhong Tang

Papers

Direct electrodeposition of reduced graphene oxide on glassy carbon electrode and its electrochemical application

Vertical single or few-layer MoS2 nanosheets rooting into TiO2 nanofibers for highly efficient photocatalytic hydrogen evolution

Direct electrodeposition of graphene enabling the one-step synthesis of graphene-metal nanocomposite films.

Amino siloxane oligomer-linked graphene oxide as an efficient adsorbent for removal of Pb(II) from wastewater

A highly efficient polyampholyte hydrogel sorbent based fixed-bed process for heavy metal removal in actual industrial effluent.