TiO₂ nanotubes with ultrathin walls for enhanced water splitting

Abstract

We demonstrate, for the first time, the synthesis of titania nanotubes with ultrathin (3-5 nm) wall thickness. As revealed by the incident photon-to-current collection efficiency (IPCE) and electrochemical impedance spectroscopy measurements, the ultrathin walls, less than the charge carrier diffusion length, were essential to ensure fast and efficient charge carrier collection.One-dimensional metal oxide nanoarchitectures have demonstrated great performance in many technologies including solar energy conversion. [1][2][3] In particular, TiO 2 nanotube arrays formed by electrochemical anodization have demonstrated outstanding performance in solar fuel generation and solar cell applications. 4,5 Although the use of nanotubular form decouples the light absorption and the transfer of charge carriers, enhancing the dynamics of charge carriers in TiO 2 is still a challenge. 6,7 In this regard, many studies have been devoted to improve the dynamics including passivation of defects, 8 the use of co-catalysts among others. 9,10 As many studies have shown the dependence of the functionality of the material on its physical dimensions, the best way to improve the transport and collection of charge carriers is to optimize the inherent intrinsic properties of the material. 7,11,12 To this end, controlling the length and the diameter of TiO 2 nanotubes has shown tremendous positive effects on the performance of the materials in solar energy conversion. [12][13][14] However, the effect of wall thickness was poorly discussed in the literature, despite the fact that it is one of the determinant factors controlling the dynamics of charge carriers, especially in photoelectrochemical water splitting systems. 7,11,14,15 Most of the published articles deal with wall thicknesses that are greater than the diffusion length of charge carriers in titania. 7 Herein, we report the first demonstration of the fabrication of vertically aligned titania nanotube arrays with very thin wall thickness (3-5 nm) and their use in solar water splitting. The thin-walled nanotubes facilitate the diffusion of the photogenerated holes to the semiconductor/ electrolyte interface during water splitting, allowing for efficient separation of charge carriers.The diffusion length of charge carriers in titania is around 10 nm, 7 and until now there has been no reproducible method to produce titania nanotubes with a wall thickness that is considerably lower than the diffusion length. Inspired by the work of Amer et al., 16 who were able to fabricate thin-walled ZrO 2 nanotubes, we used a mixture of non-aqueous (glycerol) and aqueous (water) electrolytes to anodize titanium in order to achieve titania nanotubes with ultrathin walls. The detailed experimental setup and conditions are summarized in the ESI †. In order to show the effect of the wall thickness, two sets of samples were fabricated; namely thick-walled nanotubes (NT1) using the conventional anodization method and thin-walled nanotubes (NT2) using our modified fabrication method, see the ESI † for more d...

CuO Nanoparticles Supported on TiO2 with High Efficiency for CO2 Electrochemical Reduction to Ethanol

“…From this figure, we can clearly see that the main peak of Cu 2p 1/2 at 953.8 eV and its satellite peak at 962.5 eV were separated by about 9.0 eV, which also confirms the presence of CuO [42]. Both Ti 2p 3/2 and Ti 2p 1/2 peaks at 458.7 eV and 464.4 eV, respectively, were observed ( Figure 2c) with a separation of 5.7 eV, indicating that TiO 2 existed in this catalyst [43]. The obtained XPS spectrum of O 1 s was presented in Figure 2d.…”

Improvement on electrochemical performances of nanoporous titania as anode of lithium-ion batteries through annealing of pure titanium foils

“…3 (c). The formation of oxide is evident from the O 1 s and Ti 2 p peaks with the Ti/O molar ratio being close to the stoichiometric proportion [49] . It is noted that we did not estimate the stoichiometry of nanopores, as XPS does not provide the composition of nanoporous samples accurately [50] .…”

10-fold enhancement in light-driven water splitting using niobium oxynitride microcone array films

“…Also, with thin films, the light absorption and carrier collection are in competition, i.e. while thick films are needed to harvest a reasonable amount of the solar Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat spectrum, thicker films than the carrier diffusion length will result in poor carrier collection efficiency [7]. It is noteworthy mentioning that ordered nanostructured and/or microstructured niobium oxynitrides have been investigated.…”

“…However, the long-standing bottleneck is the fact that metal oxides are wide band gap materials, with limited absorption activity to the ultraviolet spectral region, which contains only 3-5% of the incident solar energy. Thanks to the progress in the fabrication techniques, the precise control of the morphological and structural features of metal oxides that can enhance their optical and structural properties, rendering such structured materials useful in solar spectrum conversion devices [6,7]. However, the structured metal oxide systems demonstrated to date are still circumscribed by the minimal visible light absorption, which can be expounded predictably based on the fact that the valence bands of oxide semiconductor materials are mainly formed by O 2p orbitals, which are fixed at an extremely positive level of around 3.0 V vs. the normal hydrogen electrode (NHE) [8].…”