Brumbach Michael, Veneman P Alex, Marrikar F Saneeha, Schulmeyer Thomas, Simmonds Adam, Xia Wei, Lee Paul, Armstrong Neal R
Department of Chemistry, University of Arizona, Tucson, Arizona 85721, USA.
Langmuir. 2007 Oct 23;23(22):11089-99. doi: 10.1021/la701754u. Epub 2007 Sep 20.
We compare the near-surface composition and electroactivity of commercial indium tin oxide (ITO) thin films, activated by plasma cleaning or etching with strong haloacids, with ITO films that have been freshly deposited in high vacuum, before and after exposure to the atmosphere or water vapor. Conductive-tip AFM, X-ray photoelectron spectroscopy (XPS), and the electrochemistry of probe molecules in solution were used to compare the relative degrees of electroactivity and the near-surface composition of these materials. Brief etching of commercial ITO samples with concentrated HCl or HI significantly enhances the electrical activity of these oxides as revealed by C-AFM. XPS was used to compare the composition of these activated surfaces, focusing on the intrinsically asymmetric O 1s line shape. Energy-loss processes associated with photoemission from the tin-doped, oxygen-deficient oxides complicate the interpretation of the O 1s spectra. O 1s spectra from the stoichiometric indium oxide lattice are accompanied by higher-binding-energy peaks associated with hydroxylated forms of the oxide (and in some cases carbonaceous impurities) and overlapping photoemission associated with energy-loss processes. Characterization of freshly sputter-deposited indium oxide (IO) and ITO films, transferred under high vacuum to the surface analysis environment, allowed us to differentiate the contributions of tin doping and oxygen-vacancy doping to the O 1s line shape, relative to higher-binding-energy O 1s components associated with hydroxyl species and carbonaceous impurities. Using these approaches, we determined that acid activation and O2 plasma etching create an ITO surface that is still covered with an average of one to two monolayers of hydroxide. Both of these activation treatments lead to significantly higher rates of electron transfer to solution probe molecules, such as dimethyferrocene in acetonitrile. Solution electron-transfer events appear to occur at no more than 4x10(7) electroactive sites per cm2 (each with diameters of ca. 50-200 nm) (i.e., a small fraction of the geometric area of the electrode). Electron-transfer rates correlate with the near-surface tin dopant concentration, suggesting that these electroactive sites arise from near-surface tin enrichment.
我们将经过等离子体清洗或用强卤代酸蚀刻活化的商用铟锡氧化物（ITO）薄膜的近表面成分和电活性，与在高真空下新鲜沉积、暴露于大气或水蒸气前后的ITO薄膜进行比较。使用导电尖端原子力显微镜（AFM）、X射线光电子能谱（XPS）以及溶液中探针分子的电化学来比较这些材料的相对电活性程度和近表面成分。用浓盐酸或氢碘酸对商用ITO样品进行短暂蚀刻，如C-AFM所示，显著增强了这些氧化物的电活性。XPS用于比较这些活化表面的成分，重点关注本质上不对称的O 1s线形。与掺锡缺氧氧化物的光发射相关的能量损失过程使O 1s光谱的解释变得复杂。化学计量比氧化铟晶格的O 1s光谱伴随着与氧化物的羟基化形式（以及某些情况下的碳质杂质）相关的高结合能峰以及与能量损失过程相关的重叠光发射。对在高真空下转移到表面分析环境的新鲜溅射沉积氧化铟（IO）和ITO薄膜进行表征，使我们能够区分锡掺杂和氧空位掺杂对O 1s线形的贡献，相对于与羟基物种和碳质杂质相关的高结合能O 1s成分。使用这些方法，我们确定酸活化和O2等离子体蚀刻产生的ITO表面平均仍覆盖有一到两个单层的氢氧化物。这两种活化处理都导致电子转移到溶液探针分子（如乙腈中的二甲基二茂铁）的速率显著提高。溶液中的电子转移事件似乎发生在每平方厘米不超过4×10^7个电活性位点（每个位点直径约为50 - 200 nm）（即电极几何面积的一小部分）。电子转移速率与近表面锡掺杂剂浓度相关，表明这些电活性位点源于近表面锡富集。
