The double-walled nature of TiO2 nanotubes and formation of tube-in-tube structures – a characterization of different tube morphologies
Introduction
One-dimensional (1D) nanostructures, in particular in the form of nanotubes (NTs), provide an exceptional combination of optical, electrical and chemical properties with a distinct and defined geometry and represent, therefore, ideal arrays for application in fields such as energy conversion and storage, and medicine [1], [2], [3].
Several methods are reported for the synthesis of 1D TiO2 nanotubes, such as hydrothermal, sol-gel or template-assisted approaches [4], [5], [6]. The most common and straightforward is self-organizing electrochemical anodization (SOA) [3], [7], [8].
Traditionally, anodization is carried out in a simple two-electrode configuration, where the metal of interest (M, the anode) under a suitable voltage is oxidized (M → Mz+ + ze–) and converted to the corresponding (mostly compact) metal oxide, by a reaction with O2– ions derived from H2O in the electrolyte [3].
For an optimized set of experimental parameters (e.g., electrolyte composition and pH, temperature, applied voltage, time etc.), self-organizing conditions can be established and one-dimensional TiO2 NTs can be grown as highly aligned, size-controlled, and back-contacted (i.e., anchored to a metallic substrate) nanostructure arrays. These arrays provide a broad range of functionalities and properties e.g. for a use in solar cells [9], [10], as well as in photo-electrochemical [11], energy storage [12] and biochemical devices [13]. Key to the formation of nanoporous/nanotubular oxide arrays is the presence of F– ions in the electrolyte that, under optimized conditions, establishes a steady-state equilibrium between oxide formation and dissolution required for self-organization [14], [15].
Since the first demonstration of self-ordered anodic TiO2 nanostructures by anodization [16], tremendous research on the structural design and properties of anodic TiO2 tubes has been carried out.
The first generation of anodic TiO2 NTs was grown in acidic-aqueous solutions [16], [17], [18]. For these tubes, only a limited maximum length (∼ 2.5 μm, due to high chemical etching rate promoted by excess water) and a low self-ordering degree (ripple-walled tubes were typically reported) could be achieved.
The introduction of organic electrolytes represents a keystone in the growth of smooth, ordered and longer (up to several 100 μm) TiO2 nanotubes [19]. Among organic electrolytes, most classic are glycerol and the nowadays widely used mixtures of ethylene glycol (EG) containing ammonium fluoride (NH4F) and low amounts of water (∼ 1–5 vol%). Tubes grown in these electrolytes – due to the growth mechanism – typically feature a double-walled (DW) structure, with an inner (shell) wall rich in carbon-species embedded from the electrolyte during the anodic growth, while an outer shell of nearly pure TiO2 is present [20], [21], [22].
Owing to its composition [22], the inner shell is notoriously detrimental for TiO2 NT applications e.g. as photo-anode in solar cells and photo-electrochemical devices, among others, as carbon retards the electron mobility .
Most recent findings show that a simple chemical treatment in combination with low-temperature annealing can selectively etch out the inner part of DW NTs leaving behind a so-called single-walled (SW) nanostructure; as a result, SW tubes typically have larger inner diameter, a smoother and better-defined wall surface and, most notably, improved electronic properties compared to the original DW tubes [22].
A most remarkable step-forward in the fabrication of TiO2 nanotubes has been achieved through the introduction of weak organic acids (e.g., lactic acid (LA), glycolic acid, citric acid, and EDTA) in the traditional EG-based electrolyte. By preventing the occurrence of dielectric breakdown events in the growing oxide, these additives allow anodization at a significantly higher voltage, ultimately yielding a ultra-fast growth that drastically (positively) affects the tubes’ properties. Specifically, while preserving a double-walled morphology, EG/LA-TiO2 NTs exhibit a larger diameter, superior mechanical resistance, and improved robustness, compared to classic EG-tubes. All these features contribute to an amelioration of the performance of such nanotubes in several applications [22], [23], [24], [25].
In spite of the wide use of DW-TiO2 NTs fabricated in EG/LA-based electrolytes, their structural and chemical properties as well as their differences to SW tubes have only sparingly been investigated. Moreover, no study on the effect of different anodization conditions, also combined with specific post-growth treatments, on the morphology of such tubes has been reported so far.
Thus, within this work, we investigate the effect of anodizing temperature on the growth of EG/LA-based TiO2 nanotubes, and characterize composition, structure and differences in properties of such tubes.
In particular, we find that, for tubes grown in EG/LA electrolyte using low anodizing temperatures, a significant thickening of the NT walls is observed; this then represents a precursor state to form a remarkable tube-in-tube (T-in-T) morphology that develops during an annealing treatment.
Section snippets
Preparation of TiO2 nanotubes
TiO2 nanotubes (16 μm-long) were grown on titanium foils (0.125 mm thick, 99.6+ % purity, Advent, England) that, prior to anodization, were degreased by sonication in acetone, ethanol and isopropanol, rinsed with deionized water, and then dried with a nitrogen jet. Electrochemical anodization was performed with a high-voltage potentiostat (Jaissle IMP 88 PC) at 120 V in a two-electrode configuration with a Pt foil as counter electrode, and using an electrolyte composition of 1.5 M lactic acid (LA,
Results and Discussion
Nanotube layers, as shown in Fig. 1, were fabricated in an organic-based electrolyte (0.1 M NH4F–5 wt.% H2O–EG), also containing lactic acid (1.5 M).
This electrolyte was selected as lactic acid is reported to shift the “burning” voltage to a significantly higher value [26] and, thus, allows a high ion flux through the oxide film and therefore provides an extremely fast growth of the anodic oxide without affecting the tubes’ functional properties [23], [27], [28]. Fig. 1(j) shows the current
Conclusions
In the present manuscript, we confirm the concept that anodizing Ti in an organic electrolyte leads to nanotubular structures composed of an inner (low-quality and C-enriched) and an outer (high-quality and TiOx-based) oxide shell. An optimized chemical treatment in piranha solution leads to the dissolution of the C-rich inner shell with the consequent formation of a single-walled nanotube morphology.
We also show that for anodization performed at relatively low temperature (10 °C), followed by
Acknowledgements
The authors would like to acknowledge ERC, DFG and the DFG Cluster of Excellence “Engineering of Advanced Materials” (EAM) for financial support.
References (48)
One-dimensional nanostructures: Chemistry, physics & applications
Solid State Commun.
(1998)- et al.
TiO2 nanotubes: Self-organized electrochemical formation, properties and applications, Curr
Opin. Solid State Mater. Sci.
(2007) - et al.
Robust free standing flow-through TiO2 nanotube membranes of pure anatase
Electrochem. Commun.
(2016) - et al.
Electrical and mechanical breakdown of anodic films on tungsten in aqueous electrolytes
J. Electroanal. Chem.
(1988) - et al.
The origin for tubular growth of TiO2 nanotubes: A fluoride rich layer between tube-walls
Surf. Sci.
(2011) - et al.
Transition of TiO2 nanotubes to nanopores for electrolytes with very low water contents
Electrochem. Commun.
(2010) - et al.
From anodic TiO2 nanotubes to hexagonally ordered TiO2 nanocolumns
Appl. Surf. Sci.
(2011) The surface science of titanium dioxide
Surf. Sci. Rep.
(2003)- et al.
Photocatalysis of dicarboxylic acids over TiO2: An in situ ATR-IR study
J. Catal.
(2007) - et al.
Size-effects in TiO2 nanotubes: Diameter dependent anatase/rutile stabilization
Electrochem. Commun.
(2011)
One-dimensional titanium dioxide nanomaterials: Nanotubes
Chem. Rev.
Formation of Titanium Oxide Nanotube
Langmuir.
Template-assisted fabrication of dense, aligned arrays of titania nanotubes with well-controlled dimensions on substrates
Adv. Mater.
Aligned arrays of nanotubes and segmented nanotubes on substrates fabricated by electrodeposition onto nanorods
J. Am. Chem. Soc.
TiO2 nanotubes: Synthesis and applications
Angew. Chemie - Int. Ed.
Aligned metal oxide nanotube arrays: key-aspects of anodic TiO2 nanotube formation and properties
Nanoscale Horizons.
Removing structural disorder from oriented TiO2 nanotube arrays: Reducing the dimensionality of transport and recombination in dye-sensitized solar cells
Nano Lett.
Dye-sensitized solar cells based on thick highly ordered TiO(2) nanotubes produced by controlled anodic oxidation in non-aqueous electrolytic media
Nanotechnology
Nb doped TiO2 nanotubes for enhanced photoelectrochemical water-splitting
Nanoscale
High-contrast electrochromic switching using transparent lift-off layers of self-organized TiO2 nanotubes
Small
Nanosize and vitality: TiO2 nanotube diameter directs cell fate
Nano Lett.
250 μm long anodic TiO2 nanotubes with hexagonal self-ordering
Phys. Status Solidi – Rapid Res. Lett
Oxide morphology and adhesive bonding on titanium surfaces
J. Mater. Sci.
Cited by (34)
Intrinsic properties of anodic TiO<inf>2</inf> nanotube layers: In-situ XRD annealing of TiO<inf>2</inf> nanotube layers
2023, Ceramics InternationalCitation Excerpt :The growth of this crystallites is limited by the available space. Additionally, the inner wall has a different chemical composition than the outer wall as it contains impurities of carbon and fluoride species and is probably more defective than the outer wall which is composed of almost pure TiO2 [13–17]. Thus, average crystallite sizes of the DW TNT layers reveal crystallites of the inner and outer wall, while the average crystallite sizes of the SW TNT layers take just the high purity outer TiO2 wall into account (see also the scheme in Fig. 5 for better visualization).
Anodic TiO<inf>2</inf> nanotubes: A promising material for energy conversion and storage
2022, Applied Materials TodayEffectiveness of physicochemical techniques on the activation of Ti6Al4V surface with improved biocompatibility and antibacterial properties
2022, Surface and Coatings TechnologyCitation Excerpt :The most common physical method is plasma oxidation [25], whereas wet treatments and electrochemical anodization can be found among chemical oxidation methods [26,27]. Even so, regarding their surface oxidation and hydroxylation effectiveness, there is a lack of knowledge due to the contradictory results reported throughout last decades [28–36]. For this reason, it is necessary to investigate thoroughly the effectiveness of surface activation processes since it is expected that the highest Ti-OH/TiO2 ratio along with the widest TiO2 layer could improve Ti6Al4V performance in various aspects: (i) surface reactivity to create subsequently bioactive coatings if needed, (ii) biocompatibility, and (iii) antibacterial activity [37,38].
Nanostructured photocatalysts for the abatement of contaminants by photocatalysis and photocatalytic ozonation: An overview
2022, Science of the Total EnvironmentCitation Excerpt :The key to produce these types of layers is related to an increase in the anodization potentials without reaching the nanotube's dielectric breakdown (high local current densities and significant temperature increase on the anodized Ti), which is irreversible (Alijani et al., 2021). The introduction of weak acids (as the lactic acid, citric acid, glycol acid or EDTA) in the electrolyte solution prevents the dielectric breakdown, allowing to use higher voltages leading to a faster growth that affects the nanotubes properties (So et al., 2017). As example, Alijani et al. (2021) obtained high aspect ratio TNT using an electrolyte solution composed by NH4F, H2O and EG with lactic acid and a potential of 160 V.
- 1
These authors contributed equally to this work.