Shape-controlled Synthesis Of “Order-junction” Photocatalysts And Metal Co-catalysts

It is urgently desired to find novel renewable and cleaning energy to address the globe projected depletion of the fossil fuel and the resulted energy and environment crises.As an ideal approach,photocatalytic water splitting for hydrogen production using solar energy,by which solar energy can be converted to clean and storable hydrogen energy,has attracted great interest.As a bottleneck,the inefficient photocatalyst greatly restricts the development of the technology.Thanks to the efforts of many researchers,it is now realized that the key to increase the efficiency is to accelerate the separation of photoexcited charge carriers,and further the reduction and oxidation reactions at the surface of semiconductor nanocrystals.Starting from the viewpoint of strengthening the charge separation and the use of Cu₂WS₄ single crystal which enclosed by different facets and Cd_1-xZn_x S twin nanosphere as well as one-dimension?1D?nanorod,we put forward a new concept of “order-junction” design in terms of the enhanced physical process during photocatalytic hydrogen production and the well-established relationship between the junction structure and hydrogen production capability.Moreover,from the perspective of accelerating interface reaction and revealing the true role of metal cocatalysts during photocatalytic reactions,together with the systemic study of mechanism on surface diffusion and oxidative-etching-regrowth involved in kinetic controlled growth of metal nanocrystals,we have quantitatively established the kinetic theory for shape-controlled metal cocatalysts,which will lay the foundation for studying the true role of metal cocatalysts in the future.Firstly,Cu₂WS₄ decahedral crystal enclosed by {001} and {101} facets was prepared by a simple one-step hydrothermal method.The synthesis was carried out with CuCl and Na₂WO₄ as metal precursors,and TAA as the source of sulfur.According to the experimental observation by in-situ photodepositing noble metal and innovatively in-situ photocorrosion,spatial charge separation was testified over the decahedral photocatalyst.The charge separation effect is similar to that arising from the conventional junction.Therefore,it was named “quasi-homojunction”.After photodepositing 1.5 wt% Ru,the photocatalytic activity was greatly improved,about 30 times in comparison to that non-loaded one,with a quantum efficiency of 11% at 425 nm,which is among the best results reported.With the qualitative understanding of the reaction kinetics on the coupled photocatalytic system,the impact was further investigated taken by changing the surface-ratio of {001} and {101} facets.The reactive area?site?for oxidation or reduction thus could be regulated.It is interesting that highest H₂ evolution rate was reached on larger {001}/{101} ratio.In particular,with facet engineering in single crystal,such quasi-homojunction may be achieved with ordered distribution,regulated microstructure,which provides a new approach not only to quantitatively studying the reactions,but also to the design of photocatalysts with higher photocatalytic activity.Therefore,the ordered facet-induced quasi-homojunctions in single crystals were fabricated.It can be noted that the only limited quasi-homojunction could be formed duo to the numbered crystal facets in the single crystals.It is urgent to find such a structure that not only introduces orderred junctions,but also makes the junctions densely available.Based on the understanding of structure-difference induced quasi-homojunction,it can be expected that crystal defects,which will change the local structure of the crystal,may have shown the similar performance.It can be noticed that such a special structure,namely parallel coherent twin planes in face-centered-cubic crystals,will also introduce different lattice constructions.It is easy to think that homojunctions can also be formed in such special ordered structures.Therefore,Cd_1-xZn_x S solid solution photocatalyst was taken as a model.Zinc blende?ZB?Cd_1-xZn_x S nanocrystals was synthesized in the form of nanosphere and containing large amount of twin planes.Interestingly,by analyzing their structures,it was found that a twin plane in the nanocrystal introduce a 3-bilayer wurtzite?WZ?segment.Computationally,a type II staggered band alignment can be formed at the interface of WZ/ZB,therefore yielding the formation of a novel homojunction known for the ability of charge separation: with photogenerated electrons directed to the center place between two twin planes,and photogenerated holes moved to the twin planes.Thanks to the well-matched band positions of the solid solution,the twinning Cd_1-xZn_x S photocatalysts indeed exhibited superior photocatalytic performances compared to that non-twin contained ones.The highest efficiency occurred at x = 0.5,i.e.,Cd_0.5Zn_0.5S,with a visible-light-driven hydrogen production rate of 1.79 mmol h^-1 and the corresponding quantum efficiency of 43% at 425 nm,which was the highest value among the co-catalyst-free powder photocatalysts.As aforementioned,twin boundaries are parallel to each other and 1D design vertical to the twin boundary will make higher photocatalytic activity available as twin super lattice with better synergistic effect can be achieved.Based on a kinetic controlled synthesis,twinned nanorods with average aspect ratios of 1.9 and 4.0 were successfully prepared.The prediction was certified by the photocatalytic test over those nanorods.The twinned nanorod with an aspect ratio of 4.0 showed the highest visible-light-driven hydrogen production rate,reaching 2.58mmol h^-1 with a quantum efficiency of 62% at 425 nm.This is the highest efficiency reported so far over co-catalyst-free powder photocatalyst.This result thus provides a direct evidence that twin induced 1D homojunction is critical to the improvement of photocatalytic activity.The concept of single-crystal and nanotwin based?quasi-?homojunctions,similar to conventional heterojunction,can not only be simply extended to other semiconductor photocatalyst design,but also offers a new approach towards the enhancement of the charge separation process.As we known,photocatalytic reaction occurs at the interface between the photocatalyst and the reaction solution.Only with coupling the interface reaction and charge separation can make further improvement of photocatalytic activity available.Metal co-catalyst is crucial during photocatalytic hydrogen production.On the other hand,the reaction mechanism is closely influenced by the microstructure of metal particles.Then lots of efforts were devoted to quantitatively study the growth mechanisms including surface diffusion,etching and regrowth.Together with the use of well-shape Pd models,it can be realized the quantitative understanding of metal co-catalyst growth kinetics.Moreover,a general strategy was developed to quantitatively control the synthesis of various single-element based nanocrystals,or bimetallic?including alloyed?nanocrystals in the form of cube,octahedron,and many other high-indexed crystals.Starting from interface reaction,the origin of photocatalysis,together with the quantitative control of co-catalyst microstructure,is the fundamental way to mechanistically understanding as well as maximizing the effect taken by co-catalysts.