Controllable Synthesis And Their Organic Pollutant Degradation Performances Of BiOX（X=Cl, I）/TiO₂Nanocomposite Arrays

Semiconductor photocatalysis promises great potential in solving environmentalpollution and energy shortage problems, and has become the research forefront andhotspot in the fields of materials, environment and energy. However, shortage of highperformance photocatalyst has been the bottleneck in booming the photocatalysisapplications. Hence, how to enhance the photocatalytic performance and expand thevisible-light response range of the catalyst have been the two key scientific issues.This research work is mainly focused on the controllable synthesis and organicpollutant degradation performance of novel high performance photocatalysts byhybridizing layered bismuth-based compounds of BiOX(X=Cl, Br, I) andhighly-ordered TiO2nanotube arrays (TNTAs) for photocatalysis application. Novelhigh performance film catalyst of BiOX(X=Cl, I)/TiO2nanocomposite arrays aredeveloped by introducing nanostructured BiOX(X=Cl, I) into highly-ordered TNTAs.Considering the application bottleneck of BiOCl/TiO2system which only responses toUV light, Ag nanoparticles are introduced into the BiOCl/TiO2system to obtainAg-BiOCl/TiO2nanocomposite arrays with highly enhanced visible-lightphotocatalytic performance. In addition, a facile and environment friendly approach tosynthesize unique hierarchical BiOCl photocatalyst is developed with the assistance ofa biodegradable surfactant Polyvinyl Alcohol (PVA). Main research results in thiswork can be summarized as following:1. Novel flake-tube structured film catalyst of BiOCl/TiO2nanocomposite arrays(BiOCl/TNTAs) is successfully prepared by uniformly loading BiOCl nanoflakes ontoboth outer and inner walls of well-separated TiO2nanotubes (NTs) via anodization inthe10vol.%H2O electrolyte followed by sequential chemical bath deposition(S-CBD) method. The loading effect can be fine-tuned by adjusting depositiontemperature, deposition time and cycle times. The organic pollutant degradationperformance of as-prepared BiOCl/TNTAs samples is evaluated by the photocatalytic(PC) degradation test toward the methyl orange (MO) solutions under UV lightirradiation. In addition, the UV light PC performance of BiOCl/TNTAs samples isfurther confirmed by the transient photocurrent response test. The results from thecurrent study reveal that the BiOCl/TNTAs-2sample prepared with depositiontemperature of60℃, single deposition time of5min and2deposition cycles exhibits the best PC activity, favorable stability and the highest photocurrent density among allthe BiOCl/TNTAs samples, revealing that loading proper amount of BiOCl nanoflakescan effectively improve the PC performance and photoelectric properties of TNTAs,and over loading may lead to the decline of PC activity and photoelectric performance.Synergistic effect of the following factors may contribute to the remarkably enhancedPC activity and photoelectric property for the BiOCl/TNTAs-2sample including:①the formation of BiOCl/TiO2heterojunction for promoting the separation ofphoto-induced electron-hole pairs;②3D connected intertube spacing system andopen tube-mouth structure may benefit the light absorption and utilization, as well asthe convenient immersion and diffusion of the solution to be degraded;③largerspecific surface area resulted from the dispersion of tiny nanosized BiOCl nanoflakesonto the surface of TiO2NTs for increasing the contact area between the organicmolecules and catalyst.2. Flake-tube structured film catalyst of BiOI/TiO2nanocomposite arrays(BiOI/TNTAs) is successfully prepared by loading large amounts of narrow band gapBiOI nanoflakes onto both outer and inner walls of well-separated TiO2NTs usinganodization followed by S-CBD method. The loading effect can be fine-tuned byadjusting deposition cycle times. The results from the current study reveal that undervisible-light irradiation (λ>420nm), the5-BiOI/TNTAs sample with five depositioncycles exhibits the best photoelectrocatalytic (PEC) activity and efficiency, favorablestability in the MO degradation test, and shows the highest photocurrent densityamong all the BiOI/TNTAs heterostructured samples. The combined effects of severalfactors may contribute to the remarkable visible-light PEC performance for the5-BiOI/TNTAs sample including:①strong visible-light absorption by BiOI forraising the carriers yield;②the formation of BiOI/TiO2heterojunction for promotingthe separation of photo-induced electron-hole pairs;③3D connected intertubespacing system and open tube-mouth structure benefit the light absorption andutilization, as well as the convenient immersion of the organic solution;④largerspecific surface area resulted from the dispersion of tiny BiOI nanoflakes on thesurface of TiO2NTs for increasing the contact area between the organic molecules andcatalyst;⑤the applied external electrostatic field for promoting the separation andtransfer of carriers.3. Novel film photocatalyst of Ag-BiOCl/TiO2nanocomposite arrays(Ag-BiOCl/TNTAs) is successfully prepared by loading Ag nanoparticles and BiOCl nanoflakes successively onto the surface of TiO2NTs via in situ light reductionfollowed by S-CBD method. Ag-BiOCl/TNTAs film catalyst contains Ag/TiO2,Ag/BiOCl and BiOCl/TiO2composite structures comprised of fcc structure Ag,anatase phaseTiO2and tetragonal phase BiOCl. Ag nanoparticle size is about10~15nm, and the BiOCl nanflake size is about20~30nm. Compared with TNTAs,BiOCl/TNTAs-2and Ag/TNTAs samples, Ag-BiOCl/TNTA sample shows the highestoptical absorption properties in the visible light region, and exhibits the highest PCactivity under both UV and simulated sunlight irradiation towards the degradation ofMO solution. Main causes are proposed as follows:①SPR effect and electroncapturing function of Ag nanoparticles enhance the visible light absorption and PCactivity of Ag/TNTAs;②The contact area of Ag/semiconductor increases due to theformation of Ag/BiOCl junction in the Ag-BiOCl/TNTAs by further introducingBiOCl nanoflakes into Ag/TNTAs, thus full playing the SPR effect and electroncapturing function of Ag nanoparticles;③larger specific surface area and theformation of BiOCl/TiO2junction may further enhance the PC activity of theAg-BiOCl/TNTAs by further introducing BiOCl nanoflakes into Ag/TNTAs.4. A facile and environment friendly approach to synthesize unique hierarchicalBiOCl photocatalyst is developed with the assistance of a biodegradable surfactantPolyvinyl Alcohol (PVA). Compared with common BiOCl nanosheets synthesized inthe absence of PVA, the resulting hierarchical BiOCl photocatalyst demonstratessignificantly enhanced PC activity towards the degradation of MO solution due to itslarge specific surface area, porosity, and suitable band gap for improving the lightabsorption and utilization, dye molecules adsorption, and the transportation ofreactants and products. The amount of PVA has obvious influence on themicrostructures and PC performance of the target hierarchical BiOCl catalytst. Theoptimized amount of PVA added into the reaction system is determined according tothe results of the PC performance test.