Hard-templating of chiral TiO₂ nanofibres with electron transition-based optical activity

Optically active materials respond differently to left and right-handed circularly polarized light in terms of propagation speed and extinction. Optically active inorganic materials have aroused general interest from synthetic, structural, and functional standpoints because of their potential applications in chiral separations, chemical and biological sensors, and optical devices [1–11]. As is known, electronic transition-based absorptions in the ultraviolet (UV) or visible range are very common in compounds, and organic materials with electron transition-based optical activity (ETOA) have been investigated extensively. However, so far, inorganic materials with ETOA have been scarcely reported [12–15]. By comparison, chiral inorganic materials with ETOA have a clear advantage for application over both chiral organic assemblies and chiral nanoparticles supported by organics due to their increased physical robustness, resulting in permanent molecule or block chirality.

On the other hand, self-organization of entities in multidimensional scales can lead to unexpected emergence of collective intrinsic properties, which are neither those of the corresponding bulk phase nor those of the isolated particles. Collective optical and magnetic properties have been observed for the dipolar interactions between inorganic nanocrystals when they are organized into 2D superlattices. To find new physical properties as a result of the ordering, it is prerequisite for the nanocrystals to be ordered at the mesoscopic scale [16]. Furthermore, the monodispersed blocks of colloid nanocrystals of silver, CdSe, cobalt, and gold have been built into ordered superstructures by surfactant-templated approaches [17–19]. However, soft-template methods often suffer from the elaborate choice of template, solvent and precursor, because precursors of many materials either condense too fast, destroy the assembly of the surfactant, or are not compatible with aqueous synthesis. For the preparation of ordered nanoarchitecture, hard templating has some advantages compared with soft templating, especially in its stability, predictability, controllability and versatility [20–22].

Herein, we present a simple hard-templating method for constructing porous helical nanofibre superstructures using inorganic anisotropic nanocrystals, which features the collective intrinsic property of ETOA. This method involves directly casting metal oxides on the helical carbonaceous nanotubes, and subsequently removing the template along with the formation of metal oxide nanocrystals by calcination at high temperature in air. The resulting materials with helical stacking of nanocrystals have distinct and durable ETOA, responding to circularly polarized light. Moreover, such hierarchical materials possess high special surface area and large pore volume, and pores come from the removal of the hard template in the axial direction as well as the looser packing of nanocrystals in the wall, respectively.

In this investigation, TiO₂ was selected as an example for illustrating our proposal, because the anatase nanocrystals typically show ultraviolet absorption in the region of 200–400 nm, which would exhibit ETOA if they had an ordered spatial arrangement with suitable pitch [12, 23]. Here, the synthesis process of chiral TiO₂ nanocrystals is as follows: firstly, chiral polypyrrole nanotube (CPPyN) and chiral carbon nanotube (CCN) were synthesized by the method of our previous report [24–26]. Both of the core/shell coaxial chiral nanotubes of CPPyN/TiO₂ and CCN/TiO₂ were obtained by nanocasting with a thin TiO₂ layer. Upon removal of the CPPyN or CCN templates, TiO₂ nanotubes were obtained as free-standing stacks of anatase nanocrystals that have a long-range chiral helical structure, resulting in ETOA properties.

2.1. Chemicals

2.2. Preparation of chiral templates of CPPyN and CCN

2.3. Preparation of chiral TiO₂ nanotubes

2.4. Characterization

Both of the core/shell coaxial chiral nanotubes of antipodal L/R-TiO₂@CPPyN and L/R-TiO₂@CCN were obtained by nanocasting of a thin TiO₂ layer on the chiral nanotubes of CPPyN and CCN by controlled hydrolysis and condensation of TBOT with H₂O under conditions of ambient pressure and temperature, where the helical nanotubes of CPPyN or CCN can serve as hard templates. Figure 1 shows SEM and TEM images of L-TiO₂@CPPyN, L-TiO₂@CCN, L-P-TiO₂ and L-C-TiO₂ derived from L-CPPyN and L-CCN (figure S1). Here, the L-CPPyN with ∼90 nm in diameter and L-CCN with ∼85 nm in diameter were used as templates. With uniform encapsulation of the TiO₂ nanoshell around L-CPPyNs or L-CCNs, the double-helical morphology of the templates was well maintained (figure 1). The as-prepared L-TiO₂@CPPyN or L-TiO₂@CCN hybrids synthesized with left-handed CPPyNs or CCNs were exclusively composed of left-handed double-helical fibres. The right-handed R-TiO₂@CPPyN hybrid nanotubes gave mirror image profiles with the same size and quality (figure S2). The diameters of the fibres increased from ∼90 nm of L-CPPyNs to ∼100 nm in L-TiO₂@CPPyNs, and from ∼85 nm of L-CCNs to ∼95 nm in L-TiO₂@CCNs (figure S3), indicating that such mineralization yielded highly full coverage of about ∼5 nm average thickness of TiO₂ nanoshell over the entire length of the templates. In addition, the fibres possess inner tubes with a diameter of ∼20 nm that are constant along the central axis of each individual tube (figures 1(a-2), (b-2)), which implied the presence of the helical lipid aggregation of N-acyl-l-glutamic acid (C₁₈-l-Glu) in the CPPyNs or CCNs. From the distinct helical morphology, the pitch length along the axis of the fibre was estimated to be ∼100 nm (shown as arrows in figure 1). TiO₂ and carbon could not be distinguished from TEM images (figures 1(a-2), (b-2)), because TiO₂ attached uniformly on the surface of CPPyNs and CCNs is in the form of an amorphous structure, which is the same as helically arranged carbonaceous nanostructured building blocks of CPPyNs and CCNs consisting of graphitic and amorphous carbon [25].

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As shown in figures 1(a-3) and (b-3), upon removal of CPPyNs or CCNs, TiO₂ nanofibres with the helical morphology were obtained, which indicated those free-standing TiO₂ nanofibres have long-range chiral helical structure with the arrangement of nanoparticles. The diameters of fibres decreased from ∼100 nm of L-TiO₂@CPPyNs to ∼60 nm in L-P-TiO₂, and from ∼95 nm of L-TiO₂@CCNs to ∼55 nm in L-C-TiO₂. However, the rotational misplaced arrangement of TiO₂ nanocrystals still remained and further enhanced circular dichroism.

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Figure 2 shows XRD patterns of L-TiO₂@CPPyN, L-TiO₂@CCN, L-P-TiO₂ and L-TiO₂@CCN. As shown in figure 2(a), the XRD patterns of the as-prepared L-TiO₂@CPPyN and L -TiO₂@CCN show only a broad band at 2θ = 25°, indicating that the TiO₂ coated on CPPyN or CCN was amorphous. However, after calcination at 550 °C in the air, the XRD patterns exhibited identical reflections indexed as the anatase phase of TiO₂ [JCPDS cards, No. 21-1272] with the space group of I4₁/amd and lattice parameters of a = 3.79 Å and c = 9.51 Å, [23, 27, 28] and no other peaks related to impurities, such as rutile or brookite were detected. The mean crystallite sizes of L-P-TiO₂ and L-C-TiO₂ were under ∼10 nm, which were evaluated using Debye–Scherrer's formula [29]. By comparison, the size of the anatase nanocrystals formed by the hard templating route here is smaller than that of the chiral TiO₂ nanofibres synthesised by the surfactant-templating method, which could be attributed to their confined growth on the helical surface of templates as well as a small amount of coating of TBOT as the titanium precursor [12].

The HRTEM image in figure 3 shows that the TiO₂ nanotubes were composed of loosely packed anatase nanocrystals. The diameters of the nanocrystals are less than 10 nm, which agreed with the XRD results. The possible reason for these anatase 2D nanosheets is that the interface TiO₂ coating close to the carbonaceous tubes has to grow restrictively and crystallize along the template surface, and different conditions with the outside TiO₂ cause their anisotropic growth into nanosheet structures. These nanocrystals exhibit the contrast close to 〈111〉 zone axis of the anatase structure, which indicates that the nanocrystals must have a preferential growth direction during the crystallization process. Additionally, it has been observed that such anatase 2D nanosheet structures are not favoured to be directly formed according to the Wulff construction, because these anatase crystals will be prone to grow into the octahedral shape with the majority of the exposed surface of (101) facets for the energetic stability [30–35]. Nevertheless, the adjacent nanocrystals can grow and crystallize with a rotational arrangement according to the demand of the helical template surface, even if the hard templates have been removed in the stage with elevated temperature in air.

High-surface-area TiO₂ nanocrystals are of particular interest for applications such as dye-sensitized solar cells, photocatalysts, gas sensors, and batteries [36–39]. Because of the extraordinarily large surface areas and abundant channels or pores, the nanoporous materials can greatly facilitate mass diffusion within frameworks. Nitrogen adsorption measurements were performed on the chiral TiO₂ samples (figure 4). The Brunauer–Emmett–Teller surface areas of TiO₂ nanofibres obtained from L-TiO₂@CPPyN and L-TiO₂@CCN are 194 and 224 m² g⁻¹, respectively, with corresponding pore volumes of 0.539 and 0.689 cm³ g⁻¹ and peak pore diameters of 15 nm. According to the results by the above SEM and TEM characterization, their pores would be created from the removal of the hard template in the axial direction as well as the looser packing of nanocrystals in the hierarchical wall, respectively. The specific surface area and pore volumes of TiO₂ templated by chiral carbonaceous nanotubes compare well with TiO₂ that has been templated by other mesoporous hosts [40–42]. Therefore, the above results demonstrate that such a helical template can also be a new candidate for the synthesis of porous materials with higher specific surface area and larger pore volume, but not just limited to mesoporous templates.

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As is known, it still remains a fundamental challenge to organize the anisotropic nanocrystals into ordered structures with collective properties, such as chemical, physical (optical, electrical, magnetic, etc) properties. The optical activity of materials shows their ability to have differential interactions with left- and right-handed circularly polarized light, resulting from their interaction between their chiral structure and light passing through, which can be detected by solid-state DRCD. Figure 5(a) shows the DRCD and UV–vis spectra of two pairs of antipodal L/R-TiO₂@CPPyN and L/R-TiO₂@CCN. For the above hybrid materials, the DRCD and UV–vis spectra show the composite signals, which are attributed to TiO₂ and carbonaceous materials of L/R-CPPyN or L/R-CCN. The signals in the range of less than 350 nm are mostly attributed to the outside TiO₂ on the template, while a broad absorption band in the range of 200–800 nm is attributed to CPPyN or CCN resulting from the combination of nanoeffect, polarization, and π–π conjugated structure, as described in our previous work [25]. After the removal of the carbonaceous template, the antipodal L/R-P-TiO₂ and L/R-C-TiO₂ showed strong mirror-image CD signals with a peak at 330–350 nm corresponding to the UV absorption edges of TiO₂ (figure 5(b)). This ETOA can be considered a result of the dynamic coulomb interaction under the dissymmetric field electron transition from the valence band to the conduction band of TiO₂ through a vicinal effect of helically arranged TiO₂ species, resulting in their selective reflection of left- or right-handed circularly polarized light in the ultraviolet absorption region [12–14]. In addition, the CD signal of L/R-TiO₂@CPPyN is stronger than L/R-TiO₂@CCN. The DRCD intensity of L/R-P-TiO₂ is much stronger than that of L/R-C-TiO₂. One possible reason is the poor helical morphology of CCNs, the other might be the stronger interaction between Ti(IV) and amino groups of CPPyNs than physical attachment with CCNs, resulting in more closely stacked TiO₂ nanocrystals in CPPyN and further inducing stronger dynamic coulomb interaction and CD signals [10, 43]. A recent report indicates that the chiral nematic organization of TiO₂ nanocrystals into a mesoporous film can also endow them with new a new ability to reflect circularly polarized light, however, which can be ascribed to the scattering at the inorganic/air interfaces of the helical arrays rather than the electron transition [5].

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In summary, chiral TiO₂ nanofibres with ETOA can be easily synthesized by chiral helical hard templates. Such materials with hierarchical structure possess high special surface area and large pore volume from the removal of the hard template in the axial direction as well as the looser packing of the nanocrystals in the wall, respectively. The materials selectively reflect left- and right-handed circularly polarized light, which indicates a chiral organization of the titania crystallites. These novel highly porous titania nanotubes may be excellent materials for applications in dye-sensitized solar cells, photocatalysts, and lithium ion batteries. Therefore, one can reasonably expect that other porous materials with ETOA could be prepared by this simple hard templating method.

We acknowledge the support from the National Natural Science Foundation of China (Grant No. 21471099) and the National Basic Research Program (2013CB934101) of China. We thank the Instrumental Analysis Centre of Shanghai Jiao Tong University for their collaboration on the DRCD measurements.