| Summary: | 碩士 === 國立臺灣科技大學 === 材料科學與工程系 === 106 === Two Dimensional (2D) materials offer a new opportunity to explore fundamental physics and overcome the scaling limit of bulk materials. The most extensively studied materials are 2H phase in group VI TMDCs such as molybdenum disulfide (MoS2). MoS2 is one of TMDs with adjustable optical properties by tuning the layer number. Bulk MoS2 has an indirect bandgap, which increases monotonically as the number of layer decreases. This ability makes MoS2 as a promising material for photocatalytic CO2 reduction. Light absorption performance is one of the keys to determine the performance metric of the photo-catalyst for CO2 photo-reduction.
This thesis studies MoS2 ultrathin film for CO2 photo-reduction. To make CO2 photo-reduction happen, there are 6 important factors to be considered. Bandgap (Absorption), charge separation and transportation, band position vs redox potential, reaction between adsorbed species and charge carries, adsorption of chemical species on the photo-catalyst surface, and desorption of chemical products. This thesis is to investigate the thickness dependence and substrate effect on the CO2 reduction of MoS2. MoS2 ultrathin film was successfully prepared by post sulfurization.
First step was to deposit MoO3 then converted to MoO2 and sulfurized into MoS2. AFM confirm the thickness, were named as MS3, MS7, and MS17 corresponding to the original oxide layer of 3 nm, 7 nm, 17 nm and 25 nm. UV visible measurement was performed to find out the bandgap and absorption property. From tauc plot calculated from absorbance data, the bandgap are 1.7 eV, 1.65 eV and 1.58 eV, and the absorption performance are 10%, 12% and 30% for MS3, MS7 and MS17 respectively. Raman Spectroscopy confirm the frequency differences between A1g and E2g1 with 23.27, 24.76, 25.76 and 26.24 cm-1 are for MS3, MS7, MS17 and bulk respectively. The grain size was ranging from 40 to 235Å.
The gas chromatography has been performed to understand the productivity and selectivity. 2 x 2 sample inserted to the 7 cm3 chamber with nitrogen, CO2 and H2O entered later. From the GC measurement, 3 products obtained from all samples are methane (0.095 µmole), acetaldehyde (0.12 µmole) and acetone (0.04 µmole). From the thickness dependent measurement, the production rate increase proportional to increased thickness but decreases after exceeding 17 nm. The production rate also revealed if the C1 product not sensitive to thickness inversely proportional with C2 and C3 product.
Finally, MoS2 ultrathin film successfully demonstrated for CO2 photo-reduction. Thickness dependent and substrate effect play a role for selectivity and productivity. Detailed mechanism need further investigation.
|
|---|
