| Tungsten trioxide(WO3)and its composite structure can be widely used in the fields of gas sensors,photocatalytic degradation of organic dyes,electrochromic and photochromic devices,varistors,solar devices.In this thesis,four kinds of WO3samples were prepared by a simple hydrothermal method with the aid of polyvinylpyrrolidone(PVP),sodium citrate,trisodium citrate and without surfactant.A series of graphene/WO3 composites with different graphene content were successfully prepared by a two-step method.Gas sensing properties of the WO3 nanoplates were studied systematically and the gas sensing mechanism was proposed.The photocatalytic activity of two kinds of WO3 nanoplates and commercial WO3 powder was investigated and the photocatalytic mechanism was proposed.The sensing properties of a series of graphene/WO3 composites for ammonia gas were studied,and the optimum content of graphene was determined.The details are as follows:1.Three WO3 samples of S1,S2 and S3 have been synthesized by a simple hydrothermal method with the aid of three surfactants of PVP,sodium citrate and trisodium citrate.The WO3 samples were characterized by SEM,XRD,TEM and XPS.The WS-30A gas sensor test system was used to analyze and compare the sensing properties of three WO3 samples towards acetone gas,and the best sample,S1 was determined.Then gas sensing performance of S1 was investigated.The results show that,at 300°C,S1 has an excellent linear relationship(1-500 ppm),a lower detection limit(1 ppm),a fast response and recovery(3 s and 7 s),a higher selectivity(the response value of acetone is 3.8-19.5 times that of the other six gases)and a good recycling stability(no significant change was observed in 5 cycles)and long-term stability(the response value decreased only 2.1%on the 9th day)towards acetone gas.At 140°C,S1 also has a wide linear relationship(5-500 ppm),a low detection limit(5ppm),fast response and recovery(39 s and 10 s),high selectivity(the response value of ammonia is 2.6-5.7 times that of the other six gases)and a good recycling stability(the resistance of the fifth cycle decreased only 1.1%)towards ammonia gas.The gas sensing mechanisms of S1 towards acetone and ammonia gas were discussed,respectively.The results show that S1 has similar gas sensing mechanism towards acetone and ammonia gases,while the types of active oxygen have changed due to the different operating temperatures.At 300°C,the active O-plays a leading role in the reaction with acetone gas molecules,while at 140°C,the active oxygen species turns into the active O2-.2.Two kinds of WO3 nanoplates samples,WO3-1 and WO3-2,have been prepared without surfactant and with PVP,respectively.The products were characterized by SEM,XRD,XPS,Raman spectrum and BET.The optical properties of two kinds of WO3 nanoplates and commercial WO3 powders were analyzed by UV-Vis-NIR diffuse reflectance spectroscopy.The photocatalytic activities of WO3 samples were studied by the photodegradation of methylene blue(MB).The results show that WO3-2 sample exhibits excellent photodegradation performance.At 20 min,the photodegradation rate of 10 mg L-1 MB solution was 97.1%.The photodegradation rate of 70 mg L-1 high concentration MB solution can reach 67.3%at 80 min and 98.9%at120 min,indicating that WO3-2 catalyst has excellent photodegradation activity for MB dye.The mechanism of photodegradation was explored by radical-trapping experiments of the WO3-2 catalyst and the main active substance was determined to be O2-..In addition,·OH and h+also contribut to the photooxidation of MB to certain extent.The photodegradation efficiency can be significantly improved with the increase of reaction temperature and the addition of a small amount of salt ions.At room temperature and 80 min,the photodegradation rate of 50 mg L-1 high concentration MB solution is 92.1%,which is 20.6%higher than that at 6°C,and up to 99.1%at 30°C.Under the condition of room temperature and visible light,at 80 min,the photodegradation rates of 50 mg L-1 high concentration MB solutions added with 3 m M KNO3,3 m M Na NO3,3 m M KCl and 1.5 m M K2CO3 are 56%,54%,49.3%and 55%,respectively,which were increased by 14.6%,12.6%,7.9%and 13.6%,compared with that of the blank group(without salt).3.A series of graphene/WO3 composites with different graphene content have been successfully prepared by a two-step method,and the samples were characterized by SEM,XRD,EDS and IR spectrum.Gas sensing performance of series of graphene/WO3 composite samples has been compared by WS-30A gas sensor test system.The results show that 11.1%graphene/WO3 composite has the best gas sensing performance for ammonia gas. |
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