Preparation of novel organometallic precursor-based printing inks and control of phase and microstructure of printed films

Abstract

Printing technology based on inkjet, microcontact, offset, and gravure printing techniques has attracted much attention as an alternative to conventional vacuum technology due to its fast, low-cost, large-scale manufacturability for solid-state devices, especially flexible devices. In accordance with these technological developments, the controlled preparation of ink with suitable rheological properties for specific printing processes represents a new issue. Initial studies focusing on printed electronics have focused primarily on the formation of metal electrodes using the inks based on Au, Ag, and Cu nanoparticles. However, such nanoparticle-based inks have serious limitations in industrial production owing to the difficulty in large scale synthesis, homogeneous dispersion and stable storage of the nanoparticles and the increased costs due to the use of surfactants. Therefore, the atmospheric stable, large-scale synthesizable, novel inks based on organometallic precursors is prepared and their applicability is investigated in this dissertation. However, no studies on the formation of solid particles or films and their sintering behavior and microstructural evolution have been attempted previously in the printed films with the organometallic precursor-based inks, unlike the case in nanoparticle-based inks. The poor adhesion of metal electrodes, including especially severe Cu, to the substrates, particularly glass, makes it harder for the printed films with organometallic precursor-based inks to be investigated. Accordingly, in this study, the novel Cu(II) complex inks based on Cu(II) formate tetrahydrate, hexylamine and ethyl cellulose were prepared with the high adhesion to substrates and the controlled viscosity, and the ink films were directly printed and heat-treated on glass, SiO2/Si, and SnO2:F (FTO) substrates for various device applications – the printed Cu metallization, p-type CuO gas sensor and photocathode for photoelectrochemical water splitting. The detailed studies on thermal decomposition behavior, the formation of solid particles, phase and microstructural evolution, and their sintering behavior of the Cu(II) complex inks enabled the optimization of the printing features, suitable for each device application. In the part I, it was found that change in the molar concentration ratio between Cu(II) formate tetrahydrate precursor and hexylamine solvent in the Cu(II) complex ink induces the difference in Cu nucleation behavior during thermal decomposition of the ink through Fourier-transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and field emission scanning electron microscopy (FE-SEM). Specifically, Cu nanoparticles were produced with slow nucleation rate and longer nucleation time period during thermal decomposition of the film printed using the ink with the relatively low hexylamine content, resulting in the film microstructure consisting of large particles with wide size distribution. The Cu particle packing density inside this film was low and the resulting electrical resistivity was high. In contrast, Cu nanoparticles were produced with high nucleation rate and shorter nucleation time period during thermal decomposition of the film printed using the ink with the high hexylamine content, resulting in the densely packed structure of small particles with narrow size distribution inside the film. As a result, lower electrical resistivity was achieved in this film. Based on these results, it is concluded that the metal-ion complex ink for the printed metallization requiring high electrical conductivity is desirable to be prepared with high concentration of reactive solvent to organometallic precursor in the ink. In addition, the reductive sintering of printed films under a formic acid (HCOOH) atmosphere was performed to obtain better electrical conductivity in the films by removing the oxide scales on the surfaces of the Cu nanoparticles produced during thermal decomposition of the films. As a result, the growth of Cu nanoparticles inside the films was promoted through such a reductive sintering and the electrical resistivity of the films could be reduced by an order of magnitude. Finally, an increase in the sintering temperature further decreased the film resistivity by inducing the necking and densification of the Cu nanoparticles. As a consequence, the lowest electrical resistivity of ~5.2 μΩcm was achieved in the optimized film using selective ink formulation and reductive and higher temperature sintering processes. In the part II, pure oxide-phase films with high crystallinity and high surface-to-volume ratio were fabricated via novel Cu(II) complex ink printing routes for functional gas sensor and photoelectrochemical device applications. First, the Cu(II) complex ink with high hexylamine content to Cu(II) precursor was used for the facile preparation of mesoporous p-type CuO sensing films. Pure polycrystalline CuO films were formed with porous structure via thermal heating from 200 °C to 600 °C in air. Such porous structures were formed by interconnected pore channels due to thermal evolution of the empty space between densely packed, small Cu nanoparticles. In particular, the mesoporous CuO thin film calcined for 1 h at 500 °C in air revealed the best H2 and C2H5OH gas responses, attributed to sufficient hole concentration, good crystallinity and the highest surface-to-volume ratio. The porous film calcined at 600 °C exhibited the fastest gas response rates, due to both the excellent crystallinity and the low grain boundary density. These results obtained using neat CuO provide the right direction for the development of a high-performance p-type gas sensor. Furthermore, the novel ink solution route used for fabricating the mesoporous structure in this work can provide the realization of the printed sensor devices as well as the facile approach for ion doping or alloying in oxides to improve the sensor performance. Second, another novel, facile printing processing for preparation of pure oxide-phase films with high crystallinity and high surface-to-volume ratio was introduced. The direct printing synthesis of metal oxide hollow spheres in the form of film on a substrate is reported for the first time. This method offers facile, scalable high-throughput production and device fabrication processes. The printing was carried out by a doctor-blade method using Cu(II) complex ink with low hexylamine content. Following only thermal heating in air, well-defined polycrystalline copper oxide hollow spheres with a submicron diameter (≤1 μm) were formed spontaneously while being assembled in the form of film with good adhesion on the substrate. This spontaneous hollowing mechanism was found to result from the Kirkendall effect during oxidation at an elevated temperature. The CuO films with hollow spheres, prepared via direct printing synthesis at 500 °C, led to the creation of superior p-type gas sensor and photocathode for photoelectrochemical water splitting with completely hollow cores, a rough/porous shell structure, a single phase, high crystallinity, and no organic/polymer residue. As a result, the CuO hollow-sphere films showed high gas responses and permissible response speeds to reducing gases and high photocurrent density, compared to conventional CuO powder films and to the values previously reported. These results exemplify the successful realization of a high-throughput printing fabrication method for the creation of superior nanostructured devices.