A phenomenon of degradation of methyl orange observed during the reaction of NH₄TiOF₃ nanotubes with the aqueous medium to produce TiO₂ anatase nanoparticles

Abstract

The phenomenon of decolourization of MO was observed when NH₄TiOF₃ nanotubes, in water, underwent a transformation toward anatase due to the effect of hydroxyl radicals. NH₄TiOF₃ nanotubes were synthesized using the electrochemical anodization method of titanium. They react within an aqueous solution producing nanoparticulated nanotubes formed of anatase crystals with a size of about 4–6 nm. A second reaction process was observed when the solution contained methyl orange (MO), which decomposed losing color, resulting in smaller molecules. The modified layer on the Ti anodized substrate was immersed in the aqueous solution in the absence of light. The degradation of MO was monitored by UV-Vis absorption. The highest degradation efficiency was obtained after 96 h. Sample analysis was conducted using liquid chromatography coupled with electrospray ionization ion-trap mass spectrometry. Six intermediate molecules were found during the degradation process of MO. Therefore, a degradation pathway of MO was proposed. The composition and surface structure of the compound were characterized. The analysis shows that NH₄TiOF₃ nanotubes change in morphology and crystal structure, showing nanotubes with nanoparticle walls and its evolution to anatase structure.

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Introduction

Purification of water from hazardous chemicals is an important research area. Organic contaminants, such as industrial dyes, halocarbons and phenol derivatives, are among the main contaminants that demand completely safe removal.¹

Azo dyes, which are characterized by the presence of one or more azo groups (N=N) bound to aromatic rings, are the largest and most important class of synthetic organic dyes. It has been estimated that more than 50% of all dyes commonly used are azo dyes because of their chemical stability and versatility.² Methyl orange (MO) is an azo dye and its removal has been studied by adsorption on activated carbon,³ graphene oxide⁴ and its degradation by heterogeneous photocatalysis.⁵

A variety of physical, chemical and biological methods are available for the treatment of wastewater discharged from various industries. Biological treatment is a cost effective and proven technology, but it suffers from a number of disadvantages, such as cleaning difficulties, susceptibility to temperature changes and most plants need at least three tanks.⁶

Physical methods, such as liquid–liquid extraction, ion-exchange, adsorption, air or steam stripping, are ineffective because of pollutants, which are not readily adsorbable or volatile. A further disadvantage is that it simply transfers the pollutants to another phase rather than destroying them. In contrast, chemical oxidation methods can result in almost complete mineralization of organic pollutants and are effective for a wider range of organics.⁷

Mesocrystals exist as intermediates of crystallization.⁸ The NH₄TiOF₃ phase as a mesocrystal is transformed to the anatase form of TiO₂, as had been processed by L. Zhou et al.,⁹ after washing with aqueous H₃BO₃.

Identical results were obtained by sintering in air. The nature of these particles was examined after washing with diluted boric acid (H₃BO₃, 35 °C for 2 h) or sintering in air (450 °C for 2 h). The synthesis of NH₄TiOF₃ nanotubes and the transformation of this compound in anatase nanoparticles have not been reported.

The principle objective of the present study was to investigate the effect of decolouration of MO during the transformation of NH₄TiOF₃ nanotubes in anatase with the absence of light at atmospheric pressure and room temperature. The change in morphology and crystal structure evolution was studied. Moreover, a change in the nanotube walls and its crystallinity was found.

Experimental

Ti anodization for obtaining NH₄TiOF₃ nanotubes layers

The preparation of industrial grade Ti plates began by polishing them to remove the corrosion. The substrates were subjected to sandblasting using Al₂O₃ as the abrasive. Subsequently, they were treated with ultrasonic washing and drying.

The anodization of Ti was carried out in an organic medium initially consisting of 98% v/v ethylene glycol, 1700 ppm ammonium fluoride as a source of fluoride, and 2.5 ppm oxygen. The oxygen concentration was controlled by continuously dripping water, and measurements were obtained for the experiments in the range of 2.3–1.9 ppm. A potential of 60 V was applied for 2 h. This potential was reached by steps with increments of 6 V per minute. Ti plates were used as both the anode and cathode. The measurements of fluorine and oxygen were carried out using Thermo Scientific Orion™ probes, 9609BNWP and 083005MD, respectively. After the Ti anodization, there was a layer of NH₄TiOF₃ nanotubes formed on the surface.

Degradation of methyl orange

Methyl orange (MO) was dissolved in deionized water to obtain an MO solution (20 mg L⁻¹). A Ti plate anodized on both sides was immersed for 96 h in the MO solution in the absence of light. Their geometric surfaces were typically 9 cm² per side.

UV-Vis absorption

The reaction was monitored by measuring the UV-Vis absorption spectra of the sample solution taken at intervals of 6, 12, 72, and 96 h. For the UV-Vis spectrophotometric analysis, 2 mL of the sample solution was taken and filtered through a 0.22 μm filter membrane to separate any possible nanotubes detached from the anodized Ti plate. The absorbance spectra were obtained over a wavelength range of 190–600 nm.

UHPLC-ESI analysis

Electrospray ionization (ESI) analysis was done on a Bruker micrOTOF-QII (Bruker Daltonics, Billerica, MA). The samples were dissolved in methanol and were injected directly into the spectrometer. The capillary potential was 4.5 kV, the dry gas temperature was 180 °C and the drying gas flow was 4 L min⁻¹. Total ion chromatograms from m/z 500 to 3000 were obtained.

XRD analysis and Rietveld refinement

X-ray diffraction analysis was performed by means of a Bruker D8 Advance diffractometer. X-ray diffraction (XRD) patterns were collected over 2 h between the 2θ values of 10° and 80° using a CuKα1 radiation with 1.5405 Å. Rietveld analysis was carried out using the software MAUD v. 2.55.¹⁰

HRTEM analysis

High Resolution Transmission Electron Microscopic studies (HRTEM, JEOL 2100) and Selected Area Electron Diffraction (SAED) patterns were obtained from Fast Fourier Transformation (FFT); the HRTEM images were obtained at 120 kV. The sample was prepared by scratching the nanotube layer into ethanol, which was already placed on the carbon coated copper grid.

XPS analysis

XPS was conducted using a Thermo Fisher Scientific K-Alpha X-ray photoelectron spectrometer with a monochromatized AlKα X-ray source (1487 eV). The O 1s peak position at 531.0 eV was used as an internal standard, instead of C 1s, to detect and compensate for the charge shift of the core level peaks. The preceding was conducted such that C was not the main component of the surfaces. Both the Ti 2p and F 1s core level spectra were fitted using a Gaussian–Lorentzian mix function and Shirley type background subtraction. Throughout the measurements, the base pressure in the analysis chamber was 10⁻⁹ mbar. The survey and high resolution core level spectra were obtained at 160 and 60 eV pass energy analyzer, respectively. An X-ray beam, with a 400 μm spot size, was employed to analyze three different regions located onto the sample surfaces.

Results and discussion

Synthesis of NH₄TiOF₃ nanotubes

Fig. 1 shows the current density vs. time. A progressive drop in current density was observed. This is due to the formation of a denser oxide layer that poses higher resistivity.¹¹ After 60 minutes, the current density increased due to chemical etching by F⁻ ions.¹¹ The inset in Fig. 1 shows the ramp used for progressively increasing the voltage with 6 V steps per minute. The current density increases at the beginning of each step, followed by a sudden drop, indicating the formation of a thin barrier oxide layer.¹²

Fig. 1 Profile of current density vs. time during the anodization of Ti in organic medium for the formation of NH₄TiOF₃ nanotubes.

The formation of the nanotubes was limited by the electrochemical oxidation of Ti and chemical dissolution of TiO₂. The anodizing medium was composed of 2% water, and an imposed potential caused its electrolysis, breaking the molecules, as presented in reaction (1). With the imposed potential and the presence of oxygen, oxidation of the metal occurred at the surface, as presented in reaction (2). The F⁻ ions were concentrated around the anode under the influence of the electric field. They reacted with the titanium oxide layer causing the formation of titanium complexes [TiOF₄]²⁻ (3), which in turn lead to the chemical etching to form NH₄TiOF₃ nanotubes (4). Thus, a self-organized porous layer was obtained. The tube growth stopped, in the solution, when the oxidation or the dissolution was absent.

2H₂O → O₂ + 4H + 4e⁻ (1)

Ti + O₂ → TiO₂ (2)

TiO₂ + 4F⁻ + 2H⁺ → [TiOF₄]²⁻ + H₂O (3)

[TiOF₄]²⁻ + NH₄⁺ → NH₄TiOF₃ + F⁻ (4)

Fig. 2 shows an atypical view of TiO₂ nanotubes prepared in an organic medium over anodized Ti plates. The SEM image shows unordered tubular structures due to the applied sandblasting, which was chosen instead of a typical mirror finishing to increase the surface areas and the mechanical resistance of the nanostructured layer and. The micrograph proves that nanotube formation was normal to the surface plane on a microscopic scale, and at this highly rough surface, it showed an average roughness of about ten microns.

Fig. 2 SEM image of NH₄TiOF₃ nanotubes grown in anodized Ti, previously treated by sandblast.

Catalytic activity and determination of MO residues

The degradation of methyl orange in the absence of irradiation was monitored. The substrate was analysed after 96 h. Nevertheless, at this time, complete discolouration had not occurred.

The –N=N– double bond in the azo dyes is the chromophoric group for color.¹³ Typically, the absorption band of methyl orange shows a red shift with a decrease in pH, which results in a color change from yellow to red, and an increase in wavelength suggests an increase in delocalization in the methyl orange molecules.¹⁴ Fig. 3 shows the UV/Vis absorption measurement of methyl orange degradation. It is observed that the dye degradation occurred at 6 h after the addition of the NH₄TiOF₃ nanotubes into the dye solution. Even considering that the discolouration was not completed in 96 h, the curves represent the absorption spectra of methyl orange solution in contact with NH₄TiOF₃ nanotubes.

Fig. 3 UV/Vis absorption measurement of methyl orange degradation.

A decrease in the intensity of the absorption band at ∼465.6 nm is observed. This indicates the possible cleavage of the azo bond, which is the chromophoric group, and thereby the solutions were decolorized. The formation of aromatic products is indicated by the peak ∼235 nm and its intensity decreased over the reaction time. This suggests the successful decomposition of methyl orange dye in the solution by the reduction of the azo bond to two or more possible chemical structures with amines (–NR₂). This peak indicates the discoloration of the methyl orange dye due to dye degradation from the solution, instead of only physical adsorption.¹⁵

Fig. 4a–e reports the chromatograms monitored in a MS scan from m/z 50 to 500, corresponding to solutions of methyl orange degraded at 0, 6, 12, 72, and 96 h, respectively. The results indicate that the number of degradation products present at different times was variable. The significant peaks present at different degradation times are labelled with the corresponding m/z values. Note that in Fig. 4a–e, there are a total of three intense species signals having m/z values higher than the original molecule but only one of them (m/z 408) remained for a longer period of time. The other species correspond to higher m/z values so their structure may be considered present in the original molecule. The structural details of the following discussion are reported in Fig. 4. The species of m/z 233 (no more methyl groups bonded to nitrogen atom of amino group, Fig. 4b) has the highest retention time, followed by the species of m/z 129 (one amide group) and the parent molecule m/z 306 (both methyl groups present). The species with m/z 149 and 306 were fragments (Fig. 4d and e, respectively) from the ion shown in Fig. 4b.

Fig. 4 Chromatograms monitored in a scan from m/z 50 to 500 corresponding to solutions of MO degraded to (a) 0, (b) 6, (c) 12, (d) 72 and (e) 96 h, some peaks were characterized by its m/z value.

The structural details of the following discussion are reported in Fig. 5. The species of m/z 233 has the highest retention time, followed by the species of m/z 129 (one amide group) and the parent molecule m/z 306 (both methyl groups present). The species with m/z 261 and 306 were fragments of the original molecule when the azo bond was broken. The structures m/z 149 and m/z 129 are fractions from the structure m/z 328, with the loss of phenyl sulfone, and m/z 129 is the result of the cleavage in the methyl group and the insertion of the ion OH⁻ on the phenyl group.

Fig. 5 Fragmentation scheme of methyl orange degradation products.

The structure with m/z 149 is proposed as a result of the breaking off in the molecule with m/z 261, and the oxygen loss from the sulfone group. The chromatogram in Fig. 4 shows that the signal m/z 149 was increased; therefore, the concentration of amine groups increased as well.

The reactions were not reversible. Methyl orange molecules were segmented in their azo bond with a continuous decrease in color. The disappearing and appearing signals go with the process. No additional solution was incorporated. The process was not a photocatalytic one; this process was associated with oxidation–reduction of the surface and the solution by the reaction of the ammonium oxofluorotitanate with oxygen. This certainly decolorizes but also may lead to form new bonds, increasing the size of the subproducts, which can be observed in the progressive rise of signals higher than m/z = 328 in Fig. 4c–e.

Structure of the catalyst

Crystal structure of the catalyst. The crystalline phase present in the nanotubes was identified by X-ray diffraction. The nanotubes were initially amorphous after anodizing Ti. The structure underwent a crystallization process as it was progressively oxidized. Fig. 6a corresponds to a sample exposed to laboratory room moisture for a month (about 40–60% RH). The diffractogram of the crystalline phase of the nanotubes formed by anodizing untreated Ti is shown. This phase corresponds to ammonium oxotrifluorotitanate (NH₄TiOF₃), as indicated with the JCPDS 52-1674. The presence of the Ti phase corresponds to the Ti substrate used for the formation of nanotubes (JCPDS 65-3362).

Fig. 6 (a) Diffractogram of anodized Ti before the catalytic test, corresponding to NH₄TiOF₃, and (b) Rietveld analysis after the catalytic process, corresponding to TiO₂ as the anatase phase.

Fig. 6b shows a diffractogram that corresponds to a sample of nanotubes after being immersed in an aqueous MO solution for 96 h. The Rietveld fitting was computed. The identified phase corresponds to the anatase structure; these data were indexed using the PDF card # 21-272. High-intensity peaks of the titanium substrate can be observed. Moreover, in the range 20–80° 2θ, peaks attributable to the nanotubes with nanoparticulate anatase walls were present. Moreover, in the same graph, it was possible to observe peaks that corresponded to Al₂O₃. The presence of this compound was due to the sandblasting treatment applied to the Ti surface. Rietveld refinement results indicated a crystallite size of 4 nm, with lattice parameters corresponding to a = 3.9 Å, b = 4.0 Å, c = 9.9 Å; α = β = γ = 90°, cell volume 145.2 ų, space group I4₁/amd, and tetragonal symmetry.

Fig. 7a shows an HRTEM image of the nanotubes as prepared after anodizing titanium in an organic medium, corresponding to the NH₄TiOF₃ compound, and before undergoing further oxidization with sizes within the range described above. Fig. 7b shows the structures that underwent an oxidization process in an aqueous MO solution. Anatase nanoparticles grew and emerged from the nanotube walls causing a distortion of the structure and leaving fuzzy nanotubes or clusters. Their crystalline orientation is depicted in Fig. 7c, where the family of planes (101) was identified and labeled having a distance of 0.33 nm. The inset corresponds to (i) profile of the interplanar distance, (ii) the inverse FFT from SAED, and (iii) the SAED obtained from FFT indexed. The resulting nanotubes were difficult to detach from the substrate that resembled the Ti substrate with a light whitish tonality, whereas the resulting nanoparticulated nanotubes were easy to detach and displayed a totally white tonality.

Fig. 7 TEM images of the nanotubes: (a) corresponding to the NH₄TiOF₃ compound, (b) nanoparticulated after further oxidization in an aqueous MO solution, and HRTEM (c) anatase TiO₂ with its (101) crystalline orientation having d = 0.33 nm, the inset corresponds to (i) profile of the interplanar distance, (ii) the inverse FFT from SAED, and (iii) the SAED obtained from FFT indexed.

Surface of the catalyst. In Fig. 8a-i, the XPS survey spectrum is an average of three measurements with a spot size of 400 μm² and the table of composition in percentages by element for the sample corresponding to the NH₄TiOF₃ nanotubes.

Fig. 8 XPS spectra for (a) NH₄TiOF₃ nanotubes and (b) anatase TiO₂ nanoparticulated nanotubes. (a-i and b-i) XPS survey spectrum and table of composition. Core level corresponding to Ti (a-ii and b-ii), O (a-iii and b-iii), and (a-iv and b-iv).

In Fig. 8a-ii, the peak at 460.2 ± 0.2 eV corresponds to the link Ti–O and the band at 461.0 ± 0.2 eV is indicative of Ti–F in TiF₄, which is very close to the signal of Ti⁴⁺, of the Ti–F link, which corresponds to TiOF₃⁻.

The O 1s XPS spectrum in Fig. 8a-iii can be separated into two peaks at 530.9 and 532.4 eV. The signal at 532.4 eV is due to surface OH groups,¹² whereas 530.9 eV corresponds to Ti–O, confirming the formation of the Ti–O structure.^16–18

In Fig. 8a-iv, the signal at 686.1 eV is assigned as interstitial F. The relative energies of the two binding states are explained by the qualitatively different charge exchange between F and either Ti or O depending on the insertion site.¹⁹ The contribution near 685.3 eV is attributed to the F-atoms in TiOF₂.^20–22 This result is in agreement with the link Ti–F in the compound NH₄TiOF₃. Table 1 lists the results for each NH₄TiOF₃ nanotube species.

Table 1 Shows the species assigned for NH₄TiOF₃ nanotubes

Chemical species	Position (eV)	Area	FWHM (eV)	% GL
Ti (2p3/2) Ti–O	460	37 769.2	2.1	30
Ti (2p3/2) TiF4	461	6541.8	2.2	30
Satellite	474	4139.6	3.5	30
O (1s) Ti–O	530.9	13 455.9	1.3	30
O (1s) OH	532.4	17 442.9	2.9	30
F (1s) TiF2	685.3	45 761.4	1.5	30
F (1s) Ti–F	686.1	53 362.1	2.4	30

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Fig. 8b-i shows the survey spectrum and the table of composition in percentages by elements for the sample corresponding to the TiO₂ nanotubes. Fig. 8b-ii indicates that the Ti 2p_3/2 band in the TiO₂ nanotubes could be found at 458.5 eV, whereas the signal at 457.1 eV corresponds to the trivalent state of Ti.²³ Table 2 lists the results for each TiO₂ nanotube species.

Table 2 Shows the species assigned for TiO₂ nanotubes

Chemical species	Position (eV)	Area	FWHM (eV)	% GL
Ti 2p3 (1) Ti^3+	457.1	8842.8	1.5	30
Ti 2p3 (2) anatase	458.5	109 550.1	1.3	35
Ti 2p (3) satellite	471.4	12 562.1	2.2	25
O (2s) anatase	529.9	97 003.5	1.2	30
O (2s) OH	530.7	62 622.8	3.1	30
F 1s (1) F (ads)	685.2	39 658.8	1.8	30

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Conclusions

The phenomenon of decolourization of MO was observed when NH₄TiOF₃ nanotubes, in water, underwent a transformation toward anatase under dark conditions without heat treatment. This novel effect of decolourization was related directly to the reactions taking place on the surface, with the nanotubes evolving from NH₄TiOF₃ to anatase TiO₂. The effect of decolourization ends once equilibrium on the surface is reached, which implies that this process is unsuitable for water remediation on a large scale. The structural information about the degradation intermediates achieved from the MS and MS2 studies was quite compatible with the degradation steps already reported in the literature for other molecules, particularly the introduction of OH groups to aromatic rings.

Anodizing Ti in organic media and controlling its oxygen content led to amorphous NH₄TiOF₃ nanotubes being obtained. Subsequently, they evolved into TiO₂ nanotubes having nanoparticulate walls that caused distortion of the structure, leaving fuzzy nanotubes or clusters.

Acknowledgements

The authors gratefully acknowledge the financial support from the Mexican Council for Science and Technology through the projects QRO-2014-C03-250295 and CONACYT-SENER CEMIE-Sol No. 207450. Moreover, the first author acknowledges CONACyT for his graduate fellowship. Thanks to the Center of Nanoscience and Micro and Nanotechnologies for their valuable help, especially to Dr Hugo Martinez Gutierrez and Dr Hector Mendoza de León for SEM analysis and Dr Nicolas Cayetano Castro and Dr Raúl Borja Urby for HRTEM analysis. Thanks to Eric Albert Huston for his comments about this written work.

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