Preparation of nanoparticulate TiO₂ containing nanocrystalline phases of anatase and brookite by electrochemical dissolution of remelted titanium components

2.1 Electrochemical preparation of nanoparticulate TiO₂

Nanoparticulate TiO₂ was galvanostatically prepared in a 2-electrode cell having an 800 cm² cylindrical tube of SAE-AISI 1018 steel (cathode). The anode was an 80 cm² square plate, hand-cutted from a sample of remelted titanium component (see elemental composition in Table 1) positioned in the center of the cylindrical cathode. Prior to use, the Ti-containing anode was immersed in concentrated HF (J.T. Baker, 49.9 wt%) for 10 s and then washed with copious amounts of deionized water (having a resistivity of 18.18 MΩ cm). The electrolysis cell was filled with 350 mL of an aqueous solution containing 50 v/v% ethanol (J.T. Baker, 94.7%), plus 0.13 M NaCl (Fermont, 99.6%) and 0.01 M sodium dodecyl sulfate (SDS, Sigma-Aldrich, 99%). The ionic conductance of this electrolytic solution was 10 µS cm⁻¹ at 25 °C.

A current density of 4 mA cm⁻², with continual stirring, was passed through the anode to produce nano-particulate TiO₂. Electro-generation of TiO₂ was experimentally observed as the Ti-containing anode dissolved according to Reaction (1) (Bratsch, 18 (1989)). The reaction began immediately upon current onset. After 1 h of anodic dissolution, the current was turned off and the white TiO₂ nano-particles were filtered from the electrolyte. The filtrate product was washed and dried at room temperature prior to characterization.

2.2 Characterization of electro-generated TiO₂

3.1 Faradaic efficiency of nanoparticulate TiO₂ electrogeneration

Faradic efficiency (ξ) was estimated at 84% using Relationship 2. Here, j (set at 4 mA cm⁻²) is the total current passed through the cell during a time t (in sec), A_g (80 cm²) is geometric area of the Ti-containing anode, n (equal to 4) is the number of electrons involved in Reaction (1), and F is Faraday’s constant (96500 C mol⁻¹). Additionally, $m_{{\mathit{TiO}}_2}$ (0.20 g) and $M_{{\mathit{TiO}}_2}$ (79.86 g mol⁻¹) are the mass and molecular weight of the electro-generated TiO₂, respectively (Pletcher, 1991). This important result clearly indicates that the electrochemical preparation of nano-particulate TiO₂ was very efficient.

3.2 Particle size distribution and ζ-potential measurements

Fig. 1 shows the particle size distribution obtained from an aqueous suspension of electro-generated TiO₂, which clearly reveals a colloidal system constituted by TiO₂ nano-particles having an average size of 29 nm. Complementarily, ζ-potential was measured in −62 mV at the same TiO₂ colloidal suspension, an indicating of very good suspension stability (Salopek et al., 1992).

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Fig. 1

Particle size distribution measurements obtained for an aqueous suspension of electro-generated TiO₂ material.

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3.3 X-Ray diffraction (XRD) and Raman spectroscopy analysis

Crystalline phases were identified in the electro-generated TiO₂ by XRD Fig. 2A) and Raman spectroscopy Fig. 2B) analyses. During XRD analysis, the interplanar spacing was calculated using JCPDS files 21-1272 (anatase) and 29-1360 (brookite). A detailed review of Fig. 2A reveals a set of 8 characteristic signals for 2θ at 38°, 47°, 53°, 55°, 67°, 68°, 70° and 75°, which seem to be the anatase planes A(1 0 1), A(0 0 4), A(2 0 0), A(1 0 5), A(2 1 1), A(2 0 4), A(1 1 6), A(2 2 0), and A(2 1 5), respectively. Furthermore, the same review indicate another set of 8 characteristic signals localized at 2θ of 31°, 42°, 45°, 49°, 57°, 60°, 62°, and 66°, which should correspond to brookite planes B(1 2 1), B(2 2 1), B(0 3 2), B(1 3 2), B(1 1 3), B(1 2 3), B(0 5 2), and B(1 3 3), respectively. Additionally, an intense signal was localized at 2θ = 25° in Fig. 2A, which can be associated with the overlapping of the diffraction of planes A(1 0 1) and B(1 2 0), for anatase and brookite, respectively.

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Fig. 2

(A) X-ray diffractogram and (B) Raman spectrum obtained from a sample of electro-generated TiO₂ material.

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Reviewing Fig. 2B confirmed the presence of a set of anatase characteristic bands at 406, 512 and 634 cm⁻¹, corresponding to the vibrational modes B_1g, A_1g, and E_g (Kaplan et al., 2016; Leyva-Porras et al., 2015). It also confirmed the presence of a set of brookite characteristic bands at 246, 318, 362 and 452 cm⁻¹, corresponding to brookite’s vibrational modes A_1g, B_3g, B_2g, and B_2g (Cao et al., 2016; Shen et al., 2013; Tran et al., 2017). Additionally, one observes the very intense band localized at 150 cm⁻¹, corresponding to the overlap of the vibrational modes E_g and A_1g of anatase and brookite phases, respectively. This observation supports the results obtained from the XRD analysis and strongly suggests the existence of anatase|brookite junctions within the lattice of the electro-generated TiO₂ nano-particles (El-Sheikh et al., 2017).

3.4 Electron microscopy studies

Fig. 3 shows HR-SEM Fig. 3A) and EDS studies Fig. 3B) of a sample of the electro-generated TiO₂ nano-particles. On first viewing of Fig. 3A one observes sub-100 nm TiO₂ agglomerates, whereas a detailed examination of Fig. 3B revealed an elemental composition consisting of Ti and O (as the main inorganic elements) and C (as an organic impurity). It was reasonable to expect detection of the presence of C-based impurities since SDS was employed as a tenso-active additive in the electro-synthesis electrolytic solution.

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Fig. 3

(A) SEM images and (B) EDS analysis obtained from a sample of electro-generated TiO₂ material. SEM images are presented at magnifications of 100 and 400 kX (for comparative purposes).

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A comparison between TEM images Fig. 4A) and SAED pattern Fig. 4B-inset) of cross-sections of TiO₂ particles confirmed the nano-crystalline structure of the electro-generated TiO₂ powders, supporting the results of the particle size distribution reported in Section 3.2. A detailed exploration of Fig. 4A and Table 2 reveal the presence of TiO₂ nanostructures that conformed to 7 nm (anatase) and 13 nm (brookite) crystallites, distributed in a crystalline fraction of 40% anatase and 60% brookite. These results are well supported by the BET surface areas reported for the electro-generated TiO₂ (200.13 ± 1.27 m² g⁻¹) in Section 2.2.2, and for the commercial Degussa® P25 TiO₂ (56 m² g⁻¹, nominal crystallites size of 21 nm (Raj and Viswanathan, 2009). Moreover, a comparison between Fig. 4A and B revealed the presence of the characteristic A(2 0 0) and B(1 2 1) planes, but also confirmed the existence of nano-metric anatase|brookite junctions in the TiO₂ lattice. This conclusion was supported by the appearance of overlapped A(1 1 0) and B(1 2 0) planes at TEM images and SAED pattern (El-Sheikh et al., 2017).

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Fig. 4

TEM images and SAED patterns (inset) of cross-sections of electro-generated TiO₂ particles. The interplanar distances of the anatase and brookite crystalline phases were identified.

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3.5 X-ray photoelectron spectroscopy (XPS) analysis

Fig. 5 shows the results of the XPS analysis obtained from a sample of the electro-generated TiO₂ material. In particular, Fig. 5A shows the survey XPS spectrum with two signals detected at 457 and 529 eV, which correspond to the emitted electrons’ energies from the Ti_2p and O_1s orbitals, respectively. Additionally, the single signal localized at 278 eV was associated with the emitted electrons’ energy from C_1s orbitals. These data clearly confirmed the presence of Ti, O, and C as the main elements contained in the TiO₂ lattice.

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Fig. 5

XPS spectra obtained for electro-generated TiO₂ material where, (A) survey spectrum, (B, C and D) high-resolution core level spectra corresponding to O, Ti, and C, respectively.

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Fig. 5B and C shows the high-resolution core level XPS spectra for O and Ti. Reviewing Fig. 5B reveals two signals: at 529 and 530 eV, which were associated with the emitted electrons’ energies from the O_1s orbitals in an O—Ti—O and Ti—OH binding scheme, respectively (Si et al., 2018; Naseema et al., 2018). Additionally, a spin-orbital splitting of the emitted electrons’ energies from the Ti_2p orbitals can be observed in Fig. 5C at 458 and 463 eV, which correspond to the signals-coupling ${\mathrm{T}\mathrm{i}}_{{2\mathrm{p}}_{3/2}}^{4+}$ and ${\mathrm{T}\mathrm{i}}_{{2\mathrm{p}}_{1/2}}^{4+}$ , respectively. Since there was an absence of the ${\mathrm{T}\mathrm{i}}_{{2\mathrm{p}}_{1/2}}^{3+}$ characteristic signal around 460 eV in Fig. 5C, the team concluded that Ti⁴⁺ is predominant within the TiO₂ lattice (Atuchin et al., 2006; Bharti et al., 2016).

A further comparative exploration of Fig. 5B and D reveals the weak intensities of the signals localized at 530 eV (in Fig. 5B) and 286 eV (in Fig. 5D) suggesting the presence of very few Ti—OH and C—O—C bindings at the TiO₂ lattice. However, a detailed review of Fig. 5C, clearly reveals that the content of O—C⚌O and C—C bindings in the TiO₂ lattice indicated by significantly strong intensities of their respective signals (288 and 284 eV). Again, this observation could be reasonably expected following the addition of tenso-active SDS into the electro-synthesis electrolytic solution which could leave residuals (Yi et al., 2017). However, we have shown earlier that the presence of C-based impurities do not significantly affect the electrical and photo-catalytic properties of nano-crystalline TiO₂ powders (Manríquez and Godínez, 2007; Peralta-Hernández et al., 2007).