Simple sonochemical synthesis, characterization of TmVO₄ nanostructure in the presence of Schiff-base ligands and investigation of its potential in the removal of toxic dyes

Abstract

Thulium vanadate (TmVO4) nanorods were successfully prepared by a simple sonochemical approach using Schiff-base ligands. Additionally, TmVO₄ nanorods were employed as a photocatalyst. The most optimal crystal structure and morphology of TmVO4 have been determined and optimized by varying Schiff-base ligands, the molar ratio of H2Salen, the sonication time and power, and the calcination time. A Eriochrome Black T (EBT) analysis revealed that the specific surface area was 24.91 m2/g. A bandgap of 2.3 eV was determined by diffuse reflectance spectroscopy (DRS) spectroscopy, which makes this compound suitable for visible photocatalytic applications. In order to assess the photocatalytic performance under visible light, two anionic dyes (EBT) and cationic dyes (Methyl Violet (MV)) were used as models. A variety of factors have been studied in order to improve the efficiency of the photocatalytic reaction, including dye type, pH, dye concentration, and catalyst loading. Under visible light, the highest efficiency was achieved (97.7%) when 45 mg TmVO4 nanocatalysts were present in 10 ppm Eriochorome Black T at pH = 10.

1. Introduction

As a result of population growth, increasing urbanization, rapid industrialization, overuse of natural resources, and widespread use of unsustainable resources, the atmosphere suffers irreversible, detrimental, and severe damage. Various industries and human activities produce wastewater and effluents that endanger our natural resources [1], [2]. Every day, millions of gallons of sewage are released by industries such as paper, textiles, chemicals, pulp, pesticides, fertilizers, metal plating, food processing, batteries, pharmaceutical industries, and refineries, contaminating the terrestrial ecosystem and water bodies around us. A variety of contaminants can be found in sewage, including organic (pesticides, dyes, fertilizers, surfactants, pharmaceutical ingredients, phenols, organohalides, etc.), inorganic (metal oxides, heavy metal ions, salts, metal complexes, etc.), pathogens, agricultural runoff, and nutrients [3], [4]. In recent years, organic pollutants have received extensive attention because of their widespread use and consequent depletion of terrestrial lands and water bodies, their ability to persist for an extended period of time, and their significant effects on the environment and human health [5], [6]. Persistent organic pollutants (POPs) are very resistant to decomposition and degradation, and therefore, they may have detrimental effects on the health of humans and other living organisms [7]. It has been hypothesized that POPs may cause cancer, genetic disabilities, immune and reproductive dysfunction, which will adversely affect the growth of children and infants [8]. Among the most significant categories of organic contaminants are pesticides, dyes, phenolic substances, pharmaceutical ingredients, plasticizers, hydrocarbons, fertilizers, etc. [9], [10]. The use of dyes as coloring agents is prevalent in a wide variety of industries, including plastics, textiles, paper, leather, carpets, food, cosmetics, and printing [11]. The textile industry alone consumes about 10,000 tons of dyes each year, of which approximately 10–15% are released as effluents [12]. Consequently, the untreated discharge of effluents from these enterprises into rivers, lakes, and seas is the primary cause of water pollution. As well as being unsightly, highly colored trash blocks light from entering water bodies, thereby disrupting the environmental balance [13].Scheme 1.

Scheme 1.

Schematic diagram of the mechanism for the photodegradation of TmVO₄ nanostructures over EBT and MV.

In the field of water treatment and environmental decontamination, semiconductor-based photocatalysis is one of the most popular green methods. This manufacturing technique relies on exposing an appropriate semiconductor - usually some transition metal oxides or sulfides - to UV or visible photons of sufficient intensity to form electron and hole pairs (e^-/h⁺) in the semiconductor's conduction and valence bands, respectively. The potent and non-selective superoxide and hydroxyl radicals can be produced by the photogenerated electrons and holes reacting with the dissolved oxygen and water molecules (or OH anions), respectively. The organic contaminants present in media can be attacked by these radicals and the photogenerated e^-/h⁺ pairs, which can then fragment them into smaller pieces and mineralize them into water and carbon dioxide [14], [15], [16].

Among the most significant azo dyes, Eriochrome Black T (EBT), is used to dye silk, wool, nylon, and multifiber materials and as a complexometric titrant for the determination of Ca²⁺, Mg²⁺, and Zn²⁺ in laboratories. Even the intermediate component of the extremely harmful dye EBT, naphthoquinone, has a higher level of carcinogenicity. There has been only a limited amount of research conducted in the literature on the effective decolorization of this dye due to an important concern for the effective treatment of wastewater, including EBT and other organic pollutants [17]. Methyl Violet (MV) or pentamethylpararosaniline chloride (C₂₄H₂₈N₃Cl) is one of the most common triphenylmethane dyes. In addition to being highly soluble in water, this cationic dye is also highly resistant to photolysis caused by sunlight, oxidizing agents, and aerobic attacks. It is commercially available as a mixture containing tetramethyl- and hexamethylpararosanilines and is primarily used as a source of intense violet color in copying papers and printing inks, as well as a source of deep colors suitable for dyeing cotton, silk, paper, bamboo, weed, straw, and leather [18]. The use of MV is also common in chemical laboratories as a pH indicator, in microbiological laboratories as an antiallergenic and bactericidal agent, and in analysis of Gram's stain for primary bacterial classification and Flemming triple stain with iodine [19].

One of the most significant compounds is vanadate, which is used in a variety of applications, including implantable cardiac defibrillators (ICDs) [20], optical devices [21], catalysts [22], [23], and cathodes in batteries [24], [25]. A vanadium atom can exist in four adjacent oxidation states as a result of its chemistry. A variety of methods have been employed to prepare thulium vanadate (TmVO₄), a type of rare earth (RE) orthovanadate [26], [27]. TmVO₄ powders are well-known for their electrical, photocatalytic, catalytic, photoluminescence, laser, and optic properties [28], [29], [30]. The study of TmVO₄ preparation is crucial because TmVO₄ is a novel photocatalyst with high catalytic activity in the visible light region [31]. In this regard, we have reported the production of TmVO₄ nanoparticles by sonochemical means using Schiff-base ligands H₂Salen and H₂Salophen as capping and precipitating agents for the control of particle size and shape. Having successfully fabricated TmVO₄ nanoparticles and evaluated their optical properties, the photocatalytic behavior of TmVO₄ was examined by degrading toxic dyes under visible light. A variety of parameters including pH, dye types, dye concentrations, and catalyst dosage were studied for their effect on photocatalytic performance.

2. Materials and methods

2.1. Materials

Ammonium metavanadate (NH₄VO₃), Thulium Nitrate Hexahydrate (Tm(NO₃)₃·6H₂O), N, N'-Bis(salicylidene)ethylenediamine (H₂Salen), N, N′-bis(salicylidene)-1,2-phenylenediamine (H₂Salophen), and methanol, were acquired from Merck company and applied without further purification. In addition, the organic dyes utilized as pollutants, such as eriochrome black T (EBT), methyl violet (MV), benzoic acid (BA), benzoquinone (BQ), and Ethylenediaminetetraacetic acid (EDTA) were acquired from Sigma-Aldrich.

2.2. Fabrication of TmVO₄

250 mg (0.7 mmol) Tm(NO₃)₃, 82 mg (0.7 mmol) NH₄VO₃, were separately dissolved in 10 mL of distilled water. 188 mg H₂Salen (0.7 mmol) (or 190 mg H₂Salophen) was dissolved in 20 mL of methanol. H₂Salen (or H₂Salophen) solution was added to Tm(NO₃)₃ solution under stirring. The homogenized solution was placed in an ultrasound device (MPI Ultrasonics; welding, 1000 W, 20 KHz, Switzerland) under ultrasound waves with a power of 60 W for 3 min. The solution containing NH₄VO₃ was added drop-wise to the solution under ultrasonic waves for 30 min until the reaction was complete. After the addition of the solution containing NH₄VO₃, the color of the solution changed from pale yellow to dark green. After completion of the reaction, the precipitate was centrifuged and washed several times with distilled water and then dried in an oven at 70° C for 12 h. Finally, the powder was calcined at 550 ℃ for 6 h. Different conditions were changed to obtain the desired sample (Table 1).

Table 1.

Different conditions for preparation of TmVO_4.

Sample No.	Types of Ligands	Molar ratio Tm:V:Ligand	Method	Sonication Time (min)	Sonication Power (W)	Calcination Time (h)
1	H2Salen	1:1:1	Ultrasound	30	60	6
2	H2Salophen	1:1:1	Ultrasound	30	60	6
3	H2Salen	1:1:0.5	Ultrasound	30	60	6
4	H2Salen	1:1:2	Ultrasound	30	60	6
5	H2Salen	1:1:2	Ultrasound	15	60	6
6	H2Salen	1:1:2	Ultrasound	45	60	6
7	H2Salen	1:1:2	Ultrasound	30	80	6
8	H2Salen	1:1:2	Ultrasound	30	40	6
9	H2Salen	1:1:2	Ultrasound	30	60	7
10	H2Salen	1:1:2	Ultrasound	30	60	8
11	H2Salen	1:1:2	Ultrasound	30	60	9
12	H2Salen	1:1:2	Ultrasound	30	60	5

2.3. Photocatalytic process

The catalytic behavior of the TmVO₄ nano photocatalyst was assessed by employing two different dyes as toxic contaminants under visible rays. 50 mg of TmVO₄ and 15 ppm of dye solution (methyl violet and eriochrome black T) were mixed and moved to a quartz beaker. The suspension was irradiated by visible light (Osram light, Irradiation power; 150 W, Light intensity: 4,000 lm, Volts: 120 V, Luminous flux: 2,400 lm, Wavelength: 400–700 nm) after reaching the adsorption equilibrium of toxic contaminant molecules on the TmVO₄ surface. The light source and quartz vessel were housed in an opaque box with a fan to bypass radiation leakage. Influential factors, including dye types, catalyst content, dye concentration, and pH dye adjustment, were assessed for improving the photodegradation ability of TmVO₄. The absorbance of samples was measured using a UV–Vis spectrometer at specific time intervals. The dye's degradation percentage (D.P.) was calculated using equation (1). The initial and t-time concentrations of the dye solution should be monitored in this equation.

$$D.P.=\frac{(C_0-C_t)}{C_0}\times 100$$ (1)

3. Results and discussion

3.1. Crystal structure

XRD patterns for TmVO₄ prepared with H₂Salen and H2Salophene as ligands are shown in Fig. 1a and 1b. These patterns are consistent with the characteristic peaks of TmVO₄ with reference code 082–1975 and a tetragonal structure. With a molar ratio of 1:1, H₂Salen and H₂Salophene can produce pure TmVO₄ when calcined at 600 °C. Fig. 1c and 1d illustrate the effect of different molar ratios of H2Salen to V and Tm. As can be seen in the figure, TmVO4 with the JCPDS card number 082–1975 and tetragonal structure forms when the molar ratios are 1:1:0.5 and 1:1:2. Figs. Fig. 2a and 2b demonstrate how sonication power affects the formation of TmVO₄. There is a good match between the patterns and the reference code 082–1975. Further, calcination time had no effect on the crystallinity degree of TmVO₄ (Fig. 2c and 2d).

Fig. 1.

XRD patterns of TmVO₄ preprared in the presence of different ligands a) H₂Salen, b) H₂Salophene, of different molar ratios of Tm:V: ligands c) 1:1:0.5, and d) 1:1:2 with sonication power and time 60 W and 30 min at 550℃ for 6 h.

Fig. 2.

XRD patterns of TmVO₄ preprared with different sonication powers a) 80 W, b) 40 W for 30 min sonication time, different calcination times c) 7 h, and d) 5 h.

3.2. Mechanism of formation

In general, water is pyrolyzed into H• and OH• radicals during ultrasonic processing. The sonochemical method relies on these highly active radicals. According to the mechanism proposed in this work, VO₃^- may diminish in the presence of H₂O₂ to form VO₄^3-. During this sequence of reactions, each radical OH• or H• is capable of generating H₂O₂. OH• and H• radicals can interact in a variety of ways, as shown in the following examples:

$$H_{2}O\underrightarrow{\mathit{ultrasonic}irradiayion}H·+OH·$$ (2)

$$H·+H·\to H_2$$ (3)

$$\mathit{OH}·+OH·\to H_2{O}_2$$ (4)

$$H·+O_2\to HO{·}_2$$ (5)

$$H·+HO{·}_2\to H_2{O}_2$$ (6)

$$\mathit{HO}{·}_2+HO{·}_2\to H_2{O}_2+O_2$$ (7)

In the reaction between OH• and H• radicals, HO•₂ radicals can be formed. In addition, two HO•₂ radicals may react to produce H₂O₂. It is also possible to generate hydrogen peroxide by reacting two OH• radicals with ten OH• radicals in the opposite reaction equation. In spite of this, both OH radicals and H radicals appear to have an influence on the production process [32]. It is possible for the first precursors to dissolve into ions that produce water. In the proposed study, the initial precursors are ionic and non-volatile. Thus, the majority of the solution and the cavitation bubbles undergo a reaction. Due to their low vapor pressure, ionic precursors are unable to reach the inner zone of cavitation bubbles. By using the following sonication technique, the ions listed above may react under the appropriate circumstances to form nanosized TmVO₄. In Equation (5), VO₃ ions can be reduced to VO4^3- ions using an H₂O₂ solution during sonication. At the end of this process, Tm³⁺ interacts with VO4^3- ions to form TmVO₄ nanoparticles [33]. According to the reaction shown below, it appears that hydrogen peroxide and the end product react directly, and the amount of TmVO₄ nanostructures and hydrogen peroxide are positively correlated. Consequently, increasing the production of H₂O₂ increases the nucleation rate. Rapid nucleation produces small particles, resulting in a large number of nuclei, and shortening the crystallization process. Ultrasonic techniques can assist in the rapid production of hydrogen peroxide, resulting in a smaller size and shorter time for the development of products [34].

$$N{H}_{4}V{O}_3+H_{2}O\to N{H}_4++V{O}_3^{-}+H^{+}+O{H}^{-}$$ (8)

$$V{O}_3^{-}+H_2{O}_2\underrightarrow{\mathit{ultrasonic}irradiayion}V{O}_4+H_{2}O$$ (9)

$$\mathit{Tm}{\left(N{O}_3\right)}_3.6H_{2}O\to T{m}^{3+}+3N{O}_3+H_{2}O$$ (10)

$$T{m}^{3+}+V{O}_4\to TmV{O}_4$$ (11)

$$N{O}_3^{-}+N{H}_4^{+}\to N{H}_{4}N{O}_3$$ (12)

$$N{H}_{4}V{O}_3+Tm{\left(N{O}_3\right)}_3+H_2{O}_2\to TmV{O}_4+N{H}_{4}N{O}_3+2N{O}_3^{-}+O{H}^{-}+H^{+}$$ (13)

Furthermore, numerous bubbles formed during the sonochemical process, which led to the pulse waves popping and obliterating. The surfaces of particles and nanoparticles created by the release of bubble-defeating energy may be affected by these phenomena. In response to particle collisions, morphology, scale, and structure have all been significantly altered.

3.3. Bond structure

We present in Fig. 3a and 3b the FT-IR spectra of TmVO₄ before and after calcination to study the bonding structures and chemical interactions. Fig. In Fig. 3a, TmVO₄ is shown before it has been calcined. As can be seen in Fig. 3a, different peaks can be seen in the TmVO₄ spectrum that indicate the presence of H₂Salen on the surface of the TmVO₄ nanostructures. In this spectrum, it is evident that all characteristic peaks of H₂Salen are present. The stretching vibration of O^-H groups of adsorbed water can be attributed to the absorption at 3425 cm⁻¹ [35]. The intense peak at 1637 cm⁻¹ was caused by C = N, and the peak that was placed at 2925 cm⁻¹ belongs to CH₂ in the H₂Salen ligand structure. The absorption bands at 1210 and 1287 cm⁻¹ are assigned to C^-O stretching modes. V^-O band and VO₄ stretching vibration are detected at 847 cm⁻¹ and 454 cm⁻¹, respectively [36]. The band at 1384 cm⁻¹ belongs to C^-H bending vibration [37]. A FTIR spectrum of TmVO₄ after calcination can be seen in Fig. 3b, which shows that the peaks associated with the compound H₂Salen have disappeared. In Fig. 3c, the results of EDS analyses of TmVO₄ nanostructures are presented in order to identify the elemental composition and chemical purity. Tm, V, and O lines can be seen clearly in Fig. 3c without any impurities.

Fig. 3.

FTIR spectra a) before, b) after calcination, and c) EDS spectrum of TmVO₄ prepared at 30 min, 60 W sonication and 7 h calcination time (Sample 9).

3.4. Morphology studies

FESEM microscopy was used to determine the particle diameter as well as the morphology of TmVO₄ structures (Fig. 4). After thermal treatment, the morphology, uniformity, size, and shape of the samples are influenced by a number of factors, including ligand type, precursor molar ratio, sonication power and duration, and calcination time.

Fig. 4.

FESEM micrographs of TmVO₄ prepared in different conditions a) H₂Salen, b) H₂Salophene ligands, c)1:1:0.5, d) 1:1:2 Tm:V:H₂Salen molar ratios, e) 15 min, and f) 45 min sonication times.

Fig. 4a and 4b illustrate the surface morphology of TmVO₄ in the presence of H₂Salen and H₂Salophene. In these micrographs, it can be observed that Nanoparticles produced in the presence of H₂Salen are more uniform and smaller in size than those produced in the presence of H₂Salophene. Among the most important parameters that can affect the morphology is the molar ratio of the ligand to the metal. Figs. As shown in Fig. 4a, 4c, and 4d, Tm:V:H₂Salen has several molar ratios, such as 1:1:1, 1:1:0.5, and 1:1:2. The figures demonstrate that homogenous nanoparticles are formed in the presence of all three molar ratios. TmVO₄ particles in the presence of a 1:1:2 M ratio are smaller than two other particles. Therefore, a 1:1:2 M ratio was selected as the desired ratio. Schiff-base ligands, such as H₂Salen, can be bound to the crystal surfaces of TmVO₄ and inhibit crystal growth, resulting in a structure of tiny particles composed of TmVO₄. Due to the steric hindrance effect, increasing the concentration of H2Salen ligand by a factor of two results in increased particle sizes, whereas lower concentrations of ligand (1:1 and 1:0.5) result in smaller particle sizes. These phenomena are caused by unfinished ligand coverings on crystal surfaces, which cause particles to enlarge into large structures. Additionally, the time of sonication can have a significant impact on the shape and morphology of products. The figures in 4d, 4e, and 4f illustrate the morphology of TmVO₄ prepared after 30 min, 15 min, and 45 min of sonication at constant power. In the presence of ultrasound, a certain threshold of time (30 min) causes the destruction of large particles and the formation of nanoparticles. When the ultrasound time is increased from 30 min to 45 min, the particle size grows due to an increase in the reaction temperature. FESEM images of TmVO₄ are shown in Fig. 4d, 5a, and 5b at various sonication powers (60 W, 80 W, and 40 W) and for a constant period of time (30 min). In Fig. 4d and 5a the trend of increasing particle size is clearly visible when the sonication power is increased from 60 W to 80 W. Micro-sized particles are formed due to the increasing reaction temperature that leads to particle agglomeration. In contrast, reducing the ultrasound power can also result in bulk structures. Specifically, increasing the power from 40 W (Fig. 5b) to 60 W (Fig. 4d) results in a reduction in particle diameter and an increase in uniformity. An extreme situation occurs when a cavitation bubble collapses, inducing both chemical and physical reactions. As a result of the implosive collapse of bubbles, which spreads out into the liquid medium, a shock wave is created. The shock wave can accelerate solid particles that are suspended in liquid. A particle's morphology, particle size distribution, and surface composition can be altered by collisions between particles, which may occur at speeds exceeding hundreds of meters per second. It is therefore evident that particles are fragmented. By varying the sonication power, nanostructures can be altered as far as their homogeneity and size are concerned. Consequently, the optimum conditions were obtained at 60 W, 30 min of sonication, and a 1:2 M ratio of metal to ligand at 600 °C for 6 h. Likewise, controlling the morphology and particle size of a product depends on the calcination time. In Fig. 5c, 5d, and 5e, the morphology of TmVO₄ at a variety of calcination temperatures (7 h, 8 h, 9 h, and 5 h) can be seen. An increase in calcination time from 6 h to 7 h results in very uniform nanorods (Fig. 5c). As calcination time is increased to 8 h and 9 h, the diameter of the nanorods increases. Consequently, increasing the time to 9 h resulted in the formation of microrods adjacent to the nanoparticles (Fig. 5e). Fig. 5f illustrates how sphere-like nanoparticles are produced by decreasing the calcination time to 5 h. Using comprehensive preparation status control, the most efficient parameters for the fabrication of TmVO₄ were determined. In view of its size and uniformity for photocatalytic application, sample No. 9 was determined to be the most efficient sample. It is evident from these figures that when ultrasonic irradiation is used, particle structure is unified. Ultrasonic irradiation causes high pressure and temperature in a solution through the formation of bubbles and the collapse of bubbles. As a result of cavitation surrounding solid particles, the bubble degradation symmetry is altered when the cavitation occurs around the particle. Conversely, the collapse of a bubble that occurs far from the particle causes extremely turbulent flow in the solution as a result of cavitation [38], [39]. Thus, the different structures of nanoparticles are created by ultrasonic irradiation under various conditions due to cavitation. TEM micrographs were taken (Fig. 6) to determine the surface morphology of the best sample (sample 9). As can be seen in the photographs taken with the TEM, nanorods are clearly visible.

Fig. 5.

FESEM micrographs of TmVO₄ prepared in different conditions a) 80 W, b) 40 W sonication powers, different calcination times c)7h, d) 8 h, e) 9 h, and f) 5 h.

Fig. 6.

TEM micrographs of TmVO₄ prepared at 30 min, 60 W sonication and 7 h calcination time (sample 9).

3.5. BET isotherm

Using the BET method, a standard instrumentation for measuring nitrogen adsorption isotherms at 77 K, the surface area and pore volume of samples can be determined. In Fig. 7a and 7b, the adsorption–desorption (ADS-DES) isotherm and BJH curve of TmVO₄ can be seen. In accordance with IUPAC categories, the N₂ ADS-DES isotherm belongs to type IV isotherms with H₃ hysteresis loops, which are regarded as being characteristic of ordered mesoporous materials. The pore size distribution for TmVO₄ can be seen in Fig. 7b, which shows a broad distribution with a maximum in the vicinity of pores of 29 nm diameter. This sample has a mean pore diameter of 30.73 nm and a total pore volume of 0.1914 cm³/g, respectively. In addition, the BET analysis indicates a specific surface area of 24.91 m²/g.

Fig. 7.

A) n₂ absorption–desorption, b) BJH plot of TmVO₄ prepared at 30 min, 60 W sonication and 7 h calcination time (sample 9), (c, e) UV–Vis DRS spectrum, (d, f) the plot of bandgap energies of samples 4 and 9, and g) VSM plot of TmVO₄ prepared at 30 min, 60 W sonication and 7 h calcination time (sample 9).

3.6. Optical absorption and magnetic properties

The electronic excitation from the VB to the CB is usually related to the optical absorption, or absorption edge. Therefore, the DRS data can be used to estimate the optical bandgap energy of the subject sample. It is most effective to achieve this objective by using the Kubelka-Maunk (K-M) equation and the Tauc graphs that result from it. In a typical K-M equation, the absolute R-values (in %) are converted into K-values, a measure of the transformed R: [40].

$$K={\left(1-R\right)}^2/2R$$ (14)

Using the Tauc relation, it is possible to calculate the samples' energy band gaps:

$$\left(\alpha hv\right)=\beta {\left(\mathit{hv}-Eg\right)}^n$$ (15)

In Eq. (15), ʋ and h (6.626 × 10⁻³⁴ J s) represent the light frequency and Planck constant, respectively. α is the absorption coefficient which depends on the sample thickness (d) and the sample absorbance (A), α= ([2.303 × A]/d). β is the sample absorbance. Depending on the type of electronic transition, the power “n” will vary. In this way, it obtains values of 2 and 1/2 for the permitted indirect and direct electronic transitions, as well as 3 and 3/2 for the forbidden indirect and direct transitions [41]. Based on the diffuse reflectance spectrum (DRS) of samples No. 4 and No. 9, the bandgap energy was calculated. Both samples (Samples No. 4 and 9) have an estimated bandgap of 2.3 eV as the best-prepared samples. A bandgap may be used to determine what kind of light source is required to destroy dyes.

A vibrating sample magnetometer (VSM) is used to characterize the magnetization hysteresis of TmVO₄ (sample No. 9) at room temperature (25 °C) (Fig. 7g). During the ideal conditions, the hysteresis loop of the sample exhibits paramagnetic behavior, with a maximum saturation magnetization of 1.417 emu/g.

3.7. Photoactivity of catalyst

3.7.1. Dye effect

The Fig. 8a illustrates the degradation of Eriochrome Black T (EBT) and Methyl Violet (MV) in the presence of TmVO₄ nanorods. Using TmVO₄ nanorods can induce 50.6% of decoloration of EBT as an anionic dye and 46.6% of decoloration of MV as a cationic dye after 90 min, respectively. As an excellent oxidizing agent, TmVO₄ has the capacity to oxidize the anionic dye, Eriochrome Black T, more efficiently and effectively than the cationic dye, methyl violet. Since EBT has a negative charge, it is more susceptible to oxidation by TmVO₄ than MV, which has a positive charge. Moreover, the anionic dye has a larger surface area than the cationic dye, which makes it easier for TmVO₄ to react with and degrade EBT.

Fig. 8.

Photocatalytic degradation of TmVO₄ prepared at 30 min, 60 W sonication and 7 h calcination time (sample 9) in the presence of a) Eriochrome black T and methyl violet, b) effect of different pH of the solution, c) different dye concentrations, d) different dosages of catalyst, e) scavenger effect, and f) recyclability under visible radiation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

3.7.2. pH effect

In photodegradation efficiency, pH plays a significant role since it influences both the charged species of the pollutant and the accumulated charges on the catalyst surface [42], [43], [44]. Photocatalytic efficiencies were investigated in relation to pH of the EBT solutions (Fig. 8b). EBT degradation efficiencies range from 65.6% at pH 10, 7.4% at pH 4, and 38.78% at pH 4. In the photocatalytic process, the pH of the initial solution plays an important role in the uptake of the adsorption capacity. Diprotic dye EBT uptake is most effective at pH = 10, which is the optimal condition for its removal. As a result of monitoring the photocatalytic action between pH 4 and 10, TmVO₄ nanorods performed best in alkaline media, while their photocatalytic activity increased at pH 10. We are concerned with two issues in alkaline media: increasing the density of the •OH radical and acquiring a more effective position on the surface of TmVO₄. As pH is raised, a greater concentration of •OH functions is produced. In consequence, the generation of •OH radicals in solution increased, and toxic dye was eliminated more effectively. Upon interaction of two hydroxyl radicals, the H₂O₂ and HO₂ radicals were produced. When pH levels are higher, radical reactions occur due to the presence of a large number of •OH radicals. Therefore, a rise in pH is associated with an increase in the efficiency of photocatalytic degradation of toxic dyes. Accordingly, it is possible to infer that pH and decolorization efficacy are directly related.

3.7.3. Dye concentration effect

The photocatalytic activity of TmVO₄ nanorods was subsequently assessed over a range of EBT concentrations. According to Fig. According to Fig. 8c, TmVO₄ nanorods exhibit approximately 47.1%, 65.6%, and 57.5% activity when EBT is present at 5 ppm, 10 ppm, and 15 ppm under visible light, respectively. A 15 ppm concentration of EBT results in a greater number of EBT molecules saturating binding sites on the surface of TmVO₄ nanorods. The removal efficiency percentage decreased as the initial dye concentration increased. Conversely, when the dye concentration is less than a certain level, the number of molecules is too small to be placed on the surface of the catalyst in order to eliminate it.

3.7.4. Catalyst loading effect

An example of a photocatalytic test for verifying the effect of catalyst loading can be found in Fig. 8d. With an increase in catalyst dosage from 15 mg to 30 mg, the photocatalytic efficiency increased from 40.5% to 65.6%. Furthermore, the photocatalytic efficiency was improved from 65.6% to 97.7% by increasing the TmVO₄ content from 30 mg to 45 mg. When the TmVO₄ loading was increased to 60 mg, the efficiency was decreased to 81.8%%.

Increasing the TmVO₄ content led to an increase in TmVO₄ surface area and enhanced dye absorption on TmVO₄ surfaces. Nevertheless, the increase in catalyst content is only practical to a certain extent (45 mg), and then the nanorod coatings were saturated. The photocatalytic efficiency decreases as TmVO₄ nanorods in dye solutions become thicker since they are unable to reach the solution for irradiation. Consequently, loading 45 mg of TmVO₄ is the ideal condition for Eriochorme black T degradation..

3.7.5. Comparison study

In Table 2, a comparison of vanadate photocatalysts is provided. It has been demonstrated that nanostructures of TmVO₄ are capable of degrading organic dyes with a suitable level of photocatalytic activity.

Table 2.

Photocatalytic performance comparison of different vanadate photocatalysts.

Photocatalyst	Method	Targeted Pollutants	Light Source	Reaction Time (min)	Degradation (%)	Ref.
GdVO4/silica fiber	Alcohol -thermal treatment	MB	UV	90	64.5	[45]
CeVO4/silica fiber	Alcohol -thermal treatment	MB	UV	90	48.5	[45]
NdVO4/silica fiber	Alcohol -thermal treatment	MB	UV	90	32.3	[45]
CeVO4	Hydrothermal	RhB	UV	80	65.0	[46]
HoVO4	Sonochemical	MV	UV	90	67.6	[47]
YbVO4	Sonochemical	MB	Visible	120	65.0	[34]
PrVO4	Sonochemical	Erythrosine	UV	90	50. 7	[48]
SmVO4	Sol-gel	carbamazepine	Solar simulator	300	26.6	[49]
NdVO4	Sonochemical	EBT	UV	75	77.4	[36]
TmVO4	Sonochemical	Phenol red	Visible	90	71.7	This work

3.7.6. Scavenger test

A combination of three scavengers, including BA, EDTA, and BQ, were employed to capture, respectively, •OH, h⁺, and •O₂^–. A comparison of the degradation rates of BA and EDTA to those of no scavenger is shown in Fig. 8e; these degradation rates were approximately 85.3%, 88.1%, and 97.7%, respectively. Therefore, while •OH and h⁺ have little effect on the deterioration of TmVO₄ nanostructures, •O₂^– has a significant effect.

3.7.7. Recyclability test

TmVO₄ nanocatalyst was tested for repeatability in an EBT photodegradation recycling test. After the initial visible exposure, TmVO₄ nanorods were typically separated and washed with distilled water. We began the second run of the test by combining the new dye solution with the previously used nanocatalysts. In a similar manner to the initial run, the following four runs of photocatalytic recycling were conducted. As shown in Fig. 8f, the adsorption abilities decreased slightly from 97.7% to 86.6% after five repetitions of the photoactivity test. It is evident from the results that TmVO₄ is highly stable. Fig. 9 depicts the XRD pattern of TmVO₄ after the recycling test. The diffraction peaks are all attributed to TmVO₄ (JCPDS No.082–1975, tetragonal structure). The pattern illustrates the high stability of the TmVO₄ structure after five cycles.

Fig. 9.

XRD pattern of TmVO4 after recycling test.

3.7.8. Mechanism of photodegradation

On the basis of molecular excitation processes, reasonable mechanisms for dye photodegradation can be proposed. TmVO4 nanorods are excited by light, which causes electrons and holes to form, which can then react with water and oxygen in the environment to create a series of active sites, including •O2 − and •OH radials (Eq. (14) ^- (16)). As a result of the electron combining with an oxygen molecule, it produces superoxide radicals, and as a result of the hole oxidizing water, it produces hydroxyl radicals [43], [44], [45], [46]. As stated in Eqs. (17), (18), (19), (20), (21), these radicals are necessary for the oxidation of pollutants.

$$\mathit{TmV}{O}_4\underrightarrow{\mathit{hv}}TmV{O}_4\left(h_{\mathit{VB}}^{+}+e_{\mathit{CB}}^{-}\right)$$ (16)

$$h_{\mathit{VB}}^{+}+H_{2}O\to H^{+}+·OH$$ (17)

$$e_{\mathit{CB}}^{-}+O_2\to ·O_2^{-}$$ (18)

$$e_{\mathit{CB}}^{-}+H^{+}+·O_2^{-}\to H_2{O}_2$$ (19)

$$2H^{+}+2·O_2^{-}\to H_2{O}_2+O_2$$ (20)

$$e_{\mathit{CB}}^{-}+H_2{O}_2\to ·OH+O{H}^{-}$$ (21)

$$\mathit{EBT}+\left(\mathit{OH}·or·O_2^{-}\right)\to Decolorizationproducts+C{O}_2+H_{2}O$$ (22)

3.7.9. Kinetic of the process

In order to evaluate the kinetics of typical heterogeneous photocatalytic processes, the Langmuir-Hinshelwood (L-H) model, which is based on the monolayer adsorption of both reactants and products at the solid–liquid interface, has been widely used. A photodegradation process is based on equilibrium adsorption in which the degree of oxidant and reductant adsorption has a significant impact on the contaminant's and its degradation intermediates' photodegradation rates. It should also be noted that the adsorption of these species on the surface of the catalyst is the determining factor in the entire photodegradation procedure [47]. Eq.(3) represents the L-H model, in which r is the reaction rate of oxidation of the reactant/pollutant in mg/L min, k is the specific rate constant of the reaction in mg/L min, K is the equilibrium constant of the reactant in L/mg, and C is the concentration of the reactant/pollutant.

$$r=\left(dC/dt\right)=\left(\mathit{kKC}\right)/\left(1+KC\right)$$ (23)

The Eq.(23), which is commonly used by researchers, can be simplified to its integrated version (Eqs. (24) and 25). Because the catalyst is one of the reactants located in the solid phase and its concentration is constant, this equation is well known for its apparent first-order equation, which is frequently used to describe the kinetics of heterogeneous photodegradation. Here, K' is the apparent first-order rate constant.

$$\ln \left(C_0/C\right)-k\left(C_0-C\right)=kKt$$ (24)

$$\ln \left(C_0/C\right)=kKt=K^{\prime }t$$ (25)

In Fig. 10, Hinshelwood plots are shown under a variety of photocatalytic reaction conditions. Therefore, a higher reaction rate constant (k = 0.0259 min⁻¹) was required to obtain superior photocatalytic performance.

Fig. 10.

The resulted Hinshelwood plots of TmVO₄ to study the kinetic of the EBT photodegradation process: a) effect of different pH of the solution, b) different dye concentrations, and c) different dosages of catalyst.

4. Conclusions

The purpose of this study was to evaluate promising nano-photocatalysts that were homogeneous and small in size. Thus, H₂Salen was used as a capping agent during the reaction in order to prevent product agglomeration. A number of factors have been altered in order to achieve the desired size and uniformity of the TmVO₄ nanorods as they were made. This includes types of Schiff-base ligands, the molar ratio of H₂Salen to metal, sonication power and time, and calcination time. The nanorod morphology of TmVO₄ was obtained using a 2:1 ratio of H₂Salen to Tm using a sonication power of 60 W and a calcination time of 30 min at 600 °C. In accordance with BET data, the specific surface area of the sample was 24.91 m²/g. The bandgap of TmVO₄ was calculated by DRS spectroscopy to be 2.3 eV, making it a suitable photocatalyst for visible applications. Various factors, including dye types, pH, TmVO4 loading, and dye concentration, were examined to determine how they affected TmVO4 nanorod photocatalysis. The 45 mg of TmVO₄ demonstrates 97.7% photocatalytic yield over the degradation of 10 ppm EBT at pH = 10 under visible light, suggesting it could be a promising catalyst for improving the catalytic capability for the removal of pollutants. In accordance with our findings, we suggest that TmVO₄ nanorods could be used as nano-photocatalysts for dye degradation from effluents. Due to this, we believe TmVO₄ nanorods can be utilized as an excellent catalyst in a variety of industries.

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.