A voltage-dependent investigation on detachment process for free-standing crystalline TiO2 nanotube membranes

TiO2 nanotube (TNT) arrays have attracted considerable scientific interests because of their photoelectric properties and technological importance for diverse applications as solar cells [1–4], photocatalysis [5–7], gas sensors [8], hydrogen generation from water splitting [6], and composite nano-membranes [9, 10]. Among various synthesis methods, electrochemical anodization is an excellent approach to fabricate TNT arrays due to its simplicity, low cost, and tunable morphology [11]. However, the nature of as-prepared TNT membrane adhered on opaque Ti foil restricts their feasibility for the applications such as tube filling [9, 10], biofiltration, flow-through photocatalytic reactions [5], and front illumination in solar cells [2–4]. After detachment from the substrate, free-standing TNT membranes show high performance due to the absence of a blocking layer [2, 4]. The TNT membrane can be directly used or attached on foreign substrates for the above applications. So far, different techniques have been proposed for preparing free-standing TNT membranes from Ti substrates, including ultrasonic splitting [8], solvent evaporation [12, 13], chemically assisted delamination [6, 14–16], selective metal dissolution [5, 17], and voltage transition at the end of anodization [18–22]. Generally speaking, these processes fabricated amorphous and fragile TNT membranes. The amorphous characteristic of these membranes hampered their applications and the crystallized TNT membrane is desirable for practical devices. To get the crystallized TNT membranes, attempts such as selective dissolution of the amorphous layer between the desired crystalline TNT arrays and Ti substrate [3], adjusting electrolyte temperature in multi-step anodic processes [23] have also been explored. The success of the former is due to the different resolvability of materials, while the different mechanical stability and etching contrast between the layers account for the latter’s detachment. Nevertheless, the fabrication of large-area and free-standing crystalline TNT membranes is still a challenge. In this article, a handy experimental procedure was reported to fabricate free-standing crystalline TNT membranes. The concept is outlined in Fig. 1 (1–4). The ordered TNT arrays are firstly fabricated under optimized parameters, after an annealing and followed by a second (detachment) anodization step, free-standing crystalline TNT membranes are detached from the substrate without any cracks. This method is a reliable technique for fabricating free-standing TNT membranes without the needs for any complicated processes or dangerous chemicals. At the same time, different from previous studies, we have systematically investigated the effects of voltage on the detachment process of free-standing crystalline TNT membranes.