Study On The Fabrication And Photocatalytic Performance Of ZnO-PNIPAM Thermosensitive Composites Via SI-ATRP

Zinc oxide(ZnO)has extensive application prospect in many fields due to its good photocatalytic,chemical and thermal stability during use.However,nanometer sized ZnO materials have a high level of agglomeration,as well as,are difficult to recovery and reuse.Hence,it is of great value on theory and practice to solve these problems by composite modification,as well as maintain the mtrinsic characteristics of ZnO.In this work,ZnO nanorods(ZnO NRs)were successfully grown by solution method onto seed layers of ZnO that fabricated by Langmuir-Blodgett(LB)method.Meanwhile,ZnO microspheres(ZnO MS)assembled by nanoflaskes were fabricated by solution method.On this basis,thermo-responsive polymer of poly(Nisopropylacrylamide)(PNIPAM)was grafted onto the surfaces of ZnO nano particles(ZnO NPs),ZnO NRs and ZnO MS via surface-initiated atom transfer radical polymerization(SI-ATRP).The structures of ZnO-PNIPAM composites were confirmed by Fourier-transform infrared spectroscopy(FT-IR)and X-ray diffraction(XRD).X-ray photoelectron spectroscopy(XPS)and thermogravimetric analyses(TGA)were used to determine the components and the interactions between the two surface components.The morphology of composites were observed by scanning electron microscopy(SEM)and transmission electron microscopy(TEM).Meanwhile,the molecular weights(Mn),molecular weight distributions(Mw/Mn)and the distribution of polymers were detected by gel permeation chromatograph analyses(GPC)and energy dispersive X-ray spectrometry(EDS)mapping images.Moreover,contact angles(CA)measurement,dynamic light scattering(DLS)measurement and optical photographs were carried out to study the temperaturesensitive behavior of composites.Furthermore,the degradation experiments we re employed to estimate the photocatalytic ability of composites.ZnO seed layers were fabricated by LB method and annealing treatment.The optimized parameters for Zn2+/stearic acid monolayer formation and multilayers deposition with the transfer ration closed to 1.0 by LB method were determined,with znic acetate dehydrate/ultrapure water solution concentration of 0.10 ?mol·m L-1,stearic acid/chloroform spreading liquid solution concentration of 0.28 mg·m L-1,spreading volume of 80 ?L,still time of 10 min,barrier speed of 6 mm·min-1,surface pressure of 32 m N·m-1 and lifting speed of 5 mm·min-1.ZnO seed layers were obtained by following annealing treatment at 550 oC.ZnO NRs were grown by solution method onto seed layers of ZnO.The influence of growing parameters(temperature,time and solution concentration)on morphology of ZnO NRs were discussed.Furthermore,ZnO MS assembled by nanoflaskes were synthetized by solution method and the effect of the sodium citrate solution concentration and annealing temperature on the morphology and structure of microspheres were studied.The results of XRD and SEM showed that the particles of ZnO seed layers were close-packed and the size distribution was narrow.ZnO NRs grown on seed layers of ZnO by solution method had well-defined and close-packed structures,with the uniform diameter distribution of about 100 nm,preferred-oriented along c axis direction and high crystallinity.In addition,the obtained ZnO MS by solution method implied high crystallinity and purity,with the uniform diameter distribution of about 5 ?m.The microsphere was assembled by many nanosheets with a thickness of 20 nm,which were intercrossed with each other.The results of nitrogen adsorptiondesorption analysis revealed that the existence of abundant mesopores in the prepared ZnO MS and the specific surface area of the microspheres was calculated to be 29.118 m2/g by the Brunauer-Emmett-Teller(BET)equation.ZnO materials with different morphologies were selected to graft thermo-responsive polymers of PNIPAM by SI-ATRP.According to TGA results,the grafting amount of polymers grafted onto the ZnO NRs were 18.34 % with the grafting amount of ATRP initiator of 0.1 mmol,and those of the ZnO NPs and ZnO MS were 26.37 % and 17.14 % with the grafting density of ATRP initiator of 0.7 and 0.54 mmol·g-1,respectively.The FT-IR results showed that PNIPAM polymers chains was grafted from the SI-ATRP initiator pre-installed onto the surface of ZnO materials.XRD results suggested that SI-ATRP polymerization did not cause any detectable change on the structure of ZnO.SEM and TEM results demonstrated that,after the grafting treatment,a thin layer(2~15 nm)of polymer had been introduced to the surface of ZnO with no obvious change of the original morphology.The GPC results and EDS mapping images revealed that the grafted polymers were dispersed evenly on the surface of ZnO,with the relatively high molecular weights(Mn =24300 Daltons)and narrow molecular weight distributions(Mw/Mn =1.19).This indicated that the controlled thickness and well-defined polymer layers could be conducted in a controlled/living radical polymerization for the synthesized procedures.Around the corresponding relationship between photocatalytic ability and temperature,the performance test of three composites were investigated,respectively.The decrease in the atom concentration of Zn on the surface of ZnO-PNIPAM composites indicated that some active sites onto composites' surface were occupied partly by XPS results.Furthermore,the degradation experiments results revealed that the grafted ZnO-PNIPAM composites still maintained photocatalysis.At 25 oC,ZnO NPs composites could degrade 23 % rhodamine B under UV light,while 14 % and 40 % of rhodamine B photodegradation efficiency of ZnO NRs and ZnO MS composites,respectively.Moreover,the photocatalytic ability of composites was controlled by the temperature.At 45 oC,the rhodamine B photodegradation efficiency of ZnO NPs composites was only 8 %,while the photocatalytic activity almost unchanged of ZnO NRs and ZnO MS composites.In addition,the results of recycling tests showed that ZnO NPs-PNIPAM composites presented good recycling performance,with the degradation efficiency of 1.16 % for the fifth cycle.Also,there were no significant decrease in the degradation efficiency for ZnO NRs and ZnO MS during the five cycles.Moreover,the CA values showed that the surface of ZnO NRs-PNIPAM composites exhibited temperature-induced hydrophilicity/hydrophobicity transition.At 25 oC,the surface of composites presented hydrophilicity.At 45 °C,the value of the surface angles increased to 124 o,suggesting the hydrophobicity of the surface of composites.In addition,the size of ZnO NPs-PNIPAM and ZnO MS-PNIPAM composites exhibited thermo-modulated increasing/decreasing transition.The average diameter of NPs-PNIPAM composites and MS-PNIPAM composites were 52 nm and 5340 nm at 25 oC,respectively,wherever the value was 42 nm and 5030 nm at 45 oC.Meanwhile,the dispersion of NPs-PNIPAM and MS-PNIPAM composites was influenced by the temperature.At 25 oC,composites dispersed well in aqueous solution,resulting that the aqueous solution became muddy and the transmittance was closed to zero.Instead,the agglomeration of composites at 45 °C resulting the transparent solution and the increased transmittance.This temperature controlled desperison behavior of ZnO-PNIPAM composites can significantly prevent agglomeration and simplify the recycle process of ZnO used as the catalyst for pollutant degradation system and avoid secondary pollution.