Preparation of MgTi₂O₅ nanoparticles for sonophotocatalytic degradation of triphenylmethane dyes

Graphical abstract

Highlights

• •MgTi₂O₅ nanoparticles were prepared by simple hydrothermal assisted post-annealing method.
• •Sonophotocatalytic activity was examined for triphenylmethane dyes degradation.
• •Sonophotocatalytic results exhibit a strong synergetic effect for crystal violet degradation.
• •Proposed mechanism of triphenylmethane dyes degradation over MgTi₂O₅ nanoparticles.

Abstract

MgTi₂O₅ (magnesium dititanate) nanoparticles were prepared by a simple hydrothermal assisted post-annealing method and characterized with various analytical techniques. The catalytic properties (sonocatalytic, photocatalytic and sonophotocatalytic activity) were evaluated using the degradation of triphenylmethane dyes (crystal violet, basic fuchsin, and acid fuchsin). The sonophotocatalytic activity of MgTi₂O₅ nanoparticles towards crystal violet was found to be ~2.9 times higher than the photocatalytic activity and ~20 times higher than that of the sonocatalytic processes. In addition, the sonophotocatalytic efficiency of MgTi₂O₅ nanoparticles was found to be remarkable for the degradation of basic fuchsin (cationic dye) and acid fuchsin (anionic dye). The mechanism of these catalytic activities has been discussed in detail.

1. Introduction

Magnesium oxide (MgO) with an energy bandgap of >5.4 eV is found to show excellent photocatalytic activity due to the presence of reactive sites (defects or low coordinated sites) on the surface [1], [2], [3]. To tune, the energy bandgap of composite materials such as TiO₂/MgO heterostructures and magnesium titanates (MgTiO₃, Mg₂TiO_4, and MgTi₂O₅) have been developed [4], [5], [6], [7], [8], [9], [10]. Among these materials, MgTi₂O₅ (magnesium dititanate) has a theoretical energy bandgap of ~3.4 eV with orthorhombic phase and pseudobrookite-type structures [9], [10]. The formation of pure MgTi₂O₅ phase is found to be difficult due to the formation of mixed phases such as MgTiO₃-MgTi₂O₅. Hence, recent studies are focus on the controlled growth of the pure MgTi₂O₅ phase [10]. MgTi₂O₅ has been found useful in several applications such as white pigment, thermistor, water purification, sodium-ion batteries, and photocatalyst for hydrogen production by water splitting [9], [10], [11], [12], [13], [14], [15]. Triphenylmethane dyes (methyl violet, crystal violet, malachite green, basic fuchsin, acid fuchsin, and cresol red) are mostly employed in the textile industry for colouring and as a food preservative in seafood industries. The discharge of these dyes into water bodies poses serious harmto human health [16], [17]. In the past decades, numerous techniques (adsorption, membrane filtration, coagulation, incineration, and microbial degradation) have been employed for the removal of organic contaminants from wastewater [18], [19]. Unfortunately, the conventional processes are ineffective for the removal of pollutants [20], [21]. Advanced oxidation processes (AOPs) are used for the mineralization of harmful toxic pollutants present in wastewater. In these processes, highly reactive oxidizing radicals (e.g., ^•OH) are produced that react with organic pollutants and convert them into harmless products [22]. Acoustic cavitation is classified as an AOP since it generates highly reactive oxidizing radicals within cavitation bubbles that have been used in environmental remediation for the degradation of pollutants in aqueous solutions [23], [24], [25], [26]. Photocatalysis, Fenton process, ozonation, electro-chemical/catalytic techniques, etc, also generated highly reactive radicals that are used for the degradation of organic pollutants [27], [28], [29], [30], [31], [32], [33]. The photocatalytic process involves the use of solar energy to initiate oxidation/reduction reactions over semiconductors. Both sonolysis (acoustic cavitation) and photocatalysis have some disadvantages that could be overcome by combining these techniques. The combined/hybrid technique is known as sonophotocatalysis and has been shown to provide some synergistic advantages compared to individual techniques [24], [34], [35], [36]. In the current study, we prepared MgTi₂O₅ nanoparticles by a simple hydrothermal assisted post-annealing method and used them for the degradation of triphenylmethane dyes (crystal violet, basic fuchsin, and acid fuchsin) by sonocatalytic, photocatalytic and sonophotocatalytic processes.

2. Experimental procedure

2.1. Materials

Magnesium(II) nitrate hexahydrate (ACS reagent, 98.0–102.0%), titanium(IV) isopropoxide (99%), urea (ACS reagent, 99.0–100.5%), crystal violet (CV; ACS reagent, ≥99.0%), basic fuchsin (BF; >85%), acid fuchsin (AF; ~70%), nitroblue tetrazolium (NBT; 90.0–11.0%) and terephthalic acid (reagent grade, 98%) were received from Sigma-Aldrich and used without further purification. Ethylene glycol (≥99%) was purchased from Merck

2.2. MgTi₂O₅ nanoparticles preparation

About 3.84 g of magnesium (II) nitrate hexahydrate dissolved in 30 mL of aqueous urea solution (~12 g) was added to 40 mL of ethylene glycol containing 6 mL of titanium isopropoxide in a 250 mL round bottom flask. Then, the solution mixtures were kept at 90 °C for 24 h under ambient conditions with constant stirring. Subsequently, the product was collected and washed with distilled water and dried at 110 °C for 4 h. Finally, the calcination of this product was carried out at 600 °C for 2 h produces MgTi₂O₅ as nanoparticles.

2.3. MgTi₂O₅ nanoparticles characterization

The crystalline structure of the MgTi₂O₅ sample was characterized by a powder X-ray diffractometer (XRD) (Rigaku D/max-2500 utilizing Cu Kα radiation). The chemical structure of the sample was analyzed by Fourier transform infrared (FTIR) spectra using Nicolet iS50 infrared spectrophotometer by KBr pellet technique. Raman spectra of the sample were recorded using the EnSpectr RS-532 Raman system. Surface morphology and structural analysis were done by field emission scanning electron microscopy (FESEM model JEOL 7401F) and transmission electron microscopy (TEM model FEI Tecnai F30 microscope operated at 300 kV). The elemental analysis was carried out by an energy-dispersive X-ray spectrometer (EDS, Oxford Inc attached with FESEM). The chemical states of the elements were analyzed by X-ray photoelectron spectroscopy (XPS) (Physical Electronics PHI 5600 spectrophotometer with monochromatic Al Kα (1486.6 eV)). The UV–visible absorption spectrum was measured by an ultraviolet–visible (UV–vis) spectrophotometer (T90+, PG Instruments, UK). The photoluminescence (PL) spectra of the samples were recorded on Shimadzu (RF5301PC) spectrofluorometer. The surface potential (ζ-zeta) was measured by Zetasizer Nano Zs/ZEN3600, Malvern instrument. Herein, the sample (about 1–2 mg in 5 mL) is prepared using deionized water followed by sonication for dispersion. The pH of solutions has been adjusted by using dilute HCl and NaOH.

2.4. Experimental setup and catalytic activity of triphenylmethane dyes degradation

The ultrasonic reactor was operated at 38 kHz frequency and 100 W power (Kaijo 30110, Japan). The reactor was operated in its maximum volume capacity of 1 L. The photocatalytic activity was performed with a 6 W UV lamp (Heber Scientific, India, emitting light radiation of 365 nm as the light source) immersed in quartz well placed at the middle of the reactor [37]. A typical batch photoreactor was used with 100 mL of 5 × 10⁻⁵ mol L⁻¹ triphenylmethane dyes. About 50 mg of catalyst (0.5 g L⁻¹) was added and stirred for 30 min under dark conditions to attain adsorption/desorption equilibrium between the dye molecules and surface of the catalyst. Following this process, the solution mixture was illuminated with a UV lamp (for the photocatalytic process) or US irradiation (sonocatalytic process) or a combination of both for sonophotocatalytic processes (both UV illumination + US irradiation). At regular time intervals, 4 mL of dye solution was collected and the catalyst was removed by filtration by a PVDF syringe filter (0.45 µm). The degradation efficiencies [ (η, %) were calculated using Eq. (1)]

η = [A₀ – A_t / A₀] × 100% (1)

where, A_o and A_t are absorbance and concentration of the dye solutions before and after degradation treatment, respectively.

2.5. Generation of superoxide (O₂^•−) radical species analysis

The quantity of superoxide radical species generated by the sono and photocatalytic processes were measured by UV–vis spectroscopy. For this experiment, about 100 mg of catalyst was taken in 100 mL (conc. 5 × 10⁻⁵ mol L⁻¹) of nitroblue tetrazolium solution. Then, the reaction mixture was subjected to the photocatalytic process. At regular time intervals, 4 mL of solution were collected and the catalyst was filtered by PVDF syringe filter (0.45 µm). The absorbance was analyzed by UV–vis spectrophotometer.

2.6. Generation of hydroxyl radical (^•OH) analysis

The detection of hydroxyl radicals under the sono and photocatalytic processes was carried out by a simple photoluminescence technique. About 100 mg of catalyst was added to 100 mL of terephthalic acid (conc. 5 × 10⁻⁴ mol L⁻¹) in a base solution (conc. 2 × 10⁻³ mol L⁻¹of NaOH). The reaction mixture was subjected to the photocatalytic process. At regular time intervals, 4 mL of solutions were collected and the catalyst was filtered by PVDF syringe filter (0.45 µm). Then, the PL measurement was carryout with an excitation wavelength at 315 nm.

3. Results and discussion

The main aim of this work was to synthesize a highly active photocatalyst MgTi₂O₅. In order to confirm the formation of MgTi₂O₅ several characterization techniques were used. Fig. 1a shows the powder XRD pattern of the as-prepared catalyst. The diffraction peaks at 2θ = 25.4°, 26.9°, 31.2°, 32.5°, 36.5°, 37.2°, 40.6°, 41.2°, 46.1°, 48.5°, 49.3°, 52.2°, 55.1°, 56.2°, 57.3°, 59.6°, 60.2°, 63.1°, 65.4°, 68.0°, 69.1°, 73.0°, 75.1° and 76.5° are indexed to the (1 0 1), (1 1 1), (1 2 1), (2 3 0), (3 0 1), (1 3 1), (2 4 0), (4 2 0), (4 3 0), (0 0 2), (2 5 0), (2 0 2), (0 6 0), (5 2 1), (6 1 0), (2 3 2), (5 3 1), (6 3 0), (4 2 2), (3 6 1), (4 3 2), (7 1 1), (7 2 1) and (0 6 2) planes of orthorhombic phase for the Karroite (MgTi₂O₅; JCPDS card no. 35-0792) [10], [14]. Further, no other peaks were observed in the XRD spectrum suggesting the formation of the pure phase of MgTi₂O₅. The FTIR spectrum of this sample is shown in Fig. 1b. The observed peaks at 3410 cm⁻¹ and 1660 cm⁻¹ correspond to O–H vibration due to the presence of water (chemisorbed/physisorbed) on the surface. The characteristic peaks at 1500 cm⁻¹ and 1402 cm⁻¹ can be assigned to surface bicarbonate species [3]. Raman spectrum of the sample is presented in Fig. 1c. The sample exhibits several bands at 146, 204, 254, 330, 396, 516, 640 and 788 cm⁻¹ that could be assigned to ν₁, ν₆, ν₇, ν₉, ν₁₀, ν₁₄, ν₁₆ and ν₁₇ vibration modes (A_g, B_1g, B_1g, and B_3g), respectively [38], [39]. Thus, the above results provide support to the formation of MgTi₂O₅ with a crystalline orthorhombic phase of Karroite [39].

Fig 1.

(a) Powder XRD pattern, (b) FTIR spectrum, and (c) Raman spectrum of MgTi₂O₅.

The morphological features of the as-synthesized MgTi₂O₅ sample are shown in Fig. 2a-c. At low magnification, the FESEM image of MgTi₂O₅ appears in honeycomb-like structures, consisting of a large number of nanoparticles (Fig. 2a). The higher magnification image shows a clearer image of nanoparticles (Fig. 2b). The TEM image provides additional information on the structure of MgTi₂O₅ (Fig. 2c). The EDS pattern of the MgTi₂O₅ sample is shown in Fig. 2d. Herein, the peaks of Mg, Ti, and O elements can be found. The HRTEM image indicates that a lattice fringe of 0.34 nm could be ascribed to the (1 0 1) plane for the orthorhombic phase of MgTi₂O₅ (Fig. 2e and inset the corresponding FFT image) [10].

Fig 2.

(a-b) FESEM image, (c) TEM image, (d) EDS spectrum and (e) HRTEM image of MgTi₂O₅ nanoparticles (inset the corresponding FFT image).

The chemical states and elemental composition of MgTi₂O₅ nanoparticles were evaluated by XPS analysis (Fig. 3). As shown in the survey spectrum (Fig. 3a), the presence of Mg, Ti, and O is confirmed and the results are in good agreement with EDS results. Further, in this core-level signal at 49.6 eV for Mg 2p as the strong peak is ascribed to Mg 2p_3/2 (Fig. 3b) and the Ti 2p spectrum displayed into two peaks at 458.3 and 464.1 eV for Ti 2P_3/2 and Ti 2P_1/2, respectively (Fig. 3c). Then, the O 1s spectrum can be fitted into three spectral peaks at 529.6, 531.5, and 532.7 eV due to lattice oxygen, adsorbed oxygen, and hydroxyl oxygen, respectively (Fig. 3d) [40], [41]. Besides, surface-coordinated bicarbonate species (hydrocarbon, carbon–oxygen contaminants, and carbonate ion) consist of three peaks at 279.5, 283.4, and 288.7 eV for C 1S present on the MgTi₂O₅ nanoparticles [41], [42].

Fig 3.

XPS spectrum of MgTi₂O₅ nanoparticles (a) survey scan, (b) Mg 2p, (c) Ti 2p, (d) O1s and (e) C1s peaks.

To evaluate the catalytic activity of MgTi₂O₅ nanoparticles for sonocatalytic, photocatalytic, and sonophotocatalytic degradation of triphenylmethane dyes was performed. Here, the aqueous solution of CV dye showed a strong absorption peak at 591 nm. Further, the temporal evolution of CV degradation for sonocatalytic, photocatalytic and sonophotocatalytic processes showed a decrease in that absorption with reaction time (Fig. 4a-c). However, the absorption peak intensities of sonocatalytic process (only US irradiation) indicate that the slight degradation (relatively lower) when compared with photocatalytic and sonophotocatalytic processes (Fig. 4a). Whereas, in the case of photocatalytic process (only UV illumination), a higher degradation was observed when compared with sonocatalytic and lesser degradation when compared with sonophotocataltic processes (Fig. 4b). Whilst, the sonophotocatalytic process (both US + UV) showed complete degradation of CV at 75 min (Fig. 4c). For comparison, the adsorption (equivalently about ~16%) and degradation profiles for different experiments are presented in Fig. 4d. The result of sonophotocatalytic process shows maximum degradation efficiency (~96%) followed by photocatalytic (72%) and sonocatalytic (28%) at 75 min (Fig. 4d-e). Also, the degradation kinetics is shown by plotting -ln(C/C₀) against irradiation time (Fig. 4f). The observed pseudo-first-order degradation rate constants (k) are found to be 0.0402 min⁻¹ (sonophotocatalytic), 0.0142 min⁻¹ (photocatalytic), and 0.00197 min⁻¹ (sonocatalytic).The higher efficiency of sonophotocatalytic process is due to the synergistic effect (more reactive radicals produced) [43].

Fig 4.

Time dependant UV–vis absorption spectra of (a) sonocatalytic, (b) photocatalytic, and (c) sonophotocatalytic degradation of crystal violet using MgTi₂O₅ nanoparticles. (d) Adsorption/degradation profile, (e) degradation curves, and (f) corresponding kinetic plots of crystal violet solution. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

In general, the nanoparticle surface generates positively charged holes (h⁺) under ultrasonic irradiation, which will react with water molecules to produces more ^●H, ^●OH, ^●HO_2, etc. On the other hand, e⁻ might be involved in the generation of O₂^●− radicals by reacting with molecular oxygen (adsorbed O₂). Similarly, the reactive radicals are generated under direct photocatalytic illumination on the MgTi₂O₅ nanoparticle surface. Only the sonocatalytic process showed a lower catalytic activity for CV degradation (k = 0.00197 min⁻¹) due to acoustic cavitation Further, the photocatalytic process alone revealed a moderate catalytic activity (k = 0.0142 min⁻¹) for CV degradation that could be attributed to a reduction in the accessibility of surface sites for the formation of reactive radicals. The combined US irradiation and UV illumination (sonophotocatalytic process) enhanced the formation of reactive radicals and mass transfer between the bulk liquid and dispersion of MgTi₂O₅ nanoparticles leading to a higher degradation of CV (k = 0.0402 min⁻¹). Further, the percentage of synergy was calculated using the rate constants values using Eq. (2).

$$\mathrm{S}\mathrm{y}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{y}(\%)=\frac{{\mathrm{k}}_{\mathrm{s}\mathrm{o}\mathrm{n}\mathrm{o}\mathrm{p}\mathrm{h}\mathrm{o}\mathrm{t}\mathrm{o}\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{t}\mathrm{i}\mathrm{c}}-({\mathrm{k}}_{\mathrm{p}\mathrm{h}\mathrm{o}\mathrm{t}\mathrm{o}\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{t}\mathrm{i}\mathrm{c}}+{\mathrm{k}}_{\mathrm{s}\mathrm{o}\mathrm{n}\mathrm{o}\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{t}\mathrm{i}\mathrm{c}})}{{\mathrm{k}}_{\mathrm{s}\mathrm{o}\mathrm{n}\mathrm{o}\mathrm{p}\mathrm{h}\mathrm{o}\mathrm{t}\mathrm{o}\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{t}\mathrm{i}\mathrm{c}}}\times 100$$ (2)

Therefore, the results indicate that the huge synergistic effect play under sonophotocatalytic process using MgTi₂O₅ nanoparticles for CV dye degradation. The enhancement of synergy during the sonophotocatalytic process was found to be about 60% when compared with the pristine photocatalytic and sonocatalytic process in the present work. In addition, the sonophotocatalytic activity for degradation of triphenylmethane dyes (cationic; BF and anionic; AF) was also studied. Fig. 5a and b show the sonophotocatalytic degradation data of these dyes (BF and AF) using MgTi₂O₅ nanoparticles. After 60 min, these dyes are almost completely removed. Then, the sonophotocatalytic degradation kinetic is shown in Fig. 5c and d. The results exhibit pseudo-first-order kinetic and observed rate constants (k) 0.0475 min⁻¹ and 0.0277 min⁻¹ for BF and AF dyes, respectively.

Fig 5.

Time dependant UV–vis absorption spectra of (a) basic fuchsin and (b) acid fuchsin solutions during the sonophotocatalytic degradation by MgTi₂O₅ nanoparticles. (c) change in concentration with time curves and (d) first-order kinetic plots of corresponding samples.

The results revealed for the cationic dye (BF) under dark adsorption was 13.5%, whereas the anionic dye (AF) showed 60% indicating that MgTi₂O₅ nanoparticles (Fig. 5a and b). The ζ-potential for MgTi₂O₅ nanoparticles was measured as a function of solution pH. Under acidic conditions, the ζ-potential was found to be positive potential. The ζ-potential decreased at alkaline pH (inset in scheme 1). The as-prepared MgTi₂O₅ nanoparticles have a net positive charge on the surface, which led to the strong adsorption of AF. The cationic dyes (CV and BF) were repelled from the surface MgTi₂O₅ nanoparticles. The degradation efficiency for triphenylmethane dyes with different catalysts or processes is compared with these results and summarized in Table 1.

Scheme 1.

The schematic diagram for adsorption and desorption of triphenylmethane dyes with MgTi₂O₅ nanoparticles (inset shows the ζ-potential data).

Table 1.

Degradation efficacy of triphenylmethane dyes with different catalysts.

Catalyst	Irradiation source	Concentration of catalyst	Concentration of dyes	Degradation efficacy	Reference
Nanocrystalline Bi2WO6	20 W (UV-365 nm lamps)	0.05 g L−1	100 mL of CV (50 ppm)	80.1% at 24 h	[44]
Ga2Zr2-xWxO7	82 lm W−1 (visible metal halide lamp)	1 g L−1	4 mg L−1 of CV	Disappears after 360 min	[45]
Mesoporous titania	Two 20 W UV lamps	0.5 g L−1	50 mL of BF (10 ppm)	69.4% at 8 h	[46]
–	Ultrasonic frequency (25 kHz; 500 W)	–	10 μmol L−1 of BF	75% at 60 min	[47]
ZnS nanoparticles	250 W Infrared light lamp	10 mg	20 mL of AF (1 × 10−5 mol L−1)	Disappears after 30 min	[48]
Zn doped SnO2	500 W Hg lamp	0.1 g L−1	4 mg L−1 of AF	96% at 80 min	[49]
CuFe2O4 nanotubes	175 W tungsten lamp	30 mg	50 mL of AF (15 mg L−1)	Disappears after 90 min	[50]
MgTi2O5 nanoparticles	100 W power (38 kHz frequency) and 6 W UV lamp	50 mg	100 mL of CV, BF, AF (5 × 10−5 mol L−1)	Disappears after 75 min in CV and 60 min in BF and AF	This work

To understand, the mechanism of this photocatalytic process was further investigated. The optical absorption of MgTi₂O₅ nanoparticles was studied (Fig. 6a) and the bandgap energy (E_g) was calculated by plotting the values of (αhν)² vs the photon energy (hν) (inset Fig. 6a). The relationship between the absorption coefficient and the reflectance of the sample is calculated by the Tauc approach using Eq. (3).

$$\alpha =\frac{{\mathrm{C}\left(\mathrm{h}\nu -{\mathrm{E}}_{\mathrm{g}}^{\mathrm{b}\mathrm{u}\mathrm{l}\mathrm{k}}\right)}^{\mathrm{n}}}{\mathrm{h}\nu }$$ (3)

where α is the absorption coefficient. C is a constant, hν is the photon energy and E_g^bulk is the bandgap energy of the sample. The value of n is found to be dependent upon the type of transitions involved in the semiconductors (n = 2 for a direct transition and n = ½ for an indirect transition). Based on the above, the estimated value of the bandgap energy (E_g) is found to be 3.35 eV of MgTi₂O₅ nanoparticles for the direct transition. The observed bandgap energy of MgTi₂O₅ nanoparticles is almost close to the theoretical value (3.4 eV) [10]. The conduction band (E_CB) and the valence band (E_VB) positions of MgTi₂O₅ nanoparticles are calculated using Eqs. (4), (5) (E_g = 3.35 eV).

E_VB = χ – E_e + 0.5 E_g (4)

E_CB = E_VB- E_g (5)

Fig 6.

(a) UV–vis absorption spectrum (inset Tauc plot), (b) PL emission spectrum, and (c) Transient absorption vs. time curve of MgTi₂O₅ nanoparticles at different excitation wavelengths.

The absolute electronegativity (χ) of MgTi₂O₅ is calculated as the geometric mean of the absolute electronegativity of constituent atoms Mg (3.75 eV), Ti (3.45 eV), and O (7.54 eV), respectively [51]. E_e is the energy of free electrons on the hydrogen scale (E_e = 4.5 eV). The value of χ is calculated to be 5.68 eV for MgTi₂O₅ and the estimated E_VB and E_CB of about 2.85 eV and −0.49 eV, respectively. The PL spectrum of MgTi₂O₅ nanoparticles (Fig. 6b) exhibits a strong emission band at 358 nm (excitation wavelength of 286 nm) due to the recombination of photo-generated charge carriers (electron (e) and hole (h)). In addition, the emission peaks at 397, 420, 440, 452, 470, 483, 493, and 523 nm are attributed to surface defects and trapped electrons present on the MgTi₂O₅ nanoparticles. Laser flash photolysis data show transient absorption signals on MgTi₂O₅ nanoparticles at different excitation wavelengths (Fig. 6c). The decay of transient absorption signals indicates the internal electron/hole recombination [52], [53]. Further, the E_CB position of MgTi₂O₅ nanoparticles is calculated and found to be more negative than that of O₂/O₂^•− (-0.33 eV vs. NHE) which allows the generation of electrons for photoreduction. The generation of superoxide O₂^•− radical was analyzed with nitroblue tetrazolium as a probe molecule under UV illumination and sonochemical irradiation processes. However, the UV–vis spectrum in Fig. 7a shows a gradual decrease in nitroblue tetrazolium absorption at 259 nm during photocatalytic illumination [54]. Whereas, the pristine sonocatalytic irradiation displays a negligible amount decrease in absorption (259 nm) (Fig. 7b). This illustrates that the superoxide O₂^•− radical has been generated only under UV illumination in the presence of MgTi₂O₅ nanoparticles (however absolutely negligible in the sonochemical irradiation).

Fig 7.

UV-vis absorption spectra of nitroblue tetrazolium solution (a) photocatalytic illumination and (b) sonocatalytic irradiation using MgTi₂O₅ nanoparticles; PL emission spectra of terephthalic acid in alkaline solution under (c) photocatalytic illumination and (d) sonocatalytic irradiation withMgTi₂O₅ nanoparticles.

Then, the E_VB position of MgTi₂O₅ nanoparticles was also calculated and the value is close to that of ^•OH/H₂O (2.8 eV vs. NHE) oxidation potential. Furthermore, the MgTi₂O₅ nanoparticles were tested for ^•OH radical generation by PL studies with terephthalic acid as a probe molecule (photooxidation through-hole) [55]. The PL spectrum shows that the emission peak at 425 nm gradually increases due to the formation of 2-hydroxy terephthalic acid via ^•OH radical oxidation (photocatalytic illumination) (Fig. 7c). Besides, the ultrasonic irradiation was also tested for the production of reactive radicals using terephthalic acid probe molecule (^•OH radical generation) [56]. In this case, the PL results indicate that ^•OH radicals were generated during ultrasonic irradiation using MgTi₂O₅ nanoparticles (relatively one-half of photocatalytic illumination) (Fig. 7d). Hence, under the sonophotocatalytic processes both superoxide O₂^•− radical and ^•OH radical generation takes place which results in enhanced degradation of triphenylmethane dyes (CV, BF, and AF) overMgTi₂O₅ nanoparticles as shown in Eqs. (6), (7), (8), (9), (10), (11). Based on the above discussion, a mechanism for the sonophotocatalytic degradation of triphenylmethane dyes is proposed as depicted in Scheme 2.

MgTi₂O₅ + hν → MgTi₂O₅ (e_CB⁻) + MgTi₂O₅(h_VB⁺) (6)

MgTi₂O₅ + )))) → MgTi₂O₅ (e_CB⁻) + MgTi₂O₅(h_VB⁺) (7)

MgTi₂O₅ (e_CB⁻) + O₂ → MgTi₂O₅ + O₂^●− (8)

MgTi₂O₅(h_VB⁺) + H₂O → ^●OH + H⁺ + MgTi₂O₅ (9)

4H₂O + )))) → 5^●H + 2^●OH+ ^●HO₂ (10)

^●OH+ ^●H + ^●HO₂ + O₂^●−+ Triphenylmethanedyes + hν + )))) → Degradation products (11)

Scheme 2.

Schematic illustration of sonophotocatalytic degradation for triphenylmethane dyes over MgTi₂O₅ nanoparticles.

4. Conclusions

In conclusion, phase pure MgTi₂O₅ catalytic nanoparticles were synthesized by a simple hydrothermal post-annealing method. The morphological features confirm that a large number of MgTi₂O₅ nanoparticles aggregate into honeycomb-like structures.The MgTi₂O₅ nanoparticles are crystalline in-nature containing defects with trapped electrons on the surface. The optical bandgap energy of as-prepared MgTi₂O₅ nanoparticles was found to be 3.35 eV for the direct transition. The sonophotocatalytic activity towards the degradation of crystal violet revealed a strong synergetic effect compared to that of photocatalytic and sonocatalytic processes. In addition, the basic fuchsin and acid fuchsin were completed degraded by the sonophotocatalytic process within 60 min using MgTi₂O₅ nanoparticles as a catalyst. The as-prepared MgTi₂O₅ nanoparticles can be used as a sonophotocatalyst for environmental remediation processes such as waste-water treatment.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.