| dc.description.abstract | In our previous studies, we have prepared photocatalyst thin films using Ultrasonic Chemical Bath Deposition (UCBD). By controlling [Ag]/[In] molar ratios in the precursors, we can obtain a single phase AgIn5S8, mixtures of AgIn5S8 and AgInS2 and a single phase AgInS2 thin films. Our studies focused on preparing AgInS2 films with different thickness and studying their electrochemical properties.
All the AgInS2 films after 400 °C thermal treatment have the orthorhombic structure and the direct energy band gap in the range of 1.93 to 1.98 eV. In order to understand the photoelectrochemical properties, AgInS2 films with different coatings were prepared. Xe lamp with an intensity of 100 mW/cm2 was then used to illuminate our samples. The photocurrent densities as a function of applied potential were measured. It was found that homogeneous AgInS2 films were obtained with increasing coatings. In addition, these dense films can effectively suppress the the dark current. In particular, the AgInS2 thin film of deposition two times (485.2 ± 28.2 nm) has the highest photocurrent density of 1.8 mA/cm2 under a bias of 1 V vs. SCE.
The fermi level (flat band potential) of films can be estimated from open circuit potential (OCP) measurements, as well as electrochemical impedance spectroscopic (EIS) analysis. The fermi levels of films in the sacrificial reagent consisted of Na2S and K2SO3 measured using OCP and EIS were varied from -0.845 V~ -1.020 V and -0.8 V~ -1.2 V, respectively. More information, such as charge transfer resistance and capacitance, can be retrieved from EIS analysis by fitting the experimental data to the model. In fact, Randle’ s model fitted the data better than other complicated models, which suggested that carriers transfer to the electrolyte directly from valence band under illumination. When depositing times increase, the resistance R1 (solution resistance and film resistance) will increase. When the applied potential decreases, the capacitance of the semiconductor will increase due to the thinner depletion layer. R2 (charge transfer resistance) will decrease dramatically under illumination, perhaps due to much higher carrier density. At -1.0 V vs. SCE, the AgInS2 film (D3) has the highest capacitance and the logest lifetime.
In the future, the EIS analysis can be used to investigate Ag-In-S thin film photoelectrode with different [Ag]/[In] molar ratios, to realize the physical original of charge transfer process of such materials. | en_US |