| Titanium dioxide(TiO2)has been considered as a reliable and efficient photocatalyst in environmental treatment due to its chemical stability and environmental friendly characteristics.However,two obstacles greatly limit the practical application of TiO2,the poor utilization of solar energy and the low quantum efficiency.Therefore,it is vital to develop visible response photocatalyst systems to meet the practical need in environmental cleaning.Tin monoxide(SnO)is a p-type semiconductor,has attracted much attention due to its stability at room temperature,prominent hole mobility and adjustable band structure.Due to the adjustable band structure,the improved light absorption and redox ability could be acquired by adjusting the size and morphology of SnO.In addition,the coupling of SnO and TiO2 can form a p-n junction at the interface,which further facilitate the enhanced photocatalytic activity.Unfortunately,SnO is thermodynamically unstable and hard to acquire through solution processes due to the fact that divalent tin ions(Sn2+)are inclined to their highly-charged state(Sn4+).Meanwhile,due to the lack of a profound understanding of the growth mechanism,it is a challenge to control the morphology and particle size of SnO during the hydrothermal process.In this respect,we focus on the control of the size,morphology of SnO and design a series of SnO microstructures with reduced transmission pathways and optimized band structure based on the research of SnO growth mechanism,ensuring the improved charge separation efficiency and the stronger redox ability when it is excited by visible light.Based on the good band structure matching characteristics between TiO2 and SnO,TiO2/SnO p-n heterojunction was designed and prepared.The influence of SnO morphology on light absorption,carrier separation and redox ability was explored,and the mechanism for the enhanced photocatalytic activity of heterojunctions was revealed.Plate-like SnO with side length of 10 ?m and thickness of 1-2 ?m was prepared via the hydrothermal method.A possible growth mechanism of plate-like SnO was proposed based on the evolution of the phase and morphology.It is indicated that the growth process of the plate-like SnO was governed by a dissolution-recrystallization growth mechanism.Plate-like SnO has excellent visible light response capability,its degradation efficiency of methylene blue(MB)and methyl orange(MO)is 25%and 24%,respectively,in 10 h under light irradiation.The unsatisfying photocatalytic properties depend on the morphology and particle size.Therefore,the research focuses on the synthesis of SnO with controllable size and microstructure.SnO sheets with tunable thickness were successfully synthesized via a PEG assisted hydrothermal method.It was found that the thickness of SnO sheets greatly reduced with the increase of PEG.As PEG was preferentially adsorbed on the(001)facets of SnO thus inhibited the growth of the<001>crystal direction,which determined the final shape to be sheets with reduced thickness.The reduced thickness of SnO offered a shorter pathway thus induced a rapid charge transfer and high separation efficiency.Meanwhile,the reduced thickness of SnO sheets induced the enlarged band gap,and correspondingly increased the redox ability of SnO.Benefiting from the improved charge separation efficiency and the stronger redox ability,enhanced photocatalytic degradation efficiency was acquired in SnO sheets with reduced thickness.The SnO sheets with thickness of 150 nm exhibited the optimal photocatalytic activity in the degradation of MB,which is about 2.8 times higher than SnO sheet with 1 ?m thick.Moreover,the sample still exhibited high efficiency(-70%)after three recycling runs,demonstrating the good recyclability.Hierarchical SnO architectures assembled by nanosheets with tuned thickness were prepared using three kinds of alkali(NaHCO3,Na2CO3 and NaOH solution)with different electrolytic dissociation rate(?).It was found that the morphology and the thickness of assembling nanosheets in SnO could be delicately controlled by adjusting the electrolytic dissociation rate.When the thickness of the assembled nanosheets reduced from 200 nm to 30 nm,the specific surface area increased from 8.75 m2/g to 15.89 m2/g,providing stronger adsorb ability as well as sufficient active site.Meanwhile,the reduced thickness induced an enlarged band gap(from 2.64 eV to 2.94 eV)of SnO,endowing the increased valence band potential and correspondingly improved oxidation capacity.The above critical features ensured an enhanced photocatalytic activity.After 8 h of light irradiation,the photocatalytic degradation efficiency of MB reached 90%,and the catalyst still maintained good catalytic activity after three cycles.The TiO2/SnO p-n heterojunctions with different morphologies(including plates and microflowers)were designed and prepared via a hydrothermal route.Different dyes such as TBO,MB and MO were selected as the targeted pollutants to study the photocatalytic performance of different samples.Compared with SnO and TiO2 samples,the TiO2/SnO heterojunction exhibited enhanced photocatalytic activity,which is attributed to the extending optical absorption and the promoted charge separation efficiency induced by the formation of p-n junction.Meanwhile,it is found that TiO2/SnO micro flowers showed the highest photocatalytic activities during all the samples as the hierarchical structure provides larger specific surface area,sufficient active sites and the stronger redox capacity.Moreover,the photocatalytic mechanism for the TiO2/SnO heterojunction was proposed based on band alignments calculation and the active species trapping experiments.This study provided a new idea for the design of high-efficiency heterojunction photocatalysts,which is of significance for photocatalytic applications. |
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