| Summary: | 碩士 === 國立中央大學 === 化學研究所 === 91 ===
In-situ Scanning Tunneling Microscopy (In-situ STM) and Cyclic Voltammetry (CV) are used to study the bulk deposition process of mercury on iodine-modified and bare Ir(111) electrodes. We also perform similar deposition experiments on Pt(111) and Rh(111) to investigate the effect of substrate on the electrodeposition of Hg films.
The compositions of Ir(111) surface can dominate the morphology of Hg film. An iodine adlayer render layer-by-layer deposition of Hg till the 7th layer when 3D islands growth prevail. At the mean time, high-quality STM atomic resolution identifies a well-ordered iodine overlayer residing on top of the Hg film, strongly suggesting that the Hg film is crystalline. At a bare Ir(111), Hg deposition was also layer-by-layer but only up to two atomic layers. Without the iodine adlayer, the Hg film was however seriously pitted and it was not possible to achieve atomic resolution.
The coverage and real-space structure of iodine adatoms on Ir(111)-supported Hg thin films vary with potential, similar to the “electrocompression” previously observed at Au(111). Making potential positively render a series of structures, starting with (Ö3 x Ö3)R30° , □ = 0.33, transforming to (11 x Ö3-R30°), □ = 0.36 at 0.3 V, and finally ends up with (4 x 4), □ = 0.44 at 0.5 V. This Ir-supported Hg film began to oxide to produce islands consisting of multilayers of Hg2I2. On the other hand, since iodine adatoms on Hg film is highly ordered and close-packed, the spacing between two nearest iodine atoms is 4.5 Å, from which the Hg interatomic spacing of 3.1 Å is deduced.
The iodine overlayer on Pt(111) results in layer-by-layer deposition of Hg till the 3th layer, where 3D islands growth takes control to produce nanometer-scaled Hg drops. The iodine adlayer floating atop the Hg films is highly ordered, identified as (Ö13 x Ö13)R13.9° . However, Hg film begins to oxide at 0.25 V, which occurs anomalously 250 mV more negative than that of Ir(111). This oxidation event does not produce well packed Hg2I2 on Pt(111). Hg electrodeposited on Rh(111) is uniform for only one layer before 3D islands growth predominates. It’s not possible to achieve atomic resolution on this system. In summary, the identity of substrates dominates the mechanisms of Hg electrodeposition, as Hg form uniform films amount to 7, 3 and 1 layer thick on Ir(111), Pt(111) and Rh(111), respectively.
Electrodeposition of Pb on Ir-supported monolayer Hg films has been examined with STM. Pb thin films are atomically smooth up to a thickness of about 4 layers. This result suggests that the resolution of Hg thin film electrodes used for heavy metal stripping analysis can produce higher resolution than traditional mercury drop electrode, because Pb adatoms tend to stay at the surface of Hg films. As the use of anodic stripping voltammetry (ASV) and thicker Hg film in analyzing Pb in aqueous samples, the results showed the peak current of Pb oxidative stripping was directly proportional to the Pb2+ concentration in the solution. Further improvement of this method include optimization of deposition, the concentration and time of deposited Hg, the rotating speed in depositing solution, and potential of reduced Pb2+.
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