Gas Sensing Properties And Mechanism Of Zinc Oxide Films Based On Quantum Dots

Acting as functional devices which collect the information,Metal oxide semiconductor gas sensors with its advantages of high sensitivity and simple preparation have been attracting the researchers' attention.It is thought all the time that the smaller the oxide grain size is,and the larger specific surface area is,the higher sensitivity of the gas sensor will be.In recently years,with the development of nanotechnology,preparation of nanomaterials has become perfect.However,it was found that the sensitivity of gas sensitive films based on semiconductor quantum dots with very small grain were rather low.For the abnormal phenomenon,ZnO quantum dots and reduction gas H2 S were chose as research objects in this paper.The paper presents all kinds of factors affecting the gas sensitivity of pure phase ZnO film and the methods of how to optimize the gas sensitivity.The main research contents and conclusions are as follows:(1)ZnO quantum dots were prepared through a sol-gel method.ZnO quantum dots of the hexagonal wurtzite structure were synthesized out with grain size from 3.2nm to 16.6nm by controlling the preparation conditions,such as reaction time,reaction temperature,precursor concentration and the mole ratio of reactant.ZnO quantum dots were steady dispersed in the ethanol solution.(2)ZnO film has been prepared on alumina ceramic substrate with spin coating method using ZnO quantum dot with grain size of 4.9nm at room temperature.The electrical properties and gas-sensitive properties of ZnO films were analyzed.The experimental results suggested that the resistance of ZnO film as synthesized reached up to 12G? and the response to 68.5ppm H2 S was only 1.85 at room temperature,which reached its highest value 3.5 at 90 °C.In order to improve the gas sensitive properties of the ZnO thin films,ZnO films undergo a low temperature heat treatment.Then,the sensitivity of ZnO film toward H2 S increased significantly to 75 at room temperature.Micro measurements indicated that neck connections were formed between ZnO grains after heat treatment,which make it easy for electron to transport,while there is no electronic channel formed in ZnO films based on quantum dots before heat treatment.The energy flat band model of quantum dot was revised by introducing a contact barrier,and the model give a good explanation to the experimental results.(3)In the base of the former work,heat treatment temperature increased further to study the influence of grain size on gas-sensitive properties of ZnO films,when the Electronic channels have been formed already.The experiment results showed that the gas sensitivity of ZnO film didn't monotonous increase with decreasing the grain size at room temperature and the value of sensitivity reached its maximum of 131 when the grain size of ZnO was 13.6nm.In order to eliminate the influence of heat treatment,ZnO films were prepared on Al2O3 substrate using different grain sized ZnO quantum dots at room temperature and the gas sensitive properities of ZnO films were studied.The experiment results indicated that the sensitivity of ZnO toward H2 S firstly increased and then decreased with the increase of grain size of ZnO.There was an optimized grain size of ZnO which can maximize the sensitivity.The optimized grain size is 14.1nm and is very close to the grain size above.Through the analyses of results above,a Debye size effect model was built to analyze the effect of grain size on the gas sensitive properties of ZnO films under different conditions.According to the sensitivity expression derived,the sensitivity of ZnO films reaches the theoretical maximum when the grain size is equal to 2 times Debye length,which is in good agreement with the experimental result.(4)According to the experimental results and the theoretical analysis above,the formation of carrier channel and the grain size close to twice Debye length are key factors to obtain high sensitivity ZnO film based on quantum dot.In order to prepare high gas sensitive properties films,graphene with conduction band matching and good conductivity was compounded with ZnO quantum dots to prepare composite film at room temperature.The electrical property and gas-sensitive properties of composite film were also studied in this paper.In complex system,graphite may take ZnO QDs the carrier,can significantly enhance carrier mobility and enlarge the area with gas,which are conducive to enhance the sensitivity of films.As the results showed,the sensitivity of ZnO toward H2 S firstly increased and then decreased with the increase of graphite content,and reached its highest value when graphene content is 5wt%.Taking the grain size of 14.1nm ZnO quantum dot for example,the sensitivity of composite film increased from 14.2 before graphene doped to 33.2.What's more,the response time and recovery time obviously reduced.The response time reduced from 55 s before graphene doped to 41 s,while the recovery time reduced from 246 s to 154 s.