Interface Engineering in 2D/2D Heterogeneous Photocatalysts.

Assembling different 2D nanomaterials into heterostructures with strong interfacial interactions presents a promising approach for novel artificial photocatalytic materials. Chemically implementing the 2D nanomaterials' construction/stacking modes to regulate different interfaces can extend their functionalities and achieve good performance. Herein, based on different fundamental principles and photochemical processes, multiple construction modes (e.g., face-to-face, edge-to-face, interface-to-face, edge-to-edge) are overviewed systematically with emphasis on the relationships between their interfacial characteristics (e.g., point, linear, planar), synthetic strategies (e.g., in situ growth, ex situ assembly), and enhanced applications to achieve precise regulation. Meanwhile, recent efforts for enhancing photocatalytic performances of 2D/2D heterostructures are summarized from the critical factors of enhancing visible light absorption, accelerating charge transfer/separation, and introducing novel active sites. Notably, the crucial roles of surface defects, cocatalysts, and surface modification for photocatalytic performance optimization of 2D/2D heterostructures are also discussed based on the synergistic effect of optimization engineering and heterogeneous interfaces. Finally, perspectives and challenges are proposed to emphasize future opportunities for expanding 2D/2D heterostructures for photocatalysis.

1D/2D Heterostructured Photocatalysts: From Design and Unique Properties to Their Environmental Applications.

With the progress of dissimilar dimensional materials, 1D and 2D materials have been extensively investigated as heterogeneous photocatalysts, which realize the unique dimensionality-dependent advantages and mitigate the disadvantages during the environmental and sustainable energy applications. The progress in 1D/2D heterogeneous photocatalysts stems from the combination of different growth modes between 1D and 2D nanostructures and the judicious control to establish the oriented 1D/2D interface. To promote this field, it is necessary to gain insights into the interface engineering in the 1D/2D heterogeneous photocatalysts. This mini-review summarizes the designed synthesis and application of dimensional heterogeneous photocatalysts from 1D and 2D materials. Some typical research to overview the advantages of different types of interface engineered 1D/2D heterogeneous photocatalysts for various photocatalytic processes is highlighted in detail. At last, this review ends by drawing on more design strategies for such 1D/2D heterogeneous photocatalysts, which may inspire further developments of efficient dissimilar dimensional heterogeneous photocatalysts.

Facile preparation of protonated hexaniobate nanosheets and its enhanced photocatalytic activity.

Exfoliated hexaniobate nanosheets E-H2K2Nb6O17-x (E-HKNO) with broad light absorption (up to 850 nm) and high adsorption properties were prepared via ion exchange and transient annealing processes with micron-size K4Nb6O17 powders as the precursor. The as-prepared E-HKNO nanosheets show excellent visible light photodegradation performances when compared to degussa P25, which was evaluated in terms of degradation of Rhodamine B (Rh B). High adsorption and broad light absorption characteristics could be attributed to the exfoliation behavior and the reduction of surface Nb5+ to Nb4+, which was confirmed by x-ray photoelectron spectroscopy (XPS) and Raman spectra. From the Mott-Schottky analysis, the E-HKNO is an n-type semiconductor and has a higher flat band voltage (-0.46 V versus RHE at pH = 7), compared with K4Nb6O17. In addition, the electrochemical impedance spectroscopy (EIS) indicates that the E-HKNO nanosheets have an increased semiconductor-electrolyte charge transfer resistance, which is not conducive to the separation of photogenerated carriers (e--h+). Accordingly, a small amount of holes scavenger (EDTA) was added to improve the photodegradation performance of the E-HKNO, since the holes scavenger can inhibit the recombination of the photogenerated carriers. This work provides not only a facile method for the preparation of an efficient E-HKNO nanosheets photocatalyst, but also new insights for further enhancing the photodegradation performance by adding trace scavenger.

Photoreversible [2] Catenane via the Host-Guest Interactions between a Palladium Metallacycle and ß-Cyclodextrin.

We report the efficient preparation of an A2D2 (A = acceptor and D = donor) metallacycle 2 = [(en)2Pd2(1)2](NO3)4, using the coordination driven self-assembly of trans-azobenzene based bispyridyl ligand 1 and (en)Pd(NO3)2 (en = ethylenediamine). In the metallacycle, the trans-azobenzene units serve both as a structural element and as sites for subsequent host-guest chemistry with ß-cyclodextrin, leading to the formation of a [2] catenane 3. This catenation process is reversible and can be switched off and on in a photocontrollable manner via the trans-cis isomerization of the azobenzene units.

Anatase TiO2 nanotubes as photoanode for dye-sensitized solar cells.

To achieve higher power conversion efficiency of dye-sensitized solar cells, anatase TiO2 nanotubes anodized and transferred onto fluorine doped tin oxide glass. This technique is a promising candidate to improve the efficiency due to its outstanding properties, such as high light scattering effect, high surface-to-volume ratio, which result in enhancing light harvesting, minimum trapping sites, and low recombination rate. In this review, the structure, fabrication, and property of the TiO2 nanotube photoanode is compared with other photoanodes. In addition, the integration of a heterojunction and other advancements into the TiO2 nanotubes for getting better performance is also briefly discussed.

Surface scattering and reflecting: the effect on light absorption or photocatalytic activity of TiO2 scattering microspheres.

In this article, nanoplate-assembled flower-like TiO2 scattering microspheres were prepared via a microwave solvothermal process. X-ray diffraction analysis, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, N2 adsorption-desorption isotherms and UV-Vis spectroscopy were used to characterize the as-prepared TiO2 scattering microspheres. The results indicate that the scattering microspheres exhibit extremely high photocatalytic activity due to their unique structural and optical properties. Since the nanoplates are assembled on the surface of the microspheres, when the incident light irradiates the scattering surface, the light will be scattered. The scattered light has a longer optical path and thus is more easily absorbed. In addition, the doped F atoms lead to an enhancement of the surface acidity, thus increasing the adsorption of the reactant and improving visible light adsorption, thereby enhancing the number of photons available to take part in the photocatalytic reaction.

Building heterogeneous nanostructures for photocatalytic ammonia decomposition.

Ammonia is an important chemical for human beings that is used in the synthesis of chemical fertilizers and products; meanwhile, it is also a hazardous compound which causes undesirable odors, several diseases, and environmental problems. Therefore, there is an urgent need to control and remove ammonia pollutants from water, air and soil. Hence, clean processes using photocatalysis to convert ammonia into H2 and N2 have been an important research topic in recent years. To date, only some metal-loaded common photocatalysts, such as TiO2, ZnO, C3N4, graphene and other carbon-based materials together with their hybrid materials, have been reported as active photocatalysts for the decomposition of aqueous ammonia solutions. In this review, we summarize the recent advances in heterogeneous nanostructures for photocatalytic ammonia decomposition. Particular emphasis is also given to metal-loading along with the resulting heterojunctions. Furthermore, the recent efforts toward the development of heterogeneous nanostructures for photocatalytic ammonia decomposition in this direction are discussed and appraised. Finally, perspectives and future opportunities regarding the challenges and future directions in the area of heterogeneous photocatalysts for ammonia decomposition are also provided.

Sensitivity Improvement of Stretchable Strain Sensors by the Internal and External Structural Designs for Strain Redistribution.

Fiber strain sensors that are directly woven into smart textiles play an important role in wearable systems. These sensors require a high sensitivity to detect the subtle strain in practical applications. However, traditional fiber strain sensors with constant diameters undergo homogeneous strain distribution in the axial direction, thereby limiting the sensitivity improvement. Herein, a novel strategy of internal or external structural design is proposed to significantly improve the sensitivity of fiber strain sensors. The fibers are produced with directional increases in diameter (internal design) or polydimethylsiloxane (PDMS) microbeads attached to surfaces (external design) by combining hollow glass tubes used as templates with PDMS drops. The structural modification of the fiber significantly impacts the sensing performance. After optimizing structural parameters, the highest gauge factor reaches 123.1 in the internal-external structure design at 25% strain. A comprehensive analysis reveals that the desirable scheme is the internal structural design, which features a high sensitivity of 110 with a 100% improvement at ∼5-20% strain. Because of the sufficiently robust interface, even at the 800th cycle, fiber sensors still possessed an excellent stable performance. The morphology evolution mechanism indicates that the resistance increase is closely related with the increased peak width and distance, and the appearance of gaps. Based on the finite element modeling simulation, the quantified effective contributions of different strategies positively correlate with the improved sensitivity. The proposed fiber strain sensors, which are woven into the two-dimensional network structure, exhibit an excellent capability for displacement monitoring and facilitate the traffic control of crossroads.

Highly stretchable multi-walled carbon nanotube/thermoplastic polyurethane composite fibers for ultrasensitive, wearable strain sensors.

Here, we report a novel highly sensitive wearable strain sensor based on a highly stretchable multi-walled carbon nanotube (MWCNT)/Thermoplastic Polyurethane (TPU) fiber obtained via a wet spinning process. The MWCNT/TPU fiber showed the highest tensile strength and ultra-high sensitivity with a gauge factor (GF) of approximately 2800 in the strain range of 5-100%. Due to its high strain sensitivity of conductivity, this CNT-reinforced composite fiber was able to be used to monitor the weight and shape of an object based on the 2D mapping of resistance changes. Moreover, the composite fiber was able to be stitched onto a highly stretchable elastic bandage using a sewing machine to produce a wearable strain sensor for the detection of diverse human motions. We also demonstrated the detection of finger motion by fabricating a smart glove at the joints. Due to its scalable production process, high stretchability and ultrasensitivity, the MWCNT/TPU fiber may open a new avenue for the fabrication of next-generation stretchable textile-based strain sensors.

Self-assembly of metal-ion-responsive supramolecular coordination complexes and their photophysical properties.

Herein, we describe the synthesis and characterization of two newly self-assembled supramolecular coordination complexes (SCCs) by using the cis-{Pt(PEt3)2}2+ center and two different kinds of pyridyl-derivatized ligands. The photophysical properties of the resulting SCCs in the presence of metal ions revealed that these dipyridyl-containing SCCs hold potential as metal-ion-responsive materials.

Enhanced Conversion Efficiencies in Dye-Sensitized Solar Cells Achieved through Self-Assembled Platinum(II) Metallacages.

Two-component self-assembly supramolecular coordination complexes with particular photo-physical property, wherein unique donors are combined with a single metal acceptor, can be utilized for many applications including in photo-devices. In this communication, we described the synthesis and characterization of two-component self-assembly supramolecular coordination complexes (SCCs) bearing triazine and porphyrin faces with promising light-harvesting properties. These complexes were obtained from the self-assembly of a 90° Pt(II) acceptor with 2,4,6-tris(4-pyridyl)-1,3,5-triazine (TPyT) or 5,10,15,20-Tetra(4-pyridyl)-21H,23H-porphine (TPyP). The greatly improved conversion efficiencies of the dye-sensitized TiO2 solar cells were 6.79 and 6.08 respectively, while these SCCs were introduced into the TiO2 nanoparticle film photoanodes. In addition, the open circuit voltage (Voc) of dye-sensitized solar cells was also increased to 0.769 and 0.768 V, which could be ascribed to the inhibited interfacial charge recombination due to the addition of SCCs.

Double-layer electrode based on TiO2 nanotubes arrays for enhancing photovoltaic properties in dye-sensitized solar cells.

The present work reports a rapid and facile method to fabricate a novel double-layer TiO2 photoanode, which is based on highly ordered TiO2 nanotube arrays and monodispersive scattering microspheres. This double-layer TiO2 sphere/TNTA photoanode have got many unique structural and optical properties from TiO2 scattering microspheres, such as high specific surface area, multiple interparticle scattering, and efficient light-harvesting. Results indicate that this as-fabricated double-layer TiO2 sphere/TNTA front-illumination dye-sensitized solar cell, which is fabricated from the TiO2 nanotube arrays with a 17.4 µm length after TiCl4 treatment, exhibits a pronounced power conversion efficiency of 7.24% under an AM1.5 G irradiation, which can be attributed to the increased incident photon-to-current conversion and light-harvesting efficiency.

Photocatalytic degradation of methyl orange over nitrogen-fluorine codoped TiO2 nanobelts prepared by solvothermal synthesis.

Anatase type nitrogen-fluorine (N-F) codoped TiO(2) nanobelts were prepared by a solvothermal method in which amorphous titania microspheres were used as the precursors. The as-prepared TiO(2) nanobelts are composed of thin narrow nanobelts and it is noted that there are large amount of wormhole-like mesopores on these narrow nanobelts. Photocatalytic activity of the N-F codoped TiO(2) nanobelts was measured by the reaction of photocatalytic degradation of methyl orange. Results indicate that the photocatalytic activity of the N-F codoped TiO(2) nanobelts is higher than that of P25, which is mainly ascribed to wormhole-like mesopores like prison, larger surface area, and enhanced absorption of light due to N-F codoping. Interestingly, it is also found that the photocatalytic activity can be further enhanced when tested in a new testing method because more photons can be captured by the nanobelts to stimulate the formation of the hole-electron pair.

Characterization and activity of mesoporous titanium dioxide beads with high surface areas and controllable pore sizes.

Mesoporous titanium dioxide beads with high surface areas (over 90 m(2)/g) and tunable pore sizes (from 12.8 to 16.5 nm) were synthesized via a solvothermal process heating by microwave irradiation, with ammonia being used as both a source of nitrogen and a control agent for the mesoporous structure. Structural characterization indicated that the mesoporous TiO(2) beads were composed of nanocrystals and pores and the beads possess a optical band gap energy of 3.11 eV. The doping nitrogen was in the form of NH(x) or NO(x) species and was adsorbed on surface of the beads, which caused changes to the surface electronic structure. The results show that the samples which possess higher-order structure, large surface area and well-defined crystallinity have the best performance in photocatalytic activities exhibited as evaluated in the degradation of methylene blue.