First-principles Study Of CuO-based And CdS-based Molecular And Solids Compounds

In recent years, with the development of nanotechnology, the exploration and utilization of novel, highly efficient nanostructured semiconductor materials have attracted more and more attention. Nanostructured semiconductor materials have great potential applications in the areas of electronics, catalysis, sensors, biomedical and so on. Two kinds of transition metal semiconductor materials copper oxide and cadmium sulfide are popular and wildly studied. Especially, cuprous oxide (Cu2O) is a p-type semiconductor material with narrow band gap (Eg=2.0 eV), which can directly absorb visible light. Because of non-toxic, pollution-free, and simple preparation, it has been well studied in the semiconductor and catalytic domain. Recently, experimental studies have found that these materials can surprisingly show ferromagnetic or paramagnetic properties. Besides, Hybrid organic-inorganic materials copper hydroxide acetate Cu2(OH)3(CH3COO)·H2O, which is mainly made of copper oxide and organic material (such as acetic acid), has shown an unusual magnetic properties. However, these special magnetic properties have not been well explained. In addition, cadmium sulfide (CdS), which belongs to the family of traditionalⅡ-Ⅵsemiconductor, has a band gap of 2.4 eV. Due to its high redox of conduction band, it has been regarded as an effective photocatalyst for water splitting and CO2 reduction under VL and can be used in many other areas. However, the poor quantum efficiency and antiphotocorrosive property inhibit the wide application of CdS. Therefore, a great number of investigations are dedicated on the improvement of its properties.In this thesis, copper oxide and cadmium sulfide compound have been consisdered as two main research objects. We have investigated the two materials' geometry, electronic structure, spin topology, magnetic properties, photocatalytic activity and so on by first-principles, with which many of new materials with different dimensions (solid, molecules and clusters) were studied. Based on reasonable physical and chemical models, we devoted to the investigation of the relationship between structure and native electronic properties of the materials, which will shed light on understanding the physical and chemical properties of materials and thus benefit the design of novel and highly efficient nanostructured materials. The detail research works are as follows.1. By using density-functional theory in the framework of first-principles molecular dynamics, we carry out a dynamical annealing to identify the stable structures of copper hydroxide acetate Cu2(OH)3(CH3COO)·H2O, a fundamental compound in the field of hybrid organic-inorganic materials, for which accurate crystallographic data are not available. For the total spin value S=0, we obtain a large set of stable structures having very close sets of coordinates and differing in the spatial distribution of the spin densities. Only some of these structures (～20%) feature spin topologies consistent with the in-plane ferromagnetic character experimentally established. An electron localization analysis through the electron localization function ELF shows that the different atomic and molecular units composing the systems (Cu2(OH)3(CH3COO)-, OH- and H2O) can be associated with different localization basins and connect to each other through non-covalent interactions. The relationship between the appearance of sizeable spin densities on specific O atoms and the magnitude of the spin densities on the neighboring Cu atoms is also discussed.2. Experimental evidence shows that small Cu2O nanoparticles exhibit ferromagnetic or paramagnetic properties, which demonstrates the promising possibility to recycle catalyst Cu2O easily in wastewater treatment. So, the theoretical calculation has been performed to study the magnetic property of copper/oxide clusters further. That is, the structural and electronic properties of a series of CumOn ((m, n)=(4,1); (4,2); (4,5); (16,15); (28,15); (44,15); (28,27)) clusters were investigated by using generalized gradient approximation (GGA) and Hubbard U method within density functional theory (DFT). It is found that the electronic structures of bulk Cu2O calculated by the GGA and GGA+U are similar. The structures of CumOn ((m, n)=(4,1); (4,2); (4,5)) are all planar. For the bulk-product CumOn ((m, n)=(16,15); (28,15); (44,15); (28,27)), O atoms prefer to be the outmost atoms. We classified two types of clusters based on their O to Cu atomic ratios. One is O-rich clusters, i.e. Cu4O5, Cu16O15 and Cu28O27. The other is O-poor clusters, i.e. Cu4O, Cu4O2, Cu28O15 and Cu44O15. The calculation results show that the O-rich clusters (Cu4O5, Cu16O15 and Cu28O27) have longer average Cu-Cu bonds and larger binding energy than those of the O-poor ones. More interestingly, the former are magnetic and give ferromagnetic ordering while the latter are non-magnetic. The hydrogenation of O-terminated clusters can improve its stability whereas depress its magnetism. The study may be extremely useful for the potential applications of Cu2O nanopaticles in catalysis and semiconductor field.3. The adsorption of monomer and dimer CdS clusters on graphene, (3,3) and (5,5) single-walled carbon nanotubes are investigated by density functional theory. We firstly calculated the adsorbed properties of single S and Cd atoms. It is found that the adsorption energy of single S shows stronger dependence on substrate curvature than Cd. The adsorption energies for Cd on different substrates are no more than 0.2 eV. Our results show that the different adsorption behavior of single S and Cd atoms impacts on adsorption of CdS clusters. For CdnSn cluster, their adsorption behavior relies on the interaction of S, Cd and C surface. The cluster with small size and C surface with small radius leads to high stability for composite systems. Maximum stability is encountered for bridge site of CdnSn on substrates. The adsorption behavior of CdnSn cluster can be further explained by orbital hybridization between cluster and substrates, which shows the lower Fermi level of CNTs, charges transfer from CNTs to cluster and a small band gap appears for CNT in the most stable composite systems. This study is extremely hepful for understanding and promoting applications of CdS/CNTs composite system in catalysis and other areas.4. We devoted to design of highly photoeclectrochemical activity CdS by metallic doping approach. First-principles density-functional theory was performed to investigate the atomic structures and electronic properties of dopant complexes involving Cu, Ag, Zn, Ga, In in wurtzite CdS. Based on the mondoping calculated results, we examined codoping of (Cu+Ag), (Ag+In) and (Cu+In) for CdS. The photoelectrochemical activity of doped CdS systems were analyzed. It is found that mondoping of Zn and Ga can lower the VB of CdS in a great deal, implying good catalytic activity of Zn- and Ga-monodoped CdS for the degeneration of organic object. Further more, (Cu+Ag) and (Cu+In) codoping may improve the photoeclectrochemical activity for H2 production from water splitting under VL. In addition, the effect of doping concentration (include modoping Zn, and codoping (Cu+In)) on CdS photoelectrochemical activity were also examined theoretical and experimentally. The improved high photoelectrochemical activity of CdS have been obtained. So, the calculation results can be used to guide the design and preparation of novel photocatalysts based on CdS or other semiconductors.