Kraut Max, Pantle Florian, Winnerl Julia, Hetzl Martin, Eckmann Felix, Sharp Ian D, Stutzmann Martin
Walter Schottky Institut and Physik-Department, Technische Universität München, Am Coulombwall 4, 85748 Garching, Germany.
Nanoscale. 2019 Apr 23;11(16):7967-7975. doi: 10.1039/c8nr10021g.
Nanowire (NW) based devices for solar driven artificial photosynthesis have gained increasing interest in recent years due to the intrinsically high surface to volume ratio and the excellent achievable crystal qualities. However, catalytically active surfaces often suffer from insufficient stability under operational conditions. To gain a fundamental understanding of the underlying processes, the photochemical etching behavior of hexagonal and round GaN NWs in deionized water under illumination are investigated. We find that the crystallographic c-plane remains stable, whereas the m-planes are photochemically etched with rates up to 11 nm min-1, depending on the applied UV light intensity. By investigating nanowalls, we achieve control of the exposed crystallographic facets and find an enhanced stability of the a-plane compared to the m-plane. Photo-excited holes, which drift to the side facets due to the upward surface band bending in nominally n-type (not intentionally doped) GaN, are identified as the driving force of the process, which allows the development of concepts for the stabilization of the nanostructures. A geometrically enhanced absorption of periodic NW arrays is correlated with a dependence of the etch rate on the NW pitch and diameter. Further, we find selective photochemical etching of the NW base in the presence of sub-band gap illumination, which is attributed to defect-related absorption in this region. These results provide improved understanding of the roles of inhomogeneous defect distribution, light excitation profiles, and different surface facets on the photochemical stability of nanostructures and provide viable strategies for improving stabilities under light-driven reaction conditions.
近年来，基于纳米线（NW）的太阳能驱动人工光合作用器件因其固有的高表面积与体积比以及出色的可实现晶体质量而受到越来越多的关注。然而，催化活性表面在操作条件下往往稳定性不足。为了深入了解其潜在过程，我们研究了六边形和圆形GaN纳米线在去离子水中光照下的光化学蚀刻行为。我们发现，晶体学c面保持稳定，而m面则以高达11 nm/min的速率进行光化学蚀刻，这取决于所施加的紫外光强度。通过研究纳米壁，我们实现了对暴露的晶体学面的控制，并发现a面相对于m面具有更高的稳定性。光激发空穴由于名义上n型（非故意掺杂）GaN中向上的表面能带弯曲而漂移到侧面，被确定为该过程的驱动力，这使得能够开发纳米结构稳定化的概念。周期性NW阵列的几何增强吸收与蚀刻速率对NW间距和直径的依赖性相关。此外，我们发现在子带隙光照下NW基部会发生选择性光化学蚀刻，这归因于该区域与缺陷相关的吸收。这些结果有助于更好地理解不均匀缺陷分布、光激发分布以及不同表面面对纳米结构光化学稳定性的作用，并为改善光驱动反应条件下的稳定性提供可行策略。
