TiO₂ powder [P25, (99.5%, 21 nm)] and potassium hydroxide (KOH) were purchased from Sigma-Aldrich, USA. HCl for acid washing was purchased from S.D. Fine Chemicals, India. TNTs were synthesized via the hydrothermal method, where a mixture of 0.5 g of TiO₂ powder was dispersed in 30 mL of concentrated aqueous KOH solution. The mixture was stirred for 30 min and then transferred into a Teflon-lined stainless-steel autoclave, where the hydrothermal treatment was performed for 24 h at 150˚C [34] . The precipitate was collected and washed with deionized water and dilute HCl until the pH was 6.5. The TNTs were then dried for 10 h at 90˚C, followed by annealing at 400˚C for 2 h. In the case of doping, the molar ratio of titanium/dopant was 20:1 when Nd(NO₃)₃ and/or Gd(NO₃)₃ were added to the TiO₂ in the KOH solution. The hydrothermal and post-synthetic treatments were then performed as described above for the un-doped TNTs.

The samples were accordingly labelled TNTs, Nd-TNTs, Gd-TNTs, and Nd-Gd-TNTs. Finally, the TNTs became a white powder after drying and grinding. They were characterized with respect to surface morphology and crystallographic structure.

In Figure 1(a), a spherical shape was observed for the P25 TiO₂ powder. The un-doped TNTs samples calcined at 400˚C (Figure 1(b)) were aggregated thread networks. Figures 1(c)-(e) are SEM images of in situ lanthanide-doped TNTs calcined at 400˚C. These nanotubes were heavily aggregated.

The approximate compositions of P25 and the un-doped and doped TNTs were calculated from EDX spectra (Figures 1(a')-(e')). Uniform distributions of Nd and Gd nanoparticles were observed in the TNT lattice. As shown in Table 1, the EDX patterns had nearly 3.37% Nd loading in the Nd-TNTs, 2.92% Gd loading in the Gd-TNTs, and 9.42% Nd and 3.48% Gd loading in the Nd-Gd TNTs.

Figure 2(a) displays a TEM image of un-doped TNTs that were aggregated and formed a large composite fiber-like structure, in agreement with the SEM images. The nanotubes were uniform and hollow, with multiwall sheets. The outer diameters were 6.5 - 10.6 nm, and the lengths were 51 nm. The open-ended fiber-like structures of uniform, multiwall, straight tubes are shown in Figures 2(b)-(d). The well-dispersed Nd and/or Gd oxide nanoparticles in the TNT lattice also can be seen. The outer diameter of the Nd-TNT multi-layered sheets was in the range of 5.7 - 8 nm, that for the Gd-TNTs was in the range of 6.1 - 14.2 nm, and that for the Nd-Gd-TNTs was in range of 9.4 nm - 14.2 nm.

The mechanisms for multiwall TNT formation by hydrothermal treatment could be understood as follows. When the Ti precursors were mixed with the concentrated aqueous KOH at high temperature and pressure, Ti-O-K bonds formed on the TiO₂ surface instead of “Ti-O-Ti” bonds because of the concentrated KOH [35] . These salts formed nanosheets by:

Ti-O-Ti + 2 KOH → 2 ( Ti-O − , K + ) + H 2 O (1)

Then, when the samples were treated with deionized water, the Ti-O-K bonds in the ( K 2 Ti n O 2 n + 1 ) multilayer crystal gradually converted into Ti-OH bonds in sheets of hydroxyl titanate ( H 2 Ti n O 2 n + 1 ) . After the sample was treated by dilute acid, Ti-O-Ti bonds [36] or Ti-O-H ⋯ O-Ti hydrogen bonds were generated according to:

( Ti-O − , K + ) + H + → Ti-OH + K + (2)

These bonds significantly decreased the Ti-O length, thus leading to the folding of the sheets into a multiwall tube structure, as shown in Figure 3.

The crystal phases of P25 TiO₂ and un-doped and doped TNTs are shown in Figure 4. The XRD patterns in Figure 4(e) for P25 TiO₂ indicated anatase phase planes with main peaks located at 25.3˚, 37.8˚, 48.0˚, and 55.1˚, respectively corresponding to the (101), (004), (200), and (211) planes, in good convention with the standard spectrum [Joint Committee on Powder Diffraction Standards (JCPDS), card no.: 21-1272] [37] . The XRD patterns of un-doped TNTs are shown in Figure 4(a), where diffraction peaks were observed at 24.90˚, 48.10˚, and 55.91˚, corresponding to the (101), (200), and (211) crystal planes of anatase TiO₂, respectively. These data suggested an anatase phase of TiO₂ (JCPDS, card no.: (00-021-1272). However, these XRD patterns were typical of layered materials, suggesting that the TNTs were multi-walled [38] . The diffraction peak with a low 2θ value of 11.5˚ (020) corresponded to the TNT interlayer distance. An intense peak at 2θ = 48.10˚ was due to the edge-sharing TiO₆ octahedral [18] .

The diffraction planes of anatase in Figure 4(b) were sharp, indicating good crystallization of Nd-TNTs, and the other four peaks had the same positions of 22.70˚, 32.45˚, 46.65˚, and 57.28˚ assigned to (002), (200), (220), and (132) reflections, indicating well-embedded incorporation of Nd ions in the TiO₂ lattice as NdTiO₃ (JCPDS, card no.: (00-029-0922). Meanwhile, in Figure 4(c), there were characteristic peaks of Gd observed at 29.28˚, 31.64˚, 47.89˚, and 58.72˚ assigned to (201), (040), (341), and (611) planes, respectively, indicating Gd ions in the form of Gd₂TiO₅ (JCPDS, card no.: (00-021-0342). The XRD patterns recorded for co-doped Nd-Gd-TNTs are shown in Figure 4(d). They revealed anatase phases, with new peaks for Nd and/or Gd.

The occasional absence of sharp peaks corresponding to Nd and/or Gd in the XRD patterns could be attributed to small or highly dispersed concentrations below the detection limit, or they were located inside the TiO₂ lattice (i.e., TNT doping) [39] .

Grain sizes of the P25 TiO₂ and the un-doped and doped TNT powders were calculated using the Scherrer equation. The crystalline sizes are shown in Table 2, where there was a slight change when Nd and/or Gd were incorporated.

The slight expansion of the TiO₂ crystal volume after lanthanide modification suggested that the Nd and Gd oxide were located at the crystal boundaries rather than inside the TiO₂ unit cell [38] [39] . For the Nd-Gd-TNTs, the crystallite size was reduced from 7.06 nm to 6.35 nm. Zhang et al. [40] and Meksi et al. [20] explained this decrease of the TiO₂ anatase phase in terms of dopant cations located at the grain boundary hindering grain growth.

The BET surface area in Table 2 of un-doped TNTs was 160 m²/g, which increased to 175 m²/g for (20:1) for Nd-loading, 207 m²/g for (20:1) Gd-loading, and 231 m²/g (20:1) for Nd-Gd-loading. The massive increases compared to P25 were attributed to the tubular form of the TNTs [41] . The doped TNTs had a larger mesoporous volume than that of the undoped TNTs, while no change was observed in pore size. The larger mesoporous volume of all rare-earth-loaded TNTs samples has been attributed to the enhanced mesoporosity [40] .

A Raman spectrum of un-doped TNTs is shown in Figure 5. The peaks observed in Table 3 at 143 cm⁻¹ (E_1g), 198 cm⁻¹ (E_2g), 394 cm⁻¹ (B_1g), and 638 cm⁻¹ (E_3g) were attributed to anatase TiO₂, in agreement with previous studies [42] [43] [44] . The strongest E_1g mode at 143 cm⁻¹, attributed to the external vibration of the anatase structure, was well resolved and indicated that the anatase phase was formed in the nanotube [45] . This was in good agreement with the XRD data and previous work. The E_g peak was attributed mainly to the symmetric stretching vibration of O-Ti-O linkages in TiO₂, the symmetric bending vibration of O-Ti-O was confirmed by B_1g, and the A_1g peak was associated with the antisymmetric bending vibration of the O-Ti-O linkage [33] . The same strong peaks observed in Figures 5(b)-(d) for the doped TNTs had anatase characteristics, indicating that Nd and Gd may be introduced into the lattice or interstitial sites of TiO₂.

The band at 278 cm⁻¹ was assigned to the stretching vibration of Ti-O-K bonds [45] , while the bands at 198 cm⁻¹ and 394 cm⁻¹ corresponded to anatase Ti-O-Ti [43] . The bands at 448 cm⁻¹ were related to Ti-O-Ti crystal phonons [46] . The bands at 704 cm⁻¹ and 820 cm⁻¹ corresponded to covalent Ti-O-H bonds, and the band at 920 cm⁻¹ was assigned to surface Ti-O-K vibrations [47] (Table 3).

In FT-IR spectra of TNTs (Figure 6), three absorption bands cantered at 3432.53 cm⁻¹, 1627.87 cm⁻¹, and 955.87 cm⁻¹ were assigned to stretching vibrations of OH groups, and bending vibrations of H-O-H and Ti-O bonds, respectively [48] . The strong intensity of the hydroxyl groups at 3432.53 cm⁻¹ indicated a large concentration on the un-doped TNT surface. This helped conjunction and dispersion of the metal particles on the TNT walls [49] [50] [51] [52] , and also captured photoexcited electrons and holes to produce reactive oxygen species for photocatalysis of organic wastewater. The peaks at 1627.87 cm⁻¹ and 1349.52 cm⁻¹ corresponded to undoped TNTs [50] . The unchanged intensity of the 3432.53 cm⁻¹ band may have been due to the interactions of doped Nd and Gd in the TiO₂ lattice that did not replace -OH groups on the oxide surface. Moreover, the band at 446 ± 4 cm⁻¹ corresponded to the asymmetric vibration of M-Ti-O (M = K, Nd, Gd) groups inside the lattice [27] . In all cases, the FT-IR spectra kept their general features, which confirmed that the metals and the TNTs had strong chemical interactions, consistent with the XRD and Raman structural analyses.