Biomedical Engineering and Environment Sciences, National Tsing Hua University, Hsinchu, Taiwan 30013.
ACS Appl Mater Interfaces. 2013 Aug 28;5(16):7937-49. doi: 10.1021/am402024q. Epub 2013 Aug 1.
It has been suggested that a high concentration of Fe(3+) in solution, a low pH, and noncomplexing ions of high ionic strength are all essential for developing a high-quality hematite array. Our curiosity was piqued regarding the role of the electrolyte ions in the hydrothermal synthesis of hematite photoanodes. In this study, we prepared hematite photoanodes hydrothermally from precursor solutions of 0.1 M FeCl3 at pH 1.55 with a background electrolyte of 1.0 M sodium halide (NaF, NaCl, NaBr, or NaI). We compared the structures and properties of the as-obtained hematite photoanodes with those of the material prepared in 1.0 M NaNO3, the most widely adopted electrolyte in previous studies. Among our studied systems, we found that the hematite photoanode prepared in NaCl solution was the only one possessing properties similar to those of the sample obtained from the NaNO3 solution-most importantly in terms of photoelectrochemical performance (ca. 0.2 mA/cm(2) with +0.4 V vs SCE). The hematites obtained from the NaF, NaBr, and NaI solutions exhibited much lower (by approximately 2 orders of magnitude) photocurrent densities under the same conditions, possibly because of their relatively less ordered crystallinity and the absence of rodlike morphologies. Because the synthetic protocol was identical in each case, we believe that these two distinct features reflect the environments in which these hematite photoanodes were formed. Consistent with the latest studies reported in the literature of the X-ray photoelectron spectra of fast-frozen hematite colloids in aqueous solutions, it appears that the degree of surface ion loading at the electrolyte-hematite interface (Stern layer) is critical during the development of hematite photoanodes. We suspect that a lower ion surface loading benefits the hematite developing relatively higher-order and a rodlike texture, thereby improving the photoelectrochemical activity.
有人认为，溶液中高浓度的 Fe(3+)、低 pH 值和非配位离子的高离子强度对于形成高质量赤铁矿阵列都是必不可少的。我们对电解质离子在赤铁矿光阳极的水热合成中的作用感到好奇。在这项研究中，我们使用 0.1 M FeCl3 的前驱体溶液在 pH 值为 1.55 的条件下，以 1.0 M 卤化钠（NaF、NaCl、NaBr 或 NaI）作为背景电解质，水热合成赤铁矿光阳极。我们将所得赤铁矿光阳极的结构和性能与之前研究中采用的最广泛的电解质 1.0 M NaNO3 制备的材料进行了比较。在所研究的体系中，我们发现只有在 NaCl 溶液中制备的赤铁矿光阳极具有类似于从 NaNO3 溶液中获得的样品的性质-最重要的是光电化学性能（约 0.2 mA/cm(2)，相对于 SCE 为 +0.4 V）。在相同条件下，从 NaF、NaBr 和 NaI 溶液中获得的赤铁矿的光电流密度要低得多（约低 2 个数量级），这可能是由于它们的结晶度相对较低且不存在棒状形态。由于每种情况下的合成方案都是相同的，我们认为这两个截然不同的特征反映了形成这些赤铁矿光阳极的环境。与文献中报道的水溶液中快速冻结赤铁矿胶体的 X 射线光电子能谱的最新研究一致，似乎在赤铁矿光阳极的形成过程中，电解质-赤铁矿界面（斯特恩层）上的表面离子负载程度至关重要。我们怀疑较低的离子表面负载有利于赤铁矿形成相对较高阶和棒状结构，从而提高光电化学活性。
Zotero 插件