Semiconductor-mediated photocatalysis has received tremendous attention as it holds great promise to address the worldwide energy and environmental issues. To overcome the serious drawbacks of fast charge recombination and the limited visible-light absorption of semiconductor photocatalysts, many strategies have been developed in the past few decades and the most widely used one is to develop photocatalytic heterojunctions. This review attempts to summarize the recent progress in the rational design and fabrication of heterojunction photocatalysts, such as the semiconductor–semiconductor heterojunction, the semiconductor–metal heterojunction, the semiconductor–carbon heterojunction and the multicomponent heterojunction. The photocatalytic properties of the four junction systems are also discussed in relation to the environmental and energy applications, such as degradation of pollutants, hydrogen generation and photocatalytic disinfection. This tutorial review ends with a summary and some perspectives on the challenges and new directions in this exciting and still emerging area of research.

Semiconductor heterojunction photocatalysts: design, construction, and photocatalytic performances

The science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3¯5 Wh/kg with a power density of 300¯500 W/kg for high efficiency (90¯95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1¯2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 cm2. Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.

Ultracapacitors: Why, How, and Where is the Technology

Electrospinning is a versatile and viable technique for generating ultrathin fibers. Remarkable progress has been made with regard to the development of electrospinning methods and engineering of electrospun nanofibers to suit or enable various applications. We aim to provide a comprehensive overview of electrospinning, including the principle, methods, materials, and applications. We begin with a brief introduction to the early history of electrospinning, followed by discussion of its principle and typical apparatus. We then discuss its renaissance over the past two decades as a powerful technology for the production of nanofibers with diversified compositions, structures, and properties. Afterward, we discuss the applications of electrospun nanofibers, including their use as "smart" mats, filtration membranes, catalytic supports, energy harvesting/conversion/storage components, and photonic and electronic devices, as well as biomedical scaffolds. We highlight the most relevant and recent advances related to the applications of electrospun nanofibers by focusing on the most representative examples. We also offer perspectives on the challenges, opportunities, and new directions for future development. At the end, we discuss approaches to the scale-up production of electrospun nanofibers and briefly discuss various types of commercial products based on electrospun nanofibers that have found widespread use in our everyday life.

Electrospinning and Electrospun Nanofibers: Methods, Materials, and Applications

We developed two-step solution-phase reactions to form hybrid materials of Mn3O4 nanoparticles on reduced graphene oxide (RGO) sheets for lithium ion battery applications. Mn3O4 nanoparticles grown selectively on RGO sheets over free particle growth in solution allowed for the electrically insulating Mn3O4 nanoparticles wired up to a current collector through the underlying conducting graphene network. The Mn3O4 nanoparticles formed on RGO show a high specific capacity up to ~900mAh/g near its theoretical capacity with good rate capability and cycling stability, owing to the intimate interactions between the graphene substrates and the Mn3O4 nanoparticles grown atop. The Mn3O4/RGO hybrid could be a promising candidate material for high-capacity, low-cost, and environmentally friendly anode for lithium ion batteries. Our growth-on-graphene approach should offer a new technique for design and synthesis of battery electrodes based on highly insulating materials.

Mn3O4-Graphene Hybrid as a High Capacity Anode Material for Lithium Ion Batteries

Rare-earth upconversion nanophosphors (UCNPs), when excited by continuous-wave near-infrared light, exhibit a unique narrow photoluminescence with higher energy. Such special upconversion luminescence makes UCNPs promising as bioimaging probes with attractive features, such as no auto-fluorescence from biological samples and a large penetration depth. As a result, UCNPs have emerged as novel imaging agents for small animals. In this critical review, recent reports regarding the synthesis of water-soluble UCNPs and their surface modification and bioconjugation chemistry are summarized. The applications of UCNPs for small-animal imaging, including tumor-targeted imaging, lymphatic imaging, vascular imaging and cell tracking are reviewed in detail. The exploration of UCNPs as multifunctional nanoscale carriers for integrated imaging and therapy is also presented. The biodistribution and toxicology of UCNPs are further described. Finally, we discuss the challenges and opportunities in the development of UCNP-based nanoplatforms for small-animal imaging (276 references).

Upconversion nanophosphors for small-animal imaging

Nanoporous polystyrene fibers for oil spill cleanup

In this study, we conducted a subtle regulation of micro- and nanostructures of electrospun polystyrene (PS) fibers via tuning the molecular weights of the polymers with different sources, solvent compositions, and solution concentration. The surface morphology and porous structures of as-prepared PS fibers were characterized, and a full and intuitive observation of the porous structures as well as a tentative account of the formation of porous structures was presented. Additionally, the porous PS fibrous mats showed much higher oil absorption capacities than those of commercial polypropylene fibers in the form of a non-woven fabric, which displays a bight future for oil spill cleanups. We believe that such regulation of micro- and nanostructures of the PS fibers will widen the range of their applications in self-cleaning materials, ultra-high sensitivity sensors, tissue engineering, ion exchange materials, etc.

Subtle regulation of the micro- and nanostructures of electrospun polystyrene fibers and their application in oil absorption

Hierarchically structured, superhydrophobic filter medium exhibiting robust filtration performance to airborne particulate were prepared by a facile deposition of electrospun polysulfone/titania nanoparticles (PSU/TiO2 NPs) on a conventional nonwoven substrate. The air permeability, tensile strength and abrasion resistance of pristine PSU fibrous membranes could be finely controlled by regulating the solvent composition and number ratios of jets. By employing the TiO2 NPs incorporation, the pristine PSU fibers were endowed with promising superhydrophobicity with a water contact angle of up to 152°. The quantitative hierarchical roughness analysis using N2 adsorption method has confirmed the major contribution of TiO2 NPs on enhancing the porous structure and surface fractal features with irregular rough structure. Filtration performance studies have revealed that the filtration efficiency and pressure drop of resultant hybrid membranes could be manipulated by tuning the surface composition as well as the hierarchical structures. Furthermore, the as-prepared PSU/TiO2-5 membrane exhibited improved filtration efficiency (99.997%) and pressure drop (45.3 Pa) compared with pristine PSU membrane, which would make them a promising media for fine particle filtration, and a new insight was also provided into the design and development of high performance filter medium based on hierarchical structured fibers.

Hierarchically structured polysulfone/titania fibrous membranes with enhanced air filtration performance.

Hollow micro-/nanostructured electrode materials with high active surface area are highly desirable for achieving outstanding electrochemical properties. Herein, we report the successful synthesis of hierarchical hollow NiCo2O4 nanospheres with high surface area as electrode materials for supercapacitors. Electrochemical measurements prove that such electrode materials exhibit excellent electrochemical behavior with a specific capacitance reaching 1229 F/g at 1 A/g, remarkable rate performance (∼83.6% retention from 1 to 25 A/g) and good cycling performance (86.3% after 3000 cycles). Furthermore, the asymmetric supercapacitor is fabricated with hollow NiCo2O4 nanospheres electrode and activated carbon (AC) electrode as the positive and negative, respectively. This device exhibits a maximum energy density of 21.5 W h/kg, excellent cycling performance and coulombic efficiency. The results show that hollow NiCo2O4 nanosphere electrode is a promising electrode material for the future application in high performance supercapacitors.

Jianmao Yang

Papers

Nanoporous polystyrene fibers for oil spill cleanup

Subtle regulation of the micro- and nanostructures of electrospun polystyrene fibers and their application in oil absorption

Hierarchically structured polysulfone/titania fibrous membranes with enhanced air filtration performance.

Synthesis of hollow NiCo2O4 nanospheres with large specific surface area for asymmetric supercapacitors

CuCo2O4 nanowire arrays wrapped in metal oxide nanosheets as hierarchical multicomponent electrodes for supercapacitors