Fabrication And Photoelectrochemical Properties Study Of Bi₂S₃/PbS-Sensitized TiO₂Nanotube Arrays

Nowadays, the world is facing serious energy and environmental problems, so it is very urgent to produce a clean, sustainable and renewable energy source to solve such problems. Titanium dioxide (TiO2) is one of the most important wide gap semiconductors and has been attracted the attention in energy conversion, such as photocatalytic degradation of pollutants in water, hydrogen production from photoelectrocatalytic water splitting, and solar cells. With advances in nanoscience, development of various forms of TiO2nanomaterials has made great progress. Compared with the traditional mesoporous TiO2film, the highly ordered TiO2nanotube arrays (TiO2NAs) has better alignment characteristics and larger specific surface area. Simltaneously, TiO2NAs allows a fast and efficient transfer of the photogenerated charge carriers. Unfortunately, TiO2is restricted in the application as solar cell materials due to its wide band gap and low quantum efficiency. The band gap of anatase TiO2is3.2eV, which is too large for efficient absorption of energy from sunlight. Therefore, we try to seek an efficiently approach of modifying TiO2to improve the overlap of the absorption spectrum with the solar spectrum. The main content in this work is as follows:1. Morphology controllable TiO2nanostructures were fabricated on the Ti substrate in an ethylene glycol solution of0.25wt%NH4F via anodic oxidation method at different temperatures (10～70℃). The morphology of TiO2nanostructures can be controlled by varying temperature of the electrolyte, such as TiO2nanotube array or nanotube/nanowire composite film. Temperature between40℃and50℃is the turning area of changing nanotube to nanowire. In the above-mentioned temperature range, the amorphous TiO2begins to change into anatase. The effect of temperature of the electrolyte during the anodic oxidation process was mainly studied.2. Compositing with narrow bandgap semiconductor is an important way to expand the TiO2light response range. Bismuth sulfide (Bi2S3) has a narrow band gap (1.3eV) and a large absorption coefficient. It is an ideal candidate for the photosensitization of TiO2. Amorphous a-TiO2NAs and anatase TiO2NAs were chose as templates to synthesis nanocomposite. A novel heterostructure of nanoscale Bi2S3-sensitized TiO2NAs was fabricated by a conventional hydrothermal method. The result shows that the coverage of Bi2S3in Bi2S3/a-TiO2NAs is larger than corresponding coverage in Bi2S3/TiO2NAs, leading to stronger light absoption and surface photovoltage response are obtained from Bi2S3/a-TiO2NAs than Bi2S3/TiO2NAs. However, in the case of Bi2S3/TiO2NAs, the Bi2S3distributed both the inside and outside rather than the top surface of TiO2nanotubes, and the size of Bi2S3is much smaller than that in Bi2S3/a-TiO2NAs. Thus, the interfacial electric field in Bi2S3/Ti02NAs is stronger than that in the case of Bi2S3/a-TiO2NAs. The results demonstrate that photoelectrochemical solar cells based on Bi2S3/TiO2NAs has short-circuit current JSc of4.54mA/cm2and photoelectric conversion efficiency η of1.86%.3. The coverage of Bi2S3in TiO2NAs can be tuned by concentration of precursor solution in hydrothermal process. Bi2S3fill the TiO2nanotubes in and do not accumulate on the top of nanotubes when the concentration of precursor solution (Bi(NO3)3) is5mmol/L. Compared with other cases, its light absoption and surface photovoltage response are strongest. And then the photoelectric conversion efficiency η of2.65%is obtained.4. Lead sulfide (PbS) is a good candidate for solar cells, because it can be made to overlap the solar spectrum optimally. By controlling its size, the absorption wavelength of the first exciton peak can easily be extended into the infrared. PbS-sensitized TiO2heterostructure nanotube arrays were synthesised by Successive Ionic Layer Adsorption and Reaction (SILAR) method. Ultrasound (PbS(u)/TiO2NAs) has an important function for the formation of PbS nanoparticles. Compared with non-ultrasound PbS/TiO2NAs, PbS(u)/TiO2NAs have small PbS nanoparticles uniformly distributed on both the outside and inside rather than the top surface of TiO2NAs. A stronger separate efficiency of photogenerated charge carriers is found in PbS(u)/TiO2NAs than that in PbS/TiO2NAs, resulting in a better photoelectrochemical property is obtained.5. PbS nanoparticles as an efficient sensitizer for TiO2nanotube arrays (TiO2NAs) were fabricated by the SILAR method under ultrasound. The coverage and size of PbS nanoparticles can be tuned by changing the repeated cycles (n) of SILAR process. UV-vis diffuse-reflectance spectra and surface photovoltage spectra were used to investigate the light absorption properties and the transfer behavior of photogenerated charges in PbS-modified TiO2NAs heterostructures. The results show that the absorption range of TiO2NAs is widened from ultraviolet to the visible region by PbS nanoparticles modifying. And a heterojunction is formed between PbS nanoparticles and TiO2NAs, which facilitates the separation of photogenerated charge carriers. TiO2NAs can be fully covered with PbS NPs with size from below4nm to25nm and large aggregates inside and outside of nanotubes when the repeated cycles (n) reach15, which exhibits the best photoelectrochemical performance in all PbS-sensitized TiO2NAs electrodes. With AM1.5G illumination at100mW/cm2, its short-circuit current density Jsc, open-circuit voltage Foe and photoelectric conversion efficiency η is9.55mA/cm2,0.95V and2.83%, respectively.6. PbS and Bi2S3co-sensitized TiO2NAs electrode was fabricated. The photoelectric conversion efficiency η is only1.13%.