Anodic TiO 2 nanotubes (NTs) have been studied extensively for many years. However, the growth kinetics still remains unclear, because it is hardly derived by direct in situ methods. Here, an interesting approach is proposed to overcome this challenge. A combinatorial anodization was exploited to monitor the pore initiation and nanotube growth under a preformed compact surface layer (CSL). The preformed CSL and the NTs under the CSL (UCSL-NTs) were formed in fluoride-free and fluoride-containing electrolytes, respectively. The forming process of UCSL-NTs was discussed as compared with that of the general NTs, mainly focusing on the differences of current-time curves and electric charge quantity (Coulomb). The results show that pore embryos of UCSL-NTs have already been achieved under the CSL before the CSL is dissolved. There are five stages in the current-time curve of UCSL-NTs, which is significantly different from three stages of the general NTs. A new growth model, based on a comprehensive review of the existing theories, is proposed to explain the current decrease and increase. And the forming process of TiO 2 NTs is considered to be dominated by the oxide plastic flow around the oxygen bubbles.Anodic TiO 2 nanotubes (NTs) and other porous anodic oxides have attracted considerable scientific interests due to their various applications (e.g., solar energy materials, magnetic semiconductors and biosensors) 1-3 and mysterious formation mechanisms. 4,5 Different mechanisms of TiO 2 NTs have been reported in many electrochemical journals in recent years. 4-8 It is well known that field-assisted dissolution (FAD) (TiO 2 + 6F − + 4H + → [TiF 6 ] 2− + 2H 2 O) of the oxide leads to pore formation in anodic titania films, 8-10 similar to that in porous anodic alumina (PAA) films (Al 2 O 3 + 6H + → 2Al 3 + + 3H 2 O), 11-14 despite a lack of direct experimental evidence that confirms this expectation. 14 As the formation mechanism is impossible to be derived by direct in situ experimental methods, much remains to be done along these directions. 15 Garcia-Vergara et al. 16,17 proposed the field-assisted 'plastic flow' model, the constant thickness of the barrier layer is maintained by flow of oxide from the pore bottom toward the pore wall, driven by compressive stresses from electrostriction and possibly through volume expansion. 16 In fact, the plastic flow is contrary to expectations of the FAD. 16 The behavior of incorporated species in PAA is always incompatible with the FAD model. 16 The flow model has been recognized and exploited for explaining the formation of TiO 2 NTs and serrated nanochannels. 18,19 However, Zhou et al. 12 indicated that both the FAD and the flow models cannot explain the formation of gaps among nanotubes. In recent tracer studies on Ti thin films, the expansion factors increase from 1.5 to 3.0, 20,21 these findings cannot be clarified. Furthermore, anodized TiO 2 NTs have been achieved in an aqueous H 2 SO 4 solution as well as other fluoride free solutions, 12,22,23 this fact puts the flu...

No abstract

Anodic TiO2 nanotubes have been studied extensively for many years. However, the growth kinetics still remains unclear. The systematic study of the current transient under constant anodizing voltage has not been mentioned in the original literature. Here, a derivation and its corresponding theoretical formula are proposed to overcome this challenge. In this paper, the theoretical expressions for the time dependent ionic current and electronic current are derived to explore the anodizing process of Ti. The anodizing current-time curves under different anodizing voltages and different temperatures are experimentally investigated in the anodization of Ti. Furthermore, the quantitative relationship between the thickness of the barrier layer and anodizing time, and the relationships between the ionic/electronic current and temperatures are proposed in this paper. All of the current-transient plots can be fitted consistently by the proposed theoretical expressions. Additionally, it is the first time that the coefficient A of the exponential relationship (ionic current j(ion) = A exp(BE)) has been determined under various temperatures and voltages. And the results indicate that as temperature and voltage increase, ionic current and electronic current both increase. The temperature has a larger effect on electronic current than ionic current. These results can promote the research of kinetics from a qualitative to quantitative level.

Light management is of paramount importance to improve the performance of optoelectronic devices including photodetectors, solar cells, and light-emitting diodes. Extensive studies have shown that the efficiency of these optoelectronic devices largely depends on the device structural design. In the case of solar cells, threedimensional (3-D) nanostructures can remarkably improve device energy conversion efficiency via various light-trapping mechanisms, and a number of nanostructures were fabricated and exhibited tremendous potential for highly efficient photovoltaics. Meanwhile, these optical absorption enhancement schemes can benefit photodetectors by achieving higher quantum efficiency and photon extraction efficiency. On the other hand, low extraction efficiency of a photon from the emissive layer to outside often puts a constraint on the external quantum efficiency (EQE) of LEDs. In this regard, different designs of device configuration based on nanostructured materials such as nanoparticles and nanotextures were developed to improve the out-coupling efficiency of photons in LEDs under various frameworks such as waveguides, plasmonic theory, and so forth. In this Perspective, we aim to provide a comprehensive review of the recent progress of research on various light management nanostructures and their potency to improve performance of optoelectronic devices including photodetectors, solar cells, and LEDs. O ptoelectronic devices, such as photodetectors, solar cells, and light-emitting diodes (LEDs), are essentially light to electricity or vice versa energy conversion devices. Utilization of efficient optoelectronic devices cannot only produce clean energy but can also help with energy conservation. Recent extensive studies have shown that the efficiency of these optoelectronic devices largely depends on the device structural design. In the case of solar cells, which involve conversion from solar radiation to electricity, it has been discovered that nanostructures can remarkably improve the energy conversion efficiency via various light-trapping mechanisms. Therefore, a number of nanostructures including nanowires, nanopillars, nanoholes, and so forth were fabricated, and their effectiveness for photovoltaic (PV) performance improvement has been examined based on different material systems. Meanwhile, these optical absorption enhancement schemes can benefit photodetectors as well. It is worth pointing out that due to the different application scale requirement, low-cost approaches are desired for effective light management in PV applications. However, performance is the primary concern for photodetectors in most circumstances. On the other hand, a LED is a device that converts electrical energy to optical radiation. High quantum efficiency and photon extraction efficiency not only help energy conservation but also minimize the overheating of the device, thus prolonging its lifetime. In recent decades, numerous material systems and techniques were extensively studied to improve the internal quantum ...

The weak adhesion of anodic TiO2 nanotube arrays (TNTAs) to the underlying Ti substrate compromises many promising applications. In this work, a compact oxide layer between TNTAs and Ti substrate is introduced by employing an additional anodization in a fluoride-free electrolyte. The additional anodization results in an about 200 nm thick compact layer near the nanotube bottoms. Scratch test demonstrates that the critical load of TNTAs with the compact oxide layer is a more than threefold increase in comparison with those without the compact layer. Moreover, this facile method can also improve the photoactivity and supercapacitor performances of TNTAs markedly.

No abstract

Three-dimensional (3-D) structures have triggered tremendous interest for thin-film solar cells since they can dramatically reduce the material usage and incident light reflection. However, the high aspect ratio feature of some 3-D structures leads to deterioration of internal electric field and carrier collection capability, which reduces device power conversion efficiency (PCE). Here, we report high performance flexible thin-film amorphous silicon solar cells with a unique and effective light trapping scheme. In this device structure, a polymer nanopillar membrane is attached on top of a device, which benefits broadband and omnidirectional performances, and a 3-D nanostructure with shallow dent arrays underneath serves as a back reflector on flexible titanium (Ti) foil resulting in an increased optical path length by exciting hybrid optical modes. The efficient light management results in 42.7% and 41.7% remarkable improvements of short-circuit current density and overall efficiency, respectively. Meanwhile, an excellent flexibility has been achieved as PCE remains 97.6% of the initial efficiency even after 10 000 bending cycles. This unique device structure can also be duplicated for other flexible photovoltaic devices based on different active materials such as CdTe, Cu(In,Ga)Se2 (CIGS), organohalide lead perovskites, and so forth.

The formation mechanism of porous anodic TiO 2 nanotubes (PATNT) still remains unclear. A special approach is proposed in this paper to investigate the forming process of nanopores in the preformed nanotubes. A novel and not easily brittle nanostructure, called triple-layered TiO 2 nanotube array, has been fabricated by changing the electrolytes during the electrochemical anodizing processes. The first porous layer was fabricated in fluoride-containing electrolyte, the middle compact layer was formed in fluoridefree electrolyte and the second porous layer was formed in the same fluoride-containing electrolyte. The results show that middle compact layer becomes thicker with the increase of the third time anodizing voltage. At the same time, it needs more time for the fourth time anodization to reach the equilibrium current, where the nanotubes begin to develop steadily. Furthermore, a possible mechanism for the growth of the triple-layered nanotubes is discussed by comparison with the normal PATNT. The present results may be helpful to understand the mechanism of PATNT and facilitate assembling diverse nanostructures for extensive applications in photocatalysis, dye-sensitized solar cells, and biomedical devices.Self-ordering porous anodic alumina (PAA) 1-3 and porous anodic TiO 2 nanotubes (PATNT) 4,5 have been extensively investigated due to their various applications. The formation mechanisms of PAA 6-8 and PATNT 9,10 have received considerable attention. Several models proposed include the field-assisted dissolution (FAD) model, 6,9 the oxide viscous flow model, 8,10,11 oxygen bubble and electronic current model, 7,12,13 etc. For more than 60 years, it has been assumed that fieldassisted dissolution leads to pore formation in PAA, despite a lack of direct experimental evidence that confirms this expectation, 14 because the formation mechanism is hardly derived by in situ experimental methods. 15 In fact, the viscous flow model and anionic incorporation into PAA are both contrary to expectations of the FAD model. 8,11 Moreover, as Hebert et al. indicated that the relationships between porous morphology and the processing parameters (current-time or voltage-time transients) were not yet well understood. 11 It is well known that there are two different types of anodic oxide films for aluminum and titanium, the compact-type film and poroustype film. 16,17 For a constant voltage anodization, the current-time transients of the compact-type and porous-type films are very different. 16,17 Initially both transients are identical; as the initially formed barrier layer thickens, the electric field strength decreases and the current density decreases rapidly. At a special point D p , the two curves now begin to diverge; the compact-type film current continues to decrease exponentially, while the porous-type film current, after a short period of continuing decrease, begins to increase. 16 To the best of our knowledge, most of the researchers assembling the PATNT take fluoride-containing solutions as the anodizing electroly...