Interplay between Fe-Titanate Nanotube Fragmentation and Catalytic Decomposition of C 2 H 4 : Formation of C/TiO 2 Hybrid Interfaces

: This paper reports the synthesis of Fe-titanate nanotubes by means of the conventional ion-exchange method with iron nitrate solutions. As the iron-rich nanotubes were found to contain the catalyst precursor intrinsically dispersed in their structures, the unprecedented possibility of using this kind of materials for building carbon nanostructures, ﬁrmly attached at the surface of the forming TiO 2 nanoparticles, was veriﬁed. The catalytic decomposition of ethylene, used as a carbon source, was performed at a relatively high temperature (750 ◦ C) when the nanotubes undergo an irreversible phase transformation to form anatase and rutile nanoparticles. Due to the different amounts of Fe ions in the nanotubes, distinct types of carbon/TiO 2 hybrid interfaces were formed, ranging from amorphous (lower Fe 3+ concentration) to the more crystalline graphitic domains (higher Fe 3+ concentrations), as documented by the microstructure of the treated samples. The present approach is of potential interest for (photo)catalytic and energy conversion/transport applications.


Introduction
The families of one-dimensional (1D) and two-dimensional (2D) nanomaterials have grown noticeably since the discoveries of carbon nanotubes (1991) and of graphene (2004). These 1D/2D nanostructures have opened new perspectives due to their unique properties, which are very different from that of the three-dimensional counterpart [1,2] and to the fact that such nanostructures can be assembled to form composites, hybrids, and van der Waals heterostructures [3][4][5][6][7]. The possibility of building or assembling carbon allotropes and carbon analogues at the surface of another inorganic system, like TiO 2 [8][9][10][11][12], provides the opportunity for building interfaces, matching phases with different properties, thus creating domains with predetermined characteristics, but exhibiting new functionalities.
On the other hand, a plethora of opportunities is also available when layered titanates (2D) and/or titanate nanotubes (1D) are taken into consideration [3,13,14]. The formation of anatase nanotubes, obtained from the hydrothermal reaction of TiO 2 nanoparticles and NaOH, was initially described by Kasuga et al. [15,16]. Through systematic structural analyses of samples, before and after HCl washing, subsequent studies suggested different structures for the hydrothermally formed nanotubes, including: H 2 Ti 3 O 7 [17], H 2 Ti 4 O 9 ·xH 2 O [18], H 2 Ti 2 O 4 (OH) 2 nanotubes [19], and today the consensus of opinion is that the fabricated nanotubes are made of layered titanate or lepidocrocite-type (H x Ti 2−x/4 x/4 O 4 ·H 2 O, where indicates a vacancy) structures. Both titanate nanotubes, trititanate and lepidocrocite-type, have multilayered structures with walls separated by 0.7-0.8 nm, where H + are located and can be exchanged with cations [20]. The trititanate nanotubes are composed of steps of corrugated layers, while the lepidocrocite-type is made of a more regular and flat surface, consisting of a continuous and planar 2D array [20]. The reason for investigating these TiO 2 -based tubular structures comes from their physical and chemical properties which area combination of the distinctive characteristics of the more conventional TiO 2 particles (i.e., wide-bandgap, semiconductor-type character) with those of layered titanates (i.e., ion-exchange, proton conductivity) and/or of nanotubes (i.e., small tubular cavities, high surface area and porosity properties).
In this work, Fe-titanate nanotubes were used as a support to decompose a carbon source (ethylene) at the surface of anatase and rutile nanoparticles under formation, where Fe is expected to play a role as a catalyst. Moreover, by controlling the catalyst content, when the nucleation of larger catalyst particles is promoted, curved graphitic domains are formed. This approach presents the unprecedented possibility of building carbon systems, from amorphous patches to more graphitic domains, firmly attached at the surface of TiO 2 .

UV-Vis and FTIR Spectroscopies of Fe (III)-Exchanged Titanate Nanotubes
UV-Vis spectroscopy is a fundamental technique for the characterization of transition-metal ions that allows distinguishing highly dispersed cations on/in a solid matrix from more isolated species. Spectral features (intensity, position and shape of bands) can reveal the nature (i.e., symmetry, oxidation state) and the aggregation state of metal ions. Along with the oxidation and coordination states, accurate details are usually given for the d-d transition bands. Such transitions can be assumed as fingerprints for cations having a well-defined oxidation state in the tetrahedral and octahedral coordination, according to Ligand Field Theory (i.e., transitions of e − within orbitals placed on a central ion). Unfortunately, for Fe cations the explanation is not straightforward due to the fact that: (i) iron in the d 5 configuration (Fe 3+ ) has very weak transitions (forbidden by the Laporte rule); (ii) ligand-to-metal CT transitions in the tetrahedral and octahedral coordination give bands with similar positions, as documented in the controversial literature on this matter. However, the degree of clustering of Fe cations may be semi-empirically estimated from the UV-Vis features, as the bands are red shifted by increasing the degree of nuclearity [39]. The literature on this subject is well documented by studies on Fe-zeolites (e.g., ZSM-5, beta structures), in which the evolution of Fe species in the UV-Vis spectra, from the more isolated to polynuclear and oxide, is shown [39].
In this context, diffuse reflectance (DR) UV-Vis spectroscopy is adopted here to investigate the nature of the Fe cations on/in titanate nanotubes (HTITs) by using different precursor concentrations (0.001 M, 0.0025 M and 0.01 M). Figure 1 shows the DR UV-Vis spectra obtained on the HTITs before and after ion-exchange in ethanol and the subsequent filtering and drying steps. The spectrum of hydrogen titanate nanotubes (spectrum a), is composed of a strong absorption edge located at~28,000 cm −1 (~357 nm), becoming dominated by the more complex envelope of absorptions in the 28,000-10,000 cm −1 range at the increasing concentrations of the Fe(NO 3 ) 3 solutions (b, c and d spectra). More precisely, the spectra are constituted by a first strong absorption extending up to~25,000 cm −1 (500 nm), with a weak shoulder at about 24,000 cm −1 (417 nm), an absorption band centered at~21,000 cm −1 and a wider tail up to 10,000 cm −1 (1000 nm). All these spectral features, ascribed to nucleation centers (i.e., formation of larger aggregates), develop at the increasing Fe (III) solution concentrations. Hence, the formation of oxide species (those at lower energy) and polymeric Fe species (band at~21,000 cm −1 ) is inferred. Other features at lower energy can be plausibly ascribed to dinuclear Fe (III) species (28,000-29,000 cm −1 ) and more isolated Fe cations (tetrahedral coordination), but a more detailed description of Fe species is very difficult here due to the presence of the absorption edge of the titanate nanotubes obscuring the 46,000-33,000 cm −1 range, where ligand-to-metal CT transitions of Fe (III) in the octahedral coordination with oxygens, are found [40].
In the inset of Figure 1, (αhν) 1/2 is plotted versus the energy for the indirect transitions as obtained by the Tauc plot relation (Tauc and Davis-Mott model): where h is the Planck's constant, ν is the wavenumber, α = absorption coefficient, A is a constant and E g is the optical band gap (in eV) for direct (n = 1 2 ) or indirect (n = 2) transitions, respectively. By assuming F(R) is proportional to the absorption coefficient α, it is clearly shown in this plot that the incorporation of iron into the structure of the nanotube alters its lattice and electronic structure.
In more detail, by extrapolating the linear portion of the (F(R) × hν) 1/2 curves vs. the photon energy hν, the reduction in the energy bandgap from hydrogen titanate nanotubes (3.3 eV) is well illustrated for the Fe-titanate sample series. By increasing the Fe 3+ content the value of the bandgap energy progressively decreases from 3.2 eV (curve b: sample ion-exchanged with 0.001 M Fe(NO 3 ) 3 solution) to 3.1 eV and 2.8 eV (curve c and curve d: samples ion-exchanged with 0.0025 M and 0.01 M solutions of Fe 3+ ). The decrease in the energy bandgap may be ascribed to the progressive doping iron ions that can form dopant levels near the valence band of the titanate nanotubes.
In conclusion, the UV-Vis spectra of our Fe-TIT samples are similar to those shown for the iron-silicalite, zeolitic structures and mesoporous frameworks [39].
FTIR spectra of the hydrogen titanate nanotubes and of the Fe-TITs#0.01 (exchanged with 0.01 M Fe 3+ ) are shown in Figure 2. In Figure 2, the bands in the 3600-3000 cm −1 and 1690-1550 cm −1 intervals (spectrum a) are assigned to stretching and bending vibrations of the -OH groups, respectively, interacting through hydrogen bonds, while those in the 1000-400 cm −1 region (inset of Figure 2) are assigned to Ti-O surface and Ti-O-Ti broad phonon bands due to TO vibrational modes [41]. Additionally, the intense band centered at 490 cm −1 can be attributed to Ti-O-Ti skeletal modes of the interconnected octahedra forming the TNT walls [41]. In the Fe-titanate sample (spectrum b) an additional peak at about 950 cm −1 can be attributed to the Fe-O stretching mode. However, the general erosion of the wide absorption observed in the 880-550 cm −1 interval can be explained with the Fe-doping that causes interruptions of the Ti-O-Ti bond linkage [27].

Morphology, Structure and Chemical Composition of Fe-Titanate Nanotubes
The chemical composition, morphology and structure of the hydrogen titanate nanotubes were investigated before and after the ion-exchange with the Fe(NO 3 ) 3 solutions. The results are reported in Figure 3.
The prepared hydrogen titanate nanotubes are constituted by well-shaped and elongated nanostructures 7-10 nm in diameter and several hundred nanometers in length with an internal cavity (Figure 3a). The nanotubes have a multiwalled structure with walls~7.5 Å spaced, which are parallel to the axis of the nanotube. A second family of fringes~3.5 Å spaced can be observed at about 74 • (Figure 3b). The observed crystal features are in agreement with a lepidocrocite-type titanate structure. The Fe-TIT sample (exchanged with 0.0025 as a molar solution) shows tubular multiwalled nanostructures a few hundred nanometers in length, open-endings, outer and inner diameters of 7-10 nm and of 3-4 nm, respectively (inset of Figure 3c). The walls are~7.6 Å spaced and the different wall thicknesses, as shown on the different wall sides (Figure 3c), suggest that they are formed through scrolling-up of 1 L sheets, as reported in other studies [13,34]. EDAX spectrum, taken for the Fe-titanate nanotubes shown in Figure 3c, reveals that Fe is present in the structure of the nanotubes (Figure 3d), but no Fe-containing nanoparticles or clusters were imaged by the TEM observations of several nanotubes. This fact is in agreement with the Fe doping in substitutional positions of the lattice [31] observed at low Fe contents, which does not affect the interlayer distance of the walls [20]. Despite the fact that the substitution of Ti 4+ with Fe 3+ ions [31,34] or of the hydrated iron complexes of [Fe x II (OH) 2x−1 (OH 2 )] + [32], have been shown at low Fe 3+ concentrations, the presence of isolated Fe x O y clusters in our samples escaping from the TEM detection could not be ruled out for the more concentrated Fe-TIT sample, based on the previously discussed UV-Vis spectra. The Fe contents, as obtained from EDAX spectra of the different Fe-titanate nanotubes, are summarized in Table 1. Although EDAX analysis is only semi-quantitative, it is worth noting that different Fe compositions (~0.4 wt %, 1 wt % and 3 wt %) were found for the Fe-titanate nanotubes ion-exchanged with the different concentrations (0.001 M, 0.0025 M and 0.01 M, respectively) of FeNO 3 .  and cementite (PDF card n. #35-0772), respectively. Finally, the XRD peak at 2θ ≈ 44.7 • responds to the (110) planes of ferrite (α-Fe, bcc-phased Fe, PDF card n. #06-0696). All these minor features reveal the role played by the Fe catalyst in assembling carbon domains (from amorphous to the more crystalline) via the decomposition of C 2 H 4 . Ethylene is known in catalysis not only for the catalytic polymerization/oligomerization [42], but also for the formation of carbon nanostructures, including single-walled carbon nanotubes (SWCNTs) [43], multi-walled carbon nanotubes (MWCNTs) [44], graphene [45], and curved graphitic domains [46]. To gain more insight into the structure and morphology of samples after being contacted with C 2 H 4 at 750 • C, TEM images were acquired. The morphological properties of Fe-TITs obtained from the 0.001M solution of Fe(NO 3 ) 3 and treated at 750 • C are shown in Figure 5a. HRTEM images reveal that elongated (or rod-like) nanoparticles with sizes of 10-30 nm are formed and that such nanocrystals expose fully decorated edges, but nothing more can be inferred at this magnification level. At this point, from a chemical point of view it is important to recall the fact that titanate nanostructures are thermodynamically metastable phases forming TiO 2 polymorphs under thermal treatment (with the release of H 2 O), while from a morphological viewpoint the disappearance of the internal cavity is accompanied by the formation of rod-like nanostructures having a size very close to the external diameter of the initial nanotubes [20,47].
In Figure 5b the amorphous nature of this thin film, covering anatase nanoparticles, is illustrated. Furthermore, an anatase nanocrystal, oriented along the [001] zone axis (see fast-Fourier transform: FFT, and the simulated SAED pattern in Figure 5c,d, respectively), is shown in this figure.
Finally, the amorphous nature of the thin film at the surface of the nanocrystals is shown (Figure 5e) in a selected region (of Figure 5b).
The situation is completely different if TEM images of Fe-TITs, obtained from the more concentrated solutions of Fe(NO 3 ) 3 (0.0025 M and 0.01 M) are taken into consideration (Figure 6a-d, respectively). In this figure, the higher crystallinity of carbon (i.e., graphitic stacking) is well documented by regular or more curved structures (see arrows in Figure 6a,c), whose stacking numbers, symmetry and quantities increase with the increase in the amount of nanotube-supported catalyst (higher for titanate nanotubes ion-exchanged with iron nitrate 0.01 M). In the same images, the presence of anatase nanocrystals is evidenced by the regularly 0.35 Å spaced fringes, while the role played by the catalyst in forming graphitic structures is illustrated in Figure 6d, where carbon-encapsulated nanoparticles (see arrows in panel d) have spacing, measured between the adjacent lattice fringes of 2.0 Å, which is in good agreement with the spacing determined from XRD measurements for (110) planes of ferrite (α-Fe).
The yields of carbon have been obtained from the weight loss of the TGA curves for the samples contacted with C 2 H 4 for 1 min (Figure 7). From this figure, it is clear that the catalyst content (see Table 1) plays a key role in the production of carbons occurring at the surface of the thermally treated Fe-TITs. From a quantitative viewpoint the carbon quantity has been calculated to be~5 wt %,~11 wt % and~21 wt % (for Fe-TITs with about 0.4 wt %, 2 wt % and 3 wt % of Fe, respectively).
These C/TiO 2 -based hybrid structures show magnetic properties when an external magnet is used more than two years after the preparation (Figure 8 and Supplementary Materials), thus testifying to the good stability to oxidation in air of the encapsulated metal catalyst.
The magnetic character, together with semiconducting properties of C/TiO 2, may find application in the photodegradation of pollutants [48,49] due to the easier recovery of the photocatalyst from the solution. Moreover, C-TiO 2 interfaces have been shown to be effective in building functional systems for energy conversion applications [50].

Synthesis of Materials
Titanate nanotubes were prepared following the procedure described in a previous work [28]. Briefly, TiO 2 anatase (Sigma-Aldrich, Milan, Italy) was used as a precursor material and treated with a NaOH solution (10 M) in a PTFE autoclave for 18 h at 150 • C. The as-obtained sodium titanate nanotubes, washed several times by using distilled water and filtered to remove the excess of reactants, were then treated with HCl (0.1 M) for 5 h under stirring. The obtained material was washed, filtered and dried at 120 • C to obtain protonated titanate nanotubes (hereafter H-TITs). Thereafter, H-TITs were treated with Fe(NO 3 ) 3 ·9H 2 O in ethanol solutions (0.001 M, 0.0025 M and 0.01 M) for 20 h. The as-obtained samples (hereafter Fe-TIT) were then filtered with EtOH several times, to remove the excessive Fe(NO 3 ) 3 and dried at 80 • C. Fe-TIT samples were inserted in a quartz tube reactor and thermally treated adopting the following steps: (i) a ramp from RT to 750 • C (heating rate: 15 • C/min) under N 2 flow (0.5 L/min); (ii) reaction with a C 2 H 2 /H 2 gas mixture (4:1) at 750 • C for 1 min.

Conclusions
The preparation of Fe-titanate nanotubes from hydrogen titanate and Fe(NO 3 ) 3 solutions by means of the ion-exchange method is reported. As the Fe-rich titanate nanotubes contain the catalyst precursor intrinsically dispersed in their structures, the unprecedented possibility of this kind of system of building carbon nanostructures, firmly attached at the surface of the forming TiO 2 nanoparticles was verified. The catalytic decomposition of ethylene, used as a carbon source, was performed at a relatively high temperature (750 • C), at which the titanate nanotubes undergo an irreversible phase transformation into anatase and rutile nanoparticles. Due to the different amounts of Fe ions, different types of carbon/TiO 2 hybrid interfaces were obtained, ranging from amorphous (lower Fe 3+ concentration) to the more crystalline graphitic domains (higher Fe 3+ concentration), as documented by the microstructure of the treated samples. The present approach is of potential interest for (photo)catalytic and energy conversion/transport applications.