| TiO2 aerogel is a unique three-dimensional porous nano-particle material with the characteristics of low density,high specific surface area and good light transmittance.In the field of photocatalysis,it has a very promising application prospect.Due to the presence of too much liquid water molecules in the gel and surface free non-bridging hydroxyl in the preparation process,the three-dimensional gel structures of TiO2 aerogels are easily damaged for capillary force.And the use of supercritical drying technology is expensive,complicated and dangerous,which limited its industrial application.Therefore,in this paper,a novel method involving ultrasonic-assisted sol-gel,solvent exchange,and vacuum drying was successfully developed to synthesis the TiO2 aerogel.The effects of alcohol consumption（VEtOH）,ultrasonic frequency（F）and titanium/water mole ratio（n（Ti:H2O））on TiO2 aerogel’s structure and properties were researched.The hydrolysis polycondensation mechanism of tetrabutyl titanate,the mechanism of action of ultrasound wave,and the transformation law of micro-mesoporous pores in the ultrasonic-assisted sol-gel synthesis process were researched.The morphology and properties of the prepared TiO2 aerogels were characterized by Fourier transform infrared spectroscopy（FTIR）,the Brunauer-Emmett-Teller theory（BET）,thermogravimetric analysis differential thermal analysis（TGA-DTA）,x-ray diffraction（XRD）,field-emission scanning electron microscopy（FESEM）,energy dispersive spectroscopy（EDS）,transmission electron microscope（TEM）,and Raman spectroscopy.The adsorption and photocatalytic activity of TiO2 aerogels was evaluated by monitoring the degradation of tetracycline hydrochloride solution.Our results indicated that:（1）TiO2 aerogel was synthesized via a ultrasonic-assisted sol-gel method using tetrabutyl titanate,hydrochloric acid,water and absolute ethyl alcohol as raw material for the first time.The preparation process included the preparation of homogeneous TiO2 alcogel by ultrasonic assisted sol gel method,solvent-exchange of n-hexane,vacuum drying at 60℃ for 24 h,and calcination for 2 h after grind.（2）The effect of VEtOH on ultrasound-assisted sol-gel system was researched.The results indicated that ethanol molecules were playing an role to hinder the growth,condensation and crosslinking of sol particles.The gelation time follows an exponentially increasing with increasing VEtOH,while SBET and VTP are inversely associated with VEtOH.VEtOH does not evidently affect microporous structure,but it plays a significant influence on mesoporous structure of TiO2 aerogel.The less VEtOH is added,the value of Smes-t-method and Vmes is higher,and better adsorption-catalytic activity degrade the tetracycline hydrochloride solution.The prepared TiO2 aerogel is an amorphous micro-mesoporous complex material with five distribution peaks in about 1.2,1.6,1.8,2.8,and 3.6 nm.（3）The action mechanism of ultrasound wave in ultrasound-assisted sol-gel system was researched for the first time.The results indicated that ultrasound wave has a significant influence on structure property of TiO2 aerogel.The conditions of high temperature and high pressure,the shock waves and micro fluidic caused by cavitation effect contributed to removing too much liquid water molecules and the surface free of bridging hydroxyl in the gel inside,which improved the TiO2 aerogel’s SBET,VTP and DP.With increasing of ultrasonic frequency,SBET,Smic-t-method,VTP and Vmic of TiO2 aerogel increased first and then decreased.When ultrasonic frequency f=40 kHz,the optimized structure is SBET=563.6 m2·g-1,VTP=0.4241 cc·g-1,DP=3.01 nm,and calcined SBET=106 m2·g-1（500 ℃ for 2 h）.The Raman spectrum indicated that the ultrasonic frequency affected the deformation vibration of TiO2 aerogel.The full width at half maximum of Eg（1）increased first and then decreased with increase of ultrasonic frequency.（4）Titanium/water mole ratio significantly affected hydrolysis polycondensation reaction of tetrabutyl titanate.TiO2 aerogel’s SBET,Smes-t-method,VTP and Vmes increased first and then decreased with the increase of n（Ti:H2O）,and the optimal value was obtained in n（Ti:H2O）=8:1,while the highest Smic-t-method,Vmic and DP were found to advance to n（Ti:H2O）=7:1.The n（Ti:H2O）significantly influenced the micro-mesoporous structure and the adsorption and desorption isotherms.When n（Ti:H2O）=4:1～10:1,all the isotherms seemed to display a behavior similar to type IV（a）isotherms of mesoporous materials in the H2（b）hysteresis.When n（Ti:H2O）=15:1～25:1,especially n（Ti:H2O）=25:1,the isotherms were agreed well with type Ⅰ（b）of microporous materials in H4 hysteresis loop.（5）TiO2 aerogels possessed controllable crystal form depending on the calcining temperature.The crystal face（101）of anatase,complete and high crystallinity anatase,mixed state of 38.13%of anatase and 61.87%of rutile,and complete rutile were obtained by increasing.the temperature from 200～300℃,from 400～500 ℃,600 ℃,and from 700～1000 ℃,respectively.Calcined TiO2 aerogel displayed a typical IV type isotherms of mesoporous materials in the H2（a）hysteresis.FESEM and TEM showed that TiO2 aerogel calcined at 500 ℃ after 2 h was composed of 10 to 20 nm clusters particle,and it had a well-developed porosity network of three-dimensional skeleton structure.（6）TiO2 aerogel has a excellent photocatalytic performance as high as 96%degradation for 20 mg·L-1 tetracycline hydrochloride in under 15W UV lamp due to adsorption-photocatalytic synergistic effect.The maximum saturated adsorption capacity of TiO2 aerogel on tetracycline hydrochloride was about 25 mg·g-1,which was about 7 times compared with the adsorption content of commercial P25-TiO2(1～3.5 mg·g-1).The adsorption kinetics conformed to the pseudo-second-order model,while the kinetics of photocatalytic degradation were agreed well with the pseudo-first-order model.The best adsorption-photocatalytic degradation conditions was that substrate concentration was 20～30 mg·L-1,the dosage of photocatalyst was 30 mg/50 mL,and initial pH was 9.0. |
