Preparation And Photocatalytic Activities Of CoAl-LDH-Based Composite Materials

With the ever-increasing urgent demand of solving environmental contamination, the technology of photocatalytic degradation using semiconductors has received widespread attention. Nevertheless, the low quantum yield and low solar energy utilization efficiency have been two major stumbling blocks inhibiting the catalytic applications of most semiconductors. Therefore, design and synthesize new photocatalytic materials with higher activities have always been the focus in the photocatalytic field. Some of layered compounds with photocatalytic activity can be exfoliated into two-dimensional nanosheets, which subseuently act as building blocks assembled with other semiconductors to form heterojunction structures, which leads to the facts that the spectral response range can be effectively expanded, the rapid recombination of the charge carriers can be restrained, and that the photocatalytic activities of nanocomposites are thus enhanced. It was shown that layered compounds have abundant applications in the semiconductor photocatalytic field.Co-Al layered double hydroxides(CoAl-LDH) are a kind of anionic lamellar compounds, which can be exfoliated into positively charged nanosheets. Assembled with other suitable semiconductors, the resulted materials exhibit visible-light driven catalytic activity. In this dissertation, CoAl-LDH was chosen as the research object. Using the exfoliation-restacking method, we have successfully synthesized various mesoporous nanohybrids via electrostatically derived self-assembly of positively charged CoAl-LDH nanosheets with the mixed negatively charged CdS nanoparticles, TiNbO5- nanosheets and Ti3O72- nanosheets, respectively. XRD, SEM, TEM, PL, UV-vis DRS, XPS, N2 adsorption-desorption isotherms and photoelectric chemical tests were adopted to characterize the as-prepared materials. The photocatalytic activities of the as-prepared materials were measured by photodegradating the target pollutants under visible light irradiation. The mechanisms of photocatalytic processes were also studied.(1) Preparation and characterization of CdS/CoAl-LDH pillared nanohybrid. The CdS-pillared nanohybrid CdS/CoAl-LDH was synthesized by mixing the exfoliated LDHs nanosheets with CdS nanosol. It was revealed that the as-synthesized solids are mesoporous with an expanded specific surface areas of 62 m2·g-1, the gallery height was supposed to be 2.14 nm. Under visible-light irradiation, the hybrids showed higher photodegradation activity towards acid red 88, the degradation degree reached up to near 90% within 180 min, which is more than two times those of the pristine CoAl-LDH and CdS. Meanwhile, the nanohybrids exhibited good photocatalytic stability.(2) Preparation and characterization of CoAl-TiNbO5 interstratified nanohybrid. Using the exfoliated nanosheets of narrow-band semiconductor CoAl-LDH and broad-band semiconductor HTiNbO5 as the building blocks, we obtained the nanocompounds with two kinds of nanosheets restacking in a layer-by-layer ordered arrangement. The as-prepared materials have a hierarchical porous structure with an expanded specific surface areas of 46 m2·g-1. The two components in the hybrids contact compactly with each other, forming strong electron coupling, which expanded the response range of light absorption and accelerated the conduction and seperation rate of the charge carriers. The CoAl-TiNbO5 composites displayed a preferable photodegration activity towards acid red under visible-light exposure, which is about 3.2 times that of CoAl-LDH and 17.8 times that of HTiNbO5, together with a good performance in the recycling experiments.(3) Preparation and characterization of CoAl-Ti3O7 sandwich-like nanohybrid. The CoAl-Ti3O7 hybrids were synthesized by compositing CoAl-LDH nanosheets and H2Ti3O7 nanosheets via electrostatic self-assembly route. The compactly contacted two components formed house-of-cards structure when accumulated disorderly, bringing multilevel distributed mesopore diameters and expanding specific surface areas to 67 m2·g-1. Due to the expanded surface area and the synergistic effect of the formed Z-type heterojunction, the CoAl-Ti3O7 composites possessed of remarkable photocatalytic activity in degradating rhodamin B under visible-light irradiation, which is 6.6 times that of CoAl-LDH and 3.8 times that of H2Ti3O7, it was found that the hybrids exhibited high recycling stability performance.