Fabrication and Characterization of Lanthanide-TiO₂ Nanotube Composites

Khadijah M. Emran^1*, Hessah E. Alanazi²

¹Department of Chemistry, College of Science, Taibah University, Madinah, KSA.
²Department of Chemistry, College of Science and Art, Al Jouf University, Qurayyat, KSA.

DOI: 10.4236/ojpc.2023.132002                

Abstract

Titanium dioxide (TiO₂) doped with neodymium (Nd) and/or Gadolinium (Gd) rare-earth elements were fabricated into nanotubes via the hydrothermal method in a KOH solution and in-situ doping. Titanium dioxide nanotubes (TNTs) and in-situ Nd-doped and/or Gd-doped TNTs were characterized with transmission and scanning electron microscopy, energy-dispersive X-ray analysis, X-ray diffraction, Raman spectroscopy, and Fourier-transform infrared spectroscopy. Morphologies indicated a network of aggregated nanotubes. The phase and composition analyses revealed that the lanthanide TNTs had anatase phases with Nd and/or Gd nanoparticles in the TNT lattice. The nanoparticles were uniformly deposited on the surface because of hydroxyl groups on the TNT surfaces, resulting in a very high loading density. The outer diameter and the length of the TNTs increased with doping. The mechanisms for the formation of multiwall TNTs are discussed.

Keywords

TiO₂ Nanotube, Lanthanide Doped, Hydrothermal, XRD, TEM, Raman Spectroscopy

1. Introduction

In the effort to improve the performance of fuel cells, dye-sensitized solar cells [1] , photocatalysis [2] [3] , and solar driven processes [4] , the fabrication and electrocatalytic properties [5] [6] of metals deposited on nanomaterials have been intensely investigated. Nanoscale materials derived from titanium oxide (TiO₂) have been extensively investigated for various applications, including solar-cells and batteries [7] [8] , hydrogen generation [9] [10] [11] , drug delivery [12] [13] , photolysis [14] [15] , and electrocatalysis [16] [17] . Titanium nanotubes (TNT) can be fabricated via the template-free hydrothermal method, which is simple, cost-effective, and environmentally friendly [18] . Furthermore, TiO₂ is a low-cost abundant resource known for its long-term stability, non-toxicity, and resistance to photo-corrosion.

Lanthanide ions have a special 4f^x5d^y electronic structure that enables them to form complexes with various compounds, such as TiO₂, that can donate a pair of nonbonding electrons. Incorporation of lanthanide ions in a TiO₂ matrix could concentrate organic pollutants on the surface and thus enhance electrocatalytic and/or photocatalytic activity [19] [20] [21] [22] . The specific surface area of the nanoscale catalyst and its particle size are very important parameters that strongly affect its catalytic activity.

In recent years, researchers used many Lanthanide elements to modify the TiO₂ nanostructure (such as La, Pr, Nd, Eu, Gd, Tb, Ho, Er and Yb) by different method to improve its photocatalytic and others applications [20] - [31] . In particular, Lanthanide elements (such as La, Eu, Gd and Ce) doped TNTs were prepared by hydrothermal method using NaOH to improve its photocatalytic and photoelectrocatalytic activity for oxidation of organic dye and organic compounds [20] [23] [29] [31] . Doping methods include wetness impregnation method [20] , electrochemical anodization [21] [22] [27] [30] , precipitation methods [24] , sol-gel method [19] [25] , hydrothermal treatment [26] , wetness impregnation method [20] , ultrasonic hydrothermal [29] , ion exchange [31] .

Here, the hydrothermal method for TNT syntheses and an in-situ doping strategy was used to incorporate Nd and/or Gd nanoparticles in TNT scaffolds. The effects of these lanthanides on the structural, textural, and morphological properties of TiO₂ nanomaterials obtained after calcination of hydrogen titanate nanotubes were investigated. The synthesized TNT, Nd and/or Gd-TNT show different applications, such as great performance of electro-oxidation of hydrazine by electrosensory [32] and determination of pharmaceutical compound as Alendronate sodium [33] .

2. Experimental

2.1. Synthesis of TNTs

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.

2.2. Apparatus

2.2.1. Morphology and Compound Composition

The morphological characterization of un-doped TNTs and doped TNTs prepped by hydrothermal method was achieved using Transmission Electron Microscope (TEM, JEOL JEM 1400, Japan) at 110 kV. The samples were prepared by dropping the ethanol solution of TNTs and Nd-TNTs, Gd-TNTs and Nd-Gd-TNTs catalysts on the Cu coating by carbon grids. The scanning electron microscopy (SEM) images were taken at different magnifications, without coating with conductive material, by (SEM, Super scan SS-550, Shimadzu, Japan). EDX was used to characterize the distribution of elements in un-doped TNTs and doped-TNTs synthesis by both hydrothermal using (EDX, Superscan SS-550, Shimadzu, Japan).

2.2.2. Crystalline Phase

Crystalline phases were acquired via X-ray diffraction (XRD, Shimadzu, XRD-7000, Japan) at 40 kV and 30 mA, using a Cu Kα incident beam (0.154 nm). The TNT molecular structures were characterized with Fourier-transform infrared spectroscopy (FT-IR, Thermo Fisher Scientific Inc., Madison, WI, USA) over the range 400 - 4000 cm⁻¹. Raman spectroscopy was performed with 532-nm excitation on a Raman microscope (Sentrarra, Bruker, USA) coupled to a Leica microscope (Olympus BX series, USA). The spectra were analyzed over the range 50 - 1200 cm⁻¹, with Rayleigh rejection via a 532 nm.

2.2.3. Surface Area and Porosity

Brunauer-Emmett-Teller specific surface areas (S_BET) of the TNTs were determined by a multipoint method using adsorption data for a relative pressure of 0.1515 (Micromeritics analyzer, Gemini VII, 2390 Surface Area and Porosity, USA).

3. Results and Discussion

3.1. Nanotube Morphologies

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.

(a) (a') (b) (b') (c) (c') (d) (d') (e) (e')

Figure 1. SEM images of ((a), (b), (c), (d)) P25 TiO₂ Powder, undoped TNTs, Nd-TNTs, Gd-TNTs, and Nd-Gd-TNTs; ((a'), (b'), (c'), (d')) EDX of undoped TNTs, Nd-TNTs, Gd-TNTs, and Nd-Gd-TNTs respectively.

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:

Table 1. EDX analysis of P25 TiO₂, undoped TNTs, Lanthanides-doped TNTs.

(a) (b) (c) (d)

Figure 2. TEM images of (a) undoped TNTs, (b) Nd-TNTs, (c) Gd-TNTs, and (d) Nd-Gd-TNTs.

$\text{Ti-O-Ti}+2\text{KOH}\to 2\left({\text{Ti-O}}^{-},{\text{K}}^{+}\right)+{\text{H}}_2\text{O}$ (1)

Then, when the samples were treated with deionized water, the $\text{Ti-O-K}$ bonds in the $\left({\text{K}}_2{\text{Ti}}_n{\text{O}}_2{}_{n+1}\right)$ multilayer crystal gradually converted into Ti-OH bonds in sheets of hydroxyl titanate $\left({\text{H}}_{\text{2}}{\text{Ti}}_n{\text{O}}_2{}_{n+1}\right)$ . After the sample was treated by dilute acid, $\text{Ti-O-Ti}$ bonds [36] or $\text{Ti-O-H}\cdots \text{O-Ti}$ hydrogen bonds were generated according to:

$\left({\text{Ti-O}}^{-},{\text{K}}^{+}\right)+{\text{H}}^{+}\to \text{Ti-OH}+{\text{K}}^{+}$ (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.

3.2. Crystalline Phase Characterization

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.

Figure 3. Hydroxyl titanate nanosheets ( ${\text{H}}_2{\text{Ti}}_n{\text{O}}_{2n+1}$ ), (a) mechanism for breaking of ${\text{Na}}_2{\text{Ti}}_2{\text{O}}_4{\left(\text{OH}\right)}_2$ , (b) rolling up the nanosheet to nanotubes.

Figure 4. XRD image of (a) P25 TiO₂, (b) undoped TNTs, (c) Nd-TNTs, (d) Gd-TNTs, and (e) Nd-Gd-TNTs.

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.

3.3. BET Surface Area

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] .

3.4. Resonance Raman Spectra

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.

Table 2. Particle sizes, BET surface area, pore volume and pore size of synthesis TNTs

(A)(B)

Figure 5. Raman spectra of (a) undoped TNTs, (b) Nd-TNTs, (c) Gd-TNTs, and (d) Nd-Gd-TNTs.

Table 3. Raman shift of vibration mode for different catalyst supports.

Figure 6. FT-IR spectroscopy of (a) TNTs, (b) Nd-TNTs, (c) Gd-TNTs, and (d) Nd-Gd -TNTs.

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).

3.5. Fourier-Transform Infrared Spectra

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.

4. Conclusion

The fabrication of un-doped TNTs and Nd- and/or Gd-doped TNTs was demonstrated via the environmentally-friendly hydrothermal treatment of TiO₂ P25 in a concentrated KOH solution and in-situ doping. The morphologies of all the samples from SEM, EDX, and TEM data confirmed that the structure of the multiwall nanotubes was fiber-like with a uniform distribution of Nd and/or Gd ions in the TNT lattice. The nanotubes were in an anatase phase, even after doping with Nd and/or Gd oxide. The mixed Nd-Gd-TNTs had smaller grain sizes because the dopant cations were located at the grain boundary, thus hindering grain growth. It was found that the specific surface area of the nanotubes (50 - 210 m²/g) considerably exceeded that of the initial TiO₂ (12 m²/g).

Acknowledgements

The authors thank Prof. Abdullah Al-Mayouf from King Saud University for his helpful advice on various technical issues. In addition, the authors would like to thank the technicians, Mr. Abdallah Jaber (physics department) and Miss. Fatemah Abdalraheem (chemistry department) for conducting surface measurements of the study samples.

Contribution

Emran and Alanazi carried out the experiments. Emran developed the theory and performed the computations with Alanazi. Emran encouraged Alanazi to investigate the preparation of TNTA and supervised the findings of this work. Both authors discussed the results and contributed to the final manuscript.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

References

[1]	Madian, M., Eychmüller, A. and Giebeler, L. (2018) Current Advances in TiO2-Based Nanostructure Electrodes for High Performance Lithium Ion Batteries. Batteries, 4, Article No. 7. https://doi.org/10.3390/batteries4010007
[2]	Beydoun, D., Amal, R., Low, G. and McEvoy, S. (1999) Role of Nanoparticles in Photocatalysis. Journal of Nanoparticle Research, 1, 439-458. https://doi.org/10.1023/A:1010044830871
[3]	Clarizia, L., Russo, D., Di Somma, I., et al. (2018) Multifunctional Photocatalytic Materials for Energy. Woodhead Publishing, Sawston.
[4]	Wang, W., Li, G., Xia, D., et al. (2017) Photocatalytic Nanomaterials for Solar-Driven Bacterial Inactivation: Recent Progress and Challenges. Environmental Science: Nano, 4, 782-799. https://doi.org/10.1039/C7EN00063D
[5]	Wang, X., Li, X., Liu, D., et al. (2012) Highly Luminescent Carbon Nanodots by Microwave-Assisted Pyrolysis. Chemical Communications, 48, 7955-7957. https://doi.org/10.1039/c2cc33869f
[6]	Mahmood, A., Guo, W., Tabassum, H. and Zou, R. (2016) Metal-Organic Framework-Based Nanomaterials for Electrocatalysis. Advanced Energy Materials, 6, Article ID: 1600423. https://doi.org/10.1002/aenm.201600423
[7]	Byranvand, M.M., Kharat, A.N. and Bazargan, M.H. (2012) Titania Nanostructures for Dye-Sensitized Solar Cells. Nano-Micro Letters, 4, 253-266. https://doi.org/10.1007/BF03353723
[8]	Salari, M., Aboutalebi, S.H., Aghassi, A., et al. (2015) Disorder Engineering of Undoped TiO2 Nanotube Arrays for Highly Efficient Solar-Driven Oxygen Evolution. Physical Chemistry Chemical Physics, 17, 5642-5649. https://doi.org/10.1039/C4CP03177F
[9]	Lee, Y.-L., Chi, C.-F. and Liau, S.-Y. (2009) CdS/CdSe Co-Sensitized TiO2 Photoelectrode for Efficient Hydrogen Generation in a Photoelectrochemical Cell. Chemistry of Materials, 22, 922-927. https://doi.org/10.1021/cm901762h
[10]	Guayaquil-Sosa, J.F., Calzada, A., Serrano, B., Escobedo, S. and de Lasa, H. (2017) Hydrogen Production via Water Dissociation Using Pt-TiO2 Photocatalysts: An Oxidation-Reduction Network. Catalysts, 7, 324-345. https://doi.org/10.3390/catal7110324
[11]	Naldoni, A., Altomare, M., Zoppellaro, G., et al. (2018) Photocatalysis with Reduced TiO2: From Black TiO2 to Cocatalyst-Free Hydrogen Production. ACS Catalysis, 9, 345-364. https://doi.org/10.1021/acscatal.8b04068
[12]	Song, Y.-Y., Schmidt-Stein, F., Bauer, S. and Schmuki, P. (2009) Amphiphilic TiO2 Nanotube Arrays: An Actively Controllable Drug Delivery System. Journal of the American Chemical Society, 131, 4230-4232. https://doi.org/10.1021/ja810130h
[13]	Wang, T., Jiang, H., Wan, L., et al. (2015) Potential Application of Functional Porous TiO2 Nanoparticles in Light-Controlled Drug Release and Targeted Drug Delivery. Acta Biomaterialia, 13, 354-363. https://doi.org/10.1016/j.actbio.2014.11.010
[14]	Enachi, M., Guix, M., Braniste, T., et al. (2015) Photocatalytic Properties of TiO2 Nanotubes Doped with Ag, Au and Pt or Covered by Ag, Au and Pt Nanodots. Surface Engineering and Applied Electrochemistry, 51, 3-8. https://doi.org/10.3103/S1068375515010044
[15]	Perera, S.D., Mariano, R.G., Vu, K., et al. (2012) Hydrothermal Synthesis of Graphene-TiO2 Nanotube Composites with Enhanced Photocatalytic Activity. ACS Catalysis, 2, 949-956. https://doi.org/10.1021/cs200621c
[16]	Lei, B., Xue, J.J., Jin, D.P., et al. (2008) Fabrication, Annealing, and Electrocatalytic Properties of Platinum Nanoparticles Supported on Self-Organized TiO2 Nanotubes. Rare Metals, 27, 445-450. https://doi.org/10.1016/S1001-0521(08)60160-6
[17]	Zhang, Q., Wei, Y., Wang, Y., et al. (2017) Fabrication and Electrocatalytic Activity of TiO2 Nanotubes Based Electrode with High Oxygen Evolution Potential. Journal of Nanoscience and Nanotechnology, 17, 1950-1956. https://doi.org/10.1166/jnn.2017.12935
[18]	Liu, N., Chen, X., Zhang, J. and Schwank, J.W. (2014) A Review on TiO2-Based Nanotubes Synthesized via Hydrothermal Method: Formation Mechanism, Structure Modification, and Photocatalytic Applications. Catalysis Today, 225, 34-51. https://doi.org/10.1016/j.cattod.2013.10.090
[19]	Reszczyńska, J., Grzyb, T., Sobczak, J.W., et al. (2014) Lanthanide Co-Doped TiO2: The Effect of Metal Type and Amount on Surface Properties and Photocatalytic Activity. Applied Surface Science, 307, 333-345. https://doi.org/10.1016/j.apsusc.2014.03.199
[20]	Meksi, M., Turki, A., Kochkar, H., et al. (2016) The Role of Lanthanum in the Enhancement of Photocatalytic Properties of TiO2 Nanomaterials Obtained by Calcination of Hydrogenotitanate Nanotubes. Applied Catalysis B: Environmental, 181, 651-660. https://doi.org/10.1016/j.apcatb.2015.08.037
[21]	Mazierski, P., Lisowski, W., Grzyb, T., et al. (2017) Enhanced Photocatalytic Properties of Lanthanide-TiO2 Nanotubes: An Experimental and Theoretical Study. Applied Catalysis B: Environmental, 205, 376-385. https://doi.org/10.1016/j.apcatb.2016.12.044
[22]	Parnicka, P., Mazierski, P., Lisowski, W., et al. (2018) A New Simple Approach to Prepare Rare-Earth Metals-Modified TiO2 Nanotube Arrays Photoactive under Visible Light: Surface Properties and Mechanism Investigation. Results in Physics, 12, 412-423. https://doi.org/10.1016/j.rinp.2018.11.073
[23]	Li, H., Sheng, Y., Zhang, H., et al. (2011) Synthesis and Luminescent Properties of TiO2:Eu3+ Nanotubes. Powder Technology, 212, 372-377. https://doi.org/10.1016/j.powtec.2011.06.019
[24]	Hewer, T.L.R., Souza, E.C.C., Martins, T.S., Muccillo, E.N.S. and Freire, R.S. (2011) Influence of Neodymium Ions on Photocatalytic Activity of TiO2 Synthesized by Sol-Gel and Precipitation Methods. Journal of Molecular Catalysis A: Chemical, 336, 58-63. https://doi.org/10.1016/j.molcata.2010.12.010
[25]	Nassoko, D., Li, Y.-F., Li, J.-L., Li, X. and Yu, Y. (2012) Neodymium-Doped TiO2 with Anatase and Brookite Two Phases: Mechanism for Photocatalytic Activity Enhancement under Visible Light and the Role of Electron. International Journal of Photoenergy, 2012, Article ID: 716087. https://doi.org/10.1155/2012/716087
[26]	Li, L., Zhou, Z., Lei, J., He, J., Liu, P. and Pan, F. (2012) Nd2O3-Decorated TiO2 Nanotube Arrays with High Photoelectrocatalytic Activity. Materials Letters, 79, 252-255. https://doi.org/10.1016/j.matlet.2012.04.017
[27]	Nie, J., Mo, Y., Zheng, B., Yuan, H. and Xiao, D. (2013) Electrochemical Fabrication of Lanthanum-Doped TiO2 Nanotube Array Electrode and Investigation of Its Photoelectrochemical Capability. Electrochimica Acta, 90, 589-596. https://doi.org/10.1016/j.electacta.2012.12.049
[28]	Zheng, Y. and Wang, W. (2014) Electrospun Nanofibers of Er3+-Doped TiO2 with Photocatalytic Activity beyond the Absorption Edge. Journal of Solid State Chemistry, 210, 206-212. https://doi.org/10.1016/j.jssc.2013.11.029
[29]	Chai, Y., Lin, L., Zhang, K., Zhao, B. and He, D. (2014) Efficient Visible-Light Photocatalysts from Gd-La Codoped TiO2 Nanotubes. Ceramics International, 40, 2691-2696. https://doi.org/10.1016/j.ceramint.2013.10.054
[30]	Wang, X., Jia, H., Wang, Y., Li, Y. and Bao, Z. (2016) The Photocatalytic Performance Research of La2O3 Modified TiO2 Nanotube Arrays. Journal of Materials Science: Materials in Electronics, 27, 7073-7078. https://doi.org/10.1007/s10854-016-4665-4
[31]	Tuyen, L.T.T., Quang, D.A., Toan, T.T.T., et al. (2018) Synthesis of CeO2/TiO2 Nanotubes and Heterogeneous Photocatalytic Degradation of Methylene Blue. Journal of Environmental Chemical Engineering, 6, 5999-6011. https://doi.org/10.1016/j.jece.2018.09.022
[32]	Emran, K.M., Ali, S.M. and Alanazi, H.E. (2020) Novel Hydrazine Sensors Based on Pd Electrodeposited on Highly Dispersed Lanthanide-Doped TiO2 Nanotubes. Journal of Electroanalytical Chemistry, 856, Article ID: 113661. https://doi.org/10.1016/j.jelechem.2019.113661
[33]	Amro, A.N., Emran, K.M. and Alanazi, H.E. (2020) Voltammetric Determination of Itopride Using Carbon Paste Electrode Modified with Gd Doped TiO2 Nanotubes. Journal of Chemistry, 44, 1122-1133. https://doi.org/10.3906/kim-2003-56
[34]	Emran, K.M. (2020) Catalytic Activity of Strontium Modified TiO2 Nanotubes for Hydrogen Evolution Reaction. International Journal of Electrochemical Science, 15, 4218-4231. https://doi.org/10.20964/2020.05.02
[35]	Harsha, N., Ranya, K.R., Babitha, K.B., et al. (2011) Effect of Silver and Palladium on Dye-Removal Characteristics of Anatase-Titania Nanotubes. Journal of Nanoscience and Nanotechnology, 11, 1175-1187. https://doi.org/10.1166/jnn.2011.3048
[36]	Zhang, S., Li, W., Jin, Z., et al. (2004) Study on ESR and Inter-Related Properties of Vacuum-Dehydrated Nanotubed Titanic Acid. Journal of Solid State Chemistry, 177, 1365-1371. https://doi.org/10.1016/j.jssc.2003.11.027
[37]	Ohno, T., Sarukawa, K., Tokieda, K. and Matsumura, M. (2001) Morphology of a TiO2 Photocatalyst (Degussa, P-25) Consisting of Anatase and Rutile Crystalline Phases. Journal of Catalysis, 203, 82-86. https://doi.org/10.1006/jcat.2001.3316
[38]	Alves, W., Ribeiro, A.O., Pinheiro, M.V.B., et al. (2011) Quenching of Photoactivity in Phthalocyanine Copper(II)-Titanate Nanotube Hybrid Systems. The Journal of Physical Chemistry C, 115, 12082-12089. https://doi.org/10.1021/jp202101r
[39]	Parnicka, P., Mazierski, P., Grzyb, T., et al. (2018) Influence of the Preparation Method on the Photocatalytic Activity of Nd-Modified TiO2. Beilstein Journal of Nanotechnology, 9, 447-459. https://doi.org/10.3762/bjnano.9.43
[40]	Zhang, Y., Zhang, H., Xu, Y. and Wang, Y. (2004) Significant Effect of Lanthanide Doping on the Texture and Properties of Nanocrystalline Mesoporous TiO2. Journal of Solid State Chemistry, 177, 3490-3498. https://doi.org/10.1016/j.jssc.2004.05.026
[41]	Subramaniam, M.N., Goh, P.S., Ismail, A.F. and Lau, W.J. (2016) Effect of Titania Nanotubes on the Flux and Separation Performance of Polyethersulfone Membranes. IOP Conference Series: Earth and Environmental Science, 36, Article ID: 012024. https://doi.org/10.1088/1755-1315/36/1/012024
[42]	Huang, S., Si, Z., Li, X., et al. (2016) A Novel Au/R-GO/Tnts Electrode for H2O2, O2 and Nitrite Detection. Sensors and Actuators B: Chemical, 234, 264-272. https://doi.org/10.1016/j.snb.2016.04.167
[43]	Van Viet, P., Phan, B.T., Mott, D., Maenosono, S., Sang, T.T. and Thi, C.M. (2018) Silver Nanoparticle Loaded TiO2 Nanotubes with High Photocatalytic and Antibacterial Activity Synthesized by Photoreduction Method. Journal of Photochemistry and Photobiology A: Chemistry, 352, 106-112. https://doi.org/10.1016/j.jphotochem.2017.10.051
[44]	Tahir, B., Tahir, M. and Amin, N.A.S. (2018) Tailoring Performance of La-Modified TiO2 Nanocatalyst for Continuous Photocatalytic CO2 Reforming of CH4 to Fuels in the Presence of H2O. Energy Conversion and Management, 159, 284-298. https://doi.org/10.1016/j.enconman.2017.12.089
[45]	Ali, I. and Kim, J.O. (2018) Visible-Light-Assisted Photocatalytic Activity of Bismuth-TiO2 Nanotube Composites for Chromium Reduction and Dye Degradation. Chemosphere, 207, 285-292. https://doi.org/10.1016/j.chemosphere.2018.05.075
[46]	Zhang, S.P., Lin, J.S, Lin, R.K., et al. (2020) In Situ Raman Study of the Photoinduced Behavior of Dye Molecules on TiO2 (hkl) Single Crystal Surfaces. Chemical Science, 11, 6431-6435. https://doi.org/10.1039/D0SC00588F
[47]	Monçäo, M.M., Barreto, I.C., Miguel, F.B., et al. (2022) Raman Spectroscopy Analysis of Wollastonite/Tricalcium Phosphate Glass-Ceramics after Implantation in Critical Bone Defect in Rats. Materials Sciences and Applications, 13, 317-333. https://doi.org/10.4236/msa.2022.135017
[48]	Dong, B., He, B.-L., Huang, J., Gao, G.-Y., Yang, Z. and Li, H.-L. (2008) High Dispersion and Electrocatalytic Activity of Pd/Titanium Dioxide Nanotubes Catalysts for Hydrazine Oxidation. Journal of Power Sources, 175, 266-271. https://doi.org/10.1016/j.jpowsour.2007.08.090
[49]	Dong, B., He, B., Chai, Y. and Liu, C. (2010) Novel Pt Nanoclusters/Titanium Dioxide Nanotubes Composites for Hydrazine Oxidation. Materials Chemistry and Physics, 120, 404-408. https://doi.org/10.1016/j.matchemphys.2009.11.022
[50]	Wang, Y.-F., Peng, C.-F. and Chao, H.-P. (2015) Sorption of Volatile Organic Compounds on Organic Substance-Modified Titanate Nanotubes. Aerosol and Air Quality Research, 15, 2688-2699. https://doi.org/10.4209/aaqr.2015.10.0592
[51]	Qu, X.F., Yuan, J.J., Da Deng, X., Hou, Y.C., Wang, Y.F. and Song, H.B. (2017) An Efficient Method to Form TiO2/CdS Nanotube Arrays Using Anodic Aluminum Oxide (AAO) Templates. Key Engineering Materials, 727, 374-380. https://doi.org/10.4028/www.scientific.net/KEM.727.374
[52]	Zhao, F., Rong, Y., Wan, J., Hu, Z., Peng, Z. and Wang, B. (2018) High Photocatalytic Performance of Carbon Quantum Dots/TNTs Composites for Enhanced Photogenerated Charges Separation under Visible Light. Catalysis Today, 315, 162-170. https://doi.org/10.1016/j.cattod.2018.02.019