Analysis of TiO2 Atomic Layer Deposition Surface Chemistry and Evidence of Propene Oligomerization using Surface-Enhanced Raman Spectroscopy,Journal of the American Chemical Society

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Analysis of TiO2 Atomic Layer Deposition Surface Chemistry and Evidence of Propene Oligomerization using Surface-Enhanced Raman Spectroscopy
Journal of the American Chemical Society ( IF 14.4 ) Pub Date : 2018-12-11 , DOI: 10.1021/jacs.8b10689
Ryan A. Hackler ₁ , Gyeongwon Kang ₁ , George C. Schatz ₁ , Peter C. Stair ₁ , Richard P. Van Duyne ₁

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Atomic layer deposition (ALD) of TiO2 was performed in tandem with in situ surface-enhanced Raman spectroscopy (SERS) to monitor changes in the transient surface species across multiple ALD cycles. A self-assembled monolayer of 3-mercaptopropionic acid was used as a capture agent to ensure that nucleation of the titanium precursor (titanium tetraisopropoxide [TTIP]) occurs. Comparisons between the Raman spectra of the neat precursor and the SER spectra of the first ALD cycle of TiO2 reveal typical ligand exchange chemistry taking place, with self-limiting behavior and intact isopropoxide ligands. However, subsequent cycles show drastically different chemistry, with no isopropoxide ligands remaining at any point during the second and third cycles. Continuous exposure of either TTIP or isopropyl alcohol after the first cycle shows unlimited chemical vapor deposition (CVD)-type growth. Comparisons with alternative precursors (aluminum isopropoxide, titanium tert-butoxide, and titanium propoxide) and DFT calculations reveal that, for the TTIP precursor, isolated TiO2 sites play a role in the dehydration of off-gassing isopropyl alcohol. The resulting propene then undergoes oligomerization into six-carbon olefins before polymerizing into indistinguishable carbon products that accumulate on the surface. The emergence of the dehydration chemistry is expected to be exclusively the result of these isolated TiO2 sites and, as such, is expected to occur on other surfaces where TiO2 ALD is feasible. This work showcases how seemingly innocuous ALD can evolve into a CVD process when the products can participate in various side reactions with newly made surface sites.

中文翻译：

使用表面增强拉曼光谱分析 TiO2 原子层沉积表面化学和丙烯低聚的证据

TiO2 的原子层沉积 (ALD) 与原位表面增强拉曼光谱 (SERS) 协同进行，以监测多个 ALD 循环中瞬态表面物质的变化。3-巯基丙酸的自组装单层用作捕获剂，以确保钛前体（四异丙醇钛 [TTIP]）的成核发生。纯前体的拉曼光谱与 TiO2 的第一次 ALD 循环的 SER 光谱之间的比较揭示了典型的配体交换化学发生，具有自限行为和完整的异丙醇配体。然而，随后的循环显示出截然不同的化学性质，在第二次和第三次循环期间的任何一点都没有残留异丙醇配体。在第一个循环后连续接触 TTIP 或异丙醇显示出无限制的化学气相沉积 (CVD) 型生长。与替代前体（异丙醇铝、叔丁醇钛和丙醇钛）的比较和 DFT 计算表明，对于 TTIP 前体，孤立的 TiO2 位点在排气异丙醇的脱水中起作用。得到的丙烯然后低聚成六碳烯烃，然后聚合成难以区分的碳产物，这些产物积聚在表面上。预计脱水化学的出现完全是这些孤立的 TiO2 位点的结果，因此，预计会发生在其他 TiO2 ALD 可行的表面上。

更新日期：2018-12-11

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