Modification Of Surfacial Electronic Structures Of Nanomatierials For Applications In Catalysis And Other Fields

The improvement of preparation methods and characterization technologies during the past thirty years has driven the nano technology to achieve an unprecedented rapid development. The surface area of one solid material increases as its size decreases. When the size is reduced to the nanometer range, its surface structure and electronic property will play an important role in the practical applications. With the development of two dimensional materials, such as graphene, their excellent properties have been used widely in the catalytic chemistry, surface spectroscopy, energy storage and other hot fields. On the other hand, based on the existing knowledge, researchers have begun to control the surface properties through various surfacial modifications, to change their original electronic structures and achieve better performance in the practical applications. Therefore, it is significant to carry out different investigations on the modifications on the surface of nanomaterials not only for the basic research, but also for their potential values of applications.This article aims to control the electronic structure and surface configuration of graphene (also and other materials) via doping and other surfacial modification methods, exploring their potential applications in surface catalysis, surface enhanced Raman scattering (SERS) and energy storage. This will help us understand and grasp the mechanism and value of the influence of surfacial modification on the electronic structure of various functional materials. The details are summarized as follow:1. First, graphene oxides were prepared using an improved Hummer method. Then, the nitrogen atoms were doped into graphene as "graphitic","pyridinic","pyrrolic" and "amine" forms by the post treatment of graphene oxides in a solution of ammonia in an autoclave at180degree celsius. The reduction of4-nitrophenol is employed to test the catalytic ability of the obtained sample. It is found that when the sodium borohydride is taken as the reducing reagent, the prepared nitrogen doped graphene could take the catalytic role in reducing4-nitrophenol, with the reaction rate of7.34×10-8mol*L-1s-1, and conversion rate and selectivity both almost100%. This is the first report of metal free catalysts employed in this reaction under mild conditions without light radiation. Based on the in situ FTIR characterization and density functional theory calculations, we find the4-nitrophenol will adsorb on the carbon atom next to the doped nitrogen atom on graphene surface, via the oxygen atom of its hydroxyl group. In addition, we also have studied the application of boron doped graphene and hydrogen decorated graphene in the oxygen reduction reactions based on simulations. It is found both of these two midified graphene have good catalytic activities. There will be local high spin and high charge areas on the graphene surface induced by the doped foreign elements, and these special areas could be taken as high active sites for the adsorption of reactant molecules, further promoting the catalytic reaction to be carried out continuously.2. There are plentiful oxygenated chemical groups on the surface of graphene oxides. They combine with graphene in different formats, leading to different modifications on the original electronic structure of graphene sheets. Via the improved Hummer method, we obtain graphene oxides. Through the hydrothermal treatment at120degree celsius, we find the obtained sample have good catalytic ability for the reduction of4-nitrophenol, with the reaction rate of0.147min-1. However, after the sodium hydroxide treatment at high temperature, its catalytic performance will be greatly reduced. Through the FTIR and X ray photoelectron spectroscopy characterizations, we find the relative content of surfacial hydroxyl groups will decrease after the sodium hydroxide treatment. Combined with the density functional theory simulations, it is found that hydroxyl and alkoxyl groups could contribute to the adsorption of4-nitrophenol, but carboxyl and epoxy groups are not conducive to its adsorption. This is in line with the experimental results. Therefore, hydroxyl and alkoxyl groups are beneficial for the catalytic ability of the obtained sample while carboxyl and epoxy groups should be avoided in this reaction. Apart from this, we have prepared a film electrode using the prepared graphene oxides and found that it has weak ferroelectricity. This is the first case about ferroelectricity found on graphene based materials, which may be due to the abundant hydroxyl groups. The hydroxyl groups on the surface and at the edge of graphene sheets tend to form hydrogen bonds, which will form one dimensional hydrogen bond chains further and produce spontaneous polarizations. They will transfer following the applied electric field, contributing to its ferroelectric property.3. Taking the prepared silver nanoparticles as the substrate and selecting pyridine and Rhodamine6G as the probe molecules, respectively, we find the applied magnetic field has a negative influence on SERS. This is the first report about the influence of magnetic field on the Raman spectra. By virtue of calculations, it is found the free electrons on surface of silver nanoparticles tend to transfer into their body center regions, decreasing the surface electrons and weakening the surface plasmon resonance. This will reduce the electromagnetic mechanism of SERS. Simutaneously, the5s orbital of silver tend to decrease and the π anti bond orbital will increase under the applied magnetic field, broadening the energy gap for electron transfer and reducing the probability of charge transferring at the interface, which will weaken the chemical enhancement of SERS. In addition, based on the existing experimental reports and our simulatiaons, we have found boron doped graphene could also enhance the SERS signals with pyridine taken as the probe molecule. The doped boron atom is positive charged and the nitrogen atom of pyridine is negative charged, so the combined interaction at the interface will be strengthened after boron doping, facilitating the adsorption of pyridine molecule on graphene substrates and contributing to the electron transferring from the substrate to the probe molecule under excitation, and this is benefical for the chemical enhancement mechanism.4. We have found the pyridinic like nitrogen doping within graphene lattice could improve its performance in Li ion batteries. When the doped atoms are four and they form a hole structure, this new material will have high Li storage capacity, good stable structure and high mobility for the adsorbed Li atoms. Because of the stronger catch force for Li caused by the doped nitrogen element, More Li atoms will remain on the doped graphene surface during the charge and discharge process. Therefore, compared with the pristine graphene, the discharge capacity of nitrogen doped graphene will decrease faster, as seen in the related experimental result. Moreover, the nitrogen doping can increase the orbital hybrid degree between various carbon materials and the transition metal Pd, strengthening its adsorption for Pd. This is beneficial for its use in hydrogen storage. The stronger catch force induced by doping could prevent the leaching and aggregation of the supported Pd, facilitating their uniform distribution on the carbon materials and contributing to their performance in hydrogen storage and catalysis.