Yang Quanhong

Reduced-sized monolayer carbon nitride nanosheets for highly improved photoresponse for cell imaging and photocatalysis

Two-dimensional graphitic carbon nitride (g-C3N4) nanosheets (GCNNs) have been considered as an attractive metal-free semiconductor because of their superior catalytic, optical, and electronic properties. However, it is still challenging to prepare monolayer GCNNs with a reduced lateral size in nanoscale. Herein, a highly efficient ultrasonic technique was used to prepare nanosized monolayer graphitic carbon nitride nanosheets (NMGCNs) with a thickness of around 0.6 nm and an average lateral size of about 55 nm. With a reduced lateral size yet monolayer thickness, NMGCNs show unique photo-responsive properties as compared to both large-sized GCNNs and GCN quantum dots. A dispersion of NMGCNs in water has good stability and exhibits strong blue fluorescence with a high quantum yield of 32%, showing good biocompatibility for cell imaging. Besides, compared to the multilayer GCNNs, NMGCNs show a highly improved photocatalysis under visible light irradiation. Overall, NMGCNs, characterized with monolayer and nanosized lateral dimension, fill the gap between large size (very high aspect ratio) and quantum dot-like counterparts, and show great potential applications as sensors, photo-related and electronic devices.摘要不含金属的二维石墨相氮化碳纳米片由于具有优异的催化、光学及电学性能而受到研究者的广泛关注. 然而制备纳米级尺寸的单层石墨相氮化碳纳米片仍然存在挑战. 本文采用一种高效超声方法制备了横向尺寸约为55 nm, 厚度约为0.6 nm的单层石墨相氮化碳纳米片(NMGCNs). 由于其纳米级尺寸及单层片状结构, NMGCNs表现出与大尺寸纳米片和量子点显著不同的光响应特性. NMGCNs的水分散溶液具有良好的稳定性能和优异的荧光性能, 荧光量子产率可达32%, 所以可用于细胞荧光成像. 此外, NMGCNs表现出比多层石墨相氮化碳纳米片更优异的可见光催化性能. 独特的小尺寸及单层超薄结构使得NMGCNs在传感器和光电子等领域都具有潜在应用前景.

Graphene conductive additives for lithium ion batteries: Origin, progress and prospect

This paper gives a critical review on the scientific origin, current research progresses and application prospects of graphene conductive additives applied in lithium-ion batteries (LIBs) and emphasizes that the electron transport (enhancement of electron conductance by graphene) and ion diffusion (steric effect of graphene for ion diffusion) should be considered comprehensively for a real mass application. Due to the high electronic conductivity, two-dimensional planar geometrical structure and the ″most flexible and thinnest″ character, our group proposed that graphene is a very promising conducting additive for LIBs. Through ″plan-to-point″ contact model with active material particles, graphene can improve the electronic conductivity of the electrode laminate with much less addition fraction, and hence enhance the energy density of the LIBs. Therefore, graphene shows much better performance than other types of conductive additives, such as carbon black, conducting graphite and even carbon nanotube. Due to the size difference of active material particles, the optimized addition fraction of graphene is different. For example, 2 wt% of graphene works best for LiFePO4, while the most suitable addition amount of graphene in LiCoO2 is only 1 wt%. At the same time, graphene and conducting carbon black can work together to construct a much more efficient conductive network. This kind of binary conductive additive can build high speed pathway for electron both in long and short distance (that is, electrons can transport in the whole electrode and gain access to the whole surface of the active material particle simultaneously), and further improve the electrochemical performance of active materials. Hence, hybrid material containing graphene and carbon black is fabricated to use directly as a novel binary conductive additive. Other than the improvement of the electron conduction, the introduction of graphene brings a bottleneck for its application in LIBs, which has been reported by our group in 2012 based on a 10 Ah LIB using graphene as conductive additive, that is, the steric effect for lithium ion diffusion. Ion diffusion in the porous electrode will be retarded by planar graphene because it is very hard to penetrate through the hexagonal carbon ring for lithium ions. Many efforts have been made by our group to illustrate this effect. We found that it is the thickness and tortuosity of the electrode laminate that determines and the situation varies with different active material. When it comes to LiFePO4, the ion steric effect is not obvious when the electrode is thin (e.g. thinner than 26 μm). When the electrode sheet is thicker (>39 μm), the rate performance of LiFePO4 becomes worse when the graphene addition increases. However, when it comes to LiCoO2, which is large in particle size, the steric effect from graphene disappears. It can be attributed to the pore tortuosity of the electrode. Since the particle size of LiCoO2 is much larger, the electrode is not as tortuous as that of LiFePO4 electrode. Therefore, ions diffuse easily along the electrode thickness direction. Nevertheless, it should be noted that the steric effect in LiFePO4 electrode can be elimated by introducing pores on the surface of graphene or making ribbon-like graphene to provide more diffusion path for lithium ions in the porous electrode. From the practical points, the dispersion of graphene in the active materials is a vitally important but a tough job. In order to construct an efficient conducting network, graphene should be well dispersed and cover every particle of the active material in the electrode. Agglomeration of graphene will not construct a good conducting network, and even bring adverse effects for the ion diffusion. Normal mixing equipments alone cannot ensure good dispersion of graphene, and ultrasonication is an efficient pre-treatment step. New techniques with higher dispersion efficiency are expected to realize really uniform and monolayered dispersion of graphene in active materials, which is very important for mass application of graphene additives in LIBs.