STM Study Of The Adsorption And Reaction Of Phenol And Catechol On The ZnO Surface

Zinc oxide(ZnO)is widely used and plays an important role in people’s life.Compared with other wide band gap semiconductors,the excition binding energy of ZnO is as high as 60 meV,which has been used in the fields of light,electricity,and catalysis for a long time.With the continuous development of technology,the demand for innovative design of functional materials is getting higher and higher.Therefore,the molecular functionalization of ZnO surfaces has attracted widespread attention.These molecules adsorbed on the surfaces provide customizability in terms of size,skeleton and function,and can feasibly control the physical and chemical properties of surfaces and interfaces on the molecular scale.A large amount of researches have been devoted to investigating the interactions between functional molecules and the ZnO surface,which has provided important information for understanding the principles of how to tailor the surface properties of ZnO and thus optimizing the function of the combinational systems.However,the direct experimental researches at the atomic level are still lacking and hence in highly demand.As the commonly used anchoring groups,catechol and phenol both possess rather simple molecular structures,and have been frequently used as model systems to study their absorption behaviors as well as electronic interactions on the surfaces of various materials.The research results have provided valuable information for deepening people’s understandings of the surface functionalization process.In this thesis,we mainly used low-temperature scanning tunneling microscopy(LT-STM)in combination with density functional theory calculations(DFT,together with collaborators),to investigate systematically the adsorption,diffusion,reaction,and desorption of catechol and phenol molecules on the ZnO(10-10)surface.The main findings are summarized in the following:1.For the first time we obtained the high-resolution STM images of catechol on the surface of ZnO(10-10)at liquid nitrogen temperature(LN2,77 K)and systematically investigated its morphologies under different scanning biases.Combined with the theoretical calculations,we concluded that catechol molecules possess mostly a bidentate configuration and bind to the surface through the adjacent two OH groups.According to the different degrees of dissociation,the catechol molecules can be further divided into three adsoption configurations,i.e.fully-protonated(FP),half-protonated(HP)and fully-deprotonated(FD)ones.Annealing the LN2 sample to room temperature(RT)drives the catechol molecules to diffuse on the surface and assemble into a one-dimensional chain structure orienting along the[0001]direction.When the coverage also increases,a c(2×2)superstructure is gradually developed and finally becomes dominating on the surface.Detailed theoretical calculations pointed out that the charge transfer between catechol and ZnO surface strongly depends on the adsorption configuration and organization state of the former.At low temperature,isolated catechol molecules are mainly acceptors,receiving electrons from the surface.At room temperature,the catechol transformed into donors after aggregation,and the degree of electron transfer per molecule decreases along with the increasing coverage.2.For the phenol molecule we also investigated its single molecular adsorption on the ZnO(10-10)surface occurring at low temperature and observed an obvious different morphology compared with catechol.Combining the high-resolution STM images with theoretical calculations,we concluded that the adsorbed phenol molecules also hold three different configurations,i.e.type-S,type-L-Ⅰ and typeL-Ⅱ.All these phenol molecules can migrate prefe rentially along the[1-210]direction under the tip interactions.Moreover,sometimes the tip pulses can also stimulate the structural transformation between type-L-Ⅰ and type-L-Ⅱ phenols.When annealing the low-temperature sample to RT or directly perform adsorption at RT,we found the type-L-Ⅰ phenol molecules diffuse readily and assemble into a chain structure orienting along the[0001]direction,whereas the type-L-Ⅱ and type-S phenol molecules first aggregate into a more stable "three-point" dimer structure before transforming further into the chains composed of type-L-Ⅰ.The theoretical calculations also revealed the charge transfer occurring between phenol and ZnO surface which is similar to the case of catechol,i.e.the single phenol molecule plays mainly as an acceptor while the aggregated phenol changes into electron donors relative to the ZnO surface.3.After the detailed study of the adsorption,diffusion and organization processes of catechol and phenol on the ZnO(10-10)surface,we further investigated their surface reactions under the stimilations of either light or thermal energy.Our experiments found that both the adsorbed catechol and phenol may transform into phenoxy group at elevated temperature,which will reduce the ZnO surface during the desorption process and create line defects upon removing the left Zn ions.Under the illumination of ultraviolet(UV)light at low temperature,the isolated catechol molecules were observed to partially change into more stable configurations whereas the phenol molecules more or less remain intact.When the UV illumination was performed at RT,both the aggregated catechol and phenol molecules were found to partially desorb directly from the surface,and leave oxygen vacancies.Comparing the thermal reactions of the catechol and phenol molecules with and without the UV illumination,we concluded that the reaction products are almost the same but the UV light obviously increases the transform ratio.These results may bring insights into the photothermal stability of the catechol-or phenol-functionalized ZnO surfaces in practical applications.